We describe synchrotron x-ray diffraction measurements of strain in Cu and Pd metal nanoparticles (1.7–40 nm diameter) both with an air-formed oxide shell and after reduction of the oxide by treatment in a hydrogen-containing atmosphere. Oxide removal is evident from x-ray diffraction (for Cu) and x-ray absorption spectroscopy (for Pd). A simple model that uses bulk elastic properties is applied to each system. In the Pd case the model predictions agree well with the experiment. For Cu the observed strains are much smaller than predicted. This discrepancy is attributed to (a) the presence of multiple grains within the Cu particles and (b) the incoherency of the oxide with the metal core.

The evaporation of gold onto SiN or SiOx leads to the formation of nanoscale islands separated by tunneling barriers. As the deposited thickness of Au increases, there is a linear increase in island size and an exponential decrease in film resistance, resulting from coalescence of neighboring islands. This understanding of film morphology allows the construction of a simple model of film resistivity. The model also accounts for the exponential decrease of resistance observed during deposition of Pd nanoparticles onto Au island films. The results suggest the use of discontinuous Au island films as a sensitive way of controlling nanoparticle deposition.

We describe the formation of hydrogen sensors by deposition of Pd clusters onto silicon dioxide coated silicon substrates with electrical contacts defined by a simple shadow masking technique. The clusters are prepared by sputtering in a gas aggregation source. The sensors are characterized by exposure to hydrogen in a simple flow chamber and by measuring the temperature dependence of the sensor resistance. Sensors with cluster coverage greater than the percolation threshold form "thin film" type sensors which exhibit a small increase in resistance on exposure to hydrogen, consistent with the increase in resistivity of bulk Pd on absorption of hydrogen. Sensors with coverage smaller than the percolation threshold form sensors which exhibit a much larger decrease in resistance on exposure to hydrogen. The response of these "percolating-tunneling" sensors is due to the absorption of hydrogen by the Pd clusters, which causes the tunnel gaps in the film to decrease in size, leading to an increase in conductance. Finally we describe tunneling sensors, where gold islands are grown on the substrate prior to cluster deposition, and which exhibit similar characteristics to the percolating-tunneling sensors.

Low-energy multiple-collisional-excitation experiments have been performed on C70+ and C70− at the ClusterTrap apparatus. The ions are stored in a Penning trap where they are excited via radial dipolar excitation before undergoing collisions with neutral argon atoms. The dominant decay mechanism for C70+ (sequential loss of C2-units) is compared with the dominant decay process of C70− (thermionic electron emission). A simple model based on the decay rates of the clusters is found to be in reasonable agreement with the experimental data obtained for the fragmentation process of the cationic fullerenes. The same model, when applied to the anions is observed to be in less agreement with the experimental results.

We have prepared sensitive hydrogen and ammonia sensors from thin films of tin nanoclusters with diameters between 3 and 10 nm. By baking the samples at 200 °C in ambient air the clusters were oxidized, resulting in very stable films of tin oxide clusters with similar diameters to the original Sn clusters. By monitoring the electrical resistance, it is shown that the cluster films are highly responsive to hydrogen and ammonia at relatively low temperatures, thereby making them attractive for commercial applications in which low power consumption is required. Doping of the films by depositing Pd on top of the clusters resulted in much improved sensor response and response times. It is shown that optimal sensor properties are achieved for very thin cluster films (a few monolayers of clusters).

Palladium nanoclusters produced in an inert gas aggregation/magnetron sputtering source are being used as building blocks for the construction of nano electronic devices with large surface to volume ratios that can be used as sensitive hydrogen gas sensors in fuel cells and in petrochemical plants. Grazing incidence X-ray diffraction (GIXRD) at the Stanford Synchrotron Radiation Laboratory (SSRL) and high resolution transmission electron microscopy (HRTEM) have been used to characterise the structure, lattice constant, particle diameter and oxide thickness of the palladium nanoclusters in order to understand the operation of these sensors.

The operation conditions of a double pulsed field mass filter were studied using both experiment and simulation. The mass filter consists of two pairs of parallel plates and operates on the time-of-flight principle. The study showed that the ions’ beam deflection angle is a critical factor in optimizing the mass filter transmission efficiency. This angle is dependent on the accelerating voltage, ion mass, and horizontal velocity of the ions. The optimum operating conditions for the mass filter were found and used to study the mass distribution of palladium ions produced by a magnetron sputtering source. The study shows that this mass filter is suitable for technological applications because of its high transmission and wide mass range.

We describe nanocluster based tunneling sensors for detecting hydrogen. Pd clusters with diameters ranging from 3.5to6nm are deposited between a pair of contacts until a predetermined resistance between the contacts is obtained. We demonstrate that the conduction through the cluster film is dominated by tunneling gaps. Upon exposure to hydrogen, the clusters expand, reducing the average size of the gaps and reducing the measured resistance. The sensor response as a function of external hydrogen pressure is described, for different cluster sizes and different operating temperatures, by a simple model with a single physically meaningful fitting parameter.

An experimental study of Bi, Sb, and Cu clusters incident at velocities ≳50m∕s on SiO2, Si3N4, polymethylmethacrylate, and photoresist surfaces shows that the clusters adhere much more strongly to SiO2 and Si3N4 than to the polymer materials. The differences in adhesion properties allow assembly of a range of nanowire-based electronic devices from cluster building blocks using lithographically patterned polymer layers. Clusters adhere to the substrate but not to the surface of the polymer template, eliminating parasitic conduction. Molecular dynamics simulations show that differing cluster-surface interactions affect adhesion most strongly when high incident velocities cause significant plastic deformation of the clusters.

Atomic clusters have been deposited from an inert gas aggregation source to form cluster-assembled nanowires with a view to electronic and chemical sensor applications. The fabrication methods employed make use of many of the desirable features of both bottom-up and top-down approaches to nanotechnology. We have produced electrically conducting nanowires through the deposition of atomic clusters onto lithographically prepared templates. These nanowires are being commercialized through a start-up company in collaboration with US-based partners. The application examples presented here are i) working hydrogen sensors based on Pd clusters and ii) trenches filled with copper clusters for use as interconnects.

Thin films and bulk samples of endohedral fullerenes Li@C-60 are studied using current-voltage (I-V) measurements and electron paramagnetic resonance (EPR). Electrical measurements show a linear behaviour for the I-V curves and give an average resistivity of ca. 1.5 kOmegacm for thin Li@C-60 films deposited in vacuum, four orders of magnitude lower than C-60 samples. A drastic effect on the conductance, lowering it to the values typical for C-60, is observed when the Li@C-60 samples are exposed to ambient atmosphere. No additional paramagnetic centres (PCs) are found for the Li@C-60 compared to C-60 that can be related to the formation of dimers or trimers of the endohedral fullerene molecules. However, the presence of the Li atoms in the fullerene cages contributes to a change of the spin-spin and spin-lattice relaxation times. The spin-lattice relaxation time becomes four orders of magnitude longer compared to the spin-spin relaxation time. (C) 2004 Elsevier Ltd. All rights reserved.

Fullerene dianions in the range C-70(2-) to C-90(2-) have been created by subjecting trapped fullerene monoanions to low energy electrons in a Penning trap. The dianion production was found to be a function of the trapping-potential depth and the time of interaction between the simultaneously stored monoanions and electrons. Under similar conditions the dianion yield depends on the size of the fullerenes with more than 10% of the trapped C-90(-) ions forming dianions while the corresponding relative yield for C-70(2-) was less than 0.1%. The large difference can be explained by the repulsive Coulomb barrier and the second electron affinity of the fullerenes.

S. A. Brown, J. G. Partridge, S. Scott, S. Hendy£, A. Ayesh, R. Reichel, K. C. Tee, M. Kaufmann, A. Awasthi£, P. Zoontjens£, A. Lassesson, J. van Lith, M. Schulze. # MacDiarmid Institute of Advanced Materials and Nanotechnology, † Nano Cluster Devices Ltd, Department of Physics and Astronomy, Rutherford Building, University of Canterbury, Christchurch, New Zealand Industrial Research Ltd, PO Box 31-310, Lower Hutt, New Zealand, Contact email: Simon.Brown@nanoclusterdevices.com

The endohedral fullerenes La@C-82 and Li@C-60 have been studied in the gas phase with femtosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C-82 fragmenting via the shrink-wrap mechanism and Li@C-60 predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C-82 agree well with a recently developed model for fs laser ionisation of C-60.

Neutral and charged phosphorus clusters of a wide size range have been produced by pulsed laser ablation (PLA) in vacuum at 532, 337, and 193 nm ablating wavelengths and investigated by time-of-flight mass spectrometry. The neutral P_n clusters are even-numbered with local abundance maxima at n = 10 and 14, while the cationic and anionic clusters are preferentially odd-numbered with (P_7)+, (P_21)+, and (P_17)- being the most abundant ions. The dominance of the magic clusters is more pronounced at 337-nm ablation that is explained by efficient direct ejection of their building blocks under these conditions. Nanocrystalline phosphorus films have been produced by PLA in ambient helium gas.

Photofragmentation experiments on molecules and clusters often involve multiple photon absorption. The distributions of the absorbed number of photons are frequently approximated by Poisson distributions. For realistic laser beam profiles, this approximation fails seriously due to the spatial variation of the mean number of absorbed photons across the laser beam. We calculate the distribution of absorbed energy for various laser and molecular-beam parameters. For a Gaussian laser beam, the spatially averaged distributions have a power-law behavior for low energy with a cutoff at an energy which is proportional to fluence. The power varies between -1 for an almost parallel laser beam and -5/2 for a divergent beam (on the scale of the molecular beam). We show that the experimental abundance spectra of fullerenes and small carbon clusters can be used to reconstruct the distribution of internal energy in the excited C60 molecule prior to fragmentation and find good agreement with the calculated curves.

IR and Raman investigations have been carried out on purified Li@C-60 and Li@C-70 produced by low-energy ion implantation. The structural interpretations, based on theoretical considerations and comparison of the obtained vibrational spectra with those of different known fullerene oligomers, have shown that neither of the chromatographically isolated endohedral species (Li@C-60(70)) correspond to monomeric nonderivatized molecules. We ascribe the two different fractions of Li@C-60 to a double-bonded dimer and a trimer, presumably with the shape of closed triangle. The infrared and Raman spectra of the Li@C-70 compound are similar to those Of (C-70)(2), and we therefore propose for this species the structure of a double-bonded dimer (Li@C-70)(2). The IR spectra of the Li@C-60 fractions prepared with Li-6 and Li-7 isotopes are identical except for the position of broad bands at about 450-600 cm(-1), originating from vibrational-rotational movement of the Li+ cation inside the carbon cage.

The fragmentation behavior of the endohedral metallofullerene La@C82 was studied using gas phase time-of-flight mass spectrometry. The results were compared with the fragmentation of C60. When the metallofullerene was excited by a 337 nm ns laser, small lanthanum–carbide fragments LaCn+, n=0–6, were detected. A simple statistical maximum entropy model was used to simulate the excitation energy dependence of the loss of the LaCn+ molecules as well as C2 evaporation to form smaller metallofullerenes. By comparing experimental ion intensities with the model, the appearance energies of LaCn+, n=0–6, were found to lie above 65 eV. The lanthanum–carbide fragments ejected from La@C82 decrease in size with increasing internal energy. The ejection of La+ and LaC2+ is preferred at the expense of larger fragments such as LaC4+ and metallofullerenes at the higher excitation.

The conductivity of thin films of endohedral fullerenes has been studied using four-probe measurements. The conductivity under vacuum conditions is orders of magnitude better than that of C-60. On exposure to air all samples show degradation. The effect is strongest for Li@C-60 films. Laser desorption mass spectrometry studies on thin films of La@C-82 also show evidence for degradation of the material on exposure to air.

Laser desorption mass spectrometry is used to study thin films of purified La@C82. The films are changed on exposure to atmosphere due to oxidation reactions with molecular oxygen. The endohedral fullerene oxides fragment more readily on laser desorption leading to the formation of small lanthanum carbide species and LaO. The half life of the oxidation reaction is found to be on the order of a few days.