Research

Monte Carlo

Monte Carlo Simulation: A Monte Carlo code is developed to simulate bulk and thin-film heterostructure semiconductor based devices. This code now includes phonon scatterings (inter and intra valley acoustic and polar optical phonon scatterings ), impurity scatterings (neutral and ionized), Plasmon scattering and electron-electron interactions through a 1D Poisson solver. Pauli exclusion principle is included in the code with a new algorithm, which allows us to calculate local electron temperature and thermal current. The picture shows the distribution of electrons in the k space as they form a Fermi sphere.

Zebarjadi M., Bulutay C., Esfarjani K., and Shakouri A., Monte Carlo simulation of electron transport in degenerate and inhomogeneous semiconductors , Appl. Phys. Lett. 90 092111 (2007)

Zebarjadi M., Shakouri A., and Esfarjani K., Thermoelectric transport perpendicular to thin-film heterostructures calculated using the Monte Carlo technique, Phys. Rev. B 74 195331 (2006)

Nonlinear Peltier

Nonlinear Peltier coefficient of a doped InGaAs semiconductor is calculated numerically using the Monte Carlo technique. The Peltier coefficient is also obtained analytically for single parabolic band semiconductors assuming a shifted Fermi-Dirac electronic distribution under an applied bias. Analytical results are in agreement with numerical simulations. Key material parameters affecting the nonlinear behavior are doping concentration, effective mass, and electron-phonon coupling. Current density thresholds at which nonlinear behavior is observable are extracted from numerical data. It is shown that the nonlinear Peltier effect can be used to enhance cooling of thin film microrefrigerator devices especially at low temperatures.

Zebarjadi M., Esfarjani K., and Shakouri A., Nonlinear Peltier effect in semiconductors, Appl. Phys. Lett. 91 2785154 (2007)

Embedded nanoparticles

Embedded nano-particles can enhance the performance of thermoelectric devices by scattering phonons more effectively than electrons and by blocking low-energy electrons. Scattering of electrons from nanoparticles were modeled successfully in different limits when the nano-particle potential is weak (the Born approximation), when the nano-particle potential is strong but the nanoparticle concentration is low (partial wave method, within the single site approximation) and when the nano-particle concentration is high (coherent potential approximation). Results show that at low temperatures, nano-particle doped samples can have efficiencies up to 4 times higher than that of regular impurity doped samples if they are designed properly.

Zebarjadi M., Esfarjani K., Shakouri A., Bahk JH., Bian ZX., Zeng G., Bowers JE., Lu H., Zide JMO. and Gossard A., Effect of nano-particle scattering on thermoelectric power factor, App. Phys. Lett 94, 202105 (2009).

-Zebarjadi M., Esfarjani K., Shakouri A., et. al., Effect of nano-particle scattering on electronic and thermoelectric transport, J. of Electronic Materials 38, 954 (2009)

Zebarjadi M., Esfarjani K., Bian ZX. and Shakouri A., Low-Temperature Thermoelectric Power Factor Enhancement by Controlling Nanoparticle Size Distribution, Nanoletters, 11, 225-230 (2011)

3w measurements

3w measurement:

This is a technique to measure the Seebeck coefficient as well as thermal conductivity in the cross plane direction. We are doing these measurements in a cryostat (CCS-400H/204) under the vacuum of ~10-5 torr. Using a helium compressor and two heater stages, with this setup we can control the temperature in the range of 30K to 800K using Janis cryostat.

CPA- approx

Coherent potential approximation is used to study the effect of adding doped spherical nanoparticles inside a host matrix on the thermoelectric properties. This takes into account electron multiple scatterings that are important in samples with relatively high volume fraction of nanoparticles (>1 %) We show that with large fraction of uniform small size nanoparticles (similar to 1 nm), the power factor can be enhanced significantly. The improvement could be large (up to 450% for GaAs) especially at low temperatures when the mobility is limited by impurity or nanoparticle scattering. The advantage of doping via embedded nanoparticles compared to the conventional shallow impurities is quantified. At the optimum thermoelectric power factor, the electrical conductivity of the nanoparticle-doped material is larger than that of impurity-doped one at the studied temperature range (50-500 K) whereas the Seebeck coefficient of the nanoparticle doped material is enhanced only at low temperatures (similar to 50 K).

Zebarjadi M., Esfarjani K., Bian ZX. and Shakouri A., Low-Temperature Thermoelectric Power Factor Enhancement by Controlling Nanoparticle Size Distribution, Nanoletters, 11, 225-230 (2011)

MD simulation-Cage type structures

The relations between the thermal conductivity of cagelike structures and their crystal parameters are investigated using a two-dimensional toy model. The model consists of host atoms on a rectangular lattice with fillers at the center of each rectangle. The effect of mass and size of the filler on thermal conductivity is investigated using equilibrium molecular-dynamics simulations. We show that the thermal conductivity decreases with increasing atomic displacement parameter while it has local minima versus the filler to host mass ratio. Similar trends were observed in experiments on filled skutterudites. The trends are explained by analyzing the effect of the filler on the phonon dispersion and relaxation times of the host material.

Zebarjadi M., Esfarjani K., Yang J., Ren ZF., and Chen, G., Effect of filler mass and binding on thermal conductivity of fully filled skutterudites, Phys. Rev. B 82, 195207 (2010)

Modulation-Doping

Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si80Ge20)(70)(Si100B5)(30) by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si70Ge30 instead of Si as the nanoparticles and Si95Ge5 as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 +/- 0.1 at 900 degrees C.

Yu B.*, Zebarjadi M.*, Lukas K., Wang H., Wang H., Opeil C., Dresselhaus M.S., Chen G. and Ren Z.F., Enhancement of thermoelectric properties by modulation doping in silicon germanium alloy nanocomposites, NanoLetters. 12, 2077 (2012) * equal contribution. Highlighted in News.

Zebarjadi M., Joshi G., Zhu G.H., Yu B., Minnich A., Lan Y.C., Wang X.W., Dresselhaus M., Ren Z.F. and Chen G., Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites, Nanoletters, 11, 2225-2230 (2011)

Cloaking

Cloaking Nanoparticles: We aim at making nanoparticles embedded in a host semiconductor with a size comparable to electronic wavelengths "invisible" to the electron transport. Inspired by the recent progress made in optics and working within the framework of the expansion of partial waves, we demonstrate that the opposite effects imposed by potential barriers and wells of a core-shell nanoparticle on the phase shifts associated with the scattered electron wave could make the scattering cross section of the first two partial waves vanish simultaneously. We show that this is sufficient to cloak the nanoparticle from being detected by electrons with specific energy in the sense that a total scattering cross section smaller than 0.01% of the physical cross section can be obtained and a 4 orders of magnitude difference in the total scattering cross section can be presented within an energy range of only 40 meV, indicating possible applications of the "electron cloaks" as novel electronic switches and sensors, and in efficient energy harvesting and conversion technologies.

Liao B., Zebarjadi M., Esfarjani K., and Chen G., Isotropic and energy-selective electron cloaks on graphene, Physical Review B 88 (15), 155432, 2013.

Liao B.*, Zebarjadi M.*, Esfarjani K. and Chen G., Cloaking Core-Shell Nanoparticles from Conducting Electrons in Solids, Physical Review Letters, 109, 126806 (2012)

Invisible Dopants

Invisible Dopants: Nanoparticle dopants that are invisible to conduction electrons and have sharp dips in their electron scattering rate versus electron energy close to the Fermi level, are designed. Replacement of such dopants with traditional impurities results in simultaneous enhancement of the Seebeck coefficient and the electron mobility and therefore a large enhancement in the thermoelectric power factor can be achieved.

-Zebarjadi M., Liao B., Esfarjani K., Dresselhaus MS., and Chen G., Enhancement of thermoelectric power factor by invisible dopants, Adv. Mat. 15, 2013.

-Shen W., Tian T., Liao B., and Zebarjadi M., Combinatorial approach to identify electronically cloaked hollow nanoparticles, PRB 90, 075301, 2014.

-M. Zebarjadi, W. Shen, An algorithm to reduce combinatorial search in design of invisible dopants, Computational Materials Science, 113, Pages 171-177, 2016. Editor Choice

Thermoelectric Devices

Thermoelectric devices:

Active and Passive cooling: We show materials with high power factor and high thermal conductivity and not traditionally used high-Z materials should be used for applications in which heat is pumped from hot to cold such as electronic cooling.

M. Zebarjadi, Electronic Cooling Using Thermoelectric Devices, APL. 106 (20), 203506, 2015.

Heat management: We show that when poor cooling is used in power generation mode, it is beneficial to purposely open heat loss channels from side-walls of thermoelectric modules.

M. Zebarjadi, Heat Management in Thermoelectric Power Generators, Scientific Reports 6,20951,2016 (available on arXive :1509.02407)

Thermionic Devices

Thermionic devices based on van der Waals heterostructures

van der Waals (vdW) heterostructures based on 2D layered materials have great potential for thermionic energy conversion due to the following two advantages: a) thermionic transport barriers can be tuned by changing the number of layers of 2D materials, and b) thermal conductance across these non-covalent heterostructures is very weak. We show that graphene-phosphorene-graphene vdW heterostructure sandwiched by gold electrodes is a potential device to achieve efficient thermionic cooling.

X. Wang, M. Zebarjadi, K. Esfarjani, First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures, Nanoscale, 2016 (available on arXiv:1510.03783).

Semimetallic Thermoelectrics

Semimetals as potential thermoelectric materials

The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a band gap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. We propose semi-metals with large asymmetry between conduction and valence bands as an alternative class of thermoelectric materials.

M. Markov, X. Hu, HC Liu, N. Liu, J. Poon, K. Esfarjani, and M. Zebarjadi, Semi-metals as potential thermoelectric materials: case of HgTe, Nature scientific reports8, 2018.

Hybrid organic-inorganic thermoelectrics

Hybrid organic-inorganic materials for thermoelectric applications

Hybrid organic-inorganic materials are the latest class of thermoelectric materials. We are interested in studying charge transfer at the interface of organic-inorganics and the response function of the 2D electron/hole gas trapped at the interface.

X Wang, KEsfarjani, M Zebarjadi, Charge transfer at hybrid inorganic-organic interfaces,The Journal of Physical Chemistry C, 121,29,2017. also on: arXiv:1610.0624

N. Liu, J. Peters, A. Ramu, J.A. Floro, J. E. Bowers, M. Zebarjadi, Thermoelectric Transport at F4TCNQ-Silicon interface, invitedpaper, APL Materials, 7,021104, 2019

Heat diffusion imaging

Heat diffusion imaging: in-plane thermal conductivity of supported thin film

This method combines the principles of heat spreader method and the thermoreflectance imaging techniques. It is an electrical-pump optical-probe method that measures the in-plane thermal conductivity of thin films or 2D materials on a substrate. We couple the system with an optical cryostat for temperature dependent measurement.

Tianhui Zhu, David H. Olson, Patrick E. Hopkins, and Mona Zebarjadi. "Heat diffusion imaging: In-plane thermal conductivity measurement of thin films in a broad temperature range." Review of Scientific Instruments 91, no. 11 (2020): 113701.

2D Thermoelectrics

Few-layer 2D materials are promising for thermoelectric applications, especially for nanoelectronics. A sharp density of states (DOS) profile at the band edge and a superior electron mobility in many 2D crystals are desirable for high power factor (PF). Their electrical properties can be tuned by mechanical strain, applied field, and the number of layers.

We have characterized graphene and are looking to optimize 2D transition metal dichalcogenides (TMDCs) for thermoelectric applications.

Qin-Yi Li, Qing Hao, Tianhui Zhu, and Mona Zebarjadi. "Nanostructured and heterostructured 2D materials for thermoelectrics." Engineered Science (2020).

Junxi Duan, Xiaoming Wang, Xinyuan Lai, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Mona Zebarjadi, and Eva Y. Andrei. "High thermoelectricpower factor in graphene/hBN devices." Proceedings of the National Academy of Sciences 113, no. 50 (2016): 14272-14276.

Tianhui Zhu, Peter M. Litwin, Md Golam Rosul, Devin Jessup, Md Sabbir Akhanda, Farjana F. Tonni, Sergiy Krylyuk et al. "Transport properties of few-layer NbSe2: From electronic structure to thermoelectric properties." Materials Today Physics 27 (2022): 100789.

Nanostructured Si thin film thermoelectrics

An efficient thermoelectric device has high ZT, which requires high power factor and low thermal conductivity. Highly doped bulk Si has a power factor larger than commercial thermoelectric material Bi2Te3. We added nanostructures to Si thin films to introduce more frequent phonon scattering and to suppresses the thermal conductivity.

Naiming Liu*, Tianhui Zhu*, Md Golam Rosul, Jon Peters, John E. Bowers, and Mona Zebarjadi. "Thermoelectric properties of holey silicon at elevated temperatures." Materials Today Physics 14 (2020): 100224. *equal contribution

Research: