Structural Study of Diketopyrrolopyrrole Derivative Thin Films: Influence of Deposition Method, Substrate Surface, and Aging.

We report the morphology and microstructure of n-dialkyl side-chain-substituted thiophene DPP end-capped with phenyl groups (Ph-TDPP-Ph) thin films and compare the influence of deposition method and substrate surface using thermally oxidized Si and graphene substrates as well as monolayer graphene surfaces with an underlying self-assembled octadecyltrichlorosilane monolayer, complemented by an aging study of spin-coated films over a 2 weeks aging period. A distinct difference in morphology was observed between spin-coated and vacuum-deposited thin films, which formed a fiber-like morphology and a continuous layer of terraced grains, respectively. After an initial film evolution, all combinations of deposition method and substrate type result in well-ordered thin films with almost identical crystalline phases with slight variations in crystallinity and mosaicity. These findings point toward strong intermolecular forces dominating during growth, and the templating effect observed for other oligomer films formed on graphene is consequently ineffective for this material type. Upon aging of spin-coated films, a noticeable evolution involving two different morphologies and crystalline phases were observed. After several days, the thin film evolved into a more stable crystal phase and a fiber-like morphology. Moreover, slight variation in optical spectra were elucidated on the basis on density functional theory calculations. These results demonstrate that thin-film properties of DPP derivatives can be tailored by manipulating the film formation process.

Surface temperature determination using long range thermal emission spectroscopy based on a first order scanning Fabry-Pérot interferometer.

Determination of the surface temperature of different materials based on thermographic imaging is a difficult task as the thermal emission spectrum is both temperature and emissivity dependent. Without prior knowledge of the emissivity of the object under investigation, it makes up a temperature-emissivity underdetermined system. This work demonstrates the possibility of recognizing specific materials from hyperspectral thermal images (HSTI) in the wavelength range from 8-14 µm. The hyperspectral images were acquired using a microbolometer sensor array in combination with a scanning 1st order Fabry-Pérot interferometer acting as a bandpass filter. A logistic regression model was used to successfully differentiate between polyimide tape, sapphire, borosilicate glass, fused silica, and alumina ceramic at temperatures as low as 34.0 ± 0.05 °C. Each material was recognized with true positive rates above 94% calculated from individual pixel spectra. The surface temperature of the samples was subsequently predicted using pre-fitted partial least squares (PLS) models, which predicted all surface temperature values with a common root mean square error (RMSE) of 1.10 °C and thereby outperforming conventional thermography. This approach paves the way for a practical solution to the underdetermined temperature-emissivity system.

Semiconducting Supramolecular Organic Frameworks Assembled from a Near-Infrared Fluorescent Macrocyclic Probe and Fullerenes.

We report here a new extended tetrathiafulvalene (exTTF)-porphyrin scaffold, 2, that acts as a ball-and-socket receptor for C60 and C70. Supramolecular interactions between 2 and these fullerenes serve to stabilize 3D supramolecular organic frameworks (SOFs) in the solid state formally comprising peapod-like linear assemblies. The SOFs prepared via self-assembly in this way act as "tunable functional materials", wherein the complementary geometry of the components and the choice of fullerene play crucial roles in defining the conductance properties. The highest electrical conductivity (σ = 1.3 × 10-8 S cm-1 at 298 K) was observed in the case of the C70-based SOF. In contrast, low conductivity was seen for the SOF based on pristine 2 (σ = 5.9 × 10-11 S cm-1 at 298 K). The conductivity seen for the C70-based SOF approaches that seen for other TTF- and fullerene-based supramolecular materials despite the fact that the present systems are metal-free and constructed entirely from neutral building blocks. Transient absorption spectroscopic measurements corroborated the formation of charge-transfer states (i.e., 2δ+/C60δ- and 2δ+/C70δ-, respectively) rather than fully charge separated states (i.e., 2•+/C60•- and 2•+/C70•-, respectively) both in solution (toluene and benzonitrile) and in the solid state at 298 K. Such findings are considered consistent with an ability to transfer charges effectively over long distances within the present SOFs, rather than, for example, the formation of energetically trapped ionic species.

Surface-Controlled Crystal Alignment of Naphthyl End-Capped Oligothiophene on Graphene: Thin-Film Growth Studied by in Situ X-ray Diffraction.

We report on the microstructure, morphology, and growth of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) thin films deposited on graphene, characterized by grazing incidence X-ray diffraction (GIXRD) and complemented by atomic force microscopy (AFM) measurements. NaT2 is deposited on two types of graphene surfaces: custom-made samples where chemical vapor deposition (CVD)-grown graphene layers are transferred onto a Si/SiO2 substrate by us and common commercially transferred CVD graphene on Si/SiO2. Pristine Si/SiO2 substrates are used as a reference. The NaT2 crystal structure and orientation depend strongly on the underlying surface, with the molecules predominantly lying down on the graphene surface (face-on orientation) and standing nearly out-of-plane (edge-on orientation) on the Si/SiO2 reference surface. Post growth GIXRD and AFM measurements reveal that the crystalline structure and grain morphology differ depending on whether there is polymer residue left on the graphene surface. In situ GIXRD measurements show that the thickness dependence of the intensity of the (111) reflection from the crystalline edge-on phase does not intersect zero at the beginning of the deposition process, suggesting that an initial wetting layer, corresponding to 1-2 molecular layers, is formed at the surface-film interface. By contrast, the (111) reflection intensity from the crystalline face-on phase grows at a constant rate as a function of film thickness during the entire deposition.

Structural Evaluation of 5,5'-Bis(naphth-2-yl)-2,2'-bithiophene in Organic Field-Effect Transistors with n-Octadecyltrichlorosilane Coated SiO2 Gate Dielectric.

We report on the structure and morphology of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) films in bottom-contact organic field-effect transistors (OFETs) with octadecyltrichlorosilane (OTS) coated SiO2 gate dielectric, characterized by atomic force microscopy (AFM), grazing-incidence X-ray diffraction (GIXRD), and electrical transport measurements. Three types of devices were investigated with the NaT2 thin-film deposited either on (1) pristine SiO2 (corresponding to higher surface energy, 47 mJ/m2) or on OTS deposited on SiO2 under (2) anhydrous or (3) humid conditions (corresponding to lower surface energies, 20-25 mJ/m2). NaT2 films grown on pristine SiO2 form nearly featureless three-dimensional islands. NaT2 films grown on OTS/SiO2 deposited under anhydrous conditions form staggered pyramid islands where the interlayer spacing corresponds to the size of the NaT2 unit cell. At the same time, the grain size measured by AFM increases from hundreds of nanometers to micrometers and the crystal size measured by GIXRD from 30 nm to more than 100 nm. NaT2 on OTS/SiO2 deposited under humid conditions also promotes staggered pyramids but with smaller crystals 30-80 nm. The NaT2 unit cell parameters in OFETs differ 1-2% from those in bulk. Carrier mobilities tend to be higher for NaT2 layers on SiO2 (2-3 × 10-4 cm2/(V s)) compared to NaT2 on OTS (2 × 10-5-1 × 10-4 cm2/(V s)). An applied voltage does not influence the unit cell parameters when probed by GIXRD in operando.

Modeling temperature dependent singlet exciton dynamics in multilayered organic nanofibers.

Organic nanofibers have shown potential for application in optoelectronic devices because of the tunability of their optical properties. These properties are influenced by the electronic structure of the molecules that compose the nanofibers and also by the behavior of the excitons generated in the material. Exciton diffusion by means of Förster resonance energy transfer is responsible, for instance, for the change with temperature of colors in the light emitted by systems composed of different types of nanofibers. To study in detail this mechanism, we model temperature dependent singlet exciton dynamics in multilayered organic nanofibers. By simulating absorption and emission spectra, the possible Förster transitions are identified. Then, a kinetic Monte Carlo model is employed in combination with a genetic algorithm to theoretically reproduce time-resolved photoluminescence measurements for several temperatures. This procedure allows for the obtainment of different information regarding exciton diffusion in such a system, including temperature effects on the Förster transfer efficiency and the activation energy of the Förster mechanism. The method is general and may be employed for different systems where exciton diffusion plays a role.

The interplay between localized and propagating plasmonic excitations tracked in space and time.

In this work, the mutual coupling and coherent interaction of propagating and localized surface plasmons within a model-type plasmonic assembly is experimentally demonstrated, imaged, and analyzed. Using interferometric time-resolved photoemission electron microscopy the interplay between ultrashort surface plasmon polariton wave packets and plasmonic nanoantennas is monitored on subfemtosecond time scales. The data reveal real-time insights into dispersion and localization of electromagnetic fields as governed by the elementary modes determining the functionality of plasmonic operation units.

Organic surface-grown nanowires for functional devices.

Discontinuous organic thin film growth on the surface of single crystals results in crystalline nanowires with extraordinary morphological and optoelectronic properties. By way of being generated at the interface of organic and inorganic materials, these nanowires combine the advantages of flexible organic films with the defectless character of inorganic crystalline substrates. The development of destruction-free transfer and direct growth methods allows one to integrate the organic nanowires into semiconductor, metallic electronic or photonic platforms. This article details the mechanisms that lead to the growth of these nanowires and exemplifies some of the linear as well as non-linear photonic properties, such as optical wave guiding, lasing and frequency conversion. The article also highlights future potential by showing that organic nanowires can be integrated into optoelectronic devices or hybrid photonic/plasmonic platforms as passive and active nanoplasmonic elements.

Measurement of surface plasmon autocorrelation functions.

In this paper we demonstrate the realization of an autocorrelator for the characterization of ultrashort surface plasmon polariton (SPP) pulses. A wedge shaped structure is used to continuously increase the time delay between two interfering SPPs. The autocorrelation signal is monitored by non-linear two-photon photoemission electron microscopy. The presented approach is applicable to other SPP sensitive detection schemes that provide only moderate spatial resolution and may therefore be of general interest in the field of ultrafast plasmonics.

Surface plasmon polariton propagation in organic nanofiber based plasmonic waveguides.

Plasmonic wave packet propagation is monitored in dielectric-loaded surface plasmon polariton waveguides realized from para-hexaphenylene nanofibers deposited onto a 60 nm thick gold film. Using interferometric time resolved two-photon photoemission electron microscopy we are able to determine phase and group velocity of the surface plasmon polariton (SPP) waveguiding mode (0.967c and 0.85c at λ(Laser) = 812nm) as well as the effective propagation length (39 µm) along the fiber-gold interface. We furthermore observe that the propagation properties of the SPP waveguiding mode are governed by the cross section of the waveguide.

Morphological tuning of the plasmon dispersion relation in dielectric-loaded nanofiber waveguides.

Understanding the impact of lateral mode confinement in plasmonic waveguides is of fundamental interest regarding potential applications in plasmonic devices. The knowledge of the frequency-wave vector dispersion relation provides the full information on electromagnetic field propagation in a waveguide. This Letter reports on the measurement of the real part of the surface plasmon polariton dispersion relation in the near infrared spectral regime for individual nanoscale plasmonic waveguides, which were formed by deposition of para-hexaphenylene (p-6P) based nanofibers on top of a gold film. A detailed structural characterization of the nanofibers provides accurate information on the dimensions of the investigated waveguides and enables us to quantify the effect of mode confinement by comparison with experimental results from continuous p-6P films and calculations based on the effective index method.

Mapping surface plasmon polariton propagation via counter-propagating light pulses.

In an interferometric time-resolved photoemission electron microscopy (ITR-PEEM) experiment, the near-field associated with surface plasmon polaritons (SPP) can be locally sensed via interference with ultrashort laser pulses. Here, we present ITR-PEEM data of SPP propagation at a gold vacuum interface recorded in a counter-propagating pump-probe geometry. In comparison to former work this approach provides a very intuitive real-time access to the SPP wave packet. The quantitative analysis of the PEEM data enables us to determine in a rather direct manner the propagation characteristics of the SPP.

Acquisition and Analysis of Hyperspectral Thermal Images for Sample Segregation.

This study presents the first results of a new type of hyperspectral imager in the long-wave thermal radiation range from 8.0 to 14.0 µm which is simpler than readily available Fourier transform infrared spectroscopy-based imagers. Conventional thermography images the thermal radiation from hot objects, but an accurate determination of temperature is hampered by the often unknown emissivities of different materials present in the same image. This paper describes the setup and development of a hyperspectral thermal camera based on a low-order scanning Fabry-Pérot interferometer acting as a bandpass filter. A three-dimensional hyperspectral data cube (two spatial and one spectral dimension) was measured by imaging a high-emissivity carbon nanotube-coated surface (Vantablack), black painted aluminum, borosilicate glass, Kapton tape, and bare aluminum. A principal component analysis (PCA) of the hyperspectral thermal image clearly segregates the individual samples. The most distinguishable sample from the PCA is the borosilicate Petri dish of which the Si-O-Si bond in borosilicate glass was the most noticeable. Additionally, it was found that the relatively large 1024 × 768 × 70 data cube can be reduced to a much smaller cube of size 1024 × 768 × 5 containing 92% of the variance in the original dataset. The possibility of discriminating between the samples by their spectroscopic signature was tested using a logistic regression classifier. The model was fitted to a chosen set of principal components obtained from a PCA of the original hyperspectral data cube. The model was used to predict all pixels in the original data cube resulting in estimates with very high true positive rate (TPR). The highest TPR was obtained for borosilicate glass with a value of 99% correctly predicted pixels. The remaining TPRs were 94% for black painted aluminum, 81% for bare aluminum, 79% for Kapton tape, and 70% for Vantablack. A standard thermographic image was acquired of the same objects where it was found that the samples were mutually indistinguishable in this image. This shows that the hyperspectral thermal image contains sample characteristics which are material related and therefore outperforms standard thermography in the amount of information contained in an image.

Thermo-optic control of dielectric-loaded plasmonic waveguide components.

We report preliminary results on the development of compact (length < 100 microm) fiber-coupled dielectric-loaded plasmonic waveguide components, including Mach-Zehnder interferometers (MZIs), waveguide-ring resonators (WRRs) and directional couplers (DCs), whose operation at telecom wavelengths is controlled via the thermo-optic effect by electrically heating the gold stripes of dielectric-loaded plasmonic waveguides. Strong output modulation (> 20%) is demonstrated with MZI- and WRR-based components, and efficient (approximately 30%) rerouting is achieved with DC switches.

The surface microstructure controlled growth of organic nanofibres.

A combination of top down fabrication of microstructured growth templates and bottom up growth of oriented organic nanofibres is demonstrated. At appropriate surface temperatures blue light emitting para-hexaphenylene nanofibres are grown on gold coated silicon substrates, perpendicularly oriented to the edges of micron-sized ridges. These nanofibres have well-defined maximum lengths of a few micrometres and typical heights and widths of around a hundred and a few hundred nanometres, respectively. Previously such oriented growth has been reported solely on specific, single-crystalline growth substrates. The present finding opens up a route to direct growth of oriented organic nanofibres on microstructured substrates that can act as device platforms.

On the suitability of carbon nanotube forests as non-stick surfaces for nanomanipulation.

A carbon nanotube forest provides a unique non-stick surface for nanomanipulation, as the nanostructuring of the surface allows micro- and nanoscale objects to be easily removed after first being deposited via a liquid dispersion. A common problem for smooth surfaces is the strong initial stiction caused by adhesion forces after deposition onto the surface. In this work, carbon nanotube forests fabricated by plasma-enhanced chemical vapour deposition are compared to structures with a similar morphology, silicon nanograss, defined by anisotropic reactive ion-etching. While manipulation experiments with latex microbeads on structured as well as smooth surfaces (gold, silicon, silicon dioxide, Teflon, diamond-like carbon) showed a very low initial stiction for both carbon nanotube forests and silicon nanograss, a homogeneous distribution of particles was significantly easier to achieve on the carbon nanotube forests. Contact-angle measurements during gradual evaporation revealed that the silicon nanograss was superhydrophic with no contact-line pinning, while carbon nanotube forests in contrast showed strong contact-line pinning, as confirmed by environmental scanning electron microscopy of microdroplets. As a consequence, latex microbeads dispersed on the surface from an aqueous solution distributed evenly on carbon nanotube forests, but formed large agglomerates after evaporation on silicon nanograss. Lateral manipulation of latex microbeads with a microcantilever was found to be easier on carbon nanotube forests and silicon nanograss compared to smooth diamond-like carbon, due to a substantially lower initial stiction force on surfaces with nanoscale roughness. Nanomanipulation of bismuth nanowires, carbon nanotubes and organic nanofibres was demonstrated on carbon nanotube forests using a sharp tungsten tip. We find that the reason for the remarkable suitability of carbon nanotube forests as a non-stick surface for nanomanipulation is indeed the strong contact-line pinning in combination with the nanostructured surface, which allows homogeneous dispersion and easy manipulation of individual particles.

Correlating Charge Transport with Structure in Deconstructed Diketopyrrolopyrrole Oligomers: A Case Study of a Monomer in Field-Effect Transistors.

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 Å, b = 13.02 Å, c = 5.85 Å, α = 101.4°, ß = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with a = 24.92 Å, b = 25.59 Å, c = 5.42 Å, α = 80.3°, ß = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm2/V s and an on/off ratio of 106 are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph.

Three-point bending setup for piezoresistive gauge factor measurement of thin-film samples at high temperatures.

We present a new method for measuring the piezoresistive gauge factor of a thin-film resistor based on three-point bending. A ceramic fixture has been designed and manufactured to fit a state-of-the-art mechanical testing apparatus (TA Instruments Q800). The method has been developed to test thin-film samples deposited on silicon substrates with an insulating layer of SiO2. The electrical connections to the resistor are achieved through contacts in the support points. This insures that the influence of the electrical contacts is reduced to a minimum and eliminates wire-bonding or connectors attached to the sample. During measurement, both force and deflection of the sample are recorded simultaneously with the electrical data. The data analysis extracts a precise measurement of the sample thickness (<1% error) in addition to the gauge factor and the temperature coefficient of resistivity. The sample thickness is a critical parameter for an accurate calculation of the strain in the thin-film resistor. This method provides a faster sample evaluation by eliminating an additional sample thickness measurement or alternatively an option for cross checking data. Furthermore, the method implements a full compensation of thermoelectrical effects, which could otherwise lead to significant errors at high temperature. We also discuss the magnitude of the error sources in the setup. The performance of the setup is demonstrated using a titanium nitride thin-film, which is tested up to 400 °C revealing the gauge factor behavior in this temperature span and the temperature coefficient of resistivity.

A joint theoretical and experimental characterization of two acene-thiophene derivatives.

Acene-thiophenes compounds have been used successfully as active materials in thin-film transistors and photodetectors. This work aims at obtaining an adequate theoretical framework to accurately characterize the optical and electronic properties of two such compounds: NaT2 and NaT3. This is done by comparing the results of simulations with experimental absorption spectra. Basis size effects are investigated as well as the role of intramolecular vibrations in the simulated spectra. It is shown that the most important feature of a DFT calculation is the appropriate selection of long-range corrected functionals, which allows for the accurate description of the first absorption band of these molecules.

Mechanical properties of organic nanofibers.

Intrinsic elastic and inelastic mechanical properties of individual, self-assembled, quasi-single-crystalline para-hexaphenylene nanofibers supported on substrates with different hydrophobicities are investigated as well as the interplay between the fibers and the underlying substrates. We find from atomic-force-microscopy-based rupture experiments a rupture shear stress of about 2 x 10(7) Pa for an individual fiber. Deflecting a nanofiber suspended across a gap results in a Young's modulus of 0.65 GPa. Translational motion of intact nanofibers across the surface is demonstrated for fibers on a silicon substrate with a low-adhesion coating, whereas such motion on a noncoated substrate is limited to very short (sub-micrometer) nanofiber pieces due to strong adhesive forces.