Browsing by Subject "Two dimensional materials"

Now showing 1 - 7 of 7

Open Access
Alkali metal intercalation in MXene/Graphene heterostructures: a new platform for ion battery applications
(American Chemical Society, 2019) Demiroğlu, İ.; Peeters, F. M.; Gülseren, Oğuz; Çakır, D.; Sevik, C.
The adsorption and diffusion of Na, K, and Ca atoms on MXene/graphene heterostructures of MXene systems Sc2C(OH)2, Ti2CO2, and V2CO2 are systematically investigated by using first-principles methods. We found that alkali metal intercalation is energetically favorable and thermally stable for Ti2CO2/graphene and V2CO2/graphene heterostructures but not for Sc2C(OH)2. Diffusion kinetics calculations showed the advantage of MXene/graphene heterostructures over sole MXene systems as the energy barriers are halved for the considered alkali metals. Low energy barriers are found for Na and K ions, which are promising for fast charge/discharge rates. Calculated voltage profiles reveal that estimated high capacities can be fully achieved for Na ion in V2CO2/graphene and Ti2CO2/graphene heterostructures. Our results indicate that Ti2CO2/graphene and V2CO2/graphene electrode materials are very promising for Na ion battery applications. The former could be exploited for low voltage applications while the latter will be more appropriate for higher voltages.
Open Access
Atomically thin materials
(Springer, 2020) Kasırga, T. Serkan
In this chapter, I will provide a brief overview of atomically thin materials that are formed by layers held together by van der Waals forces or weak covalent bonding. These materials provide a unique and cheap way of studying plethora of phenomena. Perhaps, the relative simplicity of the methods that are commonly used in the studies of two dimensional (2D) materials are one of the main reasons why they attracted attention at this level since the advent of graphene. After an introduction to the properties of 2D materials, I will talk about the methods to obtain 2D materials and conclude the chapter with the possibilities of heterostructures of 2D materials.
Open Access
Defect states in monolayer hexagonal BN: A comparative DFT and DFT-1/2 study
(Elsevier, 2020) Aksu-Korkmaz, Yağmur; Bulutay, Ceyhun; Sevik, C.
Hexagonal boron nitride (h-BN) acts like a semiconductor vacuum to point defects enabling stable and controllable spin states at room temperature which qualifies them for quantum technological applications. To characterize their properties first-principles techniques constitute indispensable tools. The currently established paradigm for such solid-state electronic structure calculations is the density functional theory (DFT). Recently its variant, so-called DFT-1/2 method was introduced with the promise of accurate band gaps without a computational overhead with respect to ordinary DFT. Here, for the monolayer h-BN we contrast DFT and DFT-1/2 results for carbon substitutional impurities (CB, CN), boron and nitrogen single vacancies (VB, VN), divacancy, and Stone-Wales defects. Comparisons with more sophisticated, yet computationally costly techniques namely, hybrid functional DFT and the GW are also made, where available. From the standpoint of defect states embedded in the band gap region we demonstrate a clear advantage of DFT-1/2 in revealing the localized states otherwise buried within either the valence or conduction band continuum due to well-known gap underestimation syndrome of the standard DFT implementations. Thus, DFT-1/2 can serve for the rapid screening of candidate defect systems before more demanding considerations.
Open Access
Electronic structure and optical properties of monolayer semiconductors: a computational study
(2021-09) Korkmaz, Yağmur Aksu
Interest on monolayer semiconductors is rapidly growing in recent years. One of the prominent members is hexagonal boron nitride (h-BN). At room tem-perature, it harbors an environment similar to semiconductor vacuum for point defects which is crucial for stable and controllable spin states. This qualiﬁcation makes h-BN a suitable medium for quantum technological applications. First-principles calculations are essential in order to characterize such systems. Density functional theory (DFT) is one of the most reliable methods used for these type of calculations. Recently, a variant called as DFT-1/2, has been proposed to calculate the band gaps of the materials more accurately without a signiﬁcant additional computational cost. In the ﬁrst part of thesis, we have compared the results of DFT and DFT-1/2 for carbon impurities (CB, CN ), single vacan-cies (VB, VN ), double vacancy (divacancy) and Stone-Wales defect in monolayer h-BN. Subsequently, results from computationally expensive techniques such as hybrid or GW are presented and compared with the obtained DFT-1/2 results. Especially for the defect states seemingly hidden in valence or conduction band, DFT-1/2 technique is instrumental in revealing these states while widening the band gap. Thus, we recommend the DFT-1/2 method for a quick screening of candidate band gap defect states. Another outstanding group of semiconductors is transition metal dichalco-genides (TMDs). They owe their advantages to optically addressable valley and bringing optics and mechanics together as in valleytronics, thanks to their high ﬂexibility. In the second part of this thesis, ten TMDs including their janus coun-terparts (JTMDs), namely, MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2, MoSSe, MoSeTe, WSSe, and WSeTe have been computationally studied. To begin with, the electronic band structure of the speciﬁed materials have been computed using DFT followed by hybrid calculations over these, with the addition of spin-orbit coupling. Biaxial and uniaxial strain calculations are subsequently performed. JTMDs were previously proclaimed to have a good piezoelectric characteristic. According to our DFT results, JTMDs exhibit band structure and electronic properties in between its constituent TMDs, and in this respect they do not dis-play an outstanding behaviour. Based on the acquired DFT data, spinless and spinful k · p parameters are extracted by ﬁtting around optically active K valley. With the help of k·p parametrization, linear and circular dichroic behaviours are studied for unstrained and strained cases. In consideration of all these materials, WTe2 displays the largest linear dichroic responsitivity for uniaxial strain, since it has the smallest band gap and the greatest uniaxial deformation potential at the K valley. Thus, we propose monolayer WTe2 membranes to be considered for optical polarization based strain measurements, as well as, strain adjustable optical polarizers.
Open Access
Investigation of new polymorphs of borophene and their functionalization
(2017-12) Khanifaev, Jamoliddin
The realization of buckled monolayer sheets of boron (i.e., borophene) and its other polymorphs has attracted signi cant interest in the eld of two-dimensional systems. Motivated by their chameleonic behavior we analyzed di erent polymorphs of borophene and discovered two new phases with unprecedented crystal structures namely symmetric washboard and asymmetric washboard using ab initio methods based on density functional theory. While symmetric washboard borophene is a metal with high electronic density of states in the vicinity of Fermi level asymmetric washboard borophene is a narrow band gap semiconductor. Asymmetric washboard structure is actually a 2 1 reconstructed form of symmetric structure with in plane and out of plane Peierl's distortion along the chains of boron atoms which is the key reason for the contrasting electronic behavior of these phases. Phonons dispersion calculations based on density functional perturbation theory reveal that both structures are stable at 0 K however ab ini- tio molecular dynamics simulations showed that symmetric washboard structure is stable only at temperatures close to absolute zero and at nite temperatures this structure gets deformed transforming into asymmetric washboard structure. Moreover we discovered that asymmetric washboard structure has a positive Poissons's ratio however symmetric one has a negative Poisson's ratio. In the next work, motivated by buckled borophene's tendency to donate electrons, we analyzed the interaction of single halogen atoms (F, Cl, Br, I) with borophene. The possible adsorption sites are tested and the top of the boron atom is found as the ground state con guration. The nature of bonding and strong chemical interaction is revealed by using projected density of states and charge di erence analysis. The migration of single halogen atoms on the surface of borophene is analyzed and high di usion barriers that decrease with atomic size are obtained. The metallicity of borophene is preserved upon adsorption but anisotropy in electrical conductivity is altered. The variation of adsorption and formation energy, interatomic distance, charge transfer, di usion barriers, and bonding character with the type of halogen atom are explored and trends are revealed. Lastly, the adsorption of halogen molecules (F2, Cl2, Br2, I2), including the possibility of dissociation, is studied. The obtained results are substantial for fundamental understanding and possible device implementations of borophenes and their halogenated derivatives.
Open Access
Mechanical and electrical monitoring in the dynamics of twisted phosphorene nanoflakes on 2D monolayers
(American Chemical Society, 2019) Görkan, T.; Kadıoğlu, Y.; Üzengi-Aktürk, O.; Gökoğlu, G.; Aktürk, E.; Çıracı, Salim
We investigated the rotational and translational dynamics of hydrogen-passivated, black phosphorene and blue phosphorene nanoflakes of diverse size and geometry anchored to graphene, black phosphorene, blue phosphorene, and MoS2 monolayer substrates. The optimized attractive interaction energy between each nanoflake and monolayer substrates are harmonic for small angular displacements, leading to libration frequencies. We showed that the relevant dynamical parameters and resulting libration frequencies, which vary with the size/geometry of nanoflakes, as well as with the type of substrate, can be monitored by charging, external electric field, pressure, and also by a molecule anchored to the flake. The optimized energy profiles and energy barriers thereof have been calculated in translational and in large angle rotational dynamics. Owing to the weak interaction between the flakes and monolayers the energy barriers are particularly small for incommensurate systems and can renders nearly frictionless rotation and translation, which is crucial for nanoscale mechanics. Even if small for particular combined nanoflake + monolayer heterostructures, the energy band gaps exhibit variations with angular and linear displacements of nanoflakes. However, these band gaps undergo considerable reduction under pressure. With tunable dynamics, electronic structure, and low friction coefficients, individual or periodically repeating nanoflakes on a monolayer substrate constitute critical composite structures offering the design of novel detectors, nanomechanical, electromechanical, and electronic devices.
Open Access
Multilayer mXene heterostructures and nanohybrids for multifunctional applications: a review
(American Chemical Society, 2022-05-17) Tasnim Mahmud, S.; Bain, S; Hasan, Md Mehdi; Rahman, S.T.; Rhaman, M.; Hossain, M.M.; Ordu, Mustafa
MXenes (transition metal carbides and nitrides) have experienced exponential growth over the last two decades, thanks to their excellent physical, chemical, and mechanical properties. Intriguing properties like high conductivity, wear, and corrosion resistance while maintaining flexibility are the strong motivation behind the exploration of MXenes. Moreover, the large surface area and unique layered structure enhance the functionality of multilayer-MXene heterostructures and hybrids. This paper reviews the synthesis chemistry, structure properties of multilayer MXenes, and their multifunctional applications. MXene synthesis under different conditions, their hybrids and composites, intercalation, and structural geometries are discussed. The electrical, mechanical, optical, and magnetic properties of MXenes are briefly presented. Recent progress and development in MXene-based heterostructures and nanohybrids for supercapacitors, batteries, environmental and water treatment, antibacterial and tissue engineering, and electromagnetic absorption and shielding are systematically discussed. Finally, research challenges and a perspective in this specified area are addressed for potential developments.