Che-Hung Kuo

National Taiwan University, T’ai-pei, Taipei, Taiwan

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Publications (14)75.91 Total impact

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    ABSTRACT: Dynamic secondary ion mass spectrometry (D-SIMS) analysis of poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) was conducted using a quadrupole mass analyzer with various combinations of continuous C(60)(+) and Ar(+) ion sputtering. Individually, the Ar(+) beam failed to generate fragments above m/z 200, and the C(60)(+) beam generated molecular fragments of m/z ~1000. By combining the two beams, the auxiliary Ar(+) beam, which is proposed to suppress carbon deposition due to C(60)(+) bombardment and/or remove graphitized polymer, the sputtering range of the C(60)(+) beam is extended. Another advantage of this technique is that the high sputtering rate and associated high molecular ion intensity of the C(60)(+) beam generate adequate high-mass fragments that mask the damage from the Ar(+) beam. As a result, fragments at m/z ~900 can be clearly observed. As a depth-profiling tool, the single C(60)(+) beam cannot reach a steady state for either PET or PMMA at high ion fluence, and the intensity of the molecular fragments produced by the beam decreases with increasing C(60)(+) fluence. As a result, the single C(60)(+) beam is suitable for profiling surface layers with limited thickness. With C(60)(+)-Ar(+) co-sputtering, although the initial drop in intensity is more significant than with single C(60)(+) ionization because of the damage introduced by the auxiliary Ar(+), the intensity levels indicate that a more steady-state process can be achieved. In addition, the secondary ion intensity at high fluence is higher with co-sputtering. As a result, the sputtered depth is enhanced with co-sputtering and the technique is suitable for profiling thick layers. Furthermore, co-sputtering yields a smoother surface than single C(60)(+) sputtering.
    Rapid Communications in Mass Spectrometry 10/2011; 25(19):2897-904. DOI:10.1002/rcm.5181 · 2.25 Impact Factor
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    ABSTRACT: It has been shown that the application of self-assembled monolayers (SAMs) to semiconductors or metals may enhance the efficiency of optoelectronic devices by changing the surface properties and tuning the work functions at their interfaces. In this work, binary SAMs with various ratios of 3-aminopropyltrimethoxysilane (APTMS) and 3-mercaptopropyltrimethoxysilane (MPTMS) were used to modify the surface of Si to fine-tune the work function of Si to an arbitrary energy level. As an electron-donor, amine SAM (from APTMS) produced outward dipole moments, which led to a lower work function. Conversely, electron-accepting thiol SAM (from MPTMS) increased the work function. It was found that the work function of Si changed linearly with the chemical composition and increased with the concentration of thiol SAMs. Because dipoles of opposite directions cancelled each other out, homogeneously mixing them leads to a net dipole moment (hence the additional surface potential) between the extremes defined by each dipole and changes linearly with the chemical composition. As a result, the work function changed linearly with the chemical composition. Furthermore, the amine SAM possessed a stronger dipole than the thiol SAM. Therefore, the SAMs modified with APTMS showed a greater work function shift than did the SAMs modified with MPTMS.
    Physical Chemistry Chemical Physics 09/2011; 13(33):15122-6. DOI:10.1039/c1cp20590k · 4.49 Impact Factor
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    ABSTRACT: Cluster ion sputtering has been proven to be an effective technique for depth profiling of organic materials. In particular, C(60)(+) ion beams are widely used to profile soft matter. The limitation of carbon deposition associated with C(60)(+) sputtering can be alleviated by concurrently using a low-energy Ar(+) beam. In this work, the role of this auxiliary atomic ion beam was examined by using an apparatus that could analyze the sputtered materials and the remaining target simultaneously using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS), respectively. It was found that the auxiliary 0.2 kV Ar(+) stream was capable of slowly removing the carbon deposition and suppresses the carbon from implantation. As a result, a more steady sputtering condition was achieved more quickly with co-sputtering than by using C(60)(+) alone. Additionally, the Ar(+) beam was found to interfere with the C(60)(+) beam and may lower the overall sputtering rate and secondary ion intensity in some cases. Therefore, the current of this auxiliary ion beam needs to be carefully optimized for successful depth profiling.
    The Analyst 03/2011; 136(5):941-6. DOI:10.1039/c0an00642d · 4.11 Impact Factor
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    ABSTRACT: This study demonstrated that the work function (Φ) of Au substrates can be fine-tuned by using series ratios of binary self-assembled monolayers (SAMs). By using pure amine- and carboxylic acid-bearing alkanethiol SAM on gold substrates, Φ of Au changed from 5.10 to 5.16 and 5.83, respectively, as determined by ultra-violet photoelectron spectrometry (UPS). The shift in Φ due to the use of different functional groups was rationalized by considering the dipole moments of the molecules anchored on the Au surface. A series of binary SAMs were fabricated by mixing carboxylic acid- and amine-terminated alkanethiols in the deposition solution. By mixing these functional groups in SAMs, a linear correlation between Φ with respect to chemical composition (hence the effective dipole moment on the Au surface) was observed. It was found that arbitrary Φ between extremes (5.16 and 5.83) controlled by respective functional groups can be obtained by changing the chemical composition of SAMs. The Scanning Kelvin Probe (SKP) was also used to measure the contact potential difference (CPD) between SAMs and referencing Au on a patterned substrate prepared by photo-lithography. It was found that the CPD of SAMs with different chemical compositions correlates to their Φ. However, the magnitude of the CPD was smaller than the difference in Φ measured by UPS that was possibly due to the adsorption of contaminants in air.
    Physical Chemistry Chemical Physics 03/2011; 13(10):4335-9. DOI:10.1039/c0cp02437f · 4.49 Impact Factor
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    ABSTRACT: Self-assembled monolayer (SAM)-modified nano-materials are a new technology to deliver drug molecules. While the majority of these depend on covalently immobilizing molecules on the surface, it is proposed that electrostatic interactions may be used to deliver drugs. By tuning the surface potential of solid substrates with SAMs, drug molecules could be either absorbed on or desorbed from substrates through the difference in electrostatic interactions around the selected iso-electric point (IEP). In this work, the surface of silicon substrates was tailored with various ratios of 3-aminopropyltrimethoxysilane (APTMS) and 3-mercaptopropyltrimethoxysilane (MPTMS), which form amine- and thiol-bearing SAMs, respectively. The ratio of the functional groups on the silicon surface was quantified by X-ray photoelectron spectrometry (XPS); in general, the deposition kinetics of APTMS were found to be faster than those of MPTMS. Furthermore, for solutions with high MPTMS concentrations, the relative deposition rate of APTMS increased dramatically due to the acid-base reaction in the solution and subsequent electrostatic interactions between the molecules and the substrate. The zeta potential in aqueous electrolytes was determined with an electro-kinetic analyzer. By depositing SAMs of binary functional groups in varied ratios, the surface potential and IEP of silicon substrates could be fine-tuned. For <50% amine concentration in SAMs, the IEP changed linearly with the chemical composition from <2 to 7.18. For higher amine concentrations, the IEP slowly increased with concentration to 7.94 because the formation of hydrogen-bonding suppressed the subsequent protonation of amines.
    Physical Chemistry Chemical Physics 03/2011; 13(9):3649-53. DOI:10.1039/c0cp02615h · 4.49 Impact Factor
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    ABSTRACT: Over the past years, organic light-emitting diodes (OLEDs) have attracted increasing interest because of their great potential for use in high-quality flat-panel displays and solid-state lighting. One of the basic requirements in any emissive device is to provide adequate stability to ensure a sufficiently long lifetime. Recently, it was observed that small molecules migrate toward the ITO anode under a direct driving voltage while retaining their original structures. To prevent this bias-driven migration of small molecules, a chemical structure with a higher steric hindrance could be introduced as a blockade, thus molecular migration could be suppressed and the device half-life increased. In this work, OLED devices with different hosts, including CBP, mCP, SimCP2, and SimCP, with increasing steric hindrances are fabricated. The spatial distribution of the tracking molecules after operation for different lengths of time is examined by using X-ray photoelectron spectroscopy (XPS) with in situ high-energy C60+ and low-energy Ar+ co-sputtering for depth profiling. It is found that the bias-driven molecular migration is suppressed and the device half-life prolonged as the steric hindrance of the host increases.
    Organic Electronics 02/2011; 12(2):376-382. DOI:10.1016/j.orgel.2010.12.005 · 3.83 Impact Factor
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    ABSTRACT: The nanostructure of the light emissive layer (EL) of polymer light emitting diodes (PLEDs) was investigated using force modulation microscopy (FMM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) excited with focused Bi(3)(2+) primary beam. Three-dimensional nanostructures were reconstructed from high resolution ToF-SIMS images acquired with different C(60)(+) sputtering times. The observed nanostructure is related to the efficiency of the PLED. In poly(9-vinyl-carbazole) (PVK) based EL, a high processing temperature (60 °C) yielded less nanoscale phase separation than a low processing temperature (30 °C). This nanostructure can be further suppressed by replacing the host polymer with poly[oxy(3-(9H-9-carbazol-9-ilmethyl-2-methyltrimethylene)] (SL74) and poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (RS12), which have similar chemical structures and energy levels as PVK. The device efficiency increases when the phase separation inside the EL is suppressed. While the spontaneous formation of a bicontinuous nanostructure inside the active layer is known to provide a path for charge carrier transportation and to be the key to highly efficient polymeric solar cells, these nanostructures are less efficient for trapping the carrier inside the EL and thus lower the power conversion efficiency of the PLED devices.
    The Analyst 10/2010; 136(4):716-23. DOI:10.1039/c0an00335b · 4.11 Impact Factor
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    ABSTRACT: By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.
    ACS Nano 05/2010; 4(5):2547-54. DOI:10.1021/nn901593c · 12.88 Impact Factor
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    ABSTRACT: Amine-terminated self-assembled monolayers (SAMs) on Au surfaces are commonly used to immobilize various types of molecules, including DNA and proteins. However, little is known about the stability of these types of surfaces. In this work, it was observed that the surface potential (as well as the isoelectric point) of amine-bearing SAMs on flat gold substrates changed significantly with time, indicating that the surface functional group is not stable under ambient conditions (standard temperature and pressure). Contact angle analysis indicated that after degradation, the polar component of the interfacial force decreased and the dispersion component increased. These results indicate that the surface has undergone a chemical transformation. X-ray photoelectron spectroscopy (XPS) was used to detect changes in the chemical state of the surface nitrogen atoms. The chemical shift of the binding energy indicates that the nitrogen is partially oxidized. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), the oxidation of the amino groups to nitroso groups was evident, as was the previously reported oxidation of the thiol groups to sulfonate groups. Two methods for retarding the oxidation of the amine functional group are presented in this work. By isolating the SAM from either light or air, the oxidation is suppressed and the surface properties are preserved. In other words, the shelf life of the amine-modified gold substrates is prolonged. This result suggests that the oxidation is either photocatalyzed or photoinduced.
    The Journal of Physical Chemistry C 05/2010; 114(23). DOI:10.1021/jp101426h · 4.77 Impact Factor
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    ABSTRACT: Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.
    ACS Nano 02/2010; 4(2):833-40. DOI:10.1021/nn9014449 · 12.88 Impact Factor
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    ABSTRACT: Alkanethiol anchored self-assembled monolayers (SAMs) on gold are widely used to immobilize and detect molecules including DNA and proteins. Most of these molecules are covalently bonded with the SAM on the Au surface and cannot be released easily. By using different functional groups, the interfacial charge of SAMs can be selected, and thus, they can be considered as adaptors for immobilizing and releasing materials selectively through electrostatic interaction under given conditions. In this work, as an additional factor to control the surface charge, SAMs with mixed functional groups are presented, and it is demonstrated that the isoelectric point (IEP) can be tailored by the ratio of functional groups. Using carboxylic acid- and amine-SAM on gold substrates as an example, isoelectric points (IEPs) from 3.5 to 6.5 can be obtained arbitrarily. The ratio between the functional groups on the surface was quantified by X-ray photoelectron spectrometry (XPS) and was found to be slightly different from the deposition solution. The homogeneous spatial distribution of the functional groups was determined with scanning electrical potential microscopy (SEPM). The interfacial charge of SAMs with mixed functional groups on gold was investigated by electrokinetic analysis in aqueous electrolyte solutions.
    Physical Chemistry Chemical Physics 09/2009; 11(29):6199-204. DOI:10.1039/b902044f · 4.49 Impact Factor
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    ABSTRACT: Solution-processable fullerene and copolymer bulk-heterojunctions are widely used as the active layer of solar cells. It is known that the controlled phase-separation in the film provides a pathway for carrier transportation and is crucial to efficiency. In this work, scanning electrical potential microscopy (SEPM) is used to examine the surface distribution of [6,6]phenyl-C61-butyric acid methyl ester and poly(3-hexylthiophene), which form the bulk-heterojunction. Because the two components have different energies in the highest occupied molecular orbital (HOMO), the differences in contact potential yield strong contrast in SEPM. A cluster ion beam (C(60)(+)) is used to remove the surface in order to determine the structure below, and SEPM is used to analyze the newly exposed surface. With the SEPM images acquired from different depth through the material stacked, a 3D volume image is obtained. It is demonstrated that using SEPM with cluster ion slicing is an effective tool for studying the 3D nanostructures of soft materials.
    Analytical Chemistry 09/2009; 81(21):8936-41. DOI:10.1021/ac901588t · 5.64 Impact Factor
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    ABSTRACT: Self-assembled monolayer (SAM)-modified gold nanoparticles can be used to immobilize and transport molecules including DNA and proteins. However, these molecules are usually covalently bound to the surface and chemical reactions are required to cleave and release them. Therefore, immobilizing molecules using electrostatic interactions might be beneficial. In this work, Au nanoparticles modified by SAMs with mixed carboxylic acid and amine functional groups are presented. The surface potential and the iso-electric point (IEP) of the nanoparticles can be tailored by the ratio of these functional groups and arbitrary IEPs between 3.2 and 7.3 can be achieved. As a result, based on electrostatic interactions, molecules could be triggered to adsorb/desorb by changing the environmental pH around this tunable IEP. These engineered nanoparticles were synthesized in a single-phase system based on the reduction of HAuCl4 by NaBH4 in ethanol with a mixture of 16-mercaptohexadecanoic acid and 8-amino-1-octanethiol that forms the SAM on the synthesized nanoparticles. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrophoresis light scattering revealed the particle size, ratio of the functional groups, and zeta-potential of the particles as a function of pH, respectively.
    Journal of Colloid and Interface Science 09/2009; 340(1):126-30. DOI:10.1016/j.jcis.2009.08.014 · 3.37 Impact Factor
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    ABSTRACT: The change of the chemical states of inorganic oxoanion salts by low-energy single atomic projectiles (0.5 kV Ar(+)), high-energy cluster ion beams (10 kV C(60)(+)), and mixed 0.2 kV Ar(+) and 10 kV C(60)(+) are presented. Although the sputtering conditions of C(60)(+) and C(60)(+)-Ar(+) mixed sputtering used in this work provide more accurate results for profiling organic films than Ar(+) sputtering, the difference in profiling inorganic materials is not as dramatic. For inert oxoanions like carbonate and phosphate, both Ar(+) and C(60)(+) sputtering cause unappreciable sputter-induced chemical transformation to the remaining surface. For reactive oxoanions like nitrate, perchlorate, chlorate, and chlorite, although C(60)(+) yields slightly better results than Ar(+) sputtering due to the thinner disturbed layer, all the ion beams altered the chemical state significantly. As a result, none of these techniques can be used to provide true information below the surface. For intermediate oxoanions like nitrite and thio-S in thiosulfate, C(60)(+) yields slightly better results than Ar(+) sputtering. However, for sulfonate and the core-S in thiosulfate, C(60)(+) causes more sputter-induced chemical transformation than Ar(+) sputtering.
    The Analyst 06/2009; 134(5):945-51. DOI:10.1039/b814729a · 4.11 Impact Factor