Xu-Qian Zheng

Environmental, thermal, and electrical susceptibility of black phosphorus field effect transistors

Atomic layers of black phosphorus (P) isolated from its layered bulk make a new two-dimensional (2D) semiconducting crystal with sizable direct bandgap, high carrier mobility, and promises for 2D electronics and optoelectronics. However, the integrity of black P crystal could be susceptible to a number of environmental variables and processes, resulting in degradation in device performance even before the device optical image suggests so. Here, the authors perform a systematic study of the environmental effects on black P electronic devices through continued measurements over a month under a number of controlled conditions, including ambient light, air, and humidity, and identify evolution of device performance under each condition. The authors further examine effects of thermal and electrical treatments on inducing morphology and performance changes and failure modes in black P devices. The results suggest that procedures well established for nanodevices in other 2D materials may not directly apply to black P devices, and improved procedures need to be devised to attain stable device operation.

Resolving and Tuning Mechanical Anisotropy in Black Phosphorus via Nanomechanical Multimode Resonance Spectromicroscopy

Black phosphorus (P) has emerged as a layered semiconductor with a unique crystal structure featuring corrugated atomic layers and strong in-plane anisotropy in its physical properties. Here, we demonstrate that the crystal orientation and mechanical anisotropy in free-standing black P thin layers can be precisely determined by spatially resolved multimode nanomechanical resonances. This offers a new means for resolving important crystal orientation and anisotropy in black P device platforms in situ beyond conventional optical and electrical calibration techniques. Furthermore, we show that electrostatic-gating-induced straining can continuously tune the mechanical anisotropic effects on multimode resonances in black P electromechanical devices. Combined with finite element modeling (FEM), we also determine the Youngs moduli of multilayer black P to be 116.1 and 46.5 GPa in the zigzag and armchair directions, respectively.

Two-dimensional nanoelectromechanical systems (2D NEMS) via atomically-thin semiconducting crystals vibrating at radio frequencies

We report on the initial explorations of engineering atomically-thin semiconducting crystals into a new class of two-dimensional nanoelectromechanical systems (2D NEMS) that are attractive for realizing ultimately thin 2D transducers for embedding in both planar and curved systems. We describe the first resonant NEMS operating at radio frequencies (RF), based on MoS2, a hallmark of 2D semiconducting crystals derived from layered materials in transition metal dichalcogenides (TMDCs). Through a series of careful measurements and analyses, we demonstrate a family of MoS2 2D NEMS resonators possessing very high fundamental-mode frequencies (f0~120MHz, in the VHF band), very broad dynamic range (DR~70-110dB), rich nonlinear dynamics, and outstanding electrical tunability.

Hexagonal boron nitride (h-BN) nanomechanical resonators with temperature-dependent multimode operations

We present the first experimental demonstrations of ultrathin hexagonal boron nitride (h-BN) circular drumhead vibrating resonators with multiple modes, operating in a wide temperature range from -7°C to 141°C, by characterizing flexural-mode resonances of the devices. We fabricate h-BN resonators with down to 10nm-thick h-BN crystalline flakes using a wet-chemistry-free, all-dry transfer technique. We observe distinct multimode resonances up to 5 modes, and resonance frequencies up to ~70MHz. In addition to spatial mapping of multimode thermomechanical motions, we observe very large temperature coefficients of frequency (TCfs) in these h-BN resonators, up to -2850ppm/K.

Hexagonal boron nitride nanomechanical resonators with spatially visualized motion

Atomic layers of hexagonal boron nitride (h-BN) crystal are excellent candidates for structural materials as enabling ultrathin, two-dimensional (2D) nanoelectromechanical systems (NEMS) due to the outstanding mechanical properties and very wide bandgap (5.9 eV) of h-BN. In this work, we report the experimental demonstration of h-BN 2D nanomechanical resonators vibrating at high and very high frequencies (from ~5 to ~70 MHz), and investigations of the elastic properties of h-BN by measuring the multimode resonant behavior of these devices. First, we demonstrate a dry-transferred doubly clamped h-BN membrane with ~6.7 nm thickness, the thinnest h-BN resonator known to date. In addition, we fabricate circular drumhead h-BN resonators with thicknesses ranging from ~9 to 292 nm, from which we measure up to eight resonance modes in the range of ~18 to 35 MHz. Combining measurements and modeling of the rich multimode resonances, we resolve h-BN’s elastic behavior, including the transition from membrane to disk regime, with built-in tension ranging from 0.02 to 2 N m−1. The Young’s modulus of h-BN is determined to be EY≈392 GPa from the measured resonances. The ultrasensitive measurements further reveal subtle structural characteristics and mechanical properties of the suspended h-BN diaphragms, including anisotropic built-in tension and bulging, thus suggesting guidelines on how these effects can be exploited for engineering multimode resonant functions in 2D NEMS transducers.

mm-Scale and MEMS piezoelectric energy harvesters powering on-chip CMOS temperature sensing for IoT applications

We report on the design and demonstration of a self-powering sensor microsystem that exploits vibrational energy harvesting to fully supply power for an integrated, ultra-low-power, CMOS temperature sensor. This system features a novel circuit interface, capable of handling input harvested power down to the sub-μW level, enabled by an application-specific integrated circuit (ASIC) that performs energy conversion from mm-scale and MEMS piezoelectric harvesters. The ASIC exhibits efficient voltage rectification and electrical energy storage, supplying sufficient energy for on-chip temperature sensing, by employing a single mm-scale piezoelectric harvester or multiple piezoelectric MEMS cantilevers.

Characterization of thin film lead zirconate titanate (PZT) multimode piezoelectric cantilevers vibrating in ultrasonic band

We report on characterization of microsale lead zirconate titanate (PZT) thin film cantilevers, by both optical and electrical measurements, for up to the 7th resonance mode in 2kHz to 2MHz frequency range, with quality (Q) factors between 380 and 600. We also calibrate the PZT thin films inverse piezoelectric coefficient (e31 ≈ -5C/m2). This work presents a multiphysical characterization of important material and device properties of microscale PZT cantilevers for enabling piezoelectric MEMS sensors and resonant piezoelectric transducers, which hold promises for multiphysics sensing, on-chip energy scavenging, and wireless power transmission.