High Power Factor of Ag2Se/Ag/Nylon Composite Films for Wearable Thermoelectric Devices.

A flexible thermoelectric device has been considered as a competitive candidate for powering wearable electronics. Here, we fabricated an n-type Ag2Se/Ag composite film on a flexible nylon substrate using vacuum-assisted filtration and a combination of cold and hot pressing. By optimising the Ag/Se ratio and the sequential addition and reaction time of AA, an excellent power factor of 2277.3 µW∙m-1 K-2 (corresponding to a ZT of ~0.71) at room temperature was achieved. In addition, the Ag2Se/Ag composite film exhibits remarkable flexibility, with only 4% loss and 10% loss in electrical conductivity after being bent around a rod of 4 mm radius for 1000 cycles and 2000 cycles, respectively. A seven-leg flexible thermoelectric device assembled with the optimised film demonstrates a voltage of 19 mV and a maximum power output of 3.48 µW (corresponding power density of 35.5 W m-2) at a temperature difference of 30 K. This study provides a potential path to design improved flexible TE devices.

An in situ characterization technique for electron emission behavior under a photo-electric-common-excitation field: study on the vertical few-layer graphene individuals.

The in situ characterization on the individuals offers an effective way to explore the dynamic behaviors and underlying physics of materials at the nanoscale, and this is of benefit for actual applications. In the field of vacuum micro-nano electronics, the existing in situ techniques can obtain the material information such as structure, morphology and composition in the process of electron emission driven by a single source of excitation. However, the relevant process and mechanism become more complicated when two or more excitation sources are commonly acted on the emitters. In this paper, we present an in situ nano characterization technique to trigger and record the electron emission behavior under the photo-electric-common-excitation multiple physical fields. Specifically, we probed into the in situ electron emission from an individual vertical few-layer graphene (vFLG) emitter under a laser-plus-electrostatic driving field. Electrons were driven out from the vFLG's emission edge, operated in situ under an external electrostatic field coupled with a 785 nm continuous-wave laser-triggered optical field. The incident light has been demonstrated to significantly improve the electron emission properties of graphene, which were recorded as an obvious decrease of the turn-on voltage, a higher emission current by factor of 35, as well as a photo-response on-off ratio as high as 5. More importantly, during their actual electron emission process, a series of in situ characterizations such as SEM observation and Raman spectra were used to study the structure, composition and even real-time Raman frequency changes of the emitters. These information can further reveal the key factors for the electron emission properties, such as field enhancement, work function and real-time surface temperature. Thereafter, the emission mechanism of vFLG in this study has been semi-quantitatively demonstrated to be the two concurrent processes of photon-assisted thermal enhanced field emission and photo field emission.

Controllable two-dimensional movement and redistribution of lithium ions in metal oxides.

Rechargeable lithium batteries are the most practical and widely used power sources for portable and mobile devices in modern society. Manipulation of the electronic and ionic charge transport and accumulation in solid materials has always been crucial for rechargeable lithium batteries. The transport and accumulation of lithium ions in electrode materials, which is a diffusion process, is determined by the concentration distribution of lithium ions and the intrinsic structure of the electrode material and thus far has not been manipulated by an external force. Here, we report the realization of controllable two-dimensional movement and redistribution of lithium ions in metal oxides. This achievement is one kind of centimeter-scale control and is achieved by a magnetic field based on the 'current-driving model'. This work provides additional insight for building safe and high-capacity rechargeable lithium batteries.

van der Waals Transition-Metal Oxide for Vis-MIR Broadband Photodetection via Intercalation Strategy.

Defects engineering can broaden the absorption band of wide band gap van der Waals (vdW) materials to the visible or near-IR regime at the expense of material stability and photoresponse speed. Herein, we introduce an atomic intercalation method that brings the wide band gap vdW α-MoO3 for vis-MIR broadband optoelectronic conversion. We confirm experimentally that intercalation significantly enhances photoabsorption and electrical conductivity buts effects negligible change to the lattice structure as compared with ion intercalation. Charge transfer from the Sn atom to the lattices induces an optoelectrical change. As a result, the Sn-intercalated α-MoO3 shows room temperature, air stable, broadband photodetection ability from 405 nm to 10 µm, with photoresponsivity better than 9.0 A W-1 in 405-1500 nm, ∼0.4 A W-1 at 3700 nm, and 0.16 A W-1 at 10 µm, response time of ∼0.1 s, and peak D* of 7.3 × 107 cm Hz0.5 W-1 at 520 nm. We further reveal that photothermal effect dominates in our detection range by real-time photothermal-electrical measurement, and the materials show a high temperature coefficient of resistance value of -1.658% K-1 at 300 K. These results provide feasible route for designing broadband absorption materials for photoelectrical, photothermal, or thermal-electrical application.

A Plasmon-Mediated Electron Emission Process.

Light-driven electron emission plays an important role in modern optoelectronic devices. However, such a process usually requires a light field with either a high intensity or a high frequency, which is not favorable for its implementations and difficult for its integrations. To solve these issues, we propose to combine plasmonic nanostructures with nanoelectron emitters of low work function. In such a heterostructure, hot electrons generated by plasmon resonances upon light excitation can be directly injected into the adjacent emitter, which can subsequently be emitted into the vacuum. Electron emission of high efficiency can be obtained with light fields of moderate intensities and visible wavelengths, which is a plasmon-mediated electron emission (PMEE) process. We have demonstrated our proposed design using a gold-on-graphene (Au-on-Gr) nanostructure, which can have electron emission with light intensity down to 73 mW·cm-2. It should be noted that the field electron emission is not involved in such a PMEE process. This proposal is of interest for applications including cold-cathode electron sources, advanced photocathodes, and micro- and nanoelectronic devices relying on free electrons.

Resonance Coupling in Heterostructures Composed of Silicon Nanosphere and Monolayer WS2: A Magnetic-Dipole-Mediated Energy Transfer Process.

Light-matter resonance coupling is a long-studied topic for both fundamental research and photonic and optoelectronic applications. Here we investigated the resonance coupling between the magnetic dipole mode of a dielectric nanosphere and 2D excitons in a monolayer semiconductor. By coating an individual silicon nanosphere with a monolayer of WS2, we theoretically demonstrated that, because of the strong energy transfer between the magnetic dipole mode of the nanosphere and the A-exciton in WS2, resonance coupling evidenced by anticrossing behavior in the scattering energy diagram was observed, with a mode splitting of 43 meV. In contrast to plexcitons, which involve plasmonic nanocavities, the resonance coupling in this all-dielectric heterostructure was insensitive to the spacing between the silicon nanosphere core and the WS2 shell. Additionally, the two split modes exhibited distinct light-scattering directionality. We further experimentally demonstrated the resonance coupling effect by depositing silicon nanospheres with different diameters onto a WS2 monolayer and collecting the scattering spectra of the resulting heterostructures under ambient conditions. We further demonstrated active control of the resonance coupling by temperature scanning. Our findings highlighted the potential of our all-dielectric heterostructure as a solid platform for studying strong light-matter interactions at the nanoscale.

Plasmonic Sensing Characteristics of Gold Nanorods with Large Aspect Ratios.

Plasmonic gold nanorods play important roles in nowadays state-of-the-art plasmonic sensing techniques. Most of the previous studies and applications focused on gold nanorods with relatively small aspect ratios, where the plasmon wavelengths are smaller than 900 nm. Gold nanorods with large aspect ratios are predicted to exhibit high refractive-index sensitivity (Langmir 2008, 24, 5233⁻5237), which therefore should be promising for the development of high-performance plasmonic chemical- and bio-sensors. In this study, we developed gold nanorods with aspect ratios over 7.9, which exhibit plasmon resonances around 1064 nm. The refractive index (RI) sensitivity of these nanorods have been evaluated by varying their dielectric environment, whereby a sensitivity as high as 473 nm/RIU (refractive index unit) can be obtained. Furthermore, we have demonstrated the large-aspect-ratio nanorods as efficient substrate for surface enhanced Raman spectroscopy (SERS), where an enhancement factor (EF) as high as 9.47 × 108 was measured using 4-methylbenzenethiol (4-MBT) as probe molecule. Finally, a type of flexible SERS substrate is developed by conjugating the gold nanorods with the polystyrene (PS) polymer. The results obtained in our study can benefit the development of plasmonic sensing techniques utilized in the near-infrared spectral region.

Tetragonal Single-Crystalline Boron Nanowires with Strong Anisotropic Light Scattering Behaviors and Photocurrent Response in Visible-Light Regime.

Boron is a narrow-bandgap (1.56 eV) semiconductor with high melting-point, low-density, large Young's modulus and very high refractive index (3.03) close to silicon. Therefore, boron nanostructures is expected to possess strong visible-light scattering properties. However, photonic and optoelectronic properties of the boron nanostructures are seldom studied until now. In this paper, we have successfully prepared single-crystalline boron nanowire (BNW) arrays with high-density on Si substrate. All the BNWs are found to possess strong light-scattering behaviors in the visible regime. Most of all, the scattered light is found to polarize along the longitudinal direction of the nanowire. They also have excellent second-harmonic generation (SHG) properties under ultrafast laser irradiation. Further optoelectronic measurements show that an individual BNW device exhibits notable photocurrent responses in the visible-light range at ambient conditions, which can be attributed to the strong coupling effect between individual BNW and the visible light. The maximum photoresponsivity of an individual BNW can reach up to 12.12 A W-1 at a voltage of 10 V, and the response time is only 18 ms. Therefore, it unveils that the BNWs have a promising future in visible-light communications and detections.

Room-Temperature Strong Light-Matter Interaction with Active Control in Single Plasmonic Nanorod Coupled with Two-Dimensional Atomic Crystals.

Strong light-matter coupling manifested by Rabi splitting has attracted tremendous attention due to its fundamental importance in cavity quantum-electrodynamics research and great potentials in quantum information applications. A prerequisite for practical applications of the strong coupling in future optoelectronic devices is an all-solid-state system exhibiting room-temperature Rabi splitting with active control. Here we realized such a system in heterostructure consisted of monolayer WS2 and an individual plasmonic gold nanorod. By taking advantages of the small mode volume of the nanorod and large transition dipole moment of the WS2 exciton, giant Rabi splitting energies of 91-133 meV can be achieved at ambient conditions, which only involve a small number of excitons. The strong light-matter coupling can be dynamically tuned either by electrostatic gating or temperature scanning. These findings can pave the way toward active nanophotonic devices operating at room temperature.

Tailoring the thermal and electrical transport properties of graphene films by grain size engineering.

Understanding the influence of grain boundaries (GBs) on the electrical and thermal transport properties of graphene films is essentially important for electronic, optoelectronic and thermoelectric applications. Here we report a segregation-adsorption chemical vapour deposition method to grow well-stitched high-quality monolayer graphene films with a tunable uniform grain size from ∼200 nm to ∼1 µm, by using a Pt substrate with medium carbon solubility, which enables the determination of the scaling laws of thermal and electrical conductivities as a function of grain size. We found that the thermal conductivity of graphene films dramatically decreases with decreasing grain size by a small thermal boundary conductance of ∼3.8 × 109 W m-2 K-1, while the electrical conductivity slowly decreases with an extraordinarily small GB transport gap of ∼0.01 eV and resistivity of ∼0.3 kΩ µm. Moreover, the changes in both the thermal and electrical conductivities with grain size change are greater than those of typical semiconducting thermoelectric materials.

Molybdenum Nanoscrews: A Novel Non-coinage-Metal Substrate for Surface-Enhanced Raman Scattering.

ABSTRACT: For the first time, Mo nanoscrew was cultivated as a novel non-coinage-metal substrate for surface-enhanced Raman scattering (SERS). It was found that the nanoscrew is composed of many small screw threads stacking along its length direction with small separations. Under external light excitation, strong electromagnetic coupling was initiated within the gaps, and many hot-spots formed on the surface of the nanoscrew, which was confirmed by high-resolution scanning near-field optical microscope measurements and numerical simulations using finite element method. These hot-spots are responsible for the observed SERS activity of the nanoscrews. Raman mapping characterizations further revealed the excellent reproducibility of the SERS activity. Our findings may pave the way for design of low-cost and stable SERS substrates. GRAPHICAL ABSTRACT: Mo nanoscrews are for the first time cultivated as a novel type of SERS substrate. The SERS activity is originated from the electromagnetic field enhancements on the individual Mo nanoscrew, which is corroborated by single-particle optical characterizations.

Resonance Coupling in Silicon Nanosphere-J-Aggregate Heterostructures.

Due to their optical magnetic and electric resonances associated with the high refractive index, dielectric silicon nanoparticles have been explored as novel nanocavities that are excellent candidates for enhancing various light-matter interactions at the nanoscale. Here, from both of theoretical and experimental aspects, we explored resonance coupling between excitons and magnetic/electric resonances in heterostructures composed of the silicon nanoparticle coated with a molecular J-aggregate shell. The resonance coupling was originated from coherent energy transfer between the exciton and magnetic/electric modes, which was manifested by quenching dips on the scattering spectrum due to formation of hybrid modes. The influences of various parameters, including the molecular oscillation strength, molecular absorption line width, molecular shell thickness, refractive index of the surrounding environment, and separation between the core and shell, on the resonance coupling behaviors were scrutinized. In particular, the resonance coupling can approach the strong coupling regime by choosing appropriate molecular parameters, where an anticrossing behavior with a mode splitting of 100 meV was observed on the energy diagram. Most interestingly, the hybrid modes in such dielectric heterostructure can exhibit unidirectional light scattering behaviors, which cannot be achieved by those in plexcitonic nanoparticle composed of a metal nanoparticle core and a molecular shell.

Quantitative Analysis of Scattering Mechanisms in Highly Crystalline CVD MoS2 through a Self-Limited Growth Strategy by Interface Engineering.

The electrical performance of highly crystalline monolayer MoS2 is remarkably enhanced by a self-limited growth strategy on octadecyltrimethoxysilane self-assembled monolayer modified SiO2 /Si substrates. The scattering mechanisms in low-κ dielectric, including the dominant charged impurities, acoustic deformation potentials, optical deformation potentials), Fröhlich interaction, and the remote interface phonon interaction in dielectrics, are quantitatively analyzed.

All Chemical Vapor Deposition Synthesis and Intrinsic Bandgap Observation of MoS2 /Graphene Heterostructures.

A facile all-chemical vapor deposition approach is designed, which allows both sequentially grown Gr and monolayer MoS2 in the same growth process, thus allowing the direct construction of MoS2 /Gr vertical heterostructures on Au foils. A weak n-doping effect and an intrinsic bandgap of MoS2 are obtained from MoS2 /Gr/Au via scanning tunneling microscopy and spectroscopy characterization. The exciton binding energy is accurately deduced by combining photoluminescence measurements.

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.

Lateral Built-In Potential of Monolayer MoS2-WS2 In-Plane Heterostructures by a Shortcut Growth Strategy.

Lateral WS2-MoS2 heterostructures are synthesized by a shortcut one-step growth recipe with low-cost and soluble salts. The 2D spatial distributions of the built-in potential and the related electric field of the lateral WS2-MoS2 heterostructure are quantitatively analyzed by scanning Kelvin probe force microscopy revealing the fundamental attributes of the lateral heterostructure devices.