Anisotropic Optical Properties Research Articles

Microstructure-Induced Anisotropic Optical Properties of YF3 Columnar Thin Films Prepared by Glancing Angle Deposition.

Yttrium fluoride (YF3) columnar thin films (CTFs) were fabricated by electron beam evaporation with the glancing angle deposition method. The microstructures and optical properties of YF3 CTFs were studied systematically. The YF3 films grown at different deposition angles are all amorphous. As the deposition angle increases, the columns in YF3 CTFs become increasingly separated and inclined, and the volume fraction of YF3 decreases, resulting in lower refractive indices. This phenomenon is attributed to the self-shadowing effect and limited adatom diffusion. The YF3 CTFs are optically biaxial anisotropic with the long axis (c-axis) parallel to the columns, the short axis (b-axis) perpendicular to the columns, and the other axis (a-axis) parallel to the film interface. The principal refractive index along the b-axis for the 82°-deposited sample is approximately 1.233 at 550 nm. For the 78°-deposited sample, the differences of principal refractive indices between the c-axis and the b-axis and between the a-axis and the b-axis reach the maximum 0.056 and 0.029, respectively. The differences of principal refractive indices were affected by both the deposition angle and the volume fraction of YF3.

First principles calculation of metal (Ni and Cu) contact on the electronic transport properties of 2D GeP semiconductor

Two-dimensional (2D) IV-V semiconductors are shown to possess suitable bandgap, strongly anisotropic electronic and optical properties for nanoelectronic and optoelectronic applications. Motivated by the experimental demonstration of the 2D IV-V semiconductors, we have computationally designed the monolayered GeP nanodevice consisting of nickel (Ni) and copper (Cu) metal contacts, and evaluated their transport properties. Using first principles calculations combined with non-equilibrium Green’s function, we have calculated the current density (J) versus bias (Vb), and the transmission coefficient. Both Ni and Cu metals could form excellent metallization contact with the GeP semiconductor channel, leading to effective carrier injection to the semiconductor and yielding significant improvements on the transport properties. The current is determined by the potential difference at the interface and the left/right electrode density of states (L/RDOS) coupling of the device. These two factors play predominant role at low and moderate biases, respectively. Our simulations have shown that the 2D GeP electronic device could be significantly improved by appropriate metal contact, which can be helpful to the 2D IV-V semiconductors for future electronic applications.

Theoretical study of structural, electronic, and optical properties of mixed chalcogenide-halide bismuth perovskites CH3NH3BiChI2 (Ch = S, Se, Te)

We investigate the structural, electronic, and optical properties of the mixed chalcogenide-halide bismuth perovskites CH3NH3BiChI2 (Ch = S, Se, Te) using the relativistic all-electron calculations. We use the structural model where the Ch atoms are placed at the apical position in the BiCh2I4 octahedron. It is found that the structures of CH3NH3BiChI2 are contracted in the apical direction and the Bi–Ch bond lengths decrease in the order of Ch = S, Se, and Te. CH3NH3BiChI2 exhibit anisotropic optical properties and show higher absorption coefficients than CH3NH3PbI3. Furthermore, employing the spectroscopically limited maximum efficiency as a metric for quantifying the photovoltaic performance, we find that CH3NH3BiSI2 and CH3NH3BiSeI2 are promising candidates for thin-film solar cells.

Multi-Functional Film Based on Aligned CNT Sheet for Liquid Crystal Display Application

Carbon nanotube (CNT), an extremely long hollow cylinder with nanometer-sized diameter, is a promising carbon material with unique thermal, electrical, and physical properties. Due to their peculiar and fascinating properties, CNTs have been studied and used in various industries since their discovery. The CNTs are potentially viable alternatives for conventional materials in electric devices considering their mechanical resilience, high electrical conductivity, and optical transparency. The alignment of CNTs into ordered structures garnered a special interest as it enhances electrical conductivity and optical transparency, and generates a new intriguing anisotropic optical absorption property. The optical properties of highly oriented CNT sheets have been investigated to apply as optical polarizer and polarized light source. Further, the flexible electrodes derived from an aligned CNTs have unveiled applications in super-capacitors, solar cell, and lithium-ion battery. Therefore, to utilize CNTs in LCD and many other flexible electronic device applications it is essential to attain highly ordered CNTs which should be static even under external stresses such as bending, pressing, and strain.In this work, we fabricated the aligned CNT sheet films (a-CSFs) in which the grown CNTs are stretched unidirectionally on the substrate and its orientation is fixed via polymerization of a coated monomer and then measured its optical and mechanical properties, testing feasibility of its application for multi-functioning films such as polarizer, electrode, and liquid crystal (LC) alignment. The fabricated a-CSF exhibits polarization efficiency (PE) (87.23% of 10 layers of a-CSF), sheet resistance (361.7 Ω/sq), and an excellent aligning capability of LC molecules as shown in Fig. 1 (a~c). Besides, flexibility and stability test of a-CSF via repeating bending test of 1,000 cycles at bending radius of 2 mm exhibited very low variation in the change ratio of sheet resistance and transmittance less than 8% and 10%, respectively.Utlilizing the mutifunctionalty of a-CSF, a liquid crystal display (LCD) with a twisted nematic(TN) mode was fabricated and exhibited feasibility of dynamic swicthing between bright and dark states on application of voltage off/on even without using conventional components of LCD such as transparent electrode, film-type polarizer, and polyimide-type alignment layer. We measured the voltage-dependent transmittance of both devices based on the a-CSF with 5 layers and 8 layers. The threshold voltage (Vth) and operating voltage (Vop) are defined as the voltage required to achieve 90% and 10% of transmittance relative to maximum transmittance, respectively. The measured Vth of both devices with 5 and 8 layers are 1.7 Vrms and 1.3 Vrms, respectively and the measured Vop of both devices are 7.6 Vrms and 4.3 Vrms, respectively. In the case of 8 layers, it shows slightly lower Vth and Vop, which is mainly due to the lower sheet resistance of a-CSF with 8 layers than 5 layers as shown Fig 2.In conclusion, we fabricated the multi-functional a-CSF with properties of the electrode, polarizer, and alignment layer based on the a-CSF using the simple fabrication process. We confirmed the a-CSFs exhibit better conductivity and PE in the visible regime. Furthermore, it has the excellent grooved morphology along the drawn direction of a-CS for homogeneous alignment of LC molecules. We carried out the bending test to confirm the stability of the films from mechanical stress. It showed excellent stability after bending tests of 1,000 cycles. The performance of the fabricated TN cells using a-CSFs were considerably working good. Figure 1

First principles prediction of exceptional mechanical and electronic behaviour of Titanite (CaTiSiO5)

Titanite (CaTiSiO5) is a naturally occurring silicate phase, recently recognised as a potential material for immobilization of nuclear wastes, high-end ceramic lining and optical device development. The silicate undergoes a pressure-induced structural transition (P21/c to C2/c) at ~3.5 GPa, which we confirm from density function perturbation theory. Using first principles calculations pressure-dependent structural parameters of both the phases are presented. This article features the mechanical property of C2/c titanite characterized by a negative component (C36 = -16.41 GPa) of the elastic constant tensor (Cij), an enigmatic phenomenon seen not only in low symmetry (monoclinic), but also in the high symmetry system (cubic). We propose rotational kinematics of bonds, controlled by the valence charge distributions as a crucial atomic scale mechanism for structural collapse of the lattice under strain resulting in negative C36. This kinematic model allows us to predict the necessary and sufficient condition for the pressure dependent softening of their shear elastic constants (C44 and C55). The present study also sheds a new light upon the electronic properties of titanite, accounting for the intraband and interband transitions that influence the optical activity. Its anisotropic optical properties of titanite evaluated in the range 0–60 eV are marked by an attractive optical behaviour of this phase. It is transparent in the visible spectra, but showing excellent absorption and reflectivity in the UV region. We thus project titanite as an industrially potential UV shield material. Our theoretical estimate yields the highest value of anisotropic refractive index, 2.21 in [001].

Bideposited silver nanocolloid arrays with strong plasmon-induced birefringence for SERS application

Silver nano-rod, nano-zigzag, nano-saw, and nano-particle arrays are fabricated with glancing angle bideposition. The structure-dependent anisotropic optical properties of those bideposited nanostructured arrays are measured and investigated. The equivalent birefringence values of nano-rod and nano-zigzag arrays are much larger than crystals found in nature and liquid crystal used in display products. The fact that induced localized plasmon-magnetic field between nanorods dominates the strong phase retardation between p-polarized and s-polarized transmitted wave. For the nano-saw, the strong localized electric field induced between the saw teeth leads to strong SERS signals. Although the bideposited nanoparticles own weak morphological anisotropy, strong optical phase retardation is still detected at wavelengths near 400 nm.

Au-Encapsulated Fe Nanorods in Oxide Matrix with Tunable Magneto-Optic Coupling Properties.

Materials with magneto-optic coupling properties are highly coveted for their potential applications ranging from spintronics and optical switches to sensors. In this work, a new, three-phase Au-Fe-La0.5Sr0.5FeO3 (LSFO) hybrid material grown in a vertically aligned nanocomposite (VAN) form has been demonstrated. This three-phase hybrid material combines the strong ferromagnetic properties of Fe and the strong plasmonic properties of Au and the dielectric nature of the LSFO matrix. More interestingly, the immiscible Au and Fe phases form Au-encapsulated Fe nanopillars, embedded in the LSFO matrix. Multifunctionalities including anisotropic optical dielectric properties, plasmonic properties, magnetic anisotropy, and room-temperature magneto-optic Kerr effect coupling are demonstrated. The single-step growth method to grow the immiscible two-metal nanostructures (i.e., Au and Fe) in the complex hybrid material form opens exciting new potential opportunities for future three-phase VAN systems with more versatile metal selections.

Theoretical investigation of the electronic structure and anisotropic optical properties of quasi-1D Sb2Se3 photovoltaic absorber materials

Sb2Se3 is a nontoxic, environmentally friendly photovoltaic absorber material with both a narrow bandgap and high absorption coefficient. The optical properties of Sb2Se3 crystals are theoretically studied herein using first-principles methods. The crystal structure, electronic structure, dielectric function, and absorption coefficient are calculated. The results show that the quasi-one-dimensional structure of Sb2Se3 has an indirect bandgap with a value of 0.953 eV. The bottom of the conduction band and the top of the valence band are mainly formed by Sb 5p and Se 4p hybridized orbitals. For light incident along the [100], [010], and [001] directions, the real and imaginary parts of the dielectric function exhibit clearly anisotropic characteristics. The absorption coefficient is greater than 105 cm−1.

Additional excitonic features and momentum-dark states in ReS2

Unidirectional in-plane structural anisotropy in Rhenium-based transition metal dichalcogenides (TMDs) introduces a new class of 2-D materials, exhibiting anisotropic optical properties. In this work, we perform temperature dependent, polarization-resolved photoluminescence and reflectance measurements on several-layer ReS$_{2}$. We discover two additional excitonic resonances (X$_{3}$ and X$_{4}$), which can be attributed to splitting of spin degenerate states. Strong in-plane oscillator strength of exciton species X$_{1}$ and X$_{2}$ are accompanied by weaker counterparts X$_{3}$ and X$_{4}$ with similar polarization orientations. The in-plane anisotropic dielectric function has been obtained for ReS$_{2}$ which is essential for engineering light matter coupling for polarization sensitive optoelectronic devices. Furthermore, our temperature dependent study revealed the existence of low-lying momentum-forbidden dark states causing an anomalous PL intensity variation at 30 K, which has been elucidated using a rate equation model involving phonon scattering from these states. Our findings of the additional excitonic features and the momentum-dark states can shed light on the true nature of the electronic band structure of ReS$_{2}$.

Highly In‐Plane Anisotropic 2D PdSe2 for Polarized Photodetection with Orientation Selectivity

AbstractPolarized photodetection based on anisotropic two‐dimensional materials display promising prospects for practical application in optical communication and optoelectronic fields. However, most of the reported polarized photodetection are limited by the lack of valid tunable strategy and low linear dichroism ratio. A peculiar noble metal dichalcogenide—PdSe2 with a puckered pentagonal structure and abnormal linear dichroism conversion—potentially removes these restrictions and is demonstrated in this study. Herein, azimuth‐dependent reflectance difference microscopy combined with anisotropic electrical transport measurements indicate strong in‐plane anisotropic optical and electrical properties of two‐dimensional PdSe2. Remarkably, the typical polarization‐resolved photodetection exhibits anisotropic photodetection characteristics with a dichroic ratio up to ≈1.8 at 532 nm and ≈2.2 at 369 nm, and their dominant polarization orientation differs by 90° corresponding to the a‐axis and b‐axis, respectively. The unique orientation selection behavior in polarization‐dependent photodetection can be attributed to the intrinsic linear dichroism conversion. The results make 2D PdSe2 a promising platform for investigating anisotropic structure–property correlations and integrated optical applications for novel polarization‐sensitive photodetection.

Two-dimensional carbon allotropes with tunable direct band gaps and high carrier mobility

Using first-principles calculations, two sp2 + sp3-hybridized two-dimensional (2D) carbon allotropes with stable orthorhombic structures are predicted in this work. Both structures are composed of tetra-, penta-, and hexa-rings of carbon atoms, known as TPH-I carbon and TPH-II carbon. We demonstrate that they can be obtained from penta-graphene by the Stone-Wales (SW) transformation. TPH-I carbon has an inherent direct band gap of 2.704 eV, while TPH-II carbon has a quasi-direct band gap of 2.361 eV. After applying uniaxial strain, TPH-I and TPH-II carbon will have tunable direct band gaps in the range of 2.441–2.812 eV and 2.468–2.7562 eV, respectively. Their electrons and holes mobilities are found to be anisotropic. TPH-I carbon shows extraordinary room temperature in-plane electron mobility of ~7.9 × 104 cm2 V−1 s−1 and TPH-II carbon shows high hole mobility of ~1.5 × 104 cm2 V−1 s−1. Both novel carbon sheets exhibit strong absorption of visible light. And their intrinsic anisotropic optical properties allow optical determination of the crystalline orientation. In addition, TPH-I carbon is a potential photocatalyst for the water splitting reaction.

First principles study on the effect of Bi3+ and Y3+ doping on electronic structure, thermal and optical properties of KSr2Nb5O15

The electronic structure, thermal and optical properties of KSr2Nb5O15 with Bi3+ doping (KSN-Bi) and Y3+ doping (KSN-Y) are studied using first principles. When Bi3+ and Y3+ doping in the KSN, the band gap becomes smaller and satisfies to KSN > KSN-Bi > KSN-Y, the original tetragonal structure changes to be orthorhombic due to the great distortion of Nb-O octahedron affected by Bi3+ and Y3+. The specific heat capacity under constant volume Cv, specific heat capacity under constant pressure Cp, entropy Svib, thermal expansion coefficient α(T) and volume change decreased after doping. The calculated Gibbs results suggest that KSN-Y is more stable than KSN-Bi. Furthermore, the dielectric constant is increased and the imaginary part of dielectric function decreased, while the refraction property is increased and reflectivity is decreased after Bi3+ and Y3+ doping. Both the KSN-Bi and KSN-Y show ultraviolet absorption in 100–400 nm, refractivity in 0–5 eV and reflectivity 5–8 eV. KSN-Bi and KSN-Y show optical anisotropy properties and KSN-Bi has better optical properties than that of KSN-Y.

A brush-coating approach to achieve anisotropic optical film via field-induced stretching of carbon nanotube clusters in a reactive mesogen

We present a novel brush coating method for fabricating a coatable polarizer that utilizes a mixture of functionalized single-walled carbon nanotubes (h-SWCNTs) grafted with carboxyl and hydroxyl groups and a reactive mesogen (RM), which possess a long-range orientational ordering of their rod-shaped molecules with photo-sensitive functional groups. The h-SWCNTs are shortened to a length of around 150 nm by an acid sonochemical oxidation process and then dispersed in an RM solution. The brush-coated thin layer initially consists of h-SWCNT clusters, but applying an in-plane electric field induces large-scale stretching of these along the field direction, after which the layer is photo-polymerized by ultraviolet irradiation to form a film embedding the stretched nanotubes. The uniaxially aligned carbon nanotubes (CNTs) produce a broadband absorption spectrum that enables the film to exhibit an optical anisotropic property that absorbs incident light from the ultraviolet to the visible spectrum selectively depending on the polarization direction, thus acting as a coatable CNT polarizer. The dispersibility and elongation of h-SWCNT clusters induced by applying the electric field, as well as the anisotropic light-absorption properties of the h-SWCNT film, are investigated.

Self‐Assembled, 10 nm‐Tailored, Near Infrared Plasmonic Metasurface Acting as Broadband Omnidirectional Polarizing Mirror

AbstractPlasmonic metasurfaces tailored at the few‐nanometers scale and fabricated by high‐throughput lithography‐free methods are needed to efficiently produce optical components with optimal performance and small footprint. Herein, the design of such metasurfaces and their fabrication over cm2 areas by self‐assembly are reported. They present an optical cavity structure, in which a 2D array of parallel Au nanorods is coupled to a metal mirror through a thin spacer. The nanorods present a quasi‐monodisperse ≈10 nm diameter and separation, and a broad polydisperse length distribution. These remarkable nanostructural features, combining periodicity in one direction with randomness in another, endow the nanorod array with unconventional plasmonic and anisotropic effective optical properties. Thanks to these properties, for light polarized along the nanorods, the metasurface displays strong destructive optical interference and thus near‐perfect absorption over a broad range of angles of incidence and of wavelengths in the near infrared. In contrast, for light polarized perpendicular to the nanorods, no such interference occurs and thus the metasurface behaves as a metal mirror. In sum, the metasurface can be used as broadband omnidirectional polarizing mirror, an original thin and monolithic optical component merging the functionalities of a metal mirror and of a wire‐grid polarizer.

Anisotropic exciton excitations and optical properties of Hittorf's phosphorene

This paper presents a first-principles approach to the quasiparticle electronic structure, exciton, and optical properties in two dimensional Hittorf's phosphorene. The authors show the onset of exciton-enhanced optical absorption and strong polarization-dependent electron-hole excitation.

Emerging Low‐Dimensional Nanoagents for Bio‐Microimaging

AbstractNanomaterials with unique physical and biological properties have attracted increased attention for bio‐microimaging applications. Among them, low‐dimensional nanomaterials have quantum confinement effect in zero, one, or two dimensions, which have high specific surface area for cargo (such as dye) loading, anisotropic optical properties for polarization, and enhanced infrared absorption for efficient photoluminescence. In this review, recent efforts on bio‐microimaging of low‐dimensional nanomaterials, including confocal imaging, multiphoton imaging, Raman imaging, and super‐resolution imaging, will be highlighted. Bio‐microimaging principle, respective advantages and bottlenecks, and applications of low‐dimensional materials in various imaging technologies will be discussed. Furthermore, the review will shed light on the common synthetic methods of low‐dimensional nanomaterials. Finally, the classification and comparison of various nanomaterials, including carbon, black phosphorus, gold, metal composites, and 2D graphene analogues will also be elaborated.

Light-Induced Anisotropic Morphological Dynamics of Black Phosphorus Membranes Visualized by Dark-Field Ultrafast Electron Microscopy.

Black phosphorus (BP) is an elemental layered material with a strong in-plane anisotropic structure. This structure is accompanied by anisotropic optical, electrical, thermal, and mechanical properties. Despite interest in BP from both fundamental and technical aspects, investigation into the structural dynamics of BP caused by strain fields, which are prevalent for two-dimensional (2D) materials and tune the material physical properties, has been overlooked. Here, we report the morphological dynamics of photoexcited BP membranes observed using time-resolved diffractograms and dark-field images obtained via ultrafast electron microscopy. Aided by 4D reconstruction, we visualize the nonequilibrium bulging of thin BP membranes and reveal that the buckling transition is driven by impulsive thermal stress upon photoexcitation in real time. The bulging, buckling, and flattening (on strain release) showed anisotropic spatiotemporal behavior. Our observations offer insights into the fleeting morphology of anisotropic 2D matter and provide a glimpse into the mapping of transient, modulated physical properties upon impulsive excitation, as well as strain engineering at the nanoscale.

2D Materials Based on Main Group Element Compounds: Phases, Synthesis, Characterization, and Applications

Abstract2D materials based on main group element compounds have recently attracted significant attention because of their rich stoichiometric ratios and structure motifs. This review focuses on the phases in various 2D binary materials including III–VI, IV–VI, V–VI, III–V, IV–V, and V–V materials. Reducing 3D materials to 2D introduces confinement and surface effects as well as stabilizes unstable 3D phases in their 2D form. Their crystal structures, stability, preparation, and applications are summarized based on theoretical predictions and experimental explorations. Moreover, various properties of 2D materials, such as ferroelectric effect, anisotropic optical and electrical properties, ultralow thermal conductivity, and topological state are discussed. Finally, a few perspectives and an outlook are given to inspire readers toward exploring 2D materials with new phases and properties.

High pressure effect on the phase transition and in-plane anisotropy of SnSe

The phase transition and the in-plane anisotropy of semiconductor compound SnSe under pressure were studied by optical and electronical methods. Our result showed that an electronic transition from semiconductor to semimetal was observed above 12 GPa, followed by Pnma to Bbmm structural transition. A structure transition was also observed at ∼8 GPa, corresponding to the phase transition from layered to bulk structure. The anisotropic optical property of SnSe reduces when increasing the pressure and disappeared at ∼12 GPa. Our results confirmed that the pressure can induce structural and electronic structure transition of SnSe. And the study of anisotropic optical property of SnSe under pressure supplies a viable route to developing the strain-based anisotropic optics, optoelectronics and electronics in devices.

Alloy engineered germanium monochalcogenide with tunable bandgap for broadband optoelectrical applications

Germanium monochalcogenides (GeSe and GeS) are promising materials for various optoelectronic applications because of their solar range bandgaps, high carrier mobilities, high stabilities, earth abundance, and anisotropic optical properties. Precise control of germanium monochalcogenide bandgaps is critical to applications in continuously tunable optoelectronics. In this paper, we combine first-principles calculations and experiments to predict and confirm that alloy engineering is a significant strategy for tailoring germanium monochalcogenide (${\mathrm{GeS}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$) optoelectronic properties. When the Se content $x$ increases from 0.0 to 1.0, the bandgap decreases from 1.23 to 0.89 eV. In addition, there is a direct-indirect bandgap transition when $x$ is approximately 0.3. Tunable ${\mathrm{GeS}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$ bandgaps can open up exciting opportunities for the development of various electronic and optoelectronic devices.