Photoluminescence in quantum-confined SnO 2 nanocrystals: Evidence of free exciton decay E. J. H. Lee, C. Ribeiro, T. R. Giraldi, E. Longo, E. R. Leite, and J. A. Varela Citation: Applied Physics Letters 84, 1745 (2004); doi: 10.1063/1.1655693 View online: http://dx.doi.org/10.1063/1.1655693 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/10?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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J. H. Lee, C. Ribeiro, T. R. Giraldi, E. Longo, and E. R. Leitea) Laboratório Interdı́sciplanar de Electroquimica & Ceraˆmica–Centro Multı́disciplinar de Desenvolvimento de Mâteriais Cerâmicos–Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, Rodovia Washington Luiz, km 235, Caı´xa Postal 676, Codigo de Enderecamento Postal 13565-905, São Carlos, Sa˜o Paulo, Brazil J. A. Varela Laboratório Interdı́sciplanar de Electroquimica & Ceraˆmica–Centro Multı́disciplinar de Desenvolvimento de Mâteriais Cerâmicos–Instituto de Quı´mica, Universidade Estadual Paulista, R. Professor Francisco Degni, s/n Araraquara, Sa˜o Paulo, Brazil ~Received 13 October 2003; accepted 7 January 2004! Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2 . The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process. ©2004 American Institute of Physics. @DOI: 10.1063/1.1655693# Research on semiconductor quantum dot~QD! systems has been of great interest for the last several years. This is mainly due to novel physical and chemical properties dis- played by these nanometer-scale structures. The extensive experimental and theoretical studies done on III–V~e.g., GaAs and InP! and II–VI ~e.g., Cd chalcogenides! semicon- ductors resulted in solid knowledge of the theory of QDs.1–3 Recently, there has been growing interest in the properties of wide-band gap oxide QD semiconductors, such as nanocrys- talline SnO2. This size-dependent linear and nonlinear opti- cal properties are desirable for applications such as photonic devices and UV-lasing systems. Several studies have re- ported the optical properties of SnO2 QDs, but none has presented photoluminescent emission related to free exciton decay, which is necessary for application to UV-lasing systems.4–7 On the other hand, luminescence related to free exciton decay in bulk SnO2 has already been reported.8 When the dimensions of nanocrystalline particles ap- proach the exciton Bohr radius (aB), a blueshift in energy is observed due to the quantum confinement phenomenon. The effective mass model is commonly used to study the size dependence of optical properties of QD systems. In this ap- proach the exciton is treated analogously to a hydrogen atom, but is limited by spatial confinement. Therefore the energy of the system is obtained by solving the proper Schro¨- dinger equation. In this manner two regimes are defined ac- cording to the coupling of motion of the electron and the hole in the exciton: weak and strong confinements. In the former the particle size is larger thanaB , and the electron and the hole are treated as a correlated pair. The blueshift of the band gap energy is described by Eq.~1!, whereEg eff is the effective band gap energy,Eg is the bulk band gap energy,R is the particle radius,\ is the Planck constant over 2p ~i.e., approximately 1.0546310234 J s in MKS units!, andm is the effective reduced mass. Eg eff5Eg1 \2p2 2mR2 . ~1! In the latter, the QD radius is belowaB , the electron and hole motion is not correlated and the shift in the band gap energy is given by Eq.~2!, calculated by the variational method given by Brus,9 Eg eff5Eg1 \2p2 2mR22 1.8e2 «R 1¯ . ~2! Tin dioxide has bulk band gap energyEg of 3.6 eV, static dielectric constant« of approximately 14 and the effective reduced massm may be replaced by the electron effective mass (me* 50.275m0), sinceme* !mh* (me* andmh* are the electron and hole effective masses, respectively!.8,10 There- fore the calculated electron Bohr radiusaB is approximately 2.7 nm. In this work we demonstrate the size dependent pho- toluminescence properties of SnO2 nanocrystals and the good agreement between the blueshifts in energy predicted by the effective mass model and the experimental results. The results indicate that the luminescence emitted is related to a free exciton decay process. Details of the experimental procedure to obtain SnO2 nanocrystals is described elsewhere.11 The synthesis reaction a!Author to whom correspondence should be addressed; electronic mail: derl@power.ufscar.br APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 10 8 MARCH 2004 17450003-6951/2004/84(10)/1745/3/$22.00 © 2004 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:29:33 http://dx.doi.org/10.1063/1.1655693 consisted of the hydrolysis of SnCl2 2H2O ~Mallinckrodt! in ethanolic solution~Merck absolute ethanol!. This procedure enables one to obtain nanocrystalline SnO2 at room tempera- ture. Cleaning of the chloride ions was performed by dialysis in de-ionized water. The final product is a colloidal suspen- sion of nanocrystalline tin dioxide, which we further refer to as pure synthesized SnO2. Nanocrystals of different sizes were synthesized by making small changes in the procedure. First, the addition of surfactant tetrabutyl ammonium hy- droxide ~TBA, aqueous solution 0.4 M from J. T. Baker! enabled us to obtain smaller particles, since its stereochemic and electrostatic effects prevented particle growth by the grain rotation induced grain coalescence~GRIGC! mechanism.11–13The product of this synthesis procedure will be referred to as TBA-added SnO2. In order to prepare par- ticles of larger size, hydrothermal treatment was carried out at 200 °C for different periods of time~5, 24, 48 and 100 h!. The dependence of the photoluminescence properties on the particle size was studied using six specimens. The morphol- ogy and particle size distribution were characterized by a 200 kV transmission electron microscope~Philips CM200!. The samples for transmission electron microscopy~TEM! were prepared by wetting carbon-coated copper grids with a drop of the colloidal suspensions for 20 s, followed by drying in air. Photoluminescence~PL! spectra were collected by a Jobin-Yvon, Inc. Fluorolog model FL3-12 fluorimeter. A Xe lamp was used for excitation with the photon wavelength fixed at 250 nm. The PL spectra were collected in the 250– 400 nm range with a photomultiplier tube detector. Optical spectra in the ultraviolet and visible light ranges~UV-vis! were collected by Perkin-Elmer equipment. All optical mea- surements were done in the colloidal suspensions at room temperature. Figure 1 shows high-resolution transmission electron mi- croscopy~HRTEM! images and a selected-area diffraction ~SAD! pattern of the pure synthesized SnO2 nanoparticles. The SAD patterns of all the samples were characteristic of polycrystalline samples and could be indexed to the rutile structure of the cassiterite phase of tin dioxide. From the HRTEM images it can be observed that there are two types of morphology displayed by the nanoparticles:~1! nearly spherical and~2! elongated oriented particles. The presence of elongated oriented particles is an indication that particle growth occurs by the GRIGC mechanism. The TBA-added SnO2 sample had a smaller mean particle radius and almost no elongated particles at all, due to the stereochemic and electrostatic effects of TBA. Table I presents the values of the mean particle radii, which were estimated from the par- ticle size distributions obtained by measurement of over 200 particles in the TEM images. The lower left inset of Fig. 1 shows a HRTEM image of an isolated nearly spherical tin dioxide particle. The separation between lattice planes esti- mated from this image was approximately 3.35 Å, which corresponds to the$110% plane family, and 2.64 Å, which can be ascribed to the$101% plane family. Figure 1~c! illustrates a SnO2 nanoparticle with an elongated morphology due to hydrothermal-induced growth. The results of optical characterization are displayed in Fig. 2. It can be observed from the UV-vis measurements @Fig. 2~a!# that the TBA-added sample shows a blueshift at the onset of absorption, compared to the purely synthesized sample. The opposite occurred for the hydrothermally treated samples. This effect can readily be ascribed to the effect of particle size ~i.e., a quantum confinement phenomenon!, since the mean particle size is close to the value of the exci- ton Bohr radius. The same behavior can be seen in the pho- toluminescence characterization@Fig. 2~b!#. The decrease in particle radius created a blueshift in the photon energy emit- ted. The PL spectra consisted of single broad bands peaked at energies greater than the bulk band gap energy for all samples. The PL was recorded up to 400 nm, and no other band could be observed. Previous work in the literature has reported PL emission at energies below 3 eV, indicating that the emission process did not occur by free exciton electron- hole recombination. It is usually thought that no band-to- band transition luminescence is observed due to the presence of point defects, such as oxygen vacancies.6,7 Assuming that the maximum intensity photon energy in the PL spectra corresponds to the band gap energy, it was possible to study the effect of the particle radius on the band gap energy of the samples~Table I!. This assumption is quite reasonable, since we are interested in observing the behavior of the mean particle size. Figure 3 shows the experimental and theoretical size dependence of the effective band gap FIG. 1. TEM characterization:~a! Low magnification TEM image overview; ~b! HRTEM image of an isolated nearly spherical nanoparticle;~c! HRTEM image of an elongated grown particle, and~d! SAD pattern of an agglom- eration of nanoparticles. The SAD pattern can be indexed to the rutile phase of SnO2 . TABLE I. Effect of modification in the experimental procedure on the mean particle radius and effective band gap energy of SnO2 nanoparticles. Specimen Mean particle radius ~nm! Effective band gap energy ~eV! Purely synthesized 1.75 4.01 TBA added 1.50 4.05 Hydrothermal 200 °C/5 h 1.99 3.94 Hydrothermal 200 °C/24 h 2.59 3.71 Hydrothermal 200 °C/48 h 4.26 3.69 Hydrothermal 200 °C/100 h 4.16 3.68 1746 Appl. Phys. Lett., Vol. 84, No. 10, 8 March 2004 Lee et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:29:33 energy. It can be observed that there is very good agreement between the experimental results and the theoretical predic- tions. Most of the samples fit the weak confinement regime curve very well. The main difficulty in separating the weak and strong confinement limits in SnO2 is due to the relatively small exciton Bohr radius compared to that in most II–VI and III–V semiconductors. Some of the samples presented a mean particle radius close to that of the exciton Bohr radius. In this region the confinement regimes do not behave signifi- cantly different and both fit the experimental data well. To observe characteristic strong confinement behavior it would be necessary to prepare smaller particles, which is very dif- ficult. The effective mass model assumes that a blueshift in the band gap energy occurs due to spatial confinement of an exciton. Hence to generate a free exciton, energy higher than the effective band gap energy must be available. In the ab- sence of additional levels introduced by defects, radiative electron-hole recombination of this free exciton should result in photon emission with energy equivalent to the band gap energy. Therefore, the agreement between the predictions of this model and the experimental results indicates that the PL observed for the SnO2 QDs is related to a free exciton decay process. The nanocrystalline tin dioxide particles prepared in this work showed PL emission compatible with the needs of UV- lasing application. The size dependence of the PL emitted photon and the band-to-band~free exciton decay! lumines- cence are desired characteristics for this kind of application, since it would be possible to tailor the lasing wavelength. The size dependence behavior of the band gap energy indi- cated that the photoluminescence occurred due to a free ex- citon decay process. It was also shown that it is possible to have a certain amount of control over the particle size of QDs. Further studies on the growth mechanisms of SnO2 nanoparticles are currently being done in order to enable the production of these quantum dots. The authors would like to acknowledge the Brazilian funding agencies FAPESP and CNPq. 1S. V. Gaponenko,Optical Properties of Semiconductor Materials~Cam- bridge University Press, Cambridge, 1998!, Chaps. 2 and 3. 2T. J. Bukowski and J. H. Simmons, Crit. Rev. Solid State Mater. Sci.27, 119 ~2002!. 3N. N. Ledentsov and D. Bimberg, J. Cryst. Growth255, 68 ~2003!. 4B. Yu, C. Zhu, and F. Gan, Opt. Mater.~Amsterdam, Neth.! 7, 15 ~1997!. 5X. Wu, B. Zou, J. Xu, B. Yu, G. Tang, G. Zhang, and W. Chen, Nano- struct. Mater.8, 179 ~1997!. 6N. Chiodini, A. Paleari, D. DiMartino, and G. Spinolo, Appl. Phys. Lett. 81, 1702~2002!. 7F. Gu, S. F. Wang, C. F. Song, M. K. Lu¨, Y. X. Qi, G. J. Zhou, D. Xu, and D. R. Yuan, Chem. Phys. Lett.372, 451 ~2003!. 8V. F. Agekyan and Y. A. Stepanov, Sov. Phys. Solid State34, 266 ~1992!. 9L. Brus, J. Phys. Chem.90, 2555~1986!. 10K. J. Button, C. G. Fonstad, and W. Dreybrodt, Phys. Rev. B4, 4539 ~1971!. 11E. R. Leite, T. R. Giraldi, F. M. Pontes, E. Longo, A. Beltran, and J. Andrés, Appl. Phys. Lett.83, 1566~2003!. 12R. Lee Penn and J. F. Banfield, Science281, 969 ~1998!. 13D. Moldovan, V. Yamakov, D. Wolf, and S. R. Phillport, Phys. Rev. Lett. 89, 206101~2002!. FIG. 2. Optical characterization of the SnO2 colloidal suspensions:~a! Ab- sorbance spectra measured by UV-vis of nanoparticles of different size and ~b! photoluminescence emission spectra of excitation ofl5250 nm. All measurements were carried out at room temperature. FIG. 3. Experimental and theoretical predictions of the behavior of the band gap energy as a function of the QD size. Experimental mean particle sizes were estimated from TEM images. The exciton Bohr radius (aB) separates the strong and weak confinement regimes. 1747Appl. Phys. Lett., Vol. 84, No. 10, 8 March 2004 Lee et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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