J. Phys. Chem. C 2007, 111, 11843-11848
11843
Raman Spectroscopy of Ultranarrow CdS Nanostructures L. Zeiri,*,† I. Patla,† S. Acharya,‡,§ Y. Golan,‡,⊥ and S. Efrima†,|,⊥ Departments of Chemistry and Materials Engineering and Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion UniVersity of the NegeV, Beer-SheVa 84105, Israel ReceiVed: March 13, 2007; In Final Form: May 22, 2007
CdS semiconductor nanoparticles, with dimensions above and below the Bohr radius (∼2.5 nm) of bulk material, were prepared in a single-step benchtop procedure. The degree of quantum confinement in these nanoparticles was determined from their optical absorption and photoluminescence spectra. The size-dependent properties of the nanocrystals were studied by resonance Raman spectroscopy as a function of excitation wavelength and temperature. The spectra were composed of the fundamental longitudinal optical (LO) mode around 300 cm-1, along with the first and second overtones. The shapes and positions of the Raman peaks exhibited only a weak dependence on particle size even for the two extreme cases of bulk and nanostructures. We show that the ratio of the overtone to the fundamental LO frequency was sensitive to the particle diameter and decreased upon reduction of the particle diameter to values below the Bohr radius. Temperature-dependent Raman measurements of ultranarrow nanorods showed a small red shift with decreasing temperature. Very high anti-Stokes intensities were observed for the CdS nanoparticles.
1. Introduction Quantum confinement effects in semiconductor nanocrystals with reduced dimensions and different shapes have attracted considerable attention over the past decade. A number of studies have shown quantitatively that the confinement effect depends on the ratio of the nanocrystal radius to the Bohr radius of the electron-hole pairs.1-5 For different confinement regimes (nanocrystal sizes), confinement modifications occur in the quantization of the center of mass of the electron-hole pair. The situation appears to be even more complex for ultranarrow nanocrystals whose radii are smaller than the Bohr radius. In the very strong confinement regime, size quantization is expected to influence both electronic and vibrational states and their coupling. The confinement effects are dependent not only on the size but also on the shape of the nanoparticles. CdS is an important direct-band gap (2.5 eV) semiconductor material with a broad range of potential applications such as light-emitting diodes and optical devices. Considerable efforts have therefore been devoted to the synthesis of CdS nanostructures of various sizes and shapes. These efforts have been accompanied by a large number of characterization studies designed to clarify the effect of the size of semiconductor nanoparticles on their properties.6-8 Among the later studies, there are several reports on Raman measurements of CdS nanostructures of different sizes.9-13 These studies have produced contradictory findings regarding the influence of the nanoparticle radius on the electron-phonon interaction, measured as the ratio between the overtone and the fundamental frequencies. Shiang et al.9 observed that the net strength of electron-vibration coupling in CdS nanoparticles decreased * To whom correspondence should be addressed: e-mail
[email protected]. † Department of Chemistry. ‡ Department of Materials Engineering. § Present address: International Center for Young Scientists, National Research Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan. | Deceased March 26, 2005. ⊥ Ilse Katz Center for Meso and Nanoscale Science and Technology.
when their sizes went down from 7 to 1 nm, while an opposite behavior was reported by Scamarcio et al.,12 who reported an increase of the electron longitudinal optic (LO)-phonon coupling with decreasing particle size in the strong confinement limit for CdSxSe1-x nanocrystals embedded in a glass matrix. Their finding contradicted the theoretical formalism of SchmittRink et al.13 and other theoretical studies, which predicted an increase in electron-vibration coupling with increasing nanoparticle size in the region below 10 nm.14,15 Despite the immense amount of research on CdS nanoparticles, the radii of the narrowest reported one-dimensional CdS nanoparticles still exceeded the Bohr radius, thus limiting expected quantum confinement effects.16-18 Moreover, the shape asymmetry of the one-dimensional nanoparticles along the c-axis was reported to produce a band structure that differed from that of spherical nanoparticles, which apparently influenced electronhole interaction and electron-vibration coupling.19 Recently, our group has reported on the synthesis and assembly properties of ultranarrow CdS wires and rods.20 Here we describe the Raman studies of CdS nanoparticles, ultranarrow rods, and spherical prolate particles, below and above the Bohr radius. The Raman measurements of these different nanostructures were carried out at different excitation wavelengths and temperatures, and compared to bulk CdS. 2. Experimental Section All syntheses (nanorods and spherical prolate nanoparticles) were carried out at relatively low temperatures in a one-step benchtop decomposition procedure. The morphologies of the nanocrystals were tuned only by changing the precursor amount. Cadmium hexadecylxanthate (0.13 g, ∼1.7 × 10-4 mol for nanorods; 0.013 g, ∼1.7 × 10-5 mol for spherical prolate) was added to molten hexadecylamine (HDA, Aldrich, >98%, 2 g, 0.0083 mol) at 60 °C with continuous stirring under N2 purging. When a clear and homogeneous yellowish color appeared, the temperature was increased and then held at 90 °C for 30 min to facilitate the decomposition reaction. Thereafter, the product
10.1021/jp072015q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007
11844 J. Phys. Chem. C, Vol. 111, No. 32, 2007
Figure 1. TEM micrographs of the CdS nanoparticles of different morphologies: (a) rods; (b) spherical prolate particles.
was allowed to anneal under a temperature regime that produced different nanoshapes at 200 °C for 1 h. After this annealing stage, the temperature was slowly decreased to 70 °C, and methanol was added to flocculate the product. The flocculated solid was washed several times by centrifugation at 3000 rpm for 3 min. Finally, the flocculated solid was washed with 50 mL of a mixture of spectral-grade dichloromethane and methanol (1:3) to remove any excess HDA. The final product was redispersed in dichloromethane, and the suspension was used for the different measurements. Note that quality optimization of the rods sometimes required slight variations of the reaction time, temperature, and concentrations. CdS bulk powder was purchased from Aldrich (99.995% purity) and used as received. Characterization. The synthesized CdS nanoparticles were characterized by transmission electron microscopy (TEM; JEOL JEM-2010 microscope equipped with a high-resolution pole piece), UV-vis spectroscopy (Jasco V-560 UV-vis diode-array spectrophotometer), and photoluminescence (PL) spectroscopy (Jobin Yvon Fluorolog-3). Samples for TEM were prepared by placing a drop of a colloidal suspension of the nanomaterial in dichloromethane on a lacey carbon-coated copper grid of 300 mesh (Ted Pella 01883-F). Raman System. The Raman system comprised a Jobin-Yvon LabRam HR 800 micro-Raman system, equipped with a liquid-N2-cooled detector. The following excitation wavelengths were used: A Melles-Griot He-Cd laser (325 and 442 nm), a Melles-Griot air-cooled Ar ion laser (457, 488, and 514 nm), and a He-Ne laser (633 nm; supplied with the JY Raman spectrometer). Each excitation line had its own interference filter (for filtering out the plasma emission) and a suitable Raman notch filter (for laser light rejection). The laser power on the sample was reduced by neutral density (ND) filters to 1-100 µW to prevent photodegradation of the samples. Most measurements were taken with a 600 g mm-1 grating and a confocal microscope with a 100 µm aperture, giving a resolution of 4-8 cm-1. 3. Results and Discussion TEM images of HDA-coated nanorods (diameter 1.7-2.9 nm; length 15-40 nm), and spherical prolate nanoparticles (diameter ∼5 nm) showed that the synthetic route20 does indeed enable fine control of the morphology of the nanoparticles (Figure 1). The spherical prolate nanoparticles will be referred to from here on as spherical nanoparticles. High-resolution TEM and X-ray diffraction analyses of rods and spherical nanoparticles (not shown) indicated that the wurtzite structure of CdS was predominant in these samples.20 Room-temperature UV-vis absorption and PL spectra of the nanorods and spherical nanoparticles suspended in dichloromethane are shown in Figure 2, panels a and b, respectively. For the nanorods, the UV-vis spectrum shows a sharp discrete absorbance peak at 376 nm and a broad shoulder at around 420 nm. This low-energy peak maybe attributed to surface or defect
Zeiri et al. states20 or to aggregated nanoparticles.21 The sharp, blue-shifted high-energy peak suggests that the ultranarrow width of the nanorods (that is smaller than or equal to the Bohr radius of CdS) is the major quantum confining dimension of the nanorods. The PL spectrum of the nanorods showed a strong band-edge emission at 432 nm and a weaker peak at 587 nm. Similar PL spectra with peaks around 390 nm21-23 or 430 nm24 and broad emissions centered around 500-550 nm were reported for CdS nanoparticles produced by various techniques. The sharp peak around 390 nm was assigned as a band to band transition, while the broad peak around 500-550 nm was attributed to surface states. The sharpness of the high-energy peak was attributed to the narrow distribution of the particles. The sharp peaks disappeared for aggregated nanoparticles.21,25 We found no dependence of the PL spectra on the excitation wavelength as reported by Sarkar et. al.,22 showing the homogeneity of our samples. The absorption spectrum of the spherical nanoparticles (Figure 2b) exhibited a weak peak at ∼462 nm, and the PL spectrum showed a high-energy peak at around 440 nm and surface peaks at 635 and 675 nm, indicating the presence of different surface sites. The 462 nm cannot be assigned to the excitonic peak, being lower in energy than the PL peak, and it is attributed to surface states or aggregated particles. The excitonic peak should be in the range 400-450 nm, and therefore we assume it is hidden under the background. We expect the excitonic peak to be broader than the nanorod peak due to the increase in the particle sizes. The red-shifting of the spherical nanoparticles (diameter 5 nm) in both the absorption and PL spectra vis-avis the nanorods (diameter 1.7-2.9 nm) is in agreement with their relatively larger diameters. As expected, the excitonic peaks of both the rods and spherical nanoparticles were blue-shifted compared to those of bulk CdS.26 The relative amount of coating by HDA molecules was calculated for both nanoparticles and it was found that the rods contain about 50% more. By comparing their spectra, we notice that the nanorods have higher absorption and lower surface peaks, probably due to their better passivation. The different shapes of these two nanoparticle samples (rods and spherical) may also contribute to the observed shifts as was reported earlier.22 Raman measurements of the rods and the spherical CdS nanostructures were performed on cast films prepared by placing a drop of suspension on a glass slide and allowing it to dry. The strongest Raman spectra were obtained for excitation at 325 and 488 nm, whereas weaker spectra were obtained for 514 nm, and a very weak (mainly HDA peaks) signal resulted from 633 nm excitation. The measurements with the bulk sample resulted in strong Raman spectra for 514 and 633 nm excitation, whereas solely fluorescence was observed with 325 and 488 nm excitation. The blue shift of the resonance excitation condition for CdS nanoparticles relative to the bulk sample is also an indication of the quantum confinement effect in these nanoparticles. We have measured the PL spectra for all samples by extending the Raman measurements up to 8000 cm-1 for the different excitation wavelengths. The PL spectra of the dried nanoparticle films are somewhat different from those of dichloromethane suspensions. The spectra for the spherical particles (not shown) are similar to those of the nanorods and differ only in the ratio between the Raman and the PL intensities, which is higher for the rods. With 325 nm excitation, the rods and spheres showed similar PL spectra (Figure 3a) with two broad peaks at around 410 and 450 nm, respectively, in line with the spectra obtained from dichloromethane solution as described in Figure
Raman Spectroscopy of Ultranarrow CdS Nanostructures
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11845
Figure 2. Room-temperature UV-vis absorption and photoluminescence spectra (excitation at 370 nm) of CdS in dichloromethane: (a) nanorods; (b) spherical particles.
Figure 3. Photoluminescence spectra of nanorods and bulk CdS sample with excitation at (a) 325 nm and (b) 514 nm.
Figure 4. Raman spectra of CdS nanorods at different excitation wavelengths.
2. However, with 488 nm (not shown) and 514 nm excitation (only the low-energy PL peaks are presented; Figure 3b), broad emission centered between 700 and 750 nm was observed. For the bulk CdS sample, intense emission was observed for all excitation wavelengths below 514 nm (Figure 3a). The observed PL was 3 orders of magnitude more intense than that obtained from CdS nanostructures, thus masking the Raman peaks for bulk CdS. However, excitation at 514 nm (shown in Figure 8b) and 633 nm yielded strong Raman spectra superimposed with the PL spectra. The PL spectrum excited at 325 nm has a very intense, broad peak at 380 nm; for 488 nm excitation it is shifted to 510 nm; and at 514 and 633 nm excitation the two peaks at 720 and 750 nm (typical also for the nanostructures) dominate the PL spectra. Similar low-energy peaks appeared in our PL measurements (Figure 2) in agreement with other reports; peaks around 560 nm21,23,26 and at 758 nm25 were assigned to transitions from surface and surface defects states. The Raman spectra were very strong for 514 and 633 nm excitation in the bulk, while for the nanostructures strong Raman spectra were obtained for lower wavelength excitations (488 nm and less). The shift in the absorption edge is as
expected, indicating a blue shift of the band gap energy due to confinement.9,11,27 Figure 4 shows the Raman spectra for the nanorods with different excitation wavelengths. The typical peaks of the longitudinal optical modes of CdS are evident from the spectra. The fundamental frequency at 300 cm-1 (1LO) and the first overtone at 600 cm-1 (2LO) were observed. The third harmonic at 900 cm-1 was very weak and could be observed only for the 325 and 488 nm excitations. Contributions from HDA molecules (the capping surfactant molecules) appeared mainly for nonresonance conditions where the CdS Raman intensity was low. For 325, 457, and 488 nm excitations, strong spectra showing the two characteristic CdS peaks, together with a weak peak of the third harmonic at 900 cm-1, were observed. For excitation at 514 nm, the HDA peaks appeared together with the typical CdS peaks. At 633 nm excitation, the HDA peaks dominated the spectrum; the CdS peak at 300 cm-1 was weak and that at 600 cm-1 was not visible. All nanosamples exhibited similar Raman spectra, showing mainly the two typical LO modes. No marked change in the peak position as a function of particle size (2-5 nm), shape
11846 J. Phys. Chem. C, Vol. 111, No. 32, 2007
Zeiri et al. TABLE 1: Ratio of I2LO/I1LO for Various Samples at Different Excitation Wavelengths
Figure 5. Raman spectra of nanorods and spherical nanoparticles with excitation at 488 nm.
(rods, spheres) or excitation wavelength was observed. These findings are in agreement with those of Shiang et al.9 but in contradiction with earlier study on CdS nanowires11 reporting a large red shift of the frequency with a decrease in particle size from 22 to 9 nm. Larger nanowires with 50-200 nm diameters28 showed a smaller red shift in comparison with CdS bulk samples. A different study27 showed a small shift of the LO frequency in response to changing the excitation wavelength, with the shift being attributed to selective enhancement of crystallites of certain sizes and not to phonon confinement. The Raman spectra of the surfactant-coated nanoparticles exhibit some contribution from the stabilizer molecule (HDA). The HDA signature decreases when CdS is in resonance with the laser line (325 and 488 nm) and increases for out-ofresonance conditions (514 and 633 nm). The difference in spectra between the nanorods and the spherical nanoparticle is reflected in the ratio between the CdS and the HDA contribution, as shown in Figure 5. The rods spectrum has mostly CdS contributions (300 and 600 cm-1) while that of the spheres shows a noticeable contribution from HDA in the range 7001500 cm-1, owing to the higher absorption cross-section of the 488 nm laser light by the nanorods as is shown in Figure 2. Bulk CdS yielded only very strong photoluminescence for excitation wavelengths of 325, 457, and 488 nm, respectively, and no Raman lines were observed for these excitations. However, upon changing the excitation wavelength to 514 and 633 nm, very strong Raman spectra were observed for the bulk sample (Figure 6). The Raman spectra with 514 nm excitation exhibit the typical 300 and 600 cm-1 peaks with similar intensities. A very weak third harmonic at 900 cm-1 was observed as well. At 633 nm excitation, the nonresonance conditions resulted in a weaker Raman spectrum showing the two typical CdS peaks along with some additional weaker peaks that were not detected for other excitation wavelengths. These weaker peaks probably originated from surface impurities present in the sample (note the unpas-
wavelength (nm)
spheres
rods
bulk
325 457 488 514 633
0.38 ( 0.02 0.25 ( 0.04 0.16 ( 0.02 0.42 ( 0.05 -
0.35 ( 0.02 0.25 ( 0.04 0.14 ( 0.02 0.40 ( 0.05 -
0.88 ( 0.01 1.16 ( 0.04
sivated nature of the bulk sample surface). Their spectra were observed only under conditions where the CdS was not in resonance with the laser line (633 and 785 nm). The extra peaks disappeared for 514 nm excitation as a result of enhancement of the CdS peaks under resonance conditions. For 633 nm excitation, the 600 cm-1 peak became stronger than the 300 cm-1 peak, and the 600/300 cm-1 intensity ratio grew from 0.88 to 1.6 upon changing the excitation from 514 to 633 nm. The ratio of the 2LO mode intensity to the 1LO mode intensity, I2LO/I1LO,9,10 which reflects exciton-phonon coupling, was calculated for the different samples at different excitation wavelengths (Table 1) with repetitive measurements. It is evident from Table 1 that the 600/300 cm-1 ratio depended on the size and the shape of the nanoparticles. Clearly, the I2LO/I1LO values for the two types of nanoparticles are low in comparison with those for the bulk material due to quantum confinement. The ratio varied from 1.16 in the bulk down to 0.14 in the nanostructures. A smaller difference in the ratio has been observed between the rods and the spherical particles. Although the error bars are masking the effect, it seems that the nanorods yielded consistently smaller ratios in comparison to the spheres owing to their smaller widths. Our observations are in agreement with an earlier reported decrease in ratio from 0.92 to 0.2 upon decrease in particle size from 6 to 1 nm.9 The phenomenon may be attributed to lowering of the exciton-vibration coupling due to quantum confinement, in agreement with many theoretical studies.13-15 Similar behavior was found for different crystal phases of CdS,29 where a strong decrease in the I2LO/I1LO ratio was observed for the hexagonal phase and a weaker one was observed for the cubic phase. Similarly, a big decrease of the ratio for nanoparticles in comparison with a single crystal is observed where factors of 4 and even 14 are reported,10,30 depending on the nanoparticle sizes. In contradiction to others and to our results, Pan et al.28 found an increase in ratios between the two LO modes (I2LO/ I1LO) from bulk (0.15) to nanowires (1.6) with bigger dimensions (50-200 nm in diameter and micrometers in length). The exciton-vibration coupling reflected by this ratio is very sensitive to particle sizes and excitation wavelengths, being very low for the smallest particles under resonance conditions. The confinement effect on the overtone to fundamental ratio is attributed to two main sources.31 The first is the increase of the
Figure 6. Raman spectra of CdS bulk sample at different excitation wavelengths.
Raman Spectroscopy of Ultranarrow CdS Nanostructures
Figure 7. Temperature dependence of the Raman spectra of CdS nanorods with 488 nm excitation.
overlap of the spatial wave functions of the electron and the hole in the electronic excited states in the nanoparticles, and the second is a big decrease of the lifetime of the excited state due to trapping on defects on the surface A weak third harmonic LO peak at 900 cm-1 was observed for both nanostructures (rods and spheres) at 488 and 514 nm excitations. At 488 nm, the value of the 300/900 cm-1 ratio was 0.02. In bulk CdS for 514 nm excitation, this ratio was found to be 0.05. Thus, the LO mode ratios also depended on the excitation wavelength, being the lowest for resonance Raman conditions, that is, at 488 nm for the nanostructures and at 514 nm for the bulk material. A previous comprehensive study for CdS nanowires showed a strong influence of the size and the excitation energy on the I2LO/I1LO ratio.11 The I2LO/I1LO ratios decreased with decreasing particle size and excitation energy, varying from 0.2 for 9 nm particles to 1.0 for 22 nm particles. The band gap energy calculated from this data showed a clear increase with decreasing particle sizes, as expected. In another study, the changes of I1LO and the I2LO for different excitation wavelengths32 were correlated with resonance of the 1LO mode (around 496 nm) and the 2LO mode (around 488 nm) with the photoluminescence band. We have also studied the effect of temperature on the Raman spectrum of the nanostructures and bulk samples over a broad spectral range (100-8000 cm-1). The temperature dependence of the Raman spectra in the range of 80-300 K for the nanorods with 488 nm excitation is shown in Figure 7. The shapes of the Raman peaks and the 600/300 cm-1 ratios remained unchanged over the entire temperature range. However, a red shift of 4-5 cm-1 in the peak maxima was observed when the temperature was lowered down to 80 K as shown in Figure 7. Varying temperature for the CdS bulk sample did not show any significant shifts in the Raman peak positions. The red shift of the peak maxima for the rods might be attributed to reduced thermal motion at low temperature.
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11847 When the Raman measurements were extended up to 3000 cm-1, two sharp Raman peaks at 2850 and 2887 cm-1 were found for different excitations (488, 514, and 633 nm). For 488 nm the peaks are visible around 565 nm (Figure 8a), which appeared for the entire temperature range studied (80-300 K). The absence of the peaks for bulk CdS samples (for this excitation wavelength, 3000 cm-1 Raman shift should appear around 605 nm in Figure 8b) related them to the quantum confinement of the nanostructures. Broad low-energy peaks around 2900 cm-1 were reported by Blandin et al.26 for 3.8 nm CdS particles that were attributed to interband transitions in the conduction band. The peak breadth was attributed to the wide range of sizes of the nanocrystals. Our low-energy peaks were somewhat shifted vis-a`-vis those of Blandin et al.,26 possibly due to the different dimensions and shapes of our CdS nanoparticles. Cooling of the CdS nanorods down to 80 K caused a decrease in the intensity of the short-wavelength emission, as well as in the intensity of the Raman peaks (Figure 8a). The broad emissions around 580 and 700 nm gained intensity upon cooling. For bulk CdS (Figure 8b), the short-wavelength emission and the Raman peaks became weaker upon cooling but with no changes in the Raman peak positions. A new strong emission around 560 nm appeared, and the 750 nm emission increased. A similar increase in the low-energy surface state emission intensity upon cooling was reported earlier33 and assigned to defects and irregularities in the samples. For the cubic phase of CdS,34 the opposite behavior was reported, where sample cooling caused narrowing of the photoluminescence peaks along with a blue shift. An interesting phenomenon was observed for the differentshaped nanostructures, when the intensities of the two typical CdS peaks in the anti-Stokes spectra were compared to those for the same peaks in the Stokes spectra. We obtained very high anti-Stokes intensities, which were the same or even higher than the Stokes intensities. The ratios of anti-Stokes to Stokes intensities were found to largely exceed those predicted according to the Boltzmann factors.35 Classical Boltzmann distribution predicts values much smaller than unity, while we obtained values closer to, or even higher than, unity. The strongest effects were observed with 514 and 488 nm excitations, that is, under resonance conditions with the laser line. The phenomenon of very high anti-Stokes intensities was less pronounced for bigger particles and disappeared for bulk samples. These findings are probably due to a specific photonexciton interaction, which is strong for small particles where quantum confinement plays a dominant role. Large anti-Stokes intensities were also observed for surface-enhanced Raman scattering (SERS) in the presence of silver nanoparticles and were attributed to selective enhancements in SERS.36,37 The
Figure 8. (a) CdS nanorods excited at 488 nm; (b) CdS bulk excited at 514 nm.
11848 J. Phys. Chem. C, Vol. 111, No. 32, 2007 phenomenon has been analyzed in detail by our group for different temperatures, wavelengths, sample thickness, etc. (L.Z. et al., unpublished data) and will be reported separately. The exact position of the Raman peaks can be determined by averaging the anti-Stokes and Stokes peak positions. Any shift of the Stokes peaks due to the nonideality of the system will be expressed as an opposite shift of the anti-Stokes peaks. Application of this procedure for the maximum Raman intensity at 488 nm yielded a value of 300.6 cm-1 for the main peak (1LO) for the nanospheres and 301.5 cm-1 for the nanorods. For the secondary peaks (2LO), these values were 601.8 cm-1 for the nanospheres and 602.8 cm-1 for the nanorods. These small shifts might have been caused by the size differences between the nanoparticles; the width of the rods is around 2 nm, while the width of the spheres is larger (5 nm). For CdS bulk, even lower values at 299.2 and 601.5 cm-1 were obtained with 514 nm excitation. These very small shifts appeared systematically in many repetitive measurements. Shifts of Raman peaks toward smaller values for bigger particles have been observed in other studies.11,28 The blue shift of 7 cm-1 measured for particles of 9-22 nm11 is even greater than the differences we obtained between the nanoparticles and the bulk. We were able to prepare nanowires having a diameter similar that of the nanorods but a larger length of 80-150 nm.20 The different Raman measurements for the nanowires yielded results very similar to those of the nanorods, indicating the importance of the diameter rather than the length as the critical dimension in this system. 4. Summary Our Raman measurements of ultranarrow CdS nanorods showed a higher confinement effect in comparison to bulk samples, while the difference was smaller between the ultranarrow nanorods and slightly larger spherical nanoparticles. While the Raman peaks were not very sensitive to particle size, confinement effects were evident in the absorption and PL spectra, in the Raman intensity relative to that obtained from the HDA capping surfactant, and in the ratio of the overtone to the fundamental LO modes. For nanorods narrower than the Bohr radius, we observed a blue shift in the absorption and PL relative to the spheres and the smallest 2LO/1LO ratio. Cooling of the nanoparticles resulted in an increased PL emission intensity and a red shift of the Raman frequency. The coupling between LO modes and optically generated electrons and holes decreased with decreasing nanoparticle sizes. The coupling, which depended on the excitation wavelength, fell to a minimum at resonance conditions. The decrease in the vibronic character of the lowest electronic excitation was in accord with most theoretical and experimental studies. Both rods and spherical particles showed a large red shift of the Raman peak position relative to bulk CdS, while a smaller difference was observed between the two nanoparticle samples. Evaluating the exact positions of the peaks by averaging both Stokes and anti-Stokes peak positions showed a weak red shift of the peaks with decreasing nanoparticle size. The extension of the Raman measurements to the anti-Stokes region yielded the surprising observation of very strong anti-Stokes intensities. The phenom-
Zeiri et al. enon was not observed for bulk CdS samples, relating the effect to the nanoscale dimensions of these nanoparticles. Acknowledgment. This work has been partially supported by the European Commission FP-6 Project SEMINANO under Contract NMP4-CT-2004-505285, and by the U.S.-Israel Binational Science Foundation, Grant 2002059. References and Notes (1) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060. (2) Banyai, L.; Koch, S. W. Phys. ReV. Lett. 1986, 57, 2722. (3) Empedocles, S.; Neuhauser, A. R.; Bawendi, M. G. Nature 1999, 399, 126. (4) Efros, A. L.; Rosen, M. Annu. ReV. Mater. Sci. 2000, 30, 475. (5) Empedocless, S. A.; Norris, D. J.; Bawendi, M. G. Phys. ReV. Lett. 1996, 77, 3873. (6) Zhang, J. Z. Acc. Chem. Res. 1997, 30, 423. (7) Weller, H.; Eychmu¨ler, A. In AdVances in Photochemistry; Wiley, New York, 1995; pp 165-215. (8) Alivisatos, A. P. J. Phys. Chem. B 1996, 100, 13226. (9) Shiang, J. J.; Risbud, S. H.; Alivisatos, A. P. J. Chem. Phys. 1993, 98, 8432. (10) Shen, W. Z. Physica B 2002, 322, 201. (11) Routkevitch, D.; Haslett, T. L.; Ryan, L.; Bigioni, T.; Douketis, C.; Moskovits, S. Chem. Phys. 1996, 210, 343. (12) Scamarcio, G.; Spagnolo, V.; Ventruti, G.; Lugara´, M.; Righini, G. C. Phys. ReV. B 1996, 53, 10489. (13) Schmitt-Rink, S.; Miller, D. A. B.; Chemla, D. S. Phys. ReV. B 1987, 35, 8113. (14) Efros, A. L.; Ekimov, A. I.; Kozolwski, F.; Petrova-Koch, V.; Schmidbar, H.; Shumilov, S. Solid State Commun. 1991, 78, 853. (15) Nomura, S.; Kobayashi, T. Phys. ReV. B 1992, 45, 1305. (16) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (17) Warner, J. H.; Tilley, R. D. AdV. Mater. 2005, 17, 2997. (18) Cao, C. Y.; Wang, J. J. Am. Chem. Soc. 2004, 126, 14336. (19) Alivisatos, A. P.; Harris, T. D.; Carrol, P. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 90, 3463. (20) Acharya, S.; Patla, I.; Kost, J.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2006, 128, 9294. (21) Cao, L.; Miao, Y.; Zhang, Z.; Xie, S.; Yang, G.; Zou, B. J. Chem. Phys. 2005, 123, 24702. (22) Sarkar, R.; Shaw, A. K.; Narayanan, S. S.; Rothe, C.; Hintschich, S.; Monkman, A.; Pal, S. K. Opt. Mater. 2006 (in press). (23) Miao, Y.; Wu, Z.; Cao, L.; Fu, L.; He, Y.; Xie, S.; Zou, B. Opt. Mater. 2004, 26, 71. (24) Pan, D.; Jiang, S; An, L.; Jiang, B. AdV. Mater. 2004, 16, 982. (25) Wang, Z. Q.; Gong, J. F.; Duan, J. H.; Huang, H. B.; Yang, S. G.; Zhao, X. N.; Zhang, R.; Du, Y. W. Appl. Phys. Lett. 2006, 89, 033102. (26) Blandin, A.; Wang, K. L.; Kouklin, N.; Bandyopadhyay, S. Appl. Phys. Lett. 2000, 76, 137. (27) Schreder, B.; Dem, C.; Schmitt, M.; Materny, A.; Kiefer, W.; Winkler, U.; Umbach, E. J. Raman Spectrosc. 2003, 34, 100. (28) Pan, A.; Liu, R.; Yang, Q.; Zhu, Y.; Yang, G.; Zou, B.; Chen, K. J. Phys. Chem. B 2005, 109, 24268. (29) Sivasubramanian, V.; Arora, A.; K. Premila, M.; Sundar, C. S.; Sastry, V. S. Physica E 2006, 31, 93. (30) Kostic, R.; Romcevic, N. Phys. Status Solidi C 2004, 1, 2646. (31) Shaing, J. J.; Goldstein, A. N.; Alivisatos, A. P. J. Chem. Phys. 1990, 92, 3232. (32) Abdi, A.; Titova, L. V.; Smith, L. M.; Jackson, H. E.; YarrisonRice, J. M.; Lensch Appl. Phys. Lett. 2006, 88, 043118. (33) Titova, L. V.; Hoang, T. B.; Jackson, H. E.; Smith, L. M.; YarrisonRice, J. M.; Lensch, J. L.; Lauhon, L. J. Condens. Matter 2006 1-3, arXiv: cond-mat/0608057; Los Alamos National Laboratory Preprint Archive. (34) Yu, Y. M.; Choi. Y. D. Phys. Status Solidi C 2006, 3, 1180. (35) Long, D. A. Raman Spectroscopy; McGraw-Hill International New York, 1977; p 84. (36) Kneipp, K.; Wang, Y.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1996, 76, 2444. (37) Maher, R. C.; Etchegoin, P. G.; Cohen, L. F. J. Phys. Chem. B 2006, 110, 11757.
