2D ferroelectric material has emerged as an attractive building block for high‐density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a ...key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α‐In2Se3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out‐of‐plane (OOP) and in‐plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α‐In2Se3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric‐field‐induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α‐In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure‐based nanoelectronics/optoelectronics.
The thinnest layered ferroelectric is demonstrated for the first time at room temperature. The semiconducting hexagonal α‐In2Se3 nanoflakes exhibit out‐of‐plane and in‐plane ferroelectricity that are closely intercorrelated. The polarization switching and hysteresis loops can be realized in the thickness as thin as ≈2.3 nm (bilayer) and ≈1.2 nm (monolayer). Two types of ferroelectric switchable devices are proposed to show the potential application in nonvolatile memories.
Light-regime variability is an important limiting factor constraining tree growth in tropical forests. However, there is considerable debate about whether radiation-induced green-up during the dry ...season is real, or an apparent artifact of the remote-sensing techniques used to infer seasonal changes in canopy leaf area. Direct and widespread observations of vertical canopy structures that drive radiation regimes have been largely absent. Here we analyze seasonal dynamic patterns between the canopy and understory layers in Amazon evergreen forests using observations of vertical canopy structure from a spaceborne lidar. We discovered that net leaf flushing of the canopy layer mainly occurs in early dry season, and is followed by net abscission in late dry season that coincides with increasing leaf area of the understory layer. Our observations of understory development from lidar either weakly respond to or are not correlated to seasonal variations in precipitation or insolation, but are strongly related to the seasonal structural dynamics of the canopy layer. We hypothesize that understory growth is driven by increased light gaps caused by seasonal variations of the canopy. This lightregime variability that exists in both spatial and temporal domains can better reveal the drought-induced green-up phenomenon, which appears less obvious when treating the Amazon forests as a whole.
Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. However, graphene has no band gap ...and the transistors are "leaky"; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of "on" to "off" currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li et al. succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary Science, this issue p. 524 Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered
. Atomically thin two-dimensional semiconductors have great ...potential for realizing high-performance electronic devices
. However, owing to metal-induced gap states (MIGS)
, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far
. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS
; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS
, WS
and WSe
. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.
For the first time, the transient nucleation stages were possible to record during electrodeposition from molten salt electrolytes. Specific to La electrodeposition from LiCl-KCl melts, it was ...clearly confirmed that the nucleation and subsequent growth lead to dendritic morphology, contradicting the theoretical prediction for 3D-semispherical growth by Scharifker-Hill model. In addition to the evaluation of standard rate constant for quasi-reversible reduction reaction of La(III) on Mo substrate, several thermodynamic La(III)/La(0) properties were determined, as well.
•Chronological verification by SEM of La dendritic nucleation and growth from molten LiCl-KCl.•Evaluation of exchange current density of La(III)/La(0) on molybdenum and lanthanum substrates in LiCl–KCl melts.•Use of Nicholson and Matsuda-Ayabe methods to determine the standard rate constant of La(III)/La(0) redox reaction.•Electrochemical kinetics and thermodynamics of La(III)/La(0) on Mo-substrate in molten eutectic LiCl-KCl
The electrochemical behavior of LaCl3 dissolved in molten LiCl–KCl eutectic salt was studied in the temperature range of 693–823K by using inert electrodes, Mo as the cathode, and high density graphite as the anode. Cyclic voltammetry, chronopotentiometry and square wave voltammetry were used to determine major kinetic parameters. The standard reaction rate constant of the order ≈ 10−3cm s−1, determined by Nicholson method, placed the redox reaction of lanthanum in the quasi-reversible range per Matsuda-Ayabe criteria for practical concept of electrochemical reversibility. Sand's equation was used to determine the diffusion coefficient of La(III) ions at four different temperatures. The effect of temperature on diffusion coefficient obeyed the Arrhenius law, according to which the activation energy for diffusion of La(III) ions was 33.5±0.5kJmol−1. The exchange current density of La(III)/La(0) redox reaction, evaluated at three different temperatures by linear polarization method on Mo and La substrates, was consistently somewhat higher on the later.
For each temperature, the equilibrium potential of La(III)/La(0) redox couple was determined by using open circuit chronopotentiometry, with subsequent calculation of the apparent standard potential, ELa(III/La(0))*0, and the apparent Gibbs free energy, ΔGLaCl3*0 The activity coefficients for LaCl3, γLaCl3 was determined from the difference of apparent and standard Gibbs free energies, ΔGLaCl3*0−ΔGLaCl3(SC)0.
The nucleation mechanism of lanthanum deposition on a molybdenum substrate according to the electrochemical model of Scharifker-Hill indicated the instantaneous nucleation with three-dimensional growth of the hemispherical nuclei. Contrary to this, the SEM studies of electrode surface morphology as a function of electrodeposition time clearly showed that La nucleation and growth follows the mechanisms responsible for dendritic growth. For the first time, the transient dendritic morphology events were possible to record, which is the major contribution of this work.
Developing solid state near-IR (NIR) emitters and simultaneously discriminative detection of trace water in organic solvents has long been a significant challenge. In this work, a novel ...diketopyrrolopyrrole-based luminogen (DPP1) with excited state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) characteristics has been designed and synthesized. Its amorphous and crystal solids show red and NIR-emissive fluorescence at 625 and 675 nm, respectively. When DPP1 reacts with fluoride anion, the resulting system (DPP1·F) can discriminatively detect the water content in aprotic solvents with colorimetric and fluorescent dual modes. Distinct fluorescent responses of “turn-on”, “ratiometric turn-off”, and “ratiometric turn-on” and low limits of detection of 0.0064, 0.042, and 0. The water-induced sensitive and fast change in THF was applied to the determination of water in foodstuffs in practical solid state indicator paper strips.
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well‐known Anderson's ...model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect‐free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
The band alignment of stacked 2D material heterojunctions is experimentally proven to follow the Anderson's model. Based on this discovery it is demonstrated that electron affinity and band gap values are sufficient to construct the band alignment of stacked 2D heterojunctions.
The clustered regularly interspaced short palindromic repeat (CRISPR) has great potential to revolutionize biomedical research and disease therapy. The specific and efficient genome editing strongly ...depends on high efficiency of delivery of the CRISPR payloads. However, optimization of CRISPR delivery vehicles still remains a major obstacle. Recently, various non-viral vectors have been utilized to deliver CRISPR tools. Many of these vectors have multi-layer structures assembled. In this review, we will introduce the development of CRISPR-Cas9 systems and their general therapeutic applications by summarizing current CRISPR-Cas9 based clinical trials. We will highlight the multi-layer nanoparticles (NPs) that have been developed to deliver CRISPR cargos in vitro and in vivo for various purposes, as well the potential building blocks of multi-layer NPs. We will also discuss the challenges in making the CRISPR tools into viable pharmaceutical products and provide potential solutions on efficiency and biosafety issues.
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Radiocesium remediation is desirable for ecological protection, human health and sustainable development of nuclear energy. Effective capture of Cs
from acidic solutions is still challenging, mainly ...due to the low stability of the adsorbing materials and the competitive adsorption of protons. Herein, the rapid and highly selective capture of Cs
from strongly acidic solutions is achieved by a robust K
-directed layered metal sulfide KInSnS
(InSnS-1) that exhibits excellent acid and radiation resistance. InSnS-1 possesses high adsorption capacity for Cs
and can serve as the stationary phase in ion exchange columns to effectively remove Cs
from neutral and acidic solutions. The adsorption of Cs
and H
O
is monitored by single-crystal structure analysis, and thus the underlying mechanism of selective Cs
capture from acidic solutions is elucidated at the molecular level.
In this paper, we present a single-chip 65 × 42 element ultrasonic pulse-echo fingerprint sensor with transmit (TX) beamforming based on piezoelectric micromachined ultrasonic transducers directly ...bonded to a CMOS readout application-specific integrated circuit (ASIC). The readout ASIC was realized in a standard 180-nm CMOS process with a 24-V high-voltage transistor option. Pulse-echo measurements are performed column-by-column in sequence using either one column or five columns to TX the ultrasonic pulse at 20 MHz. TX beamforming is used to focus the ultrasonic beam at the imaging plane where the finger is located, increasing the ultrasonic pressure and narrowing the 3-dB beamwidth to 50 μm, a factor of 6.4 narrower than nonbeamformed measurements. The surface of the sensor is coated with a poly-dimethylsiloxane (PDMS) layer to provide good acoustic impedance matching to skin. Scanning laser Doppler vibrometry of the PDMS surface was used to map the ultrasonic pressure field at the imaging surface, demonstrating the expected increase in pressure, and reduction in beamwidth. Imaging experiments were conducted using both PDMS phantoms and real fingerprints. The average image contrast is increased by a factor of 1.5 when beamforming is used.
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