Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS 2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS 2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS 2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipoleallowed transitions in Fe:MoS 2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS 2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.

We describe a facile technique based on polymer encapsulation to apply several percent (>5%) controllable strains to monolayer and few-layer transition metal dichalcogenides (TMDs). We use this technique to study the lattice response to strain via polarized Raman spectroscopy in monolayer WSe 2 and WS 2 . The application of strain causes mode-dependent red shifts, with larger shift rates observed for in-plane modes. We observe a splitting of the degeneracy of the in-plane E′ modes in both materials and measure the Gruneisen parameters. At large strain, we observe that the reduction of crystal symmetry can lead to a change in the polarization response of the A′ mode in WS 2 . While both WSe 2 and WS 2 exhibit similar qualitative changes in the phonon structure with strain, we observe much larger changes in mode positions and intensities with strain in WS 2 . These differences can be explained simply by the degree of iconicity of the metal−chalcogen bond.

affects both its electrical conductivity and its absorption spectrum. These changes have obvious application as a sensor of relative humidity (RH).Structural colors found in nature [9] have received great scientific interest and been reproduced by diverse structural colors. [10][11][12] The simplest structural coloration mechanism consists of planar multilayered arrangements. [13][14][15][16][17] In particular, the metal-insulator-metal (MIM) configuration based on a Fabry-Pérot resonator has a high quality factor efficient band-pass filtering with scalability and cost-effective fabrication. [18][19][20] However, conventional planar MIM resonators lack tunable function, because the resonance depends only on the geometry and optical parameters of the insulating layer.Structural colors have been tuned using chemical, [21][22][23][24][25][26] mechanical, [27,28] or electrical stimuli, [29,30] polarization, [31,32] and phase-change materials. [33] However, tunable structural colors have a slow response, [21] complicated mechanism [27,28,33] and insufficient dynamic change. For those reasons, practical application of structural colors has been limited.In this work, we propose tunable color filter composed of MIM multilayer, in which the insulator is chitosan hydrogel. This color filter can serve as a humidity sensors when combined with a photovoltaic (PV) cell. The structure uses chitosan film sandwiched between two ultrathin silver (Ag) layers deposited on a glass substrate. The key element is the chitosan insulating layer, in which the effective optical thickness t eff and refractive index n c change in response to RH; this trait can be exploited to obtain optical tunability of the resonance wavelengths. The corresponding resonance peak shift induces output current change of a PV cell, which is proportional to a change in the RH value of the environment. The special features of the proposed sensor are simple development, incorporation into PV cell, and potentially zero power usage, that make it a promising material for devices that monitor RH in enclosed spaces, workplaces and storage areas. Results and Discussion Transfer-Matrix Method (TMM) Simulation of Ag-Chitosan-Ag Multilayer StructuresWe present a tunable MIM bandpass filter in which the sensitive insulating layer is composed of chitosan, which can adsorb A tunable Fabry-Pérot resonator is realized using metal-insulator-metal structure, in which the insulator is chitosan hydrogel. The chitosan swells in response to changes in relative humidity; this change affects transmissive structural color of the multilayer structure. This tunable resonator is utilized for a humidity sensor combined with a photovoltaic cell. The change in current through the photovoltaic cell provides rapid precise measurement of relative humidity, and the change in color of the multilayer provides an approximate, remotely-readable estimate. The response requires no power, so the device has numerous sensing applications.

Transition metal dichalcogenides (TMDs) have emerged as promising materials to complement graphene for advanced optoelectronics. However, irreversible degradation of chemical vapor deposition-grown monolayer TMDs via oxidation under ambient conditions limits applications of TMD-based devices. Here, the growth of oxidation-resistant tungsten disulfide (WS ) monolayers on graphene is demonstrated, and the mechanism of oxidation of WS on SiO , graphene/SiO , and on graphene suspended in air is elucidated. While WS on a SiO substrate begins oxidation within weeks, epitaxially grown WS on suspended graphene does not show any sign of oxidation, attributed to the screening effect of surface electric field caused by the substrate. The control of a local oxidation of WS on a SiO substrate by a local electric field created using an atomic force microscope tip is also demonstrated.

We present a controlled chemical vapor deposition (CVD) growth and transfer of arrayed MoS 2 monolayers on predetermined locations. The patterned transition metal oxide (e.g. MoO 3 ) source substrate was contacted face-to-face with an SiO 2 growth substrate, where localized MoS 2 flakes were synthesized on both source and growth substrates, following a CVD procedure. This growth technique enabled the growth of both single crystalline and polycrystalline MoS 2 monolayer arrays with controlled size and location, exclusively on predetermined locations on the growth substrates. As-grown MoS 2 arrays were transferred using a unique process that combines the wet and stamping transfer processes and dramatically enhanced the integrity of transferred MoS 2 on microstructures, while protecting the microstructures during the transfer process. This fabrication technique can be applied to different transition metal dichalcogenides (TMDs) and allows the formation of TMDs on select locations, potentially eliminating a post-lithography step for device fabrication.

Valley-selective optical selection rules and a spin-valley locking in transition-metal dichalcogenide (TMDC) monolayers are at the heart of "valleytronic physics", which exploits the valley degree of freedom and has been a major research topic in recent years. In contrast, valleytronic properties of TMDC bilayers have not been in the focus so much by now. Here, we report on the valleytronic properties and optical characterization of bilayers of WS 2 as a representative TMDC material. In particular, we study the influence of the relative layer alignment in TMDC homo-bilayer samples on their polarization-dependent optical properties. Therefore, CVD-grown WS 2 bilayer samples have been prepared that favor either the inversion symmetric AA' stacking or AB stacking without inversion symmetry during synthesis. Subsequently, a detailed analysis of reflection contrast and photoluminescence spectra under different polarization conditions has been performed. We observe circular and linear dichroism of the photoluminescence that is more pronounced for the AB stacking configuration. Our experimental findings are supported by theoretical calculations showing that the observed dichroism can be linked to optical selection rules, that maintain the spin-valley locking in the AB-stacked WS 2 bilayer, whereas a spin-layer-locking is present the inversion symmetric AA' bilayer instead. Furthermore, our theoretical calculations predict a small relative shift of the excitonic resonances in both stacking configurations, which is also experimentally observed.The ability to obtain van-der-Waals (vdW) materials as monolayers has rendered them an emerging novel material class. Transition metal dichalcogenides (TMDCs) as two-dimensional semiconductors have attracted considerable attention, because of their extraordinary strong light-matter interaction and excitonic effects, but also because of their potential application in valleytronic devices.Common to TMDCs and other layered vdW materials is the honeycomb geometry in real space as well as in reciprocal space with direct band gaps occuring at the corners of the Brillouin zone. Opposite corners are related by the parity or time-reversal symmetry and are referred to as K and K' valleys. The broken spatial inversion symmetry in a TMDC monolayer allows addressing opposite valleys separately with circular polarized light. This opens up the possibility of using the valley index -or pseudo-spin -for information storage and processing, to name but a few applications.Many optical investigations on TMDC monolayers have focused on the aspect of valley coherence and found a pronounced optical helicity [1-7] as well as a linear-polarization anisotropy for emitted light after

We report a surface energy-controlled low-pressure chemical vapor deposition growth of WS 2 monolayers on SiO 2 using pre-growth oxygen plasma treatment of substrates, facilitating increased monolayer surface coverage and patterned growth without lithography. Oxygen plasma treatment of the substrate caused an increase in the average domain size of WS 2 monolayers by 78% ± 2% while having a slight reduction in nucleation density, which translates to increased monolayer surface coverage. This substrate effect on growth was exploited to grow patterned WS 2 monolayers by patterned plasma treatment on patterned substrates and by patterned source material with resolutions less than 10 µm. Contact anglebased surface energy measurements revealed a dramatic increase in polar surface energy. A growth model was proposed with lowered activation energies for growth and increased surface diffusion length consistent with the range of results observed. WS 2 samples grown with and without oxygen plasma were similar high quality monolayers verified through transmission electron microscopy, selected area electron diffraction, atomic force microscopy, Raman, and photoluminescence measurements. This technique enables the production of large-grain size, patterned WS 2 without a post-growth lithography process, thereby providing clean surfaces for device applications.

Doping of two-dimensional (2D) semiconductors has been intensively studied toward modulating their electrical, optical, and magnetic properties. While ferromagnetic 2D semiconductors hold promise for future spintronics and valleytronics, the origin of ferromagnetism in 2D materials remains unclear. Here, we show that substitutional Fe-doping of MoS2 and WS2 monolayers induce different magnetic properties. The Fe-doped monolayers are directly synthesized via chemical vapor deposition. In both cases, Fe substitutional doping is successfully achieved, as confirmed using scanning transmission electron microscopy. While both Fe:MoS2 and Fe:WS2 show PL quenching and n-type doping, Fe dopants in WS2 monolayers are found to assume deep-level trap states, in contrast to the case of Fe:MoS2, where the states are found to be shallow. Using μm- and mm-precision local NV− magnetometry and superconducting quantum interference device, we discover that, unlike MoS2 monolayers, WS2 monolayers do not show a magnetic phase transition to ferromagnetism upon Fe-doping. The absence of ferromagnetism in Fe:WS2 is corroborated using density functional theory calculations.