Effect of structural modifications in the laser properties of polymer films doped with perylenebisimide derivatives

In this work we study the effect of different types of modifications of the chemical structure of perylenebisimide derivatives (PBIs) in the laser properties of PBI-doped polystyrene films at various concentrations. In particular, we focus on controlling the wavelength of emission, in order to tune the laser wavelength, as well as on increasing the amount of PBI in the films, aiming to decrease the laser thresholds, while keeping a good photostability. Amplified spontaneous emission (ASE) was observed for all compounds, the best performance being obtained for films doped with PBIs symmetrically substituted at the imide positions, that emitted at 580 nm with a threshold of 20 kW/cm² and a photostability half-life of 30,000 pump pulses. Substitution at the bay positions of the PBI core and replacement of imide functions by anhydride groups allow to red-shift the emission wavelength up to 645 nm, but the thresholds increase considerably and the photostability is reduced. PBIs are among the most photostable materials reported in the literature and show very reasonable thresholds. In addition they have a great potential for their application in the field of data communications based on poly(methyl methacrylate) optical fibers, with low-loss transmission windows between 530 and 590 nm and at 650 nm.     

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

Laser beam dispersion affects the resolution of Raman and photo-stimulated luminescence piezo-spectroscopy measurements of transparent materials. In this paper, we investigate the lateral spreading of the laser beam and the axial sampling depth of Raman spectroscopy measurements within thermal sprayed yttria-stabilized zirconia (YSZ) thin coatings. The lateral diameters of the laser beams (λ = 632.8 nm and 514 nm) reach approximately ～160 μm after travelling through a thickness of 200 μm of air plasma sprayed (APS) YSZ and ～80 μm after travelling through 120 μm of electron beam physical vapour deposited YSZ. The Raman spectroscopy sampling depth was found to be between 30 and 40 μm in APS YSZ. The beam dispersions within these two coatings were simulated using the ray-tracing software ZEMAX to understand the observed scattering patterns. The results are discussed with respect to the application of these two spectroscopic techniques in multi-layered thermal barrier coating systems.     

Top gate copper phthalocyanine thin film transistors with laser-printed dielectric

Dielectric layers involved in top gate organic thin film transistors (TG-OTFTs) have been fabricated by using laser induced forward transfer (LIFT) technique. Poly(methyl methacrylate) (PMMA) as insulating polymer was spin-coated on a quartz substrate and transferred by laser on an acceptor substrate to form a dielectric layer on top of an organic semiconducting layer and source/drain contacts both previously vapour phase deposited. Copper phthalocyanine (CuPc) has been chosen to form p-type organic active layers. The nature of transferred patterns and the efficiency of LIFT confirm the important potential of a laser printing technique in the development of plastic microelectronics. Electrical characterizations in TG configuration demonstrated that transistors are fully operative with hole mobility up to 8.6 × 10^?3 cm²/V s. A comparative study with others dielectric layers in bottom gate transistors (BG-OTFTs), as PMMA spin-coated and silicon dioxide SiO₂, points out more precisely the limiting parameters to an efficient charge transport in the conducting channel created at the interface between PMMA and CuPc.     

The effect of film thickness on the failure strain of polymer-supported metal films

We perform uniaxial tensile tests on polyimide-supported copper films with a strong (1 1 1) fiber texture and with thicknesses varying from 50 nm to 1 μm. Films with thicknesses below 200 nm fail by intergranular fracture at elongations of only a few percent. Thicker films rupture by ductile transgranular fracture and local debonding from the substrate. The failure strain for transgranular fracture exhibits a maximum for film thicknesses around 500 nm. The transgranular failure mechanism is elucidated by performing finite element simulations that incorporate a cohesive zone along the film/substrate interface. As the film thickness increases from 200 to 500 nm, a decrease in the yield stress of the film makes it more difficult for the film to debond from the substrate, thus increasing the failure strain. As the thickness increases beyond 500 nm, however, the fraction of (1 0 0) grains in the (1 1 1)-textured films increases. On deformation, necking and debonding initiate at the (1 0 0) grains, leading to a reduction in the failure strain of the films.     

Carbazole based electroluminescent devices obtained by vacuum evaporation

Transparent conductive oxide (SnO₂)/organic layers/aluminum thin film sandwich structures have been obtained by vacuum evaporation. The organic component was either a thin carbazole film or a bilayer. In that case, the carbazole film was deposited on a thin insulating polymer film. The polymer used was the poly(tetrabromo-p-phenylenediselenide) (PBrPDSe). Photoluminescence measurements have shown that the carbazole thin films emit blue light. The I–V measurements have shown that the structures exhibit diode characteristics. The forward direction is obtained when the TCO is positively biased. However, the reproducibility of the results obtained with a single carbazole layer is poor. It appears that the stability of the sample is improved when a thin PBrPDSe film (50 nm) is introduced between the carbazole and the SnO₂. The polymer film avoids short circuit effect. In that case, the turn-on voltage of the diode is about 3 V, when the thickness of the carbazole film is around 250 nm and the electroluminescence appears at a voltage of about 4 V.     

Polymer infrared photo-detector with high sensitivity up to 1100 nm

We reported a low band-gap conjugated polymer, poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine] (PDTTP), was studied for the near infrared (NIR) photo-detector application. PDTTP shows intense absorption in NIR wavelength (to 1000 nm) and the estimated optical and electrochemical band-gaps of PDTTP are quite small around 1.15 eV and 1.08 eV, respectively. The low band-gap and the extended long wavelength absorption originates from the introduction of alternating TP units when its parent polythieno[3,4-b]pyrazine shows excellent narrow band-gap properties. Therefore, the relatively low band-gap and intense absorption in long wavelength of PDTTP make itself a promising candidate for near-infrared photo-detector. The hole mobility of the PDTTP measured from the bottom contact field effect transistor is around 1.40 × 10^?3 cm²/V s with a on/off ratio of 2100. The photo-detector based on bulk hetero-junction PDTTP and (6,6)-phenyl-C61-butyric acid methyl ester blend (PCBM) has the incident photon-to-electron conversion efficiency 28.9% at 1000 nm (?5 V) and 6.2% at 1100 nm (?5 V). This photo-detector can be operated at a high-speed of 1 MHz. The experimental result suggests the potential applications of low band-gap conjugated polymers on near-infrared photo-detectors.     

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

Direct emissive pattern formation in PPV-type polymer with built-in photoresist properties and the application to light emitting devices

The principle of photobleaching in the conjugated polymer material didodecylstilbenevinylene (didodecyl-PSV) was used as a built-in photoresist property to control the extent of the emissive layer in polymer light emitting devices (PLEDs). Illumination of thin films of the conjugated polymer material on optically transparent conducting substrates through a positive mask with short exposure times (20–60 s) using a 150 W Xenon lamp and subsequent application of the and the second electrode enabled the formation of an electroluminescent image of the mask. We demonstrate a direct method where no photoresist, intermediate steps or mechanical manipulation is required to generate a complex emissive pattern employing a homogenous film and homogenous electrodes. The fluorescence of the film showed complex lifetime decays. We found that the lifetime increased with emission wavelength indicating the presence of energetically disordered excitons. The fine structure observed in the solution emission is absent in the film emission. However, after photobleaching the film emission becomes very similar to the solution emission. The appearance of fine structure could indicate that only one state became responsible for the photoluminescence spectrum. Infrared spectroscopy was used to confirm the presence of a degradation route similar to that reported for dialkoxypolyphenylenevinylene materials where the vinylene bonds are react with molecular oxygen and form carboxylic acid groups. Finally the effect of using different electrode metals was established and a comparison made between the photoluminescence and electroluminescence properties. The electroluminescence emission maximum (580 nm) of didodecyl-PSV light emitting devices was found to be redshifted by 20 nm compared to the photoluminescence, exhibited turn-on voltages below 3 V and reached a luminance of 100 cd m⁻² at current densities below 10 mA cm⁻².     

Alternating phenylenevinylene and thienylenevinylene copolymers with cyano groups: Synthesis,photophysics and photovoltaics

Two alternating conjugated copolymers TTP and TTT were synthesized by Heck coupling of 2,3-bis-(5-bromothiophene-2-yl)acrylonitrile with 1,4-dihexyloxy-2,5-divinylbenzene and 3-hexyl-2,5-divinylthiophene, respectively. The absorption spectra of the copolymers in THF solution showed three maxima around 270, 370 and 460 nm with optical band gaps of 2.30–2.34 eV. The electrochemically estimated band gaps of copolymers were 2.04–2.10 eV. The thin film absorption spectra were broad and extended about from 250 to 600 nm with a long wavelength maximum near 470 nm. The copolymers emitted yellow-orange light with maximum at 528–551 nm in THF solution and 567–610 nm in thin film. TTP showed the most red shifted emission maximum between the copolymers. The performance of the photovoltaic cells which were fabricated using blends of the copolymers with 6,6-phenyl C61-butyric acid methyl ester (PCBM) (1:1 and 1:4, w/w) as well as pure copolymers was investigated.     

Synthesis,characterization and photovoltaic properties of poly{[1′,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene}

A new conjugated polymer, poly{[1′,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene} (PTVMEH-PPV) was synthesized via Grim polymerization. The polymer is soluble in common organic solvents such as chloroform and tetrahydrofuran, and possesses adequate thermal stability (T_d > 246 °C). The absorption spectrum of PTVMEH-PPV film shows a broader absorption peak covering the wavelength range from 380 nm to 620 nm, which is red-shifted and broadened in comparison with that of MEH-PPV. The onset oxidation potential of the polymer is 0.12 V versus Ag/Ag⁺, ca. 0.2 V lower than that of MEH-PPV. The band gap of the polymer measured by cyclic voltammetry is 1.82 eV, which basically agrees with that obtained from the onset wavelength of the absorption spectra. Polymer solar cell was fabricated based on the blend of PTVMEH-PPV and PCBM with a weight ratio of 1:1. The device shows the maximum external quantum efficiency of 14% at ca. 520 nm, an open circuit voltage of 0.67 V and a power conversion efficiency of 0.32% under the illumination of AM1.5, 80 mW/cm².     

Angle-resolved XPS study of carbon steel passivity and chloride-induced depassivation in simulated concrete pore solution

The passivity and chloride-induced depassivation of carbon steel in simulated concrete pore (CP) solution (pH = 13.3) was investigated using angle-resolved X-ray photoelectron spectroscopy (XPS). After full passivation, the thickness of the oxide film on steel specimens was approximately 5 nm. The oxide films close to the steel substrate were mainly composed of protective Fe(II) oxides while near the free surface the film consisted mostly of Fe(III) oxides. The addition of chloride to the CP solution decreased the thickness of the oxide film and changed its stoichiometry such that near the film/substrate interface the Fe(II)/Fe(III) decreased.     

Epitaxial Ni–Mn–Ga/MgO(1 0 0) thin films ranging in thickness from 10 to 100 nm

Thin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L2₁ austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization.     

Composites of low bandgap conducting polymer-wrapped MWNT and poly(methyl methacrylate) for low percolation and high transparency

The composites of multi-walled carbon nanotubes (MWNT) wrapped with low bandgap conjugated polymer and poly(methyl methacrylate) (PMMA) were prepared for transparent conductive films. NIR-absorbing poly(ethyl thieno[3,4-b]thiophene-2-carboxylate) (PTTEt) with E_g of 1.0 eV was used in this study. Upon hybridization with MWNT, PTTEt in an insulating state became partially conductive due to electron transfer from PTTEt to MWNT, meaning that PTTEt can function as conductive glue interconnecting MWNT in a PMMA matrix. The electrical conduction of the composites (PTTEt-MWNT/PMMA), consisting of PTTEt-wrapped MWNT (PTTEt-MWNT/PMMA) and PMMA, showed the percolation at 0.10 wt% MWNT loading, which was ca. 0.18 wt% lower than the composites of MWNT and PMMA (MWNT/PMMA). The maximum conductivity of PTTEt-MWNT/PMMA, on the other hand, was one order of magnitude lower than that of MWNT/PMMA, suggesting that PTTEt incorporation onto MWNT for transparent conductive films is effective within a specific range of MWNT loadings (i.e., between percolation thresholds of MWNT/PMMA and PTTEt–MWNT/PMMA). The comparison of transmittance of PTTEt–MWNT/PMMA (0.18 wt% MWNT) with MWNT/PMMA (0.32 wt% MWNT), possessing the same conductivities (3 × 10^?3 S cm^?1), showed ca. 10% enhanced transmittance at 550 nm. These results imply that hybridization of low bandgap conjugated polymers with carbon nanotubes can be utilized for the reduction of percolation threshold and the increase of optical transparency without sacrificing conductivities at low MWNT loadings.     

Broadband anti-reflection and enhanced field emission from catalyst-free grown small-sized ITO nanowires at a low temperature

Small-sized indium tin oxide (ITO) nanowires were fabricated using the electron beam evaporation (EBE) technique at low temperature (～150 °C) without adding any catalyst. The ITO nanowires have a typical diameter of around 10 nm and a length of more than 100 nm, with body-centered cubic crystal structures that grow along the 〈1 0 0〉 directions, as revealed by transmission electron microscopy. The growth mechanism of the branched ITO nanowires was found to be a vapor–solid process. The nanowire films show a broadband anti-reflection property due to the graded refraction index from the film surface to the substrate. Enhanced field emission properties with a low turn-on electric field and a high field enhancement factor were also observed in the ITO nanowires.     

Laser curing of silver-based conductive inks for in situ 3D structural electronics fabrication in stereolithography

A hybrid stereolithography (SL)/direct print (DP) system has been developed and previously described that fabricates three-dimensional (3D) structural electronic devices in which component placement, interconnect routing, and system boundaries are not confined to two dimensions as is the case with traditional printed circuit boards (PCBs). The resulting increased level of design and fabrication freedom provides potential for a reduction in both volume and weight as well as the capability of fabricating systems in arbitrary and complex shapes as required to conform to unique application requirements (e.g., human anatomy, airframe structures, and other volumetrically-constrained mechanical systems). The fabrication of these devices without intermediate part removal between SL and DP processes requires in situ curing of DP-dispensed silver-based inks to sufficiently cure the inks prior to continuing additional SL fabrication. This paper describes investigations into the laser curing process using the two laser wavelengths, 355 nm and 325 nm, which have been used in commercial SL machines. Various laser curing parameters, including energy (laser power and scan speed), scanning location, and laser wavelength were investigated. The trace resistances and structural changes in the SL substrate and printed trace were compared for each experiment to determine the most preferred laser ink curing method. Furthermore, oven curing of partially laser cured ink traces was investigated as a means for minimizing the number of in situ laser passes required to embed ink traces during SL fabrication. The laser curing process was repeated for a wide variety of conductive inks, having different structure, composition, and curing properties to determine if certain inks were more responsive to laser curing and if the ink curing results could be generalized. A statistical study was conducted under the hypothesis that laser curing of inks at 325 nm wavelength would be better, due to lower silver reflectance, as compared to 355 nm wavelength. Results indicated that particulate silver based conductive inks can be successfully cured in situ using SL lasers with various laser curing parameters. Curing ink traces at high laser power and slow scan speeds with the laser beam located on the substrate adjacent to the ink channel resulted in the most effective ink curing but resulted in discoloration of the ink and/or charring of the SL substrate. Comparatively, when laser power was reduced sufficiently to eliminate the charring, lower effective ink curing was achieved. Irradiating the laser directly on the ink did not damage the ink or the substrate, while providing low trace resistances, and represents the most viable laser curing alternative to achieve acceptable trace resistance without charring the SL substrate. The results further indicated that partial laser curing of ink using a reduced number of laser passes with subsequent oven cure seem to work effectively and may decrease overall manufacturing time. SL lasers with low power (<100 mW) may not be effective for curing conductive inks that have high viscosities and require high curing temperatures. Finally, a statistical study determined that 325 nm is more effective for direct curing of the ink as compared to the 355 nm laser.     

Pulsed laser deposition of functionally gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser

In this study, functionally gradient diamond-like carbon (FGDLC) films are fabricated using a novel pulsed laser deposition technique to enhance adhesion strength. A 355 nm picosecond laser beam is split into two beams, and the power of each split beam is changed individually by a motorized beam attenuator as a function of time. In this way, two laser beams with customized time-varying powers are available for ablating two different target materials. Two beams are irradiated on graphite and 316L stainless steel targets, respectively, in a vacuum chamber, and the produced dissimilar plasmas are mixed in space before they are deposited on a stainless steel 316L substrate. Using this method, we have built FGDLC films with a thickness of ～510 nm, where the composition changes gradually from stainless steel to DLC in the direction of deposition. We have confirmed that FGDLC films show much higher adhesion strength than normal DLC films.     

Nondestructive evaluation of creep and fatigue damages in nickel-base superalloys using a scanning blue laser microscope

A new nondestructive evaluation method for detecting both creep and fatigue damages of Ni-base superalloy used in advanced gas turbine systems has been proposed by applying a scanning blue laser microscope. The change of the microtexture in a grain of the alloy due to creep and fatigue damages can be observed clearly by using this microscope. The reflectance of the γ′ (Ni₃Al) phase of the alloy was found to be less than that of the γ (Ni-base alloy) phase, when a laser beam of wavelength shorter than 500 nm was radiated to the alloy. Since the microtexture of the alloy varies significantly during damage progress, it is possible to evaluate the damages quantitatively and nondestructively by observing the change of the microtexture using the laser beam with wavelength 410 nm.     

Electrophoretic deposition of conjugated polymer: Deposition from dilute solution and PEDOT coating effect

It has been shown that the films of a soluble conjugated polymer poly[(9,9-dioctyl-2,7-divinylene fluorenylene)-alt-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}] for electronic devices can be prepared by the electrophoretic deposition with polymer suspensions derived from dilute polymer solutions which are so dilute that the conventional spin-coating technique is not applicable. For example, a 100 nm-thick film can be prepared on an indium-tin-oxide (ITO) electrode from a suspension from solution containing 0.1 g/l of the polymer. The thickness of the polymer film deposited is found to be almost proportional to the concentration of the polymer, and the linearity down to 5.0 × 10^?3 g/l is confirmed. On the other hand, it has been found that coating the ITO electrode with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) salt results in low and nonlinear deposition rate.     

Self-organized gold nanostructures on laser patterned PET

Irradiation of polyethyleneterephtalate (PET) with linearly polarized light from a pulsed 248 nm KrF laser with 6000 pulses per area and fluences between 2.0 and 8.4 mJ·cm^? 2 resulted in formation of coherent ripple patterns with a lateral periodicity in the order of laser light wavelength and structure depth of several 10 nm. The structure period could be controlled by variation of the incidence angle of the laser beam. Subsequently, thin gold layers were deposited onto the laser patterned PET surface either by sputtering and evaporation. The resulting structures were analyzed by atomic force microscopy (AFM) and focused ion beam cuts in combination with scanning electron microscopy (FIB-SEM). Evaporation leads to a continuous metal coverage copying the nanostructured polymer surface, while sputtering leads to formation of isolated nano-wires on the ridge of the laser-induced ripples. The width of the gold nano-wires could be tailored by the ripple periodicity formed before the deposition process.     

Synthesis and characterization of conjugated copolymer containing 2,5-bis(3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole and 2,6-[(1,5-didecyloxy)naphthalene for polymer solar cells

A new alternating donor–acceptor conjugated copolymer, poly{(2,5-bis(3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl)-alt-(2,6-[(1,5-didecyloxy)naphthalene])} (PNTzTz) based on thiazolothiazole and didecyloxynaphthalene units was synthesized using a typical Suzuki coupling polymerization method and was explored as a donor polymer in bulk heterojunction (BHJ) solar cells. The copolymer was characterized using differential scanning calorimetry, thermogravimetric analysis, UV–Vis absorption and cyclic voltammetry. It possesses moderate molecular weights and excellent thermal properties with a 5% weight loss around 340 °C. It also exhibits good optical absorption with an absorption peak at 475 nm and an absorption onset at 560 nm in the film state. Photovoltaic properties of the copolymer were investigated blended with electron acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The optimized photovoltaic device with a device structure of ITO/PEDOT:PSS/PNTzTz:PC₆₁BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 0.99% with a short circuit current density of 3.3 mA cm^?2, an open circuit voltage of 0.96 V and a fill factor of 0.33 under the illumination of AM 1.5, 100 mW cm^?2.