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Optoelectronic excitations in monolayer ${\mathrm{MoS}}_{2}$ manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena---critical to both many-body physics exploration and device applications---presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. Here, optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies. The results explicitly disentangle the competing effects and highlight longstanding theoretical predictions of large carrier-induced band gap and exciton renormalization in two-dimensional semiconductors.

Chemical vapor deposition and growth dynamics of highly anisotropic 2D lateral heterojunctions between pseudo-1D ReS2 and isotropic WS2 monolayers are reported for the first time. Constituent ReS2 and WS2 layers have vastly different atomic structure, crystallizing in anisotropic 1T′ and isotropic 2H phases, respectively. Through high-resolution scanning transmission electron microscopy, electron energy loss spectroscopy, and angle-resolved Raman spectroscopy, this study is able to provide the very first atomic look at intimate interfaces between these dissimilar 2D materials. Surprisingly, the results reveal that ReS2 lateral heterojunctions to WS2 produce well-oriented (highly anisotropic) Re-chains perpendicular to WS2 edges. When vertically stacked, Re-chains orient themselves along the WS2 zigzag direction, and consequently, Re-chains exhibit six-fold rotation, resulting in loss of macroscopic scale anisotropy. The degree of anisotropy of ReS2 on WS2 largely depends on the domain size, and decreases for increasing domain size due to randomization of Re-chains and formation of ReS2 subdomains. Present work establishes the growth dynamics of atomic junctions between novel anisotropic/isotropic 2D materials, and overall results mark the very first demonstration of control over anisotropy direction, which is a significant leap forward for large-scale nanomanufacturing of anisotropic systems.

In this work, the first use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based optical silver sensors is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric support materials. Silver sensing materials were fabricated by electrospinning technique. A fiber-optic bundle was used for measurements. Sensors were based on the change in the fluorescence signal intensity of methoxy azomethine ionophore (M-AZM). The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect silver ions are 10-100-fold higher than those of the thin film based sensors. The extraordinary sensitivities can be attributed to the high surface area of the nanofibrous membrane structures. It was found that the stability of the sensing agent in the employed matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 5 months. Our stability tests are still in progress. (C) 2010 Elsevier B.V. All rights reserved.

We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational and chemical properties for potential applications in flexible optoelectronics.

Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light–matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1−x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4–300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2., Single atomic layers of transition metal dichalcogenides are semiconductors with possible applications in spintronics. Here, the authors demonstrate tuning of the spin-orbit splitting in molybdenum tungsten diselenide by altering the alloy's composition, impacting valley polarization and light emission yield.

Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon dispersions obtained based on density function perturbation theory, the high anisotropy is attributed mainly to direction-dependent phonon dispersion and partially to phonon–phonon scattering. Our results revealing the intrinsic, orientation-dependent thermal conductivity of black phosphorus are useful for designing devices, as well as understanding fundamental physical properties of layered materials., Understanding the flow of heat in materials just one or a few atoms thick is vital for harnessing them in compact electronic devices. Here, the authors present the temperature-dependent thermal conductivity of black phosphorus ribbons and demonstrate an intrinsic orientation dependence.

Ca(OH)2 crystals, well known as portlandite, are grown in layered form, and we found that they can be exfoliated on different substrates. We performed first principles calculations to investigate the structural, electronic, vibrational, and mechanical properties of bulk, bilayer, and monolayer structures of this material. Different from other lamellar structures such as graphite and transition-metal dichalcogenides, intralayer bonding in Ca(OH)2 is mainly ionic, while the interlayer interaction remains a weak dispersion-type force. Unlike well-known transition-metal dichalcogenides that exhibit an indirect-to-direct band gap crossover when going from bulk to a single layer, Ca(OH)2 is a direct band gap semiconductor independent of the number layers. The in-plane Young's modulus and the in-plane shear modulus of monolayer Ca(OH)2 are predicted to be quite low while the in-plane Poisson ratio is larger in comparison to those in the monolayer of ionic crystal BN. We measured the Raman spectrum of bulk Ca(OH)2 and identified the high-frequency OH stretching mode A1g at 3620cm-1. In this study, bilayer and monolayer portlandite [Ca(OH)2] are predicted to be stable and their characteristics are analyzed in detail. Our results can guide further research on ultrathin hydroxites., Flemish Science Foundation (FWO-Vl); Methusalem foundation of the Flemish government; FWO Pegasus Long Marie Curie Fellowship

Angle-resolved photoemission spectroscopy experiments reveal that the valence band splitting at $K$ point in transition metal dichalcogenides is primarily due to the spin-orbit coupling rather than inter-layer interactions. The authors have achieved convincing agreement between experimental results and theoretical calculations over a wide energy range.

Bone tissue engineering literature conveys investigations regarding biodegradable polymers where bioactive inorganic materials are added either before or after electrospinning process. The goal is to mimic the composition of bone and enhance the biocompatibility of the materials. Yet, most polymeric materials are hydrophobic in nature; therefore, their surfaces are not favorable for human cellular adhesion. In this sense, modifications of the hydrophobic surface of electrospun polymer fibers with hydrophilic and bioactive nanoparticles are beneficial. In this work, dispersion of hydroxyapatite (HAp), which is similar to the mineral component of natural bone, within biodegradable and biocompatible polymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with the aid of a surfactant has been investigated. Non-ionic TWEEN20 and 12-hydroxysteric acid (HSA), cationic dodecyl trimethyl ammonium bromide (DTAB) and anionic sodium deoxycholate and sodium dodecyl sulfate (SDS) surfactants were used for comparison in order to prepare stable and homogenous nanocomposite suspensions of HAp/PHBV for the electrospinning process. Continuous and uniform composite nanofibers were generated successfully within a diameter range of 400-1,000 nm by the mediation of all surfactant types. Results showed that incorporation of HAp and any of the surfactant types strongly activates the precipitation rate of the apatite-like particles and decreases percent crystallinity of the HAp/PHBV mats. Mineralization was greatly enhanced on the fibers produced by using DTAB, HSA, and especially SDS on where also osteoblastic metabolic activity was similarly increased. The produced HAp/PHBV nanofibrous composite scaffolds would be a promising candidate as an osteoconductive bioceramic/polymer composite material for tissue engineering applications.

In this work, the use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based copper sensitive chemosensor is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric support materials. Sensing slides were fabricated by electrospinning technique. Copper sensors based on the change in the fluorescence signal intensity of fluoroionophore; N'-3-(4-(dimethylamino phenly)allylidene)isonicotinohydrazide. The sensor slides exhibited high sensitivities due to the high surface area of the nanofibrous membrane structures. The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect Cu(II) ions are 6-20-fold higher than those of the continuous thin films. By this way we obtained linear calibration plots for Cu(II) ions in the concentration range of 10(-12)-10(-5) M. The response times of the sensing slides were less than 1 min. Stability of the employed ionophore in the matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 6 months. Our stability tests are still in progress. (C) 2012 Elsevier B.V. All rights reserved.

Ethyl cellulose (EC) based electrospun nanofibers were exploited for sub-nanomolar level optical chemical sensing of ionic mercury. An azomethine ionophore was used as Hg (I) and Hg (II) sensing material. Ethyl cellulose nanofibers with varying amounts of the ionic liquid; 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) were prepared and characterized. The nanofibers were fabricated by electrospinning technique. The offered chemosensor allow determination of mercury ions in a large linear working range between 1.0 × 10 −10 and 1.0 × 10 −4 mol L −1 . Limit of detection was found to be 0.07 nM which makes this technique alternative to cold-vapor atomic absorption spectrometry (CV-AAS), flame emission methods and to inductively coupled plasma-mass spectrometry (ICP-MS). The electrospun nanofibers exhibited excellent sensitivity for Hg (II) with respect to the continuous thin films prepared with same composition. The observed high sensitivity can be attributed to the high surface area of the nanofibrous materials and enhanced diffusibility of the mercury ions to the ionophore.

The first use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based optical CO(2) sensors is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric materials. Sensing slides were fabricated by electrospinning technique. A fiber-optic bundle was used for the gas detection. CO(2) sensors based on the change in the fluorescence signal intensity of ion pair form of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS). The sensor slides showed high sensitivities due to the high surface area-to-volume ratio of the nanofibrous membrane structures. The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect CO(2) are 24 to 120 fold higher than those of the thin film based sensors. The response times of the sensing reagents were short and the signal changes were fully reversible. The stability of ion pair form of HPTS in the employed matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 7 months. Our stability tests are still in progress.