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Kartick Tarafder

Publications and source records attributed to Kartick Tarafder.

13 recordsLinked to original sources
arXiv2025

Graphene Straintronics by Molecular Trapping

Here, we report on controlling strain in graphene by trapping molecules at the graphene-substrate interface, leveraging molecular dipole moments. Spectroscopic and transport measurements show that strain correlates with the dipole moments of trapped molecules, with a dipole range of 1.5 D to 4.9 D resulting in a 50-fold increase in strain and a substantial rise in the residual carrier density. This has been possible by charge transfer between graphene and trapped molecules, altering the C=C bond length, and causing biaxial strain. First-principles density functional theory calculations confirm a consistent dependence of bending height on molecular dipole moments.

cond-mat.mes-hall↗
arXiv2024

Realtime observation of a tungsten-promoted size regulation mechanism in a rhodium catalyst at atomic resolution

The static and genuine structure of small rhodium and rhodium/tungsten nanoparticles on an alumina support can be imaged with atomic resolution even if single digit atom clusters are investigated. Low dose rate electron microscopy is key to the achievement and can generally be applied to investigate any similar material. In such conditions it becomes feasible to identify the chemical composition of nanocrystals from quantitative contrast analyses alone by counting atoms. The ability to fully characterize an unaltered, initial state of the objects allows targeting structural excitations or conformational changes induced by the electron beam itself. For the specific case of catalytic Rh:W particles we stimulate a tungsten-promoted size regulation mechanism in real time that is driven by Oswald ripening and can be understood by a strong binding of tungsten atoms to the oxygen atoms of the support, which builds up strain as the cluster sizes increase.

cond-mat.mtrl-sci↗
arXiv2020

Theoretical investigation of quantum capacitance in the functionalized MoS$_2$-monolayer

In this work, we investigated the electronic structure and the quantum capacitance of the functionalized MoS$_2$ monolayer. The functionalizations have been done by using different ad-atom adsorption on Mo$S_2$ monolayer. Density functional theory calculations are performed to obtain an accurate electronic structure of ad-atom doped MoS$_2$ monolayer with a varying degree of doping concentration. The quantum capacitance of the systems was subsequently estimated. A marked quantum capacitance above 200 $μ$F/cm$^2$ has been observed. Our calculations show that the quantum capacitance of MoS$_2$ monolayer is significantly enhanced with substitutional doping of Mo with transition metal ad-atoms. The microscopic origin of such enhancement in quantum capacitance in this system has been analyzed. Our DFT-based calculation shows that generation of new electronic states at the proximity of the band-edge and the shift of Fermi level caused by the ad-atom adsorption results in a very high quantum capacitance in the system.

cond-mat.mtrl-sci↗
arXiv2019

Enhanced photocatalytic efficiency of layered CdS/CdSe heterostructures: Insights from first principles electronic structure calculations

Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. CdS in this class of materials, have a direct band gap of 2.4 eV, have gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strategy for improving the photocatalytic performance. In this work we have done a detail theoretical investigation based on hybrid density functional theory calculation to get insight into the energy band structure, mobility and charge transfer across the CdS/CdSe heterojunction. The results indicate that CdS/CdSe forms type-II heterostructure that has several advantages in improving the photocatalytic efficiency under visible light irradiation.

cond-mat.mtrl-sci↗
arXiv2019

Enhancement of quantum capacitance in graphene electrodes by chemical modifications using aliphatic/aromatic molecules and their radicals

We have carried out the systematic study of quantum capacitance (C$_Q$) in functionalized graphene. The functionlization of graphene has been done by doping with different aliphatic and aromatic molecules and their radicals. Using density functional theory calculations, we first analyze the electronic band structure of the functionalized graphene and subsequently obtained the quantum capacitance in each system. We observed that the quantum capacitance can be enhanced by doping aliphatic and aromatic molecules and their radicals on graphene sheet, especially radical functionalized graphene shows significant enhancement in C$_Q$. Our theoretical investigation reveals that aromatic and aliphatic radicals generates localized density of states near the Fermi level, due to a charge localization. As a result, a very high quantum capacitance (above 200 $μF/cm^2$) compared to pristine graphene has been estimated. Effects of atomic dislocation and the vacancy defect on graphene during functionalization has also been incorporated in our investigation. Obtain results predict that the formation of molecular radicals while functionalization of graphene could be an efficient way to synthesize highly efficient graphene based supercapacitor electrode materials which in turn may significantly improve the performance of supercapacitors.

cond-mat.mtrl-sci↗
arXiv2019

Theoretical Investigations of Electronic Structure, Magnetic and Optical Properties of Transition Metal-dinuclear Molecules

The work presents the electronic structure, spin state and optical properties of TM-dinuclear molecules (TM = Cr, Mn, Fe, Co, and Ni) which was modelled according to the recently reported Pt$^{II}$-dinuclear complex data\cite{kar_colour_2016}. The molecules were geometrically optimized in the gas phase and their stability were analyzed from vibrational spectra study using density functional theory (DFT) calculations. The ground spin state of the tetra-coordinated TM atom in the modeled molecules was predicted based on the relative energies between the possible spin states of the molecules. We further performed DFT+U calculations to investigate the precise ground state spin configuration of molecules. Interestingly, optical characterization of these molecules shows that the absorption spectra have a large peak in the blue-light wavelength range, therefore could be suitable for blue-LED application. Our work promotes further computational and experimental studies on TM-dinuclear molecules in field of molecular spintronics and optoelectronics.

cond-mat.mtrl-sci↗
arXiv2019

Route to Achieving Enhanced Quantum Capacitance in Functionalized Graphene based Supercapacitor Electrodes

We have investigated the quantum capacitance ($C_Q$) in functionalized graphene, modified with ad-atoms from different groups in the periodic table. Changes in the electronic band structure of graphene upon functionalization and subsequently the quantum capacitance ($C_Q$) of the modified graphene were systematically analyzed using density functional theory(DFT) calculations. We observed that the quantum capacitance can be enhanced significantly by means of controlled doping of N, Cl and P ad-atoms in the pristine graphene surface. These ad-atoms are behaving as magnetic impurities in the system, generates a localized density of states near the Fermi energy, which intern increases charge(electron/hole) carrier density in the system. As a result, a very high quantum capacitance was observed. Finally, the temperature dependent study of $C_Q$ for Cl and N functionalized graphene shows that the CQ remains very high in a wide range of temperature near the room temperature.

cond-mat.mtrl-sci↗
arXiv2018

Route to Achieving Giant Magnetoelectric Coupling in BaTiO$_3$/Sr$_2$CoO$_3$F Perovskite Heterostructures

Polarization induced spin switching of atoms in magnetic materials opens for possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density-functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride Sr$_2$CoO$_3$F (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic BaTiO$_3$ (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by altering the polarization direction of the BTO, specifically, to switch from high-spin state to low-spin state by changing the relative orientation of the ferroelectric polarization of BTO with respect to SCOF, leading to an unexpected, giant magnetoelectric coupling, $α_s \approx 21 \times 10 ^{-10}$ Gcm$^2$/V.

cond-mat.mtrl-sci↗
arXiv2010

Charge-induced spin polarization in non-magnetic organic molecule Alq$_{3}$

Electrical injection in organic semiconductors is a key prerequisite for the realization of organic spintronics. Using density-functional theory calculations we report the effect of electron transfer into the organic molecule Alq$_3$. Our first-principles simulations show that electron injection spontaneously spin-polarizes non-magnetic Alq$_3$ with a magnetic moment linearly increasing with induced charge. An asymmetry of the Al--N bond lengths leads to an asymmetric distribution of injected charge over the molecule. The spin-polarization arises from a filling of dominantly the nitrogen $p_z$ orbitals in the molecule's LUMO together with ferromagnetic coupling of the spins on the quinoline rings.

cond-mat.mtrl-sci↗
arXiv2007

Electronic, magnetic and optical properties of random Fe-Cr alloys

In this communication we have studied the electronic structure, magnetic and optical properties of bcc \fecr alloys in the ferromagnetic phase. We have used the augmented space recursion technique coupled with tight-binding linearized muffin-tin orbital technique (TB-LMTO-ASR) as well as the coherent-potential approximation based on the Korringa-Kohn-Rostocker method (KKR-CPA). Also the plane wave projector augmented wave (PAW) method has been used with the disorder simulated by the special quasi-random structure method f or configuration averaging (SQS). This was to provide a comparison between the different methods in common use for random alloys. Moreover, using the self-consistent potential parameters from TB-LMTO-ASR ca lculations we obtained the spin resolved optical conductivity using the generalized recursion technique proposed by Müller and Vishwanathan.

cond-mat.dis-nn↗
arXiv2006

Effect of short-range order on the electronic structure and optical properties of the CuZn alloy : an augmented space approach

In this work we have combined the generalized augmented space method introduced by one of us with the recursion method of Haydock et al (GASR), within the framework of the local density functional based linear muffin-tin orbitals basis (TB-LMTO). Using this we have studied the effect of short-range ordering and clustering on the density of states, optical conductivity and reflectivity of 50-50 CuZn alloys. Our results are in good agreement with alternative techniques. We argue that the TB-LMTO-GASR is a feasible, efficient and quantitatively accurate computational technique for the study of environmental effects in disordered binary alloys.

cond-mat.dis-nn↗
arXiv2005

Optical conductivity in disordered alloys : an approach via the augmented space recursion

We present here a calculation of the configuration averaged optical conductivity of random binary alloys CuAu and AgAu. Our formulation is based on the augmented space formalism proposed by Mookerjee [J. Phys. C : Solid State Phys. {\bf 6} 1340 (1973)] and the optical conductivity is obtained directly through a recursive procedure suggested by Viswanath and Müller ["The user friendly recursion method", Troisieme Cycle de la Physique, en Suisse Romande (1993)].

cond-mat.dis-nn↗