Enhanced thermoelectric performance of defected silicene nanoribbons

doi:10.1016/j.ssc.2015.11.012

Solid State Communications

Volume 227, February 2016, Pages 1-8

https://doi.org/10.1016/j.ssc.2015.11.012 Get rights and content

Highlights

•
Based on non-equilibrium Green׳s function method, we investigate the thermoelectric performance for both zigzag (ZSiNRs) and armchair (ASiNRs) silicene nanoribbons with central or edge defects.
•
For various types of edges and defects, with increasing defect numbers in longitudinal direction, ZT_M increases monotonously while the phononic thermal conductance decreases.
•
Comparing with ZSiNRs, defected ASiNRs possess higher thermoelectric performance due to higher Seebeck coefficient and lower thermal conductance.
•
In particular, about 2.5 times enhancement to ZT values is obtained in ASiNRs with edge defects.
•
Our theoretical simulations indicate that by controlling the type and number of defects, ZT values of SiNRs could be enhanced greatly which suggests their very appealing thermoelectric applications.

Abstract

Based on non-equilibrium Green׳s function method, we investigate the thermoelectric performance for both zigzag (ZSiNRs) and armchair (ASiNRs) silicene nanoribbons with central or edge defects. For perfect silicene nanoribbons (SiNRs), it is shown that with its width increasing, the maximum of ZT values (ZT_M) decreases monotonously while the phononic thermal conductance increases linearly. For various types of edges and defects, with increasing defect numbers in longitudinal direction, ZT_M increases monotonously while the phononic thermal conductance decreases. Comparing with ZSiNRs, defected ASiNRs possess higher thermoelectric performance due to higher Seebeck coefficient and lower thermal conductance. In particular, about 2.5 times enhancement to ZT values is obtained in ASiNRs with edge defects. Our theoretical simulations indicate that by controlling the type and number of defects, ZT values of SiNRs could be enhanced greatly which suggests their very appealing thermoelectric applications.

Introduction

Thermoelectric materials which can convert dissipated heat into electric energy at nanoscale have attracted increasing attention from both theoretical and experimental research due to their potential technological applications [1], [2], [3]. The performance of thermoelectric materials is measured by the figure of merit ZT=S²GT/(κ_e+κ_p), where S, G, T, κ_e and κ_p represent the Seebeck coefficient, electronic conductance, absolute temperature, electronic and phononic thermal conductance, respectively. Explicitly, an optimal thermoelectric material should possess high Seebeck coefficient, high electronic conductance, and low thermal conductance.

Both experimental and theoretical research have indicated that nanoscale materials could exhibit much higher ZT values compared with those of bulk materials, which lead to a very important prospect of thermoelectric applications. Experimentally, in particular, it was found that the thermal conductivity of Si nanowires (SiNWs) can be 100 times smaller than that of bulk silicon, which suggests the possibility of using silicon-based nanostructures as efficient thermoelectric materials [4], [5]. Theoretically, improved thermoelectric performance of SiNWs was predicated [6], [7], [8], [9], as a result of reduced thermal conductivity caused by phonon surface scattering and enhanced power factor due to quantum confinement effect. Subsequently, other silicon-based nanostructures like nanotubes [10] and nanomembranes [11], [12] have been suggested, and their interesting thermoelectric properties have also been reported.

Very recently, new two-dimensional materials like graphene and silicene, especially one-dimensional graphene (GNRs) and silicene (SiNRs) nanoribbons, have been attracting a great interest due to their unique properties [13], [14], [15], [16]. Though the thermopower of pristine graphene is not very high, it can be considerably enhanced in GNRs, especially in nanostructures consisting of nanoribbons of various types. Indeed, in a properly designed nanoribbon with alternating zigzag and armchair sections, thermoelectric figure of merit exceeding unity at room temperature has been found [17]. The efficiency can be also enhanced by randomly distributed hydrogen vacancies in almost completely hydrogenated GNRs [18]. Furthermore, structural defects, especially in the form of antidots, also appear a promising way to enhance thermoelectric efficiency [19], [20], [21]. Meanwhile, giant spin related thermoelectric phenomena have been predicted for ferromagnetic ZGNRs with antidotes [22].

It is generally accepted that decreasing the characteristic size of nanostructures and introducing defects are two effective ways to further improve their ZT values. So higher thermoelectric performance can be expected for SiNRs with defects, which is due to lower thermal conductance than GNRs. In this paper, we investigate the thermoelectric performance for both ZSiNRs and ASiNRs with central or edge defects by using non-equilibrium Green׳s function method. For perfect silicene nanoribbons (SiNRs), it is shown that with its width increasing, the maximum of ZT values (ZT_M) decreases monotonously while the phononic thermal conductance increases linearly. For various types of edges and defects, with increasing defect numbers in longitudinal direction, ZT_M increases monotonously while the phononic thermal conductance decreases. Comparing with ZSiNRs, defected ASiNRs possess higher thermoelectric performance due to higher Seebeck coefficient and lower thermal conductance. In particular, about 2.5 times enhancement to ZT values is obtained in ASiNRs with edge defects. Our theoretical simulations indicate that by controlling the type and number of defects, ZT values of SiNRs could be enhanced greatly which suggests their very appealing thermoelectric applications.

Section snippets

Model and method

In order to enhance the ZT values of SiNRs, we introduce central and edge cavities in ZSiNRs and ASiNRs respectively, as shown in Fig. 1. Following a common convention, we refer to the SiNRs with N dimer lines in width as N-SiNRs. The system is composed of a central junction of length L and width W and two semi-infinite ideal leads of the same width. The central junction is formed by removing hexagonal carbon rings from perfect SiNRs, in which the numbers of hexagons along the transversal and

Results and discussions

We first calculate the thermoelectric properties of perfect ZSiNRs and ASiNRs, as shown in Fig. 2. The phononic thermal conductance of both ZSiNRs and ASiNRs increases linearly with increasing the width N in Fig. 2(a), which is due to the increase in number of effective phononic channels. Comparing with the thermal conductivity of graphene [34], this result implies that silicene devices should have higher thermoelectric performance. In Fig. 2(b), the phononic thermal conductance of SiNRs raises

Conclusions

In summary, by using NEGF method, we have investigated numerically the the thermoelectric performance for both ZSiNRs and ASiNRs with central or edge defects within tight-binding approximation. For perfect SiNRs, it is shown that with its width increasing, ZT_M decreases monotonously while the phononic thermal conductance increases linearly. For various types of edges and defects, with increasing defect numbers in longitudinal direction, ZT_M increases monotonously while the phononic thermal

Acknowledgments

This work is supported by National Natural Science Foundation of China (Grant no. 11204259 and No. 11374252), and partially by SPCSIRT (IRT1080).

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Enhanced thermoelectric performance of defected silicene nanoribbons

Highlights

Abstract

Introduction

Section snippets

Model and method

Results and discussions

Conclusions

Acknowledgments

NPG Asia Mater.

Phys. Lett. A

Superlattices Microstruct.

Adv. Mater.

Nat. Mater.

Nature

Nature

Nano Lett.

Phys. Rev. B

Phys. Rev. Lett.

Nano Lett.

Nano Lett.

Nano Lett.

J. Heat. Transf.

Nature

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. Lett.

Phys. Rev. B