Electro‐optical Materials: Switching of Electrically Responsive,Light‐Sensitive Colloidal Suspensions of Anisotropic Pigment Particles (Adv. Funct. Mater. 3/2011)

An unusual electro‐optical behavior of colloidal suspensions of dichroic, elongated (rod‐shaped) pigment particles is reported. These suspensions exhibit nematic liquid crystal order at low volume fraction of the suspended particles (<15 wt%) and show a strong electric and optical response to an external electric field. Additionally, the characteristics of the optical response can be reversibly manipulated by illuminating the sample with light in its absorption band. The suspensions show a number of interesting phenomena like homeotropic‐planar orientational transitions and light‐induced pattern formation.     

LaPO4 Mineral Liquid Crystalline Suspensions with Outstanding Colloidal Stability for Electro‐Optical Applications

Mineral liquid crystals are materials in which mineral's intrinsic properties are combined with the self‐organization behavior of colloids. However, the use of such a system for practical application, such as optical switching, has rarely been demonstrated due to the fundamental drawbacks of colloidal systems such as limited dispersion stability. Studying colloidal suspensions of LaPO₄ nanorods, it is found that drastic improvement of colloidal stability can be obtained through a transfer of particles from water towards ethylene glycol, thus enabling the investigation of liquid crystalline properties of these concentrated suspensions. Using polarization microscopy and small‐angle x‐ray scattering (SAXS), self‐organization into nematic and columnar mesophases is observed enabling the determination of the whole phase diagram as a function of ionic strength and rod volume fraction. When an external alternative electric field is applied, a very efficient orientation of the nanorods in the liquid‐crystalline suspension is obtained, which is associated with a significant optical birefringence. These properties, combined with the high colloidal stability, are promising for the use of such high transparent and athermal material in electro‐optical devices.     

Colloidal Suspensions of Nanometer‐Sized Mesoporous Silica

Nanometer‐sized surfactant‐templated materials are prepared in the form of stable suspensions of colloidal mesoporous silica (CMS) consisting of discrete, nonaggregated particles with dimensions smaller than 200 nm. A high‐yield synthesis procedure is reported based on a cationic surfactant and low water content that additionally enables the adjustment of the size range of the individual particles between 50 and 100 nm. Particularly, the use of the base triethanolamine (TEA) and the specific reaction conditions result in long‐lived suspensions. Dynamic light scattering reveals narrow particle size distributions in these suspensions. Smooth spherical particles with pores growing from the center to the periphery are observed by using transmission electron microscopy, suggesting a seed‐growth mechanism. The template molecules could be extracted from the nanoscale mesoporous particles via sonication in acidic media. The resulting nanoparticles give rise to type IV adsorption isotherms revealing typical mesopores and additional textural porosity. High surface areas of over 1000 m² g^–1 and large pore volumes of up to 1 mL g^–1 are obtained for these extracted samples.     

Directed Electric Field Z‐Alignment Kinetics of Anisotropic Nanoparticles for Enhanced Ionic Conductivity

In this study, the fast transient evolution of the electric field assisted thickness Z‐direction orientation and assembly of clay particles is studied using a instrumented real time system that simultaneously measures in‐plane and out of plane birefringence. The optical anisotropy master curves are developed, connecting the exposure time and electric field strength with orientation, using a superposition principle. Z‐oriented nanocomposite films manufactured through the R2R process show enhancement through thickness ionic conductivity, useful for membranes of batteries and fuel cells.     

Femtochemistry of Guest Molecules Hosted in Colloidal Zeolites

The ultrafast deprotonation of 2‐(2′‐hydroxyphenyl)benzothiazole (HBT) hosted in nanometer‐sized FAU and MFI zeolites is reported. Samples are prepared via in‐situ incorporation of HBT in the precursor colloidal solutions resulting in the formation of nanometer‐sized zeolites under hydrothermal treatment. The diameter of the zeolite particles formed in the crystalline suspensions is determined by dynamic light scattering and high‐resolution transmission microscopy to lie in the range 40–100 nm. It is shown that the HBT loading does not influence the degree of the zeolite crystallinity but does change the size and the morphology of the individual zeolite nanoparticles. Colloidal suspensions containing the crystalline nanoparticles are well suited for optical investigations since they are sufficiently transparent and clear. The photochemical properties of the HBT guest in the zeolite‐host systems are studied with femtosecond transient transmission spectroscopy. Depending on the acid–base properties either the enol or the keto tautomer of HBT is found to be hosted in the internal voids of the zeolites; upon UV excitation, the HBT‐keto tautomer is converted to the enol form in both MFI‐ and FAU‐type hosts. The HBT photoconversion takes place via an ultrafast deprotonation within 1.5 ps as detected by femtosecond transient absorption spectroscopy.     

Graphene Heterostructure Integrated Optical Fiber Bragg Grating for Light Motion Tracking and Ultrabroadband Photodetection from 400 nm to 10.768 µm

Integrated photonics and optoelectronics devices based on graphene and related 2D materials are at the core of the future industrial revolution, facilitating compact and flexible nanophotonic devices. Tracking and detecting the motion of broadband light in millimeter to nanometer scale is an unfold science which has not been fully explored. In this work, tracking and detecting the motion of light (millimeter precision) is first demonstrated by integrating graphene with an optical fiber Bragg grating device (graphene‐FBG). When the incident light moves toward and away from the graphene‐FBG device, the Bragg wavelength red‐shifts and blue‐shifts, indicating its light motion tracking ability. Such light tracking capability can be further extended to an ultrabroad wavelength range as all‐optical photodetectors show the robust response from 400 nm to 10.768 µm with a linear optical response. Interestingly, it is found that graphene‐Bi₂Te₃ heterostructure on FBG shows 87% higher photoresponse than graphene‐FBG at both visible and telecom wavelengths, due to stronger phonon‐electron coupling and photo‐thermal conversion in the heterostructure. The device also shows superior stability even after 100 d. This work may open up amazing integrated nanophotonics applications such as astrophysics, optical communication, optical computing, optical logic gating, spectroscopy, and laser biology.     

Enhanced Photoluminescence of Oxygen Sensing Films through Doping with High Dielectric Constant Particles

A uniquely simple approach to increase the intensity of the photoluminescence (PL) of dye‐doped sensor films is demonstrated for oxygen sensors, where the sensor film, i.e., Pt or Pd octaethylporphyrin (PtOEP or PdOEP, respectively)‐doped polystyrene, is additionally doped with small‐size particles that have a high dielectric constant, such as 360 nm‐diameter titania (TiO₂) particles. When excited by an organic light emitting device (OLED), the dye PL intensity increases up to ～ 10 fold, depending on the TiO₂ concentration and the excitation source. The enhanced PL is attributed to light scattering by the embedded particles and possibly by voids in the film. The particles scatter the light that excites the PL, increasing the optical path of the exciting light and consequently the absorption of that light and the PL. The particles can also result in an increase in the PL outcoupling, reducing waveguiding to the film edges. The increased PL results in an improved signal‐to‐noise (S/N) ratio in oxygen monitoring, without any deterioration or change in the response time or the long‐term stability of the sensor films. In addition, at a given O₂ level, the dye PL decay time τ increases in the presence of the particles, but is independent of their concentration in the measured range. The improved S/N can improve the analyte limit of detection, allow shortened data acquisition times, and enable the use of low‐intensity excitation sources to minimize potential dye photobleaching. In particular, it improves the performance of structurally integrated OLED‐based chemical and biological sensors, which are drawing increasing attention due to their uniquely simple and flexible integration geometry.     

Anisotropic Optical Properties of Semitransparent Coatings of Gold Nanocaps

An ordered array of cap‐shaped gold nanoparticles has been prepared by vapor deposition onto polystyrene nanospheres supported on a glass substrate. The method of fabrication used imparts a significant anisotropy to the geometric and optical properties of the coating. The optical‐absorption properties of these deposits have been measured using UV‐vis spectrometry and simulated using a code based on the discrete dipole approximation. Because the nanocaps are not interconnected, they interact with incident light as individual particles with a plasmon resonance that depends upon wavelength and the polarization vector of the light. The resulting extinction peaks manifest in the upper visible and near‐infrared regions of the electromagnetic spectrum. Surprisingly, varying the angle of incidence of the light (for a fixed polarization) has no effect on the optical properties of individual nanocaps. Calculations show that these phenomena may be readily interpreted in terms of dipole resonances excited across the longitudinal, transverse, and short‐transverse directions of the nanocaps. Coatings comprised of arrays of these particles have the potential to serve as angularly and spectrally selective filters.     

Response of the Internal Electric Field in CdZnTe to Illumination at Multiple Optical Powers

Manipulation of CdZnTe (CZT) crystals using illumination is a useful tool for altering the internal electric field present under normal bias conditions. The interactions with carriers that are trapped at either terminal are visualized by the electric field distribution through polarization. In this report, we demonstrate an ability to selectively manipulate the internal electric field of CZT using multiple-wavelength light illumination at various optical powers. The internal electric field polarization can be controlled using changes in optical power. We also investigate the electric field distributions using multiple optical powers to examine the light response as a function of light penetration depth.     

Self‐Assembly of Tunable Nanocrystal Superlattices Using Poly‐(NIPAM) Spacers

Understanding and controlling 3D nanocrystal self‐assembly is a fundamental challenge in materials science. Assembly enables the unique optical and electronic properties of nanocrystals to be exploited in macroscopic materials, and also opens up the possibility to couple the optical response of nanocrystals to the optical modes of the superlattice. To date, assembly of such nanocrystal superlattices (NCSL) has focussed on fixed, close packed structures with particle separations of just 1–3 nm. To achieve highly crystalline structures with tunable optical response, the nanocrystal interparticle separation needs to be precise and easily variable but >50 nm. Here, we show the preparation of nanocrystal superlattices with spacings of 50–500 nm assembled from gold‐poly‐N‐isopropylacrylamide core‐shell particles and the characterization of their fascinating diffraction behavior by means of UV‐vis spectroscopy. These nanocrystal superlattices exhibit pronounced diffraction in the visible (440‐560 nm) with peak half‐widths of the order of 10 nm. The position of the Bragg peak is simply tuned by adjusting the particle volume fraction. Due to the thermoresponsive nature of the polymer shell, temperature is used to initiate crystallization or melting of the superlattice. Heating and cooling cycles cause highly reversible melting/recrystallization in less than a minute.     

Holographic Recording and Hierarchical Surface Patterning on Periodic Submicrometer Pillar Arrays of Azo Molecular Glass via Polarized Light Irradiation

Periodic submiocrometer pillar arrays are fabricated from a photoresponsive azo molecular glass (IA‐Chol) by soft‐lithographic hot embossing with elastomeric poly(dimethylsiloxane) (PDMS) modes. Through deformation of each submicrometer‐sized pillar in response to the local amplitude and polarization of the superimposed electric waves, optical holograms are recorded on the IA‐Chol pillar arrays. When the interference pattern is formed by two polarized beams with opposite‐circular polarizations, the recorded patterns mainly reflect the polarization state variations with spatial phase difference of the interfering waves. When two plane waves with the same linear polarizations are superimposed, where the polarization direction is almost the same as the writing beams, the intensity variation of the superimposed electric waves is recorded by the pillar arrays changing spatially with the phase variations. Various ordered surface patterns with distinct hierarchical configurations are successfully developed by the intensity and polarization modulations of the interfering waves. This approach not only allows to directly visualize the intensity and polarization of the coherent light captured by the holograms, but also provides a powerful platform to fabricate various complex surface patterns. The submicrometer pillar arrays can also be used to record polarization hologram and the images are reconstructed by reference light in diffracted spots.     

A Single‐Step Dual Stabilization of Smart Window by the Formation of Liquid Crystal Physical Gels and the Construction of Liquid Crystal Chambers

Smart windows are very attractive because they not only provide comfortable indoor conditions for cars and buildings, but also protect privacy. However, current smart windows have problems such as high energy consumption, slow response time, and poor stability. To solve these problems, a single‐step dual stabilization (SSDS) is newly proposed for the fabrication of robust liquid crystal (LC) smart windows switching fast at low voltage. Upon irradiating ultraviolet light on the selected area of the nematic (N) LC optical cell with photoisomerizable macrogelators (B3AZ) and photopolymerizable monomers, NLC physical gels (LCPGs) and partition walls are simultaneously constructed. LCPGs play a role of light shutter under a low electric field (9.6 V_pp) which is ten times lower than that of the conventional polymer‐stabilized LC‐based smart windows. Partition walls constructed by the selected area photopolymerization significantly enhance the mechanical stabilities. Based on the experimental results, it is realized that the NLC layer generated near the partition walls makes the LCPGs respond to a low voltage. Robust SSDS smart windows could open new doors for the development of high‐performance smart windows.     

Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light‐Emitting Diodes

In this study, nanoparticles (NPs) of various types and sizes are arranged to enhance both the omnidirectional light harvesting of solar cells and the light extraction of light emitting diodes (LEDs). A graded‐refractive‐index NP stack can minimize reflectance, not only over a broad range of wavelengths but also at different incident angles; the photocurrent of silicon‐based solar cells an also be significantly improved omnidirectionally. In addition, the optical gradient of an NP stack can also enhance the light‐extraction efficiency of LEDs, due to both the graded refractive index and the moderate surface roughness. Large particles having sizes on the same order of the wavelength of the incident light roughen the LED surfaces further and extract light from beyond the critical angle, as supported by three‐dimensional finite‐difference time‐domain simulations. Using this approach, the photoluminescence intensity can be increased by up to sevenfold. The developed technique: arranging sequences of different NPs in graded‐refractive‐index stacks, and considering their ability to scatter light due to their sizes and optical constants, may also significantly improve the performance of various optoelectronic devices.     

Electrofluorescent colloid light switch

A low field electro-optic light switch mechanism is proposed, based on changes in the fluorescence from dye molecules absorbed to sepiolite clay particles. When tagged with acridine orange and suitably oriented in an electric field, sepiolite suspensions give fluorescence effects orders of magnitude greater than those for dye-tagged liquid crystals. Uses of the system are discussed.     

An Autonomous Soft Actuator with Light‐Driven Self‐Sustained Wavelike Oscillation for Phototactic Self‐Locomotion and Power Generation

Mimicking the intelligence of biological organisms in artificial systems to design smart actuators that act autonomously in response to constant environmental stimuli is crucial to the construction of intelligent biomimetic robots and devices, but remains a great challenge. Here, a light‐driven autonomous carbon‐nanotube‐based bimorph actuator is developed through an elaborate structural design. This curled droplet‐shaped actuator can be simply driven by constant white light irradiation, self‐propelled by a light‐mechanical negative feedback loop created by light‐driven actuation, time delay in the photothermal response along the actuator, and good elasticity from the curled structure, performing a continuously self‐oscillating motion in a wavelike fashion, which mimics the human sit‐up motion. Moreover, this autonomous self‐oscillating motion can be further tuned by controlling the intensity and direction of the incident light. The autonomous actuator with continuous wavelike oscillating motion shows immense potential in light‐driven biomimetic soft robots and optical‐energy‐harvesting devices. Furthermore, a self‐locomotive artificial snake with phototaxis is constructed, which autonomously and continuously crawls toward the light source in a wave‐propagating manner under constant light irradiation. This snake can be placed on a substrate made of triboelectric materials to realize continuous electric output when exposed to constant light illumination.     

Liquid Phase Exfoliated Indium Selenide Based Highly Sensitive Photodetectors

Layered semiconductors of the IIIA–VIA group have attracted considerable attention in (opto)electronic applications thanks to their atomically thin structures and their thickness‐dependent optical and electronic properties, which promise ultrafast response and high sensitivity. In particular, 2D indium selenide (InSe) has emerged as a promising candidate for the realization of thin‐film field effect transistors and phototransistors due to its high intrinsic mobility (>10² cm² V^?1 s^?1) and the direct optical transitions in an energy range suitable for visible and near‐infrared light detection. A key requirement for the exploitation of large‐scale (opto)electronic applications relies on the development of low‐cost and industrially relevant 2D material production processes, such as liquid phase exfoliation, combined with the availability of high‐throughput device fabrication methods. Here, a β polymorph of indium selenide (β‐InSe) is exfoliated in isopropanol and spray‐coated InSe‐based photodetectors are demonstrated, exhibiting high responsivity to visible light (maximum value of 274 A W^?1 under blue excitation 455 nm) and fast response time (15 ms). The devices show a gate‐dependent conduction with an n‐channel transistor behavior. Overall, this study establishes that liquid phase exfoliated β‐InSe is a valid candidate for printed high‐performance photodetectors, which is critical for the development of industrial‐scale 2D material‐based optoelectronic devices.     

Growth Control and Optics of Organic Nanoaggregates

Light‐emitting organic nanofibers made of phenyl molecules like para‐hexaphenyl (p‐6P) and grown on muscovite mica form a model system well‐suited for the study of optics in the sub‐wavelength regime. We demonstrate that p‐6P nanofibers can be grown with high control of the morphology of individual nanoaggregates and also of the mutual alignment of aggregates by the use of appropriate growth conditions and substrate surfaces. The nanofibers can be detached from the substrate, thus allowing one to study the optical response under a huge variety of fundamentally different conditions, from individual floating aggregates to dense bunches of interacting aggregates. We show examples of linear and nonlinear optical properties of the blue‐light‐emitting aggregates and mention possible applications in future submicrometer‐sized optoelectronics.     

Solution‐Processed Self‐Powered Transparent Ultraviolet Photodetectors with Ultrafast Response Speed for High‐Performance Communication System

Transparent ultraviolet (UV) photodetectors are an essential component of next‐generation “see‐through” electronics. However, the current photodetectors often suffer from relatively slow response speeds and high driving voltages. Here, all‐solution‐processed UV photodetectors are reported that are facilely prepared from environmentally friendly and abundant materials. The UV photodetectors are composed of a titanium dioxide thin film as the photosensitive layer sandwiched between two different transparent electrodes to form asymmetric Schottky junctions. The photodetector with high optical transparency can operate at zero bias because of spontaneous separation of photogenerated electron–hole pairs by the built‐in electric field. The resulting self‐powered photodetector displays high sensitivity to broadband UV light (200–400 nm). In particular, an ultrafast response speed up to 44 ns is obtained, representing a significant improvement over those of the conventional transparent photodetectors. Moreover, the photodetector has been successfully applied, for the first time, in a UV communication system as the self‐powered signal receiver. This work uniquely combines the features of high optical transparency and self‐power ability into UV photodetectors and would enable a broad range of optoelectronic applications.     

光学表面粒子污染物散射的单层薄膜调控特性

为了降低超精密低损耗光学元件表面粒子污染物的光散射损耗,文中提出通过在光学表面沉积单层薄膜来调控表面场强分布,从而降低散射损耗的方法。理论分析了K9玻璃超光滑光学表面不同厚度单层二氧化硅（SiO₂）和单层二氧化钛（TiO₂）薄膜表面上方半径为100 nm粒子污染物所在处的电场强度,理论分析结果发现,当SiO₂薄膜厚度为137.4 nm,TiO₂薄膜厚度为12.3 nm时,表面粒子污染物所在处的电场强度最小。在此基础上分别计算了光学元件表面沉积厚度为137.4 nm的单层SiO₂薄膜以及厚度为12.3 nm的单层TiO₂薄膜,表面粒子污染物的总散射损耗(S)和双向反射分布函数(BRDF),计算结果表明,在波长为632.8 nm的光垂直入射时,单层SiO₂薄膜和单层TiO₂薄膜可有效降低其表面粒子的BRDF,且可将K9玻璃表面的总散射分别降低12.40%和25.04%。实验验证了单层SiO₂薄膜对于表面粒子污染物散射降低的有效性。     

Selective Enhancement of Inner Tube Photoluminescence in Filled Double‐Walled Carbon Nanotubes

A highly selective enhancement of the optical response of the inner tubes of double‐walled carbon nanotubes has been identified upon transformation of the residual C atoms inside the hollow core to linear carbon chains (LCC). By varying the growth conditions and using standardized suspensions, it has been observed that this optical response depends sensitively on the tube diameter and LCC growth yield. It is reported how the formation of LCC by postsynthesis annealing at 1400 °C leads to an increase of the photoluminescence (PL) signal of the inner tubes up to a factor of 6 for tubes with (8,3) chirality. This behavior can be attributed to a local charge transfer from the inner tubes to the carbon chains, counterbalancing quenching mechanisms induced by the outer tubes. These findings provide a viable pathway to enhance the low PL quantum yield of double‐walled carbon nanotubes and proof the capability of inner tubes to exhibit photoluminescence.