【作者】 王鹏；

【导师】 刘文军； 高波；

【作者基本信息】 哈尔滨工业大学 ， 物理学， 2021， 博士

【摘要】 单层MoS₂,因其具有特殊的光学、电学、力学性质,在集成电路和逻辑器件的应用上有巨大的应用潜力,被认为是最有可能实现下一代电子和光电子器件的材料。但制备大尺寸、高质量的样品仍然具有挑战性,目前所生长的单层MoS₂在实际应用中具有较大的局限性。近年来,利用化学气相沉积（CVD）法生长单层MoS₂的过程中引入碱金属盐作为促进物被广泛关注,但现阶段对促进机制缺乏合理的解释。本论文,从碱金属化合物辅助大尺寸单层MoS₂的CVD生长出发,研究了碱金属化合物对生长单层MoS₂的促进机制,并研究了单层MoS₂光学性质的同步掺杂、基底和温度调控。首先,揭示了碱金属化合物在CVD生长单层MoS₂中的促进机制。选用各种碱金属盐或氢氧化物作为促进物,无论阴离子如何都可以获得几十微米至上百微米大尺寸的单层MoS₂,而非碱金属则没有促进作用。通过对样品的表面形貌、晶格结构、光谱特性和成分分析,表明生长的MoS₂具有较高的质量。碱金属化合物促进单层MoS₂生长过程中,共熔中间体的形成是关键。利用两步生长法,分析中间体产物的表面形貌和组成成分,并在此基础上提出共熔中间体理论模型,揭示碱金属化合物促进单层MoS₂生长的机制。Raman和EDS分析表明,无硫气氛下生长的共熔中间体包含碱金属钼酸盐和钼氧化物。共融中间体硫化后,形成大尺寸单层MoS₂。热重分析（TGA）表明,碱金属化合物的引入降低前驱体熔点。生长温度下,熔融的共熔中间体迁移、碰撞,融合后降低成核密度,随后促使单层MoS₂的横向生长。共熔中间体理论模型的提出,不仅合理地解释碱金属化合物促进单层MoS₂生长的机制,同时还为2D TMDs材料的低温生长提供新的参考和启发。其次,发现和研究了碱金属化合物促进单层MoS₂生长过程中的同步掺杂效应。利用3 mol/L的NaOH作为辅助物,生长出具有同步掺杂效应的单层MoS₂。由拉曼光谱和荧光光谱分析,该掺杂效应为n型掺杂效应。通过X射线光电子能谱（XPS）分析,研究引起同步掺杂效应的掺杂物和掺杂方式。结果表明,生长结束后由碱金属阳离子生成的残留物以Na-O团簇的形式物理吸附在单层MoS₂表面,但Na-O团簇不稳定,极易与空气中的水蒸气反应生成NaOH,进而对MoS₂产生掺杂效应,转移实验和NaOH后处理实验也证实了这一点。利用飞行时间二次离子质谱（TOF-SIMS）深度剖析手段,证明实现同步掺杂的掺杂物位于MoS₂之上。密度泛函理论（DFT）计算表明,单层MoS₂吸附NaOH后呈现出n型掺杂效应。随后,研究NaOH浓度和布局对掺杂程度的影响,实现进单层MoS₂中同步掺杂效应的可控性。最后,研究了单层MoS₂光学性质的基底和温度调控。首先,揭示了铌酸锂（Li Nb O₃）自发极化方向对单层MoS₂荧光特性的影响。结果表明,自发极化方向向上的Li Nb O₃基底表面正向极化电荷累积,对单层MoS₂产生p型掺杂效应,进而使单层MoS₂荧光强度增强;自发极化方向向下的Li Nb O₃基底表面聚集负向极化电荷,对单层MoS₂产生n型掺杂效应,进而抑制MoS₂的荧光强度。随后,利用激光辐照和热退火的方式,研究极化方向向上的Fe₂O₃:Li Nb O₃基底上单层MoS₂的光学性质。激光照射下,Fe:Li Nb O₃中光激发载流子的扩散和迁移,减弱基底表面极化电荷对单层MoS₂的初始掺杂效应,进而使单层MoS₂的荧光强度减弱。热退火后,扩散和迁移的光激发载流子重新回到初始状态,单层MoS₂的掺杂效应也将恢复到初始状态。通过激光辐照和热退火的方式,实现了单层MoS₂光学性质的可逆调控。更多还原

【Abstract】 Monolayer MoS₂,processing remarkable optical,electrical,mechanical properties derived from its direct bandgap feature,has been endowed to be one of the potential materials for practical applications in next generation electronics and optoelectronics such as integrated circuits and logic devices.The effective synthesis of atomically thin MoS₂ of high quality and uniformity over a large-size is essential for the use in electronic and optical devices.Developing reliable and scalable synthesis methods for monolayer MoS₂ is highly desired.Recently,a facile CVD method using alkali metal salts as promoters attracted a lot of attention.However,it has not come to a consensus on the role of promoters in facilitating the growth of monolayer MoS₂.In this thesis,the role of alkali metal compounds in growing monolayer MoS₂ was systematically investigated and a eutectic intermediates model was proposed.Spontaneous n-doping monolayer MoS₂ was grown by alkali metal compounds-promoted CVD.The optical properties of monolayer MoS₂ were modulated via changing the dielectric property of substrate and temperature.Firstly,the role of alkali metal compounds in growing monolayer MoS₂ was systematically investigated by CVD.In the presence of alkali metal compounds,large monolayer MoS₂ with hight quality was obtained,regardless of the anions.However,non-alkali metal compounds did not promote the growth of monolayer MoS₂.We considered eutectic intermediates could formed during growth.Raman and EDS measurements showed that eutectic intermediates,containing alkali metal molybdate and molybdenum oxides,were formed in the absence of sulfur during growth.These eutectic intermediates could transform to monolayer MoS ₂ in subsequent sulfurization.Thermogravimetric analysis（TGA）of precursor and promotor mixture shows a prominent weight loss at much lower temperature in contrast to pure Mo O ₃.Low melting point of eutectic intermediates facilitates their mobility,favor ing less nuclei and lateral growth for large monolayer MoS₂.The proposal of eutectic intermediates model could not only contribute to growing ultra-large monolayer MoS₂ and other 2D materials,but also inspire new ideas about growing 2D materials based on low melting point and high mobility of eutectic intermediates.Secondly,the residues and their influence on optical properties of monolayer MoS₂ in alkali metal compounds-promoted CVD growth were explored.When using NaOH and other alkali metal compounds as promoters,the Raman peak of A_1g mode redshifted with a broadening width and the PL intensity of the A peak decreased with a redshift,which was attributed to spontaneous n-doping effect during growth.Moreover,the growth using amount gradient of NaOH promoter suggests that the n-doping level could be controlled by the amount of promoter.X-ray photo-electron spectroscopy（XPS）and time-of-flight secondary-ion mass spectroscopy（TOF-SIMS）showed the existence of cation-derived residues in the form of Na-O cluster physiosorbed on top of monolayer MoS₂,which was also confirmed by the transfer experiment.The NaOH treatment experiment and density functional theory（DFT）calculations demonstrate that sodium hydroxide cluster,which could be converted from the combination of Na-O cluster and water vapor,could produce n-doping effect.Finally,a novel approach to reversibly modulate the optical properties of monolayer MoS₂ was presented via changing the dielectric property of substrate by laser irradiation and thermal annealing.Our approach is based on Li Nb O₃,a widely known ferroelectric material for various optical and acoustic applications.Due to the inherent surface charges,the polarized Li Nb O₃ was found to change the doping and hence the PL of the supported monolayer MoS₂.Afterwards,using laser we irradiated Fe₂O₃-doped Li Nb O₃ with positive ferroelectric polarity to excite and accumulate electrons on surface under spontaneous polarization field,and found that the PL intensity of the A peak of the supporting monolayer MoS₂ decreased with irradiation time.After thermally annealing the irradiated sample to erase the charge redistribution,the PL intensity of the A peak was restored.In this section,a reversible PL modulation of monolayer 2D materials was demonstrated by optically irradiating and thermally annealing the beneath ferroelectric substrates.更多还原