Chongling Sun

Chemical sensing by band modulation of a black phosphorus/molybdenum diselenide van der Waals hetero-structure

We report on a new chemical sensor based on black phosphorus/molybdenum diselenide van der Waals hetero-junctions. Due to the atomically thin nature of two-dimensional (2D) materials, surface adsorption of gas molecules can effectively modulate the band alignment at the junction interface, making the device a highly sensitive detector for chemical adsorptions. Compared to sensors made of homogeneous nanomaterials, the hetero-junction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide. Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance, proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors. The study demonstrates the potential of van der Waals hetero-junction as a new platform for sensing applications, and provides more insights into the interaction between gaseous molecules and 2D hetero-structures.

Acoustically induced current in graphene by aluminum nitride transducers

We report on the excitation of acousto-eletric (AE) charge transport in monolayer graphene by acoustic transducers based on aluminum nitride thin films. The acoustic waves induced macroscopic current flow that linearly scaled with input power. The AE current exhibited unique frequency dependence due to special configuration and piezoelectric properties of the transducer, which led to transitions between traveling and standing acoustic waves across a characteristic frequency. A Finite Element model was built to investigate and understand the phenomena and the underlying mechanisms.

Acoustic charge transport induced by the surface acoustic wave in chemical doped graphene

A graphene/LiNbO3 hybrid device is used to investigate the acoustic induced charge transport in chemical doped graphene. The chemical doping of graphene via its physisorption of gas molecules affects the surface acoustic wave (SAW) charge carrier transport in a manner different from electric field drift. That transport induces doping dependent macroscopic acoustoelectric current. The chemical doping can manipulate majority carriers and induces unique acoustoelectric features. The observation is explained by a classical relaxation model. Eventually the device based on acoustoelectric current is proved to outperform the common chemiresistor for chemicals. Our finding provides insight into acoustic charge carrier transport during chemical doping. The doping affects interaction of carriers with SAW phonon and facilitates the understanding of nanoscale acoustoelectric effect. The exploration inspires potential acoustoelectric application for chemical detection involving emerging 2D nanomaterials.

On-chip surface modified nanostructured ZnO as functional pH sensors

Zinc oxide (ZnO) nanostructures are promising candidates as electronic components for biological and chemical applications. In this study, ZnO ultra-fine nanowire (NW) and nanoflake (NF) hybrid structures have been prepared by Au-assisted chemical vapor deposition (CVD) under ambient pressure. Their surface morphology, lattice structures, and crystal orientation were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Two types of ZnO nanostructures were successfully integrated as gate electrodes in extended-gate field-effect transistors (EGFETs). Due to the amphoteric properties of ZnO, such devices function as pH sensors. We found that the ultra-fine NWs, which were more than 50 μm in length and less than 100 nm in diameter, performed better in the pH sensing process than NW-NF hybrid structures because of their higher surface-to-volume ratio, considering the Nernst equation and the Gouy-Chapman-Stern model. Furthermore, the surface coating of (3-Aminopropyl)triethoxysilane (APTES) protects ZnO nanostructures in both acidic and alkaline environments, thus enhancing the device stability and extending its pH sensing dynamic range.

Contact Engineering of Molybdenum Ditelluride Field Effect Transistors through Rapid Thermal Annealing

Understanding and engineering the interface between metal and two-dimensional materials are of great importance to the research and development of nanoelectronics. In many cases the interface of metal and 2D materials can dominate the transport behavior of the devices. In this study, we focus on the metal contacts of MoTe2 (molybdenum ditelluride) FETs (field effect transistors) and demonstrate how to use post-annealing treatment to modulate their transport behaviors in a controlled manner. We have also carried out low temperature and transmission electron microscopy studies to understand the mechanisms behind the prominent effect of the annealing process. Changes in transport properties are presumably due to anti-site defects formed at the metal-MoTe2 interface under elevated temperature. The study provides more insights into MoTe2 field effect devices and suggests guidelines for future optimizations.

Modulation of acousto-electric current using a hybrid on-chip AlN SAW/GFET device

We fabricated a hybrid on-chip acousto-electric (AE) and field-effect device to investigate the modulation of acoustic carrier transportation by gate voltage. The device fabrication exploited a surface micromachining aluminum nitride process on a silicon wafer, facilitating an integration of a surface acoustic wave (SAW) delay line and a graphene field-effect transistor. The SAW device induced an AE current in graphene, which scales linearly with the input power and remains essentially constant when subtracting the offset current at different DC biases. At a constant DC bias, the AE current can be modulated by the gate voltage, due to the change of the carrier mobility in graphene. A four-fold enhancement in the AE current was realized when ∼35 V voltage was applied to the gate electrode. The highly integrated device proves to be a powerful tool to understand the AE current in graphene, and since it supports integration for versatile functionality, it opens an avenue to explore the properties of diverse n...

Bioparticle manipulations using lamb wave resonator array

We apply the Lamb wave resonators (LWRs) in microfluidic field, and design a novel MEMS device consisting of 4 LWRs, that can efficiently drive multiple cylindrical vortices and locally enrich the bioparticles in liquid. The acoustic streaming effects in the fluid induced by the LWR is deduced. A numerical simulation method to compute the acoustic streaming effect is presented, and based on which, a 3D finite element model for the LWR inducing acoustic streaming is built. An LWR array consisting of 4 resonators is designed, simulated and fabricated. The simulation model predicts the distributions of multiple micro-vortices induced by the LWR array and the concentration of the particles in the vortices. Experimentally, we demonstrate that, the LWR array device, can efficiently drive multiple horizontal cylindrical vortices in a 1 μL drop and further trapped the bioparticles at the center of the vortex, which shows a great potential for biomolecular manipulations and biosensing applications.

Directly trapping of nanoscale biomolecules using bulk acoustic wave resonators

Techniques that can manipulate micro or macro biomaterials like cells and organisms have been of interest for a wide range of applications from biochemistry to clinical diagnostics and many clever techniques have been developed. However, methods with the ability to directly manipulate nanoscale biomaterials (e.g. proteins or DNAs) are still challenging due to physical limitations like Brownian motions. Here, according to theoretical design of the stagnation point in the medium which is formed by bulk-acoustic-wave-resonator-induced acoustic streaming, we experimentally realize trapping of biomolecules characterized by length scales at nanometer. We expect bulk acoustic wave resonator (BAWR) to become a powerful tool (e.g. biosensor and bioactuator) to revolutionize the fundamental and applied research for nanoscale biomaterials manipulation.