Pressure Tuning Spectroscopy of Mn 2+ in Bulk and Nanocrystalline Sulfide Semiconductors
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Pressure Tuning Spectroscopy of Mn2+ in Bulk and Nanocrystalline Sulfide Semiconductors Randy J. Smith, Yongrong Shen, and Kevin L. Bray Department of Chemistry, Washington State University, Pullman, WA 99164 ABSTRACT We report the results of high pressure luminescence studies of the emission of Mn2+ in bulk Zn0.55Ga0.30S up to ~214 kbar, bulk ZnS up to ~184 kbar, and nanocrystalline ZnS (~8 nm particle sizes) up to ~300 kbar. We observed a strong redshift with pressure (-30(3) cm-1/kbar) for the 4T1→6A1 emission transition of Mn2+ in all three systems. We also observed emission quenching at high pressure in all three systems and attribute the quenching to a pressure induced phase change of the ZnS host lattice to a rocksalt (NaCl) phase. INTRODUCTION Mn2+:ZnS is widely used as a phosphor in cathode-ray tubes and electroluminescent displays. The high efficiency of Mn2+:ZnS has motivated efforts to shift or broaden the yellow 4 T1→6A1 emission of Mn2+ to achieve a wider range of colors for phosphor or filtered white light displays. Most attempts to tune the emission properties of Mn2+ in sulfide semiconductors have focused on chemically modifying the ZnS host lattice or preparing the nanocrystalline form. Recent work has shown that alloying ZnS with Ga leads to a red shift and broadening of the Mn2+ emission [1] and that higher excitation efficiencies are achievable in nanocrystalline Mn2+:ZnS [2]. In this paper, we discuss high pressure luminescence studies of the emission of Mn2+ in bulk ZnS, Zn1-3x/2GaxS (x = 0.30) and nanocrystalline ZnS. Our objective is to use pressure to systematically vary the coordination geometry, crystal field strength and electronic states of Mn2+ as well as the band structure of the host lattice in an attempt to understand the chemical and physical factors responsible for determining the emission properties of Mn2+ in sulfide semiconductors. In nanocrystalline material, pressure also provides a way to vary particle size and to explore size-dependent properties. MATERIAL SYNTHESIS Nanocrystalline Mn2+:ZnS powders were synthesized by a solution phase method [4,5]. We first prepared an aqueous solution containing 380 mL of 0.13 M Zn(CH3COO)2, 50 g sodium polyphosphate (Na(PO3)n) as a particle stabilizer, and 20 mL of 0.05 M Mn(CH3COO)2. After stirring at room temperature for ~15 min., a stoichiometric amount of 1 M Na2S was slowly added. The solution became opaque and white as nanocrystalline Mn2+:ZnS formed. After centrifuging and washing several times, the resulting nanocrystalline Mn2+:ZnS powder was dried in a rotovap at ~45 degrees. X-ray diffraction analysis indicated that the synthesized nanoparticle powders had a cubic zincblende structure. Bulk 1% Mn2+:Zn0.55Ga0.3S and 1% Mn2+:ZnS powders were prepared by a high temperature solid state reaction technique [3] and were kindly provided by Prof. D. A. Keszler. X-ray powder diffraction measurements on the asreceived samples indicated that both powders possessed a hexagonal wurtzite structure. G4.9.1
EXPERIMENTAL DETAILS Luminescence e
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