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.

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.

Improved field emission properties of α-Fe2O3 nanoflakes with current aging treatment and morphology optimization.

α-Fe2O3 nanomaterials were synthesized by thermal oxidation of pure iron foil and the effects of current aging treatment and morphology on their field emission properties were systematically investigated. The current aging treatment was found to be an efficient method to improve the field emission properties of α-Fe2O3 nanoflakes. The emission current density was largely enhanced from 0.05-5.70 mA cm-2 under an applied electrical field of 7.8 MV m-1, and their threshold field decreased from than 11.0-6.6 MV m-1 after the current aging treatment. The mechanism of the improvement in the field emission performance of α-Fe2O3 nanoflakes induced by the current aging treatment is discussed. α-Fe2O3 nanostructures with various morphologies were synthesized by adjusting the growth temperatures between 300 °C-450 °C to optimize their morphologies. α-Fe2O3 nanoflakes synthesized at 350 °C were superior field emitters with a low threshold field of 5.1 MV m-1, high current density of 63.4 mA cm-2, and stable emission, which demonstrated that α-Fe2O3 nanoflakes could be a promising material for application as field emitters.

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.

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.

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.

Growth of large-scale boron nanowire patterns with identical base-up mode and in situ field emission studies of individual boron nanowire.

Boron nanowires (BNWs) are considered as an ideal optoelectronic nanomaterial, but controlling them in identical growth mode and large-area patterns is technically challenging. Here, large-scale BNW patterns with a uniform base-up growth mode are successfully fabricated by choosing Ni film as the catalyst. Moreover, they exhibit low turn-on field (4.3 V/µm) and excellent field emission uniformity (88%).

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure.

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Realization of High Current Gain for Van der Waals MoS2/WSe2/MoS2 Bipolar Junction Transistor.

Two-dimensional (2D) materials have attracted great attention in the past few years and offer new opportunities for the development of high-performance and multifunctional bipolar junction transistors (BJTs). Here, a van der Waals BJT based on vertically stacked n+-MoS2/WSe2/MoS2 was demonstrated. The electrical performance of the device was investigated under common-base and common-emitter configurations, which show relatively large current gains of α ≈ 0.98 and ß ≈ 225. In addition, the breakdown characteristics of the vertically stacked n+-MoS2/WSe2/MoS2 BJT were investigated. An open-emitter base-collector breakdown voltage (BVCBO) of 52.9 V and an open-base collector-emitter breakdown voltage (BVCEO) of 40.3 V were observed under a room-temperature condition. With the increase in the operating temperature, both BVCBO and BVCEO increased. This study demonstrates a promising way to obtain 2D-material-based BJT with high current gains and provides a deep insight into the breakdown characteristics of the device, which may promote the applications of van der Waals BJTs in the fields of integrated circuits.

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.

Imitating and exploring the human brain's resting and task-performing states via brain computing: scaling and architecture.

A computational human brain model with the voxel-wise assimilation method was established based on individual structural and functional imaging data. We found that the more similar the brain model is to the biological counterpart in both scale and architecture, the more similarity was found between the assimilated model and the biological brain both in resting states and during tasks by quantitative metrics. The hypothesis that resting state activity reflects internal body states was validated by the interoceptive circuit's capability to enhance the similarity between the simulation model and the biological brain. We identified that the removal of connections from the primary visual cortex (V1) to downstream visual pathways significantly decreased the similarity at the hippocampus between the model and its biological counterpart, despite a slight influence on the whole brain. In conclusion, the model and methodology present a solid quantitative framework for a digital twin brain for discovering the relationship between brain architecture and functions, and for digitally trying and testing diverse cognitive, medical and lesioning approaches that would otherwise be unfeasible in real subjects.

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.

Firing feature-driven neural circuits with scalable memristive neurons for robotic obstacle avoidance.

Neural circuits with specific structures and diverse neuronal firing features are the foundation for supporting intelligent tasks in biology and are regarded as the driver for catalyzing next-generation artificial intelligence. Emulating neural circuits in hardware underpins engineering highly efficient neuromorphic chips, however, implementing a firing features-driven functional neural circuit is still an open question. In this work, inspired by avoidance neural circuits of crickets, we construct a spiking feature-driven sensorimotor control neural circuit consisting of three memristive Hodgkin-Huxley neurons. The ascending neurons exhibit mixed tonic spiking and bursting features, which are used for encoding sensing input. Additionally, we innovatively introduce a selective communication scheme in biology to decode mixed firing features using two descending neurons. We proceed to integrate such a neural circuit with a robot for avoidance control and achieve lower latency than conventional platforms. These results provide a foundation for implementing real brain-like systems driven by firing features with memristive neurons and put constructing high-order intelligent machines on the agenda.

In-plane hyperbolic polariton tuners in terahertz and long-wave infrared regimes.

One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon - polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO3, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO3. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices.

High luminescent Li2CaSiO4:Eu2+ cyan phosphor film for wide color gamut field emission display.

Li(2)CaSiO(4):Eu(2+) cyan phosphor screen for enlarging the color gamut of field emission display has been prepared and characterized. The luminance of Li(2)CaSiO(4):Eu(2+) phosphor film can reach as high as about 12000 cd/m(2) under the conditions of V(a) = 7 kV and J(a) = 2.8 mA/cm(2). The cathodoluminescent spectrum, luminance, saturation current density and degradation property are compared with another cyan phosphor Mg(2)SnO(4):Ti(4+),Mn(2+). It is found that Li(2)CaSiO(4):Eu(2+) phosphor exhibits narrower emission band, higher luminance, higher saturation current density, higher resistance to electron bombardment, higher thermal stability and conductivity as well as purer color. Thus, Li(2)CaSiO(4):Eu(2+) has great potential in application in field emission display as well as light emitting diode.

Optimize the field emission character of a vertical few-layer graphene sheet by manipulating the morphology.

Vertical few-layer graphene (FLG) sheets have been fabricated by using microwave-plasma-enhanced chemical vapour deposition. Their shape was manipulated through adjusting the growth time and hydrocarbon gas ratio. The growth mechanism during different growth stages is discussed. The field emission characteristics for different FLG shapes were tested and found to be strongly influenced by the tip shape, the height and the amorphous carbon content. The optimal shape of vertical FLG for field emission had fewer layers, sharp corners, large height and was free of amorphous carbon. The best field emission properties with the optimal shape were observed with a turn-on field of 1:8 V µm(-1) and maximum current density of 7 mA cm(-2).

Controlling and Focusing In-Plane Hyperbolic Phonon Polaritons in α-MoO3 with a Curved Plasmonic Antenna.

Hyperbolic phonon polaritons (HPhPs) sustained in polar van der Waals (vdW) crystals exhibit extraordinary confinement of long-wave electromagnetic fields to the deep subwavelength scale. In stark contrast to uniaxial vdW hyperbolic materials, recently emerged biaxial hyperbolic materials, such as α-MoO3 and α-V2 O5 , offer new degrees of freedom for controlling light in two-dimensions due to their distinctive in-plane hyperbolic dispersions. However, the control and focusing of these in-plane HPhPs remain elusive. Here, a versatile technique is proposed for launching, controlling, and focusing in-plane HPhPs in α-MoO3 with geometrically designed curved gold plasmonic antennas. It is found that the subwavelength manipulation and focusing behaviors are strongly dependent on the curvature of the antenna extremity. This strategy operates effectively in a broadband spectral region. These findings not only provide fundamental insights into the manipulation of light by biaxial hyperbolic crystals at the nanoscale but also open up new opportunities for planar nanophotonic applications.

Engineering Graphene Grain Boundaries for Plasmonic Multi-Excitation and Hotspots.

Surface plasmons, merging photonics and electronics in nanoscale dimensions, have been the cornerstones in integrated informatics, precision detection, high-resolution imaging, and energy conversion. Arising from the exceptional Fermi-Dirac tunability, ultrafast carrier mobility, and high-field confinement, graphene offers excellent advantages for plasmon technologies and enables a variety of state-of-the-art optoelectronic applications ranging from tight-field-enhanced light sources, modulators, and photodetectors to biochemical sensors. However, it is challenging to co-excite multiple graphene plasmons on one single graphene sheet with high density, a key step toward plasmonic wavelength-division multiplexing and next-generation dynamical optoelectronics. Here, we report the heteroepitaxial growth of a polycrystalline graphene monolayer with patterned gradient grain boundary density, which is synthesized by creating diverse nanosized local growth environments on a centimeter-scale substrate with a polycrystalline graphene ring seed in chemical vapor deposition. Such geometry enables plasmonic co-excitation with varied wavelength diversification in the nanoscale. Via using high-resolution scanning near-field optical microscopy, we demonstrate rich plasmon standing waves, even bright plasmonic hotspots with a size up to 3 µm. Moreover, by changing the grain boundary density and annealing, we find the local plasmonic wavelengths are widely tunable, from 70 to 300 nm. Theoretical modeling supports that such plasmonic versatility is due to the grain boundary-induced plasmon-phonon interactions through random phase approximation. The seed-induced heteroepitaxial growth provides a promising way for the grain boundary engineering of two-dimensional materials, and the controllable grain boundary-based plasmon co-generation and manipulation in one single graphene monolayer will facilitate the applications of graphene for plasmonics and nanophotonics.