Search

In our study, we report Near-field electrospinning as a novel method of patterning and tuning suspended graphene channel. As a proof of concept, a resistive sensor has been fabricated for the detection of air carrying NH3 gas vapors. The suspended graphene channel, performing as the primary sensing element, changes its resistivity when NH3 molecules are absorbed on the surface of the graphene, effectively functioning as scattering centers. By utilizing the Near-field electrospinning for the deposition of the functional fibers, not only the width of the bridge but also the chemical doping effect on graphene can be readily tuned through the adjustment of process parameters and implementation of a wide range of polymers. About 78 % boost in sensitivity is confirmed when the width of suspended graphene channel is decreased from 15 μm to 5 μm. The presented process is expected to provide a simple, cost-effective and one-step precise manufacturing method by easily creating suspended graphene channels for a wide range of sensing applications not just limited to gas detection. more...

In electrospinning, a liquid polymer meniscus deforms into a conical structure called the Taylor cone when an excessive electric field is applied to the air-to-polymer interface. This Taylor cone i... more...

Lithography plays a key role in advancing manufacturing as well as the semiconductor industry. However, the currently available state-of-the-art lithography methods still require access to expensive tools and facilities. Herein, we suggest a novel lithography method based on electromagnetic phase modulation of ultraviolet using a highly birefringent electrospun fiber to overcome such limitations. The optical birefringent effect, by which the phase of incident ultraviolet electromagnetic fields is retarded when passing through optically anisotropic media, is combined with semicrystalline poly(ethylene oxide) (PEO)-poly(ethylene glycol) (PEG) polymeric microfibers patterned in a programmable form using near-field electrospinning. By positioning the mask between two linear polarizers that are perpendicular to each other, only the UV waves that are passing through the fibers can be selectively utilized to exhibit lithographic property. Therefore, the UV intensity on the photoresist (PR) surface follows the shape of the fiber pattern, enabling precisely controlled patterning of the photoresist. Zero- to two-dimensional key features of lithography are achieved, including straight, curved, array, and isolated patterns. Facile optical alignments without using dedicated alignment marks are successfully demonstrated, as well as various applications including micro- to macroscale serpentine, tree, and antenna circuit patterns on a flexible substrate. The presented approach, packed in a table-top scale, is expected to provide a practical and affordable lithography solution by leveraging the direct-writing capability and tunable optical functionality of polymers, scalability, and the simple optical alignment method. more...

Paraffin is a phase change material that can store thermal energy, and it has been used in a variety of applications. However, the low thermal conductivity of paraffin hinders the further expansion of its use in many applications. Thus, it is of great interest to identify and develop composites that can improve the thermal conductivity of paraffin to maximize its heat capacity. Here, we report a facile fabrication method for producing spherical paraffin/graphene composites with thermal conductivities enhanced by 57%. We used fused deposition 3D printing to build a prototype mold in which spherical paraffin/graphene composites were successfully fabricated. more...

Ultraviolet (UV) is widely used in daily life as well as in industrial manufacturing. In this study, a single-step postprocess to improve the sensitivity of a graphene-based UV sensor is studied. We leverage the advantage of electric-field-assisted on-demand printing, which is simply applicable for mounting functional polymers onto various structures. Here, the facile printing process creates optical plano-convex geometry by accelerating and colliding a highly viscous droplet on a micropatterned graphene channel. The printed transparent lens refracts UV rays. The concentrated UV photon energy from a wide field of view enhances the photodesorption of electron-hole pairs between the lens and the graphene sensor channel, which is coupled with a large change in resistance. As a result, the one-step post-treatment has about a 4× higher sensitivity compared to bare sensors without the lenses. We verify the applicability of printing and the boosting mechanism by variation of lens dimensions, a series of UV exposure tests, and optical simulation. Moreover, the method contributes to UV sensing in acute angle or low irradiation. In addition, the catalytic lens provides about a 9× higher recovery rate, where water molecules inside the PEI lens deliver fast reassembly of the electron-hole pairs. The presented method with an ultimately simple fabrication step is expected to be applied to academic research and prototyping, including optoelectronic sensors, energy devices, and advanced manufacturing processes. more...

Complicated microscale patterns of a continuous nanofiber were precisely fabricated via direct-write near-field electrospinning (NFES). Although NFES could eliminate the bending instability effect by shortening tip-to-collector distance (TTCD) less than the critical length, reported patterns up to the date have been limited to simply aligned straight-lines because of the ignorance of the importance of jet impact speed. To realize the jet impact speed for precise microscale patterning, experimental study of the effect of the process parameters such as applied voltage, TTCD, and an electric field was performed. It has been found that lowering the applied voltage and the TTCD simultaneously could reduce the jet impact speed while keeping the process stable and continuous, resulting in considerable improvement of the accuracy of the deposition. Having the ability to control jet impact speed in NFES will provide the essential foundation to broaden applications of NFES to micro and nanoscale engineering such as an optical waveguide, customized cell culture matrix fabrication to name a few. more...

Silver dimetal chalcogenide (Ag-V-VI2 ) ternary quantum dots (QDs) are emerging lead-free materials for optoelectronic devices due to their NIR band gaps, large absorption coefficients, and superior electronic properties. However, thin film-based devices of the ternary QDs still lag behind due to the lack of understanding of the surface chemistry, compared to that of lead chalcogenide QDs even with the same crystal structure. Herein the surface ligand interactions of AgSbS2 QDs, synthesized with 1-dodecanethiol used as a stabilizer, are studied. For nonpolar (1 0 0) surfaces, it is suggested that the thiolate ligands are associated with the crystal lattices, thus preventing surface oxidation by protecting sulfur after air-exposure, as confirmed through optical and surface chemical analysis. Otherwise, silver rich (1 1 1) surfaces are passivated by thiolate ligands, allowing ligand exchange processes for the conductive films. This in-depth investigation of the surface chemistry of ternary QDs will prompt the performance enhancement of their optoelectronic devices. more...

Electrospinning, one of the most effective ways of producing nanofibers, has been applied in as many fields throughout its long history. Starting with far-field electrospinning (FFES) and advancing to the near-field, the application area has continued to expand, but lack of understanding of the exact jet speed and fiber deposition rate is a major obstacle to entry into precision micro- to nano-scale manufacturing. In this paper, we, for the first time, analyze and predict the jet velocity and deposition rate in near-field electrospinning (NFES) through novel image analysis process. Especially, analog image is converted into a digital image, and then, the area occupied by the deposited fiber is converted into a velocity, through which the accuracy of the proposed method is proved to be comparable to direct jet speed measurement. Finally, we verified the proposed method can be applied to various process conditions without performing delicate experiments. This research not only will broaden the understanding of jet speed and fiber deposition rate in NFES but also will be applicable to various areas including patterning of the sensor, a uniform arrangement of nanofibers, energy harvester, reinforcing of composite, and reproducing of artificial tissue. more...

Silver dimetal chalcogenide (Ag-V-VI