Mechanical-electrical triggers and sensors using piezoelectric micowires/nanowires

We demonstrate a mechanical-electrical trigger using a ZnO piezoelectric fine-wire (PFW) (microwire, nanowire). Once subjected to mechanical impact, a bent PFW creates a voltage drop across its width, with the tensile and compressive surfaces showing positive and negative voltages, respectively. The voltage and current created by the piezoelectric effect could trigger an external electronic system, thus, the impact force/pressure can be detected. The response time of the trigger/sensor is approximately 10 ms. The piezoelectric potential across the PFW has a lifetime of approximately 100 s, which is long enough for effectively "gating" the transport current along the wire; thus a piezoelectric field effect transistor is possible based on the piezotronic effect.     

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high‐index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c‐axis‐oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO–ZnO core–shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high‐index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO–ZnO core–shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core–shell nanowires are largely non‐responsive to varying O₃ concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect‐rich high‐index polar surfaces.     

Optical and bio-sensing characteristics of ZnO nanotubes grown by hydrothermal method

ZnO nanostructures were fabricated on copper substrates by hydrothermal method at an optimized growth temperature of -95 degrees C. Structural properties were investigated by field emission scanning electron and transmission electron microscopy. Distinct morphologies were found to be formed at different growth times. The formation of nanotubes mainly involved the initial nucleation followed by the growth of nanorods at 95 degrees C, and then with the increase of dissolution time at room temperature, the preferential chemical dissolution of the metastable Zn-rich [0001] polar surfaces resulted in removing the atoms from the surfaces, thus leading to the thinning of the wall of the nanostructures. Completely hollow ZnO nanotubes could be obtained at a high dissolution time. The room temperature photoluminescence and optical absorption properties of ZnO nanotubes have been studied as a function of dissolution time. The efficacy of ZnO nanotubes for glucose sensing applications has been studied.     

High-yield self-assembly of flower-like ZnO nanostructures

High-yield three-dimensional (3D) flower-like nanostructures self-assembled from 1 D ZnO nanorods and nanotubes are experimentally demonstrated. The Zn and O terminated crystal planes of ZnO nanorods results in positively and negatively charged top (001) and bottom (00-1) surfaces, respectively. The nanorods self-assembled into 3D nanostructures via the electrostatic interaction between the crystal planes with opposite charges. Moreover, on the basis of the different stability of polar and nonpolar planes in wurtzite-type ZnO, the nanorods based 3D nanostructures transformed into nanotubes based ones spontaneously. This provides a new approach to prepare multi-dimensional materials without the necessity to employ any external intervention.     

Temperature-dependent <Emphasis Type="Italic">C-V</Emphasis> characteristics of Au/ZnO/n-Si device obtained by atomic layer deposition technique

Since the importance of Schottkky devices, Au/ZnO/n-Si device were obtained, and the capacitance–voltage (C-V) and conductance-voltage (G-V) characteristics of Au/ZnO/n-Si device were studied using admittance spectroscopy at changing temperature from 160 to 340 K with 20 K intervals and ?1 to +2 V bias voltage range. The interface thin film ZnO layer was deposited on the n-type Si wafer by atomic layer deposition technique (ALD) in order to obtain homogenous interface layer. The layer thickness of ZnO was taken as 10 nm by the resulting ZnO film growth rate at about 1.45 Å per cycle. This thin film layer was characterized with XRD and AFM analyses. It can be seen from the C-V curves of the device that the capacitance values increased in depletion region with increasing temperature and exhibited peaks towards to forward biases after 240 K temperature. The changing of capacitance values confirmed re-ordering and re-structuring of charges in the interface of the device with changing temperature. The G-V curves of the device also increased with increasing temperature and towards to forward bias voltages due to increasing free charges in the interface. The series resistance (\({R}_{s}\)) of the device was taken into account to understand its effect on main electrical parameters, and it could be seen from these results that the \({R}_{s}\) strongly depends on the device temperature. The impedance (Z) values decreased with changing from ?1 to +2 V bias voltages and increasing temperature. The barrier height which was obtained from the C ^?2 -V plots increased a slope of 0.00108 eV/K with a decrease in temperature from 160 to 340 K. It can be concluded that the Au/ZnO/n-Si device may be used and improved for next technological applications such as capacitor and memristor.     

Cl‐Doped ZnO Nanowire Arrays on 3D Graphene Foam with Highly Efficient Field Emission and Photocatalytic Properties

An environmentally friendly, low‐cost, and large‐scale method is developed for fabrication of Cl‐doped ZnO nanowire arrays (NWAs) on 3D graphene foam (Cl‐ZnO NWAs/GF), and investigates its applications as a highly efficient field emitter and photocatalyst. The introduction of Cl‐dopant in ZnO increases free electrons in the conduction band of ZnO and also leads to the rough surface of ZnO NWAs, which greatly improves the field emission properties of the Cl‐ZnO NWAs/GF. The Cl‐ZnO NWAs/GF demonstrates a low turn‐on field (≈1.6 V μm⁻¹), a high field enhancement factor (≈12844), and excellent field emission stability. Also, the Cl‐ZnO NWAs/GF shows high photocatalytic efficiency under UV irradiation, enabling photodegradation of organic dyes such as RhB within ≈75 min, with excellent recyclability. The excellent photocatalytic performance of the Cl‐ZnO NWAs/GF originates from the highly efficient charge separation efficiency at the heterointerface of Cl‐ZnO and GF, as well as improved electron transport efficiency due to the doping of Cl. These results open up new possibilities of using Cl‐ZnO and graphene‐based hybrid nanostructures for various functional devices.     

Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method

A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy.     

氮掺杂ZnO纳米阵列的制备和电学特性

采用化学气相沉积 （CVD） 法,以高纯ZnO和活性C混合粉末为原料,以NH3为掺杂气体,在Si （111）衬底上制备了N掺杂的ZnO纳米线阵列,用X射线衍射仪 （XRD）、扫描电镜 （SEM）、拉曼光谱对样品进行分析,结果表明, 氮气的掺杂过程对生长N掺杂的ZnO纳米线阵列有一定的影响。除此之外,N掺杂的ZnO纳微米p-n结被合成,表现出很明显的整流特性。     

Magnetoelastic behavior of glass-covered amorphous ferromagneticmicrowire

Particular aspects concerning the magnetoelastic behavior of an Fe-rich glass-covered amorphous ferromagnetic microwire (diameter about 20 μm) are reported in both the as-prepared state and after various heat treatments. The main feature of such behavior is that related with the bistable behavior exhibited by the microwire. The magnetization process takes place by a single and large Barkhausen jump at a given applied field. For the as-prepared material, the value of this switching field (H*≈112 Am^-1) is about one order of magnitude larger than that reported for Fe-rich amorphous wires with a diameter of about 125 μm (H*≈8 Am^-1). The effect of the thermal treatment as well as the stress dependence of H* for as-obtained and treated microwire are also reported and discussed in terms of the stress distribution within the microwire     

Enhanced field-emission from a mixture of carbon nanotubes, ZnO tetrapods and conductive particles

We report the enhancement of field-emission current from a mixture of carbon nanotubes, ZnO tetrapod-like nano structures, and conductive particles. Carbon nanotubes are deposited on the electrode as the field emitters. A MgO layer is printed around the cathode electrode, and ZnO tetrapod-like nano structures are deposited on this layer for the generation of secondary emission electrons. A few conductive particles are also distributed on the MgO layer by spraying or screen-printing. These conductive particles enhance the transverse electric field around the cathode electrode. Consequently, more primary electrons emitted from the carbon nanotubes bombard on the ZnO tetrapods, and secondary emission electrons and scattered electrons are yielded. Finally, the field-emission current is enhanced obviously. As experimental results shown, a high field-emission current about 32 mA in a direct current emission mode has been obtained from a 0.5 cm2 emission site when an electric field of 9 V/microm is applied between cathode and anode. Compared with a conventional carbon nanotube cathode, the field-emission current has been improved about 80%.     

氧化锌/氧化亚铜异质结太阳能电池的研究进展

因氧化亚铜(Cu2O)、氧化锌(ZnO)能级和晶格匹配较好,近年来较多的研究者将两者复合制备异质结太阳能电池。异质结的形成可提高光生电子-空穴对的分离效率,同时拓展复合结构的光响应范围,从而有效提高太阳能电池性能。介绍了3类主流的ZnO/Cu2O异质结结构,分别阐述主要的进展,综述了异质结结构中Cu2O、ZnO的制备方法以及制备条件对电池效率的影响,讨论了电池性能的改进措施,并对ZnO/Cu2O异质结太阳能电池未来的发展前景进行展望。     

化学溶液法外延组装ZnO分级纳米结构

采用气相法、液相法相结合的方法外延组装了一种形貌新颖的复杂ZnO分级纳米结构--"纳米毛刷".首先用热蒸发的方法制备了宽面为极性面的ZnO纳米带,然后采用化学溶液法,在强碱溶液中在ZnO纳米带的极性面上外延生长Zno纳米棒阵列,实现了ZnO分级纳米结构"由下而上"地外延组装.采用负离子配位多面体生长基元模型讨论了ZnO分级纳米结构的外延组装机理.这种ZnO分级结构的实现,可望作为ZnO纳米器件的原型材料构建新型光电器件.     

Bicrystalline zinc oxide nanocombs

Bicrystalline ZnO nanocombs have been prepared by zinc powder evaporation at 650 degrees C. Structural analysis showed that as-synthesized samples are composed of two crystals that form a twin structure parallel to the (113) plane with the growth direction of the branching nanowires and the main stem closely parallel to (0001) and (0110), respectively. Due to the unique twin structures, both sides of the main stems could be Zn-terminated ZnO(0001) polar surfaces. The chemically active surfaces make the aligned branching nanowires grow from both sides of the main stems, which is consistent with the structure of the obtained bicrystalline nanomaterials. The growth of bicrystalline ZnO nanocombs can be explained by polar-surface dominated growth and twins induced growth mechanisms.     

Novel Structure for High Performance UV Photodetector Based on BiOCl/ZnO Hybrid Film

A novel type of high performance ultraviolet (UV) photodetector (PD) based on a ZnO film has been prepared by incorporating a BiOCl nanostructure into the film. The responsivity of the BiOCl/ZnO hybrid film PD in UV region can reach 182.87 mA W^?1, which is about 2.72 and 6.87 times for that of TiO₂/ZnO hybrid film PD and pure ZnO film PD. The rise/decay time of BiOCl/ZnO hybrid film PD is 25.83/11.25 s, which is much shorter than that of TiO₂/ZnO hybrid film PD (51.94/26.05 s) and pure ZnO film PD (69.34/>120 s). The BiOCl nanostructure can inject photogenerated electrons into the ZnO film under UV light illumination, leading to the increase of photocurrent, and forms barriers to block the straight transmission of electrons between electrodes, resulting in the decrease of decay time. The results of control experiment show that the transfer path of photogenerated electrons formed by p–n junction will be cut off after depositing gold nanoparticles on the film surface, which means this hybrid film is a unique and novel structure to improve the optoelectronic performance of photodetectors. This novel BiOCl/ZnO hybrid structure paves new route for the development of film PDs based on ZnO film.     

Synergistic graphene/aluminum surface plasmon coupling for zinc oxide lasing improvement

Collective oscillations of free electrons generate plasmons on the surface of a material.A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall.When these two kinds of electromagnetic waves meet each other,the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and A1 nanoparticles.The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/Al surface plasmons with ZnO excitons.The lasing characteristics and the coupling mechanism were investigated systematically.     

Correlated piezoelectric and electrical properties in individual ZnO nanorods

Resistivity and piezoelectric response of individual ZnO nanorods were measured using scanning force microscopy. We found a variation in resistivity of 3 orders of magnitude, from 0.1 to 155 Omegacm and in piezoelectric coefficient ranging from 0.4 to 9.5 pm/V in ZnO nanorods grown from solution at the same time on the same substrate. However, there exists a clear correlation between these two properties: nanorods with low piezoelectric response display low resistivity. The relationship is explained by the reduction of the Madelung constant due to free electrons. The results highlight that slight differences in the local environment during synthesis can cause large variation in physical properties found among similar nanostructures. These variations cannot be revealed through ensemble measurements and may contribute to the confusion in the literature of individual nanostructure properties. We demonstrate that correlating multiple physical properties on individual nanostructures provides an insight into the origin of the varying physical properties.     

Semi-Infinite Anti-Plane Crack in a Piezoelectric Material

The problem of an impermeable semi-infinite crack in a piezoelectric material is considered. The electroelastic field due to a pair of concentrate forces and free charges applied at the crack surfaces is obtained by using the Mellin transform method, and the intensity factors of the quantities of concern and the mechanical strain energy release rate are given. The obtained results may be used as the fundamental solution, from which some general solutions can be calculated by superposition. The well-known solution for a purely elastic material will be recovered from the present solution if letting the coupling constant vanish.     

X-ray induced electrostatic charging of helium films

It is well known that free electrons can be held onto the free surface of liquid helium through either their own image charges or through the effect of an externally applied electric field. The resultant electrostatic pressure causes films to thin. We have recently measured x-ray reflectivity from static films of isotopic mixtures of helium with an intense x-ray beam in the temperature range between 0.37 K and 1.3 K. Normally, no significant thickness variation with x-ray intensity is expected over a wide range of temperatures when the film is superfluid. We have found that even modest x-ray intensities affect the thickness of films containing only trace amounts of³He. We believe that the effect is due to x-ray produced photoelectrons, which thermalize in the vapor and then reside on the surface, attracted by both the film and a charged substrate. The temperature and concentration dependence is then due to the transport properties of the electrons at the surface. It may be possible to study the 2-D electron gas produced in this way by diffraction techniques.     

Optical and field-emission properties of ZnO nanostructures deposited using high-pressure pulsed laser deposition

ZnO nanostructures were deposited on GaN (0001), Al2O3 (0001), and Si (100) substrates using a high-pressure pulsed laser deposition (PLD) method. Vertically aligned hexagonal-pyramidal ZnO nanorods were obtained on the Al2O3 and Si substrates whereas interlinked ZnO nanowalls were obtained on the GaN substrates. A growth mechanism has been proposed for the formation of ZnO nanowalls based on different growth rates of ZnO polar and nonpolar planes. Both ZnO nanorods and nanowalls exhibit a strong E2H vibration mode in the micro-Raman spectra. The corresponding fluorescence spectra of ZnO nanorods and nanowalls showed near band emission at 3.28 eV. The ZnO nanorods grown on the Si substrates exhibited better crystalline and optical properties compared with the ZnO structures grown on the GaN and Al2O3 substrates. The high aspect ratio, good vertical alignment, and better crystallinity of the ZnO nanorods with tapered tips exhibited promising field emission performance with a low turn-on field of 2 V/μm, a high current density of 7.7 mA/cm2, and a large field enhancement factor.     

Piezoelectric potential output from ZnO nanowire functionalized with p-type oligomer

We have studied the piezoelectric potential output of a ZnO wire/belt functionalized with p-type oligomer (2,5-Bis(octanoxy)-1,4-bis(4-formyl phenylene vinylene) benzene) (OPV2) when it was deflected by an atomic force microscope (AFM) tip in contact mode. In comparison to the ZnO wire/belt without oligomer coating, an extra positive voltage peak was observed prior to the appearance of a negative potential peak. The paired positive and negative voltage peaks are the results of tip contact to the stretched and the compressed side of the wire/belt, corresponding to the positive and negative local piezoelectric potential, respectively. The p-n junction between OPV2 and ZnO serves as a "diode" that controls the flow of current. When the nanowire/nanobelt is first bent by the AFM tip, the diode is reversely biased and the piezoelectric charges are stored in the ZnO wire/belt. As the AFM tip further bends the wire/belt, the local piezoelectric potential is continuously accumulated to a value that is large enough to break through the diode. Then the free charges from the external circuit can flow in and neutralize/screen part of the piezoelectric charges, resulting in a positive pulse in the output signal. When the AFM tip continues to scan to reach the compressed side of the ZnO wire/belt, the p-n junction is forwardly biased. Neutralizing/screening the residual and the newly created piezoelectric charges leads to the flow of current from the tip to the ZnO wire/belt, resulting in a negative voltage pulse. This study supports the charging and discharging model proposed for the piezoelectric nanogenerator.