By adapting the concept of epitaxy to two-dimensional space, we show the growth of a single-atomic-layer, in-plane heterostructure of a prototypical material system--graphene and hexagonal boron nitride (h-BN). Monolayer crystalline h-BN grew from fresh edges of monolayer graphene with atomic lattice coherence, forming an abrupt one-dimensional interface, or boundary. More important, the h-BN lattice orientation is solely determined by the graphene, forgoing configurations favored by the supporting copper substrate.

We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief in situ treatment, namely, melting and subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting-resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations.

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable improvement in the device performance of organic thin films, wherein charge transport, exciton diffusion, and dissociation are currently limited by grain size effects and molecular orientation.

Motivated from the recent success in synthesizing bismuth-based double perovskites (J. Am. Chem. Soc. 2016, 138, 2138–2141), we perform a comprehensive study of interfacial properties of bismuth-based double perovskites Cs2AgBiX6 (X = Br, Cl) and TiO2 interfaces. The bismuth-based double perovskites possess desirable electronic and optical properties as excellent light absorber and thus may serve as lead-free alternatives to the organic–inorganic perovskites. On the basis of density functional theory computation, we systematically study the Cs2BBiX6 (B = Ag, Cu; X = Br, Cl)/TiO2 interfaces and analyze the trend of charge transfer across the interfaces. We find that the Cs4X4 (X = Br and Cl)/TiO2-mediated interfaces are prospective interfaces for charge extraction and separation due largely to the withdrawn trap states for the TiO2 part when in contact with the Cs4X4 termination. Moreover, the ionic interaction and charge redistribution across the specific interfaces can lead to the appropriate band alignment, reduced band gap for the rock-salt double perovskite part, and smooth gradient distribution for the locally projected density of states along the normal direction to the interfaces, further facilitating the charge transfer. Overall, we predict that bismuth-based double perovskites Cs2AgBiX6 (X = Br, Cl) and TiO2 interfaces are highly efficient for charge extraction, suggesting high potential for interfacial engineering optoelectronic devices.

Double perovskites Cs2AgSbCl6 have been synthesized via the solution state for applications as a promising photovoltaic absorber. Considering TiO2 as an electron transport layer (ETL), Cs2AgSbCl6/TiO2 heterojunction nanoparticles have also been prepared by the hydrothermal process to study the interface effect. Experimental measurements show that Cs2AgSbCl6 has a cubic structure with the lattice constant of 10.699 Å. The absorption peaks in the optical spectrum of the Ag and Sb-based double perovskites agree well with our density functional theory calculations. The Cs2AgSbCl6/TiO2 heterostructure exhibits enhanced optical absorption in the visible-light region compared to that of Cs2AgSbCl6, which is caused by the formation of the interface states and the decreased bandgap, thus facilitating the photo-induced optical transition in the visible-light region. From the charge transfer analysis of two interfaces (Ag2Sb2Cl8/TiO2 and Cs4Cl4/TiO2 interfaces), we find that the efficient separation of photo-induced carriers can be achieved at the Cs4Cl4/TiO2 interface, with electron flowing from the double perovskite layer to the TiO2 ETL, which is beneficial for improving the power conversion efficiency of solar cells. The combined study of theory and experiments indicates that the double perovskites Cs2AgSbCl6 would be a promising light-absorbing material in contact with TiO2 for the lead-free perovskite-based solar cell devices.