Sooyeoun Oh

Fabrication of a stretchable and patchable array of high performance micro-supercapacitors using a non-aqueous solvent based gel electrolyte

In this study, we report the fabrication of a stretchable and patchable array of micro-supercapacitors (MSCs) using a gel-type electrolyte of poly(methyl methacrylate)–propylene carbonate–lithium perchlorate. As electrodes, a layer-by-layer-assembled thin film of multi-walled carbon nanotubes with a top layer of Mn3O4 nanoparticles was used. The fabricated MSC maintained over 85% of its performance for 2 weeks in ambient air without encapsulation owing to the use of a non-aqueous solvent based gel electrolyte. Dry-transferred MSC arrays on a specially designed stretchable polymer substrate exhibited stable electrochemical performance under various deformations, including bending, twisting, both uniaxial and biaxial stretching up to 50%, and winding around the curved substrate. Furthermore, the encapsulated MSC array with a thin polymer film directly attached to skin maintained its electrochemical performance under repeated body movement, cycles of attachment–detachment, and even in water. This study clearly demonstrates a stretchable and patchable MSC array for practical use as an energy storage device that can be attached to the body for electronic function, even under wet conditions.

Effect of front and back gates on β-Ga2O3 nano-belt field-effect transistors

Field effect transistors (FETs) using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively, were fabricated on exfoliated two-dimensional (2D) β-Ga2O3 nano-belts transferred to a SiO2/Si substrate. The mechanical exfoliation and transfer process produced nano-belts with smooth surface morphologies and a uniform low defect density interface with the SiO2/Si substrate. The depletion mode nanobelt transistors exhibited better channel modulation with both front and back gates operational compared to either front or back-gating alone. The maximum transconductance was ∼4.4 mS mm−1 with front and back-gating and ∼3.7 mS mm−1 with front-gating only and a maximum drain source current density of 60 mA mm−1 was achieved at a drain-source voltage of 10 V. The FETs had on/off ratios of ∼105 at 25 °C with gate-source current densities of ∼2 × 10−3 mA mm−1 at a gate voltage of −30 V. The device characteristics were stable over more than a month for storage in air ambient and the results show the ...

Development of solar-blind photodetectors based on Si-implanted β-Ga(2)O(3).

β-Ga(2)O(3) films grown on Al(2)O(3) by a metalorganic chemical vapor deposition technique were used to fabricate a solar-blind photodetector with a planar photoconductor structure. The crystal structure and quality of the β-Ga(2)O(3) films were analyzed using X-ray diffraction and micro-Raman spectroscopy. Si ions were introduced into the β-Ga(2)O(3) thin films by ion implantation method and activated by an annealing process to form an Ohmic contact between the Ti/Au electrode and the β-Ga(2)O(3) film. The electrical conductivity of the β-Ga(2)O(3) films was greatly improved by the implantation and subsequent activation of the Si ions. The photoresponse properties of the photodetectors were investigated by analyzing the current-voltage characteristics and the time-dependent photoresponse curves. The fabricated solar-blind photodetectors exhibited photoresponse to 254 nm wavelength, and blindness to 365 nm light, with a high spectral selectivity.

Quasi-two-dimensional β-gallium oxide solar-blind photodetectors with ultrahigh responsivity

Solar-blind photodetectors have received a great deal of interest owing to their high selectivity for deep ultra-violet light in the presence of visible light. The development of alternative materials and innovative device designs are necessary for such solar-blind photodetectors, as the currently available commercial devices have issues pertaining to chemical and thermal instability, cost, and material handling due to their rigidity. Here, we fabricated solar-blind photodetectors based on exfoliated quasi-two-dimensional β-Ga2O3 flakes with optimal opto-electrical properties (direct bandgap of ∼4.9 eV), chemical and thermal stability, and then systematically characterized their photoresponsive properties. The fabricated device structures were based on back-gated field-effect transistors, allowing us to control the dark currents. These photodetectors exhibit extraordinary photoresponsive properties including the highest responsivity (1.8 × 105 A W−1) among reported semiconductor thin-film solar-blind photodetectors.

Tuning the thickness of exfoliated quasi-two-dimensional β-Ga2O3 flakes by plasma etching

We demonstrated the thinning of exfoliated quasi-two-dimensional β-Ga2O3 flakes by using a reactive ion etching technique. Mechanical exfoliation of the bulk β-Ga2O3 by using an adhesive tape was followed by plasma etching to tune its thickness. Since β-Ga2O3 is not a van der Waals material, it is challenging to obtain ultra-thin flakes below a thickness of 100 nm. In this study, an etch rate of approximately 16 nm/min was achieved at a power of 200 W with a flow of 50 sccm of SF6, and under these conditions, thinning of β-Ga2O3 flakes from 300 nm down to ∼60 nm was achieved with smooth morphology. We believe that the reaction between SF6 and Ga2O3 results in oxygen and volatile oxygen fluoride compounds, and non-volatile compounds such as GaFX that can be removed by ion bombardment. The opto-electrical properties were also characterized by fabricating solar-blind photodetectors using the plasma-thinned β-Ga2O3 flakes; these detectors showed fast response and decay with excellent responsivity and selectiv...

Effect of 5 MeV proton irradiation damage on performance of β-Ga2O3 photodetectors

Planar thin film β-Ga2O3 photodetectors were irradiated with 5 MeV protons at doses from 1013 to 1015 cm−2, and the resulting effects on photocurrent, responsivity, quantum efficiency, and photo-to-dark current ratio at 254 nm wavelength were measured at both 25 and 150 °C. The photocurrent increased with dose due to the introduction of damage from nonionizing energy loss by the protons. The total calculated vacancy concentration increased from 5 × 1015 to 5 × 1017 cm−3 over the dose range investigated. The dark current increased in proportion with the implant dose, leading to a decrease in the ratio of photocurrent to dark current. The photocurrent induced by 254 nm illumination increased with dose, from ∼0.3 × 10−7 A at 25 °C for a dose of 1013 cm−2 to ∼10−6 A at a dose of 1015 cm−2 at a fixed light intensity of 760 μW/cm2. The photo-to-dark current ratio decreased from ∼60 in the control samples to ∼9 after proton doses of 1015 cm−2, with corresponding external quantum efficiencies of ∼103% in control ...

AuCl3 chemical doping on defective graphene layer

This study investigated the effects of defects on chemical doping of graphene layer. Graphene grown by the chemical vapor deposition method on copper foil was subjected to ultraviolet treatments to introduce defects, including sp3 bonding and vacancies. The chemical doping process was performed using a gold chloride (AuCl3)/nitromethane solution at a concentration of 20 mM. Raman spectroscopy and four-point probe measurement were used to analyze the effects of AuCl3 doping on the electrical and optical properties of defective graphene. AuCl3 doping was effective for lowering sheet resistance even for the highly damaged graphene. Additionally, the 2D-peak of the defective graphene was partially recovered after AuCl3-based chemical doping. The authors believe that the defect engineering in graphene can enhance the electrical properties and the long-term stability of chemical doping.

Elevated temperature performance of Si-implanted solar-blind β-Ga2O3 photodetectors

The temperature dependent photoresponse of planar photodetectors fabricated on β-Ga2O3 films grown on Al2O3 by metalorganic chemical vapor deposition to 254 nm wavelength, and blindness to 365 nm light, are reported over the range of 25–350 °C. Ohmic contacts were formed by Si-implantation and annealing at 900 °C, followed by deposition of Ti/Au metallization. The photocurrent induced by 254 nm illumination increased monotonically with temperature, from ∼2.5 × 10−7 A at 25 °C to ∼2.2 × 10−6 A at 350 °C at a fixed 254 nm light intensity of 760 μW/cm2. The photosensitivity decreases at high temperatures in many photoconductors (thermal quenching), in sharp contrast to the photosensitivity increase with high temperatures in this study. This is ascribed to the presence of states in the gap of Ga2O3, whose presence was proven by exposure to below band-gap energy. In this case, the current still increased due to the presence of defect levels in the band gap and the generation of photocurrent is due to a transit...

Selective deposition of graphene sheets on a flexible substrate by a nonuniform electric field

This study reports on the precise positioning of high-quality graphene sheets on a flexible polyethylene naphthalate substrate using a dielectrophoretic (DEP) force. Positive DEP assembly is carried out using a 100 kHz frequency AC signal with a peak-to-peak voltage of 10 V. The presence and quality of the graphene flakes aligned around the prepatterned electrodes are characterized by scanning electron microscopy, optical microscopy, and micro-Raman spectroscopy. Current–voltage measurements were also used to evaluate the electrical properties of these flexible devices under varying compressive (−) and tensile (+) strain conditions up to ±0.6%, where the currents decreased with increasing strains.

Effects of defect density on ultrathin graphene-based metal diffusion barriers

The authors investigated the effects of defect density on the performance of monolayer graphene as a barrier to metal diffusion. The defects were introduced to the graphene by controlled ultraviolet-ozone irradiation. The barrier performance of pristine graphene was found to be superior to that of defective graphene at temperatures up to 700 °C. Changes in surface morphology were more prevalent in the defective graphene-based films than in the pristine graphene-based film; the thermal stability of graphene films depends on their defect density. Defect density was found to be a determining factor in the barrier performance of graphene.