Intrinsic control of interlayer exciton generation rate in van der Waals materials via Janus layers

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. This has important consequences for the charge separation efficiency. We investigate different MoS$_2$@Janus layer combinations using first-principles methods including electron-hole interactions (excitons) and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between interlayer exciton states -- which usually have a very low oscillator strength and hence are almost dark in absorption -- and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS$_2$@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient electron-hole pair generation with efficient charge carrier separation

Tuning the magnetic anisotropy in single-layer crystal structures

The effect of an applied electric field and the effect of charging are investigated on the magnetic anisotropy (MA) of various stable two-dimensional (2D) crystals such as graphene, FeCl2, graphone, fluorographene, and MoTe2 using first-principles calculations. We found that the magnetocrystalline anisotropy energy of Co-on-graphene and Os-doped-MoTe2 systems change linearly with electric field, opening the possibility of electric field tuning MA of these compounds. In addition, charging can rotate the easy-axis direction of Co-on-graphene and Os-doped-MoTe2 systems from the out-of-plane (in-plane) to in-plane (out-of-plane) direction. The tunable MA of the studied materials is crucial for nanoscale electronic technologies such as data storage and spintronics devices. Our results show that controlling the MA of the mentioned 2D crystal structures can be realized in various ways, and this can lead to the emergence of a wide range of potential applications where the tuning and switching of magnetic functionalities are important.Flemish Science Foundation (FWO-Vl); Methusalem Foundation of the Flemish government; Hercules Foundation; FWO Pegasus Marie Curie Fellowship; TUBITAK (111T318

Physicochemical and Antioxidant Responses of St. John’s Wort (Hypericum perforatum L.) under Drought Stress

This study investigated the effects of drought stress on the physiological and biochemical responses of the medicinal and aromatic plant Hypericum perforatum (St. John’s Wort). Changes were determined in leaf length, relative water content (RWC), osmotic potential, chlorophyll fluorescence (Fv/Fm), lipid peroxidation (TBARS), hydrogen peroxide (H2O2), and proline content as well as in the antioxidant system enzyme activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), and glutathione reductase (GR). These responses were examined in relation to the tolerance of drought stress in H. perforatum. Ninety-day-old seedlings were subjected to drought for three weeks. The physiological parameters of leaf length, RWC, Fv/Fm, and osmotic potential were reduced under drought. The H2O2, TBARS, and proline levels were increased significantly under drought stress. Moreover, the proline content increase was greatly pronounced (25.9-fold) compared to the control groups. The high accumulation of proline may have resulted from the 83.8% leaf RWC still remaining under drought stress. On the other hand, the SOD, CAT, and GR enzyme activities were enhanced, whereas the POX and APX activities were reduced. The results indicate that improved tolerance to drought stress in H. perforatum plants may be accomplished through increased capacity of the antioxidative defense syste

Şeker Otu (Stevia rebaudiana Bertoni) Bitkisinde Kuraklık Stresinin Fizyolojik ve Biyokimyasal Etkileri

Kuraklık son yıllarda endişe verici bir şekilde artmakta olup tarımsal ürünlerin verimliliğini sınırlandırmaktadır. Bu durum, kurak koşullara dayanıklı bitkilerin tespit edilmesine yönelik araştırmaların önemini artırmıştır. Bu çalışmada, Stevia rebaudiana Bertoni bitkisine ait iki çeşidin (Yalova ve STF-4) kuraklık stresi altında fizyolojik ve biyokimyasal tepkileri araştırılmıştır. Bitkiler kontrollü sera koşullarında 3 ay boyunca yetiştirilmiş ve sonrasında 3 hafta boyunca kuraklığa maruz bırakılmıştır. Üç hafta sonunda hasat edilen bitkilerden yaprak uzunluğu, ozmotik potansiyel, nisbi su içeriği (RWC), klorofil floresansı (Fv/Fm), prolin miktarı, hidrojen peroksit (H2O2) miktarı ve lipid peroksidasyonu seviyesi ölçülmüştür. S. rebaudiana bitkisinin her iki çeşidinde de kuraklık stresi altında kontrol grubundaki bitkiler ile kıyaslandığında yaprak uzunluğunda azalma belirlenirken en çok azalma Yalova çeşidinde (%25,7) saptanmıştır. Bununla birlikte, her iki çeşit de kuraklık stresi altında su durumlarını korumuşlardır. Fv/Fm değeri STF-4 çeşidinde kuraklıktan etkilenmezken Yalova çeşidinde kontrole oranla düşüş göstermiştir. Prolin miktarında ise çeşitler arasında fark kaydedilmiştir. Kurak koşullar altında STF-4 çeşidinde prolin miktarında değişim gözlenmezken Yalova çeşidinde %42,9 artış meydana gelmiştir. Diğer taraftan, kuraklık stresi, yapraklardaki lipid peroksidasyon seviyesini önemli ölçüde arttırmıştır. Bu artış, Yalova çeşidinde %41,2 iken STF-4 çeşidinde %21,1 olarak belirlenmiştir. İki çeşit arasında kuraklık stresine karşı farklı tepki H2O2 içeriğinde gözlenmiştir. Kuraklık stresi altında H2O2 miktarı Yalova çeşidinde %42,7 oranında azalırken STF-4 çeşidinde %5,5 artmıştır. Sonuç olarak, S. rebaudiana bitkisinin STF-4 çeşidinin ölçülen parametreler ışığında kuraklığa daha toleranslı olduğu ortaya konulmuştur

Photoinduced Phase Transitions in Ferroelectrics

Ferroic materials naturally exhibit a rich number of functionalities, which often arise from thermally, chemically, or mechanically induced symmetry breakings or phase transitions. Based on density functional calculations, we demonstrate here that light can drive phase transitions as well in ferroelectric materials such as the perovskite oxides lead titanate and barium titanate. Phonon analysis and total energy calculations reveal that the polarization tends to vanish under illumination, to favor the emergence of nonpolar phases, potentially antiferroelectric, and exhibiting a tilt of the oxygen octahedra. Strategies to tailor photoinduced phases based on phonon instabilities in the electronic ground state are also discussed

Ab initio and semiempirical modeling of excitons and trions in monolayer TiS3

We explore the electronic and the optical properties of monolayer TiS3, which shows in-plane anisotropy and is composed of a chain-like structure along one of the lattice directions. Together with its robust direct band gap, which changes very slightly with stacking order and with the thickness of the sample, the anisotropic physical prop- erties of TiS3 make the material very attractive for various device applications. In this study, we present a detailed investigation on the effect of the crystal anisotropy on the excitons and the trions of the TiS3 monolayer. We use many-body perturbation theory to calculate the absorption spectrum of anisotropic TiS3 monolayer by solving the Bethe-Salpeter equation. In parallel, we implement and use a Wannier-Mott model for the excitons that takes into account the anisotropic effective masses and Coulomb screening, which are obtained from ab initio calculations. This model is then extended for the investigation of trion states of monolayer TiS3. Our calculations indicate that the absorption spectrum of monolayer TiS3 drastically depends on the polarization of the incoming light, which excites different excitons with distinct binding energies. In addition, the binding energies of positively and the negatively charged trions are observed to be distinct and they exhibit an anisotropic probability density distribution

Interlayer and intralayer excitons in MoS2/WS2 and MoSe2/WSe2 heterobilayers

Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems possess an inter- or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single layers. In this paper, we report the electronic structure and the absorption spectra of MoS2/WS2 and MoSe2/WSe2 HBLs from first-principles calculations. We explore the spectral positions, binding energies, and the origins of inter- and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a G0W0 calculation, which corrects the independent-particle eigenvalues obtained from density-functional theory. Our calculations reveal that the lowest energy exciton in both HBLs possess an interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of interlayer excitons might be expected to be considerably smaller than that of intralayer ones. However, according to our calculations, the binding energy of lowest energy interlayer excitons is only ∼20% lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible

Optical control of polarization in ferroelectric heterostructures

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ferroelectric heterostructures utilizing a two-dimensional narrow-gap semiconductor MoS2 as a top electrode. This effect is attributed to the redistribution of the photogenerated carriers and screening charges at the MoS2/BaTiO3 interface. Specifically, a twostep process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the positive charge accumulation at the interface forcing the polarization reversal from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. It is suggested that the discovered effect is of general nature and should be observable in any heterostructure comprising a ferroelectric and a narrow gap semiconductor

Optical control of polarization in ferroelectric heterostructures

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ferroelectric heterostructures utilizing a two-dimensional narrow-gap semiconductor MoS2 as a top electrode. This effect is attributed to the redistribution of the photo-generated carriers and screening charges at the MoS2/BaTiO3 interface. Specifically, a two-step process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the positive charge accumulation at the interface forcing the polarization reversal from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. It is suggested that the discovered effect is of general nature and should be observable in any heterostructure comprising a ferroelectric and a narrow gap semiconductor

Güneş pili uygulamaları için boya molekülleri adsorbe edilmiş anataz-titanya yüzeyinin elektronik özellikleri.

Wide band gap metal oxides have recently become one of the most investigated materials in surface science. Among these metal oxides especially TiO2 attracts great interest, because of its wide range applications, low cost, biocompatibility and ease of analysis by all experimental techniques. The usage of TiO2 as a component in solar cell technology is one of the most investigated applications of TiO2 . The wide band gap of TiO2 renders it ine cient for isolated use in solar cells. TiO2 surface are therefore coated with a dye in order to increase e ciency. This type of solar cells are called dye sensitized solar cells . The e ciency of dye sensitized solar cells is directly related with the absorbed light portion of the entire solar spectrum by the dye molecule. Inspite of the early dyes, recent dye molcules, which are called wider wavelength response dye molecules, can absorb a larger portion of entire solar spectrum. Thus, the e ciency of dye sensitized solar cells is increased by a considerably amount. In this thesis the electronic structure of organic rings, which are the fundamental components of the dye molecules, adsorbed on anatase (001) surface is analyzed using density functionaltheory. The main goal is to obtain a trend in the electronic structure of the system as a function of increasing ring number. Electronic structure analysis is conducted through band structure and density of states calculations. Results are presented and discussed in the framework of dye sensitized solar cells theory.M.S. - Master of Scienc