Raman polarization studies of highly oriented organic thin films

In this work we report on the capability of polarized Raman spectroscopy to investigate the structure of thin organic films. Diindenoperylene (DIP) thin films on (1 × 1)‐rutile(110) were prepared via organic molecular beam deposition (OMBD). Raman spectra of DIP thin films showed several strong Raman modes in the wavenumber region from 1200 to 1650 cm⁻¹. The A_g mode at 1284 cm⁻¹ shows two contributions, thereby indicating the coexistence of at least two DIP film structures. Polarized Raman spectroscopy was applied to characterize the molecular orientation and the dominance of the σ‐configuration (i.e. upright standing DIP molecules) was found. Copyright © 2009 John Wiley & Sons, Ltd.     

Study on solid structure of pentacene thin films using Raman imaging

We present a 532‐nm excited Raman imaging study of pentacene thin films (thickness, 2, 5, 10, 20, 50, 100, and 150 nm) prepared on an SiO₂ surface. The structure of the pentacene films has been investigated by images and histograms of the ratio (R) of intensity of the 1596‐cm⁻¹ band (b_3g) to that of the 1533‐cm⁻¹ band (a_g), which can be used as a marker of solid‐state phases: 1.54‐nm and 1.44‐nm phases. The Raman images showed that island‐like 1.44‐nm phase domains are grown on the 1.54‐nm phase layer from 50 nm, and all the surface of the 1.54‐nm phase layer is covered with the 1.44‐nm phase layer from 100 nm. The structural disorders have been discussed on the basis of the full width at half maximum of a band in the histogram of the R values for each film. Copyright © 2012 John Wiley & Sons, Ltd.     

Self‐assembly and structure formation in liquid crystalline phthalocyanine thin films studied by Raman spectroscopy and AFM

This work presents an investigation of films prepared by doctor blade casting, the formation of self‐assembled microstructures of a liquid crystalline phthalocyanine with highly oriented molecules. Raman Spectroscopy in combination with atomic force microscopy is applied to study the structures within the films. By keeping the substrate at room temperature or at 353 K during coating, different geometric structures namely rods and islands form. Rod‐like structures are growing in coating direction, whereas directional growth of the islands is not observed. The distribution of the rod lengths varies widely, whereas the width appears more uniform. Annealing of the samples shows a different behavior of the two textures. Islands tend to melt, and rods smooth their structural form, which is extracted from Raman imaging in combination with atomic force microscopy. Additionally, Raman imaging gives insight into laterally different relative crystallinity. These observations are discussed in the context of the molecular orientation as probed by polarized Raman spectroscopy. These polarized Raman spectra indicate azimuthal alignment of the molecules within the rods (edge on alignment). This alignment occurs along and also perpendicular to the growth direction. In contrast to the alignment in the rods, the molecules inside the islands occurring at higher temperature do not show preferential molecular orientation. After annealing, no preferential molecular orientation is observed in rods because of the loss of anisotropy, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Real‐time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low‐density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C C symmetrical stretching vibration of polyethylene (PE) at 1132 cm⁻¹ was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c‐axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (〈P₂(cosθ)〉) and fourth (〈P₄(cosθ)〉) moments of the orientation distribution function. The orientation parameters (P₂, P₄) were found to increase along the axial distance in the film line even past the frost‐line height (FLH). The P₂ values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I₁₁₃₂/I₁₀₆₄) obtained from a single, ZZ‐back‐scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical in‐situ characterisation of interface stabilised organic thin‐film transistors

We report on the electrical in‐situ characterisation of organic thin film transistors under high vacuum conditions. Model devices in a bottom‐gate/bottom‐contact (coplanar) configuration are electrically characterised in‐situ, monolayer by monolayer (ML), while the organic semiconductor (OSC) is evaporated by organic molecular beam epitaxy (OMBE). Thermal SiO₂ with an optional polymer interface stabilisation layer serves as the gate dielectric and pentacene is chosen as the organic semiconductor. The evolution of transistor param‐ eters is studied on a bi‐layer dielectric of a 150 nm of SiO₂ and 20 nm of poly((±)endo,exo‐bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylic acid, diphenylester) (PNDPE) and compared to the behaviour on a pure SiO₂ dielectric. The thin layer of PNDPE, which is an intrinsically photo‐patternable organic dielectric, shows an excellent stabilisation performance, significantly reducing the calculated interface trap density at the OSC/dielectric interface up to two orders of magnitude, and thus remarkably improving the transistor performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Nanoporous Ag–GaN thin films prepared by metal‐assisted electroless etching and deposition as three‐dimensional substrates for surface‐enhanced Raman scattering

Three‐dimensional (3D) nanoporous gallium nitride (PGaN) scaffolds are fabricated by Pt‐assisted electroless hydrofluoric acid (HF) etching of crystalline GaN followed by in situ electroless deposition of Ag nanostructures onto the interior surfaces of the nanopores, yielding a large surface area substrate for surface‐enhanced Raman scattering (SERS). The resulting 3D SERS‐active substrates have been optimized by varying reaction parameters and starting material concentration, exhibiting enhanced Raman signals 10–100× more intense than either (1) sputtered Ag‐coated porous GaN or (2) Ag‐coated planar GaN. The increase in SERS signal is attributed to a combination of the large surface area and the inherent transparency of PGaN in the visible spectral region. Overall, Ag‐decorated PGaN is a promising platform for high sensitivity SERS detection and chemical analysis, particularly for reaction and metabolic products that can be trapped inside the highly anisotropic nanoscale pores of PGaN. The potential of this sampling mode is illustrated by the ability to acquire Raman spectra of adenine down to 5 fmol. Additionally, correlated SERS and laser desorption/ionization mass spectrometry spectra can be acquired from same sample spot without further preparation, opening new possibilities for the investigation of surface‐bound molecules with substantially enhanced information content. Copyright © 2012 John Wiley & Sons, Ltd.     

Measuring stresses in thin metal films by means of Raman microscopy using silicon as a strain gage material

Mechanical stresses in microelectronics and micro‐electromechanical systems may influence the reliability of applications and devices. The origin of the stresses can be because of the joining of dissimilar materials with regard to the thermal expansion coefficient, electromigration or the deposition process utilized. Stresses can lead to delamination, crack formation and stress migration and therefore to a failure of the device. Identifying the locations of highest stresses in a device is crucial for reliability improvement. Currently, both Laue X‐ray micro diffraction and convergent‐beam electron diffraction are able to locally determine the stresses in thin metal films. Here, we propose a modified method of indirect Raman microspectroscopy to measure stresses with a lateral resolution in the submicrometer range at a laboratory scale. The method encompasses the crystallization of an amorphous silicon layer by local laser annealing and its subsequent usage as a strain gage. Stresses in an aluminum thin film were determined as a function of temperature. In addition to the average stress, the stress distribution could be monitored. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman micro‐spectroscopy for quantitative thickness measurement of nanometer thin polymer films

The sensitivity of far‐field Raman micro‐spectroscopy was investigated to determine quantitatively the actual thickness of organic thin films. It is shown that the thickness of organic films can be quantitatively determined down to 3 nm with an error margin of 20% and down to 1.5 nm with an error margin of 100%. Raman imaging of thin‐film surfaces with a far‐field optical microscope establishes the distribution of a polymer with a lateral resolution of ~400 nm and the homogeneity of the film. Raman images are presented for spin‐coated thin films of polysulfone (PSU) with average thicknesses between 3 and 50 nm. In films with an average thickness of 43 nm, the variation in thickness was around 5% for PSU. In films with an average thickness of 3 nm for PSU, the detected thickness variation was 100%. Raman imaging was performed in minutes for a surface area of 900 µm². The results illustrate the ability of far‐field Raman microscopy as a sensitive method to quantitatively determine the thickness of thin films down to the nanometer range. Copyright © 2015 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman study of PrFeO3thin film

Low‐temperature Raman study of (001)‐oriented PrFeO₃ thin film of around 200 nm thickness deposited on a LaAlO₃ (001) substrate by using the pulsed‐laser deposition technique is presented. X‐ray diffraction analysis of this film shows an orthorhombic structure with Pbnm space group. The observed substrate‐induced strain is found to be small. In the room temperature Raman spectra, different Raman modes were observed that were classified according to the orthorhombic structure. All the observed modes show a decrease in wavenumber with rise in temperature, except the B_1g mode (624 cm⁻¹) which shows some anomalous behavior. We tried to correlate the variations in linewidth and position with temperature for the observed modes with the octahedral disorder of FeO₆. Many possibilities are presented to explain the observed results. Copyright © 2010 John Wiley & Sons, Ltd.     

纳米级润滑膜分子排列取向的拉曼光谱表征技术

利用激光拉曼散射技术,对剪切作用下受限于钢球与石英盘之间的纳米级液晶5 CB的分子排列取向进行研究. 结果表明,在特定的实验条件下,可以得到高信噪比的纳米级润滑膜的拉曼散射信号(20∶1). 同时发现,当激光偏振方向与剪切运动方向平行(垂直)时,所得拉曼信号强度达到最大(小)值,表明纳米级液晶5 CB分子在剪切诱导作用下,沿剪切运动方向趋于定向排列. 另外,当钢球与石英盘之间的剪切速度逐渐增大时,受限的纳米级液晶5 CB的拉曼信号强度也逐渐增大. 最后,利用根据相对光强干涉原理研制的纳米膜厚测量仪对纳米级 关键词： 薄膜润滑 分子排列取向 拉曼散射     

Raman spectroscopy of sol–gel derived titanium oxide thin films

Raman spectra of TiO₂ films prepared via the sol–gel process were studied by UV and visible Raman spectroscopy. The evolution of the phases of TiO₂ films during annealing was investigated, and the relative intensities of the Raman bands excited with 325 nm were found to be distinct from those of the bands excited with 514 nm. The transmittance and FTIR spectra of the films annealed at different temperatures were characterized. The crystallization process of the powders and thin films treated by different annealing methods were also studied with Raman spectroscopy. The results show that the change in the relative intensities is caused by the resonance Raman effect. The anatase to rutile transition of the powder occurs at 700 °C, while that of the thin film occurs at 800 °C. The analysis of Raman band shape (peak position and full width at half‐maximum) after conventional furnace annealing and rapid thermal annealing indicates the influence of the non‐stoichiometry and phonon confinement effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman polarization analysis of highly crystalline polyethylene fiber

The complex orientation dependence in space of Raman active vibrations in the orthorhombic structure of polyethylene (PE) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. Building upon the symmetry assignment of these vibrational modes, we systematically studied, from both theoretical and experimental viewpoints, the changes of polarized intensity for the A_g and the B_2g + B_3g vibrational modes with respect to PE molecular orientation. After explicitly expanding the Raman selection rules associated with the A_g and the B_2g + B_3g modes, introducing them into general expressions of the orientation distribution function, and validating them by means of a least‐square fitting procedure on experimental data, we compare here two mesostructural models for a highly crystallized and self‐aligned PE fiber structure. Stereological arguments are shown concerning the arrangement of orthorhombic fibrils in such a sample that unfold the correct values of five independent Raman tensor elements for orthorhombic PE. Copyright © 2010 John Wiley & Sons, Ltd.     

Photodissociation effects on pulsed laser deposited silver oxide thin films: surface‐enhanced resonance Raman scattering

Temporal Raman scattering measurements with 488, 532 and 632 nm excitation wavelengths and normal Raman studies by varying the power (from 30 W/cm² to 2 MW/cm²) at 488 nm were performed on silver oxide thin films prepared by pulsed‐laser deposition. Initially, silver oxide Raman spectra were observed with all three excitation wavelengths. With further increase in time and power, silver oxide photodissociated into silver nanostructures. High‐intensity spectral lines were observed at 1336 ± 25 and 1596 ± 10 cm⁻¹ with 488 nm excitation. No spectral features were observed with 633 nm excitation. Surface‐enhanced resonance Raman scattering theory is used to explain the complex behavior in the intensity of the 1336/1596 cm⁻¹ lines with varying power of 488 nm excitation. Copyright © 2011 John Wiley & Sons, Ltd.     

对以并五苯和酞菁铜为不同有源层的有机薄膜晶体管特性研究

通过扫描电镜和X射线衍射对SiO₂衬底上生长并五苯和酞菁铜薄膜的表面形貌进行表征,并得到在SiO₂衬底上生长的并五苯薄膜是以岛状结构生长,其大小约为100nm,且薄膜有较好的结晶取向,呈多晶态存在. 酞菁铜薄膜则没有表现出明显的生长机理,其呈非晶态存在. 还对通过掩膜的方法制作得以酞菁铜和并五苯为有源层的顶栅极有机薄膜晶体管的特性进行了研究. 有源层的厚度为40nm,绝缘层SiO₂的厚度为250nm,器件的沟道宽长比(W/关键词： 有机薄膜晶体管 并五苯薄膜 酞菁铜薄膜 μ_EF)')" href="#">场效应迁移率(μ_EF)     

有机半导体薄膜生长原位实时测量方法的研究

针对原位实时监测有机半导体薄膜生长情况的需求, 提出了差分反射光谱法与场效应晶体管法结合的光电联合测量方法, 设计研制了测量系统. 以并五苯有机分子为例, 通过自制底栅底接触式场效应管微结构, 实验测试了热蒸发法生长导电膜层过程中光电信号的演变与相互关联. 光谱信号显示, 并五苯以薄膜态结构进行生长, 光谱随生长进程变化显著. 实验数据与四相结构模型仿真结果的良好吻合, 表明因薄膜增厚引起干涉条件的改变是光谱变化的主因, 由此推算出薄膜生长速率为0.23 nm/min. 当薄膜等效厚度达到28 nm时, 场效应管的导电性显著增强, 标志着并五苯有效传输层的形成. 此后, 薄膜厚度持续增加, 但测试电流增长缓慢, 说明该结构进入电学特性饱和区. 光电联合法不仅有助于研究有机半导体薄膜的光谱信息、电学特性和薄膜结构之间的相互对应关系, 也为发展原位监测有机半导体薄膜制备过程, 探索最佳工艺提供了新的研究手段.     

Surface‐sensitive Raman microscopy with total internal reflection illumination

A Raman microscope using a total internal reflection (TIR) annular illumination geometry through a ZnSe solid immersion lens (SIL) is described. Spectra of a thin‐film sample of the transparent organic conductor poly(3,4‐ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) on a polyethylene terephthalate (PET) substrate are presented and compared with those from a conventional confocal Raman configuration. These spectra demonstrate a significant increase in surface selectivity upon the use of TIR illumination, as the decay length of the evanescent excitation field limits the depth of sample probed in this configuration. Spectral interference from the underlying PET substrate layer is thus greatly reduced. An increase in surface selectivity is also demonstrated for spectra acquired through the SIL with uniform illumination. Raman images of a micropatterned PEDOT:PSS film acquired with TIR illumination are also reported. Enhanced lateral resolution is realized in this configuration because of the immersion effect of the SIL, and the sampling depth is limited to 150 nm by the choice of illumination geometry. This results in analysis volumes on the order of tens of femtoliters, nearly two orders of magnitude smaller than typically achieved in conventional confocal Raman microscopes. This approach yields Raman spectra and images with surface selectivity significantly greater than can be achieved in confocal Raman, and provides a valuable tool for the microanalysis of thin surface films. Published in 2010 by John Wiley & Sons, Ltd.     

氮化铝薄膜的红外吸收光谱和喇曼光谱的研究

本文报道用直流平面磁控溅射法在Si片上生长c轴高度择优取向AIN薄膜的光学特性.俄歇谱分析表明薄膜是高纯的.从红外吸收光谱上分析获得晶格振动纵、横模的频率分别为2.5×10~(13)HZ和1.8×10~(13)Hz.从喇曼光谱上分析获得AIN薄膜的光学声子频率为297、512、607、656、832cm~(-1).与几种已知的纤锌矿结构二元化合物的声子频率模式类比获得AIN的光学声子模式.进一步分析表明AIN是一种静电力大于原子间各向异性力的晶体,且声子的最高频率与r~(-3/2)N~(-1/2)成正比.     

Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

In this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm⁻¹ which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A₁‐transverse optical (A₁‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A₁‐TO mode is forbidden, while the 379 cm⁻¹ peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm⁻¹ is the E_1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.     

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

The techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N₂ Ar with concentrations varying from 0 to 79% N₂. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Photoconductivity of thin organic films

Thin organic films were deposited on silicon oxide surfaces with golden interdigitated electrodes (interelectrode gap was 2 μm), and the film resistivities were measured in dark and under white light illumination. The compounds selected for the measurements include molecules widely used in solar cell applications, such as polythiophene (PHT), fullerene (C₆₀), pyrelene tetracarboxylic diimide (PTCDI) and copper phthalocyanine (CuPc), as well as molecules potentially interesting for photovoltaic applications, e.g. porphyrin-fullerene dyads. The films were deposited using thermal evaporation (e.g. for C₆₀ and CuPc films), spin coating for PHT, and Langmuir-Schaeffer for the layer-by-layer deposition of porphyrin-fullerene dyads. The most conducting materials in the series are films of PHT and CuPc with resistivities 1.2 × 10³ Ω m and 3 × 10⁴ Ω m, respectively. Under light illumination resistivity of all films decreases, with the strongest light effect observed for PTCDI, for which resistivity decreases by 100 times, from 3.2 × 10⁸ Ω m in dark to 3.1 × 10⁶ Ω m under the light.