Morphological work function dependence of rare-earth disilicide metal nanostructures
(Result of the month 04/2009)

The work function of various DySi2-x nanostructures epitaxially grown on a Si(001) surface was correlated with structure using high resolution Kelvin probe force microscopy and scanning tunneling microscopy in ultrahigh vacuum. DySi2-x nanowires showed a 0.2 to 0.23 eV lower work function than that of DySi2-x nanoislands which can be attributed to confinement of electrons along the surface normal that induces a surface dipole when the film thickness approaches the Fermi wavelength. The ability to tune the work function of metal nanostructures should be useful for understanding how electronic structure affects catalytic activity.
Kelvin Probe Force Microscopy (KPFM) measurements over 500 nm × 500 nm regions of self-assembled DySi2-x nanostructures on Si(001) showing simultaneous (a) topography and (b) contact potential difference (CPD) after post-growth annealing at 680 °C and (c) topography and (d) CPD after post-growth annealing at 800 °C. Topography (CPD) line profiles, blue open circles (red lines), across white lines in images of the NW in (a) are labeled 1 (1’) and shown in (e), of the thin-NI in (a) are labeled 2 (2’) and shown in (f), and the thick-NI in (c) are labeled 3 (3’) and shown in (g), respectively. Note: white lines in topography and CPD images are taken along the same region.
Histograms of measured CPD values of substrate and DySi2-x nanostructures on Si(001); 2×1 reconstructed Si(001): open purple circles, NW: closed blue triangles, thin-NI: open red triangles, and thick-NI: closed green circles.
Image 2 displays histograms of measured CPD values for different DySi2-x nanostructures on Si(001) as well as the CPD distribution of clean 2×1 reconstructed Si substrate before Dy deposition as a reference. The measured CPD values of NWs was -0.50±0.04(1σ) eV (closed blue triangles), of thin-NIs was -0.73±0.03(1σ) eV (open red triangles) and of thick-NIs was - 0.76±0.02(1σ) eV (closed green circles). First, from the statistical analysis shown in image 2 one can conclude that the mean CPD of thick-NIs (closed green circles) was slightly lower than that of thin- NIs (open red triangles) which means the work function of thick-NIs is slightly larger than that of thin-NIs. In order to explain the differing values of work function between a NW and NI, theoretical calculations predict that electron motion along the surface normal of a NW is quantized when the film thickness is comparable to the Fermi wavelength, whereas electron motion along the surface normal of a NI is not. By comparing theoretical studies to this experimental data, the observation that DySi2-x NWs have a lower work function than DySi2-x NIs can be attributed to electron motion confinement along the NW thickness since this dimension is comparable to the Fermi wavelength.


Authors
Sangyeob Lee1, Aniketa Shinde 1,2, Regina Ragan1

1Department of Chemical Engineering and Materials Science, University of California, Irvine, California 92697, USA
2Department of Physics and Astronomy, University of California, Irvine, California 92697, USA

Name and email of corresponding author
rragan@uci.edu

Institute web-pages
http://ragan-group.eng.uci.edu/
http://www.eng.uci.edu/dept/chems

Publication(s)
submitted to Nanotechnology 2008

 
This result has been obtained with :
Variable Temperature UHV SPM

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