Fengrui Yao

BN‐Enabled Epitaxy of Pb1–xSnxSe Nanoplates on SiO2/Si for High‐Performance Mid‐Infrared Detection

By designing a few-layer boron nitried (BN) buffer layer, topological crystalline insulator Pb(1-x)Sn(x)Se nanoplates are directly grown on SiO2/Si, which shows high compatibility with current Si-based integrated circuit technology. Back-gated field-effect transistors of Pb(1-x)Sn(x)Se nanoplates exhibit a room-temperature carrier mobility of 0.73-4.90 cm(2) V(-1) s(-1), comparable to layered materials and molecular crystals, and high-efficiency mid-IR detection (1.9-2.0 μm).

High Conversion Efficiency Carbon Nanotube-Based Barrier-Free Bipolar-Diode Photodetector

Conversion efficiency (CE) is the most important figure of merit for photodetectors. For carbon nanotubes (CNT) based photodetectors, the CE is mainly determined by excitons dissociation and transport of free carriers toward contacts. While phonon-assisted exciton dissociation mechanism is effective in split-gate CNT p-n diodes, the CE is typically low in these devices, approximately 1-5%. Here, we evaluate the performance of a barrier-free bipolar diode (BFBD), which is basically a semiconducting CNT asymmetrically contacted by perfect n-type ohmic contact (Sc) and p-type ohmic contact (Pd) at the two ends of the diode. We show that the CE in short channel BFBD devices (e.g., 60 nm) is over 60%, and it reduces rapidly with increasing channel length. We find that the electric-field-assisted mechanism dominates the dissociation rate of excitons in BFBD devices at zero bias and thus the photocurrent generation process. By performing a time-resolved and spatial-resolved Monte Carlo simulation, we find that there exists an effective electron (hole)-rich region near the n-type (p-type) electrode in the asymmetrically contacted BFBD device, where the electric-field strength is larger than 17 V/μm and exciton dissociation is extremely fast (<0.1 ps), leading to very high CE in the BFBD devices.

Carbon Nanotubes as an Ultrafast Emitter with a Narrow Energy Spread at Optical Frequency

Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow ultrafast dynamics to be characterized in materials. However, the pursuit of simultaneous ultimate spatial and temporal resolution of microscopy and spectroscopy is largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. Field-driven photoemission from metal tips provides high light-phase synchronization, but suffers large electron energy spreads (3-100 eV) as driven by a long wavelength laser (>800 nm). Here, ultrafast electron emission from carbon nanotubes (≈1 nm radius) excited by a 410 nm femtosecond laser is realized in the field-driven regime. In addition, the emitted electrons have great monochromaticity with energy spread as low as 0.25 eV. This great performance benefits from the extraordinarily high field enhancement and great stability of carbon nanotubes, superior to metal tips. The new nanotube-based ultrafast electron source opens exciting prospects for extending current characterization to sub-femtosecond temporal resolution as well as sub-nanometer spatial resolution.

High‐Throughput Determination of Statistical Structure Information for Horizontal Carbon Nanotube Arrays by Optical Imaging

Optical multicolor imaging is used as a high-throughput statistical tool to determine the structure information of horizontally aligned carbon nanotube arrays on various substrates and in diverse environments. This high-throughput ability is achieved through the direct use of optical image information and an over 10-fold enhancement of the optical contrast by polarization manipulation.

SWCNT‐MoS2‐SWCNT Vertical Point Heterostructures

A vertical point heterostructure (VPH) is constructed by sandwiching a two-dimensional (2D) MoS2 flake with two cross-stacked metallic single-walled carbon nanotubes. It can be used as a field-effect transistor with high on/off ratio and a light detector with high spatial resolution. Moreover, the hybrid 1D-2D-1D VPHs open up new possibilities for nanoelectronics and nano-optoelectronics.

Structure–property relations in individual carbon nanotubes [Invited]

After more than a quarter century’s intense research and exploration for their distinctive physical properties and potential applications, carbon nanotubes remain an active research field with many surprises and opportunities. Recent advances in nano-optics provide a powerful tool to optically characterize carbon nanotubes with a defined chiral index at the single-nanotube level. Here we review our recent effort along this direction, including (1) combining transmission electron microscopy and single-nanotube optical spectroscopy to establish an atlas for carbon nanotube optical transitions and (2) developing a high-contrast polarization microscope for real-time optical imaging and in situ spectroscopy of individual nanotubes in devices. We will also discuss the importance of such characterizations for controlled nanotube growth and for understanding chirality-dependent device behaviors.

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 t...

Measurement of complex optical susceptibility for individual carbon nanotubes by elliptically polarized light excitation

The complex optical susceptibility is the most fundamental parameter characterizing light-matter interactions and determining optical applications in any material. In one-dimensional (1D) materials, all conventional techniques to measure the complex susceptibility become invalid. Here we report a methodology to measure the complex optical susceptibility of individual 1D materials by an elliptical-polarization-based optical homodyne detection. This method is based on the accurate manipulation of interference between incident left- (right-) handed elliptically polarized light and the scattering light, which results in the opposite (same) contribution of the real and imaginary susceptibility in two sets of spectra. We successfully demonstrate its application in determining complex susceptibility of individual chirality-defined carbon nanotubes in a broad optical spectral range (1.6–2.7 eV) and under different environments (suspended and in device). This full characterization of the complex optical responses should accelerate applications of various 1D nanomaterials in future photonic, optoelectronic, photovoltaic, and bio-imaging devices.One-dimensional materials such as carbon nanotubes have many applications, but not all of their properties can be described in the same way as for conventional media. Here, the authors devise a method to measure the complex optical susceptibility in a 1D nanomaterial and demonstrate it for carbon nanotubes.

Ultrafast and highly sensitive infrared photodetectors based on two-dimensional oxyselenide crystals

Infrared light detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of various photoelectric materials. The rise of two-dimensional materials, thanks to their distinct electronic structures, extreme dimensional confinement and strong light–matter interactions, provides a material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which are rare to meet at the same time in one two-dimensional material. Herein we demonstrate an infrared photodetector based on two-dimensional Bi2O2Se crystal, whose main characteristics are outstanding in the whole two-dimensional family: high sensitivity of 65 AW−1 at 1200 nm and ultrafast photoresponse of ~1 ps at room temperature, implying an intrinsic material-limited bandwidth up to 500 GHz. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of two-dimensional oxyselenide.Two-dimensional (2D) bismuth oxyselenide crystals with suitable electronic band-gap and ultrahigh carrier mobility enable near-infrared photodetection. Here, the authors report an infrared photodetector based on 2D-bismuth oxyselenide with high responsivity, ultrafast photoresponse of ~ 1 ps at room temperature and a detectable frequency limit of up to 500 GHz.

Ultrafast Broadband Charge Collection from Clean Graphene/CH3NH3PbI3 Interface

Photocarrier generation in a material, transportation to the material surface, and collection at the electrode interface are of paramount importance in any optoelectronic and photovoltaic device. In the last collection process, ideal performance comprises ultrafast charge collection to enhance current conversion efficiency and broadband collection to enhance energy conversion efficiency. Here, for the first time, we demonstrate ultrafast broadband charge collection achieved simultaneously at the clean graphene/organic-inorganic halide perovskite interface. The clean interface is realized by directly growing perovskite on graphene surface without polymer contamination. The tunable two-color pump-probe spectroscopy, time-resolved photoluminescence spectroscopy, and time-dependent density functional theory all reveal that the clean-interfacial graphene collects band-edge photocarriers of perovskite in an ultrashort time of ∼100 fs, with a current collection efficiency close to 99%. In addition, graphene can extract deep-band hot carriers of perovskite within only ∼50 fs, several orders faster than hot carrier relaxation and cooling in perovskite itself, due to the unique Dirac linear band structure of graphene, indicating a potential high energy conversion efficiency exceeding the Shockley-Queisser limit. Adding other graphene superiority of good transparency, high carrier mobility, and extreme flexibility, clean-interfacial graphene provides an ideal charge collection layer and electrode candidate for future optoelectronic and photovoltaic applications in two dimensions.