Phonon Polaritons in Twisted Double-Layers of Hyperbolic van der Waals Crystals.

Controlling the twist angle between two stacked van der Waals (vdW) crystals is a powerful approach for tuning their electronic and photonic properties. Hyperbolic media have recently attracted much attention due to their ability to tailor electromagnetic waves at the subwavelength-scale which, however, usually requires complex patterning procedures. Here, we demonstrate a lithography-free approach for manipulating the hyperbolicity by harnessing the twist-dependent coupling of phonon polaritons in double-layers of vdW α-MoO3, a naturally biaxial hyperbolic crystal. The polariton isofrequency contours can be modified due to this interlayer coupling, allowing for controlling the polaritonic characteristics by adjusting the orientation angles between the two layers. Our findings provide opportunities for control of nanoscale light flow with twisted stacks of vdW crystals.

Non-uniaxial stress-assisted fabrication of nanoconstriction on vertical nanostructured Si.

Vertically aligned Si nanoconstrictions have potential for applications of electronic, photonic and phononic nanodevices. Herein, we report a featured method by utilizing the non-uniaxial tangential tension stress (σ T ) at the Si surface of a vertical hyperbolic Si/SiO2 core-shell nanostructure during thermal oxidation to achieve well defined Si nanoconstrictions. A thermal oxidation model was proposed to describe the correlations between σ T and the structural parameters of the hyperbolic nanostructure, i.e. oxide thickness (t ox ), sidewall curvature radius (R 0) and neck diameter (2r A0). Numerical simulations indicated that the Si surface at the position with the narrowest diameter (neck position) has the highest σ T (∼GPa) and presents a gradient distribution at both ends. By means of stress regulation, an array of well defined Si nanoconstrictions about 10 nm in diameter and about 34 nm in length was obtained. The experimental findings demonstrated that the high σ T would induce a nanofracture and thus a local oxidation to form a nanoconstriction, self-aligned at the neck position. The finding notably extends the capability of stress-assisted 'nanofabrication' of Si via thermal oxidation.

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.

Epitaxial growth of multiwall carbon nanotube from stainless steel substrate and effect on electrical conduction and field emission.

The epitaxial growth of carbon nanotubes (CNTs) is an important subject of research. Recent attention has been paid to finding new strategies for the controlled growth of single-wall CNTs with a defined chirality. In addition, many potential applications require multiwall CNTs (MWCNTs) to grow vertically from the substrate and the interface property is crucial. Here, we report for the first time that MWCNTs can grow directly from the surface of a substrate by epitaxy, based on the experimental study of individual multiwall carbon nanotubes on a large-area stainless steel substrate, which is a very useful system for electrical and mechanical applications. In particular, evidence is given of the lattice matching between the MWCNT and the lattice of a hexagonal Cr2O3: (Fe, Mn) film formed on the surface of the substrate. Furthermore, a method is developed to increase the density of the MWCNTs; a mechanism of simultaneous top and bottom growth is proposed. The resultant significantly improved electrical transport and field emission properties are also presented, showing the Ohmic contact for electrical conduction and high performance in resisting the catastrophic cold-cathode vacuum breakdown of the CNTs.

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.

Highly reproducible surface-enhanced Raman scattering substrate for detection of phenolic pollutants.

The ordering degree of nanostructures is the key to determining the uniformity of surface-enhanced Raman scattering (SERS). However, fabrication of large-area ordered nanostructures remains a challenge, especially with the ultrahigh-density (>1010 cm-2). Here, we report a fabrication of large-area ultrahigh-density ordered Ag@Al2O3/Ag core-shell nanosphere (NS) arrays with tunable nanostructures. The ultrahigh-density (2.8 × 1010 cm-2) ordered NS arrays over a large-area capability (diameter >4.0 cm) enable the uniform SERS signals with the relative standard deviation of less than 5%. The as-fabricated highly reproducible SERS substrate can be applied to detect trace phenolic pollutants in water. This work does not only provide a new route for synthesizing the ultrahigh-density ordered nanostructures, but also create a new class of SERS substrates with high sensitivity and excellent reproducibility.

Effect of Crystallinity on the Field Emission Characteristics of Carbon Nanotube Grown on W-Co Bimetallic Catalyst.

Carbon nanotube (CNT) is an excellent field emission material. However, uniformity and stability are the key issues hampering its device application. In this work, a bimetallic W-Co alloy was adopted as the catalyst of CNT in chemical vapor deposition process. The high melting point and stable crystal structure of W-Co helps to increase the grown CNT diameter uniformity and homogeneous crystal structure. High-crystallinity CNTs were grown on the W-Co bimetallic catalyst. Its field emission characteristics demonstrated a low turn-on field, high current density, stable current stability, and uniform emission distribution. The Fowler-Nordheim (FN) and Seppen-Katamuki (SK) analyses revealed that the CNT grown on the W-Co catalyst has a relatively low work function and high field enhancement factor. The high crystallinity and homogeneous crystal structure of CNT also reduce the body resistance and increase the emission current stability and maximum current. The result provides a way to synthesis a high-quality CNT field emitter, which will accelerate the development of cold cathode vacuum electronic device application.

A Flexible and Wearable Photodetector Enabling Ultra-Broadband Imaging from Ultraviolet to Millimeter-Wave Regimes.

Flexible and miniaturized photodetectors, offering a fast response across the ultraviolet (UV) to millimeter (MM) wave spectrum, are crucial for applications like healthcare monitoring and wearable optoelectronics. Despite their potential, developing such photodetectors faces challenges due to the lack of suitable materials and operational mechanisms. Here, the study proposes a flexible photodetector composed of a monolayer graphene connected by two distinct metal electrodes. Through the photothermoelectric effect, these asymmetric electrodes induce electron flow within the graphene channel upon electromagnetic wave illumination, resulting in a compact device with ultra-broadband and rapid photoresponse. The devices, with footprints ranging from 3 × 20 µm2 to 50 × 20 µm2, operate across a spectrum from 325 nm (UV) to 1.19 mm (MM) wave. They demonstrate a responsivity (RV) of up to 396.4 ± 5.1 mV W-1, a noise-equivalent power (NEP) of 8.6 ± 0.1 nW Hz- 0.5, and a response time as small as 0.8 ± 0.1 ms. This device facilitates direct imaging of shielded objects and material differentiation under simulated human body-wearing conditions. The straightforward device architecture, aligned with its ultra-broadband operational frequency range, is anticipated to hold significant implications for the development of miniaturized, wearable, and portable photodetectors.

High Crystallinity Vertical Few-Layer Graphene Grown Using Template Method Assisted ICPCVD Approach.

Controllable synthesis of high crystallinity, low defects vertical few-layer graphene (VFLG) is significant for its application in electron emission, sensor or energy storage, etc. In this paper, a template method was introduced to grow high crystallinity VFLG (HCVFLG). A copper mask acted as a template which has two effects in the high-density plasma enhanced deposition which are protecting VFLG from ion etching and creating a molecular gas flow to assist efficient growth. Raman and TEM results confirmed the improved crystallinity of VFLG with the assistance of a copper mask. As a field emitter, the HCVFLG has a large field emission current and a low turn-on field. The maximum field emission current of a single HCVFLG sheet reaches 93 µA which is two orders of magnitude higher than VFLG grown without a mask. The maximum current density of HCVFLG film reached 67.15 mA/cm2 and is 2.6 times of VFLG grown without a mask. The vacuum breakdown mechanism of HCVFLG was contacted interface damage resulting in VFLG detaching from the substrate. This work provides a practical strategy for high-quality VFLG controllable synthesis and provides a simple method to realize the pattern growth of VFLG.

Effects of Substrates on Nucleation, Growth and Electrical Property of Vertical Few-Layer Graphene.

A key common problem for vertical few-layer graphene (VFLG) applications in electronic devices is the solution to grow on substrates. In this study, four kinds of substrates (silicon, stainless-steel, quartz and carbon-cloth) were examined to understand the mechanism of the nucleation and growth of VFLG by using the inductively-coupled plasma-enhanced chemical vapor deposition (ICPCVD) method. The theoretical and experimental results show that the initial nucleation of VFLG was influenced by the properties of the substrates. Surface energy and catalysis of substrates had a significant effect on controlling nucleation density and nucleation rate of VFLG at the initial growth stage. The quality of the VFLG sheet rarely had a relationship with this kind of substrate and was prone to being influenced by growth conditions. The characterization of conductivity and field emissions for a single VFLG were examined in order to understand the influence of substrates on the electrical property. The results showed that there was little difference in the conductivity of the VFLG sheet grown on the four substrates, while the interfacial contact resistance of VFLG on the four substrates showed a tremendous difference due to the different properties of said substrates. Therefore, the field emission characterization of the VFLG sheet grown on stainless-steel substrate was the best, with the maximum emission current of 35 µA at a 160 V/µm electrostatic field. This finding highlights the controllable interface of between VFLG and substrates as an important issue for electrical application.

Self-Assembly Vertical Graphene-Based MoO3 Nanosheets for High Performance Supercapacitors.

Supercapacitors have been extensively studied due to their advantages of fast-charging and discharging, high-power density, long-cycling life, low cost, etc. Exploring novel nanomaterial schemes for high-performance electrode materials is of great significance. Herein, a strategy to combine vertical graphene (VG) with MoO3 nanosheets to form a composite VG/MoO3 nanostructure is proposed. VGs as transition layers supply rich active sites for the growth of MoO3 nanosheets with increasing specific surface areas. The VG transition layer further improves the electric contact and adhesion of the MoO3 electrode, simultaneously stabilizing its volume and crystal structure during repeated redox reactions. Thus, the prepared VG/MoO3 nanosheets have been demonstrated to exhibit excellent electrochemical properties, such as high reversible capacitance, better cycling performance, and high-rate capability.

High Current Field Emission from Large-Area Indium Doped ZnO Nanowire Field Emitter Arrays for Flat-Panel X-ray Source Application.

Large-area zinc oxide (ZnO) nanowire arrays have important applications in flat-panel X-ray sources and detectors. Doping is an effective way to enhance the emission current by changing the nanowire conductivity and the lattice structure. In this paper, large-area indium-doped ZnO nanowire arrays were prepared on indium-tin-oxide-coated glass substrates by the thermal oxidation method. Doping with indium concentrations up to 1 at% was achieved by directly oxidizing the In-Zn alloy thin film. The growth process was subsequently explained using a self-catalytic vapor-liquid-solid growth mechanism. The field emission measurements show that a high emission current of ~20 mA could be obtained from large-area In-doped sample with a 4.8 × 4.8 cm2 area. This high emission current was attributed to the high crystallinity and conductivity change induced by the indium dopants. Furthermore, the application of these In-doped ZnO nanowire arrays in a flat-panel X-ray source was realized and distinct X-ray imaging was demonstrated.

Realizing the large current field emission characteristics of single vertical few-layer graphene by constructing a lateral graphite heat dissipation interface.

With the potential to be an excellent field electron emitter, few-layer graphene (FLG) has to avoid Joule heat induced vacuum breakdown during high current field electron emission. Creating a good heat dissipation path is the key factor maintaining the heat equilibrium of a field emitter. In this work, a graphite interlayer was grown between the FLG and the tungsten substrate. The graphite interlayer with its good in plane electrical and thermal conductivities helps FLG dissipate the heat in the lateral direction efficiently and broaden the heat dissipation path. As a result, both the temperature of the FLG and the chance of vacuum breakdown were reduced. The destructive in situ TEM field emission test of a single FLG showed that the breakage of the graphite interlayer during field emission blocks up the lateral heat dissipation path, causes heat accumulation and finally induces the vacuum breakdown of FLG. Benefiting from the graphite interlayer, the high current field emission characteristics of a single FLG were achieved. The maximum field emission current of six single FLG samples was between 78 and 233 µA with the corresponding current densities in the range of 1.2 × 107-5.85 × 108 A cm-2. This finding demonstrates that interface heat engineering is crucial for nanomaterial-based field emitters that work under high current and high temperature conditions.

Optimization Strategies for High Photoluminescence Quantum Yield of Monolayer Chemical Vapor Deposition Transition Metal Dichalcogenides.

Chemical vapor deposition (CVD) is a promising method to obtain monolayer transition metal dichalcogenides (TMDCs) with high quality and enough size to meet the requirements of practical photoelectric devices. However, the as-grown monolayers often exhibit a lower PL performance due to the stress between the as-grown TMDCs flakes and the substrate. Therefore, finding a facile method to effectively promote the photoluminescence quantum yield (PL QY) of CVD monolayer TMDCs with a clean surface is highly desirable for practical applications. In this work, based on the CVD monolayers MoS2 and MoSe2, the effect of various stress relaxation methods on the TMDCs PL enhancement is systemically studied. By comparing the different kinds of volatile solution treatment processes, as well as the traditional transfer process, it can be found that the volatile solution with a moderate volatilization rate such as ethanol or IPA is a preferred option to improve the PL performance of the CVD monolayer TMDCs, which also surpasses the traditional transfer method by avoiding wrinkles, defects, and contamination to the samples. PL QY of ethanol-treated CVD samples could increase by 6 times on average. Significantly, PL QY of CVD MoSe2 treated by ethanol can reach ∼16%, which is at the forefront of the previous reports of 2D MoSe2. Our study demonstrated an optimized method to enhance the PL QY of CVD monolayer TMDCs, which would facilitate TMDCs optoelectronics.

Polariton waveguide modes in two-dimensional van der Waals crystals: an analytical model and correlative nano-imaging.

Two-dimensional van der Waals (vdW) crystals can sustain various types of polaritons with strong electromagnetic confinements, making them highly attractive for nanoscale photonic and optoelectronic applications. While extensive experimental and numerical studies have been devoted to the polaritons of the vdW crystals, analytical models are sparse. Particularly, applying the model to describe polariton behaviors that are visualized by state of the art near-field optical microscopy requires further investigations. In this study, we develop an analytical waveguide model to describe polariton propagations in vdW crystals. The dispersion contours, dispersion relations, and localized electromagnetic field distributions of polariton waveguide modes are derived. The model is verified by real-space optical nano-imaging and numerical simulation of phonon polaritons in α-MoO3, which is a vdW biaxial crystal. Although we focus on α-MoO3, the proposed model is valid for other polaritonic crystals within the vdW family given the corresponding dielectric substitutions. Our model therefore provides an analytical rationale for describing and understanding the localized electromagnetic fields in vdW crystals that are associated with polaritons.

In Situ Ultrafast and Patterned Growth of Transition Metal Dichalcogenides from Inkjet-Printed Aqueous Precursors.

Chemical vapor deposition (CVD) has been widely used to synthesize high-quality 2D transition-metal dichalcogenides (TMDCs) from different precursors. At present, quantitative control of the precursor with high precision and good repeatability is still challenging. Moreover, the process to synthesize TMDCs with designed patterns is complicated. Here, by using an industrial inkjet-printer, an in situ aqueous precursor with robust usage control at the picogram (10-12 g) level is achieved, and by precisely tuning the inkjet-printing parameters, followed by a rapid heating process, large-area patterned TMDC films with centimeter size and good thickness controllability, as well as heterostructures of the TMDCs, are achieved facilely, and high-quality single-domain monolayer TMDCs with millimeter-size can be easily synthesized within 30 s (corresponding to a growth rate up to 36.4 µm s-1 ). The resulting monolayer MoS2 and MoSe2 exhibits excellent electronic properties with carrier mobility up to 21 and 54 cm2 V-1 s-1 , respectively. The study paves a simple and robust way for the in situ ultrafast and patterned growth of high-quality TMDCs and heterostructures with promising industrialization prospects. Moreover, this ultrafast and green method can be easily used for synthesis of other 2D materials with slight modification.

Synthesis and Characterization of Metallic Janus MoSH Monolayer.

Janus transition-metal dichalcogenides (TMDCs) are emerging as special 2D materials with different chalcogen atoms covalently bonded on each side of the unit cell, resulting in interesting properties. To date, several synthetic strategies have been developed to realize Janus TMDCs, which first involves stripping the top-layer S of MoS2 with H atoms. However, there has been little discussion on the intermediate Janus MoSH. It is critical to find the appropriate plasma treatment time to avoid sample damage. A thorough understanding of the formation and properties of MoSH is highly desirable. In this work, a controlled H2-plasma treatment has been developed to gradually synthesize a Janus MoSH monolayer, which was confirmed by the TOF-SIMS analysis as well as the subsequent fabrication of MoSSe. The electronic properties of MoSH, including the high intrinsic carrier concentration (∼2 × 1013 cm-2) and the Fermi level (∼ - 4.11 eV), have been systematically investigated by the combination of FET device study, KPFM, and DFT calculations. The results demonstrate a method for the creation of Janus MoSH and present the essential electronic parameters which have great significance for device applications. Furthermore, owing to the metallicity, 2D Janus MoSH might be a potential platform to observe the SPR behavior in the mid-infrared region.

Observation of thickness-tuned universality class in superconducting ß-W thin films.

The interplay between quenched disorder and critical behavior in quantum phase transitions is conceptually fascinating and of fundamental importance for understanding phase transitions. However, it is still unclear whether or not the quenched disorder influences the universality class of quantum phase transitions. More crucially, the absence of superconducting-metal transitions under in-plane magnetic fields in 2D superconductors imposes constraints on the universality of quantum criticality. Here, we observe the thickness-tuned universality class of superconductor-metal transition by changing the disorder strength in ß-W films with varying thickness. The finite-size scaling uncovers the switch of universality class: quantum Griffiths singularity to multiple quantum criticality at a critical thickness of tc⊥1~8nm and then from multiple quantum criticality to single criticality at tc⊥2~16nm. Moreover, the superconducting-metal transition is observed for the first time under in-plane magnetic fields and the universality class is changed at tc‖~8nm. The observation of thickness-tuned universality class under both out-of-plane and in-plane magnetic fields provides broad information for the disorder effect on superconducting-metal transitions and quantum criticality.

A Universal Method to Weld Individual One-Dimensional Nanostructures with a Tungsten Needle Based on Synergy of the Electron Beam and Electrical Current.

One-dimensional (1D) nanostructures are extensively used in the design of novel electronic devices, sensors, and energy devices. One of the major challenges faced by the electronics industry is the problem of contact between the 1D nanostructure and electrode, which can limit or even jeopardize device operations. Herein, a universal method that can realize good Ohmic and mechanical contact between an individual 1D nanostructure and a tungsten needle at sub-micron or micron scale is investigated and presented in a scanning electron microscope (SEM) chamber with the synergy of an electron beam and electrical current flowing through the welded joint. The linear I‒V curves of five types of individual 1D nanostructures, characterized by in-situ electrical measurements, demonstrate that most of them demonstrate good Ohmic contact with the tungsten needle, and the results of in-situ tensile measurements demonstrate that the welded joints possess excellent mechanical performance. By simulation analysis using the finite element method, it is proved that the local heating effect, which is mainly produced by the electrical current flowing through the welded joints during the welding process, is the key factor in achieving good Ohmic contact.

Self-Optimizing Effect of a Few-Layer Graphene's Top-Edge Structure during Field Electron Emission Observed by In Situ TEM.

A phenomenon that a few-layer graphene's (FLG) top transformed to single-layer graphene (SLG) with some bilayer/triple-layer patches on its surface during field electron emission was observed using in situ transmission electron microscopy (TEM). During field electron emission with high emission current, the FLG's top five layers split and finally transformed to SLG with some bilayer/triple-layer patches on its surface with a better crystallinity. It was due to thermal exfoliation and atom recombination at high temperatures induced by joule heat. The heat-induced structural self-transformation optimizes the field electron emission from the graphene's top edge. After transformation, the emission current increased with an order of magnitude at high field region (>307 V/µm). A modified field emission theory of graphene with curves of ln (I/E3/2)∼1/E and ln (I/E3)∼1/E2 in high and low field regimes, respectively, has been used to analyze the phenomenon. The graphene's line current density of two-dimensional (2D) structure and its special energy-dispersion relation at K state of Dirac point makes the curves of ln (I/E3/2)∼1/E and ln (I/E3)∼1/E2 to show up-bending features, which leads to the improvement of the field electron emission tunneling efficiency as the applied electric field increases. These results revealed that the intrinsic field emission characteristics of graphene can be achieved after a structural self-optimizing transformation of FLG during high current field electron emission. It offers an efficient post-treatment method to achieve high performance of graphene field emitter.