Won Il Park

Hanyang University · Division of Materials Science and Engineering (MSE)

This Perspective presents a review of current research activities on one-dimensional (1D) semiconductor nanostructures grown on two-dimensional (2D) nanomaterials for flexible electronic and optoelectronic device applications. For hybrid-dimensional nanostructures, 1D nanostructures exhibit excellent material characteristics, including high carrier...

We investigated the effects of different thermal treatment conditions on the surface and internal residual strains of bulk GaN grown by hydride vapor phase epitaxy (HVPE). Thermal annealing was performed at 700–1000 °C for 1–5 h in nitrogen atmosphere. The GaN was characterized by atomic force microscopy, energy dispersive spectrometry, X-ray photo...

In this study, we propose a rational route to create wrinkling patterns with individually controllable location and direction in thin polymer films. Optical and atomic force microscopy analysis confirmed the formation of straight wrinkles with a typical width of 1.51 to 1.55 μm and a height of 60 to 65 nm. Confocal fluorescence microscopy revealed...

We propose a novel wallpapering-inspired strategy to create wrinkles in a desired location and direction of single-layer graphene (Gr). The key to controlling the wrinkles is to regulate the permeation and release of solution through a regular array of holes between the substrate surface and Gr-based thin membrane (Gr/Cu in the first stage and Gr i...

There have been many efforts to grow high-quality, single-crystalline graphene on a cheap substrate, such as a metal foil. However, control of nucleation and growth of individual flakes, as well as their seamless stitching, remains elusive. Here we demonstrate a new strategy to fast and facile synthesis of large scale single-crystal graphene on cop...

Despite the important role of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) binders in graphite electrodes for Li-ion batteries, the direct analysis of these binders remains challenging, particularly at very low concentrations as in practical graphite anodes. In this paper, we report the systematic investigation of the physiochem...

Vertically aligned ZnO nanorods have been widely investigated for use in piezoelectric generators, photovoltaic devices, nanotemplates, photoelectrochemical applications, etc. In addition to vertical alignment, intentional arrangement of ZnO nanorods according to a careful design can enhance the device performance. In this work, we used metal nanot...

The electrical double layer (EDL), consisting of two parallel layers of opposite charges, is foundational to many interfacial phenomena and unique in atomically-thin materials. An important but unanswered question is how the ‘transparency’ of atomically thin materials to their substrates influences the formation of the EDL. Here, we report that the...

InGaN/GaN micro-crystals (μ-crystals) are self-contained and individually-addressable light emitting crystals that have unique potential in the development of ultra-small and ultra-high resolution pixels for next-generation displays. In this study, we explore the electrically-driven light emission behavior of vertically standing InGaN/GaN micro-cry...

Ultralow power chemical sensing is essential toward realizing the Internet of Things. However, electrically driven sensors must consume power to generate an electrical readout. Here, a different class of self‐powered chemical sensing platform based on unconventional photovoltaic heterojunctions consisting of a top graphene (Gr) layer in contact wit...

Diverse strategies have been used to address the irreversible structural collapse issues of Ge-based anodes associated with lithium insertion/extraction. In this research, the hollow C-Ge and C-Si core-shell ([email protected] and [email protected]) anode materials are successfully synthesized through a facile chemical vapor deposition (CVD) method...

With increasing demand for high-capacity and rapidly rechargeable anodes, problems associated with unstable evolution of a solid-electrolyte interphase on the active anode surface become more detrimental. Here, we report the near fatigue-free, ultrafast, and high-power operations of lithium-ion battery anodes employing silicide nanowires anchored s...

Graphene has attracted great attention as an alternative to conventional metallic or transparent conducting electrodes. Despite its similarities with conventional electrodes, recent studies have shown that a single-atom layer of graphene possesses unique characteristics, such as a tunable work function and transparencies for electric potential, rea...

The increasing demands for three-dimensional (3D) electronic and optoelectronic devices have triggered interest in epitaxial growth of 3D semiconductor materials. However, most of the epitaxially-grown nano- and micro-structures available so far are limited to certain forms of crystal arrays, and the level of control is still very low. In this revi...

Herein, we report the ultraviolet (UV) photoresponse characteristics of vertically-grown ZnO rod arrays and graphene hybrid detectors. Graphene serves as a UV transparent electrode for efficient light absorption and a Schottky electrode for promoting the separation of charge carriers photogenerated in ZnO. In particular, when the ZnO rods were pass...

Large area growth of single- and few-layer graphene has been established, yet its prerequisite of metallic catalysts has impeded direct graphene growth on three-dimensional (3D) dielectric substrates. Here, we report a new strategy for direct chemical-vapor-deposition (CVD) growth of graphene films on 3D patterned sapphire substrates by placing the...

Well-ordered nanostructure arrays with controlled densities can potentially improve material properties; however, their fabrication typically involves the use of complicated processing techniques. In this work, we demonstrate a uniaxial alignment procedure for fabricating poly(vinylidene fluoride) (PVDF) electrospun nanofibers (NFs) by introducing...

We report the synthesis, fabrication, and characterization of a nanotube-on-graphene (NT-on-Gr) field-effect sensor array for electrical detection of the biological activity of living cells. In order to form vertical nanotubes on a graphene surface, Ge/Si core-shell nanowires were vertically grown on graphene, followed by cap opening and Ge core-et...

Porous transition metal oxides are widely used in energy storage applications owing to their unique physicochemical properties. However, the development of simple synthetic routes using cheap precursors without sacrificial templates still remains challenging. Herein we report a synthesis of hierarchical porous TiNb2O7 nanotubes via dual nozzle elec...

We demonstrate the epitaxial growth of three-dimensional (3D) GaN single crystal arrays through metal-organic vapor phase epitaxy (MOVPE) on lattice-matched ZnO templates that were achieved via hydrothermal growth in which we controlled the position, size and morphology of the layer. To prevent collapse of the crystals during MOVPE growth, graphene...

We report the role of defects in enzymatic graphene field-effect transistor sensors by introducing engineered defects in graphene channels. Compared with conventional graphene sensors (Gr sensors), sensors based on graphene mesh (GM sensors) with an array of circular holes initially exhibited a higher irreversible response to glucose involving stro...

Gate-tunable Schottky junction solar cells (SJSCs) based on graphene and graphene mesh electrodes on n-type Si are fabricated and the effect of the external gate voltage (Vg) on the photovoltaic characteristics is investigated. The power conversion efficiencies (PCEs) of both devices continuously increase with increasing absolute values of Vg. Impo...

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphe...

Graphene has been intensively studied for applications to high-performance sensors, but the sensing characteristics of graphene devices have varied from case to case, and the sensing mechanism has not been satisfactorily determined thus far. In this review, we describe recent progress in engineering of the defects in graphene grown by a silica-assi...

Despite recent advances in three-dimensional (3D) fabrication, the epitaxial growth of finely controlled, single crystalline 3D structures is still constrained. In this study, we demonstrate the step-by-step growth of hierarchical 3D architectures composed of single crystalline semiconductors. To achieve this goal, we first established control of t...

We have studied the role of defects in electrolyte-gated graphene mesh (GM) field-effect transistors (FETs) by introducing engineered edge defects in graphene (Gr) channels. Compared with Gr-FETs, GM-FETs were characterized as having large increments of Dirac point shift (~30-100 mV/pH) which even sometimes exceeded the Nernst limit (59 mV/pH) by m...

We fabricated flexible and transparent hydrogen gas sensors based on a palladium-decorated graphene mesh. Electron microscopy analysis confirmed that ∼2-3 nm diameter Pd nanoparticles were uniformly dispersed on the graphene mesh surfaces. The sensors were highly transparent with an average optical transmission of >90% to visible light and they exh...

Managing interfacial instability is crucial for enhancing cyclability in lithium-ion batteries (LIBs), yet little attention has been devoted to this issue until recently. Here, we introduce graphene as an interfacial layer between the current collector and the anode composed of Si nanowires (SiNWs) to improve the cycling capability of LIBs. The ato...

Ventricular repolarization is assessed using the QT interval corrected by the heart rate (QTc) via an electrocardiogram (ECG). Prolonged QTc is associated with an increased risk of arrhythmias and cardiac mortality. As there have been few reports regarding the effects of hyperthyroidism on ventricular repolarization, we studied the association betw...

Nanoscale time-dependent mechanical-electrical coupled behavior of single crystal ZnO nanorods was systematically explored, which is essential for accessing the long-term reliability of the ZnO nanorod-based flexible devices. A series of compression creep tests combined with in-situ electrical measurement was performed on vertically-grown single cr...

The inherently low electrical conductivity of TiO2-based electrodes as well as the high electrical resistance between an electrode and a current collector represents a major obstacle to their use as an anode for lithium ion batteries. In this study, we report on high-density TiO2 nanotubes (NTs) branched onto a carbon nanofiber (CNF) "tree" that pr...

Three-dimensional (3D), tubular-structured monolayer graphene networks were hybridized with TiO2 nanoparticular layer for futuristic and robust electrode applications. A continuous form of 3D graphene with good carrier mobility provides a direct pathway for electrons to the current collector for a photoanode in dye-sensitized solar cells. This char...

The nanostructures found in nature sometimes have elaborate, three-dimensional structures that consist of soft and flexible constituents, and which exhibit diverse mechanical and optical functions. Here, we introduce a facile, low-cost and scalable nanofabrication approach based on a hot embossing process that can replicate sub-micron to nanoscale...

Wet-chemical synthesis is a promising alternative to the conventional vapour-phase method owing to its advantages in commercial-scale production at low cost. Studies on nanocrystallization in solution have suggested that growth rate is commonly affected by the size and density of surrounding crystals. However, systematic investigation on the mutual...

Transparent colloidal ZnO quantum-dot (QD)/graphene nanocomposites were formed on poly(ethylene terephthalate) (PET) substrates. Ultraviolet (UV)–visible absorption spectra showed a shoulder peak around 350 nm corresponding to the absorption of ZnO QDs. Optical transmittance of the ZnO QD/graphene/PET multilayer was approximately 80%. High-resoluti...

Graphene, a single-atomic-thick planar sheet of sp2-bonded carbon atoms, has been widely investigated for its potential applications in many areas, including biological interfaces, due to its superb electromechanical, optical and chemical properties. In particular, its mechanical flexibility and biocompatibility allow graphene to be configured and...

Chemically-synthesized semiconductor nanowires (NWs) have demonstrated substantial promise for nanoelectronics, nano-energy and nano-bio technology, but the lack of an effective and controllable assembly process has limited the wide adoption of NWs in these areas. Here we demonstrate a facile, robust and controllable approach to assembling and dens...

Si-Ge composites have recently been explored as an anode material for lithium-ion batteries due to their stable cycle performance and excellent rate capability. Although previous reports show the benefits of Si-Ge composites on electrochemical performance, the specific mechanism and structural effects have been overlooked. Here, the structural effe...

We investigated the effects of thermal annealing of graphene on its electrical properties with the aim of improving the electrical contacts between graphene and the p-GaN layer. Compared with pristine graphene which shows ambipolar characteristics, graphene thermally annealed under NH3 atmosphere exhibited obvious p-type characteristics and increas...

Zn‐polar (0001) surfaces are more chemically reactive than other surfaces of ZnO crystals and drive preferential anisotropic and asymmetric growth along the [0001] direction, which facilitates growth of c‐axis oriented, one‐dimensional ZnO nanostructures. Accordingly, capping the top (0001) surface of ZnO crystals can impede c‐axis growth and thus...

External stresses are applied during operation or storage in flexible electronics, which makes understanding time-dependent plastic deformation of nanobuilding blocks more crucial for ensuring the reliability of the devices. Here, we systematically explored the time-dependent nanoscale plasticity of single-crystal ZnO nanorods and its size effects....

The development of various technologies has led to the advent of a variety of deformable devices. Despite significant technological advancement in this area, it is still challenging to integrate different devices due to limitations such as substrate issues and differences among growth and deposition conditions. Creating an interconnection between t...

Here we describe a new method for the direct patterned synthesis of graphene meshes on Cu foils that use self-assembled silica sphere arrays as growth masks. Structural analyses based on electron microscopy and Raman spectroscopy showed that the graphene meshes are mostly single- or double-layer necks with empty holes that have abrupt edges. On the...

We propose a facile method for synthesizing a novel Si membrane structure with good mechanical strength and three-dimensional (3D) configuration that is capable of accommodating the large volume changes associated with lithiation in lithium ion battery applications. The membrane electrodes demonstrated a reversible charge capacity as high as 2,414...

Objective: To clarify the clinical features of severe postpartum hemorrhage (PPH) resulting from a ruptured pseudoaneurysm and to determine the efficacy of selective arterial embolization as a method of management. Study design: Eleven women who underwent selective arterial embolization for treatment of severe hemorrhage due to a ruptured pseudo...

Single crystalline indium oxide (In2O3) nanowires (NWs), formed via a simple vapor transport route, were synthesized by a selective growth approach on patterned SiO2/Si substrates. The selective growth approach enabled us to have a more systematic analysis of the effects of the synthesizing parameters on the growth of the NWs. In this study, the am...

We report that a metal-coated silicon nanowire functions as a plasmonic waveguide. Measurements showed that plasmonic waveguide modes propagated efficiently through a chemically synthesized silicon nanowire with a diameter of similar to 80nm coated with silver. The propagation lengths for transverse-magnetic and transverse-electric modes were estim...

Ultraviolet (UV) photodetectors were fabricated using the wet spin-coating for ZnO quantum dots (QDs) and the transfer method for the graphene sheet. High-resolution transmission electron microscopy images showed that the ZnO QDs were uniformly distributed between the voids of the surface circumferences on the graphene layers. Current-voltage measu...

We report a new facile route to synthesize the ZnO nanotubes by thermal annealing of solid nanorods in ambient NH3. The unique characteristic of this approach allows achievement of ultrathin nanotubes with well-organized hexagonal nanowalls and sealed layouts. On the basis of our experimental observations, we developed a nanotube formation mechanis...

Recent theoretical and experimental studies demonstrated that breaking of the sublattice symmetry in graphene produces an energy gap at the former Dirac point. We describe the synthesis of graphene sheets decorated with ultrathin, Si-rich two-dimensional (2D) islands (i.e., Gr:Si sheets), in which the electronic property of graphene is modulated by...

Active nanophotonic devices are attractive due to their low-power consumption, ultrafast modulation speed and high-density integration. Although electrical operation is required for practical implementation of these devices, it is not straightforward to introduce a proper current path into such a wavelength-scale nanostructure without affecting the...

Supplementary Figures S1-S7

ZnO nanocrystalline networks (NCNWs) consisting of percolating nanocrystals with irregular shape and size were synthesized using Al seed layers in a hydrothermal process. Various thicknesses of Al films were used to assess the effects of film thickness on the formation of ZnO NCNWs; the coverage and size of the ZnO nanocrystals increased with an in...

A facile route to synthesize GaAs nanowires by simply heating Au-coated GaAs substrates to 700 °C under vacuum (∼10−3 Torr) was developed in this study. Detailed structural analyses showed that ultrathin single crystal GaAs nanowires with an average diameter of ∼15 nm were grown outward from the Au metal droplets remaining on the surface of the GaA...

Biosilification is of interest due to its capability to produce a highly intricate structure under environmentally friendly conditions. Despite the considerable effort that has been devoted toward biomimetic silification, the synthesis of highly complex silica structures, as found in the structures of diatom cell walls, is still in its infancy. Her...

Here we describe a simple, versatile technique to produce large-scale arrays of highly ordered ZnO nanorods. Patterning of three distinct ZnO crystal morphologies is demonstrated through use of different ZnO seed layers. Array formation is accomplished through a simple variation on nanosphere lithography that imprints a thickness variation across a...

One-dimensional and two-dimensional hybrid structures, composed of vertical ZnO nanorods grown on large-area graphene, are successfully integrated onto the GaN/InGaN light emitting diodes (LEDs). Compared with GaN LED without transparent conducting electrode, current injection and light emission increased almost 2–3 times, respectively, by the intr...

We review current research activities on the hybrid system that combines nanostructured semiconductor materials with two-dimensional (2D) graphene materials as emerging building blocks for future electronics and photonics. Within this category are three distinct subgroups: (1) 0D-2D nanoparticle-graphene hybrids, (2) 1D-2D nanorod-graphene hybrids,...

Purpose: Recently, public interest in obesity and earlier pubertal development has been increasing. The purpose of this study was to analyze the relationship between obesity and pubertal development in girls. Methods: A total of 158 girls presenting with earlier pubertal development from July 2008 to June 2010 were included in the study. Their mean...

Vertically oriented ZnO hexagonal rod arrays were produced by site-specific hydrothermal growth. These ZnO nanostructures possess well-defined surfaces composed of Zn-terminated (0001) planes (for the top surface) and nonpolar {101̅0} planes (for the rectangular side faces), thereby providing an opportunity to investigate the correlation between su...

Problems related to tremendous volume changes associated with cycling and the low electron conductivity and ion diffusivity of Si represent major obstacles to its use in high-capacity anodes for lithium ion batteries. We have developed a group IVA based nanotube heterostructure array, consisting of a high-capacity Si inner layer and a highly conduc...

Hutchinson-Gilford progeria syndrome (HGPS) is a rare condition originally described by Hutchinson in 1886. Death result from cardiac complications in the majority of cases and usually occurs at average age of thirteen years. A 4-yr old boy had typical clinical findings such as short stature, craniofacial disproportion, alopecia, prominent scalp ve...

The performance of a polymer photovoltaic device using multilayered graphene on an amorphous PET substrate as the electrode was studied. The changes in surface morphology of graphene coated polyethylene terephthalate (PETG) substrate upon thermal annealing were investigated by atomic force microscopy (AFM), field emission scanning electron microsco...

Herein we describe the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoarchitectures via a facile hydrothermal route. In this approach, vertical and in-plane crystallographic orientations of ZnO crystals were tuned by appropriate patterning of growth masks and seed layer control, which enabled the formation of three types of ZnO n...

We have investigated the optical properties of laterally aligned Si nanowire (SiNW) arrays in order to explore their potential applicability in transparent electronics. The SiNW array exhibited good optical transparency in the visible spectral range with a transmittance of ∼90% for a NW density of ∼20–25 per 10 μm. In addition, polarization-depende...

Sinanowires (SiNWs)–carbon nanofibers (CNFs) branched structures with variation in carbon densities were synthesized. SiNWs with critical density of CNFs show the best electrochemical performance, which is attributed to increase in free volume around the SiNWs as well as a buffering role of the branched CNFs against large volumetric change during c...

In this study, we introduce a new strategy to control silicon nanowire (SiNW) morphology (cone shape and cylindrical shape) by exploiting adatom diffusion at the nanostructured surface. In our approach, by using chemical treatment with a mixture of HF and H2SO4 acids, ordinary flat Si substrates were changed to nanoscale-faceted, corrugated surface...

We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼5.5 to ∼0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene...

Quantum dot sensitized solar cells (QDSCs) are currently receiving increasing attention as an alternative to conventional dyes. The efficiencies of QDSCs have experienced a fast growth in the last years, mainly due to an increase in the reported photocurrents and fill factors. Despite this increase, further enhancement of QDSCs needs an improvement...

This article presents a review of current research activities on the hybrid heterostructures of inorganic nanostructures grown directly on graphene layers, which can be categorized primarily as zero-dimensional nanoparticles; one-dimensional nanorods, nanowires, and nanotubes; and two-dimensional nanowalls. For the hybrid structures, the nanostruct...

We present data, which were obtained before bending and after bending, for the electrical bistabilities, memory stabilities, and memory mechanisms of three-layer structured flexible bistable organic memory (BOM) devices, which were fabricated utilizing the ultrathin graphite sheets (UGS) sandwiched between insulating poly(methylmethacrylate) (PMMA)...

We present the fabrication and characterization of new type of flexible gas sensors, composed mainly of a bottom ZnO conductive layer on metal foil, vertically aligned ZnO nanorod channel, and graphene-based top conductive electrode. Multiple cycling tests demonstrated the ZnO nanorods (NRs) and graphene (Gr) hybrid architectures accommodated the f...

Gonadotropin-releasing hormone-agonist (GnRH-Ag) used in controlled ovarian hyperstimulation (COH) for in vitro fertilization and embryo transfer (IVF-ET) has been known to directly affect apoptosis of human ovarian cells, but its mechanism is not clearly understood. Therefore, the purpose of the present study was to investigate whether caspase-8,...

We report vertical growth of ferromagnetic and metallic Fe1.3Ge nanowire (NW) arrays on few-layer graphene in a large area, induced by a relatively good epitaxial lattice match. Integrating well-aligned NW arrays onto graphene would offer a good opportunity to combine superb material properties of graphene with versatile properties of NWs into nove...

Despite many efforts to advance the understanding of nanowire mechanics, a precise characterization of the mechanical behavior and properties of nanowires is still far from standardization. The primary objective of this work is to suggest the most appropriate testing method for accurately determining the mechanical performance of silicon nanowires....

The electrical and the optical properties of polymer light-emitting devices (PLEDs) fabricated utilizing a graphene hole injection layer (HIL) were investigated to clarify the hole injection enhancement caused by the insertion of the graphene HIL. The graphene was synthesized by using chemical vapor deposition, and the PLEDs with a graphene HIL wer...

In this study, hybrid composite films (HCFs) consisting of aligned Si nanowires (SiNWs) and a photoresist polymer were prepared and their applicability to nanomaterial assembly and pattern transfer onto various substrates, including curved surfaces, was demonstrated. The synthesis and transfer of the HCFs can be scaled up to large areas. Transfer o...

We report large-area synthesis of few-layer graphene films by chemical vapor deposition (CVD) in a cold-wall reactor. The key feature of this method is that the catalytic metal layers on the SiO2/Si substrates are self-heated to high growth temperature (900°C to 1000°C) by high-current Joule heating. Synthesis of high-quality graphene films, whose...