Tian-Ling Ren

A Graphene-Based Resistive Pressure Sensor with Record-High Sensitivity in a Wide Pressure Range

Pressure sensors are a key component in electronic skin (e-skin) sensing systems. Most reported resistive pressure sensors have a high sensitivity at low pressures (<5 kPa) to enable ultra-sensitive detection. However, the sensitivity drops significantly at high pressures (>5 kPa), which is inadequate for practical applications. For example, actions like a gentle touch and object manipulation have pressures below 10 kPa, and 10–100 kPa, respectively. Maintaining a high sensitivity in a wide pressure range is in great demand. Here, a flexible, wide range and ultra-sensitive resistive pressure sensor with a foam-like structure based on laser-scribed graphene (LSG) is demonstrated. Benefitting from the large spacing between graphene layers and the unique v-shaped microstructure of the LSG, the sensitivity of the pressure sensor is as high as 0.96 kPa−1 in a wide pressure range (0 ~ 50 kPa). Considering both sensitivity and pressure sensing range, the pressure sensor developed in this work is the best among all reported pressure sensors to date. A model of the LSG pressure sensor is also established, which agrees well with the experimental results. This work indicates that laser scribed flexible graphene pressure sensors could be widely used for artificial e-skin, medical-sensing, bio-sensing and many other areas.

Carbonized Silk Fabric for Ultrastretchable, Highly Sensitive, and Wearable Strain Sensors

A carbonized plain-weave silk fabric is fabricated into wearable and robust strain sensors, which can be stretched up to 500% and show high sensitivity in a wide strain range. This sensor can be assembled into wearable devices for detection of both large and subtle human activities, showing great potential for monitoring human motions and personal health.

Graphene-on-Paper Sound Source Devices

We demonstrate an interesting phenomenon that graphene can emit sound. The application of graphene can be expanded in the acoustic field. Graphene-on-paper sound source devices are made by patterning graphene on paper substrates. Three graphene sheet samples with the thickness of 100, 60, and 20 nm were fabricated. Sound emission from graphene is measured as a function of power, distance, angle, and frequency in the far-field. The theoretical model of air/graphene/paper/PCB board multilayer structure is established to analyze the sound directivity, frequency response, and efficiency. Measured sound pressure level (SPL) and efficiency are in good agreement with theoretical results. It is found that graphene has a significant flat frequency response in the wide ultrasound range 20-50 kHz. In addition, the thinner graphene sheets can produce higher SPL due to its lower heat capacity per unit area (HCPUA). The infrared thermal images reveal that a thermoacoustic effect is the working principle. We find that the sound performance mainly depends on the HCPUA of the conductor and the thermal properties of the substrate. The paper-based graphene sound source devices have highly reliable, flexible, no mechanical vibration, simple structure and high performance characteristics. It could open wide applications in multimedia, consumer electronics, biological, medical, and many other areas.

Graphene/semiconductor heterojunction solar cells with modulated antireflection and graphene work function†

Theoretical and experimental studies have been performed to simulate and optimize graphene/semiconductor heterojunction solar cells. By controlling graphene layer number, tuning graphene work function and adding an antireflection film, a maximal theoretical conversion efficiency of ∼9.2% could be achieved. Following the theoretical optimization proposal, the Schottky junction solar cells with modified graphene films and silicon pillar arrays were fabricated and were found to give a conversion efficiency of up to 7.7%.

Giant electrocaloric effect in lead-free thin film of strontium bismuth tantalite

Giant electrocaloric effect was observed in lead-free material for the first time. A demonstration of large electrocaloric effect in 200 nm sol-gel thin film SrBi2Ta2O9 is described here. The ferroelectric hysteresis loops and the film permittivity were measured. The extracted data characterized the electrocaloric temperature change, up to 4.93K in 12V (i.e., 0.41 K V−1) near the Curie point. It is concluded that the giant electrocaloric effect exists in lead-free materials, and a large family of layered perovskite oxides may exhibit analogical property. The absence of lead allows eco-friendly application and enhances compatibility with integrated circuit process in future applications.

Large-Area, Transparent, and Flexible Infrared Photodetector Fabricated Using P-N Junctions Formed by N-Doping Chemical Vapor Deposition Grown Graphene

Graphene is a highly promising material for high speed, broadband, and multicolor photodetection. Because of its lack of bandgap, individually gated P- and N-regions are needed to fabricate photodetectors. Here we report a technique for making a large-area photodetector on the basis of controllable fabrication of graphene P-N junctions. Our selectively doped chemical vapor deposition (CVD) graphene photodetector showed a ∼5% modulation of conductance under global IR irradiation. By comparing devices of various geometries, we identify that both the homogeneous and the P-N junction regions contribute competitively to the photoresponse. Furthermore, we demonstrate that our two-terminal graphene photodetector can be fabricated on both transparent and flexible substrates without the need for complex fabrication processes used in electrically gated three-terminal devices. This represents the first demonstration of a fully transparent and flexible graphene-based IR photodetector that exhibits both good photoresponsivity and high bending capability. This simple approach should facilitate the development of next generation high-performance IR photodetectors.

Dielectric, magnetic, and magnetoelectric properties of La and Ti codoped BiFeO3

The authors report on the dielectric, magnetic, and magnetoelectric (ME) properties of La and Ti codoped BiFeO3 (LBFTO). Codoping changes the structure of BiFeO3 from rhombohedral to tetragonal and the ferromagnetic properties of LBFTO are remarkably improved. More interestingly, the dielectric constant of LBFTO shows a linear increase with magnetic field and the slope decreases linearly with increasing temperature. The electric polarization of LBFTO also increases upon applying a magnetic field. The ME coupling coefficients of different orders were obtained by analyzing these data. The results were discussed by considering the doping induced destruction of the cycloidal structure in LBFTO.

Resistance switching and white-light photovoltaic effects in BiFeO3/Nb–SrTiO3 heterojunctions

BiFeO3/Nb–SrTiO3 heterojunctions with room-temperature resistance switching (RS) and white-light photovoltaic (PV) effects were fabricated by pulsed laser deposition. The current-voltage characteristics of these heterojunctions show a good rectifying property with a large rectifying ratio of 104. Nonvolatile bipolar RS effect was observed with an ON/OFF-state current ratio of about 102. The heterojunctions also exhibit a substantial white-light PV effect. Both the RS and PV behaviors can be modulated by additional pulsed voltages, which control the electric polarization of the heterojunctions. This letter is helpful for exploring the multifunctional heterojunctions and their applications in memory devices and solar cells.

Enhanced photovoltaic properties in graphene/polycrystalline BiFeO3/Pt heterojunction structure

We report the enhanced photovoltaic properties in polycrystalline BiFeO3 (BFO) thin films with graphene as top electrodes. The short circuit current density (Jsc) and open circuit voltage of the heterojunction are measured to be 25 μA/cm2 and 0.44 V, respectively, much higher than the reported values for polycrystalline BFO with indium tin oxide (ITO) as top electrodes. Influence of HNO3 treatment on the photovoltaic properties is studied, and a significant photocurrent density improvement from 25 μA/cm2 to 2.8 mA/cm2 is observed. A metal-intrinsic semiconductor-metal model is proposed to explain the graphene induced enhancement comparing with traditional ITO.

Novel Field-Effect Schottky Barrier Transistors Based on Graphene-MoS2 Heterojunctions

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.