The unique transport and optical properties of graphene find interesting applications in optoelectronics such as transparent conductive electrode substituting ITO and as active electrode in Schottky junctions with conventional semiconductors [1]. Unlike conventional metal electrodes, graphene allows the modulation of the Schottky barrier since its work function can be tailored by electrostatic gating or chemical doping. Several doping methodologies have been studied and, currently, it is more challenging to achieve stable n-doped graphene than the easily obtained p-doped one (with the exception of epitaxial graphene that is intrinsically n-doped)[2].This contribution focuses on the n-doping of CVD-graphene by ammonia treatments. In contrast to substitutional doping, the proposed "post-growth" doping methodology exploits charge transfer from adsorbed ammonia that allows the Fermi level modulation without important effect on carriers mobility. The degree of charge transfer between graphene and adsorbed ammonia as well as the strength of this interaction (chemisorption or physisorption) are still under debate as demonstrated by the many theoretical and experimental studies reported in literature. Thus, the main aim of this work is to evaluate the feasibility of ammonia treatment for achieving effective and stable n-doping.Ammonia adsorption on graphene was studied by real time monitoring of graphene sheet resistance (Rxx) upon exposure to high NH3 pressures. To point up how high electron-doping affects graphene electronic properties, complementary measurements of Hall resistance (Rxy) in magnetic field (B) were also carried out. This provided a direct evaluation of the influence of chemical doping on the charge carriers density (n=1/Rxxe?) and mobility (?=Rxy/RxxB). We demonstrate that ammonia adsorption is sensitive to functional groups and defects on the CVD-graphene surface such as epoxyl, idroxyl or carbonyl groups at defects and grain edges [3]. Specifically, a direct correlation between the degree of n-doping and the oxidation degree of different graphene samples has been found.In order to evaluate the stability of the achieved chemical n-doping and to provide a better understanding of graphene/ammonia interaction, the effect of temperature on the ammonia adsorption and desorption processes was also investigated. Experimental data attest for different adsorption configurations of ammonia on graphene characterized by different interaction strengths [4]. In particular, the role of temperature in promoting reactive interaction of ammonia with defects and functionalities on CVD graphene is demonstrated.

Attaching functional molecules such as thiols and proteins to semiconductor surfaces is increasingly exploited in functional devices such as sensors. Despite extensive research to understand this interface and demonstrate a robust protocol for attachment, the bonding chemistry of thiolates to III-V surfaces has been under great debate in the literature. This study provides a comprehensive chemical model for the attachment of thiols to InAs, an increasingly device-relevant III-V semiconductor, using cysteamine as a model molecule. We examine the attachment of cysteamine to InAs via the thiol group using X-ray photoelectron spectroscopy and spectroscopic ellipsometry and confirm that thiolate bonding to the substrate occurs preferentially to As sites over In sites as a limit These experiments explore the interplay of the native oxide chemical properties, the cysteamine concentration, and the evolving InAs surface chemistry with functionalization. The thiol-InAs interaction can be framed as a general acid-base reaction, where the nucleophilic and/or electrophilic attack of the surface (i.e., binding to In sites and/or As sites) depends on the acidity of the thiol. The roles of the initial oxide composition, the solvent of the functionalizing solution, and the cysteamine as a limiting reagent in fully displacing the oxide and creating In-S and As-S bonds are highlighted.

Metal nanoparticle (NP)-graphene multifunctional platforms are of great interest for exploring strong light graphene interactions enhanced by plasmons and for improving performance of numerous applications, such as sensing and catalysis. These platforms can also be used to carry out fundamental studies on charge transfer, and the findings can lead to new strategies for doping graphene. There have been a large number of studies on noble metal Au-graphene and Ag-graphene platforms that have shown their potential for a number of applications. These studies have also highlighted some drawbacks that must be overcome to realize high performance. Here we demonstrate the promise of plasmonic gallium (Ga) nanoparticle (NP) graphene hybrids as a means of modulating the graphene Fermi level, creating tunable localized surface plasmon resonances and, consequently, creating high-performance surface-enhanced Raman scattering (SERS) platforms. Four prominent peculiarities of Ga, differentiating it from the commonly used noble (gold and silver) metals are (1) the ability to create tunable (from the UV to the visible) plasmonic platforms, (2) its chemical stability leading to long-lifetime plasmonic platforms, (3) its ability to n-type dope graphene, and (4) its weak chemical interaction with graphene, which preserves the integrity of the graphene lattice. As a result of these factors, a Ga NP-enhanced graphene Raman intensity effect has been observed. To further elucidate the roles of the electromagnetic enhancement (or plasmonic) mechanism in relation to electron transfer, we compare graphene-on-Ga NP and Ga NP-on-graphene SERS platforms using the cationic dye rhodamine B, a drug model biomolecule, as the analyte.

Stable gold nanoparticles with surface plasmonresonance tunable from visible (Vis) to near-infrared (NIR)are deposited via a direct sputtering methodology on largearea polyethylene terephthalate (PET) to be used as effective,flexible NIR surface-enhanced Raman scattering (SERS) substrates.AnO2 plasma treatment of PET is used to tailor growthdynamics, geometry, and plasmonic properties of nanoparticles.The O2 plasma treatment of PET results also in effectiveimprovement of nanoparticle anchoring on the plastic substrate,providing more stable, flexible SERS systems. Thefunctionality of fabricated SERS substrates has been testedusing benzylthiol, and SERS enhancement factors in the range104 have been achieved, which are comparable with thosereported in literature for gold nanostructures fabricated onsilicon substrate. These results attest the great potentiality ofthis methodology for the production of cost-effective flexibleand reusable large-scale SERS substrates.

The direct chemical vapor deposition of WS2 by W(CO)(6) and elemental sulfur as precursors onto epitaxial-graphene on SiC and CVD-graphene transferred on SiO2/Si substrate is presented. This methodology allows the epitaxial growth of continuous WS2 films with a homogeneous and narrow photoluminescence peak without inducing stress or structural defects in the graphene substrates. The control of the WS2 growth dynamics for providing the localized sulfide deposition by tuning the surface energy of the graphene substrates is also demonstrated. This growth methodology opens the way towards the direct bottom up fabrication of devices based on TMDCs/graphene van der Waals heterostructures.

For device integration purposes plasmonic metal nanoparticles must be supported/deposited on substrates. Therefore, it is important to understand the interaction between surfactant-free plasmonic metal nanoparticles and different substrates, as well as to identify factors that drive nanoparticles nucleation and formation. Here we show that for nanoparticles grown directly on supports, the substrate/nanoparticle interfacial energy affects the equilibrium shape of nanoparticles. Therefore, oblate, spherical and prolate Au nanoparticles (NPs) with different shapes have been deposited by radiofrequency sputtering on substrates with different characteristics, namely a dielectric oxide Al2O3 (0001), a narrow bandgap semiconductor Si (100), and a polar piezoelectric wide bandgap semiconductor 4H-SiC (0001). We demonstrate that the higher the substrate surface energy, the higher the interaction with the substrate, resulting in flat prolate Au nanoparticles. The resulting localized surface plasmon resonance characteristics of Au NPs/Al2O3, Au NPs/Si and Au NPs/SiC have been determined by spectroscopic ellipsometry and correlated with their structure and shape studied by transmission electron microscopy. Finally, we have demonstrated the diverse response of the tailored plasmonic substrates as ultrasensitive SERS chemical sensors. Flat oblates Au NPs on SiC result in an enhanced and more stable SERS response. The experimental findings are validated by numerical simulations of electromagnetic fields.

Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a great challenge to control the thickness of graphene especially in real-time during the growth. So far, there are no reports on the real-time monitoring Of graphene growth. Here, we show for the first time real-time in situ kinetic monitoring of graphene deposition by CVD on nickel. We demonstrate an optical nondestructive method of,dynamic spectroscopic ellipsometry for controlling and optimizing the,catalyst cleaning and annealing and, Consequently, the graphene deposition and properties the kinetic ellipsometry monitoring also highlights the mechanism of graphene formation. Discussion shows the applicability and industrial scalability of this ellipsometric method to the control of large-area graphene formation on any substrate. This approach opens a Way thin-line real-time graphene metrology and is helpful in guiding the graphene growth process as we try to achieve reproducible and controllable research as well as industry processes for quality graphene formation.

A large variety of applications ranging from plasmonic sensingto plasmonic enhanced solar cells, photonics, and optics can benefit from areliable method to enhance chemical and time stability of silver-basedplasmonic nanostructures and metamaterials. Therefore, here we demonstrateand discuss the effectiveness of a low-temperature (100 °C) hydrogen atomprocessing of silver to inhibit its oxidation and stabilize surface plasmonresonances in silver nanostructure suitable for plasmonics, metamaterials,sensing, and photovoltaics. Interestingly, no dielectric overlayer encapsulatingAg is used to protect the silver nanostructure, differently from the commonapproach, because these overlayers typically lead to a red shift of the opticalresonances due to their refractive index. Conversely, we demonstrate that thesilver deoxidation by the hydrogen treatment results in a slight blue shift ofresonances, which is useful for preserving resonances in the visible range. Thechemical mechanism rationalizing the validity of this processing is discussed. The optical properties of the fabricated sampleswere measured by means of transmission, reflection, and ellipsometry spectroscopies. Theoretical support to the interpretation ofthe optical properties demonstrates the advantages of this advanced processing. Therefore, this work is an important step towardnovel and breakthrough applications of stable silver-based nanostructures for plasmonics and metamaterials exploiting visiblelight.

Graphene is emerging as a promising material for plasmonics applications due to itsstrong light-matter interactions, most of which are theoretically predicted but not yetexperimentally realized. Therefore, the integration of plasmonic nanoparticles to createmetal nanoparticle/graphene composites enables numerous phenomena important for arange of applications from photonics to catalysis. For these applications it is importantto articulate the coupling of photon-based excitations such as the interaction betweenplasmons in each of the material components, as well as their charge-based interactionsdependent upon the energy alignment at the metal/graphene interface. These coupledphenomena underpin an active application area in graphene-based composites due tonanoparticle-dependent surface-enhanced Raman scattering (SERS) of graphene phononmodes. This study reveals the coupling of a graphene/SiC support with Ga-nanoparticlelocalizedsurface plasmon resonance, which is of particular interest due to its ability to betuned across the UV into the near-IR region. This work is the fi rst demonstration of theevolving plasmon resonance on graphene during the synthesis of surface-supported metalnanoparticles, thus providing evidence for the theoretically predicted screening revealed bya damped resonance with little energy shift. Therefore, the role of the graphene/substrateheterojunction in tailoring the plasmon resonance for nanoplasmonic applicationsis shown. Additionally, the coupled phenomena between the graphene-Ga plasmonproperties, charge transfer, and SERS of graphene vibrational modes are explored.

Despite the large number of papers on the NH3 doping of graphene, the achievement of stable n-doped large area CVD (chemical vapor deposition) graphene, which is intrinsically p-doped, is still challenging. A control of the NH3 chemisorption and of the N-bond configuration is still needed. Here it is shown the feasibility of a room temperature NH3 high pressure treatment of CVD graphene to achieve n-type doping. We use and correlate data of (a) sheet resistance, Rsh, and Hall coefficient, RH, in Van der Pauw configuration, acquired in real time during the NH3 doping of CVD-graphene on a glass substrate, (b) optical measurements of the effect of doping on the graphene Van Hove singularity point at 4.6 eV in the dielectric function spectra by spectroscopic ellipsometry, and of (c) N-bond configuration by XPS to better understand and, finally, control the NH3 doping of graphene. The discussion is focused on the thermal and time stability of the n-doping after air exposure. A chemical rationale is provided for the NH3 n-doping based on the interaction of (i) NH3 with intrinsic oxygen functionalities and defects of CVD graphene and of (ii) C-NH2 doping centers with acceptor species present in the air.

ConceptThe fabrication of substrates for surface enhanced Raman spectroscopy (SERS) responding to specific analytical (sensitivity, selectivity and reproducibility) and/or technical needs (cost, large area, stability) represents a highly active field of research. Supported gold nanostructure SERS substrates are interesting because they can accomplish plasmon resonance both in the near infrared (NIR) and visible range, matching standard laser sources for Raman analysis. In this contribute, we explore two different strategies to fabricate SERS substrates based on gold nanostructures: (i) two dimensional periodic arrays of gold nano-patches fabricated by means of nano-lithographic technique; and (ii) ensembles of gold nanoparticles grown by mask-less sputtering methodology.Motivations and ObjectivesThe exploitation of SERS as a routine analytical technique depends on development of plasmonic substrates which have to provide significant enhancement factor, stability upon interaction with analytes and a low fabrication cost. Thus, this work is aimed to (i) the engineering of plasmonic structures in terms of size, shape, and surface distribution (random or periodic) maximizing the interaction with both analytes and excitation laser source, (ii) to the optimization of the explored fabrication routes for the production of stable array of gold nanostructures on different substrates (silicon, glass, PET, PI, and supported graphene).Results and DiscussionSERS substrates based on periodic array of gold nano-patches (fig. a) and ensemble of gold nanoparticles (fig. b) supported on silicon have been fabricated. The effectiveness of these SERS substrates has been investigated and tested by a probe molecule (the benzyl thiol) in order to point up and to compare specific advantages and limits.

ConceptAmong the several synthesis methodologies for graphene, chemical vapor deposition (CVD) on copper foils allows the production of graphene on large area. A strong correlation has been reported between grain sizes and sheet resistance in CVD-graphene1. Thus, improving the quality of CVD-graphene requires the optimization of the synthesis process to provide an increase in average grain size1,2. Therefore, diagnostic methodologies for the accurate monitoring of grain sizes and distribution directly on the copper substrate is needed. In this work we exploit a wet chemistry approach for the selective oxidation of the copper substrate through graphene grains rendering their boundaries visible by optical microscopy. We use the Fenton reaction as a source of hydroxyl radicals to functionalize graphene grain boundaries.Motivations and ObjectivesThe aim of this work is the development of an effective methodology for analyzing grain size and distribution in graphene as grown on copper by optical microscopy. In contrast to transmission electron microscopy3,4 and scanning tunneling microscopy3,5, this optical technique is not expensive and time-consuming and, above all, effective for the diagnostic on large scale. We also demonstrate the feasibility of this methodology for highlighting defect sites in graphene also depending post-growth processing such as transferring on other substrates.Results and DiscussionWe have optimized this diagnostic approach by using Raman spectroscopy to probe and confirm chemical and structural changes in graphene and copper substrate upon the wet treatment. Optical images of graphene on copper (a) and graphene on copper after Fenton reaction (b) show that graphene grain boundaries became clearly visible after wet oxidation treatment.

This review is an overview of our previous work on the synthesis and propertiesof poly(p-phenylenevinylene)s (PPVs) selectively fluorinated in different positions of theconjugated backbone. Both the synthetic challenges and the effects of functionalization withfluorine atoms on the optical behavior are discussed, highlighting the peculiarities and theinterest of this class of conjugated polymers. A general polymerization protocol for PPVs,that is based on the Pd-catalyzed Stille cross-coupling reaction of bis-stannylated vinylenemonomers with aromatic bis-halides, has been successfully extended to the synthesis ofselectively fluorinated poly(p-phenylenevinylene)s. The properties of a series of these PPVsdiffering in the number and positions of the fluorine atoms on the conjugated backbone havebeen studied, even in comparison with the non-fluorinated counterparts. The intriguingoptical features of the resulting materials are discussed considering not only the role of theelectronic and steric effects induced by the fluorine substituents, but also the impact of thefluorination on the solid state organization and intermolecular interactions.

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The solar cells with the fluorinated polymer show better photovoltaic performances than those recorded with the non-fluorinated material. Comparative spectroscopic and computational studies, together with morphological, electrical and optical characterization of thin films, have been carried out to shed light on the reasons for the improvement of performances as induced by the double bond fluorination. Our study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The solar cells with the fluorinated polymer show better photovoltaic performances than those recorded with the non-fluorinated material. Comparative spectroscopic and computational studies, together with morphological, electrical and optical characterization of thin films, have been carried out to shed light on the reasons for the improvement of performances as induced by the double bond fluorination. Our study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.

Metal nanoparticle (NP) -- graphene multifunctional platforms are of great interest for numerous applications, such as sensing and catalysis, and for fundamental studies on charge transfer and light-matter interactions. To understand platform-photon interactions, it is important to articulate the coupling of photon-based excitations, such as the interaction between plasmons in each of the material components, as well as their charge-based interactions dependent upon the energy alignment at the metal/graphene interface. Herein, we use liquid metal Ga nanoparticles, which can be deposited at 300K on graphene, to explore the surface-enhanced Raman spectroscopy modulation induced by the NPs,. The localized charge transfer between Ga NPs and graphene are investigated, and enhancement of the graphene Raman modes is correlated with metal coverage the transfer of electrons from Ga to graphene creating local regions of enhanced electron concentration which modify the electron-phonon interaction in graphene.

The growth and properties of alloys in the alternative quaternary alloy system GaAs1yzPyBizwere explored. This materials system allows simultaneous and independent tuning of latticeconstant and band gap energy, Eg, over a wide range for potential near- and mid-infrared optoelectronicapplications by adjusting y and z in GaAs1yzPyBiz. Highly tensile-strained, pseudomorphicfilms of GaAs1yPy with a lattice mismatch strain of 1.2% served as the host for the subsequentaddition of Bi. Lattice-matched alloy materials to GaAs were generated by holding y3.3z inGaAs1yzPyBiz. Epitaxial films with both high Bi content, z0.0854, and a smooth morphologywere realized with measured band gap energies as low as 1.11-1.01 eV, lattice-matched to GaAssubstrates. Density functional theory calculations are used to provide a predictive model for theband gap of GaAs1yzPyBiz lattice-matched to GaAs.

The role of graphene in enabling deoxidation of silver nanostructures, therebycontributing to enhance plasmonic properties and to improve the temporalstability of graphene/silver hybrids for both general plasmonic and metamaterialsapplications, as well as for surface enhanced Raman scattering(SERS) substrates, is demonstrated. The chemical mechanism occurring atthe graphene-silver oxide interface is based on the reduction of silver oxidetriggered by graphene that acts as a shuttle of electrons and as a kind of catalystin the deoxidation. A mechanism is formulated, combining elements ofelectron transfer, role of defects in graphene, and electrochemical potentialsof graphene, silver, and oxygen. Therefore, the formulated model representsa step forward from the simple view of graphene as barrier to oxygen diffusionproposed so far in literature. Single layer graphene grown by chemicalvapor deposition is transferred onto silver thin fi lms, a periodic silver fi shnetstructure fabricated by nanoimprint lithography, and onto silver nanoparticleensembles supporting a localized surface plasmon resonance in the visiblerange. Through the study of these nanostructured graphene/Ag hybrids, theeffectiveness of graphene in preventing and reducing oxidation of silver plasmonicstructures, keeping silver in a metallic state over months at air exposure,is demonstrated. The enhanced and stable plasmonic properties of thesilver-fi shnet/graphene hybrids are evaluated through their SERS responsefor detecting benzyl mercaptane.

The improvement of technologies towards a more sustainable development and use of energy resources and devices has received an increasing interest worldwide in recent decades. There is consensus that this improvement needs corroborating materials synthesis and device fabrication processes with in-situ real-time characterization. Elucidating the synthesis-processing-material-functionality interplay is also a key issue in fundamental science, necessary to advance basic knowledge of materials and to manipulate their properties at the nanoscale. Therefore, this contribution overviews how spectroscopic ellipsometry, which is a non-destructive, nonintrusive, noninvasive, and contactless optical technique, has evolved into an efficient characterization tool for graphene-metal based hybrids for plasmonics, optical sensors, and SERS devices. The scope of this contribution is to highlight experimental achievements obtained using real-time spectroscopic ellipsometry on the CVD growth on nickel and copper substrates of graphene, and its interactions with metals. Specifically, it will be shown improvements in the properties of graphene achieved by optimizing CVD growth and establishing correlations between process kinetics and graphene thickness. It will also be provided for the first time the direct evidence of the tunability of the plasmon resonance of graphene coupled to plasmonic metal nanoparticles (NPs). New opportunities for the design of plasmonic graphene/metal (Ga, Au, Ag) systems based on charge transfer are presented exploiting the real time tailoring of the plasmon resonance by spectroscopic ellipsometry. Charge transfer is crucial for characterizing metal/graphene interfaces and understanding plasmon-electron coupling across the materials components. The role of graphene in the charge transfer, between the metal NPs and the substrate, on the plasmon resonance energy and amplitude is discussed and elucidated.

Understanding the chemical vapor deposition (CVD) kinetics of graphene growth is important for advancing graphene processing and achieving better control of graphene thickness and properties. In the perspective of improving large area graphene quality, we have investigated in real-time the CVD kinetics using CH(4)-H(2) precursors on both polycrystalline copper and nickel. We highlighted the role of hydrogen in differentiating the growth kinetics and thickness of graphene on copper and nickel. Specifically, the growth kinetics and mechanism is framed in the competitive dissociative chemisorption of H(2) and dehydrogenating chemisorption of CH(4), and in the competition of the in-diffusion of carbon and hydrogen, being hydrogen in-diffusion faster in copper than nickel, while carbon diffusion is faster in nickel than copper. It is shown that hydrogen acts as an inhibitor for the CH(4) dehydrogenation on copper, contributing to suppress deposition onto the copper substrate, and degrades quality of graphene. Additionally, the evidence of the role of hydrogen in forming C-H out of plane defects in CVD graphene on Cu is also provided. Conversely, resurfacing recombination of hydrogen aids CH(4) decomposition in the case of Ni. Understanding better and providing other elements to the kinetics of graphene growth is helpful to define the optimal CH(4)/H(2) ratio, which ultimately can contribute to improve graphene layer thickness uniformity even on polycrystalline substrates.

We investigate the chemistry and kinetics of the surface functionalization of InAs with hemin aimed at demonstrating novel hemin-functionalized InAs planar resistors or molecularly controlled resistors (MOCSERs) with subparts per billion sensitivity to NO at 300 K. The high performance is a result of the strong coupling of the intrinsic surface-confined two-dimensional electron gas (2DEG) at the InAs surface with the surface-attached hemin molecules and their selective interactions with NO. The presence of the surface 2DEG in InAs is highly advantageous for sensing and obviates the need for high surface area to volume geometries conferred by nanowire structures. The chemistry and kinetics of surface attachment processes are investigated using X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry, showing that the covalent attachment of hemin to both In and As sites modifies the band-bending of the InAs. In addition, the selective, reversible electronic interaction between hemin and NO molecules increases the InAs resistance, reducing the NO during the interaction. In addition to presenting the sensor performance characteristics, a chemical model based on the electron transfer from the 2DEG of InAs to the iron center of hemin, which reduces Fe(III) to the higher NO affinity Fe(II) state, is proposed.

We explore the effects of substrate, grain size, oxidation and cleaning on the optical properties of chemical vapor deposited polycrystalline monolayer graphene exploiting spectroscopic ellipsometry in the NIR-Vis-UV range. Both Drude-Lorentz oscillators' and point-by-point fit approaches are used to analyze the ellipsometric spectra. For monolayer graphene, since anisotropy cannot be resolved, an isotropic model is used. A prominent absorption peak at approximately 4.8 eV, which is a mixture of pi-pi* interband transitions at the M-point of the Brillouin zone and of the pi-plasmonic excitation, is observed. We discuss the sensitivity of this peak to the structural and cleaning quality of graphene. The comparison with previous published dielectric function spectra of graphene is discussed giving a rationale for the observed differences. (C) 2014 Elsevier B.V. All rights reserved.

Nowadays considerable efforts are devoted to the synthesis of low bandgap conjugated polymers forapplication in organic polymer solar cells. A large variety of low bandgap polymers are prepared byalternating copolymerization of electron-donating donor and electron-withdrawing acceptor units. Theinteraction between these two units can reduce the polymer bandgap, increasing the sunlight absorption.Benzothiadiazole is commonly used as acceptor block unit in low bandgap polymers. In this contributionwe investigate the supramolecular organization and optical properties of thin films of conjugated polymersconsisting of benzothiadiazole and thiophene with electron-withdrawing difluorovinylene, andelectron-donating vinylene substituents. Atomic force microscopy and spectroscopic ellipsometry areexploited for the analysis of the morphology and optical transitions, respectively. It is found that F-atomsin the vinylene unit yield a blue-shift of the absorption peaks of 0.2 eV respect to the hydrogenated polymerand an increase in the absorption coefficient of fluorinated polymers, which indicates their potentialapplication as photovoltaic material. The morphology evolution of the conjugated polymers blended witha fullerene derivate ([6,6]-phenyl C61-butyric acid methyl ester, PCBM) is also investigated by atomic forcemicroscopy.

The electronic properties of graphene sheets have recently attracted much experimental and theoretical interest. A single graphene layer is a semimetal or zero-gap semiconductor, and has excellent electronic properties, such as high mobility (200 000 cm2 V-1 s-1), room-temperature quantum Hall effect, and high mechanical elasticity (elastic modulus of about 1 TPa). In addition, high flexibility, optical transmittance, and chemical stability are other technological advantages of single graphene layers. These superb characteristics open new potential applications of graphene in flexible and transparent electronic devices. Among other properties, the work function is an important factor governing the application of graphene as an electrode, for instance, in solar cells and light emitting diodes. The work function determines the band alignment in the contact to facilitate selective electron and hole transport.The work function of graphene also depends on the interaction graphene-substrate and Kelvin-Probe Electrostatic Force Microscopy (KP-EFM) is a suitable approach to measure the graphene work function on various supports. Kelvin-Probe electrostatic force microscopy (EFM) has been widely used to probe graphene layers with different thickness on different substrates, SiO2/Si, SiC, and KBr.In this contribution, we apply the KP-EFM characterization to determine the work function of graphene samples on:-different substrate (SiC, glass, a-Si and SiO2);-with different doping induced by metals ( Al, Au, In, Ga), and by molecular functionalization with self-assembled monolayers (thiols, thienylene-phenylene polymers).

P-doped microcrystalline silicon films (?c-Si:H(p)) have been deposited by plasma-enhanced chemical vapour deposition(PECVD) using low RF power, from a silane and trimethylboron (SiH4/B(CH3)3) mixture diluted in helium (He) and hydrogen (H2). Thematerial has been optically, electrically and structurally characterised. This work focuses on the effect of depletion regime on the propertiesof ?c-Si:H(p). Four deposition parameters have been explored, all of them determining the amount of energy supplied per gas molecule,specifically the pressure (P), the total mass-flow (?T), the He- and H2-dilution – D(He) and D(H2) – and the applied radio-frequency power(RFP). The material developed here is intended to be used in the future as a BSF (back surface field) in silicon-heterojunction (SHJ) solarcells with a metal/TCO/a-Si:H(n)/c-Si(p)/?c-Si:H(p+)/metal configuration.

A new class of photovoltaic devices including graphene and plasmonics is emerging, leading to a new trend in the development of both inorganic and organic solar cells.Graphene is mainly used as semitransparent conductive electrode and as antireflection layer. It is characterized by many magical properties, but one of the most important peculiarity of graphene is that it is an anphoteric material. Its work function can be changed by doping through charge transfer. This is a very important property to increase device efficiencies because graphene can be used as a well-matched (macd) electrode for different materials sets.Localized surface plasmon resonance tuning in metal nanoparticles is used for light trapping and enhancement of light absorption.In this context, our research aims at improving photovoltaic performance of organic polymer-based and inorganic silicon-based solar cells by: -the integration of plasmonic gold nanoparticles to harvest photon energy; nanoparticles are deposited by r.f. sputtering and their plasmon resonance tailored in real time by spectroscopic ellipsometry.-the integration of graphene as semitransparent contact; the graphene is grown by CVD on Ni and Cu and then transferred on a large variety of materials including plastics and solar cells.

Electromagnetic enhancement of metallic nanoparticles on substrates is assessed by an exhaustive numerical analysis. The potential of each technologically promising metal for UVplasmonic applications may be ascertained.© OSA 2014.

In this work we demonstrate for the first time the micro- and nanostructuring of graphene bymeans of UV-nanoimprint lithography. Exfoliated graphene on SiO2 substrates, as well asgraphene deposited by chemical vapor deposition (CVD) on polycrystalline nickel and copper,and transferred CVD graphene on dielectric substrates, were used to demonstrate that ourtechnique is suitable for large-area patterning (2  2 cm2) of graphene on various types ofsubstrates. The demonstrated fabrication procedure of micrometer as well as nanometer-sizedgraphene structures with feature sizes down to 20 nm by a wafer-scale process opens up anavenue for the low-cost and high-throughput manufacturing of graphene-based optical andelectronic applications. The processed graphene films show electron mobilities of up to4:6  103 cm2 V-1 s-1, which confirms them to exhibit state-of-the-art electronic quality withrespect to the current literature.

Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions must be carefully controlled to obtain large grain size. Specifically, a strong correlation has been reported between grain sizes and sheet resistance in CVD-graphene1. Improving the quality of CVD-graphene requires the optimization of nucleation and growth rates in the synthesis process, in order to provide an increase in grain average size1,2. Therefore, diagnostic methodologies for the accurate monitoring of grain sizes and distribution directly on the growing substrate is needed.In this work we present an effective methodology for analyzing grain size and distribution in graphene as grown on copper by optical microscopy. In contrast to transmission electron microscopy3,4 and scanning tunneling microscopy3,5, this optical technique is not expensive and time-consuming and, above all, effective for the diagnostic on large scale. We exploit a wet chemistry approach for the selective oxidation of the metal substrate through graphene grains making their boundaries visible by optical microscopy. Specifically, we use the Fenton's reaction as a source of hydroxyl radicals to functionalize graphene grain boundaries. This diagnostic approach has been optimized by using Raman spectroscopy to probe and confirm chemical and structural changes in graphene and the Cu substrate upon the wet treatment. We also demonstrate the feasibility of this methodology for highlighting defect sites in graphene and, hence, for probing the retention of its structural quality upon post-growth processing such as transferring on other substrate.

Gold nanoclusters are deposited directly on silicon by sputtering of a target of metallic gold usingan argon plasma to provide a semiconductor-based plasmonic platform. The effects of annealingand substrate temperatures during the nanoparticles deposition and of the silicon surface energy onthe shape of the nanoparticles and resulting surface plasmon resonance are investigated. The Aunanoparticles are characterized optically, structurally and morphologically using spectroscopic ellipsometry,transmission electron microscopy and atomic force microscopy to establish a correlationamong the Au/Si interface reactivity, the Au nanoparticles shape and plasmonic resonance properties.It is found that post-growth annealing up to 600 C of nanoparticles deposited at 60 C causesaggregation of nanoparticles. Increasing the temperature of the substrate during the sputtering ofgold on Si yields pancake-like nanoparticles with a large Si/Au interface reactivity forming a goldsilicidesinterface layer. The O2 plasma treatment of the Si surface forming a thin intentional SiO2interface layer prevents the Au/Si interdiffusion yielding polyedrical nanoparticles whose plasmonresonance can be shifted down to 1.5 eV

Fluorination of graphene enables tuning of its electronic properties, provided that control of the fluorination degree and of modification of graphene structure can be achieved. In this work we demonstrate that SF6 modulated plasma fluorination of monolayer graphene yields polyene-graphene hybrids. The extent of fluorination is determined by the plasma exposure time and controlled in real time by monitoring the change in the optical response by spectroscopic ellipsometry. Raman spectroscopy reveals the formation of polyenes in partially fluorinated graphene (F/C < 0.25), which are responsible for changes in conductivity and for opening a transport gap of similar to 25 meV. We demonstrate that the cis- and trans-isomers of the polyenes in graphene are tunable using the photothermal switching. Specifically, the room temperature fluorination results in the cis- isomer that can be converted to the trans-isomer by annealing at T > 4 150 degrees C, whereas photoirradiation activates the trans-to-cis isomerization. The two isomers give to the polyene-graphene hybrids different optical and conductivity properties providing a way to engineer electrical response of graphene.

Achieving a pinhole-free CdS layer is necessary to produce high performance thin film CdTe solar cells. Pinholes in the CdS layer can compromise the efficiency of a CdTe solar cell by causing shunts. We have investigated the use of a plasma treatment of a fluorine doped tin oxide coated glass substrate (NSG TEC 15) and its effect on pinhole reduction in thin film CdS layers grown by Chemical Bath Deposition. CdS films, <100nm thickness, were deposited on both O-2/Ar plasma cleaned and conventionally cleaned substrates. We show that the O-2/Ar plasma treatment of the TEC 15 substrate reduced the water contact angle from similar to 55 degrees to less than 12 degrees indicating a substantial increase in the surface energy. The CdS deposited on the plasma treated TEC 15 was pinhole free, very smooth and homogenous in morphology and composition. Scanning electron microscopy images show that the O-2/Ar plasma treatment is effective in increasing film density and grain size. Corresponding spectroscopic ellipsometry measurements show an increase in the refractive index from 2.18 to 2.43 at 550 nm wavelength. (C) 2013 AIP Publishing LLC.

This paper reports on the growth of Si nanowires (NWs) by SiH4/H2 plasmas using the non-noble Gananoparticles(NPs) catalysts. A comparative investigation of conventional Si-NWs vapour-liquid-solid(VLS) growth catalyzed by Au NPs is also reported. We investigate the use of a hydrogen plasma and of aSiH4/H2 plasma for removing Ga oxide shell and for enhancing the Si dissolution into the catalyst, respectively.By exploiting the Ga NPs surface plasmon resonance (SPR) sensitivity to their surface chemistry,the SPR characteristic of Ga NPs has been monitored by real time spectroscopic ellipsometry in orderto control the hydrogen plasma/Ga NPs interaction and the involved processes (oxide removal and NPsdissolution by volatile gallium hydride). Using in situ laser reflectance interferometry the metal catalyzedSi NWs growth process has been investigated to find the effect of the plasma activation on the growthkinetics. The role of atomic hydrogen in the NWs growth mechanism and, in particular, in the SiH4 dissolutioninto the catalysts, is discussed. We show that while Au catalysts because of the re-aggregationof NPs yields NWs that do not correspond to the original size of the Au NPs catalyst, the NWs grown bythe Ga catalyst retains the diameter dictated by the size of the Ga NPs. Therefore, the advantage of GaNPs as catalysts for controlling NWs diameter is demonstrated.

The control of In adlayer during plasma-assisted molecular beam epitaxy (PAMBE) of InGaN/GaN multiquantum wells (MQWs) is critical to achieve good structural and optical properties. This contribution focuses on the investigation by real-time spectroscopic ellipsometry, corroborated by reflection high energy electron diffraction (RHEED), of In adlayer during the PAMBE growth of InGaN/GaN MQWs. The ellipsometric data reveal In accumulation during InGaN growth, which results in thicker quantum well than designed. We address the effect of the indium adlayer on the growth of InGaN/GaN with an indium composition between 7% and 13%, and on their optical and structural properties determined by high-resolution X-ray diffraction and photoluminescence

In this work, we report the fabrication of NiO films from two different nickel b-doketonate complexes as precursors for MOCVD. Volatile and thermally stable Ni complexes were prepared by the reaction of nickel acetate suspended in dicloromethane with b-diketonates (Hhfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedione and Htta=2-thenoyl-trifluoroacetone) and N-donor ligand tmeda (tmeda=N',N',N,N,-tetramethyl-ethylendiamine) to obtain respectively the complexes Ni(hfa)2tmeda and Ni(tta)2tmeda. These cpmplexes were applied as precursors in MCVD experiments directed to NiO deposition. The films grown on quartz were characterized using X-ray diffraction analysis (XRD), field emission scannng electron microscopy (FE-SEM) and atomic force microscopy (AFM). The optical proerties were determined by UV and ellipsometric measurements.

InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10-12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of InxAl(1-x)N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template. (C) 2014 AIP Publishing LLC.

Spectroscopic ellipsometry combined with localized surface plasmon resonance (LSPR) of gold nanoparticles (Au NPs) is exploited to design label-free bionsensors. We demonstrate that the size of Au NPs significantly affects the sensitivity of the ellipsometry analysis. Additionally, functionalizing Au NPs of different sizes with molecules/proteins of different sizes and shapes, such as dodecanethiol, hemin, human albumin and its antibody, we show that the size of nanoparticles can strongly influence the binding activity of adsorbed proteins and, consequently, the sensor functioning. Specifically, Au NPs with a diameter in the range 30-50 nm exhibit higher sensitivity to the change in the optical properties, and the variations of the ellipsometric parameter Psi allow discerning phenomena of aggregation of Au NPs of the sensor, of detachment of Au NPs and of protein chemisorption on Au NPs. The data are discussed in terms of two main factors affecting the ellipsometry sensitivity, i.e., the dependence of the LSPR electromagnetic enhancement on the Au NP size, and the strength of the interaction of the functionalizing molecule with Au NPs. (C) 2013 Elsevier B.V. All rights reserved.

From fundamental and technological points of view, there is interest in developing opticaltransducers exploiting the localized surface plasmon resonance (LSPR) of gold nanoparticles(Au NPs) for the study of interaction of biomolecules as well as of their functionalizationmechanism of inorganic surfaces.Spectroscopic ellipsometry monitoring the LSPR change gives the possibility to detect withgood resolution and accuracy time-dependent changes of amplitude ?(t) and phase ?(t) [1]during the immobilization of biomolecules at the solid-liquid interfaces. The simultaneousmeasurements of kinetics of both ellipsometric parameters ?(?,t) and phase ?(?,t) enable toreach advanced sensitivity in a wide range of the binding process, even up to the completeformation of a biomolecular layer [2].One of the problems encountered in developing Au NPs LSPR sensor is the change of thesensor itself due to mobility of the Au NPs depending on the interaction with thefunctionalizing molecules.As an example in the case of the most investigatedthiols functionalization, depending on themolecular structure of the thiol and Au NPs size,interparticle aggregation may occur resulting ininstability of the sensor itself.The aim of this work is to demonstrate theadvantage of simultaneous measurements of ?(t)and ?(t) for the evaluation of the interaction ofbiomolecules with Au NPs inducing theiraggregation and temporal instability. Onceellipsometry has been useful to identify a stable AuNPs sensing substrate (in various conditions ofsolvents, pH, molecules, etc..), parameterscharacterizing the immobilization kinetics ofporphirins and antigen interaction withimmobilized antibodies is shown. Theellipsometric characterization is corroborated bymorphological analysis with atomic forcemicroscopy (AFM) also operating in electric forcemode (Kelvin probe) and by structural analysis ithRaman spectroscopy.

Fully fluorinated arylenevinylene polymers havebeen synthesized via a methodology based on the Stillecross-coupling reaction and characterized by FTIR spectroscopyand MALDI-TOF mass spectrometry. Investigation ofthin film properties by cyclic voltammetry and ellipsometryshows that complete substitution of hydrogen atoms withfluorine atoms on the conjugated backbone of the poly(arylenevinylene)s results in a strong increase of the band gap.

Here we discuss the use in solar cells of graphene grown by chemical vapor deposition (CVD) and of plasmonic gold nanoparticles (Au NPs) deposited by sputtering. The Au NPs have been coupled with a-Si heterojunction solar cells, with an organic active layer used in organic photovoltaics, and with graphene. Extensive characterization of those three systems by the optical technique of spectroscopic ellipsometry, which is suitable to monitor and analyze the plasmon resonance of the Au NPs, by the microstructural technique of Raman spectroscopy, which is suitable to analyze graphene properties and doping, and by atomic force microscopy has been carried out. Those techniques highlighted interactions between Au NPs and silicon, polymer and graphene, which lead to variation in the plasmon resonance of Au NPs and consequently in the characteristics of the Au NPs/Si, Au NPs/polymer and Au NPs/graphene hybrids. Specifically, we found that an optimal size and density of Au NPs are able to enhance the efficiency of c-Si/a-Si heterojunction solar cells and that exceeding with Au NPs size and density causes device shortcut because of interface interdiffusion between silicon and gold. To discuss organic photovoltaics, Au NPs have been combined with an electro-donating conjugated polymer, the poly[1,4bis(2-thienyl)-2,5-bis-(2-ethyl-hexyloxyphenylenes)]. We found that there is a strong correlation between the thickness and morphology of the organic active layer, which affects the energy and amplitude of the Au NPs plasmon resonance. Finally, Au NPs have been deposited on graphene. We found that Au NPs show the plasmon resonance in the region where graphene is transparent and also yield p-type doping of graphene decreasing its sheet resistance.

Here we discuss the use in solar cells of graphene grown by chemical vapor deposition (CVD) and ofplasmonic gold nanoparticles (Au NPs) deposited by sputtering. The Au NPs have been coupled with a-Siheterojunction solar cells, with an organic active layer used in organic photovoltaics, and with graphene.Extensive characterization of those three systems by the optical technique of spectroscopic ellipsometry,which is suitable to monitor and analyze the plasmon resonance of the Au NPs, by the microstructuraltechnique of Raman spectroscopy, which is suitable to analyze graphene properties and doping, and byatomic force microscopy has been carried out. Those techniques highlighted interactions between AuNPs and silicon, polymer and graphene, which lead to variation in the plasmon resonance of Au NPsand consequently in the characteristics of the Au NPs/Si, Au NPs/polymer and Au NPs/graphene hybrids.Specifically, we found that an optimal size and density of Au NPs are able to enhance the efficiency of c-Si/a-Si heterojunction solar cells and that exceeding with Au NPs size and density causes device shortcutbecause of interface interdiffusion between silicon and gold. To discuss organic photovoltaics, Au NPshave been combined with an electro-donating conjugated polymer, the poly[1,4bis(2-thienyl)-2,5-bis-(2-ethyl-hexyloxyphenylenes)]. We found that there is a strong correlation between the thickness andmorphology of the organic active layer, which affects the energy and amplitude of the Au NPs plasmonresonance. Finally, Au NPs have been deposited on graphene. We found that Au NPs show the plasmon resonancein the region where graphene is transparent and also yield p-type doping of graphene decreasingits sheet resistance.

We investigate the optical and morphological effects of silver island films exposed to atmosphere. Silver nanoparticles with an average radius below 5 nm were highly tarnished after one week, showing significant morphological changes. Using x-ray photoelectron spectroscopy we demonstrate that the tarnishing is predominantly caused by oxidation and not sulfidation for exposed silver nanostructures under normal conditions. Post-deposition annealing of tarnished nanoparticle films results in further morphological changes which are highly dependent on the exposure time and nominal film thickness. Our results suggest that the size and density of the nanoparticles can be pre-designed by controlling the deposition thickness, tarnishing and annealing. The processing causes a red-shift in the localized surface plasmon resonance due to reduced particle interactions.

Graphene is defined as a single atomic layer of sp2 bonded carbons in a honeycomb lattice. This emerging material posses peculiar properties such as high carrier mobility, optical transparency, flexibility and high chemical resistance that have stimulated a vast amount of research in several scientific fields. The widespread investigation of graphene properties begins since the first isolation of a graphene flakes by the well-known mechanical exfoliation method (demonstrated in 2004 by Novoselov et al). Whereas mechanical exfoliation of graphene allows the production of high quality graphene on the laboratory scale for characterization and fundamental studies, the graphene technology mainly relies on two other fabrication techniques: (i) the growth of epitaxial graphene (E-graphene) on SiC for substituting silicon in high value-added electronic devices (with operating speeds up to the terahertz range), and (ii) the growth of graphene on large area metal substrates by chemical vapor deposition (CVD) for applications as flexible conducting transparent electrodes for replacing ITO technology and developing new flexible electronics.Currently, the production of CVD- and E-graphene is characterized by a very high cost and a poor control of the graphene polycrystalline nature (grain sizes and orientations) and thickness (the growth of an uniform single- or bi-layer graphene is still challenging). These factors as well as the presence of structural defects and impurities (also deriving from the transfer process in the case of CVD-G) strongly affect the graphene transport properties. Additionally, the opening of an optical gap is fundamental for graphene applications in transistors, circuits and photonic devices.This contribution discusses the role of chemistry in addressing the graphene research challenges. Chemical routes for optimizing the growth of E-graphene and CVD-graphen in terms of quality (grain size, number of layer, presence of defects, etc.) and reproducibility are presented. These routes are based on the control of the graphene growth kinetics. Moreover, results on the covalent and non-covalent chemical functionalization of graphene for band-gap engineering, doping, creation of magnetism, and for anchoring organic molecules and metal nanoparticles.

Self-assembled, irregular ensembles of hemi, spherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled T deposition time, exhibited. localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet.

The practical efficacy of technologically promising metals for use in ultraviolet plasmonics (3-6 eV) is assessed by an exhaustive numerical analysis. This begins with estimates of the near- and far-field electromagnetic enhancement factors of isolated hemispherical and spherical metallic nanoparticles deposited on typical dielectric substrates like sapphire, from which the potential of each metal for plasmonic applications may be ascertained. The ultraviolet plasmonic behavior of aluminum, chromium, copper, gallium, indium, magnesium, palladium, platinum, rhodium, ruthenium, titanium, and tungsten was compared with the well-known behavior of gold and silver in the visible. After exploring this behavior for each metal as a function of nanoparticle shape and size, the deleterious effect caused by the metal's native oxide is considered, and the potential for applications such as surface-enhanced Raman spectroscopy, accelerated photodegradation and photocatalysis is addressed.