Eleanor E. B. Campbell

Laser ablation of dielectrics with temporally shaped femtosecond pulses

A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.

Increasing cost of pentagon adjacency for larger fullerenes

The cost of a pentagon adjacency in a fullerene cage grows linearly from 72 kJ mol−1 for C30 to 111 kJ mol−1 for C60, according to systematic QCFF/PI model calculations on a set of 2624 structural isomers.

Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes

Classical continuum mechanics is used extensively to predict the properties of nanoscale materials such as graphene. The bending rigidity, κ, is an important parameter that is used, for example, to predict the performance of graphene nanoelectromechanical devices and also ripple formation. Despite its importance, there is a large spread in the theoretical predictions of κ for few-layer graphene. We have used the snap-through behavior of convex buckled graphene membranes under the application of electrostatic pressure to determine experimentally values of κ for double-layer graphene membranes. We demonstrate how to prepare convex-buckled suspended graphene ribbons and fully clamped suspended membranes and show how the determination of the curvature of the membranes and the critical snap-through voltage, using AFM, allows us to extract κ. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be 35.5−15.0 +20.0 eV. Monolayers are shown to have significantly lower κ than bilayers.

Optical emission studies of laser desorbed C60

The optical emission spectra of laser desorbed C60 have been investigated as a function of laser fluence for desorption with a XeCl excimer laser (pulse length 20 ns). The observed spectra show close similarities to black‐body radiation and can be fitted with the Planck black body formula (modified for small particles) thus giving information on the temperature of the desorbed species. The temperatures obtained (2300–3000 K) are in good qualitative agreement with previous, indirect temperature estimates. Spatially and temporally resolved measurements provide additional insight into the desorption mechanisms. An estimate of cooling rates indicates that thermionic electron emission and C2 fragmentation dominate for temperatures above about 3000 K but below this value the dominant cooling mechanism is black‐body radiation.

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF2 are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.

Schottky barriers in carbon nanotube-metal contacts

Semiconducting carbon nanotubes (CNTs) have several properties that are advantageous for field effect transistors such as high mobility, good electrostatics due to their small diameter allowing for aggressive gate length scaling and capability to withstand high current densities. However, in spite of the exceptional performance of single transistors only a few simple circuits and logic gates using CNTs have been demonstrated so far. One of the major obstacles for large scale integration of CNTs is to reliably fabricate p-type and n-type ohmic contacts. To achieve this, the nature of Schottky barriers that often form between metals and small diameter CNTs has to be fully understood. However, since experimental techniques commonly used to study contacts to bulk materials cannot be exploited and studies often have been performed on only single or a few devices there is a large discrepancy in the Schottky barrier heights reported and also several contradicting conclusions. This paper presents a comprehensive ...

Particle size dependence and model for iron-catalyzed growth of carbon nanotubes by thermal chemical vapor deposition

The catalytic particle size dependence of chemical vapor deposition growth of multiwall carbon nanotubes was systematically investigated using two different molecules, C2H2 and C60, as carbon feedstock gases. In the particle size range between 25 and 500 nm, the use of C2H2 leads exclusively to growth of carbon nanotubes. The nanotube diameters increase with increasing catalytic particle sizes but do not scale 1:1. In contrast, nanotube formation from C60 is observed only if the particle sizes are sufficiently small with an optimum between 20 and 30 nm. For catalyst samples with considerably larger diameters, C60 is transformed into a nontubular deposit. A growth model is given that explains the different behavior.

A simple method for the production of large arrays of aligned carbon nanotubes

Aligned carbon nanotubes are a technologically relevant member of the family of novel carbon materials, which find applications, e.g., as field emitters in flat panel displays. Different strategies of various complexities for their production have been demonstrated. Here we present an efficient and versatile but simple method for the production of large arrays of aligned carbon nanotubes based on thermal chemical vapor deposition from common precursor molecules. Iron catalyst particles are obtained from thermal decomposition of Fe(CO)5, while C2H2 serves as carbon feedstock. Growth of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and acetylene.

AN INTENSE, SIMPLE CARBON CLUSTER SOURCE

An intense source of positive, negative, and neutral carbon clusters using excimer laser ablation of polyimide is described. The detection probability for large masses using a standard channel plate configuration is shown to increase exponentially with ion velocity. The large clusters are formed via aggregation of atomic carbon or small carbon molecules. The small ‘‘clusters’’ may be molecular fragments from the polymer which rearrange on leaving the surface to form stable structures.

Laser mass spectroscopic investigations of purified, laboratory-produced C60/C70

Abstract Laser desorption mass spectra of Kratschmers purified “fullerite” have been studied in a reflectron time-of-flight mass spectrometer. Desorption with 308 or 248 nm yields direct ions emerging from the substrate with high efficiency. At low fluence, a distribution of pure C + 60 (76%), C + 70 (23%) and, perhaps, a small amount C + 84 (⩽ 1.5%) ions are observed. As the fluence is increased, fragmentation of these species and aggregation to larger clusters are observed. Post-ionisation of neutral desorbed species with 248 or 308 nm is much less efficient and leads to substantial fragmentation both immediately on ionisation as well as in the field-free region of the mass spectrometer (a timescale of up to 100 μs).