ReX2 (X=S, Se): 二維各向異性材料發(fā)展的新機遇

王人焱, 甘霖, 翟天佑

ReX2(X=S, Se): 二維各向異性材料發(fā)展的新機遇

王人焱, 甘霖, 翟天佑

(華中科技大學材料科學與工程學院, 材料成型與模具技術國家重點實驗室, 武漢 430074)

二維材料因其不同于體相的超薄原子結構、大的比表面積和量子限域效應等受到了人們的廣泛關注。二維各向異性材料作為二維材料家族的一員, 其取向依賴的物理和化學性質, 使得對該類材料性能的選擇性優(yōu)化成為可能。過渡金屬Re基硫屬化合物作為各向異性材料的典型代表, 具有可調的可見光波段吸收帶隙, 極弱的層間耦合作用力, 以及各向異性的光學、電學性能, 現(xiàn)已成為電子和光電子領域的研究熱點之一。本文主要介紹了ReX2(X=S, Se)的晶體結構和基本性質, 總結目前該材料體系主流的合成方法, 研究其各向異性物理特性及優(yōu)化的手段和條件, 并對ReX2的制備和發(fā)展進行了展望。

各向異性; ReS2; ReSe2; 綜述

超薄的原子結構和巨大的比表面積賦予二維材料不同于體相的光學、電子學、磁學等方面獨特的物理性質。石墨烯、MoS2、h-BN作為最常見的一類二維材料, 擁有高度對稱的晶體結構和晶格取向, 并表現(xiàn)出各向同性的物理性質, 在傳統(tǒng)大面積均勻器件的性能平衡上有很大的應用優(yōu)勢。而另外一類二維材料, 如黑磷(BP)、WTe2、IV族硫屬化合物(SnS、GeS)、過渡金屬Re基硫屬化合物(ReS2、ReSe2)等擁有不對稱的原子排布, 在不同取向上表現(xiàn)出差異顯著的電學、光學、熱學和機械性質[1-3], 為發(fā)展新一代多功能低維電子及光電子器件提供了可能。基于BP的場效應晶體管在“Z”方向和“扶手椅”方向的空穴霍爾遷移率相差1.6倍, 這種各向異性的電子輸運行為可用于神經(jīng)突觸器件、反向器等[4]; 各向異性的SnSe納米片沿、軸的熱電品質因子相差近3倍, 體現(xiàn)了各向異性的熱傳導, 有望應用于散熱器、熱阻器件等方面[5]; 此外, Wan等[6]報道了基于GeSe2的光電行為, 在450 nm波長激光的激發(fā)下, 角度依賴的光電流最大比值達到3.4, 在偏振光電探測器方面有極大的應用潛力。二維各向異性材料開始在各個領域嶄露頭角, 并且在有特定需求的器件設備上有了新的研究突破。

ReX2(X=S, Se)作為二維各向異性材料的一族, 憑借其較低的對稱結構和優(yōu)異的光學、電學、光電子和機械性能, 現(xiàn)已在微電子器件[7-12]、光電探測[9,13-18]、能源存儲[19-22]、光催化[23-29]等領域展現(xiàn)出良好的應用前景。

本文綜述了過渡金屬Re基硫屬化合物ReX2(X=S, Se)這一類各向異性材料。盡管已有ReS2[30]和ReX2[31]等的相關總結, 但對于該材料體系各向異性的深入探究還未有報道。本文從介紹ReX2的基本結構出發(fā), 研究結構對其性質的影響, 總結目前已報道的合成方法, 并討論了各向異性的結構所導致的各向異性的物理特性, 最后針對該材料的合成及未來的應用發(fā)展等提出了建議。

1 晶體結構與基本性質

1.1 晶體結構

ReS2和ReSe2都具有Re-TMDs半導體層狀結構, 有著相似的原子結構和空間排列。以ReS2為例, ReS2為三斜晶系, 屬于P-1空間群[32]。如圖1(a)所示, ReS2單胞包含4個Re原子和8個S原子[33], Re和S擁有不同的原子占位環(huán)境, 與其他具有高度對稱結構的TMDs(如MoS2、WS2等)不同的是, 由于Re原子核外未成鍵價電子的存在, 單胞中的4個Re原子會形成Re4團簇, Re-Re金屬鍵導致了Re鏈的形成[34], 致使結構對稱性降低。圖1(b)為單層ReS2原子結構的頂部視圖, 沿著[010]軸方向能夠發(fā)現(xiàn)明顯的Re4鏈。,方向夾角約為119.8°。ReS2層間間距約0.67 nm, 值得一提的是, ReS2為中心對稱結構[35]。表1為ReS2和ReSe2晶胞參數(shù)[36-37]。

通常來說, TMDs材料的電子結構和相結構類型均依賴于過渡金屬原子的配位環(huán)境和d軌道電子數(shù)目[38]。在八面體配位場中, 如圖1(c)所示, d能級分裂成eg和t2g能級, eg能級的兩個軌道dz2和dx2–y2與硫屬原子的p軌道雜化而形成σ成鍵軌道和σ*反鍵軌道, 并構成相應能量最高和最低的兩個能帶, 未成鍵的t2g軌道則形成介于σ和σ*帶之間的能帶, 若過渡金屬價電子填充半滿則呈現(xiàn)金屬性[39]。

在ReX2中, Re原子d軌道價電子與s原子p態(tài)為八面體配位類型簡并態(tài)。如圖1(d)所示, d軌道價電子填充至未成鍵的t2g能帶, 理論上應呈現(xiàn)金屬 性[40]。但是, Re4鏈的形成改變了ReX2的電子分布, 導致ReX2為扭曲的八面體結構, 并將帶隙打開, 最終呈現(xiàn)半導體特性。ReS2的費米能級更靠近導帶底, 為n型半導體; 而ReSe2費米能級更靠近價帶頂, 表現(xiàn)為p型半導體。

1.2 基本性質

在MoS2[41-44]、MoSe2[45]、WS2[42]等結構對稱的TMDs中, 電子的帶隙結構有著很強的層數(shù)依賴性。當層數(shù)減少至單層, 由于量子限域效應, 帶隙類型從間接帶隙轉變?yōu)橹苯訋? 進而引起PL和光響應等的顯著增強[38]。相比傳統(tǒng)的2D半導體材料, 由于Re–Re金屬鍵, ReX2表現(xiàn)出更弱的層間范德華力, 帶隙結構與層數(shù)的變化關系稍有不同。理論計算表明, ReSe2在塊體狀態(tài)時表現(xiàn)為直接帶隙半導體, 單層尺度下轉變?yōu)殚g接帶隙半導體[46], 而ReS2則一直保持直接帶隙的電子結構[47-48]。如圖1(e), Wolverson等[49]也通過理論計算得到塊體ReSe2為直接帶隙類型(1.09~1.31 eV), 在單層為間接帶隙電子結構(1.34 eV)。ReS2在塊體狀態(tài)下?lián)碛幸粋€直接類型帶隙結, 并且隨著厚度減薄至單層時保持不變[50]。Liu等[51]也對ReS2電子結構進行了研究, 圖1(f)中顯示為單層、3層、5層的ReS2能帶計算結果, 發(fā)現(xiàn)在層數(shù)變化下都保持直接帶隙結構。總結ReX2的電子結構特點, 層數(shù)會影響ReSe2的電子結構性質, 單層情況下表現(xiàn)為間接半導體帶隙, 塊體狀態(tài)下表現(xiàn)為直接帶隙半導體結構。ReS2電子結構形態(tài)稍有不同, 一直保持不隨層數(shù)變化的直接帶隙類型。

圖1 (a)ReS2單胞結構圖[33]; (b)ReX2晶體結構頂視圖; (c) ~(d)1T相結構軌道成鍵和能級示意圖[39-40]; (e)ReSe2塊體(上)和單層(下)帶隙結構[50]; (f)1層、3層、5層的ReS2能帶模擬圖[51]

表1 ReS2和ReSe2單胞晶格參數(shù)[36–37]

光致發(fā)光譜(PL)和Raman光譜等手段可以對ReX2獨特的電子結構進行表征。隨著厚度減薄至單層, MoS2[43,52-53]、WS2[54-55]等會由間接帶隙向直接帶隙轉變, 使得PL光譜信號大大增強; 而隨著層數(shù)的減少, ReX2帶隙增大, 發(fā)光卻明顯變弱。Zhao等[56]分別對單層, 少層, 10層和塊體的ReSe2進行PL測試, 并發(fā)現(xiàn)隨著層數(shù)減少到單層, 材料的帶隙逐漸增大, 峰強逐漸減弱, 單層的ReSe2表現(xiàn)出間接帶隙半導體的性質。相比于ReSe2, 單層和塊體的ReS2都具有直接帶隙的電子結構, 層間弱的范德華耦合使塊體材料具有更高的吸光和激子躍遷機率, 從而使PL大大增強。Tongay等[50]研究了室溫下不同層數(shù)ReS2的微區(qū)PL, 如圖2(a)和(b)所示, 不同層數(shù)ReS2的PL峰位未發(fā)生明顯變化, 均位于1.5~1.6 eV之間, 再次證明了弱的層間耦合; 且隨著層數(shù)的增加, 峰強度會逐漸增大, 直到6層以后達到飽和。圖2(b)還對比了其他二維材料, 如MoS2[40]、MoSe2[57]、WS2、WSe2[54]與ReS2的PL隨層數(shù)變化關系的差異。具體來說, 隨著層數(shù)的增加, M、W基過渡金屬硫屬化合物的PL強度降低了幾個數(shù)量級, 這是由于PL來源于直接帶隙的熱激發(fā)激子發(fā)光[53], 而熱激發(fā)激子的數(shù)量會隨著層數(shù)的增加呈數(shù)量級的減小。

HPLC法同時測定蒙藥達烏里芯芭中梓醇和益母草苷的含量 ……………………………………………… 包玉秋等（11）：1542

Raman光譜是分析分子振動、轉動, 反映分子本征結構的散射光譜。ReX2層間弱相互作用也體現(xiàn)在與層數(shù)相關的Raman光譜上。圖2(c)和(d)分別代表在2.33 eV平行偏振光激發(fā)下, ReS2、ReSe2的層內Raman振動。可以看出, 隨著層數(shù)的變化, 代表Re4鏈方向的分子振動Raman峰位未發(fā)生偏移[58]。Feng等[35]通過計算模擬了ReS2的Raman光譜, 并發(fā)現(xiàn)幾乎所有的Raman峰位都不隨層數(shù)變化, 說明ReS2層間具有極弱的耦合作用。

圖2 (a)ReS2不同層數(shù)的PL光譜; (b)ReS2, MoS2, MoSe2, WS2和WSe2的PL強度和層數(shù)依賴關系[50]; (c)ReS2和 (d) ReSe2單層到塊體厚度的拉曼光譜[58]; (e)ReS2納米卷自組裝機制[59]; (f)單層ReS2納米墻熱彎曲示意圖[60]

層間弱的耦合作用不僅表現(xiàn)在材料本征性質上, 也在實驗現(xiàn)象中有所體現(xiàn)。Fu等[59]通過電化學Li離子插層化學氣相沉積(CVD)合成的垂直ReS2納米片, 邊對邊自組裝形成ReS2納米卷。圖2(e)顯示納米卷自組裝機制示意圖, 首先通過Li離子插層和剝離獲得分散納米片, Li離子的電荷排斥和ReS2弱的層間耦合共同作用, 導致層間滑移形成自組裝納米片, 最后卷曲成納米卷結構。他們還對自組裝過程進行了模擬, 層間滑移模式顯示了最有效和最合理的生成途徑, 并且自卷曲降低了整個過程的總能量。另外, 通過熱引入的手段也可以觀察到垂直ReS2納米片的熱彎曲現(xiàn)象, 由于極弱的層間相互作用, 熱引入會導致(001)面滑移, 最終導致材料面外彎曲[60]。圖2(f)簡單地描述了ReS2熱彎曲機制。

2 合成方法與表征

目前, 針對ReX2的合成策略, 主要包括氣相輸運、化學輔助剝離、氣相沉積和其他的一些材料制備手段。

2.1 氣相輸運法

大部分機械剝離的樣品都是經(jīng)過化學氣相輸運(CVT)的方法獲得的單晶, 再經(jīng)過膠帶反復地剝離, 最后得到比較薄的二維材料。Jariwala等[61]通過氣相輸運法得到ReS2和ReSe2單晶。他們將Re和S/Se源放入處理過的密封石英管中, 在高真空條件下先緩慢升溫1100℃超過72 h, 退火24 h后以1℃/h降溫至900℃, 最后冷卻到室溫。氣相輸運的關鍵在于極其緩慢的組分交換, 利于形成高質量的單晶, 但缺點就是耗時較長。Hu等[62]通過氣相輸運法一步合成二維超薄的層狀ReS2, 大大縮短了合成時間, 而且省去了機械剝離2D材料的過程, 避免了膠帶剝離帶來的層數(shù)不可控性和有機物的污染。一步合成相比于傳統(tǒng)的CVT方法不同的地方在于封閉石英管細口處的優(yōu)化, 更細和更長的瓶頸有利于氣相更加緩慢平穩(wěn)的輸送達到生長2D的目的。他們也研究了載氣對實驗的影響, 發(fā)現(xiàn)在沒有輸運載氣的情況下并沒有樣品生成, 說明了輸運載氣在CVT實驗中的重要性。

2.2 輔助剝離法

二維材料輔助剝離一般包括溶液超聲離心分散[63]、溶劑輔助液相剝離[52,64-69]、干法化學反應剝離[70]等。Chen等[70]利用無溶劑的化學剝離方法成功剝離ReS2粉末形成2D納米片。圖3(a)分別為ReS2處理前后掃描電子顯微鏡(SEM)和透射電子顯微鏡(TEM)照片, 剝離過后納米片尺寸變小, 大約50~100 nm。插圖為剝離納米片的水溶液照片, 呈現(xiàn)典型的暗棕色。

為了檢驗剝離樣品的質量, 用STEM來表征化學剝離的單層ReS2納米片。如圖3(b)所示, Re4原子鏈排列清晰, 證明了高質量的1T’相ReS2。Kang等[63]利用等密度梯度高速離心得到了不同厚度的ReS2納米片, 并且在溶液中呈梯度排列, 實驗的關鍵是在高粘性的碘克沙醇溶劑中添加CsCl, 增加了溶液最大懸浮密度。圖3(c)顯示通過等密度梯度高速離心法(iDGU)分離ReS2的機制, 在不同的密度下分別懸浮不同層數(shù)的樣品。原子力顯微鏡(AFM)照片(圖3(d))和Raman光譜(圖3(e))也證明了該分散樣品為ReS2納米片。

2.3 氣相沉積法

氣相沉積是一種比較常見的制備大尺寸高質量2D材料的合成方法, 相比于其他合成方法, 其優(yōu)點在于合成方便、樣品厚度和形貌可控, 缺點是不能完全實現(xiàn)大規(guī)模的樣品制備, 是材料基礎研究比較青睞的合成方法之一。氣相沉積包括物理氣相沉積和化學氣相沉積, 兩者的區(qū)別在于合成過程中原料在氣相狀態(tài)下有沒有發(fā)生化學反應。近年來, ReX2的氣相合成已經(jīng)有很多文獻報道[10,14,29,34,71-96]。

Qi等[88]通過直接使用ReS2粉末作為反應源, 如圖4(a)~(c)所示, 通過物理氣相沉積在SiO2/Si襯底上得到了厚度均一的ReS2薄膜, Raman復雜的峰位來源于低對稱的晶體結構, 插圖中TEM表征也證明成功合成了ReS2, AFM測試厚度約為2.3 nm, 對應3層。Keyshar等[34]通過CVD首次合成了單層ReS2, 低熔點高錸酸銨(NH4ReO4)的引入降低了反應所需要的溫度。Hafeez等通過CVD方法合成了ReS2[74]和ReSe2[78], 如圖4(d)~(e), 對比純藍寶石襯底可以看出, 在藍寶石襯底上生長的2層ReS2薄膜呈現(xiàn)均一的顏色襯度, 在氧化硅基底也可以得到六方形的ReS2納米片結構。

大尺寸超薄2D合成是CVD的主要特點, 但是材料生長存在很多的影響因素, 比如氣流、溫度、源量、襯底類型等都會對材料的合成產(chǎn)生影響, 所以, 在CVD合成過程中各個參數(shù)的摸索和探究一直是研究者們所關注的問題。為了優(yōu)化Re-TMDs的CVD合成, 研究者們采用了很多手段來輔助調控實驗參數(shù), 以便獲得更好的生長效果[97-102]。Cui等[77]通過Te輔助CVD合成控制源蒸發(fā)的手段在云母襯底上合成了厚度均一的單層高質量ReS2納米片。圖5(a)為Te輔助合成ReS2示意圖, 采用云母襯底倒扣的方式, Te粉和Re粉進行混合會在高溫時形成Re–Te合金相, 大大降低了Re的蒸發(fā)熔點, 提高Re蒸氣壓促進反應的進行。有趣地是, 得到的樣品中并沒有表征出Te的存在, 說明Te并不會造成產(chǎn)物污染, 這在之前WS2和MoS2的CVD生長中也被證實[103]。圖5(b)為轉移到SiO2/Si襯底上的ReS2樣品, 表現(xiàn)出均一的顏色襯度和一致的形貌大小, 插圖中AFM表征顯示0.7 nm, 表明合成的樣品為單層ReS2。

圖3 (a)分離前ReS2納米片SEM照片(左)和分離后TEM照片(右), 插圖：剝離樣品水溶液光學照片; (b)HRTEM照片[70]; (c)等密度梯度超速離心分離不同密度梯度ReS2納米片示意圖; (d)AFM照片; (e)ReS2拉曼光譜圖[63]

圖4 (a)PVD制備ReS2薄膜示意圖; (b)ReS2薄膜Raman光譜圖; (c)光學照片, 插圖：AFM照片和TEM照片[88]; (d)藍寶石襯底和ReS2薄膜光學照片; (e)SiO2/Si襯底CVD生長ReS2納米片光學照片[74]

圖5 (a)Te輔助CVD合成ReS2示意圖; (b)轉移至SiO2/Si襯底ReS2光學照片, 插圖為AFM照片[77]; (c)夾層限域生長ReSe2表面反應機制示意圖; (d)A、B面的ReSe2形貌光學照片[89]

除了利用合金化的手段降低源熔點, 提高反應源供給量以外, 采用襯底夾層限域空間也是一種控制材料生長的手段[104]。Li等[90]采用兩片云母夾層的方式構造限域空間, 在夾層間能夠生長原子層平整尺寸達到幾十微米的ReS2薄片。Xu等[89]也通過夾層的方法合成了ReSe2。首先, 云母原子級平整的表面有利于ReSe2原子的表面遷移促進均勻生長; 其次, 云母夾層的限域空間抑制了低對稱ReSe2面外生長活性。圖5(c)顯示了夾層限域生長ReSe2表面反應機制, 由于傳質和反應之間的相互關系, 在夾層內的A面能夠得到完整平坦的ReSe2納米片和薄膜, 而在夾層外的B面, 會產(chǎn)生不平整的島狀納米片結構。圖5(d)轉移的ReSe2納米片和薄膜光學照片證明了限域手段的可行性。

另外一種控制源的手段是源的吸附和緩釋放, 大多采用無機多孔的材料, 如(陶瓷、碳粉、多孔Al2O3)去吸附在高溫過程中過多的蒸發(fā)源。Xu等[76]采用分子篩輔助控制源蒸發(fā)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確控制S、Se反應的濃度以外, Re源的濃度也至關重要。多孔分子篩的加入有效緩釋了氣相ReO3, 保證了云母襯底在合適的生長溫度條件下也擁有均勻的源供應, 達到平衡大尺寸生長的目的。通過此種方法生長的合金最大能夠達到206 μm。

2.4 其他合成方法

傳統(tǒng)的2D材料CVD合成除了以上方法以外, 還有原子層沉積(ALD)[105-106]、熱蒸鍍[107]、磁控濺射[108]、分子束外延(MBE)[109]等。H?m?l?inen等[110]通過ALD沉積技術探究了不同沉積參數(shù)對ReS2生長形貌的影響。實驗結果表明合適的沉積溫度在120℃到150℃之間, ReCl5源的增加抑制了垂直ReS2納米片的產(chǎn)生, 而H2S含量的增加會使ReS2納米片寬化。

3 各向異性特性

3.1 各向異性光學和拉曼振動

ReS2和ReSe2各向異性電子結構已經(jīng)通過角分辨光電子能譜得到證實[47-48, 111-112], 各向異性的介電性導致不同取向的折射率差異, 進而體現(xiàn)在光學各向異性上。而ReX2線性二色性的光學各向異性, 已經(jīng)在角度依賴的光吸收[112-115]、激子[116-122]中得到證明。Sim等[122]在少層ReS2中發(fā)現(xiàn)激子光學斯塔克效應, 圖6(a)展示了泵浦探測少層ReS2納米片各向異性激子示意圖。他們利用線性偏振光極化ReS2觀察到角度依賴的光激子吸收, 如圖6(b)和(c)所示, 非簡并激子隨極化角度會呈線性變化, 通過擬合得到, X1, X2各向異性激子最大極化光吸收分別在19°和87°。

另外, 類似于黑磷[123], 利用泵浦探測ReS2也發(fā)現(xiàn)激子存在光學斯塔克位移和超快的量子震蕩現(xiàn)象[121], 為將來ReS2各向異性超快光學的應用提供了可能。

ReX2低對稱的晶格取向也造成了各向異性Raman振動和聲子耦合。角度依賴的Raman振動是確定物質Raman振動模式的有效手段之一[124]。盡管不同激光波長[125]、不同溫度[126]、樣品厚度[35]和分子修飾[127]、疊層取向[46,125]都會對ReX2的Raman測量產(chǎn)生影響, 但對于各向異性本征特性而言, 并不會隨著條件變化而消失。ReX2的Raman振動模式包括層內振動模式和低頻層間耦合模式。圖6(d)上圖顯示了塊體ReS2在100~450 cm–1之間18種具有Raman活性的層內振動模式[128]。類似于之前的報道[129], 這里把所有的振動模式全都定義為Ag模式, 在150 cm–1處的面內振動模式為Ag1, 在437 cm–1處的S原子面外振動模式為Ag2, 418 cm–1處的S原子面內外共振模式為Ag3, 這里要指出的是, 在約213 cm–1處的Raman峰振動對應面內Re–Re鏈(軸方向)的振動方向, 這在很多關于ReS2研究中已有報道[50,130]。表2列舉了在633 nm激光下單層和塊體18種的Raman振動頻率和振動來源。低對稱性結構造就了復雜的Raman模式, 而弱范德華耦合導致了不隨厚度變化的Raman振動。除了層內振動模式以外, ReX2弱的層間耦合在低頻Raman振動中也有所體現(xiàn)。圖6(d)下圖為少層ReS2低頻模式Raman光譜, 與MoS2相同, 層間呼吸模式在更高的振動頻率, 非簡并滑移模式位于比較低的振動頻率位置[131]。Chenet等[130]研究了不同厚度和偏振光下的ReS2Raman振動, 他們發(fā)現(xiàn)幾個主要的Raman振動峰隨層數(shù)的變化都沒有明顯的峰位偏移, 但都表現(xiàn)出極化角度依賴性。單層ReS2從0~180°極化Raman光譜顯示在圖6(e)中, 可以看出, 對于比較突出的振動模式, 峰強度都擁有角度依賴關系, 隨著偏振角度發(fā)生改變。圖6(f)為塊體ReS2在150和213 cm–1振動峰位下的極化Raman曲線擬合, 兩種模式都隨角度呈現(xiàn)180°周期性的變化, 并且 213 cm–1的極化取向也剛好對應高分辨中軸的方向, 進一步證明了之前的觀點[50]。

圖6 (a)泵浦探測少層ReS2納米片實驗示意圖, 插圖：剝離ReS2光學照片; (b)角度依賴的少層ReS2納米片光吸收譜; (c)X1, X2激子角度依賴的光吸收極化圖[122]; (d)ReS2 Raman光譜[128]和低頻Raman光譜[131]; (e)ReS2不同旋轉角度的Raman光譜; (f)少層ReS2納米片高分辨TEM照片和對應偏振Raman極化圖[130]

表2 633 nm激光激發(fā)塊體和單層ReS2的18種Raman振動光譜[35]

3.2 各向異性電子輸運

2D各向異性材料的不對稱晶格結構影響了不同取向上電子和空穴的有效質量, 進而反映出不同的電子輸運行為。例如黑磷(BP), “扶手椅”方向的 電子空穴有效質量小于“Z”方向, 表現(xiàn)出更高的電 導[132-134]和載流子遷移率[135]。Zhai等[136]報道的層狀雙金屬硫化物Ta2NiS5各向, 異性電子輸運, 在80 K條件下, 沿軸的電導是軸方向的1.78倍。ReX2作為各向異性材料中的一類, 在電子輸運上也表現(xiàn)出取向差異。圖7(a)為機械剝離的ReS2不同取向的場效應晶體管(FET)器件光學照片, 圖7(b)中的高分辨透射照片對應圖7(a)中不同電極的原子取向, 1-4和2-3電極對應[010]方向的電子傳導, 1-3和2-4電極電子傳導垂直于方向。20 V柵壓下的-測試表明, 如圖7(c), 在1 V偏壓下, 不同原子取向的電子輸運有明顯的差異, 沿軸方向的電導為 0.82 μS, 垂直方向的電導為0.075 μS, 相差了10.9倍。他們也測試了不同取向的ReS2的轉移特性曲線, 如圖7(d), 在–3 V偏壓下, 1-4電極的ReS2開關比達到106, 遷移率為23.1 cm2/(V·s), 而1-2電極遷移率為14.8 cm2/(V·s), 相差1.56倍[137]。Xing等[51]制備了單層和少層ReS2場效應晶體管, 研究各向異性的電導和遷移率。如圖7(e)中、軸方向的轉移特性曲線,、方向電導有明顯區(qū)別, 不同柵壓的各向異性電導比表現(xiàn)出柵壓依賴的電學性質。插圖中關于ReS2接觸電阻的測試也證明了ReS2固有的各向異性本征特性。另外, 為了進一步研究各向異性的電子輸運, 對角度極化的遷移率進行了計算, 如圖7(f)。通過理論和實驗得出, 不同取向的極化遷移率最大比值達到3.1, 是目前2D各向異性材料中最大的極化遷移率比值。除了ReS2以外, ReSSe合金的各向異性電學輸運也有人研究, Xu等[76]利用CVD方法合成了ReS1.23Se0.77合金并構筑了FET器件。如 圖7(g)所示, FET開關比達到105, 沿軸方向和垂直軸方向的遷移率分別為0.34 cm2/(V·s)和0.12 cm2/(V·s), 相差接近3倍。

3.3 各向異性光電子器件

高性能光電子器件一直是未來研究和發(fā)展趨勢之一[138-142]。近年來, 關于各向異性材料光電探測也漸漸引起人們的關注, ReX2光電探測一直是各向異性材料中所關注和研究的熱點。Liu等[112]報道了ReS2各向異性的光電子效應, 隨著極化光的偏轉, 光電流發(fā)生顯著的變化。如圖8(a)和(b)所示, 在同一偏壓下, 極化角度至90°對應ReS2樣品Re4鏈方向時, 光電流達到最大值, 產(chǎn)生電流差異的主要原因是取向性的光吸收差異以及材料本征的結構不對稱。Xu等[76]也報道了ReSSe合金的光電各向異性, 他們測試了ReS1.06Se0.94合金納米片角度依賴光電性能, 如圖8(c)和(d)所示,-曲線顯示光電流隨角度呈現(xiàn)周期性的變化, 證明了ReX2光電子的各向異性輸運。另外, 和之前Raman峰的極化振動類似, 角度依賴的極化擬合曲線表明最大的光電流取向也完美地對應Re4鏈的晶格方向。此外, 關于ReSe2納米片各向異性光電子也有人報道[15], 如圖8(e)和(f)所示, 圖8(e)顯示ReSe2極化光電探測器示意圖, 0至90°的光電流成像直觀地表明了光電流來自于溝道而不是電極, 排除了肖特基勢壘的影響。在=0時, 偏振光沿著軸, 溝道內電流最大,=90°時, 偏振光垂直軸, 溝道內電流最小, 電流最大值分布取向也和偏振光基本一致, 證明了光電流的差異本質上來源于晶格各向異性, 顯示了光電流的線性二色行為。

圖7 (a)ReS2晶體管光學照片; (b)對應(a)圖ReS2的TEM照片; (c)不同電極的I-V曲線; (d)不同電極之間的ReS2轉移特性曲線[137]; (e)各向異性ReS2晶體管轉移特性曲線, 插圖為器件光學照片和R-Vbg曲線; (f)角度依賴的 FET遷移率極化圖, 插圖：器件光學照片[51]; (g)ReS1.23Se0.77合金各向異性轉移特性曲線, 插圖為器件光學照片[76]

圖8 (a)角度依賴的ReS2極化光電流圖; (b)極化光電流曲線[112]; (c)偏振光I-t曲線; (d)偏振光電流極化圖[76]; (e)ReSe2偏振光電器件示意圖; (f)器件SEM照片和不同角度的入射光電流分布圖[15]

3.4 各向異性熱傳導

材料的熱傳輸也是人們比較關注的話題, 良好的熱傳導材料在一些器件諸如CPU、存儲器等使用中能有效地降低整個器件的能耗和溫度, 提高器件的性能。各向異性的熱導材料為材料的熱傳導提供了新的選擇和發(fā)展方向。2D各向異性材料諸如BP[142-144]、WTe2[145-146]、SnSe[147-149]、SnS2[150]等, 都在熱傳輸方面表現(xiàn)出優(yōu)異的取向差異。關于ReX2的熱傳導研究目前報道很少, 但作為各向異性材料, 在不同取向上晶格振動造成聲子耦合差異必然會導致熱傳導不同。Cahill等[151]報道了3D ReS2各向異性的熱傳導性質, 他們發(fā)現(xiàn)沿著Re4鏈方向的面內熱導率系數(shù)為(70±18)W×m–1×K–1, 垂直Re4鏈方向的熱導系數(shù)為(50±13) W×m–1×K–1, 相差1.4倍, 而相比于面內熱傳導, 垂直于面內的面外熱傳導表現(xiàn)出近乎熱阻的性質, 熱導率系數(shù)僅有(0.55±0.07) W×m–1×K–1, 這也是目前2D材料中關于面外熱導報道最低的數(shù)值。低的面外熱傳導系數(shù)主要歸因于ReS2極弱的層間耦合作用。圖9(a)展示了測試熱導率剝離的ReS2的光學照片。熱導率的測試通過時間分辨的熱致反射光譜(TDTR)來完成, 主要是利用一定頻率的光進行熱滲透, 測量不同偏移位置的輸入和輸出相位電壓來確定熱導大小。圖9(b)顯示了面外熱傳導不同頻率的TDTR數(shù)值擬合曲線, 相比高頻光熱輻照, 低頻熱傳導顯示更大的輸入輸出相位電壓比, 更能體現(xiàn)材料熱導的本征差異性。面內不同取向的熱傳導如圖9(c)和(d)所示, 利用1.1 MHz頻率熱輻照, 熱信號隨位移呈規(guī)律性減弱, 沿(Re4鏈)方向和沿(垂直Re4鏈)方向的等值熱信號呈現(xiàn)橢圓形, 表明在不同取向上熱傳導的各向異性。

圖9 (a)剝離層狀ReS2光學照片; (b)面外熱傳導曲線; (c)極化方向的熱傳導曲線; (d)面內熱傳導分布圖; (e)不同厚度的面內極化熱導率[151]

另外, 不同厚度ReS2的面內熱導也表現(xiàn)在圖9(e)中, 可以看出, 在不同的厚度情況下, ReS2同樣表現(xiàn)出取向依賴的熱導現(xiàn)象, 并且平行Re4鏈方向熱導率要高于垂直方向。

4 修飾和改性

盡管擁有獨特本征性質的ReX2已經(jīng)在很多方面表現(xiàn)出優(yōu)異的性能, 但通過其他手段提高和改進材料的性能仍舊是一個大家比較感興趣的方向。研究者們已經(jīng)通過應力工程[152-164]、分子修飾[16,165-168]和元素摻雜[169-171]等手段研究了ReX2性質和性能的變化。對于各向異性而言, 外部環(huán)境對材料本征各向異性的影響是人們較為關注的問題。目前, ReX2相轉變是材料各向異性性質變化的一種明顯反映。Li等[152]研究了在應力條件下, 單層ReS2電子空穴有效質量和遷移率的變化, 通過理論計算, 他們發(fā)現(xiàn)在軸方向的應力作用會導致ReS2直接帶隙向間接帶隙的轉變。各向異性的載流子遷移率也會受到應力作用的影響, 當在軸方向施加5%應力時, 理論計算表明軸方向的電子遷移率會從338.83 cm2/(V·s)增加到3940.21 cm2/(V·s),軸方向電子遷移率則從799.64 cm2/(V·s)增加到了4300.22 cm2/(V·s); 同樣地,軸方向空穴遷移率從239.80 cm2/(V·s)增加 到1439.84 cm2/(V·s),軸方向空穴遷移率從 30.90 cm2/(V·s)增加到1100.76 cm2/(V·s)。可見, 應力調控是一種改變ReX2各向異性的有效手段。

除了應力調控之外, 分子吸附也是調節(jié)材料各向異性的一種方法。Sahin等[165]研究發(fā)現(xiàn), H原子吸附ReS2會形成強的S-H鍵, 導致1T-ReS2結構變成更具有動力學穩(wěn)定性的Re–Re二聚體結構1T-ReS2H2。H原子吸附之前, 計算表明ReS2沿軸方向的硬度為159 J/m2, 楊氏模量為477 GPa, H原子吸附之后, 由于形成Re–Re二聚體打破了Re4團簇的穩(wěn)定性, 硬度和楊氏模量都有所降低, 沿軸方向的硬度降為97 J/m2, 楊氏模量則降為250 GPa。該工作表明H原子的吸附改變了ReS2各向異性的性能參數(shù), 為調控ReX2各向異性的特殊性質提供了方向。

5 總結和展望

與普通的2D材料相比, 各向異性的2D材料擁有更為突出的材料特性和更廣闊的應用前景。本文總結了2D各向異性材料中的典型代表：過渡金屬Re基硫屬化合物ReX2, 介紹了其獨特的晶體結構和由結構導致的特殊材料性質, 并分析了性質產(chǎn)生的原因。另外, 對于當前ReX2的主要合成方法包括機械剝離、輔助剝離、氣相沉積等做了簡單的總結; 最主要的是針對ReX2獨特的各向異性性質和改進手段進行了討論, 希望能夠為將來ReX2的應用和發(fā)展提供有價值的指導。

雖然ReX2已經(jīng)在很多領域有所應用, 但都只是處于實驗階段, 對于材料目前的研究發(fā)展仍然沒有達到真正應用化的水平。首先需要開發(fā)新的合成和制備方法來滿足工業(yè)化應用需求, 比如優(yōu)化實驗合成裝置, 采用卷軸襯底手段進行大面積合成; 其次, 針對材料各向異性性能的優(yōu)化仍然需要研究者們不斷的努力和改進, 例如尋找新材料來與之進行復合、摻雜, 或者利用異質結來調控各向異性性能; 另外, 各向異性基礎研究也還需要更深入地發(fā)掘和探索, 能夠從本質出發(fā)研究各向異性基本特性, 比如研究ReX2的電子結構變化引起的相變過程, 提出一些新的科學問題, 進而將會從根本上優(yōu)化和解決材料各向異性發(fā)展存在的難題, 這也將是今后發(fā)展各向異性材料ReX2新的趨勢和機遇。

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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

國家自然科學基金(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