Optical properties of hybrid systems Based on WS2 and metastructures

Semiconductors are crucial components of electronic devices that influences almost every aspect of modern society. Efforts from generations have been put into this field, especially in searching for and studying new semiconducting materials. Nowadays, silicon is the most critical element for fabricating most electronic circuits. But along with the development of manufacturing technology, the Si transistors have approached to their limits and thus hinders the development of new Si-based devices.The 2D semiconductors are believed to be an alternative to Si for next-generation semiconductor devices because of their novel properties. Therefore, the study of 2D semiconductors is very meaningful for new devices.In my thesis, we focuses on the optical properties of 2D material heterostructures and the WS2 monolayer integrated with plasmonic structures. First, the CVD-grown twisted bilayer WS2 and the mechanically-stacked WS2/perovskite heterostructure were studied. Then the surface plasmon enhanced PL and Raman scattering of a WS2 monolayer were investigated. In addition, technical skills such as obtaining WS2 monolayers, preparing plasmonic metastructures, and transferring 2D materials have also been discussed. And we then study the optical properties of plasmonic structures and their hybrid system with WS2 by measuring and analyzing the spectra of PL, Raman, reflection/absorption, and angle-resolved reflection/absorption. At last, we introduce numerical simulations (FDTD) to understand and confirm our results from theoretical aspects.Although our results enrich the content of 2D TMDs in photonics and optoelectronics, challenges still remain in commercializing 2D TMD photonic and optoelectronic devices

Nanostructured Fe 2

In the present work, a method combining arc plasma evaporation of a metal followed by oxidation in air was developed to produce nanosized metal oxide based composites in large scale. As an example, Fe2O3 based nanocomposites were prepared through such a method. With increasing the oxidation temperature, α-Fe2O3 content in the composites increases, while γ-Fe2O3 and residual α-Fe contents decrease. As anode materials for lithium batteries, the electrochemical properties of nanosized Fe2O3 composites were tested. It was found that the anode materials changed to tiny crystallites and then followed by grain growth during the galvanostatic charge/discharge cycles. A capacity rising was observed for the composites obtained at 400°C and 450°C, which was more prominent with increasing the oxidation temperature. Among these composites, the one obtained at 450°C showed the best performance: a specific capacity of 507.6 mAh/g remained after 150 cycles at a current density of 200 mA/g, much higher than that of the commercial nano-Fe2O3 powder (~180 mAh/g after 30 cycles)

Strong anisotropic enhancement of photoluminescence in WS₂ integrated with plasmonic nanowire array

Layered transition metal dichalcogenides (TMDCs) have shown great potential for a wide range of applications in photonics and optoelectronics. Nevertheless, valley decoherence severely randomizes its polarization which is important to a light emitter. Plasmonic metasurface with a unique way to manipulate the light-matter interaction may provide an effective and practical solution. Here by integrating TMDCs with plasmonic nanowire arrays, we demonstrate strong anisotropic enhancement of the excitonic emission at different spectral positions. For the indirect bandgap transition in bilayer WS2, multifold enhancement can be achieved with the photoluminescence (PL) polarization either perpendicular or parallel to the long axis of nanowires, which arises from the coupling of WS2 with localized or guided plasmon modes, respectively. Moreover, PL of high linearity is obtained in the direct bandgap transition benefiting from, in addition to the plasmonic enhancement, the directional diffraction scattering of nanowire arrays. Our method with enhanced PL intensity contrasts to the conventional form-birefringence based on the aspect ratio of nanowire arrays where the intensity loss is remarkable. Our results provide a prototypical plasmon-exciton hybrid system for anisotropic enhancement of the PL at the nanoscale, enabling simultaneous control of the intensity, polarization and wavelength toward practical ultrathin photonic devices based on TMDCs

Probing and Tuning the Spin Textures of the K and Q Valleys in Few-Layer MoS2

The strong spin-orbit coupling along with broken inversion symmetry in transition metal dichalcogenides (TMDs) results in spin polarized valleys, which are the origins of many interesting properties such as Ising superconductivity, circular dichroism, valley Hall effect, etc. Herein, it is shown that encapsulating few-layer MoS2 between hexagonal boron nitride (h-BN) and gating the electrical contacts by ionic liquid pronounce Shubnikov-de Haas (SdH) oscillations in magnetoresistance. Notably, the SdH oscillations remain unchanged in tilted magnetic fields, demonstrating that the spins of the Q/Q ' valleys are firmly locked to the out-of-plane direction; therefore, Zeeman energy is insensitive to the in-plane magnetic field. Ionic liquid gating induces superconductivity on the surface of unencapsulated MoS2. The spins of Cooper pairs are strongly pinned to the out-of-plane direction by the effective Zeeman field, hence are protected from being realigned by an in-plane magnetic field, namely, Ising protection. As a result, superconductivity persists in an in-plane magnetic field up to 14 T, in which T-c only decreases by approximate to 0.3 K from T-c0 as approximate to 7 K. By applying back gate, the strength of Ising protection can be effectively tuned, where an increase in 70% is observed when back gate changes from +90 to -90 V

A Flip-Over Plasmonic Structure for Photoluminescence Enhancement of Encapsulated WS₂ Monolayers

Transition metal dichalcogenide (TMD) monolayers, with their direct band gaps, have attracted wide attention from the fields of photonics and optoelectronics. However, monolayer semiconducting TMDs generally suffer from low excitation absorption and emission efficiency, limiting their further applications. Here a flip-over plasmonic structure comprised of silver nano-disk arrays supporting a WS2 monolayer sandwiched by hexagonal boron nitride (h-BN) layers is demonstrated. The flip-over configuration optimizes the optical process with a free excitation/emission path from the top and a strong plasmonic interaction from the bottom. As a result, the photoluminescence from the TMD monolayers can be greatly enhanced more than tenfold by optimizing the metasurface, which can be further improved nearly tenfold by optimizing the thickness of bottom h-BN. This study shows the advantages of using the flip-over structure, where the plasmonic interaction between the metasurface and TMDs can be tuned by introducing optimized plasmonic arrays and h-BN layers with suitable thickness. This hybrid device configuration paves a reliable platform to study the light–matter interaction, achieving highly efficient plasmonic TMD devices

Correlated States in Strained Twisted Bilayer Graphenes Away from the Magic Angle

Graphene moiré superlattice formed by rotating two graphene sheets can host strongly correlated and topological states when flat bands form at so-called magic angles. Here, we report that, for a twisting angle far away from the magic angle, the heterostrain induced during stacking heterostructures can also create flat bands. Combining a direct visualization of strain effect in twisted bilayer graphene moiré superlattices and transport measurements, features of correlated states appear at "non-magic"angles in twisted bilayer graphene under the heterostrain. Observing correlated states in these "non-standard"conditions can enrich the understanding of the possible origins of the correlated states and widen the freedom in tuning the moiré heterostructures and the scope of exploring the correlated physics in moiré superlattices

Orbital Fulde–Ferrell–Larkin–Ovchinnikov state in an Ising superconductor

In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum1,2. In superconductors lacking (local) inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin–orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram3–5. However, when the Zeeman effect is suppressed because of spin locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries6–8. Here we report the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite-momentum Cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of a normal metal, a uniform Ising superconducting phase and a six-fold orbital FFLO state. This study highlights an alternative route to achieving finite-momentum superconductivity and provides a universal mechanism to preparing orbital FFLO states in similar materials with broken inversion symmetries.</p

Nonradiative Energy Transfer and Selective Charge Transfer in a WS₂/(PEA)₂PbI₄Heterostructure

van der Waals heterostructures are currently the focus of intense investigation; this is essentially due to the unprecedented flexibility offered by the total relaxation of lattice matching requirements and their new and exotic properties compared to the individual layers. Here, we investigate the hybrid transition-metal dichalcogenide/2D perovskite heterostructure WS2/(PEA)2PbI4 (where PEA stands for phenylethylammonium). We present the first density functional theory (DFT) calculations of a heterostructure ensemble, which reveal a novel band alignment, where direct electron transfer is blocked by the organic spacer of the 2D perovskite. In contrast, the valence band forms a cascade from WS2 through the PEA to the PbI4 layer allowing hole transfer. These predictions are supported by optical spectroscopy studies, which provide compelling evidence for both charge transfer and nonradiative transfer of the excitation (energy transfer) between the layers. Our results show that TMD/2D perovskite (where TMD stands for transition-metal dichalcogenides) heterostructures provide a flexible and convenient way to engineer the band alignment

Structural colors and enhanced resolution at the nanoscale: Local structuring of phase-change materials using focused ion beam

In the past few years, phase-change materials have become increasingly important in nano-photonics and optoelectronics. The advantages of sizeable optical contrast between phases and the additional degree of freedom from phase switching have been the driving force. From multilevel reflectance to dynamic nanoprinting and structural colors, phase-change materials have achieved outstanding results with prospects for real-world applications. The local crystallization/amorphization of phase-change materials and the corresponding reflectance tunning by the crystallized/amorphized region size have potential applications for future dynamic display devices. Although the resolution is much higher than current display devices, the pixel sizes in those devices are limited by the locally switchable structure size. Here, we reduce the spot sizes further by using ion beams instead of laser beams and dramatically increase the pixel density, demonstrating the capability of having superior resolution. In addition, the power to sputter away materials can be utilized in creating nanostructures with relative height differences and local contrast. Our experiment focuses on one archetypal phase-change material, Sb$_2$Se$_3$, prepared by pulsed-laser deposition on a reflective gold substrate. We demonstrate that we can produce structural colors and achieve reflectance tunning by focused ion beam milling/sputtering of phase change materials at the nanoscale. Furthermore, we show that the local structuring of phase-change materials by focused ion beam can be used to produce high pixel density display devices with superior resolutions