A practical vacuum sensor based on a ZnO nanowire array

We report a practical vacuum pressure sensor based on a ZnO nanowire array (NWA). An oriented single-crystal ZnO NWA was synthesized by electrodeposition. The device consists of two ITO glass plates coated with a ZnO NWA. Scanning electron microscopy (SEM) and the x-ray diffraction (XRD) pattern show that the as-grown ZnO NWAs are single-crystal and roughly oriented with the ZnO(002) plane parallel to the substrate. Through measuring the pressure dependent resistance of the sensor at different gas species and temperatures, we discovered that the resistance increases monotonically with vacuum pressure. This demonstrates that a practical vacuum sensor could be fabricated since measurements were carried out with a normal multimeter, with no need for the high sensitivity and costly equipment as routinely required in nanotechnology for extremely weak signals. Measurement at elevated temperature (300?°C) showed that the vacuum sensor is much stabler and more sensitive to O(2) pressure. The principle of the device relates to the adsorbed oxygen species on the large surface area of a ZnO NWA to form a resistive depletion layer at the nanowire (NW) surface.     

A ZnO nanowire vacuum pressure sensor

In this study, we report the growth and characterization of lateral ZnO nanowires (NWs) on ZnO:Ga/glass templates. Using x-ray diffraction and micro-Raman spectroscopy, it was found that crystal quality of the as-grown ZnO NWs is good. It was also found that the average length and average diameter of the laterally grown ZnO NWs were 5?μm and 30?nm, respectively. A vacuum pressure sensor was then fabricated using a single NW bridging across two electrodes. By measuring the current-voltage characteristics of the samples at low pressure, we found that the currents were of 17, 34.28, 57.37 and 96.06?nA for the ZnO NW measured at 1 × 10(-3)?Torr, 1 × 10(-4)?Torr, 3 × 10(-5)?Torr and 5 × 10(-6)?Torr, respectively. These values suggest that the laterally grown ZnO NWs prepared in this study are potentially useful for vacuum pressure sensing.     

共平面低功耗CO气体传感器的设计及制作

设计了一种新型的共平面微结构气体传感器,利用MEMS技术制作了传感器样品.实验测得该传感器对于体积分数为50×10~(-6)CO气体的灵敏度为8.6(R0/R),功耗为82mW,响应时间约5s,恢复时间约22s,该传感器的灵敏度是同种敏感材料烧结型传感器(测体积分数50×10~(-6) CO气体)的2.5倍,而功耗却是同种敏感材料烧结型传感器的1/2,是一种具备了体积小、重量轻、功耗低、灵敏度高、便于与其它器件集成等特点,而且制作工艺简单,成本较低的微结构CO气体传感器.     

Improving the NH(3) gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration

We report a method to improve the sensitivity of a zinc oxide (ZnO) nanowire gas sensor towards ammonia (NH(3)) without the use of catalyst nanoparticles on the nanowire surface. This improvement is achieved by lowering the nominal carrier concentration in the as-grown ZnO nanowires. The carrier concentration in the as-grown ZnO nanowires can be tuned by treating these nanowires to either an oxidizing gas plasma or a reducing gas plasma, as observed from the measured current-voltage (I-V) characteristics response. We demonstrate that a ZnO nanowire sensor device that has been subjected to oxygen plasma treatment, thereby having a reduced carrier concentration, exhibits a sensitivity towards 0.75% NH(3) gas that is improved by approximately four times. The origin of this gas sensitivity improvement is discussed based on x-ray photoelectron spectroscopy analysis results of the plasma-treated ZnO nanowires.     

A ZnO-nanowire phototransistor prepared on glass substrates

The fabrication of a phototransistor via the bridging of two prefabricated electrodes with a laterally grown ZnO nanowire is reported. It was found that the fabricated device is an n-channel enhancement-mode phototransistor with a dark carrier concentration of 6.34 × 10(17) cm(-3) when the gate voltage is biased at 5 V. With an incident-light wavelength of 360 nm and a zero gate bias, it was found that the noise equivalent power and normalized detectivity (D*) of the fabricated ZnO phototransistor were 6.67 × 10(-17) W and 1.27 × 10(13) cm Hz(0.5) W(-1), respectively. It was also found that the current in the device can be modulated efficiently by tuning the wavelength of the excitation source.     

真空度对MBE GaAs光阴极激活结果的影响

采用分子束外延(MBE)技术生长的GaAs光阴极材料,按照常规方法进行高-低温两步激活时,总是出现低温灵敏度比高温低的反常现象.研究中,当激活时的系统真空度从1×10~(-7)Pa提升到1×10~(-8)Pa时,发现结果能够重新出现低温灵敏度比高温灵敏度高30%的预期规律.此外,在系统真空度为10~(-7)Pa条件下,由于变掺杂材料的表面掺杂浓度较低,其出现光电流时的首次进Cs时间也较均匀掺杂材料长,而在真空度为约10~(-9)Pa条件下,这一情况也不再明显.初步分析造成该现象的原因,是与MBE材料的掺杂元素及其低温处理特性对真空度比较敏感有关.MBE阴极激活结果受系统真空度条件影响较大,因此对MBE变掺杂光阴极的制备工艺应随系统真空度条件不同而调整.     

Exciting Dilute Magnetic Semiconductor: Copper-Doped ZnO

The present study is focused on the copper-doped ZnO system. Bulk copper-doped ZnO pellets were synthesized by a solid-state reaction technique and used as target material in pulsed laser deposition. Thin films were grown for different Cu doped pellets on sapphire substrates in vacuum (5×10^?5 mbar). Thin films having (002) plane of ZnO showed different oxidation states of dopants. M–H curves exhibited weak ferromagnetic signal for 1–3 % Cu doping but for 5 % Cu doped thin film sample showed the diamagnetic behavior. For deeper information, thin films were grown for 5 % Cu doped ZnO bulk pellet in different oxygen ambient pressures and analyzed. PL measurement at low temperature showed the emission peak in thin films samples due to acceptor-related transitions. XPS results show that copper exists in Cu²⁺ and Cu⁺¹ valence states in thin films and with increasing O₂ ambient pressure the valence-band maximum in films shifts towards higher binding energy. Furthermore, in lower oxygen ambient pressure (1×10^?2 mbar) thin films showed magnetic behavior but this vanished for the film grown at higher ambient pressures of oxygen (6×10^?2 mbar), which hints towards the decrease in donor defects.     

基于气动加热原理的火星大气密度传感器可行性研究

火星大气密度测量需要传感器灵敏度高、功耗低、响应时间短,为此提出了基于气动加热原理的新型传感器方案。通过对火星轨道气体流动状态进行判断、对分子与热敏器件碰撞产生的热功率密度和正压强的计算和对噪声及性能的分析,新型传感器可行性从理论上得到了论证。传感器测量火星轨道高度范围为90 km到400 km,理论热响应时间为122 ms,最小可探测压强为2.05×10-8Pa。     

MWCNTs/多孔ZnO纳米材料的制备及室温NO气敏性研究

为开发室温气敏传感器材料,以Zn(NO3)2.6H2O为锌源、尿素为沉淀剂,在制备水合碱式碳酸锌(Zn4CO3(OH)6.H2O)的过程中加入羧基化的MWCNTs(MWCNT-COOH),焙烧制备了MWCNTs/ZnO复合材料.采用XRD,SEM和TEM等对其进行了分析.结果表明:复合材料中MWCNTs分散均匀,ZnO呈多孔纳米片状,纳米片由多个尺寸在10～20 nm的ZnO颗粒组成;在室温、空气湿度为50%的氛围中测试复合材料对NO的气敏响应发现,复合材料对体积浓度1×10-4的NO气敏响应灵敏度大约是MWCNT-COOH的3倍,明显高于MWCNT-COOH;对比加入不同量MWCNT-COOH制备的3种复合材料对NO的气敏性可知,加入200 mg MWCNT-COOH所制备的复合材料对低浓度(体积浓度≤50×10-6)的NO气体表现出较高的灵敏度.     

Structure and physical properties of undoped ZnO and vanadium doped ZnO films deposited by pulsed laser deposition

Undoped ZnO films were deposited using pulsed laser deposition technique on Si and glass substrates in different O2 partial pressures (ranging from 10(-5) mbar to 3 mbar) and substrate temperatures. When the substrate temperature is 500 degrees C and O2 partial pressure (pp) approximately 3 mbar, randomly oriented ZnO hexagons were observed on glass substrate, whereas, dense ZnO hexagonal rod like structures (diameter ranging from 200-500 nm) were observed on Si substrate. The photoluminescence (PL) characterization of ZnO film grown on Si exhibited an intense defect free narrow excitonic emission in the UV region (Full width half maximum (FWHM) approximately 11.26 nm) as compared to broad emission (FWHM approximately 57.06 nm) from that grown on glass. The parent film emission was found to shift from UV to blue region on doping ZnO with Vanadium.     

Self-powered system with wireless data transmission

We demonstrate the first self-powered system driven by a nanogenerator (NG) that works wirelessly and independently for long-distance data transmission. The NG was made of a free cantilever beam that consisted of a five-layer structure: a flexible polymer substrate, ZnO nanowire textured films on its top and bottom surfaces, and electrodes on the surfaces. When it was strained to 0.12% at a strain rate of 3.56% S(-1), the measured output voltage reached 10 V, and the output current exceeded 0.6 μA (corresponding power density 10 mW/cm(3)). A system was built up by integrating a NG, rectification circuit, capacitor for energy storage, sensor, and RF data transmitter. Wireless signals sent out by the system were detected by a commercial radio at a distance of 5-10 m. This study proves the feasibility of using ZnO nanowire NGs for building self-powered systems, and its potential application in wireless biosensing, environmental/infrastructure monitoring, sensor networks, personal electronics, and even national security.     

Microstructured vacuum gauges and their future perspectives

New prototypes and concepts of microstructured vacuum gauges have been developed by using the fabrication technologies for micro electro mechanical systems (MEMS). The realization of such microstructured gauges requires sophisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS vacuum gauges is demonstrated by a few examples.Micro-Pirani gauges are based on the principle that the heat transfer between two surfaces is proportional to the number of molecules (and hence the pressure) transferring the heat, when the mean free path in the gas is larger than the distance between the surfaces. In contrast to conventional Pirani gauges with heated thin wires, in a micro-Pirani gauge the heat transfer takes place between an extremely thin heated membrane and the surrounding. The thin membrane (200-nm thick) is realized by deposition of siliconoxide/siliconnitride, photolithography and anisotropic etching of silicon wafers. Heating is performed by a meander-shaped aluminum thin film heater in the center of the membrane. This micro-Pirani gauge shows a high sensitivity in the pressure range between 10⁻⁴ and 100 mbar. By using a silicon “microbridge” with 10 μm small gap between heated membrane and surrounding, the pressure sensitivity of the chip is extended up to 1000 mbar.Similar concepts are presented and discussed with respect to the miniaturization of spinning rotor gauges. The new concept is based on the application of microfabricated disks (instead of spheres) and of electrostatic instead of magnetic driving forces. The extension of the sensitivity range for miniaturized spinning rotor gauges is also discussed.Finally, new perspectives for mechanical vacuum gauges are demonstrated. By application of micromechanical processes, very thin stress-compensated membranes can be fabricated which enable sensitive mechanical gauges even for pressures in the high vacuum range. First, experimental results with respect to these membranes are represented.     

The Micro Pirani™: A solid-state vacuum gauge with wide range

For measuring pressures in the range 1 × 10^?2 to 100 mbar, thermal conductivity gauges are commonly used. The widespread Pirani gauge with a thin wire suffers from problems with temperature drift, low accuracy, requirement for adjustment, large measuring volume, filament contamination, and sensitivity to mounting position. Recently, Wenzel Electronics has introduced the Micro Pirani?, which is based on a newly developed solid-state sensor element and which eliminates or significantly reduces the problems. A temperature compensation improves the stability and extends the useful range of operation to the high vacuum range at 1 × 10^?5 mbar. The geometry of the basic sensor element, close to “infinite slab geometry”, facilitates the introduction of a fairly accurate mathematical model of the sensor. The sensor is insensitive to mounting position since no convection can take place in the sensor due to its small dimensions. Furthermore, the internal volume (“dead volume”) has been significantly reduced as compared to traditional Pirani sensors. The conversion from transmitter output voltage to pressure is achieved by a simple and fast algorithm. The selection of a particular gas species is simple, since the algorithm contains only 3 gas-dependent constants. An excellent reproducibility in production eliminates the need for an individual calibration of individual sensors in most applications.     

A novel gas sensor based on field ionization from ZnO nanowires: moderate working voltage and high stability

We report a kind of gas sensor using ZnO nanowires as the field ionization anode. The sharp tips of nanowires generate very high electric fields at relatively low voltages. The sensors show good sensitivity and selectivity. Moreover, the detection limitation of the field ionization based ZnO nanowire gas sensors is about 5%. More importantly, a sensor with ZnO nanowires as the anode exhibits an impressive performance with respect to stability and anti-oxidation behavior, which are significantly better than those of carbon nanotubes (CNTs) as electrodes. Therefore, the simple, low-cost, sensors described here could be deployed for a variety of applications.     

ZnO nanowire UV photodetectors with high internal gain

ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G approximately 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (tau approximately 20 ns) and slow (tau approximately 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than approximately 10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.     

Electrical actuation and readout in a nanoelectromechanical resonator based on a laterally suspended zinc oxide nanowire

In this paper, we present experimental results describing enhanced readout of the vibratory response of a doubly clamped zinc oxide (ZnO) nanowire employing a purely electrical actuation and detection scheme. The measured response suggests that the piezoelectric and semiconducting properties of ZnO effectively enhance the motional current for electromechanical transduction. For a doubly clamped ZnO nanowire resonator with radius ~10 nm and length ~1.91 μm, a resonant frequency around 21.4 MHz is observed with a quality factor (Q) of ~358 in vacuum. A comparison with the Q obtained in air (~242) shows that these nano-scale devices may be operated in fluid as viscous damping is less significant at these length scales. Additionally, the suspended nanowire bridges show field effect transistor (FET) characteristics when the underlying silicon substrate is used as a gate electrode or using a lithographically patterned in-plane gate electrode. Moreover, the Young's modulus of ZnO nanowires is extracted from a static bending test performed on a nanowire cantilever using an AFM and the value is compared to that obtained from resonant frequency measurements of electrically addressed clamped–clamped beam nanowire resonators.     

A study of Joule heating-induced breakdown of carbon nanotube interconnects

We investigate breakdown of carbon nanotube (CNT) interconnects induced by Joule heating in air and under high vacuum conditions (10(-5) mbar). A CNT with a diameter of 18 nm, which is grown by chemical vapor deposition to connect opposing titanium nitride (TiN) electrodes, is able to carry an electrical power up to 0.6 mW before breaking down under vacuum, with a corresponding maximum current density up to 8 × 10(7) A cm(-2) (compared to 0.16 mW and 2 × 10(7) A cm(-2) in air). Decoration with electrochemically deposited Ni particles allows protection of the CNT interconnect against oxidation and improvement of the heat release through the surrounding environment. A CNT decorated with Ni particles is able to carry an increased electrical power of about 1.5 mW before breaking down under vacuum, with a corresponding maximum current density as high as 1.2 × 10(8) A cm(-2). The Joule heating produced along the current carrying CNT interconnect is able to melt the Ni particles and promotes the formation of titanium carbon nitride which improves the electrical contact between the CNT and the TiN electrodes.     

Formation of networked ZnO nanowires by vapor phase growth and their sensing properties with respect to CO

Using a selective growth of ZnO nanowires on patterned electrode layers by using the vapor-phase-growth method, networked ZnO nanowires were fabricated. These networked nanowires were tested as sensors for detecting CO at ppm levels. Good sensitivity and a dynamic property with respect to CO were confirmed in the networked ZnO nanowire sensor. The sensing mechanism is attributed to the change not only in the width of the space charge region along the length direction of each nanowire, but also in the height of the potential barrier at the junction during adsorption and desorption of chemical gaseous species. This novel method to fabricate gas sensors may circumvent the drawbacks of single nanowire gas sensors.     

Mesoporous thin-film on highly-sensitive resonant chemical sensor for relative humidity and CO2 detection

Distributed sensing of gas-phase chemicals is a promising application for mesoporous materials when combined with highly sensitive miniaturized gas sensors. We present a direct application of a mesoporous silica thin film on a highly sensitive miniaturized resonant chemical sensor with a mass sensitivity at the zeptogram scale for relative humidity and CO(2) detection. Using mesoporous silica thin-film, we report one of the lowest volume resolutions and a sensitive detection of 5.1 × 10(-4)% RH/Hz to water vapor in N(2), which is 70 times higher than a device with a nontemplated silica layer. In addition, a mesoporous thin-film that is functionalized with an amino-group is directly applied on the resonant sensor, which exhibits a volume sensitivity of 1.6 × 10(-4)%/Hz and a volume resolution of 1.82 × 10(-4)% to CO(2) in N(2).     

室温下射频磁控溅射制备ZnO:Al透明导电薄膜及其性能研究

采用射频磁控溅射技术,在室温下,以ZnO:Al2O3(2%Al2O3(质量比))为靶材,在石英玻璃基底上,采用不同工艺条件制备了ZnO:Al(AZO)薄膜。使用扫描电子显微镜观察了薄膜的表面形貌,X射线衍射分析了薄膜的结构,四探针测量仪得到薄膜的表面电阻,轮廓仪测量了薄膜厚度,并计算了电阻率,最后采用分光光度计测量了薄膜的透过率;研究了溅射功率、溅射气压与薄膜厚度对薄膜电阻率及透过率的影响。结果表明:所制备的AZO薄膜具有(002)择优取向,并且发现薄膜厚度对薄膜的光电性能有明显影响,溅射气压和溅射功率对薄膜电学性能有较大影响,但是对薄膜透过率影响不大。当功率为1kW、溅射气压0.052Pa、AZO薄膜厚度为250nm时,其电阻率为8.38×10-4Ω·cm,波长在550nm处透过率为89%,接近基底的本底透过率92%。当薄膜厚度为1125 nm时薄膜的电阻率降至最低(6.16×10-4Ω·cm)。