San Yu

Microanalysis of single-phase AlN nanocrystals and AlN-Al nanocomposites prepared by DC arc-discharge

AlN nanocrystals and AlN-Al nanocomposites of < 100 nm size were prepared by the direct current (DC) arc-discharge plasma evaporation of aluminum in an ambient of N2 + NH3. The analysis of these two kinds of nanoparticles revealed that the AlN nanocrystals were single phase and single crystal. The AlN-Al nanocomposites were composed of AlN single-crystals embedded in amorphous aluminum phase. The crystallographic features and formation mechanisms for these two kinds of nanoparticles are discussed.

Synthesis, extraction and electronic structure of Ce@C2n

In view of the growing interest in endohedral lanthanide fullerenes, Ce, as a typical +4 oxidation state lanthanide element, has been systematically studied. The synthesis, extraction and electronic structure of Ce@C-2n are investigated. Soot containing Ce@C-2n was synthesized in high yield by carbonizing CeO2-containing graphite rods and are back-burning the CeC2-enriched cathode deposit in a DC are plasma apparatus. Ce@C-2n, dominated by Ce@C-82, can be efficiently extracted from the insoluble part of the soot after toluene Soxhlet extraction by pyridine at high temperature and high pressure in a closed vessel. About 60% Ce@C-2n (2n = 82,80,78,76) and 35% Ce@C-82 can be enriched in the pyridine extract. This fact is identified by desorption electron impact mass spectrometry (DEI MS). The electronic structure of Ce@C-2n is analyzed by using X-ray photoemission spectroscopy (XPS) of pyridine-free film. It is suggested that the encapsulated Ce atom is in a charge state close to +3 and was effectively protected from reaction with water and oxygen by the enclosing fullerene cage. Unlike theoretical expectation, the electronic state of Ce@C-82 is formally described as Ce+3@C-82(3-)

Twin-step-related growth phenomena of thick diamond film

Abstract A thick diamond film prepared by glow discharge plasma assisted chemical vapor deposition (CVD) was investigated by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The degeneration of the {111} faces into three {110} faces was found to be widespread. Both the {110} and {100} faces grow via a step mechanism, with the steps generally originating from the re-entrant corner formed by twinning. The {100} penetration twin on {100} surfaces is discussed in detail. A twin-step growth mechanism of thick diamond film, which explains the formation of cleavage steps on fracture surface, is proposed.

Microanalysis of wurtzite-GaN single crystals prepared by d.c. arc discharge

Abstract Gallium nitride crystals with definite faces have been fabricated by d.c. arc discharge using gallium and H2 + NH3 as starting materials. Transmission electron microscope, selected area diffraction and X-ray diffraction investigation of the as-grown GaN crystals show that the well faceted crystals are single crystalline GaN having a wurtzite structure with lattice constants a 0 = 3.18 A and c 0 = 5.18 A . Both X-ray microanalysis and nano-probe diffraction techniques have been used to determine the composition and microstructure of the samples.

Zn nanocrystals discovered from pencil-core

Nanoparticles or powders [1–3] show many novel properties different from those of bulk materials and have inspired extensive experimental and theoretical researches. Many physical and chemical methods have been developed to produce metal, alloy, compound or composite nanoparticles. The most basic way for metal nanocrystal growth is to evaporate bulk metal by heating in an inert gas environment. Then nanocrystals could be obtained by condensation of the metal vapor at a cooled collector in the system. Metal nanocrystals including Zn have long been prepared and studied [4–7]. In this letter, we report a TEM investigation of Zn nanocrystals discovered by evaporating of pencilcore in a vacuum chamber. Growth mechanism of the Zn nanocrystals is discussed. A schematic diagram of the experimental apparatus is shown in Fig. 1. A commercial pencil-core (Hi.Uni, HB 0.5 mm, Mitsubish, Japan) about 2 cm long is fixed between two electrodes. Copper grids covered with collodium film are placed about 10 cm under the pencilcore. When vacuum reaches 10−5–10−6 τ , apply an alternating voltage to the electrodes and then gradually increase the currents. The pencil-core will be illuminating with changing color from red to white. Carbon is likely to be evaporated out from the pencil-core at higher temperature. When current reaches 16–20 A, the pencil-core will break apart at midpoint. Observations were then made on the copper grids using TEMs (HITACHI-8100 , CM200-FEG with GIF). In one experiment, a kind of nanocrystals was found on the copper grids as shown in Fig. 2a. The crystals are several-hundred nanometers in size with different welldefined geometric shapes such as triangle, rectangle and hexagon with clear sharp edges. Many hexagons show concave corners. The rectangular crystal is in fact an edge-view of the hexagonal plate standing on the cover film with one side-face. From the edge-view we could know the thickness of the crystal plates. In another experiment, smaller crystals around 100 nm were found, as shown in Fig. 2b. These crystals have hexagonal and rectangular shapes with rough edges. The core region of the hexagonal crystals shows different contrast from that of the outer region. The crystals found in the two experiments show the same EELS and ED results, which indicates that they belong to the same kind of crystals but with different size and morphology as a result of relatively different preparation conditions. EELS spectrum of the crystals are shown in Fig. 3, in which carbon and oxygen peaks comes from the cover film of the grid. So the crystals are thought to be metal Zn. Fig. 4 shows two ED patterns and corresponding crystals with orientations perpendicular between each other. From the ED, we know that the crystal has hexagonal structure with cell parameters a= 2.74 A and c= 5.08 A, which is in agreement with that of Zn. The crystals are bounded by {10-10} faces. From the above results including crystal morphology (hexagonal thin plate as reported before [4]), composition and ED pattern, the nanocrystal discovered from pencil-core could be confirmed to be Zn. Satellite spots with hexagonal symmetry were found around the center spot and the main diffraction spots in the [001] ED patterns. The satellite spots are in fact short arcs with intensity distribution varies from one main spot to another. They roughly occupy the 1/6 commensurate positions in reciprocal space. Certain satellite spots are strong and others are so weak that they are hard to be discerned from the graph. Similar ED pattern was also found from Zn crystals in Refs. 4 and 5. The satellite spots indicate certain long period superstructure in the Zn nanocrystals. It is interesting to

Dissociation of {1 1 1} faces into {1 1 0} faces in thick diamond films

Abstract Surface morphology of thick diamond films was investigated using scanning electron microscopy (SEM). The dissociation of {1 1 1} faces into {1 1 0} faces is positively confirmed by the geometrical consideration of the crystal shapes, such as the V-shaped penetration twins and some other unusual crystal shapes. Growth mechanisms of the diamond {1 1 1} and {1 1 0} faces are discussed.

Optical properties of boron-doped diamond films

Boron doped and undoped diamond films were prepared by a hot filament method of chemical vapor deposition (CVD). Up to 1020 cm-3 boron concentration was obtained in doped specimens. Infrared (IR) absorption and luminescent emission of diamond films were measured and discussed. Absorption peaks of 1971.3 cm-1, 2020.0 cm-1, and 2161.4 cm-1 were observed in undoped diamond films, which are assigned to two-phonon lattice vibration absorption of diamond. Two absorption peaks at 2850 cm-1 and 2910 cm-1 were usually observed in CVD diamond films, which is very similar to that of C-H vibration absorptions in CH2-radicals. In the range of 300 nm - 800 nm, four typical luminescent emissions were observed, which are 2.76 eV broad emission, 2.34 eV broad emission, 2.16 eV sharp emission with a low- energy shoulder and a sharp emission at 1.675 eV. The 2.34 eV emission is originated from the donor-acceptor (D-A) pairs, the others are originated from defect related centers.