Electronic States of Mn 2+ in ZnS:Mn Nanoparticle Examined by EPR Measurements and Molecular Orbital Calculation
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Electronic states of Mn2+ in ZnS:Mn nanoparticle examined by EPR measurements and molecular orbital calculation *Sanshiro Nagare, Tetsuhiko Isobe, Mamoru Senna Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Takahiro Igarashi SONY corporation, 4-16-1, Okata, Atsugi-shi 243-0021, Japan ABSTRACT The discrete variational Xα method was introduced to elucidate the possible factors of luminescence enhancement of ZnS:Mn modified by poly acrylic acid. The effective charge calculation of Mn indicated that the ionicity of Mn and its first neighbors increase when Mn atom is located at the surface site rather than in the bulk, in accordance with the signals observed from EPR measurements. The bond order between Mn and S increased by the stronger exchange interaction when Mn was oxidized by coordination of carboxyl group rather than by adsorption of OH or a SH group. The bond order between Mn and O is higher for the same situation, which may attribute to the energy transfer from poly acrylic acid to ZnS:Mn. As a consequence, the symmetry of the d orbital decreased, judging from the contour maps of the molecular orbital. This relaxed the forbidden d-d transition of Mn2+ and resulted in the enhanced photo luminescence.
INTRODUCTION Research in the field of doped semiconductor nanocrystals has attracted interest in the recent years because of their unique optical properties and for novel applications to display devices [1]. The present authors’ group reported the enhanced photoluminescence (PL) of ZnS:Mn nanocrystal by modifying the surface with poly acrylic acid (PAA) [2]. EPR measurement showed two different signals for ZnS:Mn nanocrystal, i.e. signal I (g=2.0024, |A|=6.9mT) corresponding to an isolatedly substitutional site, and signal II (g=2.0013, |A|=9.0mT) corresponding to a surface site [2,3]. Energy transfer from PAA to ZnS:Mn plays one of the significant roles on increasing the PL intensity in addition to a quantum confinement effect. In this work, the electronic state of Mn2+ was observed in detail by using molecular orbital calculations, to elucidate the possible factors of the luminescence enhancement of ZnS:Mn nanocrystal modified by PAA. We focused on the bond order between Mn2+ and its first W6.4.1
neighbors to evaluate the efficiency of energy transfer processes from ZnS to Mn2+ and from PAA to Mn2+.
CALCULATION Molecular orbital calculations were performed by discrete variational X alpha method (DV-Xα) [4]. Three surface cluster models were used to calculate the electronic state of Mn2+ (Fig.1-a). Since the PL intensity increases when ZnS:Mn nanocrystal is modified by PAA [2], the on top sulfur was replaced by oxygen to elucidate the effect of oxygen on the luminescence efficiency. From infrared absorption and X-ray photoelectron spectroscopies, we concluded that ZnS:Mn nanocrystal modified by PAA is more likely oxidized by a coordination of carboxyl group, and not simply by atmospheric oxygen [2,3]. To take this into acco
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