Plasmonic hot-electron reconfigurable photodetector based on phase-change material Sb2S3.

Hot-carrier based photodetectors and enhanced by surface plasmons (SPs) hot-electron injection into semiconductors, are drawing significant attention. This photodetecting strategy yields to narrowband photoresponse while enabling photodetection at sub-bandgap energies of the semiconductor materials. In this work, we analyze the design of a reconfigurable photodetector based on a metal-semiconductor (MS) configuration with interdigitated dual-comb Au electrodes deposited on the semiconducting Sb2S3 phase-change material. The reconfigurability of the device relies on the changes of refractive index between the amorphous and crystalline phases of Sb2S3 that entail a modulation of the properties of the SPs generated at the dual-comb Au electrodes. An exhaustive numerical study has been realized on the Au grating parameters formed by the dual-comb electrodes, and on the SP order with the purpose of optimizing the absorption of the device, and thus, the responsivity of the photodetector. The optimized photodetector layout proposed here enables tunable narrowband photodetection from the O telecom band (λ = 1310 nm) to the C telecom band (λ = 1550 nm).

Layered gallium sulfide optical properties from monolayer to CVD crystalline thin films.

Interest in layered van der Waals semiconductor gallium monosulfide (GaS) is growing rapidly because of its wide band gap value between those of two-dimensional transition metal dichalcogenides and of insulating layered materials such as hexagonal boron nitride. For the design of envisaged optoelectronic, photocatalytic and photonic applications of GaS, the knowledge of its dielectric function is fundamental. Here we present a combined theoretical and experimental investigation of the dielectric function of crystalline 2H-GaS from monolayer to bulk. Spectroscopic imaging ellipsometry with micron resolution measurements are corroborated by first principle calculations of the electronic structure and dielectric function. We further demonstrate and validate the applicability of the established dielectric function to the analysis of the optical response of c-axis oriented GaS layers grown by chemical vapor deposition (CVD). These optical results can guide the design of novel, to our knowledge, optoelectronic and photonic devices based on low-dimensional GaS.

Synthesis and Investigation of Electro-Optical Properties of H-Shape Dibenzofulvene Derivatives.

We have synthetized two classes of dibenzofulvene-arylamino derivatives with an H-shape design, for a total of six different molecules. The molecular structures consist of two D-A-D units connected by a thiophene or bitiophene bridge, using diarylamino substituents as donor groups anchored to the 2,7- (Group A) and 3,6- (Group B) positions of the dibenzofulvene backbone. The donor units and the thiophene or bithiophene bridges were used as chemico-structural tools to modulate electro-optical and morphological-electrical properties. A combination of experiments, such as absorption measurements (UV-Vis spectroscopy), cyclic voltammetry, ellipsometry, Raman, atomic force microscopy, TD-DFT calculation and hole-mobility measurements, were carried out on the synthesized small organic molecules to investigate the differences between the two classes and therefore understand the relevance of the molecular design of the various properties. We found that the anchoring position on dibenzofulvene plays a crucial key for fine-tuning the optical, structural, and morphological properties of molecules. In particular, molecules with substituents in 2,7 positions (Group A) showed a lower structural disorder, a larger molecular planarity, and a lower roughness.

Sustainable and Tunable Mg/MgO Plasmon-Catalytic Platform for the Grand Challenge of SF6 Environmental Remediation.

Sulfur hexafluoride (SF6) is one of the most harmful greenhouse gases producing environmental risks. Therefore, developing ways of degrading SF6 without forming hazardous products is increasingly important. Herein, we demonstrate for the first time the plasmon-catalytic heterogeneous degradation of SF6 into nonhazardous MgF2 and MgSO4 products by nontoxic and sustainable plasmonic magnesium/magnesium oxide (Mg/MgO) nanoparticles, which are also effective as a plasmon-enhanced SF6 chemometric sensor. The main product depends on the excitation wavelength; when the localized surface plasmon resonance (LSPR) is in the ultraviolet, then MgF2 forms, while visible light LSPR results in MgSO4. Furthermore, Mg/MgO platforms can be regenerated in few seconds by hydrogen plasma treatment and can be reused in a new cycle of air purification. Therefore, this research first demonstrates effectiveness of Mg/MgO plasmon-catalysis enabling environmental remediation with the concurrent functionalities of monitoring, degrading, and detecting sulfur and fluorine gases in the atmosphere.

Optically addressing interaction of Mg/MgO plasmonic systems with hydrogen.

Magnesium-based films and nanostructures are being studied in order to improve hydrogen reversibility, storage capacity, and kinetics, because of their potential in the hydrogen economy. Some challenges with magnesium (Mg) samples are their unavoidable oxidation by air exposure and lack of direct in situ real time measurements of hydrogen interaction with Mg and MgO surfaces and Mg plasmonic nanoparticles. Given these challenges, the present article investigates direct interaction of Mg with hydrogen, as well as implications of its inevitable oxidation by real-time spectroscopic ellipsometry for exploiting the optical properties of Mg, MgH2 and MgO. The direct hydrogenation measurements have been performed in a reactor that combines a remote hydrogen plasma source with an in situ spectroscopic ellipsometer, which allows optical monitoring of the hydrogen interaction and results in optical property modification. The hydrogen plasma dual use is to provide the hydrogen-atoms and to reduce barriers to heterogeneous hydrogen reactions.

Thermally stable coexistence of liquid and solid phases in gallium nanoparticles.

Gallium (Ga), a group III metal, is of fundamental interest due to its polymorphism and unusual phase transition behaviours. New solid phases have been observed when Ga is confined at the nanoscale. Herein, we demonstrate the stable coexistence, from 180 K to 800 K, of the unexpected solid γ-phase core and a liquid shell in substrate-supported Ga nanoparticles. We show that the support plays a fundamental role in determining Ga nanoparticle phases, with the driving forces for the nucleation of the γ-phase being the Laplace pressure in the nanoparticles and the epitaxial relationship of this phase to the substrate. We exploit the change in the amplitude of the evolving surface plasmon resonance of Ga nanoparticle ensembles during synthesis to reveal in real time the solid core formation in the liquid Ga nanoparticle. Finally, we provide a general framework for understanding how nanoscale confinement, interfacial and surface energies, and crystalline relationships to the substrate enable and stabilize the coexistence of unexpected phases.

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.

Directional Scattering Switching from an All-Dielectric Phase Change Metasurface.

All-dielectric metasurfaces are a blooming field with a wide range of new applications spanning from enhanced imaging to structural color, holography, planar sensors, and directionality scattering. These devices are nanopatterned structures of sub-wavelength dimensions whose optical behavior (absorption, reflection, and transmission) is determined by the dielectric composition, dimensions, and environment. However, the functionality of these metasurfaces is fixed at the fabrication step by the geometry and optical properties of the dielectric materials, limiting their potential as active reconfigurable devices. Herein, a reconfigurable all-dielectric metasurface based on two high refractive index (HRI) materials like silicon (Si) and the phase-change chalcogenide antimony triselenide (Sb2Se3) for the control of scattered light is proposed. It consists of a 2D array of Si-Sb2Se3-Si sandwich disks embedded in a SiO2 matrix. The tunability of the device is provided through the amorphous-to-crystalline transition of Sb2Se3. We demonstrate that in the Sb2Se3 amorphous state, all the light can be transmitted, as it is verified using the zero-backward condition, while in the crystalline phase most of the light is reflected due to a resonance whose origin is the contribution of the electric (ED) and magnetic (MD) dipoles and the anapole (AP) of the nanodisks. By this configuration, a contrast in transmission (ΔT) of 0.81 at a wavelength of 980 nm by governing the phase of Sb2Se3 can be achieved.

Stability of Nanometer-Thick Layered Gallium Chalcogenides and Improvements via Hydrogen Passivation.

The gallium monochalcogenides family, comprising gallium sulfide (GaS), gallium selenide (GaSe), and gallium telluride (GaTe), is capturing attention for its applications in energy storage and production, catalysis, photonics, and optoelectronics. This interest originates from their properties, which include an optical bandgap larger than those of most common transition metal dichalcogenides, efficient light absorption, and significant carrier mobility. For any application, stability to air exposure is a fundamental requirement. Here, we perform a comparative study of the stability of layered GaS, GaSe, and GaTe nanometer-thick films down to a few layers with the goal of identifying the most suitable Ga chalcogenide for future integration in photonic and optoelectronic devices. Our study unveils a trend of decreasing air stability from sulfide to selenide and finally to telluride. Furthermore, we demonstrate a hydrogen passivation process to prevent the oxidation of GaSe with a higher feasibility and durability than other state-of-the-art passivation methods proposed in the literature.

Supported MOCVD TiO2 Thin Films Grown on Modified Stainless Steel Mesh for Sensing Applications.

Among semiconductor metal oxides, that are an important class of sensing materials, titanium dioxide (TiO2) thin films are widely employed as sensors because of their high chemical and mechanical stability in harsh environments, non-toxicity, eco-compatibility, and photocatalytic properties. TiO2-based chemical oxygen demand (COD) sensors exploit the photocatalytic properties of TiO2 in inducing the oxidation of organic compounds to CO2. In this work, we discuss nanostructured TiO2 thin films grown via low-pressure metal organic chemical vapor deposition (MOCVD) on metallic AISI 316 mesh. To increase the surface sensing area, different inorganic acid-based chemical etching protocols have been developed, determining the optimal experimental conditions for adequate substrate roughness. Both chemically etched pristine meshes and the MOCVD-coated ones have been studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) microanalysis, and X-ray photoelectron spectroscopy (XPS). We demonstrate that etching by HCl/H2SO4 at 55 °C provides the most suitable surface morphology. To investigate the behavior of the developed high surface area TiO2 thin films as COD sensors, photocatalytic degradation of functional model pollutants based on ISO 10678:2010 has been tested, showing for the best performing acid-etched mesh coated with polycrystalline TiO2 an increase of 60% in activity, and degrading 66 µmol of MB per square meter per hour.

Roadmap for phase change materials in photonics and beyond.

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their diverse applications in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum. It discusses various device configurations and optimization techniques, including deep learning-based metasurface design. The integration of PCMs with Photonic Integrated Circuits and advanced electric-driven PCMs are explored. PCMs hold great promise for multifunctional device development, including applications in non-volatile memory, optical data storage, photonics, energy harvesting, biomedical technology, neuromorphic computing, thermal management, and flexible electronics.

Erratum: Roadmap for phase change materials in photonics and beyond.

[This corrects the article DOI: 10.1016/j.isci.2023.107946.].

Evidence of plasmonic coupling in gallium nanoparticles/graphene/SiC.

Graphene is emerging as a promising material for plasmonics applications due to its strong light-matter interactions, most of which are theoretically predicted but not yet experimentally realized. Therefore, the integration of plasmonic nanoparticles to create metal nanoparticle/graphene composites enables numerous phenomena important for a range of applications from photonics to catalysis. For these applications 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. These coupled phenomena underpin an active application area in graphene-based composites due to nanoparticle-dependent surface-enhanced Raman scattering (SERS) of graphene phonon modes. This study reveals the coupling of a graphene/SiC support with Ga-nanoparticle-localized surface plasmon resonance, which is of particular interest due to its ability to be tuned across the UV into the near-IR region. This work is the first demonstration of the evolving plasmon resonance on graphene during the synthesis of surface-supported metal nanoparticles, thus providing evidence for the theoretically predicted screening revealed by a damped resonance with little energy shift. Therefore, the role of the graphene/substrate heterojunction in tailoring the plasmon resonance for nanoplasmonic applications is shown. Additionally, the coupled phenomena between the graphene-Ga plasmon properties, charge transfer, and SERS of graphene vibrational modes are explored.

Cysteamine-based functionalization of InAs surfaces: revealing the critical role of oxide interactions in biasing attachment.

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.

Nano- and microstructuring of graphene using UV-NIL.

In this work we demonstrate for the first time the micro- and nanostructuring of graphene by means of UV-nanoimprint lithography. Exfoliated graphene on SiO(2) substrates, as well as graphene deposited by chemical vapor deposition (CVD) on polycrystalline nickel and copper, and transferred CVD graphene on dielectric substrates, were used to demonstrate that our technique is suitable for large-area patterning (2 × 2 cm(2)) of graphene on various types of substrates. The demonstrated fabrication procedure of micrometer as well as nanometer-sized graphene structures with feature sizes down to 20 nm by a wafer-scale process opens up an avenue for the low-cost and high-throughput manufacturing of graphene-based optical and electronic applications. The processed graphene films show electron mobilities of up to 4.6 × 10(3) cm(2) V (-1) s(-1), which confirms them to exhibit state-of-the-art electronic quality with respect to the current literature.

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.

Interplay between Thickness, Defects, Optical Properties, and Photoconductivity at the Centimeter Scale in Layered GaS.

From the group-III monochalcogenide (MX, M = Ga, In; X = S, Se, Te) layered semiconductors, gallium monosulfide, GaS, has emerged as a promising material for electronics, optoelectronics, and catalysis applications. In this work, GaS samples of various thicknesses in the range from 38 to 1665 nm have been obtained by mechanical exfoliation to study the interplay between structural, morphological, optical, and photoresponsivity properties as a function of thickness. This interplay has been established by analyzing the structure through Raman spectroscopy and X-ray diffraction, the morphology through scanning electron microscopy and atomic force microscopy, the density and optical properties through spectroscopic ellipsometry, and the photoresponsivity through current-voltage measurements under UV light. This work shows that photoresponsivity increases with increases in GaS thickness, resulting in a UV photoresponsivity of 1.5·10-4 AW-1 stable over several on/off cycles.

Interlaboratory study on Sb2S3 interplay between structure, dielectric function, and amorphous-to-crystalline phase change for photonics.

Antimony sulfide, Sb2S3, is interesting as the phase-change material for applications requiring high transmission from the visible to telecom wavelengths, with its band gap tunable from 2.2 to 1.6 eV, depending on the amorphous and crystalline phase. Here we present results from an interlaboratory study on the interplay between the structural change and resulting optical contrast during the amorphous-to-crystalline transformation triggered both thermally and optically. By statistical analysis of Raman and ellipsometric spectroscopic data, we have identified two regimes of crystallization, namely 250°C ≤ T < 300°C, resulting in Type-I spherulitic crystallization yielding an optical contrast Δn ∼ 0.4, and 300 ≤ T < 350°C, yielding Type-II crystallization bended spherulitic structure with different dielectric function and optical contrast Δn ∼ 0.2 below 1.5 eV. Based on our findings, applications of on-chip reconfigurable nanophotonic phase modulators and of a reconfigurable high-refractive-index core/phase-change shell nanoantenna are designed and proposed.

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.

Exploring the Thickness-Dependence of the Properties of Layered Gallium Sulfide.

Group III layered monochalcogenide gallium sulfide, GaS, is one of the latest additions to the two-dimensional (2D) materials family, and of particular interest for visible-UV optoelectronic applications due to its wide bandgap energy in the range 2.35-3.05 eV going from bulk to monolayer. Interestingly, when going to the few-layer regime, changes in the electronic structure occur, resulting in a change in the properties of the material. Therefore, a systematic study on the thickness dependence of the different properties of GaS is needed. Here, we analyze mechanically exfoliated GaS layers transferred to glass substrates. Specifically, we report the dependence of the Raman spectra, photoluminescence, optical transmittance, resistivity, and work function on the thickness of GaS. Those findings can be used as guidance in designing devices based on GaS.