C. Goletti

Characterization and design of porphyrins-based broad selectivity chemical sensors for electronic nose applications

Abstract The absolute selectivity of chemical sensors ceased in the last years to be a fundamental feature due to the uprising of many applications, such as electronic noses, which require the presence of broad selectivity sensors. On the other hand for these applications the attention is paid to the development of sensors whose broad selectivity can be oriented towards different classes of molecules representing a given environment in order to optimize the interface between the sensitive layers deposited on transducers and the chemical complexity of the environment itself. In this paper the main problems in the design and characterization of such sensors are presented and discussed in the case of a class of molecules, such as metallo-porphyrins, which have been recently exploited as sensitive layers in electronic nose applications.

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.

Chemical sensing materials characterization by Kelvin probe technique

Abstract This paper deals first of all with an improvement of the Kelvin probe (KP) theory taking into consideration the series resistance of the input circuit. Then it illustrates a number of work function measurements performed on self-assembled monolayers interacting with varieties of analytes, and on Langmuir–Blodgett (LB) films of porphyrins of different thickness. The output intensities of the work function have been investigated and comments are given of the results obtained. The link between the work function and thickness of material under test has been analyzed and discussed as a method for the coverage factor estimation of absorbing surfaces.

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

Direct observation of the epitaxial growth of molecular layers on molecular single crystals

In this letter the authors use the evolution of reflectance anisotropy spectra with film thickness during the growth of organic molecular films of a prototype molecular system (α-quarterthiophene grown onto a single crystal of the same material) to demonstrate homoepitaxy. The real time monitoring of the optical anisotropy of a thin film during deposition by organic molecular beam epitaxy is assessed as a powerful tool to achieve an effective in situ control of the growth starting from the very early deposition stages.

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.

Kelvin probe investigation of the thickness effects in Langmuir–Blodgett films of pyrrolic macrocycles sensitive to volatile compounds in gas phase

Abstract Langmuir–Blodgett (LB) films are utilized as sensing materials for many different chemical sensors. This technique allows the fabrication of very thin films, down to one single monolayer. As is well known from classical material science, there is a minimum number of elementary blocks (atoms or molecules) that have to be aggregated before the material properties can be assessed. To this regard, molecular films tend to behave in a different way; since the basic block is a complex molecule, most of the properties are found in the single molecule. This opens the possibility of obtaining materials for which the surface to volume ratio can be maximized in order to increase, for instance, the sensor sensitivity. In this paper, some aspects related to the electronic structure of LB films of porphyrinoids are studied by the Kelvin probe technique as a function of the number of monolayers. Results show that the material properties are, in the case of corrole, confined to the single molecule, while in the case of sapphyrin, a build-up process seems to take place, so that many monolayers are necessary to establish the bulk material properties.

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.