Oriented growth of pentacene films on vacuum-deposited
  polytetrafluoroethylene layers aligned by rubbing technique
Microsoft Word - Oriented_growth___for_publish.doc
Oriented growth of pentacene films on vacuum-deposited 
polytetrafluoroethylene layers aligned by rubbing technique 
Marius Prelipceanu∗,1, Otilia – Sanda Prelipceanu1, Ovidiu-Gelu Tudose1,  
Konstantin Grytsenko1,2 , Sigurd Schrader 1  
1University of Applied Sciences Wildau, Faculty of Engineering, Department of Engineering 
Physics, Friedrich-Engels-Strasse 63, D-15475 Wildau, Germany. 
2Institute of Semiconductor Physics, 45 Nauki pr., Kyiv, 03028, Ukraine. 
Abstract 
A new method for preparation of high quality dielectric thin films made of 
polytetrafluoroethylene (PTFE) is described. This method includes film formation by 
means of a special kind of vacuum deposition polymerization (VDP) of PTFE, assisted 
by electron cloud activation. Rubbing of those layers makes them orienting substrate 
materials which induce spontaneous ordering of deposited chromophore layers. We 
investigated structure and morphology of PTFE layers deposited by vacuum process in 
dependence on deposition parameters: deposition rate, deposition temperature, electron 
activation energy and activation current. Pentacene (PnC) layers deposited on top of 
those PTFE films are used as a tool to demonstrate the orienting ability of the PTFE 
layers. The molecular structure of the PTFE films was investigated by use of infrared 
spectroscopy. By means of ellipsometry, values of refractive index between 1.33 and 
1.36 have been obtained for PTFE films in dependence on deposition conditions. Using 
the cold friction technique orienting PTFE layers with unidirectional grooves are 
obtained. On top of these PTFE films oriented PnC layers were grown. The obtained 
order depends both on the PTFE layer thickness and on PnC growth temperature. 
Keywords: polytetrafluoroethylene, vacuum deposition polymerisation, orientation, 
organic insulator, semiconductor, pentacene. 
                                                 
∗ Marius Prelipceanu, Tel: +49 3375 508 524, Fax: +49 3375 508 503, E-Mail: mariusp@igw.tfh-wildau.de 
1. Introduction 
Organic electronics, in particular, organic field effect transistors (OFET) is a fast 
developing field of research and technological development [1-3]. Pentacene (PnC) is 
one of the most extensively studied organic semiconductors for OFETs due to its 
relatively high carrier mobility [2]. Ordered molecular materials are used in electronic 
and photonic organic devices for obtaining anisotropic properties. Therefore, techniques 
for formation of high-quality films play an important role in the development of organic 
thin film devices. For such applications, uniform films with the thickness range from 
nanometers to submicrons are required. For electronic applications, film purity and 
interface characteristics influence the charge transport and energy transfer processes. 
For optical applications, controlling of dipole orientation is required as well as uniform 
thickness and low scattering loss. It is not easy to fulfill all these requirements by the 
wet processing. On the other hand, stable polymers like polytetrafluoroethylene (PTFE) 
do not dissolve in any solvent. Therefore, vacuum-based dry processing is the only 
possible method for deposition of such polymers. Some polymers can be evaporated by 
heating in vacuum, but for complex polymers low temperature plasma polymerisation 
should be used. Primary polymer degradation products are generated by the scission of 
the molecular chain at various sites and/or the cleavage of side groups or atoms. 
Depending on the nature of the polymer structure, the scission of polymer chains can 
occur either randomly or in an ordered depolymerisation mechanism. PTFE films were 
deposited in vacuum, but with a modified technique, which includes electron cloud 
activation of the decomposition products [4]. Since the discovery of the friction transfer 
method of PTFE hot friction transfer has been used extensively to prepare substrates 
materials on top of which deposited chromophores form oriented layers by self-
organization [5-7]. Recently it was found that vacuum deposited and rubbed PTFE films 
also support growth of oriented dye layers [8-10]. Using a series of measuring 
techniques (e.g. ellipsometry, optical and infrared spectroscopy and atomic force 
microscopy) we investigated physical and optical properties of vacuum deposited PTFE 
and PnC thin films formed on top of these PTFE layers in order to find optimal 
conditions for deposition of highly oriented PnC films. 
2. Experimental 
2.1. Description of PTFE and PnC 
PTFE is a linear polymer having the chemical structure shown in figure 1a. 
(a)  polytetrafluoroethylene (PTFE) (b)  pentacene
Fig.1. Chemical structure of: (a) polytetrafluoroethylene (PTFE), (b) pentacene 
(PnC). 
PTFE can be considered to be a suitable organic material serving as gate dielectric in 
organic field-effect transistor (OFET) devices because of its physical and chemical 
properties: very good chemical, photochemical and thermal stability, low dielectric 
constant, very low conductivity and high breakdown filed strength. PTFE is one of the 
most thermally stable plastic materials manifesting no appreciable decompositions 
below 260°C.  
The chemical structure of pentacene which consists of five annulated benzene rings 
is shown in fig. 1b. Due to its flat conformation it can easily form crystals, which show 
highly anisotropic transport properties. Pentacene has a molar mass of 278.35 grams. 
The melting point is at about 300°C and the heat of vaporization is 74.4 kJ/mol. 
2.2. Deposition technique 
The preparation of PTFE films was carried out by use of a special vacuum 
deposition technique. The films were obtained by evaporation of bulk PTFE pellets in 
the temperature range between 300° and 450°C with electron cloud-assisted activation 
with typical process pressure of 10-2 Pa, an accelerating voltage of 1-3 kV and an 
electron activation current of 0 – 5 mA as proposed before [4,11]. The electron cloud 
was produced by an electron gun with a ring cathode. A computer equipped with a 
quartz oscillator card Sigma SQM-242 was monitoring the film thickness and deposition 
rate. The temperature of the crucible was monitored by a chromel-alumel thermocouple. 
The deposition rate depends both on the electron current used for activation and on the 
temperature of the crucible. At the start of a deposition run, the increase of PTFE 
temperature in the evaporator causes an increase of both pressure and deposition rate. 
Fragments are colliding with each other before reaching the substrate, losing their 
chemical reactivity by forming stable gaseous species, which will not be incorporated 
into the deposited layer on the substrate. The evaporation rate is limited by the fact that 
the pressure can rise only to a certain value at which a breakdown of the electrical gun 
occurs. Hence, there exists an operation heating temperature [4, 11], which strongly 
depends on the pumping speed of the vacuum system and should be determined for each 
installation. This method gives the possibility to have a fast control of the evaporation 
rate, that in general is limited by the thermal inertia of the crucible, but in this method it 
is controlled instantly by the electrical power, that produces the activating cloud of 
electrons and, therefore, changing the quantity of active species. All PTFE films were 
deposited at a substrate temperature of 200C. Fig. 2 shows schematically the deposition 
installation used for the PTFE film deposition. 
Fig.2. Deposition set-up used for PTFE and pentacene deposition. 
PnC for fluorescence was purchased from Sigma-Aldrich and used as received. PnC 
films were deposited onto rubbed PTFE layers using conventional a tantalum boat 
heated by electric current. Important parameter which governs film formation is the 
temperature of the substrate. Related to this temperature, kinetic limitations such as 
molecular mobility, crystallization speed, and other thermodynamic factors are 
controlling the structure and morphology of the film. The substrate temperature was 
kept constant at room or elevated temperature and monitored by a chromel-alumel 
thermocouple. The deposition rate was chosen in the range between 0.05 and 0.2 nm/s. 
The distance between evaporators and substrate was 0.15 m. 
2.3. Mechanical rubbing method 
The PTFE layers deposited onto different substrates were rubbed in an unidirectional 
mode on a cotton surface used to clean optical systems. The cotton for friction was 
placed in a fixed position on an optical table. The samples were rubbed 3- 6 times on a 
cotton surface with a constant force and speed. The scheme for cold friction is shown in 
Fig. 3. 
Fig.3. Schematic representation of the cold friction technique applied to a PTFE 
layer. 
2.4. Studies of the deposited films 
The surface morphology of the films was obtained using an Atomic Force 
Microscope (Autoprobe VP 2 Park Scientific Instruments), operating in non-contact 
mode in air at room temperature. The mean thickness and index of refraction (n) of the 
PTFE films were determined by means of ellipsometry using a Plasmos SD2000 
Automatic Ellipsometer operating at a wavelength of 632,8 nm. The thickness of the 
investigated thin films was measured using a Dektak Profilometer (DEKTAK 3 from 
Veeco Instruments) device, which has the capability of measuring the step height down 
to a few nm. Polarized absorption spectra of pentacene films were obtained with a 
UV/VIS Spectrometer (Lambda 16 Perkin Elmer). Measurements of infrared spectra of 
PTFE films have been carried out by use of a Perkin-Elmer Spectrum 2000 fourier 
transform infrared (FT-IR) spectrometer. 
3. Results and discussions 
The analysis of the results, obtained using electron cloud assisted activation 
evaporation revealed that only a few important processes determine the film properties. 
Fig. 4 shows the influence of the evaporator temperature on the layer thickness at 
constant deposition time of 10 minutes. The presented curve stops well below the 
limiting pressure above which a decrease of deposition rate occurs due to the reason 
described above [4]. With increase of electron activation current the deposition rate and 
resulting film thickness are increasing. The maximum deposition rate obtained at a 
limiting pressure of 5-6 x10–2 Pa was 0.18 nm/s at an activation current of 10 mA and a 
voltage of 3 kV. The surface relief of PTFE films deposited at different conditions onto 
silicon substrate is shown in Fig. 5. 
Fig.4. Dependence of layer thickness on crucible temperature after 10 minutes of 
deposition, I=2mA, V=1.5 kV. 
Fig.5. Surface morphology and profile of PFTE films: (a) 2 mA and (b) 1 mA 
electron current activation, respectively. 
The surface of all PTFE films is smooth. For smaller electron activation current, a 
larger granular structure on the surface is detected. Root mean square (RMS) roughness 
is 1 nm and 3 nm, respectively. The obtained RMS values indicate a smoother surface 
occurs at higher electron activation energy. Ellipsometry results confirm the AFM 
investigations: thicknesses determined by both methods are comparable. In addition, a 
change of refractive index in dependence on electron activation energy and on current 
density was found, as determined by means of ellipsometry. Thus, electron activation 
parameters affect the surface morphology and refractive index of the PTFE films. 
Table 1. Refractive index of PTFE films versus activation conditions. 
The IR spectra of deposited films under different activation are depicted in Fig. 6. 
The bands at 1161 and 1258 cm-1 was assigned to the -CF2- groups, the band at 1350 cm-
1 to groups with a double bond. The intensity of the bands at 524 and 556 cm-1 is lower, 
than the intensity of the band at 736 cm-1, thus indicating, that the material of the films 
is almost amorphous [11-13]. Normally at low electron activation PTFE layers are 
crystalline [4, 13]. An increase of electron activation current makes the films amorphous 
and increases the content of double bonds and side branches. Here at low activation 
power almost amorphous films with some double bonds but almost without branches 
were deposited. 
Fig.6. IR-spectra of a 500 nm-thick films, deposited by PTFE evaporation under 
following conditions: 1 with activation current 1,5 mA; 2 with activation current 2 mA. 
Inset: the magnification of IR spectra in the range from 500 to 1000 cm-1 is shown. 
After rubbing with a cotton cloth, the film surfaces were investigated by AFM and 
profilometry techniques. The film surface acquired ordered relief oriented in the 
direction of friction. Fig. 7 shows the relief and the profile of the PTFE layer after 
friction. The PTFE grooves have 10 -100 µm length, and about 300 nm height. Also, we 
can see that the spectral lines show exactly the linear structure of PTFE. 
Fig.7. The 1 µm × 1 µm AFM scan of rubbed PTFE films:  (a) 3D image of the film 
relief; (b) profile of a series of grooves. The groove length is about 100 nm, and the 
height is 300 nm.  
Fig.8. Polarized absorption spectra of pentacene films for the parallel (dotted 
curves) and perpendicular (solid curves) orientation of the electric vector of light in 
respect to the PTFE layer alignment.  Films were deposited at the following substrate 
conditions: a – onto 36 nm PTFE at 200C, b – onto50 nm PTFE at 200C, c – onto 90 nm 
PTFE at 750C, d – onto50 nm PTFE at 750C. Film thicknesses by quartz monitor: a) 
and b) – 75 nm, c) and d) – 80 nm. Band splitting at the main absorption is 30, 34, 40 
and 40 nm for a), b), c) and d) respectively. 
Fig.9. Surface relief of PnC film onto rubbed PTFE sublayer. 
Measurements of electronic absorption spectra of the pentacene films deposited onto 
rubbed PTFE layers of different thickness have shown that orientation of the PnC films 
depend on both the PTFE film thickness and on substrate temperature. Optical spectra of 
some PnC films are presented in Fig.8. They are in a good agreement with spectra of α- 
and β- phase of PnC films, deposited onto both inorganic and polymer substrates, 
including PTFE [7, 14]. Rubbed PTFE films of about 50 nm thickness lead to the best 
oriented PnC films. Absorption measurements with polarized light have shown that the 
deposited PnC films show a pronounced dichroism. A dichroic ratio of about 2 was 
measured even at deposition temperature of 20°C. This dichroic ratio is larger than 
obtained for PnC films deposited onto friction transferred PTFE layers, and for 
deposition at 20°C there no dichroism was observed at all. The temperature elevation 
from 20°C to 75°C slightly enhances the PnC film orientation and changes the spectral 
shape. The latter two effects can be explained by the PnC molecular mobility 
enhancement. The former one is subject for further detailed studies. A little difference in 
the spectral shape indicates different molecular interactions inside of the PnC crystals 
dependent on deposition conditions. The crystal size and structure is also sensitive to the 
deposition conditions and results in modification of the absorption spectra. The optical 
spectra of the PnC films deposited at 75°C show a small shift of all bands towards to red 
region and an increase of band splitting in comparison with bands of the films, deposited 
at 20°C, thus evidencing better intermolecular interactions in films, deposited at elevated 
temperature. Both PTFE film thickness and substrate temperature allow controlling this 
parameter in order to deposit PnC films with predetermined properties. The absorption 
of films deposited at 75°C is smaller than the absorption of films deposited at 20°C, 
although the quartz monitor thickness was the same for both samples. Obviously, a re-
evaporation took place already at 75°C as mentioned before. By AFM no preferred 
crystal orientation was found in all PnC films. The typical relief of a PnC film on a 
PTFE aligned layer is shown in Fig.9a. The crystal size is in the range of 80 to 200 nm, 
depending on deposition conditions. Sometimes freely distributed needle-like PnC 
crystal with long axis up to 500 nm appeared (Fig.9b). Such films have low optical 
anisotropy. Therefore, the optical anisotropy of PnC films is due to the unidirectional 
arrangement of PnC molecules inside all crystals. Comparison of the obtained PnC 
crystals with those grown on friction-transferred PTFE layers shows that the crystals 
grown on the vacuum deposited, rubbed PTFE layers have smaller size and more round 
shape. The like effect was found for the growth of squarylium dyes on such vacuum 
deposited PTFE films [10]. This effect is caused by a smaller relief of the surface of the 
vacuum-deposited and rubbed PTFE film in comparison to the friction transferred films. 
In addition, some differences in the structure of friction-transferred and vacuum-
deposited PTFE also plays a role. The PnC nucleation directed by PTFE edges are the 
main mechanism of growth of oriented PnC film as it was proposed by Brinkmann et. 
al.[7]. They observed that the top material domains have been enforced to grow parallel 
to the ledge direction due to the confinement by the PTFE nanofibrils. Only when the 
height of these domains exceeds that of the ledge the lateral growth of the domains is 
possible. The opinion about the prevailing influence of PTFE aligned on the molecular 
level onto dye oriented growth was expressed previously by Tanaka et. al. [8] and 
Wittmann et al. [5, 6, 7]. Our results seem to support the latter opinion, but the 
amorphous structure of our PTFE films should be taken into account. Perhaps, both 
mechanisms are taking place with different contributions in dependence on both the 
sublayer properties and deposition conditions. But even this suggestion does not explain 
all peculiarities, so further research should be carried out. 
4. Conclusions 
Amorphous PTFE films with RMS roughness of 1-3 nm were deposited by electron 
cloud-assisted deposition in vacuum. Aligned grooves and ridges on the PTFE film 
surface were obtained by rubbing with a cotton cloth. PTFE film thickness and growth 
temperature elevation influence anisotropy of pentacene film. A dichroic ratio about of 2 
was obtained even when the substrate was held at room temperature. The pentacene film 
is oriented on the molecular level.  
The strength of this technique is that the vacuum deposited, and rubbed PTFE layers 
have a higher orienting power than friction transferred PTFE layers so that they may 
favorably be used in OFETs as bottom gate dielectric which induces enhanced order in 
the channel material deposited on top of them. In addition, the vacuum deposited PTFE 
layers can also be used in top gate geometry, i.e. by deposition on top of OFET channel 
materials on plastic substrates.  
5. Acknowledgements 
The authors would like to thanks to Dagmar Stabenow (University of Potsdam), 
Ramakrishna Velagapudi (University of Applied Sciences Wildau) for the AFM and 
optical measurements and Dr. Oleg Dimitiriev (Institute of Semiconductor Physics, 
Kyiv) for the fruitful discussions. Financial support of the European Commission under 
contract number: HPRN-CT-2002-00327-RTN EUROFET and of Federal Ministry of 
Education and Research (BMBF) Project under no. Ukr 04/004 is gratefully 
acknowledged. 
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