(Invited) Microscopic Studies of Black Phosphorus and Its Field-Effect Transistors

An increasing number of alternative two-dimensional (2D) materials are being explored in the “post-graphene age”, such as transition metal dichalcogenides (TMDs), silicene, and germanene. These materials are studied with the hope of overcoming graphene’s deficiencies such as its zero bandgap. Among these 2D layered materials, layered black phosphorus (phosphorene) is expected to exhibit superior mechanical, electrical, and optical properties due to an intrinsic and tunable bandgap. It is, besides graphene, the only stable elemental 2D material that can be mechanically exfoliated. Its high hole mobility and direct semiconducting bandgap hold the hope for the development of new electronic devices in the postsilicon era. However, before phosphorene can be integrated in everyday electronics, many aspects of the material still remain to be elucidated. Here we report atomic scale studies of electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (x) and heavy (y) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction and the related edge states are also determined. Besides, we also investigate electrical transport and optoelectronic properties of field effect transistors (FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements. Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed to the photovoltaic effect in the off-state and photo-thermoelectric effect in the on-state, and their anisotropic feature primarily results from the directional-dependent absorption of BP crystals.

1. Liangbo Liang, Jun Wang, Bobby G. Sumpter, Vincent Meunier, and Minghu Pan, “Electronic Bandgap and Edge Reconstruction in Phosphorene Materials”, Nano Letters, 2014, 14 (11), 6400–6406

2. Tu Hong, Bhim Chamlagain, Wenzhi Lin, Hsun-Jen Chuang, Minghu Pan, Zhixian Zhou, and Ya-Qiong Xu, “Polarized Photocurrent Response in Black Phosphorus Field-Effect Transistors”, Nanoscale, 6, 8978-8983(2014).