Tunable Response Research Articles

Spectrally Selective Full-Color Single-Component Organic Photodetectors Based on Donor-Acceptor Conjugated Molecules.

Photodetectors based on organic materials are attractive due to their tunable spectral response and biocompatibility, meaning that they are a promising platform for an artificial human eye. To mimic the photoelectric response of the human eye, narrowband spectrally-selective organic photodetectors are in great demand, and single-component organic photodetectors based on donor-acceptor conjugated molecules are a noteworthy candidate. In this work, we present single-component selective full-color organic photodetectors based on donor-acceptor conjugated molecules synthetized to mimic the spectral response of the cones and rods of a human eye. The photodetectors demonstrated a high responsivity (up to 70 mA/W) with a response time of less than 1 µs, which is three orders of magnitude faster than that of human eye photoreceptors. Our results demonstrate the possibility of the creation of an artificial eye or photoactive eye "prostheses".

Electrothermal and water dual-mode-responsive flexible polydiacetylene chiroptical films for multiplex laser displays

A composite film with remarkable optical activity and exceptional mechanical deformation ability was fabricated through the twisted stacking strategy, which can be applied in laser display and anti-counterfeiting fields.

Admirable stability achieved by ns2 ions Co-doping for all-inorganic metal halides towards optical anti-counterfeiting.

Optical materials play a momentous role in anti-counterfeiting field, such as authentication, currency and security. The development of tunable optical properties and optical responses to a range of external stimuli is quite imperative for the growing demand of optical anti-counterfeiting technology. Metal halide perovskites have attracted much attention of researchers due to their excellent optical properties. In addition, co-doping methods have been gradually applied to the research of metal halide perovskites, by which more abundant luminescence phenomena can be introduced into the host perovskite. Herein, the ns2 ions of bismuth (Bi3+) and antimony (Sb3+) ions co-doped zero-dimensional Cs2SnCl6 metal halide with an excitation-wavelength-dependent emission phenomenon is synthesized as an efficient multimodal luminescent material, the luminescence of which is tunable and covers a wide region of color. What's more, a dynamic dual-emission phenomenon is captured when the excitation wavelength changes from 320 nm to 420 nm for Cs2SnCl6:Bi0.08Sb0.12 crystals. Moreover, the Bi3+ and Sb3+ doped metal halide material shows great enhancement in solvent resistance and thermal stability compared to the pristine Cs2SnCl6. The admirable stability and distinguishable photoluminescence (PL) phenomenon of this all-inorganic metal halide has great potential to be applied in optical anti-counterfeiting technology. Furthermore, the co-doping method can accelerate the discovery of new luminescence phenomena in original metal halide perovskites.

Facile aqueous synthesis of hollow dual plasmonic hetero-nanostructures with tunable optical responses through nanoscale Kirkendall effects.

Herein, we report the colloidal synthesis of hollow dual-plasmonic nanoparticles (NPs) using Au@Cu2O core-shell NPs as templates and exploiting the nanoscale Kirkendall effect. In our synthesis, we used organic compounds as a source of chalcogenide ions for an anion exchange reaction at elevated temperatures using polyvinylpyrrolidone (PVP) as a capping reagent to transform the solid Cu2O shell into a hollow copper chalcogenide shell. The resulting structures possess different features depending on the chalcogenide precursor employed. TEM images confirm the complete transformation of Au@Cu2O templates when 1,1-dimethyl-2-selenourea was added and the formation of hollow Au@Cu2-x Se nanostructures. In contrast, residues of Cu2O attached to the Au core were present when thioacetamide was used for the synthesis of Au@Cu2-x S with all other conditions kept the same. The divergence of architectures caused distinct optical properties of Au@Cu2-x S and Au@Cu2-x Se NPs. This synthetic approach is an effective pathway for maneuvering the size of interior voids by varying the concentration of chalcogenide ions in the reaction mixture. The insights gained from this work will enrich the synthetic toolbox at the nanoscale and guide us on the rational design of multicomponent plasmonic nanoparticles with precisely controlled hollow interiors and sophisticated geometries, further enhancing our capabilities to fine-tune the electronic, optical, compositional, and physicochemical properties.

Facile preparation of fluorescent non-conjugated polymer films with tunable multicolor photoluminescencevialayer-by-layer assembly

Fluorescent non-conjugated polymer films of poly(acrylic acid)/polyethylenimine are prepared by the layer-by-layer assembly, and these films exhibit tunable multicolor photoluminescence and sensitive response to metal ions.

A graphene-like BeS monolayer as a promising gas sensor material with strain and electric field induced tunable response: a first-principles study.

A comprehensive investigation of the gas sensing potential of BeS monolayer has been conducted using DFT calculations. Twelve common pollutant gases: NH3, NO2, NO, CO, CO2, CH4, H2, O2, N2, H2S, H2O and SO2, have been studied. Our analysis reveals defect states in the band structure near the Fermi level and strong hybridization between gas molecule orbitals and the BeS monolayer. We observe higher adsorption energies for NH3 and CO compared to other popular gas sensing materials. The optical properties of CO2 and NO2 adsorbed on the BeS monolayer show increased reflectivity and absorption coefficient in the UV and far infrared region. Tensile strain has minimal impact on adsorption energy, while biaxial compressive strains enhance the gas sensing capability of the BeS monolayer. The application of an electric field offers control over gas adsorption and desorption. We propose the BeS monolayer as a promising candidate for future gas molecule sensing applications due to its high adsorption energy, rapid recovery time, and distinct optical properties.

Patterned Graphene-Based Metamaterials for Terahertz Wave Absorption

Graphene-based metamaterials have been widely applied in optoelectronic devices, optical modulators, and chemical sensors due to the outstanding tunability and optical response in the terahertz (THz) region. Here, tunable THz metamaterial absorbers based on patterned graphene are designed, fabricated, and modulated. The proposed metamaterial absorbers are constructed by the top layer of patterned graphene arrays and the aluminum (Al) film separated by polyimide (PI). The different THz absorption spectra can be acquired by changing the patterns of graphene. In order to verify the simulation results, a series of tests were conducted by THz time-domain spectrometer (THz-TDS) systems. The proposed absorbers are able to be insensitive to the angle of the incident wave. Besides, chemical doping is applied to turn the Fermi level of graphene and the absorption performance is promoted with the increase of the Fermi level. The experimental results have been demonstrated to have associated resonant peaks with the simulation results. The aim of this paper is to exhibit a systematic study on graphene-based THz metamaterial absorbers, including the simulation and experiments. By comparing the simulation and experimental results, it is useful to clarify the relevant theories and manufacturing processes. The work will provide a further step in the development of high-performance terahertz devices, including tunable absorbers, sensors, and electro-optic switches.

Shape-Programmable Liquid Metal Fibers

Conductive and stretchable fibers are the cornerstone of intelligent textiles and imperceptible electronics. Among existing fiber conductors, gallium-based liquid metals (LMs) featuring high conductivity, fluidity, and self-healing are excellent candidates for highly stretchable fibers with sensing, actuation, power generation, and interconnection functionalities. However, current LM fibers fabricated by direct injection or surface coating have a limitation in shape programmability. This hinders their applications in functional fibers with tunable electromechanical response and miniaturization. Here, we reported a simple and efficient method to create shape-programmable LM fibers using the phase transition of gallium. Gallium metal wires in the solid state can be easily shaped into a 3D helical structure, and the structure can be preserved after coating the wire with polyurethane and liquifying the metal. The 3D helical LM fiber offered enhanced stretchability with a high breaking strain of 1273% and showed invariable conductance over 283% strain. Moreover, we can reduce the fiber diameter by stretching the fiber during the solidification of polyurethane. We also demonstrated applications of the programmed fibers in self-powered strain sensing, heart rate monitoring, airflow, and humidity sensing. This work provided simple and facile ways toward functional LM fibers, which may facilitate the broad applications of LM fibers in e-skins, wearable computation, soft robots, and smart fabrics.

Dual thermo-responsive multifunctional ionic conductive hydrogel by salt modulation strategy for multilevel encryption and visual monitoring

Despite the current progress in thermo-responsive materials, it is still a significant challenge to develop superior multiple temperature-sensitive ionic conductive hydrogels with programmable regulation capability for interactive wearable iontronics. Here, a dual thermo-responsive ionic conductive hydrogel (DTICH) is constructed based on a salt modulation strategy, which cleverly integrates low-temperature sensitive sodium dodecyl sulfate (SDS) and high-temperature sensitive poly(N-isopropylacrylamide) PNIPAm-based polymer. Owing to the common ion effect and salting-out effect, the resultant DTICH hydrogel visually presents wide and tunable dual temperature response behaviors of 0-18℃ (Tk) and 37-80℃ (LCST) by controlling NaCl concentration for the first time. Meanwhile, benefiting from the synergy of NaCl and thermo-sensitive materials, the hydrogel is concurrently endowed with excellent stretchability (>900 %), adhesion (adhesion strength >30 kPa), frost resisting (-40℃) and multilevel encryption functions. More importantly, the DTICH hydrogel can ingeniously perceive mechanical force and external temperature via the changes of relative resistance and transparency, which can be applied as human–computer interfaces to interactively monitor human motions and temperatures in real time. Therefore, our study establishes a brand new and effective route for the rational design and fabrication of high-level visual interactive devices and smart wearable iontronics.

Double-Layer Hydrogels with Tunable Mechanofluorochromic Response for Smart Display

Double-Layer Hydrogels with Tunable Mechanofluorochromic Response for Smart Display

Living Cellulose Materials with Tunable Viscoelasticity through Probiotic Proliferation.

Probiotic cellulose (PC), a living material (LM) consisting of probiotics integrated into bacterial cellulose, is the first example where life (probiotic proliferation) is the input to tune the viscoelasticity of the biomaterial. The gradual proliferation of probiotics within the matrix acts as a key modulator of the cellulose viscoelasticity, providing from celluloses with lower-than-matrix viscoelasticity to celluloses with viscoelastic moduli closer to those of elastic solids. This concept is a promising approach to producing living bio-ink with tunable viscoelastic response of special interest for specific applications such as 3D printing. In contrast to the most common hydrogels with stimuli-tunable mechanical properties, which require external stimuli such as mechanical stress, UV radiation, or heat, this living bio-ink only requires time to tune from a fluid-like into a solid-like biomaterial.

Tunable circular polarization responses of twisted black phosphorus metamaterials.

As one of the most significant 2D materials, black phosphorus (BP) offers a promising way to manipulate the polarization state of light due to its in-plane anisotropy, however, reconfigurable polarization manipulation is still challenging in simple BP structure. Here, we propose a multilayer metamaterial with twisted BP nanostructures and numerically study its circular dichroism (CD) and circular birefringence (CB) responses. The dependences of the circular polarization responses in the twisted BP metamaterial have been fully investigated on geometrical and material parameters. The giant tunability enables the twisted BP nanostructure to be attractive for constructing BP-based metamaterials devices, such as polarizers, biosensors and modulators.

Photonic band-edge assisted enhanced nonlinear absorption of carbon encapsulated gold nanostructures in polymeric multilayers

Integration of nonlinear optical materials in nanophotonic structures offers unprecedented opportunities to tailor the light-matter interaction, resulting in tunable optical responses. Periodic dielectric structures are particularly the desired optical platform to incorporate nonlinear materials to enhance their weak nonlinearity for practical applications. In this work, we report the giant enhancement in the nonlinear absorption of a 1-D polymer photonic crystal containing gold-carbon (Au@C) core–shell structure. The Au@C nanostructures synthesized by pulsed laser ablation technique were dispersed in spin-coated alternate layers of photonic crystal comprised of polyvinyl carbazole and cellulose acetate. The long-wavelength photonic band-edge of the polymeric Bragg mirror was designed at 532 nm to enhance the non-resonant nonlinear absorption of the core–shell nanostructure. The resultant nonlinearity is ascribed to slow light propagation at the band-edges and consequent local field confinement of optical pulses. Angle-dependent tunability of photonic band-edge effects and nonlinear transmission characteristics were analyzed experimentally and correlated with numerical simulations. These findings open up new ways to implement cost-effective, highly tunable and low input threshold all-optical devices.

Many Facets of Photonic Crystals: From Optics and Sensors to Energy Storage and Photocatalysis

AbstractThe ability to selectively redirect specific wavelengths of light has attracted a lot attention for photonic crystal materials. Presently, there is a wealth of research relating to the fabrication and application of photonic crystal materials. There are a number of structures that fall into the category of a photonic crystal; 1D, 2D, and 3D ordered structures can qualify as a photonic crystal, provided there exists ordered repeating lattices of dielectric material with a sufficient refractive index contrast. The optical responses of these structures, namely the associated photonic bandgap or stopband, are of particular interest for any application involving light. The sensitivity of the photonic bandgap to changes in lattice size or refractive index composition creates the possibility for accurate optical sensors. Optical phenomena involving reduced group velocity at wavelengths on the edge of the photonic bandgap are commonly exploited for photocatalytic applications. The inherent reflectivity of the photonic bandgap has created applications in optical waveguides or as solar cell reflector layers. There are countless examples of research attempting to exploit these facets of photonic crystal behavior for improved material design. Here, the role of photonic crystals is reviewed across a wide variety of disciplines; cataloging the ways in which these structures have enhanced specific applications. Particular emphasis is placed on providing an understanding of the specific function of the tunable optical response in photonic crystals in relation to their application.

Artificial Visual Systems With Tunable Photoconductivity Based on Organic Molecule‐Nanowire Heterojunctions

AbstractThe visual system, one of the most crucial units of the human perception system, combines the functions of multi‐wavelength signal detection and data processing. Herein, the large‐scale artificial synaptic device arrays based on the organic molecule‐nanowire heterojunctions with tunable photoconductivity are proposed and demonstrated. The organic thin films of p‐type 2,7‐dioctyl[1]benzothieno[3,2‐b][1] benzothiophene (C8‐BTBT) or n‐type phenyl‐C61‐butyric acid methyl ester (PC61BM) are used to wrap the InGaAs nanowire parallel arrays to configure two different type‐I heterojunctions, respectively. Due to the difference in carrier injection, persistent negative photoconductivity (NPC) or positive photoconductivity (PPC) are achieved in these heterojunctions. The irradiation with different wavelengths (solar‐blind to visible ranges) can stimulate the heterojunction devices, effectively mimicking the synaptic behaviors with two different photoconductivities. The long‐term and multi‐state light memory are also realized through synergistic photoelectric modulation. Notably, the arrays with different photoconductivities are adopted to build the hardware kernel for the visual system. Due to the tunable photoconductivity and response to multiple wavelengths, the recognition rate of neural networks can reach 100% with lower complexity and power consumption. Evidently, these photosynaptic devices are illustrated with retina‐like behaviors and capabilities for large‐area integration, which reveals their promising potential for artificial visual systems.

Photothermal and Photoacoustic Response of a VO2@Au Nanoshell Irradiated by a Nanosecond Laser Pulse

Plasmonic nanoparticles (PNPs) are considered as a proper mediator in photothermal therapy due to their capability to efficiently convert the absorbed light energy into localized heat. As the localized heat diffuses during therapy processes, it is crucial to control and detect the temperature in a non-invasive way. Synthesizing a new generation of PNPs that utilize optical phase-change materials leads to a tunable photothermal response without geometrical variation. This tunability originates from the prompt variation of optical and thermal properties during the phase-change transition. In this paper, we numerically study the photothermal response of a VO2@Au nanoshell in an aqueous medium when irradiated by a nanosecond pulsed laser (5 ns). For the temperature profiles, we simultaneously solve a self-consistent multiphysics problem consisting of electromagnetism and thermodynamics. We do our calculations for two wavelengths of 658 and 737 nm where the absorption spectrum of the nanoshell has two extrema during the VO2 core’s phase-change transition. Finally, for the two selected wavelengths, by coupling the structural dynamic physics to the problem, we calculate the acoustic pressure signals generated by the photothermal expansion of both the nanoshell and its surrounding medium. This photoacoustic signal could be considered as a non-invasive method to measure the local temperature in deep tissues accurately.

"Enzyme-Like" Spatially Fixed Polyhydroxyl Microenvironment-Activated Hydrochromic Molecular Switching for Naked Eye Detection of ppm Level Humidity.

The detection and monitoring of ultralow humidity (<100ppm) are critical in many important industries, such as high-tech manufacturing, scientific research, and aerospace. However, the development of ppm level humidity sensors with portability, low cost, and ease of regeneration remains a significant challenge. Herein, an innovative "enzyme-like" construction strategy is proposed to address this problem by employing suitable molecular-level humidity-sensitive units and chemically constructing a multilevel spatial synergistic sensitization microenvironment around it. The as-prepared ultralow humidity-sensitive paper (UHSP) achieved a naked eye recognition humidity of 0.01-100ppm. UHSP not only is simple to prepare, handy and low-cost, but can also be simply and efficiently regenerated as well as recycled many times by skillfully utilizing the "unconventional sublimation" and "lime slaked" of calcium oxide. The molecular reaction mechanisms involved in the humidity response and regeneration of UHSP have been demonstrated in detail. UHSP can provide a promising new method for ultralow humidity detection in the form of portable kits or sirens. The demonstrated "enzyme-like" construction strategy can bring unlimited ideas and implications to the design and development of sensors with tunable response thresholds, particularly high sensitivity.

Dielectric loss reduction and validation of P(VDF-HFP) films via sandwiching film fabrication for capacitive energy storage

Poly(vinylidene fluoride) (PVDF) series polymers have a high dielectric constant (k) among polymer families but a high intrinsic loss tangent disqualifying their capacitor application. In this work, Poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer as the high-k dielectric layer is sandwiched between two low-k polyetherimide (PEI and PEI/BaTiO3) layers to construct the composite films with tunable dielectric responses. By changing the low-k layer thickness in the PEI/P(VDF-HFP)/PEI tri-layer structure, its characteristic of capacitors in series results in the moderate dielectric constant and dramatic reduction of loss tangent to a meaningful level for capacitor design. The loss tangent can be dramatically reduced to below 0.005 at 1 kHz using the thickness ratio of 1:1:1 from about 0.03 for P(VDF-HFP) film alone. The extraordinary dielectric response of the tri-layer structure is also accompanied by more than 2 times the higher energy density than that of the PEI film under the same electric field. The ingenious design of the low-k/high-k/low-k sandwiched polymers synergizes the high-k advantage of P(VDF-HFP) and low tangent loss of the low-k layer as well as their high dielectric strength. This work provides a meaningful and effective method for realizing PVDF-based polymers with reduced loss tangent for actual capacitor application.

Tunable surface plasmon resonance wavelengths response from Au/Ag nanocomposite system

In this study, Au/Ag nanocomposite films with localized surface plasmon resonance (LSPR) tunability were prepared by thermal annealing under ambient conditions. The effects of thermal annealing on the structure, morphology, and optical properties of the samples were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–VIS-NIR double-beam spectroscopy and Raman spectroscopy. The results show that thermal annealing transforms Au/Ag films from continuous to discontinuous porous or gaping nanostructures with red-shifted properties, which greatly increases the sensitivity of surface-enhanced Raman scattering (SERS). In addition, finite-difference time-domain (FDTD) simulations confirm the correctness of the Raman scattering results.

Multiple Effects of an Anionic Cyclodextrin Macrocycle on the Reversible Isomerization of a Photoactive Guest Dye.

Understanding and controlling the reversible isomerization of photoactive molecules in order to obtain a tunable optical response is desirable for many photofunctional applications. This study describes the interesting effects of an anionic cyclodextrin host (sulfated-βCD, SCD) on the photoisomerization and protonation equilibrium of an important hemicyanine dye (trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide, DSP). The SCD host assists in unlocking the photoisomerization potential of DSP by promoting protonation of the dye. It also assists in stabilizing the cis isomer of the protonated dye, thereby significantly delaying the reverse cis to trans isomerization of DSPH+. Furthermore, the interplay of both hydrophobic and electrostatic interactions in the complex formation of SCD with DSPH+ makes the reverse cis to trans isomerization of DSPH+ amenable to influence by the added salt. The stimuli-responsive reversible isomerization of SCD-DSPH+ is an interesting case from the perspective of chemical sensing or light operated functional materials with host-guest systems.