Bidirectional mid-infrared communications between two identical macroscopic graphene fibres.

Among light-based free-space communication platforms, mid-infrared (MIR) light pertains to important applications in biomedical engineering, environmental monitoring, and remote sensing systems. Integrating MIR generation and reception in a network using two identical devices is vital for the miniaturization and simplification of MIR communications. However, conventional MIR emitters and receivers are not bidirectional due to intrinsic limitations of low performance and often require cryogenic cooling. Here, we demonstrate that macroscopic graphene fibres (GFs) assembled from weakly-coupled graphene layers allow room-temperature MIR detection and emission with megahertz modulation frequencies due to the persistence of photo-thermoelectric effect in millimeter-length and the ability to rapidly modulate gray-body radiation. Based on the dual-functionality of GFs, we set up a system that conducts bidirectional data transmission by switching modes between two identical GFs. The room-temperature operation of our systems and the potential to produce GFs on industrial textile-scale offer opportunities for simplified and wearable optical communications.

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors.

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultrabroadband photodetection of our QD/graphene hybrid phototransistors features ultrahigh responsivity (up to ∼109 A/W), gain (up to ∼1012), and specific detectivity (up to ∼1013 Jones).

3D Atomic-Scale Insights into Anisotropic Core-Shell-Structured InGaAs Nanowires Grown by Metal-Organic Chemical Vapor Deposition.

III-V ternary InGaAs nanowires have great potential for electronic and optoelectronic device applications; however, the 3D structure and chemistry at the atomic-scale inside the nanowires remain unclear, which hinders tailoring the nanowires for specific applications. Here, atom probe tomography is used in conjunction with a first-principles simulation to investigate the 3D structure and chemistry of InGaAs nanowires, and reveals i) the nanowires form a spontaneous core-shell structure with a Ga-enriched core and an In-enriched shell, due to different growth mechanisms in the axial and lateral directions; ii) the shape of the core evolves from hexagon into Reuleaux triangle and grows larger, which results from In outward and Ga inward interdiffusion occurring at the core-shell interface; and iii) the irregular hexagonal shell manifests an anisotropic growth rate on {112}A and {112}B facets. Accordingly, a model in terms of the core-shell shape and chemistry evolution is proposed, which provides fresh insights into the growth of these nanowires.

A Broadband Fluorographene Photodetector.

High-performance photodetectors operating over a broad wavelength range from ultraviolet, visible, to infrared are of scientific and technological importance for a wide range of applications. Here, a photodetector based on van der Waals heterostructures of graphene and its fluorine-functionalized derivative is presented. It consistently shows broadband photoresponse from the ultraviolet (255 nm) to the mid-infrared (4.3 µm) wavelengths, with three orders of magnitude enhanced responsivity compared to pristine graphene photodetectors. The broadband photodetection is attributed to the synergistic effects of the spatial nonuniform collective quantum confinement of sp2 domains, and the trapping of photoexcited charge carriers in the localized states in sp3 domains. Tunable photoresponse is achieved by controlling the nature of sp3 sites and the size and fraction of sp3 /sp2 domains. In addition, the photoresponse due to the different photoexcited-charge-carrier trapping times in sp2 and sp3 nanodomains is determined. The proposed scheme paves the way toward implementing high-performance broadband graphene-based photodetectors.

Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions.

After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, we investigate the experimental efforts in interfacing 2D layers with 3D materials and analyze the properties of the heterojunctions formed between them. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the physical models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.

Methodology exploration of specimen preparation for atom probe tomography from nanowires.

Semiconductor nanowires have been intensively explored for applications in electronics, photonics, energy conversion and storage. A fundamental and quantitative understanding of growth-structure-property relationships is central to applications where nanowires exhibit clear advantages. Atom Probe Tomography (APT) is able to provide 3 dimensional quantitative elemental distributions at atomic-resolution and is therefore unique in understanding the growth-structure-property relationships. However, the specimen preparation with nanowires is extremely challenging. In this paper, two ion beam free specimen preparation methods for APT are presented which are efficient for various nanowires.

Spatial decomposition of molecular ions within 3D atom probe reconstructions.

Two methods for separating the constituent atoms of molecular ions within atom probe tomography reconstructions are presented. The Gaussian Separation Method efficiently deconvolutes molecular ions containing two constituent atoms and is tested on simulated data before being applied to an experimental HSLA steel dataset containing NbN. The Delaunay Separation Method extends separation to larger complex ions and is also tested on simulated data before being applied to an experimental GaAs dataset containing many large (>3 atoms) complex ions. First nearest neighbour (1NN) distributions and images of the reconstruction before and after the separations are used to show the effect of the algorithms and their validity and practicality are also discussed.

Magnetic properties of fluffy Fe@α-Fe2O3 core-shell nanowires.

: Novel fluffy Fe@α-Fe2O3 core-shell nanowires have been synthesized using the chemical reaction of ferrous sulfate and sodium borohydride, as well as the post-annealing process in air. The coercivity of the as-synthesized nanowires is above 684 Oe in the temperature range of 5 to 300 K, which is significantly higher than that of the bulk Fe (approximately 0.9 Oe). Through the annealing process in air, the coercivity and the exchange field are evidently improved. Both the coercivity and the exchange field increase with increasing annealing time (TA) and reach their maximum values of 1,042 and 78 Oe, respectively, at TA = 4 h. The magnetic measurements show that the effective anisotropy is increased with increasing the thickness of theα-Fe2O3 by annealing. The large values of coercivity and exchange field, as well as the high surface area to volume ratio, may make the fluffy Fe@α-Fe2O3 core-shell nanowire a promising candidate for the applications of the magnetic drug delivery, electrochemical energy storage, gas sensors, photocatalysis, and so forth.

Full tip imaging in atom probe tomography.

Atom probe tomography (APT) is capable of simultaneously revealing the chemical identities and three dimensional positions of individual atoms within a needle-shaped specimen, but suffers from a limited field-of-view (FOV), i.e., only the core of the specimen is effectively detected. Therefore, the capacity to analyze the full tip is crucial and much desired in cases that the shell of the specimen is also the region of interest. In this paper, we demonstrate that, in the analysis of III-V nanowires epitaxially grown from a substrate, the presence of the flat substrate positioned only micrometers away from the analyzed tip apex alters the field distribution and ion trajectories, which provides extra image compression that allows for the analysis of the entire specimen. An array of experimental results, including field desorption maps, elemental distributions, and crystallographic features clearly demonstrate the fact that the whole tip has been imaged, which is confirmed by electrostatic simulations.

Quantitative dopant distributions in GaAs nanowires using atom probe tomography.

Controllable doping of semiconductor nanowires is critical to realize their proposed applications, however precise and reliable characterization of dopant distributions remains challenging. In this article, we demonstrate an atomic-resolution three-dimensional elemental mapping of pristine semiconductor nanowires on growth substrates by using atom probe tomography to tackle this major challenge. This highly transferrable method is able to analyze the full diameter of a nanowire, with a depth resolution better than 0.17 nm thanks to an advanced reconstruction method exploiting the specimen's crystallography, and an enhanced chemical sensitivity of better than 8-fold increase in the signal-to-noise ratio.