Jukka Lukkari

XPS study of irradiation damage and different metal–sulfur bonds in dodecanethiol monolayers on gold and platinum surfaces

The X-ray induced irradiation damage is studied in the CH3(CH2)11SH monolayers on gold and platinum surfaces to get information on the metal–sulfur bonds. This is important because the sulfur–substrate bond is one of the crucial factors in thiol SAMs. The X-ray photoelectron spectra (XPS) of the S 2p and C 1s emission were analyzed. During the modification of the surface the thiol/platinum system behaves differently than the thiol/gold system. In the latter case the S 2p spectrum changes as a function of time during the measurement and two chemically different sulfur species, associated with Au-thiolates and irradiation induced sulfur species, can be identified in the spectra. On platinum, there are two chemically different sulfur species in pristine monolayers and their ratio remains unchanged as a function of measurement time. This trend is obviously surface induced and is related to the nature of the metal–sulfur bonds in the self-assembled thiol layers.

Effect of anions on the transient redox behavior of polypyrrole in anhydrous acetonitrile

Abstract The transient redox behavior of polypyrrole in acetonitrile containing 10 ± 1 ppm water has been studied in the presence of the spherical anions ClO 4 − , BF 4 − and PF 6 − by a spectroelectrochemical method. The rate of absorbance changes as well as the transferred charge values registered after different potential steps have been correlated to the radii and association tendencies of the anions participating in the redox process. It is concluded that ‘net’ effect of the dopant can be observed only when hydration of the anions is excluded as far as possible.

A thermodynamic approach to the interpretation of anodic and cathodic doping of poly(3-methylthiophene)

Abstract Anodically and cathodically conducting poly(3-methylthiophene) have been studied by transient and steady-state spectroelectrochemical methods in the presence of different anions (ClO−4, PF−6) and cations (BU4N+, Dec4N+) in acetonitrile solution of low water content. Spectral differences obtained with different ions are interpreted by assuming segments of different effective conjugation lengths in the neutral polymer determined by an interaction between the anions and the chains during the electrochemical preparation of the film. A redox mechanism based on thermodynamic considerations is suggested to describe the anodic and cathodic doping of thiophene-type conducting polymers. Both dopings start with the electrochemical transformation of species of longer effective conjugation length. Cyclic measurements are strongly controlled by non-equilibrium or quasi-reversible effects explained by the assumption of a two-phase system resulting in the undoping not being the simple inverse process of doping.

In situ video recording of the nucleation enhancement in the electropolymerization of 3-methylthiophene

Abstract A new electrochemical technique, in situ video recording of electrochemical processes, is described and applied to the study of the nucleation of 3-methylthiophene on indium-tin oxide (ITO). The method utilizes a video camera attached to a microscope for recording processes occurring at optically transparent electrodes. The recordings are then later analysed with an image processing program. Application to the polymerization reveals that on bare clean ITO only a small amount of nuclei are formed and they are unevenly distributed on the surface, the process being enhanced by surface irregularities. Nucleation on ITO can be artificially enhanced either by using high initial current pulses or by modifying the electrode surface with thiophene-containing species.

Cyclic spectrovoltammetry: a new method to study the redox processes in conductive polymers

Abstract We describe a new spectroelectrochemical method called cyclic spectrovoltammetry. The essence of the method is the application of factor analysis to the spectra recorded during a voltammetric measurement. Factor analysis enables us to use all the information contained in the spectra, a considerable improvement to former techniques employing data at only some wavelengths. When applied to poly(3-methylthiophene), two successive oxidation processes can be observed during the single oxidation peak in the cyclic voltammogram. The usefulness of the method in cases where voltammetric measurements are difficult to interpret is also demonstrated.

Evolutionary spectral factor analysis of doping-undoping processes of thin conductive polymer films

Abstract Mathematical methods based on evolutionary factor analysis are applied to the doping-undoping processes of poly(3-methylthiophene). Two new methods are developed in this work: differential evolutionary factor analysis (DEFA), which is sensitive to changes in the composition of the mixture during the evolution process, and the projection matrix method, which can be used to annihilate the influence of a known spectrum from a set of measured spectra in order to obtain information about the unknown components.

Redox processes in thick films of polypyrrole/dodecylsulfate in the presence of alkali and tetramethylammonium chlorides

A complete understanding of the mechanism of charge transport in conducting polymers is essential to the design and optimization of these materials. In this work we present a systematic study of the redox behavior of thick polypyrrole/dodecylsulfate films in aqueous solutions of alkali and tetramethylammonium chlorides. The results suggest that under these experimental conditions alkali cations retain most of their solvation shell when entering the polymer. The transformation of the redox state starts from the solution side of the polymer and moves towards the electrode. The size of the hydrated cation determines whether the counter ions move as a uniform conductive front or whether they prefer moving through some pores in the polymer. The polymer was found to be irreversibly altered after the first redox cycle.

Self-Assembled Carbon Nanotubes on Gold: Polarization-Modulated Infrared Reflection−Absorption Spectroscopy, High-Resolution X-ray Photoemission Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectroscopy Study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.

Ultrathin polyelectrolyte multilayers: in situ ESR/UV-Vis-NIR spectroelectrochemical study of charge carriers formed under oxidation

Redox cycling of self-assembled polyelectrolyte multilayers, consisting of well-defined water-soluble electronically conducting poly-3-(3′-thienyloxy)propyltriethylammonium (P3TOPA) and insulating PSS (poly(sodium-4-styrenesulfonate)) on indium–tin oxide (ITO) electrode, or poly-3-(3′-thienyloxy)propanesulfonate) (P3TOPS) and insulating poly(diallyldimethylammonium chloride) (PDADMA) on Au electrode, were investigated using in situ ESR/UV-Vis-NIR cyclovoltammetry and modulated electroreflectance. The spectroelectrochemical responses of (P3TOPA/PSS)n multilayers on ITO were compared with the spectroelectrochemistry of P3TOPA dissolved in aqueous or DMSO solutions. We found that both in solution and in the film the radical cationic structures (polarons) were primarily formed upon oxidation in chains of high conjugation length. However, they fully disproportionate into neutral and dicationic segments and the spinless charge carriers dominate, similarly to recent results with well defined long chain oligothiophenes and ladder type indenofluoranthene oligomers. A similar spectroelectrochemical behaviour was observed for the (P3TOPS/PDADMA)n system and its electroreflectance data indicate that the generation of polarons was markedly slower than the production of bipolarons. We compare these new results on well-defined polymer chains with the previous in situ spectroelectrochemical studies of the electrochemically prepared conducting polymer layers (PT, PMeT, PPy, PANI) in order to find a general scheme to describe the redox behaviour of different films with various degree of structure homogenity.

Characterisation of poly(3,4-ethylenedioxythiophene) films polymerised in aqueous media

Highly conducting poly(3,4-ethylenedioxythiophene) films have been electrosynthesised in aqueous medium. The highest in situ conductivity of the films obtained in pure aqueous medium was of the same order than with the films polymerised in organic solvents. The UV-Vis characterisation of the films showed that the average conjugation length of the polymer chains was affected only slightly by selecting water as the electrosynthesis medium. Furthermore, FTIR measurements indicated that no ring opening or overoxidation of the films had occurred in either media.