Direct visualization of structural defects in 2D semiconductors

Direct visualization of the structural defects in two-dimensional (2D) semiconductors at a large scale plays a significant role in understanding their electrical/optical/magnetic properties, but is challenging. Although traditional atomic resolution imaging techniques, such as transmission electron microscopy and scanning tunneling microscopy, can directly image the structural defects, they provide only local-scale information and require complex setups. Here, we develop a simple, non-invasive wet etching method to directly visualize the structural defects in 2D semiconductors at a large scale, including both point defects and grain boundaries. Utilizing this method, we extract successfully the defects density in several different types of monolayer molybdenum disulfide samples, providing key insights into the device functions. Furthermore, the etching method we developed is anisotropic and tunable, opening up opportunities to obtain exotic edge states on demand.     

ULTRAHIGH DATA DENSITY STORAGE WITH SCANNING TUNNELING MICROSCOPY

Ultrahigh density data storage devices made by scanning probe techniques based on various recording media and their corresponding recording mechanisms, have attracted much attention recently, since they ensure a high data density in a non-volatile, erasable form in some kinds of ways. It is of particular interest to employ organic polymers with novel functional properties within a single molecule (or a single molecular complex) for fabricating electronic devices on a single molecular scale. Here, it is reported that a new process for ultrahigh density and erasable data storage, namely, molecular bistability on an organic charge transfer complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine (NBMN-pDA) switched by a scanning tunneling microscope (STM). Data density exceeds 10¹³ bits/cm² with a writing time per bit of ～1μs. Current-voltage (I/V) measurements before and after the voltage pulse from the STM tip, together with optical absorption spectroscopy and macroscopic four-probe I/V measurements demonstrate that the writing mechanism is conductance transition in the organic complex. This mechanism offers an attractive combination of ultrahigh data density coupled with high speed. The ultimate bit density achievable appears to be limited only by the size of the organic complex, which is less than 1nm in our case, corresponding to 10¹⁴ bits/cm². We believe that provided the lifetime can be improved, molecular bistability may represent a practical route for ultrahigh density data storage devices.     

Structural transformation of Ge dimers on Ge（001） surfaces induced by bias voltage

Scanning tunnelling microscopy is utilized to investigate the local bias voltage tunnelling dependent transformation between （2×1） and c（4×2） structures on Ge（001） surfaces, which is reversibly observed at room temperature and a critical bias voltage of -0.80 V. Similar transformation is also found on an epitaxial Ce islands but at a slightly different critical bias voltage of -1.00V. It is found that the interaction between the topmost atoms on the STM tip and the atoms of the dimers, and the pinning effect induced by Sb atoms, the nacancies or the epitaxial clusters, can drive the structural transformation at the critical bias voltage.     

Graphene的物理性质与器件应用

Graphene因其新奇的物理性质和广泛的应用前景已迅速成为国际新材料领域的研究前沿和热点.文章详细介绍了Graphene奇特的物理性质(多体相互作用、量子霍尔效应、双极场效应、弱局域化效应等),并对其在微纳米器件、分子电子学、自旋电子学等领域的应用进展给出了综述.     

原子尺度上的异质催化

现实中的催化剂是个相当复杂的系统体系，且为粉末状，限制了多种表面科学表征手段的应用,科学家通过建立简化的催化剂模型，充分利用目前多种有力的科技分析手段，如扫描隧道显微镜（STM）、透射电镜（TEM）、光电子能谱（XPS）、傅里叶变换红外吸收谱（FTIR）、电子能量损失谱（EELS）等，直观地在原子尺度上研究催化反应的机理，从而使得人们能够设计出选择性能更高、催化性能更好的催化剂．     

有机单体3-phenyl-1-ureidonitrile薄膜的超高密度信息存储

采用扫描隧道显微镜(STM)在3-phenyl-1-ureidonitrile(PUN)有机单体薄膜上进行了超高密度信息存储的研究.通过在STM针尖和高定向裂解石墨(HOPG)衬底之间施加一系列的电压脉冲,在薄膜上写入了一个稳定的5×6信息点阵,信息点的大小是0.8nm.电流电压(I-V)曲线表明,施加电压脉冲前后薄膜的导电性质发生了变化.信息点的写入机制可能是强电场作用下引发的PUN分子的局域聚合,从而导致薄膜由高电阻态向低电阻态转变. 关键词： 超高密度信息存储 有机薄膜 扫描隧道显微镜(STM)     

石墨烯封装单层二硫化钼的热稳定性研究

将单层二硫化钼用石墨烯进行封装,构造了石墨烯和二硫化钼的范德瓦耳斯异质结构,并且分别在氩气（Ar）和氢气（H₂）氛围下,详细研究了被封装的二硫化钼的热稳定性.结果表明：在氩气氛围中,石墨烯封装的二硫化钼在400–1000℃下一直保持稳定,而石墨烯和氧化硅上裸露的二硫化钼在1000℃时几乎全部分解;在氢气氛围中,石墨烯封装的二硫化钼在400–1000℃下一直稳定存在,而石墨烯和氧化硅上裸露的二硫化钼在800℃下已经完全分解.综上可得,在氩气和氢气的氛围下,被石墨烯封装的二硫化钼的热稳定性得到了显著的提高.该研究通过用石墨烯将单层的二硫化钼进行封装以提高其热稳定性,在未来以单层二硫化钼作为基础材料的电子器件中,可以保证其在高温下能够正常工作.该研究也为提高其他二维材料的热稳定性提供了一种可行的方法和思路.     

退火对单层二硫化钼荧光特性的影响

单层二硫化钼是继石墨烯后的一种新型二维材料. 它是一种直接带隙半导体, 具有优异的光电特性, 从而受到人们的广泛关注. 之前的研究报道过单层二硫化钼在氩气中退火后可以提升其A激子峰的荧光强度, 但我们发现, 空气中退火较氩气退火效果更为明显. 本文重点研究了在空气中退火对二硫化钼的荧光特性的影响. 不同条件下制备的单层二硫化钼样品, 经过在空气中退火处理后, 荧光峰位均发生了蓝移, 荧光强度提升了一个数量级. 我们认为, 这是由于空气退火造成二硫化钼缺陷的形成, 大量氧气分子被缺陷束缚并发生电荷转移. 氧气分子充当受主的角色, 起着P型掺杂的作用. 电荷的抽取造成二硫化钼的负电激子减少, 中性激子增多, 提升了其荧光量子效率. 我们在对照实验中发现, NH₃吸附在二硫化钼表面时, 荧光强度下降, 峰位红移, 这是由于NH₃分子充当施主的角色, 造成负电激子增多, 中性激子减少. 本文为提高单层二硫化钼的荧光量子效率提供了一种简单有效的方法.     

单层二硫化钼的制备及在器件应用方面的研究

二硫化钼(MoS₂)作为一种新兴的二维半导体材料,它具有天然原子级的厚度以及优异的光电特性和机械性能,在未来超大规模集成电路中具有巨大的应用潜力.本文综述了我们课题组在过去几年中在单层MoS₂薄膜研究方面所取得的进展,具体包括:在MoS₂薄膜制备方面,通过氧辅助气相沉积方法,实现了大尺寸MoS₂单晶的可控生长;通过独特的多源立式生长方法,实现了4 in晶圆级大晶粒高定向的单层MoS₂薄膜的外延生长,样品显示出极高的光学和电学质量,是目前国际上报道的质量最好的晶圆级MoS₂样品;通过调节MoS₂薄膜的氧掺杂浓度,可以实现对其电学和光学特性的有效调控.在MoS₂薄膜器件与应用方面,利用制备的高质量单层MoS₂薄膜,实现了高性能柔性晶体管的集成,这种大面积柔性逻辑和存储器件显示出优异的电学性能;在集成多层场效应晶体管的基础上,设计,加工了垂直集成的多层全二维材料的多功能器件,充分发挥器件的组合性能...     

Precisely controlling the twist angle of epitaxial MoS2/graphene heterostructure by AFM tip manipulation

Two-dimensional (2D) moiré materials have attracted a lot of attention and opened a new research frontier of twistronics due to their novel physical properties. Although great progress has been achieved, the inability to precisely and reproducibly manipulate the twist angle hinders the further development of twistronics. Here, we demonstrated an atomic force microscope (AFM) tip manipulation method to control the interlayer twist angle of epitaxial MoS₂/graphene heterostructure with an ultra-high accuracy better than 0.1°. Furthermore, conductive AFM and spectroscopic characterizations were conducted to show the effects of the twist angle on moiré pattern wavelength, phonons and excitons. Our work provides a technique to precisely control the twist angle of 2D moiré materials, enabling the possibility to establish the phase diagrams of moiré physics with twist angle.