Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2

Continuous tuning of material properties is highly desirable for a wide range of applications, with strain engineering being an interesting way of achieving it. The tuning range, however, is limited in conventional bulk materials that can suffer from plasticity and low fracture limit due to the presence of defects and dislocations. Atomically thin membranes such as MoS2 on the other hand exhibit high Young's modulus and fracture strength, which makes them viable candidates for modifying their properties via strain. The bandgap of MoS2 is highly strain-tunable, which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect, whereas a piezoelectric effect was also observed in odd-layered MoS2 with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS2 a very promising material for nanoelectromechanical systems (NEMS). Here, we incorporate monolayer, bilayer, and trilayer MoS2 in a nanoelectromechanical membrane configuration. We detect strain-induced band gap tuning via electrical conductivity measurements and demonstrate the emergence of the piezoresistive effect in MoS2. Finite element method (FEM) simulations are used to quantify the band gap change and to obtain a comprehensive picture of the spatially varying bandgap profile on the membrane. The piezoresistive gauge factor is calculated to be -148 +/- 19, -224 +/- 19, and -43.5 +/- 11 for monolayer, bilayer, and trilayer MoS2, respectively, which is comparable to state-of-the-art silicon strain sensors and 2 orders of magnitude higher than in strain sensors based on suspended graphene. Controllable modulation of resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a novel approach for implementing an important class of NEMS transducers, flexible and wearable electronics, tunable photovoltaics, and photodetection.

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

Nowadays, the interest in 2D materials has gone far beyond graphene. Specially, monolayers of transition metal dichalcogenides (TMDs) offer a broad spectrum of electronic and optical properties, and show the potential to revolutionize the electronics industry. The promising electronic properties of 2D semiconductors combined with their mechanical strength and flexibility, makes them ideal candidates for nanoelectromechanical systems (NEMS). This thesis focuses on realizing NEMS based on graphene and MoS2, which is one of the most appealing TMDs. First, we present the realization of graphene NEMS by fabricating single and bilayer graphene transistors featuring a doubly clamped suspended channel. The electromechanical response of monolayer graphene nanoribbons show a strain-induced increase in their electrical resistance, making it possible to estimate an upper limit for the piezoresistive gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of the two graphene layers with respect to each other. Our results report on the rare observation of room temperature electronic interference in bilayer graphene. Next, we investigate the static electromechanical response of atomically thin MoS2. MoS2 exhibits high youngâs modulus and fracture strength. Furthermore, the bandgap of MoS2 is highly strain-tunable. This coupling between electrical and mechanical properties makes MoS2 a promising material for NEMS. Here we incorporate monolayer, bilayer and trilayer MoS2 in a suspended membrane configuration with the electrodes acting as mechanical clamps. Strain-induced bandgap tuning is detected via electrical conductivity measurements and the emergence of the piezoresistive effect in MoS2 is demonstrated. We observe a reversible bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element method (FEM) simulations are used to obtain the spatially varying bandgap profile on the membrane and to quantify the rate of bandgap change. The piezoresistive gauge factor is calculated for single layer, bilayer and trilayer MoS2. Our results reveal that monolayer and bilayer MoS2 show a piezoresistive effect which is comparable to the state-of-the-art silicon strain sensors and two orders of magnitude higher than in graphene. Finally, we present the investigation of MoS2 NEMS resonators working in the VHF range and featuring piezoresistive transduction. The atomic thickness of monolayer MoS2 places it as a promising candidate for miniaturization of electromechanical devices to the limits of vertical scaling. While the small mass of MoS2 leads to an increased resonant frequency and a higher mass sensitivity, the presence of piezoresistivity offers a transduction mechanism in addition to the traditional capacitive transduction. Operating in the tension-dominated regime, monolayer MoS2 NEMS resonators not only allow tunability of the resonant frequency using an external voltage, but also show the strain-induced enhancement of their dynamic range. Furthermore, the resonators are driven into the nonlinear regime allowing the study of nonlinear effects. This work sheds light on the potential of TMD based NEMS as ultra-low power switches, sensors and resonators for applications in RF range.

EPFL2018

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

Electromechanical oscillations in bilayer graphene

Gabriel Albert Autes, Mohamed Malik Benameur, Fernando Gargiulo, Andras Kis, Sajedeh Manzeli, Mahmut Tosun, Oleg Yazyev

Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.