Baoping Wang

ZnO-based amperometric enzyme biosensors.

Nanostructured ZnO with its unique properties could provide a suitable microenvironment for immobilization of enzymes while retaining their biological activity, and thus lead to an expanded use of this nanomaterial for the construction of electrochemical biosensors with enhanced analytical performance. ZnO-based enzyme electrochemical biosensors are summarized in several tables for an easy overview according to the target biosensing analyte (glucose, hydrogen peroxide, phenol and cholesterol), respectively. Moreover, recent developments in enzyme electrochemical biosensors based on ZnO nanomaterials are reviewed with an emphasis on the fabrications and features of ZnO, approaches for biosensor construction (e.g., modified electrodes and enzyme immobilization) and biosensor performances.

Field emission characteristic of screen-printed carbon nanotube cathode

Abstract The fabrication of carbon nanotube emitters with excellent emission properties is described. The multi-walled carbon nanotubes (MWNTs) produced by chemical vapor deposition (CVD) method were purified with oxidation method and mixed with organic binding pastes and then screen-printed on glass substrates with ITO film. We applied anode voltage gradually to refine the emission behavior of the emitter by cleaning the top surface of screen-printed carbon nanotubes (CNTs). The density of the carbon nanotubes is about 2.5×108/cm2. Diode field emission experiments were performed in dynamic vacuum system to study the emission current, the emission uniformity, etc. Bright and stable character emission images were obtained in the diode structure and the emission current could approach 1xa0mA/cm2.

High emission current density, vertically aligned carbon nanotube mesh, field emitter array

A vertically aligned carbon nanotube mesh emitter array has been fabricated and tested, giving a current density of up to 1.5u2002A/cm2, and a threshold field of 1.5u2002V/μm for a current density 1u2002mA/cm2. Low temperature carbon nanotube growth is used to fabricate the carbon nanotube mesh emitter arrays significantly reducing the cost of the fabrication of large area electron emitters. This system exhibits ultralong lifetime.

Enhanced field emission from injector-like ZnO nanostructures with minimized screening effect

Injector-like zinc oxide (ZnO) nanostructures have been synthesized on Si substrate by the vapour phase transport method. Samples with different areal densities were obtained by controlling the temperature. The field emission properties of the injector-like ZnO nanostructures showed a clear dependence on the areal density of the nanostructures, which is due to the screening effect. The samples with a needle length of 850xa0nm and an areal density of 1 × 10(8)xa0cm(-2) showed the lowest field emission turn-on field of 1.85xa0Vxa0µm(-1) at a current density of 10xa0µAxa0cm(-2), and the current density reaches 1xa0mAxa0cm(-2) at an applied field of 4.7xa0Vxa0µm(-1).

Stable, self-ballasting field emission from zinc oxide nanowires grown on an array of vertically aligned carbon nanofibers

A structure composed of zinc oxide nanowires (ZNWs) grown hydrothermally on an array of vertically aligned carbon nanofibers (CNFs) was fabricated and its field emission properties determined and compared with bare CNF arrays. The combination produced a macroscopic turn-on field of 1.2u2002V/μm which was found to be the lowest reported from ZNWs deposited on a two-dimensional substrate and much less than the equivalent CNFs array (5.2u2002V/μm). Crucially, field emission was found to be much more stable at higher pressures of 5×10−6u2002mbar without exhibiting current degradation for a fixed external field, while emitting with a current density of 1u2002mA/cm2, the current density typically required for backlighting and field emission displays. We propose a self-ballasting mechanism, in which the low carrier density in the zinc oxide prevents current runaway in the presence of adsorbed species.

Efficient surface-conducted field emission from ZnO nanotetrapods

We report a surface-conducted field emission cathode using ZnO nanotetrapods as the surface conduction emitter. The dependences of electron emission efficiency on ZnO nanotetrapod film thickness and the spacing between adjacent electrodes of surface emitter were studied. A maximum electron emission efficiency of 60% was obtained for an optimal device with film thickness of 8μm and electrode spacing of 0.1mm. A low turn-on voltage of about 100V (corresponding to a field of 1V∕μm) at an emission current density of 0.6mA∕cm2 was obtained which is good enough for field emission displays. Good stability and uniformity were also demonstrated.

Manganese-doped zinc oxide tetratubes and their photoluminescent properties

Based on vapor-phase transport method, manganese-doped zinc oxide (ZnO:Mn) tetropod whiskers were fabricated. The pods of the ZnO:Mn whiskers show hexagonal hollow shape with multitips at the front. X-ray diffraction and high-resolution transmission electron microscopy demonstrate that the tube pods and the tips are composed of wurtzite ZnO growing along [0001] direction. The generation of the luminescent center in ZnO whiskers and electron transport between the ground state and the excitation states of Mn2+ are analyzed by Raman-scattering, photoluminescence, and photoluminescent excitation measurements.

Enhanced field emission from ZnO nanotetrapods on a carbon nanofiber buffered Ag film by screen printing

Field emission properties of screen-printed ZnO nanotetrapods on a carbon nanofiber buffered Ag electrode were studied. The turn-on electric field (at a current of 0.1u2002μA/cm2) and maximum emission current are 0.6u2002V/μm and 2.8u2002mA/cm2 (at a field of 2.2u2002V/μm), which were significantly improved compared to the control device made of ZnO nanotetrapods on Ag directly without a carbon nanofiber buffer. The improved field emission is due to the better contact (both mechanical and electrical) formed with a carbon nanofiber buffer.

High efficiency surface-conducted field emission from a ZnO nanotetrapod and MgO nanoparticle composite emitter

We report a surface-conducted field emitter made of a ZnO nanotetrapod and MgO nanoparticle composites with a high emission efficiency (∼100%) and current (3.77 mA at a gate voltage of 100 V and anode voltage of 1800 V). The fabrications of the triode structure with a 10×10pixel array and corresponding driving method have been proposed. The electron trajectories are simulated according to the structure. Individual pixel addressing can be achieved by a sequential scanning mode. Display of moving images employing this triode structure was demonstrated. The results are of significance to the development of ZnO based triode field emitters.

Probing local electric field distribution of nanotube arrays using electrostatic force microscopy

The local electric field distribution of nanotube arrays has been studied by using the electrostatic force microscopy (EFM) technique. The nanotube arrays were fabricated using the anodic alumina template method. Good electric contact has been proofed using contact mode conductive atomic force microscopy. The experiment shows that the EFM can provide a quantitative mapping tool to measure three-dimensional distribution of local electric field with resolution down to several nanometers. The finite difference method has been applied to calculate the electric field distribution near the surface of the nanotube array induced by a conductive tip. The results show that the field decays in a power law with exponent varies for nanotubes of different packing environments as the tip was lifted away from the top of nanotubes. The protrusion of nanotubes causes a much higher enhanced field than packing geometry. Medium packing density may enable the maximum collective emission current for such nanotube arrays of narr...