Dielectric Properties of Cadmium Selenide (CdSe) Nanoparticles synthesized by solvothermal method
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Synthesis of nanoparticles of cadmium selenide (CdSe) was carried out using solvothermal method with cadmium nitrate and sodium selenite as precursors. Hydrazine hydrate and ethylenediamine tetra acetic acid were used as the capping agent to control the size of the nanoparticles. As their size decreases to their Bohr radius (usually around a few nanometers), all electronic properties change, and equally important, become dependent on size. In this size, a semiconductor nanoparticle transition occurs in which the electrons and holes are confined beyond their natural Bohr radius. The properties become dependent not only on size, but also on shape. The crystalline nature and particle size of the samples were characterized by Powder X-ray diffraction analysis (XRD). The morphology of prepared CdSe nanoparticles was studied by scanning electron microscope. Dielectric studies were carried out for the pelletized sample of CdSe nanoparticles. The ac conductivity of CdSe nanoparticle has been studied. The obtained results are discussed.
KeywordsNanomaterials CdSe Solvothermal method Electrical studies
The semiconductor nanoparticles belong to the state of matter in transition between molecules and bulk solids in which the relevant physical dimensions changes on the length of a few to a few hundred nanometers. Both equilibrium and dynamic properties of nanomaterials can be very different from those of their corresponding bulk materials or isolated atoms and molecules. The dielectric constant of a semiconductor is one among its most important properties. Its magnitude and temperature dependence are significant in both fundamental and technological considerations. Recently, many extensive studies are going on in the semiconductor nanocrystals because they exhibit strong size dependent optical properties. These will be the key structural parameters in the fabrication of novel electronic nanodevices and nanocircuits. Semiconductor particles exhibit size dependent properties such as the scaling of the energy gap and corresponding change in the optical properties. CdSe is one of such materials, shows strong fluorescence which can be tuned according to the particle size. CdSe has been considered in many applications such as optoelectronic devices (Nazzal et al. 2003), light sensors (Bruchez et al. 1998), biological labels (Colvin et al. 1994), chemical libraries (Gaponik et al. 2002), etc. The nanopowder of CdSe provides excellent and unique properties which depend upon the shape and size of the nanostructures (Haram et al. 2001; Wang et al. 2004; Datta and Das 1990; Peng et al. 2000). Various methods such as hydrothermal, sol–gel approach, surfactant-assisted approach, etc. had been utilized for the synthesis of nanoparticles (Tang et al. 2003; Busbee et al. 2003). Synthesis, structural, and optical properties of CdSe nanoparticles have been reported (Dwivedi et al. 2011). In the present study, the main focus is on the electrical properties of pellets of nanoparticles of CdSe at different temperatures. The frequency dependence of dielectric constant, dielectric loss and ac conductivity was also investigated.
Materials and methods
Nanoparticles of CdSe were synthesized by solvothermal method. The chemicals such as cadmium nitrate and sodium selenite were used as the precursor materials to prepare CdSe nanoparticles. Cadmium chloride and sodium selenite in the molar ratio of 2:1 was dissolved in de-ionized water and stirred well by magnetic stirrer for 30 min. Hydrazine hydrate and ethylenediamine tetraacetic acid were used as complexing agents. Then the solution was centrifuged, filtered, and washed. Finally, the dark red product was collected by centrifugation and dried at 50 °C for 1 h. The dielectric measurements using HIOKI 3532-50 LCR HITESTER in the frequency range of 100 Hz and 1 MHz at various temperatures were carried out. The electrodes on either side with air-dying silver paste were made on the pelletized sample which behaves like a parallel plate capacitor. The dielectric studies were carried out at different temperatures and different frequencies. In this experiment, palletized nanoparticles using very high pressure have been used, so interfaces with this kind of large volume fraction in the nanosized samples must contain a great deal of defects including micro porosities, vacancies, vacancy clusters and dangling bonds. These defects can cause a change of positive and negative space-charge distributions in the interfaces. The pallets exposed to an external electric field, positive and negative charges on interfaces moves towards the negative and positive poles of electric field, respectively. Meanwhile, a great number of dipole moments are unavoidably formed after they have been trapped by defects. Consequently, space-charge polarization occurs in the interfaces of CdSe nanoparticles, which results in the much larger dielectric constant for CdSe nanoparticles. For conventional CdSe powders, it is impossible to observe space-charge polarization due to the smaller specific surface.
Results and discussion
X-ray diffraction analysis
Electrical conductivity studies
In the case of nanoparticles, due to the small grain size and large grain boundaries, the electronic state close to Fermi level is localized. When the states are localized, the conduction occurs by hopping of carriers between occupied and unoccupied localized which depends on the density of state and the position of Fermi level. Mott (Elahi and Ghobadi 2008) established a dependence of conductivity with the temperature for such systems. The gradual decrease in the activation energy with the decrease in the temperature suggested that the conduction is by hopping of carriers among localized states (Johna et al. 2005). The high surface stress in the nanoparticles causes a lattice contraction which may not be symmetrical (Solliard and Flueli 1985; Gamarnik and Sidorin 1989). This may result in a lattice disorder which can be equivalent to a plastic deformation causing dislocations. It is not reasonable to argue that these dislocations may be considered to be one dimensional. It has been shown that, smaller the semiconductor particles, the greater the chance of the charge carriers to escape onto the semiconductor surface where electron transfers can occur (Fendler 1987). Experimental evidence strongly indicate that in small particles, the confinement of charge carriers perturb the band structure resulting in a series of discrete states in the conduction and valence bands and also causes an increase in the effective band gap. In case of pellets of small particles, the boundary between the particles must play an important role in determining the conductivity as in the case of polycrystalline semiconductor films (Kamins 1971). According to grain-boundary trapping theory, free carriers are trapped by the trapping states at the boundary causing a depletion of charges in the grain region nearest to the boundary (Seto 1975). Therefore, the region near the surface of the particle becomes depleted of charges causing a space charge which should establish an energy barrier between adjacent particles.
Nanoparticles of CdSe were synthesized using solvothermal method. The Powder X-ray analysis confirmed the nanoparticle size and the fundamental diffraction patterns of CdSe. The SEM reveals the morphology in the synthesized samples. The dielectric studies at different temperatures showed that the dielectric constant and dielectric loss have low values at higher frequencies and are independent of the temperature. The ac conductivity of the pellet form of CdSe nanoparticles were determined and observed that it increases with the increase in temperature. The enhancement in the conductivity and change in dielectric properties have been attributed to the special properties of nanometer-sized particles.
- Barsoum M (1977) Fundamentals of ceramics. Mc Graw Hill, New York, p 543Google Scholar
- Dwivedi DK, Kumar V, Dubey M, Pathak HP (2011) Structural, electrical and optical investigations of CdSe nanoparticles. Chalcogenide Lett 8:521–527Google Scholar
- Elahi M, Ghobadi N (2008) Structural, optical and electrical properties of CdSe nanocrystalline films. Iran Phys J 2:27–31Google Scholar
- Kamins TI (1971) Hall mobility in chemically deposited polycrystalline silicon. J Appl Phys 42(4357):9Google Scholar
- Seto JYW (1975) The electrical properties of polycrystalline silicon films. J Appl Phys 46(5247):8Google Scholar
- Smyth CP (1965) Dielectric behaviour and structure. Mc Graw Hill, New YorkGoogle Scholar
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