Synthesis and Study of the Structural and Electrical Conductivity for Nanocrystalline PBS Thin Films

Structural and electrical properties of prepared nanocrystalline lead sulfide thin films were studied which deposited on glass substrates by chemical bath deposition (CBD) techniques. The films were obtained in a reaction bath at times of (15, 30, 45,60and90) min. The X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements confirmed that the thin films grown by this technique had good crystalline cubic structures and homogeneous surfaces. The dc electrical conductivity found in the range of 10 -6 -10 -5 (Ω.cm) -1 .


EXPERIMENTAL DETAILS
The deposition of PbS films was done in a reactive solution prepared in a 100 ml beaker by the sequential addition of 5 ml of 0.25 M lead acetate, 5 ml of 1M KOH, 6 ml of 0.5M thiourea and 1 ml of 0.1M triethanolamine.The solution was added with deionized water to complete a total volume of 100 ml.The films were deposited on glass slide substrates.The substrates were immersed into the solution vertically.The substrates were subsequently retired from the beaker at different times: 15, 30, 45, 60, and 90 min.After deposition of the films , substrates were taken out and thoroughly washed in doubly distilled water and then dried in air.The resultant films were homogeneous, well adhered to the substrate, and secularly reflecting with a varying color depending on the deposition time.Film thickness was measured by optical interferometer method, using He-Ne laser and the thickness (d) were determined using the formula[6]: . 2 Where ∆x is the distance between two fringes, x is the fringe width and λ is the wavelength of He-Ne laser light (0.632 µm).The thickness of films varies with in the range (90 to162) nm.Structural characteristics of the films were determined by using X-ray diffraction system (Philips-PW1840) at room temperature with Cukα radiation and 0.15405 nm wavelength.The two-dimensional images of the surface samples were obtained by using atomic force microscopy (AFM) a scanning prop microscopy (CSPM-5000) instrument.The electrical resistivity of the deposited films was determined by equation respectively.Resistance change of thin film with temperature is achieved, and putting the samples under vacuum to prevent oxidation of the films.K type thermocouple was used to measure the temperature via digital meter of the type (Digit-sense).Sample resistivity measured with Keithley (602) electrometer as a function of temperature.The electrical conductivity of nano PbS films was measured in the 313-321 K temperature range using dc two-point probe method and its temperature dependence can be expressed by usual Arrheneous equation as [8]: Where ρ is the resistivity at temperature (T), where ρₒ is the resistivity at room temperature, K B is the Boltzmann´s constant and E a is the activation energy.

RESULTS AND DISCUSSION XRD Analysis
Structural identification of PbS films was carried out with x-ray diffraction in the range of angle 2θ between (10 to 60). Figure ( 1) shows the XRD patterns for PbS thin films, which were nanocrystalline in nature.All PbS films have cubic structure as confirmed by standard ASTM data.The well defined ( 111), ( 200), ( 220), ( 311), ( 113) and ( 222) peaks were observed in the XRD patterns.The average crystalline size (G.S) of nano PbS in the film was determined from the (200) by using Scherrer formula [9]: cos Where A is a constant taken to be 0.94 [9], λ is the wavelength of X-ray used which is Cukα radiation, and ∆θ is full width at half maximum (FWHM) of the diffraction peak corresponding to a particular crystal plane .The value of the crystallite size (G.S) as shown in Table .1 varies with in the range (4.9 to 11.3) nm.The value of G.S as determined by Scherrer s formula which gives the size of the crystallites in a direction perpendicular to the respective planes, FWHM of whose diffraction profile is taken for calculation of G.S, is found to be different in different directions for all the crystallites the size is small in a direction perpendicular to (200) plane [10].From Figure (2a,b), it is clear that the PbS films show a decreasing of lattice constant with time(aₒ=0.5936nm).The decreasing of the lattice constant is more floral due to the enhanced of the degree of crystallity of films and indicating that films grow on the glass substrate without stresses at the interface.However, increasing time deposition produce an increase in the grain size of the PbS films from approximately 4.9 to 11.3 nm for the films deposited at 15 to 90 min, respectively.

Atomic Force Microscopy (AFM) Analysis
The surface images of the PbS films deposited at different time , obtained by AFM, are shown in Figure (3).It can be observed that the surface of the films is very compact.The films are constituted by round shaped rectangular pyramids packed together with a regular size distribution.A lot of empty spaces can be seen between these pyramids.The average size of the clusters increased with time.The larger pyramids in the surface of the films exhibit clearly an aggregate structure, that is, they are composed of small crystallites or grains.The changes in the structural and surface morphology properties of the PbS films produced by the deposition time can be related to the larger number of both Pb 2+ and S 2ions in the reaction solution at higher time.The release of metal ions by the complexing agent is a thermal activated process and the double hidrolisation of thiourea to produce S 2ions is also more effective at higher temperatures [12].Our results show that both processes can take place in a reaction solution at time as 15min. .We can notice from this figure that the conductivity of PbS films increased with increasing deposition time which is due to the increasing in the grain sizes [13].In almost all deposited films showed two different regions corresponding to two activation energies as shown in Figure ( 4).The activation energies of an electrical conduction have been determined in both low and high temperature regions [14].

CONCLUSIONS
Nanostructured PbS thin films were prepared by a simple chemical bath deposition method.The results of XRD study shows nanocrystalline structure with cubic phase (galena) and AFM show that the deposited PbS film consists of nano-sized grains.The grain size increases with increasing deposition time.The deposits are dark chocolate in color with conductivity of range 10 -6 -10 -5 (Ω.cm) -1 .
is the electrical resistivity of the film and l, b and d are the distance between two centers of electrodes , width of electrode and thickness of the film Figure (1) X-ray patterns of nano PbS thin films for different time: (A) 15min (B) 30min (C) 45min (D) 60min.
Figure (2a,b) Variation of (a) lattice constant (b) grain size with deposition time for nano PbS films .

Tech. Journal .Vol31,Part (B), No. Electrical Conductivity for Nanocrystalline PbS Thin Films 704 nonlinear
optics.Most studied nanocrystalline semiconductors belong to the II-VI and IV-VI groups as they are relatively easy to synthesize and are generally prepared as particles or in thin film form[2].PbS is a semiconductor with band gap about 0.41 eV at room temperature and 0.29 eV at liquid-N 2 temperature.For PbS, the size effect can be observed for a crystallite as large as 180Å, which contains over 105 atoms[3].

Table ( 1) Structural parameters of nano PbS thin films for different time. G.S (nm) d (nm) a XRD (nm) hkl d(nm) t(min)
Where h, k ,l are Miller indices, d is the inter-planer spacing.