Fabrication of pbse nanostructures by employing chemical bath deposition (CBD) for photonics applications

Abstract

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

Description

Technical Field

[0001] Related applications

[0002] This application claims the benefit and priority of U.S. Provisional Application No. 63 / 282,389, filed November 23, 2021, entitled "Fabrication of PbSe Nanostructures by Chemical Bath Deposition (CBD) for Photonic Applications". The entire subject matter and contents of Application No. 63 / 282,389 are incorporated herein by reference.

[0003] This disclosure relates to the fabrication of PbSe nanostructures using chemical bath deposition (CBD), a technique employed for photonic applications. Prior Technology

[0004] Many applications employ photosensitive materials as detectors. However, conventional techniques are expensive and complex to manufacture, and their absorption range can be narrow. Therefore, photosensitive materials with higher sensitivity produced through cheaper and simpler processes are ideal.

[0005] By comparing such systems with the rest of this disclosure as illustrated with reference to the accompanying drawings, the further limitations and disadvantages of conventional and traditional methods will become apparent to those skilled in the art. Summary of the Invention

[0006] A system and / or method are provided for homogeneous, single-crystal, conductive, and narrow-bandgap PbSe nanostructures, synthesized using chemical bath deposition on, for example, a quartz substrate, and including a tunable iodine doping process to select the size and / or shape of the nanostructures. The single-crystal PbSe nanostructures can be exposed after an etching process, and the concentration and / or distribution of iodine across multiple PbSe nanostructures (e.g., on a quartz substrate) can be adjusted during post-processing steps (including thermal treatment).

[0007] These and various other advantages, features and novel characteristics of this disclosure, as well as the details of its illustrative embodiments, will become more fully understood from the following description and the accompanying drawings. Simple Explanation of the Diagram

[0008] Figure 1 illustrates an exemplary method for processing single-crystal lead selenide (PbSe) nanostructures according to an exemplary embodiment of the present disclosure.

[0009] Figure 2A shows images of various PbSe nanostructures according to this disclosure.

[0010] Figure 2B shows a matrix of multiple possible nanostructure angles or crystal orientations of PbSe materials according to various morphologies disclosed herein.

[0011] Figure 3A shows images of various PbSe nanostructures produced by the method disclosed herein, according to the present disclosure.

[0012] Figure 3B is a detailed image of the PbSe nanostructure of Figure 3A in various forms according to this disclosure.

[0013] Figure 4 is a graph showing the carrier concentration levels in PbSe nanostructures for various samples according to this disclosure.

[0014] Figures 5A and 5B are graphs providing photoluminescence measurement standards for various PbSe nanostructures according to this disclosure.

[0015] The accompanying drawings are not necessarily drawn to scale. Where appropriate, similar or identical reference numerals are used to refer to similar or identical components. Implementation

[0016] In some instances, PbSe nanostructures are formed by processing single-crystal PbSe nanostructures, a method comprising one or more of the following: substrate preparation, chemical preparation and mixing with a solvent, chemical deposition (e.g., via chemical bath deposition), vacuum baking, thin film oxidation, thin film iodization, annealing, nanostructure isolation (e.g., chemical and / or electrochemical etching to remove oxides and separation processes), and / or post-processing.

[0017] In this disclosure, homogeneous, single-crystal, conductive, and narrow-bandgap PbSe nanostructures are synthesized using chemical bath deposition on, for example, a quartz substrate, and include tunable iodine doping processes to select the size and / or shape of the nanostructures. The single-crystal PbSe nanostructures can be exposed after a nanostructure isolation process (e.g., an etching process), and the iodine concentration and / or distribution across multiple PbSe nanostructures (e.g., on a quartz substrate) can be adjusted during post-processing steps (including heat treatment).

[0018] In the disclosed examples, iodine-doped PbSe nanostructures are applied to thin film samples. These PbSe nanostructures are synthesized using chemical bath deposition and techniques including oxygen sensitization, iodination, and one or more post-treatments (e.g., heat treatment and / or baking). The disclosed PbSe nanostructures, including methods for producing such nanostructures, thin films containing such nanostructures, and applications employing such nanostructures, demonstrate 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 produced 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, for example, appears as a thin film on a quartz substrate, exhibiting an increased distributed iodine concentration to provide enhanced sensitivity and a wider range of absorption characteristics.

[0019] As used in this article, the term "nanometer" refers to the nanometer (nm) measurement scale and can be used to describe structures, particles, distances, wavelengths, etc., measured at the nanometer scale.

[0020] As used herein, "and / or" means any one or more items in a list connected by "and / or". For example, "x and / or y" means any element in the three-element set {(x), (y), (x, y)}. Similarly, "x, y and / or z" means any element in the seven-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 can be implemented in hardware, software, firmware, or any combination thereof. As used herein, the terms "instance" or "illustrative" are used as non-limiting examples, examples, or illustrations.

[0021] In the disclosed example, a method for forming a single-crystal lead selenide (PbSe) nanostructure is provided. The method includes the following steps: preparing a substrate; preparing chemical lead and selenium precursors; depositing the precursors onto the substrate via a chemical bath deposition (CBD) process to form a thin film containing a PbSe alloy; vacuum baking the film at a temperature above 100°C to remove residual solvent; exposing the film to an oxygen-containing gas to induce recrystallization and generate an oxide passivation layer; doping the film with a vapor containing a predetermined concentration of iodine for a predetermined time period; applying a nanostructure isolation technique using a chemical etchant (and / or an electrochemical etching process) to the film to expose the oxide passivation layer and the single-crystal PbSe nanostructure beneath the polycrystalline PbSe; and post-processing the film with uncovered single-crystal PbSe to redistribute iodine to the PbSe nanostructure.

[0022] In some instances, the deposition step produces two PbSe crystalline layers, including a first exposed layer comprising a substantially iodine-free polycrystalline PbSe oxide form; and a second underlying layer comprising an iodine-containing single-crystal PbSe nanostructure, wherein the amount of iodine in the single-crystal PbSe is controlled by one or more of the time or temperature during doping.

[0023] In this example, the deposition process includes the following steps: depositing a precursor for the first time during the CBD process to form a first layer; and depositing the precursor and a carrier solution containing a predetermined amount of iodine for the second time during the CBD process to form a second layer.

[0024] In some instances, the method further includes the step of changing the time used for deposition during a first or second time period, the time variation corresponding to a change in the thickness or morphology of the single-crystal PbSe film.

[0025] In some instances, the method further includes the step of annealing prior to etching.

[0026] In some instances, the first or second time is approximately 30 minutes.

[0027] In some instances, the first or second time may vary depending on the required specifications.

[0028] In some instances, oxygen-containing gas comprises a mixture of oxygen and nitrogen.

[0029] In some instances, the method further includes the step of adjusting a critical temperature to control the crystallization of the PbSe nanostructure, the critical temperature corresponding to the size or shape of the PbSe nanostructure.

[0030] In some instances, PbSe nanostructures comprise one or more of the following: PbSe nanoprisms, PbSe nanoplates, PbSe nanoribbons, or PbSe nanodisks.

[0031] In some instances, PbSe nanostructures are formed beneath the oxide layer in various sizes that can be controlled according to manufacturing parameters.

[0032] In some instances, the predetermined temperature corresponds to approximately 420°C, resulting in a roughly rectangular shape for the nanostructure.

[0033] In some instances, the substrate comprises a quartz substrate having one or more rough surfaces.

[0034] In some instances, the iodine doping step further includes the following step: introducing iodine vapor into the furnace using nitrogen as a carrier gas.

[0035] In some instances, the method further includes the following steps: post-processing by adjusting one of the baking temperature or time to control the size of the PbSe nanostructures and the redistribution of iodine concentration.

[0036] In some instances, the method further includes the following steps: controlling the rate, time, or temperature of the cooling process to control the shape of the PbSe nanostructure.

[0037] In some instances, the method further includes one or more additional annealing procedures, including uniformly or homogeneously distributing the dopant in the PbSe matrix at a predetermined time or temperature.

[0038] In some instances, the etching step of the polycrystalline PbSe material is performed using a hydrogen fluoride (HF) solution or a NaOH:IPA:DI solution.

[0039] In some instances, polycrystalline growth can occur via one of two common deposition mechanisms: ion-by-ion growth or hydroxide cluster growth.

[0040] In some of the disclosed examples, the photoconductive thin film includes a substrate; and a plurality of single-crystal lead selenide (PbSe) nanoparticles arranged on the substrate, the nanoparticles containing a limited amount of iodine dopant.

[0041] In some instances, the thin film has electrical properties that can be varied by light or heat excitation caused by electromagnetic radiation impacting the thin film.

[0042] In some instances, the thin film has a circuit that is electrically connected to the thin film.

[0043] In some instances, the circuit is used to measure changes in the electrical properties of a thin film in response to light or heat excitation caused by electromagnetic radiation striking the film.

[0044] In some instances, electromagnetic radiation includes infrared radiation.

[0045] This disclosure describes the production of nanoscale, flat colloidal synthetic lead selenium (PbSe) nanostructures (e.g., nanoprisms, nanodisks, nanorods, etc.) via a chemical bath deposition process. It also discloses a method for preparing photosensitive PbSe nanostructures for detecting electromagnetic energy (e.g., infrared and / or mid-IR wavelengths).

[0046] In the disclosed examples, the PbSe nanoparticles produced by the disclosed method can be used as collectors and / or detectors in photosensitive thin films, thereby providing low-cost, nano-sized single-crystal thin films for 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., undetectable), including in the manufacture of solar cells, light-emitting diodes, and / or lasers, as a non-limiting list of examples.

[0047] PbSe is a polar semiconductor, exhibiting both ionic and covalent bonds, with the atomic nuclei sharing electrons unequally. However, covalent bonds dominate in the resulting PbSe crystals. Crystalline PbSe has a face-centered cubic lattice structure with a lattice constant of approximately 6.12 Å (but may be smaller or larger depending on the desired properties). In some instances, the lattice structure can have a direct energy band gap of approximately 0.27 eV for bulk materials at room temperature and an intrinsic carrier density of 3 x 10¹⁶ cm⁻³ (although it may exhibit smaller or larger band gaps and / or carrier densities, depending on the desired properties). Due to this narrow band gap, the structure is sensitive to radiation in the infrared (IR) spectrum. For this reason, some lead chalcogenides have been widely used, at least to some extent, in various applications, including IR sensors, photoresistors, photodiodes, IR lasers, and / or thermoelectric generators. For example, PbSe can provide detection of longer wavelengths in the IR spectrum, ranging from approximately 4 micrometers to approximately 6 micrometers.

[0048] In single-junction solar cells, photons with incident energy exceeding the bandgap of the affected material are absorbed. However, the excess energy of the photons is lost as heat through thermal relaxation. To reduce energy loss due to thermal relaxation and thus improve conversion efficiency, multi-junction solar cells can be integrated using different semiconductor materials with varying bandgap energies. Therefore, high-energy photons can be collected with higher efficiency in multi-junction solar cells.

[0049] However, the increased material usage and manufacturing complexity of multi-junction semiconductors raise the overall equipment cost. On the other hand, PbSe has a narrower bandgap, allowing it to absorb a wider range of the solar spectrum; most notably in the low infrared region, which some semiconductor photovoltaic cells cannot absorb. Furthermore, the revealed PbSe nanostructures and corresponding thin films are significantly cheaper to produce than conventional semiconductor materials used in photovoltaic cells.

[0050] Furthermore, PbSe is a direct bandgap semiconductor, capable of absorbing solar radiation in materials or layers as thin as a few micrometers, much thinner than conventional solar cells, even those with narrower light absorption ranges. Moreover, advantageously, the revealed PbSe nanostructures can be fabricated relatively easily over large areas at lower temperatures using low-cost fabrication techniques such as chemical bath deposition (e.g., compared to conventional semiconductor materials).

[0051] In this disclosure, polycrystalline PbSe thin films are produced as mixtures of nanoparticles (e.g., including nanostructures, implanted nanoprisms, and / or nanodisks). Such films can be used to increase energy absorption over a wider 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 solar cell products.

[0052] Furthermore, by employing one or more etching and / or alignment techniques on nanoparticles, low-cost single particles for specific detectors and / or solar cells can be produced. In some instances, carrier multiplication and / or quantum size effects are evident in PbSe nanocrystals. For example, carrier multiplication is a process that generates multiple excitons from a single incident photon. For instance, single incident photons generating up to seven excitons have been observed in PbSe quantum dots. In this disclosure, PbSe nanostructures are provided that can efficiently generate and separate these excitons, thereby contributing to improved conversion efficiency of solar cells and / or sensitivity of detectors. In some instances, the band gap of PbSe increases as the size of the nanostructure decreases. Therefore, the effect of quantum confinement allows tuning the band gap in PbSe quantum dots, facilitating targeted absorption in one or more specific regions of the electromagnetic spectrum.

[0053] In exemplary gas sensor applications, the revealed PbSe nanostructures can be used in sensors for measuring carbon dioxide (CO2) or ammonia (NH3) as part of fundamental environmental applications for monitoring air pollution. Due to the single-crystal structure and small size of PbSe nanostructures (e.g., nanoprisms), sensors employing PbSe nanoprisms can be directly integrated into small devices, such as smartphones or other devices, for real-time monitoring of air and / or environmental pollution both indoors and outdoors.

[0054] Currently, bulky polycrystalline PbSe thin films are used in some detection systems, such as nondispersive infrared spectroscopy. For example, commercially available thin-film detectors have a detection limit of about 50 ppm for CO2 concentrations in the range of 0 to 4000 ppm.

[0055] Advantageously, the disclosed sensors offer a higher degree of sensitivity, and thus provide detection capability, as well as cost-effectiveness. For example, air pollution monitoring systems employing PbSe nanostructures can be miniaturized for integration with a variety of devices, including smartphones, personal medical devices, tablets, and / or wearable consumer products, as a non-limiting list of examples. [Nanostructures were separated via ultrasound and centrifugation, via] [DEP] [Dimensional separation during manufacturing process] [ / ] [operate] []

[0056] Nanocrystals or nanostructures are typically separated from the growth and preparation medium by adding a polar solvent. Lead chalcogenide nanostructures mechanically exfoliated from a substrate are carefully transferred to a polar solvent container. Ultrasonic precipitation and centrifugation in the solvent are used to collect the nanostructures. However, other techniques can be used to separate monodisperse nanostructures, such as dieletrophoresis (DEP). This technique is based on the movement of dielectric or polarizable nanostructures in a non-uniform electric field due to the interaction between the dipoles of the nanostructure and the spatial gradient of the electric field. Because lead chalcogenides have different dielectric properties, DEP can be used to manipulate, transport, separate, and classify different types of lead chalcogenide nanocrystals / nanostructures. DEP wafers consist of electrodes, typically microarrays, separated by gaps forming microfluidic channels. The lead chalcogenide nanostructure to be separated is introduced, and an appropriate electric field is applied to separate the target nanostructure according to its size and shape. Sensor-based DEP devices can be used to monitor the separation process of captured nanostructures, thereby enabling simultaneous detection of concentration changes in the nanostructure-containing solution. Two staggered electrode devices are designed to perform dielectric capture of the nanostructures and measure changes in the dielectric conductivity (e.g., impedance).

[0057] In some disclosed examples, as illustrated in Figure 1, a method 100 is provided for processing single-crystal PbSe nanostructures (e.g., nano-prisms, etc.). This method 100 comprises one or more of the following: substrate preparation 102, chemical preparation and solvent mixing 104, chemical deposition (e.g., via chemical bath deposition) 106, vacuum baking 108, thin film oxidation (e.g., oxygen sensitization) 110, thin film iodization (e.g., iodine sensitization) 112, annealing 114, nanostructure isolation (e.g., chemical and / or electrochemical etching to remove oxides) 116, and / or post-processing 118. As provided herein, this process can be performed as listed and can be performed by one or more of the listed actions in an optional and / or different arrangement.

[0058] In some instances, the quartz substrate undergoes pre-cleaning, which may include additional or optional plasma cleaning and / or surface treatments to roughen one or more surfaces of the substrate during the substrate fabrication steps. In step 4, vacuum baking 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 an oxidation process is performed at a predetermined temperature above the initial vacuum baking temperature (e.g., about 420°C). In some embodiments, iodine sensitization may be performed in a quartz tube furnace using a mixed gas consisting of a carrier solution (e.g., nitrogen) and iodine vapor at a predetermined temperature (e.g., 450°C). Chemical etching is performed in a mixed solution of hydrogen fluoride (HF) and deionized (DI) water (e.g., NaOH:IPA:DI), wherein the HF solution concentration may be 50:1 (per volume ratio) and / or the deionized water solution concentration may be 1:25:50 (per volume ratio). As a result of the revealed method, PbSe nanostructures (including nanoprisms) are formed beneath the oxide PbSe phase and have a variety of different sizes.

[0059] By using inexpensive materials, the overall cost of fabricating PbSe nanostructures will be reduced. These and other revealed methods, and the resulting PbSe nanostructure products, offer advantages over other technologies because they allow for the production of large quantities of single-crystal nanoprisms by varying the iodine doping level in a low-cost, low-temperature fabrication technique using the CBD method. [PbSe] [Growth Mechanism] []

[0060] To produce PbSe nanostructures including nanoprisms, nanodisks, and / or nanorods, polycrystalline and / or amorphous PbSe layers or films can first be formed. The growth rate of the PbSe film depends largely on the rate of release of Pb²⁺ ions from their wedge state and the decomposition rates of lead acetate trihydrate (C₄H₆O₄Pb∙3H₂O) and selenourea (CH₄N₂Se). PbSe forms when the ion product of Pb²⁺ and Se²⁻ ions exceeds the solution solubility product of PbSe (e.g., approximately 10⁻³⁸ at 300 K). Therefore, the concentrations of lead and selenium ions are controlled during film growth.

[0061] In lead chalcogenide deposition, lead acetate trihydrate is typically used as a lead precursor, ligated to control the release of the metal cation Pb²⁺ and prevent the precipitation of Pb(OH)₂. In the case of PbSe, the hydrolysis of the chalcogenide precursor selenourea (CH₄N₂Se) provides the anion Se²⁻. If the ion product is greater than the solubility product (e.g., the Ksp of PbSe is approximately 10⁻³⁸ at 300 K), the lead chalcogenide will precipitate. Although a significant amount of precipitation is expected in CBD processes, a large amount of precipitate adhesion to the substrate is not anticipated. Therefore, polycrystalline growth can occur via one of two common deposition mechanisms: ion-by-ion growth or hydroxide cluster growth.

[0062] Ion-by-ion growth is the result of ionic reactions, typically occurring during homogeneous nucleation. Therefore, collisions between ions form nuclei, which are then adsorbed onto the substrate. Ion-by-ion growth usually produces relatively large crystals, with crystal size proportional to film thickness.

[0063] Hydroxide cluster growth occurs in the presence of metal hydroxides. Therefore, if Pb(OH)₂ exists in precipitate or colloidal form, a deposition mechanism will occur. Compared to ion-by-ion growth, hydroxide cluster growth typically produces smaller crystals. Unlike ion-by-ion growth, film thickness does not significantly affect the crystal size of hydroxide cluster growth.

[0064] This disclosure describes the use of selenourea to provide Se²⁻ ions for PbSe deposition, and that the polycrystalline PbSe thin film mechanism follows an ion-by-ion growth pattern. Therefore, if the ion product is greater than the solubility product, PbSe will be formed via the ionic reaction described in Equation 1: Equation 1

[0065] The chemical reaction described in Equation 1 causes PbSe to precipitate onto the substrate in solution. [Lead selenide deposition] []

[0066] Once formed, PbSe can be deposited using a variety of processing techniques, including pyrolysis, vacuum evaporation, sputtering, chemical vapor deposition (CVD), molecular-beam epitaxy (MBE), and chemical bath deposition (CBD). Based on the disclosed examples, CBD technology offers particular advantages in forming PbSe nanostructures, such as being a simple and low-cost technique that can be performed at relatively low temperatures.

[0067] The chemicals used to prepare PbSe films are analytical grade and can be used without special purification. In some instances, lead acetate trihydrate ((CH3COO)Pb∙3H2O) and selenourea (CH4N2Se) are used as Pb2+ and Se2- ion sources, respectively. Trisodium citrate (TSC) acts as a slowly releasing metal ion complexing agent, thereby promoting the formation of nanocrystalline PbSe films.

[0068] Before initiating chemical bath deposition, lead acetate solution, selenourea solution, and / or iodine solution 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 g) of lead acetate trihydrate crystals is added to a container (e.g., a 1000 mL plastic bottle), and then deionized water is slowly added until the weight of the bottle reaches the critical limit (e.g., about 800 g). Another container (e.g., a 2000 mL beaker) contains a predetermined amount of liquid (e.g., 500 mL of deionized water), which has been heated (e.g., placed at a high temperature on a hot plate), and the container containing lead acetate is placed in it.

[0069] In some instances, the reading at the top surface of the hot plate was approximately 64°C. Heating will completely dissolve the lead acetate crystals within approximately 30 minutes. After this step, the bottle containing the lead acetate solution is placed in a thermostat set to 30°C. To prepare the selenourea solution, 1355 g of deionized water is added to a 2000 mL volumetric flask, which is then placed on a hot plate. It takes approximately one hour for the water to boil at a high position on the hot plate. The bottom temperature of the flask reaches approximately 90°C. After the water in the flask has boiled for 10 minutes, the volumetric flask is placed in a container of tap water to cool to 70°C.

[0070] After cooling, add the amount of deionized water lost due to boiling to ensure recovery of approximately 1355 grams. Then, pour approximately 0.050 kg of selenourea into the flask and shake several times until the selenourea is completely dissolved. Once the selenourea solution is prepared, place it in a dark environment at room temperature for a predetermined period of time before using it for CBD.

[0071] To prepare the iodine solution, a pre-cleaned 250 mL graduated cylinder containing 13 g of potassium iodide (KI) crystals was mixed with approximately 50 mL of deionized (DI) water, and approximately 50 mL of isopropanol was added to the graduated cylinder. To completely dissolve the crystals, the graduated cylinder was placed in an ultrasonic vibrator at up to 100% power for approximately 20 minutes, and thoroughly mixed by inverting the graduated cylinder several times. If not completely dissolved, this step was repeated 2 or 3 times. After the KI crystals were completely dissolved, deionized water was added to the graduated cylinder to the 250 mL mark, and then the graduated cylinder was inverted 20-25 times to mix thoroughly. The solution was stored at room temperature for at least overnight.

[0072] In some instances, PbSe deposition consists of two layers: a first layer with little or no iodine added, and a second layer doped with an adjustable amount or concentration of iodine, controlled to produce the desired PbSe nanostructure (e.g., nano-prisms), with the doping amount considered as a donor impurity. Therefore, the carrier concentration can be varied by adding the desired amount of iodine during the CBD process. In examples, one or two layers undergo chemical bath deposition for a predetermined amount of time (e.g., 30 minutes per layer, for a total of one hour), which can be the same for the resulting layers but can be varied. By changing the deposition time, the thickness and morphology of the thin film PbSe will change, thereby optimizing the structure and / or properties of PbSe. As illustrated in the example of Figure 4, the carrier concentration varies (e.g., decreases) depending on one or more sensitization treatments (e.g., oxidation and / or iodination) applied to the PbSe nanostructure (e.g., p-type PbSe particles).

[0073] In this example, one or more substrates are mounted in a container of deionized water, which is used to heat the substrates by tungsten lamps immersed in the deionized water, with the lamp voltage provided by a lab-made procedure. The peak deposition temperature of the solution gradually reaches 90°C during the chemical deposition process. A first volume of lead acetate solution is measured in a 100 mL graduated beaker, and a second volume of selenurea solution is prepared separately using a separate 250 mL graduated beaker. Based on experimentally obtained single-crystal PbSe nanoprism growth data, the x and y values ​​will vary; typically, for PbSe detector applications, the x / y ratio will be 75 / 150. A Teflon-coated stir bar is mounted at the bottom of the solution pan, rotating at 100-300 rpm. The first and second volumes of each solution are simultaneously poured into the solution pan with the stir bar, and 100 mL of deionized water is poured into the substrate pan as a medium to transfer heat from the tungsten (W) lamp to the substrates, which are mounted on the bottom of the pan facing the solution mixture. This is a method of indirectly and uniformly transferring heat to the substrate. The bath temperature is measured by a thermocouple placed at the midpoint depth of the container. At the end of the deposition cycle, the bath temperature is concentrated at its peak temperature, and the tray containing the substrate is suddenly separated from the container containing the solution. The substrate tray is then immersed in deionized water to restore the temperature to room temperature.

[0074] For the second layer deposition, the first layer is repeated by adding a given amount of iodine solution to a selenourea and lead acetate solution in a container. This alters the donor concentration upon completion of the chemical deposition, incorporating it into the PbSe crystal matrix. In some additional or alternative examples, the amount of iodine added for the disclosed PbSe detector may be approximately 12-60 mL (e.g., approximately 24 mL). The manner in which iodine is added will affect the morphology of the polycrystalline PbSe film and the distribution of doping throughout the PbSe film. In this invention, after adding the iodine solution, a magnetic stirrer is immediately rotated for 1 minute before starting the actual CBD. After completing the second layer, the disk containing the substrate is placed in a container filled with deionized water until the temperature cools down, followed by the normal cleaning procedure.

[0075] The prepared PbSe thin film will be inherently polycrystalline and undergo a vacuum baking step to remove any solvents that may be trapped in the film or substrate, which has been etched to a rough surface morphology. The vacuum baking step is performed at a predetermined temperature (e.g., 105°C) for a given time period (e.g., overnight). This temperature will not alter the crystal structure of the PbSe thin film.

[0076] In some instances (represented by block 110), during the oxidation step, the initially doped iodine induces surface recrystallization on the film surface and within the PbSe matrix, which will form a series of single-crystal PbSe nanoprisms, nanorods, and nanodisks. During the oxidation step, gases (e.g., oxygen and / or nitrogen) are purged from top to bottom into a vertical quartz tube furnace for oxidation. For example, the gas ratio of the oxygen and nitrogen mixture is set to approximately 20% and 75%, respectively, which corresponds to an oxygen flow rate of approximately 1.05 L / min and a nitrogen flow rate of approximately 1.95 L / min, resulting in a total flow rate of approximately 3.0 L / min for the entire vertical furnace. The peak temperature within the tube furnace is related to the degree of PbSe crystallinity and can be adjusted according to the required size and shape of the nanoparticles. A typical peak furnace temperature for obtaining a rectangular shape of nanoprisms is believed to be approximately 420 °C (see, for example, Figures 5A and 5B). Because the deposited thin-film PbSe alloy (with a Pb 0.55, Se 0.45 composition ratio) is subjected to high temperatures (e.g., 100°C) above the melting point of Pb, other phases of nanoparticles (e.g., nanoprisms, nanoribbons, and / or nanodisks) can be grown by mixing solid and liquid phases. Subsequent cooling allows determination of the particle shape (e.g., cooling rate, time to reach the desired temperature, presence of gas, etc.).

[0077] In some instances (represented by block 112), iodine is subsequently used as a donor impurity for the doping process. In this block, the carrier concentration of the single-crystal PbSe nanoparticles (e.g., nanoprisms, nanobelts, nanodisks) can be varied according to the requirements of a specific application. In other words, it is directly evident that adding iodine in a simple tube furnace can adjust the impurity level in the band structure. The set temperature of the central region of the tube furnace can be set at a predetermined point (e.g., approximately 347°C), which generates an elevated exposure temperature (e.g., approximately 395°C) within the tube furnace.

[0078] In some instances, the iodine crystal sublimation process is performed using a laboratory-manufactured water-circulating Gramm tube containing iodine crystals to supply iodine vapor to the furnace. Iodine-rich vapor is introduced into the furnace using nitrogen as a carrier gas. When the solution does not carry iodine vapor, the application of the nitrogen solution can be set to a predetermined rate (e.g., 1.2 standard cubic feet per hour (SCFH)), while when the solution does carry vapor, a higher predetermined rate (e.g., 12.0 SCFH) can be set. By optimizing the iodine deposition time at relatively high temperatures, the doping level of the PbSe crystal can be adjusted. For example, at or near the peak temperature at the center of the furnace, the doping time can be set to approximately 70 seconds.

[0079] In some instances, the doping time can be varied to provide a desired sensitivity corresponding to a given doping concentration. As disclosed herein, the doping process is performed while the sample (e.g., a quartz substrate) is heat-treated (e.g., in a quartz furnace). Figure 2A shows an image of a PbSe thin film (and thus the nanostructure contained therein), where the carrier concentration level can be adjusted by two sensitization process conditions (e.g., oxidation and / or iodization), where the adjustable parameters correspond to one or more of the temperature, time, and / or the amount and / or concentration of induced gas during the heat treatment (e.g., in a furnace). In this disclosure, the carrier concentration in the resulting PbSe nanostructure can be modified exponentially by modifying the parameters of one or more of the two sensitization processes. Figure 2B illustrates many possible nanostructure angles or crystal growth directions for the PbSe material.

[0080] In some instances (represented by block 114), annealing is performed to lower the temperature of the nanoparticles after iodination. [PbSe] [Manufacturing of Nanoparticles] []

[0081] At this stage, (e.g., under SEM or other research tools) nanoparticles cannot be detected because the single-crystal nanoparticles are not exposed, as they are embedded in and / or located beneath the polycrystalline PbSe film.

[0082] In the disclosed example (represented by block 116), PbSe nanoparticles can be exposed via a surface etching process using an etchant, such as a diluted hydrogen fluoride (HF) solution and a diluted HF solution (volume ratio of DI water:HF = 50:1) or a solution of NaOH:IPA:Di water (volume ratio of 1:25:50). Figure 3A shows exemplary nanoparticles obtained via the disclosed method. Figure 3B illustrates the PbSe nanoprism within the PbSe nanostructure image in Figure 3A, employing the particle shape with angular profiles 101 and 110 provided in the matrix.

[0083] As confirmed by SEM studies, in the case of rectangular nanoprisms, the first dimension (e.g., length) is estimated to be in the range of approximately 400 nm to close to 1 μm, and the second dimension (e.g., thickness) is estimated to be in the range of several nanometers to 50 nanometers. Energy-dispersive X-ray spectroscopy (EDS) analysis reveals PbSe nanocrystal / nanostructures containing iodine as a dopant. For example, Figure 3B shows a large number of nanoparticles with dimensions of 1 μm x 1 μm. These particles can be collected using solution-based methods.

[0084] In some instances (represented by block 118), post-processing, such as long-term post-baking, performed at a predetermined temperature (e.g., about 150°C), will provide one or more of the following benefits: the doped iodine will be uniformly redistributed into the PbSe nanoparticles, the size of which may increase during the post-baking step.

[0085] The method disclosed is designed to synthesize substantially flat PbSe nanostructures (e.g., nanoprisms, nanoplates) using CBD technology and iodine doping processes, and ultimately control the electrical properties of the PbSe nanostructures, as shown in the example diagram in Figure 4.

[0086] The abundance of single-crystal PbSe nanoprisms throughout the PbSe matrix enhances the sensitivity and detection performance of thin films, thereby bringing technical benefits beyond the detection range in other applications, including, 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 nanoscale detectors in the mid-infrared region (e.g., 3-5 micrometers).

[0087] While this disclosure refers to applications of PbSe, such as thin-film photodetectors employing PbSe, the methods disclosed are applicable to other Pb-based chalcogenide and / or semiconductor materials, and / or other metal alloys (e.g., including other post-transition metals).

[0088] Figures 5A and 5B are graphs providing photoluminescence measurement levels for various PbSe nanostructures according to this disclosure. For example, photoluminescence (PL) can be measured via a process that tests the electronic structure of a particular material. PL is a phenomenon in which electromagnetic energy is absorbed and then emitted within a certain wavelength range, which may differ from the wavelength of the absorbed electromagnetic energy.

[0089] In some instances, monochromatic energy (e.g., laser) is directed to the material to excite the sample. Electrons excited in response to this energy move from the ground state to a higher excited energy state. The material then emits energy as a combination of phonons (vibrations) and photons (light) upon returning to the ground state. Sensors can measure the emitted light to perform spectral and / or spatial analysis to generate information about the material's properties.

[0090] In some instances, the excited state is in the conduction band (CB). It should be shown that the excitation photon excites the electron into the CB. Then, nonradiative relaxation brings the electron into the conduction band minimum (CBM).

[0091] The emitted energy is less than the absorbed energy because a certain amount of energy is consumed during the interaction with the material. The energy of the emitted light due to radiative recombination is related to the difference between the two energy levels involved in the transition between the excited and equilibrium states.

[0092] This process can be used to generate information in various situations, such as defect detection, measurement of impurity levels, recombination mechanisms, analysis of material quality, determination of band gap energy, molecular structure and crystallinity, as listed as a non-limiting example.

[0093] Figure 5A shows an example of PL intensity juxtaposed between the unprepared material and the nano-prism material prepared by the revealing method. The unprepared material reaches a low level (e.g., less than 10 au), while the revealed nano-prism material reaches a peak of nearly 140 au. Therefore, the PL intensity is approximately 140 times higher than that of the deposited unprepared material, indicating a significant enhancement in PbSe crystallinity. Figure 5B shows the filtered or smoothed lines of the PL plot in Figure 5A.

[0094] Although several examples and / or embodiments of PbSe nanostructures are described herein, the principles and / or advantages disclosed herein can be applied using techniques not limited to specific types of materials and / or applications.

[0095] While this disclosure has been described with reference to certain embodiments, those skilled in the art will understand that various changes can be made and equivalents can be substituted without departing from the scope of this disclosure. Furthermore, many modifications can be made to adapt particular situations or materials to the teachings of this disclosure without departing from the scope of this disclosure. Therefore, this disclosure is intended to be limited to the specific embodiments disclosed, but rather to include all embodiments falling within the scope of the invention claims.

[0096] 100: Method 102: Substrate Preparation 104: Chemical Mixing Preparation 106: Chemical Bath Deposition (CBD) 108: Vacuum Baking 110: Oxygen sensitization 112: Iodine sensitization 114: Annealing 116: Nanostructure isolation 118: Post-processing

[0097] Domestic storage information (please note in order of storage institution, date, and number) none Overseas storage information (please note in the order of storage country, institution, date, and number) none

Claims

1. A method for forming a single-crystal lead selenide (PbSe) nanostructure, comprising the following steps: preparing a substrate; preparing a plurality of chemical lead and selenium precursors; depositing the precursors onto the substrate via a chemical bath deposition (CBD) process to form 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 oxygen-containing gas to induce recrystallization and generate an oxide passivation layer; doping the thin film with a vapor containing a predetermined concentration of iodine for a predetermined time period; applying a plurality of nanostructure isolation techniques using a chemical etchant or an electrochemical etching process to the thin film to expose a single-crystal PbSe nanostructure located beneath the oxide passivation layer and the polycrystalline PbSe; and post-processing the thin film having uncovered single-crystal PbSe to redistribute iodine into the PbSe nanostructure.

2. The method as claimed in claim 1, wherein the deposition step produces two PbSe crystalline layers, comprising: a first exposed layer comprising a polycrystalline PbSe oxide form that is substantially free of iodine; and a second underlying layer comprising the iodine-containing single-crystal PbSe nanostructures, wherein the amount of iodine in the single-crystal PbSe is controlled by one or more of a time or a temperature during the doping process.

3. The method as described in claim 2, wherein the deposition step comprises the following steps: depositing the precursors during the CBD process for a first time to form the first layer; and depositing the precursors and a carrier solution having a predetermined amount of iodine during the CBD process for a second time to form the second layer.

4. The method as described in claim 3 further comprises the step of: changing a time for the deposition during the first or second time period, the change in time corresponding to a change in the thickness or morphology of the single-crystal PbSe film.

5. The method as described in claim 1 further includes the step of annealing prior to the step of applying multiple nanostructure isolation techniques.

6. The method as described in claim 3, wherein the first time or the second time is approximately 30 minutes.

7. The method as described in claim 3, wherein the first time or the second time may vary according to the required specifications.

8. The method as described in claim 1, wherein the oxygen-containing gas comprises a mixture of oxygen and nitrogen.

9. The method as described in claim 1 further comprises the step of: adjusting a threshold temperature to control the crystallization of the PbSe nanostructure, the threshold temperature corresponding to a size or shape of the PbSe nanostructure.

10. The method as described in claim 1, wherein the PbSe nanostructure comprises one or more of a PbSe nanoprism, a PbSe nanoplate, a PbSe nanoband, or a PbSe nanodisk.

11. The method as described in claim 1, wherein the PbSe nanostructure is formed beneath the oxide layer at various sizes that can be controlled according to multiple manufacturing parameters.

12. The method as described in claim 1, wherein the film is exposed to the oxygen-containing gas at a predetermined temperature of about 420°C, resulting in a generally rectangular shape of the nanostructures.

13. The method as claimed in claim 1, wherein the substrate comprises a quartz substrate having one or more rough surfaces.

14. The method as claimed in claim 1, wherein the iodine doping step further comprises the step of introducing an iodine vapor into the furnace using nitrogen as a carrier gas.

15. The method as described in claim 1 further comprises the step of: performing post-processing by adjusting one of a temperature or a time to control a size and redistribution of iodine concentration in the PbSe nanostructures.

16. The method as described in claim 1 further comprises the step of controlling a rate, time, or temperature of a cooling process to control the shape of the PbSe nanostructures.

17. The method as described in claim 1, further comprising one or more additional annealing processes, including uniformly or homogeneously distributing the dopants in the PbSe matrix at a predetermined time or temperature.

18. The method as described in claim 1, wherein the etching step of the polycrystalline PbSe material is performed using a hydrogen fluoride (HF) solution or a NaOH:IPA:DI solution.

19. The method as described in claim 1, wherein the polycrystalline growth can occur by one of two general deposition mechanisms: ion-by-ion growth or hydroxide cluster growth.

Images