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

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Kapitel 5. Experimental procedures

In this chapter the experimental methodologies which are used for the work presented below are discussed. The chapter is divided into 5 paragraphs:

5.1 Preparation of the substrates

The preparation of the substrate plays a key role for the self-assembly of molecules at surfaces. In order to obtain highly reproducible results, very flat substrates with precisely controlled chemical functionalities, freshly prepared just before the chemical deposition, have been used.

5.1.1 Layered substrates

Layered substrates are widely utilized for the adsorption of the organic materials and SPM studies.

For the present investigations two different commercially available supports were chosen:

Both of them can be easily cleaved with adhesive tape. The ease and reproducibility of the sample preparation as well as the chemical inertness and the atomical flatness on the


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micrometer scale that can be achieved, render this type of preparation commonly used for Scanning Probe Microscopy investigations of physisorbed samples.

Muscovite mica has been used widely as support for both self-assembly of molecules from solutions and sublimation of a metallic layer in high-vacuum (HV). Slices, 120 µm thick, were cut into discs of variable sizes with a punch and die set (Precision Brand, Downers Grove, Illinois U.S.A.) to maintain nicely cleavable edges.

HOPG was used for STM studied at the solid-liquid interface and also as conductive support for producing dry layers of HBC-C12 from solution.

5.1.2 Amorphous substrates

Non-crystalline insulating substrates have been also utilized for understanding the role of the crystallinity of the support (mica) in the growth of PPE from solution. Thin glass discs (120 µm thick and with a diameter of 1 cm) have been first cleaned at 60°C for twice 5 minutes in trichloroethylene, acetone and ethanol respectively. Then they have been rinsed with deionized H2O (Milli-Q) and dried with a gentle blow of N2.

In addition for the TEM investigations of PPE thin films 400-mesh carbon coated copper grids have been utilized as amorphous substrates.

5.1.3 Metallic substrates

Metallic substrates are useful supports for self-assembly and in particular for chemisorptions. They can be produced by sublimation in HV with a chamber pressure of ~ 10-6 mbar or in ultra high-vacuum (UHV) with a pressure of < 10-9 mbar. The goal of this kind of preparation is to obtain an epitaxial or pseudo-epitaxial substrate that exhibits a very high flatness. In collaboration with Dr. P. Thiele (Department of Physics, Humboldt University Berlin) thin films of Ag and Au have been sublimed onto freshly cleaved mica discs in HV at a chamber pressure of ~ 10-5 - 10-7 mbar [Chi88]. The temperature of the substrate during the sublimation and the nominal thickness of the metallic adlayer (between 50 and 200 nm) were varied systematically in order to find conditions yielding a pseudo epitaxial film. The


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nominal thickness of the sublimed film was detected with a quartz oscillator that was calibrated according to the Au or Ag parameters. The temperature of the substrate during the sublimation was measured with a thermocouple that was placed on the edge of the surface that was got coated. Inside the vacuum chamber the crucible and the substrate holder have got two separate heating systems. For the case of silver, since its surface gets oxidized very rapidly, some precautions have been taken too minimize its contact with air: after the sublimation, the vacuum chamber was vented to room pressure with an argon flow and as soon as the chamber was open, the films were immersed in organic solutions that were also under Ar reflux. On the other hand, in order to increase the epitaxiality of the Au surfaces prepared by sublimation, post annealing (followed by quenching in EtOH or millipore H2O saturated with N2) has been applied to the 200 nm thick metallic films. This post treatment has been carried out both by flame annealing and shock annealing with an halogen lamp from the front and from the rear side of the sample. The temperatures during all these processes cannot be easily determined but are more likely within a range of 500-600 °C; the duration of the process is about one second. STM imaging has been used to characterize the film structures, in particular their roughnesses, on a micrometer scale.

5.1.3.1 Template Stripped Gold

Alternatively, in collaboration with Dr. J. Diebel (from the group of Dr. H. Löwe, Institute of Microtechnology Mainz) we have designed a method to grow Template Stripped Gold (TSG) substrates. The sample preparation scheme is shown in Fig. 5.1. [Sam99b]

A 200 nm thick gold film has been evaporated in high-vacuum (~ 10-6 mbar) onto freshly cleaved muscovite mica kept at a T=300 °C. Subsequently a layer of Ni of about 200 µm was electroplated on the free Au surface. A rubber ring was then attached to the upper Ni surface. At this point the mica can be easily stripped off mechanically by pulling the rubber ring with tweezers. The Au/Ni film was immersed with the gold face up into a 3.5 mM solution of undecanthiol (CH3(CH2)10SH) (Lancaster) or nonanthiol (CH3(CH2)8SH) (Aldrich) in methanol or benzene and left for 20 hours. The chemisorbed adlayer after immersion was rinsed several times with the same solvent and blown dry under a gentle flow of Ar or N2.


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Figure 5.1: Schematic representation of the Ni-TSG preparation

5.2 Scanning Tunneling Microscopy

5.2.1 Apparatus

STM investigations have been carried out using a home-made STM interfaced to Omicron Electronics (Omicron Vakuumphysik GmbH,Taunusstein,Germany). This apparatus, was built by Dr. P. E. Hillner (group of Prof. Dr. J. P. Rabe, Department of Physics, Humboldt University Berlin) according to the Besocke beetle-type design (Fig. 5.2) [Bes87, Bes88, Fro89]. Four identical piezoelectric legs expand and contract with temperature at the same rate. Three of these legs are placed at the edges of an equilateral triangle and behave as the “feet“ of the STM head and the fourth leg in the middle ends with the STM tip. The sample holder (Fig 5.3) is a equilateral triangle of either glass or quartz at whose 3 edges steel discs have been glued. The sample is also glued at the center of the triangle and silver paint was used to make the contact with the back foot.


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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.

Two additional features make of this home made apparatus ideal for the investigation of organic adsorbates:

  1. the possibility to detect currents down to 8 pA; this is very important for imaging materials with a large resistivity, such as organic films;
  2. the possibility to achieve rather high scan rates (200 Hz/line); this is essential since the mobility of molecules and molecular clusters at surfaces can be quite high.

The microscope is supplied with a piezo system that allowed to do investigations that span from a 5 µm scan length to the nm scale.


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5.2.2 Tip preparation

Pt/Ir (80:20) wire has been selected as material because it is chemically stable in air and sufficiently stiff.

A 0.25 mm wire has been electrochemically etched in a solution of 2N KOH + 6N NaCN. Keeping the voltage constant at 6-10 V (50 Hz, alternate current) the etching takes place as long as the current through the wire is bigger than 40 mA. At this threshold the voltage gets interrupted automatically. The tip is then rinsed with Millipore water and dried with a gentle flux of Ar or N2. This recipe leads to reproducible atomically sharp stable metallic needles ready to be used [Mel91, Wei95].

5.2.3 Vibration isolation

The STM is placed on a “bungy“ cord set-up, which consists of a large mass, M, attached to bungy cords (four total, one in each corner in our configuration), firmly anchored to the building ceiling (Fig. 5.4) [Par87].

Figure 5.4: Scheme of the bungy set-up.

The equation of motion governing the movement of the AFM in a box, B, if the one of a damped harmonic oscillator, with the solution

where:

z is the vertical deflection away from equilibrium;


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zo is the maximum deviation from equilibrium;

omegao is the angular resonance frequency of the mass;

tau is the damping time constant of the bungy cord.

For our system, the bungy cords have a length of L = 0.7 m and a diameter of 6 mm. They are attached to a granite slab with a mass of about M = 50 kg. The mass extends the length of the bungy by about DeltaL=0.8 m. From this extension of the bungy cord we can determine its spring constant, k=Mg/DeltaL ~ 600 N/m and the resonance frequency ~ 0.5 Hz Þ k=100 N/m

Noteworthy, the resonance frequency can be independent of the mass M, depending only on the stretch of the bungy [2]. The damping of the oscillation can be attributed to rubbing of the rubber fibers inside of the bungy cord against the outside lining material. Ideally one seeks a high damping situation, that is, where tau~1 second. This can be optimized (damping can be enhanced) by greater weight on the bungy cords up to a point before plastic deformation of the bungy material, after which it is no longer useful for vibration isolation. Since, this will also lead to a shortened life of the cords, care must be taken to support the mass when the system is not in operation.

Box B (Fig. 5.4) is an environmental control chamber that completely engulfs the microscope. Such a box enables the user to remove all moisture from the microscope by fluxing inert gases through the chamber. Between the low resonance frequency of the bungy cord system and the high resonance frequency of the microscope head (> 10 kHz), the SPM effectively comprises a band pass filter. This allowed us to safely image samples in the intermediate range of about 1-100 Hz and achieve atomic resolution.

5.2.4 STM on dry films

The STM set-up can be used to investigate the morphology and the structure of a thin dry organic film. The essential requirement is a sufficiently low thickness of the non-conductive organic layer. Using the apparatus described above, the morphology and structure of metallic films (Au, Ag, TSG) has been studied before and after chemisorption. In the latter case the


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tunneling parameters have to be selected in order not to damage the adsorbate (Fig 5.5).

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).

The SAMs grown on TSG have been imaged first on a large (micrometer) scale in order to select a flat crystallite; then zooming in on a terrace, it was tried to resolve the molecular packing on a sub-nanometer scale. Typical scan rates were 1-3 Hz per line for topographical images (constant current mode) on the micrometer scale and 50-200 Hz for molecular imaging on a sub-nanometer scale (constant height mode).

5.2.4.1 SAMs of saturated alkanethiols

SAMs of fully saturated commercial alkanethiols have been produced on several surfaces of Ag(111) and Au (111), which were prepared as previously presented in this chapter. 1 mM solutions in ethanol (EtOH) or benzene have been prepared and the fresh surfaces of the metallic supports have been immersed face up in the organic solution for ~1-3 hours for the case of Ag and ~18-24 hours for the case of Au. After this, the coated surface was rinsed several times with the pure solvent and dried using a gentle flux of N2 or Ar. Subsequently the samples have been investigated with STM.

5.2.4.2 SAMs of unsaturated alkenethiols and mixtures

In collaboration with Dr A. Wei (group of Prof. Dr. J. -M. Lehn, Univ. Strasbourg) alkenethiols and mixtures of alkanethiols and alkenethiols have self-assembled on metallic


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surfaces.

In all cases concentrated organic solutions were received frozen and have been defrozen just before use. Although they did not show any insoluble particles floating in the solutions, they have been passed through a disposable membrane filter and diluted. Both the solutions and the assembled films have been stored in the dark.

Four types of experiments have been carried out:

Ag (111) films (substrate temperature (Tsub)=275 °C and nominal thickness of the metallic film (Z=50 nm)) have been immersed for 6 hours in different ratios of saturated/unsaturated C9 , C11 , C12 chains in 5mM solutions in benzene, where in Cn n is the number of carbons on the main chain.

Ag (111) films (Tsub=275 °C and Z=50 nm) have been immersed for 1 hour and 6 hours in different ratios of saturated/unsaturated C9 , C11 , C12 chains in 1mM solutions in benzene;

TSG and sublimed Au have been immersed for 22 hours in different ratios of saturated/unsaturated C11 chains in 0.66 mM or 2 mM solutions in benzene;

sublimed Au surfaces have been immersed for 22 hours in different ratios of saturated/unsaturated C11 chains in 0.66 mM or 2 mM solutions in benzene.

5.2.5 Investigations at the solid-liquid interface

The STM set-up can be used to perform investigations at the interface between a solid conductive substrate, like HOPG, and an almost saturated organic solution (Fig. 5.6) [Rab91a].

The proper selection of the tunneling parameters, tunneling current, It (~1 nA), and voltage between tip-sample, Ut (~1V), allows to control the distance tip-sample and therefore to choose to image only the first layer physisorbed on HOPG. By varying the tunneling parameters, in order to decrease the distance tip-sample, it has been possible to visualize the HOPG lattice underneath and therefore to calibrate the piezo in situ. Molecular resolution at the solid-liquid interface have been achieved using high scan rates (~ 60-200 Hz/line) in the constant-current mode.


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Figure 5.6: Scheme of the solid-liquid interface STM studies.

These kinds of measurements require the use of a solvent with a low volatility:

Both solvents are available from Aldrich Chemicals and both are suitable for this kind of measurements; in addition the second one is also rather aggressive and more toxic. When it has been possible it was preferred to use the first one. The solvents allowed to measure for several hours having the tunneling tip immersed in the solution.

During the experiments, first the HOPG lattice is resolved for 2-4 hours until the images exhibit a good stability due to an absence of drift. Only at this point a drop of the organic solution is applied to the basal plane of the substrate.

Crystalline powder of alpha-iodo-omega -[(2,5-dihexyl-4-ethynyl)phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)] , alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl] ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)] (PPE) or alpha-phenylethynyl-omega-phenyl-ter[(2,5-dihexylphenylene-1,4)ethynylene)] was solubilized in 1-phenyloctane and a drop of the almost saturated solution was applied to the freshly cleaved highly oriented pyrolitic graphite (HOPG) substrate.

5.3 Scanning Force Microscopy

5.3.1 Apparatus


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Scanning Force Microscopy investigations have been executed on a commercial Nanoscope IIIa set-up (Fig. 5.7) produced by Digital Instruments, Santa Barbara, CA that can run both in contact and in Tapping mode. We have used different commercial microfabricated cantilevers available either from Digital Instruments, Santa Barbara, CA or from Olympus Opt. Co. LTD.

Figure 5.7: Nanoscope Multimode IIIa (Digital Instruments).

k (N/m)

Cantilever length (?m)

Mode

Brand

Material/shape

17 - 64

125

Tapping mode

DI

Si/rectangular

0.06

200

Contact mode

DI

Si3N4/triangular

0.12

200

Contact mode

DI

Si3N4/triangular

0.38

100

Contact mode

DI

Si3N4/triangular

0.58

100

Contact mode

DI

Si3N4/triangular

0.02

200

Contact mode

Olympus

Si3N4/triangular

k= Force constant

Three different scanners have been used to explore different ranges of scan lengths:

Scanner

Maximum Scan length (?m)


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A

1

E

13

J

130

Most of the studies have been performed with scanner E. On the other hand scanner A is useful to explore samples on a molecular scale, while the J scanner allows to get an overview of the surface on a hundred micrometers scale. For distance calibration of the piezo controller, images of mica and gold calibration gratings were employed routinely. The samples have been attached with a double sided tape or with epoxy glue to a steel disc that is held magnetically on the piezo.

It was possible to obtain an overview of the cantilever and of the sample surface on a hundreds of micrometers scale by using an optical microscope (Nikon-Japan) that is interfaced with a CCD camera and is therefore able to produce images on a monitor. This set-up renders possible the selection of the investigated area with a good precision in the tens of µm range.

An analogous bungy set-up described for the STM has been used with the SFM, alternatively to a heavy table made from a 1m*2m granite slab, which stands on a pneumatic system that keeps it floating with N2 pressure.

5.3.2 Investigations on polymeric phenyleneethynylenes

PPE solutions have been applied to different non conductive substrates, which were either amorphous (glass, carbon copper grids) or crystalline (mica). Mica was freshly cleaved before use while the other two substrates have been cleaned chemically. The concentration of the solutions (from 3.0 g/l to 0.01 g/l of PPE), the type of solvent and size of the macromolecular system (molecular weight) have been varied systematically. The solvent has been chosen in order to vary the rate of evaporation and of crystallization of the organic adsorbate. Pure tetrahydrofurane (THF , boiling temperature 69 °C), methanol (MeOH , b.p. 65 °C), a mixture of THF and 1-phenyloctane and pure 1-phenyloctane have been used.


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Molecular self-assembly was achieved in two different ways:

  1. spin coating: a drop of solution is placed on the substrate that is rotated very fast for 30 sec (Fig. 5.8a). The adsorption therefore occurs very rapidly.
  2. solution casting: applying a drop of solution onto a freshly cleaved mica surface and letting the solvent evaporate (Fig. 5.8b). Making use of different solvents the self-assembly can take place in a few hours (2-3 for THF) or in a few days (2-3) for the mixture. The evaporation of the solvent for the case of pure 1-phenyloctane required more than one month time and therefore is considered impractical.

The thin dried films have been observed by SFM in air environment both in contact and in Tapping Mode using mainly the E-scanner in a range of scan lengths from 10 µm to 0.3 µm.

Figure 5.8: a) Spin coating deposition; b) Solution casting.

5.3.3 Investigations on hexakis-dodecyl-hexabenzocoronene (HBC-C12)

The exact quantity of HBC-C12 dissolved in 1,2,4 trichlorobenzene for covering homogeneously the HOPG substrate with 1 layer (solution I) and 1000 layers (solution II) have been deposited, using different procedures discussed here below, following the hypothesis that HBC-C12 molecules pack on HOPG with the disc oriented flat on the basal plane of the substrate, as observed in previous studies at the solid-liquid interface [Sta95b]. The according quantity can be calculated as follows:

Molecular Weight of HBC-C12: 1540 g/mol

3.8 mg of HBC-C12 in 1.5 ml of TCB _ concentration = 2.53 g/l

Avogadro Number: 6.022*1023 mol-1

Surface covered by a single molecule: 1.94nm*2.64nm*sin80°=5.044 nm2 (area unit cell calculated according to [Sta95b])


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Surface of the HOPG substrate: 1014 nm2 = 1 cm2

Number of molecules required to make a single layer on the substrate: 1014nm2/5.044 nm2=1,983*1013 molecules

Concentration=3.8 mg/1.5 ml=2.533 g/l à 2.533g/l / 1540 g/mol= 1.645*10-3 mol/l

Volume of solution cast on the substrate = 20 µl

Solution I: using 20 µl of a 1.645*10-3 mol/l solution, one nominally creates 103 layers on the HOPG basal plane.

Solution II: using 20 µl of a 1.645*10-6 mol/l solution, one nominally creates a single layer on the HOPG basal plane.

The self-assembly was carried out in the two ways described for PPE in chapter 5.3.2. An alternative route of solution casting was applied with an almost sealed environment of the vapors of the organic solvent. The substrate freshly coated with a drop of solution and a beaker of solvent are placed under a big beaker (Fig. 5.9). This procedure leads to a further decrease of the rate of adsorption.

Figure 5.9: Solution casting in an ambient saturated with the solution vapors.

The rate of the adsorption processes are :

Method

Process

Rate

A

Spin coating

~ 30 sec

B

Solution casting

~ 3-5 hours

C

Solution casting in sealed ambient

~ 12 hours


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The films, prepared according to the different routes, have been investigated with SFM in Tapping Mode (detecting the amplitude signal and the phase lag) and in contact mode, using the E scanner in air ambient. Also STM investigations have been carried out in order to estimate the thickness of the adsorbate by measuring the average film resistance.

Nominal contour sizes of the molecules have been computed by means of molecular mechanics calculations using a commercial software, DISCOVER VERSION 4.0.0, Biosym Technologies Inc., San Diego, CA.

5.4 Image Processing

SPMs are local probe techniques that reveal local features, which are not necessarily representative for the whole sample surface. Because of this reason it is appropriate to record and process several images. This allows to minimize the influence of a particular sample area and to determine an average behavior. All the evaluations that are described in the following paragraph have been carried out quantitatively and averaged over a large number of samples making use of different image processing software.

STM: Making use of the HOPG micrographs as reference, STM images of the adlayer have been corrected one by one for the piezo drift with the software of the STM instrument. The dimensions of the unit cells have been determined utilizing a public domain software, Scanning Probe Image Processor (SPIP), vers. 3.0, developed by J.F. Jorgensen. On the other hand the software package Image Tools 1.27 produced by University of Texas - Health Science Center in Saint Antonio has been used as an electronic ruler to measure the spacing between parallel backbones from STM micrographs. The surface roughnesses have been evaluated with the SPIP software on raw data constant current images.

For the Ostwald ripening studies, measurements of the area of the molecular crystallites have been computed by drawing manually the contours of the molecular domains from several images [Bis95] using NIH-Image software (National Institutes of Health, Bethesda, Maryland). An analysis of the evolution of the crystallite areas as a function of time on the minute time scale has been executed on several individual domains from different images. A linear fitting of the area of the crystals vs. time has been computed for the last 2-4 minutes of each island life time, and was averaged.

SFM: The height of features like the ribbons of PPE or the layer thicknesses for HBC-C12


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have been evaluated from singular profiles using the software of the Nanoscope IIIa instrument. Besides, the ribbon widths have been surveyed with Image Tools 1.27 software.

5.5 UPS, XPS

Photoelectron spectroscopies studies have been carried out in Linköping together with Dr. Matthias Keil within the group of Prof. W.R. Salaneck. The X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy investigations have accomplished using an ultra-high vacuum (UHV) apparatus with a base pressure of better than 10-9 mbar. The X-ray source was a 1254.6 eV Mg(K?) radiation while UPS was performed using monochromatized HeI (21.2 eV) or HeII (40.8 eV) photons from an He discharge source. The beam was usually oriented at 45° from the normal to the substrate except for the case of angle resolved measurements of C12-HBC where also spectra at 0° have been recorded. The thermal annealing of the films was performed inside the vacuum chamber.

5.5.1 Photoelectron spectroscopies on phenyleneethynylene derivatives

Films of the trimer and the polymer of phenyleneethynylene have been grown on 200 nm thick Au(111) films sublimed on silicon wafers. The trimer, namely (alpha-phenylethynyl-omega-phenyl-ter[(2,5-dihexylphenylene-1,4)ethynylene)]), possesses 74 carbon atoms (20 carbon atoms per “monomer“). It has been processed in thin films both by sublimation in UHV and by spin-coating using THF as solvent. The UHV-sublimation of the organic compound has been executed at 250 °C for 11 minutes at a pressure that ranged between 1.6 and 2.0*10-9 mbar, while spin-coating of a trimer solution (2.4 g/liter) in THF has been carried out at 2000 round/minutes for 2 minutes.

The polymer, (alpha-phenylethynyl-omega-phenyl-poly[(2,5-dihexylphenylene-1,4)ethynylene)]), exhibits a number averaged molecular weight of Mn= 3542 g/mol (from GPC investigation poly(para-phenylene), PPP, calibrated) which indicates an average degree of polymerization (number average of repeating units) of ~13 repeat units, and a polydispersity (U=Mw/Mn)=2.12. The average number of carbon atoms for each macromolecule is ~ 274 (20 carbon atoms per “monomer“). Films of this macromolecular system have been prepared by


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spin-coating on Au films using THF as a solvent.

Before the deposition of the organic layer, the Au substrates have been cleaned by acetone and iso-propyl alcohol (known also as 2-propanol) successive baths in a sonicator apparatus for 5 minutes each step. The samples have been dried after each step with a gentle flow of N2.

Evaporation of Na on the bare Au surfaces have been carried out in order to calibrate the Na source, and the intensity and the shape of the peaks in XPS. This has been executed both on a Au surface cleaned by Ne sputtering and with a Au surface cleaned by acetone and 2-propanol. The sodium was evaporated from a getter source which was heated with a current of 7 A. The Na doping of the interface have been performed with successive increasing evaporation time of the same sample. After each step (one or two minutes of evaporation) XPS (survey scan, C 1s and Na 1s) and UPS (HeI, HeII) spectra have been recorded.

5.5.2 Photoelectron spectroscopies on hexakis-dodecyl-hexabenzocoronene (HBC-C12)

Thin films of hexakis-dodecyl-hexabenzocoronene prepared from solution (as described in paragraph 5.3.3) have been investigated complementary by means of XPS and angle resolved UPS.

5.6 Current-voltage (I-V) measurements

The deprotection of alpha-[[4-[(N,N-dimethylcarbamoyl)thio]phenyl]ethynyl]-omega-[4-[(N,N-dimethylcarbamoyl)thio]phenyl]-poly[(2,5-dihexylphenylene-1,4)ethynylene)] has been accomplished following the route described in Fig. 5.10. The polymeric polycrystalline powder was dissolved in a solution of THF or in a mix of phenyloctane/THF (volume ratio 1:10). The pH of the solution was increased by adding KOH pellets dissolved in MeOH. The solution was stored in a well controlled environment inside a glass balloon. Successive cycles of vacuum and Argon venting were executed in order to obtain an oxygen free ambient; after this, the balloon was kept under a gentle flow of Ar. All solvents have been degased before the use. Following to this first step HCl was added in order to protonize the end groups leading to thiols (-SH).


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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)].

The Au nanoelectrodes, after testing their I-V behaviour one by one, were immersed for 18-22 hours in the solution either when the polymer exhibited ionic sulphide groups at their edges (Fig. 5.10b) or after acidification (Fig. 5.10c). In both cases the self-assembly was performed in two different modes:

1) applying a voltage between the two electrodes

2) without voltage between the two electrodes.

The first approach was executed in order to help the dielectric matter (PPE) to get pulled into the position where the electric field is most intense, that is in the center of the gap. The drawback could be that if the intensity of the electric field is too high, a random and strong molecular precipitation between the electrodes can take place. To avoid this risk a small voltage (1 Volt) was applied between the electrodes. In the latter case the molecular self-assembly is expected to be governed by the chemisorption of the functionalized molecules on Au.

The I-V characteristics of the electrodes and of the molecular aggregate adsorbed between them have been probed using a Keithley 487 picoammeter - voltage source commercial set-up interfaced to a personal Computer. This allowed to detect currents down to the 10-14 A range.


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