Graphene Overlayer Research Articles

Confinement Effects of Two-dimensional Surfaces on Water Adsorption and Dissociation over Pt(111).

It has been established that the confined space created by stacking a two dimensional (2D) surface atop a metal catalyst serves as a nano-reactor. According to recent research, when a graphene (Gr) overlayer encloses a Pt catalyst from above, the activation barrier for the water dissociation reaction, a process with major industrial significance, decreases. In order to investigate how the effect of confinement varies among different two-dimensional (2D) materials, we study the adsorption and dissociation barriers of water molecule on Pt(111) under graphene, hexagonal boron nitride (h-BN), and heptazine-based graphitic carbon nitride (g-C3N4) layers using density functional theory calculations. Our findings reveal that the strength of adsorption does not decrease consistently with a reduction in the height of the 2D overlayer. Furthermore, a smaller barrier is not always the consequence of poorer adsorption of the reactant. We also examine the effect of confinement on the shape of the reaction path, on the frequencies of vibrational modes, and on the rate constants derived using the harmonic transition state theory. Overall, all three of the 2D surfaces cause a decrease in barrier height and a weakening of adsorption, though to differing degrees due to a mix of mechanical, geometric and electronic variables.

Adsorption of water, hydrogen and oxygen on graphene/Pt(111) surfaces: 2D graphene cover effect studied by temperature programmed desorption

Understanding the interaction between reactive gas and graphene overlayers on metal substrates is prerequisite to study the surface reactions confined at the interfaces between graphene cover and metal surfaces. Here, we report H2O, H2 and O2 adsorption on graphene overlayer covered Pt(111) surfaces by using temperature programmed desorption (TPD). H2O has weak interaction with graphene overlayers and does not intercalate under graphene overlayers under UHV adsorption conditions. Submonolayer H2O desorption on graphene is about 20 K lower directly compared to that on Pt(111) on the submonolayer graphene/Pt(111) surfaces with well-defined metal-graphene interfaces. Both H2 and O2 can intercalate on submonolayer graphene/Pt(111) but not on the graphene fully covered Pt(111) surfaces under UHV adsorption conditions. Graphene cover weakens H2 adsorption at the graphene-Pt(111) interfaces with the H2 desorption temperature lowered by about 30 K and desorption energy lowered by 8.4 ± 0.6 kJ/mol . The intercalated oxygen under graphene reacts with the graphene overlayers above 600 K with CO2 and CO as the oxidation products.

Highly sensitive plasmonic biosensor enhanced by perovskite-graphene hybrid configuration

This study theoretically proposes a highly sensitive plasmonic biosensor comprising a silver film, perovskite nanosheets, bi-layer silicon nanosheets, and a graphene overlayer. The introduction of perovskite-graphene hybrid structure can reduce the optical reflectivity of the proposed biosensor configuration so that photons are converted into energy to support surface plasmon resonance (SPR) enhancement. Unlike conventional angular modulation, our proposed biosensor configuration is designed using phase modulation, offering sensitivity enhancements as high as 3 orders of magnitude. Excellent biosensing performances have been examined by optimizing the silver film thickness and the number of perovskite nanosheets. Considering a tiny refractive index change of 0.0012 refractive index unit (RIU) caused by biomolecules adsorption behavior, the optimal plasmonic biosensor configuration of 46-nm silver film–5.7-nm silicon interlayer-1-layer CH3NH3PbI3 perovskite film–0.7-nm silicon interlayer-1-layer graphene can produce the highest detection sensitivity of 1.2314 × 105 degree/RIU. In addition, an excellent linear response in the differential phase can be obtained, as the refractive index variation of the sensing interface can be as low as 10−6 RIU, showing great potential in performing ultrasensitive biosensing.

Lifshitz transition in titanium carbide driven by a graphene overlayer

Lifshitz transition in titanium carbide driven by a graphene overlayer

Highly Selective Photoelectroreduction of Carbon Dioxide to Ethanol over Graphene/Silicon Carbide Composites

AbstractUsing sunlight to produce valuable chemicals and fuels from carbon dioxide (CO2), i.e., artificial photosynthesis (AP) is a promising strategy to achieve solar energy storage and a negative carbon cycle. However, selective synthesis of C2compounds with a high CO2conversion rate remains challenging for current AP technologies. We performed CO2photoelectroreduction over a graphene/silicon carbide (SiC) catalyst under simulated solar irradiation with ethanol (C2H5OH) selectivity of>99 % and a CO2conversion rate of up to 17.1 mmol gcat−1 h−1with sustained performance. Experimental and theoretical investigations indicated an optimal interfacial layer to facilitate the transfer of photogenerated electrons from the SiC substrate to the few‐layer graphene overlayer, which also favored an efficient CO2to C2H5OH conversion pathway.

Highly Selective Photoelectroreduction of Carbon Dioxide to Ethanol over Graphene/Silicon Carbide Composites.

Using sunlight to produce valuable chemicals and fuels from carbon dioxide (CO2 ), i.e., artificial photosynthesis (AP) is a promising strategy to achieve solar energy storage and a negative carbon cycle. However, selective synthesis of C2 compounds with a high CO2 conversion rate remains challenging for current AP technologies. We performed CO2 photoelectroreduction over a graphene/silicon carbide (SiC) catalyst under simulated solar irradiation with ethanol (C2 H5 OH) selectivity of>99 % and a CO2 conversion rate of up to 17.1 mmol gcat -1 h-1 with sustained performance. Experimental and theoretical investigations indicated an optimal interfacial layer to facilitate the transfer of photogenerated electrons from the SiC substrate to the few-layer graphene overlayer, which also favored an efficient CO2 to C2 H5 OH conversion pathway.

Energetics and Kinetics of Hydrogen Electrosorption on a Graphene-Covered Pt(111) Electrode.

The Angstrom-scale space between graphene and its substrate provides an attractive playground for scientific exploration and can lead to breakthrough applications. Here, we report the energetics and kinetics of hydrogen electrosorption on a graphene-covered Pt(111) electrode using electrochemical experiments, in situ spectroscopy, and density functional theory calculations. The graphene overlayer influences the hydrogen adsorption on Pt(111) by shielding the ions from the interface and weakening the Pt-H bond energy. Analysis of the proton permeation resistance with controlled graphene defect density proves that the domain boundary defects and point defects are the pathways for proton permeation in the graphene layer, in agreement with density functional theory (DFT) calculations of the lowest energy proton permeation pathways. Although graphene blocks the interaction of anions with the Pt(111) surfaces, anions do adsorb near the defects: the rate constant for hydrogen permeation is sensitively dependent on anion identity and concentration.

Multifunctional graphene metadevice using dual metamaterial induced transparency in terahertz regime

The dual metamaterial induced transparency (dual-MIT) phenomenon, which can be significantly tailored via the depolarization field effect of the graphene overlayer, is demonstrated in our fabricated multifunctional terahertz metastructures composed of unequal-lengthed 3-bar arrays. Those metasurfaces could actively manipulate the phase and group delay at various frequencies, as well as the different polarization conversion capability. The introduction of graphene reinforces the amplitude modulation of dual MITs and further reduces the quenched power by 4 times with a rotated angle. Combined with carrier dynamics, the dual-MIT transient regulation can realize an ultrafast all-optical switch with 2-time enhancement in modulation speed. Based on the advantage of the dual-MIT windows with different frequencies and magnitudes, we have illustrated the encrypted imaging on the erasure and reappearance of the target pattern. Our work provides a lower power consumption option for ultrafast all-optical switch and opens up the opportunities for future communication and information security in terahertz regime.

An Effective Charge Neutralization Enabled by Graphene Overlayer in Ambient Pressure XPS Measurements of Insulators

AbstractAmbient pressure XPS is a powerful technique capable of performing measurements with samples kept at mbar pressure. The filled gas, also X‐ray ionized, provides electrons to neutralize positive charges built up on insulating samples. However, this convenient neutralization scheme does not solve the charging problem completely. In this study, the effectiveness of how the electrons generated from conducting mechanical parts mounted in the immediate front of insulating samples neutralize positive charges on the samples during XPS measurements is tested. The mechanical parts range from fine gold mesh, holey carbon film to graphene monolayer, and measured insulating samples are polished sapphire and rough pellets of CaCO3 powder. All these mechanical add‐ons reduce the charging to different degrees at the expense of sample signal. Amazingly, the graphene monolayer is found to effect perfect charge neutralization for both smooth sapphire and rough CaCO3 pellet, evidenced by realistic spectral profile and binding energy values. The finding that an untreated CaCO3 pellet is often terminated with Ca(OH)(HCO3) is consistent with earlier reports. It is speculated that a likely conformal covering with graphene on the sample with rough morphology holds the key to its superb performance in charge compensation.

Green-Graphene Protective Overlayer on Optical Microfibers: Prolongs the Device Lifetime.

Optical microfibers find new applications in various fields of industry, which in turn require wear resistance, environmental friendliness and ease of use. However, optical microfibers are fragile. Here we report a new method to prolong the microfiber lifetime by modifying its surface with green-extracted graphene overlayers. Graphene films were obtained by dispergation of shungite mineral samples in an aqueous medium. For this, we tapered optical fibers and sculptured them with graphene films mixed with gold nanoparticles. We observed that due to the surface modification the lifetime and survivability of the microfiber increased 5 times, as compared to the bare microfiber. The embedded gold nanoparticles can also be utilized for enhanced sensitivity and other applications.

Graphene-Encapsulated Silver Nanoparticles for Plasmonic Vapor Sensing.

Graphene-covered silver nanoparticles were prepared directly on highly oriented pyrolytic graphite substrates and characterized by atomic force microscopy. UV–Vis reflectance spectroscopy was used to measure the shift in the local surface plasmon resonance (LSPR) upon exposure to acetone, ethanol, 2-propanol, toluene, and water vapor. The optical responses were found to be substance-specific, as also demonstrated by principal component analysis. Point defects were introduced in the structure of the graphene overlayer by O2 plasma. The LSPR was affected by the plasma treatment, but it was completely recovered using subsequent annealing. It was found that the presence of defects increased the response for toluene and water while decreasing it for acetone.

Dynamics of Li deposition on epitaxial graphene/Ru(0001) islands

Li metal has been deposited on the surface of a Ru(0001) single crystal containing patches of monolayer-thick epitaxial graphene islands. The use of low-energy electron microscopy and diffraction allowed us to in situ monitor the process by measuring the local work function as well as to study the system in real and reciprocal space, comparing the changes taking place on the graphene with those on the bare Ru(0001) surface. It is found that Li deposition decreases the work function of the graphene islands but to a much smaller degree than of the Ru(0001) surface, as corresponds to its intercalation below the graphene overlayer. Finally, the diffusion process of Li out of the graphene islands has been monitored by photoelectron microscopy using a visible-light laser.

Near-infrared surface plasmon resonance sensor with a graphene-gold surface architecture for ultra-sensitive biodetection

Binding behaviors of proteins are important for applications in the field of biochemistry. Though a standard assay has a favorable limit of detection (LOD), it is mainly limited to indirect observation via fluorescence labeling. We reported and demonstrated a novel label-free sensing approach based on a near-infrared (NIR) surface plasmon resonance (SPR) sensing chip modified with a graphene-gold surface architecture in this paper. The NIR excitation wavelength can greatly improve the sensitivity of SPR sensing derived from the wavelength modulation-based methodology. Moreover, benefiting from the excellent electro-optical properties of graphene in NIR range, the graphene-gold surface architecture was built to further improve the sensing sensitivity. Experimental results proved the superiority over most of those reported previously in terms of ultra-sensitivity (39,160 nm/RIU) and resolution (5.107 × 10−7 RIU). We detected human immunoglobulin G (IgG) to confirm the ability to enhanced-sensitive detection with a graphene overlayer. This sensor provides surface-specific detection schemes with a large linear dynamic range of ng/ml (pM) to fg/ml (aM) and a LOD of 7.2 fg/ml (48 aM) using gold nanoparticles (GNPs) as amplification labels. The proposed method provides a simple and effective strategy to improve sensitivity and LOD for biochemical detection in a rapid, ultrasensitive, and nondestructive manner.

Scanning probe analysis of twisted graphene grown on a graphene/silicon carbide template

Overlayer growth of graphene on an epitaxial graphene/silicon carbide (SiC) as a solid template by ethanol chemical vapor deposition is performed over a wide growth temperature range from 900 °C to 1450 °C. Structural analysis using atomic force and scanning tunneling microscopies reveal that graphene islands grown at 1300 °C form hexagonal twisted bilayer graphene as a single crystal. When the growth temperature exceeds 1400 °C, the grown graphene islands show a circular shape. Moreover, moiré patterns with different periods are observed in a single graphene island. This means that the graphene islands grown at high temperature are composed of several graphene domains with different twist angles. From these results, we conclude that graphene overlayer growth on the epitaxial graphene/SiC solid at 1300 °C effectively synthesizes the twisted few-layer graphene with a high crystallinity.

Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111)

The production of hydrogen via water electrolysis using renewable electricity is a promising carbon-neutral technology. In this contribution, we report insights into the hydrogen evolution reaction (HER) in H2SO4 on Pt(111) and graphene-covered Pt(111), in addition to the electrochemical properties of graphene overlayers. As-prepared graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between graphene and Pt(111) while excluding SO42– and other anions. We demonstrate that defects in these as-prepared graphene overlayers can be generated from oxidation at high overpotentials or reduction from the production of H2 bubbles and postulate that HER occurs locally at only Pt(111) in the proximity of defects in graphene overlayers on as-prepared G/Pt(111) electrodes, and as defects in graphene increases, more of the Pt(111) surface becomes utilized for HER. Kinetically, the addition of defective graphene overlayers can increase the geometric HER rate by up to 200%, while Tafel slopes and [H+] reaction orders remain unchanged. These results shed kinetic insight into the nature of graphene overlayers and their effect on HER catalysis and also demonstrate the promise of confinement modifications in designing catalysts with properties closer to achieving optimum rates.

Electrocatalysis Under Cover: Enhanced Hydrogen Evolution Reaction (HER) Via Defective Graphene Covered Pt(111)

The production of hydrogen through water electrolysis using renewable electricity is a promising carbon-neutral technology. We present insights into the hydrogen evolution reaction (HER) in aqueous H2SO4 (0.005 to 0.5 M) on Pt(111) and Graphene/Pt(111) under ambient temperatures and pressures. Graphene was grown on Pt(111) using chemical vapor deposition (CVD) of hydrocarbons [1]. Cyclic voltammograms (CV) and HER currents of Pt(111) in H2SO4 were quantitatively benchmarked to literature to ensure reproducibility and comparability of kinetic measurements [1-3]. CV sweeps (0.005 to 0.5 M, 23°C, 0.1 to 0.8 VRHE) indicate that graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between Pt(111) and graphene while excluding SO4 2- and other anions [2]. The extent of defects in graphene was quantified by integrating the anion adsorption region from CVs. The anion adsorption coverage on electrochemically pre-treated Graphene/Pt(111) (250 cycles between 0.1 to 1.2 VRHE in 0.005 M H2SO4) increases with HER reaction time while untreated Graphene/Pt(111) exhibits a slower increase. Though the defect generation rate is slower on untreated Graphene/Pt(111), a sudden order-of-magnitude increase in anion coverage concurrent with transformation of the Pt(111) to polycrystalline Pt is observed after ~10 hours of HER. In contrast, no sudden increase in the anion capacity is observed on electrochemically pre-treated Graphene/Pt(111) for up to 15 hours of HER, though a transition from Pt(111) to polycrystalline Pt is also observed after ~10 hours of HER.Initial HER currents on both pre-treated and un-treated Graphene/Pt(111) are slower than Pt(111) by 1 to 2 orders of magnitude (0.005 to 0.5 M, 23°C, -0.08 VRHE). HER currents on both pre-treated and un-treated Graphene/Pt(111) increase with HER reaction time to values equal to or higher than that of Pt(111), concurrent with increasing graphene defects as probed using CV and SEM. Unchanged HER Tafel slopes and [H+] reaction orders suggest the same HER mechanism between Pt(111) and Graphene/Pt(111). We postulate that untreated graphene is mass transfer limited and HER currents transition from mass transfer limited to kinetics limited with increasing graphene defects. Li et al. computes that a graphene overlayer would weaken the hydrogen binding energy to Pt(111), shift the reaction closer to the peak of the HER volcano plot, and thus increase HER currents [6,7]. Though our results agree with prediction, we cannot negate other possible explanations for the observed increase in HER rate. These results demonstrate the promise of overlayer and confinement modifications in designing catalysts with properties closer to achieving optimum currents.

Heterointerface-Enhanced Ultrafast Optical Switching via Manipulating Metamaterial-Induced Transparency in a Hybrid Terahertz Graphene Metamaterial.

We have demonstrated the active manipulation of metamaterial-induced transparency (MIT) in a terahertz hybrid metamaterial with graphene overlayer under photoexcitation. It is found that the introduction of graphene can greatly modify the resonant dips and transparency window through the formed depolarization field around unequal-length double bars to weaken dipole resonances and their destructive interference. Transient control of MIT behaviors is determined by the photogenerated carrier dynamics, which influences the distributions of currents and electric fields in the resonant region to hinder the near-field coupling of two bright modes. Optical modulation depth is sensitive to bar spacing due to an anomalous increased double-bar coupling involving intracell and intercell interaction. Heterointerface formed by the added graphene with substrate could further enhance terahertz response via effective separation of the photoexcited carriers. Theoretical calculation based on the coupled Lorentz oscillator model reveals that the photoinduced terahertz response mainly originates from the coupling and damping in hybrid structures. Our findings could facilitate the development of graphene-based dynamical terahertz modulators and optoelectronic devices.

Electronic structure of graphene/TiO2 interface: Design and functional perspectives

We propose the design of low strained and energetically favourable mono and bilayer graphene overlayer on anatase TiO2 (001) surface and examined the electronic structure of the interface with the aid of first principle calculations. In the absence of hybridization between surface TiO2 and graphene states, dipolar fluctuations govern the minor charge transfer across the interface. As a result, both the substrate and the overlayer retain their pristine electronic structure. The interface with the monolayer graphene retains its gapless linear band dispersion irrespective of the induced epitaxial strain. The potential gradient opens up a few meV bandgap in the case of Bernal stacking and strengthens the interpenetration of the Dirac cones in the case of hexagonal stacking of the bilayer graphene. The difference between the macroscopic average potential of the TiO2 and graphene layer(s) in the heterostructure lies in the range 3–3.13 eV, which is very close to the TiO2 bandgap (~3.2 eV). Therefore, the proposed heterostructure will exhibit enhanced photo-induced charge transfer and the graphene component will serve as a visible light sensitizer.

Approaches for deep-ultraviolet surface plasmon resonance sensors.

Aluminum (Al) is a preferred metal for designing deep-ultraviolet (DUV) surface plasmon resonance (SPR)-based sensors. The native oxide layer (alumina), which grows when the Al film is exposed to air, adds an extra layer to the multilayer stack and consequently affects the DUV-SPR sensing performance. To mitigate the performance loss in DUV-SPR-based sensing, new, to the best of our knowledge, approaches are considered here. We first consider chromium, indium (In), nickel, and platinum as alternative plasmonic materials to Al. In-film-based DUV-SPR sensors exhibit the best performance parameters compared to these alternative materials. We next consider the approach of replacing the native oxide layer by an ultrathin gold (Au) layer on top of bare Al or In. With an optimal Au thickness, higher sensitivity as compared to oxidized metals is observed. The next approach adds one or more graphene layers on top of the bare metal film. In this case, the performance depends on the number of graphene layers, but improvement in sensor characteristics in the DUV is also obtained. The use of Au or graphene overlayers increases the refractive index sensing dynamic range, which can be significant for In with these overlayers under certain operating conditions.

Defect formation in a graphene overlayer on ruthenium under high pressure

Due to its highly corrugated moir\'e structure and the nanometer modulation of its electronic properties, graphene deposited on ruthenium substrates (G-Ru for short) is a versatile playground for molecular deposition and chemical reactivity. Under local ultrahigh pressure conditions, one can expect an even richer behavior, reinforced by the likely appearance of defects. We have theoretically investigated the formation of such defects on G-Ru by using density functional theory methods. We show that defects can be produced in the high areas of the moir\'e, either by generating reactive centers in graphene through the creation of vacancies or by modifying the relative carbon-ruthenium positions through conformational changes. The energetic cost to induce these defects is rather small, of the order of a few eV, so that defects are expected to appear when impacting a STM tip or applying high pressure by a diamond AFM tip in these regions. The different types of defects can be clearly distinguished from each other in our simulated STM images.