• GIRIDHAR U KULKARNI

Articles written in Bulletin of Materials Science

• Twisted multilayer graphene exhibiting strong absorption bands induced by van Hove Singularities

Twisted bilayer graphene exhibits several angle-dependent properties due to the emergence of the van Hove Singularities in its density of states. Among them, twist-angle-dependent optical absorption has gained a lot of attention due to its presence in the visible spectral region. However, observation of such absorption is experimentally tricky due to large transmittance. In this study, we use highly decoupled twisted multilayer graphene to observe such absorption in thevisible region using a simple spectrometer. A large number of twisted graphene layers in the system enable observation of such absorption evident in the visible region; the absorption band position correlates with the twist angle measuredusing selective area electron diffraction pattern as well as predictions from theory. While the Raman spectra were akin to those of the decoupled graphene system, at specific twist angle of $\sim$13$^{\circ}$, the spectrum contained clear signatures of G-band enhancement.

• A spring network simulation in three dimensions for designing optimal crack pattern template to fabricate transparent conducting electrodes

Desiccation cracks in colloids are of the order of µm width or less. Therefore, such connected crack networks in a desiccating colloidal ﬁlm can provide a very ﬁne template for the fabrication of transparent conducting surfaces by vapour deposition of a metal onto the crack network (Rao $et al$ 2014 $Adv. Mater. Interfaces$ 1 140009). The colloidal layer is removed leaving a connected metallic network invisible to the eye. So the surface becomes conducting, while retaining its transparency. The challenge lies in maximizing electrical conductivity while retaining the transparency as far as possible. An optimal combination of the system parameters, which affect the morphology of the crack network is necessary to meet this challenge. In this work, we simulate crack pattern in desiccating colloidal ﬁlms in three dimensions using a spring network model. We look for the optimal combination of system parameters, such as ﬁlm thickness, material stiffness and polydispersity, which can produce the best template for producing a conducting network on transparent surfaces.

• Metal mesh-based transparent electrodes as high-performance EMI shields

Electromagnetic interference (EMI) shields in the form of coatings and films are useful for blocking radiations in various household and industrial settings. Being transparent and flexible would enhance their utility domain. In this study, we have fabricated transparent and flexible EMI shields made of metal meshes produced using the crack templating method pioneered in the laboratory. A Cu metal mesh with polyethylene terephthalate (PET) sheet as its substrate exhibited a visible transmittance of $\sim$85% and a sheet resistance of $\sim$0.83 $\Omega$ per square. The shielding efficiency was tested over a wide spectral range of the K$_{\rm u}$ band (12–18 GHz), relevant to communication electronics. The Cu mesh/PET film showed a remarkably high value for total EMI shielding (SE$_{\rm T}$) with the average value being $\sim$41 dB. The film could be laminated using a commonly available method, thus protecting exposure of the mesh to the environment. The laminated film is multifunctional, and this aspect was demonstrated by fabricating a large area (3.5 $\times$ 2.2 cm$^ 2$) Joule heater for defrosting and defogging applications.

• Shape-controlled template-driven growth of large CuS hexagonal nanoplates

Ultrathin 2D metal sulphide nanostructures have revealed extraordinary properties, applications and thus, the subject matter of current research. However, synthesis of shape-controlled large-sized (approximately few $\mu$ms) plates/sheets are not well known, specifically due to the colloidal synthesis process resulting in the distribution of shape/size. Here, we report the template-assisted growth of large-sized CuS nanoplates ($\sim$0.5–9 $\mu$m) by simple wet chemistry, where the Au microplates serve as a template. The Au microplates act as seed and facilitate the Au-Cu via epitaxial growth of Cu. Later, the Au-Cu acts as the source of Cu and in presence of aqueous S$^{2-}$, the CuS hexagons form, however, the edge of the hexagons are parallel to the Au seed and thus, hinting to be controlled by the seed. Interestingly, the substrateindependent Au-Cu can even provide the platform for the spiral nanoplates, by simply decorating the outer surface of the Cu with pillar-like features.

• Supercapacitor fabricated with 30 M KOH electrolyte and highly stable Pd$_4$S electrodes

In this work, supercapacitors have been fabricated using Pd$_4$S electrode material which is highly conducting and chemically robust to the acid and alkali media. The synthesis strategy is simple and involves a single step thermolysis of palladium octanethiolate film in the H$_2$ atmosphere at 250°C for 1 h. Besides its conducting nature, its porous morphology makes it a suitable electrode material for supercapacitors. The electrochemical properties of the Pd4S were examined in harsh alkali media (30 M KOH). Typically, in the literature, only lower electrolyte concentration (1–6 M KOH) is preferred as the electrode may not withstand higher concentrations. Using the prepared Pd$_4$S film as supercapacitor electrode, a specific capacitance of 570 F g$^{-1}$ with energy and power densities of 28 Wh kg$^{-1}$ and 1.5 kW kg$^{-1}$, respectively, were achieved with 30 M KOH electrolyte. After 1500 cycles of charging and discharging with a constant current density of 1.75 A g$^{-1}$, more than 98% capacitance retention was observed.

• # Bulletin of Materials Science

Volume 44, 2021
All articles
Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020