Browsing by Subject "2D materials"

Now showing 1 - 20 of 25

Open Access
An ab initio study of vertical heterostructures formed by CdO and SnC monolayers
(Elsevier, 2024-01-30) Seyedmohammadzadeh, Mahsa; Mobaraki, Arash; Tanatar, B.; Gülseren, Oğuz
Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.
Open Access
Characterization of two-dimensional Ga1−xAlxN ordered alloys with varying chemical composition
(Elsevier, 2019) Kanlı, Muammer; Önen, Abdullatif; Mogulkoc, A.; Durgun, Engin
Similar to bulk semiconductors, alloying suggests a promising strategy to tailor the fundamental properties oftwo-dimensional (2D) systems with constituent composition. In that sense, detailed understanding of atomicstructure and stability analysis are required to predict and design new 2D alloys. In this paper, we analyze thestructural, mechanical, electronic, thermal, and optical properties of monolayer Ga1−xAlxN ordered alloys forvarying concentration by usingab initiomethods. Following the determination of ground state geometries bytaking into account the possibility of segregation, we investigate the stability of the considered structures byphonon spectrum analysis and high temperature molecular dynamics calculations. Our results indicate that theproperties of 2D Ga1−xAlxN can be modified continuously by controlling the Al concentration. Tunability of thedesired properties broadens the possible usage of 2D semiconductors in nanoscale applications.
Open Access
Chemical vapor transport synthesis of a selenium-based two-dimensional material
(TÜBITAK, 2018) Kasırga, Talip Serkan
Selenium-based layered materials, and in particular transition-metal diselenides (TMDSs), have intriguing properties in the monolayer limit. Materials such as MoSe2, WSe2, and NbSe2 display striking features such as spin-valley coupling at the valence-band edges and offer great potential for optoelectronics applications. Although a dozen of other TMDSs have been realized or proposed, whether two-dimensional chalcogens are possible or not is still an open challenge. In this work, we show the chemical vapor transport synthesis of a novel, atomically thin selenium-based material on oxidized silicon substrates. This new member of the two-dimensional materials family has a unique Raman spectrum similar to that of bulk selenium and has an optical gap of ∼1.57 eV at room temperature determined by the photoluminescence. No transition metals are found in the stoichiometry of the crystals. Analysis of high-resolution transmission electron micrographs of the monolayers reveals a distinctive set of hexagonal spots indicating a sixfold symmetry of the lattice. Atomic force microscopy measurements show the monolayer thickness to be ∼0.75 nm.
Open Access
Development of force fields for novel 2D materials for temperature dependent vibrational properties
(2019-09) Mobaraki, Arash
A new era of nanodevice engineering has been started after fabricating graphene. This motivated vast number of researches for predicting, fabricating and utilizing 2D materials. Temperature dependent properties are essential for device applications. Although rigorous density functional theory based approaches are able to predict electronic and mechanical properties accurately, but they are mostly limited to zero temperature and ab initio based molecular dynamics are computationally very demanding. Classical molecular dynamics is a very powerful alternative, however its accuracy is basically depend on the interatomic potential used for describing the considered system and therefore constructing accurate force fields is always an open problem, especially for the emerging 2D materials with extra ordinary properties. Single-layer transition metal dichalcogenides (TMDs) are new class of 2D materials which are shown to be good candidates for thermoelectric applications, flexible electronic and optoelectronic devices. In order to investigate thermal properties of TMDs, Stillinger-Weber type potentials are developed using particle swarm optimization method. These potentials are validated by comparing the resulted phonon dispersion curves and thermal conductivities with available first principle and experimental results. In addition, for understanding the anharmonic effects imposed by the generated force fields the trends of the shifts of the optical phonon frequencies at 􀀀 point with variation in the temperature are compared with available experimental data. In all cases, optimized potentials generate results which are in agreement with the target data. In the second step, spectral energy density method together with phonon mode decomposition is used for obtaining temperature dependent phonon frequencies and lifetimes in entire Brillouin zone. The contribution of each phonon branch in thermal conductivity is predicted utilizing the obtained phonon lifetimes and group velocities within the framework of relaxation time approximation. Eventually, with the aim of constructing transferable potentials for describing 2D and bulk structures, a very fast and reliable optimization method is presented. Combining local and global optimization methods and utilizing the energy curves obtained from first principle method, novel Stillinger-Weber type potentials for graphene, silicene and group III nitrides are developed. The proposed approach provides a solid framework for parameter selection and investigating the role of each parameter in the resulted phonon dispersion curves.
Open Access
Electric-field-induced reversible phase transitions in a spontaneously ion-Intercalated 2D metal oxide
(American Chemical Society, 2021-05-12) Rasouli, Hamid Reza; Kim, J.; Mehmood, Naveed; Sheraz, Ali; Jo, M. K.; Song, Seungwoo; Kang, K.; Kasırga, Talip Serkan
Electric field driven reversible phase transitions in two-dimensional (2D) materials are appealing for their potential in switching applications. Here, we introduce potassium intercalated MnO2 as an exemplary case. We demonstrate the synthesis of large-area single-crystal layered MnO2 via chemical vapor deposition as thin as 5 nm. These crystals are spontaneously intercalated by potassium ions during the synthesis. We showed that the charge transport in 2D K-MnO2 is dominated by motion of hydrated potassium ions in the interlayer space. Under a few volts bias, separation of potassium and the structural water leads to formation of different phases at the opposite terminals, and at larger biases K-MnO2 crystals exhibit reversible layered-to-spinel phase transition. These phase transitions are accompanied by electrical and optical changes in the material. We used the electric field driven ionic motion in K-MnO2 based devices to demonstrate the memristive capabilities of two terminal devices.
Open Access
Enhanced photoresponse of PVP:GaSe nanocomposite thin film based photodetectors
(Institute of Physics Publishing Ltd., 2022-02-21) Demirtaş, T.; Odacı, C.; Aydemir, Umut
Two-dimensional materials have become the focus of attention of researchers in recent years. The demand for two-dimensional materials is increasing day by day, especially with the inadequacy of graphene in optical applications. In this context, the optical and electrical characteristics of the PVP:GaSe thin film nanocomposites were investigated. The surface morphologies of the samples were characterized by SEM, the thin film thicknesses and refractive index parameters were measured by the Ellipsometer method, the structural characteristics were obtained by XRD, and Raman and PL spectroscopy was used to determine the optical characteristics. Critical parameters of Au/PVP:GaSe/n-Si photodetector were calculated under various illumination intensities. It is observed that photodetector with PVP:%5GaSe thin film has the best performance results. According to the experimental results, its responsivity, external quantum efficiency, and detectivity values are 0.485 A W−1, %86, and 1.14 × 107 cm Hz1/2 W−1 respectively.
Open Access
First-principles investigation of armchair stanene nanoribbons
(Elsevier B.V., 2018) Fadaie, M.; Shahtahmassebi, N.; Roknabad, M. R.; Gülseren, Oğuz
In this study, we systematically investigated the structural, electronic and optical properties of armchair stanene nanoribbons (ASNRs) by using the first-principles calculations. First, we performed full geometry optimization calculations on various finite width ASNRs where all the edge Sn atoms are saturated by hydrogen atoms. The buckled honeycomb structure of two dimensional (2D) stanene is preserved, however the bond length between the edge Sn atoms is shortened to 2.77 Å compared to the remaining bonds with 2.82 Å length. The electronic properties of these nanoribbons strongly depend on their ribbon width. In general, band gap opens and increases with decreasing nanoribbon width indicating the quantum confinement effect. Consequently, the band gap values vary from a few meV exhibiting low-gap semiconductor (quasi-metallic) behavior to ∼0.4-0.5 eV showing moderate semiconductor character. Furthermore, the band gap values are categorized into three groups according to modulo 3 of integer ribbon width N which is the number of Sn atoms along the width. In order to investigate the optical properties, we calculated the complex dielectric function and absorption spectra of ASNRs, they are similar to the one of 2D stanene. For light polarized along ASNRs, in general, largest peaks appear around 0.5 eV and 4.0 eV in the imaginary part of dielectric functions, and there are several smaller peaks between them. These major peaks redshifts, slightly to the lower energies of incident light with increasing nanoribbon width. On the other hand, for light polarized perpendicular to the ribbon, there is a small peak around 1.6 eV, then, there is a band formed from several peaks from 5 eV to ∼7.5 eV, and the second one from 8 eV to ∼9.5 eV. Moreover, the peak positions hardly move with varying nanoribbon width, which indicates that quantum confinement effect is not playing an essential role on the optical properties of armchair stanene nanoribbons. In addition, our calculations of the optical properties indicate the anisotropy with respect to the type of light polarization. This anisotropy is due to the quasi-2D nature of the nanoribbons.
Open Access
Formation and functionalization of boron phosphide monolayers
(2015-09) Hallıoğlu, Lütfiye
Since the synthesis of graphene with its unique properties has increased the focus on novel two dimensional (2D) materials, successively new 2D materials from either layered or non-layered materials have been synthesized following the advances in thin film growth and characterization techniques. Hexagonal boron nitride (h-BN) is the runner-up material, which is structurally stable in hexagonal honeycomb form. h-BN is an insulator whereas, it is a good thermal conductor. However, the electronic and structural properties of these 2D materials are very susceptible to doping and adsorption, as such, these properties can be altered extensively. Therefore, we have examined the phosphorization of h-BN with varying concentrations, which leads to stable 2D boron phosphide at the ultimate limit. The lattice constant of the BN16 gap semiconductor with impurity characteristics of adsorbants. Also, we have shown that except for Al and Ga, these impurity adatoms carry small amount of magnetic moment in moderate temperatures. In addition, we have studied the substitution of monolayer BP with Group III-IVV atoms. Based on our calculations, we have found that C and N can substitute P atom under ambient conditions. Nonetheless, only N atom selectively substitute for P atom, whereas C atom substitutes both for B and P giving rise to possible chemical etching of monolayer BP in the presence of excess C atom. Substitution of C for B/P results in metallic state in monolayer BP, while substitution of N for P leaves monolayer BP direct gap semiconductor. It is also found that none of these substitutions makes substrate magnetic. Using state-of-the-art computational tools based on the Density Functional Theory( DFT), we have calculated the structural and electronic properties of phosphorization of monolayer h-BN and doped monolayer h-BP.
Open Access
Functionalization of group V monolayers and their compounds: alloying, doping and surface modification
(2020-11) Kanlı, Muammer
There has been growing interest during the last decade in two-dimensional (2D) materials due to their important roles in various scientific and technological applications such as detectors, lasers and light emitting diodes. In this thesis we present a theoretical investigation of a couple of such 2D materials from group V monolayers and their compounds. Firstly, ordered alloys of GaxAl1−xN hexagonal monolayer are studied and the effect of Al content on mechanical, electronic, thermal and optical properties are investigated. The optimized lattice constants and band gaps change in accordance to Vegard’s Law. Low barrier energies and favorable substitution of Ga by Al may show feasibility of fabrication. Segregation is also checked with mixing energy calculations. The dynamical stability of alloys is shown by phonon spectrum analysis and MD simulations. GaxAl1−xN alloys give lower in-plane stiffness than h-BN or graphene, but higher Poisson’s ratio than most realized 2D systems. Heat capacity of alloys delivers a decrease with Al content at low temperatures but it converges to the classical limit at high temperatures. The absorption onset of GaxAl1−xN alloys remain in the near UV range and prominent absorption peaks blue-shifts with increasing x in compliance with the variation of the band gap. The considered systems, in regard to their stability and tunable fundamental properties seem to be very promising 2D semiconductors for wide range of applications at reduced scales. Then, the interaction of alkali metal atoms (Li, Na, and K) with single layer and periodic structures of hb-As and sw-As phases are revealed by first-principles methods. Arsenene phases are considered to be used as electrodes (anode) for ion-batteries. Strong alkali-electrode binding and low diffusion energy barriers gives out better cycling stability and faster diffusion, respectively. hb-As shows better storage capacity than sw-As. However, deviations from ordered pattern and decline of formation energy with increasing doping level point out a possible structural transformation. By MD calculations, crystalline to amorphous phase transition is seen even for low concentrations level at ambient temperature. The average open-circuit voltages of 0.68-0.88 V (0.65-0.98 V) with specific capacity up to 715 mAhg−1 (358 mAhg−1) are calculated for single layer (periodic) configurations. Overall, non-crystalline phases are calculated to offer more favorable structures than crystalline configurations and they provide more coherent results when compared with experimental data. The obtained voltage profile together with low diffusion barriers and strong metal-electrode binding suggests arsenene as a promising anode material to be used in for alkali-ion battery applications. Lastly, the formation of dumbbell (DB) geometry upon adsorption of Ga, N adatoms to GaN monolayer is investigated. While Ga-N DBs are unstable, Ga-Ga and N-N DB geometries are predicted to form in an exothermic and spontaneous scheme. Cohesive energy of hexagonal GaN monolayer decreases when a DB is formed on its surface. Electronic structures for Ga-Ga DBs for 2×2, 3×3, 4×4 and 5×5 phases show spinpolarized and degenerate bands mainly contributed by p-orbitals of the atoms in impurity zone. Degenarated bands are not observed for N-N dumbbell for HDP, TDP, 2×2 and 3×3 phases. Upon DB formation, semiconductor GaN monolayer become spin-polarized semiconductor with varying band gap, where this functionalization allows electronic structure to be tuned substantionally. This would be highly desired for nanoscale electronic and optical devices. These Ga-Ga and N-N DB geometries may also be used for the synthesis of layered GaN structures.
Open Access
Geometric band properties in strained monolayer transition metal dichalcogenides using simple band structures
(American Institute of Physics, 2019) Aas, Shahnaz; Bulutay, Ceyhun
Monolayer transition metal dichalcogenides (TMDs) bare large Berry curvature hotspots readily exploitable for geometric band effects. Tailoring and enhancement of these features via strain is an active research direction. Here, we consider spinless two- and three-band and spinful four-band models capable to quantify the Berry curvature and the orbital magnetic moment of strained TMDs. First, we provide a k⋅p parameter set for MoS2, MoSe2, WS2, and WSe2 in the light of the recently released ab initio and experimental band properties. Its validity range extends from the K valley edge to about one hundred millielectron volts into valence and conduction bands for these TMDs. To expand this over a larger part of the Brillouin zone, we incorporate strain to an available three-band tight-binding Hamiltonian. With these techniques, we demonstrate that both the Berry curvature and the orbital magnetic moment can be doubled compared to their intrinsic values by applying typically a 2.5% biaxial tensile strain. These simple band structure tools can find application in the quantitative device modeling of the geometric band effects in strained monolayer TMDs.
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
Many-body theory explored optical properties of selected 2D group II-VI monochalcogenides
(2022-09) Seyedmohammadzadeh, Mahsa
Two-dimensional (2D) metal oxides (MOs) and metal chalcogenides (MChs) are emerging classes of 2D materials. Depending on the constituent elements, these materials can display various electronic and optical properties making them promising candidates in many device applications, such as solar cells and transparent circuits. Binary graphene-like structures of II-VI are the most straightforward structures of 2D MOs and MChs. We systematically examined the electronic and optical properties of selected 2D structures from this category: BeO, BaTe, CdO, CaO, CaS, MgO, SrS, SrSe and ZnO. The dynamical stability of these materials has been reported in previous studies. In 2D semiconductors, excitonic effects dominate the optical properties. Theoretical investigation of such phenomena requires employing many-body approaches beyond standard density functional theory. We utilized a single shot of GW approximation to predict the electronic band structure and solved the Bethe- Salpeter equation in the Tamm-Dancoff approximation to consider excitonic effects. Our results show that all structures possess indirect band gaps except ZnO and CdO. Furthermore, the considered structures have large exciton binding energies ranging from 0.72 eV in CdO to 2.84 eV in BeO. CdO has the smallest calculated optical band gap with a value of 1.43 eV. Analyzing the optical absorption spectra reveals that the CdO can absorb 7.9 % of the incident light in its optical band gap. The maximum amount of absorption appears in BeO, which can absorb 28% of incident light in the ultraviolet region. Among the structure mentioned above, there is a close matching between the lattice constants of ZnO and MgO, promising for creating lateral and vertical heterostructures. Due to the enhanced performance resulting from mixing distinct properties of individual monolayers, van der Waals heterostructures (vdWHs) are regarded as a revolutionary class among a plethora of presently fabricated or predicted 2D materials. Alongside vdWHs, recent studies have also reported 2D heterostructures with interlayer bonding. Motivated by the flourishing properties of vertical heterostructures, we comprehensively examined the mechanical, electronic and optical properties of ZnO/MgO structures in four different stackings. Structural relaxation has indicated two vdWHs and two structures with interlayer binding. All considered structures are mechanically stable. In addition, phonon dispersion curves show that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. The s orbital of Zn atom dominates the minimum of the first conduction band of these structures. The optical absorbance spectra show that strong excitonic effects reduce the optical band gap to the visible light spectrum range, and all structures can absorb around 8% of incident light.
Open Access
The mechanical characterization of two-dimensional materials (WS2, MoS2, and graphene) and the effect of defects on young`s modulus of CVD grown single-layer graphene
(2017-11) Adilbekova, Begimai
With the first isolation of graphene two-dimensional (2D) materials attracted the enormous interest of many researchers. Owing to extraordinary properties and atomic thickness 2D materials have many applications in gas detection, electrodes, energy storage devices, field effect devices, sensors, photodetectors, solar cells, nanocomposites, actuators/ resonators, biological membranes,cancer detectors, piezoresistive pressure sensors, gas impermeable membranes, gas or liquid separation. Despite the atomic thickness, the gapless character of the graphene limits its applications in modern electronic devices. There are two strategies for solving the problem, first one is to open the band-gap in graphene and second is to explore new 2D materials. Generation of defects and applying an electrical field can increase the band-gap of graphene but defects can affect other properties of it. Therefore, there is a need for new analogs and transition metal dichalcogenides (TMDs) are the most promising ones. TMDs drew the attention of many researchers because of the remarkable electronic and optical properties. TMD materials exhibit a semiconducting nature owing to the presence of the band-gap, which is essential for the logical operations. Besides electronic properties, the mechanical properties( Young`s Modulus) play a significant role in applications of 2D materials. 2D materials are most promising candidates for flexible electronic devices, which received enormous interest in recent years. But the applied strain and other external forces can modify the structure of crystalline graphene and TMDs, consequently affect the performance and lifetime of devices. In this work, we aimed to measure the Young`s Modulus of graphene, MoS2 film, MoS2 and WS2 flakes with an Atomic Force Microscope (AFM). In addition, we sought the relation between the defect intensity and Young`s Modulus of graphene. The defects in graphene were generated by Ga+ with different doses in Focused Ion Beam (FIB). We found Young`s Moduli of graphene, MoS2 film, MoS2 and WS2 flakes to be 270 N/m, 330 N/m, 90 N/m and 140 N/m, respectively. These values are lower than those given in a literature, what might be caused by the pre-existence of unwanted defects. Also, it appeared that the introduction of defects leads to the fall in Young`s Modulus values of the graphene.
Open Access
MoS2 phototransistor sensitized by colloidal semiconductor quantum wells
(Wiley-VCH Verlag, 2020-12) Sar, H.; Taghipour, Nima; Lisheshar, İ. W.; Delikanlı, Savaş; Demirtaş, M.; Demir, Hilmi Volkan; Ay, F.; Perkgöz, N. K.
A phototransistor built by the assembly of 2D colloidal semiconductor quantum wells (CQWs) on a single layer of 2D transition metal dichalcogenide (TMD) is displayed. This hybrid device architecture exhibits high efficiency in Förster resonance energy transfer (FRET) enabling superior performance in terms of photoresponsivity and detectivity. Here, a thin film of CdSe/CdS CQWs acts as a sensitizer layer on top of the MoS2 monolayer based field‐effect transistor, where this CQWs–MoS2 structure allows for strong light absorption in CQWs in the operating spectral region and strong dipole‐to‐dipole coupling between MoS2 and CQWs resulting in enhanced photoresponsivity of one order of magnitude (11‐fold) at maximum gate voltage (VBG = 2 V) and two orders of magnitude (≈ 5 × 102) at VBG = −1.5 V, and tenfold enhanced specific detectivity. The illumination power‐dependent characterization of this hybrid device reveals that the thin layer of CQWs dominates the photogating mechanism compared to the photoconductivity effect on detection performance. Such hybrid designs hold great promise for 2D‐material based photodetectors to reach high performance and find use in optoelectronic applications.
Open Access
A novel method for thermal conductivity measurement of two dimensional materials
(2019-09) Çakıroğlu, Onur
Thermal conductivity is a quantity which governs the heat transfer in a material. After increasing importance of efficiency in power generation systems and cooling mechanisms in micro-structures, many measurement methods have been developed to explore the thermal conductivity in micro and nano-sized materials. However, complexity in experimental setups, difficulties in the fabrication of devices required for measurements, and lacking exact solutions to thermal equations limit the usability of the methods to a class of materials. It is particularly challenging to study atomically thin metallic materials. To tackle this challenge, we have developed a new thermal conductivity measurement method based on the temperature dependent electrical resistance change and analyzed our method analytically and numerically by finite element method. We applied our method to 2H-TaS2 and found thermal conductivity as 9.55 1.27 W/m.K. Thermal conductivity value of TaS2, a metallic transition metal dichalcogenide was measured for the first time. This is supported by Wiedemann-Franz law and thermal conductivity of similar materials such as 2H-TaSe2 and 1T-TaS2. The method can be applied to semiconducting thin materials as well and is superior to other methods in various ways.
Open Access
Optimized stillinger-weber potentials for 1H, 1T and 1T′ phases of WS2 for molecular dynamics studies: thermal transport as an example
(2024-01) Waheed, Alim Mohamed
The advent of graphene has poured numerous amount of research effort into the study 2D materials and utilizing it for device fabrication. Monolayer Transition Metal Dichalcogenides are one such class of polymorphic material with high prospect in versatile device applications due to its unique properties exhibited across the various phases. Classical Molecular Dynamics is a powerful tool that can be utilized to study the thermal and mechanical properties of these phases. Considering this, we optimise Stillinger-Weber type Potential for the seperate 1H, 1T and 1T′ phases of WS2 using Particle Swarm Optimization. These potentials are validated by comparison of phonon dispersion curves, Density Functional Theory (DFT) based target characteristic data and through an accuracy assessment conducted using Non-Equilibrium Molecular Dynamic (NEMD) simulations to evaluate thermal conductivity of the polymorphic structures. Thermal conductivity results obtained for 1H and 1T′ are in good agreement with first principle predictions calculated using Boltzmann Transport Equation. NEMD simulation of 1T phase prove to be challenging due to its dynamic instability with incoherent buckle structure formation along the symmetric directions.
Embargo
Promising anisotropic mechanical, electronic, and charge transport properties of 2D InN alloys for photocatalytic water splitting
(Elsevier, 2023-11-30) Özbey, Doğukan Hazar; Kilic, M. E.; Durgun, Engin
Two-dimensional (2D) materials with unique physical properties lead to new possibilities in future nanomaterial-based devices. Among them, 2D structures suitable to be the solar-driven catalyst for water-splitting reactions have become excessively important since the demand for clean energy sources has increased. Apart from the conventional crystals with well-known symmetries, recent studies showed that materials with exotic decorations could possess superior features in these kinds of applications. In this respect, we report novel 2D tetrahexagonal (th-) InN crystal and its ordered alloys In0.33 X0.67N (X = Al, Ga) that can be utilized as effective catalysts for water splitting reactions. Proposed structures possess robust energetic, dynamical, thermal, and mechanical stability with a versatile mechanical response. After a critical tensile strain value, all monolayers exhibit strain-induced negative Poisson's ratio in a particular crystal direction, making them half-auxetic materials. The examined materials are indirect semiconductors with desired band gaps and band edge positions for water-splitting applications. Due to their structural anisotropy, they have direction-dependent mobility that can keep the photogenerated charge carriers separated by reducing their recombination probability, which boosts the photocatalytic process. High absorption capacity in the wide spectral range underlines their potential performance. The versatile mechanical, electronic, and optical properties of 2D th-InN and its alloys, together with their remarkable structural stability, indicate that they can appropriately be exploited in the future for water splitting applications.
Open Access
Real time optical observation of the synthesis of novel 2D materials and investigation of their fundamental properties
(2021-03) Rasouli, Hamid Reza
Two-dimensional transition metal dichalcogenide (2D TMDC) with superb phys-ical and chemical properties, used as the active material for various devices. The on-going primary focus is their reliable high-throughput synthesis using processes compatible with the current semiconductor technology. At present, among the common approaches, chemical vapor deposition (CVD) has been considered as the most promising method for preparing large-area high-quality 2D materials. However, the lack of in-situ information during the growth in conventional CVD systems, makes it impractical to realize high-temperature phenomena. In this thesis, we developed a novel CVD chamber that allows real time optical obser-vation and control of the crystal growth. Using this new CVD method, which we call real time optical-CVD, RTO-CVD in short, we elaborated the involved mechanisms in salt-assisted synthesis of TMDCs and their vertical/lateral het-erostructures. Through direct visualization of WSe2 monolayer growth, we iden-tiﬁed that both vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Then, we focused our attention to synthesize novel 2D materials such as V2O3 and K-MnO2 nanosheets. We succeeded synthesis route in favor of high-quality single-crystalline V2O3 nanoplates whose 2D characteristic allows us to study their peculiar electrical and physical properties such as metal-insulator transition (MIT) and supercritical state. The electrical properties of both as-grown and transferred V2O3 crystals were investigated with respect to the V2O3 phase-stability diagram. We observed emergence of a novel crystal structure upon electron beam heating in selected area electron diﬀraction (SAED) experi-ments and correlated it to the supercritical state by means of high-temperature Raman spectroscopy. Finally, we introduced large-area ultra-thin layered MnO2 crystals, spontaneously intercalated by potassium ions during the synthesis. The charge transport in 2D K-MnO2 devices was shown to be dominated by the in-plane ionic conductivity through the motion of hydrated K ions in the interlayer space. The K-MnO2 crystals exhibited reversible layered-to-spinel phase tran-sition accompanied by an optical contrast change based on the electrical and optical modulation of the potassium and the interlayer water concentration. We used the electric-ﬁeld driven ionic motion in K-MnO2 devices to demonstrate the memristive properties and elucidated the resistance-switching mechanisms via real-time analyses upon the measurements. K-MnO2 memristors were artiﬁcially able to emulate neuromorphic synapse-like behaviors, namely short and long-term potentiation/depression as well as ionic coupling eﬀects.
Open Access
Strain-induced structural phase transition in GeN monolayer
(Elsevier BV, 2021-11-30) Abboud, Mohammad; Özbey, D.H.; Durgun, Engin
The recent synthesis of SiP, SiAs, GeP, and GeAs monolayers has brought two-dimensional (2D) group IV–V systems into the limelight. To date, all the fabricated structures of this class belong to the C2/m space group which has a low structural symmetry, while the class could exist in more symmetric phases (i.e., P3m1 and P6m2). The realization of more symmetric phases can enhance the intrinsic properties of these materials and increase their potential field of usage. In this study, the possibility of a structural phase transition in GeN monolayer by application of mechanical strain is investigated. Based on ab initio simulations, we first confirm the stability of the GeN monolayer in all phases, then demonstrate how a large enough compressive strain (~%12) can transform C2/m into P3m1 phase. The results are interpreted by analyzing the geometry, bond order, electron localization functions, and net atomic charges of the structures. Upon transition into the P3m1 phase, tensile strength and in-plane stiffness double, while the compressive strength quadruples. On the other hand, the effect of the phase transition on the electronic properties is not substantial and similar band structure profiles with narrowed band gap are obtained. Our study provides insight on how to experimentally achieve the P3m1 phase of the GeN monolayer, which is in principle applicable to other group IV–V monolayers under suitable conditions involving the optimization of pressure, temperature, and impurity concentration. These unique features of the GeN monolayer render them ideal candidates for a variety of high technological nanoscale applications.
Open Access
Trion-mediated förster resonance energy transfer and optical gating effect in WS2/hBN/MoSe2 heterojunction
(American Chemical Society, 2020) Hu, Z.; Hernández-Martínez, P. L.; Liu, X.; Amara, M. R.; Zhao, W.; Watanabe, K.; Taniguchi, T.; Demir, Hilmi Volkan
van der Waals two-dimensional layered heterostructures have recently emerged as a platform, where the interlayer couplings give rise to interesting physics and multifunctionalities in optoelectronics. Such couplings can be rationally controlled by dielectric, separation, and stacking angles, which affect the overall charge or energy-transfer processes, and emergent potential landscape for twistronics. Herein, we report the efficient Förster resonance energy transfer (FRET) in WS2/ hBN/MoSe2 heterostructure, probed by both steady-state and timeresolved optical spectroscopy. We clarified the evolution behavior of the electron−hole pairs and free electrons from the trions, that is, ∼59.9% of the electron−hole pairs could transfer into MoSe2 by FRET channels (∼38 ps) while the free electrons accumulate at the WS2/hBN interface to photogate MoSe2. This study presents a clear picture of the FRET process in two-dimensional transition-metal dichalcogenides’ heterojunctions, which establishes the scientific foundation for developing the related heterojunction optoelectronic devices.