AVS2001 Session OF+TF-ThP: Aspects of Organic Films Poster Session
Thursday, November 1, 2001 5:30 PM in Room 134/135
Thursday Afternoon
Time Period ThP Sessions | Topic OF Sessions | Time Periods | Topics | AVS2001 Schedule
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OF+TF-ThP-2 Tuning Supramolecular Self-assembly of Trimesic Acid Molecules on Cu(100) by Copper Adatoms
A. Dmitriev, N. Lin, J. Weckesser (Max-Planck-Institut for Solid State Physics, Germany); J.V. Barth (EPFL, Switzerland); K. Kern (Max-Planck-Institut for Solid State Physics, Germany and EPFL, Switzerland) Recently, it was reported that one-dimensional supramolecular nanostructures can be realized via intermolecular hydrogen bonding.1 Here we demonstrate that, for the system trimesic acid (TMA) on Cu(100), one can tune the intermolecular hydrogen bonding by adjusting the density and mobility of Cu adatoms and hence fabricate various supramolecular nanostructures. In our experiments submonolayers of TMA on Cu(100) have been prepared by organic molecular beam epitaxy (OMBE) under ultrahigh vacuum and in situ characterized by scanning tunneling microscopy (STM). The TMA molecules are parallel to the surface (flat-lying) as deposited and form hydrogen-bonded chicken-wire networks via dimerization of their carboxylic acid groups.2 These networks are only stable at low-temperatures (< 250 K) and at room-temperature rapidly transform into a stripe-like structure, where the TMA molecules are perpendicular to the surface (up-standing). By means of CO predosing, Cu coevaporation or adjustment of the substrate temperature the Cu adatom density and mobility have been controlled, which allows to convert the hydrogen bonded carboxylic acid dimers into copper-carboxylate complexes.3 Sequences of STM images directly demonstrate the real time formation and dissociation of single chemical bonds in copper-coordinated supermolecules. Cu adatoms of high density and mobility finally leads to well-ordered two-dimensional supramolecular nanostructures of large domain size on the surface. |
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OF+TF-ThP-3 Current Response of Nickel Phthalocyanine Ozone Sensing Films Formed by Plasma-activated Evaporation
S. Takeda (Takushoku University, Japan) Nickel phthalocyanine(NiPc) sensing films for O3 gas have been fabricated by r.f.- plasma-activated evaporation and common vacuum deposition techniques. Sensor structure as a detecting element has a inter digital type Au or Pt electrode deposited on a glass substrate. Sensing NiPc films formed on the electrode are about 400 and 800 Å in thickness. Original value of electric resistance of the NiPc films between the electrodes is about 10-10-10-11 Ω , but exposing the sensing film to O3 gas it immediately changes down to 5-6 order of magnitude under the applied voltage d.c.1.5 V. Response time of the current corresponding to 63% of the saturated value (time constant 1 τ) is about 3.5 min. Due to more thinner NiPc films response time can be shorten and saturate more quickly. In this report we showed the data of only with 150 ppm but this kind of sensors can be detectable it more lower than 5 ppm. From the results of response characteristics, it suggests that absorbed O3 gases on the NiPc film diffuse inside the film, so that the response time of cause, depends on the film thickness and structure. Comparing the response times with 400 and 800 Å, thinner one is shorter several times than the other, and also the films prepared by plasma-activated evaporation respond quickly than that the films without plasma-activation. To clarify the reason of different responses, morphologic images of the NiPc films were observed by Atomic Force Microscope. Surface of the films formed by common vacuum deposition is likely needle crystals but the other films show very flatness. |
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OF+TF-ThP-4 Mobility, Binding Transition and Ordering of C60 on Pd(110): Investigations at the Local and Mesoscopic Scale
J. Weckesser (Max-Planck-Institut fuer Festkoerperforschung, Germany); C. Cepek (Laboratorio Nazionale TASC-INFM, Italy); R. Fasel (Swiss Federal Laboratories for Materials Testing and Research, Switzerland); J.V. Barth (Ecole Polytechnique Federale de Lausanne, Switzerland); T. Greber, J. Osterwalder (Universitaet Zuerich, Switzerland); K. Kern (Max-Planck-Institut fuer Festkoerperforschung, Germany) We present a comprehensive study of C60 on a Pd(110) surface using scanning tunneling microscopy (STM), low-eneregy electron diffraction (LEED), x-ray photoelectron spectroscopy and diffraction (XPS and XPD). The mobility and interactions of C60 on a Pd(110) surface have been characterized by variable temperature scanning tunneling microscopy. The motion of isolated C60 molecules was directly monitored and the corresponding tracer diffusion barrier was determined to (1.4±0.2) eV. Upon annealing to 700 K the C60 molecules undergoe an irreversible bonding transition resulting in a second, more strongly bound C60 species. This is associated with a local substrate reconstruction whereupon C60 molecules sink into the formed microscopic pits. The rearrangement of Pd substrate atoms turns out to play a crucial role similarly in the formation of C60 thin films. In STM three well-ordered structures consisting of alternating dark and bright molecular rows are found, in combination with LEED and XPS their real space structures are determined. The height difference in STM is attributed to a local reconstruction of the Pd substrate. Whereas the C60 molecules of the bright molecular rows are adsorbed in one layer deep microscopic pits, C60 accommodated in two layer deep pits account for the dark molecular rows. XPD results complete the understanding of the film structure and reveal the orientation of the C60 cages. |
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OF+TF-ThP-7 In-situ Synthesis of Self-assembled Polyaniline/ Poly(styrene sulfonic acid) Composite Ultra Thin Films
T. Koga, H. Otsuka, A. Takahara (Kyushu University, Japan) In-situ preparation method of self-assembled polyaniline (PANI)/ poly(styrene sulfonic acid) (PSS) composite ultra thin films on aminosilane grafted surfaces was proposed. PSS was adsorbed onto the aminosilane grafted Si-wafers, then PANI/ PSS composite ultra thin films were prepared in-situ by template polymerization utilizing ionic interaction between PSS and aniline. PANI/ PSS composite ultra thin films were characterized by time of flight secondary ion mass spectroscopy (TOF-SIMS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible-near infrared (UV-Vis-near IR) spectroscopy. Composition of PANI/ PSS composite ultra thin films estimated by XPS gave good agreement with the calculated value based on the chemical structures. AFM images and UV-Vis-near IR spectra revealed that the progress of conjugation of PANI/ PSS composite ultra thin films. From these results, it can be inferred that the formation of ultra thin films is closely related to the electrostatic character of functionalized substrate surfaces. In this method, PSS plays an important role as a template for polymerization of aniline. Therefore, the positively charged surfaces facilitated the formation. |
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OF+TF-ThP-8 Growth and Nucleation of 2-methyl-4-nitroaniline (MNA)
K. Nam, K.I. Seo, L. Wang, C.E. Bonner (Norfolk State University); E.S. Gillman (Jefferson Lab) Despite the intense interest in the microstructure of MNA over the last decade, a detailed understanding of its structure with respect to growth conditions is still not clear. A relationship between structure and growth of thin films is often sought, however in many instances these relations are vague and complex. One reason is that thin film properties generally vary by orders of magnitudes depending upon preparation conditions. Thin films are, in general, not ideal materials systems in terms of bond distortion, coordination, point defects, dislocations, compositional inhomogeneities and impurities, grain boundaries and disordered low-density void regions. These defects are created by a complex array of parameters that are directly related to the surface interface. Thus, it is expected that any quantitative description of preparation-property relations must first start with a quantitative description of its internal and external morphology. Here we present results for MNA thin films that are grown using an organic molecular beam deposition (OMBD) technique. |
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OF+TF-ThP-9 The Dielectric Properties of P(VDF-TrFE) Copolymer Thin Films by Physical Vapor Deposition
G.B. Park (Yuhan College, Korea); M.Y. Chung, J.H. Yoon (Inha University, Korea); S.H. Park (Unix Electric Co. Ltd, Korea); S.H. Lee (Korea Institute of Industrial Technology, Korea); D.C. Lee (Inha University, Korea) The copolymer thin films of poly(vinylidene fluoride-trifluoroethylene), which shows excellent piezoelectricity and pyroelectricity, were fabricated by physical vapor deposition method in the compositions 70/30 and 80/20 mol%. The effect of the substrate temperature and the mol% rates on the molecular structure, the dielectric and piezoelectric properties was investigated. In deposition process, the substrate temperature(Ts) was varied from 30°C to the Curie transition temperature(Tc). From the results of deposition rate, molecular and crystal structure of the films, it was found that 300°C was the optimal evaporation temperature in the process. The β-form characteristic peaks increased with increasing the substrate temperature on account of the increase of the crystallinity, but over 90°C the peaks decreased according to the phase transition from the ferroelectric crystal phase to the paraelectric crystal phase. From this results, the copolymer films were fabricated from 30°C to 90°C substrate temperature. The relative dielectric constant decreased as a form of anomalous dispersion with increasing measurement frequency. With increasing Ts, the relative dielectric constant increased from 3.643 to 5.29 on the 70/30mol% films, and from 4.367 to 10.77 on the 80/20mol% films, respectively. The α relaxation by interfacial polarization and the β relaxation by dipole orientation polarization were observed in low and high frequency region, respectively. The α relaxation decreased and the β relaxation increased with increasing Ts. From the results, it is suggested that the crystallinity of the films increased with increasing Ts. The effect of increasing Ts on the piezoelectric properties will also be discussed in detail. |
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OF+TF-ThP-11 Optical Characterization of ZnO-CdO Thin Films Grown by Sol-Gel Method
T. Hata, H. Komaki (Miyazaki University, Japan) Wide bandgap oxide-semiconductors have attracted much attention for liquid crystal displays and solar cells. Recently, ZnO based materials are much respected for UV light-emitting devices. ZnO is a semiconductor with a large bandgap of 3.4 eV and a large excitonic binding energy of 60 meV. CdO is an also semiconductor with a direct bandgap of 2.3 eV and small indirect bandgap of 0.8 - 1.1 eV. The bandgap energy can be changed from 2.3 to 3.4 eV on ZnO and CdO mixed crystal. Moreover, although CdO is known to have a poor optical transmittance in the visible spectral region in comparison with those of ZnO films, CdO thin films are enough to be used for the window material for solar cells. Pure ZnO and CdO films have been studied many research groups. However, ZnCdO films have been few studied previously.In this work, ZnO-CdO thin films were grown by sol-gel dip coat method on glass and Silicon (Si) substrates at 100 ~ 600 °C under air atmosphere. The sol-gel technique is known to have the distinct advantages of process simplicity, lower cost and ease of composition control. Precursor solutions of ZnO-CdO are prepared by dissolving both 5 wt.% zinc acetate dihydrate and 5 wt.% cadmium acetate dihydrate into anhydrous ethanol for the solutions to have the desired Cd/Zn at.%. Poly-ZnO and -CdO thin films are obtained on the glass substrate more than 500 ¡C and 200 °C, respectively. A value of full width at half maximum of (100) peak at the XRD spectra become small with the increasing the growth temperature. By using the Si substrate (100) instead of the glass substrate, poly-ZnO and -CdO thin films have (100) orientation. Moreover, an optical transmittance and bandgap energy of the ZnXCd1-XO thin films decrease nonlinearly with the decreasing X values. |
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OF+TF-ThP-12 XPS Study of Conducting Polymer Film Growth on Si(111) by Electrochemical Deposition Method
H. Kato, S. Takemura (Kanto Gakuin University, Japan) Electochemically deposited conducting polymer polythiophene (PT) films on Si(111) were investigated by XPS. PT/Si(111) interface at the initial stage of electrochemical polymer growth was closely investigated by analyzing the core-level energies and spect ral profiles of the atomic components. Spectral profiles of Si core-level spectra showed that both Si 2s and 2p spectra were basically composed of different three Gaussian components correspond to different valence states of Si in contrast to the core-lev el spectra of non-deposited Si(111). The lower peaks (LS1 and LS2) observed in Si 2p spectra of PT/Si correspond to the Si states with strong interaction between Si and PT. The peak height of LS1 slightly increases and LS2 drastically grows in the case of PT polymer growth on Si substrate. The C 1s core-level spectrum was composed of a higher energy component and a lower energy component originated from the polymer backbone and oxidized Si layers, respectively. Affinity between a deposited polymer PT film and Si substrate was strong compared with the case of a PT film deposited on ITO substrate expecting bondings between polymer chains and Si substrate layers. |
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OF+TF-ThP-13 XPS Studies of Conducting Polymer Hybrid Films Incorporated with Dye Molecules
H. Kato, S. Takemura, N. Aragaki (Kanto Gakuin University, Japan) Conducting polymer polythiophene (PT) films incorporated with dye molecule safranine T (ST) prepared by electrochemical doping and diffuse injection methods were investigated by XPS. Polymeric structure, charge transfer and interaction between dye molecul e and PT backbone in the hybrid films were closely investigated by analyzing the core-level energies and spectral profiles of the atomic components. XPS core-level analysis of N 1s showed that dye molecules were injected into polymer matrix in hybrid fil ms. In the case of PT film incorporated with ST, drastic change of C 1s, S 2p, and Cl 2p core-level spectra between electrochemically as-grown and reduced films was observed. In the case of electrochemically reduced sample core-level energies of C1s and S 2p were shifted to the lower energy side indicating the creation of n-type polymer chain while in the case of electrochemically as-grown sample the polymer chain is p-type. The splitting of S 2p reflects the strong interaction betwen ST molecule and PT backbone. The C 1s core-level spectrum varied with the degree of electrochemical reduction. The C 1s spectrum is basically composed of two different Gaussian components. In the highly reduced case lower energy component grows while in the low reduced case higher energy component becomes dominant. The spectral change reflects the existence of different charged states in polymer backbone. |
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OF+TF-ThP-14 Surface Freezing and Self-ordering Phenomena of Long Chain n-Alkanes Studied by Newly Developed High Vacuum DTA Apparatus
Y. Ouchi (Nagoya University, Japan); Y. Yamamoto (Nagoya Institute of Technology, Japan); M. Kuroi, N. Yamaguchi, H. Ishii, K. Seki (Nagoya University, Japan) We developed a new apparatus of differential thermal analysis (DTA) capable of simultaneous measurement of UPS, XPS, NEXAFS etc to investigate phase transitions of ultra-thin films of organic molecular systems. The apparatus is installed in a high vacuum chamber for thermal isolation and measurements of photoelectron emission. The performance of the apparatus was examined using a 650 Å-thick pentacontane (n-C50H102) film, which may exhibit a unique monolayer phase transition at air (vacuum)/liquid interface, known as a surface freezing self-organization effect. We observed two anomalies of DTA curve around the bulk melting temperature, one of which is apparently due to thebulk melting. Since the temperature dependence of the surface specific UV photoemission measurements showed corresponding changes in photoemission current, we could conclude that the other phase transition peak originates from the formation of frozen (crystallized) single molecular layer located at the top alkane liquid surface. We will report the detailed analysis on the transition entropy and the chain length dependence of the temperature region of the existence of this unique surface freezing layer up to n=72. Though it was believed from grazing X-ray reflection measurements that the effect will disappear around n=60, our DTA system could confirm their existence over n=60. This demonstrates that our DTA apparatus is sufficiently sensitive to examine the 2D phase transitions of monolayers. Simultaneous measurement with other photoelectron emission measurement will be beneficial for the investigation of temperature dependence of organic thin films and its self-organization process. |
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OF+TF-ThP-15 Comparison of Alkyl-phosphonic Acid (APA) and Alkyl-carboxylic Acid (ACA) Self Assembled Monolayers on the Hydrated Alumina Surface of Aluminium
T.A. Lewington, G.E. Thompson, M.R. Alexander (UMIST, UK); E. McAlpine (Alcan International, UK) The interaction of organics with metal oxide surfaces is important in many areas including corrosion protection and adhesion promotion. Good corrosion protection has been obtained using APA SAMs deposited on Al prior to painting.1 The phosphonic acid head-group interacts with the surface of alumina to form a phosphonate bond.2 It is presumed that resin components react with the phosphonic acid tail-group when di-functional molecules are used. Detailed characterisation of ACAs on Al has been reported but not of APAs.3 The formation of alkyl-thiol SAMs on Au has been extensively studied.4 In contrast to the stable Au metal surface Al forms an oxide, the outer region of which is readily hydrated on exposure to ambient conditions. Two approaches have been employed to obtain reproducible Al surface chemistry for self-assembly: solution pre-treatment or atmospheric exposure. Here, SAMs have been formed on sputtered Al by immersion in 5 mM ACA and APA solutions in n-hexadecane and ethanol respectively. Surface pre-treatment with acetic or methyl-phosphonic acid was compared with 24 h atmospheric exposure. Both XPS and contact angle measurement of SAMs on Al indicate relationships between assembly, time and alkyl chain length similar to alkyl-thiols on Au. Notable differences include changes in the oxide surface, found by XPS to occur in ethanoic solutions, and the importance of pre-treatment. FTIR provides information on the orientation of the SAM and the bonding to the surface. This enables a model of the APA-Al system to be proposed, which may be compared with existing models of ACA-Al. The electrochemical properties of uncoated SAMs are investigated using cyclic voltammetry and rationalised in terms of the models. |
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OF+TF-ThP-17 Giant Polarization in Organic Heterostructures
T.U. Kampen, I. Thurzo, D.R.T. Zahn (TU Chemnitz, Germany) Deep levels or polarization effects can influence the charge transport through organic layers. Both effects can be identified by charge deep-level transient spectroscopy (Q-DLTS). Here, deep levels or polarization effects cause the charge transient signal Q(t) to be dependent or independent on the bias voltage, respectively. Q-DLTS, accompanied by feedback charge capacitance (FCM) measurements, has been used in the present work to investigate an organic heterostructure grown on an inorganic semiconductor. GaAs(100) substrates (n = 0.3 4x1018 cm-3) were sulfur passivated by wet chemical etching and additional annealing under ultra-high vacuum (UHV) conditions. Organic molecular beam deposition was used for the growth of 20 nm of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA, Lancaster) and 27 nm of tris-(8-hydroxyquinoline) aluminum (Alq3, Syntec). Silver was evaporated on the Alq3 film through a shadow mask resulting in an array of circular contacts with an area of A = 2.1x10-7 m2. The back contact to the GaAs(100) was achieved by an In-Ga-alloy resulting in a series resistance of less than 20Ω. The electrical characterisations were done at room temperature and in situ in the UHV system. The Q-DLTS measurements show a well resolved maximum in Q(t). The amplitude remains almost constant as a function of the bias voltage, which is a clear indication of a polarization in the organic heterostructure. From the experimental results the permittivity dispersion is determined to 2.37. The permittivity dispersion is independently obtained from FCM measurements. With the experimentally determined excess capacitance ΔC of 90 pF and the thickness of the organic heterostructure the permittivity dispersion is determined to 2.42. The FCM scans show no hysteresis due to the absence of deep levels. The ΔC/C = 1 presented here clearly indicates the presence of a giant polarization. |