Jae-Hyuck Yoo

Low‐Cost Facile Fabrication of Flexible Transparent Copper Electrodes by Nanosecond Laser Ablation

Low-cost Cu flexible transparent conducting electrodes (FTCEs) are fabricated by facile nanosecond laser ablation. The fabricated Cu FTCEs show excellent opto-electrical properties (transmittance: 83%, sheet resistance: 17.48 Ω sq(-1)) with outstanding mechanical durability. Successful demonstration of a touch-screen panel confirms the potential applicability of Cu FTCEs to the flexible optoelectronic devices.

Graphene folds by femtosecond laser ablation

We report the production of graphene folds induced by femtosecond laser ablation. A single laser pulse irradiation on graphene produced an ablated spot featuring in its proximity circumferentially periodic graphene folds. The graphene fold structure was constructed through folding of a single layer graphene segment. We investigated the laser fluence effect on the graphene fold structure. We also performed ablation on suspended graphene and verified that interaction with the underlying substrate is required for the formation of graphene folds. We expect this one-step folding method may provide a controlled process to explore properties of graphene folds.

Laser-Induced Direct Graphene Patterning and Simultaneous Transferring Method for Graphene Sensor Platform

General methods utilized in the fabrication of graphene devices involve graphene transferring and subsequent patterning of graphene via multiple wet-chemical processes. In the present study, a laser-induced pattern transfer (LIPT) method is proposed for the transferring and patterning of graphene in a single processing step. Via the direct graphene patterning and simultaneous transferring, the LIPT method greatly reduces the complexity of graphene fabrication while augmenting flexibility in graphene device design. Femtosecond laser ablation under ambient conditions is employed to transfer graphene/PMMA microscale patterns to arbitrary substrates, including a flexible film. Suspended cantilever structures are also demonstrated over a prefabricated trench structure via the single-step method. The feasibility of this method for the fabrication of functional graphene devices is confirmed by measuring the electrical response of a graphene/PMMA device under laser illumination.

In Situ TEM Near-Field Optical Probing of Nanoscale Silicon Crystallization

Laser-based processing enables a wide variety of device configurations comprising thin films and nanostructures on sensitive, flexible substrates that are not possible with more traditional thermal annealing schemes. In near-field optical probing, only small regions of a sample are illuminated by the laser beam at any given time. Here we report a new technique that couples the optical near-field of the laser illumination into a transmission electron microscope (TEM) for real-time observations of the laser-materials interactions. We apply this technique to observe the transformation of an amorphous confined Si volume to a single crystal of Si using laser melting. By confinement of the material volume to nanometric dimensions, the entire amorphous precursor is within the laser spot size and transformed into a single crystal. This observation provides a path for laser processing of single-crystal seeds from amorphous precursors, a potentially transformative technique for the fabrication of solar cells and other nanoelectronic devices.

On demand shape-selective integration of individual vertical germanium nanowires on a Si(111) substrate via laser-localized heating.

Semiconductor nanowire (NW) synthesis methods by blanket furnace heating produce structures of uniform size and shape. This study overcomes this constraint by applying laser-localized synthesis on catalytic nanodots defined by electron beam lithography in order to accomplish site- and shape-selective direct integration of vertically oriented germanium nanowires (GeNWs) on a single Si(111) substrate. Since the laser-induced local temperature field drives the growth process, each NW could be synthesized with distinctly different geometric features. The NW shape was dialed on demand, ranging from cylindrical to hexagonal/irregular hexagonal pyramid. Finite difference time domain analysis supported the tunability of the light absorption and scattering spectra via controlling the GeNW shape.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

Optical damage performance of conductive widegap semiconductors: spatial, temporal, and lifetime modeling

The optical damage performance of electrically conductive gallium nitride (GaN) and indium tin oxide (ITO) films is addressed using large area, high power laser beam exposures at 1064 nm sub-bandgap wavelength. Analysis of the laser damage process assumes that onset of damage (threshold) is determined by the absorption and heating of a nanoscale region of a characteristic size reaching a critical temperature. This model is used to rationalize semi-quantitatively the pulse width scaling of the damage threshold from picosecond to nanosecond timescales, along with the pulse width dependence of the damage threshold probability derived by fitting large beam damage density data. Multi-shot exposures were used to address lifetime performance degradation described by an empirical expression based on the single exposure damage model. A damage threshold degradation of at least 50% was observed for both materials. Overall, the GaN films tested had 5-10 × higher optical damage thresholds than the ITO films tested for comparable transmission and electrical conductivity. The route to optically robust, large aperture transparent electrodes and power optoelectronics may thus involve use of next generation widegap semiconductors such as GaN.

Single Pass Laser Process for Super-Hydrophobic Flexible Surfaces with Micro/Nano Hierarchical Structures

Wetting has been studied in various fields: chemical industry, automobile manufacturing, food companies, and even life sciences. In these studies, super-hydrophobic surfaces have been achieved through complex steps and processes. To realize super-hydrophobicity, however, we demonstrated a simple and single pass laser process for the fabrication of micro/nano hierarchical structures on the flexible polytetrafluoroethylene (PTFE, Teflon) surface. The fabricated hierarchical structures helped increase the hydrophobicity by augmenting the surface roughness and promoting air-trapping. In addition, we employed a low-cost and high-throughput replication process producing numerous polydimethylsiloxane (PDMS) replicas from the laser-processed PTFE film. Thanks to the anti-adhesive characteristics of PTFE and the elasticity of PDMS, the structure perfectly transferred to the replica without any mechanical failure. Moreover, our designed mesh patterns offered the possibility of large area applications through varying the process parameters (pitch, beam spot size, laser fluence, and scan speed). Even though mesh patterns had relatively large pitch (190 μm), we were able to achieve high contact angle (>150°). Through pneumatically deformed structure, we clearly showed that the shape of the droplets on our laser-processed super-hydrophobic surface was spherical. Based on these outcomes, we can expect our single laser pulse exposure process can overcome many drawbacks and offer opportunities for advancing applications of the wetting phenomena.

Localized planarization of optical damage using laser-based chemical vapor deposition

We present a method to repair damaged optics using laser-based chemical vapor deposition (L-CVD). A CO2 laser is used to heat damaged silica regions and polymerize a gas precursor to form SiO2. Measured deposition rates and morphologies agree well with finite element modeling of a two-phase reaction. Along with optimizing deposition rates and morphology, we also show that the deposited silica is structurally identical to high-grade silica substrate and possesses high UV laser damage thresholds. Successful application of such a method could reduce processing costs, extend optic lifetime, and lead to more damage resistant laser optics used in high power applications.

Rapid feedback of chemical vapor deposition growth mechanisms by operando X-ray diffraction

An operando x-ray diffraction system is presented for elucidating optimal laser assisted chemical vapor deposition growth conditions. The technique is utilized to investigate deposition dynamics of boron-carbon materials using trimethyl borate precursor. Trimethyl borate exhibits vastly reduced toxicological and flammability hazards compared to existing precursors, but has previously not been applied to boron carbide growth. Crystalline boron-rich carbide material is produced in a narrow growth regime on addition of hydrogen during the growth phase at high temperature. The use of the operando x-ray diffraction system allows for the exploration of highly nonequilibrium conditions and rapid process control, which are not possible using ex situ diagnostics.