Search

Crystallization is at the heart of many industrial processes in pharmaceuticals, dyes and pigments, microelectronics, and emerging wearable sensors. This paper reviews nucleation and early-stage crystal growth activated by an electrical pulse at microelectrodes and nanoelectrodes. We review thermodynamic and kinetic theories of electrochemistry developed around microelectrodes. We describe various methods to make microelectrodes and nanoelectrodes. Fundamental understanding is still needed for predicting and controlling nucleation and early-stage crystal growth. Using nanoelectrodes, nucleation and growth kinetics can be studied on one nucleation site at a time. In contrast, on macroelectrodes, nanoparticles are nucleated at random sites and at different times. This gives rise to overlapping growth zones resulting in inhomogeneous particle deposition and growth. The random size and density distributions prevent electrodeposition from being widely adopted as a manufacturing tool for making nanodevices. We describe advances in electrodeposition of metal nanoparticles and organic charge-transfer complexes on micro/nanoelectrodes. We anticipate increased interests in applying electrochemistry for making nanodevices particularly nanosensors and nanosensor arrays. These electrochemically fabricated nanosensor arrays will in turn fulfill the promise of nanoelectrodes as the most advanced analytical tools for medical diagnostics, environmental monitoring, and renewable energy.

The stripe phase formed by long-chain alkane derivatives on the graphite lattice provides a unique opportunity for the study of molecular adsorption, aggregation, and reaction on patterns. Fatty acids, such as arachidic acid (AA), self-assemble on graphite into a sheet of parallel stripes with a periodicity of twice its molecular chain length. The molecular pattern is thus defined precisely by the size and functionality of the headgroup and tailgroup of the amphiphile. Complexation of metal ions to AA fixes the number and location of the ion, which can serve as a precursor to semiconductor nanocrystal arrays. In order to understand the effect of the ion complexation, we carry out atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) investigations of AA self-assembly in the presence of various metal ions. While the stripe orientation is dictated by the graphite lattice and the stripe periodicity is determined by the AA chain length, the size, shape, and degree of order of the stripe crystalline domain are influenced by the metal ion bond strength to the carboxylic ligand. The change of morphology in the self-assembled pattern shows a trend along the Irving-Williams series.

Forces between colloidal particles determine the performances of many industrial processes and products. Colloidal force measurements conducted between a colloidal particle AFM probe and particles immobilized on a flat substrate are valuable in selecting appropriate surfactants for colloidal stabilization. One of the features of inorganic fillers and extenders is the prevalence of rough surfaces-even the polymer latex particles, often used as model colloidal systems including the current study, have rough surfaces albeit at a much smaller scale. Surface roughness is frequently cited as the reason for disparity between experimental observations and theoretical treatment but seldom verified by direct evidence. This work reports the effect of nanoscale surface roughness on colloidal force measurements carried out in the presence of surfactants. We applied a heating method to reduce the mean surface roughness of commercial latex particles from 30 to 1 nm. We conducted force measurements using the two types of particles at various salt and surfactant concentrations. The surfactants used were pentaethylene glycol monododecyl ether, Pluronic F108, and a styrene/acrylic copolymer, Joncryl 60. In the absence of the surfactant, nanometer surface roughness affects colloidal forces only in high salt conditions when the Debye length becomes smaller than the surface roughness. The adhesion is stronger between colloids with higher surface roughness and requires a higher surfactant concentration to be eliminated. The effect of surface roughness on colloidal forces was also investigated as a function of the adsorbed surfactant layer structure characterized by AFM indentation and dynamic light scattering. We found that when the layer thickness exceeds the surface roughness, the colloidal adhesion is less influenced by surfactant concentration variation. This study demonstrates that surface roughness at the nanoscale can influence colloidal forces significantly and should be taken into account in colloidal dispersion formulations.

The paper describes immobilization methods of bacteriophage P22 and tailspike gp9 proteins isolated from P22 on atomic force microscope (AFM) probes. The paper also reports single molecule force spectroscopy (SMFS) using AFM of the immobilized P22 (or gp9) interactions with substrate-supported O-antigenic lipopolysaccharides (LPS) bilayers. LPS covers the outer membrane of gram-negative bacteria, such as Salmonella typhimurium. Evidence from AFM imaging and SMFS shows that immobilized P22 (or gp9) are capable of strong and multivalent binding to supported LPS. The most common rupture forces between P22 and LPS were identified to be 72, 130, 206, and 279 pN at force loading rate of 12,000 pN/s. The quantized unbinding force was found to decrease with decreasing force loading rate as predicted by the Bell model. By fitting the force data with the Bell model, an energy barrier of 55 kJ/mol was obtained. Evidence is also provided that demonstrates the resilience of phage to pH and temperature fluctuation as well as dehydration/rehydration cycles. The biospecific interactions between P22 and the LPS are relevant to cell infection, inflammation, cancer progression and metastasis, food safety, pharmaceuticals, and biosensor development.

Dendrimers, with their well-defined globular shape and a high density of functional groups, are ideal nanoscale materials for templating sensor surfaces. This work exploits dendrimers as a versatile platform for capturing biomarkers with improved sensitivity and specificity. Synthesis, characterization, fabrication, and functional validation of the dendrimer-based assay platform are described. Bifunctional hydroxyl/thiol functionalized G4-polyamidoamine (PAMAM) dendrimer is synthesized and immobilized on to the polyethylene-glycol (PEG)-functionalized assay plate by coupling PEG-maleimide and dendrimer thiol groups. Simultaneously, part of the dendrimer thiol groups are converted to hydrazide functionalities. The resulting dendrimer-modified surface is coupled to the capture antibody in the Fc region of the oxidized antibody. This preserves the orientation flexibility of the antigen binding region (Fv) of the antibody. To validate the approach, the fabricated plates are further used as a solid phase for developing a sandwich type ELISA to detect IL-6 and IL-1β, important biomarkers for early stages of chorioamnionitis. The dendrimer-modified plate provides assays with significantly enhanced sensitivity, lower nonspecific adsorption, and a detection limit of 0.13 pg ml-1 for IL-6 luminol detection and 1.15 pg ml-1 for IL-1β TMB detection, which are significantly better than those for the traditional ELISA. The assays were validated in human serum samples from normal (non-pregnant) woman and pregnant women with pyelonephritis. The specificity and the improved sensitivity of the dendrimer-based capture strategy could have significant implications for the detection of a wide range of cytokines and biomarkers since the capture strategy could be applied to multiplex microbead assays, conductometric immunosensors and field effect biosensors.

Controlled intracellular disassembly of polyelectrolyte complexes of polycations and DNA (polyplexes) is a crucial step for the success of nonviral gene delivery. Motivated by our previous observation of different gene delivery performances among multiblock reducible copolypeptide vectors (Manickam, D. S., and Oupický, D. Bioconjugate Chem. 2006, 17, 1395−1403), atomic force microscopy is used to visualize plasmid DNA in various decondensed states from reducible polypeptide polyplexes under simulated physiological reducing conditions. DNA decondensation is triggered by reductive degradation of disulfide-containing cationic polypeptides. Striking differences in DNA release dynamics between polyplexes based on polypeptides of histidine-rich peptide (HRP, CKHHHKHHHKC) and nuclear localization signal (NLS, CGAGPKKKRKVC) peptide are presented. The HRP and NLS polyplexes are similar to each other in their initial morphology with a majority of them containing only one DNA plasmid. Upon reductive degradation by d...

Phages are promising alternatives to antibodies as the biorecognition element in a variety of biosensing applications. In this study, a monolayer of bacteriophage P22 whose tailspike proteins specifically recognize Salmonella serotypes was covalently bound to glass substrates through a bifunctional cross linker 3-aminopropyltrimethoxysilane. The specific binding of Salmonella typhimurium to the phage monolayer was studied by enzyme-linked immunosorbent assay and atomic force microscopy. Escherichia coli and a Gram-positive bacterium Listeria monocytogenes were also studied as control bacteria. The P22 particles show strong binding affinity to Salmonella typhimurium. In addition, the dried P22 monolayer maintained 50% binding capacity to Salmonella typhimurium after a one-week storage time. This is a promising method to prepare phage monolayer coatings on surface plasmon resonance and acoustic biosensor substrates in order to utilize the nascent phage display technology.

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.

We investigate the making of semiconductor nanorods using well-defined molecular templates. The films are characterized by AFM and XPS. The spin-coated monolayer of arachidic acid assembles on graphite as 1D nanostripes with bilayer periodicity. The copper arachidate film shows curved stripes, which become straight after reacting with hydrogen sulfide. The new template scheme may facilitate the fabrication of nanorod arrays of a variety of inorganic and semiconducting materials.

Atomic force microscopy (AFM) was used to study the aggregation structure and kinetics of nonionic surfactant penta(oxyethylene) dodecyl ether (C12E5) on mica as a function of temperature. Surface forces and topographical images of surfactant aggregates at the liquid/solid interface were captured in the vicinity of 21 °C. The surfactant molecular aggregates were imaged by choosing an imaging force in the steric repulsion region so that the AFM tip was just outside the surface of the aggregates. At below 21 °C, C12E5 adsorbed as fragments of tens of nanometers in size. The fragments consist of 1−2 nm thick strongly adsorbed species and weakly adsorbed species extending as far as 15 nm from the substrate surface. The fragments gradually increased in number and attached to each other to form clusters connected in one dimension (strings) and two dimensions (networks). With increasing surface coverage, the surface force changed from a purely attractive one to one showing a repulsive force barrier. At above 21 ...

Ultrathin organic multilayer films are appealing to scientists not only because they serve as molecular level models but also because they can be transformed into superlattices through successive deposition of alternating layers of different materials. Cationic bolaform amphiphile and anionic polyelectrolyte multilayers were self-assembled from their aqueous solutions via electrostatic attraction. The bolaform amphiphile layers are photopolymerizable by UV irradiation. The mechanical and surface electrical properties of the multilayer film were studied by surface force measurements, and the surface topography was studied by atomic force microscopy. We observed an average increase in film thickness of 75 with each amphiphile and polyelectrolyte double layer deposition, a molecularly smooth first anchoring amphiphile layer, and nonuniform polyelectrolyte layers with a surface roughness of 30 A. The surface roughness was preserved in the multilayer deposition process and was reduced by adding salt to the polyelectrolyte solution. Photopolymerization increases the integrity of the first anchoring layer as well as the subsequent multilayers. It took less than 5 min to dissolve a monomeric amphiphile film in chloroform, but immersion of the UV-irradiated film in chloroform for 2 h only resulted in partial removal of film materials. The multilayer film is quite fragile especially for the first few layers. Controlling of the monolayer structure by selective variation of the amphiphile chemical structure may help to optimize the photopolymerization process.

We have prepared self-assembled monolayers of novel cationic bolaform amphiphiles on negatively charged substrates. Most of these amphiphiles form smooth, defect-free monolayers which can be used to reverse the substrate surface charge and thus allow subsequent adsorption of anionic molecules and construction of multilayers. Atomic force microscopy, surface force measurement, and surface plasmon spectroscopy were combined to probe the molecular orientation and ordering, mechanical properties, and surface electrical properties of the monolayers. In addition, the amphiphile aggregation behavior at an air-water interface was studied by surface tension measurement, and lyotropic phase behavior was studied by polarization microscopy. Our study suggests that monolayer interfacial and bulk properties can be controlled to a certain degree by selective variation of amphiphile chemical structure, in particular, the alkyl chain length and the type of counterions. A n increase in alkyl chain length assists close-packing at the liquid-solid interface and self-assembly in a liquid medium due to a favorable hydrophobic free energy change. Exchange of halide ions with the strongly associating salicylate ions reduces electrostatic repulsion between head groups and also favors self-assembly and close-packing. Our study suggests that it is possible to overcome the dominance and limitation of the substrate electrostatic effect on monolayer structure by using amphiphiles with a strong inherent tendency for close-packing. Our observations contribute to the understanding of two-dimensional topochemical photopolymerization, multilayer deposition of alternating surface charges, modification of hydrophilic surface electrical properties, and in general, the dependence of monolayer architecture on molecular chemical structure and intermolecular forces.

Monoand multilayers of photopolymerizable bolaform amphiphiles and polyelectrolytes self-assembled onto mica from aqueous solution were studied by surface force measurement and atomic force microscopy. Multiple layers of positively charged amphiphiles self-assemble if a negatively charged polyelectrolyte is inserted between them. The layers can be photopolymerized with a high-pressure UV lamp. The atomic force microscopy images indicate that the photopolymerized amphiphile monolayer has a higher degree of order than the unpolymerized monolayer, although the latter possesses some long-range order. Mechanical properties of the system measured by the surface force apparatus indicate that a photopolymerized layer is more robust and demonstrate that the rigidity and integrity of the first anchoring layer greatly influence the stability of the multilayer. The interaction between polyelectrolyte layers in water can be either attractive or repulsive depending on the coverage of the polyelectrolyte on the amphiphile layer. The mica separation of a pair of opposed monolayers in contact in the presence of water is surprisingly small (25 A) compared to the thickness (78 A) of a bilayer composed of fully extended amphiphiles.

DNA release dynamics from layer-by-layer (LbL) films is an important aspect to consider with regards to localized gene delivery systems. The rate of DNA release and the condensation state of DNA during release are of particular interest in the field of gene delivery. A hyperbranched poly(amido amine) (RHB) containing bioreducible disulfide bonds is used to form interpolyelectrolyte complexes with DNA during LbL film assembly. During film disassembly, DNA is released in physiologic conditions due to the reducing nature of the RHB. Uncondensed DNA deposited on the surface was compared to DNA condensed by RHB in polyplex form by using two types of LbL films, RHB/DNA/RHB and polyplex terminated films, RHB/DNA/polyplex. LbL films with up to three layers are used in order to facilitate high-resolution atomic force microscopy (AFM) imaging. X-ray reflectivity, ellipsometry, and Fourier transform infrared spectroscopy are also used. The film disassembly, rearrangement, and release of molecules from the surface due to thiol-disulfide exchange is conducted in reducing dithiothreitol (DTT) solutions. Salt is found to accelerate the overall rate of film disassembly. Additionally, it was found that the polyplex layer disassembles faster than the DNA layer. The predominant intermediate structure is the toroid structure for the polyplex layer and the fiber bundle structure for the DNA layer during film disassembly. This study offers a simple means to modulate DNA release from LbL films by utilizing both condensed and uncondensed DNA in different layers. The study highlights nanostructures, toroids, and bundles as dominant intermediate DNA structures during DNA release from LbL films.

This article describes the stability and reversibility of ultrathin films of N-isopropylacrylamide (NIPA)-vinylimidazole (VI)-poly(ethylene glycol) (PEG) graft terpolymer adsorbed at the solid-liquid interface upon temperature cycling from below to above its phase transition temperature. The coil-to-globule and globule-to-coil phase transitions were captured by in situ fluid tapping atomic force microscopy (AFM). The film thickness of 1 nm was determined by AFM, X-ray photoelectron spectroscopy, and X-ray reflectivity. The concentration required to reach full coverage was found to be higher when adsorption occurred below the phase transition temperature. From 23 to 42 degrees C, the adsorbed NIPA terpolymer film was observed to be molecularly smooth, corresponding to the close-packed structure of flexible polymer coils. Particles containing between one and a few globules appeared abruptly at the interface at 42-43 degrees C, the same temperature as the solution phase transition temperature, which was determined by dynamic light scattering. The size of the particles did not change with temperature, whereas the number of particles increased with increasing temperature up to 60 degrees C. The particles correspond to the collapsed and associated state of the globules. The film morphological changes were found to be reversible upon temperature cycling. Subtle differences were observed between dip-coated and spin-coated films that are consistent with a higher degree of molecular freedom for spin-coated films. The study contributes to the fundamental understanding and applications of smart ultrathin films and coatings.

Aspirin and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) are deposited from their alcoholic mixed solution onto highly oriented pyrolytic graphite (HOPG) by spin coating. The film structure and morphology are characterized by atomic force microscopy (AFM). The barely soluble DMPE forms a highly oriented stripe phase as a result of its one-dimensional epitaxy with the HOPG lattice. The bilayer stripe pattern exposes the cross section of the lipid bilayer lamellae and enables the direct visualization of the molecular interactions of drug or biological molecules with either the hydrophobic or the hydrophilic part of the phospholipid bilayer. The bilayer pattern affects the aspirin molecular deposition and aggregation. AFM shows that the aspirin molecules prefer to deposit and aggregate along the aliphatic interior part of the bilayer pattern, giving rise to parallel dimer rods in registry with the underlying pattern. The nonpolar interactions between aspirin and the phospholipid bilayer are consistent with the lipophilic nature of aspirin. The bilayer pattern not only stabilizes the rodlike aggregate structure of aspirin at low aspirin concentration but also inhibits crystallization of aspirin at high aspirin concentration. Molecular models show that the width of the DMPE aliphatic chain interior can accommodate no more than two aspirin dimers. The bilayer confinement may prevent aspirin from reaching its critical nucleus size. This study illustrates a general method to induce a metastable or amorphous form of an active pharmaceutical ingredient (API) by chemical confinement under high undercooling conditions. Metastable and amorphous solids often display better solubility and bioavailability than the stable crystalline form of the API.

We have created nanospirals by self-assembly during droplet evaporation. The nanospirals, 60–70 nm in diameter, formed when solvent mixtures of methanol and m-cresol were used. In contrast, spin coating using only methanol as the solvent produced epitaxial films of stripe nanopatterns and using only m-cresol disordered structure. Due to the disparity in vapor pressure between the two solvents, droplets of m-cresol solution remaining on the substrate serve as templates for the self-assembly of carboxylic acid molecules, which in turn allows the visualization of solution droplet evaporation one molecule at a time.

Molecular aggregates formed by polymethine dyes in solutions, at surfaces, and in crystals can be used to modulate the intensity and wavelength of light absorption.1 The applications for such dye aggregates are manifold: photography, xerography, photovoltaic, molecular photonic, and microelectronic devices.2 The internal structure of such aggregates is widely perceived as arrays of tightly packed molecules with their molecular planes stacked against each other. Several packing models have been proposed based on the epitaxy match between dye crystalline layers and the lattice of a mica substrate: staircase, brickwork, and ladder structure.3 A special case of the staircase packing in single crystals of polymethine dye 1,7-bis(dimethylamino)heptamethine perchlorate or BDHClO4 is illustrated in Figure 1. The aggregate structure has been quantitatively linked to its spectroscopic properties.4 The strong coupling of molecular transition dipoles generates excitonic states4 whose absorption energy is significantly shifted to shorter (Haggregates) or to longer (J-aggregates) wavelength with respect to the nonaggregated state. When there are two molecular orientations in the unit cell, the molecular transition dipoles, M1 and M2, couple with each other as vectors, which results in two excitonic transitions: the Davydov sum, m+, and difference m-. These two components are perpendicular to each other.5 InBDHClO4 crystals, the m+ component parallels [201] and absorbs light at 639 nm, and the mcomponent parallels [010] and exhibits an absorption maximum at 592 nm.6 Both transitions are red-shifted from the monomer absorption at 510 nm. In addition to the exciton propagation, the absorption behavior of three-dimensional dye arrays is also altered by polaritons.7 The polaritons are generated by the coupling of the light wave with the polarization of themediumand lead to a splitting in the longitudinal and transverse exciton energy. The absorptionwavelength of the dye crystal varies within the two energy boundaries according to the angle, â, between the transition dipole moment and the crystal face normal. It is conceivable that the color of a transparent thin film of threedimensionally oriented J-aggregates can be tuned precisely if one finds a way to rotate the molecular axis in such thin films. A method called the thin layer aggregation (TLA) was recently developed to prepare dye filmswith various color patterns.8 The TLA method exploits the aggregation of polymethine dyes in strongly undercooled amorphous layers. While themacroscopic properties of thedye layers have been investigated by the UV/vis spectroscopy and thermal analysis, almost nothing is known about the dye thin film surface coverage, roughness, morphology, and molecular packing at the nanoscale. Atomic force microscopy (AFM)9 has beenwidely used to study the lattice structure of ordered self-assembled and LangmuirBlodgett thin films as well as the microstructure of amorphous films and has recently been used to study J-aggregatestructure.10 Thegoalsof this ongoingresearch are to directly link the color to structure of the aggregate film and to control the film quality and color by intermolecular forces. Structural characterization is carried outbypolarizedoptical spectroscopyandmicroscopy,X-ray diffraction, and thermal analysis, in addition to AFM. BDHClO4wasobtainedbyexchangingClwithClO4 in BDH+Cl-. BDH+Clwas synthesized according to the literature.11 Single crystals with green golden color were grown by repeated crystallization from acetone. HPLC grade acetone with purity of>99.7%was purchased from Fisher Scientific and used as received. A 20 μL saturated acetone solution of dissolved BDHClO4 single crystals was spin-coated onquartz substrates. Later, the filmwas (1) Nakatsu, K.; Yoshioka, H.; Morishita, H.Acta Crystallogr. 1977, B33, 2181.Daltrozzo,E.; Scheibe,G.;Gschwind,K.;Haimerl,F.Photogr. Sci. Eng. 1974, 18, 441. (2) Thirtle, J. R.; Zwick, D. M. Color Photography In Kirk-Othmer: Encyclopedia ofChemicalTechnology, 3rd ed.; JohnWileyS Vol. 6, p 617. Law,K.-Y.Chem.Rev. 1993, 93, 449.McEvoy, A. J.; Graetzel,M.Sol. EnergyMater. Sol. Cells1994, 32, 221. Lovinger, A. J.; Rothberg, L. J. J. Mater. Res. 1996, 11, 1581. (3) Czikkely,V.; Forsterling,H.D.;Kuhn,H.Chem.Phys. Lett.1970, 6, 11; 6, 207. (4) Kasha, M.; McRae, E. G. In Physical Processes in Radiation Chemistry; Augenstein, L., Ed.; Academic Press: NewYork, 1964; p 23. (5) Davydov, A. S.Theory ofMolecular Excitons; McGraw-Hill: New York, 1962; p 23. (6) Daehne, L.; Horvath, A.; Weiser, G. Chem. Phys.1995, 196, 307. (7) Small, G. J.; Connolly,M. A.; Stevenson, S. H.Chem. Phys. 1988, 157, 157. Daehne, L.; Reck, G.; Horvath, A.; Weiser, G. Adv. Mater. 1996, 8, 486. (8) Daehne, L. J. Am. Chem. Soc. 1995, 117, 12855. (9) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 12, 930. (10) Higgins, D. A.; Kerimo, J.; Vanden Bout, D. A.; Barbara, P. F. J. Am. Chem. Soc. 1996, 118, 4049. Shiojiri, M.; Saijo. H. J. Crystal Growth 1996, 166, 930. Wolthaus, L.; Schaper, A.; Mobius, D. Chem. Phys. Lett. 1994, 225, 322. (11) Nikolajewski,H.E.; Dahne, S.;Hirsch, B.Chem.Ber.1967, 100, 2616. Figure 1. Model of molecular packing in J-aggregates: the distorted herringbone structure of one dimensional staircases in single crystals of BDHClO4. Absorption and reflection spectra of J-aggregates depend on the following parameters: the intermolecular distance d, the stacking angle R, the angle τ between the two molecule directions, the Davydov sum and differencem+ andm-, due to coupling of molecular transition dipole moments M1 and M2, and the angle â (not shown in the figure) between the molecular axis and the face normal of the crystal. 565 Langmuir 1998, 14, 565-568