Session W12: Focus Session: Thermoelectrics Nanomaterials I

Thermoelectric performance of chemically exfoliated n-Bi$_2$Te$_3$

Bi$_{2}$Te$_{3}$ based thermoelectric (TE) devices are of interest because of their high thermoelectric figure of merit (ZT) near room temperature, and ability to be utilized in both refrigeration and power generation modes. Recently, nano-structuring has shown promise in improving the TE performance of $p$-type Bi$_{2}$Te$_{3}$, however $n$-type counterparts are still lagging in this respect. Here, we display high ZT values ($\sim$ 0.9) in exfoliated $n$-Bi$_{2}$Te$_{3}$ at elevated temperatures (400-- 500 K). The chemically exfoliated samples were prepared by an ultra-sonication technique with subsequent spark plasma sintering to obtain dense pellets. Our transport results showed improved compatibility and a shift in the ZT maximum towards a higher temperature ($\sim$ 430 K) than commercially available ingots. The experimental details and transport data will be discussed within the frame work of exfoliation-induced structural modifications.   

Room Temperature Thermoelectric Properties of Porous BiSbTe Thin Films

Bi$_{(2-x)}$Sb$_{x}$Te$_{3}$ is currently the best known room temperature p-type thermoelectric material, with a ZT value $\sim$ 0.75. We report synthesis of Bi$_{(2-x)}$Sb$_{x}$Te$_{3}$ thin films via pulsed laser deposition using a Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ target. We have investigated the effect of deposition parameters, including substrate, laser power and inert gas pressure, and annealing conditions on the microstructure, composition and thermoelectric properties of the films. We find a strong dependence of film characteristics on background pressure: The Sb content of the films increases with deposition pressure. Low pressure (1-2 mTorr) depositions yield highly conducting and amorphous films deficient in Te. In addition, we will present a comparison of the thermoelectric properties of porous and dense BiSbTe films, to evaluate film porosity as a means for increasing confinement and improving the thermoelectric power factor.   

Thermoelectric properties of electrolessly etched silicon nanowire arrays

Patterning silicon as nanowires with roughened sidewalls enhances the thermoelectric figure-of-merit ZT by order of magnitude compared to the bulk at 300 K [1]. The enhancement is mainly achieved by the remarkable reduction in the thermal conductivity below 5 W/mK at 300 K with only a negligible effect on the power factor of these nanowires. While the focus remained on understanding the implications of surface disorder on the thermal conductivity, the phonon transport effects on the Seebeck coefficient of these wires remains largely unexplored. We developed an electroless etching technique to generate nanowire arrays (NWAs) with controlled surface roughness, morphology, porosity and doping [2]. We conduct the simultaneous device-level measurements of the Seebeck coefficient and thermal conductivity of the NWAs using frequency domain techniques. We observe that nano-structuring quenches the phonon drag [3] in NWAs thereby reducing the Seebeck coefficient by $\sim$25{\%} compared to the bulk at degenerate doping levels. Further, we observe that the sidewall roughness greater than 3 nm roughness height lowers the thermal conductivity 75{\%} below the Casimir limit [4] with 10{\%} - 15{\%} increase in Seebeck coefficient. The porous NWAs show thermal conductivity close to the amorphous limit of Si with enhancement in the Seebeck coefficient primarily due to the carrier depletion. References: [1] A. I. Hochbaum et al, Nature 451, 163-167 (2008). [2] K. Balasundaram et. al., Nanotechnology 23, 305304 (2012). [3] C. Herring, Phys. Rev. 96, 1163 (1954). [4] H. G. B. Casimir, Physica 5, 495 (1938).   

Low temperature phonon boundary scattering in slightly rough Silicon nanowires

Nanostructured materials [1-3] have lower thermal conductivities than the bulk and are promising candidates for thermoelectric applications. In particular, measurements on single silicon nanowires show a reduction in thermal conductivity below the Casimir limit. This reduction increases with surface roughness [4] but the trend and its connection to phonon boundary scattering are still elusive. Here, we measure the thermal conductivity of single silicon nanowires fabricated using metal-assisted chemical etching. High resolution TEM imaging shows crystalline wires with slightly rough surfaces. Their statistical correlation lengths (5-15 nm) and RMS heights (0.8-1.5 nm) are in a range where perturbation-based wave scattering theory is still applicable. We use the thermal conductivity data to extract the frequency dependence of phonon boundary scattering at low temperatures (10-40 K) and show agreement with multiple scattering theory. This work provides insight into enhancing the thermoelectric performance of nanostructures. 1-A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008). 2-A. J. Minnich et al, Energy Environ. Sci. 2, 466-479 (2009). 3-L. Shi, Nanoscale Microscale Thermophys. Eng. 16, 79--116 (2012). 4-J. Lim et al, Nano Lett. 12, 2475$-$2482 (2012).   

Thermal conductivity of disordered porous Silicon

Nanostructuring bulk materials is a promising approach for engineering high-efficiency thermoelectric devices thanks to its ability to decoupling the thermal and electrical transport. Among different approaches, porous Silicon has been attracting much attention due to its ability of strongly suppressing heat transport. Recent experimental works show that classical size effects of phonons can be further enhanced by having staggered pores, as opposed to the aligned pores case. Motivated by these results, we solve the phonon Boltzmann Transport Equation to compute heat transport across an arbitrary pores arrangement. The model has been discretized by means of the Discontinuous Galerkin method, which allows complex simulation domains. We focus on triangular, circle and square pores where the orientation is allowed to change stochastically. In order to compute the ZT, the electrical conductivity and the Seebeck coefficients are computed by means of diffusive theory. Our main finding is that pore disorder can play a crucial rule in optimizing thermoelectric materials. Indeed, in the special case of triangular pores we predict an increasing in ZT of up to ten times the value found for the aligned case.   

Strong suppression of near-surface thermal transport by metal-assisted chemical etching of Si

Recently, we reported that the thermal conductivity of Si nanowire arrays roughened by metal-assisted chemical etching (MAC-etch) is strongly correlated to both the magnitude of the roughness and a broadening of the one-phonon Raman linewidth. We hypothesized that microstructural disorder induced by the etching chemistry leads to changes in the Raman linewidth and reduced thermal conductivity. Here, we simplify the study of such effects by chemically roughening Si wafers instead of nanowires. We have studied the effects of various roughening procedures on the near-surface thermal transport properties using time-domain thermoreflectance. We find that the thermal conductance of the near-surface region is systematically reduced by the MAC-etch process, despite the expectation that pristine roughened surfaces should have increased conductance due to enhanced surface area. In addition, highly roughened surfaces show strong picosecond acoustic echoes with reflection coefficient indicative of a soft interface. These features are consistent with the presence of strong disorder or nanoporosity in the near-surface region created by the MAC-etch process.   

Evaluating Heat Dissipation in Si/SiGe Nanostructures using Raman Scattering

Bulk SiGe alloys and SiGe nanostructures exhibit relatively low thermal conductivity and have found applications in efficient thermoelectric devices. Practical measurements of thermal conductivity involve a sophisticated device design, which may not be applicable to sub-micrometer structures and devices. Raman scattering can be used to measure local temperature with a high accuracy, and it allows calculations of thermal conductivity. In this work, we present Raman data obtained for three sets of samples: partially-relaxed SiGe alloy layers with thickness close to 50 nm; planar Si/SiGe superlattices (SL) with $\sim$ 30{\%} Ge content; and three-dimensional (3D) Si/SiGe cluster multilayers with different Ge concentration and degrees of vertical self-ordering. Despite a high signal-to-noise ratio (better than 1000 to 1), quantitative analysis of Raman spectra requires proper baseline modeling and subtraction. By measuring multi-modal Stokes/anti-Stokes Raman signals and performing base line correction, we calculate local temperatures and develop a model of heat dissipation in the different SiGe and Si/SiGe nanostructures.   

Probing Large-Wavevector Phonons in Silicon Nanomembranes using X-ray Thermal Diffuse Scattering

Phonons play a critical role in determining physical properties of crystalline materials. Phonon dispersions can be modified via nanoscale engineering, by introducing boundaries separated by distances comparable to phonon wavelengths. In free-standing nanowires and sheets, theoretical and experimental investigations have been largely restricted to studying small-wavevector phonons lying within the central 1$\%$ of the Brillouin Zone. Large-wavevector phonons, important for transport in nanostructures, cannot be modeled using continuum physics, and are difficult to probe using conventional optical techniques. Synchrotron x-ray thermal diffuse scattering (TDS) collects information from the scattering of x-rays by phonons with wavevectors spanning the entire Brillouin zone. We adopt this technique to probe the dispersion of large-wavevector acoustic phonons in the nanoscale regime. TDS measurements were performed on silicon nanomembranes, from 315 nm thick sheets exhibiting bulk Si dispersions, to membranes as thin as 6 nm, where deviations from bulk-like behavior are observed. Systematic examinations of the variation of scattered intensity with crystallographic orientation, wavevector, and membrane thickness will be presented.   

Atomistic Monte Carlo simulations of heat transport in Si and SiGe nanostructured materials

Efficient thermoelectric energy conversion depends on the design of materials with low thermal conductivity and/or high electrical conductivity and Seebeck coefficient [1]. Semiconducting nanostructured materials are promising candidates to exhibit high thermoelectric efficiency, as they may have much lower thermal conductivity than their bulk counterparts [1]. Atomistic simulations capable of handling large samples and describing accurately phonon dispersions and lifetimes at the nanoscale could greatly advance our understanding of heat transport in such materials [2]. We will present an atomistic Monte Carlo method to solve the Boltzmann transport equation [3] that enables the computation of the thermal conductivity of large systems with both empirical and first principles Hamiltonians (e.g. up to several thousand atoms in the case of Tersoff potentials). We will demonstrate how this new approach allows one to rationalize trends in the thermal conductivity of a range of Si and SiGe based nanostructures, as a function of size, dimensionality and morphology [3]. [1] See e.g. A. J. Minnich et al. Energy Environ. Sci. 2, 466 (2009). [2] Y. He, I. Savic, D. Donadio, and G. Galli, accepted in Phys. Chem. Chem. Phys. [3] I. Savic, D. Donadio, F. Gygi, and G. Galli, submitted.   

Thermoelectric Power Factor Engineering of Low-Dimensional and Nanocomposite Si Nanostructures

By employing nanostructured materials the thermoelectric figure of merit ZT has been raised to unprecedented large values, with a present record of ZT$=$2.4. Even in traditionally poor thermoelectric materials such as Si, high ZT values were achieved. The improvement was a result of the drastic reduction in the thermal conductivity, which could be suppressed close or even below the amorphous limit. Since thermal conductivity reduction is reaching its limits, additional benefits resulting from electronic structure engineering have to be investigated. In this work we theoretically provide design directions for the thermoelectric power factor (comprising Seebeck coefficient and electrical conductivity) and thermal conductivity in nanostructured Si channels. We consider 1D nanowires, 2D ultra-thin layers, and nanocomposite Si-based materials. We employ semiclassical Boltzmann transport and use both atomistic and continuum calculations for the electronic and phononic structure of the materials. This study examines how length scale can be exploited as a degree of freedom in designing the nanoscale thermoelectric material properties.   

Enhanced Power Factor in Strained Silicon Nanomesh Thin Film

The power factor $S^{2}\sigma $ ($S$ is the Seebeck coefficient and $\sigma $ is the electrical conductivity) of n-type silicon thin films is increased by utilizing both tensile lattice strain and nanomesh structures. The tensile strained lattice in n-type silicon splits the six-fold degenerate conduction band, which results in reduced inter-valley scattering and consequently enhanced electron mobility. The nanomesh feature structure decreases the thermal conductivity due to increased phonon scattering. The nanomesh was patterned onto both strained and unstrained silicon on insulator (SOI) using reactive ion etching with self-assembled block copolymers as masks. The Seebeck coefficient and electrical conductivity measurements were then performed on both strained and unstrained nanomesh SOI in vacuum over a wide temperature range. Increases in $S$ and $\sigma $ were observed and an enhanced power factor was obtained.   

Studies on Seebeck Coefficient of Individual Bismuth Telluride Nanotube

We have studied on Seebeck coefficient (S) of freestanding individual Bismuth Telluride nanotubes using micro-fabricated thermoelectric workbench at the temperatures from 300 K to 25 K. The thermoelectric workbench is composed of three main elements: heater, thermocouple, and platinum pad. A polysilicon-gold thermocouple accurately measures the temperature, arising from the heat generated at the tips of the test sites from the polysilicon heater located 2 $\mu $m apart from the thermocouple. Platinum pads placed on top of the heater and thermocouple structures and electrically isolated from these constitute S measurement circuit. IPA solution containing Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ nanotubes was drop-cast on the workbench and the Ebeam Induced Deposition of platinum was used to improve the electrical and thermal contacts between nanotube and platinum pads. The inner and outer diameter of nanotube is 50 nm and 70 nm, respectively. The sign of obtained S was positive which is indicating the nanotube is p-type. And the magnitude was increased compared to the bulk and nanowire types. The measured S (364 $\mu $V/K) of nanotube at T $=$ 300 K is 1.65 times larger than that (220 $\mu $V/K) of bulk and 1.4 times larger than the previously reported value (260 $\mu $V/K) of nanowire.   

Signatures of 1D Electron Subbands in the Thermoelectric Properties of InAs Nanowire

We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with diameter around 20nm is obtained over $T=$ 40 to 300K. At low temperatures (less than c.a.100K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) sub-bands. This work experimentally shows the possibility to modulate semiconductor nanowire's thermoelectric properties through the peaked 1D density of states in the diffusive transport regime, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the importance of scattering (or disorder) induced energy level broadening in smearing out the 1D confinement enhanced thermoelectric power factor at practical temperatures (e.g. 300K). The authors acknowledge NSF CAREER Award (DMR-1151534), ACS PRF (48800-DNI10) and the NSF of China for support of this research.   

Thermoelectric effects in disordered branched nanowires

We shall develop formalism of thermal and electrical transport in $Si_{1-x}Ge_x$ and $BiTe$ nanowires. The key feature of those nanowires is the possibility of dendrimer type branching. The branching tree can be of size comparable to the short wavelength of phonons and by far smaller than the long wavelength of conducting electrons. Hence it is expected that the branching may suppress thermal and let alone electrical conductance. We demonstrate that the morphology of branches strongly affects the electronic conductance. The effect is important to the class of materials known as thermoelectrics. The small size of the branching region makes large temperature and electrical gradients. On the other hand the smallness of the region would allow the electrical transport being ballistic. As usual for the mesoscopic systems we have to solve macroscopic (temperature) and microscopic ((electric potential, current)) equations self-consistently. Electronic conductance is studied via NEGF formalism on the irreducible electron transfer graph. We also investigate the figure of merit $ZT$ as a measure of the suppressed electron conductance.   

Electrochemical Deposition of Lanthanum Telluride Thin Films and Nanowires

Tellurium alloys are characterized by their high performance thermoelectric properties and recent research has shown nanostructured tellurium alloys display even greater performance than bulk equivalents [1-2]. Increased thermoelectric efficiency of nanostructured materials have led to significant interests in developing thin film and nanowire structures. Here, we report on the first successful electrodeposition of lanthanum telluride thin films and nanowires. The electrodeposition of lanthanum telluride thin films is performed in ionic liquids at room temperature. The synthesis of nanowires involves electrodepositing lanthanum telluride arrays into anodic aluminum oxide (AAO) nanoporous membranes. These novel procedures can serve as an alternative means of simple, inexpensive and laboratory-environment friendly methods to synthesize nanostructured thermoelectric materials. The thermoelectric properties of thin films and nanowires will be presented to compare to current state-of-the-art thermoelectric materials. The morphologies and chemical compositions of the deposited films and nanowires are characterized using SEM and EDAX analysis. References: [1] D. M. Rowe, \textit{CRC Handbook of Thermoelectrics}, CRC Press (1995). [2] A. May \textit{et al.,} \textit{Phys. Rev. B} \textbf{78}, 125205 (2008).