Elsevier

Applied Surface Science

Volume 256, Issue 4, 30 November 2009, Pages 1061-1064
Applied Surface Science

Application of X-ray photoelectron spectroscopy to characterization of Au nanoparticles formed by ion implantation into SiO2

https://doi.org/10.1016/j.apsusc.2009.05.104Get rights and content

Abstract

In X-ray photoelectron spectroscopy (XPS) of Au nanoparticles, the width of 5d valence band changes with Au particle size. This enables us to estimate the size of Au nanoparticles by using XPS. In this work, the 5d-band width has been measured for Au nanoparticles formed by ion implantation into SiO2. The 5d-band width is found to be correlated strongly with the Au concentration. As the Au concentration increases, the 5d-band width becomes larger, indicating that the Au nanoparticles with the larger size tend to be formed in the vicinity of the projected range of Au ions. This correlation agrees very well with the results from transmission electron microscopy.

Introduction

In X-ray photoelectron spectroscopy (XPS) of Au nanoparticles, Au 4f-binding energy shifts toward higher energy from a bulk value, depending on Au particle size [1], [2], [3], [4], [5], [6], [7], [8]. The Au particle size can be, therefore, deduced from the Au 4f-binding energy shift. In the case of XPS of Au nanoparticles embedded in a non-conductive matrix, e.g., Al2O3 and SiO2, it is very difficult to measure the Au 4f-binding energy because of the surface charging. During emission of photoelectron in XPS of such samples, the positive charges resulting from the photoionization are not immediately neutralized. Thus the sample surface acquires a positive potential. The surface level of Au nanoparticles is, therefore, not in equilibrium with the Fermi level of the spectrometer and is shifted by the unknown surface potential. The C 1s line (284.5–285.0 eV [9]) of adsorbed hydrocarbon is sometimes used as the reference for the determination of a binding energy. This technique cannot be, however, applied for the present study because most of hydrocarbon is sputtered away when sputter etching is conducted for depth analysis. When sputter etching is applied, the Ar 2p line of implanted Ar atoms can be the candidate for charging reference. The binding energy of an Ar 2p3/2 line of 241.82 eV for Ar implanted in graphite has been recommended [10]. The Ar 2p3/2-binding energy, however, vary depending on the material in which the ions are implanted [11]. This will give rise to fatal uncertainty for binding energy measurements.

Alternatively, we focused on the shape of a valence band photoemission spectrum of Au nanoparticles. The spin–orbit energy splitting in a valence band, which is essentially insensitive to the sample charging, is a measure of the size of nanoparticles if the charge transfer between the nanoparticles and their surroundings is negligibly small. In the Au 5d valence band, for example, 5d5/2 and 5d3/2 levels are well separated for a bulk Au [12], [13] and the splitting between the two levels decreases as the size of Au nanoparticles decreases [1], [4], [14], [15], [16], [17], [18], [19]. For very small particles, such as a cluster composed of several atoms, the separation between 5d5/2 and 5d3/2 levels is too narrow to resolve the doublet. In this case, the measurement of 5d-band width can be alternative. Compagnini et al. [20] showed, in fact, that the 5d-band width was reduced linearly with the inverse of Au particle size.

In the present work, the 5d-band width was measured on Au nanoparticles formed by ion implantation into SiO2 matrix to estimate the Au particle size as a function of depth in the matrix. The depth distributions of the size obtained from XPS are compared with the results from direct observations with transmission electron microscopy (TEM).

Section snippets

Experiment

Au+ ions with an energy of 500 keV were implanted into fused silica (SiO2) to a fluence of 4 × 1016 cm−2. Au implantation was performed using a tandem accelerator of IMR, Tohoku University. The depth distributions of implanted Au atoms were analyzed by using Rutherford backscattering spectrometry (RBS) with 2 MeV-He ions from a single-ended accelerator at Takasaki Ion Accelerator for Radiation Application (TIARA) of the Japan Atomic Energy Agency. The etching rate by Ar ions in XPS analysis was

Results and discussion

Fig. 1 shows a cross-sectional transmission electron micrograph of Au-implanted SiO2. The Au-implanted layer at depths 70–250 nm contains a lot of spherical particles of 2–5 nm, depending on the depth. The spherical precipitates with size smaller than 1 nm cannot be clearly seen in Fig. 1. RBS analysis reveals that Au concentrations are 1.5–7.5  at.% in the particle-precipitated layer (or band). The smallest precipitates (∼2 nm) disperse in the vicinity of the deeper and shallower edges of the

Conclusion

Valence band photoemission spectra of Au nanoparticles formed by 500 keV-Au+ implantation into SiO2 have been measured to analyze their size depending on depth. The depth profiles of Au particle size estimated by measuring the 5d-band width agree well with the results obtained from TEM. Thus the measurement of the 5d-band width is found to be useful to estimate the Au particle size. The measurements of 5d-band width for a wide range of Au particle size are required to derive the 5d width–size

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research (C) (19510116), and was performed under the cooperative research program of TIARA of Japan Atomic Energy Agency and IMR of Tohoku University.

References (21)

  • A. Howard et al.

    Surf. Sci.

    (2002)
  • C.J. Powell

    Appl. Surf. Sci.

    (1995)
  • A. Jablonski

    Surf. Interface Anal.

    (1995)
  • S.B. DiCenzo et al.

    Phys. Rev. B

    (1988)
  • D.M. Cox et al.

    Z. Phys. D: Atoms Mol. Clusters

    (1991)
  • H.-G. Boyen et al.

    Phys. Rev. Lett.

    (2005)
  • G.K. Wertheim et al.

    Phys. Rev. Lett.

    (1983)
  • M. Quinten et al.

    Z. Phys. D: Atoms Mol. Clusters

    (1991)
  • T. Ohgi et al.

    Phys. Rev. B

    (2002)
  • J.N. O'Shea et al.

    Appl. Phys. Lett.

    (2002)
There are more references available in the full text version of this article.

Cited by (8)

View all citing articles on Scopus
View full text