Ultrasensitive and Self-Powered Terahertz Detection Driven by Nodal-Line Dirac Fermions and Van der Waals Architecture

Terahertz detection has been highly sought to open a range of cutting-edge applications in biomedical, high-speed communications, astronomy, security screening, and military surveillance. Nonetheless, these ideal prospects are hindered by the difficulties in photodetection featuring self-powered operation at room temperature. Here, this challenge is addressed for the first time by synthesizing the high-quality ZrGeSe with extraordinary quantum properties of Dirac nodal-line semimetal. Benefiting from its high mobility and gapless nature, a metal-ZrGeSe-metal photodetector with broken mirror symmetry allows for a high-efficiency photoelectric conversion assisted by the photo-thermoelectric effect. The designed architecture features ultrahigh sensitivity, excellent ambient stability, and an efficient rectified signal even above 0.26 THz. Maximum responsivity larger than 0.11 A W-1 , response time of 8.3 µs, noise equivalent power (NEP) less than 0.15 nW Hz-1/2 , and demonstrative imaging application are all achieved. The superb performances with a lower dark current and NEP less than 15 pW Hz-1/2 are validated through integrating the van der Waals heterostructure. These results open up an appealing perspective to explore the nontrivial topology of Dirac nodal-line semimetal by devising the peculiar device geometry that allows for a novel roadmap to address targeted terahertz application requirements.

Imaging quantum spin Hall edges in monolayer WTe2

A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe₂. Here, we directly image the local conductivity of monolayer WTe₂ using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe₂. Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform.

Engineering the Complex-Valued Constitutive Parameters of Metamaterials for Perfect Absorption

We theoretically studied how to directly engineer the constitutive parameters of metamaterials for perfect absorbers of electromagnetic waves. As an example, we numerically investigated the necessary refractive index n and extinction coefficient k and the relative permittivity ε and permeability μ of a metamaterial anti-reflection layer, which could cancel the reflection from a hydrogenated amorphous silicon (α-Si:H) thin film on a metal substrate, within the visible wavelength range from 300 to 800 nm. We found that the metamaterial anti-reflection layer should have a negative refractive index (n < 0) for short-wavelength visible light but have a positive refractive index (n > 0) for long-wavelength visible light. The relative permittivity ε and permeability μ could be fitted by the Lorentz model, which exhibited electric and magnetic resonances, respectively.

Synaptic Plasticity and Learning Behaviors Mimicked in Single Inorganic Synapses of Pt/HfOx/ZnOx/TiN Memristive System

In this work, a kind of new memristor with the simple structure of Pt/HfO_x/ZnO_x/TiN was fabricated completely via combination of thermal-atomic layer deposition (TALD) and plasma-enhanced ALD (PEALD). The synaptic plasticity and learning behaviors of Pt/HfO_x/ZnO_x/TiN memristive system have been investigated deeply. Multilevel resistance states are obtained by varying the programming voltage amplitudes during the pulse cycling. The device conductance can be continuously increased or decreased from cycle to cycle with better endurance characteristics up to about 3 × 10³ cycles. Several essential synaptic functions are simultaneously achieved in such a single double-layer of HfO_x/ZnO_x device, including nonlinear transmission properties, such as long-term plasticity (LTP), short-term plasticity (STP), and spike-timing-dependent plasticity. The transformation from STP to LTP induced by repetitive pulse stimulation is confirmed in Pt/HfO_x/ZnO_x/TiN memristive device. Above all, simple structure of Pt/HfO_x/ZnO_x/TiN by ALD technique is a kind of promising memristor device for applications in artificial neural network.

Interfacial, Electrical, and Band Alignment Characteristics of HfO2/Ge Stacks with In Situ-Formed SiO2 Interlayer by Plasma-Enhanced Atomic Layer Deposition

In situ-formed SiO₂ was introduced into HfO₂ gate dielectrics on Ge substrate as interlayer by plasma-enhanced atomic layer deposition (PEALD). The interfacial, electrical, and band alignment characteristics of the HfO₂/SiO₂ high-k gate dielectric stacks on Ge have been well investigated. It has been demonstrated that Si-O-Ge interlayer is formed on Ge surface during the in situ PEALD SiO₂ deposition process. This interlayer shows fantastic thermal stability during annealing without obvious Hf-silicates formation. In addition, it can also suppress the GeO₂ degradation. The electrical measurements show that capacitance equivalent thickness of 1.53 nm and a leakage current density of 2.1 × 10^-3 A/cm² at gate bias of V_fb + 1 V was obtained for the annealed sample. The conduction (valence) band offsets at the HfO₂/SiO₂/Ge interface with and without PDA are found to be 2.24 (2.69) and 2.48 (2.45) eV, respectively. These results indicate that in situ PEALD SiO₂ may be a promising interfacial control layer for the realization of high-quality Ge-based transistor devices. Moreover, it can be demonstrated that PEALD is a much more powerful technology for ultrathin interfacial control layer deposition than MOCVD.

Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber-based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m(-2)). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.