S. L. Johnson

Effects of the absorption coefficient on resonant infrared laser ablation of poly(ethylene glycol)

We describe experiments on resonant infrared laser ablation of poly(ethylene glycol) (PEG) at two different resonant excitation wavelengths and for different molecular weights of PEG. The two resonant wavelengths correspond to different stretching vibrations of the polymer and have absorption coefficients that differ by roughly an order of magnitude. Ablation via excitation of the O–H terminal group stretching mode at 2.94 μm, the weaker of the two absorptions, is delayed in time by several microseconds with respect to ablation at 3.47 μm, the more strongly absorbing C–H stretching mode of the polymer. Time-resolved plume shadowgraphs along with ablation rate measurements for the two modes reveal that the absorption coefficient strongly affects the physical characteristics of the ejecta and plume, as well as the time scale for material removal. Temperature-rise calculations demonstrate that phase explosion is likely the operative mechanism in ablation at the C–H mode, while normal boiling may play a role ...

Fabrication of polymer LEDs by resonant infrared pulsed laser ablation

Polymer light emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant infrared laser ablation of the light emitting layer. The light emitting polymer used was poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and was deposited into the device structure ITO/MEH-PPV/Al. Fourier transform infrared (FTIR) spectroscopy confirmed that the laser-deposited polymer was not drastically altered by the deposition process. Laser-fabricated devices displayed similar properties such as electroluminescence spectra and IV characteristics as conventional spin-coated devices. The dependence of these device properties on laser fluence was investigated, and showed no strong dependency. Peak emission wavelengths of electroluminescence spectra were all within 10 nm of electroluminescence spectra of spin coated devices and showed only slight peak broadening. These results are technologically important in that shadow mask technology can be incorporated into this method to arbitrarily pattern substrates with light emitting polymers.

Deposition of polymer barrier materials by resonant infrared pulsed laser ablation

We describe resonant infrared pulsed laser deposition (RIR-PLD) of cyclic olefin copolymer, a barrier and protective layer; for comparison, we describe RIR-PLD of polystyrene and poly(ethylene dioxythiophene) about which we already have significant knowledge. Film deposition based on resonant infrared laser ablation is a low-temperature process leading to evaporation and deposition of intact molecules. In this paper, we focus on deposition of this model barrier and protective material that is potentially useful in the fabrication of organic light emitting diodes. The films were characterized by scanning electron microscopy and Fourier-transform infrared spectroscopy. We also compared the properties of films deposited by a free electron laser and a picosecond optical parametric oscillator.

On the mechanism of resonant infrared polymer ablation: the case of polystyrene

We investigate the fundamental mechanisms of resonant-infrared laser ablation of polymers using polystyrene as a model material. Time-resolved plume shadowgraphy coupled with laser-induced temperature-rise calculations indicate that spinodal decomposition of a superheated surface layer is the primary mechanism for the initial stages of material removal. The majority of the ablated material is then released by way of recoil-induced ejection of liquid which proceeds for some tens of microseconds following a ~μs laser pulse excitation. The recoil-induced ejection of liquid material as the dominant ablation mechanism helps to explain previous observations of laser deposition of intact polymeric material.

Effects of laser wavelength, fluence, and pulse duration on infrared pulsed laser deposition of a conducting polymer

Thin films of a conducting polymer have been grown by resonant infrared matrix-assisted pulsed-laser evaporation (RIR-MAPLE). Properties of the thin films such as surface morphology and electrical conductivity have been investigated as a function of laser wavelength, fluence, and pulse structure. Using a free-electron laser whose wavelength is continuously tunable throughout the mid-infrared region (2-10 μm), we are able to deposit polymer films from various liquid matrices by resonantly exciting selective vibrational modes of the solvent. An Er:YAG laser operating at 2.94 μm is used to study the effects of different laser pulse durations. In the case of poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), it is found that only specific excitation wavelengths and pulse durations lead to the deposition of smooth and functional polymer films.

Mechanism of resonant infrared laser vaporization of intact polymers

Experiments on pulsed laser vaporization of many different kinds of polymers have demonstrated that it is possible to eject intact polymers into the ambient, whether air or vacuum, by resonant pulsed laser excitation, using both neat and matrix targets. Two recent studies of resonant infrared ablation - one on polystyrene, the other on poly(amic acid), the precursor for the thermoset polyimide - show moreover that the ablation process is both wavelength selective and surprisingly non-energetic, especially compared to ultraviolet laser ablation. We propose a wavelength-selective photothermal mechanism involving breaking of intermolecular hydrogen bonds that is consistent with these observations.

Thermodynamics of resonant infrared matrix-assisted pulsed laser evaporation of luminescent dendrimers

The successful demonstration of resonant infrared laser ablation and deposition of intact polymers and thermally labile organic molecules raises complex questions about the thermo dynamics of the process. Here we present results of resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) of luminescent dendrimers, successfully deposited as thin films using cryogenic matrices of chloroform and toluene and analyzed by atomic-force microscopy and nuclear magnetic resonance of the films. The properties of the two matrices have definite effects on the dynamics of the ablation process, resulting in relatively rough films with little damage to the dendrimer for the chloroform matrix, and smoother films with more structural damage to the dendrimer in the case of the toluene matrix. Thermodynamic modeling suggests that RIR-MAPLE in the chloroform matrix proceeds via explosive vaporization, while the ablation process from the toluene matrix is normal boiling and vaporization.

Mechanism of resonant infrared laser ablation of polystyrene

Although resonant infrared (RIR) pulsed laser deposition (PLD) was discovered in 2001 [1] and has since been applied to produce technologically interesting thin films and rudimentary opto-electronic devices [2], the mechanism governing RIR-PLD has been primarily a matter of conjecture. Understanding these mechanisms at the microscopic level has significant implications for process control in thin-film deposition, by linking laser parameters to film characteristics such as surface roughness. In this paper, we describe new experiments and calculations on the model material polystyrene that facilitate quantitative conclusions about these issues.

Fabrication of a Multilayer Polymer Light-Emitting Diode by Resonant Infrared Laser Ablation

Multi-layer polymer light-emitting diodes have been fabricated in vacuum by infrared laser ablation of conducting and light-emitting polymers. The spectral output of the devices resembles that of similar spin-coated devices, but shows some fluence dependence.

Fabrication of multi-layered polymer LEDs by resonant infrared pulsed-laser deposition

Multi-layered polymer light-emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant infrared pulsed-laser deposition of the polymer layers. The light emitter used was poly[2-methoxy-5-(2- ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and in some cases a layer of the hole-transport polymer poly(3,4 etylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) was also laser deposited, resulting in a device structure of ITO/PEDOT:PSS/MEH-PPV/Al. Fourier transform infrared (FTIR) spectroscopy confirmed that neither of the laser-deposited polymers was significantly altered by the deposition process. Laser-fabricated devices displayed electroluminescent spectra similar to those of conventional spin-coated devices, but the differences in electrical characteristics and device efficiency were substantial. These discrepancies can probably be attributed to surface roughness of the deposited polymer layers. With the appropriate refinement of the deposition protocols, however, we believe that this process can be improved to a level that is suitable for routine fabrication of organic electronic components.