Physics Department, University of California, Davis, USA.
Materials Science Division, Argonne National Laboratory, Lemont, IL, 60439, USA.
Sci Rep. 2017 Aug 1;7(1):7071. doi: 10.1038/s41598-017-06497-1.
Progress has been rapid in increasing the efficiency of energy conversion in nanoparticles. However, extraction of the photo-generated charge carriers remains challenging. Encouragingly, the charge mobility has been improved recently by driving nanoparticle (NP) films across the metal-insulator transition (MIT). To simulate MIT in NP films, we developed a hierarchical Kinetic Monte Carlo transport model. Electrons transfer between neighboring NPs via activated hopping when the NP energies differ by more than an overlap energy, but transfer by a non-activated quantum delocalization, if the NP energies are closer than the overlap energy. As the overlap energy increases, emerging percolating clusters support a metallic transport across the entire film. We simulated the evolution of the temperature-dependent electron mobility. We analyzed our data in terms of two candidate models of the MIT: (a) as a Quantum Critical Transition, signaled by an effective gap going to zero; and (b) as a Quantum Percolation Transition, where a sample-spanning metallic percolation path is formed as the fraction of the hopping bonds in the transport paths is going to zero. We found that the Quantum Percolation Transition theory provides a better description of the MIT. We also observed an anomalously low gap region next to the MIT. We discuss the relevance of our results in the light of recent experimental measurements.
在提高纳米粒子能量转换效率方面已经取得了快速进展。然而，提取光生电荷载流子仍然具有挑战性。令人鼓舞的是，最近通过驱动纳米粒子（NP）薄膜穿过金属-绝缘体转变（MIT），电荷迁移率得到了提高。为了在 NP 薄膜中模拟 MIT，我们开发了一个分层的动力学蒙特卡罗输运模型。当 NP 能量差超过重叠能时，电子通过激活跳跃在相邻 NP 之间转移，但如果 NP 能量小于重叠能，则通过非激活量子离域化转移。随着重叠能的增加，新兴的渗透簇支持整个薄膜的金属输运。我们模拟了温度依赖的电子迁移率的演化。我们根据 MIT 的两个候选模型（a）作为有效能隙趋近于零的量子临界点；和（b）作为量子渗流转变，其中随着输运路径中跳跃键的分数趋近于零，形成了跨越样品的金属渗流路径，对数据进行了分析。我们发现，量子渗流转变理论提供了对 MIT 的更好描述。我们还在 MIT 附近观察到一个异常低的能隙区域。我们根据最近的实验测量结果讨论了我们结果的相关性。
