Myoungho Pyo

SnO2/Graphene Composites with Self‐Assembled Alternating Oxide and Amine Layers for High Li‐Storage and Excellent Stability

An alternating stack (SG/GN) consisting of SnO₂-functionalized graphene oxide (SG) and amine-functionalized GO (GN) is prepared in solution. The thermally reduced SG/GN (r(SG/GN)) with decreased micro-mesopores and completely eliminated macropores, results in a high reversible capacity and excellent capacity retention (872 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹) when compared to a composite without GN.

Highly crystalline Prussian blue/graphene composites for high-rate performance cathodes in Na-ion batteries

We report the synthesis of highly crystalline Prussian Blue (PB) embedded in graphene oxide (GO) layers and its superior electrochemical properties. Highly crystalline PB is prepared from Fe2O3 nanoparticles anchored on GO (Fe2O3/GO). Regulated Fe3+-ion release and slow crystallization with [Fe(CN)6] in the vicinity of Fe2O3/GO produce a GO-interconnected PB (HC-PB/GO) with fewer [Fe(CN)6] vacancies and H2O molecules. When compared with PB synthesized under identical conditions without GO, the HC-PB/GO delivers a noticeably higher reversible capacity and better cyclability as a cathode in Na-ion batteries (SIBs). The improvement in high-rate performance is rather striking. While the energy density of PB at a charge/discharge (C/D) rate of 2.0 A g−1 is negligible, the HC-PB/GO delivers 280 mW h g−1. The increase of electronic conduction and Na+ ion diffusion in HC-PB/GO contribute to a substantial improvement in rate capability.

Key Issues With Printed Flexible Thin Film Transistors and Their Application in Disposable RF Sensors

This paper addresses the key issues that must be overcome to realize fully printed TFT-based flexible devices via commercially viable methods. In particular the threshold voltage (Vth) variation in printed TFTs is a serious impediment to the successful launch of fully printed TFT-based devices in the market. The underlying causes of the Vth variation in fully printed TFTs were analyzed by considering the misalignment of printed drain-source to gate electrodes, the rheology of electronic inks and effects from external sources of charge. By alleviating the influences of external sources of charge using a printed passivation layer, Vth variation is maintained below 30% using a fully printed process. Based on the attainable variation range, the required number of integrated TFTs was estimated to fabricate a fully printed TFT-based radio frequency (RF) sensor device. A practical compromise enables fully printed RF sensors to be realized via the scalability of printing processes that mitigate Vth variation by minimizing the level of TFT integration. Prototypes of fully printed RF sensors with human interactive capability-an RF sensor label, and an RF e-sensor (cyclic voltammetry) tag-are enabled with as few as 26 printed TFTs, demonstrating that low-cost and high throughput manufacturing of printed electronics is feasible.

Y6+x/3Si11−yAlyN20+x−yO1−x+y:Re3+ (Re = Ce3+, Tb3+, Sm3+) phosphors identified by solid-state combinatorial chemistry

Solid-state combinatorial chemistry was implemented to develop new oxynitride phosphors for use in white light-emitting diodes. A two-step, high-throughput screening was implemented in a YN–AlN–SiN4/3 ternary system that was based on random choices. We pinpointed a host structure, Y6+x/3Si11−yAlyN20+x−yO1−x+y (x = 0.939, y = 2), the refined structure of which was a trigonal (P31c) with lattice parameters a = b = 9.84196(6) A, c = 10.68317(6) A, α = β = 90°, and γ = 120°. Y6+x/3Si11−yAlyN20+x−yO1−x+y:Re3+ (Re = Ce3+, Tb3+, Sm3+) phosphors exhibited an acceptable emission in the green to red color range at blue or ultraviolet excitations.

Suppression of Phase Transition in LiTb0.01Mn1.99O4 Cathodes with Fast Li+ Diffusion

The structural characteristics of terbium-doped spinel LiTb(x)Mn(2-x)O(4) related to the electrochemical performance were studied as the cathode in lithium-ion batteries. We chose terbium as the dopant, which is a well-known mixed-valent cation (3+/4+), expecting that it would provide structural stabilization and improve the power density. LiTb(x)Mn(2-x)O(4) revealed that terbium doping significantly affected the lattice structure and lithium-ion diffusion during charge-discharge cycles, resulting in an enhanced capacity retention and rate capability at an extremely small amount of terbium doping (LiTb(0.01)Mn(1.99)O(4)). The absence of two-cubic phase formation in the delithiated state and a tetragonal phase in the overlithiated state, along with a reduced dimensional change of the main cubic phase during charge-discharge, provided LiTb(0.01)Mn(1.99)O(4) with structural stability at both room temperature and 60 °C. The fast lithium-ion diffusion resulted in reduced polarization, which became more conspicuous as the C rates increased. As a result, the power density of LiTb(0.01)Mn(1.99)O(4), which was similar to that of LiMn(2)O(4) at 1C (476.1 W·kg(-1) for LiMn(2)O(4) vs 487.0 W·kg(-1) for LiTb(0.01)Mn(1.99)O(4)), was greatly improved at higher C rates. For example, the power density of LiTb(0.01)Mn(1.99)O(4) was improved to 4000 and 6000 W·kg(-1) at 10 and 20, respectively, compared with 3120 and 3320 W·kg(-1) for pristine LiMn(2)O(4).

Nonradiative energy transfer between two different activator sites in La 4−x Ca x Si 12 O 3+x N 18−x :Eu 2+

Energy transfer, which affects the entire performance of luminescent material, has been generally treated as an averaged parameter by assuming the host material to be a homogeneous continuum. However, energy transfer should be investigated in association with the crystallographic local structure around an activator site. To accomplish this, we established an analytical model and derived comprehensive rate equations, elucidating the relationship between the local structure and energy transfer behavior of La(4-x)Ca(x)Si12O(3+x)N(18-x):Eu2+, which is a recently discovered luminescent material for use in light-emitting diodes. Using the rate-equation model with the assistance of particle swarm optimization, the full-scale decay curves of donors and acceptors located at different crystallographic sites was computed.

Graphene oxide self-assembled with a cationic fullerene for high performance pseudo-capacitors

Control of the microstructures of graphene oxide (GO) is realized by introducing a cationic fullerene (CFU), resulting in a high-performance pseudo-capacitor. The strong electrostatic interaction between anionic GO and the CFU produces a self-assembled composite (GO/CFU), in which the CFU units intervene to form randomly stacked GO layers. The CFU acts as a spacer between GO layers, allowing a significant fraction of the oxygen-functional groups of GO to be redox-active. When tested as a pseudo-capacitor in 1.0 M H2SO4, the optimized GO/CFU composite delivers a capacitance of 357 F g−1 at 0.4 A g−1, in contrast to 160 F g−1 for GO alone, which is one of the greatest values reported for graphene composites with electro-inactive carbonaceous entities. The improvement in the capacitance by CFU incorporation is also evidenced at a high charge/discharge rate (285 and 137 F g−1 at 5 A g−1 for GO/CFU and GO, respectively). As a result, the GO/CFU composite delivers an energy density of 40 W h kg−1 and a power density of 2793 W kg−1 at 5 A g−1, in contrast to 19 W h kg−1 and 2748 W kg−1 for GO alone. During 5000 charge/discharge cycles at 5 A g−1, the capacitance of the GO/CFU composite increases slightly (4% increase in GO/CFU vs. 4% decrease in GO), which validates the effectiveness of a self-assembly strategy for high performance supercapacitor applications.

Classification of crystal structure using a convolutional neural network

A deep-machine-learning technique based on a convolutional neural network (CNN) is introduced. It has been employed for the classification of crystal system, extinction group and space group for given powder X-ray diffraction patterns of inorganic materials.

Systematic Approach To Calculate the Band Gap Energy of a Disordered Compound with a Low Symmetry and Large Cell Size via Density Functional Theory

An ab initio calculation based on density functional theory (DFT) was used to verify the disordered structure of a novel oxynitride phosphor host, La4–xCaxSi12O3+xN18–x, with a large unit cell (74 atoms), low level of symmetry (C2), and large band gap (4.45 eV). Several Wyckoff sites in the La4–xCaxSi12O3+xN18–x structure were randomly shared by La/Ca and O/N ions. This type of structure is referred to as either partially occupied or disordered. The adoption of a supercell that is sufficiently large along with an infinite variety of ensemble configurations to simulate such a random distribution in a partially occupied structure would be an option that could achieve a reliable DFT calculation, but this would increase the calculation expenses significantly. We chose 5184 independent unit cell configurations to be used as input model structures for DFT calculations, which is a reduction from a possible total of 20 736 unit cell configurations for C2 symmetry. Instead of calculating the total energy as well as the band gap energy for all 5184 configurations, we pinpointed configurations that would exhibit a band gap that approximated the actual value by employing an elitist nondominated sorting genetic algorithm (NSGA-II) wherein the 5184 configurations were represented mathematically as genomes and the calculated total and band gap energies were represented as objective (fitness) functions. This preliminary screening based on NSGA-II was completed using a generalized gradient approximation (GGA), and thereafter, we executed a hybrid functional calculation (HSE06) for only the most plausible GGA-relaxed configurations with higher band gap energies and lower total energies. Finally, we averaged the HSE06 band gap energy over these selected configurations using the Boltzmann energy distribution and achieved a realistic band gap energy that more closely approximated the experimental measurement.

Genetic Algorithm-assisted optimization of partially dyed-TiO2 for room-temperature printable photoanodes of dye-sensitized solar cells

The processing parameters for photoanodes that are printable in a room-temperature (RT) continuous process for dye-sensitized solar cells (DSCs) were optimized by the use of Genetic Algorithm (GA). The photoanodes were prepared at RT from mixtures of partially dyed-TiO2 (PDT) with various sizes and dye-loadings and were subsequently compressed for the fabrication of PDT films with different compositions, nanoporous structures, and thicknesses. According to our decision parameter design, there were 220 different cases, from which only 30 chromosomes for each generation were selected for the testing of photovoltaic performance to determine the global optimal point in power conversion efficiency (η). After 7 generations (i.e., only 210 chromosomes tested out of 220), η reached 6.02%, which was 22% higher than the value reported previously without the aid of a GA. The photoanode composed of a mixture of 14, 21, and 40 nm TiO2 with different dye-loadings and compressed to a thickness of 8.0 μm under 94 MPa showed the highest η (short circuit current = 12.03 mA cm−2, open circuit voltage = 685 mV, fill factor = 72.8%). A comparison of the photovoltaic parameters and characteristic charge transport properties revealed that sparsely dyed small TiO2 provided an efficient electron transport route to increase the short circuit current. The role of large TiO2 dyed up to monolayer coverage, on the other hand, was also disclosed to partially increase the short circuit current by light scattering. We hope that the GA-assisted photoanode optimization, presented here, will encourage new directions in research while providing an efficient RT printing process for the continuous fabrication of DSCs.