Peerasak Paoprasert

University of Wisconsin, Madison, Mississippi, United States

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Publications (16)55.28 Total impact

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    ABSTRACT: We demonstrate that hydroxyl containing bipyridine-based molecules can be efficiently grafted onto SiO2 and TiO2 substrates to create a stable monolayer. These surface-grafted ligands are relevant to several optoelectronic applications as well as catalysis since many metal–bipyridine complexes have been used as dye sensitizers, light emitters, and catalysts. Control experiments were carried out to infer that the grafting occurs through the formation of an ether link with available hydroxylgroups on the surface. The coverage was found to be a function of annealing temperature, annealing time, and availability of surface hydroxylgroups. Further complexation with rhenium salts resulted in a rhenium–bipyridine complex, a common electron injector in dye-sensitized solar cells. All samples were characterized by X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS). This simple grafting method leads to monolayer coverage on the surface, effectively avoiding crosslinking, and shows good stability in acetonitrile and aqueous solutions for at least four weeks. We further expand the scope of this approach by grafting an electroactive ferrocene unit onto indiumtin oxide. Based on these results, the formation of an ether bond via thermal annealing of small molecules offers a versatile strategy for preparing stable organic layers on a variety of oxide surfaces, and therefore, expands the tool box for functionalizing organic/inorganic interfaces.
    Journal of Materials Chemistry 12/2011; 22(3):1046-1053. · 5.97 Impact Factor
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    ABSTRACT: In this proceeding, transient 2D IR spectroscopy is used to study Re dye sensitized TiO2 nanocrystalline thin films. Multiple conformations of the dye on the interfaces are found by equilibrium 2D IR spectrum and transient 2D IR spectrum indicates these different binding conformations have different electron transfer kinetics.
    Proc SPIE 09/2011;
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    ABSTRACT: Photoinduced charge transfer modifies the device properties of illuminated pentacene field effect transistors (FETs) incorporating ZnO quantum dots at the gate insulator/pentacene interface. The transferred charge is trapped on electronic states associated with the ZnO quantum dots, with a steady state population approximately proportional to the rate of organic-inorganic charge transfer. Trapped charge shifts the threshold voltage of the FETs, providing the means to evaluate the rate of organic/inorganic charge transfer and the effects of interface modification. Monolayers of the wide-gap alkane stearic acid and the conjugated oligomer terthiophene attached to the ZnO suppress or permit charge transfer, respectively.
    Applied Physics Letters 04/2011; · 3.79 Impact Factor
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    ABSTRACT: The transfer of charge at organic-inorganic semiconductor interfaces is relevant for a range of applications such as organic light emitting diodes and photovoltaics. We investigate exciton dissociation and charge transfer using a novel probe based on a modified pentacene phototransistor with ZnO nanoparticles at the gate dielectric interface. We observe a large threshold voltage shift (+33 V) upon illumination of samples with ZnO, and no comparably large shift in samples without ZnO. This large threshold voltage shift arises from photoinduced charge transfer at the pentacene-ZnO interface and charge trapping on the nanoparticles. We further find that modifying the ZnO surface with a layer of organic molecules using a carboxylic acid binding chemistry can tune the threshold voltage, and therefore the density of charge transferred to ZnO. The effect of absorbing incident photons with varying energy will also be discussed.
    03/2010;
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    ABSTRACT: Poly(3-hexylthiophene) (P3HT) brushes on silicon dioxide (SiO2) were prepared using a click reaction between ethynyl-terminated P3HT and an azide self-assembled monolayer (SAM). Regioregular ethynyl-terminated P3HT with molecular weight of 5900 g mol(-1) and polydispersity of 1.2 was synthesized by catalyst-transfer polycondensation using Grignard metathesis mediated by a nickel-based catalyst. The azide SAM was prepared from bifunctional molecules containing azide and siloxane as click reaction precursor and surface linker, respectively. The P3HT brushes were characterized by atomic force microscopy, ellipsometry, X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and UV-visible spectroscopy. The grafting of P3HT brushes was studied as a function of click reaction time and the growth of the brushes is governed by a diffusion-controlled process. P3HT brushes were prepared on pre-fabricated field-effect transistor structures in order to probe the electrical properties of the brushes. The versatile synthetic methodology developed in this work can be generalized to prepare a wide variety of semiconducting conjugated polymer brushes on oxide surfaces relevant to organic electronic devices.
    Journal of Materials Chemistry 01/2010; 20(13):2651-2658. · 5.97 Impact Factor
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    ABSTRACT: We have measured the electron injection kinetics of four rhenium bipyridine complexes (Re1C, ReEC, Re1TC, and Re2TC) on TiO2 nanocrystalline films using transient infrared spectroscopy. The self-assembled monolayer formation of these complexes was characterized by UV-visible spectroscopy, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy. These complexes bind to the TiO2 surface through the formation of carboxylate groups, and these self-assembled layers are approximately a monolayer. The kinetics studies address the effect of insulating and conjugated spacers and the length of conjugation on the electron-transfer process. The insulating bridge leads to a slower injection rate and poorer injection yield compared with the conjugated spacers. The electron injection of Re2TC was found to be a fast, high-yielding, and multiple electron injector process. The ground and electronically excited states of the dye complexes were characterized using ground-state and time-dependent density functional theory. We present the role of electronic conjugation in modulating electron injection using a combination of computational and experimental work and find that these metal-based complexes adsorbed on a semiconductor surface can be used to read out the electron injection kinetics through tailored molecular bridges.
    Journal of Physical Chemistry C. 01/2010; 114(21):9898-9907.
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    ABSTRACT: We use nonlinear 2D IR spectroscopy to study TiO(2) nanocrystalline thin films sensitized with a Re dye. We find that the free electron signal, which often obscures the vibrational features in the transient absorption spectrum, is not observed in the 2D IR spectra. Its absence allows the vibrational features of the dye to be much better resolved than with the typical IR absorption probe. We observe multiple absorption bands but no cross peaks in the 2D IR spectra, which indicates that the dyes have at least three conformations. Furthermore, by using a pulse sequence in which we initiate electron transfer in the middle of the infrared pulse train, we are able to assign the excited state features by correlating them to the ground state vibrational modes and determine that the three conformations have different time scales and cross sections for electron injection. 2D IR spectroscopy is proving to be very useful in disentangling overlapping structural distributions in biological and chemical physics processes. These experiments demonstrate that nonlinear infrared probes are also a powerful new tool for studying charge transfer at interfaces.
    Journal of the American Chemical Society 11/2009; 131(50):18040-1. · 10.68 Impact Factor
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    ABSTRACT: Linear-dendritic block copolymer hosts were synthesized by end-functionalizing poly(methylmethacrylate) with dendrons that acted as hydrogen-bonding acceptors for nonlinear optical chromophores. Second harmonic generation experiments indicate that the d33 coefficients and maximum chromophore loading are increased in linear-dendritic block copolymer hosts over comparable homopolymer hosts. Transmission electron microscopy shows 5–10 nm chromophore domains, confirming the effective spatial dispersion of the chromophores. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5017–5026, 2009
    Journal of Polymer Science Part A Polymer Chemistry 08/2009; 47(19):5017 - 5026. · 3.54 Impact Factor
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    ABSTRACT: A linear-dendritic block copolymer was synthesized by end-functionalizing PMMA homopolymer with dendrons that acted as H-bond acceptors for thiophene-based nonlinear optical chromophores. The polymer forms excellent quality films which can be spin coated and corona poled. Electro-optic measurements show r<sub>33</sub> = 25 pm/V at 1550 nm. The relative simplicity of the H-bonded scheme can be readily extended to more highly nonlinear chromophores.
    Lasers and Electro-Optics, 2009 and 2009 Conference on Quantum electronics and Laser Science Conference. CLEO/QELS 2009. Conference on; 07/2009
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    ABSTRACT: A C <sub>60</sub> -terminated self-assembled monolayer can be used to place molecular acceptor states at the interface between the semiconductor and gate insulator of an organic field effect transistor. The time dependence of the photoinduced charge transfer between pentacene and C <sub>60</sub> has a fast component with a characteristic time of 1.9 s and slower component with a time constant of 32 and 48 s at the beginning and end of a transient increase in illumination, respectively. Variation in the threshold voltage shift with the thickness of the pentacene results from the competing length scales for light absorption and exciton diffusion.
    Applied Physics Letters 03/2009; · 3.79 Impact Factor
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    ABSTRACT: Factors affecting the electric-field-induced poling of nonlinear optical chromophores in block copolymer domains were investigated by encapsulating the chromophores in a linear−diblock copolymer [poly(styrene-b-4-vinylpyridine)] and linear−dendritic (poly(methyl methacrylate)−dendron) block copolymer via hydrogen bonding. Temperature-dependent Fourier transform infrared spectroscopy and morphology evaluation by X-ray scattering and transmission electron microscopy were used with in situ second harmonic generation to correlate domain architectures, processing conditions such as thermal history, and chromophore concentrations with poling efficiency. Poling of chromophores encapsulated in the minority domain (spheres or cylinders) of a linear−diblock copolymer was inhibited by the increasing chromophore concentration within the domain and the chemical nature of the majority domain. Chromophore encapsulation in the majority domain produced the most favorable conditions for poling as measured by in situ second harmonic generation. Thermal annealing of the linear−diblock copolymer/chromophore composites resulted in chromophore aggregation with a corresponding decrease in nonlinear optical activity. The linear−dendron/chromophore system presented the most effective architecture for spatially dispersing chromophores. These findings suggest that while well-ordered phase-separated systems such as block copolymers enhance chromophore isolation over homopolymer systems, a more effective approach is to explore polymer chains end functionalized with chromophores.
    Macromolecules. 06/2008; 41(13).
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    ABSTRACT: A layer of functionalized disperse red 19 (DR19) chromophore molecules leads to novel reversible photoelectrical characteristics in organic thin film field effect transistor. Photoassisted poling, along with the change in the dipole of the DR19 under illumination, and photoinduced charge transfer to electron traps in the DR19 layer combine to change the threshold voltage of transistors by up to 100 V.
    Advanced Materials 01/2008; 20(21):4180-+. · 14.83 Impact Factor
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    ABSTRACT: The grafting of organophosphonic acids to the oxidized GaN(0 0 0 1) surface was investigated using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The stability of the monolayers was characterized by immersion in buffer solutions at pH 5, pH 7, and pH 9 for one week. The results demonstrate excellent stability under acidic and neutral conditions, but decreased stability under basic conditions. While photochemical grafting of alkenes directly to the unoxidized GaN surface appears to provide slightly better stability under basic conditions, the versatility of phosphonic acids makes this approach a potentially attractive alternative method for integrating molecular and/or biomolecular layers with GaN and other wide-bandgap semiconductors.
    Surface Science 01/2008; · 1.84 Impact Factor
  • Journal of Polymer Science Part A Polymer Chemistry 06/2007; 45(15):3166 - 3177. · 3.54 Impact Factor
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    ABSTRACT: [GRAPHICS] A C-60-terminated self assembled monolayer at the semiconductor/gate insulator interface of an organic thin film field effect transistor provides acceptor states optimized for photoinduced charge transfer. Charge densities of up to 10(13) charges cm(-2) can be trapped in the C-60 acceptors.
    Advanced Materials. 01/2007; 19(24):4353-+.
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    ABSTRACT: The accumulation layer of organic thin film field-effect transistors extends away from the gate insulator/semiconductor interface by only a few molecular layers into the semiconductor thin film. The structure of these first few layers are thus crucially important to the electrical properties of devices and to the design of other functional structures incorporating organic/inorganic interfaces. We have used scanning tunneling microscopy to image the vacancies and grain boundaries in pentacene thin films on Si (0 0 1) substrates chemically terminated by styrene. The styrene termination allows pentacene molecules to form in a crystal structure similar to that of pentacene layers on the insulating substrates. In addition, by incorporating specially designed molecular monolayers at the interface between the gate insulator and the pentacene thin film, it is possible to form novel photoinduced charge-transfer structures. Tuning the chemical properties of the self-assembled monolayers affords a new degree of control in tuning the properties of these devices. (C) 2007 Elsevier B.V. All rights reserved.
    Physica B Condensed Matter 01/2007; 401:686. · 1.33 Impact Factor