Au Monolayer Research Articles

Electronic structure of monolayer Cu, Ag and Au structures

Within the framework of the density functional theory, the features of the electronic structure of copper, silver and gold monolayers are investigated. Changes in the electronic states of slabs are investigated depending on the number of monolayers and the concentration of defects. The tendencies in the formation of the total density of electronic states curve during the transformation from a monolayer to a bulk sample for copper and gold are of a similar character. It has been established that the shape of the total density of electronic states curve of gold slabs with a thickness of 1–3 monolayers is mainly determined by the components of the d-states associated with the z axis. In the case of three monolayers, the shape of the total density of electronic states curve is specified by the d-states of the atoms of the outer layers. Point defects, which are smaller than the atomic size and are observed in STM images of gold monolayers, are determined by the peculiarities of atomic ordering. The presence of topological neighbors of the atoms of the metals under study lead to the broadening of the band and the formation of the lower part of the valence band. The loss of a larger number of atoms in the slab leads to a significant shift of the main maximum of the d-states of the metal towards lower binding energies, while a smaller number of atomic defects in a monolayer lead to the splitting of the initial features.

A theoretical study of the ability of 2D monolayer Au (111) to activate gas molecules

The adsorption and activation of gas molecules are investigated substantially in solid-gas heterogeneous catalysis. Here we investigated the interaction between gas molecules and unique two-dimensional monolayer Au (111) structure using density functional theory. It is found that CO2, H2O, N2 and CH4 molecules are weakly adsorbed on the surface with the adsorption energies between −0.150 and −0.250 eV due to van der Waals interaction. While CO, NO, NO2, and NH3 molecules are adsorbed more stably with the adsorption energies between −0.300 and −0.470 eV. Especially, the bond length of CO is stretched by 0.038 Å and the bond angle of NO2 is obviously enlarged by 10.460°. The activation originates from the rearrangement of molecule orbitals and the orbitals hybridization between the partial orbitals of gas molecules and Au-5d orbitals. The fundamental analyses of adsorption mechanism and electronic properties may provide guidance for the applications of two-dimensional monolayer metal catalysis.PACSnumbers 73.22.-f, 73.61.-r

Controllable preparation of the Au–MoS2 nano-array composite: optical properties study and SERS application

A kind of controllable material constructed with Au nano-disk array and monolayer MoS2 had been developed. The optical properties of the composite were studied and the Raman enhancement was obtained.

Proximity-induced magnetization in graphene: Towards efficient spin gating

Gate-tunable spin-dependent properties could be induced in graphene at room temperature through magnetic proximity effect by placing it in contact with a metallic ferromagnet. Because strong chemical bonding with the metallic substrate makes gating ineffective, an intervening passivation layer is needed. Previously considered passivation layers result in a large shift of the Dirac point away from the Fermi level, so that unrealistically large gate fields are required to tune the spin polarization in graphene. We show that a monolayer of Au or Pt used as the passivation layer between Co and graphene brings the Dirac point closer to the Fermi level. In the \Co/\Pt/\Gr system the proximity-induced spin polarization in graphene and its gate control are strongly enhanced by the presence of a surface band near the Fermi level. Furthermore, the shift of the Dirac point could be eliminated entirely by selecting submonolayer coverage in the passivation layer. Our findings open a path towards experimental realization of an optimized two-dimensional system with gate-tunable spin-dependent properties.

First Au monolayer formation on Cu(110) surface

We study in this work the growth of a gold monolayer on a Cu (110) surface thanks to the kinetic Monte Carlo method. We considered a coverage of 0.1% of the monolayer. Indeed, the growth of a monolayer is done through the intermediary of several diffusion processes. All activation energies for each process were calculated using the static method coupled with the KMC method. This coupling is established by the ABBM anisotropic bond breaking model. The study of the results obtained in the temperature range (100-300K) at different deposition rates shows the formation of small 1D islands and certain 2D islands at high temperature (280 k-300 K).

Polarization-Dependent Light Emission and Charge Creation in MoS2 Monolayers on Plasmonic Au Nanogratings.

We fabricated plasmonic hybrid nanostructures consisting of MoS2 monolayer flakes and Au nanogratings with a period of 500 nm. The angle-resolved reflectance and photoluminescence spectra of the hybrid nanostructures clearly indicated a coupling between surface plasmon polaritons (SPPs) and incoming photons. The surface photovoltage (SPV) maps could visualize the spatial distribution of net charges while shining light on the sample. Considerable polarization and wavelength dependence of the SPV signals suggested that the SPP mode enhanced the light-matter interaction and resulting exciton generation in the MoS2 monolayer. From the photoluminescence spectra and the morphology of the suspended MoS2 region, it could be noted that light irradiation did not much raise the temperature of the MoS2 monolayers on the nanogratings. Nanoscopic SPV and surface topography measurements could reveal the local optoelectronic and mechanical properties of MoS2 monolayers. This work provided us insights into the proposal of a high-performance MoS2/metal optoelectronic devices, based on the understanding of the SPP-photon and SPP-exciton coupling.

Photonic Spin Hall Effect Modified by Ultrathin Au Films and Monolayer Transition Metal Dichalcogenides in One-Dimensional Photonic Crystal

When the metal film becomes far thinner than the electron mean free path (EMFP) in their bulk counterpart, many exotic optical properties accompanied with numbers of novel applications may emerge. Herein, the photonic spin Hall effect (PSHE) occurring on the surface of one-dimensional photonic crystal consisting of ultrathin Au films, monolayer transition metal dichalcogenides (TMDCs), and one defective layer is investigated. The Au film thickness ranges from 2.4 to 40 nm which covers the thickness range below the EMFP (37.7 nm) in bulk Au. Results show that ultrathin Au films are more competent to enhance the PSHE than bulk Au film. In particular, the maximum spin-dependent transverse displacement (37.94 times of the incident wavelength) at the Au film thickness of 4.4 nm is more than 12-fold higher than that at the bulk Au film with thickness of 40 nm. Besides it may be due to the coupling between the short-range surface plasmon polaritons and the B excitonic resonance in monolayer WS2, among four monolayer TMDCs (i.e., MoS2, MoSe2, WS2, WSe2), the photonic crystal with monolayer WS2 gives the strongest PSHE. Additionally, evolutions of photonic spin Hall shifts with the refractive index of defective layer, incident wavelength, and incident angle are also studied in detail. These findings shed light on the superiority of ultrathin metal films for light manipulation in spinoptics and may provide new routes to tailor the optical helicity in nanophotonic devices.

Enhanced Oxidation of Uric Acid at Thiourea-Modified Gold Electrode in Alkaline Media

The surface of polycrystalline gold (Au (poly)) electrode was ex situ modified with self-assembled monolayer (SAM) of thiourea (TU) with a view of applying the electrode for the electrochemical oxidation of uric acid (UA) in alkaline media. UA undergoes an enhanced electrochemical oxidation at the monolayer of TU-modified Au (poly) (TU|Au (poly)) electrode at a remarkably lower potential with a higher current density as compared to the bare Au (poly) electrode. The enhanced activity of the electrode was achieved due to the effective blocking of harmful adsorption of UA that takes place strongly at the bare Au (poly) electrode surface. The TU|Au (poly) electrode was also found to be very effective towards simultaneous oxidations of UA, ascorbic acid (AA) and dopamine (DA) in their mixture with well separation of the peak current of the individual substance. This technique may be advantageously utilized for developing sensor for the purpose of detection of UA.

Atom probe tomography of nanoporous gold formed by dealloying lean noble alloys

Nanoporous gold (NPG) has numerous applications in (electro)catalysis, (bio)sensing, actuation, and photonics. The economic viability, however, remains a barrier to its widespread applicability. In this study, lean noble alloys (5 at. % Au) were designed and fabricated while aiming for the minimum gold content to ensure a fully Au covered surface after electrochemical dealloying. Moreover, the morphological effects of alloying with Pt were elucidated via electrochemical, microscopical, and tomographical characterization. The true surface area after dealloying was found to increase by a factor of 240 and 900 in the case of the binary (Ag95Au5) and the ternary (Ag94Au5Pt1) systems, respectively. To enable atom probe tomography (APT) on such highly porous samples, the nanoporous structures were filled by Cu electrochemical depositon. APT data analysis provided various composition profiles according to which Au enrichment on ligament edges, which was more pronounced in the case of the binary samples, was detected. Future experiments on monolayer Au covered model samples are planned to obtain conclusive atomic-scale insight into the near surface composition of lean nanoporous gold structures.

Free-standing 2D metals from binary metal alloys

Recent experiments have demonstrated the formation of free-standing Au monolayers by exposing the Au–Ag alloy to electron beam irradiation. Inspired by this discovery, we used semi-empirical effective medium theory simulations to investigate monolayer formation in 30 different binary metal alloys composed of late d-series metals such as Ni, Cu, Pd, Ag, Pt, and Au. In qualitative agreement with the experiment, we find that the beam energy required to dealloy Ag atoms from the Au–Ag alloy is smaller than the energy required to break the dealloyed Au monolayer. Our simulations suggest that a similar method could also be used to form Au monolayers from the Au–Cu alloy and Pt monolayers from Pt–Cu, Pt–Ni, and Pt–Pd alloys.

Tailoring the Electronic Structure of Transition Metals by the V2C MXene Support: Excellent Oxygen Reduction Performance Triggered by Metal–Support Interactions

The enhancement of oxygen reduction reaction (ORR) activity can significantly boost the performance of fuel cells. MXene-supported transition metals with strong metal-support interactions (SMSI) are an effective strategy to increase the catalytic activity and durability while decreasing the usage of noble metals. Herein, a series of composites of transition-metal atoms (Ni, Pd, Pt, Cu, Ag, and Au) deposited on V2C MXene are designed as potential catalysts for ORR using density functional theory. The calculation results demonstrate that all the transition metals prefer to form a monolayer on V2C (TMML/V2C) with high thermodynamic stability because of SMSI, in which the Pd, Pt, Ag, and Au monolayers exhibit high chemical stability during the ORR process. PtML/V2C exhibits the highest activity toward ORR with the overpotential down to 0.38 V and the largest energy barrier of 0.48 eV. The excellent catalytic performance originates from the modification of the electronic structure by the V2C support because of SMSI. Our studies elucidate the SMSI between transition-metal atoms and V2C MXene from the atomic level and thus rationally design the ORR catalyst at the cathode of fuel cells to enhance the activity while possessing high stability and less Pt usage.

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.

Surface Plasmon Resonance of Au Thin Films Modified By Catalyst Monolayer

In this talk, we present the results and modeling o the catalyst monolayer modification effects on plasmon resonance of Au thin films. The experiments are conducted using an attenuated total reflection arrangement in which p-polarized light is incident through a prism onto a thin (-400 A) Au film deposited on the mica substrate. (Kretschmann configuration [[1]]). The angle of incidence on the base of the prism is greater than the critical angle for the prism: electrolyte interface and SPs are excited by the evanescent wave generated under these conditions of total internal reflection. The excitation is manifested by a decrease in the intensity of reflected light. The Pd monolayer on Au were electrodeposited using procedure reported by Kolbe et al [[2],[3]], while the Pt submonolayer/monolayers were deposited on Au using Surface Limited Redox Replacement (SLRR) of Pb UPD monolayer [[4]]. The plasmon resonance frequency of Au(hkl), PdML/Au(hkl) and PtML/Au(hkl) and PtSML/Au(hkl) were investigated as a function of the applied potential in the range between -0.24 V to 0.4 V vs. SCE. The comparative analysis, between Au and catalyst monolayer modified Au(hkl) samples indicate that SP resonant frequency change is induced by both; surface charge and dipole moment of the surface as well as he adsorption of H on the catalyst monolayer surface in the UPD region. The latter effect is quantified using subtractive analysis between the Au and catalyst modified Au surface and solutions with different pH. In conclusion, we present a simple model describing the catalyst monolayer effect on plasmon resonance of Au films. [1] E. Kretschmann and H. Raether, Z. Natutforsch. 23a (1968) 2135. [2] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Surface Science, 443 (1999) 19. [3] J. Tang, M. Petri, L.A. Kibler, D.M. Kolb, Electrochimica Acta, 51, (2005) 125 [4] K. Sasaki et al, Electrochim. Acta, 55, (2010) 2645.

Improvement of valley splitting and valley injection efficiency for graphene/ferromagnet heterostructure**Project supported by the National Key R&D Program of China (Grant No. 2017YFF0206104), the National Natural Science Foundation of China (Grant No. 51871018), Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, the Opening Project of Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics of Chinese

The valley splitting has been realized in the graphene/Ni heterostructure with the splitting value of 14 meV, and the obtained valley injecting efficiency from the heterostructure into graphene was 6.18% [Phys. Rev. B 92 115404 (2015)]. In this paper, we report a way to improve the valley splitting and the valley injecting efficiency of the graphene/Ni heterostructure. By intercalating an Au monolayer between the graphene and the Ni, the split can be increased up to 50 meV. However, the valley injecting efficiency is not improved because the splitted valley area of graphene moves away from the Fermi level. Then, we mend the deviation by covering a monolayer of Cu on the graphene. As a result, the valley injecting efficiency of the Cu/graphene/Au/Ni heterostructure reaches 10%, which is more than 60% improvement compared to the simple graphene/Ni heterostructure. Then we theoretically design a valley-injection device based on the Cu/graphene/Au/Ni heterostructure and demonstrate that the valley injection can be easily switched solely by changing the magnetization direction of Ni, which can be used to generate and control the valley-polarized current.

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity.

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

Predicting metal-metal interactions. I. The influence of strain on nanoparticle and metal adlayer stabilities.

Strain-engineering of bimetallic nanomaterials is an important design strategy for developing new catalysts. Herein, we introduce an approach for including strain effects into a recently introduced, density functional theory (DFT)-based alloy stability model. The model predicts adsorption site stabilities in nanoparticles and connects these site stabilities with catalytic reactivity and selectivity. Strain-based dependencies will increase the model's accuracy for nanoparticles affected by finite-size effects. In addition to the stability of small nanoparticles, strain also influences the heat of adsorption of epitaxially grown metal-on-metal adlayers. In this respect, we successfully benchmark the strain-including alloy stability model with previous experimentally determined trends in the heats of adsorption of Au and Cu adlayers on Pt (111). For these systems, our model predicts stronger bimetallic interactions in the first monolayer than monometallic interactions in the second monolayer. We explicitly quantify the interplay between destabilizing strain effects and the energy gained by forming new metal-metal bonds. While tensile strain in the first Cu monolayer significantly destabilizes the adsorption strength, compressive strain in the first Au monolayer has a minimal impact on the heat of adsorption. Hence, this study introduces and, by comparison with previous experiments, validates an efficient DFT-based approach for strain-engineering the stability, and, in turn, the catalytic performance, of active sites in bimetallic alloys with atomic level resolution.

Adsorption of small gas molecules on strained monolayer WSe2 doped with Pd, Ag, Au, and Pt: A computational investigation

Herein, we report on the doping of monolayer WSe2 with Pd, Ag, Au, and Pt as a promising approach to improve the gas sensing performance of the 2D material. The adsorption of CO2, NO2 and SO2 gas molecules on Pd, Ag, Au, and Pt-doped monolayer WSe2 are studied in terms of adsorption energy, charge transfer and band structure. Results suggest that NO2 adsorption on Ag-WSe2 monolayer is energetically favorable giving rise to the most stable configuration. NO2 is chemisorb on the doped monolayer surface due to a strong interaction. We study the influence of applying strain on the gas molecule/TM-WSe2 adsorption systems. Increasing the stain, the adsorption properties decrease for NO2. Our results indicate that Ag-WSe2 monolayer is a great candidate for the development of efficient NO2 gas sensors.

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.

Au-induced reconstructions of the Si(111) surface with ordered and disordered domain walls

Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) observations combined with the density functional theory (DFT) calculations were used to elucidate atomic arrangement of Au/Si(111) reconstructions forming at Au coverage around one monolayer. They include the Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au with ordered domain walls, the Si(111)$\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au and the Si(111)$\ensuremath{\beta}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au surfaces with disordered domain walls of various density, and the Si(111)$2\sqrt{21}\ifmmode\times\else\texttimes\fi{}2\sqrt{21}$-Au surface which structure is relevant to those of other reconstructions, albeit cannot be ascribed solely to the domain walls. Using detailed comparison of the STM and ARPES data with the results of DFT calculations, we explicitly proved the atomic model of Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au surface proposed by Grozea et al. [Surf. Sci. 418, 32 (1998)]. This model provided us a hint for constructing an atomic model of domain walls which demonstrates a proper coincidence between experimental and simulated STM images. The model contains five Au atoms per $2\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ unit cell, i.e., 1.25 monolayers of Au. For the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au and $\ensuremath{\beta}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au surfaces, this finding allows an overall description of their atomic arrangement and, hence, accurate determination of their Au coverages. Knowledge on the atomic arrangement of the Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au surface and domain walls also allowed us to construct an atomic model of the Si(111)$2\sqrt{21}\ifmmode\times\else\texttimes\fi{}2\sqrt{21}$-Au surface. The obtained data were unified in the refined formation phase diagram for the Au/Si(111) system.

First-principles study of vacancy defects at interfaces between monolayer MoS2 and Au.

The performance of MoS2 based devices is closely related to the quality and defect morphology of the monolayer MoS2 deposited on metal. First-principles calculations were performed to investigate the vacancy effects of Au–mMoS2 contact. Four possible S-vacancy and a Mo-vacancy were considered in our calculations. Energetic studies show that S-vacancies are easier to form than Mo-vacancy in Au–mMoS2 contact, while S-vacancy (hollow site at interface, VS4) has the lowest formation energy under Mo-rich environments. Electron and charge redistribution analysis of defective Au–mMoS2 contact indicate that the lower contact resistance and higher electron injection efficiency of defective Au–MoS2 contact than perfect ones. Notably, the S-vacancy at top layer showed better electronic performance than that at bottom layer of monolayer MoS2 in the contact. High quality n-type Au–mMoS2 contact can therefore be expected through defect engineering.