Hee-Tae Jung

Layer-by-layer assembly of graphene and gold nanoparticles by vacuum filtration and spontaneous reduction of gold ions

Layer-by-layer films comprised of alternating graphene and gold nanoparticle layers are readily produced by the two-step procedure involving the use of vacuum filtration of a reduced graphene oxide solution to fabricate the graphene thin film on the quartz substrate, followed by gold nanoparticle formation by spontaneous reduction of gold ions in a gold salt solution on the graphene films.

Helical Nanofilament Phases

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Molecules lacking handedness can form layered, mesoporous helical structures. In the formation of chiral crystals, the tendency for twist in the orientation of neighboring molecules is incompatible with ordering into a lattice: Twist is expelled from planar layers at the expense of local strain. We report the ordered state of a neat material in which a local chiral structure is expressed as twisted layers, a state made possible by spatial limitation of layering to a periodic array of nanoscale filaments. Although made of achiral molecules, the layers in these filaments are twisted and rigorously homochiral—a broken symmetry. The precise structural definition achieved in filament self-assembly enables collective organization into arrays in which an additional broken symmetry—the appearance of macroscopic coherence of the filament twist—produces a liquid crystal phase of helically precessing layers.

Direct visualization of large-area graphene domains and boundaries by optical birefringency.

The boundaries between domains in single-layer graphene strongly influence its electronic properties. However, existing approaches for domain visualization, which are based on microscopy and spectroscopy, are only effective for domains that are less than a few micrometres in size. Here, we report a simple method for the visualization of arbitrarily large graphene domains by imaging the birefringence of a graphene surface covered with nematic liquid crystals. The method relies on a correspondence between the orientation of the liquid crystals and that of the underlying graphene, which we use to determine the boundaries of macroscopic domains.

Tunable Volatile Organic Compounds Sensor by Using Thiolated Ligand Conjugation on MoS2

One of the most important issues in the development of gas sensors for breath analysis is the fabrication of gas sensor arrays that possess different responses for recognizing patterns for volatile organic compounds (VOCs). Here, we develop a high-performance chemiresistor with a tunable sensor response and high sensitivity for representative VOC groups by using molybdenum disulfide (MoS2) and by conjugating a thiolated ligand (mercaptoundecanoic acid (MUA)) to MoS2 surface. Primitive and MUA-conjugated MoS2 sensing channels exhibit distinctly different sensor responses toward VOCs. In particular, the primitive MoS2 sensor presents positive responses for oxygen-functionalized VOCs, while the MUA-conjugated MoS2 sensor presents negative responses for the same analytes. Such characteristic sensor responses demonstrate that ligand conjugation successfully adds functionality to a MoS2 matrix. Thus, this will be a promising approach to constructing a versatile sensor array, by conjugating a wide variety of thiolated ligands on the MoS2 surface. Furthermore, these MoS2 sensors in this study exhibit high sensitivity to representative VOCs down to a concentration of 1 ppm. This approach to fabricating a tunable and sensitive VOC sensor may lead to a valuable real-world application for lung cancer diagnosis by breath analysis.

Enhanced Solar‐Cell Efficiency in Bulk‐Heterojunction Polymer Systems Obtained by Nanoimprinting with Commercially Available AAO Membrane Filters

The power-conversion efficiency of an organic photovoltaic cell composed of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester is enhanced significantly by controlling the surface morphology of the photoactive layer. The hierarchical micro-/nanostructures of the active-layer surface are fabricated by nanoimprinting the active layer with a commercially available, anodic aluminum oxide (AAO) membrane filter.

Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layer

The (dark) diode characteristics of the organic bulk heterojunction solar cell based on the phase separated blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole)] with [6,6]-phenyl C70-butyric acid methyl ester have been analyzed with a focus on the effect of the titanium suboxide (TiOx) electron transport layer. The addition of the TiOx layer into the device structure causes the saturation current density to decrease by a factor of 26 and the shunt resistance to increase by a factor of 12. The diode ideality factor and series resistance are, respectively, almost the same for diodes made with and without the TiOx layer. The results indicate that the TiOx layer increases the energy barrier for hole transport and reduces the minority carrier density.

Gaussian curvature and the equilibrium among bilayer cylinders, spheres, and discs

In mixtures of cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate (FC7) in aqueous solution, novel bilayer cylinders with hemispherical end caps and open, flat discs coexist with spherical unilamellar vesicles, apparently at equilibrium. Such equilibrium among bilayer cylinders, spheres, and discs is only possible for systems with a spontaneous curvature, Ro, and a positive Gaussian curvature modulus, κ̄. We have measured the size distributions of the spherical vesicles, cylinders, and discs by using cryo-electron microscopy; a simple analysis of this length distribution allows us to independently determine that the mean curvature modulus, κ ≈ 5 ± 1 kBT and κ̄ ≈ 2 ± 1 kBT. This is one of the few situations in which Ro, κ, and κ̄ can be determined from the same experiment. From a similar analysis of the disk size distribution, we find that the edges of the discs are likely stabilized by excess CTAB. The fraction of discs, spherical vesicles, and cylinders depends on the CTAB/FC7 mole ratio: increasing CTAB favors discs, while decreasing CTAB favors cylinders. This control over aggregate shape with surfactant concentration may be useful for the design of templates for polymerization, mesoporous silicates, etc.

Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers

In this work, we demonstrate that gas adsorption is significantly higher in edge sites of vertically aligned MoS2 compared to that of the conventional basal plane exposed MoS2 films. To compare the effect of the alignment of MoS2 on the gas adsorption properties, we synthesized three distinct MoS2 films with different alignment directions ((1) horizontally aligned MoS2 (basal plane exposed), (2) mixture of horizontally aligned MoS2 and vertically aligned layers (basal and edge exposed), and (3) vertically aligned MoS2 (edge exposed)) by using rapid sulfurization method of CVD process. Vertically aligned MoS2 film shows about 5-fold enhanced sensitivity to NO2 gas molecules compared to horizontally aligned MoS2 film. Vertically aligned MoS2 has superior resistance variation compared to horizontally aligned MoS2 even with same surface area exposed to identical concentration of gas molecules. We found that electrical response to target gas molecules correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. Density functional theory (DFT) calculations corroborate the experimental results as stronger NO2 binding energies are computed for multiple configurations near the edge sites of MoS2, which verifies that electrical response to target gas molecules (NO2) correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. We believe that this observation extends to other 2D TMD materials as well as MoS2 and can be applied to significantly enhance the gas sensor performance in these materials.

Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films

The use of carbon nanotubes (CNTs) as transparent conducting films is one of the most promising aspects of CNT-based applications due to their high electrical conductivity, transparency, and flexibility. However, despite many efforts in this field, the conductivity of carbon nanotube network films at high transmittance is still not sufficient to replace the present electrodes, indium tin oxide (ITO), due to the contact resistances and semi-conducting nanotubes of the nanotube network films. Many studies have attempted to overcome such problems by the chemical doping and hybridization of conducting guest components by various methods, including acid treatment, deposition of metal nanoparticles, and the creation of a composite of conducting polymers. This review focuses on recent advances in surface-modified carbon nanotube networks for transparent conducting film applications. Fabrication methods will be described, and the stability of carbon nanotube network films prepared by various methods will be demonstrated.

Gold nanolayer-encapsulated silica particles synthesized by surface seeding and shell growing method: near infrared responsive materials

Gold nanolayer-encapsulated silica particles whose optical resonance is located in 750-900 nm spectral region were synthesized by combining Sn (tin)-surface seeding and a shell growing process. The synthesized composite particles can be potentially used in wide biological fields, due to biocompatibility and a well-known bioconjugation technique of gold layer. Sn atoms, which can act not only as a catalytic surface for reduction of gold but also as a linker between silica surface and gold nanoparticles, were chemically deposited on hydroxylated silica particles. Then, we introduced another reductant with gold chloride in order to produce a multilayer of Au shell. In the process, Au shells grew by the reduction of additional gold ions on the Sn-functionalized silica surface and resulted in the subsequent coalescence and growth of the deposited gold nanoparticles. Finally, a complete gold nanoshell was formed on the silica surface by the one-step method, without a repeated coating process. The deposition of a gold nanolayer on the silica particles was easily controlled by the concentration ratio of Sn-functionalized silica particles and gold chloride solutions. Transmission electron microscopy (TEM) images and optical extinction spectra clearly showed that gold nanolayers were successfully deposited on the silica surface by the novel method. As the gold colloids attached on the silica surface grew, their optical plasmon peak became red-shifted until complete a gold shell was formed. After the gold shell was completed, the optical plasmon resonance became blue-shifted and the extinction spectra were functions of a relative ratio of the core to shell thickness.