NANO LETTERS
Steady-State Photoluminescence Characteristics of Sb-Doped AgI Thin Films
2002 Vol. 2, No. 9 975-978
P. Senthil Kumar and C. S. Sunandana* School of Physics, UniVersity of Hyderabad, Hyderabad 500 046 India Received April 30, 2002; Revised Manuscript Received June 5, 2002
ABSTRACT The photoluminescence (PL) spectra of vapor quenched Ag−Sb thin films progressively iodized under ambient conditions have been measured for various excitation energies. At lower concentrations (1% and 5%), Sb doping enhances the emission intensity by way of binding to surface defect sites that otherwise would trap excited electrons and prevent fluorescence. At higher concentration (13%), excess Sb is available, even after saturating the initial traps, to quench the radiative emission and thus decrease the quantum efficiency. In this way, Sb addition effectively blocks direct charge recombination but without affecting the decay characteristics relative to that of undoped AgI.
Introduction. The optical properties of semiconductor microand nanocrystals are of much current interest, as they provide information about the evolution of physical characteristics from atom to bulk and about the three-dimensional confinement of carriers and excitons.1,2 Further, when the particle size is reduced to exciton Bohr radius dimensions, electrons and holes may become individually confined.3 At these dimensions, the size and shape of the particles invariably determines the extent of confinement and plays an important role in determining the electronic energy states of the semiconductor.3 Most interestingly, surface effects on excitons by way of quantum confinement and exciton trapping occur, which may be observed even at ambient preparative conditions.4 Recent studies have used experimental techniques such as pico- and femtosecond laser spectroscopy5,6 and pulse radiolysis7 to study the dynamics of silver atom cluster formation on the surface of the silver halide nanoparticles, a phenomenon basic to the photographic process.8 Thus, understanding the different pathway mechanisms of the excited state electrons is a singularly important task in further improving the efficiency of modern photographic films. Fluorescence is a useful tool for monitoring electronic changes at the nanoparticle surface. Both steady state and dynamic fluorescence spectroscopy have been used in the past to elucidate the electronic structure of I-VII semiconductor nanoparticles as a function of changing the solvent system or surface adsorbates.5-8 One way to study the electronic properties in AgI nanoparticles is to observe changes in their luminescence properties in the presence of electron or hole acceptors. Since nanoparticles by nature have * Corresponding author. E-mail:
[email protected]. 10.1021/nl020206t CCC: $22.00 Published on Web 07/27/2002
© 2002 American Chemical Society
a high surface-to-volume ratio, many of their optoelectronic characteristics are related to the nature of the surface. Mochizuki’s group have synthesized both quasi-free AgI nanoparticles (3-9 nm diameter) and thin films onto a substrate held at a temperature > 420 K (the wurtzite/zinc blende to disordered bcc superionic structural phase transition temperature) and attributed the maxima in their in situ photoluminescence characteristics to free exciton emission and other weak shoulder peaks arising from shallow exciton traps involving electron-phonon interactions or crystalline defects and impurities.9,10 In a similar way, carefully prepared Cu-doped AgI thin films also facilitates the excitonic luminescence.11 In gelatin- and polymer-stabilized AgI colloids (>30 nm diameter), the radiative excitonic recombination is dramatically reduced and donor-acceptor recombination is favored, which implies that the gelatin/polymer synthesis of AgI might introduce a large concentration of donor-acceptor states or lattice imperfections causing the excitons to become trapped more quickly in the smaller samples.12-14 In the femtosecond study of photoinduced electron dynamics of AgI and AgI/ Ag2S colloids, the electrons decay by a double exponential with time constants of 2.5 ps and >0.5 ns.6 While these are examples of direct synthesis of AgI colloidal nanoparticles, there are thin film strategies in which the precursor films could be carefully synthesized first for subsequent halogenation11,15,16 analogous with the “bottomup” procedure for ease in preparation of semiconductor nanoparticles.17 It has been demonstrated that surface-active additives can alter the thin film morphology for both semiconductors as well as metals.18 The additive can act to change either the surface free energy of the growing layers or the growth kinetics. In the present case, Sb is chosen as
Figure 1. SEM micrographs clearly depicting the effect of Sb doping in the island formation of AgI clusters. (a) 260 Å thick AgI film, (b) 150 Å thick Ag0.99Sb0.01I film, (c) 150 Å thick Ag0.95Sb0.05I film, and (d) 275 Å thick Ag0.87Sb0.13I film. All the films were iodized for 5 h. Different thickness films have been chosen for (a) and (d) to conveniently show the cluster formation.
a substitutional additive to Ag, because of their fundamentally different dielectric properties and nonlinear response in metal nanocluster glass composites.19 It turns out that the randomly deposited Sb atoms act as nucleation centers for the formation of Ag islands,20 which demonstrates clearly the importance of alloy thin films in AgI nanoparticle growth utilizing effectively the reversible place-exchange mechanism of Sb and Ag.18 The purpose of this work was to investigate the ambient excitation energy dependent, steady-state cw PL spectra of AgI- and Sb-doped AgI thin films, their dopant (Sb) concentration dependence, and to discuss the observed charge carrier decay and relaxation phenomena. Experimental Section. Three compositions (wt %) of Ag-Sb, viz. 99%Ag-1%Sb, 95%Ag-5%Sb, and 87%Ag13%Sb, respectively, were chosen from the Ag-Sb phase diagram21 so that a repeated random condensation procedure results in Ag-rich islands incorporated with Sb.18,20 The fabrication procedure and the details of structural, microstructural, and optical characterization of both the precursor and the iodized films were discussed in our earlier work.22 The photoluminescence spectra were collected using a high-pressure Xenon lamp as the excitation source on a HITACHI F-3010 model spectrophotometer for various excitation energies under ambient conditions. Results and Discussion. As the slower attack of halogen vapors on silver surface yields strongly oriented layers of the halide lattice, Sb addition unmistakably favors the oriented γ-AgI cluster growth right from the nucleation stage itself, which is strongly supported by the average distribution of particle sizes (40-70 nm) calculated separately from SEM (Figure 1) and XRD using the Debye-Scherrer formula.22 Retarded iodization rates (∼3 times the duration compared to that of undoped Ag films) and nanoparticle “decoration” emerge as the most spectacular effects of Sb-doping on AgI particle growth. The 420 nm exciton peak, sharp rise in 976
Figure 2. UV-vis absorption spectra clearly showing the increased absorbance for 1% and 5% Sb-doped AgI films along with the drastically reduced absorbance for 13% Sb-doped AgI films as compared with undoped AgI films. Inset shows the spectra in exciton absorption region. All films are of 150 Å thickness and iodized for 24 h.
absorption below 320 nm, and the long wavelength tail due to the intrinsic Frenkel disorder, all systematically characterize the very basic process of the band structure formation of AgI starting from the nanocluster level (Figure 2). In Figure 3a-d displayed are the PL spectra of the representative thin film samples AgI, Ag0.99Sb0.01I, Ag0.95Sb0.05I, and Ag0.87Sb0.13I obtained for various excitation energies. A careful examination of these spectra reveals that the enhanced emission intensity of 1% and 5%Sb-doped AgI films relative to that of undoped AgI is due to the binding of Sb to surface defect sites that otherwise would trap excited state electrons and prevent fluorescence. At 13% Sb concentration, the excess Sb that remained after saturating the initial traps could further quench the radiative emission, but did not affect the lifetimes effectively. In this way, Sb effectively blocks charge recombination and decreases the fluorescence quantum efficiency at higher concentrations but does not affect the decay characteristics at all concentrations. This is in accordance with the fact that presence of higher valent dopant cations strongly reduces the iodination rate of silver under normal conditions. Iodide film growth is mainly controlled by the migration of electron holes across the growing film on both Ag- and Sb-doped Ag systems.23 The luminescence spectra exhibit five prominent emission bands, the first of its kind to the best of our knowledge obtained in AgI thin films under ambient conditions. The first weak fluorescence band at around 440 nm is red shifted from the excitonic absorption peak and does not appear to be from the bottom of the conduction band, thus possibly corresponding to a shallow trap state or intrinsic near band edge state slightly below the conduction band. As the quantum yield associated with this weak fluorescence is estimated to be e0.05%,8,22 the vast majority of the charge carriers, i.e., electron-hole pairs, decay primarily via nonradiative pathways (shown by the remaining subbands). Due to strong binding of the electron-hole pair,6,9 the nonradiative decay is attributed to fast electron-hole (D-A) recomNano Lett., Vol. 2, No. 9, 2002
Figure 3. PL emission spectra of the AgI- and Sb-doped AgI thin films (same as that in Figure 2) for four different excitation energies. In a-d, the plots are represented by Ag0.99Sb0.01I, Ag0.95Sb0.05I, AgI, and Ag0.87Sb0.13I films from top to bottom, showing clearly the most intense, inhomogeneously broadened subbands for 1% Sb-doped AgI films.
bination mediated by high density of deep trap states involving exciton-phonon interactions or some crystalline defects or impurities. Also, the relatively large surface-tovolume ratio of these clusters may create unsaturated dangling bonds that act as surface traps and influences effectively the PL properties.24 Similar spectra with unresolved subbands were obtained in the case of polymer stabilized AgI aquasols.8 Excitation at 300 nm (4.13 eV) (Figure 3a) yields very few thermalized electrons from the conduction band, and thus the resulting D-A recombination rate is very low, the lowest being for 13% Sb doped AgI films due to the proximity of the surface traps induced by the maximum Sb content, which strongly influences the mobility of Ag ions to reach the halide Nano Lett., Vol. 2, No. 9, 2002
surface. The 325 nm (3.82 eV) excitation involves sufficient contribution from the thermalized electrons in the conduction band and thus fast trapping from the shallow and deep trap states occurs, providing a sufficient D-A recombination rate (Figure 3b), even though the excitation fluences used in all our measurements were very low. Room-temperature emission measurements on AgI colloids8,12 have also quantified that the rate of formation of small silver clusters upon irradiation of AgI was inversely related to the size of the samples. The higher rates from smaller samples were explained by the room-temperature migratory tendency of the silver ions in silver halides to the surface where they collide to form clusters, and these then quench the excitonic emission. Thus, the surface could play a role in exciton decay or breakup prior to radiative recombination.8,12,13 During excitation at 325 nm (3.82 eV), a hump starts developing around 400 nm (Figure 3b), which may be attributed to the Z3 excitonic luminescence probably arising from that of the γ-AgI structure formed during the iodization process. The lack of measured data for the γ-phase of AgI such as exciton binding energy, polaron masses of the electron, and hole and dielectric constants makes the use of Brus equation a qualitative tool at best.13 The red shift of the emission peaks following increased excitation energy is indicative of a Coulombic interaction between the donor and acceptor sites. The intrinsic Frenkel defects such as interstitial silver ions (Agi) and silver ion vacancies (VAg) would have the proper Coulombic interaction. Additionally, divalent cation impurities at the level of 1015-1016 cm-3 are common, even using the purest reactants.13 Excitation at 360 nm (3.45 eV) (Figure 3c) produces the maximum D-A recombination rate. The enhanced trapping of the shallow and deep trap states and their limit of saturation can be visualized from the increase in full width at half-maximum (fwhm) of the inhomogeneously broadened subbands, which also corroborates with the fact that maximum binding of almost all the surface defect sites at low Sb concentration and quenching of radiative emission at high Sb concentration takes place. Correspondingly, the emission around 400 nm also steeply increases. At 385 nm (3.22 eV) excitation (Figure 3d) AgI absorbs much more strongly.6 Thus, almost all the thermalized electrons in the conduction band participate in the process, contributing to the steep rise in emission at around 410 nm. Thus, the strong PL features with large red shift and multiphonon structure in Sb-doped AgI films suggest a larger radiative lifetime and a higher recombination rate with respect to bulk. The donors and acceptors in AgI could well be Ag+ vacancies (hole traps) or Ag+ interstitials (electron traps) as well as trace Sb impurities. The effect of 1% Sb doping into AgI is seen as an increase in the PL intensity (Figure 3) of the deep trap states induced by recombination of the charge carriers. As the dopant Sb has a higher dielectric constant than Ag, there will be a nonnegligible e2/r term in the expression for recombination energy given by13 E(r) ) Eg - (ED + EA) - e2/r
(1) 977
where Eg is the band gap energy, ED and EA are the binding energies of the donor and acceptor, and the last term is the Coulombic interaction of the donor-acceptor pair separated by a distance r. When the Sb concentration is increased to 5% and 13%, the number and the cluster density gradually decrease, as evidenced from SEM (Figure 1). Thus, the PL intensity is reduced for the same excitation wavelength relative to that of undoped AgI, with low emission edge and inhomogeneous broadening of the D-A recombination peaks. This luminescence quenching is primarily due to the formation of defect complexes that result in the formation of deeper traps that could not be accessed at room temperature. Thermoluminescence experiments at temperatures higher than ambient would probably reveal the nature of these traps, as has been done on ZnS nanoparticles.25 Recent PL work on CdSe nanoparticles26 has emphasized clearly the role of impurity in enhancing and quenching the luminescence at lower and higher concentrations, respectively, but without changing their decay lifetimes. Sb is thus quite like a molecular impurity in controlling the PL characteristics of AgI. Being a facile substitutional impurity in AgI, Sb is sufficiently mobile to alter the surface microstructure and particle size to some extent. However, it is neither reactive nor compound forming like Rb+, K+, etc., and also not like Cu+ which only reinforces the cation sublattice but maintains the intrinsic electronic properties of AgI.11 On the other hand, Sb can contribute to band filling and also exercise an effective control on trap density. It emerges that 1% or 2% Sb is an optimal concentration that can enhance the PL intensity, beyond which Sb can interfere with the trapping process, which is understandable as Sb by itself does not produce any visible PL. The role of Sb, then, in quenching the excitonic emission and introducing the deep trap states becomes significant. This metamorphological role of Sb starts right from inducing the nucleation of large density of smaller Ag islands18-20 thereby reducing considerably its surface mobility in the precursor films, which in turn also strongly favors the retarded growth of less denser AgI clusters with increasing Sb concentration during the iodization process, as shown in Figure 2. Thus, following photoexcitation, charge carriers first thermalize and are then trapped in shallow trap states or other near band edge states, which subsequently decay into deep trap states. These deep-trapped electrons finally react with interstitial Ag+ ions to form Ag atoms. The use of excited-state trapped electrons to reduce silver ions to silver atoms is the foundation of the photographic process.27 The decay time appears to be slow for this reaction (calculated to be > 0.5 ns from the femtosecond photoinduced electron dynamics measurements).6 This might be partly due to the interstitial nature of some of the silver ions participating in the reaction, the reaction rate being limited by the migration time of these ions to reach the particle surface assuming that the electrons are trapped by surface traps. Thus, unlike the predominance of excitonic PL in bulk crystals and other specifically processed AgI nanoparticles,
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in the present case, the excitonic band does not appear to be actinic with regard to different excitation energies, although an initial fast decayed emission is observed at the corresponding wavelength with a red shift. Then, we could assign the emission to a surface recombination process, where it is likely to be self-activated at the large variety of surface defects. Thus the conditions for confinement of electrons and the realization of surface states are simultaneously satisfied in the present samples. Acknowledgment. The authors sincerely thank Mrs. Swati Ray for help in the sample preparation. One of the authors, P.S.K., acknowledges CSIR, India for a research fellowship. We are thankful to the referee for his critical and thorough reading of the manuscript. References (1) Alivisatos, A. P. Science 1996, 271, 933. Ho, J.; Li, L. S.; Yang, W.; Manna, L.; Wang, L. W.; Alivisatos, A. P. Science 2001, 292, 2060. (2) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. Landes, C.; Braun, M.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 10554. (3) Yoffe, A. D. AdV. Phys. 2000, 50, 1. (4) Mochizuki, S. J. Lumin. 1996, 70, 60. (5) Sahyun, M. R. V.; Serpone, N.; Sharma, D. K. J. Imaging Sci. Technol. 1993, 37, 261. (6) Brelle, M. C.; Zhang, J. Z. J. Chem. Phys. 1998, 108, 3119. (7) Schmidt, K. H.; Patel, R.; Meisel, D. J. Am. Chem. Soc. 1988, 110, 4882. (8) Serpone, N.; Lawless, D.; Lenet, B. J. Imaging Sci. Technol. 1993, 37, 517. Micic, O. I.; Meglic, M.; Lawless, D.; Sharma, D. K.; Serpone, N. Langmuir 1990, 6, 487. (9) Mochizuki, S.; Umezawa, K. Phys. Lett. A 1997, 228, 111. (10) Mochizuki, S.; Ohta, Y. J. Lumin. 2000, 87-89, 299. Mochizuki, S. Physica B 2001, 308-310, 1042. (11) Senthil Kumar, P.; Sunandana, C. S. Nano Lett. 2002, 2, 431. (12) Freedholf, M. H.; Marchetti, A. P.; McLendon, G. L. J. Lumin. 1996, 70, 400. (13) Rodney, P. J. Ph.D. Thesis. University of Rochester, 1998. (14) Henglein, A.; Gutierrez, M.; Weller, H.; Fotjik, A.; Jirkovsky, J. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 593. (15) Senthil Kumar, P.; Sunandana, C. S. Thin Solid Films, 1998, 323, 110. (16) Senthil Kumar, P.; Babu Dayal, P.; Sunandana, C. S. Thin Solid Films, 1999, 357, 111. (17) Penner, R. M. Acc. Chem. Res. 2000, 33, 78. (18) Meyer, J. A.; van der Vegt, H. A.; Vrijmoeth, J.; Vlieg, E.; Behm, R. J. Surf. Sci. 1996, 355, L375. (19) Zuhr, R. A.; Magruder, R. H., III.; Anderson, T. S. Surf. Coat. Technol. 1998, 103-104, 401. (20) van der Vegt, H. A.; Huisman, W. J.; Howes, P. B.; Vlieg, E. Surf. Sci. 1995, 330, 101. (21) Hansen, M. Constitution of Binary Alloys; McGraw-Hill: New York, 1958. (22) Senthil Kumar, P.; Swati Ray; Sunandana, C. S. Mater. Phys. Mech. 2001, 4, 39. (23) Kuiry, S. C.; Roy, S. K.; Bose, S. K. Metall. Mater. Trans. B 1997, 28, 1189. (24) Nayak, J.; Sahu, S. N. Appl. Surf. Sci. 2001, 182, 407. (25) Chen, W.; Wang, Z.; Lin, Z.; Lin, L. Appl. Phys. Lett. 1997, 70, 1465. (26) Landes, C.; Burda, C.; Braun, M.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 2981. (27) Gurney, R. W.; Mott, N. F. Proc. Royal Soc. London, A 1938, 164, 151.
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