Chemical force microscopy of mixed self-assembled monolayers of alkanethiols on gold: evidence for phase separation

Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.     

Micrometer and nanometer scale patterning using the photo-fries rearrangement: toward selective execution of molecular transformations with nanoscale spatial resolution

The photolithographic modification of monolayers provides a versatile and powerful means of fabricating functionalized nanostructured surfaces. In this contribution, we present photosensitive thiol-bearing aryl ester groups which are capable of undergoing the so-called photo-Fries rearrangement to yield hydroxyketones. Phenyl 16-mercaptohexadecanoate was prepared by a three-step synthesis. This molecule undergoes a photoisomerization reaction upon illumination with UV light at ca. 250 nm. Subsequently this molecule was applied as a self-assembled monolayer on gold. Following photochemical modification, the adsorbates were selectively derivatized to yield amino-functionalized surfaces using a simple two-step reaction. This reaction was monitored by X-ray photoelectron spectroscopy and contact angle measurements and friction force microscopy. Micrometer-scale patterned surfaces were produced using a contact mask in conjunction with a frequency-doubled argon ion laser (lambda=244 nm). Near-field optical exposure was carried out by coupling the laser to a scanning near-field optical microscope and yielded nanometer-scale resolution. Following derivatization, the resulting structures were analyzed by friction force microscopy. Clear contrast was observed in the friction signal following surface modification.     

Influence of the solvent environment on the contact mechanics of tip-sample interactions in friction force microscopy of poly(ethylene terephthalate) films

Friction force microscopy measurements have been carried out on free-standing films of poly(ethylene terephthalate) in a variety of different media. In ethanol, the adhesion force was small, and the friction-load relationship was linear. In perfluorodecalin, nonlinearity was observed in the friction-load relationship, and the data have been found to fit the Johnson-Kendall-Roberts model of contact mechanics. The behavior in hexadecane was also characterized by a single-asperity contact model, but in this case, the data were found to fit the Derjaguin-Müller-Toporov model. It is suggested that these differences are due to the different strengths of tip-sample adhesion, which arise from the differences in the dielectric constants of the media: in ethanol, which has a high dielectric constant, the friction force varies linearly with the load, whereas in media of low dielectric constant, adhesion-limited behavior is observed.     

Scanning near-field photolithography--surface photochemistry with nanoscale spatial resolution

This tutorial review describes recent advances that have challenged the traditional view that the Rayleigh limit, of approximately lambda/2, represents the ultimate resolution accessible using optical methods. Near-field optical methods offer a powerful capability for optical measurement and manipulation of materials. Using a scanning near-field optical microscope coupled to a UV laser it is possible to create photopatterned molecular structures with dimensions nearly 15 times smaller than the Rayleigh limit. Near-field methods offer the possibility for selective initiation of surface chemical transformations with exquisite spatial resolution, bringing the prospect of unifying top-down and bottom-up nanofabrication into view.     

On the Superfluid Fraction of an Arbitrary Many-Body System at T=0

I derive a set of sufficient and (barring certain pathologies) necessary conditions for a one-component system with velocity-independent forces to have a super-fluid fraction ρ _s /ρ equal to unity at zero temperature: In addition to a condition closely related (but not obviously equivalent) to the usual one of off-diagonal long-range order, the ground state should possess unbroken invariance under both spatial translation and time reversal. Some generalizations are made to the case of multicomponent systems.     

Is “relative quantum phase” transitive?

I discuss the question: Is it possible to prepare, by purely thermodynamic means, an ensemble described by a quantum state having a definite phase relation between two component states which have never been in direct contact? Resolution of this question requires us to take explicit account of the nature of the correlations between the system and its thermal environment.     

Machine computation of equilibrium concentrations-some practical considerations

The calculation of equilibrium concentrations in solution by means of seven published computer programs is critically evaluated. Six equilibrium systems were chosen to illustrate the capabilities of these programs. It was found, in general, that the choice of algorithm determines the efficiency of program execution. In all cases studied the problems were most rapidly solved by a modified Newton-Raphson approach that employs Choleski factoring.