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The worldwide proliferation of COVID-19 poses the urgent need for sterilizable and transparent air filters to inhibit virus transmission while retaining ease of communication. Here, we introduce copper nanowires to fabricate transparent and self-sterilizable air filters. Copper nanowire air filter (CNAF) allowed visible light penetration, thereby can exhibit facial expressions, helpful for better communication. CNAF effectively captured particulate matter (PM) by mechanical and electrostatic filtration mechanisms. The temperature of CNAF could be controlled by Joule-heating up to 100 °C with thermal stability. CNAF successfully inhibited the growth of E. coli because of the oligodynamic effect of copper. With heat sterilization, the antibacterial efficiency against G. anodireducens was greatly improved up to 99.3% within 10 min. CNAF showed high reusability with stable filtration efficiency and thermal antibacterial efficacy after five repeated uses. Our result suggests an alternative form of active antimicrobial air filter in preparation for the current and future pandemic situations.

A photothermal conversion of sol precursor by selective laser sintering (SLS) process has opened a novel patterning way for advanced electronic applications. The SLS process bases on the photo-thermal-chemical effect and features for high precision, fast processing, and room temperature processability without inert gas. Therefore, the effectiveness of the interaction between the nanomaterials and laser has examined on the various conductive metal nanomaterials such as gold, silver, and copper. Although the laser sintering process has shown many achievements on the metal nanomaterials, it has rarely studied for the interaction for the metal oxide nanomaterials. In this study, WOx thin film layer employs the SLS process for post-processing which enables patterning and annealing simultaneously with inducing photochemical redox reaction as well as photothermal effect. The SLS-processed WO3 thin film has shown similar electrochemical performances for the electrochromic application to the thermal annealed WO3 thin film. We have demonstrated a facile and fast fabrication of the electrochromic device (ECD) with the SLS-processed WO3 thin film layer.

The patterning of polydimethylsiloxane (PDMS) into complex two-dimensional (2D) or 3D shapes is a crucial step for diverse applications based on soft lithography. Nevertheless, mould replication that incorporates time-consuming and costly photolithography processes still remains the dominant technology in the field. Here we developed monolithic quasi-3D digital patterning of PDMS using laser pyrolysis. In contrast with conventional burning or laser ablation of transparent PDMS, which yields poor surface properties, our successive laser pyrolysis technique converts PDMS into easily removable silicon carbide via consecutive photothermal pyrolysis guided by a continuous-wave laser. We obtained high-quality 2D or 3D PDMS structures with complex patterning starting from a PDMS monolith in a remarkably low prototyping time (less than one hour). Moreover, we developed distinct microfluidic devices with elaborated channel architectures and a customizable organ-on-a-chip device using this approach, which showcases the potential of the successive laser pyrolysis technique for the fabrication of devices for several technological applications. A laser-based patterning method enables the fast fabrication of high-quality two- and three-dimensional features in polydimethylsiloxane for microfluidics and biomedical applications.

Early researches have actively attempted to measure the thermal conductivity since the thermal conductivity of graphene was predicted to be extremely higher than that of any other existing materials. At the beginning of graphene discovery, the characterization of thermal properties of graphene was extremely challenging work due to its atomic-layer thickness. In order to resolve problems regarding its sub-nanoscale thickness, many experimental techniques have been designed to demonstrate properties of graphene. Among various techniques, however, Raman-based techniques are regarded as highly effective tools for characterizing thermal properties of various types of graphene. In addition, techniques based on Raman spectroscopy can be similarly applied to unveil secrets of newly emerging two-dimensional materials like silicene, MoS2, h-BN. In this paper, we will review several representative experimental techniques for investigating thermal properties of graphene. In particular, Raman-based techniques will be introduced in detail. Furthermore, we will discuss on how each technique has been developed to supplement the shortcoming of the initial techniques.

One of the exceptional properties of graphene is its extremely high thermal conductivity, which makes graphene a promising material in thermal engineering. The superlative thermal conductivity of graphene itself provides excellent functions, however, advantages from controlling it are so immense that there has been a rapid growth of interest in adjusting it for recent years. In this respect, defect engineering in graphene has become a critical and strong technique to manipulate the thermal conductivity of graphene and thus gives graphene great opportunities to be used for innovative applications totally different from previous thermal applications. Furthermore, the thermal conductivity of graphene can be effectively adjusted by defected engineering since the type of imposed defect and its concentration determine the thermal conductivity of modified graphene. Therefore, it is necessary to thoroughly examine the properties of diverse defected graphene. In this paper, we will introduce various types of defects applicable to graphene and discuss how the thermal conductivity is altered by the induced defects. Moreover, we will also review a variety of engineering applications pertaining to the inhibition of heat dissipation using defected graphene.

Spin-coated silver nanowires (AgNWs) on graphene show a significantly improved thermal conductance of the composite in comparison with pristine graphene with no nanowires. CVD-synthesized graphene is transferred onto an 8-nm thin TEM grid substrate, and AgNWs (average diameter 150-nm and average length 30-μm) are chemically grown from an AgNO3 reagent solution. The AgNW bridging overrides the negative effect of the grain boundary scattering of the electron/phonon energy carriers propagating in the polycrystalline CVD graphene and ultimately enhances the grain-to-grain heat transport by widening their passages. This boosting contribution of AgNWs is quantitatively assessed by measurement of thermal conductance for synthesized AgNW/graphene composite samples. The Raman thermometry measurement locations are selected to be beside a single AgNW (G-1), two AgNWs (G-2), and three or more AgNWs (G-3), so that the effect of AgNW density can be examined. The average enhanced thermal conductance values for the three AgNW-laid graphene samples are 319.27 nW/K, 343.66 nW/K, and 455.26 nW/K, respectively.

Flexible micro/nano metal grid transparent conductors emerged as an alternative to the fragile/rigid indium tin oxide electrode. They are usually fabricated by the combination of the conventional photolithography and the vacuum deposition of regular metal grid patterns, however, seriously suffer from moiré and starburst problems induced by periodic regular pattern structures. In this paper, we demonstrated flexible and imperceptible random copper microconductors with an extremely high figure-of-merit (∼2000) by the thermal conduction layer-assisted photonic sintering of copper nanoparticles without damages in the plastic substrate. This process can be easily applied to complicated structures and surfaces including a random pattern which is imperceptible and free of interferences. As a proof-of-concept, a transparent windshield defogger in a car was demonstrated with a Cu transparent random conductor at an extreme and reversible fogging state.

The thermal conductivities of monolayer graphene with 4.1 μm grain size and multilayer graphene with 0.3 μm grain sizes are measured by optothermal Raman technique. The number of layers and average grain sizes of graphene are controlled by copper thickness and synthesis conditions in chemical vapor deposition system. In addition, the graphene samples are suspended on a thorough 8-μm hole substrate via PMMA transfer method to avoid substrate effects. As a result, the thermal conductivity graphene is significantly reduced from 3000 to 660 W/m K at 320 K with increasing graphene layers and decreasing grain sizes due to the enhanced phonon scattering. The multilayer graphene with fine grain sizes that has the suppressed thermal conductivity can be useful for application in various field, such as thermoelectric material and thermal rectification.

Generally, a high-temperature postannealing process is required to enhance the photoelectrochemical (PEC) performance of hematite nanorod (NR) photoanodes. However, the thermal annealing time is limited to a short duration as thermal annealing at high temperatures can result in some critical problems, such as conductivity degradation of the fluorine-doped tin oxide film and deformation of the glass substrate. In this study, selective laser processing is introduced for hematite-based PEC cells as an alternative annealing process. The developed laser-induced phase transformation (LIPT) process yields hematite NRs with enhanced optical, chemical, and electrical properties for application in hematite NR-based PEC cells. Owing to its improved properties, the LIPT-processed hematite NR PEC cell exhibits an enhanced water oxidation performance compared to that processed by the conventional annealing process. As the LIPT process is conducted under ambient conditions, it would be an excellent alternative annealing technique for heat-sensitive flexible substrates in the future.

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Based on its outstanding mechanical, thermal, and chemical properties, a Polyimide (PI) is useful in a wide range of applications. Its usage in biomedicine is drawing great attention owing to the recent confirmation of the biocompatibility of various PIs. However, the conventional patterning of a PI, based on photolithographic methods, which is expensive and time-consuming, hampers the rapid advancement of research-oriented fields that require frequent design changes. To resolve this problem, we introduce the method of the monolithic digital patterning of PI up to the quasi-three-dimensional (3D) structures at the microscale resolution by laser-induced jetting of highly porous Laser-induced graphene (LIG) from the PI matrix. Pyrolytic jetting of the LIG is dependent not only on the laser-induced temperature but also on its temporal and spatial gradients. However, the surfaces of the remaining PI can be exceptionally smooth at the optimum laser condition, comparable to the pristine surface at the microscopic level, as confirmed by Raman spectroscopy and Atomic force microscopy (AFM) measurements. On-demand microfluidic channels and multilevel imprinting molds are created in a single step using the proposed method as a proof-of-concept, substantiating its potential application in relevant research.

The in-plane thermal conductivity ( k S i G ) of silicon-doped graphene (SiG) was greatly suppressed primarily due to increased phonon scattering associated with the large mass difference of Si from its host C atoms. For SiG as supported on an 8 nm-thick SiO2 substrate, the measured k S i G represents progressive decrease and saturation with the increase of Si dopants concentration, showing more than an order-of-magnitude reduction from that of supported pristine graphene (PG) and nearly two order-of-magnitude reductions when compared with suspended PG at about 2% doping concentration. The enhanced graphene-substrate conformity through thermal annealing in a vacuum additionally lowers k S i G from that of ambient annealing. The substitutional Si dopants tend to suppress the contribution of temperature-sensitive phonons with long mean free paths and weaken the temperature dependence of k S i G . The presence of Si dopants seems to allow for faster attainment of thermal equilibrium between different heat carriers due to the reduced phonon mean free paths. We believe that SiG holds the possibility of exclusively controlling the thermal properties of graphene, since the substitutional dopants do not violently destruct the hexagonal lattice structure of graphene and may possibly have minimal effects on graphene's electrical properties.

Among the various techniques for micro patterning of metal oxide nanowire, the resistive micro heater is promising technique due to the merit of localized and selective growth of sensing nanowire. In this study, we introduce a facile maskless fabrication of resistive micro heater using selective laser sintering of silver nanoparticle without using any conventional photolithography or vacuum deposition. By localized hydrothermal synthesis of ZnO nanowire using Joule heating of the resistive micro heater, the digitally patterned resistive micro heater is applied to the ZnO nanowire based micro UV sensor.

Measuring the thermal conductivity kg of supported graphene is inherently complicated due to uncertainties associated with the heat dissipation into the substrate. We innovate the use of an ultra-thin 8-nm SiO2 substrate to alleviate these uncertainties and thus improve the accuracy of optothermal Raman technique to measure kg of supported graphene. As a result, we present an extensive kg database for a wide temperature range from 325 K to 575 K. Furthermore, we have found that the thermal conductivity of supported graphene before annealing is close to that of suspended graphene at 3000 W m−1 K−1, which is attributable to graphene “suspension” lightly on the substrate roughness, and then progressively decreases over repeated thermal annealing. We elaborate on this annealing-induced kg to occur mainly because of the thermally enhanced graphene-substrate conformity and interfacial scattering by probing the Raman spectroscopic characterization of charge carrier density in graphene and the thermal expansion mismatching strain between graphene and substrate. Repeated thermal annealing also expedites the depletion of intercalated impurities to reduce the graphene-substrate separation distance, which acts to further reduces kg, ultimately to its lower bound under vacuum-annealing. Therefore, manipulating the thermo-mechanical affiliation can offer an alternative route to control the in-plane thermal conductivity of supported graphene.

Graphene was regarded as inappropriate material for practical thermoelectric application due to its low thermoelectric conversion efficiency (ZT) from its superlative thermal conductivity and gapless electronic structure. In this study, for the first time, we experimentally investigate the significant thermoelectric conversion efficiency enhancement in graphene by introducing substitutional Si dopants during the CVD growth of single layer graphene. The thermal conductivity of Si-doped graphene (SiG) shows the significant suppression due to the large molecular weight mismatch between host C atoms and Si dopants in the same group V. On the other hand, the Si dopants do not severely suppress the electrical properties compared to thermal conductivity due to the narrow minimum in the electron transmission spectrum of SiG near Fermi Level. The incorporated Si dopants provide the progressive reduction in Seebeck coefficient due to the increased scattering of charge carriers in SiG, but we found nearly 17 times thermoelectric conversion efficiency enhancement (ZT/ZT0) in the suspended graphene when the 2.59% of Si dopants are incorporated, which is much higher than graphene with other dopants for lattice defects. We believe that substitutional Si dopants can provide a promising method to break the limit in the current graphene based thermoelectric materials.

The electrical and mechanical robustness of thin metal film electrodes can be improved by adding an atomically thin interlayer, such as graphene, between the metal and the flexible substrate.

Exploration of the electronics solely composed of bottom-up synthesized nanowires has been largely limited due to the complex multistep integration of diverse nanowires. We report a single-step, selective, direct, and on-demand laser synthesis of a hierarchical heterogeneous nanowire-on-nanowire structure (secondary nanowire on the primary backbone nanowire) without using any conventional photolithography or vacuum deposition. The highly confined temperature rise by laser irradiation on the primary backbone metallic nanowire generates a highly localized nanoscale temperature field and photothermal reaction to selectively grow secondary branch nanowires along the backbone nanowire. As a proof-of-concept for an all-nanowire electronics demonstration, an all-nanowire UV sensor was successfully fabricated without using conventional fabrication processes.

Inspired by steel-wire reinforced concrete, which is a strong building material, a novel nanocomposite of a nanowire reinforced nanoparticle matrix film was developed for flexible and transparent electrode applications. The transparent electrode made in this study exhibits superior mechanical characteristics under tensile as well as compressive bending, in comparison to conventional sintered metal nanoparticle based metallic grids.

Selective laser sintering of metal nanoparticle ink is an attractive technology for the creation of metal layers at the microscale without any vacuum deposition process, yet its application to elastomer substrates has remained a highly challenging task. To address this issue, we introduced the shear-assisted laser transfer of metal nanoparticle ink by utilizing the difference in thermal expansion coefficients between the elastomer and the target metal electrode. The laser was focused and scanned across the absorbing metal nanoparticle ink layer that was in conformal contact with the elastomer with a high thermal expansion coefficient. The resultant shear stress at the interface assists the selective transfer of the sintered metal nanoparticle layer. We expect that the proposed method can be a competent fabrication route for a transparent conductor on elastomer substrates.

Laser has long been used for material processing, and its applications to nanomaterials for their direct synthesis, positioning and processing are currently active fields of study. The main mechanism of typical laser processes is photothermal reaction by a focused laser that remotely generates confined temperature field at a desired position with high controllability. The laser-induced elevated temperature enables direct synthesis of nanomaterials in both gas and liquid environment as well as photophysical processing of nanomaterials through melting or vaporization, represented by laser sintering and ablation processes, in spatially selective manners. On the other hand, recent advances in laser process further incorporates not only different optical responses such as optical forces and photochemical reactions for more advanced manipulation of nanomaterials, but also the interaction between electromagnetic waves, nanostructures and underlying substrates to facilitate novel processing those cannot be achieved by any other means including laser nanowelding for percolation network and laser thinning for two dimensional nanomaterials. At the same time, the shortcomings of laser process in nanomaterial processing such as limited resolution and low throughput are tackled through introducing different optical schemes together with the integration with other systems. In this review, we summarize the development and current status of digital selective laser methods for nanomaterials in broad aspects that cover from nanomaterial synthesis to its processing.

Hydrothermal growth of zinc oxide (ZnO) nanowire (NW) enables a facile synthesis of versatile 1D nanostructure, but the conventional bulk heating procedure is assessed to be unsuitable for microsensor application in terms of selectivity and power consumption. In this study, we introduce selective electro-thermal growth of ZnO NW on photolithographically patterned electrode in the liquid environment. By applying constant current to the targeted electrode, densely packed ZnO NWs are successfully synthesized only at the desired electrode with very small power consumption. It is also confirmed that the electro — thermally grown ZnO NW network connecting two distinct electrode can be utilized as photosensitive channel, proving that this process as well as the resultant ZnO NW has a high potential for microsensor applications.

Selective laser sintering of metal nanoparticle ink is a low-temperature and non-vacuum technique developed for the fabrication of patterned metal layer on arbitrary substrates, but its application to a metal layer composed of large metal area with small voids is very much limited due to the increase in scanning time proportional to the metal pattern density. For the facile manufacturing of such metal layer, we introduce micropatterning of metal nanoparticle ink based on laser-induced thermocapillary flow as a complementary process to the previous selective laser sintering process for metal nanoparticle ink. By harnessing the shear flow of the solvent at large temperature gradient, the metal nanoparticles are selectively pushed away from the scanning path to create metal nanoparticle free trenches. These trenches are confirmed to be stable even after the complete process owing to the presence of the accompanying ridges as well as the bump created along the scanning path. As a representative example of a metal layer with large metal area and small voids, dark-field photomask with Alphabetic letters are firstly created by the proposed method and it is then demonstrated that the corresponding letters can be successfully reproduced on the screen by an achromatic lens.

In this Letter, we report the development of a continuously tunable color filter based on a self-assembled isotropically stretchable microbead monolayer. Spreading equidistantly upon the application of lateral strain, the isotropically stretchable monolayer serves as a dynamic diffraction grating whose diffraction angle can be mechanically modulated. Combined with a simple spatial filtering scheme, the spectra of the filtered light are solely controlled by external strain (up to 32% radial strain) to cover a broad visible spectrum. Through a finite-difference time-domain far-field diffraction simulation, we validate the working principle of the proposed color filter. The proposed continuously tunable color filter is expected to open original applications in next-generation display field.

The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 &micro, m, 2.2 &micro, m, and 0.5 &micro, m was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of &sigma, S, and k. Electrical conductivity (&sigma, ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (VG). Mobility (&micro, ) values of 529, 459, and 314 cm2/V&middot, s for holes and 1042, 745, and 490 cm2/V&middot, s for electrons for the three grain sizes of 4.1 &micro, m, respectively, were obtained from the slopes of the measured &sigma, vs. VG graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 &micro, m to 0.5 &micro, m.

As several innovative technologies for flexible electric devices are being realized, demand for in-situ strain monitoring for flexible electric devices is being emphasized. Because flexible devices are commonly influenced by substrate strain, suitable strain sensors for flexible devices are essential for the sophisticated maneuvering of flexible devices. In this study, a flexible strain sensor based on an Ag electrode is prepared on a polyimide substrate using the LDW (laser direct writing) process. In this process, first, the Ag nanoparticles are coated on the substrate and selectively sintered using a focused laser. Because of the advantages of the LDW process (such as being mask-less, using low temperatures, and having non-vacuum characteristics), the entire fabrication process has been dramatically simplified; as a final outcome, a highly reliable strain sensor has been fabricated. Using this strain sensor, various strain conditions that arise from different bending radii can be detected by measuring real-time electrical signals.

We introduce a facile method to enhance the functionality of a patterned metallic transparent conductor through selective laser ablation of metal nanowire percolation network. By scanning focused nanosecond pulsed laser on silver nanowire percolation network, silver nanowires are selectively ablated and patterned without using any conventional chemical etching or photolithography steps. Various arbitrary patterns of silver nanowire transparent conductors are readily created on the percolation network by changing various laser parameters such as repetition rate and power. The macroscopic optical and electrical properties of the percolation network transparent conductor can be easily tuned by changing the conductor pattern design via digital selective laser ablation. Further investigation on the silver nanowire based electrode line prepared by the ablation process substantiates that the general relation for a conducting thin film fails at a narrow width, which should be considered for the applications that requires a high resolution patterns. Finally, as a proof of concept, a capacitive touch sensor with diamond patterns has been demonstrated by selective laser ablation of metal nanowire percolation network.

A flexible all-solid-state supercapacitor was demonstrated with a flexible Ag nanoparticle current collector which is prepared by a roll-to-roll (R2R) gravure printing process combined with a fast, low temperature laser annealing process. The laser annealing could yield good electrical conductivity of the printed Ag nanoparticle (NP) very rapidly without any noticeable polymer substrate damage and with outstanding adhesion to the underlying polymer substrate which is essential for the fabrication of stable energy devices. The laser annealed Ag NP films are subsequently sandwiched with a carbon slurry and a polymer layer as the active material and the electrolyte to assemble flexible all solid-state supercapacitors that can be bent up to 135° without any severe decrease of the electrochemical performance. By combining the proposed laser process with the existing R2R system, we expect that the printing process for flexible electronic devices could be greatly improved in terms of processing time and space.

We investigate the mechanical durability and environmental stability of laser annealed silver (Ag) nanoparticle (NP) film. Roll-to-roll printed Ag NP film on polyethylene terephthalate substrate is annealed with a rapid laser annealing process in ambient conditions as an alternative to the conventional thermal annealing process. The laser annealed Ag NP film exhibits superior electrical and mechanical properties, with fast annealing time and no damage on the substrate. The outer/inner bending test results demonstrate that the flexibility of the laser annealed Ag film is excellent. The failure bending radii in the outer/inner bending tests are 3 mm. The laser annealed film can withstand 10,000 bending cycles. A nano-scratch test indicates that the adhesion strength of the laser annealed film is comparable to that of the thermal annealed film. The environmental reliability of Ag NP film is investigated under different high-temperature and high-humidity conditions, while being subjected to cyclic bending fatigue stress. The durability of printed Ag film is found to be influenced by temperature and humidity. The laser annealed film shows relatively large increase in resistance during the bending fatigue test under high temperature and humidity condition (60 °C/90% RH), which is attributed to the oxidation of Ag nanoparticles and initiation of cracks. Generation of cracks is accelerated owing to the combinational effects of the cyclic stress and humidity. These results suggest that, even though the laser annealed Ag film demonstrates sufficient mechanical durability, further improvement of the film properties is required for use in extreme mechanical and environmental conditions.

We experimentally demonstrate a fiberized saturable absorber using a cladding-etched optical fiber as a platform. A wet etching technique based on a mixed solution of NH4F and (NH4)2SO4, which is not toxic, is used. Using a graphene oxide (GO)-deposited cladding-etched fiber into an erbium fiber ring cavity, it is shown that femtosecond pulses can readily be produced.

Laser is a very unique and useful tool for materials processing with the various parameter choice range. The main mechanism of typical laser processes is photo-induced thermal reaction that generates confined temperature field at a desired position with high controllability. The laser-induced selective temperature rise enables selective chemical reaction for synthesis of nanomaterials in various environment including both gas and liquid environment as well as photophysical processing of nanomaterials through melting or vaporization, represented by laser sintering and ablation processes. In this review, recent progresses in the selective digital laser processing for nanomaterial process and synthesis on various heat sensitive substrate. This laser based process can achieve novel device fabrication which have never been possible with conventional photolithography based electronics fabrication process.

Flexible polymer electrolyte fuel cells are made of novel current collectors including highly conductive metal nanowires with extremely high stretchability. Endplates with flexible current collectors maximize clamping forces under bent conditions, which is beneficial in reducing ohmic resistances. These cells show an increased power density as the radius of curvature of the cell decreases. Cells in an asymmetric configuration, where the thicknesses of plates at the anode and cathode sides are different, shows higher power density than symmetric cells because of enhanced normal pressure, and thus, decreased ohmic loss under a given cell curvature. The peak power density of a highly bent stack with a bending radius of 15.6 cm is measured to be actually larger than a flat stack by 95%.

We demonstrate selective laser sintering of silver (Ag) nanoparticle (NP) ink using a digital micromirror device (DMD) for the facile fabrication of 2D electrode pattern without any conventional lithographic means or scanning procedure. An arbitrary 2D pattern at the lateral size of 25 μm × 25 μm with 160 nm height is readily produced on a glass substrate by a short exposure of 532 nm Nd:YAG continuous wave laser. The resultant metal pattern exhibits low electrical resistivity of 10.8 uΩ · cm and also shows a fine edge sharpness by the virtue of low thermal conductivity of Ag NP ink. Furthermore, 10 × 10 star-shaped micropattern arrays are fabricated through a step-and-repeat scheme to ensure the potential of this process for the large-area metal pattern fabrication.

We introduce a rapid and low-temperature laser annealing process of roll-to-roll (R2R) printed metal nanoparticle (NP) ink on a polymer substrate as an alternative to the conventional thermal annealing process using expensive and time consuming lengthy furnace. Due to the confined heating induced by the focused laser, R2R printed Ag NP film is selectively turned into a continuous conductive film without heating the whole substrate. As a result, the damage on the underlying polymer substrate, as well as the processing time required for the annealing, is significantly reduced by the laser annealing compared to the conventional thermal annealing. The resultant laser annealed Ag NP film also exhibits superior electrical and mechanical properties in comparison to the thermal annealed sample. The laser annealed Ag NP film is further applied to the flexible supercapacitor as the current collector in order to confirm the excellence of laser annealed Ag NP film in the fabrication of practical devices.

The detrimental effect of nanoscale hole defects on the in-plane thermal conductivity (k) was first examined for supported CVD graphene. A focused ion beam punctured equally spaced 50-nm diameter holes with different hole spacings (200, 400, and 800 nm) in supported graphene on an 8-nm thin SiO2 substrate. For the relatively low 4.91% porosity, the thermal conductivity showed a significant reduction to 212.6 W/mK from 1045 W/mK in supported graphene with no holes and even more dramatically so from 3500 W/mK in suspended pristine graphene. The thermal conductivity showed an order-of-magnitude faster reduction with increasing porosity compared to the Eucken model, which is based on the diffusive thermal transport reduction due to the void holes on the macroscale. This is believed to be attributed to the enhanced phonon scattering by the nanoscale hole edges and also by the reduced phonon passage length-scale that became comparable to the phonon mean-free-paths. Furthermore, a phenomenological fitting model is presented to comprehensively describe the k dependence on porosity, hole spacing, and the spectral dependence of the phonon mean-free-path in nanoscale holey graphene.

Organic non-volatile memory devices with significantly enhanced retention are explored with C60 thin-film transistors containing silver nanoparticles (Ag-NPs) within gate dielectrics as charge storage nodes. Dipolar self-assembled monolayers covering Ag-NPs effectively prevent stored charges from being lost by providing an additional energy threshold for back-tunneling process. This enables long retention even with ultrathin tunneling dielectric layers, providing a simple means to realize long retention without causing an excessive increase in operation voltage.

For functional nanowire based electronics fabrication, conventionally, combination of complex multiple steps, such as (1) chemical vapor deposition (CVD) growth of nanowire, (2) harvesting of nanowire, (3) manipulation and placement of individual nanowires, and (4) integration of nanowire to circuit are necessary. Each step is very time consuming, expensive, and environmentally unfriendly, and only a very low yield is achieved through the multiple steps. As an alternative to conventional complex multistep approach, original findings are presented on the first demonstration of rapid, one step, digital selective growth of nanowires directly on 3D micro/nanostructures by developing a novel approach; laser induced hydrothermal growth (LIHG) without any complex integration of series of multiple process steps such as using any conventional photolithography process or CVD. The LIHG process can grow nanowires by scanning a focused laser beam as a local heat source in a fully digital manner to grow nanowires on arbitrary patterns and even on the non-flat, 3D micro/nano structures in a safer liquid environment, as opposed to a gas environment. The LIHG process can greatly reduce the processing lead time and simplify the nanowire-based nanofabrication process by removing multiple steps for growth, harvest, manipulation/placement, and integration of the nanowires. LIHG process can grow nanowire directly on 3D micro/nano structures, which will be extremely challenging even for the conventional nanowire integration processes. LIHG does not need a vacuum environment to grow nanowires but can be performed in a solution environment which is safer and cheaper. LIHG can also be used for flexible substrates such as temperature-sensitive polymers due to the low processing temperature. Most of all, the LIHG process is a digital process that does not require conventional vacuum deposition or a photolithography mask.

A novel direct method of micro/nano quantum dot (QD) patterning via one-step imprinting over a large area at low temperatures and low pressures was demonstrated as an alternative to conventional vacuum deposition and photolithography methods. More complex QD patterning could be demonstrated by expanding the QD direct imprinting process for multiple colored QD and patterning on the multiple layers. Additionally, a self-alignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. Our approach may be useful for QD-based optoelectronic device fabrication and patterning on large flexible substrates due to the low-temperature requirements of this process.

Organic photovoltaics (OPVs) have recently received increasing attention as alternatives to inorganic solar cells because of their light weight, compatibility with flexible electronics, and low production cost. In this work, an ideal device structure for ordered bulk heterojunction (OBHJ) organic photovoltaics is proposed. Also suggested is a parameter related to the power conversion efficiency (PCE) of the devices. Such a parameter could serve as a rule of thumb for researchers. For cases in which it is difficult to reduce the pillar size and spacing, the proposed parameter dictates that an increase in the active layer thickness could be one way to increase the PCE of an OBHJ. To generate maximum PCE, (1) the pillar size and spacing must be two to three times less than the exciton diffusion length and (2) the thickness of the active layer has to be greater than half of the photon mean free path, which is the inverse of the absorption coefficient.

Indium tin oxide (ITO) coatings are widely used as transparent electrodes for optoelectronic devices. The most common preparation methods are sputtering, evaporation, and wet chemical deposition. ITO coatings can also be manufactured by solution deposition of ITO nanoparticles followed by furnace thermal annealing with the major motivation to reduce equipment investment. However, conventional furnace annealing is energy intensive, slow, and limited by the peak processing temperature. To overcome these constraints, we suggest using a laser beam for ITO nanoparticle annealing over a large area. It is shown in the present study that a low cost, high power, and high efficiency laser can yield large area functional ITO films in a process that carries substantial promise for potential industrial implementation. Furthermore, laser annealing generates higher electrical conductivity than conventional, thermally annealed nanoparticle films. The optical and electrical properties of the annealed ITO films can also be altered by adjusting laser parameters and environmental gases.

A plastic-compatible low-temperature metal deposition and patterning process is essential for the fabrication of flexible electronics because they are usually built on a heat-sensitive flexible substrate, for example plastic, fabric, paper, or metal foil. There is considerable interest in solution-processible metal nanoparticle ink deposition and patterning by selective laser sintering. It provides flexible electronics fabrication without the use of conventional photolithography or vacuum deposition techniques. We summarize our recent progress on the selective laser sintering of metals and metal oxide nanoparticles on a polymer substrate to realize flexible electronics such as flexible displays and flexible solar cells. Future research directions are also discussed.

We experimentally demonstrate a simple and stable all-polarization maintaining fiber (PMF) nonlinear amplifying loop mirror (NALM)-based burst pulse fiber laser with a pulse number tuning capability, which can readily generate bursts of linearly-polarized femtosecond pulses at 1030 nm. The laser was based on an NALM that was operated to produce burst-mode, dissipative soliton pulses at a wavelength of 1030 nm, and these were then compressed into 400 fs Gaussian pulses using a grating pair-based compressor. The laser was constructed with the figure-eight configuration incorporating ytterbium-doped fiber as gain medium. It was shown that the number of burst pulses was readily tunable through the adjustment of the pump power. Further, the output-pulse characteristics were quantitatively investigated and the laser stability was checked by observing the temporal characteristic variation of the output pulses for one hour.

Nanoparticle solutions are considered promising for realizing low cost printable high performance flexible electronics. In this letter, excimer laser annealing (ELA) was employed to induce melting of solution-deposited ZnO nanoparticles and form electrically conductive porous films. The properties of the films were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, DC conductance, and photoluminescence measurements. Thin-film field-effect transistors have been fabricated by ELA without the use of conventional vacuum or any high temperature thermal annealing processes. The transistors show n-type accumulation mode behavior with mobility greater than 0.1 cm2/V s and current on/off ratios of more than 104. Optimization and control of the laser processing parameters minimized thermal impact on the substrate. This technique can be beneficial in the fabrication of metal oxide based electronics on heat sensitive flexible plastic substrates using low-cost, large-area solution processing combined with direct printing techniques.

Inkjet direct writing of functional materials provides a promising pathway towards realization of ultra-low-cost, large-area printed electronics, albeit at the expense of lowered resolution (∼20–50 μm). We demonstrate that selective laser sintering and ablation of inkjet-printed metal nanoparticles enables low-temperature metal deposition as well as high-resolution patterning. Combined with an air-stable carboxylate-functionalized polythiophene, all-inkjet-printed and laser-processed organic field effect transistors with micron to submicron critical feature resolution were fabricated in a fully maskless sequence, eliminating the need for any lithographic processes. All processing and characterization steps were carried out at plastic-compatible low temperatures and in air under ambient pressure.

Nanomaterials show various interesting unique thermal characteristics such as size dependent melting temperature drop, which can be used to develop plastic compatible low temperature metal patterning process. Focused laser as a local heat source can further reduce the processing temperature or induced localized thermochemical reaction. In this talk, recent research development and trend in nanomaterial based low temperature laser thermal engineering as well as applications will be discussed.