The ability to understand and control the electronic properties of individual molecules in a device environment is crucial for developing future technologies at the nanometre scale and below. ...Achieving this, however, requires the creation of three-terminal devices that allow single molecules to be both gated and imaged at the atomic scale. We have accomplished this by integrating a graphene field effect transistor with a scanning tunnelling microscope, thus allowing gate-controlled charging and spectroscopic interrogation of individual tetrafluoro-tetracyanoquinodimethane molecules. We observe a non-rigid shift in the molecule's lowest unoccupied molecular orbital energy (relative to the Dirac point) as a function of gate voltage due to graphene polarization effects. Our results show that electron-electron interactions play an important role in how molecular energy levels align to the graphene Dirac point, and may significantly influence charge transport through individual molecules incorporated in graphene-based nanodevices.
The problem of electron-electron interactions in graphene is reviewed. Starting from the screening of long-range interactions in these systems, the existence of an emerging Dirac liquid of Lorentz ...invariant quasiparticles in the weak-coupling regime is discussed, as well as the formation of strongly correlated electronic states in the strong-coupling regime. The analogy and connections between the many-body problem and the Coulomb impurity problem are also analyzed. The problem of the magnetic instability and Kondo effect of impurities and/or adatoms in graphene is also discussed in analogy with classical models of many-body effects in ordinary metals. Lorentz invariance is shown to play a fundamental role and leads to effects that span the whole spectrum, from the ultraviolet to the infrared. The effect of an emerging Lorentz invariance is also discussed in the context of finite size and edge effects as well as mesoscopic physics. The effects of strong magnetic fields in single layers and some of the main aspects of the many-body problem in graphene bilayers are briefly reviewed. In addition to reviewing the fully understood aspects of the many-body problem in graphene, a plethora of interesting issues are shown to remain open, both theoretically and experimentally, and the field of graphene research is still exciting and vibrant.
We demonstrate a straightforward and effective laser pruning approach to reduce multilayer black phosphorus (BP) to few-layer BP under ambient condition. Phosphorene oxides and suboxides are formed ...and the degree of laser-induced oxidation is controlled by the laser power. Since the band gaps of the phosphorene suboxide depend on the oxygen concentration, this simple technique is able to realize localized band gap engineering of the thin BP. Micropatterns of few-layer phosphorene suboxide flakes with unique optical and fluorescence properties are created. Remarkably, some of these suboxide flakes display long-term (up to 2 weeks) stability in ambient condition. Comparing against the optical properties predicted by first-principle calculations, we develop a “calibration” map in using focused laser power as a handle to tune the band gap of the BP suboxide flake. Moreover, the surface of the laser patterned region is altered to be sensitive to toxic gas by way of fluorescence contrast. Therefore, the multicolored display is further demonstrated as a toxic gas monitor. In addition, the BP suboxide flake is demonstrated to exhibit higher drain current modulation and mobility comparable to that of the pristine BP in the electronic application.
The scattering of two-dimensional massless Dirac fermions from local spin-orbit interactions with an origin in dilute concentrations of physisorbed atomic species on graphene is theoretically ...investigated. The hybridization between graphene and the adatoms' orbitals lifts spin and valley degeneracies of the pristine host material, giving rise to rich spin-orbit coupling mechanisms with features determined by the exact adsorption position on the honeycomb latticeemdashbridge, hollow, or top positionemdashand the adatoms' outer-shell orbital type. Effective graphene-only Hamiltonians are derived from symmetry considerations, while a microscopic tight-binding approach connects effective low-energy couplings and graphene-adatom hybridization parameters. Within the T-matrix formalism, a theory for (spin-dependent) scattering events involving graphene's charge carriers, and the spin-orbit active adatoms is developed. Spin currents associated with intravalley and intervalley scattering are found to tend to oppose each other. We establish that under certain conditions, hollow-position adatoms give rise to the spin Hall effect, through skew scattering, while top-position adatoms induce transverse charge currents via trigonal potential scattering. We also identify the critical Fermi energy range where the spin Hall effect is dramatically enhanced, and the associated transverse spin currents can be reversed.
Heteroepitaxy of two-dimensional (2D) crystals, such as hexagonal boron nitride (BN) on graphene (G), can occur at the edge of an existing heterointerface. Understanding strain relaxation at such 2D ...laterally fused interface is useful in fabricating heterointerfaces with a high degree of atomic coherency and structural stability. We use in situ scanning tunneling microscopy to study the 2D heteroepitaxy of BN on graphene edges on a Ru(0001) surface with the aim of understanding the propagation of interfacial strain. We found that defect-free, pseudomorphic growth of BN on a graphene edge “substrate” occurs only for a short distance (<1.29 nm) perpendicular to the interface, beyond which misfit zero-dimensional dislocations occur to reduce the elastic strain energy. Boundary states originating from a coherent zigzag-linked G/BN boundary are observed to greatly enhance the local conductivity, thus affording a new avenue to construct one-dimensional transport channels in G/BN hybrid interface.
To evaluate the surface and wettability characteristics and the microbial biofilm interaction of graphene coating on titanium.
Graphene was deposited on titanium (Control) via a liquid-free ...technique. The transfer was performed once (TiGS), repeated two (TiGD) and five times (TiGV) and characterized by AFM (n=10), Raman spectroscopy (n=10), contact angle and SFE (n=5). Biofilm formation (n=3) to Streptococcus mutans, Enterococcus faecalis, Pseudomonas aeruginosa and Candida albicans was evaluated after 24h by CV assay, CFU, XTT and confocal microscopy. Statistics were performed by one-way Anova, Tukey’s tests and Pearson’s correlation analysis at a pre-set significance level of 5 %.
Raman mappings revealed coverage yield of 82 % for TiGS and ≥99 % for TiGD and TiGV. Both TiGD and TiGV presented FWHM>44cm−1 and ID/IG ratio<0.12, indicating multiple graphene layers and occlusion of defects. The contact angle was significantly higher for TiGD and TiGV (110° and 117°) comparing to the Control (70°). The SFE was lower for TiGD (13.8mN/m) and TiGV (12.1mN/m) comparing to Control (38.3mN/m). TiGD was selected for biofilm assays and exhibited significant reduction in biofilm formation for all microorganisms compared to Control. There were statistical correlations between the high contact angle and low SFE of TiGD and decreased biofilm formation.
TiGD presented high quality and coverage and decreased biofilm formation for all species. The increased hydrophobicity of graphene films was correlated with the decreased biofilm formation for various species.
Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been ...achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer‐space‐confined chemical design (ICCD) is reported for the synthesis of dilute single‐atom‐doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H‐TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single‐atom doping via co‐intercalated cobalt ions, resulting in the formation of quasi‐monolayer Co‐doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital‐selected p–d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal‐size TaS2 molecular superlattices.
Ferromagnetism is successfully introduced in superconducting TaS2 molecular superlattices with single Co atom doping, which is achieved by an interlayer‐space‐confined chemical design (ICCD). This ICCD approach can be applied to various metal ions, opening up new avenues for designing artificial 2D material superlattices with exotic phases and desired functionalities.
Graphene has shown great application potential as the host material for next-generation electronic devices. However, despite its intriguing properties, one of the biggest hurdles for graphene to be ...useful as an electronic material is the lack of an energy gap in its electronic spectra. This, for example, prevents the use of graphene in making transistors. Although several proposals have been made to open a gap in graphene's electronic spectra, they all require complex engineering of the graphene layer. Here, we show that when graphene is epitaxially grown on SiC substrate, a gap of approximately 0.26 eV is produced. This gap decreases as the sample thickness increases and eventually approaches zero when the number of layers exceeds four. We propose that the origin of this gap is the breaking of sublattice symmetry owing to the graphene-substrate interaction. We believe that our results highlight a promising direction for bandgap engineering of graphene.
Titanium implants present 2 major drawbacks—namely, the long time needed for osseointegration and the lack of inherent antimicrobial properties. Surface modifications and coatings to improve ...biomaterials can lose their integrity and biological potential when exposed to stressful microenvironments. Graphene nanocoating (GN) can be deposited onto actual-size dental and orthopedic implants. It has antiadhesive properties and can enhance bone formation in vivo. However, its ability to maintain structural integrity and quality when challenged by biologically relevant stresses remains largely unknown. GN was produced by chemical vapor deposition and transferred to titanium via a polymer-assisted transfer technique. GN has high inertness and did not increase expression of inflammatory markers by macrophages, even in the presence of lipopolysaccharides. It kept high coverage at the top tercile of tapered dental implant collars after installation and removal from bone substitute and pig maxilla. It also resisted microbiologically influenced corrosion, and it maintained very high coverage area and quality after prolonged exposure to biofilms and their removal by different techniques. Our findings show that GN is unresponsive to harsh and inflammatory environments and that it maintains a promising level of structural integrity on the top tercile of dental implant collars, which is the area highly affected by biofilms during the onset of implant diseases. Our findings open the avenues for the clinical studies required for the use of GN in the development of implants that have higher osteogenic potential and are less prone to implant diseases.
Realistic relaxed configurations of triaxially strained graphene quantum dots are obtained from unbiased atomistic mechanical simulations. The local electronic structure and quantum transport ...characteristics of y-junctions based on such dots are studied, revealing that the quasi-uniform pseudomagnetic field induced by strain restricts transport to Landau level- and edge state-assisted resonant tunneling. Valley degeneracy is broken in the presence of an external field, allowing the selective filtering of the valley and chirality of the states assisting in the resonant tunneling. Asymmetric strain conditions can be explored to select the exit channel of the y-junction.
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