Qualification: 
Ph.D, M.Tech, B-Tech
r_abhilash@blr.amrita.edu

Dr. Abhilash Ravikumar currently serves as Assistant Professor (Sr.Gr.) at the department of Electronics and Communication Engineering, School of Engineering, Amrita Vishwa Vidyapeetham, Bengaluru Campus.

He completed his bachelors in Electronics and Communication Enginnering from Amrita School of Engineering, Bangalore (2011). Later his interest towards fundamental physics led him to write GATE in Physics and did his M.Tech from National Institute of Technology-Surathkal and Indian Institute of Science, Bangalore in Materials Science (2014). Here he mainly worked on Density Functional Theory models to understand the adsorption of graphene on silicon(111). Later he joined as a Marie Curie doctoral Fellow under Prof. Brivio at University of MilanoBicocca where he mainly worked his spin-dependent, conductive properties of core excited and functionalized graphene. He continued his post-doctoral research at University of Milano-Bicocca in the same group and worked on Green’s function techniques to study quantum transport in 2-D heterostructure interfaces.

Education

  • 2017: Ph. D. in Materials Science and Nanotechnology
    University of Milano-Bicocca
  • 2014: M.Tech. in Materials Science
    National Institute of Technology-Surathkal
  • 2011: B.Tech. in Electronics and Communication Engineering
    School of Engineering, Amrita Vishwa Vidyapeetham, Bengaluru

Publications

Publication Type: Journal Article

Year of Publication Publication Type Title

2018

Journal Article

Abhilash Ravikumar, Kladnik, G., Müller, M., Cossaro, A., Bavdek, G., Patera, L. L., Sánchez-Portal, D., Venkataraman, L., Morgante, A., Brivio, G. Paolo, Cvetko, D., and Fratesi, G., “Tuning Ultrafast Electron Injection Dynamics at Organic-Graphene/Metal Interfaces”, Nanoscale, vol. 10, pp. 8014-8022, 2018.[Abstract]


We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4′-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

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2018

Journal Article

Abhilash Ravikumar, Brivio, G. Paolo, and Fratesi, G., “Core Level Spectra of Organic Molecules Adsorbed on Graphene”, Materials , vol. 11, no. 4, p. 518, 2018.[Abstract]


We perform first principle calculations based on density functional theory to investigate the effect of the adsorption of core-excited organic molecules on graphene. We simulate Near Edge X-ray absorption Fine Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) at the N and C edges for two moieties: pyridine and the pyridine radical on graphene, which exemplify two different adsorption characters. The modifications of molecular and graphene energy levels due to their interplay with the core-level excitation are discussed. We find that upon physisorption of pyridine, the binding energies of graphene close to the adsorption site reduce mildly, and the NEXAFS spectra of the molecule and graphene resemble those of gas phase pyridine and pristine graphene, respectively. However, the chemisorption of the pyridine radical is found to significantly alter these core excited spectra. The C 1s binding energy of the C atom of graphene participating in chemisorption increases by ∼1 eV, and the C atoms of graphene alternate to the adsorption site show a reduction in the binding energy. Analogously, these C atoms also show strong modifications in the NEXAFS spectra. The NEXAFS spectrum of the chemisorbed molecule is also modified as a result of hybridization with and screening by graphene. We eventually explore the electronic properties and magnetism of the system as a core-level excitation is adiabatically switched on.

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2018

Journal Article

G. Fratesi, Achilli, S., Manini, N., Onida, G., Baby, A., Abhilash Ravikumar, Ugolotti, A., Brivio, G., Milani, A., and Casari, C., “Fingerprints of sp1 Hybridized C in the Near-Edge X-ray Absorption Spectra of Surface-Grown Materials”, Materials , 2018.[Abstract]


Carbon structures comprising sp1 chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp1/sp2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp1/sp2 materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp2 systems. We then consider a mixed polymer alternating sp1 C4 units with sp2 biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp1 hybridization in surface-grown C structures.

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2018

Journal Article

A. Baby, Lin, H., Abhilash Ravikumar, Bittencourt, C., Wegner, H. A., Floreano, L., Goldoni, A., and Fratesi, G., “Lattice Mismatch Drives Spatial Modulation of Corannulene Tilt on Ag (111)”, The Journal of Physical Chemistry C , vol. 122 , no. 19, pp. 10365–10376, 2018.[Abstract]


We investigated the adsorption of corannulene (C20H10) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM), X-ray photoemission (XPS), and near-edge X-ray absorption fine structure (NEXAFS). Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and the core excited spectra evaluated within the transition-potential approach. Corannulene is physisorbed in a bowl-up orientation displaying a very high mobility (diffusing) and dynamics (tilting and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close-compact phase with an average intermolecular spacing of ∼10.5 ± 0.3 Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which, however, could not be identified with a specific superlattice periodicity. DFT calculations indicate that the structural and spectroscopic properties are intermediate between those predicted for the limiting cases of an on-hexagon geometry (with a 3-fold, ∼8.6 Å unit mesh) and an on-pentagon geometry (with a 4-fold, ∼11.5 Å unit mesh). We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the 3-fold and 4-fold phases.

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2017

Journal Article

B. Javvaji, Shenoy, B. Maithry, D Mahapatra, R., Abhilash Ravikumar, Hegde, G. M., and Rizwan, M. R., “Stable configurations of graphene on silicon”, Applied Surface Science , vol. 414, 31 , pp. 25-33, 2017.[Abstract]


Integration of graphene on silicon-based nanostructures is crucial in advancing graphene based nanoelectronic device technologies. The present paper provides a new insight on the combined effect of graphene structure and silicon (001) substrate on their two-dimensional anisotropic interface. Molecular dynamics simulations involving the sub-nanoscale interface reveal a most favourable set of temperature independent orientations of the monolayer graphene sheet with an angle of ∽15° between its armchair direction and [010] axis of the silicon substrate. While computing the favorable stable orientations, both the translation and the rotational vibrations of graphene are included. The possible interactions between the graphene atoms and the silicon atoms are identified from their coordination. Graphene sheet shows maximum bonding density with bond length 0.195 nm and minimum bond energy when interfaced with silicon substrate at 15° orientation. Local deformation analysis reveals probability distribution with maximum strain levels of 0.134, 0.047 and 0.029 for 900 K, 300 K and 100 K, respectively in silicon surface for 15° oriented graphene whereas the maximum probable strain in graphene is about 0.041 irrespective of temperature. Silicon–silicon dimer formation is changed due to silicon–carbon bonding. These results may help further in band structure engineering of silicon–graphene lattice.

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2016

Journal Article

Abhilash Ravikumar, Baby, A., Lin, H., Brivio, G. Paolo, and Fratesi, G., “Femtomagnetism in Graphene Induced by Core Level Excitation of Organic Adsorbates and the Role of Electron Transfer”, Nanophotonics: principles and applications, 2016.

2016

Journal Article

Abhilash Ravikumar, Baby, A., Lin, H., Brivio, G. Paolo, and Fratesi, G., “Femtomagnetism in Graphene Induced by Core Level Excitation of Organic Adsorbates”, Scientific reports, vol. 6, p. 24603, 2016.[Abstract]


We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numerical simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

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Publication Type: Thesis

Year of Publication Publication Type Title

2017

Thesis

Abhilash Ravikumar, “Electronic, Spin Dependent Conductive Properties of Modified Graphene”, 2017.[Abstract]


Understanding the adsorption mechanisms of organic molecules on graphene and their subsequent influence on the electronic and magnetic properties of this interface is essential in designing graphene based devices. In this thesis we perform first principles calculations based on density functional theory (DFT) in an effort to understand these phenomena. Most organic electronic devices are composed of interfaces formed by the organic overlayer and a metallic electrode. Understanding the charge transfer dynamics at the interface would help engineer efficient organic devices. With this in mind, the first part of research we present is the adsorption of core-excited organic molecules on graphene. We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. We consider three organic molecules: Pyridine - whose interaction with graphene is mainly facilitated by van der Waals forces, Picoline radical - an intermediate case where there is a strong van der Waals interaction of the pyridine π ring with graphene but a covalent bonding of the molecule and pyridine radical - where the interaction is mainly through covalent bonding, and study the ground state and N 1s core excited state electronic properties for these systems. 

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Publication Type: Conference Paper

Year of Publication Publication Type Title

2016

Conference Paper

Abhilash Ravikumar, Baby, A., Lin, H., Brivio, G., and Fratesi, G., “Transient Magnetism in Graphene Induced by Core Level Excitation of Organic Adsorbates”, in APS March Meeting, 2016.[Abstract]


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2016

Conference Paper

Abhilash Ravikumar, Baby, A., Lin, H., Brivio, G. Paolo, and Fratesi, G., “Transient Magnetization of Core Excited Organic Molecules Adsorbed on Graphene”, in APS Meeting Abstracts, 2016.[Abstract]


This work presents a density functional theory based computational investigation of electronic and magnetic properties of physisorbed and chemisorbed organic molecules on graphene in the ground state and core excited one at low molecular coverage. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, it is found that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the contrary, when graphene is covalently functionalized, the system is magnetic in the ground state presenting two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

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2016

Conference Paper

G. Fratesi, Baby, A., Lin, H., Abhilash Ravikumar, Muller, M., Sànchez-Portal, D., Selloni, A., and Brivio, G., “Electron Transfer with Core-Level Excitations at Hybrid Interfaces”, in Workshop on Surfaces, Interfaces and Functionalization Processes in Organic Compounds and Applications (SINFO), 2016.

2015

Conference Paper

Abhilash Ravikumar, Lin, H., Baby, A., Fratesi, G., and Brivio, G., “Adsorption of Organic Molecules on Graphene”, in School on Organic Electronics, 2015.

2015

Conference Paper

G. Paolo Brivio, Fratesi, G., Lin, H., Abhilash Ravikumar, Adak, O., Venkataraman, L., Kladnik, G., Cvetko, D., and Morgante, A., “Lifetimes for Fast Charge Transfer of Core Excited Molecules on Gold and Graphene”, in APS Meeting Abstracts, 2015.[Abstract]


The charge transfer time from an excited organic molecule both adsorbed on gold and graphene is studied in terms of the resonant linewidth of the molecular orbital energy levels interacting with the valence band of the substrate. The calculations are performed by density functional theory including the van der Waals contribution. Experiments are carried out by the core level resonant spectroscopies with fs resolution. The core valence exciton is described by a static perturbation of the atomic potential. The calculated widths are consistent with the experimental transfer times. They display a dependence on the molecular adsorption angle both in theory and experiments, and this effect is predicted to be function of the excited orbital.

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2014

Conference Paper

Abhilash Ravikumar, Lin, H., Fratesi, G., and Brivio, G., “Adsorption of Pyridine on Graphene”, in XIX ETSF Workshop on Electronic Excitations, 2014.

2014

Conference Paper

B. Javvaji, Abhilash Ravikumar, Shenoy, B. M., D Mahapatra, R., Rahman, M. R., and Hegde, G. M., “Electronic Band Structure and Photoemission Spectra of Graphene on Silicon Substrate”, in Physics and Simulation of Optoelectronic Devices XXII, SPIE OPTO, 2014, San Francisco, California, United States, 2014.[Abstract]


Synergizing graphene on silicon based nanostructures is pivotal in advancing nano-electronic device technology. A combination of molecular dynamics and density functional theory has been used to predict the electronic energy band structure and photo-emission spectrum for graphene-Si system with silicon as a substrate for graphene. The equilibrium geometry of the system after energy minimization is obtained from molecular dynamics simulations. For the stable geometry obtained, density functional theory calculations are employed to determine the energy band structure and dielectric constant of the system. Further the work function of the system which is a direct consequence of photoemission spectrum is calculated from the energy band structure using random phase approximations.

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