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March 2023. PhD and postdoc positions available
One PhD and one postdoc position are available immediately to work on machine learning and conductance. The PhD candidate will enroll in the doctoral program at a Czech university. The postdoc position is fully funded by a research grant and can be extended up to 3 years (until the end of 2025) upon mutual agreement. Interested candidates should describe their experience in ML methods and/or electronic structure calculations. Informal inquiries are welcome.
Oct. 2022. Mechanically tuning the thermopower of molecular junctions
Together with experimental colleagues from the Fujii group in Japan, we showed how the thermopower could be tuned via STM compression for a series of paradigmatic molecular junctions. A thermoelectric voltage arises, at zero bias, when applying a temperature difference across the junction since the Fermi distributions at both electrodes open new channels for transmission. For junctions containing benzene diamine, fullerene and bipyridine, the thermopower changed as the size of the nanojunction was changed by modifying the STM separation. Our contribution was to model and rationalize these results. We revealed how the energy of the conducting molecular resonance was tuned by changing the size of the tunneling gap. Our results showed, depending on the choice of molecular backbone, positive or negative thermoelectric coefficients which increased or decreased in absolute value as the junction was mechanically compressed. Link
Sept. 2022. DFT-based calculation of molecular conductance for thousands of junction geometries
Conductance calculations, which use DFT-NEGF methods, are very expensive computationally. This paper describes a way to calculate conductance from a small subsystem. The idea is (very broad) that Au contact atoms effectively act as source and drain, and that changes in backbone structure will be reflected in changes in conductance. The method had been developed in the past for small clusters, whose geometry was optimized while a single parameter was scanned. Now, however, there are no constraints or optimizations on the geometry, which is taken from room-temperature MD simulations of the junction with adatom tip structures and several Au layers. At each timestep, the geometry of the Au-molecule-Au cluster is extracted and its electronic properties calculated. A scaling factor is needed to bring the computed quantities to actual conductance values, which comes from a few DFT-NEGF calculations. With these, tens of thousands of DFT-computed conductance values were obtained, without resorting to Hückel Hamiltonians, which opens the way to calculating structure-conductance relationships. In addition, this paper was also a very satisfying experience, since this was my first (and, so far, only) single-author publication. It was nice to see the manuscript through all stages: conception, calculations, figures, and writing. Link
July 2022. Classical force field parameters for metal-molecule interfaces
Narendra’s paper on classical force field interface parameters is published. The high computational cost of DFT simulations of molecular junctions clearly motivates the use of classical MD simulations. These are much faster since the junction is described using a “balls and springs” model that replaces quantum chemical calculations. However, for a proper description, accurate parameters are clearly needed. Parameters sets are available for the molecule and for the bulk metal, but not for the metal-molecule interface. Here we described a method to calculate the parameters of bond, angular and dihedral interactions of interfaces with amine, methyl-sulfide and Au-C metal-molecule linkers. These were fitted to DFT-based cluster calculations. The novelty of the work likes in explicitly considering binding of the molecules to Au structures adsorbed on the surface, in contrast to binding on the more idealized Au(111) surface. With these parameter sets, we were able to perform nanosecond MD simulations of the junction at room temperature. These provide insight that is not accessible in DFT-based MD simulations, typically restricted to the picosecond timescale. Link
Feb. 2022. Congratulations Narendra for your PhD defense!
Narendra successfully defends his PhD thesis and is now a Doctor. Narendra published 5 papers in the group, and one more has been submitted. Well done, well earned, and congratulations!
Dec. 2021. Goodbye Wudmir!
The GAČR grant comes to an end. With the end of this cycle, Wudmir leaves our group.
Nov. 2021. Our DFT+Σtot method to calculate molecular frontier orbital energies at interfaces
In this paper we describe an extension to the DFT+Σ method, which is a self-energy based scheme to correct the deficiencies of DFT in the calculation of the energy of molecular resonances at metal interfaces. In the DFT+Σtot extension, we take into account the hybridization of molecular orbital with metal states and scale corrections to MO energies accordingly. We first compute the molecular character of each junction state through the dot product of all junction and all molecular states. We then apply corrections onto each junction state scaling them by how strongly they resemble molecular orbitals. We find that, even for the paradigmatic weakly-interacting interfaces considered here, there is substantial metal-molecule hybridization for some MOs, which motivates the DFT+Σtot approach. DFT+Σtot corrections act on the full junction Hamiltonian. This contrasts with DFT+Σ, which modifies only the molecular subspace. Within DFT+Σtot, molecular frontier levels are comparable to G0W0 and WKM, and closer to the Fermi level than in DFT+Σ. Link
Aug. 2021. Adsorption of carboranes on Si surfaces
In a paper with Antonín Fejfar, we computed the electronic structure of thiol-terminated carboranes on hydrogen-passivated Si(100) surfaces. We found that that the S atom in the carborane can displace the passivating H atoms and create strong, covalent S-Si bonds. Carborane chemisorption does not lead to a surface reconstruction or gap states. Upon adsorption, the value of the semiconducting gap is almost unchanged and still determined by Si states. However, the large electrostatic dipole of carborane shifts the spectrum downwards, effectively doping the interface. Our paper suggests that adsorption of molecules having a strong dipole such as carborane is a promising strategy to non-destructively dope semiconducting surfaces. Link
April 2021. Does antiaromaticity lead to increased conductance?
The higher conductance of antiaromatic molecules compared to their aromatic analogues was predicted in the 1970s by Breslow. We noticed a pattern in the results of our previous paper (the first study of a genuinely antiaromatic molecule!) and here we explored how general a phenomenon it is. We compared the π electronic spectrum and conducting properties of several aromatic, antiaromatic and quinoidal analogues. We found that the symmetry and nodal pattern of molecular orbitals is largely preserved between aromatic and antiaromatic counterparts: π states are (roughly) the same in both species, but in the aromatic species (4n+2 electrons) one more MO is fully occupied than in the aromatic molecule (4n electrons). Thus, we claim that antiaromaticity can be thought of as double p-doping of the analogue aromatic spectrum. Concerning conductance, for similar attachment points to the electrodes, either the aromatic or the antiaromatic molecule will exhibit destructive quantum interference features. The spectra and transmission properties of quinoids resemble those of antiaromatic analogues plus some MOs localized in the C=O groups. Antiaromaticity favors increased conductance compared to aromatic analogues due to the smaller energy gap, but interference features (present in one of the species) plays an important role. We showed how these, and thus the relative conductance of aromatic and antiaromatic molecules, could be tuned though the choice of linker groups. Altogether, our work illustrates a general relationship between the spectra of aromatic, antiaromatic and quinoidal molecules, and provides a unified picture showing the relationship between (anti)aromaticity, connectivity to the electrodes and quantum interference. Link
Jan. 2021. Calculated conductance of BDA junctions with different tip structures
This paper is part of a special issue edited by friend and colleague Linda Zotti. We studied the electronic structure and conductance of BDA junctions. Although this interface has been investigated many times, here we discussed the effect of atomistic tip structure in detail. We focused on the HOMO energy at the DFT level and also on its correction in the DFT+Σ formalism. We found that the contribution from metal screening, approximated here within a classical image-charge model, was most sensitive to tip termination. We also showed that, for BDA junctions, a Lorentzian approximation to the HOMO-derived transmission peak was a good approximation for all tips considered. Link
Jan. 2021. A donor-acceptor dyad adsorbed on a reactive metal surface
In collaboration with the STM group of Silvia Karthäuser and the synthetic group of Thomas J.J. Müller, we studied the electronic properties of a fused donor-acceptor dyad adsorbed on a Pt(111) surface. The novelty of our approach is to introduce a flexible bridge between donor and acceptor units, which allows for steric and electronic decoupling on the metal surface. The relatively few existing studies of donor and acceptor molecules on metallic substrates had focused on Au(111), whose weakly-interacting nature had helped preserve molecular electronic structure. In contrast, the highly-reactive Pt(111) surface we considered represents a more realistic description of actual catalyst conditions. Our work shows that, despite the reactive Pt surface, the donor-acceptor character can still be identified after adsorption due to the introduction of the flexible bridge. Link
Jan. 2021. Conductance of polyferrocenylene chains.
In a collaboration with our friends and neighbors from Pavel Jelínek’s group and Ivo Starý’s group we created and studied long polyferrocenylene chains. On-surface synthesis allowed us to grow on Ag(111) polyferrocenylene chains of up to 50 nm, where such lengths were not possible using traditional methods. The chains were lifted with the STM/AFM and their transport properties measured in I/V scans. My contribution was to design a model to interpret charge transport across the polyferrocenylene chains, which showed metallic-like behavior during the initial stages of the lifting process, and semiconducting-like characteristics when a longer segment of the chain had been picked up. Link
Nov. 2020. A new carbene on the Au surface
Together with the groups of Alberto Morgante, Latha Venkataraman, Dean Cvetko and Xavier Roy, we created a new stable carbene on the Au surface. Many recent studies of carbenes on surfaces, including our own, have focused on N-Heterocyclic carbenes (NHCs). Here we synthesize and adsorb the smallest possible carbene, cyclopropenylidene, on a Au(111) surface and fully characterize its structure using X -ray absorption spectroscopy, STM, AFM and DFT simulations. Our group carried out the calculations. We found that cyclopropenylidenes bind to Au even more strongly than NHCs, and that they adopt a tilted orientation on the surface. This study is the first surface modification with a carbene other than NHC and points to cyclopropenylidenes as suitable new surface ligands. Link
March 2020. Covid.
Like much of the world, we go into lockdown amid uncertainty and fear while he pandemic rages on. Home office, remote work and many video calls to friends and family.
Aug. 2019. Welcome Wudmir! Wudmir joins the group as a postdoc. He will work on developing approximations to single molecule conductance.
June 2019. New methodology for characterizing the adsorption site of molecules. In single molecule circuits, conductance depends strongly on the specific geometry at the interfaces. However, it is very difficult to characterize the geometry directly since some techniques used for films (eg. X-ray spectroscopy) cannot be applied to junctions, or due to the low temperature required by other techniques (~5K for IETS) which is much lower than usual room temperature experiments, and because the junction has a short lifetime. In collaboration with the Kiguchi group link from Tokyo Institute of Technology, we combined I-V measurements, surface enhanced Raman scattering (SERS), and DFT-based calculations to characterize the adsorption site of two paradigmatic molecules. Variations in conductance and in Raman frequencies characteristic of the molecule were measured in the experiment. DFT calculations of electronic and vibrational properties assigned these variations to changes in the interface geometry of the metal-molecule junction. The combination of these techniques makes it possible to distinguish between hollow, bridge or atop binding sites, and to monitor changes between these configurations. Through the application of a small voltage, it was possible to induce changes between the different binding sites. This new methodology enables the determination of the molecular binding site, thus addressing one of the main challenges in the realization of stable and reproducible single molecule circuits. Link pdf
Feb. 2019. Outstanding Reviewer for Phys. Chem. Chem. Phys. in 2018! I was named Outstanding Reviewer for Physical Chemistry Chemical Physics (PCCP) in 2018. I enjoy reviewing as it is interesting to read all these manuscripts, and because it allows me to help the community (after all, when I submit a paper, I would like it to be given proper attention). It does take time but I find it very rewarding. I appreciate the recognition by PCCP and I am very happy for this news! Link
Feb. 2019. Strong dipoles from carboranes adsorbed on Au surfaces. Carboranes are cage-like molecules where 2 C atoms and 10 B atoms are arranged in an icosahedral shape. Carboranes have delocalized orbitals and the different electronegativities of C and B can lead to strong permanent dipoles. When thiol-functionalized, carboranes can adsorb on surfaces and their large dipole is an obvious strategy to tune the work function. Here we consider two such carboranes, whose dipole can be pointing towards the surface or towards the vacuum. We sample a large number of adsorption geometries and calculate the junction electronic structure and work function changes. Carborane adsorption can modify the work function by as much as 1 eV in either direction, depending on dipole orientation. Interestingly, we find that the structure of the Au surface layer can be deformed by the adsorbed carboranes, and these changes can strongly influence interface electronic properties. Link pdf
Jan. 2019. Inelastic dips and peaks of Au-C bonded polyphenyls. Interaction of tunneling electrons and localized vibrations leads to characteristic IETS features. Inelastic spectra of organic molecules generally exhibit peaks but, in some cases where conductance is very high, dips can also appear. Here we consider conjugated molecules having one to four benzene rings bonded to electrodes with highly-conducting Au-C links. The shortest molecule (with 1 benzene ring) shows near unit transmission and IETS dips. As more rings are added to the backbone, conductance is reduced and peaks appear in IETS. Our calculations thus predict a crossover from IETS dips to peaks by simply extending the molecular backbone. While in the main result is that dips are replaced by peaks as the length of the molecule increases, a detailed analysis shows that some dips disappear, others evolve into peaks, and yet other peaks emerge. Our paper explains this in terms of the character of the mode, and rationalizes the behavior of IETS features as the length of the molecule is increased. Link pdf
Sept. 2018. Congratulations Enrique! Enrique wins a postdoctoral fellowship in the International Mobility Mezinárodní mobilita MSCA-IF II program, which is co-funded by the EU and the Czech Ministry of Education, Youth and Sports. Enrique will work for the next 2 years on electronic structure and transport modeling beyond a DFT description. We look forward to a productive fellowship!
July 2018. Single molecule circuits with N-Heterocyclic Carbene linkers. In this paper with the Roy and Venkataraman groups at Columbia University, we demonstrate for the first time NHCs as linkers for single molecule junctions. In JACS we show how NHC-bonded single molecule circuits can be made from a series of air-stable metal-NHC complexes that are reduced in situ. The conductance of the molecule is modulated by the identity of this single metal atom bonded to the NHC. Calculations substantiate the important role of the NHC group to the electronic properties and transport characteristics of the molecule, and demonstrate the strong electronic coupling of the NHC termination to the leads. Link pdf
The group in June 2018. Héctor, Giuseppe, Enrique and Narendra (not in picture: Martin).
June 2018. Goodbye, Giuseppe! Giuseppe’s Marie Skłodowska-Curie fellowship has come to an end. He joined the group at the start of 2015 under Czech (GAČR) funding. He then won the Marie Skłodowska-Curie fellowship, which lasted from July 2016 until the end of June 2018. In the 3.5 years he was in the group, he published 6 papers. In June we had a barbeque at the Institute together with our friends from the Nanosurf group to say goodbye! The grill was great, and we enjoyed some Bohemian champagne. Thanks Shayan Ed for the pictures!
May 2018. Workshop on European grants at the Institute. As supervisor of a Marie Skłodowska-Curie fellowship, I collaborated in a seminar on European grants at the Institute organized by the new grant support unit. The goal was to give an overview of Marie Skłodowska-Curie and ERC grants and to provide insight on the evaluation process. I contributed to the training session by steering a group of participants in evaluating some sections of a Marie Skłodowska-Curie application. The participants in my group were very stern reviewers! The workshop included Marie Skłodowska-Curie and ERC grantees, supervisors and evaluators. Hopefully this will result in many successful European grants!!!
May 2018. New predictions for current-induced vibrational instabilities in single molecule circuits. An electrical current can release energy to molecular vibrational modes and excite vibrations localized at the molecule. If this is not balanced by cooling mechanisms, no steady-state solution for the population of vibrational modes can be found, and the junction breaks. Previous works had shown that vibrational instabilities could be expected in donor -acceptor molecules where the donor state was lifted above the acceptor state by the applied bias, a regime known as population inversion. In J. Phys. Chem. Lett. we now show that vibrational instabilities are a much more general phenomenon than previously reported and that instabilities can occur in a broader class of molecules in a less restrictive physical regime. We calculate the rates of absorption and emission of vibrations and demonstrate vibrational instabilities for a small LUMO-conducting oligophenyl, without the requirements of donor-acceptor structure or population inversion. Bulky side groups were added, which result in a large twist angle between the benzene rings due to steric repulsion. This is important since it reduces conjugation and electronically separates states at both ends of the molecule, allowing for the tuning of their relative position under bias. In addition to DFT-NEGF calculations, we develop a two-site model which generalizes these findings, disentangles the effects of electronic properties and internal structure of vibrational modes, and maps junction (in)stability as a function of the different interface parameters. Link pdf
April 2018. Stable bipodal platform for single molecule transport based on biphenylene. In this paper we proposed a new molecular platform for single molecule conductance based on biphenylene. It is a bipodal platform with stable mechanical properties and highly-conducting Au-C bonds. Biphenylene is an antiaromatic molecule consisting of two benzene rings connected via single bonds: the chemical structure has 6-4-6 member rings. The conjugated system has 12 pi electrons, making it antiaromatic (4n pi electrons in Hückel’s rule). Since antiaromatic structures are unstable, in our work we considered that the antiaromatic instability of biphenylene leads to the breaking of Carbon-Carbon bonds inside the molecule. These Carbon atoms then form covalent bonds with the Au substrate. The resulting geometry has benzene rings with a large twist angle that stand almost upright on the surface, a good platform for contacting this molecule with an STM tip. For the top contact we investigated a series of chemical linkers spanning low to high interaction with the tip. We showed how the proposed molecular architecture based on biphenylene is very stable mechanically and highly transparent electronically, demonstrating its potential in single molecule transport studies. Link pdf
April 2018. Outreach and recruitment in the Physics Career Day. On April 24, I presented the group's research and PhD student activities in the Physics Career Day. This event was organized by Careermarket at the Charles University Faculty of Mathematics and Physics. Some of us from the Institute of Physics gave short presentations of PhD work in our groups intended for physics undergrads. I outlined for students what molecular transport theory consists on and mentioned the main research areas in the group. I also described the knowledge desirable for DFT-based simulations and what range of skills one might learn in a PhD.
Jan. 2018. Conductance of helical molecules and demonstration of the converse piezoelectric effect. In the converse piezoelectric effect, an applied voltage results in mechanical strain. Together with our colleagues from the groups of Pavel Jelinek here at the Institute of Physics and Ivo Starý at the Institute of Organic Chemistry and Biochemistry, we investigated this effect and the electron transport properties in helicene derivatives on a Ag substrate. These molecules consist of 7 benzene rings coupled together forming a helix. In contrast to many other conjugated molecules, their geometry is not planar but three-dimensional: helicene molecules have a spring-like structure, with each ring spiralling ‘upwards’ and where the seventh ring is positioned above the first one. These terminal rings have acetylsufanyl (-SCOCH3) groups and one of them binds to the Ag(111) surface. Individual molecules were contacted using simultaneous AFM/STM at 5K. Topographic images showed both bright and dark helicene molecules, whose appearance is related to the geometry of the linker group. A conformer with the top CO group pointing up (towards vacuum) appears bright on the STM, while if it points down it appears dark. At contact, the tip binds to the S atom and dark molecules have a higher conductance due to their shorter tunnelling distance. The formation of the metal-molecule bond was studied by AFM/STM at various applied voltages. We saw that the distance at which tip-molecule contact was established varied linearly with applied voltage, and showed that this is due to the bias-induced vertical deformation of the spring-like scaffold. Link pdf
Oct 2017. High-resolution submolecular imaging at 5K using the IETS amplitude. This paper combining measurements and simulations is a collaboration with our colleagues from Pavel Jelinek’s group here at the Institute and from the DIPC. In a seminal paper, W. Ho’s group showed that submolecular structure could be imaged from the spatial variation of the IETS frequencies. However, the high resolution needed to track the changes in mode energies at different tip positions was only possible at sub-Kelvin temperatures, limiting the wider application of this technique. Here we show how submolecular resolution is possible at 5K from the spatial variation of the IETS intensity. We approached a FePc molecule on Au with CO-functionalized tip: as the lateral position of the CO+tip was changed, the amplitude of the FR modes of CO remained almost constant, while that of the FT modes changed substantially. The contribution of our group was to explain the sensitivity of these modes to tip position. We used a technique we had developed previously to map the spatial origin of the IETS peaks and found clear differences between FT and FR modes. FR modes are generated mostly on the CO molecule and tip, and the contribution from other atoms is small. To correctly describe the intensity of FT modes, on the other hand, it is necessary to also include the interaction of the CO molecule with the molecular substrate. FT modes will therefore be more sensitive to the lateral position of the CO molecule relative to the molecule. Our colleagues also extended their probe-particle model to describe the common imaging mechanism for all three STM, AFM and IETS channels. Altogether, this paper demonstrates that it is possible to achieve submolecular resolution at 5K comparable to AFM from the spatial variation of the IETS amplitude. Link pdf
Oct 2017. Paper on the role of molecular adsorbates in current-induced cooling. In the past we had studied how, when applying a voltage across a molecular junction, the electrical current could heat (but also cool!) the junction. In this new paper, we again consider a biscarbene molecule but this time with an electron-withdrawing NH2 species adsorbed on only one of the tip terminations. We compare the changes in current-induced energy exchange of this system with the ‘clean’ junction (which has no co-adsorbed species). Despite the absence of chemical bonds between the bridging molecule and the adsorbate, the NH2 group in the vicinity results in the cooling of biscarbene modes at all applied voltages. The population and energy stored in biscarbene modes are smaller with co-adsorbed NH2 than for the clean junction, and NH2 modes are heated while biscarbene modes cooled with applied voltage. We find that this is a rather indirect effect, caused by the shift in the biscarbene DOS induced by the presence of the NH2 species, while the effect of the adsorbate DOS itself is small. By changing the nature of the adsorbate (from electron-donating to withdrawing), it should be possible to tune the heating and cooling of molecular junctions. Link pdf
July 2017. Conductance of an antiaromatic molecule measured for the first time! Together with our Japanese collaborators the Kiguchi group from Tokyo Institute of Technology and the Shinokubo group from Nagoya University, we study the conductance of a genuinely antiaromatic molecular circuit for the first time. Cyclic and planar molecules are said to be aromatic if they have 4n+2 pi electrons, and antiaromatic if they have 4n pi electrons. There are many aromatic species in molecular nanoscience (oligophenyls, thiophenes, porphyrins…) but antiaromatic molecules are very unstable and hard to synthesize. Antiaromatic molecules had been predicted by Breslow in the 70s to be highly conducting based on electrochemical measurements of oxidation/reduction potentials. In single molecule circuits it was recently shown that less aromatic molecules are more conducting but no genuinely antiaromatic molecule had been measured up to now. In our work we compare porphyrin (aromatic) and norcorrole (antiaromatic) units. The norcorrole core has two fewer Carbon atoms than the porphyrin (thus going from 4n+2 to 4n) but is structurally similar. The antiaromatic molecule is ~25× more conducting than its aromatic counterpart due to a more favorable alignment of frontier orbitals at the interface. Link pdf
May 2017. Our paper on Au-C bonds in fullerenes is out! We form fullerene-based structures on Au(111) which are stable at room temperature against diffusion on the surface. We do this by low-energy Ar⁺ sputtering of fullerene films. Sputtering at these low energies (120 eV) was shown in Carbon materials to result in predominantly single vacancy defects. After sputtering we see bright spots on the surface that do not diffuse at room temperature. DFT calculations show that molecules with one missing atom are much more stable at the surface than fullerenes or than molecules with double vacancies. This increased binding comes from the saturation of the bonds around the defect by the Au surface. We interpret the observed features as adsorbed fullerene-based molecules with C vacancies. This is a collaboration with the Nanosurf group here at the Institute, who did the measurements. Link pdf
Jan. 2017. Welcome Enrique! Enrique joins the group as a postdoctoral researcher. He will work on single molecule transport theory. Group
Dec. 2016. Our paper on current-induced cooling is out! We calculate the current-induced excitation and damping of vibrational modes in a carbene-based molecular circuit. Previously, we had seen that for this system, the width and position of the LUMO resonance was determined by the atomistic details of the tip (adatom, pyramid,...). Here we calculate the energy exchange between electronic and vibrational degrees of freedom. Energy transferred to vibrational modes will heat the junction, while in the opposite case it is released in the electrodes far from the junction, which is effectively cooled. We find that for chain-like tip structures (which can result from the stretching of strong Au-C bonds), current can cool the junction at high applied voltage. Current-induced heating and cooling processes are important as they strongly influence the stability of the molecular junction under an applied voltage. Link pdf
July 2016. Postdoc position available! A postdoc position is available in the group. Candidates should have experience with first-principles simulations. Preference will be given to candidates with expertise in (Tran)SIESTA codes and FORTRAN programming. Informal inquiries are welcome.
July 2016. Our paper on the local contributions to IETS is out! Here we completely characterize the different contributions to the inelastic spectrum of a representative molecular junction. These contributions encode information on both the vibrational modes and the electronic structure and thus go beyond the usual figures of the modes. Since we use a local orbital basis, we can determine the spatial origin of all contributions to the vibrational modes, mapping the origin of the inelastic signal. We thereby provide, across all vibrational modes, a quantitative relation between the degree of symmetry of each mode, its inelastic signal, and the locality of selection rules. Link pdf
July 2016. Giuseppe starts his Marie Skłodowska-Curie fellowship! In this project we will study current-induced vibrational heating and cooling of molecular junctions. The interaction of vibrational degrees of freedom with the tunneling electrons can result in the heating and, under certain circumstances, even cooling of the junction, and the endless number of chemical structures opens many possibilities to tune and control these processes. To study the electronic structure and the inelastic processes associated to the emission and absorption of molecular vibrations, we will carry out first-principles simulations based on DFT. We look forward to a fruitful fellowship!
Feb. 2016. Our paper on tip-induced gating of carbene-based junctions is out! We calculate the transport properties of N-heterocyclic carbene-based (NHC) molecules on gold and see a very strong dependence of the LUMO position on the atomistic details of the tip. Depending on whether the tips have tetrameric, pyramidal, chain-like or adatom terminations, the position of the LUMO shifts by almost ~0.8 eV. We explain these changes through an analysis of the electron density difference. Since transport is LUMO-derived, the calculated transmission at the Fermi level changes by a factor 8× and the tip is effectively gating the carbene-based circuit. Link pdf
Feb. 2016. Welcome Narendra! Narendra joins the group. He will work on using and developing DFT-based methods to calculate conductance single in molecular junctions. Group
Nov. 2015. Welcome Martin! In addition to his experimental work on nanoprobes in the group of Antonín Fejfar, we are happy Martin is interested in carrying out ab-initio simulations. He will calculate the interface properties of carboranes at metal surfaces. Group

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