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The nature of optical excitations and the spatial extent of excitons in organic semiconductors, both of which determine exciton diffusion and carrier mobilities, are key factors for the proper understanding and tuning of material performances. Using a combined experimental and theoretical approach, we investigate the excitonic properties of meso- tetraphenyl porphyrin-Zn(II) crystals. We find that several bands contribute to the optical absorption spectra, beyond the four main ones considered here as the analogue to the four frontier molecular orbitals of the Gouterman model commonly adopted for the isolated molecule. By using many-body perturbation theory in the GW and Bethe-Salpeter equation approach, we interpret the experimental large optical anisotropy as being due to the interplay between long- and short-range intermolecular interactions. In addition, both localized and delocalized excitons in the ?-stacking direction are demonstrated to determine the optical response, in agreement with recent experimental observations reported for organic crystals with similar molecular packing.

Corrole molecules deposited from aqueous solution on Au(111) arrange flat forming mesa-like clusters (islands), as evidenced by in situ scanning tunneling microscopy (STM). A morphology quantitative assessment of the entire image is given by STM data analysis, evaluating the islands’ fractal dimension as a whole. To this end, two methods are employed: the first exploits the power law that binds the perimeter and the area of all the islands; the second is simply a value of an appropriate average of the fractal dimensions of the islands. In fact the two methods return very close results.

A terbium(iii)-bis(phthalocyaninato) neutral complex was deposited on the rutile TiO2(110) surface, and their interaction was studied by Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the TiO2 rutile surface favours the adsorption of isolated molecules adopting a lying down configuration with the phthalocyanine planes tilted by about 30° when they lie in the first layer. The electronic and chemical properties of the molecules on the surface were studied by XPS as a function of the TiO2(110) substrate preparation. This study evidences that strong molecule-substrate interactions are present and a charge transfer process occurs from the molecule to the surface.

The self-assembly of corrole molecules dissolved in ethanol and then adsorbed onto an immersed Au(111) surface was investigated by scanning tunneling microscopy (STM) at the solution-solid interface. The physisorption and the subsequent organization of the molecules in multilayers were followed as a function of time at two concentrations, differing by about a factor of 500. In both cases, molecules adsorb flat on the gold surface. For the low concentration, we demonstrate that the first disordered phase is followed, rather abruptly, by a more ordered one, essentially composed of extremely smooth layers of molecules. Even the sharp and winding steps, which mark the layers, describe an evolution toward order. In fact, their rims tend to flatten out, as testified by the decrease in time of their fractal dimension. In the case of high concentration, we have monitored the kinetics of coverage, showing that its evolution for the uppermost layer can be described by Langmuirian kinetics, which provides the adsorption and desorption rate constants, ka = (20.7 ± 1.6) L/(mol s) and kd = (6.5 ± 0.1) × 10-4 s-1.

Ultra-thin organic films must be screened when deposited onto freshly prepared metal surfaces in view of preserving the molecular physical-chemical properties. A current promising strategy prescribes the oxidation of the substrate and, when possible, the creation of a single oxide layer. This strategy was successfully exploited with metal-tetra-phenyl porphyrins (M-TPP) on Fe(0 0 1)-p(1 × 1)O. Here, our study is extended to the oxidized Cu(1 1 0)-(2 × 1)O. Having free coordination sites, the Co-TPP is perturbed when deposited onto the freshly prepared copper surface and the HOMO-LUMO levels quenched, as reported in the literature. Here, we prove that the oxide copper surface reduces the molecule-substrate interaction and photoemission spectroscopy is able to reveal the characteristic electronic features of the molecules.

In situ atomic force microscopy (AFM) measures, with the scanner head immersed inside a liquid (namely an electrolyte), are—generally speaking—not easy to be acquired in real time, i.e., during electrochemical processes, the immersion of a massive and wide system (with respect to the electrochemical cell mean radius) can significantly perturb the electrolyte and the I/V profile of the electrochemical characterization (CV). Despite this fact, the information that can be obtained from an electrode morphological inspection is very precious, and the coupling between an AFM and an electrochemical cell is in any case successful. In this work, we discuss the surface morphological evolution of a graphite electrode when immersed inside an aqueous NaOH electrolyte. 1.0 M sodium hydroxide added inside electrochemical baths was shown to promote the intercalation of ions between the stratified graphite structure or graphene layers. Possible consequences of diluted NaOH electrolyte on the quality of the graphite basal plane have been never considered in detail yet. By means of an in situ AFM, we show the effects of CVs on the surface morphology of the graphite basal plane. Our result shows that a 1.0 M NaOH solution, generally used for promoting intercalation, changes the electrode surface quality as a consequence of a partial carbon corrosion.

Tetra-phenyl prophyrins (TPP) are an interesting class of organic molecules characterized by a ring structure with a metal ion in their centre. An ordered growth of such molecules can be obtained even on metallic substrates by means of a proper modification of the reactive interface, as we demonstrated for ZnTPP molecules coupled to oxygen-passivated Fe(001) [G. Bussetti et al. Appl. Surf. Sci. 390, 856 (2016)]. More recently, we focused on CoTPP molecules, characterized by a not nil magnetic moment and therefore of potential interest for magnetic applications. As in the ZnTPP case, our results for one monolayer coverage report the formation of an ordered assembly of flat-lying molecules. However, some differences between the two molecular species are observed in the packing scheme and in the degree of electronic interaction with the substrate. With the aim of reaching, also for CoTPP, a comprehensive view of molecular organization on Fe, we complement here our previous investigations by following the growth of the CoTPP film for increasing coverage, showing that an ordered stacking of such molecules is indeed realized at least up to four molecular layers.

We present a multitechnique investigation of the structural and electronic properties of the prototypical system composed by ultra-thin films of magnetic molecules [Co-tetraphenyl-porphyrins (Co-TPP)] grown on a ferromagnetic substrate [oxygen passivated Fe(0 0 1), namely the Fe(0 0 1)-p(1 × 1)O surface]. Low Energy electron diffraction (LEED) and scanning tunneling microscopy (STM), coupled with first-principles calculations, reveal the formation of a commensurate superstructure at monolayer coverage, made by a square array of flat-lying TPP molecules. UV–photoemission and inverse photoemission spectroscopies (UPS and IPES) are used to investigate their electronic structure. Similar to our previous results on the Zn–TPP growth on Fe(0 0 1)–p(1 × 1)O, the passivation of the metallic surface is able to preserve the photoemission features characteristic of quasi-free molecules, opening the route towards an exploitation of single oxide layers as protective films in organic/inorganic junctions. X-ray photoemission (XPS) and near edge X-ray adsorption fine structure spectroscopies (NEXAFS), are used to reveal the details of the Co–TPP interaction with the substrate., We acknowledge the CINECA award under the ISCRA initiative, for the availability of high performance computing resources and support (Application No. HP10C2SVDP).

In the present work, we used two different electrochemical (EC) techniques, namely, cyclic voltammetry and normal pulsed voltammetry, applied to a highly oriented pyrolytic graphite (HOPG) electrod...

Graphite intercalation via chemical strategies is a common procedure to delaminate stratified crystals and obtain a suspension of graphene flakes. The intercalation mechanism at the molecular level is still under investigation in view of enhancing graphene production and reducing damage to the original pristine crystal. The latter, in particular, can undergo surface detriment due to both blister evolution and carbon dissolution. The role of the electrolyte temperature in this process has never been investigated. Here, by using an in-situ atomic force microscopy (AFM) apparatus, we explore surface morphology changes after the application of fast cyclic-voltammetries at 343 K, in view of de-coupling the crystal swelling phenomenon from the other electrochemical processes. We find that blisters do not evolve as a consequence of the increasing temperature, while the quality of the graphite surface becomes significantly worse, due to the formation of some adsorbates on possible defect sites of the electrode surface. Our results suggest that the chemical baths used in graphite delamination must be carefully monitored in temperature for avoiding undesired electrode detriment.

Ion intercalation inside graphite crystal is a well-established method for graphene production. Pure acid electrolytes swell the stratified crystal structure and a gentle sonication disperses graphene flakes inside the solution. In view of a better environment-friendly graphite delamination procedure, diluted electrolytes can represent a viable option. Unfortunately, the presence of solvated ions between graphite planes induces oxidation of carbon atoms and the following development of gases in the buried layers of the crystal. Being not permeable, graphite entraps the gases that deform the crystal by producing characteristic bubbles called blisters. The latter represent a detriment of the original specimen and influence the quality of the produced graphene flakes. Here, we will address key properties of blister formation and growth by proving, in particular, that blisters evolve when the applied electrochemical potential value is reduced (cathodic regime), i.e. after the intercalation, conversely to what has been believed for long time. This finding requires a refinement of the current anion intercalation interpretative model.

An extended formation of faceted pit-like defects on Ge(001) and Ge(111) wafers was obtained by thermal cycles to T> 750 {\deg}C. This temperature range is relevant in many surface-preparation recipes of the Ge surface. The density of the defects depends on the temperature reached, the number of annealing cycles performed and correlates to the surface-energy stability of the specific crystal orientation. We propose that the pits were formed by preferential desorption from the strained regions around dislocation pile-ups. Indeed, the morphology of the pits was the same as that observed for preferential chemical etching of dislocations while the spatial distribution of the pits was clearly non-Poissonian in line with mutual interactions between the core dislocations., Comment: European Union (EU), Horizon 2020, Far-Infrared Lasers Assembled using Silicon Heterostructures,FLASH,Grant number: 766719

When light is reflected by a solid sample, the collected signal includes the optical response of all the matter within the light penetration depth. Studying films thinner than this length, the substrate thus contributes to the reflected beam, and affects optical spectroscopy measurements. This is particularly true when polarized light is used, as in reflectance anisotropy spectroscopy (RAS), where substrate symmetry properties play a crucial part. A detailed comprehension of the substrate role in the optical response of a sample has important consequences for correctly extracting the optical contribution of the layer grown on it. This problem becomes delicate in organics, where the choice of the substrate is often fundamental for obtaining an overlayer with a desired structure. In particular, the use of organic single crystal substrates, often structurally anisotropic, is a powerful tool for obtaining highly ordered crystalline films by organic epitaxy. To achieve the film optical properties from an experimental RAS spectrum, an interpretative key is necessary: but the most used model describing the interaction of light with the layered system (the so called three-layer model) fails when the substrate is anisotropic. This care in the application of RAS to organics has not been discussed in details yet, in particular to understand the contribution of the substrate to the total anisotropy signal. Within a phenomenological approach to RAS, we have here weighed the substrate contribution, by considering five different and carefully chosen samples, i.e. a crystalline overlayer of the same material and thickness but on five different substrates. The data show that the substrate clearly influences the readability of the spectra by influencing their lineshape, in some complex cases limiting the possibility of extracting the film properties. Consequently, the correct choice of the substrate - that is a fundamental task for obtaining specific properties of the growing layer - is also a delicate element for a correct interpretation of RAS results.

Corrole derivatives have been recently employed in many applications at the solid-liquid interface. Therefore, the structural arrangement of the molecular layers in direct contact with the liquid is of fundamental interest. We investigated in solution the deposition of molecular layers of the previously prepared water-soluble phosphorus complex of a 2-sulfonato-10-(4-sulfonatophenyl)-5,15-dimesitylcorrole [see synthesis in our previous paper, M. Naitana et al., Chem. Eur. J. 2017, 23, 905-916]. The layer formation of P corroles onto the Au(111) surface was monitored by STM in situ, that is, with the substrate immersed in the solution. Marked differences in the morphology between the organic layer formed on the substrate and that deposited after solvent evaporation (drop casting) are reported. In particular, the coating of gold was more effective and stable in the presence of liquid. Preservation of functionality of the corrole molecules after adsorption was verified. This result validates the relevance of corrole layers at the solid-liquid interface to exploit the peculiar properties of these molecules in real-world applications.

On-surface polymerization realized at the solid-liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of ?-conjugated polymer formation at the interface between an iodine-modified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and ?-conjugated final product. The I/Au(111) substrate enhances the formation of ?-conjugated polymers, as established comparing their production on the surface and in the bulk solution.

Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium.

We have applied reflectance anisotropy spectroscopy (RAS) to investigate the rebonded-step (RS) reconstructed strained Ge/Si(105) surface. RAS spectra display a marked peak at 2.1 eV that is completely cancelled after exposure to about 1200 L of molecular oxygen. Ab initio calculations of the RAS signal explain the experimental spectra in terms of optical transitions between filled bulk-like states and empty surface bands. The early stages of exposure of the clean surface to molecular oxygen have been monitored by RAS and scanning tunneling microscopy (STM), correlating the evolution of the optical anisotropy to the bonding configuration of the adsorbed gas. Oxygen atoms are proposed to first adsorb onto the arms and later on the head dimers of the “horseshoe” motifs that comprise the RS reconstruction, thus gradually destroying the local order.

How can school pupils be introduced to materials science and nanotechnology? Pierfrancesco Riccardi and Claudio Goletti illustrate the advantages of extracurricular activities designed by researchers and teachers.

Reflectance anisotropy spectroscopy (RAS) is demonstrated to be particularly suitable for studying the deposition of organic epitaxial layers in ultrahigh vacuum by organic molecular beam epitaxy, thanks to its high sensitivity and applicability in situ. In the case of α-quaterthiophene, both homoepitaxy and heteroepitaxy have been monitored, demonstrating the crystallinity of the films up to tens of monolayers and the epitaxial relation to the substrate. In both cases, optical RAS data are compared to the results of ex situ characterization of the same samples by atomic force microscopy.© 2009 American Vacuum Society

We have recently measured for the first time–by using Reflectance Anisotropy spectroscopy- the optical gap between dangling-bond states on a single-domain diamond C(111)-2 × 1 surface. The experimental result we have obtained (1.47±0.02 eV ) represents the conclusion of the long-standing effort to determine if this surface is metallic or semiconducting. The large surface gap (even larger for possible excitonic effects) opens a new scientific question: at the moment neither state-of-the-art band calculations have succeeded in obtaining it, nor has its physical origin been explained.

The dispersion of quasi-one-dimensional dangling-bond electrons in pi-bonded chains at the Si(111)-2 x 1 and Ge(111)-2 x 1 surfaces has been experimentally investigated by angle-resolved photoemission spectroscopy, in the direction perpendicular to the chains, with a high energy and angle precision. The results show a very small dispersion in the case of Si(111)-2 x 1 and instead a much larger (downward) dispersion (156 meV) in the case of Ge(111)-2 x 1. Accurate density-functional calculations with GW corrections are in very good agreement with the experimental results. Then the surface chains are somewhat interacting in Ge(111)-2 x 1 - the coupling occurring mainly through the subsurface region - while in Si(111)-2 x 1 they are essentially decoupled. Therefore the one-dimensional character of electrons in surface chains is enhanced in Si(111)-2 x 1 with respect to Ge(111)-2 x 1. (c) 2008 Elsevier B. V. All rights reserved.

For organic molecular materials the definition of the condition of epitaxial growth has been subjected to several misconceptions and a great debate, leading in any case to less restrictive requirements than for inorganic materials. Here, the deposition of oligothiophene films by molecular beam epitaxy on properly grown organic single crystals is discussed and all-organic rigorous epitaxy demonstrated for both sub-monolayer and several-monolayer thick films. The possibility of growing all-organic nanostructures directly follows from these results. © 2006 Elsevier B.V. All rights reserved.

The pivotal importance of TiO2 as a technological material involves most applications in an aqueous environment, but the single-crystal TiO2/bulk-water interfaces are almost completely unexplored, since up to date solid/liquid interfaces are more difficult to access than surfaces in ultrahigh vacuum (UHV). Only a few techniques (as scanning probe microscopy) offer the opportunity to explore these systems under realistic conditions. The rutile TiO2(110) surface immersed in high-purity water is studied by in situ scanning tunneling microscopy. The large-scale surface morphology as obtained after preparation under UHV conditions remains unchanged upon prolonged exposure to bulk water. Moreover, in contrast to UHV, atomically resolved images show a twofold periodicity along the [001] direction, indicative of an ordered structure resulting from the hydration layer. This is consistent with density-functional theory based molecular dynamics simulations where neighboring interfacial molecules of the first water layer in contact with the bulk liquid form dimers. By contrast, this dimerization is not observed for a single adsorbed water monolayer, i.e., in the absence of bulk water.

Using cyclic voltammetry (CV), reflectance anisotropy spectroscopy (RAS), and in situ electrochemical scanning tunneling microscopy (EC-STM), we have studied the structure and structural transition...

The surface or bulk origin of the optical anisotropies detected by reflectance anisotropy spectroscopy (RAS) at GaAs(001)(2 x 4) surfaces has been extensively investigated in the last years and a quite general agreement has been reached that the dominating character would be bulk-like. Nevertheless, a very recent paper [F. Arciprete et al., Phys. Rev. B 69, 081308(R) (2004)] has again issued the presence of surface states contributions in optical anisotropies, revealing a structure at 2.5 eV due to surface states, in addition to the well known features around 2.9 eV and 4.5 eV related to the bulk critical points E 1 and E' 0 . We have carried out a new experiment to prove this conclusion by following the changes in the optical anisotropy of a GaAs(001)(2 x 4) surface in the range 2.0-5.0 eV induced by Ag/Sb-codeposition. The interface Ag/GaAs(001) is known to be not reactive. Due to its surfactant effect, codeposition of Sb leads to a nearly epitaxial growth of the Ag overlayer. We show that at the early stages of deposition (nominally at 0.25 monolayer) an evident modification of the RAS spectrum is detected at 2.5 eV, well below the photon energy (2.9 eV) where bulk-like anisotropies appear. We relate this modification to the disappearance of surface states characteristic of the (2 x 4) reconstruction, in excellent agreement with previous conclusions and experiments.

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.

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.

Reflectance anisotropy spectroscopy (RAS) has been recently used to investigate organic compounds, similarly to what has been already done in the case of inorganic semiconductors and metals. An important development is the application of RAS to monitor in real time the growth of thin organic layers in ultra-high vacuum (UHV). In this paper, we discuss the main characteristics of RAS spectroscopy, underlining its peculiar advantages with respect to other techniques. Then, we present recent results obtained by following in situ the deposition of an ordered organic film in UHV, namely α-sexithiophene films onto potassium acid phthalate.

Reflectance anisotropy spectroscopy (RAS) has been applied to study the oxidation of decapped InAs(0 0 1) surfaces at roomtem perature. (2 � 4) and (4 � 2) surfaces were prepared in ultra high vacuumby annealing, and then exposed to increasing amounts of molecular oxygen. During exposure to gas, RAS spectral structures at 2.4 eV in (2 � 4) and at 1.7 eV in (4 � 2) are progressively reduced. After about 1 � 10 4 L, a broad residual anisotropy spectrumis still m easured for (2 � 4), while relic anisotropy is nearly null for (4 � 2). Residual features are visible at bulk critical points (E1, E1 þ D1 and E 0 ). The different behavior versus gas contamination evidences the origin of RAS spectral structures, allowing interpretation in terms of surface-state or bulk transitions (the latter modified by the surface). By using Kramers–Kronig relations, the anisotropy of the imaginary part of the surface dielectric function (De 00 ) between [ 1 ] and [1 1 0] directions of the substrate has been obtained fromRAS data. We conclude that the main RAS structures (at 2.4 eV in (2 � 4), at 1.7 eV in (4 � 2)) are related to the surface reconstruction, although only the latter is a transition involving true surface states. We observe bulk contributions in coincidence with critical points (E1, E1 þ D1 and E 0 ). 2002 Elsevier Science B.V. All rights reserved.

We have studied the optical anisotropy of porphyrin layers with different thicknesses deposited onto gold substrates by the Langmuir−Schaefer technique. In coincidence with the Soret band of the molecule, the optical spectral line shape as determined by reflectance anisotropy spectroscopy exhibits a characteristic, large structure, which changes from a “peaklike” to a “derivative-like” shape at a well-defined thickness (8−10 monolayers). We interpret this result in the framework of electronic effects due to structural changes in the layers, yielding solid-state effects originating from the coupling of the π orbitals of the porphyrin macrocycles.

The controlled adsorption of molecules of volatile compounds on Langmuir–Schäfer 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.

The use of porphyrins as the sensing elements in technical applications (e.g. electronic nose and electronic tongue) has anticipated the real comprehension of the mechanisms determining at the microscopic level the interaction between the analyte molecules and the layer itself. A huge bulk of phenomenological data has been accumulated, showing the high efficiency of these organic materials versus specific gaseous particles. It has been also reported that the sensitivity to gas is higher for highly disordered layers, probably due to the enhanced surface area of the resulting assembly. A better comprehension of the microscopic behavior would definitely allow a deeper exploitation of the properties of the layer in view of the most efficient electronic devices . Consequently, recently there has been an effort to gain a higher level of accuracy in characterizing the electronic properties and the morphology of the thin (sometimes ultrathin) layers deposited.

Porphyrins represent molecular building blocks for the development of a large variety of functional materials. The control of subtle intramolecular and intermolecular processes in solid porphyrin aggregates is promising for the establishment of organic nanotechnology, which requires the knowledge of the properties of nanometric aggregates down to single atoms. Within this framework, the optical anisotropy in thin and ultrathin metal-free tetraphenylporphyrin (H2TPP) films deposited on graphite carries interesting information related to their molecular aggregation. In this work, we compare the surface morphology and the optical anisotropy of H2TPP 2D layers, obtained following a recently developed experimental strategy, with thin films where both 2D and 3D aggregates are present. After reaching a fine control of the relative amount of the two phases present in films of different thicknesses, we propose a precise physical model for the optical response of the pure 2D phase, therefore correlating its optical anisotropy with the electronic properties of single H2TPP molecules, the occurrence of intramolecular chemical reactions, and the molecular arrangement. The response of thicker films, characterized by the coexistence of the 2D and 3D phases, is described by a phenomenological model that considers the main spectroscopic features. © 2014 American Chemical Society.

Until now, with a few exceptions, Reflectance Anisotropy Spectroscopy (RAS) has been applied mostly to inorganic semiconductors and metals, proving to be a powerful tool to investigate their optical properties, via anisotropies introduced by the surface in otherwise isotropic bulk materials. Recently, we have shown that RAS can be successfully used to study the optical anisotropies of organic Langmuir-Blodgett films. In this paper we have extended our study to thick Langmuir-Blodgett films of porphyrins, a class of molecules widely studied for the importance of their physico-chemical properties.

The sum rule for the imaginary part of the dielectric function e″ can help in discriminating surface and bulk-related optical transitions, by means of the number of electrons taking part in the transitions. This is particularly simple in the region below the main critical points of the bulk band structure (below 3.5 eV for Si, below 2.2 eV for Ge), since there the surface differential reflectivity is roughly proportional to e″ of the surface. We discuss the cases of Ge(1 1 1)-2×1 and Si(1 1 1)-2×1 investigated by surface differential reflectivity in the energy range 0.3–3.5 eV, with light linearly polarized parallel and perpendicular to the chains of the Pandey model. A key point is that the sum rule holds for each polarization separately (termed α and β), implying that: ∫ωe α ″ d ω=∫ωe β ″ d ω . In these results, the effect of the sum rule can be easily recognized (crossing of the curves and seemingly equal areas on both sides of the crossing point). Similar considerations should also hold for reflectance anisotropy spectroscopy (RAS). It will be shown, however, that a compensation of positive and negative signals, in compliance with the sum rule, is actually expected only if RA spectra are multiplied by a suitable function. Moreover, the compensation is not exact, when the bulk is strongly absorbing. The results of a numerical simulation valid for various materials and surface oscillators are reported. They help to understand why in some cases (for instance in GaAs(0 0 1)-2×4 and −4×2), the sum rule is apparently not obeyed.

This paper presents experimental results of surface differential reflectivity for Ge(111)-(2×1) in the energy range 0.3–3.5 eV, obtained with light polarized with the electric vector parallel and perpendicular to the chains of the Pandey model. The process of oxidation of the clean surface is followed in the range 0–3×104 Langmuir (L). Two distinct oxidation regimes are observed: one in connection with the spectrum below 2.5 eV and a second for energies above 2.5 eV. The former shows a saturation at about 1000 L, the same exposure where the (2×1) LEED pattern disappears. The latter does not show a saturation even at 3×104 L. This different behavior demonstrates that two distinct groups of electrons are involved. Sum rules for anisotropic surfaces applied to experimental results show that the dangling-bond electrons have not exhausted their oscillator strength at 1 eV and that they contribute in a significant way to the spectrum above the bulk gap. The comparison with Si(111)-(2×1) is also discussed. The effective numbers of electrons associated with the transitions below 2.5 eV in germanium and 3.5 eV in silicon show that approximately 1.5 and 1 electron per surface atom contribute to the spectra in germanium and silicon, respectively.

We have investigated the growth of thin indium layers on As- and Ga-terminated GaAs(001) surfaces by reflectance anisotropy spectroscopy (RAS), low energy electron diffraction (LEED), and auger electron spectroscopy. Room temperature deposition of indium on the $(2\ifmmode\times\else\texttimes\fi{}4)/c(2\ifmmode\times\else\texttimes\fi{}8)$ surface and subsequent annealing at 450 \ifmmode^\circ\else\textdegree\fi{}C leads to the formation of an In-terminated surface showing a $(4\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern, accompanied with strong changes in the measured surface optical anisotropy. When indium is deposited onto the $(4\ifmmode\times\else\texttimes\fi{}2)/c(8\ifmmode\times\else\texttimes\fi{}2)$ surface, on the contrary, the $(4\ifmmode\times\else\texttimes\fi{}2)$ In-terminated surface is already formed at room temperature deposition without needing annealing, as demonstrated by the RAS spectra. The finding that almost identical RAS spectra and $(4\ifmmode\times\else\texttimes\fi{}2)$ LEED patterns are obtained in both cases shows that the same final atomic structure is achieved. Finally, we conclude that the structure of the In-terminated surface is similar to that of the clean Ga-rich surface, although a more detailed model would need accurate calculations of the microscopic origin of the measured anisotropy.

We present a detailed experimental investigation of the room temperature optical anisotropy of clean GaAs(001) surfaces, and of the change of this anisotropy after exposure to oxygen. On the As-rich (2×4) surface, this anisotropy consists of two main signals, situated, respectively, near 3 and 4.5 eV. The results, together with the analysis of the anisotropy of Ga1−xAlxAs, lead us to the conclusion that the anisotropy near 3 eV originates mainly from localized states related to surface dimers. However, the anisotropy near 4.5 eV is related to bulk-like optical transitions, since this anisotropy is chemically insensitive, and since its spectral shape is similar to that of the bulk dielectric constant.

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.

The growth of thin indium-layers on the GaAs(100) As-rich (2 × 4) c (2 × 8) surface has been investigated by reflectance anisotropy spectroscopy (RAS), LEED and AES. Clean surfaces of the (2 × 4) c (2 × 8) reconstruction were prepared in UHV by thermal desorption of a protective arsenic layer deposited on homoepitaxially grown MBE layers. Room temperature deposition of indium on the (2 × 4) c (2 × 8) surface and subsequent annealing at 450°C leads to a 90° rotation of symmetry in the LEED pattern at a threshold coverage of 0.5 monolayers, i.e. a change from the (2 × 4) c (2 × 8) to the (4 × 2) c (8 × 2) reconstruction. The RAS spectra show the evolution of a distinct negative feature at 1.8 eV, that shifts to 2.1 eV after annealing, corresponding to optical transitions attributed to In-dimers orientated along the [011]-direction. AES analysis shows a change in growth mode beyond 0.5 ML indicating saturation of all available adsorption sites at this coverage and RAS spectra show a contribution from additional disordered In. The AES spectra display no evidence of a surface exchange reaction between gallium and arsenic atoms, thus indicating a surface termination by In-dimers adsorbed on a layer of As.

Dopo avere capito come funziona una cella fotovoltaica, e avere approfondito alcuni aspetti costruttivi, affrontiamo il passo successivo, ovvero il controllo sperimentale delle sue prestazioni dopo averla realizzata. Questa attivita sperimentale si articola a sua volta su due livelli: i) da una parte la verifica del funzionamento della cella, ed eventualmente la misurazione di parametri importanti che permettono di valutarne e anzi quantificarne l’efficacia; ii) dall’altra, uno studio attento dei particolari processi che avvengono durante l’utilizzo del dispositivo: l’assorbimento della luce, la produzione delle cariche elettriche, il loro trasferimento al circuito esterno. Seguendo questo secondo metodo, si vuole comprendere cosa realmente avvenga nell’interno della cella: a tale scopo, talvolta si analizzano sistemi anche notevolmente semplificati rispetto al dispositivo originale, disarticolando le fasi successive del fenomeno con l’obiettivo di comprenderlo, e quindi riprodurlo in modo controllato e consapevole.

Results on the chemisorption of molecular oxygen on Si(111)-2 × 1 at different temperatures are reported. They show that oxygen is initially physisorbed in a precursor state, from which it can be thermally activated into the chemisorbed state. An activation energy of 20 meV is found. The dependence of the sticking coefficient upon coverage deviates from the classical Langmuir theory. A model of chemisorption fitting the experimental observations is presented. In the frame of this model, the heights of the barriers for chemisorption and desorption are obtained.

We have revisited the issue of (empty) intrinsic surface states at the GaP(110) surface and related Fermi-level position, since previous results based on photoemission experiments were uncertain due to the effect of surface photovoltage. The photovoltage problem has been overcome by combining kelvin-probe measurements and photoemission spectroscopy. We confirm that on well-cleaved samples almost flat bands are observed, while the usual pinning position of the Fermi level at 1.5--1.7 eV above the top of the valence band is due to extrinsic surface states associated with cleavage defects. Therefore, intrinsic surface states of GaP(110) are very close to the bottom of the conduction band.

Reflectance anisotropy spectroscopy (RAS) has been employed to study in situ the growth of thin alpha-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. (C) 2003 American Institute of Physics