Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C₆₀ molecular crystals. Transmission electron microscopy and theoretical analysis of the C₆₀ /BP vdW heterostructure clearly confirm that the BP template results in oriented C₆₀ assembly with higher-order commensurism. Lateral and vertical devices with C₆₀ /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C₆₀ /BP junctions. Effective tuning of the C₆₀ /BP junction barrier from 0.2 to 0.5 eV and maximum on-current density higher than 10⁴  mA cm^-2 are achieved with graphite/C₆₀ /BP vertical vdW transistors. Due to the formation of high-quality C₆₀ film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W^-1 as well as a fast response time under visible light illumination.

Modification of an ultrathin C60 interlayer on the electronic structure and molecular packing of C8-BTBT on HOPG

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM) and X-ray diffraction (XRD) were applied to investigate the electronic structure and molecular packing of C8-BTBT on HOPG with an ultrathin C₆₀ interlayer. It was found that C8-BTBT displays a Vollmer-Weber (V-W) growth mode on HOPG, with an ultrathin C₆₀ interlayer (0.7 nm). Compared to the uniform lying-down growth mode as directly grown on HOPG, the C8-BTBT molecules here adopt a lying-down orientation at low coverage with some small tilt angles because the π-π interaction between C8-BTBT and HOPG is partly disturbed by the C₆₀ interlayer, delivering a higher highest occupied molecular orbital (HOMO) in C8-BTBT. An interface dipole of 0.14 eV is observed due to electron transport from C8-BTBT to C₆₀. The upward and downward band bending in C8-BTBT and C₆₀, respectively, near the C8-BTBT/C₆₀ interface reduces the hole transport barrier at the interface, facilitating the hole injection from C₆₀ to C8-BTBT, while a large electron transfer barrier from C₆₀ to C8-BTBT is detected at this interface, which effectively limits electron injection from C₆₀ to C8-BTBT. The HOMO of C8-BTBT near the interface is largely lifted up by the C₆₀ insertion layer, which causes a p-doping effect and increases the hole mobility in C8-BTBT. Furthermore, owing to the lowest occupied molecular orbital (LUMO) of C₆₀ residing in the gap of C8-BTBT, charge transfer occurs between C₆₀ and the trap states in C8-BTBT to effectively passivate the trapping states. Our efforts aid a better understanding of the electron structure and film growth of anisotropic molecules and provide a useful strategy to improve the performance of C8-BTBT-based devices.