Molecularly specific detection towards trace nitrogen dioxide by utilizing Schottky-junction-based Gas Sensor.

Trace NO2 detection is essential for the production and life, where the sensing strategy is appropriate for rapid detection but lacks molecular specificity. This investigation proposes a sensing mechanism dominated by surface-scattering to achieve the molecularly-specific detection. Two-dimensional Bi2O2Se is firstly fabricated into a Schottky-junction-based gas-sensor. Applied with an alternating excitation, the sensor simultaneously outputs multiple response signals (i.e., resistance, reactance, and the impedance angle). Their response times are shorter than 200 s at room temperature. In NO2 sensing, these responses present the detection limit in ppt range and the sensitivity is up to 16.8 %·ppb-1. This NO2 sensitivity presents orders of magnitude higher than those of the common gases within the exhaled breath. The impedance angle is involved in the principle component analysis together with the other two sensing signals. Twelve kinds of typical gases containing NO2 are acquired with molecular characteristics. The change in dipole moment of the target molecule adsorbed is demonstrated to correlate with the impedance angle via surface scattering. The proposed mechanism is confirmed to output ultra-sensitive sensing responses with the molecular characteristic.

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.

Step-Climbing Epitaxy of Layered Materials with Giant Out-of-Plane Lattice Mismatch.

Heteroepitaxy with large lattice mismatch remains a great challenge for high-quality epifilm growth. Although great efforts have been devoted to epifilm growth with an in-plane lattice mismatch, the epitaxy of 2D layered crystals on stepped substrates with a giant out-of-plane lattice mismatch is seldom reported. Here, taking the molecular-beam epitaxy of 2D semiconducting Bi2 O2 Se on 3D SrTiO3 substrates as an example, a step-climbing epitaxy growth strategy is proposed, in which the n-th (n = 1, 2, 3…) epilayer climbs the step with height difference from out-of-plane lattice mismatch and continues to grow the n+1-th epilayer. Step-climbing epitaxy can spontaneously relax and release the strain from the out-of-plane lattice mismatch, which ensures the high quality of large-area epitaxial films. Wafer-scale uniform 2D Bi2 O2 Se single-crystal films with controllable thickness can be obtained via step-climbing epitaxy. Most notably, one-unit-cell Bi2 O2 Se films (1.2 nm thick) exhibit a high Hall mobility of 180 cm2 V-1 s-1 at room temperature, which exceeds that of silicon and other 2D semiconductors with comparable thickness. As an out-of-plane lattice mismatch is generally present in the epitaxy of layered materials, the step-climbing epitaxy strategy expands the existing epitaxial growth theory and provides guidance toward the high-quality synthesis of layered materials.