Au Intercalation Research Articles

Gold intercalation of different 6H-SiC(0001) surface reconstructions

We report on in situ investigation of Au intercalation of different zero-layer graphene surface reconstructions of 6H-SiC(0001): (3×3)R30∘+(63×63)R30∘, (63×63)R30∘+(5×5)R0∘, (63×63)R30∘+(5×5)R0∘+Gr. We show that the most appropriate reconstruction for graphene formation via Au intercalation is a one with some fraction of initial graphene monolayer. A well-ordered epitaxial Au(111) islands on the zero-layer graphene were revealed with the same crystallographic orientation as the SiC surface. Ab initio calculations approve that the value of Rashba splitting in graphene increases with the density of Au atoms per unit cell and with the reduction of average distance between Au and graphene.

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity.

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Defect Induced Unconventional Ferromagnetism in Au Intercalated Bi2Se3 Nanocrystals for Spintronic Applications

AbstractBreaking time‐reversal symmetry through magnetic impurities has been a useful strategy for achieving exotic physical phenomena in topological insulators for a long time. In contrast, reports on achieving ferromagnetism in topological insulators by introducing non‐magnetic elements are very scarce. The present article illustrates Au ion implantation as an alternative tool to study the structural and magnetic properties in the Bi2Se3 nanocrystals grown by the chemical vapor deposition. The as‐grown pristine Bi2Se3 exhibits triangular and hexagonal plate‐like morphology, which undergoes structural distortion with increased Au ions. The room temperature Raman spectra show defect‐induced phonon softening with ion fluence. The non‐magnetic Bi2Se3 exhibit room temperature ferromagnetic hysteresis that originates from Bi vacancies which is verified from the density functional theory (DFT)calculations. The incorporation of Au ions initially decreases the magnetic moment up to the fluence 1 × 1014 ions cm−2 and enhances at higher fluences due to the formation of Au nanoclusters. The experimental and DFT results suggest a competition between the Se “4p” states and Au “5d” states for the non‐monotonic behavior of magnetic properties. The present work on Au intercalation in Bi2Se3 can pave the way to design artificial topological insulator devices for potential technological applications for future spintronics and quantum computations.

Electrochemical performance of gold-decorated graphene electrodes integrated with SiC

Here we investigate the interface properties of gold (Au) decorated graphenized surfaces of 4H-SiC intended for electrochemical electrodes. These are fabricated using a two-step process: discontinuous Au layers with a nominal thickness of 2 nm are sputter-deposited onto 4H-SiC substrates with different graphenization extent—zero-layer graphene (ZLG) and monolayer epitaxial graphene) —followed by thermal annealing. By performing combined morphometric analysis, Raman mapping analysis, conductive atomic force microscopy, and electrochemical impedance spectroscopy measurements, we shed light on the relationship between physical processes (Au intercalation, particle re-shaping, and de-wetting) caused by thermal annealing and the intrinsic properties of graphenized SiC (vertical electron transport, charge-transfer properties, vibrational properties, and catalytic activity). We find that the impedance spectra of all considered structures exhibit two semicircles in the high and low frequency regions, which may be attributed to the graphene/ZLG/SiC (or Au/graphene/ZLG/SiC) and SiC/ZLG/graphene/electrolyte (or SiC/ZLG//Au/electrolyte) interfaces, respectively. An equivalent circuit model is proposed to estimate the interface carrier transfer parameters. This work provides an in-depth comprehension of the way by which the Au/2D carbon/SiC interaction strength influences the interface properties of heterostructures, which can be helpful for developing high performance catalytic and sensing devices.

Влияние интеркаляции золота на электронную структуру графена на Co-Si/SiC(0001)

Investigation of graphene on an ultrathin CoSi/CoSi2 cobalt silicide layer on a SiC(0001) substrate after the Au intercalation has been carried out. It was shown that deposition of Au and subsequent annealing of the system at a temperature of 500°C leads to the intercalation of Au atoms and the formation of gold silicide under graphene with a stoichiometry close to that of Au2Si. The study of the electronic structure of the system in the region of the K point of the surface Brillouin zone revealed the quasi-freestanding character of graphene with a linear spectrum of π states and a Dirac point near the Fermi level. The measurements using photoelectron microscopy showed the homogeneity of the work function along the surface of the sample on a micrometer scale.

Femtosecond Laser Irradiation-Mediated MoS2-Metal Contact Engineering for High-Performance Field-Effect Transistors and Photodetectors.

2D materials exhibit intriguing electrical and optical properties, making them promising candidates for next-generation nanoelectronic devices. However, the high contact resistance of 2D materials to electrode material often limits the ultimate performance and potential of 2D materials and devices. In this work, we demonstrate a localized femtosecond (fs) laser irradiation process to substantially minimize the resistance of MoS2-metal contacts. A reduction of the contact resistance exceeding three orders of magnitude is achieved for mechanically exfoliated MoS2, which remarkably improves the overall FET performance. The underlying mechanisms of resistance reduction are the removal of organic contamination induced by the transfer process, as well as the lowering of Schottky barrier resistance (RSB) attributed to interface Fermi level pinning (FLP) by Au diffusion, and the lowering of interlayer resistance (Rint) due to interlayer coupling enhancement by Au intercalation under fs laser irradiation. By taking advantage of the improved MoS2-metal contact behavior, a high-performance MoS2 photodetector was developed with a photoresponsivity of 68.8 A W-1 at quite a low Vds of 0.5 V, which is ∼80 times higher than the pristine multilayer photodetector. This contamination-free, site-specific, and universal photonic fabrication technique provides an effective tool for the integration of complex 2D devices, and the mechanism of MoS2-metal interface modification reveals a new pathway to engineer the 2D material-metal interface.

Clustering and Morphology Evolution of Gold on Nanostructured Surfaces of Silicon Carbide: Implications for Catalysis and Sensing

A fundamental understanding of the behavior of gold (Au) nanostructures deposited on functional surfaces is imperative to discover and leverage interface-related phenomena that can boost the efficiency of existing electronic devices in sensorics, catalysis, and spintronics. In the present work, Au layers with nominal thickness of 2 nm were sputter-deposited on graphenized SiC substrates represented by buffer layer (BuL)/4H-SiC and monolayer epitaxial graphene (MLG)/4H-SiC. Morphometric analysis by means of scanning electron microscopy shows that Au on BuL self-assembles in nearly round-shaped plasmonically active islands, while on MLG, a fractal growth of considerably larger and ramified islands is observed. To correlate the experimentally established differences in surface morphology on the two types of graphenized substrates with energetics and kinetics of Au nanostructure growth, the deposit–substrate interaction strength was studied using density functional theory (DFT) calculations, molecular dynamics simulations, and optical measurements. The theoretical considerations involve participation of Au clusters with different sizes and energetics at the initial stages of the metal nanostructure formation. The results indicate that gold exhibits a considerably stronger interaction with BuL than with MLG, which can be considered as a key aspect for explaining the experimentally observed morphological differences. From the statistical analysis of Raman spectra, indications of Au intercalation of MLG are discussed. The current research shows that, due to its unique surface chemistry, buffer layer has peculiar affinity to gold when compared to other atomically flat surfaces, which is beneficial for boosting high-performance catalytic and sensing technologies based on low-dimensional materials.

Graphene p-n junction formed on SiC(0001) by Au intercalation

We propose a method to fabricate the chemical p-n junction in wafer-scale epitaxial graphene. In the case of Au intercalation in between graphene and SiC(0001), there exist two structurally distinct phases that result in p-type and n-type doping in the graphene layer, respectively. In the process of in situ Au deposition on our samples, we used a shadow mask to form a sharp junction of different Au coverage. The intercalation of Au atoms induced by thermal annealing leads to the abrupt p-n junction in the graphene layer, which is characterized by angle-resolved photoemission spectroscopy. This p-n junction of graphene is abrupt in the scale comparable to the beam size of approximately 50 μm. This p-n junction of graphene is expected to be atomically abrupt, since there exist only two structurally distinct phases by self-assembly. The proposed method may be useful not only to fabricate a wafer-scale p-n junction of graphene, but also for a fundamental study on atomically abrupt graphene p-n junction.

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 {\deg}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.

Intercalation of Au Atoms into SiC(0001)/Buffer Interfaces-A First-Principles Density Functional Theory Study.

The process of Au intercalation into a SiC/buffer interface has been theoretically investigated here by using density functional theory (DFT) and the nudged elastic band (NEB) method. Energy barriers were at first calculated (using NEB) for the transfer of an Au atom through a free-standing graphene sheet. The graphene sheet was either of a nondefect character or with a defect in the form of an enlarged hexagonal carbon ring. Defects in the form of single and double vacancies were also considered. Besides giving a qualitative prediction of the relative energy barriers for the corresponding SiC/buffer interfaces, some of the graphene calculations also proved evidence of energy minima close to the graphene sheet. The most stable Au positions within the SiC/buffer interface were, therefore, calculated by performing geometry optimization with Au in the vicinity of the buffer layer. Based on these NEB and DFT calculations, two factors were observed to have a great influence on the Au intercalation process: (i) energy barrier and (ii) preferential bonding of Au to the radical C atoms at the edges of the vacancies. The energy barriers were considerably smaller in the presence of vacancies. However, the Au atoms preferred to bind to the edge atoms of these vacancies when approaching the buffer layer. It can thereby be concluded that the Au intercalation will only occur for a nondefect buffer layer when using high temperature and/or by using high-energy impacts by Au atoms. For this type of Au intercalation, the buffer layer will become completely detached from the SiC surface, forming a single layer of graphene with an intact Dirac point.

Characterization of Au intercalation at the interface of graphene/polycrystalline Ni substrate

Intercalation of metals at a graphene/substrate interface can modify the electronic and vibration properties of the graphene. We investigated the intercalation of Au at the graphene/polycrystalline Ni interface by scanning electron microscopy, transmission electron microscopy, and Raman scattering spectroscopy. Graphene coverage induces step bunching on the poly-crystalline Ni surface. Au intercalation selectively occurs at the graphene/Ni(111) facets and changes the secondary electron contrast as well as recovering intrinsic graphene Raman signals. Au nanoparticles are observed only on the Ni(111) facets and are absent on the narrow step-bunched regions. High-angle annular dark field images indicates the presence of monolayer Au only at the graphene/Ni(111) interface.

Au intercalation under epitaxial graphene on Ru(0001): The role of graphene edges

The intercalation mechanism of Au at the graphene-Ru (0001) interface is studied at elevated temperature using spectromicroscopy with high lateral resolution. Above 970 K, Au is shown not to stick to graphene-covered regions as expected from the weak Au-graphene binding, which is crucial to single out the graphene edges as the only intercalation channel. The modified Stranski-Krastanov Au growth on Ru (0001) with two atomic wetting layers followed by 3D islands is observed on the Ru (0001) surface partially-covered with micron sized graphene flakes. The Au layers nucleate in and out of the graphene islands following a precise order with a direct correlation to the Au adsorption energy expected in the various configurations. In particular, the adsorption energy is found to decrease going from monolayer Au on Ru (0001) to monolayer Au intercalated at the graphene-Ru interface, and further to Au bilayer on Ru (0001). Moreover, the chemical maps at different stages during Au growth point to a pronounced kinetic barrier at the graphene edges preventing intercalation before the Au monolayer is complete outside the graphene-covered regions.

The morphology of an intercalated Au layer with its effect on the Dirac point of graphene

This is a theoretical investigation where Density Functional Theory (DFT) has been used in studying the phenomenon of Au intercalation within the 4H-SiC/graphene interface. The electronic structure of some carefully chosen morphologies of the Au layer has then been of special interest to study. One of these specific Au morphologies is of a more hypothetical nature, whilst the others are, from an experimental point of view, realistic ones. The latter ones were also found to be energetically stable. Band structure calculations showed that intercalated Au layers with morphologies different from a planar Au layer will induce a band gap at the Dirac point of graphene (with up to 174 meV for the morphologies studied in the present work). It should here be mentioned that this bandgap size is four times larger than the energy of thermal motion at room temperature (26 meV). These findings reveal that a wide bandgap at the Dirac point of graphene comes from an inhomogeneous staggered potential on the Au layer, which non-uniformly breaks the sublattice symmetry. The presence of spin-orbit (SO) interactions have also been included in the present study, with the purpose to find out if SO will create a bandgap and/or band splitting of graphene.

Controlled one step thinning and doping of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have drawn intensive attention due to their ultrathin feature with excellent electrostatic gating capability, and unique thickness-dependent electronic and optical properties. Controlling the thickness and doping of 2D TMDCs are crucial toward their future applications. Here, we report an effective HAuCl4 treatment method and achieve simultaneous thinning and doping of various TMDCs in one step. We find that the HAuCl4 treatment not only thins thick MoS2 flakes into few layers or even monolayers, but also simultaneously tunes MoS2 into p-type. The effects of various parameters in the process have been studied systematically, and an Au intercalation assisted thinning and doping mechanism is proposed. Importantly, this method also works for other typical TMDCs, including WS2, MoSe2 and WSe2, showing good universality. Electrical transport measurements of field-effect transistors (FETs) based on MoS2 flakes show a big increase of On/Off current ratios (from 102 to 107) after the HAuCl4 treatment. Meanwhile, the subthreshold voltages of the MoS2 FETs shift from −60 to +27 V after the HAuCl4 treatment, with a p-type doping behavior. This study provides an effective and simple method to control the thickness and doping properties of 2D TMDCs, paving a way for their applications in high performance electronics and optoelectronics.

First-principles study of electronic structure modulations in graphene on Ru(0001) by Au intercalation

First-principles calculations based on density functional theory are used to explore the electronic-structure modulations in graphene on Ru(0001) by Au intercalation. We first use a lattice-matched model to demonstrate that a substantial band gap is induced in graphene by sufficiently strong A-B sublattice symmetry breaking. This band gap opening occurs even in the absence of hybridization between graphene $\ensuremath{\pi}$ states and Au states, and a strong sublattice asymmetry is established for a small separation ($d$) between the graphene and Au layer, typically, $dl3.0\phantom{\rule{0.28em}{0ex}}\AA{}$, which can actually be achieved for a low Au coverage. In realistic situations, which are mimicked using lattice-mismatched models, graphene $\ensuremath{\pi}$ states near the Dirac point easily hybridize with nearby (in energy) Au states even for a van der Waals distance, $d\ensuremath{\sim}3.4\phantom{\rule{0.28em}{0ex}}\AA{}$, and this hybridization usually dictates a band gap opening in graphene. In that case, the top parts of the intact Dirac cones survive the hybridization and are isolated to form midgap states within the hybridization gap, denying that the band gap is induced by sublattice symmetry breaking. This feature of a band gap opening is similar to that found for the so-called ``first'' graphene layer on silicon carbide (SiC) and the predicted band gap and doping level are in good agreement with the experiments for graphene/Au/Ru(0001).

Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: role of the dipolar field.

Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour.

Electronic structure of graphene on Ni surfaces with different orientation

An experimental study of the graphene, synthesized by propylene cracking on Ni surfaces with different orientation: (100) and (111), using angle-resolved photoemission, has been performed. It has been shown that graphene on Ni(111) had a perfect lateral structure due to consistency of their lattices, whereas graphene/Ni(100) consisted of a lot of domains. For both systems electronic structure was quite similar and demonstrated a strong bonding of graphene to the underlying Ni surface. After Au intercalation the electronic structure of graphene in both systems was shifted to the Fermi level and became linear in the vicinity of the K point of the Brillouin zone.

Growth of graphene monolayer by “internal solid-state carbon source”: Electronic structure, morphology and Au intercalation

Synthesis of a graphene monolayer atop Ni(111) film deposited on HOPG using method of “internal solid-state carbon source” due to low-temperature segregation of carbon atoms through Ni film from the Ni/HOPG interface was studied by core level and valence band photoelectron spectroscopy. The atomic structure and morphology were studied by STM. It was established that the synthesis of graphene occurs via a stage of transformation from the surface carbide phase (with stoichiometry Ni2C) as a result of the surface carbide decomposition. We consider that such transformation is a necessary stage of the graphene synthesis. We have shown that the method of “solid-state carbon source” indeed results in a graphene monolayer formation on top of Ni film. A graphene monolayer begins to form even under annealing at 180°C. Formation of practically full graphene covering with well-ordered structure takes place even at temperature of 260–270°C. The formed graphene is strongly coupled to the Ni substrate. STM images show formation of well-ordered atomic hexagonal surface structure characteristic for graphene monolayer. Intercalation of Au atoms leads to formation of the p(9×9) superstructure and blocks strong coupling. It testifies that the main part of the surface is covered by epitaxial graphene on Ni.

Rashba splitting of 100 meV in Au-intercalated graphene on SiC

Intercalation of Au can produce giant Rashba-type spin-orbit splittings in graphene, but this has not yet been achieved on a semiconductor substrate. For graphene/SiC(0001), Au intercalation yields two phases with different doping. We observe a 100 meV Rashba-type spin-orbit splitting at 0.9 eV binding energy in the case of p-type graphene after Au intercalation. We show that this giant splitting is due to hybridization and much more limited in energy and momentum space than for Au-intercalated graphene on Ni.

Comparative study of ternary graphite-potassium-metal (M = Tl, Hg, Au) intercalation compounds

Potassium–thallium, potassium–mercury, and potassium–gold alloys can be intercalated into graphite, leading to the formation of poly-layered ternary compounds. Different ways of synthesis are involved for the preparation of the corresponding Graphite Intercalation Compounds (GICs). The as-prepared bulk GICs are described in a comparative study, regarding their chemical formula, their formation mechanism, and their structural and superconducting properties. Analogies and differences are pointed out. [TANSO 2015 (No. 268), 145–53.]