Jiuxiang Dai

a) Atomic resolution HADF STEM image showing stacking fault. b) Electrical resistivity fitted by adiabatic small polaron hopping model ρ(T) = ATexp(E _ρ/k _B T) with E _ρ being the activation energy. c) S(T) versus 1000/T curve fitted by polaron model S(T) = (k _B/e)(α + E _S/k _B T) with E _S being the activation energy.

Abstract

Intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr₂Ge₂Te₆ retains long-range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr₂Ge₂Te₆ are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr₂Ge₂Te₆—based devices by electron–phonon- and spin–orbit coupling-based tailoring of polaron properties.

Developing heat-insensitive memory is crucial for future electronics which are facing the severe self-heating effect. By using 2D ferroelectric α-In₂Se₃ with a high Curie temperature, the authors report a van der Waals ferroelectric tunnel junction memory that delivers outstanding and reliable performance at high temperatures, which demonstrates an exciting promise of α-In₂Se₃ for antagonizing unavoidable temperature rising due to self-heating.

Abstract

Facing the constant scaling down and thus increasingly severe self-heating effect, developing ultrathin and heat-insensitive ferroelectric devices is essential for future electronics. However, conventional ultrathin ferroelectrics and most 2D ferroelectric materials (2DFMs) are not suitable for high-temperature operation due to their low Curie temperature. Here, by using few-layer α-In₂Se₃, a special 2DFM with high Curie temperature, van der Waals (vdW) ferroelectric tunnel junction (FTJ) memories that deliver outstanding and reliable performance at both room and high temperatures are constructed. The vdW FTJs offer a large on/off ratio of 10⁴ at room temperature and still reveal excellent on/off ratio at an ultrahigh temperature of 470 K, which will fail down other 2DFMs. Moreover, long retention and reliable cyclic endurance at high temperature are achieved, showing robust thermal stability of the vdW FTJ memory. The observations of this work demonstrate an exciting promise of α-In₂Se₃ for reliable service in high temperature either from self-heating or harsh environments.

Intercalation chemistry is extensively investigated to synthesize and modulate 2D transition metal dichalcogenides (TMDs) with atomic thickness due to the diverse host–guest configurations and impact in their layered framework. Recent advances regarding 2D TMD materials, through intercalation exfoliation, from the views of host, guest, and solvent, are reviewed.

Abstract

Intercalation chemistry is of great importance in solid-state physics and chemistry for the ability to modulate electronic structures for constructing new materials with exotic properties. This ancient and versatile discipline has recently become prevailing in the synthesis and regulation of 2D transition metal dichalcogenides (TMDs) with atomic thickness due to diverse host–guest configurations and their impact on layered frameworks, which bring in extensive applications in electronics, optoelectronics, and other energy-based devices. In order to prepare 2D TMD materials with desired structure and properties, it is essential to gain in-depth understanding of the key role the intercalation chemistry plays in the preparation process. A focused review on recent advances regarding 2D TMD materials through intercalation exfoliation from the view of host, guest, and solvent interactions is provided. The effect of intercalation chemistry on TMD nanosheets synthesis and modification is comprehensively reviewed. The interactions between host and guest from the aspects of lattice strain, interlayer distance, and carrier density are considered. Finally, a prospectus of the future research opportunities for the intercalation chemistry of 2D materials is provided.

Moiré patterns of van der Waals heterostructures offer a powerful platform for engineering excitonic states, leading to the formation of moiré excitonic states. However, interactions between moiré excitonic states and magnetic elementary excitations are not studied. In this work, a moiré excitonic system interacting with an antiferromagnetic order in a monolayer MoSe₂ and an antiferromagnetic MnPS₃ heterostructure is reported.

Abstract

Moiré fringe patterns created by stacking different 2D layered materials as artificial van der Waals (vdW) heterostructures have become a novel platform to study and engineer optically generated excitonic properties. The moiré patterns contribute to the formation of spatially ordered excitonic states (excitons and trions), which can be used in the quantum simulation of many-body systems and ensembles of coherent quantum light emitters. The intriguing moiré excitonic properties are affected by and controlled via the interaction with magnetic elements. Here, a moiré excitonic system interacting with the magnetic elementary excitation of antiferromagnetic orders in MoSe₂/MnPS₃ vdW heterostructures is reported. The low-temperature photoluminescence spectra with additional fine spectral structures on the low-energy side, which are coupled magnon–trion peaks below the Néel temperature of MnPS₃, are carefully investigated. The fine spectral structures with long lifetime and coherence time are assigned to intralayer trion–magnon complexes trapped in the moiré potentials (moiré trion–magnon complexes). These findings highlight the emergence of moiré trion–magnon complexes and provide a new way to explore novel quantum phenomena in moiré excitonic systems with magnetic functionalities.

Light: Science & Applications, Published online: 01 March 2022; doi:10.1038/s41377-022-00734-7

Nature, Published online: 02 March 2022; doi:10.1038/s41586-021-04105-x

MXenes are promising for future electronics and optoelectronics; however, previously reported patterning methods lack efficiency, resolution, and compatibility with mainstream semiconductor processing. Here, a wafer-scale combination patterning method with a resolution up to the micrometer scale is developed, resulting in an integrated array of 1024-pixel Ti₃C₂T _x /Si photodetectors with a record-high detectivity of 7.73 × 10¹⁴ Jones.

Abstract

As a rapidly growing family of 2D transition metal carbides and nitrides, MXenes are recognized as promising materials for the development of future electronics and optoelectronics. So far, the reported patterning methods for MXene films lack efficiency, resolution, and compatibility, resulting in limited device integration and performance. Here, a high-performance MXene image sensor array fabricated by a wafer-scale combination patterning method of an MXene film is reported. This method combines MXene centrifugation, spin-coating, photolithography, and dry-etching and is highly compatible with mainstream semiconductor processing, with a resolution up to 2 µm, which is at least 100 times higher than other large-area patterning methods reported previously. As a result, a high-density integrated array of 1024-pixel Ti₃C₂T _x /Si photodetectors with a detectivity of 7.73 × 10¹⁴ Jones and a light–dark current ratio (I _light/I _dark) of 6.22 × 10⁶, which is the ultrahigh value among all reported MXene-based photodetectors, is fabricated. This patterning technique paves a way for large-scale high-performance MXetronics compatible with mainstream semiconductor processes.

A gradual transition from plastic to purely elastic lattice relaxation is found in InAs quantum dots on the tops of GaAs(111)A nanopillars with decreasing dot size by means of scanning transmission electron microscopy and molecular static simulations. The strained region at the heterointerface has a 20–80% width of ≤1.2 nm, covering only a small part of the dots.

Abstract

The size-dependent strain relaxation in InAs quantum dots on the top face of GaAs(111)A nanopillars is studied experimentally by scanning transmission electron microscopy (STEM) and theoretically using molecular static simulations. In the experiment, a dislocation-free, coherent state is observed for InAs dimensions below 10 nm in width and 7 nm in height, while 60° misfit dislocations occur for larger sizes. Experimental strain maps reveal the presence of a narrow-strained region at the heterointerface with a 20–80% width of 1.2 nm in the coherent case and 0.4 nm in the dislocated state, in agreement with simulations. Moreover, an analysis of the amount of misfit relaxed by the observed dislocations shows that the transition between the purely elastic and plastic relaxation regimes appears to be gradual. Intensity profiles of STEM images reveal that the misfit dislocations are located directly at the GaAs/InAs heterointerface.

Boron phosphide (BP) holds promise as an unconventional p-type transparent conductor. This work employs carbon doping and annealing of reactively sputtered BP films to achieve bipolar conductivity and the highest p-type carrier concentration reported to date. Modifications to the deposition process are suggested to obtain better transparency and higher hole mobilities.

Abstract

With an indirect band gap in the visible and a direct band gap at a much higher energy, boron phosphide (BP) holds promise as an unconventional p-type transparent conductor. This work reports on reactive sputtering of amorphous BP films, their partial crystallization in a P-containing annealing atmosphere, and extrinsic doping by C and Si. The highest hole concentration to date for p-type BP (5 × 10²⁰ cm⁻³) is achieved using C doping under B-rich conditions. Furthermore, bipolar doping is confirmed to be feasible in BP. An anneal temperature of at least 1000 °C is necessary for crystallization and dopant activation. Hole mobilities are low and indirect optical transitions are stronger than that predicted by theory. Low crystalline quality probably plays a role in both cases. High figures of merit for transparent conductors might be achievable in extrinsically doped BP films with improved crystalline quality.

Author(s): I. S. Burmistrov, V. Yu. Kachorovskii, M. J. Klug, and J. Schmalian

We develop the theory of anomalous elasticity in two-dimensional flexible materials with orthorhombic crystal symmetry. Remarkably, in the universal region, where characteristic length scales are larger than the rather small Ginzburg scale ∼10  nm, these materials possess an infinite set of flat pha...

[Phys. Rev. Lett. 128, 096101] Published Tue Mar 01, 2022

Incorporation of III-N semiconductors with optical resonances has attracted considerable interests. Here, the recent progress of III-N optoelectronic devices like light emitted diodes, photodetectors, solar cells and light photocatalysis, integrated with different optical resonances, including surface plasmons, distributed Bragg reflectors and micro cavities, is reviewed. The authors also give a prospect for the development of future GaN-based optoelectronic devices.

Abstract

Being direct wide bandgap, III-nitride (III-N) semiconductors have many applications in optoelectronics, including light-emitting diodes, lasers, detectors, photocatalysis, etc. Incorporation of III-N semiconductors with high-efficiency optical resonances including surface plasmons, distributed Bragg reflectors and micro cavities, has attracted considerable interests for upgrading their performance, which can not only reveal the new coupling mechanisms between optical resonances and quasiparticles, but also unveil the shield of novel optoelectronic devices with superior performances. In this review, the content covers the recent progress of GaN-based optoelectronic devices integrated with plasmonics and/or micro resonators, including the LEDs, photodetectors, solar cells, and light photocatalysis. The authors aim to provide an inspiring insight of recent remarkable progress and breakthroughs, as well as a promising prospect for the future highly-integrated, high speed, and efficient GaN-based optoelectronic devices.

The large-area uniform growth of monolayer, bilayer and few-layer 1T-SnSe₂ single-crystal flakes is presented via a chemical vapor deposition route. The thickness-tunable 1T-SnSe₂ flakes allow to establish a good criterion to identify the thickness and its unique optical properties. Intriguingly, we find obvious second harmonic generation in the inversion symmetry structure, with its intensity showing linear dependence.

Abstract

As an important member of the IVA–VIA group compounds, 2D SnSe₂ has emerged as a perfect platform for developing diverse applications, especially in high-performance optoelectronic devices and data storage, etc. However, the bottom-up synthesis of large-area uniform, atomically thin SnSe₂ crystals with controlled thicknesses has not yet been realized. Herein, we report the large-area uniform growth of monolayer (1L), bilayer (2L), and few-layer (FL) 1T-SnSe₂ single-crystal flakes on mica substrates via a facile chemical vapor deposition (CVD) route. The feeding amount of Sn precursor and flow rate of hydrogen carrier are found to be the key parameters for the thickness-controlled growth of uniform SnSe₂ flakes. More intriguingly, obvious second harmonic generation (SHG) is revealed in the retained inversion symmetry structure of 1T-SnSe₂, with its intensity showing linear dependence with the thickness from monolayer to multilayers. The new findings reported herein should pave the ways for the thickness-tunable growth of atomically thin SnSe₂ crystals, and their unique optical property explorations and applications in nonlinear optics.

Real-time electron microscopy observations reveal that the surface passivation of aluminum occurs via a two-stage process starting with intralayer atomic disordering upon the incorporation of oxygen into both the surface and subsurface region of the metal lattice, followed by interlayer disordering that leads to full amorphization of the oxide layer.

Abstract

Despite the ubiquitous presence of passivation on most metal surfaces, the microscopic-level picture of how surface passivation occurs has been hitherto unclear. Using the canonical example of the surface passivation of aluminum, here in situ atomistic transmission electron microscopy observations and computational modeling are employed to disentangle entangled microscopic processes and identify the atomic processes leading to the surface passivation. Based on atomic-scale observations of the layer-by-layer expansion of the metal lattice and its subsequent transformation into the amorphous oxide, it is shown that the surface passivation occurs via a two-stage oxidation process, in which the first stage is dominated by intralayer atomic shuffling whereas the second stage is governed by interlayer atomic disordering upon the progressive oxygen uptake. The first stage can be bypassed by increasing surface defects to promote the interlayer atomic migration that results in direct amorphization of multiple atomic layers of the metal lattice. The identified two-stage reaction mechanism and the effect of surface defects in promoting interlayer atomic shuffling can find broader applicability in utilizing surface defects to tune the mass transport and passivation kinetics, as well as the composition, structure, and transport properties of the passivation films.

To suppress the dark current of the SnSe-based photodetector, a SnSe/InSe vertical-van der Waals heterojunction with type-II band alignment is designed. The achieved heterojunction behaves as a self-driven photodetector with a typical rectification behavior, photovoltaic effect, and broadband detection ranging from visible to near infrared light (405–808 nm). At near infrared 808 nm it still exhibits excellent and balanced photoresponse performance.

Abstract

SnSe as a van der Waals (vdW) layered material has an extremely low light-dark current ratio and renders its application in high-performance optoelectronics. Herein, a SnSe/InSe vertical-vdW heterojunction with built-in electrical field favorable for suppressing the dark current is achieved. The SnSe/InSe heterojunction behaves as a self-driven photodetector with a typical rectification behavior, photovoltaic effect, and broadband detection ranging from visible to near infrared light (405–808 nm). Moreover, the SnSe/InSe photodetector at near infrared 808 nm still exhibits excellent and balanced photoresponse performance with R of 350 mA W⁻¹, external quantum efficiency of 48% and detectivity of 5.8 × 10¹⁰ Jones, respectively. The results pave the way for the applications of the novel SnSe/InSe vdW heterostructures in broadband photodetectors and photovoltaic devices.

The recent progress on the fundamental properties of different bottom-up synthesized nanographenes, and the collective properties upon forming nanographene-2D material-based van der Waals heterostructures, toward their state-of-the-art applications in optoelectronics and energy storage is summarized. Forthcoming challenges and opportunities of this emerging field are highlighted, and perspectives in boosting the progress toward advanced device applications are offered.

Abstract

Bottom-up synthesized graphene nanostructures, including 0D graphene quantum dots and 1D graphene nanoribbons, have recently emerged as promising candidates for efficient, green optoelectronic, and energy storage applications. The versatility in their molecular structures offers a large and novel library of nanographenes with excellent and adjustable optical, electronic, and catalytic properties. In this minireview, recent progress on the fundamental understanding of the properties of different graphene nanostructures, and their state-of-the-art applications in optoelectronics and energy storage are summarized. The properties of pristine nanographenes, including high emissivity and intriguing blinking effect in graphene quantum dots, superior charge transport properties in graphene nanoribbons, and edge-specific electrochemistry in various graphene nanostructures, are highlighted. Furthermore, it is shown that emerging nanographene-2D material-based van der Waals heterostructures provide an exciting opportunity for efficient green optoelectronics with tunable characteristics. Finally, challenges and opportunities of the field are highlighted by offering guidelines for future combined efforts in the synthesis, assembly, spectroscopic, and electrical studies as well as (nano)fabrication to boost the progress toward advanced device applications.

npj 2D Materials and Applications, Published online: 28 February 2022; doi:10.1038/s41699-022-00288-7

npj 2D Materials and Applications, Published online: 28 February 2022; doi:10.1038/s41699-022-00286-9

Nature Communications, Published online: 28 February 2022; doi:10.1038/s41467-022-28480-9

Nature Electronics, Published online: 28 February 2022; doi:10.1038/s41928-022-00730-0

Nature Electronics, Published online: 28 February 2022; doi:10.1038/s41928-022-00721-1