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Transparent conductive oxides (TCO) have the unique characteristics of combining optical transparency with high electrical conductivity; such a property makes them uniquely alluring for applications in visible and infrared photonics. One of their most interesting features is the large sensitivity of their optical response to the doping level. We performed the active electrical manipulation of the dielectric properties of aluminum-doped ZnO (AZO), a TCO-based on Earth-abundant elements. We actively tuned the optical and electric performances of AZO films by means of an applied voltage in a parallel-plate capacitor configuration, with SrTiO

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

Doped semiconductor nanocrystal-based thin films are widely used for many applications, such as screens, electrochromic windows, light emitting diodes, and solar cells. Herein, we have employed spectroscopic ellipsometry to measure and model the complex dielectric response of indium tin oxide films fabricated by nanocrystal deposition and sintering. The films could be modelled as Bruggemann effective media, allowing to estimate the nanoscale interstitial porosity of the structure. The effective dielectric constant show the possibility of tuning the plasma frequency and the epsilon-near zero condition of the film.

Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS2 grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS2 flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS2, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate.

We investigated the temperature-dependent optical response of ordered lattices of noninteracting gold-core/poly(N-isopropylacrylamide)-shell nanoparticles (NPs), a system with proven photothermal a...

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

The study of nonthermal electrons, generated upon photoexcitation of plasmonic nanostructures, plays a key role in a variety of contexts, from photocatalysis and energy conversion to photodetection and nonlinear optics. Their ultrafast relaxation and subsequent release of energy to a low energy distribution of thermalized hot electrons has been the subject of a myriad of papers, mostly based on femtosecond transient absorption spectroscopy (FTAS). However, the FTAS signal stems from a complex interplay of different contributions arising from both nonthermal and thermal electrons, making the disentanglement of the two a very challenging task, so far accomplished only in terms of numerical simulations. Here a combined approach is introduced, based on a post-processing of the FTAS measurements guided by a reduced semiclassical model, the so-called extended two-temperature model, which has allowed the purely nonthermal contribution to the pump-probe experimental map recorded for 2D arrays of gold nanoellipsoids to be isolated. This approach displays the intimate correlation between electron energy and probe photon energy on the ultrafast time-scale of electron thermalization. It also sheds new light on the ultrafast transient optical response of gold nanostructures, and will help the development of optimized plasmonic configurations for nonthermal electrons generation and harvesting.

The development of low-dimensional transparent conducting systems is nowadays gaining interest in view of novel optoelectronic applications. In this paper, we investigate the evolution of optical and electronic properties of Ta-doped TiO2 films when their thickness is decreased down to 5 nm and as a function of Ta doping (5-10 at%), and we correlate the observed behavior with the structural properties, showing a high degree of tunability. Ta:TiO2 polycrystalline anatase films are synthetized via pulsed laser deposition, followed by vacuum annealing. For films of thickness 50-200 nm, the electrical resistivity is ∼8 × 10-4-1 × 10-3 Ω cm and the charge carrier density increases with the doping content while the mobility decreases. Below a thickness of 20 nm, the electrical properties partially deteriorate, but still conductive ultra-thin films can be obtained down to 5 nm. The optical response changes with Ta addition, i.e. the absorption band in the UV range blue-shifts, according to the Moss-Burstein effect, while absorption in the IR range increases because of free carriers. Finally, we provide estimates of the effective mass and the plasma energy in the IR range. The fine tunability of the optoelectrical properties of Ta:TiO2 films makes them suitable as transparent conductive components for devices and for photonic or plasmonic applications in the visible and IR ranges.

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

Transparent Conductive Oxides (TCOs) are a class of materials with high optical transparency and electrical conductivity. This combination makes them extremely appealing for solar cells, optoelectronics and infrared-plasmonics applications. In this work, we report the active tuning of the optical response of aluminium-doped ZnO (AZO) films upon electrical gating within a parallel-plate capacitor configuration. We investigated the electrical-bias-dependent optical response of thin AZO films fabricated by pulsed laser deposition by means of spectroscopic ellipsometry (SE). Calculations based on an exponentially-decaying spatial distribution of injected/depleted charge density in the AZO film are in accordance with experimental observations, allowing the extraction of the Debye length.

We report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8 nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting.

Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer growth on Ag(001). The effective correlation energy U$_{\textrm{eff}}$, usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e$_g$ or t$_{2g}$) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e$_g$ orbital and of the exchange interaction for the t$_{2g}$ orbital with respect to the Fe/Ag case that can be associated to the stronger electron confinement and to the exchange coupling between the two layers, respectively., Comment: 37 pages, 6 figures

Silver represents, by and large, the best plasmonic metal available, due to its very low optical losses in a broad photon-energy range encompassing all the visible optical spectrum. Its performances are, more often than not, severely hampered by the presence of a few-nanometer thick surface-tarnish layer; thermal annealing under high-vacuum (HV) conditions may however lead to its decomposition, thereby allowing to attain the clean-metal response. Here, we report an experimental investigation of the temperature dependence of the plasmonic response of Ag nanoparticles, either clean or tarnished, by means of in situ optical spectroscopies under HV conditions. For tarnished nanoparticles, we observed the temperature dynamics of thermal decomposition of the contamination layer in real time and compared it with the corresponding behavior of spatially extended, flat surfaces. For clean Ag nanoparticles we witness instead a remarkable temperature invariance of the localized-plasmon response, indicating Ag as a potential candidate for temperature-invariant thermoplasmonics applications.

The excitation of plasmonic nanoparticles by ultrashort laser pulses sets in motion a complex ultrafast relaxation process involving the gradual re-equilibration of the system's electron gas, lattice, and environment. One of the major hurdles in studying these processes is the lack of direct measurements of the dynamic temperature evolution of the system subcomponents. We measured the dynamic optical response of ensembles of plasmonic Au nanoparticles following ultra-short-pulse excitation, and we compared it with the corresponding static optical response as a function of the increasing temperature of the thermodynamic bath. Evaluating the two sets of data, the optical fingerprints of equilibrium or off-equilibrium responses could be clearly identified, allowing us to extract a dynamic thermometric calibration scale of the relaxation process, yielding the experimental ultrafast temperature evolution of the plasmonic particles as a function of time.

A set of electron-correlation energies as large as 10 eV have been measured for a magnetic 2ML Fefilm deposited on Ag(001). By exploiting the spin selectivity in angle-resolved Auger-photoelectroncoincidence spectroscopy and the Cini-Sawatzky theory, the core-valence-valence Auger spectrumof a spin-polarized system have been resolved: correlation energies have been determined for eachindividual combination of the two holes created in the four sub-bands involved in the decay: majorityand minority spin, as well asegandt2g. The energy difference between final states with paralleland antiparallel spin of the two emitted electrons is ascribed to the spin-flip energy for the final ionstate, thus disentangling the contributions of Coulomb and exchange interactions., Comment: 5 pages, 2 figures, 1 table

Silver is an extremely appealing metal for plasmonics due to its very low optical losses in the visible and near-ultraviolet range and its relatively low reactivity. Within the emerging field of thermoplasmonics, where light-metal interactions are exploited to generate heat on the nanometric scale, knowledge of temperature-dependent complex permittivities of plasmonic materials is indispensable. We extracted the temperature-dependent complex permittivity of silver ${\ensuremath{\varepsilon}}_{Ag}$ by spectroscopic ellipsometry under high-vacuum conditions. For rising $T$, we observed an increase of the free-electron contribution to the imaginary part of the permittivity $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ and a temperature-dependent absorption band splitting off the interband absorption edge in the 320--360-nm range. Around 340 nm the relative increase of $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ at 600 K with respect to its room-temperature value is around 500%. In order to understand the implications of this behavior on silver thermoplasmonics, we computed the temperature-dependent extinction efficiency of oblate Ag ellipsoids with localized plasmon resonance within the 320--360-nm range. We predict that dramatic damping of the plasmon resonance occurs for increasing temperature, possibly leading to intriguing self-limiting effects in Ag thermoplasmonics.

Monolayer tungsten disulfide ($WS_2$) has recently attracted large interest as a promising material for advanced electronic and optoelectronic devices such as photodetectors, modulators, and sensors. Since these devices can be integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the stability of monolayer $WS_2$ in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal $WS_2$ at typical BEOL conditions is investigated; namely (i) heating temperature of $300$ $^\circ C$, (ii) pressures in the medium- ($10^{-3}$ mbar) and high- ($10^{-8}$ mbar) vacuum range; (iii) heating times from $30$ minutes to $20$ hours. Structural, optical and chemical analyses of $WS_2$ are performed via scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). It is found that monolayer single-crystal $WS_2$ is intrinsically stable at these temperature and pressures, even after $20$ hours of thermal treatment. The thermal stability of $WS_2$ is also preserved after exposure to low-current electron beam ($12$ pA) or low-fluence laser ($0.9$ $mJ/\mu m^2$), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and in-line monitoring procedures that allow the integration of $WS_2$ in device fabrication flows without compromising the material quality., Comment: 21 pages, 5 figures

We investigated the effects of ultrafast laser excitation of Au nanoparticles (NPs) having strong interparticle electromagnetic coupling by irradiating the NPs either at interband or plasmon-resonance wavelengths (13-100 J/m(2) fluence regime). We observed that interband excitation is significantly more efficient than plasmonic excitation in reshaping, coalescing, and ultimately sublimating the NPs, despite the light-absorption cross section of interband excitation being almost half that of plasmonic irradiation. We ascribed this to the different localizations of radiation-induced heat sources in the strongly coupled NPs in the two cases. Interband excitation induces homogeneous heat generation in Au, and so the conventional NP heating pathway is followed, eventually leading to overall melting, coalescence, and ablation of Au. Plasmonic irradiation, on the other hand, promotes strong localization of the heat sources within small energetic hot spots, a fact that we suggest may lead to nonthermal effects that melt and reshape the NPs only on the local scale, leaving the system otherwise relatively unscathed.

We report a real time, in-situ spectroscopic ellipsometry study of the temperature-induced solid-state dewetting of Au nanowires into nanoparticles. Very large spectral variations are observed at different temperatures. Analysis of the key features in the acquired spectra reveals two different regimes: up to 300 °C the variation in the optical response is dominated by solid-state dewetting, while above that temperature, smaller variations not compatible with such mechanism are visible. Therefore our ellipsometry measurements allow us to determine in real time at which temperature the solid-state dewetting ceases and the morphology of our sample becomes stable. We point out that this observation is possible thanks to the higher sensitiviy of ellipsometry with respect to reflectance/transmittance measurements.

Electromagnetically-heated metal nanoparticles can be exploited as efficient heat sources at the nanoscale. The assessment of their temperature is, however, often performed indirectly by modelling their temperature-dependent dielectric response. Direct measurements of the optical properties of metallic nanoparticles in equilibrium with a thermodynamic bath provide a calibration of their thermo-optical response, to be exploited for refining current thermoplasmonic models or whenever direct temperature assessments are practically unfeasible. We investigated the plasmonic response of supported Au nanoparticles in a thermodynamic bath from room temperature to 350 °C. A model explicitly including the temperature-dependent dielectric function of the metal and finite-size corrections to the nanoparticles' permittivity correctly reproduced experimental data for temperatures up to 75 °C. The model accuracy gradually faded for higher temperatures. Introducing a temperature-dependent correction that effectively mimics a surface-scattering-like source of damping in the permittivity of the nanoparticles restored good agreement with the data. A finite-size thermodynamic effect such as surface premelting may be invoked to explain this effect.

We report an investigation of the graphene/substrate interface morphology in large-area polycrystalline graphene grown by chemical-vapour deposition and wet-transferred onto Si wafers. We combined spectroscopic ellipsometry, X-ray photoelectron spectroscopy and atomic-force microscopy in order to yield morphological and chemical information about the system. The data showed that wet-transferred samples may randomly exhibit nanosized relief patterns indicative of small water nanopockets trapped between graphene and the underlying substrate. These pockets affect the adhesion of graphene to the substrate, but can be efficiently removed upon a mild annealing in high vacuum. We show that ellipsometry is capable of successfully and reliably detecting, via multilayer dielectric modelling, both the presence of such a spurious intercalation layer and its removal. The fast, broadly applicable and non-invasive character of this technique can therefore promote its application for quickly and reliably assessing the degree of adhesion of graphene transferred onto target substrates, either for ex-post evaluation or in-line process monitoring., 11 pages, 4 figures

We report a combined X-ray photoelectron spectroscopy and atomic force microscopy study of ultrafine (

The evaluation of thickness, refractive index, and optical properties of biomolecular films and self-assembled monolayers (SAMs) has a prominent relevance in the development of label-free detection techniques (quartz microbalance, surface plasmon resonance, electrochemical devices) for sensing and diagnostics. In this framework Spectroscopic Ellipsometry (SE) is an important player. In our approach to SE measurements on ultrathin soft matter, we exploit the small changes of the ellipsometry response (\(\delta \varDelta \) and \(\delta \varPsi \)) following the addition/removal of a layer in a nanolayered structure. So-called \(\delta \varDelta \) and \(\delta \varPsi \) difference spectra allow to recognize features related to the molecular film (thickness, absorptions) and to the film-substrate interface thus extending SE to a sensitive surface UV-VIS spectroscopy. The potential of ellipsometry as a surface spectroscopy tool can be boosted when flanked by other characterizations methods. The chapter deals with the combined application of broad-band Spectroscopic Ellipsometry and nanolithography methods to study organic SAMs and multilayers. Nanolithography is achieved by the accurate removal of molecules from regularly shaped areas obtained through the action of shear forces exerted by the AFM tip in programmed scans. Differential height measurements between adjacent depleted and covered areas provide a direct measurement of film thickness, which can be compared with SE results or feed the SE analysis. In this chapter we will describe the main concepts behind the SE difference spectra method and AFM nanolithograhy. We will describe how SE and AFM can be combined to strengthen the reliability of the determination of thickness and, as a consequence, of the optical properties of films. Examples will be discussed, taken from recent experiments aimed to integrate SE and AFM nanolithography applied to SAMs and nano layers of biological interest. By analysing in detail the changes of the spectroscopic features of compact versus non-compact layers and correlating such changes with the post-lithography AFM analysis of surface morphology SE unravels the specific versus unspecific adsorption of biomolecules on gold surfaces functionalized with suitable SAMs.

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bioorganic molecules (L-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 +/- 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity `` relaxes'' to 6.2 +/- 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.

In situ spectroscopic ellipsometry is employed to investigate the adsorption of Yeast Cytochrome c (YCC) on SiO 2 /Si substrates. In order to highlight the slight variations induced by protein adsorption, difference spectra ( δ Δ and δ Ψ ) have been considered, following the approach introduced in our previous studies on self-assembled monolayers. The difference spectra show a sharp dip at about 410 nm, the Soret or B band, related to the heme optical absorption, whose fine position is sensitive to the protein environment and to the oxidation state of the iron ion within the heme. Remarkably, the analysis of the difference spectra allowed us to detect two lower intensity dips in the 590–650 nm range, the Q bands, whose position and lineshape provide additional information on protein conformation and redox state. Quantitative reproduction of experimental data obtained by using a simple isotropic optical model, accounting for the molecular absorption spectrum, is presented. Estimates of the film thickness and determination of the position and shape of the heme-related features were obtained from calculations. The results are compared with those previously obtained in a study on YCC adsorption on Au substrates. Complementary ex-situ atomic force microscopy measurements are also presented.

Small aluminum nanoparticles have the potential to exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum, however technical and scientific challenges make it difficult to attain this limit. We report the fabrication of arrays of Al/Al 2O3 core/shell nanoparticles with a metallic-core diameter between 12 and 25 nm that display sharp plasmonic resonances at very high energies, up to 5.8 eV (down to ? = 215 nm). The arrays were fabricated by means of a straightforward self-organization approach. The experimental spectra were compared with theoretical calculations that allow the correlation of each feature to the corresponding plasmon modes.

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev, Phys. Rev. Lett. 107, 165003 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.165003], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior.

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS2) on epitaxial graphene (EG) on SiC(0001). The WS2 is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS2/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin–orbit splitting of monolayer WS2 on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS2/EG to be 4.5 ± 0.1 eV. A large shift of the WS2 VB maximum is observed as well, due to the lowering of the WS2 work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS2 crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin–orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

The proper exploitation of the plasmon resonance typical of metallic nanoparticles can allow for the confinement of the electromagnetic field in nanometric volumes, thus creating the so called "hot spots". These nanometric volumes are characterized by high field, remarkably useful characteristic for a huge variety of applications in photonics and optics. The most commonly employed plasmonic metals, Au and Ag, yield resonances only reaching up to the near-UV electromagnetic range, in fact stretching upwards the energy of plasmon resonances requires the use of different materials. Deep-ultraviolet plasmon resonances were indeed predicted exploiting one of the cheapest and most abundant materials available on earth. Aluminium holds the promise of a broadly-tuneable plasmonic response, theoretically extending far into the deep-ultraviolet (DUV). Complex fabrication issues, including the strong Al reactivity, have however stood in the way of achieving this ultimate DUV response. We report the successful realization of 2-dimensional arrays of ultrafine aluminium nanoparticles that exhibit a remarkable plasmonic response up to the DUV electromagnetic range. Careful nanofabrication allowed to maintain the mean NP size below 20 nm, preserving a purely-metallic core. These systems exhibit a striking high-energy plasmon resonance up to 6.8 eV photon energy, and preserve their DUV plasmon response when exposed to atmosphere [1,2]. These observations pave the way to the full exploitation of aluminium plasmonic tunability, hence extending the numerous applications of plasmonics to the high-energy side of the spectral range.

Spectroscopic ellipsometry (SE) has been used to investigate the adsorption of yeast cytochrome c (YCC) on gold films. YCC is a prototypical molecular system to evaluate the sensitivity and reliability of optical methods in probing conformational changes upon adsorption at surfaces. Measurements have been performed in situ in aqueous solutions at room temperature. We have applied to monolayer-thick YCC films the same scheme of analysis of SE spectra used in our recent works on thiolate self-assembled monolayers on gold, exploiting the so-called δΨ and δΔ difference spectra. A comparison between δΨ and δΔ spectra obtained from YCC adsorption on flat and cluster-assembled nanostructured gold films, obtained by supersonic cluster beam deposition is proposed. For both kinds of substrate δΨ and δΔ, difference spectra show well-defined features related to molecular optical absorptions typical of the so-called heme group, in particular the Soret band at about 410 nm. These features appear in the same position found for molecules in solution, suggesting that YCC native conformation is maintained upon adsorption also in the case of a substrate with typical roughness at the nanoscale.

Spectroscopic ellipsometry (SE) has been applied to study the optical properties of thiolate polydiacetylene (PDA) self-assembled monolayers (SAMs) deposited on low-roughness polycrystalline gold. A systematic investigation of methyl-terminated diacetylene (dihexacosa-7,9-diyn disulfide, DS9) SAMs is presented. The results have been compared with experiments on carbazolyl-derivatized diacetylene (14-(9H-9-carbazolyl)tetradeca-10,12-diyn-1-yl disulfide, CDS9) SAMs and with findings recently obtained on SAMs of alkanethiols. The SE measurements have been complemented with X-ray photoelectron spectroscopy and atomic force microscopy data. The difference between SE spectra measured after and before the monolayer assembly (δΨ = ΨSAM − ΨAu and δΔ = ΔSAM − ΔAu) showed specific absorptions of the adsorbed molecules, including narrow features in the 500−700 nm wavelength range which have been interpreted as markers of the SAM polymerization state. To our knowledge, these results represent the first unambiguous opt...

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.

Plasmonic color-graded systems are devices featuring a spatially variable plasmonic response over their surface. They are widely used as nanoscale color filters; their typical size is small enough to allow integration with miniaturized electronic circuits, paving the way to realize novel nanophotonic devices. Currently, most plasmonic color-graded systems are intrinsically discrete because their chromatic response exploits the tailored plasmon resonance of microarchitectures characterized by different size or geometry for each target color. Here, we report the realization of multifunctional plasmon-graded devices where continuously graded chromatic response is achieved by smoothly tuning the composition of the resonator material while simultaneously maintaining an achromatic nanoscale geometry. The result is a new class of versatile materials: we show their application as plasmonic filters with a potential pixel size smaller than half of the exciting wavelength but also as multiplexed surface-enhanced Raman spectroscopy (SERS) substrates. Many more implementations, such as photovoltaic efficiency boosters or color routers, await and will benefit from the low fabrication cost and intrinsic plasmonic flexibility of the presented systems.

We report first results about the interaction of a He ∗ beam with nanometer thick layers of l -cysteine, a fundamental amino-acid, deposited under ultra high vacuum conditions on Cu and Au surfaces. Electronic properties of adsorbed molecules have been investigated by metastable deexcitation spectroscopy (MDS). Deexcitation of He ∗ on molecular layers gives rise to well defined Penning ionization spectra. In these spectra, we distinguish contributions of the molecules strongly bound to the surface through the thiol group and of molecules weakly bound to the surface, as for example those belonging to multilayers. The beam (whose intensity on the sample is of the order of 10 11 at/s with a spot diameter of 1 mm) induces slow rate desorption of weakly bound molecules. Mild annealing to 330 K induces the desorption of weakly bound molecules as well. Cleavage of intra-molecular bonds is observed for annealing temperatures above 420 K. Beam-induced damage of first layer molecules appears not significant on the time scale (tens of seconds) of the MDS experiment. The data demonstrate the full applicability of MDS to the investigation of self-assembled monolayers of small biomolecules.

The effects of a chemical alteration of the antiferromagnet surface in the exchange-biased ultrathin epitaxial antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001) have been investigated. The formation of a nanometer-thick paramagnetic oxide at the surface of the Mn layer was induced through the controlled exposure to oxygen. Although the buried Mn/Co interface was not affected by the oxidation, strong changes of the exchange-bias properties of the system, which cannot be accounted for by the mere reduction of the antiferromagnetic Mn thickness, were observed. We propose that the produced surface modification affects the magnetic configuration of the near-surface region and the domain structure in the whole antiferromagnet layer which, in turn, influences the pinned uncompensated spin density responsible for the exchange bias.

Structural and electronic properties of ultrathin Mg films on Ag(1 0 0) have been investigated by means of metastable de-excitation spectroscopy (MDS), work function change (Δφ), electron diffraction and helium reflectivity (RHe) measurements. At room temperature Mg grows on Ag(1 0 0) following the Stranski–Krastanov growth mode. Successive annealing of a 10 ML thick Mg film leads to its almost complete desorption at about 390 K and to the formation of a Mg–Ag alloy. RHe and electron diffraction measurements indicate that for annealing temperatures between 480 and 570 K, the alloy orders with a c(2×2) periodicity. Further annealing at higher temperatures restores the initial Ag(1 0 0) surface.

In this work we present a simple pathway to obtain large single-crystal graphene on copper (Cu) foils with high growth rates using a commercially available cold-wall chemical vapour deposition (CVD) reactor. We show that graphene nucleation density is drastically reduced and crystal growth is accelerated when: i) using ex-situ oxidised foils; ii) performing annealing in an inert atmosphere prior to growth; iii) enclosing the foils to lower the precursor impingement flux during growth. Growth rates as high as 14.7 and 17.5 micrometers per minute are obtained on flat and folded foils, respectively. Thus, single-crystal grains with lateral size of about one millimetre can be obtained in just one hour. The samples are characterised by optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy as well as selected area electron diffraction (SAED) and low-energy electron diffraction (LEED), which confirm the high quality and homogeneity of the films. The development of a process for the quick production of large grain graphene in a commonly used commercial CVD reactor is a significant step towards an increased accessibility to millimetre-sized graphene crystals., Comment: Article: 7 pages, 6 figures. Supplementary Information: 5 pages, 7 figures

The surface magnetic properties of ultrathin Cr films grown on O(1×1)-Fe(0 0 1) have been studied by means of spin-polarised metastable de-excitation spectroscopy (SPMDS). The spin-selected surface density of states extracted from the experimental SPMDS data provides information on both composition and magnetic properties of the topmost layer. We find that oxygen floats on the surface of the growing Cr film and that Cr/Fe intermixing is restricted to the first Cr layer. Concerning the magnetic properties, a strong polarisation of Cr3d electrons is found in agreement with theoretical calculations. Oxygen 2p states are polarised too and their polarisation is opposite to that of Cr3d states. Surface magnetisation as a function of the Cr film thickness shows an oscillatory behaviour induced by the layered antiferromagnetic structure of Cr. At variance with the results of other experimental investigation in this case the layered antiferromagnetic structure of Cr is already present in the 3 ML thick film.

Codeposition of oxygen and Fe on Cu 3 Au(0 0 1) has been investigated in order to look at the modifications of the growth regime induced by the chemical additive. Oxygen deeply modifies the growth modes both at relatively low temperature (150 K) and at room temperature (RT). The bilayer growth mode active at earlier stages of growth in absence of the surfactant element is suppressed. We concentrated our investigation on RT deposition. A relatively ordered, pseudomorphic iron film has been found only at low thickness (up to 4 ML). This film is covered with an oxygen layer, whose coverage and order depends on the post-growth annealing treatment. The annealed oxygen layer has been used as a surfactant for further Fe deposition cycles. During deposition, part of the oxygen remains trapped into the film. Restoration of the chemisorbed layer is obtained after annealing at moderate temperature (700 K). As the film thickness increases as a result of successive depositions, the film is first characterized by coexistence of pseudomorphic and bcc patches. The pseudomorphic patches seem stabilized not only by presence of the oxygen, but also by Cu and Au impurities in the film. For relatively large thickness (above 25 ML) bcc domains become majority.