Marcello Campione

Low power, non-coherent sensitized photon up-conversion: modelling and perspectives

In the last few years, non-coherent sensitized photon up-conversion (SUC) in multi-component systems has been developed to achieve significantly high quantum yields for various chromophore combinations at low excitation powers, spanning from the ultraviolet (UV) to near infrared (NIR) spectrum. This promising photon energy management technique became indeed suitable for wide applications in lighting technology and especially in photovoltaics, being able to recover the sub-bandgap photons lost by current devices. A full and general description of the SUC photophysics will be presented, with the analysis of the parameter affecting the photon conversion quantum yield and the quantities which define the optimal working range of any SUC system, namely the threshold and saturation excitation intensity. It will be shown how these quantities depend on intrinsic photophysical properties of the moieties involved and on the SUC solid host matrix. The model proposed represents a powerful tool for evaluation of a newly proposed system, and its reliability will be discussed in respect to an optimized system with SUC yield of 0.26 ± 0.02. The results obtained will outline the research guidelines which must be pursued to optimize the SUC efficiency for its perspective technological applications.

Glycine-spacers influence functional motifs exposure and self-assembling propensity of functionalized substrates tailored for neural stem cell cultures.

The understanding of phenomena involved in the self-assembling of bio-inspired biomaterials acting as three-dimensional scaffolds for regenerative medicine applications is a necessary step to develop effective therapies in neural tissue engineering. We investigated the self-assembled nanostructures of functionalized peptides featuring four, two or no glycine-spacers between the self-assembly sequence RADA16-I and the functional biological motif PFSSTKT. The effectiveness of their biological functionalization was assessed via in vitro experiments with neural stem cells (NSCs) and their molecular assembly was elucidated via atomic force microscopy, Raman and Fourier Transform Infrared spectroscopy. We demonstrated that glycine-spacers play a crucial role in the scaffold stability and in the exposure of the functional motifs. In particular, a glycine-spacer of four residues leads to a more stable nanostructure and to an improved exposure of the functional motif. Accordingly, the longer spacer of glycines, the more effective is the functional motif in both eliciting NSCs adhesion, improving their viability and increasing their differentiation. Therefore, optimized designing strategies of functionalized biomaterials may open, in the near future, new therapies in tissue engineering and regenerative medicine.

Measured Davydov splitting in oligothiophene crystals.

The polarized absorption spectra of single crystals of oligothiophenes in a wide spectral range are reported. The experimental procedure is discussed, underlying several details which are relevant to obtain reliable spectra particularly for samples of increasing thickness. On the basis of these considerations, it has been possible to fully detect the transition to the upper Davydov exciton originating from the first molecular state. The position and shape of the main exciton peak in these materials are compared and discussed, taking into consideration the molecular arrangement and the longitudinal contribution which depends on the transition moment orientation. The Davydov splitting values as deduced from the experimental data at room temperature are also reported either for the first vibronic replica or for the electronic transition as a whole. The difference between the purely transverse and the measured Davydov splitting is discussed.

Growth and characterisation of centimetre-sized single crystals of molecular organic materials

The growth of high quality single crystals and the control of their size and shape are crucial issues for the study of the fundamental physical properties of materials. A novel floating-drop technique is here employed for the growth of centimetre-sized single crystals of different organic molecular materials, namely oligothiophenes and oligocenes dissolved in organic solvents, using water as liquid substrate. The static properties of the two-liquid system are described and their tuning for the control of the thickness of the grown crystals discussed. The quality of the samples is assessed by atomic force microscopy and optical absorption spectroscopy measurements.

Organic Metal‐Semiconductor Field‐Effect Transistor (OMESFET) Fabricated on a Rubrene Single Crystal

2010 WILEY-VCH Verlag Gm While some applications are becoming a commercial reality, the basics of organic electronic devices still present many unclear aspects. Understanding their mode of operation is not straightforward, because the high density of defects in the active layer generally obscures the intrinsic mechanisms of the devices. However, as in the inorganic case, single crystals of conjugated molecules can be easily grown and efficient, flexible and large-scale devices can be fabricated and characterized. Many of the effects linked to the non-periodicity of the elemental structure and presence of defects can be worked around and issues regarding the intrinsic properties of a device elucidated. Therefore, in order to clarify the operating mechanism of the organic metal–semiconductor field effect transistor (OMESFET), we used rubrene, a well-know organic semiconductor that forms single crystals with an extremely low defect density. An OMESFET is a non-conventional organic field-effect device, already proposed for use as a low-operating-voltage variable resistor, which differs from the conventional organic field-effect transistor (OFET) by the absence of a gate dielectric. Instead, the gate is directly deposited on the semiconductor layer, on which it forms a blocking contact. This simple architecture requires fewer fabrication steps than a traditional insulated-gate OFET. The device performance depends on the nature of the non-injecting gate metal electrode. Even if its practical integration into complex electronic circuits has still to be demonstrated, its importance for fundamental investigations is obvious, as underlined in a recent paper published during the course of this work. The formation of a metal–organic interface and the injection of electrical charges from a ‘‘non-ohmic’’ contact into an organic active layer involve complex and still unresolved processes, and the analysis of these devices, combined with the characterization of single crystal asymmetric diodes, should help in clarifying some of the aspects underlying organic electronics. Besides its importance in many optoelectronic applications, such as solar cells, light emitting diodes, and logic circuits, the metal– organic interface is also the basic ingredient of OMESFETs; its analysis is therefore a required preliminary step. From the study of asymmetric metal–semiconductor–metal structures, we did not find any evidence for the formation of a depletion layer in the rubrene single crystal close to the non-ohmic contact. Instead, the dependence of the impedance on the voltage, combined with the analysis of the current-voltage (I–V) characteristics, reveals the effect of a built-in potential, arising from the difference between the work function of the two metal electrodes at both sides of the structure. As long as the diode is under reverse bias, that is, when a negative bias is applied to the high work function electrode, no current flow is registered, while when the system is polarized in forward bias, injection of holes occurs from the high work function electrode. The small-signal impedance response of the diode clearly reveals the contribution of a second RC element in series with the expected high resonance geometrical one. On this basis, efficient single crystal metal–semiconductor field effect transistors have been fabricated and characterized. Thanks to the high resistance of an Au/rubrene/Al structure under reverse bias, these voltage-controlled resistors show negligible leakage current and a high ON/OFF ratio, comparable to that found in other low-voltage field effect devices. The absence of a depletion layer in an organic single crystal diode suggests that the physical principles underlying the device operation must be revisited. A first simple attempt in this direction is given here, suggesting that the current modulation is an interface process in which charge injection is controlled by the gate potential. By laying a thin single crystal on a metal substrate and evaporating a top electrode on the opposite side of the crystal, we fabricated asymmetric metal–semiconductor–metal structures, through which the fundamentals of organic diodes can be investigated. Figure 1 shows the semi-logarithmic and linear plots of a typical I–V response of an ITO/rubrene/Al system. These single crystal diodes are hole-only devices with no detectable current in the reverse direction. A measurable current only flows when a positive bias is applied to the anode (high work function electrode, ITO or Au), in which case positive carriers are injected into the active layer thanks to a suitable energy level alignment between the metal Fermi level and the rubrene highest occupied molecular orbital (HOMO) level, while practically no electrons can pass over the high injection barrier between the aluminum and the lowest unoccupied molecular orbital (LUMO) level of the semiconductor. The curve in Figure 1 is qualitatively similar to that of an inorganic diode. However, the principles governing the operation of both devices are quite different. In the organic diode, the rectifying effect is not controlled by a single metal–semiconductor barrier; instead, it is a consequence of the work function difference between the metal electrodes at both sides of the rubrene single crystal and of the resulting built-in potential established in the organic semiconductor layer. Figure 2 shows a close view of the current onset of

Effect of functionalization on the self-assembling propensity of β-sheet forming peptides

The mechanism underlying self-assembly of short peptides has not been fully understood despite the fact that a few decades have passed since their serendipitous discovery. RADA16-I (AcN-RADARADARADARADA-CONH2), representative of a class of self-assembling peptides with alternate hydrophobic and hydrophilic residues, self-assembles into β-sheet bilayer filaments. Though a sliding diffusion model for this class of peptides has been developed in previous works, this theory need further improvements, supported by experimental investigations, to explain how RADA16-I functionalization with biological active motifs, added at the C-terminus of the self-assembling core sequence, may influence the self-assembling tendency of new functionalized peptides (FPs). Since FPs recently became a promising class of biomaterials for cell biology and tissue engineering, a better understanding of the phenomenon is necessary to design new scaffolds for nanotechnology applications. In this work we investigated viaatomic force microscopy and Raman spectroscopy the assembly of three RADA16-I FPs that have different hydrophobic/hydrophilic profiles and charge distributions. We performed molecular dynamics simulations to provide further insights into the experimental results: functionalizing self-assembling peptides can strongly influence or prevent molecular assembly into nanofibers. We also found certain vibrational molecular modes in Raman spectroscopy to be useful indicators for elucidating the assembly propensity of FPs. Preliminary FP designing strategies should therefore include functional motif sequences with balanced hydrophobicity profiles avoiding hydrophobic patches, causing fast hydrophobic collapses of the FP molecules, or very hydrophilic motifs capable of destabilizing the RADA16-I double layered β-sheet structure.

Structural characterisation of single crystals and thin films of α,ω-dihexylquaterthiophene

Single crystals and thin films of the p-type organic semiconductor α,ω-dihexylquaterthiophene have been studied using X-ray diffraction and TEM, respectively. The single crystal analysis reveals the presence of monomolecular layers of quaterthiophene cores with terminally bound hexyl chains. A herringbone molecular packing akin to that exhibited by the parent quaterthiophene molecule is observed. Electron diffraction of thin films on silica clearly shows the crystalline polymorph of the solution-grown crystals and the crystallites deposited by organic molecular beam deposition are identical. However, thin film crystallites are severely affected by multiple twinning.

Directional dispersion in absorbance spectra of oligothiophene crystals

Due to the large oscillator strength of the first molecular transition in oligothiophenes, a strong directional dispersion of the b(u) exciton transition is expected originating from the macroscopic polarization field. Examining such dispersion unambiguously usually requires different faces to be accessible for the optical measurements. Alternatively, measurements carried out at different angles of incidence are met with intrinsic limits due to the peculiarities of wave propagation in such anisotropic systems. In order to demonstrate these limits along with the experimental difficulties involved, we examine refraction and absorption of light in these crystals and discuss the effects of directional dispersion on the absorbance spectra of quaterthiophene crystals.

Thickness measurements by quartz microbalance during thin-film growth by organic-molecular-beam deposition

The problem of in situ monitoring the film thickness by quartz microbalance during vacuum deposition of organic-molecular semiconductors is addressed herein by setting a procedure for sensor calibration based on ex situ analysis of the deposited molecular film by atomic-force microscopy measurements. The procedure is applied to the growth of molecular-organic thin films on silica. Some physical parameters of the materials are deduced.

Crystal structure of polycrystalline films of quaterthiophene grown by organic molecular beam deposition

Abstract The growth of thin films of oligothiophenes by organic molecular beam deposition (OMBD) can produce highly ordered polycrystalline samples. Recently, quaterthiophene (4T) thin films were grown on different substrates (silica, graphite, potassium acid phthalate, silicon). In some cases, their optical behavior was found to resemble that of 4T single crystals, with a macroscopic anisotropy close to that of the bulk crystal. A careful structural characterization of these films was undertaken. A morphological and optical characterization was performed by atomic force microscopy (AFM) and optical absorption. Electron microscopic examination in both bright field and diffraction was carried out on 4T thin films deposited on two selected substrates: silica and single crystals of potassium phthalate acid salt. The crystal polymorph, contact plane, and coherent orientation obtained on these 4T micro-crystalline films fully agree with the optical behavior of the samples.