Entire Visible Spectrum Research Articles

PECVD Silicon Nitride-Based Multilayers with Optimized Mechanical Properties

Silicon nitride (SiN) based films deposited by plasma-enhanced chemical vapor deposition (PECVD) have interesting optical, mechanical, and chemical properties. They are used for applications such as anti-reflective coatings and surface passivation layers in solar cells. Amorphous SiN-based films also are frequently used to create multilayer structures of alternatingly high-index and low-index films. This approach is very promising to fabricate narrow-bandwidth notch filters on photovoltaic cells to reflect on demand a wide variety of colors over the entire visible spectrum [1]. However, these multilayer structures need to be highly durable and mechanically stable since the lifecycle of photovoltaic cells in some applications [2] is very long, and the coated surfaces are very large. To satisfy these specific functional requirements, we need to adjust the films' optical and mechanical properties by changing the deposition parameters of a PECVD reactor. For instance, it is possible to strongly influence the chemical composition of amorphous SiN-based films, such as their stoichiometry, by tuning the gas flows [3]. This work investigates the effect of the deposition pressure, power, and source gas ratio on SiN-based monolayers during plasma deposition. We have deposited SiN and silicon oxynitride (SiON) films in an electron cyclotron resonance (ECR) PECVD reactor using a SiH4/N2/O2/Ar precursor mixture. We have measured the refractive index and the absorption of the films using a variable angle spectroscopic ellipsometry (VASE) to assess their optical quality. The mechanical properties of the films, such as residual stress and coefficient of thermal expansion, were measured ex-situ on a KLA-Tencor FLX-2320 film stress measurement system. The Young's modulus and hardness of the films were evaluated using nanoindentation. After studying the properties of monolayers, we have made numerical simulations and further characterizations to optimize the design of multilayer structures considering both the optical and mechanical properties. Through this discussion, we try to better understand the interactions taking place when amorphous SiN-based films with different structural properties and compositions are stacked on top of each other. Finally, we suggest methods to predict and control the residual stress in multilayer structures without affecting the optical properties.

Characteristics investigation of composite electrochromic films based on Ni(Oh)2, polyvinyl alcohol, and polyvinylpyrrolidone

The cost of electrochromic “smart” windows ranges from 200–800 USD/m2. The high cost is determined by the small volume of production and the high cost of vacuum methods for applying device layers. The development of other methods could lead to lower costs and further mass use of “smart” windows. The paper presents the work on improving the electrochemical method of deposition of electrochromic films based on Ni(OH)2. Composite electrochromic films were deposited from Ni(NO3)2 solutions with the addition of water-soluble polymers – polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and mixtures thereof. Two PVA grades and one PVP grade were used in the experiments. As a result, six films were obtained from solutions of different compositions and concentrations of polymers. An investigation of the optical properties of the films in the initial state showed that there is no intense absorption of light in the entire visible spectrum and the transmission corresponds to a glass substrate. Thus, the formed electrochromic films will not produce tints and visible light absorption in the finished electrochromic element. The electrochemical and electrochromic characteristics of the films differed significantly depending on the amount and specific polymers used. The film obtained from a solution containing PVA grade 24-99 was the most electrochemically active, while the best electrochromic characteristics were in the film deposited from a solution of a mixture of PVP grade 24-99 and PVP. Moreover, the films obtained in solutions containing separately PVA grade 24-99 and PVP had worse electrochromic characteristics, and the electrochemical characteristics were the same or worse relative to the film obtained in a mixture of polymers. The results of the method of electrochemical impedance spectroscopy showed compliance with the found electrochromic and electrochemical characteristics of the formed films.

Realizing full-color perovskite quantum dots light-emitting diodes via contemporary surface ligand/anion engineering

Unprecedentedly, cesium lead halide (CsPbX3, X = Cl, Br, I) quantum dots (QDs) with tunable spectral emissions and photophysical properties are used as a common emissive material for the fabrication of full-color perovskite QDs light-emitting diodes. An efficient, non-destructive ligand/anion exchange strategy is developed to fine-tune the optical properties of CsPbBr3 QDs solid-state films. Specifically, the surface of the CsPbBr3 QDs films is passivated by phenethylammonium halides, PEAX (X = Cl, Br, I) salts via a facile spin-coating technique, producing PEAX-CsPbBr3 QDs (X = Cl, Br, I) films with tunable (blue/green/red) color emissions that cover the entire visible spectrum. During passivation treatment, the PEAX passivators supply additional halide (Cl, Br, I) anions to fill in the intrinsic Br vacancies, allowing contemporary defect-passivation and emission-tuning effect in the QDs film. More importantly, this surface passivation strategy enables the fabrication of full-color perovskite QDs light-emitting diodes using PEAX-CsPbBr3 QDs as a common emissive film.

Extracellular Optogenetics at the Interface of Synthetic Biology and Materials Science.

We review fundamental mechanisms and applications of OptoGels: hydrogels with light-programmable properties endowed by photoswitchable proteins (“optoproteins”) found in nature. Light, as the primary source of energy on earth, has driven evolution to develop highly-tuned functionalities, such as phototropism and circadian entrainment. These functions are mediated through a growing family of optoproteins that respond to the entire visible spectrum ranging from ultraviolet to infrared by changing their structure to transmit signals inside of cells. In a recent series of articles, engineers and biochemists have incorporated optoproteins into a variety of extracellular systems, endowing them with photocontrollability. While other routes exist for dynamically controlling material properties, light-sensitive proteins have several distinct advantages, including precise spatiotemporal control, reversibility, substrate selectivity, as well as biodegradability and biocompatibility. Available conjugation chemistries endow OptoGels with a combinatorially large design space determined by the set of optoproteins and polymer networks. These combinations result in a variety of tunable material properties. Despite their potential, relatively little of the OptoGel design space has been explored. Here, we aim to summarize innovations in this emerging field and highlight potential future applications of these next generation materials. OptoGels show great promise in applications ranging from mechanobiology, to 3D cell and organoid engineering, and programmable cell eluting materials.

Dynamic Control of the Shielding Effectiveness of Optically Transparent Screens

This article presents the design, fabrication, and characterization of an active electromagnetic shield intended to dynamically protect optical and electromagnetic sensors against high intensity radiated fields. The shield exhibits high and constant optical transparency level over the entire visible light spectrum thanks to a micrometric mesh metal thin film printed on a glass substrate. The central micrometric mesh area is separated from the peripheral ground plane of the shield by a peripheral slot. This slot is fitted out with p-i-n diodes and resistors, connecting electrically the central micrometric mesh area and the ground plane. The aim of these components is to dynamically control the shielding effectiveness of the screen, by using the conducting ( <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> ) or blocking ( <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> ) states of the p-i-n diodes. Accordingly, the shielding effectiveness can be set at high level to protect the system against high intensity radiated fields, and conversely at low level to both prevent system electromagnetic self-perturbation and increase the sensitivity of the internal electromagnetic sensors. Dynamic control of the shielding effectiveness of the fabricated screen, whose optical transparency is close to 85% over the entire visible light spectrum, is fully demonstrated. A shielding effectiveness contrast ranging from 5 dB to 24 dB between the <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> diode states was measured in the 2–10.5 GHz frequency range in a reverberation chamber.

Polarization‐Selective Ultra‐Broadband Reflective Diffuser as a Smart Projection Screen

Herein, active smart screens are increasingly investigated to enhance projection‐type transparent displays owing to the high contrast ratio generated by their switchability between projection and transparent states. Despite the development of various active smart screens, most screens employ the scattering state as the projection state, which suffers from low reflectance of the projection state and optical interference with background light. Herein, an active smart screen composed of a passive polymer‐stabilized cholesteric liquid crystal (PSCLC) diffuser and an active polarization modulator (PM) is proposed. The PSCLCs enabled by the chiral monomer have a pitch‐gradient helical structure, wherein liquid crystal molecules form a multi‐domain planar texture and exhibit a broadband reflection band over the entire visible spectrum; their natural circular polarization selectivity facilitates the operation between projection and transparent states when the PSCLC is observed using opposite‐handed circular polarized light. Unlike traditional active smart screens, both transparent and projection states of the PSCLC diffuser can be switched by driving the PM, thereby eliminating the issue of background interference. The proposed active projection screen based on the polarization‐selective PSCLC can potentially improve the display quality of existing projection‐type transparent displays and form the basis for see‐through windows and near‐eye display applications.

Fast Self-Assembly of Photonic Crystal Hydrogel for Wearable Strain and Temperature Sensor.

Structural colors from photonic crystals (PCs) have attracted emerging attention in the research area of wearable sensors. Conventional self-assembly of PC takes days to weeks. Here, a fast self-assembly method of PC with horizontal precipitation of silica nanoparticles (NPs) in a polydimethylsiloxane fence, which can be completed within 1-4 h depending on the fence parameters, is introduced. The resultant PC exhibits tunable structural colors in the entire visible spectrum. With infiltration of composite hydrogels containing acrylic acid, acrylamide, chitosan, and carbon nanotubes (CNTs) into the gaps of NPs to form an inverse opal PC, a structural color hydrogel that can quickly respond to different stimuli, including strain and temperature, is obtained. Moreover, with the addition of CNTs, the composite PC hydrogel can also output an electronic signal together with optical color changes. Based on these extraordinary responsive behaviors, the PC hydrogel sensor for quantitative feedback to external stimuli of stretching, bending, pressing, and thermal stimuli, with brilliant color change and electronic signal outputs simultaneously, is demonstrated. This fast-assembled PC hydrogel with excellent responsive properties has great potential for applications in wearable devices, mechanical sensors, temperature sensors, and colorimetric displays.

Preparation and Characterization of Black Titanium by Chemical Reduction of TiO2 and its Photocatalytic Activity

Background: Monitoring of the chemical synthesis of black titanium. Objective: In this study, we prepared a black titanium nanomaterial by chemical reduction (NaBH4 treatment). Control of the black TiO2 nanomaterial synthesis followed by a thermal analysis from 100°C to 400°C under azote atmosphere is presented. We used a commercial dye, Reactive Bezactiv Yellow (RBY) degradation, to examine the photocatalytic activity of the black titanium. Method: The thermal analysis of WT and a mixture of treated TiO2(WT+NaBH4) was examined by thermogravimetric analysis (TGA). The obtained nanoparticle is analyzed by X-Ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), UV–visible spectrophotometry, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Results: A deformation of the crystalline lattice is extended beyond the entire visible spectrum. The thermal property reveals that the black titanium is more stable than the white titanium, and BT indicated a more photocatalytic performance than WT. Conclusion: We have successfully synthesized black titanium via chemical reduction employing a synthesis of white titanium. The thermal analysis reveals that BT has a high resistance than WT that offers a promising opportunity for several photocatalytic applications.

MoS2-based absorbers with whole visible spectrum coverage and high efficiency

To design highly efficient and broadband nanometer-sized absorbers based on the atomically thin transition metal dichalcogenides (TMDCs), we propose utilizing inclined gold gratings on MoS2 monolayer. In the case of gold gratings with zero inclination, coverage of the absorption spectrum in the entire visible range occurs between the values of 42% to 73%. Considerable increase in the absorbed light occurs by introducing 13 nm inclination to the gold gratings with equal values of the grating’s period and width as 60 nm. With the application of this grating, maximum absorption of 88% is reached and the absorption bandwidth covers the entire visible spectrum with only 12% variation of the absorption value relative to this maximum (88%). Footprints of resonant excitation of two different modes in the absorber structure are evident: the named “reflection” mode and localized surface plasmons (LSPs). Inclination of the gratings leads the LSP modes to slide toward the MoS2 and causes a remarkable increment in the absorption efficiency. An impressive absorption value of 56% in MoS2 monolayer is gained by the gold grating’s inclination of 17 nm. The designed absorber paves a new way in designing TMDC-based absorbers with extended bandwidths and higher efficiencies.

Design and analysis of a miniaturized UWB plasmonic absorber in visible light spectrum

This paper proposes a miniaturized UWB resonating THz unit-cell absorber in the visible light spectrum 400–700 THz with an average absorption of more than 95%. This novel structure is tuned to desired photonic spectrum by plasmonic electric and magnetic resonance with the suitable choice of the metal-insulator-metal stacked layers. The arrangement has a metal of thickness 15 nm on top and bottom layers constituted on a SiO2 substrate of 50 nm thickness. The stronger energy concentration due to plasmonic resonance provides higher absorbing efficiency. In this work, an investigation on absorption, S11, and surface impedance for various metals like Chromium (Cr), Gold (Au), Magnesium (Mg), Platinum (Pt), and Titanium (Ti) is presented. It is identified that Chromium (Cr) presents stable and maximum absorption in the entire visible spectrum. A study on different incident and polarization angles is also reported for TE and TM modes. Lumped equivalent models (T and π) for the proposed absorber are also presented. This structure can be utilized to harvest photonic energy for evoking low-power enabled IoT devices and sensors in biomedical applications.

Identification of dyes and matrices for dye doped polymer waveguide emitters covering the visible spectrum

Polymer based photonic devices offer the possibility cost effective roll-to-roll manufacture of photonic devices. The incorporation of luminescent dopants within a solid polymer waveguide allows for the generation of light within the device avoiding tedious mechanical light coupling. However, when a dopant is embedded in a solid matrix, depending on its concentration and the nature of materials involved, the emitted light may be quenched due to aggregation effects. In this work, thin films and ridge waveguides processed by UV-photolithography have been successfully obtained from a selection of standard photopolymerizable organic monomers, SU8, EpoCore and OrmoStamp doped with a selection of standard dyes like Rhodamine-B, Coumarin-540A and Pyrromethene-580. All structures were manufactured on glass substrates. An analysis of the solubility and optical properties including band gap energy, absorption coefficient (alpha ) and fluorescence of the doped photoresists at different concentrations has been performed. Photoresists doped with Rhodamine-B shows a higher energy of indirect allowed band gap transition (2.04–2.09 eV) compared to previously reported pure Rhodamine-B thin films (1.95–1.98 eV). Fabrication protocols of dye doped photoresists covering the entire visible spectrum is established.

Wide-angle broadband polarization independent bend-able nano-meta absorber employed in optical wavelength

A new wide-angle, bend-able, three-layer nano-meta-absorber (NMA) unit cell structure, consisting of tungsten (W)-silicon dioxide (SiO2)-tungsten (W) for the entire visible spectrum (445 nm–780nm) has been proposed. The NMA has an average absorption of 97.58% and peak absorption of 99.99% has been achieved at 581.3194 nm under a normal incidence angle. This unit cell shows a good impedance match, good capacitive and inductive coupling along with plasmonic resonance features, as well as high electromagnetic (EM) fields with power loss distribution. The design was numerically simulated by the finite integration technique (FIT) and validated with the finite element method (FEM). The proposed NMA can be implemented as a solar energy harvester, optical sensor, light detector, or imaging application.

Luminescence of Reichardt's dye in polyelectrolyte-modified saponite colloids

Reichardt’s dye (RD) exhibits solvatochromic properties across the entire visible spectrum. This work reports, for the first time, luminescence of RD in colloidal systems of layered silicate saponite (Sap) modified with poly(diallyldimethylammonium) polycations (PDDA), and provides an interpretation of this unique phenomenon. The application of tailored theoretical chemistry methods such as DFT and other approaches supplies a theoretical interpretation of the phenomenon to supplement the results of the experiments. RD luminescence was achieved by preventing the movement of molecules and suppressing non-radiative relaxation processes. This effect only occurred if the negatively-charged phenolate group in the RD molecule interacted with the positively-charged groups of the PDDA cations, while at the same time the positively-charged pyridinium group was also adsorbed onto the negatively-charged Sap siloxane surface. Only at small coverage of the Sap surface by PDDA cations is there sufficient fixation of the RD molecules for them to exhibit luminescent properties. The results suggest that the application of this dye may be extended to luminescent probes, which could be used to characterize the distribution of charge centers on the surfaces of various materials, biopolymers, biological tissues, etc.

Broadband absorption enhancement of metal-dielectric structures in the optical range and their application in solar energy storage

In this work, scattering properties of a metal-dielectric structure are carried out in the optical range. In the proposed structure design, the absorption coefficient reaches more than 80% and has an almost constant character in the entire visible spectrum. Based on the obtained simulation results, such structures can be utilized in the field of solar energy storage applications. A review of works in the field of plasmonic solar cells is given, where the efficiency is usually increased due to the confinement of incident radiation in the active region of photoelectric conversion using localized modes, which are concentrated at the surface of conducting nanoparticles. That is, such batteries are operated in narrow wavelength intervals. However, in this article, the assumed solar cell efficiency enhancement can be achieved by incorporating the entire optical range due to the proposed metamaterial structure.

Quadrupolar Ultrafast Charge Transfer in Diaminoazobenzene-Bridged Perylenediimide Triads.

Strong push-pull interactions between electron donor, diaminoazobenzene (azo), and an electron acceptor, perylenediimide (PDI), entities in the newly synthesized A-D-A type triads (A=electron acceptor and D=electron donor) and the corresponding A-D dyads are shown to reveal wide-band absorption covering the entire visible spectrum. Electrochemical studies revealed the facile reduction of PDI and relatively easier oxidation of diaminoazobenzene in the dyads and triads. Charge transfer reversal using fluorescence-spectroelectrochemistry wherein the PDI fluorescence recovery upon one-electron oxidation, deterring the charge-transfer interactions, was possible to accomplish. The charge transfer state density difference and the frontier orbitals from the DFT calculations established the electron-deficient PDI to be an electron acceptor and diaminoazobenzene to be an electron donor resulting in energetically closely positioned PDIδ- -Azoδ+ -PDIδ- quadrupolar charge-transfer states in the case of triads and Azoδ+ -PDIδ- dipolar charge-transfer states in the case of dyads. Subsequent femtosecond transient absorption spectral studies unequivocally proved the occurrence of excited-state charge transfer in these dyads and triads in benzonitrile wherein the calculated forward charge transfer rate constants, kf , were limited to instrument response factor, meaning >1012 s-1 revealing the occurrence of ultrafast photo-events. The charge recombination rate constant, kr , was found to depend on the type of donor-acceptor conjugates, that is, it was possible to establish faster kr in the case of triads (∼1011 s-1 ) compared to dyads (∼1010 s-1 ). Modulating both ground and excited-state properties of PDI with the help of strong quadrupolar and dipolar charge transfer and witnessing ultrafast charge transfer events in the studied triads and dyads is borne out from the present study.

Achieving transmissive-to-black chromism via engineered dual electro-thermoresponsive single-molecule for full modulation of solar transmittance

Pursuit of complete and null transmissive optical material in smart windows for full control of the intensity and spectrum of incoming solar light has led to the advancement of transmissive-to-black chromism. While most reported transmissive-to-black devices are electrically stimulated, we report here an alternative approach based on dual stimulation of a new dual responsive molecule. By integrating asymmetric viologen onto a newly engineered poly(2-isopropyl-2-oxazoline)-based molecule, one can achieve transmissive-to-black chromism. The synergistic effect of its thermochromic and electrochromic response when stimulated simultaneously shows full absorption of the entire visible solar spectrum resulting to a truly black state that is ideal for building facades requiring absolute privacy. Autonomous chromic switchability function is also possible by independently stimulating the material either electrically or thermally where it displays unique colored state of vibrant magenta and a hazy state, respectively. Tunability of the material’s overall performance in terms of optical contrast, switching kinetics, coloration efficiency and cyclic stability based on the lattice water concentration was elucidated in this work. Furthermore, the fabricated device displays self-bleaching property and has a short-term “memory” effect that may broaden its potential for other applications.

Tuning optical properties of CsPbBr3 by mixing Nd3+ trivalent lanthanide halide cations for blue light emitting devices

In recent years, significant progress has been made in the red and green perovskite quantum dots (PQDs) based light-emitting devices. However, a scarcity of blue-emitting devices that are extremely efficient precludes their research and development for optoelectronic applications. Taking advantage of tunable bandgaps of PQDs over the entire visible spectrum, herein we tune optical properties of CSPbBr3 by mixing Nd3+ trivalent lanthanide halide cations for blue light-emitting devices. The CsPbBr3 PQDs doped with Nd3+ trivalent lanthanide halide cations emitted strong photoemission from green into the blue region. By adjusting their doping concentration, a tunable wavelength from (515 nm) to (450 nm) was achieved with FWHM from (37.83 nm) to (16.6 nm). We simultaneously observed PL linewidth broadening thermal quenching of PL and the blue shift of the optical bandgap from temperature-dependent PL studies. The Nd3+ cations into CsPbBr3 PQDs more efficiently reduced non-radiative recombination. As a result of the efficient removal of defects from PQDs, the photoluminescence quantum yield (PLQY) has been significantly increased to 91% in the blue-emitting region. Significantly, Nd3+ PQDs exhibit excellent long-term stability against the external environment, including water, temperature, and ultraviolet light irradiation. Moreover, we successfully transformed Nd3+ doped PQDs into highly fluorescent nanocomposites. Incorporating these findings, we fabricate and test a stable blue light-emitting LED with EL emission at (462 nm), (475 nm), and successfully produce white light emission from Nd3+ doped nanocomposites with a CIE at (0.32, 0.34), respectively. The findings imply that low-cost Nd3+ doped perovskites may be attractive as light converters in LCDs with a broad color gamut.

Electrotunable 180° achromatic linear polarization rotator based on a dual-frequency liquid crystal.

Linear polarization rotators have been widely used in optical systems. Commonly used polarization rotators are still beset by strong dispersion and thus restricted spectral bandwidth of operation. This leads to the development of achromatic or broadband alternatives, but most of them incorporate multiple waveplates for retardation compensation, which comes at the cost of increased complexity and reduced flexibility in operation and system design. Here, we demonstrate a single-element achromatic polarization rotator based on a thin film of dual-frequency chiral liquid crystal. The angle of polarization rotation is electrically tunable from 0° to 180° with low dispersion (±3°) in the entire visible spectrum, and a high degree of linear polarization (>95%) at the output.

Antireflective and nanocolumnar-shaped Mn:ZnO films grown by chemical bath deposition

In the present study, intriguing nanocolumnar-shaped Mn-doped ZnO (Mn:ZnO) films have been prepared by a simple and scalable two-step chemical bath deposition technique. Firstly, the ZnO nano-seeds were deposited on a glass substrate, and in the second step, the hexagonal pyramidal-shaped Mn:ZnO arrays were grown regularly as well as vertically based on the ZnO seeds. The optical properties of the nanocolumnar-shaped Mn:ZnO films have been investigated in detail. Interestingly, the reflectance measurements demonstrate that the nanocolumnar-shaped Mn:ZnO films are an excellent candidate for antireflection coatings, showing a significantly low reflectance over the entire visible spectrum. The inverse hexagonal pyramidal-shaped Mn:ZnO films with antireflection and photon-trapping properties have great potential for their integration into nanoscale devices, especially thin-film solar cells.

White light emission of wide‐bandgap silicon carbide: A review

AbstractWhite light‐emitting diodes (LEDs) are the most promising alternative to the conventional lighting sources due to their high efficacy and energy saving in illumination. Silicon carbide (SiC) has a wide optical bandgap and could be tailored to emit light at different wavelengths across the entire visible spectrum by introducing different dopants. Donor and acceptor (DA) co‐doped fluorescent SiC (f‐SiC) is a potential candidate for replacing phosphor material in white LEDs, as it has been observed as a good wavelength converter overcoming the disadvantages of rare earth‐containing phosphors, such as poor color‐rendering index (CRI), short lifetime, and short degradation time. The current study attempts to present an overview on the available approaches to fabricate f‐SiC for generating the white light emission and challenges in fundamental research issues to enhance quantum efficiency, color rendering performance, stability, reproducibility of color quality, and lifetime of f‐SiC.