AVS1996 Session NS-TuP: Properties of Nanostructures Poster Session
Tuesday, October 15, 1996 6:30 PM in Ballroom A
Tuesday Evening
Time Period TuP Sessions | Topic NS Sessions | Time Periods | Topics | AVS1996 Schedule
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NS-TuP-1 Influence of Electrostatic Forces on the Investigation of Dopant Atoms in Layered Semiconductors by Scanning Tunneling Microscopy/Spectroscopy and Atomic Force Microscopy
R. Schlaf, D. Louder, M. Nelson, B. Parkinson (Colorado State University) Investigation of the atomic scale topography and electronic structure of dopant sites in semiconductor materials is a promising application of scanning probe microscopies. Due to the shrinking dimensions of electronic circuits the determination of local dopant densities and their influence on the local electronic properties is becoming more and more important. Dopants have been routinely imaged with STM on and near the surface of conventional semiconductor materials as well as on layered compounds. On both kinds of materials dopants are detected as either protrusions or depressions. The comparison of the measured heights between the materials shows that the values on layered materials are considerably larger than those on the conventional (3D) semiconductors. We interpret this as the influence of dopant induced electrostatic forces between tip and sample leading to a structural deformation of the surface around dopant atoms. In order to investigate the influence of electrostatic forces we performed STM measurements on p-type MoS2 at different bias voltages. The bias dependence of the images indicates the presence of electrostatic forces and demonstrates the influence of screening due to the surrounding electron density. Additional measurements with Current Imaging Tunneling Spectroscopy (CITS) show that changes in the density of states at dopant sites plays only a minor role and cannot account for the large protrusions observed. Atomic Force Microscopy (AFM) measurements, with an applied DC voltage between cantilever and sample, also confirm the role of electrostatic forces since voltage dependent changes in the topograpy were observed. |
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NS-TuP-2 STM Study of Defects and Multiple Layer Formation Upon the Addition of Squaraine Dye to Liquid Crystals
D. Sampson, B. Parkinson (Colorado State University) Scanning tunneling microscopy has been used to image surface and near surface phases formed on several layered substrates from bulk liquid crystals (LCs) with the addition of a squaraine dye (bis (4-(N-ethyl-N-octadecylamino)-2-hydroxyphenyl) squaraine (2,18-SQ)) to the bulk LC phase. Addition of small amounts (0.05 - 5%) of 2,18-SQ in the bulk phase produced surface phases different from those observed from the bulk LC alone. Samples studied included various cyano biphenyls and cyano benzoate LCs. The surface phases produced were independent of the substrate used (HOPG, MoS/sub 2/ and WSe/sub 2/). Multilayer formation is explained by the long alkyls tails of the 2,18-SQ 'crosslinking' the stacked layers and allowing for STM imaging of smectic LC domains several molecular layers above the surface. Molecular sized defects found in these layers are unique to thi system and are believed to be from the incorporation of 2,18-SQ into the LC lattice. |
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NS-TuP-3 Investigating the Surface Structure of Conjugated Polymer Layers by Scanning Tunneling Microscopy in View of Application in a Polymeric Light-emitting Diode
H. Razafitrimo (University of Rochester); B. Hsieh (Xerox Wilson Center for Research & Technology); Y. Gao (University of Rochester) Scanning tunneling microscopy has been used to examine the surfaces of poly (p-phenylene vinylene) oligomers and derivatives. The surface of the indium-tin-oxide (ITO) substrate, on which the organic layers are deposited, and that of the Ag electrode, which was evaporated on to the organic layer, were also examined in order to obtain the profile of the ITO/organic layer/Ag structure which constitutes a light-emitting diode. Every organic layer smears out completely the granular structure of ITO: 50 \Ao\ of 1,4-bis[4- (3,5-di-tert-butyl-styryl)styryl]benzene deposited by vacuum evaporation on to ITO forms a continuous film made of well- rounded grains; poly(2-methoxy-5-(2U-ethyl)hexyloxy-p-phenylene vinylene) drop cast on to ITO forms less than 250 \Ao\ thick film and presents a smooth granular structure; poly(2,3-diphenyl phenylene vinylene) (DP-PPV) spin cast onto ITO forms 450-500 \Ao\ thick film that presents one-dimensional structures but does not exhibit any features at atomic scale. The flatness of the organic layers contrasts with the roughness of the substrate and the top Ag layer. The 100 \Ao\ thick Ag layer that was evaporated on to the DP-PPV film exhibits large coalescing clusters. Every surface was characterized and can be differentiated by its height difference correlation function. Work supported in part by NSF under grant DMR-9303019. |
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NS-TuP-4 Template Directed Growth of Polymer Nanostructures on Organothiol Self-assembled Mixed Monolayers
C. Shannon, W. Hayes, X. Yue (Auburn University) We report the template-directed growth of polyaniline nanostructures at Au electrodes modified with two-component self-assembled monolayers consisting of 4-aminothiophenol (4-ATP) and n-octadecanethiol (ODT). The 4-ATP template molecules, which are oxidized to the cation radical at potentials 260 mV less oxidizing than that required to oxidize aniline itself, serve as nucleation sites for the growth of polyaniline. We find that if a 4-ATP/ODT mixed monolayer is oxidized in the presence of aniline, nanometer scale polyaniline features are formed. The size and distribution of these features were characterized by AFM and can be controlled through a combination of monolayer composition and the concentration of aniline in solution. The use of this approach to visualize monolayer structure will also be discussed. |
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NS-TuP-5 Local Probe Microscopy of Liposomes and Proteoliposomes
M. Gallyamov (Moscow State University, Russia); Y. Ivanov (Institute of Biomedical Chemistry, Russia); O. Kiselyova (Moscow State University, Russia); V. Uvarov (Institute of Biomedical Chemistry, Russia); I. Yaminsky (Moscow State University, Russia) Liposomes of dimiristoilphosphatidylcholine with incorporated molecules of heme-containing protein cytochrome P were studied on solid substrate by means of scanning tunneling and atomic force microscopy (STM and AFM). Images of liposomes prepared by two different chemical methods were obtained on metalic films, grafite, mica. In the spectroscopy mode dI/dU(x,y) at positive tip/sample polarity liposomes on grafite were observed as depletions and at the negative one - as hills. The possibility to identify separate molecules of cytocrome incorporated into the liposome bilayer membrane is demonstrated. At the negative tip polarity protein molecules are observed as protrusions on the liposome surface. Under positive tip polarity depletions of the same diameter are observed in the same positions. During the successive voltage inversions the images are completly restored. The proposed theoretical model of the observed STM image contrast is discussed. |
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NS-TuP-6 Ex Situ STM Study of Surfactant-induced DNA Condensation in Aqueous and Organic Media
M. Gallyamov, V. Kabanov, O. Pyshkina, V. Sergeyev, A. Zezin, I. Yaminsky (Moscow State University, Russia) DNA is typically tightly packaged in many biological systems. Model systems that can produce DNA condensation in solution are of great interest for comprehension of DNA behavior inside cells and viruses. We study DNA (from chicken erythrocytes) interaction with lipid-like surfactants (distearoyldimethylammonium cloride, cetyltrimethylammonium bromide) in water solution and in chlorophorm and the formation of toroid-like structures. The condensed form of DNA was immobilized on the graphite surface by physical adsorption. An inert graphite surface was chosen to minimize the substrate effect on the topography of DNA-surfactant complex. The average DNA toroid dimensions were found to be about 200 nm and dependent on the type of surfactant. We estimated that the observed toroid particles in aqueous solution consist of 5-15 DNA molecules and in chloroform - only of 1 DNA molecule. So, we assume that the aggregation of DNA-surfactant complex is stabilized by hydrophobic interaction of the alkyl radicals of surfactants, which don't exist in organic media. The observed STM data coincide with the fluorescent microscopy observations [1]. The results on surfactant-induced DNA condensation are of practical importance for modeling of DNA transport through cells membranes. [1] S.Mel'nikov, V. Sergeyev, K. Yoshikawa, J.Amer.Chem.Soc., 117, 9951 (1995). |
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NS-TuP-7 Raman and Photoluminescence Studies of Chemically Deposited CdSe Thin Films
B. Rai, R. Katiyar (University of Puerto Rico); M. Nair, P. Nair (Universidad Nacional Autonoma de Mexico) We have performed Raman and Photoluminescence measurements on as-prepared and thermally annealed chemically deposited CdSe thin films using 488.0 and 514.5 nm laser lines. With the 514.5 nm Ar\super +\ laser radiation, the Raman spectra of the as-prepared CdSe thin films with different deposition times show resonance enhancement of the LO mode at 205 cm\super -1\, with the first and second overtones of LO modes clearly observed. However, the Raman spectra of the thermally annealed CdSe films at 400 \super 0\C for 1 hr in air, are of very weak intensities and they don't show any resonance enhancement due to the red sh ift and the sharpening of the luminescence band upon annealing. The band at 170 cm\super -1\ has been assigned to the surface optic (SO) modes. In addition, appearance of two wide bands with higher laser powers at 580 and 1100 cm\super -1\ has been attributed to [SS]\super -\ and [SO] vibrational modes, owing to the small amount of sulfur present in the film. The x-ray results suggest the as-prepared samples to be of lower crystalline quality, while the annealed samples tend to assume the hex agonal structure of CdSe. The luminescence spectra of the as- prepared samples are weak and broad, but these increase in intensity and sharpen after annealing. With the higher laser powers, the luminescence spectra of annealed samples show a red shift from a peak position of 1.71 eV at 2 mW laser power and a decrease in the intensity. This phenomenon has been attributed to the quantum confinement of CdSe nano-clusters deposited on the glass plate. Due to the nano-size of the CdSe clusters, the absorption probability becomes very large and in principle it is possible to observe the "saturation effect" in optical transitions with higher laser powers, at which photons of the same wavelength can no longer be absorbed. However, we did not observe any saturation effect in optical transition with in our laser power range of 2- 12 mW. Acknowledgment: This work was supported by EPSCOR-NSF and NASA grant NCCW-0088 |
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NS-TuP-8 Synthesis and Characterization of Nanocrystalline Bi\sub 4\Ti\sub 3\O\sub 12\
J. Meng, R. Katiyar (University of Puerto Rico) Bismuth titanate, Bi\sub 4\Ti\sub 3\O\sub 12\(BTO), is a typical ferroelectric material with a layered perovskite structure and shows excellent electro-optic properties for optical memory and electro-optic devices. In this paper, nanocrystallites BTO with several grain sizes have been prepared by sol-gel method, using bismuth nitrate Bi(NO\sub 3\) \sub 4\5H\sub 2\O and Ti(SO\sub 4\)\sub 2\ as starting materials. In the process of synthesis, we found that the crystallites of BTO depend on the temperature and duration of heating as well as the PEG contents. The BTO with smallest grain size, whose structure was confirmed to be in pseudo- tetragonal phase by powder X-ray diffraction, is about 26 nm. The Raman spectra at room temperature indicated that the lowest frequency phonon mode of BTO(soft mode) decreases in frequency as the grain size is reduced and the phase transition temperature moves towards a higher temperature. This is different from that of PbTiO\sub 3\, where the phase transition temperature decreases with reducing grain size. We also discuss the effects of Bi ions with dangling bonds on Raman phonon modes of BTO as well as its structure. Acknowledgment: this work was supported by NASA-NCCW-0088, DE- FG02-94ER75764, and NSF-OSR-9452893 Grants. |
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NS-TuP-9 Tin Implanted in Fused Silica: Optical Response from the Far-infrared to the Ultraviolet
D. Henderson, A. Ueda, R. Mu, Y. Tung (Fisk University); C. White, J. Zhu (Oak Ridge National Laboratory) Metal nanocrystals have drawn much attention recently for reasons ranging from their unique linear and nonlinear optical properties to serving as catalytic agents in chemical reactions. Several fabrication techniques have been developed for synthesizing these nanocrystals, but they are limited to some extent in a way that restricts the size, size dispersion, and very importantly the nature of its surrounding environment, the host. Ion implantation is technique that circumvents many of these limitations allowing for one to select a host and implant the ion of choice for a particular investigation or application. To this end, we have applied ion implantation to form tin nanocrystals in optical grade fused silica. The ion beam damage to the host was revealed in the infrared reflectance measurements showing an intensity decrease in the longitudinal and transverse optical modes of silica located between 1350 and 1050 cm\super -1\ and the appearance of a new peak near 1050 cm\super -1\. Annealing the implanted substrates at temperatures of 900 C healed the resulted in a disappearance of the 1050 cm\super -1\ together with a concomitant increase in the LO and TO intensities. These experiments indicate that the 1050 cm\super -1\ peak is due to an Si-O dangling bond. The UV-Vis spectra show after annealing the samples a peak near 300 nm and it is tentatively assigned to the surface plasmon absorption of tin nanocrystals. |
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NS-TuP-10 Impedance Spectroscopy of Nanosized Tetragonal Zirconia
Z. Zhou, C. Slough, R. Soltis, E. Logothetis (Ford Motor Company); S. Ferrell, M. Mayo (Pennsylvania State University) Nanosized tetragonal zirconia samples were prepared by chemical co-= precipitation and sintered at 1050 C for 3-5 hours in air. The crystalli= te sizes of the samples (as determined by x-ray diffraction and atomic fo= rce microscopy) are in the range of 55 - 110 nm. The sample densities ar= e in the range of 90 - 99 % of theoretical density. Impedance spectrosco= py was performed on the zirconia samples in the temperature range from 20= 0 - 700 C. Analysis of the impedance spectroscopy data shows that the ac= tivation energies for intragrain and grain boundary conduction are 0.900 = \+-\ 0.005 eV and 1.15 \+-\ 0.02 eV, respectively. The activation energie= s for the oxygen vacancy conduction in the grains and in grain boundaries= are independent of crystallite size over this range. The results for th= e activation energies are consistent with the published data of 0.92\+-\0= =2E02 eV for intragrain resistivity and 1.08\+-\0.02 eV for grain boundar= y resistivity in micron zirconia ceramics (by Badwal et al.), and 0.92\+-= \0.01 eV for single crystals (by N. Bonanos et al.). The impedance spect= roscopy study is being extended to samples with crystallite sizes of less= than 55 nm. |
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NS-TuP-11 Onset of Nanoscale Wear on Polystyrene
D. Woodland, W. Unertl (University of Maine) We describe the onset of nanoscale wear on polystyrene (PS) surfaces studied using an atomic force microscope (AFM). 0.5 \mu\m thick PS films (molecular weights 24k and 210k) were cast from toluene solution and dried in vacuum. Some samples were also heated to about 25 K above the glass transition temperature. The initial roughness was \<=\ 0.8 nm rms, including a few small pits. Wear was produced by raster scanning a Si\sub 3\N\sub 4\ AFM tip under applied loads of 10-210 nN at speeds in the range 80-320 \mu\m/s). Unheated samples develop characteristic parallel ridges oriented perpendicular to the scan direction. For 24k PS, the roughness of the abraded areas increases with the number of abrasion cycles and, at constant load, the ridge spacing is proportional to scan speed. Small pits (\<\ 50 nm wide and 10 nm deep) appear to have no effect on the abrasion process. 210k PS is substantially more resistant to wear and has smaller ridge spacing. Heated samples of both molecular weights show substantially enhanced resistance to the formation of wear. The qualitative features of the observed wear behavior are consistent with continuum mechanics analysis using bulk mechanical properties. |
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NS-TuP-12 Unusual Magnetic Behavior in Sputtered FeO and \alpha\ - Fe\sub 2\O\sub 3\
D. Dimitrov, G. Hadjipanayis (University of Delaware); V. Papaefthymiou (University of Ioannina, Greece) Thin FeO and FeO and \alpha\-Fe\sub 2\O\sub 3\ were prepared by reactive dc magnetron sputtering in a mixture of O\sub 2\ and Ar gases. The samples were characterized by Transmission electron microscopy (TEM), Selected area diffraction (SAD), Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD), Mossbauer spectroscopy and SQUID magnetometry. All films were polycrystalline with random orientation of the crystallographic axes. Ferromagnetic-like behavior with large coercivities at low temperature and significant saturation magnetization was observed in both films. The observation is in contrast to the antiferromagnetic behavior of bulk FeO and \alpha\-Fe\sub 2\O\sub 3\. At room temperature the coercivity of FeO films is negligible and the samples are superparamagnetic, but below 150 K the coercivity increases rapidly. The saturation magnetization of the films is large and thickness dependent. Mossbauer spectra revealed that the FeO films were offstoichiometric ( Fe\sub x\O ) with x about 0.9, containing clusters of Fe\super 3+\ ions in an environment similar to that of Fe\sub 3\O\sub 4\. The Mossbauer spectra of \alpha\-Fe\sub 2\O\sub 3\ could be analyzed in terms of two components; one with parameters close to that of bulk \alpha\-Fe\sub 2\O\sub 3\ and the other one, with much lower hyperfine field, identified as surface \alpha\-Fe\sub 2\O\sub 3\. The relative amount of the two components is in agreement with previous studies on ultrafine \alpha\-Fe\sub 2\O\sub 3\ particles. The anomalous hysteresis behavior is believed to be due to the Fe\super 3+\ in Fe\sub x\O films, and to uncompensated spins in thesurface component of \alpha\-Fe\sub 2\O\sub 3\ films. Large shifts in the hysteresis loop in field cooled samples indicate strong exchange coupling between the ferromagnetic regions with the antiferromagnetic matrix. Acknowledgments: Work supported by NSF DMR - 9307676 |
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NS-TuP-13 Enhanced Magneto-optical Properties and Low-temperature Properties of Co-Pt Alloy Film
G. Chang, C. Whang (Yonsei University, Korea); K. Kim, Y. Lee (Sunmoon University, Korea) The research on Co-Pt alloy films as a magneto-optical (MO) recording medium attracts attention recently because of their excellent MO Kerr effect at shorter wavelength and easier fabrication than Co/Pt multilayered (ML) films. The properties of CoPt and CoPt\sub3\ films prepared by ultrahigh-vacuum cosputtering were investigated at both room temperature (RT) and low temperatures (LT), and also correlated with the structures and stoichiometry measured by x-ray diffraction, atomic and magnetic force microscopies, and photoemission spectroscopy. The Kerr rotation angles (\theta\\sub k\) at RT of the CoPt\sub3\ films were increased from 0.214 degree at a photon energy of 1.96 eV to 0.338 degree at 2.71 eV, and these values for \theta\\sub k\ are all larger than those of Pt(10\Ao\) / Co(3\Ao\) ML film in the photon energy range. On the other hand, the measurements of \theta\\sub k\ at LT showed a larger increase rate with respect to photon energy at lower temperature, and \theta\\sub k\ at 2.71 eV turned out to be 0.361 degree at 200 K, 0.385 degree at 77 K, and 0.426 degree at 5 K. The detailed discussion will be presented, together with other measurement and analysis results. |
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NS-TuP-14 Structure, Resistivity and Soft Magnetic Property of RF Diode and DC Magnetron Sputtered NiFe Films
L. Xie, V. Versteeg, A. Paranjpe (CVC Products, Inc.) Permalloy (NiFe) thin films were deposited using both rf diode and high flux cathode dc magnetron sputtering systems. Magnetic properties and crystal structures were investigated and compared using a BH looper and an x-ray diffractometer. Compared to conventional dc magnetron sputtering, the cathode magnetic field for high flux cathode penetrates much thicker magnetic material targets. This provides a deposition rate as high as 160 nm/min., which is 6~7 times that using rf diode sputtering.A sample stage equipped with an electric resistance heater and an electromagnet was used to control sample temperature and orienting magnetic field. Magnetic anisotropy of the films was studied as a function of substrate temperature and orienting magnetic field. The resistivity of permalloy is sensitive to contamination. The influence of Ar/N\sub 2\ and Ar/O\sub 2\ sputtering gases on resistivity of the films was explored. AES, XPS and RBS were used for the film chemical composition analysis.Smooth, uniform films with easy axis coercivity < 1.5 Oe, hard axis coercivity < 0.1 Oe, low skew < 2deg, resistivity < 24 \mu\\Omega\-cm at 300 \Ao\ and MR coefficient > 2.2% were obtained using both rf diode and high flux cathode dc magnetron sputtering systems. |
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NS-TuP-15 State-Resolved and Circular Dichroic Photoelectron Diffraction on O/W(110) (1x1)-(1x12)
R. Ynzunza (Lawrence Berkeley National Laboratory); F. Palomares (ICMM-CSIC, Spain); E. Tober, Z. Wang (Lawrence Berkeley National Laboratory); H. Daimon (Osaka University, Japan); C. Fadley (Lawrence Berkeley National Laboratory) We have measured full-hemisphere interface-resolved W 4f photoelectron diffraction from clean W(110) and (1x1)-(1x12) O/W(110). By comparing experimental data with multiple scattering calculations, various structural conclusions can be drawn, such as the degree of surface relaxation for the clean tungsten surface layer and the location of the overlayer oxygen atoms on the oxide covered tungsten. In addition, we measured the circular dichroism in the W 4f photoelectron diffraction with the oxide overlayer. The diffraction pattern is found to show both peak "rotations" (as seen and explained previously by Daimon et al. for Si(001)) and other distortions of features; all of these effects are well predicted by multiple scattering diffraction calculations. Implications for the study of magnetic surfaces and interfaces will be discussed. \super **\ Work supported by ONR (Contracts N00014-90-5-1457 and N00014-94-1-0162) and DOE,BES, Mat. Sci. Div. (Contract DOE-AC03- 76SF00098). |