Our research program aims to expand our understanding of materials, surfaces, and interfaces and to apply this knowledge to make advances in a number of technologically emerging and societally critical areas. Currently, at the fundamental level, we are interested in low-dimensional materials, hybrid organic/inorganic materials, metal oxides and silicates. In these materials systems, we aim to establish synthesis-structure-property relationships, and to understand and control their surfaces and interfaces. These insights are helping to guide our interest in impacting areas such as environment, health, sustainability and energy as detailed below.
Fundamental understanding of MOX-based sensors:Chemiresistive sensors, which transduce target gas concentrations based on the change in resistance of a sensing material, provide sensitive, low-cost detection of gaseous analytes in applications such as environmental monitoring, quality control, and clinical diagnostics. Semiconducting metal oxides (MOX) such as SnO2 are an industry-standard material system for chemiresistive sensing. We aim to develop MOX-based sensors with stable long-term characteristics by investigating how the structural and electronic properties of MOX respond to different operating temperatures, durations, and environments. To enhance the stability, sensitivity, and selectivity of our sensors, we are also investigating catalytically active noble metals, such as Pt, Au, and Pd, loaded onto MOX materials to form noble metal-loaded MOX nanocomposites. These nanocomposites are integrated with interdigited electrodes to form fully functioning sensors. To understand the sensing mechanisms of pristine MOX and metal-loaded MOX nanocomposites, we are correlating charge transport processes elucidated by impedance spectroscopy with structural and chemical information conveyed by X-ray photoelectron spectroscopy. Mature sensors will leverage energy-efficient (~15 mW to reach 500 °C) poly-Si and SiC microheater platforms previously developed by our group to enable robust, low-power gas microsensors for a variety of applications.
Materials innovations:A classic challenge in gas sensing is the tunability of the sensing material for the selective absorption of target gases without interference from unwanted species. Metal-organic frameworks (MOFs), made up of metal-cluster nodes connected by organic linkers, can achieve selective adsorption owing to their high chemical and structural tunability. Their selectivity and flexibility make MOFs attractive for gas sensing, as realized in novel low-power, low-footprint, on-chip devices such as the chemical-sensitive field-effect transistor, previously demonstrated by our group. In this project, we aim to explore the large library of MOFs (consisting of different metal nodes and organic linkers) towards novel electronic sensor arrays. This effort includes investigating the underlying electronic transduction mechanisms of MOFs through resistance measurement, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and infrared and Raman spectroscopies, targeting important polluting and toxic gases such as CO2, CH4, CO, CH2O.
Device innovations:Developing cheap and efficient sensors for monitoring different gases (such as CO2) is of great importance in many industries, and to environmental and human health, including food storage, microbial investigation, air-quality assessment, and capnography. The most common CO2 gas sensor is nondispersive infrared (NDIR) sensors which, while very effective, have proven difficult to miniaturize and to reduce cost. Among reported methods for gas detections, colorimetric sensors stand out for their simplicity, passive nature, and capability of exhibiting color changes detectable to human eyes, providing a user-friendly and convenient platform in practical applications. We aim to develop sensing materials based on MOFs with specific chemical entities to generate strong and reversible gas adsorptions. For example, the porous, robust ZIF-8, which is constructed by Zn2+ ions and 2-methylimidazole linkers can be modified with ethylenediamine (ED) post-synthesis. ED reacts with CO2 (in methanol) to form a zwitterion intermediate, which further protonates the incorporated pH indicator (phenol red, PSP) to generate a color change. With the rise of interest in the Internet of Things (IoT), the need for low-power sensors for monitoring the working environment has been in spotlight. Considering the number of sensors required to provide real time monitoring, creating sustainable and self-powered sensors is essential. Triboelectric nanogenerator (TENG), which converts mechanical motion to electrical energy, is one of the most promising candidates for realizing self-powered sensors due to its sensitivity to surface material properties and ability to generate consistent signals depending on mechanical input. We aim to develop functionalized TENG devices whose characteristics change in response to a target gas, enabling self-powered chemical sensing.
- May 2022 – Jeffrey graduates with highest honors and heads to Stanford for graduate studies!
- May 2022 – Adrian receives the UC Dissertation fellowship!
- March 2022 – Zhou successfully completes his PhD and joins Stanford as a postdoc!