**2. Experimental**

new process. Its first practical use was developed in the 1880s in the production of incandescent lamps to improve the strength of filaments by coating them with carbon or metal [27]. CVD may be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapour phase. CVD has several important advantages over the liquid phase chemistry which make it the preferred process in many cases: absence of solvent, high deposition rate, good surface conformity. CVD equipment does not normally require ultrahigh vacuum and generally can be adapted to many process variations. Its flexibility is such that it allows many changes in composition during deposition and the co-deposition of elements or compounds is readily achieved. In this scenario, several kinds of CVD techniques are currently used to

The first one is the so called plasma-assisted or plasma-enhanced CVD (PE-CVD), introduced in the fabrication of semiconductor from 1960s and for long time restricted to the synthesis of inorganic compounds, becoming an essential factor in the manufacture of semiconductors and other electronic components. This technique is based on electrical energy rather than thermal energy to initiate homogeneous reactions for the production of chemically active ions and radicals that can participate in heterogeneous reactions, which, in turn, lead to layer formation on the substrate. A major advantage of PE-CVD over thermal CVD processes is that deposition can occur at very low temperatures, even close to ambient, which allows the use of temperature sensitive substrates [28]. Unfortunately, monomers injected into the glow discharge are often irregular and fragmentized by the electrons, ions/ radicals present into the glow discharge. Furthermore such plasma polymers are often irregular and have rather short chain lengths. Plasma polymerisation leads to a random poly-recombination of radicals and fragments of monomers. A variation could be represented by pulsed-PE-CVD. In this case, the glow discharge is pulsed and when it is switched on (plasma-on), the activate molecules can produce radicals which initiate the polymerisation mechanism. When the glow discharge is switched off (plasma-off), the residual radicals initiate a conventional chemical chain-growth reaction. In this case a more 'conventional' product is obtained but the chain propagation is restricted by the low probability of attaching a new molecule monomer to the radical at the growing chain-end. Moreover the deactivation of chain propagation is expected under the common vacuum conditions (0.1 Torr) owing to the recombination of neighbouring radicals. This leads to a significant loss of the active radical sites during the plasma-off period. Glow discharge is thus re-ignited to reproduce fresh initial radicals. The resulting polymer film has a structure and composition closer to its counterpart produced by liquid phase but it shows repeatable irregularities induced

Another known CVD method is the vapour phase polymerisation (VPP), where the solubility of the conjugated monomer is no longer required. VPP directly translates the step-growth mechanism established for the liquid phase synthesis via solvent-less environment [30]. VPP is usually performed by two-step process. The first one consists of the pre-application of the oxidant to the substrate, which is typically carried out by spin coating, while the second step consists of the exposure of the pre-treated substrate to monomer vapours under vacuum reaction chamber. The VPP delivers conducting polymers, like polypyrrole and PEDOT, using iron(III) chloride as oxidant [31, 32]. Spreading the oxidant all over the surface limits the

obtained PEDOT films.

142 Recent Research in Polymerization

by the plasma-on period [29].

PEDOT has been investigated as case study to prove the effectiveness of PRAP-CVD.

PRAP-CVD polymerisation results from surface reaction of the monomer and oxidant flows, here 3,4-ethylenedioxythiophene (EDOT) and bromine, respectively [41]. The oxidation of EDOT to form PEDOT shows some similarities to the oxidative polymerisation mechanism of pyrrole suggested by Diaz [42, 43]. The proposed polymerisation mechanism of EDOT to form PEDOT is illustrated in **Figure 1** [41].

A preliminary step takes place in a plasma chamber, where bromine radicals are generated while the glow discharge is switched on. Progressively they reach the reaction chamber where EDOT monomer is parallel injected. Bromination of EDOT molecules occurred in α position by the formation of a radical that shows several resonance forms. A dimer is formed by combination of two of these radicals. Substitution of the EDOT thiophene ring at the 3,4-positions blocks β-coupling, allowing new bonds only at the 2,5-positions. The dimer can be oxidised to form another radical following the steps described above. The alternating single and double bonds of the oligomers are π-conjugated, which delocalizes the electrons

**Figure 1.** Polymerisation mechanism of EDOT to form PEDOT. (1) Ignition of glow discharge fed with bromine gas allows the production of bromine radicals. (2) Bromination of EDOT molecules occurs in α position. (3) Radical polymerisation propagates. (4) Polymerisation stops when also α′ position is brominated [41].

and decreases the oxidation potential [34]. In PRAP-CVD the bromine acts as radical initiator as well as oxidant, and the final electronic arrangement presented in the polymer is depicted in **Figure 2**.

PRAP-CVD PEDOT, which is carried out in PRODOS-200 PVPD™ R&D System (AIXTRON SE, Herzogenrath, Germany), owing to the facilities of the machine and the mild experimental conditions, can be deposited on a wide range of substrates, both in terms of dimension and in terms of composition: from silicon wafer to fabrics and up to 15 × 15 cm<sup>2</sup> , as reported in **Figure 3**.

Raman studies were conducted to determine the vibrational modes of the different regions in the wavenumber range of 250–1800 cm−1, and to prove the obtained EDOT polymerisation.

**Figure 2.** Electronic arrangement of Br-PEDOT films. The bromine acts as radical initiator as well as oxidant [41].

**Figure 3.** Examples of substrates treated via PRAP-CVD to obtain PEDOT films: (a) silicon wafer (20 cm in dia), (b) glass (15 × 15 cm<sup>2</sup> ), and (c) PET fabric (12 × 12 cm<sup>2</sup> ) [41, 44].

PEDOT was deposited on silicon wafer, prevented covered with a thin layer of SiO<sup>2</sup> (deposited in house by Rapid Thermal-Chemical Vapour Deposition – RT-CVD) to avoid any interferences during the electrical measurements, as reported in **Figure 4**.

The spectrum of monomer was compared with that one of polymer. Six strong Raman bands were detected in the spectrum of EDOT, **Figure 4a**, at 1486, 1423, 1185, 892, 834, and 766 cm−1 [45]. The shape of spectra was completely different and the spectrum of PEDOT showed much more bands related to different vibration modes owing to presence of the polymer chain. In **Table 1** the assignment related to the bands are reported. A good agreement with literature data was found to support the accuracy of the proposed polymerisation mechanism [46, 47].

Furthermore, the confirmation that the polymerisation took place in the right way, i.e. without thiophene opening ring, in favour of correct PEDOT film formation, came also from the absence of the band at 1705 cm−1 [48].

and decreases the oxidation potential [34]. In PRAP-CVD the bromine acts as radical initiator as well as oxidant, and the final electronic arrangement presented in the polymer is depicted

**Figure 1.** Polymerisation mechanism of EDOT to form PEDOT. (1) Ignition of glow discharge fed with bromine gas allows the production of bromine radicals. (2) Bromination of EDOT molecules occurs in α position. (3) Radical polymerisation

PRAP-CVD PEDOT, which is carried out in PRODOS-200 PVPD™ R&D System (AIXTRON SE, Herzogenrath, Germany), owing to the facilities of the machine and the mild experimental conditions, can be deposited on a wide range of substrates, both in terms of dimension and in

Raman studies were conducted to determine the vibrational modes of the different regions in the wavenumber range of 250–1800 cm−1, and to prove the obtained EDOT polymerisation.

, as reported in **Figure 3**.

terms of composition: from silicon wafer to fabrics and up to 15 × 15 cm<sup>2</sup>

propagates. (4) Polymerisation stops when also α′ position is brominated [41].

in **Figure 2**.

144 Recent Research in Polymerization

The mild experimental conditions carried out in the PRAP-CVD allow even the treatment of sensible substrates as papers of fabrics without any damages. To prove this advantage of PRAP-CVD compared to the other CVDs, a few temperature sensitive substrates have been treated and a PEDOT thin layer was deposited. SEM micrographies were recorded and shown in **Figure 5** where (a) PET fabric, (b) cotton fabric, (c) linen fabric and (d) paper cellulose fibres are reported.

**Figure 4.** Raman spectrum of (a) EDOT and (b) PRAP-CVD PEDOT films recorded at an excitation wavelength of 633 nm.


**Table 1.** Observed frequencies of doped poly(3,4-ethylenedioxythiophene) with assignment of principal bands.

Samples have been investigated after the deposition without any further manipulation. Not only by PRAP-CVD it is possible to treat fibres but PRAP-CVD benefits of a quite unique property in terms of conformal deposition on complex substrates, as of the matter of fact, fibres are, **Figure 6**.

PET fabrics have been chosen as reference substrate for this series of characterizations. Correspondence of the textile morphology between the uncoated and coated sample illustrated the high conformal coverage of the fibres with polymer thin film, **Figure 6a** and **b**. The presence of PEDOT was confirmed by Energy Dispersive X-ray (EDX) spectroscopy investigation reported in **Figure 6c** and **d**, respectively. In the spectrum of PET, peaks of carbon (C) Ka at 0.3 keV and oxygen (O) at 0.5 keV were detected while the spectrum of fibres treated via PRAP-CVD highlighted the presence of sulphur (S) at 2.3 keV and bromine (Br) at 11.9 keV. Sulphur as well as carbon and oxygen could be attributed to the chemical composition of EDOT while bromine to the dopant. PEDOT film resulted in defect/pin-hole free, as attested by the high magnification SEM micrographs. In order to deeply prove this

**Figure 5.** SEM microscopies of PRAP-CVD PEDOT film deposited on (a) PET fabric, (b) cotton fabric, (c) linen fabric and (d) paper cellulose fibres. Samples have been investigated after the deposition without any further manipulation [44].

peculiarity of PRAP-CVD, a stripe of PET fabric covered with PEDOT was cut by means of a scalpel and analysed. In **Figure 7**, micrographs of the same PEDOT/PET area, recorded at different magnifications, are reported (**Figure 7a** and **c**).

Samples have been investigated after the deposition without any further manipulation. Not only by PRAP-CVD it is possible to treat fibres but PRAP-CVD benefits of a quite unique property in terms of conformal deposition on complex substrates, as of the matter of fact,

**Figure 4.** Raman spectrum of (a) EDOT and (b) PRAP-CVD PEDOT films recorded at an excitation wavelength of 633 nm.

**Table 1.** Observed frequencies of doped poly(3,4-ethylenedioxythiophene) with assignment of principal bands.

**Wavenumber (cm−1) Assignments**

**1366** νCβ-Cβ

**1096** νC-O-C

**439** δSO<sup>2</sup>

**1509** Asymmetric νCα = Cβ **1432** Symmetric νCα = Cβ (–O)

**1266** Inter-ring νCα-Cα

**699** Symmetric νC-S-C

**990, 573** Oxyethylene ring deformation

PET fabrics have been chosen as reference substrate for this series of characterizations. Correspondence of the textile morphology between the uncoated and coated sample illustrated the high conformal coverage of the fibres with polymer thin film, **Figure 6a** and **b**. The presence of PEDOT was confirmed by Energy Dispersive X-ray (EDX) spectroscopy investigation reported in **Figure 6c** and **d**, respectively. In the spectrum of PET, peaks of carbon (C) Ka at 0.3 keV and oxygen (O) at 0.5 keV were detected while the spectrum of fibres treated via PRAP-CVD highlighted the presence of sulphur (S) at 2.3 keV and bromine (Br) at 11.9 keV. Sulphur as well as carbon and oxygen could be attributed to the chemical composition of EDOT while bromine to the dopant. PEDOT film resulted in defect/pin-hole free, as attested by the high magnification SEM micrographs. In order to deeply prove this

fibres are, **Figure 6**.

146 Recent Research in Polymerization

The cut induced a physical convergence of fibres. The lighter part of image could be attributed to the PEDOT which responded differently from PET under the irradiation of the electron beam. In the same experimental conditions, PET, as all non-conductive materials, suffers of charging effects, revealing a darker image than PEDOT/PET. The same images were also artificially coloured by Image J software as reported in **Figure 5b** and **d** to easily identify the layer of PEDOT which conformably surrounded each fibre although the convergence of fibres. By SEM it was possible on one side to investigate the conformality of the film and on the other anticipated its electrical conductivity properties too. Following FIB cross-sectioning step, an accurate thickness measurement of film could be determined. A thickness of (215 ± 10) nm allowed to establish a deposition rate of 6 nm/min. This level of coating quality is unequalled to current wet and dry PEDOT synthesis. PRAP-CVD allows to modify only the surface of substrate without impacting on the bulk properties, as demonstrated by XPS spectra in **Figure 8** [41].

Chemical states of carbon (C 1s) in PET and PEDOT/PET C 1s were identified by XPS. In **Figure 8a** three chemical states of C 1s, reflecting the presence of the three peaks, were

**Figure 6.** SEM images of textile fibres (a) before and (b) after the PRAP-CVD process. Owing to the induced conductivity of PEDOT, the two images resulted in different colours, darker the raw PET and lighter the PEDOT/PET fibre. Respective EDX spectra identified the presence of (c) carbon and oxygen in raw PET and (d) also sulphur and bromine residues in the PEDOT coated fabric [44].

identified in the PET. They could be attributed to C–C/H bond (aliphatic/aromatic carbon atoms) (C1 - BE: 284.5 eV), C–O bond (methylene carbon atoms singly bonded to oxygen) – (C2 – BE: 286.0 eV), –COO– bond (ester carbon atoms) (C3 – BE: 288.5 eV) and π–π\* shake-up transition associated with the aromatic ring at 291 eV. By performing the best fitting procedure, the following percentage areas: 54.2% C1, 24.6% C2, 14.6% C3, and 6.6% shake-up were obtained [49]. In **Figure 8b**, the distribution of functional groups of PEDOT/PET C 1s showed three different components: C–C/H bonds (C1 – BE: 284.4 eV), C–S bonds (C2 – BE: 285.5 eV) and C–O bonds (C3 – BE: 286.1 eV). Additionally, an asymmetrical peak at about 288 eV was also identified and attributed to the contribution from the π–π\* shake-up transition and positively polarised or charged carbon [50]. Peak areas were calculated as 34.8% for C–C/H, 42.5% for C–S, 17.3% for C–O and 5.4% for the shake-up, respectively [51]. Bestfitting procedure was also applied to S 2p core-level, which corresponded to single sulphur bonding environment in PEDOT with a spin-split doublet, 2 p1/2 and 2 p3/2 separated by 1.18 eV in binding energy and an area ratio 1:2, as shown in **Figure 9**.

The pick presented an asymmetric tail at higher binding energy, which could be related to the doping process, where the delocalized π-electrons in thiophene ring broaden the binding

**Figure 7.** SEM micrographs of PET fibres cross-section with PEDOT film (a, c) at different magnification; (b, d) corresponded coloured picture at same magnification. The darker area corresponded to PET fibres. Conductive PEDOT film responded differently from PET while irradiated by the electron beam [44].

energy spectrum of the sulphur atom [52]. The quantitative analysis of PEDOT film showed the followed atomic percentage of 66% C, 21% O, 9% S, and 4% Br [41].

identified in the PET. They could be attributed to C–C/H bond (aliphatic/aromatic carbon atoms) (C1 - BE: 284.5 eV), C–O bond (methylene carbon atoms singly bonded to oxygen) – (C2 – BE: 286.0 eV), –COO– bond (ester carbon atoms) (C3 – BE: 288.5 eV) and π–π\* shake-up transition associated with the aromatic ring at 291 eV. By performing the best fitting procedure, the following percentage areas: 54.2% C1, 24.6% C2, 14.6% C3, and 6.6% shake-up were obtained [49]. In **Figure 8b**, the distribution of functional groups of PEDOT/PET C 1s showed three different components: C–C/H bonds (C1 – BE: 284.4 eV), C–S bonds (C2 – BE: 285.5 eV) and C–O bonds (C3 – BE: 286.1 eV). Additionally, an asymmetrical peak at about 288 eV was also identified and attributed to the contribution from the π–π\* shake-up transition and positively polarised or charged carbon [50]. Peak areas were calculated as 34.8% for C–C/H, 42.5% for C–S, 17.3% for C–O and 5.4% for the shake-up, respectively [51]. Bestfitting procedure was also applied to S 2p core-level, which corresponded to single sulphur bonding environment in PEDOT with a spin-split doublet, 2 p1/2 and 2 p3/2 separated by

**Figure 6.** SEM images of textile fibres (a) before and (b) after the PRAP-CVD process. Owing to the induced conductivity of PEDOT, the two images resulted in different colours, darker the raw PET and lighter the PEDOT/PET fibre. Respective EDX spectra identified the presence of (c) carbon and oxygen in raw PET and (d) also sulphur and bromine residues in

The pick presented an asymmetric tail at higher binding energy, which could be related to the doping process, where the delocalized π-electrons in thiophene ring broaden the binding

1.18 eV in binding energy and an area ratio 1:2, as shown in **Figure 9**.

the PEDOT coated fabric [44].

148 Recent Research in Polymerization

In the last years the demand of transparent flexible and conductive electrodes increased suddenly, in particular the research of materials to replace indium tin oxide (ITO). The main drawbacks presented by ITO are on one side the cost of indium itself and the remaining amount of this element the earth and on the other the brittleness of ITO based electrodes. PRAP-CVD PEDOT could answer to these requirements because as demonstrated it can be deposited on flexible substrate as plastic and it shows optical transparency properties, as reported in **Figure 3**. More in detail, transmittance and absorbance values of PEDOT deposited on glass in UV range were recorded and presented in **Figure 10**.

In **Figure 10a**, the transmittance of PEDOT thin films decreased while wavelength increased beyond 500 nm, this is owing to the presence of the free carrier tail but PEDOT still showed a value of transmittance over 70%.

In **Figure 10b**, the peculiar absorption feature, which characterises PEDOT film, known in the literature as a 'free carrier tail', is confirmed and can be attributed to the conductivity of the polymer films [53]. The presence of this band has been found and investigated for the first time in the doped polyaniline. It corresponds to the polymer having a longer conjugation length and greater order, which allows for greater mobility of charge carriers [54].

**Figure 8.** XPS C 1s high resolution spectra of (a) PET and (b) PEDOT/PET and relatively chemical states [44].

**Figure 9.** XPS HR high resolution spectra of S 2p.

PRAP-CVD: A Novel Technique to Deposit Intrinsically Conductive Polymers http://dx.doi.org/10.5772/intechopen.71736 151

**Figure 10.** UV-Vis-NIR spectrum of thin PEDOT films as a function of (a) transmittance and (b) absorbance.

**Figure 11.** (a) Conductivity of the 50 nm thick PEDOT film deposited on BOROFLAT® 33 as a function of the sample temperature. The measurements were performed both during cooling-down (red) and warming-up (blue) the sample. (b) Natural logarithm of the conductivity plotted as a function of 1000/T (black points) for the data measured during the cooling-down. The red curve is a fit of the data by Eq. (1), with the power α = 0.63 ± 0.07.

**Figure 9.** XPS HR high resolution spectra of S 2p.

150 Recent Research in Polymerization

**Figure 8.** XPS C 1s high resolution spectra of (a) PET and (b) PEDOT/PET and relatively chemical states [44].

**Figure 12.** Raman spectra of PEDOT film recorded all along 1 year [41].

Performances of films have been investigated also in terms of conductivity as function of temperature. **Figure 11** shows the conductivity as a function temperature in the range 130–350 K, both when cooling-down and warming-up the sample.

The good reproducibility showed that there was no structural changes in the film, and that the temperature-dependence of the conductivity was owing to the intrinsic carrier transport mechanism in the film. The observed decreasing in conductivity, while the temperature decreases, is typical for an organic conductor, where carrier transport usually occurs by hopping conduction. In order to further investigate the carrier transport mechanism, in **Figure 11b** the natural logarithm of the conductivity as a function of the inverse temperature, 1/T have been reported. It was observed a good fit with a power, implying the following dependence for the conductivity as reported in Eq. (1):

$$
\sigma = \sigma\_o \exp\left[-\left(\frac{T\_0}{T}\right)\alpha\right] \tag{1}
$$

The best fit was obtained with α = 0.63 ± 0.07. This value is far from 1/4, already reported for 3D variable range hoping (VRH), far from 1, as reported for nearest neighbour hoping, and far from 1/3, as reported for 2D variable range hoping [25, 55, 56]. The value is closer to 1/2, already reported for PEDOT:PSS and which corresponds to 1D VRH or hopping in a granular material where, the exponent α for 3D VRH varies from1/4 to 1/2 due to charging energylimited tunnelling between grains [57, 58].

Another requirement that PEDOT film has to show is the stability of the time. PEDOT films were exposed to ambient conditions over 1 year and its chemical structure was followed by Raman spectroscopy investigation, as reported in **Figure 12**.

Previous studies suggested that exposure to water vapour in the atmosphere could induce an increase in PEDOT, in particular PEDOT:PSS, sheet resistance as the hygroscopic polymer absorbs moisture over time [59, 60]. As shown in **Figure 12**, the main Raman peaks did not suffer of changing during the year. This different behaviour can be explained by the absence of PSS, which reveals all its acid character when exposed to the moisture.
