Defects in conventional semiconductors substantially lower the photoluminescence (PL) quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device ...efficiency. Two-dimensional transition-metal dichalcogenides (TMDCs), such as monolayer MoS
, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. We show that the PL QY of as-processed MoS
and WS
monolayers reaches near-unity when they are made intrinsic through electrostatic doping, without any chemical passivation. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased.
Gold‐mediated exfoliation of ultralarge optoelectronically perfect monolayers with lateral dimensions up to ≈500 μm is reported. Electrical, optical, and X‐ray photoelectron spectroscopy ...characterization show that the quality of the gold‐exfoliated flakes is similar to that of tape‐exfoliated flakes. Large‐area flakes allow manufacturing of large‐area monolayer transition metal dichalcogenide electronics.
The structure of human cortical bone evolves over multiple length scales from its basic constituents of collagen and hydroxyapatite at the nanoscale to osteonal structures at near-millimeter ...dimensions, which all provide the basis for its mechanical properties. To resist fracture, bone’s toughness is derived intrinsically through plasticity (e.g., fibrillar sliding) at structural scales typically below a micrometer and extrinsically (i.e., during crack growth) through mechanisms (e.g., crack deflection/bridging) generated at larger structural scales. Biological factors such as aging lead to a markedly increased fracture risk, which is often associated with an age-related loss in bone mass (bone quantity). However, we find that age-related structural changes can significantly degrade the fracture resistance (bone quality) over multiple length scales. Using in situ small-angle X-ray scattering and wide-angle X-ray diffraction to characterize submicrometer structural changes and synchrotron X-ray computed tomography and in situ fracture-toughness measurements in the scanning electron microscope to characterize effects at micrometer scales, we show how these age-related structural changes at differing size scales degrade both the intrinsic and extrinsic toughness of bone. Specifically, we attribute the loss in toughness to increased nonenzymatic collagen cross-linking, which suppresses plasticity at nanoscale dimensions, and to an increased osteonal density, which limits the potency of crack-bridging mechanisms at micrometer scales. The link between these processes is that the increased stiffness of the cross-linked collagen requires energy to be absorbed by "plastic" deformation at higher structural levels, which occurs by the process of microcracking.
Electrolyte cation size is known to influence the electrochemical reduction of CO2 over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that ...these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pK a for cation hydrolysis decreases and is sufficiently low for hydrated K+, Rb+, and Cs+ to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO2. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H2 and CH4, and an increase in Faradaic efficiencies for CO, C2H4, and C2H5OH, in full agreement with experimental observations for CO2 reduction over Ag and Cu.
The application of strain to semiconductors allows for controlled modification of their band structure. This principle is employed for the manufacturing of devices ranging from high-performance ...transistors to solid-state lasers. Traditionally, strain is typically achieved via growth on lattice-mismatched substrates. For two-dimensional (2D) semiconductors, this is not feasible as they typically do not interact epitaxially with the substrate. Here, we demonstrate controlled strain engineering of 2D semiconductors during synthesis by utilizing the thermal coefficient of expansion mismatch between the substrate and semiconductor. Using WSe
as a model system, we demonstrate stable built-in strains ranging from 1% tensile to 0.2% compressive on substrates with different thermal coefficient of expansion. Consequently, we observe a dramatic modulation of the band structure, manifested by a strain-driven indirect-to-direct bandgap transition and brightening of the dark exciton in bilayer and monolayer WSe
, respectively. The growth method developed here should enable flexibility in design of more sophisticated devices based on 2D materials.Strain engineering is an essential tool for modifying local electronic properties in silicon-based electronics. Here, Ahn et al. demonstrate control of biaxial strain in two-dimensional materials based on the growth substrate, enabling more complex low-dimensional electronics.
Electrochemical reduction of CO2 in aqueous media is a strategy for sustainable production of fuels and commodity chemicals. Cu is the only catalyst which converts CO2 to significant quantities of ...hydrocarbons and oxygenates. Here we demonstrate that oxygenate products can be favored over hydrocarbons by positioning a local source of CO generated by a CO producing catalyst (Au or Ag) in close proximity to a Cu catalyst. Use of a bimetallic device comprising interdigitated and independently controllable lines of Au and Cu allows the local CO concentration to be modulated. Notably, diffusional simulations show that the saturation concentration of CO can be exceeded locally. Actuating both the Au and Cu lines increases the oxygenate to ethylene ratio compared to actuating Cu only. Increasing the relative area of CO-producing Au relative to Cu also increases this ratio. These insights are translated into a second bimetallic system comprising Cu dots/lines patterned directly onto a Ag substrate, allowing for the distance between Cu and the CO generating metal to be precisely controlled. Controlling the relative areas of Ag and Cu allows for tuning of the oxygenate to ethylene ratio from 0.59 to 2.39 and an increase in oxygenate faradaic efficiency from 21.4% to 41.4%, while maintaining the selectivity to C2/C3 products in the 50–65% range. We attribute this change in selectivity to be due to an increased *CO coverage on Cu. By utilizing diffusional transport of CO to the Cu, a sequential catalysis pathway is created which allows for the control of oxygenate selectivity in aqueous CO2 reduction.
Exploring robust catalysts for water oxidation in acidic electrolyte is challenging due to the limited material choice. Iridium (Ir) is the only active element with a high resistance to the acid ...corrosion during water electrolysis. However, Ir is rare, and its large-scale application could only be possible if the intrinsic activity of Ir could be greatly enhanced. Here, a pseudo-cubic SrCo
Ir
O
perovskite, containing corner-shared IrO6 octahedrons, is designed. The Ir in the SrCo
Ir
O
catalyst shows an extremely high intrinsic activity as reflected from its high turnover frequency, which is more than two orders of magnitude higher than that of IrO
. During the electrochemical cycling, a surface reconstruction, with Sr and Co leaching, over SrCo
Ir
O
occurs. Such reconstructed surface region, likely contains a high amount of structural domains with corner-shared and under-coordinated IrO
octahedrons, is responsible for the observed high activity.
Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable ...their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
Hybrid organic–inorganic halide perovskite based semiconductor materials are attractive for use in a wide range of optoelectronic devices because they combine the advantages of suitable ...optoelectronic attributes and simultaneously low-cost solution processability. Here, we present a two-step low-pressure vapor-assisted solution process to grow high quality homogeneous CH3NH3PbI3–x Br x perovskite films over the full band gap range of 1.6–2.3 eV. Photoluminescence light-in versus light-out characterization techniques are used to provide new insights into the optoelectronic properties of Br-containing hybrid organic–inorganic perovskites as a function of optical carrier injection by employing pump-powers over a 6 orders of magnitude dynamic range. The internal luminescence quantum yield of wide band gap perovskites reaches impressive values up to 30%. This high quantum yield translates into substantial quasi-Fermi level splitting and high “luminescence or optically implied” open-circuit voltage. Most importantly, both attributes, high internal quantum yield and high optically implied open-circuit voltage, are demonstrated over the entire band gap range (1.6 eV ≤ E g ≤ 2.3 eV). These results establish the versatility of Br-containing perovskite semiconductors for a variety of applications and especially for the use as high-quality top cell in tandem photovoltaic devices in combination with industry dominant Si bottom cells.
Transition-metal dichalcogenide monolayers have naturally terminated surfaces and can exhibit a near-unity photoluminescence quantum yield in the presence of suitable defect passivation. To date, ...steady-state monolayer light-emitting devices suffer from Schottky contacts or require complex heterostructures. We demonstrate a transient-mode electroluminescent device based on transition-metal dichalcogenide monolayers (MoS
, WS
, MoSe
, and WSe
) to overcome these problems. Electroluminescence from this dopant-free two-terminal device is obtained by applying an AC voltage between the gate and the semiconductor. Notably, the electroluminescence intensity is weakly dependent on the Schottky barrier height or polarity of the contact. We fabricate a monolayer seven-segment display and achieve the first transparent and bright millimeter-scale light-emitting monolayer semiconductor device.
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