Samorí , Paolo : Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics
Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics
Dissertation

zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Chemie

eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
der Humboldt-Universität zu Berlin

von Paolo Samorí ,
geb. 3.5.1971 in Imola (Bologna)

Präsident der Humboldt-Universität zu Berlin
Prof. Dr. Dr. h.c. H. Meyer

Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I
Prof. Dr. B. Ronacher

Gutachter:
1. Prof. Dr. Frans C. De Schryver
2. Prof. Dr. Jürgen P. Rabe
3. Prof. Dr. Klaus Rademann

Eingereicht am: 14.07.2000

Tag der mündlichen Prüfung: 24.10.2000

Abstract

In this thesis the self-assembly of pi-conjugated (macro)molecular architectures, either through chemisorption or via physisorption, into highly ordered supramolecular nanoscopic and microscopic structures has been studied. On solid substrates structure and dynamics has been investigated on the molecular scale making use primarily of Scanning Probe Microscopies, in particular Scanning Tunneling Microscopy and Scanning Force Microscopy. This allowed to characterize a variety of phenomena occurring both at the solid-liquid interface, such as the dynamics of the single molecular nanorods (known as Ostwald ripening), the fractionation of a solution of rigid-rod polymers upon physisorption on graphite; and in dry films, i.e. the self-assembly of rigid-rod polymers into nanoribbons with molecular cross sections which can be epitaxially oriented at surfaces and the formation ordered layered architectures of disc-like molecules. In addition the electronic properties of the investigated moieties have been studied by means of Photoelectron Spectroscopies. The nanostructures that have been developed are not only of interest for nanoconstructions on solid surfaces, but also exhibit properties that render them candidates for applications in the field of molecular electronics, in particular for building molecular nanowire devices.

Keywords:
Conjugated Molecules, Molecular Electronics, Scanning Probe, Microscopy, Self-assembly

Abstrakt

In dieser Dissertation wird die Selbstorganisation von pi-konjugierten (makro)molekularen Architekturen durch Chemisorption oder Physisorption in hochgeordnete supramolekulare nanoskopische und mikroskopische Strukturen auf festen Trägern untersucht.

Ihre Struktur und Dynamik wurden auf molekularer Skala hauptsächlich mit Rastersondenmikroskopien, insbesondere mit Rastertunnel- und Rasterkraftmikroskopie, untersucht. Dies erlaubte die Charakterisierung einer Reihe von Phänomenen, die sowohl an Fest-Flüssig-Grenzflächen auftreten, wie beispielsweise die Dynamik der einzelnen molekularen Nanostäbchen (Ostwald Reifung) und die Fraktionierung steifer Polymerstäbchen durch Physisorption an der Grafitoberfläche aus der Lösung heraus, als auch in trockenen Filmen vorkommen wie die Selbstorganisation steifer Polymerstäbchen zu Nanobändern mit molekularen Querschnitten, die sich epitaktisch auf Oberflächen orientieren lassen und auch die Ausbildung gestapelter Architekturen von diskförmigen Molekülen. Außerdem wurden die elektronischen Eigenschaften der untersuchten Systeme mit Hilfe von Photoelektronenspektroskopie charakterisiert. Die entwickelten Nanostrukturen sind nicht nur für Nanokonstruktionen auf festen Oberflächen von Interesse, sondern besitzen auch Eigenschaften, die sie für Anwendungen in einer zukünftigen molekularen Elektronik prädestiniert, etwa für den Aufbau molekularer Drähte.

Schlagwörter:
Konjugierten Molekulen, Molekularen Elektronik, Rastersondenmikroskopien, Selbstorganisation


Seiten: [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166] [167] [168] [169] [170] [171] [172] [173] [175] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189]

Inhaltsverzeichnis

TitelseiteSelf-assembly of conjugated (macro)molecules: nanostructures for molecular electronics
Abkürzungsverzeichnis List of Abbreviations
1 Introduction
2 Scanning Probe Microscopies
2.1The techniques
2.2Scanning Tunneling Microscopy
2.2.1STM modes
2.2.2Applications of STM
2.3Atomic Force Microscopy
2.3.1Classification of forces
2.3.2Contact mode AFM
2.3.3Vibrating Modes
2.3.3.1Non Contact AFM (NC-AFM)
2.3.3.2Tapping Mode™
2.3.4Applications of AFM
3 Conjugated molecular systems
3.1Introduction
3.2Application in molecular electronics
3.3Phenyleneethynylenes
3.3.1Kinetics of the polycondensation reaction
3.3.2Molecular weight distribution
3.3.2.1Schulz-Zimm distribution
3.3.2.2Schulz-Flory distribution
3.3.2.3Poisson Distribution
3.4Hexa-peri-hexabenzocoronenes
4 Self-assembly of molecules at surfaces and nanoelectrode fabrication
4.1Physisorption
4.1.1Conductive substrate
4.1.2Insulating substrates
4.2Chemisorption
4.3Metallic nanoelectrodes for a molecular nanowire device
5 Experimental procedures
5.1Preparation of the substrates
5.1.1Layered substrates
5.1.2Amorphous substrates
5.1.3Metallic substrates
5.1.3.1Template Stripped Gold
5.2Scanning Tunneling Microscopy
5.2.1Apparatus
5.2.2Tip preparation
5.2.3Vibration isolation
5.2.4STM on dry films
5.2.4.1SAMs of saturated alkanethiols
5.2.4.2SAMs of unsaturated alkenethiols and mixtures
5.2.5Investigations at the solid-liquid interface
5.3Scanning Force Microscopy
5.3.1Apparatus
5.3.2Investigations on polymeric phenyleneethynylenes
5.3.3Investigations on hexakis-dodecyl-hexabenzocoronene (HBC-C12)
5.4Image Processing
5.5UPS, XPS
5.5.1Photoelectron spectroscopies on phenyleneethynylene derivatives
5.5.2Photoelectron spectroscopies on hexakis-dodecyl-hexabenzocoronene (HBC-C12)
5.6Current-voltage (I-V) measurements
6 Results and discussions
6.1Self-assembly of thiols on metallic substrates
6.1.1Introduction
6.1.2Sublimed Au and Ag substrates
6.1.3Template Stripped Gold substrates
6.1.3.1SAMs on Template Stripped Gold substrates
6.1.4Conductivity of SAMs of Alkenes and Alkanes
6.2Role of the substrate in physisorption
6.3Phenyleneethynylene trimers
6.3.1Introduction
6.3.2STM on physisorbed monolayers
6.3.3XRD on single crystals
6.3.4Discussion
6.3.5Dynamics of molecules at the solid-liquid interface
6.4Visualization of single macromolecules in monolayers
6.4.1Macromolecular fractionation
6.5PPE on insulating substrates
6.5.1Introduction
6.5.2Morphology at different concentrations of the solution
6.5.3Morphology at different molecular weight
6.5.4Morphology on different substrates
6.5.5Morphology at different rate of the deposition process
6.5.6Morphology with different side chains
6.5.7Morphology of thiol free end functionalized PPE
6.6Electronic structure of phenyleneethynylene derivatives
6.6.1Introduction
6.6.2Work functions of pristine and doped phenylenethynylene trimer and polymer
6.6.2.1Spin coated PPE trimer
6.6.2.2Spin coated polymer
6.6.3Optical absorption investigation of PPE
6.6.4Density of valence states calculations
6.7Current-Voltage (I-V) Measurements
6.7.1Visualization of the nanoelectrodes
6.7.2Bridging metallic nanogaps with molecules
6.8Growth of dry hexakis-dodecyl-hexabenzocoronene films from solutions
7 Conclusions
8 Zusammenfassung
Bibliographie Bibliographic references
Danksagung
Lebenslauf
Anhang A List of publications, awards and conferences presentations
Selbständigkeitserklärung

Tabellenverzeichnis

Table 2.1: Main Scanning Probe Microscopies.
Table 6.1: Sample of unsaturated C11A mixed with fully saturated C11H23SH
Table 6.2: Sample of C11A mixed with saturated ones
Table 6.3: Sample of C11A mixed with the fully saturated analogue: electric properties
Table 6.4: Crystallographic data and details of the structure refinements of alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl]ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-ter[(2,5-dihexylphenylene-1,4)ethynylene)] (1) , alpha-phenylethynyl-omega-phenyl-ter[(2,5-dihexylphenylene-1,4)ethynylene)] (2) and 1,4-Bis[2-[4-[(N,N-dimethylcarbamoyl)thio] phenyl]ethynyl-2,5-dihexylbenzene (3). The structures were solved by direct methods (SIR92) and refined by full-matrix least squares analysis.
Table 6.5: Samples of PPEs investigated 4 with STM
Table 6.6: PPE samples investigated: average number of repeating units and contour lengths according to 1H-NMR results; polydispersity ( ) from GPC measurements with PPP calibration.
Table 6.7: n-doping of the PPE trimer: evolution of the work function phis with the doping level.
Table 6.8: n-doping of the PPE: evolution of the work function phis with the doping level.

Abbildungsverzeichnis

Figure 1.1: Spatial resolution of some microscopical techniques
Figure 2.1. Scanner motion during data acquisition
Figure 2.2: Scheme of the STM junction.
Figure 2.3: Energy diagram explaining tunneling in STM experiments
Figure 2.4: Comparison of constant-height and constant-current mode for STM.
Figure 2.5: The beam-bounce detection scheme.
Figure 2.6: Interatomic force vs. distance curve.
Figure 2.7: Vibrating modes: the tip-sample interaction affects the amplitude and phase of the swing.
Figure 3.1: Chemical formulae of several conjugated polymers.
Figure 3.2: Chemical formula of hexa-peri-benzocoronenes (HBC).
Figure 3.3: Synthesis of poly(para-phenyleneethynylene).
Figure 3.4: Synthesis of para-phenyleneethynylene trimer.
Figure 3.5: Columnar stacking of HBC-C12.
Figure 3.6: Synthesis of HBC-C12.
Figure 4.1: Chemisorption reaction of a surfactant on a substrate.
Figure 4.2: Junctions for probing electronic properties of a single molecule or molecular object.
Figure. 4.3: Scheme of the e-beam lithography procedure used for producing Au nanogaps.
Figure 4.4: Method for developing “ Mechanically controllable break junctions“.
Figure 5.1: Schematic representation of the Ni-TSG preparation
Figure 5.2: Scheme of the STM set-up: a) Side view of the apparatus; b) Top view of the piezo system: possible displacements in which the drift minimized.
Figure 5.3: Scheme of the sample holder.
Figure 5.4: Scheme of the bungy set-up.
Figure 5.5: STM measurement of a dry film. a) suitable tunneling parameters; b) invasive mode due to inappropriate parameters (too low resistance set-point).
Figure 5.6: Scheme of the solid-liquid interface STM studies.
Figure 5.7: Nanoscope Multimode IIIa (Digital Instruments).
Figure 5.8: a) Spin coating deposition; b) Solution casting.
Figure 5.9: Solution casting in an ambient saturated with the solution vapors.
Figure 5.10: Deprotection reaction of alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl] ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)].
Figure 6.1: STM images of Ag(111). Flat crystallites with sharp angles of 120°.
Tip bias (Ut)= 1.5 V; Average tunneling current (It) =0.03 nA.
Figure 6.2: STM constant current image of uncoated Ag surface after 30 min.exposure to air.
Figure 6.3: STM Constant current image of uncoated Au surface (Tsub=350 °C, Z= 50nm), exhibiting a polycrystalline structure. The gray scale height is (h)=20 nm.
Figure 6.4: STM constant current image of uncoated Au surface (Tsub=400 °C, Z= 200nm),
exhibiting epitaxial terraces. The gray scale height is (h)=1.78 nm.
Figure 6.5: STM constant current images of Au flame-annealed sample recorded in different areas of the surface. A) Ut = 0.1 V; It =0.3 nA; scan rate = 7.4 Hz; scan length (L) = 1.5 µm, Rrms=4.94 Å resolution 250X250 px; B) 0.13 V; 0.2 nA; 10.8 Hz; L = 1 µm, Rrms=6.14 Å; 512X512 px.
Figure 6.6: STM constant current image of Au shock-annealed sample (from the front) recorded in different areas of the surface. A) Ut = 0.12 V ; It = 0.2 nA; scan rate = 11.1 Hz; L = 1 µm, Rrms=5.52 Å; 250X250 px; B) 0.13 V ; 0.2 nA ; 10.3 Hz; L = 1 µm, Rrms=4.56 Å; 512X512 px.
Figure 6.7: STM constant current image of Au shock-annealed sample (from the back).
Ut = 0.13 V ; It = 0.348 nA ; scan rate = 9.8Hz; L = 0.5 µm, Rrms=1.88 Å; 512X512 px.
Figure 6.8: Unfiltered topographical STM image of a coated Ni supported-
TSG surface (Ut=20 mV, average It= 800 pA). Height range= 11.4 Å; Rrms=2.3 Å.
Figure 6.9: STM constant current image of undecanthiol on Ni supported-TSG surface. Ut = 800 mV; I = 40 pA. a) Unfiltered image with the 2D-Fourier Transform showing the periodicity of the hexagonal lattice (alpha-phase); b) band pass filtered image. Although a high frequency noise blurs the raw image (as evident also from the FFT), the lattice of the adlayer can be seen in both images.
Figure 6.10: STM constant current image of TSG coated surface (Ut = 800 mV; I =30 pA) (sample C11A1). Typical holes proving the reaction of the thiols with gold. Height range= 7.2 Å; Rrms=1.4 Å.
Figure 6.11: STM constant current image of TSG coated surface (Ut = 800 mV; I = 20 pA) (sample C11A1). Typical holes proving the reaction of the thiols with gold. Height range= 7.2 Å; Rrms=1.5 Å.
Figure 6.12: STM constant current image of C11A1 on TSG (Ut = 800 mV; I = 50 pA). Typical holes proving the reaction of the thiols with gold. Height range = 49.6 Å; Rrms=12.0 Å .
Figure 6.13: STM constant current image of C11A3 on TSG (Ut = 500 mV; I = 50 pA). Different Au crystallites with different alignments of adsorbed molecules indicated with black arrows.
Figure 6.14: STM image of sample C11A3 on Ni supported-TSG surface. Ut = 500 mV; I = 50 pA. a) Unfiltered image; b) band pass filtered image. Although a high frequency noise blurs the raw image, the lattice of the adlayer can be recognized in both images.
Figure 6.15: HOPG crystallographic structure.
Figure 6.16: Layer structure of muscovite mica structure: A) Side view; B) Top view: the cleaved plane is the basal plane, composed of hexagonal array of oxygen ions with regular vacancies which are randomly filled with potassium. Unit cell: a= 5.2Å , b=9.0 Å.
Figure 6.17: alpha-phenylethynyl-omega-phenyl-ter[1,4-(2,5-dihexylphenylene)ethynylene)] (2).
Figure 6.18: STM current image of 2 in 1-phenyloctane imaged at the solid-liquid interface on HOPG (Ut=1.2 V , average It= 1.0 nA). 2-D crystal structure with its unit cell, averaged over several images amounts to a=(1.78±0.09) nm, b=(2.90±0.18) nm, alpha=(113±5)°. The distance between neighboring backbones is in this case DeltaL=(1.46±0.11) nn.
Figure 6.19: 1,4-Bis-[[2,5-dihexyl-4-[[4-[(N,N-dimethylcarbamoyl)
thio]phenyl]ethynyl]phenyl]ethynyl]-2,5-dihexylbenzene (1).
Figure 6.20: STM constant current image of 1 in 1-phenyloctane imaged at the solid-liquid interface on HOPG. Ut = 1.2 V , It = 1.0 nA. 2-D crystal structure with its unit cell, averaged on several images: a = (1.83 ± 0.11) nm, b = (3.42 ± 0.12) nm, alpha= (108 ± 5)°. The distance between parallel backbones is DeltaL = (1.52 ± 0.08) nm. The angle between the backbones and the lamella main direction is (67 ± 2)°. Superimposed are two molecular models.
Figure 6.21: STM constant current image of 1 in 1-phenyloctane imaged at the solid-liquid interface on HOPG. Ut = 1.2 V , It = 1.0 nA. Polycrystalline structure made of single crystallites with different molecular orientations. The arrow indicates a defect (two missing molecules) in a crystal lattice.
Figure 6.22: Chemical formula of the monomer: 1,4-Bis[2-[4-[(N,N-dimethylcarbamoyl)thio] phenyl]ethynyl-2,5-dihexylbenzene (3).
Figure 6.23: Single molecular structure in single crystals of a) (3), b) (2), c) (1).
Figure 6.24: Crystallographic structure of alpha-phenylethynyl-omega-phenyl-
ter[(2,5-dihexylphenylene-1,4)ethynylene)] (2).
Figure 6.25: Crystallographic structure of Crystallographic intermolecular structure of alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl]ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-ter[(2,5-dihexylphenylene-1,4)ethynylene)] (1).
Figure 6.26: STM constant current images. Evolution of the 2-D polycrystal structure of 1 at the solid-liquid interface during 22 minutes: Ostwald ripening phenomena brought about by the reorientation of single molecules in island I, II, III, IV. The white scale bar is 20 nm. Arrow in a) indicates a domain boundary of the type shown in Fig. 6.27a.
Figure 6.27: Scheme of different types of molecular packing in neighbor domains.
Single molecules are represented as a rod. a) translated; b) rotated.
Figure 6.28: Scheme of the molecular packing at the crystallite frontiers.
Figure 6.29: Evolution of domain areas with time. Filled symbols correspond
to STM images in Fig. 6.26. Open symbols: images not shown.
Figure 6.30: Chemical formula of (4): alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl] ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4) ethynylene)].
Figure 6.31: STM constant current image of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. Ut=1.2 V , average It= 1.0 nA. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length 7.9 nm The molecular backbones exhibit a nematic orientation along preferred directions, as evidenced by the hexagonal pattern in the 2D-Fourier Transform. The distance between the backbones averaged over several images amounts to DeltaL=(1.62±0.10) nm.
Figure 6.32: STM current images of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length of 11.2 nm. The molecular backbones exhibit a nematic orientation along preferred directions, as evidenced by the hexagonal pattern in the 2D-Fourier Transform. a) Ut=1.7 V , average It= 0.2 nA; b) 1.35 V, average It=0.7 nA.
Figure 6.33: STM current images of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length 20.3 nm long. Ut=1.4 V , average It= 0.55 nA. Besides the molecularly resolved areas there are zones with high molecular dynamics where the molecules are not immobilized at surface.
Figure 6.34: Histograms of the distribution of lengths of physisorbed rods. The PPEs posses an average contour length according to 1H-NMR analysis on the end groups, a polydispersity determined by GPC using poly(para-phenylene)s for calibration and a number of physisorbed rods as measured: (a) 7.95 nm / 1.92 / 593; (b) 11.2 nm / 1.84 / 253. The mole fraction of the Schulz-Zimm distribution is plotted in solid lines and in (a) Monte Carlo simulation (dashed and dotted lines). Dashed line: ratio of the potential of the interactions molecule-substrate and molecule-molecule = 2; dotted line: interaction molecule-molecule is reduced to zero. The normalization of the curves have been carried out on the tail of the distribution. Simulations have been performed using a repulsive hard core potential and London attractive forces. The model system was designed forming linear rods by overlapping several spheres linearly. The procedure was computed for a distribution of rods according to the Schulz-Zimm distribution. Calculations have been executed varying systematically the ratio between the potential of the interactions molecules-substrate and molecules-molecules in a range that spans from 2 to infinite.
Figure 6.35: Molecular Dynamics simulation of the evolution of the end-to-end distance of a single PPE in vacuum as a function of the increasing polymer length (expressed either in # of repeat units or contour length). The simulations have been computed at room temperature in vacuum applying the pcff force field (Molecular modelling package DISCOVER VERSION 4.0.0, Biosym Technologies Inc., San Diego, CA.) for a time scale of 100 ps for polymer up to 28 r.u., 150 ps for the molecule made of 35 and 42 r.u., and 250 ps for the polymer composed of 49 and 56 r.u.; these turned out to be enough for achieving a constant regime of energy fluctuations. The error bars represents the standard deviation of the end-to-end distance in the final 100 ps calculated.
Figure 6.36: TM-SFM images representing the evolution of the morphology of the PPE on mica as a function of the concentration of the polymer. a) 2.0 g/l in MeOH, amplitude image, height range (peak-valley) (h) in the correspondent height image of ~ 70nm; b) 0.14 g/l in THF, height image, h=50 nm; c) 0.07 g/l in THF, height image, h=15 nm.
Figure 6.37: Scanning Electron Microscope (SEM) micrograph (courtesy of Dr. Rogaschewski) of commercial Si tip (Digital Instruments). Typical shape of the cantilever with a conical tip attached at its end.
Figure 6.38: Transmission Electron Microscope (TEM) micrograph (courtesy of Dr. C. Böttcher) of commercial Si tips (Digital Instruments). Tips displayed in a) and b) present a different shape. The average of the terminal tip radius of several tips has been evaluated as R = (13±7) nm.
Figure 6.39: Geometrical model for the broadening of the image of a ribbon due to the tip radius. R is the terminal tip radius, h is the mean height of the needles. Delta = = (8±3) nm. The effective broadening results as 2Delta=(16±6) nm.
Figure 6.40: Chemical formula of 5, namely alpha-iodo-omega -[(2,5-dihexyl-4-ethynyl)phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)].
Figure 6.41: SFM Tapping Mode height images representing the evolution of the morphology of the PPE on mica as a function of the polymer length. Average contour length of the molecule according to 1H-NMR results: a) 7.9 nm; b) 16.4; c) 20.3 nm. Height range of images: a) 20 nm; b) 30 nm; c) 20 nm. In case a) a mixture of THF and phenyloctane was used as a solvent while in b and c) pure THF was used.
Figure 6.42: Schematic representation of molecular ribbons adsorbed on the mica surface. a) Ribbons are made of several rods packed parallel to each other. b) Each rod is typically made of two PPE molecules packed with the hexyl lateral chains perpendicular to the basal plane of the substrate.
Figure 6.43: Histograms of the distribution of ribbon widths for PPE batches A through D with increasing length. Average contour length of the molecule according to 1H-NMR results (Table 6.6) and number of ribbons measured: (A) 7.9 nm / 846; (B) 9.2 nm / 160; (C) 13.5 nm / 264;(D) 16.4 nm / 334. The Schulz-Zimm distribution is plotted in terms of number counting (solid lines) and weight function (dotted line). 2Delta=(16±7) nm is the effective broadening of the tip. Dashed lines are experimental GPC data obtained from PS calibrated measurements.
Figure 6.44: A) Tapping Mode SFM height image of an on average 11.2 nm long PPE on glass. Singular nanoribbon assembled on the grainy glass surface. B) TEM micrograph showing webs of ribbons on a carbon copper mesh support made by casting an on average 16.4 nm long PPE from a 0.1 g/l solution in THF.
Figure 6.45: Histograms of the distribution of ribbon widths for PPE from profiles on TEM micrographs. Contour length of the molecule according to 1H-NMR results and number of ribbons measured: (A) 7.9 nm / 164; (B) 16.4 nm / 167. Mole fraction (solid line) and weight fraction (dashed line) of the theoretical Schulz-Zimm distribution are in fairly good agreement with the experimental data.
Figure 6.46: Schematic representation of the molecular packing in the nanoribbon: a) Top view: a ribbon is composed by parallel, fully extended backbones, stacked perpendicular to the main ribbon axis; b) Side view: the lateral alkyl chains are disordered between adjacent backbones in a bilayer/trilayer aggregate standing on the substrate.
Figure 6.47: Topographical Tapping Mode Scanning Force Microscopy images of PPE cast on freshly cleaved muscovite mica. a) Solvent : THF, height range (peak-valley) h=30 nm; b) Solvent:mixture THF-phenyloctane (5:1), h=20 nm.
Figure 6.48: Chemical formulae of the CPPE and EHPPE.
Figure 6.49: Tapping Mode SFM height images of: a) EHPPE from 0.013 g/l in THF, h=40 nm; b) EHPPE from 0.0033 g/l in THF, h=40 nm; c) CPPE from 0.017 g/l in H20+KOH, h=7 nm.
Figure 6.50: Tapping Mode SFM height image of PPE with free thiol end-groups
assembled on a SiO2 wafer. Height of the gray scale h=50nm.
Figure 6.51: Principle of photoelectron spectroscopy of thin organic films on a conductive substrate.
Figure 6.52: Electronic levels in a pi-conjugated molecule. EF=Energy of the Fermi level; EVac=Energy of the vacuum level; EKin=Kinetic energy = hny-Ii; phis=work function. Bottom of the figure: schematic cartoon of the electronic levels. Top: corresponding PES spectra.
Figure 6.53: XPS survey spectra. a) Au surface cleaned with isopropanol and aceton bath; b) spin coated PPE sample #1 (nominal thickness d=3.1 nm); c) spin coated PPE sample #2 (d= 2.7 nm); d) spin coated PPE trimer (d= 3.0 nm); e) sublimed PPE trimer (d= 4.1 nm).
Figure 6.54: UPS HeII spectra of a) Au surface cleaned with isopropanol and aceton bath (work function phis=4.1 eV); and pristine samples of: b) spin coated PPE sample #1 (phis =4.2 eV); c) spin coated PPE sample #2 (phis = 4.3 eV); d) spin coated PPE trimer (phis= 4.5 eV); e) sublimed PPE trimer (phis = 4.0 eV).
Figure 6.55: XPS survey spectra of spin coated sample of phenyleneethynylene trimer at
increasing level of doping. a) Na/C/Au=0/100/17 ; b) 1.9/100/19.6 (0.38 Na-at/mon.); c)
Figure 6.56: UPS HeII spectra of spin coated film of phenyleneethynylene trimer at increasing level of doping. The spectra are plotted relative to the Fermi level (left) and to the vacuum level (right). a) pristine: phis=4.5 eV ; b) phis= 3.2 eV (0.38 Na-at/mon.); c) phis= 2.8 eV (0.84 Na-at/mon.); d) phis= 2.7 eV (0.98 Na-at/mon.); e) phis= 2.6 eV (1.38 Na-at/mon.).
Figure 6.57: XPS spectra zoomed on the Na 1s peak of spin coated sample of phenyleneethynylene trimer at increasing level of doping. The spectra are plotted relative to the Fermi level (left) and to the vacuum level (right). a) phis= 3.2 eV (0.38 Na-at/mon.); b) phis= 2.8 eV (0.84 Na-at/mon.); c) phis= 2.6 eV (1.38 Na-at/mon.).
Figure 6.58: XPS spectra zoomed on the C 1s peak for spin coated sample of phenyleneethynylene trimer at increasing level of doping. The spectra are plotted relative to the Fermi level (left) and to the vacuum level (right). a) phis= 4.5 eV (pristine); b) phis= 3.2 eV (0.38 Na-at/mon.); c) phis= 2.8 eV (0.84 Na-at/mon.); d) phis= 2.6 eV (1.38 Na-at/mon.).
Figure 6.59: XPS survey spectra of spin coated sample of PPE at increasing level of doping. a) Na/C/Au=0/100/23.4 (pristine); b) 2.1/100/24.8 (0.42 Na-at/mon.); c) 4.7/100/28.1 (0.94 Na-at/mon.); d) 5.5/100/29.3 (1.1 Na-at/mon.).
Figure 6.60: UPS HeII spectra of spin coated sample of PPE at increasing level of doping. The spectra are plotted relative to the Fermi level (left) and to the vacuum level (right). a) pristine: phis=4.3 eV ; b) phis= 3.5 eV (0.42 Na-at/mon.); c) phis= 3.0 eV (0.94 Na-at/mon.); d) phis= 2.6 eV (1.1 Na-at/mon.).
Figure 6.61: UV-Visible optical absorption spectra of PPE. a) Degree of polymerization (DP)= 8; b) DP=35. (The spectra have been recorded by Dr. V. Francke in MPI for Polymer Research, Mainz, Germany).
Figure 6.62: Valence Effective Hamiltonian (VEH) of the density of valence states (DOVS) simulated spectra and UPS HeII spectra of a spin coated film of pristine PPE trimer. The spectra are plotted relative to the Fermi level.
Figure 6.63: Valence Effective Hamiltonian (VEH) of the density of valence states (DOVS) simulated spectra of a doubly charged system and UPS HeII spectra of spin coated film of a doped PPE trimer (1.38 Na at/mon). The spectra are plotted relative to the Fermi level.
Figure 6.64: Tapping Mode SFM height image of Au nanoelectrodes produced by Dr. S. Rogaschewski. The height of the gray scale (h) is: a) h= 50 nm ; b) h= 40 nm.
Figure 6.65: Tapping Mode SFM height image of Au nanoelectrodes produced by Dr. S. Rogaschewski. The lift off process was not perfect; therefore there is no gap between the two electrodes. h= 70 nm.
Figure 6.66: Tapping Mode SFM height image of Au nanoelectrodes
produced by Dr. K. Kragler. h= 40 nm.
Figure 6.67: Tapping Mode SFM height image of Au nanoelectrodes
produced by Dr. A.C.F. Hoole. h= 40 nm.
Figure 6.68: Adsorption of sulphur free PPE. a) Electrochemical adsorption induced by the applied voltage between the two electrodes; b) the self-assembly is likely to be governed by the chemisorption of sulphurs on Au nanoelectrodes.
Figure 6.69: Tapping Mode SFM image of a PPE with an average contour length of 14 r.u. . The height image shows electrodes strongly coated. a) Height image: h=70 nm; b) Phase image.
Figure 6.70: Current-voltage curve: A) non coated electrodes; b) coated electrodes. The curve
appears to be symmetric and the maximum current is on the order of 10 nA at ± 1V.
Figure 6.71: hexakis-dodecyl-hexabenzocoronene (HBC-C12)
Figure 6.72: Tapping Mode SFM images of HBC-C12 film prepared by spin coating the quantity to form ~ 1000 layers coating the HOPG (method A described in paragraph 5.3.3). a) Height image: z- scale (h)=10 nm; b) Phase image: z- scale=7°.
Figure 6.73: Tapping Mode SFM micrographs of HBC-C12 grown on HOPG by solution casting (method B in paragraph 5.3.3) a quantity sufficient to coat the HOPG with one layer. a) Height image: h=20 nm; b) Phase image: z- scale=6°; c) Height image: h=20 nm.
Figure 6.74: Topographical Tapping Mode SFM image of a HBC-C12 surface produced by slow solution casting of one layer in controlled environment (route C in paragraph 5.3.3). h=15 nm. The white arrows indicate the preferential directions along which the layers grow anisotropic. The angle between them is 60° in perfect agreement with the three-fold symmetry of the HOPG substrate.
Figure 6.75: UPS HeII spectra of HBC-C12 plotted relative to the Fermi level of samples prepared at decreasing rates of physisorption: a) casting 1 layer; b) casting ~ 1000 layers; c) slow casting of 1 layer.
Figure 6.76: Angle Resolved UPS He(II) spectra of HBC-C12 plotted relative to the Fermi level of sample prepared by slow casting 1 layer. a) sample as prepared; b) thermally annealed sample at 200°C for 2 hours.
Figure 6.77: Scheme of the packing of HBC-C12 into columnar arrays (from [Her96]).

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