Microscopic spectrophotometry applied to quasifractal gold particle clusters Jacob Jonssona, Juan Sotelob, Gunnar Nikiassona, Arne Roosa, and Bengt Nilssonc aDept Materials Science, Uppsala University, P0 Box 534, Uppsala, Sweden bDpto de Fisica y Matematicas, Universidad Peruana Cayetano Heredia, Apto. 5045, Lima, Peru CDept Physics, Chalmers University of Technology, Gothenburg, Sweden ABSTRACT An optical measurement system composed of an optical microscope (Olympus BX6O) and an optical multichannel analyzer (EG&G OMA 1460) has been assembled and tested. The optical microscope allows the user to make measurements on a small and well defined area of the sample. The light source, a 100 W halogen lamp, and the diode array detector, result in high sensitivity in the wavelength region 450-750 urn. The spectral resolution of the instrument is listed as 0.59 nm/channel. The full width at half maximum (FWHM) of the strongest peaks in calibration measurements on a mercury lamp is 5 channels corresponding to 3 nm. Quasifractal clusters of gold particles have been produced with electron beam lithography. The clusters consist of different numbers of particles, giving a cluster size variation from 1.6 pm to 50 pm. The individual gold particles are 50 nm in diameter each. The measurement system has been used to measure both absolute transmittance and the relative transmittance using the uncoated substrate as a reference. Keywords: Microscopy, Spectrophotometry, Optical Measurements, Fractal Aggregates 1. INTRODUCTION Most commercial instruments that measure transmittance and reflectance use a large beam size (approximately 0.1 to 2 cm2). This can only give macroscopic information about the material's properties. Effects of local microstructures cannot be measured separately. Microscopic structures, such as those studied in this paper, is a field where traditional spectrophotometers fail to give relevant information. Traditional instruments, however, have several advantages over the microscopic set-up described here. They measure over a wider energy range, scattering samples can be measured with integrating spheres, and it is possible to measure at oblique angles of incidence. The measurement set-up described in this paper provides a way of studying the optical properties of microscopic structures such as quasifractal clusters. A different situation where the set-up is well suited, is for studying defects in e. g. thin films. Integrating the steps of both localisation of the area of interest on the sample and characterisation in one instrument is a great advantage. The main weaknesses of microscopic spectroscopy are the difficulty to measure absolute levels, and that only the specular signal is measured. A fractal structure is a structure that is repeated over several length scales, i.e. it is scale invariant.1 Random fractal structures are often used to describe surface roughness,2 colloidal aggregates,3 and other inhomogeneous materials. The optical properties of fractal structures have been studied theoretically and calculations have been performed using several different methods.47 The theoretical work has recently been reviewed by Shalaev.8 There are, however, few experimental results presented and this limits the comparability between theory and experiment. Lithographically produced square arrays of nanoparticles have been previously studied9'3 . Some of the experimental results from these studies showed absorption peaks described by theory. There is however little experimental work on fractal structures. In Sect. 2 below we describe our measurement system for microscopic spectrophotometry. Section 3 describes the preparation of our samples consisting of quasifractal gold particle clusters situated on a planar substrate. Theo- retical computations of the optical properties of small gold clusters show prominent absorption peaks in the visible wavelength range, where our measurement set-up is most sensitive. Experimental results and theoretical calculations are presented in Sect. 4. Further author information: (Send correspondence to J.J.) J J.: E-mail: jacob .jonsson©angstrom .uu.se Optical Diagnostic Methods for Inorganic Materials II, Leonard M. Hanssen, Editor, 98 Proceedings of SPIE Vol. 4103 (2000) 2000 SPIE. . 0277-786X/O0/$1 5.00 Downloaded from SPIE Digital Library on 22 Feb 2010 to 130.238.21.142. Terms of Use: http://spiedl.org/terms