Dongchen Qi

Room temperature ferromagnetism in partially hydrogenated epitaxial graphene

We report room temperature ferromagnetism in partially hydrogenated epitaxial graphene grown on 4HSiC(0001). The presence of ferromagnetism was confirmed by superconducting quantum interference devices measurements. Synchrotron-based near-edge x-ray absorption fine structure and high resolution electron energy loss spectroscopy measurements have been used to investigate the hydrogenation mechanism on the epitaxial graphene and the origin of room temperature ferromagnetism. The partial hydrogenation induces the formation of unpaired electrons in graphene, which together with the remnant delocalized π bonding network, can explain the observed ferromagnetism in partially hydrogenated epitaxial graphene.

Mutual ferromagnetic-ferroelectric coupling in multiferroic copper-doped ZnO.

A mutual ferromagnetic and ferroelectric coupling (multiferroic behavior) in Cu‐doped ZnO is demonstrated via deterministic control of Cu doping and defect engineering. The coexistence of multivalence Cu ions and oxygen vacancies is important to multiferroic behaviors in ZnO:Cu. The samples show clear ferroelectric and ferromagnetic domain patterns. These domain structures may be written reversibly via electric and magnetic bias.

Quasi-free-standing epitaxial graphene on SiC (0001) by fluorine intercalation from a molecular source.

We demonstrated a novel method to obtain charge neutral quasi-free-standing graphene on SiC (0001) from the buffer layer using fluorine from a molecular source, fluorinated fullerene (C(60)F(48)). The intercalated product is stable under ambient conditions and resistant to elevated temperatures of up to 1200 °C. Scanning tunneling microscopy and spectroscopy measurements are performed for the first time on such quasi-free-standing graphene to elucidate changes in the electronic and structural properties of both the graphene and interfacial layer. Novel structures due to a highly localized perturbation caused by the presence of adsorbed fluorine were produced in the intercalation process and investigated. Photoemission spectroscopy is used to confirm these electronic and structural changes.

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).

Surface transfer doping of diamond by MoO3 : a combined spectroscopic and Hall measurement study

Surface transfer doping of diamond has been demonstrated using MoO3 as a surface electron acceptor material. Synchrotron-based high resolution photoemission spectroscopy reveals that electrons are transferred from the diamond surface to MoO3, leading to the formation of a sub-surface quasi 2-dimensional hole gas within the diamond. Ex-situ electrical characterization demonstrated an increase in hole carrier concentration from 1.00 × 1013/cm2 for the air-exposed hydrogen-terminated diamond surface to 2.16 × 1013/cm2 following MoO3 deposition. This demonstrates the potential to improve the stability and performance of hydrogen-terminated diamond electronic devices through the incorporation of high electron affinity transition metal oxides.

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.

Chemical vapor deposition graphene as structural template to control interfacial molecular orientation of chloroaluminium phthalocyanine

Chemical vapor deposition (CVD) graphene has been used as an effective structural template to manipulate molecular orientation of organic thin film of chloroaluminium phthalocyanine (ClAlPc) on indium tin oxide (ITO) electrode. As revealed by in-situ near-edge x-ray adsorption fine structure measurement, ClAlPc molecules on the CVD graphene modified ITO electrode adopt a well-aligned lying-down configuration with their molecular π-plane nearly parallel to the electrode surface, in contrast to the random orientation of ClAlPc molecules on the bare ITO electrode. This lying-down configuration results in an optimized stacking of the molecular π-plane perpendicular to the electrode surface and hence facilitates efficient charge transport along this direction.

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.