QCM-2013-2-0037: Ab-initio calculations of thermoelectric properties in nanostructures: silicene, germanene, Si/Ge heterostructu

Thermoelectric materials that convert waste heat into useful electric energy has been attracted tremendous interests both theoretically and experimentally due to significant potential industrial applications [1-4]. The efficiency of the thermoelectric conversion is characterized by a dimensionless parameter which is called figure of merit: ZT=sigma S^2 * T/(kappa_e+kappa_p), where sigma is the electric conductance, S is the Seebeck coefficient, T is the absolute temperature, kappa_e and kappa_p are the thermal conductances of electron and phonon contributions, respectively. Obviously, an ideal thermoelectric material should have the electric conductance and Seebeck coefficient as high as possible, and the thermal conductance as low as possible. Generally, materials with ZT~1 are regarded as good thermoelectrics, while ZT approaching to or larger than 3 can then compete with conventional energy conversion techniques. Unfortunately, because of the Wiedemann-Franz law [kappa_e/\sigma=(k_B \pi)^2 T/3e^2, where k_B and e are respectively the Boltzmann constant and carrier charge] [5], an increase in the electric conductivity normally leads to an increase in the electrical thermal conductance. Although some works focused on trying to decrease the phonon thermal conductance have been studied [6-8], the maximum ZT obtained up to now is still not satisfactory.
Recently, the existence of graphene-like 2D-hexagonal silicene has been demonstrated experimentally [9-12]. The linear dispersion of the band structure near the Dirac point (i.e., massless Dirac fermions as in graphene) conveys some remarkable properties such as the quantum spin Hall effect (QSHE) to silicene, which continues to attract extensive attention. In comparison to graphene, silicene has the extra advantage of being directly compatible with current Si-based technology and thus is a promising material for different applications in nanoelectronics.
Besides C- and Si-based systems, a 2D-honeycomb structure has been predicted for germanium (the so-called germanene) as well, which also suggests a linear dependency of the massless Dirac fermions energy dispersion on momentum [13,14]. Similar to fully hydrogenated graphene, silicene and germanene hydrides have also been predicted theoretically. The heterostructure reduces their conductivity, opening a finite band gap and, as a consequence, paving the way for practical applications in future in the semiconductor technology.
The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness [15-17]. Although TMDs have been studied for decades, recent advances in nanoscale material characterization and device fabrication have opened up new opportunities in nanoelectronics and optoelectronics. TMDs such as MoS2, MoSe2, WS2 and WSe2 have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices.

[1] A. I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature (London) 451, 163 (2008).
[2] X. Yan, G. Joshi, W. Liu, Y. Lan, H. Wang, S. Lee, J. W. Simonson, S. J. Poon, T. M. Tritt, G. Chen, and Z. F. Ren, Nano Lett. 11, 556 (2011).
[3] Y. M. Zuev, W. Chang, and P. Kim, Phys. Rev. Lett. 102, 096807 (2009).
[4] P. Wei, W. Bao, Y. Pu, C. N. Lau, and J. Shi, Phys. Rev. Lett. 102, 166808 (2009).
[5] M. Jonson and G. D. Mahan, Phys. Rev. B 10, 4223 (1980).
[6] K. Yang, Y. Chen, R. D'Agosta, Y. Xie, J. Zhong and A. Rubio, Phys. Rev. B 86, 045425(2012).
[7] G. Seol and J. Guo, Appl. Phys. Lett. 98, 143107 (2011).
[8] E. J. Kan, X. Wu, Z. Li, X. C. Zeng, J. Yang, and J. G. Hou, J. Chem. Phys. 129, 084712 (2008).
[9] B. Lalmi, H. Oughaddou, H. Enriquez, A. Kara, S. Vizzini, B. Ealet and B. Aufray, Appl. Phys. Lett. 97, 223109 (2010).
[10] B. Aufray, A. Kara, S. Vizzini, H. Oughaddou, C. Léandri, B. Ealet and G. Le Lay, Appl. Phys. Lett. 96, 183102 (2010).
[11] F. Bechstedt, L. Matthes, P. Gori and O. Pulci, Appl. Phys. Lett. 100, 261906 (2012).
[12] S. Cahangirov, M. Topsakal, E. Aktürk, H. Şahin and S. Ciraci, Phys. Rev. Lett. 102, 263804 (2009).
[13] S. Lebègue and O. Eriksson, Phys. Rev. B: Condens. Matter Mater. Phys. 79, 115409 (2009).
[14] H. Şahin, S. Cahangirov, M. Topsakal, E. Bekaroglu, E. Aktürk, R. T. Senger and S. Ciraci, Phys. Rev. B: Condens. Matter Mater. Phys. 80, 155453 (2009).
[15] Mattheis, L. F, Phys. Rev. B 8, 3719–3740 (1973).
[16] Wilson, J. A. & Yoffe, A. D, Adv. Phys. 18, 193–335 (1969).
[17] Osada, M. & Sasaki, T. Adv. Mater. 24, 210 (2012).

Grant References: Consolider Nanotherm No. CSD2010-00044