Patent Description:
The present application claims priority pursuant to Patent Cooperation Treaty Article <NUM> and <NUM> U. § <NUM>(e) to <CIT>.

The present invention relates to composite nanoparticles and associated assemblies and, in particular, to composite nanoparticles and assemblies exhibiting enhanced thermoelectric properties.

Solid-state energy conversion utilizing thermoelectric (TE) materials has attracted increasing interest due to their unparalleled properties to convert waste heat to electric energy. The efficiency of TE materials is expressed by a dimensionless figure of merit ZT, which is governed by electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ) that mainly includes the lattice thermal conductivity κL and carrier thermal conductivity κc(κL>>κc). An ideal efficient TE material necessarily to possesses high σ and a low κ. Unfortunately, for most TE systems, these three parameters are interdependent, thus maximizing one normally counteracts or reduces the other two. This has ultimately prevented the widespread application of TE materials as noise-free power generators or scalable solid-state Peltier coolers. <CIT> discloses a semiconductor nanocrystal-metal complex and method of preparing the same. Furthermore, the publication<NPL> discloses a chalcogenide nanocomposite with nanoscale metal particles.

Composite nanoparticle compositions and associated nanoparticle assemblies are described herein which, in some embodiments, exhibit enhancements to one or more thermoelectric properties including increases in electrical conductivity and/or Seebeck coefficient and/or decreases in thermal conductivity. In one aspect, a composite nanoparticle composition comprises a semiconductor nanoparticle including a front face and a back face and sidewalls extending between the front and back faces. Metallic nanoparticles nucleated and grown on at least one of the sidewalls, are bonded to at least one of the sidewalls establishing a metal-semiconductor junction, wherein the composite nanoparticle has a platelet, a pyramidal, or a bi-pyramidal morphology. In some embodiments, the metallic nanoparticles are bonded to a plurality of the semiconductor nanoparticle sidewalls establishing multiple metal-semiconductor junctions.

In another aspect, composite nanoparticle assemblies are described herein. Briefly, a composite nanoparticle assembly comprises semiconductor nanoparticles comprising front and back faces and sidewalls extending between the front and back faces, wherein spacing between the semiconductor nanoparticles is bridged by metallic nanoparticles bonded to the sidewalls of the semiconductor nanoparticles. As described further herein, the bridging metallic nanoparticles establish metal-semiconductor junctions with sidewalls of the semiconductor nanoparticles.

In a further aspect, methods of enhancing metal chalcogenide thermoelectric performance are provided. In some embodiments, a method of enhancing metal chalcogenide thermoelectric performance comprises providing metal chalcogenide nanoparticles comprising front and back faces and sidewalls extending between the front and back faces, said metal chalcogenide nanoparticles having a platelet, a pyramidal, or a bi-pyramidal morphology.

At least one of electrical conductivity and Seebeck coefficient of the chalcogenide nanoparticles is increased via nucleation and growth of metallic nanoparticles on the sidewalls, wherein the metallic nanoparticles bridge spacing between the chalcogenide nanoparticles. Moreover, thermal conductivity of the chalcogenide nanoparticles can be decreased by the metal nanoparticles bridging spacing between adjacent nanoparticles.

These and other embodiments are described in greater detail in the following detailed description.

Embodiments described herein can be understood more readily by reference to the following detailed description and examples and their previous and following descriptions.

In one aspect, a composite nanoparticle composition comprises a semiconductor nanoparticle including a front face and a back face and sidewalls extending between the front and back faces. Metallic nanoparticles are bonded to at least one of the sidewalls establishing a metal-semiconductor junction. In some embodiments, the metallic nanoparticles are bonded to a plurality of the semiconductor nanoparticle sidewalls establishing multiple metal-semiconductor junctions. The semiconductor nanoparticle can comprise any semiconductor not inconsistent with the thermoelectric principles and electronic structures described herein. Suitable semiconductor nanoparticles can include various chalcogenides, such as metal sulfides, metal selenides and/or metal tellurides. Moreover, semiconductor nanoparticles can be p-type or n-type. For example, semiconductor nanoparticles can comprise molybdenum sulfide (MoS<NUM>), antimony telluride (Sb<NUM>Te<NUM>) or bismuth telluride (Bi<NUM>Te<NUM>). Additionally, semiconductor nanoparticles of the composite composition provide sidewalls for bonding and nucleation of the metallic nanoparticles. In some embodiments, semiconductor nanoparticles have two-dimensional (2D) morphology. A semiconductor nanoparticle, for example, can be a platelet wherein the metal nanoparticles are bonded to one or more sidewalls of the platelet. Semiconductor nanoparticles, in some embodiments, exhibit a pyramidal or bi-pyramidal structure. Non-limiting examples of pyramidal or bi-pyramidal structures are illustrated in <FIG>.

Metallic nanoparticles bonded to one or more sidewalls of a semiconductor nanoparticle can comprise any metal not inconsistent with the thermoelectric principles and electronic structures described herein. Suitable metals include various transition metals, such as metals selected from Groups IVA-VIIIA and Group IB of the Periodic Table. In some embodiments, the metallic nanoparticles are formed of noble metal(s). The metal nanoparticles can nucleate and self-assemble on sidewall surfaces of the semiconductor nanoparticles. In being bonded to the semiconductor nanoparticle sidewall, an interfacial transition region can be established between the metal nanoparticle and semiconductor. In some embodiments, the interfacial transition region comprises metal atoms chemically bonded to atoms of the semiconductor nanoparticle. In one example, silver nanoparticles are bonded to sidewalls of a Sb<NUM>Te<NUM> nanoparticle, wherein an interfacial transition region comprises Sb<NUM>Te<NUM>-Ag<NUM>Te-Ag. Metal nanoparticles bonded to semiconductor sidewalls can have any size not inconsistent with the objectives of the present invention. In some embodiments, metal nanoparticle size is governed by spacing between the semiconductor nanoparticles in a composite assembly. As described further herein, the metal nanoparticles can bridge spacing between adjacent semiconductor nanoparticles, binding to sidewalls of the semiconductor nanoparticles. In such embodiments, composite nanoparticle assemblies can be formed.

<FIG> illustrates a top plan view of a Bi<NUM>Te<NUM> nanoparticle having platelet morphology according to some embodiments described herein. The Bi<NUM>Te<NUM> nanoparticle <NUM> comprises a front face <NUM> and an opposing back face (not shown). Sidewalls <NUM> extend between the front face <NUM> and back face. <FIG> illustrates metallic nanoparticle <NUM> nucleation and growth on sidewalls <NUM> of the Bi<NUM>Te<NUM> nanoparticle <NUM>.

Bonding and growth of metallic nanoparticles on sidewalls of the semiconductor establishes a metal-semiconductor junction. In some embodiments, a Schottky barrier is formed at the metal semiconductor junction. <FIG> illustrates the band diagram of a Sb<NUM>Te<NUM> nanoparticle prior to nucleation and growth of silver nanoparticles along one or more sidewalls. The work function of silver is also illustrated in <FIG>. Before contact, the initial Fermi level of Ag is located above the intrinsic Sb<NUM>Te<NUM>. After nucleation and growth, the presence of Ag nanoparticles pins the effective Fermi level of the present nanocomposites around the work function of silver due to the large carrier density in the metallic layer. As can be seen in <FIG>, the blend band gap between the host Sb<NUM>Te<NUM> semiconductor and metallic Ag nanoparticles forms a Schottky barrier, which is believed to be much better than Ohmic contact. Moreover, the potential barrier height (~<NUM> meV) is around the theoretical optimized height of <NUM> meV. Therefore, interfaces in Ag-Sb<NUM>Te<NUM> nanoplates induce energy-dependent carrier scattering by introducing a Schottky barrier to filter carriers with low energy, i.e. the carrier filtering technique prevents the transport of the lower-energy carriers, which results in an increase in the moment of the differential conductivity about the Fermi level.

The metallic nanoparticles bridge spacing between adjacent semiconductor nanoparticles to provide composite nanoparticle assemblies. A metal nanoparticle, for example extends from a sidewall of a first semiconductor nanoparticle to bond to a sidewall of a second adjacent semiconductor nanoparticle. When occurring over multiple sidewalls, nanocomposite assemblies are formed as illustrated in <FIG>. Spacing between the platelet Sb<NUM>Te<NUM> nanoparticles <NUM> is filled with metallic nanoparticles <NUM> bound to sidewalls of the Sb<NUM>Te<NUM> nanoparticles. Schottky barriers can be established at metal-semiconductor interfaces along the sidewalls permitting filtering of low energy carriers as described herein. Filtering of the low energy carriers can enhance electrical conductivity of the composite nanoparticle assembly. In some embodiments, a nanocomposite assembly has an electrical conductivity of at least <NUM> × <NUM><NUM> S/m or at least <NUM> × <NUM><NUM> S/m. Additionally, the metal nanoparticles can enhance phonon scattering, thereby lowering thermal conductivity of the nanocomposite assembly. Seebeck coefficient of the semiconductor nanoparticles can also be improved by the presence of the metal nanoparticles. In some embodiments, a nanocomposite assembly has a room temperature Seebeck coefficient of at least <NUM>µV/K. The foregoing enhancements also increase the power factor of composite nanoparticle assemblies described herein. In some embodiments, a nanoparticle assembly has a power factor greater than <NUM>µW/mK<NUM> or a power factor greater than <NUM>µW/mK<NUM>.

Composite nanoparticle assemblies can be formed into thin flexible films for various thermoelectric applications. Composite nanoparticle assemblies, in some embodiments, are stacked to provide thin film architectures. Cross-sectional structure of the stacked composite assemblies can include porosity or open spaces between composite nanoparticle assemblies. Such porosity and/or open spaces are illustrated in <FIG> provides scanning electron microscopy (SEM) images of pellet based Bi<NUM>Te<NUM> nanoplates after Ag decoration at different magnifications according to some embodiments.

In a further aspect, methods of enhancing chalcogenide thermoelectric performance are provided. In some embodiments, a method of enhancing chalcogenide thermoelectric performance comprises providing chalcogenide nanoparticles comprising front and back faces and sidewalls extending between the front and back faces. At least one of electrical conductivity and Seebeck coefficient of the chalcogenide nanoparticles is increased via nucleation of metallic nanoparticles on the sidewalls, wherein the metallic nanoparticles bridge spacing between the chalcogenide nanoparticles. Moreover, thermal conductivity of the chalcogenide nanoparticles can be decreased the metal nanoparticles bridging spacing between adjacent nanoparticles.

These and other embodiments are further illustrated in the following non-limiting examples.

V-VI Sb<NUM>Te<NUM> was selected because of its state-of-the-art performance that exhibits the highest ZT near <NUM>. Silver was chosen as the metallic nanoparticle phase due to its low work function (<NUM>-<NUM> eV) needed for efficient carrier injection into the Sb<NUM>Te<NUM> conduction band. In detail, using the ultrathin/active Sb<NUM>Te<NUM> edge as the nucleation sites, Ag can be reduced from AgNO<NUM> in ethyl alcohol (EG) at room temperature. As a result, Ag nanoparticles with diameters around <NUM> were found to grow uniformly at the edge of the Sb<NUM>Te<NUM> nanoplates.

Actually, a slight layer of second phase (n-type Ag<NUM>Te) was also introduced in this process. These interfaces among Sb<NUM>Te<NUM>-Ag<NUM>Te-Ag act as a low-energy carrier and phonon scattering center, which facilitates the enhancement of the Seebeck coefficient (from <NUM> to <NUM>µV/K) and the suppression of thermal conductivity. Meanwhile, the electrical conductivity was also improved from <NUM>×<NUM><NUM> to <NUM>×<NUM><NUM> S/m due to the increased carrier concentration with a slight decrease of carrier mobility. This simultaneous enhancement of electrical conductivity and Seebeck coefficients demonstrates that these self-assembled Ag nanoparticles are able to inject charge carriers and facilitate charge transport between Sb<NUM>Te<NUM> nanoplates; at the same time, the generated the energy barrier among Ag nanoparticles, the introduced Ag<NUM>Te second phase and Sb<NUM>Te<NUM> nanoplatelets also assist in blocking charge carriers with lower energy, facilitate the decoupling of the Seebeck coefficient and electrical conductivity.

In a typical synthesis, <NUM> ethylene glycol (EG) solution containing mixed antimony trichloride (SbCl<NUM>, <NUM> mmol), tellurium dioxide (TeO<NUM>, <NUM> mmol), sodium hydroxide (NaOH, <NUM>), and polyvinylpyrrolidone (PVP, Ms ≈ <NUM>/mol, <NUM>) are heated to <NUM>. <NUM> hydrazine hydrate (N<NUM>H<NUM>) was injected (with injection rate <NUM>/min), and the solution were maintained at <NUM> for <NUM> hours. After which, the mixture are heated at <NUM> under reflux for another <NUM> hours. The precipitates were collected by centrifugation, washed using ethanol for at least three times. Finally, a simple and efficacious vacuum filtration process is adopted to fabricate the Sb<NUM>Te<NUM>-based thin film using water as solution. In details, the Sb<NUM>Te<NUM>-based nanocomposites were dispersed in water via homogenization and sonication, subsequently, the resulting aqueous suspension was vacuum-filtered through a poly(vinylidene difluoride) (PVDF) filter (<NUM> pore size) to form a silver gray film (shiny metallic appearance) on the filter surface. The fabricated thin film can finally be transform to different substrates (like Silicon or PET) for different applications. For the fabrication of Ag-decorated Sb<NUM>Te<NUM> nanocomposites, <NUM> mmol as-fabricated Sb<NUM>Te<NUM> was dispersed in <NUM> EG, and proper amount of AgNO<NUM> was added with gently and stirred over night at room temperature. The precipitates were collected by centrifugation, washed using ethanol for at least three times. Finally, the vacuum filtration process is adopted to fabricate the flexible thin film based on Ag-decorated Sb<NUM>Te<NUM> using water as solution. Ag nanoparticles with high uniformity were observed to embed regularly around the edges of the Sb<NUM>Te<NUM> nanoplates, as evidenced by TEM images of <FIG>. Freestanding flexible Ag-decorated Sb<NUM>Te<NUM> thin film is illustrated in <FIG>.

The active Sb<NUM>Te<NUM> edges with exposed Te dangling bonds are believed to act as heterogeneous nucleation sites, first reacting with Ag+ and then facilitating the growth of Ag nanoparticles with the help of a reducing agent (EG). No dissociate Ag nanoparticles were found in the solution (dynamically unstable) or on the surface of the Sb<NUM>Te<NUM> nanoplates, suggesting that laterally selective growth is more preferable (calculated as -<NUM> eV/Å<NUM> compared with -<NUM> eV/Å<NUM> that growth on the face). The strain in the crystal lattice is beneficial to the decrease of the lattice thermal conductivity contribution by strain field scattering. By using XRD measurement, a slight amount of Ag<NUM>Te was also found beyond Ag (peaks <NUM>° and <NUM>° that corresponds to the main peak of monoclinic phase Ag<NUM>Te), which likely occurs in the beginning of nucleation as illustrated in <FIG>. That is in agreement with the slight layer (~<NUM>) of second phase (n-type Ag<NUM>Te, P2/n, and PDF No. <NUM>-<NUM>) observed by HRTEM in <FIG>, implying the uniformly generated Sb<NUM>Te<NUM>-Ag<NUM>Te-Ag interfaces. Here, by using self-assembled nanoengineering, a uniform p-n junction was generated around each Sb<NUM>Te<NUM> nanoplate, which might be one of the main reasons this unique heterojunction has extremely high electrical conductivity while maintaining a decent Seebeck coefficient. XPS of Sb<NUM>Te<NUM> and Ag-decorated Sb<NUM>Te<NUM> have been conducted to study the chemical environment of each element as provided in <FIG>. The <NUM>d<NUM>/<NUM> and <NUM>d<NUM>/<NUM> peaks of Ag are located at <NUM> and <NUM> eV, respectively. Interestingly, a slight shoulder peak appears in the Sb<NUM>Te<NUM> sample due to the oxidation states of Te compared to the sharp peak of the Ag-decorated Sb<NUM>Te<NUM> sample. This means the oxidative stability of the nanocomposites is strengthened after Ag-decoration. TGA and DSC analyses of Sb<NUM>Te<NUM> nanoplates and Ag-decorated Sb<NUM>Te<NUM> nanocomposites are also given in <FIG>.

Ag-decorated Sb<NUM>Te<NUM> nanocomposites possess significantly higher electrical conductivities around <NUM>×<NUM><NUM> S/m, which is eight times larger than that of the Sb<NUM>Te<NUM> based thin film and even comparable to the bulk Sb<NUM>Te<NUM> pellet. At the same time, the Seebeck coefficient also increases from <NUM> to <NUM>µV/K at <NUM> (><NUM>%) after nanoengineering. As a result, with the enhanced electrical conductivity and improved Seebeck coefficient, Ag-decorated Sb<NUM>Te<NUM> nanocomposite based films give a power factor of <NUM>µW/mK<NUM>, which is much higher than our previous Bi<NUM>Se<NUM>-based thin films and other Sb<NUM>Te<NUM>-based films. The electrical conductivity and Seebeck coefficient of the present films exhibited high stability against the bending test, demonstrating no apparent change in performance upon repeated bending for up to <NUM> cycles under bending radius <NUM>. The boost in performance results from the simultaneously enhanced electrical conductivity and Seebeck coefficients. Here, the enhanced electrical conductivity is explained by an efficient injection of carriers from the metallic Ag to the conduction band of the Sb<NUM>Te<NUM> semiconductor. To clarify the above mechanism, a room temperature Hall measurement was performed, which reveals a remarkable increase in carrier concentration from <NUM>×<NUM><NUM> cm-<NUM> to <NUM>×<NUM><NUM> cm-<NUM> with a slight decrease of mobility from <NUM> to <NUM><NUM>V-<NUM>S-<NUM>, as can be seen in Table <NUM>. This is also in agreement with the estimated results from the increase of equivalent conductivity (in unit of the relaxation time τ) for Ag-decorated Sb<NUM>Te<NUM> nanocomposites compared with Ag-free Sb<NUM>Te<NUM> based on first-principles calculations.

A beneficial energy barrier is introduced between the metallic nanoparticles and semiconductor nanoplates to maintain a decent Seebeck coefficient. The band alignment between Sb<NUM>Te<NUM> and Ag nanoparticles is shown in <FIG>. The detailed electrical information of Sb<NUM>Te<NUM> and the work function of silver nanoparticles (~<NUM> eV) are taken from experiments. The assumption of bulk is reasonable since the thickness of the bottom layer of Sb<NUM>Te<NUM> is around <NUM>, which corresponds to <NUM> QL. The Fermi level (EF) is positioned near the top of valence band maximum with a small gap at the Γ-point according to previous study. Before contact, the initial Fermi level of Ag is located above the intrinsic Sb<NUM>Te<NUM>. After Ag decoration, the presence of Ag nanoparticles pins the effective Fermi level of the present nanocomposites around the work function of silver due to the large carrier density in the metallic layer. As can be seen, the blend band gap between the host Sb<NUM>Te<NUM> semiconductor and metallic Ag nanoparticles forms a Schottky barrier, which is believed to be much better than Ohmic contact. Moreover, the potential barrier height (~<NUM> meV) is around the theoretical optimized height of <NUM> meV. Therefore, interfaces in Ag-Sb<NUM>Te<NUM> nanoplates induce energy-dependent carrier scattering by introducing a Schottky barrier to filter carriers with low energy, i.e. the carrier filtering technique described above is preventing the transport of the lower-energy carriers, which results in an increase in the moment of the differential conductivity about the Fermi level.

In summary, increasing the electrical conductivity while maintaining or even enhancing the Seebeck coefficient by chopping the distribution cold carriers is achieved with the introduced self-assembled heterojunction architectures, leading to a dramatically increased power factor for the present flexible thermoelectric fabrics. Further, to roughtly estimate the thermal conductivity of the present system, we made samples with thickness aournd <NUM>. The room temperature thermal conductivity of Ag-decorated Sb<NUM>Te<NUM> nanocomposites was determined around <NUM> W/m·K, which gives a ZT of <NUM>.

In order to fabricate Bi<NUM>Te<NUM> nanoplates, <NUM> mmol Bi(NO<NUM>)<NUM> and <NUM> mmol Na<NUM>TeO<NUM> was dissolved in <NUM> ethylene glycol, <NUM> NaOH was added with vigorous stirring, and followed by <NUM> polyvinylpyrrolidone (PVP, Ms=<NUM>/mol) and refluxing the mixture solution at <NUM> over night. After the mixture cool down to room temperature, Acetone was used to precipitate the fabricated Bi<NUM>Te<NUM> nanoplates and then re-dissolve by Ethanol. This process was repeated three times to remove any unreacted chemicals and ethylene glycol from the surface.

In a typical synthesis, <NUM> ethylene glycol (EG) solution containing mixed antimony trichloride (SbCl<NUM>, <NUM> mmol), tellurium dioxide (TeO<NUM>, <NUM> mmol), sodium hydroxide (NaOH, <NUM>), and polyvinylpyrrolidone (PVP, Ms ≈ <NUM>/mol, <NUM>) are heated to <NUM>. <NUM> hydrazine hydrate (N<NUM>H<NUM>) was injected (with injection rate <NUM>/min), and the solution were maintained at <NUM> for <NUM> hours. After which, the mixture are heated at <NUM> under reflux for another <NUM> hours. The precipitates were collected by centrifugation, washed using ethanol for at least three times to remove any unreacted chemicals and ethylene glycol from the surface.

For the fabrication of Ag-decorated Bi<NUM>Te<NUM>/Sb<NUM>Te<NUM>, <NUM> mmol as-fabricated Bi<NUM>Te<NUM>/Sb<NUM>Te<NUM> was dispersed in <NUM> EG, and proper amount of AgNO<NUM> was added with gently and stirred over night at room temperature. The precipitates were collected by centrifugation, washed using ethanol for at least three times. The fabrication of Cu-decorated Bi<NUM>Te<NUM>/Sb<NUM>Te<NUM> is similar as that of Ag, except CuI/CuCl was used to with a reaction temperature around <NUM>. This synthesis protocol enabled a high batch-to-batch reproducibility and a high material yield larger than <NUM>%.

The dried nanocomposites were loaded into a graphite die and compacted into pellets (Ø10 mm × ~<NUM>). The process was carried out in Ar atmosphere, using a custom-made hot press to simultaneously apply a pressure of <NUM> MPa and a temperature of <NUM>-<NUM> during <NUM>. In this system, the heat was provided by an induction coil operated at <NUM>-<NUM> and it was applied directly to a graphite die acting as a susceptor. Fast heating ramps of <NUM> s-<NUM> are reached by this method with a <NUM> kW induction heater. All the pellets were mechanically robust enough to endure polishing.

The synthesized Bi<NUM>Te<NUM> nanoplates (powder and bulk) were analyzed by X-ray diffraction (XRD) using Cu Kα radiation with a scanning step <NUM>° (λ=<NUM>Å, Bruker D2 Phaser). Transmission Electron Microscope (TEM) and High Resolution TEM techniques including the selected area electron diffraction (SAED) images were performed using a JEM-<NUM> electron microscope. Element mapping and energy dispersive X-ray spectroscopy (EDX) were also performed. The morphology and thickness of single NPs were measured by the Atomic Force Microscope (AFM). X-ray photoelectron spectroscopy (XPS) were used to study the quality. The morphology of the Bi<NUM>Te<NUM>/Sb<NUM>Te<NUM> nanoplates and Ag-decorated Bi<NUM>Te<NUM>/Sb<NUM>Te<NUM> nanocomposites (cross section and top-view) were measured by the Scanning Electron Microscope (SEM, JEOL, JSM-6330F). The Thermogravimetric (TG) and differential scanning calorimetric (DSC) were measured with a TG-DTA/DSC thermal analyzer (Netzsch, Germany) with a heating rate of <NUM>/min in flowing N<NUM> gas.

For the measurement of thermoelectric properties, Seebeck coefficients were measured using a static DC method, and electrical resistivity data was obtained by a standard four-probe method. Both the Seebeck coefficient and the electrical resistivity were measured simultaneously in a LSR-<NUM> LINSEIS system in the temperature range between room temperature and <NUM>, under helium atmosphere. At each temperature at least three consecutive measurements were performed to get rid of the minor variations. Taking into account the system accuracy and the measurement precision, we estimate an error of ca. <NUM> % in the measurement of the electrical conductivity and Seebeck coefficient. The thermal diffusivity coefficient (D) was measured between <NUM>-<NUM> by the A XFA <NUM> Xenon Flash apparatus from Linseis. The heat capacity (Cp) was measured using differential scanning calorimetry (DSC, Netzsch DSC-404C) with an associated error of ca. The thermal conductivity was calculated from the expression κ=DCpd, where d is the density of the sample. The density (ρ) was determined using the dimensions and mass of the sample and then reconfirmed using the Archimedes method. The Hall coefficient (RH) was determined using a physical properties measurement system (PPMS-9T) under a magnetic field of 2T and electrical current. The carrier concentration (n) was calculated as n=<NUM>/eRH, where e is the proton charge. The Hall mobility µ=RHσ, where σ is the electrical resistivity.

To determine the effect of self-assembled metal nanoparticles on thermoelectric (TE) performance of these nanocomposites, temperature dependent electrical and thermal transport properties for a series of metal-decorated Bi<NUM>Te<NUM> pellets are provided. <FIG> illustrates temperature dependent electrical properties of Ag-decorated Bi<NUM>Te<NUM> pellets with different Ag concentrations. <FIG> illustrates temperature dependent thermal transport properties of Ag-decorated Bi<NUM>Te<NUM> pellets with different Ag concentrations. <FIG> illustrates temperature dependent electrical and thermal transport properties of Cu-decorated Bi<NUM>Te<NUM> pellets with different Cu concentrations. The concentrations of Ag or Cu on Bi<NUM>Te<NUM> nanoplates can be fine-adjusted by AgNO<NUM> or CuI/CuCl precursors. Typically, metal-free Bi<NUM>Te<NUM> exhibits a relatively low electrical conductivity (σ) around <NUM>×<NUM><NUM> S/m at room temperature (RT). One reason for this low value compared with Bi<NUM>Te<NUM> ingot is the thermal degradation residue from the capping PVP ligand. The σ gradually increases with the self-assembled metallic nanoparticles. For example, nanocomposites with <NUM> at% Ag exhibit a significantly increased σ up to <NUM>×<NUM><NUM> S/m at RT, which is nearly three times compared with pure Bi<NUM>Te<NUM>. With temperature, all the nanocomposites show a nondegenerate semiconductor behavior, i.e. the σ increases slightly with the temperature over the studied measurement range <NUM>-<NUM>. This is in agreement with the nanostructured Bi<NUM>Te<NUM> observed previously. This behavior can be attributed to the thermal excitation carrier concentrations (n) across the band gap and the increased extrinsic n with the increasing temperature.

At the same time, it is interesting to see a simultaneous <NUM>% increase in S (negative) with the decorated nanoparticles, which enhances from <NUM> to <NUM>µV/K at RT for Ag-free Bi<NUM>Te<NUM> and Ag-decorated Bi<NUM>Te<NUM>, respectively. This causes the power factor (PF= σS<NUM>) keeping increasing until the amount of Ag reaches <NUM>%, with a champion value of <NUM>µW/mK<NUM> at RT. Further increase of Ag failed to enhance the σ. This is likely caused by the increased micro-voids in the nanocomposite due to the different Young's modulus between Ag (<NUM> GPa) and Bi<NUM>Te<NUM> (<NUM> GPa), or the introduced impurities on the surface of the nanoplates instead of the lateral edges (See the saturated density, the cross section SEM images of Ag-decorated Bi<NUM>Te<NUM> and the generated XRD patterns in Figure SI). In short, with the decoupled σ and S, the overall PF exhibits a five times increase for the nanocomposites over the original Bi<NUM>Te<NUM>.

For Cu-decorated Bi<NUM>Te<NUM> system, this decoupled phenomenon is similar but become much more strengthened, i.e. the absolute value of S shows a <NUM>% increase from <NUM> to <NUM>µV/K as the content of Cu increases, meanwhile, the σ enhanced dramatically from <NUM> × <NUM><NUM> to <NUM>×<NUM><NUM> S/m (with Cu concentration around <NUM> at%). Therefore, the highest PF of Cu-decorated Bi<NUM>Te<NUM> reaches <NUM>µW/mK<NUM> at RT, which is nearly nine times higher than the original Bi<NUM>Te<NUM>. The maximum PF of <NUM>µW/mK<NUM> at <NUM> was achieved for the Cu-decorated Bi<NUM>Te<NUM> containing <NUM> at% Cu.

The RT electrical performance for both Cu- and Ag- decorated Bi<NUM>Te<NUM> were summarized in <FIG>, demonstrating obviously decoupled σ and S i.e. the S and σ increase in parallel with the introduced self-assembled heterojunction architectures. This noticeable trend is fundamentally different to the coupled relationship between the σ and S in traditional inorganic systems with multiphases, in which two or more phases are either simply mixing together after separately synthesized or combined by the traditional doping strategy. In most previous reports, the addition of a second phase results in an increased σ with a decreased S or vice versa. To explore the detailed mechanism behind this, Hall Effect measurement results were studied at RT. First of all, compared with the Ag-free Bi<NUM>Te<NUM> matrix, a gradual increase in carrier concentrations (n) in the Ag-decorated Bi<NUM>Te<NUM> nanocomposites was observed. In other words, nanocomposites with self-assembled metal nanoparticles increases the n, which partially compensating the reduced µ. In general, S tends to decrease with the increased n. For the present self-assembled heterojunction system, the parallel enhancement of σ and S might arise from the simultaneous occurrence of parabolic bands across the Fermi surface and flat bands near the Fermi surface of Bi<NUM>Te<NUM>. Meanwhile, the interactions among the electron-electron, electron-phonon, and the porous three dimensional structure that is distinct from the tightly stacked bulk materials can also contribute to the increased S.

Energy filtering effect is normally qualitatively to explain the increased S by based on simple band gap alignment. However, introduced chemical bonding at the interface plays important roles in determining the electrical band structures and the Schottky-barrier height (SHB), thus detailed first principles calculations involved in a more accurate treatment of the interface dipole is necessary. Here, DFT calculations were carried out in order to uncover the mechanism behind the decoupled phenomenon. On one hand, from the increased electrical conductivity (σ=neµ) discussed above, the introduced metal nanoparticles are capable to inject charge carriers and facilitate promising efficient charge transport across neighboring nanoplates and the whole pellet. On the other hand, as the phonon scattering in nanostructured materials is strongly dependent on the numerous interfaces between the nanostructures, the self-assembled metal nanoparticles might also assist in blocking phonon propagation that transport large fraction of heat. This interface scattering in nanostructures originated from the enhanced phonon boundary scattering and the low energy electrons filtering, with which the compression of κL is expected. Nanoscale precipitates and mesoscale grains acting as multi-wavelength phonon scattering centers were indeed preserved. Based on the Rayleigh scattering regime, the scattering cross section is defined as σ~b<NUM>/λ<NUM>, where b and λ are the size of the scattering particles and phonon wavelength, respectively. Here, the size of the Ag nanoparticles is around <NUM>, which is sufficiently large enough to create a scattering regime that does not overlap Rayleigh scattering on the atomic scale (~<NUM>Å). Therefore, both Ag/Cu nanoparticles and the second phase Ag<NUM>Te/Cu<NUM>Te are thought to assist in blocking propagation of phonons with mid to long wavelengths that transport a large fraction of heat; thus, significantly reducing κL in nanocomposites is achieved.

The temperature dependence of κ and RT κL for the nanocomposites confirm the self-assembled heterojunction has a great influence on the thermal transport. Since bipolar thermal conductivity is negligible near RT, κL is calculated from the difference between κ and κe (κe=LσT, where L is the Lorentz number calculated using Fermi integral function. The calculation details are provided in SI). As a result of the low density, the thermal conductivity of Ag-free Bi<NUM>Te<NUM> pellet (<NUM> W·m-<NUM>·K-<NUM>) is much lower than that of ingot Bi<NUM>Te<NUM> (<NUM>-<NUM> W·m-<NUM>·K-<NUM>). It is seen that κL, shows a continuous decrease with Ag concentrations, for Ag-free Bi<NUM>Te<NUM> and Ag-decorated Bi<NUM>Te<NUM> at RT, respectively. These values lie within the minimum range of lattice thermal conductivity (<NUM>-<NUM> W·m-<NUM>·K-<NUM>) defined by Slack.

Claim 1:
A composite nanoparticle composition comprising:
a semiconductor nanoparticle (<NUM>) including a front face (<NUM>) and a back face and sidewall (<NUM>) extending between the front and back faces; and
metallic nanoparticles (<NUM>) nucleated and grown on at least one of the sidewalls, whereby the metallic nanoparticles are bonded to at least one of the sidewalls establishing a metal-semiconductor junction;
wherein the semiconductor nanoparticle has a platelet, a pyramidal, or a bi-pyramidal morphology.