Atomic Dynamics of Multi-Interfacial Migration and Transformations.

Redox-induced interconversions of metal oxidation states typically result in multiple phase boundaries that separate chemically and structurally distinct oxides and suboxides. Directly probing such multi-interfacial reactions is challenging because of the difficulty in simultaneously resolving the multiple reaction fronts at the atomic scale. Using the example of CuO reduction in H2 gas, a reaction pathway of CuO → monoclinic m-Cu4 O3 → Cu2 O is demonstrated and identifies interfacial reaction fronts at the atomic scale, where the Cu2 O/m-Cu4 O3 interface shows a diffuse-type interfacial transformation; while the lateral flow of interfacial ledges appears to control the m-Cu4 O3 /CuO transformation. Together with atomistic modeling, it is shown that such a multi-interface transformation results from the surface-reaction-induced formation of oxygen vacancies that diffuse into deeper atomic layers, thereby resulting in the formation of the lower oxides of Cu2 O and m-Cu4 O3 , and activate the interfacial transformations. These results demonstrate the lively dynamics at the reaction fronts of the multiple interfaces and have substantial implications for controlling the microstructure and interphase boundaries by coupling the interplay between the surface reaction dynamics and the resulting mass transport and phase evolution in the subsurface and bulk.

Uneven Oxidation and Surface Reconstructions on Stepped Cu(100) and Cu(110).

How defects such as surface steps affect oxidation, especially initial oxide formation, is critical for nano-oxide applications in catalysis, electronics, and corrosion. We posit that surface reconstruction, a crucial intermediate oxidation step, can highlight initial oxide formation preferences and thus enable bridging the temporal and spatial scale gaps between atomistic simulations and experiments. We investigate the surface-step-induced uneven surface oxidation on Cu(100) and Cu(110), using atomic-scale in situ environmental transmission electron microscopy experiments with dynamical gas control and advanced data processing. We show that the Cu(100)-O (2√2 × âˆš2)R45° missing row reconstruction strongly favors upper terraces over lower terraces, while Cu(110)-O (2 × 1) "added row" reconstructions indicate slight preferences for upper or lower terraces, depending on oxygen concentration. The observed formation site preference and its variation with surface orientation and oxygen concentration are mechanistically explained by Ehrlich-Schwöbel barrier differences for oxygen diffusion on stepped surfaces.

First-Principles Phonon Quasiparticle Theory Applied to a Strongly Anharmonic Halide Perovskite.

Understanding and predicting lattice dynamics in strongly anharmonic crystals is one of the long-standing challenges in condensed matter physics. Here, we propose a first-principles method that gives accurate quasiparticle (QP) peaks of the phonon spectrum with strong anharmonic broadening. On top of the conventional first-order self-consistent phonon (SC1) dynamical matrix, the proposed method incorporates frequency renormalization effects by the bubble self-energy within the QP approximation. We apply the developed methodology to the strongly anharmonic α-CsPbBr_{3} that displays phonon instability within the harmonic approximation in the whole Brillouin zone. While the SC1 theory significantly underestimates the cubic-to-tetragonal phase transition temperature (T_{c}) by more than 50%, we show that our approach yields T_{c}=404-423 K, in excellent agreement with the experimental value of 403 K. We also demonstrate that an accurate determination of QP peaks is paramount for quantitative prediction and elucidation of the phonon linewidth.

Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles via In Situ Transmission Electron Microscopy.

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H2) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H2 environment was investigated by in situ gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. In situ chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H2 reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H2 formation and storage, catalytic hydrogenation, and corrosion removal.

Tuning electrical and interfacial thermal properties of bilayer MoS2via electrochemical intercalation.

Layered two-dimensional (2D) materials such as MoS2have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to modulate material properties of such layered 2D films reversibly. We probe both the electrical and thermal properties of Li-intercalated bilayer MoS2nanosheets by combining electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8 × 1012to 1.5 × 1014cm-2), and we simultaneously obtain the thermal boundary conductance between the bilayer and its supporting SiO2substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MW m-2K-1before intercalation to 1.8 ± 0.9 MW m-2K-1when the MoS2is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

Size-dependent polarizabilities and van der Waals dispersion coefficients of fullerenes from large-scale complex polarization propagator calculations.

While the anomalous non-additive size-dependencies of static dipole polarizabilities and van der Waals C6 dispersion coefficients of carbon fullerenes are well established, the widespread reported scalings for the latter (ranging from N2.2 to N2.8) call for a comprehensive first-principles investigation. With a highly efficient implementation of the linear complex polarization propagator, we have performed Hartree-Fock and Kohn-Sham density functional theory calculations of the frequency-dependent polarizabilities for fullerenes consisting of up to 540 carbon atoms. Our results for the static polarizabilities and C6 coefficients show scalings of N1.2 and N2.2, respectively, thereby deviating significantly from the previously reported values obtained with the use of semi-classical/empirical methods. Arguably, our reported values are the most accurate to date as they represent the first ab initio or first-principles treatment of fullerenes up to a convincing system size.

First-principles exploration of oxygen vacancy impact on electronic and optical properties of ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites.

ABO3-δ perovskites are utilized in many applications including optical gas sensing for energy systems. Understanding the opto-electronic properties allows rational selection of the perovskite-based sensors from a diverse family of ABO3-δ perovskites, associated with the choices of A and B cations and range of oxygen concentrations. Herein, we assess the impact of oxygen vacancies on the electronic structure and optical response of pristine and oxygen-vacant ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites via first-principles calculations. The endothermic formation energy for oxygen vacancies shows that the generation of ABO3-δ defect structures is thermodynamically possible. LaCrO3 and LaMnO3 have direct and indirect ground-state band gaps, respectively, whereas SrCrO3 and SrMnO3 are metallic. In the presence of an oxygen mono-vacancy, however, the band gap decreases in LaCrO3-δ and vanishes in LaMnO3-δ. In contrast to the decrease in the band gaps, the oxygen vacancies in ABO3-δ are found to increase optical absorption in the visible to near-infrared wavelength regime, and thus lower the onset energy of absorption compared with the pristine materials. Our assessments emphasize the role of the oxygen vacancy, or other possible oxygen non-stoichiometry defects, in perovskite oxides with respect to the opto-electronic performance parameters that are of interest for optical gas sensors for energy generation process environments.

Diverse electronic properties of 2D layered Se-containing materials composed of quasi-1D atomic chains.

The two-dimensional (2D) atomically thin layered materials have attracted significant attention for constructing next-generation integrated electronic and optoelectronic devices. A special class of 2D materials composed of quasi one-dimensional (1D) atomic chains that show intriguing properties are less studied. Here, two Se-containing 2D layered materials α-Se and Sb2Se3 that have quasi-1D atomic chains are investigated via first-principles electronic structure calculations. Results shows that the electronic properties of n-monolayers (n-MLs) stacked α-Se and Sb2Se3 exhibit distinct layer-dependence electronic properties. The band gap of 2D α-Se remarkably decreases with increasing thickness, whereas the band gap of 2D Sb2Se3 show negligible change with thickness. The evolution of lattice phonon frequencies with thickness also show similar distinction. The underpinnings of the diverse electronic properties are attributed to the different electronic coupling among the layers of α-Se and Sb2Se3 that results in different van der Waals interactions among chains/layers. Our study demonstrates the rich diversity in the properties of 2D layered materials composed of lower-dimensional structural motifs.

In situ environmental TEM observation of two-stage shrinking of Cu2O islands on Cu(100) during methanol reduction.

The structural dynamics of Cu catalyst regeneration from Cu2O under methanol is poorly understood. In situ Environmental TEM on Cu(100)-supported Cu2O islands reveals a transition from anisotropic to isotropic shrinking during reduction. Two-stage reduction is statistically supported and explained by preferential methanol reactivity on Cu2O nano-islands with DFT simulations.

Optimization and validation of a deep learning CuZr atomistic potential: Robust applications for crystalline and amorphous phases with near-DFT accuracy.

We show that a deep-learning neural network potential (DP) based on density functional theory (DFT) calculations can well describe Cu-Zr materials, an example of a binary alloy system, that can coexist in as ordered intermetallic and as an amorphous phase. The complex phase diagram for Cu-Zr makes it a challenging system for traditional atomistic force-fields that cannot accurately describe the different properties and phases. Instead, we show that a DP approach using a large database with ∼300k configurations can render results generally on par with DFT. The training set includes configurations of pristine and bulk elementary metals and intermetallic structures in the liquid and solid phases in addition to slab and amorphous configurations. The DP model was validated by comparing bulk properties such as lattice constants, elastic constants, bulk moduli, phonon spectra, and surface energies to DFT values for identical structures. Furthermore, we contrast the DP results with values obtained using well-established two embedded atom method potentials. Overall, our DP potential provides near DFT accuracy for the different Cu-Zr phases but with a fraction of its computational cost, thus enabling accurate computations of realistic atomistic models, especially for the amorphous phase.

Soft Lattice and Defect Covalency Rationalize Tolerance of ß-CsPbI3 Perovskite Solar Cells to Native Defects.

Although all-inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic-inorganic MHPs in efficiency. Recently, a conceptually new ß-CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non-adiabatic molecular dynamics to show that native point defects in ß-CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect-mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low-frequency vibrations. Both factors decrease notably the non-adiabatic coupling and slow down the dissipation of energy to heat.

Delocalized Impurity Phonon Induced Electron-Hole Recombination in Doped Semiconductors.

Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron-hole (e-h) recombination centers, where the determining element for e-h recombination is still unclear. Taking doped TiO2 as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e-h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e-h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e-h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e-h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control.

Phonon-Assisted Ultrafast Charge Transfer at van der Waals Heterostructure Interface.

The van der Waals (vdW) interfaces of two-dimensional (2D) semiconductor are central to new device concepts and emerging technologies in light-electricity transduction where the efficient charge separation is a key factor. Contrary to general expectation, efficient electron-hole separation can occur in vertically stacked transition-metal dichalcogenide heterostructure bilayers through ultrafast charge transfer between the neighboring layers despite their weak vdW bonding. In this report, we show by ab initio nonadiabatic molecular dynamics calculations, that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly promoted by an adiabatic mechanism through phonon excitation occurring on 20 fs, which is in good agreement with the experiment. The atomic level picture of the phonon-assisted ultrafast mechanism revealed in our study is valuable both for the fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as for the design of novel quasi-2D devices for optoelectronic and photovoltaic applications.

Defect-Induced Near-Infrared Photoluminescence of Single-Walled Carbon Nanotubes Treated with Polyunsaturated Fatty Acids.

Single-walled carbon nanotubes (SWCNTs) have been incorporated in many emerging applications in the biomedical field including chemical sensing, biological imaging, drug delivery, and photothermal therapy. To overcome inherent hydrophobicity and improve their biocompatibility, pristine SWCNTs are often coated with surfactants, polymers, DNA, proteins, or lipids. In this paper, we report the effect of polyunsaturated fatty acids (PUFAs) on SWCNT photoluminescence. A decrease in the SWCNT bandgap emission (E11) and a new red-shifted emission (E11-) were observed in the presence of PUFAs. We attribute the change in SWCNT photoluminescence to the formation of oxygen-containing defects by lipid hydroperoxides through photo-oxidation. The observed changes in near-infrared emission of SWCNTs are important for understanding the interaction between SWCNTs and lipid biocorona. Our results also indicate that photoexcited SWCNTs can catalyze lipid peroxidation similarly to lipoxygenases.

Publisher's Note: Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane [Phys. Rev. Lett. 118, 186101 (2017)].

This corrects the article DOI: 10.1103/PhysRevLett.118.186101.

Effects of Cr-doping on the adsorption and dissociation of S, SO, and SO2 on Ni(111) surfaces.

Nickel-based alloys are widely applied materials in high-temperature applications because they exhibit superior corrosion resistance and mechanical properties. The effects of sulfur, which is invariably present in industrial atmospheres, on the early stages of oxidation of Ni-based surfaces are not well understood. Here we use density functional theory to investigate the interactions of sulfur, SO, and SO2 with the Ni(111) and Cr-doped Ni(111) surface and elucidate their electronic interactions and potential energy surfaces. The results show that Cr doping of the Ni(111) surface increases the adsorption energies of sulfur, oxygen on the sulfur pre-adsorbed condition, SO and SO2. Further, this increase positively correlates with Cr concentration on top of the Ni(111) surface, although sulfur does not have any preferential interaction with Cr. This explains why Cr doping has little effect on the activation energy of sulfur for the most preferable diffusion path. Nevertheless, the increase in adsorption energies indicates a strong interaction with Cr-doped surfaces, which is due to the Cr-enhanced charge transfer to sulfur adsorbates. The existence of pre-adsorbed sulfur is shown to have a destabilizing effect on the oxygen interactions with the surfaces. Our results show that Cr doping helps to stabilize the protective oxide scale on Ni(111) surfaces and enhances its corrosion resistance.

Ultrafast Dynamics of Photongenerated Holes at a CH3OH/TiO2 Rutile Interface.

Photogenerated charge carrier dynamics near molecule/TiO2 interfaces are important for the photocatalytic and photovoltaic processes. To understand this fundamental aspect, we performed a time-domain ab initio nonadiabatic molecular dynamics study of the photogenerated hole dynamics at the CH3OH/rutile TiO2(110) interface. We studied the forward and reverse hole transfer between TiO2 and CH3OH as well as the hole energy relaxation to the valence band maximum. First, we show that the hole-trapping ability of CH3OH depends strongly on the adsorption structure. Only when the CH3OH is deprotonated to form chemisorbed CH3O will ∼15% of the hole be trapped by the molecule. Second, we find that strong fluctuations of the HOMO energies of the adsorbed molecules induced by electron-phonon coupling provide additional channels, which accelerate the hole energy relaxation. Third, we demonstrate that the charge transfer and energy relaxation processes depend significantly on temperature. When the temperature decreases from 100 to 30 K, the forward hole transfer and energy relaxation processes are strongly suppressed because of the reduction of phonon occupation. These results indicate that the molecule/TiO2 energy level alignment, thermal excitation of a phonon, and electron-phonon coupling are the key factors that determine the photogenerated hole dynamics. Our studies provide valuable insights into the photogenerated charge and energy transfer dynamics at molecule/semiconductor interfaces.

Tunability of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface by strain-induced ferroelectricity.

The two-dimensional electron gas (2DEG) formed at the interface between two insulating materials LaAlO3 (LAO) and SrTiO3 (STO) has recently generated a lot of interest. Here, based on first-principles density functional theory calculations, we investigate the existence and stability of the 2DEG under the application of a biaxial strain on the LAO/STO(001) heterostructure. The compressive strain induces ferroelectric (FE) polarization in STO, which allows for the tunability of the 2DEG by reversing the STO polarization orientation. We show that the formation of the 2DEG is unstable when LAO and STO have the same polarization direction. On the other hand, the 2DEG will always form if the two polarizations are in the opposite directions regardless of the LAO thickness, which is in contrast to the unstrained interface that has a critical thickness for stabilizing the 2DEG. We show that the underpinnings of this behavior are due to charge passivation and band gap alignment.