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Surface-supported arrays of Fe(4)-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe(4)(L)(2)(dpm)(6)] (1) on Au, an isomorphous compound [Fe(3)Cr(L)(2)(dpm)(6)] was synthesized and structurally and magnetically characterized (H(3)L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr(3+) ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L(2,3) edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L(2,3) spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe(3+) ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.

Here we report the experimental observation of circular dichroism in the second-harmonic field (800--400 nm conversion) generated by self-organized gold nanowire arrays with subwavelength periodicity (160 nm). Such circular dichroism, raised by a nonlinear optical extrinsic chirality, is the evident signature of the sample morphology. It arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence. The results were compared, both in the optical linear and nonlinear regime, with a reference sample composed of straight wires. Despite the weak extrinsic optical chirality of our samples (not observable by our optical linear measurements), high visibility (more than 50%) was obtained in the second-harmonic generated field.

The controlled production of nanostructures endowed with well-selected shapes and reduced dimensionality represents a multidisciplinary key activity in nanotechnology. In particular, a great effort is currently oriented at the development of metal nanoparticle arrays in order to exploit the strong field-enhancement effects associated with the excitation of localized surface plasmon resonances. Here we demonstrate that ion-beam sputtering can be successfully employed to convert a polycrystalline metal film into a disconnected array of nanowires supported on a flat dielectric substrate. The nanowire arrays exhibit spectrally selective dichroic absorption due to the excitation of localized plasmon oscillations. The possibility to tailor the broadband plasmonic response has been demonstrated by finely tuning the morphological parameters of the arrays.

The self-organized formation of high aspect ratio dielectric nanostructures can be guided and sped-up recurring to a sacrificial metal film during ion beam sputtering. Following ion irradiation, the metal film evolves into a disconnected array of laterally ordered nanowires, which guide etching of the dielectric substrate. While the amplification rate of large scale features can be described simply in terms of the ratio of the sputtering yields of substrate and film, for small scale features the amplification rate depends on the interplay and lateral range of the smoothing and erosive mechanisms of the two materials.

Nano-patterned metal surfaces on dielectric substrates are become very important in nanoscale optical devices such as molecular sensors, second harmonic emitters, negative index materials. Among these, self-organised approaches are a promising alternative to the conventional lithographic or scanning lithography methods, since allow low cost patterning of large macroscopic surface in a single step event. The possibility to construct tailored artificial nanostructures by acting on simple experimental parameters opens new exciting possibilities in the nanoscale technology. In this work we explored the second harmonic emission properties in transmission of self-organized gold nanowires on dielectric substrates. Second harmonic emission from thin metal surfaces is limited by several constrains: for example the emission is forbidden at normal incidence and the emission is always polarized along the plane of incidence of the pumping light. In our samples selected morphology allows to overcome such constrains leading to high emission at normal incidence, polarization of the emission guided by the wires orientation. The angular dependence of the generation efficiency was measured on test samples whose thicknesses of the wires range from 5 nm to 40 nm. The efficiency results of a value larger with respect to samples with equivalent thickness and different morphology.

A tetrairon(III) single-molecule magnet is deposited using a thermal evaporation technique in high vacuum. The chemical integrity is demonstrated by time-of-flight secondary ion mass spectrometry on a film deposited on Al foil, while superconducting quantum interference device magnetometry and alternating current susceptometry of a film deposited on a kapton substrate show magnetic properties identical to the pristine powder. High-frequency electron paramagnetic resonance spectra confirm the characteristic behavior for a system with S = 5 and a large Ising-type magnetic anisotropy. All these results indicate that the molecules are not damaged during the deposition procedure keeping intact the single-molecule magnet behavior.

Here we report the second harmonic emission properties of self-organized gold nanowires arrays supported on dielectric substrates with a sub-wavelength periodic pattern. The peculiar morphology of the nanowires, which are locally tilted with respect to the average plane of the substrate, allows to generate maximum second harmonic signal at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The generation efficiency was increased by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency and also by increasing the local tilt of the nanowires.

Here we report the second harmonic emission properties of self-organized gold nanowires arrays supported on dielectric substrates with a sub-wavelength periodic pattern. The peculiar morphology of the nanowires, which are locally tilted with respect to the average plane of the substrate, allows to generate maximum second harmonic signal at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The generation efficiency was increased by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency and also by increasing the local tilt of the nanowires.

Here we report on the development of an unconventional approach for the physical synthesis of laterally ordered self-organized arrays of metallic nanowires supported on nanostructured dielectric templates. The method, based on a combination of nanoscale patterning of the glass substrate by ion beam sputtering with shadow deposition of the metal nanoparticles, provides a viable alternative to time consuming serial nanopatterning approaches. Far-field optical characterization demonstrates that the nanowire arrays exhibit tunable anisotropic properties in the visible range due to the excitation of localized plasmon resonances.

We report on the self-organized patterning of polycrystalline metal films supported on dielectric substrates using defocused Ar{sup +} ion-beam sputtering at glancing incidence. The data demonstrate that a nonflat initial surface morphology does plays a critical role in triggering self-organization during the early stages, where the distribution of polycrystalline grains initially imposes a nonstochastic spatial modulation of the ion impact sites. A crossover to the conventional erosive regime observed for single crystals sets in at longer sputtering times. Surprisingly, the vertical and lateral modulation of the pristine surface profile of the nonflat films results in a substantial reduction in the ion dose required for the formation of the nanoscale patterns, in comparison to the case of an initially flat film.

Large area MoS2 films with tunable physical-chemical properties are grown on dielectric substrates by annealing of ultrathin Mo layers in the presence of a sulfur-containing gaseous precursor. Different growth conditions are found to have a significant impact on material properties, including chemical composition, roughness, and grain sizes, thus shedding light on critical parameters that govern sulfurization processes for the synthesis of large area 2D transition metal dichalcogenides. Optimized growth conditions in combination with the use of single crystal sapphire substrates with atomically flat interface result in the formation of oriented MoS2 films with improved quality and electrical performance. On the basis of this versatile synthesis technique, an original double-step process is presented for the synthesis of WS2/MoS2 vertical heterostructures. Good uniformity of layers over large area has enabled first isolation of defects by electron spin resonance spectroscopy with densities correlated with mobility degradation and the first experimental characterization of the band alignment at the interfaces of MoS2, WS2, and their vertical stacks with the underlying SiO2 insulator.

Silicene grown on Ag(1 1 1) is characterized by several critical parameters. Among them, the substrate temperature plays a key role in determining the morphology during growth. However, an unexpected important role is also equally played by the post-deposition annealing temperature which determines the self-organization of silicene domains even in the submonolayer coverage regime and consecutive transitions between silicene with different periodicity. These temperature-driven phase transitions can be exploited to select the desired majority silicene phase, thus allowing for the manipulation of silicene properties.

We report on the formation of high aspect ratio anisotropic nanopatterns on crystalline GaAs (100) and Si (100) substrates exploiting defocused Ion Beam Sputtering assisted by a sacrificial self-organised Au stencil mask. The tailored optical properties of the substrates are characterised in terms of total reflectivity and haze by means of integrating sphere measurements as a function of the morphological modification at increasing ion fluence. Refractive index grading from sub-wavelength surface features induces polarisation dependent anti-reflection behaviour in the visible-near infrared (VIS-NIR) range, while light scattering at off-specular angles from larger structures leads to very high values of the haze functions in reflection. The results, obtained for an important class of technologically relevant materials, are appealing in view of photovoltaic and photonic applications aiming at photon harvesting in ultrathin crystalline solar cells.

We report on nanoscale patterning of GaAs (100) semiconductor substrates employing an ion projection through a self-organized stencil mask. The nanostructured mask, formed by ion beam sputtering of a polycrystalline Au film, allows driving the GaAs morphology strongly out of equilibrium. In a second stage, after the stencil mask is removed, we quantitatively follow the dynamical evolution of the forced system toward equilibrium by analyzing the evolution of the power spectral density of the height profiles and their slope and curvature distribution. Strong differences are observed by comparing the relaxation dynamics of GaAs surfaces which tend to smoothen with that of glass, a material which instead tends to non-linearly amplify the pattern once driven out of equilibrium by the stencil mask.

A sessile (water) drop deposited onto a superhydrophobic surface can stay in two different regimes: the heterogeneous (or Cassie–Baxter) regime, where air or vapour is trapped at the interface, or the homogenous (or Wenzel) regime, where the interface is completely wet. Often, a spontaneous transition from the first to the second regime may occur in time. We report on a study of this phenomenon in a new kind of superhydrophobic surfaces, characterized by a relevant spatial disorder. The observed transition follows a behaviour quite different respect to the one measured in the commonly used periodic artificial arrays.

We report on self-organized patterning of polycrystalline noble metal films, supported on dielectric substrates, by defocused Ar+ ion beam irradiation. The initial surface morphology affects the formation of nanostructures, forcing the growth of ripples with a lateral periodicity imposed by the pristine polycrystalline grain distribution. At the early stages, the self-organization process is dominated by the shadowing of taller grains, while a crossover to the conventional erosive regime, observed for single crystals, sets in at longer sputtering times. The grain boundaries, although providing an additional bias for diffusion of mobile defects, do not affect the propagation of nanoscale ripples across individual grains.

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and compreh...

Artificial circular dichroism [1] is investigated for developing novel devices for active polarization controllers, like rotators and modulators and high efficient molecular sensors. Here we show the chiral behavior raised by metal (Au) nanostructures on dielectric (polystyrene, PS) nanosphere substrate produced by a self assembled procedure that guarantees large area fabrication with low time consuming procedure (figure 1). In this material the artificial chirality [2,3] arises from the curvature of the gold layer covering the nanospheres and their reciprocal orientation [4].

Interplay of optical effects with the surface morphology at the nanometer scale in samples of Au nanowires is exploited in optical near-field microscopy with polarization control in the mode collection.

We demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations.

Metal nanostructures supported on dielectric substrates have attracted great interest as building blocks of nanoscale optical devices such nano-plasmonic devices or planar metamaterials. In this framework artificial circular dichroism [1] is investigated for developing novel devices for active polarization controllers, like rotators and modulators and high efficient molecular sensors. Here we report the experimental observation of nonlinear extrinsic chirality [2] (circular dichroism) of the second harmonic (SH) field generated by self-organized gold nanowire arrays with sub-wavelength periodicity [3]. In this material the chirality arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence [2]. Such circular dichroism in the SH field is the evident signature of the sample morphology and can be used in order to develop more efficient molecular sensors, based on metal enhanced fluorescence or surface enhanced Raman scattering.

Using internal photoemission of electrons from few-monolayer thin MoS2 films into SiO2 we found that the MoS2 layer transfer processing perturbs electroneutrality of the interface, leading to an increase of the electron barrier height by ≈0.5-1 eV as compared to the case of the same films synthesized directly on SiO2. This effect is associated with the formation of an interface dipole, tentatively ascribed to interaction of H2O molecules with the SiO2 surface resulting in the incorporation of silanol (SiOH) groups. This violation of the interface electroneutrality may account for additional electron scattering in ultrathin transferred films and threshold voltage instabilities. Post-transfer annealing in H2S is shown to reduce the transfer-induced interface degradation. ispartof: NANOTECHNOLOGY vol:30 issue:5 ispartof: location:England status: published