Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

We report detailed temperature dependent photoluminescence (PL) spectra of pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown on SiO(2). PEN and PFP are particularly suitable for this study, since they are structurally compatible for good intermixing and form a model donor/acceptor system. The PL spectra of PEN are discussed in the context of existing literature and compared to the new findings for PFP. We analyze the optical transitions observed in the spectra of PEN and PFP using time-dependent density functional theory calculations. Importantly, for the mixed PEN:PFP film we observe an optical transition in PL at 1.4 eV providing evidence for coupling effects in the blend. We discuss a possible charge-transfer (CT) and provide a tentative scheme of the optical transitions in the blended films.     

Structure and morphology of coevaporated pentacene-perfluoropentacene thin films

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.     

Quantitative chemical composition of thin films with infrared spectroscopic ellipsometry: application to hydrated oxide films on aluminium

A method to quantify the composition of thin films using infrared spectroscopic ellipsometry (IRSE), supplemented by visible spectroscopic ellipsometry (VISSE), is proposed. Because ellipsometry measures the thickness and optical constants of a surface layer simultaneously, the absorption coefficient of the film as a function of wavelength can be obtained. Using values of the absorption coefficients for the pure components of the film, the percentages (mol.% or wt.%) of each component in the film can be calculated. The method is demonstrated in a study of the hydration of oxide films on electropolished aluminium and the anodically formed barrier oxide film. The IRSE technique shows that hydration of the films by immersion in boiling water results in the conversion of aluminium oxide to pseudoboehmite. Copyright © 2003 John Wiley & Sons, Ltd.     

Optical absorption of tetraphenylporphyrin thin films in UV-vis-NIR region

The optical absorption of thermally-evaporated tetraphenylporphyrin (TPP), in the UV-vis-NIR region has been studied. The absorption spectra recorded in the UV-vis region for the as-deposited and annealed films showed different absorption bands, namely the Soret, B, at region 360-490 nm, Q-band region consist of four bands in the region 500-720 nm and two other bands labeled N and M in UV region. The Soret band always shows splitting in all the TPP thin films and the effect of annealing on the intensities of these components have been observed. The spectra of the infrared absorption allow characterization of vibrational modes for the powder, as deposited and annealed thin films. Some of the optical absorption parameters, namely molar extinction coefficient (epsilon), half band width (Deltalambda), electronic dipole strength (q2), and oscillator strength (f), of the principal optical transitions have also been evaluated.     

Optical absorption of tetraphenylporphyrin thin films in UV-vis-NIR region

The optical absorption of thermally evaporated tetraphenylporphyrin (TPP) in the UV-vis-NIR region have been studied. The absorption spectra recorded in the UV-vis region for the as deposited and annealed films showed different absorption bands, namely the Soret(B) at region 360-490nm, Q-band region consist of four bands in the region 500-720nm and two other bands labeled N and M in UV region. The Soret band always shows its characteristic effect splitting in all the TPP thin films and the effect of annealing on the intensities of these components have been observed. The spectra of the infrared absorption allow characterization of vibrational modes for the powder, as deposited and annealed thin films. Some of the optical absorption parameters, namely molar extinction coefficient, varepsilon, half band width, Deltalambda, electronic dipole strength, q(2) and oscillator strength, f, of the principle optical transitions have also been evaluated.     

热释电Langmuir-Blodgett膜结构的光谱研究

利用Langmuir-Blodgett(LB)技术对偶氮苯衍生物进行极性分子组装. 紫外-可见吸收光谱显示该膜具有类H聚集效应, 利用红外线性二向色性光谱获得了分子的主要极性功能团的跃迁矩与膜表面法线的夹角, 并结合其它光谱信息, 给出了膜结构模型, 就此对其优越的热释电性能做出解释.     

Adsorption-induced intramolecular dipole: correlating molecular conformation and interface electronic structure

The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable.     

Structural and optical characterization of thermally evaporated cadmium sulfide thin films

The article reports the structural and optical properties of vacuum‐evaporated cadmium sulfide (CdS) films with different thicknesses at room temperature. The structural investigations performed by means of X‐ray diffraction (XRD) technique have showed that all the films have the zinc‐blende structure, a face‐centered cubic form with lattice constants a = b = c = 5.82 Å and point group F4 3m. Crystallite sizes calculated from Scherrer relation are in the range of 173–345 Å. So far, because the optical parameters of the metastable cubic CdS have not been so well known, we apply spectroscopic ellipsometry to determine the thickness, optical constants and energy band gap of CdS thin film deposited by thermal evaporation onto opaque gold substrate, a perfect reflectivity and inert metal. As shown the measured spectral behavior of the optical constants and the band gap value of CdS thin film are in agreement with those obtained by the reflectance and transmittance methods. The energy band gap of CdS thin film determined from the spectral behavior of the absorption coefficient is about 2.46 eV. Copyright © 2008 John Wiley & Sons, Ltd.     

A polarised ATR study of structural orientation in dry and humid thin HPMC films

The structure of thin films of the polymer hypromellose (HPMC) have been investigated under dry and ambient humidity conditions using polarised attenuated total reflectance (ATR) infrared spectroscopy. In particular spectra were recorded in the C–H and O–H stretching regions for spin coated films deposited on silicon, germanium and zinc selenide internal reflection elements (IREs). A recent development in the field of polarised ATR has demonstrated a simple quantitative relationship between complementary s- and p-polarised spectra of orientationally ordered monolayer films, which yields the spectrum uniquely in the direction perpendicular to the surface, the “z-polarised” spectrum. As well as recording s- and p-polarised spectra this work examines the z-spectra derived from the experimental s- and p-polarisation spectra. For the C–H band all three polarisation spectra reveal no change in the preferred orientation of the transition dipole between humid and dry films on germanium or silicon but with a marginally increased orientation parallel to the surface on zinc selenide. On the other hand the O–H band spectra show an increased orientation of the transition dipole parallel to the surface for both humid and dried films for all three IREs when compared to the corresponding C–H band spectra. This effect was greater in the dried film, i.e. with free water removed, than in the humid film.     

Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy

We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV--visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.     

Optical absorption of sodium copper chlorophyllin thin films in UV-vis-NIR region

The optical absorption studies of sodium copper chlorophyllin thin films (SCC), prepared by spray pyrolysis, in the UV-vis-NIR region was reported for the first time. Several new discrete transitions are observed in the UV-vis region of the spectra in addition to a strong continuum component in the IR region. The spectra of the infrared absorption allow characterization of vibration modes for the powder and thin films of SCC. The absorption spectrum recorded in the UV-vis region showed different absorption bands, namely the Soret (B) in the region 340-450 nm and Q-band in the region 600-700 nm and other band labeled N in the 240-320 region. Some important spectral parameters namely optical absorption coefficient (alpha), molar extinction coefficient (epsilon(molar)), oscillator strength (f), electric dipole strength (q(2)) and absorption half bandwidth (Deltalambda) of the principle optical transitions were evaluated. The analysis of the absorption coefficient in the absorption region revealed direct transitions and the energy gap was estimated as 1.63 eV. Discussion of the obtained results and their comparison with the previous published data are also given.     

Spectroscopic and excited-state properties of tri-9-anthrylborane II: Electroabsorption and electrofluorescence spectra

Electroabsorption and electrofluorescence spectroscopies were conducted for tri-9-anthrylborane (TAB) doped in poly(methyl methacrylate) films (1.0 mol %) to reveal the spectroscopic and excited-state properties of the compound. TAB showed three distinct absorption bands: bands I [(19 - 25) x 10(3) cm(-1)], II [(25-31) x 10(3) cm(-1)], and III (>31 x 10(3) cm(-1)). The electroabsorption spectrum demonstrated that the electronic transitions in bands I and III accompanied electric dipole moment changes (Deltamu), while the change in the molecular polarizability contributed mainly to electroabsorption band II. Because of the similarities of the electroabsorption spectrum of band II with that of anthracene itself, band II was assigned to the electronic transition to the locally excited (LE) state of the anthryl group. On the other hand, bands I and III were best described by the electronic transitions to the excited charge-transfer (CT) states. The study demonstrated furthermore that the Deltamu value of TAB accompanied by the lowest-energy electronic transition was as large as 7.8 D, which agreed very well with that determined by the solvent dependences of the absorption and fluorescence maximum energies of TAB (approximately 8.0 D, ref 1): Deltamu = 7.8-8.0 D. The results proved explicitly that the excited state of TAB was localized primarily on the p orbital of the boron atom. Despite the dipole moment change (Deltamu = 7.8-8.0 D) for the lowest-energy electronic transition (band I), the electrofluorescence of TAB accompanied the change in the molecular polarizability. The spectroscopic and excited-state properties of TAB including the curious behavior of the electrofluorescence spectrum as mentioned above were discussed on the basis of theoretical considerations.     

Crystalline ice grown on the surface of the ferroelectric polymer poly(vinylidene fluoride) (70%) and trifluoroethylene (30%)

Water ice is observed to order at the copolymer ferroelectric poly(vinylidene difluoride-trifluoroethylene) surface. The successful growth of crystalline thin films of water on these polymer surfaces implicates water to polymer dipole interactions. These ice thin films are sufficiently ordered for experimental identification of the wave vector dependence in the electronic band structure of hexagonal ice. The significant band dispersion, of about 1 eV, suggests strong overlap of molecular orbitals between adjacent water molecules in the ice film. The presence of dipole interactions with adsorbate water is consistent with the possibility of water acting as a spectator to surface ferroelectric transitions in this system.     

Vibronic fine structure in the absorption spectrum of oligothiophene thin films

A multimode Holstein Hamiltonian is used to describe optical excitations in quaterthiophene pinwheel aggregates. The Hamiltonian includes the coupling of excitons originating from the 1A(g)-->1B(u) electronic transition to phonons originating from the five intramolecular vibrational modes known from oligothiophene solution absorption/emission spectroscopy. The resulting eigenstates with lowest energy are best described as hybrid polaron phonons. The polarons are formed by coupling excitons with the higher frequency (688, 1235, and 1551 cm(-1)) vibrational modes, while the (optical) phonons arise from the lower frequency (161 and 333 cm(-1)) modes. The polaron phonons are responsible for the fine structure defining the A(1) band in the low-energy region of the absorption spectrum, ranging from the band origin to approximately 1500 cm(-1) beyond. The calculated A(1) band of quaterthiophene aggregates agrees favorably with that observed from thin films.     

Fabrication and optical properties of gallium phosphide nanoparticulate thin film

The gallium phosphide (GaP) nanoparticulate thin films were fabricated by colloidal suspensions deposition with GaP nanoparticles dispersed in N,N-dimethylformamide (DMF). The microstructure and optical properties of the film have been studied by scanning electron microscopy, high resolution transmission electron microscope, and optical absorption and fluorescence spectra. The morphology of the film was found to be composed of nanoparticle aggregates, and with an irregularly rough surface. From the result of fluorescence, it can be established that the film not only retains the violet and blue light emissions which ascribed to transition from conduction band to valence band of gallium phosphide particles, but has an excellent luminescence property. The correlation between the optical properties and the microstructure of the thin film is discussed.     

Structural order in perfluoropentacene thin films and heterostructures with pentacene

Synchrotron x-ray diffraction reciprocal space mapping was performed on perfluoropentacene (PFP) thin films on SiO2 in order to determine the crystal structure of a novel, substrate-induced thin film phase to be monoclinic with unit cell parameters of a = 15.76 +/- 0.02 A, b = 4.51 +/- 0.02 A, c = 11.48 +/- 0.02 A, and beta = 90.4 +/- 0.1 degrees . Moreover, layered and co-deposited heterostructures of PFP and pentacene (P) were investigated by specular and grazing-incidence x-ray diffraction, atomic force microscopy, and Fourier-transform infrared spectroscopy. For a ca. three-monolayers-thick PFP film grown on a P underlayer, slightly increased lattice spacing was found. In contrast, co-deposited P/PFP films form a new mixed-crystal structure with no detectable degree of phase separation. These results highlight the structural complexity of these technically relevant molecular heterojunctions for use in organic electronics.     

衬底温度对HfOxNy薄膜光学性质的影响

采用射频反应磁控溅射法制备了HfOxNy栅介质薄膜,并研究了HfOxNy栅介质薄膜的光学特性随淀积温度的变化而发生变化的规律．椭偏仪模拟结果显示HfOxNy薄膜的折射率和消光系数都是随衬底温度的升高而增加,根据消光系数和吸收系数还得到了HfOxNy薄膜的光学带隙值,随着衬底温度的升高,带隙减小,这主要是由于N含量的增加所致．     

掺硼p型非晶硅薄膜的制备及光学性能的表征

以高氢稀释的硅烷(SiH₄ )为反应气体,硼烷(B₂H₆)为掺杂气体,利用RF-PECVD方法,在玻璃衬底上制备出掺硼的氢化非晶硅(a-Si:H)薄膜,研究了硼掺杂量对氢化非晶硅(a-Si:H)薄膜的光学性能的影响.利用NKD-7000 W光学薄膜分析系统测试薄膜的透射谱和反射谱,并利用该系统的软件拟合得出薄膜的折射率、消光系数、吸收系数等光学性能参数,利用Tauc法计算掺硼的非晶硅薄膜的光学带隙.实验结果表明,随着硼掺杂量的增加,掺杂非晶硅薄膜样品在同一波长处的折射率先增大后减小,而且每一样品均随着入射光波长的增加而减小,在波长500 nm处的折射率均达到4.3以上;薄膜的消光系数和吸收系数随着硼掺杂量的增大而增大,在500 nm处的吸收系数可高达1.5×10⁵cm^-1.在实验的硼掺杂范围内,光学带隙从1.81 eV变化到1.71 eV.     

Optical properties of thermally evaporated 4-(4-nitrobenzalideneamino) antipyrine Schiff base thin films

4-(4-nitrobenzalideneamino) antipyrine (NBAA) have been synthesized by refluxed ethanolic solution of 4-aminoantipyrine and p-nitrobenzaldehyde for 4 h and characterized with various physico-chemical techniques. Thin films of NBAA have been prepared by the thermal deposition technique in a vacuum of 1.5 × 10⁻⁵ mbar onto optical flat glass substrates. X-ray diffraction patterns show amorphous nature for all NBAA thin films except UV irradiated thin film which shows amorphous nature with some crystallinity but with small amount. The optical constants of thermally deposited NBAA thin film were investigated in the wavelength range 200–2500 nm. The type of optical transition near the edge of the band gap is found to be indirect allowed transition. The effect of UV irradiation as well as the effect of annealing on the optical properties of NBAA thin films was investigated. The value of the energy gap for thin films under investigation is calculated and found to be 1.56 eV for as-deposited, 1.45 eV for annealed and 1.82 eV for UV irradiation.