With the miniaturization of a solid, quantum and interface effects become increasingly important. As a result, the band structure of a nanometric semiconductor changes: the band gap expands, the core level shifts, the bandwidth revises, and the sublevel separation within a band increases. Unfortunately, such a thorough change goes beyond the scope of currently available models such as the `quantum confinement' theory. A consistent understanding of the factors dominating the band-structure change is highly desirable. Here we present a new approach for the size-induced unusual change by adding the effect of surface-coordination deficiency-induced bond contraction to the convention of an extended solid of which the Hamiltonian contains the intraatomic trapping interaction and the interatomic binding interaction. Agreement between modelling predictions and the observed size dependency in the photoluminescence of Si oxides and some nanometric III-V and II-VI semiconductors, and in the core-level shift of Cu-O nanosolids has been reached. Results indicate that the spontaneous contraction of chemical bonds at a surface and the rise in the surface-to-volume ratio with reducing particle size are responsible for the unusual change of the band structure of a nanosolid.

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Carbon nanotubes (CNTs) with supreme field emission properties were synthesized by depositing Co-containing amorphous carbon (a-C:Co) composite films using filtered cathodic vacuum arc technique with a 15at% Co-containing graphite target and subsequently growing CNTs using biased thermal chemical vapor deposition at 580°C with the a-C:Co composite film as a catalyst layer. The as-grown CNTs with a thin diameter of about 10nm have a low threshold field of 1.6V∕μm and a stable current density of 2.1mA∕cm2 at 3V∕μm. Thus an x-ray source was built in a diode configuration using the CNTs as its cold electron source showing good potential in x-ray radiography.

Tetrahedral amorphous carbon films with 70%–88% sp3 content are studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy as a function of annealing temperature in the range 25–1100°C. Using a high-resolution AFM current imaging, we directly image the formation and growth of conducting graphitic (sp2-bonded) nanoclusters in the ta-C films. Overall results from all the techniques used show that the structural and electronic changes in the films depend sensitively on the initial sp3 content. Cross-sectional TEM confirms that the clusters appear not only at the surface of the films but in the bulk as well. The growth and, perhaps, the partial orientation of the sp2-bonded nanoclusters in the size range of 1–3nm is accompanied by a large reduction in the film stress, which decreases sharply in the temperature range 500–600°C.