The time evolution of the Fermi level of the graphene channel during a gas sensing process is systematically investigated. A Fermi level converging behavior at negative back-gate voltage and a Fermi level pinning behavior at positive back-gate voltage are observed during NH3 gas sensing process for an originally p-type doped graphene channel. The experimental results confirm that the original p-type doping level has a significant effect on the energy level where the Fermi level converges (CFL). An empirical model is proposed for the Fermi level converging and pinning behavior: the up-shift of Fermi level suppresses the electron injection from NH3 to graphene while it enhances the electron extraction from graphene to the original p-type doping agent such as H2O. At positive back-gate voltage, the significantly suppressed electron injection from the freshly absorbed NH3 is complemented by the electron extraction from graphene to the original p-type doping agent. That is why a graphene channel shows no response to NH3 when the Fermi level is pushed above CFL even though it is still significantly below the donor level of NH3. The results in this report reveal a general interplay behavior between the freshly introduced testing gas and the original dopant of the graphene.Download high-res image (248KB)Download full-size image

An electric pulse output by a nanogenerator upon a strain-and-release event resembles a neural impulse. Cutaneous receptors imbedded in skin transduce mechanical forces impinging the skin into neural impulses and the tactile information is encoded into the firing rates of the neural impulses. Here, we report a nanogenerator-type tactile sensor, which records the texture and sliding motion by outputting a sequence of electric pulses. The sensitive component of the device is an NG embedded in a polydimethylsiloxane package. An artificial finger-print serving as a strain introducer mimicking finger prints is integrated over the NG. The electric pulses outputted by the device transmit the texture and sliding motion information. The device demonstrates a capability of detecting punch holes with depth less than 200 μm on a nonwoven cloth. It also shows a perfect reproducibility of the electric pulses as it scans the same area of a band wire and a piece of nonwoven cloth. The artificial finger-print is the key element in transferring the strain direction, which allows the active sensor (a nanogenerator) beneath to detect the bumpy structure during a touch and sliding motion.

We present a simple processing method for synthesizing well-aligned millimeter-sized tetragon-shaped graphene domains on a polycrystalline copper substrate via low-pressure chemical vapor deposition. The tetragonal shape is achieved simply by wet loading the copper substrate with processing conditions previously used for the growth of millimeter-sized hexagon-shaped graphene domains. Electron backscatter diffraction (EBSD) shows that the wet loaded copper substrate is uniformly textured with a surface plane between Cu (1 0 0) and Cu (1 1 0). The in-plane rotation of the crystalline orientation across the Cu grains is very small. However, the EBSD showed that the surface orientation of the dry loaded substrate is close to the (1 1 1) crystal plane. The different surface orientation of the wet and dry loaded samples is attributed to the different surface oxygen concentration, which changes the relative stability of the (0 0 1), (1 1 0), and (1 1 1) plane during copper sublimation and recrystallization. These results provide an approach to tune the surface crystal orientation and thus the shape and orientation of the graphene domains.

A copper substrate soaking-treatment with FeCl3 solution is introduced to significantly reduce the initial nucleation density of graphene (up to 6-fold from 0.29 to 0.05 μm−2), and the overall graphene coverage increase-rate is successfully increased. The reduction in nucleation density is attributed to the oxidization of copper by treatment with the FeCl3 solution according to the X-ray photoelectron spectroscopy results. The soaking-treatment results in a rougher surface and consequently significant surface morphology rebuilding during the chemical vapor deposition. Pretreatment of copper substrate by soaking in FeCl3 solution is a simple and economical approach to control graphene growth. Most importantly, the technique is compatible with the common patterning technique of graphene.

Laser was coupled into an optical fiber, on which covered a layer of well-aligned carbon nanotubes (CNTs) serving as cathode, to tune the field emission of the cathode. CNT arrays as field emission cathode were synthesized by chemical vapor deposition (CVD) on a naked fiber core. When the laser was coupled into the fiber, the turn-on voltage (Vto at a current density of 1 mA cm−2) decreased from 1.0 to 0.9 kV and the emission current density increased from 0.83 mA cm−2 (at a 1 kV bias voltage) to 3.04 mA cm−2 on 40 μm diameter fiber. A photon absorption mechanism is attributed to the field emission improvement. The estimated effective work function of CNT arrays on the optical fiber decrease from 4.89 to 4.29 eV. The results show the possibility of constructing a waveguide type laser modulated field emission cathode.

Substrates with and without catalyst layers have been tested for the capacitively coupled radio-frequency plasma-enhanced chemical-vapor deposition of carbon nanosheets (CNSs) at a temperature as low as 500 °C. Glass coated with Ni, Ni/Co, Co/Ni, and Ni/Zn and glass without a catalyst layer were used as the substrates. CNSs were only obtained on substrate samples with a catalyst layer. The morphology of the CNSs was catalyst dependent. Both flat and standing CNSs were obtained. Transmission electron microscopy analysis revealed that the CNSs deposited on a Ni/Zn catalyst layer have densely distributed catalyst-induced wrinkles, while still sustaining a planar structure. The numerous wrinkles mean that these CNSs may serve as catalyst supports, ultracapacitor electrodes and gas-sensor materials. Electron diffraction and Raman studies showed that the CNSs obtained in this experiment are reasonably graphitized.Graphical abstractResearch highlights► Flat and standing carbon nanosheets are obtained on glasses by PECVD under 500 °C. ► Densely distributed wrinkles are induced into flat carbon nanosheets by catalyst. ► carbon nanosheets with wrinkles have a lot of potential applications.

Aging of the field emission performance of the printed carbon nanotubes (CNTs) cathode is studied. A continuous increase of the field emission current as well as the density of field emission sites under a constant voltage is observed. It is revealed that the resistant heating may play an important role in the activation of the potential emitters during the aging process. A technique of activating the printed CNT cathode with an aging process is suggested. F–N curves before and after the aging processes are analyzed and it is revealed that the effective emission area increases during the aging process.

The relationships between the cathode roughness such as protuberances and the trace of the electron beams were given by numerical result for the first time. Simulation results tell that a micron or sub-micron protuberances on the cathode will affect the electron trace remarkably and the divergence of the electron beam is sensitive to the size of protuberance. Further more, a quantitative result of effecting region of the protuberance was given. It's also revealed that the cathode roughness near the edge will give rise to the opportunity of the charge up of the insulator layer and miss addressing because of a raised divergent angle. And this conclusion was confirmed in luminescent pixels by the trails of electron beams, whose divergent angle was greatly increased by the edge field. According to the simulation result, a special treatment was adopted to enhance the uniformity of field emission, and a uniform luminescent pixels array was achieved.

•The response of graphene channel to the charges on a stimulator and the in situ generated charges are different.•The in situ generated triboelectric charges on a polymer passive layer induce a stepwise change of the channel resistance.•The results in this work suggest two possible operation models for the tribo-charge top-gate graphene channel.A graphene channel with polymer polymethyl methacrylate (PMMA) or Polydimethylsiloxane (PDMS) passive layer were fabricated on a Si/SiO2 substrate. The gate-effect of triboelectric charges on a foreign stimulator and in situ generated triboelectric charges on the passive layer are tested by controlling the foreign stimulator (PDMS or Acrylic plate) touch or not with the passive layer. When the negatively charged PDMS (positively charged Acrylic) stimulator approach and leave the device without touching the passive layer, the channel resistance shows a pulsed decrease (increase) with amplitude less than 1%. When the stimulator has a temporary touch with the passive layer, the in situ generated triboelectric charges on the PMMA (PDMS) passive layer cause stepwise increase (decrease) of the channel resistance. The amplitude of the resistance change steps of the device with PMMA passive layer (2 μm in thickness) is about one order larger for than that of the device with PDMS passive layer (about 60 μm in thickness). The significant different response of the device depending on whether the stimulator touches or not with the passive layer suggests two possible operation models for the tribo-charge top-gated graphene channel. It may enable diverse design of tactile sensors and pressure sensors based on the tribotronic effect.