P. Chiaradia

Highly sensitive optical monitoring of molecular film growth by organic molecular beam deposition

Reflectance anisotropy spectroscopy (RAS) has been employed to study in situ the growth of thin α-sexithiophene films by organic molecular beam deposition onto an organic substrate. A large anisotropy can be detected by following the line shape evolution of the RAS spectrum; in addition, the signal variation at a fixed wavelength is used to monitor the film growth. The signal intensity scales with the deposited thickness, demonstrating a very high sensitivity of RAS to less than 1/50 of a monolayer. Evidence of the advantages of RAS to monitor in real time the growth of molecular films and to probe in situ their properties is therefore obtained.

Optical anisotropy of Langmuir–Blodgett sapphyrin films

The polarization dependence of the optical reflectivity for sapphyrin layers deposited by the Langmuir–Blodgett technique onto a gold substrate has been measured. The experimental results show that characteristic reflectance anisotropy spectroscopy (RAS) spectra are related to layers of different thicknesses. In order to interpret the measured spectral features, the anisotropy of the optical properties of the sapphyrin molecule has been evaluated by using a semi-empirical quantum chemistry approach. The results show clearly that the main RAS structures are related to the electronic properties of the sapphyrin molecules. In particular, two different regimes are observed. Below one (true) monolayer, the optical signal remains very low and structureless, in agreement with the sapphyrin molecules being stacked with their planes almost perpendicular to the substrate plane. Above one monolayer, instead, a strong RAS signal related to the Soret band develops, suggesting that sapphyrin molecules lay more parallel...

Optical anisotropy of porphyrin Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of 5,10,15,20-tetrakis-[4-(1-Heptyloxy)phenyl]porphyrin (H2THOPP) have been deposited onto quartz and silver substrates. The deposited layers (20 monolayers thick) have been characterised by reflectance anisotropy spectroscopy, a technique that recently has been successfully used to study the optical anisotropies of organic materials, For porphyrin LB films, the experimental results show that a large anisotropy signal is measured in the Soret band energy region. We show that this anisotropy is clearly connected to the derivative of the dielectric function of the layer. We propose an interpretation of the derivative-like line shape as due to either strain or similar perturbations on the otherwise symmetric top-layer molecules induced by the lower-lying tilted molecules aligned along a direction related to the growth

Kelvin probe and scanning tunneling microscope characterization of Langmuir–Blodgett sapphyrin films

The work function of solid layers of increasing thicknesses of E2M8− sapphyrin, deposited on a gold substrate by the Langmuir–Blodgett method, has been measured by the Kelvin-probe technique. The results show that the contact-potential-difference values depend upon the layer thickness, reaching saturation after a certain amount of deposited sapphyrin. Scanning tunneling microscope images taken at the same coverages show that corresponding with this threshold, sapphyrin forms a true continuous layer on gold, completely covering the substrate. Evolution of the layer towards its completion is accompanied by a continuous variation of the work-function value, consistent with an increasing dipole term due to the interaction of sapphyrin with the metal substrate.

In situ optical investigation of oligothiophene layers grown by organic molecular beam epitaxy

Reflectance anisotropy spectroscopy (RAS) has recently been used to monitor in situ and in real time the growth of thin organic layers in ultra high vacuum (UHV) (Goletti et al 2003 Appl. Phys. Lett, 83 4146). In this paper, after a short discussion of RAS, with particular attention to the application to organics, we present recent results on the deposition of ordered oligothiophene films by organic molecular beam epitaxy (OMBE), namely α-sexithiophene and quaterthiophene films onto potassium acid phthalate substrates.

Pseudomorphic growth of organic semiconductor thin films driven by incommensurate epitaxy

A stable pseudomorphic phase of α-quaterthiophene, a well known organic semiconductor, is obtained by growing films with organic molecular beam epitaxy (OMBE) on a single crystal of another organic semiconductor, namely, tetracene. The structural characteristics of the new phase are investigated by monitoring in situ the OMBE process by reflectance anisotropy spectroscopy; thus assessing that incommensurate epitaxy is in this case, the driving force for tuning the molecular packing in organic molecular films and in turn, their solid state properties.

Optical anisotropy readout in solid-state porphyrins for the detection of volatile compounds

The controlled adsorption of molecules of volatile compounds on Langmuir–Schafer films of tetraphenylporphyrins produces a modification of the film optical anisotropy, as revealed by reflectance anisotropy spectroscopy (RAS). These experiments allow a better understanding of the molecular package occurring in the film as well as of its alteration upon the interaction with volatile compounds, helping in the comprehension of the mechanisms responsible for the binding of molecules. The results recommend RAS as a potential transducer technique for chemical sensing.

Optical transitions and sum rules at clean semiconductor surfaces

The experimental results of surface differential reflectivity and reflectance anisotropy spectroscopy show that in Si(111)-2 × 1, Ge(111)-2 × 1 and GaAs(001)-2 × 4 a sum rule for the imaginary part of the (surface) dielectric function is verified both for the isotropic and anisotropic parts of . It is shown that the sum rule together with the dependence of the spectra upon oxygen contamination are useful in the interpretation of the optical transitions of the above surfaces. In particular, for the case of GaAs(001)-2 × 4 the above analysis has allowed the distinction between optical transitions associated to true surface states and bulk states modified by the surface near the 3 eV critical point.

Langmuir–Blodgett films of a modified tetraphenylporphyrin

Abstract Langmuir–Blodgett (LB) films of 5,10,15,20-tetrakis-[4-(1-Heptyloxy)phenyl]porphyrin (H2THOPP) in a 1/4 molar ratio with arachidic acid have been deposited onto quartz and silver substrates at the surface pressure of 25 mN/m. A detailed analysis of the Langmuir isotherms is presented. Optical absorption spectra show aggregation of the porphyrins in the LB film. The deposited layers have been characterised by Reflectance Anisotropy Spectroscopy (RAS), showing a large anisotropy signal measured in the Soret band energy region.

Structure-dependent optical anisotropy of porphyrin Langmuir-Schaefer films

Abstract We have studied the optical anisotropy of Langmuir–Schaefer layers of PdC10OAP porphyrin, deposited onto gold substrates with thickness in the range 0–16 monolayers (ML). Deposition has been carried out at two values of the surface pressure Π, corresponding to different layer structures. In one case (Π=30 mN/m), molecules are well ordered in stacks oriented edge-on with respect to the substrate. In the other (Π=10 mN/m), a complex reorganization of the system happens several days after deposition, to form a mesoscopic two-dimensional lattice. The spectra measured by reflectance anisotropy spectroscopy (RAS) in two cases are clearly characterized. In the former, the line shape is dominated by a characteristic, large structure appearing in coincidence with the Soret band of the molecule, the development of which from a “peak-like” to a “derivative-like” appearance occurs at a well-defined critical thickness Θc (8 ML). In the latter, the line shape is always “peak-like”. We explain both line shapes in terms of morphological characteristics of the layer, occurring at different thickness values. The present results clearly show the potential of RAS to characterize efficiently the deposition of organic materials, and suggest that in short time it will be used as an in situ and real time spectroscopy, as already done in inorganic growth.