Additive manufacturing system with NC fusion and related methods

Abstract

An AM system includes a printing chamber having printing ink, the printing ink having inorganic NCs. The AM system also includes a laser source configured to emit a pulsed laser output, one or more optical elements coupled downstream from the laser source and configured to direct the pulsed laser output to the printing chamber, and a controller coupled to the laser source. The controller is configured to cause the pulsed laser output to print one or more layers of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption / transformation in the inorganic NCs.

Description

ADDITIVE MANUFACTURING SYSTEM WITH NC FUSION AND RELATED METHODSGovernment Rights

[0001] This invention was made with government support under award Nos. 2054098 and 2213693 awarded by National Science Foundation. The government has certain rights in the invention.Related Application

[0002] This application is based upon prior filed copending Application No. 63 / 682,070 filed August 12, 2024, the entire subject matter of which is incorporated herein by reference in its entirety.Technical Field

[0003] The present disclosure relates to the field of manufacturing, and, more particularly, to additive manufacturing and related methods.Background

[0004] Additive manufacturing (AM), colloquially known as three-dimensional (3D) printing, is a collective term describing an emerging set of technologies that enable the production of complex functional components by successively joining materials layer-by-layer from digital model data. AM technologies have witnessed notable advancements over the past two decades, resulting in increasing their level of maturity and transitioning many of them from prototyping tools to viable manufacturing methods used to produce end-use parts in the aerospace, automotive, and biomedical industries.Summary

[0005] Generally, an AM system comprises a printing chamber having printing ink. The printing ink comprises a plurality of inorganic nanocrystals (NCs). The AM system also includes a laser source configured to emit a pulsed laser output, at least one optical element coupled downstream from the laser source and configured to direct the pulsed laser output to the printing chamber, and a controller coupled to the laser source. The controller is configured to cause the pulsed laser output to print at least one layer of an object based upon a flux of photoexcited hot electrons toinduce NC fusion via ligand desorption / transformation in the plurality of inorganic NCs. In particular, the plurality of inorganic NCs may comprise one of a plurality of metal NCs, a plurality of semiconductor NCs, and a plurality of metal oxide NCs.

[0006] The controller may be configured to cause the pulsed laser output to print the at least one layer of the object based upon multiple excitations of hot electrons in the NCs during each laser pulse for ligand desorption / transformation through interaction with ligand states, for example, the lowest unoccupied molecular orbital (LUMO). The laser source may be configured to emit the pulsed laser output based upon a material relaxation time in the plurality of inorganic NCs. The laser source may be configured to generate the pulsed laser output so that an interval time between two successive photoexcited hot electrons is less than the material relaxation time.

[0007] For example, the plurality of metal inorganic NCs may comprise at least one of copper NCs, gold NCs, platinum NCs, nickel NCs, cobalt NCs, and silver NCs. The laser source may have an operating wavelength based upon the printing ink. The laser source may be configured to emit the pulsed laser output based upon a repetition rate in a range of 0.5kHz-5MHz. The laser source may be configured to emit the pulsed laser output with pulse width in a range of Ifs to Ips. The laser source may be configured to emit the pulsed laser output with averaged intensity during each laser pulse in the range of lx 106to 1 x 1012W / cm2at the laser focus spot. The laser source may be configured to combine two laser sources with one emitting a shorter pulse (100 fs) and the other one emitting longer (e.g., 10ns) pulse width. The two pulses may be spatially overlapped and temporally overlapped or separated with a controlled time.

[0008] In some embodiments, the controller may be configured to form at least one additional layer of the printing ink after formation of the at least one layer of the object, and cause the pulsed laser output to print the at least one additional layer of the object. Alternatively, the controller may be configured to directly 3D scan the laser output in a serial manner and form 3D structure. The laser source may be configured to emit the pulsed laser output comprising one of a pulsed femtosecond laser output and a pulsed nanosecond (ns) laser output. Also, the controller may be configured to directly scan a 3D trajectory to form free standing 3D structures, such as pillar, spiral structures and other structures without layer-by-layer formation. The laser source may be configured to operate at a laser intensity between 1 x 106to 1x1012W / cm2at a focal spot.

[0009] Another aspect is directed to an AM method. The method comprises providing printing ink in a printing chamber. The printing ink comprises a plurality of inorganic NCs, and operatinga laser source configured to emit a pulsed laser output. The method also comprises operating at least one optical element coupled downstream from the laser source and directing the pulsed laser output to the printing chamber, and operating a controller coupled to the laser source. The controller is configured to cause the pulsed laser output to print at least one layer of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption / transformation in the plurality of inorganic NCs.Brief Description of the Drawings

[0010] FIG. 1 is a schematic diagram of an AM system, according to a first example embodiment of the present disclosure.

[0011] FIG. 2 is an enlarged schematic diagram of an AM system, according to a second example embodiment of the present disclosure.

[0012] FIG. 3 is an electron microscope image of an object fabricated by the AM system from FIG. 1.

[0013] FIG. 4 is a schematic diagram of a Z-scan study related to the AM system of FIG. 1.

[0014] FIG. 5 is a transmission curve diagram for the Z-scan study of FIG. 4.

[0015] FIGS. 6A-6J are electron microscope images of objects fabricated by the AM system from FIG. 1.

[0016] FIGS. 7A-7C are schematic diagrams of a printing process in an AM system, according to a third example embodiment of the present disclosure.

[0017] FIGS. 8A-8F are diagrams of an underlying mechanism for the AM system of FIGS. 7A- 7C.

[0018] FIG. 9 is an electron microscope image of an object fabricated by the AM system from FIGS. 7A-7C.

[0019] FIG. 10 is a schematic diagram of sub-diffraction limited hot-electron driven laser sintering of NCs for metasurface fabrication in an AM system, according to a fourth example embodiment of the present disclosure.

[0020] FIGS. 11 A-l 1C are diagrams for dependence of linewidth on laser fluence, and summary of linewidth fabricated with different polarizations, according to the fourth example embodiment of the present disclosure.

[0021] FIGS. 12A-12D are diagrams for Raman spectrum of as-deposit NC fdms, Raman spectrum for a laser sintered NC fdms, the NC sintering mediated by ligand diffusion and desorption, and electrical conductivity of laser sintered NC fdms, respectively, according to the fourth example embodiment of the present disclosure.

[0022] FIGS. 13A-13D are diagrams for laser fluence in an AM system, according to the fourth example embodiment of the present disclosure.

[0023] FIGS. 13E-13F are diagrams for the transport phenomena during the thermally assisted hot electron driven laser sintering process and for total density of state of the methylamine-gold complex and partial density of state of the methylamine, respectively, according to the fourth example embodiment of the present disclosure.

[0024] FIGS. 14A-14B are diagrams for the metasurface supercell consisting of rotated gold rectangular bars on a glass substrate and simulated electric field profiles showing bidirectional refraction for lefthand circular polarization and righthand circular polarization incident light, respectively, according to the fourth example embodiment of the present disclosure.Detailed Description

[0025] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which several embodiments of the invention are shown. This present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like numbers refer to like elements throughout, and base 100 reference numerals are used to indicate similar elements in alternative embodiments.

[0026] Referring initially to FIG. 1, an AM system 100 according to the present disclosure is now described. The AM system 100 illustratively includes a printing chamber 101 configured to hold printing ink. In some embodiments, the printing chamber 101 is preloaded with the printing ink, and in other embodiments, the printing ink is fed into the printing chamber during the AM process. The printing ink comprises a plurality of inorganic metal NCs. The plurality of metal inorganic NCs may comprise at least one of copper NCs, gold NCs, and silver NCs, for example.

[0027] The AM system 100 illustratively includes a laser source 104 configured to emit a pulsed femtosecond output. In particular, the laser source 104 may be configured to generate the pulsedfemtosecond output with a pulse width in a range of 300-400 femtoseconds. The laser source 104 may have an operating wavelength based upon the printing ink, and an operating averaged laser power less than 2 pW. The AM system 100 illustratively includes a plurality of optical elements 105a-105e coupled downstream from the laser source 104, and a scanner 106 coupled downstream from the plurality of optical elements and configured to direct the pulsed femtosecond output to the printing chamber 101.

[0028] Another aspect is directed to an AM method. The method includes filling a printing chamber 101 with printing ink, the printing ink comprising a plurality of inorganic metal NCs, operating a laser source 104 to emit a pulsed femtosecond output, and operating a scanner 106 coupled downstream from a plurality of optical elements 105a-105e and the laser source, the scanner to direct the pulsed femtosecond output to the printing chamber. The method further includes causing the pulsed femtosecond output to print a plurality of layers of an object by causing multiple excitations of hot electrons based upon the plurality of metal inorganic NCs.

[0029] In FIG. 2, the AM system illustratively comprises a controller coupled to the scanner and configured to cause the pulsed femtosecond output to print a plurality of layers of an object on a substrate 206. As will be appreciated, the object is printed in-situ. In other words, the object is printed layer-by-layer from the bottom up within the printing ink and on the substrate 206.

[0030] Referring now additionally to a diagram 1000 of FIG. 3, multiple excitation of hot electrons induced printing facilitates high-resolution structures, with feature sizes achievable below 200 nm. The ink concentration and laser wavelength were optimized to enable bottom-up printing via laser beam propagation and tight focus on the substrate. A 0.02 wt% metal NC ink was utilized, confined within a 120 pm spacer thickness chamber (FIG. 2). Ink concentrations exceeding 0.05 wt% or increased spacer thicknesses attenuate laser beam power due to enhanced linear absorption, thereby compromising print quality as linear absorption surpasses non-linear absorption — the latter being critical for high-resolution printing. Conversely, ink concentrations below 0.01 wt% exhibit elevated power thresholds to initiate the printing process, attributable to insufficient metal NCs within the focal volume, thus losing the advantages of low-dose printing. Therefore, optimizing ink concentration, optimizing laser wavelength, and optimizing chamber height are critical for the feasibility and efficiency of this printing technology. These parameters must be carefully considered within the context of the overall printing system to achieve optimal performance.

[0031] Laser three-dimensional (3D) nanoprinting is a technique for fabricating complex nanostructures. This method enables direct printing of arbitrary 3D nanostructures through focused laser radiation. One of the advantages of laser 3D nanoprinting is its maskless printing with high resolutions beyond the optical diffraction limit. Currently, many of the works in 3D laser nanoprinting are based on the two-photon polymerization (2PP) mechanism, a mature technique that is at the stage of developing fast scanning strategies for generating macroscopic structures with nanoscale features [1, 2]. However, even though 2PP has shown such progress, the printing media is still limited to polymer-based materials, diminishing its functionalities [3], In recent years, the nano-manufacturing of metals has gained much attention due to the versatility and potential of metallic nanostructures in various applications, including plasmonic devices, biosensors, metamaterials, and catalysts [4-6], The unique optical, electrical, and mechanical characteristics of metals at the nanoscale have driven research efforts to develop novel methods to overcome the barriers of materials limitation. Several innovative techniques have been designed to address the challenges of metal nanoprinting. Charged aerosol jets enable the 3D printing of multiple metals with feature sizes below 100 nm [7], This approach shows high resolution but requires a mask and suffers from low translation speeds, making printing arbitrary complex structures challenging — the need for a mask constrains the favorable geometric freedom in 3D nanoprinting applications.

[0032] Focused electron beam-induced deposition (FEB ID) enables the printing of complex structures through a layer-by-layer approach. It can achieve feature sizes below 100 nm while exhibiting the excellent quality of printed structures [8], Also, FEB ID offers the advantage of maskless fabrication and can produce high-quality metal structures. However, the growth rate and the mandatary of vacuum conditions raise concerns about its potential for large-scale production and commercialization. Localized pulsed electrodeposition (LPED) demonstrates the capability of printing metals at sub-micrometer resolutions in the 500-1000 nm [9], This method offers high precision and is feasible for multi-metals printing, making it attractive for fabricating composite nanostructures. The pulsed nature of the deposition process allows reasonable control of the structural growth compared to continuous electrodeposition methods. However, the feature size is limited by the nozzle size. Moreover, the printing rate and compatibility with true 3D printing of complex geometries remain questioned. The capability of LPED to create overhanging structures is still ongoing research and development, which is often required inadvanced 3D nanodevices. Other laser-based printing techniques, such as laser-induced photoreduction and laser-induced forward transfer, have demonstrated the potential of 3D printing of metal structures [10, 11], Sufficient pulse energy is required for both techniques to overcome the printing power threshold, resulting in feature size only being down to a few microns due to inevitable thermal accumulation.

[0033] Recently, laser-triggered direct chemical bonding between inorganic NCs shows printed 3D nanostructures with decent material purity and properties [12, 13], This technique demonstrates the feasibility of printing multi-materials, including metal and some semiconductors. The inorganic NCs are capped with ligands as a suspension in liquid vehicles to avoid agglomeration and stabilization of NCs. As the laser irradiated, ligands absorbed multiphoton and generated radicals, further bridging the NCs. In this case, the NCs do not participate in the photon-triggered excitation process, but their surface ligands. This technique can be ideal for some applications where NCs can preserve intrinsic properties without fusion. However, it cannot be implemented in the applications where fusion of NCs is crucial and necessary to improve electrical, chemical, and mechanical properties of printed structures.

[0034] The disclosed printing technique is based on fusion of metal NCs via hot-electron induced ligand desorption and / or transformation. The hot electrons transition from the conduction band of NCs to the LUMO of the ligands, leading to ligand desorption or transformation. Once the ligands are desorbed, the NCs in the liquid can be accumulated and fused to form larger particles, initiating the printing process. The present disclosure may be called a wavelength-selected printing technique. For multiple excitations of hot electrons, a specific wavelength is required for printing. For instance, for gold printing, a femtosecond laser with a green wavelength (515 nm) is used.

[0035] To provide evidence of the feasibility of the printing process enabled by multiple excitation of hot-electrons, a z-scan study was conducted. (See diagram 1005, FIGS. 4-5). This study, which is an approach to characterize the optical nonlinear response (multi-electron excitation) of the material, involves translating a sample 1006 through the beam waist of a focused beam and then measuring the power transmitted through the sample via a lens 1007 and camera 1008, as shown in FIG. 4. As the sample approaches the focus region of the laser beam, the laser intensity increases, reaching a maximum at the focus spot position (z=0 mm). From the transmission curve in a diagram 1010 of FIG. 5, a valley at the center position is shown,indicating that multi -electron absorption had occurred, thereby confirming the effectiveness of the multiple excitations of hot-electron printing process.

[0036] Existing micro / nano additive manufacturing can only print polymer structures. In this invention, a method and apparatus to print multiple types of non-polymer materials are disclosed. The following describes the specifics about laser setup, laser parameters, nanomaterial, i ligand that are needed to achieve the multi-material 3D printing.

[0037] In the illustrative example of FIG. 1, the laser 3D nanoprinting system includes a femtosecond (fs) pulse laser featuring adjustable wavelengths spanning from near-infrared, visible to ultraviolet (UV) with a pulse width of 350 fs. Following emission, the laser beam undergoes frequency selection to output desirable wavelengths, proceeds to an attenuator for precise modulation of laser power, and traverses a beam expander for broadening the diameter of the laser beam, finally focusing inside the printing chamber by a high numerical aperture oil immersion objective (e.g., with numerical aperture = 1.3). To generate arbitrary 3D structures, the Galvo scanner is synchronized with a piezo-stage and an optical shutter, these components are integrated and controlled by computer. The reflective mirrors are applied to direct the trajectory of the laser beam path. A camera and a white light source are utilized for monitoring the printing process in real time.

[0038] This technique demonstrates the capacity to fabricate a wide variety of non-polymeric materials at the nanoscale, including metals, semiconductors, and metal oxides. All materials exist in the form of NCs, with sizes ranging from 1-10 nm. These NCs are capped with specially selected organic ligands and suspended within organic or water-based liquid media, which are then sealed within the chamber. Ligands are selected based on material system and laser parameters according to modeling and numerical simulations.

[0039] 3D nanoprinting system is engineered to implement a bottom-up printing approach and serves as a direct method for producing arbitrary 3D structures. Achieving low roughness and durable printed structures requires careful selection of printing parameters. The scanning speed of the laser beam typically falls within the range of 0.5-5 pm / s, while ensuring that the gap between each layer does not exceed 100 nm. Utilizing an excessive scanning speed or wider layer gaps may result in the formation of distorted printing structures or incomplete printing outcomes.

[0040] The laser pulse duration, laser wavelength, repetition rate, laser intensity at focal spot, and averaged power are essential parameters that significantly influence the quality of 3D nanostructure fabrication. Laser wavelength has been selected based on laser-material interaction which is determined by modeling and numerical simulations. Optimal fabrication conditions involve maintaining proper repetition rate (e.g., 0.5kHz - 5MHz) and laser peak intensity. Specifically, for green wavelength, suitable fs laser peak intensity power falls within the range of 5xlO10to 5xl0nW / cm2The integration of extremely low repetition rates and minimal laser averaged power <2pW yields a distinctive processing parameter, which is unique and unprecedented. It is called “low-dose 3D nanoprinting”. Over-dose processing may lead to forming porous structures and unexpected growth of excessive printing parts.

[0041] Referring now to FIGS. 6A-6J, scanning electron microscope (SEM) images 1020, 1030, 1040, 1050, 1060, 1070, 1080, 1090, 1100, 1105 of printed 3D structures are shown. FIG. 6A shows the 10 x 10 tilted pillars array of gold metal with diameter of 270-280 nm and height of 2 pm. FIGS. 6B & 6C show the spiral pillars array of gold with height of 6 pm and diameter of 1 pm. FIG. 6D & 6E show the spiral pillars array of cadmium selenide (CdSe) with height of 8 pm and diameter of 2 pm. FIG. 6F shows the spiral pillars array of iron oxide with height of 8 pm and diameter of 2 pm. FIG. 6G shows the single-layer of complex structure of gold with size of 10 pm xlO pm and height of 1.5 pm. FIG. 6H shows the multi-layers of complex structure of gold with size of 10 pm x 10 pm and height of 15 pm . FIG. 61 shows a side view and FIG. 6J a top view of a building structure of CdSe with size of 10 pm x 10 pm and height of 15 pm. FIG. 6K shows a side view and FIG. 6L a top view of the building structure of gold with size of 10 pm x 10 pm and height of 15 pm.

[0042] Referring now to FIGS. 7A-7C, 8A-8F, & 9, another example embodiment of the AM system 100 is now described. In the present work, non-thermal or hot electrons generated in metal NCs in rapid succession by ultrafast laser excitation are employed to induce ligand desorption and subsequent NC fusion and printing, which is called multi-electron 3D nanoprinting (diagrams 1110, 1120, 1130 of FIGS. 7A-7C). Photoexcited hot electrons decay rapidly in metals, within 5-10 femtoseconds

[0014] , through collisions with ground-state electrons. Due to the spatial confinement of hot electrons within the NCs and the temporal confinement of photons within a single femtosecond pulse, the hot electrons can be continuously re-excited by incoming photons that are closely separated in time. The nonlinear buildup of sufficientlyenergetic hot electrons by multiple excitations in the NCs during each laser pulse facilitates ligand desorption through strong interaction with the LUMO of the ligands, which is called nonlinear multiple electron desorption (FIGS. 8A-8C). Unlike linear desorption, the nonlinear desorption enables strongly localized fusion of metal NCs and 3D printing with exceptional radial and axial resolutions (FIGS. 8A-8C). This technique enables 3D nano-printing of a wide range of metals at extremely low power (down to approximately 50 pW, ±5%), which may be ideal for a high-throughput printing process.

[0043] In FIGS. 8A-8B, diagrams 1140, 1150 show the underlying mechanism, where reexcitation hot electrons facilitate ligand desorption. FIG. 8C includes a diagram 1160, which shows Oleylamine (OA) ligand bonding to metal NCs and the mechanism of laser-induced ligand desorption. FIGS. 8D-8E include diagrams 1170, 1180, which shows schematic representations of nanostructures printed using 343 nm and 15 nm wavelengths, highlighting differences in axial and lateral printing resolution. FIG. 8F includes a diagram 1190, which shows Z-scan curves of gold NCs under 515nm wavelength illustrating the nonlinear absorption behavior due to multi -electron absorption.

[0044] To demonstrate this concept, organic phase metal NCs capped with OA (FIG. 8C) are used. OA bonds to metal NCs through electron donation from its nitrogen atom

[0025] , forming a binding energy of 0.3-0.5 eV across a range of metals. The OA-capped NCs of various metals were synthesized in-house, and NC ink was prepared by dissolving the dry particles in solvents, such as xylene or toluene, and used for laser printing. The feature size of the printed metal structures by employing femtosecond laser pulses at 343 nm and 515 nm were evaluated. The laser beam is focused and dwelled on the surface of the substrate. The pulse energy for both wavelengths is adjusted to around the printing threshold. In the case of 343nm irradiation, it was determined that the printed Au structure continues growing at axial direction up to >2.5pm after 30s of laser exposure, forming a single pillar. On the other hand, under 515nm irradiation, the structure remained as dots (approximately (±5%) 200nm) even after 30s of laser exposure, showing exceptional axial and lateral resolution (FIG. 8D). The high axial resolution observed in 515nm is also observed for Ag, Pt, Cu, Ni, Co. Also, the width of the structures printed by 515nm wavelength can be down to less than 200nm, meanwhile structures printed by 343nm is greater than 250nm (FIG.8E). Z-scan shows a strong nonlinear absorption of NCs at 515nm wavelength (FIG. 8F), while no nonlinear responses were observed with laser intensity up to theprinting threshold at 343nm wavelength. The observed nonlinear reversed saturable absorption arises from the transient absorption of hot electrons in the conduction band before they lose energy and thermalize in Ag

[0016]

[0017] , Au

[0016]

[0018] , Pt

[0019] , Co, and Ni. Although the d-band of various metals can lie below Fermi level (Au, Ag) or above the Fermi level (Pt, Ni, Co), the nonlinear absorption is independent of d-band level, which enables the high-resolution nanoscale printing of a wide range of metals.

[0045] The transfer of photoexcited hot electrons to molecules attached on metal surface has been corroborated for plasmonic [20, 21] (Au, Ag, Cu, Ni, Co, Fe) and non-plasmonic metals [22, 23] (Pt). Transient exchange of photoexcited hot carriers, with a lifetime of a few fs, between the NP and chemically attached molecules through hybridized electronic states lead to vibrational excitation of the attached molecules

[0024] and reactions

[0025] , Under high intensity photoexcitation, ligand desorption and fragmentation in NCs occur

[0026] , The hybridized LUMO of amine ligand with metals is located 3-4eV above Fermi level for different metals. Timedependent density -functional theory (TDDFT) simulation shows at 1 x 1012W / cm2laser intensity (515nm), there will be multiple hot electrons formed within the NC over the duration of laser pulse of about 250 fs. The interval time between two successive photoexcited hot electrons is shorter than the electron-electron relaxation time T (approximately (±5%) lOOfs), which allows re-excitation of hot electrons as evident by the RSA. The re-excitation within laser pulse is responsible for the nonlinear desorption. Comparably, laser irradiation with 343nm shows stronger linear absorption. At the printing threshold, no obvious nonlinear absorption can be observed, which compromises the printing resolution near the printing threshold.

[0046] The features of this embodiment relating to printing structures are now described. To demonstrate the capability of multi-electron 3D printing, a z-axis piezo stage was synchronized with a 2D galvo scanner to fabricate arbitrary 3D objects. A layer-by-layer stacking strategy was employed for all prints, consistent with the approach used in commercial selective laser melting systems. The metal NC inks were dispersed in a custom-built, sealed chamber with a thickness of 120 pm. By focusing laser beam on the substrate, photons penetrate the metal NC ink and reach to the surface of the substrate, initiating multi -electron printing process. First, 2D nanopatterning was performed at the speed of 100 pm / s, generating a portrait over an area of 22.5 pm *35 pm (SEM image 1200, FIG. 9). The grayscale contrast of the image was achieved by tuning the laser dwell time of each pixel location, modulating the printing of nano-dots. Additionally, the pixel-art portrait was produced by arranging the space of freestanding nanopillars, each with an average diameter of approximately (±5%) 288 nm. The nanopatterning exhibits high spatial resolution and precision control of the printing system, demonstrating the robustness of the multi-electron nanoprinting approach.

[0047] For 3D nanoprinting, a 10 * 10 array of nanopillars was fabricated with heights of 1 pm and center-to-center spacing of 1 pm. A 4 x 4 array of spiral structures with diameters of 800 nm was also printed, showing excellent uniformity across the array. In addition to these pillar-based arrays, complex, volumetric 3D structures, were fabricated including a boat-shaped architecture, an Eiffel Tower-like structure, and a hierarchical mechanical metamaterial (HMM), demonstrating the feasibility and versatility of the printing system in constructing arbitrarily designed, functional nanoscale geometries.

[0048] Helpfully, this embodiment is versatile and can be used with multiple materials. The 3D nanoprinting platforms enables printing arbitrary designed structures with extended dimensions, while preserving the intrinsic properties of metals. The underlying printing mechanism is broadly applicable to a wide range of metal materials, driven by the re-excitation of hot electrons within metallic NCs and generating strong nonlinear absorption. To demonstrate the versatility and robustness of the system, complex boat-shaped structures were fabricated with dimensions of 15 pm *7.7 pm*21 pm (LxWxH) using various metals, including Au, Ag, Pt, Cu, Ni, and Co. The boat structure contains diverse geometrical features, such as overhanging structures, sharp curvatures, and 90-degree comers, serving as benchmarks for evaluating printing performance across different metals. Printing parameters were individually optimized for each metal to achieve high-fidelity structures with minimal defects. Notably, the laser pulse energy required for printing all metals is extremely low, ranging from 50 pj to 75 pj, indicating highly efficient ligand desorption and nanoparticle fusion during the printing process. Despite minor variations in morphology among the different metals, the high-resolution printing quality is sufficient for further advanced applications.

[0049] Referring now to FIGS. 10, 11A-11C, 12A-12D, 13A-13F, & 14A-14B, another example embodiment of the AM system 100 is now described. As shown in a diagram 1210 of FIG. 10, a 355nm ns laser (20ns pulse duration and 100kHz repetition rate) is focused on the spin-coated gold NC films (OA capped NCs with size around 3-4 nm) with an oil-immersion objective lens. The localized laser energy deposition, even at ultralow averaged laser power level (i.e.,approximately a few pW (1 -10 pW)), is sufficient to induce NC fusion and sintering. The process enables the direct writing of functional micro- and nanoscale patterns with subdiffraction limited features. Concentric antennas were fabricated, which is used in actively controlled anisotropic near-field coupling

[0080] , Phase shifting antennas were fabricated, which discontinuously shift the phases of propagating light to generate anomalous refraction and reflection

[0081] , Chiral structures were fabricated, which provide chiral optical resonant responses

[0082] , Bowtie antennas were fabricated, which produce enhanced localized resonance fields from infrared exposure to harvest energy

[0083] , All the four metasurfaces are with linewidth of approximately (±5%) 200nm which is in the size region of IR metasurface and can be used in the plasmonic optics.

[0050] A series of horizontal lines were fabricated using various laser fluences and scanning speeds. The linewidth decreases with reducing laser fluence. The smallest linewidth as fabricated is approximately (±5%) 70nm (near or about X / 5) suggesting sub-diffraction-limited laser sintering capability. FIG. 11 A includes a diagram 1220, which shows the dependence of linewidth d on laser fluence and laser scanning speed, which can be modeled based on the laser- driven photochemical reaction model as shown below:where a>0is the radius of the focused laser spot (nm) and F is the laser fluence (mJ / cm2), v is the laser scanning speed (pm / s), and Ct is a parameter indicating sintering threshold. The relationship between d and laser fluence F is fitted using the model, from which the parameters <z>o, Ct and n are extracted. The fitting results yield 2m0= 442nm and n = 3.59. The fitted spot size 442nm is comparable to the diffraction-limited spot size of a circular and uniform beam 1.22X / NA=333nm. The exponent n>l signifies a super-linear dependence of the laser-driven photochemical reaction rate on fluence, following a P1relationship. Details of the photochemical reactions and the sintering mechanism will be discussed herein.

[0051] It is found that the polarization of the beam has profound effects on the linewidth of the laser sintered lines. Line sintering with 0°, 45°, 90°, 135° angle to the polarization direction were fabricated. Line patterns were fabricated under identical power and scanning speed conditions, with varying polarization. Line features written with linear polarization parallel to the scan direction exhibited narrower linewidths compared to those written with perpendicularpolarization. When using higher laser power to obtain larger linewidth, the polarizationdependent linewidth difference becomes more evident. Such phenomenon is caused by the electric field intensity concentration in the polarization direction resulting in directionally determined intensity distribution and larger linewidth. Meanwhile, the 45° and 135° lines show almost equal linewidths, and the linewidth is between lines fabricated with perpendicular and parallel polarizations. Similar result was reported in laser ablation study

[0079] , The polarization dependence on linewidth is summarized in diagrams 1230, 1240 of FIGS. 1 IB-11C. When considering printing resolution or linewidth, the polarization effect needs to be considered. To minimize the anisotropic effect induced by the polarization, the laser beam was modulated with a % X waveplate to generate circular polarization. An SEM image of the spiral structures fabricated with the circularly polarized beam exhibits significantly more uniform lines in both the horizontal and vertical directions.

[0052] Now, the sintering mechanism is described. Raman spectroscopy was employed to elucidate the evolution of the ligands during the laser sintering process, providing insights into the underlying mechanisms. FIGS. 12A-12B include diagrams 1250, 1260, which illustrate the Raman spectra of NC films subjected to conventional thermal sintering, accompanied by their visual appearance as indicated by optical reflectance. The Raman spectra of as-deposit film shows characteristic peaks around 2800-3000 cm ', corresponding to C-H vibrations in OA, and a peak at 261 cm associated with the Au-N bond between metal core and the ligands. Thermal sintering at 200 °C imparts a reflective appearance to the film, while strong peaks at approximately (±5%) 1340 cm-1 and 1590 cm'1, corresponding to the D-band and G-band of graphitic carbon, respectively, appear in the Raman spectra. These bands are attributed to carbonaceous byproducts formed during ligand decomposition

[0070] whose intensities increase with sintering time. The carbonaceous byproducts can be further removed by subjecting to thermal sintering at 35O°C in ambient air

[0084] , In contrast, laser sintering with 35mJ / cm2and 280mJ / cm2laser fluences produces a highly reflective film comparable to that obtained by thermal sintering, while the D-band and G-band are absent in the relevant range suggesting a different sintering mechanism. It is postulated that incident photons (3.5eV) generate energetic hot electrons in gold NCs, which subsequently scatter with the ligand, leading to ligand desorption and diffusion that drive the NC sintering. In the hot electron driven process, NC sintering is accomplished by ligand desorption and diffusion (diagram 1270, FIG. 12C), whileligand thermal decomposition and degradation, which commonly occur in conventional sintering

[0084] , are largely avoided. To further validate the photoexcited hot electron driven nature of the sintering process, a laser operating at 1064 nm with identical pulse duration and repetition rate was used for comparison. No sintering was observed, and strong D and G bands emerged, indicating significant carbonaceous residue.

[0053] The ligand desorption and diffusion promote the formation of long-range conductive inter-NC networks, leading to approximately 1-10% of bulk electrical conductivity as measured by the nanoelectrode fabricated by laser sintering with fluence 66-90mJ / cm2(diagram 1280, FIG. 12D). It is worth noting that this level of electrical conductivity was achieved despite the presence of residual ligands in the films that are not degraded by laser irradiation. These residual ligands can be removed through subsequent thermal decomposition, if necessary. This is evidenced by the further improved electrical conductivity of laser-sintered films subjected to additional thermal sintering, which reaches up to approximately (±5%) 50% of the bulk conductivity - comparable to that of films sintered exclusively by thermal means (FIG. 12D). Further support comes from Raman spectroscopy, where the spectra of laser-sintered films treated with additional thermal sintering at 200°C and 350°C show ligand evolution similar to that observed in films sintered purely thermally. The results indicate that the hot electron driven laser sintering, in conjunction with optional thermal sintering, enables excellent patterning resolution while maintaining superior sintering quality.

[0054] Now, the super-linear hot electron driven process is described. To characterize the kinetics of the hot electron driven ligand reaction process, an in-situ transmission measurement were conducted

[0084] , Firstly, the transmission measurement was performed without externally heating the substrate (Tsub=25°C). The relationship between laser fluence and sintering rate is shown in a diagram 1290 of FIG. 13A. The sintering rate exhibits a power-law dependence on laser fluence with an exponent of n~6.7. Secondly, the sintering rate measurements were conducted at elevated substrate temperature (Tsub = 88°C) and compared with sintering rate at Tsub = 25°C. The diagram 1300 in FIG. 13B demonstrates that substrate temperature enhances the sintering rate by approximately 3-6 times, thereby supporting a thermally assisted, hot electron-driven sintering mechanism.

[0055] FIG. 13A shows a super-linear dependence of laser sintering rate on laser fluence. FIG. 13B shows thermally assisted hot electron driven sintering as evident by the increase of sinteringrate by raising substrate temperature. FIG. 13C includes a diagram 1310, which shows quantummechanical model predicted super-linear dependence of laser sintering rate on laser fluence. FIG. 13D includes a diagram 1320, which shows a quantum-mechanical model predicted dependence of sintering rate on temperature. FIG. 13E includes a diagram 1330, which shows the transport phenomena during the thermally assisted hot electron driven laser sintering process. FIG. 13F includes a diagram 1340, which shows the total density of state (DOS) of the methylamine-gold complex and partial density of state (PDOS) of the methylamine.

[0056] The observed super-linear dependence in FIG. 13 A suggests the interval time between successive photoexcited hot electron driven events becomes shorter than certain characteristic relaxation time T in photoexcited NC fdms

[0085] , At high laser intensity, desorption induced by multi-electron transitions (DIMET) is possible wherein the interval time between hot electrons is shorter than the vibrational relaxation time Tvib (approximately a few ps) of adsorbates resulting in a super-linear desorption rate [86, 87], However, the laser intensity applied in this study is unlikely to drive the interval time below Tvib. Another relevant characteristic time in photoexcited NC films is the heat dissipation time Tt (approximately 500ps - a few ns) due to heat dissipation to the substrate

[0085] , Heat generated in the NC from a preceding hot electron excitation elevates the NC temperature. Before the thermal energy dissipates over time it, the increased probability of the ligand occupying higher vibrational energy states, as described by the Bose-Einstein distribution, lowers the activation barrier for subsequent hot electron-induced reactions [88, 89],

[0057] Based on this analysis, the microscopic mechanism of the thermally assisted hot electron driven sintering is illustrated in FIG. 13E. During the laser pulse of approximately (±5%) 20ns, a flux of photoexcited hot electrons is generated, which scatters with the ligands, transferring quanta of vibrational energy to induce ligand desorption. These hot electrons decay rapidly, followed by thermalization of hot electrons and electron-phonon relaxation that raises the temperature of the NC which in turn assists the subsequent hot electron-driven desorption processes. To compute the ligand desorption rate driven by the hot electrons, a quantum mechanical desorption model is developed, wherein the OA molecule is modeled as a methylamine molecule absorbed on gold (111) surface. The hybridization of the LUMO level of the methylamine with gold atoms form states approximately (±5%) 4.1 eV above the Fermi level (FIG. 13F). According to time dependent density functional theory (TDDFT) calculation, hot electrons with energies up to approximately (±5%) 3.5 eV above the Fermi level are transientlyformed in the NC, as shown in FIG. 13F, prior to undergoing rapid decay. Transient hot electrons engage in non-adiabatic coupling with the hybridized amine-gold states, inducing vibrational excitation and driving amine desorption. The reaction rate coefficient k of the ligand desorption on the NC can be written as

[0086] :wherein J is the flux of hot electrons in a NC, # is the number of ligands (or absorption sites) per NC,is the probability of ligand desorption per adsorption site per hot electron scattering, R is the desorption probability induced by an electron with energy s derived based on a finite temperature electron scattering model

[0089] and f(e) is the energy distribution of photoexcited hot electrons (FIG. 13F). The calculated desorption probability is shown as:induced by a photoexcited (355nm) hot electron generated. Temperature significantly enhances the desorption probability, increasing it by 3-4 orders of magnitude as the temperature rises from 300 K to 800 K. The hot electron flux per NC Jean be computed by TDDFT. At a laser fluence of approximately 10-100mJ / cm2(averaged laser intensity 5xl05-5xl06W7cm2), the hot electron flux per NC is found to be 8xl08-8xl09 / s corresponding to 1.25ns-125ps interval time between hot electrons. The interval time is comparable to the heat dissipation time rt, supporting a thermally assisted mechanism driven by hot electrons.

[0058] The finite-temperature quantum mechanical desorption model is integrated with a thermal model to quantitatively predict the sintering rate Renter as a function of laser fluence F, enabling direct comparison with sintering rates measured by the in-situ transmission measurement. FIG. 13B shows the calculated super-linear relationship characterized by an exponent of approximately n~6.2. The model suggests that the temperature of the NC film rises to approximately (±5%) 600 K with laser fluence of 60mJ / cm2, which plays a critical role in enabling the super-linear sintering rate. Furthermore, the model predicts an increase (by 3-4 times) in the sintering rate by raising the substrate temperature from 25°C to 88°C (FIG. 13D).The predicted super-linear relationship (and exponent n) and the sintering rate dependence on substrate temperature are is in reasonable agreement with experimental results verifying the thermally assisted hot electron driven sintering mechanism.

[0059] The exponent n = 6.7 as measured by the in-situ transmission measurement appears to be higher than the n = 3.59 obtained by fitting the linewidth with laser fluence (FIG. 11A). The difference is attributed to variation in temperature response during laser irradiation. In the in-situ transmission measurement, the sintering rate was evaluated at the center of the laser spot, where the temperature is assumed to be linearly proportional to the laser fluence Fo (i.e., T~Fo). In contrast, the linewidth is influenced by the spatial distribution of temperature across the laser spot, which may deviate from the laser profile due to thermal diffusion. The thermal diffusivity of glass substrate is approximately 5xl0-7m2 / s which corresponds to about lOOnm heat diffusion length (20ns laser heating). The thermal diffusion length is comparable to the laser spot radius (coo), resulting in a broader temperature distribution than the original laser spot size. Since the sintering rate Rsinter (or k) is a function of both local laser fluence and local temperature, a broader temperature distribution effectively reduces the power law dependence, leading to a lower exponent n.

[0060] Now, the metasurface fabrication is described. Detailed investigations were made into the resolution limits and precision capabilities of laser nano-sintering, focusing on the line gap and end-to-end spacing performance. An SEM image result of a sintered serpentine line with gradient line gaps in between the parallel lines, ranging from 216nm down to as small as 40nm. This gradient arrangement provides a direct visualization of the achievable minimum feature spacing. The clear distinction between adjacent lines, even at sub-100 nm gaps, highlights the strong super-linear effect of the laser-nanoparticle interaction. The progression of line-to-line spacing offers a visual record of the transition from well-resolved individual lines.

[0061] In a high-magnification view of line end to end gap test, the spacing between the terminations of horizontally oriented sintered lines is gradually varied, ranging from 155 nm down to an impressive 35 nm. This test shows the laser process has a great ability to resolve fine terminations without unintended bridging, particularly at extremely small feature gaps. The formation of clean ends with no observable merging or deformation indicates a sharp fluence threshold which comes from the strong super-linear effect.

[0062] With the knowledge of the line and line gap (or interline gap) limit, several metasurface nanostructures were made with this technique, the periodic concentric circular antennas were made to demonstrate the ability of making precise and uniform periodical structures. Such patterns are ideal for plasmonic metasurfaces, which can be used as resonant antennas for absorbers [52, 89], besides the antenna, plasmonic nanodots, by alternating the dwelling time of the laser beam on the fixed position, nanometer-sized metal plasmonic dots can be made with different size.

[0063] To verify the quality of the metasurface fabricated by the laser sintering, a series of bidirectional metasurfaces were made with periodic arrays of gold rectangular bars as subwavelength nanostructures as shown in a diagram 1350 of FIG. 14A. These subwavelength nanostructures enable precise control over the propagation of electromagnetic waves to lead the metasurface exhibits polarization-dependent refraction and the rotation profile imparts a phase gradient light refraction. Each gold bar undergoes strategic rotation to induce a specific phase shift, thereby forming a spatially varying phase profile across the supercell. The design leverages the polarization state of incident circularly polarized light to dictate the direction of refraction. The electric field distribution using finite-difference time-domain (FDTD) techniques was simulated using Tidy3D software. As demonstrated in the simulation results in a diagram 1360 of FIG. 14B, the metasurface refracts incident light differently depending on whether it is left or right circularly polarized (LCP / RCP). The color-coded phase responses in red and green indicate distinct output angles for left and right circular polarizations, respectively. These results confirm the device’s capacity for polarization-selective bidirectional refraction.

[0064] The designed metasurfaces were fabricated with sintering on a Ik rpm coated gold nanoparticle ink film on glass slide substrate. The SEM image captures its resulting nanostructure. To characterize the optical performance, a refraction performance test was conducted. By varying the number of unit cells in a supercell, the refraction angle can be influenced. A higher number of unit cells (N) leads to a reduced phase gradient, resulting in smaller refraction angles, consistent with the generalized Snell’s law

[0064] , Quantitatively, the phase gradient is defined as c / 0 / dx, where d0 denotes the phase change per unit cell and dx represents the pitch of each meta-atom in the supercell. The results show that a supercell with N=3 exhibits a steeper phase gradient and, thus, a larger refraction angle than one with N=4. Simulated refraction angles are marked with yellow stars in the plot, matching experimentalobservations. The metasurface maintains efficient light manipulation across a narrow broadband wavelength range from 950 nm to 1050 nm. The spectral dependence of the refraction angle may be calculated, which increases with wavelength. This phenomenon adheres to the generalized Snell’s law, expressed as:A dcp n,- sin 0; H - — = ntsin 0t1 12n dxfhere, m and nt denote the refractive indices of the incident and transmission media, A is the wavelength, and 0tare the incident and refracted angles, respectively. As A increases, the term increases, thereby enhancing the refracted angle. This test demonstrated the broadband capability of the metasurface.

[0065] The experimental results in momentum space affirm the bidirectional behavior of the metasurface. Depending on the circular polarization of the incident light, the beam diffracts into the +1 or -1 order, validating the structure’s capability to route light dynamically based on polarization input. Collectively, these results underscore the potential of polarization-engineered metasurfaces for compact, tunable photonic devices that exploit phase-gradient control to manipulate light direction with high fidelity. The disclosed fabrication methodology demonstrates a robust and scalable platform capable of producing intricately patterned metasurfaces tailored for advanced photonic applications.

[0066] Referring again to FIG. 1, an AM system 100 according to the present disclosure is now described. The AM system 100 includes a printing chamber 101 having printing ink. In this illustrated embodiment, the printing chamber 101 illustratively includes movable stage 102 (e.g., x-y movable piezoelectric stage), and a substrate 103 (e.g., glass substrate) thereon. The printing ink comprises a plurality of inorganic NCs. In some embodiments, this printing ink is formed onto the substrate one layer at a time (e.g., spin coating process).

[0067] The AM system 100 also includes a laser source 104 configured to emit a pulsed laser output, and a plurality of optical elements 105a-105e coupled downstream from the laser source and configured to direct the pulsed laser output to the printing chamber 101. In this illustrated embodiment, the plurality of optical elements 105a-105e sequentially includes a first mirror 105a, a shutter 105b, an attenuator 105c, a beam expander 105d, and a second mirror 105e. It should be appreciated that the plurality of optical elements 105a-105e, which are noted with dashed boxes, may vary in order and constituency based upon the needed optical path.

[0068] The AM system 100 also illustratively includes a scanner 106 between the beam expander 105d and the second mirror 105e, an objective lens 107 (e.g., an oil immersed objective lens) between the second mirror and the printing chamber 101, and a camera 110 and light source 111 coupled upstream of the second mirror and to illuminate the printing chamber. The AM system 100 also includes a controller 112 coupled to the laser source 104, the shutter 105b, the scanner 106, and the movable stage 102. The controller 112 is configured to control the pulsed laser output based upon settable operational characteristics of the laser source 104 and control of the shutter 105b.

[0069] The controller 112 is configured to cause the pulsed laser output to print one or more layers of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption / transformation (i.e., one or both of ligand desorption and ligand transformation) in the plurality of inorganic NCs. In some embodiments, the controller 112 may be configured to, subsequently to lasing a first layer, form one or more additional layers of the printing ink, and cause the pulsed laser output to print the one or more additional layers of the object (i.e., lasing each layer individually before forming the next layer). Alternatively, the controller 112 may be configured to directly 3D scan the laser output in a serial manner and form 3D structure.

[0070] In some embodiments, the controller 112 may be configured to cause the pulsed laser output to print the one or more layers of the object based upon metal SPR. The controller 112 may be configured to cause the pulsed laser output to print the one or more layers of the object based upon buildup of hot electrons (multiple excitations of hot electrons / photo excited electrons) in the NCs during each laser pulse for ligand desorption through interaction with ligand states, for example, the LUMO. The laser source 104 may be configured to emit the pulsed laser output based upon a material relaxation time in the plurality of inorganic NCs. The laser source 104 may be configured to generate the pulsed laser output so that an interval time between two successive photoexcited hot electrons is less than the material relaxation time (the relaxation time can range from femtoseconds to nanoseconds depending on the relaxation mechanism). The relaxation time can range from femtoseconds to nanoseconds depending on the relaxation mechanism.

[0071] For example, the plurality of metal inorganic NCs may comprise at least one of copper NCs, gold NCs, platinum NCs, nickel NCs, cobalt NCs, and silver NCs. The laser source 104may have an operating wavelength based upon the printing ink. The laser source 104 may be configured to emit the pulsed laser output based upon a repetition rate in a range of 0.5kHz- 5MHz. For example, the pulsed laser output may have a 20 ns pulse with a 1 kHz repetition rate (i.e., each second of the pulsed laser output comprises 1000 pulses with pulse width of 20 ns, pulse spacing may be equal or variable). For example, for printing metal NCs, wavelength in the range of 450nm-550nm may be desirable.

[0072] The laser source 104 may be configured to emit the pulsed laser output comprising one of a pulsed femtosecond laser output, a pulsed ns laser output, a pulsed fs laser, and a pulsed ns laser. The laser source 104 may be configured to operate at a power level of 50 pW or less and / or at a laser intensity between 1 * 106to 1 x 1012W / cm2at a focal spot. The laser source 104 may be configured to emit the pulsed laser output with pulse width in a range of Ifs to 1 ps. The laser source 104 may be configured to emit the pulsed laser output with averaged intensity during each laser pulse in the range of 1x106to 1 x 1012W / cm2at the laser focus spot. The laser source 104 may be configured to combine two laser sources with one emitting a shorter pulse (100 fs) and the other one emitting longer (e.g., 10ns) pulse width. The two pulses may be spatially overlapped and temporally overlapped or separated with a controlled time.

[0073] Another aspect is directed to an AM method. The method comprises providing printing ink in a printing chamber 101, the printing ink comprising a plurality of inorganic NCs, and operating a laser source 104 configured to emit a pulsed laser output. The method also comprises operating a plurality of optical elements 105a-105e coupled downstream from the laser source 104 and to direct the pulsed laser output to the printing chamber 101, and operating a controller 112 coupled to the laser source. The controller 112 is configured to cause the pulsed laser output to print one or more layers of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption in the plurality of inorganic NCs.

[0074] Referring now additionally to FIG. 2, another embodiment of the printing chamber 201 is now described. In this embodiment of the printing chamber 201, those elements already discussed above with respect to FIG. 1 are incremented by 100 and most require no further discussion herein. This embodiment differs from the previous embodiment in that this printing chamber 201 illustratively includes a substrate 206, first and second spacers 220a-220b, and a cover glass 221 over the first and second spacers to define an ink receiving cavity 222 for the printing ink comprising a plurality of inorganic NCs 223a-223c. The laser source and theplurality of optical elements are configured to generate a focused laser beam 224, which lases a voxel 225 of fused NCs on top of a layer of conductive material 226. The plurality of inorganic NCs and the layer of conductive material 226 comprise a same metallic material, for example, gold.

[0075] Other features relating to additive manufacturing are disclosed in the following papers: titled “Direct Writing of Nanostructured Metasurfaces by Hot-electron-driven Laser Sintering,” Chang et al.; and titled “Time and Space Co-confined Multi -electron 3D Nanoprinting of Metals,” Chang et al., all incorporated herein by reference in their entirety.

[0076] Many modifications and other embodiments of the present disclosure will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. In particular, features from the disclosed embodiments may be combined with each other. Therefore, it is understood that the present disclosure is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.References (the contents of each reference are hereby incorporated by reference in their entirety)1. L. Zhang, C. Wang, C. Zhang, Y. Xue, Z. Ye, L. Xu, Y. Hu, J. Li, J. Chu, and D. Wu, "High- Throughput Two-Photon 3D Printing Enabled by Holographic Multi-Foci High-Speed Scanning," Nano Letters 24, 2671-2679 (2024).2. A. Balena, M. Bianco, F. Pisanello, and M. 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Claims

CLAIMS:

1. An additive manufacturing (AM) system comprising: a printing chamber having printing ink, the printing ink comprising a plurality of inorganic nanocrystals (NCs); a laser source configured to emit a pulsed laser output; at least one optical element coupled downstream from the laser source and configured to direct the pulsed laser output to the printing chamber; and a controller coupled to the laser source and configured to cause the pulsed laser output to print at least one layer of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption / transformation in the plurality of inorganic NCs.

2. The AM system of claim 1 wherein the plurality of inorganic NCs comprises one of a plurality of metal NCs, a plurality of semiconductor NCs, and a plurality of metal oxide NCs.

3. The AM system of claim 2 wherein the plurality of metal NCs comprises at least one of copper NCs, gold NCs, platinum NCs, nickel NCs, cobalt NCs, and silver NCs.

4. The AM system of claim 1 wherein the controller is configured to cause the pulsed laser output to print the at least one layer of the object based upon multiple excitations of hot electrons in the NCs during each laser pulse for ligand desorption / transformation through interaction with ligand states.

5. The AM system of claim 1 wherein the laser source is configured to emit the pulsed laser output based upon a material relaxation time in the plurality of inorganic NCs.

6. The AM system of claim 5 wherein the laser source is configured to generate the pulsed laser output so that an interval time between two successive photoexcited hot electrons is less than the material relaxation time.

7. The AM system of claim 1 wherein the laser source has an operating wavelength based upon the printing ink.

8. The AM system of claim 1 wherein the laser source is configured to emit the pulsed laser output based upon a repetition rate in a range of 0.5kHz-5MHz.

9. The AM system of claim 1 wherein the controller is configured to form at least one additional layer of the printing ink after formation of the at least one layer of the object, and cause the pulsed laser output to print the at least one additional layer of the object.

10. The AM system of claim 1 wherein the controller is configured to directly scan a 3D trajectory to form free standing three-dimensional (3D) structures without layer-by- layer formation.11 . The AM system of claim 1 wherein the laser source is configured to emit the pulsed laser output comprising one of a pulsed femtosecond laser output and a pulsed nanosecond laser output; and wherein the laser source is configured to operate at a laser intensity between I O6to l * 1012W / cm2at a focal spot.

12. An additive manufacturing (AM) method, the method comprising: providing printing ink in a printing chamber, the printing ink comprising a plurality of inorganic nanocrystals (NCs); operating a laser source configured to emit a pulsed laser output; operating at least one optical element coupled downstream from the laser source and to direct the pulsed laser output to the printing chamber; and operating a controller coupled to the laser source, the controller configured to cause the pulsed laser output to print at least one layer of an object based upon a flux of photoexcited hot electrons to induce NC fusion via ligand desorption / transformation in the plurality of inorganic NCs.

13. The AM method of claim 12 wherein the plurality of inorganic NCs comprises one of a plurality of metal NCs, a plurality of semiconductor NCs, and a plurality of metal oxide NCs.

14. The AM method of claim 13 wherein the plurality of metal NCs comprises at least one of copper NCs, gold NCs, platinum NCs, nickel NCs, cobalt NCs, and silver NCs.

15. The AM method of claim 12 wherein the controller is configured to cause the pulsed laser output to print the at least one layer of the object based upon multiple excitations of hot electrons in the NCs during each laser pulse for ligand desorption / transformation through interaction with ligand states.

16. The AM method of claim 12 wherein the laser source is configured to emit the pulsed laser output based upon a material relaxation time in the plurality of inorganic NCs; and wherein the laser source is configured to generate the pulsed laser output so that an interval time between two successive photoexcited hot electrons is less than the material relaxation time.

17. The AM method of claim 12 wherein the laser source has an operating wavelength based upon the printing ink; and wherein the laser source is configured to operate at a laser intensity between 1 x 106to 1 x 1012W / cm2at a focal spot.

18. The AM method of claim 12 wherein the laser source is configured to emit the pulsed laser output based upon a repetition rate in a range of 0.5kHz-5MHz.

19. The AM method of claim 12 wherein the controller is configured to form at least one additional layer of the printing ink after formation of the at least one layer of the object, and cause the pulsed laser output to print the at least one additional layer of the object.

20. The AM method of claim 12 wherein the laser source is configured to emit the pulsed laser output comprising one of a pulsed femtosecond laser output and a pulsed nanosecond laser output.

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