Unlocking the Potential of Van der Waals 2D Materials: A Surface and Interface Engineering Approach

Abstract

The critical quest of high-quality interfaces and surfaces of atomically thin and dangling bond free van der Waals two dimensional (2D) materials is essential in the field of mechanical and electronic devices with the potential to revolutionize silicon-based industry. Multiple surface and interface engineering methods, like lithography and printing techniques for standard nanopatterning, mechanical cleaning, surface doping, plasma treatment, thermal annealing, dielectric engineering, and covalent-noncovalent modification have been utilized to tune quality of the surface and interfaces of materials. However, these techniques involve multistep processes to optimize scalability, specific control in the feature size, and introduce defects in 2D materials, which hampers the controlled and rapid prototyping of preferred surface and interface manipulation techniques. Addressing these bottlenecks, the thesis work focuses on the controlled surface and interface engineering of van der Waals 2D materials, using reliable and simple approaches for improving electrical properties and inducing exotic characteristics. Even though large areas of 2D materials are successfully synthesized, achieving high-quality of material remains challenging due to inherent issues like Crystal Defects and Wrinkles. which often occurs during synthesis, transfer, and handling. In spite of using various strategies devised by researchers to eliminate the wrinkles from 2D flakes, the issue is far from resolved. In this work, we explore a systematic electron beam-based de-wrinkling process of 2D flakes by employing the electron beam (e-beam) to remove the ripples and defects on the flake during in-situ measurements of lattice properties. Experimental results show that the optimal electron beam exposure (20 to 30 min of exposure at fluence rate = 1.02 × 1029 m−2 s−1) de‐stresses/relaxes the lattice and the maximum ordering of lattice planes is achieved; beyond which, the stress in lattice rises again and lattice planes start buckling. Analysis of selected area electron diffraction (SAED) patterns with tilting of sample before and after exposure reveals the removal of surface corrugations with nanoscale precision. Moreover, a notable ~2-fold enhancement in the conductance of the optoelectronic black phosphorus (BP) device is observed. This method not only ensures precise surface de wrinkling but also allows for real-time monitoring of the process, contributing to a significant advancement in 2D material fabrication and characterization. vii Since the de-wrinkling of 2D flakes under e-beam occurs primarily through lattice relaxation induced rather than chemical changes, the proposed protocol is likely to be universal in nature and applicable to other 2D materials, as previously demonstrated on 2D BP. Similar observations with the electron beam-based de-wrinkling process on the 2D molybdenum disulfide (MoS2) are observed where the uniform parallel lattice planes facilitated by the removal of disclinations and line defects upon irradiation are formed. Moreover, the focus of this study lies on the electrical properties of de-wrinkled 2D materials. For that, we have done the MoS2-based field-effect transistors (FETs) measurement on 300 nm gate oxide SiO2/Si substrates under the exposure of e-beam with a fluence rate of 7 × 1024 m-2s-1 to probe the performance and transport at the de-wrinkled surface. Notably, a systematic increase in irradiation time (~15 min) has induced significant changes in the characteristics of FETs including shifting the threshold voltage negatively by ~2 orders and increasing the mobility by around 3-fold due to the uniform surface of 2D MoS2. Interestingly, the electrical properties of the MoS2 FETs continue to evolve even after 30 min of e-beam irradiation, with the threshold voltage shifting towards the positive side and the mobility decreasing. This suggests a decrease in crystallinity and buckling of the flakes over time. Thus, the universal nature of our proposed protocol for flattening materials using electron beams holds promise for removing surface corrugations across various 2D materials and enhancing their electrical properties. As discussed earlier, crystal defects inherently exist in 2D materials and pose significant challenges to avoid, yet strategically introducing and exploiting these defects can yield beneficial applications. So, we demonstrate a unique capability to create artificial edges acting as localized piezoelectric facets on the surface of 2D BP flakes using a low-power focused laser irradiation. This leads to the selective piezoelectric response at facets such as basal plane, artificial edges, and natural edges, addressing the challenge of mixed facets in contemporary semicrystalline piezoelectric materials. Our experimental and theoretical investigations reveal that in-plane piezoelectric response is prominent at the edges, while the out-of-plane response is significant at the basal plane of the flakes. Thinner flakes exhibit a stronger ferroelectric response, as indicated by lower ∼9-fold coercive voltage (VC) and ∼6 fold voltage at spontaneous polarization (VS). These findings pave an innovative path for understanding the active piezoelectric coefficient along different crystal facets of 2D materials, offering exciting prospects for research beyond current piezoelectric materials. viii The interface between gate dielectric materials and the semiconductor channel in FETs holds significant importance, as it not only modulates carrier concentration but also induces lattice perturbations within the channel, thereby influencing device performance. Conventional dielectric materials such as silicon dioxide, SiO2 and aluminium oxide Al2O3 often introduce disorders such as charge scattering and trap states, posing challenges in achieving desired device characteristics. To address these limitations, atomically thin van der Waals 2D materials have emerged as promising alternatives. In this study, Raman spectroscopy is employed to explore the impact of hexagonal boron nitride (hBN) and air as gate dielectrics on the lattice structure of 2D materials devices such as MoS2, BP, and BP/MoS2 heterostructures. The experimental findings establish a correlation between lattice strain induced by gate dielectrics and external voltage and provide insights into the phonon characteristics and electronic band structure. Additionally, hBN/BP/MoS2 heterostructure devices exhibit a more rapid increase in the strain-induced electrostrictive response under applied voltage compared to MoS2, BP, and BP/MoS2 heterostructures. In short, the thesis work explores the properties of van der Waals 2D materials through controlled surface and interface engineering, offering advancements in piezoelectric response, de-wrinkling by electron beam irradiation, and interface design, paving the way for improved nanoelectronics device design and performance