Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 2(11), 51-54, November (2012) Res.J.Chem. Sci. International Science Congress Association 51 Quantum Confinement effects on the Band Gap of Bi3 Thin Films using Chemical Bath DepositionBalasubramanian V.1*, Suriyanarayanan N. and Kannan R.3 *1Department of Physics, Tamilnadu College of Engineering, Karumathampatti, Coimbatore, Tamil Nadu, INDIA Department of Physics, Government College of Technology, Coimbatore, Tamil Nadu, INDIA Department of Physics, KSR College of Engineering, Tiruchengode-637215, Tamil Nadu, INDIA Available online at: www.isca.in Received 1st August 2012, revised 13th August 2012, accepted 17th August 2012Abstract Chemical bath deposition technique has been used to prepare Bi thin films for various applications. Optical absorption studies were carried out using UV-VIS-NIR Spectrophotometer (Model-V-570) in the wavelength range 190 nm to 2500 nm at room temperature. Transmittance spectra of Bi3 thin films increases monotonically with the increase in wavelength; but decreases monotonically with the increase in film thickness. Optical band gap energy was decreased with increase in thickness. Keywords: Bismuth sulphide, thin films, chemical bath deposition, optical properties, quantum confinement effect, band gap. IntroductionQuantum-conned semiconductor structures, including quantum wells, quantum rods, and quantum dots, have been extensively investigated in the past few years. One of the most interesting effect of low-dimensional semiconductor structures is the size-dependent band gap. Semiconductors, which have changed properties resulting from quantum confinement, have drawn considerable interest and are currently being investigated. Semiconducting thin films exhibit size-dependent electronic band gap energies melting temperatures, solid phase transition temperatures and pressures. In addition to these, doped semiconducting thin films have tremendous potential for use in light emitting applications. These properties of nanomaterial make them an interesting category of material for opto-electronic applications. These semiconducting materials may find applications in nonlinear optical devices, photo catalysis, etc. Semiconductors have immense technological importance in different applied branches of science and technology. Bismuth sulphide (Bi) is an important binary semiconducting material and it has been studied due to its wide applications as solar absorbers and in optoelectronics. The Bi thin film is prepared using various methods such as spray pyrolysis, sputtering, sol-gel spin coating, pulsed laser deposition (PLD), chemical vapor deposition, and chemical bath deposition4–10. In spite of few studies regarding to the chemical bath deposition method, the chemical bath method has some merits, such as the easy control of chemical components, and fabrication of thin film at a low cost to investigate structure and optical properties of Bi thin films. In this paper we report some optical properties and quantum confinement effect on of Bi thin films prepared by chemical method. Material and MethodsThe film preparation was previously described in our previous paper. Different thickness Bithin film samples are prepared on to well cleaned glass substrates by chemical bath deposition method. Optical absorption studies were carried out using UV-VIS-NIR spectrophotometer (Model-V-570) in the wavelength range 190 nm to 2500 nm at room temperature using unpolarised lights from deuterium and tungsten lamps which are used at near normal incidence. Results and Discussion Chemically deposited Bi films were characterized by optical absorption and transmittance measurements. The measurements have been taken in the wavelength range 190-2500 nm. The optical absorption and transmittance spectra of Bi films of three different thicknesses 246 nm to 899 nm are shown in figures-1-6. It can be observed that in general a increase in film thickness improves the absorption. 07501500225030000.250.500.75 Absorption (%)Wavelength (nm) Bi246 nmFigure-1 Absorption spectra of Bi thin films of thickness 246 nm Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 51-54, November (2012) Res. J. Chem. Sci. International Science Congress Association 52 07501500225030000.250.500.751.001.251.501.75 Absorption (%)Wavelength (nm) Bi473 nmFigure-2 Absorption spectra of Bi thin films of thickness 473 nm 07501500225030000.250.500.751.001.251.501.75 Absorption (%)Wavelength (nm) Bi 899nmFigure-3 Absorption spectra of Bi thin films of thickness 899 nm 0750150022503000182430364248546066 Transmittance (%)Wavelength (nm) Bi246 nmFigure-4 Transmittance spectra of Bi thin films of thickness 246 nm 07501500225030001530 Transmittance (%)Wavelength (nm) Bi473 nmFigure-5 Transmittance spectra of Bi thin films of thickness 473 nm 0750150022503000153045 Transmittance (%)Wavelength (nm) Bi899 nmFigure-6 Transmittance spectra of Bi thin films of thickness 899 nm Transmittance spectra show that Bi3 thin films increases monotonically with the increase in wavelength; but decreases monotonically with the increase in film thickness. Absorption coefficient of Bi3 thin films has been calculated using transmittance spectra. The high value of optical absorption coefficient confirms that Bi3 has direct band gap. Killedar et al.11 have also observed that Bihas direct band gap 1.8 eV. Depending on the band structure of material, direct or indirect optical transitions are possible12. The relationship between the absorption coefficient and the incident photon energy can be written as = a (h-E / h (1) Where “a” is a constant, Eg is the band gap, n is a constant. The value of is obtained from the relation Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 51-54, November (2012) Res. J. Chem. Sci. International Science Congress Association 53 = 2.303 A / t (2) where ‘A’ is the absorbance and ‘t’ is the thickness of the film. A plot of ( versus (h) in figures-7-9 are used to determine band gap of Bi. It is found that the band gap of Bithin films is thickness dependent. It decreases with the increase in film thickness. Bithin films are polycrystalline. In thin films the particle size of crystallites is of the order of film thickness and proportional to thickness of films. Since grain size influences the energy level of electrons, the band gap will be dependent on thickness of films Band gap of thin films as a function of reciprocal of square of thickness 12. It is linear. The linear extrapolation of ( versus (h) curve to the energy axis gives the value of band gap of Bi as 2.6 eV, 2.16 eV and 2.1 eV which agrees with the reported value13. Table-1 gives the optical band of Bi thin films of different thicknesses. The optical band gap increases from 2.1 eV to 2.6 eV as thickness varied from 899 nm to 246 nm. The decrease in and increase in crystalinity of the film as a function of the film thickness suggest that quantum confinement effect exists in chemically deposited Bi thin films13-15. 1234560.00.10.20.30.4 n )X1014 -2(eV)nnn (eV))Bi246 nm Eg=2.6eVFigure-7 Band gap plot of Bi thin films of thickness 246 nm Table-1 Optical band of Bi thin films of different thicknesses Thickness (nm) Band gap (eV) 246 2.6 473 2.16 899 2.1 1234560.00.10.20.30.40.5 n )X1014 -2(eV)nnn (eV))Bi473 nm Eg=2.16eV Figure-8 Band gap plot of Bi thin films of thickness 473 nm 01234560.00.10.20.30.40.50.60.70.80.91.0 n )X1014 -2(eV)nnn (eV))Bi899 nm Eg=2.1eV Figure-9 Band gap plot of Bi thin films of thickness 899 nm Conclusion Transmittance spectra of Bi3 thin films increases monotonically with the increase in wavelength; but decreases monotonically with the increase in film thickness. Optical band gap energy was decreased with increase in thickness. 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