Steven L. Bernasek

Improved organic thin-film transistor performance using novel self-assembled monolayers

Pentacene-based organic thin-film transistors have been fabricated using a phosphonate-linked anthracene self-assembled monolayer as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Vast improvements in the subthreshold slope and threshold voltage are observed compared to control devices fabricated without the buffer. Both observations are consistent with a greatly reduced density of charge trapping states at the semiconductor-dielectric interface effected by introduction of the self-assembled monolayer.

Molecular beam study of the mechanism of catalyzed hydrogen–deuterium exchange on platinum single crystal surfaces

The hydrogen–deuterium exchange reaction was studied by molecular beam scattering on low and high Miller index crystal faces of platinum in the surface temperature range of 300–1300°K. Under the condition of the experiments which put strict limitation on the residence time of the detected molecules, the reaction product, HD, was readily detectable from the high Miller index, stepped surfaces (integrated reaction probability, defined as total desorbed HD flux divided by D2 flux, is ∼10−1) while HD formation was below the limit of detectability on the Pt(111) low Miller index surface (reaction probability <10−5). Atomic steps at the platinum surface must play a controlling role in dissociating the diatomic molecules. The exchange reaction is first‐order in D2 beam pressure and half‐order in H2 background pressure. This observation indicates that an atom–molecule reaction or possibly an atom–atom reaction is the rate‐limiting step in the exchange. The absence of beam kinetic energy dependence of the rate ind...

Activation of carbon monoxide on clean and sulfur modified Fe(100)

Chemisorption of CO on clean and sulfur modified Fe(100) has been investigated using X-ray photoelectron spectroscopy, temperature programmed desorption, high resolution electron energy loss spectroscopy and near edge X-ray absorption fine spectroscopy. On the clean iron surface, CO adsorbs in three sequentially filled molecular states. Two of these states desorb at or below 300 K. The third state ( α 3 ) partially desorbs at 440 K and is the precursor to CO dissociation on the Fe(100) surface. This precursor occupies the 4-fold hollow sites and has an extremely weak CO bond ( ν CO =1210 cm −1 ). The geometry of the adsorbed CO is a major factor in determining the unusually weak carbon-oxygen bond in the precursor state. Sulfur modification of the surface reduces CO dissociation by simple site blocking; no evidence for a long range electronic interaction was obtained.

Adsorption of CO on the clean and sulfur modified Fe(100) surface

Abstract The interaction of CO with clean and sulfur modified Fe(100) surfaces has been investigated using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). On the clean Fe(100) surface, CO adsorbs molecularly in three sequentially filled states. Dissociation of CO occurs at 440 K and is correlated with the most tightly bound molecular state. Investigation of the CO adsorption as a function of sulfur coverage on the Fe(100) surface indicated that sulfur reduces the CO dissociation by blocking the sites for dissociated carbon and oxygen atoms. No evidence for long-range electronic effects was obtained.

Anodized Indium Metal Electrodes for Enhanced Carbon Dioxide Reduction in Aqueous Electrolyte

The interactions of CO2 with indium metal electrodes have been characterized for electrochemical formate production. The electrode oxidation state, morphology, and voltammetric behaviors were systematically probed. It was found that an anodized indium electrode stabilized formate production over time compared to etched indium electrodes and indium electrodes bearing a native oxide in applied potential range of -1.4 to -1.8 V vs SCE. In addition, it was observed that formate is the major product at unprecedentedly low overpotentials at the anodized surface. A surface hydroxide species was observed suggesting a mechanism of formate production that involves insertion of CO2 at the indium interface to form an electroactive surface bicarbonate species.

Reactivity of vibrationally excited methane on nickel surfaces

Four-dimensional variational calculations have been performed for modeling energy flow between methane (CH4) stretching vibrational energy states as the molecule adiabatically approaches a metallic surface. The model is based on a local mode Hamiltonian for an isolated CH4 molecule and a London-Eyring-Polanyi-Sato potential describing surface–molecule interactions. The results suggest the possibility of mode specific effects on chemical reactivity. Specifically, the symmetric A1 stretch fundamental adiabatically correlates with the localized excitation in the unique CH bond pointing towards the surface. Conversely, the antisymmetric F2 stretch fundamental excitation correlates with A and E vibrations in the CH3 radical, and therefore this degree of freedom is localized away from the reactive CH bond. Landau–Zener semiclassical analysis of nonadiabatic curve crossings predicts a significant velocity dependence to the state specific energy flow dynamics. Since excitation localized in active versus spectator...

State selective vibrational (2ν3) activation of the chemisorption of methane on Pt (111)

The state-selective dissociative sticking coefficient on Pt(111) surfaces for CH4 in the 2ν3 J=1,2 ro-vibrational levels has been measured using thermal energy atomic scattering. Continuous wave laser excitation of a molecular beam of CH4 seeded in He with a 1.5 μm color center laser, tunable around 6000 cm−1 and coupled to the beam by means of a resonant build-up cavity, allows pumping of up to 11% of the molecules to the excited ro-vibrational state. The laser/molecular beam combination allows precise control over the translational energy as well as the vibrational state of the methane that impinges on the clean Pt(111) surface. The intensity of the specular reflection of the incident helium beam is used to quantitatively monitor the coverage of chemisorbed methane on the platinum surface as a function of time (exposure). The sticking coefficient (S0) of CH4 with 5.4 kJ/mol normal translational energy is found to increase from 6.2×10−6 to 1.8×10−4 upon 2ν3 excitation (the overtone of the asymmetric stre...

Direct detection of vibrational excitation in the CO2 product of the oxidation of CO on a platinum surface

Abstract Infrared chemliuminescence techniques are applied to the study of vibrationally excited surface reaction products. The method is demonstrated by the detection of infrared emission from highly excited CO 2 product molecules in the reaction of CO with O on a platinum surface.

Electronic and Structural Factors in Modification and Functionalization of Clean and Passivated Semiconductor Surfaces with Aromatic Systems

The chemistry of organic molecules on solid surfaces has been a central issue of surface and interfacial chemistry. The technologically important surfaces mainly include catalytic transition metal surfaces such as Pt(111),1 semiconductor surfaces such as Si(100),2 and insulator surfaces such as MgO.3 The interactions between organic molecules and solid surfaces are quite diverse.4 For example, the weak van der Waals interactions between organic molecules and solid surfaces such as graphite make the adsorbed organic molecules have the capability of self-assembling into various 2-D or even 3-D functional structures;5,6 the metastable interactions of organic molecules such as cyclohexene and benzene on noble metals make heterogeneous catalysis such as hydrogenation and dehydrogenation possible;7,8 the strong covalent binding of organic molecules on semiconductor surfaces makes those surfaces ideal for the immobilization of functional organic materials for the development of new hybridmaterialsandsemiconductor-basedmoleculardevices.2,9-15 Thus, from weak van der Waals interactions to strong covalent binding the diverse interfacial chemistry between organic molecules and solid surfaces contributes to a wide spectrum of technological needs and the development of new hybrid materials, sensors, and electronic devices. * To whom correspondence should be addressed. † Princeton University. ‡ University of California, Berkeley and Lawrence Berkeley National Laboratory. § National University of Singapore. Chem. Rev. 2009, 109, 3991–4024 3991

A tilted precursor for CO dissociation on the Fe(100) surface

Near edge X-ray absorption fine structure (NEXAFS) has been used to study the molecular orientation of the α3 state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45° ± 10° with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 ± 0.02 A in the α3 state, relative to the other molecularly adsorbed CO states on this surface.