Acoustic Actuation Of Suspended Graphene For Linear Excitation Of 2D Nems

In this paper, we present the fabrication of on-chip piezoelectric acoustic actuation for 2D materials. Acoustic actuation is achieved at device level through patterning of aluminum nitride (AlN) thin film at wafer scale. Two piezoelectric stacks consisting of a 100 nm-thick AlN layer sandwiched between 25 nm-thick platinum (Pt) and molybdenum (Mo) electrodes are fabricated close to the suspended 2D material such as graphene. The graphene nanoelectromechanical resonator is first actuated electrostatically using highly doped silicon (Si) back gate. Due to the effect of damping on amplitude, the frequency response curves do not lie on top of each other. Nonetheless, when actuated through the on-chip piezoelectric stack, the various frequency response curves lie on top of each other, indicating a linearly damped system. We also find that the acoustic actuation technique to be less dissipative than the electrostatic one.

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

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.

EPFL2017

Molybdenum Disulfide (MoS2) Nanoelectromechanical Resonators With On-Chip Aluminum Nitride (AlN) Piezoelectric Excitation

Muhammad Faizan, Tom Larsen, Luis Guillermo Villanueva Torrijo

We report on the first experimental demonstration of vibrating two-dimensional nanoelectromechanical systems (2D NEMS) with on-chip piezoelectric excitation. Combining a wafer-scale aluminum nitride (AlN) thin film technology with an all-dry transfer technique for atomic layer 2D semiconductors, we fabricate and piezoelectrically excite few-atomic-layer molybdenum disulfide (MoS2) NEMS resonators in the high frequency (HF) band. Multimode resonances up to 38MHz are observed, with efficient electromechanical drive from the AlN layer off the vibrating 2D NEMS device region (to avoid compromising the movable 2D device by electrodes needed for on-chip excitation and readout). The piezoelectrically excited 2D NEMS resonators may enable remotely driven, ultrasensitive transducers. Combined with on-chip electrical readout techniques (e.g., mixing), this device platform also holds promise for future radio frequency (RF) electronics and integrated systems.