Structural and thermal studies of silver nanoparticles and electrical transport study of their thin films
© Majeed Khan et al; licensee Springer. 2011
Received: 26 January 2011
Accepted: 22 June 2011
Published: 22 June 2011
This work reports the preparation and characterization of silver nanoparticles synthesized through wet chemical solution method and of silver films deposited by dip-coating method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDX) have been used to characterize the prepared silver nanoparticles and thin film. The morphology and crystal structure of silver nanoparticles have been determined by FESEM, HRTEM, and FETEM. The average grain size of silver nanoparticles is found to be 17.5 nm. The peaks in XRD pattern are in good agreement with that of face-centered-cubic form of metallic silver. TGA/DTA results confirmed the weight loss and the exothermic reaction due to desorption of chemisorbed water. The temperature dependence of resistivity of silver thin film, determined in the temperature range of 100-300 K, exhibit semiconducting behavior of the sample. The sample shows the activated variable range hopping in the localized states near the Fermi level.
Metal nanoparticles with at least one dimension approximately 1-100 nm have received considerable attention in both scientific and technological areas due to their unique and unusual physico-chemical properties compared with that of bulk materials . Phenomena at the nanoscale are likely to be a completely new world, where properties may not be predictable from those observed at large size scales, on account of quantum size effect and surface effects. Synthesis of nanoparticles has been a rapidly growing field in solid state chemistry . Metal nanoparticles are particularly interesting because they can easily be synthesized and modified chemically as well as can suitably be applied for device fabrication [3–5]. Due to the specific size, shape, and distribution, nanoparticles are used in the production of novel systems such as nanosensors , nanoresonators , nanoactuators , nanoreactors , single electron tunneling devices , plasmonics , and nanowire based devices  etc.
Among the various metal nanostructures, noble metal nanoparticles have attracted much attention, due to their superior physical and chemical properties. Nowadays, a lot of researches have been focused on silver nanoparticles because of their important scientific and technological applications in color filters [13, 14], optical switching , optical sensors [16, 17], and especially in surface-enhanced Raman scattering [18–20]. Such properties and applications strongly depend on the morphology, crystal structure, and dimensions of silver nanostructures. Over recent years, silver thin films have been a subject of intensive investigations because of excellent optical, electrical, catalytic, sensing, and antibacterial properties [21, 22] and subsequent applications. The synthesis of silver nanoparticles with controlled morphology is important for uncovering their specific properties and for achieving their practical applications.
Silver nanoparticles are of current importance because of its easy preparation process and unique optical, electrical, and thermal properties. The electrical conductivity of polyaniline-silver nanocomposite increases with increase in silver nanoparticles content than that of pure polyaniline [23, 24]. Pillai et al.  demonstrated that solar cells employing metallic nanoparticles can dramatically enhance the near infrared absorption due to the presence of surface plasmons. The excited surface plasmons can eject electrons into a surrounding conductive medium resulting in effective charge separation. D. Basak et al.  observed significant modifications in the electrical properties of poly (methyl methacrylate) thin films upon dispersion of silver nanoparticles. So far as the electrical properties are concerned, it is necessary to throw some light on the structural and morphological characteristics of silver nanoparticles.
In the synthesis of nanoparticles, it is very important to control not only the particle size but also the particle shape and particle size distribution as well. In the present investigation, the synthesis of silver nanoparticles and thin films by wet chemical solution route  has been discussed. The prepared silver nanoparticles have been examined using X-ray diffraction (XRD), field emission transmission scanning electron microscope (FESEM), field emission transmission electron microscope (FETEM), high-resolution transmission electron microscope (HRTEM), two-probe direct-current (dc) resistivity measurement and thermogravimetric analysis/differential thermal analysis (TGA/DTA) thermal system.
All the chemicals employed in the synthesis have been of analytical reagent grade. We used them without further purification. The nanoparticles of silver have been prepared according to the conventional procedure . The aqueous solution (20 ml) containing glucose (9 mmol), polyvinylpyrrolidone (12 mmol), and sodium hydroxide (7 mmol) has been heated at 60°C for 30 min under vigorous stirring at 3,000 rpm. After that, 10 ml aqueous solution of AgNO3 (1 mol/l) has been dropped in the previous solution. After refluxing for 60 min, the colloidal solution has been allowed to cool slowly to room temperature. The resultant solution has been undertaken to centrifugation at 8,000 rpm for 90 min. After filtration, the precipitate so obtained has been washed many times with deionized water using centrifugation for 15 min each time. Finally, the precipitate has been collected and powdered finely, and identified as silver nanoparticles using characterization tools. These silver nanoparticles have been re-dispersed in ethanol for the preparation of silver film. The films have been deposited on ultra-clean quartz substrates using dip-coating method. The quartz substrate has been immersed vertically into the ethanol solution of silver nanoparticles (25 mg/ml). After that, the container has been placed in a vacuum chamber (10-3 torr) at room temperature for 24 h; the smooth, uniform, and bright silver film has been obtained on the quartz substrate due to the evaporation of the solvent (ethanol) under reduced pressure. The film shows good adhesion to the substrate. Prior to the deposition of silver film, the quartz slide has been immersed in chromic-sulfuric acid for a day in order to clean the surface and to enhance its hydrophilicity; and then rinsed many times with deionized water and dried in air.
The morphology and crystal structure of silver nanoparticles powder has been evaluated by FESEM, FETEM, HRTEM, SAED, energy dispersive spectroscopy (EDX), and XRD. SEM images were obtained using a field emission scanning electron microscope (JSM-7600F, JEOL, Tokyo Japan) at an accelerating voltage of 15 kV. The fine powder of silver nanoparticles has been dispersed in ethanol on a carbon coated copper grid and TEM images were obtained with ultra-high resolution FETEM (JEOL, JEM-2100F) at an accelerating voltage of 200 kV. The reaction type and weight loss have been confirmed using TGA/DTA thermal system (DTG-60, Shimadzu, Kyoto, Japan). The XRD pattern was recorded by X-ray diffractometer (PANalytical X'Pert, Almelo, The Netherlands) equipped with Ni filter and CuKα (λ = 1.54056 Å) radiation source. For dc resistivity measurements, silver film with deposited contacts has been mounted in a specially designed metallic sample holder where a vacuum of about 10-3 torr could be maintained throughout the measurements. A voltage (1.5 V, DC) was applied across the film and the resulting current was measured by a digital electrometer (Keithley 617, Keithley Instruments, Inc., Cleveland OH, USA). The temperature was measured by mounting a calibrated copper-constantan thermocouple near the sample.
Results and discussion
where β is FWHM in radian, t is the grain size in nm, ε is the strain, λ is X-ray wavelength in nanometers, and C is a correction factor taken as 0.94. The grain size and strain of the sample have been found to be 16.37 nm and 3.98 × 10-3, respectively.
Here, Y is the Young's modulus of Ag (83 GPa), a is the lattice constant (in nanometers) measured from XRD data, a 0 is the bulk lattice constant (0.5406 nm) and γ is the Poisson's ratio (0.37) for Ag.
The X-ray line profile analysis has been was used to determine the intrinsic stress and dislocation density of silver nanoparticles and found to be as 0.275 GPa and 7.0 × -14 m-2 respectively.
The elemental analysis of sample has been performed using EDX spectroscopy. Inset of Figure 2 shows EDX spectrum of silver nanoparticles. The peaks observed at 3.0, 3.2, and 3.4 keV correspond to the binding energies of Ag L α , Ag L β , and Ag L β 2respectively; while the peaks situated at the binding energies of 0.85, 1.0, 8.05, and 8.95 keV belong to CuL 1, CuL α , CuK α , and CuK β , respectively. In addition, a peak at 0.25 keV corresponding to carbon (CK α ) has been observed. The copper and the carbon peaks correspond to the carbon coated copper grid of TEM. No peaks of other impurity have been detected. Therefore, the EDX profile of sample (inset of Figure 2) indicates that the silver nanoparticles sample contain pure silver, with no oxide.
where ρo is the high temperature limit of resistivity (in Ω-m)and T o is Mott's characteristic temperature (in kelvin) associated with the degree of localization of the electronic wave function.
where k B is the Boltzmann constant (in electronvolt per kelvin), N(E F) is the density of states (in per electron volt per cubic meter), 1/γ is the decay length of electronic wave function which typically varies in the range 3-30 Å and C o is a dimensionless constant, which has a value in the range 16-310 . It is clear from Figure 5 and its inset that the sample exhibits a good fitting over the entire temperature range 100-300 K. Here, we have taken the localization length γ = 3 Å as reported by Maddison et al. . From the fitted values of T o, we have found the value of density of states at the Fermi level N(E F) approximately 3.732 × 1024 eV-1 m-3 for silver nanoparticles.
The present wet chemical solution method for the preparation of silver nanoparticles and their thin films is simple, convenient, and viable which allows nanocrystalline silver particles of spherical shape and almost narrow size distribution. The x-ray diffraction pattern of sample shows a face-centered cubic crystalline phase of silver with lattice constant 0.4085 nm. The average particle size, as obtained from FETEM analysis, is 17.5 nm that agreed with XRD results. TGA/DTA study shows that the dominant weight loss occurs between 200°C and 300°C; and the reaction is of exothermic type. The temperature dependence of resistivity of silver film exhibits semiconducting behavior of the sample. The electrical conduction is due to the activated VRH in the localized states near the Fermi level.
Thanks are due to King Abdul Aziz City for Science and Technology (KAACST), Riyadh, Saudi Arabia (Grant No.: 10-NAN1001-02) for providing financial assistance in the form of major research project.
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