huangpanqi

The atomic structure changes from 2H MoTe₂ to 1T′ MoTe₂ are monitored directly by in situ transmission electron microscopy observation and the seamless interface can be formed between these two phases. By patterning hexagonal boron nitride on MoTe₂, a lateral 1T′-enriched MoTe₂/2H MoTe₂ homojunction photodetector can be built, which exhibits fast photo response due to the gradient bandgap alignment.

Abstract

Direct atomic-scale observation of the local phase transition in transition metal dichalcogenides (TMDCs) is critically required to carry out in-depth studies of their atomic structures and electronic features. However, the structural aspects including crystal symmetries tend to be unclear and unintuitive in real-time monitoring of the phase transition process. Herein, by using in situ transmission electron microscopy, information about the phase transition mechanism of MoTe₂ from hexagonal structure (2H phase) to monoclinic structure (1T′ phase) driven by sublimation of Te atoms after a spike annealing is obtained directly. Furthermore, with the control of Te atom sublimation by modulating the hexagonal boron nitride (h-BN) coverage in the desired area, the lateral 1T′-enriched MoTe₂/2H MoTe₂ homojunction can be one-step constructed via an annealing treatment. Owing to the gradient bandgap provided by 1T′-enriched MoTe₂ and 2H MoTe₂, the photodetector composed of the 1T′-enriched MoTe₂/2H MoTe₂ homojunction shows fast photoresponse and ten times larger photocurrents than that consisting of a pure 2H MoTe₂ channel. The study reveals a route to improve the performance of optoelectronic and electronic devices based on TMDCs with both semiconducting and semimetallic phases.

2D Materials

In article number 2110119, Bin Wang, Shaojuan Li, Dabing Li, and co-workers discuss the state of the art of using 2D materials in optoelectronic integration. They also provide general advice for future development in this field.

A higher lanthanide concentration of 80% sensitizer (Yb³⁺) and 20% activator (Er³⁺) increases the absorption rate of upconversion nanoparticles. A NaYF₄ shell blocks the energy migration pathways to the particle surface, which enhances the upconversion luminescence upon near-infrared excitation significantly. An investigation of particle diameter and shell thickness reveals significantly enhanced luminescence in the red (660 nm) for bioanalytical applications.

Abstract

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background-free luminescence. The progress in synthesis nowadays enables access to complex core-shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core-only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near-infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF₄(20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF₄ shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb³⁺ and 2% Er³⁺. The impact of size, composition, and core-shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.

The chemical vapor deposition-grown hexagonal WS₂ monolayer always exhibits alternating bright and dark photoluminescence domains. The correlation between the patterned photoluminescence emission and the details of defects is explored consistently by experiments and density functional theory calculations. The results indicate that the WS-vacancy is the most likely vacancy that matters.

Abstract

Monolayer transition-metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD-grown hexagonal WS₂ monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect-related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD-grown hexagonal WS₂ monolayer are revealed. The temperature-dependent PL spectra show the correlation between the defect-trapped and band-edge exciton emission. The high-resolution scanning transmission electron microscopy identifies the positive correlation between the density of WS _x -vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p- and n-doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS₂ monolayer, which provides important guidance in the regulation of electronic and optical properties of transition-metal dichalcogenides via defect engineering.

On epitaxial substrates with moderate surface interactions, density functional theory calculations show that a step with appropriate height can not only promote simultaneous nucleation of thickness-matched MoS₂ domains with aligned edges but also suppresses the nucleation of thinner MoS₂ domains, by which uniform multilayer MoS₂ with targeted thickness can be grown.

Abstract

Multilayer MoS₂ shows superior performance over the monolayer MoS₂ for electronic devices while the growth of multilayer MoS₂ with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS₂ domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS₂ domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS₂ domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS₂ on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS₂ with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS₂ films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Nature Nanotechnology, Published online: 04 April 2022; doi:10.1038/s41565-022-01102-7

A two-dimensional transition metal dichalcogenide-on-compound-semiconductor fabrication method enables the realization of an active matrix micro-LED display.

Nature Physics, Published online: 11 April 2022; doi:10.1038/s41567-022-01556-5

The presence or absence of a strange metal phase in twisted bilayer graphene has been controversial. Now, measurements over a wide range of temperature and doping give much stronger evidence for its existence.

Layered CrZr₄Te₁₄ is constructed of [CrTe₂] and [ZrTe₃] chains along the b-axis. The intrachain magnetic coupling of the [CrTe₂] chain is ferromagnetic, while the opposite magnetic ordering between the chains makes CrZr₄Te₁₄ an antiferromagnet. Moreover, CrZr₄Te₁₄ shows large intrinsic negative magnetoresistence of −56%, and the in-plane anisotropic factor reaches 8.2 owing to the ferromagnetic [CrTe₂] chains.

Abstract

The discovery of 2D van der Waals (vdW) magnetic materials is of great significance to explore intriguing 2D magnetic physics and develop innovative spintronic devices. In this work, a new quasi-1D vdW layered compound CrZr₄Te₁₄ is successfully synthesized. Owing to the existence of 1D [CrTe₂] and [ZrTe₃] chains along the b-axis, CrZr₄Te₁₄ crystals show strong anisotropy of phonon vibrations, electrical transport, and magnetism. Density functional theory calculations reveal the ferromagnetic (FM) coupling within the [CrTe₂] chain, while the interchain and interlayer couplings are both weakly antiferromagnetic (AF). Notably, a large intrinsic negative magnetoresistance (nMR) of −56% is achieved at 2 K under 9 T, and the in-plane anisotropic factor of nMR can reach up to 8.2 in the CrZr₄Te₁₄ device. The 1D FM chains and anisotropic nMR effect make CrZr₄Te₁₄ an interesting platform for exploring novel polarization-sensitive spintronics.

The transient absorption in monolayers of transition metal dichalcogenides grown on fused silica by the laser-assisted synthesis technique exhibits very unusual decay dynamics that feature long-lived components progressively increasing with the excitation fluence. The experimental observations are interpreted as resulting from the strain-induced modification of the electronic bands and concomitant interplay of the intervalley and intravalley excitons’ populations.

Abstract

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions. Moreover, these systems exhibit a dramatic slowing down of exciton dynamics (100s of picoseconds to few nanoseconds) with an increase in carrier densities, which strongly contrasts with the monolayers in their freestanding form as well as in comparison with more traditionally grown TMDs. The observations are related to the modifications of the electronic bands as expected from the strain and associated population of the intervalley dark excitons that can now interplay with intravalley excitations. These findings are consistent across both the Mo- and W-based TMD families, providing key information about the influence of the growth conditions on the nature of optical excitations and fostering emerging optoelectronic applications of monolayer TMDs.

Atomically resolved multiple 2D phase transformations (MoS₂ → Mo₄S₆, MoSe₂ → L-, Z-Mo₆Se₆) is observed in monolayer transition metal dichalcogenides under in situ heating with stoichiometry control by electron beam irradiation. Through chalcogen sliding and reconstruction mechanisms, phase transformations are well manipulated to fabricate diphase heterostructures with atomically sharp interfaces, which will pave the way to phase engineered optoelectronics.

Abstract

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS₂ and MoSe₂. The multiphase transformations: MoS₂ → Mo₄S₆ and MoSe₂ → Mo₆Se₆ which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor–metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.

An atom-programming on semiconducting oligomers tunes NIR-II emission beyond 1500 nm with high brightness. With this strategy, whole-body vessel, biliary tract, and bladder of living mice are clearly monitored at emission wavelengths over 1500 nm. Utilizing NIR-II nanoparticles, NIR-II microscopic imaging of in vivo deep-brain at a high depth of 870 µm and ultrafast hemodynamics are achieved in high resolution.

Abstract

Materials with long-wavelength second near-infrared (NIR-II) emission are highly desired for in vivo dynamic visualizating of microstructures in deep tissues. Herein, by employing an atom-programming strategy, a series of highly fluorescent semiconducting oligomers (SOMs) with tunable NIR-IIb emissions are developed for bioimaging applications. After self-assembly into nanoparticles (NPs), they show good brightness, high photostability, and satisfactory biocompatibility. The SOM NPs are applied as probes for high-resolution imaging of whole-body and hind-limb blood vessels, biliary tract, and bladder with their emissions over 1500 nm. This work demonstrates an atom-programming strategy for constructing semiconducting small molecules with enhanced NIR-II fluorescence for deep-tissue imaging, affording new insight for advancing molecular design of NIR-II fluorophores.

2D field-effect-transistors (FETs) are affected by time-dependent changes in their performance, which must be minimized for industrial-scale applications. In this work, the stability of circuits based on 2D transistors is evaluated. The results suggest that the performance parameters of certain material combinations for 2D FETs are already close to Si technologies. Furthermore, parameter variability criteria are formulated to evaluate the future development of 2D technologies.

Abstract

Within the last decade, considerable efforts have been devoted to fabricating transistors utilizing 2D semiconductors. Also, small circuits consisting of a few transistors have been demonstrated, including inverters, ring oscillators, and static random access memory cells. However, for industrial applications, both time-zero and time-dependent variability in the performance of the transistors appear critical. While time-zero variability is primarily related to immature processing, time-dependent drifts are dominated by charge trapping at defects located at the channel/insulator interface and in the insulator itself, which can substantially degrade the stability of circuits. At the current state of the art, 2D transistors typically exhibit a few orders of magnitude higher trap densities than silicon devices, which considerably increases their time-dependent variability, resulting in stability and yield issues. Here, the stability of currently available 2D electronics is carefully evaluated using circuit simulations to determine the impact of transistor-related issues on the overall circuit performance. The results suggest that while the performance parameters of transistors based on certain material combinations are already getting close to being competitive with Si technologies, a reduction in variability and defect densities is required. Overall, the criteria for parameter variability serve as guidance for evaluating the future development of 2D technologies.

2D Nanomaterials

In article number 2106110, Joohoon Kang and co-workers demonstrate various electronic devices such as field-effect transistors, photodetectors, diodes, and logic gates in wafer-scale based on building-block assembly for van der Waals heterostructure formation. The building blocks consist of solution-processed nanomaterials including metallic graphene, semiconducting MoS₂ and CNT, and oxidized HfS₂ (i.e., HfO₂) as an insulator.

Controllable and reversible photoconductance polarity switching can enhance broadband photodetection performance besides enabling multi-level optoelectronic logic and memory applications. In this article, wavelength-controlled negative (NPC) and positive photoconductance (PPC), along with reversible switching between the two, have been demonstrated in a BP-MoS₂ heterostructure phototransistor. The NPC-to-PPC crossover wavelength can be tuned by varying MoS₂ thickness.

Abstract

Layered 2D van der Waals semiconductors and their heterostructures have been shown to exhibit positive photoconductance (PPC) in many studies. A few recent reports have demonstrated negative photoconductance (NPC) as well that can enable broadband photodetection besides multi-level optoelectronic logic and memory. Controllable and reversible switching between PPC and NPC is a key requirement for these applications. This report demonstrates visible-to-near infrared wavelength-driven NPC and PPC, along with reversible switching between the two, in an air stable, high mobility, broadband black phosphorus field effect transistor covered with a few layer MoS₂ flake. The crossover switching wavelength can be tuned by varying the MoS₂ bandgap through its flake thickness and the NPC and PPC photoresponsivities can be modulated using electrostatic gating as well as laser power. Recombination-driven NPC and PPC, as supported by density functional theory calculations, allows for reversible switching. Further, gate voltage-dependent negative persistent photoconductance is well-suited for optosynaptic applications.

A red phosphor SrLaScO₄:Eu is successfully synthesized, and the photoluminescence quantum yield (PLQY) is improved by employing (NH₄)₂SO₄-assisted synthesis. The improved PLQY is attributed to reducing Eu³⁺ to Eu²⁺ via a competitive site occupation. This work reveals a reduction strategy to develop Eu²⁺-doped high-efficiency red phosphor for practical applications in solid-state lighting.

Abstract

The discovery of Eu²⁺-doped high-efficiency red phosphors remains a vital challenge for white light-emitting diode (WLED) applications. It is therefore urgent to find effective strategies managing the oxidation state to help reduce Eu³⁺ to Eu²⁺ and accordingly increase the photoluminescence quantum yield (PLQY). Herein, a new red-emitting SrLaScO₄:Eu phosphor is designed, and the PLQY is enhanced from 13% to 67% under 450 nm excitation by employing (NH₄)₂SO₄-assisted sintering. Combined structural analysis, optical spectroscopy, and theoretical calculation reveal that predominant Eu²⁺ prefers to occupy the Sr²⁺ sites in the SrLaScO₄ enabling red emission, and a competitive site occupation of Eu³⁺ in La³⁺ can be restrained, and the reduction mechanism of Eu³⁺ to Eu²⁺ originating from the (NH₄)₂SO₄ addition is analyzed. The fabricated WLED device using red-emitting SrLaScO₄:Eu and yellow-emitting Y₃(Al,Ga)₅O₁₂:Ce³⁺ exhibits a high color-rendering index of 86.7 at a low correlated color temperature of 4005 K. This work provides a feasible reduction strategy for guiding the development of high-efficiency Eu²⁺-doped red phosphor for WLED applications.

In response to ultraviolet–blue light, Eu²⁺-doped layered double borate phosphor with ultrabroadband near-infrared (NIR) emission peak at 720 nm is reported. This work extends the perception of the d–f transition of Eu²⁺ and provides a strategy to design Eu²⁺ doped phosphors with ultrabroadband long wavelength or NIR emission by occupying irregular and large sites in solids.

Abstract

Ultrabroadband near-infrared (NIR) luminescent materials are important components for compact light sources used in food testing, medical and biosensing applications. A major long-term challenge facing NIR luminescent materials is to design ultrabroadband phosphors activated by the rare-earth ions characterized by the f–d transition. Here, an Eu²⁺-doped Ba₃Lu(BO₃)₃ phosphor, showing peculiar luminescence characteristics such as a long emission wavelength ≈720 nm, a broader full width at half maximum of ≈197 nm, a large Stokes shift of ≈8300 cm^–1, and a longer decay time ≈5.58 µs, is reported. The smaller Eu²⁺ occupy the irregular and larger Ba²⁺ sites further reducing the symmetry, resulting in the increased splitting of the degenerate 5d level, thereby lowering the energy of Eu²⁺ emission. The longer decay time is also associated with the irregular Ba sites. This work improves the NIR mechanism of Eu²⁺ and opens up a new idea for exploring promising broadband NIR luminescent materials.

Nature Nanotechnology, Published online: 14 March 2022; doi:10.1038/s41565-021-01068-y

This Review discusses the recent progress in the emerging field of exciton phenomena in semiconductor moiré superlattices.