Color-tunable anisotropic optical films fabricated using perylene diimide mixed with naphthalene benzimidazole

Naphthalene benzimidazole compound (NBI) was synthesized and mixed with perylene diimide (PDI) in order to modulate the absorbance wavelength of an optically anisotropic film between 400 and 550 nm. Equimolar mixture of NBI and PDI could form an ordered lyotropic liquid crystal (LLC) phase in 16.5 N formic acid solution when the solid content was larger than 20 wt.%, even though NBI salt itself did not show any LLC behaviors in all range of concentration. With increasing the content of NBI in NBI/PDI solution, the intensity of absorbance bands between 400 and 450 nm increased but polarization efficiency (P_eff) of the oriented films decreased. Based on the polarized optical microscopy morphological observations, it was realized that the orientation of PDI was hindered by the formation of NBI aggregates. When the molar ratio of NBI and PDI was 0.25, P_eff of the oriented film exhibited 84.1% at 475 nm and 49.8% at 400 nm, which can be compared with those (90.4% at 486 nm and 30.2% at 400 nm) of the oriented film without NBI.     

Magneto-optical Kerr-effect studies on copper oxide thin films produced by atomic layer deposition on SiO2

This work demonstrates the sensitivity of magneto-optical Kerr-effect (MOKE) spectroscopy to ultra-thin nonmagnetic films using the example of copper oxide. The films with an effective thickness between 0.6 nm and 6 nm are produced by atomic layer deposition (ALD) on silicon oxide substrates based on the Cu(I) β-diketonate precursor [(ⁿBu₃P)₂Cu(acac)] (acac = acetylacetonate) at a process temperature of 120 °C. The copper oxide films exhibit magneto-optical activity in the spectral ranges around 2.6 eV and above 4 eV. The evolution of the spectral features as a function of the number of ALD cycles is simulated numerically using the dielectric function and the Voigt constant of Cu₂O as input parameters. The comparison between experimental and simulated MOKE spectra strengthens the conclusion drawn from spectroscopic ellipsometry studies that the thin film optical constants differ markedly from the bulk ones.     

Tuning morphology and fluorescence of aggregated nanostructures of derived perylene diimide molecules

In this paper, we report on the self-assembly of water-soluble N,N'-di(N,N'-dimethyl-dodecane-1, 12-diamide)-perylene-3,4,9,10-tetracarboxylic diimide (PDDoAM) in formic acid and chloride salts for producing varied nano-aggregates with different optical properties. Interestingly, the self-assembly can lead to nanocubic, microsheet and "tower-like" nanostructures respectively, as demonstrated by Scanning Electron Microscopy (SEM) images. The optical properties of molecular aggregates were investigated by means of Confocal Raman Microscopy, indicating the morphologies and fluorescence of these nanomaterials are dependent on acids, acid concentrations and casting methods.     

Field emission studies of CVD diamond thin films: effect of acid treatment

Field emission from CVD diamond thin films deposited on silicon substrate has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. Field emission studies of as-deposited and acid-treated films were carried out using ‘diode’ configuration in an all metal UHV chamber. Upon acid treatment, the field emission current is found to decrease by two orders of magnitude with increase in the turn-on voltage by 30%. This has been attributed to the removal of sp² content present in the film due to acid etching. Raman spectra of both the as-deposited and acid-treated films exhibit identical spectral features, a well-defined peak at 1333 cm⁻¹ and a broad hump around 1550 cm⁻¹, signatures of diamond (sp³ phase) and graphite (sp² phase), respectively. However upon acid treatment, the ratio (I_d/I_g) is observed to decrease which supports the speculation of removal of sp² content from the film. The surface roughness was studied using atomic force microscopy (AFM). The AFM images indicate increase in the number of protrusions with slight enhancement in overall surface roughness after acid etching. The degradation of field emission current despite an increase in film surface roughness upon acid treatment implies that the sp² content plays significant role in field emission characteristics of CVD diamond films.     

Photoconductive relaxation studies of SnSe thin films

The compound tin selenide was prepared from the constituent elements (Sn and Se) using the standard fusing technique and from X-ray diffraction studies it was identified as tin selenide. Thin films of SnSe were obtained on thoroughly cleaned glass substrates by vacuum sublimation on substrates maintained at 301 K. The photoconductive relaxation of these films was studied with oxidation. The results have been explained with the help of grain boundary potential barrier model.     

Steering rigidified nonplanar conjugation based perylene diimide supramolecular for enhancing photocatalytic activity

π-Conjugated supramolecular with higher delocalization of electrons has attracted considerable attention in enhancing the charge transfer in photocatalysis. However, those conjugated macromolecules often possess varied rotational geometries, which will significantly deteriorate charge mobility but still inexplicitly. Herein, we reported diversified PDI polymers with intramolecular angles of 94.7°, 149.7° and 176.3° to explore the role of π-conjugated non-planar molecules. Density functional theory (DFT) calculations and experimental results show that vertical structural PDIMH has antibonding in anisotropic polarizable monomer to generate a macro-dipole, which greatly expands the built-in electric field and facilitates charge transfer and exciton dissociation. On the other hand, the vertical angle is favorable for the face-to-face overlap of the homogeneous molecules, which will create a carrier migration channel and promote carrier separation. Notably, PDIMH exhibited highly effective photocatalytic sterilization (near 100% in 2 h) and benzylamine oxidation (conversion rate up to 300 mmol g⁻¹h⁻¹), which is superior to other ever reported catalysts. This work provides a new interpretation for regulating molecular geometry in developing highly efficient photocatalyst to solve future sustainable issues.     

TEM studies of RF magnetron-sputtered thin films

Lead scandium tantalate (PST) thin films sputtered onto sapphire substrates have been studied by using transmission electron microscopy. Samples in transverse section were used to characterize the microstructure of the thin films as a function of distance from the PST-sapphire interface whereas samples in plan section allowed a more detailed, structural investigation of the PST. A liquid nitrogen cold stage was used to induce the paraelectric-ferroelectric phase transitionin situ and to perform heating and cooling experiments on the thin films.In general, samples which had the lowest dielectric constants were found to have a layer of unreacted ScTaO₄ at the film-substrate interface, whereas the highest dielectric constants were associated with fully perovskite-structured films. Films prepared by the sequential deposition of ScTaO₄ and PbO, followed by heat treatment, readily spalled from the sapphire. They also exhibited an increase in the Sc and Ta cation order with distance from the film-sapphire interface. A sample prepared by the simultaneous deposition of ScTaO₄ and PbO did not spall and showed an increase in structural order at both the PST-sapphire interface and at the film surface.     

Low reflection and photo-sensitive organic light-emitting device with perylene diimide and double-metal structure

In this paper, an organic light-emitting device (OLED) with low reflection for the entire visible range and at different viewing angles by using an absorptive and photo-sensitive material, N,N'-Bis(2,6-diisopropylphenyl)-1,7-bis(4-methoxyphenyl)perylene-3,4,9,10-tetracarboxydiimide (MPPDI), as a black layer (BL) structure was demonstrated. A semitransparent double-metal structure, Al/Ag, was used to reduce the driving voltage and enhance the destructive interference of the BL structure. Combined with the broad absorption band of the MPPDI, the reflectance of such a device shows a 12-times decrease compared to the conventional OLED. Also, a photocurrent enhancement effect was observed in our BL-OLED due to the photosensitive quality of the MPPDI.     

Magneto-optical properties of obliquely evaporated Ni films

Longitudinal magneto-optical Kerr effect (MOKE) configuration and scanning electron microscopy (SEM) were used to investigate the effects of deposition angle, on magnetic properties and morphology of obliquely evaporated Ni films. The results show that the angle of deposition has a critical effect on the magnetic anisotropy of the films. These effects are due to the microstructure of the films, which is controlled by the angle of deposition with respect to the sample normal. The results show the presence of shape anisotropy governing the demagnetization of the magnetic fields. The anisotropy, coercivity and squareness of the hysteresis loops increased with an increase in of the vapour flux. These properties and surface roughness became marked for off-normal deposition angles larger than 50°. At low s, the easy axis of magnetization lay perpendicular to the incidence plane. At large s the easy axis changed parallel to the incidence plane. The results may be quantitatively understood from the presence of an inclined columnar microstructure with shape anisotropy governing the demagnetization of the magnetic fields.     

Magneto-optical properties of barium ferrite sputtered films

Barium ferrite thin films with perpendicular anisotropy were grown on (111) oriented GGG substrate by rf diode sputtering method. Their magnetic properties were measured. Faraday rotation was measured in the wavelength range 460–800 nm for such sputtered thin films. These films were found quite suitable for magneto-optical recording applications.     

Self diffusion studies on cobalt thin films

The study of lateral diffusion in thin metallic films is important from the application point of view, especially in electromigration reliability studies. Lateral self diffusion in cobalt thin films is studied using a non-destructive tracer scanning method. Neutron irradiation is employed to make a well-defined radioactive (⁶⁰Co) region in the middle of a continuous cobalt thin film stripe of width 3 mm. The experimental data are fitted to the appropriate solution of the diffusion equations by means of a non-linear least square fitting procedure using a computer. The diffusion experiments are conducted in the temperature range 300–600°C in argon atmosphere. This thin film data are compared with the diffusion data available on bulk cobalt. The activation energy for surface diffusion obtained (0·14 eV) is very much smaller than the reported activation energy for grain boundary diffusion in cobalt.     

Electrical conductivity studies on carbazole thin films

Thin films of carbazole have been prepared using vacuum evaporation technique. The electrical conductivity studies are carried out in both low and high temperature regions and the activation energies have been determined. In the low temperature region the electrical conduction is due to hopping of charge carriers in a coulomb gap. Carbazole thin films have been used to fabricate capacitors and the variation of capacitance, dielectric constant; conductivity and dielectric loss in the frequency range between 100 Hz and 3.16 MHz are investigated. The effect of annealing on the dielectric properties is also investigated. The surface topography of the deposited films is studied using scanning electron microscopy. Film morphologies are found to change by annealing.     

Ellipsometric studies on cupric telluride thin films

The optical properties of cupric telluride (CuTe) thin films have been studied in the wavelength range 310-800 nm using spectroscopic ellipsometry (SE). Thin films of thickness between 30 and 150 nm were prepared by thermal evaporation at the rate of 15.6 Å/s on well cleaned glass substrates kept at 300 K under the vacuum better than 2×10⁻⁵ mbar. It has been found that the optical band gap increases with the thickness of the films. The refractive index of the films increases with the energy but the extinction coefficient first increases and then decreases gradually with energy. The analysis of the absorption coefficient determined from the extinction coefficient reveals that there is allowed direct transition with a band gap of about 1.5 eV. The increase in the band gap with the increase in the film thickness has been ascribed to defect levels in the band gap formed by defects in the films.     

Dielectric studies on tellurium oxide thin films

Tellurium has been evaporated in an r.f. glow discharge of oxygen onto negatively-biased glass substrates at room temperature. These tellurium oxide thin films have been used as dielectrics in capacitors. The variations of capacitance, C, and dielectric loss, , with frequency in the range of 0.5 to 30 kHz at various temperatures (300 to 443 K) have been studied. The temperature coefficient of capacitance and the dielectric constant for the material have been evaluated. The ' spectrum reveals a loss peak and a loss minimum, both shifting towards higher frequences with increasing temperatures. The maximum in the dissipation factor is explained on the basis of a dipolar relaxation phenomenon. The activation energy for the process has also been evaluated.     

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly <111> oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 10⁶ to 2.5 × 10⁷ V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 10⁶ V/m. In the higher field range, from 2.5 × 10⁶ to 2.5 × 10⁷ V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.     

Magneto-optical studies of low-dimensional organic conductors

Our periodic orbit resonance (POR) results on quasi-two-dimensional (q2D), highly anisotropic q2D and quasi-one-dimensional (q1D) organic conductors are reviewed together with our rotational cavity magneto-optical measurement system. Higher order POR up to seventh order has been observed in the q2D system (BEDT-TTF)₂Br(DIA), and the experimental conditions to observe POR and the cyclotron resonance (CR) are discussed. Highly anisotropic q2D Fermi surface (FS) in β″-(BEDT-TTF)(TCNQ), which was considered to have q1D FS previously, is proposed by our POR measurements, and the possible interpretations of other experimental results of β″-(BEDT-TTF)(TCNQ) are discussed assuming the highly anisotropic q2D FS. Finally, detailed q1D FS of (DMET)₂I₃, obtained from our POR results, is discussed in connection with the typical q1D system (TMTSF)₂ClO₄.