Theoretical Study On Transport Properties Of Titanium Trisulfide And Other Two-dimensional Materials

Since the discovery of graphene,two-dimensional（2D）layered materials have at-tracted widespread interest as a class of materials that may have great influence on future electronic industry due to their novel electrical,thermoelectric,mechanical and optical properties.Very recently,single-layer titanium trisulfide（TiS₃）,a new 2D material,has been successfully fabricated by the exfoliation of its layered bulk phase down to a single layer.In contrast to the indirect optical band gap in its bulk counterpart,the single-layer TiS₃has a direct band gap of about 1 eV as predicted by hybrid density functionals cal-culations,manifesting itself as a possible candidate for future electronic applications.We perform first-principles calculations to investigate contact,thermoelectric and electronic characteristics of single-layer TiS₃and some other two-dimensional materials.Generally speaking,the quality of electrical contact is as important as the semicon-ductor itself to the performance of the entire devices,which becomes particularly crucial in the 2D cases because of the large contact resistance at the interface between the 2D semiconductor and any three-dimensional（3D）metal electrodes.This significantly re-strains the efficiency of injected charge carriers to the 2D semiconductors.To this end,a great quantity of theoretical and experimental studies have been devoted to the optimal design of the metal-2D semiconductor interface,in order to find the best candidate with the excellent Ohmic contact.Much less attention,however,has been paid to preliminarily judge whether a contact is Schottky or Ohmic type prior to the intensive first-principles calculations on each combination of metal electrodes and 2D semiconductors.In this work,we propose a simple and convenient criteria to distinguish between Schottky and Ohmic contacts according to the separation between metal electrode and 2D semicon-ductor,based on our first-principles calculations about several typical 2D semiconductors adsorbed on Au（111）,Ag（111）,Al（111）,Cu（111）and Sc（111）.By taking TiS₃as an example,we first illustrate the interfacial properties in detail,including the geometry,bonding,electronic structure,charge transfer and local potential.We observe the Ohmic contact only at TiS₃/Sc（111）interface,originating from the strong hybridization between TiS₃and Sc（111）.In comparison with calculated results of other five typical 2D semi-conductors（namely,graphene,MoS₂,WS₂,MoSe₂and black phosphorene）,we reveal an interesting dependence of the contact type on the separation d between metal and 2D semiconductor,which exhibits a transition from Schottky contact to Ohmic contact at around d=2.3?.In thermoelectric materials applications,the enhancement of figure of merit（ZT）of thermoelectric materials is significantly important.Low dimensional materials have attracted great attention as a new class of materials with high ZT.Using first-principles calculations method,we intensively investigate the thermoelectric characteristics of the monolayer TiS₃and 1D（one-dimensional）TiS₃nanoribbon.We find that the thermoelec-tric figure of merit of TiS₃is dependent of doping concentration,direction and tempera-ture.The maximum thermoelectric figure of merit and Seebeck coefficient are obtained around valance band maximum（VBM）and conduct band minimum（CBM）due to the drastic change of density of states（DOS）there.In order to further study the change of the thermoelectric properties induced by quantum confinement effect,we calculate the thermoelectric properties of one-dimensional TiS₃nanoribbon.we find that the figure of merit of TiS₃nanoribbon is significantly enhanced when compared with that of the monolayer TiS₃.High mobility 2D semiconductor have attracted widespread interest for future high performance metal-oxide-semiconductor field-effect transistors,considered as candidates to replace strained-Si.Meanwhile,strain engineering is considered as one of the most promising technologies to improve the mobility of 2D semiconductor.In this paper,we use first-principles calculations method to intensively investigate the mobility properties of the monolayer TiS₃and 1D（one-dimensional）TiS₃nanoribbon.The electron and hole mobility of monolayer TiS₃are 21801 and 94.432 cm²/V·s,respectively,while the values for 1D TiS₃nanoribbon are much smaller（383.75 and 20.23 cm²/V·s for electron and hole,respectively）.To simulate the compressive strain applied on the 2D semiconductor,the lattice constants in corresponding directions are reduced by the same amount.We find that the hole mobility of monolayer TiS₃increases while the electron mobility decreases after inducing 1.0%and 2.0%compressive strains.