Jiuxiang Dai

Recently reported new methods to induce strain are summarized and discussed, and the latest developments on the modification of electrical, magnetic, and optical properties of 2D materials are updated by strain engineering (in particular properties such as the electrochemical, magnetic and superconducting characteristics, and their strain tuning have received little attention previously), and future perspectives are presented.

Abstract

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2D materials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

A universal strategy for constructing 2D photovoltaic devices with high performances is demonstrated. The device based on 2D WS₂ exhibits nearly ideal external quantum efficiency (EQE) of 92% and high power conversion efficiency (PCE) of 5.0%, which are attributed to the defect-free interface and recombination-free channel. The nearly ideal EQE provides great potential for PCE approaching the Shockley–Queisser limit.

Abstract

2D transition metal dichalcogenides (TMDs) are promising candidates for realizing ultrathin and high-performance photovoltaic devices. However, the external quantum efficiency (EQE) and power conversion efficiency (PCE) of most 2D photovoltaic devices face great challenges in exceeding 50% and 3%, respectively, due to the low efficiency of photocarrier separation and collection. Here, this study demonstrates photovoltaic devices with defect-free interface and recombination-free channel based on 2D WS₂, showing high EQE of 92% approaching the theoretical limit and high PCE of 5.0%. The high performances are attributed to the van der Waals metal contact without interface defects and Fermi-level pinning, and the fully depleted channel without photocarrier recombination, leading to intrinsic photocarrier separation and collection with high efficiency. Furthermore, this study demonstrates that the strategy can be extended to other TMDs such as MoSe₂ and WSe₂ with EQE of 92% and 94%, respectively. This work proposes a universal strategy for building high-performance 2D photovoltaic devices. The nearly ideal EQE provides great potential for PCE approaching the Shockley–Queisser limit.

Nature Nanotechnology, Published online: 11 August 2022; doi:10.1038/s41565-022-01187-0

A ‘dual-ligand passivation system’ is designed and synthesized to functionalize colloidal quantum dots to realize ultra-high resolution patterns by direct photolithography.

Nature Nanotechnology, Published online: 11 August 2022; doi:10.1038/s41565-022-01186-1

A 2D material based liquid-crystal shows an extremely large optical anisotropy factor in the deep ultraviolet region, showing magnetically tunable birefringence.

Nature Nanotechnology, Published online: 11 August 2022; doi:10.1038/s41565-022-01183-4

Double ionic gated transistors enable excellent control of the band structure of atomically thin semiconductors. Perpendicular electric fields as large as 3 V nm−1 can fully quench the gap of bi- and few-layer WSe2.

The piezoelectric responses at 2D-material phase boundaries are studied and a strong piezoelectric response at the 2H–1T′ junction is observed, owing to the charge transfer across the semiconducting and metallic junctions, resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.

Abstract

Piezoelectricity in low-dimensional materials and metal–semiconductor junctions has attracted recent attention. Herein, a 2D in-plane metal–semiconductor junction made of multilayer 2H and 1T′ phases of molybdenum(IV) telluride (MoTe₂) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H–1T′ junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.

Abstract

The layer-dependent properties are still unclarified in two-dimensional (2D) vertical heterostructures. In this study, we layer-by-layer deposited semimetal β-In₂Se₃ on monolayer MoS₂ to form vertical β-In₂Se₃/MoS₂ heterostructures by chemical vapor deposition. The defect-mediated nucleation mechanism induces β-In₂Se₃ nanosheets to grow on monolayer MoS₂, and the layer number of stacked β-In₂Se₃ can be precisely regulated from 1 layer (L) to 13 L by prolonging the growth time. The β-In₂Se₃/MoS₂ heterostructures reveal tunable type-II band alignment arrangement by altering the layer number of β-In₂Se₃, which optimizes the internal electron transfer. Meanwhile, the edge atomic structure of β-In₂Se₃ stacking on monolayer MoS₂ shows the reconstruction derived from large lattice mismatch (∼ 29%), and the presence of β-In₂Se₃ also further increases the electrical conductivity of β-In₂Se₃/MoS₂ heterostructures. Attributed to abundant layer-dependent edge active sites, edge reconstruction, improved hydrophilicity, and high electrical conductivity of β-In₂Se₃/MoS₂ heterostructures, the edge of β-In₂Se₃/MoS₂ heterostructures exhibits excellent electrocatalytic hydrogen evolution performance. Lower onset potential and smaller Tafel slope can be observed at the edge of monolayer MoS₂ coupled with 13-L β-In₂Se₃. Hence, the outstanding conductive layers coupled with edge reconstruction in 2D vertical heterostructures play decisive roles in the optimization of electron energy levels and improvement of layer-dependent catalytic performance.

Nature, Published online: 10 August 2022; doi:10.1038/s41586-022-04937-1

A cascade of gate-tunable correlated insulating and metallic phases is observed in trigonally warped Bernal bilayer graphene at large electric fields.

Nature, Published online: 10 August 2022; doi:10.1038/s41586-022-04961-1

Fabrication of a low-dimensional metal halide perovskite superlattice by chemical epitaxy is reported, with a criss-cross two-dimensional network parallel to the substrate, leading to efficient carrier transport in three dimensions.

Nature Electronics, Published online: 08 August 2022; doi:10.1038/s41928-022-00800-3

Vanadium diselenide van der Waals contacts made with a controlled crack formation process can be used to fabricate tungsten diselenide transistors with channel lengths of less than 100 nm, on-state current densities of up to 1.7 mA μm–1 and on-state resistances down to 0.50 kΩ μm.

The 2D semiconductor of MoS₂ has great potential for advanced electronics technologies beyond silicon. So far, high-quality monolayer MoS₂ wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer. However, achieving high-quality multi-layer MoS₂ wafers remains a challenge. Here we report the growth of high-quality multi-layer MoS₂ 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements in device performances were found in thicker-layer field-effect transistors (FETs), as expected. For example, the average field-effect mobility (μ_FE) at room temperature (RT) can increase from ∼80 cm²·V^–1·s^–1 for monolayers to ∼110/145 cm²·V^–1·s^–1 for bilayer/trilayer devices. The highest RT μ_FE of 234.7 cm²·V^–1·s^–1 and record-high on-current densities of 1.70 mA·μm^–1 at V_ds = 2 V were also achieved in trilayer MoS₂ FETs with a high on/off ratio of >10⁷. Our work hence moves a step closer to practical applications of 2D MoS₂ in electronics.

Continuous progress in flexible electronics is bringing more convenience and comfort to human lives. In this field, interconnection and novel display applications are acknowledged as important future directions. However, it is a huge scientific and technical challenge to develop intrinsically flexible displays due to the limited size and shape of the display panel. To address this conundrum, it is crucial to develop intrinsically flexible electrode materials, semiconductor materials and dielectric materials, as well as the relevant flexible transistor drivers and display panels. In this review, we focus on the recent progress in this field from seven aspects: background and concept, intrinsically flexible electrode materials, intrinsically flexible organic semiconductors and dielectric materials for organic thin film transistors (OTFTs), intrinsically flexible organic emissive semiconductors for electroluminescent devices, and OTFT-driven electroluminescent devices for intrinsically flexible displays. Finally, some suggestions and prospects for the future development of intrinsically flexible displays are proposed.

Abstract

Elementary excitations, such as in-plane anisotropic phonons and phonon polaritons (PhPs), in α-MoO₃ play key roles in its outstanding physical properties like high carrier mobility and ultralow phonon thermal conductivity (κ_p). Understanding the excitation mechanisms like phonon-phonon interactions is the most fundamental step to further applications. Here, we report on the systematic Raman investigations on phonon anisotropy and anharmonicity of representative Mo-O stretching vibration phonon modes (SVPMs) in physical vapor deposition (PVD)-grown α-MoO₃ flakes. Polarizations of SVPMs verify the phonon anisotropy. The abnormal temperature dependence of SVPMs reveals that giant quartic-phonon decay dominates the phonon anharmonicity in α-MoO₃. An ultrashort phonon lifetime of ∼ 0.34 ps gives evidence of theoretically predicted ultralow κ_p in α-MoO₃. Our findings give deep insight into the phonon-phonon interactions in a-MoO₃ and provide an indicator for its extreme thermal device applications.

Nature Materials, Published online: 04 August 2022; doi:10.1038/s41563-022-01320-3

The authors investigate tunnelling magnetoresistance in Fe3GeTe2/hBN(WSe2)/Fe3GeTe2 magnetic tunnel junctions and report strong variations with bias including polarization reversals.