1.School of Basic Medicine, Changsha Medical University, Changsha 410219, China 2.Hunan Key Laboratory Cultivation Base of Research and Development of Novel Pharmaceutical Preparations, Changsha Medical University, Changsha 410219, China 3.School of Science, Hubei University of Technology, Wuhan 430068, China
Fund Project:Project supported by the Research Foundation of Education Bureau of Hunan Province, China (Grant Nos. 17A024, 17B034), the Hunan Key Laboratory Cultivation Base of Research and Development of Novel Pharmaceutical Preparations, China (Grant No. 2016TP1029), and the Construct Program of the Key Discipline in Hunan Province, the Training Program for Excellent Young Innovators of Changsha, China (Grant No. kq1802024).
Received Date:04 January 2019
Accepted Date:10 March 2019
Available Online:01 May 2019
Published Online:05 May 2019
Abstract:Two-dimensional (2D) materials exhibit massive potential in research and development in the scientific world due to their unique electrical, optical, thermal and mechanical properties. Graphene is an earliest found two-dimensional material, which has many excellent properties, such as high carrier mobility and large surface area. However, single layer graphene has a zero band gap, which limits its response in electronic devices. Unlike graphene, the transition metal sulfides (TMDs) have various band structures and chemical compositions, which greatly compensate for the defect of zero gap in graphene. The WS2 is one of the 2D TMDs exhibiting a series of unique properties, such as strong spin-orbit coupling, band splitting and high nonlinear susceptibility, which make it possess many applications in semiconducting optoelectronics and micro/nano-electronics. The 2D semiconductors along with semimetallic graphene are seen as basic building blocks for a new generation of nanoelectronic devices. In this way, the artificially designed TMD heterostructure is a promising option for ultrathin photodetectors. There are few reports on the physical mechanism of carrier mobility and charge distribution at the interface of WS2/graphene heterostructure, by varying the interfacial distance of WS2/graphene heterostructure to investigate the effect on the electronic properties. Here in this work, the corresponding effects of interface cohesive interaction and electronic properties of WS2/graphene heterostructure are studied by first-principles method. The calculation results indicate that the lattice mismatch between monolayer WS2 and graphene is low, the equilibrium layer distance d of about 3.42 ? for the WS2/graphene heterostructure and a weak van der Waals interaction forms in interface. Further, by analyzing the energy band structures and the three-dimensional charge density difference of WS2/graphene, we can identify that at the interface of the WS2 layer there appears an obvious electron accumulation: positive charges are accumulated near to the graphene layer, showing that WS2 is an n-type semiconductor due to the combination with graphene. Furthermore, the total density of states and corresponding partial density of states of WS2/graphene heterostructure are investigated, and the results show that the valence band is composed of hybrid orbitals of W 5d and C 2p, whereas the conduction band is comprised of W 5d and S 3p orbitals, the orbital hybridization between W 5d and S 3p will cause photogenerated electrons to transfer easily from the internal W atoms to the external S atoms, thereby forming a build-in internal electric field from graphene to WS2. Finally, by varying the interfacial distance for analyzing the Schottky barrier transition, as the interfacial distance is changed greatly from 2.4 ? to 4.2 ?, the shape of the band changes slightly, however, the Fermi level descends relatively gradually, which can achieve the transition from a p-type Schottky contact to an n-type Schottky contact in the WS2/graphene. The plane-averaged charge density difference proves that the interfacial charge transfer and the Fermi level shift are the reasons for determining the Schottky barrier transition in the WS2/graphene heterostructure. Our studies may prove to be instrumental in the future design and fabrication of van der Waals based field effect transistors. Keywords:heterostructure/ band structure/ Schottky contact/ first-principles
异质结的界面相互作用会直接影响电子性质, 因此计算了异质结能带结构, 图2分别为单层二硫化钨、石墨烯和二硫化钨/石墨烯异质结的能带结构. 图 2 单层二硫化钨(a)、石墨烯(b)和二硫化钨/石墨烯异质结(c)的能带结构, 其中费米能级处在0 eV, 用红色的虚线表示 Figure2. Energy band structures of (a) WS2 monolayer, (b) graphene and (c) WS2/graphene heterostructure. The Fermi levels are set to zero and marked by red dashed lines.
从图2可以看出, 单层的二硫化钨为直接带隙, 其价带顶和导带底均位于G点, 带隙宽度为1.76 eV, 这与Ding等[38]计算的结果相差很小. 计算表明石墨烯的${\text{π}}$带(bonding)和${\text{π}}$*带(antibonding)在K点即所谓的狄拉克点处交叉, 这表明石墨烯是一种无间隙的半导体, 并保持其金属特性, 这与先前的理论研究一致[39]. 形成异质结后, 其能带结构整体形状基本上为单层石墨烯和单层二硫化钨能带结构的简单叠加, 而异质结的电子结构在很大程度上也保留了石墨烯层和二硫化钨层各自独立的电子结构, 其中石墨烯层的狄拉克点依然处于费米能级处, 基本没有发生移动. 这说明石墨烯导带是满带, 相比单层的二硫化钨, 异质结中的二硫化钨, 其导带底和价带顶依然位于G点, 只是导带底下移, 价带顶上移, 费米能级也由原来靠近价带顶转化为靠近导带底附近, 从而使二硫化钨层显示出典型的n型半导体特征. 此外, 我们计算了异质结在层间平衡距离为3.42 ?时的总态密度及分态密度. 如图3所示, 靠近费米能级异质结的价带顶主要是由W 5d和C 2p轨道组成, 而导带底则是由W 5d和S 3p轨道占据, 并且费米能级向导带边靠近, 从而形成以电子导体为主的半导体. 根据以上分析, 异质结之间的接触是n型半导体, 这和能带分析结果一致. 从图3还发现导带中W 5d 和S 3p轨道存在能级重叠现象, 说明两者之间形成了杂化轨道, 从而有利于处于激发态的电子从W 5d轨道向S 3p轨道跃迁, 使更多的电子趋向界面的S原子层. 图 3 二硫化钨/石墨烯异质结的总态密度以及相应的分态密度 Figure3. Calculated total density of states (TDOS) and the corresponding partial density of states (PDOS) of WS2/graphene heterostructure.
由上述得知异质结的形成会改变二硫化钨和石墨烯之间的电荷转移, 为了进一步阐明二硫化钨/石墨烯层间电荷转移具体情况, 计算了在平衡距离时异质结的三维电荷密度差分, 如图4所示. 图 4 二硫化钨/石墨烯异质结的三维电子密度差分图 (a)侧视图; (b)顶视图 Figure4. Three-dimensional charge density difference plots WS2/graphene heterostructure: (a) Side view; (b) top view.
在实验和理论上界面的电荷转移、肖特基势垒以及功函数可以通过电场调控、应力调控、掺杂、空位、表面修饰等[40-43]来调控, 而事实上, 二维范德瓦耳斯异质结的界面距离对其电子性质有着至关重要的影响. 因此, 可以通过改变界面的距离来调控二硫化钨/石墨烯的异质结对电子性质和肖特基势垒类型的影响. 本文中选取层间距2.4 ?到4.2 ?, 分析了异质结能带的变化. 如图5, 当层间距由2.4 ?增加到3.4 ?时, 异质结能带的整体形状没发生太大变化. 这说明碳原子未成键的p电子仍然处于大${\text{π}}$键内, 并以费米速度运动[44]. 此时, WS2对石墨烯的导电类型没有影响, 但导带底和价带顶的位置却发生了变化, 即n型和p型肖特基势垒的大小. 很明显, 层间距为2.4 ?时的肖特基接触为p型, 当界面距离从2.4 ?到3.4 ?变化时, 导带底的数值逐渐减少, 向费米能级靠近, 异质结的肖特基势垒逐渐转变为n型. 当层间距增加到3.4 ?时, 导带底的数值几乎为0, 而价带顶的数值逐渐增大, 离费米能级越远, 则表明异质结之间肖特基接触为n型, 且肖特基势垒大小也逐渐减少. 而当层间距由3.6 ?增加到4.2 ?时, 导带底和价带顶的数值基本没发生变化, 而此种情况下肖特基势垒的大小也基本没变, 但费米能级位置相对于层间距为3.4 ?时都明显下移, 使石墨烯狄拉克点位于其上方, 此时界面的肖特基势垒接触类型依然保持为n型. 因此, 界面距离不仅可以调节肖特基势垒的大小, 还可以实现从p型肖特基接触到n型肖特基接触的转变. 图 5 不同层间距下的二硫化钨/石墨烯异质结的能带图, 其中蓝色曲线代表石墨烯部分的贡献 (a)?(j)分别代表层间距为2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2 ?, 费米能级处在0 eV, 用红色虚线表示 Figure5. Band structures of WS2/graphene heterostructure under different interface distances. Blue curves denote the contributions from graphene. Panels (a)?(j) correspond to the interface distances of 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2 ?, respectively. The Fermi level is set to zero and marked by red dotted line.
为了定量描述不同层间距下界面的肖特基势垒高度和肖特基类型, 给出了不同层间距下二硫化钨部分的导带底、价带顶和带隙的数据图, 结果如图6所示. 在不同层间距作用下, 二硫化钨/石墨烯异质结中二硫化钨带隙基本不变. 当层间距为2.4 ?时, 界面的p肖特基势垒高度为0.361 eV; 当层间距为2.6, 2.8, 3.0, 3.2, 3.4 ?时, 界面的n肖特基势垒高度分别为0.638, 0.489, 0.327, 0.189, 0.084 eV; 而当层间距由3.4 ?增加到3.6 ?时, 由于出现了费米能级钉扎效应, 使得层间距为3.6, 3.8, 4.0, 4.2 ?时, 其肖特基势垒高度不变, 均为0.524 eV, 界面电荷转移几乎为零, 此时肖特基接触仍为n型. 图 6 二硫化钨/石墨烯异质结中二硫化钨部分的导带底、价带顶和带隙在不同层间距的值 Figure6. Conduction band minimum (CBM), valence band maximum (VBM) and band gap of WS2 monolayer in the WS2/graphene heterostructure as a function of interfacial distance.