1.School of Physics and Optoelectronics, Xiangtan University, Xiangtan 411105, China 2.Hunan Key Laboratory for Micro-Nano Energy Materials and Device, Xiangtan University, Xiangtan 411105, China
Fund Project:Project supported by the Research Foundation of Education Bureau of Hunan Province, China (Grant No. 17B252)
Received Date:22 August 2019
Accepted Date:26 September 2019
Available Online:27 November 2019
Published Online:01 December 2019
Abstract:Thermoelectric materials, which can convert heat energy into electric energy and also from electric energy into heat energy, have aroused widespread interest of both theoretical and technological researches recently. Graphene is a typical two-dimensional carbon nanomaterial and regarded as a competitive candidate for the next-generation micro/nano-devices. Unfortunately, graphene is an inefficient thermoelectric material due to the extremely high thermal conductivity. To overcome this drawback, exploring an effective way to improve the thermoelectric performance is of critical importance. In this paper, using the nonequilibrium Green’s function approach, we systematically investigate the effects of grain boundary on the thermoelectric properties of graphene nanoribbons. The results show that owing to the existence of grain boundary, the phonons and electrons encounter great scatterings when they transmit through the polycrystalline graphene nanoribbons. These scatterings cause the phononic and electronic transmission coefficient to decrease dramatically, and thus leading the thermal conductance (including both electron and phonon parts) of graphene nanoribbons to be evidently suppressed. Meanwhile, such scatterings induce more intense transmission peaks and pits in the electronic transmission spectrum of polycrystalline graphene nanoribbons. Generally, the Seebeck coefficient depends on the derivative of electronic transmission coefficient. The larger the logarithmic derivative of transmission, the higher the Seebeck coefficient can be obtained. Therefore Seebeck coefficient is improved obviously in the polycrystalline graphene nanoribbons. Based on such two positive effects, the thermoelectric performance of polycrystalline graphene nanoribbons is significantly enhanced. At room temperature, the thermoelectric figure of merit of polycrystalline graphene nanoribbons can approach to 0.3, which is about 6 times larger than that of pristine graphene nanoribbon (figure of merit is about 0.05). It is also found that the quantity of grain boundaries and length of system can further improve the thermoelectric properties of the polycrystalline graphene nanoribbons, while the width of system has a limited influence on it. This is because the quantity of grain boundaries and length of polycrystalline graphene nanoribbons can give rise to more intense phonon and electron scatterings and further decreasing of thermal conductance and enhancement of Seebeck coefficient. The results presented in this paper demonstrate that polycrystalline structure is indeed an effective way to improve the thermoelectric conversion efficiency of graphene nanoribbons, and provide a theoretical guideline for designing and preparing thermoelectric devices based on graphene nanoribbons. Keywords:polycrystalline graphene/ thermoelectric properties/ nonequilibrium Green’s function
${T_{\rm{e}}}\left( E \right) = {T_{\rm{r}}}{\rm{[}}{G^{\rm{r}}}\left( E \right){{{\varGamma }}_{\rm{L}}}{G^{\rm{a}}}\left( E \right){\varGamma _{\rm{R}}}{\rm{]}},$
结合声子和电子输运性质的计算结果, 可以根据(11)式获得多晶石墨烯纳米带的热电品质因子, 如图4所示. 在室温下完美石墨烯纳米带的热电品质因子约为0.049, 这与先前的理论计算预测基本一致. 当结构中引入晶界后, 可以看到热电品质因子得到了显著的提升. 在室温下, 晶界的引入可将完美石墨烯纳米带的品质因子提高6倍左右 (N = 7). 然而, 从图4还可以发现温度对于提升多晶石墨烯纳米带的热电转换效率并没有明显的积极作用, 这与先前对石墨炔热电性质的研究略有不同[34]. 随着晶界数目的增加, 多晶石墨烯纳米带热电性能还能得到进一步的增强. 该结果表明多晶化的确是提升石墨烯纳米带热电性质的有效途径. 最后研究了系统的结构尺寸对于多晶石墨烯纳米带热电性能的影响. 如图5(a)所示, 在固定晶界数目前提下, 增加石墨烯纳米带的长度会进一步提高石墨烯纳米带的热电性能. 这主要是由于单晶石墨烯晶粒长度的增加会使得低频(长波长)声子受到更多的散射. 该行为将进一步削弱多晶石墨烯纳米带的热导, 进而增强其热电转换效率. 对于宽度效应, 从图5(b)中可以发现多晶石墨烯纳米带的热电品质因子会随着宽度的增加而逐渐下降. 这主要归因于较宽的完美石墨烯纳米带本征热电性能低下所引起的. 但是值得一提的是, 多晶石墨烯热电性能提升的倍率并未随着宽度的增加而显著下降. 综上所述, 为了获得更为优异的热电性能, 应该选取较长的且宽度适中的多晶石墨烯纳米带. 图 4 完美和多晶石墨烯纳米带的热电品质因子随温度的变化 Figure4. Peak values of ZT as a function of temperature for perfect graphene nanoribbons and polycrystalline graphene nanoribbons.
图 5 室温下(300 K), 完美与多晶石墨烯纳米带(N = 5)热电品质因子随系统(a)长度L和(b)宽度W的变化(阴影部分为标准偏差) Figure5. Peak values of ZT of perfect and polycrystalline graphene nanoribbons (N = 5) at room temperature as a function of (a) nanoribbon length or (b) nanoribbon width. The shading part corresponds to the standard deviation.