【作者】 周楠；

【导师】 翟天佑； 甘霖；

【作者基本信息】 华中科技大学 ， 材料学， 2018， 博士

【摘要】 二维过渡金属硫属化合物（TMDCs）,因其具有特殊的晶体结构、极大的比表面积和纳米尺度效应,而表现出优异的电学、光学性能,未来在光电领域有巨大的应用前景。MoS₂是其中最为典型的代表,单层MoS₂的直接带隙半导体属性,赋予其极高的光电应用前景。基于MoS₂的场效应晶体管和光探测器已经证实,它具有较高的电流开光比（可达10⁸）和较高的响应度(880 A W^-1)等优异的光电性能。但是MoS₂也存在光谱响应范围受限及光谱响应时间较长等缺点,这些缺点极大地降低了其应用潜力。众所周知,由于二维材料超薄的原子结构,使得其各项性质对于外界环境非常敏感,因而可以通过改变其局部的外界环境来实现对其性能的可控调节。我们发现通过构筑基于二维MoS₂的异质结,可以有效地调控其光电性能,实现对二维MoS₂光电性能的提升和拓展。基于上述,本文主要采用化学修饰和原位生长异质结的方法对其光电性能进行优化。主要研究内容和成果如下:（1）由于O₂/H₂O等杂质的吸附影响而造成的响应速度慢是二维MoS₂应用于光电器件的一大障碍。本工作通过范德华外延生长的方法合成了p-GaSe/n-MoS₂异质结,并结合理论计算和实验,对该异质结进行了研究。GaSe的存在,有效地促进了电子-空穴对的分离,同时材料界面的存在也一定程度上减少了MoS₂表面O₂/H₂O的吸附,使得少数载流子空穴的寿命很大程度上被抑制,因而实现了MoS₂的响应速度近三个数量级的提升。通过化学气相沉积外延生长GaSe/MoS₂垂直P-N结制备方法简单,且能有效调节材料的光电性能,是目前较为简单的提升MoS₂响应速度的方法之一。这种通过范德华外延生长的P-N结来提升响应速度的方法,为优化二维材料光电性能的研究提供了新的思路。（2）为了拓宽MoS₂的光谱响应范围,利用“堆垛模式限制生长”的方法制备了MoTe₂/MoS₂双层异质结,该方法有效地增加了异质结的界面接触并提升了界面清洁度。PL揭示了MoTe₂和MoS₂层之间的电荷转移,导致在异质结区域有明显的PL淬灭。光探测试结果证实,该异质结的生成,有效地拓展MoS₂的光谱响应范围至近红外区域（1100 nm）,且表现出高响应度4.71 A/W（1100 nm）和优异的EQE 1.935×10³%（300 nm）,该值优于MoS₂及相关的TMDCs的异质结器件的性能。（3）为实现MoS₂在近红外区域的选择性的光响应,我们采用上转换纳米颗粒（UCNPs）来修饰二维MoS₂纳米片。Ln系掺杂的上转换材料是一种独特的发光材料,能够吸收特定波段红外光,发出可见光。在该复合体系中,二维MoS2纳米片因其超高的比表面积和优越的光电性能作为复合器件的载体,UCNPs作为红外光的吸收者,利用UCNPs孤立分布的吸收光谱赋予MoS₂近红外区波长选择的光响应。此外,UCNPs吸收峰的位置可以随掺杂的稀土元素的种类而改变;MoS₂也可以被其他TMDCs所替代,所以这种器件设计概念可以拓展至其他材料体系的响应范围调控。更多还原

【Abstract】 Two-dimensional（2D）transition metal chalcogenides（TMDCs）exhibit excellent electrical and optical properties etc.,due to their special crystalline structures,large specific surface area,and nano-scale effects,and thus they have great application prospect in optoelectronics.MoS₂ is the most typical TMDCs,and the direct bandgap semiconducting characters of the single-layer MoS₂ endow itself with excellent optical and electrical properties.Field-effect transistors and photodetectors based on MoS₂ have confirmed its excellent photoelectric performance,such as high current on-off ratio（up to 10⁸）and high responsivity(880 A W^-1),and so on.However,the drawbacks,for instance,UV-Vis spectral response range limited by the intrinsic band gap and long response time,etc.,greatly degrade its application potential at the same time.As it is well known,the properties of 2D materials are extremely sensitive to the environment,and thus one can modulate the properties of 2D materials via changing the local environment.We found that the optoelectronic properties of2D MoS₂ can be effectively enhanced and extended by building various MoS₂-based heterojunctions.Based on the above,this article mainly adopts chemical modification and epitaxial growth of heterojunction to optimize the optoelecetronic properties of 2D MoS₂.The main research contents and results are as follows:（1）Slow photoresponse is one of the key drawbacks for 2D MoS₂ due to the influence from the adsorption of impurities such as O₂/H₂O.In this work,p-GaSe/n-MoS₂ was synthesized by van der Waals epitaxial growth,then the heterojunction was studied by combining theoretical calculation and experiment.The presence of GaSe effectively facilitates the separation of electron-hole pairs and suppresses the absorption of impurities as well,which significantly decrease the lifetime of minor carriers in MoS₂,thus enhancing the reponse rate for three orders of magnitudes.The epitaxial growth of the vertical p-GaSe/n-MoS₂ junction by chemical vapor deposition is simple,but can effectively adjust the optoelectronic properties,which is one of the simplest ways to improve the response speed of MoS₂.This method of improving the response speed through the p-n junction grown by van der Waals expitaxial growth provides a new idea for the study on optimizing the optoelectronic properties of 2D materials.（2）In order to broaden the spectral response range of MoS₂,MoTe₂/MoS₂ bilayer heterojunction was prepared by using the method of“Stacking-mode confined growth”.This method effectively enhances the heterojunction interface contact and improves the interface cleanliness.PL revealed charge transfer between the MoTe₂ and MoS₂ layers,resulting in a significant PL quenching in the heterojunction region.The results of photodetection have confirmed that the formation of the heterojunction effectively extended the spectral response range of the MoS₂ to the near-infrared region（1100 nm）,and exhibited high responsivity of4.71 A/W（1100 nm）and excellent EQE of 1.935×10³%（300 nm）,which were better than the performances of MoS₂ and the related TMDCs heterojunction devices.（3）To achieve spectrally selective response based on MoS₂ in the near-infrared region,we used up-converting nanoparticles（UCNPs）to modify 2D MoS₂,and successfully realized photodetection for specific wavelength bands in the near-infrared region.The Ln-doped UCNPs is a unique luminescent material,which absorbs specific wavelength infrared light and emits visible light.In this composite system,2D MoS₂ is used as the supporter of the composite device because of its ultra-high specific surface area and superior optoelectronic properties,UCNPs are used as the absorbers of infrared light.Specially,the isolated absorption spectrum of the UCNPs grants the wavelength selection of the MoS₂ for the photoresponse in the near-infrared region.In addition,the position of the UCNPs absorption peak can be changed with the types of the doped rare earth elements;and MoS₂can also be replaced by other TMDCs,and thus this device design concept could be extended to other similar systems for modulation of response range.更多还原