Ethylenediamine-Mediated Wurtzite Phase Formation in ZnS

Jan 24, 2013 - Department of Physics, RTM Nagpur University, Nagpur 440033, M.S, India. ‡. Department of Physics, Indian Institute of Science Educat...
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Ethylenediamine-Mediated Wurtzite Phase Formation in ZnS S. A. Acharya,† Neeraj Maheshwari,‡ Laxman Tatikondewar,§ Anjali Kshirsagar,§ and S. K. Kulkarni*,‡ †

Department of Physics, RTM Nagpur University, Nagpur 440033, M.S, India Department of Physics, Indian Institute of Science Education and Research, Pune 411008, India § Department of Physics and Centre for Modeling and Simulation, University of Pune, Pune 411007, India ‡

S Supporting Information *

ABSTRACT: The usual high temperature wurtzite phase of ZnS was successfully obtained at low temperature (170 °C) in the presence of ethylenediamine (EN) as the soft template. X-ray diffraction and Raman spectroscopy analysis confirmed the EN-mediated phase transformation (zinc blende to wurtzite) of ZnS. X-ray photoelectron spectroscopy (XPS) showed that all the samples were sulfur deficient. A high temperature X-ray diffraction (XRD) study showed that ZnS samples, both EN-mediated and without EN, retained their phases except small changes in the unit cell dimension. Besides the EN-mediated phase transition, morphology transformations from nearly spherical shape to nanorods are also observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coupling between EN molecules with ZnS is confirmed by Fourier transform infrared spectroscopy. A significant reduction in the phase transition temperature of ZnS has been achieved as compared to the bulk transition temperature (1020 °C). Mechanisms of phase transformation have been discussed. The density functional theory (DFT) supports the rod formation and wurtzite structure in the presence of nitrogen-terminated ZnS surface.

predominantly associated with two main structures: the cubic zinc-blende structure and the hexagonal wurtzite structure.20 Among these, the cubic structure of zinc sulfide is the most stable form in the bulk, which transforms into a thermodynamically metastable state called wurtzite structure at 1020 °C and melts at 1650 °C.21 In both structures, Zn and S are tetrahedrally bonded with the only difference in the stacking sequence of atomic layers in cubic and hexagonal structures. The properties of materials change dramatically with size, including thermodynamic stability. The depression of the melting point temperature has been reported in various nanomaterials.22 Also, structural transformations have been demonstrated to take place in nanoscale materials at lower temperatures.23 In an earlier report, we have shown that, under certain conditions, nanoparticles of CdS both in the wurtzite and sphalerite structures simultaneously existed in a lowtemperature synthesis.24 Unlike our previous work on ZnS,25 Quadari et al. found the lowering of the structural transformation temperature in the case of vacuum-annealed nanocrystallites from cubic ZnS to wurtzite ZnS.26 We did not find the structural transformation even up to ∼710 °C in vacuum-annealed 1.4 nm sized ZnS nanoparticles. This difference may be related to the state of aggregation during annealing of nanomaterials.

Physical properties of materials such as mechanical, thermal, magnetic, etc. strongly depend upon the structure. Therefore, the effect of pressure and temperature on phase transformation is widely investigated in condensed matter physics or material science. In nanoscale materials additional factors like interfacial energies or surfactants, solvents, templates, if any, and other experimental parameters also can influence (manipulate) the structural transformations due to aggregation and atomic motions at nanoscale.1−3 It is also possible to grow nanomaterials of desired structure on the seeds.4,5 In semiconductor nanoparticles, band gap energy can easily be manipulated by slight tuning in size and composition, which enables them to be used in various applications like photocatalysis, imaging, solar cells, etc. with an increase in efficiency.6,7 Particularly, the differences in their crystalline structure and size can lead to considerable changes in the effective masses of electrons, holes, and energy gaps. The surface morphology also plays an important role in determining the properties of the system, especially at nanoscale due to their large surface-to-volume ratio. A simultaneous control of structure and morphology of semiconductor nanocrystallites provides opportunities to tune and explore their optical properties. Therefore, structure and morphology control is of great interest in the development of semiconductor nanocrystals.8−16 ZnS, with its large direct band gap of 3.66 eV at 27 °C, is one of the important II−VI group semiconductors due to its unique applications in optoelectronic and luminescent devices.17−19 ZnS has been observed in a variety of polytypes, which are © 2013 American Chemical Society

Received: March 3, 2012 Revised: January 24, 2013 Published: January 24, 2013 1369

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(CH4N2S) procured from Sigma-Aldrich and ethlynediamine procured from Merck, Germany. All reagents were used as received without any purification. Equal moles (0.8 M) of zinc acetate and thiourea were mixed with the EN [C2H4(NH2)2)] solution. The concentrations of EN were 12%, 40%, 67%, 90%, and 100% (to the total volume of solution, 100 mL) as a x:(100 − x) volumetric ratio of EN:H2O. The solutions were introduced in a round-bottom flask, and the reaction was carried out at 170 °C for 8 h using a hot plate and magnetic stirrer. The final products were filtered and dried in vacuum. The same procedure is repeated without EN only, using double-distilled water as a solvent to synthesize ZnS without EN. A Bruker AXS D8 Advance X-ray diffractometer equipped with a copper target (λ, CuKαl = 1.5405 Å) was used for analyzing the dried powder samples. The morphology and energy dispersive X-ray (EDAX) analysis were carried out by using a JEOL, JSM, 6380 Analytical SEM. The transmission electron microscopy (TEM) images were obtained using a Technai 20G2 operated at 200 kV. The samples were prepared by dispersing the samples in N,N dimethylformamide (DMF) and placing a drop of liquid on carbon-coated Formvar grids. The Raman Spectroscopy analysis of as-synthesized ZnS samples was made using Raman spectroscopy (LABRAMAR4R) using a He−Ne laser operating at 632 nm (15 mW) as the source in the backscattering geometry. The EN-ZnS couplings were determined using a Thermo Scientific Nicolet-6700 Fourier transform infrared spectrometer (FTIR). Thermal behavior of the samples was determined by thermogravimetric analysis (TGA) using a Perkin-Elmer STA 6000 TGA analyzer with a heating rate of 10 °C/min, heated up to 800 °C to a constant weight in the nitrogen flow. The X-ray photoemission spectroscopy (XPS) analysis was made using an Omicron-EA 125 hemispherical analyzer using an Al Kα (1486.6 eV) radiation source. The XRD patterns of samples prepared using different concentrations of EN are shown in Figure 1. The XRD patterns

Interestingly, the lowering of temperature from the sphalerite to the wurtzite structure transition is possible for nanosized ZnS using surfactants. Molecular dynamics simulations and experimental work by Zhang et al. suggests that wurtzite particles smaller than about 7 nm, in vacuum, are more stable than sphalerite in vacuum at room temperature.27 The same group also has reported water-driven structure transformation in nanoparticles at room temperature.28 Synthesis and organization of nanoscale CdS and ZnS semiconductors also has been achieved using peptides and viral capsid assembly.29 However, the synthesis protocols are rather complex. Besides, there is also a problem with the wurtzite structure being transformed spontaneously to the zinc-blende structure when it comes into contact with some organic molecules at ambient temperature.30 Several attempts have been made to lower the phasetransition temperature of ZnS from sphalerite to wurtzite by surface active molecules. Deng et al. prepared phase-pure hexagonal wurtzite ZnS by annealing the corresponding precursors, which had been formed by forming intercalates between ethylenediamine and ZnS crystallites through a solvothermal route using ethylenediamine as solvent, in an inert atmosphere at temperatures above 350 °C.31 Platelet morphologies of ZnS precursors, which were formularized as ZnS.0.5EN, has been produced. However, Jang et al. obtained the porous plates composed of wurtzite ZnS nanocrystallites which are in the amorphous ZnS.0.5(EN), and after calcinations at temperatures of 400−500◦C, remnants are ZnO, ZnS, and ZnS.0.5(EN) of solvothermally synthesized samples.32 Ouyang et al. and R. Mosca et al. also got ZnS.0.5(EN), having layered the wurtzite ZnS fragment using the same synthesis route in the presence of EN.33,34 There are some more references on the synthesis of ZnS with EN;35−42 the as-synthesized product has been reported to be a ZnS-EN complex, and high temperature annealing is needed to stabilize the wurtzite phase. Therefore, it is challenging to directly synthesize wurtzite ZnS and stabilize it at low temperature by controlling its transformation to the cubic phase. Morphology control of ZnS nanocrystallites is another important factor in the manipulation of optical properties. The research efforts in the field of morphology control of ZnS have been mostly focused on fabricating one-dimensional (1D) nanowires or nanorods and two-dimensional nanosheets.43−48 Therefore, the synthesis of ZnS nanoparticles of desired size, shape, crystalline phase, and composition by a simple synthesis method is interesting. In the present work, we report the synthesis of ZnS in the wurtzite phase at a very low temperature in the presence of EN by a simple chemical route. The phase and morphology transformations of ZnS in the presence of EN are systematically studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform Infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscope (SEM), and transmission electron microscope (TEM). High temperature XRD of the samples demonstrated the phase stability at high temperature. Density functional theory (DFT) calculations support the transformation from a cubic to a hexagonal structure and rod formation in the presence of nitrogen-terminated ZnS nanoparticles or quantum dots. This is the first report on phase transition of ZnS from zinc blendeto-wurtzite at 170 °C in the presence of EN. The starting precursors for the synthesis of ZnS were zinc acetate dihydrate [Zn (CH3COO)2·2(H2O)] and thiourea

Figure 1. X-ray diffraction of as-synthesized ZnS.

clearly show broadened diffraction peaks compared to those of the bulk ZnS crystals, signifying the finite size of these crystallites. For the sample without EN, the diffraction peaks are observed at 28.79°, 47.87°, and 56.30°. These peaks are identified due to (111), (220), and (311) planes of cubic or zinc blende structures of ZnS (JCPDS no. 5-566). The effect of EN on the phase transition of ZnS is clearly evident from Figure 1. With an addition of EN into an aqueous solution, the XRD pattern shows the emergence of new additional peaks. For 1370

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12% EN, four new peaks are emerging out at 2θ = 27.06°, 30.88°, 39.89°, and 52.29° with three old peaks overlapping at 28.79°, 47.87°, and 56.30°. All the peaks are recognized with planes (100), (002), (101), (102), (110), (103), and (112) of the wurtzite phase of ZnS (JCPDS card no. 36-1450). All diffraction peaks of the sample are matched well to that of the wurtzite phase of ZnS crystal, indicating that the wurtzite phase has been formed in the sample in the presence of EN. With increasing EN concentration, the relative intensity (I(hkl)/I(002)) of the new peaks due to (100), (101), (102), and (103) planes is found to be increasing (Figure 2). Generally, peak intensity is

Figure 3. Phase transformation mechanism in the presence of EN. The (a) zinc blende structure without EN, (b) ethylenediamine (EN) molecule, (c) (ZnS)·En complex, (d) ZnS phase with stacking and twinning fault due to release of En, and (e) Wurtzite phase.

Figure 2. Increase of Relative Intensity of XRD peak with EN concentration.

3a) with EN as a soft template (Figure 3b), the EN molecule is active as a bidentate ligand, and two lone pairs of electrons associated with the two nitrogen atoms interact with cationic zinc ions. In the presence of the S2− ions, a complex layer of intermediate structure of ZnS·(EN) is formed (Figure 3c), which may lead to stacking faults. The process still continues to develop a stable phase of ZnS (Figure 3d). Further, the wurtzite particles (or rods as observed in SEM below) get bonded with EN molecules through surface ions. It is also possible that such an intermediate EN layer would not be stable as in the PbSe nanorod structures formed by the fusion of nanoparticles, which were capped with oleic acid.53 In any case, the surfactant-mediated transformation is quite complicated and more detailed experiments would be needed to conclude which mechanism is responsible for the phase and morphology transformation observed here. However, we have done some DFT calculations, which bring about some interesting results, albeit for small particles, in the support of our structural and morphological transformation, which is possible in the case of ZnS (in file 003 of the Supporting Information). It has been shown that when the surface atoms of ZnS are passivated with N atoms, a rodlike geometry is most stable, and when the ZnS is in the rodlike geometry, the preferred structure is Wurtzite. Thus, it is likely that the presence of a molecule like EN (with the amine end providing passivation through N ions) is able to introduce phase and morphology transformation. Thermogravimetry analaysis (TGA) of all the samples along with XRD was carried out from room temperature up to 750 °C. It was found that despite the small weight loss (related to the EN decomposition and subsequent loss) and very small changes in the lattice constant, there were no significant changes in the samples (see file 004 of the Supporting Information for TGA and XRD of all the samples). The surface morphology of the as-synthesized ZnS nanocrystallites in the zinc blende and wurtzite phases are shown in

a measure of the total scattering from each plane and is directly dependent on the distribution of atoms in the structure. It reflects the degree of crystallinity of the particular plane. The trend of increasing relative intensity indicates that the degree of crystallinity of the wurtzite phase of the ZnS is becoming stronger with increasing EN concentration. The crystallinity is also related to the concentration, reaction temperature, time, and the particle size measured. In the present work, there is high uniformity maintained in reaction conditions, while synthesizing all the samples, except a variation of EN concentration in water. It implies that the variation in the degree of crystallinity observed here is only due to the different percentage of EN used for sample preparation. Growth of nanoparticles in the rod form or 1D structure in the case of II−VI semiconductors has been reported earlier and reviewed by Chakrobarrtty and Chan.49 Some have suggested selective adhesion of surfactants (EN in the present case) leading to the growth of a least passivated surface. Another possibility also discussed in ref 49 is that the small particles are formed initially, which attach to each other growing into a 1D structure without any hint of grain boundaries. One more possibility also can be considered as follows. Two phases of ZnS viz. zinc blende and wurtzite differ in their stacking structures, and the transformation is mediated by the EN molecules which are involved in the intermediate stage and again act as capping molecules. The stacking sequence of the close-packed planes of zinc blend [the (111) planes] is represented by the ABCABCABCABC repeating pattern. However, if the close-packed planes stack like the ABABABABAB repeating pattern, they form the (001) [equivalently to (002)] planes of the wurtzite structure.50 In such a case, the stacking or twinning fault may develop during formation or growth of the nuclei initiate phase transition.51,52 Such a phase transformation mechanism in the presence of EN is schematically represented in Figure 3. In the synthesis of ZnS (Figure 1371

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Figure 4(a−e). As can be seen in Figure 4a, highly agglomerated but nearly spherical particles are obtained for as-synthesized ZnS without EN. However, with EN the ZnS sample shows rodlike morphology. There is little observable

change in the diameter as well as length of the rods with increasing concentration of EN from 12 to 67%. However, 100% of the EN sample shows a noticeable change in the length of the rods. EN with water as a solvent probably interrupts the length of the chain of EN, which is acting as a soft template in the reaction. It limits the length of ZnS rods in the EN−water sample. The comparative increase in the length of the rods of ZnS for 100% EN as a solvent is clear evidence for this. Close scrutiny of the SEM images of ZnS−100% EN shows some residues in between the rods. TGA (S1) of the ZnS−100% EN sample also shows drastic loss in the weight of the sample (ZnS−100% EN) which could be due to such EN residue loss. Transmission electron microscopy (TEM) measurements (Figure 5) confirm the morphology of ZnS with and without

Figure 4. SEM of (a) ZnS-0% EN, (b) ZnS-12% EN, (c) ZnS-40% EN, (d) ZnS-67% EN, (e) ZnS-100% EN.

Figure 5. TEM images of (a) ZnS-0% EN, (b) ZnS-40% EN, and (c) ZnS-100% EN. 1372

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EN. The TEM measurements of ZnS−0% EN also clearly show the agglomerated, nearly spherical shape particles having a size variation between 10 to 15 nm, while for ZnS−40% EN and 100% EN they show a rod-shape morphology. Diameters of the rods are observed to be around 11 nm in both the samples. For more in-depth size analysis, the particles size distribution for all the samples was determined by manual image analysis of approximaterly 100 particles from SEM. Particle size was determined by matching the digital pixel scale with the diameter of both types of the morphology (i.e., near spherical and rods). The quantitative studies of the obtained data from the image analysis were done by plotting the histograms of a number of particles versus particle size. It can be seen that the near spherical particles are ∼14 nm in size, and the rods are ∼11 nm in size. The details are given in file 004 of the Supporting Information. The mechanism of morphology transformations in the presence of EN are widely discussed in the literature.54−61 Most of the reports on the EN-induced morphology transformations show the formation of the (ZnS)·En0.5 complex structure and not the pure wurtzite ZnS phase. In the present study, XRD clearly demonstrates the formation of the pure wurtzite phase of ZnS samples. Hence, EN induced morphology transformation observed in the present study is interesting. The observed EN-induced phase and morphology transformation in ZnS is the clear evidence to relate growth directions of nuclei with crystallographic phases, particularly in the presence of surface-active molecules. There are certain reports on the controlling of the isotropic or anisotropic growth of nuclei by crystallographic phases.62−65 For the wurtzite ZnS nuclei, a close-packed (001) face has the highest surface energy (0.91−1.52 Jm2−) and thermodynamically fastest growth direction.66,52 The fastest growth along the (002) face results in higher atomic density and number of dangling bonds along the (002) direction. Therefore, growth along the c axis has occurred. It is also confirmed from XRD that the peak intensity of (002) is the largest. This may be attributed to 1D growth and gives rise to rodlike morphology of ZnS−EN, while ZnS without EN shows isotropic morphology. The elemental analysis of the samples was carried out by energy dispersive X-ray diffraction (EDAX). The EDAX spectrum of ZnS-100%EN is presented as an example in Figure 6. The peaks due to Zn, S, Si, C, N, and O can easily be detected. The strongest peak of Zn and S with the very weak peak of C, N, Si, and O are observed in the spectrum. The peak of Si is due to the substrate, while O is due to oxidation of the ZnS surface in air. The peaks at C and N indicate the presence of EN with ZnS. Additionally, X-ray photoelectron analysis was also carried out and discussed in the following section. The XPS data of the ZnS without and with a different percentage of EN, over a wide range of binding energy (0− 1200 eV), are shown in Figure 7a. It is evident that the elements Zn, S, C, O, and N (except in ZnS sample without EN) are present in the samples. The N 1s, Zn 2p, S 2p, and C 1s detailed regions are shown in Figure 7 (panels b, c, d, and e, respectively). Au 4f7/2 at 84.3 eV is used as an external and C 1s at 284.6 eV as internal standards. There is not much variation in the binding energy values of the XPS peaks and therefore not given here. The spin−orbit splitting of Zn 2p3/2 and Zn 2p1/2 is found to be 23.0 eV, characteristic of ZnS.67 Further, it is found that the ZnS ratio for the 0% EN sample is ∼3.8, whereas for other samples it is ∼2.0. Thus, all the samples are excess in Zn

Figure 6. EDAX spectrum of ZnS-100% EN.

or sulfur deficient. Figure 7b also indicates that the sample without EN does not have any nitrogen, but other samples indeed have an increase of nitrogen with increasing EN, as expected. It indicates that the surface of ZnS gets more covered with EN%. The Raman spectra of the as-synthesized ZnS samples are shown in Figure 8. The wave numbers of the identified phonon modes for all the samples are marked on the Raman spectrum. The spectrum shows strong and broad peaks at 267 cm−1 and 351 cm−1 for ZnS without EN, while at 254 cm−1, 307 cm−1, and 348 cm−1 for samples with EN. To compare the results of the Raman spectra obtained for ZnS under different synthesis conditions to previously reported data,68−70 we present an overview of Raman scattering of ZnS. Zinc blende ZnS belongs to the point group Td (43 m) with two atoms per unit cell and 3-fold optic branches that degenerate at k = 0. Due to the polarization field in ionic and partially ionic crystals, the degeneracy breakdown is observed immediately away from the center of the Brillouin Zone. Thus, in ZnS having a partial ionic nature of Zn−S bonding where there are one LO and one TO mode at k = 0 is expected, both of these are Raman active. The two spectra obtained in the present work at 267 and 350 cm−1 are identical TO and LO modes, respectively. Compared to the previous Raman study for zinc blende ZnS (Tables 1 and 2), the red shift is observed in the TO modes, while the result of the LO mode is consistent. The basic structure of the Wurtzite phase of ZnS has four atoms per unit cell and belongs to the space group C6v (6 mm) and gives rise to nine optical branches as Γopt = A1 + E1 + 2E2 + 2B1. The B1 modes are silent modes, A1 and E1 modes are polar modes and are both Raman and infrared active. The frequencies of the Raman-active modes of wurtzite ZnS are listed in Tables 1 and 2. The lower E21 mode is not seen in our experimental results because the measurement range of our spectrometer is from 200 cm−1. Compared to the previous results, the first order TO phonon peak of both phases of ZnS shows a red shift, while the second order TO peak of wurtzite ZnS shows a blue shift. A red shift is attributed to the size-induced optical phonon softening, which commonly occurs in a nanostructure due to the phonon-confinement effect.71 The blue shift in the second order TO phonon peak in wurtzite ZnS is observed for the first 1373

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Figure 7. XPS data of the as-synthesized ZnS-EN samples, (a) survey scan, (b) N 1S, (c) Zn 2p, (d) S 2p, and (e) C 1s.

Table 2. Comparative Raman Spectra for Wurtzite ZnS phonon

this work

ref (68)

ref (69)

ref (70)

E2 A1 (TO) E1 (TO) EL2 E1 (LO) A1 (LO)

− 254 254 307 348 348

− 275 279 285 353 353

72 273 273 286 351 351

69 272 276 286 351 −

Figure 8. Raman spectrum of ZnS.

Table 1. Comparative Raman Spectra for Cubic ZnS phonon

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ref (68)

ref (68)

ref. (68)

T2 (TO) T2 (LO)

267 cm−1 351 cm−1

278 cm−1 351 cm−1

271 cm−1 352 cm−1

275 cm−1 350 cm−1

Figure 9. FTIR spectra of ZnS.

quite different. Clearly, spectra without EN exhibit one broad peak near 3250 cm−1, which corresponds to the O−H stretching mode of water, indicating the presence of moisture in the sample. The peak at 669 cm−1 is assigned to a Zn−S bond. The spectra of samples formed in the presence of EN additionally have peaks at 1740 cm−1, 1380 cm−1, and 1120

time. The observed shift is a consequence of the combined effect of size and morpholgy. To further investigate the interaction between ZnS and EN, FTIR spectra were measured. The FTIR spectra of the assynthesized ZnS with a different percentage of EN are shown in Figure 9. The spectra for ZnS without EN and with EN are 1374

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cm−1, which respectively match with the rocking mode of NH2, the CC stretching bonds, and the CH2 twist band. It confirms the coupling between EN molecules with ZnS. It can also be seen that with the increasing concentration of EN in the synthesis, the peaks at 1740, 1380, and 1120 cm−1 increase in intensity. In conclusion, the phase transformation of ZnS from zinc blende-to-wurtzite, in the presence of EN, has been demonstrated experimentally and by DFT calculations. There are many reports on the synthesis of ZnS with surface-active molecules like CTAB, TTAB, DTAB, SL, SDBS, Triton X-100 (t-octyl-(OCH2CH2)xOH, where x = 9 and 10), ammonium salt of 2-undecyl-1-dithioureido-ethyl-imidazoline (SUDEI), EN, EDTA, and the list goes on.72−78 In most cases, the zinc blende structure or some complex intermediate phases of ZnS were developed. Very few reports are on the direct synthesis of the wurtzite phase. High temperature annealing treatment is needed to reduce the complex intermediate phase to the wurtzite phase of ZnS. Our present study is on the phase and morphology transformation and systematic crystalline growth of the wurtzite phase of ZnS from XRD by minutely changing the EN percentage. The structural stabilities of the phases at high temperature are confirmed by high temperature XRD and supported by TGA. Both the studies conclude that high temperature loss of EN does not affect phase stability. XPS analysis ensures the presence of EN at the surface of the wurtzite phase. The coupling between EN molecules with ZnS has been confirmed by FTIR. The SEM and TEM study confirm the shape transformation from a nearly spherical particle to rodlike morphology of ZnS in the presence of EN. Diameters of the nanorods are around 11 nm. Further experiments with high resolution TEM at different stages of growth (i.e., aliquots investigated at different timings), EN%, and different N-terminated surfactants would reveal the detailed growth mechanism. The present investigations show that in the presence of EN, the low-temperature transition from a zinc blende to a wurtzite structure accompanied by a spherical zerodimensional (0D) to rod shape (1D) change takes place. The growth is accelerated by an increase of EN concentration.

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ASSOCIATED CONTENT

S Supporting Information *

Computational details and results, thermo gravimetry analysis, high temperature XRD, and particle size analysis using SEM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.M. and S.K.K. acknowledge the Nano Unit, DST, India, at IISER, Pune, India. S.K.K also thanks UGC, India, for the constant support. UGC-DAE CSR at Indore, India, is acknowledged for providing the XPS and TEM facilities.

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