Observation of chiral and slow plasmons in twisted bilayer graphene.

Moiré superlattices have led to observations of exotic emergent electronic properties such as superconductivity and strong correlated states in small-rotation-angle twisted bilayer graphene (tBLG)1,2. Recently, these findings have inspired the search for new properties in moiré plasmons. Although plasmon propagation in the tBLG basal plane has been studied by near-field nano-imaging techniques3-7, the general electromagnetic character and properties of these plasmons remain elusive. Here we report the direct observation of two new plasmon modes in macroscopic tBLG with a highly ordered moiré superlattice. Using spiral structured nanoribbons of tBLG, we identify signatures of chiral plasmons that arise owing to the uncompensated Berry flux of the electron gas under optical pumping. The salient features of these chiral plasmons are shown through their dependence on optical pumping intensity and electron fillings, in conjunction with distinct resonance splitting and Faraday rotation coinciding with the spectral window of maximal Berry flux. Moreover, we also identify a slow plasmonic mode around 0.4 electronvolts, which stems from the interband transitions between the nested subbands in lattice-relaxed AB-stacked domains. This mode may open up opportunities for strong light-matter interactions within the highly sought after mid-wave infrared spectral window8. Our results unveil the new electromagnetic dynamics of small-angle tBLG and exemplify it as a unique quantum optical platform.

Single-nanowire-morphed mechanical slingshot for directional shooting delivery of micro-payloads.

Stretching elastomer bands to accumulate strain energy, for a sudden projectile launching, has been an old hunting skill that will continue to find new applications in miniaturized worlds. In this work, we explore the use of highly resilient and geometry-tailored ultrathin crystalline silicon nanowires (SiNWs) as elastic medium to fabricate the first, and the smallest, mechanical slingshot. These NW-morphed slingshots were first grown on a planar surface, with desired layout, and then mounted upon standing pillar frames, with a unique self-hooking structure that allows for a facile and reliable assembly, loading and shooting maneuver of microsphere payloads. Impressively, the elastic spring design can help to store 10 times more strain energy into the NW springs, compared with the straight ones under the same pulling force, which has been strong enough to overcome the sticky van der Waals (vdW) force at the touching interfaces that otherwise will hinder a reliable releasing onto soft surface with low-surface energy or adhesion force, and to achieve a directional shooting delivery of precise amount of tiny payload units onto delicate target with the least impact damage. This NW-morphing construction strategy also provides a generic protocol/platform to fast design, prototype, and deploy new nanoelectromechanical and biological applications at extremely low costs.

Atomic-Scale Confinement and Negative Refraction of Plasmons by Twisted Bilayer Graphene.

Moiré superlattices provide in-plane quantum restriction for light-matter interactions in twisted bilayer graphene (tBLG), leading to the exotic photon-Moiré physics and potential applications for light manipulation. Recently, our experiment identified a highly confined slow surface plasmons polaritons (SPPs) mode in tBLG. Here, we demonstrate that the propagation of the slow SPPs mode in tBLG is spatially tailored and steered at deep subwavelengths. Analysis by the perturbation theory indicates that the coupling between the slow SPPs mode and the Moiré system is greatly strengthened, which regulates the wavefront at the atomic scale and makes tBLG serve as a universal optical metamaterial. Consequently, the negative refraction is achieved at the interface of monolayer graphene and tBLG, by which a metalens with a controllable focal length and an extremely high resolution up to 1/150 of wavelength is devised. Our work paves the way for constructing optical metamaterial at the atomic scale and develops future photon-Moiré interaction systems.

Designable Integration of Silicide Nanowire Springs as Ultra-Compact and Stretchable Electronic Interconnections.

Stretchable electronics are finding widespread applications in bio-sensing, skin-mimetic electronics, and flexible displays, where high-density integration of elastic and durable interconnections is a key capability. Instead of forming a randomly crossed nanowire (NW) network, here, a large-scale and precise integration of highly conductive nickel silicide nanospring (SiNix -NS) arrays are demonstrated, which are fabricated out of an in-plane solid-liquid-solid guided growth of planar Si nanowires (SiNWs), and subsequent alloy-forming process that boosts the channel conductivity over 4 orders of magnitude (to 2 × 104 S cm-1 ). Thanks to the narrow diameter of the serpentine SiNix -NS channels, the elastic geometry engineering can be accomplished within a very short interconnection distance (down to ≈3 µm), which is crucial for integrating high-density displays or logic units in a rigid-island and elastic-interconnection configuration. Deployed over soft polydimethylsiloxane thin film substrate, the SiNix -NS array demonstrates an excellent stretchability that can sustain up to 50% stretching and for 10 000 cycles (at 15%). This approach paves the way to integrate high-density inorganic electronics and interconnections for high-performance health monitoring, displays, and on-skin electronic applications, based on the mature and rather reliable Si thin film technology.

Ultra-Confined Catalytic Growth Integration of Sub-10 nm 3D Stacked Silicon Nanowires Via a Self-Delimited Droplet Formation Strategy.

Fabricating ultrathin silicon (Si) channels down to critical dimension (CD) <10 nm, a key capability to implementing cutting-edge microelectronics and quantum charge-qubits, has never been accomplished via an extremely low-cost catalytic growth. In this work, 3D stacked ultrathin Si nanowires (SiNWs) are demonstrated, with width and height of Wnw  = 9.9 ± 1.2 nm (down to 8 nm) and Hnw  = 18.8 ± 1.8 nm, that can be reliably grown into the ultrafine sidewall grooves, approaching to the CD of 10 nm technology node, thanks to a new self-delimited droplet control strategy. Interestingly, the cross-sections of the as-grown SiNW channels can also be easily tailored from fin-like to sheet-like geometries by tuning the groove profile, while a sharply folding guided growth indicates a unique capability to produce closely-packed multiple rows of stacked SiNWs, out of a single run growth, with the minimal use of catalyst metal. Prototype field effect transistors are also successfully fabricated, achieving Ion/off ratio and sub-threshold swing of >106 and 125 mV dec-1 , respectively. These results highlight the unexplored potential of versatile catalytic growth to compete with, or complement, the advanced top-down etching technology in the exploitation of monolithic 3D integration of logic-in-memory, neuromorphic and charge-qubit applications.

Superfast Growth Dynamics of High-Quality Silicon Nanowires on Polymer Films via Self-Selected Laser-Droplet-Heating.

Growing high quality silicon nanowires (SiNWs) at elevated temperature on cooler polymer films seems to be contradictive but highly desirable for building high performance flexible and wearable electronics. In this work, we demonstrate a superfast (vnw > 3.5 µm·s-1) growth of high quality SiNWs on polymer/glass substrates, powered by self-selected laser at 808 nm heating of indium catalyst droplets that absorb amorphous Si layer to produce SiNWs. Because of the tiny heat capacity of the nanodroplets, the SiNW growth can be quickly heated up and frozen via rapid laser ON/OFF switching, enabling a deterministic diameter modulation in the ultralong SiNWs. Finally, prototype field effect transistors are also fabricated upon the laser-droplet-heating grown SiNWs with a high Ion/Ioff ratio of >104 and reasonable subthreshold swing of 386 mV·dec-1, opening a generic new route to integrate high-quality NW channels directly upon large area and lightweight polymer substrates for developing high-performance flexible electronics.

Ab Initio Design, Shaping, and Assembly of Free-Standing Silicon Nanoprobes.

Free-standing silicon nanoprobes (SiNPs) are critical tools for intracellular bioelectrical signal recording, while a scalable fabrication of these tiny SiNPs with ab initio geometry designs has not been possible. In this work, we demonstrate a novel growth shaping of slim Si nanowires (SiNWs) into SiNPs with sharp tips (curvature radii <300 nm), tunable angles of 30°, 60°, to 120° and even programmable triangle/circular shapes. A precise growth integration of orderly single, double, and quadruple SiNPs at prescribed locations enables convenient electrode connection, transferring and mounting these tiny tips onto movable arms to serve as long-protruding (over 4-20 µm) nanoprobes. Mechanical flexibility, resilience, and field-effect sensing functionality of the SiNPs were systematically testified in liquid nanodroplet and cell environments. This highly reliable and economic manufacturing of advanced SiNPs holds a strong potential to boost and open up the market implementations of a wide range of intracellular sensing, monitoring, and editing applications.

Terrace-confined guided growth of high-density ultrathin silicon nanowire array for large area electronics.

Ultrathin silicon nanowires (SiNWs) are ideal 1D channels to construct high performance nanoelectronics and sensors. We here report on a high-density catalytic growth of orderly ultrathin SiNWs, with diameter down toDnw=27±2nmand narrow NW-to-NW spacing of onlySnw âˆ¼80 nm, without the use of high-resolution lithography. This has been accomplished via a terrace-confined strategy, where tiny indium (In) droplets move on sidewall terraces to absorb precoated amorphous Si layer as precursor and produce self-aligned SiNW array. It is found that, under proper parameter control, a tighter terrace-step confinement can help to scale the dimensions of the SiNW array down to the extremes that have not been reported before, while maintaining still a stable guiding growth over complex contours. Prototype SiNW field effect transistors demonstrate a highIon/Ioffcurrent ratio ∼107, low leakage current of ∼0.3 pA and steep subthreshold swing of 220 mV dec-1. These results highlight the unexplored potential of catalytic growth in advanced nanostructure fabrication that is highly relevant for scalable SiNW logic and sensor applications.

Cylindrical Line-Feeding Growth of Free-Standing Silicon Nanohelices as Elastic Springs and Resonators.

Three-dimensional (3D) construction of free-standing silicon (Si) nanohelices has been a formidable challenge for planar lithography and etching technology. We here demonstrate a convenient 3D growth and integration of Si nanohelices (SiNHs) upon bamboolike cylinders with corrugated sidewall grooves, where the indium catalyst droplets grow around the cylinders in a helical fashion, while consuming precoated amorphous Si (a-Si) thin film to produce crystalline Si nanowires on the sidewalls. At the end of each groove cycle, the droplets are enforced to linefeed/switch into the neighbor groove to continue a spiral growth of SiNHs with readily tunable diameter, pitch, aspect-ratio, and chiral/achiral symmetries. In addition, the SiNHs can be reliably released as free-standing units to serve as elastic links, supports and vibrational resonators. These results highlight the unexplored potential of high precision 3D self-assembly growth in constructing a wide range of sophisticated electromechanical, sensor, and optoelectronic functionalities.

Unprecedented Uniform 3D Growth Integration of 10-Layer Stacked Si Nanowires on Tightly Confined Sidewall Grooves.

Bottom-up catalytic growth offers a high-yield, versatile, and powerful tool for the construction of versatile 3D nanocomplexes, while the major challenge is to achieve a precise location and uniformity control, as guaranteed by top-down lithography. Here, an unprecedented uniform and reliable growth integration of 10-layer stacked Si nanowires (SiNWs) has been accomplished, for the very first time, via a new groove-confined and tailored catalyst formation and guided growth upon the truncated sidewall of SiO2/SiNx multilayers. The SiNW array accomplishes a narrow diameter of Dnw = 28 ± 2.4 nm, NW-to-NW spacing of tsp = 40 nm, and extremely stable growth over Lnw > 50 µm and bending locations, which can compete with or even outperform the state-of-the-art top-down lithography and etching approaches, in terms of stacking number, channel uniformity at different levels, fabrication cost, and efficiency. These results provide a solid basis to establish a new 3D integration approach to batch-manufacture various advanced electronic and sensor applications.

Germanium quantum dot infrared photodetectors addressed by self-aligned silicon nanowire electrodes.

Germanium quantum dots (GeQDs), addressed by self-aligned and epitaxial silicon nanowires (SiNWs) as electrodes, represent the most fundamental and the smallest units that can be integrated into Si optoelectronics for 1550 nm wavelength detection. In this work, individual GeQD photodetectors have been fabricated based on a low temperature self-condensation of uniform amorphous Si (a-Si)/a-Ge bilayers at 300 °C, led by rolling indium (In) droplets. Remarkably, the diameter of the GeQD nodes can be independently controlled to achieve wider GeQDs for maximizing infrared absorption with narrower SiNW electrodes to ensure a high quality Ge/Si hetero-epitaxial connection. Importantly, these hetero GeQD/SiNW photodetectors can be deployed into predesigned locations for scalable device fabrication. The photodetectors demonstrate a responsivity of 1.5 mA W-1 and a photoconductive gain exceeding 102 to the communication wavelength signals, which are related to the beneficial type-II Ge/Si alignment, gradient Ge/Si epitaxial transition and a larger QD/NW diameter ratio. These results indicate a new approach to batch-fabricate and integrate GeQDs for ultra-compact Si-compatible photodetection and imaging applications.

Monolithic Integration of Silicon Nanowire Networks as a Soft Wafer for Highly Stretchable and Transparent Electronics.

Assembling nanoscale building blocks into an orderly network with a programmable layout and channel designs represents a critical capability to enable a wide range of stretchable electronics. Here, we demonstrate the growth-in-place integration of silicon nanowire (SiNW) springs into highly stretchable, transparent, and quasicontinuous functional networks with a close to unity interconnection among the discrete electrode joints because of a unique double-lane/double-step guiding edge design. The SiNW networks can be reliably transferred to a soft elastomer substrate, conformally attached to highly curved surfaces, or deployed as self-supporting/movable membranes suspended over voids. A high stretchability of >40% is achieved for the SiNW network on an elastomer, which can be employed as a transparent and semiconducting thin-film material endowed with a high carrier mobility of >50 cm2/(V s), Ion/Ioff ratio >104, and a tunable transmission of >80% over a wide spectrum range. Reversibly stretchable and bendable sensors based on the SiNW network have been successfully demonstrated, where the local strain distribution within the spring network can be directly observed and analyzed by finite element simulations. This SiNW network has a unique potential to eventually establish a new generically purposed waferlike platform for constructing soft electronics with Si-based hard performances.

Plasmon Excited Ultrahot Carriers and Negative Differential Photoresponse in a Vertical Graphene van der Waals Heterostructure.

Photogenerated nonequilibrium hot carriers play a key role in graphene's intriguing optoelectronic properties. Compared to conventional photoexcitation, plasmon excitation can be engineered to enhance and control the generation and dynamics of hot carriers. Here, we report an unusual negative differential photoresponse of plasmon-induced "ultrahot" electrons in a graphene-boron nitride-graphene tunneling junction. We demonstrate nanocrescent gold plasmonic nanostructures that substantially enhance the absorption of long-wavelength photons whose energy is greatly below the tunneling barrier and significantly boost the electron thermalization in graphene. We further analyze the generation and transfer of ultrahot electrons under different bias and power conditions. We find that the competition among thermionic emission, the carrier-cooling effect, and the field effect results in a hitherto unusual negative differential photoresponse in the photocurrent-bias plot. Our results not only exemplify a promising platform for detecting low-energy photons, enhancing the photoresponse, and reducing the dark current but also reveal the critically coupled pathways for harvesting ultrahot carriers.

Coupled boron-doping and geometry control of tin-catalyzed silicon nanowires for high performance radial junction photovoltaics.

Geometry and doping control in silicon nanowires (SiNWs) are both crucial aspects in fabricating three-dimensional (3D) radial junction thin film solar cells, while the coupling between them remains a peculiar aspect to be better understood. In this work, we focus on the geometry evolution and the doping effects realized in tin-catalyzed SiNWs grown via a plasma-enhanced vapor-liquid-solid procedure by using different diborane (B2H6) dopant flows. It is shown that with the increase of B2H6 flow rate from 0.3 to 2.1 SCCM, the radial growth of SiNWs is greatly accelerated by more than 30%, while the length is shortened to 50%. This can be related to the enhanced chemisorption probability of SiHx radicals, with the addition of B2H6, on the SiNW sidewall during silane (SiH4) plasma deposition in PECVD system, which leads to easier nucleation directly on the sidewalls and faster radial expansion of the SiNWs. A trade-off has to be sought between seeking a strong light trapping and ensuring a sufficient doping for high-quality PIN junction with the increase of B2H6 doping flow. These new understandings lay a critical basis for understanding and searching for an optimal growth control for constructing high-performance 3D radial junction thin-film solar cells.

Advanced radial junction thin film photovoltaics and detectors built on standing silicon nanowires.

Three-dimensional (3D) construction of radial junction hydrogenated amorphous silicon (a-Si:H) thin film solar cells on standing silicon nanowires (SiNWs) is a promising strategy to maximize the light harvesting performance and improve the photocarrier collection in an optimized junction configuration. The unique light in-coupling and absorption behaviour in the antenna-like 3D photonic structures also necessitates a set of new theoretical models and simulation tools to design, predict and optimize the photovoltaic performance of radial junction solar cells, which can be rather different from planar junction solar cells. Recently, the performance of radial junction a-Si:H thin film solar cells has progressed steadily to a level comparable or even superior to that of their planar counterparts, with plenty of room for further improvement. This review will first address the growth strategy and critical parameter control of SiNWs produced via a plasma-assisted low-temperature vapour-liquid-solid procedure using low-melting-point metals as the catalyst. Then, the construction of high-performance radial junction thin film solar cells over the standing SiNW matrix, as well as their optimal structural designs, will be introduced. At the end, the new applications of 3D radial junction units will be summarized, which include, for example, the construction of very flexible, low-cost and efficient a-Si:H solar cells with the highest power-to-weight ratio, the demonstration of highly sensitive solar-blind photodetectors operating at the ultraviolet wavelength spectrum and the development of novel biomimetic radial tandem junction photodetectors with an intrinsic red-green-blue (RGB) colour distinguishing capability.

Nanodroplet Hydrodynamic Transformation of Uniform Amorphous Bilayer into Highly Modulated Ge/Si Island-Chains.

Geometric and compositional modulations are the principal parameters of control to tailor the band profile in germanium/silicon (Ge/Si) heteronanowires (NWs). This has been achieved mainly by alternating the feeding precursors during a uniaxial growth of Ge/Si NWs. In this work, a self-automated growth of Ge/Si hetero island-chain nanowires (hiNWs), consisting of wider c-Ge islands connected by thinner c-Si chains, has been accomplished via an indium (In) droplet-mediated transformation of uniform amorphous a-Si/a-Ge bilayer thin films. The surface-running In droplet enforces a circulative hydrodynamics in the nanoscale droplet, which can modulate the absorption depth into the amorphous bilayer and enable a single-run growth of a superlattice-like hiNWs without the need for any external manipulation. Meanwhile, the separation and accumulation of electrons and holes in the phase-modulated Ge/Si superlattice leads to a modulated surface potential profile that can be directly resolved by Kelvin probe force microscopy. This simple self-assembly growth and modulation dynamics can help to establish a powerful new concept or strategy to tailor and program the geometric and compositional profiles of more complex hetero nanowire structures, as promising building blocks to develop advanced nanoelectronics or optoelectronics.

Electrically tunable optical properties of few-layer black arsenic phosphorus.

Black arsenic phosphorus (b-AsP) is a promising two-dimensional material for various optoelectronic applications, bridging the wavelength gap between two-dimensional molybdenum disulfide and graphene. In particular, it has intriguing potential in photodetectors and great advantages in the mid-infrared field. However, its optoelectronic modulation has yet to be elucidated, which requires a fundamental understanding of its field-effect optical modulation. Here, we report the measurements of the lower-energy infrared anisotropic optical response of thin b-AsP under different electrical gating. We reveal that in addition to band edge absorption, amplitude modulation of sub-band absorption up to ten percent is also obtained in reflection extinction. These in-gap absorptions are attributed to spin-orbital coupling and free carrier absorption. Our results suggest the important potential for use of b-AsP in mid-infrared optoelectronic modulator applications.

Deterministic Line-Shape Programming of Silicon Nanowires for Extremely Stretchable Springs and Electronics.

Line-shape engineering is a key strategy to endow extra stretchability to 1D silicon nanowires (SiNWs) grown with self-assembly processes. We here demonstrate a deterministic line-shape programming of in-plane SiNWs into extremely stretchable springs or arbitrary 2D patterns with the aid of indium droplets that absorb amorphous Si precursor thin film to produce ultralong c-Si NWs along programmed step edges. A reliable and faithful single run growth of c-SiNWs over turning tracks with different local curvatures has been established, while high resolution transmission electron microscopy analysis reveals a high quality monolike crystallinity in the line-shaped engineered SiNW springs. Excitingly, in situ scanning electron microscopy stretching and current-voltage characterizations also demonstrate a superelastic and robust electric transport carried by the SiNW springs even under large stretching of more than 200%. We suggest that this highly reliable line-shape programming approach holds a strong promise to extend the mature c-Si technology into the development of a new generation of high performance biofriendly and stretchable electronics.

Heteroepitaxial Writing of Silicon-on-Sapphire Nanowires.

The heteroepitaxial growth of crystal silicon thin films on sapphire, usually referred to as SoS, has been a key technology for high-speed mixed-signal integrated circuits and processors. Here, we report a novel nanoscale SoS heteroepitaxial growth that resembles the in-plane writing of self-aligned silicon nanowires (SiNWs) on R-plane sapphire. During a low-temperature growth at <350 °C, compared to that required for conventional SoS fabrication at >900 °C, the bottom heterointerface cultivates crystalline Si pyramid seeds within the catalyst droplet, while the vertical SiNW/catalyst interface subsequently threads the seeds into continuous nanowires, producing self-oriented in-plane SiNWs that follow a set of crystallographic directions of the sapphire substrate. Despite the low-temperature fabrication process, the field effect transistors built on the SoS-SiNWs demonstrate a high on/off ratio of >5 × 104 and a peak hole mobility of >50 cm2/V·s. These results indicate the novel potential of deploying in-plane SoS nanowire channels in places that require high-performance nanoelectronics and optoelectronics with a drastically reduced thermal budget and a simplified manufacturing procedure.

Boosting light emission from Si-based thin film over Si and SiO(2) nanowires architecture.

Silicon (Si)-based light emitting thin film has been a key ingredient for all-Si-based optoelectronics. Besides material engineering, adopting a novel 3D photonic architecture represents an effective strategy to boost light excitation and extraction from Si-based thin film material. We here explore the use of a nanowires (NW) framework, grown via vapor-liquid-solid mode, to achieve strongly enhanced yellow-green luminescence from SiN(x)O(y)/NW core-shell structure, with an order of magnitude enhancement compared to co-deposited planar references. We found that choosing geometrically-identical but different NW cores (Si or SiO(2)) can lead to profound influence on the overall light emission performance. Combining parametric investigation and theoretical modeling, we have been able to evaluate the key contributions arising from different mechanisms that include near-field enhancement, 3D light trapping and enhanced light extraction. These new findings indicate a new and effective strategy for strong Si-based thin film light emitting source, while being generic enough to be applicable in a wide variety of other thin film materials.