ReX2 (X=S, Se): 二维各向异性材料发展的新机遇

王人焱, 甘霖, 翟天佑

ReX2(X=S, Se): 二维各向异性材料发展的新机遇

王人焱, 甘霖, 翟天佑

(华中科技大学材料科学与工程学院, 材料成型与模具技术国家重点实验室, 武汉 430074)

二维材料因其不同于体相的超薄原子结构、大的比表面积和量子限域效应等受到了人们的广泛关注。二维各向异性材料作为二维材料家族的一员, 其取向依赖的物理和化学性质, 使得对该类材料性能的选择性优化成为可能。过渡金属Re基硫属化合物作为各向异性材料的典型代表, 具有可调的可见光波段吸收带隙, 极弱的层间耦合作用力, 以及各向异性的光学、电学性能, 现已成为电子和光电子领域的研究热点之一。本文主要介绍了ReX2(X=S, Se)的晶体结构和基本性质, 总结目前该材料体系主流的合成方法, 研究其各向异性物理特性及优化的手段和条件, 并对ReX2的制备和发展进行了展望。

各向异性; ReS2; ReSe2; 综述

超薄的原子结构和巨大的比表面积赋予二维材料不同于体相的光学、电子学、磁学等方面独特的物理性质。石墨烯、MoS2、h-BN作为最常见的一类二维材料, 拥有高度对称的晶体结构和晶格取向, 并表现出各向同性的物理性质, 在传统大面积均匀器件的性能平衡上有很大的应用优势。而另外一类二维材料, 如黑磷(BP)、WTe2、IV族硫属化合物(SnS、GeS)、过渡金属Re基硫属化合物(ReS2、ReSe2)等拥有不对称的原子排布, 在不同取向上表现出差异显著的电学、光学、热学和机械性质[1-3], 为发展新一代多功能低维电子及光电子器件提供了可能。基于BP的场效应晶体管在“Z”方向和“扶手椅”方向的空穴霍尔迁移率相差1.6倍, 这种各向异性的电子输运行为可用于神经突触器件、反向器等[4]; 各向异性的SnSe纳米片沿、轴的热电品质因子相差近3倍, 体现了各向异性的热传导, 有望应用于散热器、热阻器件等方面[5]; 此外, Wan等[6]报道了基于GeSe2的光电行为, 在450 nm波长激光的激发下, 角度依赖的光电流最大比值达到3.4, 在偏振光电探测器方面有极大的应用潜力。二维各向异性材料开始在各个领域崭露头角, 并且在有特定需求的器件设备上有了新的研究突破。

ReX2(X=S, Se)作为二维各向异性材料的一族, 凭借其较低的对称结构和优异的光学、电学、光电子和机械性能, 现已在微电子器件[7-12]、光电探测[9,13-18]、能源存储[19-22]、光催化[23-29]等领域展现出良好的应用前景。

本文综述了过渡金属Re基硫属化合物ReX2(X=S, Se)这一类各向异性材料。尽管已有ReS2[30]和ReX2[31]等的相关总结, 但对于该材料体系各向异性的深入探究还未有报道。本文从介绍ReX2的基本结构出发, 研究结构对其性质的影响, 总结目前已报道的合成方法, 并讨论了各向异性的结构所导致的各向异性的物理特性, 最后针对该材料的合成及未来的应用发展等提出了建议。

1 晶体结构与基本性质

1.1 晶体结构

ReS2和ReSe2都具有Re-TMDs半导体层状结构, 有着相似的原子结构和空间排列。以ReS2为例, ReS2为三斜晶系, 属于P-1空间群[32]。如图1(a)所示, ReS2单胞包含4个Re原子和8个S原子[33], Re和S拥有不同的原子占位环境, 与其他具有高度对称结构的TMDs(如MoS2、WS2等)不同的是, 由于Re原子核外未成键价电子的存在, 单胞中的4个Re原子会形成Re4团簇, Re-Re金属键导致了Re链的形成[34], 致使结构对称性降低。图1(b)为单层ReS2原子结构的顶部视图, 沿着[010]轴方向能够发现明显的Re4链。,方向夹角约为119.8°。ReS2层间间距约0.67 nm, 值得一提的是, ReS2为中心对称结构[35]。表1为ReS2和ReSe2晶胞参数[36-37]。

通常来说, TMDs材料的电子结构和相结构类型均依赖于过渡金属原子的配位环境和d轨道电子数目[38]。在八面体配位场中, 如图1(c)所示, d能级分裂成eg和t2g能级, eg能级的两个轨道dz2和dx2–y2与硫属原子的p轨道杂化而形成σ成键轨道和σ*反键轨道, 并构成相应能量最高和最低的两个能带, 未成键的t2g轨道则形成介于σ和σ*带之间的能带, 若过渡金属价电子填充半满则呈现金属性[39]。

在ReX2中, Re原子d轨道价电子与s原子p态为八面体配位类型简并态。如图1(d)所示, d轨道价电子填充至未成键的t2g能带, 理论上应呈现金属 性[40]。但是, Re4链的形成改变了ReX2的电子分布, 导致ReX2为扭曲的八面体结构, 并将带隙打开, 最终呈现半导体特性。ReS2的费米能级更靠近导带底, 为n型半导体; 而ReSe2费米能级更靠近价带顶, 表现为p型半导体。

1.2 基本性质

在MoS2[41-44]、MoSe2[45]、WS2[42]等结构对称的TMDs中, 电子的带隙结构有着很强的层数依赖性。当层数减少至单层, 由于量子限域效应, 带隙类型从间接带隙转变为直接带隙, 进而引起PL和光响应等的显著增强[38]。相比传统的2D半导体材料, 由于Re–Re金属键, ReX2表现出更弱的层间范德华力, 带隙结构与层数的变化关系稍有不同。理论计算表明, ReSe2在块体状态时表现为直接带隙半导体, 单层尺度下转变为间接带隙半导体[46], 而ReS2则一直保持直接带隙的电子结构[47-48]。如图1(e), Wolverson等[49]也通过理论计算得到块体ReSe2为直接带隙类型(1.09~1.31 eV), 在单层为间接带隙电子结构(1.34 eV)。ReS2在块体状态下拥有一个直接类型带隙结, 并且随着厚度减薄至单层时保持不变[50]。Liu等[51]也对ReS2电子结构进行了研究, 图1(f)中显示为单层、3层、5层的ReS2能带计算结果, 发现在层数变化下都保持直接带隙结构。总结ReX2的电子结构特点, 层数会影响ReSe2的电子结构性质, 单层情况下表现为间接半导体带隙, 块体状态下表现为直接带隙半导体结构。ReS2电子结构形态稍有不同, 一直保持不随层数变化的直接带隙类型。

图1 (a)ReS2单胞结构图[33]; (b)ReX2晶体结构顶视图; (c) ~(d)1T相结构轨道成键和能级示意图[39-40]; (e)ReSe2块体(上)和单层(下)带隙结构[50]; (f)1层、3层、5层的ReS2能带模拟图[51]

表1 ReS2和ReSe2单胞晶格参数[36–37]

光致发光谱(PL)和Raman光谱等手段可以对ReX2独特的电子结构进行表征。随着厚度减薄至单层, MoS2[43,52-53]、WS2[54-55]等会由间接带隙向直接带隙转变, 使得PL光谱信号大大增强; 而随着层数的减少, ReX2带隙增大, 发光却明显变弱。Zhao等[56]分别对单层, 少层, 10层和块体的ReSe2进行PL测试, 并发现随着层数减少到单层, 材料的带隙逐渐增大, 峰强逐渐减弱, 单层的ReSe2表现出间接带隙半导体的性质。相比于ReSe2, 单层和块体的ReS2都具有直接带隙的电子结构, 层间弱的范德华耦合使块体材料具有更高的吸光和激子跃迁机率, 从而使PL大大增强。Tongay等[50]研究了室温下不同层数ReS2的微区PL, 如图2(a)和(b)所示, 不同层数ReS2的PL峰位未发生明显变化, 均位于1.5~1.6 eV之间, 再次证明了弱的层间耦合; 且随着层数的增加, 峰强度会逐渐增大, 直到6层以后达到饱和。图2(b)还对比了其他二维材料, 如MoS2[40]、MoSe2[57]、WS2、WSe2[54]与ReS2的PL随层数变化关系的差异。具体来说, 随着层数的增加, M、W基过渡金属硫属化合物的PL强度降低了几个数量级, 这是由于PL来源于直接带隙的热激发激子发光[53], 而热激发激子的数量会随着层数的增加呈数量级的减小。

HPLC法同时测定蒙药达乌里芯芭中梓醇和益母草苷的含量 ……………………………………………… 包玉秋等（11）：1542

Raman光谱是分析分子振动、转动, 反映分子本征结构的散射光谱。ReX2层间弱相互作用也体现在与层数相关的Raman光谱上。图2(c)和(d)分别代表在2.33 eV平行偏振光激发下, ReS2、ReSe2的层内Raman振动。可以看出, 随着层数的变化, 代表Re4链方向的分子振动Raman峰位未发生偏移[58]。Feng等[35]通过计算模拟了ReS2的Raman光谱, 并发现几乎所有的Raman峰位都不随层数变化, 说明ReS2层间具有极弱的耦合作用。

图2 (a)ReS2不同层数的PL光谱; (b)ReS2, MoS2, MoSe2, WS2和WSe2的PL强度和层数依赖关系[50]; (c)ReS2和 (d) ReSe2单层到块体厚度的拉曼光谱[58]; (e)ReS2纳米卷自组装机制[59]; (f)单层ReS2纳米墙热弯曲示意图[60]

层间弱的耦合作用不仅表现在材料本征性质上, 也在实验现象中有所体现。Fu等[59]通过电化学Li离子插层化学气相沉积(CVD)合成的垂直ReS2纳米片, 边对边自组装形成ReS2纳米卷。图2(e)显示纳米卷自组装机制示意图, 首先通过Li离子插层和剥离获得分散纳米片, Li离子的电荷排斥和ReS2弱的层间耦合共同作用, 导致层间滑移形成自组装纳米片, 最后卷曲成纳米卷结构。他们还对自组装过程进行了模拟, 层间滑移模式显示了最有效和最合理的生成途径, 并且自卷曲降低了整个过程的总能量。另外, 通过热引入的手段也可以观察到垂直ReS2纳米片的热弯曲现象, 由于极弱的层间相互作用, 热引入会导致(001)面滑移, 最终导致材料面外弯曲[60]。图2(f)简单地描述了ReS2热弯曲机制。

2 合成方法与表征

目前, 针对ReX2的合成策略, 主要包括气相输运、化学辅助剥离、气相沉积和其他的一些材料制备手段。

2.1 气相输运法

大部分机械剥离的样品都是经过化学气相输运(CVT)的方法获得的单晶, 再经过胶带反复地剥离, 最后得到比较薄的二维材料。Jariwala等[61]通过气相输运法得到ReS2和ReSe2单晶。他们将Re和S/Se源放入处理过的密封石英管中, 在高真空条件下先缓慢升温1100℃超过72 h, 退火24 h后以1℃/h降温至900℃, 最后冷却到室温。气相输运的关键在于极其缓慢的组分交换, 利于形成高质量的单晶, 但缺点就是耗时较长。Hu等[62]通过气相输运法一步合成二维超薄的层状ReS2, 大大缩短了合成时间, 而且省去了机械剥离2D材料的过程, 避免了胶带剥离带来的层数不可控性和有机物的污染。一步合成相比于传统的CVT方法不同的地方在于封闭石英管细口处的优化, 更细和更长的瓶颈有利于气相更加缓慢平稳的输送达到生长2D的目的。他们也研究了载气对实验的影响, 发现在没有输运载气的情况下并没有样品生成, 说明了输运载气在CVT实验中的重要性。

2.2 辅助剥离法

二维材料辅助剥离一般包括溶液超声离心分散[63]、溶剂辅助液相剥离[52,64-69]、干法化学反应剥离[70]等。Chen等[70]利用无溶剂的化学剥离方法成功剥离ReS2粉末形成2D纳米片。图3(a)分别为ReS2处理前后扫描电子显微镜(SEM)和透射电子显微镜(TEM)照片, 剥离过后纳米片尺寸变小, 大约50~100 nm。插图为剥离纳米片的水溶液照片, 呈现典型的暗棕色。

为了检验剥离样品的质量, 用STEM来表征化学剥离的单层ReS2纳米片。如图3(b)所示, Re4原子链排列清晰, 证明了高质量的1T’相ReS2。Kang等[63]利用等密度梯度高速离心得到了不同厚度的ReS2纳米片, 并且在溶液中呈梯度排列, 实验的关键是在高粘性的碘克沙醇溶剂中添加CsCl, 增加了溶液最大悬浮密度。图3(c)显示通过等密度梯度高速离心法(iDGU)分离ReS2的机制, 在不同的密度下分别悬浮不同层数的样品。原子力显微镜(AFM)照片(图3(d))和Raman光谱(图3(e))也证明了该分散样品为ReS2纳米片。

2.3 气相沉积法

气相沉积是一种比较常见的制备大尺寸高质量2D材料的合成方法, 相比于其他合成方法, 其优点在于合成方便、样品厚度和形貌可控, 缺点是不能完全实现大规模的样品制备, 是材料基础研究比较青睐的合成方法之一。气相沉积包括物理气相沉积和化学气相沉积, 两者的区别在于合成过程中原料在气相状态下有没有发生化学反应。近年来, ReX2的气相合成已经有很多文献报道[10,14,29,34,71-96]。

Qi等[88]通过直接使用ReS2粉末作为反应源, 如图4(a)~(c)所示, 通过物理气相沉积在SiO2/Si衬底上得到了厚度均一的ReS2薄膜, Raman复杂的峰位来源于低对称的晶体结构, 插图中TEM表征也证明成功合成了ReS2, AFM测试厚度约为2.3 nm, 对应3层。Keyshar等[34]通过CVD首次合成了单层ReS2, 低熔点高铼酸铵(NH4ReO4)的引入降低了反应所需要的温度。Hafeez等通过CVD方法合成了ReS2[74]和ReSe2[78], 如图4(d)~(e), 对比纯蓝宝石衬底可以看出, 在蓝宝石衬底上生长的2层ReS2薄膜呈现均一的颜色衬度, 在氧化硅基底也可以得到六方形的ReS2纳米片结构。

大尺寸超薄2D合成是CVD的主要特点, 但是材料生长存在很多的影响因素, 比如气流、温度、源量、衬底类型等都会对材料的合成产生影响, 所以, 在CVD合成过程中各个参数的摸索和探究一直是研究者们所关注的问题。为了优化Re-TMDs的CVD合成, 研究者们采用了很多手段来辅助调控实验参数, 以便获得更好的生长效果[97-102]。Cui等[77]通过Te辅助CVD合成控制源蒸发的手段在云母衬底上合成了厚度均一的单层高质量ReS2纳米片。图5(a)为Te辅助合成ReS2示意图, 采用云母衬底倒扣的方式, Te粉和Re粉进行混合会在高温时形成Re–Te合金相, 大大降低了Re的蒸发熔点, 提高Re蒸气压促进反应的进行。有趣地是, 得到的样品中并没有表征出Te的存在, 说明Te并不会造成产物污染, 这在之前WS2和MoS2的CVD生长中也被证实[103]。图5(b)为转移到SiO2/Si衬底上的ReS2样品, 表现出均一的颜色衬度和一致的形貌大小, 插图中AFM表征显示0.7 nm, 表明合成的样品为单层ReS2。

图3 (a)分离前ReS2纳米片SEM照片(左)和分离后TEM照片(右), 插图：剥离样品水溶液光学照片; (b)HRTEM照片[70]; (c)等密度梯度超速离心分离不同密度梯度ReS2纳米片示意图; (d)AFM照片; (e)ReS2拉曼光谱图[63]

图4 (a)PVD制备ReS2薄膜示意图; (b)ReS2薄膜Raman光谱图; (c)光学照片, 插图：AFM照片和TEM照片[88]; (d)蓝宝石衬底和ReS2薄膜光学照片; (e)SiO2/Si衬底CVD生长ReS2纳米片光学照片[74]

图5 (a)Te辅助CVD合成ReS2示意图; (b)转移至SiO2/Si衬底ReS2光学照片, 插图为AFM照片[77]; (c)夹层限域生长ReSe2表面反应机制示意图; (d)A、B面的ReSe2形貌光学照片[89]

除了利用合金化的手段降低源熔点, 提高反应源供给量以外, 采用衬底夹层限域空间也是一种控制材料生长的手段[104]。Li等[90]采用两片云母夹层的方式构造限域空间, 在夹层间能够生长原子层平整尺寸达到几十微米的ReS2薄片。Xu等[89]也通过夹层的方法合成了ReSe2。首先, 云母原子级平整的表面有利于ReSe2原子的表面迁移促进均匀生长; 其次, 云母夹层的限域空间抑制了低对称ReSe2面外生长活性。图5(c)显示了夹层限域生长ReSe2表面反应机制, 由于传质和反应之间的相互关系, 在夹层内的A面能够得到完整平坦的ReSe2纳米片和薄膜, 而在夹层外的B面, 会产生不平整的岛状纳米片结构。图5(d)转移的ReSe2纳米片和薄膜光学照片证明了限域手段的可行性。

另外一种控制源的手段是源的吸附和缓释放, 大多采用无机多孔的材料, 如(陶瓷、碳粉、多孔Al2O3)去吸附在高温过程中过多的蒸发源。Xu等[76]采用分子筛辅助控制源蒸发的方法成功在云母表面合成了ReSSe合金相。不同配比的合金相除了需要精确控制S、Se反应的浓度以外, Re源的浓度也至关重要。多孔分子筛的加入有效缓释了气相ReO3, 保证了云母衬底在合适的生长温度条件下也拥有均匀的源供应, 达到平衡大尺寸生长的目的。通过此种方法生长的合金最大能够达到206 μm。

2.4 其他合成方法

传统的2D材料CVD合成除了以上方法以外, 还有原子层沉积(ALD)[105-106]、热蒸镀[107]、磁控溅射[108]、分子束外延(MBE)[109]等。Hämäläinen等[110]通过ALD沉积技术探究了不同沉积参数对ReS2生长形貌的影响。实验结果表明合适的沉积温度在120℃到150℃之间, ReCl5源的增加抑制了垂直ReS2纳米片的产生, 而H2S含量的增加会使ReS2纳米片宽化。

3 各向异性特性

3.1 各向异性光学和拉曼振动

ReS2和ReSe2各向异性电子结构已经通过角分辨光电子能谱得到证实[47-48, 111-112], 各向异性的介电性导致不同取向的折射率差异, 进而体现在光学各向异性上。而ReX2线性二色性的光学各向异性, 已经在角度依赖的光吸收[112-115]、激子[116-122]中得到证明。Sim等[122]在少层ReS2中发现激子光学斯塔克效应, 图6(a)展示了泵浦探测少层ReS2纳米片各向异性激子示意图。他们利用线性偏振光极化ReS2观察到角度依赖的光激子吸收, 如图6(b)和(c)所示, 非简并激子随极化角度会呈线性变化, 通过拟合得到, X1, X2各向异性激子最大极化光吸收分别在19°和87°。

另外, 类似于黑磷[123], 利用泵浦探测ReS2也发现激子存在光学斯塔克位移和超快的量子震荡现象[121], 为将来ReS2各向异性超快光学的应用提供了可能。

ReX2低对称的晶格取向也造成了各向异性Raman振动和声子耦合。角度依赖的Raman振动是确定物质Raman振动模式的有效手段之一[124]。尽管不同激光波长[125]、不同温度[126]、样品厚度[35]和分子修饰[127]、叠层取向[46,125]都会对ReX2的Raman测量产生影响, 但对于各向异性本征特性而言, 并不会随着条件变化而消失。ReX2的Raman振动模式包括层内振动模式和低频层间耦合模式。图6(d)上图显示了块体ReS2在100~450 cm–1之间18种具有Raman活性的层内振动模式[128]。类似于之前的报道[129], 这里把所有的振动模式全都定义为Ag模式, 在150 cm–1处的面内振动模式为Ag1, 在437 cm–1处的S原子面外振动模式为Ag2, 418 cm–1处的S原子面内外共振模式为Ag3, 这里要指出的是, 在约213 cm–1处的Raman峰振动对应面内Re–Re链(轴方向)的振动方向, 这在很多关于ReS2研究中已有报道[50,130]。表2列举了在633 nm激光下单层和块体18种的Raman振动频率和振动来源。低对称性结构造就了复杂的Raman模式, 而弱范德华耦合导致了不随厚度变化的Raman振动。除了层内振动模式以外, ReX2弱的层间耦合在低频Raman振动中也有所体现。图6(d)下图为少层ReS2低频模式Raman光谱, 与MoS2相同, 层间呼吸模式在更高的振动频率, 非简并滑移模式位于比较低的振动频率位置[131]。Chenet等[130]研究了不同厚度和偏振光下的ReS2Raman振动, 他们发现几个主要的Raman振动峰随层数的变化都没有明显的峰位偏移, 但都表现出极化角度依赖性。单层ReS2从0~180°极化Raman光谱显示在图6(e)中, 可以看出, 对于比较突出的振动模式, 峰强度都拥有角度依赖关系, 随着偏振角度发生改变。图6(f)为块体ReS2在150和213 cm–1振动峰位下的极化Raman曲线拟合, 两种模式都随角度呈现180°周期性的变化, 并且 213 cm–1的极化取向也刚好对应高分辨中轴的方向, 进一步证明了之前的观点[50]。

图6 (a)泵浦探测少层ReS2纳米片实验示意图, 插图：剥离ReS2光学照片; (b)角度依赖的少层ReS2纳米片光吸收谱; (c)X1, X2激子角度依赖的光吸收极化图[122]; (d)ReS2 Raman光谱[128]和低频Raman光谱[131]; (e)ReS2不同旋转角度的Raman光谱; (f)少层ReS2纳米片高分辨TEM照片和对应偏振Raman极化图[130]

表2 633 nm激光激发块体和单层ReS2的18种Raman振动光谱[35]

3.2 各向异性电子输运

2D各向异性材料的不对称晶格结构影响了不同取向上电子和空穴的有效质量, 进而反映出不同的电子输运行为。例如黑磷(BP), “扶手椅”方向的 电子空穴有效质量小于“Z”方向, 表现出更高的电 导[132-134]和载流子迁移率[135]。Zhai等[136]报道的层状双金属硫化物Ta2NiS5各向, 异性电子输运, 在80 K条件下, 沿轴的电导是轴方向的1.78倍。ReX2作为各向异性材料中的一类, 在电子输运上也表现出取向差异。图7(a)为机械剥离的ReS2不同取向的场效应晶体管(FET)器件光学照片, 图7(b)中的高分辨透射照片对应图7(a)中不同电极的原子取向, 1-4和2-3电极对应[010]方向的电子传导, 1-3和2-4电极电子传导垂直于方向。20 V栅压下的-测试表明, 如图7(c), 在1 V偏压下, 不同原子取向的电子输运有明显的差异, 沿轴方向的电导为 0.82 μS, 垂直方向的电导为0.075 μS, 相差了10.9倍。他们也测试了不同取向的ReS2的转移特性曲线, 如图7(d), 在–3 V偏压下, 1-4电极的ReS2开关比达到106, 迁移率为23.1 cm2/(V·s), 而1-2电极迁移率为14.8 cm2/(V·s), 相差1.56倍[137]。Xing等[51]制备了单层和少层ReS2场效应晶体管, 研究各向异性的电导和迁移率。如图7(e)中、轴方向的转移特性曲线,、方向电导有明显区别, 不同栅压的各向异性电导比表现出栅压依赖的电学性质。插图中关于ReS2接触电阻的测试也证明了ReS2固有的各向异性本征特性。另外, 为了进一步研究各向异性的电子输运, 对角度极化的迁移率进行了计算, 如图7(f)。通过理论和实验得出, 不同取向的极化迁移率最大比值达到3.1, 是目前2D各向异性材料中最大的极化迁移率比值。除了ReS2以外, ReSSe合金的各向异性电学输运也有人研究, Xu等[76]利用CVD方法合成了ReS1.23Se0.77合金并构筑了FET器件。如 图7(g)所示, FET开关比达到105, 沿轴方向和垂直轴方向的迁移率分别为0.34 cm2/(V·s)和0.12 cm2/(V·s), 相差接近3倍。

3.3 各向异性光电子器件

高性能光电子器件一直是未来研究和发展趋势之一[138-142]。近年来, 关于各向异性材料光电探测也渐渐引起人们的关注, ReX2光电探测一直是各向异性材料中所关注和研究的热点。Liu等[112]报道了ReS2各向异性的光电子效应, 随着极化光的偏转, 光电流发生显著的变化。如图8(a)和(b)所示, 在同一偏压下, 极化角度至90°对应ReS2样品Re4链方向时, 光电流达到最大值, 产生电流差异的主要原因是取向性的光吸收差异以及材料本征的结构不对称。Xu等[76]也报道了ReSSe合金的光电各向异性, 他们测试了ReS1.06Se0.94合金纳米片角度依赖光电性能, 如图8(c)和(d)所示,-曲线显示光电流随角度呈现周期性的变化, 证明了ReX2光电子的各向异性输运。另外, 和之前Raman峰的极化振动类似, 角度依赖的极化拟合曲线表明最大的光电流取向也完美地对应Re4链的晶格方向。此外, 关于ReSe2纳米片各向异性光电子也有人报道[15], 如图8(e)和(f)所示, 图8(e)显示ReSe2极化光电探测器示意图, 0至90°的光电流成像直观地表明了光电流来自于沟道而不是电极, 排除了肖特基势垒的影响。在=0时, 偏振光沿着轴, 沟道内电流最大,=90°时, 偏振光垂直轴, 沟道内电流最小, 电流最大值分布取向也和偏振光基本一致, 证明了光电流的差异本质上来源于晶格各向异性, 显示了光电流的线性二色行为。

图7 (a)ReS2晶体管光学照片; (b)对应(a)图ReS2的TEM照片; (c)不同电极的I-V曲线; (d)不同电极之间的ReS2转移特性曲线[137]; (e)各向异性ReS2晶体管转移特性曲线, 插图为器件光学照片和R-Vbg曲线; (f)角度依赖的 FET迁移率极化图, 插图：器件光学照片[51]; (g)ReS1.23Se0.77合金各向异性转移特性曲线, 插图为器件光学照片[76]

图8 (a)角度依赖的ReS2极化光电流图; (b)极化光电流曲线[112]; (c)偏振光I-t曲线; (d)偏振光电流极化图[76]; (e)ReSe2偏振光电器件示意图; (f)器件SEM照片和不同角度的入射光电流分布图[15]

3.4 各向异性热传导

材料的热传输也是人们比较关注的话题, 良好的热传导材料在一些器件诸如CPU、存储器等使用中能有效地降低整个器件的能耗和温度, 提高器件的性能。各向异性的热导材料为材料的热传导提供了新的选择和发展方向。2D各向异性材料诸如BP[142-144]、WTe2[145-146]、SnSe[147-149]、SnS2[150]等, 都在热传输方面表现出优异的取向差异。关于ReX2的热传导研究目前报道很少, 但作为各向异性材料, 在不同取向上晶格振动造成声子耦合差异必然会导致热传导不同。Cahill等[151]报道了3D ReS2各向异性的热传导性质, 他们发现沿着Re4链方向的面内热导率系数为(70±18)W×m–1×K–1, 垂直Re4链方向的热导系数为(50±13) W×m–1×K–1, 相差1.4倍, 而相比于面内热传导, 垂直于面内的面外热传导表现出近乎热阻的性质, 热导率系数仅有(0.55±0.07) W×m–1×K–1, 这也是目前2D材料中关于面外热导报道最低的数值。低的面外热传导系数主要归因于ReS2极弱的层间耦合作用。图9(a)展示了测试热导率剥离的ReS2的光学照片。热导率的测试通过时间分辨的热致反射光谱(TDTR)来完成, 主要是利用一定频率的光进行热渗透, 测量不同偏移位置的输入和输出相位电压来确定热导大小。图9(b)显示了面外热传导不同频率的TDTR数值拟合曲线, 相比高频光热辐照, 低频热传导显示更大的输入输出相位电压比, 更能体现材料热导的本征差异性。面内不同取向的热传导如图9(c)和(d)所示, 利用1.1 MHz频率热辐照, 热信号随位移呈规律性减弱, 沿(Re4链)方向和沿(垂直Re4链)方向的等值热信号呈现椭圆形, 表明在不同取向上热传导的各向异性。

图9 (a)剥离层状ReS2光学照片; (b)面外热传导曲线; (c)极化方向的热传导曲线; (d)面内热传导分布图; (e)不同厚度的面内极化热导率[151]

另外, 不同厚度ReS2的面内热导也表现在图9(e)中, 可以看出, 在不同的厚度情况下, ReS2同样表现出取向依赖的热导现象, 并且平行Re4链方向热导率要高于垂直方向。

4 修饰和改性

尽管拥有独特本征性质的ReX2已经在很多方面表现出优异的性能, 但通过其他手段提高和改进材料的性能仍旧是一个大家比较感兴趣的方向。研究者们已经通过应力工程[152-164]、分子修饰[16,165-168]和元素掺杂[169-171]等手段研究了ReX2性质和性能的变化。对于各向异性而言, 外部环境对材料本征各向异性的影响是人们较为关注的问题。目前, ReX2相转变是材料各向异性性质变化的一种明显反映。Li等[152]研究了在应力条件下, 单层ReS2电子空穴有效质量和迁移率的变化, 通过理论计算, 他们发现在轴方向的应力作用会导致ReS2直接带隙向间接带隙的转变。各向异性的载流子迁移率也会受到应力作用的影响, 当在轴方向施加5%应力时, 理论计算表明轴方向的电子迁移率会从338.83 cm2/(V·s)增加到3940.21 cm2/(V·s),轴方向电子迁移率则从799.64 cm2/(V·s)增加到了4300.22 cm2/(V·s); 同样地,轴方向空穴迁移率从239.80 cm2/(V·s)增加 到1439.84 cm2/(V·s),轴方向空穴迁移率从 30.90 cm2/(V·s)增加到1100.76 cm2/(V·s)。可见, 应力调控是一种改变ReX2各向异性的有效手段。

除了应力调控之外, 分子吸附也是调节材料各向异性的一种方法。Sahin等[165]研究发现, H原子吸附ReS2会形成强的S-H键, 导致1T-ReS2结构变成更具有动力学稳定性的Re–Re二聚体结构1T-ReS2H2。H原子吸附之前, 计算表明ReS2沿轴方向的硬度为159 J/m2, 杨氏模量为477 GPa, H原子吸附之后, 由于形成Re–Re二聚体打破了Re4团簇的稳定性, 硬度和杨氏模量都有所降低, 沿轴方向的硬度降为97 J/m2, 杨氏模量则降为250 GPa。该工作表明H原子的吸附改变了ReS2各向异性的性能参数, 为调控ReX2各向异性的特殊性质提供了方向。

5 总结和展望

与普通的2D材料相比, 各向异性的2D材料拥有更为突出的材料特性和更广阔的应用前景。本文总结了2D各向异性材料中的典型代表：过渡金属Re基硫属化合物ReX2, 介绍了其独特的晶体结构和由结构导致的特殊材料性质, 并分析了性质产生的原因。另外, 对于当前ReX2的主要合成方法包括机械剥离、辅助剥离、气相沉积等做了简单的总结; 最主要的是针对ReX2独特的各向异性性质和改进手段进行了讨论, 希望能够为将来ReX2的应用和发展提供有价值的指导。

虽然ReX2已经在很多领域有所应用, 但都只是处于实验阶段, 对于材料目前的研究发展仍然没有达到真正应用化的水平。首先需要开发新的合成和制备方法来满足工业化应用需求, 比如优化实验合成装置, 采用卷轴衬底手段进行大面积合成; 其次, 针对材料各向异性性能的优化仍然需要研究者们不断的努力和改进, 例如寻找新材料来与之进行复合、掺杂, 或者利用异质结来调控各向异性性能; 另外, 各向异性基础研究也还需要更深入地发掘和探索, 能够从本质出发研究各向异性基本特性, 比如研究ReX2的电子结构变化引起的相变过程, 提出一些新的科学问题, 进而将会从根本上优化和解决材料各向异性发展存在的难题, 这也将是今后发展各向异性材料ReX2新的趋势和机遇。

[1] GONG C, ZHANG Y, CHEN W,. Electronic and optoelectronic applications based on 2D novel anisotropic transition metal dichalcogenides., 2017, 4(12): 1700231–1–19.

[2] TIAN H, TICE J, FEI R,. Low-symmetry two-dimensional materials for electronic and photonic applications., 2016, 11(6): 763–777.

[3] LIU X, RYDER C R, WELLS S A,. Resolving the in-plane anisotropic properties of black phosphorus., 2017, 1(6): 1700143–1–9.

[4] XIA F N, WANG H, JIA Y C. Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics., 2014, 5: 4458–1–6.

[5] ZHAO L D, LO S H, ZHANG Y,. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals., 2014, 508(7496): 373–377.

[6] YANG Y, LIU S C, YANG W,. Air-stable in-plane anisotropic GeSe2for highly polarization-sensitive photodetection in short wave region., 2018, 140(11): 4150– 4156.

[7] XU J, CHEN L, DAI Y W,. A two-dimensional semiconductor transistor with boosted gate control and sensing ability., 2017, 3(5): 1602246–1–8.

[8] SHIM J, OH S, KANG D H,. Phosphorene/rhenium disulfide heterojunction-based negative differential resistance device for multi-valued logic., 2016, 7: 13413–1–8.

[9] WANG X, HUANG L, PENG Y,. Enhanced rectification, transport property and photocurrent generation of multilayer ReSe2/MoS2p-n heterojunctions., 2016, 9(2): 507–516.

[10] DATHBUN A, KIM Y, KIM S,. Large-area CVD-grown sub-2 V ReS2transistors and logic gates., 2017, 17(5): 2999–3005.

[11] MOHAMMED O B, MOVVA H C P, PRASAD N,. ReS2-based interlayer tunnel field effect transistor., 2017, 122(24): 245701–1–7.

[12] CORBETT C M, MCCLELLAN C, RAI A,. Field effect transistors with current saturation and voltage gain in ultrathin ReS2., 2015, 9(1): 363–370.

[13] CHO A J, NAMGUNG S D, KIM H,. Electric and photovoltaic characteristics of a multi-layer ReS2/ReSe2heterostructure., 2017, 5(7): 076101–1–8.

[14] ZHANG E, JIN Y, YUAN X,. ReS2-based field-effect transistors and photodetectors., 2015, 25(26): 4076–4082.

[15] ZHANG E, WANG P, LI Z,. Tunable ambipolar polarization- sensitive photodetectors based on high-anisotropy ReSe2nanosheets., 2016, 10(8): 8067–8077.

[16] QIN J K, REN D D, SHAO W Z,. Photoresponse enhancement in monolayer ReS2phototransistor decorated with CdSe-CdS-ZnS quantum dots., 2017, 9(45): 39456–39463.

[17] YANG S, TONGAY S, LI Y,. Layer-dependent electrical and optoelectronic responses of ReSe2nanosheet transistors., 2014, 6(13): 7226–7231.

[18] CUI Y, LU F, LIU X. Nonlinear saturable and polarization- induced absorption of rhenium disulfide., 2017, 7: 40080–1–9.

[19] ZHANG Q, TAN S, MENDES R G,. Extremely weak Van Der Waals coupling in vertical ReS2nanowalls for high-current- density lithium-ion batteries., 2016, 28(13): 2616– 2623.

[20] QI F, CHEN Y, ZHENG B,. Hierarchical architecture of ReS2/rGO composites with enhanced electrochemical properties for lithium-ion batteries., 2017, 413: 123– 128.

[21] QI F, HE J, CHEN Y,. Few-layered ReS2nanosheets grown on carbon nanotubes: a highly efficient anode for high-performance lithium-ion batteries., 2017, 315: 10–17.

[22] QI F, CHEN Y, ZHENG B,. 3D chrysanthemum-like ReS2microspheres composed of curly few-layered nanosheets with enhanced electrochemical properties for lithium-ion batteries., 2017, 52(7): 3622–3629.

[23] ESCALONA N, LLAMBIAS F J G, VRINAT M,. Highly active ReS2/gamma-Al2O3catalysts: effect of calcination and activation over thiophene hydrodesulfurization., 2007, 8(3): 285–288.

[24] ALIAGA J A, ZEPEDA T N, PAWELEC B N,. Microspherical ReS2as a high-performance hydrodesulfurization catalyst., 2017, 147(5): 1243–1251.

[25] SEPULVEDA C, ESCALONA N, GARCIA R,. Hydrodeoxygenation and hydrodesulfurization co-processing over ReS2supported catalysts., 2012, 195(1): 101–105.

[26] ANTONIO ALIAGA J, ZEPEDA T, FRANCISCO ARAYA J,. Low-dimensional ReS2/C composite as effective hydrodesulfurization catalyst., 2017, 7(12): 7120377–1–11.

[27] QI F, WANG X, ZHENG B,. Self-assembled chrysanthemum- like microspheres constructed by few-layer ReSe2nanosheets as a highly efficient and stable electrocatalyst for hydrogen evolution reaction., 2017, 224: 593–599.

[28] HO T C, SHEN Q, MCCONNACHIE J M,. Kinetic characterization of unsupported ReS2as hydroprocessing catalyst., 2010, 276(1): 114–122.

[29] GAO J, LI L, TAN J,. Vertically oriented arrays of ReS2nanosheets for electrochemical energy storage and electrocatalysis., 2016, 16(6): 3780–3787.

[30] RAHMAN M, DAVEY K, QIAO S Z. Advent of 2D rhenium disulfide (ReS2): fundamentals to applications., 2017, 27(10): 1606129–1–21.

[31] HAFEEZ M, GAN L, BHATTI A S,. Rhenium dichalcogenides (ReX2, X = S or Se): an emerging class of TMDs family., 2017, 1(10): 1917–1932.

[32] WILDERVANCK J C, JELLINEK F. The dichalcogenides of technetium and rhenium., 1971, 24(1): 73–81.

[33] YU Z G, CAI Y, ZHANG Y W. Robust direct bandgap characteristics of one- and two-dimensional ReS2., 2015, 5: 13783–1–9.

[34] KEYSHAR K, GONG Y, YE G,. Chemical vapor deposition of monolayer rhenium disulfide (ReS2)., 2015, 27(31): 4640–4648.

[35] FENG Y, ZHOU W, WANG Y,. Raman vibrational spectra of bulk to monolayer ReS2with lower symmetry., 2015, 92(5): 054110–1–24.

[36] KAO Y C, HUANG T, LIN D Y,. Anomalous structural phase transition properties in ReSe2and Au-doped ReSe2., 2012, 137(2): 024509–1–7.

[37] TIONG K K, HO C H, HUANG Y S. The electrical transport properties of ReS2and ReSe2layered crystals., 1999, 111(11): 635–640.

[38] WANG Q H, KALANTAR-ZADEH K, KIS A,. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides., 2012, 7(11): 699–712.

[39] YANG H, KIM S W, LEE Y H,. Structural and quantum- state phase transitions in Van Der Waals layered materials., 2017, 13: 931–937.

[40] CHHOWALLA M, SHIN H S, EDA G,. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets., 2013, 5(4): 263–275.

[41] YIN Z, LI H, JIANG L,. Single-layer MoS2phototransistors., 2012, 6(1): 74–80.

[42] TONGAY S, FAN W, KANG J,. Tuning interlayer coupling in large-area heterostructures with CVD-grown MoS2and WS2monolayers., 2014, 14(6): 3185–3190.

[43] SPLENDIANI A, SUN L, ZHANG Y,. Emerging photoluminescence in monolayer MoS2., 2010, 10(4): 1271–1275.

[44] LI T, GALLI G. Electronic properties of MoS2nanoparticles., 2007, 111(44): 16192–16196.

[45] TONNDORF P, SCHMIDT R, BOTTGER P,. Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2., 2013, 21(4): 4908–4916.

[46] HART L, DALE S, HOYE S,. Rhenium dichalcogenides: layered semiconductors with two vertical orientations., 2016, 16(2): 1381–1386.

[47] HART L S, WEBB J L, DALE S,. Electronic bandstructure and Van Der Waals coupling of ReSe2revealed by high-resolution angle-resolved photoemission spectroscopy., 2017, 7: 5145–1–9.

[48] WEBB J L, HART L S, WOLVERSON D,. Electronic band structure of ReS2by high-resolution angle-resolved photoemission spectroscopy., 2017, 96(11): 115205–1–8.

[49] WOLVERSON D, CRAMPIN S, KAZEMI A S,. Raman spectra of monolayer, few-layer, and bulk ReSe2: an anisotropic layered semiconductor., 2014, 8(11): 11154–11164.

[50] TONGAY S, SAHIN H, KO C,. Monolayer behaviour in bulk ReS2due to electronic and vibrational decoupling., 2014, 5: 3252–1–6.

[51] LIU E, FU Y, WANG Y,. Integrated digital inverters based on two-dimensional anisotropic ReS2field-effect transistors., 2015, 6: 6991–1–7.

[52] EDA G, YAMAGUCHI H, VOIRY D,. Photoluminescence from chemically exfoliated MoS2., 2011, 11(12): 5111–5116.

[53] MAK K F, LEE C, HONE J,. Atomically thin MoS2: a new direct-gap semiconductor., 2010, 105(13): 136805–1–4.

[54] ZHAO W, GHORANNEVIS Z, CHU L,. Evolution of electronic structure in atomically thin sheets of WS2and WSe2., 2013, 7(1): 791–797.

[55] GUTIERREZ H R, PEREA-LOPEZ N, ELIAS A L,. Extraordinary room-temperature photoluminescence in triangular WS2monolayers., 2013, 13(8): 3447–3454.

[56] ZHAO H, WU J, ZHONG H,. Interlayer interactions in anisotropic atomically thin rhenium diselenide., 2015, 8(11): 3651–3661.

[57] TONGAY S, ZHOU J, ATACA C,. Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2versus MoS2., 2012, 12(11): 5576–5580.

[58] LORCHAT E, FROEHLICHER G, BERCIAUD S. Splitting of interlayer shear modes and photon energy dependent anisotropic raman response in N-layer ReSe2and ReS2., 2016, 10(2): 2752–2760.

[59] ZHANG Q, WANG W, KONG X,. Edge-to-edge oriented self-assembly of ReS2nanoflakes., 2016, 138(35): 11101–11104.

[60] ZHANG Q, WANG W, ZHANG J,. Thermally induced bending of ReS2nanowalls., 2018, 30(3): 1704585–1–7.

[61] JARIWALA B, VOIRY D, JINDAL A,. Synthesis and characterization of ReS2and ReSe2layered chalcogenide single crystals., 2016, 28(10): 3352–3359.

[62] HU D, XU G, XING L,. Two-dimensional semiconductors grown by chemical vapor transport., 2017, 56(13): 3611–3615.

[63] KANG J, SANGWAN V K, WOOD J D,. Layer-by-layer sorting of rhenium disulfidehigh-density isopycnic density gradient ultracentrifugation., 2016, 16(11): 7216– 7223.

[64] JAWAID A, NEPAL D, PARK K,. Mechanism for liquid phase exfoliation of MoS2., 2015, 28(1): 337–348.

[65] ZHENG J, ZHANG H, DONG S,. High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide., 2014, 5: 2995–1–7.

[66] COLEMAN J N, LOTYA M, O'NEILL A,. Two-dimensional nanosheets produced by liquid exfoliation of layered materials., 2011, 331(6017): 568–571.

[67] NICOLOSI V, CHHOWALLA M, KANATZIDIS M G,. Liquid exfoliation of layered materials., 2013, 340(6139): 1226419–1226419.

[68] HUO C, YAN Z, SONG X,. 2D materialsliquid exfoliation: a review on fabrication and applications., 2015, 60(23): 1994–2008.

[69] ZHAO X, MA X, SUN J,. Enhanced catalytic activities of surfactant-assisted exfoliated WS2nanodots for hydrogen evolution., 2016, 10(2): 2159–2166.

[70] FUJITA T, ITO Y, TAN Y,. Chemically exfoliated ReS2nanosheets., 2014, 6(21): 12458–12462.

[71] XU K, DENG H X, WANG Z,. Sulfur vacancy activated field effect transistors based on ReS2nanosheets., 2015, 7(38): 15757–15762.

[72] KIM Y, KANG B, CHOI Y,. Direct synthesis of large-area continuous ReS2films on a flexible glass at low temperature.D, 2017, 4(2): 025057–1–10.

[73] QIN J K, SHAO W Z, XU C Y,. Chemical vapor deposition growth of degenerate p-type Mo-doped ReS2films and their homojunction., 2017, 9(18): 15583–15591.

[74] HAFEEZ M, GAN L, LI H,. Large-area bilayer ReS2film/multilayer ReS2flakes synthesized by chemical vapor deposition for high performance photodetectors., 2016, 26(25): 4551–4560.

[75] CHEN B, WU K, SUSLU A,. Controlling structural anisotropy of anisotropic 2D layers in pseudo-1D/2D material heterojunctions., 2017, 29(34): 1701201–1–8.

[76] CUI F, FENG Q, HONG J,. Synthesis of large-size 1T ' ReS2xSe2(1–x)alloy monolayer with tunable bandgap and carrier type., 2017, 29(46): 1705015–1–9.

[77] CUI F, WANG C, LI X,. Tellurium-assisted epitaxial growth of large-area, highly crystalline ReS2atomic layers on mica substrate., 2016, 28(25): 5019–5024.

[78] HAFEEZ M, GAN L, LI H,. Chemical vapor deposition synthesis of ultrathin hexagonal ReSe2flakes for anisotropic Raman property and optoelectronic application., 2016, 28(37): 8296–8301.

[79] AL-DULAIMI N, LEWIS E A, LEWIS D J,. Sequential bottom-up and top-down processing for the synthesis of transition metal dichalcogenide nanosheets: the case of rhenium disulfide (ReS2)., 2016, 52(50): 7878–7881.

[80] YELLA A, THERESE H A, ZINK N,. Large scale MOCVD synthesis of hollow ReS2nanoparticles with nested fullerene-like structure., 2008, 20(11): 3587– 3593.

[81] KIM S, YU H K, YOON S,. Growth of two-dimensional rhenium disulfide (ReS2) nanosheets with a few layers at low temperature., 2017, 19(36): 5341–5345.

[82] CHATURVEDI A, SLABON A, HU P,. Rapid synthesis of transition metal dichalcogenide few-layer thin crystals by the microwave-induced-plasma assisted method., 2016, 450: 140–147.

[83] AL-DULAIMI N, LEWIS D J, ZHONG X L,. Chemical vapour deposition of rhenium disulfide and rhenium-doped molybdenum disulfide thin films using single-source precursors., 2016, 4(12): 2312–2318.

[84] AL-DULAIMI N, LEWIS E A, SAVJANI N,. The influence of precursor on rhenium incorporation into Re-doped MoS2(Mo1–xReS2) thin films by aerosol-assisted chemical vapour deposition (AACVD)., 2017, 5(35): 9044–9052.

[85] SIMCHI H, WALTER T N, CHOUDHURY T H,. Sulfidation of 2D transition metals (Mo, W, Re, Nb, Ta): thermodynamics, processing, and characterization., 2017, 52(17): 10127–10139.

[86] BOROWIEC J, GILLIN W P, WILLIS M A C,. Room temperature synthesis of ReS2through aqueous perrhenate sulfidation.:, 2018, 30(5): 055702–1–11.

[87] HU S Y, CHEN Y Z, TIONG K K,. Growth and characterization of molybdenum-doped rhenium diselenide., 2007, 104(1): 105–108.

[88] QI F, CHEN Y, ZHENG B,. Facile growth of large-area and high-quality few-layer ReS2by physical vapour deposition., 2016, 184: 324–327.

[89] CUI F, LI X, FENG Q,. Epitaxial growth of large-area and highly crystalline anisotropic ReSe2atomic layer., 2017, 10(8): 2732–2742.

[90] LI X, CUI F, FENG Q,. Controlled growth of large-area anisotropic ReS2atomic layer and its photodetector application., 2016, 8(45): 18956–18962.

[91] ZHANG T, JIANG B, XU Z,. Twinned growth behaviour of two-dimensional materials., 2016, 7: 13911– 1–7.

[92] CHEN P, WANG J, LU Y,. The fabrication of ReS2flowers at controlled locations by chemical vapor deposition., 2017, 89: 115–118.

[93] QIN J K, SHAO W Z, LI Y,. Van der Waals epitaxy of large-area continuous ReS2films on mica substrate., 2017, 7(39): 24188–24194.

[94] HE X, LIU F, HU P,. Chemical vapor deposition of high-quality and atomically layered ReS2., 2015, 11(40): 5423–5429.

[95] YANG S, TONGAY S, YUE Q,. High-performance few-layer Mo-doped ReSe2nanosheet photodetectors., 2014, 4: 5442–1–6.

[96] WU K, CHEN B, YANG S,. Domain architectures and grain boundaries in chemical vapor deposited highly anisotropic ReS2monolayer films., 2016, 16(9): 5888–5894.

[97] CHEN Y, GAN L, LI H,. Achieving uniform monolayer transition metal dichalcogenides film on silicon wafersilanization treatment: a typical study on WS2., 2017, 29(7): 160550–1–6.

[98] YAN C, GAN L, ZHOU X,. Space-confined chemical vapor deposition synthesis of ultrathin HfS2flakes for optoelectronic application., 2017, 27(39): 1702918– 1–9.

[99] JIN B, HUANG P, ZHANG Q,. Self-limited epitaxial growth of ultrathin non-layered CdS flakes for high-performance photodetectors.., 2018, 28: 1800181– 1–9.

[100] JU M, LIANG X, LIU J,. Universal substrate-trapping strategy to grow strictly monolayer transition metal dichalcogenides crystals., 2017, 29(14): 6095–6103.

[101] ZHANG Q, XIAO Y, ZHANG T,. Iodine-mediated chem­ical vapor deposition growth of metastable transition metal di­chalcogenides., 2017, 29(11): 4641– 4644.

[102] HUANG W, GAN L, YANG H,. Controlled synthesis of ultrathin 2DIn2S3with broadband photoresponse by chemical vapor deposition., 2017, 27(36): 1702448– 1–9.

[103] GONG Y, LIN Z, YE G,. Tellurium-assisted low-temperature synthesis of MoS2and WS2monolayers., 2015, 9(12): 11658–11666.

[104] ZHOU S, GAN L, WANG D,. Space-confined vapor deposition synthesis of two dimensional materials., 2017: 12274–1–23.

[105] SONG J G, PARK J, LEE W,. Layer-controlled, wafer- scale, and conformal synthesis of tungsten disulfide nanosheets using atomic layer deposition., 2013, 7(12): 11333–11340.

[106] JANG Y, YEO S, LEE H B R,. Wafer-scale, conformal and direct growth of MoS2thin films by atomic layer deposition., 2016, 365: 160–165.

[107] MEMARIAN N, ROZATI S M, CONCINA I,. Deposition of nanostructured CdS thin films by thermal evaporation method: effect of substrate temperature., 2017, 10(7): 773–1–8.

[108] MAZUR M, HOWIND T, GIBSON D,. Modification of various properties of HfO2thin films obtained by changing magnetron sputtering conditions., 2017, 320: 426–431.

[109] ZHAN L, WAN W, ZHU Z,. MoS2materials synthesized on SiO2/Si substratesMBE., 2017, 864: 012037–1–5.

[110] HAMALAINEN J, MATTINEN M, MIZOHATA K,. Atomic layer deposition of rhenium disulfide., 2018, 30(24): 1703622–1–6.

[111] BISWAS D, GANOSE A M, YANO R,. Narrow-band anisotropic electronic structure of ReS2., 2017, 96(8): 085205–1–7.

[112] LIU F, ZHENG S, HE X,. Highly sensitive detection of polarized light using anisotropic 2D ReS2., 2016, 26(8): 1169–1177.

[113] HUANG C C, KAO C C, LIN D Y,. A comprehensive study on the optical properties of thin gold-doped rhenium disulphide layered single crystals., 2013, 52(4): 04CH11–1–6.

[114] HO C H, HSIEH M H, WU C C,. Dichroic optical and electrical properties of rhenium dichalcogenides layer compounds., 2007, 442(1/2): 245–248.

[115] CUI Q, HE J, BELLUS M Z,. Transient absorption measurements on anisotropic monolayer ReS2., 2015, 11(41): 5565–5571.

[116] ASLAN O B, CHENET D A, VAN DER ZANDE A M,. Linearly polarized excitons in single- and few-layer ReS2crystals., 2016, 3(1): 96–101.

[117] ZHENG J Y, LIN D Y, HUANG Y S. Piezoreflectance study of band-edge excitons of ReS2:Au., 2009, 48(5): 052302–1–6.

[118] WU S, SHAN Y, GUO J,. Phase-engineering-induced generation and control of highly anisotropic and robust excitons in few-layer ReS2., 2017, 8(12): 2719– 2724.

[119] HO C H, LEE H W, WU C C. Polarization sensitive behaviour of the band-edge transitions in ReS2and ReSe2layered semiconductors., 2004, 16(32): 5937–5944.

[120] ARORA A, NOKY J, DRUEPPEL M,. Highly anisotropic in-plane excitons in atomically thin and bulklike 1T’-ReSe2., 2017, 17(5): 3202–3207.

[121] SIM S, LEE D, TRIFONOV A V,. Ultrafast quantum beats of anisotropic excitons in atomically thin ReS2., 2018, 9(1): 351–1–7.

[122] SIM S, LEE D, NOH M,. Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2., 2016, 7: 13569–1–6.

[123] CHAVES A, LOW T, AVOURIS P,. Anisotropic exciton Stark shift in black phosphorus., 2015, 91(15): 155311–1–7.

[124] SAITO R, TATSUMI Y, HUANG S,. Raman spectroscopy of transition metal dichalcogenides., 2016, 28(35): 353002–1–37.

[125] ZHANG S, MAO N, ZHANG N,. Anomalous polarized raman scattering and large circular intensity differential in layered triclinic ReS2., 2017, 11(10): 10366–10372.

[126] TAUBE A, LAPINSKA A, JUDEK J,. Temperature dependence of Raman shifts in layered ReSe2and SnSe2semiconductor nanosheets., 2015, 107(1): 013105– 1–5.

[127] MIAO P, QIN J K, SHEN Y,. Unraveling the Raman enhancement mechanism on 1T'-phase ReS2nanosheets., 2018: 1704079–1–8.

[128] MCCREARY A, SIMPSON J R, WANG Y,. Intricate resonant Raman response in anisotropic ReS2., 2017, 17(10): 5897–5907.

[129] PRADHAN N R, MCCREARY A, RHODES D,. Metal to insulator quantum-phase transition in few-layered ReS2., 2015, 15(12): 8377–8384.

[130] CHENET D A, ASLAN O B, HUANG P Y,. In-plane anisotropy in mono- and few-layer ReS2probed by Raman spectroscopy and scanning transmission electron microscopy., 2015, 15(9): 5667–5672.

[131] HE R, YAN J A, YIN Z,. Coupling and stacking order of ReS2atomic layers revealed by ultralow-frequency Raman spectroscopy., 2016, 16(2): 1404–1409.

[132] WU J, MAO N, XIE L,. Identifying the crystalline orientation of black phosphorus using angle-resolved polarized Raman spectroscopy., 2015, 127(8): 2396– 2399.

[133] TAO J, SHEN W, WU S,. Mechanical and electrical anisotropy of few-layer black phosphorus., 2015, 9(11): 11362–11370.

[134] XIA F, WANG H, JIA Y. Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics., 2014, 5: 4458–1–6.

[135] QIAO J, KONG X, HU Z X,. High-mobility transport anisotropy and linear dichroism in few-layer black phosphorus., 2014, 5: 4475–1–7.

[136] LI L, GONG P, WANG W,. Strong in-plane anisotropies of optical and electrical response in layered dimetal chalcogenide., 2017, 11(10): 10264–10272.

[137] LIN Y C, KOMSA H P, YEH C H,. Single-layer ReS2: two-dimensional semiconductor with tunable in-plane anisotropy., 2015, 9(11): 11249–11257.

[138] LI L, PI L, LI H,. Photodetectors based on two-dimensional semiconductors: progress, opportunity and challenge., 2017, 62(27): 3134–3153.

[139] YAN K, WEI Z, ZHANG T,. Near-infrared photoresponse of one-sided abrupt MAPbI3/TiO2heterojunction through a tunneling process., 2016, 26(46): 8545– 8554.

[140] ZENG L, TAO L, TANG C,. High-responsivity UV-Vis photodetector based on transferable WS2film deposited by magnetron sputtering., 2016, 6: 20343–1–8.

[141] LI L, WANG W, CHAI Y,. Few-layered PtS2phototransistor on h-BN with high gain., 2017, 27: 1701011–1–8.

[142] JIANG J W. Thermal conduction in single-layer black phosphorus: highly anisotropic?, 2015, 26(5): 055701–1–6.

[143] LUO Z, MAASSEN J, DENG Y,. Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus., 2015, 6: 8572–1–8.

[144] LEE S, YANG F, SUH J,. Anisotropic in-plane thermal conductivity of black phosphorus nanoribbons at temperatures higher than 100 K., 2015, 6: 8573–1–7.

[145] MA J L, CHEN Y N, HAN Z,. Strong anisotropic thermal conductivity of monolayer WTe2., 2016, 3(4): 045010–1–8.

[146] LIU G, SUN H Y, ZHOU J,. First-principles study of lattice thermal conductivity of Td–WTe2.., 2016, 18(3): 033017–1–9.

[147] CARRETE J, MINGO N, CURTAROLO S. Low thermal conductivity and triaxial phononic anisotropy of SnSe., 2014, 105(10): 101907–1–5.

[148] LI Y L, SHI X, REN D D,. Investigation of the anisotropic thermoelectric properties of oriented polycrystalline SnSe., 2015, 8(7): 6275–6285.

[149] GUO R Q, WANG X J, KUANG Y D,. First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS., 2015, 92(11): 115202–1–13.

[150] SUN B Z, MA Z, HE C,. Anisotropic thermoelectric properties of layered compounds in SnX2(X = S, Se): a promising thermoelectric material., 2015, 17(44):29844–29853.

[151] JANG H, RYDER C R, WOOD J D,. 3D anisotropic thermal conductivity of exfoliated rhenium disulfide., 2017, 29(35): 1700650–1–6.

[152] YU S, ZHU H, ESHUN K,. Strain-engineering the anisotropic electrical conductance in ReS2monolayer., 2016, 108(19): 191901–1–6.

[153] MENG M, SHI C G, LI T,. Magnetism induced by cationic defect in monolayer ReSe2controlled by strain engineering., 2017, 425: 696–701.

[154] MIN Y M, WANG A Q, REN X M,. Defect formation and electronic structure regulated by strain engineering in ReS2., 2018, 427: 942–948.

[155] ZHOU Z H, WEI B C, HE C Y,. Anisotropic Raman scattering and mobility in monolayer 1Td-ReS2controlled by strain engineering., 2017, 404: 276–281.

[156] ZHANG X O, LI Q F. Strain-induced magnetism in ReS2mon­olayer with defects., 2016, 25(11): 117103–1–5.

[157] LI T H, ZHOU Z H, GUO J H,. Raman scattering modification in monolayer ReS2controlled by strain engineering., 2016, 33(4): 046201–1–5.

[158] LI Y L, LI Y, TANG C. Strain engineering and photocatalytic application of single-layer ReS2., 2017, 42(1): 161–167.

[159] KAO Y C, HUANG T, LIN D Y,. Anomalous structural phase transition properties in ReSe2and Au-doped ReSe2., 2012, 137(2): 024509–1–7.

[160] YAN Y, JIN C, WANG J,. Associated lattice and electronic structural evolutions in compressed multilayer ReS2., 2017, 8(15): 3648–3655.

[161] HOU D, MA Y, DU J,. High pressure X-ray diffraction study of ReS2., 2010, 71(11): 1571–1575.

[162] NAUMOV P G, ELGHAZALI M A, MIRHOSSEINI H,. Pressure-induced metallization in layered ReSe2., 2017, 30(3): 035401–1–6.

[163] YANG S, WANG C, SAHIN H,. Tuning the optical, magnetic, and electrical properties of ReSe2by nanoscale strain engineering., 2015, 15(3): 1660–1666.

[164] ZHOU D, ZHOU Y, PU C,. Pressure-induced metallization and superconducting phase in ReS2., 2017, 2(19): 1–7.

[165] YAGMURCUKARDES M, BACAKSIZ C, SENGER R T,. Hydrogen-induced structural transition in single layer ReS2., 2017, 4(3): 035013–1–8.

[166] JO S H, PARK H Y, KANG D H,. Broad detection range rhenium diselenide photodetector enhanced by (3-aminopropyl) triethoxysilane and triphenylphosphine treatment., 2016, 28(31): 6711–6718.

[167] ALI M H, KANG D H, PARK J H. Rhenium diselenide (ReSe2) infrared photodetector enhanced by (3-aminopropyl) trimethoxysilane (APTMS) treatment., 2018, 53: 14–19.

[168] ZHANG X, LI Q. Electronic and magnetic properties of nonmetal atoms adsorbed ReS2monolayers., 2015, 118(6): 064306–1–7.

[169] LUO M, SHEN Y H, YIN T L. Structural, electronic, and magnetic properties of transition metal doped ReS2monolayer., 2017, 105(4): 255–259.

[170] LOH G C, PANDEY R. Robust magnetic domains in fluorinated ReS2monolayer.,2015, 17(28): 18843–18853.

[171] OBODO K O, OUMA C N M, OBODO J T,. Influence of transition metal doping on the electronic and optical properties of ReS2and ReSe2monolayers., 2017, 19(29): 19050–19057.

ReX2(X=S, Se): A New Opportunity for Development of Two-dimensional Anisotropic Materials

WANG Ren-Yan, GAN Lin, ZHAI Tian-You

(State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China)

Two dimensional (2D) materials have attracted wide attention due to their ultrathin atomic structure, large specific surface area and quantum confinement effect which are remarkably different from their bulk counterparts. Anisotropic materials are unique among reported 2D materials. Their orientation-dependent physical and chemical properties make it possible to selectively improve the performance of materials. As representative examples, Re-based transition metal dichalcogenides (Re-TMDs) have tunable bandgaps in visible spectrum, extremely weak interlayer coupling, and anisotropic properties in optics and electronics, which make them attractive in the application areas of electronics and optoelectronics. In this riviev, the unique crystal structures and intrinsic properties of the Re-based TMDs semiconductors are introduced firstly, and then the synthetic method is introduced, followed by discussion on the unique physical characterizations and optimized means. Finally, prospects and suggestions are put forward for the preparation and research of ReX2.

anisotropy; ReS2; ReSe2; review

1000-324X(2019)01-0001-16

10.15541/jim20180171

TQ174

A

2018-04-19;

2018-06-03

国家自然科学基金(91622117, 21825103) National Natural Science Foundation of China (91622117, 21825103)

王人焱(1994–), 男, 博士研究生. E-mail: Renyanwang@hust.edu.cn

翟天佑, 教授. E-mail: zhaity@hust.edu.cn