Hongliang Lu

Tunable interfacial properties of epitaxial graphene on metal substrates

We report on tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the graphene/metal systems. These findings may give further insights to the interfacial interactions in the graphene/metal systems and promote the use of graphene-based heterostructures in devices.

Studies of graphene-based nanoelectromechanical switches

AbstractElectromechanical switch devices employing suspended graphene as movable elements have been developed. Their on and off states can be controlled by modulating the electrostatic force applied to the graphene. The devices exhibit on-off ratios of up to 104 and lifetimes of over 500 cycles. The prototype device demonstrates the feasibility of using multilayer graphene in electromechanical systems. Measurements of the mechanical properties of the free-standing monolayer graphene gave a value of 0.96 TPa for the Young’s modulus and a van der Waals force with silicon oxide of 0.17 nN/nm2.

Reversible insulator-metal transition of LaAlO3/SrTiO3 interface for nonvolatile memory

We report a new type of memory device based on insulating LaAlO3/SrTiO3 (LAO/STO) hetero-interface. The microstructures of the LAO/STO interface are characterized by Cs-corrected scanning transmission electron microscopy, which reveals the element intermixing at the interface. The inhomogeneous element distribution may result in carrier localization, which is responsible for the insulating state. The insulating state of such interface can be converted to metallic state by light illumination and the metallic state maintains after light off due to giant persistent photoconductivity (PPC) effect. The on/off ratio between the PPC and the initial dark conductance is as large as 105. The metallic state also can be converted back to insulating state by applying gate voltage. Reversible and reproducible resistive switching makes LAO/STO interface promising as a nonvolatile memory. Our results deepen the understanding of PPC phenomenon in LAO/STO, and pave the way for the development of all-oxide electronics integrating information storage devices.

Step terrace tuned anisotropic transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on vicinal SrTiO3 substrates.

Highly epitaxial LaBaCo2O5.5+δ (LBCO) thin films were grown on different miscut (001) SrTiO3 substrates (miscut angle of 0.5°, 3.0°, and 5.0°) to study the substrate surface step terrace effect on the in-plane electrical transport properties. The microstructure studies by X-ray diffraction and transmission electron microscopy indicate that the as-grown films are A-site disordered cubic perovskite structures with the c-axis highly oriented along the film growth direction. The four-probe scanning tunneling microscopy (STM) studies show that the LBCO thin films grown on the vicinal SrTiO3 substrates have a typical semiconductor behavior with the substrate surface terrace step inducing anisotropic electronic transport properties. These results indicate that in highly epitaxial thin films the surface terrace step induced local strains can play an important role in controlling the electronic transport properties and the anisotropic nature.

Surface-step-terrace-induced anomalous transport properties in highly epitaxial La0.67Ca0.33MnO3 thin films

La0.67Ca0.33MnO3 thin films were epitaxially grown on miscut MgO(001) substrates by pulsed laser ablation. Electrical transport properties were studied by using an ultra high vacuum, four-probe STM system at different temperatures. Anomalous resistivity behavior and metal-insulator transition temperatures were found, both of which are highly dependent upon the miscut angle (1, 3, and 5°). These phenomena are attributed to the difference in residual strain that results from the difference in terrace widths of the vicinal surfaces.

Synthesis of PbTe/Pb quasi-one-dimensional nanostructure material arrays by electrodeposition

The ordered PbTe/Pb quasi-one-dimensional nanowires array was electrodeposited on the SiO2/Si substrate. There are two essential factors for the formation of such kind of change in nanowire morphology and structure. One is the charges distribution at the tips of electrodeposit, the other one is the change in ion concentration in front of growth tip. We research the current versus bias voltage characteristics of single PbTe/Pb nanowire by four-probe scanning tunneling microscopy system.

Epitaxial fabrication of two-dimensional NiSe2 on Ni(111) substrate

Two-dimensional (2D) transition metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, and catalysis. A high-quality 2D film of NiSe2, a TMD material, is grown epitaxially by a single step direct selenization of a Ni(111) substrate. X-ray photoemission spectroscopy, low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations are combined to confirm the formation and structure of the film, revealing a (√3 × √3) superlattice of the NiSe2 film formed on the (√7 × √7) superlattice of the substrate. Fabrication of this 2D NiSe2 film opens opportunities to research its applications, especially for electrocatalysis and energy storage devices.

Influence of Si Co-doping on electrical transport properties of magnesium-doped boron nanoswords

Magnesium-doped boron nanoswords were synthesized via a thermoreduction method. The as-prepared nanoswords are single crystalline and β-rhombohedral (β-rh) phase. Electrical transport measurements show that variable range hopping conductivity increases with temperature, and carrier mobility has a greater influence than carrier concentration. These results are consistent with the three dimensional Mott’s model (M. Cutler and N. F. Mott, Phys. Rev. 181, 1336 (1969)) besides a high density of localized states at the Fermi level compared with bulk β-rh boron. Conductivity of Mg-doped boron nanoswords is significantly lower than that of “pure” (free of magnesium) boron nanoswords. Electron energy loss spectroscopy studies confirm that the poorer conductivity arises from silicon against magnesium doping.

Piezoresponse Force Microscopy Study of Ferroelectric BaTiO3 Thin Film Directly Deposited on Si(001) by Magnetron Sputtering

Direct integration of ferroelectrics with semiconductors is critical to lower the cost and simplify the production procedures for data storage/processing components and miniature sensor/actuator development. By optimizing magnetron sputtering parameters, highly <001> preferential growth of BaTiO3 thin films with reproducible ferroelectric responses have been achieved on Si (001) substrates. The thin film ferroelectric characteristics were systematically studied by piezoresponse force microscopy, and a piezoelectric coefficient d33 of 24pm/V has been measured. It is found that the scanning tip sidewall angle and cantilever tilt affect the contour and size of polarized area.

Modification of the Potential Landscape of Molecular Rotors on Au(111) by the Presence of an STM Tip

Molecular rotors on solid surfaces are fundamental components of molecular machines. No matter whether the rotation is activated by heat, electric field or light, it is determined by the intrinsic rotational potential landscape. Therefore, tuning the potential landscape is of great importance for future applications of controlled molecular rotors. Here, using scanning tunneling microscopy (STM), we demonstrate that both tip-molecule distance and sample bias can modify the rotational potential of molecular rotors. We achieve the potential energy difference variations of ∼0.3 meV/pm and ∼18 meV/V between two configurations of a molecular rotor, a tetra- tert-butyl nickel phthalocyanine molecule on Au(111) substrate. Further analysis indicates that the mechanism of modifying the rotational potential is a combination of the van der Waals interaction and the interaction between the molecular dipole and an electric field. This work provides insight into the methods used to modify the effective rotational potential energy of molecular rotors.