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In a tip-enhanced Raman spectroscopy (TERS) system, using a sharp nanotip that comprises a noble metal is critical to attaining high spatial resolution and highly enhanced Raman scattering. A strongly acidic solution is typically used to fabricate gold nanotips in a quick and reliable manner. However, using an acidic solution could corrode the etching system, thereby posing hazardous problems. Therefore, both the corrosion of the etching system and human error induced by the conventional method considerably decrease the quality and reproducibility of the tip. In this study, we significantly increased the reproducibility of tip fabrication by automating the electrochemical etching system. In addition, we optimized the etching conditions for an etchant that comprised a KCl solution to which ethanol was added to overcome the limitations of the acidic etchant. The automated etching system significantly increases the yield rate of tip-fabrication reproducibility from 65 to 95%. The standard deviation of the radius of curvature decreased to 7.3 nm with an average radius of curvature of 30 nm. Accordingly, the automated electrochemical etching system might improve the efficiency of TERS.

We investigated the photophysical interaction between a conjugated polymer (CP) and a plasmonic gold nanoparticle (Au NP) using transient absorption spectroscopy. We prepared a hybrid system contai...

We demonstrated an effective poly(p-chloro-xylylene) (Parylene-C) encapsulation method for MAPbI3 solar cells. By structural and optical analysis, we confirmed that Parylene-C efficiently slowed the decomposition reaction in MAPbI3. From a water permeability test with different encapsulating materials, we found that Parylene-C-coated MAPbI3 perovskite was successfully passivated from reaction with water, owing to the hydrophobic behavior of Parylene-C. As a result, the Parylene-C-coated MAPbI3 solar cells showed better device stability than uncoated cells, virtually maintaining the initial power conversion efficiency value (15.5 ± 0.3%) for 196 h.

Methylammonium lead halide (MAPbI3) perovskites are considered as promising materials owing to their excellent optical and electrical properties. However, perovskite materials suffer from degradation in air, which limits their practical applications. Here, we demonstrate successful passivation of the MAPbI3 photodetectors through monochloro-para-xylylene (Parylene-C) deposition. The time-dependent photocurrent characteristics were systematically investigated, and we achieved significantly improved device performance and stability with Parylene-C passivation. Based on the excitation-power-dependent photoluminescence (PL) data, we confirmed that Parylene-C can reduce the carrier losses in MAPbI3, leading to the enhancement of photocurrent and PL in MAPbI3 photodetectors.

Hybrid organic–inorganic lead halide perovskites have recorded unprecedented improvement in efficiency as fourth-generation photovoltaic materials. Recently, they have attracted enormous interest in nonlinear optics stemming basically from their excellent optoelectronic properties. Here, we investigate multiphoton absorption (MPA) in high-quality MAPbX3 (MA = CH3NH3 and X = Cl, Br, I) bulk single crystals synthesized by an inverse-temperature crystallization (ITC) method. The two-photon absorption (2PA) coefficients under picosecond pulse excitation are determined to be β = 23 ± 2 cm/GW and 9 ± 1 cm/GW for MAPbI3 and MAPbBr3 at λ = 1064 nm, and 13 ± 2 cm/GW for MAPbCl3 at λ = 532 nm. The 2PA coefficients are comparable to those of conventional semiconductors having similar bandgaps and can be explained by a two-band model. Furthermore, we characterize the three-photon absorption behavior of MAPbCl3 at λ = 1064 nm, yielding γ = 0.05 ± 0.01 cm3/GW2. The polarization dependence of MPA is also probed to exper...

Understanding the photophysical interactions between the components in organic-inorganic nanocomposites is a key factor for their efficient application in optoelectronic devices. In particular, the photophysical study of nanocomposites based on organic conjugated polymers is rare. We investigated the effect of surface plasmon resonance (SPR) of gold nanoparticles (Au NPs) on the photoluminescence (PL) property of a push-pull conjugated polymer (PBDB-T). We prepared the hybrid system by incorporating poly(3-hexylthiophene)-stabilized Au NPs (P3HT-Au NPs) into PBDB-T. The enhanced and blueshifted PL was observed in the hybrid system compared to PL in a neat PBDB-T system, indicating that the P3HT chains attached to the Au NPs suppressed charge-transfer from PBDB-T to the Au NPs and relayed the hot electrons to PBDB-T (the band-filling effect). This photophysical phenomenon limited the auto-dissociation of PBDB-T excitons. Thus, the radiative recombination of the excitons occurred more in our hybrid system than in the neat system.

Despite the symbolism of TiO2 in photoelectrochemistry, its fundamental nanoscale properties in oxidation reaction have often reported inconsistent results. We demonstrated the simple model of TiO2 thin films (TFs) for photoelectrochemical (PEC) oxidation reaction (OR) of water, which is constituted of Indium Tin Oxide (ITO) and TiO2 TFs, by depositing it directly onto ITO coated glass substrates through Atomic Layer Deposition (ALD). Using ALD with a sub-nanometer thickness control, 9 to 90 nm in thickness (200 to 2000 in no. of ALD cycles) of TiO2 TFs were fabricated and annealed for crystallization into anatase phase. Anatase TiO2 TF photoanodes showed an optimum PEC OR performance at the thickness of 45 nm (corresponding to around 1000 ALD cycles) in the electrolyte solution of 0.1 M NaOH (pH = 12.9) under UV light illumination of 365 nm in wavelength. Different thicknesses of TiO2 TFs were characterized by electrochemical and optical measurements. Increasing bulk traps in TiO2 TFs up to the critical thickness improved charge transfer and PEC OR by increasing holes’ lifetime. However, it was found to be detrimental when it became dominant along with the increase of TFs’ thickness. This work provides insight in designing highly efficient TiO2 photoelectrodes with the precisely controlled thickness that can have a significant effect on charge transfer kinetics and thus PEC performances.

Molybdenum disulfide (MoS2) is rapidly emerging in a wide range of applications owing to its superior optical, electrical, and catalytic properties. In particular, aside from the current great interest in monolayer MoS2, MoS2 quantum dots (QDs) have received much attention in the electronics and optoelectronics fields owing to their inherent electrical and optical properties arising from the quantum confinement effect. Thus, various methods for producing MoS2 QDs, such as exfoliation, substrate growth, and colloidal synthesis, have been attempted. In this study, the method for manufacturing MoS2 QD with a size of 10 nm which is simpler than the conventional method was devised. On the basis of characterization of the prepared MoS2 QD samples, resistive switching devices was fabricated. These devices demonstrated stable unipolar resistive switching behavior without an electroforming process. This study provides a new approach for the mass production of MoS2 QD and one of their potential applications.