RESUMEN
Magnetic fields can have profound effects on the motion of electrons in quantum materials. Two-dimensional electron systems subject to strong magnetic fields are expected to exhibit quantized Hall conductivity, chiral edge currents and distinctive collective modes referred to as magnetoplasmons and magnetoexcitons. Generating these propagating collective modes in charge-neutral samples and imaging them at their native nanometre length scales have thus far been experimentally elusive. Here we visualize propagating magnetoexciton polaritons at their native length scales and report their magnetic-field-tunable dispersion in near-charge-neutral graphene. Imaging these collective modes and their associated nano-electro-optical responses allows us to identify polariton-modulated optical and photo-thermal electric effects at the sample edges, which are the most pronounced near charge neutrality. Our work is enabled by innovations in cryogenic near-field optical microscopy techniques that allow for the nano-imaging of the near-field responses of two-dimensional materials under magnetic fields up to 7 T. This nano-magneto-optics approach allows us to explore and manipulate magnetopolaritons in specimens with low carrier doping via harnessing high magnetic fields.
RESUMEN
Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported. Micrometre-scale single-crystal domains formed as isolated faceted islands or merged together to achieve full monolayer coverage. The crystal structure of this phase has ten boron atoms and two hexagonal vacancies in its unit cell. First-principles calculations indicate that charge transfer, rather than covalent bonding, binds this two-dimensional boron to the Cu(100) surface. The electronic band structure of this material features multiple anisotropic tilted Dirac cones.
RESUMEN
Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on NixPd1-x(111) alloy substrates was systematically investigated experimentally as a function of Si surface coverage, annealing time and temperature, O2 partial pressure, and substrate composition and the results were compared with thermodynamic predictions based on density functional theory (DFT) calculations and thermochemical data for O2. Experimentally, 2D Ni silicate was exclusively observed at higher O2 pressures (â¼10-6 Torr), higher annealing temperatures (1000 K), and more prolonged annealing (10 min) if the substrate contained any Ni and for initial Si coverages up to 2 monolayers. In contrast, decreasing the O2 pressure to â¼10-8 Torr and restricting the annealing temperature and time enabled 2D VDW silica formation. Amorphous 2D VDW silica was observed even when the substrate composition was tuned to lattice match crystalline 2D VDW silica. The trend of decreased O2 pressure favoring 2D VDW silica was consistent with the theoretical predictions; however, theory also suggested that sufficient Si coverage could avoid Ni silicate formation. The effect of epitaxial strain on 2D Ni silicate was investigated by modifying the solid solution alloy substrate composition. It was found that 2D Ni silicate will stretch to match the substrate lattice constant up to 1.12% tensile strain. When the lattice mismatch was over 1.40%, incommensurate crystalline domains were observed, indicating relaxation of the overlayer to its favored lattice constant. The limited epitaxial strain that could be applied was attributed to a combination of the 2D silicate stiffness, the insensitivity of its bonding to the substrate to its alignment with the substrate, and its lack of accessible structural rearrangements that can reduce the strain energy. The results demonstrate how the resulting 2D material can be manipulated through the growth conditions and how a solid solution alloy substrate can be used to maximize the epitaxial strain imparted to the 2D system.
RESUMEN
Borophene, a theoretically proposed two-dimensional (2D) boron allotrope1-3, has attracted much attention4,5 as a candidate material platform for high-speed, transparent and flexible electronics6-9. It was recently synthesized, on Ag(111) substrates10,11, and studied by tunnelling and electron spectroscopy12. However, the exact crystal structure is still controversial, the nanometre-size single-crystal domains produced so far are too small for device fabrication and the structural tunability via substrate-dependent epitaxy is yet to be proven. We report on the synthesis of borophene monitored in situ by low-energy electron microscopy, diffraction and scanning tunnelling microscopy (STM) and modelled by ab initio theory. We resolved the crystal structure and phase diagram of borophene on Ag(111), but found that the domains remain nanoscale for all growth conditions. However, by growing borophene on Cu(111) surfaces, we obtained large single-crystal domains, up to 100 µm2 in size. The crystal structure is a novel triangular network with a concentration of hexagonal vacancies of η = 1/5. Our experimental data, together with first principles calculations, indicate charge-transfer coupling to the substrate without significant covalent bonding. Our work sets the stage for fabricating borophene-based devices and substantiates the idea of borophene as a model for development of artificial 2D materials.
