Dapeng Wei

Wearable temperature sensor based on graphene nanowalls

We demonstrate an ultrasensitive wearable temperature sensor prepared using an emerging material, graphene nanowalls (GNWs), and its ease of combination with polydimethylsiloxane (PDMS). Fabrication of the sensor allows for a polymer-assisted transfer method making it considerably facile, biocompatible and cost effective. The resultant device exhibits a positive temperature coefficient of resistivity (TCR) as high as 0.214 °C−1, which is three fold higher than that of conventional counterparts. We attribute this to the excellent stretchability and thermal sensitivity of GNWs together with the large expansion coefficient of PDMS. Moreover, the sensor is capable of monitoring body temperature in real time, and it presents a quite fast response/recovery speed as well as long term stability. Such wearable temperature sensors could constitute a significant step towards integration with the next frontier in personalized healthcare and human–machine interface systems.

Direct Growth of Graphene Films on 3D Grating Structural Quartz Substrates for High-Performance Pressure-Sensitive Sensors.

Conformal graphene films have directly been synthesized on the surface of grating microstructured quartz substrates by a simple chemical vapor deposition process. The wonderful conformality and relatively high quality of the as-prepared graphene on the three-dimensional substrate have been verified by scanning electron microscopy and Raman spectra. This conformal graphene film possesses excellent electrical and optical properties with a sheet resistance of <2000 Ω·sq(-1) and a transmittance of >80% (at 550 nm), which can be attached with a flat graphene film on a poly(dimethylsiloxane) substrate, and then could work as a pressure-sensitive sensor. This device possesses a high-pressure sensitivity of -6.524 kPa(-1) in a low-pressure range of 0-200 Pa. Meanwhile, this pressure-sensitive sensor exhibits super-reliability (≥5000 cycles) and an ultrafast response time (≤4 ms). Owing to these features, this pressure-sensitive sensor based on 3D conformal graphene is adequately introduced to test wind pressure, expressing higher accuracy and a lower background noise level than a market anemometer.

Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction

Van der Waals heterostructures built from two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for next-generation information devices. Here we report that by simply stacking a vapor-phase-synthesized multilayer n-type WS2 film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature coefficient of the breakdown voltage. The WS2/Si heterojunction working in the Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340 to 1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm and a fast response speed of 670 μs. Such high performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p-n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photogenerated electron-hole pairs.

Direct growth of graphene nanowalls on the crystalline silicon for solar cells

We developed a simple approach to fabricate graphene/Si heterojunction solar cells via direct growth of graphene nanowalls on Si substrate. This 3D graphene structure was outstanding electrode network and could form fine interface with Si substrate. Moreover, direct growth method not only simplified manufacturing process, but also avoided damages and contaminants from graphene transfer process. The short-circuit current (Jsc) increased greatly and could reach 31 mA/cm2. After HNO3 doping, the energy conversion efficiency was increased up to 5.1%. Furthermore, we investigated the influence of growth time on the cell performance.

High-efficiency, stable and non-chemically doped graphene–Si solar cells through interface engineering and PMMA antireflection

In graphene–Si (Gr–Si) solar cells, chemical doping could remarkably enhance the performance of the cells, but weakens their stability, which limits their further application. However, in terms of the efficiency of pristine cells, the interfacial defect states and the increased thickness of the oxide layer in air also make high-efficiency and stable cells more difficult to achieve. Here we directly grew carbon nanowalls (CNWs) as a passivation layer onto the Si surface, which could obviously increase the efficiency. On the other hand, a poly(methyl-methacrylate) (PMMA) film was retained after transferring graphene, which could not only keep the graphene intact, but could also serve as an efficient antireflection layer for greater light absorption of the Si. A maximum PCE of 8.9% was achieved for a PMMA-bilayer Gr-CNWs-Si solar cell. Our cell’s efficiency showed a slight degradation after being stored in air for 4 months. This result is far superior to other previously reported stability data for chemically doped Gr–Si solar cells. The PMMA-Gr-CNWs-Si solar cell, with high efficiency and stability, possesses important potential for practical photovoltaic applications.

Double-induced-mode integrated triboelectric nanogenerator based on spring steel to maximize space utilization

Integrated multilayered triboelectric nanogenerators (TENGs) are an efficient approach to solve the insufficient energy problem caused by a single-layered TENG for achieving high output power density. However, most integrated multilayered TENGs have a relatively large volume. Here, a double-induced-mode integrated triboelectric nanogenerator (DI-TENG) based on spring steel plates is presented as a cost-effective, simple, and high-performance device for ambient vibration energy harvesting. The unique stackable rhombus structure, in which spring steel plates act both as skeletons and as electrodes, can enhance the output performance and maximize space utilization. The DI-TENG with five repeated units in a volume of 12 cm × 5 cm × 0.4 cm can generate a short-circuit current of 51 μA and can transfer charges of 1.25 μC in a half period. The contrast experiment is conducted systematically and the results have proved that the DI-TENG has a great advantage over the single-induced-mode TENG (SI-TENG) with only one side of a friction layer on its electrode. Besides, the DI-TENG can easily power a commercial calculator and can be used as a door switch sensor.

Flexible solar cells based on graphene-ultrathin silicon Schottky junction

We developed a flexible graphene–silicon (Gr–Si) photovoltaic device with high reliability and stability. Ultrathin Si film was fabricated via an anisotropic Si etching method, and exhibited excellent flexibility. Different from the traditional graphene transfer approach, polymethylmethacrylate (PMMA) film remained, by which the physical damage of graphene resulting from the PMMA dissolution process is avoided. Moreover, PMMA film could serve as an antireflection layer that reduces the reflectance from 40% to lower than 20%. The power conversion efficiency of a PMMA–Gr–Si film solar cell reached 5.09%, which far exceeds the efficiency of a Gr–Si solar cell with the same thickness of Si film of 10.6 μm. More importantly, the PMMA film worked as a packaging material to improve the device stability. The PMMA–Gr–Si solar cell could keep 93% of the original efficiency after bending 60 times. The simple, low-cost and flexible photovoltaic device shows promising prospects in potential applications for portable and wearable electronic products.

Embedding variable micro-capacitors in polydimethylsiloxane for enhancing output power of triboelectric nanogenerator

Polydimethylsiloxane (PDMS) is an excellent material for investigating the mechanism of triboelectricity as it can easily be used to construct various microstructures. In this study, micro-capacitors (MCs) and variable microcapacitors (VMCs) were embedded in PDMS by filling PDMS with silver nanoparticles (NPs) and constructing an internal cellular structure. The output performance of the triboelectric nanogenerators (TENGs) based on MCs@PDMS and VMCs@PDMS films was systematically investigated, with variation of the filling content of silver NPs and the pore ratio and size. The microstructure, permittivity, dielectric loss, and capacitance of the VMCs@PDMS films were well characterized. The output current of the TENG based on the VMCs@PDMS film was respectively 4.0 and 1.6 times higher than that of the TENGs based on the pure PDMS film and MCs@PDMS film, and the output power density of the former reached 6 W·m–2. This study sheds light on the physical nature of conductive nanoparticle fillings and cellular structures in dielectric triboelectric polymers.

Composite Transparent Electrode of Graphene Nanowalls and Silver Nanowires on Micropyramidal Si for High-Efficiency Schottky Junction Solar Cells

The conventional graphene-silicon Schottky junction solar cell inevitably involves the graphene growth and transfer process, which results in complicated technology, loss of quality of the graphene, extra cost, and environmental unfriendliness. Moreover, the conventional transfer method is not well suited to conformationally coat graphene on a three-dimensional (3D) silicon surface. Thus, worse interfacial conditions are inevitable. In this work, we directly grow graphene nanowalls (GNWs) onto the micropyramidal silicon (MP) by the plasma-enhanced chemical vapor deposition method. By controlling growth time, the cell exhibits optimal pristine photovoltaic performance of 3.8%. Furthermore, we improve the conductivity of the GNW electrode by introducing the silver nanowire (AgNW) network, which could achieve lower sheet resistance. An efficiency of 6.6% has been obtained for the AgNWs-GNWs-MP solar cell without any chemical doping. Meanwhile, the cell exhibits excellent stability exposed to air. Our studies show a promising way to develop simple-technology, low-cost, high-efficiency, and stable Schottky junction solar cells.

Three-dimensional conformal graphene microstructure for flexible and highly sensitive electronic skin

We demonstrate a highly stretchable electronic skin (E-skin) based on the facile combination of microstructured graphene nanowalls (GNWs) and a polydimethylsiloxane (PDMS) substrate. The microstructure of the GNWs was endowed by conformally growing them on the unpolished silicon wafer without the aid of nanofabrication technology. Then a stamping transfer method was used to replicate the micropattern of the unpolished silicon wafer. Due to the large contact interface between the 3D graphene network and the PDMS, this type of E-skin worked under a stretching ratio of nearly 100%, and showed excellent mechanical strength and high sensitivity, with a change in relative resistance of up to 6500% and a gauge factor of 65.9 at 99.64% strain. Furthermore, the E-skin exhibited an obvious highly sensitive response to joint movement, eye movement and sound vibration, demonstrating broad potential applications in healthcare, body monitoring and wearable devices.