New semiconducting multi-branched conjugated molecules based on π-extended triphenylene and its application to organic field-effect transistor

Triphenylene derivatives are the most well-known representatives of discotic liquid crystals forming columnar mesophases. In this work, the authors report on the design and synthesis of new semiconducting materials, based on a triphenylene core, those are solution processable. These molecules have a good solubility and are applicable to soluble processing for organic field-effect transistor (OFET). Their thermal stabilities and dynamic behaviors are suitable for OFET device fabrication. In comparison with all of previous reported triphenylenes derivatives, these molecules have lower bandgap energies. As far as we know, there are few reports on the application of π-extended triphenylene systems to OFET. The semiconducting properties of these molecules were quite promising for further designs of π-extended triphenylene derivatives.     

Highly conductive single molecular junctions by direct binding of π-conjugated molecule to metal electrodes

We have investigated electrical conductance of the single C₆₀ and benzene molecules bridging between metal electrodes. The single C₆₀ and benzene molecular junctions were prepared in ultra high vacuum. The single molecular junctions showed the high conductance values (around 0.1–1 G₀: G₀ = 2e²/h), which were comparable to that of the metal atomic contact. The highly conductive single molecular junctions could be prepared by direct binding of the π-conjugated organic molecule to the metal electrodes without the use of anchoring groups. For comparison, the single 1,4-benzenediamine molecular junction was investigated in solution. The benzene molecule was bound to the Au electrodes via amine (anchoring group) for the single 1,4-benzenediamine molecular junction. The conductance of the single 1,4-benzenediamine molecular junction was 8 × 10⁻³G_0. It was suggested that the anchoring groups acted as resistive spacers between the molecule and metal.     

Alkali halide surfaces near melting: Wetting and nanofriction properties

Alkali halide (1 0 0) crystal surfaces are poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(1 0 0) within the well tested BMHFT model potential. Calculations of the solid–vapor, solid–liquid and liquid–vapor free energies showed that solid NaCl(1 0 0) is a non-melting surface, and explain its bad wetting in detail. The extreme stability of NaCl(1 0 0) is ideal for a study of the nanofriction in the high temperature regime, close to and even above the bulk melting temperature (T_M). Our simulations reveal in this regime two distinct and opposite phenomena for plowing and for grazing friction. We found a frictional drop close to T_M for deep ploughing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena we obtain a fresh microscopic understanding, relating the former to “skating” through a local liquid cloud, the latter to softening of the free substrate surface. It is argued that both phenomena, to be pursued experimentally, should be much more general than the specific NaCl surface case. Most metals in particular possessing one or more close packed non-melting surface, such as Pb, Al or Au(1 1 1), should behave quite similarly.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.     

Near-Infrared Spectral Emissivity of Cu, Ag, and Au in the Liquid and Solid States at Their Melting Points

Normal spectral emissivities of liquid and solid Cu, Ag, and Au have been determined at their melting points over a wavelength range 1000 to 2500 nm using an apparatus that consists of a cold crucible and a diffraction grating spectrometer. For the noble metals, the emissivities of liquid phases are systematically larger than those of solid phases over the measured wavelength range, and the wavelength dependence of the liquid is similar to that of the solid. The measured emissivities for the liquid metals are compared with those deduced from the optical constants measured by Miller and Krishnan et al. The present results for liquid Cu and Au are in good agreement with the data of Krishnan et al., but not with those of Miller for Cu, which suggests that the optical constants measured by Krishnan et al. for liquid Cu are more accurate than those of Miller. The present data for liquid Ag and Au are in excellent agreement with all previously reported data. For the solid metals at their melting point, a semi-empirical estimation of the emissivity was carried out based upon the Drude model incorporating the effects of interband absorption and a frequency-dependent scattering rate, ^–1()= ^–1 ₀+b ². The values of ^–1 ₀ and b at the melting point are obtained by fitting the modified Drude model to the measurement results for the solid noble metals.     

Synthesis and characterization of bent-shaped azobenzene monomers: Guest–host effects in liquid crystals with azo dyes for optical image storage devices

Six novel bent-shaped monomers were synthesized such as substituted/or non-substituted 1,3-phenylene bis-{4-[(4-allyloxy)phenylazo]benzoate} (4a–c) and substituted/or non-substituted 1,3-phenylene bis-{4-[3-(4-allyloxy-3-fluoro)phenylazo]benzoate} (4d–f) in which azobenzene moiety in the periphery and substituted/or non-substituted resorcinol as central unit with polymeriable double bonds are linked at both ends of all the molecules. The mesophase behavior was investigated using polarizing optical microscopy, DSC and XRD measurements. Four members of the family show an intercalated smectic (Sm_intercal) phase and two were crystalline in nature. The trans-form of azo compounds (4a–f) showed a strong band in the UV region (355–366 nm), which was attributed to the π–π^* transition, and a weak band in the visible region at 455–465 nm due to the n–π^* transition. When one of the azo dye (4d) is mixed with liquid crystal as a guest, showed greater increase in thermal back relaxation time which is useful for creation of optical image storage devices.     

Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings

This study characterizes the all-optical switching effect in holographic polymer-dispersed liquid crystal transmission gratings. The light-induced switching behaviors of these structures are due to the doped azobenzene-derived LC (azo-LC), which changes the refractive index of phase-separated LC within the polymer composite. This study also optimizes the polymer-dispersed liquid crystal formulation containing 15 wt.% azo-LC and 35 wt.% nematic LC to achieve a grating performance with a tunable diffraction efficiency of 78% and a fast switching-on time (0.5 s) with a relatively small light stimulus of 9 mW/cm².     

All-plastic organic dye laser with distributed feedback resonator structure

We demonstrated an all-plastic waveguide organic dye laser with distributed feedback (DFB) resonator. We fabricated DFB structure on a surface of SU-8 2002 photoresist polymer using the interference of two beams of a frequency-tripled Nd:YAG pulse laser at 355 nm. The typical grating pitch of fabricated DFB structure was 190, 380 and 570 nm, corresponding to the number of mode m, and each corresponding grating amplitude was ca. 1, 4-5, and 25 nm, respectively. Shallow amplitude of 1 or 4-5 nm is ascribed to the wide incidence angle of the interference beams. Threshold of lasing for m = 3 is lower than that for m = 1 or m = 2 depending on the emission wavelength. DFB structure with the smaller amplitude of gratings at m = 1 and 2 is required for the higher threshold of laser emission. By controlling the grating pitch with nanometer scale, we can tune the wavelength of laser emission with 40 to 60 nm tunability. Effective energy transfer via nonradiative Förster transfer mechanism assists in lowering the threshold of laser emission.     

Formation process of mixed silicon and titanium carbide from copyrolysis of polysilane and metallic titanium: Part II

In part I we reported a copyrolysis of polysilane with metallic tungsten [1]. Now copyrolysis of chlorine containing polysilane with metallic titanium addition in the range of 0–10 at % Ti was performed. A significant decrease of the mass loss during the polysilane pyrolysis was observed. Also the specific surface area, gas evolution and occuring phase formation changes. The titanium forms preferently carbides and temporary silicides depending on the applied pyrolysis temperature. The reactions are controlled by transport phenomena of carbon and titanium. In the end of the process silicon carbide (SiC) and titanium carbide (TiC_x) are stable.     

Extremely-high-intensity photoluminescence from CuBr films fabricated by film-substrate chemical reaction of CuCl films on KBr-crystal substrate

High quality CuBr film is fabricated by film-substrate chemical reaction of CuCl on KBr, where CuCl is vacuum-deposited film of the order of ∼ 100 nm thickness and KBr is fleshly-cleaved single crystal. The resulting CuBr films show ∼ 10³ times higher efficiency of free-exciton photoluminescence (PL) than conventional CuBr films. The efficiency exceeds that of conventional CuCl films even by a factor of ∼ 10², despite the general recognition that the free-exciton PL from CuBr is much weaker than that from CuCl. The present result encourages us to challenge to rethink the exciton PL in CuBr as well as in CuCl, because the compounds are the model materials for basic and application studies of linear/nonlinear exciton-related PL properties of solids.     

On the control of the liquid-phase distribution in multi-material assemblies processed by liquid-phase sintering

Proper control of composition gradients in components processed by liquid-phase sintering requires the control of the migration of the liquid phase during sintering. A model considering isotropic interface energies has been developed for expressing the variation of the solid–liquid interface area with liquid volume fraction during the final stage of liquid-phase sintering (i.e. when residual porosity has disappeared). The model allows the computation of the driving forces for shape accommodation as a function of the dihedral angle ψ, liquid volume fraction, u, particle volume, V_p, and average particle coordination, n_c. It particularly enlightens the coupling between the effects of ψ and of n_c. The model allows a new insight into the control of microstructural evolution during liquid-phase sintering of assemblies of dual-phase materials with different average particle size. Taking ψ = 0° for the WC–Co system, the model was found to predict with a good accuracy the distribution of Co at equilibrium in assemblies made by heat-treating at 1400 °C under protective atmosphere stacks of cylinders of WC–Co cermets differing in mean WC particle size and cobalt content.     

Influence of driving electric field on electron-ripplon resonance in 2D electron crystal

No Heading Resonance features in the conductivity of a 2D electron crystal over liquid helium are studied experimentally depending on driving electric field. The experiment has been carried out at temperature T = 75 mK for the crystal with surface electron density n_s = 8 × 10⁸ cm^–2 in the frequency range 4–7 MHz where the resonance mode (0,1) of coupled electron-ripplon oscillations can be excited in the experimental cell. It is found that the resonance amplitude decreases as the exciting voltage increases. Additional features of the resonance curve are observed at frequencies lower than the (0,1) mode frequency. The results are analyzed within Monarkhas theory which takes into account an influence of non-linearity of interaction between electrons and liquid surface on the dynamics of 2D electron crystal moving along the surface of liquid helium.PACS numbers: 73.20.Dx; 73.90.+f     

Mo/Si multilayer-coated ruled blazed gratings for the soft-x-ray region

Two Mo/Si multilayer-coated blazed gratings have been fabricated for operation at soft-x-ray wavelengths above the Si L edge, λ ≥ 12.4 nm, at (near) normal incidence. The sawtooth profile of the grating structure was mechanically ruled into a 200-nm Au film that was deposited onto a plane glass substrate. To smooth the rough Au surface and to prevent interdiffusion of the Au film with the upper Mo/Si multilayer, a carbon film was evaporated onto the Au grating surface of one of the gratings before the deposition of the multilayer coating. We matched the multilayer grating, working on blaze in the third diffraction order, in which an absolute diffraction efficiency of 3.4% at a wavelength of 14 nm was measured, whereas only 1.1% was achieved for a similar grating (without a carbon interlayer). These efficiencies are higher than those obtained for other ruled blazed gratings reported in the literature. As a result of the multilayer and grating periodicity, the wavelength of diffraction can be tuned bya rotation of the grating, which is important for application in a soft-x-ray monochromator.     

Microsegregation in directionally solidified dendritic-cellular structure of superalloy CMSX-4

A directionally solidified sample of superalloy CMSX-4 was investigated to show the effect of crystal orientation on the segregation distribution. The solute distribution of alloying elements across a dendritic cell was measured. Due to the preferred crystal growth in <100> orientation the segregation profiles in this direction is much flatter than that in <110> orientation.     

Facile synthesis of highly uniform ZnO multipods as the supports of Au and Ag nanoparticles

Through the direct reaction of the aqueous solutions of zinc salts and KOH, highly uniform ZnO multipods consisted of rod-shaped branches are synthesized in an extremely high yield. The synthesis is conducted at low temperature, does not require any organic capping agents to affect the crystal habit, and is independent on the species of zinc sources. Moreover, by changing the reaction temperature from 25 to 70 °C, the tips of the multipods can be adjusted from taper to regular hexagon. The mechanism responsible for the formation of ZnO multipods is the self-regulation growth guided by the inherently asymmetric structure of hexagonal ZnO along the c-axis. After the surface modification with amino groups, ZnO multipods show good affinity to Ag and Au nanoparticles, which results in the spontaneous migration of Au and Ag nanoparticles to the surface of ZnO to form well-defined Au/ZnO and Ag/ZnO heterostructures. The fabricated hybrids are found to be effective surface-enhanced Raman scattering (SERS) substrates because of their distinct geometrical shape, large surface area, and the ability to create a strong local electromagnetic field by the metal–semiconductor heterojunction.     

A fully biocompatible poly(ethylene glycol)–gold plasmonic crystal for optical sensing

A significant challenge in nano and biophotonics is to demonstrate fully biocompatible nano-optics devices that can perform biofunctions in vivo. Here we present a scalable, cost-effective, and large-area nanofabrication method for creating a quasi-3D plasmonic crystal using poly(ethylene glycol) (PEG) and gold (Au), both biocompatible materials. The plasmonic crystal was prepared by depositing an Au layer on the upper hemisphere of the replicated PEG nanospheres array. Additionally we demonstrated that the fabricated plasmonic crystal can behave as a label-free glucose sensor with sensitivity and figure-of-merit values comparable to other plasmonic crystal based sensors.     

Optimization of sputtered Cr/Au thin film for diaphragm-based MEMS applications

An optimization study on the sputtering of Cr/Au thin film for diaphragm-based MEMS applications is presented. The effects of the film thickness, process pressure and process power on the residual stress of the film are investigated. A low-stress silicon nitride diaphragm-based device characterization platform is fabricated to study the influence of the Cr/Au film stress on the diaphragm compliance. The fabricated devices are characterized by measuring the capacitance change under a bias voltage from 0 to 40 V. For the 8-µm and 10-µm air gap device characterization platforms, the largest capacitance changes of 5.1% and 4.3%, respectively, occur at a compressive film stress of − 200 MPa. A large capacitance change indicates a more sensitive diaphragm, which is desired in pressure sensor design.     

Deposition of low-resistivity gallium-doped zinc oxide films by low-temperature radio-frequency magnetron sputtering

The transparent and conductive gallium-doped zinc oxide (GZO) film was deposited on 1737F Corning glass using the radio-frequency (RF) magnetron sputtering system with a GZO ceramic target. (The Ga₂O₃ contents are approximately 5 wt. %). In this study, the effect of the sputtering pressure on the structural, optical and electrical properties of GZO films upon the glass or polyester film (PET) substrate was investigated and discussed in detail. The GZO film was grown under a steady RF power of 400 W and a lower substrate temperature from room temperature up to 200 °C. The crystal structure and orientation of GZO thin films were examined by X-ray diffraction. All of the GZO films under various sputtering pressures had strong c-axis (002)-preferred orientation. Optical transparency was high (> 80%) over a wide spectral range from 380 nm to 900 nm. According to the experimental data, the resistivity of a single-layered GZO film was optimized at  8.3 × 10^− 4 Ω cm and significantly influenced by the sputtering pressure. In further research, the sandwich structure of the GZO film/Au metal/GZO film was demonstrated to improve the electrical properties of the single-layered GZO film. The resistivity of the sandwich-structured GZO film was around 2.8 × 10^− 4 Ω cm.     

Polarization at metal–biomolecular interfaces in solution

Metal surfaces in contact with water, surfactants and biopolymers experience attractive polarization owing to induced charges. This fundamental physical interaction complements stronger epitaxial and covalent surface interactions and remains difficult to measure experimentally. We present a first step to quantify polarization on even gold (Au) surfaces in contact with water and with aqueous solutions of peptides of different charge state (A3 and Flg-Na3) by molecular dynamics simulation in all-atomic resolution and a posteriori computation of the image potential. Attractive polarization scales with the magnitude of atomic charges and with the length of multi-poles in the aqueous phase such as the distance between cationic and anionic groups. The polarization energy per surface area is similar on aqueous Au {1 1 1} and Au {1 0 0} interfaces of approximately −50 mJ m⁻² and decreases to −70 mJ m⁻² in the presence of charged peptides. In molecular terms, the polarization energy corresponds to −2.3 and −0.1 kJ mol⁻¹ for water in the first and second molecular layers on the metal surface, and to between −40 and 0 kJ mol⁻¹ for individual amino acids in the peptides depending on the charge state, multi-pole length and proximity to the surface. The net contribution of polarization to peptide adsorption on the metal surface is determined by the balance between polarization by the peptide and loss of polarization by replaced surface-bound water. On metal surfaces with significant epitaxial attraction of peptides such as Au {1 1 1}, polarization contributes only 10–20% to total adsorption related to similar polarity of water and of amino acids. On metal surfaces with weak epitaxial attraction of peptides such as Au {1 0 0}, polarization is a major contribution to adsorption, especially for charged peptides (−80 kJ mol⁻¹ for peptide Flg-Na₃). A remaining water interlayer between the metal surface and the peptide then reduces losses in polarization energy by replaced surface-bound water. Computed polarization energies are sensitive to the precise location of the image plane (within tenths of Angstroms near the jellium edge). The computational method can be extended to complex nanometre and micrometer-size surface topologies.     

Optical properties of tungsten disulfide single crystals doped with gold

Single crystals of WS₂ doped with gold have been grown by the chemical vapour transport method using iodine as a transporting agent. X-ray diffraction (XRD) pattern analysis revealed presence of mixed three-layer rhombohedral (3R) and two-layer hexagonal (2H) polytypes for the doped crystals while the undoped one shows only 2H form. Hall measurements indicate that the samples are p-type in nature. The doping effects of the materials are characterized by surface photovoltage (SPV), photoconductivity (PC) and piezoreflectance (PzR) measurements. Room temperature SPV and PC spectra reveal a feature located at 60 meV below the A exciton and has been tentatively assigned to be an impurity level caused by Au dopant. Excitonic transition energies of the A, B, d and C excitons detected in PzR spectra show red shift due to the presence of a small amount of Au and the broadening parameters of the excitonic transition features increase due to impurity scattering. The values of the parameters that describe the electron (exciton)–phonon interaction of excitonic transitions of A–B are about two times larger than that of d–C excitonic pairs. The possible assignments of the different origins of A–B and d–C excitonic pairs have been discussed.