Yicong Chen

Vacuum gap dependence of field electron emission properties of large area multi-walled carbon nanotube films

Field electron emission properties of aligned multi-walled carbon nanotube films were studied with variation of the vacuum gap d between anode and cathode. With d varying in the range of 0.4-2 mm, the emission current-gap voltage characteristics and the corresponding Fowler-Nordheim (FN) plots show distinct nonlinearity and regular changes with electrode separation. Three field enhancement factors may be derived from the three linear sections of a FN plot. Their variation with gap d results in different behaviours; significantly a drop of five times in the field enhancement factor is observed. The physical process responsible for our findings is suggested to be the space charge effect and both theoretical and experimental evidence is provided to support our arguments. The implication of our findings in technical applications is also discussed.

Physical origin of non-linearity in Fowler–Nordheim plots of aligned large area multi-walled nitrogen-containing carbon nanotubes

Abstract Field electron emission properties of multi-walled nitrogen-containing carbon nanofibers were studied with particular interest in the mechanism responsible for their non-linearity of a Fowler–Nordheim (FN) plot. With vacuum gap d between anode and cathode varying in the range of 0.4–1.4 mm, the FN plots show distinct non-linearity and regular changes with the gap separation. Typically, two linear sections may approximately be defined in a FN plot, and thus two field enhancement factors may be derived rather than one. The variation of the field enhancement factors of the two sections with gap d show different behavior. The physical mechanisms responsible for the strong gap-dependent of I – V characteristics and their implication on technical applications are discussed.

In Situ Characterization of the Local Work Function along Individual Free Standing Nanowire by Electrostatic Deflection

In situ characterization of the work function of quasi one dimensional nanomaterials is essential for exploring their applications. Here we proposed to use the electrostatic deflection induced by work function difference between nanoprobe and nanowire for in situ measuring the local work function along a free standing nanowire. The physical mechanism for the measurement was discussed in details and a parabolic relationship between the deflection and the potential difference was derived. As a demonstration, measurement of the local work functions on the tip and the sidewall of a ZnO nanowire with Au catalyst at its end and a LaB6 nanowire have been achieved with good accuracy.

Thermal-enhanced field emission from CuO nanowires due to defect-induced localized states

The temperature dependence of the field emission properties of CuO nanowires was studied from 163 to 453 K. Large current increases were observed with increasing temperature, which cannot be explained by band to band excitation or emission from the valence band. Two distinct sections were observed from the Arrhenius plot. Activation energies of 100 meV for the high-temperature range (273 to 453 K) and 26.4 meV for the low-temperature range (163 to 273 K) were obtained. Phonon-assisted and defect-assisted thermal field emission mechanisms from p-type CuO NWs were proposed to explain the observed phenomena in the two temperature ranges, which relate to the defect-induced localized states. Numerical simulation using the proposed mechanism was carried out and a good fit with the experimental results was achieved. The results suggest that defect-induced localized states play an important role in field emission from nanowires.

In-situ determination of the flat band carrier concentration and surface charge density of individual semiconductor nanowires by a combination of electrical and field emission measurements

The carrier concentration of semiconductor nanowires is one of the most important parameters for their nanoelectronic and optoelectronic applications. Because of their nanoscale geometry, the carrier concentration of nanowires is related to their flat band carrier concentration and surface charge density. Usually, these fundamental properties can be derived from the electrical transport and capacitance measurements of a nanowire field effect transistor (FET). Considering that the FET fabrication process can easily affect the nanowire surface, in-situ determination of these properties is of great interest. In this work, a method based on the chemical adsorption-induced surface band bending and field emission penetration effect was proposed to fulfill this task. Using this technique, the flat band carrier concentration and the surface charge density of a free-standing ZnO nanowire were obtained to be 0.7–2 × 1018 cm−3 and 1.07–3.73 × 1012 e/cm2, respectively. Compared with the conventional method based on a...

Coplanar-gate ZnO nanowire field emitter arrays with enhanced gate-control performance using a ring-shaped cathode

Nanowire field emitters have great potential for use as large-area gated field emitter arrays (FEAs). However, the micrometer-scale cathode patterns in gated FEA devices will reduce regulation of the gate voltage and limit the field emission currents of these devices as a result of field-screening effect among the neighboring nanowires. In this article, a ring-shaped ZnO nanowire pad is proposed to overcome this problem. Diode measurements show that the prepared ring-shaped ZnO nanowire pad arrays shows uniform emission with a turn-on field of 5.9 V/µm and a field emission current density of 4.6 mA/cm2 under an applied field of 9 V/µm. The ZnO nanowire pad arrays were integrated into coplanar-gate FEAs and enhanced gate-controlled device characteristics were obtained. The gate-controlled capability was studied via microscopic in-situ measurements of the field emission from the ZnO nanowires in the coplanar-gate FEAs. Based on the results of both simulations and experiments, we attributed the enhanced gate-controlled device capabilities to more efficient emission of electrons from the ZnO nanowires as a result of the increase edge area by designing ring-shaped ZnO nanowire pad. The results are important to the realization of large-area gate-controlled FEAs based on nanowire emitters for use in vacuum electronic devices.

Maximum field emission current density of CuO nanowires: theoretical study using a defect-related semiconductor field emission model and in situ measurements

In this study, we proposed a theoretical model for one-dimensional semiconductor nanowires (NWs), taking account of the defect-related electrical transport process. The maximum emission current density was calculated by considering the influence of Joule heating, using a one-dimensional heat equation. The field emission properties of individual CuO NWs with different electrical properties were studied using an in situ experimental technique. The experimental results for maximum emission current density agreed well with the theoretical predictions and suggested that multiple conduction mechanisms were active. These may be induced by the concentration of defects in the CuO NW. The concentration of defects and the transport mechanisms were found to be key factors influencing the maximum field emission current density of the semiconductor NW. As is limited by the change of resistivity with temperature, only thermal runaway can trigger breakdown in CuO NWs.

Effect of CF 4 plasma treatment on morphology, composition and field emission properties of ZnO nanowires

Fluorine doping of ZnO nanowires was realized by using a biased CF4 plasma treatment. The morphology, work function, electrical characteristics and field emission properties of ZnO nanowires were investigated before and after CF4 plasma treatment. Though lowered work function and resistance were observed, the ZnO nanowires exhibit higher turn-on field after CF4 plasma treatment. The results were explained by a hot electron emission model of wide-band gap semiconductor.

Effect of the electrical properties on the field emission properties of CuO nanowires

A theoretical model for one-dimensional CuO nanowire was established considering transport mechanisms induced by different defect concentration. By using a one-dimensional heat equation, the emission current was calculated by considering the temperature induced by the emission current. The maximum emission current density and maximum applied field was studied considering different electrical properties. From the obtained results, we elucidate the relationship between electrical properties and field emission properties. Finally, the theoretical results were compared with experimental field emission data measured from individual CuO nanowires of different electrical properties.

In-situ study of surface work function, electrical characteristic and field emission property of individual ZnO nanowire

A method for in-situ measuring work function of a free standing nanowire has been developed. By using this technique, in-situ study of the work function, electrical characteristic and field emission property of individual ZnO nanowires has been carried out. This method is proven to be useful for understanding the field emission characteristics of individual nanowire.