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APPLIED PHYSICS LETTERS 89, 153126 ͑2006͒
S. Brovelli, N. Chiodini, F. Meinardi, A. Lauria, and A. PaleariDepartment of Materials Science, University of Milano-Bicocca, Consorzio Nazionale Interuniversitario perle Scienze fisiche della Materia (CNISM), Via R. Cozzi 53, I-20125 Milano, Italy ͑Received 8 June 2006; accepted 29 August 2006; published online 13 October 2006͒ SnO2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping topassivate the cluster boundaries. As a result, emission at 3.8 eV from the decay of SnO2 freeexcitons is observed in nanostructured SnO2: SiO2, besides the extrinsic 2 eV luminescence ofdefects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of theextrinsic emission competitive with the ultraviolet ͑UV͒ luminescence evidences the involvement ofa phonon mode at 210 cm−1 from a SnO-like phase. The feasibility of passivated wide-band-gapnanocrystals in silica gives interesting perspectives for UV-emitting optical devices. 2006American Institute of Physics. ͓DOI: Wide-band-gap nanostructures are attracting consider- systems,obtaining the partial passivation of the nanocrystal able attention in the perspective of applications as light- surfaces. The analysis of the competitive extrinsic lumines- emitting devices in the ultraviolet ͑UV͒ region.Excitonic cence coexisting with the exciton emission clearly identifies emission in nanostructured materials is the key function in the involvement of an understoichiometric SnO-like phase that respect, together with the possibility of obtaining an op- tical material highly workable and compatible with the Optical-grade bulk samples ͑1 mm thick͒ of Er-doped silica-based technology. In this framework, tin dioxide—with silica with SnO2 nanoclusters were prepared by sol-gel tech- an optical gap of 3.6 eV—is a promising system for nique, cogelling tetraethoxysilane, dibutyl tin diacetate, and the production of wide-band-gap nanostructures in optical- erbium nitrate with dopant concentration of 1.0 mol % er- grade materials, since the growth of nanometer-sized SnO bium and 8 mol % Sn. After gelation and drying, the xerogel crystals in transparent silica glass has been recently proven sample was heated ͑about 3 ° C / h͒ in oxygen up to 1050 ° C with interesting optical results as regards to nonlinear to induce SnO2 nanoclustering and silica response,photosensitivity,laser photowriting,and rare Nanostructure morphology was analyzed by means of trans- mission electron microscopy ͑TEM͒ showing SnO2 nano- electron-hole recombination has been observed only in crys- crystals of few nanometers in size dispersed in the amor- phous silica matrix ͑Fig. . High resolution images of suspensions.A broad luminescence at about 2 eV is instead nanoclusters evidence single domain crystalline features with the only emission observed in nanostructured SnO2: SiO2 up to now.As a matter of fact, the opportunity to take advan- analysis of cluster sizes from several TEM images of differ- tage of nanometer-size effects for a high light-emission effi- ent sample regions ͑inset of Fig. shows a narrow distribu- ciency and tunability depends on the relative rate of exci- tion largely lying below the exciton Bohr radius ͑about tonic luminescence over all other decay paths activated in the system. Exciton self-trapping and competitive decay drivenby point defects and localized states at the boundaries of thenanostructures are the main effects to be minimized for atechnological application. In the case of SnO2 nanocrystalsin silica, a source of interphase defects is the structural mis-match arising from the different coordination features ofglassy silica ͑SiO tahedra͒, probably with the formation of an undercoordinatedtin oxide at the nanocrystal boundaries. Indeed, suboxideboundaries have been recently identified in other nanostruc-tured silica-based systems.In these cases, the identificationof the interphase and the passivation of the nanoparticle sur-faces may be the key for a substantial improvement of thematerial.
In the present work, we give the evidence of free-exciton emission from SnO2 nanocrystals embedded in a silica ma-trix. The result is achieved exploiting the erbium tendency ofdistributing at the interphase of nanostructured silica-based FIG. 1. TEM image of 1.0 mol % Er, 8 mol % SnO2 doped silica. Inset: ͑right͒ high resolution image of a SnO2 quantum dot and ͑left͒ histogram of a͒Electronic mail: alberto.paleari@mater.unimib.it the distribution of cluster size from TEM analysis.
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Appl. Phys. Lett. 89, 153126 ͑2006͒
FIG. 3. PL spectra in the visible range of 1.0 mol % Er, 8 mol % SnO2doped silica excited at 3.5 eV at different temperatures ͑from 80 to 290 K, FIG. 2. PL spectrum ͑open circles͒ of 1.0 mol % Er, 8 mol % SnO steps of about 25 K͒. Inset: thermal behavior of the normalized M ͑ glass ceramic excited at 4.7 eV. The Gaussian fits of the main spectral open circles͒ moments of the PL band.
components are drawn in solid curves.
of the competitive decay process, the extrinsic PL band has Photoluminescence ͑PL͒ measurements were carried out been analyzed as a function of the temperature, evidencing in two excitation configurations by means of ͑i͒ the fourth the coupling with a specific phonon mode of the responsible harmonic of a Nd:YAG ͑yttrium aluminum garnet͒ laser to phase. PL spectra excited at 3.5 eV at increasing temperature excite at 4.7 eV within the SnO2 band-to-band transition en- from 80 to 290 K are reported in Fig. The broad band of ergy range and ͑ii͒ the third harmonic of a Nd:YAG laser to defectlike luminescence dominates the spectra and it is ac- excite at 3.5 eV in the low-energy SnO2 absorption tail companied by a series of positive and negative narrow com- caused by localized and defect states in the nanoparticle in- ponents due to absorption and emission at the erbium transi- terphase. In the first configuration, UV and visible lumines- tions. The defect-related PL band shows a broadening and a cence were dispersed by a single grating monochromator shift to higher energies with increasing temperature. Infor- with 1.5 nm bandwidth and detected by a photomultiplier mation on the decay dynamics may be extracted from the tube; in the second configuration, the emission signal was analysis of the energy of the emission band fitted detected by a charge-coupled device camera coupled with a by a Gaussian band. The energy moments M0, M1, and M2 of polychromator with 1.5 nm bandwidth. All measurements the spectral distribution f͑E͒ of PL intensity are obtained, were corrected for the overall spectral response. Raman respectively, by calculating ͐f͑E͒dE ͑integrated inten- spectra were obtained at 300 K in backscattering configura- tion by means of an Ar+ laser at 488 nm with a resolution of E2f͑E͒dE M1 energy bandwidth͒.
The normalized zeroth moment M0 in inset of Fig. Figure shows the PL spectrum excited at 4.7 eV. The ͑filled circles͒ follows the expected temperature dependence, outstanding feature in the spectrum is the UV emission cen- with a decrease of luminescence efficiency at growing tem- tered at 3.9 eV, just as it is expected for the radiative recom- perature due to the activation of nonradiative decay bination of electron-hole pairs confined in SnO2 nanocrystals channels.A blueshift of the PL band is evidenced by M1 with 2 nm average according to data in inset of Fig.
values ͑open circles͒. The thermal behavior of the second In fact, from the values of nanocrystal radius R and exci- spectral moment M2 may be analyzed in the approximation ton reduced mass m* ͑about 0.27m of broadening dominated by electron-phonon coupling. In shift ⌬E of the exciton recombination energy with respect to this case, the total PL bandwidth is given by the following the bulk gap energy ͑3.6 eV͒ is ⌬E = ͑ប2/ 2m*͒ / ͑␲2/ R2͒ Ϸ0.3 eV. The 3.9 eV emission, never observed in undopedsamples, is thus the evidence of free-exciton decay in tin dioxide nanocrystals embedded in silica. The present data do not allow to give a reliable estimation of the conversion ef- ficiency. Anyway, this result suggests that erbium ions might where kB is the Boltzmann constant, ប␻ph is the mean energy play a passivating function on the defects responsible for the of the local mode coupled with the electronic transition, A the number and the linear coupling constant of vibrational Nevertheless, below 2.5 eV, the spectrum in Fig. also modes in Einstein’s oscillator model, and ␴inh accounts for shows the broad PL band from exciton trapping and decay in the temperature independent inhomogeneous broadening due localized with the superposition of narrow peaks to site-to-site disorder at the cluster boundaries. Looking at phonon mode involved in the decay process. Figure shows A weak emission band at 3.1 eV is ascribable to Sn-related M2 as a function of the temperature, together with the fit to oxygen vacancies in the silica matrix ͑␤ band͒as expected Eq. The resulting phonon energy ប␻ph is about from the dispersion of a small fraction of Sn atoms from 200 cm−1, consistent with the A1g mode of tin SnO2 nanocrystals to silica during UV laser excitation.
at 211 cm−1. Raman spectra on the same sample confirm the The presence of the defect-related PL component indi- presence of a vibrational peak at 210 cm−1 ͑inset of Fig. .
cates that the passivation is incomplete. To identify the origin These evidences concur in identifying a SnO-like phase Downloaded 08 Jan 2007 to 200.136.226.189. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Appl. Phys. Lett. 89, 153126 ͑2006͒
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