Intrinsic carrier multiplication in layered Bi2O2Se avalanche photodiodes with gain bandwidth product exceeding 1 GHz

Emerging layered semiconductors present multiple advantages for optoelectronic technologies including high carrier mobilities, strong light-matter interactions, and tunable optical absorption and emission. Here, metal-semiconductor-metal avalanche photodiodes (APDs) are fabricated from Bi₂O₂Se crystals, which consist of electrostatically bound [Bi₂O₂]²⁺ and [Se]²⁻ layers. The resulting APDs possess an intrinsic carrier multiplication factor up to 400 at 7 K with a responsivity gain exceeding 3,000 A/W and bandwidth of ~ 400 kHz at a visible wavelength of 515.6 nm, ultimately resulting in a gain bandwidth product exceeding 1 GHz. Due to exceptionally low dark currents, Bi₂O₂Se APDs also yield high detectivities up to 4.6 × 10¹⁴ Jones. A systematic analysis of the photocurrent temperature and bias dependence reveals that the carrier multiplication process in Bi₂O₂Se APDs is consistent with a reverse biased Schottky diode model with a barrier height of ~ 44 meV, in contrast to the charge trapping extrinsic gain mechanism that dominates most layered semiconductor phototransistors. In this manner, layered Bi₂O₂Se APDs provide a unique platform that can be exploited in a diverse range of high-performance photodetector applications.

     

Visualizing Thermally Activated Memristive Switching in Percolating Networks of Solution-Processed 2D Semiconductors

Memristive systems present a low-power alternative to silicon-based electronics for neuromorphic and in-memory computation. 2D materials have been increasingly explored for memristive applications due to their novel biomimetic functions, ultrathin geometry for ultimate scaling limits, and potential for fabricating large-area, flexible, and printed neuromorphic devices. While the switching mechanism in memristors based on single 2D nanosheets is similar to conventional oxide memristors, the switching mechanism in nanosheet composite films is complicated by the interplay of multiple physical processes and the inaccessibility of the active area in a two-terminal vertical geometry. Here, the authors report thermally activated memristors fabricated from percolating networks of diverse solution-processed 2D semiconductors including MoS₂, ReS₂, WS₂, and InSe. The mechanisms underlying threshold switching and negative differential resistance are elucidated by designing large-area lateral memristors that allow the direct observation of filament and dendrite formation using in situ spatially resolved optical, chemical, and thermal analyses. The high switching ratios (up to 10³) that are achieved at low fields (≈4 kV cm⁻¹) are explained by thermally assisted electrical discharge that preferentially occurs at the sharp edges of 2D nanosheets. Overall, this work establishes percolating networks of solution-processed 2D semiconductors as a platform for neuromorphic architectures.     

Parallel fabrication of RNA microarrays by mechanical transfer from a DNA master

A new method for fabrication of RNA microarrays is described. The approach involves cohybridization of a short, biotinylated DNA oligonucleotide and an RNA probe sequence to DNA templates spotted onto a master array. Next, the short DNA sequence and the RNA probe are linked using a T4 DNA ligase. Finally, a poly(dimethylsiloxane) (PDMS) monolith modified on the surface with streptavidin is brought into conformal contact with the master array. This results in binding of the biotinylated DNA/RNA oligonucleotides to the PDMS surface. When the two substrates are mechanically separated, the DNA/RNA oligonucleotides transfer to the PDMS replica, and the DNA oligonucleotides remaining on the master array are ready to template another RNA replica array. This sequence can be repeated for at least 18 cycles using a single master array. RNA arrays consisting of up to three different oligonucleotide sequences and consisting of up to 2500 individual approximately 70 microm spots have been prepared.     

Aqueous dispersion of 1D van der Waals Mo6S3I6 crystal using biocompatible tri-block copolymer

To achieve the stable dispersion of 1D van der Waals crystal Mo₆S₃I₆ in aqueous media, the tri-block copolymer (Poloxamer) is used as dispersant. The head group of Poloxamer, hydrophobic polypropylene oxide parts can be adsorbed to Mo₆S₃I₆ surface by hydrophobic interaction and the tail group with hydrophilic polyethylene oxide exposed to the outside of the Mo₆S₃I₆ is soluble in water and can form sufficient steric hindrance, resulting in stable aqueous dispersion in nm scale. The excellent biocompatibility of aqueous dispersed nm scale 1D Mo₆S₃I₆ was demonstrated by effective proliferation of C2C12 cells.     

Top-down nanofabrication approaches toward single-digit-nanometer scale structures

Journal of Mechanical Science and Technology - Sub-10 nm nanostructures have received broad interest for their intriguing nano-optical phenomena, such as extreme field localization and enhancement,...     

Toxicological Profiling of Highly Purified Single‐Walled Carbon Nanotubes with Different Lengths in the Rodent Lung and Escherichia Coli

Carbon nanotubes (CNTs) exhibit a number of physicochemical properties that contribute to adverse biological outcomes. However, it is difficult to define the independent contribution of individual properties without purified materials. A library of highly purified single‐walled carbon nanotubes (SWCNTs) of different lengths is prepared from the same base material by density gradient ultracentrifugation, designated as short (318 nm), medium (789 nm), and long (1215 nm) SWCNTs. In vitro screening shows length‐dependent interleukin‐1β (IL‐1β) production, in order of long > medium > short. However, there are no differences in transforming growth factor‐β1 production in BEAS‐2B cells. Oropharyngeal aspiration shows that all the SWCNTs induce profibrogenic effects in mouse lung at 21 d postexposure, but there are no differences between tube lengths. In contrast, these SWCNTs demonstrate length‐dependent antibacterial effects on Escherichia coli, with the long SWCNT exerting stronger effects than the medium or short tubes. These effects are reduced by Pluronic F108 coating or supplementing with glucose. The data show length‐dependent effects on proinflammatory response in macrophage cell line and antibacterial effects, but not on collagen deposition in the lung. These data demonstrate that over the length scale tested, the biological response to highly purified SWCNTs is dependent on the complexity of the nano/bio interface.     

Design of Wavy Ag Microwire Array for Mechanically Stable,Multimodal Vibrational Haptic Interface

A vibrotactile interface is an actuator device to convey haptic information intuitively from electronics to users. For the next‐generation of user‐friendly interface applications, the vibrotactile actuator is required to be vibration intensity/frequency controllable, mechanically stable, transparent, and have large scalability. Previously, although these requirements are satisfied via several approaches using a random network film of Ag wires or a mixture with conductive polymers, the random‐network‐based materials only have limited control on material density and uniformity, which in turn hinders precise control over vibration intensity and device transparency. Here, a new approach to assemble large‐scale Ag microwire arrays is demonstrated by involving an evaporative assembly method and is presented to overcome the current limitations. In particular, the 1D wavy structure derived from fractal designs promotes vibration intensity and cycling due to greater areal coverage and improved mechanical stability. Furthermore, by taking advantage of the precisely aligned microwires array, tunable multimode vibration frequencies are obtained by generating two different voltage frequencies. The large‐scale wavy Ag microwire array with precise spatial controllability will be directly adaptable as a user‐friendly interface in electronic applications like wearable devices, computer interfaces, and flexible mobile phones.