Hyungduk Ko

Highly stable and imperceptible electronics utilizing photoactivated heterogeneous sol-gel metal-oxide dielectrics and semiconductors.

Incorporation of Zr into an AlOx matrix generates an intrinsically activated ZAO surface enabling the formation of a stable semiconducting IGZO film and good interfacial properties. Photochemically annealed metal-oxide devices and circuits with the optimized sol-gel ZAO dielectric and IGZO semiconductor layers demonstrate the high performance and electrically/mechanically stable operation of flexible electronics fabricated via a low-temperature solution process.

Additive-free hollow-structured Co3O4 nanoparticle Li-ion battery: the origins of irreversible capacity loss.

Origins of the irreversible capacity loss were addressed through probing changes in the electronic and structural properties of hollow-structured Co3O4 nanoparticles (NPs) during lithiation and delithiation using electrochemical Co3O4 transistor devices that function as a Co3O4 Li-ion battery. Additive-free Co3O4 NPs were assembled into a Li-ion battery, allowing us to isolate and explore the effects of the Co and Li2O formation/decomposition conversion reactions on the electrical and structural degradation within Co3O4 NP films. NP films ranging between a single monolayer and multilayered film hundreds of nanometers thick prepared with blade-coating and electrophoretic deposition methods, respectively, were embedded in the transistor devices for in situ conduction measurements as a function of battery cycles. During battery operation, the electronic and structural properties of Co3O4 NP films in the bulk, Co3O4/electrolyte, and Co3O4/current collector interfaces were spatially mapped to address the origin of the initial irreversible capacity loss from the first lithiation process. Further, change in carrier injection/extraction between the current collector and the Co3O4 NPs was explored using a modified electrochemical transistor device with multiple voltage probes along the electrical channel.

Simultaneous enhancement of upconversion and downshifting luminescence via plasmonic structure.

We describe a metal nanodisk-insulator-metal (MIM) structure that enhances lanthanide-based upconversion (UC) and downshifting (DS) simultaneously. The structure was fabricated using a nanotransfer printing method that facilitates large-area applications of nanostructures for optoelectronic devices. The proposed MIM structure is a promising way to harness the entire solar spectrum by converting both ultraviolet and near-infrared to visible light concurrently through resonant-mode excitation. The overall photoluminescence enhancements of the UC and DS were 174- and 29-fold, respectively.

A Plasmonic Platform with Disordered Array of Metal Nanoparticles for Three-Order Enhanced Upconversion Luminescence and Highly Sensitive Near-Infrared Photodetector.

Three-order enhanced upconversion luminescence from upconversion nanoparticles is suggested by way of a promising platform utilizing a disordered array of plasmonic metal nanoparticles. Its application toward highly sensitive NIR photodetectors is discussed.

Structural origin of the band gap anomaly of quaternary alloy Cd x Zn 1-x S y Se 1-y nanowires, nanobelts, and nanosheets in the visible spectrum

Single-crystalline alloy II-VI semiconductor nanostructures have been used as functional materials to propel photonic and optoelectronic device performance in a broad range of the visible spectrum. Their functionality depends on the stable modulation of the direct band gap (Eg), which can be finely tuned by controlling the properties of alloy composition, crystallinity, and morphology. We report on the structural correlation of the optical band gap anomaly of quaternary alloy CdxZn1-xSySe1-y single-crystalline nanostructures that exhibit different morphologies, such as nanowires (NWs), nanobelts (NBs), and nanosheets (NSs), and cover a wide range of the visible spectrum (Eg = 1.96-2.88 eV). Using pulsed laser deposition, the nanostructures evolve from NWs via NBs to NSs with decreasing growth temperature. The effects of the growth temperature are also reflected in the systematic variation of the composition. The alloy nanostructures firmly maintain single crystallinity of the hexagonal wurtzite and the nanoscale morphology, with no distortion of lattice parameters, satisfying the virtual crystal model. For the optical properties, however, we observed distinct structure-dependent band gap anomalies: the disappearance of bowing for NWs and maximum and slightly reduced bowing for NBs and NSs, respectively. We tried to uncover the underlying mechanism that bridges the structural properties and the optical anomaly using an empirical pseudopotential model calculation of electronic band structures. From the calculations, we found that the optical bowings in NBs and NSs were due to residual strain, by which they are also distinguishable from each other: large for NBs and small for NSs. To explain the origin of the residual strain, we suggest a semiempirical model that considers intrinsic atomic disorder, resulting from the bond length mismatch, combined with the strain relaxation factor as a function of the width-to-thickness ratio of the NBs or NSs. The model agreed well with the observed optical bowing of the alloy nanostructures in which a mechanism for the maximum bowing for NBs is explained. The present systematic study on the structural-optical properties correlation opens a new perspective to understand the morphology- and composition-dependent unique optical properties of II-VI alloy nanostructures as well as a comprehensive strategy to design a facile band gap modulation method of preparing photoconverting and photodetecting materials.

White light emission from polystyrene under pulsed ultra violet laser irradiation

This paper reports for the first time the luminescent property of polystyrene (PS), produced by pulsed ultra violet laser irradiation. We have discovered that, in air, ultra-violet (UV) irradiated PS nanospheres emit bright white light with the dominant peak at 510 nm, while in vacuum they emit in the near-blue region. From the comparison of PS nanospheres irradiated in vacuum and air, we suggest that the white luminescence is due to the formation of carbonyl groups on the surface of PS by photochemical oxidation. Our results potentially offer a new route and strategy for white light sources.

Copper thin films on PET prepared at ambient temperature by ECR-CVD

Metallized polymers were prepared at ambient temperature by an electron-cyclotron-resonance (ECR) chemical vapor deposition system equipped with (-) DC bias from the Cu (hfac)/sub 2/-Ar-H/sub 2/ system. X-ray difraction (XRD) results showed that the Cu (111) peaks were clearly observed when H/sub 2/ was introduced to the plasma. The surface morphology showed that larger Cu grains were formed in the metal-organic composite films with the introduction of H/sub 2/ to the plasma. AES depth profiles showed that H/sub 2/ gas introduction to the plasma led to the formation of copper-rich films with a homogeneous composition. Also, the sheet resistance was strongly dependent on the H/sub 2/ content of the plasma. This means that hydrogen may lead to both the formation of stable volatile organic compounds and the reduction of copper, which influences both the crystallographic structure and the composition of films. As a result, crystalline copper films with a sheet resistance of 2-3/spl Omega//sup 2/ can be prepared on poly ethylene terephthalate with the addition of H/sub 2/ to the plasma.

A multifunctional fullerene interlayer in colloidal quantum dot-based hybrid solar cells

Chemically modifying the surfaces of colloidal quantum dots (QDs) offers an effective approach to improve their photovoltaic performances. Ligand exchange processes, however, tend to cause nanoscale cracks throughout the QD-based films, which increases the leakage current and magnitude of recombination losses. Here, we have developed a multifunctional [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) cathode interlayer for use in polymer–QD hybrid bulk heterojunction (BHJ) solar cells. The PC60BM layer deposited onto the hybrid BHJ film via solution uniformly covered the nanocracks produced by QD surface ligand exchange with thiol and facilitated electron transport to the cathode. The PC60BM layer also improved photon harvesting at short wavelengths and formed an efficient vertical donor–acceptor/acceptor′ (D–A/A′) junction in the interfacial areas between the hybrid blend and PC60BM. The efficient vertical junction increased the probability of ultrafast exciton dissociation, provided pathways for effective transport and extraction of photo-generated electrons, and blocked holes to reduce recombination losses. These combined advantages significantly enhanced the overall efficiency of the hybrid solar cells over the current state-of-the-art efficiency.

The formation of a functional pentacene/CH3NH3PbI3−xClx perovskite interface: optical gating and field-induced charge retention

We fabricated a functional pentacene/CH3NH3PbI3-xClx perovskite interface where optical gating and field assisted charge retention occur. Using a pentacene/perovskite field effect transistor (FET) test platform, we investigated the interfacial charge transfer associated with optical gating through threshold voltage measurements under illumination. Importantly, bistable electrical conduction in pentacene/perovskite FET devices was achieved as a result of field-induced charge retention at the interface and the origin is discussed to be associated with interfacial charging at the pentacene/perovskite interface. Interfacial contact modification associated with ion migration and other possible effects in the perovskite layer plays a crucial role in forming a functional interface involving organic semiconducting materials.

Plasmonic nanobump-assembled platform for absorption enhancement of upconversion materials

We numerically investigate a plasmonic nanobump and upconversion (UC) layer incorporated metal–insulator–metal (MIM) platform as a light absorber. The hemispherical nanobump array situated over a substrate can serve as an optical nanoantenna in a broadband wavelength range. By precisely engineering the design and optical parameters of the insulating spacer layer sandwiched by the top nanobumps and back reflecting metal film, we can manipulate the light absorption inside the upconversion layer. The optical near-field distribution of the nanobump-assembled plasmonic platform is studied using the finite-difference time-domain (FDTD) method to probe the origin of enhanced absorption within the thin UC layer. A suggested mathematical model considering plasmonic and quenching effects of the MIM configuration to analyze the near-field maximum as a function of an insulator thickness is in good agreement with the FDTD result. The 30-fold enhanced light absorption within the UC layer is observed for the MIM plasmonic platform compared to the reference sample. Well-established optical field confinement at the nanoscale gap and excitation of surface plasmons near the nanobumps can be attributed to increased light absorption inside the plasmonic MIM platform. The plasmonic nanobump array platform can be an alternative strategy to apply a highly efficient light absorber to an UC device.We numerically investigate a plasmonic nanobump and upconversion (UC) layer incorporated metal–insulator–metal (MIM) platform as a light absorber. The hemispherical nanobump array situated over a substrate can serve as an optical nanoantenna in a broadband wavelength range. By precisely engineering the design and optical parameters of the insulating spacer layer sandwiched by the top nanobumps and back reflecting metal film, we can manipulate the light absorption inside the upconversion layer. The optical near-field distribution of the nanobump-assembled plasmonic platform is studied using the finite-difference time-domain (FDTD) method to probe the origin of enhanced absorption within the thin UC layer. A suggested mathematical model considering plasmonic and quenching effects of the MIM configuration to analyze the near-field maximum as a function of an insulator thickness is in good agreement with the FDTD result. The 30-fold enhanced light absorption within the UC layer is observed for the MIM plasmon...