Xingyu Gao

Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene

In situ low-temperature scanning tunneling microscopy is used to study the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on epitaxial graphene (EG) on 6H-SiC(0001), as well as on HOPG for comparison. PTCDA adopts a layer-by-layer growth mode, with its molecular plane lying flat on both surfaces. The PTCDA films grow continuously over the EG step edges, but not on HOPG. STS performed on single-layer PTCDA on monolayer EG shows a wide band gap larger than 3.3 eV, consistent with pristine PTCDA films. Synchrotron-based high-resolution photoemission spectroscopy reveals weak charge transfer between PTCDA and EG. This suggests weak electronic coupling between PTCDA and the underlying EG layer.

Surface transfer hole doping of epitaxial graphene using MoO3 thin film

Synchrotron-based in situ photoelectron spectroscopy investigations demonstrate effective surface transfer p-type doping of epitaxial graphene (EG) thermally grown on 4H–SiC(0001) via the deposition of MoO3 thin film on top. The large work function difference between EG and MoO3 facilitates electron transfer from EG to the MoO3 thin film. This leads to hole accumulation in the EG layer with an areal hole density of about 1.0×1013 cm−2, and places the Fermi level 0.38 eV below the graphene Dirac point.

Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation

Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

Molecular orientation dependent interfacial dipole at the F16CuPc∕CuPc organic heterojunction interface

In situ synchrotron-based near-edge x-ray absorption fine structure measurements and photoemission spectroscopy have been used to investigate the effect of molecular orientation on the interfacial dipole and the energy level alignment at the interfaces of organic heterojunctions comprising copper-hexadecafluoro-phthalocyanine (F16CuPc) on both standing-up and lying-down copper(II) phthalocyanine (CuPc) thin films. It is found that F16CuPc thin films adopt the same molecular orientation of the underlying CuPc thin films. An interfacial dipole of 0.45eV forms at the interface of lying-down F16CuPc∕CuPc on highly ordered pyrolytic graphite. In contrast, a much larger interfacial dipole of 1.35eV appears at the interface of standing-up F16CuPc∕CuPc on octane-1-thiol terminated Au(111).

Molecular orientation of 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride thin films at organic heterojunction interfaces

In situ low-temperature scanning tunneling microscopy and near-edge x-ray absorption spectroscopy measurements have been used to investigate the molecular orientation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films at the interface of organic heterojunctions of PTCDA on copper(II) phthalocyanine (CuPc). On the CuPc monolayer on highly oriented pyrolitic graphite, PTCDA molecules form a well-ordered in-plane herringbone structure with their molecular plane parallel to the substrate surface. The formation of multiple in-plane hydrogen bonding between neighboring PTCDA molecules is responsible for the flat-lying PTCDA on CuPc monolayer, and gives rise to the lying-down orientation of PTCDA thin films on both standing-up and lying-down CuPc thin films, as well as on Au(111) passivated by a self-assembled monolayer of octane-1-thiol.

Orientation-controlled charge transfer at CuPc/F16CuPc interfaces

Molecular orientation-controlled charge transfer has been observed at the organic-organic heterojunction interfaces of copper-hexadecafluoro-phthalocyanine (F16CuPc) or copper(II) phthalocyanine (CuPc) on both standing-up and lying-down CuPc or F16CuPc thin films. In situ synchrotron-based photoemission spectroscopy reveals that the charge transfer at the standing F16CuPc/CuPc or CuPc/F16CuPc interface is much larger than that at the lying F16CuPc/CuPc or CuPc/F16CuPc interface. This can be explained by the orientation-dependent ionization potentials of well-ordered organic thin films, which place the highest-occupied molecular orbital of the standing CuPc film much closer to the lowest-unoccupied molecular orbital of the standing F16CuPc film, facilitating stronger charge transfer as compared to that at the lying OOH interfaces. Our results suggest the possibility of manipulating interfacial electronic structures of organic heterojunctions by controlling the molecular orientation, in particular for appli...

Probing the ultrafast electron transfer at the CuPc∕Au(111) interface

Core-hole clock spectroscopy and near-edge x-ray-absorption fine structure measurements have been used to investigate the ultrafast electron transfer dynamics at the Copper(II) phthalocyanine (CuPc)∕Au(111) interface. It was found that the strong electronic coupling between the first layer of CuPc molecules and Au(111) substrate favors ultrafast electron transfer from the lowest unoccupied molecular orbital of the CuPc molecules to the conduction band of Au(111) in the time scale of ∼6fs. In contrast, the intermolecular electron transfer within multilayers of CuPc molecules via the weak van der Waals interaction was much slower.

Molecular orientation of CuPc thin films on C60/Ag(111)

The molecular orientation of copper(II) phthalocyanine (CuPc) thin films on monolayer C60 on Ag(111) was studied by in situ near-edge x-ray absorption fine structure measurements and low-temperature scanning tunneling microscopy. It is found that in densely packed thin films, CuPc molecules adopt a standing-up configuration with the molecular π-plane tilting slightly from the surface normal on monolayer C60.

Thickness-dependent energy level alignment of rubrene adsorbed on Au(111)

Energy level alignment of rubrene adsorbed on Au(111) was studied by photoemission spectroscopy. After rubrene adsorption, the work function is reduced from 5.24eV for clean Au to 4.31eV, suggesting the invalidity of vacuum level alignment and the presence of a strong interfacial dipole. The frontier molecular orbital energies of rubrene are modified by electrode surface polarization in the submonolayer regime. As a consequence, the hole injection barrier is thickness dependent and varies from about 0.4eV for a monolayer of rubrene to 0.9eV for a thick layer.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.