Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order.

When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moirésuperlattices form. Small lattice rotations between the two materials in contact strongly modify the moirélattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree-and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111). We find that selecting a substrate temperature between 1530 and 1000 K during the growth of graphene leads to distinct relative rotational angles of 0°, ± 0.6°, ±1.1°, and ±1.7°. When modeling the moirésuperlattices as two-dimensional coincidence networks, we can ascribe the observed rotations to favorable low-strain graphene structures. The dissimilar thermal expansion of the substrate and graphene is regarded as an effective compressive biaxial pressure that is more easily accommodated in graphene by small rotations rather than by compression.

Articles you may be interested inWell-width dependence of in-plane optical anisotropy in (001) GaAs/AlGaAs quantum wells induced by in-plane uniaxial strain and interface asymmetry Strain-induced splitting of the valence band in epitaxially lifted-off GaAs films Low temperature photoluminescence and reflectance measurements on epitaxially lifted-off ͑ELO͒ bulk GaAs and GaAs/AlGaAs multiple quantum wells ͑MQWs͒ bonded to Si and MgO substrates are reported. Photoluminescence measurements indicate no strain at room temperature for the ELO bulk GaAs film but show biaxial strain at 10 K. Si-bonded films undergo tensile strain, while films with MgO host substrates experience compressive strain. Reflectance measurements at 10 K show that light hole band is closer to the conduction band for the tensile strained film. In GaAs MQW ELO films, the separation of the heavy hole and light hole band is reduced in tensile strained films by 4.7 meV, corresponding to a strain = −0.7Ϯ 0.05ϫ 10 −3 and stress X = 0.9Ϯ 0.05 kbar ͑90Ϯ 5 MPa͒. For compressively strained films, this separation is enhanced by 3.9 meV, equivalent to a strain = 0.6Ϯ 0.05ϫ 10 −3 and X = 0.8Ϯ 0.05ϫ 10 −3 kbar ͑80Ϯ 5 MPa͒. The findings demonstrate that ELO is an effective technique to introduce tensile and compressive strain in GaAs heterostructures and is appropriate for strain-related spectroscopy.

A broad, bell-shaped intensity component is observed in low-energy electron diffraction from high-quality epitaxial 2D-systems. Three 2Dsystems, graphene on Ir(111), graphene on SiC(0001), and hexagonal boron nitride on Ir(111), have been prepared in situ under ultra-high vacuum conditions. In all three systems-independent of substrate material-similar strong diffuse intensity is observed, exhibiting a width as large as 50% of the Brillouin zone and an integrated intensity more than 10 times the intensity of the Bragg spots. The presented experimental results provide evidence for a common origin of such diffuse diffraction intensity in different atomically thin 2D-materials.

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