Seungmin Hyun

Polypyrrole–MnO2-Coated Textile-Based Flexible-Stretchable Supercapacitor with High Electrochemical and Mechanical Reliability

Carbon-nanotube (CNT)-based textile supercapacitors with MnO2 nanoparticles have excellent power and energy densities, but MnO2 nanoparticles can be delaminated during charge-discharge cycles, which results in significant degradation in capacitance. In this study, polypyrrole conductive polymer was coated on top of MnO2 nanoparticles that are deposited on CNT textile supercapacitor to prevent delamination of MnO2 nanoparticles. An increase of 38% in electrochemical energy capacity to 461 F/g was observed, while cyclic reliability also improved, as 93.8% of energy capacity was retained over 10u2009000 cycles. Energy density and power density were measured to be 31.1 Wh/kg and 22.1 kW/kg, respectively. An in situ electrochemical-mechanical study revealed that polypyrrole-MnO2-coated CNT textile supercapacitor can retain 98.5% of its initial energy capacity upon application of 21% tensile strain and showed no observable energy storage capacity change upon application of 13% bending strain. After imposing cyclic bending of 750u2009000 cycles, the capacitance was retained to 96.3%. Therefore, the results from this study confirmed for the first time that the polypyrrole-MnO2-coated CNT textile can reliably operate with high energy and power densities with in situ application of both tensile and bending strains.

Photoresponsive Smart Coloration Electrochromic Supercapacitor

Electrochromic devices have been widely adopted in energy saving applications by taking advantage of the electrode coloration, but it is critical to develop a new electrochromic device that can undergo smart coloration and can have a wide spectrum in transmittance in response to input light intensity while also functioning as a rechargeable energy storage system. In this study, a photoresponsive electrochromic supercapacitor based on cellulose-nanofiber/Ag-nanowire/reduced-graphene-oxide/WO3 -composite electrode that is capable of undergoing smart reversible coloration while simultaneously functioning as a reliable energy-storage device is developed. The fabricated device exhibits a high coloration efficiency of 64.8 cm2 C-1 and electrochemical performance with specific capacitance of 406.0 F g-1 , energy/power densities of 40.6-47.8 Wh kg-1 and 6.8-16.9 kW kg-1 . The electrochromic supercapacitor exhibits excellent cycle reliability, where 75.0% and 94.1% of its coloration efficiency and electrochemical performance is retained, respectively, beyond 10 000 charge-discharge cycles. Cyclic fatigue tests show that the developed device is mechanically durable and suitable for wearable electronics applications. The smart electrochromic supercapacitor system is then integrated with a solar sensor to enable photoresponsive coloration where the transmittance changes in response to varying light intensity.

A hybridized graphene carrier highway for enhanced thermoelectric power generation

The decoupling and enhancement of both Seebeck coefficient and electrical conductivity were achieved by constructing the c-axis preferentially oriented nanoscale Sb(2)Te(3) film on monolayer graphene. The external graphene layer provided a highway for charge carriers, which were stored in the thicker binary telluride film, due to the extremely high mobility.

The effect of plasma pre-cleaning on the Cu-Cu direct bonding for 3D chip stacking.

The effect of bonding temperature and plasma treatment on the interfacial adhesion energy of the Cu-Cu direct bonding layers was investigated under 4-point bending test method. Interfacial adhesion energies with increasing bonding temperature, Good-quality Cu to Cu bonding is achieved at the low bonding temperature of 250°C with surface treatment.

Effect of Post-Annealing Conditions on Interfacial Adhesion Energy of Cu-Cu Bonding for 3-D IC Integration

m-thick copper films deposited on silicon wafers were successfully bonded at 415 C/25 kN for 40 minutes in a thermo-compression bonding method that did not involve a pre-cleaning or pre-annealing process. The original copper bonding interface disappeared and showed a homogeneous microstructure with few voids at the original bonding interface. Quantitative interfacial adhesion energies were greater than 10.4 J/m as measured via a four-point bending test. Post-bonding annealing at a temperature that was less than 300 C had only a slight effect on the bonding energy, whereas an oxygen environment significantly deteriorated the bonding energy over 400 C. This was most likely due to the fast growth of brittle interfacial oxides. Therefore, the annealing environment and temperature conditions greatly affect the interfacial bonding energy and reliability in Cu-Cu bonded wafer stacks. Key word Adhesion, 3-D Integration, 4-point bending test, Cu-Cu bonding, post annealing

Freestanding silicon microparticle and self-healing polymer composite design for effective lithiation stress relaxation

Self-healing features that mimic the biological mechanisms for self-repair have recently been applied to high-capacity but extreme volume expansion electrode materials such as silicon anodes to overcome the short cycle-life caused by electrical contact loss and active material pulverization. In this study, we adopt a freestanding composite design for effective relaxation of lithiation induced stresses and enhancement of electrochemical reliability. Silicon microparticles are homogenously dispersed and embedded within a self-healing polymer matrix that enables free volume expansion and contraction during lithiation and delithiation. The freestanding electrode, which does not require a separate current collector, demonstrated 91.8% capacity retention after 100 cycles at C/10 rate with an average specific capacity and gravimetric capacity, including current collector mass, of ∼2100 mA h g−1 and ∼1050 mA h g−1 respectively, which is a significant improvement compared to the conventional design of simple self-healing polymer coatings on silicon particle embedded current collectors. The fabricated freestanding silicon microparticle and self-healing polymer composite electrode demonstrated stable electrochemical performance after being completely cut, reattached, and cycled and retained at most 95% of its initial capacity. Overall, the proposed freestanding silicon microparticle and self-healing polymer composite design demonstrated excellent gravimetric capacity, cycle life, and self-healing capability without employing expensive and complex nanostructures.

Conversion Reaction of Nanoporous ZnO for Stable Electrochemical Cycling of Binderless Si Microparticle Composite Anode

Binderless, additiveless Si electrode design is developed where a nanoporous ZnO matrix is coated on a Si microparticle electrode to accommodate extreme Si volume expansion and facilitate stable electrochemical cycling. The conversion reaction of nanoporous ZnO forms an ionically and electrically conductive matrix of metallic Zn embedded in Li2O that surrounds the Si microparticles. Upon lithiation, the porous Li2O/Zn matrix expands with Si, preventing extensive pulverization, while Zn serves as active material to form Li xZn to further enhance capacity. Electrodes with a Si mass loading of 1.5 mg/cm2 were fabricated, and a high initial capacity of ∼3900 mAh/g was achieved with an excellent reversible capacity of ∼1500 mAh/g (areal capacity ∼1.7 mAh/cm2) beyond 200 cycles. A high first-cycle Coulombic efficiency was obtained owing to the conversion reaction of nanoporous ZnO, which is a notable feature in comparison to conventional Si anodes. Ex situ analyses confirmed that the nanoporous ZnO coating maintained the coalescence of SiMPs throughout extended cycling. Therefore, the Li2O/Zn matrix derived from conversion-reacted nanoporous ZnO acted as an effective buffer to lithiation-induced stresses from volume expansion and served as a binder-like matrix that contributed to the overall electrode capacity and stability.

Characterization of Adhesion Properties of a UV-Curable Nanoimprint Resin with Different Amounts of Release Agents

In ultraviolet (UV) nanoimprint lithography, the adhesion between the UV-curable resin and the stamp must be carefully controlled to avoid deformation and breakage of the imprinted patterns during the stamp separation process. In this work, UV-curable resins with different amounts of release agent were prepared, and the adhesion properties of the resins were characterized quantitatively using a four-point bending test. The interfacial fracture energy between the UV-curable resin with a release agent and a glass wafer was measured based on the concept of interfacial fracture mechanics. The results were used to investigate the effect of the release agent on the reduction of the interfacial adhesion. The interfacial fracture energy between the resin containing the release agent and the glass wafer decreased with increasing concentration of the release agent. After the test, the fractured surfaces were analyzed using scanning electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy, and the effect of the release agent on the adhesion was examined.

In-situ synchrotron X-ray diffraction measurement of epitaxial FeRh thin films

The magnetic properties and structure of FeRh thin film epitaxially grown onto MgO(001) substrate were studied by MPMS(Magnetic Properties Measure System) and in-situ temperature synchrotron XRD(X-ray Diffraction). The transition temperature of FeRh thin films was around 380K. Both M-T curve and d-spacing changes correspond to each other very closely. Abrupt changes in the lattice constants can be observed from the in-situ analysis. Also, there is the likelihood of existence of a new phase.

Characterization of Interfacial Adhesion of Cu-Cu Bonding Fabricated by Thermo-Compression Bonding Process

Four-point bending tests were performed to investigate the interfacial adhesion of Cu-Cu bonding fabricated by thermo-compression process for three dimensional packaging. A pair of Cu-coated Si wafers was bonded under a pressure of 15 kN at 350 °C for 1 h, followed by post annealing at 350 °C for 1 h. The bonded wafers were diced into 30 mm × 3 mm pieces for the test. Each specimen had a 400-�m-deep notch along the center. An optical inspection module was installed in the testing apparatus to observe crack initiation at the notch and crack propagation over the weak interface. The tests were performed under a fixed loading speed, and the corresponding load was measured. The measured interfacial adhesion energy of the Cu-to-Cu bonding was 9.75 J/m 2 , and the delaminated interfaces were analyzed after the test. The surface analysis shows that the delamination occurred in the interface between SiO2 and Ti.