CdSe nanoparticles

The synthetic approach is very simple and does not require any special set up. In a typical room temperature reaction, 1.0 mL aqueous solution of cadmium chloride was added to 20 mL aqueous solution of soluble starch in a 50 mL one-necked round-bottom flask with constant stirring at room temperature. The pH of the solution was adjusted from 6 to 11 using 0.1 M ammonia solution. This was followed by a slow addition of 1.0 mL colourless selenide ion stock solution. The mixture was further stirred for 2 h and aged for 18 h. The resultant solution was filtered and extracted with acetone to obtain a red precipitate of CdSe nanoaprticles. The precipitate was washed several times and dried at room temperature to give a material which readily dispersed in water. The same procedure was repeated for the synthesis of PVA and PVP - capped CdSe nanoparticles by replacing the starch solution with the PVA and PVP polymers while the synthesis of elongated nanoparticles was achieved by changing the Cd Se precursor ratio from 1 1 to 1 2. The synthesis of polymer capped ZnSe nanoparticles also follows the same procedure except that ZnCb solution was used instead of CdCb solution. 

Fig. 1. (a) The TEM image and (b) particle size distribution of starch-capped CdSe nanoparticles at 1 1 precursor molar ratio. 

Fig. 2. The TEM images of starch-capped CdSe nanoparticles at 0.5 1 precursor molar ratio showing (A) nanowires with interspersed spherical particles and (B) a fraction of the interspersed spherical particles at low magnification.

As discussed earlier the whole process is a redox reaction. Selenium is reduced using sodium borohydride to give selenide ions. In the above reaction, the metal ion reacts with the polymer (PVP or PVA) solution to form the polymer-metal ion solution. Addition of the selenide ion solution to the polymer-metal ion solutions resulted in instantaneous change in the colour of the solutions from colourless to orange (PVA) and orange red (PVP). This indicates the formation of CdSe nanoparticles. The addition of the selenide solution to the polymer - metal ion solution resulted in gradual release of selenide ion (Se -) upon hydrolytic decomposition in alkaline media (equation 4). The released selenide ions then react with metal ion to form seed particles (nucleation). 

Fig. 4. Absorption and photoluminescence (inlet) spectra of PVA-capped CdSe nanoparticles at (a) 1 hr, (b) 3 hrs, (c) 5 hrs and (d) 24 hrs reaction times.

We have reported a simple, green, bench top, economical and environmentally benign room temperature synthesis of MSe (M=Cd or Zn) nanoparticles using starch, PVA and PVP as passivating agents. The whole process is a redox reaction with selenium acting as the oxidant and MSe as the reduction product. An entire "green" chemistry was explored in this synthetic procedure and it is reproducible. The optical spectroscopy showed that all the particles are blue shifted from the bulk band gap clearly due to quantum confinement. Starch capped CdSe nanoparticles showed the presence of monodispersed spherical... 

Bozanic, D. K. Djokovic, V. Bibic, N. Sreekumari Nair, P. Georges, M. K. and Radhakrishnan, T. (2009). Biopolymer-protected CdSe nanoparticles. Carbohydrate Research, 344, 2383-2387. 

Oluwafemi, O. S. and Revaprasadu, N. (2009). Study on Growth Kinetics of Hexadecylamine capped CdSe Nanoparticles using its electronic properties. Physical B Condense matter, 404,1204-1208. 

Oluwafemi, O. S. Revaprasadu N and Adeyemi O. O. (2010). A new synthesis of hexadecylamine-capped Mn-doped wurtzite CdSe nanoparticles. Material Letter, 64, 1513-1516. 

FIG. 14 Measurements on monolayers and LB films of CdSe nanoparticles of narrow size distribution (a) II-A isotherms for Langmuir monolayers of CdSe nanoparticles of diameter 2.5 run (curve a), 3.0 mn (curve b), 3.6 mn (curve c), 4.3 mn (curve d), and 5.3 mn (curve e). The area per nanoparticle was determined by dividing the trough area by the estimated number of particles deposited on the surface, (b) Absorbance and photoluminescence spectra of the nanoparticles in solution (A, B) and in monolayers on sulfonated polystyrene-coated glass sbdes (C. D). The nanoparticle diameters are 2.5 nm (curves labeled a), 3.6 nm (curves labeled b), and 5.3 nm (curves labeled c). The excitation wavelengths are (a) 430 nm, (b) 490 nm, and (c) 540 nm. (Reproduced with permission from Ref. 158. Copyright 1994 American Chemical Society.)... 

Coulomb blockade effects have been observed in a tunnel diode architectme consisting of an aluminum electrode covered by a six-layer LB film of eicosanoic acid, a layer of 3.8-nm CdSe nanoparticles capped with hexanethiol, and a gold electrode [166]. The LB film serves as a tunneling barrier between aluminum and the conduction band of the CdSe particles. The conductance versus applied voltage showed an onset of current flow near 0.7 V. The curve shows some small peaks as the current first rises that were attributed to surface states. The data could be fit using a tunneling model integrated between the bottom of the conduction band of the particles and the Fermi level of the aluminum electrode. 

Mann JR, Watson DP (2007) Adsorption of CdSe nanoparticles to thiolated TiOa surfaces Influence of intralayer disulfide formation on CdSe surface coverage. Langmuir 23 10924-10928... 

Research on semiconductor nanoparticle technology by chemists, materials scientists, and physicists has already led to the fabrication of a number of devices. Initially, Alivisatos and co-workers developed an electroluminescence device from a dispersion of CdSe nanoparticles capped with a conducting polymer349 and then improved on this by replacing the polymer with a layer of CdS, producing a device with efficiency and lifetime increased by factors of 8 and 10, respectively. 0 Chemical synthetic methods for the assembly of nanocrystal composites, consisting of II-VI quantum dot polymer composite materials,351 represent one important step towards the fabrication of new functional devices that incorporate quantum dots. 

Figure 10.4. (a) Schematic energy diagrams of clusters, nanoparticles, and bulk semiconductors. (b) Manifestation of the size quantization effect as a color change of aqueous colloidal solutions of CdSe nanoparticles (courtesy of A. Rogach). The particle size changes from left to right from -1.5 to -4.5 nm. (c) Bulk CdSe crystal. (See color insert.)... 

Talapin, D. V. Shevchenko, E. V. Kornowski, A. Gaponik, N. Haase, M. Rogach, A. L. Weller, H. 2001. A new approach to crystalhzation of CdSe nanoparticles into ordered three-dimensional superlattices. Adv. Mater. 13 1868-1871. 

Fig. 20 CdSe nanoparticles residing in the pores of a template prepared from a PS-PMMA block copolymer. Reproduced from [86]... 

Ziegler J, Merkulov A, Grabolle M, Resch-Genger U, Nann T (2007) High-quality ZnS shells for CdSe nanoparticles rapid microwave synthesis. Langmuir 23 7751-7759... 

Kopping JT, Patten TE (2008) Identification of acidic phosphorus-containing ligands involved in the surface chemistry of CdSe nanoparticles prepared in tri-n-octylphosphine oxide solvents. J Am Chem Soc 130 5689-5698... 

Gautam UK, Seshadri R, Rajamathi M, Meldrum F, Morgan P (2001) A solvothermal route to capped CdSe nanoparticles. Chem Commun 629-630... 

Cassagneau T, Mallouk TE, Fendler JH (1998) Layer-by-layer assembly of thin film zener diodes from conducting polymer and CdSe nanoparticles. J Am Chem Soc 120 7848-7879... 

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