Controllable Synthesis of Transferable Ultrathin Bi2Ge(Si)O5 Dielectric Alloys with Composition-Tunable High-κ Properties.

Two-dimensional (2D) alloys hold great promise to serve as important components of 2D transistors, since their properties allow continuous regulation by varying their compositions. However, previous studies are mainly limited to the metallic/semiconducting ones as contact/channel materials, but very few are related to the insulating dielectrics. Here, we use a facile one-step chemical vapor deposition (CVD) method to synthesize ultrathin Bi2SixGe1-xO5 dielectric alloys, whose composition is tunable over the full range of x just by changing the relative ratios of the GeO2/SiO2 precursors. Moreover, their dielectric properties are highly composition-tunable, showing a record-high dielectric constant of >40 among CVD-grown 2D insulators. The vertically grown nature of Bi2GeO5 and Bi2SixGe1-xO5 enables polymer-free transfer and subsequent clean van der Waals integration as the high-κ encapsulation layer to enhance the mobility of 2D semiconductors. Besides, the MoS2 transistors using Bi2SixGe1-xO5 alloy as gate dielectrics exhibit a large Ion/Ioff (>108), ideal subthreshold swing of ∼61 mV/decade, and a small gate hysteresis (∼5 mV). Our work not only gives very few examples on controlled CVD growth of insulating dielectric alloys but also expands the family of 2D single-crystalline high-κ dielectrics.

Single-crystalline van der Waals layered dielectric with high dielectric constant.

The scaling of silicon-based transistors at sub-ten-nanometre technology nodes faces challenges such as interface imperfection and gate current leakage for an ultrathin silicon channel1,2. For next-generation nanoelectronics, high-mobility two-dimensional (2D) layered semiconductors with an atomic thickness and dangling-bond-free surfaces are expected as channel materials to achieve smaller channel sizes, less interfacial scattering and more efficient gate-field penetration1,2. However, further progress towards 2D electronics is hindered by factors such as the lack of a high dielectric constant (κ) dielectric with an atomically flat and dangling-bond-free surface3,4. Here, we report a facile synthesis of a single-crystalline high-κ (κ of roughly 16.5) van der Waals layered dielectric Bi2SeO5. The centimetre-scale single crystal of Bi2SeO5 can be efficiently exfoliated to an atomically flat nanosheet as large as 250 × 200 µm2 and as thin as monolayer. With these Bi2SeO5 nanosheets as dielectric and encapsulation layers, 2D materials such as Bi2O2Se, MoS2 and graphene show improved electronic performances. For example, in 2D Bi2O2Se, the quantum Hall effect is observed and the carrier mobility reaches 470,000 cm2 V-1 s-1 at 1.8 K. Our finding expands the realm of dielectric and opens up a new possibility for lowering the gate voltage and power consumption in 2D electronics and integrated circuits.

Tunable Topological States in Stacked Chern Insulator Bilayers.

The emergence of intrinsic quantum anomalous Hall (QAH) insulators with a long-range ferromagnetic (FM) order triggers unprecedented prosperity for combining topology and magnetism in low dimensions. Built upon atom-thin Chern insulator monolayer MnBr3, we propose that the topologically nontrivial electronic states can be systematically tuned by inherent magnetic orders and external electric/optical fields in stacked Chern insulator bilayers. The FM bilayer illustrates a high-Chern-number QAH state characterized by both quantized Hall plateaus and specific magneto-optical Kerr angles. In antiferromagnetic bilayers, Berry curvature singularity induced by electrostatic fields or lasers emerges, which further leads to a novel implementation of the layer Hall effect depending on the chirality of irradiated circularly polarized light. These results demonstrate that abundant tunable topological properties can be achieved in stacked Chern insulator bilayers, thereby suggesting a universal routine to modulate d-orbital-dominated topological Dirac fermions.

Size-tunable energy gaps of hydrogen-terminated biphenylene segments.

The electronic properties of hydrogen-terminated biphenylene (BP) segments of different sizes on the sub-nanoscale are explored using density functional theory, and the size dependence of the energy gap is evaluated using a structural parameter as a function of the bond lengths and the electronic density contributions. More importantly, the energy gap is observed to decrease linearly with the reduced hydrogen-to-carbon ratio of the corresponding structures, while the decrease-rate undergoes a diminution of four times at a gap of 0.5 eV due to the transformed distribution of the lowest unoccupied molecular orbital. The results give a deep insight into the size-tunable energy gaps of BPs and provide a possibility for the preparation of hydrogen-terminated carbon materials with a desirable energy gap.

High Mobility and Quantum Oscillations in Semiconducting Bi2O2Te Nanosheets Grown by Chemical Vapor Deposition.

Bi2O2Te has the smallest effective mass and preferable carrier mobility in the Bi2O2X (X = S, Se, Te) family. However, compared to the widely explored Bi2O2Se, the studies on Bi2O2Te are very rare, probably attributed to the lack of efficient ways to achieve the growth of ultrathin films. Herein, ultrathin Bi2O2Te crystals were successfully synthesized by a trace amount of O2-assisted chemical vapor deposition (CVD) method, enabling the observation of ultrahigh low-temperature Hall mobility of >20 000 cm2 V-1 s-1, pronounced Shubnikov-de Haas quantum oscillations, and small effective mass of ∼0.10 m0. Furthermore, few nm thick CVD-grown Bi2O2Te crystals showed high room-temperature Hall mobility (up to 500 cm2 V-1 s-1) both in nonencapsulated and top-gated device configurations and preserved the intrinsic semiconducting behavior with Ion/Ioff ∼ 103 at 300 K and >106 at 80 K. Our work uncovers the veil of semiconducting Bi2O2Te with high mobility and brings new blood into Bi2O2X family.

In-Situ Manipulation of the Magnetic Anisotropy of Single Mn Atom via Molecular Ligands.

Magnetic anisotropy is essential for permanent magnets to maintain their magnetization along specific directions. Understanding and controlling the magnetic anisotropy on a single-molecule scale are challenging but of fundamental importance for the future's spintronic technology. Here, by using scanning tunneling microscopy (STM), we demonstrated the ability to control the magnetic anisotropy by tuning the ligand field at the single-molecule level. We constructed a molecular magnetic complex with a single Mn atom and an organic molecule (4,4'-biphenyldicarbonitrile) as a ligand via atomic manipulation. Inelastic tunneling spectra (IETS) show that the Mn complex has much larger axial magnetic anisotropy than individual Mn atoms, and the anisotropy energy can be tuned by the coupling strength of the ligand. With density functional theory calculations, we revealed that the enhanced magnetic anisotropy of Mn arising from the carbonitrile ligand provides a prototype for the engineering of the magnetism of quantum devices.

Visualizing coexisting surface states in the weak and crystalline topological insulator Bi2TeI.

Dual topological materials are unique topological phases that host coexisting surface states of different topological nature on the same or on different material facets. Here, we show that Bi2TeI is a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band, which classify it as a weak topological insulator with metallic states on its 'side' surfaces. The mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. We investigated Bi2TeI spectroscopically to show the existence of both two-dimensional Dirac surface states, which are susceptible to mirror symmetry breaking, and one-dimensional channels that reside along the step edges. Their mutual coexistence on the step edge, where both facets join, is facilitated by momentum and energy segregation. Our observation of a dual topological insulator should stimulate investigations of other dual topology classes with distinct surface manifestations coexisting at their boundaries.

Probing Nonequilibrium Dynamics of Photoexcited Polarons on a Metal-Oxide Surface with Atomic Precision.

Understanding the nonequilibrium dynamics of photoexcited polarons at the atomic scale is of great importance for improving the performance of photocatalytic and solar-energy materials. Using a pulsed-laser-combined scanning tunneling microscopy and spectroscopy, here we succeeded in resolving the relaxation dynamics of single polarons bound to oxygen vacancies on the surface of a prototypical photocatalyst, rutile TiO_{2}(110). The visible-light excitation of the defect-derived polarons depletes the polaron states and leads to delocalized free electrons in the conduction band, which is further corroborated by ab initio calculations. We found that the trapping time of polarons becomes considerably shorter when the polaron is bound to two surface oxygen vacancies than that to one. In contrast, the lifetime of photogenerated free electrons is insensitive to the atomic-scale distribution of the defects but correlated with the averaged defect density within a nanometer-sized area. Those results shed new light on the photocatalytically active sites at the metal-oxide surface.

Photoexcitation Induced Quantum Dynamics of Charge Density Wave and Emergence of a Collective Mode in 1T-TaS2.

Photoexcitation is a powerful means in distinguishing different interactions and manipulating the states of matter, especially in charge density wave (CDW) materials. The CDW state of 1T-TaS2 has been widely studied experimentally mainly because of its intriguing laser-induced ultrafast responses of electronic and lattice subsystems. However, the microscopic atomic dynamics and underlying electronic mechanism upon photoexcitation remain unclear. Here, we demonstrate photoexcitation induced ultrafast dynamics of CDW in 1T-TaS2 using time-dependent density functional theory molecular dynamics. We discover a novel collective oscillation mode between the CDW state and a transient state induced by photodoping, which is significantly different from thermally induced phonon mode and attributed to the modification of the potential energy surface from laser excitation. In addition, our finding validates nonthermal melting of CDW induced at low light intensities, supporting that conventional hot electron model is inadequate to explain photoinduced dynamics. Our results provide a deep insight into the coherent electron and lattice quantum dynamics during the formation and excitation of CDW in 1T-TaS2.

Low Residual Carrier Concentration and High Mobility in 2D Semiconducting Bi2O2Se.

The air-stable and high-mobility two-dimensional (2D) Bi2O2Se semiconductor has emerged as a promising alternative that is complementary to graphene, MoS2, and black phosphorus for next-generation digital applications. However, the room-temperature residual charge carrier concentration of 2D Bi2O2Se nanoplates synthesized so far is as high as about 1019-1020 cm-3, which results in a poor electrostatic gate control and unsuitable threshold voltage, detrimental to the fabrication of high-performance low-power devices. Here, we first present a facile approach for synthesizing 2D Bi2O2Se single crystals with ultralow carrier concentration of ∼1016 cm-3 and high Hall mobility up to 410 cm2 V-1 s-1 simultaneously at room temperature. With optimized conditions, these high-mobility and low-carrier-concentration 2D Bi2O2Se nanoplates with domain sizes greater than 250 µm and thicknesses down to 4 layers (∼2.5 nm) were readily grown by using Se and Bi2O3 powders as coevaporation sources in a dual heating zone chemical vapor deposition (CVD) system. High-quality 2D Bi2O2Se crystals were fabricated into high-performance and low-power transistors, showing excellent current modulation of >106, robust current saturation, and low threshold voltage of -0.4 V. All these features suggest 2D Bi2O2Se as an alternative option for high-performance low-power digital applications.