Abstract:
This disclosure provides systems, methods, and apparatus related to thermoelectric materials. In one aspect, a method includes providing a plurality of nanostructures. The plurality of nanostructures comprise a thermoelectric material, with each nanostructure of the plurality of nanostructures having first ligands disposed on a surface of the nanostructure. The plurality of nanostructures is mixed with a solution containing second ligands and a ligand exchange process occurs in which the first ligands disposed on the plurality of nanostructures are replaced with the second ligands. The plurality of nanostructures is deposited on a substrate to form a layer. The layer is thermally annealed.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/215,352, filed Sep. 8, 2015, to U.S. Provisional Patent Application Ser. No. 62/247,310, filed Oct. 28, 2015, to U.S. Provisional Patent Application Ser. No. 62/256,786, filed Nov. 18, 2015, and to U.S. Provisional Patent Application Ser. No. 62/333,409, filed May 9, 2016, all of which are herein incorporated by reference. 
     
    
     STATEMENT OF GOVERNMENT SUPPORT 
       [0002]    This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy and under Grant No. FA9550-12-1-0002 awarded by the Air Force Office of Scientific Research. The government has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0003]    This disclosure relates generally to thermoelectric materials. 
       BACKGROUND 
       [0004]    Performance of thermoelectric materials is quantified by a dimensionless thermoelectric figure of merit, ZT=S 2 ×σ×T/λ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, and λ is the thermal conductivity. The term S 2 ×σ is known as the power factor. Increasing the numerator or decreasing the denominator of ZT will improve the performance of a thermoelectric material. Historically, the focus has been on reducing the thermal conductivity because in bulk materials, the Seebeck coefficient and the electrical conductivity are inversely coupled. Nanostructuring of bulk thermoelectric materials has enabled the lowering of the thermal conductivity. This strategy has been performed using energy intensive techniques (e.g., spark plasma sintering (SPS)). 
       SUMMARY 
       [0005]    An alternative strategy to producing nanostructured thermoelectric materials with properties comparable to those of bulk materials of the same composition is described herein. The method involves mild thermal annealing of thin films or layers of solution-processed inorganic nanomaterials. A simultaneous increase in both the Seebeck coefficient and the thermal conductivity through an annealing process, which retains the nanoscale material features that are beneficial for thermal conductivity reductions by avoiding macroscale sintering and grain growth, has been demonstrated. The annealing process also leads to molecular electronic coupling, resulting in high power factors. Further, this method allows for facile film transfer from the original substrate to another substrate. The thermoelectric material design strategy is compatible with any solution-based nanoscale active material, including inorganic, organic, and hybrid organic/inorganic/molecular components. 
         [0006]    In some embodiments, copper selenide (Cu 2 Se) nanoparticle films were treated by thermal annealing. The thermal annealing increased the electric conductivity and Seebeck coefficient of the Cu 2 Se nanoparticle films. 
         [0007]    One innovative aspect of the subject matter described in this disclosure can be implemented in a method including providing a plurality of nanostructures. The plurality of nanostructures comprise a thermoelectric material, with each nanostructure of the plurality of nanostructures having first ligands disposed on a surface of the nanostructure. The plurality of nanostructures is mixed with a solution containing second ligands and a ligand exchange process occurs in which the first ligands disposed on the plurality of nanostructures are replaced with the second ligands. The plurality of nanostructures is deposited on a substrate to form a layer. The layer is thermally annealed. 
         [0008]    Another innovative aspect of the subject matter described in this disclosure can be implemented in a method including providing a plurality of nanostructures. The plurality of nanostructures comprises a thermoelectric material, with each nanostructure of the plurality of nanostructures having first ligands disposed on a surface of the nanostructure. The plurality of nanostructures is deposited on a substrate to form a layer. The layer is contacted with a solution containing second ligands and a ligand exchange process occurs in which the first ligands disposed on the plurality of nanostructures are replaced with the second ligands. The layer is thermally annealed. 
         [0009]    Another innovative aspect of the subject matter described in this disclosure can be implemented in a material including a plurality of nanostructures. The plurality of nanostructures comprises a thermoelectric material. Nanostructures of the plurality of nanostructures have dimensions of less than about 100 nanometers. The material has a thermoelectric figure of merit (ZT) of at least about 0.3. 
         [0010]    Details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A and 1B  show examples of flow diagrams illustrating manufacturing processes for a nanostructured layer of a thermoelectric material. 
           [0012]      FIGS. 2A and 2B  show examples of schematic illustrations of a nanostructured layer of a thermoelectric material at various stages in the manufacturing process. 
           [0013]      FIGS. 3A-3D  show examples of scanning electron microscopy (SEM) images of a layer of Cu 2-x Se nanoparticles. 
           [0014]      FIGS. 4A-4J  show examples of the results of the structural characterization of layers of Cu 2-x Se nanoparticles. 
           [0015]      FIGS. 5A-5D  show examples of plots showing the transport properties of layers of Cu 2-x Se nanoparticles as a function of annealing temperature. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Reference will now be made in detail to some specific examples of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0017]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Particular example embodiments of the present invention may be implemented without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
         [0018]    Various techniques and mechanisms of the present invention will sometimes be described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless noted otherwise. 
         [0019]    The terms “about” or “approximate” and the like are synonymous and are used to indicate that the value modified by the term has an understood range associated with it, where the range can be ±10%, ±5%, or ±1%. The term “substantially” is used to indicate that a value is close to a targeted value, where close can mean, for example, the value is within 90% of the targeted value, within 95% of the targeted value, or within 99% of the targeted value. 
         [0020]      FIGS. 1A and 1B  show examples of flow diagrams illustrating manufacturing processes for a nanostructured layer of a thermoelectric material.  FIGS. 2A and 2B  show examples of schematic illustrations of a nanostructured layer of a thermoelectric material at various stages in the manufacturing process. 
         [0021]    Starting at block  105  of the method  100  shown in  FIG. 1A , a plurality of nanostructures is provided. In some embodiments, the largest dimension of a nanostructure is about 5 nanometers (nm) to 50 nm. For example, the largest dimension of a spherical nanostructure is the diameter of the nanostructure. In some embodiments, the nanostructures are monodisperse; i.e., in some embodiments, the nanostructures have a narrow size range. In some embodiments, the nanostructures comprise nanostructures selected from a group consisting of nanorods, nanowires, nanoparticles, nanosheets, nanoplates, and quantum dots. In some embodiments, the nanostructures comprise nanoparticles. In some embodiments, the nanostructures comprise copper selenide (Cu 2 Se), copper telluride, copper sulfide, bismuth selenide, bismuth telluride, bismuth sulfide, lead selenide, lead telluride, lead sulfide, bismuth, tellurium, silicon, silver telluride, silver selenide, antimony selenide, antimony telluride, or alloys thereof. In some embodiments, the nanostructures comprise a nonstoichiometric composition. For example, the nanostructures may comprise Cu 2-x Se, with x generally being about 0.01 and 0.20. In some embodiments, after the nanostructures are fabricated, each nanostructure of the plurality of nanostructures has ligands disposed on a surface of the nanostructure. For example, the ligands generally may comprise amines, phosphines, phosphonic acids, phosphine oxides, or carboxylic acids. For example, the ligands may comprise oleylamine, oleic acid, octylamine, hexylamine, trioctylphosphine, tributylphosphine, trioctylphosphine oxide, hexadecylamine, octadecylamine, dodecylphosphonic acid, or tetradecylphosphonic acid. 
         [0022]    In some embodiments, the nanostructures are fabricated by mixing a first solution a metal salt in an organic solvent with a second solution of a chalcogen (e.g., sulfur, selenium, tellurium) in an organic solvent. In some embodiments, the metal salt comprises a copper salt (e.g., copper chloride, copper nitrate, copper sulfate, copper acetate, copper acetylacetonate, copper methoxide, copper ethylhexanoate, copper tetrafluoroacetate, copper hexafluoroacetylacetonate). In some embodiments, the organic solvent of the first solution is the same as the organic solvent of the second solution. In some embodiments, the organic solvent of the first solution and the organic solvent of the second solution are miscible. 
         [0023]    At block  110 , the plurality of nanostructures is mixed with a solution containing second ligands. A ligand exchange process occurs in which the first ligands disposed on the plurality of nanostructures are replaced with second ligands. In some embodiments, the first ligands have a longer chain length than the second ligands. For example, in some embodiments, the first ligands are long, insulating ligands that are useful for solubilizing the nanostructures, and the second ligands are shorter ligands. The second ligands may allow the nanostructures to pack closer together when the nanostructures are deposited a substrate. In some embodiments, the second ligands comprise ethanedithiol, ethylenediamine, succinic acid, propionic acid benzenedithiolate, benzenediamine, hydrazine, ammlonium thiocyanate, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, sodium hydrosulfide, potassium hydrosulfide, ammonium sulfide, sodium telluride, potassium telluride, sodium hydrotelluride, potassium hydrotelluride, sodium selenide, sodium hydroselenide, potassium selenide, potassium hydroselenide, nitrosonium tetrafluoroborate, mercaptopropionic acid, formic acid, chalcogenidometallates (ChaMs), or polyoxometalates (POMs). 
         [0024]    At block  115 , the plurality of nanostructures is deposited on a substrate to form a layer of nanostructures. In some embodiments, the nanostructures are deposited on the substrate using a solution processing technique. For example, solution processing techniques include spin coating, dip coating, spray coating, doctor blade, and inkjet printing. In some embodiments, the substrate comprises a silicon substrate or a glass substrate. In some embodiments, the as-deposited layer of nanostructures has a thickness of about 50 nm to 200 nm. In some embodiments, the layer of nanostructures is disposed on about 1 centimeter (cm) by 1 cm of the substrate surface. 
         [0025]    In some embodiments, the ligand exchange process occurs at block  115 . For example, the plurality of nanostructures can be mixed with the solution containing the second ligands during the deposition process. 
         [0026]    In some embodiments, block  115  is repeated to generate a layer of nanostructures having a specified thickness. For example, block  115  is performed to form a first layer of nanostructures. Block  115  could then be repeated to form a second layer of nanostructures disposed on the first layer of nanostructures. 
         [0027]    In some embodiments, Cu 2 Se nanoparticles mixed in hexane and octane are deposited on a substrate using spin coating. The spacing, electronic coupling, and thermal coupling between nanoparticles in the layer are mediated by the ligands attached to the Cu 2 Se nanoparticles. Typical thermoelectric properties of a non-annealed layer were electrical conductivity of 0.03 S/cm, thermal conductivity of 0.4 W/m×K, and Seebeck coefficient of 10 −4  V/K. 
         [0028]      FIG. 2A  shows an example of schematic illustrations of a nanostructured layer of a thermoelectric material after block  115 . As shown in  FIG. 2A , a plurality of nanostructures  210  is disposed on a substrate  205 . The second ligands  215  attached to surfaces of each nanostructure of the plurality of nanostructures control the spacing between the nanostructures. 
         [0029]    At block  120 , the layer of nanostructures is thermally annealed. In some embodiments, the thermal annealing is performed at a temperature of about 50° C. to 600° C. In some embodiments, the thermal annealing is performed at a temperature at or below about 500° C. In some embodiments, the thermal annealing is performed at a temperature of about 300° C. or about 400° C. In some embodiments, the thermal annealing is performed for about 15 minutes to 1 hour, or about 45 minutes. In some embodiments, the thermal annealing is performed in air. In some embodiments, the thermal annealing is performed in a specified atmosphere, such as an inert atmosphere. For example, in some embodiments, the thermal annealing is performed in an atmosphere comprising a gas selected from a group consisting of nitrogen, argon, hydrogen, oxygen, and mixtures thereof. 
         [0030]    In some embodiments, the thermal annealing removes at least some of the second ligands from the surface of each nanostructure of the plurality of nanostructures. In some embodiments, the thermal annealing removes all of the second ligands from the surface of each nanostructure of the plurality of nanostructures. In some embodiments, some of the surfaces of the nanostructures are in contact with each other after the thermal annealing. For example, a thermoelectric material surface of a first nanostructure may be in contact with a thermoelectric material surface of a second nanostructure. In some embodiments, the thermal annealing modifies the second ligands and the modified second ligands form a layer of a material on a surface of each nanostructure of the plurality of nanostructures. For example, during the thermal annealing, the second ligands may become mobile and form a layer of ligand molecules on a surface of each nanostructure of the plurality of nanostructures. In some embodiments, after the annealing, the layer of nanostructures has a thickness of about 25 nm to 100 nm, or about 75 nm. In some embodiments, after the annealing, the nanostructures have dimensions of about 100 nm or less. In some embodiments, after the annealing, the layer of nanostructures has a thermoelectric figure of merit (ZT) of at least about 0.3 or at least about 1. 
         [0031]    In some embodiments, a Cu 2 Se nanoparticle layer is thermally annealed by placing the substrate with a Cu 2 Se nanoparticle layer disposed thereon in an oven with temperature set below about 500° C. for about 45 minutes in a nitrogen atmosphere. The thermal annealing results in modification and/or decomposition of the original ligand. Further, the Cu 2 Se nanoparticles may change shape and/or sinter. 
         [0032]      FIG. 2B  shows an examples of schematic illustrations of a nanostructured layer of a thermoelectric material after block  120 . As shown in  FIG. 2B , the plurality of nanostructures  210  is disposed on a substrate  205 . The second ligands  215  that were attached to the surfaces of the nanostructures are modified to form a layer of a material  220  on a surface of each nanostructure  210  of the plurality of nanostructures. 
         [0033]    Typical thermoelectric properties of a Cu 2 Se nanoparticle layer annealed at 200° C. were electrical conductivity of 150 S/cm, thermal conductivity of 0.9 W/m×K, and Seebeck coefficient of 10 −4  V/K. The highest power factor of a Cu 2 Se nanoparticle layer was about 730 μ W/mK 2  at room temperature, which is comparable to the power factor of dense, spark plasma sintering (SPS) copper selenide at room temperature. 
         [0034]      FIG. 1B  shows an example of a flow diagram illustrating a manufacturing process for a nanostructured layer of a thermoelectric material. In some embodiments, the operations of the method  150  shown in  FIG. 1B  are similar to the operations of the method  100  shown in  FIG. 1A . 
         [0035]    Starting at block  155  of the method  150 , a plurality of nanostructures is provided. In some embodiments, the same operation performed at block  105  shown in  FIG. 1A  is performed at block  155 . For example, the nanostructures may comprise copper selenide, copper telluride, bismuth selenide. bismuth telluride, lead selenide, lead telluride, bismuth, or tellurium. 
         [0036]    At block  160 , the plurality of nanostructures is deposited on a substrate to form a layer of nanostructures. In some embodiments, the same operation performed at block  115  shown in  FIG. 1A  is performed at block  160 . For example, the plurality of nanostructures may be deposited with a solution processing technique. 
         [0037]    At block  165 , the layer of nanostructures is contacted with a solution containing the second ligands. In some embodiments, the layer of nanostructures is contacted with the solution containing the second ligands for about 15 seconds to 2 minutes, or about 30 seconds. When the solution is in contact with the layer of nanostructures, a ligand exchange occurs in which first ligands disposed on a surface of the each of the nanostructures are replaced with second ligands. In some embodiments, the solution containing the second ligands is removed from contact with the layer of nanostructures using a physical technique (e.g., spinning the substrate to remove the solution with a centrifugal force) or a solvent. 
         [0038]    In some embodiments, blocks  160  and  165  are repeated to generate a layer of nanostructures having a specified thickness. For example, blocks  160  and  165  are performed to form a first layer of nanostructures. Blocks  160  and  165  could then be repeated to form a second layer of nanostructures disposed on the first layer of nanostructures. 
         [0039]    At block  170 , the layer of nanostructures is thermally annealed. In some embodiments, the same operation performed at block  120  shown in  FIG. 1A  is performed at block  170 . For example, the thermal annealing may be performed at a temperature of about 50° C. to 600° C. 
         [0040]    The methods  100  ( FIG. 1A ) and  150  ( FIG. 1B ) have advantages over energy intensive SPS techniques. For example, with SPS techniques, samples can be produced by heating copper and selenium shot in a tube furnace to about 1150° C. for about 12 hours followed by an about 7 day hold at 800° C. This thermal processing is followed by powdering (i.e., reducing the material to a powder). The powder is then subjected to SPS at about 437° C. under about 65 MPa of pressure. 
         [0041]    The nanostructured layers of a thermoelectric material described herein are applicable to both power generation applications and active cooling applications. 
       EXAMPLES 
       [0042]    The following examples are intended to be examples of the embodiments disclosed herein, and are not intended to be limiting. 
         [0043]    Copper selenide (Cu 2-x Se) nanocrystals were synthesized following a modified version of the method described in Kriegel, I. et al., Tuning the Exitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals,  J. Am. Chem. S.  134, 1583-1590 (2012). Briefly, a solution of copper (I) chloride in oleylamine (OAm) and 1-octadecene (ODE) was injected into a solution of selenium dissolved in the same solvents at a temperature of 310° C. The reaction was allowed to proceed for 20 minutes at 300° C. 11.8±1.1 nm diameter nanocrystals were formed. The nanocrystals were washed by precipitating with ethanol and resuspending in hexane twice. The nanocrystals were kept in hexane for future use. 
         [0044]    Ligand-exchanged thin films of Cu 2-x Se were prepared by spin coating sub-monolayers of OAm-coordinated nanocrystals from a solution of hexane and octane. After each deposition of OAm-coordinated NCs, 60 μL of an ethanedithiol (EDT) solution (1% by volume in acetonitrile, ACN) was pipetted onto the film. The EDT solution was left in contact with the film for approximately 30 seconds while the original OAm ligands were exchanged for the smaller EDT ligands. After the ligand exchange step, the film was spun to remove the excess solution and was rinsed once with neat ACN. As a result of the ligand exchange, the nanocrystals in the film were no longer soluble in the original non-polar solvents, allowing for multiple depositions of OAm-coordinated NCs to reach a specified thickness. 
         [0045]    After deposition, the films were annealed at different temperatures. 100 nm thick gold contacts were then deposited at the corners of the films via thermal evaporation. Following contact deposition, a series of measurements were made on the films to characterize their electrical and thermal properties, the results of which are set forth in  FIGS. 3A-3D, 4A-4J, and 5A-5D . DC electrical conductivity was determined via the van der Pauw method. Thermopower was determined by applying a temperature gradient across the film and measuring the steady state open circuit voltage. Cross-plane thermal conductivity was measured by time domain thermal reflectance (TDTR). 
         [0046]      FIGS. 3A-3D  show examples of scanning electron microscopy (SEM) images of a layer of Cu 2-x Se nanoparticles.  FIG. 3A  shows an example of a top-view SEM image of an as-cast thin film composed of Cu 2-x Se nanoparticles. The film was deposited in three spin casting steps. After each step the original oleylamine ligands were replaced with ethanedithiol ligands.  FIG. 3B  shows an example of a cross-sectional SEM image of the film shown in  FIG. 3A . The film is approximately 75 nm thick.  FIG. 3C  shows an example of a top-view SEM image of a Cu 2-x Se thin film after annealing at 400° C. for 45 minutes. Prior to annealing, this film was prepared the same way as the film shown in  FIG. 3A .  FIG. 3D  shows an example of a cross-sectional SEM image of the film shown in  FIG. 3C . The film is approximately 54 nm thick. The scale bars in all images shown in  FIGS. 3A-3D  are 100 nm. 
         [0047]      FIGS. 4A-4J  show examples of the results of the structural characterization of layers of Cu 2-x Se nanoparticles.  FIG. 4A  shows examples of x-ray diffraction (XRD) spectra for drop-cast and annealed films of Cu 2-x Se. The low-temperature structure is maintained after annealing.  FIGS. 4B-4H  shows examples of top-view SEM images of thin films that were annealed at the same temperature as the drop-cast samples used for generating the XRD spectra shown in  FIG. 4A .  FIG. 41  shows an example the results of a crystal size analysis. Crystal size of the nanocrystals in the films was determined by fitting the highlighted peak at q˜30 nm −1  in  FIG. 4A  with a Voigt profile and using the Scherrer formula.  FIG. 4J  shows an example of a high resolution transmission electron microscopy (TEM) image of a Cu 2-x Se nanocrystal. 
         [0048]      FIGS. 5A-5D  show examples of plots showing the transport properties of layers of Cu 2-x Se nanoparticles as a function of annealing temperature.  FIG. 5A  shows the Seebeck coefficient of thin films of Cu 2-x Se.  FIG. 5B  shows the electrical conductivity of thin films of Cu 2-x Se.  FIG. 5C  shows the thermal conductivity, as determined by time domain thermal reflectance, of thin films of Cu 2-x Se.  FIG. 5D  shows the thermoelectric figure of merit, ZT, at room temperature, of thin films of Cu 2-x Se. 
       Conclusion 
       [0049]    Further details regarding the methods and materials described herein can be found in J. Lynch et al., “Ligand Coupling Symmetry Correlates with Thermopower Enhancement in Small-Molecule/Nanocrystal Hybrid Materials,” ACS Nano, 2014, 8 (10), pp 10528-10536, which is herein incorporated by reference. 
         [0050]    In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention.