Synthesis and Raman Spectra of Cupric Oxide Quantum Dots
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100 ppm of any impurity content was detected. The crystal structure and composition of the specimens were analyzed by X-ray diffraction (XRD) using a Philips PW 1710 diffractometer equipped with a Cu K. X-ray generator under ambient conditions. The feature micrographs were obtained using a JEM-200CX transmission electron microscope (TEM), with a 160-kV voltage applied to the measurements. The average size of CuO quantum dots was measured and determined by TEM and XRD. The Raman spectra were recorded with a micro-Raman
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Two Theta (deg.) Figure 1. XRD spectrum of the CuO quantum dots. system, which was modified by coupling an Olympus microscope to a Spex 1704 spectrometer with a CCD detector. The excitation was provided by an argon ion laser operated at 488 nm. The laser power at the sample was estimated to be below 10 mW and the spectral resolution of the spectrometer is 0.5 cm"1. Heating of the sample was achieved by conduction using resistively heated tantalum wires. Cooling was achieved by thermal contact with a liquid nitrogen reservoir. The sample temperature was measured with a K-type thermocouple seated near the sample and controlled by a YCC1612 APC programmable temperature controller. RESULTS As shown in Figure 1, the XRD pattern from the sample is identical to that of pure CuO, without signals from CuCI2, NaOH, or other precursor compounds, indicating the formation of single phase CuO with a monoclinic structure. From the width of the XRD
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diffraction peaks we deduced a mean size of 12.1 nm using Scherrer's formula. The size distribution was determined by TEM measurements and is in good agreement with the results obtained from the XRD analysis. A TEM image of the CuO particles is presented in Figure 2, showing that the samples are composed of polyhedral nanocrystals.
Figure 2. A TEM micrograph of the CuO particles.
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Raman Shift (cm-1 ) Figure 3. Raman spectra of a) CuO quantum dots and b) after annealing in air at 1000 'C (average size >100 nm).
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Cupric oxide (CuO) belongs to the C 6 2h space group with two molecules per primitive cell. One can find for the zone center normal modes F =-4Au + 5Bu + Ag + 2Bg
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There are three acoustic modes (Au+2Bu), six infrared active modes (3Au+3Bu), and three Raman active modes (Ag+2Bg).
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Raman Shift (cm-) Figure 4. The Raman spectra of the CuO quantum dots at different temperatures, a) 77, b) 123, c) 173, d) 223, e) 300, f) 373,g) 473, h) 573, i) 673, j) 773, and k) 873 K. Figure 3a presents the Raman spectrum of nanocrystal CuO at room temperature. It can be seen that there are three Raman peaks at 282, 332, and 618 cm', with the second one being the weakest and the third being the most broad. In comparison with the vibrational spectra of single crystal CuO [9], we can assign the peak at 282 cm"1 to the Ag mode and the peaks at 332, and 618 cm' to the Bg modes. However, we note
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