• M Ghanashyam Krishna

      Articles written in Bulletin of Materials Science

    • Optical, structural and electrical properties of Mn doped tin oxide thin films

      Rajeeb Brahma M Ghanashyam Krishna A K Bhatnagar

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      Mn doped SnO𝑥 thin films have been fabricated by extended annealing of Mn/SnO2 bilayers at 200°C in air for 110 h. The dopant concentration was varied by controlling the thickness of the metal layer. The overall thickness of the film was 115 nm with dopant concentration between 0 and 30 wt% of Mn. The films exhibit nanocrystalline size (10–20 nm) and presence of both SnO and SnO2. The highest transmission observed in the films was 75% and the band gap varied between 2.7 and 3.4 eV. Significantly, it was observed that at a dopant concentration of ∼ 4 wt% the transmission in the films reached a minimum accompanied by a decrease in the optical band gap. At the same value of dopant concentration the resistivity also reached a peak. This behaviour appears to be a consequence of valence fluctuation in Sn between the 2+ and 4+ states. The transparent conductivity behaviour fits into a model that attributes it to the presence of Sn interstitials rather than oxygen vacancies alone in the presence of Sn2+.

    • Effect of microstructural evolution on magnetic properties of Ni thin films

      Prashant Kumar M Ghanashyam Krishna A K Bhattacharya

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      The magnetic properties of Ni thin films, in the range 20–500 nm, at the crystalline–nanocrystalline interface are reported. The effect of thickness, substrate and substrate temperature has been studied. For the films deposited at ambient temperatures on borosilicate glass substrates, the crystallite size, coercive field and magnetization energy density first increase and achieve a maximum at a critical value of thickness and decrease thereafter. At a thickness of 50 nm, the films deposited at ambient temperature onto borosilicate glass, MgO and silicon do not exhibit long-range order but are magnetic as is evident from the non-zero coercive field and magnetization energy. Phase contrast microscopy revealed that the grain sizes increase from a value of 30–50 nm at ambient temperature to 120–150 nm at 503 K and remain approximately constant in this range up to 593 K. The existence of grain boundary walls of width 30–50 nm is demonstrated using phase contrast images. The grain boundary area also stagnates at higher substrate temperature. There is pronounced shape anisotropy as evidenced by the increased aspect ratio of the grains as a function of substrate temperature. Nickel thin films of 50 nm show the absence of long-range crystalline order at ambient temperature growth conditions and a preferred [111] orientation at higher substrate temperatures. Thin films are found to be thermally relaxed at elevated deposition temperature and having large compressive strain at ambient temperature. This transition from nanocrystalline to crystalline order causes a peak in the coercive field in the region of transition as a function of thickness and substrate temperature. The saturation magnetization on the other hand increases with increase in substrate temperature.

    • Interface controlled growth of nanostructures in discontinuous Ag and Au thin films fabricated by ion beam sputter deposition for plasmonic applications

      R Brahma M Ghanashyam Krishna

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      The growth of discontinuous thin films of Ag and Au by low energy ion beam sputter deposition is reported. The study focuses on the role of the film–substrate in determining the shape and size of nanostructures achieved in such films. Ag films were deposited using Ar ion energy of 150 eV while the Au films were deposited with Ar ion energies of 250–450 eV. Three types of interfaces were investigated in this study. The first set of film–substrate interfaces consisted of Ag and Au films grown on borosilicate glass and carbon coated Cu grids used as substrates. The second set of films was metallic bilayers in which one of the metals (Ag or Au) was grown on a continuous film of the other metal (Au or Ag). The third set of interfaces comprised of discontinuous Ag and Au films deposited on different dielectrics such as SiO2, TiO2 and ZrO2. In each case, a rich variety of nanostructures including self organized arrays of nanoparticles, nanoclusters and nanoneedles have been achieved. The role of the film–substrate interface is discussed within the framework of existing theories of thin film nucleation and growth. Interfacial nanostructuring of thin films is demonstrated to be a viable technique to realize a variety of nanostructures. The use of interfacial nanostructuring for plasmonic applications is demonstrated. It is shown that the surface Plasmon resonance of the metal nanostructures can be tuned over a wide range of wavelengths from 400 to 700 nm by controlling the film–substrate interface.

    • Effects of metal doping on photoinduced hydrophilicity of SnO2 thin films

      Debarun Dhar Purkayastha Rajeeb Brahma M Ghanashyam Krishna V Madhurima

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      The influence of metal dopants (Mn2+, Al3+ and Cu2+) on the wetting properties of SnO2 thin films deposited by thermal evaporation is reported. The undoped and doped SnO2 films crystallize into the orthorhombic structure upon annealing at 200°C for 110 h. It is shown that wettability behaviour, before and after ultraviolet (UV) irradiation, is dependent on the ionic radius of the dopant. The contact angle of un-irradiated samples increases with increase in ionic radius of the dopant and also in comparison with the undoped sample. It is 54° for pure SnO2 and increases to 77.5°, 92.3° and 95.9° for the Al3+, Mn2+ and Cu2+ doped samples, respectively. After UV irradiation, the value is 5.4° for the undoped sample. This increases to 21.2° for Al doping reaching a minimum of 6.4° for the Mn-doped sample increasing thereafter to 63.3° for the Cu-doped sample. It is observed that pre-irradiation contact angle behaviour can be correlated with the change in roughness of the films with increasing ionic radius. In contrast, photoinduced hydrophilicity of the films correlates with their optical bandgap. The contact angle is lowest for the lowest bandgap material, i.e., Mn-doped SnO2, with a bandgap of 2.48 eV. Thus, the band structure of SnO2 that can be controlled by dopant ionic radius can in turn be employed to manipulate the wettability of these surfaces.

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