Showing papers by "Zheyu Fang published in 2017"

Self-Assembled Au/CdSe Nanocrystal Clusters for Plasmon-Mediated Photocatalytic Hydrogen Evolution.

Abstract: Plasmon-mediated photocatalytic systems generally suffer from poor efficiency due to weak absorption overlap and thus limited energy transfer between the plasmonic metal and the semiconductor. Herein, a near-ideal plasmon-mediated photocatalyst system is developed. Au/CdSe nanocrystal clusters (NCs) are successfully fabricated through a facile emulsion-based self-assembly approach, containing Au nanoparticles (NPs) of size 2.8, 4.6, 7.2, or 9.0 nm and CdSe quantum dots (QDs) of size ≈3.3 nm. Under visible-light irradiation, the Au/CdSe NCs with 7.2 nm Au NPs afford very stable operation and a remarkable H2-evolution rate of 73 mmol gCdSe−1 h−1 (10× higher than bare CdSe NCs). Plasmon resonance energy transfer from the Au NPs to the CdSe QDs, which enhances charge-carrier generation in the semiconductor and suppresses bulk recombination, is responsible for the outstanding photocatalytic performance. The approach used here to fabricate the Au/CdSe NCs is suitable for the construction of other plasmon-mediated photocatalysts.

Plasmonics of 2D Nanomaterials: Properties and Applications.

Abstract: Plasmonics has developed for decades in the field of condensed matter physics and optics. Based on the classical Maxwell theory, collective excitations exhibit profound light-matter interaction properties beyond classical physics in lots of material systems. With the development of nanofabrication and characterization technology, ultra-thin two-dimensional (2D) nanomaterials attract tremendous interest and show exceptional plasmonic properties. Here, we elaborate the advanced optical properties of 2D materials especially graphene and monolayer molybdenum disulfide (MoS2), review the plasmonic properties of graphene, and discuss the coupling effect in hybrid 2D nanomaterials. Then, the plasmonic tuning methods of 2D nanomaterials are presented from theoretical models to experimental investigations. Furthermore, we reveal the potential applications in photocatalysis, photovoltaics and photodetections, based on the development of 2D nanomaterials, we make a prospect for the future theoretical physics and practical applications.

Plasmonic Chiral Nanostructures: Chiroptical Effects and Applications

Abstract: The plasmonic chiroptical effect has been used to manipulate chiral states of light, where the strong field enhancement and light localization in metallic nanostructures can amplify the chiroptical response. Moreover, in metamaterials, the chiroptical effect leads to circular dichroism (CD), circular birefringence (CB), and asymmetric transmission. Potential applications enabled by chiral plasmonics have been realized in various areas of nanoscience and nanotechnology. In this review, both basic theories and state-of-the-art studies on plasmonic chiroptical effects are summarized. Molecular chiroptical effects are drastically enhanced by metallic nanostructures that can generate a “superchiral” field, which arises from the strong electromagnetic interactions. Both intrinsic and extrinsic plasmonic chiral metamaterials formed by the periodic arrangement of metallic nanostructured units show high levels of CB, CD, and asymmetric transmission. Consequent applications including photo detection, molecular sensing, and chirality tuning are discussed, and a perspective of emerging concepts such as Pancharatnam−Berry (PB) phase in this booming research field is presented.

Tailoring MoS2 Exciton–Plasmon Interaction by Optical Spin–Orbit Coupling

Abstract: Molybdenum disulfide (MoS2) monolayer as one of the atomic thickness two-dimensional materials has remarkable electronic and optical properties, which is an ideal candidate for a wide range of optoelectronic applications. However, the atomic monolayer thickness poses a significant challenge in MoS2 photoluminescence emission due to weak light–matter interaction. Here, we investigate the MoS2 exciton–plasmon interaction with spin–orbit coupling of light. The plasmonic spiral rings with subwavelength dimensions are designed and fabricated on hybrid substrates. MoS2 photoluminescence enhancement can be actively controlled by changing the incident optical spin states, laser powers, and the nanospiral geometries, which is arising from the change of field enhancement at near-field region. Planar light-emitting devices based on spin–orbit coupling (SOC) effect were further realized and flexibly controlled by changing the polarization of light. The SOC effect is discussed by the accumulation of geometric and dyna...

Single-Nanoparticle Plasmonic Electro-optic Modulator Based on MoS2 Monolayers.

Abstract: The manipulation of light in an integrated circuit is crucial for the development of high-speed electro-optic devices. Recently, molybdenum disulfide (MoS2) monolayers generated broad interest for the optoelectronics because of their huge exciton binding energy, tunable optical emission, direct electronic band-gap structure, etc. Miniaturization and multifunctionality of electro-optic devices further require the manipulation of light–matter interaction at the single-nanoparticle level. The strong exciton–plasmon interaction that is generated between the MoS2 monolayers and metallic nanostructures may be a possible solution for compact electro-optic devices at the nanoscale. Here, we demonstrate a nanoplasmonic modulator in the visible spectral region by combining the MoS2 monolayers with a single Au nanodisk. The narrow MoS2 excitons coupled with broad Au plasmons result in a deep Fano resonance, which can be switched on and off by applying different gate voltages on the MoS2 monolayers. A reversible disp...

Controlled growth and shape-directed self-assembly of gold nanoarrows.

Abstract: Self-assembly of colloidal nanocrystals into complex superstructures offers notable opportunities to create functional devices and artificial materials with unusual properties. Anisotropic nanoparticles with nonspherical shapes, such as rods, plates, polyhedra, and multipods, enable the formation of a diverse range of ordered superlattices. However, the structural complexity and tunability of nanocrystal superlattices are restricted by the limited geometries of the anisotropic nanoparticles available for supercrystal self-assembly. We show that uniform gold nanoarrows (GNAs) consisting of two pyramidal heads connected by a four-wing shaft are readily synthesized through controlled overgrowth of gold nanorods. The distinct concave geometry endows the GNAs with unique packing and interlocking ability and allows for the shape-directed assembly of sophisticated two-dimensional (2D) and 3D supercrystals with unprecedented architectures. Net-like 2D supercrystals are assembled through the face-to-face contact of the GNAs lying on the pyramidal edges, whereas zipper-like and weave-like 2D supercrystals are constructed by the interlocked GNAs lying on the pyramidal {111} facets. Furthermore, multilayer packing of net-like and weave-like 2D assemblies of GNAs leads to non-close-packed 3D supercrystals with varied packing efficiencies and pore structures. Electromagnetic simulation of the diverse nanoarrow supercrystals exhibits exotic patterns of nanoscale electromagnetic field confinement. This study may open new avenues toward tunable self-assembly of nanoparticle superstructures with increased complexity and unusual functionality and may advance the design of novel plasmonic metamaterials for nanophotonics and reconfigurable architectured materials.

Plasmonic hot electron tunneling photodetection in vertical Au–graphene hybrid nanostructures

Abstract: Plasmon-induced hot-electron generation provides an efficient way to convert light into electric current. The investigation of the optoelectronic response in two-dimensional materials and metallic hybrid nanostructure attracts increasing research interest. Here, we present a tunneling effect of plasmonic hot electrons that is generated from Au nanoparticles, which can vertically tunnel through graphene monolayers. A strong photocurrent induced by the hot electrons was measured in this graphene-based vertical photodetector with its intensity maximum reached at the plasmon resonance wavelength. The tunneling effect of plasmonic hot electrons was investigated by gradually increasing the incident laser power and bias voltage between the top and bottom electrodes. The dynamic attenuation of plasmonic hot electrons in an excited state was further investigated with multilayered graphene sheets. These results show that our vertical hybrid structure can function as an effective design for the tunneling photodetector, and enable the realization of complex nanophotonic devices that are based on graphene and other 2D materials, such as optical transistors and plasmonic hot-electron sensors.

Near-Field Raman Spectroscopy with Aperture Tips

Abstract: In this paper, we review nano-Raman techniques based on aperture scanning near-field optical microscopy (SNOM). Fundamentals of SNOM and aperture-tip-based near-field Raman spectroscopy and their applications in key technical issues, including Raman signal intensity and collection time, are introduced. Recent advances in the tip design are discussed, and applications of the aperture-SNOM-based nano-Raman technique are presented. We attempt to identify the most pressing open questions in this field. We believe that, by improving the power transmission efficiency and combining the local field enhancing technique with the tip-enhanced spectroscopy, the performance of aperture-SNOM can be significantly improved. Its nanometer-scale excitation volume and the consequent low background make the aperture-tip technique feasible for many important samples that cannot be measured by other optical nanospectroscopies.

Enhanced optical performance of multifocal metalens with conic shapes

Abstract: A multifocal metalens, which focuses incident light at multiple foci, has many applications in imaging systems and optical communications. However, the traditional design strategy of a multifocal metalens combines several lenses that have different focal points into a planar integrated unit, resulting in low imaging quality because of the high background noise. Here we show that the defects of the traditional method can be overcome by designing a metalens with conic shapes (the ellipse and the hyperbola); this approach could improve the imaging performance and substantially decrease the background noise of multifocal metalenses. These benefits arise from the intrinsic properties of the two conic curves, which can focus incident light constructively at all of the foci of the metalens. We further demonstrate that the proposed conic-shaped metalens can function well within a broadband operation wavelength that ranges from 600 to 900 nm with the dual polarity actively controlled by the incident circular polarized light. The great agreement between the experimental and simulation results demonstrates that our proposed metalens has significant potential for use in future integrated nanophotonic devices.

Revealing the spin optics in conic-shaped metasurfaces

Abstract: Ellipse and hyperbola are two well-known curves in mathematics with numerous applications in various fields, but their properties and inherent differences in spin optics are less understood. Here, we investigate the peculiar optical spin properties of the two curves and establish a connection between their foci and the spin states of incident light by introducing a geometric-phase distribution along the conic curve. We show that the optical spin Hall effect is the intrinsic optical spin property of ellipse, where photons with different spin states can be exactly separated to each of its two foci, while a hyperbola exhibits optical spin-selective effect, where only photons with one particular spin state can be accumulated at its foci. These properties are then experimentally demonstrated in near field by arranging nanoslits in conic shapes. Based on the spin properties of the curves, we design spin-based plasmonic devices with various functionalities. Our results reveal the intrinsic optical spin properties behind conic curves and provide a route for designing spin-based plasmonic devices.

Valley Pseudospin with a Widely Tunable Bandgap in Doped Honeycomb BN Monolayer

Abstract: Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra...

Temperature dependent Raman and photoluminescence of vertical WS2/MoS2 monolayer heterostructures

Abstract: Heterostructures from two-dimensional transition-metal dichalcogenides MX2 have emerged as a hot topic in recent years due to their various fascinating properties. Here, we investigated the temperature dependent Raman and photoluminescence (PL) spectra in vertical stacked WS2/MoS2 monolayer heterostructures. Our result shows that both E12g and A1g modes of WS2 and MoS2 vary linearly with temperature increasing from 300 to 642 K. The PL measurement also reveals strong temperature dependencies of the PL intensity and peak position. The activation energy of the thermal quenching of the PL emission has been found to be equal to 69.6 meV. The temperature dependence of the peak energy well follows the band-gap shrinkage of bulk semiconductor.

Radiative energy transfer from MoS 2 excitons to surface plasmons

Abstract: In this work, we demonstrated the energy transfer process from few-layer MoS2 to gold dimer arrays via ultrafast pump-probe spectroscopy. With the overlap between the MoS2 exciton and the designed plasmon dipolar modes in the frequency domain, the exciton energy can be radiatively transferred to plasmonic structures, excited the localized surface plasmon resonance, and then enhanced the oscillation of coherent acoustic phonons. Power-dependent differential reflection signals and an analytical model based on the rate equation of exciton density were carried out to quantitatively study the energy transfer process. Our finding explores the energy flow between MoS2 excitons and surface plasmons, and can be contributed to the design of exciton-plasmon structures utilizing ultrathin materials.

Plasmonic Spiral Nanostructure for Analyzing the Angular Momentum of Light

Abstract: A compound plasmonic light analyzer with an elliptical Archimedean spiral nanostructure is proposed analytically and analyzed numerically. The elliptical Archimedean spiral is analogous to an ellipse but has varying semimajor axes. When the structure is illuminated by the incident light with different angular momentum, surface plasmon polaritons (SPPs) are excited, and various plasmonic vortexes indicating the angular momentum of the light are generated. Incident light with different spin angular momentum can create plasmonic vortexes centered at different points, and the orbital angular momentum of light determines the size of the vortex, which manifests its topological charge. Thus, both spin and orbital angular momentum states of the incident light can be estimated simultaneously by observing the position and the size of the plasmonic vortex. We provide an analytical description of the structure and it is well consistent with the finite-difference time-domain (FDTD) numerical simulation results. This plasmonic light analyzer may provide various potential applications in nanophotonics device and optical information technology.