Non-galvanic template synthesis of CdSe nanowires using Anodic AluminaMembrane and their optical band gap determination
Ranjeet Singh a, Rajesh Kumar b, S.K. Chakarvarti c,⁎
a Department of Physics, Govt Post Graduate College, Karnal-132001, Indiab Department of Physics, Haryana College of Technology & Management, Kaithal-136027, India
c Department of Applied Physics, National Institute of Technology, Deemed University, Kurukshetra-136119, India
Received 24 March 2007; accepted 3 July 2007Available online 17 July 2007
Keywords: CdSe nanowires; Anodic Alumina Membrane; Optical band gap
1. Introduction
Over a decade, much attention has been paid towardsynthesis and characterization of one dimensional nanostruc-tures mainly due to their unique magnetic, electronic andoptical properties and their potential myriad applications indifferent fields including magnetic, electronic and opticaldevices [1–6]. There are various techniques used in thefabrication of one dimensional structures but template synthesisis an elegant, versatile and economic method for synthesizingvariety of these nanostructures including metal, semiconduc-tors, hetero-junctions, conducting polymers, carbon nanotubes(CNT's) etc [5–15].
CdSe is one of the II–IV semiconductors and because of highphotosensitivity it has been widely used in photoconductivedevices [16–19]. CdSe nanowires have been fabricated bydirect current (dc) as well as alternating current (ac)electrodeposition into the pores of Anodic Alumina Membrane
(AAM) using CdSO4 and Se dissolved in DMSO (dimethylsulfoxide) at high temperature [20,21]. CdSe nanowires alsohave been fabricated through the pores of AAM from alkalinesolution containing CdSO4 and SeO2 by electrodeposition atroom temperature [22]. Highly precise conditions like concen-tration, pH and cathodic potential are required to have goodstoichiometry ratio for Cd and Se.
We report here a non-galvanic method (chemical method) forpreparing an ordered and crystalline (ensembled from nano-particles) array of CdSe nanowires at room temperature usingAAM as template which is sandwiched in a two-compartmentcell. CdSO4 complexed with tartaric acid (TA) is employedas Cd2+ source and Na2SeSO3 in the presence of OH− ionsemployed as Se2− source. Electron microscopy, EDAX, XRDand UV–Vis characterization were performed for morpholog-ical, quantitative composition, structural and optical band gapanalysis.
2. Experimental section
All the chemical reagents used were RA grade and with-out further purification. CdSO4, Na2SO3 and Se powder were
Fig. 1. SEM image of aligned and ordered CdSe nanowires.
Fig. 2. XRD spectrum of CdSe nanowire arrays.
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obtained from s.d.fine-Chem Ltd. Mumbai, India, and allsolutions were prepared in de-ionized water. AAM anodisc-21 (Whatman, UK) with pore diameter 200 nm was used astemplate. For the deposition of CdSe nanowires, anodisc issandwiched in a two-compartment cell. 50 mM solution ofCdSO4 complexed with TA was filled in one compartmentand 50 mM solution of Na2SeSO3 with a pH value 12,adjusted by KOH, was filled in other compartment.Na2SeSO3 aqueous solution (0.5 M) was prepared byrefluxing 0.05 mol Se powder in 100 ml Na2SO3 aqueoussolution (1.00 M) for 3 h [23]. After filling the cell, it wasleft for 12 h at room temperature so that CdSe nanowiresare formed inside the pores of AAM.
The morphology of CdSe nanowires was examined by SEMby liberating them from the matrix after dissolving the AAOtemplate in 1 M NaOH solution at 25 °C for 1 h followedby subsequent washing. The cleaned and dried samples weremounted on special designed aluminum stub and coated withgold by using JEOL, FINE SPUTTER JFC-1100 sputter coaterand viewed under JEOL, JSM 6100 SEM. EDAX analysisof gold coated nanowires was carried out by RENTEC: ModelQX-1 instrument.
The CdSe nanowires embedded in AAM template wereplaced on glass plate and crystallographic studies were carriedout using Philips PW 1710 X-ray diffractometer in 2θ rangefrom 10° to 70° using CuKα radiation.
The optical absorption spectra were recorded with inthe range 200–850 nm using SIMAZDU; 2500 UV–Visspectrophotometer.
3. Result and discussion
In one of the compartments of cell, the cationic precursor solution,[Cd (Tartaric Acid)]2+ complex release Cd2+ ions as
½CdðTartaric AcidÞ�2þ→Cd2þ þ tartaric acid:
In the other compartment of cell, the anionic precursor solutionNa2SeSO3 hydrolyses in the presence of OH− ions to give Se2− as
Na2SeSO3 þ OH−→Na2SO4 þ HSe−
HSe− þ OH−→H2O þ Se2−:
In the pores of AAM, Cd2+ combined with Se2− to give CdSeprecipitate as
Cd2þ þ Se2−→CdSe:
When the cell is left for adequate time (12 h for CdSe), the aboveprocess continues till the pores are completely filled with the CdSe.This results into the formation of CdSe nanowires.
Fig. 1 shows SEM images of CdSe nanowires. It can be seenthat diameter of nanowires is about 200 nm that closely correspondsto the diameter of pores of template used and also all the CdSenanowires have parallel orientation and the length, diameterand direction of growth of CdSe are quite uniform which is dueto the confined growth of nanowires in the ordered pores of AAMtemplate.
An XRD pattern of CdSe nanowires embedded in AAM templateis shown in Fig. 2. The spectrum shows three peaks at 2θ=25.36°,2θ=42.16° and 2θ=50.05° that correspond to (111) d=3.51 Å, (220)d=2.14 Å and (311) d=1.82 Å planes, respectively. A carefulanalysis of broadening of XRD peaks suggest that CdSe nanowires sosynthesized are probably not single crystalline but are assembledfrom nanocrysallites (nanoparticles) [24]. Comparison of observed dvalues with standard d values [25,26] suggests the sphalerite cubic
Fig. 3. EDAX Spectrum of CdSe nanowires.
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(zinc blende type) nature of nanocrystallites. The broad hump may bedue to the amorphous nature of AAM template. The averagecrystallite size in CdSe nanowire for cubic phase can be calculatedusing relation [27]
b2hcosh ¼ ðKk=DÞ þ 4e Vsinh;
where β2θ is FWHM, θ is Bragg's angle, λ=1.5406 Å for CuKαradiation, D is average diameter of crystallite, K is shape factor(approximate value is unity) and ε′ is residual strain. The value ofaverage crystallite size is found less than 4 nm.
The chemical composition of CdSe nanowires was determinedusing energy dispersive EDAX technique. The EDAX spectrum shownin Fig. 3 and Table 1 clearly reveals that nanowires are composed of Cdand Se and quantitative analysis indicates that the atomic ratio of Cdand Se is nearly 1:1. The chemical composition determination isimportant because the excess of tartaric acid as complexing agent canresult in the precipitation of elemental Se. Therefore the optimumconcentration of tartaric acid is determined. The Gold (Au) peak is dueto the coating of Au thin layer on CdSe nanowires during taking EDAXspectrum.
The Fig. 4 shows the UV–Vis absorption spectrum of CdSenanowires. The optical band gap of CdSe nanowires is estimated fromUV–Vis absorption spectrum and Tauc plot [19,27]. The optical bandgap obtained from this fit is 2.25 eV, which is greater than standardband gap (1.7 eV) for bulk CdSe [28], showing a blue shift of 0.55 eVin band gap. The blue shift in band gap is due to quantum confinement
Table 1EDAX results of the sample showing element(El), atomic number(AN), series,wt.%, at.% of elements present
effect [29,30]. It is known that the size quantization is because of thelocalization of electrons and holes in the semiconductor nanocrystal-lites (nanosize in the range of 30–110 Å) which causes a change inelectronic band structure and hence an optical band gap larger ascompared with that of the bulk. Similar blue shift in optical band gapvalue is reported by Kale et al. [27] in case of thin films. Murray et al.[24] have also reported blue shift in band gap in case of nanocrystallitesof CdS and CdSe.
Fig. 4. (a) Plot of absorption (α) versus wavelength (λ), (b) plot of (αhυ)2 versushõ (Tauc plot with dotted line is a theoretical fit) for CdSe nanowires.
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4. Conclusion
Highly ordered CdSe nanowires composed of nanocrystal-lites are template synthesized into the pores of AAM using non-galvanic (chemical) method at room temperature. The SEManalysis shows that nanowires are highly ordered and uniformin diameter. XRD and EDAX analysis indicate nanocrystallinenature and good stoichiometry ratio of Cd and Se in CdSe,respectively. From the optical absorption spectrum and Taucplot, a blue shift in the band gap in CdSe is observed. Themechanism of the grown nanowires is also demonstrated. Thismethod can be possibly used in the synthesis of other chalco-genides also.
Acknowledgements
One of the authors, Ranjeet Singh, acknowledges the helpand encouragement from Prof. G. V. Prakash I.I.T. Delhi, Indiaand Prof. Shyam Kumar, K.U. Kurukshetra, India.
References
[1] K. Liu, K. Nagodawithana, P.C. Searson, C.L. Chien, Phys. Rev., B 51(1995) 7381.
[2] W. Fritszsche, K.J. Bohm, E. Unger, J.M. Kohler, Appl. Phys. Lett. 75(1999) 2854.
[3] Y. Kondo, K. Takayanang, Science 289 (2000) 606.[4] X. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298.[5] G. Fasol, Science 280 (1998) 545.[6] C.R. Martin, Science 266 (1994) 1961.[7] S.K. Chakarvarti, J. Vetter, Radiat. Meas. 29 (1998) 149.
[8] A. Huczko, Appl. Phys., A 70 (2000) 365.[9] S.K. Chakarvarti, Proceedings of SPIE, San Diego, California, USA,
vol. 6172, 2006, p. 61720G1.[10] A. Blondel, B. Ddoudin, J.-Ph. Ansermet, J. Magn. Magn. Mater. 165
Razavi, T.S. Mayer, J. Phys. Chem., B 106 (2002) 7458.[13] T. Iwasaki, T. Motoi, T. Den, Appl. Phys. Lett. 75 (1999) 2044.[14] U. Lunz, M. Keim, G. Reuscher, K. Schull, A. Waag, G. Landwehr,
J. Appl. Phys. 80 (1996) 6329.[15] R. Kumar, M. Chaudhari, S.K. Chakavarti, Mater. Lett. 61 (2007) 1580.[16] L.E. Brue, J. Chem. Phys. 80 (1984) 4403.[17] A.P. Alivisatos, T.D. Harris, P.J. Caroll, M.L. Stiegerwald, L.E. Brus,
J. Chem. Phys. 90 (1989) 3463.[18] M.T. Gutierrez, J. Ortega, J. Electrochem. Soc. 136 (1989) 2316.[19] G.V. Parkash, R. Singh, A. Kumar, R.K. Mishra, Mater. Lett. 60 (2006)
1744.[20] X.U. Dongsheng, D. Chen, Y. Xu, X. Shi, G. Guo, L. Gui, Y. Tang, Pure
Appl. Chem. 72 (1,2) (2000) 127.[21] D. Routkoyitch, A.A. Tager, J. Haruyama, D. Almawlawi, M. Moskorvits,
