Optical Sheet Research Articles

Free-standing paper-like heat spreading films based on graphene oxide-aromatic molecule composites

Graphene has recently attracted great attention as a heat spreading material due to its unique thermal transfer property. Structural defects introduced into graphene unavoidably during graphene growth or processing are significantly affect its thermal transfer property. In this study, we choose four aromatic molecules (naphthalene, anthracene, tetracene and perylene) as “molecular patches” to heal structural defects in graphene, and free-standing paper-like heat spreading films based on graphene oxide (GO)-aromatic molecule composites have been fabricated via a synchronous reduction and assembly procedure, showing tunable optical transmittance and low sheet resistance. The Raman and X-ray diffraction characterizations of the as-prepared films indicate that the aromatic molecules have been adsorbed successfully by the GO and have partially restored the intrinsic quality of graphene. Preliminary studies on heat spreading materials using GO-aromatic molecule composite films demonstrate that the films can lead to obvious reduction in temperature of the hot source, indicating that these films are promising heat spreading materials for commercial portable electronics.

Diffraction-free space–time light sheets

Diffraction-free optical beams propagate freely without change in shape and scale. Monochromatic beams that avoid diffractive spreading require two-dimensional transverse profiles, and there are no corresponding solutions for profiles restricted to one transverse dimension. Here, we demonstrate that the temporal degree of freedom can be exploited to efficiently synthesize one-dimensional pulsed optical sheets that propagate self-similarly in free space. By introducing programmable conical (hyperbolic, parabolic, or elliptical) spectral correlations between the beam's spatio-temporal degrees of freedom, a continuum of families of axially invariant pulsed localized beams is generated. The spectral loci of such beams are the reduced-dimensionality trajectories at the intersection of the light-cone with spatio-temporal spectral planes. Far from being exceptional, self-similar axial propagation is a generic feature of fields whose spatial and temporal degrees of freedom are tightly correlated. These one-dimensional `space-time' beams can be useful in optical sheet microscopy, nonlinear spectroscopy, and non-contact measurements.

Crystal structure of tetra-iso-butyl-thiuram di-sulfide.

Tetra-kis(2-methyl-prop-yl)thio-per-oxy-dicarbonic di-amide, or tetra-iso-butyl-thiuram di-sulfide, C18H36N2S4, crystallizes in a general position in the triclinic space group P-1 but shows pseudo-C2 symmetry about the di-sulfide bond. The C-S-S-C torsion angle [-85.81 (2)°] and the dihedral angle between the two NCS2 mean planes [85.91 (5)°] are within the range observed for this compound type. Multiple intra- and inter-molecular S⋯H-C close contacts appear to play a role in assisting the specific conformation of the pendant isobutyl groups and the packing arrangement of mol-ecules within the cell. Tetra-iso-butyl-thiuram di-sulfide mol-ecules of one optical configuration form sheets in the plane of the a and b axes. Inversion centers exist between adjoining sheets, which stack along the c axis and alternate in the handedness of their constituent mol-ecules.

Organic Light-Emitting Diodes with a Perylene Interlayer Between the Electrode–Organic Interface

The performance of an organic light-emitting diode (OLED) with a vacuum-deposited perylene layer over a fluorine-doped tin oxide (FTO) surface is reported. To investigate the effect of the perylene layer on OLED performance, different thicknesses of perylene are deposited on the FTO surface and their current density–voltages (J–V), luminance–voltages (L–V) and device efficiency characteristics at their respective thickness are studied. Further analysis is carried out with an UV–visible light double-beam spectrophotometer unit, a four-probe resistivity unit and a field emission scanning electron microscope set up to study the optical transmittance, sheet resistance and surface morphology of the bilayer anode film. We used N,N′-bis(3-methyl phenyl)-N,N′(phenyl)-benzidine (TPD) as the hole transport layer, Tris(8-hydroxyquinolinato)aluminum (Alq3) as a light-emitting layer and lithium fluoride as an electron injection layer. The luminance efficiency of an OLED structure with a 9-nm-thick perylene interlayer is increased by 2.08 times that of the single-layer FTO anode OLED. The maximum value of current efficiency is found to be 5.25 cd/A.

Solution-Processable transparent conducting electrodes via the self-assembly of silver nanowires for organic photovoltaic devices

Solution-processed transparent conducting electrodes (TCEs) were fabricated via the self-assembly deposition of silver nanowires (Ag NWs). Glass substrates modified with (3-aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTES) were coated with Ag NWs for various deposition times, leading to three different Ag NWs samples (APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH)). Controlling the deposition time produced Ag NWs monolayer thin films with different optical transmittance and sheet resistance. Post-annealing treatment improved their electrical conductivity. The Ag NWs films were successfully characterized using UV–Vis spectroscopy, field emission scanning electron microscopy, optical microscopy and four-point probe. Three Ag NWs films exhibited low sheet resistance of 4–19Ω/sq and high optical transmittance of 65–81% (at 550nm), which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device by using Ag NWs as the anode instead of ITO electrode, and optimized device with Ag NWs exhibited power conversion efficiency of 1.72%.

Long-term air-stable Au doping of graphene by layer-by-layer assembly with graphene oxide for flexible transparent electrodes

In order to realize the most suitable hybrid structure for air-stable graphene-based transparent and flexible electrodes, we systematically compared graphene oxide (GO)/Au/graphene and graphene/Au/GO hybrid films fabricated by layer-by-layer assembly. The sheet resistance of graphene/Au/GO was proportional to the concentration of AuCl3 (1–5 mg/ml), which can be understood by a significant blue-shift in the G-band associated with the phonon stiffening. The size and density of formed Au nanoparticles strongly influenced their reduction reaction, which is a crucial factor for maximizing the doping effects. The optimized optical transmittance and sheet resistance of GO/Au/graphene were 94.9% and 198 ± 29 Ohm/sq, respectively. Furthermore, the hybrid films revealed stable doping effects (sheet resistance variation: 23–55%) against ambient conditions after 1 month. The variation in sheet resistance for GO/Au/graphene hybrid films corresponds to 30% with a bending radius of 10 mm after repeated bending tests (bending cycles of 105).

High aspect ratio silver grid transparent electrodes using UV embossing process

This study presents a UV embossing process to fabricate high aspect ratio silver grid transparent electrodes on a polymer film. Transparent electrodes with a high optical transmittance (93 %) and low sheet resistance (4.6 Ω/sq) were fabricated without any high temperature or vacuum processes. The strong adhesion force between the UV resin and the silver ink enables the fabrication of silver microstructures with an aspect ratio higher than 3. The high aspect ratio results in a low sheet resistance while maintaining a high optical transmittance. Multi-layer transparent electrodes were fabricated by repeating the proposed UV process. Additionally, a large-area of 8-inch touch panel was fabricated with the proposed UV process. The proposed UV process is a relatively simple and low cost process making it suitable for large-area production as well as mass production.

Tethered Magnets Are the Key to Magnetotaxis: Direct Observations of Magnetospirillum magneticum AMB-1 Show that MamK Distributes Magnetosome Organelles Equally to Daughter Cells

ABSTRACTMagnetotactic bacteria are a unique group of bacteria that synthesize a magnetic organelle termed the magnetosome, which they use to assist with their magnetic navigation in a specific type of bacterial motility called magneto-aerotaxis. Cytoskeletal filaments consisting of the actin-like protein MamK are associated with the magnetosome chain. Previously, the function of MamK was thought to be in positioning magnetosome organelles; this was proposed based on observations via electron microscopy still images. Here, we conducted live-cell time-lapse fluorescence imaging analyses employing highly inclined and laminated optical sheet microscopy, and these methods enabled us to visualize detailed dynamic movement of magnetosomes in growing cells during the entire cell cycle with high-temporal resolution and a high signal/noise ratio. We found that the MamK cytoskeleton anchors magnetosomes through a mechanism that requires MamK-ATPase activity throughout the cell cycle to prevent simple diffusion of magnetosomes within the cell. We concluded that the static chain-like arrangement of the magnetosomes is required to precisely and consistently segregate the magnetosomes to daughter cells. Thus, the daughter cells inherit a functional magnetic sensor that mediates magneto-reception.

Structural Engineering of Metal-Mesh Structure Applicable for Transparent Electrodes Fabricated by Self-Formable Cracked Template.

Flexible and transparent conducting electrodes are essential for future electronic devices. In this study, we successfully fabricated a highly-interconnected metal-mesh structure (MMS) using a self-formable cracked template. The template—fabricated from colloidal silica—can be easily formed and removed, presenting a simple and cost-effective way to construct a randomly and uniformly networked MMS. The structure of the MMS can be controlled by varying the spin-coating speed during the coating of the template solution or by stacking of metal-mesh layers. Through these techniques, the optical transparency and sheet resistance of the MMS can be designed for a specific purpose. A double-layered Al MMS showed high optical transparency (~80%) in the visible region, low sheet resistance (~20 Ω/sq), and good flexibility under bending test compared with a single-layered MMS, because of its highly-interconnected wire structure. Additionally, we identified the applicability of the MMS in the case of practical devices by applying it to electrodes of thin-film transistors (TFTs). The TFTs with MMS electrodes showed comparable electrical characteristics to those with conventional film-type electrodes. The cracked template can be used for the fabrication of a mesh structure consisting of any material, so it can be used for not only transparent electrodes, but also various applications such as solar cells, sensors, etc.

Investigation of physical, electrical and optical properties of sol-gel deposited Sc-loaded CdO films

For the first time, Sc-loaded CdO thin layers have been deposited with a sol-gel process by using Cd(CH3COO)2·2H2O and ScCl3 salts. The Sc-contribution effect on the crystalline, surface, electrical, and optical features have been inquired by means of XRD, AFM, SEM, Four Probe method, and UV/VIS spectrophotometer. The polycrystalline cubic CdO nano-particles with (111) preferential direction were observed from XRD studies. The average crystallite size, micro-strain values of Sc-loaded CdO samples changed between 22.05–17.16nm and 2.46×10−3–3.16×10−3. The SEM and AFM images indicated that CdO samples have been composed of nano-sized spherical particles. The most homogeneous particle distribution was observed for 1 at.% Sc-contribution level and the particle sizes of pure and Sc-loaded CdO samples varied between about 30–200nm. The sheet resistance values of films were found in the range of 5.0×103Ω and 2.1×102Ω. The optical transmittance and reflectance values for samples have been found between 65 and 90% and 5–22% over visible region. The optical band gap for pure CdO firstly went up from 2.63eV to 2.66eV with 1 at.% Sc-doping ratio and then it continuously decreased. The Urbach tail, extinction coefficient, dielectric constant and refractive index values were also determined as depending on Sc-doping content. As a result of the present research, it was seen the Sc-contribution level affected the crystallite size, lattice constant, micro-strain, optical band gap and sheet resistance values of CdO.

Transparent, conductive, and superhydrophobic nanocomposite coatings on polymer substrate

Transparent, conductive, and superhydrophobic nanocomposite coatings were fabricated on the polyethylene terephthalate substrate by a spray method. Different concentrations of multi-walled carbon nanotubes (MWCNTs) entwined with SiO2 nanoparticles, which originated from the hydrolysis and condensation of tetraethyl orthosilicate, were sprayed to form MWCNTs/SiO2 nanocomposite coatings. The coatings were characterized by scanning electron microscopy, contact angle measurements, and other analytical techniques. The surface morphology, hydrophobicity, transparency, and conductivity of the nanocomposite coating were found to be strongly dependent on the MWCNT concentration. With increasing MWCNT concentration, the hydrophobicity increased first and then decreased, and the optical transmittance and sheet resistance decreased. The enhanced hydrophobicity was associated with the surface microstructure and chemical composition of the coating. The decreased hydrophobicity resulted mainly from the decrease in the trapped air between the water droplet and the nanocomposite coating. Owing to the hierarchically porous 3-dimensional microstructure and opportune fluorinated MWCNT content, the nanocomposite coating with 0.2wt% MWCNTs exhibited the best hydrophobicity with a contact angle of 156.7°, good transparency with 95.7% transmittance and relatively high conductivity with a sheet resistance of 3.2×104Ωsq−1.

Effects of laser parameters on optoelectronic properties of polycrystalline silicon films prepared by two-step annealing process

A two-step method combining furnace and laser annealing is proposed for improving the electrical and optical properties of amorphous silicon thin films on glass substrates. It is shown that the optical transmittance of the as-deposited silicon film increases from 27 to 31% following furnace annealing at 600 °C. However, the electrical resistance is too high to be measured using a four-point probe. The sheet resistance can be reduced to 270 kΩ/□ by annealing the as-deposited film using an ultra-violet (UV) laser. However, the resistance is still too high for TFT applications. The as-deposited silicon film is first furnace annealed at 600 °C for 12 h and then annealed using a UV laser with a laser power of 41 mW and a scanning speed of 60 mm/s. The optical transmittance and sheet resistance of the annealed film are found to be 29% and 1.17 kΩ/□, respectively. The X-ray diffraction (XRD) results suggest that the improved optical and electrical properties are the result of an amorphous-to-crystalline transformation of the silicon microstructure.

Optical performance of the honeycomb texture – a cell and module level analysis using the OPTOS formalism

A comprehensive analysis of the optical properties of different surface textures for silicon solar cells must include (i) the optical performance at cell level as well as encapsulated to a module stack and (ii) a varying angle of incidence of the solar irradiation, as it influences the optical yield. Within this work, the OPTOS formalism (Optical Properties of Textured Optical Sheets) was used to compare a honeycomb texture featuring a high aspect ratio with state of the art random pyramids and the isotexture. The honeycomb texture exhibits an increased absorbed photocurrent density of 2.3mA/cm2 before and 0.7mA/cm2 after encapsulation compared to the isotexture for a 180µm thick solar cell. The simulated absolute value of 42.3mA/cm2 after encapsulation is even superior to the random pyramid texture with 41.5mA/cm2. Furthermore, a simulation-based optical yield analysis reveals that the angular irradiance of three exemplary terrestrial locations plays a minor role for the relative performance between the textures compared to influence of encapsulation effects.

Fabrication of a transparent conducting electrode based on graphene/silver nanowires via layer-by-layer method for organic photovoltaic devices

A solution-processed transparent conducting electrode was fabricated via layer-by-layer (LBL) deposition of graphene oxide (GO) and silver nanowires (Ag NWs). First, graphite was oxidized with a modified Hummer’s method to obtain negatively-charged GO sheets, and Ag NWs were functionalized with cysteamine hydrochloride to acquire positively-charged silver nanowires. Oppositely-charged GO and Ag NWs were then sequentially coated on a 3-aminopropyltriethoxysilane modified glass substrate via LBL deposition, which provided highly controllable thin films in terms of optical transmittance and sheet resistance. Next, the reduction of GO sheets was performed to improve the electrical conductivity of the multilayer films. The resulting GO/Ag NWs multilayer was characterized by a UV–Vis spectrometer, field emission scanning electron microscope (FE-SEM), optical microscope (OM) and sheet resistance using a four-point probe method. The best result was achieved with a 2-bilayer film, resulting in a sheet resistance of 6.5Ω sq−1 with an optical transmittance of 78.2% at 550nm, which values are comparable to those of commercial ITO electrodes. The device based on a 2-bilayer hybrid film exhibited the highest device efficiency of 1.30% among the devices with different number of graphene/Ag NW LBL depositions.

Fabrication of solution-processable, highly transparent and conductive electrodes via layer-by-layer assembly of functional silver nanowires

Highly transparent and conductive electrodes were prepared on glass substrates via layer-by-layer (LBL) deposition of functionalized silver nanowires (AgNWs). First, commercial nanowires were functionalized with cysteamine hydrochloride (CA) and 3-mercaptopropionic acid (MPA) to obtain positively and negatively charged AgNWs, respectively. Second, oppositely charged AgNWs were sequentially coated on a 3-aminopropyltriethoxysilane (APTES) modified glass substrate via LBL deposition, which provided highly controllable thin films in terms of optical transmittance and sheet resistance. Next, the multilayer film was subjected to thermal annealing to improve the electrical conductivity. Finally, AgNW multilayer film was characterized by UV–Vis spectrometer, field emission scanning electron microscope (FE-SEM), optical microscope (OM) and sheet resistance measurement by four-point probe method. The best result was achieved with 6-bilayer film providing a sheet resistance of 18.3Ω/□ with an optical transmittance of 83.8% at 550nm, which is comparable to commercial ITO electrode.

Diode-based additive manufacturing of metals using an optically-addressable light valve.

Selective Laser Melting (SLM) of metal powder bed layers, whereby 3D metal objects can be printed from a digital file with unprecedented design flexibility, is spurring manufacturing innovations in medical, automotive, aerospace and textile industries. Because SLM is based on raster-scanning a laser beam over each layer, the process is relatively slow compared to most traditional manufacturing methods (hours to days), thus limiting wider spread use. Here we demonstrate the use of a large area, photolithographic method for 3D metal printing, using an optically-addressable light valve (OALV) as the photomask, to print entire layers of metal powder at once. An optical sheet of multiplexed ~5 kW, 20 ms laser diode and ~1 MW, 7 ns Q-switched laser pulses are used to selectively melt each layer. The patterning of near infrared light is accomplished by imaging 470 nm light onto the transmissive OALV, which consists of polarization-selective nematic liquid crystal sandwiched between a photoconductor and transparent conductor for switching.

Direct Nanoimprinting of a Colloidal Self‐Organizing Nanowire Ink for Flexible, Transparent Electrodes

A semicontinuous process is used to nanoimprint a nanowire ink and form transparent conductive electrodes. Ultrathin gold nanowires are confined in the features of an elastomeric stamp, where they spontaneously assemble upon drying into hierarchical, percolating superstructures. This templated self‐organization yields grids with defined line widths down to 0.9 µm and high pattern fidelity. Metal grids with square, hexagonal, and linear features are printed over 30 cm2 on different substrates and gently sintered in hydrogen plasma. Meshes on polyethylene terephthalate foil show high optical transmittances (>92%) and low sheet resistances (106–168 Ω sq−1). Their resistance is changed by only 10% after 500 bending cycles at a radius of 5 mm. The printed electrodes are used to build capacitive and resistive touch sensor devices.

Magnetic and thermal properties of amorphous TbFeCo alloy films

Amorphous TbFeCo material with perpendicular magnetic anisotropy is currently attracting more attention for potential applications in spintronic devices and logic memories. We systematically investigate magnetic, structural, thermal, optical and electrical properties of TbFeCo alloy films. It shows out-of-plane easy axis of the films turns into in-plane orientation after annealing. Significant increase in saturation magnetization in the temperature range between 400 and 450°C is revealed by thermomagnetic measurements. The occurrence of crystallization and oxidation at high temperatures is confirmed by X-ray diffraction measurements. Pronounced changes in optical reflectance and sheet resistance are observed with temperature, in line with structural relaxation and change. The activation barriers for crystallization and oxidation are determined to be 1.01eV and 0.83eV, respectively, for FeCo-rich and Tb-rich samples. Better thermal stability against crystallization and oxidation is demonstrated in the FeCo-rich sample than the Tb-rich type. Our results provide some useful information for the alloy used in device fabrication.

Novel Concept for Fabricating a Flexible Transparent Electrode Using the Simple and Scalable Self-Assembly of Ag Nanowires

We introduce a new concept for transparent electrodes via the self-assembly of a silver nanowire (Ag NW) network with a cell shape. A transparent conductive network was achieved by forming an array of Ag NWs around droplets of a solvent with higher vapor pressure in Ag NWs ink. The difference in vapor pressure and viscosity of the solvent causes an Ag NWs network with a cell shape, and the cell size can be easily controlled from 10 to 100 μm using the solvent ratio. The cell network of Ag NWs with a high optical transmittance (>92%) and low sheet resistance (40 Ω/sq) was simply fabricated on flexible polymer films of large scale using a Meyer rod coating. In addition, we also studied and demonstrated the figure of merit of the transparent electrode between our method and a random Ag NWs network from the general method. The performance of the transparent electrode may be applied to a wide array of optoelectronic devices and can replace transparent conductive oxides such as Al-doped ZnO and indium tin oxide.

Integration of Prism Sheet on Quantum Dot Film With Bridge Patterns to Enhance Luminance of LED Backlight Unit

An integrated prism sheet on quantum dot (QD) film with bridge patterns has been proposed to increase the optical efficiency of an LED backlight unit. The bridge patterns were designed, such that they not only extract the light trapped in a QD film but also change the optical path effectively toward the vertical direction. We verified our design concept and geometrical parameters by optical simulation, and fabricated the integrated optical sheet using a lithography and UV imprinting process. The simulation results show the luminance enhancement of 33.6% after applying the bridge patterns while keeping the same Commission Internationale de l'Éclairage (CIE) after careful controlling the concentration of QDs. The enhancement of luminance toward the vertical directionwas also confirmed experimentally after the fabrication of an integrated prism sheet on a QD enhancement film.