Fabrication of PbSe nanostructures by using chemical bath deposition (CBD) for photonics applications

Abstract

Methods and systems are provided for homogeneous, single-crystalline, conductive, narrow-bandgap PbSe nanostructures, synthesized using, for example, chemical bath deposition onto a quartz substrate, including a tunable iodine doping process to select the size and / or shape of the nanostructures. Single-crystalline PbSe nanostructures can be exposed after an isolation process (e.g., an etching process), and the concentration and / or distribution of iodine across multiple PbSe nanostructures (e.g., on a quartz substrate) can be tuned during post-processing steps, including thermal treatment.

Description

[Technical field]

[0001] [Related Applications] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 282,389, entitled "Fabrication Of PbSe Nanostructures By Employing Chemical Bath Deposition (CBD) For Photonics Applications," filed November 23, 2021. The entire subject matter and content of No. 63 / 282,389 is hereby incorporated by reference in its entirety. [Background technology]

[0002] Many applications use photosensitive materials as detectors. However, conventional techniques can be expensive and complex to manufacture and have narrow absorption ranges. Therefore, photosensitive materials with improved sensitivity that can be manufactured using lower cost, less complex processes are desirable.

[0003] By comparing such systems with the present disclosure described in the remainder of this application with reference to the drawings, further limitations and disadvantages of the conventional and traditional approaches will become apparent to one skilled in the art. Summary of the Invention

[0004] Systems and / or methods are provided for homogeneous, single crystalline, conductive, narrow band gap PbSe nanostructures synthesized, for example, using chemical bath deposition onto a quartz substrate, which include a tunable iodine doping process to select the size and / or shape of the nanostructures. The single crystalline PbSe nanostructures can be exposed after an etching process, and the concentration and / or distribution of iodine across a plurality of PbSe nanostructures (e.g., on a quartz substrate) can be tuned during post-processing steps including thermal treatment.

[0005] These and various other advantages, aspects and novel features of the present disclosure, as well as details of illustrated embodiments thereof, will become more fully understood from the following description and drawings. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 illustrates an exemplary method for processing single crystal lead selenide (PbSe) nanostructures according to an exemplary embodiment of the present disclosure.

[0007] [Figure 2A] 1 is an image of a PbSe nanostructure according to an embodiment of the present disclosure.

[0008] [Figure 2B] FIG. 1 illustrates a matrix of PbSe material with multiple possible nanostructure angles or crystallographic orientations, according to an embodiment of the present disclosure.

[0009] [Figure 3A] 1 is an image of PbSe nanostructures produced by the disclosed method, according to an embodiment of the present disclosure.

[0010] [Figure 3B] 3B is a detailed image of the PbSe nanostructure of FIG. 3A according to an embodiment of the present disclosure.

[0011] [Figure 4] 1 is a graph providing carrier concentration levels in PbSe nanostructures, according to an embodiment of the present disclosure.

[0012] [Figure 5A] 1 is a graph providing measured photoluminescence levels of PbSe nanostructures, according to an embodiment of the present disclosure. [Figure 5B] 1 is a graph providing measured photoluminescence levels of PbSe nanostructures, according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The figures are not necessarily to scale. Wherever possible, similar or identical reference numbers will be used to refer to similar or identical components.

[0014] In some examples, PbSe nanostructures are produced by a method of processing single crystalline PbSe nanostructures that includes one or more of substrate preparation, chemical preparation and mixing with a solvent, chemical deposition (e.g., by chemical bath deposition), vacuum baking, thin film oxidation, thin film iodination, annealing, nanostructure isolation (e.g., chemical and / or electrochemical etching and separation processes to remove oxides), and / or post-treatment.

[0015] In the present disclosure, homogeneous, single-crystalline, conductive, narrow bandgap PbSe nanostructures are synthesized using, for example, chemical bath deposition onto a quartz substrate, and the method includes a tunable iodine doping process to select the size and / or shape of the nanostructures. The single-crystalline PbSe nanostructures can be exposed after a nanostructure isolation process (e.g., an etching process), and the concentration and / or distribution of iodine across multiple PbSe nanostructures (e.g., on a quartz substrate) can be tuned during post-processing steps including thermal treatment.

[0016] In the disclosed examples, iodine-doped PbSe nanostructures are applied to thin film samples, which are synthesized using techniques including chemical bath deposition and oxygen sensitization, iodination, and one or more post-treatments (e.g., thermal treatment and / or calcination). The disclosed PbSe nanostructures, including methods for making such nanostructures, thin films containing the nanostructures, and applications using such nanostructures, show a correlation between the size, shape, orientation, and / or layer thickness of the PbSe nanostructures and the sensitivity of the sensor itself. For example, iodine-doped PbSe single crystal nanostructures are created after a series of surface treatments including one or more of chemical bath deposition, oxygen sensitization, iodine sensitization, post-annealing, and / or etching processes. The resulting single crystal PbSe is provided as a thin film on a quartz substrate, e.g., with an increased and distributed iodine concentration to provide enhanced sensitivity and broader absorption characteristics.

[0017] As used herein, the term "nano" refers to measurements on the nanometer (nm) scale and can be used to describe structures, particles, distances, wavelengths, etc. that are measured on the nanoscale.

[0018] As used herein, "and / or" means any one or more of the items in the list joined by "and / or." For example, "x and / or y" means any element of the 3-element set {(x),(y),(x,y)}. Similarly, "x, y, and / or z" means any element of the 7-element set {(x),(y),(z),(x,y),(x,z),(y,z),(x,y,z)}. As used herein, the term "module" refers to a function that may be implemented in hardware, software, firmware, or any combination of one or more thereof. As used herein, the term "example" or "exemplary" means serving as a non-limiting example, instance, or illustration.

[0019] In a disclosed example, a method for forming single-crystalline lead selenide (PbSe) nanostructures is provided, the method including preparing a substrate, preparing chemical lead and selenium precursors, depositing the precursors on the substrate via a chemical bath deposition (CBD) process to obtain a thin film including PbSe alloy, vacuum-baking the thin film at a temperature above 100° C. to remove residual solvent, exposing the thin film to an oxygenated gas to induce recrystallization and create an oxide passivation layer, doping the thin film with a vapor including a predetermined concentration of iodine for a predetermined time, applying a nanostructure isolation technique to the thin film using a chemical etchant (and / or an electrochemical etching process) to expose the single-crystalline PbSe nanostructures underlying the oxide passivation layer and polycrystalline PbSe, and post-treating the thin film with the exposed single-crystalline PbSe to redistribute iodine into the PbSe nanostructures.

[0020] In some examples, the depositing results in a bi-layer PbSe crystalline layer including a first exposed layer comprising a morphology of polycrystalline PbSe oxide substantially free of iodine and a second underlayer comprising single-crystalline PbSe nanostructures that include iodine, where the amount of iodine in the single-crystalline PbSe is controlled by one or more of time or temperature during doping.

[0021] In an example, depositing includes depositing a precursor for a first time during the CBD process to produce a first layer, and depositing a carrier solution having the precursor and an amount of iodine for a second time during the CBD process to produce a second layer.

[0022] In some examples, the method further includes varying a time for deposition during the first time or the second time, the change in time corresponding to a change in thickness or morphology of the single crystal PbSe thin film.

[0023] In some examples, the method further includes annealing prior to etching.

[0024] In some examples, the first time period or the second time period is about 30 minutes.

[0025] In some examples, the first time or the second time can be varied depending on the required specifications.

[0026] In some examples, the oxygenated gas includes a mixture of oxygen and nitrogen.

[0027] In some examples, the method further includes adjusting a threshold temperature to control crystallization of the PbSe nanostructures, the threshold temperature corresponding to a size or shape of the PbSe nanostructures.

[0028] In some examples, the PbSe nanostructures include one or more of PbSe nanoprisms, PbSe nanoplates, PbSe nanoribbons, or PbSe nanodisks.

[0029] In some instances, PbSe nanostructures are formed beneath the oxide layer with a variety of different sizes that can be controlled based on fabrication parameters.

[0030] In some examples, the predetermined temperature corresponds to about 420° C., resulting in a generally rectangular shape of the nanostructures.

[0031] In some examples, the substrate includes a quartz substrate having one or more roughened surfaces.

[0032] In some examples, doping with iodine further includes introducing iodine vapor into the furnace using nitrogen gas as a carrier.

[0033] In some examples, the method further includes post-treating by adjusting one of the temperature or time of post-baking to control the size and redistribution of iodine concentration of the PbSe nanostructures.

[0034] In some examples, the method further includes controlling the rate, time, or temperature of the cooling process to control the shape of the PbSe nanostructures.

[0035] In some examples, the method further comprises one or more additional annealing steps involving uniformly or homogeneously distributing the dopant throughout the PbSe matrix for a predetermined time or temperature.

[0036] In some examples, etching of polycrystalline PbSe material uses a hydrogen fluoride (HF) solution or NaOH:IPA:DI water.

[0037] In some instances, polycrystalline crystal growth can occur by one of two general deposition mechanisms: ion-by-ion growth, or hydroxide cluster growth.

[0038] In some disclosed examples, the photoconductive thin film includes a substrate and a plurality of single-crystalline lead selenide (PbSe) nanoparticles disposed on the substrate, the nanoparticles containing a threshold amount of an iodine dopant.

[0039] In some examples, the thin film has electrical properties that change upon optical or thermal excitation caused by electromagnetic radiation impinging on the thin film.

[0040] In some examples, the membrane has circuitry in electrical communication with the membrane.

[0041] In some examples, the circuitry is configured to measure a change in an electrical property of the thin film in response to optical or thermal excitation caused by electromagnetic radiation impinging on the thin film.

[0042] In some examples, the electromagnetic radiation includes infrared radiation.

[0043] In the present disclosure, nano-thick and flat colloidally synthesized lead selenide (PbSe) nanostructures (e.g., nanoprisms, nanodisks, nanorods, etc.) are fabricated via a chemical bath deposition process. Also disclosed is a method for preparing photosensitive PbSe nanostructures for detection of electromagnetic energy (e.g., infrared and / or mid-IR wavelengths).

[0044] In disclosed examples, PbSe nanoparticles made by the disclosed methods can be employed in photosensitive thin films for use as collectors and / or detectors to provide low-cost nano-sized single crystal thin films for use in a variety of applications (e.g., infrared detectors, direct sensors, process control, gas analysis, defense, and / or temperature control). The disclosed PbSe nanoparticles can be used in other applications, products, and / or use cases (e.g., beyond detection), including the manufacture of solar cells, light emitting diodes, and / or lasers, as a non-limiting list of examples.

[0045] PbSe is a polar semiconductor that exhibits both ionic and covalent chemical bonds, with electrons shared unequally by the nuclei that form these bonds. In the resulting PbSe crystals, however, covalent bonds predominate. Crystalline PbSe has a face-centered cubic lattice structure and can have a lattice constant of about 6.12 Å (although it can be smaller or larger depending on the desired properties). In some instances, the lattice structure can have a direct energy band gap of about 0.27 eV relative to the bulk material at room temperature, a factor of 3×10 16 cm -3 (although smaller or larger bandgaps and / or carrier concentrations may be exhibited depending on the desired properties). Due to this narrow bandgap, the structure is sensitive to radiation in the infrared (IR) spectrum. For this reason, at least in part, some lead chalcogenides have been employed in a wide variety of applications, including IR sensors, photoresistors, photodiodes, IR lasers, and / or thermoelectric generation devices. For example, PbSe provides detection at longer wavelengths in the IR spectrum, ranging from about 4 microns to about 6 microns.

[0046] In the example of a single-junction solar cell, photons with incident energy greater than the band gap of the bombarded material are absorbed. However, the excess energy of the photons is lost as heat due to thermal relaxation. To reduce the energy loss due to thermal relaxation and thereby increase the conversion efficiency, multi-junction solar cells can be integrated by using semiconductor materials with different band gap energies. Thus, high energy photons can be collected with higher efficiency in multi-junction solar cells.

[0047] However, the composition and fabrication of multijunction semiconductors increases the overall cost of the device due to increased material usage and manufacturing complexity. On the other hand, PbSe has a narrow band gap that allows light absorption over a broader range of the solar spectrum, most notably in the low infrared region where some semiconductor photovoltaic cells cannot absorb. Furthermore, the disclosed fabrication of PbSe nanostructures and corresponding thin films is significantly less costly than conventional semiconductor materials used in photovoltaic cells.

[0048] In addition, PbSe is a direct bandgap semiconductor capable of absorbing solar radiation in materials or layers having thicknesses of only a few microns, which is significantly thinner than conventional solar cells, even though it has a narrower optical absorption range. Moreover, advantageously, the disclosed PbSe nanostructures can be relatively easily fabricated over large areas at low temperatures (e.g., compared to conventional semiconductor materials) and by low-cost fabrication techniques such as chemical bath deposition.

[0049] In the present disclosure, polycrystalline PbSe thin films are produced that are a mixture of nanoparticles (e.g., including nanostructures such as nanoprisms and / or nanodisks). Such thin films can be used to increase energy absorption from a broader range of the electromagnetic spectrum (e.g., the solar spectrum) while reducing costs associated with the manufacture and / or maintenance of other semiconductor materials used in photovoltaic products.

[0050] In addition, by employing one or more etching and / or alignment techniques for nanoparticles, low-cost individual particles for specific detectors and / or solar cells can be created. In some examples, carrier multiplication effects and / or quantum size effects are prominent in PbSe nanocrystals. For example, carrier multiplication is a process in which multiple excitons are generated from a single incident photon. For example, a single incident photon generating up to seven excitons has been observed in PbSe quantum dots. In the present disclosure, PbSe nanostructures are provided that can effectively generate and separate these excitons, which in turn can contribute to increasing the conversion efficiency of solar cells and / or the sensitivity of detectors. In some examples, the band gap of PbSe increases with decreasing size of the nanostructures, so that quantum confinement effects allow tuning of the band gap in PbSe quantum dots, which can aid in targeted absorption of one or more specific regions of the electromagnetic spectrum.

[0051] In an exemplary gas sensor application, the disclosed PbSe nanostructures can be used in sensors for carbon dioxide (CO2) or ammonia (NH3) measurement, which are part of essential environmental applications for monitoring air pollution. Because PbSe nanostructures (e.g., nanoprisms) have a single crystal structure and small size, sensors using PbSe nanoprisms can be directly integrated into small devices, such as smartphones or other devices, to monitor air and / or environmental pollution in real time, both indoors and outdoors.

[0052] Currently, some detection systems, such as non-dispersive infrared spectroscopy, use bulky, large polycrystalline PbSe thin films. For example, commercially available thin film detectors have a detection limit of approximately 50 ppm for CO2 concentrations ranging from 0 ppm to 4000 ppm.

[0053] Advantageously, the disclosed sensors provide greater sensitivity and therefore detectability, and are cost-effective. For example, air pollution monitoring systems using PbSe nanostructures can be miniaturized for integration into a variety of devices, including smartphones, personal medical devices, tablets, and / or wearable consumer products, as a non-limiting list of examples.

[0054] Isolation of nanostructures by sonication and centrifugation, size separation / manipulation by DEP process Nanocrystals or nanostructures are typically isolated from their growth and preparation medium by the addition of a polar solvent. Lead chalcogenide nanostructures, mechanically peeled off from the substrate, are carefully transferred to a polar solvent container. Sonication in the solvent and centrifugation sedimentation are used to collect the nanostructures. However, other techniques such as dieletrophoresis (DEP) can also be employed to separate monodisperse nanostructures. This technique is based on the migration of dielectric or polarizable nanostructures in a non-uniform electric field due to the interaction of the dipoles of the nanostructures with the spatial gradient of the electric field. Since lead chalcogenides have diverse dielectric properties, DEP can be used to manipulate, transport, separate, and sort different types of lead chalcogenides in nanocrystals / nanostructures. A DEP chip consists of electrodes, typically a microarray, separated by gaps that form a microfluidic channel. Lead chalcogenide nanostructures to be separated are introduced and an appropriate electric field is applied to separate the target nanostructures by their size and shape. To monitor the separation process of the trapped nanostructures, a sensor-based DEP device can be used to simultaneously detect and measure concentration changes in the nanostructure-containing solution. Two interconnected electrode devices are designed to perform the dielectric trapping of the nanostructures and measure the change in the conductivity (e.g., impedance) of the medium.

[0055] In some disclosed examples, a method 100 for processing single crystal PbSe nanostructures (e.g., nanoprisms, etc.) is provided that consists of one or more of substrate preparation 102, chemical preparation and mixing with a solvent 104, chemical deposition (e.g., by chemical bath deposition) 106, vacuum baking 108, thin film oxidation (e.g., oxygen sensitization) 110, thin film iodination (e.g., iodine sensitization) 112, annealing 114, nanostructure isolation (e.g., chemical and / or electrochemical etching to remove oxide) 116, and / or post-treatment 118, as shown in Figure 1. As provided herein, the process can proceed as enumerated, or can proceed with one or more of the enumerated actions being optional and / or configured differently.

[0056] In some examples, the quartz substrate is subjected to a pre-cleaning step in the substrate preparation, which may include additional or optional plasma cleaning and / or surface treatment to roughen one or more surfaces of the substrate. In step 4, the vacuum bake is performed at a predetermined temperature and / or temperature range (e.g., about 105° C.) for a predetermined time (e.g., about 16 hours), and the oxidation process is performed at a predetermined temperature higher than the initial vacuum bake temperature (e.g., about 420° C.). In some examples, the iodine sensitization can be performed by a quartz tube furnace at a predetermined temperature (e.g., 450° C.) using a gas mixture composed of a carrier solution (e.g., nitrogen) and iodine vapor. The chemical etching is performed in a mixture solution of hydrogen fluoride (HF) and deionized (DI) water (e.g., NaOH:IPA:DI), which can have a solution concentration of HF of 50:1 (by volume) and / or DI water of 1:25:50 (by volume). As a result of the disclosed method, PbSe nanostructures containing nanoprisms are formed beneath the oxide PbSe phase with a variety of different sizes.

[0057] The use of low cost reduces the overall cost of producing PbSe nanostructures. These and other disclosed methods and the resulting PbSe nanostructure products offer advantages over other techniques because they enable the production of large quantities of single crystal nanoprisms with varying iodine doping levels in a low cost and low temperature manufacturing technique using CBD methods.

[0058] Growth mechanism of PbSe To fabricate PbSe nanostructures, including nanoprisms, nanodisks, and / or nanorods, a polycrystalline and / or amorphous PbSe layer or thin film can be formed first. The growth rate of the PbSe film depends on the dissociation of Pb from the complex state. 2+ It depends primarily on the rate of release of the ions and the decomposition of lead acetate trihydrate (C4H6O4Pb 3H2O) and selenourea (CH4N2Se). 2+ Ion and Se 2- The ionic product with the ion is the solution solubility product of PbSe (e.g., about 10 at 300 K). -38 ) The concentrations of lead and selenium ions are therefore controlled during film growth.

[0059] In the deposition of lead chalcogenides, the metal cation Pb is generally complexed with lead acetate trihydrate, which acts as the lead precursor. 2+ This controls the release of ions and prevents the precipitation of Pb(OH)2. In the case of PbSe, hydrolysis of the chalcogenide precursor selenourea (CH4N2Se) produces the anion Se 2- Lead chalcogenides provide ions with an ionic product greater than the solubility product (e.g., K for PbSe at 300 K). sp about 10 -38 ) precipitates. Even if significant precipitation is expected during the CBD process, it is not expected to adhere to the substrate in significant amounts. Thus, polycrystalline crystal growth can occur by one of two general deposition mechanisms: ion-by-ion growth, or hydroxide cluster growth.

[0060] Ion-by-ion growth typically occurs as a result of ionic reactions when homogeneous nucleation occurs. Collisions between ions form nuclei that are adsorbed onto the substrate, and ion-by-ion growth typically results in larger crystals, with the crystal size being directly proportional to the film thickness.

[0061] Hydroxide cluster growth occurs in the presence of metal hydroxides. Thus, the deposition mechanism can occur when Pb(OH)2 is present either as a precipitate or as a colloid. Hydroxide cluster growth typically results in smaller crystals compared to ion-by-ion growth. Unlike ion-by-ion growth, film thickness does not greatly affect the crystal size from hydroxide cluster growth.

[0062] This disclosure describes a method for the deposition of Se 2- They describe the use of selenourea to provide ions, and the polycrystalline PbSe thin film mechanism follows an ion-by-ion growth. Thus, PbSe is formed by an ionic reaction as described by Equation 1, provided that the ionic product is greater than the solubility product. Pb 2+ +Se 2- ⇔PbSe formula 1

[0063] The chemical reaction given in Equation 1 results in the precipitation of PbSe onto the substrate in solution.

[0064] Lead selenide deposition Once formed, PbSe can be deposited by a variety of processing techniques, including pyrolysis, vacuum evaporation, sputtering, chemical vapor deposition (CVD), molecular-beam epitaxy (MBE), and chemical bath deposition (CBD). According to the disclosed examples, the CBD technique is particularly advantageous for forming PbSe nanostructures, such as being a simple, low-cost technique that can be performed at relatively low temperatures.

[0065] The chemicals used in the preparation of PbSe thin films are of analytical grade and can be used without further purification. In some instances, lead acetate trihydrate ((CH3COO)Pb 3H2O) and selenourea (CH4N2Se) were used to prepare PbSe thin films, respectively. 2+ and Se 2- Used as an ion source, trisodium citrate (TSC) acts as a complexing agent for the slow release of metal ions, thus facilitating the formation of nanocrystalline PbSe thin films.

[0066] Prior to commencing chemical bath deposition, the lead acetate, selenourea and / or iodine solutions are prepared separately, which can be done some time in advance (e.g., about 24 hours in advance). To prepare the selenourea solution, an initial amount (e.g., about 455.2 grams) of lead acetate trihydrate crystals is added to a container (e.g., a 1000 ml plastic bottle) and DI water is slowly added until the weight of the bottle reaches a threshold amount (e.g., about 800 grams). A predetermined amount of fluid (e.g., 500 ml of DI water) is provided in another container (e.g., a 2000 ml beaker), which is heated (e.g., placed on a high setting on a hot plate) and the container with the lead acetate is placed therein.

[0067] In some instances, at the top of the hot plate, the reading is approximately 64°C. The heat supply completely dissolves the lead acetate crystals within approximately 30 minutes. Once this step is complete, the bottle of lead acetate solution is kept in a thermostatic bath set at 30°C. For the preparation of the selenourea solution, 1355 grams of DI water is added to a 2000 ml volumetric flask, which is then placed on the hot plate. With the hot plate dial turned to high, it takes approximately one hour to bring the water to a boil. The base of the flask has reached a temperature of approximately 90°C. Once the water in the flask has boiled for 10 minutes, the volumetric flask is placed in a container of tap water and allowed to cool to 70°C.

[0068] After cooling, add the amount of DI water lost due to boiling to ensure that approximately 1355 grams is recovered. Next, approximately 0.050 kg of selenourea is poured into the flask and shaken several times until the selenourea is completely dissolved. Once the selenourea solution is complete, place it in a dark environment at room temperature for a period of time before using it for CBD.

[0069] For the preparation of iodine solution, a pre-cleaned 250 ml graduated cylinder containing 13 grams of potassium iodide (KI) crystals is mixed with approximately 50 ml of deionized (DI) water and approximately 50 ml of isopropanol is added to the graduated cylinder. To completely dissolve the crystals, place the graduated cylinder in an ultrasonic vibrator with power up to 100% supplied for approximately 20 minutes and mix thoroughly by inverting the graduated cylinder several times. If not completely dissolved, repeat this process two or three times. Once dissolution of the KI crystals is complete, add DI water to the graduated cylinder up to the 250 ml mark and then mix thoroughly by inverting the graduated cylinder 20 to 25 times. Store this solution at room temperature for at least overnight.

[0070] In some examples, the PbSe deposition consists of two layers, the first layer having little or no iodine added to it, and the second layer is doped with an adjustable amount or concentration of iodine, which is controlled to create the desired PbSe nanostructures (e.g., nanoprisms) with doping in an amount considered a donor impurity. Thus, the carrier concentration can be varied by adding a desired amount of iodine during the CBD process. In examples, one or both layers are subjected to chemical bath deposition for a predetermined time (e.g., 30 minutes per layer, 1 hour total), which may be the same for the leech layer, but may vary. Varying the deposition time changes the thickness and morphology of the thin film PbSe, allowing for optimization of the PbSe structure and / or properties. As shown in the example of FIG. 4, the carrier concentration varies (e.g., decreases) as a function of one or more sensitization treatments (e.g., oxygenation and / or iodination) applied to the PbSe nanostructures (e.g., p-type PbSe particles).

[0071] In the example, one or more substrates are mounted in a container of DI water and a tungsten lamp immersed in the DI water is used to heat the substrates, with the lamp voltage being supplied by a laboratory-made program. The peak deposition temperature of the solutions during chemical deposition gradually reaches 90°C. A first amount of lead acetate solution is measured into a 100 ml graduated beaker and a second amount of selenourea solution is prepared separately using another 250 ml graduated beaker. Depending on the experimentally obtained data for single crystal PbSe nanoprism growth, the x and y amounts vary, typically for PbSe detector applications, the x / y ratio is 75 / 150. At the bottom of the solution pan is a Teflon-coated stir bar used at 100 rpm to 300 rpm. The first and second amounts of each solution are simultaneously poured into the solution pan containing the stir bar and 100 ml of DI water is poured into the substrate pan as a medium to transfer thermal energy from the tungsten (W) lamp bulb to the substrate attached to the bottom of the pan facing the solution mixture. This is one way to indirectly transfer heat energy uniformly to the substrate. The bath temperature is measured by a thermocouple placed at the midpoint depth in the vessel. At the end of the deposition cycle, the bath temperature is centered at the peak temperature, the pan with the substrate is abruptly removed from the container with the solution, and the substrate pan is then immersed in deionized water, allowing the temperature to return to room temperature.

[0072] For the deposition of the second layer, the first layer is repeated by adding a given amount of iodine solution to the selenourea and lead acetate solution in the container, which changes the donor concentration upon completion of the chemical deposition to be incorporated into the PbSe crystal matrix. In some additional or alternative examples, the amount of iodine added for the disclosed PbSe detector can be about 12 ml to 60 ml (e.g., about 24 ml). The manner of iodine addition omission affects the morphology of the polycrystalline PbSe thin film, and the distribution of the doping amount throughout the PbSe thin film. In the present invention, the magnetic stirrer is rotated for 1 minute immediately after adding the iodine solution and before starting the actual CBD. Once the second layer is completed, the pan containing the substrate is placed in a container of DI water until it cools down, followed by the usual cleaning procedure.

[0073] The prepared PbSe thin film is essentially polycrystalline in structure and is subjected to a vacuum baking step to remove any type of solvent that may be trapped in the thin film or substrate and has an etched rough surface morphology. The vacuum baking step is performed at a given temperature (e.g., 105° C.) for a given period of time (e.g., overnight). This temperature does not change the crystal structure of the PbSe thin film.

[0074] In some examples (represented by block 110), during the oxidation step, the initially doped iodine induces a surface recrystallization phenomenon on the surface of the thin film as well as inside the PbSe matrix, forming a series of single-crystalline PbSe nanoprisms, including nanoribbons, nanorods, and nanodisks. In the oxidation step, gas (e.g., oxygen and / or nitrogen) is used and purged from top to bottom in a vertical oxidation quartz tube furnace. For example, the gas ratio of the mixed gas of oxygen and nitrogen is set to about 20%, 75%, which may correspond to an oxygen flow rate of about 1.05 liters / min and a nitrogen flow rate of about 1.95 liters / min, resulting in a total of about 3.0 liters / min in the vertical furnace. The peak temperature in the tube furnace is related to the degree of crystallization of PbSe, which can be adjusted according to the size and shape requirements of the nanoparticles. A typical peak furnace temperature to obtain a rectangular shape of nanoprisms is considered to be about 420° C. (see, for example, FIG. 5A and FIG. 5B). As the deposited thin film PbSe alloy (with a composition ratio of Pb0.55, Se0.45) is exposed to high temperatures above the melting point of Pb (e.g., 100°C), other phases of the nanoparticles (e.g., nanoprisms, nanoribbons, and / or nanodisks) can grow through mixed solid and liquid phases. Subsequent cooling can determine the shape of the particles (e.g., cooling rate, time to reach the desired temperature, presence of gas, etc.).

[0075] In some examples, a doping process using iodine as a donor impurity (represented by block 112) follows. In this block, the carrier concentration of the single crystal PbSe nanoparticles (e.g., nanoprisms, nanoribbons, nanodisks) can be varied depending on the requirements of a particular application. In other words, it is directly obtained that the impurity level in the energy band diagram can be adjusted by adding iodine in a simple tube furnace. The set temperature of the central zone of the tube furnace can be set to a predetermined point (e.g., about 347°C) that results in a high exposure temperature (e.g., about 395°C) in the tube furnace.

[0076] In some examples, the iodine crystal sublimation process is carried out using a laboratory-made water-circulating Graham tube containing iodine crystals to deliver iodine vapor into the furnace. Nitrogen gas is used as a carrier to introduce the iodine-rich vapor into the furnace. The application of the nitrogen gas solution can be set at a predetermined rate (e.g., 1.2 standard cubic feet per hour (SCFH)) if the solution does not carry iodine vapor, and at a higher predetermined rate (e.g., 12.0 SCFH) if the solution carries vapor. The doping level of the PbSe crystals can be adjusted by optimizing the iodine deposition time at a relatively high temperature. For example, the doping time can be set to about 70 seconds at or near the peak temperature at the center of the furnace.

[0077] In some examples, the doping time can be varied to provide the desired sensitivity corresponding to a given doping concentration. As disclosed herein, the doping process is implemented when the sample (e.g., quartz substrate) is thermally treated (e.g., in a quartz furnace). FIG. 2A shows an image of a PbSe thin film (and thus the nanostructures contained therein) where the carrier concentration level can be adjusted by two sensitization process conditions (e.g., oxygenation and / or iodination), where the adjustable parameters correspond to one or more of the temperature, time, and / or amount and / or concentration of the induced gas volume during the thermal treatment (e.g., in the furnace). In the present disclosure, the carrier concentration in the resulting PbSe nanostructures can be changed by an exponential amount by changing one or more parameters of the two sensitization processes. FIG. 2B shows a number of possible nanostructure angles or crystallographic growth orientations for the PbSe material.

[0078] In some instances (represented by block 114), annealing is performed to reduce the temperature of the nanoparticles after iodination.

[0079] Preparation of PbSe nanoparticles At this stage, the single crystalline nanoparticles are not exposed, but are embedded within and / or beneath the polycrystalline PbSe thin film, and therefore the nanoparticles are not detectable upon investigation (e.g., under SEM or other investigation tools).

[0080] In the disclosed example (represented by block 116), the PbSe nanoparticles can be exposed by a surface etching process using an etchant such as a diluted hydrogen fluoride (HF) solution, a diluted HF solution (DI water:HF=50:1 by volume), or a solution of NaOH:IPA:DI water (1:25:50 by volume). An example of nanoparticles obtained by the disclosed method is shown in Figure 3A. Figure 3B shows a PbSe nanoprism in the image of the PbSe nanostructure of Figure 3A, taking the shape of a particle with angular profiles 101 and 110 as provided in the matrix.

[0081] For rectangular shaped nanoprisms, the first dimension (e.g., length) is estimated to be in the range of 400 nm to about 1 micron, and the second dimension (e.g., thickness) is estimated to be in the range of a few nm to 50 nm, as confirmed by SEM investigation. Under energy-dispersive X-ray spectroscopy (EDS) analysis, PbSe nanocrystals / nanostructures containing iodine as a dopant are presented. For example, Figure 3B shows a number of nanoparticles with a size of 1 micron x 1 micron. These particles can be collected by solution-based methods.

[0082] In some examples, a post-treatment such as an extended post-baking (represented by block 118) is performed at a predetermined temperature (e.g., about 150° C.) that provides one or more of the following advantages: The doped iodine is uniformly redistributed within the PbSe nanoparticles, which can grow in size during the post-baking step.

[0083] The disclosed method is designed to synthesize substantially flat PbSe nanostructures (e.g., nanoprisms, nanoplates) using CBD techniques and iodine doping processes, and ultimately control the electrical properties of the PbSe nanostructures, as shown in the exemplary graph in FIG.

[0084] Substantial numbers of single crystal PbSe nanoprisms throughout the PbSe matrix increase the sensitivity and detection performance of the thin film, providing technical advantages for applications other than detection, such as, for example, solar cells, light emitting diodes, gas analysis, medical services, industrial processes, emissions monitoring, spectroscopy, process control systems, thermal imaging, defense and security technologies, and / or nano-sized detectors in the mid-wave IR range (e.g., 3 microns to 5 microns).

[0085] Although this disclosure refers to applications of PbSe, such as thin film photodetectors employing PbSe, the disclosed methods may be applicable to other Pb-based chalcogenide and / or semiconductor materials, and / or other metal alloys (e.g., including other post-transition metals).

[0086] 5A and 5B are graphs providing photoluminescence measurement levels of PbSe nanostructures according to embodiments of the present disclosure. For example, photoluminescence (PL) can be measured through a process that examines the electronic structure of a particular material. PL is a phenomenon when electromagnetic energy is absorbed and then emitted at a range of wavelengths that may be different from the absorbed electromagnetic energy.

[0087] In some examples, a monochromatic energy source (e.g., a laser) is directed at a material to excite the sample. In response to this energy, excited electrons move from a ground state to a higher excited energy state. The material then releases energy as a combination of phonons (vibrations) and photons (light) as it returns to the ground state. Sensors can measure the emitted light for spectral and / or spatial analysis to obtain information about material properties.

[0088] In some cases, the excited state is in the conduction band (CB). An excitation photon must be triggered to excite an electron higher into the CB. Then, by non-radiative relaxation, the electron moves to the conduction band edge (CBM).

[0089] The amount of energy released is less than the energy absorbed, since an amount of energy is consumed during the interaction with the matter. The energy of the light released by radiative recombination is related to the difference between the two energy levels involved in the transition between the excited state and the equilibrium state.

[0090] This process can be used to generate information in a variety of contexts, such as defect detection, measurement of impurity levels, recombination mechanisms, analysis of material quality, determination of band gap energy, molecular structure and crystallinity, to list a non-limiting number of examples.

[0091] FIG. 5A shows an example of the PL intensity for both unprepared material juxtaposed against nanoprism material prepared by the disclosed method. The unprepared material reaches low levels (e.g., less than 10 arbitrary units), while the disclosed nanoprism material reaches a peak approaching 140 arbitrary units. Thus, the PL intensity increases by about 140 times better than the deposited unprepared material, indicating that the degree of PbSe crystallinity is substantially enhanced. FIG. 5B shows a filtered or smoothed line of the PL graph of FIG. 5A.

[0092] Although several examples and / or embodiments are described with respect to PbSe nanostructures, the principles and / or advantages disclosed herein can be employed in techniques that are not limited to any particular type of material and / or application.

[0093] Although the present disclosure has been described with reference to certain specific embodiments, those skilled in the art will recognize that various modifications can be made and equivalents can be substituted without departing from the scope of the present disclosure. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present disclosure without departing from its scope. Therefore, the present disclosure is not limited to the particular embodiments disclosed, but rather, the present disclosure is intended to include all embodiments falling within the scope of the appended claims.

Claims

1. 1. A method for forming single crystalline lead selenide (PbSe) nanostructures, comprising: Preparing a substrate; preparing chemical lead and selenium precursors; depositing the precursor onto the substrate via a chemical bath deposition (CBD) process to obtain a thin film comprising a PbSe alloy; vacuum baking the thin film at a temperature above 100°C to remove residual solvent; exposing the thin film to an oxygenated gas to induce recrystallization and create an oxide passivation layer; doping the thin film with a vapor containing iodine at a predetermined concentration for a predetermined time; applying a nanostructure isolation technique using a chemical etchant or an electrochemical etching process to the thin film to expose single-crystalline PbSe nanostructures underlying the oxide passivation layer and polycrystalline PbSe; post-treating the thin film with the exposed single-crystalline PbSe to redistribute iodine into the PbSe nanostructures; A method comprising:

2. A photoconductive thin film, A substrate; a plurality of single-crystalline lead selenide (PbSe) nanoparticles disposed on the substrate; wherein the nanoparticles contain a threshold amount of iodine dopant. Photoconductive thin film.

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