Research

One of the important features of graphene and related low-dimensional objects is the relatively simple tunability of their electronic structure, an asset that extends the usability of these materials even further beyond present experience. A direct injection of charge carriers into the conduction or valence bands, that is, doping, represents a viable way of shifting the Fermi level. In particular, electrochemical doping should be the method of choice, when higher doping levels are desired and when a firm control of experimental conditions is needed. The electrochemical experiments can be coupled in-situ to spectroscopic methods like Raman spectroscopy, photoluminescence or UV/Vis/NIR absorption spectroscopy. The in-situ spectroelectrochemistry is a very powerful combination of methods, potentially delivering direct information on the band filling, disorder and related phenomena as the function of charge.

Enhanced Raman spectroscopy is a valuable tool for detection of low concentrations of molecules. For this reason, surface-enahnced Raman spectroscopy (SERS) it is a suitable method for inspecting functionalized graphene. However, for a proper understanding of the metal-graphene interactions we focused also on the the enhancement of graphene itself. For example, we investigated the SERS effect with isotopically labelled bilayer graphene, and its behaviour under different excitation wavelengths and plasmonic metals. The Raman signal of molecules is enhanced also in the vicinity of graphene by graphene-enhanced Raman scattering (GERS). In our work we experimentally probed the theoretical expectaitons fro GERS using functionalized graphene.

Magnetism in carbon-based materials is in the spotlight of current science; however, still a matter of controversy. While ferromagnetism has been confirmed in defected graphite, magnetism enthusiastically reported for carbon nanotubes was later excluded by synchrotron radiation experiments, which revealed presence of metallic magnetic impurities. The magnetic myths also rised for graphene, for which several scenarios of apparently exotic magnetic order appeared, without a serious experimental evidence and relevant theoretical support. We aim to fair approach to this issue by employing ultra-sensitive micro-squid and spin resonance techniques, as well as by using spin-resolved AR PES, neutron scattering and synchrotron scattering methods, such as XMCD and LMD.

Energy storage is one of the most important topics in today’s R&D, where carbon-based materials play an irreplaceable role. Graphite is widely used as intercalation material for anodes in Li-ion batteries, different kinds of sp2 carbons are employed as conductive additives in both anodes and cathodes in these batteries, and various porous carbon materials found their way into supercapacitors. Graphene has a prominent place in this regard, because its relative surface area (2630 m2/g) is essentially unrivalled. Even though the main focus in the Department of Electrochemical materials in the field of Li-ion (or Na-ion) batteries is on the binary and ternary oxides, phosphates and similar ones, the mutual interactions between those and the conductive additives is still of our concern. Furthermore, we are studying porous carbon-based materials based on nanodiamond for the supercapacitors.

In the Department of Low-dimensional Systems focus is also kept on sensorics. Graphene, carbon nanotubes and other nanocarbon material evince promising electronic and chemical properties which could be efficiently used in sensorics. Detection of ppm concentration of target gas species was sufficiently realized using carbon nanostructures. Graphene functionalized with polymers showed promising gas adsorption/desorption behavior as well. In the presence research is mostly focused on fundemental study of gas adsorption to graphene and its impact to graphene’s electronic band structure. For that purpose graphene field-effect-transistors are fabricated and their transport characteristics are measured in-situ during gas adsorption/desorption. These effects could be also studied at different temperatures. The lab is also equiped with microfluidics devices for liquid sensing.

The use of the 2D materials in different types of solar cells has been shown to improve some of the designs, or at least to provide an alternative to expensive materials in the others. They can be potentially implemented in the silicon technology to replace the top layer, during the production of which (by switching of the dopant) “dead” layers on the interface are formed. Graphene can form Schottky junctions with silicon, while the common TMDCs like MoS2 form p-n junctions. In dye-sensitized solar cells (DSSC) graphene can be used at the counter electrode, with both the conductivity and catalytic activity at least on par with the commonly used platinum, for some electrolyte/redox mediators even surpassing it. In the so much promising perovskites graphene can serve both as electron and hole conductor.

Our activities are spread into all the above mentioned topics: we try to apply and modify the concept of 2D material implementation in silicon solar cells by looking into cheaper alternatives to bulk crystalline Si, and then by tweaking and controlling the 2D/3D interface. In DSSCs, the search for more stable and efficient electrolyte/mediator/dye combinations needs to be accompanied by optimization of the whole design and graphene finds the use for example when combined with Co-based redox mediators. In relation with two dimensional (2D) materials perovskite can also have a “layered” structure, the simplest of which is called Ruddlesden-Popper phase. In this phase the organic and inorganic part of the perovskite form alternate layers. Smart choices of the perovskite precursors lead to fine modifications of the structure and the related properties like tuning the distance between the layers, the photoluminescence and the affinity to certain substrates. In this context we are studying some classes of these layered perovskites and trying to combine them with graphene foreseeing possible develops in optoelectronic devices.

Mechanical deformation is capable of modifying the crystal structure and in turn also the (opto)electronic structure of 2D materials both globally and locally. It is thus crucial to monitor and control the deformation for any envisioned application of these materials, in a way that their unique properties are not compromised. On the other hand, their applicability can be even boosted by a smart choice of the deformation type and its level. For example, in graphene, uniaxial in-plane strain leads to symmetry breaking, elongating the C-C bonds in one direction and shortening them in the perpendicular one. This can – theoretically – lead to band gap opening when high enough elongation is reached, albeit the threshold is at the very limit of graphene’s predicted strain at failure. Bilayer graphene is more susceptible to the effects of stress: inhomogeneous uniaxial strain can lead to band-gap opening at much lower levels then those needed for monolayer graphene. Uniaxial out-of-plane compression in twisted bilayer graphene causes changes in the optical transition energies, otherwise controlled only by the misorientation angle between the two layers. In transition metal dichalcogenides, in-plane tension closes their band-gap, while compression opens it. Uniaxial out-of-plane compression in TMDCs closes the band-gap very quickly, and transition from semiconductor to semimetal is possible at quite low pressures. The stress can be relaxed by out-of-plane corrugations like ripples, wrinkles or folds, which alter the global mechanical properties as well as conductivity (for the worse), adhesion to the substrate (for the better) or local chemical potentials where large stress is accumulated. Sharp corrugations can even lead to phonon confinement.

We are looking both into the changes of the lattice under various deformations, ideally in-situ using Raman spectroscopy and photoluminescence, and into the possible ways of manipulating the structure permanently, mostly through substrate modification by thermal processing, patterning etc.

2D materials are fascinating emerging materials with amazing features for an exhaustive range of fields. Tuning of the material’s properties is crucial to unlock the potential for each particular application. It can be realized by several methods, among which covalent chemical functionalization provides unlimited pool of species to be grafted onto the surface. These species inherently modify the material and also introduce moieties which offer new functionalities to the material.

Functionalization of graphene is one of the most efficient methods allowing modification of its properties what may broaden the range of its applications. It has been already found that the hydrogenation of graphene leads to the opening of the bandgap, enhancement of the spin-orbit coupling and appearance of the magnetic moment.

Moreover, the prior hydrogenation of graphene can open the possibility for its further functionalization with compounds that do not react with pristine graphene.

We are developing approaches for covalent functionalization of graphene, MoS2 and related materials. We have demonstrated successful attachment of molecules by three mechanistically different protocols: radical C-C bond formation, nucleophilic exchange on fluorinated graphene, and electrophilic substitution of partly hydrogenated graphene. These three methods are orthogonal in terms of processing and compatibility with functional chemical groups.

The large scale production of graphene for electronic devices relies on catalytic chemical vapor deposition (CVD). Therefore, main attention is dedicated to understand the mechanism of the graphene formation and also to control the growth. Nevertheless, in spite of many efforts put into the graphene CVD research, there are still many challenges to be solved. Cu or Ni are the most widely used catalysts due to their low cost, etchability and large grain size. Depending on the catalyst, two mechanisms of the graphene growth are proposed. In the case of Ni, the precursor is decomposed at the surface and carbon is dissolved in the metal. When the substrate is cooled down, the solubility of C in Ni decreases and graphene first segregates and then grows on Ni surface. Hence, it is very important to control the cooling conditions to reach a monolayer graphene (1-LG). On the other hand, in the case of copper catalyst, the carbon intermediate is not dissolved in the metal since the solubility of C in Cu is negligible even at a very high temperature. Instead, the carbon atoms form graphene directly on the surface already at high temperature, i.e. there is no need to precisely control the cooling of the metal. The CVD on copper is suggested to be surface mediated and self-limiting, once the monolayer is completed, the process does not propagate any more, since the catalytic Cu surface is blocked. Hence, only 1-LG should be formed by the Cu-catalyzed CVD, but in many cases small regions with double- or multilayers are observed. The mechanism of the formation of a multilayer regions is not well understood yet. These multilayer regions may impede the fabrication of graphene devices on large scale, because the multilayer areas disturb the uniformity of the graphene film.

Graphene is a one-atom-thick planar sheet of sp2-bondedcarbon atoms densely packed in a hexagonal honeycomb crystal lattice. Although graphene had been studied theoretically for decades, its actual existence wasn't proven until 2004, when Kostya Novoselov and Andrei Geim from the Manchester University managed to isolate a monolayer for the first time. In 2010 they were awarded the Nobel Prize. In our group we are interested in exploring and exploiting some of the exceptional properties of graphene - at the moment we focus on mechanical and electrochemical alteration of its electronic structure and we try to observe the response to these changes by various methods, but mainly with Raman spectroscopy. We prepare our samples either ourselves by CVD or mechanical exfoliation.