Two-dimensional Transition Metal Dichalcogenides (TMDs) possessing extraordinary physical properties at reduced dimensionality have attracted interest due to their promise in electronic and optical ...device applications. However, TMD monolayers can show a broad range of different properties depending on their crystal phase; for example, H phases are usually semiconductors, while the T phases are metallic. Thus, controlling phase transitions has become critical for device applications. In this study, the energetically low-lying crystal structures of pristine and Janus TMDs are investigated by using
Nudged Elastic Band and molecular dynamics simulations to provide a general explanation for their phase stability and transition properties. Across all materials investigated, the T phase is found to be the least stable and the H phase is the most stable except for WTe
, while the T' and T'' phases change places according to the TMD material. The transition energy barriers are found to be large enough to hint that even the higher energy phases are unlikely to undergo a phase transition to a more stable phase if they can be achieved except for the least stable T phase, which has zero barrier towards the T' phase. Indeed, in molecular dynamics simulations the thermodynamically least stable T phase transformed into the T' phase spontaneously while in general no other phase transition was observed up to 2100 K for the other three phases. Thus, the examined T', T'' and H phases were shown to be mostly stable and do not readily transform into another phase. Furthermore, so-called mixed phase calculations considered in our study explain the experimentally observed lateral hybrid structures and point out that the coexistence of different phases is strongly stable against phase transitions. Indeed, stable complex structures such as metal-semiconductor-metal architectures, which have immense potential to be used in future device applications, are also possible based on our investigation.
The global optimization of subnanometer Ru–Pt binary nanoalloys in the size range 2–8 atoms is systematically investigated using the Birmingham Parallel Genetic Algorithm (BPGA). The effect of size ...and composition on the structures, stabilities and mixing properties of Ru–Pt nanoalloys are discussed. The results revealed that the maximum mixing tendency is achieved for 40–50% Ru compositions. Global minimum structures show that the Ru atoms prefer to occupy central and core positions and maximize coordination number and the number of strong Ru–Ru bonds.
A range of models of free standing and Ag(1 1 1)-supported stoichiometric ZnO films with coverages between 2-3 monolayers are studied using density functional calculations. Following experimental ...observations we focus on stoichiometric hexagonal and triangular ad-layer islands grown on top of two complete ZnO monolayers. The adlayer islands display distinct edge and corner reconstructions and are found to induce a structural transition extending from the island core to the layered phase below. Based on our results we propose a general model of ad-layer triangular island structure based on seven regions exhibiting four distinct polymorphs.
Long‐Term Stability Control of CVD‐Grown Monolayer MoS2 Şar, Hüseyin; Özden, Ayberk; Demiroğlu, İlker ...
Physica status solidi. PSS-RRL. Rapid research letters,
July 2019, 20190701, Letnik:
13, Številka:
7
Journal Article
Recenzirano
The structural stability of 2D transition metal dichalcogenide (TMD) formations is of particular importance for their reliable device performance in nano‐electronics and opto‐electronics. Recent ...observations show that the CVD‐grown TMD monolayers are likely to encounter stability problems such as cracking or fracturing when they are kept under ambient conditions. Here, two different growth configurations are investigated and a favorable growth geometry is proposed, which also sheds light onto the growth mechanism and provides a solution for the stability and fracture formation issues for TMDs specifically for MoS2 monolayers. It is shown that 18 months naturally and thermally aged MoS2 monolayer flakes grown using specifically developed conditions, retain their stability. To understand the mechanism of the structural deterioration, two possible effective mechanisms, S vacancy defects and growth‐induced tensile stress, are assessed by the first principle calculations where the role of S vacancy defects in obtaining oxidation resistant MoS2 monolayer flakes is revealed to be rather more critical. Hence, these simulations, time‐dependent observations and thermal aging experiments show that durability and stability of 2D MoS2 flakes can be controlled by CVD growth configuration.
The proposed growth geometry provides a solution for the stability–fracture issue for CVD‐grown transition metal dichalcogenides. A clear difference is unveiled regarding the mechanical stability of the flakes grown by horizontal (HO, proposed by the authors) and face down (FD, commonly used) growth approaches, where the FD‐grown flakes spontaneously crack but HO flakes remain unchanged and keep their as‐grown structures for the duration of their observation, exceeding 18 months.
Two-dimensional materials have been attracting increasing interests because of their outstanding properties for Lithium-ion battery applications. In particular, a material family called MXenes ...(Mn+1Cn, where n = 1, 2, 3) have been recently attracted immense interest in this respect due to their incomparable fast-charging properties and high capacity promises. In this article, we review the state-of-the-art computational progress on Li-ion battery applications of MXene materials in accordance with our systematical DFT calculations. Structural, mechanical, dynamical, and electrical properties of 20 distinct MXene (M: Sc, Ti, V, Cr, Nb, Mo, Hf, Ta, W, and Zr) have been discussed. The battery performances of these MXene monolayers are further investigated by Li-ion binding energies, open circuit voltage values, and Li migration energy barriers. The experimental and theoretical progress up to date demonstrates particularly the potential of non-terminated or pristine MXene materials in Li ion-storage applications. Stability analyses show most of the pristine MXenes should be achievable, however susceptible to the development progress on the experimental growth procedures. Among pristine MXenes, Ti2C, V2C, Sc2C, and Zr2C compounds excel with their high charge/discharge rate prospect due to their extremely low Li diffusion energy barriers. Considering also their higher predicted gravimetric capacities, Sc, Ti, V, and Zr containing MXenes are more promising for their utilization in energy storage applications.
Thermal expansion behavior of two-dimensional (2D) nitrides and graphene were studied by ab initio molecular dynamics (MD) simulations as well as quasiharmonic approximation (QHA). Anharmonicity of ...the acoustic phonon modes are related to the unusual negative thermal expansion (NTE) behavior of the nitrides. Our results also hint that direct ab initio MD simulations are a more elaborate method to investigate thermal expansion behavior of 2D materials than the QHA. Nevertheless, giant NTE coefficients are found for h-GaN and h-AlN within the covered temperature range 100–600 K regardless of the chosen computational method. This unusual NTE of 2D nitrides is reasoned with the out-of-plane oscillations related to the rippling behavior of the monolayers.
High-performance rechargeable batteries are becoming very important for high-end technologies with their ever increasing application areas. Hence, improving the performance of such batteries has ...become the main bottleneck to transferring high-end technologies to end users. In this study, we propose an argon intercalation strategy to enhance battery performance via engineering the interlayer spacing of honeycomb structures such as graphite, a common electrode material in lithium-ion batteries (LIBs). Herein, we systematically investigated the LIB performance of graphite and hexagonal boron nitride (h-BN) when argon atoms were sent into between their layers by using first-principles density-functional-theory calculations. Our results showed enhanced lithium binding for graphite and h-BN structures when argon atoms were intercalated. The increased interlayer space doubles the gravimetric lithium capacity for graphite, while the volumetric capacity also increased by around 20% even though the volume was also increased. The
molecular dynamics simulations indicate the thermal stability of such graphite structures against any structural transformation and Li release. The nudged-elastic-band calculations showed that the migration energy barriers were drastically lowered, which promises fast charging capability for batteries containing graphite electrodes. Although a similar level of battery promise was not achieved for h-BN material, its enhanced battery capabilities by argon intercalation also support that the argon intercalation strategy can be a viable route to enhance such honeycomb battery electrodes.
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.
Improving the activity and stability of Pt-based electrocatalysts is still crucial for hydrogen generation applications. In this study, we investigated the catalytic properties of Cu-doped Pt alloy ...nanostructures synthesized by the modified polyol method, and compared the performance of PtCu catalyst with that of commercially available Pt/C for the hydrogen evolution reaction (HER) at room temperature. Structural analyses revealed that the PtCu catalysts exhibited fcc-Fm3¯m crystal structure with an average particle size below 5 nm. The HER performance of the catalysts showed overpotential of around −1.0 V (vs. Ag/AgCl) and Pt0.25Cu0.75 and Pt0.75Cu0.25 catalysts exhibited enhanced performance in 1 M KOH. The Pt0.75Cu0.25 catalyst exhibited distinct performance, with the highest mass activity found to be 62.80 mA mg−1Pt. The most active Pt0.75Cu0.25 catalysis for the HER process had the lowest onset potential of 0.989 V and Tafel slope of 35.5 mV dec−1, which were an improvement compared to commercially available Pt/C catalysts. First principle DFT calculations confirmed the stabilities of Pt1Cu3, Pt1Cu1 and Pt3Cu1 compositions as ordered alloy structures. Nudged elastic band method calculations and the d-band model verified the higher catalytic activity of Pt–Cu nanoparticles compared to pure nanoparticles and point out the relevance of a synergistic effect of Pt and Cu atoms on water dissociation. According to DFT calculations, Cu and Pt sites significantly influenced the adsorption of H2O and H, respectively, for splitting water molecules. While the Pt59Cu20 cluster has the highest H2O adsorption energy at the (100) atop site of Cu atoms, H adsorption occurs at the (111) site of Pt atoms.
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•PtxCu1-x (x: 0.25, 0.55 and 0.75) NPs were synthesized with narrow size distribution between 5 nm and cubic structure.•The Tafel slope of Pt0.75Cu0.25 catalyst is 35.5 mV dec−1 was revealed nearly equal to the Pt/C catalyst (31 mV dec−1).•The ordered alloying morphologies are more favorable for the PtCu crystal which found by excess energy calculation.•When compared pure Pt and Cu clusters, all the alloy structures give an upshift of the d-band center.
The surface termination of MXenes greatly determines the electrochemical properties and ion kinetics on their surfaces. So far, hydroxyl-, oxygen-, and fluorine-terminated MXenes have been widely ...studied for energy storage applications. Recently, sulfur-functionalized MXene structures, which possess low diffusion barriers, have been proposed as candidate materials to enhance battery performance. We performed first-principles calculations on the structural, stability, electrochemical, and ion dynamic properties of Li-adsorbed sulfur-functionalized groups 3B, 4B, 5B, and 6B transition-metal (M)-based MXenes (i.e., M2CS2 with M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W). We performed phonon calculations, which indicated that all of the above M2CS2 MXenes, except for Sc, are dynamically stable at T = 0 K. The ground-state structure of each M2CS2 monolayer depends on the type of M atom. For instance, while sulfur prefers to sit at the FCC site on Ti2CS2, it occupies the HCP site of Cr-based MXene. We determined the Li adsorption configurations at different concentrations using the cluster expansion method. The highest maximum open-circuit voltages were computed for the group 4B element (i.e., Ti, Zr, and Hf)-based M2CS2, which are larger than 2.1 V, while their average voltages are approximately 1 V. The maximum voltage for the group 6B element (i.e., Cr, Mo, W)-based M2CS2 is less than 1 V, and the average voltage is less than 0.71 V. We found that S functionalization is helpful for capacity improvements over the O-terminated MXenes. In this respect, the computed storage gravimetric capacity may reach up to 417.4 mAh/g for Ti2CS2 and 404.5 mAh/g for V2CS2. Ta-, Cr-, Mo-, and W-based M2CS2 MXenes show very low capacities, which are less than 100 mAh/g. The Li surface diffusion energy barriers for all of the considered MXenes are less than 0.22 eV, which is favorable for high charging and discharging rates. Finally, ab initio molecular dynamic simulations performed at 400 K and bond-length analysis with respect to Li concentration verify that selected promising systems are robust against thermally induced perturbations that may induce structural transformations or distortions and undesirable Li release.
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