Silicon based low-dimensional materials receive significant attention as new generation thermoelectric materials after they have demonstrated record low thermal conductivities. Very few works to-date, however, report significant advances with regards to the power factor. In this review we examine possibilities of power factor enhancement in: (i) low-dimensional Si channels and (ii) nanocrystalline Si materials. For low-dimensional channels we use atomistic simulations and consider ultra-narrow Si nanowires and ultra-thin Si layers of feature sizes below 15 nm. Room temperature is exclusively considered. We show that, in general, low-dimensionality does not offer possibilities for power factor improvement, because although the Seebeck coefficient could slightly increase, the conductivity inevitably degrades at a much larger extend. The power factor in these channels, however, can be optimized by proper choice of geometrical parameters such as the transport orientation, confinement orientation, and confinement length scale. Our simulations show that in the case where room temperature thermal conductivities as low as κ l = 2 W/mK are achieved, the ZT figure of merit of an optimized Si low-dimensional channel could reach values around unity. For the second case of materials, we show that by making effective use of energy filtering, and taking advantage of the inhomogeneity within the nanocrystalline geometry, the underlying potential profile and dopant distribution large improvements in the thermoelectric power factor can be achieved. The paper is intended to be a review of the main findings with regards to the thermoelectric performance of nanoscale Si through our simulation work as well as through recent experimental observations.
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机译:硅基低尺寸材料在表现出创纪录的低热导率之后,作为新一代热电材料受到了广泛的关注。迄今为止,很少有作品报道了功率因数方面的重大进步。在这篇综述中,我们研究了在以下方面提高功率因数的可能性:(i)低维Si沟道和(ii)纳米晶Si材料。对于低维通道,我们使用原子模拟,并考虑特征尺寸低于15 nm的超窄硅纳米线和超薄硅层。仅考虑室温。我们表明,通常,低维数不提供改善功率因数的可能性,因为尽管塞贝克系数可能会略有增加,但电导率不可避免地会在更大范围内降低。但是,可以通过适当选择几何参数(例如运输方向,限制方向和限制长度比例)来优化这些通道中的功率因数。我们的仿真表明,在达到κl = 2 W / mK的室温热导率的情况下,优化的Si低维沟道的ZT品质因数可以达到大约1的值。对于材料的第二种情况,我们表明,通过有效利用能量过滤,并利用纳米晶体几何结构内的不均匀性,可以实现潜在的电势分布和掺杂剂分布,从而大大提高热电功率因数。本文旨在通过我们的模拟工作以及最近的实验观察,对有关纳米Si热电性能的主要发现进行综述。
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