NANO LETTERS
Strain-Induced Confinement of Excitons in Quasi-free AgI Nanoparticles
2002 Vol. 2, No. 4 431-434
P. Senthil Kumar and C. S. Sunandana* School of Physics, UniVersity of Hyderabad, Hyderabad 500 046 India Received January 24, 2002; Revised Manuscript Received February 4, 2002
ABSTRACT The optical spectra of vapor quenched metastable Ag−Cu thin films progressively iodized under ambient conditions have been measured. Delayed evolution and inhomogeneous broadening of the exciton absorption at 420 nm of the spatially confined γ-AgI nanoparticles were clearly seen by the development of Z1,2 exciton peak − iodization being controlled by the as-quenched Ag−Cu clusters. Cu addition not only restricts the particle size, but also favors the island type growth mode.
Introduction. Evolution and harnessing of semiconductor nanoparticles is an assiduously pursued theme in the physics/ chemistry and technology of advanced materials.1-3 The physics of such materials relate to the low dimensional systems, popularly called the quantum wires and quantum dots of semiconductors, such as II-VI and III-V compounds typified by CdS, CdSe, and GaAs.4-6 To this family belongs the class of I-VII compounds (e.g., AgI, CuI, and CuBr), which are ambient large band gap semiconductors with zinc blende structure having in-built cation disorder (liquid or glasslike) coexisting with long range order.7 Cationic Frenkel defects and excitons dictate the ion dynamics as well as characterize the optical behavior in these systems, which makes their particle growth study fundamentally important as well as interesting from the technological point of view.8-10 The relatively weak covalent bond in AgI makes it mechanically rather soft and nonthermodynamic defects such as stacking faults occurring as a consequence stabilize various polymorphic structures depending on their stacking energies. Thus, AgI is trimorphic under normal pressure: the stable hexagonal wurtzite (β-AgI) structure and the metastable cubic zinc blende (γ-AgI) structure below 420 K and the disordered body centered (R-AgI) structure above 420 K. R-AgI is a well-known superionic conductor. Because of such instability against phase transitions, the production of AgI thin films requires special precautions.11 The substrate and the preparative conditions may also reflect strongly on the AgI polymorphic behavior.12 Our attention was drawn toward the formation of γ-AgI layers, as they are potential structures in energy transmission through hollow fibers13 and have electrochemical sensor applications.14 Earlier, a very simple method of preparation and characterization of γ-AgI films * To whom correspondence should be addressed. E-mail: csssp@ uohyd.ernet.in. 10.1021/nl025511t CCC: $22.00 Published on Web 02/21/2002
© 2002 American Chemical Society
at ambient has been discussed by us.15 In this letter, optical absorption and luminescence measurements are utilized in studying the stepwise growth of γ-AgI nanoparticles in thin films. In these ZnS-type structural systems, polymorphic modifications unrealizable in bulk (e.g., γ-AgI) are rather easily obtained in thin films.16 Analogously, the feasibility of introducing high concentrations of impurities in thin films is more than in bulk. Cu doping in ZnS type systems considerably decreases the grain size, thereby stabilizing the corresponding cubic structure to a greater extent.17 Individual bare atoms/ions in AgI and CuI can be used as precursors for their total synthesis, allowing the direct reaction of the corresponding elements.18 This is the key to controlling the nucleation and growth steps, as well as ensuring that many bonding geometries are sampled during the premature growth stage itself.19 In the present work, the Ag-Cu solid solutions were obtained in the metastable state by vapor quenching.20 Furthermore, the smaller ionic radius of Cu+-ion (0.96 Å) compared to that of the Ag+-ion (1.16 Å) makes it feasible for the Ag+-ions to be substituted by the Cu+-ions in the amorphous environment. Clustering in these Ag-rich alloys can be conveniently studied by exciton spectroscopy, as demonstrated by the progressive iodination of these precursor alloy films. Unlike conventional thin film formation, the precursor films in the present case are “reactive” so that the film formation involves, among other things, a chemical reaction site at the surface itself. It is quite reasonable to assume that the reaction occurs at the Ag surface or Ag/AgI interface by penetration or diffusion of iodine molecules/atoms from the gas to the metal through the pores between crystallites on the precursor film. In this paper, we present the preliminary study of the growth of AgI nanoparticles from the stage of
formation of molecular species to the fully developed crystallites possessing bulk properties, where time resolution poses no technical limitation. But, it is very difficult to identify the various intermediate complex products formed (at least under the application of the present experimental conditions) ultimately limiting the complete understanding of the evolutionary process. Experimental Section. Metastable Ag-Cu solid solutions in the composition range (90:10, 80:20, 70:30, and 60:40) are obtained as thin films by coevaporation of the thoroughly ground mixtures of the two metal powders from the same molybdenum boat onto amorphous silica glass substrates held at room temperature. The base pressure of the vacuum chamber was maintained at ∼1 × 10-6 Torr throughout. The microscopic silica glass substrates were held at a distance of 15 cm from the heating element and the deposition was maintained at a constant rate of about 3 Å/min during the entire process. The above conditions were found favorable for the formation of homogeneous, amorphous alloy films, although the resulting composition ratio was not analyzed. The thickness of the films were determined using a quartz crystal thickness monitor, with an accuracy of (1 Å. Without any further postdeposition heat treatment, these amorphous films of different compositions were then subjected to progressive iodination (1 min-7 h) as described earlier.21 The ambient optical absorption measurements were carried out ex-situ using JASSCO 7800 spectrophotometer in the UV-vis wavelength range of 300-600 nm. Steadystate photoluminescence measurements were recorded using xenon lamp as the excitation source in HITACHI F-3010 model spectrophotometer (excitation wavelength, λ ) 330 nm). Results and Discussion. Figure 1 shows the room temperature absorption spectra of the progressively iodized metastable quasi-amorphous precursor Ag-Cu films. A careful comparison of these optical spectra for isochronal iodization (up to 7 h) of the four Ag-Cu composite films of 150 nm thickness reveals that the increasing Cu content considerably retards the AgI “nucleation and growth” thereby effectively controlling the resultant product particle sizes. In comparison with Cardona,22 the evolution of 420 nm peak (due to the dipole forbidden 4d10-4d95s transition in Ag, allowed by the tetrahedral symmetry of Ag+ ion in the zinc blende AgI structure and the resultant p-d hybridization) and the 320 nm peak (due to spin-orbit split I- valence of the spin-orbit interaction) with increasing iodination time have been attributed to Z1,2 and Z3 excitons, respectively. The immediate development of the Z1,2 exciton peak at 420 nm even for 1 min. iodination and its gradual broadening with progressive iodination clearly signals the nanoparticle formation. At the early stages of iodination (1-30 min) this exciton peak appears to have split into two faint bands, even though not well resolved. This faint double peak structure may possibly arise from the fundamental change in chemical bonding responsible for the short-range order and thus in the density of states due to the quasi-amorphous nature of the precursor films.11 With progressive iodination, these bands grow to a single band with a red shift. In-situ or low 432
Figure 1. Evolution of the exciton absorption band at around 420 nm during the progressive iodination of the as-quenched Ag-Cu films of various compositions. (a) Ag90Cu10, (b) Ag80Cu20, (c) Ag70Cu30, and (d) Ag60Cu40 films. The iodination time increases from bottom to top as given.
Figure 2. Peak wavelength shift of the Z1,2 exciton peak of the AgI nanocrystallites formed during iodination. The saturation of the exciton peak shift above 1 h. iodination is clearly seen for all the compositions.
temperature measurements are necessary to quantify clearly the origin of this double peak structure in the small crystallite region. The plot of the Z1,2 peak wavelength vs the iodination time for all Cu concentrations as shown in Figure 2 clearly reveals the saturation of the excess peak-wavelength above 1 h iodination suggesting that the surrounding inorganic matrix strongly restrict the aggregation and ripening of the as-formed crystallites. More interestingly, this saturation behavior clearly demonstrates strong particle size control particularly during the long term iodination regime and inhibits invariably the anisotropic growth and coalescence of the special crystal plane of the crystallites through “Ostwald ripening” proNano Lett., Vol. 2, No. 4, 2002
cesses.23,24 Thus, the presence of Cu results in the renormalization of the static dielectric constant of AgI and thereby stabilizing the low-dimensional quasi-free nanoparticles in this matrix-free approach. Significantly, the common interesting feature in all these optical spectra is the appearance of a “steplike” band centered at around 360 nm (3.45 eV) during the early stages of iodination, i.e., below 1 h. With increasing copper concentration as well as the iodination duration, this step changes its shape and becomes less symmetrical; its low energy part becomes steeper and turns into an absorption edge. This feature is in agreement with the fact that in Ag-rich Ag-Cu alloys, the absorption spectra essentially has the shape of the matrix Ag along with a supplementary absorption band due to Cu impurities.25 This steplike absorption, clearly absent in the optical spectra of undoped AgI films,21 is attributed to the presence of unreacted mixed (Ag + Cu) clusters uniformly dispersed in the metal nanoclusterdielectric composite. These unreacted clusters are consumed in the progressive iodination process. This transient steplike absorption is thus like a nonresonant electromagnetic absorption possibly contributing to the dielectric function of the growing “composite” films, which eventually merges with the dielectric function of the host films and turns into an absorption edge. The exciton absorption observed even in the amorphous state reflects the very small exciton radius of AgI (∼0.51 nm), which is so small that could exist within the shortrange order itself.26 However, no trace of CuI excitons was observed in the present case as seen from the Figure 1. Presumably, Cu+ ions are dispersed as monomers in the amorphous environment, thus maintaining the electrical neutrality of the system i.e., each Ag+ ion is replaced by a Cu+ ion. These Cu+ ions may also absorb light at their respective exciton energies,27 but are probably hidden under the strong absorption of the host Ag+ ions. Furthermore, the smaller ionic radius of the Cu+ ions is considered favorable for the heavy doping in such a unique manner. Figure 3 shows the photoluminescence (PL) spectra of AgI nanoparticles on the glass substrate at room temperature. From the early stage of iodination itself, the a peak appears at the same wavelength as that of the Z1,2 exciton band as described in the absorption spectra. This spectrum indicates that photoexcited electrons at the conduction band edge recombine directly with holes at the valence band edge. This process results in the emission of a PL photon having energy equal to that of the band gap. It is also well-known that in silver halides, the interstitial silver atoms and clusters are produced under illumination. Such defects may act as shallow exciton traps and cause some series of the multi-phonon structure in the photoluminescence spectrum as observed.26 As the reliable phonon frequencies of AgI are available only at 4.2 K,28 the assignment of these phonon subbands at room temperature is deferred. The absence of any characteristic features of γ-CuI or of β-CuI in the optical spectra of Ag(Cu)I- films is quite significant. This reflects the fact that the amorphous AgI formed at the initial stages of iodization relaxes to achieve Nano Lett., Vol. 2, No. 4, 2002
Figure 3. Typical photoluminescence spectra of AgI nanocrystals formed on the glass substrate during the iodination of as-quenched Ag-Cu films. The iodination time increases bottom to top as given in Figure 1. Series of multiple phonon structures are seen clearly on either side of the intense PL peak at 425 nm.
much better-defined short-range order than that of CuI before the crystallization process takes place and a complete nanocrystalline film is obtained. Essentially, the role of Cu is to drastically change the thin film growth mode from a simple layered type growth to a layered plus island type growth (S-K growth) mode29 through the formation of mixed Ag-Cu clusters, as evidenced from the optical spectra (Figure 1). This latter growth mode involves negligible interparticle diffusion as observed in the recent electrochemical synthesis of meso-scale metal nanoparticles.30 Thus, the strain-induced size control by Cu helps in the confinement of excitons in AgI nanoparticles and the inhomogenity of the PL peak observed even at RT represents the particle size distribution31,32 as reflected by its fwhm (full width at halfmaximum). The essentially iodine deficient γ-AgI can then be visualized as an n-type direct band gap semiconductor.8,21,24 Therefore, the observed continuous wave steady-state PL is a manifestation of the donor-acceptor recombination.32 This recombination seems to be a linear process inasmuch as there is apparently a rough linear correlation between the intensity of luminescence and the particle size.31 The fwhm of the most intense luminescence peak is ∼130 meV thus corresponds to a particle size of ∼40 Å. Eventually, the photoluminescence saturates as the particle size crosses the nanometer regime. The transitions responsible for emission involves trap states located below the conduction band edge.31 However, precise characterization of the exciton 433
energy levels and wave functions and the identification of the traps responsible for luminescence must await sub-helium temperature measurements. Acknowledgment. The authors sincerely thank Ms. Ch. Sujatha for help in regarding the optical spectra. One of the authors P.S.K. acknowledges CSIR, India for a research fellowship. The authors are grateful to the Referee for his critical comments on the manuscript. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Trindade, T.: Paul O’Brien; Pickett, N. L. Chem. Mat. 2001, 13, 3843. (3) Lucjan, J. Eur. J. Phys. 2000, 21, 487. (4) Yoffe, A. D. AdV. Phys. 2000, 50, 1. (5) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (6) Uchida, H.; Curtis, C. J.; Nozik, A. J. J. Phys. Chem. 1991, 95, 5382. (7) Gurevich, Yu. Ya.; Ivanov-Shits, A. K. Semiconductors and Semimetals; Academic Press: New York, 1988; 26, 230. (8) Grahn, H. T. Introduction to Semiconductor Physics; World Scientific: Singapore, 1999. (9) Aniya, M. Solid State Ionic Materials; World Scientific: Singapore, 1994; p 223. (10) Lee, J. S.; Adams, S.; Maier, J. J. Phys. Chem. Solids 2000, 61, 1607. (11) Kondo, S.; Itoh, T.; Saito, T. Phys. ReV. B 1998, 57, 13 235. (12) Mochizuki, S.; Ohta, Y. J. Lumin. 2000, 87-89, 299. (13) Dahan, R. J.; Dror, J.; Croitoru, N. Mater. Res. Bull. 1992, 27, 761. Tianfa Wen; Jianping Gao; Jiuming Zhang; Beiya Bian; Juyun Shen Infrared Phys., & Techn. 2001, 42, 501. (14) Wada, K.; Sumioka, K.; Mizuma, T. Research Report, Sasebo Coll. Techn. Jpn. 1990, 27, 15.
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NL025511T
Nano Lett., Vol. 2, No. 4, 2002
