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Author

L.K. Malhotra

Other affiliations: Indian Institutes of Technology
Bio: L.K. Malhotra is an academic researcher from Indian Institute of Technology Delhi. The author has contributed to research in topics: Thin film & Amorphous solid. The author has an hindex of 19, co-authored 76 publications receiving 1391 citations. Previous affiliations of L.K. Malhotra include Indian Institutes of Technology.


Papers
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TL;DR: In this paper, a spray pyrolysing solution of zinc acetate and aluminium chloride was used to produce transparent conducting thin films of aluminium-doped zinc oxide, which have a resistivity of 10 −3 ω cm and a visible transmission of 85%.

175 citations

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TL;DR: In this article, an analysis of the electronic structure of Pd nanoparticles synthesized by inert gas evaporation technique is given, which provides a direct evidence of interplay of quantum confinement (a size effect) and coordination reduction (a surface effect).
Abstract: An analysis is given of the electronic structure of Pd nanoparticles synthesized by inert gas evaporation technique. A study of the effect of size on various core and valence electrons in Pd nanoparticles reveals a varied dependence of binding energy of electrons in different electronic levels. The shift in the Pd x-ray photoelectron spectroscopy 4d valence band centroid is more than the core level shift. The results of the present study provide a direct evidence of interplay of quantum confinement (a size effect) and coordination reduction (a surface effect).

109 citations

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TL;DR: In this paper, a low frequency spur has been observed in the impedance diagram at medium and high humidity levels and attributed to the migration of adsorbed ions towards the electrode sample contac...
Abstract: Impedance spectroscopy has been used to investigate the properties of sol-gel derived ultrafine grained tin oxide thin films subjected to various ambients and temperatures. The impedance measurements have been carried out in the frequency range 250 kHz–10 mHz and Cole–Cole plots drawn. Equivalent circuit models have been suggested and fitting of the experimentally obtained data done on the basis of the approach of “universal dielectric behavior” introduced by Jonscher. The films have shown strong interaction with water vapor in the atmosphere at room temperature (300 K) as reflected from the changes in the values of resistive and capacitive components in the equivalent circuits at different humidity levels. The conduction mechanism appears to be due to the transfer of protons through the physisorbed layer of water molecules. A low frequency spur has been observed in the impedance diagram at medium and high humidity levels and attributed to the migration of adsorbed ions towards the electrode sample contac...

102 citations

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TL;DR: In this article, an upper layer of Co3O4 and subsequent layers of CoO down to the substrate were used for spray pyrolysis on stainless steel substrates kept at 300°C.

77 citations

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TL;DR: The influence of various paratneters such as viscosity and pH of the sol; pulling speed and sintering temperature on thickness of the coatings and their microstructure have been investigated as mentioned in this paper.

77 citations


Cited by
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TL;DR: Transparent conductors (TCs) have a multitude of applications for solar energy utilization and for energy savings, especially in buildings as discussed by the authors, which leads naturally to considerations of spectral selectivity, angular selectivity, and temporal variability of TCs, as covered in three subsequent sections.

1,471 citations

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TL;DR: In this article, the state of the art in the field of experimental techniques possible to be applied to the study of conductometric gas sensors based on semiconducting metal oxides is reviewed.
Abstract: The paper critically reviews the state of the art in the field of experimental techniques possible to be applied to the study of conductometric gas sensors based on semiconducting metal oxides. The used assessment criteria are subordinated to the proposed R&D approach, which focuses on the study, and subsequent modelling, of sensors’ performance in realistic operation conditions by means of a combination of phenomenological and spectroscopic techniques. With this viewpoint, the paper presents both the to-date achievements and shortcomings of different experimental techniques, describes – by using selected examples – how the proposed approach can be used and proposes a set of objectives for the near future.

1,416 citations

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TL;DR: In this article, a comparison of the band gap energy estimated from UV-vis reflectance spectra of TiO2 powders prepared by sol-gel route versus commercial TiO 2 powders, nanopowder, bulk powder and P25 is reported.
Abstract: A comparison of the band gap energy estimated from UV–vis reflectance spectra of TiO2 powders prepared by sol–gel route versus commercial TiO2 powders, nanopowder, bulkpowder and P25 is reported. The experimental results obtained from the optical absorption spectra were reported for all the TiO2 samples. Graphic representations were used to calculate Eg: absorbance versus λ; F(R) versus E; (F(R) hν)n versus E, with n = ½ for an indirect allowed transition and n = 2 for a direct allowed transition. From the results, it could be seen that Eg strongly varied according to the equation used for the graphic representation. Differences in Eg up to 0.5 eV for the same semiconductor depending on the transition chosen were observed. Accurate Eg estimation in the four semiconductors studied was obtained by using the general equation α (hν) ≈ B (hν − Eg)n (where α ~ F(R)) and indirect allowed transition.

1,290 citations

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TL;DR: There are an immense number of sensors reported in the literature for hydrogen detection and in this article these sensors are classified into eight different operating principles, such as measuring range, sensitivity, selectivity and response time.
Abstract: Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

1,217 citations

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TL;DR: In this paper, a conceptual model for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal ammorphous state, is given for thin film deposition by sputtering, electronic band structure and ion diffusion.
Abstract: Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

1,156 citations