MAGNETIC BEAM RESEARCH

 

Studies of adsorbed gas layers and ultrathin layers of Metals and Semiconductors

The ability of PAES to determine the elemental content of the topmost atomic layer has been exploited in a number of investigations aimed at the study of the growth and ability of ultrathin metal layers on metal substrates including Pd on Cu, Au on Cu, Fe on Cu, Rh on Ag, Au on Si, Cs on Cu, and Si on Ge. These systems are of interest because of their unique chemical, electronic, and magnetic properties. Another application of the magnetic beam system was the positron work function measurement from an energy analysis of the re-emitted positrons when the surface was bombarded with keV positrons.

B-beam System Objects Journal
Clean Surface Ni(100), Cu(100)

Au,Pd,Fe,Cu

PAES Mechanism

Relative Auger Intensities

PRL 61 (1988)

S. Sci. (1992)

Ultra-thin Film Growth Pd/Cu(100)

Cs/Cu(100)

Au/Cu(100)

Rh/Ag(100)

Fe/Cu(100)

Au/Si(100)

Au/Si(111)

Si/H/Ge(100)

Top layer selectivity/Intermixing

Positron trapping at interface

Stability of Au films on Cu

Ag migration (Ag/Rh/Ag(100))

Cu migration (Cu/Fe/Cu(100))

Depth profile analysis

Thin film growth characterization

H effect on Ge segregation

PRB (1993)

PRL (1992)

J. Mater (1994)

J. Vac. Sci (1994)

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Surf. Sci (1996)

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A. S. Sci. (1997)

Gas overlayer S/Cu(100)

H2,O2/Si(100)

Surface sensitivity

Sticking coefficients/Surface sensitivity

PRB(1990)

S. Sci (1997)

Work function Fe,Mo,Ni,Pt,Ti,V

6H-SiC(0001)

GaAs(100)

Polycrystalline materials

Positron related characterizations

Positron reemission

PRB(1991)

Materials Sci. Forum

Materials Sci. Forum

Catalysis Au/O2/Si(100) Identify the adsorption sites A. S. Sci (1995)

 


 

Identification of Active Sites on Catalytic surfaces

Bimetallic surfaces, which are finding increased use as catalysts, the correct identification of prefered adsorption sites for gas molecules is a crucial starting point in the study of reactivity and selectivity. Preliminary measurements on a model system have demonstrated that PAES is able to identify sites of preferential adsorption.

(left) Schematic Diagram of "Blocking" of the positron wave function by Adsorbates

The preferential adsorption of molecules to a particular type of site (such as at only one of the elements of the alloy) will displace positrons from those sites leading to substantial and readily identifiable changes in PAES intensities.

(right) PAES spectra for a Si (100) surface covered with 0.3 ML of Au (open circles) before and (closed circles) after exposure to 300 Langmuir of 02.

The oxygen exposure caused the counts in the region of the Si peak to decrease by 60% while the Au peak maintained the same level. This indicates that the positrons are being pushed away from the Si but not the Au due to selective adsorption of oxygen at exposed Si sites. A schematic drawing indicating this mechanism is shown in this Figure.

 


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