Photonics / Fotonik
Permanent URI for this collectionhttps://hdl.handle.net/11147/2590
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Article Citation - WoS: 24Citation - Scopus: 24Defect Tolerant and Dimension Dependent Ferromagnetism in Mnse2(Royal Society of Chemistry, 2019) Eren, İsmail; İyikanat, Fadıl; Şahin, HasanBy performing density functional theory-based calculations, we investigate the structural, vibrational, electronic and magnetic properties of 2D monolayers, nanoribbons and quantum dots of MnSe2. Vibrational spectrum analysis reveals the dynamical stability of not only ferromagnetic but also antiferromagnetic phases of single layer MnSe2 crystal structures. Electronically, calculations show that 1T-MnSe2 is a ferromagnetic structure displaying metallic behavior. It is also found that the structure preserves its dynamical stability and metallic behavior even under the presence of high density Se vacancies. Moreover, it was predicted that, differing from the 2D MnSe2, metal-metal interaction driven reconstructions result in ferromagnetic-to-antiferromagnetic crossover in the ground state of nanoribbons and quantum dots. With its robust ferromagnetic metallic character in the 2D ultra-thin limit and dimension-dependent magnetic properties, MnSe2 is an important candidate for spintronic device applications.Article Citation - WoS: 3Citation - Scopus: 3Hydrogenated Derivatives of Hexacoordinated Metallic Cu2si Monolayer(Royal Society of Chemistry, 2018) Ünsal, Elif; İyikanat, Fadıl; Şahin, Hasan; Senger, Ramazan TuğrulHerein, we carried out first-principles calculations based on density functional theory to investigate the effects of surface functionalization with hydrogen atoms on structural, dynamical and electronic properties of Cu2Si monolayer. Pristine Cu2Si, a metallic monolayer, has a planar hexacoordinate structure. Calculations revealed that the most favorable position of a single H atom on the Cu2Si monolayer is at the top of a Si site. Derivatives of Cu2Si monolayer with various H concentrations were investigated, and by performing phonon calculations, it was found that there are three stable hydrogenated structures. Specific heat of these monolayers was found to increase with the hydrogen concentration at temperatures higher than 100 K. Electronically, the hydrogenated derivatives of Cu2Si monolayer preserve the metallic character.Article Citation - WoS: 15Citation - Scopus: 16Monitoring the Crystal Orientation of Black-Arsenic Via Vibrational Spectra(Royal Society of Chemistry, 2019) Kandemir, Ali; İyikanat, Fadıl; Şahin, HasanIn this study, the structural, mechanical, and vibrational properties of a recently discovered anisotropic ultra-thin material, black-arsenic (b-As), are investigated by using density functional theory. Direction dependent elastic constants such as in-plane stiffness, Young's modulus and Poisson's ratio of single-layer b-As are calculated and compared with those of the structural cousin black-phosphorus (b-P). The calculated Poisson's ratio of b-As for the zigzag direction is nearly 1, which is quite higher than that of b-P, 0.65. Besides, it is found that all the three elastic constants are highly anisotropic and their values in the zigzag direction are almost three times higher than that of the armchair direction. The mechanical strength of the material is also calculated and high-toughness is seen in both armchair and zigzag directions. It is revealed that the material is quite stiff against straining along the zigzag direction; in contrast, it is quite flexible along the armchair direction. Vibrational stability analysis shows that the material is stable up to 9% biaxially applied strain, and 12% and 45% uniaxially applied strain in the zigzag and armchair directions, respectively. Furthermore, the prominent Raman active peaks of the b-As structure show strong anisotropy in the strain dependent vibrational spectra and they can also be used for easy-determination of the crystal orientation of b-As from Raman measurements.Article Citation - WoS: 67Citation - Scopus: 66Bilayers of Janus Wsse: Monitoring the Stacking Type: Via the Vibrational Spectrum(Royal Society of Chemistry, 2018) Kandemir, Ali; Şahin, HasanMotivated by the recent successful synthesis of Janus type single layers of transition metal dichalcogenides, we investigate the stability, vibrational and electronic properties of the Janus single layer structure of WSSe and its bilayers by means of density functional theory. The structural and vibrational analysis show that the Janus single layer of WSSe forms a dynamically stable structure in the 2H phase. Owing to its non-centrosymmetric structure, the Janus WSSe single layer has two in-plane (E) and two out-of-plane (A) Raman active phonon modes. The eigen-frequencies of the prominent Raman active modes are calculated to be 277 (A) and 322 (E) cm-1. Similar to single layer WS2 and WSe2, Janus WSSe is a direct band gap semiconductor that has two electronically different faces. In addition, the possible bilayer stacking orders of the Janus WSSe single layers are investigated. It is found that there are 3 stacking types of bilayer Janus WSSe and each stacking type has distinctive Raman characteristics in its vibrational spectrum. Our results show that thanks to the vibrational characteristics, which stem from the distinctive interlayer interactions at different sides, the stability and stacking types of the bilayer of WSSe Janus structure can be monitored.Article Citation - WoS: 46Citation - Scopus: 49Tuning Electronic and Magnetic Properties of Monolayer ?-Rucl3 by In-Plane Strain(Royal Society of Chemistry, 2018) İyikanat, Fadıl; Yağmurcukardeş, Mehmet; Senger, Ramazan Tuğrul; Şahin, HasanBy employing density functional theory-based methods, the structural, vibrational, electronic, and magnetic properties of monolayer α-RuCl3 were investigated. It was demonstrated that ferromagnetic (FM) and zigzag-antiferromagnetic (ZZ-AFM) spin orders in the material have very close total energies with the latter being the ground state. We found that each Ru atom possesses a magnetic moment of 0.9 μB and the material exhibits strong magnetic anisotropy. While both phases exhibit indirect gaps, the FM phase is a magnetic semiconductor and the ZZ-AFM phase is a non-magnetic semiconductor. The structural stability of the material was confirmed by phonon calculations. Moreover, dynamical analysis revealed that the magnetic order in the material can be monitored via Raman measurements of the crystal structure. In addition, the magnetic ground state of the material changes from ZZ-AFM to FM upon certain applied strains. Valence and conduction band-edges of the material vary considerably under in-plane strains. Owing to the stable lattice structure and unique and controllable magnetic properties, monolayer α-RuCl3 is a promising material in nanoscale device applications.Article Citation - WoS: 11Citation - Scopus: 11Stable Monolayer ?-Phase of Cdte: Strain-Dependent Properties(Royal Society of Chemistry, 2017) Ünsal, Elif; Senger, Ramazan Tuğrul; Şahin, HasanCdTe is a well known and widely used binary compound for optoelectronic applications. In this study, we propose the thinnest, free standing monolayer of CdTe which has a tetragonal-PbO (α-PbO) symmetry. The structural, electronic, vibrational and strain dependent properties are investigated by means of first principles calculations based on density functional theory. Our results demonstrate that monolayer α-CdTe is a dynamically stable and mechanically flexible material. It is found that the thinnest monolayer crystal of CdTe is a semiconductor with a direct band gap of 1.95 eV, which corresponds to red light in the visible spectrum. Moreover, it is found that the band gap can be tunable under biaxial strain. With its strain-controllable direct band gap within the visible spectrum, the stable α-phase of monolayer CdTe is a suitable candidate for optoelectronic device applications.Article Citation - WoS: 65Citation - Scopus: 67Angle Resolved Vibrational Properties of Anisotropic Transition Metal Trichalcogenide Nanosheets(Royal Society of Chemistry, 2017) Kong, Wilson; Bacaksız, Cihan; Chen, Bin; Wu, Kedi; Blei, Mark; Fan, Xi; Shen, Yuxia; Şahin, Hasan; Wright, David; Narang, Deepa S.; Tongay, SefaattinLayered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS3, ZrS3, and HfS3 have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar angle resolved studies on the novel TiNbS3 TMTC alloy reveal that determination of anisotropy/crystalline direction is rather difficult possibly due to loss of anisotropy by randomization distribution of quasi-1D MX6 chains by the presence of defects which are commonly found in 2D alloys and also due to the complex Raman tensor of TMTC alloys. Overall, the experimental and theoretical results establish non-destructive methods used to identify the direction of anisotropy in TMTCs and reveal their vibrational characteristics which are necessary to gain insight into potential applications that utilize direction dependent thermal response, optical polarization, and linear dichroism.Article Citation - WoS: 68Citation - Scopus: 71Strong Dichroic Emission in the Pseudo One Dimensional Material Zrs3(Royal Society of Chemistry, 2016) Pant, Anupum; Torun, Engin; Chen, Bin; Bhat, Soumya; Fan, Xi; Wu, Kedi; Wright, David P.; Peeters, François M.; Soignard, Emmanuel; Şahin, Hasan; Tongay, SefaattinZirconium trisulphide (ZrS3), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS3 are highly anisotropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D (planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.Article Citation - WoS: 10Citation - Scopus: 11Single layer PbI2: Hydrogenation-driven reconstructions(Royal Society of Chemistry, 2016) Bacaksız, Cihan; Şahin, HasanBy performing density functional theory-based calculations, we investigate how a hydrogen atom interacts with the surfaces of monolayer PbI2 and how one- and two-side hydrogenation modifies its structural, electronic, and magnetic properties. Firstly, it was shown that the T-phase of single layer PbI2 is energetically more favorable than the H-phase. It is found that hydrogenation of its surfaces is possible through the adsorption of hydrogen on the iodine sites. While H atoms do not form a particular bonding-type at low concentration, by increasing the number of hydrogenated I-sites well-ordered hydrogen patterns are formed on the PbI2 matrix. In addition, we found that for one-side hydrogenation, the structure forms a (2 × 1) Jahn-Teller type distorted structure and the bandgap is dramatically reduced compared to hydrogen-free single layer PbI2. Moreover, in the case of full hydrogenation, the structure also possesses another (2 × 2) reconstruction with a reduction in the bandgap. The easily tunable electronic and structural properties of single layer PbI2 controlled by hydrogenation reveal its potential uses in nanoscale semiconducting device applications.Article Citation - WoS: 25Citation - Scopus: 26Solution Processable Neutral State Colourless Electrochromic Devices: Effect of the Layer Thickness on the Electrochromic Performance(Royal Society of Chemistry, 2016) Tahtalı, Gürhan; Has, Zeynep; Doyranlı, Ceylan; Varlıklı, Canan; Koyuncu, SermetIn this study, a neutral state colourless electrochromic device fabricated through a solution process is introduced. The device contains indium doped transparent oxide coated glass as the transparent conductive electrode, poly(N-(2-ethylhexyl)carbazol-3,6-diyl) (PCbz) as the anodically colouring material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the ion storage layer and a lithium perchlorate based conducting gel electrolyte. A maximum optical contrast (ΔT%) of 58% at 800 nm is achieved by optimizing the film thickness of both the organic layers. The device that has layers of 220 nm PCbz and 120 nm PEDOT:PSS thickness revealed a high coloration efficiency (1246 cm2 C-1) and colouring and bleaching response times of 5.6 s and 2.3 s, respectively.