One-pot synthesis of oleic acid-capped cadmium chalcogenides (CdE: E = Se, Te) nano-crystals
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- Khanna, P.K., Srinivasa Rao, K., Patil, K.R. et al. J Nanopart Res (2010) 12: 101. doi:10.1007/s11051-008-9581-y
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Surface-capped CdSe and CdTe nano-crystals (NCs) have been synthesized using cadmium acetate, oleic acid and respective tri-octylphosphine chalcogenide (TOPE; E = Se/Te) in diphenyl ether (DPE). Well-dispersed CdSe particles showed two absorption bands at the region of 431–34 and 458–60 nm in optical absorption study. A band-edge emission resulted at 515 nm with an excitation energy of 400 nm, in its photoluminescence (PL) spectrum. Similarly, UV–visible absorption study of CdTe revealed an absorption band at <700 nm. The broadened X-ray diffraction (XRD) pattern showed that at higher reaction temperature cubic CdSe but hexagonal CdTe can be obtained with crystallite size of <10 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that agglomerated particles are of spherical nature. The inter-planar spacing in CdTe was measured to be 0.406 nm, a characteristic of (100) lattice plane in hexagonal CdTe. X-ray photoelectron spectroscopy (XPS) showed that CdSe NCs have better air stability stable than CdTe. Presence of organic moiety around the semiconductor particles was confirmed by infra-red (IR) spectroscopy.
KeywordsSemiconductorsSurface cappingChemical synthesisPhotoluminescence
The quantum dots (QDs) are the tiny particles broadly with particle size below 20 nm, as the optimum size quantization can be attained below this range. It is reported that optimum size variation in CdSe nano-crystals (NCs) due to quantum confinement is from 01 to 11 nm that means it contain 10–10,000 atoms. As the size decreases, number of atoms on the surface increases, e.g. an absorption at about 400 nm for crystal size of 1.0–1.5 nm may have about 80–90% atoms on the surface. Therefore, less number of atoms on the surface may result loss of optical properties and shift in absorption frequency to red region of the visible spectrum due to increase in particle size (Yen et al. 2003; Rosenthal et al. 2007; Colvin et al. 1994).
Further, it is well-known that band gap of semiconductor particles can be tuned by varying the particle size and surface morphology; thus, better optical properties can be achieved with decreasing particle size of the QDs. The energy gap between the valence and conduction band can be thus manipulated by variation in the particle size of the QDs. There are several area of applications for high-quality NCs of semiconductors, e.g. in luminescent devices, as biological markers, in lasers, light emitting diodes and in telecommunication including nano-electronics, photonics, photovoltaics, etc. (Yen et al. 2003; Rosenthal et al. 2007; Raevskaya et al. 2006; Sashchiuk et al. 2004; Colvin et al. 1994; Alivisatos 1998). Quantum dots emitting range of colours into the entire visible spectrum have been reported by researchers, but the red-light emitting CdSe NCs are perhaps the most straight forward when it comes to their synthesis (Wang and Seo 2006).
One of the biggest challenges is to isolate free standing QDs that show light emission even after long storage. Normally organic or polymeric surfactants are utilized to synthesized quantum dots of excellent quality. Quantum dots can be isolated from the preparation vessel via single-stage or multi-staged centrifugation through size-selective precipitation (generally through combination of lower alcohols and non-solvent such as hexane, toluene, etc.). By adopting advanced synthetic methodology, one can prepare QDs of hydrophilic and hydrophobic nature thereby extending the scope of their applications. Amongst the binary semiconductors, CdSe and CdTe QDs are best candidates for tuned band-gap energy in the entire visible region (Lee et al. 2007; Han et al. 2006; Firth et al. 2004; Liu et al. 2008; Diao et al. 2007). Recent advances have shown that CdTe QDs are excellent candidates in probing applications, e.g. determination of cationic surfactants (CS), specifically cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB) and cetylpyridinium chloride (CPC), via a fluorescence quenching method which employed water-soluble luminescent CdTe QDs modified with thioglycolic acid (TGA). Large number of surfactants have been reported for preparing QDs of excellent quality that include long carbon chain carboxylic acids (e.g. oleic acid, myristic acid and stearic acid), alkyl amines and trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) (Qu et al. 2004; Nie et al. 2004; Murray et al. 1993, 2001). There are a number of synthetic strategies reported in the literature including simple beaker chemistry to advanced organometallic chemistry as well ultrasonic techniques for synthesis of CdSe and CdTe in aqueous as well as organic medium (Murray et al. 1993, 2001; Chen and Gao 2002; Deng et al. 2003; Peng and Peng 2001; Afzaal et al. 2003; Trindale et al. 1999; Chu and Liu 2006; Khanna et al. 2004a, b, 2008 unpublished results).
Present study is based on our chemistry expertise where a range of reactions can be adopted and be further simplified to make QD synthesis more attractive and reachable to common researchers and users. In this article, we have directed efforts to isolate surface-capped CdE nano-particles via use of oleic acid and elemental chalcogen in TOP/diphenyl ether. Use of TOPE (E = Se or Te) by normal dissolution of the chalcogen in TOP at less than 100 °C or by direct use of the element along with addition of TOP can be exploited.
Cadmium acetate (anhydrous, 99.9%) and selenium powder (99%) were purchased from Qualigen India and were used as received. Oleic acid was obtained from Sigma-Aldrich. UV–visible measurements were done in toluene on JASCO V-570 UV-Visible-NIR Spectrophotometer. Photoluminescence spectra were measured in toluene using Hitachi F-2500 Fluorescence Spectrophotometer. SEM was carried out on a Philips XL-30 instrument by dispersing NCs on an aluminium substrate. Powder XRD pattern was obtained on a Mini Flex Rigaku instrument using Cu-Kα radiation (λ = 1.5406 Å). XPS analysis of powdered sample was done on a 9 channeltron CLAM4 analyzer under a vacuum better than 1 × 10−8 Torr, using Mg-Kα radiation with a constant pass energy of 50 eV. Thermo-gravimetric analysis (TGA) was performed on a Mettler Toledo instrument with nitrogen as a purging gas at a scanning rate of 10 °C/min. TEM analysis was carried out on Tecnai-G20 high-resolution transmission electron microscopy (HRTEM) working at 200 kV.
Synthesis of surface-capped CdSe and CdTe
Cadmium acetate (2.46 g) was dissolved in oleic acid (10 mL). After complete dissolution, diphenyl ether was added to make the volume to about 50 mL. To this, was added TOPE (E = Se/Te) prepared from respective element (2 g) in 10 mL of TOP was added under argon. The reaction mixtures were heated to 150 and 200 °C with constant stirring for about 5–8 h to cause an orange to brown precipitation. The orange–red–brown reaction mixture was cooled to room temperature followed by addition of 30–50 mL of toluene and stirring at room temperature for half-an-hour. The reaction mixture was then centrifuged and was washed with the same solvent to obtain orange or brown powder.
Results and discussion
TEM of CdSe showed that the individual particles were stacked over several other similar particles and they appeared as a cluster. However, this was not the case for CdTe. The spherical nano-particles of CdTe were well dispersed and were not so much agglomerated. In fact the high-resolution TEM of CdTe prepared at 150 or 200 °C showed good crystallinity, and the electron diffraction showed that the concentric rings are different for the sample that was prepared at 150 °C than the one prepared at 200 °C. The XRD has shown that the two samples have different crystal structures, so this was also confirmed by electron diffraction pattern and HRTEM. The average size of the particles in sample of CdTe prepared at 150 °C was 5 nm. HRTEM bright field image showed nearly spherical NCs. The HRTEM image of the particles showed lattice fringes with the inter-fringe distance measured to be 0.379 nm, close to the lattice spacing of (111) planes at 0.374 nm in the cubic-structured CdTe. HRTEM image clearly indicated that no crystal defects such as point defects, line defects and stacking faults were present in CdTe QDs synthesized at 150 °C. The selected area electron diffraction (SAED) pattern showed successive inter-planar distances corresponding to the zinc blend (cubic) structure. Similarly, the average particle size in the case of samples synthesized at 200 °C was 9 nm. The lattice planes were visualized within the crystal structure. The inter-planar spacing in CdTe was measured to be 0.406 nm, which corresponds to the characteristic (100) lattice plane in hexagonal (wurtzite) CdTe (0.398 nm). The SAED pattern showed successive inter-planar distances for wurtzite crystal structure. Figure 8 shows a SAED pattern recorded at an accelerating voltage of 200 kV. The diffuse ring immediate to the transmitted beam in the SAED pattern in Fig. 8 is indexed as the (102) plane. The second, third and fourth rings in Fig. 8 correspond to (103), (203) and (110) planes, respectively. If CdTe in the present synthesis procedure crystallized in zinc blende structure, then no ring corresponding (102) plane would be observed in the SAED pattern. The appearance of (102) plane confirms that CdTe crystallizes in wurtzite structure at 200 °C.
The composition of CdTe was qualitatively determined by means of energy disperse X-ray analysis (EDS) measurements. The ratio of Cd:Te was found to be 1:1.06, confirming correct ratio of the two elements. SEM of CdTe showed that the particles were well scattered but were aggregated due to presence of surfactant.
Re-dispersible CdSe and CdTe nano-particles have been synthesized with effective capping of oleic acid in presence of TOP. Optical studies showed that the absorption and emission properties are well matched with the reported values for the particle diameter of less than 5 nm. A blue shift of more than 250 nm was found in CdSe indicating an increased band gap of so-prepared material. Formation of cubic crystal structure was predominant at 150 °C for both; however, at 200 °C hexagonal CdTe was obtained as against cubic CdSe. XPS analysis showed no peak due to SeO2 indicating that the particles are stable for a long time. The current work highlights the usefulness of one-pot synthesis of semiconductor QDs and allows the researchers to adopt the method for their day-to-day research in nano-technology.
PKK thanks Dr. T. L. Prakash, Director, C-MET, Hyderabad, for permitting KRSR to work in his laboratory at Pune and DST (Government of India) for financial support through grant no. SR/S1/PC-17/2006.