Size Effects of Raman and Photoluminescence Spectra of CdS

Article pubs.acs.org/JPCC

Size Effects of Raman and Photoluminescence Spectra of CdS Nanobelts Chuan Hu, Xianghua Zeng,* Jieya Cui, Haitao Chen, and Junfeng Lu College of Physics Science and Technology & Institute of Optoelectronic Technology, Yangzhou University, Yangzhou 225002, China ABSTRACT: Different sizes of CdS nanobelts were synthesized at 800, 850, and 900 °C by the thermal evaporation of CdS powders on Au-coated silicon substrates and were used to study the size effects of Raman scattering and photoluminescent spectra. The Raman spectra of CdS nanobelts clearly exhibit first- and second-order longitudinal optical (LO) Raman peaks, surface phonon peaks, and multiphonon processes when excited using a wavelength of 532 nm. The mechanism of exciton−phonon coupling was observed to be mainly associated with the Fröhlich interaction, and the coupling strength of the exciton−phonon increases with increasing lateral size. Compared with a larger CdS nanobelt, a narrower nanobelt exhibits a larger tensile strain. Recombination of free excitons (FX), excitons bound to neutral impurities (A0X), and donor−acceptor pairs (DAP) were identified from a low-temperature PL spectrum. At temperatures below ∼123 K, a red shift of the FX energy occurs with decreasing lateral size due to a larger uniaxial tensile strain; at temperatures above ∼123 K, a red shift of the FX energy occurs with increasing lateral size because of the reabsorption of the emitted light inside the thicker belt, indicating that the FX energy is affected by both the tensile strain and the surface-depletion-induced quantum confinement (the reabsorption of the emitted light) in the nanobelt. troscopy. Liu et al.4 conducted exciton-related optical studies of cadmium sulfide (CdS) nanobelts based on temperaturedependent PL spectra, where the recombination of free excitons, excitons bound to neutral impurities, and donor−acceptor pairs were resolved and a random lasing action was observed at room temperature. By studying the relations of the PL peaks with the thickness of the CdS nanobelts, Li et el.5 observed a systematic blue shift of the free exciton A (EA) emission of CdS nanobelts, with the thickness decreasing from 230 to 31 nm for different temperatures and ascribed the red shift of the thick CdS nanobelt to the reabsorption of the emitted light. In addition, these authors explained the thickness dependence of emission energy beyond the quantum confinement regime as surface-depletioninduced quantum confinement. In contrast, in ref 6, the redshift of the emission peak energy was attributed to the uniaxial tensile strain and the enhanced exciton−phonon coupling strength in the bent ZnO nanowires (NWs). On the one hand, a stronger tensile strain in the narrow nanobelt will lead to a red shift; on the other hand, the reabsorption of the emitted light in the thicker nanobelt will lead to a red shift. However, we do not understand the competition between the tensile strain and the surfacedepletion-induced quantum confinement (the reabsorption of the emitted light) or how this behavior changes. Furthermore, the electron phonon (EP) coupling in polar semiconductor NWs is an important issue, as it has a significant

1. INTRODUCTION One-dimensional semiconductor nanostructures have attracted significant attention because their anisotropic geometry results in unique physical properties, which offer great potential application in nanolasers,1 thermal conductors,2 and photonic integration. As an important II−VI semiconductor material, CdS has a direct band gap of 2.45 eV at room temperature and is regarded as a possible candidate for optoelectronic applications in the visible spectrum range. The detailed optical investigation on CdS nanomaterials in terms of their light emission mechanism remains of interest, especially the exciton-related emission that has not been firmly identified due to the complicated exciton− phonon coupling for different samples. The recent controlled growth of one-dimensional nanostructures provides the conditions necessary to investigate the optical properties associated with exciton-related emission, as the exciton-related emission is strongly connected with the size confinement, strain, crystallinity, and exciton longitudinal optical longitudinal optical (LO) phonon interaction. The coupling between nuclear vibrations and electronic excitations (electron−phonon coupling) in semiconductor nanocrystals affects many photophysical processes in nanomaterials. Recently, the exciton dynamic processes in CdS nanostructures attracted significant attention3−5 because defects in CdS consist of sulfur vacancies (VS) and cadmium vacancies (VCd), interstitials, such as CdI and SI, and anti-sites, such as SCd and CdS; these defects can trap electrons, holes, or excitons and form bound complexes, which can be studied using temperaturedependent and time-resolved photoluminescence (PL) spec© XXXX American Chemical Society

Received: July 22, 2013 Revised: September 10, 2013

A

dx.doi.org/10.1021/jp407272u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

film was obliquely put downstream of the CdS powder at a distance of 10.5 cm. Before heating, the system was purged with 522 sccm high-purity argon (Ar 99.999%) for 30 min. Then, the pressure was reduced to 7.5 × 10−2 Torr for the duration of the reaction. Next, the furnace was heated to the desired temperature at a heating rate of 10 °C/min and maintained at this temperature for 30 min of deposition with the Ar flow maintained at 50 sccm. Finally, three types of nanobelts were obtained with deposition temperatures of 800, 850, and 900 °C and were labeled as A, B, and C, respectively. The as-synthesized products were characterized by X-ray diffraction (XRD-7000, Shimadzu) using Cu Kα radiation (λ = 0.15406 nm). The XRD data were collected with a scanning speed of 7° (2θ)/min in the 2θ range from 10° to 65° using the continuum scanning method. The morphology and microstructure of the samples were observed using a field-emission scanning electron microscope (FESEM, s-4800II, Hitachi) equipped with an X-ray energy dispersive spectrometer (EDS) under an acceleration voltage of 15 kV and a high-resolution transmission electron microscope (HRTEM, Tecnai F30, FE). The absorption measurements were performed using a UV−vis− NIR spectrophotometer (UV−vis, Cary-5000, Varian) with an integrating sphere. The PL measurements were performed using a Britain Renishaw In Via micro-Raman spectrophotometer, with a 325 nm line of a He−Cd laser as the excitation light source in a closed-cycle He cryostat. The Raman measurements were performed using Renishaw In Via equipment.

effect on the optical and electrical properties, such as nonlinear optical processes and the energy relaxation mechanism of excited carriers. Raman spectroscopy has been regarded as a powerful tool for the investigation of material properties, such as lattice defect identification, crystal orientation, and doping concentration. From Raman scattering, the intensity ratio of 2LO to 1LO provides an indication of the exciton−phonon coupling, where a larger intensity ratio of 2LO to 1LO corresponds to a stronger exciton−phonon coupling. The 1LO and 2LO phonon peaks for a single crystal of bulk CdS were reported to be positioned7 at 305 cm−1 and 600 cm−1, respectively, while for CdS nanostructures, the 1LO phonon peak was observed at 298−305 cm−1.8−12 The red shift of the Raman peaks compared with that of bulk CdS was considered to be due to the effect of phonon confinement. Although there are some reports on the electron−phonon coupling,14−19 the size effects of the electron− phonon coupling are still not clear. For example, some results indicate that the exciton-LO phonon coupling strength increases with the size of CdS quantum dots14 or in nanowire and nanobelts.15 In contrast, some results indicate that the excitonLO phonon coupling strength decreases with size. For example, Pan et al.16 reported that the exciton-LO phonon coupling strength in NWs is much stronger than in the bulk; Fan et al.17 demonstrated that the strength of electron−phonon coupling increases with decreasing size in ZnO nanocombs, and similar results were observed in CdS:Mn nanoparticles11 and CdS nanoparticles.20 Thus, the size dependence of the exciton-LO phonon coupling strength in CdS nanostructures remains unresolved. In this paper, single crystal CdS nanobelts are prepared via a catalyst-assisted vapor−liquid−solid (VLS) method, and Raman and PL spectrum measurements are performed to study their optical properties. The Raman measurement using an excitation wavelength of 532 clearly reveals that the 1LO and 2LO phonons are located near 302 cm−1 and 604 cm−1, respectively. Meanwhile, the surface optical (SO) phonon peaked at 283 cm−1, and multiphonon processes are observed. The intensity ratio of the 2LO phonon mode to the 1LO phonon mode (I2LO/ I1LO) was observed to be large and sensitive to the size of the nanobelts, indicating strong exciton−phonon coupling in CdS nanobelts. The 1LO phonon energy systematically increases with increasing lateral size, implying a strong tensile strain in a narrow naonobelt. The exciton-related optical properties of CdS nanobelts are investigated using temperature-dependent PL spectra, where the band-edge recombination in the visible region can be assigned as a free exciton (FX), an exciton bound to neutral donor (D0X) impurities, an exciton bound to neutral acceptor (A0X) impurities, and a donor−acceptor pair (DAP) through systematic analysis. The analysis reveals that, below ∼123 K, a red shift of the PL peak energy occurs with decreasing lateral size. In contrast, above ∼123 K, a blue shift occurs because all of the excitons become mobile at higher temperatures, and more emitted light contributed from the recombination of excitons could be trapped inside the thick nanobelt due to the internal reflection and could be absorbed again, resulting in the red shift, a broadening of the emission peak.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The FESEM micrographs of the as-grown products were acquired to investigate the effect of temperature on the morphology of the hexagonal wurtzite CdS nanostructures. As shown in the FESEM images in Figure 1a−c, a large amount of CdS nanobelts is distributed randomly on the substrate, and most of the nanobelts are uniform with a smooth edge, except for a few with saw-like edges along the belts. From Figure 1, it is clear that the size of the nanobelt is determined by the growth temperature. At 800 °C, the typical width of the nanobelts is in the range of 150−400 nm, and the

2. EXPERIMENT CdS nanobelts of different widths and thicknesses were prepared via the thermal evaporation of CdS powders onto Au-coated Si substrates at 800, 850, and 900 °C. First, a 0.2 g sample of CdS powder was used as the source material, which was placed at the center of the heating zone. A Si (100) substrate coated with a Au

Figure 1. Low- and high-resolution (inset) FESEM micrographs of the as-grown CdS nanobelts for samples (a) A (800 °C), (b) B (850 °C), and (c) C (900 °C); (d) EDS spectrum for CdS nanobelts with a deposition temperature of 800 °C. B



Article

3.2. Raman Spectra. The Raman spectra of the CdS nanobelts were collected using an excitation wavelength of 532 nm and an incident laser light power of 300 mW; a micro-Raman setup was used to focus a spot size diameter of 5 μm on the surface of a NC sample or a mesa structure, where only 5% incident laser light power was used. Each of the phonon wavenumbers was extracted by fitting the spectrum to a Lorentz line shape, and the 1LO and 2LO phonons and multiphonon processes can be clearly observed in Figure 3a. For samples A, B, and C, the 1LO phonons are located at 302, 303, and 303.7 cm−1, respectively, and their full-width at half-maxima (FWHMs) are 11.2, 11.2, and 12.3 cm−1, respectively. In addition to the LO phonon and its replicas, several peaks are resolved as 214, 238, 255, 283, 323, 347, 367, and 559 cm−1, suggesting that the samples have better crystalline quality.13 The peaks at 238 and 255 cm−1 reported by Fan et al.21 can be assigned to A1 (TO) and E2H, respectively. The peaks at approximately 214, 347, 367, and 559 cm−1 can be assigned to multiphonon scattering, which is consistent with other reports.22 From Figure 3, one can easily determine that the intensity of 1-LO phonon decreases while that of 2-LO phonon increases greatly with the lateral size increasing (from sample A to C), and the integrated intensity ratios of 2LO to 1LO (I2LO/I1LO) are 0.91, 1.4, and 3.8 for samples A, B, and C, respectively. The I2LO/I1LO ratio, which is proportional to the Huang−Rhys factor S, could be an indicator of the strength of EPC; that is, a larger integrated ratio of 2LO to 1LO corresponds to a stronger electron−phonon coupling interaction. Therefore, we can conclude that the electron−phonon interaction strength increases with an increase in the width of the nanobelt. When the size increases, the extent of the excitonic wave function can spread over a larger region with a longer electron−hole separation, and these excitons couple strongly with the polarization field of LO phonons at the surface region, leading to a larger S. This finding is consistent with other reported works14,15 but is in contrast to the results of refs 16, 17, and 20. The e−p coupling mechanism can be described as the deformation potential and the Fröhlich potential,17,23 with the deformation potential involving the short-range interaction between the lattice displacement and the electrons and determining the TO Raman scattering cross section, and the LO Raman scattering cross-section including contributions from both the deformation potential and the Fröhlich potential, where the Fröhlich potential involves the long-range interaction generated by the displacement of partial ion nuclei. From Figure 3, one observes that the TO peak does not change with the lateral size; however, the LO intensity is sensitive to the lateral size and also stronger than the TO intensity. Therefore, we conclude that the strong electron−phonon coupling in the CdS nanobelts is mainly associated with the Fröhlich interaction, which is consistent with other results.14 A previous study on the sizedependent e−p coupling of ZnO nanocrystals reported that the Fröhlich coupling decreases with decreasing size due to the Fröhlich potential;17 this phenomenon can be attributed to the increasing defects or vacancies on the surface of the ZnO nanocomb with decreasing size. A hole can be localized in traps such as defects or vacancies on the crystalline surface, which will result in incomplete overlap, causing the increase of e−p coupling with decreasing size. For the Raman peak position of the 1LO phonon mode, a small wavenumber shift occurs, as indicated in Figure 3b, and the A1(LO) phonon mode corresponds to atomic oscillations along the c-axis. The peak value of this mode is sensitive to lattice strain along the c-axis;

thickness is on the scale of a few tens of nanometers (Figure 1a); at 850 °C, widths between 400 nm and 3 μm and thicknesses of approximately 60−80 nm are observed (Figure 1b); at 900 °C, widths of 1−4 μm, thicknesses between 80−110 nm, and a typical length of tens to hundreds of micrometers are observed, and some of the nanobelts exhibit a sawtooth-like shape (Figure 1c). The EDS spectrum of sample A indicates that Cd and S are the major elements (Figure 1d), and the atomic percentage of S is less than that of Cd, implying that sulfide vacancies are present in the nanobelts. The XRD patterns and TEM and HRTEM images were employed to further reveal the structure details and growth orientation of the CdS nanobelts, as shown in Figure 2. Figure 2a

Figure 2. (a) TEM images of CdS nanobelts with deposition temperature of 800 °C. (b, d) HRTEM images of the rectangle area, the inset is the FFT analysis of the HRTEM image in d. (c) XRD patterns of CdS nanobelts with different substrate temperatures.

and b presents the collective TEM images of a typical CdS nanobelt, where the HRTEM images are shown in Figure 2d acquired from the areas marked by rectangles in Figure 2a and b. One can find a perfect single crystal structure from the HRTEM images with lattice spacings of 0.36 and 0.21 nm, which correspond to the (1010̅ ) and (1120̅ ) planes of wurtzite CdS, respectively; this result is consistent with the XRD powder diffraction file (PDF no. 65-3414) (Figure 2c). Both the HRTEM image (Figure 2d) and the corresponding FFT pattern (inset) indicate the growth direction (along ⟨101⟩) and better crystalline quality. Typical XRD patterns of the products deposited on the Si substrates are presented in Figure 2c, where the standard spectra for the hexagonal wurtzite CdS are presented for reference. The diffraction peaks of all of the products are almost identical and consistent with the peaks of the hexagonal wurtzite CdS in the JCPDS Card (No. 65-3414), with the lattice constants of a = 4.132 Å and c = 6.734 Å. Only the peaks at 2θ = 38.187°, 44.385°, and 64.576° correspond to the (111), (200), and (220) planes of Au (JCPDS no. 65-2870). For three samples, the XRD patterns have the most intense peak at (101), indicating preferential growth along the ⟨101⟩ direction and good crystalline quality. C



Article

Figure 3. (a) Room-temperature Raman scattering spectra of samples A, B, and C; (b) first-order Raman spectra for samples A, B, and C; (c) calculated SO phonon dispersions for samples A and B, with the symmetric (S) and asymmetric (AS) modes presented.

Then, the lattice contractions Δc/c are determined to be −0.36%, −0.24%, and −0.16% for samples A, B, and C, respectively. Thus, we conclude that the tensile strain becomes stronger for a narrow nanobelt. In our previous work,24 the existing tensile strain was released by regulating the phase structure in the nanosheet. Here, the narrow nanobelt is under a strong tensile strain, and the

therefore, the strain associated with a lattice contraction of the caxis can be calculated using Δω = (1 + 3Δc /c)−γ − 1

(1)

where γ is the Grueneisen parameter (1.1 for CdS) and Δω is the 1LO phonon energy shift from its bulk value ω0 (305 cm−1). D



Article

lattice contraction decreases as the lateral width increases; this result is consistent with the observation in ref 18, where the firstorder longitudinal optical (LO) phonon energy was observed to systematically increase with increasing lateral size from nanowires to nanobelts to nanosheets. There is a peak at 283 cm−1 for samples A and B, which can be attributed to a surface phonon. Using the formula13 ωSO = [(ε∞ω2LO + 2εhω2TO)/(ε∞ + 2εh)]1/2 and parameters of ε∞ = 5.32 and εh = 1, we obtain ωSO values of 286.6 and 287.5 cm−1, respectively, which are very close to the experiment result of 283 cm−1. In ref 13, the Fröhlich surface optical phonon mode at 278 cm−1 was observed from IR absorption measurement. The surface phonon can be calculated further by considering the nanobelt as a rectangle shape. For the cross-section shape of a rectangular nanostructure, the SO phonon dispersion relations for belt-like nanostructures have symmetric and asymmetric solutions given by the following equations:25 ωSO

2 2 ⎤1/2 ⎡ ) ε∞(ωLO − ωTO 2 ⎥ = ⎢ωTO + ε∞ + εMf (x) ⎦ ⎣

(2)

f (x) = I0(x) K1(x)/[I1(x) K 0(x)]

(3)

where I and K are Bessel functions. In the limit where the phonon wavevector q ≫ ω/c, the dispersion relation for the SO phonon mode for a cylindrical nanobelt can be expressed as18 S ωSO

⎡ ε ω 2 tanh(qr /2) + ε ω 2 ) ⎤1/2 M TO ⎥ = ⎢ ∞ LO ε∞ tanh(qr /2) + εM ⎣ ⎦ ⎡ε ω2 ∞ LO

AS ωSO =⎢ ⎣

ωSSO

−1

tanh (qr /2) +

Figure 4. UV−vis spectra (a) and their relations with band gaps (b) for samples A (800 °C), B (850 °C), and C (900 °C). (4)

where A is the parameter that relates to the effective masses associated with the valence and conduction bands, and hν is the photon energy. The optical absorption in the edge region can be well-fit using eq 6 (shown in Figure 4b); from the fitting, the band gaps are extracted as 2.455, 2.441, and 2.422 eV for samples A (at 800 °C), B (at 850 °C), and C (at 900 °C), respectively. Compared with sample A, there are approximately 14 and 33 meV red shifts for samples B and C, respectively. The red shift of the band gap can be explained by the differences in the thickness direction of the CdS nanobelts. To determine the origins of the emission, we conducted temperature-dependent PL spectroscopy for A (grown at 800 °C) and C (grown at 900 °C), respectively. Figure 5a and b is the normalized temperature-dependent PL spectra of samples A and C, respectively. For both samples, with increasing measurement temperature, a red shift in the peak energy occurs, and the peak broadens and decreases in intensity. To further understand the size-dependent emission, we present the spectra at 5 K for samples A and C fitted with Lorentzian line shape decompositions in Figure 5c and d, where the emission peaks can be well-resolved, similar to the PL spectrum of the ensemble CdS nanobelts.3,4 Due to the spin−orbit coupling and crystal-field interaction, the valence band in CdS splits into three sub-bands, yielding three exciton levels with characteristic energies of EA = 2.550 eV, EB = 2.568 eV, and EC = 2.629 eV.28,29 Therefore, the emission peaks at 2.550, 2.537, and 2.525 eV for sample A are ascribed to EA, excitons bound to a neutral acceptor (D0X), and excitons bound to a neutral donor (A0X), respectively. The next peak near 2.507 eV on the lower energy side of A0X can be reasonably attributed to the first LO phonon replica of D0X with the 30 meV energy of the LO phonon of CdS. In addition, the two peaks near 2.485 and 2.455 eV can be ascribed to the 1LO and 2LO replicas of A0X. The peak at 2.412 eV is assigned to

2 ⎤1/2 ) εMωTO

ε∞ tanh−1(qr /2) + εM

⎥ ⎦

(5)

ωAS SO

where and are the symmetric and asymmetric SO phonon wavenumbers, respectively, q is the phonon wavevector, and εM = 4 is the dielectric constant of the surrounding medium. By substituting the ωLO and ωTO values of samples A and B into the above equations, we can plot the curves of the symmetric and asymmetric SO phonon wavenumbers with qr (as shown in Figure 3c). The values corresponding to the intersection position are ωSO phonon wavenumbers, that is, 276.7 and 277.3 cm−1, which are slightly smaller than the experimental values of 283 cm−1 and very similar to the values reported in ref 25, where the surface-optical modes were located at ∼240 cm−1 for single rods and ∼270 cm−1 for nanorod tracks. Unlike the case of the Zn/ ZnO core−shell, where the SO phonon mode exhibited a significant size confinement effect,26 no size effect on the SO phonon mode was observed, as the thickness of the nanobelt is significantly larger than the Bohr radius (6 nm) of CdS. 3.3. Photoluminescence. To examine the band gap and optical properties as a function of the size of the nanobelts, we performed UV−vis absorption and PL spectra measurements. The UV−vis absorption spectra were obtained by measuring the optical absorption spectra using an UV−visible spectrophotometer (Cary-5000) with an integrating sphere to guarantee a better measurement result, as shown in Figure 4a. The relationship between the absorption coefficient (R) near the absorption edge and the optical band gap (Eg) for direct interband transitions obeys the following formula:27 (αhν)2 = A(hν − Eg )

(6) E



Article

the redshift in the emission peak energy of the bent NWs compared with the as-grown NWs was considered to be the uniaxial tensile strain and the enhanced exciton−phonon coupling strength in the bent ZnO NWs. Therefore, the red shift could be regarded as the effect of larger uniaxial tensile strains because sample A has a larger uniaxial tensile strain than sample C, as previously discussed. At lower temperatures, the PL peak energies of samples A and C exhibit blue shifts due to the delocalization of excitons; at higher temperatures, as the excitons become mobile, the emissions attributed to these FX lines, their replicas become smaller, and the energies exhibit a red shift. Comparing the spectra of A and C, at temperatures lower than 100 K, the spectra of sample A are wider and the DAP emissions are stronger than that of sample C, indicating that sample C has better crystalline quality; in contrast, at temperatures higher than 150 K the spectra of sample C become wider, and a red shift occurs compared with that of the sample A. The electron−phonon interaction effect can cause a shift of the conduction and valence bands, which will result in a quadratic variation of the energy gap at low temperature and a linear shift at high temperature. The band gap or the energy of FXA as a function of temperature can be described by the empirical Varshni formula: E(T ) = E(0) −

αT 2 T + β′

(7)

where E(0) is the energy of FXA at 0 K, and α and β are Varshni’s coefficients. As shown in Figure 6a, the Varshni fitting curves (solid red line) fit our experimental results very well; the fitting parameters for sample A are E(0) = 2.552 ± 0.001 eV, α = 0.628 ± 0.061 meV K−1, and β = 243.77 ± 50.7 K, and the parameters for sample B are E(0) = 2.558 ± 0.001 eV, α = 0.985 ± 0.203 meV K−1, and β = 351.70 ± 130.07 K, which are consistent with previous results.5,29 From Figure 6a, at approximately 123 K, there is an intersection point for the two curves, which means that the energy of FXA of the narrower nanobelt is smaller than that of the wider one below 123 K but larger than that of the wider one above 123 K. To clearly observe the shift of the energy, the room temperature PL measurements are shown in Figure 6b for the three samples. There have been several discussions on the shift of the PL spectra; in ref 5, the authors observed that, with an increasing thickness of the nanobelt (larger than half of the wavelength of the emitted light in nanobelts), the emitted light could be trapped inside the belt due to internal reflection and absorbed again, and the reabsorption would lead to a red shift, a broadening of the emission peak, and a long tail at the longer wavelength side at higher temperatures. In Figure 5a and b, one can observe that, with the temperature increasing to ∼100 K, all of the excitons become mobile, and more emitted light originates from the recombination of excitons; in addition, more emitted light could be trapped inside the thick nanobelt due to internal reflection and could be absorbed again, leading to a red shift, a broadening of the emission peak. Therefore, the red shift for the wider and thicker nanobelt above 123 K can be ascribed to the reabsorption of the emitted light in the nanobelt, and the red shift for the narrow nanobelt below 123 K can be ascribed to the larger uniaxial tensile strains. Here, the discussion on exciton dynamics is considered only at one exciton wavelength, and more information about the tensile strain and surface depletion could be obtained by considering different excitation energies, as discussed for ZnO.30

Figure 5. Temperature-dependent PL spectra of the CdS nanobelts in the range from 5 to 300 K for samples A (a) and C (b); PL spectra taken at 5 K with spectral features for samples A (c) and C (d). The color curves are Lorentzian line shape decompositions with each peak clearly labeled. The spectra in a and b are normalized and shifted in the vertical direction for clarity.

DAP, the following peaks at 2.374, 2.340, and 2.303 eV with an energy spacing of approximately 37 meV are assigned to the higher-order LO phonon replicas of DAP, DAP-1LO, DAP-2LO, and DAP-3LO. Furthermore, the spectrum of sample C measured at 5 K consists of several distinct peaks at 2.556, 2.541, 2.528, 2.510, 2.490, 2.471, 2.450, 2.423, 2.385, and 2.345 eV. The latter four peaks can be assigned to DAP emission and its third-order phonon replicas. The peaks at 2.556, 2.541, 2.528, 2.510, 2.490, 2.471, and 2.450 eV can be ascribed to free exciton EA, D0X, A0X, D0X-1LO, A0X-1LO, D0X-2LO, and A0X-2LO, respectively. Compared with the emission peak energies of sample A, the spectra of sample C exhibits a systematic blueshift of 6 meV for free exciton EA and A0X and 9 meV for DAP. In ref 6, F



Article

under different excitation wavelengths, which will be helpful to the applications of nanostructures.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the financial support for this work from Yangzhou Science and Technology Development (No. YZ2011150); we would also like to acknowledge the technical support received at the Testing Center of Yangzhou University.

■

REFERENCES

(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 1897−1899. (2) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Highly Polarized Photoluminescence and Photodetection from Single Indium Phosphide Nanowires. Science 2001, 293, 1455−1457. (3) Xu, X. L.; Zhao, Y. Y.; Sie, E. J.; Lu, Y. H.; Liu, B.; Ekahana, S. A.; Ju, X.; Jiang, Q. K.; Wang, J. B.; Sun, H. D.; et al. Dynamics of Bound Exciton Complexes in CdS Nanobelts. ACS Nano 2011, 5, 3660−3669. (4) Liu, B.; Chen, R.; Xu, X. L.; Li, D. H.; Zhao, Y. Y.; Shen, Z. X.; Xiong, Q. H.; Sun, H. D. Exciton-Related Photoluminescence and Lasing in CdS Nanobelts. J. Phys. Chem. C. 2011, 115, 12826−12830. (5) Li, D. H.; Zhang, J.; Xiong, Q. H. Surface Depletion Induced Quantum Confinement in CdS Nanobelts. ACS Nano 2012, 6, 5283− 5290. (6) Chen, R.; Ye, Q. L.; He, T. C.; Wu, T.; Sun, H. D. Uniaxial Tensile Strain and Exciton−Phonon Coupling in Bent ZnO Nanowires. Appl. Phys. Lett. 2011, 98, 241916−241916−3. (7) Tell, B.; Damen, T. C.; Porto, S. P. S. Raman Effect in Cadmium Sulfide. Phys. Rev. 1966, 144, 771−774. (8) Phuruangrat, A.; Thongtem, T.; Thongtem, S. Characterization and Photonic Absorption of Hierarchical Tree-like CdS Nanostructure Synthesized by Solvothermal Mmethod. Mater. Lett. 2012, 80, 114−116. (9) Chuu, D. S.; Dai, C. M.; Hsieh, W. F.; Tsai, C. T. Raman Investigations of the Surface Modes of the Srystallites in CdS Thin Films Grown by Pulsed Laser and Thermal Evaporation. J. Appl. Phys. 1991, 69, 8402−8404. (10) Kumar, P.; Saxena, N.; Singh, F.; Agarwal, A. Nanotwinning in CdS Quantum Dots. Physica B 2012, 407, 3347−3351. (11) Chawla, P.; Lochab, S. P.; Singh, N. Photoluminescence, Thermoluminescence and Raman Studies of CdS Nanocrystalline Phosphor. J. Alloys Compd. 2010, 492, 662−666. (12) Gupta, K.; Kripal, R. EPR and Photoluminescence Properties of Mn2+ Doped CdS Nanoparticles Synthesized via Co-precipitation Method. Spectrochim. Acta, Part A 2012, 96, 626−631. (13) Sivasubramanian, V.; Arora, A. K.; Premila, M.; Sundar, C. S.; Sastry, V. S. Optical properties of CdS Nanoparticles upon Annealing. Physica E 2006, 31, 93−98. (14) Shiang, J. J.; Risbud, S. H.; Alivisatos, A. P. Resonance Raman Studies of the Ground and Lowest Electronic Excited state in CdS Nanocrystals. J. Chem. Phys. 1993, 98, 8432−8442. (15) Lee, K. Y.; Lim, J. R.; Rho, H. Evolution of Optical Phonons in CdS Nanowires, Nanobelts, and Nanosheets. Appl. Phys. Lett. 2007, 91, 201901−201901−3. (16) Pan, H.; Xing, G. C.; Ni, Z. H.; Ji, W.; Feng, Y. P. Stimulated Emission of CdS Nanowires Grown by Thermal Evaporation. Appl. Phys. Lett. 2007, 91, 193105−193105−3. (17) Fan, D. H.; Zhang, R. Z.; Zhu, Y. F.; Peng, H. R. Size Dependence of Surface Optical Mode and Electron−Phonon Coupling in ZnO Nanocombs. Physica B 2012, 407, 3510−3514.

Figure 6. (a) Temperature dependence of the free exciton peak energy of samples A (800 °C) and C (900 °C), where the curves represent the fitted results, and the dots represent the experimental results; (b) roomtemperature PL spectra of samples A, B, and C.

4. CONCLUSIONS By studying the optical properties of nanobelts of different widths and thicknesses, we have obtained some interesting results. First, in addition to the first-order longitudinal optical phonon mode (1LO) at ∼302 cm−1 and the first overtone mode (2LO) at ∼604 cm−1, multiphonon processes and a surface phonon that peaked at 283 cm−1 were observed when excited using a wavelength of 532 nm. A strong exciton−phonon coupling interaction was observed, which increases with increasing lateral size, and a narrow nanobelt is under a larger tensile strain. As the TO intensity does not change with the lateral size, the LO intensity is not only sensitive to the lateral size but also stronger than the TO; therefore, we conclude that the strong electron−phonon coupling in the CdS nanobelts is mainly associated with the Fröhlich interaction. The PL spectrum at 10 K exhibits rich spectral features, which are identified as being the recombination of free excitons (EA), excitons bound to neutral impurities (A0X), and donor−acceptor pairs (DAP). Below ∼123 K, the PL spectra of the narrow nanobelt exhibit a red shift due to the larger uniaxial tensile strains; above ∼123 K, there is a red shift for the wider nanobelt because the emitted light could be trapped inside the belt due to internal reflection and absorbed again. We observed that competition between the tensile strain and the surface-depletion-induced quantum confinement (the reabsorption of the emitted light) exists in the nanobelt, and more details on the competition between the tensile strain and the surface depletion can be determined through further studies G



Article

(18) Kim, J. H.; Rho, H.; Kim, J.; Choi, Y.-J.; Park, J.-G. Raman Spectroscopy of ZnS Nanostructures. J. Raman Spectrosc. 2012, 43, 906−910. (19) Sahoo, S.; Arora, A. K. Laser-Power-Induced Multiphonon Resonant Raman Scattering in Laser-Heated CdS Nanocrystal. J. Phys. Chem. B. 2010, 114, 4199−4203. (20) Singha, V.; Sharmab, P. K.; Chauhan, P. Surfactant Mediated Phase Transformation of CdS Nanoparticles. Mater. Chem. Phys. 2010, 121, 202−207. (21) Fan, H. M.; Ni, Z. H.; Feng, Y. P.; Fan, X. F.; Kuo, J. L.; Shen, Z. X.; Zou, B. S. Anisotropy of Electron-Phonon Coupling in Single Wurtzite CdS nanowires. Appl. Phys. Lett. 2007, 91, 171911−171911−3. (22) Kar, S.; Satpati, B.; Satyam, P. V.; Chaudhuri, S. Synthesis and Optical Properties of CdS Nanoribbons. J. Phys. Chem. B 2005, 109, 19134−19138. (23) Zhang, Q.; Zhang, J.; Utama, M. I. B.; Peng, B.; de la Mata, M.; Arbiol, J.; Xiong, Q. H. Exciton-phonon Coupling in Individual ZnTe Nanorods Studied by Resonant Raman Spectroscopy. Phys. Rev. B 2012, 85, 085418−085426. (24) Lu, J. F.; Zeng, X. H.; Liu, H. F.; Zhang, W.; Zhang, Y. Preparation and Photoluminescence of (3C-ZnS)/(2H-ZnS) Superlattice in Mn -doped ZnS Nanoribbons. J. Phys. Chem. C 2012, 116, 23013−23018. (25) Giugni, A.; Das, G.; Alabastri, A.; Zaccaria, R. P.; Zanella, M.; Franchini, I.; Di Fabrizio, E.; Krahne, R. Optical Phonon Modes in Ordered Core-shell CdSe/CdS Nanorod Arrays. Phys. Rev. B 2012, 85, 115413−115418. (26) Zeng, H. B.; Cai, W. P.; Cao, B. Q.; Hu, J. L.; Li, Y.; Liu, P. S. Surface Optical Phonon Raman Scattering in Zn/ZnO Core-shell Structured Nanoparticles. Appl. Phys. Lett. 2006, 88, 181905. (27) Li, L.; Yang, Y. W.; Li, G. H.; Zhang, L. D. Conversion of a Bi Nanowire Array to an Array of Bi-Bi2O3 Core-Shell Nanowires and Bi2O3 Nanotubes. Small 2006, 2, 548−553. (28) Imada, A.; Ozaki, S.; Adachi, S. Photoreflectance Spectroscopy of Wurtzite CdS. J. Appl. Phys. 2002, 92, 793−1798. (29) Satoru, S. Photoluminescence, Reflectance and Photoreflectance Spectra in CdS Epilayers on Si(111) Substrates. Jpn. J. Appl. Phys. 2005, 44, 5913−5917. (30) Zeng, H. B.; Duan, G. T.; Li, Y.; Yang, S. K.; Xu, X. X.; Cai, W. P. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561−572.

H


Size Effects of Raman and Photoluminescence Spectra of CdS

Recommend Documents