Huayong Pan

Microstructural and compositional characterization of a new silicon carbide nanocables using scanning transmission electron microscopy

Abstract Composite nanocables with peculiar structure were synthesized using a solid–liquid–solid mechanism. Each of the nanocables consists of a crystalline core sheathed with an amorphous layer ( 40 nm in average diameter). The crystalline core of the nanocables is so fine (1– 6 nm in diameter), that it is nearly impossible to characterize them using other methods. Fortunately, the powerful high resolution electron energy loss spectroscopy (EELS) technique (with minimum beam size nm ) allowed us to analyze this peculiar nanomaterial. The fine crystalline core was proved to be a hexagonal silicon carbide, while the sheathing layer was silicon oxide. High angle dark field technique was employed to map the nanocable structures. Our results show that the EELS is powerful in nanometric regime characterization, while the SiC nanocables reported here may be useful in future nanotechnology.

InAs/GaSb core-shell nanowires grown on Si substrates by metal-organic chemical vapor deposition

We report the growth of InAs/GaSb core-shell heterostructure nanowires with smooth sidewalls on Si substrates using metal-organic chemical vapor deposition with no assistance from foreign catalysts. Sb adatoms were observed to strongly influence the morphology of the GaSb shell. In particular, Ga droplets form on the nanowire tips when a relatively low TMSb flow rate is used, whereas the droplets are missing and the radial growth of the GaSb is enhanced due to a reduction in the diffusion length of the Ga adatoms when the TMSb flow rate is increased. Moreover, transmission electron microscopy measurements revealed that the GaSb shell coherently grew on the InAs core. The results obtained here show that the InAs/GaSb core-shell nanowires grown using the Si platform have strong potential in the fabrication of future nanometer-scale devices and in the study of fundamental quantum physics.

Amazing ageing property and in situ comparative study of field emission from tungsten oxide nanowires

In this work, needle-shaping of tungsten oxide nanowires occurred during field emission characterization. Compared with nanowires with a flat apex, needle-shaped emitters showed a lower threshold field of 11.9 V µm(-1) for 1 mA cm(-2) and a higher emission current of 1120 µA at 16.2 V µm(-1). Most notably, the measured ageing current dramatically increased by more than four times until it slightly decreased, tending towards stability. In addition, the samples showed striking difference in their nonlinear Fowler-Nordheim plot before and after ageing tests. Selected area diffraction and transmission electron microscope characterizations were used to further study these amazing results.

InAs-mediated growth of vertical InSb nanowires on Si substrates

In this work, InSb nanowires are grown vertically on Si (111) with metal organic chemical vapor deposition using InAs as seed layer, instead of external metal catalyst. Two groups of InSb nanowires are fabricated and characterized: one group presents Indium droplets at the nanowires free end, while the other, in contrast, ends without Indium droplet but with pyramid-shaped InSb. The indium-droplet-ended nanowires are longer than the other group of nanowires. For both groups of InSb nanowires, InAs layers play an important role in their formation by serving as a template for growing InSb nanowires. The results presented in this work suggest a useful approach to grow catalyst-free InSb nanowires on Si substrates, which is significant for their device applications.

Controlled-Direction Growth of Planar InAsSb Nanowires on Si Substrates without Foreign Catalysts

We describe the controlled growth of planar InAsSb nanowires (NWs) on differently oriented Si substrates without any foreign catalysts. Interestingly, the planar InAsSb NWs grew along four criss-crossed ⟨110⟩ directions on an [100]-oriented substrate, two ⟨100⟩ directions plus four ⟨111⟩ directions on an [110]-oriented substrate, and six equivalent ⟨112⟩ directions on an [111]-oriented substrate, which correspond to the projections of ⟨111⟩ family crystal directions on the substrate planes. High-resolution transmission electron microscopy (HRTEM) reveals that the NWs experienced a transition from out-of-plane to in-plane growth at the early growth stage but still occurred on the {111} plane, which has the lowest surface energy among all the surfaces. Furthermore, the NWs exhibit a pure zinc-blende crystal structure without any defects. A growth model is presented to explain growth of the NWs. In addition, conductive atomic force microscopy shows that electrically rectifying p-n junctions form naturally between the planar InAsSb NWs and the p-type Si substrates. The results presented here could open up a new route way to fabricate highly integrated III-V nanodevices.

Room temperature synthesis of K2Mo3O10·3H2O nanowires in minutes

Polyoxometalates have been widely used in the fields of catalysis, analytical chemistry, biochemistry, medicine and synthesis of novel organic-inorganic materials. It is difficult to synthesize pure polymolybdate products from a solution because several kinds of molybdenum-based anions may coexist. As a result, varied acidification methods are commonly used for solution synthesis of polymolybdates. In this paper we report an approach for the synthesis of [001]-oriented K(2)Mo(3)O(10)x3H(2)O nanowires from an aqueous solution of (NH(4))(6)Mo(7)O(24)x4H(2)O and KCl at low temperatures. The reaction occurs even at temperatures as low as 0 degrees C, and at 30-90 degrees C the whole procedure needs only a few minutes. Without any additional acidification treatments, the pH value of the solution is maintained in a narrow range of +/- 0.1 between 4.9 and 5.5 during the whole synthesis procedure. The starting pH depends on the reaction temperature. Crystalline structure and purity of the final products have been characterized with x-ray diffraction, electron diffraction and dehydration measurements. This simple and rapid method provides a unique case for studying the growth mechanism of polymolybdate nanostructures, and has a promising potential in the mass production of low-cost, pure-phase polymolybdates for a variety of applications.

Growth of High Material Quality Group III-Antimonide Semiconductor Nanowires by a Naturally Cooling Process

We report on a simple but powerful approach to grow high material quality InSb and GaSb nanowires in a commonly used tube furnace setup. The approach employs a process of stable heating at a high temperature and then cooling down naturally to room temperature with the nanowire growth occurred effectively during the naturally cooling step. As-grown nanowires are analyzed using a scanning electron microscope and a transmission electron microscope equipped with an energy-dispersive X-ray spectroscopy setup. It is shown that the grown nanowires are several micrometers in lengths and are zincblende InSb and GaSb crystals. The FET devices are also fabricated with the grown nanowires and investigated. It is shown that the grown nanowires show good, desired electrical properties and should have potential applications in the future nanoelectronics and infrared optoelectronics.

Microstructure of epitaxial superconductive YBa2Cu3O7 thin films prepared at different deposition rates

A study is reported on the growth mechanism of YBa2Cu3O7 with different growth speeds by high resolution transmission microscopy (HRTEM) and analysis of the interface and thin film microstructure. Two thin films were synthesized by pulse laser deposition on [100], miscut 5°, SrTiO3 substrate at 820 °C, one with a pulse laser frequency of 1 Hz and one with 6 Hz. Cross-sections were studied by an H-9000 NAR HRTEM along the [010] direction. The growth process of the sample made at 1 Hz was as follows. First, distorted step flow growth occurred on a step-mediated substrate surface of 3–4 cells thickness. Second, about a 15 nm thickness of island shape growth becomes superimposed on the area of the step flow layer. Finally, thin film growth occurred but with growth fluctuation. The sample made at 6 Hz showed the characteristics of island growth; the growth area of island or ball shape was of small size and dense distribution, and seemed to be a confused mosaic stack. The influence of growth speed on YBCO epitaxial film microstructure was studied explicitly by HRTEM.

Dissolvable Trimolybdate Nanowires as Ag Carriers forHigh-Efficiency Antimicrobial Applications

Elimination of bacteria and other microbes effectively is important to our daily life and a variety of medical applications. Here, we introduce a new kind of trimolybdate nanowires, namely Ag2−x(NH4)xMo3O10⋅3H2O, that carry a large amount of Ag atoms in the lattice and Ag-rich nanoparticles on the surface. These nanowires can eliminate bacteria of E. coli, Staphylococcus aureus, and unknown microbes in raw natural water with high efficiency. For example, they can inactivate more than 98% of E. coli with a nanowire concentration of only 5 ppm in the solution. The excellent sterilization performance is attributed to the combined effects of Ag ions, Mo ions, and Ag-rich nanoparticles of the Ag2−x(NH4)xMo3O10⋅3H2O nanowires. These nanowires are not dissolvable in deionized water but can be dissolved by the metabolic materials released from bacteria, making them attractive for many biological applications.

High FE current density from tungsten oxide nanowires

In this paper, quasi-aligned tungsten oxide (W18O49) nanowires were grown from commercially available tungsten powders in a horizontal tube furnace. And field emission (FE) performance of the prepared tungsten oxide was also studied.