Dino Novko

Changing character of electronic transitions in graphene: From single-particle excitations to plasmons

In this paper we clarify the nature of π and π + σ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from

Optical absorption and conductivity in quasi-two-dimensional crystals from first principles: Application to graphene

This paper gives a theoretical formulation of the electromagnetic response of the quasi-two-dimensional crystals suitable for investigation of optical activity and polariton modes. The response to external electromagnetic field is described by current-current response tensor Πμν calculated by solving the Dyson equation in the random phase approximation, where current-current interaction is mediated by the photon propagator Dμν. The irreducible current-current response tensor Π0μν is calculated from the ab initio Kohn-Sham orbitals. The accuracy of Π0μν is tested in the long-wavelength limit where it gives correct Drude dielectric function and conductivity. The theory is applied to the calculation of optical absorption and conductivity in pristine and doped single-layer graphene and successfully compared with previous calculations and measurements.

On the tautomerisation of porphycene on copper (111)

We use density-functional theory (DFT) to analyse the interaction of trans- and cis-porphycene with Cu(111) and their interconversion by intramolecular H-transfer. This tautomerisation reaction is characterised by small values for the reaction energy and barrier, on the order of ∼0.1 eV, where the trans configuration is thermodynamically more stable upon adsorption according to the experiments [J. N. Ladenthin et al., ACS Nano 9, 7287-7295 (2015)]. To gain even a qualitatively correct description of this reaction at the DFT level, an accurate treatment of dispersion interactions and a careful choice of the exchange contribution are required in order to predict the subtle energetics. Analysis of the electronic structure shows that adsorption is contributed by a van der Waals (vdW) interaction, mainly responsible for stabilising the polyaromatic fragments, and by a significant charge redistribution localised between Cu and the unsaturated N atoms of the molecule central cavity. We find that different vdW functionals can produce qualitatively different electronic structures, while yielding small trans vs. cis energy differences. Unlike other functionals surveyed here, vdW-DF with PBE exchange satisfactorily reproduces not only the experimental energetics but also the scanning tunneling microscopy images. This gives us confidence that this functional achieves a reliable balance between the two mechanisms contributing to the adsorption of porphycene.

Intermode Coupling Drives the Irreversible Tautomerization in Porphycene on Copper(111) Induced by Scanning Tunnelling Microscopy

In this contribution, we develop a nonadiabatic theory that explains, from first-principles, the recently reported irreversible trans → cis tautomerization of porphycene on Cu(111) induced by a scanning tunnelling microscope at finite bias. The inelastic contribution to the STM current is found to excite a large number of skeletal vibrational modes of the molecule, thereby inducing a deformation of the potential energy landscape along the hydrogen transfer coordinate. Above a threshold bias, the stability of the tautomers is reversed, which indirectly drives the reaction via intermode coupling. The proposed potential deformation term accounts effectively for the excitation of all internal vibrational modes without increasing the dimensionality of the problem. The model yields information about reaction rates, explains the reaction irreversibility at low temperatures, and accounts for the presence of resonant processes.