Wei-Bin Su

A Direct and Polymer-Free Method for Transferring Graphene Grown by Chemical Vapor Deposition to Any Substrate

We demonstrate a polymer-free method that can routinely transfer relatively large-area graphene to any substrate with advanced electrical properties and superior atomic and chemical structures as compared to the graphene sheets transferred with conventional polymer-assisted methods. The graphene films that are transferred with polymer-free method show high electrical conductance and excellent optical transmittance. Raman spectroscopy and X-ray/ultraviolet photoelectron spectroscopy also confirm the presence of high quality graphene sheets with little contamination after transfer. Atom-resolved images can be obtained using scanning tunneling microscope on as-transferred graphene sheets without additional cleaning process. The mobility of the polymer-free graphene monolayer is as high as 63,000 cm(2) V(-1) s(-1), which is 50% higher than the similar sample transferred with the conventional method. More importantly, this method allows us to place graphene directly on top of devices made of soft materials, such as organic and polymeric thin films, which widens the applications of graphene in soft electronics.

Scanning tunneling microscopy/spectroscopy of picene thin films formed on Ag(111)

Using ultrahigh-vacuum low-temperature scanning tunneling microscopy and spectroscopy combined with first principles density functional theory calculations, we have investigated structural and electronic properties of pristine and potassium (K)-deposited picene thin films formed in situ on a Ag(111) substrate. At low coverages, the molecules are uniformly distributed with the long axis aligned along the [112̄] direction of the substrate. At higher coverages, ordered structures composed of monolayer molecules are observed, one of which is a monolayer with tilted and flat-lying molecules resembling a (11̄0) plane of the bulk crystalline picene. Between the molecules and the substrate, the van der Waals interaction is dominant with negligible hybridization between their electronic states; a conclusion that contrasts with the chemisorption exhibited by pentacene molecules on the same substrate. We also observed a monolayer picene thin film in which all molecules were standing to form an intermolecular π stacking. Two-dimensional delocalized electronic states are found on the K-deposited π stacking structure.

Magnetic properties and microstructure of ultrathin Co∕Si(111) films

The magnetic properties and microstructure of ultrathin Co films grown on a Si(111)-7×7 surface were investigated. The experimental results observed by surface magneto-optic Kerr effect (SMOKE) and scanning tunneling microscopy show that the surface morphological evolution of x ML (monolayer) Co∕Si(111) films is strongly related to their magnetic properties. Due to the formation of a CoSi2 layer, no magnetic signal could be detected by SMOKE for x=2.1. Both longitudinal and polar hysteresis loops appear for 4.2–8.5 ML Co∕Si(111) films because of their rougher surfaces. When the Co thickness is increased to 11 ML, a magnetic hysteresis loop only occurs in the longitudinal configuration, which can be attributed to the contribution of volume anisotropy. After annealing an 11 ML Co∕Si(111) film at 400 and 500K, the surface becomes rougher, inducing magnetic anisotropy on the polar configuration. When the annealing temperature was increased to 600K, however, the Co could react with Si to form a nonmagnetic cob...

Step edge growth of Co nanoislands on Cu(111) surface

Step edge growth of Co nanoislands on Cu(111) surface have been investigated by scanning tunneling microscopy (STM). The cobalt atoms cluster at the upper step edges and form bilayer islands of 2nm in diameter (about nine Co atoms in width) initially during the initial stage of Co deposition. This result is in accordance with the total energy calculations within density functional theory. Besides, the size and amount of nanoislands increase with increasing coverage. The average number of Co atoms contained in one island increases with a rate of 375 atoms per monolayer (ML). The statistics data on the STM images indicate that the cobalt nanoislands preferentially grow at the upper step edge during the first stage of Co deposition, then toward terrace, and finally, the growth rate of islands in edge is almost the same as that in terrace for Co thickness above 0.78–1.42 ML.

A Comparative Study on the Adsorption Behavior of Pentacene and Perfluoropentacene Molecules on Au(111) Surfaces

In this study, we use low-temperature scanning tunneling microscopy and X-ray photoemission spectroscopy to study two closely related molecules, pentacene (PEN) and perfluoropentacene (PFP), adsorbed on a herringbone reconstructed Au(111) surface. PEN molecules are mobile under the probe tip at an elevated positive sample bias voltage with the direction of diffusion being correlated to the surface structure and the initial molecular orientation. Moreover, an induced rearrangement of the herringbone reconstruction is observable after manipulation. PFP molecules rearrange into flat, densely packed islands and the herringbone structure is undisturbed by the adsorbed PFP molecules. In addition, the X-ray photoelectron spectroscopy (XPS) C 1s and F 1s core level spectra of PFP show a shift toward high binding energy at high coverage. In comparison only a subtle shift for the C 1s core level of PEN at high coverage is seen. This indicates a different molecular arrangement for PFP in the bulk and in close proximity to the gold substrate.

Determining the Thickness of Pb Film Similar to Bulk with Energy Dispersion Derived from Quantum Well States

It is known that the energy spacing between adjacent empty quantum well (QW) states in Pb islands on Cu(111) would reveal the shrinking characteristic originating from the effect of the image potential. Using the phase accumulation model, including a phase factor contributed from the image potential, the shrinking energy spacing can be quantitatively explained with the assumption of the parabolic energy versus wave vector (E–k) dispersion. However, an experimental dispersion acquired from analyzing the energies of the QW state reveals a linear E–k relationship corresponding to the Pb bulk band structure, implying the assumed parabolic dispersion is not appropriate. By combining the linear dispersion with the image potential effect in the calculation, it is found that the calculated values of energy spacing of island thickness below eight atomic layers are not in agreement with the experimental measurements. This implies that the electronic structure of Pb islands would be similar to that of the bulk when their thicknesses reach eight-atomic layers. # 2013 The Japan Society of Applied Physics

Field enhancement factors and self-focus functions manifesting in field emission resonances in scanning tunneling microscopy.

Field emission (FE) resonance (or Gundlach oscillation) in scanning tunneling microscopy (STM) is a phenomenon in which the FE electrons emitted from the microscope tip couple into the quantized standing-wave states within the STM tunneling gap. Although the occurrence of FE resonance peaks can be semi-quantitatively described using the triangular potential well model, it cannot explain the experimental observation that the number of resonance peaks may change under the same emission current. This study demonstrates that the aforementioned variation can be adequately explained by introducing a field enhancement factor that is related to the local electric field at the tip apex. The peak number of FE resonances increases with the field enhancement factor. The peak intensity of the FE resonance on the reconstructed Au(111) surface varies in the face-center cubic, hexagonal-close-packed, and ridge regions, thus providing the contrast in the mapping through FE resonances. The mapping contrast is demonstrated to be nearly independent of the tip-sample distance, implying that the FE electron beam is not divergent because of a self-focus function intrinsically involved in the STM configuration.

Measurement of work function difference between Pb/Si(111) and Pb/Ge/Si(111) by high-order Gundlach oscillation

Ge films can be grown between the Pb overlayer and Si(111) substrate by the surfactant-mediated epitaxy. We detect the high-order Gundlach oscillation revealed in scanning tunneling microscopy (STM) to measure the work function difference between Pb/Si(111) and Pb/Ge/Si(111). Owing to different dielectric responses of Si and Ge, the tunneling current on Pb/Si has to be larger than that on Pb/Ge/Si by a factor of 2–3 to establish the same electric field in STM gap on both regions. This condition leads us to obtain a work function difference of 200 meV from observing Gundlach oscillation. It is believed that the method developed in this work can be extended to measure the surface work function difference of bulk conductors as well.

Quantum Size Effects in Low-Temperature Growth of Pb Islands on Si(111)7×7 Surfaces

Flat-top Pb islands with critical and magic thickness have been observed in the Pb/Si(111)7×7 system at 200 K by scanning tunneling microscopy. The growth behavior, different from that in the Stranski-Krastanov mode, arises from a quantum size effect. Quantized states are detected in the current–voltage (I–V) spectra on the Pb islands of varying thickness. Our observation of asymmetrical and oscillatory relaxation in the island thickness reveals that the charge distribution of confined electrons can influence the interlayer spacing. A simple model based on the infinite potential well can explain well all of our results.

Sharpness-induced energy shifts of quantum well states in Pb islands on Cu(111)

We elucidate that the tip sharpness in scanning tunneling microscopy (STM) can be characterized through the number of field-emission (FE) resonances. A higher number of FE resonances indicates higher sharpness. We observe empty quantum well (QW) states in Pb islands on Cu(111) under different tip sharpness levels. We found that QW states observed by sharper tips always had lower energies, revealing negative energy shifts. This sharpness-induced energy shift originates from an inhomogeneous electric field in the STM gap. An increase in sharpness increases the electric field inhomogeneity, that is, enhances the electric field near the tip apex, but weakens the electric field near the sample. As a result, higher sharpness can increase the electronic phase in vacuum, causing the lowering of QW state energies. Moreover, the behaviors of negative energy shift as a function of state energy are entirely different for Pb islands with a thickness of two and nine atomic layers. This thickness-dependent behavior results from the electrostatic force in the STM gap decreasing with increasing tip sharpness. The variation of the phase contributed from the expansion deformation induced by the electrostatic force in a nine-layer Pb island is significantly greater, sufficient to effectively negate the increase of electronic phase in vacuum.