Presentation Title

Angular Dependence of the Neutralization Probability of Low Energy Alkali Ions Scattered off of Au Nanoclusters with Adsorbed Br

Faculty Mentor

Jory Yarmoff

Start Date

18-11-2017 2:15 PM

End Date

18-11-2017 3:15 PM

Location

BSC-Ursa Minor 44

Session

Poster 3

Type of Presentation

Poster

Subject Area

physical_mathematical_sciences

Abstract

Gold (Au) nanoclusters, on the order a few nm in diameter, have been heavily studied because of their intriguingly high catalytic activity, especially when compared to bulk gold. To assist in understanding how these clusters are involved in surface chemical reactions, we employ a specialized method of Low Energy Ion Scattering (LEIS) to probe the electronic properties of Au nanoclusters grown by direct deposition onto thin SiO2 films in ultra-high vacuum. As more Au is deposited, the average size of the clusters increases. The experiments measure the neutralization probability of singly scattered low energy (1-5 keV) alkali ions, which is acutely sensitive to the local electrostatic potential a few Å’s above the surface. Because the Au atoms are much more massive than the substrate atoms, the signal from the nanoclusters can easily be separated from that of the substrate so that the neutralization reflects the local properties of the cluster surfaces. Earlier work had demonstrated that the neutralization is a function of cluster size, and that it is enhanced for the smallest clusters. Here, the clusters are exposed to Br2 gas emitted from a solid-state electrochemical cell, which causes the bromine molecules to dissociate and adsorb onto the surface as Br atoms. It is found that the neutralization of Na+ scattered from the Au clusters decreases with Br coverage, and has a maximum value when the angle between the detector and the surface normal is in the range of 20-40°. These maxima are symmetric on both sides of the surface normal. Possible explanations for these observations will be discussed.

Summary of research results to be presented

Using low energy alkali ion scattering (LEIS), we measured the neutralization of the ions. By comparing the ratio of neutralized ions to total ions that were scattered, we produce a neutral fraction, which is related to its catalytic activity. The neutral fraction was seen to decrease as the amount of Br exposed increased. When the scattering angle was changed, we also found a change in neutral fraction. The neutral fraction increased as the angle was changed until it hit a maximum in the 20-40 degree range, then would decrease as the angle increased. The mechanism behind this result is still being investigated at this moment.

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Nov 18th, 2:15 PM Nov 18th, 3:15 PM

Angular Dependence of the Neutralization Probability of Low Energy Alkali Ions Scattered off of Au Nanoclusters with Adsorbed Br

BSC-Ursa Minor 44

Gold (Au) nanoclusters, on the order a few nm in diameter, have been heavily studied because of their intriguingly high catalytic activity, especially when compared to bulk gold. To assist in understanding how these clusters are involved in surface chemical reactions, we employ a specialized method of Low Energy Ion Scattering (LEIS) to probe the electronic properties of Au nanoclusters grown by direct deposition onto thin SiO2 films in ultra-high vacuum. As more Au is deposited, the average size of the clusters increases. The experiments measure the neutralization probability of singly scattered low energy (1-5 keV) alkali ions, which is acutely sensitive to the local electrostatic potential a few Å’s above the surface. Because the Au atoms are much more massive than the substrate atoms, the signal from the nanoclusters can easily be separated from that of the substrate so that the neutralization reflects the local properties of the cluster surfaces. Earlier work had demonstrated that the neutralization is a function of cluster size, and that it is enhanced for the smallest clusters. Here, the clusters are exposed to Br2 gas emitted from a solid-state electrochemical cell, which causes the bromine molecules to dissociate and adsorb onto the surface as Br atoms. It is found that the neutralization of Na+ scattered from the Au clusters decreases with Br coverage, and has a maximum value when the angle between the detector and the surface normal is in the range of 20-40°. These maxima are symmetric on both sides of the surface normal. Possible explanations for these observations will be discussed.