The effect of surface morphology upon the optical response of Au(1 1 0)

doi:10.1016/S0039-6028(03)00131-6

Surface Science

Volumes 532–535, 10 June 2003, Pages 1-7

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Abstract

The effect of surface morphology upon the optical response of Au(1 1 0) has been studied using reflection anisotropy spectroscopy (RAS) and scanning tunnelling microscopy (STM). Starting from a clean and well-characterised Au(1 1 0)-(1 × 2) surface, changes in surface morphology due to increasing Ar ion bombardment have been observed using STM and correlated to changes in the RA spectrum. It is found that the RA spectrum is particularly sensitive to surface morphology in the photon energy region 3.0–5.0 eV. The study highlights the sensitivity of surface optical properties to surface structure and reveals the length scale on which surface roughness starts to influence the RA response.

Introduction

The electronic structure of a surface is correlated with its atomic arrangement; for example, point defects and steps are known to cause perturbations in surface electronic structure [1]. Consequently it is expected that surface optical properties will be influenced by surface morphology. Reflection anisotropy spectroscopy (RAS), an optical probe of surfaces [2], has shown sensitivity to surface morphology [3], [4], [5]. For example, a Cu(1 1 0) surface consisting of an array of monoatomic steps aligned along the [0 0 1] direction produced a characteristic RA spectrum [5]. In the work presented here, we investigate the sensitivity of the optical RA of Au(1 1 0) to surface roughness created by ion bombardment and observed using scanning tunnelling microscopy (STM).

RAS measures the difference in normal incidence reflection of two perpendicular directions in the surface plane (Δr) normalised to the mean reflection (r). For Au(1 1 0) we define the RA as: $Δ r r = 2(r_{[11̄0]} −r_{[001]}) r_{[11̄0]} +r_{[001]}$ where the reflections r are complex Fresnel reflection amplitudes. The clean, room temperature Au(1 1 0) surface exhibits the (1 × 2) missing row reconstruction where every second [1 1̄ 0] row of atoms in the top layer is absent. Starting from a clean, well-ordered Au(1 1 0)-(1 × 2) surface, changes induced in surface morphology due to increasing doses of Ar ion bombardment have been observed using STM and correlated to changes in the RA spectrum.

Section snippets

Experimental procedure

The experiment was carried out in an ultra-high vacuum (UHV) environment at a base pressure in the 10⁻¹⁰ mbar region. A clean Au(1 1 0) surface was prepared in UHV by repeated cycles of Ar ion bombardment and annealing. Surface order was confirmed by a sharp (1 × 2) low-energy electron diffraction (LEED) pattern and from the observation of (1 × 2) reconstructed terraces in the STM data. Cleanliness was monitored using X-ray photoelectron spectroscopy. The clean, well-ordered Au(1 1 0) surface following

Results

The RA spectrum of the clean, well-ordered Au(1 1 0)-(1 × 2) surface is shown in Fig. 1a and the corresponding STM image of the surface structure is shown in Fig. 2a. The STM results showed large (1 × 2) reconstructed terraces that were terminated by monoatomic height steps (Fig. 2a). The majority of steps observed were aligned broadly along the [1 1̄ 0] direction. The RA spectrum of the surface shows negative-going peaks at 2.5 and 3.5 eV (Fig. 1a) and the profile is similar to previous RAS

Discussion

The RA spectrum of Cu(1 1 0) [10] and Ag(1 1 0) [11] include features identified with transitions between surface states located at the $Y$ point of the surface Brillouin zone. For Au(1 1 0) a similar contribution to the RA profile might be anticipated. A small contribution to the RA response at 1.9 eV of the clean Au(1 1 0)-(1 × 2) surface has been associated [8] with transitions between surface states at $Y$ . However there is disagreement between inverse photoemission, photoemission, and theoretical

Conclusion

The effect of changes in surface morphology induced by ion bombardment upon the optical response of Au(1 1 0) has been studied. It is found that the RA spectrum is particularly sensitive to surface morphology in the region 3.0–5.0 eV. The RA peak at 2.5 eV appears relatively insensitive to increasing roughness. The STM data shows the length scale on which surface roughness starts to influence the RA response.

Acknowledgements

The authors are pleased to acknowledge financial support from the UK EPSRC.

References (20)

J. Bremer et al.
Surf. Sci.
(2000)
J. Bremer et al.
Surf. Sci. Lett.
(1999)
J.-K. Hansen et al.
Physica A
(2001)
R. Drube et al.
Surf. Sci.
(1989)
N.V. Smith et al.
Surf. Sci.
(1990)
M. Sastry et al.
Surf. Sci.
(1992)
J.C. Hansen et al.
Solid State Commun.
(1989)
J.D.E. McIntyre et al.
Surf. Sci.
(1971)
M.F. Crommie et al.
Nature
(1993)
D.S. Martin et al.
Surf. Interface Anal.
(2001)

There are more references available in the full text version of this article.

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The effect of surface morphology upon the optical response of Au(1 1 0)

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Conclusion

Acknowledgements

Surf. Sci.

Surf. Sci. Lett.

Physica A

Surf. Sci.

Surf. Sci.

Surf. Sci.

Solid State Commun.

Surf. Sci.

Nature

Surf. Interface Anal.