Surface modification on time-resolved fluorescences of Fe 2 O 3 nanocrystals B.S. Zou * , V. Volkov Laser Dynamics Laboratory, School of chemistry and Biochemistry, Georgia, Institute of Technology, Atlanta, GA 30332-0400, USA Received 2 December 1998; received in revised form 11 May 1999; accepted 18 August 1999 Abstract The time-resolved studies on the fluorescence properties of Fe 2 O 3 nanoparticles prepared by forcing hydrolysis and micro- emulsion techniques were carried out for the first time. Two different systems of different sized nanocrystals were characterized. Specifically Fe 2 O 3 5, 10 and 20 nm particles in water and Fe 2 O 3 5, 10 nm particles in toluene demonstrated different fluores- cence behavior. We discussed the observed phenomena in terms of surface related effects, attributing the crucial role in optical time response to the surface polarization in these systems. At the same time we clarify some controversial conclusions made in other related studies. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Time-resolved fluorescences 1. Introduction Most studies on nanoparticles dealt with their quantum confinement effect. In spite of the obvious contribution to the optical properties, surface effect is still a minor factor taken into consideration. In fact we expect the surface to be even more crucial in defining some optical properties for specific condensed systems in the nanosize domain. Metal and transition metal oxides (TMO) in low-dimension fabri- cations provide a unique opportunity to examine such surface related contributions. Being one of the new quantum solid materials with strong electron–electron correlation and electron–phonon coupling, TMOs are of the intense research interest in modern days [1]. Compared with other low-dimension materials they show unique properties different from their bulk physical and chemical properties, especially in their magnetic [2], band and/or electron transport characteriza- tion. There were several reports recently on the electronic structure, optical and phase properties of Fe 2 O 3 nanosize particles capped by a surfactant with strong quantum and dielectric confinement [3–5]. However, until now there are still some problems related to their carrier dynamics description. Fe 2 O 3 is a representative TMO compound. The contributions of their quantum confinement and surface effects are still unknown, so it is necessary to characterize them carefully. a-Fe 2 O 3 is a uniaxial crystal with electronic anisotropy. There are two octahedra connected face by face along the c- axis [6]. The optical bandgap of bulk ferric oxide is about 2.1–2.2 eV [7]. However, there is still weak absorption [8] below the bandgap. The 400–900 nm absorption was assigned to the local crystal field transition and pair excita- tions [7], whereas 250–400 nm absorption was considered to be a result of the charge-transfer transitions. Particularly Fujimori et al. [9] and Drager et al. [10] determined charge- transfer phenomena in Fe 2 O 3 , and related that to the Fe–Fe interaction between two FeO 6 octahedra through X-ray photoelectron spectra. Such interactions are considered as a result of the 4sp orbital populations and Fe valence reduc- tion. The enhancement of the 4sp populations in the valence band and increase of the electron–phonon coupling for nanoparticles [11], especially that capped by ionic surfac- tants, were observed recently by Zou et al. [4,5]. All these studies indicate that the valence band and conduction band of Fe 2 O 3 are made up of O(2p), Fe(3d) and Fe(4sp) orbitals accompanied by some structural distortions. Besides the transition below the bandgap, state mixing also exists above the bandgap due to the 4sp electron’s participation. As a result of such complicated electronic structures and Journal of Physics and Chemistry of Solids 61 (2000) 757–764 0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(99)00266-8 www.elsevier.nl/locate/jpcs * Corresponding author.