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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...

The advancement of ion transport applications will require the development of functional materials with a high ionic conductivity that is stable, scalable, and micro-patternable. We report unusually high ionic conductivity of Li+, Na+, and K+ in 2D MoS2 nanofilm exceeding 1 S/cm, which is more than 2 orders of magnitude higher when compared to that of conventional solid ionic materials. The high ion conductivity of different cations can be explained by the mitigated activation energy via percolative ion channels in 2H-MoS2, including the 1D ion channel at the grain boundary, as confirmed by modeling and analysis. We obtain field-effect modulation of ion transport with a high on/off ratio. The ion channel is large-scale patternable by conventional lithography, and the thickness can be tuned down to a single atomic layer. The findings yield insight into the ion transport mechanism of van der Waals solid materials and guide the development of future ionic devices owing to the facile and scalable device fabrication with superionic conductivity.

Graphite is currently utilized as anode materials for Li-ion batteries, but it is well-known that graphite does not show good electrochemical performances as the anode material for sodium-ion batteries (SIBs). It was also reported that the low electrochemical performances of graphite originated from the larger ionic radius of the sodium ion due to the required higher strain energy for sodium-ion intercalation into graphite leading to an unstable sodium-ion intercalated graphite intercalation compound (GIC). In this work, using first-principles calculations, we introduce pillaring effects of Na n X (n = 3 and 4; X = F, Cl, or Br) halide clusters in GICs, which become electrochemically active for Na redox reactions. Specifically, to enable sodium-ion intercalation into graphite, the interlayer spacing of graphite is required to increase over 3.9 A, and Na n X halide cluster GICs maintain an expanded interlayer spacing of >3.9 A. This enlarged interlayer spacing of Na n X halide cluster GICs facilitates stable intercalation of sodium ions. Na3F, Na4Cl, and Na4Br halide clusters are identified as suitable pillar candidates for anode materials because they not only expand the interlayer spacing but also provide reasonable binding energy for intercalated sodium ions for reversible deintercalation. Based on the model analysis, theoretical capacities of Na3F, Na4Cl, and Na4Br halide cluster GICs are estimated respectively to be 186, 155, and 155 mA h g-1. These predictions would provide a rational strategy guiding the search for promising anode materials for SIBs.

Pump it up Carbon nanotube yarns can be used as electrochemical actuators because infiltration with ions causes a contraction in length and an expansion in diameter. Either positive or negative ions can cause this effect. Chu et al. constructed an all-solid-state muscle that eliminated the need for an electrolyte bath, which may expand the potential for its use in applications. By infiltrating the yarns with charged polymers, the fibers start partially swollen, so the length can increase through the loss of ions. It is thus possible to increase the overall stroke of the muscle. Further, these composite materials show a surprising increase in stroke with scan rate. Science , this issue p. 494

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.

Reducing sulfur poisoning is significant for maintaining the catalytic efficiency and durability of heterogeneous catalysts. We screened PdAu nanoclusters with specific Pd : Au ratios based on Monte Carlo simulations and then carried out density functional calculations to reveal how to reduce sulfur poisoning via alloying. Among various nanoclusters, the core-shell structure Pd13Au42 (Pd@Au) exhibits a low adsorption energy of SO2 (-0.67 eV), comparable with O2 (-0.45 eV) and lower than CO (-1.25 eV), thus avoiding sulfur poisoning during the CO catalytic oxidation. Fundamentally, the weak adsorption of SO2 originates from the negative d-band center of the shell and delocalized charge distribution near the Fermi level, due to the appropriate charge transfer from the core to shell. Core-shell nanoclusters with a different core (Ni, Cu, Ag, Pt) and a Pd@Au slab model were further constructed to validate and extend the results. These findings provide insights into designing core-shell catalysts to suppress sulfur poisoning while optimizing catalytic behaviors.

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.

Density functional theory (DFT) is employed to investigate ferroelectric (FE) hafnium-zirconium oxide stack models for both metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. The role of dielectric (DE) interlayers at the ferroelectric interfaces with metals and semiconductors and the effects of thickness scaling of FE and DE layers were investigated using atomic stack models. A high internal field is induced in the FE and DE layers by the FE polarization field which can promote defect generation leading to limited endurance. It is also shown that device operation will be adversely affected by too thick DE interlayers due to high operating voltage. These DFT models elucidate the underlying mechanisms of the lower endurance in experimental MIS devices compared to MIM devices and provide insights into the fundamental mechanisms at the interfaces.

The role of catalysts in aprotic Li–O2 batteries remains unclear. To identify the exact catalytic nature of oxide catalysts, a precisely surface-engineered model catalyst, perovskite (LaMnO3), was ...

Degradation in CH3NH3PbI3 (MAPbI3), when in contact with commonly used metal oxide transport layer materials in optoelectronic devices, is examined experimentally and theoretically. On the basis of...

Exploring ultrathin two-dimensional (2D) solid electrolytes with fast ion transport is highly desirable in nanoelectronics, ionic devices and various energy storage systems, following the rapid scaling of devices to the nanometer scale. Herein, two-dimensional (2D) metal trihalides MX3 (ScCl3, ScBr3, AsI3, ScI3, YBr3, SbI3, YI3 and BiI3) with intrinsic atomic pore structures have been examined and found to be promising as realistic 2D solid electrolytes. Through examining the binding interactions and the diffusion barriers of monolayer MX3-ion (Li+, Na+, K+, Mg2+, and Ca2+) systems by utilizing first principles calculations, it is found that MX3-ion complexes are energetically favorable and the energy barriers of some MX3-ion systems are comparable to or even smaller than those of the conventional solid electrolyte systems. More significantly, the short diffusion time of Na+ and K+ ions in some monolayers MX3 at the nanosecond (ns) or even at the sub-ns scale indicates fast ion transport. In terms of practical applications, ultrafast Li+ travelling in the timescale of sub-ns to ns and Na+ in several tens ns in few-layer MX3 is achieved. In addition, the insulating nature of wide band gaps for MX3 is maintained during the ion transport, which is essential for solid electrolytes. These theoretical results provide fundamental guidance that MX3 materials with natural atomic pores are realistic candidates for 2D solid electrolytes with broad applications in ionic devices and energy storage devices.

A two-dimensional (2D) WOx/ZnO stack reveals a unique carrier transport behavior, which can be utilized as a novel device element to achieve a very high on/off ratio (>106) and an off current density lower than 1 nA cm−2. These unique behaviors are explained by a dynamic band alignment between WOx and ZnO, which can be actively modulated by a gate bias. The performance of FET utilizing the WOx/ZnO stack is comparable to those of other 2D heterojunction devices; however, it has a unique benefit in terms of process integration because of very low temperature process capability (T < 110 °C). The high on/off switching with extremely low off current density utilizing the dynamic band alignment modulation at the WOx/ZnO stack can be a very useful element for future device applications, especially in monolithic 3D integration or flexible electronics.

Recently, spintronics devices using MoS2 and ferromagnetic electrode have been in the spotlight. However, strong Fermi level pinning (FLP) is known to occur between MoS2 and ferromagnetic electrode, resulting in a large Schottky barrier height (SBH), which makes it difficult to inject electron from ferromagnetic electrode to semiconductor. To resolve this problem, we study the reduction of FLP occurring between two materials by investigating the effect of atomic passivation at Ni-MoS2 interfaces on contact behaviors. Such atomic passivation can reduce the FLP and magnetic moments induced at S atoms of MoS2. Especially, the largest decrease in the FLP occurs in the case of H atom passivation. Besides, the N, O, and F atom passivation confirms the possibility of ohmic contact, indicated from small SBHs (~0.2 eV). As a result, the SBH and thus the efficiency of the device can be controlled by atomic passivation at metal-semiconductor interfaces.

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.

Building supercapacitors that can provide high energy density over a wide range of temperatures, where traditional energy storage devices fail to operate, requires tailoring of electrolyte and/or electrode material. Here, we show that record gravimetric capacitances of 164 and 182 F g−1 can be attained at −100 and 60 °C, respectively, nearly equivalent to the room-temperature value of 177 F g−1, when activated carbon-based electrodes with predominantly slit-shaped micropores and a low freezing-point electrolyte are used. Experimental data and density functional theory calculations suggest that electrode material characteristics, such as pore size and shape, matched with the effective size of partially solvated ions of the electrolyte, are the key factors in achieving such performance. This study provides evidence for the effective design of robust supercapacitors with sustained performance at both low and high temperatures.

The electronic transport properties of amorphous In$_{2} \mathrm{O}_{3}(\mathrm{a} - In _{2} \mathrm{O}_{3})$ are investigated from first principles simulations for BEOL transistor application. It is determined that local atomic and electronic structure disorders in a-In 2 O 3 are the fundamental origin of reduced mobility in amorphous phase. Medium range order In-In connectivity is responsible for the electron conduction pathway. It is found that amorphous disorder present in a-In 2 O 3 could induce shallow donor states and acceptor states that are responsible for the device threshold voltage instability. Nonstoichiometric defects including indium vacancy and interstitial will further increase the density of these defect states intrinsic to a-In 2 O 3 . The results could provide a better understanding of the electronic transport behavior in a-In 2 O 3 and useful insights for future defect controlling for better device performance.

The emergent magnetic two-dimensional (2D) materials provide ideal solid-state platforms for a broad range of applications including miniaturized spintronics, nonreciprocal optics, and magnetoelectric sensors. Owing to the general environmental sensitivity of 2D magnets, the understanding of ambient effects on 2D magnetism is critical. Apparently, the nature of itinerant ferromagnetism potentially makes metallic 2D magnets insensitive to environmental disturbance. Nevertheless, our systematic study showed that the Curie temperature of metallic 2D Fe3GeTe2 decreases dramatically in the air but thick Fe3GeTe2 exhibits self-protection. Remarkably, we found the air exposure effectively promotes the formation of multiple magnetic domains in 2D Fe3GeTe2, but not in bulk Fe3GeTe2. Our first-principles calculations support the scenario that substrate-induced roughness and tellurium vacancies boost the interaction of 2D Fe3GeTe2 with the air. Our elucidation of the thickness-dependent air-catalyzed evolution of Curie temperatures and magnetic domains in 2D magnets provides critical insights for chemically decorating and manipulating 2D magnets.

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...

Hafnium zirconium oxide films have been utilized in negative capacitance (NC) field-effect transistors (FETs). The band alignment of semiconductor and HfZrOx film is critical to obtain high device performance. The band alignment of Hf0.5Zr0.5O2/SiOx/Ge system before and after post deposition annealing at 500 °C is studied via angle resolved X-ray photoelectron spectroscopy, synchrotron radiation photoemission spectroscopy and UV–Visible spectroscopy. The band gap of Hf0.5Zr0.5O2 is seen narrowed 0.27 ± 0.05 eV, and the valence band offset between Hf0.5Zr0.5O2 and Ge decreases 0.25 eV ± 0.05 eV after PDA at 500 °C. Therefore, the conduction band offset is nearly unchanged. This work gives insights into the interface physics about Hf0.5Zr0.5O2/SiOx and is valuable for Ge-based NC pFETs.

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.

By combining experimental and theoretical approaches, we investigate the quantitative relationship between molecular desorption temperature and binding energy on d and f metal oxide surfaces. We demonstrate how temperature-programmed desorption can be used to quantitatively correlate the theoretical surface chemistry of metal oxides (via on-site Hubbard U correction) to gas surface interactions for catalytic reactions. For this purpose, both CO and NO oxidation mechanisms are studied in a step-by-step reaction process for perovskite and mullite-type oxides, respectively. Additionally, we show solutions for over-binding issues found in COx, NOx, SOx, and other covalently bonded molecules that must be considered during surface reaction modeling. This work shows the high reliability of using TPD and density functional theory in conjunction to create accurate surface chemistry information for a variety of correlated metal oxide materials.

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.

Atomic layer deposition is a chemical deposition technology that provides ultimate control over the conformality of films and their thickness, even down to Angstrom-scale precision. Based on the marked superficial character and gas phase process of the technique, metal sources and their ligands shall ideally be highly volatile. However, in numerous cases those ligands corrode the substrate or compete for adsorption sites, well-known as side reactions of these processes. Therefore, the ability to control such side reactions might be of great interest, since it could achieve synchronous coating and alteration of a substrate in one process, saving time and energy otherwise needed for a post-treatment of the sample. Consequently, advances in this way must require understanding and control of the chemical processes that occur during the coating. In this work, we show how choosing an appropriate ligand of the metal source can unveil a novel approach to concertedly coat and reduce γ-Fe2O3 nanoparticles to form a final product composed of Fe3O4/TiO2 core/shell nanoparticles. To this aim, we envisage that appropriate design of precursors and selection of substrates will pave the way for numerous new compositions, while the ALD process itself allows for easy upscaling to large amounts of coated and reduced particles for industrial use.

Quasi-two-dimensional 1T-MoS2 is a promising pseudocapacitance (Credox) electrode material due to its large specific surface area, superior electrical conductivity and mechanical stability. However, the microscopic mechanism of Credox and its further manipulation via modulating the structures and electronic structures are still unclear. Thus, the Credox of monolayer 1T-MoS2 has been explored based on first-principles calculations. For monolayer 1T-MoS2 adsorbed by H+ ions on one side or both sides, a band gap opens, decreases and even disappears with the coverage increase in H+ ions up to 100%. In this process, the charge transfer from the monolayer 1T-MoS2 to the adsorbed H+ ions almost linearly increases with the coverage increase in the H+ ions. In contrast, the potential change rate of the monolayer 1T-MoS2 reduces, resulting in the enhancement of Credox. Herein, the maximum values of Credox reached ∼76.7 μF cm−2 (252.8 F g−1) and ∼213.7 μF cm−2 (704.5 F g−1) for the 100% coverage of H+ ions on one side and both sides, respectively. Furthermore, the manipulation of Credox in the monolayer 1T-MoS2 could be realized through intrinsic defects engineering. Particularly, the Credox was greatly improved in the system with S vacancies. These results would provide significantly fundamental insights for understanding the correlation between the Credox and the electronic structures of 1T-MoS2 and other similar quasi-two dimensional materials.

By studying their surface chemistry, metal–oxygen bond strength, and critical energy barrier heights, we elucidate the differences in the NO oxidation catalytic performance of PrMn2O5 and SmMn2O5 mullite-type oxides. The 50% conversion temperature is lower (230 °C vs. 275 °C) and the maximum conversion efficiency is higher (81% at 282 °C vs. 68% at 314 °C) for PrMn2O5 compared to SmMn2O5, despite having a ∼15% lower specific surface area. Furthermore, PrMn2O5 exhibits higher maximum efficiency compared to Pt/Al2O3. Combined experimental and theoretical findings indicate that the superior catalytic performance of PrMn2O5 at low temperatures arises from the presence of more labile and reactive surface lattice oxygen due to weaker Mn–O bond strength and lower thermal stability of surface NOx ad-species.

The interfacial structure of metal-oxide composite catalysts plays a vital role in heterogeneous catalysis, which is crucial to the adsorption and activation of reactants. Herein, the interfacial effects of bare and Fe/Co/Ni doped SmMn2O5 mullite oxide supported Pt clusters on CO oxidation have been investigated by first-principles based microkinetics analysis. A robust formation of Pt/Mn2 trimer structures is demonstrated at the bifunctional interfaces irrespective of the Ptn cluster's size, which can provide spatially separated sites for CO adsorption and O2 dissociation. The binding strength of CO at the interfacial Pt sites is in the optimal range due to the charge transfer from Pt clusters to oxide, while the strong polarization of Mn2 dimers induced by Pt clusters with stable three-dimensional morphologies can lower the energy barrier of O2 dissociation. Based on the microkinetics analysis, the O2 dissociation is the rate-determining step in the full CO oxidation cycle, and the introduction of Mn-Fe hetero-dimers at the interface is predicted to further enhance the low temperature CO oxidation activity of Pt/SmMn2O5 catalysts.

MXenes are experimentally proved to be one of the promising supercapacitor materials; nevertheless, the theoretical limit of their supercapacitance and its redox origin are not accessible yet, which largely inhibits the future explorations on MXenes to be applied in the capacitors. Here, via first-principles calculations and our developed rigid band approximation (RBA), we systematically investigate charged electrodes Ti3C2T2 (T = bare, O, S) to reveal a theoretical limit of supercapacitance and the relevant origin of the capacitance. It is found that the theoretical gravimetric capacitance value of pure Ti3C2 surprisingly reaches 2131 F g−1 due to the high continuous density of states near the Fermi level (EF) and large difference of electronegativity between surface atoms and solvent ions. When considering the groups O and S with H coverage from 0 to 100%, we note that these groups significantly suppress the density of states around EF, i.e., the capability of storing charge. To explore the ion binding influence on the capacitance, the phase diagram of hydrogen binding energy and capacitance is provided for the screening and optimization of the next generation of electrode materials. These findings offer a comprehensive understanding of the supercapacitance of MXenes and guidance of further improvements of the capacitance in realistic charged environments.

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

Direct, atomic-scale visualization of polarization switching in a functional, polycrystalline, binary oxide via insitu high-resolution transmission electron microscopy (HRTEM) biasing is reported for the first time. Antiferroelectric (AFE) ZrO 2 was used as the model system, which is important for commercial DRAMs and as emerging NVMs (through work-function engineering). We observed (1) clear shifting and coalescing of domains within a single grain, and (2) dramatic changes of the atomic arrangements and crystalline phases-both at voltages above the critical voltage measured for AFE switching. Similar synergistic in-situ structural-electrical characterization can pave the way to understand and engineer microscopic mechanisms for retention, fatigue, variability, sub-coercive switching and analog states in ferroelectric and AFE-based memory devices.

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

Substitutional doping in 2D semiconductor MoS2 was investigated by charge transition level (CTL) calculations for Nitrogen group (N, P, As, Sb) and Halogen group (F, Cl, Br, I) dopants at the S site of monolayer MoS2. Both n-type and p-type dopant levels are calculated to be deep mid-gap states (~1 eV from band edges) from DFT total energy-based CTL and separate DFT + GW calculations. The deep dopant levels result from the giant renormalization of hydrogen-like defect states by reduced dielectric screening in ultrathin 2D films. Theoretical analysis based on Keldysh formulation provides a consistent impurity binding energy of ~1 eV for dielectric thin films. These findings of intrinsic deep impurity levels in 2D semiconductors MoS2 may be applicable to diverse novel emerging device applications.

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.

A GeP 3 compound is introduced here for the first time as a promising anode for sodium-ion batteries (SIBs). The compound shows a high capacity and good cyclic stability, which well agree with the first-principle calculation results regarding its soft bonding nature and related innovative mechanical endurance. The binding energies of Ge–P and P–P verify that Ge–P has softer bonding feature with smaller energy variations compared to P–P when the bonding length is changed. In order to confirm the bonding natures and their effect on the mechanical and electrochemical properties, two layered GeP n compounds with different Ge–P content (i.e., GeP 3 and GeP 5 ) have been synthesized using a high energy mechanical ball-milling (HEMM) method. GeP 3 maintains high discharge and charge capacities of 1.274 and 1.269 Ah g −1 even after 150 cycles, respectively, which correspond to capacity retentions of 87.6% and 88.0% from the 5th cycle, respectively. A comparative study on the elastic moduli of GeP 3 and GeP 5 demonstrates how the superior electrochemical performance of GeP 3 is correlated with its more softened bonding feature compared to GeP 5 both in the pristine state and during charge/discharge. The elastic modulus of GeP 3 shows significantly softer features than that of GeP 5 , implying that mechanical stress or strain can be more easily alleviated in GeP 3 than in GeP 5 during charge/discharge. A morphological comparison between GeP 3 and GeP 5 electrodes reveals that GeP 5 electrode undergoes very serious morphology and volume changes, whereas GeP 3 electrode does not show any significant change in good accordance with the elastic modulus comparison from the first-principle calculations.

Ni-rich layered oxides have been identified as promising candidates for commercial cathodes in Li-ion batteries. However, the commercialization has been hindered by severe cyclic degradation and mechanical failure induced by severe phase transformations and fractures. To resolve these challenges by understanding their fundamental mechanisms, we present mechanics-based multiscale investigations to elucidate the fundamental mechanisms of mechanical failure including deformations and fractures. We have also suggested a practical solution to the failure, which involves enhancing electronic interactions between transition metal layers. The methodological framework for our investigations was developed from first-principles atomic calculations, electronic structure, thermodynamics and kinetics for combined phase transformation, phase field modeling, finite element methodology for mechanical deformation, and phase field crack modeling. Our practical solution addresses the electronic interactions that can be strengthened when O ions are reduced by substituting strongly oxidizing elements such as Ti. Our multiscale framework shows that the reduced O ions are responsible for higher fracture toughness, reduced volume changes, stable deformation, mitigated stress generation, and suppressed fractures. Thus, this study proposes a practical solution for the improvement and design of Ni-rich layered oxide cathode materials. Furthermore, the mechanics-based multiscale methodology employed herein could be applied to a number of other solid-state energy materials suffering from mechanical failures.

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 maki...

Uniform p-type CuBi2O4 thin film was prepared through a spin coating method on fluorine-doped tin oxide (FTO) coated glass substrate, with a subsequent hypoxic post-annealing process under semi-sealed condition to enhance its photoelectrochemical efficiency for hydrogen evolution reaction. Compared to CuBi2O4 specimen annealed in open-air environment, the semi-sealed annealed CuBi2O4 photocathode presents a remarkable improvement in cathodic photocurrent, from 0.42 mA/cm2 to 0.7 mA/cm2 at 0.25 VRHE. X-ray photoelectron spectroscopy study revealed that the electronic structure of CuBi2O4 film was significantly modified during the post-annealing process and higher carrier concentration was obtained through Mott-Schottky measurement on semi-annealed CuBi2O4. We also demonstrate that the synthesized CuBi2O4 film with a thin overlayer of sputtered TiO2 can retain good stability and efficiency as a photocathode. This work provides insights into the mechanism of the high efficiency CuBi2O4 photocathode achieved from the unique post-annealing treatment.

Monolayer doping (MLD) is a relatively new method to incorporate shallow dopants from an adsorbed organic monolayer. To prevent evaporation of the dopant-containing organic layer during thermal pro...

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.

Ternary oxide nano-clusters compared to unary metallic and binary ones potentially exhibit more remarkable properties due to their higher stoichiometric flexibility in addition to cluster size variations. Herein, by combining with the structural searching scheme CALYPSO, we have built a series of Mn-mullite oxide clusters (SmxMnyOz)n {(xyz) = (125); (115); n = 1-4, 8} prior to investigation of their geometric and electronic structures via first-principles calculations. In small size regime (n < 4), (SmxMnyOz)n prefer nonstoichiometric (Sm1Mn1O5)n phases composed of nonmagnetic MnO4 tetrahedrons. When n ≧ 4, the clusters tend to develop as stoichiometric (Sm1Mn2O5)n species, including magnetic MnOn polyhedrons and Mn-Mn dimers, which contribute 3d-orbitals (dz2 and/or dx2-y2) around the Fermi levels. The different magnetic behaviors of nonstoichiometric and stoichiometric species originate from the distinct couplings of MnOn polyhedronal units, wherein Mn atoms experience different ligand fields and thus display different spin states. Such findings enable the tuning of electronic properties and potential applications in heterogeneous catalysis, electrochemical catalysis, and the related fields via engineering cluster size and stoichiometry.

To eliminate the aggregation of Co3O4 in the methane combustion process at high temperature, a thermally stable mullite structure, SmMn2O5 (SMO), was utilized as a support to improve the catalytic durability of Co3O4 particles. In detail, Co/SMO composite catalysts were prepared using the deposition–precipitation method with distinct Co/SMO nominal weight ratios of 5%, 30% and 50%. Later, their methane combustion performances under an oxygen-rich atmosphere were evaluated and compared. Meanwhile, their physical and chemical properties were characterized by XRD, Raman spectroscopy, the BET method, SEM, HRTEM, XPS, H2-TPR and O2-TPD. H2-TPR and O2-TPD results illustrated that the Co/SMO-50% catalyst showed the highest reducibility and it enhanced the mobility of oxygen species. As a result, the Co/SMO-50% sample exhibited the highest CH4 combustion catalytic activity among all the composite catalysts. Specifically, the T10, T50, and T90 for methane combustion were measured to be 334 °C, 390 °C, and 437 °C, very similar to those of Co3O4 catalyst. At the same time, the Co/SMO-50% catalyst showed improved durability, better performance after recycling, thermal aging and long-term experiments when compared to the Co3O4 catalyst.

Combining experimental and theoretical studies, we investigate the role of R-site (R = Y, Sm, Bi) element on the phase formation and thermal stability of R2(Mn1−xFex)4O10−δ (x = 0, 0.5, 1) mullite-type oxides. Our results show a distinct R-site dependent phase behavior for mullite-type oxides as Fe is substituted for Mn: 100% mullite-type phase was formed in (Y, Sm, Bi)2Mn4O10; 55% and 18% of (Y, Sm)2Mn2Fe2O10−δ was found when R = Y and Sm, respectively, for equal Fe and Mn molar concentrations in the reactants, whereas Bi formed 54% O10- and 42% O9-mixed mullite-type phases. Furthermore, when the reactants contain 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to Bi2Fe4O9 O9-phase was found for R = Bi. Thermogravimetric analysis and density functional theory (DFT) calculation results show a decreasing thermal stability in O10-type structure with increasing Fe incorporation; for example, the decomposition temperature is 1142 K for Bi2Mn2Fe2O10−δ vs. 1217 K for Bi2Mn4O10. On the other hand, Bi2Fe4O9 O9-type structure is found to be thermally stable up to 1227 K. These findings are explained by electronic structure calculations: (1) as Fe concentration increases, Jahn–Teller distortion results in mid band-gap empty states from unstable Fe4+ occupied octahedra, which is responsible for the decrease in O10 structure stability; (2) the directional sp orbital hybridization unique to Bi effectively stabilizes the mullite-type structure as Fe replaces Mn.

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.

The thermal stability of Ge/Al 2 O 3 and Ge/AlN/Al 2 O 3 stacks has been systematically studied upon post deposition annealing (PDA) at 400, 500 and 600 °C with nitrogen gas flow. X-ray photoelectron spectroscopy (XPS) with the incident photon energy of 1486.7 eV and synchrotron radiation photoemission spectroscopy (SRPES) with the incident photon energy of 720, 500 and 200 eV have been used to characterize the interface chemistry and the diffusion of Ge-oxides. More aggressive “clean-up” effect takes place with a higher substrate temperature during the atomic layer deposition (ALD) process for the growth of Al 2 O 3 and AlN thin films. A competitive process among the Ge-oxides growth at the Ge/high-k dielectrics interface, the Ge-oxides diffusion, and GeO desorption has been suggested upon PDA treatments. The effective suppression for formation of Ge-oxides by an ultrathin AlN layer has been observed for the samples before PDA and after PDA at 400 °C. Ge-oxide diffusion in proximity to the gate oxide surface has been characterized from the Ge2 p 3/2 spectra by XPS after PDA at 400 °C for Ge/Al 2 O 3 and Ge/AlN/Al 2 O 3 stacks. The diffusion mechanism is hypothesized by diffusion of oxygen vacancy. Moreover, a significant desorption of GeO occurs after PDA at 600 °C for the AlN passivated sample.