Anja Schröter

Photothermally Induced Microchemical Functionalization of Organic Monolayers

Photopatterning of organic coatings represents a key step in many technological applications ranging from microchip fabrication to the design of bioarrays and microfluidic devices. Fundamentally, these applications rely on photochemical proACHTUNGTRENNUNGcesses, in which chemical reactions are initiated via direct or substrate-mediated electronic excitations. In the simplest case decomposition of the coating takes place. A broad range of photochemical routines, though, also allows for local functionalization of organic coatings. The lateral resolution, in turn, usually is limited by optical diffraction, that is, the fabricated structures are not much smaller than the wavelength even when highly focusing optics is used. Scanning near-field photolithography, of course, allows for sub-wavelength patterning. Processing, though, is very slow and restricted to small areas. A means to enhance the lateral resolution of far-field optical techniques takes advantage of nonlinear effects. In photothermal laser processing, for example, a focused laser beam is used to locally heat the substrate surface and to thermally initiate chemical reactions. For this reason, photothermal processing is highly nonlinear in laser power density and facilitates sub-wavelength patterning. In recent years, organic monolayers have gained particular attraction as photothermally patternable platforms. Generally, the lateral resolution depends on the thermal and chemical stability of the coating. Strongly bound coatings, for example silane-based monolayers, can be patterned from the micrometer range down to the sub 100 nm range. Such patterns have been used as chemical templates to build up functional surface architectures from nanoscopic components. These results emphasize the capabilities of photothermal routines in microand nanofabrication of organic interfaces. Commonly, though, photothermal processing of organic monolayers results in local decomposition of the coating. In analogy to photochemical routines, of course, it is tempting to explore photothermal procedures which allow to locally functionalize organic monolayers. Such procedures open up a facile avenue towards more complex chemical surface structures such as multifunctional templates and chemical gradients. As a prototype example, we here address a photothermal procedure for local functionalization of alkylsiloxane monolayers on surface-oxidized silicon substrates in a gaseous bromine ambient. As outlined below, this procedure takes advantage of some characteristic features of common photobromination reactions. Photobromination of hydrocarbons represents a classical reaction in organic synthesis. Previous contributions also investigated large-area photobromination of polymer interfaces and organic monolayers. In conjunction with other chemical transformations this provides an efficient route to a broad variety of functional groups. Reactions (1)–(5) recapitulates the underlying radical reaction mechanism.

Substrate-mediated effects in photothermal patterning of alkanethiol self-assembled monolayers with microfocused continuous-wave lasers

Summary In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear laser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate subwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate surface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics, the overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing ordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the nature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this contribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon substrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic simulations were carried out in order to clarify the processes that determine the performance of the patterning technique. Because of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very narrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 µm, fabrication of subwavelength structures with diameters of 300–400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on Au/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This results in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on Au/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for patterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall patterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings.

Photothermally induced bromination and decomposition of alkylsiloxane monolayers on surface-oxidized silicon substrates

Photothermal laser processing of alkylsiloxane monolayers in gaseous bromine is investigated. Surface-oxidized silicon samples are coated with octadecylsiloxane monolayers and locally irradiated with a focused beam of an Ar+-laser at λ=514 nm and a 1/e2 spot diameter of 3 μm. For characterization, atomic force microscopy, scanning electron microscopy, and optical microscopy in conjunction with labeling techniques and condensation experiments are used. At low laser powers, monolayer bromination in micron-sized areas is observed. Additionally, at high laser powers, decomposition of the monolayer takes place at the center of the brominated areas. Prospects and limitations of this procedure in fabrication of multifunctional templates are discussed.

Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization.

Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e(2) spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates.

Micro-patterning of self-assembled organic monolayers by using tunable ultrafast laser pulses

We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer (SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers, which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of 480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated glass substrates with a film thickness of 1-2 nm and 40 nm respectively.

Sub-wavelength patterning of self-assembled organic monolayers via non-collinear optical parametric amplifier

Self-assembled monolayers (SAMs) are ultra-thin organic monolayers, which can be used in different ways to assemble functionalized surface structures. This potential is caused by the ability of the SAMs to tie further molecules and components through the terminal groups of the organic layers. Additional applications for microfluidics and micromechanics require micro and nano structuring of the SAMs. In combination with multi-photon lithography (MPL) SAMs are offering advantageous properties as ultra thin layers. Thus, the processing with single pulses is feasible and results in very short processing times without the appearance of bubbles and formation of particles compared to photo resists. For our experiments, we used a non-collinear optical parametric amplifier (NOPA) which has the ability to generate short pulses of sub-30fs in the visible and near-infrared (NIR) range of light. The NOPA can be tuned in the range of 480 nm to 950 nm without spectral gaps.We used thiol based SAMs as ultra thin layers on gold substrates. The selected laser power offers the possibility of ablation of the SAMs without damaging the gold layer. Thereby we investigate the characteristics of thiol-based SAMs as monomolecular resists during etching of gold. We also investigate the wavelength dependencies of the substrate to get an optimal process window for the ablation process. Minimum structure sizes at a 1/e laser spot diameter of about 1.6 µm are close to 1/5 of the spot diameter.Self-assembled monolayers (SAMs) are ultra-thin organic monolayers, which can be used in different ways to assemble functionalized surface structures. This potential is caused by the ability of the SAMs to tie further molecules and components through the terminal groups of the organic layers. Additional applications for microfluidics and micromechanics require micro and nano structuring of the SAMs. In combination with multi-photon lithography (MPL) SAMs are offering advantageous properties as ultra thin layers. Thus, the processing with single pulses is feasible and results in very short processing times without the appearance of bubbles and formation of particles compared to photo resists. For our experiments, we used a non-collinear optical parametric amplifier (NOPA) which has the ability to generate short pulses of sub-30fs in the visible and near-infrared (NIR) range of light. The NOPA can be tuned in the range of 480 nm to 950 nm without spectral gaps.We used thiol based SAMs as ultra thin layers o...