Two-dimensional semiconductor | EPFL Graph Search

A two-dimensional semiconductor (also known as 2D semiconductor) is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms. This coupling could enable applications. One research focus is on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor. Materials Graphene Graphene Graphene, consisting of single sheets of carbon atoms, has high electron mobility and high thermal conductivity. One issue regarding graphene is its lack of a band gap, which poses a problem in particular with digital electronics because

Optoelectronic and magnetic properties induced by atomic defects in two-dimensional semiconductors

Alessio Scavuzzo

Two-dimensional (2D) materials have attracted increasing attention over the last decade owing to their remarkable mechanical, electrical and optical properties. Following the groundbreaking discovery of graphene, a plethora of other atomically-thin materials have emerged. They cover a broad spectrum of electronic character including semiconductors, metals and insulators. The functionalities of 2D materials can be further extended by stacking individual layers into vertical heterostructures. The resulting interlayer electrical coupling enables harnessing complementary properties of the constituent materials and exploring novel fundamental physics phenomena. Their versatility and intriguing optoelectronic properties make 2D materials promising alternatives and compliments to current semiconductor technologies which suffer from scale-down limitations. Among the 2D semiconductors, transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 stand out due to their direct band gap, strong spin-orbit coupling and valley degree of freedom. This class of materials could thus enable a new generation of 2D optoelectronic devices in which information is processed through the electron spin or valley pseudospin, and accessed by optical means. Another promising 2D material is hexagonal boron nitride (h-BN), which is a transparent wide band gap insulator with excellent mechanical properties. Furthermore, the absence of dangling bonds and its strong dielectric screening capability make h-BN an ideal substrate for 2D materials. In general, crystalline defects such as vacancies, adatoms, and substitutional impurities have a significant impact on the properties of atomically-thin materials. Defects in TMDCs can trap free excitons, thus creating single-photon sources, while in h-BN they can introduce localized electronic states deep within its large band gap, resulting in stable quantum emission at room temperature. Defects can also be exploited to modulate the characteristics of the host 2D material, thus introducing functionalities that are absent in the pristine form. One option is the controlled introduction of substitutional impurities in TMDCs in order to induce tunable magnetic properties. The present thesis aims at exploring the optoelectronic properties of quantum emitters in h-BN and the magnetic properties arising from substitutional V doping in WSe2. Along these lines, a further task is to identify novel approaches to control the functionalities of the two different 2D semiconductors. In the first experimental part, the electrical tuning of single photon emitters in atomically-thin h-BN is investigated by applying a vertical or horizontal electric field. The large and robust Stark shift attained with both configurations testifies the efficiency of the electrical tuning to control the emission and underscores the ability of this method to address fundamental properties of the quantum emitters. The second experimental part describes the results of circular polarization-resolved PL and magnetotransport measurements performed with the aim of probing ferromagnetic order in V-WSe2. Although the transport measurements on few-layer flakes point toward a layered antiferromagnetic phase, the PL data gained on monolayer sheets suggest the presence of small local ferromagnetic domains. Taken together, these findings provide a valuable basis for further investigations on tailoring the properties of 2D materials via the controlled introduction of defects.

EPFL2020

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

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