Zinc Selenide Quantum Dots

Chapter 3 Zinc Selenide Quantum Dots 3.1 Introduction: Nanoscale materials have been extensively studied in recent years to study the evolution of the electronic structure from molecules to bulk. The properties or the electronic structure of nanoparticles is not a simple extrapolation of its corresponding molecular or bulk state. Quantum dots show unique features arising from the variation in crystal structure parameters, discretization of electron energy levels, concentration of oscillator strength for particular transitions, high polarizability of electron energy levels and increased surface to volume ratio [1-4]. Quantum confinement in semiconductors leads to discrete transitions that are blue shifted in energy from the bulk. Inhomogeneous broadening of the optical spectra due to size distribution and shape variations of the nanoparticles, conceal the fine structure in the energy states of quantum dots. In order to study the evolution of the electronic, optical and structural properties of material with size it is essential to synthesize nanoparticles of different sizes (with narrow size distribution) and controlled surface properties. II-VI semiconductors are relatively easy to synthesize, yielding free standing colloidal quantum dots by wet chemical routes. The evolving electronic structure as a function of size can be determined by non-contact optical probes in case of direct band gap II-VI semiconductors. The quantum size effects in CdSe nanoparticles with wurtzite structure are thoroughly investigated [5], while relatively less attention is being paid to other II-VI nanocrystals such as zinc selenide or zinc oxide. ZnSe is a II-VI direct band gap, zinc blende semiconductor with a bulk band gap of 2.7 eV at room temperature. ZnSe has a bulk Bohr exciton radius of 4.5 nm and bulk 49 exciton binding energy of 18 meV [6]. ZnSe is an attractive material to fabricate optoelectronic devices such as light emitting diodes or laser diodes in the blue to UV spectral range. Few reports demonstrate the possibility of fabricating such devices [7, 8]. The synthesis and characterization of ZnSe nanoparticles [9-23] have been reported. The earliest report on ZnSe nanoparticles by Chestnoy et al. [9] describes the synthesis of the nanocrystallites in alcoholic as well as aqueous media. Along with a strong transition for the lowest excited state, a weaker transition at higher energy was observed which was attributed to spin orbit splitting in the valence band. These nanoparticles were stable only at low temperatures. Li and Nogami [10] prepared ZnSe nanoparticles by dip-coating method for sol-gel derived complex solution. A strong photoluminescence peak observed at around 600 nm is attributed to Se vacancies. ZnSe nanoparticle formation embedded in glass prepared from supersaturated glass solution by annealing and rapid quenching are reported [12, 17]. The quantum confined band edge emission and the size dependence of two excited state energies, the spin orbit interaction, and the red shifted emission were observed for the first time in the ZnSe quantum dots in a glass matrix. Naturally formed self organized quantum dot arrays (QDAs) of ZnSe were grown by Stranski-Krastanov mode by Zhang et al. [13]. The confinement of carriers results from the difference in the band gaps of the strained ZnSe layer and the strain-relaxed ZnSe QDA. Liao et al. [14] have fabricated ZnSe quantum dots in a Volmer-Weber mode (an island growth) employing metalorganic chemical vapor deposition. Band gap emission at about 3.1 eV has been observed at 10 K for smaller crystallites. Kumbhojkar et al. [15] prepared free standing ZnSe quantum dots in an aqueous media capped by sodium hexameta phosphate showing luminescence at about 440 nm. Self activated luminescence due to Zn vacancies was seen in aged and uncapped ZnSe quantum dots. Highly luminescent in UV-blue region, and relatively monodisperse, colloidal ZnSe nanocrystals having HOMO-LUMO gap between 2.8 eV and 3.4 eV were synthesized from organometallic precursors in a hot alkylamine 50 coordinating solvent by Hines et al.[\6]. Lomascolo et al. [19] followed the same method to synthesize ZnSe/ZnS core shell nanoparticles. Quinlan et al. [20] have used inverse micelles technique using ion exchange reaction to produce monodisperse, cubic ZnSe quantum dots. Zhan et al. [21] have carried out a reaction in which zinc powder reacts with equivalent amount of elemental selenium to produce a complex which can be converted into wurtzite ZnSe nanoparticles. Zhu et al. [22] synthesized ZnSe nanoparticles of about 3 nm diameter by sonochemical irradiation of an aqueous solution of selenourea and zinc acetate under argon. Kim et al. [23] have grown self- assembled ZnSe quantum dots on a ZnS matrix in the Stranski-Krastanov mode using metalorganic chemical vapor deposition technique. Quantum confined carriers in the ZnSe quantum dots showed narrowing of the photoluminescence line width and a large red shift of the emission peak energy with increasing temperature. In the present study, ZnSe nanoparticles were synthesized by multiple injections of Zn and Se precursors in a hot coordinating solvent hexadecyl amine (HDA). Different sizes of the nanoparticles were obtained by injecting the precursors at different temperatures. Cubic zinc blende crystallites with sizes ranging from 2.5 nm to 4.5 nm with well passivated surfaces and hence showing only band gap luminescence were obtained. Low temperature photoluminescence excitation (PLE) measurements are carried out to determine the energy of the higher excited states in ZnSe nanoparticles. PLE studies at 10 K reveal up to three different excited states showing size dependent variation. Tentatively, these transitions were attributed using effective mass approximation. 3.2 Experiment: Study of quantum size effects requires synthesis of highly monodisperse nanocrystallites with good crystallinity, and electronic as well as chemical passivation of its surface. In the present study, ZnSe nanoparticles were synthesized according to the method described by Hines et al. [16] and modified by the method of Peng et al. 51 [24] to achieve different sizes. In this procedure a high boiling point solvent, hexadecylamine (HDA) was dried and degassed under vacuum at 125°C for few hours. This solution was purged with nitrogen gas during this process. HDA was heated to 310°C under nitrogen atmosphere. Appropriate amounts of diethyl zinc and tri octyl phosphine selenide were diluted with 2 ml of tri octyl phosphine (TOP). The Zn and Se precursors in TOP solution was divided into two parts of equal amount and injected into the hot HDA at different temperatures depending on the desired size. The smallest size was obtained by injecting at a temperature of 260°C while the largest size was obtained by injecting at 300°C. In the case of the smallest sized samples heating was stopped within few seconds of the injection whereas for higher sizes heating was continued for few minutes after which the solution was allowed to cool slowly. At 80°C butanol was added to the solution to prevent HDA from solidifying at room temperature. Proper amount of methanol was added to the cooled solution for precipitation of the nanocrystals. The ZnSe nanocrystals were isolated and washed with butanol. The isolated nanocrystals were dried in vacuum and dispersed in hexane for characterization. The solvents butanol, methanol and hexane were used after distillation by standard procedures. Elemental analysis of ZnSe quantum dots was carried out using energy dispersive x-ray analysis. Studies indicate presence of Zn and Se in the samples. X ray diffraction (XRD) measurements were obtained using Philips PW 1840 powder x ray diffractometer, using Cu K<j (A. = 1.542 A) as the incident radiation. Transmission electron microscopic (TEM) measurements were carried out on JEOL- 2000FX at 200 kV. The measurements were carried out by putting a drop of ZnSe dispersed in hexane on a Cu mesh. Optical absorption measurements were carried out to study the quantum size effects in the ZnSe nanoparticles using a Hitachi - 330, double beam spectrophotometer at room temperature. Luminescence measurements were carried out using an assembled photoluminescence set up with Jobin-Yvon 450 W Xe lamp, TRIAX 180 monochromators, photomultiplier tube and Janis CCS-150 cryostat. The schematic of the complete photoluminescence setup is shown in figure 3.1. 52 Computer Xe Lamp Excitation Monochromator 4 Sample chamber and Cryostat f Emission He Lines Data PMT Monochromator Acquisition System Compressor Figure 3.1: Schematic diagram of the photoluminescence setup. The focal length of both the monochromators was 0.19 m while their resolution was 0.1 nm. Light from the Xenon arc lamp is incident on the entrance slit of excitation monochromator. Monochromatic light from the excitation monochromator is focused on the sample kept in the cryostat using quartz lenses. Light emitted from the sample is collected by quartz lenses and focused onto the entrance slit of emission monochromator. The PMT placed at the exit slit of the emission monochromator detects the light and the signal from the PMT is input to the data acquisition system. In all the experiments the overall resolution of the photoluminescence setup was kept at -0.5 nm. The sample was placed between two quartz plates mounted on a closed cycle helium 53 cryostat (Janis CCS-150). The photoluminescence (PL) and photoluminescence excitation (PLE) measurements were carried out at 10 K. 3.3 Results and Discussion:

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