Reconfigurable and optically transparent microwave absorbers based on deep eutectic solvent-gated graphene.

Electrolytically tunable graphene "building blocks" for reconfigurable and optically transparent microwave surfaces and absorbers have been designed and fabricated by exploiting Deep Eutectic Solvents (DESs). DESs have been first explored as electrolytic and environmentally friendly media for tuning sheet resistance and Fermi level of graphene together with its microwave response (reflection, transmission and absorption). We consider the tunability of the reconfigurable surfaces in terms of transmittance, absorption and reflectance, respectively, over the X and Ku bands when the gate voltage is varied in the -1.4/+1.4 V range. The numerical simulations and experimental measurements also show the ability of the absorber, in the Salisbury screen configuration, to achieve near perfect absorption with a modulation of about 20%. These results could find applications in several technological fields, ranging from electromagnetic pollution to integrated multi-physical regulation systems, thereby helping the advance of the performance of microwave cloaking systems, stealth windows, frequency selective surfaces, modulators and polarizers.

Optically Transparent Microwave Polarizer Based On Quasi-Metallic Graphene.

In this paper, we report on the engineering and the realization of optically transparent graphene-based microwave devices using Chemical Vapour Deposition (CVD) graphene whose sheet resistance may be tailored down to values below 30 Ω/sq. In particular, we show that the process was successfully used to realize and characterize a simple, optically transparent graphene-based wire-grid polarizer at microwave frequencies (X band). The availability of graphene operating in a quasi-metallic region may allow the integration of graphene layers in several microwave components, thus leading to the realization of fully transparent (and flexible) microwave devices.

Photothermally controlled structural switching in fluorinated polyene-graphene hybrids.

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 ∼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 > 150 °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.

Optical properties of silicon semiconductor-supported gold nanoparticles obtained by sputtering.

Gold nanoclusters are deposited directly on silicon by sputtering of a target of metallic gold using an argon plasma to provide a semiconductor-based plasmonic platform. The effects of annealing and substrate temperatures during the nanoparticles deposition and of the silicon surface energy on the shape of the nanoparticles and resulting surface plasmon resonance are investigated. The Au nanoparticles are characterized optically, structurally and morphologically using spectroscopic ellipsometry, transmission electron microscopy and atomic force microscopy to establish a correlation among the Au/Si interface reactivity, the Au nanoparticles shape and plasmonic resonance properties. It is found that post-growth annealing up to 600 degrees C of nanoparticles deposited at 60 degrees C causes aggregation of nanoparticles. Increasing the temperature of the substrate during the sputtering of gold on Si yields pancake-like nanoparticles with a large Si/Au interface reactivity forming a gold-silicides interface layer. The O2 plasma treatment of the Si surface forming a thin intentional SiO2 interface layer prevents the Au/Si interdiffusion yielding polyedrical nanoparticles whose plasmon resonance can be shifted down to 1.5 eV.

Graphene CVD growth on copper and nickel: role of hydrogen in kinetics and structure.

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.

Tailoring density and optical and thermal behavior of gold surfaces and nanoparticles exploiting aromatic dithiols.

Self-assembled monolayers (SAMs) derived of 4-methoxy-terphenyl-3'',5''-dimethanethiol (TPDMT) and 4-methoxyterphenyl-4''-methanethiol (TPMT) have been prepared by chemisorption from solution onto gold thin films and nanoparticles. The SAMs have been characterized by spectroscopic ellipsometry, Raman spectroscopy and atomic force microscopy to determine their optical properties, namely the refractive index and extinction coefficient, in an extended spectral range of 0.75-6.5 eV. From the analysis of the optical data, information on SAMs structural organization has been inferred. Comparison of SAMs generated from the above aromatic thiols to well-known SAMs generated from the alkanethiol dodecanethiol revealed that the former aromatic SAMs are densely packed and highly vertically oriented, with a slightly higher packing density and a absence of molecular inclination in TPMT/Au. The thermal behavior of SAMs has also been monitored using ellipsometry in the temperature range 25-500 degrees C. Gold nanoparticles functionalized by the same aromatic thiols have also been discussed for surface enhanced Raman spectroscopy applications. This study represents a step forward tailoring the optical and thermal behavior of surfaces as well as nanoparticles.