IEEETRANSACTIONS ON MAGNETlCS, VOL. 30, NO. 2, MARCH 1994 739
UV-VIS Spectra Of Small Iron Particles
M.C. Blanco, A. Mcira, *D. Baldomir, *J. Rivas, and M.A. L6pcz-Quintcla Chcmical Physics and *Applied Physics Dpis.,
University of Santiago E-15706 Santiago de Compostcla, Spain
A b s t r n c t - - UV-Vis absorption spectra of
u l t r a f i n e i ron p a r t i c l e s o b t a i n e d in
microemulsions were measured from 290 nm up
to 550 nm. It was observed the existence o f
three bands and a band shift towards higher
energy region by 0.1-0.2 eV compared with that
of bulk iron. This energy shift can be
interpreted in terms of a quantum size effect.
The electron-hole effective mass is evaluated to
be 0.20 times the mass of a bare electron.
and oxygen). According to our previous results [ 111 the size
of the particles produced in this way is controlled by the
radius of the droplets of the microemulsions used, and this
in turns depends on the ratio R = [H2O]/[AOT] through the
relationship: r (nm) = 1.00 + 0.24R [ 12.131.
Fig. 1 shows a TEM picture for particles obtained in a
microemulsion with R = 13. The mean size of the particles
obtained, r = 3.0nm, is approximately equal to the size of
the droplets used (r = 4.1 nm).
I. INTRODUCTION
There has been a great interest in the conductive and
magnetic properties of ultrafine particles and iron clusters in
recent years [l-41. A major feature of these studies is the
great difference observed between the properties of the bulk
material and these reduced systems.
On the other hand, the optical properties of different
kind of fine particles have been studied by many authors [5-
101 concluding that when the particle size becomes
comparable to the Bohr radius for electron-hole pairs,
quantization effects can be observed due to the quantum
confinement of charge carriers in the finite volume of the
particles.
In this paper we report the experimental results of the
optical absorption spectra of ultrafine (nanometer range) iron
particles obtained in microemulsions and discuss the
diferences between the optical properties of the small
particles obtained and those of the bulk iron from the view
point of the quantum size effect.
11. SAMPLE PREPARATION
Details of the sample preparation are described elsewhere
[ 1 I ] . In essential it consists in the mixture of two AOT-
based microemulsions (AOT = aerosol OT = bis-2-ethyl
hexyl sodium sulphosuccinatc) containing Feel2 and
NaBH4 respectively. To avoid oxidation of the particles the
solutions were deoxygenaicd and thc overall process was
carried out in an ineri glove box (less than lppm in water
Manuscript received May 3 1. 1993
Fig. 1: TEM micrograph of iron particles obtained in micmmdsions.
Magnification 250,000.
The structural characterization of the particles was
carried out by X-ray diffraction and the magnetic properties
were measured by SQUID and VSM magnetometries in the
temperaturc range 2 < T(K) < 350. The results werc
published elscwherc 1141. It is noticed that due IO the small
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