Controlled Preparation of High Quality Bubble-Free and Uniform Conducting Interfaces of Vertical van der Waals Heterostructures of Arrays

Sharp and clean interfaces of van der Waals (vdW) heterostructures are highly demanded in two-dimensional (2D) materials-based devices. However, current assembly methods usually cause interfacial bubbles and wrinkles, hindering carrier interlayer transport. The preparation of a large-scale vdW heterostructure with a bubble-free interface is still a challenge. Although many efforts have been made to eliminate bubbles, the evolution processes of the interfacial bubbles are rarely studied. Here, the interface bubble formation and evolution of the transferred 2D materials and their vdW heterostructure are systemically studied by the atomic force microscopy (AFM) technique and high-resolution surface current mapping. A thermal annealing procedure is developed to reduce the number of bubbles and to improve the quality of interfaces. In addition, influences of the interface residues and nanosteps on bubble evolution are also discussed. Further, we develop the polystyrene (PS)-mediated polydimethyl­siloxane (PDMS) transfer technique to realize the high-quality transfer of heterostructure arrays. Finally, high-resolution surface current mapping results confirm that we can now produce highly uniform electrical conduction interfaces of heterojunctions. This study provides guidance for assembling high quality interfaces and paves the way for production of bubble-free heterostructure-based electronic devices with high performance and good uniformity

Flexible, Transparent, and Free-Standing Silicon Nanowire SERS Platform for in Situ Food Inspection

We demonstrated a flexible transparent and free-standing Si nanowire paper (SiNWP) as a surface enhanced Raman scattering (SERS) platform for in situ chemical sensing on warping surfaces with high sensitivity. The SERS activity has originated from the three-dimension interconnected nanowire network structure and electromagnetic coupling between closely separated nanowires in the SiNWP. In addition, the SERS activity can be highly improved by functionalizing the SiNWP with plasmonic Au nanoparticles. The hybrid substrate not only showed excellent reproducibility and stability of the SERS signal, but also maintained the flexibility and transparency of the pristine SiNWP. To demonstrate its potential application in food inspection, the Au nanoparticles-modified SiNWP was directly wrapped onto the lemon surface for in situ identification and detection of the pesticide residues. The results showed that the excellent SERS activity and transparency of the hybrid substrate enabled the detection of the pesticides down to 72 ng/cm², which was much lower than the permitted residue dose in food safety

Plasmon-Controlled Förster Resonance Energy Transfer

The localized plasmons of metal nanocrystals have been widely utilized to control a variety of optical signals, such as Raman, fluorescence, and circular dichroism, from proximal dye molecules. We show, on the single-particle level, that the Förster resonance energy transfer between two different fluorophores can be modulated by adjacent plasmonic nanocrystals. The donor and acceptor fluorophore molecules are embedded in a mesostructured silica shell that is uniformly coated on Au–Ag core–shell nanocrystals. The longitudinal plasmon wavelengths of the core–shell metal nanocrystals are synthetically tailored by varying the aspect ratio. Comparison of the scattering and fluorescence spectra taken from the different hybrid nanostructures indicates that the energy transfer efficiency can be controlled by the plasmon wavelength. When the plasmon peak overlaps with the emission peak of the donor, the energy transfer channel is turned off. When the plasmon peak is red-shifted to be in between the emission peak of the donor and the absorption peak of the acceptor or right at the intrinsic emission peak of the acceptor, the energy transfer channel is turned on

Distinct Plasmonic Manifestation on Gold Nanorods Induced by the Spatial Perturbation of Small Gold Nanospheres

The plasmon coupling between a Au nanorod and a small Au nanosphere has been studied with scattering measurements, electrodynamic simulations, and model analysis. The spatial perturbation of the nanosphere leads to distinct spectral changes of the heterodimer. The plasmonic responses, including Fano resonance, are remarkably sensitive to the nanosphere position on the nanorod, the gap distance, and the nanocrystal dimensions. The nanosphere dipole is intriguingly found to rotate around the nanorod dipole to achieve favorable attractive interaction for the bonding dipole–dipole mode. The sensitive spectral response of the heterodimer to the spatial perturbation of the nanosphere offers an approach to designing plasmon rulers of two spatial coordinates for sensing and high-resolution measurements of distance changes

Directional Fano Resonance in a Silicon Nanosphere Dimer

Fano resonance arising from the interaction between a broad “bright” mode and a narrow “dark” mode has been widely investigated in symmetry-breaking structures made of noble metals such as plasmonic asymmetric oligomers or other well-designed nanostructures. However, Fano resonance in nanoscale all-dielectric dimers has not been experimentally demonstrated so far. We report the first experimental observation of directional Fano resonance in silicon nanosphere dimers (both homodimer and heterodimer) and clarify that the coupling between magnetic and electric dipole modes can easily generate Fano resonance in all-dielectric oligomers, distinctly differing from conventional Fano resonances based on electric responses or artificial optical magnetism. A silicon nanosphere dimer, exhibiting a strong magnetic response inside and an electric enhancement in the gap, is an excellent structure to support magnetic-based Fano scattering. Interactions between magnetic and electric dipoles can suppress backward scattering and enhance forward scattering at Fano wavelengths. This directional scattering is much more prominent than that from a single silicon sphere and shows promising applications in areas such as directional nanoantenna or optical switching, opening up avenues for developing all-dielectric low-loss metamaterials or nanophotonic devices at visible wavelengths

Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer MoS₂ Film with Spatial Homogeneity and the Visualization of Grain Boundaries

MoS₂ monolayer attracts considerable attention due to its semiconducting nature with a direct bandgap which can be tuned by various approaches. Yet a controllable and low-cost method to produce large-scale, high-quality, and uniform MoS₂ monolayer continuous film, which is of crucial importance for practical applications and optical measurements, remains a great challenge. Most previously reported MoS₂ monolayer films had limited crystalline sizes, and the high density of grain boundaries inside the films greatly affected the electrical properties. Herein, we demonstrate that highly crystalline MoS₂ monolayer film with spatial size up to centimeters can be obtained via a facile chemical vapor deposition method with solid-phase precursors. This growth strategy contains selected precursor and controlled diffusion rate, giving rise to the high quality of the film. The well-defined grain boundaries inside the continuous film, which are invisible under an optical microscope, can be clearly detected in photoluminescence mapping and atomic force microscope phase images, with a low density of ∼0.04 μm^–1. Transmission electron microscopy combined with selected area electron diffraction measurements further confirm the high structural homogeneity of the MoS₂ monolayer film with large crystalline sizes. Electrical measurements show uniform and promising performance of the transistors made from the MoS₂ monolayer film. The carrier mobility remains high at large channel lengths. This work opens a new pathway toward electronic and optical applications, and fundamental growth mechanism as well, of the MoS₂ monolayer

Janus Magneto–Electric Nanosphere Dimers Exhibiting Unidirectional Visible Light Scattering and Strong Electromagnetic Field Enhancement

Steering incident light into specific directions at the nanoscale is very important for future nanophotonics applications of signal transmission and detection. A prerequisite for such a purpose is the development of nanostructures with high-efficiency unidirectional light scattering properties. Here, from both theoretical and experimental sides, we conceived and demonstrated the unidirectional visible light scattering behaviors of a heterostructure, Janus dimer composed of gold and silicon nanospheres. By carefully adjusting the sizes and spacings of the two nanospheres, the Janus dimer can support both electric and magnetic dipole modes with spectral overlaps and comparable strengths. The interference of these two modes gives rise to the narrow-band unidirectional scattering behaviors with enhanced forward scattering and suppressed backward scattering. The directionality can further be improved by arranging the dimers into one-dimensional chain structures. In addition, the dimers also show remarkable electromagnetic field enhancements. These results will be important not only for applications of light emitting devices, solar cells, optical filters, and various surface enhanced spectroscopies but also for furthering our understanding on the light–matter interactions at the nanoscale

Electronic Properties of MoS₂–WS₂ Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy

Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS₂–WS₂ heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX₂ heterostructure with monolayer MoS₂ core wrapped by multilayer WS₂ is obtained by the technique, which is entirely different from MX₂ heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS₂ (monolayer)/WS₂ (multilayer) heterojunctions serve as intrinsic lateral p–n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor

1T′ Transition Metal Telluride Atomic Layers for Plasmon-Free SERS at Femtomolar Levels

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T′ transition metal telluride atomic layers (WTe₂ and MoTe₂) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T′-W­(Mo)­Te₂ reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T′-W­(Mo)­Te₂, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 10⁹ (1.6 × 10⁸). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 10¹⁰ (6.2 × 10⁹), respectively, when 1T′-W­(Mo)­Te₂ is integrated on the Bragg reflector. The strong interaction between the analyte and 1T′-W­(Mo)­Te₂ and the abundant density of states near the Fermi level of the semimetal 1T′-W­(Mo)­Te₂ in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex

Characteristics of a Silicon Nanowires/PEDOT:PSS Heterojunction and Its Effect on the Solar Cell Performance

The interfacial energy-level alignment of a silicon nanowires (SiNWs)/PEDOT:PSS heterojunction is investigated using Kelvin probe force microscopy. The potential difference and electrical distribution in the junction are systematically revealed. When the PEDOT:PSS layer is covered at the bottom of the SiNW array, an abrupt junction is formed at the interface whose characteristics are mainly determined by the uniformly doped Si bulk. When the PEDOT:PSS layer is covered on the top, a hyperabrupt junction localized at the top of the SiNWs forms, and this characteristic depends on the surface properties of the SiNWs. Because the calculation shows that the absorption of light from the SiNWs and the Si bulk are equally important, the bottom-coverage structure leads to better position matching between the depletion and absorption area and therefore shows better photovoltaic performance. The dependence of <i>J</i>_SC and <i>V</i>_OC on the junction characteristic is discussed