W. A. de Heer

Evidence of anisotropic metallic behaviour in the optical properties of carbon nanotubes

We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet, in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. The fair agreement between our experimental data and the phenomenological MG-approach suggests that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

The optical response of carbon nanotubes

Abstract We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet (from 20 cm −1 up to 3× 10 4 cm −1 ), in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. We establish that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

Magnetic properties of free alkali and transition metal clusters

The Stern-Gerlach deflections of small alkali clusters (N<6) and iron clusters (10<N<500) show that the paramagnetic alkali clusters always have a non-deflecting component, while the iron clusters always deflect in the high field direction. Both of these effects appear to be related to spin relaxation however in the case of alkali clusters it is shown that they are in fact caused by avoided level crossing in the Zeeman diagram. For alkali clusters the relatively weak couplings cause reduced magnetic moments where levels cross. For iron clusters however the total spin is strongly coupled to the molecular framework. Consequently this coupling is responsible for avoided level crossings which ultimately cause the total energy of the cluster to decrease with increasing magnetic field so that the iron clusters will deflect in one direction when introduced in an inhomogeneous magnetic field. Experiment and theory are discussed for both cases.

Electronic properties of aligned carbon nanotubes

Abstract The electronic properties of aligned carbon nanotube films have been investigated by conduction electron spin resonance (CESR) and transport measurements. Hall effect and CESR reveal that the nanotubes behave like a semimetal with 10 19 carriers/cm 3 at room temperature. The intrinsic resistivity is an order of magnitude lower than the dc resistivity of the films. Strong deviations in the physical characteristics suggest a charge-carrier localisation below 40 K. Low level of doping with Potassium strongly modifies the electronic properties of the nanotubes.

An improved laser vaporization cluster source and time-of-flight mass spectrometer

We briefly describe an improved laser vaporization cluster source, which produces intense, stable and cold cluster beams, and a new time-of-flight mass spectrometer, which in several respects is more versatile and better suited for cluster studies than traditional designs. The mass spectrometer has a high resolution mode with a very large effective ionization region as well as position and velocity sensitive detection modes.

Electronic properties of aluminum clusters compared with the jellium model

The experimental polarizabilities, ionization potentials and electron affinities of aluminum clusters are compared with jellium predictions. It is found that the clusters have radii and work functions which are close to the jellium model predictions for clusters with more than 13 atoms. The polarizabilities of Aln correspond with the jellium only forn>40 and the shell structure features in the ionization potentials are anomalous up to 37. We conclude that nonjellium effects are important up ton=40.

Generation of Graphitic Onions

We present a structural study of nanometric graphitic particles formed by nested fullerenes (onion-like). Carbon onions generated by high energy electron irradiation of carbonaceous materials present a remarkable spherical shape. As the electron bombardment is realized in situ in a electron microscope, the dynamic of the formation may be followed in real time. Macroscopic quantity of onion-like graphitic particles (3-10 nm in diameter) may be obtained by a simple thermal treatment of carbon soot.

Carbon Nanotubes and Aligned Carbon Nanotube Films

Carbon nanotube are slowly coming to the forefront of fullerene materials especially since mass quantities can be produced rather easily. A new methods to make aligned nanotube films hold many promises, not only for basic research but also for eventual applications. With this method the tubes can be aligned either parallel or perpendicular to the surface. Here we give a short review of the field with several examples of the microscopic properties of single tubes and the bulk properties of the aligned films.

Simple Models for Metal Clusters

The electronic properties of simple metal clusters are examined in several simple models. The electronic shell effects, polarizabilities, ionization potentials and plasma resonances are calculated using the Sommerfeld model and the Drude model in an ellipsoidal potential well with a shape derived in the Nilsson-Clemenger model. Despite their simplicity these models describe and predict electronic properties surprisingly well, far better than might be expected considering the simplifications, and in some cases even surpass much more sophisticated treatments.