Sub‐bandgap activated charges transfer in a graphene‐MoS 2 ‐graphene heterostructure

Transition metal (Ti, Cu, Zn, Pt) single-atom modified graphene/AS2 (A = Mo, W) van der Waals heterostructures for removing airborne pollutants

Air pollution is a worldwide issue that affects human health and the environment. The scientific community tries to control it through different approaches, from experimental to theoretical assessments. Here, we perform DFT calculations to describe CO2, NO2, and SO2 detection on a single-atom (Ti, Cu, Zn, Pt) graphene supported on 2D molybdenum disulfide (MoS2) and tungsten disulfide (WS2). Transition metal single atoms on graphene improve the monolayer reactivity by generating an effective way to remove airborne pollutants. Results indicate that SO2 and NO2 chemically adsorb on all tested transition metals, whereas CO2 stands on top of the incorporated atoms through van der Waals interactions. Since strong Ti-O interactions appear, the Ti single-atom graphene/MoS2(WS2) systems efficiently remove CO2 from the environment. Compared to pristine graphene, our proposed heterostructures improve the SO2, NO2, and CO2 adsorption energies. The heterostructures' electronic properties change once the molecules interact with the transition metals, generating sensible and selective pollutant molecule detection and removal.

Interlayer charge transfer in supported and suspended MoS2/Graphene/MoS2 vertical heterostructures

In this letter, we report on the optical and structural properties of supported and suspended MoS2/Graphene/MoS2 vertical heterostructures using Raman and photoluminescence (PL) spectroscopies. Vertical heterostructures (VH) are formed by multiple wet transfers on micro-sized holes in SiO2/Si substrates, resulting in VH with different configurations. The strong interlayer coupling is confirmed by Raman spectroscopy. Additionally, we observe an enhancement of the PL emission in the three-layer VH (either support or suspended) compared with bare MoS2 or MoS2/Graphene. This suggests the formation of a spatial type-II band alignment assisted by the graphene layer and thus, the operation of the VH as a n++/metal/n junction.

A Voltage-Tuned Terahertz Absorber Based on MoS2/Graphene Nanoribbon Structure

Terahertz frequency has promising applications in communication, security scanning, medical imaging, and industry. THz absorbers are one of the required components for future THz applications. However, nowadays, obtaining a high absorption, simple structure, and ultrathin absorber is a challenge. In this work, we present a thin THz absorber that can be easily tuned through the whole THz range (0.1-10 THz) by applying a low gate voltage (<1 V). The structure is based on cheap and abundant materials (MoS₂/graphene). Nanoribbons of MoS₂/graphene heterostructure are laid over a SiO₂ substrate with an applied vertical gate voltage. The computational model shows that we can achieve an absorptance of approximately 50% of the incident light. The absorptance frequency can be tuned through varying the structure and the substrate dimensions, where the nanoribbon width can be varied approximately from 90 nm to 300 nm, while still covering the whole THz range. The structure performance is not affected by high temperatures (500 K and above), so it is thermally stable. The proposed structure represents a low-voltage, easily tunable, low-cost, and small-size THz absorber that can be used in imaging and detection. It is an alternative to expensive THz metamaterial-based absorbers.

Hot Carrier Transfer in PtSe2/Graphene Enabled by the Hot Phonon Bottleneck

The charge transfer (CT) process of two-dimensional (2D) graphene/transition metal dichalcogenides (TMDs) heterostructures makes the photoelectric conversion ability of TMDs into a wider spectral range for the light harvester and photoelectric detector applications. However, the direct in situ investigation of the hot carrier transport in graphene/TMDs heterostructures has been rarely reported. Herein, using the optical pump and a terahertz (THz) probe (OPTP) spectroscopy, the CT process from graphene to five-layer PtSe₂ in the PtSe₂/graphene (P/G) heterostructure is demonstrated to be related to the pump fluence, which is enabled by the hot phonon bottleneck (HPB) effect in graphene. Furthermore, the frequency dispersion conductivity and the THz emission spectroscopy of the P/G heterostructure confirmed the existence of interlayer CT and its pump fluence-dependent behavior. Our results provide in-depth physical insights into the CT mechanism at the P/G van der Waals interface, which is crucial for further exploration of optoelectronic devices based on P/G heterostructures.

Large interfacial contribution to ultrafast THz emission by inverse spin Hall effect in CoFeB/Ta heterostructure

Ultrafast THz radiation generation from ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures generally exploits the spin-charge conversion within the nonmagnetic layer and its interface with the ferromagnetic layer. Various possible sub-contributions to the underlying mechanism need to be exploited not only for investigating the intricacies at the fundamental level in the material properties themselves but also for improving their performance for broadband and high-power THz emission. Here, we report ultrafast THz emission from (CoFeB,Fe)/(Ta,Pt) bilayers at varying sample temperatures to unravel the role of intrinsic and extrinsic spin-charge conversion processes through the extracted values of spin-Hall conductivities. An enhanced THz emission along with temperature-dependent THz signal polarity reversal is observed in the case of annealed CoFeB/Ta. These results demonstrate a large interfacial contribution to the overall spin-Hall angle arising from the modified interface in the annealed CoFeB/Ta.

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Temperature-dependent THz emission from spintronic heterostructures

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THz polarity reversal at a certain temperature in the annealed CoFeB/Ta

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Determination of spin-Hall conductivity from the efficiency of THz emission

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Distinguishing the intrinsic/extrinsic contributions to spin-charge conversion

Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon

We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0° and 45° angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photo-excited electrons and holes account for the change of the dielectric constant of the photo-excited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range.