Piezoelectric and semiconducting coupled power generating process of a single ZnO belt/wire. A technology for harvesting electricity from the environment

This paper presents the experimental observation of piezoelectric generation from a single ZnO wire/belt for illustrating a fundamental process of converting mechanical energy into electricity at nanoscale. By deflecting a wire/belt using a conductive atomic force microscope tip in contact mode, the energy is first created by the deflection force and stored by piezoelectric potential, and later converts into piezoelectric energy. The mechanism of the generator is a result of coupled semiconducting and piezoelectric properties of ZnO. A piezoelectric effect is required to create electric potential of ionic charges from elastic deformation; semiconducting property is necessary to separate and maintain the charges and then release the potential via the rectifying behavior of the Schottky barrier at the metal-ZnO interface, which serves as a switch in the entire process. The good conductivity of ZnO is rather unique because it makes the current flow possible. This paper demonstrates a principle for harvesting energy from the environment. The technology has the potential of converting mechanical movement energy (such as body movement, muscle stretching, blood pressure), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as flow of body fluid, blood flow, contraction of blood vessels) into electric energy that may be sufficient for self-powering nanodevices and nanosystems in applications such as in situ, real-time, and implantable biosensing, biomedical monitoring, and biodetection.     

Enhancing light emission of ZnO microwire-based diodes by piezo-phototronic effect

Light emission from semiconductors depends not only on the efficiency of carrier injection and recombination but also extraction efficiency. For ultraviolet emission from high band gap materials such as ZnO, nanowires have higher extraction efficiencies than thin films, but conventional approaches for creating a p-n diode result in low efficiency. We exploited the noncentral symmetric nature of n-type ZnO nanowire/p-type GaN substrate to create a piezoelectric potential within the nanowire by applying stress. Because of the polarization of ions in a crystal that has noncentral symmetry, a piezoelectric potential (piezopotential) is created in the crystal under stress. The piezopotential acts as a "gate" voltage to tune the charge transport and enhance carrier injection, which is called the piezo-phototronic effect. We propose that band modification traps free carriers at the interface region in a channel created by the local piezoelectric charges. The emission intensity and injection current at a fixed applied voltage have been enhanced by a factor of 17 and 4, respectively, after applying a 0.093% compressive strain and improved conversion efficiency by a factor of 4.25. This huge enhanced performance is suggested arising from an effective increase in the local "biased voltage" as a result of the band modification caused by piezopotential and the trapping of holes at the interface region in a channel created by the local piezoelectric charges near the interface. Our study can be extended from ultraviolet range to visible range for a variety of optoelectronic devices that are important for today's safe, green, and renewable energy technology.     

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.     

Electrostatic potential in a bent piezoelectric nanowire. The fundamental theory of nanogenerator and nanopiezotronics

We have applied the perturbation theory for calculating the piezoelectric potential distribution in a nanowire (NW) as pushed by a lateral force at the tip. The analytical solution given under the first-order approximation produces a result that is within 6% from the full numerically calculated result using the finite element method. The calculation shows that the piezoelectric potential in the NW almost does not depend on the z-coordinate along the NW unless very close to the two ends, meaning that the NW can be approximately taken as a "parallel plated capacitor". This is entirely consistent to the model established for nanopiezotronics, in which the potential drop across the nanowire serves as the gate voltage for the piezoelectric field effect transistor. The maximum potential at the surface of the NW is directly proportional to the lateral displacement of the NW and inversely proportional to the cube of its length-to-diameter aspect ratio. The magnitude of piezoelectric potential for a NW of diameter 50 nm and length 600 nm is approximately 0.3 V. This voltage is much larger than the thermal voltage ( approximately 25 mV) and is high enough to drive the metal-semiconductor Schottky diode at the interface between atomic force microscope tip and the ZnO NW, as assumed in our original mechanism for the nanogenerators.     

Controlled positive and negative surface charge injection and erasure in a GaAs/AlGaAs based microdevice by scanning probe microscopy

In this paper, we show that positive and negative charges can be injected into the surface of SiO(2)/Si(3)N(4)/SiO(2)/GaAs/AlGaAs heterostructure material by using a biased tip of a scanning probe microscope. Furthermore, the injected charges can be erased with the same tip once grounded, working in slow scan and contact mode. Surface potential measurements by quantitative analysis of Kelvin probe force microscopy after drawing and erasing charges at room temperature are presented and discussed.     

Polar charges induced electric hysteresis of ZnO nano/microwire for fast data storage

We demonstrate an innovative memory device made of a single crystalline ZnO nanowire/microwire that works with a different mechanism from the p-n junction based memristor. A nonsymmetric, Schottky-Ohmic contacted ZnO nano/microwire can serve as a memristor if the channel length is short and the applied frequency is high. The observed phenomena could be explained based on a screening model of the polar charges at the two ends of the wire owing to the crystal structure of ZnO. The polar charges are usually fully screened by free electrons coming from the metal sides. But when the magnitude of the externally applied field exceeds a threshold value, the free electrons that screen the polar surfaces can be pulled away from the interface region, leading to a transient change in the effective height of the local Schottky barrier height owing to the electrical field formed by the polar surfaces of ZnO nanowires, which acts as a resistor with its magnitude depending on the total charges being transported. Such a phenomenon could be used for high density and fast writing/erasing data storage.     

Temperature dependant characteristics of zinc oxide nanorods based heterojunction diode

Temperature-dependant characteristics of heterojunction diode made by n-ZnO nanorods grown on p-silicon substrates has been characterized and demonstrated in this paper. ZnO nanorods were grown onto the silicon substrate via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen at approximately 550 degrees C without the use of any metal catalysts or additives. The as-grown ZnO nanorods were characterized in terms of their structural and optical properties. The detailed structural studies by XRD, TEM, HRTEM and SAED revealed that the grown nanorods are well-crystalline with the wurtzite hexagonal phase and preferentially grown along the [0001] direction. The as-grown n-ZnO nanorods grown on p-Si substrate were used to fabricate p-n heterojunction diode. The fabricated p-n junction diode attained almost similar turn-on voltage of approximately 0.6 V. The values of turn-on voltage and least current are same with the variations of temperature (i.e., 27 degrees C, 70 degrees C and 130 degrees C).     

Piezotronic effect on the output voltage of P3HT/ZnO micro/nanowire heterojunction solar cells

We report the first observation of piezotronic effect on the output voltage of a flexible heterojunction solar cell. The solar cell was fabricated by contacting poly(3-hexylthiophene) (P3HT) with one end of a ZnO micro/nanowire to form a p-n heterojunction on a flexible polystyrene (PS) substrate. The open-circuit voltage V(oc) of the solar cell was characterized by tuning the strain-induced polarization charges at the interface between ZnO and P3HT. The experimental data were understood based on the modification of the band structure at the p-n junction by the piezopotential, which is referred as a result of the piezotronic effect. This study not only provides an in-depth understanding about the effect but also is useful for maximizing the output of a solar cell using wurtzite structured materials.     

Carrier density and Schottky barrier on the performance of DC nanogenerator

By assembling a ZnO nanowire (NW) array based nanogenerator (NG) that is transparent to UV light, we have investigated the performance of the NG by tuning its carrier density and the characteristics of the Schottky barrier at the interface between the metal electrode and the NW. The formation of a Schottky diode at the interface is a must for the effective operation of the NG. UV light not only increases the carrier density in ZnO but also reduces the barrier height. A reduced barrier height greatly weakens the function of the barrier for preserving the piezoelectric potential in the NW for an extended period of time, resulting in little output current. An increased carrier density speeds up the rate at which the piezoelectric charges are screened/neutralized, but a very low carrier density prevents the flow of current through the NWs. Therefore, there is an optimum conductance of the NW for maximizing the output of the NG. Our study provides solid evidence to further prove the mechanism proposed for the piezoelectric NG and piezotronics. The output current density of the NG has been improved to 8.3 microA/cm2.     

Construction and evaluation of high-quality n-ZnO nanorod/p-diamond heterojunctions

Vertically-aligned ZnO nanorods (NRs) arrays were synthesized by a low-temperature solution method on boron-doped diamond (BDD) films. The morphology, growth direction, and crystallinity of the ZnO NRs were studied by scanning electron microscopy, X-ray diffraction and cathodoluminescence. Electrical characterization of the ZnO NR/BBD heterostructures revealed characteristic p-n junction properties with an on/off ratio of about 50 at +/- 4 V and a small reverse leakage current approximately 1 microA. Moreover, the junctions showed an ideality factor around 1.0 at a low forward voltage from 0 to 0.3 V and about 2.1 for an increased voltage ranging from 1.2 to 3.0 V, being consistent with that of an ideal diode according to the Sah-Noyce-Shockley theory.     

Piezoelectric potential in vertically aligned nanowires for high output nanogenerators

In this work we analyze the coupled piezoelectric and semiconductive behavior of vertically aligned ZnO nanowires under uniform compression. The screening effect on the piezoelectric field caused by the free carriers in vertically compressed zinc oxide nanowires (NWs) has been computed by means of both analytical considerations and finite element calculations. We predict that, for typical geometries and donor concentrations, the length of the NW does not significantly influence the maximum output piezopotential because the potential mainly drops across the tip, so that relatively short NWs can be sufficient for high-efficiency nanogenerators, which is an important result for wet-chemistry fabrication of low-cost, CMOS- or MEMS-compatible nanogenerators. Furthermore, simulations reveal that the dielectric surrounding the NW influences the output piezopotential, especially for low donor concentrations. Other parameters such as the applied force, the sectional area and the donor concentration have been varied in order to understand their effects on the output voltage of the nanogenerator.     

Photovoltaic device on a single ZnO nanowire p-n homojunction

A photovoltaic device was successfully grown solely based on the single ZnO p-n homojunction nanowire. The ZnO nanowire p-n diode consists of an as-grown n-type segment and an in situ arsenic-doped p-type segment. This p-n homojunction acts as a good photovoltaic cell, producing a photocurrent almost 45 times larger than the dark current under reverse-biased conditions. Our results demonstrate that the present ZnO p-n homojunction nanowire can be used as a self-powered ultraviolet photodetector as well as a photovoltaic cell, which can also be used as an ultralow electrical power source for nanoscale electronic, optoelectronic and medical devices.     

Fundamental theory of piezotronics

Due to polarization of ions in crystals with noncentral symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created in the crystal when stress is applied. Electronics fabricated using the inner-crystal piezopotential as a gate voltage to tune or control the charge transport behavior across a metal/semiconductor interface or a p-n junction are called piezotronics. This is different from the basic design of complimentary metal oxide semiconductor (CMOS) field-effect transistors and has applications in force and pressure triggered or controlled electronic devices, sensors, microelectromechanical systems (MEMS), human-computer interfacing, nanorobotics, and touch-pad technologies. Here, the theory of charge transport in piezotronic devices is investigated. In addition to presenting the formal theoretical frame work, analytical solutions are presented for cases including metal-semiconductor contact and p-n junctions under simplified conditions. Numerical calculations are given for predicting the current-voltage characteristics of a general piezotronic transistor: metal-ZnO nanowire-metal device. This study provides important insight into the working principles and characteristics of piezotronic devices, as well as providing guidance for device design.     

Playing with dimensions: rational design for heteroepitaxial p-n junctions

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and it's capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co(3)O(4)) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co(3)O(4) nanoplates from the orientation of β-Co(OH)(2) nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co(3)O(4) were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co(3)O(4) nanoplates were experimentally obtained. Using this epitaxial p-n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10(4), respectively.     

An improved AFM cross-sectional method for piezoelectric nanostructures properties investigation: application to GaN nanowires

We present an improved atomic force microscopy (AFM) method to study the piezoelectric properties of nanostructures. An AFM tip is used to deform a free-standing piezoelectric nanowire. The deflection of the nanowire induces an electric potential via the piezoelectric effect, which is measured by the AFM coating tip. During the manipulation, the applied force, the forcing location and the nanowire's deflection are precisely known and under strict control. We show the measurements carried out on intrinsic GaN and n-doped GaN-AlN-GaN nanowires by using our method. The measured electric potential, as high as 200 mV for n-doped GaN-AlN-GaN nanowire and 150 mV for intrinsic GaN nanowire, have been obtained, these values are higher than theoretical calculations. Our investigation method is exceptionally useful to thoroughly examine and completely understand the piezoelectric phenomena of nanostructures. Our experimental observations intuitively reveal the great potential of piezoelectric nanostructures for converting mechanical energy into electricity. The piezoelectric properties of nanostructures, which are demonstrated in detail in this paper, represent a promising approach to fabricating cost-effective nano-generators and highly sensitive self-powered NEMS sensors.     

外电场下氧化石墨烯薄膜的微观摩擦行为

氧化石墨烯是一种准二维片层结构炭材料,具有独特的物理化学性能,其中摩擦性能的好坏对其构成的一些微／纳机电体系的稳定应用起着重要作用。为了研究外加电场作用对氧化石墨烯薄膜微观摩擦行为的影响,采用原子力显微镜,对针尖施加一定电压,测量其与氧化石墨烯薄膜表面的微观摩擦力大小,实验结果表明,针尖外加负电压时摩擦力不受影响,而正电压下摩擦力会明显上升,且随外加正电压值的增加而增大。同时结合针尖在外加电压时与氧化石墨烯薄膜表面的粘附力和静电力的测量,对外加电压的影响机制进行了探讨。     

A bootstrap voltage reference based upon an N-type negativeresistance device

A circuit that develops a reference voltage based upon the stability of the current of an N-type negative-resistance device (NNRD) in the neighborhood of the current peak is described. The NNRD is self-driven in a bootstrap configuration utilizing a single operational amplifier, with the bias circuit acting as a stiff voltage source having a negative Thevenin equivalent DC resistance. The analysis is specialized to the case of a particular NNRD, a p-n junction tunnel diode (TD), and a series of matching networks coupling the TD to the remainder of the bootstrap circuit are analyzed to demonstrate that a properly matched TD, biased by this bootstrap circuit, can be made stable against oscillation. The regulation of bias fluctuations, noise performance, temperature stability, and immunity to neutron damage are reviewed. Comparison is made to avalanche-diode-based bootstrap references. NNRD-based voltage references may have an impact in circuits and material systems where fabrication of high-quality Zener devices is not possible     

Robust optimization of the output voltage of nanogenerators by statistical design of experiments

Nanogenerators were first demonstrated by deflecting aligned ZnO nanowires using a conductive atomic force microscopy (AFM) tip. The output of a nanogenerator is affected by three parameters: tip normal force, tip scanning speed, and tip abrasion. In this work, systematic experimental studies have been carried out to examine the combined effects of these three parameters on the output, using statistical design of experiments. A statistical model has been built to analyze the data and predict the optimal parameter settings. For an AFM tip of cone angle 70° coated with Pt, and ZnO nanowires with a diameter of 50 nm and lengths of 600 nm to 1 μm, the optimized parameters for the nanogenerator were found to be a normal force of 137 nN and scanning speed of 40 μm/s, rather than the conventional settings of 120 nN for the normal force and 30 μm/s for the scanning speed. A nanogenerator with the optimized settings has three times the average output voltage of one with the conventional settings.     

Investigation on light emission in light-emitting diodes constructed with n-ZnO and p-Si nanowires

The light emission was investigated in light-emitting diodes (LEDs) constructed with n-ZnO and p-Si nanowires (NWs). ZnO NWs were synthesized by thermal chemical vapor deposition and Si NWs were formed by crystallographic wet etching of a Si wafer. The LEDs were fabricated using the NWs via dielectrophoresis (DEP) and direct transfer methods. The DEP method enabled to align the ZnO NW at the position that led to p-n heterojunction diodes by crossing with the transferred Si NW. The I-V curve of the p-n heterojunction diode showed the well-defined current-rectifying characteristic, with a turn-on voltage of 3 V. The electroluminescence spectrum in the dark showed the strong emission at approximately 385 nm and the broad emission centered at approximately 510 nm, at a forward bias of 30 V. Under the illumination of 325-nm-wavelength light, the luminescence intensity at 385 nm was dramatically enhanced, compared to that in the dark, probably due to the electric-field-induced enhancement of luminescence.     

Charge injection on insulators via scanning probe microscopy techniques: towards data storage devices

The idea of contact electrification aiming the development of nano-scale data storage devices has been explored through a careful investigation of charge injection on insulating films (SiO2 and PMMA) via scanning probe microscopy techniques. A complete route for data storage, showing simple and effective ways to write (inject the charge with an atomic force microscopy (AFM) tip), to read (detect the charge with electric force microscopy), to store (keep sample charge by changing ambient and surface conditions) and to erase the information (make the discharge process faster) is proposed and discussed. A detailed study of the influence of several parameters like AFM mode, bias voltage, relative humidity and surface hydrophobicity is also presented to optimize both charge injection and discharge processes. Results show that monitoring parameters such as ambient relative humidity and surface hydrophobic/hydrophilic character enable the control of pattern size, lateral dispersion, and storage time. The charge polarity is also dependent on the surface hydrophobicity and either positive or negative charges can become more appropriate for storage depending on the surface hydrophobic/hydrophilic character.