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J. Phys. Chem. B 2001, 105, 7490-7498
In2S3 Nanocolloids with Excitonic Emission: In2S3 vs CdS Comparative Study of Optical and Structural Characteristics Dattatri K. Nagesha,† Xiaorong Liang,† Arif A. Mamedov,† Gordon Gainer,‡ Margaret A. Eastman,† Michael Giersig,§ Jin-Joo Song,‡ Tong Ni,† and Nicholas A. Kotov*,† Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, Department of Physics, Center for Laser and Photonics Research, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, and Hahn-Meitner-Institut, Abt. Physikalische Chemie, Glienickerstr. 100, D-15109, Berlin, Germany ReceiVed: April 4, 2001
Stable aqueous colloids of 2-3 nm In2S3 nanocrystals have been prepared by using the classical method of nanoparticle stabilization by low molecular weight thiols. TEM crystal lattice spacing, X-ray diffraction, EDAX data, and electron diffraction indicate that the nanoparticles are predominantly in β-In2S3 form. They exhibit relatively strong excitonic emission at 360-380 nm with a quantum yield of 1.5%. The excitonic radiative lifetime is 350 ns, which indicates that a direct allowed electronic transition is responsible for this emission. The NMR lines of the stabilizer are strongly broadened and shifted as a result of deshielding induced by electron withdrawing by positively charged metal ions. This effect quickly wears off as the carbon chain becomes longer and the separation between the hydrogen atoms of the stabilizer and the semiconductor surface increases. The broadening is attributed to the reduced mobility of the stabilizer in the nanoparticle shell. For CdS nanoparticles of the same size, this effect was found to be substantially stronger than for In2S3. The lower density of metal centers in In2S3 than in CdS, which serve as anchor points for the stabilizer, promotes greater mobility of the stabilizer moieties.
I. Introduction Semiconductor nanoparticles (NPs) offer a rich palette of optical, electronic, and catalytic properties, which can be tuned by their size. Their uniqueness, as compared to the bulk materials stems from their large surface area and size quantization effect.1-3 For the past decade, a large variety of semiconductors have been prepared in nanocrystalline form. Among others, metal chalcogenides are the materials for which the quantum confinement effect is most pronounced. The properties of NPs (quantum dots) have been extensively investigated for the II-VI class of compounds (CdS, CdSe, etc.).4 A large amount of work has also been done for III-V (InP, GaAs etc.),5 I-VI (Ag2S, Cu2S, etc.),6 and I-VII semiconductors (AgI, AgCl, etc.).7 A high degree of understanding of the nature of electronic processes has been attained, particularly in II-VI quantum dots.4,8,9 The stabilization of the NP colloids is typically achieved by protecting them with a (mono)layer of organic Lewis bases, such as thiols, phosphines, acids, or amines, binding to positively charged metal sites on the surface of the NP core. The stronger the metal-stabilizer bonds, the denser the coating and the greater the stability of NPs against agglomeration and oxidation becomes. Note that all nanocrystalline materials that have been mentioned above have 1:1 or greater stoichiometric ratio of metal atoms to atoms of chalcogenide in the unit cell. There is also a large number of other semiconductors for which the total number of chalcogen atoms in the unit cell is greater than the * Author to whom correspondence should be addressed. E-mail: kotov@ okstate.edu. † Department of Chemistry, Oklahoma State University. ‡ Department of Physics, Center for Laser and Photonics Research, Oklahoma State University. § Hahn-Meitner-Institut, Abt. Physikalische Chemie.
number of metal atoms, such as MoS2, WS2, ReS2, FeS2, Sb2S3, In2S3, Bi2S3, with special optical, mechanical, and catalytic properties. For most of them, the nanocrystalline form was reported only in the presence of polymers10-15 or in a physically constrained environment, such as porous solids and reverse micelles, where the growth of the NPs is arrested by the phase boundaries.16,17 The presence of the polymeric or solid matrix greatly complicates optical studies and utilization of the nanocolloids, and broadens NP size distribution. The synthesis of stable redispersable colloids that can be used as regular chemicals in different kinds of organic and inorganic synthetic processes, would significantly aid their studies. The expansion of the spectrum of available redispersable semiconductor colloids to a new family of nanoparticles with chalcogen-rich surfaces may lead to several new directions in NP research. The dominance of chalcogen atoms will lead to (1) different photophysical behavior of electron/hole pairs in the quantum dots,18 and (2) novel surface chemistry, including new ways of building NP supramolecules. In this paper, we investigate the properties of In2S3, which has received little attention so far.10,13,14,19 In addition to the novel features mentioned above, three other special qualities make In2S3 an interesting candidate with strong motivation both fundamental and practical aspects of NP research: (1) In2S3 is a rare case of ordered crystalline material with a large amount of vacancies. Owing to tetragonal sites formed by incompletely coordinated sulfur atoms, indium sulfide can serve as a host for a number of metal ions to form semiconducting and/or magnetic materials.20,21 Doping In2S3 produces materials with exceptional optical, electrical, and magnetic properties, which can be adjusted not only by the NP diameter, but also by the concentration of the guest ion.22,23 This is in contrast to II-VI NPs, which tend to expel guest ions.24,25
10.1021/jp011265i CCC: $20.00 © 2001 American Chemical Society Published on Web 07/14/2001
In2S3 Nanocolloids with Excitonic Emission (2) Unlike most of the semiconductors currently being used for nanocolloids, In2S3 and related materials display both direct and indirect conduction-to-valence band transitions, which can be observed by different modalities of UV-vis spectroscopy at 2.0-2.2 and 1.0-1.1 eV, respectively. This opens the door for the investigation of the effect of quantum confinement on direct and indirect excitonic transitions. (3) In2S3 NP bioconjugates can have medical applications, such as cancer diagnosis. A future communication will address this aspect of In2S3 colloids in greater detail. In this context, we report a synthetic procedure demonstrating the possibility of a stable hydrophilic In2S3 nanocolloid, which can be a stepping stone to the other studies. Since it resembles the synthesis of CdS NPs stabilized by thiol,26 we point out similarities and differences in the optical characteristics and structure of these two types of NPs and relate them to intrinsic properties of the corresponding bulk semiconductors. Two findings we believe to be particularly significant, which single out this work among many others concerned with NP dispersions. First, the obtained In2S3 clusters exhibit relatively strong band-gap (excitonic) luminescence in the ultraviolet part of the spectrum with a quantum yield comparable to that of regular II-VI semiconductor dispersions. This expands the spectrum of the NP emission down to 360 nm. The lifetime measurements demonstrate that this luminescence is an allowed optical transition with a lifetime significantly shorter than that for IIVI semiconductor quantum dots with a similar quantum yield. Second, we report here the effect of the distance between a hydrocarbon group and the surface of NP on 1H NMR signal of the stabilizer coating the NP in solution. The shift of the NMR peak relative to that in the free stabilizer molecule diminishes with the distance from the NP. The influence of the crystal lattice with greater chalcogen/metal ratio can also be clearly seen in the NMR data. Narrower peaks point to the lower density of thioglycerol moieties attached to the surface of In2S3 than that for CdS. II. Experimental Section Chemicals. Indium chloride, InCl3, (Alfa Aesar), 1-thioglycerol (Aldrich), sodium sulfide (Ultra puris, Fluka), 1-amino-2methyl-2-propanethiol hydrochloride (Aldrich), and p-quaterphenyl (Aldrich) were used as received without further purification. Nanopure water (18.2 MΩ) was obtained from an E-Pure (Barnstead) deionization system. The sample for NMR was prepared in deuterium oxide, 99.9% atom D (Aldrich). Synthesis of Thiol-Stabilized Nanoparticles. The synthesis of indium sulfide NPs followed the scheme of arrested precipitation that worked well for II-VI colloids (CdS, CdSe, and CdTe).26-28 Deoxygenated indium chloride aqueous solution and thiol stabilizer were mixed together, and then, sodium sulfide was rapidly added to the vigorously stirred solution. Typically, 50 mg of InCl3 and 0.18 mL of thioglycerol (TG) or 320 mg of 1-amino-2-methyl-2-propanethiol hydrochloride (AMPT) were dissolved in 20 mL of deoxygenated water (N2, bubbling for 20 min). The molar concentration of stabilizer was 10 times that of the metal. The pH of the solution was adjusted to 3.0-3.5. This mixture was vigorously stirred and 3.39 mL of 0.1 M aqueous solution of sodium sulfide was added. Extended heating of the reaction mixture yields only a slight growth of NPs resulting in the end in the precipitation of an off-white solid. Indium sulfide NPs in the solution were sedimented as a white powder from the mother liquor by 2-propanol. Their spectroscopic characterization was carried out with reconstituted solutions in deionized water. A total of 4.3
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7491 mg of the nanopowder can be dissolved completely in 20 mL of water. The pH of these solutions was adjusted with diluted HCl or NaOH. The time passing from the moment of mixing the ingredients to the moment when a precipitate of purified NPs is obtained can be as short as 25 min. CdS NPs stabilized by thioglycerol were prepared and purified according to the recipe described in ref 26 by using TG as a stabilizer. Briefly, 0.989 g of Cd(CH3COO)2‚2H2O and 0.4 mL of 1-thioglycerol were dissolved in 80 mL DMF and heated to 70° under argon. Five milliliters of 0.4 g thiourea DMF solution was added to the mixture and the temperature was raised to 100 °C. The mixture was then refluxed at 100-105 °C under argon for 1-2 h until the color changed to the light yellow, cooled to the room temperature, and concentrated to 20 mL using rotary evaporator. Acetone was added dropwise under vigorous stirring to the concentrated NP solution until it became cloudy. The precipitated large size NPs were centrifuged out and washed with acetone. This procedure was repeated until the appropriate size distribution was reached, as indicated by optical absorption spectrum. Quantum Yield. The quantum yield of NP luminescence was determined by using two standards: p-quaterphenyl (PQP) and Rhodamine B. PQP in ethanol exhibited an emission peak at λmax ) 390 nm, while the emission maximum of Rhodamine B in ethylene glycol was at λmax ) 580 nm. On the basis of the literature data, the standard quantum yields integrated over the corresponding emission peaks were considered to be 1.0 for Rhodamine B and 0.71 for PQP.29,30 Optical density of In2S3 dispersion in water and the standard samples in their respective solvents at the excitation wavelength of 240 nm were adjusted to be within 0.05 absorbance units within each other. The overall optical density of both solutions did not exceed 0.10. The quantum yield was calculated according to the formula φIn2S3 ) φS (IIn2S3/IS)*(ODS/ODIn2S3), where φIn2S3 and φS are, respectively, the quantum yields of NPs and standards, IIn2S3 and IS are, respectively, the integrated area of the luminescence peaks of NPs and standards, and ODIn2S3 and ODS are, respectively, the optical densities of the NPs and standards at the wavelength of excitation light (240 nm). The use of both standards gave comparable quantum yields. In this paper, we report, however, the data only for PQP as the closest to indium sulfide in emission spectrum. Instrumentation. Optical absorption spectra were obtained on a Hewlett-Packard 8453 diode array spectrophotometer using 1-cm quartz cuvettes. Fluorolog 3 and Fluoromax 2 from JY SPEX were used to register the luminescence spectra of the particles. The right angle registration mode with no intermediate filters was utilized in these measurements. The time-resolved photoluminescence setup is based on a Coherent Antares Nd:YAG laser that emits infrared at a wavelength of 1.064 µm with a pulse repetition rate of 76 MHz, a pulse width of 70 ps, and 20 W power averaged over both the on and off pulse times. External to the laser cavity, but inside the laser head, a second harmonic generation crystal converts much of this infrared to green with a wavelength of 532 nm and an average power of 5 W. This green light goes into a Coherent dye laser with a cavity dumper. The output of the dye laser and cavity dumper is red light with a tunable wavelength, an average power of about 100 mW, and a pulse width of about 5 ps. The pulse repetition rate of the cavity dumper can be varied between 38 MHz and 147 kHz. This red light is then frequency doubled into ultraviolet, which is directed to the sample. The sample photoluminescence is focused into a small spectrometer, which focuses the light into a Hamamatsu
7492 J. Phys. Chem. B, Vol. 105, No. 31, 2001 C5680 streak camera with a resolution of 2 ps. Although the excitation pulses have a width of 5 ps, the optics and spectrometer spread out the light, so that we cannot obtain a resolution smaller than 10 ps. The actual resolution we obtain depends on the streak camera sweep range. For example, if the streak camera is set to a sweep range of 150 ps, we can measure a 10 ps FWHM for the diffusely reflected excitation laser light. However, when we use a 4 ns sweep range, we may measure a 50 or 100 ps FWHM for the diffusely reflected light. We measured TRPL in the photon counting mode, in which each lit-up pixel area represents one photon in an exposure time of 33 ms. A typical sample measurement has 20,000 exposures and lasts about 11 min. Excitation wavelength used in these experiments was 330 nm. A Phillips CM 300 operating at 200 kV was used for transmission electron microscopy (TEM). For the preparation of samples for electron microscopy, a drop of the indium sulfide sol was placed onto a carbon coated aluminum grid (200 mesh). The grid was allowed to dry for one minute, and then blotted with filter paper to remove excess solution. The dried grids were transferred into a nitrogen-filled container and then to the cell compartment of the TEM microscope, equipped with a Phillips EDAX 9800 analyzer. A number of grids were prepared from each sample in order to check the reproducibility of the preparative procedure. Bright-field images were taken under conditions of minimum phase contrast.31 Quantitative EDAX analysis (Element Detection and Analysis by X-rays) of individual particles was carried out with the electron microscope in nanoprobe mode (beam spot size reducible down to 1 nm) using a spot size equal to the average diameter of the particles. The size distribution curves were obtained by analyzing crystal lattice-resolved TEM images. For each curve ∼70-80 NPs were counted. The reported diameters were the diameter of the areas of continuous crystallinity approximated by a circle to match the total area of unbroken crystal lattice. Large aggregates with indistinguishable interparticle boundaries were neglected in the total count. The total number of those did not exceed 1-2 per 10-15 clearly legible NPs. A JEOL 2000-FX scanning transmission research electron microscope (200 keV, bright LaB6 source) was used for obtaining an electron diffraction pattern. It was registered at 200 000/300 000 magnification and calibrated by the diffraction spacings of gold obtained at identical conditions. Proton nuclear magnetic resonance (NMR) spectra were obtained on a Varian Inova 400 MHz instrument operating at 1H frequency of 399.96 MHz. The nanoparticle sample was obtained by sedimentation with 2-propanol, thoroughly washed with an excess of 2-propanol, and dried in a vacuum, then dissolved in 99.9% D2O to produce a clear saturated solution. Presaturation of the residual H2O line was used. Peaks were referenced to H2O at 4.8 ppm. The light scattering measurements were performed by using Malvern Zetasizer 3000 HS operating with internal 10 mW, 633 nm He-Ne laser in the right angle geometry. The standard 1 × 1 cm cuvette was used for these measurements. III. Results and Discussion Following the addition of sodium sulfide to the mixture of indium chloride and stabilizer, an absorption band at 320-393 nm for TG and a band at 318-365 nm for AMPT develop in the UV-vis spectrum of the reaction mixture within a period of time of 5 min (Figures 1 and 2, traces 1 and 2). The characteristic steplike shape of the bands indicates that they should be attributed to the valence-to-conduction-band transition
Nagesha et al.
Figure 1. Optical properties of In2S3 NPs stabilized by TG. (1) and (2) UV-vis absorption spectra of reaction mixture before and after addition of sodium sulfide. (3) Luminescence spectrum of the redispersed In2S3 NPs stabilized by TG.
Figure 2. Optical properties of In2S3 NPs stabilized by AMPT. (1) and (2) UV-vis absorption spectra of reaction mixture before and after addition of sodium sulfide, respectively. (3) Luminescence spectrum of redispersed In2S3 NPs stabilized by AMPT.
in indium sulfide. In bulk In2S3, the band gap (Eg) is reported to be between 2.0 and 2.2 eV with the corresponding UV band from 620 to 550 nm.32-35 Considering the position of the UVvis onsets estimated as the step median, 357 and 341 nm in Figures 1 and 2, the band-gap becomes equal to Eg ) 3.5 eV and Eg ) 3.6 eV for TG- and AMTP-stabilized NPs, respectively, which indicates the strong quantum confinement of the excitonic transition expected for In2S3 NPs.36 The position of the adsorption bands correlates well with the UV-vis characteristics of In2S3 specimens prepared by other groups using polymers or in physically constrained media.37-40 Indium sulfide NPs stabilized with TG or AMPT can be easily separated from the synthetic liquor by sedimentation with 2-propanol and then redispersed in many polar solvents. Solubility of the redispersed NP colloid in water was determined to be 4.3 mg per 20 mL of water. Purified reconstituted NPs can be easily seen in the TEM images (Figure 3a). EDAX data reveal that the ratio of indium to sulfur atoms in the particles is 4.1:5.9, which is close to the theoretical 2:3 ratio expected for In2S3. In the high-resolution close-ups (Figure 3b), one can see the crystalline nature of the prepared nanocolloid. The Fourier transforms of the crystalline areas yield lattice spacings equal to 2.6 and 2.9 Å. These spacings are attributed to (220) and (206) crystal planes of tetragonal β-In2S3.41;42 The observation of the 2.6 Å TEM spacing, which is unique for the β-form of In2S3, gives evidence that NPs crystallize in this allotropic modification. Electron diffraction data reveal the presence of lattice spacings of 2.09, 1.21, 1.05, 0.79, and 0.70 Å (Figure
In2S3 Nanocolloids with Excitonic Emission
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7493
Figure 4. Electron diffraction pattern of indium sulfide NPs.
Figure 3. Survey (a) and high-resolution (b) transmission electron microscopy images of redispersed In2S3 NPs stabilized by TG.
4). The diffraction circles at 2.09 and 1.21 Å correspond very well to expected 2.06 and 1.20 reflexes43 with relative intensity of 75 and 67 originating from (309) and (840) lattice planes in β-In2S3. The other signals from high index planes are difficult to assign.43 Note that 1.90, 1.64, and 1.40 Å lattice with expected relative intensity in β-In2S3 of 100, 58, and 88, respectively, cannot be seen. In the same time, the other possible modification of indium sulfide, R-In2S3, should demonstrate strong signals at 1.89, 1.55, 1.09, and 0.94 Å with relative intensities of 100, 50, 80, and 50,43 while none of them can be seen in the electron diffraction. Instability of the prepared NPs under focused e-beam prevented us from obtaining single-crystal diffraction pattern. To clarify the assignment of the crystal lattice, X-ray powder diffraction spectrum for the TG-stabilized NPs was also obtained (Figure 5a). The quite broad diffractogramsas expected for NPs of 2-3 nm in diameterswas compared against the library spectrum of β-In2S3 and some correspondence between them could be seen (Figure 5a). As one may notice, some peaks are less pronounced than the others such as (103) and (309) being
hardly noticeable over the background although in the library spectrum they appear with relatively high intensity. Interestingly, (103) peak was also not visible in β-In2S3 films of micrometer thickness of obtained by spray pyrolysis44 and in β-In2S3 NPs prepared by hydrothermal technique,45 too. For thin films, (109), (220) and (400) reflexes dominated the diffractogram,44 while the (103) reflex at 2θ ) 14° became visible only for the biggest clusters of 10-13 nm in diameter.45 The deviation between the relative intensity of X-ray and electron diffraction peaks for the NPs and the corresponding library spectrum and expected e-beam diffraction intensities can be rationalized assuming a platelet-like morphology of the indium sulfide NPs, which is known to develop for tetragonal crystals when the growth along one axis is significantly slower than for the others due to reduction of surface energy by adsorbed Lewis bases.46,47 as well as in other growth conditions.48,49 The diffraction from the lattice planes perpendicular to the platelet normal, where the number of the crystals cells can be as few as 3,50 is much reduced as compared to the others because of insufficient correlation length, while the library spectrum is calculated for the fully developed bulk crystals. Indeed, the presence of very thin sheetlike crystallites can be seen in the TEM image of ripened for two weeks colloid (Figure 5b). Note that CdS (cubic) typically forms NPs in the shape of tetrahedrons,51 while CdSe (hexagonal) tends to produce spherical or slightly oblong particles.52 Greater geometrical asymmetry of In2S3 nanocrystals brings about stronger X-ray and electron diffraction distortions.53 The comparison of observed diffraction patterns with library spectra of InS and In2S3 of other crystalline modifications, revealed stronger discrepancies (see Supporting Information). Considering presented TEM, electron and X-ray diffraction data in their entirety, one can conclude that there is more evidence for the existence of NPs in the form of β-In2S3 than in the others. Alongside, β-In2S3 is the most thermodynamically stable allo-
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Figure 6. Size distribution graphs of In2S3 NPs stabilized by TG (a) and AMPT (b).
Figure 5. (a) X-ray diffraction pattern of TG-stabilized In2S3 NPs. Bars represent the library spectrum of β-In2S3. Lattice plane spacings are (103) - 6.232 Å, (200) - 3.782 Å, (109) - 3.241 Å, (206) 3.097 Å, (220) - 2.712 Å, (309) - 2.074 Å, (400) - 1.912 Å, (533) - 1.641 Å, (444) - 1.559 Å. (b) TEM image of an In2S3 platelet in ripened reconstituted solution of NPs. The terrace-like fine structure of the platelet can be seen in the lower left part of the platelet. Small NPs are visible in the background of the image.
tropic modification of In2S3 at room temperature, which further substantiates this hypothesis. However, the data discussed above show the presence of the β-In2S3 NPs, but they do not completely rule out the possibility of other forms. We believe that the possibility of mixed crystalline state as well as a new crystalline for specific for NPs,54 must still be considered. Both R-In2S3,55 γ-Ins2S356 and even the high-pressure -phase56 have been reported for nanocrystalline In2S3 thin films. To that effect, sheetlike morphology of the NPs is also consistent with the layered crystal lattice of γ-In2S3.41 Therefore, although we believe that β-In2S3 is the prevailing modification in the thiolstabilized NP dispersions, the presence of other allotropic forms needs to be further examined in cryo-TEM setup, when e-beam damage to the NPs is minimized. Size distribution graphs obtained from TEM (Figure 6) show that 75-80% of the particles are contained within 0.5 nm of the maximum.57 There is a good correlation between the maximum of the size distribution and the onset of UV absorption. The decrease of the average particle size from 3.5 to 3.0 nm when TG is replaced with AMPT (Figure 6) is accompanied by the red shift of the absorption spectra (Figures
1, 2). The diameter of indium sulfide semiconductor core does not significantly change upon repeated redispersion; however, light scattering measurement revealed the existence of species with the size range of 150-350 nm for all solution. We attribute this to the formation of dynamic (loose) aggregates formed from several dozens of NPs, otherwise retaining their identity and exchanging with free NPs in solution. Formation of such aggregates contributes to broadening of the absorption spectrum. For both stabilizers, the prepared In2S3 NPs exhibit relatively strong luminescence (Figures 1 and 2), which had not been reported for the previously made indium sulfide nanocomposites.10,13-15,55,56 Complexes and organometallic compounds of In3+ which are forming as byproducts in the reaction of nanoparticle synthesis may interfere with the optical spectra of In2S3 clusters. Thus, to verify the origin of the luminescence peak, the redispersed in deionized water nanocolloid was exhaustively dialyzed against pure water for 3 days (3 × 24 h). The cutoff molecular weight of the dialysis membrane was 15 kDa, which was sufficiently small to retain the NPs, while all possible In3+ complexes could freely escape through a membrane to the outside volume. The luminescence spectra remain virtually unchanged before and after the dialysis (Figure 7). Thus, the observed luminescence should indeed be attributed to the nanocolloid, rather than to a low molecular weight compound.58 The slight reduction of luminescence intensity and shift of the peak from 365 to 375 nm should be attributed to the increased aggregation of the nanoclusters related to partial depletion of the protective stabilizer layer. Formation of a small amount of a solid white precipitate can be observed on the walls of the dialysis tube. The quantum yield of the blue band gap emission of In2S3 NPs was determined to be φ )1.2-1.5% with respect to p-quaterphenyl. Typical CdS and CdSe nanoclusters without
In2S3 Nanocolloids with Excitonic Emission
Figure 7. UV-vis absorption spectra before (1) and luminescence spectra of In2S3 NPs stabilized by TG before (2) and after (3) dialysis against deionized water, Mw cutoff 15 kDa.
special epitaxial coating59 give comparable (or lower) quantum yields of photoluminescence for thiol-stabilized nanocolloids.60 The closeness of the emission peaks to the absorption onset of the semiconductor indicates that this emission comes from the interband electron-hole recombination. To be correct with notations, it is necessary to note that the interband transition may actually involve electronic levels in close vicinity to the bottom of the lowest empty band and the top of the highest occupied band, as it is the case for CdSe.61-63 The UV peak of In2S3 NPs is almost always accompanied by a shoulder 0.2-0.3 eV to the red from the emission maximum. Its intensity and position in respect to the main peak is affected by the choice of stabilizer and by the media. However, we never see the so-called “red” luminescence positioned 1.5-2 eV lower in energy than the band gap, which is very characteristic for CdS and which is typically attributed to the exciton recombination from charge carrier traps and surface states.64 There is a significant controversy about the nature of the valence-to-conduction-band transition at 2.0-2.2 eV in the bulk β-In2S3. A few authors associate it with an indirect electronic transition.65-68 At the same time, there are publications treating it as a direct allowed transition,13,69-73 while some consider it a direct forbidden transition.74 Indirect band gap transitions involve some assistance from quanta of the vibrational energy of the semiconductor crystal lattice (phonons) and are normally nonemissive75 because of the fast nonradiative recombination via vibrational states, and thus, the nature of the transition is much important for the understanding of the photophysics of NPs. However, when nonradiative recombination is greatly reduced, for instance in oxidized silicon, indirect band gap NP can also emit strong luminescence.76-78 The lifetime of the UV emission of β-In2S3 dialized colloids was determined to be τ ) 5.3 ( 0.5 ns (Figure 8). Interestingly, we do not see any variation in the NP luminescence decay when varying the spectral window of the luminescence. It is also quite monoexponential after subtracting the laser pulse signal in sharp contrast to the luminescence kinetics of thiol-stabilized CdS, CdSe, and CdTe NPs, which are multiexponential and vary strongly with the experimental parameters.79,80 From the basic photochemistry it is known that the lifetime of the emissive state is determined by both radiative and radiationless transitions to the ground state;81 the nature of the radiationless processes is not essential at the moment. Assuming that there is only one emissive state, which correlates well with the monoexponential character of the luminescence kinetics, the observed lifetime τ, true radiative lifetime τ0, and quantum yield
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7495
Figure 8. Time-resolved photoluminescence spectrum of TG-stabilized In2S3 NPs, excited at 330 nm and measured from (1) 356-390 nm, (2) 390-445 nm, and (3) 356-445 nm. Trace (4) corresponds to the laser pulse profile. The lifetime was determined in the 20-50 ns time interval. For clarity, traces (1), (2), and (3) were shifted along the vertical axis.
φ are related by81
φ ) τ/τ0 From this equation the true radiative lifetime is determined to be τ0 ) 350 ns. Similar calculations based on published data82 yield an average radiative lifetime for blue-emitting CdS of τ0 ) 1200 ns.80 Most importantly, the radiative lifetime of In2S3 nanoparticles is significantly shorter than that of forbidden transitions. For instance, in oxidized silicon, it is in the range of microseconds.83 Therefore, since τ0 ) 350 ns and since the near-band-edge luminescence of In2S3 NPs is relatively strong, one can conclude that electron-hole recombination occurs in these NPs as a direct transition regardless of the nature of the transition in bulk β-In2S3. The redispersability of the prepared colloid enables the removal of reaction byproducts as well as monitoring of surface reactions by spectroscopic methods. The most comprehensive technique for studying chemical transformation of the organic groups is nuclear magnetic resonance spectroscopy (NMR), which provides detailed information on the substitution process and resulting structures in NPs.84-86 Importantly, the NMR study was carried out in the liquid state here so that the dynamic behavior of thiol stabilizers adsorbed to the nanoparticle surface can be characterized. The 1H NMR spectrum of unbound TG consists of five groups of peaks representing the protons on methylene and methane carbons (Figure 9). Assignment of the peaks is based on their relative position and the observed multiplet structure. Protons of -OH and -SH groups are not visible in these spectra due to their fast exchange with D2O. The quintet at 3.75 ppm represents the middle -CH(OD)group because it is split by the two adjacent pairs of protons. The multiplets at 2.57 and 2.69 ppm correspond to CH2SD protons, while those at 3.56 and 3.64 ppm correspond to -CH2OD signal. Upon binding to the In2S3 NPs, the signals from all protons in TG become broader (Figure 9, trace 2). Additionally, the bands of -CH2S-In2S3 and -CH(OD)- undergo a shift from 2.70 to 2.85 ppm and from 3.77 to 3.83 ppm, respectively. The protons of -CH2OD broaden, without, however, being significantly shifted. The observed broadening of the proton NMR resonance of TG bound to In2S3 could be attributed to the frustration of rotational and vibrational moment of the stabilizer when adsorbed to heavy NPs. With limited motion of the bound TG molecules, the rotational correlation time of the NPs would be more influential in determining NMR relaxation parameters.
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Nagesha et al. the thiophenyl moiety studied in ref 85. This can be attributed to the multiple hydrogen bonds that the TG group can form with adjacent stabilizer molecules as well as with the semiconductor surface. The interaction between the phenyl rings is limited to much weaker dispersion forces, which results in NMR line narrowing. IV. Conclusion
Figure 9. NMR spectrum of TG (1), In2S3 NPs stabilized by TG (2), and 3.0 nm CdS NPs stabilized by TG (3). Stars mark the signals from residual amounts of 2-propanol used for the precipitation of NPs.
The larger rotational correlation time of the coated NPs in comparison to that of the free TG molecule would be associated with a shorter transverse relaxation time, T2, and hence a broader line. Concerning the shift, it is worth noting that its magnitude correlates well with the distance between the protons and the In2S3 surface. The protons in the immediate proximity to the surface, i.e., -CH2S-In2S3, experience the strongest shift of 0.15 ppm. The signal of the protons in the next hydrocarbon group, -CH(OD)-, is shifted by 0.06 ppm, while the frequency of the terminal end protons, -CH2OD, remains virtually unchanged (Figure 9, trace 2). This effect could be a direct consequence of binding to the positively charged metal centers on the NP surface with the high positive charge of indium withdrawing electrons from the thiol headgroup and a resulting depletion of the electron density along the chain of σ-bonds affecting the first two carbon atoms and deshielding the associated protons.84,85 The NMR spectrum of TG coating on the In2S3 quantum dots can be compared to the one obtained for 3.0 nm CdS NPs (Figure 9, trace 3). The shift pattern -CH2SH > -CH(OH) > -CH2OH is consistent with the one observed for In2S3. On the other hand, the broadening of the TG signals on 3.0 nm CdS is much greater than for TG on In2S3. The bands partially merge, which is also accompanied by the complete loss of the fine structure. Since, as noted above, the width of the NMR signal largely depends on the mobility of the TG moiety, the motion of the stabilizer molecules on CdS is significantly more frustrated than on In2S3. This result can be understood by recalling the difference in the crystal lattices of the two solids. In case of CdS, the number of metal and the chalcogen atoms forming the unit cell is equal. Unlike the group II-VI quantum dots, the metal atoms are in the minority in In2S3. In β-In2S3, indium atoms are accommodated in all octahedral sites and twothirds of the tetrahedral sites of the basic cubic lattice produced by sulfur atoms. Note that in cubic CdS, the structural motif of small CdS clusters,51 all of these sites are filled with Cd atoms. The number of metal centers available for binding the stabilizer is naturally smaller in In2S3 than in CdS. The coating of the stabilizer becomes less dense, and therefore, the greater mobility of the adsorbed TG moieties lengthens T2, resulting in less broadening of the NMR lines. It is worth pointing out that the mobility of the TG group on the surface of both types of nanoparticles studied here is significantly lower than that of
These results demonstrate that NPs from indium sulfide can be successfully prepared in a form of stable redispersable colloids. The morphology of the NPs, optical properties, and packing of stabilizer in the organic shell are shown to be significantly different from those of a typical II-VI semiconductor colloid such as CdS, and these differences are related to the change in the intrinsic properties of the parent semiconductors such as the percentage of metal centers in the crystal. Fairly strong excitonic luminescence in the 350-400 nm part of the spectrum, which occurs as a direct band gap transition, is a useful feature of the prepared colloid, which justifies its further studies. Other NPs, with a band gap in the 3.0-4.0 eV region, such as ZnS, CdS, ZnO, and TiO2, rarely exhibit band gap emission at these wavelength energies due to fast radiationless recombination and typically reveal only defect-mediated luminescence lines > 450 nm,87 i.e., the same blue region where the excitonic emission of bigger CdS and CdSe clusters occurs.80 The preparation of In2S3 nanocolloid also opens the pathway to the study of a variety of NPs doped with many different transition metals, in particular, the ions with magnetic properties. One of the important objectives of the ensuing research of In2S3 nanoparticles is the refinement of the synthetic procedure to obtain more narrow size distribution and improve the control over the particle size. In parallel, new synthetic pathways facilitated by the greater percentage of chalcogen atoms capable of forming stronger covalent bonds than metal ions should also be explored. Acknowledgment. The authors thank an anonymous reviewer for insightful comments. The funding from NSFCAREER (CHE-9876265), AFOSR (F49620-99-C-0072), OCAST and Nomadics Inc., which made this work possible is gratefully acknowledged. Funds for the 400 and 600 MHz NMR spectrometers for the Oklahoma Statewide Shared NMR facility were provided by the National Science Foundation (Grant BIR9512269), the Oklahoma State Reagents for Higher Education, the W.W. Keck Foundation, and Conoco Inc. Supporting Information Available: Diffraction patterns of different indium sulfides. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brus, L. E. J. Chem. Phys. 1983, 79, 5566-5571. (2) Brus, L. E. Jerusalem Symp. Quantum Chem. Biochem. 1984, 17, 431-435. (3) Efros, A. L.; Efros, A. L. Fiz. Tekh. PoluproVodn. (Leningrad) 1982, 16, 1209-1214. (4) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226-13239; Weller, H. Philos. Trans. R. Soc. London A 1996, 354, 757-766; Brus, L. E.; Trautman, J. K. Philos. Trans. R. Soc. London A 1995, 353, 313-321; Bawendi, M. G. Solid State Commun. 1998, 107, 709-709. (5) Guzelian, A. A.; Banin, U.; Lee, J. C.; Alivisatos, A. P. Preparation And Properties Of Inas And Inp Nanocrystals. In AdVances In Metal And Semiconductor Nanoparticles; Duncan, M. A., Ed.; Jai Press Inc: Stamford, 1998; pp 1-34; Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344; Guzelian, A. A.; Katari, J. E. B.; Kadavanich, A. V.; Banin, U.; Hamad, K.; Juban, E.; Alivisatos, A. P.; Wolters, R. H.; Arnold, C. C.; Heath, J. R. J. Phys. Chem. 1996, 100, 7212-7219; Micic, O.; Ahrenkel, S. P.; Bertram, D.; Nozik, A. J. Appl. Phys. Lett. 1999, 75,
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