Browsing by Subject "Two-dimensional materials"

Now showing 1 - 8 of 8

Open Access
CVD grown 2D MoS2 layers: a photoluminescence and fluorescence lifetime imaging study
(Wiley-VCH Verlag, 2016) Özden, A.; Şar, H.; Yeltik A.; Madenoğlu, B.; Sevik, C.; Ay, F.; Perkgöz, N. K.
In this letter, we report on the fluorescence lifetime imaging and accompanying photoluminescence properties of a chemical vapour deposition (CVD) grown atomically thin material, MoS2. µ-Raman, µ-photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM) are utilized to probe the fluorescence lifetime and photoluminescence properties of individual flakes of MoS2 films. Usage of these three techniques allows identification of the grown layers, grain boundaries, structural defects and their relative effects on the PL and fluorescence lifetime spectra. Our investigation on individual monolayer flakes reveals a clear increase of the fluorescence lifetime from 0.3 ns to 0.45 ns at the edges with respect to interior region. On the other hand, investigation of the film layer reveals quenching of PL intensity and lifetime at the grain boundaries. These results could be important for applications where the activity of edges is important such as in photocatalytic water splitting. Finally, it has been demonstrated that PL mapping and FLIM are viable techniques for the investigation of the grain-boundaries. (Figure presented.). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Open Access
Investigation of anisotropic mechanical, electronic, and charge carrier transport properties of germanium-pnictogen monolayers
(Institute of Physics Publishing Ltd., 2022-02-04) Abboud, Mohammad; Özbey, Doğukan Hazar; Kılıç, M. E.; Durgun, Engin
Recently, novel two-dimensional (2D) GeP and GeAs systems have been fabricated by mechanical exfoliation and utilized in various applications. These developments have brought the 2D germanium-pnictogens, C2/m-GeX (X = N, P, As, Sb, and Bi) structures into the limelight. In this study, we systematically investigate the structural, mechanical, electronic, and charge carrier transport properties of GeX monolayers by using first-principles methods. Our results show that the considered systems are dynamically stable and possess anisotropic physical properties. Examined structures are found to be flexible, and their mechanical strength and stiffness decrease down the group-V, in line with the trends of the bond strength, cohesive energy, charge transfer, and electron localization function. Additionally, the zigzag in-plane direction is mechanically superior to the armchair direction. The electronic band structure calculations based on HSE06 hybrid functional with the inclusion of spin–orbit coupling indicate that GeX monolayers are either direct or quasi-direct semiconductors with band gaps lying within the infrared and visible spectrum. The estimated charge carrier mobilities are highly anisotropic and also differ significantly depending on the structure and carrier type. These unique properties render GeX monolayers as suitable 2D materials for flexible nanoelectronic applications.
Open Access
Investigation of new two-dimensional materials derived from stanene
(Elsevier, 2017-09) Fadaie, M.; Shahtahmassebi, N.; Roknabad, M. R.; Gulseren, O.
In this study, we have explored new structures which are derived from stanene. In these new proposed structures, half of the Sn atoms, every other Sn atom in two-dimensional (2D) buckled hexagonal stanene structure, are replaced with a group- IV atom, namely C, Si or Ge. So, we investigate the structural, electronic and optical properties of SnC, SnGe and SnSi by means of density functional theory based first-principles calculations. Based on our structure optimization calculations, we conclude that while SnC assumes almost flat structure, the other ones have buckled geometry like stanene. In terms of the cohesive energy, SnC is the most stable structure among them. The electronic properties of these structures strongly depend on the substituted atom. We found that SnC is a large indirect band gap semiconductor, but SnSi and SnGe are direct band gap ones. Optical properties are investigated for two different polarization of light. In all structures considered in this study, the optical properties are anisotropic with respect to the polarization of light. While optical properties exhibit features at low energies for parallel polarization, there is sort of broad band at higher energies after 5 eV for perpendicular polarization of the light. This anisotropy is due to the 2D nature of the structures. © 2017 Elsevier B.V.
Open Access
Kinetic analysis of 2D Mo2C crystal growth via CVD
(Institute of Physics Publishing Ltd., 2023-09-28) Buke, G. C.; Çaylan, Ömer Refet; Ogurtani, O. T.
We investigated the growth mechanism of 2D Mo2C crystals by chemical vapor deposition (CVD) under various time and temperature conditions. The growth kinetics and mechanism of Mo2C on Cu via chemical vapor deposition (CVD) were investigated using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. To analyze the surface coverage, we employed scanning electron microscopy (SEM) and applied the modified JMAK model to determine the growth rate and activation energy. The growth rate of Mo2C exhibited temperature-dependent behavior, described by the Arrhenius relationship, with an apparent activation energy of 4 eV. The Avrami plot exhibited an exponent of 3 indicating a complex process with nucleation and growth.
Open Access
Nuclear spin squeezing via electric quadrupole interaction
(American Physical Society, 2016) Korkmaz, Y. A.; Bulutay, C.
Control over nuclear-spin fluctuations is essential for processes that rely on preserving the quantum state of an embedded system. For this purpose, squeezing is a viable alternative, so far that has not been properly exploited for the nuclear spins. Of particular relevance in solids is the electric quadrupole interaction (QI), which operates on nuclei having spin higher than 1/2. In its general form, QI involves an electric-field gradient (EFG) biaxiality term. Here, we show that as this EFG biaxiality increases, it enables continuous tuning of single-particle squeezing from the one-axis twisting to the two-axis countertwisting limits. A detailed analysis of QI squeezing is provided, exhibiting the intricate consequences of EFG biaxiality. The initial states over the Bloch sphere are mapped out to identify those favorable for fast initial squeezing, or for prolonged squeezings. Furthermore, the evolution of squeezing in the presence of a phase-damping channel and an external magnetic field are investigated. We observe that dephasing drives toward an antisqueezed terminal state, the degree of which increases with the spin angular momentum. Finally, QI squeezing in the limiting case of a two-dimensional EFG with a perpendicular magnetic field is discussed, which is of importance for two-dimensional materials, and the associated beat patterns in squeezing are revealed. © 2016 American Physical Society.
Open Access
Peculiar piezoelectric properties of soft two-dimensional materials
(American Chemical Society, 2016-06) Sevik, C.; Çakır, D.; Gülseren, O.; Peeters, F. M.
Group II-VI semiconductor honeycomb monolayers have a noncentrosymmetric crystal structure and therefore are expected to be important for nano piezoelectric device applications. This motivated us to perform first-principles calculations based on density functional theory to unveil the piezoelectric properties (i.e., piezoelectric stress (e11) and piezoelectric strain (d11) coefficients) of these monolayer materials with chemical formula MX (where M = Be, Mg, Ca, Sr, Ba, Zr, Cd and X = S, Se, Te). We found that these two-dimensional materials have peculiar piezoelectric properties with d11 coefficients 1 order of magnitude larger than those of commercially utilized bulk materials. A clear trend in their piezoelectric properties emerges, which originates mainly from their mechanical properties. We establish a simple correlation between the piezoelectric strain coefficients and the physical properties, as the natural elemental polarizabilities, the Bader charges, and lattice constants of the individual M/X atoms and MX monolayers.
Restricted
Prediction of new generation two-dimensional ternary structures and investigation of their fundamental properties
(2023-05) Varjovi, Mirali Jahangirzadeh
Consecutive to the isolation of graphene and uncovering its extraordinary properties, the dynasty of two-dimensional (2D) materials has expanded rapidly. A realization of every new member suggests novel features, holding the promise to be used in current and prospective nanodevices. In parallel with the attempts on exploring new 2D systems, the formation of ternary configurations has been suggested as an alternative approach to tailor the inherent properties of the already existing 2D structures. In accordance with recent advancements in ternary 2D systems, in this dissertation, we design and investigate the 2D systems which possess three types of elements in their crystal structure. In this regard, we design Janus Al2XX′ (X/X′: O, S, Se, Te) crystals, 1H, 1T, and 1T′ phases of Janus WXO (X = S, Se, and Te) monolayers and Janus BiXY (X= S, Se, Te, and Y = F, Cl, Br, I ) nanosheets and investigate their structural, vibrational, elastic, piezoelectric, and electronic properties by first-principle methods. In addition, inspired by the synthesis of penta-Si nanoribbons, advancements in 2D pentagonal systems, and recent developments on ternary structures, we propose and investigate a new ternary pentagon-based 2D monolayer, namely penta-BNSi. We study the mechanical, electronic, piezoelectric, photocatalytic, and optical properties of penta-BNSi crystal and reveal its suitability to be used in optoelectronics and photocatalytic applications. Then, we focused our attention on new family of MA2Z4 monolayers. Based on this motivation, we perform a comprehensive study on physical properties of MSi2Z4 (M: Pd and Pt, Z: N and P) monolayers and suggested novel single layer of InSiN2 (In2Si2N4). For this purpose, in both studies, the ground state configurations of the designed materials are determined, and then the dynamical and thermal stability of these nanosheets are investigated using phonon spectrum analysis and ab initio molecular dynamic (AIMD) simulations, respectively. Next, each structure’s Raman and infrared (IR) spectrum are analyzed, and the corresponding atomic displacements of the optical phonon modes are presented. Then, the mechanical properties are studied in terms of in-plane stiffness and Poisson’s ratio. The electronic band structures are computed in the electronic properties section, and the corresponding energy band gaps are reported. For MSi2Z4 (M: Pd and Pt, Z: N and P) monolayers, the optical response is examined via calculation of the complex dielectric function by taking many-body interactions into account. In the final study, the effect of an external biaxial strain on the electronic and vibrational properties of the InSiN2 nanosheet is investigated, and the variation of the obtained properties under strain is illustrated. As a result of a thorough theoretical study focusing on ternary 2D materials, it can be said that the examined crystals are stable systems with potential applications in a wide range of nanoelectronics and nanomechanical devices.
Open Access
Stable monolayer of the RuO2 structure by the Peierls distortion
(Taylor & Francis, 2019-10-29) Ersan, Fatih; Özaydın, H. D.; Üzengi Aktürk, O.
In this paper, we presented a stable two-dimensional ruthenium dioxide monolayer by using first-principles calculations within density functional theory. In contrast to ordinary hexagonal and octahedral structures of metal dichalcogenides, RuO2 is stable in the distorted phase of the structure as a result of occurring charge density wave. A comprehensive analysis including the calculation of vibration frequencies, mechanical properties, and ab initio molecular dynamics at 300 K affirms that RuO2 monolayer structure is stable dynamically and thermally and convenient for applications at room temperature. We also investigated the electronic and optical properties of RuO2 and it is found that RuO2 has of 0.74 eV band gap which is in the infrared region and very suitable for infrared detectors.