Achieving larger electromagnetic enhancement using a nanogap between neighboring metallic nanostructures has been long pursued for boosting light–matter interactions. However, the quantitative probing of this enhancement is hindered by the lack of a reliable experimental method for measuring the local fields within a subnanometer gap. Here, we use layered MoS2 as a two-dimensional atomic crystal probe in nanoparticle-on-mirror nanoantennas to measure the plasmonic enhancement in the gap by quantitative surface-enhanced Raman scattering. Our designs ensure that the probe filled in the gap has a well-defined lattice orientation and thickness, enabling independent extraction of the anisotropic field enhancements. We find that the field enhancement can be safely described by pure classical electromagnetic theory when the gap distance is no <1.24 nm. For a 0.62 nm gap, the probable emergence of quantum mechanical effects renders an average electric field enhancement of 114-fold, 38.4% lower than classical predictions.

Covalently attached organic monolayers on etched silicon nitride (SixN4; x >/= 3) surfaces were prepared by reaction of SixN4-coated wafers with neat or solutions of 1-alkenes and 1-alkynes in refluxing mesitylene. The surface modification was monitored by measurement of the static water contact angle, XPS, IRRAS, AFM, and ToF-SIMS, and evidence for the formation of Si-C bonds is presented. The etching can be achieved by dilute HF solutions and yields both Si-H and N-H moieties. The resulting etched SixN4 surfaces are functionalized by terminal carboxylic acid groups in either of two ways: (a) via attachment of a 10-undecenoic acid 2,2,2-trifluoroethyl ester (trifluoro ethanol ester) and subsequent thermal acid hydrolysis; (b) through attachment of a photocleavable ester, and subsequent photochemical cleavage, as this would allow photopatterned functionalized SixN4. The carboxylic acids are successfully used for the attachment of oligopeptides (aspartame) and complete proteins using EDC/NHS chemistry. Finally, an amino-terminated organic monolayer can be formed by reaction of HF-treated SixN4 surfaces with a N-(omega-undecylenyl)phthalimide, which yields an amino-terminated surface upon deprotection with hydrazine.

Although polydimethylsiloxane (PDMS) transfer during microcontact printing (microCP) has been observed in previous reports, which generally focused on only one or a few different substrates, in this work we investigate the extent of PDMS transfer onto a series of surfaces with a wide range of hydrophobicities using an uninked, unpatterned PDMS stamp. These surfaces include clean silicon, clean titanium, clean gold, "dirty" silicon, polystyrene, Teflon, surfaces modified with PEG, amino, dodecyl, and hexadecyl monolayers, and also two loose molecular materials. The PDMS transferred onto planar surfaces is, in general, easily detected by wetting and spectroscopic ellipsometry. More importantly, it is detected by time-of-flight secondary ion mass spectrometry (ToF-SIMS) because of the sensitivity of this technique to PDMS. The effect of surface free energy on PDMS transfer in microcontact printing is investigated, and the relationship between the amount of PDMS in ToF-SIMS spectra and the surface tensions of initial surfaces is revealed. We show that PDMS transfer can be applied as a probe of surface free energies using ToF-SIMS, where PDMS preferentially transfers onto more hydrophilic surface features during stamping, with little being transferred onto very hydrophobic surface features. Multivariate curve resolution (MCR) analysis of the ToF-SIMS image data further confirms and clarifies these results. Our data lend themselves to the hypothesis that it is the free energy of the surface that plays a major role in determining the degree of PDMS transfer during microCP.

The chemomechanical method has emerged as a straightforward and convenient tool for simultaneously functionalizing and patterning silicon. This technique simply consists of wetting (or exposing) a silicon surface to a reactive chemical and then scribing. Scribing activates the surface and leads to monolayer formation. The properties of the monolayers are dependent on the reactive chemicals used, and mixed monolayers and funtionalized monolayers are easily produced with mixed chemicals or alpha,omega-bifunctional compounds, respectively. Both micrometer and nanometer sized functionalized features have been created. It has been shown that this technique has potential in a variety of applications.

Biomineralization is an important process, which is often assisted by biomolecules. In this paper, the effect of chondroitin sulfate on the crystallization of hydroxyapatite was examined quantitatively based on a generic heterogeneous nucleation model. It is found that chondroitin sulfate can suppress the supersaturation-driven interfacial structure mismatch between the hydroxyapatite crystal and the substrate and promote the formation of ordered hydroxyapatite nanocrystallite assemblies. The nucleation mechanism of selfaligned hydroxyapatite nanocrystallites was examined from the viewpoints of kinetics and interfacial structure and properties, which contributes to an understanding of the fundamentals of biomineralization of self-assembled structures. The results obtained from this study will provide a basic principle to design and fabricate highly orderly organic-inorganic hybrid materials.Natural materials such as bones and teeth consist of biocomposites with well organized and assembled hydroxyapatite (HAP 2 ; Ca 5 (PO 4 ) 3 OH) nanobiominerals, to perform important biological functions. Although such composites in hard tissues are formed under mild conditions, they exhibit unusual mechanical properties that outperform synthetic materials (1, 2). How can biomineral nanocrystallites be assembled to form a synergetic structure? Why are some molecules, in particular biomacromolecules, capable of fabricating different patterns of biominerals? Recently, these questions concerning the biological mechanisms that control mineralized tissue construction have attracted a great deal of attention in fields ranging from biology and chemistry to materials science and bioengineering (3-7).It is well documented that the ordered structures in mineralized tissues appeared to originate from organized assemblies of biomacromolecules, such as proteins, polysaccharides, or proteoglycans, and inorganic compound salts (8 -10). Previous studies indicate that some biomacromolecules are involved in controlling the nucleation, growth, size, and shape of the mineral phases (11), since they can act as templates through self-assembly to facilitate interaction with an insoluble matrix and to induce the desired stereochemistry for the construction of organized structures (12, 13). Normally, these molecules are functionalized with acidic groups such as carboxylic acid, sulfonate, and phosphate moieties, which enable them to become an effective metal ion chelator to combine with the inorganic matrix (14 -16). Chondroitin 4-sulfate (ChS) is just one of these biomolecules. It belongs to the family of glycosaminoglycans, which can be found on cell surfaces and in the extracellular matrix of cartilage and bone. Large amounts of it in the cartilage permit diffusion of substances between blood and vessels. ChS consists of repeated disaccharide units; one of the two monosaccharides is N-acetylgalactosamine sulfate (GalNAc-OSO 3 Ϫ ), and the other is glucuronic acid that contains a carboxylate group (cf. Fig. 1). Previous investigation indicated that ...