Zn nanocrystals discovered from pencil-core

Abstract

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