Two limiting factors for a new technology of graphene-based electronic devices are the difficulty of growing large areas of defect-free material and the integration of graphene with an atomically flat and insulating substrate material. Chemical vapor deposition (CVD) on metal surfaces, in particular on copper, may offer a solution to the first problem, while hexagonal boron nitride (h-BN) has been identified as an ideal insulating substrate material. The bottom-up growth of graphene/h-BN stacks on copper surfaces appears therefore as a promising route for future device fabrication. As an important step, we demonstrate the consecutive growth of well-aligned graphene on h-BN, both as single layers, by low-pressure CVD on Cu(111) in an ultrahigh vacuum environment. The resulting films show a largely predominant orientation, defined by the substrate, where the graphene lattice aligns parallel to the h-BN lattice, while each layer maintains its own lattice constant. The lattice mismatch of 1.6% between h-BN and graphene leads to a moiré pattern with a periodicity of about 9 nm, as observed with scanning tunneling microscopy. Accordingly, angle-resolved photoemission data reveal two slightly different Brillouin zones for electronic states localized in graphene and in h-BN, reflecting the vertical decoupling of the two layers. The graphene appears n-doped and shows no gap opening at the K[overline] point of the two-dimensional Brillouin zone.

Single atoms, and in particular the least reactive noble gases, are difficult to immobilize at room temperature. Ion implantation into a crystal lattice has this capability, but the randomness of the involved processes does not permit much control over their distribution within the solid. Here we demonstrate that the boron nitride nanomesh, a corrugated single layer of hexagonal boron nitride (h-BN) with a 3.2 nm honeycomb superstructure formed on a Rh(111) surface, can trap individual argon atoms at distinct subsurface sites at room temperature. A kinetic energy window for implantation is identified where the argon ions can penetrate the h-BN layer but not enter the Rh lattice. Scanning tunneling microscopy and photoemission data show the presence of argon atoms at two distinct sites within the nanomesh unit cell, confirmed also by density functional theory calculations. The single atom implants are stable in air. Annealing of implanted structures to 900 K induces the formation of highly regular holes of 2 nm diameter in the h-BN layer with adjacent flakes of the same size found on top of the layer. We explain this "can-opener" effect by the presence of a vacancy defect, generated during the penetration of the Ar ion through the h-BN lattice, and propagating along the rim of a nanomesh pore where the h-BN lattice is highly bent. The reported effects are also observed in graphene on ruthenium and for neon atoms.

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr 3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26 AE 0.02 m e , 50% heavier than the bare mass m 0 ¼ 0.17 m e predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α ¼ 1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

The setup of an apparatus for chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) and its characterization on four-inch wafers in ultra high vacuum (UHV) environment is reported. It provides well-controlled preparation conditions, such as oxygen and argon plasma assisted cleaning and high temperature annealing. In situ characterization of a wafer is accomplished with target current spectroscopy. A piezo motor driven x-y stage allows measurements with a step size of 1 nm on the complete wafer. To benchmark the system performance, we investigated the growth of single layer h-BN on epitaxial Rh(111) thin films. A thorough analysis of the wafer was performed after cutting in atmosphere by low energy electron diffraction, scanning tunneling microscopy, and ultraviolet and X-ray photoelectron spectroscopies. The apparatus is located in a clean room environment and delivers high quality single layers of h-BN and thus grants access to large area UHV processed surfaces, which had been hitherto restricted to expensive, small area single crystal substrates. The facility is versatile enough for customization to other UHV-CVD processes, e.g., graphene on four-inch wafers.

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