J.A. Schaefer

Understanding and tuning the epitaxy of large aromatic adsorbates by molecular design

If the rich functionality of organic molecules is to be exploited in devices such as light-emitting diodes or field-effect transistors, interface properties of organic materials with various (metallic and insulating) substrates must be tailored carefully. In many cases, this calls for well-ordered interfaces. Organic epitaxy—that is, the growth of molecular films with a commensurate structural relationship to their crystalline substrates—relies on successful recognition of preferred epitaxial sites. For some large π-conjugated molecules (‘molecular platelets’) this works surprisingly well, even if the substrate exhibits no template structure into which the molecules can lock. Here we present an explanation for site recognition in non-templated organic epitaxy, and thus resolve a long-standing puzzle. We propose that this form of site recognition relies on the existence of a local molecular reaction centre in the extended π-electron system of the molecule. Its activity can be controlled by appropriate side groups and—in a certain regime—may also be probed by molecularly sensitized scanning tunnelling microscopy. Our results open the possibility of engineering epitaxial interfaces, as well as other interfacial nanostructures for which specific site recognition is essential.

Modeling of graphene metal-oxide-semiconductor field-effect transistors with gapless large-area graphene channels

A quasianalytical modeling approach for graphene metal-oxide-semiconductor field-effect transistors (MOSFETs) with gapless large-area graphene channels is presented. The model allows the calculation of the I-V characteristics, the small-signal behavior, and the cutoff frequency of graphene MOSFETs. It applies a correct formulation of the density of states in large-area graphene to calculate the carrier-density-dependent quantum capacitance, a steady-state velocity-field characteristics with soft saturation to describe the carrier transport, and takes the source/drain series resistances into account. The modeled drain currents and transconductances show very good agreement with experimental data taken from the literature {Meric et al., [Nat. Nanotechnol. 3, 654 (2008)] and Kedzierski et al., [IEEE Electron Device Lett. 30, 745 (2009)]}. In particular, the model properly reproduces the peculiar saturation behavior of graphene MOSFETs with gapless channels.

Adsorption of H, O, and H2O at Si(100) and Si(111) surfaces in the monolayer range: A combined EELS, LEED, and XPS study

This paper is a summary of a series of experiments studying the exposure of hydrogen, oxygen, and water, on the (2×1) surfaces of Si(100) and Si(111). While the primary focus has been on high resolution electron energy loss (EELS) results, low energy electron diffraction (LEED) and x‐ray photoelectron spectroscopy (XPS) are also used in the studies. Both the (100) and cleavage (111) surfaces form a monohydride and a dihydride exhibiting a (2×1) and a (1×1) LEED pattern, respectively. These systems exhibit saturation, which is consistent with the model of hydrogen saturation of the dangling bonds. Upon water adsorption the Si–H and Si–OH vibronic modes are observed, indicating that water is decomposed. On the cleavage surface only, there is evidence of a very weak scissor mode, allowing for the possibility of a few percent of molecular water adsorption. Oxygen adsorption is complex. For samples formed at high temperatures (∼1000 K) the observed vibronic features are similar to those known for the Si–O–Si c...

Engineering polycrystalline Ni films to improve thickness uniformity of the chemical-vapor-deposition-grown graphene films

It has been shown that few-layer graphene films can be grown by atmospheric chemical vapor deposition using deposited Ni thin films on SiO(2)/Si substrates. In this paper we report the correlation between the thickness variations of the graphene film with the grain size of the Ni film. Further investigations were carried out to increase the grain size of a polycrystalline nickel film. It was found that the minimization of the internal stress not only promotes the growth of the grains with (111) orientation in the Ni film, but it also increases their grain size. Different types of SiO(2) substrates also affect the grain size development. Based upon these observations, an annealing method was used to promote large grain growth while maintaining the continuity of the nickel film. Graphene films grown from Ni films with large versus small grains were compared for confirmation.

The effect of water on friction of MEMS

Water plays a significant role in the performance of micro electro mechanical systems (MEMS). A special apparatus was employed to investigate the adhesive friction attributed to water at low coverages, i.e., in the nanometer range, where friction and adhesion are a function of the water layer thickness. In addition, the history of the sample surface also plays a significant role. The friction forces associated with hydrophobic samples are negligibly affected by humidity changes, whereas those of hydrophilic samples show a strong dependence. Sample coverage and the friction force are also influenced by the sample temperature. High forces were measured for high humidities at low sample temperatures, for hydrophilic silicon. In contrast, hydrophobic samples show an increase of the friction force with increasing temperature. Experiments performed under high vacuum demonstrated that decreasing the water layer thickness by desorption decreases the friction force with several sub‐minima and sub‐maxima. The friction signal is accompanied by sudden fluctuations. For submonolayer coverage the friction force starts to increase.

Friction of thin water films: a nanotribological study

Abstract Lubricant thickness is known to influence the frictional properties between two contacting surfaces in relative motion. In this paper, the dependence of friction on water films with various thicknesses is examined using a scanning force microscopy (SFM) between a hydrophilic silicon tip and silicon flat that was rendered either hydrophilic or hydrophobic. Results indicate that the frictional properties are influenced by the ordering effects of water. The friction of a hydrophilic surface initially covered with 2.6 nm of water was examined as a function of the film thickness, which was separately determined using scanning tunneling microscopy and dynamic SFM. Capillary effects dominate the tribological properties for films that are between 3 and about 1 nm thick. For thinner films the friction properties can be explained by the ordering effect of water on the sample and the tip, which together cause an increased resistance to shear (higher viscosity). Further reduction of film thickness due to water desorption leads to a decrease of the friction force; this regime is dominated by cohesive forces arising from the solid–solid contact. For all applied forces in this study, liquid confinement between tip and surface does not occur. Rather, the sharp SFM tip apparently penetrates the water film with the tip apex making direct contact with the surface. The ordering effect of water occurs on the sample surface and at the sides of the SFM tip. These results, together with previous microtribological studies, highlight the significant differences existing between two contacting surfaces moving in relative motion in the micro- and nanoregime.

Chemical shifts of Si-H stretching frequencies at Si(100) surfaces pre-exposed to oxygen in the submonolayer range

Abstract The Si-H stretching vibrations have been measured with high resolution electron energy loss spectroscopy (EELS) and Si(100) 2 × 1 surfaces pre-exposed at room temperature to submonolayer coverages of oxygen. The absorption of atomic hydrogen at room temperatures and subsequent isochronal annealing makes evident different stages of oxidation. The Si-H stretching frequencies vary between 261 and 283 meV, depending on the number of the adjacently bound oxygen atoms. The upward shift in frequency, empirically described in terms of Paulings electronegativity sum for the next-nearest neighbors of the hydrogen atoms, is in agreement with infrared band frequencies for oxygenated polysilane samples and for air exposed hydrogenated a-films. Empirical relations indicate that the Si-H bonding distance decreases from 1.50 to 1.46 A as the number of oxygen next-nearest neighbors increases from zero to three. Simultaneously, the s character of the Si-H bond is increased from 0.42 to 0.61.

Strong phonon-plasmon coupled modes in the graphene/silicon carbide heterosystem

We report on strong coupling of the charge carrier plasmon

ULTIMATE RESOLUTION ELECTRON ENERGY LOSS SPECTROSCOPY AT H/SI(100) SURFACES

\omega_{PL}

Water adsorption on cleaved silicon surfaces

in graphene with the surface optical phonon