Zhang Geng-Min

Field emission from an array of free-standing metallic nanowires

Arrays of single crystalline gold nanowires were synthesized electrochemically in porous polycarbonate membranes. The polycarbonate membrane was then removed to obtain free-standing nanowires for field emission measurements. The turn-on electric field strength for field emission is found to be lower than 2V/µm. The actual electric field that extracted electrons out of the gold nanowires is estimated to be about 103 times higher than the field directly expected in the model of a parallel plate condenser. The availability of the field emission is therefore attributed to the strong electric field at the tips resulting from small curvature radius of the gold nanowires.

Increased work function in PEDOT:PSS film under ultraviolet irradiation

An increase of work function (0.3 eV) is achieved by irradiating poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film in vacuum with 254-nm ultraviolet (UV) light. The mechanism for such an improvement is investigated by photoelectron yield spectroscopy, X-ray photo electron energy spectrum, and field emission technique. Surface oxidation and composition change are found as the reasons for work function increase. The UV-treated PEDOT:PSS film is used as the hole injection layer in a hole-only device. Hole injection is improved by UV-treated PEDOT:PSS film without baring the enlargement of film resistance. Our result demonstrates that UV treatment is more suitable for modifying the injection barrier than UV ozone exposure.

Electron field emission from single-walled carbon nanotube nonwoven

Field emission from single-walled carbon nanotube (SWNT) nonwoven has been investigated under high vacuum with different vacuum gaps. A low turn-on electric field of 1.05 V/μm is required to reach an emission current density of 10 μA/cm2. An emission current density of 10 mA/cm2 is obtained at an operating electric field of 1.88 V/μm. No current saturation is found even at an emission current of 5 mA. With the vacuum gap increasing from 1 to 10 mm, the turn-on field decreases monotonically from 1.21 to 0.68 V/μm, while the field amplification is augmented. The good field-emission behaviour is ascribed to the combined effects of the intrinsic field emission of SWNT and the waved topography of the nonwoven.

Field emission from zinc oxide nanowires

An array of random-oriented zinc oxide nanowires (ZnO NWs) was fabricated on silicon substrate by thermal evaporation. After a thermal evaporation process, the silicon substrate was covered with a large number of uniformly distributed ZnO islands, from which non-aligned NWs with a diameter of several ten nanometres were grown. During this process, the temperature around the substrate was intentionally kept below 500 degrees C for practical consideration. From these ZnO NWs field emission was achieved. The turn-on field, under which a 10μA/cm2 current density was extracted, was measured to be 3.0V/μm. Also, the emission site distribution was investigated using the transparent anode technique. The field emission was observed to have occurred from the whole sample surface. These results suggest that ZnO NWs have great potential application in flat panel displays.

Simulation of Space Charge Effect in Field Emission from a Nanowire

At present, field emission from nanowires (NWs) is under intense investigation. In experiments, the dependence of field emission current from an NW on applied voltage (I–V behavior) often deviates from the Fowler–Nordheim (FN) theory. We simulate the role of the space charge effect in the I–V behaviour of an NW. We determine the threshold electric field, at which the space charge effect starts to manifest itself, under different NW work functions. Our simulation result has confirmed that space charge effect is one of the possible factors responsible for the nonlinearity of the FN plots of an NW at high fields. Furthermore, our calculation reveals that the threshold field is related to the NW work function in an almost linear manner.

Structure and Electrical Properties of Gold Nanowires Grown with Electrochemical Deposition

The effect of Al substitution for Fe on the structure, magnetostriction, anisotropy and spin reorientation of a series Of Tb-0.3 Dy-0.7 (Fe1-xAlx)(1.95) alloys (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that all the compositions of Tb-0.3 Dy-0.7 (Fe1-xAlx)(1.95) substantially retained the MgCu2-type C-15 cubic Laves phase structure and the lattice constant increases with increasing x. The substitution of Al increases slightly the magnetostriction in low magnetic field ( H less than or equal to 40kA.m(-1)) when x 0.15. The intrinsic magnetostriction lambda(111) decreases sharply also with incresasing x. The Mossbauer spectra show that for 0 less than or equal to x 0.15. The measurements of relative susceptibility indicate that the temperature of spin reorientation decreases with increasing x.

Observation of MWCNTs with low-energy electron point source microscope

The low-energy electron point source (LEEPS) microscope, which creates enlarged projection images with low-energy field emission electron beams, can be used to observe the projection image of nano-scale samples and to characterize the coherence of the field emission beam. In this paper we report the design and test operation performance of a home-made LEEPS microscope. Multi-walled carbon nanotubes (MWCNTs) synthesized by the CVD method were observed by LEEPS microscope using a conventional tungsten tip, and projection images with the magnification of up to 104 was obtained. The resolution of the acquired images is ~10 nm. A higher resolution and a larger magnification can be expected when the AC magnetic field inside the equipment is shielded and the vibration of the instrument reduced.

Field-ion microscopy observation of single-walled carbon nanotubes

Field-ion microscopy (FIM), a tool for surface analysis with atomic resolution, has been employed to observe the end structure of single-walled carbon nanotubes (SWCNTs). FIM images revealed the existence of open SWCNT ends. Amorphous carbon atoms were also observed to occur around SWCNTs and traditional field evaporation failed to remove them. Heat treatment was found to be efficacious in altering the end structures of SWCNT bundles. Carbon and oxygen atoms released from heated tungsten filament are believed to be responsible for the decoration imposed on the SWCNT ends.

Coherent field emission from a multi-walled carbon nanotube with two open-ended branches

Interference fringes are obtained in a field-emission microscopy (FEM) study of a multi-walled carbon nanotube (MWCNT) with two open-ended branches. The FEM pattern, which is composed of three parallel streaks, can be interpreted by using classical Youngs double-slit interference with the ends of the two MWCNT branches treated as two secondary sources of the electron wave. The origin of the coherency of the electron beams from the two branches is discussed on the basis of the quantitative analysis of the FEM pattern. The result suggests a new approach to obtaining a coherent electron source.

Scanning Tunnelling Microscopy Observation on the 10,12-Tricosadiynoic Acid Monolayers Deposited by Schaefer's Method

The Langmuir-Blodgett monolayers of the 10,12-tricosadiynoic acid molecules were deposited onto the basal plane of highly oriented pyrolytic graphite (HOPG) by Schaefers method and then observed with the scanning tunnelling microscope (STM). With a view to achieving a parallel molecular arrangement on the graphite surface, the deposition was deliberately conducted at a relatively low surface pressure. As exhibited by the STM images, by this approach the 10,12-tricosadiynoic acid molecules could constitute an ordered structure with their molecular chains lying parallel to the substrate. The model of molecular dimer is put forward for the interpretation of the observed phenomena. PACS: 85. 65. +h, 07. 79. Cz