Yun Hee Chang

Reflectance of conducting poly(3,4-ethylenedioxythiophene)

Abstract We present the reflectance spectrum of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with PF6 with dc conductivity of 230 S/cm at room temperature, measured over a wide frequency range from 50 to 5×104 cm−1 (0.006–6 eV). The reflectance, R(ω), of PEDOT-PF6 is characterized by metal-like signatures in the infrared (IR), including high R(ω) in the far-IR and a plasma frequency of approximately 1.2 eV. The optical conductivity, σ(ω), is dominated by intraband contributions below 1 eV, characteristic of a disordered metal near the metal–insulator (M–I) transition. The reflectance spectrum is in good quantitative agreement with the “localization-modified Drude (LMD) model”. Since the onset of the interband absorption (below 1.5 eV) is at lower energy than in other conducting polymers, PEDOT-PF6 shows low absorption in the visible range between 2 and 3 eV. Our spectroscopic results demonstrate that this doped PEDOT system can be utilized as a transparent electrode for optoelectronic devices.

Monodisperse pattern nanoalloying for synergistic intermetallic catalysis.

Nanoscale alloys attract enormous research attentions in catalysis, magnetics, plasmonics and so on. Along with multicomponent synergy, quantum confinement and extreme large surface area of nanoalloys offer novel material properties, precisely and broadly tunable with chemical composition and nanoscale dimension. Despite substantial progress of nanoalloy synthesis, the randomized positional arrangement and dimensional/compositional inhomogeneity of nanoalloys remain significant technological challenges for advanced applications. Here we present a generalized route to synthesize single-crystalline intermetallic nanoalloy arrays with dimensional and compositional uniformity via self-assembly. Specific electrostatic association of multiple ionic metal complexes within self-assembled nanodomains of block copolymers generated patterned monodisperse bimetallic/trimetallic nanoalloy arrays consisting of various elements, including Au, Co, Fe, Pd, and Pt. The precise controllability of size, composition, and intermetallic crystalline structure of nanoalloys facilitated tailored synergistic properties, such as accelerated catalytic growth of vertical carbon nanotubes from Fe-Co nanoalloy arrays.

Switching and Sensing Spin States of Co–Porphyrin in Bimolecular Reactions on Au(111) Using Scanning Tunneling Microscopy

Controlling and sensing spin states of magnetic molecules at the single-molecule level is essential for spintronic molecular device applications. Here, we demonstrate that spin states of Co-porphyrin on Au(111) can be reversibly switched over by binding and unbinding of the NO molecule and can be sensed using scanning tunneling microscopy and spectroscopy (STM and STS). Before NO exposure, Co-porphryin showed a clear zero-bias peak, a signature of Kondo effect in STS, whereas after NO exposures, it formed a molecular complex, NO-Co-porphyrin, that did not show any zero-bias feature, implying that the Kondo effect was switched off by binding of NO. The Kondo effect could be switched back on by unbinding of NO through single-molecule manipulation or thermal desorption. Our density functional theory calculation results explain the observations with pairing of unpaired spins in dz(2) and ppπ* orbitals of Co-porphyrin and NO, respectively. Our study opens up ways to control molecular spin state and Kondo effect by means of enormous variety of bimolecular binding and unbinding reactions on metallic surfaces.

Intrinsic Photoluminescence Emission from Subdomained Graphene Quantum Dots

The photoluminescence (PL) origin of bright blue emission arising from intrinsic states in graphene quantum dots (GQDs) is investigated. The bright PL of intercalatively acquired GQDs is attributed to favorably formed subdomains composed of four to seven carbon hexagons. Random and harsh oxidation which hinders the energetically favorable formation of subdomains causes weak and redshifted PL.

Optical spectroscopic characterization of plasma-polymerized thin films

Abstract We report optical spectroscopic measurements of plasma-polymerized thin films coated on aluminum (Al) substrates over a spectral range from 0.01 to 6 eV. While the reflectance spectra, R (ω), remain almost unchanged in the infrared range as compared with the bare Al, R (ω) starts to decrease significantly in the higher energy above 2.5 eV. This decrease in R (ω) arises from electronic absorptions characteristic of the polymer films. Moreover, the details of R (ω) in the energy above 3 eV depend on the fabrication conditions of the polymer films. Our results, therefore, demonstrate that this optical measurement can be an excellent method to characterize the quality of polymer films deposited by the plasma polymerization process.

Probing Single-Molecule Dissociations from a Bimolecular Complex NO–Co-Porphyrin

Axial coordinations of diatomic NO molecules to metalloporphyrins play key roles in dynamic processes of biological functions such as blood pressure control and immune response. Probing such reactions at the single molecule level is essential to understand their physical mechanisms but has been rarely performed. Here we report on our single molecule dissociation experiments of diatomic NO from NO-Co-porphyrin complexes describing its dissociation mechanisms. Under tunneling junctions of scanning tunneling microscope, both positive and negative energy pulses gave rise to dissociations of NO with threshold voltages, +0.68 and -0.74 V at 0.1 nA tunneling current on Au(111). From the observed power law relations between dissociation rate and tunneling current, we argue that the dissociations were inelastically induced with molecular orbital resonances by stochastically tunneling electrons, which is supported with our density functional theory calculations. Our study shows that single molecule dissociation experiments can be used to probe reaction mechanisms in a variety of axial coordinations between small molecules and metalloporphyrins.