arXiv subjects

Rositsa Yakimova

Publications and source records attributed to Rositsa Yakimova.

13 recordsLinked to original sources

arXiv2026

Real-Space Plasmon Imaging Reveals Modified Electronic Structure of Gold at the Monolayer Limit

Atomically thin materials exhibit electronic and optical properties distinct from their three-dimensional counterparts. For metals, particularly gold, monolayer studies remain largely unexplored due to fabrication and characterisation challenges. Here we report the first optical study of a stable quasi-freestanding gold monolayer formed by Au intercalation between graphene and SiC. Mid-infrared nanoimaging reveals plasmon-polaritons with wavelengths nearly an order of magnitude shorter than free-space light. Analysis of their dispersion using a Drude model yields a relaxation time of $τ= 18\,$fs, comparable to bulk gold, and a Drude weight of $D = 1.3\,$mS$\cdot$eV, nearly twice the bulk expectation. These results establish monolayer gold as a two-dimensional metal, opening opportunities for nanoscale photonics, plasmonics and ultra-thin electronics.

physics.optics↗

arXiv2023

Scalable fabrication of edge contacts to 2D materials

We present a fabrication method for reliably and reproducibly forming electrical contacts to 2D materials, based on the tri-layer resist system. We demonstrate the applicability of this method for epitaxial graphene on silicon carbide (epigraphene) and the transition metal dichalcogenides (TMDCs) molybdenum disulfide ($MoS_2$). For epigraphene, the specific contact resistances are of the order of $ρ_c$ ~ $50$ $Ωμm$, and follow the Landauer quantum limit, $ρ_c \propto n^{-1/2}$, with $n$ being the carrier density of graphene. For $MoS_2$ flakes, our edge contacts enable field effect transistors (FET) with ON/OFF ratio of $> 10^6$ at room temperature ( $> 10^9$ at cryogenic temperatures). The fabrication route here demonstrated allows for contact metallization using thermal evaporation and also by sputtering, giving an additional flexibility when designing electrical interfaces, which is key in practical devices and when exploring the electrical properties of emerging materials.

cond-mat.mes-hall↗

arXiv2022

Nature of photoexcited states in ZnO-embedded graphene quantum dots

The combination of wide-band gap semiconductors like zinc oxide (ZnO) and graphene quantum dots (GQDs) is a promising strategy to tune optoelectronic properties of GQDs and to develop new functionalities. Here we report on a theoretical design of not-yet-synthesized hybrid materials composed of ZnO clusters surrounded by carbon moieties, hereinafter referred to as ZnO-embedded graphene quantum dots. Their structure and light absorption properties are presented, with an in-depth analysis of the nature of the photoexcited states. The stability of the (ZnO)nC96-2n system with n=1, 3, 4, 7, 12 and 27 is investigated by estimating cohesive energy and performing vibrational mode analysis. A strong dependence of the structural and optoelectronic properties of the hybrid material on the amount of ZnO pairs is revealed and discussed. A strong light absorption and unexpected enhancement of Raman modes related to the vibrations in carbon moiety are observed for highly symmetric (ZnO)27C42 system that makes it an ideal study subject. Complementary excited state analysis, charge density difference (CDD) analysis and interfragment charge transfer analysis enabled reaching deep insights into the nature of the excited states. A dominating contribution of doubly degenerate locally excited states in broadband light absorption by (ZnO)27C42 is identified. The present results are helpful to elucidate the nature of the fundamental internal mechanisms underlying the light absorption in ZnO-embedded graphene quantum dots, thereby providing a scientific background for future experimental study of low-dimensional metal-oxygen-carbon materials family.

cond-mat.mes-hall↗

arXiv2022

Highly efficient UV detection in a metal-semiconductor-metal detector with epigraphene

We show that epitaxial graphene on silicon carbide (epigraphene) grown at high temperatures (T > 1850 °C) readily acts as material for implementing solar-blind ultraviolet (UV) detectors with outstanding performance. We present centimeter-sized epigraphene metal-semiconductor-metal (MSM) detectors with peak external quantum efficiency of ~ 85% for wavelengths 250-280 nm, corresponding to nearly 100% internal quantum efficiency when accounting for reflection losses. Zero bias operation is possible in asymmetric devices, with the responsivity to UV remaining as high as R = 134 mA/W, making this a self-powered detector. The low dark currents Io ~50 fA translate into an estimated record high specific detectivity D = 3.5 x 10^15 Jones. The performance that we demonstrate, together with material reproducibility, renders epigraphene technologically attractive to implement high-performance planar MSM devices with a low processing effort, including multi-pixel UV sensor arrays, suitable for a number of practical applications.

cond-mat.mes-hall↗

arXiv2020

Electron-phonon coupling of epigraphene at millikelvin temperatures

We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as in-situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the $T^4$ dependence characteristic for clean two-dimensional conductor. Based on our measurement we predict the noise-equivalent power of $\sim 10^{-22}~{\rm W}/\sqrt{\rm Hz}$ of epigraphene bolometer at the low end of achievable temperatures.

cond-mat.mes-hall↗

arXiv2020

Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation

We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of atomically thin Au on the Bu-L followed by annealing at 850 °C in an Argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterizations and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate tunable ambipolar transport across the Dirac point, and n- or p-type doping depending on the Au content.

cond-mat.mtrl-sci↗

arXiv2020

The performance limits of epigraphene Hall sensors

Epitaxial graphene on silicon carbide, or epigraphene, provides an excellent platform for Hall sensing devices in terms of both high electrical quality and scalability. However, the challenge in controlling its carrier density has thus far prevented systematic studies of epigraphene Hall sensor performance. In this work we investigate epigraphene Hall sensors where epigraphene is doped across the Dirac point using molecular doping. Depending on the carrier density, molecular-doped epigraphene Hall sensors reach room temperature sensitivities $S_V=0.23 V/VT$,$S_I=1440 V/AT$ and magnetic field detection limits down to $B_{MIN}=27$ $nT/\sqrt{Hz}$ at 20 kHz. Thermally stabilized devices demonstrate operation up to $T=150$ $^oC$ with $S_V=0.12 V/VT$, $S_I=300 V/AT$ and $B_{MIN}\approx 100$ $nT/\sqrt{Hz}$ at 20 kHz.

cond-mat.mtrl-sci↗

arXiv2020

Chemical sensing with atomically-thin metals templated by a two-dimensional insulator

Boosting the sensitivity of solid-state gas sensors by incorporating nanostructured materials as the active sensing element can be complicated by interfacial effects. Interfaces at nanoparticles, grains, or contacts may result in non-linear current-voltage response, high electrical resistance, and ultimately, electric noise that limits the sensor read-out. Here we report the possibility to prepare nominally one atom thin, electrically continuous metals, by straightforward physical vapor deposition on the carbon zero-layer grown epitaxially on silicon carbide. With platinum as the metal, its electrical conductivity is strongly modulated when interacting with chemical analytes, due to charges being transferred to/from Pt. This, together with the scalability of the material, allows us to microfabricate chemiresistor devices for electrical read-out of chemical species with sub part-per-billion detection limits. The two-dimensional system formed by atomically-thin metals open up a route for resilient and high sensitivity chemical detection, and could be the path for designing new heterogeneous catalysts with superior activity and selectivity.

cond-mat.mtrl-sci↗

arXiv2018

Wafer scale growth and characterization of edge specific graphene nanoribbons

One of the ways to use graphene in field effect transistors is to introduce a band gap by quantum confinement effect [1]. That is why narrow graphene nanoribbons (GNRs) with width less than 50nm are considered to be essential components in future graphene electronics. The growth of graphene on sidewalls of SiC(0001) mesa structures using scalable photolithography was shown to produce high quality GNR with excellent transport properties [2-7]. Such epitaxial graphene nanoribbons are very important in fundamental science but if GNR are supposed to be used in advanced nanoelectronics, high quality thin (<50nm) nanoribbons should be produced on a large (wafer) scale. Here we present a technique for scalable template growth of high quality GNR on Si-face of SiC(0001) and provide detailed structural information along with transport properties. We succeeded to grow GNR along both [1-100] and [11-20] crystallographic directions. The quality of the grown nanoribbons was confirmed by comprehensive characterization with high resolution STM, dark field LEEM and transport measurements.

cond-mat.mes-hall↗

arXiv2018

Detection of ultra-low concentration NO2 in complex environment using epitaxial graphene sensors

We demonstrate proof-of-concept graphene sensors for environmental monitoring of ultra-low concentration NO2 in complex environments. Robust detection in a wide range of NO2 concentrations, 10-154 ppb, was achieved, highlighting the great potential for graphene-based NO2 sensors, with applications in environmental pollution monitoring, portable monitors, automotive and mobile sensors for a global real-time monitoring network. The measurements were performed in a complex environment, combining NO2/synthetic air/water vapour, traces of other contaminants and variable temperature in an attempt to fully replicate the environmental conditions of a working sensor. It is shown that the performance of the graphene-based sensor can be affected by co-adsorption of NO2 and water on the surface at low temperatures (<70 C). However, the sensitivity to NO2 increases significantly when the sensor operates at 150 C and the cross-selectivity to water, sulphur dioxide and carbon monoxide is minimized. Additionally, it is demonstrated that single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene.

physics.ins-det↗

arXiv2018

Uniform doping of graphene close to the charge neutrality point by polymer-assisted spontaneous assembly of molecular dopants

Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC - achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly (methyl-methacrylate) - proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral graphene with carrier mobilities ~70,000 cm2/Vs at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin coating wafer-scale substrates in ambient conditions, opens up a scalable technological route towards expanding the functionality of 2D materials.

cond-mat.mes-hall↗

arXiv2017

Observation of Coulomb blockade in nanostructured epitaxial bilayer graphene on SiC

We study electron transport in nanostructures patterned in bilayer graphene patches grown epitaxially on SiC as a function of doping, magnetic field, and temperature. Away from charge neutrality transport is only weakly modulated by changes in carrier concentration induced by a local side-gate. At low n-type doping close to charge neutrality, electron transport resembles that in exfoliated graphene nanoribbons and is well described by tunnelling of single electrons through a network of Coulomb-blockaded islands. Under the influence of an external magnetic field, Coulomb blockade resonances fluctuate around an average energy and the gap shrinks as a function of magnetic field. At charge neutrality, however, conduction is less insensitive to external magnetic fields. In this regime we also observe a stronger suppression of the conductance below $T^*$, which we interpret as a sign of broken interlayer symmetry or strong fluctuations in the edge/potential disorder.

cond-mat.mes-hall↗

arXiv2011

Diffusion Limited Aggregation with modified local rules

Results from a modified Diffusion Limited Aggregation (DLA) model are presented. The modifications of the classical DLA model are in the attachment to the cluster rules and in the scheme of particle generation/killing. In the classical DLA model if a particle reaches the growing cluster it sticks to it immediately and irreversibly and then the next particle is released. We will abandon this original prerequisite, and by changing the sticking probability to the cluster we will change the diffusion regime towards more kinetic one. For a growing cluster consisting of only one type of particles this variation in the sticking probability is (more or less) a rude violation of the hypothesis for diffusion limitation in the DLA model. Since in a lot of experiments different types of particles are used with different sticking probabilities (e.g. different regimes of attachment), we develop a modified DLA model with two types of particles. The second modification we introduce at that point is a scheme for particle generation/killing we call "second chance" - when a particle is killed after reaching a given limiting distance from the cluster, it is killed and then returned to the point it was originally generated. Thus the model is capable to produce a great variety of growing patterns (fractals, spirals) by changing only a single parameter and we are able to construct a morphological diagram of our generalized DLA model with two different types of particles.

cond-mat.mes-hall↗