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Monolayer transition metal dichalcogenides (TMDs) constitute a family of materials, in which coupled spin-valley physics can be explored and which could find applications in novel optoelectronic devices. However, before applications can be designed, a scalable method of monolayer extraction is required. Liquid phase exfoliation is a technique providing large quantities of the monolayer material, but the spin-valley properties of thus obtained TMDs are unknown. In this work, we employ steady-state and time-resolved photoluminescence (PL) to investigate the relaxation dynamics of localized excitons (LXs) in liquid exfoliated WS2. The results reveal that the circular polarization lifetime of the PL exceeds by at least an order of magnitude the PL lifetime. A rate equations model allows us to reproduce quantitatively the experimental data and to conclude that the observed large and long-lived PL polarization originates from efficient trapping of free excitons at localization sites hindering the intervalley relaxation. Furthermore, our results show that the depolarization process is inefficient for LXs. We discuss various mechanisms leading to this effect such as suppression of intervalley scattering of the LXs or inefficient spin relaxation of the holes. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMD) hold promise for applications in novel optoelectronic devices. There is therefore a need for materials that can be obtained in large quantities and with well understood optical properties. In this report, we present thorough photoluminescence (PL) investigations of monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown by microscopy studies, the exfoliated nanosheets have dimensions of tens of nanometers and thickness of 2.5 monolayers on average. The monolayer content is about 20%. Our studies show that at low temperature the PL is dominated by excitons localized on nanosheet edges. As a consequence, the PL is strongly sensitive to the environment and exhibits an enhanced splitting in magnetic field. As the temperature is increased, the excitons are thermally excited out of the defect states and the dominant transition is that of the negatively charged exciton. Furthermore, upon excitation with a circularly polarized light, the PL retains a degree of polarization reaching 50% and inherited from the valley polarized photoexcited excitons. The studies of PL dynamics reveal that the PL lifetime is on the order of 10 ps, which is probably limited by non-radiative processes. Our results underline the potential of liquid exfoliated TMD monolayers in large scale optoelectronic devices. [ABSTRACT FROM AUTHOR]

Multi-target-directed ligands (MTDLs) represent a promising frontier in tackling the complexity of multifactorial pathologies like Alzheimer's disease (AD). The synergistic inhibition of MAO-B, MAO-A, and AChE is believed to enhance treatment efficacy. A novel coumarin-based molecule substituted with O -phenylpiperazine via three- and four-carbon linkers at the 5- and 7-positions, has been identified as an effective MTDL against AD. Employing a medicinal chemistry approach, combined with molecular docking, molecular dynamic simulation, and ΔG bind estimation, two series of derivatives emerged as potent MTDLs: 8-acetyl-7-hydroxy-4-methylcoumarin (IC 50 : 1.52-4.95 μM for hAChE, 6.97-7.65 μM for hMAO-A) and 4,7-dimethyl-5-hydroxycoumarin (IC 50 : 1.88-4.76 μM for hMAO-B). They displayed binding free energy (ΔG bind ) of -76.32 kcal/mol ( 11 ) and -70.12 kcal/mol ( 12 ) against AChE and -66.27 kcal/mol ( 11 ) and -62.89 kcal/mol ( 12 ) against MAO-A. It is noteworthy that compounds 11 and 12 demonstrated efficient binding to both AChE and MAO-A, while compounds 3 and 10 significantly reduced MAO-B and AChE aggregation in vitro. These findings provide structural templates for the development of dual MAO and AChE inhibitors for the treatment of neurodegenerative diseases.

Fluorescent nanothermometers are positioned to revolutionize research into cell functions and provide strategies for early diagnostics. Fluorescent nanostructures hold particular promise to fulfill this potential if nontoxic, stable varieties allowing for precise temperature measurement with high thermal sensitivities can be fabricated. In this work, we investigate the performance of micelle-encapsulated CuInS 2 /ZnS core/shell colloidal quantum dots (QDs) as fluorescent nanothermometers. We demonstrate four temperature readout modes, which are based on variations in the photoluminescence intensity, energy, and lifetime and on a specific ratio of excitation efficiencies. We further leverage this multimodal readout to construct a fifth, multiparametric thermometer calibration based on the multiple linear regression (MLR) model. We show that the MLR approach boosts the thermometer sensitivity by up to 7-fold while reducing the readout error by about a factor of 3. As a result, our QDs offer the highest sensitivities among semiconducting QDs emitting in the first biological window. The obtained results indicate that CuInS 2 /ZnS QDs are excellent candidates for intracellular in vivo thermometry and provide guidelines for further optimization of their performance.

Strong tolerance to off-stoichiometry of group I-III-VI semiconductors in their nanocrystal form allows fabrication of multinary, alloyed structures of desired properties. In particular, alloyed Cu-In-Zn-S and Ag-In-Zn-S quantum dots (QDs) have recently emerged as efficient fluorophors, in which tailoring the composition allows tuning the optical properties, and achieving photoluminescence (PL) quantum yields approaching unity. However, poor understanding of the carrier recombination mechanism in these materials limits their further development. In this work, by studying transient absorption and temperature dependent PL on bare QDs and QDs conjugated with electron scavenger molecules, we obtain a detailed picture of carrier dynamics. Our results challenge the prevailing assumption that the PL is due to a donor-acceptor-pair transition. We show that the PL occurs as a result of a recombination of a delocalized electron with a localized hole.

van der Waals heterostructures are currently the focus of intense investigation; this is essentially due to the unprecedented flexibility offered by the total relaxation of lattice matching requirements and their new and exotic properties compared to the individual layers. Here, we investigate the hybrid transition-metal dichalcogenide/2D perovskite heterostructure WS 2 /(PEA) 2 PbI 4 (where PEA stands for phenylethylammonium). We present the first density functional theory (DFT) calculations of a heterostructure ensemble, which reveal a novel band alignment, where direct electron transfer is blocked by the organic spacer of the 2D perovskite. In contrast, the valence band forms a cascade from WS 2 through the PEA to the PbI 4 layer allowing hole transfer. These predictions are supported by optical spectroscopy studies, which provide compelling evidence for both charge transfer and nonradiative transfer of the excitation (energy transfer) between the layers. Our results show that TMD/2D perovskite (where TMD stands for transition-metal dichalcogenides) heterostructures provide a flexible and convenient way to engineer the band alignment.

In photodynamic therapy (PDT), photosensitizer (PS) molecules are irradiated by light to generate reactive oxygen species (ROS), the presence of which subsequently leads to cell death. At present, the modality is limited to the treatment of skin diseases because of the low tissue penetration of visible or ultraviolet light required for producing ROS. To increase tissue penetration and extend the therapeutic possibilities of PDT to the treatment of deep-seated cancer, rare-earth doped nanoparticles capable of up-converting infrared to visible light are investigated. These up-converting nanoparticles (UCNPs) are conjugated with PS molecules to efficiently generate ROS. In this work, we employ hexagonal β-NaYF 4 :Yb 3 +  ,Er 3 +  as UCNPs and Rose Bengal (RB) as PS molecules and demonstrate efficient in vitro PDT using this nanoformulation. Covalent bonding of the RB molecules is accomplished without their functionalization-an approach which is expected to increase the efficiency of ROS generation by 30%. Spectroscopic studies reveal that our approach results in UCNP surface fully covered with RB molecules. The energy transfer from UCNPs to RB is predominantly non-radiative as evidenced by luminescence lifetime measurements. As a result, ROS are generated as efficiently as under visible light illumination. The in vitro PDT is tested on murine breast 4T1 cancer cells incubated with 250 µg ml -1 of the nanoparticles and irradiated with NIR light under power density of 2 W cm -2 for 10 minutes. After 24 hours, the cell viability decreased to 33% demonstrating a very good treatment efficiency. These results are expected to simplify the protocols for preparation of the PDT agents and lead to improved therapeutic effects.

Lanthanide-doped nanoparticles enable conversion of near-infrared photons to visible ones. This property is envisioned as a basis of a broad range of applications: from optoelectronics, via energy conversion, to bio-sensing and phototherapy. The spectrum of applications can be extended if magnetooptical properties of lanthanide dopants are well understood. However, at present, there are many conflicting reports on the influence of the magnetic field on the upconverted luminescence. In this work, we resolve this discrepancy by performing a comprehensive study of β-NaYF4:Er3+,Yb3+ nanoparticles. Crucially, we show that the magnetic field impacts the luminescence only via a Zeeman-driven detuning between the excitation laser and the absorption transition. On the other hand, the energy transfer and multiphonon relaxation rates are unaffected. We propose a phenomenological model, which qualitatively reproduces the experimental results. The presented results are expected to lead to design of novel, dual-mode opto-magnetic upconverting nanomaterials.

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Because the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ∼200 μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure splitting observed in perovskite nanocrystals.

Degenerate extrema in the energy dispersion of charge carriers in solids, also referred to as valleys, can be regarded as a binary quantum degree of freedom, which can potentially be used to implement valleytronic concepts in van der Waals heterostructures based on transition metal dichalcogenides. Using magneto-photoluminescence spectroscopy, we achieve a deeper insight into the valley polarization and depolarization mechanisms of interlayer excitons formed across a MoS 2 /MoSe 2 /MoS 2 heterostructure. We account for the nontrivial behavior of the valley polarization as a function of the magnetic field by considering the interplay between exchange interaction and phonon-mediated intervalley scattering in a system consisting of Zeeman-split energy levels. Our results represent a crucial step toward the understanding of the properties of interlayer excitons with strong implications for the implementation of atomically thin valleytronic devices.

This study describes a new method of passivating ZnO nanofiber-based devices with a ZnS layer. This one-step process was carried out in H2S gas at room temperature, and resulted in the formation of core/shell ZnO/ZnS nanofibers. This study presents the structural, optical and electrical properties of ZnO/ZnS nanofibers formed by a 2 nm ZnS sphalerite crystal shell covering a 5 nm ZnO wurtzite crystal core. The passivation process prevented free carriers from capture by oxygen molecules and significantly reduced the impact of O2 on nanostructure conductivity. The conductivity of the nanofibers was increased by three orders of magnitude after the sulfidation, the photoresponse time was reduced from 1500 s to 30 s, and the cathodoluminescence intensity increased with the sulfidation time thanks to the removal of ZnO surface defects by passivation. The ZnO/ZnS nanofibers were stable in water for over 30 days, and in phosphate buffers of acidic, neutral and alkaline pH for over 3 days. The by-products of the passivation process did not affect the conductivity of the devices. The potential of ZnO/ZnS nanofibers for protein biosensing is demonstrated using biotin and streptavidin as a model system. The presented ZnS shell preparation method can facilitate the construction of future sensors and protects the ZnO surface from dissolving in a biological environment.

We study the impact of the nanowire shape anisotropy on the spin splitting of excitonic photoluminescence. The experiments are performed on individual ZnMnTe/ZnMgTe core/shell nanowires as well as on ZnTe/ZnMgTe core/shell nanowires containing optically active magnetic CdMnTe insertions. When the magnetic field is oriented parallel to the nanowire axis, the spin splitting is several times larger than for the perpendicular field. We interpret this pronounced anisotropy as an effect of mixing of valence band states arising from the strain present in the core/shell geometry. This interpretation is further supported by theoretical calculations which allow to reproduce experimental results.