Hai Liu

This paper presents a novel gas sensor based on ionization-assisted chemiresistive mechanism. Several volatile organic compounds at ppm level can be differentiated through the physical field gas ionization by CuO nanowires followed by the chemical adsorption forward a molybdenum oxide electrode. Supplemented by the pre-ionization of nanotips, an elevation of chemiresistive response toward acetone up to 16 times has been achieved at room temperature. On the other hand, the chemosensitive oxide layer provides an additional resistance signal to the gas discharge current and threshold voltage of the ionizer, resulting in a higher gas susceptibility. The gas sensing signals including gas discharge current, threshold voltage, and chemiresistance can be interpreted by the gas ionization followed by adsorption mechanism in sequence. Both the ionization energy of the target gas and adsorption energy of ionized gas fragment deduced by the first-principles calculations perform plausible counterparts to the gas selectivity, and the opposite responses have been elucidated by the different charge transfer preferences between the ionized and neutral acetone molecule. This nanowire based hybrid ionization-chemiresistance device may provide a new route to efficiently identify the organic gases.

The chemiresistive properties of oxide materials are critical to their electrical performance, which originate from the interfacial phenomena involving gaseous species and the solid substrate, e.g., adsorption and charge-transfer processes. However, their fundamental mechanisms are still insufficiently understood. In this study, we investigated the sensing behavior of p-type delafossite AgNiO₂ nanoparticles to a series of gaseous alcohols based on the chemiresistive effect, which presented a trend that the response increases with the growing carbon chain from methanol to n-pentanol but decreases from n-hexanol to n-nonanol. Combining the first-principles calculations and experimental data, we proposed a novel mechanism to elucidate this phenomenon. The growing carbon chain may alter the adsorption configuration to facilitate the supplementary electron transfer routes, which eventually boosted the chemiresistive effect. However, further elongation of the carbon chain in an alcohol can bring about the steric hindrance effect, which, in turn, suppressed the charge transfer. These findings provided new insights into the fundamental design of the functional oxide materials for advanced electrical applications.

An electromagnetically induced transparency analog (EIT-analog) metamaterial is numerically and experimentally demonstrated. The unit cell of the proposed structure is composed of two obliquely crossed cut wires (TOCCWs) and four rectangular split ring resonators (FRSRRs). Under the normal incidence of electromagnetic waves, TOCCWs and FRSRRs exhibit dipole, quadrupole, and hexapole resonances, which leads to the dual ultra-broadband and polarization-independent EIT-analog behavior. The coupling mechanism of the EIT effect is studied by analyzing transmission response and field distributions. In addition, the EIT-like transparency can be utilized for multi-channel and high-performance refractive index sensing applications.

Heart sounds contain critical information about valve activity and hemodynamics, serving as an essential basis for cardiovascular disease diagnosis. However, traditional heart sound sensors are either rigid or flexible but non-stretchable, limiting their ability to accommodate chest deformation and leading to signal distortion. This study proposes an easy-to-fabricate, stretchable piezoelectric heart sound sensor with a Kirigami-inspired design, a five-layer “sandwich” structure. Periodic Kirigami cuts significantly enhance stretchability while maintaining piezoelectric conversion efficiency. Finite element simulations reveal the Kirigami structure is more sensitive to hinge length and thickness than to hinge width. Electrical tests demonstrate a linear response to sound pressure, with output voltage rising from 0.11 V to 0.42 V (70–94 dB), and voltage amplitude increasing from 9 mV to 0.35 V (60–160 Hz). The sensor exhibits excellent stability, with a maximum amplitude variation of approximately 11% under 0–30% strain, a 17% voltage decrease at 11 mm bending radius, and less than 9% output fluctuation during 1200 s continuous excitation. Seven-day monitoring confirms reliable detection of the first (S1) and second (S2) heart sounds, with signals highly consistent with ECG and a commercial sensor, verifying its potential for wearable long-term monitoring and early cardiovascular disease screening.

Recent advances in artificial olfactory systems have attracted significant attention for their potential applications in humanoid robots and intelligent nasal devices capable of identifying objects and sensing hazards; however, the memory function is absent in traditional gas sensors, which is crucial to assess the long-term exposure risks. Meanwhile, due to the high operation temperature requirement, the gas sensors are usually difficult to integrate with the synaptic devices to form artificial olfactory systems. Here, we propose a novel artificial olfactory synaptic device to obtain and memorize formaldehyde information. The device is composed of an ion gel synaptic transistor integrated with a Ag-ZnO gas sensor, which can simulate the adverse effects of formaldehyde exposure to the human body and make an early warning. The Ag-ZnO gas sensor can detect and recognize different concentrations of formaldehyde as the chemiresistive signal at room temperature with ultraviolet irradiation instead of at high temperatures. The formaldehyde-induced resistive changes are transmitted to the gate voltage of the synaptic transistor, modulating the channel conductance to generate varying postsynaptic currents and to store gas information to realize the memory function. In addition, the postsynaptic current data of different concentrations can be imported into a support vector machine (SVM) for accurate identification, and early warning of different concentrations can be realized through the system. This bionic olfactory system provides a promising strategy for the development of advanced artificial intelligence and danger warnings.

A biomimetic multilevel microstructure coated with near-zero TCR electrode materials was developed to enable a temperature-insensitive and wide-range linear pressure sensor for reliable shape and texture recognition.

Detection capabilities similar to those of eyeballs and skin, such as pressure and light sensing, are crucial for electronic skins, bionic eyeballs, human–machine interactions, and biomimetic robots. Previous studies have primarily focused on investigating individual physical quantities, while simultaneously mapping these quantities remains challenging. This article introduces a dual-parameter sensor array based on amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). The array is capable of simultaneously and independently detecting pressure (106.9 kPa^-1, ranging from 0.5 to 10 kPa) and ultraviolet (UV) light (0.5 (μW/cm²)^-1, ranging from 0 to 76 μW/cm²). The pressure detection is achieved through a patterned conductive film connected to the source electrode of the TFTs, while the UV light is detected by photogenerated electron-hole pairs in the a-IGZO channel. Integration of TFTs increase the spatial resolution of both the pressure and light sensor array to 128 pixels per square centimeter (PPSC). The development of this dual-parameter high-density sensor array holds profound significance for applications in biomimetic eyeballs and electronic skin, such as texture and shape recognition.

The chemiresistive effect of an oxide significantly influences its electrical properties, which depend greatly on the interactions between the ambient gas molecules and the solid surface, including the gas adsorption and charge transfer still challenging to be clarified. In this work, we investigate the chemiresistive effect of the p-type delafossite CuScO2 microsheets by comparing their responses to various gaseous alcohols, which increase with an approximately linear relationship with the length of straight carbon chains from methanol to n-hexanol. A new mechanism is proposed to elucidate such a dramatic trend of observed chemiresistive change based on the first-principles calculations and test results. The increasing carbon chain length modulates the adsorption configuration and provides supplementary routes for electron transfer, which is assumed to account for the observed chemiresistive effect. This work may provide a novel perspective for the investigation and development of more advanced functional oxides for electrical applications.

As wearable and skin-friendly electronic devices become more popular, there is an increasing demand for devices that can smoothly conform to curved surfaces. However, the fabrication of electronic devices that can stretch or compress to fit these curved surfaces without wrinkles is still a challenge to the traditional planar techniques. In this work, we investigate and compare the stretchability and conformability of the “island-bridge” configuration with three different types of interconnects. The serpentine design with appropriate thickness and spacing is found to significantly excel in stretchability and conformability based on both calculations and experimental results. We propose a complete process for fabricating a stretchable interdigital electrode (IDE) array with serpentine interconnects on a polyimide base. These electrodes can sustain their electrical functionality even under 40% strain. Finally, a piezoresistive sensor array is provided on a dome using this stretchable IDE array. The sensor array exhibits outstanding performance both at the individual sensor level and across the array, demonstrating the viability of stretchable serpentine interconnects for curved electronics.