Jiuxiang Dai

A new type memristor called “sliding memristor” based on graphene/parallel-stacked hexagonal boron nitride/graphene tunneling device is fabricated with tunable memristive windows and switchable high resistance state/low resistance state transitions induced by the coupling between ferroelectric polarization and conductive filament formation, bridging the gap between sliding ferroelectricity and memory applications.

Abstract

Sliding ferroelectricity in 2D materials, arising from interlayer sliding-induced interlayer hybridization and charge redistribution at the van der Waals interface, offers a means to manipulate spontaneous polarization at the atomic scale through various methods such as stacking order, interfacial contact, and electric field. However, the practical application of extending 2D sliding ferroelectricity remains challenging due to the contentious mechanisms and the complex device structures required for ferroelectric switching. Here, a sliding memristor based on a graphene/parallel-stacked hexagonal boron nitride/graphene tunneling device, featuring a stable memristive hysteresis induced by interfacial polarizations and barrier height modulations, is presented. As the tunneling current density increases, the memristive window broadens, achieving an on/off ratio of ≈10³ and 2 order decrease of the trigger current density, attributed to the interlayer migration of positively charged boron ions and the formation of conductive filaments, as supported by the theoretical calculations. The findings open a path for exploring the sliding memristor via a tunneling device and bridge the gap between sliding ferroelectricity and memory applications.

The out-of-plane electrical transport behavior of 2D Bi₂O₂Se remains unclear so far, especially in fabricating vertical devices with high integration density for novel functionality. The developed solution-processed method can prepare 2D Bi₂O₂Se with mass production. Then the out-of-plane ferroelectric and resistant switching properties are explored by using scanning probe microscopy. This work shows the promising advantages of 2D Bi₂O₂Se for next-generation electronics.

Abstract

Although 2D Bi₂O₂Se plays an important role in the electronics and optoelectronics based on its in-plane property, its out-of-plane electrical transport behavior remains unclear, especially in fabricating vertical devices with high integration density for novel functionality. Here, a solution-processed method is developed to prepare 2D Bi₂O₂Se with mass production (e.g., hundreds of milliliter scale). The out-of-plane ferroelectric property of 2D Bi₂O₂Se is observed by piezoresponse force microscopy and the ferroelectric dipole map atom-by-atom at the Bi₂O₂Se surface, which shows an atomically resolved dipolar displacement of Se ions. The out-of-plane resistant switching property of 2D Bi₂O₂Se is revealed by conductive atomic force microscopy. Moreover, the electric field on the local polarization of Bi₂O₂Se is addressed by using ab initio simulations, which shows a broken inversion symmetry along the z-axis of Bi₂O₂Se. The working mechanism of resistant switching behavior in Bi₂O₂Se is attributed to the diffusion and shuttle of Se ions. Besides, a controllable wet-assembled method is developed to prepare Bi₂O₂Se thin film with centimeter scale and explores its application on photodetectors under 808 nm laser light. This work reveals the unique out-of-plane transport behavior of 2D Bi₂O₂Se, providing the basis for fabricating multifunctional devices with high integration based on this 2D material.

Ultrafast pump-probe optical spectroscopy is used to study the room temperature excited state dynamics of the monolayer tungsten disulfine-based exciton-Bloch surface wave strongly coupled polaritonic systems. For above-exciton energy pumping, the decay of the exciton reservoir behaves as that of a bare monolayer. The transient response at the lower polariton resonance decays on an ultrashort time scale determined by the polariton lifetime, and subsequently follows the dynamic modification of strong coupling by photoexcited reservoir states.

Abstract

The dynamics of strongly coupled polariton systems integrated with 2D transition metal dichalcogenides (TMDs) is key to enabling efficient coherent processes and achieving high-performance TMD-based polaritonic devices, such as ultralow-threshold polariton lasers and ultrafast optical switches. However, there has been a lack of a comprehensive understanding of the excited state dynamics in TMD-based polariton systems. In this work, ultrafast pump-probe optical spectroscopy is used to investigate the room temperature dynamics of the polariton systems consisting of TMD monolayer excitons strongly coupled with Bloch surface waves (BSWs) supported by all-dielectric photonic structures. The transient response is found for both above-exciton energy pumping and polariton-resonant pumping. The excited state population and ultrafast coherent coupling of the exciton reservoir and lower polariton (LP) branch are observed for resonant pumping. Moreover, it is found that the transient response of the LP first decays on a short-time scale of 0.15–0.25 ps compared to the calculated intrinsic lifetime of 0.11–0.20 ps, and is followed by a longer decay (>100 ps) due to the dynamical evolution of the exciton reservoir. The results provide a fundamental understanding of the dynamics of TMD-based polariton systems while showing the potential for achieving efficient coherent optical processes for device applications.

Chirality is a phenomenon with widespread relevance in fundamental physics, material science, chemistry, optics, and spectroscopy. In this work, we show that a free electron can be converted by the field cycles of laser light into a right-handed or left-...

Author(s): Ahmed Abouelkomsan, Emil J. Bergholtz, and Shubhayu Chatterjee

Van der Waals heterostructures have recently emerged as an exciting platform for investigating the effects of strong electronic correlations, including various forms of magnetic or electrical orders. Here, we perform an unbiased exact diagonalization study of the effects of interactions on topologic…

[Phys. Rev. Lett. 133, 026801] Published Thu Jul 11, 2024

Post-Transition Metal Chalcogenides

In article number 2400800, Sungkyu Kim, Feng Ding, Joonki Suh, and co-workers introduce so-called phase-centric approach for producing post-transition metal chalcogenides using low-temperature metal–organic chemical vapor deposition (MOCVD). High-crystalline 2D SnSe₂ films can be directly prepared in wafer scale with excellent uniformity. With theoretical inputs, the authors indirectly produce SnSe film with all benefits of MOCVD. This set of techniques will provide a new route to the BEOL-compatible preparation of high-quality post-transition metal chalcogenides.

Chiral 1T−MoS₂ nanosheets were produced via hydrothermal synthesis in the presence of tartaric acid as a chiral ligand to control mirror symmetry breaking. This novel chiral inorganic nanomaterial exhibits a chiral morphology as well as remarkably high chiroptical activity, with a g-factor of up to 0.01. The optimization of the synthetic conditions and the material's formation mechanism revealed critical details on the transfer of chirality.

Abstract

Chirality in inorganic nanostructures has recently stimulated the attention of many researchers, both to unravel fundamental questions on the origin of chirality in inorganic and hybrid materials, as well as to introduce novel promising properties that are originated by the symmetry breaking. MoS₂ is one of the most investigated among the large family of layered transition metal dichalcogenides. In particular, the metastable metallic 1T−MoS₂ phase is of large interest for potential applications. However, due to thermodynamic reasons, the synthesis of 1T−MoS₂ phase is quite challenging. Herein, we present the first synthesis of chiral 1T−MoS₂ phase which shows remarkably high chiroptical activity with a g-factor up to 0.01. Chiral 1T−MoS₂ was produced using tartaric acid as a chiral ligand to induce symmetry breaking during the material's growth under hydrothermal conditions, leading to the formation of distorted hierarchical nanosheet assemblies exhibiting chiral morphology. Thorough optimization of the synthetic conditions was carried out to maximize chiroptical activity, which is strongly related to the nanostructures’ morphology. Finally, the formation mechanism of the chiral 1T−MoS₂ nanosheet assemblies was investigated, focusing on the role of molecular intermediates in the growth of the nanosheets and the transfer of chirality.

A record-breaking 2.3 eV bandgap modulation in tellurium nanoflakes (TeNFs) via thermomechanical straining during the hot-press syntheses is reported. A non-volatile −4.01% compressive strain that surpasses most existing techniques is demonstrated. This strategy enables long-lasting strain retention and unlocks efficient blue photoemission in TeNFs, paving the way for on-demand control of nano-optoelectronic properties in 2D materials.

Abstract

Lattice deformation via substrate-driven mechanical straining of 2D materials can profoundly modulate their bandgap by altering the electronic band structure. However, such bandgap modulation is typically short-lived and weak due to substrate slippage, which restores lattice symmetry and limits strain transfer. Here, it is shown that a non-volatile thermomechanical strain induced during hot-press synthesis results in giant modulation of the inherent bandgap in quasi-2D tellurium nanoflakes (TeNFs). By leveraging the thermal expansion coefficient (TEC) mismatch and maintaining a pressure-enforced non-slip condition between TeNFs and the substrate, a non-volatile and anisotropic compressive strain is attained with ε = −4.01% along zigzag lattice orientation and average biaxial strain of −3.46%. This results in a massive permanent bandgap modulation of 2.3 eV at a rate S (ΔE _g) of up to 815 meV/% (TeNF/ITO), exceeding the highest reported values by 200%. Furthermore, TeNFs display long-term strain retention and exhibit robust band-to-band blue photoemission featuring an intrinsic quantum efficiency of 80%. The results show that non-volatile thermomechanical straining is an efficient substrate-based bandgap modulation technique scalable to other 2D semiconductors and van der Waals materials for on-demand nano-optoelectronic properties.

An individual multi-walled tungsten disulfide (WS₂) nanotube serves as a channel of a field-effect transistor, exhibiting p-type behavior. The device features slightly asymmetric Schottky barriers at drain and source contacts, enabling self-powered photoconduction. It also shows photoresponsivity of a few milliAmps per Watt that increases exponentially with the rising temperature. Moreover, driven by gate and laser pulses, the device features four well-separated current states, which make it a promising optoelectronic memory with 2-bits per cell.

Abstract

Nanotube and nanowire transistors hold great promises for future electronic and optoelectronic devices owing to their downscaling possibilities. In this work, a single multi-walled tungsten disulfide (WS₂) nanotube is utilized as the channel of a back-gated field-effect transistor. The device exhibits a p-type behavior in ambient conditions, with a hole mobility µ _p ≈  1.4 cm²V⁻¹s⁻¹ and a subthreshold swing SS ≈ 10 V dec⁻¹. Current–voltage characterization at different temperatures reveals that the device presents two slightly different asymmetric Schottky barriers at drain and source contacts. Self-powered photoconduction driven by the photovoltaic effect is demonstrated, and a photoresponsivity R ≈ 10 mAW⁻¹ at 2 V drain bias and room temperature. Moreover, the transistor is tested for data storage applications. A two-state memory is reported, where positive and negative gate pulses drive the switching between two different current states, separated by a window of 130%. Finally, gate and light pulses are combined to demonstrate an optoelectronic memory with four well-separated states. The results herein presented are promising for data storage, Boolean logic, and neural network applications.

A novel approach to synthesize tin-sulfides micropatterns in ambient conditions is achieved via direct laser writing. Sperate phases of tin sulfides, namely SnS & SnS₂, are self-arranged in either heterostructure or intermixed configuration, depending on laser intensity. The laser-synthesized tin sulfide photodetector possesses high responsivity (4 AW⁻¹) with rapid rise and fall times (1.8 and 16 ms) respectively.

Abstract

Various polytypes of van der Waals (vdW) materials can be formed by sulfur and tin, which exhibit distinctive and complementary electronic properties. Hence, these materials are attractive candidates for the design of multifunctional devices. This work demonstrates direct selective growth of tin sulfides by laser irradiation. A 532 nm continuous wave laser is used to synthesize centimeter-scale tin sulfide tracks from single source precursor tin(II) o-ethylxanthate under ambient conditions. Modulation of laser irradiation conditions enables tuning of the dominant phase of tin sulfide as well as SnS₂/SnS heterostructures formation. An in-depth investigation of the morphological, structural, and compositional characteristics of the laser-synthesized tin sulfide microstructures is reported. Furthermore, laser-synthesized tin sulfides photodetectors show broad spectral response with relatively high photoresponsivity up to 4 AW⁻¹ and fast switching time (τ _rise = 1.8 ms and τ _fall = 16 ms). This approach is versatile and can be exploited in various fields such as energy conversion and storage, catalysis, chemical sensors, and optoelectronics.

Abstract

The development of highly efficient, solution-processable, and environmentally stable perovskite quantum dots (PQDs) is crucial for their accurate high-resolution patterning and subsequently enabling the practical deployment of PQD based emissive display devices. This study presents an innovative strategy for integrating all-inorganic PQDs and ultraviolet (UV) crosslinkable acrylate polymer at a structural and functional level. The achievement is accomplished by meticulous design and one-pot synthesis of UV-crosslinkable CsPbX₃ (X = Cl, Br, I) PQDs solution, which exhibit outstanding environmental stability. Leveraging the solution-processable characteristics of the resulting UV-crosslinkable PQDs, precise patterning of high-resolution (2 µm, 7608 pixels in.⁻¹) and colorful PQDs microarrays can be readily achieved through inkjet printing and high-throughput photolithography (~ 2 µm in pitch line/space patterning). The UV cross-linked process guarantees a homogeneous distribution of PQDs, effectively mitigating coffee ring effect and improving the overall quality of stereoscopic microarrays. The photo-cured PQDs film, which undergoes free radical photopolymerization, displays an impressive photoluminescence quantum yield (PL QY) of up to 89.2%, reaching 98% of the value observed in the solution state. The approach outlined in this research is both cost-effective and pragmatic, exhibiting tremendous promise for diverse system-level integrated optoelectronic devices, such as ultra-high-resolution micro-light-emitting device (micro-LED) displays.

This study presents highly stable aluminum and fluorine co-doped zinc oxide (AFZO) films, maintaining high mobility of 60 cm² V⁻¹ s⁻¹ even after air-annealing at 600°C. Detailed analysis of defects unveil the reasons behind AFZO's durability, settling long-standing disputes and paving the way for its application in the production of (Al _x Ga_1−x )₂O₃ solar-blind Schottky photodiodes and enhancing the efficiency of perovskite solar cells.

Abstract

High-temperature stable transparent conductive oxides (TCOs) are highly desirable in optoelectronics but are rarely achieved due to the defect generation that is inevitable during high-temperature air annealing. This work reports unprecedented stability in aluminum and fluorine co-doped ZnO (AFZO) films prepared by pulse laser deposition. The AFZO can retain a mobility of 60 cm² V⁻¹ s⁻¹, an electron concentration of 4.5 × 10²⁰ cm⁻³, and a visible transmittance of 91% after air-annealing at 600°C. Comprehensive defect characterization and first principles calculations have revealed that the offset of substitutional aluminum by zinc vacancy is responsible for the instability observed in aluminum-doped ZnO, and the pairing between fluorine substitution and zinc vacancy ensures the high-temperature stability of AFZO. The utility of AFZO in enabling the epitaxial growth of (Al _x Ga_1−x )₂O₃ film within a high-temperature, oxygen-rich environment is demonstrated, facilitating the development of a self-powered solar-blind ultraviolet Schottky photodiode. Furthermore, the high-mobility AFZO transparent electrode enables perovskite solar cells to achieve improved power conversion efficiency by balancing the electron concentration-dependent conductivity and transmittance. These findings settle the long-standing controversy surrounding the instability in TCOs and open up exciting prospects for the advancement of optoelectronics.

Light: Science & Applications, Published online: 08 July 2024; doi:10.1038/s41377-024-01503-4

We demonstrate for the first time, a uniform low temperature (<250 °C) process for fabricating both high-confinement thick and low-confinement thin ultra-low loss Silicon nitride waveguides.

Facile control of the photoluminescence (PL) from monolayer molybdenum disulfide (1L MoS₂) is demonstrated. Room temperature treatment with a fluorinated or sulfonated chemical solution enhances and blueshifts the PL emission. In contrast, atomic layer deposition of a high-κ dielectric attenuates and redshifts the 1L MoS₂ PL spectrum. The peak width is also tunable.

Abstract

Monolayer molybdenum disulfide (1L MoS₂), a promising optoelectronic material, emits strong visible photoluminescence (PL). Systematic control of the intensity, energy, and spectral width of PL from 1L MoS₂ on silicon dioxide/silicon (SiO₂/Si) is demonstrated via simple external treatments. Treating MoS₂ with solutions formed from the superacid bis-(trifluoromethanesulfonyl)amide (TFSA) enhances, blueshifts, and sharpens the PL. Treatments with solutions from structurally analogous chemicals that lack sulfur, in the case of bis(trifluoroacetamide) (BTFA), or lack fluorine, in the case of methanesulfonamide (MSA), show the same trend, suggesting a two-component mechanism for TFSA involving the presence of electronegative species and sulfur vacancy passivation. Up to ≈100× enhancement of the PL intensity is achieved, with the peak blueshifted by ≈30 meV and the spectral linewidth halved. Conversely, direct thermal atomic layer deposition (ALD) of aluminum oxide (Al₂O₃) or hafnium oxide (HfO₂) is found to suppress the PL by up to a factor of ≈3, redshift by up to ≈70 meV, and broaden by ≈3×. Single-spot and mapping Raman/PL techniques are combined in a robust characterization process to associate changes in the PL character to charge doping. This work demonstrates the convenient tunability of the optical behavior of 1L MoS₂ by varying the electron density.

npj 2D Materials and Applications, Published online: 14 July 2024; doi:10.1038/s41699-024-00477-6

Relaxation effects in transition metal dichalcogenide bilayer heterostructures

npj 2D Materials and Applications, Published online: 14 July 2024; doi:10.1038/s41699-024-00482-9

Evidence of contact-induced variability in industrially-fabricated highly-scaled MoS₂ FETs

Nature Electronics, Published online: 08 July 2024; doi:10.1038/s41928-024-01202-3

Vertical metal nanosheets with atomically flat surfaces grown with a bismuth-oxide-assisted chemical vapour deposition method can be used to make metal–oxide dielectric stacks and laminated onto two-dimensional semiconductors to create transistors with sub-1 nm capacitance-equivalent thicknesses.

Nature Electronics, Published online: 08 July 2024; doi:10.1038/s41928-024-01205-0

A room-temperature approach to monolithic three-dimensional thin-film integration can be used to stack ten layers of n-channel indium oxide transistors on silicon/silicon dioxide substrates, while incorporating a range of architectures.

Nature Communications, Published online: 08 July 2024; doi:10.1038/s41467-024-49942-2

van der Waals magnets can be arranged into twisted heterostructures, with the twisting leading to the formation of new magnetic phases. Here, Li, Sun, and coauthors show via NVcentre based magnetometry small angle twisted double trilayer CrI3 exhibits a co-existing, hybrid magnetic phase with distinct phase transition temperatures.

Nature Communications, Published online: 09 July 2024; doi:10.1038/s41467-024-49994-4

Molecular rolling lubrication can control friction phenomenon like a wheel. Here, the authors find the self-curled deformation effect of graphite nanosheets at cryogenic temperature, which promotes the in-situ formation of parallel nano-rollers, and acquire molecular rolling lubrication.

Nature Communications, Published online: 15 July 2024; doi:10.1038/s41467-024-50211-5

Here authors present bottom-up fabrication of 2D large-scale organic-inorganic heterostructure with clean interface and highly-crystalline structure via a self-lifting effect with observing distinct Dirac and narrow bands.

Nature Nanotechnology, Published online: 08 July 2024; doi:10.1038/s41565-024-01710-5

Room-temperature wafer-scale thermal evaporation of 20 different polycrystalline rare-earth-metal fluoride films for their use in 2D transistors is demonstrated.

Nature Nanotechnology, Published online: 10 July 2024; doi:10.1038/s41565-024-01704-3

This Review examines conventional epitaxial growth of 2D van der Waals materials, focusing on in-plane single-crystal monolayer growth and out-of-plane homostructure fabrication. It covers nucleation and orientation control, quality control measures, and homogeneous multilayer and twisted homostructure growth techniques, providing systematic insights for on-demand fabrication of 2D van der Waals materials and their industrial device manufacturing.

Nature Nanotechnology, Published online: 12 July 2024; doi:10.1038/s41565-024-01719-w

Not only electrons but also phonons can transport angular momentum in solids. Now, in an artificial superlattice, ultrafast demagnetization induces transfer of angular momentum from the spin system to the lattice.