Nanogenerator has been a very important energy harvesting technology through directly deforming piezoelectric material. Here, we report a new magnetic force driven contactless nanogenerator (CLNG), which avoids the direct contact between nanogenerator and mechanical movement source. The CLNG can harvest the mechanical movement energy in a noncontact mode to generate electricity. Their output voltage and current can be as large as 3.2 V and 50 nA, respectively, which is large enough to power up a liquid crystal display. We also demonstrate a means by which a magnetic sensor can be built.
Response (on/off ratio) is one of the key parameters of ultraviolet (UV) sensors. In this paper, a kind of highly sensitive ZnO UV sensor with highly increased on/off current ratio was designed and developed. Under a weak UV intensity of 0.1 mW/cm 2 , this ultrathin ZnO film-based UV sensor has an on/off current ratio of 1.3 × 10 6 which is 3 times higher than the record value for ZnO-based UV sensors. In addition, it shows good flexibility and stable UV detection property during the bending process. When bending the sensor to a radius of curvature of about 18.5 mm, the sensor also shows high UV detection performance.
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 O 2 and NO 2 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 NO 2 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.
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.
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