Ruiping Gao

Work function at the tips of multiwalled carbon nanotubes

density of 10 mA/cm 2 and 10 mA/cm 2 , respectively, are in the range of 2‐5 and 4‐7 V/mm for carbon NTs. 7‐13 Recent experimental data of Pan et al. 14 show that the aligned and opened carbon NTs exhibit superior field emission performances with f to and f thr in the range of 0.6‐1 and 2‐2.7 V/mm, respectively. Another important physical quantity in electron field emission is the surface work function, which is well documented for elemental materials. For the emitters such as carbon NTs, most of the electrons are emitted from the tips of the carbon NTs, and it is the local work function that matters to the properties of the NT field emission. The work function measured from the ln(J/E 2 )v s 1/E characteristics curve ~the Fowler‐Nordheim theory!, 15 where J is the emission current density and E is the macroscopic applied electric field, is an average over all of the aligned carbon NTs that are structurally divers in diameters, lengths, and helical angles. In this letter, we present experimental measurements of tip work functions of individual carbon NTs. Our results indicate that the tip work function of ;75% of the carbon NTs is ;0.2‐0.4 eV lower than that of carbon; these nanotubes are likely to be metallic. The other 25% of the NTs have a tip work function of ;0.6 eV higher than that of carbons; these tubes are likely to be semiconductive. The multiwalled carbon NTs were synthesized by arc discharge and details have been reported elsewhere. 16 The structures of the carbon NTs are uniform and intact. The NTs have closed ends. The measurement of the tip work function of a single carbon nanotube was carried out in situ in a transmission electron microscope ~TEM! JEOL 100C ~100 kV!. 17 A specimen holder was built for applying a voltage across a NT and its counter gold electrode. The detailed experimental set up has been reported elsewhere. 18 The NTs to be used for measurements are directly imaged under TEM. The principle for work function measurement is schematically shown in Fig. 1~a!. We consider a simple case in which a carbon nanotube, partially soaked in a carbon fiber produced by arc discharge, is electrically connected to a gold ball. Due to the difference in the surface work functions between the NT and the counter Au electrode, a static charge Q0 exists at the tip of the NT to balance this potential difference even at zero applied voltage. 19 The magnitude of Q0 is proportional to the difference between work functions of the Au electrode and the NT tip ~NTT!, Q05a(WAu2WNTT), where a is related to the geometry and distance between the NT and the elec

Silica Nanotubes and Nanofiber Arrays

Sample Preparations: Multilayers of PDADMAC (Mw = 200K±350K g/ mol, Aldrich), and PSS (Mw = 70 000 g/mol, Aldrich) were deposited onto silica colloids (Snowtex, nominal diameter 70±100 nm). 3 g of silica colloid, previously dried for 12 h at 400 C, was dispersed in 500 mL of a polymer/ salt solution, comprised of 0.02 M PDADMAC and 0.1 M NaCl in Millipore Q water. This adsorption solution was left standing for 30 min, then centrifuged at 4300 rpm and the supernatant was removed. 500 mL water was added and the solution was sonicated and centrifuged. The supernatant was then removed to rinse the unadsorbed polyelectrolyte from the colloids. A total of three 500 mL washings were performed after the adsorption of each polymer layer. A small amount (~50 mg) of the coated colloid was then removed for characterization and dried at 65 C for 12 h prior to measurement. The remaining colloid was then dispersed in 500 mL of a similar solution of the oppositely charged polymer (0.02 M PSS and 0.1 M NaCl), and the adsorption and washing steps repeated, until two layers of PDADMAC and two layers of PSS had been sequentially deposited. An insoluble PEC for reference was prepared by adding 2 mL of 20 wt.-% PDADMAC aqueous solution slowly to 200 mL of a 0.01 M PSS solution, under vigorous stirring. A precipitate formed immediately as a thick milky suspension in solution. This solution was centrifuged, and the supernatant removed by pipette. To remove uncomplexed polyelectrolyte, the complex was then washed with water, agitated to disperse, and centrifuged again as described for the colloids, for a total of three washings. f-Potential Measurements: 30 mg of each dried colloid sample was suspended in 15 mL of 1 mM NaCl solution. The pH of each solution was found to be in the range of 7.4 to 7.7 for the multilayered samples, and 8.2 for the bare silica colloid. Electrophoretic mobilities were measured on a Microelectrophoresis Apparatus Mk II (Rank Brothers, Bottingham) and converted to f potentials using the Smoluchowski equation. NMR Measurements: C CP MAS NMR of the carbon spectra were recorded on a Chemagnetics CMX-300 spectrometer for the colloid coated with four layers, the precipitated complex, and both bulk polyelectrolytes. A total suppression of sidebands (TOSS) sequence with background suppression, a spinning speed of 3 kHz, and a contact time of 500 ls were used. Singleand double-quantum H MAS NMR spectra were acquired either on a Bruker ASX500 or DRX700 spectrometer equipped with a 2.5 mm fast MAS probe.

Mechanical and electrostatic properties of carbon nanotubes and nanowires

Nano-scale manipulation and property measurements of individual nanowire-like structure is challenged by the small size of the structure. Scanning probe microscopy has been the dominant tool for property characterizations of nanomaterials. We have developed an alternative novel approach that allows a direct measurement of the mechanical and electrical properties of individual nanowire-like structures by in situ transmission electron microscopy (TEM). The technique is unique in a way that it can directly correlate the atomic-scale microstructure of the nanowire with its physical properties. This paper reviews our current progress in applying the technique in investigating the mechanical and electron field emission properties of carbon nanotubes and nanowires.