A very recent experimental study [Proc. Natl. Acad. Sci. U.S.A. 123 (10) e2524790123] demonstrated that non-functionalized graphene (Gr) supported on silicon(111) catalyzed the decomposition of small sulfur-containing compounds under ultra-high vacuum. Accompanying density functional theory (DFT) calculations identify grain boundary defects (GBDs) and interfacial defects as the active sites. Functional carbon catalysts still encounter skepticism within the catalysis and surface science communities.

Graphene supported on Si(111) (short Gr/Si) is one of the very few examples of a metal-free carbon catalyst that catalyzes gas-surface reactions. Kinetics measurements indicate dissociation of SO2 and H2S but molecular adsorption of N2O. In addition, spectroscopy revealed adsorbed sulfur after SO2 and H2S adsorption. Experiments were conducted at ultrahigh vacuum conditions, using kinetics techniques [i.e., thermal desorption spectroscopy (TDS)], spectroscopy [Auger electron spectroscopy (AES), Raman, X-ray photoelectron spectroscopy (XPS)], and imaging techniques [scanning tunneling microscopy (STM), low-energy electron diffraction]. Deviations of the gas-phase fragmentation pattern and multimass TDS pattern were observed. AES revealed adsorbed sulfur after SO2 and H2S adsorption. Thus, SO2 and H2S decompose, which contrasts with N2O, where only the molecular pathway was present. Density functional theory (DFT) confirms experimental observations. Whereas pristine Gr/Si is nonreactive, DFT modeled grain boundary defects (GBD) (as seen by STM) are the active sites for the decomposition. GBD consist of interfacial defects and surface defects (as seen by XPS). Because carbon and silicon are inexhaustible, Gr-based metal-free catalysts would be a paradigm change. Moreover, breaking H2S down into H2 would allow for recycling that waste gas and synthesizing green hydrogen.

Adsorption of ethyl ether, CH3CH2-O-CH2CH3 (or C4H10O or (CH3CH2)2O), on graphene/Si(111) (hereafter Gr/ Si(111)) was characterized by kinetics (multi-mass thermal desorption spectroscopy (TDS), steady-state rate measurements) and spectroscopic (Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy) techniques as well as by density functional theory calculations (DFT) as a potential metal-free catalyst. TDS results agree with the expected fragmentation pattern of molecular ethyl ether. AES and XPS spectra collected after ethyl ether adsorption are identical with data for pristine Gr/Si(111). Therefore, ethyl ether adsorbs molecularly, consistent with large activation energies for dissociation calculated by the DFT.

To expand the possible applications, chemical vapor deposition grown graphene needs to be transferred to appropriate substrates such as a silicon wafer. Although enormous efforts have been devoted to transfer graphene to various substrates using many different methods, the quality of the final product is still insufficient. We develop a new process named semi-dry transfer, which combines wet etching and dry transfer to obtain graphene with a clean interface with the substrate. For this purpose, an adhesive tape is attached to a sacrificial polymer deposited on the synthesized graphene, which allows the graphene to be easily manipulated so that it can be carefully cleaned before being precisely transferred onto target substrates, here silicon (Si) surfaces. We used this technique to transfer up to 4 × 4 cm2 of graphene onto SiO2/Si substrates. Using various analysis techniques such as low energy electron diffraction, scanning electron microscopy, scanning tunneling microscopy/spectroscopy, Raman, Auger electron and X-ray photoelectron spectroscopies, we demonstrate that our transferred graphene on Si is continuous, clean and that it is very promising for device fabrication. Graphene transistors show transport properties comparable with the state-of-the-art.

Since graphene has a unique band structure with the valence and conduction bands touching each other at a single point called the Dirac point, this makes it extremely sensitive to the surroundings such as doping, external electric field, mechanical deformation, etc. Hence, it is very desirable for sensing applications. However, its surface inertness poses significant drawbacks. Therefore, it is necessary to treat the graphene surface to bind biomolecules. In this paper, we report the use of amine-functionalized graphene by plasma polymerization to detect the presence of biomolecules in graphene channel based on a liquid-gate field-effect transistor (LG-GFET). Taking streptavidin and biotin as an example, the binding interactions of streptavidin–biotin complexes are detected by monitoring the shift of the Dirac point. By varying the streptavidin concentrations from 0.1 nM to 1000 nM, we found that our LG-GFET achieves detection capabilities as low as 0.1 nM. Our approach can be applied for the detection of biological molecules with low detection limit, high sensitivity, and stability.

Stabilization of the 2 H phase of MoTe 2 during molecular beam epitaxy (MBE) growth on graphene terminated 6 H -SiC(0001) is highly desirable in order to take advantage of its promising properties in electronic applications. By properly adjusting the conditions, direct growth of the highly crystalline 2 H phase of MoTe 2 has been achieved. In such van der Waals heterostructure, the atomically-clean interface between graphene and MoTe 2 permits the electronic coupling between the adjacent layers and the emergence of a high variety of Moiré patterns. In this paper, we investigate a single layer of 2 H -MoTe 2 grown on graphene by MBE and we present scanning tunneling microscopy (STM) investigations combined with density functional theory (DFT) calculations and simulations of STM images. Our results show that the STM images of the MoTe 2 /graphene heterostructure surprisingly amplify the otherwise weak Moiré potential modulations leading to the appearance of unique higher-indexed Moiré patterns. These patterns are unusually rich with many Fourier-overtones and show a remarkable variety of different applied bias voltages, revealing the complex electronic features of the heterostructure.

Transporting the sample through different characterization units while maintaining its integrity is crucial if multiple surface sensitive probes are to be employed. In this context, the layered transition metal dichalcogenides (TMDs) are known to be extremely susceptible to ambient degradation, leading to the formation of protrusions and particles on the films. Compared to MoS2 and MoSe2, MoTe2 is one of the most reactive surfaces to ambient conditions. Therefore, understanding its surface oxidation and how to recover a clean surface is very important. In this paper, we report the MBE synthesis of single phase of 2H-MoTe2 with good stoichiometric films on graphene terminated 6H-SiC(0001) substrates and are able to recover the clean surface after exposure to air under properly protected conditions. Our films are analyzed in detail by reflection high energy electron diffraction (RHEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM).

The self-formation of a porous organic thin-film via corrosion inhibitor supports wide applications of carbon steel in industry. Unfortunately, serious damages could be concentrated to the pinhole and/or pore locations in the porous organic film, resulting in the localized corrosion even when an optimal concentration of organic corrosion inhibitors is used. In this work, SnO2 nanoparticles are used for producing the more robust barrier layer via the self-migration of nanoparticles, resulting in a higher corrosion resistance, smooth and uniform protective layer, as well as the existence of SnO2 in the protective layer that could directly affect the high inhibition performance. Therefore, the work suggests a new way to make a more robust thin film that could extend the use of organic corrosion inhibitors.

The interaction of molecules with surfaces plays a crucial role in the electronic and chemical properties of supported molecules and needs a comprehensive description of interfacial effects. Here, we unveil the effect of the substrate on the electronic configuration of iron porphyrin molecules on Au(111) and graphene, and we provide a physical picture of the molecule-surface interaction. We show that the frontier orbitals derive from different electronic states depending on the substrate. The origin of this difference comes from molecule-substrate orbital selective coupling caused by reduced symmetry and interaction with the substrate. The weak interaction on graphene keeps a ground state configuration close to the gas phase, while the stronger interaction on gold stabilizes another electronic solution. Our findings reveal the origin of the energy redistribution of molecular states for noncovalently bonded molecules on surfaces.

•Develop two distinct processes which achieve better layer-by-layer controllability of stoichiometric MoTe2 films.•Flat, high crystalline quality and large area of mono-layer/double-layer MoTe2 on graphene.•A coherent explanation for the Moiré patterns of both monolayer and twisted bi-layers of 2H-MoTe2 on graphene which provides insight into the atomic structure 2H-MoTe2 films together with the formation of semi-coherent twin boundaries (TBs) in the films.•Electronic structure of the 2H-MoTe2 films.

Incorporating functional atomic sites in graphene is essential for realizing advanced two-dimensional materials. Doping graphene with nitrogen offers the opportunity to tune its chemical activity, with significant charge redistribution occurring between molecules and substrate. The necessary atomic scale understanding of how this depends on the spatial distribution of dopants, as well as their positions relative to the molecule, can be provided by scanning tunneling microscopy. Here we show that a non-covalently bonded molecule such as CoPc undergoes a variable charge transfer when placed on N-doped graphene: on a nitrogen pair, it undergoes a redox reaction, with an integral charge transfer, whereas a lower fractional charge transfer occurs over a single nitrogen. Thus the charge state of molecules can be tuned by suitably tailoring the conformation of dopant atoms.

The realization of molecular electronic devices relies on the ability to perform elementary operations with functional molecules. Molecular switches are established candidates to realize basic electronic functions and data storage. Macrocyclic molecules such as phthalocyanines and their derivatives provide a family of compounds that can be switched by external stimuli between two stable states. Using scanning tunneling microscopy, we investigated tin phthalocyanine on graphene. We show that these molecules can be reversibly switched between two states and that the interaction of their electric dipole with a local electric field drives the switching yield and direction. The control at the single-molecule level of the molecular conformation in a bidimensional lattice is then used to achieve high-density data storage.

Ever since its discovery, graphene has been highly researched for its fascinating properties. However, the lack of bandgap in graphene has motivated a search for similar intrinsically semiconducting two dimensional materials. Transition metal dichalcogenides (TMDs) have been considered as an alternative owing to their unique electrical and optical properties. Bulk TMDs exhibit a wide variety of polymorphs, the important ones being 2H and 1T’. The 2H phase is semiconducting while the 1T’ phase is metallic and the controlled transition from one phase to other has been investigated and the stabilization of one phase over the other is highly desirable. Among all the TMD tellurides, molybdenum telluride (MoTe2) exhibits co-existence of both 2H and 1T’ phases at room temperature due to the small energy difference between them [1]. While this co-existence is very promising for a large number of applications, it is also a challenge for the phase-specific synthesis of large area, highly crystalline films. Although there have been several studies on the Molecular Beam Epitaxy (MBE) synthesis of MoTe2 on various substrates [24], growth mechanism, the influence of substrate temperatures on the structure and crystalline quality, the transition between the phases as well as the stabilisation of one phase over another are not fully reported yet. We present a systematic study of the growth of MoTe2 on graphene/SiC(0001) by MBE at various substrate temperatures. The synthesized films were characterized by Reflection High Energy Electron Diffraction (RHEED), Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), X-ray Photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). For a given Te:Mo flux ratio, the crystalline quality of the films depends strongly on substrate temperature. At substrate temperatures between 150°C and 250°C, films containing co-existing phases are obtained. The 2H phase can be stabilized by annealing the films with and without Te flux. Annealing without Te flux produces 2H phase with higher number of defects and the domain edges are terminated by nanowires of Mo6Te6 (Figure 1). Annealing with Te flux improves the crystalline quality with uniform domains with smooth edges. STM analysis of the domains shows a network of inversion domain boundaries. Moiré patterns confirm the rotational alignment and symmetry seen in LEED. References

Molecular switches are building blocks of potential interest to store binary information, especially when they can be organized in periodic lattices. Among the variety of possible systems, switches based on hydrogen transfer are of special importance because they allow the switching operation to occur without severe conformational change that may interfere with neighboring molecular units. We have studied the excitation process of hydrogen transfer inside porphyrin molecules assembled on a graphene surface, using a low-temperature scanning tunneling microscope. We show that this hydrogen transfer is induced by an electronic resonant tunneling process through the molecular orbitals. Using nitrogen doping of graphene, we tune the rate of hydrogen transfer by shifting the molecular orbital energies owing to the charge transfer at nitrogen dopant sites in the graphene lattice. The control of the switching process allows the storage of information inside a molecular lattice, which is demonstrated by writing an artificial pattern inside a molecular island.

This chapter contains sections titled: Introduction Electron Beam Evaporation Technique Experimental Setup Growth Mechanism Film Characterization Conclusions Acknowledgments

MoTe2 has two stable solid phases. 2H-MoTe2 is semiconducting while 1T' is semimetallic. The selective synthesis of pure-phase thin films is still challenging. In this study, we have investigated the growth temperature dependence of MoTe2 synthesized by molecular beam epitaxy and have identified the optimum temperature for growing the stoichiometric films. It is confirmed that the crystalline quality of MoTe2 strongly depends on the substrate temperature. Post-growth annealing of grown layers at 400 °C stabilizes the semiconducting phase. The structural properties and the phase change in our materials are analyzed in details by reflection high energy electron diffraction, low energy electron diffraction, auger electron spectroscopy, x-ray photoemission spectroscopy, and scanning tunneling microscopy.

The combination of graphene with molecules offers promising opportunities to achieve new functionalities. In these hybrid structures, interfacial charge transfer plays a key role in the electronic properties and thus has to be understood and mastered. Using scanning tunneling microscopy and ab initio density functional theory calculations, we show that combining nitrogen doping of graphene with an electric field allows for a selective control of the charge state in a molecular layer on graphene. On pristine graphene, the local gating applied by the tip induces a shift of the molecular levels of adsorbed molecules and can be used to control their charge state. Ab initio calculations show that under the application of an electric field, the hybrid molecule/graphene system behaves like an electrostatic dipole with opposite charges in the molecule and graphene sub-units that are found to be proportional to the electric field amplitude, which thereby controls the charge transfer. When local gating is combined with nitrogen doping of graphene, the charging voltage of molecules on nitrogen is greatly lowered. Consequently, applying the proper electric field allows one to obtain a molecular layer with a mixed charge state, where a selective reduction is performed on single molecules at nitrogen sites.

The transfer of CVD-grown graphene sheets onto arbitrary substrates is important for the development of practical applications. Unfortunately, designing a low cost and highly efficient graphene transfer technique to achieve defect-free graphene sheets with low contact resistance onto various substrates still remains a challenge. In this paper, a CVD grown monolayer graphene sheet was directly transferred on SiO2/Si substrate. We found that a combination of floating copper with graphene films on ammonium persulfate solution with an original method of supercritical CO2 fluid can effectively produce clean and dry samples without damaging the crystalline quality of graphene. This method does not require any polymeric material to be desposited on the graphene films at any stage. Samples are analyzed by optical microscopy, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. This method is very promising for cleaning graphene samples for electronic device fabrication.

Although there are several kinds of thermal interfacial materials used in the electronic semiconductor industry, such as thermal grease, thermal glue, thermal gap filler, thermal pad and thermal adhesive, the problem of heat dissipation still remains a challenge. In this context, chemical vapor deposition of graphene on copper foils in vacuum has recently become considered as a wonderful hybrid material (graphene/copper/graphene) for more demanding thermal management applications, thanks to the unique properties of graphene in comparison with other materials. We found that the thermal properties of copper films change as graphene is deposited on top of the copper surface. Especially, a single atomic plane of graphene can significantly increase the film's thermal conductivity. Our graphene on copper foil was analyzed and measured by optical microscopy, Raman spectroscopy, scanning electron microscopy and heat transfer technique. This stack of graphene/copper/graphene materials may play a very important role as a potential material with superior thermal conductivity to replace traditional copper shim thermal pads in current electronic devices.

Three-dimensional intercalated porous graphene has been formed on Si(111) by electron beam evaporation under appropriate conditions and its structural and electronic properties investigated in detail by reflection high-energy electron diffraction, x-ray photoemission spectroscopy, Raman spectroscopy, high-resolution scanning electron microscopy, atomic force microscopy, and scanning tunneling microscopy. The results show that the crystalline quality of the porous graphene depended not only on the substrate temperature but also on the SiC layer thickness during carbon atom deposition.