Monolayer Graphene Terahertz Detector Integrated with Artificial Microstructure.

Graphene, known for its high carrier mobility and broad spectral response range, has proven to be a promising material in photodetection applications. However, its high dark current has limited its application as a high-sensitivity photodetector at room temperature, particularly for the detection of low-energy photons. Our research proposes a new approach for overcoming this challenge by designing lattice antennas with an asymmetric structure for use in combination with high-quality monolayers of graphene. This configuration is capable of sensitive detection of low-energy photons. The results show that the graphene terahertz detector-based microstructure antenna has a responsivity of 29 V·W-1 at 0.12 THz, a fast response time of 7 µs, and a noise equivalent power of less than 8.5 pW/Hz1/2. These results provide a new strategy for the development of graphene array-based room-temperature terahertz photodetectors.

Ultrasensitive Self-Driven Terahertz Photodetectors Based on Low-Energy Type-II Dirac Fermions and Related Van der Waals Heterojunctions.

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22 A W-1 , with a response time of 0.6 µs, a noise-equivalent power of 18 pW Hz-0.5 , with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection.

Polarization-controlled varifocal metalens with a phase change material GSST in mid-infrared.

Detection of aldehyde carbonyl radiation plays an essential role in guaranteeing the safety of fried food. However, the radiation of low-content aldehyde carbonyl is always weak and includes polarized light. Focusing the weak radiation with polarization-sensitive configurations provides an efficient way to improve the signal-to-noise ratio of detection. The advent of dynamic metasurfaces based on phase-change materials (PCMs) have demonstrated superiorities over their traditional counterparts in tunability and miniaturization. In this paper, we propose two reflected varifocal metasurfaces, which combine Ge2Sb2Se4Te1 (GSST) with two materials that have close optical constants with amorphous and crystalline GSST. The first one realizes a four-spot focal system with linearly-polarized incidence based on polarization multiplexing. It adds a new polarization degree of freedom compared with traditional varifocal metasurfaces. Compared with traditional spatial-multiplexing method, our second metasurface enables the independent control of the polarization and phase profiles of circularly-polarized light. Remarkably, it reduces energy loss and crosstalk. We believe the novel scenarios of combing GSST with similar materials provide a new direction for tunable metasurfaces based on PCMs.

Hybrid Dirac semimetal-based photodetector with efficient low-energy photon harvesting.

Despite the considerable effort, fast and highly sensitive photodetection is not widely available at the low-photon-energy range (~meV) of the electromagnetic spectrum, owing to the challenging light funneling into small active areas with efficient conversion into an electrical signal. Here, we provide an alternative strategy by efficiently integrating and manipulating at the nanoscale the optoelectronic properties of topological Dirac semimetal PtSe2 and its van der Waals heterostructures. Explicitly, we realize strong plasmonic antenna coupling to semimetal states near the skin-depth regime (λ/104), featuring colossal photoresponse by in-plane symmetry breaking. The observed spontaneous and polarization-sensitive photocurrent are correlated to strong coupling with the nonequilibrium states in PtSe2 Dirac semimetal, yielding efficient light absorption in the photon range below 1.24 meV with responsivity exceeding ∼0.2 A/W and noise-equivalent power (NEP) less than ~38 pW/Hz0.5, as well as superb ambient stability. Present results pave the way to efficient engineering of a topological semimetal for high-speed and low-energy photon harvesting in areas such as biomedical imaging, remote sensing or security applications.

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.

High-frequency rectifiers based on type-II Dirac fermions.

The advent of topological semimetals enables the exploitation of symmetry-protected topological phenomena and quantized transport. Here, we present homogeneous rectifiers, converting high-frequency electromagnetic energy into direct current, based on low-energy Dirac fermions of topological semimetal-NiTe2, with state-of-the-art efficiency already in the first implementation. Explicitly, these devices display room-temperature photosensitivity as high as 251 mA W-1 at 0.3 THz in an unbiased mode, with a photocurrent anisotropy ratio of 22, originating from the interplay between the spin-polarized surface and bulk states. Device performances in terms of broadband operation, high dynamic range, as well as their high sensitivity, validate the immense potential and unique advantages associated to the control of nonequilibrium gapless topological states via built-in electric field, electromagnetic polarization and symmetry breaking in topological semimetals. These findings pave the way for the exploitation of topological phase of matter for high-frequency operations in polarization-sensitive sensing, communications and imaging.

The Novel of n-p-n Type Transition in the ZnSe/Ge Heterojunction Nanowire: First Principles Study.

The structure and electronic properties of the bare and hydrogen-passivated ZnSe/Ge bi-axial nanowires have been calculated by means of the first principle calculation based on density functional theory. Five different types of nanowires with different concentrations all grown along [1 1 1] direction are considered. Band gaps of bare ZnSe/Ge bi-axial nanowires are smaller than those of hydrogen-passivated ZnSe/Ge nanowires at the same doping concentrations. Both the bare and hydrogen-passivated nanowires have lower band gap at a higher Ge components. It is shown detailedly that with increasing of Ge doping concentrations, the main sources of conduction band minimum and valence band maximum of nanowires varied from the p-state of Se and Ge to the p-state of Ge. It is found clearly that there is a transition from the n-type to the p-type characteristics at the doping concentration 0.4211. Whereas, when the Ge composition is increased to 0.8421, the nanowires also have a transition from the p-type to the n-type characteristics. In addition, the structural stability and the cohesive energies of ZnSe/Ge bi-coaxial nanowires are changed obviously with different Ge components. The results offer efficiently guidance to explore their potential applications in photoelectronics.

First-principles calculations of the electronic properties of SiC-based bilayer and trilayer heterostructures.

Recently, van der Waals (vdW) two-dimensional heterostructures have attracted great attention. The combination structures demonstrate unique properties that individual layers do not possess, which foretell promising future applications. Here, we investigate the structural and electronic properties of SiC/graphene, SiC/MoS2, and graphene/SiC/MoS2 vdW heterostructures using first-principles calculations. The SiC/graphene interface forms a p-type Schottky contact, which can be turned into an n-type Schottky contact by applying an external electric field. Moreover, a transition from a Schottky to an Ohmic contact at the interface can be triggered by varying the interlayer distance or applying an external electric field. The SiC/MoS2 interface forms a type-II heterostructure, in which the recombination of photoexcited charges will be greatly suppressed. The transition from type-II to type-III band alignment can be realized in the SiC/MoS2 heterostructure by applying a biaxial strain. This heterostructure also shows excellent optical absorption abilities in the visible and far-infrared range, which merits its application as a photocatalyst. The trilayer heterostructure exhibits a tunable Schottky barrier with different stacking patterns and the assembled graphene could act as a protective encapsulating layer on SiC/MoS2. The results show that graphene and MoS2 can tune and improve the electronic performance of SiC and demonstrate the promising application of SiC-based heterostructures for nanoelectronics and nanophotonics.

The Structural and Electronic Properties of CdS/ZnS Core-Shell Nanowires.

The structural and electronic properties of the CdS/ZnS core-shell nanowires (NWs) oriented along [001] direction have been investigated by means of the first-principles calculation. It is found that CdS core suffers from the compressive strain in the CdS-core/ZnS-shell NWs, and ZnS core is stretched in the ZnS-core/CdS-shell NWs. A thicker ZnS shell can improve the NWs' stability, and a thicker CdS shell would decrease their stability. For both CdS/ZnS core-shell NWs, the band gap decreases linearly with increasing the shell when the core size is fixed. However, when the diameter of NWs is fixed, CdS-core/ZnS-shell NWs with a thicker shell would have larger band gap. The results agree well with that of red-shift or blue-shift of the spectrum in experimental observations. The partial density of states indicates that the contribution to valence band maximum mainly comes from the S-3p state, and the contribution to conduction band minimum mainly comes from Cd-5s state for CdS-core/ZnS-shell NWs. Thus the electrons would be effectively confined in CdS core, and the holes tend to distribute over both the core and shell. It can be deduced that CdS-core/ZnS-shell NWs with a thicker shell may have larger mobility.

Orbital change manipulation metal-insulator transition temperature in W-doped VO2.

A series of epitaxial V1-xWxO2 (0 ≤ x ≤ 0.76%) nanocrystalline films on c-plane sapphire substrates have been successfully synthesized. Orbital structures of V1-xWxO2 films with monoclinic and rutile states have been investigated by ultraviolet-infrared spectroscopy combined with first principles calculations. Experimental and calculated results show that the overlap of π* and d∥ orbitals increases with increasing W doping content for the rutile state. Meanwhile, in the monoclinic state, the optical band gap decreases from 0.65 to 0.54 eV with increasing W doping concentration. Clear evidence is found that the V1-xWxO2 thin film phase transition temperature change comes from orbital structure variations. This shows that, with increasing W doping concentration, the decrease of rutile d∥ orbital occupancy can reduce the strength of V-V interactions, which finally results in phase transition temperature decrease. The experimental results reveal that the d∥ orbital is very important for the VO2 phase transition process. Our findings open a possibility to tune VO2 phase transition temperature through orbital engineering.

Discrete plasmonic Talbot effect in finite metal waveguide arrays.

We introduce supermode theory into the propagation of surface plasmon polaritons (SPPs) in nanoscale metal waveguide arrays (MWGAs). The SPP supermodes in finite MWGAs are analyzed and the coefficient of excited supermodes can be determined quantitatively. The field intensity distributions in finite MWGAs can be explained by the superposition of the excited SPP supermodes. The discrete plasmonic Talbot effect in a finite MWGA is achieved successfully by adjusting different intensity for each input field. We also find that the period condition of the input fields in MWGAs is not the same with conventional dielectric waveguides. The theory is verified by the finite difference time-domain (FDTD) method.

In situ atom scale visualization of domain wall dynamics in VO2 insulator-metal phase transition.

A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.

Tunable Band Gap and Conductivity Type of ZnSe/Si Core-Shell Nanowire Heterostructures.

The electronic properties of zincblende ZnSe/Si core-shell nanowires (NWs) with a diameter of 1.1-2.8 nm are calculated by means of the first principle calculation. Band gaps of both ZnSe-core/Si-shell and Si-core/ZnSe-shell NWs are much smaller than those of pure ZnSe or Si NWs. Band alignment analysis reveals that the small band gaps of ZnSe/Si core-shell NWs are caused by the interface state. Fixing the ZnSe core size and enlarging the Si shell would turn the NWs from intrinsic to p-type, then to metallic. However, Fixing the Si core and enlarging the ZnSe shell would not change the band gap significantly. The partial charge distribution diagram shows that the conduction band maximum (CBM) is confined in Si, while the valence band maximum (VBM) is mainly distributed around the interface. Our findings also show that the band gap and conductivity type of ZnSe/Si core-shell NWs can be tuned by the concentration and diameter of the core-shell material, respectively.

The dependence of structural stability and tunable gap on the Si components of ZnSe/Si bi-coaxial nanowire heterostructures.

The bare and hydrogen-passivated ZnSe/Si bi-coaxial nanowire heterostructures along [110] direction have been investigated by using the first-principle calculations within density functional theory. The structural stability and electronic property of ZnSe/Si bi-coaxial nanowire heterostructures have been shown by changing the Si components. It is found that the ZnSe/Si nanowires have zero gaps at lower Si components, and then they have the increasing gap at higher Si components. It is seen clearly that there is the transition of band gap form zero to nonzero. With increasing Si components, the ZnSe/Si nanowires can be also achieved as n-type or p-type, in agreement qualitatively with the experimental observations. In addition, the structural stabilities and the cohesive energies of ZnSe/Si bi-coaxial nanowires are changed obviously with the different Si components.

The finite-size effect on the transport properties in edge-modified graphene nanoribbon-based molecular devices.

The size-dependence on the electronic and transport properties of the molecular devices of the edge-modified graphene nanoribbon (GNR) slices is investigated using density-functional theory and Green's function theory. Two edge-modifying functional group pairs are considered. Energy gap is found in all the GNR slices. The gap shows an exponential decrease with increasing the slice size of two vertical orientations in the two edge terminated cases, respectively. The tunneling probability and the number of conducting channel decreases with increasing the GNR-slices size in the junctions. The results indicate that the acceptor-donor pair edge modulation can improve the quantum conductance and decrease the finite-size effect on the transmission capability of the GNR slice-based molecular devices.

Transport properties of graphene nanoribbon-based molecular devices.

The electronic and transport properties of an edge-modified prototype graphene nanoribbon (GNR) slice are investigated using density functional theory and Green's function theory. Two decorating functional group pairs are considered, such as hydrogen-hydrogen and NH(2)-NO(2) with NO(2) and NH(2) serving as a donor and an acceptor, respectively. The molecular junctions consist of carbon-based GNR slices sandwiched between Au electrodes. Nonlinear I-V curves and quantum conductance have been found in all the junctions. With increasing the source-drain bias, the enhancement of conductance is quantized. Several key factors determining the transport properties such as the electron transmission probabilities, the density of states, and the component of Frontier molecular orbitals have been discussed in detail. It has been shown that the transport properties are sensitive to the edge type of carbon atoms. We have also found that the accepter-donor functional pairs can cause orders of magnitude changes of the conductance in the junctions.