6 research outputs found
Structural, electronic and phononic properties of PtSe2: From monolayer to bulk
The layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art first-principles calculations. The main findings of the study are: (i) monolayer platinum diselenide has a dynamically stable 2D octahedral structure with 1.66 eV indirect band gap, (ii) the semiconducting nature of 1T-PtSe2 monolayers remains unaffected even at high biaxial strains, (iii) top-to-top (AA) arrangement is found to be energetically the most favorable stacking of 1T-PtSe2 layers, (iv) the lattice constant (layer-layer distance) increases (decreases) with increasing number of layers, (v) while monolayer and bilayer 1T-PtSe2 are indirect semiconductors, bulk and few-layered 1T-PtSe2 are metals, (vi) Raman intensity and peak positions of the A1g and Eg modes are found to be highly dependent on the layer thickness of the material, hence; the number of layers of the material can be determined via Raman measurements.TUBITAK under the project number 117F09
Kristal yapıların difüzyon ve bozulma karakteristiklerinin Raman spekroskopisi yolu ile gözlenmesi
Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2018Includes bibliographical references (leaves: 58-76)Text in English; Abstract: Turkish and EnglishLamellar structures, having strong in-plane and weak (van der Waals) out-ofplane
bonding, exhibit extraordinary properties when thinned down to their monolayer
limit. Following the isolation of single layer graphene in 2004, there has been a rapid
increase in the number of studies focusing on other novel two dimensional (2D) materials
such as hexagonal Boron Nitride (BN), transition metal dichalcogenides (TMDs),
post transition metal chalcogenides (PTMCs), silicene and black-phosphorus. Doping
of 2D and bulk crystals is a well-known strategy that may lead to novel functionalities
and significantly alters materials’ electronic, optical, and magnetic properties. In this regard,
understanding of diffusion characteristic of dopant in a crystal via computational
simulation is vital to enlighten physical insights of the experiment. In addition, investigation
of degradation mechanisms of crystals at atomic-level is also still open question.
In this sense, the density functional theory (DFT) is one of the most powerful and commonly
used methods for such theoretical investigations. Moreover, measuring vibrational
spectra of a material via Raman spectroscopy is powerful method to understand atomic vibrations
that give information about physical properties of a material. In this regards, we
investigate diffusion characteristics and degradation mechanism of several crystal (such
as, perovskites and MoS2) by means of first-principles calculations based on density functional
theory (DFT). In addition, Raman measurements are also carried out to investigate
vibrational properties of the crystals. It is shown that few-layer MoS2 can be used for
selective nitrogenation of graphene. In addition, red shift in photoluminescence peak of
water interacted CsPbBr3 nanowires arise from detachment of surface ligand from surface
of nanowire by presense of water molecules. Lastly, time-dependent photoluminescence
measurement of Mn-doped CsPbCl3 shows that change in emission color under UV illumination
is due to segregation of Mn atoms towards crystal surface. This thesis provides
some important results for deeper understanding of degradation and diffusion mechanisms
of dopants in 2D materials and perovskites.Düzlem içi ve zayıf (van der Waals) düzlem dışı bir bağlanma yapısına sahip olan katmanlı yapılar, tek katmanlı sınırlarına kadar incelendiğinde olağanüstü özellikler sergiler. 2004 yılında tek tabakalı grafen izolasyonunu takiben altıgen Bor Nitrür (BN), geçiş metali dikalkojenler (GMD), geçiş sonrası metal kalkojen (PGMK) ve düzlem içi anizotropik tek tabakalı yapılar gibi diğer iki boyutlu (2D) malzemelere odaklanan çalışmaların sayısında hızlı bir artış oldu. İki boyutlu ve yığın kristalleri katkılama, 2D
malzemelerin uygulanabilir hale getirmeye yarayan bilinen bir stratejidir ve bu malzemelerin elektronik, optik ve manyetik özelliklerini önemli ölçüde değiştirebilir. Bu bağlamda, bir kristalde hesaplamalı simülasyon ve deneysel ölçüm yoluyla dopantın difüzyon karakteristiğinin anlaşılması, deneyin fiziksel içyüzünü aydınlatmak için önem taşımaktadır. Ayrıca, atomik seviyedeki kristallerin bozunma mekanizmalarının incelenmesi hala açık bir sorudur. Bu anlamda, yoğunluk fonksiyonel teorisi (DFT), bu tür kuramsal araştırmalar için en güçlü ve yaygın olarak kullanılan yöntemlerden biridir. Buna ek olarak, bir
malzemenin titreşim spektrumlarını Raman spektroskopisi ile ölçmek, bir malzemenin fiziksel özellikleri hakkında bilgi veren atomik titreşimleri anlamak için güçlü bir yöntemdir. Bu bağlamda, auxetik tek katmanlı pentagonal yapılar, esnek tek katmanlı delikli grafen kristalleri, ultra-esnek tek katmanlı PGMK lar, ve düzlem içi anisotropik çeşitli kristallerin difüzyon karakteristiği ve bozunma mekanizmasını (perovskites ve MoS2 gibi) ilk prensiplere dayanan yoğunluk fonksiyoneli teorisi tabanlı hesaplamalar yoluyla araştırıyoruz. Raman ölçümleri de kristallerin titreşim özelliklerini araştırmak için gerçekleştiriliyor.
Grafenin seçici nitrojenlenmesi için birkaç katman MoS2’nin kullanılabileceği gösterilmiştir. Buna ek olarak, suyun fotolüminesans pikindeki kırmızıya kayma CsPbBr3 nanotelleri, su moleküllerinin varlığıyla nanotel yüzeyinden yüzey ligandının ayrılmasından kaynaklanır. Son olarak, Mn-katkılı CsPbCl3’nın zamana bağlı fotolüminesans ölçümü, UV aydınlatması altında emisyon rengindeki değişikliğin Mn atomlarının kristal yüzeyine doğru ayrılmasından kaynaklandığını göstermektedir. Bu tez çalışmasında elde edilen sonuçlar perovskite ve iki boyutlu malzemelerin bozunma ve difüzyon karakteristiklerinin
derinlemesine anlaşılmasına olanak sağlayacak önemli temel bilgileri sağlamaktadır.TUBITAK project number: 117F09
Few-layer MoS2 as nitrogen protective barrier
We report experimental and theoretical investigations of the observed barrier behavior of few-layer MoS2 against nitrogenation. Owing to its low-strength shearing, low friction coefficient, and high lubricity, MoS2 exhibits the demeanor of a natural N-resistant coating material. Raman spectroscopy is done to determine the coating capability of MoS2 on graphene. Surface morphology of our MoS2/graphene heterostructure is characterized by using optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, density functional theory-based calculations are performed to understand the energy barrier performance of MoS2 against nitrogenation. The penetration of nitrogen atoms through a defect-free MoS2 layer is prevented by a very high vertical diffusion barrier, indicating that MoS2 can serve as a protective layer for the nitrogenation of graphene. Our experimental and theoretical results show that MoS2 material can be used both as an efficient nanocoating material and as a nanoscale mask for selective nitrogenation of graphene layer.TUBITAK (116C073); The Science Academy, Turkey, under the BAGEP program; National Science Foundation USA NSF DMR-155222
Monitoring the doping and diffusion characteristics of Mn dopants in cesium lead halide perovskites
Cesium lead perovskites, in the form of CsPbX3 or Cs4PbX6, have been widely used for various optoelectronic applications due to their exceptionally good optical properties. In this study, the effect of Mn doping on the structural and optical properties of cesium lead halide perovskite crystals are investigated from both experimental and theoretical points of view. It is found that adding MnCl2 during the synthesis not only leads to a Mn-driven structural phase transition from Cs4PbBr6 to CsPbCl3 but also triggers the Br- to Cl- halide exchange. On the other hand, it is observed that, under UV illumination, the color of Mn-doped crystals changes from orange to blue in approximately 195 h. While the intensity of Mn-originated photoluminescence emission exponentially decays in time, the intensity of CsPbCl3-originated emission remains unchanged. In addition, diffusive motion of Mn ions results in both a growing population of MnO2 at the surface and transition of the host into a cesium-rich Cs4PbCl6 phase.TUBITAK (117F095
Strain mapping in single-layer two-dimensional crystals via Raman activity
By performing density functional theory-based ab initio calculations, Raman-active phonon modes of single-layer two-dimensional (2D) materials and the effect of in-plane biaxial strain on the peak frequencies and corresponding activities of the Raman-active modes are calculated. Our findings confirm the Raman spectrum of the unstrained 2D crystals and provide expected variations in the Raman-active modes of the crystals under in-plane biaxial strain. The results are summarized as follows: (i) frequencies of the phonon modes soften (harden) under applied tensile (compressive) strains; (ii) the response of the Raman activities to applied strain for the in-plane and out-of-plane vibrational modes have opposite trends, thus, the built-in strains in the materials can be monitored by tracking the relative activities of those modes; (iii) in particular, the A peak in single-layer Si and Ge disappears under a critical tensile strain; (iv) especially in mono- and diatomic single layers, the shift of the peak frequencies is a stronger indication of the strain rather than the change in Raman activities; (v) Raman-active modes of single-layer ReX2 (X=S, Se) are almost irresponsive to the applied strain. Strain-induced modifications in the Raman spectrum of 2D materials in terms of the peak positions and the relative Raman activities of the modes could be a convenient tool for characterization.TUBITAK (116C073
Theoretical and experimental investigation of conjugation of 1,6-hexanedithiol on MoS2
We report an experimental and theoretical investigation of conjugation of 1,6-Hexaneditihiol (HDT) on MoS2 which is prepared by mixing MoS2 structure and HDT molecules in proper solvent. Raman spectra and the calculated phonon bands reveal that the HDT molecules bind covalently to MoS2. Surface morphology of MoS2/HDT structure is changed upon conjugation of HDT on MoS2 and characterized by using Scanning Electron Microscope (SEM). Density Functional Theory (DFT) based calculations show that HOMO-LUMO band gap of HDT is altered after the conjugation and two-S binding (handle-like) configuration is energetically most favorable among three different structures. This study displays that the facile thiol functionalization process of MoS2 is promising strategy for obtaining solution processable MoS2.TUBITAK (116C073); The Science Academy, Turkey under the BAGEP progra