Ivor Lončarić

Femtosecond-laser-driven molecular dynamics on surfaces: Photodesorption of molecular oxygen from Ag(110)

We simulate the femtosecond laser induced desorption dynamics of a diatomic molecule from a metal surface by including the effect of the electron and phonon excitations created by the laser pulse. Following previous models, the laser induced surface excitation is treated through the two temperature model, while the multidimensional dynamics of the molecule is described by a classical Langevin equation, in which the friction and random forces account for the action of the heated electrons. In this work, we propose the additional use of the generalized Langevin oscillator model to also include the effect of the energy exchange between the molecule and the heated surface lattice in the desorption dynamics. The model is applied to study the laser induced desorption of O

Quasiparticle spectra and excitons of organic molecules deposited on substrates: G0W0-BSE approach applied to benzene on graphene and metallic substrates

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Imaging of Optically Active Defects with Nanometer Resolution.

from the Ag(110) surface, making use of a six-dimensional potential energy surface calculated within density functional theory. Our results reveal the importance of the phonon mediated process and show that, depending on the value of the electronic density in the surroundings of the molecule adsorption site, its inclusion can significantly enhance or reduce the desorption probabilities.

Strong Anisotropic Interaction Controls Unusual Sticking and Scattering of CO at Ru(0001)

We present an alternative methodology for calculating the quasiparticle energy, energy loss, and optical spectra of a molecule deposited on graphene o

Molecular dynamics simulation of O2 adsorption on Ag(110) from first principles electronic structure calculations

Point defects significantly influence the optical and electrical properties of solid-state materials due to their interactions with charge carriers, which reduce the band-to-band optical transition energy. There has been a demand for developing direct optical imaging methods that would allow in situ characterization of individual defects with nanometer resolution. Here, we demonstrate the localization and quantitative counting of individual optically active defects in monolayer hexagonal boron nitride using single molecule localization microscopy. By exploiting the blinking behavior of defect emitters to temporally isolate multiple emitters within one diffraction limited region, we could resolve two defect emitters with a point-to-point distance down to ten nanometers. The results and conclusion presented in this work add unprecedented dimensions toward future applications of defects in quantum information processing and biological imaging.

Strong two-dimensional plasmon in Li-intercalated hexagonal boron-nitride film with low damping

Complete sticking at low incidence energies and broad angular scattering distributions at higher energies are often observed in molecular beam experiments on gas-surface systems which feature a deep chemisorption well and lack early reaction barriers. Although CO binds strongly on Ru(0001), scattering is characterized by rather narrow angular distributions and sticking is incomplete even at low incidence energies. We perform molecular dynamics simulations, accounting for phononic (and electronic) energy loss channels, on a potential energy surface based on first-principles electronic structure calculations that reproduce the molecular beam experiments. We demonstrate that the mentioned unusual behavior is a consequence of a very strong rotational anisotropy in the molecule-surface interaction potential. Beyond the interpretation of scattering phenomena, we also discuss implications of our results for the recently proposed role of a precursor state for the desorption and scattering of CO from ruthenium.

Mystique world of acrobatic molecular crystals

We perform a detailed study of the static and dynamical properties of molecular oxygen adsorption on Ag(110) based on semi-local density functional theory (DFT) calculations and compare the results to experimental studies. For the classical dynamics calculations we use two complementary approaches, ab initio molecular dynamics and dynamics on a precalculated potential energy surface. In contrast to the molecular beam experiments, at low beam incidence energies we obtain high molecular adsorption probabilities that are related to the physisorption-like adsorption wells at the bridge sites of Ag(110). Semi-local DFT seems to overbind O2 in these wells. Based on our dynamics calculations we propose a model for adsorption in the chemisorption wells via initial adsorption in the bridge wells. In this model the measured low adsorption probabilities at low incidence energies are explained by the existence of energy barriers between the physisorption-like and chemisorption wells.

Benchmarking van der Waals functionals with noncontact RPA calculations on graphene-Ag(111)

The field of plasmonics seeks to find materials with an intensive plasmon (large plasmon pole weight) with low Landau, phonon, and other losses (small decay width). In this paper, we propose a new class of materials that show exceptionally good plasmonic properties. These materials consist of van der Waals stacked “plasmon active” layers (atomically thin metallic layers) and “supporting” layers (atomically thin wide band gap insulating layers). One such material that can be experimentally realized—lithium intercalated hexagonal boron-nitride is studied in detail. We show that its 2D plasmon intensity is superior to the intensity of well-studied Dirac plasmon in heavy doped graphene, which is hard to achieve. We also propose a method for computationally very cheap, but accurate analysis of plasmon spectra in such materials, based on one band tight-binding approach and effective background dielectric function.Plasmonics: Intercalated h-BN films support 2D plasmons with low dampingLithium intercalated hexagonal boron nitride (LiB2N2) gives rise to strong 2D plasmons. A team led by V. Despoja at Universidad del Pais Vasco and Zagreb Institute of Physics developed an ab-initio theoretical investigation of a class of materials displaying superior plasmonic properties. These are van der Waals plasmonic metallic layers, in combination with supporting layers that act as wide bandgap insulating materials. In LiB2N2 multilayers, the lithium layer represents the plasmon-active material, whereas two hBN layers are the insulating supporters. In the proposed computational methodology, the optical response of the plasmon-active layer is approximated to a 2D response function, whereas the supporting layers are accounted for by means of an effective background dielectric function. LiB2N2 can support 2D plasmons with an intensity superior to that of Dirac plasmons in heavily doped graphene.

Reversible Thermosalient Effect of N′-2-Propylidene-4-hydroxybenzohydrazide Accompanied by an Immense Negative Compressibility: Structural and Theoretical Arguments Aiming toward the Elucidation of Jumping Phenomenon

Everybody enjoys mystique and mystery that goes along with it, but they are hard to find outside the brother Grimm’s fairy tales. But upon a travel to a nanoworld, not unlike Gulliver’s voyage to Lilliput, in certain cases we can witness a truly fascinating and mystique phenomenon – a thermosalient effect. Thermosalient crystals, or more colloquially called jumping crystals, are intriguing materials that during heating/cooling exhibit joyful acrobatics in the form of hopping. These jumps are exhibited during the topotactic polymorphic phase transitions which are extremely fast and energetic so the crystals are balistically projected to heights several hundred times larger than their own dimensions. Thermosalient materials are also exhibiting huge technological potential because they are very promising candidates for fabrication of actuators on the microscopic level, such as nanoswitches, thermal sensors, artificial muscles, etc [1, 2]. In the past several years, a large number of experimental studies was performed with the to unveil the origin of the thermosalient effect, but still today the real reason for this phenomenon remains foggy and mysterious. It is becoming obvious that the experimental methods solely are not enough to successfully tackle this challenge so it seems opportune to try to complement these results with the theoretical studies, employing the highly developed DFT calculations. We present the results of such, to the best of our knowledge, first collaboration between experimental and theoretical studies performed on one of the thermosalient systems – N’-2-propylidene- 4-hydroxybenzohydrazide. This system experiences even three polymorphic thermosalient transition, one of them being irreversible and the other two reversible. As with the large majority of thermosalient compounds, it is also characterised with anisotropic thermal expansion. It shows immense negative uniaxial thermal expansion (along b direction), which we suspect is the governing force for the thermosalient phenomenon in this system. Our first principle electronic structure calculations prove that it is a direct consequence of the negative uniaxial compressibility. Elastic properties are also shown to be the origin of the reversibility or irreversibility of the thermosalient phase transitions. And finally, our DFT calculations suggest that excitations of the low-energy phonons provide a necessary trigger for the thermosalient effect in N’-2- propylidene-4-hydroxybenzohydrazide, propelling the system over the energy barrier between the thermosalient phases. [1] Skoko, Ž. et al. (2010). J. Am. Chem. Soc. 132, 14191-14202. [2] Nath, N. K. et al. (2014). CrystEngComm 2014, 1850-1858.