Yumeng Liu

Direct-Write, Self-Aligned Electrospinning on Paper for Controllable Fabrication of Three-Dimensional Structures.

Electrospinning, a process that converts a solution or melt droplet into an ejected jet under a high electric field, is a well-established technique to produce one-dimensional (1D) fibers or two-dimensional (2D) randomly arranged fibrous meshes. Nevertheless, the direct electrospinning of fibers into controllable three-dimensional (3D) architectures is still a nascent technology. Here, we apply near-field electrospinning (NFES) to directly write arbitrarily shaped 3D structures through consistent and spatially controlled fiber-by-fiber stacking of polyvinylidene fluoride (PVDF) fibers. An element central to the success of this 3D electrospinning is the use of a printing paper placed on the grounded conductive plate and acting as a fiber collector. Once deposited on the paper, residual solvents from near-field electrospun fibers can infiltrate the paper substrate, enhancing the charge transfer between the deposited fibers and the ground plate via the fibrous network within the paper. Such charge transfer grounds the deposited fibers and turns them into locally fabricated electrical poles, which attract subsequent in-flight fibers to deposit in a self-aligned manner on top of each other. This process enables the design and controlled fabrication of electrospun 3D structures such as grids, walls, hollow cylinders, and other 3D logos. As such, this technique has the potential to advance the existing electrospinning technologies in constructing 3D structures for biomedical, microelectronics, and MEMS/NMES applications.

A review on chemiresistive room temperature gas sensors based on metal oxide nanostructures, graphene and 2D transition metal dichalcogenides

AbstractRoom-temperature (RT) gas sensing is desirable for battery-powered or self-powered instrumentation that can monitor emissions associated with pollution and industrial processes. This review (with 171 references) discusses recent advances in three types of porous nanostructures that have shown remarkable potential for RT gas sensing. The first group comprises hierarchical oxide nanostructures (mainly oxides of Sn, Ni, Zn, W, In, La, Fe, Co). The second group comprises graphene and its derivatives (graphene, graphene oxides, reduced graphene oxides, and their composites with metal oxides and noble metals). The third group comprises 2D transition metal dichalcogenides (mainly sulfides of Mo, W, Sn, Ni, also in combination with metal oxides). They all have been found to enable RT sensing of gases such as NOx, NH3, H2, SO2, CO, and of vapors such as of acetone, formaldehyde or methanol. Attractive features also include high selectivity and sensitivity, long-term stability and affordable costs. Strengths and limitations of these materials are highlighted, and prospects with respect to the development of new materials to overcome existing limitations are discussed. Graphical AbstractThe review summarizes the most significant progresses related to room temperature gas sensing by using hierarchical oxide nanostructures, graphene and its derivatives and 2D transition metal dichalcogenides, highlighting the peculiar gas sensing behavior with enhanced selectivity, sensitivity and long-term stability.

A flexible graphene FET gas sensor using polymer as gate dielectrics

We have successfully demonstrated a Graphene Field Effect Transistor (GFET) gas sensor on a flexible plastic substrate with a sensitivity of 0.00428ppm-1 (ΔR/R0) for ammonia. Compared with the state-of-art technologies, four distinctive advancements have been achieved: (1) first demonstration of a flexible graphene FET gas sensor; (2) a new fabrication process to achieve embedded-gate GFET on a flexible substrate; (3) proof of utilizing polymeric materials of parylene and polyethylenimine (PEI) as gate dielectrics and channel dopant for graphene FETs, respectively; and (4) validation of a gas sensing mechanism utilizing the real-time, n-type graphene doping process induced by the exposure of graphene FET to the targeted gases. As such, the proposed sensing scheme/platform could open up a new class of research in graphene-based, flexible gas sensing systems.

A Solar-Blind UV Detector Based on Graphene-Microcrystalline Diamond Heterojunctions

An ultraviolet detector is demonstrated through a whole-wafer, thin diamond film transfer process to realize the heterojunction between graphene and microcrystalline diamond (MCD). Conventional direct transfer processes fail to deposit graphene onto the top surface of the MCD film. However, it is found that the 2 µm thick MCD diamond film can be easily peeled off from the growth silicon substrate to expose its smooth backside for the graphene transfer process for high-quality graphene/MCD heterojunctions. A vertical graphene/MCD/metal structure is constructed as the photodiode device using graphene as the transparent top electrode for solar-blind ultraviolet sensing with high responsivity and gain factor. As such, this material system and device architecture could serve as the platform for next-generation optoelectronic systems.

A versatile gas sensor with selectivity using a single graphene transistor

This work reports the technique to selectively sense different gases using a single graphene field effect transistor (FET) by measuring real time conductance as a function of gate voltage. Compare to the state-of-art, three distinctive advancements have been achieved: (1) first demonstration of selective gas sensing (NO2, NH3, H2O and CH3OH) using a single graphene FET; (2) experimental proof of linear dependence between the reciprocal of carrier mobility limited by long-range scattering and the Dirac Point voltage upon gas molecule exposure; (3) utilizations of such linear characteristic for selective gas sensing. As such, the proposed sensing scheme and results could open up a new class of graphene-based, selective gas sensing devices for practical uses as well as fundamental scientific research.

Model, Design, and Testing of Field Mill Sensors for Measuring Electric Fields Under High-Voltage Direct-Current Power Lines

High-voltage direct-current (HVdc) transmission lines have been implemented in many countries, including Australia, Brazil, China, and Sweden, and the safety concerns as the result of the high electromagnetic-radiation underneath the HVdc lines have garnered increased public attentions. Here, we report on the model, design, and testing of field-mill electric field sensors to measure the electric field at the ground level under the HVdc transmission lines. This study utilized a finite-element analysis method to establish numerical simulation results based on the electrical and mechanical parameters to achieve optimal designs with experimental calibrations. Afterward, these sensors were successfully tested and utilized at the national high-voltage test base.

Memorizing UV exposure energy in resistance — A smart patch based on conductive polymer

We have successfully demonstrated a smart UV patch to memorize the accumulative energy from the exposure to UV light over a period of time. The sensing principle is based on changes in the electrical resistance of a composite made of photo-acid generator triphenyl sulfonium triflate (TST), poly aniline emeraldine (PANI-EB), polyethylene glycol (PEG) and carbon nanotubes (CNTs). The sheet resistance of the composite can drop five orders of magnitude within 30 minutes of exposure to the UV light using an LED light source of 10Mw/cm2. The photo-induced protonation process can release protons from photo acid generators. With the help of the polyethylene glycol and CNTs, protons can diffuse across the film and cause the doping of polyaniline emeraldine base into a conductive polyaniline salt state to increase the conductivity.

An AC sensing scheme for minimal baseline drift and fast recovery on graphene FET gas sensor

This work reveals a new AC sensing scheme to achieve a fast recovery and minimal baseline drift of gas sensors based on graphene FETs. Compared with the state-of-art technologies, three distinctive advancements have been achieved: (1) first demonstration of using the AC phase lag signal between channel resistance and gate voltage as a sensitive gas detection scheme on graphene FETs; (2) achieving ultrafast baseline recovery speed (∼10s) on a defect rich, chemical vapor deposition (CVD) grown monolayer graphene FET for various tested gases, including water, methanol and ethanol vapors, respectively, almost ten times faster than those of the conventional DC resistance measurements; (3) validation of the AC phase lag sensing principle by using both analytical simulation as well as experimental data. As such, the proposed sensing scheme and results could open up a new frontier of graphene FET based gas sensing devices for accelerated sensing speed in practical uses and fundamental researches.

Direct-write n- and p-type graphene channel FETs

This paper presents a maskless, direct-write process to create both n- and p-type graphene channel FETs (Field Effect Transistors) on a single substrate. There are following achievements as compared with previous works: (1) direct deposition of controllable, arbitrary fiber patterns to construct graphene-based transistor, in which near-field electrospinning process is integrated to pattern the polymer fibers; (2) a maskless doping process to make both n-and/or p-type graphene on the same substrate, in which electrospun fibers serve as both the oxygen plasma etching mask and chemical doping sources, simultaneously; and As such, the demonstrated process could open up a new class of graphene-based devices for various applications.

Foldable paper electronics by direct-write laser patterning

We report a laser-ablation aided, direct-write fabrication technique that could convert non-conductive paper rinsed with metal ions and polymer solution into conductive metal carbide and graphene with a typical sheet resistance of 45.3 Ω/□. As fabricated paper electronics inherit the microfiber network from paper and have nanoscale pores and 2D metal carbide flakes due to the laser ablation process. This conducive porous structure could be potentially utilized for sensor and capacitor applications, which usually need large specific area. As preliminary demonstrations, we show a wireless moisture sensor and a supercapacitor fabricated with this foldable paper based electronics. Experimentally, the moisture changes are successfully detected in ambient environment by a paper-based moisture sensor and the paper-based supercapacitor has a measured capacitance of 1.2 mF/cm2. As such, this laser converted paper electronics could be useful for multiple applications such as sensors and energy storage devices.