The excitonic behavior of anisotropic two-dimensional crystals is investigated using numerical methods. We employ a screened potential arising due to the system polarizability to solve the ...central-potential problem using the Numerov approach. The dependence of the exciten energies on the interaction strength and mass anisotropy is demonstrated. We use our results to obtain the exciton binding energy in phosphorene as a function of the substrate dielectric constant.
Graphenes extremely small intrinsic spinorbit (SO) interaction1 makes the realization of many interesting phenomena such as topological/quantum spin Hall states and the spin Hall effect4 (SHE) ...practically impossible. Recently, it was predicted1,57 that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp2 to sp3 bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization8, is experimentally challenging.
Two-dimensional dilute magnetic semiconductors can provide fundamental insights into the very nature of magnetic order and their manipulation through electron and hole doping. Besides the fundamental ...interest, due to the possibility of control of charge density, they can be extremely important in spintronics applications such as spin valve and spin-based transistors. In this paper, we studied a two-dimensional dilute magnetic semiconductor consisting of a phosphorene monolayer doped with cobalt atoms in substitutional and interstitial defects. We show that these defects can be stabilized and are electrically active. Furthermore, by including holes or electrons by a potential gate, the exchange interaction and magnetic order can be engineered, and may even induce a ferromagnetic-to-antiferromagnetic phase transition in p-doped phosphorene. At a Co concentration of 2.7%, we estimate a Curie temperature of T super(MFA) sub(C) = 466 K in the mean-field approximation.
van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared ...nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in other polar van der Waals crystals and may lead to new functionalities.
Strain engineering has been proposed as an alternative method for manipulating the electronic properties of graphene. However, the bottleneck for strain engineering in graphene has been the ability ...to control such strain patterns at the nanoscale. Here we show that high level of control can be accomplished by chemically modifying the adherence of graphene on metal. Using scanning tunnelling microscopy, the shape evolution of graphene Moiré blisters towards geometrically well-defined graphene bubbles was studied during the controlled, sub-layer oxidation of the ruthenium substrate. Understanding the dynamics of the oxidation process and defects generation on the Ru substrate allows us to control the size, shape and the density of the bubbles and its associated pseudo-magnetism. We also show that a modification of the same procedure can be used to create antidots in graphene by catalytic reaction of the same nanobubbles.
Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a ...crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.
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The presence of metallic species around failed implants raises concerns about the stability of titanium alloy (Ti-6Al-4V). Graphene nanocoating on titanium alloy (GN) has promising ...anti-corrosion properties, but its long-term protective potential and structural stability remains unknown. The objective was to determine GN’s anti-corrosion potential and stability over time.
GN and uncoated titanium alloy (Control) were challenged with a highly acidic fluorinated corrosive medium (pH 2.0) for up to 240 days. The samples were periodically tested using potentiodynamic polarization curves, electrochemical impedance spectroscopy and inductively coupled plasma-atomic emission spectroscopy (elemental release). The integrity of samples was determined using Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. Statistical analyses were performed with one-sample t-test, paired t-test and one-way ANOVA with Tukey post-hoc test with a pre-set significance level of 5%.
There was negligible corrosion and elemental loss on GN. After 240 days of corrosion challenge, the corrosion rate and roughness increased by two and twelve times for the Control whereas remained unchanged for GN. The nanocoating presented remarkably high structural integrity and coverage area (>98%) at all time points tested.
Graphene nanocoating protects titanium alloy from corrosion and dissolution over a long period while maintaining high structural integrity. This coating has promising potential for persistent protection of titanium and potentially other metallic alloys against corrosion.
Point defects in the binary group-IV monochalcogenide monolayers of SnS, SnSe, GeS, and GeSe are investigated using density functional theory calculations. Several stable configurations are found for ...oxygen defects, however, we give evidence that these materials are less prone to oxidation than phosphorene, with which monochalcogenides are isoelectronic and share the same orthorhombic structure. Concurrent oxygen defects are expected to be vacancies and substitutional oxygen. We show that it is energetically favorable for oxygen to be incorporated into the layers substituting for a chalcogen (O sub(S/Se) defects), and different from most of the other defects investigated, this defect preserves the electronic structure of the material. Thus, we suggest that annealing treatments can be useful for the treatment of functional materials where loss mechanisms due to the presence of defects are undesirable.
Graphene has attracted much interest in both academia and industry. The challenge of making it semiconducting is crucial for applications in electronic devices. A promising approach is to reduce its ...physical size down to the nanometer scale. Here, we present the surface-assisted bottom-up fabrication of atomically precise armchair graphene nanoribbons (AGNRs) with predefined widths, namely 7-, 14- and 21-AGNRs, on Ag(111) as well as their spatially resolved width-dependent electronic structures. STM/STS measurements reveal their associated electron scattering patterns and the energy gaps over 1 eV. The mechanism to form such AGNRs is addressed based on the observed intermediate products. Our results provide new insights into the local properties of AGNRs, and have implications for the understanding of their electrical properties and potential applications.
We have studied the optical conductivity of two-dimensional (2D) semiconducting transition metal dichalcogenides using ab initio density functional theory. We find that this class of materials ...presents large optical response due to the phenomenon of band nesting. The tendency towards band nesting is enhanced by the presence of van Hove singularities in the band structure of these materials. Given that 2D crystals are atomically thin and naturally transparent, our results show that it is possible to have strong photon-electron interactions even in 2D.
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