NanaStructured Materials. Vol. 6, pp. 831-834.1995 Copyright Q 1995 Ekevier Science Ltd Printed in the USA. All rights merved 0965~9773195$9.50 + .OO
Zhang Lide and MO Chi - mei * of Solid State Physics, Academia Sinica Hefei 230031, P . R. China * Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026. P. R. China Institute
phenomena, which are reported for
observed for conventional coarse grain materials, materials, such as nanostructured 7, 7 - AloI.
silicon and amorphous these new luminescence terms
In nitride. phenomena
appearing impurity effect,
boehmite, we discussed in
have not been nanostructured
nanostructured and defect energy
( anatase), of
materials in levels in the
INTRODUCTION Optical properties of solid materials are closely related to their microstructure, such as electronic state, defect state and energy level structure etc. For nanostructured materials, the structure is quite different from conventional crystal and amorphous materials. Namely, they are composed of nanometer - sized particles and interfaces with a large volume fraction and in interfaces, the atomic arrangement and the bond configuration are more random. Therefore, nanostructured materials exhibit many new optical properties, which have not been observed in conventional crystals and amorphous materials. In this paper, photoluminescence characteristics of nanostructured materials were described and origins of new luminescence were discussed. EXPERIMENTAL Visible Light
Caused by Fe and Cr
Previous authors (1 - 2) reported that for Cr doped or Cr impurity contained conventional AlzO, the photoluminescence occured in the wavenumber range of 14500 to 11200 cm-‘. Our experimental results show that for nanostructured 9 and 831
P w 15750 11500 15750 11500 ZaxN Wavenumber km-l) Wavenunber ( cm-l 1 Fig. 1 The luminescence spectra of nanostructured AlsOI and boehmite with different Fe amounts : excitation wavelength of 476.5 nm; curve 1 - boehmite. particle diameter, d = 50 - 60 nm, 0. 142wt 96 Fe; curve 2 - q - Al&s - (one hind of 7 -Al+&), d = 15 nm, 0. OlQwt % Fe; curve3 q-Al& 7+a-Al&.
d=15nm. O.l4wt% Fe; curve4 - q-Al& d=15 nm, 4.61wt% Fe& curve d=15nm, O.l4wt% Fe; curve 6 - a-A&O,, d=84 nm, O.l4wt% Fe.
7 - AlaOr, besides above - mentioned fluorescence phenomenon, a new fluorescent band, p, with a peak position of - 570 nm is observed in the wavenumber range of 20000 cm-’ to 14500 cm-‘, as shown in curve 2 - 5 of Fig. 1. With increasing Fe3’, this band becomes much wider. Nanostructured boehmite exhibits only the p band (curve 1 in Figl) . For nanostructured a - A1203 the p band disappears and Cr induced fluorescent band becomes two sharp peaks (curve 6 in Fig. 1).
400 500 Wavelength
600 (nm )
Fig. 2 The ultraviolet fluorescent emission spectra of the nanostructured amorphous silicon nitride specimens, heat treated at different temperatures in a vacuum of 1.36 Pa: Curve 1: excitation wavelength of 250 nm; Curve 2: excitation wavelength of 350 nm.
400 450 Wavelength
500 (n m)
3 The luminescence spectrum of Fig. nanostructured TiOs ( anatase) at room temperature : excitation wavelength of 330 nm; the particle diameter = 19.8 nm.
LUMINESCENCE IN NANOSTRUCTURED MATERIALS
Spectra in the Range of Ultraviolet
to Visible Light
Experimental results show that for nanostructured amorphous silicon nitride (particle size = 15nm) six fluorescent bands appear in the range from ultraviolet to visible light (see Fig. 2). whose peak positions are 3.2, 2.8, 2.7, 2.4, 2.3 and 2.0 eV. respectively. However, for conventional amorphous Si&II bulks only one broad fluorescent band with a peak position of 3.5 eV can be found (3) . Recently, we observed that for nanostructured TiOa (anatase) a strong photoluminescence band with a peak position of 3.leV and two small luminescence bands occur at room temperature (see Fig. 3). However, for polycrystal coarse grain TiOz the luminescence band in the same wavelength range has not been observed (4) .
DISCUSSION Why are the photoluminescence spectra of nanostructured materials quite different from that of conventional materials? Namely, why do some new luminescence bands occur only in the luminescence spectrum of nanostructured materials? In order to answer this question, we consider that it is necessary to analyze the mechanisms of new luminescence bands appearing in nanostructured materials from the three points: (1)
In interfaces with a large volume fraction of nanostructured materials exist much more different kinds of dangling bonds and unsaturated bonds than in that of conventional materials and thus some defect energy levels can be formed in the energy gap for nanostructured materials. They may induce some new luminescence bands. For example, conventional SisN, shows only one luminescence band, but nanostructured amorphous silicon nitride presents six emission bands. Their mechanisms can be attributed to some defect energy levels in the energy gap, such as E (5). Si’, CSi-, =N-and Esi-sic (2)
Weber (6) pointed out that some transition - metals, such as Fe3’, Cr3+ and V3+etc., may cause some luminescence phenomena in the disordered system. Interfaces with a large volume fraction of nanostructured materials provide a big region with very low order degree. Therefore, Fe3’ contained nanostructured AhO3 can easily give out light in comparison with conventional Al303.
Selection rule on electronic
Because conventional materials possess translation periods and thus in k space, electronic transitions must follow the vertical transition rule and nonvertical electronic transitions are forbidden. This causes some electronic transitions from the excitation state to the low energy level not to take place, thus some luminescence bands cannot appear. However, in nanostructured materials, a large number of interfaces with more random atomic arrangements leads the translation periods in these regions to be substantially broken. Therefore, it is not suitable to describe the energy state by using k space. In other words, electronic transitions no longer follow the selection rule of electronic transitions. New luminescence bands caused by some electronic transitions, which are forbidden in conventional materials, can be observed for nanostructured materials. For example, no luminescence appears in conventional TiOa, but nanostructured TiOz presents luminescence phenomena. This is probably associated with the above - mentioned mechanism. In addition, the exciton luminescence induced by quantum confinement effect is also considered, when the diameter, d, of particles of nanostructured materials becomes very small. For example, when d of crystal Si <6 nm, luminescence appearing in the wavelength range from 700 to 900 nm can be attributed to this mechanism.
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