Zn1 – xMnxS alloy nanowires with composition (x = 0.0, 0.1 and 0.3) have been successfully synthesized by a simple thermal evaporation on the silicon substrate coated with a gold film of 2 nm thickness. X-ray powder diffraction measurements reveal that as synthesized products were hexagonal wurtzite structure. The as grown nanowires have been investigated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive Analysis of X-rays (EDAX) and photoluminescence studies. The results reveal that the as grown nanowires consist of Zn, Mn, and S material and diameter ranging from 70 - 150 nm with lengths up to several tens of micrometers. Photoluminescence studies on Zn1 – xMnxS exhibited peaks at 600 and 613 nm for x = 0.1 and 0.3 respectively.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/1053

Murali, G.

Reddy, Amaranatha D.

Reddy, B.K.

Venugopal, R.

Vijayalakshmi, R.P.

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No2, P.5-10  2011 SumDU (Sumy State University)   5 PACS number: 62.23.Hj  SYNTHESIS AND PHOTOLUMINESCENCE STUDIES ON CATALYTICALLY GROWN Zn1 – xMnxS NANOWIRES  R.P. Vijayalakshmi1*, G. Murali1, D. Amaranatha Reddy1, R. Venugopal2, B.K. Reddy1  1 S.V. University, Dept. of Physics,   517 502, Tirupati, A.P., India * E-mail: vijayaraguru@gmail.com  2 Govt. Degree College, Puttur.  Dept. of Physics, Chittoor, A.P., India  Zn1 – xMnxS alloy nanowires with composition (x   0.0, 0.1 and 0.3) have been successfully synthesized by a simple thermal evaporation on the silicon substrate coated with a gold film of 2 nm thickness. X-ray powder diffraction measurements reveal that as synthesized products were hexagonal wurtzite structure. The as grown nanowires have been investigated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive Analysis of  X-rays (EDAX) and photoluminescence studies.  The results  reveal  that the as grown nanowires consist of Zn, Mn, and S material and diameter ranging from 70 - 150 nm with lengths up to several tens of micrometers. Photoluminescence studies on  Zn1 – xMnxS exhibited peaks at 600 and 613 nm for x   0.1 and 0.3 respectively.  Keywords: Zn1 – xMnxS NANOWIRES, CVD PROCESS, EDAX, TEM, XRD, PHOTO-LUMINESCENCE.  (Received 12 January 2011, in final form 16 February 2011,  published online 02 April 2011)  1. INTRODUCTION  One dimensional nanostructures have been a subject of intense research interest and have attracted considerable attention due to their novel properties and potential applications in nanoscale devices. In recent years exploring results have been achieved in nanoresearch [1-4]. Because of the ability to synthesize II-VI compounds in numerous configurations these materials attracted great research interest [5-7]. Among II-VI compounds ZnS  is  a  direct  band  gap  semiconductor  with  band  gap  of  3.7  eV  (bulk).  Under ambient conditions it has two phases, cubic Zinc blended and hexagonal wurtzite. Cubic ZnS is stable at room temperatures, where as the hexagonal ZnS is stable at high temperature (> 1000 qC). ZnS is widely studied material  for  its  wide applications as  base  material  for  cathode ray tube luminescent materials, phosphors and catalysts [8, 9]. When doped with some transition metal ions it exhibit a variety of luminescent properties such as photoluminescence, thermoluminescence and triboluminescence [10-11]. In addition to that it is a dilute magnetic semiconductor and is used in spintronic  devices  [12,  13].  Mn  doping  in  ZnS  attracts  a  great  deal  of  interest since Mn doping can not only enhance its optical transition   6 R.P. VIJAYALAKSHMI, G. MURALI, ET AL.   efficiency, increase number of luminescent centers and induces the semi magnetic behavior [12-16].The reports on Zn1 – xMnxS nanostructures are few. Yu et al. [17] reported composition dependent characterization Zn1 – xMnxS epilayers prepared by HWE. Biswas and co-worker [18] have synthesized ZnS:Mn2+ nanorods at 200 qC with a solvothermal method, and Mandel et al. [19] have obtained ZnS:Mn2+ nanorods within the pores of a polycarbonate membrane. Ge et al. [20] have reported a halide transport chemical vapor deposition method to fabricate ZnS:Mn2+ nanobelts and nanowires at 750 qC. Synthesis of ZnS:Mn2+ nanowires by a chemical vapor deposition  technique at 700 qC using Zn and S powders as precursors have been reported by Yan et al. [21]. The synthesis of nanowires with required dimension is technological importance. Zn1 – xMnxS nanowires with required diameter can be prepared by catalytic growth of CVD process. The choosing of catalyst is the prime concern; one can get particles of required size by chemical route method with the capping agent. In the present work we synthesized the nanowires by physically mixing the appropriate quantities of ZnS and MnS. By this method one can grow a nanowires of different diameters.   2. EXPERIMENTAL STUDIES  2.1 Catalytic growth of Zn1 – xMnxS   The experimental design to synthesize ZnMnS nanowires is described elsewhere [22]. In the fabrication of Zn1 – xMnxS nanowires, high purity ZnS and MnS powders (99.999 %) were used. The source materials were prepared first by physically mixing the desired quantities (x   0.0, 0.1 and 0.3) of ZnS and MnS and then sintering the mixture at 1000 qC in a vacuum sealed quartz tube for 18 hours.  The sintered mixture was cooled slowly to room temperature  in  5  hours  and  was  then  used  as  source  material  for  the  fabrication of ZnMnS alloy nanowires. The sintered mixture was put in the alumina boat and placed inside a quartz tube in the middle of the furnace. Prior to the synthetic reaction, the quartz tube was evacuated and heated up to 900 qC.  The  Au  film  (2  nm  thickness)  coated  on  a  Si  Substrate  (20 u 10 mm)used as a product collection substrate and located down stream to  the  carrier-gas  mixture  outlet  was  vacuum  annealed  at  550  qC to recrystallize into Au nanoparticles. A carrier-gas mixture of Ar (90 %) and H2 (10 %) with flow rate of 220 sccm was frequently introduced into the quartz tube till the pressure reach 250 torr. X-ray diffraction (XRD) spectra of the as synthesized nanowires were recorded using Scintag XI diffractometer with CuKĮ (O   1.5418 ')  radiation  at  scanning  speed  of  2 qC/min in 2ș ranging from 20 qC to 80 qC.Further the nanowires were also characterized by scanning electron microscopy(SEM) (LE0 1530, Field Emission SEM), transmission electron microscopy(Philips TECHNAI FE 12)  and  the  composition  of  the  Zn1 – xMnxS nanowires were determined by energy dispersive analysis of X-rays (EDAX). The PL excitation spectrum was recorded at room temperature using xenon arc lamp.  2.2 Growth Mechanism  In this  study,  the formation of  Zn1 – xMnxS nanowires  follow a vapor-liquid solid (VLS) growth mechanism as evidenced by the eutectic tips containing    SYNTHESIS AND PHOTOLUMINESCENCE STUDIES ON… 7  Au, Zn, Mn and S. At an early reaction stage in the formation of Zn1 – xMnxS nanowires, a vapour mixture of Zn, Mn and S was carried by the Ar and H2 gases and deposited onto catalytic Au nanodroplets. When the dissolution of Zn,  Mn  and  S  in  the  Au  nanodroplets  became  supersaturated  Zn1 – xMnxS nanowires extruded from the liquid eutectic Au nanodroplets and precipitated at the liquid-solid interface. This process basically complies with an ordinary VLS growth model proposed originally by [22] in which a liquid cluster of metal catalyst provides energetically favored sites for the absorption /adsorption of gas phase reactants [23]. The sizes of the catalysts are considered to be responsible for the diameters of resultant nanowires.  3. RESULTS AND DISCUSSION  3.1 EDAX analysis  The EDAX analysis showed that the nanowires were well stoichiometric with the  components  of  zinc,  sulphur  and  manganese.  EDAX  pattern  of   Zn0.9Mn0.1S is shown in Fig. 1.  Fig. 1 – EDAX Pattern of Zn0.9 Mn0.1S  3.2 Structural Analysis:  The XRD patterns of Zn1 – xMnxS (x   0.1 and 0.3) nanowires were shown in Fig. 2. All the diffraction peaks can be well indexed as hexagonal wurtzite structure and are consistent with the standard data. Similar structure was reported by Geng et al. in ZnMnS nanobelts [24] and in ZnMnS nanowires [25]. From the XRD spectra we can observe that (002) diffraction peaks is stronger and narrower than the other peaks, suggesting a preferential growth direction along the c-axis.    8 R.P. VIJAYALAKSHMI, G. MURALI, ET AL.   2 0 3 0 4 0 5 0 6 0 7 0 8 02 T  ( D e g r e e s )Intensity(a.u.) ( 1 0 0 )( 0 0 2 )( 1 0 1 )( 1 1 0 )X = 0 . 1x = 0 . 3  Fig. 2 – XRD Pattern of Zn1 – xMnxS  3.3 Morphological studies  General SEM morphologies of the synthesized Zn1 – xMnxS  (x   0.1) nanowires orienting randomly on the Si substrate are shown in Fig. 3a, the diameters of the nanowires are distributed in the range of 70 - 150 nm. Lengths of the nanowires can be up to several tens of micrometers and respond sensitively to the synthetic reaction time. The TEM images of the Zn0.9Mn0.1S nanowire is shown in Fig. 3b.   Fig. 3 – SEM images of Zn0.9Mn0.1S nanowires (a); TEM images of Zn0.9Mn0.1S nanowires (b)  a) b)    SYNTHESIS AND PHOTOLUMINESCENCE STUDIES ON… 9  3.4 Photoluminescence studies  From the luminescence spectra (See Fig. 4), no band edge emission was observed in ZnS:Mn nanowires. The peaks at around 600 and 613 nm were observed in Zn0.9Mn0.1S  and  Zn0.7Mn0.3S nanowires respectively. This so called yellow PL band originates from the 4T1 to 6A1 transition with in the 3d5 shell of Mn2+ on a cation site with tetrahedral symmetry. It is known that the Mn2+ ion electron states act as luminescence centers, because of strong interaction with the s-p electron states of the ZnS host crystals. Sooklal et al. [26] have studied the effect of the location of the Mn2+ ions on the photo physics of ZnS nanomaterials. They found that the Mn2+ ions incorporated into the ZnS lattice would lead to the yellow-orange emission, while the Mn2+ ions distributed on the surface of ZnS would yield the ultraviolet emission. By comparison  of  our  results  we  can  conclude  that  the  Mn2+ ions  are  incorporated into the ZnS lattice occupying the tetrahedral cationic site .As the Mn2+ concentration increased the intensity of yellow emission increased. Because the Mn2+ ions would capture more electron–hole pairs and emit more photons [24], leading to the enhancement of the yellow emission. Red shift was observed with the increasing Mn concentration. The origin for the peak position change as a function of Mn composition is not well understood. However it has been reported that the emission of the Mn2+ related peaks are virtually independent of x in Zn1 – xMnxS. Presumably because the Mn-S bond lengths are independent of composition [27].  Fig. 5 – Photoluminescence Spectra of Zn1 – xMnxS (x   0.1 and 0.3) nanowires  4. CONCLUSIONS  Zn1 – xMnxS  nanowires  were  synthesized  by  CVD  with  Au  as  catalyst.  Crystalline structure and lattice parameters were determined by XRD studies and the structure was hexagonal wurtzite in all the wires. TEM studies revealed that the growth direction of the nanowires is near to the c-axis. The luminescence intensity increased with Mn concentration.     10 R.P. VIJAYALAKSHMI, G. MURALI, ET AL.   ACKNOWLEDGEMENTS   The author wishes to thank the University Grant Commission, New Delhi for the financial Assistance through Major Research Project.  REFERENCES 1. C. Lu, L. Qi, J. Yang, L. Tang, D. Zhang, J. Ma, Chem. Commun. 33, 3551 (2006). 2. L.W. Yin, Y. Bando, Nature Mater. 4, 883 (2005). 3. R. Venugopal, P.I. Lin, C.C. Liu, Y.T. Chen, J. Am. Chem. Soc. 127, 11262 (2005). 4. Y. Jiang, X.M. Meng, W.C. Yiu, J. Liu, J.X. Ding, C.S. Lee, S.T. Lee J. Phys. Chem. B 108, 2784 (2004). 5. S.Q. 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Synthesis and photoluminescence studies on catalytically grown Zn1 – xMnxS nanowires

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