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Arrays of electrical devices with each comprising multiple single-walled carbon nanotubes (SWNT) bridging metal electrodes are obtained by chemical vapor deposition (CVD) of nanotubes across prefabricated electrode arrays. The ensemble of nanotubes in such a device collectively exhibits large electrical conductance changes under electrostatic gating, owing to the high percentage of semiconducting nanotubes. This leads to the fabrication of large arrays of low-noise electrical nanotube sensors with 100% yield for detecting gas molecules. Polymer functionalization is used to impart high sensitivity and selectivity to the sensors. Polyethyleneimine coating affords n-type nanotube devices capable of detecting NO2 at less than 1 ppb (parts-per-billion) concentrations while being insensitive to NH3. Coating Nafion (a polymeric perfluorinated sulfonic acid ionomer) on nanotubes blocks NO2 and allows for selective sensing of NH3. Multiplex functionalization of a nanotube sensor array is carried out by microspotti...

In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe–Mn based P2-type Na0.67Cu0.125Fe0.375Mn0.5O2 cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

Fluorine doping of a compositionally graded cathode, with an average concentration of Li[Ni0.80Co0.05Mn0.15]O2, yields a high discharge capacity of 216 mA h g−1 with unprecedented cycling stability by retaining 78% of its initial capacity after 8000 cycles. The cathode is cycled at 100% depth of discharge (DOD), unlike the currently deployed layered cathode whose DOD is limited to 60–80% to compensate for capacity fading and guarantee the required battery life. Additionally, the capacity and cycling stability of the cathode easily surpass those of the existing state-of-the-art batteries, while achieving the energy density goal of 800 W h kg−1cathode for electric vehicles (EV) with ultra-long cycle life. The structural and chemical stabilities of the cathode were provided by the compositional partitioning and unique microstructure of the compositionally graded cathode combined with the ordered site-intermixing of Li and transition metal (TM) ions discovered via transmission electron microscopy. F doping induced the formation of a 2ahex × 2ahex × chex superlattice from ordered Li occupation in TM slabs and vice versa, which has been proven to be essential for suppressing microcrack formation in deeply charged states, while maintaining the structural stability of the cathode during extended cycling. Furthermore, the proposed cathode allows for the recycling of used EV batteries in energy storage systems, thereby alleviating the negative environmental impact by reducing the CO2 emissions and cost associated with disposing of dead batteries.

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3 Co1/3 Mn1/3 O2 . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Appling density functional theory, we have theoretically predicted the electronic and structural properties of a series of novel nanoporous two-dimensional (2D) metal chalcogenophosphates of SnP2S6, SnP2Se6, SnP2Te6, GeP2S6, GeP2Se6, GeP2Te6, PbP2S6, which show high electron mobilities. Through systematically analyzing their cleavage energies and phonon spectra, it is found that MP2X6 monolayers are stable and easily exfoliated from their bulks. In terms of electronic structures, the band gaps of monolayer MP2X6 vary from 0.24 eV for GeP2Te6 to 2.4 eV for SnP2S6. Among them, GeP2X6 and PbP2S6 are direct bandgap semiconductors. Significantly, the monolayer MP2X6 possesses higher electron mobility than that of other currently popular two-dimensional materials, such as MoS2, BN, BC2N and hydrogenated graphene, indicating that monolayer MP2X6 could be applied in fast speed nano-electronic devices fields. To further provide the references for experiments and practical devices applications, the band alignments and the strain engineering effects are also examined.

A molecularly thin electrolyte is developed to demonstrate a nonvolatile, solid-state, one-transistor (1T) memory based on an electric-double-layer (EDL) gated WSe2 field-effect transistor (FET). T...

In this study, newly designed cathode material LiFeP1-xSixO4, with silicon mixed in LiFePO4 is investigated using the density functional theory. Its most optimized structure is the olivine structure of the Pnma space group. Bonding length show the anti-site defect which hinders Li diffusivity is prevented in the LiFeP1-xSixO4. Lithium migration energy barriers in the (010) path of LiFeP1-xSixO4 (x = 0, 0.5, and 1) are calculated by using nudged elastic band calculations, and the average values are determined as 0.180, 0.245, and 0.280 eV for LiFePO4, LiFeP0.5Si0.5O4, and LiFeSiO4, respectively. This signifies that the Li ionic diffusivity is degraded thermodynamically, which is contrary to that indicates by the calculated bonding length, however, the difference is negligibly small. Furthermore, the intercalation voltage increases up to 4.97 V, depending on the Si ratio to P, and is much higher than that of the pristine cathode materials LiFePO4 (∼3.47 V) enabling voltage optimization by Si substitution. The energy density is proportional to the intercalation voltage, hence the energy density is increased, respectively. Finally, the Total density of states show that the electronic conductivity of LiFeP1-xSixO4 (x = 0–1) is better than that of LiFePO4.

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe2) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH4)2S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe2 surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe2 valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe2 by (NH4)2S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe2 surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH4)2S(aq) chemical treatment, the p-branch ON-currents (ION) of back-gated few-layer ambipolar WSe2 FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe2/contact metal interface. This (NH4)2S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides.

A quantum confined transport based on a zinc oxide composite nanolayer that has conducting states with mobility edge quantization is proposed and was applied to develop multi-value logic transistors with stable intermediate states. A composite nanolayer with zinc oxide quantum dots embedded in amorphous zinc oxide domains generated quantized conducting states at the mobility edge, which we refer to as “mobility edge quantization”. The unique quantized conducting state effectively restricted the occupied number of carriers due to its low density of states, which enable current saturation. Multi-value logic transistors were realized by applying a hybrid superlattice consisting of zinc oxide composite nanolayers and organic barriers as channels in the transistor. The superlattice channels produced multiple states due to current saturation of the quantized conducting state in the composite nanolayers. Our multi-value transistors exhibited excellent performance characteristics, stable and reliable operation with no current fluctuation, and adjustable multi-level states., Designing multi-value logic transistors with stable and reliable intermediate states remains a challenge. Here, the authors report the mobility edge quantization phenomenon via resonant hybridization of ZnO QDs embedded in amorphous ZnO domains to enable adjustable multi-value intermediate states.

By combining low energy ion scattering spectroscopy and density functional theory calculation, we study the surface composition and surface formation energy of AMn2O5 (A = Sm, Bi) mullite-type oxides synthesized by different methods and their effects on NO catalytic performance. It is well-known that hydrothermal (HT) synthesis at low temperatures produces materials with higher specific surface areas (SSAs) compared with those synthesized by coprecipitation (CP) and high-temperature calcination; however, it is less clear how synthesis methods affect surface chemistry. We find that the NO oxidation performance of SmMn2O5–HT does not match the SSA increase when compared to the lower SSA SmMn2O5–CP. Combined experimental and theoretical investigation reveals that SmMn2O5–HT includes a higher fraction of inactive Sm-terminated surfaces, which explains its lower than expected activity. However, the surface chemistry change depends strongly on the A-site element. The exposed surfaces of BiMn2O5–CP are predomina...

Endurance is a key material performance metric for device and memory applications and is diminished due to defect formation. Defects alter material properties and relative phase stability, adversely affecting the reliability of devices. Ferroelectric field-effect transistors (FEFETs) have metal-insulator-semiconductor (MIS where I=ferroelectric) gate stacks which typically show unsatisfactory endurance of about 10 5 cycles, [1] while metal-insulator-metal (MIM where I=ferroelectric) capacitors can demonstrate endurance up to 10 12 . Adding a linear dielectric (DE) interlayer at the FE-semiconductor interface may enhance the endurance by suppressing defect formation due to oxygen vacancies, [1] but the electrostatic role of the DE interlayer has not been clearly examined yet. Here, DFT is employed to develop an atomic and electronic level understanding of the behavior of atoms in MIM and MIS stack models with and without DE interlayers due to polarization switching.

Hybrid organic-inorganic lead halide perovskites have attracted much attention in the field of optoelectronic devices because of their desirable properties such as high crystallinity, smooth morphology, and well-oriented grains. Recently, it was shown that thermal nanoimprint lithography (NIL) is an effective method not only to directly pattern but also to improve the morphology, crystallinity, and crystallographic orientations of annealed perovskite films. However, the underlining mechanisms behind the positive effects of NIL on perovskite material properties have not been understood. In this work, we study the kinetics of perovskite grain growth with surface energy calculations by first-principles density functional theory (DFT) and reveal that the surface energy-driven preferential grain growth during NIL, which involves multiplex processes of restricted grain growth in the surface-normal direction, abnormal grain growth, crystallographic reorientation, and grain boundary migration, is the enabler of the material quality enhancement. Moreover, we develop an optimized NIL process and prove its effectiveness by employing it in a perovskite light-emitting electrochemical cell (PeLEC) architecture, in which we observe a fourfold enhancement of maximum current efficiency and twofold enhancement of luminance compared to a PeLEC without NIL, reaching a maximum current efficiency of 0.07598 cd/A at 3.5 V and luminance of 1084 cd/m

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment ($\theta=0^\circ$), associated with flat bands in the Brillouin zone of the moir\'e pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS$_2$/WSe$_2$ system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moir\'e unit cell of a $\theta=0^\circ$ MoS$_2$/WSe$_2$ heterobilayer, and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data. We further explore the role of twist-angle by showing experimental data for a twisted MoS$_2$/WSe$_2$ heterobilayer structure with twist angle of $\theta=15^\circ$, that exhibits a moir\'e pattern but no confinement., Comment: 13 pages, 6 figures in main text. Supporting information included with 13 pages, 8 figures, and 2 tables

In this work, we demonstrate enhancement-mode field-effect transistors by atomic-layer-deposited (ALD) amorphous In2O3 channel with thickness down to 0.7 nm. Thickness is found to be critical on the materials and electron transport of In2O3. Controllable thickness of In2O3 at atomic scale enables the design of sufficient 2D carrier density in the In2O3 channel integrated with the conventional dielectric. The threshold voltage and channel carrier density are found to be considerably tuned by channel thickness. Such phenomenon is understood by the trap neutral level (TNL) model where the Fermi-level tends to align deeply inside the conduction band of In2O3 and can be modulated to the bandgap in atomic layer thin In2O3 due to quantum confinement effect, which is confirmed by density function theory (DFT) calculation. The demonstration of enhancement-mode amorphous In2O3 transistors suggests In2O3 is a competitive channel material for back-end-of-line (BEOL) compatible transistors and monolithic 3D integration applications., Comment: 21 pages, 8 figures

International audience; The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.

Among the candidates to replace Li-ion batteries, Li–S cells are an attractive option as their energy density is about five times higher (~2,600 Wh kg−1). The success of Li–S cells depends in large part on the utilization of metallic Li as anode material. Metallic lithium, however, is prone to grow parasitic dendrites and is highly reactive to several electrolytes; moreover, Li–S cells with metallic Li are also susceptible to polysulfides dissolution. Here, we show that ~10-nm-thick two-dimensional (2D) MoS2 can act as a protective layer for Li-metal anodes, greatly improving the performances of Li–S batteries. In particular, we observe stable Li electrodeposition and the suppression of dendrite nucleation sites. The deposition and dissolution process of a symmetric MoS2-coated Li-metal cell operates at a current density of 10 mA cm−2 with low voltage hysteresis and a threefold improvement in cycle life compared with using bare Li-metal. In a Li–S full-cell configuration, using the MoS2-coated Li as anode and a 3D carbon nanotube–sulfur cathode, we obtain a specific energy density of ~589 Wh kg−1 and a Coulombic efficiency of ~98% for over 1,200 cycles at 0.5 C. Our approach could lead to the realization of high energy density and safe Li-metal-based batteries. An ~10-nm-thick MoS2 layer stabilizes lithium metal anodes and the composite can be used in full-cell Li–S batteries with enhanced performances.

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.

As the next-generation high energy capacity cathode materials for Li-ion batteries, Ni-rich oxides face the problem of obtaining near-stoichiometric phases due to excessive Ni occupying Li sites. These extra-Ni-defects drastically affect the electrochemical performance. Despite of its importance, the fundamental correlation between such defects and the key electrochemical properties is still poorly understood. In this work, using density-functional-theory, we report a comprehensive study on the effects of non-stoichiometric phases on properties of Ni-rich layered oxides. For instance, extra-Ni-defects trigger charge disproportionation reaction within the system, alleviating the Jahn-Teller distortion of Ni3+ ions, which constitutes an important reason for their low formation energies. Kinetic studies of these defects reveal their immobile nature, creating a “pillar effect” that increases the structural stability. Ab initio molecular dynamics revealed Li depletion regions surrounding extra-Ni-defects, which are ultimate responsible for the arduous Li diffusion and re-intercalation, resulting in poor rate performance and initial capacity loss. Finally, the method with combination of high valence cation doping and ion-exchange synthesis is regarded as the most promising way to obtain stoichiometric oxides. Overall, this work not only deepens our understanding of non-stoichiometric Ni-rich layered oxides, but also enables further optimizations of high energy density cathode materials.

Anionic redox reactions (O2−/O−), an alternative to conventional cationic redox reactions (Mn+/M(n+1)+; M: transition metal), have recently been identified as essential to achieve high energy density cathodes for sodium-ion batteries (SIBs). To overcome the drawbacks of anionic redox reactions leading to phase change and separation in the newly discovered Na(Li1/3Mn2/3)O2 material (NLMO, ∼4.2 V vs. Na/Na+ with a high charge capacity of 190 mAh g−1), we have rationally designed high energy density Na(Li1/3Mn1/2Cr1/6)O2 (NLMCO) in which the Cr 3d-electron is coupled with the labile O 2p-electron coordinated with Mn4+ for charge compensation during desodiation processes. NLMCO exhibits reduced phase change and separation, and chemomechanical strain and stress compared to NLMO and is thus expected to show high electrochemical performance, where the formation of short O–O bonds is not observed. By correlating the thermodynamic energy behavior with the redox mechanism in NLMO, it is concluded that our systematically designed cation–anion-coupled NLMCO is an excellent cathode material, introducing advanced materials of formula Na(Li1/3M2/3(1−y)Mcy)O2 (M and Mc: transition metals with stabilized M4+ species and cationic redox active Mc4+ species) for next-generation SIBs.

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. In this work, multi-scale electron microscopy/spectroscopy techniques and theoretical calculations are applied on both polycrystalline and single-crystal NLOs. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane, while surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. A new chain stress corrosion mechanism from anisotropic elastic (001) tensile deformation, microcrack generation, nonpolar surface reconstruction, HF attack to metal dissolution is proposed to paint the full picture of the structural degradation of NLOs. This surface-dependent stress-corrosion coupling effect indicates that severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. The surface-dependent stress-corrosion cracking in both polycrystalline and single-crystal NLOs provides grain-boundary engineering clues on designing new cathode materials with high energy density and long cycle life.

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.

Based on first-principles calculations, the H 2 adsorptions onto six types of modified graphene substrates decorated with light metals (Li, Na, K, Be, Mg, Ca) are investigated to shed light on the factors affecting the H 2 binding energies. It is demonstrated that the introduction of defects and dopants into graphene substrates is essential to prevent the metal clustering and achieve dispersed metal atoms desirable for H 2 adsorption. The interaction between H 2 and alkali/alkali-earth metal decorated graphene systems is attributed to the electrostatic effect induced by polarized dipole–-dipole interaction. Via introducing defects and hetero-atoms to modify the electronegativity of the local structure, the H 2 adsorption energy can be tuned by choosing the combination of suitable metals and substrates. The calculated H 2 binding strength is positively correlated to the charge transfer from the metal to the substrates and the dipole momentum of metal decorated substrates. Compared the cases with different metals decoration, Mg and Ca are expected to the most promising candidates for multiple H 2 adsorptions.

In this work, we perform a comprehensive study of five phenomena of LiNi1-2xCoxMnxO2 (NCM) (x = 0–1/3) cathodes at the end of charge (phase reaction, crack propagation, Li-Ni exchange, phase transition, and oxygen evolution), using first-principle calculations within the DFT + U framework. Based on our results, we have located the obstacles toward unity efficiency and revealed that the degradation strongly depends on the Ni concentration and the depth of charge. The threshold capacities for degradation of LiyNi1-2xCoxMnxO2 are 130–140 mA·hg−1 (y

Conventional interatomic potential methods for Li-ion battery cathode materials normally use fixed-charge models, which are not accurate enough to model the dynamical oxidation state change of transition metals during electrochemical reactions. In order to enable more accurate large-scale simulations for battery cathode materials, here we report a semiempirical interatomic potential of the Li-Ni-O ternary system based on an advanced dynamic charge method: Charge-Transfer Modified Embedded-Atom Method (CT-MEAM). The potential is parameterized by fitting the atomic Bader charges and energy-strain curves of the LiNiO 2 cathode material under uniaxial, biaxial and hydrostatic strains to results obtained with ab initio density-functional theory (DFT) calculations. A variety of structural, electrochemical and dynamical properties derived from the fitted CT-MEAM potential are observed to be in excellent agreement with previous DFT and experimental data. The transferability of the potential is validated by comparing relative phase stabilities and charge distribution states within the ternary Li-Ni-O systems. Our results support the capability of the present CT-MEAM to model complex ternary transition metal oxides. This method will facilitate not only the optimal design of Lithium-ion battery cathode materials, but also other transition metal oxide-based applications involving electrochemical reactions.

The power characteristics of lithium-ion batteries (LIBs) are crucial for the advent of commercialized, high-power applications, such as electric vehicles. Through both first-principles multiscale simulations and experiments, here, we present fundamental understanding on the power characteristics of the high-voltage spinel cathode correlated with its elastic softness and phase transformation in nanodomains for high-power LIBs. Atomic models of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5−xTixO4 are developed for multiscale phase field modeling based on structural information for the as-prepared nanopowders. The combined computational and experimental investigations suggest that the thermodynamic phase stability of LiNi0.5Mn1.5O4 can be effectively enhanced by the incorporation of Ti into the structure without any change to the redox mechanism. Ti incorporation provides a faster ionic mobility and the improved phase stability because of the reinforced Ti4+–O bonds. Based on the multiscale phase transformation kinetics, LiNi0.5Mn1.5−xTixO4 exhibits an enhanced elastic softness and slower phase separation than LiNi0.5Mn1.5O4 in the nanodomain during Li+ insertion and extraction. Such characteristics are mainly responsible for the improved electrochemical performance at higher current rates, as confirmed by electrochemical experiments. This fundamental understanding of the power characteristics with respect to the correlations with elastic softness and phase transformation will provide a guideline to develop and design advanced materials for high-power LIBs.

Atomic substitution and doping are two of the most adopted strategies to improve the electrochemical performance of layered cathode materials for Li-ion batteries (LIBs). In this work, we report a comprehensive study on the effects of seven dopants (Al, Ga, Mg, Si, Ti, V, and Zr) on the well-known drawbacks of Ni-rich LiNi1−2yCoyMnyO2 (NCM) (y ≤ 0.1), one of the most promising next-generation cathode materials for LIBs, including phase instability, Li–Ni exchange, Ni segregation, lattice distortion, and oxygen evolution. Our results show that there is not a single dopant that can solve all the problems at the same time and, moreover, while they often improve certain properties, they may have no effect or even worsen others. By comparing different doping sites, we found a strong site preference due to the tradeoff between Mn and Co concentrations. This site preference indicates that a multicomponent-doping strategy should be adopted at both Mn and Co sites. Finally, a rationale for the optimization of the overall electrochemical performance of Ni-rich NCM is proposed, which will ultimately provide practical guidance (Ti or Zr at the Co site and Al at the Mn site) for the design of new Ni-rich layered cathode materials for LIBs.

Bio-inspired non-precious-metal catalysts based on iron and cobalt porphyrins are promising alternatives to replace costly platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. However, the exact nature of the active sites is still not clearly understood, and further optimization design is needed for practical applications. Here, we report a rational catalyst design process by combining density functional theory (DFT) calculations and experimental validations. Two sets of square-planar (MNxC4−x) and square-pyramid (MNxC5−x) active centers (M=Mn, Fe, Co, Ni) incorporated in graphene were examined using DFT. Fe-N5 and Co-N4 sites were identified theoretically to have the best performance in fuel cells, while Ni-NxC4−x sites catalyze the most H2O2 byproduct. Graphene samples with well-dispersed incorporations of metals were synthesized, and the following electrochemical measurements show an excellent agreement with the theoretical predictions, indicating that a successful design framework and systematic understanding toward the catalytic nature of these materials are established.

A mixed-phase compound that contains SmMn2O5 mullite-type oxides has been reported to display excellent catalytic activity for nitric oxide (NO) oxidation. Here we investigate the effects of calcination temperature and precipitation pH on structural, physical, chemical, and surface properties of SmMn2O5. As the calcination temperature increases from 750 °C to 1000 °C, mullite phase purity increases from 74% to 100%, while specific surface area (SSA) decreases from 23.6 m2/g to 5.1 m2/g with particle size increases correspondingly. Mullite phase purity (87%) is independent of pH between 8.5–10.4, whereas SSA monotonically increases from 12.5 m2/g at pH 8.1 to 27.4 m2/g at pH 13. X-ray photoelectron spectroscopy (XPS) studies reveal that the surface Mn/Sm ratio is similar to the bulk value and is unaffected by calcination temperature and pH values up to 10.4, whereas sample precipitated at pH 13 is surface-rich in Sm. NO chemisorption studies show that the SSA and surface Mn/Sm ratio determine NO uptake by SmMn2O5 mullite oxides.

Edge-contacted superconductor-graphene-superconductor Josephson junctions have been utilized to realize topological superconductivity, and have shown superconducting signatures in the quantum Hall regime. We perform first-principles calculations to interpret electronic couplings at the superconductor-graphene edge contacts by investigating various aspects in hybridization of molybdenum $d$ orbitals and graphene \ensuremath{\pi} orbitals. We also reveal that interfacial oxygen defects play an important role in determining the doping type of graphene near the interface.

Scanning tunneling microscopy (STM) is an important tool in surface science on atomic scale characterization and manipulation. In this work, Ti adatom manipulation is theoretically simulated by using a tungsten tip (W-tip) in STM based on first-principle calculations. The results demonstrate the possibility of inserting Ti adatoms into the atomic pores of monolayer YBr3, which is thermodynamically stable at room temperature. In this process, the energy barriers of vertical and lateral movements of Ti are 0.38 eV and 0.64 eV, respectively, and the Ti atoms are stably placed within YBr3 by >1.2 eV binding energy. These theoretical predictions provide an insight that it is experimentally promising to manipulate Ti adatom and form artificially designed 2D magnetic materials.

Anion doping is one of the most widely adopted strategies to improve the electrochemical performance of cathode materials for Li-ion batteries. However, undesirable side effects are often observed together with enhanced electrochemical properties, leading to an unsatisfactory overall performance. In order to develop an anion doping strategy which enhances the positive effects and suppresses undesirable side effects, the understanding of their origin at the atomic scale is a crucial step. In this work, using density functional theory (DFT), we report a systematic study on the effects of three common anion dopants (F, S, Cl) on a wide range of properties of a model cathode material, LiNiO2, including redox potential, ionic conductivity, Li/Ni exchange, lattice distortion, and Ni migration upon delithiation. The results show that the dopants improve certain properties but worsen others, revealing some distance-dependent features. Overall, our work shows conflicting roles of anion doping on the battery voltag...

In this work, phase stability of Ni-rich LiNi1–2xCoxMnxO2 (NCM) (x Ni2+Mn4+ > Ni3+Mn4+ > Co3+Mn4+ > Co2+Mn4+ > Ni2+Ni4+) is then predicted by bond model and subsequently used to understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle. Our results also show that Co and Mn ions segregate to form clusters within the Ni environment when x < 0.1 (Ni ≥ 80 at. %), and such segregation is responsible for the electrochemical degradation during cycling. The obtained results agree excellently with the validation experiment in the present work and also other experiments in the ...

In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

To meet the requirement of large-scale simulation technics for Li-ion battery electrode materials, we introduce the charge-transfer modified embedded-atom method (CT-MEAM) in which the complex nature of the chemical bonding in transition metal (TM) oxides is described as a balance between metallic/covalent and ionic contributions by MEAM and a variable-charge model, respectively. The method is applied to Li 2 MnO 3 , and the parameterization is performed through fitting the energy–strain curves of Li 2 MnO 3 under uniaxial, biaxial and hydrostatic strains to a training set from ab initio density-functional theory calculations. The CT-MEAM prediction of the critical physical properties such as charge states and redox potentials match quite well with the ab initio results in various Li–Mn–O compounds beyond Li 2 MnO 3 . The constructed Li–Mn–O phase diagram is also qualitatively consistent with the ab initio reference work. The excellent transferability ensures use of the present method for a wide range of oxidation states in complex ternary TM oxides. Therefore, it will facilitate large-scale atomistic calculations required for the optimal design of many TM oxide applications including lithium-ion battery cathode materials.

Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EG(FH)) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EG(FH) displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EG(PH) forms a Schottky barrier junction.

Quantum Hall effect (QHE) is a macroscopic manifestation of quantized states that only occurs in confined two-dimensional electron gas (2DEG) systems. Experimentally, QHE is hosted in high-mobility 2DEG with large external magnetic field at low temperature. Two-dimensional van der Waals materials, such as graphene and black phosphorus, are considered interesting material systems to study quantum transport because they could unveil unique host material properties due to the easy accessibility of monolayer or few-layer thin films at the 2D quantum limit. For the first time, we report direct observation of QHE in a novel low-dimensional material system, tellurene. High-quality 2D tellurene thin films were acquired from recently reported hydrothermal method with high hole mobility of nearly 3000 cm2/(V s) at low temperatures, which allows the observation of well-developed Shubnikov-de Haas (SdH) oscillations and QHE. A four-fold degeneracy of Landau levels in SdH oscillations and QHE was revealed. Quantum oscillations were investigated under different gate biases, tilted magnetic fields, and various temperatures, and the results manifest the inherent information on the electronic structure of Te. Anomalies in both temperature-dependent oscillation amplitudes and transport characteristics were observed that are ascribed to the interplay between the Zeeman effect and spin-orbit coupling, as depicted by the density functional theory calculations.

Defect engineering of2D semiconducting transition metal dichalcogenides (TMDCs) has been demonstrated to be a promising way to tune both their bandgaps and carrier concentrations. Moreover, controlled introduction of defects in the source/drain access regions of a TMDC FET can boost its performance by decreasing the contact resistance at the metallTMDC interface [1]. While chemical functionalization offers a facile route towards defect engineering in 2D TMDCs, several chemically-treated TMDCs have not been fully understood at the molecular level. In this study, chemical sulfur treatment (ST) utilizing ammonium sulfide [(NH 4 ) 2 S] solution is shown to enhance the p-type behavior in 2D WSe2 via introduction of acceptor defect states near its valence band edge (VBE), with the results verified using detailed scanning tunneling microscopy (STM)/spectroscopy (STS) studies, field-effect transistor (FET) measurements and theoretical density-of-states (DOS) calculations.

Using the Bardeen tunneling method with first-principles wave functions, computations are made of the tunneling current in graphene/hexagonal-boron-nitride/graphene (G/h-BN/G) vertical structures. Detailed comparison with prior experimental results is made, focusing on the magnitude of the achievable tunnel current. With inclusion of the effects of translational and rotational misalignment of the graphene and the h-BN, predicted currents are found to be about 15× larger than experimental values. A reduction in this discrepancy, to a factor of 2.5×, is achieved by utilizing a realistic size for the band gap of the h-BN, hence affecting the exponential decay constant for the tunneling.

Doping is the most common strategy to suppress the intrinsic structural instability of several families of cathode materials, thus improving their electrochemical performance. During the electrode synthesis, the dopants have a low probability to occupy cationic Li sites, but it is well-known that, during the normal operation of the battery, such probability increases via inter- or intralayer diffusion. In this work, we investigate the effect of 10 Li-site cationic dopants (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) on the electrochemical properties of Li2MnO3 and LiMnO2 cathode materials using density functional theory. Our results show that, although Mn sites are thermodynamically favorable over Li-site doping, the small thermodynamic barriers between both configurations can be easily overcome during the material synthesis and/or the extraction/insertion of Li during the cycling process of the battery. Also, due to charge balance and diffusion channel opening, some of the Li-site dopants were found to act as...

Through first-principles calculations and experimental observations, we first present the correlation between the Ni and Mn ratio and the redox behaviors of the layered NCM cathodes. The equilibrium potentials based on redox reactions of Ni2+/Ni3+ are highly dependent on the Mn ratio (NCM523 and NCM721: ∼3.7 and 3.5 V) because of a donor electron, in the eg band, transferred from Mn to Ni owing to their crystal field splitting (CFS) with different electronegativities, leading to oxidation states of Ni2+-like and Mn4+. Considering the electronic donor (Mn) based on CFS with electronegativity of transition metals (TMs), we finally expect V as a promising doping source to provide donor electrons for Ni redox reactions in Ni-rich layered oxides, leading to be higher delithiation potentials (NCV523: 3.8 V). From our theoretical calculations in the NCV oxide, the oxidation states of Ni and V are stable Ni2+-like and V5+, respectively, and the fractional d-band fillings of Ni are the highest value as compared wi...

Adsorption of molecules on graphene is a promising route to achieve novel functionalizations, which can lead to new devices. Density functional theory is used to calculate stabilities, electronic structures, charge transfer, and work function for a crown-4 ether (CE) molecule and a CE–Li (or CE–Li+) complex adsorbed on graphene. For a single CE on graphene, the adsorption distance is large with small adsorption energies, regardless of the relative lateral location of the CE. Because CE interacts weakly with graphene, the charge transfer between the CE and graphene is negligibly small. When Li and Li+ are incorporated, the adsorption energies significantly increase. Simultaneously, an n-type doping of graphene is introduced by a considerable amount of charge transfer in CE–Li adsorbed system. In all of the investigated systems, the linear dispersion of the pz band in graphene at the Dirac point is well-preserved; however, the work function of graphene is effectively modulated in the range of 3.69 to 5.09 e...

Due to a high voltage of 4.6 V and large practical capacity of ~260 mAh/g, the over-lithiated-oxides (OLOs) which are often represented as Li2MnO3·LiMO2 (M = Mn, Co, Ni), have been intensely investigated as a promising candidate of the next generation cathode materials for Li-ion battery. The Li2MnO3 composite structure plays an important role in providing high capacity and phase stability to the system. However, during cathode charge-discharge operation, Li2MnO3 is unstable and partly transforms into LiMnO2, indicating that the phase used in practice has mixed both Li2MnO3 and LiMnO2. In this work, the effects of Li-site doping from 10 cationic dopants (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) on the electrochemical properties of both oxides are studied using density functional theory. The calculations show that, comparing with the Mn-site doped phases, Li-site doping is thermodynamically unstable for the ground states, but the small transition barriers can be easily overcome under high thermal fluctuations during the realistic cathode synthesis process. The redox potentials of both oxides can be lowered by most of the Li-site dopants. For example, Nb strongly lowers the redox potential of the LiMnO2 phase, and Ru shows an unexpected effect on the Li2MnO3 phase: it activates the Li atoms in the Li-layer and, at the same time, it immobilizes the Li atoms in the Li-Mn mixed-layer by increasing the redox potential. These results support the experimental observations about Li-site doping and provide an explanation about the effects of Li-site doping on the electrochemical properties.

ARTICLE INFOArticle history:Received 11 September 2014Accepted 5 December 2014Available online 9 December 2014ABSTRACTAb initio predictions for the stability of different graphene oxide (GO) structures have beenshown to conflict with experimental observations. While ab initio studies predict that themost stable GOs are fully oxygen-covered (either with epoxide or hydroxyl), stable as-produced GOs arepartially oxygen-covered and predominantly epoxide-covered structures.Although this discrepancy is being examined in terms of calculations of free energies ofGOs and large diffusion energy-barriers for oxygen groups on graphene, there is still a lackof understanding on the energetic properties of GOs using classical molecular dynamics,which is able to investigate their structural distortion. Here, using the reactive empiricalbond order (REBO) molecular dynamics potential, we compute the free energy and bindingenergy of GOs at different oxygen concentrations and epoxide to hydroxyl ratios, as well asthe distortion energies of graphene lattice. Although epoxide causes more distortion on thecarbon hexagonal planar structure, it provides more stability to the GO structure. Thedifference between free energy and binding energy of GOs is shown to be independent ofoxygen coverage. These results allow gaining more insight on the issue of GO stabilityand show that REBO can capture most of experimental properties of GOs. 2014 Elsevier Ltd. All rights reserved.

The use of an interfacial passivation layer is one important strategy for achieving a high quality interface between high-k and III-V materials integrated into high-mobility metal-oxide-semiconductor field-effect transistor (MOSFET) devices. Here, we propose gallium nitride (GaN) as the interfacial layer between III-V materials and hafnium oxide (HfO2). Utilizing first-principles calculations, we explore the structural and electronic properties of the GaN/HfO2 interface with respect to the interfacial oxygen contents. In the O-rich condition, an O8 interface (eight oxygen atoms at the interface, corresponding to 100% oxygen concentration) displays the most stability. By reducing the interfacial O concentration from 100 to 25%, we find that the interface formation energy increases; when sublayer oxygen vacancies exist, the interface becomes even less stable compared with O8. The band offset is also observed to be highly dependent on the interfacial oxygen concentration. Further analysis of the electronic structure shows that no interface states are present at the O8 interface. These findings indicate that the O8 interface serves as a promising candidate for high quality III-V MOS devices. Moreover, interfacial states are present when such interfacial oxygen is partially removed. The interface states, leading to Fermi level pinning, originate from unsaturated interfacial Ga atoms.

As a promising hybrid energy storage system, lithium ion capacitors (LICs) have been intensively investigated regarding their practical use in various applications, ranging from portable electronics to grid support. The asymmetric LIC offers high-energy and high-power densities compared with conventional energy storage systems such as electrochemical double-layer capacitors (EDLCs) and lithium ion batteries (LIBs). To enable suitable operation of the LIC, the negative electrode should be pre-lithiated prior to cell operation, which is regarded as a key technology for developing self-sustainable LICs. In this work, we have demonstrated the potential use of Li6CoO4 as an alternative lithium source to metallic lithium. A large amount of Li+ can be electrochemically extracted from the structure incorporated into the positive electrode via a highly irreversible process. Most of the extracted Li+ is available for pre-lithiation of the negative electrode during the first charge. This intriguing electrochemical behaviour of Li6CoO4 is suitable for providing sufficient Li+ to the negative electrode. To obtain a fundamental understanding of this system, the electrochemical behaviour and structural stability of Li6CoO4 is thoroughly investigated by means of electrochemical experiments and theoretical validation based on first principles calculations.

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.

The effects of four typical defects on the hydroxyl groups’ migration and the conductivity of graphene have been studied using density functional theory and nonequilibrium Green’s function formalism. An obvious anisotropy of the diffusion barriers along different paths is correlated to the symmetric behavior of spin-polarized charge density around the defects. The migration energy scenario indicates that the defects effectively hinder the hydroxyl groups’ migration toward them, indicating that most hydroxyl groups could be stabilized outside the defect region in reduced graphene oxide. Through the electronic transport calculations and local density of states analysis, hydroxyl groups locating outside of the defect region will cause the transport channels near the Fermi level to disappear and reduce the conductance considerably.

Good cyclability is essential for the potential application of cathode materials. Here, we investigate the structural stability of two-dimensional (2D) Li-layered and three-dimensional (3D) structured polymorphs of Li2FeSiO4 and Li2MnSiO4 using the density functional theory calculations. We find that all 2D Li-layered polymorphs of both materials are unstable upon full delithiation owing to layer exfoliation, which can lead to an amorphous structure. However, in contrast to the fact that the amorphization of Li2FeSiO4 can be prevented by the formation of the 3D cycled structure that is energetically stable, the 3D cycled structure of Li2MnSiO4 is found to be unstable during delithiationlithiation cycling. As a result, Li2MnSiO4 easily undergoes amorphization and shows a poor cyclability.