Hyouksoo Han

Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent

The reported photocurrent density (JSC) of PbS quantum dot (QD)-sensitized solar cell was less than 19 mA/cm2 despite the capability to generate 38 mA/cm2, which results from inefficient electron injection and fast charge recombination. Here, we report on a PbS:Hg QD-sensitized solar cell with an unprecedentedly high JSC of 30 mA/cm2. By Hg2+ doping into PbS, JSC is almost doubled with improved stability. Femtosecond transient study confirms that the improved JSC is due to enhanced electron injection and suppressed charge recombination. EXAFS reveals that Pb-S bond is reinforced and structural disorder is reduced by interstitially incorporated Hg2+, which is responsible for the enhanced electron injection, suppressed recombination and stability. Thanks to the extremely high JSC, power conversion efficiency of 5.6% is demonstrated at one sun illumination.

Full Surface Embedding of Gold Clusters on Silicon Nanowires for Efficient Capture and Photothermal Therapy of Circulating Tumor Cells

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.

An electronic structure reinterpretation of the organic semiconductor/electrode interface based on argon gas cluster ion beam sputtering investigations

The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS) analysis with phased pentacene deposition) and top-down (in situ UPS analysis with Ar GCIB sputtering) methods, and the energy levels and chemical states are compared using the same sample. The Ar GCIB sputtering process causes no variation in the primary valence band structure, including the chemical state and configuration. Therefore, the energy-level alignment determined using the top-down method is comparable to that obtained using bottom-up method, since the Ar GCIB sputtering process is damage-free.The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS)...

A high performance green-sensitive organic photodiode comprising a bulk heterojunction of dimethyl-quinacridone and dicyanovinyl terthiophene

A fullerene-free bulk-heterojunction (BHJ) organic photodiode (OPD) with high efficiency and green-color selectivity is reported. Using N,N-dimethyl quinacridone (DMQA) as a donor and dibutyl-substituted dicyanovinyl-terthiophene (DCV3T) as an acceptor, a maximum external quantum efficiency (EQE) of over 67% at 540 nm was achieved at −5 V bias. The OPD performance together with their electrical and optical behaviors were investigated by varying the ratio of donor and acceptor components and measuring the absorption coefficient, charge carrier generation, and charge transport. The composition rich in DMQA exhibited a high yield of photogenerated charge carriers and a low absorption intensity, whereas the material rich in DCV3T had a high absorption intensity and low yield of charge carriers. It was found that the 1 : 1 ratio of components showed the best device performance due to its relatively high absorption and efficient photogeneration of charge carriers. Furthermore, electrical characterization of our BHJ OPDs indicated that a balance of electron and hole mobilities is important for enhancing EQE.

A novel approach for the characterization of a bilayer of phenyl-c71-butyric-acid-methyl ester and pentacene using ultraviolet photoemission spectroscopy and argon gas cluster ion beam sputtering process

The material arrangement and energy level alignment of an organic bilayer comprising of phenyl-c71-butyric-acid-methyl ester (PCBM-71) and pentacene were studied using ultraviolet photoelectron spectroscopy (UPS) and the argon gas cluster ion beam (GCIB) sputtering process. Although there is a small difference in the full width at half maximum of the carbon C 1s core level peaks and differences in the oxygen O 1s core levels of an X-ray photoemission spectroscopy spectra, these differences are insufficient to clearly distinguish between PCBM-71 and pentacene layers and to classify the interface and bulk regions. On the other hand, the valence band structures in the UPS spectra contain completely distinct configurations for the PCBM-71 and pentacene layers, even when they have similar atomic compositions. According to the valence band structures of the PCBM-71/pentacene/electrodes, the highest unoccupied molecular orbital (HOMO) region of pentacene is at least 0.8 eV closer to the Fermi level than that of ...

Field emission characteristics of self-assembled carbon nanotubes on the gold surface

A method was studied to fabricate carbon nanotube (CNT) field emitters through self-assembly of thiol-modified CNTs onto a gold-coated substrate. By adsorption through simple immersion of the gold-coated substrate into thiol-modified CNT solution overnight, deposition of CNTs was accomplished. It was found that CNTs were deposited on the substrate as random aggregates of CNT bundles. The population density of CNTs on the gold-coated substrate was varied by controlled CNT concentration in ethanol. The I–V measurement of the self-assembled CNTs shows relatively good field emission characteristics with a low turn-on field (2.5 V/μm) and a high current density (40 μA/cm2 at 3 V/μm). In addition, more-populated CNTs on the substrate produce a higher current density. Therefore, the technique of self-assembly of CNTs on a selected area is expected to be a reliable way to prepare large-size panels with easy process and low cost. These advantages recommend further investigation of the self-assembly of CNTs.

Monitoring of adipogenic differentiation at the single-cell level using atomic force microscopic analysis

Adipogenesis plays an important role in energy homeostasis by storing excess energy as lipid droplets. However, these reservoirs are implicated in a host of major human health problems, such as obesity. Elucidation of the mechanisms underlying adipogenesis is thus crucial to overcome these problems. The preadipocyte cell lines represent an optimal model to examine adipogenesis. Cells differentiate into adipocytes with various speeds of conversion and fat accumulation. Here, we have presented a novel method for detecting adipogenic differentiation at the single-cell level using atomic force microscopic analysis. Data obtained with this method revealed a good correlation between membrane stiffness and the degree of adipogenic differentiation. Although we could not determine the underlying cause for membrane stiffness reduction during adipogenic differentiation, the technique clearly offers advantages over the existing detection systems, such as lipid drop staining and extraction. Furthermore, the degree of adipogenic differentiation at the single-cell level can be detected with this method.