Mark S. Wrighton

Functional group imaging by chemical force microscopy

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH3/CH3, CH3/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

Orthogonal Self-Assembled Monolayers: Alkanethiols on Gold and Alkane Carboxylic Acids on Alumina

This work demonstrates the practicality of forming two self-assembled monolayers (SAMs), independently but simultaneously, by adsorption of two different adsorbates from a common solution onto a substrate exposing two different materials at its surface. The experimental procedure and the degree of independence achieved in the resulting SAMs are illustrated by examination of monolayers obtained by adsorption of alkanethiols on gold and alkane carboxylic acids on alumina. This procedure provides a method for modifying the surface characteristics of microlithographically generated patterns and offers a versatile technique for controlling solid-vapor and solid-liquid interfacial properties in systems having patterns with dimensions of the order of 1 micrometer.

Surface Functionalization of Electrodes with Molecular Reagents

The use of molecular reagents to manipulate the properties of electrode surfaces has broad application in areas such as electrochemical synthesis, energy conversion and storage, displays, sensors, and new kinds of microelectronic devices. Surface modification of electrodes has contributed to a revival of interest in basic and applied research in electrochemistry and electrochemical devices. This article is focused on specific examples of systems modified electrodes where basic developments provide promising opportunities for applications stemming from the properties of molecules attached to an electrode surface.

Molecular self-assembly of two-terminal, voltammetric microsensors with internal references.

Self-assembly of a ferrocenyl thiol and a quinone thiol onto Au microelectrodes forms the basis for a new microsensor concept: a two-terminal, voltammetric microsensor with reference and sensor functions on the same electrode. The detection is based on measurement of the potential difference of current peaks for oxidation and reduction of the reference (ferrocene) and indicator (quinone) in aqueous electrolyte in a two-terminal, linear sweep voltammogram in which a counterelectrode of relatively large surface area is used. The quinone has a half-wave potential, E�, that is pH-sensitive and can be used as a pH indicator; the ferrocene center has an E1/2 that is a pH-insensitive reference. The key advantages are that such sensors require no separate reference electrode and function as long as current peaks can be located for reference and indicator molecules.

Pentacarbonyliron(0) photocatalyzed reactions of trialkylsilanes with alkenes

Abstract We report results of the near-ultraviolet irradiation of Fe(CO) 5 in the presence of R 3 SiH (R  Me or Et) and an alkene. Good conversion to a mixture of alkane, (alkyl)SiR 3 , and (alkenyl)SiR 3 products obtains. For terminal alkenes, conversion to products generally exceeds 80%. The distribution of products depends on the alkene/R 3 SiH ratio and on the reaction temperature (0-50°C). Terminal, sterically unhindered alkenes react fastest, and give organosilicon products which result from addition of the R 3 Si moiety to the terminal carbon. With R 3 SiD starting silane extensive protium/deuterium exchange can be detected in the starting alkene and silane at intermediate stages of the reaction. Quantum yields for consumption of the starting materials exceed unity, implicating photocatalysis. The key photogenerated species is proposed to be (H)(R 3 Si)Fe(CO) 3 (alkene) which can be used to rationalize the formation of all products. Also, this intermediate accommodates a temperature dependent photocatalysis as well as a reagent concentration ratio dependence on the final product distribution. At low temperature and low alkene/silane ratio the alkylsilane product dominates, but at higher temperatures and high alkene/silane ratio the alkenylsilane is the dominant product. The temperature dependence in the range 0-50°C is consistent with a thermal rate-limiting step in the photocatalytic cycle which is crucial to the final product distribution.

Photogeneration of dinuclear metal carbonyls containing a metal-metal triple bond

The photogeneration of the complexes [(η5C5H5)M(CO)2]2 (M = Cr, Mo, W) and [(η5-C(in5)(CH3)5)M(CO)2]2 (M = Cr, Mo) is reported. These species are formulated as metal-metal triple bonded complexes which can be generated from the corresponding [(η5-C5H5)M(CO)3]2 and [(η5-C5(CH3)5M(CO)3]2 metal-metal single bonded complexes by either irradiation at 25° or by heating. All of the triple bonded complexes take up CO at 25° to generate the single bonded complexes, but the Cr complexes require high CO pressure, while the Mo and W species take up CO at or below one atmosphere with half-times of less than a minute. The complexes [(η5-C5H5M(CO)2]2 (M = Mo, W) react with a variety of acetylenes at 25° C to yield complexes having a M2C2 “tetrahedrane-like” core. The mechanism of the formation of the triple bonded complexes from the single bonded complexes is proposed to tint involve cleavage of the metal-metal single bond to generate 17-electron intermediates of the formula (η5-C5H5)M(CO)3 or (η5-C5(CH3)5M(CO)3. These species are coordinatively labile and lose CO dissociatively yielding 15-electron intermediates which dimerize to give the triple bonded complexes.

The nature of the lowest excited state and photosubstitution reactivity of tetracarbonyl-1,10-phenanthrolinetungsten(0) and related complexes

Abstract Absorption and emission spectral studies of M(CO) 4 L complexes (M = Cr Mo, W; L = 2,2′-bipyridine, 1,10-phenanthroline, 5-CH 3 -, 5-Cl-, 5-Br-, 5-NO 2 -1,10-phenanthroline) have been carried out and reveal that the lowest excited state in every case is charge-transfer (CT) in character, M→ CT in absorption, and in no case do the ligand field (LF) excited states cross below the CT state. Minimum energies of the LF states have been established by the spectroscopic study of cis -bis(pyridine)- and cis -bis(aliphatic amine)-tetracarbonylmetal(0) complexes which all have LF lowest excited states for M = Mo, W. For the M(CO) 4 L complexes emission is detectable for M = Mo or W and occurs in the range 14.40-15.66 kK with lifetimes of 7.9-13.3 μsec and quantum yields of 0.02–0.09 all in EPA solution at 77 K. For the bis-pyridine and -aliphatic amine complexes emission occurs only from the W complexes and is of the order of 3.0–4.0 kK higher in energy than for the M(CO) 4 L complexes. Photosubstitution of pyridine is efficient in cis -W(CO) 4 (py) 2 (py = pyridine): Φ 436nm = 0.23; Φ 405nm = 0.27; and Φ 366nm = 0.23. The M(CO) 4 L complexes have strongly wavelength dependent, but modest, quantum yields for CO substitution and show that the lowest CT state is unreactive. Typical values for CO substitution for M = W and L = 1,10-phenanthroline are: Φ 436nm = 1.6 × 10 −4 ; Φ 405nm = 1.2 × 10 −3 ; Φ 366nm = 9.2 × 10 −3 ; and Φ 313nm = 2.2 × 10 −2 .

X‐ray photoelectron and Auger electron spectroscopic study of the CdTe surface resulting from various surface pretreatments: Correlation of photoelectrochemical and capacitance‐potential behavior with surface chemical composition

The surface chemistry and stoichiometry of p‐ and n‐type CdTe photoelectrodes treated with oxidizing and reducing etches have been characterized by x‐ray photoelectron and Auger electron spectroscopies. The results of surface analysis have been correlated with the photoelectrochemical and capacitance–potential behavior of the photoelectrodes. ‘‘Oxidized’’ surfaces are covered by a thin Te0/TeO2 layer (or a thicker Te0 layer, if the etching procedure is slightly altered), resulting in Fermi level pinning: a constant photovoltage is found for a wide range of redox potentials and potential‐independent space charge layer capacitance obtains. ‘‘Reduced’’ surfaces closely resemble ion sputtered CdTe in chemical state and stoichiometry, resulting in more nearly ‘‘ideal’’ behavior: the semiconductor/electrolyte interface is rectifying in the dark; capacitance–potential behavior follows the Mott–Schottky equation near flat band conditions; and photovoltage varies with redox potential, from 0 to ∼0.7 V for p‐CdTe.

Photoactivation of cluster catalysis: a comparison of 1-pentene isomerization by tetracarbonyl(triphenylphosphine)ruthenium and 1,1,1,2,2,2,3,3,3-nonacarbonyl-1,2,3-tris(triphenylphosphine)-triangulo-triruthenium

Abstract : The photocatalyzed isomerization of 1-pentene to trans- and cis-2-pentene by Ru(CO)4PPh3 and Ru3(CO)9(PPh3)3 is reported along with data for Ru3(CO)12, Fe3(CO)12, and Fe(CO)5. The primary photoprocess in Ru(CO)4PPh3 is dissociative loss of CO giving a coordinatively unsaturated species having the same empirical formula Ru3(CO)9(PPh3)3; the trinuclear species undergoes Ru-Ru bond rupture and ultimate declusterification subsequent to photo-excitation giving a quantitative yield of Ru(CO)4PPh3 under CO or Ru(CO)3(PPh3)2 in the presence of PPh3. The crucial result is that the cluster yields a different catalytically active species compared to Ru(CO)4PPh3, since the initial ratio of trans- and cis-2-pentene is different for the two photocatalysts. The photocatalysis and primary photoprocesses suggest that the isomerization from the Ru3(CO)9(PPh3)3 results from an active form of the cluster. By way of contrast, Fe(CO)5 and Fe3(CO)12 yields the same initial ratio of photocatalytic products, implicating a common, mononuclear catalytic species. Since the clusters are good visible absorbers compared to the mononuclear species, photoactivation of cluster catalysis can be effected with low energy visible light.

Electron beam deposition of gold nanostructures in a reactive environment

Electron beam deposition (EBD) is a maskless technique suitable for the fabrication of nanometer scale structures. Metals can be deposited from an organometallic gas, but simultaneous carbon deposition typically yields grossly impure (∼25% metal) deposits. We have found that the metal content of the deposited solid is dramatically improved by performing the whole EBD process in a reactive gaseous environment containing a source of oxygen (O2 or H2O) in addition to the organometallic gas. With simple procedures we prepared Au deposits showing significantly diminished C content (up to 50% metal) as the partial pressure of O2 (or H2O) is increased in the gas.