Modeling Nonlinear Dynamics of Functionalization Layers: Enhancing Gas Sensor Sensitivity for Piezoelectrically Driven Microcantilever.

This article presents a parametrized response model that enhances the limit of detection (LOD) of piezoelectrically driven microcantilever (PD-MC) based gas sensors by accounting for the adsorption-induced variations in elastic properties of the functionalization layer (binder) and the nonlinear motional dynamics of the PD-MC. The developed model is demonstrated for quantifying cadaverine, a volatile biogenic diamine whose concentration is used to assess the freshness of meat. At low concentrations of cadaverine, an increase in the resonance frequency is observed, contrary to the expected reduction due to mass added by adsorption. The study explores the variations in the elastic modulus vis-à-vis the adsorbed mass of cadaverine and derives the resonance frequency to the adsorbed mass response function. We advance a blended technique involving the analysis of atomic force microscopy (AFM) force-distance (f-d) curves and fitting of the quartz crystal microbalance (QCM) impedance response spectrum to deduce the adsorption-induced changes in the viscoelastic properties of the functionalization layer. The findings obtained are subsequently employed in modeling the response function for a structurally nonhomogenous PD-MC, highlighting the significance of the functionalization layer to the global elastic properties. The structural composition of the PD-MC beam adopted herein features a trapezoidal base hosting the actuating piezoelectric stratum and a rectangular free end with a functionalization layer. The Euler-Bernoulli beam theory coupled with Hamilton's principle is used to develop the equation of motion, which is subsequently discretized into a set of nonlinear ordinary differential equations via Galerkin expansion, and the solutions to the first fundamental mode of vibration are determined using the method of multiple scales. The obtained solutions provide a basis for deducing the nonlinear response function model to the adsorbed mass. The derived model is validated by recorded resonance frequency changes resulting from exposure to known concentrations of cadaverine. We demonstrate that the increase in resonance frequency for low concentrations of cadaverine is due to the dominance of the variation of the elastic modulus of the functionalization layer originating from the initial binder-analyte interactions over damping due to added mass. It is concluded that the developed nonlinear response function model can reliably be used to quantify the cadaverine concentration at low concentrations with an elevated Limit of Detection.

Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films.

Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.

Fragment evolution for GPCRs: the role of secondary binding sites in optimization.

We developed a docking-based fragment evolution approach that extends orthosteric fragments towards a less conserved secondary binding pocket of GPCRs. Evaluating 13 000 extensions for the ß1- and ß2-adrenergic receptors we synthesized and tested 112 bitopic molecules. Our results confirmed the positive contribution of the secondary binding pocket to both potency and selectivity optimizations.

Determining the Dielectric Tensor of Microtextured Organic Thin Films by Imaging Mueller Matrix Ellipsometry.

Polycrystalline textured thin films with distinct pleochroism and birefringence comprising oriented rotational domains of the orthorhombic polymorph of an anilino squaraine with isobutyl side chains (SQIB) are analyzed by imaging Mueller matrix ellipsometry to obtain the biaxial dielectric tensor. Simultaneous fitting of transmission and oblique incidence reflection Mueller matrix scans combined with the spatial resolution of an optical microscope allows to accurately determine the full biaxial dielectric tensor from a single crystallographic sample orientation. Oscillator dispersion relations model well the dielectric tensor components. Strong intermolecular interactions cause the real permittivity for all three directions to become strongly negative near the excitonic resonances, which is appealing for nanophotonic applications.

Polymorphic chiral squaraine crystallites in textured thin films.

An enantiomerically pure (R)-2-methylpyrrolidine-based anilino squaraine crystallizes in two chiral polymorphs adopting a monoclinic C2 and an orthorhombic P21 21 21 structure, respectively. By various thin-film preparation techniques, a control of the polymorph formation is targeted. The local texture of the resulting textured thin films is connected to the corresponding optical properties. Special attention is paid to an unusual Davydov splitting, the anisotropic chiroptical response arising from preferred out-of-plane orientation of the crystallites, and the impact of the polymorph specific excitonic coupling.

Giant intrinsic circular dichroism of prolinol-derived squaraine thin films.

Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.

Quasi-one-dimensional cyano-phenylene aggregates: Uniform molecule alignment contrasts varying electrostatic surface potential.

The epitaxial growth of the mono-functionalized para-quaterphenylene molecule CNHP4 on muscovite mica is investigated. The vacuum deposited molecules aggregate into nanofibers of varying morphology. Due to muscovite's cm symmetry, almost mutually parallel fibers grow. Polarized light microscopy together with X-ray diffraction resolves the projected orientation of the molecules on the substrate surface and within the fibers. Several different contact planes with the substrate are detected. For all of them, the molecules orient with their long molecule axis approximately perpendicular to the grooved muscovite direction, so that the alignment of the molecules on the substrate is uniform. Kelvin probe force microscopy finds vastly different electrostatic properties of different fiber types and facets.

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation.

Oriented, fluorescing organic nanoaggregates derived from 1,4'''-dicyano-functionalized para-quaterphenylene (CNP4) are obtained upon vacuum deposition on muscovite mica. Two types of aggregates are observed with fiber- and wing-like shape, respectively, both growing along distinct substrate directions. The polarization of the emitted fluorescence, their morphology, and their electric surface potential differ, reflecting different polymorphs. The wings are chiral twins. The molecules orient within ±5° along the same direction, templated by the substrate.

Photoelectrical Stimulation of Neuronal Cells by an Organic Semiconductor-Electrolyte Interface.

As a step toward the realization of neuroprosthetics for vision restoration, we follow an electrophysiological patch-clamp approach to study the fundamental photoelectrical stimulation mechanism of neuronal model cells by an organic semiconductor-electrolyte interface. Our photoactive layer consisting of an anilino-squaraine donor blended with a fullerene acceptor is supporting the growth of the neuronal model cell line (N2A cells) without an adhesion layer on it and is not impairing cell viability. The transient photocurrent signal upon illumination from the semiconductor-electrolyte layer is able to trigger a passive response of the neuronal cells under physiological conditions via a capacitive coupling mechanism. We study the dynamics of the capacitive transmembrane currents by patch-clamp recordings and compare them to the dynamics of the photocurrent signal and its spectral responsivity. Furthermore, we characterize the morphology of the semiconductor-electrolyte interface by atomic force microscopy and study the stability of the interface in dark and under illuminated conditions.

Substrate steered crystallization of naphthyl end-capped oligothiophenes into nanofibers: the influence of methoxy-functionalization.

Naphthyl end-capped oligothiophenes are a class of materials well suited for high-performance organics based devices. The formation of nanofibers on muscovite mica from 2,5-bis(naphth-2-yl)thiophene (NaT), 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2), and 5,5''-bis(naphth-2-yl)-2,2':5',2''-terthiophene (NaT3) as well as of the methoxy-functionalized variants MONaT, MONaT2, and MONaT3 is investigated via atomic force microscopy, X-ray diffraction, polarized fluorescence microscopy, and fluorescence spectroscopy. From polarized fluorescence microscopy spatially resolved molecular orientations are deduced revealing a profound anisotropy. Fibers from lying molecules grow along distinct substrate directions. Methoxy-functionalization substantially increases the crystallization into aligned fibers. In air Ostwald ripening is observed. The morphological variations of the aggregates result in specific optical signatures, disclosed by temperature dependent and spatially resolved fluorescence spectra.

Directed self-assembled crystalline oligomer domains on graphene and graphite.

We observe the formation of thin films of fibre-like aggregates from the prototypical organic semiconductor molecule para-hexaphenylene (p-6P) on graphite thin flakes and on monolayer graphene. Using atomic force microscopy, scanning electron microscopy, x-ray diffraction, polarized fluorescence microscopy, and bireflectance microscopy, the molecular orientations on the surface are deduced and correlated to both the morphology as well as to the high-symmetry directions of the graphitic surface: the molecules align with their long axis at ±11° with respect to a high-symmetry direction. The results show that the graphene surface can be used as a growth substrate to direct the self-assembly of organic molecular thin films and nanofibres, both with and without lithographical processing.

From clusters to fibers: parameters for discontinuous para-hexaphenylene thin film growth.

All relevant steps of discontinuous thin film growth of para-hexaphenylene on muscovite mica (0 0 1) from wetting layer over small and large clusters to nanofibers are observed and investigated in detail by a combined polarized fluorescence and atomic force microscopy study. From a variation of film thickness and surface temperature, we determine effective activation energies for cluster growth of 0.17 eV, for nanofiber length growth of 0.46 eV, for width growth of 0.19 eV, and for height growth of 0.07 eV. The corresponding exponential prefactors for the nanofiber growth are 1 x 10(9), 6 x 10(4), and 3 x 10(2) nm. Polarized fluorescence studies reveal that nanofibers grow along the grooves of the mica surface and that they do not change direction if they cross an even number of mica surface steps, while they change direction by 120 degrees for an odd number of steps. These results are taken as an input for a model of the unidirectional growth process on mica. Absolute parameters allowing one to grow nanofibers of predetermined morphology via organic molecular beam epitaxy are also given.

Organic molecular nanotechnology.

A new route to bottom-up organic nanotechnology is presented. Molecular building blocks with specific optoelectronic properties are designed and grown via directed self-assembly arrays of morphologically controlled light-emitting organic nanofibers on template surfaces. The fibers can be easily transferred from the growth substrate to device platforms either as single entities or as ordered arrays. Due to the extraordinary flexibility in the design of their optoelectronic properties they serve as key elements in next-generation nanophotonic devices.

Light-emitting organic nanoaggregates from functionalized p-quaterphenylenes.

Functionalized para-phenylenes are versatile building blocks for generating aligned fiber-like nanostructures by a self-assembly growth process on muscovite mica substrates upon controlled vapor deposition (molecule epitaxy). Functional groups were implemented at the 1,4‴-para-positions of p-quaterphenylenes (p4P) using a Suzuki cross-coupling strategy. The nanoaggregates possess outstanding optical properties, which can be modified in a controlled manner by functionalization. Functionalization allows the fluorescence peak emission frequency to shift within the blue spectral range, and the nanoaggregates' three-dimensional shape alters depending on the substitution. In the case of asymmetrically functionalized phenylenes due to the intrinsic non-zero hyperpolarizability of push-pull functionalized oligomers and non-centrosymmetry of the respective nanofibers, they act as frequency doublers.

Tailoring the growth of p-6P nanofibres using ultrathin Au layers: an organic-metal-dielectric model system.

The influence of ultrathin Au cluster films on the growth of para-hexaphenyl (p-6P) fibres is investigated. Whereas p-6P at elevated temperatures forms long, mutually parallel fibres on plain mica, these fibres become shorter but taller on Au covered mica, up to a Au film thickness of approximately 8 nm. The degree to which fibres are mutually parallel decreases with increasing Au thickness. For thicker Au films the length of the fibres increases again, and their morphology changes from flat to faceted; for Au film thicknesses above 20 nm, fibre networks are formed. The spectroscopic properties of the fibres are not modified by the Au layer, enabling independent control of the fibre morphology by means of the intermediate metallic layer.

One-dimensional random lasing in a single organic nanofiber.

One-dimensional light amplification in individual p-sexiphenyl nanofibers is investigated. The influence of fiber morphology on light propagation properties is studied via optical and atomic force microscopy. Isolated nanofibers are shown to yield low-threshold random laser emission in the deep blue. Model calculations of coherent light propagation in one-dimensional random media qualitatively reproduce the experimental results. Implications for photonic nanosensors are briefly discussed.