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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Resonant and Off-Resonant Phonon Properties of Wurtzite ZnS: Effect of Morphology on Fröhlich Coupling and Phonon Lifetime Neena Prasad and Balasubramanian Karthikeyan* Nanophotonics Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India

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S Supporting Information *

ABSTRACT: We report the occurrence of morphologyinduced variation in the phonon properties of a poly(vinylpyrrolidone)-capped ZnS semiconductor. ZnS microand nanostructures such as microparticles, nanoparticles, nanorods (NRs), and nanowires (NWs) were synthesized through a single-step hydrothermal method at low temperature. Structural, morphological, and compositional analyses were performed using powder X-ray diffraction, field emission scanning electron microscopy, energy-dispersive spectra (EDS), and high-resolution transmission electron micrographic techniques. Moreover, morphology of the prepared samples was confirmed from off-resonant Raman studies. A strong first and higher orders of longitudinal optical (LO) phonon and weak fundamental and higher orders of transverse optical features were seen from UV Raman scattering. The electron−phonon coupling as a function of morphological changes of the nanostructures was analyzed by studying the differences in LO phonon linewidths among the samples. The dominance of the interaction between the electron and LO phonon (Fröhlich coupling) over the short-range electron−phonon interaction via deformation potential was observed. Exciton−LO phonon coupling was found to be strong in one-dimensional nanostructures (NR and NW) because of the transfer of excitons and phonons in the longitudinal direction and phonon confinement in the transverse direction.

1. INTRODUCTION Zinc sulfide (ZnS) is a well-known wide-band gap II−VI semiconductor with an energy band gap around 3.77 eV under ambient temperature.1 It has an importance in nonlinear, optoelectronic applications and in electronic industry because of the applicability in fabricating light-emitting diodes, infrared (IR) windows, field emitters, sensors, lasers, nonlinear optical devices, and efficient phosphors in flat panel displays.2−6 ZnS has a mixture of covalent and ionic bonds as well as high iconicity.7 These properties made ZnS a better polar semiconductor than other II−VI compound semiconductors. Raman spectroscopy is a sensitive tool that is extensively used to identify the atomic arrangements, phonon spectra of the nanostructured materials, thermal properties, lattice vibrations of the material, and structure and bonds of the material using standard nonresonant Raman conditions.8 In nonresonant Raman scattering, only the optical phonon wave vector near the Brillouin-zone (BZ) center can contribute to Raman scattering because of momentum conservation and is triply degenerate.9 In resonant Raman, the phonons near the Brillouin boundary contribute to Raman scattering because the momentum conservation is not restricted to the BZ center.9,10 When we deal with the optoelectronic, linear, and nonlinear absorption properties of semiconductor and the energy relaxation of excited charge carriers, it is necessary to consider the interaction between the electronic and vibrational excitations.11 Thus, for the development of ZnS-based © XXXX American Chemical Society

optoelectronic devices, the study of phonon interactions with free carriers and transport properties is essential. Resonant Raman scattering can provide useful information about interactions between photon−electron and electron−phonon in semiconductor materials.12 In semiconductors, electron− phonon coupling (EPC) can be mediated either by deformation potential or by Fröhlich potential.13 The microscopic deformations of the polar lattice cause the generation of the macroscopic electric field.14 The electric field within the material not only influences the columbic interaction between the excitons but also the coupling strength of electron− phonon interaction. Long-range Frö hlich interaction is generated by the interaction between an electron and a macroscopic electric field, associated with longitudinal optical (LO) phonon.14 Short-range deformation coupling is due to the interaction between lattice displacement and electron and is associated with transverse optical (TO) phonon Raman scattering.15 The deformation and Fröhlich interaction can be equal in magnitude, and its interference can be constructive or destructive. Many reports have suggested that interactions are between the electron and phonon but its dependency with size is not yet clear. If the Fröhlich interaction is a size-dependent EPC, then the ratio between the normalized relative Raman Received: May 30, 2018 Revised: July 17, 2018

A

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C intensity of second-order LO mode to fundamental LO mode can give the coupling strength.16 The electron and LO phonon coupling are closely proportional to normalized relative Raman intensity. In polar semiconductors, the coupling between the electron and LO phonons is coupled through Fröhlich interaction, which influences the transport and optical properties because this coupling strongly depends on the ionic polarization of the crystal.17 ZnS has a very low scattering efficiency with the commonly used excitation wavelengths for Raman scattering being 514 and 532 nm because of the weak photon−matter interaction in this range. Thus, Raman spectroscopy is limited to phase identification and crystalline properties. In the present investigation, we have focused on the resonant Raman scattering studies of optical phonon behaviors of ZnS microand nanostructures to extend the Raman studies apart from the usual straightforward information.

samples were observed with a Carl Zeiss field emission scanning electron microscope (Carl Zeiss FE-SEM) with an accelerating voltage of 10.00 kV, and the composition of prepared samples was ascertained from energy-dispersive spectroscopy (EDS) analysis. The morphological changes of nanostructures, average particle size, and preferential growth direction were further confirmed with transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) micrographs using a Jeol/JEM 2100 TEM. Nonresonant Raman spectra of the prepared samples were collected at an excitation of 532 nm using a LabRam HR evolution Raman spectrometer. The resonant Raman spectra were recorded with a Renishaw micro-Raman spectrometer (model: Invia) excited with a laser excitation of 325 nm and with a laser power of 5 mW. The acquisition time is very small, usually on the order of seconds, because of near-resonant scattering, and the measurement spot size was approximately 1 μm.

2. MATERIALS AND METHODS 2.1. Materials. Poly(vinylpyrrolidone) (PVP)-capped ZnS microparticles (MPs) and nanoparticles (NPs), nanorods (NRs), and nanowires (NWs) were prepared through a simple hydrothermal method. Anhydrous zinc chloride (ZnCl2) and thiourea (CS (NH2)2) were taken as the precursor salt and precipitating agent, respectively. All chemicals were purchased from Sigma-Aldrich with 99.99% purity. Hydrazine hydrate was used as the solvent for the experiment, and to enrich the growth of one-dimensional (1D) nanostructures, suitable amount of ethylene diamine was added. 2.2. Synthesis of ZnS Nanostructures. 2.2.1. Preparation of ZnS MP Is as Follows. ZnCl2 (0.1 M) was dissolved into 40 mL of hydrazine hydrate and well stirred using a magnetic stirrer for 1 h at room temperature. Thiourea (0.15 M) was dissolved in 40 mL of hydrazine hydrate and was added dropwise to the above mixture, and this reaction mixture was magnetically stirred for 1 h and poured into a Teflon-lined autoclave and kept in an electric oven, its temperature increased to 180 °C. The obtained precipitate was washed multiple times with methanol followed by double-distilled water, and the precipitate was collected and dried at 60 °C for 6 h in a hot air oven. 2.2.2. Preparation of ZnS NP Is as Follows. PVP (50 mg) was dissolved into 40 mL of hydrazine hydrate and stirred well for 2 h at room temperature. To this solution, 0.1 M ZnCl2 was added followed by 0.15 M thiourea and stirred for 1 h. Rest of the procedures is same for the preparation of ZnS MP. 2.2.3. Preparation of ZnS NRs and NWs Is as Follows. PVP (50 mg) was dissolved into 40 mL of hydrazine hydrate and stirred well for 2 h at room temperature. To this solution, 0.1 M ZnCl2 was added and stirred for 1 h. Before adding thiourea solution, 5 mL of ethylene diamine was added to the reaction mixture and it is stirred well. Then, 0.15 M thiourea was added to this solution and is stirred and poured into a Teflon-lined autoclave and kept in an electric oven, its temperature increased to 180 °C. The reaction time was varied to 24 and 48 h for NRs and NW preparation, respectively. The samples were labeled as ZSMP, ZSNP, ZSNR, and ZSNW for MPs, NPs, NRs, and NWs, respectively, based on the morphology for convenience. 2.3. Characterization Techniques. X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima III powder X-ray diffractometer in the 2θ range from 20° to 80° with a scanning speed of 2°/min. The morphologies of the prepared

3. RESULTS AND DISCUSSION 3.1. Structural Analysis. The XRD pattern of prepared samples is shown in Figure S1 (Supporting Information). Seven major and distinct peaks were indexed, which indicates that the prepared samples were crystalline in nature with a standard hexagonal wurtzite ZnS structure with lattice constants of a = 3.818 Å and c = 6.259 Å. (JCPDS card #36-1450). The crystalline nature of the samples was further confirmed from HR-TEM images shown in Figure 2a−d. The preferential growth directions of NRs and NW along the c-axis were identified from the stronger and narrower (002) plane diffraction peak. Crystallite sizes were estimated using Scherrer’s formula eq 1 and were 89.1 ± 7.8, 27.9 ± 7.3, 10.5 ± 3.5, and 9.4 ± 4.3 nm, corresponding to (002) plane for ZSMP, ZSNP, ZSNR, and ZSNW, respectively. D=

kλ β cos θ

(1)

where k is the particle shape factor (0.94), λ is the wavelength of incident X-rays (1.5406 Å), and β is the full width at halfmaximum (fwhm) of the diffraction peak. 3.2. Morphology and Elemental Analysis. Figure 1 shows the FE-SEM images of the prepared samples. The morphology of the MPs, NPs, very long NRs, and NWs can be clearly seen in Figure 1. From Figure 1b, it can be evident that the prepared samples have hexagonal structure and have relatively uniform size distribution. The average particle size of ZSMP and ZSNP is found to have size ranges of 1.8 μm with standard deviation (SD) of 28.4 and 35 nm with SD as 4.9 nm, respectively, and the diameters of ZSNR and ZSNW are around 45 (SD = 10.1) and 35 nm (SD = 3.4 nm), respectively. Their lengths are found to be around 550 nm (SD = 8.9 nm) and 3.5 μm (SD = 25.7 nm), respectively, for ZSNR and ZSNW. The aspect ratios of ZSNR and ZSNW are around 10 and 100, respectively, which confirms that ZSNR and ZSNW have NR- and NW-like morphology. These observations are further confirmed with TEM and HR-TEM results. Comparison of crystallite size from Scherer analysis with error estimates and particle size from TEM measurements with SDs of ZnS MPs, NPs, NRs, and NWs is tabulated in Table S1 (Supporting Information). TEM images of the prepared ZnS MPs, NPs, NRs, and NWs are shown Figure 2a−d. Spherical-, rod-, and wire-like shapes of NPs, NRs, and NWs can be seen in the image, and in B

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

as: ΓOpt = A1 + E1 +2E2 + 2B1.10 A1 and E1 are polar modes and are both Raman and IR active, respectively, and the nonpolar E2 mode is Raman active only. The, B1 modes are silent modes. Polar modes can polarize the unit cell, and thus the long-range macroscopic electric field associated with the polar modes can split the frequency of IR active optical phonons into longitudinal phonon, and the short-range deformation potential that exhibits the anisotropy of force constant can split optical phonon into its transverse component.18 The frequency of LO leads that of TO because this macroscopic electric field stiffens the force constant of phonon; this may cause changes to the static dielectric constant (ε0). Because static and dynamic dielectric (ε∞) constants are related to long wavelength TO and LO phonon frequencies, it can be expressed using the Lyddane−Sachs−Teller relationship as eq 2.19,20 This relation is applicable to optical lattice vibration that has associated net polarization density. Figure 1. Scanning electron microscopy image of (a) MP, (b) NP, (c) long NRs, and (d) NWs. The inset of figure (b) displays the hexagonal structure of NP.

ε0 = ε∞

ωLO2 ωTO2

(2)

where ωLO and ωTO are frequencies of longitudinal and transverse components of optical phonons, respectively. ε∞ is the dynamic dielectric constant, usually measured for frequencies well above the LO phonon frequency but below the optical absorption edge. ε0 is the static dielectric constant, which is a second-order tensor. In principle, dielectric constant can be written as

addition, a hexagonal structure can also be clearly seen from the images. Figure 2e,f shows HR-TEM images of the corresponding nanostructure, and the crystalline nature of the prepared samples is further confirmed from these images. Interplanar spacing, marked in the HR-TEM images of NRs (0.31 nm) and NWs (0.33 nm), corresponds to (002) lattice planes of wurtzite ZnS structure and shows that ZnS NRs and NWs grow along (002) direction. These observations are in good agreement with the XRD and FE-SEM results. From the EDS spectra shown in Figure S2, it is seen that in addition to Zn and S signal, the trace of Cu element was detected in ZSNR that comes from the source. This observation indicates that the products are of high purity. Moreover, the atomic ratio of Zn to S corresponds to the stoichiometric ratio of ZnS composition. 3.3. Raman Spectral Studies. ZnS that crystallizes in wurtzite-type single crystal structure belongs to C46v (C63mc) space group with two formula units per primitive cell and all atoms occupying C3v sites. Group theory predicts the irreducible representation of optical phonon at the zone center

ε0 = ε∞ +

4πNeT *2 ωTO2M

(3)

where N, M, and eT*2 are number of unit cells per unit crystal, reduced mass of the crystal, and transverse dynamic effective change, respectively. In a polar semiconductor such as ZnS, the well-known dimensionless Fröhlich coupling constant depends on the ionic polarization of the crystal, which is used to measure the strength of the exciton−phonon interaction. The strength of Fröhlich interaction can be determined by the intensity ratio between the second-order LO to first-order LO (I2LO/ I1LO).14,21

Figure 2. TEM image of (a) MP, (b) NP, (c) long NRs, and (d) NWs. HR-TEM images of (e) ZSMP, (f) ZSNP, (g) ZSNR, and (h) ZSNW. C

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. (a) Resonant Raman spectra of ZnS MPs, NPs, NRs, and NWs. Strong fundamental LO and weak TO phonon and the higher order phonon mods are indexed in the figure. The peak shift and line shape variation of 1LO and 2LO are shown in the inset of (a,b) variation of Fröhlich phonon life time and Fröhlich interaction with crystallite size, respectively.

Figure 3 shows the multiphonon resonant Raman spectra of the ZnS structures. The Raman scattering from the samples shows a strong fundamental LO and a weak TO mode along with several higher order modes. In the present study of ZnS micro- and nanostructures, observed resonant Raman peaks are consolidated in Table S2 (Supporting Information). In the present investigation, the LO−TO splitting is very small and hence the prepared samples are of the wurtzite structure. This is in good agreement with the SEM result. The characteristics LO and TO phonon modes of bulk ZnS can be observed at 346 and 263 cm−1, respectively. Virpal et al. reported the appearance of characteristic LO and TO phonon modes of ZnS NPs at 346 and 263 cm−1, respectively,22 and Xiong et al. reported these modes at 346.4 and 269 cm−1, respectively.23 In the present work, the observed fundamental TO and LO modes of A1 and E1 symmetry observed for ZSMP, ZSNP, ZSNR, and ZSNW are in agreement with these reported in Raman investigations. There is a remarkable shift in 1LO mode in the Raman spectra from 347.46 to 350.88, 349.18, and 346.63 cm−1 during the growth process from MPs to NPs and NRs and finally to NWs. An increase in 1LO wavenumber of ZSNP compared with ZSNR and ZSNW may result from the intrinsic impurity or defect in ZSNP because of relatively large particle sizes than ZSNR and ZSNW.6 A red shift of around 3 and 2 cm−1, respectively, for ZSNP and ZSNR when compared with ZSMP can be attributed to the combined effect of tensile strain and phonon confinement,24,25 whereas the blue shift of ZSNW compared to all other morphology shows that the tensile strain relaxes during NW growth.26 A large blue shift in LO phonon wavenumber of NW from NP (4 cm−1) and NRs (3 cm−1) can be attributed to lattice contraction induced by large surface tension in the NWs.27 Lattice elongation or contraction can be written as eq 427 Δω 3Δc yz i zz = jjj1 + ω0 c { k

ωn = n·ω(LO(A1))

where ω(LO(A 1 )) is the frequency position of the fundamental LO mode. Thus, higher order combinations of fundamental LO up to third order (3LO) and fifth order (5LO) are seen for ZSMP and ZSNR, ZSNW, respectively. Additionally, higher order combinations of TO up to third order (3TO) are observed for ZSNR and ZSNW. This result shows the dominance of Fröhlich interaction over the shortrange deformation potential. Interestingly, 2LO is much stronger than 1LO mode for NRs and NWs; thus, a strong exciton−LO phonon coupling is seen for NRs and NWs. The calculated values of Fröhlich interaction (I2LO/I1LO) for ZSMP, ZSNP, ZSNR, and ZSNW are 0.76, 0.81, 1.41, and 1.57, respectively. This unexpected strengthening of exciton− phonon coupling in 1D nanostructures such as NR and NW can be due to two reasons: (i) transfer of elementary excitations such as carrier, excitons, and phonons in the longitudinal direction as well as strong coupling to charged surface due to large surface-to-volume ratio and (ii) phonon confinement in the transverse direction.14,21 Likewise, the reinforcement of coupling between exciton and LO phonon in MP and NP from bulk (0.15) can be attributed to the decrease in lifetime of excited state because of defects on the surface because this coupling is very sensitive to particle sizes and excitation wavelengths.28 More significantly, along with the changes in intensity ratio between fundamental, second, and third order LO peaks, there is a change in area between these peaks, which can be seen in Figure 3a. This ratio increases when the morphology changes from MPs to NWs and can be due to the room-temperature photoluminescence emission related to the band gap energy.6 Apart from the shift, intensity, and area changes in LO modes, it is seen that under resonant excitation, the LO phonon regions of NRs and NWs show asymmetric line shapes as compared with MPs and NPs. The asymmetric line shapes can be attributed to the surface optical (SO) phonon that may be triggered in polar semiconducting 1D nanostructures with cylindrical and rectangular crossselection when the transitional symmetry of the surface potential broken at the interface.29 Surface roughness during growth process can be the reason for the symmetry breaking mechanism,30 and this surface modulation can be seen in the

−γ

−1

(5)

(4)

γ Grüneisen parameter (0.95 for ZnS). In addition to the first-order LO modes, higher orders of this mode are also seen. The nth overtone phonon frequency position can be expressed as eq 5 D

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Lorentzian fitted nonresonant Raman spectra of ZnS MPs, NPs, NRs, and NWs. The dotted line indicates the Raman spectra from each material, whereas the solid line and dashed line indicates the Lorentzian fitted Raman spectrum and the individual phonon modes, respectively.

1 ΔE = = 2πc Γ τ h̵

HR-TEM images of NRs and NWs. The appearance of SO phonon mode in the prepared ZnS nanostructures was further confirmed and discussed from the nonresonant Raman scattering spectra shown in Figure 4. Moreover, the appearance of multiple resonant peaks observed in semiconductors varies monotonically with Huang-Rhys parameter. This factor is a measure of optical quality and depends on the ionic nature of semiconductor and the separation of the Fourier transformed charged distribution.11,31 From the recorded Raman spectra, it can be inferred that ZSNR and ZSNW are of a high optical quality because both have a large value of Huang-Rhys parameter,9 which is in good agreement with the observed XRD results because the crystallite size decreases with morphology. In opto-electronic applications, the prediction of thermal transport properties of the materials can be done from the investigation of phonon lifetime measurement. Moreover, the quality of semiconductor nanostructures can be derived from the line shape and fwhm of the Raman phonon modes. The increase in line width can be attributed to enhanced phonon scattering because of defects, and this can decrease phonon lifetime.32 Because EPC contributes for optical phonons and can be recognized in the line width, thus the experimental line width provides a direct EPC measurement.33 In semiconductor materials, the Raman line width and shape may change with strain; hence, the observed line shape variation of 1LO and 2 LO shown in the inset of Figure 3a can be due to tensile strain.34 The addition of anharmonic decay of phonon with the decay time of τA and translational symmetry loss with a phonon decay time of τ1 gives the phonon lifetime eq 6. 1 1 1 = + τ τA τ1

(7)

Here, ΔE is the uncertainty in the phonon mode energy, ℏ is the reduced Planck constant, and c is the speed of light. In the present investigation, the phonon lifetime of LO phonon has been estimated and it will have an uncertainty because we neglect inhomogeneous broadening. For 1LO phonon, the Raman line width increases when morphology changes from ZSMP to ZSNW. The line width and EPC are found to increase during morphology change from NP to NRs and finally to NWs. This is in agreement with the calculated longrange Fröhlich interaction. The variations of 1LO phonon lifetime and Fröhlich interaction with crystallite size are shown in Figure 3b. The Raman line width, phonon lifetime, and Fröhlich interaction with different morphological changes in nanostructures from the experimental data are presented in Table S3 (Supporting Information). Nonresonant Raman scattering occurs when the energy of the incident or scattered photons is less than the optical band gap of the material. Recorded nonresonant Raman scattering spectra from ZnS nanostructures in the spectral range from 240 to 450 cm−1 under the excitation 532 nm are shown in Figure 4. Multiple Lorentzian function was used to fit the peaks. In nonresonant Raman scattering, the Raman modes are determined by Raman selection rule by considering the crystal symmetry and propagation and polarization directions of incident and scattered light.19 From the Raman spectra, it is seen that in addition to the main LO phonon mode around 348 cm−1, several relatively low intense peaks identified as pure acoustic, optical modes and its overtones occur. As compared with resonant Raman scattering, the LO phonon scattering cross section in resonance Raman is enhanced significantly through Fröhlich interaction, as seen in Figure 3. The Lorentzian line shape fitting gives characteristic A1/E1 (TO), E2 (TO), and A1/E1 (LO) modes of ZnS along with the second-order Raman scattering in the range 380−450 cm−1

(6)

In the absence of inhomogeneous broadening, the phonon lifetime (τ) can be calculated from the Lorentzian fitted phonon modes in terms of the fwhm as eq 7, the energy-time uncertainty relation. E

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Notes

that consists of a combination bands of TO and transverse acoustic (TO + TA) mode, an LO + TA and LO + LA.26 Ho et al. reported a shoulder peak of LO at 329 cm−1 of ZnS nanobelt as an SO mode and Xiong et al. reported the same at 335 cm−1 in wurtzite ZnS NW.23,30 Besides, Bhattacharya et al. and Gupta et al. reported the SO mode in other polar materials such as 1D GaN nanoribbons and GaP NWs at 678 and 392 cm−1, respectively.35,36 Thus, interestingly in the present investigation, the presence of a shoulder band of LO at 344 cm−1 in the prepared NRs and NWs can be attributed to SO phonon modes which originates due to the boundary between different dielectric media. Typically, SO phonons can be activated in 1D nanostructures of polar semiconductors with cylindrical and rectangular cross-selection when the transitional symmetry broken at the interface because of the surface roughness during growth process, and this observation can be clearly observed from the HR-TEM images of NRs and NWs, as shown in the inset of Figure 2c,d. This observation confirms the formation 1D nanostructure hexagonal wurtzite structure of ZnS and is in good agreement with the XRD, SEM, TEM, and HR-TEM results.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to the Ministry of Human Resource and Development, India, for financial support.

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(1) Lu, J.; Zeng, X.; Liu, H.; Zhang, W.; Zhang, Y. Preparation and Photoluminescence of (3C-ZnS)/(2H-ZnS) Superlattice in Mndoped ZnS Nanoribbons. J. Phys. Chem. C 2012, 116, 23013−23018. (2) Wang, Z.; Daemen, L. L.; Zhao, Y.; Zha, C. S.; Downs, R. T.; Wang, X.; Wang, Z. L.; Hemley, R. J. Morphology-Tuned WurtziteType ZnS Nanobelts. Nat. Mater. 2005, 4, 922−927. (3) Bi, C.; Pan, L.; Xu, M.; Yin, J.; Qin, L.; Liu, J.; Zhu, H.; Xiao, J. Q. Synthesis and Characterization of Co-Doped Wurtzite ZnS Nanocrystals. Mater. Chem. Phys. 2009, 116, 363−367. (4) Prasanth, S.; Irshad, P.; Raj, D. R.; Vineeshkumar, T. V.; Philip, R.; Sudarsanakumar, C. Nonlinear optical property and fluorescence quenching behavior of PVP capped ZnS nanoparticles co-doped with Mn2+ and Sm3+. J. Lumin. 2015, 166, 167−175. (5) Balantseva, E.; Berlier, G.; Camino, B.; Lessio, M.; Ferrari, A. M. Surface Properties of ZnS Nanoparticles: A Combined DFT and Experimental Study. J. Phys. Chem. C 2014, 118, 23853−23862. (6) Fairbrother, A.; Izquierdo-Roca, V.; Fontané, X.; Ibáñez, M.; Cabot, A.; Saucedo, E.; Pérez-Rodríguez, A. ZnS Grain Size Effects on Near-resonant Raman Scattering: Optical Non-destructive Grain Size Estimation. CrystEngComm 2014, 16, 4120−4125. (7) Malakar, A.; Das, B.; Islam, S.; Meneghini, C.; De Giudici, G.; Merlini, M.; Kolen’ko, Y. V.; Iadecola, A.; Aquilanti, G.; Acharya, S.; Ray, S. Efficient Artificial Mineralization Route to Decontaminate Arsenic ( III ) Polluted Water - the Tooeleite Way. Sci. Rep. 2016, 6, 26031. (8) Yang, C. C.; Li, S. Size-Dependent Raman Red Shifts of Semiconductor Nanocrystals. J. Appl. Phys. 2008, 112, 14193−14197. (9) Cheng, Y. C.; Jin, C. Q.; Gao, F.; Wu, X. L.; Zhong, W.; Li, S. H.; Chu, P. K. Raman Scattering Study of Zinc Blende and Wurtzite ZnS. J. Appl. Phys. 2009, 106, 123505. (10) Gillet, Y.; Kontur, S.; Giantomassi, M.; Draxl, C.; Gonze, X. Ab Initio Approach to Second- Order Resonant Raman Scattering Including Exciton−Phonon Interaction. Sci. Rep. 2017, 7, 7344. (11) Jiang, Z.-Y.; Zhu, K.-R.; Lin, Z.-Q.; Jin, S.-W.; Li, G. Structure and Raman Scattering of Mg-doped ZnO Nanoparticles Prepared by Sol−gel Method. Rare Met. 2015, 6, 1−5. (12) Milekhin, A. G.; Yeryukov, N. A.; Sveshnikova, L. L.; Duda, T. A.; Himcinschi, C.; Zenkevich, E. I.; Zahn, D. R. T. Resonant Raman scattering of ZnS, ZnO, and ZnS/ZnO Core/shell Quantum Dots. Appl. Phys. A: Mater. Sci. Process. 2012, 107, 275−278. (13) Rolo, A. G.; Vasilevskiy, M. I. Raman Spectroscopy of Optical Phonons Confined in Semiconductor Quantum Dots and Nanocrystals. J. Raman Spectrosc. 2007, 38, 618−633. (14) Ning, J. Q.; Zheng, C. C.; Zhang, X. H.; Xu, S. J. Strong quantum confinement effect and reduced Fröhlich exciton-phonon coupling in ZnO quantum dots embedded inside a SiO2matrix. Nanoscale 2015, 7, 17482−17487. (15) Peter, Y. Y.; Cardona, M. Fundamentals of Semiconductors: Physics and Materials Properties; Springer, 2010. (16) Callender, R. H.; Sussman, S. S.; Selders, M.; Chang, R. K. Dispersion of Raman Cross Section in CdS and ZnO over a Wide Energy Range. Phys. Rev. B: Solid State 1973, 7, 3788−3798. (17) Fan, H.; Zou, B.; Liu, Y.; Xie, S. Size effect on the electronphonon coupling in CuO nanocrystals. Nanotechnology 2006, 17, 1099−1103. (18) Zhang, X. B.; Taliercio, T.; Kolliakos, S.; Lefebvre, P. Influence of Electron−Phonon Interaction on the Optical Properties of III Nitride Semiconductors. J. Phys.: Condens. Matter 2001, 13, 7053− 7074.

4. CONCLUSIONS In summary, higher order longitudinal and TO modes were observed in the near-resonant Raman scattering of ZnS microand nanostructures such as MPs, NP, NRs, and NWs. Raman spectral characteristics were correlated with the structure and morphology of polar ZnS. The ZnS NPs, NRs, and NWs were found to have good optical quality than MPs, seen from the relatively larger value of Huang-Rhys parameter. A brief discussion about the EPC, the dominance of long-range interaction of electron and LO phonon over the short-range deformation potential due to the polar nature of ZnS has been provided. Asymmetric line shape in the LO phonon region suggested the evolution of SO phonon modes under resonant excitation and were further confirmed from the nonresonant Raman scattering spectra. Transfer of elementary excitations and phonons in the longitudinal direction and phonon confinement in the transverse direction, enhanced EPC in NRs and NWs, which makes these material potential candidates in optoelectronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05164. XRD patterns of prepared ZnS morphologies with peak indexing; EDS spectra of prepared samples; comparison table of crystallite size from Scherer analysis with error estimates and TEM measurements with SDs; table containing observed resonant Raman peaks of different ZnS nanostructures; and table containing Raman linewidth and calculated 1LO phonon lifetime, Fröhlich interaction, and force constant (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +80-431-2503612. Fax: +91-(0)431-2500133. ORCID

Neena Prasad: 0000-0001-8886-0192 F

DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b05164 J. Phys. Chem. C XXXX, XXX, XXX−XXX