Quantum Technology Centre, Lancaster University , LA1 4YB Lancaster, United Kingdom.
Nano Lett. 2015 Nov 11;15(11):7467-72. doi: 10.1021/acs.nanolett.5b03033. Epub 2015 Oct 15.
Understanding phonon transport at a molecular scale is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity. We have studied phonon and electron transport in alkane and oligoyne chains of various lengths and find that, due to the more rigid nature of the latter, the phonon thermal conductances of oligoynes are counterintuitively lower than that of the corresponding alkanes. The thermal conductance of oligoynes decreases monotonically with increasing length, whereas the thermal conductance of alkanes initially increases with length and then decreases. This difference in behavior arises from phonon filtering by the gold electrodes and disappears when higher-Debye-frequency electrodes are used. Consequently a molecule that better transmits higher-frequency phonon modes, combined with a low-Debye-frequency electrode that filters high-energy phonons is a viable strategy for suppressing phonon transmission through the molecular junctions. The low thermal conductance of oligoynes, combined with their higher thermopower and higher electrical conductance lead to a maximum thermoelectric figure of merit of ZT = 1.4, which is several orders of magnitude higher than that of alkanes.
理解分子尺度上的声子输运对于开发高性能热电器件以将废热转化为电能至关重要。我们研究了不同长度的烷烃和寡炔链中的声子和电子输运，发现由于后者的刚性更高，寡炔的声子热导率出人意料地低于相应的烷烃。寡炔的热导率先随着长度的增加而单调下降，而烷烃的热导率先随着长度的增加而增加，然后再下降。这种行为上的差异源于金电极对声子的过滤，而当使用更高德拜频率的电极时，这种差异就会消失。因此，一种能够更好地传输更高频率声子模式的分子，结合一个能够过滤高能声子的低德拜频率电极，是抑制分子结中声子传输的一种可行策略。寡炔的低热导率，再加上其较高的热电势和较高的电导率，导致其最大热电优值 ZT = 1.4，比烷烃高出几个数量级。
