Controllable doping and anisotropic properties of two-dimensional materials

Abstract

Two-dimensional (2D) materials are novel materials which have emerged during the last decade. Different from traditional three-dimensional (3D) materials, 2D materials can be thinned down to a single layer without surface dangling bonds. Because the interlayer interactions are weak van der Waals interactions. Due to their unique characteristics, 2D materials have applications in versatile electrical and opto-electrical devices. Novel physical phenomenon arising from 2D materials are interesting research topics. MoS₂ is one type of 2D material which has been demonstrated to have broad applications in the future. Doping ultrathin MoS₂ by traditional ion-implantation is explored. A thin layer of Poly(methyl methacrylate (PMMA) is applied as a sacrificial layer to decelerate the dopant ions. Raman and transmission electron microscope (TEM) characterizations show that the PMMA layer protects the ultrathin MoS₂ flakes from being severely damaged by high-energy dopant ions. The p-doping effect of incorporated phosphorus ions on ultrathin MoS₂ have been demonstrated by photoluminescence (PL) and electrical characterizations. The doping effect can be tuned by controlling the thickness of MoS₂ flake and the thickness of PMMA layer. Transitional metal oxides (TMO) are used to dope ultrathin MoS₂ by surface charge transfer. The oxide layers are deposited onto MoS₂ flakes by CMOS-compatible physical vapor deposition (PVD) methods. The modulation doping process introduces negligible damage of the MoS₂ lattice. PL and electrical characterizations are performed to quantify the doping effects. TiO₂ and MoO₃ are found to have n-and p-doping effects on MoS₂, respectively. The doping level is comparable with other doping methods. The doping effect is dependent on the thickness of MoS₂. AuSe is one type of material stacked by atomic chains with inter-chain van der Waals interactions. AuSe can be thinned down to the thickness of 2D atomic sheets like other 2D materials. The optical and electrical characterizations of α-AuSe are performed. AuSe is found to have strong anisotropic phonon vibrations by in-plane polarized Raman. Electrical characterizations of field-effect-transistor (FET) reveal the extraordinary high conductivity (σ2D = 0.01 S) of carriers in AuSe. The metallic transport behavior of bulk AuSe is demonstrated by four-point resistivity measurement under low-temperature. Theoretical calculations of AuSe are performed. AuSe atomic chain is calculated to a semiconductor with an indirect bandgap of 1.26 eV while bulk AuSe is calculated to have no bandgap. Further experiments and calculations are required to fully understand the unique properties of AuSe.