Wolfgang Kern

In situ characterization of the SEI formation on graphite in the presence of a vinylene group containing film-forming electrolyte additives

Abstract Acrylic acid nitrile (AAN) is introduced as a novel example out of the large class of vinylene groups containing film-forming additives for lithium-ion batteries. The electrochemical behaviour, especially the electrolyte additive reduction and the associated film formation in the presence of this compound is investigated with the in situ methods of Fourier transform infrared (FT-IR) spectroscopy and electrochemical quartz crystal micro balance (EQCMB). The results clearly point at a solid electrolyte interphase (SEI) formation mechanism, which proceeds via the cathodically induced polymerization of AAN. We suggest that the electro-polymerisation of vinylene groups is a main electrolyte reduction mechanism for a vinylene group containing electrolyte additives. The outstanding filming properties of vinylene compounds such as AAN allow the use of graphitic carbon anodes in PC-based electrolytes even when only 1% of the additive is present in the electrolyte.

Refractive index modulation in polymers bearing photoreactive phenyl and naphthyl ester units using different UV wavelengths

Tuning and patterning of the refractive index in polymeric films is of importance for modern optical applications such as wave guiding and optical data storage. In this contribution, we report on the synthesis of new photoreactive polymers bearing aryl esters which undergo the photo-Fries rearrangement to give aromatic hydroxyketones. Poly(norbornene)-based polymers bearing naphthyl ester groups (poly-2) and copolymers bearing both phenyl and naphthyl ester groups (poly(1-co-2)) were prepared by ring opening metathesis polymerization (ROMP). The photochemistry of these polymers under irradiation with UV light of either 254 nm or 313 nm was investigated. The photoreaction causes an increase of the refractive index in poly-2 between Δn589 = +0.022 and +0.048 depending on the irradiation wavelength. The copolymer poly(1-co-2) offers the possibilities of selectively exciting either the naphthyl ester with UV light of 313 nm causing an increase in the refractive index of Δn589 = 0.010, or both the naphthyl and phenyl chromophores with UV light of 254 nm (Δn589 = 0.036). When thin films of poly(1-co-2) already irradiated with 313 nm light are subjected to a second illumination step with 254 nm light, a significant decrease of the refractive index (Δn589 = −0.043) is observed. The reason for this behaviour seems to arise from the formation of different photoproducts depending on the irradiation sequence. Thus the final refractive index can be adjusted to values between n589 = 1.564 to n589 = 1.633. The two step illumination procedure provides the possibility of erasing and even inverting the index contrast generated during the first illumination step.

Organic surface emitting laser based on a deep-ultraviolet photopolymer containing thiocyanate groups

Gratings for laser applications are being optically recorded in poly(styrene-co-4-vinylbenzyl thiocyanate) (PST-co-VBT) by deep-ultraviolet lithography (λ<300 nm). The gratings writing configuration is based on interference patterning of this holographic material. Grating structures can be further stabilized and modified by selective amine reaction from the gas phase. Gratings based on refractive index contrast (Δn∼+0.02) and on thickness modulation (Δd up to 70 nm) are obtainable from this photoreactive polymer. Holographically recorded structures in films of PST-co-VBT doped with a laser dye are designed for second order distributed feedback operation resulting in low threshold organic laser devices (wL,th=0.25 μJ cm−2) with laser emission output (TE-mode: λL=632 nm) directed perpendicular to the film plane.

Photoreactive polymers of 4-vinylbenzyl thiocyanate

Abstract 4-Vinylbenzyl thiocyanate (VBT) was synthesized as a new monomer. It was copolymerized with styrene (ST) to poly(VBT-co-ST) and polymerized to the homopolymer (PVBT). The molar fraction of VBT in the copolymers varied between 0.05 and 0.45. The lithographic properties of the materials under deep u.v. irradiation (248 nm KrF excimer laser) were studied. Depending on the copolymer composition, the gel dose E g was found between 6.7 and 262 mJ cm −2 , which corresponded to quantum efficiencies Φ of intermolecular crosslinking between 12.3 · 10 −3 and 3.8 · 10 −3 . The observed variation of the sensitivity with the composition of the copolymers was analogous to results obtained for halogenated polystyrenes under electron beam irradiation.

Lasing Action in Optically Written Two-Dimensional DFB Gratings

Gratings for laser applications in poly(styrenc-co-4-vinylbenzyl-thiocyanate) (PST-co-VBT) are being optically recorded by deep UV lithography (λ = 266nm). The gratings writing configuration is based on interference patterning of this novel-type holographic material. Using lower intensity laser irradiation, refractive index gratings are produced in PST-co-VBT by an UV induced photoisomerization SCN-NCS. Laser irradiation with higher pulse energies directly produces surface relief gratings via laser ablation. Holographically recorded structures of dye-doped PST-co-VBT (DCM:PST-co-VBT) are designed for second order distributed feedback (DFB) operation resulting in low threshold organic lasers devices. 2D DFB lasers with two perpendicular gratings having different grating periods are investigated with respect to tuneable DFB lasing.

Reductive photo-patterning of conjugated thin films

Abstract The patterning of conjugated organic materials is of general interest for controlling the emission properties of light emitting devices (LEDs). We employed UV irradiation in the presence of gaseous hydrazine (N 2 H 4 ) to inscribe patterns in films of various π-conjugated polymers. Among those are derivatives of PPV and polyfluorene. UV illumination in N 2 H 4 atmosphere causes a hydrogenation of olefinic double bonds and aromatic rings, which is accompanied by the introduction of nitrogen functionalities. The impact of the reductive process on the optoelectronic properties of the materials is investigated. It is found that the photo-bleaching of the PPV type polymers results from the breaking of π-conjugation. In contrast to oxidative bleaching, the creation of non-radiative recombination centers is of minor importance. It is demonstrated that the process of reductive photo-patterning is highly efficient and can be applied for structuring the emission of thin films of conjugated polymers.

Spectral encoding of the polarization state of light in multiplexed polarization-sensitive lasing structures

We report the experimental demonstration of spectral encoding of the polarization state of light by use of multiplexed distributed-feedback lasers in a dye-doped organic thin film by exploiting the pump polarization selectivity of the superimposed resonators. Measurement of the Stokes parameters of the pump light without a priori knowledge of the lasing properties of the structure is discussed.

Photoreactive copolymers of 4-(thiocyanatoacetyl)phenyl methacrylate

Abstract 4-(Thiocyanatoacetyl)phenyl methacrylate (TPMA) was copolymerized with phenyl methacrylate (PMA) or styrene (ST) to poly(TPMA-co-PMA) or poly(TPMA-co-ST). These copolymers were photocrosslinked by laser u.v. irradiation (308 nm). The quantum efficiency o of crosslinking was determined by means of sol/gel analysis. o varied from 1.3·10 −3 to 7.6·10 −3 with the copolymer composition. A linear dependence of o on the TPMA content was found for poly(TPMA-co-PMA).

Reductive Photopatterning of Phenylene-Vinylene-Based Polymers

Photochemical methods were developed to obtain a variation of the refractive index in aromatic polymer surfaces and a change in the photoluminescence characteristics of phenylenevinylene-based polymers. Films of aromatic polymers, among them polystyrene (PS), poly(2-vinylnaphthalene) (PVN) and derivatives of poly( p -phenylene-vinylene) (PPV) were UV irradiated in the presence of gaseous hydrazine (N 2 H 4 ). The photoreaction led to a strong reduction of the refractive index of the polymers due to a hydrogenation of the aromatic units. In the case of PPV, we observed reductive photobleaching. This new technique was employed to produce photogenerated patterns in PPV. The results are compared to oxidative bleaching.

Photoreactive self assembled monolayers for tuning the surface polarity

In this contribution the modification of gold surfaces by self assembled monolayers (SAMs) of the photoreactive compound 11-mercaptoundecanoic acid, phenyl ester (MUAP) is presented. Upon irradiation with UV-light (λ = 254 nm) the phenyl ester groups photoisomerize to give hydroxyketones (photo-Fries reaction). Due to the formation of polar hydroxy groups the surface tension of the SAMs changes. The photogenerated hydroxyl groups were selectively modified with perfluorobutyryl chloride. This postexposure modification led to a significant change in wetting behaviour and surface energy and XPS indicates that patterning is possible.