School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
Nano Lett. 2012 Feb 8;12(2):1087-91. doi: 10.1021/nl204460y. Epub 2012 Jan 27.
Unlike their bulk counterpart, nanoparticles often show spontaneous fluctuations in their crystal structure at constant temperature [Iijima, S.; Ichihashi T. Phys. Rev. Lett.1985, 56, 616; Ajayan, P. M.; Marks L. D. Phys. Rev. Lett.1988, 60, 585; Ben-David, T.; Lereah, Y.; Deutscher, G.; Penisson, J. M.; Bourret, A.; Korman, R.; Cheyssac, P. Phys. Rev. Lett.1997, 78, 2585]. This phenomenon takes place whenever the net gain in the surface energy of the particles outweighs the energy cost of internal strain. The configurational space is then densely populated due to shallow free-energy barriers between structural local minima. Here we report that in the case of bismuth (Bi) nanoparticles (BiNPs), given the high anisotropy of the mass tensor of their charge carriers, structural fluctuations result in substantial dynamic changes in their electronic and conductance properties. Transmission electron microscopy is used to probe the stochastic dynamic structural fluctuations of selected BiNPs. The related fluctuations in the electronic band structure and conductance properties are studied by scanning tunneling spectroscopy and are shown to be temperature dependent. Continuous probing of the conductance of individual BiNPs reveals corresponding dynamic fluctuations (as high as 1 eV) in their apparent band gap. At 80 K, upon freezing of structural fluctuations, conductance anisotropy in BiNPs is detected as band gap variations as a function of tip position above individual particles. BiNPs offer a unique system to explore anisotropy in zero-dimension conductors as well as the dynamic nature of nanoparticles.
与大块物质不同，纳米粒子在恒温下其晶体结构常常会自发地发生波动[Iijima, S.; Ichihashi T. Phys. Rev. Lett.1985, 56, 616; Ajayan, P. M.; Marks L. D. Phys. Rev. Lett.1988, 60, 585; Ben-David, T.; Lereah, Y.; Deutscher, G.; Penisson, J. M.; Bourret, A.; Korman, R.; Cheyssac, P. Phys. Rev. Lett.1997, 78, 2585]。只要粒子表面能的净收益超过内部应变的能量成本，这种现象就会发生。由于结构局部最小值之间的自由能势垒较浅，配置空间就会充满大量结构。在这里，我们报告在铋（Bi）纳米粒子（BiNPs）的情况下，由于其载流子质量张量的各向异性，结构波动会导致其电子和电导性质发生实质性的动态变化。透射电子显微镜用于探测选定的 BiNPs 的随机动态结构波动。通过扫描隧道光谱法研究了相关的电子能带结构和电导性质波动，发现它们随温度而变化。对单个 BiNPs 电导的连续探测揭示了其明显带隙中相应的动态波动（高达 1 eV）。在 80 K 时，随着结构波动的冻结，在单个粒子上方的针尖位置作为函数检测到 BiNPs 中的电导各向异性，表现为能带隙变化。BiNPs 为探索零维导体的各向异性以及纳米粒子的动态性质提供了一个独特的系统。
