We demonstrate the fabrication of a 2D periodic arrangement of gold square nano-patches on both silicon and borosilicate glass substrates whose optical properties can be exploited for both linear and nonlinear applications. The comparison between numerical and experimental results, combined with highresolution scanning electron microscopy images highlights the importance of the geometry and the fine shape in the determination of the square gold nano-patches optical properties.

We investigate the linear and nonlinear properties of a monolayer graphene embedded in a quarter-wave one-dimensional Photonic Crystal (1D-PhC) in terms of absorption and losses. In particular, we show that it is possible to achieve near-perfect narrow-band absorption when the monolayer graphene is sandwiched between two PhC mirrors with optimized pair number. The simulations reveal that the resonant wavelength and the total absorption frequency may be tuned by tilting the angle of incidence of the impinging source for both TE and TM polarizations. We also show that the losses related to the dielectric materials constituting the 1D-PhC can degrade the optical performance of the device. Preliminary fabrication tests reveal that the sputtering process does not lead to graphene structural damages affecting the material properties. Finally, by exploiting the large nonlinear response and the saturation effects of graphene monolayers we demonstrate how the structure can be dynamically change the structure from a perfect absorber to a mirror. These features make this device attractive for different applications ranging from tunable and saturable absorbers for short-pulse lasers, to graphene-based photodetectors. © 2014 IEEE.

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.

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.

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.

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.

We investigate the fabrication and characterization of gold nano-patches 2D arrays on different substrates (glass, silicon and graphene) whose optical properties can be used to implement novel plasmonic devices

We report theoretical and experimental investigations of the optical response of two-dimensional periodic arrays of rectangular gold nanopatches grown on a monolayer graphene placed on a glass substrate. We discuss the numerical analysis and optical characterization by means of reflection spectra and show that rectangular nanopatches display a polarization-dependent response, at normal incidence, which leads to double plasmonic resonances due to the Wood anomaly. We detail the fabrication process highlighting how the resist primer and the adhesion layer can reduce and impede the graphene doping due to the environment and to the nanopatches, respectively, by means of Raman spectroscopy.

In this paper we describe the fabrication of two-dimensional plasmonic arrays of gold square and rectangular patches on a Chemical Vapour Deposition (CVD) graphene monolayer transferred onto a glass substrate. In particular, the fabrication protocol of the 2D arrangement consisted of three main steps: an electron beam lithography exposure followed by thermal evaporation and a lift-off process. Scanning Electron Microscope (SEM) analyses are also reported showing that the geometrical parameters fit very well with the nominal parameters of the simulated structures. Furthermore Raman measurements confirm that the fabrication process does not affect the graphene monolayer.Finally, as preliminary optical characterization, the reflection behaviour of the plasmonic nanostructures, in linear regime, is investigated giving evidence of the graphene influence on the measured spectra.

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.

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.

Graphene is a perfect two-dimensional (2D) carbon sheet in honeycomb lattice with unique, multi-faceted properties. Very recently plasmons have been identified in graphene thus opening a new research field called 'graphene plasmonics' which provides a suitable alternative to noble-metal plasmonics, since the associated surface plasmons (SPs) exhibit much larger light confinement abilities and relatively long propagation distances, with the advantage of being highly tunable via electrostatic gating. Moreover graphene can be efficiently integrated in photonic nanostructures in order to increase the optical absorption in the visible and infrared regions (graphene photonics). In this paper we will illustrate some applications of graphene plasmonics and graphene photonics, reported in literature, in different research fields and we will review our recent numerical and experimental results concerning the linear and nonlinear optical response of graphene plasmonic and photonic nanostructures emphasizing their interaction in terms of absorption and optical resonances.

Graphene, a 2D carbon sheet with a honeycomb lattice, is a two-dimensional material with outstanding thermal, mechanical, electronic and optical properties. In particular, graphene is a gapless material with high mobility that exhibits remarkably high absorption values (~2.3%) for the visible and near-infrared wavelengths. In this paper, we will illustrate some applications of graphene photonics and plasmonics, reported in literature, in different research fields and we will investigate theoretically and experimentally the linear and nonlinear properties of graphene-based nanostructures such as one-dimensional (1D) photonic crystals (PhCs) and 1D gratings. In particular, we will show how to exploit the large nonlinear response and the saturation effects of graphene monolayers, sandwiched in the defect layer of an asymmetric 1D photonic crystal, to dynamically change the structure from a perfect absorber (100%) to a mirror. We will also show how it is possible to tune the working wavelength by tilting the angle of incidence of the impinging electromagnetic field for both TE and TM polarizations. Finally, we will report on a 1D dielectric grating, incorporating a graphene monolayer, that resembles the 1D PhC optical response exploiting guided mode resonances. Therefore, the proposed nanostructures could efficiently exploit the linear and nonlinear properties of the graphene monolayer for the realization of tunable absorbers or saturable mirrors improving and boosting the performance of optical devices such as photo-detectors and short-pulse lasers. © 2014 IEEE.

Graphene is believed to be one of the most promising material candidates for the next generation of photonic and plasmonic devices. Graphene, a perfect two-dimensional (2D) carbon sheet in honeycomb lattice, offers unique electrical and optical properties such as electron-hole symmetry near the charge neutrality point, high mobility, high conductivity and low absorption (?2.3% for a monolayer) over a wide spectral range [1].Recently it has been demonstrated that graphene supports surface plasmons and, hence, it can be exploited for the realization of graphene-based plasmonic devices from visible to THz regimes [2]. Moreover the combination of graphene and plasmonic nanostructures has attracted a great interest since it is possible to conjugate the graphene tunability and gating with the benefits of the plasmonic nanostructures in terms of field enhancement (hot-spots), sensitivity to environmental changes and the possibility to work beyond the diffraction limit. In this paper we propose two different schemes that exploit graphene in the visible-NIR and THz ranges. In particular, we show and detail the fabrication and the optical response of two-dimensional periodic arrays of rectangular gold nanopatches grown on a monolayer graphene placed on a glass substrate (Figure 1). The 2D periodic patterns have been written using a Raith150 e-beam lithography system operating at 30kV. A bi-layer, composed of PMMA-MA and PMMA, has been adopted as positive resist to allow the fabrication of rectangular metal patches with well-controlled geometrical feature sizes. The rectangular geometry allows exciting two different plasmonic resonances (in the visible and in the NIR range, respectively) addressable with a change of the polarization. The influence of the graphene on such a system has been analyzed by means of reflection spectra and Raman spectroscopy. Then the role and the effect of the primer and the adhesion layer on the graphene monolayer behaviour is highlighted.Finally the patterning of the graphene monolayer with 2D antidot arrays, having different shapes (e.g. circles, squares, rectangles), is reported. The 2D patterns have been fabricated using the Raith150 e-beam lithography system operating at 20 kV with a 400 nm-thick PMMA layer. The patterned PMMA layer has been used as a mask for the oxygen plasma (Figure 2(a) and (d)). Figure 2(b) and (e) prove the graphene removal, after the oxygen plasma, since the charging effect due to the presence of the SiO2 substrate is clearly evident. Raman maps for the graphene 2D peak also confirm that the graphene monolayer has been successfully removed as reported in Figure 2(c) and (f). Therefore these structures can be efficiently exploited for the realization of graphene-based metasurfaces for mid-IR and THz ranges miming the behaviour of similar plasmonic devices already reported and analyzed in the visible and infrared regions.

A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to enhance light absorption in a monolayer graphene exploiting guided mode resonances. Numerical findings reveal that the optimized configuration is able to absorb up to 60% of the impinging light at normal incidence for both TE and TM polarizations resulting in a theoretical enhancement factor of about 26 with respect to the monolayer graphene absorption (~2.3%). Experimental results confirm this behavior showing CVD graphene absorbance peaks up to about 40% over narrow bands of a few nanometers. The simple and flexible design points to a way to realize innovative, scalable and easy-tofabricate graphene-based optical absorbers.

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.

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).

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.

We propose a novel bio-sensing platform based on the observation of the shift of theleaky surface plasmon mode that occurs at the edge of the plasmonic band gap of metal gratingsbased on two-dimensional gold nano-patch arrays when an analyte is deposited on the top of themetallic structure. We detail the numerical analysis, the fabrication and the characterization of thesetwo-dimensional arrangements of gold patches in linear regime showing that sensitivity of ourdevice approaches a value of 1000 nm/RIU with a corresponding Figure of Merit (FOM) of 222RIU-1. We provide experimental proof of the sensing capabilities of the device by observing colourvariations in the diffracted field when the air overlayer is replaced with a small quantity of IsopropylAlcohol (IPA). Effects of technological tolerance such as rounded corners and surface imperfectionsare also discussed. We also report proof of changes in colour intensities as a function of theair/filling ratio ad periodicity and discuss how they can be obtained by diffracted spectra. Finally wereport the numerical and experimental investigation of the non-linear behaviour of the devicehighlighting the Surface Enhanced Raman Scattering (SERS) performance.

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.

In this paper we review our recent advances in subwavelength plasmonic structures in linear and nonlinear regimes. We show several examples of subwavelength plasmonic devices in different configurations and schemes. We emphasize how these devices can be exploited for many applications, such as sensing, spectroscopy, photovoltaics and optical communications. Finally we will illustrate some challenges that are still open and need to be fulfilled.

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.

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.

In this work, single walled carbon nanotubes (SWNTs) have been chemically functionalized at their walls with a membrane protein, namely the mutated bacteriorhodopsin D96N, integrated in its native archaeal lipid membrane. The modification of the SWNT walls with the mutant has been carried out in different buffer solutions, at pH 5, 7.5 and 9, to investigate the anchoring process, the typical chemical and physical properties of the component materials being dependent on the pH. The SWNTs modified by interactions with bacteriorhodopsin membrane patches have been characterized by UV-vis steady state, Raman and attenuated total reflection Fourier transform infrared spectroscopy and by atomic force and transmission electron microscopy. The investigation shows that the membrane protein patches wrap the carbon walls by tight chemical interactions undergoing a conformational change; such chemical interactions increase the mechanical strength of the SWNTs and promote charge transfers which p-dope the nano-objects. The functionalization, as well as the SWNT doping, is favoured in acid and basic buffer conditions; such buffers make the nanotube walls more reactive, thus catalysing the anchoring of the membrane protein. The direct electron communication among the materials can be exploited for effectively interfacing the transport properties of carbon nanotubes with both molecular recognition capability and photoactivity of the cell membrane for sensing and photoconversion applications upon integration of the achieved hybrid materials in sensors or photovoltaic devices.

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.

We experimentally investigate the surface-enhanced Raman scattering (SERS) response of a 2D-periodic array of square gold nano-patches, functionalized by means of a conjugated, rigid thiol. We measure a Raman signal enhancement up to 200 times more intense compared to other plasmon-based nanostructures functionalized with the same molecule, and show that the enhancement is not strictly correlated to the presence of plasmonic resonances. The agreement between experimental and theoretical results reveals the importance of a full-wave analysis based on the inclusion of the actual scattering cross section of the molecule. The proposed numerical approach may serve not only as a tool to predict the enhancement of Raman signal scattered from strongly resonant nanostructure but also as an effective instrument to engineer SERS platforms that target specific molecules.

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.

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.