RIR-MAPLE deposition of conjugated polymers for application to optoelectronic devices

Resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a promising deposition technology for the fabrication of conjugated polymer-based optoelectronic devices for two primary reasons: (i) the ability to control film morphology, and (ii) the ability to deposit multi-layered heterostructures. This article reviews a variation of RIR-MAPLE that uses emulsified targets of organic solvents and water such that the incident laser wavelength (Er:YAG at 2.9 μm) is resonant with hydroxyl (O–H) bonds in the host matrix, which are absent from the guest material. The novelty of the approach lies in the fact that while most polymers of interest and many compatible solvents do not resonantly absorb the laser energy at 2.9 μm, the emulsion with water enables high-quality, thin-film deposition with minimal photochemical and structural degradation for almost any polymer of interest. In addition, the advantages of emulsion-based RIR-MAPLE for conjugated polymer-based optoelectronic devices are demonstrated by two important studies. First, conjugated polymer films deposited by RIR-MAPLE are shown to have higher hole drift mobilities than films deposited using traditional drop-casting and spin-casting techniques. Second, the unique capability of RIR-MAPLE to enable conjugated polymer-based optical heterostructures is demonstrated by the fabrication and characterization of a multi-layer, polymer distributed Bragg reflector.     

Resonant infrared pulsed laser deposition of cyclic olefin copolymer films

Barrier materials on thin-film organic optoelectronic devices inhibit the uptake of water, oxygen, or environmental contaminants, and fabricating them is a major challenge. By definition, these barrier layers must be insoluble, so the usual routes to polymer- or organic-film deposition by spin coating are not problematic. In this paper, we report comparative studies of pulsed laser deposition of cyclic olefin copolymer (COC), an excellent moisture barrier and a model system for a larger class of protective materials that are potentially useful in organic electronic devices, such as organic light-emitting diodes (OLEDs). Thin films of COC were deposited by resonant and nonresonant infrared pulsed laser ablation of solid COC targets, using a free-electron laser tuned to the 3.43 μm C–H stretch of the COC, and a high-intensity nanosecond Q-switched laser operated at 1064 nm. The ablation craters and deposited films were characterized by scanning-electron microscopy, Fourier-transform infrared spectrometry, atomic-force microscopy, high-resolution optical microscopy, and surface profilometry. Thermal-diffusion calculations were performed to determine the temperature rise induced in the film at the C–H resonant wavelength. The results show that resonant infrared pulsed laser deposition (RIR-PLD) is an effective, low-temperature thin-film deposition technique that leads to evaporation and deposition of intact molecules in homogeneous, smooth films. Nonresonant PLD, on the other hand, leads to photothermal damage, degradation of the COC polymers, and to the deposition only of particulates.     

Resonant infrared pulsed laser deposition of thin biodegradable polymer films

Thin films of the biodegradable polymer poly(DL-lactide-co-glycolide) (PLGA) were deposited using resonant infrared pulsed laser deposition (RIR-PLD). The output of a free-electron laser was focused onto a solid target of the polymer, and the films were deposited using 2.90 (resonant with O-H stretch) and 3.40 (C-H) μm light at macropulse fluences of 7.8 and 6.7 J/cm², respectively. Under these conditions, a 0.5-μm thick film can be grown in less than 5 min. Film structure was determined from infrared absorbance measurements and gel permeation chromatography (GPC). While the infrared absorbance spectrum of the films is nearly identical with that of the native polymer, the average molecular weight of the films is a little less than half that of the starting material. Potential strategies for defeating this mass change are discussed. Received: 22 August 2001 / Accepted: 23 August 2001 / Published online: 17 October 2001     

Fabrication and performance of polymer–nanocomposite anti-reflective thin films deposited by RIR-MAPLE

Design of polymer anti-reflective (AR) optical coatings for plastic substrates is challenging because polymers exhibit a relatively narrow range of refractive indices. Here, we report synthesis of a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrix-assisted pulsed laser evaporation. An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO₂ nanoparticles and poly(methyl-methacrylate) (PMMA), alternating with a layer of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with transmission over 97 %, scattering <3 %, and a reflection coefficient below 0.5 % across the visible range, with a much smaller number of layers than would be predicted by a standard thin film calculation. The TiO₂ nanoparticles contribute more to the enhanced refractive index of the high-index layers than can be accounted for by an effective medium model of the nanocomposite.     

Resonant infrared matrix-assisted pulsed laser evaporation of TiO2 nanoparticle films

The successful development of flexible, high performance thin films that are competitive with silicon-based technology will likely require fabricating films of hybrid materials that incorporate nanomaterials, glasses, ceramics, polymers, and thin films. Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is an ideal method for depositing organic materials and nanoparticles with minimal photochemical or photothermal damage to the deposited material. Furthermore, there are many nonhazardous solvents containing chemical functional groups with infrared absorption bands that are accessible using IR lasers. We report here results of recent work in which RIR-MAPLE has been employed successfully to deposit thin films of TiO₂ nanoparticles on Si substrates. Using an Er:YAG laser (λ=2.94 μm), we investigated a variety of MAPLE matrices containing –OH moieties, including water and all four isomers of butyl alcohol. The alcohol isomers are shown to provide effective and relatively nontoxic solvents for use in the RIR-MAPLE process. In addition, we examine the effects of varying concentration and laser fluence on film roughness and surface coverage.     

镀高折射率纳米膜的长周期光纤光栅特性研究

针对镀高折射率纳米膜的长周期光栅,建立了传感理论模型,研究了长周期光栅的谐振波长与纳米膜厚度及外界折射率的关系,给出不同纳米膜厚度下长周期光栅不同包层模式重组特性.研究发现,当长周期光栅外面镀上一层沿角向均匀分布的纳米膜时,随着膜厚变化会出现包层模分布的明显调制;适当选择镀膜参数和外界介质折射率,最低次包层模式HE1,2会成为镀膜层中的导模,其他的包层模式将会发生模式转换现象;对于较低次包层模式(如HE1.6),在模式转换的时候存在两步转换,而高次的包层模只有一步转换(如HE1.14).同时给出了包层模式转换对外界折射率响应的关系:当膜层厚度增加时,长周期光栅模式转换现象移至低折射率区域.     

Electrical and optical properties of Si-doped indium tin oxides as transparent electrode and anti-reflection coating for solar cells

We have studied the electrical and optical properties of Si-doped indium tin oxides (ITSOs) as transparent electrodes and anti-reflection coatings for Si-based solar cells. The ITSO thin films were obtained by co-sputtering of ITO and SiO₂ targets under target power control. The resistivity of the ITSO thin films deposited at 0.625 in terms of power ratio (ITO/SiO₂) were 391 Ωcm. In this condition, the ITSO thin films showed very high resistivity compared to sputted pure ITO thin films (1.08 × 10⁻³ Ωcm). However, refractive index of ITSO thin films deposited at the same condition at 500 nm is somewhat lowered to 1.97 compared to ITO thin films (2.06). The fabricated graded refractive index AR coatings using ITO, ITSO, and SiO₂ thin films kept over 80% of transmittance regardless of their thickness varing from 97 nm to 1196 nm because of their low extinction coefficient. As the AR coating with graded refractive indices using ITO, ITSO, and SiO₂ layers was applied to general silicon-based solar cell, the current level increased nearly twice more than that of bare silicon solar cell without AR coating.     

Immobilization/hybridization of amino-modified DNA on plasma-polymerized allyl chloride

The present work describes the fabrication and characterization of chloride-derivatized polymer coatings prepared by continuous wave (cw) plasma polymerization as adhesion layers in DNA immobilization/hybridization. The stability of plasma-polymerized allyl chloride (ppAC) in H₂O was characterized by variation of the thickness of polymer films and its wettability was examined by water contact angle technique. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to study polymer matrix properties and oligonucleotide/DNA binding interaction. With the same carrier gas rate and process pressure, plasma polymers deposited at different input powers show various comparable immobilization properties; nevertheless, low input power plasma-polymerized films gives a lower sensitivity toward DNA binding than that from high input power plasma-deposited films. The following DNA immobilization on chloride-functionalized surfaces was found dependence on the macromolecular architecture of the plasma films. The hybridization between probe DNA and total mismatch target DNA shows no non-specific adsorption between target and ppAC.     

The refractive index of homogeneous SiO<Subscript>2</Subscript> thin films

The effect of the through-thickness inhomogeneity of SiO₂ films on their refractive index is studied. The SiO₂ films, deposited by tetraethoxysilane pyrolysis on plane-parallel oriented crystalline quartz substrates and on silicon plates, were etched layer-by-layer in Pliskin’s solution. The refractive index and the geometric thickness of films were calculated from the measured ellipsometric angles after each etching. The determination errors of the refractive indices as functions of the film thickness, which result from the measurement errors of the polarization angles and substrate parameters, are numerically analyzed. For SiO₂ films thicker than 20nm, no effect of inhomogeneity is found. The refractive indices of thinner films were observed to depend on the thickness. The reasons for such dependences are discussed.     

Sol-gel preparation and characterization of nanoporous ZnO/SiO2 coatings with broadband antireflection properties

Nanoporous ZnO/SiO₂ bilayer coatings were prepared on the surface of glass substrates via sol-gel dip-coating process. The structural, morphological and optical properties of the coatings were characterized. The refractive indices of ZnO layer and SiO₂ layer are 1.34 and 1.21 at 550 nm, respectively. The transmittance and reflectance spectra of the coatings were investigated and the broadband antireflection performance of the bilayer structure was determined over the solar spectrum. The solar transmittances in the range of 300-1200 nm and 1200-2500 nm are increased by 6.5% and 6.2%, respectively. The improvement of transmittance is attributed to the destructive interference of light reflected from interfaces between the different refractive-index layers with an optimized thickness. Such antireflection coatings of ZnO/SiO₂ provide a promising route for solar energy applications.     

Characterization of polymer thin films obtained by pulsed laser deposition

The development of laser techniques for the deposition of polymer and biomaterial thin films on solid surfaces in a controlled manner has attracted great attention during the last few years. Here we report the deposition of thin polymer films, namely Polyepichlorhydrin by pulsed laser deposition. Polyepichlorhydrin polymer was deposited on flat substrate (i.e. silicon) using an NdYAG laser (266 nm, 5 ns pulse duration and 10 Hz repetition rate).The obtained thin films have been characterized by atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and spectroscopic ellipsometry.It was found that for laser fluences up to 1.5 J/cm² the chemical structure of the deposited polyepichlorhydrin polymer thin layers resembles to the native polymer, whilst by increasing the laser fluence above 1.5 J/cm² the polyepichlorohydrin films present deviations from the bulk polymer.Morphological investigations (atomic force microscopy and scanning electron microscopy) reveal continuous polyepichlorhydrin thin films for a relatively narrow range of fluences (1-1.5 J/cm²).The wavelength dependence of the refractive index and extinction coefficient was determined by ellipsometry studies which lead to new insights about the material.The obtained results indicate that pulsed laser deposition method is potentially useful for the fabrication of polymer thin films to be used in applications including electronics, microsensor or bioengineering industries.     

Optical properties of hybrid polymers as barrier materials

The development of high barrier films for the encapsulation of organic electronics devices onto flexible polymeric substrates is attracting a considerable scientific interest, since it is important to protect the organic semiconductor layers of these devices from corrosion due to atmospheric gas molecule permeation. The barrier layers for encapsulation consist of a sequence of inorganic and hybrid polymer thin films that are deposited onto flexible polymeric substrates, such as polyethylene terephthalate (PET). In addition to their barrier response, these multilayer systems should also exhibit high transparency and good adhesion between the hybrid polymer and inorganic layers. The knowledge of their optical properties and the correlation of the optical response with their structure and the final barrier response are of major importance since it will contribute towards the optimization of their functionality. In this work, the optical properties of hybrid polymers deposited onto silicon oxide inorganic thin films that were grown onto flexible polymeric substrates, have been investigated by the use of spectroscopic ellipsometry in a wide spectral region from the infrared to the visible-ultra violet. As it has been found, the increase of the solid content in the hybrid polymers is associated with a reduction in the refractive index values. This behavior can be correlated to a lower density of the hybrid polymer, and furthermore to a poor barrier response, due to the less cohesive inorganic-organic bonding network. Finally, from the investigation of the optical response of the hybrid polymers in the IR spectral region has revealed information on their bonding structure that has been discussed together with their barrier response.     

4H-SiC基底Al_2O_3/SiO_2双层减反射膜的设计和制备

在4H-SiC基底上设计并制备了Al2O3SiO2紫外双层减反射膜,通过扫描电镜(SEM)和实测反射率谱来验证理论设计的正确性.利用编程计算得到Al2O2和SiO2的最优物理膜厚分别为42.0 nm和96.1 nm以及参考波长λ=280 nm处最小反射率为0.09%.由误差分析可知,实际镀膜时保持双层膜厚度之和与理论值一致有利于降低膜系反射率.实验中应当准确控制SiO2折射率并使Al2O3折射率接近1.715.用电子束蒸发法在4H-SiC基底上淀积Al2O3SiO2双层膜,厚度分别为42 nm和96 nm.SEM截面图表明淀积的薄膜和基底间具有较强的附着力.实测反射率极小值为0.33%,对应λ=276 nm,与理论结果吻合较好.与传统SiO2单层膜相比,Al2O3SiO2双层膜具有反射率小,波长选择性好等优点,从而论证了其在4H-SiC基紫外光电器件减反射膜上具有较好的应用前景.     

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

Engineered omnidirectional external-reflectivity spectra from one-dimensional layered interference filters

Owing to optical refraction, external rays that are incident upon a high-refractive-index medium fall within a small internal cone angle. A one-dimensionally periodic Bragg structure can reflect over an angular acceptance range that is greater than the small internal refraction cone, if the internal refractive-index contrast is sufficient. Thus Winn et al. [Opt. Lett. (to be published)] charted the range of refractive indices at which omnidirectional external reflection occurs. A wide spectral gap requires a high-index contrast. It is proposed that, by chirping or grading the periodicity of the structure, one can cover an arbitrarily wide spectral range with only a modest index contrast and, furthermore, that arbitrary spectral shapes can be produced. The graded-periodicity approach requires only a modest index contrast, provided that the average refractive index is >2 .     

Investigations of the sorption behaviour of amorphous nitrogen-rich carbon nitride films as sensitive layers for cantilever-based chemical sensors

We present investigations of the sorption behaviour of amorphous nitrogen-rich carbon nitride films (CN_x) towards water vapour and volatile organic compounds, for example methanol, ethanol, i-propanol and acetone, in order to evaluate their potential as sensitive layers for cantilever-based chemical sensor applications. The CN_x films were deposited by inductively coupled plasma chemical vapour deposition (ICP-CVD) utilizing transport reactions from a solid carbon source. In order to study the influence of the thickness of the sensitive layer on its sensitivity and selectivity, two series of cantilevers coated with 120 nm and 240 nm CN_x films were prepared. We found that the variation of the film thickness affected the sorption process of the CN_x film quantitatively as well as qualitatively. For thin films (120 nm), the sensor dynamic responses (frequency shift) increased with increasing molecular weight of the analytes. The largest responses were observed towards acetone and i-propanol ; here, the cantilever acted as a resonant microbalance. When the film thickness was increased from 120 to 240 nm, the analytes with higher dipole moments caused stronger response signals. In this case we observed, for example, an increase of the sensitivity towards methanol by a factor of more than three. The performance of the cantilever-based sensors functionalised with CN_x coatings was compared to that of organic polymers, and the observed peculiarities were explained by the chemical nature of the sensitive materials. PACS 81.05.-t; 85.85.+j; 07.07.Df     

Effect of corona pre-treatment on the performance of gas barrier layers applied by atomic layer deposition onto polymer-coated paperboard

The effect of corona pre-treatment on the performance of Al₂O₃ and SiO₂ gas barrier layers applied by atomic layer deposition onto polymer-coated paperboards was studied. Both polyethylene and polylactide coated paperboards were corona treated prior to ALD. Corona treatment increased surface energies of the paperboard substrates, and this effect was still observed after several days. Al₂O₃ and SiO₂ films were grown on top of the polymer coatings at temperature of 100 °C using the atomic layer deposition (ALD) technique. For SiO₂ depositions a new precursor, bis(diethylamido) silane, was used. The positive effect of the corona pre-treatment on the barrier properties of the polymer-coated paperboards with the ALD-grown layers was more significant with polyethylene coated paperboard and with thin deposited layers (shorter ALD process). SiO₂ performed similarly to Al₂O₃ with the PE coated board when it comes to the oxygen barrier, while the performance of SiO₂ with the biopolymer-coated board was more moderate. The effect of corona pre-treatment was negligible or even negative with the biopolymer-coated board. The ALD film growth and the effect of corona treatment on different substrates require further investigation.     

The influence of gas flow rate on the structural,mechanical, optical and wettability of diamond-like carbon thin films

In this research, diamond-like carbon (DLC) thin films were deposited on silicon substrates by radio-frequency plasma enhanced chemical vapor deposition method using gas mixture of CH₄ and Ar. The effect of different CH₄/Ar gas ratio on the structure, refractive index, transmission and hardness of the DLC thin films were investigated by means of Raman spectroscopy, ellipsometry, Fourier transform Infrared Spectroscopy and nano-indentation methods, respectively. Nuclear resonant reaction analysis was used to measure the amount of hydrogen and carbon in the thin films. Furthermore, wettability of the thin films was achieved by measuring of water contact angle (WCA). The results indicated that the structural properties of the diamond-like carbon thin films are strongly dependent on the composition of gas mixture. Based on ellipsometry results, refractive index of the thin films varied in the range of 1.89–2.06 at 550 nm. FTIR results determined that deposition of DLC thin films on silicon substrate led to an increase of the light transmission in IR region and these films have the potential to be used in silicon optics as the antireflective coatings in this region. Nano-indentation analysis showed that the thin films hardness changed in the range of 7.5–11 GPa. On the other hand hydrogen content and fraction of C?H bonds in the samples increased by an increase in the gas ratio of CH₄/Ar. Also, WCA measurements indicated that WCA for thin films with gas ratio of 3/7 is the most and equal to 79°.     

Optical properties of aluminium nitride films obtained by pulsed laser deposition: an ellipsometric study

Aluminium nitride (AlN) films obtained by pulsed laser deposition (PLD) at different low ambient nitrogen pressure have been optically investigated by spectral ellipsometry within the 300–800 nm wavelength range, Sellmeier and Wemple–DiDomenico approximation approaches have been applied for the ellipsometric data analysis. Optical parameters such as refractive index and single oscillator energies of deposited films were estimated and their dependence on the N₂ pressure was studied. The obtained refractive index values of AlN films are around 2 in a wide wavelength region above 400 nm and suggest the growth of a polycrystalline structure. The relatively low (<3.4 eV) threshold energies indicate the formation of a defective and disordered structure, formed during the deposition process. PACS 78.20.-e; 78.20.Ci; 78.66.Fd