Valley polarization and photocurrent generation in transition metal dichalcogenide alloy MoS2x_{2x}Se2(1−x)_{2(1-x)}

Monolayer transition metal dichalcogenides (TMDCs) constitute the core group of materials in the emerging semiconductor technology of valleytronics. While the coupled spin-valley physics of pristine TMDC materials and their heterstructures has been extensively investigated, less attention was given to TMDC alloys, which could be useful in optoelectronic applications due to the tunability of their band gaps. We report here our experimental investigations of the spin-valley physics of the monolayer and bilayer TMDC alloy, MoS$_{2x}$Se$_{2(1-x)}$, in terms of valley polarization and the generation as well as electrical control of a photocurrent utilising the circular photogalvanic effect. Piezoelectric force microscopy provides evidence for an internal electric field perpendicular to the alloy layer, thus breaking the out-of-plane mirror symmetry. The experimental observation is supported by first principles calculations based on the density functional theory. A comparison of the photocurrent device, based on the alloy material, is made with similar devices involving other TMDC materials

Proximitized spin-phonon coupling in topological insulator due to two-dimensional antiferromagnet

Induced magnetic order in a topological insulator (TI) can be realized either by depositing magnetic adatoms on the surface of a TI or engineering the interface with epitaxial thin film or stacked assembly of two-dimensional (2D) van der Waals (vdW) materials. Herein, we report the observation of spin-phonon coupling in the otherwise non-magnetic TI Bi$_\mathrm{2}$Te$_\mathrm{3}$, due to the proximity of FePS$_\mathrm{3}$ (an antiferromagnet (AFM), $T_\mathrm{N}$ $\sim$ 120 K), in a vdW heterostructure framework. Temperature-dependent Raman spectroscopic studies reveal deviation from the usual phonon anharmonicity at/below 60 K in the peak position (self-energy) and linewidth (lifetime) of the characteristic phonon modes of Bi$_{2}$Te$_{3}$ (106 cm$^{-1}$ and 138 cm$^{-1}$) in the stacked heterostructure. The Ginzburg-Landau (GL) formalism, where the respective phonon frequencies of Bi$_{2}$Te$_{3}$ couple to phonons of similar frequencies of FePS$_3$ in the AFM phase, has been adopted to understand the origin of the hybrid magneto-elastic modes. At the same time, the reduction of characteristic $T_\mathrm{N}$ of FePS$_3$ from 120 K in isolated flakes to 65 K in the heterostructure, possibly due to the interfacial strain, which leads to smaller Fe-S-Fe bond angles as corroborated by computational studies using density functional theory (DFT). Besides, our data suggest a double softening of phonon modes of Bi$_\mathrm{2}$Te$_\mathrm{3}$ (at 30 K and 60 K), which in turn, demonstrates Raman scattering as a possible probe for delineating the magnetic ordering in bulk and surface of a hybrid topological insulator

Engineering Multifunctionality in MoSe2 Nanostructures Via Strategic Mn Doping for Electrochemical Energy Storage and Photosensing

To achieve advanced functionalities in nanostructured MoSe2 for enhanced electrochemical charge storage and improved photosensing, here we propose an effective strategy, i.e., the substitutional doping of the heteroatom Mn. We achieve a 313% increase in specific capacitance for 6.2% of Mn doping compared to pristine MoSe2 at the scan rate of 5 mV/s in a three electrode configuration. For a two-electrode arrangement, also superior charge-storage performance is noted. The enhanced electrode performance can be attributed to the increase of electrical conductivity arising due to an increase of electron density for the n-type nature of Mn doping realized via an X-ray photoelectron spectroscopy study and density functional theory calculation. The latter one also unveils that Mn doping introduces catalytically active sites by disrupting homogeneous charge distribution over the topology of the MoSe2 basal plane contributing to better charge-storage performance. Mn doping-induced shift in the Fermi level of MoSe2 toward the conduction band also minimizes the contact barrier height signifying its improved capabilities for a photosensor device. Additionally, Mn doping causes alleviation of the charge-recombination process resulting in increase of photocarrier separation. As a result, we observe a 187% enhancement in the photocurrent and significantly higher responsivity and detectivity for 6.2% Mn-doped MoSe2 than its pristine counterpart. Our proposed doping strategy to modulate charge storage as well as photoresponse properties demonstrates high potential for MoSe2 along with other two-dimensional transition-metal dichalcogenides in developing next-generation energy-storage and optoelectronic devices

Engineering Multifunctionality in MoSe₂ Nanostructures Via Strategic Mn Doping for Electrochemical Energy Storage and Photosensing

To achieve advanced functionalities in nanostructured MoSe2 for enhanced electrochemical charge storage and improved photosensing, here we propose an effective strategy, i.e., the substitutional doping of the heteroatom Mn. We achieve a 313% increase in specific capacitance for 6.2% of Mn doping compared to pristine MoSe2 at the scan rate of 5 mV/s in a three-electrode configuration. For a two-electrode arrangement, also superior charge-storage performance is noted. The enhanced electrode performance can be attributed to the increase of electrical conductivity arising due to an increase of electron density for the n-type nature of Mn doping realized via an X-ray photoelectron spectroscopy study and density functional theory calculation. The latter one also unveils that Mn doping introduces catalytically active sites by disrupting homogeneous charge distribution over the topology of the MoSe2 basal plane contributing to better charge-storage performance. Mn doping-induced shift in the Fermi level of MoSe2 toward the conduction band also minimizes the contact barrier height signifying its improved capabilities for a photosensor device. Additionally, Mn doping causes alleviation of the charge-recombination process resulting in increase of photocarrier separation. As a result, we observe a 187% enhancement in the photocurrent and significantly higher responsivity and detectivity for 6.2% Mn-doped MoSe2 than its pristine counterpart. Our proposed doping strategy to modulate charge storage as well as photoresponse properties demonstrates high potential for MoSe2 along with other two-dimensional transition-metal dichalcogenides in developing next-generation energy-storage and optoelectronic devices

Excited-State Energy Transfer-Associated Dual Emission of Light-Emitting Polymers Containing Sulfonated Graphene Oxide for Sensing of pH, Co(II), and Bi(III)

The design, synthesis, optimization, and development of an excited-state energy transfer (ESET)-assisted dual-light emission hybrid polymeric sensor are very much challenging, particularly when the polymer is purely aliphatic and bears nonconventional heteroatomic subfluorophores. In this work, aliphatic light-emitting polymers (LEPs) are synthesized from dimethylaminoethyl methacrylate and maleic acid monomers having −C(O)OCH2–, −N(CH3)2, and −C(O)OH/–C(O)O– subfluorophores. In aliphatic LEPs, hydrogen bond associated strong supramolecular networks facilitate n → π* transmissions and dual excitation dual emission. The optimum incorporation of subfluorophores in LEP5 is supported by Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopies, thermogravimetric profiles, and aggregation enhanced emission (AEE) studies. Thereafter, the sulfonated graphene oxide (SGO) nanoparticle is incorporated in the optimum LEP5 to fabricate hybrid light-emitting polymers (HLEPs) having increased size/surface area, noncovalent interactions, and electronic distributions. In HLEPs, electron rich polar −C(O)OH/–C(O)O– and −SO3H/–SO3– functionalities increase hydrogen bonding and dipole–dipole interactions. Among HLEPs, HLEP3 having the maximum aggregating tendency and emission capacity is optimized by AEE studies and FTIR, Raman, powder X-ray diffraction (PXRD), and thermogravimetric analyses. In the aggregation-associated ESET phenomenon of HLEP3, the SGO nanoparticle acts as an energy acceptor. The ESET-associated single-excitation dual emissions are supported by the absorption and emission maxima of HLEP3-aggregate and HLEP3, respectively; excitation spectra; and average lifetimes in time correlated single photon count studies. The aggregations of HLEP3 are supported by dynamic light scattering (DLS) studies, scanning electron microscopy photomicrographs, and UV–vis spectra. The dual-emission phenomena of HLEP3 and its pH-sensitive subfluorophores −N(CH3)2, −C(O)OH/–C(O)O–, and −SO3H/–SO3– are responsible for precise ratiometric pH sensing within 8.0–11.0. The reversibility test and PXRD data of HLEP3 at different pH values indicate the stability of HLEP3 in acidic, neutral, and basic media. The dual-light emissions facilitate rapid sensing and detections of Co(II) and Bi(III) with limits of detection below the WHO-recommended values in both aqueous and nonaqueous media. The strong coordinations of Co(II) and Bi(III) with −C(O)O–/–SO3H/–SO3– of HLEP3 and HLEP3-aggregate, respectively, are confirmed by UV–vis, FTIR, and Raman spectroscopies along with the DLS measurements