Method for obtaining a solid-state film of metal chalcogenide quantum dots doped with n-type doping and an optoelectronic device comprising the obtained film

Abstract

The present invention relates to a method for obtaining an n-type doped metal chalcogenide quantum dot solid film, the method comprising: forming a metal chalcogenide quantum dot solid film; performing an n-type doping process on a plurality of metal chalcogenide quantum dots of the metal chalcogenide quantum dot solid film to make them exhibit intraband absorption, the process including partially replacing chalcogen element atoms with halogen atoms in the plurality of metal chalcogenide quantum dots, and disposing a substance on the plurality of metal chalcogenide quantum dots to avoid oxygen p-type doping of the metal chalcogenide quantum dots. The present invention also relates to an optoelectronic device, the optoelectronic device comprising: an n-type doped metal chalcogenide quantum dot solid film (A) obtained according to the method; and a first electrode (E1) and a second electrode (E2) physically contacting two corresponding spaced-apart regions of the film (A).

Description

Technical Field

[0001] A first aspect of the present invention relates to a method for obtaining an n-type doped metal chalcogenide quantum dot solid film, the method comprising a robust n-type doping process that causes the metal chalcogenide quantum dots to exhibit intraband absorption.

[0002] A second aspect of the present invention relates to an optoelectronic device that includes an n-type doped metal chalcogenide quantum dot solid film obtained by the method according to the first aspect of the present invention. Background Art

[0003] Optical sensing in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) is crucial for numerous applications including environmental monitoring, gas sensing, hazard detection, food and product manufacturing inspection, etc. However, currently such applications are served by expensive and complex epitaxially grown HgCdTe (mercury cadmium telluride), quantum wells, and quantum dot infrared photodetectors. The possibility of utilizing low intraband transitions makes colloidal quantum dots (CQDs) an attractive low-cost alternative to the expensive low-bandgap materials used in current infrared applications. Unfortunately, the fabrication of quantum dots that exhibit intraband absorption is technically limited by the requirement for controlled heavy doping, and to date, the requirement for controlled heavy doping has restricted mid-wave infrared colloidal quantum dot (MWIR CQD) detectors and long-wave infrared colloidal quantum dot (LWIR CQD) detectors to mercury-based materials.

[0004] The possibility of utilizing low intraband transitions makes colloidal quantum dots an attractive low-cost alternative to the expensive low-bandgap materials currently employed in infrared applications [1-3]. To achieve steady-state intraband absorption, high doping of stable colloidal quantum dots (CQDs) is required [4]. However, precise control of doping is an ongoing challenge in colloidal quantum dot (CQD) technology [5], especially if long-term stability in air is required. Thus, although steady-state intraband absorption has been demonstrated in different materials (see [4]), it was not until recently that the first devices utilizing intraband transitions (MWIR photodetectors) were shown to be made using mercury-chalcogenide nanocrystals [6-8].

[0005] Previous reports on doping PbS quantum dots (QDs) have relied on aliovalent cations or aliovalent anions: Ag + substituting for Pb 2+ to induce p-type characteristics in PbS [9] and PbSe

[10] , while Bi 3+ or In 3+ substituting for Pb 2+This renders PbS

[11] and PbSe

[12] to exhibit more n-type characteristics. There is also evidence of n-type doping of PbS after ligand-halide exchange

[13] . In particular, it has been proposed that partial substitution of S - by I -2 might contribute to the formation of n-type PbS [14, 15]. Unfortunately, oxygen is an effective p-type dopant in lead-chalcogenide compounds and reduces the effectiveness of halide doping in air, such that n-type doping in PbS quantum dot solids has only been demonstrated in low doping scenarios [13, 15]. Long-range electron transfer from cobaltocene molecules is another disclosed mechanism for doping n-type PbS and PbSe quantum dots, resulting in intraband absorption

[16] . However, none of the above methods have led to robust permanent doping [10, 12, 16], thus hindering their use in devices.

[0006] US9318628B2 discloses infrared photodetectors in mid-wave and long-wave infrared based on mercury-chalcogenide quantum dots. However, in this patent, only interband excitation (i.e., above the bandgap) was considered.

[0007] Therefore, there is a need to provide an alternative to the state of the art by providing a method for obtaining an n-type doped metal chalcogenide quantum dot solid film and an optoelectronic device comprising said n-type doped metal chalcogenide quantum dot solid film - wherein the solid film is heavily n-type doped, thus covering the gaps present in the prior art. SUMMARY OF THE INVENTION

[0008] To this end, the present invention in a first aspect relates to a method for obtaining an n-type doped metal chalcogenide quantum dot solid film, the method comprising:

[0009] - forming a metal chalcogenide quantum dot solid film, and

[0010] - performing an n-type doping process on at least a plurality of metal chalcogenide quantum dots of the metal chalcogenide quantum dot solid film to cause the plurality of metal chalcogenide quantum dots to exhibit intraband absorption, wherein the n-type doping process comprises:

[0011] - in at least the plurality of metal chalcogenide quantum dots, partially substituting chalcogen element atoms with halogen atoms; and

[0012] - disposing a substance on at least the plurality of metal chalcogenide quantum dots, wherein the substance is made and arranged to avoid p-type doping of oxygen of the plurality of metal chalcogenide quantum dots.

[0013] For one embodiment, the metal chalcogenide is at least one of a Pb-chalcogenide, a Cd-chalcogenide, and a Hg-chalcogenide, the chalcogen atom is at least one of a sulfur atom, a selenium atom, and a tellurium atom, and the halogen atom is at least one of an iodine atom, a bromine atom, and a chlorine atom.

[0014] In other words, the metal chalcogenide is represented by MX, where M can be Pb, Cd, Hg, and X can be sulfur (S), selenium (Se), and tellurium (Te) or a combination thereof.

[0015] Preferably, the crystal structure of the metal chalcogenide quantum dots has a zinc blende or rock salt structure.

[0016] Also preferably, in the metal chalcogenide, the metal has a +2 oxidation state and the chalcogen has a -2 oxidation state.

[0017] According to one embodiment, the method of the first aspect of the present invention includes providing the above-mentioned substance for coating the metal chalcogenide quantum dot solid film, so as to isolate the metal chalcogenide quantum dot solid film from oxygen in the environment.

[0018] For a supplementary or alternative embodiment, the method of the first aspect of the present invention includes providing the above-mentioned substance for infiltrating into the metal chalcogenide quantum dot solid film, so as to react with the oxygen present in the metal chalcogenide quantum dot solid film to inhibit oxygen p-type doping.

[0019] For a preferred embodiment, the method of the first aspect of the present invention includes providing the above-mentioned substance by atomic layer deposition (ALD), but the method of the present invention also includes other less preferred deposition techniques for providing the substance, such as chemical bath deposition or chemical layer deposition.

[0020] According to some embodiments, the above-mentioned substance is an oxide-type substance.

[0021] For some examples of the embodiments, the above-mentioned substance is at least one of aluminum oxide, titanium dioxide, zinc oxide (ZnO), and hafnium dioxide.

[0022] For one embodiment of the method of the first aspect of the present invention, the step of forming the metal chalcogenide quantum dot film includes forming a solid film having only one type of quantum dots, the quantum dots having an exposed chalcogen side to allow halide doping and thus allow n-type doping. In the case of PbS, this typically occurs on quantum dots with a bandgap of about 1200 nm (corresponding to a diameter of about 4 nm). For this embodiment, the method includes applying the above-mentioned n-type doping process over the entire formed solid film such that all the metal chalcogenide quantum dots are heavily n-type doped.

[0023] According to an embodiment of the method of the first aspect of the present invention, the step of forming the metal chalcogenide quantum dot film includes: forming a mixture having a host matrix composed of first metal chalcogenide quantum dots and second metal chalcogenide quantum dots with a smaller bandgap embedded in the host matrix, wherein, compared with the first metal chalcogenide quantum dots, the second metal chalcogenide quantum dots are larger and have different morphologies, such that the second metal chalcogenide quantum dots have more exposed sides containing chalcogen element atoms, thereby allowing effective electron doping by halide substitution, and wherein the method includes applying the n-type doping process over the entire formed metal chalcogenide quantum dot film, such that the second metal chalcogenide quantum dots are heavily n-type doped (because they have suitable planes for doping), while the first metal chalcogenide quantum dots are not n-type doped or are only slightly n-type doped. In this way, the dark conductivity of a photodetector including the film thus obtained is suppressed, which can lead to an enhanced SNR compared to a device based on a single quantum dot size that is all doped.

[0024] For an example of the embodiment, the method of the first aspect of the present invention includes selecting the bandgaps and band alignments of the first metal chalcogenide quantum dots and the second metal chalcogenide quantum dots such that the first metal chalcogenide quantum dots and the second metal chalcogenide quantum dots form a type-I heterojunction and a band offset, and the band offset makes the energy difference in the conduction band or the valence band equal to or less than the in-band energy of the second metal chalcogenide quantum dots.

[0025] The method of the first aspect of the present invention includes forming a mixture as described below: wherein the concentration of the second metal chalcogenide quantum dots is preferably in the range of 1% to 50% by volume, and more preferably between 5% and 25% by volume.

[0026] For an alternative embodiment, the step of forming the metal chalcogenide quantum dot film includes: forming a layered structure in which layers composed of first metal chalcogenide quantum dots and layers composed of second metal chalcogenide quantum dots are alternately arranged, for example, forming a superlattice structure, wherein, compared with the first metal chalcogenide quantum dots, the second metal chalcogenide quantum dots have a smaller bandgap, and the second metal chalcogenide quantum dots are larger and have different morphologies, such that the second metal chalcogenide quantum dots have more exposed sides containing chalcogen element atoms, and wherein the method includes:

[0027] - Applying the n-type doping process over the entire formed metal chalcogenide quantum dot film such that the second metal chalcogenide quantum dots are heavily n-type doped while the first metal chalcogenide quantum dots are not n-type doped or are only slightly n-type doped; or

[0028] - Applying the n-type doping process only on one or more layers composed of the second metal chalcogenide quantum dots.

[0029] According to an embodiment of any of the above two alternative embodiments, the method of the first aspect of the present invention includes: selecting the size and morphology of the first metal chalcogenide quantum dots such that the first metal chalcogenide quantum dots do not have any chalcogen-rich exposed sides; and selecting the size and morphology of the second metal chalcogenide quantum dots such that the second metal chalcogenide quantum dots have one to six chalcogen-rich exposed sides.

[0030] For some embodiments, regarding the metal chalcogenide quantum dots that are heavily doped or will be heavily n-type doped, the size range of their diameter is from 2 nm to 30 nm, the bandgap range is from 2.5 eV to 0.2 eV, and the thickness range is from 20 nm to 10 μm, preferably between 100 nm and 1 μm.

[0031] The second aspect of the present invention relates to an optoelectronic device, which includes:

[0032] - At least one n-type doped metal chalcogenide quantum dot solid film obtained by the method according to the first aspect for any embodiment; and

[0033] - A first conductive electrode and a second conductive electrode that are physically in contact with two corresponding spaced-apart regions of the at least one n-type doped metal chalcogenide quantum dot solid film.

[0034] For one embodiment, the at least one n-type doped metal chalcogenide quantum dot solid film is a light absorption film, and the light absorption film is made to exhibit in-band absorption of light having wavelengths included in a predetermined wavelength range. The predetermined wavelength range extends beyond the absorption range of the bandgap of the metal chalcogenide quantum dots when not n-type doped.

[0035] According to the embodiment of the embodiment, the predetermined wavelength range includes mid-wave infrared radiation and long-wave infrared radiation, and preferably wavelengths of at least from 5 μm up to 12 μm.

[0036] For a preferred embodiment, the optoelectronic device implements a photodetector that is fabricated to detect light of any wavelength having a wavelength within the predetermined wavelength range and within the wavelength range of the interband absorption of the metal chalcogenide quantum dots in the n-type doped metal chalcogenide quantum dot solid film.

[0037] According to a first embodiment of the preferred embodiment, the photodetector is a planar photodetector, including a substrate on which the at least one n-type doped metal chalcogenide quantum dot solid film, the first conductive electrode, and the second conductive electrode are deposited.

[0038] For a first variant of the first embodiment, the substrate is opaque to light having a wavelength within the predetermined wavelength range, such that the photodetector detects light directly incident from the top on the at least one n-type doped metal chalcogenide quantum dot solid film.

[0039] For a second variant of the first embodiment, the substrate is transparent to light of any wavelength within the predetermined wavelength range, such that the photodetector detects light passing through the substrate from the bottom before hitting the at least one n-type doped metal chalcogenide quantum dot solid film.

[0040] According to a second embodiment of the above preferred embodiment, the photodetector is a vertical photodetector, including a substrate on which the first conductive electrode is deposited, wherein the at least one n-type doped metal chalcogenide quantum dot solid film is deposited on the first conductive electrode, and the second conductive electrode is deposited on the at least one n-type doped metal chalcogenide quantum dot solid film.

[0041] For a first variant of the second embodiment, the substrate is opaque to light having a wavelength within the predetermined wavelength range, the second conductive electrode is transparent to light having a wavelength within the predetermined wavelength range, and the first conductive electrode is reflective to light having a wavelength within the predetermined wavelength range, such that the photodetector detects light passing through the second conductive electrode from the top, hitting the at least one n-type doped metal chalcogenide quantum dot solid film, and being reflected by the first conductive electrode.

[0042] For a second variant of the second embodiment, the substrate and the first conductive electrode are both transparent to light having wavelengths included in the predetermined wavelength range, and the second conductive electrode is reflective to light having wavelengths included in the predetermined wavelength range, such that the photodetector detects light that passes through the substrate from the bottom, through the first conductive electrode, impinges on the at least one n-type doped metal chalcogenide quantum dot solid film, and is reflected by the second conductive electrode.

[0043] The following are possible material examples for making the above first conductive electrode and / or second conductive electrode transparent or semi-transparent to light having wavelengths included in the predetermined wavelength range: graphene; metal thin films or metal oxide TCOs (transparent conductive oxides), such as ITO (indium tin oxide), AZO (aluminum-doped zinc oxide), IGZO (indium gallium zinc oxide), or FTO (fluorine-doped tin oxide), the material being thin enough to allow at least 10% transmittance at the wavelengths, preferably in the infrared wavelength.

[0044] For another embodiment of the optoelectronic device according to the second aspect of the present invention, the optoelectronic device includes the above solid film - the solid film includes only heavily n-type doped quantum dots sandwiched between a first conductive electrode and a second conductive electrode.

[0045] Generally, the photodetector further includes: a biasing device for applying a bias voltage to one of the above first conductive electrode and second conductive electrode, and a readout unit for reading the current circulating through the n-type doped metal chalcogenide quantum dot solid film.

[0046] Other applications of the film obtained by the method according to the first aspect of the present invention (whether by implementing an optoelectronic device or by implementing a non-optoelectronic device) are, for example: remote sensing, surveillance, thermal imaging, spectroscopy, chemical sensing, automotive vision, process inspection, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Hereinafter, some preferred embodiments of the present invention will be described with reference to the drawings. These embodiments are provided for illustrative purposes only and do not limit the scope of the present invention.

[0048] Figure 1 The electrical and optical characteristics of the doping density of iodine-doped PbS colloidal quantum dots (CQDs) showing an embodiment of the method according to the first aspect of the present invention are shown. Among them, a shows the energy level schematic diagram of solid PbS quantum dots with iodine ligands. 1S h and the Fermi level (E F ) are obtained from the measured UPS data. 1S eis obtained by adding the measured optical bandgap to 1S h 1P e is obtained by adding the in-band energy (obtained from the fit in Figure 2 c) to 1S e All samples were made under ambient air. b shows the quantum dot doping density n - as a function of exciton wavelength, represented by e QD / dot. The dashed line is a guide to the eye. The data were obtained from FET and absorption (optical) measurements. Error bars illustrate the uncertainty in the colloidal quantum dot (CQD) packing density (see experimental section). Three adjacent wavelength regions were identified: up to 1300 nm: unoccupied 1S e ; 1300 nm - 1800 nm: partially occupied 1S e ; above 1800 nm: highly occupied 1S e . c shows absorption measurements of various (four different sizes) PbS colloidal quantum dot films measured before (dashed line) and after (solid line) alumina deposition. Excitonic absorption bleaching was observed upon filling the 1S e state. The dashed lines therein have been normalized, but the ratio of the original data is retained for each dashed / solid line pair.

[0049] Figure 2 shows the optical characteristics of the exciton and in-band transitions of doped PbS quantum dots according to an embodiment of the method of the first aspect of the present invention. Among them, a shows the transmission measurements of an iodide-replaced PbS colloidal quantum dot film before (upper line) and after (lower line) filling and covering with alumina. The inset shows the possibility of in-band absorption and exciton absorption bleaching due to filling the 1Se state. b shows the interband absorption and in-band absorption of different-sized dots. The dashed line is a Gaussian fit to the in-band absorption peak. c shows the in-band transition energy as a function of the PbS exciton energy, obtained from a fit to absorption measurements similar to a and b. White dots represent the direct measurement results of the 1S e →1P e transition, while black dots are obtained by subtracting the energies of the 1S h →1P e transition and the 1S h →1S e transition. The corresponding lines shown are linear fits to the data. d shows the 1S e state and the 1P e state (referred to as 1S hValues of the (state). The inset plots the intra - band energy as a function of the quantum dot diameter. The black line is an exponential fit to the data.

[0050] Figure 3 Shows the MWIR and LWIR intra - band photodetection spectra of heavily doped PbS quantum dots for an embodiment of the method according to the first aspect of the present invention. Among them, a shows absorption spectra of three samples with room - temperature exciton wavelengths of 1630 nm, 1790 nm, and 2000 nm measured at 80K from top to bottom. b shows the photodetection quantum efficiency of devices (i), (ii), and (iii) using the same quantum dots as those in a. Shows the applied drain - source voltage V DS and the chopping frequency f of the excitation light. Gaussian fits (dashed lines) are superimposed on the measured data. The vertical dashed line represents the bulk bandgap of PbS at 80K. c shows the modeled normalized steady - state conductance G QD of PbS quantum dots as a function of the doping level n 0 . The left - hand axis in d, the dashed line, shows the modeled conductance increase ΔG due to intra - band optical absorption; the right - hand axis in d, the solid line, shows the modeled photodetection performance expressed as ΔG / G QD as a function of n 0 . Squares (i), (ii), and (iii) represent the cases of the three devices (i), (ii), and (iii) studied in Figure 3 b. The inset shows the effect of doping on the occupancy of the 1S e state and thus on the values of G 0 and ΔG.

[0051] Figure 4 Shows UPS data of the secondary - electron cutoff and valence band of PbS colloidal quantum dot films of different sizes. For each case, the exciton wavelength is shown.

[0052] Figure 5 Shows the XPS signal at the Al2p peak of a doped PbS film covered with 5 nm of alumina deposited by ALD for an embodiment of the method according to the first aspect of the present invention. The measured data are plotted with open squares. The solid line is a Gaussian fit. Before surface analysis, the sample was subjected to an in - situ etching process that removed approximately 20 nm - 40 nm of material. This result demonstrates that alumina fills the film during the ALD process.

[0053] Figure 6a and 6bShows XPS measurement results (hollow circles) of Pb and S in PbS colloidal quantum dots (CQDs) of different sizes (different exciton wavelengths) after synthesis (oleate ligand, OA) and ligand replacement (iodide ligand, EMII), as well as individual fits (dashed lines) and total fits (solid lines). The measured Pb / S ratio supports partial iodide substitution of sulfur atoms during the ligand replacement process.

[0054] Figure 7 Shows the FET transfer characteristics (I DS -V G ) of doped PbS dots with different exciton wavelengths according to an embodiment of the method of the first aspect of the present invention. Shows the V DS value and channel length L of each measurement result. All devices exhibit n-type characteristics.

[0055] Figure 8 Shows the FTIR transmission spectra of two colloidal quantum dot films spin-coated with the same PbS dots. In one of the colloidal quantum dot films, the original oleate ligand is replaced with iodide (using EMII), and in the other colloidal quantum dot film, the original oleate ligand is replaced with thiol (using EDT). Both colloidal quantum dot films are encapsulated with alumina. The film with iodide ligand shows interband bleach and in-band absorption.

[0056] Figure 9 Shows the FTIR transmission measurement results of a 20 nm Al 2 O 3 layer deposited on a low-doped (1 - 10 ohm·cm) silicon substrate by ALD. The transmission spectrum of Al 2 O 3 is obtained using a brand-new silicon substrate as a background sample. Al 2 O 3 shows light absorption in the range of 10 μm - 25 μm.

[0057] Figure 10 Shows the absorption spectrum of a doped PbS colloidal quantum dot film according to an embodiment of the method of the first aspect of the present invention. Three different transitions are identified by Gaussian fitting (dashed lines). Shows the transition 1S h →1S e (first exciton) and the energy of the transition 1S h →1P e . As shown in the inset, the energy difference between these two transitions is equal to the energy of the in-band transition 1S e →1P e .

[0058] Figure 11 Shows the frequency-dependent in-band photocurrent of n-type doped PbS colloidal quantum dots (CQDs) of an embodiment of the method according to the first aspect of the present invention under monochromatic 6.8 μm excitation, where the exciton wavelengths are 1630 nm (black dots) and 2000 nm (white dots). The applied V DS is 0.7 V. The photocurrent is inversely proportional to frequencies as low as 30 Hz, indicating a response time of >30 ms.

[0059] Figure 12 Shows TEM images of self-assembled PbS colloidal quantum dots with oleate ligands and exciton wavelengths of 820 nm (as shown in a) and 1830 nm (as shown in b) of an embodiment of the method according to the first aspect of the present invention. c, d show TEM images of iodide-replaced PbS colloidal quantum dot films prepared according to the procedures exactly the same as those of the FET and photodetector devices of the present invention of an embodiment of the method according to the first aspect of the present invention. The quantum dots used to fabricate these films are the same as those shown in b.

[0060] Figure 13 Shows the FET output characteristics (I DS versus V DS ) of doped PbS dots with different exciton wavelengths of an embodiment of the method according to the first aspect of the present invention. All devices exhibit ohmic properties. For all measurements, the gate voltage V G is 0 V.

[0061] Figure 14 Shows a scheme of the quantum dot (QD) transport model. The transport between quantum dots can occur via 1S e channels (G S ) and via 1P e channels (G P ). The total conductance is G = G S + G P . The selected position of the Fermi level E F represents the filled 1S e and the empty 1P e .

[0062] Figure 15 Shows the influence of the ratio m = K S / K P Φ in the modeled ΔG / G 0 . m = 1, m = 10, and m = 0.1 were evaluated.

[0063] Figure 16Schematically shown is an optoelectronic device according to a second aspect of the present invention, for an embodiment in which the optoelectronic device includes an n-type doped metal chalcogenide quantum dot solid film obtained by the method according to the first aspect sandwiched between a first electrode and a second electrode. For one embodiment, the film is a mixture of heavily n-type doped metal chalcogenide low bandgap quantum dots embedded in a host matrix composed of un-n-type doped (or slightly n-type doped) metal chalcogenide high bandgap quantum dots.

[0064] Figure 17 Schematically shown is another embodiment of the optoelectronic device according to a second aspect of the present invention, in which an n-type doped metal chalcogenide quantum dot solid film has been obtained according to the method of the first aspect. For one embodiment, the film is a layered structure in which layers composed of heavily n-type doped metal chalcogenide low bandgap quantum dots and layers composed of un-n-type doped (or slightly n-type doped) metal chalcogenide high bandgap quantum dots are alternately arranged.

[0065] Figure 18 Schematically shown is, for a film obtained by the method according to the first aspect of the present invention, the mechanism of intraband carrier transition of small bandgap heavily n-type doped quantum dots and the mechanism of carrier transfer from heavily n-type doped quantum dots to un-n-type doped or slightly n-type doped quantum dots under low infrared excitation.

[0066] Figure 19 Schematically shown is an embodiment of the optoelectronic device according to a second aspect of the present invention, for which the optoelectronic device realizes a planar photodetector.

[0067] Figure 20 Schematically shown is another embodiment of the optoelectronic device according to a second aspect of the present invention, for which the optoelectronic device realizes a vertical photodetector that is fabricated to detect light incident from the bottom, i.e., light that strikes the substrate and passes through the substrate.

[0068] Figure 21 Schematically shown is another embodiment of the optoelectronic device according to a second aspect of the present invention, for which the optoelectronic device realizes a vertical photodetector that is fabricated to detect light incident from the top, i.e., light that strikes the upper electrode and passes through the upper electrode.

[0069] Figure 22It does not correspond to the present invention, but rather to a method that provides a high surface coverage by binding iodine atoms to Pb atoms on the surface. Figure a shows a schematic diagram of an iodine-passivated (111) surface; Figure b shows the calculated density of each state of the (111) surface. The figure and its corresponding description are provided below to demonstrate that surface coverage is an immaterial issue for the present invention. Detailed Description of the Invention

[0070] In this section, the inventors have demonstrated the feasibility and good results provided by the present invention through several experiments described in detail below, particularly for embodiments where the metal chalcogenide quantum dot solid film is a PbS quantum dot solid film and sulfur atoms are partially replaced by iodine atoms.

[0071] Specifically, it is demonstrated here that the present invention provides a robust doping strategy for PbS quantum dot solid films that allows the collection of mid-wave infrared radiation and long-wave infrared radiation, far exceeding the limits achievable even in the large-size form of PbS. Heavy n-type doping is achieved through the substitution of sulfur by iodine and the effective isolation from ambient oxygen, which results in the simultaneous occurrence of interband bleaching and an increase in intraband absorption. The inventors show that the doping is stable under ambient conditions, thus making it possible for the first time to achieve an intraband PbS colloidal quantum dot (CQD) photodetector with an energy lower than the large bandgap in the range of 5 μm - 12 μm.

[0072] Here, the inventors demonstrate for the first time that the intraband absorption of photon energy and photodetection are far below the large bandgap in PbS colloidal quantum dot (CQD) solids.

[0073] Here, the inventors show that by effectively substituting sulfur with iodine and combining it with the isolation of the dots from oxygen, the inventors are able to control the effective filling of the conduction band (CB) and allow for the steady-state detection of intraband transitions in PbS colloidal quantum dots (CQDs). The proposed doping strategy allows the removal of oxygen incorporated in the film during film formation, even when it is only physically adsorbed

[17] . The inventors believe that subjecting the colloidal quantum dot film to atomic layer deposition (ALD) of, for example, aluminum oxide (Al 2 O 3 ) should be doubly beneficial for the intended purpose. First, encapsulation with aluminum oxide prohibits the oxidation process in PbS colloidal quantum dots (CQDs) by isolating the film from the atmosphere

[18] . Second, the layer-by-layer ALD method allows the penetration of precursors inside the film

[18] . Therefore, it is desirable that the aluminum precursor used during deposition reacts not only with the oxygen precursor (H 2 O), but also with the highly reactive oxygen adsorbates present in the film, thereby suppressing their p-type doping effect.

[0074] The inventors have synthesized PbS colloidal quantum dots (CQDs) according to different embodiments of the method of the first aspect of the present invention and fabricated films using an optimized procedure (under ambient atmosphere) for replacing the original oleate ligands with iodides (see the experimental section). Figure 1 Panel a in Figure 4 shows the main results of the ultraviolet photoelectron spectroscopy (UPS) of the proposed iodide-replaced PbS dots. The UPS data have been analyzed according to the calibration disclosed in

[19] ( F ). Four samples with exciton wavelengths in the range of 940 nm to 1910 nm have been measured. Since the Fermi level (E e ) is closer to the CB (1S h state) rather than to the valence band (VB, 1S e state), all samples show n-type characteristics. Interestingly, as the particle size increases (exciton wavelength increases), the n-type characteristics become stronger. For example, for the largest particles measured, the Fermi level is above the 1S Figure 1 state, which means that the CB is in equilibrium. In the same way, the inventors fabricated films for absorption measurements of quantum dots (QDs) of different sizes. The inventors have carried out absorption measurements before and after alumina deposition. Figure 5 )

[0075] Both the UPS and absorption measurement results show that the n-type doping level increases as the dot size increases. This is due to the structure-dependent stoichiometry of the colloidal quantum dots (CQDs), especially related to the exposed facets of quantum dots of different sizes: small dots are octahedral in shape with eight Pb-rich (111) facets, while as the dot diameter increases, their morphology gradually evolves to cuboctahedra with six additional S-rich (100) facets

[22] (see Figure 1(embedded graph in b in). The inventors have performed XPS measurements on PbS colloidal quantum dot films without any encapsulation using the original oleate ligands and the substituted iodide ligands (see Figure 6a and Figure 6b ). Quantitative analysis of the lead and sulfur data (see Table S1 below) shows that the Pb / S ratio increases after ligand substitution, consistent with the replacement of sulfur by iodine. In addition, as the particle size increases (more sulfur atoms are available on the surface), the relative increase in the Pb / S ratio after ligand substitution is also greater. These data indicate that effective anion substitution in larger colloidal quantum dots (CQDs) due to exposed (100) side facets is crucial for achieving highly doped schemes. In contrast, small PbS colloidal quantum dots (CQDs) do not allow this doping path due to their (111) exposed side facets, which hinders the demonstration of heavy doping in these dots [13, 15].

[0076] Table S1 below shows the effect of the ligand substitution process on the Pb / S ratio of PbS colloidal quantum dots (CQDs) of different sizes (expressed as exciton wavelength). The Pb / S ratio is obtained by quantitative analysis of the XPS measurement results shown in Figure 6a and Figure 6b for the original oleate ligand (OA) and the iodide ligand (EMII).

[0077]

[0078] Table S1

[0079] The inventors have quantified the doping level of the samples used, expressed as electrons per dot (e- / dot), by two different methods - optical (absorption) and electrical (field effect transistor, FET) measurements - (see the experimental section). QD The doping level is expressed as electrons per dot (e- / dot). Figure 1 b in shows n as a function of the exciton wavelength of the QD QD . It is found that all samples are n-type doped in the FET measurements ( Figure 7 ). Both methods yield comparable n QD values. Small dots (excitons up to 1300 nm) have unfilled 1S e . From approximately 1300 nm to 1800 nm, n QD gradually increases with the size of the dots. Beyond 1800 nm, the FET results show a stagnation of n QD . Note that n QD stagnates at approximately 8 e- / dot. Since the 1S e of PbS quantum dots is eight-fold degenerate (including spin)

[23] , n QD ≈ 8 e- / dot represents an almost fully occupied 1Se , which is consistent with the complete bleaching of the exciton absorption as shown in c of Figure 1 . The following Table S2 summarizes the electrical parameters obtained by FET measurements in iodine-doped PbS colloidal quantum dot films. The high doping level of the samples results in highly conductive films, which exhibit a conductivity value close to 0.2 S·cm -1 .

[0080]

[0081] Table S2

[0082] Intraband absorption is complementary to the first exciton (or interband) bleaching upon filling of the CB [20, 21]. Figure 2 a in h shows the transmission measurement results of two samples replaced by iodide. As shown in the upper right inset, the standard sample (without alumina, black line) shows strong absorption at the interband transition (1S e ). After alumina deposition (and thus filling), as shown in the lower left inset, due to the partial occupancy of 1S e , the exciton peak (0.786 eV) is partially bleached, and a strong intraband absorption peak (0.191 eV) appears. Note that for this sample (bandgap ≈ 1580 nm), 3 < n Q D < 4 (see Figure 1 b), which illustrates the presence of the interband peak and the intraband peak. Transmission measurements performed on PbS films replaced with thiol ligands (instead of iodide) and encapsulated with alumina did not show exciton bleaching or intraband absorption (see Figure 8 ), further demonstrating the doping effect of iodine. Figure 2 b in Figure 9 shows the transmission results of doped samples with different quantum dot (QD) sizes from smaller (bottom) dots to larger (top) dots. As shown by the Gaussian fit (red dashed line), all samples show intraband absorption, and the transition peak is in the range of 6 μm - 9 μm. As the dots become larger, the intraband transition redshifts, which is due to the gradual relaxation of charge confinement. Also note that as the dot size increases, the interband peak (left side of the panel) gradually disappears. Beyond 10 μm, the alumina overlay contributes to absorption (see

[0083] Figure 2 c in e shows the relationship between the intraband transition energy and the exciton energy of doped PbS colloidal quantum dots (CQDs). As shown in the lower right inset, the intraband transition energy has been obtained in two different ways: (I) fitting the direct intraband (1S e ) absorption measurement results (red circles), as previously inFigure 2 as shown in b of; and (II) by fitting and subtracting the 1S h →1P e transition energy and the 1S h →1S e transition energy (black circles) indirectly, as Figure 10 shown. The latter method is supported by the fact that the parity selection rule is relaxed in PbS quantum dots, thus allowing the 1S h →1P e transition, which is otherwise forbidden, to occur in an additional way [24, 25]. Both methods give similar results for the dependence of the intraband energy on the exciton energy (red and black lines are linearly fitted), thus confirming the intraband nature of the low-energy absorption of the proposed quantum dots. Finally, Figure 2 d in shows the measured 1S e and 1P e energies (relative to 1S h ) as a function of the quantum dot diameter. The inset shows the relationship between the intraband energy and the dot diameter. The dot sizes were obtained from the excitonic energies measured in solution using the empirical model of oleate-capped PbS quantum dots that was published in

[26] and confirmed in

[18] and

[22] .

[0084] Heavy doping of robust colloidal quantum dots (CQDs) in solid-state films has opened the way for new optoelectronic devices and applications such as photodetection, thermoelectric, or thermophotovoltaic technologies. In this work, the inventors tested the intraband photodetection capabilities of heavily doped PbS colloidal quantum dots (CQDs) to extend their spectral range far beyond the material bandgap. To this end, the inventors have fabricated interdigitated devices for conductivity measurements (see the experimental section). Figure 3 a in shows the intraband absorption of quantum dots of different sizes measured at 80 K. Figure 3 b in shows the intraband quantum efficiency (QE) spectrum of a device fabricated using the same dots as in Figure 3 a and measured under monochromatic light at 80 K. The inventors measured intraband photodetection in the 5 μm - 9 μm (MWIR and LWIR) range using PbS colloidal quantum dots (CQDs). The vertical dashed line indicates the bandgap of bulk PbS at 80 K (~3.7 μm)

[27] . The measurements performed demonstrated the energy of intraband photodetection below the bulk bandgap, thus breaking the lower limit set by the bulk value in colloidal quantum dot (CQD) bandgap engineering. The frequency-dependent measurement results (see Figure 11 ) indicate that the intraband detection response time in the devices fabricated according to the present invention is greater than 30 ms. The inventors have measured the specific detectivity D of the proposed devices *(See the experimental section). At 80 K and 11 Hz, the inventors obtained the following peak response rates and detectivities: 1.5×10 at 6.8 μm -4 AW -1 and 4×10 4 Jones; 1.1×10 at 7.3 μm -4 AW -1 and 8×10 4 Jones; and 1.3×10 at 8.0 μm -4 AW -1 and 4×10 4 Jones. Correction by the masking factor of the interdigital electrodes (∼50%) and the reflectivity of the PbS layer will increase the detectivity value to the order of 10 5 Jones.

[0085] To gain a deeper understanding of the performance potential of the in-band PbS quantum dot photodetector, the inventors developed a quantum transport model for the proposed doped PbS quantum dots (see Supplementary Information). This method provides qualitative information on the variation of the conductance G 0 of the proposed film as a function of n QD and the increase in conductance due to in-band optical absorption ΔG. The inventors used the ratio ΔG / G 0 (n QD ) as the figure of merit for this analysis because D * is proportional to ΔG and inversely proportional to the noise spectral density, which in turn is proportional to G 0 . Thus, a higher value of ΔG / G 0 represents higher sensitivity. Figure 3 c in shows the modeled G 0 . It is maximum at n QD = 4e - / dot (half-filled 1S e state) and decreases symmetrically for lower and higher values of n QD . This dependence makes the decrease in G 0 very slow until n QD approaches values close to 0 or 8e - / dot when it experiences a very rapid decrease. This means that n QD ideally should be 0 or 8e - / dot to minimize the dark current and noise. Figure 3 The inset in c shows n QD = 4e - / dot (maximum G 0 ), n QD= 0 and n QD = 8e - / at the point (minimum G 0 ) case. However, interband absorption is only possible when the 1S e state is occupied, which makes it so that when n QD → 8e - / at the point, ΔG( Figure 3 d in, dashed line) and ΔG / G 0 ( Figure 3 d in, solid line) are both at their maximum. Figure 3 The inset of d in shows the case of n QD = 8e - / at the point (minimum ΔG) and n QD = 8e - / at the point (maximum ΔG). Colored squares represent the modeled G 0 and ΔG / G 0 corresponding to the colloidal quantum dots (CQDs) used in the device under test (see Figure 3 b). The results obtained show that by approaching closer to the occupancy limit of 8e - / at the point, the detection rate can be further significantly improved, indicating that further experimental research is needed on this.

[0086] In summary, the inventors have developed a robust doping strategy for PbS colloidal quantum dots (CQDs) that is stable under ambient conditions, enabling the inventors to demonstrate for the first time interband absorption and photoresponse from colloidal quantum dot (CQD) materials in the mid-wave infrared and long-wave infrared ranges. The size-tunable spectral linewidth of the intersubband transitions employed in this application, together with the facile integration of colloidal quantum dots of different sizes, can lead to a CMOS-compatible low-cost infrared multispectral imaging system. The present invention further extends the availability of solution-processed materials to MWIR and LWIR for sensing and thermophotovoltaic energy harvesting applications.

[0087] Experimental section:

[0088] Quantum dot (QD) synthesis and ligand replacement procedures:

[0089] PbS quantum dots were synthesized by the previously reported single injection or multiple injection method with modifications [28 - 30]. Adjust the injection temperature and the concentration of (TMS) 2 S in ODE according to the final desired QD size. The QDs were washed with acetone / ethanol and finally dispersed in toluene at a concentration of 30 mg / ml for device fabrication.

[0090] PbS colloidal quantum dot films were deposited using a layer-by-layer spin-coating process under ambient atmosphere. For each layer, the colloidal quantum dot (CQD) solution was deposited on either a substrate (Si, Si / SiO 2 or CaF 2 ) at 2500 r.p.m. Solid-state ligand exchange was performed by flooding the surface with (I) 1-ethyl-3-methylimidazolium iodide in methanol (EMII, 7 mg / ml) or (II) 1,2-ethanedithiol (EDT) in acetonitrile (ACN) (0.01% v / v) at 2500 r.p.m. for 30 s before spin-drying. The surface was washed twice with (I) methanol or (II) acetonitrile to remove unbound ligands.

[0091] Atomic layer deposition:

[0092] The deposition of Al 2 O 3 was carried out in a GEMStar XT Thermal ALD system. High-purity trimethylaluminum (TMA) purchased from STREM Chemicals Inc. was used as the Al precursor. Pure water (H 2 O) was used as the O precursor. The deposition was carried out at 80 °C. Before the process, the reaction chamber was evacuated and then filled with pure nitrogen until the pressure reached about 0.56 mbar. During gas supply, the manifolds of TMA and H 2 O were maintained at 150 °C. Each layer of Al 2 O 2 O 3 was formed by applying a 15-ms pulse of H 2 O at a partial pressure of 0.02 mbar, followed by a 50-ms pulse of TMA at a partial pressure of 0.18 mbar. The waiting times between pulses were 15 s and 20 s, respectively.

[0093] Sample and device fabrication:

[0094] For transmission measurements, films consisting of 3 to 8 layers of quantum dots replaced with EMII or EDT were spin-coated on lightly doped silicon substrates. After film growth, 3 nm to 5 nm of Al 2 O 3 was deposited by ALD on some samples.

[0095] For photoconductivity measurements, interdigitated gold electrodes were evaporated onto CaF2 substrates patterned using standard lithography methods. The area of the interdigitated devices was 1 x 1 mm 2。The width of the metal fingers is 10 μm. The distance between the fingers is 10 μm or 20 μm. The device is completed by depositing 4 to 6 layers of quantum dots replaced by EMII, followed by ALD deposition of 3 nm to 5 nm of Al 2 O 3 .

[0096] For FET measurements, gold electrodes are evaporated onto a p-Si / SiO substrate patterned using standard lithography methods. The p-type Si layer serves as the gate electrode. The length of the FET channel is in the range of 10 μm to 25 μm. The device is completed by depositing 2 layers of quantum dots replaced by EMII, followed by ALD deposition of 3 nm to 5 nm of Al 2 O 2 O 3 .

[0097] For UPS and XPS measurements, as described previously, the thin film (4 layers) is spin-coated and ligand-exchanged onto an ITO-coated glass substrate.

[0098] Transmission and absorption measurements:

[0099] Room temperature transmission measurements and room temperature absorption measurements are performed using a Cary 5000 UV-Vis-NIR spectrometer and a Cary 600 FTIR in ambient atmosphere. Temperature-variable measurements are performed in vacuum using a Cary 610 FTIR microscope coupled to a temperature-controlled Linkam HFS350EV-PB4 platform equipped with a ZnSe window.

[0100] UPS / XPS measurements and analysis:

[0101] XPS measurements and UPS measurements are performed using a Phoibos 150 analyzer (SPECS GmbH, Berlin, Germany) under ultra-high vacuum conditions (base pressure of 5×10 -10 mbar). XPS measurements are performed using a monochromatic Kalpha X-ray source (1486.74 eV), while UPS measurements are achieved using a monochromatic HeI UV light source (21.2 eV). The UPS data has been analyzed according to the correction disclosed in

[19] . All XPS peaks have been linearly fitted with GL(30), while the Pb4f and S2p peaks have been assigned according to previous work

[31] . Quantitative analysis has been performed taking into account the total contribution of lead and sulfur species corrected separately with relative sensitivity factors (RSF).

[0102] TEM and FIB measurements:

[0103] TEM images were obtained using a JEOL JEM-2100LaB6 transmission electron microscope operating at 200 kV. Samples for TEM characterization were prepared by drop-casting a diluted NC solution onto a 300-mesh carbon-coated copper grid in a saturated toluene environment. Samples for imaging PbS colloidal quantum dot films after iodine replacement were prepared by drop-casting a 30 mg / ml solution onto a copper grid and spin-coating at 2500 rpm when performing solid-state ligand replacement according to the fabrication of the above-mentioned devices.

[0104] The thickness of the colloidal quantum dot film was determined by cross-sectional SEM images of FET devices using a Zeiss Augira beam workstation. A layer of platinum was deposited by a gas injection system (in FIB mode) during cross-sectional cutting with a gallium focused ion beam (Ga-FIB). SEM imaging was performed using an Inlens detector at a voltage of 5 kV and a pore size of 30 μm.

[0105] FET measurements:

[0106] Room-temperature FET transfer characteristics were measured in a probe station inside a Faraday cage using a Keysight B1500A semiconductor device analyzer under ambient atmosphere.

[0107] Measurement of doping level by optical measurement:

[0108] Since the 1S e state of PbS degenerates by a factor of eight (including spin), the number of electrons n at each point in the CB can be directly calculated from the bleaching of the first exciton transition QD (see Figure 1 c) in 1 ). If I 2 and I QD are defined as the integrated absorption intensities of exciton transitions for undoped and doped samples, respectively, then n 2 = 8(1 - I 1 / I DS ). Note that when referring to "undoped samples", it is assumed that the doping (either p-type or n-type) of samples without alumina is low enough to consider the case of a full VB and an empty CB.

[0109] Measurement of doping level by electrical measurement:

[0110] The mobility is calculated using the gradual channel approximation. By fitting the linear part ( Figure 7 ) of the transfer characteristics (I DS -V ) of the FET device, the mobility μ of the majority carriers can be extracted from Equation (1):

[0111]

[0112] where I DS is the drain-source current; V G is the gate voltage; l is the length of the channel; w is the width of the channel; C is the capacitance of the insulator; V DS is the drain-source voltage. To calculate C, the inventors used a value of 3.9 for the relative permittivity of silicon dioxide. Since the (I DS -V DS ) characteristics of the proposed device are Ohmic (see Figure 13 ), the carrier concentration n can be derived from Ohm's law as:

[0113]

[0114] where, I DS0 and V DS0 are the values of I G and V DS when V DS = 0 V, respectively; e is the elementary charge; d is the thickness of the quantum dot layer, and the inventors measured the thickness of the quantum dot layer by both profilometry and FIB-SEM methods. To calculate the number of electrons n QD per dot, a value of β ≈ 0.75 ± 15% was used, where β is the volume packing density of the proposed nanoparticles. Note that although 0.74 is the maximum packing density of spheres and usually 0.64 is taken as the packing density for random distribution of spheres, the maximum packing density of cuboctahedral nanoparticles (as in the case of the proposed particles, see b) in

[22] and Figure 12 ) is higher, up to values higher than 0.9

[32] . Additionally, TEM images of the iodine-substituted PbS film prepared following exactly the same procedure as the proposed FET device (see c and d in Figure 12 ) show that the proposed PbS colloidal quantum dots (CQDs) are distributed in a regular close-packed manner rather than following a random distribution. n QD can be calculated as:

[0115]

[0116] where γ QD = 4 / 3πr QD 3 is the volume of a given quantum dot. r QD (E 0 ) is the radius of the quantum dot and is obtained from the measured quantum dot bandgap E 0 using the empirical model for oleate-capped PbS quantum dots disclosed in

[26] .

[0117] Photoconductivity measurement:

[0118] The device was placed inside an open-loop liquid nitrogen cryostat equipped with a ZnSe window. A 0.3-m Bentham monochromator equipped with sufficient diffraction gratings and second-order filters was used to monochromatize and modulate the light generated using a Nernst infrared source. To avoid chromatic aberration effects, a gold mirror was used to direct the light leaving the monochromator onto the sample. A low-noise transimpedance amplifier from Stanford Research was used to bias the device and amplify the measured current. Final signal detection was carried out using standard lock-in techniques. The chopping frequency used was 11 Hz.

[0119] To correct the measured optical response and obtain the absolute value of QE, a calibrated 0.5x0.5 mm 2 VigoSystems MCT detector was used to measure the spectral power density of the monochromatic light. The detector was placed at the same position where the device was during the photocurrent measurement. The detectivity D * was calculated as: where A is the device area in cm 2 units, SR is the peak spectral response in AW -1 and S n is the noise spectral density. where λ is the photon wavelength, e is the elementary charge, h is Planck's constant, and c is the speed of light in vacuum. S n was calculated by measuring the dark current of the device (exactly the same as the photocurrent measurement but with the IR source turned off) and using the corresponding measurement bandwidth (1.89 mHz).

[0120] Frequency response of photocurrent measurement:

[0121] To characterize the frequency dependence of the in-band photocurrent, the device was illuminated using a LaserTune quantum cascade laser from Block Engineering. The laser beam was mechanically chopped in the range of 30 Hz - 200 Hz. As described above, photocurrent detection was completed using a low-noise amplifier and standard lock-in techniques.

[0122] Quantum transport model:

[0123] Using this model, the inventors wished to understand the effect of doping on the in-band detection ability of the proposed PbS quantum dots. The proposed method will be evaluated according to the doping level n QD of the dots: (I) the steady-state conductance G 0 at a given applied bias before illumination; (II) the conductance change ΔG caused by in-band absorption in the quantum dots. The ratio ΔG / G 0A qualitative indication of the detectivity of the proposed device will be provided, since the detectivity is proportional to ΔG and inversely proportional to the noise, which in turn increases with G 0 The model analyzes the coherent transport between two adjacent points and assumes that the conductance of the quantum dot matrix will be proportional to the conductance between the points. The inventors draw attention to the fact that the conductance between the quantum dots and the metal contacts is excluded from the analysis, as the inventors wish to focus only on the intrinsic material properties.

[0124] At 0 K, the conductance through the different possible channels between nanostructures is described by the Landauer formula [33, 34]:

[0125]

[0126] where is the conductance quantum, e is the elementary charge, h is the Planck constant, is the product of the number of propagating modes and the electron transmission probability per mode at the Fermi energy. At finite temperature, transport occurs through multiple energy channels (in the energy range including several k F T above the Fermi energy E B and several k F T below the Fermi energy E B ), which can be obtained through the thermal redistribution of electrons. Equation (5) is the linear response formula for conductivity at finite temperature

[34] :

[0127]

[0128] where,

[0129]

[0130] is the Fermi function and determines the electron occupancy factor (ranging from 0 to 1) at each level of energy E. Equation (5) is the starting point of the proposed model and enables the inventors to evaluate how the conductance is affected by small changes in f. Note that in the experiments conducted by the inventors, the optical power density employed was low (in the range of 10 -5 -10 -4 W / cm 2 ), so relatively speaking, the number of carriers in the proposed highly doped (~10 19 cm -3 ) samples will only change slightly.

[0131] Taking into account the current state of research, Figure 14 shows the lowest energy level of the CB of the proposed quantum dots (in the case of energy E S , 1S eis an eight-fold degenerate ground state; while at an energy of E P the case, 1P e is the first excited state). Generally, it can be said that conduction occurs through the 1S e channel (G S ) or the 1P e channel (G P ). Since the conductances through the parallel channels are added, thus G = G S + G P . Before irradiation, the 1S e is partially occupied by doping. The 1P e is unoccupied because the in-band energy (150 meV - 250 meV) is much larger than k B T (which is ~7 meV at 80 K). This is represented in Figure 14 by placing E F closer to 1S e rather than 1P e .

[0132] For the finite occupation of 1S e , it can be approximated in Equation (5) where is the product of the number of propagation modes and the electron transmission probability under the condition of E S . This means that under non-irradiated steady-state conditions, the conductance is generated only through the 1S e channel, and G 0 = G S .

[0133] And, further substituting

[0134]

[0135] in Equation (5), it can be obtained that before irradiation:

[0136] G S = K S [f S (1 - f S )] (8)

[0137] where and f S = f(E S ).

[0138] When light resonant to the 1Se→1Pe transition irradiates the quantum dot, since some electrons are excited from 1S e to 1P e , thus in 1S eA (negative) Δf is generated in the state. For simplicity, it will be assumed hereinafter that electron excitation and electron relaxation occur only between the 1S e state and the 1P e state. For a low excitation photon flux F, the absorbed light is proportional to the occupancy of 1S e , and the occupancy of 1S e is in turn proportional to f S . Therefore, Δf≈αFf S , where α is a proportionality factor related to the absorption rate of the sample and the lifetime of the electron in the 1P e state. For a sufficiently low F, αFf S →0, and

[0139] G(f S -Δf)≈G S -ΔfG S ′≈G S -αFf S G S ′ (9)

[0140] where At this point, the contribution of G P to the total conductance can no longer be ignored, because although weak, the 1P e is now occupied. G P follows Equation (10) (similar to Equation (8) for G S ):

[0141] G P =K P [f P (1 - f P )] (10)

[0142] where f P =ΦΔf is the electron occupancy factor of the 1P e state, and Φ is the ratio between the degeneracy of 1S e and the degeneracy of 1P e . For a sufficiently low F, f P →0 and Equation (10) is linear, so it can be approximately obtained that:

[0143] G P ≈K P f P =K P ΦΔf≈K P α′Ff S (11)

[0144] where is the number of propagation modes and at the energy EP The product of the electron transmission probabilities under the conditions, and α′ = Φα. Finally, the total conductance under irradiation is obtained by adding equations (9) and (11):

[0145] G = G S + G P = [G S - αFf S G S ′] + [K P α′Ff S

[0146] = K S f S (1 - f S ) + Fαf S (K P Φ - K S + 2K S f S ) = G 0 + ΔG (12)

[0147] where G 0 = K S f S (1 - f S ) [Equation 8], and ΔG = Fαf S (K P Φ - K S + 2K S f S ) is the change in conductance due to irradiation. The photocurrent measured in the experiments described herein is proportional to ΔG; thus, the detectivity D * of the proposed detector is also proportional to it. However, D * is inversely proportional to the dark current of the device and thus inversely proportional to G 0 . Manipulating equation (12) gives:

[0148]

[0149] where a = 2αF and

[0150] For an eight-fold degenerate 1S e , the inventors can calculate the occupancy factor of the 1S e state as f S = 8 / n QD , where n QD is the number of electrons occupying 1S e . Thus, equation (13) can be rewritten as:

[0151] ​

[0152] Assuming Δf → 0, the model is applicable to any values of α and F, and thus to any value of a. Figure 12 ΔG / G is shown 0 versus n QD dependency. To evaluate the sensitivity of the model to , three cases are shown: K S = K P Φ, K S = 10K P Φ, and K S = 0.1K P Φ, where is related to the degeneracy difference with 1P e and 1S e and the different transmission probabilities of their respective propagation modes. It can be seen that b has a quantitative effect on ΔG / G 0 . However, it does not affect the increasing trend with the increase of n QD , especially does not affect the sharp increase when n QD → 8. The inventors have drawn the following conclusion: complete occupation of 1S e (while keeping 1P e empty) is required in all cases to maximize the detection rate.

[0153] Figure 15 ΔG / G is shown 0 to be inversely proportional to m = K S / (K P Φ). This dependency can be explained by the fact that ΔG increases with the increase of K P Φ, because G P is proportional to this product, while G 0 is independent of this product (all transmissions occur through the 1S e state under dark conditions). In all cases, when n QD → 8, high values of ΔG / G 0 are obtained.

[0154] Finally, some schematic arrangements of different embodiments of the film obtained by the method according to the first aspect of the present invention and different embodiments of the optoelectronic device according to the second aspect of the present invention are described below with reference to Figure 16 , Figure 17 , Figure 19 , Figure 20 and Figure 21 , while Figure 18 schematically shows the mechanism of intra-band carrier transition.

[0155] Specifically, Figure 16There is shown an embodiment in which the optoelectronic device comprises an n-type doped metal chalcogenide quantum dot solid film A obtained by the method according to the first aspect, sandwiched between a first electrode E1 and a second electrode E2. For the embodiment, film A is a mixture of electrically connected quantum dots of different sizes, thus forming a type-I heterojunction, in which the small-bandgap quantum dots QD1 are heavily doped, thus exhibiting intraband absorption, while the large-bandgap quantum dots QD2 are not heavily doped, thus maintaining a low dark current of the device. The energy band offset of these two groups of quantum dots makes the energy difference (e.g., the energy difference in the conduction band) equal to or less than the intraband energy of the small-bandgap quantum dots QD1. The loading range of the small-bandgap quantum dots QD1 in the matrix of the large-bandgap quantum dots QD2 can range from 1% to 50%, and preferably between 5% and 25%.

[0156] In Figure 17 Another embodiment of the optoelectronic device according to the second aspect of the present invention is schematically shown, in which the n-type doped metal chalcogenide quantum dot solid film A is a layered structure, which alternately deposits layers of heavily n-type doped metal chalcogenide low-bandgap quantum dots QD1 and layers of non-heavily n-type doped metal chalcogenide high-bandgap quantum dots QD2, thus forming a superlattice structure disposed between the first electrode E1 and the second electrode E2. The thickness of the layers can all be in the range from 5 nm to 100 nm, and the number of such layers can be from 1 to 100, but is not necessarily limited to 100.

[0157] In both cases, the mechanism is based on low-energy infrared excitation (e.g., light with a wavelength from 3 μm to 12 μm) to excite the small-bandgap doped quantum dots QD1 through the first intraband transition, so that electrons move from the first excited state to the second excited state ( Figure 18 ). Then, starting from this second excited state that is at the same level or higher than the conduction band of the large-bandgap quantum dots QD2, when an electric field is applied, electrons are injected into the matrix (in the case of mixing), or into the next layer in the case of a superlattice. In the case where an electric field is applied, the carriers will drift towards the electrodes and will thus generate a significant photocurrent. In the case of mixing ( Figure 16 ), under the action of the applied electric field, the excited carriers are transported through the matrix towards the electrodes. In the case of a superlattice ( Figure 17 ), the carriers are forced to encounter one or more layers of n-type doped quantum dots QD1, in which the carriers drop but still escape due to the applied electric field and continue to be transported towards the electrodes.

[0158] For the optoelectronic device according to the second aspect of the present invention, in Figure 19 、 Figure 20 and Figure 21Possible embodiments of a photodetector device are shown.

[0159] Specifically, Figure 19 a planar photodetector device is shown, in which electrodes E1, E2 are deposited on a substrate S and an active material, namely an n-type doped quantum dot film A, is deposited thereon. Irradiation is incident directly on the active material A from the top. Optionally, the irradiation can also come from the bottom and pass through the substrate S; in this case, the substrate S must be transparent to light of the irradiation wavelength.

[0160] Figure 20 and Figure 21 a vertical photodetector device is shown, in which light L is incident from the top ( Figure 21 ) or from the bottom ( Figure 20 ). The electrodes (E1 in Figure 20 , E2 in Figure 21 ) between the active material A and the light source must have a low absorption rate for the wavelength of interest (e.g., infrared wavelength), and can be made of, for example, one or several layers of graphene, or a thin indium tin oxide or aluminum-doped zinc oxide film.

[0161] Another electrode (E2 in Figure 20 and E1 in Figure 21 ) must be a back reflector electrode, which can be made of a metal that preferably has a high reflectivity for the wavelength of interest (e.g., infrared wavelength), and can be made of, for example, gold, palladium, platinum, silver, etc.

[0162] The detailed description provided herein demonstrates that surface coverage is an immaterial issue for the present invention. On the contrary, the key aspect of the present invention is the surface termination of the quantum dots such that chalcogen atoms (e.g., sulfur atoms) are replaced by halogen atoms (e.g., iodine atoms).

[0163] If only surface coverage is sought instead of replacing chalcogen atoms with halogen atoms, heavy doping cannot be achieved, as Figure 22 shown, Figure 22 which does not correspond to the present invention and shows the results of theoretical calculations as follows: wherein, iodide does not replace sulfur, but binds to Pb atoms on the surface. It is clearly visible from Figure 22 that, contrary to the present invention, no heavy doping is achieved at all in that case.

[0164] Those skilled in the art can introduce changes and modifications in the described embodiments without departing from the scope of the present invention as defined by the appended claims.

[0165] References:

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Claims

1. A method for obtaining an n-type doped metal chalcogenide quantum dot solid film for performing steady-state detection of intraband transitions or intraband photodetection, the method comprises: - forming a metal chalcogenide quantum dot solid film, and - performing an n-type doping process on at least a plurality of metal chalcogenide quantum dots of the metal chalcogenide quantum dot solid film to cause the plurality of metal chalcogenide quantum dots to exhibit intraband absorption, wherein the n-type doping process comprises: - partially replacing chalcogen element atoms with halogen atoms in at least the plurality of metal chalcogenide quantum dots; and - disposing a substance on at least the plurality of metal chalcogenide quantum dots, wherein the substance is made and arranged to avoid oxygen p-type doping of the plurality of metal chalcogenide quantum dots.

2. The method according to claim 1, wherein, the metal chalcogenide is at least one of a Pb-chalcogenide, a Cd-chalcogenide, and a Hg-chalcogenide, wherein the chalcogen element atom is at least one of a sulfur atom, a selenium atom, and a tellurium atom, and the halogen atom is at least one of an iodine atom, a bromine atom, and a chlorine atom.

3. The method according to claim 1, comprising disposing the substance for: - coating the metal chalcogenide quantum dot solid film so as to isolate the metal chalcogenide quantum dot solid film from oxygen in the environment; and / or - infiltrating into the metal chalcogenide quantum dot solid film so as to react with oxygen present in the metal chalcogenide quantum dot solid film to inhibit the oxygen p-type doping effect.

4. The method according to claim 1, comprising disposing the substance by atomic layer deposition.

5. The method according to claim 1, wherein, the substance is at least one of aluminum oxide, titanium dioxide, zinc oxide, and hafnium dioxide.

6. The method according to any one of the preceding claims, wherein, the step of forming the metal chalcogenide quantum dot film comprises: forming a mixture having a main matrix composed of first metal chalcogenide quantum dots and second metal chalcogenide quantum dots with a smaller bandgap embedded in the main matrix, wherein, compared with the first metal chalcogenide quantum dots, the second metal chalcogenide quantum dots are larger and have different morphologies such that the second metal chalcogenide quantum dots have more exposed sides containing chalcogen element atoms, and the method comprises applying the n-type doping process on the entire formed metal chalcogenide quantum dot film such that the second metal chalcogenide quantum dots are heavily n-type doped while the first metal chalcogenide quantum dots are not n-type doped or are only slightly n-type doped.

7. The method according to any one of claims 1 to 5, wherein, The step of forming the metal chalcogenide quantum dot film includes: forming a layered structure in which a layer composed of first metal chalcogenide quantum dots and a layer composed of second metal chalcogenide quantum dots are alternately arranged, wherein the second metal chalcogenide quantum dots have a smaller bandgap compared to the first metal chalcogenide quantum dots, and the second metal chalcogenide quantum dots are larger and have different morphologies such that the second metal chalcogenide quantum dots have more exposed sides containing chalcogen element atoms, and the method includes: - Applying the n-type doping process to the entire formed metal chalcogenide quantum dot film such that the second metal chalcogenide quantum dots are heavily n-type doped while the first metal chalcogenide quantum dots are not n-type doped or only slightly n-type doped; or - Applying the n-type doping process only to one or more layers composed of second metal chalcogenide quantum dots.

8. The method according to claim 6, including selecting the bandgaps and energy band alignments of the first metal chalcogenide quantum dots and the second metal chalcogenide quantum dots such that the first metal chalcogenide quantum dots and the second metal chalcogenide quantum dots form a type-I heterojunction and an energy band offset, and the energy difference in the conduction band or the valence band of the energy band offset is equal to or less than the in-band energy of the second metal chalcogenide quantum dots.

9. The method according to claim 6, including forming a mixture as described below: wherein, the concentration of the second metal chalcogenide quantum dots ranges from 1% to 50% by volume.

10. The method according to claim 6, including: selecting the size and morphology of the first metal chalcogenide quantum dots such that the first metal chalcogenide quantum dots do not have any chalcogen-rich exposed sides; and selecting the size and morphology of the second metal chalcogenide quantum dots such that the second metal chalcogenide quantum dots have one to six chalcogen-rich exposed sides.

11. An optoelectronic device, including: - At least one n-type doped metal chalcogenide quantum dot solid film (A) obtained by the method according to claim 1; and - A first conductive electrode (E1) and a second conductive electrode (E2) physically contacting two corresponding spaced-apart regions of the at least one n-type doped metal chalcogenide quantum dot solid film (A).

12. The optoelectronic device according to claim 11, wherein, the at least one n-type doped metal chalcogenide quantum dot solid film (A) is a light absorption film, and the light absorption film is made to exhibit in-band absorption of light having wavelengths included in a predetermined wavelength range, and the predetermined wavelength range extends beyond the absorption range of the bandgap of the metal chalcogenide quantum dots when not n-type doped.

13. The optoelectronic device according to claim 12, wherein, The optoelectronic device implements a photodetector that is configured to detect light having any wavelength included in the predetermined wavelength range and within the wavelength range of the interband absorption of the metal chalcogenide quantum dots in the n-type doped metal chalcogenide quantum dot solid film.

14. The optoelectronic device according to claim 13, wherein, the photodetector is a planar photodetector, the photodetector comprising a substrate (S) on which the at least one n-type doped metal chalcogenide quantum dot solid film (A), the first conductive electrode (E1), and the second conductive electrode (E2) are deposited, and wherein: - the substrate (S) is opaque to light having a wavelength included in the predetermined wavelength range, such that the photodetector detects light incident directly on the at least one n-type doped metal chalcogenide quantum dot solid film (A) from the top; or - the substrate (S) is transparent to light having any wavelength included in the predetermined wavelength range, such that the photodetector detects light passing through the substrate (S) from the bottom before hitting the at least one n-type doped metal chalcogenide quantum dot solid film (A).

15. The optoelectronic device according to claim 13, wherein, the photodetector is a vertical photodetector, the photodetector comprising a substrate (S) on which the first conductive electrode (E1) is deposited, wherein the at least one n-type doped metal chalcogenide quantum dot solid film (A) is deposited on the first conductive electrode (E1), and the second conductive electrode (E2) is deposited on the at least one n-type doped metal chalcogenide quantum dot solid film (A), and wherein: - the substrate (S) is opaque to light having a wavelength included in the predetermined wavelength range, the second conductive electrode (E2) is transparent to light having a wavelength included in the predetermined wavelength range, and the first conductive electrode (E1) is reflective to light having a wavelength included in the predetermined wavelength range, such that the photodetector detects light passing through the second conductive electrode (E2) from the top, hitting the at least one n-type doped metal chalcogenide quantum dot solid film (A), and being reflected by the first conductive electrode (E1); or - the substrate (S) and the first conductive electrode (E1) are transparent to light having a wavelength included in the predetermined wavelength range, and the second conductive electrode (E2) is reflective to light having a wavelength included in the predetermined wavelength range, such that the photodetector detects light passing through the substrate (S) from the bottom, passing through the first conductive electrode (E1), hitting the at least one n-type doped metal chalcogenide quantum dot solid film (A), and being reflected by the second conductive electrode (E1).

Images