Nanotwinning in CdS quantum dots

Nanotwinning in CdS quantum dots

Physica B 407 (2012) 3347–3351 Contents lists available at SciVerse ScienceDirect Physica B journal homepage: Nanotwi...

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Physica B 407 (2012) 3347–3351

Contents lists available at SciVerse ScienceDirect

Physica B journal homepage:

Nanotwinning in CdS quantum dots Pragati Kumar a,b, Nupur Saxena a,c,1, F. Singh c, Avinash Agarwal a,n a b c

Department of Physics, Bareilly College, Bareilly 243005 U.P., India Khandelwal College of Management Science and Technology, Bareilly 243001 U.P., India Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India

a r t i c l e i n f o


Article history: Received 31 July 2011 Received in revised form 3 April 2012 Accepted 13 April 2012 Available online 4 May 2012

High resolution transmission electron microscopy, X-ray diffraction and photoluminescence measurements are carried out in order to study the defects in CdS quantum dots (QDs), synthesized in cubic phase by chemical co-precipitation method. The nanotwinning structures in CdS quantum dots (  2.7 nm) are reported for the first time. Mostly CdS QDs are characterized by existence of nanotwin structures. The twinning structures are present together with stacking faults in some QDs while others exist with grain boundaries. Raman spectroscopy analysis shows intense and broad peaks corresponding to fundamental optical phonon mode (LO) and the first over tone mode (2LO) of CdS at 302 cm  1 and 605 cm  1 respectively. A noticeable shift is observed in Raman lines indicating the effect of phonon confinement. Fourier transform infrared spectroscopy analysis confirms the presence of Cd–S stretching bands at 661 cm  1 and 706 cm  1. The photoluminescence spectrum shows emission in yellow and red regions of visible spectrum. The presence of stacking faults and other defects are explained on the basis of X-rays diffraction patterns and are correlated with photoluminescence spectrum. These nanotwinning and microstructural defects are responsible for different emissions from CdS QDs. & 2012 Elsevier B.V. All rights reserved.

Keywords: CdS quantum dots Nanotwinning Stacking fault Grain boundaries Photoluminescence

1. Introduction The new vista of technology is related to quantum dots. Semiconductor nanoparticles can be synthesized with one (nanosheets), two (nanowire), three (nanoparticle) and even zero (quantum dot) dimensional confinement mainly of charge carriers which leads to many surprising, unexplored and interesting properties. Confinement of electrons, holes and vibrations to crystallites of size of a few nanometers brings about dramatic changes in their optical properties [1,2]. Confinement of charge carriers appears as a blue shift of the band gap that is observed in the absorption and emission spectra. The coupling of confined charges and confined phonons is another factor that determines the optical properties of nanoparticles. Investigation of surfaces properties of semiconductor nanocrystals is very interesting, not only for their importance in the development of electronic devices, but also for the wide variety of unexpected reconstructions that can take place during the growth process. As semiconductor materials become smaller and smaller, down to the nanoscale, the physical properties, especially the light-


Corresponding author. Tel.: þ91 581 2568844; fax: þ 91 581 2567808. E-mail addresses: [email protected] (P. Kumar), [email protected] (A. Agarwal). 1 Present Address: Inter University Accelerator Center, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India. 0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved.

emitting properties, change drastically. The optical properties of semiconductor nanocrystals are affected by quantum confinement when their typical dimensions are equal or smaller than the Bohr radius of exciton, which in case of CdS is 2.85 nm. The microstructural characterization of nanocrystals (NCs) or QDs with size comparable to Bohr’s exciton radius is useful to understand the light emission mechanism in nanocrystals. Twin structure (TS), stacking faults (SFs) and grain boundaries (GBs) are the most common microstructural defects in nanomaterials. In addition to the surface defects, CdS nanocrystals demonstrates a wide varieties of structural defects such as sulfur and cadmium vacancies associated with unsaturated bonds of the Vs þVCd type, large cluster of interstitial-vacancy cadmium, grain boundaries and adsorbed foreign atoms such as oxygen [3].These surface and structural defects are expected to have important effects on the physical properties particularly the optical properties of the QDs. Cadmium sulfide has been investigated extensively for more than four decades. The study of size selective CdS nanoparticles is of great interest recently due to the fact that various properties like optical, electrical and chemical can be tuned by varying particle size [4]. It is an important compound semiconductor material with a direct band gap  2.42 eV, lying in visible range of spectrum. It is a well known phosphor and can be used in biomedical labeling [5], solar cell [6], LED’s [7] and quantum devices. Different physical and chemical synthesis routes were adopted by


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the researchers to synthesize nanostructures of compound semiconductors. CdS nanoparticles have been synthesized by rfmagnetron sputtering, pulse laser deposition (PLD), resistive heating, electron beam evaporation, ion implantation, sol–gel, chemical precipitation etc. [8–17]. The co-precipitation synthesis technique is relatively simple and allows in forming size selective and homogeneous multicomponent nanoparticles due to the fact that different ions precipitate under different conditions of pH and temperature. The studies of the defects appeared in CdS QDs synthesized by chemical co-precipitation method are reported. The structural and optical analyses of CdS QDs are performed using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy (PL) and transmission electron microscopy (TEM) techniques. The defects as nanotwin structure, stacking faults and grain boundaries in the QDs are imaged through high-resolution transmission electron microscopy (HRTEM). The presence of surplus interstitial cadmium is confirmed from X-ray diffraction pattern. It is observed that in some QDs, there are twin structure and stacking-fault defects; while in others the twin structure coexists with grain boundaries. The observed emission peaks in PL spectrum are explained on the basis of observed defects. These studies are important for biological applications.

2. Experimental procedure All chemicals i.e. Cd(NO3)2  4H2O and Na2S of analytical reagent grade were procured from Sigma Aldrich Ltd. (USA) and used without any further purification. The CdS QDs in powder form were synthesized by chemical co-precipitation. Homogeneous and clear solutions of 0.1 M Cd(NO3)2 and 0.5 M Na2S were prepared in deionized water by stirring magnetically for 1 h. 10 mL of the Na2S solution was injected drop wise into each 50 mL of rapidly stirring Cd(NO3)2 solution. The colloidal sample was then stirred magnetically for 1 h. Finally, a yellow precipitate was obtained. Precipitate was washed 3–4 times and dried at 200 1C. The dried sample was crushed to get fine powder CdS nanocrystals(NCs). The chemical reaction can be expressed as CdðNO3 Þ2 þ Na2 S-CdSþ 2NaNO3


X-ray diffraction pattern was obtained using Bruker D8 X-ray diffractometer with Cu Ka(l ¼1.5406 A1). The sample was examined by TEM and HRTEM using Technai G20-Stwin operating at 200 kV ˚ line resolution 2.32 A˚ and line type with point resolution 1.44 A, super twin lenses. Raman spectrum of the sample was recorded with Renishaw Invia using a 514 nm excitation line of Ar ion laser. IR spectrum of sample was recorded in transmission mode using Thermo Nicolet NEXUS-870 FT-IR with a resolution of 4 cm  1. PL spectrum is recorded using Jobin Yvon Spectroflurometer (FluroMax-3) with excitation wavelength taken as 420 nm from Xe lamp.

3. Result and discussion In order to understand the exact phase, presence of different kinds of defects and particle sizes of CdS QDs, the X-ray diffraction studies are carried out for as prepared sample. Fig. 1 shows XRD pattern of CdS QDs. It reveals that the QDs exhibit a zincblende crystal structure (PCPDF WIN-751546) with lattice ˚ The four diffraction peaks appear at 26.51, parameter 5.82 A. 43.81, 51.91 and 61.81 corresponding to (111), (220), (311) and (400) planes of the cubic phase respectively. However there is no observable peak at 30.71 corresponding to (200) plane. It shows that this peak is merged with broad peak at 26.51. Pickett et al.

Fig. 1. XRD pattern of CdS QDs.

[18] reported that the broadening of preferential peak can occur, for a number of reasons, associated with polytypism, twin structures in crystal, stacking faults (SFs), or other defects within the resulting nanoparticle or due to the crystallites being nonspherical. Recently, Gautam et al. [19] reported that small concentration of SFs is sufficient to broaden the peak so much that it merges with another peak or background. Another factor that is responsible for broadening of peak is reduced particle size. The average size of particle in present study is estimated by Debye Scherrer’s formula D¼

kl b cos y


where k¼0.9, l ¼wave length of X-ray, b ¼Full width at half maximum (FWHM) and y ¼Bragg’s angle. The estimated particle size is around 3 nm. The peak at 34.71 corresponding to (100) plane of cadmium clearly shows the deficiency of sulfur (S) or excess of interstitial cadmium (Cd) in CdS crystals. Thus, it follows that SFs, twin structures and other defects may be contributing to broadening of peak. The FTIR spectra of dried CdS QD powder mixed with KBr, is presented in Fig. 2. The bond at 1590.2 cm  1 is due to CQN vibrational stretching mode. The C–O stretching bands appear at 1303.3 cm  1 and 1190.7 cm  1. These bonds may be due to the environmental and/or chemical impurities that are always present. The atmospheric oxygen may react with small amount of sulfur present in sodium sulfide and results in trace amount of SO4  as impurity. Small absorption around 1024.6 cm  1 confirms to the observation of trace amount of SO4  impurity. It is supported by the XRD result that indicates the excess of cadmium or deficiency of sulfur. CdS QDs show two medium to strong bands at 661.4 cm  1 and 706.3 cm  1 corresponding to Cd–S stretching. The result is consistent with the report of Kawar et al. [20] and Thangadurai et al. [21]. Raman spectroscopy is a versatile tool to study vibrational properties of crystals. In bulk CdS crystals, the phonon eigen state is a plane wave and the selection rule for Raman scattering is qE0, where q is the wave vector. But in nanocrystalline materials, q E0 selection rule is relaxed due to interruption of lattice periodicity. Two optical vibration modes observed in the present study (shown in Fig. 3) around 302 cm  1 and 605 cm  1 correspond to 1LO and 2LO Raman optical phonon mode respectively. The FWHM of 1LO Raman peak is obtained around 18 nm which is

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Fig. 4. TEM micrograph of CdS QDs and particle size distribution (inset). Fig. 2. FTIR spectrum of CdS QDs.

Fig. 3. Raman spectrum of CdS QDs.

a clear signature of phonon confinement. The frequency shift of the 1LO Raman peak in CdS nanoparticles, in contrast to bulk CdS has been studied before and is mainly ascribed to the grain size effect [22]. The 1LO phonon frequency for a single crystal of bulk CdS was reported at 305 cm  1 [23]. In nanometer sized particles the most prominent peak of CdS may gets shifted to 300 cm  1 [22,24,25]. It has been theoretically calculated and experimentally observed that there is noticeable asymmetry and frequency shift towards the lower frequency side as the particle size reduces [26–28]. It is seen that the first order LO Raman line is not only broadened. It also shows an asymmetric broadening towards the lower frequency side. TEM studies were performed to highlight the shape, size, size distribution, and defects in nanocrystalline particles. For this purpose, a clear solution of CdS QDs is prepared in acetone by ultrasonication of solution for 1 h and a few drops of as prepared solution are placed on carbon coated copper grid for characterization by TEM. It can be clearly seen in Fig. 4 that most of the particles are isolated and spherical in shape with narrow size distribution. There are some particles in cluster form also. This may be due to the fact that no surfactant is used during synthesis process. The size distribution of CdS QDs is shown in inset Fig. 4.

It reveals that the sizes of CdS nanoparticles range from  0.85 to  4.75 nm with an average particle size of 2.7 nm are formed. This is in well agreement with XRD analysis. Twinning is one of the most common planar defects in nanocrystals, and it is frequently observed in fcc-structured metallic nanocrystals [29]. As a major microstructural characteristic, these defects are expected to have an important role of these defects on physical (particularly the optical, electronic and mechanical) properties of the nanocrystals. It can be observed that different types of twinning structures exist in the CdS QDs synthesized by chemical co-precipitation. Some twinning structures seem to exhibit a coalescence of two or several small CdS QDs. The coalescence of the small CdS QDs by twinning may be responsible for the growth of the large CdS QDs. HRTEM was used to examine the microstructure of the CdS QDs in detail. Fig. 5(a) shows HRTEM of CdS QDs with twin structures (TS) and stacking fault (SF). A close examination of Fig. 5(a) clearly shows the presence of double-twin configuration with stacking faults. The nanotwin boundaries are indicated by smaller white arrows. White parallel lines show the deviated atomic sequence i.e. stacking fault. These are labeled with TS1, TS2 and SF respectively in the figure. It can be observed that TS1, TS2 and SF are all parallel to each other. The shape of crystal is almost elliptical. There are only three atomic plans in nanotwin (II) connecting nanocrystals I and III. It is clear that the SF may be formed by removing a layer C from the perfect-crystal layer sequence. A characteristic of an intrinsic SF with displacement of R¼ 1/3/111S is observed. It is evident from Fig. 5(b) that the intrinsic SF transforms the perfect-crystal layer sequence y..ABCABABCABCy... instead of y.. ABCABCy.. The layers with a local hexagonal-closed-packed (hcp) environment, ABAB, are highlighted by bold letters [30]. Fig. 5(b) shows the HRTEM of CdS QDs with twinning structures (TS) and grain boundaries (GB).The misorientation or mismatching between nanocrystals i.e. GB can clearly be seen. It can be observed from Fig. 5(b) that there is single twin boundary on the left side of GB in crystal, whereas double twin boundary is present on right side of GB. These are indicated by white arrows and labeled by TS1, TS2 and TS3 respectively. The grain bondary is labeled by GB. The close view of fig shows that TS1, TS2 and TS3 all are parallel to each other. There are only five atomic plans in nanotwin (II) connecting nanocrystals I and III. It can be also seen that the crystal having double twin-configuration is almost elliptical whereas the shape of crystal having single twin boundary is irregular [31]. Wang et al. [31] reported the presence of nanotwinning structures in Si nanocrystals of crystal size larger


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Fig. 6. PL spectrum of CdS QDs at 420 nm excitation wavelength.

of bound electrons from the surface states to the valence band as reported by Tasi et al. [9]. They observed only green emission at room temperature, while red and yellow emission bands were observed at relatively low temperature. We observed yellow and red emission bands at room temperature instead of green band, which shows that the high density of defect levels due to both surface states and microstructure is present.

4. Conclusion

Fig. 5. HRTEM image of a typical CdS QDs (a) nanotwin structure and stacking fault defects (b) nanotwin structure and grain boundaries.

than 6 nm whereas no planner defect is observed in crystal below 5 nm. They described the reason of absence of micrstructures in small nanocrystals is due to larger surface area-to-volume ratio and a higher total energy of small nanocrystals. The higherenergy surface in the smaller Si nanocrystal is not compatible with the high-energy defects. Here, we observed nanotwinnning structure in CdS QDs of average diameter 2.7 nm. This may be due to the low temperature during synthesis and different growth conditions. Since, in this study CdS QDs are grown at room temperature, it is expected that at the lower growth temperature the surface energy of nanocrystals is not high enough, so that the defects may glide through the small crystals and disappear. Similar to twinning defects in the metallic nanocrystals, the heavily twinned microstructure in the CdS QDs is also expected to have important effects on the optical properties of this sample. To correlate the microstructure with the optical properties of the CdS QDs, photoluminescence (PL) measurement is carried out. PL studies provide information of different energy states available between valence and conduction bands, responsible for radiative recombination. Fig. 6 shows PL spectrum of CdS QDs recorded at room temperature using excitation wavelength of 420 nm from Xe lamp. In the PL spectrum of CdS QDs no band edge emission is detected; only the surface state emission is observed. A single PL peak at 560 nm with a shoulder at 616 nm has been observed corresponding to yellow and red emission respectively. The yellow emission was endorsed to the transition of interstitial Cd donors to the valence band and red emission corresponding to the transition

Defects are studied in chemically synthesized cubic phase quantum dots of CdS. The presence of defects observed in HRTEM micrographs are explained by the results of XRD, FTIR and PL spectrum. HRTEM micrograph clearly shows the presence of nanotwin structures together with stacking faults in some CdS QDs, while in others the nanotwin structures coexist with grain boundaries. The broadening in XRD peak is due to the presence of these defects as well. The extra peak observed is assigned to interstitial cadmium. These microstructural defects are expected to play an important role in the light emission from the CdS QDs.

Acknowledgment The authors are thankful to Dr. R.P. Singh, Principal, Bareilly College, Bareilly for providing necessary facilities and moral support. One of the authors (AA) is thankful to Council of Science and Technology, Uttar Pradesh (CST-UP), Lucknow for financial support. The help received from Mr. Pawan Kulariya, Inter University Accelarator Center in XRD measurement, Prof. Shyam Kumar, Kurukshetra University, Kurukshetra for PL measurement and Prof. B.R. Mehta, Indian Institute of Technology, Delhi for HRTEM studies are gratefully acknowledged. One of authors (PK) is thankful to Dr. Amresh Kumar, Director General, Khandelwal College of Management Science and Technology, Bareilly for his interest and support. References [1] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [2] W. Wang, I. Germanenko, M.S.E. Shall, Chem. Mater. 14 (2002) 3028. [3] O. Vigil, I. Riech, M. Garcia-Rocha, O. Zelaya-Angel, J. Vac. Sci. Technol. A 15 (4) (1997) 2282. [4] Y. Wang, N. Herron, Phys. Rev. B 42 (11) (1990) 7253.

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