Probing and Functionalizing Emerging Two-Dimensional Materials at the Nanoscale

Liu, Xiaolong

doi:https://doi.org/10.21985/n2-na9w-vf50

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Probing and Functionalizing Emerging Two-Dimensional Materials at the Nanoscale

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Two-dimensional (2D) materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. In Chapter 1 of this thesis, I first introduce the surface and interface properties that govern the performance of 2D materials and review the experimental approaches that resolve surface and interface phenomena down to the atomic scale. I then experimentally explore the surface and interface characteristics of three emerging 2D materials and their heterostructures including black phosphorus (BP), transition metal dichalcogenides (e.g., MoS2), and borophene (i.e., 2D boron). The characterization tools used span across a suite of surface science techniques, but primarily involve electron microscopy and scanning probe microscopy at the atomic/molecular scale. In particular, an atomic-scale microscopic and spectroscopic study is first performed to characterize the thermal degradation of mechanically exfoliated 2D BP in Chapter 2. From in situ scanning/transmission electron microscopy, decomposition of 2D BP is observed to occur at ~400°C in vacuum, in contrast to the 550°C bulk BP sublimation temperature. Taking advantage of the chemical instability of BP and the high-spatial resolution of atomic force microscopy (AFM), I then demonstrated nanopatterning and layer-by-layer thinning of BP with conductive AFM anodic oxidation, where the liquid-phase patterning byproduct is readily removed by water rinsing. An alternating current method is developed to enable direct nanopatterning and thinning on insulating substrates such as SiO2/Si, leading to field-effect transistors with patterned channels showing significant improvements in current modulation by up to a factor of 50. The research is further extended to MoS2 monolayers grown via chemical vapor deposition on epitaxial graphene (EG) on SiC in Chapter 3, which are suitable for ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) studies. After achieving rotationally commensurate growth of MoS2, I interrogate point and line defects in monolayer MoS2 at the atomic scale. As a result of rotational commensurability, a much lower total defect density is observed. In addition, grain boundaries are limited to mostly having 30° and 60° tilt angles with band gap reductions to ~0.8 eV and ~0.5 eV, respectively. By functionalizing such heterostructures with 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT), the molecules are found to self-assemble well on MoS2, forming a well-defined mixed-dimensional heterostructure with clean interfaces. The first C8-BTBT layer on rotationally commensurate MoS2/EG is found to be insensitive to structural defects and electronic perturbations from the underlying MoS2. In Chapter 4, the focus is shifted to an emerging 2D material - borophene. I start with atomic scale UHV STM/STS characterizations of borophene line defects corroborated by density functional theory calculations. Line defects in mixed-phase borophene are found to adopt structures that match the constituent units of the other phase and energetically favor spatially periodic self-assembly that gives rise to new borophene phases, ultimately blurring the distinction between borophene crystals and defects. Low temperature measurements further reveal subtle electronic features that are consistent with a charge density wave, which are modulated by line defects. Such borophene polymorphs are then imaged with carbon monoxide-functionalized non-contact AFM (CO-AFM) and STM (CO-STM) probes revealing for the first time features that are consistent with boron-boron covalent bonds. CO-STM is identified as an equivalent and comparatively more accessible technique to unambiguously determine borophene structures. Upon deposition of an organic molecule perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on sub-monolayer borophene samples, lateral heterostructures spontaneously form with an electronically abrupt borophene/PTCDA interface. Covalent functionalization of borophene via molecular oxygen is also explored at the atomic scale, and an effective in situ passivation scheme involving Al2O3 capping layers is developed to protect borophene from ambient degradation for at least 3 months. Finally, a brief summary and future outlook is given in Chapter 5. Under the background of ever-increasing interest in low-dimensional materials worldwide, this thesis is aimed to provide some fundamental understandings at the atomic/molecular level, and hopefully contribute to the effort of identifying new technological solutions for the pressing energy, environment, and health challenges we are facing.

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