Metal nanostructure growth on molecular buffer layers of CO2

Waggoner, Philip S.; Palmer, J. S.; Antonov, V. N.; Weaver, J. H.

doi:10.1016/j.susc.2005.08.020

Cited by 25 publications

(35 citation statements)

References 20 publications

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“…According to the disorder model, for perfect crystalline semiconductors, we should predict a value of 0 K, totally contrary to what is observed. The only reports of very low values of T MN are for migration of nanoclusters on rare gas crystals [33] and on CO 2 crystals [34], for which k B T MN corresponds almost exactly to the very small phonon energies of the crystals in question.…”

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confidence: 99%

The fallacy of Meyer-Neldel temperature as a measure of disorder

Yelon

2012

Monatsh Chem

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The widespread observation of the MeyerNeldel rule has been explained as associated with large activation energies, requiring the accumulation or annihilation of a large number of excitations for a kinetic event to take place. The characteristic Meyer-Neldel or isokinetic temperature is then proportional to the energy of these excitations divided by a coupling constant. For disordered materials at low excitation energies, a similar relation has been proposed, involving the width of bandtails or of Gaussian distributions, a measure of disorder. This similarity has resulted in interpreting high activation energy results, for which it cannot be applicable, using this description. We show that this leads to contradictions with experiments as well as to logical contradictions.

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confidence: 99%

The fallacy of Meyer-Neldel temperature as a measure of disorder

Yelon

2012

Monatsh Chem

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“…Moreover, we suggested that this dependence should be a common for nanoparticle diffusion. Studies by Philip Waggoner of diffusion on CO 2 buffers did, indeed, show an increase in the characteristic energy of the elementary excitations, consistent with the more energetic phonons of CO 2 [7]. Studies of H 2 O buffers and C 60 buffers should provide important insights, as well as determining the viability of BLAG at higher temperature.…”

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confidence: 73%

“…This also led us to look for buffers that might be effective in BLAG but which would desorb at significantly higher temperatures. For CO 2 buffers, the expected increase in diffusivity was offset by an increased barrier for diffusion due to the stronger interaction with the buffer [7]. Studies that are currently underway focus on H 2 O [ice] as a buffer, and all indications are that it functions well.…”

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confidence: 99%

Final Report: Nanoscale Cluster Assembly on Compliant Substrates: A New Route to Epitaxy and Nanosctructure Synthesis

Weaver¹

2006

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From our work at the University of Minnesota prior to 2000, we knew that buffer-layerassisted growth could be used to produce abrupt interfaces where reactions were constrained by the fact that particles derived from tens to millions of atoms were brought into contact with substrates that ranged from GaAs(110) to BiSrCaCuO superconductors [1,[10][11][12][13][14][15][16][17][18][19]. In situ scanning tunneling microscopy had demonstrated that the particles increased in size with the thickness of the buffer layer [9], and we postulated that buffer desorption somehow 'tossed the particles around' so that aggregation was possible. Through access to transmission electron microscopy in the MRL at the University of Illinois, we have been able to determine the distribution of particles delivered to amorphous carbon as a function of buffer thickness, buffer material, particle material, and warm up rate so as to reveal the physics underlying diffusion, aggregation, and coalescence. Significantly, this enhanced understanding makes it possible to design experiments that produce sizes and distributions of nanoparticles of a very wide range of materials.The first experiments were designed to determine the particle densities and shapes for comparison to Monte Carlo simulations of cluster aggregation. Graduate student Christina Haley studied Au particles on amorphous carbon that had been formed by deposition onto Xe buffers of variable thickness [2]. She compared the fractal dimension for ramified particles to simulations and determined that it was consistent with 2D diffusion-limited cluster-cluster aggregation. She also showed that both the weighted average cluster size and the cluster density had a power law dependence on the buffer thickness.When dealing with particles of nanometer dimension, it is important to realize that coalescence is driven by the very large surface energy and that particle integrity is a critical issue. We reasoned that the tendency of particles to coalesce could be altered by adsorbates that would reduce the surface energy. While the coalescence/sintering of metal particles of nanometer size had been modeled with molecular dynamic simulations and continuum models [20], clean experiments were few. Graduate student Vassil Antonov [3] investigated the coalescence of Pd nanostructures produced by BLAG under normal conditions and after coating them with an adsorbed monolayer of CO. The studies of Pd showed a behavior similar to Au, with a power law dependence of particle size on buffer thickness. He then repeated the experiments with a CO coating on the particles. He found that the CO layer was not sufficient to prevent aggregation, presumably because of irregularities in the particle shape. However, coalescence was significantly impeded, giving branched islands with thinner branches. The crossover size from compact to branched growth was reduced two-fold, indicating constraints in Pd atom diffusion related to the adsorbed CO. Another important conclusion was that solid CO performed the same function as X...

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“…The utilization of other buffer materials such as water [19] and carbon dioxide [21] have recently been demonstrated for the growth of metallic and oxide clusters. This is of practical importance allowing liquid nitrogen instead of liquid helium level of sample cooling.…”

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confidence: 99%