Ultrafast field-driven monochromatic photoemission from carbon nanotubes

Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow for various forms of ultrafast microscopy and spectroscopy to elucidate otherwise challenging to observe physical and chemical transitions. However, the pursuit of simultaneous ultimate spatial and temporal resolution has been largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. State-of-the-art photon-driven sources have good monochromaticity but poor phase synchronization. In contrast, field-driven photoemission has much higher light phase synchronization, due to the intrinsic sub-cycle emission dynamics, but poor monochromaticity. Such sources suffer from larger electron energy spreads (3 - 100 eV) attributed to the relatively low field enhancement of the conventional metal tips which necessitates long pump wavelengths (> 800 nm) in order to gain sufficient ponderomotive potential to access the field-driven regime. In this work, field-driven photoemission from ~1 nm radius carbon nanotubes excited by a femtosecond laser at a short wavelength of 410 nm has been realized. The energy spread of field-driven electrons is effectively compressed to 0.25 eV outperforming all conventional ultrafast electron sources. Our new nanotube-based ultrafast electron source opens exciting prospects for attosecond imaging and emerging light-wave electronics

Tunable Interband Transitions in Twisted h-BN/Graphene Heterostructures

In twisted h-BN/graphene heterostructures, the complex electronic properties of the fast-traveling electron gas in graphene are usually considered to be fully revealed. However, the randomly twisted heterostructures may also have unexpected transition behaviors, which may influence the device performance. Here, we study the twist angle-dependent coupling effects of h-BN/graphene heterostructures using monochromatic electron energy loss spectroscopy. We find that the moir\'e potentials alter the band structure of graphene, resulting in a redshift of the intralayer transition at the M-point, which becomes more pronounced up to 0.25 eV with increasing twist angle. Furthermore, the twisting of the Brillouin zone of h-BN relative to the graphene M-point leads to tunable vertical transition energies in the range of 5.1-5.6 eV. Our findings indicate that twist-coupling effects of van der Waals heterostructures should be carefully considered in device fabrications, and the continuously tunable interband transitions through the twist angle can serve as a new degree of freedom to design optoelectrical devices

Quiver-quenched optical-field-emission from carbon nanotubes

Carbon nanotubes (CNTs) enable large electric field enhancement for an extremely broad bandwidth spanning from the optical domain down to static fields. This is due to their high aspect ratio, small tip radius, and high structural stability. CNTs therefore represent an ideal model-system for the investigation of nonlinear and strong-field phenomena. In this paper, we extend the range of optical-field-emission materials from metal nanostructures to CNTs. Quiver-quenched optical-field-emission (i.e., the transition to a sub-cycle regime) is observed for CNTs tips in a short-wavelength laser field of 820 nm that requires a mid-infrared excitation field of conventional metal tips emitters. This special property relies on the ultrasmall tips radius (∼1 nm) and the high optical-field enhancement (∼21.6) properties of CNTs. This study suggests that CNTs are excellent candidates for optically driven ultrafast electron sources with both high spatial and high temporal coherence. They also provide more freedom for the manipulation and control of electron dynamics at the attosecond timescale, which extends the bandwidth of light-wave electronic devices.Peer reviewe

Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity

| openaire: EC/H2020/834742/EU//ATOP | openaire: EC/H2020/820423/EU//S2QUIPNonlinear optical fibres have been employed for a vast number of applications, including optical frequency conversion, ultrafast laser and optical communication1–4. In current manufacturing technologies, nonlinearity is realized by the injection of nonlinear materials into fibres5–7 or the fabrication of microstructured fibres8–10. Both strategies, however, suffer from either low optical nonlinearity or poor design flexibility. Here, we report the direct growth of MoS2, a highly nonlinear two-dimensional material11, onto the internal walls of a SiO2 optical fibre. This growth is realized via a two-step chemical vapour deposition method, where a solid precursor is pre-deposited to guarantee a homogeneous feedstock before achieving uniform two-dimensional material growth along the entire fibre walls. By using the as-fabricated 25-cm-long fibre, both second- and third-harmonic generation could be enhanced by ~300 times compared with monolayer MoS2/silica. Propagation losses remain at ~0.1 dB cm–1 for a wide frequency range. In addition, we demonstrate an all-fibre mode-locked laser (~6 mW output, ~500 fs pulse width and ~41 MHz repetition rate) by integrating the two-dimensional-material-embedded optical fibre as a saturable absorber. Initial tests show that our fabrication strategy is amenable to other transition metal dichalcogenides, making these embedded fibres versatile for several all-fibre nonlinear optics and optoelectronics applications.Peer reviewe

Complete structural characterization of single carbon nanotubes by Rayleigh scattering circular dichroism

Funding Information: This work was supported by the National Natural Science Foundation of China under grant numbers 52021006 (J. Zhang), 52025023 (K.L.) and 51991342 (K.L.); the Key R&D Programme of Guangdong Province with grant numbers 2020B010189001 (K.L.), 2019B010931001 (K.L.) and 2018B030327001 (K.L.); the National Key R&D Programme of China under grant numbers 2016YFA0300903 (K.L.), 2017YFA0303800 (K.L.) and 2016YFA0300804 (P.G.); the Strategic Priority Research Programme of Chinese Academy of Sciences with grant number XDB33000000 (K.L.); Beijing Natural Science Foundation under grant number JQ19004 (K.L.); Beijing Graphene Innovation Programme under grant number Z181100004818003 (K.L.); the Pearl River Talent Recruitment Programme of Guangdong Province with grant number 2019ZT08C321 (K.L.); National Equipment Programme of China under grant number ZDYZ2015-1 (X.B.); National Postdoctoral Programme for Innovative Talents under award number BX20190016 (C.L.) and China Postdoctoral Science Foundation under award numbers 2019M660280 (C.L.) and 2019M660281 (R.Q.). Publisher Copyright: © 2021, The Author(s), under exclusive licence to Springer Nature Limited.Non-invasive, high-throughput spectroscopic techniques can identify chiral indices (n,m) of carbon nanotubes down to the single-tube level1–6. Yet, for complete characterization and to unlock full functionality, the handedness, the structural property associated with mirror symmetry breaking, also needs to be identified accurately and efficiently7–14. So far, optical methods fail in the handedness characterization of single nanotubes because of the extremely weak chiroptical signals (roughly 10−7) compared with the excitation light15,16. Here we demonstrate the complete structure identification of single nanotubes in terms of both chiral indices and handedness by Rayleigh scattering circular dichroism. Our method is based on the background-free feature of Rayleigh scattering collected at an oblique angle, which enhances the nanotube’s chiroptical signal by three to four orders of magnitude compared with conventional absorption circular dichroism. We measured a total of 30 single-walled carbon nanotubes including both semiconducting and metallic nanotubes and found that their absolute chiroptical signals show a distinct structure dependence, which can be qualitatively understood through tight-binding calculations. Our strategy enables the exploration of handedness-related functionality of single nanotubes and provides a facile platform for chiral discrimination and chiral device exploration at the level of individual nanomaterials.Peer reviewe

Robust synthesis of two-dimensional metal dichalcogenides and their alloys by active chalcogen monomer supply

Two-dimensional (2D) transition metal dichalcogenides (TMDs), with their atomic thicknesses, high carrier mobility, fast charge transfer, and intrinsic spin-valley couplings, have been demonstrated one of the most appealing candidates for next-generation electronic and optoelectronic devices. The synthesis of TMDs with well-controlled crystallinity, quality and composition is essential to fully realize their promising applications. Similar to that in III-V semiconductor synthesis, the precise precursor supply is a precondition for controllable growth of TMDs. Although great efforts have been devoted to modulate the transition metal supply, few effective methods of chalcogen feeding control were developed. Herein we report a strategy of using active chalcogen monomer supply to grow TMDs and their alloys in a robust and controllable manner. It is found that at a high temperature, the active chalcogen monomers (such as S, Se, Te atoms or their mixtures) can be controllably released from metal chalcogenides and, thus, enable the synthesis of TMDs (MX2, M = Mo, W; X = S, Se, Te) with very high quality, e.g., MoS2 monolayers exhibit photoluminescent circular helicity of ~92%, comparable to the best exfoliated single-crystal flakes and close to the theoretical limit of unity. More intriguingly, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished for the first time. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation and reaction, quick chalcogen vacancy healing, and alloy formation during the growth. The chalcogen monomer supply strategy offers more degrees of freedom for the controllable synthesis of 2D compounds and their alloys, which will greatly benefit the development of high-end devices with desired 2D materials

Visualizing the Anomalous Catalysis in Two-Dimensional Confined Space

© 2022 American Chemical Society.Confined nanospaces provide a new platform to promote catalytic reactions. However, the mechanism of catalytic enhancement in the nanospace still requires insightful exploration due to the lack of direct visualization. Here, we report operando investigations on the etching and growth of graphene in a two-dimensional (2D) confined space between graphene and a Cu substrate. We observed that the graphene layer between the Cu and top graphene layer was surprisingly very active in etching (more than 10 times faster than the etching of the top graphene layer). More strikingly, at a relatively low temperature (530 °C), the etched carbon radicals dissociated from the bottom layer, in turn feeding the growth of the top graphene layer with a very high efficiency. Our findings reveal the in situ dynamics of the anomalous confined catalytic processes in 2D confined spaces and thus pave the way for the design of high-efficiency catalysts.11Nsciescopu

Kinetic modulation of graphene growth by fluorine through spatially confined decomposition of metal fluorides

Two-dimensional materials show a variety of promising properties, and controlling their growth is an important aspect for practical applications. To this end, active species such as hydrogen and oxygen are commonly introduced into reactors to promote the synthesis of two-dimensional materials with specific characteristics. Here, we demonstrate that fluorine can play a crucial role in tuning the growth kinetics of three representative two-dimensional materials (graphene, hexagonal boron nitride and WS2). When growing graphene by chemical vapour deposition on a copper foil, fluorine released from the decomposition of a metal fluoride placed near the copper foil greatly accelerates the growth of the graphene (up to a rate of ~200?????m???s???1). Theoretical calculations show that it does so by promoting decomposition of the methane feedstock, which converts the endothermic growth process to an exothermic one. We further show that the presence of fluorine also accelerates the growth of two-dimensional hexagonal boron nitride and WS2