Jiuxiang Dai

In two-dimensional transition metal dichalcogenides, normal strain can modulate electronic band structures, yet leaving the optical selection rules intact. In contrast, a shear strain can perturb the spin-valley locked band structures and possibly induce mixing of the spin subbands which in turn can transfer oscillator strength between spin-allowed bright and spin-forbidden dark excitons. Here, we report a novel scheme to manipulate photoluminescence (PL) in a monolayer WSe 2 -MoSe 2 lateral heterostructures, controlled by an external bending method in which strong out-of-plane shear strain (OSS) of up to 5.6% accompanies weak in-plane normal strain up to 0.72%. The spectra revealed a striking dependence on the bending direction that is stagnant in the negative (compressive) strain region and then rapidly changes with increasing positive (tensile) strain. The dependency of the PL signal under tensile bending was represented not only by the large energy shift (...

Nature Nanotechnology, Published online: 15 November 2021; doi:10.1038/s41565-021-01004-0

A dual-coupling-guided growth mechanism enables the realization of wafer-scale single-crystal WS2 on vicinal a-plane sapphire.

Van der Waals (vdW) materials are an indispensable part of functional device technology. Recently, the search for magnetic vdW materials has intensified due to the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon and they rarely show high-mobility charge carriers. Using chemical reasoning, it is found that TaCo₂Te₂ is an air-stable, high-mobility, magnetic vdW material.

Abstract

Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low-dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high-mobility charge carriers, which is an essential requirement for high-speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, TaCo₂Te₂ is introduced as an air-stable, high-mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. It is found that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after 4 months of monitoring using an optical microscope. Signatures of canted antiferromagntic behavior are also observed at low-temperature. TaCo₂Te₂ shows a metallic character and a large, nonsaturating, anisotropic magnetoresistance. Furthermore, the Hall data and quantum oscillation measurements reveal the presence of both electron- and hole-type carriers and their high mobility.

Nature Electronics, Published online: 12 November 2021; doi:10.1038/s41928-021-00675-w

Semiconducting two-dimensional materials might one day be used in scaled semiconductor technology. Andras Kis recounts how the first transistor based on a single layer of molybdenum disulfide was created.

Monolayers of transition metal dichalcogenides (TMDCs) exhibit attractive properties and are promising for fabricating photonic and optoelectronic devices, while bulk multilayered structures based on the same materials only recently has revealed many properties useful for nanophotonics. In this regard, the combination of monolayer and multilayer properties in one device (on a single flake) is an important and fruitful task that needs to be solved. In this work, we demonstrate the use of local anodic oxidation to improve the optical properties of multilayer MoSe 2 flakes on a gold-covered substrate. Using this method, we fabricated nanostructures demonstrating extraordinarily enhanced photoluminescence (PL), with an intensity up to three orders of magnitude compared to that of the original structure. Low-frequency Raman spectroscopy showed that the nature of this PL enhancement is that the bindings between the layers inside the nanostructures are severely disrupted. Thi...

Proximity effects between layered materials trigger a plethora of novel and exotic quantum transport phenomena. Besides, the capability to modulate the nature and strength of proximity effects by changing crystalline and interfacial symmetries offers a vast playground to optimize physical properties of relevance for innovative applications. In this work, we use large-scale first principles calculations to demonstrate that strain and twist-angle strongly vary the spin–orbit coupling (SOC) in graphene/transition metal dichalcogenide heterobilayers. Such a change results in a modulation of the spin relaxation times by up to two orders of magnitude. Additionally, the relative strengths of valley-Zeeman and Rashba SOC can be tailored upon twisting, which can turn the system into an ideal Dirac–Rashba regime or generate transitions between topological states of matter. These results shed new light on the debated variability of SOC and clarify how lattice deformations can be used as a ...

Here, it is demonstrated that epitaxial bi-domain III–V/Si are hybrid structures, composed of bulk photo-active semiconductors with 2D topological semi metallic vertical inclusions, endowed with ambipolar properties. Operating III–V/Si photoelectrodes confirm that this hybrid material can by itself photogenerate and laterally separate carriers, which are efficiently extracted afterward from the photoelectric device.

Abstract

Hybrid materials taking advantage of the different physical properties of materials are highly attractive for numerous applications in today's science and technology. Here, it is demonstrated that epitaxial bi-domain III–V/Si are hybrid structures, composed of bulk photo-active semiconductors with 2D topological semi-metallic vertical inclusions, endowed with ambipolar properties. By combining structural, transport, and photoelectrochemical characterizations with first-principle calculations, it is shown that the bi-domain III–V/Si materials are able within the same layer to absorb light efficiently, separate laterally the photo-generated carriers, transfer them to semimetal singularities, and ease extraction of both electrons and holes vertically, leading to efficient carrier collection. Besides, the original topological properties of the 2D semi-metallic inclusions are also discussed. This comb-like heterostructure not only merges the superior optical properties of semiconductors with good transport properties of metallic materials, but also combines the high efficiency and tunability afforded by III–V inorganic bulk materials with the flexible management of nano-scale charge carriers usually offered by blends of organic materials. Physical properties of these novel hybrid heterostructures can be of great interest for energy harvesting, photonic, electronic or computing devices.

A newly partly covered PProDOT-Me₂ on MoS₂ nanosheets counter electrode is developed for a self-powered electrochromic device. SP-ECD achieves a transmittance modulation of 46% and cycling stability of 500 cycles (4% contrast attenuation) about five times stable than that based on expensive Pt or MoS₂ CE. ECD can be colorless in the bleached state that would greatly benefit in BIPVs.

Abstract

Developing a low-cost and transparent counter electrode (CE) for a self-powered electrochromic device (SP-ECD) can address the requirements of building-integrated photovoltaics (BIPVs). Herein, a new partly covered poly(3,4-(2,2-dimethylpropylenedioxy) thiophene) (PProDOT-Me₂) on MoS₂ nanosheets CE is developed for replacing an expensive Pt CE and achieving high-performance SP-ECD. The device consists of the proposed CE, an electrolyte containing Br⁻/Br₃ ⁻ redox pair, and a working electrode made with a dye-sensitized TiO₂ photoanode integrated with PProDOT-Me₂. ECD changes its color between deep blue and colorless by varying the illuminated light on and off. Measurements reveal that the ECD achieves a transmittance modulation of 46% at 580 nm with cycling stability over 500 cycles (4% contrast attenuation) that are about five times stable than that based on Pt or MoS₂ CE. In addition, the switching speed between colored and bleached states is fast under a light on and off, with a coloration time of 2.1 s and a bleaching time of 1.5 s. These characteristics result from the unique design of CE and can be beneficial in accelerating the electrochromic process and enhancing the self-powered performance for good cycling stability. This study reveals a new approach to prepare good CE for efficient SP-ECD in BIPVs.

Linear and nonlinear optical anisotropy in mechanically exfoliated nagyágite thin flakes is investigated via polarization-resolved Raman scattering, absorption, and third-harmonic generation measurements.

Abstract

Nagyágite is a naturally occurring layered van der Waals heterostructure composed of alternating layers of [Pb(Pb,Sb)S₂] and [(Au,Te)], where the component lattices are commensurately modulated. The weak van der Waals stacking between the heterolayers facilitates mechanical exfoliation. Due to its monoclinic crystal structure, nagyágite exhibits structural anisotropy which induces strong optical anisotropy. Here, the anisotropic optical properties of ultrathin nagyágite flakes mechanically exfoliated from a natural mineral are demonstrated through angle-resolved polarized Raman scattering, linear dichroism, and polarization-dependent anisotropic third-harmonic generation. The study establishes nagyágite as a new type of natural van der Waals heterostructure based 2D material, which can be exploited for realizing ultrathin anisotropic optical devices for future on-chip photonic integrated circuits.

Through the construction of a W/Cr₂Ge₂Te₆ heterostructure with annealing treatment, the Curie temperature of Cr₂Ge₂Te₆ is raised above 150 K with strong perpendicular magnetic anisotropy, which is attributed to the interfacial orbital hybridization. Due to the weak interlayer coupling, the interfacial enhancement can be effective in long distance. The enhanced ferromagnetism can be controlled by spin-orbit torque with low current density.

Abstract

Ferromagnetic semiconductors discovered in 2D materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (T _C) of existing 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect is found to effectively manipulate the 3D magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here, it is demonstrated that the T _C of a 2D ferromagnetic semiconductor Cr₂Ge₂Te₆ (CGT) can be enhanced by 130% (from ≈65 K to above 150 K) when adjacent to a tungsten layer. The interfacial W–Te bonding contributes to the T _C enhancement with a strong perpendicular magnetic anisotropy, guaranteeing efficient magnetization switching by the spin-orbit torque with a low current density at 150 K. Distinct from the rapid attenuation in conventional magnets, the interfacial effect exhibits a weak dependence on CGT thickness and a long-distance effect (more than 10 nm) due to the weak interlayer coupling inherent to 2D magnets. This work not only reveals a unique interfacial behavior in 2D materials, but also advances the process toward practical 2D spintronics.

A transition metal dichalcogenide (TMD)/cavity monolithic cavity is achieved by directly growing single-domain tungsten diselenide (WSe₂) bilayers on single silica microsphere (MS) surfaces. The thermal strain induces bilayer bandgap from indirect to direct, and the cavity confinement factor is also improved, which directly realizes room-temperature whispering-gallery-mode lasing, with a threshold nearly an order of magnitude lower than the existing records.

Abstract

Monolayer transition metal dichalcogenides (TMDs) have emerged as widely accepted 2D gain materials in the field of light sources owing to their direct bandgap and high photoluminescence quantum yield. However, the monolayer medium suffers from weak emission because only a single layer of molecules can absorb the pump energy. Moreover, the material degradation when transferring these fragile materials hinders their cooperation with the optical cavity further. In this study, for the first time, a high-quality monolithic structure is developed by directly growing single-domain tungsten diselenide (WSe₂) bilayers on single silica microsphere (MS) cavities. Such a completely wrapped structure guides the indirect-to-direct bandgap transition of WSe₂ bilayers, leading to a significantly improved photoluminescence intensity by about 60-fold. Moreover, the high-quality monolithic structure enhances the confinement factor of the cavity by more than 20-fold. Based on the above advantages, a bilayer WSe₂/MS microlaser is realized with an ultralow threshold of 0.72 W cm⁻², nearly an order of magnitude lower than the existing records. The results demonstrate the possibility of using multilayer TMD materials as 2D gain media and provide insights into a new ultracompact monolithic platform of TMD material/cavity for lasing devices.

Single layer β′- and β*-In₂Se₃ films are experimentally validated to host 2D in-plane anti-ferroelectric and ferroelectric order, respectively, with structural and spectroscopic evidences characterized by low-temperature scanning tunneling microscopy (STM)/spectroscopy. An electric field-induced phase transition is also realized via applying an STM tip pulse, demonstrating the manipulation of the ferroelectric polarization with reversible switching.

Abstract

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In₂Se₃ films: β′- and β*-In₂Se₃ are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β′-In₂Se₃ and ferroelectric order of β*-In₂Se₃ are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

The manipulation of the lattice polarity of quasi-vdW epitaxial GaN on graphene through controlling the interface atomic configuration is reported. This polarity-control rule is not affected by the growth method and is free of either crystalline or non-crystalline substrates. It makes the epitaxy of III-nitrides with preferred lattice polarity possible and improves the ability to fabricate advanced semiconductor devices.

Abstract

Quasi van der Waals epitaxy, a pioneering epitaxy of sp³-hybridized semiconductor films on sp²-hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Energy- and area-efficient hardware acceleration of simulated annealing for the Ising spin system is achieved by exploiting subthreshold conduction and programmability of 2D field-effect transistors based on p-type WSe₂ and n-type MoS₂. Search acceleration of >800× for 4 × 4 ferromagnetic, antiferromagnetic, and spin glass systems is demonstrated with a miniscule energy dissipation of ≈120 nJ.

Abstract

Metaheuristic algorithms such as simulated annealing (SA) are often implemented for optimization in combinatorial problems, especially for discreet problems. SA employs a stochastic search, where high-energy transitions (“hill-climbing”) are allowed with a temperature-dependent probability to escape local optima. Ising spin glass systems have properties such as spin disorder and “frustration” and provide a discreet combinatorial problem with a high number of metastable states and ground-state degeneracy. In this work, subthreshold Boltzmann transport is exploited in complementary 2D field-effect transistors (p-type WSe₂ and n-type MoS₂) integrated with an analog, nonvolatile, and programmable floating-gate memory stack to develop in-memory computing primitives necessary for energy- and area-efficient hardware acceleration of SA for Ising spin systems. Search acceleration of >800× is demonstrated for 4 × 4 ferromagnetic, antiferromagnetic, and spin glass systems using SA compared to an exhaustive search using a brute force trial at miniscule total energy expenditure of ≈120 nJ. The hardware-realistic numerical simulations further highlight the astounding benefits of SA in accelerating the search for larger spin lattices.

Lattice orientation heredity is shown to be a universal strategy in the transformation of highly oriented 2D van der Waals layered and nonlayered thin films. Different from the classical and direct epitaxial deposition, this process is developed based on the heredity phenomena in material science. It opens a new window for growing textured films on disordered substrates (e.g., GaN-related technology), and provides an effective route for large-area growth of epitaxial 2D MoS₂ thin films.

Abstract

The ability to control lattice orientation is often an essential requirement in the growth of both 2D van der Waals (vdW) layered and nonlayered thin films. Here, a unique and universal phenomenon termed “lattice orientation heredity” (LOH) is reported. LOH enables product films (including 2D-layered materials) to inherit the lattice orientation from reactant films in a chemical conversion process, excluding the requirement on the substrate lattice order. The process universality is demonstrated by investigating the lattice transformations in the carbonization, nitridation, and sulfurization of epitaxial MoO₂, ZnO, and In₂O₃ thin films. Their resultant compounds all inherit the mono-oriented crystal feature from their precursor oxides, including 2D vdW-layered semiconductors (e.g., MoS₂), metallic films (e.g., MXene-like Mo₂C and MoN), wide-bandgap semiconductors (e.g., hexagonal ZnS), and ferroelectric semiconductors (e.g., In₂S₃). Using LOH-grown MoN as a seeding layer, mono-oriented GaN is achieved on an amorphous quartz substrate. The LOH process presents a universal strategy capable of growing epitaxial thin films (including 2D vdW-layered materials) not only on single-crystalline but also on noncrystalline substrates.

Dion–Jacobson (DJ) and alternating-cation-interlayer (ACI) phases are two emerging types of layered 2D halide perovskites with high potential in balanced charge-transport properties and chemical stability. By tailoring the molecular, thin-film, and device chemistries, high-performance solar cells have been demonstrated.

Abstract

Layered halide perovskites (LHPs) with crystallographically 2D structures have gained increasing interest for photovoltaic applications due to their superior chemical stability and intriguing anisotropic properties, which are in contrast to their conventional 3D perovskite counterparts. The most frequently studied LHPs are Ruddlesden–Popper (RP) phases, which suffer from a carrier-transport bottleneck due to the van der Waals gap associated with their intrinsic organic interlayer structures. To address this issue, Dion–Jacobson (DJ) and alternating-cation-interlayer (ACI) LHPs have rapidly emerged, which exhibit unique structural and (opto)electronic characteristics that may resemble those of the 3D counterparts owing to the eliminated or reduced van der Waals gap. Improved photophysical properties have been achieved in DJ and ACI LHPs, leading towards better photovoltaic performance. Here we provide a comprehensive discussion on the merits and promises of DJ and ACI LHPs from a chemistry perspective. Then, we review recent progress on the synthesis and tailoring of DJ and ACI LHP crystals and thin films, as well as their optoelectronic properties and photovoltaic performance. Finally, we discuss possible pathways to overcome critical challenges to realize the full potential of DJ and ACI LHPs for high-performance solar cells and beyond.

npj 2D Materials and Applications, Published online: 09 September 2021; doi:10.1038/s41699-021-00258-5

Optical versus electron diffraction imaging of Twist-angle in 2D transition metal dichalcogenide bilayers

npj 2D Materials and Applications, Published online: 09 September 2021; doi:10.1038/s41699-021-00259-4

Recent advances of MXenes as electrocatalysts for hydrogen evolution reaction

npj 2D Materials and Applications, Published online: 30 September 2021; doi:10.1038/s41699-021-00263-8

Photoluminescence as a probe of phosphorene properties

npj 2D Materials and Applications, Published online: 24 September 2021; doi:10.1038/s41699-021-00261-w

Cross-field optoelectronic modulation via inter-coupled ferroelectricity in 2D In₂Se₃

npj 2D Materials and Applications, Published online: 11 October 2021; doi:10.1038/s41699-021-00264-7

Bottom-up water-based solution synthesis for a large MoS₂ atomic layer for thin-film transistor applications

Nature Chemistry, Published online: 11 November 2021; doi:10.1038/s41557-021-00813-z

Several polymorphs of borophene have been synthesized on metal substrates, but typically as monolayers. Now large-size, single-crystalline bilayer borophene has been grown on Cu(111)—a sufficient electron provider to enable the bonding of the second boron layer. The resulting bilayer possesses a metallic character and is less susceptible to oxidation than its monolayer counterpart.