International Journal of Latest Research in Science and Technology ISSN (Online):2278-5299 Volume 2,Issue 1 :Page No.457-459 ,January-February (2013) http://www.mnkjournals.com/ijlrst.htm

ISSN: 2278-5299 457

CHARACTERISTICS AND PROPERTIES OF CDSE QUANTUM DOTS

1S. Mahajan, 2Meenu Rani, 3R. B. Dubey and 4Jagrati Mahajan 1ECE, Hindu College of Engg., Sonepat, India,sudhirmahajanbe53@radiffmail.com

2ECE, Hindu College of Engg., Sonepat, India, meenu.sharma1234@gmail.com 3ECE, Hindu College of Engg., Sonepat, India, rbdubeyster@gmail.com

4PIET, Panipat, India, jagratimahajan2000@gmail.com

Abstract- The CdSe nanocrystals (NCs) have been synthesized by hot-injection method to improve the crystallinity of synthesized nanoparticles and reconstruct the surface of nanoparticles. The crystalline structure and optical properties of these NCs are characterized by X-ray powder diffraction (XRD), ultraviolet�visible light absorption and photoluminescence (PL) spectroscopy. The CdSe NC grains are roughly spherical, and their average size is ~ 4�5 nm. X-ray photoelectron spectroscopy (XPS) showed that nano-crystals are air-stable and the presence of TOP around the particles is confirmed by infra-red (IR) spectroscopy. The quantum gives the corresponding CdSe NCs at Cd-to-Se molar ratios of 0.1, 0.2 and 0.3 are 2.38, 3.70 and 9.32%, respectively, indicating the strong dependence of the NC size on the precursor concentration in the period immediately following the precursor injection. Keywords -CdSe, quantum dots (QDots), nanoparticles, characterstics and properties.

I. Introduction

Colloidal II-VI semiconductor nanocrystals or quantum dots (QDs) have attained a great research focus due to their advantages in optical properties including tunable emission spectra, high photostability, resistance to photobleaching and controllable surface characteristics. The luminescent properties of synthesized CdSe nanoparticles by controlling the annealing conditions at 250°C - 350 °C in air or inert atmosphere. CdSe Qds find a wide range of applications in optoelectronic devices [1], photo catalysis, solar energy conversion and biological imaging and labeling. Various methods are developed to prepare CdSe NCs. These methods can be broadly divided into two groups: namely, organic synthetic methods and methods using aqueous media. Compared with the organic phase routes, the synthesis methods using aqueous media are simple, green and highly reproducible, and the products exhibit good water solubility, stability and biological compatibility [2, 3].The nano-crystals are characterized by XRD [2] and ultraviolet visible light [4]. The main property of CdSe QDs is optical property [3, 5]. The nanoparticles are a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is not the case. The properties of materials change as their size approaches the nanoscale and the percentage of atoms at the surface of a material becomes significant. The size of the nanoparticles is finte, so the continuous energy band of the bulk crystal transforms into a series of discrete states. The nanoparticles frequently display

photoluminescence and sometimes display electroluminescence. It is well known that the quantum confinement effect modifies the electronic structure of nanocrystals when their diameter is comparable to or smaller than the diameter of the bulk exciton [6]. For biological and environmental applications, it is essential to prepare CdSe QDs in water directly. In contrast to organometallics techniques, solution growth method appears to be a safer, lower cost and more convenient approach because it does not involve special instrumentation, poisonous intermediates, and the growing-rate could be easily controlled.

II. Quantum Confinement

Quantum dots are nanoparticals of semiconductors materials ranging from 2 to 10nm in diameter, like CdSe and Zns. There electronic characteristics are closely related to the size and shape of the individual crystal. If the size of crystal is small, then band gap between the higher valence band and the lowest conduction band becomes high and more energy is require for exciting the dot and consequently, more energy is released when the crystal returns to its resting state. A principal advantage with quantum dots is that by controlling the size of crystals, the conductive properties of the material is controlled. Because of their small size, qdots displays uniqe optical and electrical properties. The most immediately apparent of these is the emission of photons under excitation, which are visible to human eyes as light. The wavelength of

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S. Mahajan ,International Journal of Latest Research in Science and Technology.

ISSN: 2278-5299 458

these photon emissions depends not on the material from which the qdot is made but its size. The ability to control the size of qdot enables the manufacturer to determine the wavelength of emission, which in turn determines the color of light the human eye perceives. The smaller the dot, closer its to the blue end of the spectrum and the larger the dot, closer to the red end as shown in Fig 1 (courtesy from [ref. 7]).. When the size of the quantum dot is smaller than the critical characteristic length called the exciton. In Bohr radius, the electrons crowding lead to the splitting of the original energy levels into smaller ones with smaller gaps between each successive level. The quantum dots that have radii larger than the exciton Bohr radius are said to be in the 'weak confinement regime' and the ones that have radii smaller than the exciton Bohr radius are said to be in the 'strong confinement regime[7]. The fluorescence of the quantum dots is a generated when valence electron excite with a certain energy and they emits energy in the form of photons as the excited electron returns to the ground state, combining with the hole. The energy of the emitted photon is determined by the size of the quantum dot due to quantum confinement effects [8]. The energy of the emitted photon is sum of the band gap energy between occupied level and unoccupied energy level, the confinement energies of the hole and the excited electron, and the bound energy of the exciton as shown in Fig 2.

2.3 5.5 Size (nanometers) Figure 1: Different size quantum dots emitting light at different frequencies.

Figure 2: Band gap energy.

III. Results and Discussion

A. X-Ray diffraction(XRD) characterization XRD analysis exhibited the crystalline phase transformation at annealing temperatures of 350 °C-450 °C from cubic Zinc Blende (ZB). The XRD patterns are shown in figure 3. The XRD pattern of nanocrystalline CdSe contains 3 main peaks at diffraction angle 2è = 26.55°,

43.87° and 51.91° corresponds to the Miller indices (101),

(110) and (112). Average crystalline size of the CdSe QDs is estimated at 3 nm. The correspondence of 2è Bragg�s angles

and diffraction intensities of different peaks of the experiment is summarized in Table 1.

Figure:3 XRD patterns of the CdSe QDs. Table 1: The 2è Bragg�s angles (è) and diffraction intensities (%) of CdSe.

B. UV-visible analysis The electronic state is an important property and can be described in terms of valence and conductivity bands and a gap between these bands. However, as the particles become smaller, the wavelength of the electrons is closer to the range of the particle sizes and the laws of classical physics have to be substituted by quantum confinement or quantum size effect. The UV-visible spectrum showed that the absorption peak of obtained CdSe QDs in aqueous solution is 543 nm (2.28 eV), relative blue-shift to the band gap of bulk cubic CdSe (1.78 eV, 698 nm). The estimated particle size is about 3nm, which is very close to the results from XRD result and TEM observation.

Figure 4: UV-Visible absorption spectra of CdSe QDs. C. Optical Properties The quantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is the quantum confinement effect. The larger dot gives low energy fluorescence spectrum. Conversely, smaller

S. Mahajan ,International Journal of Latest Research in Science and Technology.

ISSN: 2278-5299 459

dots emit bluer light. The coloration is directly related to the energy levels of the quantum dot. The band gap energy that determines the energy of the fluorescent light is inversely proportional to the size of the quantum dot. Larger quantum dots have more energy levels which are also more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e., those closer to the red end of the spectrum. The lifetime of fluorescence is determined by the size of the quantum dot. Larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer causing larger dots to show a longer lifetime.

Figure: 5 Fluorescence spectra of CdSe quantum dots of various sizes.

IV. Conclusions

The CdSe are successfully synthesized in room temperature and pressure. The inorganic CdSe nanocrystals are of uniform size and nearly monodispersive. In XRD, the broadening of peaks indicates the inorganic components are in nanometer scale. Some extra peaks in nanocomposites may be assignable to the dopants. The ability to manipulate precisely the size, the shape, and the surface of nanocrystals has opened up a number of potential applications for these new materials: light-emitting diodes, low-threshold lasers, solar cells, optical amplifiers for telecommunications and biomedical tags. Hence, the rate at which progress is made on various fronts, the nanoscience will have a greater impact in the near future.

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