单层，少层和块状WS2薄膜中声子位移随温度的变化（英文）

Temperature-Dependent Phonon Shifts in Mono-layer,Few-layer,and Bulk WS2 Films

近年来,二维过渡金属二硫化物(TMD)由于其良好的物理和化学性质而引起人们的关注。其中,石墨烯由于其高达200000 cm2·V-1·s-1高电子迁移率得到了深入研究。由于石墨烯没有带隙,因此基于石墨烯的器件难以实现高的电流开关比。对于二维过渡金属二硫化物例如MoS2、MoSe2、WSe2和WS2,这些材料可以根据层数来调节的带隙。其中,单层和多层MoS2薄膜已进行了广泛深入研究。已经有人实现具有优异电学性能的基于MoS2的场效应晶体管(FET)。与MoS2、MoSe2和MoTe2相比,WS2具有更低的面内电子质量,这表明基于WS2的FET具有更高的载流子迁移率或更高的输出电流。与MoS2相比,WS2缺乏系统的研究,需要更多的研究工作来进一步发掘基于WS2的场效应晶体管的潜能。因此,我们使用机械剥离法来制备WS2晶体单层(1L),少层(FL)和块状WS2薄膜。使用3M透明胶带转移WS2薄膜,并且使用514 nm激光器对1L、FL和块状WS2膜进行了变温的拉曼研究。随着膜厚度增加到块状,对于1L WS2,A1g(Γ)和E2g1(Γ)模式分别显示蓝移和红移。此外,当拉曼振动模式在E12g(Γ)和A1g(Γ)之间交换时,"交叉"温度被识别为1L、FL和块状WS2膜。与MoS2相比,随着膜厚度的变化,WS2在E2g1(Γ)和A1g(Γ)之间表现出较小的频率变化(Δ),并且从拉曼峰值位置随温度变化来看,WS2比MoS2少一个量级。这项工作为基于WS2的器件设计提供了物理指导。

Two-dimensional transition metal disulfides(TMDs) have recently attracted significant research attention due to their rich physical and chemical properties.Graphene has also been studied intensively due to its high electron mobility of ~200000 cm2·V-1·s-1.Since there is no band gap,it is difficult for a graphene-based device to achieve high current on/off ratio.For TMDs,such as MoS2,MoSe2,WSe2,and WS2,the band gaps of these materials can be adjusted according to the number of layers.Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects,it is also attractive for use as a channel material in Si complementary metal oxide semiconductor(CMOS) devices larger than 22 nm.Among them,MoS2 in singlelayer and multi-layer films have been intensively researched for many years.MoS2-based field effect transistors(FETs)with excellent electrical properties have been reported.WS2 has lower in-plane electronic mass than MoS2,MoSe2,and MoTe2,and therefore has potential for higher carrier mobility or higher output current for WS2-based FETs.Experimental research on WS2 is limited compared to MoS2,and more work is needed to further exploit the full potential of WS2-based FETs.Therefore,the electron-phonon interaction and vibration properties of WS2 used in nano-electronic applications and FETs must be investigated.To this end,mono-layer(1 L),few-layer(FL),and bulk WS2 films were prepared using mechanical exfoliation from a WS2 crystal.3 M scotch-tape was used for transferring the WS2 films.Detailed temperaturedependent Raman study on 1 L,FL,and bulk WS2 films has been conducted using a 514-nm excitation laser.Raman spectroscopy,as an effective and non-destructive approach for phonon vibration study,has been used to evaluate TMDs.The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change.With the film thickness increasing to bulk,the A1g(Γ) and E2g1(Γ) modes show blue-shift and red-shift,respectively,with respect to 1 L WS2.Moreover,when the dominant Raman vibration modes swaps between E2g1(Γ) and A1g(Γ),the "crossover"temperature was identified for 1 L,FL,and bulk WS2 films.WS2 shows smaller frequency change Δ between the E2g1(Γ) and A1g(Γ) modes than MoS2,with varying film thickness.The temperature coefficient of the Raman peak position was one magnitude lower for WS2 than MoS2,implying that WS2 has better thermal stability than MoS2.The results of this systematic study provide a physical guidance for WS2-based device design.