Zhengwei Pan

Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts

Gas sensors have been fabricated using the single-crystalline SnO2 nanobelts. Electrical characterization showed that the contacts were ohmic and the nanobelts were sensitive to environmental polluting species like CO and NO2, as well as to ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance due to the introduction of the gas, is 4160% for 250 ppm of ethanol and −1550% for 0.5 ppm NO2 at 400 °C. The results demonstrate the potential of fabricating nanosized sensors using the integrity of a single nanobelt with a sensitivity at the level of a few ppb.

Very long carbon nanotubes

Carbon nanotubes can now be produced in large quantities by either arc methods, or thermal decomposition of hydrocarbons,. Here we report that pyrolysis of acetylene over iron/silica substrates is an effective method with which to produce very long, multiwalled carbon nanotubes that reach about 2 mm in length, which is an order of magnitude longer than that described in most previous reports,.

Mechanical and physical properties on carbon nanotube

Abstract The aligned and very long carbon nanotubes were prepared by chemical vapor deposition (CVD) of hydrocarbon gas on various substrates. Aligned and isolated carbon nanotubes with high density, high purity and uniform diameters were achieved. Mechanical and physical property characterizations, such as tensile strength, Youngs modulus, thermal conductivity and third-order optical nonlinearity, were carried out on the aligned and long tubes. The results were discussed with the microstructure of the carbon nanotubes.

Photostimulated near-infrared persistent luminescence as a new optical read-out from Cr3+-doped LiGa5O8

In conventional photostimulable storage phosphors, the optical information written by x-ray or ultraviolet irradiation is usually read out as a visible photostimulated luminescence (PSL) signal under the stimulation of a low-energy light with appropriate wavelength. Unlike the transient PSL, here we report a new optical read-out form, photostimulated persistent luminescence (PSPL) in the near-infrared (NIR), from a Cr3+-doped LiGa5O8 NIR persistent phosphor exhibiting a super-long NIR persistent luminescence of more than 1,000 h. An intense PSPL signal peaking at 716 nm can be repeatedly obtained in a period of more than 1,000 h when an ultraviolet-light (250–360 nm) pre-irradiated LiGa5O8:Cr3+ phosphor is repeatedly stimulated with a visible light or a NIR light. The LiGa5O8:Cr3+ phosphor has promising applications in optical information storage, night-vision surveillance, and in vivo bio-imaging.

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

Fast and highly anisotropic thermal transport through vertically aligned carbon nanotube arrays

This letter reports on fast and highly anisotropic thermal transport through millimeter-tall, vertically aligned carbon nanotube arrays (VANTAs) synthesized by chemical vapor deposition on Si substrates. Thermal diffusivity measurements were performed for both longitudinal and transverse to the nanotube alignment direction, with longitudinal values as large as 2.1±0.2cm2∕s and anisotropy ratios as large as 72. Longitudinal thermal conductivities of 15.3±1.8W∕(mK) for porous 8±1vol% VANTAs in air and 5.5±0.7W∕(mK) for epoxy-infiltrated VANTAs already exceed those of phase-changing thermal interface materials used in microelectronics. Data suggest that further improvements are possible through optimization of density and defects in the arrays.

Lead oxide nanobelts and phase transformation induced by electron beam irradiation

β-PbO2 nanobelts, with a rectangular cross section, a typical length of 10–200 μm, a width of 50–300 nm, and a width-to-thickness ratio of 5–10, have been successfully synthesized by simple elevated evaporation of commercial PbO powders at high temperature. The PbO2 nanobelts are enclosed by top surfaces ±(201) and side surfaces ±(101) and their growth direction is [010]. Each PbO2 nanobelt is found to have a large polyhedral Pb tip at one of its ends, suggesting the growth is dominated by a vapor–liquid–solid mechanism. Electron beam irradiation of the PbO2 nanobelts results in the phase transformation from PbO2 to PbO and finally to Pb.

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.

Tumor Vasculature Targeted Photodynamic Therapy for Enhanced Delivery of Nanoparticles

Delivery of nanoparticle drugs to tumors relies heavily on the enhanced permeability and retention (EPR) effect. While many consider the effect to be equally effective on all tumors, it varies drastically among the tumors’ origins, stages, and organs, owing much to differences in vessel leakiness. Suboptimal EPR effect represents a major problem in the translation of nanomedicine to the clinic. In the present study, we introduce a photodynamic therapy (PDT)-based EPR enhancement technology. The method uses RGD-modified ferritin (RFRT) as “smart” carriers that site-specifically deliver 1O2 to the tumor endothelium. The photodynamic stimulus can cause permeabilized tumor vessels that facilitate extravasation of nanoparticles at the sites. The method has proven to be safe, selective, and effective. Increased tumor uptake was observed with a wide range of nanoparticles by as much as 20.08-fold. It is expected that the methodology can find wide applications in the area of nanomedicine.

Raman Studies of Semiconducting Oxide Nanobelts

Crystalline nanobelts of ZnO and SnO2 were prepared from a thermal evaporation of oxide powders inside an alumina tube in the absence of catalysts. Typical dimensions of the nanobelt samples ranged from approximately 10 to 100 microns in length, 30 to 300 nm in width, and 6 to 30 nm in thickness. Room temperature Raman spectra were obtained on pressed mats of nanobelt samples and compared with the corresponding spectra of the starting oxide powders and bulk materials. Collectively, our Raman data indicated that the as-prepared nanobelt samples used in this study were oxygen deficient. Upon annealing at 900 degrees C in flowing oxygen for 1 h, the nanobelt samples exhibited Raman features that corresponded to those expected in respective bulk semiconducting oxides. The dimensions of the nanobelts were a bit too large to expect significant quantum size effects on the phonon structure similar to those observed in carbon nanotubes and short-period semiconductor superlattices.