Adjustment of oxygen vacancy states in ZnO and its application in ppb-level NO2 gas sensor.

Oxygen vacancy (VO) is long believed as a key factor influencing the gas sensing properties. However, the concentration of VO is generally focused while the VO state is neglected, which masks the inherent mechanism of gas sensor. Using a post annealing process, the influence of VO states on the response of ZnO nanofilm to NO2 gas is investigated in this study. The systematical analysis of the results obtained by different methods indicates a transformation of VO from the neutral to the doubly ionized state during post annealing treatment. The results also imply that the gas sensing properties is not directly correlated with the VO concentration. And due to the competitive adsorption of ambient O2, the neutral VO is majorly occupied by the adsorbed O2 while the VO in doubly ionized state can promote the adsorption of NO2. Consequently, the transition of VO from the neutral to the doubly ionized state can lead to a dramatic increase of the response to NO2, from 733 to 3.34 × 104 for 100 ppm NO2. Guided by this mechanism, NO2 gas sensing in ppb-level is also achieved: the response reaches 165% to 25 ppb (0.025 ppm) NO2 with a good repeatability.

Coaxial double helix structured fiber-based triboelectric nanogenerator for effectively harvesting mechanical energy.

Harvesting energy from the surrounding environment, particularly from human body motions, is an effective way to provide sustainable electricity for low-power mobile and portable electronics. To get adapted to the human body and its motions, we report a new fiber-based triboelectric nanogenerator (FTNG) with a coaxial double helix structure, which is appropriate for collecting mechanical energy in different forms. With a small displacement (10 mm at 1.8 Hz), this FTNG could output 850.20 mV voltage and 0.66 mA m-2 current density in the lateral sliding mode, or 2.15 V voltage and 1.42 mA m-2 current density in the vertical separating mode. Applications onto the human body are also demonstrated: the output of 6 V and 600 nA (3 V and 300 nA) could be achieved when the FTNG was attached to a cloth (wore on a wrist). The output of FTNG was maintained after washing or long-time working. This FTNG is highly adaptable to the human body and has the potential to be a promising mobile and portable power supply for wearable electronic devices.

Ultrasensitive Fiber-Based ZnO Nanowire Network Ultraviolet Photodetector Enabled by the Synergism between Interface and Surface Gating Effects.

A flexible UV photodetector with a high on/off ratio is extremely important for environmental sensing, optical communication, and flexible optoelectronic devices. In this work, a flexible fiber-based UV photodetector with an ultrahigh on/off ratio is developed by utilizing the synergism between interface and surface gating effects on a ZnO nanowire network structure. The synergism between two gating effects is realized by the interplay between surface band bending and the Fermi level through the nanowire network structure, which is proved through the control experiments between the ZnO micro/nanowire photodetector and micro/nanowire junction photodetector, and the corresponding Kelvin probe force microscopy (KPFM) measurements. The on/off ratio of the fiber-based ZnO nanowire network UV photodetector reaches 1.98 × 108 when illuminated by 1.0 mW cm-2 UV light, which is 20 times larger than the largest reported result under the same UV illumination. This new UV sensor also has a high resolution to UV light intensity change in the nW cm-2 range. Furthermore, when the fiber-based photodetector is curved, it still shows excellent performance as above. This work gives a new effective route for the development of a high-performance UV photodetector or other optoelectronic detection devices.

Mechanism of Sensitivity Enhancement of a ZnO Nanofilm Gas Sensor by UV Light Illumination.

Although ultraviolet (UV) light illumination has been widely used to increase the sensitivity of semiconductor gas sensors, its underlying mechanism is still blurred and controversial. In this work, the influence of UV light illumination on the sensitivity of ZnO nanofilm gas sensors is explored experimentally and simulated based on a modified Wolkenstein's model. The influential factors on sensitivity are determined respectively: the surface band bending and Fermi level are measured by Kelvin probe force microscopy, the binding energy and extrinsic surface state are calculated by density functional theory, and the depletion of the whole semiconductor caused by the finite size is illustrated by the transfer characteristics of a field effect transistor. With all these factors taken into consideration, the surface state densities of adsorbed O2 and NO2 molecules in the dark and under UV light illumination are calculated which determine the sensitivity. Good agreement has been obtained between the experiment and simulation results. Accordingly, when NO2 is introduced into the atmosphere, the enhancement of sensitivity is ascribed to the more dramatic increase of surface state density and surface band bending activated by the UV light illumination compared with that in the dark. This finding is critical and would contribute greatly to the development of gas sensors with high sensitivity.

Clarifying the high on/off ratio mechanism of nanowire UV photodetector by characterizing surface barrier height.

The response of semiconductor nanowire UV sensors represented by ZnO nanowire UV sensor is usually explained by the adsorption and desorption of oxygen molecules, but with the great increase of these sensors' on/off ratio in recent years, this explanation is inadequate and the inner mechanism for the large on/off ratio urgently needs to be explored. Here, the distribution of carrier concentration in a ZnO nanowire is found to be determined as a function of the radius of the nanowire, using a calibrated surface photovoltage method and space charge model. A critical radius is indicated which determines the carrier concentration and photoresponse behavior of the nanowire. When the radius is below this critical value, the carrier concentration in the dark decreases dramatically compared with that of the nanowire under UV light illumination. Specifically, a decrease of carrier concentration by 4-5 orders of magnitude occurs when the radius is below 50 nm, which causes the on/off ratio to vary by the same orders of magnitude. When the radius is above the critical value, the influence of radius on carrier concentration is nonsignificant and the on/off ratio is below 100. Finally, we found that the high on/off ratio of the ZnO nanowire should be ascribed to the complete depletion of the nanowire led by the interplay of radius and surface band bending rather than the change in width of the depletion layer as most papers have suggested.