Li2S growth on graphene: Impact on the electrochemical performance of Li-S batteries.

Lithium-sulfur batteries show remarkable potential for energy storage applications due to their high-specific capacity and the low cost of active materials, especially sulfur. However, whereas there is a consensus about the use of lithium metal as the negative electrode, there is not a clear and widely accepted architectural design for the positive electrode of sulfur batteries. The difficulties arise when trying to find a balance between high-surface-area architectures and practical utilization of the sulfur content. Intensive understanding of the interfacial mechanisms becomes then crucial to design optimized carbon-hosted sulfur architectures with enhanced electrochemical performance. In this work, we use density functional theory (DFT)-based first principles calculations to describe and characterize the growing mechanisms of Li2S active material on graphene, taken as an example of a nonencapsulated carbon host for the positive electrode of Li-S batteries. We first unravel the two growing mechanisms of Li2S supported nanostructures, which explain recent experimental findings on real-time monitoring of interfacial deposition of lithium sulfides during discharge, obtained by means of in situ atomic force microscopy. Then, using a combination of mathematical tools and DFT calculations, we obtain the first cycle voltage plot, explaining the three different regions observed that ultimately lead to the formation of high-order polysulfides upon charge. Finally, we show how the different Li2S supported nanostructures can be characterized in X-ray photoelectron spectroscopy measurements. Altogether, this work provides useful insights for the rational design of new carbon-hosted sulfur architectures with optimized characteristics for the positive electrode of lithium-sulfur batteries.

Impact of intrinsic atomic defects on the electronic structure of MoS2 monolayers.

Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.

A density-functional theory study of tip electronic structures in scanning tunneling microscopy.

In this work, we report a detailed analysis of the atomic and electronic structures of transition metal scanning tunneling microscopy tips: Rh, Pd, W, Ir, and Pt pyramidal models, and transition metal (TM) atom tips supported on the W surface, by means of ab initio density-functional theory methods. The d electrons of the apex atoms of the TM tips (Rh, Pd, W, Ir, and Pt tetrahedral structures) show different behaviors near the Fermi level and, especially for the W tip, dz(2) states are shown to be predominant near the Fermi level. The electronic structures of larger pyramidal TM tip structures with a single apex atom are also reported. Their obtained density of states are thoroughly discussed in terms of the different d-electron occupations of the TM tips.

Increasing the Strain Resistance of Si/SiO2 Interfaces for Flexible Electronics.

Understanding the changes that occur in the micro-mechanical properties of semiconductor materials is of utmost importance for the design of new flexible electronic devices, especially to control the properties of newly designed materials. In this work, we present the design, fabrication, and application of a novel tensile-testing device coupled to FTIR measurements that enables in situ atomic investigations of samples under uniaxial tensile load. The device allows for mechanical studies of rectangular samples with dimensions of 30 mm × 10 mm × 0.5 mm. By recording the alternation in dipole moments, the investigation of fracture mechanisms becomes feasible. Our results show that thermally treated SiO2 on silicon wafers has a higher strain resistance and breaking force than the SiO2 native oxide. The FTIR spectra of the samples during the unloading step indicate that for the native oxide sample, the fracture happened following the propagation of cracks from the surface into the silicon wafer. On the contrary, for the thermally treated samples, the crack growth starts from the deepest region of the oxide and propagates along the interface due to the change in the interface properties and redistribution of the applied stress. Finally, density functional theory calculations of model surfaces were conducted in order to unravel the differences in optic and electronic properties of the interfaces with and without applied stress.

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity and surface stability suggests new directions to functionalize silicon for sensors, photovoltaic devices, and nanoelectronics.

Mechanisms for Graphene Growth on SiO2 Using Plasma-Enhanced Chemical Vapor Deposition: A Density Functional Theory Study.

Plasma-enhanced chemical vapor deposition (PE-CVD) of graphene layers on dielectric substrates is one of the most important processes for the incorporation of graphene in semiconductor devices. Graphene is moving rapidly from the laboratory to practical implementation; therefore, devices may take advantage of the unique properties of such nanomaterial. Conventional approaches rely on pattern transfers after growing graphene on transition metals, which can cause nonuniformities, poor adherence, or other defects. Direct growth of graphene layers on the substrates of interest, mostly dielectrics, is the most logical approach, although it is not free from challenges and obstacles such as obtaining a specific yield of graphene layers with desired properties or accurate control of the growing number of layers. In this work, we use density-functional theory (DFT) coupled with ab initio molecular dynamics (AIMD) to investigate the initial stages of graphene growth on silicon oxide. We select C2H2 as the PE-CVD precursor due to its large carbon contribution. On the basis of our simulation results for various surface models and precursor doses, we accurately describe the early stages of graphene growth, from the formation of carbon dimer rows to the critical length required to undergo dynamical folding that results in the formation of low-order polygonal shapes. The differences in bonding with the functionalization of the silicon oxide also mark the nature of the growing carbon layers as well as shed light of potential flaws in the adherence to the substrate. Finally, our dynamical matrix calculations and the obtained infrared (IR) spectra and vibrational characteristics provide accurate recipes to trace experimentally the growth mechanisms described and the corresponding identification of possible stacking faults or defects in the emerging graphene layers.

Understanding the preferential adsorption of CO2 over N2 in a flexible metal-organic framework.

The unusual uptake behavior and preferential adsorption of CO(2) over N(2) are investigated in a flexible metal-organic framework system, Zn(2)(bdc)(2)(bpee), where bpdc = 4,4'-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO(2) with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO(2) with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO(2). DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der Waals-Density Functional Theory method, showing that the more energetically favorable positions for the CO(2) molecules are closer to the C═C bond of the bpee and the C-C bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guest-host interactions and structural changes in the framework.

Core-Shell Nanocomposites for Improving the Structural Stability of Li-Rich Layered Oxide Cathode Materials for Li-Ion Batteries.

The structural stability of Li-rich layered oxide cathode materials is the ultimate frontier to allow the full development of these family of electrode materials. Here, first-principles calculations coupled with cluster expansion are presented to investigate the electrochemical activity of phase-separation, core-shell-structured xLi2MnO3·(1 - x)LiNiCoMnO2 nanocomposites. The detrimental surface effects of the core region can be countered by the Li2MnO3 shell, which stabilizes the nanocomposites. The operational voltage windows are accurately determined to avoid the electrochemical activation of the shell and the subsequent structural evolution. In particular, the dependence of the activation voltage with the shell thickness shows that relatively high voltages can still be obtained to meet the energy density needs of Li-ion battery applications. Finally, activation energies of Li migration at the core-shell interface must also be analyzed carefully to avoid the outbreak of a phase transformation, thus making the nanocomposites suitable from a structural viewpoint.

Ab Initio Study on Surface Segregation and Anisotropy of Ni-Rich LiNi1-2yCoyMnyO2 (NCM) (y ≤ 0.1) Cathodes.

Advances in ex situ and in situ (operando) characteristic techniques have unraveled unprecedented atomic details in the electrochemical reaction of Li-ion batteries. To bridge the gap between emerging evidences and practical material development, an elaborate understanding on the electrochemical properties of cathode materials on the atomic scale is urgently needed. In this work, we perform comprehensive first-principle calculations within the density functional theory + U framework on the surface stability, morphology, and elastic anisotropy of Ni-rich LiNi1-2yCoyMnyO2 (NCM) (y ≤ 0.1) cathode materials, which are strongly related to the emerging evidence in the degradation of Li-ion batteries. On the basis of the surface stability results, the equilibrium particle morphology is obtained, which is mainly determined by the oxygen chemical potential. Ni-rich NCM particles are terminated mostly by the (012) and (001) surfaces for oxygen-poor conditions, whereas the termination corresponds to the (104) and (001) surfaces for oxygen-rich conditions. Besides, Ni surface segregation predominantly occurs on the (100), (110), and (104) nonpolar surfaces, showing a tendency to form a rocksalt NiO domain on the surface because of severe Li-Ni exchange. The observed elastic anisotropy reveals that an uneven deformation is more likely to be formed in the particles synthesized under poor-oxygen conditions, leading to crack generation and propagation. Our findings provide a deep understanding of the surface properties and degradation of Ni-rich NCM particles, thereby proposing possible solution mechanisms to the factors affecting degradation, such as synthesis conditions, coating, or novel nanostructures.

Nanopatterning of Group V Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping.

Control of the electronic properties of semiconductors is primarily achieved through doping. While scaling down the device dimensions to the molecular regime presents an increasing number of difficulties, doping control at the nanoscale is still regarded as one of the major challenges of the electronic industry. Within this context, new techniques such as monolayer doping (MLD) represent a substantial improvement toward surface doping with atomic and specific doping dose control at the nanoscale. Our previous work has explained in detail the atomistic mechanism behind MLD by means of density-functional theory calculations (Chem. Mater. 2016, 28, 1975). Here, we address the key questions that will ultimately allow one to optimize the scalability of the MLD process. First, we show that dopant coverage control cannot be achieved by simultaneous reaction of several group V elements, but stepwise reactions make it possible. Second, using ab initio molecular dynamics, we investigate the thermal decomposition of the molecular precursors, together with the stability of the corresponding binary and ternary dopant oxides, prior to the dopant diffusion into the semiconductor surface. Finally, the effect of the coverage and type of dopant on the electronic properties of the semiconductor is also analyzed. Furthermore, the atomistic characterization of the MLD process raises unexpected questions regarding possible crystal damage effects by dopant exchange with the semiconductor ions or the final distribution of the doping impurities within the crystal structure. By combining all our results, optimization recipes to create ultrashallow doped junctions at the nanoscale are finally proposed.

Computational Study of MoS2/HfO2 Defective Interfaces for Nanometer-Scale Electronics.

Atomic structures and electronic properties of MoS2/HfO2 defective interfaces are investigated extensively for future field-effect transistor device applications. To mimic the atomic layer deposition growth under ambient conditions, the impact of interfacial oxygen concentration on the MoS2/HfO2 interface electronic structure is examined. Then, the effect on band offsets (BOs) and the thermodynamic stability of those interfaces is investigated and compared with available relevant experimental data. Our results show that the BOs can be modified up to 2 eV by tuning the oxygen content through, for example, the relative partial pressure. Interfaces with hydrogen impurities as well as various structural disorders were also considered, leading to different behaviors, such as n-type doping, or introducing defect states close to the Fermi level because of the formation of hydroxyl groups. Then, our results indicate that for a well-prepared interface the electronic device performance should be better than that of other interfaces, such as III-V/high-κ, because of the absence of interface defect states. However, any unpassivated defects, if present during oxide growth, strongly affect the subsequent electronic properties of the interface. The unique electronic properties of monolayer-to-few-layered transition-metal dichalcogenides and dielectric interfaces are described in detail for the first time, showing the promising interfacial characteristics for future transistor technology.

Charge-transfer modified embedded atom method dynamic charge potential for Li-Co-O system.

To overcome the limitation of conventional fixed charge potential methods for the study of Li-ion battery cathode materials, a dynamic charge potential method, charge-transfer modified embedded atom method (CT-MEAM), has been developed and applied to the Li-Co-O ternary system. The accuracy of the potential has been tested and validated by reproducing a variety of structural and electrochemical properties of LiCoO2. A detailed analysis on the local charge distribution confirmed the capability of this potential for dynamic charge modeling. The transferability of the potential is also demonstrated by its reliability in describing Li-rich Li2CoO2 and Li-deficient LiCo2O4 compounds, including their phase stability, equilibrium volume, charge states and cathode voltages. These results demonstrate that the CT-MEAM dynamic charge potential could help to overcome the challenge of modeling complex ternary transition metal oxides. This work can promote molecular dynamics studies of Li ion cathode materials and other important transition metal oxides systems that involve complex electrochemical and catalytic reactions.

Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy.

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.

Carbonation of wollastonite(001) competing hydration: microscopic insights from ion spectroscopy and density functional theory.

In this paper, we report about the influence of the chemical potential of water on the carbonation reaction of wollastonite (CaSiO3) as a model surface of cement and concrete. Total energy calculations based on density functional theory combined with kinetic barrier predictions based on nudge elastic band method show that the exposure of the water-free wollastonite surface to CO2 results in a barrier-less carbonation. CO2 reacts with the surface oxygen and forms carbonate (CO3(2-)) complexes together with a major reconstruction of the surface. The reaction comes to a standstill after one carbonate monolayer has been formed. In case one water monolayer is covering the wollastonite surface, the carbonation is no more barrier-less, yet ending in a localized monolayer. Covered with multilayers of water, the thermodynamic ground state of the wollastonite completely changes due to a metal-proton exchange reaction (also called early stage hydration) and Ca(2+) ions are partially removed from solid phase into the H2O/wollastonite interface. Mobile Ca(2+) reacts again with CO2 and forms carbonate complexes, ending in a delocalized layer. By means of high-resolution time-of-flight secondary-ion mass spectrometry images, we confirm that hydration can lead to a partially delocalization of Ca(2+) ions on wollastonite surfaces. Finally, we evaluate the impact of our model surface results by the meaning of low-energy ion-scattering spectroscopy combined with careful discussion about the competing reactions of carbonation vs hydration.