Two-dimensional (2D) layered materials have received considerable attention because of their potential applications in various fields since the experimental discovery of graphene. Two-dimensional elementary semiconductors are particularly desired owing to their superior features in terms of fabrication, purification and doping. Few-layer black phosphorus (BP) is the first 2D mono-elementary semiconductor showing high electronic carrier mobility, strong optical absorption, linear dichroism, promising figure of merit and high tunability with external fields. All the properties immediately put few-layer BP under spotlights since its discovery in 2014. However, the flawed air-stability and difficulties in largescale fabrication were found to be two remaining issues that obscure practical applications of few-layer BP in the industry. People thus get started to question if a promising or even superior successor of BP exists. The target semiconductor is preferably an elementary one. It should also allow low-cost and large-scale synthesis and offer good environmental stability without sacrificing those striking properties offered by BP.
Professor Wei Ji has led a research group at Renmin University of China to theoretically model surfaces and interfaces of emerging electronic materials and to predict physical properties of devices comprised of these materials. Recently, they collaborated with Prof. Yang Chai from the Hong Kong Polytechnic University to report a theoretical study of a novel chain-like 2D-material, namely few-layer α-Tellurium (FL-α-Te), and predicted this material to have extremely high carrier mobility with a layer-tunable bandgap, strong light absorption, mixing of vibrational modes, layer-dependent energy maps of valence and conduction bands and among other striking properties.
The FL-α-Te is a representative material of layered one-dimensional materials, which are a novel and fast developing category of 2D-materials. They first examined the stability of three likely few-layer phases using state-of-the-art density functional theory calculations. Their calculation shows α-Tellurium is the most stable phase for bilayer and thicker layers. Given the stability unveiled, they found that a covalent-like quasi-bonding (CLQB) dominates the inter-chain interaction in both intra- and inter-layer directions. This CLQB is in analogy to the found interlayer interactions in BP, PtS2 or PtSe2, in which it shows wavefunction hybridization but does not provide extra energy gain. They managed to correlate this bonding with the layer-dependent geometric and electronic structures and their resulting behaviors in terms of electric, optical, and vibrational properties. Few-layer α-Te has extremely high hole-mobility up 105 cm2/Vs exceptionally along the non-covalent-bound (CLQB) direction and 103 cm2/Vs for the covalent-bound direction, tunable bandgap from 0.31 eV (Bulk) to 1.17 eV (2L), anisotropic inter-chain (-layer) vibrational behaviors, a crossover of interlayer shear and breathing force constants, large ideal strength (over 20%) and nearly isotropic strong light absorption (up to 9% per layer) from a highly anisotropic geometry. They also found, specifically in few-layer α-Te, that the energy surfaces of both valence and conduction bands substantially develop from bulk to bilayer, exhibiting an ``M-like" line profile of the hole pocket, which was usually found in topological insulators and is ideal for thermoelectrics.
This material succeeds most of the striking properties of BP and additionally offers a better environmental stability, a much lower fabrication cost (with wet-chemistry methods) and a stronger light absorption than those of BP. In this scenario, FL-α-Te could be regarded as a superior successor of BP. The extraordinarily high carrier mobility revealed in the CLQB direction conceptually updates the understanding of the role of non-covalent interactions in carrier mobility and may open a new avenue for seeking high carrier mobility materials.
This research was supported by the National Natural Science Foundation of China (Grant Nos. 11274380, 91433103, 11622437, 61674171, and 61761166009), the Fundamental Research Funds for the Central Universities of China and the Research Funds of Renmin University of China (Grant No. 16XNLQ01), and The Hong Kong Polytechnic University (Grant Nos. G-SB53). Two of the authors, J.Q. and C.W. were supported by the Outstanding Innovative Talents Cultivation Funded Programs 2016 and 2017 of Renmin University of China, respectively.
See the article: Jingsi Qiao, Yuhao Pan, Feng Yang, Cong Wang, Yang Chai and Wei Ji, Few-layer Tellurium: one-dimensional-like layered elementary semiconductor with striking physical properties, Science Bulletin, 2018, Vol.63, No. 3: 159-168.