INFLUENCE OF REACTION TEMPERATURE ON OPTICAL ...

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271By the experimental data of the PL spectra in Figure 4, values E g (0),α and β are 1.99 eV,6.6x10 -4 eV/K, 653(K) for CdSe cores and CdSe/CdS nanotetrapods are 1.97 eV, 6x10 -4 eV/K,646(K), respectively. We assume a constant exciton binding energy against T (Temperature), theredshift of excitonic emission line is the consequence of thermal expansion of the lattice andexciton-phonon interaction [7]. In particular, the large lattice mismatch between CdSe and CdSproduces strain effects at the interface lead to a red shift of the energy gap [8].The PL spectra of CdSe cores and CdSe/CdS nanotetrapods as a function of the excitationpower density are shown in Fig. 6(a) and 6(b), respectively. The set of spectra measured athigher illumination intensities is fitted using an additional peak for CdSe cores and CdSe/CdSnanotetrapod by a convolution of the Gaussian and Lorentzian functions.In TP-shaped CdSe NCs, because of type-II alignment, electron can be transferred from thearms to the core [10]. When the excitation power density is low, we observed one emission peakat 630 nm, due to transition electron and hole in the core. The excitation power density greater,we observed the second peak at wavelength of 651nm.In type-II, an additional separation of charges between the arms and the core occurs, givingrise to electron localization in the core region, which correspond to the transition arising from therecombination of electrons in the core and holes in the arms.14001200(a)14001200(b)10001000Intensity (a.u)800600400200Intensity (a.u)80060040020000-200600 650 700Wavelength(nm)-200600 650 700Wavelength(nm)Fig.7. Spectra sets were fitted using one and two exciton peaks of CdSe cores and CdSe/CdSnanotetrapods when excitation power density low (a) and high (b), respectively.As can see Fig 7, under force puissance of excitation, the best fitting results for the PL spectra ofCdSe NCs and CdSe/CdS nanotetrapods are obtained using two peaks and one peaks underpuissance weak excitation. It confirms that peaks at high and low energy are the emission X ofthe core CdSe and X arm arms CdSe, respectively. Additionally, CdSe/CdS nanotetrapods,injection precursor Cd 2+ , S 2- , nonradiative transitions related to impurities and structural defectslead to the emission of decreasing. It confirms that the emission peak at 632 nm and 658 nmrelate to the emission of CdSe core and CdSe 1-x S x arms.IV. C<strong>ON</strong>CLUSI<strong>ON</strong>Colloidal CdSe cores and CdSe/CdS nanotetrapods were synthesized and characterized bydifferent analytical techniques. The temperature dependence of the excitonic line is in goodagreement with the phenomenological models of the band gap red shift, is related both to thethermal expansion of the lattice and to the electron-phonon interaction. The dependence of PL552
Những tiến bộ trong Quang học, Quang phổ và Ứng dụng VI ISSN 1859 - 4271spectra on excitation power density has studied. At high excitation power density leads to theformation of X arm in CdSe and CdSe 1-x S x arms, respectively.V. ACKNOWLEDGMENTSThis work was supported by Vietnamese Ministry of Education and Training, and by the KeyLaboratory on Electronic and Materials Devices (Institute of Materials Science, VAST).REFERENCES[1] A. Sitt, F.D. Sala, G. Menagen, U.Banin, Nano Letters 9, 2009, pp. 3470-3476.[2] V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, Nature 447, 2007, pp. 441-446.[3] V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, M. G. Bawendi, Science 290, 2000, pp. 314-317.[4] M. Achermann, J. A. Hollingswoth, V. I. Klimov, Phys. Rev. B 68, 2003, pp. 245302.[5] V.I. Klimov, A. A. Mikhailovsky, D. W. Mc Brache, M. G. Bawendi, Science 287, 2000, pp. 1011-1013.[6] D. V. Talapin, J. H. Nelson, E. V. Shevchenko, S. Aloni, B. Sadtler, A. P. Alivisatos, Nano Lett. 7,2007, pp. 2951–2959.[7] Y. P. Varshini, Physica 34, 1967, pp. 149.[8] J. Lee, E. S. Koteles, M. O. Vassel, Phys. Rev. B 33, 1986, pp. 5512.[9] A. P. Alivisatos, Nat. Biotechnol 22, 2004, pp. 47–52.[10] L. Bornstein, New Series, Group III; Springer; Berlin, Vol.17b 1982.[11] L. Manna, D. J. Milliron, A. Meisel, E. C. Scher, A. P. Alivisatos, Nat. Mater 2, 2003, pp. 382–385.[12] L. Manna, E. C. Scher, A.P. Alivisatos, J. Am. Chem. Soc 122, 2000, pp. 12700–12706.[13] Y. Ding, Z. L. Wang, T. J. Sun, J. S. Qiu, Appl. Phys. Lett 90, 2007, pp. 153510.[14] B. Mahler, N. Lequeux, B. Dubertret, JACS 132, 2010, pp. 953-959.[15] A. Narayanaswamy, L. F. Feiner, A. Meijerink, P. J. Vander, ACSNano 3, 2009, pp. 2539-2546.[16] A. Narayanaswamy, L. F. Feiner, P. J. Van der Zaag, J. Phys. Chem.C 112, 2008, pp. 6775-6780.553
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