Piezoresistive effect

The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. In contrast to the piezoelectric effect, the piezoresistive effect causes a change only in electrical resistance, not in electric potential. History The change of electrical resistance in metal devices due to an applied mechanical load was first discovered in 1856 by Lord Kelvin. With single crystal silicon becoming the material of choice for the design of analog and digital circuits, the large piezoresistive effect in silicon and germanium was first discovered in 1954 (Smith 1954). Mechanism In conducting and semi-conducting materials, changes in inter-atomic spacing resulting from strain affect the bandgaps, making it easier (or harder depending on the material and strain) for electrons to be raised into the conduction band. This results in a change in resistivity of the material. Within a certain range of strain this relationshi

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

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

Adrien Victor Allain, Andras Kis, Sajedeh Manzeli

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.

Amer Chemical Soc2015

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

Adrien Victor Allain, Andras Kis, Sajedeh Manzeli

Amer Chemical Soc2015

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

EPFL2018

Nano-electromechanical systems based on ultra-thin semiconductors

Contribution à l'étude des propriétés d'équilibre du modèle de Wigner

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

Contribution à l'étude des propriétés d'équilibre du modèle de Wigner

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

Nano-electromechanical systems based on ultra-thin semiconductors