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<jats:title>Abstract</jats:title> <jats:p>Transition metal dichalcogenide (TMD) monolayers (1L) in the 2H-phase are two-dimensional semiconductors with two valleys in their band structure that can be selectively populated using circularly polarized light. The choice of the substrate for monolayer TMDs is an essential factor for the optoelectronic properties and for achieving a high degree of valley polarization at room temperature (RT). In this work, we investigate the room-temperature valley polarization of monolayer WS2 on different substrates. A degree of polarization of photoluminescence (PL) in excess of 27% is found from neutral excitons in 1L-WS2 on graphite at room temperature, under resonant excitation. Using chemical doping through photochlorination we modulate the polarization of the neutral exciton emission from 27% to 38% for 1L-WS2/graphite. We show that the valley polarization strongly depends on the interplay between doping and the choice of the supporting layer of TMDs. Time-resolved PL measurements, corroborated by a rate equation model accounting for the bright exciton population in the presence of a dark exciton reservoir support our findings. These results suggest a pathway towards engineering valley polarization and exciton lifetimes in TMDs, by controlling the carrier density and/or the dielectric environment at ambient conditions.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The efficacy of two carbon-based nanomaterials, graphene oxide (GO) and Ti3C2 MXene (MX), on the radiosensitivity of the breast cancer cells (BCC) was investigated using clinical X-ray irradiation. The prepared GO and MX nanoparticles (NPs) were firstly characterized utilizing FT-IR, UV-Vis, AFM and TEM techniques and subsequently assessed in terms of their radiobiological properties. The results of the cell toxicity assay indicated that neither NPs exhibited significant cytotoxicity after 48 h incubation with BCC up to 50 µg/mL concentration without irradiation. The cell internalization results showed an approximately equivalent cellular uptake for both NPs after 6 h incubation with BCC. Our comparative studies with radiotherapy demonstrated that both NPs substantially increased cell proliferation inhibition and cell apoptosis of BCC under X-ray irradiation when compared to BCC treated with irradiation alone. Additionally, the DCFH-DA flow cytometry results and fluorescent microscopy images revealed that both NPs remarkably increased the level of intracellular reactive oxygen species (ROS) generation in BCC under X-ray irradiation. The MX nanosheets exhibited superior radiosensitization efficiency than GO under X-ray irradiation due to its higher level of intracellular ROS generation (MX = 75.2 % and GO = 65.2 %). Clonogenic cell survival assay and extracted radiobiological parameters revealed that both NPs in combination with X-ray irradiation induced more lethal damage and less sublethal damage to BCC. Generally, the obtained results demonstrate that the MX NPs, as a stronger radiosensitizer than GO, could be a promising candidate for enhancing the effectiveness of radiotherapy in breast cancer treatment.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Graphdiyne (GDY) is a new carbon allotrope with excellent properties due to its unique structure and highly conjugated system. In this work, GDY/CuMoO4/CuO tandem S-Scheme heterojunction was constructed using the cross-coupling method. Among them, CuI is not only used as a coupling catalyst to obtain easily collected graphdiyne, but also as a precursor for more active composite catalysts. 2D GDY provides a substrate for the loading of CuMoO4 and CuO, while the highly conjugated system and excellent electrical conductivity allow the composites to form a unique system of strong charge distribution and transport. The step-by-step progressive S-Scheme heterojunctions constructed based on the one-step calcination strategy have stronger reducing activity and carrier transfer capability. The intrinsic charge transfer mechanism of the catalyst was investigated by photoelectrochemical characterisation and in situ XPS analysis, and the mechanism of the photocatalytic hydrogen production reaction was proposed. This work provides a viable approach for the development of GDY in photocatalysis and the design of S-Scheme heterojunctions.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The optoelectronic properties of two-dimensional (2D) atomically thin transition metal di-chalcogenides (TMDCs) are predominantly governed by excitons and their interaction with lattice and various physical fields. Therefore, it is essential to understand the role played by excitons in the light-matter interaction processes. We introduce sulphur isotope engineering for the first time in the TMDC family, which enables us to disentangle the crucial role played by phonons in the optoelectronic properties of TMDCs. With the gradual introduction of heavier isotopes in chemical vapor deposition (CVD) growth MoS<jats:sub>2</jats:sub>, we discovered a systematic variation of lattice phonon energy. Consequently, the transient and steady-state spontaneous photoluminescence (PL) spectra were dramatically altered. Accordingly, the isotopically pure monolayers (MLs) show more intrinsic properties with enhanced emission efficiencies than isotopically mixed MoS<jats:sub>2</jats:sub> MLs. The variation of the free exciton energies with temperature for the isotopically modified MoS<jats:sub>2</jats:sub> MLs can be well described by Varshini’s equation. Along with the enormous significance for practical applications, our study provides a unique platform to understand the fundamentals of optical processes in 2D systems, where the lattice-related quasi-particles play a dominant role.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Optical spectroscopy is indispensable for research and development in nanoscience and nanotechnology, microelectronics, energy, and advanced manufacturing. Advanced optical spectroscopy tools often require both specifically designed high-end instrumentation and intricate data analysis techniques. Beyond the common analytical tools, deep learning methods are well suited for interpreting high-dimensional and complicated spectroscopy data. They offer great opportunities to extract subtle and deep information about optical properties of materials with simpler optical setups, which would otherwise require sophisticated instrumentation. In this work, we propose a computational ellipsometry approach based on a conventional tabletop optical microscope and a deep learning model called ReflectoNet. Without any prior knowledge about the multilayer substrates, ReflectoNet can predict the complex refractive indices of thin films and 2D materials on top of these nontrivial substrates from experimentally measured optical reflectance spectra with high accuracies. This task was not feasible previously with traditional reflectometry or ellipsometry methods. Fundamental physical principles, such as the Kramers-Kronig relations, are spontaneously learned by the model without any further training. This approach enables in-operando optical characterization of functional materials and 2D materials within complex photonic structures or optoelectronic devices.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantifying and controlling the coherent dynamics and couplings of optically active excitations in solids is of paramount importance in fundamental research in condensed matter optics and for their prospective optoelectronic applications in quantum technologies. Here, we perform ultrafast coherent nonlinear spectroscopy of a charge-tunable MoSe<jats:sub>2</jats:sub> monolayer. The experiments show that the homogeneous and inhomogeneous line width and the population decay of exciton complexes hosted by this material can be directly tuned by an applied gate bias, which governs the Fermi level and therefore the free carrier density. By performing two-dimensional spectroscopy, we also show that the same bias-tuning approach permits us to control the coherent coupling strength between charged and neutral exciton complexes.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Layered van der Waals materials have risen as a powerful platform to engineer artificial competing states of matter. Here we show the emergence of multiferroic order in twisted chromium trihalide bilayers, an order fully driven by the moiré pattern and absent in aligned multilayers. Using a combination of spin models and ab initio calculations, we show that a spin texture is generated in the moiré supercell of the twisted system as a consequence of the competition between stacking-dependent interlayer magnetic exchange and magnetic anisotropy. An electric polarization arises associated with such a non-collinear magnetic state due to the spin-orbit coupling, leading to the emergence of a local ferroelectric order following the moiré. Among the stochiometric trihalides, our results show that twisted CrBr<jats:sub>3</jats:sub> bilayers give rise to the strongest multiferroic order. We further show the emergence of a strong magnetoelectric coupling, which allows the electric generation and control of magnetic skyrmions. Our results put forward twisted chromium trihalide bilayers, and in particular CrBr<jats:sub>3</jats:sub> bilayers, as a powerful platform to engineer artificial multiferroic materials and electrically tunable topological magnetic textures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Transition metal dichalcogenides have been the subject of numerous studies addressing technological applications and fundamental issues. Single-layer PtSe2 is a semiconductor with a trivial bandgap, in contrast, its counterpart with 25% of Se atoms substituted by Hg, Pt2HgSe3 (jacutingaite, a naturally occurring mineral), is a 2D topological insulator with a large bandgap. Based on ab-initio calculations, we investigate the energetic stability, and the topological transition in Pt(HgxSe1-x)2 as a function of alloy concentration, and the distribution of Hg atoms embedded in the PtSe2 host. Our findings reveal the dependence of the topological phase with respect to the alloy concentration and robustness with respect to the distribution of Hg. Through a combination of our ab-initio results and a defect wave function percolation model, we estimate the random alloy concentration threshold for the topological transition to be only 9%. Our results expand the possible search for non-trivial topological phases in random alloy systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Layered materials (LMs) produced by liquid phase exfoliation (LPE) can be used as building blocks for optoelectronic applications. However, when compared with mechanically exfoliated flakes, or films prepared by chemical vapor deposition (CVD), LPE-based printed optoelectronic devices are limited by mobility, defects and trap states. Here, we present a scalable fabrication technique combining CVD with LPE LMs to overcome such limitations. We use black phosphorus (BP) inks, inkjet-printed on graphene on Si/SiO$_{2}$, patterned by inkjet printing based lithography, and source and drain electrodes printed with an Ag ink, to prepare photodetectors (PDs). These have an external responsivity (R$_{ext}$)$\sim$337A/W at 488nm, and operate from visible ($\sim$488nm) to short-wave infrared ($\sim$2.7$\mu$m, R$_{ext}\sim$48mA/W). We also use this approach to fabricate flexible PDs on polyester fabric, one of the most common used in textiles, achieving R$_{ext}\sim$6mA/W at 488nm for an operating voltage of 1V. Thus, our combination of scalable CVD and LPE techniques via inkjet printing is promising for wearable and flexible applications</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Control on spatial location and density of defects in 2D materials can be achieved using electron beam irradiation. Conversely, ultralow accelerating voltages (≤ 5kV) are used to measure surface morphology, with no expected defect creation. We find clear signatures of defect creation in monolayer (ML) MoS2 at these voltages. Evolution of Eʹ and A1ʹ Raman modes with electron dose, and appearance of defect activated peaks indicate defect formation. To simulate Raman spectra of MoS2 at realistic defect distributions, while retaining density-functional theory accuracy, we combine machine-learning force fields for phonons and eigenmode projection approach for Raman tensors. Simulated spectra agree with experiments, with sulphur vacancies as suggested defects. We decouple defects, doping and carbonaceous contamination using control (hBN covered and encapsulated MoS2) samples. We observe cryogenic PL quenching and defect peaks, and find that carbonaceous contamination does not affect defect creation. These studies have applications in photonics and quantum emitters.&amp;#xD;</jats:p>