Design Of Functional Devices Research Articles

Light-Tunable 1T-TaS2 Charge-Density-Wave Oscillators.

External stimuli-controlled phase transitions are essential for fundamental physics and design of functional devices. Charge density wave (CDW) is a metastable collective electronic phase featured by the periodic lattice distortion. Much attention has been attracted to study the external control of CDW phases. Although much work has been done in the electric-field-induced CDW transition, the study of the role of Joule heating in the phase transition is insufficient. Here, using the Raman spectroscopy, the electric-field-driven phase transition is in situ observed in the ultrathin 1T-TaS2. By quantitative evaluation of the Joule heating effect in the electric-field-induced CDW transition, it is shown that Joule heating plays a secondary role in the nearly commensurate (NC) to incommensurate (IC) CDW transition, while it dominants the IC-NC CDW transition, providing a better understanding of the electric field-induced phase transition. More importantly, at room temperature, light illumination can modulate the CDW phase and thus tune the frequency of the ultrathin 1T-TaS2 oscillators. This light tunability of the CDW phase transition is promising for multifunctional device applications.

Anisotropic purity of entangled photons from Cooper pairs in heterostructures

It was theoretically proposed that a forward-biased p-n junction with a superconducting layer (P-N-S), would produce pure, polarization entangled photons due to inherent spin-singlet pairing of electrons. However, any heterostructure interface generically induces Rashba spin-orbit coupling, which in turn generates a mixed singlet-triplet superconducting order parameter. Here we study the effect of triplet pairing on the purity of photons produced through Cooper pair recombination. A unique directional dependence of the state purity is found for a triplet superconductor with fixed $\vec{d}$: pure, entangled photons are produced when the photon polarization axis is parallel to $\vec{d}$. Induced triplet pairing in a singlet superconductor is shown to degrade the state purity, while induced singlet pairing in a triplet superconductor is shown to enhance the production of entangled pairs. These considerations may aid the design of functional devices to produce entangled photons.

A Facile Space-Confined Solid-Phase Sulfurization Strategy for Growth of High-Quality Ultrathin Molybdenum Disulfide Single Crystals.

Single-crystal transition metal dichalcogenides (TMDs) and TMD-based heterojunctions have recently attracted significant research and industrial interest owing to their intriguing optical and electrical properties. However, the lack of a simple, low-cost, environmentally friendly, synthetic method and a poor understanding of the growth mechanism post a huge challenge to implementing TMDs in practical applications. In this work, we developed a novel approach for direct formation of high-quality, monolayer and few-layer MoS2 single crystal domains via a single-step rapid thermal processing of a sandwiched reactor with sulfur and molybdenum (Mo) film in a confined reaction space. An all-solid-phase growth mechanism was proposed and experimentally/theoretically evidenced by analyzing the surface potential and morphology mapping. Compared with the conventional chemical vapor deposition approaches, our method involves no complicated gas-phase reactant transfer or reactions and requires very small amount of solid precursors [e.g., Mo (∼3 μg)], no carrier gas, no pretreatment of the precursor, no complex equipment design, thereby facilitating a simple, low-cost, and environmentally friendly growth. Moreover, we examined the symmetry, defects, and stacking phase in as-grown MoS2 samples using simultaneous second-harmonic-/sum-frequency-generation (SHG/SFG) imaging. For the first time, we observed that the SFG (peak intensity/position) polarization can be used as a sensitive probe to identify the orientation of TMDs' crystallographic axes. Furthermore, we fabricated ferroelectric programmable Schottky junction devices via local domain patterning using the as-grown, single-crystal monolayer MoS2, revealing their great potential in logic and optoelectronic applications. Our strategy thus provides a simple, low-cost, and scalable path toward a wide variety of TMD single crystal growth and novel functional device design.

The electronic and transport properties of borophane with defects: A first principles study

Recent works well confirm the stability of hydrogenated borophene, known as borophane. Here, first principles studies have performed on the electronic and transport properties of borophane with defects. The calculations indicate that the introduction of defects results in different behavior of charges redistribution along x and y directions. The intrinsic electronic structure of borophane with Dirac cone is destroyed in various degrees by each type of defect. The inducing defect states lead to the occurrence of flat bands, which are not benefit for the electronic transport properties. According to the transmission spectra and I-V characteristics, these defects decrease the transmission intensity and the current value both along two directions. However, the transport anisotropy can be efficiently tuned by defect, which may contribute to the design of functional device.

Broadband and Robust Metalens with Nonlinear Phase Profiles for Efficient Terahertz Wave Control

Metasurfaces, 2D artificial electromagnetic media, open up a new frontier of functional device design ranging from radio waves to the visible region. Particularly, metasurface‐based lenses are indispensable in various practical terahertz applications. The authors aim at achieving flexible and robust metalenses for efficient terahertz wave control. In general, resolution and efficiency are two inevitable parameters in determining the focusing and imaging abilities of lenses, which however are rarely experimentally demonstrated in the terahertz band. In this Communication, three broadband and robust metalenses with nonlinear phase profiles are proposed, all of which are experimentally investigated by using near‐field scanning terahertz microscopy (NSTM) with a spatial resolution of 50 µm. The measurement shows that the metalens can focus a 0.95 THz wave to a spot size of 580 µm and achieve a transmittance efficiency of 45%. In addition, the NSTM system facilitates an experimental investigation of the incidence angle dependence of the terahertz metalens, which proves the robust focusing feature of the proposed device. This demonstration delivers a promising metasurface design for potential applications in imaging and information processing that may be of interest for the entire electromagnetic spectrum.

Planar gradient metamaterials

Metamaterials possess exotic properties that do not exist in nature. Gradient metamaterials, which are characterized by a continuous spatial variation of their properties, provide a promising approach to the development of both bulk and planar optics. In particular, planar gradient metamaterials can be classified into three categories: gradient metasurfaces, gradient index metamaterials and gradient metallic gratings. In this Review, we summarize the progress made in the theoretical modelling of these materials, in their experimental implementation and in the design of functional devices. We discuss the use of planar gradient metamaterials for wave bending and focusing in free space, for supporting surface plasmon polaritons and for the realization of trapped rainbows. We also focus on the implementation of these materials in waveguide systems, which can enable electromagnetic cloaking, Fano resonances, asymmetric transmission and guided mode conversion. Finally, we discuss promising trends, such as the use of dielectric rather than metallic unit elements and the use of planar gradient metamaterials in 3D systems. Planar gradient metamaterials are a promising development, overcoming the limitations of both bulk and planar optics, and have been widely investigated in various domains. This Review summarizes recent progress made in the theoretical modelling, experimental implementation and design of functional devices that utilize these materials.

DNA from natural sources in design of functional devices

The role of DNA as structuring or templating agent has become more significant with the development of nanobiotechnology. Although short single and double stranded DNA have extensively been used as immobilization tool, as a template for nanoparticle preparation and in design of various devices such as nanomotors and biosensors, DNA from natural sources has an advantage of being abundant, cheap and readily available. Therefore, it is not surprising that there is a huge interest in making the use of natural DNA properties for both nano- and micro-applications. In this review we attempt to give an overview of the up to date applications of natural DNA, either from viral, marine or mammalian sources, in design of functional devices. This article is meant to be a starting point and a guide to the platforms in which natural DNA is employed such as DNA origami, optoelectronic devices and organic catalysis.

Photoinduced Formation of Bithiophene Radical Cation via a Hole-Transfer Process from CdS Nanocrystals

The exciton dynamics in semiconductor nanocrystals can be strongly affected by coupling the nanocrystals to organic ligands. A deeper understanding of the interactions in semiconductor–organic hybrid systems is important for the design of functional devices. In the present work, the interactions between CdS quantum dots and bithiophene molecules have been investigated. In particular, the photophysical behavior of CdS nanocrystals has been investigated in n-heptane in the presence of increasing bithiophene concentration by use of steady-state and time-resolved measurements. Bithiophene is a well-known electron donor (or hole acceptor), and it has a good affinity with CdS surface for the presence of sulfur atoms. The nanocrystal luminescence was efficiently quenched upon addition of increasing concentration of the thiophene derivative, and modifications in the emission decay profiles of CdS were observed; the analysis of luminescence data suggests that quenching is mainly due to static interaction able to modify the dynamics of the exciton states of the hybrid nanomaterials. The transient absorption measurements enable to detect the bithiophene radical cation upon CdS excitation, thus revealing the occurrence of an efficient hole transfer process from the nanocrystals to the organic ligand, for which a quantum efficiency of 36% has been measured. The dependence of transient signal on bithiophene concentration and the formation of tetrathiophene intermediates indicate that CdS exciton states are able to photosensitize the polymerization of bithiophene after the hole transfer processes. The data indicate that in the investigated system the decay of charged species is not determined by back-reactions.

Tunable out-of-plane slow light in resonance induced transparent grating waveguide structures

We investigate the unexplored physics of slow light effect in resonance induced transparent grating waveguide structures. We show that with a simple three-layer thin-film structure, a narrow transparent window can be obtained, enabling substantially slow down the speed of out-of-plane propagation of light. Further, we numerically demonstrate an active slow light device that potentially achieves high-speed control of slow light at the optical communication band. This work paves a way for the design of functional devices, such as slow-light chips, switches, and modulators functioning in optical and infrared regimes.

Controlling elastic waves with isotropic materials

Design of functional devices with isotropic materials has significant advantages, as regards easy fabrication and broadband application. In this letter, we present a method to derive isotropic transformation material parameters for elastodynamics under local conformal transformation. The transformed material parameters are then applied to design a beam bender, a four-beam antenna and an approximate carpet cloak for elastic wave with isotropic materials, validated by the numerical simulations.

Complex Liquid‐Crystalline Superstructure of a π‐Conjugated Oligothiophene

A good connection: A polyphilic oligothiophene derivative (see formula) forms a complex liquid-crystalline phase, in which the π-conjugated rods are organized in a honeycomb-like network of square cylinders (see picture). This arrangement opens new possibilities for the directed organization of π-conjugated organic materials into complex superstructures and patterns for the design of functional devices by means of liquid-crystal self-assembly.