Huiliang Liu

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.n 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.

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.

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.

A wireless smart UV accumulation patch based on conductive polymer and CNT composites

Accumulative ultraviolet (UV) radiation from sunlight could be harmful to human skin and a reliable detection scheme is desirable as part of the protection strategy. This article reports the fabrication of an all passive, battery-free, irreversible, wireless smart UV patch for recording the amount of UV energy accumulation by resistance change. An ultrahigh sensitivity, with 5 orders of magnitude in resistance drop within 2 hours of exposure to sunlight, has been successfully demonstrated. The photo-induced protonation of a conductive polymer is assisted by carbon nanotube (CNT) composites for improved detection sensitivity. More importantly, the whole sensor has been successfully fabricated and optimized on a patterned flexible substrate using a laser printing technique for the electrodes and a subsequent dip-coating method has been utilized to deposit the sensing composite. As such, this novel UV accumulation sensor along with its proposed sensing scheme could be further developed for potential commercial applications.

Low-frequency electronic noises in CVD graphene gas sensors

This paper reports the investigation of low-frequency noise (1/f noise) in the Polyethylenimine (PEI) doped graphene gas sensor based on the architecture of using chemical vapor deposition (CVD) to make a graphene field effect transistor (GFET). Compare to the state-of-art, three advancements have been achieved: (1) first demonstration of 1/f noise characterization in PEI doped CVD graphene FET sensor under nitrogen, water, ethanol and methanol vapor environments; (2) modeling the measured 1/f noise characteristic features through simulating random charge transfer events caused by gas adsorption-desorption processes on graphene surface; (3) rejection of the complex background noise through an advanced digital signal processing method to accurately obtain the 1/f noise in graphene FET gas sensors. As such, the noise characterization, modeling and signal processing method on CVD-grown graphene FET gas sensors could provide new guidelines for researchers to develop 1/f noise based high performance gas sensing devices for practical applications.

A Phase Sensitive Measurement Technique for Boosted Response Speed of Graphene Fet Gas Sensor

This work presents a new class of gas sensing scheme: the phase sensitive signal on the GFET (Graphene Field Effect Transistor) gas sensors aiming to boost the response speed. Compared with the state-of-art gas sensing reports on measurements of DC resistance changes which are limited by trap states, three distinctive advancements have been achieved on graphene FETs in responses to water vapor: (1) experimental validations of a faster saturation and recovery speed, (2) in-situ measurements of field effect mobility µ and DC carrier concentration NGFET, and (3) multiple charge transfer processes through the plot of NGFET versus µ-1. As such, the proposed sensing scheme and results could open up a new frontier for graphene FET-based gas sensor with fast sensing speed in practical usages.