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Kevin Critchley

Publications and source records attributed to Kevin Critchley.

8 recordsLinked to original sources
PubMed2006

Soft-UV photolithography using self-assembled monolayers.

We report thiol-on-gold self-assembled monolayers (SAMs) that can be photodeprotected using soft UV irradiation (lambda = 365 nm) to yield CO(2)H functionalized surfaces complementing those reported previously, which yielded NH(2) functionalized surfaces. The photolysis of these SAMs were monitored using a combination of surface sensitive techniques. In the SAM environment the photodeprotection yields are lower than those obtained for equivalent reactions in dilute solution. The protected carboxylic acids SAMs are shown to have a low yield approximately 50% due to competing photoreduction reactions of the nitro group. The results from infrared studies show that, as the photolysis progresses, the long chain protected residues reorganize and shield the functional COOH groups, thereby reducing the hydrophilic character of the surface.

Journal Article↗
PubMed2006

Fabrication of biological nanostructures by scanning near-field photolithography of chloromethylphenylsiloxane monolayers.

We demonstrate the fabrication of sub-100-nm DNA surface patterns by scanning near-field optical lithography using a near-field scanning optical microscope coupled to a UV laser and a chloromethylphenylsiloxane (CMPS) self-assembled monolayer (SAM). The process involves 244-nm exposure of the CMPS SAM to create nanoscale patterns of surface carboxylic acid functional groups, followed by their conversion to the N-hydroxysuccinimidyl ester and reaction of the active ester with DNA to spatially control DNA grafting with high selectivity.

Animals↗
PubMed2006

Fabrication of gold micro- and nanostructures by photolithographic exposure of thiol-stabilized gold nanoparticles.

Exposure of thiol-stabilized gold nanoparticles supported on silicon wafers to UV light leads to oxidation of the thiol molecules and coagulation of the nanoparticles, forming densified structures that are resistant to removal by solvent exposure. Unoxidized particles may, in contrast, readily be removed leaving gold structures behind at the surface. This process provides a convenient and simple route for the fabrication of gold structures with dimensions ranging from micrometers to nanometers. The use of masks enables micrometer-scale structures to be fabricated rapidly. Exposure of nanoparticles to light from a near-field scanning optical microscope (NSOM) leads to the formation of gold nanowires. The dimensions of these nanowires depend on the method of preparation of the film: for spin-cast films, a width of 200 nm was achieved. However, this was reduced significantly, to 60 nm, for Langmuir-Schaeffer films.

Journal Article↗
PubMed2005

Photooxidation of self-assembled monolayers by exposure to light of wavelength 254 nm: a static SIMS study.

Self-assembled monolayers (SAMs) of alkanethiols have been photooxidized by exposure to light from a lamp emitting light with a wavelength of 254 nm. The data confirm that SAM oxidation on exposure to UV light sources occurs in the absence of ozone, but also suggest that the mechanism is different from that observed in previous studies using broad-spectrum arc lamps. In particular, for monolayers on both gold and silver, carboxylic acid-terminated SAMs oxidize significantly faster than methyl-terminated SAMs, in contrast to earlier observations for monolayers exposed to light from a mercury arc lamp. The difference in rates of photooxidation for the two classes of monolayer is significantly greater on silver than on gold. These data support our recent suggestion that while methyl-terminated SAMs are able to pack much more closely on silver than on gold, carboxylic acid-terminated thiols are not able to adopt the same close-packed structures, and their rates of photooxidation on silver are similar to, or slightly greater than, those measured for the same adsorbates on gold. Surface potential measurements were made for carboxylic acid- and methyl-terminated SAMs using a Kelvin probe apparatus. It was found that the work functions of carboxylic acid-terminated SAMs are significantly greater than those of methyl-terminated monolayers. It is concluded that these data are consistent with the oxidation reaction being initiated by "hot" electrons generated following the interaction of photons with the metallic substrate.

Gold↗
PubMed2005

Novel cyanoterphenyl self-assembly monolayers on Au(111) studied by ellipsometry, x-ray photoelectron spectroscopy, and vibrational spectroscopies.

The self-assembled monolayers (SAMs) of two asymmetric disulfides derivatives (namely, LC1 and LC2) were prepared on Au(111). The disulfides contain a pure alkyl chain and an alkyl chain terminated by a cyanoterphenyl group. LC1 and LC2 differ by the way the cyanoterphenyl group is attached onto the alkyl chain: it is expected to be aligned with the alkyl chain in the case of LC1 and perpendicular to it in the case of LC2 (T shape). The consequences in terms of surface coverage, chemical composition, and molecular conformation of the two SAMs are studied using ellipsometry, x-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and broadband femtosecond sum-frequency generation (SFG). A model of coverage and tilt angle based on ellipsometry and XPS results shows that the SAM "manages" the large size of the terphenyl group by lowering the terphenyl containing chain coverage and by increasing the tilt. In the case of LC2, the disulfide breaks during molecular assembly, less terphenyl chains adsorb than pure alkyl chains, and the overall chain coverage is smaller than for LC1. RAIRS and SFG results show that these differences in surface coverage correspond to a drastically different orientation of the terphenyl axis, which lies nearly parallel to the surface for LC2, while it is tilted by approximately 28 degrees for LC1. This shows that the terphenyl group takes much more space on the surface in the case of LC2 and explains why the terphenyl coverage is found smaller for LC2. The anomalous SFG relative intensities observed in the region of CH stretch between CH2 and CH3 modes, and symmetric and antisymmetric modes, show that the chains are not in the fully stretched, all-trans conformation, LC2 being probably more distorted than LC1. These distorsions allow the molecules to occupy the space available below the large terphenyl group. The relative intensities of symmetric and antisymmetric modes are discussed qualitatively for some typical molecular conformations and orientations of the alkyl chain.

Journal Article↗
PubMed2005

A mild photoactivated hydrophilic/hydrophobic switch.

Surface modification using light is one of the most powerful methods for controlling the physical and chemical properties offunctionalized surfaces. In this paper, we report on systems where soft UV irradiation (lambda = 365 nm) converts a "low" activity fluorocarbon to a "high" activity amine-functionalized surface. An amine-functionalized SAM (self-assembled monolayer) is first masked using a tertiary amine catalyzed reaction with an N-hydroxysuccinimidyl carbonyl reagent. This mild, room-temperature reaction introduces a hydrophobic photocleavable nitrobenzyl "protecting group" terminated with a fluorocarbon end-chain. UV irradiation (lambda = 365 nm) of this hydrophobic/fluorocarbon surface cleaves the nitrobenzyl residue, returning the surface to the original hydrophilic/amine-functionalized state. This provides a mild, generic method of producing surfaces with hydrophilic/hydrophobic patterns or patterned with amine functional residues. Two different protecting groups, one terminated with a single and the other with three fluorocarbon end chains, are compared. In the case of the more bulky protecting group, only a small proportion of the amine residues react, but the surface is equally hydrophobic and the amine residues equally well shielded from further reaction. Surfaces are characterized by X-ray photoelectron spectroscopy, ellipsometry, surface potential, and contact angle measurements. Images of the photopatterned SAMs were obtained using scanning electron microscopy.

Journal Article↗
PubMed2004

Gold nanoparticle patterning of silicon wafers using chemical e-beam lithography.

This paper demonstrates a novel facile method for fabrication of patterned arrays of gold nanoparticles on Si/SiO2 by combining electron beam lithography and self-assembly techniques. Our strategy is to use direct-write electron beam patterning to convert nitro functionality in self-assembled monolayers of 3-(4-nitrophenoxy)-propyltrimethoxysilane to amino functionality, forming chemically well-defined surface architectures on the 100 nm scale. These nanopatterns are employed to guide the assembly of citrate-passivated gold nanoparticles according to their different affinities for amino and nitro groups. This kind of nanoparticle assembly offers an attractive new option for nanoparticle patterning a silicon surface, as relevant, for example, to biosensors, electronics, and optical devices.

Journal Article↗