A facile one-pot synthesis of highly luminescent CdS

J Nanopart Res (2012) 14:916 DOI 10.1007/s11051-012-0916-3

RESEARCH PAPER

A facile one-pot synthesis of highly luminescent CdS nanoparticles using thioglycerol as capping agent Mou Pal • N. R. Mathews • P. Santiago X. Mathew

•

Received: 15 November 2011 / Accepted: 10 May 2012 Ó Springer Science+Business Media B.V. 2012

Abstract Highly luminescent, multicolored CdS nanoparticles (NPs) with size less than 3 nm were prepared by aqueous precipitation method using thioglycerol (TG) as the capping agent. The size of the NPs could be successfully controlled by varying TG concentration in the reaction mixture. The assynthesized NPs were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy, micro Raman, UV–Vis and photoluminescence (PL) spectroscopy. The XRD results revealed the transformation of zinc-blende cubic CdS to hexagonal wurtzite phase at higher capping agent concentration, which coincides well with TEM results. Raman spectra demonstrated a notable decrease in the peak intensity and the disappearance of 2 LO phonon mode of CdS upon TG addition. UV– Vis absorption spectra showed a remarkable blue shift in absorption edge in TG-capped CdS NPs compared to the uncapped one. With an increase in the capping agent concentration from 0 to 0.2 molar, the broad PL band at 630 nm blue shifted to 600 nm accompanying a gradual increase in the intensity of PL emission. M. Pal N. R. Mathews X. Mathew (&) Centro de Investigacioń en Energıá, Universidad Nacional Autońoma de Me´xico, 62580 Temixco, Morelos, Mexico e-mail: [email protected] P. Santiago Instituto de Fı´sica, Universidad Nacional Autońoma de Me´xico, Apartado Postal 20-364, 01000 Mexico, D.F., Mexico

Keywords CdS nanoparticles Capping agent Photoluminescence Synthesis

Introduction Size-dependent optical and electronic properties of semiconductor particles in nanometer scale have contributed an enormous development in materials chemistry providing an opportunity to explore their vast potentials in optoelectronic devices. In semiconductor nanoparticles (NPs), strong confinement effect appears when the size of the NPs is comparable to the exciton Bohr radius in the bulk (Alivisatos 1996). By controlling the size and surface properties of the particles, the electronic, optical and chemical properties can be tailored for a wide range of application in several fields (Cahn 1990; Zhao et al. 2001). CdS is a II–VI semiconductor having direct band gap energy of 2.4 eV at room temperature (Bawendi et al. 1992). The confinement effect is observed for CdS particles when the sizes are equal to or less than ˚ (Wang and Herron 1990). The CdS exists in 50 A three types of crystalline structures namely hexagonal wurtzite, cubic zinc blend and high pressure rock-salt phase. Among these, hexagonal phase is the most stable and has been found in both the bulk and nanocrystalline CdS whereas cubic and rock-salt phases are observed only in ultrafine, nanosized CdS

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system (Ricolleau et al. 1999). Due to the large band gap and its inherent n-type conduction, CdS is used as window material in thin-film hetero-junction solar cells (Romeo et al. 2004). It has potential application in many devices such as field effect transistors (Neugebauer et al. 1968; Armitage 1975), light emitting diodes (Kolvin et al. 1994), laser screen materials (Al-Bassam et al 1988), solar energy converters (Huynh et al. 2002), photocatalysis (Li et al. 2008) and biological sensors (Zhou and Ghosh 2007; Kobayashi et al. 2007). In particular, its size-dependent emission-wavelength tunability is highly attractive in band gap engineering of materials and quantum dot lasers (Unni et al. 2008). A number of synthetic methods have already been employed by different research groups to prepare CdS NPs including aqueous precipitation (Singh and Chauhan 2009), non-aqueous precipitation (Pattabi et al. 2007), soft chemical reaction (Maleki et al. 2007), solid state reaction (Wang et al. 2003), sol–gel process (Hullavarad and Hullavarad 2007), microwave heating (Wada et al. 2001), sonochemical preparation (Wang et al. 2001), and reverse micelle (Zhang et al. 2002). De´kańy et al. (1996) have reported the preparation of size-quantized CdS and ZnS NPs in the nanophase reactors adsorbed from binary liquid mixtures, at the surfaces of colloidal silica particles. On the other hand, quantum-sized semiconductor particles such as ZnO were synthesized in dimethyl sulfoxide medium via alkaline hydrolysis, using layered silicates (clay minerals) as stabilizers and supports for the nanocrystals (Ne´meth et al. 2004). To stabilize the surface of colloidal nanocrystals, suitable organic molecules, known as capping agents, are commonly used during syntheses which are bound to the particle surface, thereby hindering the growth. In this context, Sobhana et al. (2011) have reported surface passivation of CdS quantum dots (QDs) with thiol groups using different capping agents such as 3-mercaptopropionic acid, mercaptosuccinic acid, and glutathione, and noted a remarkable improvement in the optical and sizedependent properties of capped CdS QDs compared to the uncapped one. Unni et al. (2008) obtained CdS NPs of different sizes by aqueous precipitation method by changing the volume ratio of thioglycerol (TG, used as a capping agent) to sulfur precursor. In this article, we report a room temperature wet chemical preparation of highly luminescent, monodisperse CdS NPs with small sizes and high crystalline quality using

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J Nanopart Res (2012) 14:916

TG as a capping agent. The amount of TG is found to have an effect on size, shape, crystalline phase, optical, and luminescence properties of as-synthesized CdS NPs. In particular, it was found that at high TG concentration, the quasi-spherical CdS NPs undergo phase transition from cubic zinc-blende to hexagonal wurtzite phase showing strong quantum confinement effect, as evidenced by UV–Vis absorption and PL results. The novelty of this work lies in the fact that we could synthesize ultrafine, monodisperse CdS NPs with size less than 3 nm and of high crystalline quality at room temperature which opens up the possibility of using these NPs for developing nanocrystalline CdS thin films by a variety of techniques such as spraying, printing, paste coating, etc. for multifunctional applications. CdS is the most used n-type layer for high efficiency hetero-junction thin film solar cells (Wu 2004; Jackson et al. 2011; Mitzi et al. 2011). Application of CdS NPs dispersed in an appropriate medium for spraying or printing is highly economical, rapid, and attractive for large volume production of thin films. The work presented in this article will contribute to the advancement of very thin CdS film technology applied in photovoltaics and other applications such as visible light emitting diodes (Schlamp et al. 1997).

Experimental All the chemicals used were of analytical grade. Nanocrystalline CdS samples were prepared at different capping agent concentrations by chemical precipitation method at room temperature. Cadmium sulfate (CdSO4, Alfa-aeser) and sodium sulfide (Alfa-aeser) were used as precursor salts, deionized water as solvent and TG (Aldrich) as the capping agent. In a typical synthesis, 0.1 M CdSO4 and 0.1 M of Na2S were dissolved in 50 mL deionized water separately with constant magnetic stirring. The required volume of TG corresponding to 0.05, 0.1, and 0.2 M concentrations were added to CdSO4 solution and the stirring was continued for another 30 min in order to facilitate the complex formation between Cd?2 and TG. Thereafter, Na2S solution was added to TG containing CdSO4 solution drop-by-drop under vigorous stirring. CdS NPs started to form as soon as the addition of Na2S was started and the color of the reaction system was changed from transparent to orange (without TG),


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(a)

(b)

5.5

(103) CdS-C

0 1000

Cubic

CdS-B

Cubic

0

Cubic

(111)

CdS-A

(220)

0 20

FWHM (2θ )

Intensity (a.u.)

Hexagonal

0 1000

0 1000

6.0

CdS-D

(101)

1000

5.0 4.5 4.0 3.5

(311)

3.0 30

40

50

60

70

2θ (ο )

80

0.00

0.05

0.10

0.15

0.20

TG concentration (molar)

Fig. 1 a Powder X-ray diffraction patterns of CdS nanoparticles at different TG concentration: CdS-A, CdS-B, CdS-C, and CdS-D correspond to CdS nanoparticles prepared at 0, 0.05, 0.1, and 0.2 molar concentrations of TG, respectively. b Dependence

of FWHM with the capping agent content. FWHM of (111) reflection is chosen for CdS-A to CdS-C samples, while for CdSD (101) reflection is selected

bright yellow, faint yellow, and greenish yellow on increasing the concentrations of TG. Nitrogen gas was flushed throughout the synthesis in order to avoid the oxidation of the particles. The NPs were then separated by centrifugation (10,000 rpm/15 min) and washed with water and ethanol several times to get rid of impurities and other byproducts. For drying, the particles were kept in a Petri dish for about 12 h. Then the powder sample were collected and preserved in an airtight vial. Four CdS samples CdS-A, CdS-B, CdSC, and CdS-D were prepared by using 0, 0.05, 0.1, and 0.2 M TG, respectively. In this study, we did not control pH of the reaction mixture. The different pH values recorded for 50 mL of 0.1 M CdSO4 solution with 0, 0.05, 0.1, and 0.2 M of TG are 4.17, 2.79, 2.55, 2.26, respectively. The powder X-ray diffraction (XRD) spectra were recorded on a Bruker Discover D-8 X-ray diffractom˚ ). eter using CuKa radiation (Wavelength = 1.5406 A Using the Scherrer equation, the particle size was calculated from the full width at half maximum (FWHM) of the diffracted lines corresponding to (111) and (101) planes of cubic and hexagonal phases, respectively. The as-prepared samples were examined by a TECNAI 30 high resolution transmission electron microscopy (HRTEM) operated at 300 keV. TEM samples were prepared by placing a drop of ethanol solution containing the NPs directly onto carboncoated copper grids and then dried under IR lamp before taking images. The energy dispersive X-ray spectroscopy (EDS) was performed using a JEOL JSM6610LV field emission scanning electron

microscope (FESEM) attached with INCA Oxford analytical system. UV–Vis absorption spectra were recorded on a Shimadzu, UV-3101 PC double beam spectrophotometer by dispersing homogeneously the powder sample in ethanol under ultrasonication. Diffuse reflectance spectra (DRS) of the powdered samples were taken by diffuse reflectance spectroscopy (DRS), using a Varian Cary 100 UV–Vis spectrophotometer with DRA-CA-30I diffuse reflectance accessory. Raman spectra of CdS samples were recorded at room temperature using a Horiba JOBINYVON spectrophotometer (LabRam HR model) fitted with a He–Ne (332.6 nm) laser as excitation source. For recording the room temperature PL spectra of the samples, a 80-cm long Science Tech monochromator, a thermoelectrically cooled Hamamatsu (PMH-04) photomultiplier and a He-Cd laser (Melles-Griot) with emission at 325 nm wavelength were used.

Results and discussion XRD pattern gives information about the crystalline phase as well as the crystallite size of the NPs. Figure 1 shows the XRD patterns of four different CdS samples designated as CdS-A, CdS-B, CdS-C, and CdS-D. As seen in Fig. 1a, the XRD peaks are very broad indicating extremely fine size of the grains. The XRD patterns of CdS-A, B and C exhibit three prominent peaks at 2h values of 26.82°, 44.42°, and 52.0° corresponding to the diffraction from (111), (220), and (311) planes, respectively, of cubic zinc blend phase of

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Fig. 2 EDS spectra of CdS nanoparticles grown with 0 (CdS-A) and 0.2 M (CdS-D) thioglycerol concentration

CdS (JCPDS card no. 89-0440). However, the X-ray diffractogram for CdS-D sample shows two broad peaks at 28.17° and 48.0° which could be indexed to the (101) and (103) planes of the hexagonal wurtzite phase of CdS (JCPDS card no. 77-2306). The FWHM for the diffraction peaks increases almost exponentially with increasing TG concentration (Fig. 1b) which suggests a significant decrease in particle size. The average grain size of the samples were determined using Debye Scherrers’ formula, D = 0.9k/b cosh, where D is the crystallite size, k is the wavelength of ˚ ), h is the diffraction angle, and b CuKa line (1.5406 A is the FWHM of the diffraction peak in radian. The (111) crystalline plane was used to calculate the zincblende nanoparticle size while (101) plane was used to determine the hexagonal crystallite size. The estimated crystallite sizes obtained in this way were 3.85 (CdS-A), 2.13 (CdS-B), 2.07 (CdS-C), and 1.76 nm (CdS-D), respectively, which are roughly coincident with TEM results discussed below. To study the atomic composition of the constituent elements, EDS was performed (Fig. 2). The results show that the present CdS nanoparticles are not stoichiometric. The atomic composition (%) ratio of sulfur and cadmium (S:Cd) was found to be 0.99, 0.95, 0.92 and 0.85 for CdS samples prepared at 0, 0.05, 0.1, and 0.2 M TG. The atomic concentration of sulfur is slightly lower than that of cadmium in TG-uncapped sample which further decreases its value greatly with increasing TG molar ratio. This indicates that sulfur

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vacancies are predominant in present CdS naonparticles and the concentration of such vacancies is increased with increasing TG concentration. The results obtained in this study are contradictory to the previous results reported by Singh et al. (2011) which may be due to the different way (or sequence) of mixing TG in the reaction medium. In our experiment, TG was mixed thoroughly to the aqueous solution of Cd salt prior to the addition of aqueous Na2S solution, while a mixture of Na2S and TG was added to the aqueous solution of cadmium nitrate as reported by Singh et al. Figure 3 shows the HRTEM images of CdS NPs grown at different TG concentrations. The images clearly show the formation of quasi-spherical particles at TG concentration of 0 and 0.05 molar and slightly elongated or ellipsoidal nanostructures at 0.1 molar TG. When TG concentration was increased to 0.2 M maintaining the other experimental conditions fixed, rhomboidal-shaped CdS nanocrystals with the minor diagonal below 3 nm are the predominant structure as seen in Fig. 3. The mean diameters of these quasi-spherical NPs are estimated to be *4.8 (CdS-A) and 3.7 (CdS-B) nm, while the minor diagonals for elongated and rhomboidal-shaped particles are about 3 (CdS-C) and 2.8 (CdSD) nm, respectively, as observed from HRTEM images. Conceivably, at a higher TG molar ratio, more TG molecules would bind to Cd?2 ions through thiol group on the nanoparticle surface, which would impede the growth of the particles to result in a smaller crystallite size.


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Fig. 3 High resolution TEM images of CdS nanoparticles at different capping agent concentrations showing morphological evolution: CdS-A, CdS-B, CdS-C, and CdS-D correspond to CdS nanocrystals prepared in 0, 0.5, 0.1, and 0.2 M concentrations of thioglycerol

To further verify the crystalline phase of assynthesized CdS NPs, HRTEM images at different magnifications are studied carefully (Fig. 4). The lattice fringes can clearly be seen indicating good crystalline quality. For CdS-A, B and C, the fringe ˚ correspond to (111) planes of cubic spacing of 3.35 A zinc blend structure when the capping agent concentration is below 0.2 M. The fast Fourier transforms (FFT) of HRTEM images prove the exclusive presence of cubic CdS. While, the presence of hexagonal phase is evident for CdS-D sample from the FFT of HRTEM image as well as through direct measurement of the interplanar spacing by applying digital image analysis technique. The calculated d-spacings on HRTEM images in CdS-D sample are 3.56, 3.34, ˚ which correspond to (100), (002), 3.14, and 2.45 A (101), and (102) planes (JCPDS card no. 80-0006), respectively. Both TEM and XRD results are in full agreement with the fact that the cubic CdS phase was

formed at TG concentration lower than 0.2 M. Interestingly, this coincides with the observation that cubic phase is formed when TG concentration is below or equal to precursor salt concentrations, while the hexagonal phase began to appear when the capping agent concentration exceeded the precursor molar concentration. To understand the phase transformation mechanism in terms of TG concentration, a possible explanation is attempted to search in order to gain an improved understanding of the chemistry involved. In the present reaction scheme, TG is used as capping agent to interact with particles in the early phase of their growth. This interaction inhibits the growth and limits the final particle size. The thiol (–SH) group in TG makes the chelating complex with Cd?2 forming the following possible structure: HÖ HÖ

Cd+2 SH

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CdS-A

Cubic (111)

CdS-C

CdS-B

d = 3.35 Å

d = 3.36 Å d = 3.35 Å

Cubic (111)

Cubic (111)

2 nm

2 nm

2 nm

CdS-D 2.40 A 0

1

3

(002)

(002)

(100)

3.34 A0

3 hexagonal

hexagonal hexagonal (002)

(101)

(100)

2

2.45 A0

4

1

4

(101)

2 5 nm

Hexagonal phase

hexagonal

Fig. 4 HRTEM images of CdS nanoparticles showing lattice fringes. CdS-A, CdS-B, CdS-C, and CdS-D are the CdS samples grown with 0, 0.05, 0.1, and 0.2 M capping agent (TG)

concentrations. The fast Fourier transforms for HRTEM images are shown as insets or besides

The formation of either quasi-spherical cubic CdS or rhomboidal-shaped hexagonal phase is largely determined from the preferential growth of (111) or (101) crystal planes, respectively. At relatively low TG concentrations, the Cd ions might not be effectively chelated by the capping agent. Under this circumstance, the nucleation and growth of CdS particles are primarily governed by the competition between surface and volume free energies. During the nucleation of CdS, a cubic crystal structure is favored over hexagonal one due to symmetry considerations. This results in the preferred growth of the low energy (111) planes as facets of the crystallites (Banerjee et al. 2000). With TG concentrations higher than that of the precursors, Cd?2 ions are strongly chelated by TG as it

has larger coordination numbers (4, 5, and 6) than its oxidation state (?2) (Sen et al. 1999). Under such a condition, the thiol groups adsorb on high energy as well as low energy (111) faces. It is important to mention that the concentration of capping agent in the reaction medium regulates the nanocrystal growth by specific adsorption on different crystal faces. Our XRD data (Fig. 1a) of the CdS NPs stabilized with different amounts of TG indicates only a weak dependence for the preferential growth or inhibition of a particular plane when the TG concentration is well below 0.2 M. However, when the TG concentration is in the range of 0.2 M, the high energy planes were effectively stabilized, and in addition the growth of low energy (111) plane is also inhibited along with

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2RCdS ðnmÞ ¼ 0:1=ð0:1338 0:0002345ke Þ;

ð1Þ

where ke is the absorption edge in nm.The particle size thus calculated are 3.78, 2.63, 2.5, and 2.38 nm for CdS-A, CdS-B, CdS-C, and CdS-D samples, respectively, indicating a good agreement with XRD and TEM results. To avoid the uncertainty about the band gap energy (Eg) values obtained from UV–Vis absorption spectroscopy of dispersed samples, diffuse reflectance measurements were performed with dry CdS powders. Effects of light scattering in the absorption spectra of powder samples dispersed in liquid media can be avoided using DRS, providing more accurate estimation of Eg (Morales et al. 2007). As CdS has direct allowed transition, the band gap of CdS NPs was determined by the following equation (Kubelka and Munk 1931): ½F ðRa Þhm2 ¼ C hm Eg ; ð2Þ where F(Ra) is the remission or Kubelka–Munk function, Ra = Rsample/Rstandard, hm is the photon energy, and C is the proportionality constant. Therefore, plotting [F(Ra)hm2] against hm, the band gap Eg of a powder sample can be extracted easily. Figure 6 shows DRS of as-grown samples. A considerable reduction in reflectance intensity was observed at around 630 nm for CdS-A which has been remarkably blue shifted with increasing the concentration of TG. The UV–Vis absorption spectra of uncapped and TGcapped CdS samples are shown in the inset of Fig. 6. For uncapped CdS NPs (CdS-A), the absorption band CdS-A CdS-B CdS-C CdS-D

2.0

Intensity (a.u.)

(220) and (311) planes, see Fig. 1a. This condition triggered a phase transformation from cubic to hexagonal lattice (CdS-D in Fig. 1a). Therefore, depending on the precursor concentration, TG at certain molarity, probably changes the free energy of the system and induces cubic zinc-blende to hexagonal wurtzite phase transformation which also gets reflected in the final shape and morphology of the nanocrystals (Fig. 3). Tai et al. (2010) reported a similar cubic-to-hexagonal transformation when studying the phase transformation of CdS under the influence of Cl- ions. They reported that with increasing the Cl- ion concentration, the growth of (111) plane of the cubic CdS is inhibited due to adsorption of Cl-, promoting the growth of the hexagonal planes (100) and (101). Being a relatively strong adsorbing anion, Cl- ions are considered to act as a capping agent on the growing CdS crystal (Filankembo et al. 2003; Tai et al. 2010). UV–Vis absorption spectroscopy is an efficient technique to monitor the optical properties of nanomaterials which are directly related to the particle size. Figure 5 displays UV–Vis absorption spectra of CdS NPs at different TG concentration. The blue shift in absorption edge is clearly observed from the spectra. CdS-A, prepared without using the capping agent, has an absorption edge at 458 nm which has been blue shifted compared to the corresponding bulk value of 515 nm (Wu et al. 2005). On increasing TG concentration from 0 to 0.2 M, the absorption edge was shifted gradually from 458 to 392 nm, suggesting that smaller CdS particles were formed at a higher capping agent concentration. Similar phenomenon was also observed by Pucci et al. (2008) for CdS/polymer nanocomposites where they reported the influence of different poly(vinyl alcohol)-based polymer matrices on the size, morphology and optical properties of mercaptoethanol stabilized CdS. It is well-known that for semiconductor particles with a few nanometers of diameter, the discrete energy levels begin to appear associated with the progressive transition from the bulk state to the molecular level and the confinement effect is found to be highly pronounced to a slight change in particle size (Green and O’Brien 1999). In literature, various models related to the quantum confinement effect were reported. Henglein (1989) has provided an empirical formula to calculate the average diameter of the particles from the wavelength of absorption onset:

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1.5

1.0

0.5

0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 5 UV–Vis absorption spectra of CdS samples grown at 0 (CdS-A), 0.05 (CdS-B), 0.1 (CdS-C), and 0.2 (CdS-D) molar TG concentration

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J Nanopart Res (2012) 14:916 10000 CdS-A

[F(Rα hν )]2

8000

CdS-B Eg =2.68 eV

Eg= 2.5 eV

6000 4000 2000 0

2.4

2.7

3.0

2.4

2.7

3.0

3.3

10000 CdS-C

Fig. 6 Diffuse reflectance spectra of as-grown CdS nanoparticles obtained at different TG concentration. UV–Vis absorption spectra of different CdS samples are shown in the inset, indicating a blue shift in the absorption band edge with the increase of TG concentration: CdS-A, CdS-B, CdS-C, and CdSD are prepared with 0, 0.05, 0.1, and 0.2 M of TG

[F(Rα hν )]2

8000

CdS-D Eg= 2.74 eV

Eg= 2.70 eV

6000 4000 2000 0

2.4

2.7

3.0

3.3

Photon energy (eV)

edge (ke) is about 577 nm, whereas TG-capped CdSB, CdS-C, and CdS-D samples exhibit well-defined absorption features at 545, 537, and 515 nm, respectively. The blue shift of the absorption band edge relative to that of the uncapped CdS suggests the presence of quantum size effect which is in accordance with DR spectra. By applying the Kubelka–Munk treatment on the DRS of such powdered samples we have extracted their Eg without any ambiguity. The DRS of the samples after Kubelka–Munk treatment are shown in Fig. 7. The intersection between the linear fit and the photon energy axis gives the exact value of Eg. Therefore, the assigned band gap values for CdS-A, B, C, and D are 2.5, 2.68, 2.70, and 2.74 eV, respectively. As mentioned earlier, the reduction in particle size causes a blue shift in the optical band gap of the material. The particle size, absorption band edge and optical band gap values for all the four samples, extracted from different techniques are shown in Table 1. Raman spectroscopy is used to study material properties such as lattice defect, crystal orientation, etc. CdS has C6v symmetry with four atoms per unit cell. Group theory predicts that out of the nine optical branches there is one A1 and one doubly degenerate E1 which are both Raman and infrared active, two doubly degenerate E2 branches which are Raman active only, and two inactive B1 branches. In addition, for the A1

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2.4

2.7

3.0

3.3

Photon energy (eV)

Fig. 7 Kubelka–Munk transformed reflectance spectra of CdS nanoparticles grown in different concentration of aqueous TG solution: CdS-A represents uncapped particles, whereas CdS-B, CdS-C, and CdS-D correspond to particles prepared with 0.05, 0.1, and 0.2 M TG

and B1 branches, the ionic displacement, i.e., phonon polarization is in the z direction, while for the doubly degenerate E1 and E2 branches the displacements are in xy plane. For infrared active A1 and E1 branches the anions move in opposite phase from the cations, while for the E2 and B1 branches, the like ions move in opposite phase (Tell et al. 1966). Figure 8 shows room temperature Raman spectra of CdS nanocrystals grown at different capping agent concentrations. In case of TG-uncapped CdS NPs, the peaks at 297 and 591 cm-1 correspond to the scattering of 1 LO and 2 LO phonons of CdS, respectively. The strong peak at 209, and two shoulders at 344 and 364 cm-1 match well with the frequencies of several Raman active multiphonon processes in CdS (Abdulkhadar and Thomas 1995). The less intense Raman peak at 249 cm-1 is attributed to the fundamental E2 vibrational mode, which is consistent with the previous report (Tell et. al 1966). The 1 LO phonon frequency for a bulk CdS was reported at 305 cm-1 (Prabhu and Abdulkhadar


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Table 1 Comparison of particle size and band gap of CdS nanoparticles calculated from the data obtained with different experimental techniques Using UV–Vis spectra

From XRD D (nm)

From HRTM D (nm)

Using DRS ke (nm)

Eg (eV)

4.8

577

2.5

3.7

545

2.68

2.07

3.0

537

2.7

1.76

2.8

515

2.74

ke (nm)

D (nm) from Henglein’s formula

CdS-A

458

3.78

3.85

CdS-B

410

2.63

2.13

CdS-C

402

2.50

CdS-D

392

2.38

2008), while the value obtained in the present case is around 297 cm-1. The frequency shift of the 1 LO Raman peak in CdS NPs is mainly attributed to the grain size effect. For TG-capped CdS NPs, although the positions of the peaks are almost unchanged, several other modifications could be observed in the Raman spectra: (i) the disappearance of 2LO phonon mode, (ii) a notable decrease in peak intensity, and (iii) noticeable asymmetry of Raman line toward the low frequency side. These changes are associated with the change in grain size which in turn, influences the vibrational properties. It is well-known that the confinement of phonons, optical as well as acoustic, affects the phonon spectra when the grain size falls to a few nanometers. Confinement of optical phonons causes an asymmetry in the line shape and a shift toward the low frequency side compared to that for bulk CdS. In a bulk crystal, the phonon momentum is very small on the scale of Brillouin zone, so light interacts only with phonons having zero momentum. Therefore, in bulk, the selection rule for Raman scattering is q & 0, due to the consequence of infinite periodicity of the crystal lattice (where q is the wavevector). But in nanocrystalline materials, this q & 0 selection rule is relaxed due to the interruption of lattice periodicity (Singh and Chauhan 2009). Raman signals from the phonon branch away from the zone center also contribute to the resultant Raman line. The net effect of such contribution is the appearance of a marked asymmetry of the Raman line toward the low frequency side (Prabhu and Abdulkhadar 2008). The sudden reduction in Raman intensity for TG encapsulated CdS NPs, particularly for 1 LO phonon mode, may be due to the breakdown of long-range translational crystal symmetry caused by the smaller crystallite size. Photoluminescence (PL) in semiconductor NPs arises from the radiative recombination of electron–

CdS-A CdS-B CdS-C CdS-D

24000

Intensity (a.u.)

Sample

20000 16000 12000 8000 4000 200

300

400

500

600

700

800

-1

Raman shift (cm ) Fig. 8 Raman spectra of as-grown CdS nanostructures formed at variable TG concentrations: CdS-A, B, C, and D correspond to CdS nanoparticles grown in 0, 0.05, 0.1 and 0.2 M TG concentrations

hole pairs. In CdS, defects consist of cadmium vacancies that act as acceptors, sulfur vacancies that are donors, deep interstitial cadmium donor impurity and deep interstitial sulfur acceptor impurity (Zhao et al. 1991). Sharp emission band at the absorption onset is due to the radiative recombination of free charge carriers while broad low energy emission is generally attributed to trap-state emissions originating from surface defect sites (Saunders et al. 2006). It has been reported that for CdS, surface complexation by different kinds of thiols prevents radiative recombination of excited carriers as they have ability to act as hole traps (Gacoinet al. 2001). The room temperature PL spectra of different CdS samples are shown in Fig. 9(top). For PL measurements, the samples were prepared as follows: 50 mg of each CdS powder sample was pressed into small pellet of 5 mm in diameter using a hydraulic press operating at 500 psi for 3 min, and the spectra were measured at room temperature. A broad emission band centered at

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The blue shift of OE in our measurements is the consequence of smaller particle size which strongly influences the energy level structure, thereby, shifting the emission peak toward higher energy. By decomposing the experimental PL spectra into their Gaussian components we get three curves for each CdS sample (Fig. 10): a weak high energy band at 2.18 eV (k = 568 nm) assigned to be yellow emission band, the dominant low energy band at 2.07 eV (OE) and a weak low energy component at 1.65 eV (k = 751 nm) which is attributed to red emission band. The main PL peak observed in between 1.98 and 2.07 eV has been originated from the radiative recombination of deep donor acceptor pairs (DAP) or transitions from S vacancies to the VB; the intensity of this emission is found to increase with the capping agent concentration. The weak yellow emission band might be originated due to the transitions from interstitial Cd to the VB or Cd related DAP transitions. In the literature it is suggested that a donor level of 0.21 eV below the CB is related to cadmium interstitial, Cdi or to a sulfur vacancy, Vs whereas the acceptor level located 0.29–0.30 eV above the VB is originated from an impurity rather than from a native defect in CdS (Abken et al. 2009). The red shoulder peak is related to

PL intensity (a.u.)

20

CdS-A CdS-B CdS-C CdS-D

15 10 5 0 400

500

600

700

800

Wavelength(nm) 5000

Integrated PL intensity (a.u.)

1.98–2.07 eV (k = 630–600 nm) dominates the PL spectra which is assigned to be orange emission (OE) band (Abken et al. 2009; Podborska et al. 2009). It is worth to note that the absorption edges of different CdS NPs are around 458–392 nm, while the emission peak wavelengths are around 630–600 nm. This indicates that the emissions of the present CdS NPs were trap-state emissions associated with electron transitions between the trap states and the edge of conduction band (CB) or that of the valence band (VB). Because of the smaller energy difference between the trap states and the edge of the CB or that of the VB than the band gap, trap state emissions occur at larger wavelengths than those of band edge emissions (Li et al. 2008). The OE band, which arises mainly due to sulfur vacancies, exhibits a low energy asymmetry and a systematic increase in luminescence intensity with increasing the capping agent concentration. The plot in Fig. 9(bottom) shows the relation between the integrated PL intensity with TG concentration. The increment in peak intensity is particularly remarkable at TG concentration of 0.2 M. As the size of CdS NPs decreases with increasing TG concentration, its surface-to-volume ratio increases, resulting in an increased concentration of defects in smaller particles, which has been reflected in their PL intensity. In literature, it has been suggested that for CdS, the PL intensity increases with a decrease in size of the QDs only when the donor sulfur vacancies are dominant (Madan et al 2010). In the present case, the increased capping agent concentration is supposed to create more sulfur vacancies into the CdS nanocrystals, thereby, producing higher luminescence intensity. This assumption is also supported by our EDS results. At the moment, it is not clear exactly how the excess TG molar ratio can help to increase sulfur vacancies in CdS. One possibility is that with increasing TG concentration, Cd?2 ions are more effectively wrapped by the thiol groups (–SH) of TG molecules which might impede sulfur ions to link with Cd?2, thereby producing non-stoichiometric, sulfurdeficient CdS system. We can further observe that the peak position which is around 630 nm for CdS-A (Fig. 9, top) has been blue shifted to *600 nm in CdS-D sample indicating a decrease in particle size (Unni et al. 2008; Qi et al. 2001). It is well-known that quantum confinement leads to an enlargement of the energy band gap, and hence the crystallite size would strongly affect the interband related transition process.


4000 3000 2000 1000 0 0.00

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TG concentration (molar) Fig. 9 Room temperature PL spectra of CdS nanoparticles obtained with 0 (CdS-A), 0.05 (CdS-B), 0.1 (CdS-C), and 0.2 M (CdS-D) capping agent concentrations (top). Variation between the integrated PL intensity and capping agent concentration is shown at the bottom


CdS-A

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peak corresponding to defect states increases greatly with increasing TG concentration which can be correlated with the combined contribution of smaller grain size as well as increased defect concentration incorporated in CdS nanocrystals. The excellent PL properties of TG-capped CdS QDs may provide the scope of utilizing them as biological tags for cellular imaging application.

PL intensity (a.u.)

CdS-B

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PL intensity (a.u.)

CdS-D

Acknowledgments We are thankful to Luis Rendon of Instituto de Fisica, UNAM for the help in taking HRTEM images of the samples. The authors are also thankful to Ultra High Resolution Electron Microscopy facilities at Instituto Mexicano del Petro´leo and CONACyT research network: Red Panamericana de Microscopia y Espectroscopia de Nanoestructuras. This work at CIE-UNAM is partially supported by the projects CONACyT 129169, ICyTDF 318/2009, and SENER-CONACyT 117891.

References 400

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800 400

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Fig. 10 Deconvoluted emission bands for uncapped (CdS-A) and TG-capped CdS samples at different concentrations: CdSB, CdS-C, and CdS-D are, obtained respectively, with 0.05, 0.1 and 0.2 M aqueous solution of TG

the transitions from sulfur vacancies at the surface states to the VB (Tsai et al. 1996; Kulp and Kelley 1960). This emission is correlated with the accumulation of crystallographic defects in CdS nanostructures when synthesized at low temperature. While the peak intensity of yellow band is found to decrease, the intensity of red band increases with increasing capping agent concentration.

Conclusion It can be concluded that ultrafine CdS nanocrystals could be successfully prepared at room temperature using TG as capping agent. Depending on the concentration of capping agent, quasi-spherical, elongated and rhomboidal-shaped nanostructures were formed. The size of the obtained NPs range from 2.8 to 5 nm depending on the capping agent concentration. TG at certain critical concentration induces the phase transition from cubic to hexagonal crystal structure. The PL spectra show that the intensity of the emission

Abdulkhadar M, Thomas B (1995) Study of Raman spectra of nanoparticles of CdS and ZnS. Nanostruct Mater 5: 289–298 Abken AE, Halliday DP, Durose K (2009) Photoluminescence study of polycrystalline photovoltaic CdS thin film layers grown by close-spaced sublimation and chemical bath deposition. J Appl Phys 105:064515–064523 Al-Bassam A, Brinkman A, Russel G, Woods J (1988) Electrical properties of ZnxCd1-xSe (x \0.45). J Cryst Growth 86: 667–672 Alivisatos AP (1996) Semiconductor clusters, nanocrystals, and quantum dots. Science 271:933–937 Armitage SA (1975) Radiation-enhanced diffusion of ionimplanted bismuth in cadmium sulphide. J Phys D 8:2034– 2042 Banerjee R, Jayakrishnan R, Ayyub P (2000) Effect of sizeinduced structural transformation on the band gap in CdS nanoparticles. J Phys 12:10647–10654 Bawendi M, Carroll D, Wilson W, Brus L (1992) Luminescence properties of CdSe quantum crystallites: resonance between interior and surface localized states. J Chem Phys 96:946–954 Cahn RW (1990) Nanostructured materials. Nature 348:389– 390 De´kańy I, Nagy L, Tu´ri L, Kira´ly Z, Kotov NA, Fendler JH (1996) Preparation and characterization of CdS and ZnS particles in nanophase reactors provided by binary liquids adsorbed at colloidal silicaparticles. Langmuir 12:3709– 3715 Filankembo A, Giorgio S, Lisiecki I, Pileni MP (2003) Is the anion the major parameter in the shape control of nanocrystals? J Phys Chem B 107:7492–7500 Gacoin T, Lahlil K, Larregaray P, Boilot JP (2001) Transformation of CdS colloids: sols, gels and precipitates. J Phys Chem B 105:10228–10235

123

Page 12 of 13 Green M, O’Brien P (1999) Recent advances in the preparation of semiconductors as isolated nanometric particles: new routes to quantum dots. Chem Commun 11:2235–2241 Henglein A (1989) Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 89:1861–1873 Hullavarad NV, Hullavarad SS (2007) Synthesis and characterization of monodispersed CdS nanoparticles in SiO2 fibers by sol–gel method. Photonics Nanostruct 5(4): 156–163 Huynh WV, Dittmer JJ, Alivisatos AP (2002) Hybrid nanorodpolymer solar cells. Science 295(5564):2425–2427 Jackson P, Hariskos D, Lotter E, Paetel S, Wuerz R, Menner R, Wischmann W, Powalla M (2011) New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%. Prog Photovolt 19:894–897 Kobayashi H, Hama Y, Koyama Y, Barrett T, Regino CAS, Urano Y, Choyke PL (2007) Simultaneous multicolor imaging of five different lymphatic basins using quantum dots. Nano Lett 7(6):1711–1716 Kolvin VL, Schlamp MC, Alivisatos AP (1994) Light emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 370(6488):354–357 Kubelka P, Munk F (1931) EinBeitragzur, Optik der Farbanstriche. Z Tech Phys 12:593–601 Kulp BA, Kelley RH (1960) Displacement of the sulfur atom in CdS by electron bombardment. J Appl Phys 31:1057–1061 Li H, Shih W, Shih WY, Chen L, Tseng S, Tang S (2008) Transfection of aqueous CdS quantum dots using polyethylenimine. Nanotechnology 19:475101–475108 Madan S, Kumar J, Singh I, Madhwal D, Bhatnagar PK, Mathur PC (2010) The effect of cadmium vacancies on the optical properties of chemically prepared CdS quantum dots. Phys Scr 82:045702–045705 Maleki M, SasaniGhamsari M, Mirdamadi Sh, Ghasemzadeh R (2007) A facile route for preparation of CdS nanoparticles. Semicond Phys Quantum Electron Optoelectron 10:30–32 Mitzi DB, Gunawan O, Todorov TK, Wang K, Guha S (2011) The path towards a high-performance solution-processed kesterite solar cell. Solar Energy Mater Solar Cells 95: 1421–1436 Morales A, Sanchez E, Pal U (2007) Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures. Revista Mexicana de Fı´sica S53:18–22 Ne´meth J, Rodrı´guez-Gattorno G, Dıáz D, Va´zquez AR, De´kany I (2004) Synthesis of ZnO nanoparticles on a clay mineral surface in dimethyl sulfoxide medium. Langmuir 20:2855–2860 Neugebauer CA, Miller DC, Hall JW (1968) Polycrystalline CdS thin film field effect transistors: fabrication, stability, and temperature dependence. Thin Solid Films 2(1–2):57–78 Pattabi M, Amma BS, Manzoor K (2007) Photoluminescence study of PVP capped CdS nanoparticles embedded in PVA matrix. Mater Res Bull 42:828–835 Podborska A, Gawel B, Pietrzak L, Syzmanska IB, Jeszka JK, Lasocha W, Szacilowski K (2009) Anomalous photocathodic behavior of CdS within the urbach tail region. J Phys Chem C 113:6774–6784 Prabhu R, Abdulkhadar M (2008) Study of optical phonon modes of CdS nanoparticles using Raman spectroscopy. Bull Mater Sci 31:511–515

123

J Nanopart Res (2012) 14:916 Pucci A, Boccia M, Galembeck F, Leite CA, Tirelli N, Ruggeri G (2008) Luminescent nanocomposites containing CdS nanoparticles dispersed into vinyl alcohol based polymers. React Funct Polym 68:1144–1151 Qi L, Co¨lfen H, Antonietti M (2001) Synthesis and characterization of CdS nanoparticles stabilized by double-hydrophilic block copolymers. Nanoletters 1:61–65 Ricolleau C, Audinet L, Gandais M, Gacoin T (1999) Structural transformations in II–VI semiconductor nanocrystals. Eur Phys J D 9:565–570 Romeo N, Bosio A, Canevari V, Podesta A (2004) Recent progress on CdTe/CdS thin film solar cells. Sol Energy 77(6):795–801 Saunders AE, Popov I, Banin U (2006) Synthesis of hybrid CdS–Au colloidal nanostructures. J Phys Chem B 110: 24529–25421 Schlamp MC, Peng X, Alivisatos AP (1997) Improved efficiencies in light emitting diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer. J Appl Phys 82:5837–5842 Sen S, Saha MK, Kundu P, Mitra S, Kruger C, Bruckmann J (1999) Synthesis and structure of a heptacoordinated cadmium(II) complex. Inorg Chim Acta 288:118–121 Singh V, Chauhan P (2009) Structural and optical characterization of CdS nanoparticles prepared by chemical precipitation method. J Phys Chem Solids 70:1074–1079 Singh V, Sharma PK, Chauhari P (2011) Synthesis of CdS nanoparticles with enhanced optical properties. Mater Charact 62:43–52 Sobhana SL, Devi MV, Sastry TP, Mandal AB (2011) CdS quantum dots for measurement of the size-dependent optical properties of thiol capping. J Nanopart Res 13: 1747–1757 Tai G, Zhao J, Guo W (2010) Inorganic salt-induced phase control and optical characterization of cadmium sulfide nanoparticles. Nanotechnology 21:175601–175607 Tell B, Damen TC, Porto SPS (1966) Raman effect in cadmium sulfide. Phys Rev 144:771–774 Tsai CT, Chuu DS, Chen GL, Yang SL (1996) Studies of grain size effects in rf sputtered of CdS thin films. J Appl Phys 79:9105–9109 Unni C, Philip D, Gopchandran KG (2008) Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots. Spectrochim Acta A 71:1402–1407 Wada Y, Kuramoto H, Anand J, Tikamura T, Sakata T, Mori H, Yanagida S (2001) Microwave-assisted size control of CdS nanocrystallites. J Mater Chem 11:1936–1940 Wang Y, Herron N (1990) Quantum size effects on the exciton energy of CdS clusters. Phys Rev B 42:7253–7255 Wang G, Li G, Liang C, Zhang L (2001) Sonochemical synthesis and phase control of nanocrystalline CdS. Chem Lett 30:344–345 Wang W, Liu Z, Zheng C, Xu C, Liu Y, Wang G (2003) Synthesis of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant. Mater Lett 57:2755–2760 Wu X (2004) High-efficiency polycrystalline CdTe thin-film solar cells. Sol Energy 77:803–814 Wu XC, Bittner AM, Kern K (2005) Synthesis, photoluminescence and adsorption of CdS/dendrimer nanocomposites. J Phys Chem B 109:230–239

J Nanopart Res (2012) 14:916 Zhang J, Sun L, Liao C, Yan C (2002) Size control and photoluminescence enhancement of CdS nanoparticles prepared via reverse micelle method. Solid State Commun 124: 45–48 Zhao XS, Schroedar J, Persans PD, Bilodeau TG (1991) Resonant-Raman-scattering and photoluminescence studies in glass-composite and colloidal CdS. Phys Rev B 43:12580– 12589

Page 13 of 13 Zhao H, Douglas EP, Harrison BS, Schance KS (2001) Preparation of CdS nanoparticles in salt-induced block copolymer micelles. Langmuir 17:8428–8433 Zhou M, Ghosh I (2007) Quantum dots and peptides: a bright future together. Biopolymers 88:325–339

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