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The chemical characterization of materials used in works of art is extremely useful for gaining a better knowledge of the artistic heritage and to guarantee its preservation. A derivatization GC/MS procedure for the identification of proteins in a microsample from painted works of art has been optimized. The amino acid fraction is derivatized using anhydrous dimethylformamide (DMF) as solvent instead of pyridine (Py), commonly used to facilitate the reaction. Although pyridine is often considered a silylation catalyst, there are many instances in which silylation reactions actually are slower in pyridine than other solvents. In addition, pyridine also may have other undesirable effects such as the promotion of secondary products and other chromatographic anomalies. Using DMF, the formation of artifacts is limited and the derivatization yield of hydrophilic amino acids such as proline and hydroxyproline has improved, thus making the identification of organic paint media more straightforward. The method has been validated and successfully applied to identify the binder of the sample taken from the pictorial cycle of the 12th century monastery of Santa Maria delle Cerrate (Lecce, Italy), thus highlighting the use of eggs as a binding medium.

The identification of physical parameters that lead to magnetism in apparent nonmagnetic semiconductor systems has continuously challenged our community. In particular, for quantum-dot systems, they are key factors that contribute to their magneto-optical properties. We report, experimentally, optical evidences of induced nanomagnetism in nonmagnetic CdSe quantum dots and assess theoretically the role played by charged and uncharged vacancies. The analysis of these effects rests upon both the chemical and strain environments where the quantum dots are embedded. The interplay of spin-orbit interaction with built-in axial strains has been demonstrated to be a key factor for the magnetic moment alignment. This has been achieved in this paper by emulating the electronic structure at atomistic levels, considering various defect configurations and taking into account both the quantum dot composition and the influence of the host lattice. Postprint

Surface functionalization emerges as an alternative pathway for electronic structure engineering in two-dimensional (2D) systems. We explore a hybrid heterostructure built from a 2D transition-metal dichalcogenide atop a layer of photochromic self-assembled azobenzene molecules. This setup can modify carrier transport through the 2D nanosheet by an effective photoactived gating control, achieved through trans to cis isomerization of the azobenzene in close contact with the 2D nanosheet. We show theoretically that this behavior can be ascribed to a tunable gating mechanism. We use a combination of atomistic calculations of the local effective potential profile with simulations of the electron scattering by such potential in a massive Dirac Hamiltonian formalism. We find that different isomers induce a local perturbation with drastically different features that explain well the experimental behavior.

We report on the tailoring of quantum dot (QD) emission efficiency by localized surface plasmon polaritons in self-organized mesoscopic rings. Ag nanoparticles (NPs) with CdSe QDs embedded in a polymeric matrix are spatially organised in mesoscopic rings and coupled in a tuneable fashion by breath figure formation. The mean distance between NPs and QDs and consequently the intensity of QD photoluminescence, which is enhanced by the coupling of surface plasmons and excitons, are tuned by acting on the NP concentration.

On-surface chemistry for atomically precise sp2 macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp2-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C–Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C–Br scission and C–C coupling processes. Further, unreacted material can be sublimed and the coupled sp2-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp2-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology., We acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374) Spanish Grants (FIS2010-21282-C02-01 and PIB2010US-00652), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), Ikerbasque and the European Commission projects CRONOS (Grant 280879-2 CRONOS CP-FP7). C.-A.P. and K.D. were funded through the ERC Advanced Grant MolArt (no. 247299). D.J.M. acknowledges funding through the Spanish “Juan de la Cireva” program (JCI-2010-08156). J.L.C acknowledges funding through the Mexican CONACyT program.

A correlated photoluminescence (PL) and electroluminescence (EL) investigation of light-emitting electrochemical cells based on [Ir(4,5-diphenyl-2-methylthiazolo)2(5-methyl-1,10-phenanthroline)]+ [PF6]− reveals the excimer nature of the EL emission. Excimers are formed when the device is biased and the EL emission energy is a fingerprint of the excimer intermolecular interactions induced by the local electric field at the applied voltage. Imaging measurements provide evidence that the molecular rearrangement into aggregates favors the formation of excimer states, which are irreversibly formed once the device is biased and are the preferential emitting states even in devices left unbiased for a long period of time. PL lifetime measurements in working devices provide unambiguous evidence of the excimer character of EL. These results show that excimers play a role in the mechanism of operation and performance degradation of organic light-emitting electrochemical cells (OLEC) based on ionic transition metal c...

Post-growth thermal annealing, (TA) in combination with mesa-fabrication technologies, was applied to tune the shape and material composition of ZnCdSe quantum dots (QDs). The QDs were studied by means of photoluminescence spectroscopy. An enhancement of QD homogeneity due to TA is demonstrated by substantial narrowing of the QD emission, observed from large QD ensembles. Energy shifts of QD luminescence, exceeding 100 meV after thermal annealing, were observed. Tracing the evolution of single exciton lines, the activation energy of an enhanced diffusion process in small mesas was determined. Distinct changes of quantum dot symmetries were identified through a reduction of the exciton fine structure splitting. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and properties of quantum dots (QDs) and related nanostructures by conventional molecular beam epitaxy (MBE) and two other variants of the technique, have been systematically investigated. While MBE growth of CdSe on ZnSe at 300 °C results in the formation of rough CdSe quantum-well-like structures, by one of the variants, which combines epitaxial growth of a CdSe layer at 230 °C and subsequent in-situ annealing at 310-340 °C, tiny but distinct three-dimensional (3D) QDs are formed. We demonstrate that QD formation by this method, developed originally by Rabe et al. (J. Cryst. Growth 184/185, 248 (1998) [30]), essentially takes place during the annealing step, by a process of “controlled roughening” of the as-grown quasi-two-dimensional (2D) CdSe layer. In comparison to conventional MBE, this process results in reduction of the areal density of the self-organized QDs by up to two orders of magnitude, thus making them distinctly discernible. In the second variant technique, similar to that applied originally for the self-organization of CdTe/ZnTe quantum dots by Tinjod et al. (Appl. Phys. Lett. 82, 4340 (2003) [34], QD-formation takes place as a result of deposition of an amorphous group-VI layer onto a strained 2D CdSe layer at ∼ 50 °C, and its subsequent re-desorption at temperatures of 280-320 °C. While deposition and desorption of amorphous Se leads to the formation of large isolated QDs, we show that use of amorphous Te results in the self-assembly and ordering of a rich variety of nanostructures. Luminescence, morphology, and composition of QDs and the underlying formation mechanism of the three processes are discussed in detail. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Layer-by-layer epitaxial growth of CdSe on ZnSe has been observed at temperatures as low as 170–240 °C. Beyond this range, Reflection high energy electron diffraction (RHEED) specular spot intensity oscillations disappear, signifying a multilayer growth mode. While conventional epitaxial growth of CdSe at ∼300 °C results in formation of a rough layer-like structure, a method combining the epitaxial growth of a quasi-two-dimensional (2D) CdSe layer at 230 °C and subsequent annealing at 310 °C induces the formation of discrete CdSe quantum dots (QDs). Here we show that the formation of QDs by this method relies essentially on the layer-by-layer growth of CdSe at 230 °C, which allows an induced roughening of the as-grown 2D layer, during the subsequent annealing step. The layer-by-layer growth mode results possibly due to the nucleation of small and irregular precursor 2D clusters during growth at low temperatures, at the kink sites of which potential barriers for downward inter-layer transport of adatoms is low/absent.

Deposition of amorphous Te on a strained epitaxial CdSe layer at room temperature, and its subsequent desorption in the temperature range of 200–310 °C results in novel morphological reorganization of the CdSe surface. Dashes, long chains of quantum dots (QDs), and lateral QD molecules have been observed in atomic force microscopy images of CdSe surfaces, after such a treatment. Te admixture of the QD-layer may not be ruled out. The morphology of the CdSe (Te) surface is strongly governed by the thickness of the deposited amorphous Te layer, but shows almost no dependence on the temperature at which this layer is desorbed. The observed self-ordering of nanostructures over length scales of ∼600 nm by this process strongly suggests that Te leads to an enormous enhancement of adatom migration. While at low temperature, photoluminescence (PL) consists of emissions from localized excitons in both the observed large QD-based nanostructures and a residual inhomogeneous CdSe(Te) layer, at room temperature the entire contribution is apparently from the former. This leads to a redshift of the spectra, with increase in temperature from 7 to 300 K, by as much as 600 meV.

Photoluminescence of ZnCdSe/ZnSe quantum dots treated by thermal annealing is investigated in magnetic fields up to 6 T. Thermal annealing leads to considerable changes of the circular polarization properties, which we attribute to different spin relaxation times and exciton lifetimes. A negative diamagnetic shift is observed for quantum dots showing the largest TA-induced shift of 80 meV. Possible origins are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self assembled CdZnSe quantum dots (QDs) embedded in a ZnSe matrix were thermally annealed and the induced interdiffusion between Zn and Cd atoms was investigated by photoluminescence spectroscopy. We observed a blue shift of the QD emission by more than 100 meV. According to a Fickian diffusion model an activation energy of 2.2 eV for the interdiffusion of Cd and Zn between the CdZnSe QDs and the ZnSe barrier is determined. The half widths of the emission lines decrease significantly with increasing annealing temperature, which allows to observe in the spectrum of strongly interdiffused CdZnSe quantum dots a second emission peak. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The main aim of this paper is to trace the use and the organization of space in Early Bronze Age Sicilian (2200-1450 BCE) societies. The purpose is to identify the connection among types of food consumed in relationship to vessel shape, in addition to characterising indicators of food preparation and the space in which these activities take place. This study incorporates both organic chemistry and GIS spatial analyses in order to understand vessel use in direct correlation to food preparation space in a settlement context. The hill site of Santa Febronia, contains the remains of an Early Bronze Age hut that was destroyed by a fire leaving a sealed deposit with a large quantity of artefacts in their original position. The deposition creats an ideal scenario for GIS spatial analysis of the artefact distribution within the structure in contrast to areas of storage. Chemical residue analysis was carried out with GC-MS to extract and identify absorbed lipids within ceramic vessels (cf. Eerkens, 2005). The gas chromatographic results are used to establish relations between the food processing and the depositional space. The study reveals a multi-functionality of both space and pottery and further confirms the distinction of storage areas and in particular areas dedicated to water storage.

The perfect transmission of charge carriers through potential barriers in graphene (Klein tunneling) is a direct consequence of the Dirac equation that governs the low-energy carrier dynamics. As a result, localized states do not exist in unpatterned graphene, but quasi-bound states \emph{can} occur for potentials with closed integrable dynamics. Here, we report the observation of resonance states in photo-switchable self-assembled molecular(SAM)-graphene hybrid. Conductive AFM measurements performed at room temperature reveal strong current resonances, the strength of which can be reversibly gated \textit{on-} and \textit{off-} by optically switching the molecular conformation of the mSAM. Comparisons of the voltage separation between current resonances ($\sim 70$--$120$ mV) with solutions of the Dirac equation indicate that the radius of the gating potential is $\sim 7 \pm 2$ nm with a strength $\geq 0.5$ eV. Our results and methods might provide a route toward \emph{optically programmable} carrier dynamics and transport in graphene nano-materials.

A switchable diode in a 2D semiconductor-molecular junction heterostructure is reported. MoS2 is exfoliated on top of a monolayer of azobenzene-substituted thiols on gold. Photoisomerization of the azobenzenes results in switching between a rectifier with rectifying ratios of 10(4) and a conductive state, as revealed by conducting atomic force microscopy.

Molecular self-assembly is a versatile nanofabrication technique with atomic precision en route to molecule-based electronic components and devices. Here, we demonstrate a three-dimensional, bicomponent supramolecular network architecture on an all-carbon sp(2)-sp(3) transparent platform. The substrate consists of hydrogenated diamond decorated with a monolayer graphene sheet. The pertaining bilayer assembly of a melamine-naphthalenetetracarboxylic diimide supramolecular network exhibiting a nanoporous honeycomb structure is explored via scanning tunneling microscopy initially at the solution-highly oriented pyrolytic graphite interface. On both graphene-terminated copper and an atomically flat graphene/diamond hybrid substrate, an assembly protocol is demonstrated yielding similar supramolecular networks with long-range order. Our results suggest that hybrid platforms, (supramolecular) chemistry and thermodynamic growth protocols can be merged for in situ molecular device fabrication.

We report on the fabrication and characterization of a two-dimensional photonic crystal (PC) embedding two InGaAs quantum dot layers as active light emitters. Light confinement was provided by both a PC GaAs slab waveguide and a distributed Bragg reflector (DBR) located at the bottom of the guiding layer. The micro-photoluminescence characterization shows that the photonic crystal can be used to alter the quantum dot emission by modifying the photonic density of states.

The photoluminescence of annealed Cd\(_x\)Zn\(_{1-x}\)Se quantum dots (QDs) under the influence of an external magnetic field has been studied in this paper. Post-growth annealing was performed for different annealing times. Above a critical annealing time, the QD luminescence shows a pronounced red-shift of the Zeeman split magnetic subcomponents. This observation is in contrast to the blue-shift caused by the diamagnetic behavior that is usually observed in non-magnetic QDs. We attribute our finding to the paramagnetism caused by the mixing of heavy and light hole states. Hence, post-growth thermal annealing treatment might be employed to render undoped epitaxial QDs intrinsically magnetic in a controlled manner. Two theoretical models were developed: a few-particle model to account for excitonic complex effects and a multiband calculation that describes the valence band hybridization. Contrasting the two models allowed us to unambiguously elucidate the nature of such an effect.

Ionic transition metal complexes (iTMCs) have already been demonstrated to be a promising type of material to fabricate low-cost light sources, which are much more competitive in terms of realization costs with respect to standard organic light emitting diodes. The device performance, optical and morphological properties of thin films of two different complexes [Ir(phenylpyridine)(2)(5-Me-1,10-phen)][PF(6)] and [Ir(phenylquinoline)(2)(5-Me-1,10-phen)][PF(6)] have been measured and compared. The use of an ionic liquid as part of the processing procedure shows advantages in terms of low operation voltage, which is as low as 3.5 Volts. However, it leads to drawbacks in terms of device lifetime, limited to t(1/2) = 2 min, and maximum achievable brightness (1425 cd m(-2) vs. 3040 cd m(-2) without ionic liquid, for the complex [Ir(phenylpyridine)(2)(5-Me-1,10-phen)][PF(6)]).

We report a systematic study of magneto-optical properties in CdZnSe/ZnSe semiconductor quantum dots QDs subjected to postgrowth thermal annealing. The theoretical and experimental characterizations are combined in order to understand the evolution of Zeeman splitting and the blueshift as well as redshift of the magnetic subcomponents with magnetic field strength of excitons confined in annealed QDs as a function of different annealing times. A combination of a multiband model as well as parameter interpolation supported by ab initio calculations is presented and points toward an inversion of the light-hole-heavy-hole states due to annealing. The band mixing of exciton states explains besides the variation in the Zeeman splitting for differently annealed QD excitons, the evolution of the diamagnetic blueshift into a paramagnetic redshift of the magnetic subcomponents. A discussion of the effects associated to the Coulomb interaction on the polarized excitonic recombinations modulated by magnetic field is given.

A negative magnetic dispersion of the degree of circular polarization is reported for the photoluminescence of self-assembled ZnCdSe quantum dots (QDs) after a postgrowth thermal annealing (TA) treatment. Multiband calculations in the framework of the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model reveal that the phenomenon is explicable considering, apart from the changes in the confinement and composition of the QDs due to TA-induced Zn-Cd interdiffusion, the presence of extra charges of one kind within the structure.

By mapping the anisotropy fine-structure splitting of the exciton ground state in the luminescence spectra of individual CdxZn1−xSe quantum dots, treated by postgrowth rapid thermal annealing (TA), a preferred in-plane axis of Zn–Cd interdiffusion has been identified. In particular, a TA-induced sign reversal of the fine-structure splitting is demonstrated. Additionally, in the annealed quantum dots, the binding energy of the charged exciton reaches a maximum value when the fine-structure splitting is minimum. The studies demonstrate that by postgrowth thermal annealing the symmetry of individual quantum dot can be modulated.

We report photoluminescence measurements performed on monolayer- and two-layer-MoS2 placed on two types of mixed self-assembled monolayers (mSAMs) of photoswitchable azobenzene molecules. The two mSAMs differ via the electronegative character of the azobenzene derivatives. Thin layers of a transition metal dichalcogenide—MoS2—were mechanically exfoliated on mSAM to allow for direct interaction between the molecules and the MoS2 layers. When the MoS2 nanosheet is in contact with the electropositive azobenzene molecules in trans configuration, an emission side band at lower energies and at low excitation powers suggest n-type doping. The photoisomerization of the molecules from trans to cis configuration lowers the doping, quenching the side band and enhancing the overall PL efficiency by a factor of ∼3. Opposite results were observed with the chlorinated, more electronegative molecules, exhibiting a reversed trend in the PL efficiency between trans and cis, but with an overall larger intensity. The type of doping induced by the two types of mSAMs was determined by Kelvin probe force microscopy technique.

We report on the fabrication and characterization of a two-dimensional photonic crystal (PC) embedding two InGaAs quantum dot layers as active light emitters. Light confinement was provided by both a PC GaAs slab waveguide and a distributed Bragg reflector (DBR) located at the bottom of the guiding layer. The micro-photoluminescence characterization shows that the photonic crystal can be used to alter the quantum dot emission by modifying the photonic density of states.

We report on the fabrication and characterization of a two-dimensional photonic crystal (PC) embedding two InGaAs quantum dot layers as active light emitters. Light confinement was provided by both a PC GaAs slab waveguide and a distributed Bragg reflector (DBR) located at the bottom of the guiding layer. The microphotoluminescence characterization shows that the photonic crystal can be used to alter the quantum dot emission by modifying the photonic density of states.

The formation mechanism of CdSe quantum dots (QDs) on ZnSe by low temperature epitaxial growth of a CdSe layer and its subsequent in situ annealing at a higher temperature has been studied. By this method, a small postgrowth residue of mobile adatoms nucleates distinct, stable, three-dimensional islands atop preexisting two-dimensional (2D) precursors. High temperature annealing activates an up-climb of these adatoms onto the 2D precursors, which, however, predominantly climb down at the low growth temperature (TG). A growth interruption at TG, therefore, smoothens the as-grown CdSe layer, and when introduced for appropriate durations, enables reducing the QD areal density.