1.College of Science, Central South University of Forestry and Technology, Changsha 410004, China 2.School of Physics and Electronics, Hunan University, Changsha 410082, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11674090, 11347022, 11447224).
Received Date:27 August 2018
Accepted Date:05 December 2018
Available Online:01 February 2019
Published Online:05 February 2019
Abstract:Chemical functionalization of two-dimensional transition metal dichalcogenides (TMDs) with hydrogen is an effective and economical method to synthesize monolayer TMDs and tune their electronic properties. We theoretically study the stabilities and electronic properties of chemisorbed H atoms on monolayer TMDs by using density-functional theory calculations. The result shows that there exists a more stable adsorption site in the layers of the monolayer MX2 (M = Mo, W; X = S, Se, Te) than its surface for hydrogen. In the case of the same cation, with the increase of the anion (X2?) atomic number, the stronger the bonding between the H atom and the MX2 layer, the more stable the structure of the hydrogenated monolayer MX2 is. However, in the case of the same anion, the binding between the H atom and the MX2 layer becomes weaker as the atomic number of the cations increases. H atoms passes through one surface of the MS2 to the other surface with a relatively small diffusion barrier of about 0.9 eV. So the H atoms can more easily go through the barrier. And for the H atom to go through the other monolayer MX2 (M = Mo, W; X = Se, Te), the diffusion barrier is about 1.2 eV. H atoms are difficult to pass through the barrier at this time. The singular diffusion behavior of H atoms in monolayer MX2 is conducible to understanding the stability of hydrogenated two-dimensional transition metal sulfide system. In addition, the surface hydrogenation and interlaminar hydrogenation have different effects on the electronic properties of monolayer MX2, and mainly manifest themselves in the fact that the surface hydrogenation induces spontaneous magnetism and sharply reduces the band gap, but still retains the semiconductor properties of the original monolayer MX2. However, interlaminar hydrogenation enables monolayer MX2 to directly realize the transition from semiconductor to metal. Interlaminar hydrogenation monolayer MX2 (M = Mo, W; X = S, Se) make the system generating magnetism, while when the anion is Te2?, the magnetism almost disappears. These results can provide theoretical guidance in understanding hydrogen functionalization of MX2 layer, and also present a certain theoretical basis for realizing the application of MX2 in nano-electronic devices. Keywords:hydrogenation/ two-dimensional transition metal dichalcogenides/ stability/ electronic properties
其中Etot是H原子吸附在单层MX2上后的总能量; ${E_{M{X_2}}}$和${\mu _{\rm{H}}}$分别表示自由独立的单层MX2和单个H原子的能量. 采用CI-NEB (climbing image-nudged elastic band)[43]方法计算H原子穿过单层MX2的扩散能垒. 图 1 单层MX2的结构模型及可能的H吸附位点示意图, 其中蓝色, 橙色和粉色分别表示阴离子(X2?)、阳离子(M4+)和氢原子; A—F表示氢原子可能的吸附位点 Figure1. Structural model and possible H adsorption sites of single-layer MX2. Blue, orange and pink balls correspond to anions (X2?), cations (M4+) and hydrogen atoms, respectively. A?F shows a possible adsorption site for hydrogen atom.
表1氢原子吸附在单层MX2上的可能吸附位点(AS)的结合能, 及氢原子与阳离子(M4+ = Mo, W)的键长 Table1.Binding energy of possible adsorption sites (AS) of hydrogen atoms adsorbed on single-layer MX2, and M—H (M4+ = Mo, W) bond lengths.
在所考虑的吸附位点中, 选择出单层MX2(M = Mo, W; X = S, Se, Te)的最稳定的吸附位点, 这些最稳定吸附位点的Eb总结在图2中. 从图2中可以很直观地看出单层MX2稳定H吸附位点的变化规律. 由于氢原子显示出较大的电负性, 阴离子S, Se, Te的电负性(离子性)是依次减弱的, 共价键逐渐变强. 因此当同阳离子($ M^{4+}=$ Mo, W)时, 随着阴离子S2?, Se2?, Te2?原子序数的增加, 氢化单层MX2结构越稳定, 即H原子与MX2层的结合越强(MoS2 < MoSe2 < MoTe2, WS2 < WSe2 < WTe2). 而同阴离子时, 随着阳离子原子序数的增加, H原子与MX2层的结合越弱(MoS2 > WS2, MoSe2 > WSe2, MoTe2 > WTe2). 这是由于阳离子Mo4+和W4+的电负性强弱非常接近, 氢原子与阳离子Mo4+的键长小于与阳离子W4+的键长(如表1), 氢原子与阳离子的键长越短, 相互作用力越强, 结合越强, 结构越稳定. 但由于键长的差异不是很大, 导致结合能的差异也不是非常明显. 为了进一步解释氢原子稳定吸附在单层MX2层间的原因, 接下来计算了氢原子从MX2层一个表面(如图1中D点所示)穿过层间(图1中E点所示)扩散到另一个表面(图1中D'点所示)的势垒分布, 计算结果如图3所示. 从图3可以发现, 氢原子穿过MoS2层有一个相对较小的扩散势垒, 势垒大小约0.9 eV (< 1 eV), 穿过其他MX2层的扩散势垒都大于1.2 eV, 这和热质子在室温下穿过单层石墨烯的势垒0.8 eV是非常接近的. 我们知道, 当势垒超过1 eV时, 扩散就难以进行, 而1 eV以下的势垒扩散相对较容易发生. 因此, 氢原子穿过单层MS2仅有一个0.9 eV的扩散势垒, 只要在一定的条件下, 该扩散过程还是相对较容易进行的. 同时也观察到一个有趣的现象, 氢原子从表面D点扩散到D'点的过程中, 有一个比表面D(D')点更低的能量点, 该点正是图1中的E吸附位点. 此结果与结合能的计算结果是一致的. 图 3 氢原子穿过单层MX2的扩散势垒(扩散路径为从单层MX2表面D点穿过层间E点扩散到另一个表面D'点) Figure3. Diffusion barrier of hydrogen atoms through a single layer of MX2. The diffusion path is from the D point of surface through the interlayer E point to the other surface D' point.
电荷转移是衡量掺杂原子和被掺杂体系之间相互作用的一个标准. 掺杂原子与被掺杂体系之间电荷转移的越多, 电子得失程度越大, 说明掺杂原子与被掺杂体系之间的电子输送越大, 体系越稳定. 采用Bader电荷分析方法, 计算了氢原子稳定吸附在单层MX2的电荷转移情况(稳定吸附在图1中的E位点), 如图4所示. 氢原子吸附在单层MoS2, MoSe2和 MoTe2层间E位点时, 氢原子从单层MoS2, MoSe2和 MoTe2体系分别得到0.235, 0.267, 0.338个电子. 因此, 可以很容易知道, 同阳离子Mo4+时, 随着阴离子原子序数的增加, 由于阴离子电负性的逐渐减弱, 氢原子从该单层体系中得到的电子数也是逐渐增加. 类似的情况在单层WS2, WSe2和 WTe2中也可以见到, 如图4(b)所示. Bader电荷计算再次验证了同阳离子(M4+)的单层体系中, 随着阴离子(X2?)原子序数依次增加, 氢化单层MX2结构越稳定这一结论的正确性. 图 4 氢原子稳定吸附在单层 MX2的电荷转移分布图(计算的电荷转移为MX2层E位点的转移情况, 箭头指示电荷转移的方向) Figure4. Charge transfer profile of hydrogen atoms adsorbed on a single-layer MX2. The calculated charge transfer is the transfer of the E site of the MX2 layer, and the arrows indicate the direction of charge transfer.
最后研究了氢化对单层MX2电子结构的影响. 为了更清晰地了解氢化时的能带结构变化, 首先计算了4 × 4超胞的单层纯MX2的电子能带结构以作为比较. 如图5所示, 单层纯MX2都是直接带隙的半导体, 其价带顶和导带底都位于K/K'高对称点, 带隙大小变化范围约为1.00—1.80 eV[45]. 图 5 4 × 4单层MX2 (1L-MX2)超胞的能带结构图(费米能级用黑色虚线表述并设为0 eV) Figure5. Band structures of 4 × 4 monolayer MX2 (1L-MX2) supercell. The fermi level is represented by dotted black line and set as 0 eV.
图6为单个H原子位于表面稳定吸附位点时的MX2能带结构, 通过对比纯单层MX2的能带结构图, 不难发现表面氢化时氢原子能级进入了纯的MX2的带隙, 从而导致体系带隙急剧减小, 其带隙也相应从1.0—1.8 eV减小到约0.3—0.6 eV, 但依然保持原单层MX2体系的半导体性质. 这其实是由于表面氢化时, 氢原子与MX2中的X原子形成共价键吸附, 此作用相当于吸附掺杂, 而能带急剧减小也正是由于杂质(氢)能级加入所导致. 此外, 从图5和图6中的能带图对比可以看出, 没氢化时, 纯MX2的能带是简并的, 即自旋向上和向下的能带具有相同的能量. 而表面氢化时, MX2的自旋向上和自旋向下的能带能量不再简并, 产生了自旋极化劈裂, 说明此时体系具有一定的磁性, 即发生了从无磁性体系到磁性体系的过渡. MX2中磁性产生的原因主要是由于H原子电负性较强于MX2中的X原子, 其有从X原子得到电子的趋势, 导致MX2体系的电荷分布将发生重新分配, 致使体系的电子云在H原子吸附位点产生小的畸变, 从而使体系表现出微弱且局域化的磁矩. 当然, 相较于直接用H原子替代掺杂X原子以使体系少电子从而产生较大的本征磁场而言(即p型替代掺杂), 这种通过吸附H原子使MX2体系诱导出的磁性是十分微弱且局域化的, 但这也不妨是一种使无磁性的MX2产生磁性的思路. 图 6 表面氢化MX2能带结构图(蓝色和红色线分别表示自旋向上和自旋向下) Figure6. Band structures of surface hydrogenation MX2. Blue and red correspond to spin-up and spin-down, respectively.
然而, 当一个H原子稳定吸附在4 × 4的单层MX2超胞中E吸附位点时, 如图7所示, 此时H原子与MX2形成更稳定的配体成键吸附, H原子的能级穿过了费米能级, 即层间氢化使得单层MX2直接实现了从半导体到金属的过渡. 有趣的是, 不同于表面氢化诱导的自发磁性, 层间氢化单层MX2 (M = Mo, W; X = S, Se)都会使体系产生磁性, 而阴离子为Te2? 时, 磁性却几乎消失. 这可能是由于Te元素的电负性弱于S和Se元素, 层间氢化时H原子与WTe2和MoTe2中形成了更稳定的配体成键, 因而导致磁性消失. 总之, 通过控制氢化的位置, 可以有效地调控单层MX2的电子性质. 图 7 层间氢化MX2能带结构图(蓝色和红色线表示自旋向上和自旋向下) Figure7. Band structures of interlayer hydrogenation MX2. Blue and red correspond to spin-up and spin-down, respectively.