Influence of structural and electrostatic disorder on transport properties of monolayers of two-dimensional semiconductors

Two-dimensional (2D) materials are under intensive investigation recently due to variety of electronic properties, ranging from insulators (h-BN) to semi-metals (graphene), semiconductors (MoS2, WSe2) with wide variability of band-gap and correlated phases (NbSe2, TaS2). Recently, focus of research has been moved from graphene, which is now a well understood material, towards less explored materials, in particular monolayers of semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 with direct band-gap in optical range (1.5 - 2 eV) and potential towards scalable electronics applications immune to short channel effects. This thesis in fact explores materials beyond exfoliated MoS2 with the focus on electrical and structural properties of monolayers of WS2, ReS2 and synthesized by means of chemical vapour deposition (CVD) monolayers of MoS2, MoSe2 and WSe2. Four main chapters discuss the following aspects of these materials. Chapter 4 studies in details transport properties of monolayer WS2 for the first time. We demonstrate wide tuning of transport in this material from insulating regime through localized states to band-like transport with insulator to metal transition (MIT) in between, with mobilities = 50 cm2/Vs at room temperature. In Chapter 5 we employ electrolyte gating for fabrication of the first electric double-layer transistor (EDLT) based on monolayers and multilayers of ReS2. Furthermore, we employ this as a system for variation of electrostatic disorder and find an unusual and strong modulation in conduction band of ReS2. The discovered modulation of conductivity via controllable introduction of electrostatic disorder can help to understand this material as well as other ultrathin 2D systems and to optimize the design of devices based on them. In Chapter 6 we use atomically smooth sapphire for demonstration of the first epitaxial monolayer MoS2 growth. We can control the orientation of single-crystalline grains on the substrate and create large area continuous films, where grain boundaries between the grains have no impact on transport properties in contrast to other reports. Finally, Chapter 7 discusses further advances in CVD growth of TMDCs, with the first demonstration of ambipolar insulator to metal transition in devices, based on monolayers of MoSe2 and WSe2. In conclusion, this thesis establishes connection between transport properties of monolayers of 2D semiconductors and structural or electrostatic disorder. These results are important for both fundamental understanding of transport in two-dimensional materials and practical applications.

Thermal scanning probe lithography for 2D material-based gas sensing

Berke Erbas

Recently, two-dimensional (2D) material based gas sensing, especially transition metal dichalcogenide-based sensing, has been widely investigated thanks to its room temperature sensing ability. Unlike metal oxide based sensors, 2D material-based sensing can be carried out without a heater, therefore it reduces the power consumption and makes them promising candidates for smartphones and wearable electronic devices. Besides gas sensing ability, these 2D materials are interesting and promising candidates for electronics and optoelectronics applications. For example, monolayer molybdenum disulfide (MoS2), which is a 2D semiconductor with direct bandgap, is one of the promising candidates for transistor but it has low mobility compared to silicon. To replace silicon-based devices, strain engineering of MoS2 is widely investigated. It is observed that tensile strain tunes the bandgap of MoS2, enhances carrier mobility and improves NO2 gas sensing by piezotronic effect. Different strategies have been used to strain MoS2 such as flexible substrates to stretch the exfoliated material on top of them, silicon/ dielectric substrates with surface roughness or silicon/dielectric substrates patterned with nanocone structure to locally strain transferred MoS2. Herein, field effect transistor with three-dimensionally patterned gate oxide is designed, fabricated and electrically characterized for sensing application. Thermal scanning probe lithography, which is a direct three-dimensional nanofabrication technique, is used for the fabrication of sinusoidal surfaces. By transferring the exfoliated MoS2 flakes on top of these sinusoidal gate oxide, biaxial strain is induced. Unlike counterparts, thermal scanning probe lithography-based patterning provides us design flexibility, which controls the direction of strain and the strain rate. As a result of Raman spectroscopy, the shifts in out-of-plane vibration (A1g) and in-plane vibration (E1 2g) modes are up to 0.87 cm−1 and 2.60 cm−1, respectively. According to the theoretical Raman spectra calculations of strained MoS2 [1], these shifts correspond to an average strain of 0.58% in a circular area with 1 μm diameter. The strain of 0.58% is obtained for a sinusoidal pattern having a 23 nm amplitude and a 440 nm period. Besides, as a result of photoluminescence characterization, it is shown that an average strain of 0.39% leads to a 35 meV redshift for A excitation peak position. Consequently, the strain of MoS2 on the surface patterned by thermal scanning probe lithography tunes the bandgap of MoS2 by 90.11 meV/%.

2021

Spin-valley optoelectronics based on two-dimensional materials

Dmitrii Unuchek

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Charge-transport properties of monolayer MoS2 at the interface with dielectric materials

Simone Bertolazzi

Two-dimensional (2D) semiconductors, consisting of single-sheets of layered transition metal dichalcogenides (TMD), are attracting enormous interest from both fundamental science and technology. Monolayer molybdenum disulfide (MoS2), a typical example from this class of materials, is currently under intense research investigation because its direct band gap, atomic-scale thickness and mechanical flexibility could enable a wide range of novel technological applications, such as low-power flexible/transparent electronics, displays and wearable sensors. Critical to all these applications are the material mechanical strength, the mobility of charge-carriers in the 2D semiconductor and its interaction with the surrounding environment. This thesis describes experimental research conducted on these critical aspects and shows a proof-of-concept device application based on heterostructures of MoS2 and graphene. The main goal of the research was to assess experimentally the potential of monolayer MoS2 for application in flexible electronics. The thesis is based on three papers. The first paper describes the measurements of the in-plane stiffness and breaking strength of free-standing membranes of monolayer MoS2. Nanoindentation experiments were performed with the tip of an atomic force microscope (AFM) to extract the material's Young's modulus (E ~ 270 GPa) and breaking strength (Ïmax ~ 23 GPa). Breaking occurred at maximum internal strain Îµint ~ 11%, which indicates that monolayer MoS2 is suitable for integration on flexible plastic substrates. The second paper represents the core work of this thesis. It explores the charge transport properties of monolayer MoS2 supported by different dielectric substrates, such as thin polymer films of parylene, atomically flat sapphire and 2D sheets of hexagonal boron nitride (h-BN). It was found that substrate surface corrugations do not represent a major limit to charge-carrier mobility in monolayer MoS2, which seems at this stage dominated by impurities and material's defects. Field-effect transistors with mobility ~ 100 cm2/Vs and on/off current ratio Ion/Ioff > 10^5 were successfully integrated on parylene substrates, whose root-mean-square roughness Rq can be more than two times larger than the thickness of the transistor channel itself. Because parylene is also a flexible material, this work showed a viable method for the realization of high-mobility and high performance flexible devices based on 2D semiconductors. Finally, the third paper describes a flash-memory cell fabricated using monolayer MoS2/graphene heterostructures and dielectric layers of HfO2 grown by atomic layer deposition (ALD). The architecture of the device resembles that of a floating-gate transistor. Monolayer MoS2 acts as the transistor channel, graphene electrodes are used to collect and inject the charge carriers, and a piece of multilayer graphene serves as ultrathin charge trapping layer. In this paper, it was shown that graphene contacts to monolayer MoS2 result in ohmic-like current-voltage characteristics at room temperature. Moreover, the use of graphene and 2D semiconductors in flash memory technology was indicated as a potential strategy for further scaling of memory cells and for their integration in flexible electronic devices.

EPFL2015