G.M. Zhang

Atomically resolved field emission patterns of single-walled carbon nanotubes

The electron emission and structural properties of single-walled carbon nanotubes (SWCNTs) were investigated by using field emission microscopy (FEM). The transmission electron microscopy micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions, an FEM image with an elliptic ring-like structure composed of separated bright dots was obtained, a reasonable interpretation of it is that it was produced from the open end for a zigzag (16,0) SWCNT protruding from the SWCNT bundle, each bright dot corresponding to a single atom at the open end. And, if true, this means that the FEM demonstrated 0.2nm resolution, which was theoretically possible for the assumed geometry. The calculated value of the magnification of the pattern was also consistent with the measured value if the value of the compression factor beta was set at 1.76.

Synthesis and characterization of tungsten oxide nanorods

Single crystalline nanorods (15–200 nm in diameter and hundreds nanometers in length) have been formed on the carbon-covered W wires by simple electric heating under a vacuum of 5 × 10−4 Pa. The chemical composition and crystalline structure of the nanorods were carefully investigated by various characterization techniques such as scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, energy dispersive x-ray spectroscopy and electron energy loss spectroscopy. After ruling out any possible existence of carbon nanotubes (CNTs), tungsten carbide, W–Fe alloying, and formation of other types of tungsten oxides, monoclinic W18O49 phase has been well identified. The mechanism of nanorod formation of sub-tungsten oxide (∼WO2.7 compared to WO3) will be discussed in relation to the sample preparation conditions.

Ultrahigh vacuum scanning probe microscopy studies of carbon onions

Abstract Carbon onions were prepared by DC arc charge method. The behavior and electronic properties of carbon onions on highly oriented pyrolytic graphite (HOPG) substrate were studied by ultrahigh vacuum atomic force microscopy and scanning tunneling microscopy (UHV AFM/STM). UHV AFM/STM images showed that these ellipsoidal carbon onions tended to aggregate into clusters on the surface of HOPG. The scanning tunneling spectroscopy indicated that the electrical properties of carbon onions were between graphite and single-shell fullerenes.

Influence of Oxygen on Field Emission from Single-Walled Carbon Nanotubes

The influence of oxygen on field emission properties of single-walled carbon nanotubes (SWCNTs) was studied experimentally. Field emission images and current–voltage curves acquired using a field emission microscope (FEM) showed that exposure to oxygen resulted in an increased work function of SWCNTs. Oxidation etching at high temperature sharpened the SWCNT ends and field emission was therefore enhanced.

Influence of heat treatment on field emissive properties of single-walled carbon nanotubes

Abstract Single-walled carbon nanotubes (SWCNTs) were assembled onto a tungsten tip and then received heat treatment. After sample heating at different temperatures, mass spectra of residual gas were recorded with a quadrupole mass spectrometer and curves of field emissive current versus anode–cathode voltage were acquired with a field emission microscope (FEM). Water molecules were found to be responsible for the instability of the field emission before the system baking out. The change in relative amount of CO and CO 2 in the vacuum was interpreted by the existence of carbon and oxygen atoms released from tungsten filament and tip at high temperatures. The alteration in work function and FEM images of SWCNTs arising from the heating treatment was also studied.

Charge Emissions from the Plasma Photogerated in Channels of SWNTs

Summary form only given. In recent years it has been reported that nanomaterials such as single-walled carbon nanotubes (SWNTs), silicon nanowires, and polyaniline nanofibers could have an abnormal photothermal effect under a camera flashlight. We have also investigated the visible-light-induced split of water confined in channels of SWNTs. In this paper, we will focus on the charge emission phenomena when the SWNT sample in ultrahigh vacuum is irradiated with a fast light exposure. We designed a special experimental procedure to monitor the positive and negative charges launched from the irradiated SWNT sample. Indeed, both positive and negative charge emissions have been recorded. Originally, such phenomena could only be observed occasionally, but now we can definitely obtain the charge emissions for each time of fast-light exposure. We found that the charge intensity is related to the residual gas adsorption, the collecting voltage, the external magnetic field, and the intensity of the input light. Some plasma characteristics of the photogenerated charges have been determined. We suppose that the nano-confinement of the adsorbed residual gases in the channels of SWNTs would play a key role in such phenomena

Quantum yield of photoemission of Ag–BaO thin films for detecting ultrashort infrared laser pulses

Abstract The Ag–BaO thin films are a new kind of photoemissive thin films, which have transient optical and ultrafast photoelectronic response. The properties of the Ag–BaO thin films establish the application foundation of detecting ultrashort infrared laser pulses. The measurement of the quantum yield of photoemission for ultrashort laser pulses is different from that for normal visible light. The experimental results present that the quantum yield of Ag–BaO thin films is of the order of magnitude of 10−6 under the action of a single laser pulse when the incident infrared laser wavelength is 1.06xa0μm. It is two orders of magnitude higher than that of pure metals at the same conditions. The quantum yield under the action of a single-chain laser pulse, which comprises 11 single pulses of duration 40xa0ps separated by 10xa0ns, increases rapidly following increasing incident energy of the laser. In this case, the Ag–BaO thin films present multiphoton emission of 2-photon process and higher quantum yield of 10−5.

The high field enhancement of photoemission from Na2KSb photocathodes

Abstract We measured the enhancement of photoemission by the application of a high electric field to the surface of Na 2 KSb photocathodes which were stimulated by ultrashort laser pulses (wavelength 1.06 μm or 0.53 μm, pulse duration 50 ps). The gain in photoyield exceeded 5 at a field of 2.7 × 10 7 V/m, and there were steps in the curve of the gain in photoyield versus the applied electric field.