Bandgap engineered quantum dot heterostructures with post synthesis colloidal atomic layer deposition

Abstract

A method, comprising: disposing a plurality of layers on a plurality of nanocrystals (NCs) disposed on a substrate, wherein a given layer comprises a matrix material that includes a compound comprising an anionic species associated with a cationic species, the plurality of NCs optionally being core-shell NCs or quantum dots, and wherein the method is performed so as to give rise to interparticle fusion between at least some of the plurality of NCs and to completely fill interstitial gaps between NCs. Also provided is a composition, comprising: a plurality of NCs exhibiting interparticle fusion therebetween and the plurality of NCs defining interstitial spaces therebetween, the interstitial spaces being completely filled with a matrix material that comprises a plurality of layers, a given layer including an anionic species associated with a cationic species, and the plurality of NCs optionally being core-shell NCs or quantum dots.

Description

BANDGAP ENGINEERED QUANTUM DOT HETEROSTRUCTURES WITHPOST SYNTHESIS COLLOIDAL ATOMIC LAYER DEPOSITIONRELATED APPLICATIONS

[0001] The present application claims priority to and the benefit of United States patent application no. 63 / 688,952, “Bandgap Engineered Quantum Dot Heterostructures With Post Synthesis Colloidal Atomic Layer Deposition” (filed August 30, 2024). All foregoing applications are incorporated herein by reference in their entireties for any and all purposes.GOVERNMENT RIGHTS

[0002] This invention was made with government support under 2019444 awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates to the field of quantum dots and to the field of atomic layer deposition.BACKGROUND

[0004] Nanocrystal (NC) heterostructures, also known as quantum dot heterostructures, have utility in a range of applications, including detector devices and emitter devices. Existing such heterostructures, however, can allow carriers to access defect sites, thereby increasing recombination pathways and decreasing exciton and carrier lifetimes. Accordingly, there is a long-felt need in the art for improved NC heterostructures.SUMMARY

[0005] In meeting the describe long-felt needs, the present disclosure provides a method, comprising: disposing a plurality of layers on a plurality of nanocrystals (NCs) disposed on a substrate, wherein a given layer comprises a matrix material that includes acompound comprising an anionic species associated with a cationic species, the plurality of NCs optionally being core-shell NCs, and wherein the method is performed so as to give rise to interparticle fusion between at least some of the plurality of NCs and to completely fill interstitial gaps between NCs.

[0006] Also provided is a composition, comprising: a plurality of NCs exhibiting interparticle fusion therebetween and the plurality of NCs defining interstitial spaces therebetween, the interstitial spaces being completely filled with a matrix material that comprises a plurality of layers, a given layer including an anionic species associated with a cationic species, and the plurality of NCs optionally being core-shell NCs.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various aspects discussed in the present document. In the drawings:

[0008] FIG. 1 : Schematic of colloidal atomic layer deposition (c-ALD) procedure on PbSe NCs (I). Ligand exchange is performed on PbSe NCs, resulting in epitaxial fusion of PbSe NCs (II). Next, a chalcogen treatment with Na?S and Na2Se is performed to enrich the surface of the NCs with reactive chalcogen sites. The PbSe NCs are submerged in Cd(OCOCH3)2 to result in one layer of a Cd chalcogenide matrix grown on available surfaces with post deposition c-ALD. The process of enrichment with chalcogen and enrichment in Cd(OCOCH3)2 is repeated for a total of six layers of Cd chalcogenide matrix grown and described as c-ALD 6x (III). HR-TEM images of PbSe NC array s(I), ligand exchanged PbSe NC arrays (II), and c-ALD 6x treated PbSe NC arrays (III) detail the filling of interstitial gaps with the c-ALD technique. Respective Selected Area Electron Diffraction (SAED) patterns for each step in the HR-TEM insets detail the increase in CdS reflections as the c-ALD 6x grows the CdS in the PbSe matrix.

[0009] FIG. 2: EDS elemental mapping of c-ALD 6x treated PbSe NCs surrounded by matrix of CdS, shown with individual elemental maps of Pb (yellow), Se (green), Cd (blue), S (red), and combined overlay maps of Pb with Cd and Se with S. Elemental maps provide evidence of PbSe cores with interstices entirely infilled by CdSdue to c-ALD treatment (A). HR-STEM image of c-ALD 6x treated PbSe NCs with PbSe cores (brighter area) and surrounding CdS matrix (dimmer area) showing difference in contrast between PbSe NCs and CdS matrix(B). The Fast Fourier Transform (FFT) pattern of the HR-STEM image shows reflections that match the (2 0 0) reflections for both PbSe and CdS (Inset of B).

[0010] FIG. 3: Absorbance spectra of oleic acid-capped PbSe NC assemblies before (black) and after NH4SCN ligand exchange (red), and after six treatment cycles of c-ALD in Na?S and Cd(OCOCH3)2 (blue). Shift of first excitonic peak of PbSe NC assembly can be seen with ligand exchange. The shift of the first excitonic peak after the c-ALD process is less than an absorbance shift for a ligand exchanged PbSe NC assembly because arrestment of fusion caused by the c-ALD process (A). An increase of the absorbance in the visible is seen with growth of CdS due to the c-ALD 6x procedure (B). Extrapolating a linear fit to the subtracted absorbance of ligand exchanged PbSe NCs from c-ALD 6x treated PbSe NCs yields an intercept of 2.24 eV, consistent with the bandgap of bulk CdS.

[0011] FIG. 4: HR-TEM Images of InAs NC assemblies(A), InAs NC assemblies with c-ALD 3x of ZnSe (B), and InAs NC assemblies with c-ALD 6x of ZnSe (C). The ZnSe shell material is formed using the c-ALD procedure with Na2Se as the chalcogen source and Zn (CHsCCL^ as the metallic salt.

[0012] FIG. 5: HR-TEM Images of PbSe NC assemblies (A), PbSe with c-ALD 3x of CdS (B), and PbSe with c-ALD 6x of CdS (C) ordered over large areas using the liquid-air interface and ligand exchange of NH4SCN in the liquid subphase. The CdS shell material is formed using the c-ALD procedure with Na2S as the chalcogen source and Cd(OCOCH3)2 as the metallic salt and can be seen as the lighter contrast material surrounding the darker contrast material (PbSe) as the number of c-ALD treatment cycles increases.

[0013] FIG. 6: EDS elemental mapping of c-ALD 3x treated PbSe NCs surrounded by a partially grown matrix of CdS, shown with individual element maps of Pb (yellow), Se (green), Cd (blue), S (red), and combined overlay maps of Pb with Cd and Se with S. Elemental maps provide evidence of PbSe NC cores with interstices not entirely infilled by CdS due to c-ALD treatment being only 3 cycles. Dark areas in the elemental mapping indicate void spaces not occupied by material.

[0014] FIG. 7: Similar to FIG.6 but offers a lower magnification to view entire areas not filled and filled by the c-ALD treatment.

[0015] FIG. 8 Same as FIG.7, but without corresponding STEM image.

[0016] FIG. 9: EDS elemental mapping of c-ALD 3x treated PbSe NCs in a 3 x 3 matrix surrounded by a partially grown matrix of CdS, indicating local order of the PbSe NC assembly. A missing space in the top left comer of the 3x3 matrix is reflected properly in the corresponding EDS elemental map.

[0017] FIG. 10: HR-STEM images of a PbSe NC assembly (top left), PbSe NC assembly with 3 cycles of c-ALD treatment (top right), and PbSe NC assembly with 6 cycles of c-ALD treatment (bottom). The brighter area corresponds to PbSe, consistent with its higher relative mass to CdS, which appears as the lighter area surrounding the bright PbSe NC cores. The progression shows the infilling of the matrix with the lighter CdS material, filling interstitial gaps between cubic-packed PbSe NCs.

[0018] FIG. 11 : Similar to FIG. 10, with the progression of the matrix filling seen as the number of c-ALD cycles increases.

[0019] FIG. 12: Application of the liquid-air interface to provide well-ordered superlattices with long range order. The process involves a TEM grid or substrate used to transfer a film of PbSe NCs ordered using the liquid-air interface. The corresponding HR- TEM images demonstrate the span of the fused PbSe NCs over longer ranges via the liquid-air interface and NBLSCN treatment.

[0020] FIG. 13: Treatment of PbSe NC superlattices formed over long-range scales with the c-ALD procedure. HR-TEM images show the successful growth of the CdS matrix around PbSe NC superlattices with c-ALD 3x and c-ALD 6x treatments. The interstitial voids of the superlattice are fully filled by c-ALD 6x treatments.

[0021] FIG. 14: HR-TEM images of PbSe NC superlattices (A) achieved via the liquid-air interface. To progressively fill the interstitial void, c-ALD treatments of 3x (B) and 6x (C) are performed on the superlattice structure.

[0022] FIG. 15: Bandgap-engineered QD / matrix heterostructure assemblies. (A) Process schematic to prepare PbSe QD / CdS matrix assemblies starting with PbSe QD assemblies capped with oleic acid, solid-state ligand exchange with an NH4SCN solution, and N-cycles of colloidal atomic layer deposition (c-ALD) via treatment with Na?S and Cd(OCOCH3)2 solutions. HR-STEM images and (inset) SAED patterns of (B) 6 nmdiameter, oleic acid capped PbSe QD assemblies, (C) after ligand exchange with NH4SCN and a first treatment with a Na?S solution, and (D,E) after ligand exchange with NH4SCN and N=6 sequential c-ALD treatments with Na?S and Cd(OCOCH3)2 solutions to yield epitaxially-fused PbSe QD / CdS matrix heterostructure assemblies. NH4SCN and Na?S treatments are carried out at room temperature and Cd(OCOCH3)2 treatment is carried out at 65 °C.

[0023] FIG. 16: Compositional and Structural Characterization of PbSe QD / CdS Matrix Heterostructures. EDS elemental mapping of N=6 c-ALD treated epitaxially-fused PbSe QD arrays surrounded by a CdS matrix, shown by individual maps of (white) Pb, (white) Se, (gray) Cd, and (gray) S, and overlay maps of Pb with Cd and Se with S. Cd(OCOCH3)2 treatment is carried out at a temperature of 65 °C.

[0024] FIG. 17: Flowchart of exemplary process for the disclosed technology. As shown, one can provide NCs, optionally perform cation exchange, and perform a desired number of ALD cycles to place a matrix material. A user can, if desired, performed ALD of additional matrix materials, with a desired number of cycles.

[0025] FIG. 18: UV-VIS-NIR Absorbance of Bandgap-Engineered QD Heterostructure Assemblies. Absorbance spectra at (A) low and (B) high energies for PbSe QD assemblies, (black) capped with oleic acid, (red, dotted) after NH4SCN ligand exchange and a first Na2S treatment, and (dark blue, dash-dotted) N=6 c-ALD treatments with Na2S and Cd(OCOCH3)2 solutions. (Inset, B) Linear fit of the difference in the absorbance between PbSe QD assemblies after N=6 c-ALD cycles and after NH4SCN ligand exchange and a first Na2S treatment. (C) Absorbance spectra at low energies (before, after 1 month air exposure) for PbSe QD assemblies (black, gray) capped with oleic acid, (red, light red, dashed) after NH4SCN ligand exchange and a first Na2S treatment, and (blue, light blue, dotted-dashed) N=6 c-ALD treatments with Na2S and Cd(OCOCH3)2 solutions. (D) Spectral shift and FWHM of the first excitonic resonances in absorbance spectra as a function of air exposure time compared to PbSe QDs after NH4SCN ligand exchange and a first Na2S treatment. NH4SCN and Na2S treatments are carried out at room temperature and the Cd(OCOCH3)2 treatment is carried out at 65 °C. Red (blue) shifts are indicated by a negative (positive) shift and marks of ‘x’ indicate loss of a measurable first excitonic resonance after exposure.

[0026] FIG. 19: HR-STEM of PbSe QD / CdS Matrix Assemblies Before and After Annealing. HR-STEM images (A) before and (B) after annealing at 150 °C for 10 min of 6 nm diameter, PbSe QD assemblies upon (left column) solid-state ligand exchange with a NH4SCN solution and a first treatment with a Na?S solution, (middle column) N=3 cycles of c-ALD, and (right column) N=6 cycles of c-ALD. NH4SCN and Na?S treatments are carried out at room temperature and Cd(OCOCH3)2 treatment is carried out at 65 °C.

[0027] FIG. 20: Device Characterization and Thermal Stability of c-ALD Treated PbSe QD FETs and Photoconductors. (A) Optical absorption spectra and corresponding, (B) FET transfer characteristics at VDS = 50 V and (C) photoconductor transients at V= 10 V and excited at = 1550 nm for 6 nm diameter PbSe QD assemblies after (red, solid) NH4SCN and Na?S ligand exchange, (light blue, dashed) N=3 c-ALD cycles, and (dark blue, dotted-dashed) N=6 c-ALD cycles. NH4SCN and Na?S treatments are carried out at room temperature and Cd(OCOCH3)2 treatment is carried out at 65 °C. (Top) unannealed and (bottom) annealed, at 150 °C for 10 min.

[0028] FIG. 21 : STEM of PbSe QD assemblies and upon exchange with different ligand solutions. Lower and higher resolution STEM images of (A) oleic acid capped, PbSe QD assemblies followed by room temperature ligand exchange with (B) 1% w / v methanolic NH4SCN and (C) 1 mM methanolic Na2S solutions.

[0029] FIG. 22: STEM of epitaxially-fused PbSe QD assemblies treated with N=3 c-ALD cycles. High resolution STEM images of a 6 nm diameter PbSe QD assembly after ligand exchange with NH4SCN and N=3 cycles of sequential c-ALD with Na2S and Cd(OCOCH3)2 solutions, where NH4SCN and Na2S solutions are held at room temperature and Cd(OCOCH3)2 solutions are held at 65 °C.

[0030] FIG. 23: Epitaxially-fused PbSe QD assemblies c-ALD treated with Na2Se. (A) High resolution STEM images of a 6 nm diameter PbSe QD assembly after ligand exchange with NH4SCN and N=3 cycles of sequential c-ALD with Na2Se and Cd(OCOCH3)2 solutions. (B) EDS elemental map of N=3 c-ALD treated epitaxially-fused PbSe QD arrays surrounded by a CdSe matrix. (C) Absorbance at high energies for PbSe QD assemblies, (black) capped with oleic acid, (red, dashed) after NH4SCN ligand exchange and a first Na2Se treatment, and (light blue, circle dashed) N=3, (dark blue, dotted dashed) N=6 c-ALD treatments. Inset shows at high energies the difference in the absorbance between PbSe QD assemblies after (light blue, circle dashed) N =3 c-ALDcycles, (dark blue, dotted dashed) N=6 c-ALD cycles, from the absorbance of PbSe QD assemblies after NH4SCN ligand exchange and a first Na?Se treatment. The intercept yields a band edge at 1.58 eV, consistent with the bandgap of CdSe. NH4SCN and Na?Se solutions are held at room temperature and Cd(OCOCH3)2 solutions are held at 65 °C.

[0031] FIG. 24: EDS Spectra Analysis and Method of Calculation. HR-STEM EDX spectra of (black) oleic acid capped PbSe QD assemblies treated with (red) NH4SCN ligand exchange and a first Na?S treatment, (light blue) N=3 c-ALD cycles, and (blue) N=6 c-ALD cycles. Insets showing counts from Pb L, S K, Pb M, Se K, and Cd L regions.

[0032] FIG. 25: Compositional and Structural Characterization of PbSe QD / CdS Matrix Heterostructures. EDS elemental mapping of N=3 c-ALD treated epitaxially-fused PbSe QD arrays surrounded by a CdS matrix, shown by individual maps of (white) Pb, (white) Se, (gray) Cd, and (gray S, and overlay maps of Pb with Cd and Se with S. The temperature of the Cd(OCOCH3)2 treatment is 65 °C.

[0033] FIG. 26 depicts an exemplary structure according to the present disclosure. As shown, the interstitial space between NCs can be filled with a single matrix material, which matrix material can be present in multiple layers.

[0034] FIG. 27 depicts an exemplary structure according to the present disclosure. As shown, the interstitial space between NCs can be filled with multiple matrix materials, which matrix materials can be present in multiple layers of different materials. It should be understood that FIG. 27 is a qualitative representation, and the multiple materials can be added sequentially.

[0035] FIG. 28: Optical absorption spectra of QD assemblies measured as a function of storage time in the nitrogen filled glovebox. (A) Normalized absorbance spectra for PbSe QD assemblies, (black, circle) capped with oleic acid; after NH4SCN ligand exchange, assemblies are measured (green, star) immediately and (light green triangle) after 2 h in a glovebox; and after NH4SCN ligand exchange and a first Na?S treatment, assemblies are measured (red, pentagon) immediately, (light red, square) after 2 h in a glovebox, and (maroon, hexagon) after 24 h in a glovebox. (B) Analysis of the spectra center energy (filled circles) and FWHM (purple circles) as a function of time in the glovebox.

[0036] FIG. 29: Optical absorbance spectra of QD assemblies treated with N=l, 3, and 6 c-ALD cycles. Absorbance spectra at low and high energies for PbSe QDassemblies, (black) capped with oleic acid, (green, dashed) after NH4SCN ligand exchange, (red, dotted dashed) after NH4SCN ligand exchange and a first Na?S treatment, and (very light blue, square) N=l, (light blue, triangle) N=3, (dark blue, circle) N=6 c- ALD treatments with Na?S solutions held at room temperature and Cd(OCOCH3)2 solutions held at (A) room temperature and (B) 65 °C. Spectra are measured immediately after completion of the treatments. Insets show at high energies the difference in the absorbance between PbSe QD assemblies after (very light blue) N=1 c-ALD cycle, (light blue) N =3 c-ALD cycles, (dark blue) N=6 c-ALD cycles, from the absorbance of PbSe QD assemblies after NFLSCN ligand exchange and a first Na?S treatment.

[0037] FIG. 30: Optical absorbance spectra of QD assemblies treated with N=1 c-ALD cycle following NFLSCN ligand exchange and a first Na?S treatment at various times of storage in the nitrogen filled glovebox. Absorbance spectra at low energies for PbSe QD assemblies, (black, circle) capped with oleic acid, (green, triangle) after NH4SCN ligand exchange, (red, square) after NFLSCN ligand exchange and a first Na?S treatment, and (blue) N=1 c-ALD treatment with Na?S and a Cd(OCOCH3)2 solution held at room temperature. Progression of N=1 c-ALD absorbance spectra to lighter blue colors and other shapes indicates progression of time of storage (immediately (hexagon), 1 min (pentagon), 5 min(left arrow), 10 min (right arrow), 15 min (diamond), 30 min (star), 1 h (down arrow) in the glovebox after NFLSCN ligand exchange and a first Na2S treatment and before immersion in Cd(OCOCH3)2 solution. Black dotted line indicates center of N=1 c-ALD spectra immediately after NH4SCN ligand exchange and a first Na2S treatment.

[0038] FIG. 31 : Optical absorbance spectra of QD assemblies treated with N=6 c-ALD cycles following NFLSCN ligand exchange and a first Na2S treatment immediately and after 24 h of storage in the nitrogen filled glovebox. Absorbance spectra at low and high energies for PbSe QD assemblies, (black) capped with oleic acid, (green, dashed) after NFLSCN ligand exchange, (red, dotted dashed) after NFLSCN ligand exchange and a first Na2S treatment, and (dark blue, circle) N=6 c-ALD treatments with Na2S and a Cd(OCOCFL)2 solution held at (A) room temperature and (B) 65 °C. Inset of absorbance spectra at high energies show the difference in the absorbance between PbSe QD assemblies after (dark blue, circle) N=6 c-ALD cycles immediately after NFLSCN ligandexchange and a first Na?S treatment, and (purple, square) N=6 c-ALD cycles 24 h after NH4SCN ligand exchange and a first Na?S treatment.

[0039] FIG. 32: Optical absorption spectra of QD assemblies stored in ambient air. Normalized absorbance spectra for PbSe QD assemblies, (A) capped with oleic acid (black, circle), (B) ligand exchanged with NH4SCN (green, triangle), and (C) ligand exchanged with NFLSCN and a first Na?S treatment (red, square). Progression of darker to light color represents time exposed to air with measurement at time increments of immediately and after 10 min, 30 min, 45 min, 1 h, 1 h and 30 min, and 2 h after exposure to air. (D) Analysis of the spectra center energy and (E) FWHM as a function of air exposure.

[0040] FIG. 33: Optical absorbance spectra of QD assemblies treated with N=l,3, and 6 cycles of c-ALD after Ih of exposure to air. Absorbance spectra at low and high energies for PbSe QD assemblies, (black) capped with oleic acid, (green, dashed) after NFLSCN ligand exchange, (red, dotted dashed) after NFLSCN ligand exchange and a first Na?S treatment, and (very light blue, square) N=l, (light blue, triangle) N=3, (dark blue, circle) N=6 c-ALD treatments with Na?S solutions held at room temperature and Cd(OCOCH3)2 solutions held at (A) room temperature and (B) 65 °C measured after exposure to air for Ih. Spectra are measured immediately after completion of the treatments. Insets show at high energies the difference in the absorbance between PbSe QD assemblies after (very light blue) N=1 c-ALD cycle, (light blue) N =3 c-ALD cycles, (dark blue) N=6 c-ALD cycles, from the absorbance of PbSe QD assemblies after NH4SCN ligand exchange and a first Na?S treatment.

[0041] FIG. 34: Optical absorbance spectra of QD assemblies treated with N=l,3, and 6 cycles of c-ALD after 24 h of exposure to air. Absorbance spectra at low and high energies for PbSe QD assemblies, (black) capped with oleic acid, (green, dashed) after NFLSCN ligand exchange, (red, dotted dashed) after NFLSCN ligand exchange and a first Na?S treatment, and (very light blue, square) N=l, (light blue, triangle) N=3, (dark blue, circle) N=6 c-ALD treatments with Na?S solutions held at room temperature and Cd(OCOCH3)2 solutions held at (A) room temperature and (B) 65 °C measured after exposure to air for 24h. Spectra are measured immediately after completion of the treatments. Insets show at high energies the difference in the absorbance between PbSe QD assemblies after (very light blue) N=1 c-ALD cycle, (light blue) N =3 c-ALD cycles,(dark blue) N=6 c-ALD cycles, from the absorbance of PbSe QD assemblies after NH4SCN ligand exchange and a first Na?S treatment.

[0042] FIG. 35: Optical absorbance spectra of QD assemblies treated with N=6 cycles of c-ALD after 1 m of exposure to air. Absorbance spectra at low energies (before, after 1 month air exposure) for PbSe QD assemblies (black / solid, gray circle / solid) capped with oleic acid, (red / dash, light red triangle / dash) after NH4SCN ligand exchange and a first Na?S treatment, and (blue / dotted dashed, light blue square / dotted dash) N=6 c-ALD treatments with Na?S and Cd(OCOCH3)2 solutions. NH4SCN, Na?S, and Cd(OCOCH3)2 treatments are carried out at room temperature.

[0043] FIG. 36: Spectral shift and FWHM of QD assemblies with various times of exposure to air. Representative analysis of the spectral shift and FWHM of the first excitonic resonances in absorbance spectra at various air exposure times of (A) PbSe QD assemblies (black, square) capped with oleic acid, (green, triangle) after NH4SCN ligand exchange, (red, circle) after NH4SCN ligand exchange and a first Na2S treatment, after (very light blue, square) N=l, (light blue, triangle) N=3, (dark blue, circle) N=6 c-ALD treatments with Na2S and a Cd(OCOCH3)2 solution held at (B) room temperature and (C) 65 °C. Red shifts are indicated by a negative shift and blue shifts are indicated by positive shift. Marks with ‘x’ indicate loss of measurable first excitonic resonance after oxidation for all samples up to the color of the ‘x’ .

[0044] FIG. 37: Statistics of spectral shift and FWHM of QD assemblies with various times of exposure to air. Statistics of first excitonic resonances for absorbance spectra at various air exposure times of multiple sets of (A) PbSe QD assemblies, (black, square) capped with oleic acid, (green, triangle) after NH4SCN ligand exchange, (red, circle) after NH4SCN ligand exchange and a first Na2S treatment, and (very light blue, square) N=l, (light blue, triangle) N=3, (dark blue, circle) N=6 c-ALD treatments with Na2S and a Cd(OCOCH3)2 solution held at (B) room temperature and (C) 65 °C. Red shifts are indicated by a negative shift and blue shifts are indicated by positive shift. Marks with ‘x’ indicate loss of measurable first excitonic resonance after oxidation for all samples up to the color of the ‘x’.

[0045] FIG. 38: Photoluminescence of PbSe QD Assemblies treated with c-ALD. Photoluminescence of (black) oleic-acid capped PbSe QD assemblies treated with (red,dashed) NH4SCN and a first Na?S treatment and (dark blue, dotted dashed) N=6 c-ALD cycles with Cd(OCOCH3)2 solution at 65 deg. C, excited at energy of 1.18 eV.

[0046] FIG. 39: Linear Regime FET Characteristics. FET transfer characteristics at VDS = 2 V of PbSe QD assemblies after (red, solid) NH4SCN and Na?S ligand exchange, (light blue, dashed) N=3 c-ALD treated PbSe QD assemblies, and (dark blue, dotted dashed) N=6 c-ALD.DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0047] The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments and the examples included therein.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting.

[0049] The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

[0050] As used in the specification and in the claims, the term "comprising" can include the embodiments "consisting of' and "consisting essentially of.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named ingredients / steps and permit the presence of other ingredients / steps. However, such description should be construed as also describing compositions or processes as "consisting of' and "consisting essentially of' the enumerated ingredients / steps, which allows the presence of only the named ingredients / steps, along with any impurities that might result therefrom, and excludes other ingredients / steps.

[0051] As used herein, the terms “about” and “at or about” mean that the amount or value in question can be the value designated some other value approximately or about the same. It is generally understood, as used herein, that it is the nominal value indicated ±10% variation unless otherwise indicated or inferred. The term is intended to convey that similar values promote equivalent results or effects recited in the claims. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but can be approximate and / or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about” or “approximate” whether or not expressly stated to be such. It is understood that where “about” is used before a quantitative value, the parameter.

[0052] Unless indicated to the contrary, the numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0053] All ranges disclosed herein are inclusive of the recited endpoint and independently of the endpoints. The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and / or values.

[0054] As used herein, approximating language can be applied to modify any quantitative representation that can vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified, in some cases. In at least some instances, the approximating language can correspond to the precision of an instrument for measuring the value. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” can refer to plus or minus 10% of the indicated number. For example, “about 10%” can indicate a range of 9% to 11%, and “about 1” can mean from 0.9-1.1. Othermeanings of “about” can be apparent from the context, such as rounding off, so, for example “about 1” can also mean from 0.5 to 1.4.

[0055] Further, the term “comprising” should be understood as having its open- ended meaning of “including,” but the term also includes the closed meaning of the term “consisting.” For example, a composition that comprises components A and B can be a composition that includes A, B, and other components, but can also be a composition made of A and B only. Any documents cited herein are incorporated by reference in their entireties for any and all purposes.

[0056] Any embodiment or aspect provided herein is illustrative only and does not limit the scope of the present disclosure or the appended claims. Any part or parts of any one or more embodiments or aspects can be combined with any part or parts of any one or more other embodiments or aspects.

[0057] Conventional optoelectronic and electronic devices (e.g., light emitting or lasing, photodetectors, solar photovoltaic devices, and transistors) are composed of semiconductors. Quantum-based devices (e.g. qubits, interconnects, and sensors) also rely on semiconductors for operation. Bandgap engineering is known in the design of semiconductor heterostructures to spatially control the energy and thus band offsets of the conduction and valence bands and to achieve high-performance optoelectronic and electronic devices. Two-dimensional, semiconductor heterostructures, i.e., stacked layers of different semiconductors tailored in composition and thickness and typically grown by vacuum-based molecular-beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD), have the longest history and are used in commercial devices. In Type-I heterostructures, the conduction band minimum and valence band maximum are in the same semiconductor layer, whereas in Type-II heterostructures, the conduction band minimum and valence band maximum are in different semiconductor layers. These ideas extend to zero-dimensional nanocrystals (NCs). NCs can be prepared by MBE, but also as colloids by scalable, wet-chemical synthetic methods. These NC surface can be coated with ligands to allow their dispersion in solvents as colloids, and thus are known as colloidal NCs.

[0058] Colloidal NCs can be tailored in size, shape, composition, and internal structure, for example, as III-V In- and Ga-pnictides; II- VI and IV-VI Zn-, Cd-, Hg-, Pb-, and Sn-chalcogenides; I-III-VE CuxIni.x-chalcogenides; I2-VI Cu- and Ag-chalcogenides,and I- VII Cu-halides. Bandgap engineering is used to create colloidal NCs with different compositions of cores and shells (Type-I, Type-II, and quasi-Type-II) and can be achieved using wet-chemical synthetic methods. Often pursued in applications of colloidal NCs is the growth of a shell material to passivate surface states on the core, allowing high (approaching unity) photoluminescence quantum yield emitters. These colloidal NCs have been commercialized as down-conversion materials in high-definition televisions (HDTVs). Colloidal NCs are also being commercialized as sensitizing layers for photodetectors.

[0059] Light-emitting and photo-detecting NC diodes and photoconductors are both of academic and industrial interest for translation. Here, the colloidal NCs are deposited, advantageously by scalable solution-based processing techniques, to form thin- film NC solids. However, the advantageous core-shell architecture created during NC synthesis, used to passivate surface states for high quantum yield emission and long carrier lifetimes useful for light emission and detection, establishes an electronic structure that limits carrier transport once the NCs are deposited to form thin-film assemblies. As a result, most light-emitting devices are made from monolayer to few layers of NCs to be able to inject charge and most photodetectors use core-only NCs to transport and extract charge.

[0060] The present disclosure provides, among other things, a novel structure and a process to create all-inorganic bandgap-engineered NC heterostructures via postdeposition colloidal atomic layer deposition (c-ALD). These heterostructures can include of thin films of core semiconductor NCs surrounded by a matrix of a different or similar semiconductor material. The process allows the deposition of core-only NCs and treatments to control interparticle distance, to improve charge transport (e.g., from 1 nm to allowing core NC fusion), followed by the c-ALD process to grow a bandgap- engineered matrix material useful for surface passivation and the design of Type-I and Type-II heterostructures. The heterostructures can include electronically coupled cores, which can be selected for increased mobility and lifetime with the c-ALD process for use in optoelectronic devices.

[0061] The post deposition c-ALD process can be a sequential process of various treatments on thin-film assemblies of core NCs to grow a semiconductor matrix resulting in heterostructures. Core-only NCs with an initial ligand capping their surface, typically anorganic ligand used in NC synthesis, are deposited on a substrate by a coating or printing technique to form the NC assembly. The core NC thin-film assembly can be treated with a solvent (e.g., methanol) or solution containing a new ligand (e.g., inorganic chalcogenide, halide, or pseudohalide) to strip or exchange the initial ligands and thus control interparticle distance and assembly structure. An inorganic ligand such as a metal chalcogenide is subsequently used to enrich the NC surface in ions creating potential binding sites upon subsequent treatment. The surface-enriched NC can be immersed in a metallic salt (e.g. metal halide or metal acetate or metal nitrate) treatment that enriches the surface and provides ions that bind to the ions from the inorganic ligand to stabilize the NC surface and produce a single layer of an inorganic shell. To grow the matrix material, the NC surface can be enriched with excess chalcogen that reacts with the metallic salt. This entire process is similar to atomic layer deposition (ALD), a bulk semiconductor growth process that can precisely engineer layered growth of multiple semiconductor materials.

[0062] As a non-limiting example of the disclosed technology, we have prepared thin films of oleic-acid capped lead selenide (PbSe) NCs and treated the assemblies with methanolic solutions of ammonium thiocyanate (NH4SCN) to drive ligand exchange and reduce interparticle spacing to <1 nm and to the point of forming epitaxially-fused arrays of NCs. These NC assemblies are subsequently treated using a sequential, post deposition c-ALD process to grow a cadmium (Cd) chalcogenide matrix around the PbSe NC assemblies.

[0063] The process for c-ALD deposition upon PbSe NC assemblies is depicted in FIG. 1. First, the PbSe NCs are deposited from a dispersion in toluene on a substrate by spin coating. This step is followed by a solid-state ligand exchange with NH4SCN (i.e., by immersing the NC assembly in the ligand solution) to reduce interparticle spacing and remove bulky organic ligands on the NC surface. In the case of Pb chalcogenide systems, ligand exchange promotes structural reorganization and epitaxial fusion of the NCs, i.e., structurally transforming the assembly from hexagonally-packed, oleic acid capped NCs with an ~2 nm interparticle distance to square-packed NCs, thiocyanate-capped NCs with 1 nm interparticle spacing to include fused particle structures.

[0064] The interparticle spacing of PbSe NCs decreases upon ligand exchange and can be controlled through ligand type, exchange time, concentration, and temperature,with longer exchange times allowing for particle fusion, i.e., there is no interparticle spacing. This is understood as the rock salt structure of PbSe leads to truncation of the NCs in the <111> direction, exposing the { 100} facets. Oleic acid ligands are not as strongly bound on the { 100} facets and can be replaced easily. The ligand exchange preferentially replaces the oleic acid ligands on the { 100} facets of the NCs with the shorter ligand, leading to the most stable energetic configuration being fusion between PbSe NCs which results in the square-packed orientation.

[0065] The interparticle distance and fusion and necking of the NCs can be arrested by treatment of the NC assembly with a metal salt, i.e., by immersing the assembly in a metal salt solution. For example, treatment of the NC assembly with a Cd- salt (i.e., CdX2) from the c-ALD process stops the ligand exchange reaction and prevents further epitaxial fusion of particles.

[0066] NH4SCN is a preferred ligand exchange reagent as it results in well- ordered epitaxial fusion of PbSe NCs. Other ligand exchange agents include tetrabutyl ammonium halides (chloride, bromide, and iodide), ethylenediamine, and sodium azide and hydroxide, as well as sodium and potassium chalcogenides. After ligand exchange, the sample is washed with methanol or the solvent in which the ligand exchange agent is dissolved.

[0067] The NC assemblies are immersed in methanolic solutions of Na?S or Na?Se to enrich the NC surface in excess chalcogen. As noted above, treatment with Na?S or Na?Se solutions can also be used for ligand exchange directly, though NH4SCN results in greater order of PbSe NCs over larger areas. After immersion of the NC assembly in Na?S or Na?Se solutions, it is rinsed with methanol three times to remove excess / unbound salts. The substrate is subsequently submerged in a solution of a metal salt, such as Cd(OCOCH3)2. The surface chalcogens (Se or S) react to form the cadmium chalcogenide outer shell material. Afterwards, the substrate can be rinsed in methanol or other solvent to remove excess salt. The step of chalcogen enrichment and submersion in metallic salt to form the cadmium chalcogenide matrix are referred to as one layer of post-deposition c- ALD.

[0068] We have accomplished fully filling the interstices after six sequences of c-ALD to grow six layers of CdS or CdSe, forming a Cd-chalcogenide matrix around epitaxially-fused PbSe NCs. Tailoring the time and temperature of the metallic saltprecursor treatment allows for control of the speed of the self-limiting reactions that affect growth of the outer matrix material. The time of reaction with the metallic salt can range from one minute to twenty minutes at temperatures that range from room temperature to 65 °C. We have found the optimal time of the reaction to be 10 minutes as the reaction is self-limiting. At 10 minutes, additional growth of the matrix is not substantial. The optimal temperature of the reaction depends on the desired control over the degree of fusion between PbSe NCs, with room temperature resulting in more fusion between NCs and 65 °C increasing the reaction rate to arrest fusion of the NCs. Time and temperature parameters can be controlled to yield precise growth of the heterostructure matrix, akin to the control over thickness of layered materials that can be grown with atomic layer deposition.

[0069] We have investigated the crystallinity and composition of the bandgap- engineered heterostructures with aberration-corrected scanning transmission electric microscopy (STEM), high resolution transmission electron microscopy (HR-TEM), and energy dispersive X-ray spectroscopy (EDS). The presence of PbSe NCs surrounded by an encompassing matrix of CdS is confirmed by overlaying EDS elemental maps of Pb with Cd and Se with S (FIG. 2A). With STEM and HR-TEM, we have resolved lattice fringes and a significant difference in contrast between the PbSe NCs and the surrounding CdS matrix (FIG. 2B). Visible lattice fringes in both the core PbSe NCs and the surrounding CdS matrix indicate crystallinity across the whole NC heterostructure. Additionally, electron diffraction and Fast Fourier Transforms (FFT) are consistent with the (2 0 0) and (2 2 0) planes of PbSe and CdS (FIG. 2B Inset). Selected Area Electron Diffraction (SAED) patterns for c-ALD treated samples indicate an increase in the number of CdS reflections corresponding to growth in the CdS matrix when compared with SAED patterns of only PbSe NCs (FIG. 1).

[0070] Optical absorption spectra for c-ALD treated heterostructures compared to untreated PbSe NCs show an increase in absorbance at visible wavelengths that corresponds to growth of the CdS matrix upon the PbSe NC assembly. By subtracting the absorbance of PbSe NC assemblies from that of c-ALD grown PbSe NC / CdS matrix heterostructures, we see the onset and growth in visible absorbance characteristic of the absorbance of the CdS matrix. Using a linear fit to the absorbance, we extrapolate the x- intercept to find the bandgap of the CdS matrix, which is similar to that of bulk CdS.Additionally, absorption spectra detail the change in the first excitonic peak of PbSe NC assembly with ligand exchange and c-ALD treatment. In FIG. 3, the first excitonic peak of the oleic acid-capped PbSe NC assembly (black) red shifts after ligand exchange with NH4SCN (red) as interparticle distance decreases. The six-layer c-ALD grown PbSe NC / CdS matrix heterostructure (blue) absorption spectrum show the first excitonic peak at wavelengths in between that of the red and black curves, as the c-ALD treatment with metal salts arrests the progressive fusion and thus spectral evolution of the PbSe NC assembly. This phenomenon is seen in Cd(OCOCH3)2 treatment at 65 °C, as the increased temperature promotes the rate of the c-ALD reaction, arresting PbSe NC fusion more quickly than treatments carried out at room temperature.

[0071] The photoluminescence of oleic-acid capped PbSe NC assemblies decreases after ligand exchange as the increase in electronic coupling between NCs provides for greater charge transport, allowing carriers to access defect sites, increasing recombination pathways and decreasing the exciton lifetime. However, growing the CdS matrix to encapsulate the PbSe NCs in assemblies creates heterostructures that passivates surface defect states, causing the photoluminescence to be partially recovered, but while maintaining the electronic coupling of the fused PbSe NC network. We have obtained results that demonstrate recovery of the photoluminescence after ligand exchange with growth of the matrix via c-ALD.

[0072] The unique capability of Pb chalcogenide NCs to epitaxially fuse creates more conductive pathways for carrier transport, leading to increased mobilities. However, epitaxially-fused PbSe NCs have unpassivated surface sites and interstitial spaces that can be filled with another semiconductor material via the post deposition c-ALD process. Although thin films of epitaxially-fused PbSe NCs results in increased carrier transport, defect states are easily accessed as a result, leading to a reduction in photoluminescence and subsequent utility for optoelectronics. The c-ALD technique presents an opportunity to fill and passivate defect states (e.g., of epitaxially-fused PbSe) with semiconductor matrix (e.g., Cd-chalcogenide) and recover photoluminescence. The Cd chalcogenide matrix, with a larger bandgap than that of the PbSe NCs, adds to the volume of the heterostructure and collects additional photons of wavelengths above the bandgap of the Cd chalcogenide matrix, improving viability for optoelectronics when combined with the increased transport offered by the epitaxially-fused PbSe NC assembly.

[0073] The benefits of post-deposition c-ALD provide increased mobilities and lifetimes for optimized use in optoelectronics. Especially for the case of infrared optoelectronics, there is a need for materials with high mobilities and long lifetimes that could be satisfied with post-deposition c-ALD on PbSe NC assemblies.

[0074] The utility for post deposition c-ALD extends to other NC and matrix material systems. For example, we have grown CdS and CdSe matrices around both PbS and PbS NC assemblies. This can be extended to III-V semiconductor NCs, e.g. we have evidence of growing ZnSe matrices uniformly around InAs NCs (FIG. 4). Cation exchange can be used to change the composition of the core NCs in assemblies (e.g., to transform PbSe NC assemblies to CuxS NC assemblies to CdSe NC assemblies), and different compositions of matrices can be grown. One can also effect graded matrix growth with two or more different semiconductor materials for the semiconductor matrix. An example is PbSe core NC assemblies surrounded by a layer of CdSe and then by additional layers of ZnSe. Graded shelling is used in wet-chemical synthesis of core-shell NCs to engineer the band alignment and achieve highly luminescent NC dispersions. The use of post-deposition c-ALD in developing these materials is unique and allows for strong electronic coupling of core NCs. The c-ALD approach can also be used to create matrices of size and shape tailored NCs to tune the electronic structure and create anisotropy for charge transport and interactions with light.

[0075] Techniques, such as liquid-air interface assembly, allow large area, ligand-exchanged NC superlattices. On these NC assemblies, it is then possible to perform the c-ALD procedure to yield superlattices of NC heterostructures with fully-filled matrices, as seen in FIG. 5. There is a variety of NC superlattices, including large-area PbSe NC superlattices. Performing the c-ALD on these superlattices provides higher optoelectronic performance as the order of these superlattice heterostructures is longer range and features more conductive pathways compared to heterostructures with short range order.

[0076] Additional Disclosure

[0077] We report all-inorganic, bandgap-engineered epitaxially-fused PbSe QD / CdS matrix heterostructures achieved through post-deposition sequential colloidal atomic layer deposition (c-ALD). The CdS matrix grows epitaxially on the PbSe QDs to ultimately fully infill the interstitial space in and between the fused PbSe QD arrays. Thelow-energy excitonic absorbance of the epitaxially-fused PbSe QD assembly is maintained and the absorbance increases at energies above the CdS matrix bandgap. The c-ALD CdS matrix enhances the oxidative and thermal stability of the QD assemblies, allowing us to preserve the QD / matrix heterostructure upon annealing. By controlling the number of c- ALD cycles and by thermal annealing, we control carrier type, concentration, and mobility probed in the platform of field-effect transistors and the dark current density and responsivity of infrared-absorbing PbSe QD / CdS matrix heterostructure photoconductors. Photoconductors treated with c-ALD show up to a 50 times increase in photocurrent modulation, enhancing infrared photoresponsivity.

[0078] Semiconductor heterostructures integrate two or more dissimilar semiconductor materials with band-engineered energy offsets to spatially control carrier energy, type, and concentration and enhance carrier mobility and lifetime to achieve high performance electronic and optoelectronic devices. Two-dimensional semiconductor heterostructures are typically grown using molecular-beam epitaxy or metal-organic chemical vapor deposition to realize fine control over layered deposition. The type and function of the heterostructure is dependent on the band alignment between the two semiconductor materials. In type-I heterostructures, the conduction band minima and valence band maxima are in the same semiconductor, confining electrons and holes to increase exciton formation and in devices improve the efficiency of light emitting and laser diodes. Type-II heterostructures consist of semiconductors where the conduction and valence band extrema are staggered, increasing carrier separation, particularly useful in solar cells and photodetectors. Both type-I and type-II junctions are used to control carrier injection and collection in high-performance, high-speed transistors.

[0079] Analogously, wet-chemical synthetic methods are used to prepare bandgap-engineered colloidal semiconductor quantum dot (QD) heterostructures. Methods include seeded growth of a second semiconductor on a QD core; colloidal atomic-layer deposition (c-ALD), also known as successive ion layer adsorption and reaction (SILAR), where cations and anions sequentially adsorb and react with surface sites on the QD core, progressively growing a second semiconductor shell; and ion-exchange reactions to replace cations or anions with those of a different composition forming heterojunctions. These reactions can be used to create radial core-shell or axial segmented QD heterostructures, which depend on the composition and dimensions of the semiconductors,form type-I or type-II band alignments or quasi type-II structures, where one carrier is confined to the QD core and, with a small band offset, the other carrier is delocalized across the core and shell. However, with exception to using mono- to few-layer core-shell QD assemblies in light emitting devices, which limit the need for carrier transport between QDs, these as-synthesized QD heterojunctions are not used in other electronic and optoelectronic devices. In thin-film assemblies, the shell in type-I or type-II core-shell QDs restricts transport of one or both carriers and segmented QDs require oriented assembly to create bi-continuous semiconductor heterostructures for carrier transport. Typically, in electronic transistors and circuits and optoelectronic photodetectors and solar cells, core only QDs are assembled and ligand exchange is introduced to strip or replace the long insulating ligands used in synthesis with shorter ligands to decrease interparticle spacing, even to the point of QD fusion, and enhance carrier transport and device speed. For example, while QD photodetectors have been realized by ligand exchange with short organic ligands, the low mobility of carrier transport limits device speed, motivating the need for ligand exchange with more compact inorganic moieties or ligand stripping to increase carrier mobility and device bandwidth required for applications such as motion sensing and imaging. However, ligand exchange and stripping may increase the surface state density and the increase in carrier mobility also leads to greater access to defect sites, increasing carrier recombination and decreasing carrier lifetime.

[0080] Once QDs are assembled to form thin films in the solid-state, c-ALD has previously been used to add ions to QD surfaces and thereby, control stoichiometry and introduce impurities to create n- and p-type QD layers for QD electronic and optoelectronic devices. This method has also been used sequentially to grow matrices around core-shell QDs to increase photoluminescence quantum yield, but with no known electronic transport as there is a lack of coupling between the QD cores. In epitaxially- fused QD assemblies, the process has been used to increase the neck width and connectivity between QDs, yielding high conductance, degenerately-doped QD thin films.

[0081] Here, we demonstrate sequential, post-deposition c-ALD treatments on epitaxially-fused PbSe QD assemblies to grow an epitaxial shell of and ultimately infill interstitial voids with CdS and produce PbSe QD / CdS matrix heterostructures. The chemical and physical structure of the PbSe QD / CdS matrix heterostructures is characterized by imaging and elemental analysis in the scanning transmission electronmicroscope (STEM). Optical absorption spectroscopy shows the excitonic resonances of the epitaxially-fused QDs are maintained upon c-ALD, while absorption in the blue increases, characteristic of the c-ALD grown CdS matrix. Growth of the CdS matrix desirably and significantly enhances the thermal and oxidative stability of the epitaxially- fused PbSe QD assemblies, preventing QD sintering upon thermal annealing at 150 °C and maintaining quantum confinement for up to one month after exposure to air; in contrast to core-only PbSe QD assemblies which sinter upon annealing at temperatures above 110 °C or oxidize when exposed to air for more than a few minutes. By controlling the number of c-ALD treatment cycles and upon annealing, we tailor the carrier type, concentration, and mobility which we study through electronic transport measurements of field effect transistors (FETs) and transient current response measurements of photoconductors featuring the PbSe QD / CdS matrix heterostructures.

[0082] Results and Discussion

[0083] We synthesize and assemble QDs and introduce a c-ALD process to form all-inorganic QD / matrix heterostructures (FIG. 15 A). We synthesize 5 nm and 6 nm diameter (d) PbSe QDs following methods previously reported in literature,32,39-41varying injection temperature to control QD size. 5 nm diameter PbSe QDs are selected to allow spectroscopic measurements using detectors with responsivity at E > 0.775 eV (X < 1600 nm). 6 nm diameter PbSe QDs are chosen to increase the neck width of epitaxially-fused PbSe QDs to create more conductive pathways for electrical and optoelectronic measurements. Oleic acid capped PbSe QD dispersions are deposited to form thin-film assemblies with thicknesses of 10 nm via spin-coating, unless otherwise noted. High resolution-scanning transmission electron microscopy (HR-STEM) images and selected- area electron diffraction (SAED) patterns (FIG. 15B and FIG. 21A) show the oleic acid capped QDs have a rock salt crystal structure, are truncated in the <111> direction exposing { 100} facets, and are hexagonally packed with a 2.06 ± 0.517 nm interparticle distance.

[0084] We treat the QD assemblies by immersion in 1% w / v methanolic NH4SCN solutions for 30 s at room temperature to exchange weakly bound oleic acid ligands with more compact SCN' ligands. Ligand exchange drives epitaxial fusion between atoms on the { 100} facets of the PbSe QDs and structural reorganization to form square-packed QD assemblies over domain sizes exceeding 50 nm and containing morethan 100 QDs (FIG. 2 IB). Epitaxially-fused 6 nm diameter QDs have a neck width (Z) of 2.90 ± 0.34 nm, equivalent to a neck width to QD diameter ofof 0.46 ± 0.06, consistent with previous literature reports.

[0085] We utilize the c-ALD approach in the solid state and show that it can be carried out sequentially to grow an inorganic matrix. First, the SCN-exchanged PbSe QD assemblies are treated at room temperature with 1 mM methanolic Na?S solutions for 10 s, enriching the QD surfaces in chalcogenides (FIG. 15 A, C) The epitaxially-fused QD neck width increases to I of 3.47 ± 0.56 nm. The increase in neck width after a first Na?S treatment is consistent with the addition of two sulfide anions to the QD neck, with each anion having a diameter of 0.34 nm.47We note that treatment of oleic acid capped PbSe QD assemblies directly with Na?S, z.e., without the intermediate NEUSCN ligand exchange, also results in oleic acid replacement and chalcogenide enrichment, however the more reactive chalcogenides lead to smaller ordered domains (FIG. 21C). The NEUSCN ligand exchange step is added to achieve larger ordered domains.

[0086] After NH4SCN ligand exchange and treatment with Na?S solutions, the PbSe QD assemblies are soaked in 50 mM methanolic Cd(OCOCH3)2 solutions for 10 min at room temperature or at 65 °C, allowing Cd2+ions to bind to surface chalcogenide ions and form a Cd-chalcogenide layer on exposed PbSe QD surfaces. Raising the temperature of the Cd(OCOCH3)2 solution to 65 °C during growth drives the reaction and shortens the time to produce the Cd chalcogenide shell, as described further below. The enrichment with the chalcogenide from Na2S and the subsequent reaction with the Cd(OCOCH3)2 to produce the Cd-chalcogenide layer represents one cycle of the c-ALD treatment. Increasing the number (N) of repeated cycles of the c-ALD procedure (FIG. 15 A) dictates the extent of Cd-chalcogenide growth around and between the PbSe QDs in the assemblies. For example, for 6 nm PbSe QD assemblies, sequential c-ALD with N=3 cycles grows the Cd-chalcogenide on the QD surfaces but does not completely infill the interstitial spaces between the QDs (FIG. 22). Increasing to N^6 cycles grows the Cd- chalcogenide to the point of completely infilling the interstitial spaces, forming epitaxially-fused PbSe QD assemblies with an inorganic Cd-chalcogenide matrix. We also show through calculations that N^6 cycles completely infill the interstitial space between epitaxially-fused PbSe QD assemblies with CdS. We study the c-ALD process with Cd(OCOCH3)2 and Na2S solutions to create CdS matrices that are compositionally distinctfrom the core PbSe QDs to differentiate and study matrix growth on the QDs; however, the process may be generalized to bandgap engineer QD / matrix heterostructures; e.g., treatment with Na?Se solutions leads to formation of a CdSe matrix (FIG. 23).

[0087] FIG. 15D,E shows lower and higher resolution STEM images of epitaxially-fused PbSe QDs infilled with a CdS matrix. The square-packed higher atomic number (Z) PbSe QDs appear brighter and the infilled lower Z CdS matrix appears darker. In higher resolution STEM images, visible lattice fringes from the CdS matrix are seen indicating that the matrix is poly crystalline and that it grows heteroepitaxially on the { 111 } facets of PbSe QDs, studied in greater detail below. The inset SAED pattern shows the crystallinity of the PbSe QD / CdS matrix heterostructure; however, the small difference in lattice constants is reflected in the closeness of the PbSe and CdS reflections.

[0088] Data throughout the manuscript is color coded, as it is for the HR-STEM images in FIG. 15B-D, to represent PbSe QD assemblies that are oleic acid capped (black), are treated by ligand exchange in a NEUSCN solution (green) and a first treatment in a Na?S solution (red), and N-cycles of c-ALD shown for N=1 (very light blue), N=3 (light blue), and N=6 (dark blue). We note these samples serve as good comparisons of the structural and physical properties of the assemblies through different stages of the c-ALD process, with additional data provided herein.

[0089] Table 1: Elemental atomic ratios of Pb:Se and Cd:S for oleic acid capped PbSe QDs treated with NH4SCN and Na?S, N=3 c-ALD cycles, and N=6 c-ALD cycles.Ratios are measured via EDS in the STEM and SEM and calculated for a theoretical model of assembly and growth.

[0090] We characterize the c-ALD PbSe QD / CdS matrix heterostructure assembly growth using energy dispersive x-ray spectroscopy (EDS) in the TEM (FIG. 16 and Table 1). FIG. 16 shows individual elemental maps and their overlays for N=6 c-ALD PbSe QD / CdS matrix heterostructure assemblies. Pb and Se are found in the fused array of PbSe QD cores and Cd and S form the surrounding CdS matrix. Oleic acid capped PbSe QD assemblies have a Pb:Se ratio of 1.26± 0.03. Ligand exchange with NFLSCN results in removal of oleic acid ligands and replacement with SCN' moieties. After ligand exchange, the QD surface is treated with Na?S and enriched in chalcogen. The Pb:Se ratio remains the same, but the addition of S2' decreases the metal to chalcogen ratio to 0.81 ± 0.04. After c-ALD treatment, the excess chalcogen on the QD surface reacts with the Cd(OCOCH3)2, maintaining the Pb:Se ratio and introducing CdS with a Cd:S ratio of 1.01 ± 0.09 for N=3 c-ALD cycles and 0.71 ± 0.04 for N=6 c-ALD cycles. We model the epitaxially-fused PbSe QD assembly and infilling with the CdS matrix (detailed in FIG. 25), with stoichiometric control of PbSe QD assemblies using ligand exchange and a single c-ALD cycle. The PbSe QDs are 5.9 nm diameter, truncated octahedra terminated with eight, Pb-terminated { 111 } and six, stoichiometric { 100} facets, consistent with a Pb:Se of 1.14: 1. Upon ligand exchange, the QDs fuse along { 100} facets, maintaining a Pb:Se of 1.14: 1. In each c-ALD cycle, we assume S2' (Cd2+) ions binds to surface metals (chalcogenides) on exposed { 111 } facets, yielding a Pb:Se ratio that is unchanged at 1.14: 1 and a Cd:S ratio of 1 : 1 for N=3 and N=6 c-ALD cycles. EDS measured compositions are consistent with the modeled stoichiometry through the c-ALD growth process to form PbSe QD / CdS matrix heterostructures with the exception of a lower Cd:S ratio for N=6 c-ALD cycles. We hypothesize that upon complete infilling for N=6 c-ALD cycles, insufficient space limits addition of the larger Cd2+ions to the matrix. Similar trends in composition are found using EDS measurements in the SEM of ~1 pm thick films. We perform spectroscopic studies of 5 nm diameter PbSe QD assemblies to follow the c-ALD process and realize PbSe QD / CdS matrix heterostructures. We monitor the optical absorbance spectra at low energies near the first excitonic resonance of the PbSe QDs (FIG. 18A) and at high energies where the CdS matrix also absorbs light (FIG. 18B). At low energies (FIG. 18 A), optical absorbance spectra show 5 nm diameter, oleic acidcapped, PbSe QD assemblies (black) with a first excitonic peak at 0.839 eV. After NFLSCN-ligand exchange drives QD fusion, the first excitonic resonance of the PbSe QD assemblies red shifts by 0.043 eV and broadens by 0.035 eV. After NILSCN-ligand exchange and a first Na?S treatment (red), the first excitonic resonance of the PbSe QD assemblies further red shifts by 0.010 eV and broadens by 0.024 eV compared to PbSe QD assemblies treated with only NH4SCN (FIG. 28).

[0091] The amount by which the first excitonic absorbance resonance red shifts and broadens depends on the time between sample preparation and measurement, as ligand exchange with NH4SCN and / or with Na?S leaves reactive surfaces that continue to drive further QD fusion, evolving even in the nitrogen-filled environment of the glovebox (FIG. 28). The first excitonic resonance energy further red shifts by a 0.015 eV and the peak broadening increases by 0.048 eV over the first 24 h in the glovebox. As a result, a sample’s absorbance depends on the timescale over which the QDs progressively fuse prior to further treatment.

[0092] For PbSe QD assemblies quickly immersed in the Cd(OCOCH3)2 solution after ligand exchange and chalcogenide enrichment, treatment of the QD surfaces with Cd(OCOCH3)2 solutions stabilizes the surfaces and arrests QD fusion, consistent with previous studies using Pb-salt solutions. Arrestment of fusion is seen upon the initial N=1 c-ALD cycle in optical absorption spectra as Cd(OCOCH3)2 treated assemblies show excitonic resonances at shorter wavelengths, compared to PbSe QD assemblies treated with NH4SCN and Na2S solutions. These fused, Cd(OCOCH3)2-treated QD assemblies have excitonic absorption resonances that are red shifted compared to the parent oleic acid capped PbSe QD assemblies (FIG. 29); the magnitude of the shift depends on the temperature of the Cd(OCOCH3)2 treatment, as described below. Controlling the time before immersion of the QD assemblies in the Cd(OCOCH3)2 solution is important, with fusion of most PbSe QD thin-film assemblies in this work progressing for one to five min before initial arrestment via the c-ALD procedure, a time scale referred to as immediately in this work (FIG. 30). Allowing the samples to sit in the glovebox for 1 to 24 h after NFLSCN-ligand exchange and a first Na2S treatment allows fusion to progress, reflected in a red-shift in the first excitonic resonance, compared to samples treated immediately, after N=1 and N=6 c-ALD treatment cycles (FIGs. 30-31).

[0093] For typical immediate initiation of the c-ALD process, we monitor the PbSe QD excitonic resonance as we increase the number N of c-ALD cycles and grow the CdS matrix. For Cd(OCOCH3)2 solution treatments carried out at room temperature (FIG. 29), the excitonic resonance peak position shows, upon N=1 c-ALD cycles, a 0.027 eV red-shift and 0.042 eV broadening relative to the parent oleic acid capped PbSe QD assemblies. N=3 and N=6 samples are similar and show a further red-shift from N=1 samples of 0.015 eV and broadening of 0.039 eV of their first excitonic resonances (FIG. 29).

[0094] When the temperature of the Cd(OCOCH3)2 solution used in the c-ALD process cycles is raised to 65 °C (FIG. 29), the first excitonic peak of the PbSe QD assemblies with N=1 c-ALD cycles red-shifts by 0.012 eV and broadens by 0.045 eV from the first excitonic peak for oleic acid capped PbSe QD assemblies. The increase in linewidth of the first excitonic peak largely remains the same for different temperature Cd(OCOCH3)2 treatments. However, the smaller red-shift for the higher Cd(OCOCH3)2 temperature is consistent with the faster addition of Cd2+ions to the QD surface arresting the QD fusion. For assemblies further treated with N=6 c-ALD cycles (dark blue), where the Cd(OCOCH3)2 temperature is held at 65 °C, the PbSe QD excitonic resonance blue shifts 0.006 eV and broadens 0.043 eV compared to N=1 treated QD assemblies (FIG. 18 A). The small blue shift between N=l, N=3, and N=6 c-ALD cycles suggests that the higher reaction temperature may also promote partial cation exchange (FIG. 31).

[0095] At higher energies of 1.75 eV to 3.00 eV (FIG. 18B and FIG. 29), the absorbance of the c-ALD treated PbSe QD assemblies increases as the CdS matrix grows on the PbSe QD surfaces and fills in the interstitial gaps. PbSe QD assemblies after ligand exchange also exhibit an increase in absorbance at higher energies due to increased fusion of PbSe QDs resulting in a larger optical absorption cross sectional area. We isolate the absorbance component of the c-ALD grown CdS matrix by subtracting the absorbance of the ligand-exchanged PbSe QD assembly (inset, FIG. 18B). By extrapolating a linear fit to the difference spectrum, in a manner similar to a Tauc plot, we find an intercept of 2.18 eV, consistent with the CdS bulk bandgap at 2.42 eV.

[0096] We measure the photoluminescence (PL) spectra of the PbSe QD assemblies (FIG. 38). However, upon NH4SCN ligand exchange and / or a first Na2S treatment, the PL is lost entirely. The reduction in PL is consistent with ligand exchangeincreasing the density of surface defect sites; the Na?S treatment enriching the QDs in chalcogenides and heavily p-doping the assemblies, increasing the Auger process; and fusion of the PbSe QDs increasing carrier and exciton transport and access to defect sites driving recombination. PbSe QD assemblies treated by N=6 cycles of c-ALD exhibit no measurable PL recovery, consistent with an anticipated quasi or type-II band alignment, as quantum confinement reduces and / or changes the sign of the conduction band offset from the similar bulk PbSe and CdS electron affinities.

[0097] The effects of oxidation are studied by removing and exposing the PbSe QD assemblies treated with NH4SCN, a first treatment of Na?S, and N=l,3, and N=6 c- ALD cycles to ambient air (FIG. 18C, 18D, FIGs. 32-36). Measuring assemblies after exposure to air for short times up to 2 h shows the first excitonic resonance for PbSe QD assemblies after NH4SCN and / or a first Na?S treatment progressively blue shifts (FIG. 32), consistent with their rapid oxidation compared to oleic acid capped PbSe QD assemblies. Measurements of the oleic acid capped, NH4SCN exchanged, and NH4SCN and a first Na?S treated PbSe QD assemblies over a period of a month all show a progressive blue shift and broadening and eventually the loss of the PbSe QD first excitonic resonance (FIG. 18C). However, for PbSe QD assemblies treated with N=l, N=3, and N=6 c-ALD CdS layers, the first excitonic resonance blue shift and broadening with air exposure becomes increasingly smaller as N and the Cd(OCOCH3)2 temperature increases (FIGs. 33-36). FIG. 18D and FIGs. 36, 37 show statistical measurements following the spectral shift and broadening for multiple PbSe QD assemblies stored in ambient air for 1 month and a comparison of the initial and 1 month air exposed samples optical absorption spectra. After 1 month stored in ambient air, samples treated by N=6 c-ALD cycles maintain the excitonic resonances of the PbSe QDs, only showing a blue shift of 0.028 eV and 0.021 eV for Cd(OCOCH3)2 treatments at room temperature and 65 °C, respectively.

[0098] To examine the effects of annealing on the physical structure of the PbSe QD assemblies, HR-STEM images are collected for samples that are (A) unannealed and (B) annealed, at 150 °C for 10 min in a nitrogen glove box (FIG. 19). The PbSe QD assemblies with only NH4SCN ligand exchange and a first Na2S treatment, which unannealed form epitaxially-fused square lattices (FIG. 15B), sinter upon annealing to yield polycrystalline PbSe films with grain sizes of 36.08 ± 10.8 nm. In contrast, PbSe QD assemblies with N=3 and N=6 c-ALD cycles, which unannealed have epitaxially-fusedsquare lattices of PbSe QDs partially and completely filled by CdS matrices, maintain their nanoscale structures upon annealing, importantly demonstrating the thermal stability of QD / matrix heterostructured assemblies. The center-to-center distance of N=3 and N=6 c-ALD treated PbSe QD assemblies before annealing is 6.31± 0.45 nm and after annealing is 6.49±0.45 nm, showing no signs of densifying after annealing. The HR-STEM images of c-ALD treated PbSe QD assemblies after annealing do reveal a higher degree of crystallinity of the CdS matrix.

[0099] We study charge generation and transport in c-ALD PbSe QD / CdS matrix heterostructures in the device platforms of field effect transistors (FETs) and photoconductors. FETs are fabricated on heavily n-doped, Si wafers with 250 nm SiCL layers serving as the device gate electrode and gate dielectric layers, respectively. Photoconductors are fabricated on quartz substrates. The thermally-oxidized Si wafers and the quartz substrates are treated with (3-mercaptopropyl)trimethoxylsilane prior to spin coating 6 nm diameter, oleic acid capped PbSe QDs and treating the assemblies with NH4SCN ligand exchange and a first Na?S solution. The QD deposition and treatment is repeated twice to produce 20 nm thick films. Subsequently, Au is deposited by thermal evaporation to form device top electrical contacts and complete the FET and photoconductor fabrication. After electrode deposition, the FETs and photoconductors are treated with N=3 and N=6 c-ALD cycles to grow CdS matrices and studied before and after annealing at 150 °C for 10 min in a nitrogen glovebox.

[0100] Optical absorption spectra (FIG. 20A) of the corresponding (top) unannealed and (bottom) annealed PbSe QD assemblies are consistent with their corresponding structures shown in HR-STEM images in FIG. 19. Unannealed 6 nm diameter PbSe QD assemblies show first excitonic resonances at 0.689 eV. Like for 5 nm diameter QD assemblies, after NH4SCN ligand exchange and a first Na?S treatment and after N=3 cycles and N=6 cycles of c-ALD, the first excitonic resonance red-shifts and broadens, compared to untreated PbSe QD assemblies. Annealed assemblies of PbSe QD assemblies exchanged with NH4SCN solutions and a first treatment in a Na?S solution have first excitonic resonances that are nearly completely lost, consistent with thermal annealing leading to QD sintering forming polycrystalline PbSe films. PbSe QD assemblies treated by N=3 and N=6 c-ALD cycles to form CdS matrices maintain a first excitonic resonance upon annealing, with blue shifts of 0.036 eV and 0.023 eV andbroadening of 0.012 eV and 0.008 eV when compared to their pre-annealed first excitonic resonances, respectively. We hypothesize that the blue shift originates from alloying and incorporation of Cd into the PbSe QD assemblies. The minimal broadening and maintenance of the first excitonic resonance serves as evidence of the greater thermal stability of the QD / matrix nanostructures.

[0101] FETs are measured using a probe station mounted in a nitrogen-filled glovebox to study charge carrier type, concentration, and mobility after ligand exchange and c-ALD treatment (FIG. 20B, Table 2). Unannealed PbSe QD assemblies treated by NFUSCN ligand exchange and a first Na?S treatment are enriched in chalcogenide, and are thus degenerately p-doped, as seen by the small on / off current modulation2.14 for their FETs. These assemblies have a hole mobility fj.p= 0.17 cm2 / P ■ s and a threshold voltage VT> 100 V, outside of the window of measurement consistent with large hole concentrations of 1018~ to 1019cm-3. After N=3 or N=6 c-ALD treatments of the PbSe QD assemblies, the FETs remain p-type. The ^0N / [ increases by nearly an order of magnitude to 9.33 and 7.72 and VTshifts negatively, within the measurement window, to 37.7 V and 37.3 V, respectively (FIG. 39). We use the VTshift in the FET linear regime (VDS = 2V) characteristics to calculate the change in excess carrier concentration (Ap or An) from electronic doping as An =where A VTis thedifference between threshold voltages, Coxis the unit capacitance of the gate oxide, and d is the thickness of the film. After N=3 and N=6 c-ALD treatments of the PbSe QD assemblies, Ap decreases by 4.74 x 1018cm-3and 4.77 x 1018cm-3, respectively, and / / p decreases to= 37 x 10~3cm2 / V ■ s, respectively. The loweris consistent with the c-ALD process reducing the degenerate hole concentration to ~1017cm-3and shifting the Fermi level EF) away from the valence band and the high density, high mobility states.21,49,50The carrier type and concentration is dominated by the initial N=1 c-ALD cycle, enriching the QD surface with Cd2+ions, consistent with our previous study using a single c-ALD layer to control PbSe QD stoichiometry, i.e., by adding excess Pb2+ions on the QD surface to reduce p-doping and even n-doping PbSe QD thin films.

[0102] After annealing, PbSe QD FETs treated by NFUSCN and a first Na?S treatment remain degenerately p-doped, exhibiting little change with at 4.53, VT> 100 V, and fpat 0.43 cm2 / V ■ s. Interestingly, annealed c-ALD PbSe QD / CdS matrix FETs show a change in carrier type and concentration, as the heterostructures are more crystalline (FIG. 19) and as surface atoms / ions reconfigure and doping is thermally activated. As the doping depends on the metal: chalcogen ratio, the N=6 c-ALD treated films have a smaller metal: chalcogen ratio and thus, the FETs are less n-type than the N=3 c-ALD treated FETs. Annealed N=3 c-ALD devices show predominantly n-type ambipolar characteristics with an electron mobility )J.n= 99 X 10~3cm2 / V ■ s and / J.p= 3.5 x 10~3cm2 / V • s and an increased ^°N / f0FFof 102. The change in polarity is consistent with a VTof -15.9 V and an increase in the electron carrier concentration to ~1017cm-3. Devices treated with N=6 c-ALD cycles exhibit predominantly p-type ambipolar characteristics with fin= 2.0 x 10~3cm2 / V ■ s and fip= 21.3 x 10-3cm2 / V ■ s. N / increases to 14.4 and a positive VT shift results in a hole carrier concentration ' ‘OFF of ~1016cm-3.

[0103] Photoconductor current transients are measured under a bias of 10 V and laser excitation at 1550 nm, with powers averaging 3 mW, periodically modulated on and off at time intervals of 30 s by using a mechanical shutter (FIG. 20C). Photocurrent modulation inthe transient device characteristics is calculated from thephotogenerated currentfound by subtracting device currents in the dark from those under illumination, and dividing by dark current ( / 0). Before annealing, photoconductors composed of epitaxially-fused NFUSCN and Na?S treated PbSe QD assemblies have higher than those of N=3 or N=6 c-ALD grown PbSe QD / CdS matrixheterostructures, consistent with the higher mobility hole transport of the assemblies dominating device photocurrents. After annealing, the QDs sinter in the ligand-exchanged PbSe QD photoconductors and and device responsivity is greatly diminished. Incontrast, the thermal stability of the c-ALD treated heterostructures leads to maintenance or an increase of the ^phL depending on the number of c-ALD cycles. The dark currentsand photocurrents of photoconductors are dependent on the carrier concentration and mobility following the current equations, Io= qAn0[iE and Iph= qA TqiE, where q is the charge of an electron, A is the device area, and E is the electric field across the device channel. However, j = ^n / n0, assuming the carrier-concentration dependentmobility difference is small in the dark and under illumination as would be expected for the small excess photogenerated carrier concentrations. for c-ALD N =3photoconductors increases by nearly 50-fold after annealing and originates from the decrease in dark current levels and increase of the majority n-type carrier concentration and mobility. Assuming the optical generation rate is constant, an increase in An is also consistent with an increase in the majority carrier lifetime. EAiQphl of c-ALD N=6 photoconductors remains nearly the same after annealing. Though dark current levels decrease for c-ALD N=6 photoconductors, the majority p-type carrier concentration and mobility also drops.

[0104] Further engineering of large-area ordered25,60’61and thickness controlled PbSe QD / CdS matrix heterostructures and of semiconductor-metal and semiconductor dielectric interfaces40promises to allow expanded measurements of materials physics and to optimize and more greatly characterize electrical and optoelectronic device performance.

[0105] Conclusion

[0106] We achieved fully inorganic QD / matrix heterostructures via a postsynthetic c-ALD process to infill epitaxially-fused PbSe QD assemblies with a CdS matrix. The QD / matrix heterostructures increase the oxidative and thermal stability of the assemblies, and by controlling the number of c-ALD cycles, allows control over carrier type and concentration, and thus carrier mobility and lifetime measured in FETs and photoconductors. This work sets the stage for continued studies to investigate the structure of the PbSe QD / CdS matrix interface and carrier statistics, transport, and dynamics important to engineering device stacks and optimizing the performance of QD electrical and optoelectronic devices.

[0107] This approach can be used to expand the compositional library of epitaxially-fused QDs and matrix materials and even to create multimaterial matrices and tailored alloying and interfacial strain. QD / matrix engineering can be used to design theband alignment, interface potential, and QD / matrix dielectric contrast and the carrier energy, type, concentration, mobility, and lifetime of QD / matrix heterostructures. For example, the c-ALD technique can be used to create environmentally and thermally stable heterostructures such as type-I QD heterostructures that regain PL after epitaxial fusion or type-II QD heterostructures that feature optimized mobility -lifetime products for high optoelectronic device performance.

[0108] Materials

[0109] Lead oxide and extra dry acetone are purchased from Acros.Tri octylphosphine (TOP, 90%), octadecene (ODE), oleic acid (OA, 90%) amorphous selenium pellets (99.999%), diphenylphosphine (DPP), (3- mercaptoprpyl)trimethoxylsilane (MPTS, 95%), ammonium thiocyanate (NFLSCN), sodium sulfide (Na?S), anhydrous cadmium acetate (Cd(OCOCH3)2), anhydrous isopropyl alcohol, anhydrous methanol, and anhydrous toluene are purchased from Aldrich.

[0110] Methods

[0111] PbSe QD Synthesis

[0112] Colloidal PbSe QDs, 5 nm and 6 nm diameter, are synthesized according to a previously reported procedure.32 39 41QD synthesis is carried out under a nitrogen atmosphere using Schlenk line techniques. 446 mg of lead oxide is dissolved in 10 mL of ODE and 1.5 mL of OA and degassed under vacuum at 120 °C for 2 h. The solution is then heated to either 140 °C (for 5 nm diameter QDs) or 180 °C (for 6 nm diameter QDs) under nitrogen. A 4 mL solution of 1 M TOP-Se solution and 40 pL DPP is then injected into the heated solution. After a reaction time of 30 to 60 s, the heating mantle is removed and the dispersion is quickly cooled down and transferred to a Schlenk tube via a double- ended cannula. The PbSe QDs are purified in a nitrogen glovebox with the addition of 2 mL of the solvent hexane and then 40 mL of the anti solvent isopropyl alcohol, leading to precipitation of the QDs, which are collected by centrifugation at 6000 rpm for 5 min. The PbSe QDs are redispersed in 2 mL of hexane and the washing process is repeated two more times with 20 mL of isopropyl alcohol and then a final time with 20 mL of acetone as the antisolvents. The PbSe QDs are then dispersed in 4 mL of hexane and stored in a glovebox.

[0113] Colloidal Atomic Layer Deposition (c-ALD) Procedure

[0114] The PbSe QDs are dried, weighed, and redispersed in toluene to produce 10 mg / mL PbSe QD dispersions. Substrates (e.g. glass, quartz, sapphire, silicon) are treated with 5% MPTS in toluene overnight to form a self-assembled monolayer. Inside a nitrogen glovebox, the PbSe QD dispersion is deposited by spin coating onto the substrates at 800 rpm for 20 s and 2000 rpm for 5 s, producing thin films with thicknesses of ~10 nm. The films are then treated with 10 mg / mL methanolic NH4SCN solutions for 30 s and rinsed three times with methanol. The films are then treated with 1 mM methanolic Na?S solutions for 10 s and rinsed three times with methanol. The films are then immersed for 10 min in 50 mM Cd(OCOCH3)2) methanolic solutions and removed and rinsed with methanol, completing one c-ALD cycle (N=l). To achieve multiple c- ALD cycles, the Na?S and Cd(OCOCH3)2) treatments are repeated N times to achieve N cycles of c-ALD.

[0115] Structural and Optical Characterization

[0116] TEM samples are prepared by spin coating PbSe QDs onto 300 mesh Cu TEM grids adhered to a Si wafer with Kapton tape. Subsequent ligand exchange and c- ALD treatments are carried out on TEM grids on Si substrates. TEM grids are purchased from EMS. EDS and TEM / STEM imaging is taken using JEOL F200 and JEOL NEOARM microscopes with Cold-FEG emission sources and operating at 200 kV.

[0117] EDS and SEM imaging is taken using a Quanta ESEM operating at 30 kV.

[0118] Absorbance measurements are taken on glass and quartz substrates on a Cary Series UV-Vis Spectrophotometer. Photoluminescence spectra are taken on quartz and sapphire substrates on an Edinburgh Fluorimeter FLS with excitation at 400 nm and 425 nm.

[0119] FET and Photoconductor Device Fabrication

[0120] FETs and photoconductors are fabricated by spin coating 10 mg / mL PbSe QD dispersions in toluene at 800 rpm for 20 s and 2000 rpm for 5 s on n+doped singleside polished silicon wafers with a 250 nm SiCL oxide and on quartz substrates for electrical and optoelectronic measurements, respectively. The PbSe QD thin films are treated using ligand exchange and c-ALD processes, as described above. Thicker, ~20 nm PbSe QD films are also prepared by spin-coating a second layer of PbSe QDs and following the same ligand exchange and c-ALD processes. An evaporator mounted in thenitrogen-filled glovebox is used to deposit 100 nm Au electrodes through shadow masks. FETs have channel lengths of 100 jim and widths of 1500 jim. Photoconductors feature contact pads in a dog-bone shape with a channel length of 1 j m and width of 6800 jim.

[0121] Device Characterization

[0122] FET and photoconductor electrical measurements are conducted at room temperature using a Karl Suss PM5 probestation mounted inside a glovebox and a HP 4516C semiconductor parameter analyzer. For linear and saturation regime FET characteristics, we fit the forward sweep for n-type characteristics and the reverse sweep for p-type characteristics. Photoconductors are excited with a tunable, femtosecond Coherent Monaco 1035 laser light that is carried by optical fiber into a photoconductor measurement setup inside a nitrogen glovebox. The laser has a circular spot size with 1500 jim diameter. Photoconductor transients are taken by turning excitation off and on in 30 s periods.

[0123] Theoretical Calculations of c-ALD Treatment Infilling in the Interstitial Void Spaces of Epitaxially-Fused PbSe QD Assemblies

[0124] We model the epitaxial fusion of 6 nm diameter PbSe QDs and the c- ALD of the CdS matrix. The PbSe QDs are epitaxially fused along the { 100} facets and thus atoms on these facets do not contribute to the c-ALD process. We base our calculations on a theoretical model reported by Oh et al. for epitaxial fusion of PbSe QD assemblies.1We assume the 6 nm diameter PbSe QDs are truncated octahedra, with an edge length of 1.897 nm and a total volume of approximately 77.23 nm3, and form epitaxially-fused QD assemblies with a QD center-to-center distance of 6 nm.

[0125] We calculate the interstitial volume in the void spaces between a 2 x 2 x 2 array of eight, PbSe QDs by forming a cube using the 6 nm center-to-center distance to give a volume of 216 nm3. The interstitial volume is calculated by subtracting the volume occupied in the cube by the 1 / 8 of each of the eight octahedral PbSe QDs from the total volume of the cube itself. Because we have eight octahedra and 1 / 8 of each octahedron is in the cube, the total volume of the PbSe QDs inside the cube is the volume of one octahedron. Therefore, the total interstitial volume is 216 nm3- 77.23 nm3= 138.77 nm3.

[0126] To calculate the number of reactive sites for the growth of the CdS matrix, we consider a 6 nm PbSe QD to consist of 2190 Pb atoms and 1925 Se atoms. The QDs fuse along the { 100} facets and thus the atoms on these facets are assumed not tocontribute to the c-ALD reaction. The { 111 } facets contribute to the growth of the CdS matrix. The { 111 } facets are Pb-terminated and after a first Na?S treatment, there are 64 S2' ions on each { 111 } facet. There are a total of 8 { 111 } facets for each QD but we only consider 1 / 8 of those facets facing in and contributing to the interstitial void volume. Thus, there is one { 111 } facet per QD that contributes to the c-ALD process filling the interstitial volume with CdS.

[0127] We then look at the formation of the CdS matrix. We use the lattice constant of zinc-blende CdS, 5.832 A, for our calculations. The volume of a FCC CdS unit cell is then the cube of the lattice constant, 0.1984 nm3. A FCC unit cell consists of four S atoms and four Cd atoms, i.e., there are 4 CdS units per FCC unit cell. Thus, the volume of a single CdS unit, representing the bond between a unit of a Cd2+and a S2' on the surface of the QD, is 0.050 nm3. Thus, the first c-ALD cycle produces 64 reactive sites multiplied by the volume of one CdS unit, giving us a volume of 3.17 nm3occupied by the CdS matrix. For the eight PbSe QDs, the volume occupied by CdS after one cycle is 25.40 nm3. As an ALD process, we assume each Cd2+ions binds to one S2' ion and vice versa. As N increases and the interstitial space is filled, the number of reactive sites as well as the unfilled space will decrease, requiring deposition of fewer CdS units.

[0128] To calculate the number of c-ALD cycles required to fill the interstitial void volume between a 2 x 2 x 2 array of PbSe QDs, we assume the volume occupied by the CdS matrix after one cycle of c-ALD occupies a trapezoidal prism configuration. The angle between the { 111 } and { 100} facet is calculated as 54.74°, the length of the initial { 111 } facet is the edge length of the truncated octahedron, 1.897 nm, and we take the bond distance between Cd and S, 2.55 A, to be the height of the trapezoid. We calculate the ratio of the longer base to the shorter base of the trapezoid to be 0.81 for the first c- ALD cycle, resulting in 52 reactive sites and a length of 1.54 nm for the second c-ALD cycle. Repeating the same calculation of the trapezoidal ratio for 52 reactive sites and the longer trapezoidal edge length of 1.54 nm yields 40 reactive sites with a length of 1.18 nm. We repeat this calculation and find that the number of reactive sites is exhausted after N=6 c-ALD cycles, as it takes 6 cycles of c-ALD to fully infill the interstitial void. This confirms our experimental observation of fully infilling the interstitial void with N=6 cycles of c-ALD.

[0129] QD / Matrix Stoichiometry

[0130] Model:

[0131] We begin by calculating the QD / matrix stoichiometry, and thus Pb:Se and Cd:S ratios. As synthesized, oleic acid capped, 6 nm diameter PbSe QDs consist of 2190 Pb atoms and 1925 Se atoms, yielding a Pb:Se of 1.14: 1. The Pb:Se ratio is expected to remain unchanged throughout the c-ALD process, with exception to the possible loss of Pb2+during the methanolic NH4SCN and Na2S treatments, as methanol is known to strip surface metals.

[0132] After S2' treatment, there are a total of 8 {111 } facets with 64 S2' on each facet. When we react Cd2+with surface S2' using the c-ALD treatment, we assume they add one-to-one to yield a Cd:S ratio of 1: 1. The second cycle adds 52 S2' and an additional 52 Cd2+as infilling progresses and the number of available sites decreases. As a result, we assume the Cd:S ratio to be 1 : 1 and the Pb:Se ratio as remaining the same, independent of N. The metal: chalcogen ratio after N=1 c-ALD cycle is 1.11, after N=3 c-ALD cycles is 1.08, and after N=6 c-ALD cycles is 1.07. In practice, in each layer and as the matrix grows to infill the interstices, the number of added anions and cations may differ. Since Cd2+is added to S2' sites and since Cd2+ions are larger than S2', we expect the number of Cd2+added to be smaller than that of S2', especially as infilling approaches completion, resulting in a smaller metal: chai cogen ratio than that calculated.

[0133] EDS Elemental Ratios '.

[0134] We use EDS measurements in the HRTEM to calculate the Pb:Se and Cd:S ratios through the ligand exchange and c-ALD processes. Proper k-factors are used in the fitting and integration of counts for calculating elemental ratios. The close proximity of the S (2.31 keV) and Pb (2.34 keV) elemental peaks make it difficult to accurately analyze elemental contributions. To address this interference, we anchor all EDS spectra to an oleic acid capped PbSe sample by integrating the Pb, L (10.55 keV) and Se, K (11.22 keV) peaks and use it as a standard in scaling the counts of Pb, L and Se, K peaks for other EDS spectra. We assume the Pb:Se ratio does not change significantly throughout the subsequent ligand exchange and c-ALD treatments. We integrate the Pb, M (2.342 keV) and S, K ((2.307 keV) region in samples treated with S2' and then subtract the counts from the Pb, L region of the PbSe standard to isolate the sulfur counts. This allows us to obtain elemental ratios of Pb:Se and Cd:S with spectral fitting and analysis. The sulfur countshave higher error due to the fitting and subtraction procedure. We maintain the EDS measurement conditions for each sample, though the number of particles featured in each EDS measurement can vary.

[0135] Detrending Photoconductor Transients

[0136] Photoconductor transients measured with light on and off at various times show sloping currents due to carrier nonequilibrium. A common solution is to bias photoconductors under no optical stimulation for time scales of a few hours to reach steady state dark current levels. However, photoconductors may still exhibit falling dark currents at a constant slope. In order to accurately and reliably measure photoconductor characteristics such as lifetime, rise and fall times, or modulation, we detrend the photoconductor transient measurements by using a Python detrend tool that removes the sloped baseline from transients. Photoconductor characteristics are measured using detrended photoconductor transients. Average current is used due to noise in current signals.

[0137] Table 2 - Example FET Device Characteristics

[0138] Aspects

[0139] The following Aspects are illustrative only and do not limit the scope of the present disclosure or the appended claims. Any part or parts of any one or more Aspects can be combined with any part or parts of any one or more other Aspects.

[0140] Aspect 1. A method, comprising: disposing a plurality of layers on a plurality of NCs disposed on a substrate, wherein a given layer comprises a matrix material that includes a compound comprising an anionic species associated with a cationic species, the plurality of NCs optionally being core-shell NCs or quantum dots, and wherein the method is performed so as to give rise to interparticle fusion between at least some of the plurality of NCs and to completely fill interstitial gaps between NCs. This is shown, for example, in section III of FIG. 1. Adjacent NCs can, as described, be electronically coupled. It should be understood that the disclosed technology can completely fill all available interstital gaps between NCs in a given layer. This is not a requirement, however, as the disclosed technology can in some embodiments completely fill some but not all available interstitial gaps between NCs in a given layer.

[0141] In some embodiments, interparticle distance - between neighboring NCs - can be less than or equal to 5, 4, 3, 2, or even 1 nm. Neighboring NCs can be epitaxially fused; without being bound to any particular theory or embodiment, epitaxial fusion can be a suitable embodiment for charge transport that achieves comparatively high mobilities. The growth of the matrix material - which can be inorganic - can be performed such that there is an all-inorganic matrix between assembled NCs.

[0142] Aspect 2. The method of Aspect 1, wherein the plurality of NCs bear an initial ligand thereon, and further comprising removing the initial ligand.

[0143] Aspect 3. The method of Aspect 2, wherein the initial ligand comprises an organic ligand, the organic ligand optionally comprising any one or more of oleic acid, octadecene, and oleylamine. It should be understood that the foregoing listing of ligands is not exhaustive and is exemplary only.

[0144] Aspect 4. The method of any one of Aspects 2-3, wherein the removing the initial ligand comprises exchanging the initial ligand for a first ligand.

[0145] Without being bound to any particular theory, the use of ligand exchange allows one to control interparticle distance - for example to < 1 nm between core NCs and epitaxially fuse particular NCs to create more electrically conductive NC assemblies.These ligand exchanged QD assemblies have unpassivated surface sites. The sites can be passivated with the c-ALD process to grow additional semiconductor materials around the core QDs and passivate electronic defect states, which can be useful in increasing carrier lifetime and luminescence.

[0146] Aspect 5. The method of Aspect 4, wherein the first ligand comprises any one or more of a halide, a pseudohalide, a hydroxide, a pnictide, and a chalcogenide. Exemplary ligands that comprise a halide include tetrabutyl ammonium chloride, ammoinium chloride, tetrabutyl ammonium bromide, tetrabutyl ammonium iodide. Example ligands that comprise a pseudohalide include thiocyanate and azide. An example ligand that comprises a pnictide is arsenic chloride. Example ligands that include a chalcogenide include sodium sulfide and sodium selenide. An example ligand that comprises hydroxide is tetramethyl ammonium hydroxide.

[0147] Aspect 6. The method of Aspect 4, wherein the first ligand comprises any one or more of NEUSCN and ethylenediamine.

[0148] Aspect 7. The method of any one of Aspects 1-6, further comprising enriching surfaces of the plurality of NCs with at least one chalcogenide, wherein the at least one chalcogenide comprises at least one of S2' and Se2'.

[0149] Aspect 8. The method of any one of Aspects 1-7, wherein disposing a layer comprises contacting the anionic species with the cationic species. The cationic species can be, for example, a metal.

[0150] Aspect 9. The method of Aspect 8, wherein the anionic species comprises any one or more of a chalcogenide and a pnictogenide. Example, non-limiting such species include sodium sulfide and sodium selenide

[0151] Aspect 10. The method of Aspect 9, wherein the chalcogenide comprises at least one of S2' and Se2'.

[0152] Aspect 11. The method of Aspect 8, wherein the cationic species is a metal. The cationic species can, for example, be introduced as a metal salt.

[0153] Aspect 12. The method of Aspect 11, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In3+, Pb2+, and Zn2+.

[0154] Aspect 13. The method of any one of Aspects 1-12, wherein the method comprises disposing a plurality of layers, the method optionally being performed so as to give rise to from 2 to 10 layers. It should be understood, however, that the disclosedmethods can be performed to give rise to from 2 to 10 layers, or from 2 to 9 layers, or from 2 to 8 layers, or from 2 to 7 layers, from 2 to 6 layers, from 2 to 5 layers, from 2 to 4 layers, or even from 2 to 3 layers.

[0155] Aspect 14. The method of any one of Aspects 1-13, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers. As a non-limiting example, the first two layers can include the same anion, and the next four layers can include a different anion than the first two layers.

[0156] Aspect 15. The method of any one of Aspects 1-14, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers. As a non-limiting example, the first two layers can include the same cation, and the next four layers can include a different cation than the first two layers.

[0157] Aspect 16. The method of any one of Aspects 1-15, further comprising effecting cation exchange so as to change a cation of the NCs.

[0158] Aspect 17. The method of Aspect 16, wherein the cation of the NCs comprises Pb, and wherein the cation exchange is performed so as to exchange Pb for Cd.

[0159] Aspect 18. The method of Aspect 17, wherein the core-shell NCs comprise PbSe, and wherein the cation exchange is performed so as to exchange Pb for Cd.

[0160] Aspect 19. A composition, comprising: a plurality of NCs exhibiting interparticle fusion therebetween and the plurality of NCs defining interstitial spaces therebetween, the interstitial spaces being filled with a matrix material that comprises a plurality of layers, a given layer including an anionic species associated with a cationic species, and the plurality of NCs optionally being core-shell NCs or quantum dots. The interstitial spaces can be, in some embodiments, be completely filled by the matrix material.

[0161] As described elsewhere herein, the matrix material can comprise a metal and a chalcogenide. CdS and CdSe are example - but non-limiting - matrix materials.

[0162] Aspect 20. The composition of Aspect 19, wherein the anionic species comprises a chalcogenide.

[0163] Aspect 21. The composition of Aspect 20, wherein the chalcogenide comprises at least one of S2' and Se2'.

[0164] Aspect 22. The composition of any one of Aspects 19-21, wherein the cationic species comprises a metal.

[0165] Aspect 23. The composition of Aspect 22, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In2+, and Zn2+.

[0166] Aspect 24. The composition of any one of Aspects 19-23, wherein a NC comprises PbSe or InAs. An NC can also comprises CdSe, CdS, CdTe, PbS, PbTe, InP, InSb. Binary compounds are suitable, as are ternary compounds. One such ternary compound is InAsxSbi-x.

[0167] Aspect 25. The composition of any one of Aspects 19-24, wherein the matrix material comprises any one or more of CdS, CdSe, ZnS, ZnSe, ZnTe, and CdTe. The foregoing are non-limiting.

[0168] Aspect 26. The composition of any one of Aspects 19-25, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers. As a non-limiting example, the first two layers can include the same anion, and the next four layers can include a different anion than the first two layers.

[0169] Aspect 27. The composition of any one of Aspects 19-26, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers. As a non-limiting example, the first two layers can include the same cation, and the next four layers can include a different cation than the first two layers.

[0170] Aspect 28. The composition of any one of Aspects 19-27, wherein a first layer of the plurality of layers comprises a first material, wherein a second layer of the plurality of layers comprises a second material, and wherein the first material and the second material differ in terms of at least one of anionic species and cationic species.

[0171] Aspect 29. The composition of any one of Aspects 19-28, wherein the composition exhibits a first exci tonic resonance that changes by less than about 10% after 30 days of exposure to ambient air, optionally wherein the composition exhibits a first excitonic resonance that changes by less than about 8% after 30 days of exposure to ambient air.

[0172] A composition according to the present disclosure can, for example, exhibit a first excitonic peak of 0.8 eV and a peak shift change of 0.031 eV for a peakfollowing about 30 days of exposure to ambient air. The bandgap of a given composition can, of course, influence the peak and change.

[0173] As an example, for an exemplary PbSe - SCN - Na?S composition, over 2 weeks of oxidation exposure in ambient air, there is an about 22% change of the first excitonic peak position; for a month of oxidation for a N=6 c-ALD treated sample, there can be a change of 8% of the first excitonic peak. The magnitude of the PbSe - SCN - Na?S peak shift after 2 weeks of exposure to air is 0.1725 eV, and for N=6 c-ALD sample, it is 0.0631 eV. After 1 month of such exposure, the first excitonic peak of the PbSe - SCN - Na?S disappears, and for the N=6 c-ALD sample, its shift is 0.0679 eV.

[0174] Aspect 30. The composition of any one of Aspects 19-29, comprising a structure that exhibits an epitaxial configuration which is maintained following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box. Such a structure can comprise the plurality of NCs exhibiting interparticle fusion therebetween, as such a structure can exhibit an epitaxial configuration. Layers grown thereon can likewise exhibit an epitaxial configuration.

[0175] Aspect 31. The composition of any one of Aspects 19-30, wherein the diameter of a NC of the composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

[0176] Aspect 32. The composition of any one of Aspects 19-31, wherein the distance between adjacent NCs of composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

[0177] Aspect 33. The wcomposition of any one of Aspects 19-32, wherein the composition essentially maintains quantum confinement following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box.

[0178] Aspect 34. The composition of any one of Aspects 19-33, wherein the composition is characterized as a Type I heterostructure.

[0179] Aspect 35. The composition of any one of Aspects 19-33, wherein the composition is characterized as a Type II heterostructure.

[0180] As explained herein, the disclosed post deposition c-ALD technology allows for infilling of a QD matrix with same or different semiconductor materials. Wehave demonstrated that the interstitial space between epitaxially-fused QDs can be fully infilled with the c-ALD technology. The c-ALD technology is unique to QD structures and allows for flexible tailoring of the surface chemistry to produce desired QD heterostructures. For the c-ALD process to occur, the surface of a QD is first chemically enriched with a ligand treatment that provides ions, creating potential binding sites. The enriched surface is then immersed in a metal salt that provides metal ions to bind to the ions provided by the ligand. This constitutes a cycle of the c-ALD process and grows a single layer of an inorganic shell.

[0181] Typically, QD heterostructures include of QD core / shell systems. However, the QD cores of these core / shell systems are not coupled, resulting in limited charge transport. We demonstrate here the successful use of the c-ALD technology on epitaxially-fused QDs, achieved post deposition, in the solid state, to produce QD / matrix heterostructures. Epitaxially-fused QDs demonstrate increased carrier mobilities. However, the enhanced mobility leads to increased access of carriers to defect states, decreasing lifetime and feasibility for optoelectronics. The c-ALD technology can be used on epitaxially-fused QDs to passivate surface defects in via the infilling of the QD matrix through multiple c-ALD cycles in a layer-by-layer fashion with another semiconductor material, increasing lifetime while maintaining electronic coupling between epitaxially- fused QDs. The resulting structures can be both oxidatively- and thermally-stable.

[0182] The c-ALD technology is modular, meaning various cycles of c-ALD can be performed. Variance of the number of c-ALD cycles provides a unique route to achieve tunability and tailoring of device metrics, such as carrier concentration, mobility, and lifetime. We demonstrate the modularity of device characteristics with various cycles of infilling the QD matrix, using the c-ALD technology.

[0183] The c-ALD technology can be extended to other material systems. The flexibility in using different materials to perform c-ALD upon and with allows for bandgap-engineering of QD-based heterostructures for tailored optoelectronic performance.

[0184] Some QDs exhibit environmental instability, such as oxidation upon exposure to air and thermal instability in the form of sintering. These environmental instabilities lead to loss of quantum confinement and optical responsivity. As shown herein, structures achieved with the c-ALD technology demonstrate maintenance ofquantum confinement and enhanced environmental stability in response to exposure to air and thermal annealing. Further the c-ALD technology, as compared to other methods of achieving QD heterostructures, can be effected at lower temperatures. This is relevant to scalability of the technology to industrial processes.

Claims

What is Claimed:

1. A method, comprising: disposing a plurality of layers on a plurality of nanocrystals (NCs) disposed on a substrate, wherein a given layer comprises a matrix material that includes a compound comprising an anionic species associated with a cationic species, the plurality of NCs optionally being core-shell NCs or quantum dots, and wherein the method is performed so as to give rise to interparticle fusion between at least some of the plurality of NCs and to completely fill interstitial gaps between NCs.

2. The method of claim 1, wherein the plurality of NCs bear an initial ligand thereon, and further comprising removing the initial ligand.

3. The method of claim 2, wherein the initial ligand comprises an organic ligand, the organic ligand optionally comprising any one or more of oleic acid, octadecene, and oleylamine.

4. The method of claim 2, wherein the removing the initial ligand comprises exchanging the initial ligand for a first ligand.

5. The method of claim 4, wherein the first ligand comprises any one or more of a halide, a pseudohalide, a hydroxide, a pnictide, and a chalcogenide.

6. The method of claim 4, wherein the first ligand comprises any one or more of NH4SCN and ethylenediamine.

7. The method of any one of claims 1-6, further comprising enriching surfaces of the plurality of NCs with at least one chalcogenide, wherein the least one chalcogenide comprises at least one of S2' and Se2'.

8. The method of any one of claims 1-6, wherein disposing a layer comprises contacting the anionic species with the cationic species.

9. The method of claim 8, wherein the anionic species comprises any one or more of a chalcogenide and a pnictogenide.

10. The method of claim 9, wherein the chalcogenide comprises at least one of S2' and Se2'.

11. The method of claim 8, wherein the cationic species is a metal.

12. The method of claim 11, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In3+, Pb2+, and Zn2+.

13. The method of any one of claims 1-6, wherein the method comprises disposing a plurality of layers, the method optionally being performed so as to give rise to from 2 to 10 layers.

14. The method of any one of claims 1-6, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers.

15. The method of any one of claims 1-6, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers.

16. The method of any one of claims 1-6, further comprising effecting cation exchange so as to change a cation of the NCs.

17. The method of claim 16, wherein the cation of the NCs comprises Pb.

18. The method of claim 17, wherein the cation exchange is performed so as to exchange Pb for Cd.

19. A composition, comprising:a plurality of NCs exhibiting interparticle fusion therebetween, and the plurality of NCs further defining interstitial spaces therebetween, the interstitial spaces being completely filled with a matrix material that comprises a plurality of layers, a given layer including an anionic species associated with a cationic species, and the plurality of NCs optionally being core-shell NCs or quantum dots.

20. The composition of claim 19, wherein the anionic species comprises a chalcogenide.

21. The composition of claim 20, wherein the chalcogenide comprises at least one of S2' and Se2'.

22. The composition of any one of claims 19-21, wherein the cationic species comprises a metal.

23. The composition of claim 22, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In2+, and Zn2+.

24. The composition of any one of claims 19-21, wherein a NC comprises PbSe or In As.

25. The composition of any one of claims 19-21, wherein the matrix material comprises any one or more of CdS, CdSe, ZnS, ZnSe, ZnTe, and CdTe .

26. The composition of any one of claims 19-21, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers.

27. The composition of any one of claims 19-21, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers.

28. The composition of any one of claims 19-21, wherein a first layer of the plurality of layers comprises a first material, wherein a second layer of the plurality of layers comprises a second material, and wherein the first material and the second material differ in terms of at least one of anionic species and cationic species.

29. The composition of any one of claims 19-21, wherein the composition exhibits a first excitonic resonance that changes by less than 10% after 30 days of exposure to ambient air, optionally wherein the composition exhibits a first excitonic resonance that changes by less than about 8% after 30 days of exposure to ambient air.

30. The composition of any one of claims 19-21, comprising a structure that exhibits an epitaxial configuration which is maintained following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box.

31. The composition of any one of claims 19-21, wherein a diameter of a NC of the composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

32. Aspect 32. The composition of any one of claims 19-21, wherein a distance between adjacent NCs of the composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

33. The composition of any one of claims 19-21, wherein the composition essentially maintains quantum confinement following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box.

34. The composition of any one of claims 19-21, wherein the composition defines a heterostructure that forms a Type I band alignment.

35. The composition of any one of claims 19-21, wherein the composition defines a heterostructure that forms a Type II band alignment.What is Claimed:

1. A method, comprising: disposing a plurality of layers on a plurality of nanocrystals (NCs) disposed on a substrate, wherein a given layer comprises a matrix material that includes a compound comprising an anionic species associated with a cationic species, the plurality of NCs optionally being core-shell NCs or quantum dots, and wherein the method is performed so as to give rise to interparticle fusion between at least some of the plurality of NCs and to completely fill interstitial gaps between NCs.

2. The method of claim 1, wherein the plurality of NCs bear an initial ligand thereon, and further comprising removing the initial ligand.

3. The method of claim 2, wherein the initial ligand comprises an organic ligand, the organic ligand optionally comprising any one or more of oleic acid, octadecene, and oleylamine.

4. The method of claim 2, wherein the removing the initial ligand comprises exchanging the initial ligand for a first ligand.

5. The method of claim 4, wherein the first ligand comprises any one or more of a halide, a pseudohalide, a hydroxide, a pnictide, and a chalcogenide.

6. The method of claim 4, wherein the first ligand comprises any one or more of NH4SCN and ethylenediamine.

7. The method of any one of claims 1-6, further comprising enriching surfaces of the plurality of NCs with at least one chalcogenide, wherein the least one chalcogenide comprises at least one of S2' and Se2'.

8. The method of any one of claims 1-6, wherein disposing a layer comprises contacting the anionic species with the cationic species.

9. The method of claim 8, wherein the anionic species comprises any one or more of a chalcogenide and a pnictogenide.

10. The method of claim 9, wherein the chalcogenide comprises at least one of S2' and Se2'.

11. The method of claim 8, wherein the cationic species is a metal.

12. The method of claim 11, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In3+, Pb2+, and Zn2+.

13. The method of any one of claims 1-6, wherein the method comprises disposing a plurality of layers, the method optionally being performed so as to give rise to from 2 to 10 layers.

14. The method of any one of claims 1-6, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers.

15. The method of any one of claims 1-6, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers.

16. The method of any one of claims 1-6, further comprising effecting cation exchange so as to change a cation of the NCs.

17. The method of claim 16, wherein the cation of the NCs comprises Pb.

18. The method of claim 17, wherein the cation exchange is performed so as to exchange Pb for Cd.

19. A composition, comprising:a plurality of NCs exhibiting interparticle fusion therebetween, and the plurality of NCs further defining interstitial spaces therebetween, the interstitial spaces being completely filled with a matrix material that comprises a plurality of layers, a given layer including an anionic species associated with a cationic species, and the plurality of NCs optionally being core-shell NCs or quantum dots.

20. The composition of claim 19, wherein the anionic species comprises a chalcogenide.

21. The composition of claim 20, wherein the chalcogenide comprises at least one of S2' and Se2'.

22. The composition of any one of claims 19-21, wherein the cationic species comprises a metal.

23. The composition of claim 22, wherein the metal comprises any one or more of Bi2+, Cd2+, Hg2+, In2+, and Zn2+.

24. The composition of any one of claims 19-21, wherein a NC comprises PbSe or In As.

25. The composition of any one of claims 19-21, wherein the matrix material comprises any one or more of CdS, CdSe, ZnS, ZnSe, ZnTe, and CdTe .

26. The composition of any one of claims 19-21, wherein a layer of the plurality of layers comprises an anion that differs from an anion of another layer of the plurality of layers.

27. The composition of any one of claims 19-21, wherein a layer of the plurality of layers comprises a cation that differs from a cation of another layer of the plurality of layers.

28. The composition of any one of claims 19-21, wherein a first layer of the plurality of layers comprises a first material, wherein a second layer of the plurality of layers comprises a second material, and wherein the first material and the second material differ in terms of at least one of anionic species and cationic species.

29. The composition of any one of claims 19-21, wherein the composition exhibits a first excitonic resonance that changes by less than 10% after 30 days of exposure to ambient air, optionally wherein the composition exhibits a first excitonic resonance that changes by less than about 8% after 30 days of exposure to ambient air.

30. The composition of any one of claims 19-21, comprising a structure that exhibits an epitaxial configuration which is maintained following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box.

31. The composition of any one of claims 19-21, wherein a diameter of a NC of the composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

32. Aspect 32. The composition of any one of claims 19-21, wherein a distance between adjacent NCs of the composition changes by less than about 5% following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box, optionally by less than about 4%, optionally by less than about 3%.

33. The composition of any one of claims 19-21, wherein the composition essentially maintains quantum confinement following annealing at 150 deg. C. for 10 minutes in a nitrogen glove box.

34. The composition of any one of claims 19-21, wherein the composition defines a heterostructure that forms a Type I band alignment.

35. The composition of any one of claims 19-21, wherein the composition defines a heterostructure that forms a Type II band alignment.

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