Exploring in-plane interactions beside an adsorbed molecule with lateral force microscopy.

Atomic force microscopy with a CO-functionalized tip can be used to directly image the internal structure of a planar molecule and to characterize chemical bonds. However, hydrogen atoms usually cannot be directly observed due to their small size. At the same time, these atoms are highly important, since they can direct on-surface chemical reactions. Measuring in-plane interactions at the sides of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) molecules with lateral force microscopy allowed us to directly identify hydrogen atoms via their repulsive signature, which we confirmed with a model incorporating radially symmetric atomic interactions. Additional features were observed in the force data and could not be explained by H-bonding of the CO tip with the PTCDA sides. Instead, they are caused by electrostatic interaction of the large dipole of the metal apex, which we verified with density functional theory. This calculation allowed us to estimate the strength of the dipole at the metal tip apex. To further confirm that this dipole generally affects measurements on weakly polarized systems, we investigated the archetypical surface adsorbate of a single CO molecule. We determined the radially symmetric atomic interaction to be valid over a large solid angle of 5.4 sr, corresponding to 82°. We therefore find that in both the PTCDA and CO systems, the underlying interaction preventing direct observations of H-bonding and causing a collapse of the radially symmetric model is the dipole at the metal apex, which plays a significant role when approaching closer than standard imaging heights.

Neural functional theory for inhomogeneous fluids: Fundamentals and applications.

We present a hybrid scheme based on classical density functional theory and machine learning for determining the equilibrium structure and thermodynamics of inhomogeneous fluids. The exact functional map from the density profile to the one-body direct correlation function is represented locally by a deep neural network. We substantiate the general framework for the hard sphere fluid and use grand canonical Monte Carlo simulation data of systems in randomized external environments during training and as reference. Functional calculus is implemented on the basis of the neural network to access higher-order correlation functions via automatic differentiation and the free energy via functional line integration. Thermal Noether sum rules are validated explicitly. We demonstrate the use of the neural functional in the self-consistent calculation of density profiles. The results outperform those from state-of-the-art fundamental measure density functional theory. The low cost of solving an associated Euler-Lagrange equation allows to bridge the gap from the system size of the original training data to macroscopic predictions upon maintaining near-simulation microscopic precision. These results establish the machine learning of functionals as an effective tool in the multiscale description of soft matter.

A first principles analysis of potential-dependent structural evolution of active sites in Fe-N-C catalysts.

Fe-N-C (iron-nitrogen-carbon) electrocatalysts have emerged as potential alternatives to precious metal-based materials for the oxygen reduction reaction (ORR). However, the structure of these materials under electrochemical conditions is not well understood, and their poor stability in acidic environments poses a formidable challenge for successful adoption in commercial fuel cells. To provide molecular-level insights into these complex phenomena, we combine periodic density functional theory (DFT) calculations, exhaustive treatment of coadsorption effects for ORR reaction intermediates, including O and OH, and comprehensive analysis of solvation stabilization effects to construct voltage-dependent ab initio thermodynamic phase diagrams that describe the in situ structure of the active sites. These structures are further linked to activity and stability descriptors that can be compared with experimental parameters such as the half-wave potential for ORR and the onset potential for carbon corrosion and CO2 evolution. The results indicate that pyridinic Fe sites at zigzag carbon edges, as well as other edge sites, exhibit high activity for ORR compared to sites in the bulk. However, edges neighboring the active sites are prone to instability via overoxidation and consequent site loss. The results suggest that it could be beneficial to synthesize Fe-N-C catalysts with small sizes and large perimeter edge lengths to enhance ORR activity, while voltage fluctuations should be limited during fuel cell operation to prevent carbon corrosion of overoxidized edges.

Aqueous Titania Interfaces.

Water-metal oxide interfaces are central to many phenomena and applications, ranging from material corrosion and dissolution to photoelectrochemistry and bioengineering. In particular, the discovery of photocatalytic water splitting on TiO2 has motivated intensive studies of water-TiO2 interfaces for decades. So far, a broad understanding of the interaction of water vapor with several TiO2 surfaces has been obtained. However, much less is known about liquid water-TiO2 interfaces, which are more relevant to many practical applications. Probing these complex systems at the molecular level is experimentally challenging and is sometimes possible only through computational studies. This review summarizes recent advances in the atomistic understanding, mostly through computational simulations, of the structure and dynamics of interfacial water on TiO2 surfaces. The main focus is on the nature, molecular or dissociated, of water in direct contact with low-index defect-free crystalline surfaces. The hydroxyls resulting from water dissociation are essential in the photooxidation of water and critically affect the surface chemistry of TiO2.

The structural origin of the efficient photochromism in natural minerals.

SignificanceNatural photochromic minerals have been reported by geologists for decades. However, the understanding of the photochromism mechanism has a key question still unanswered: What in their structure gives rise to the photochromism's reversibility? By combining experimental and computational methods specifically developed to investigate this photochromism, this work provides the answer to this fundamental question. The specific crystal structure of these minerals allows an unusual motion of the sodium atoms stabilizing the electronic states associated to the colored forms. With a complete understanding of the photochromism mechanism in hand, it is now possible to design new families of stable and tunable photochromic inorganic materials-based devices.

Homogeneous ice nucleation in an ab initio machine-learning model of water.

Molecular simulations have provided valuable insight into the microscopic mechanisms underlying homogeneous ice nucleation. While empirical models have been used extensively to study this phenomenon, simulations based on first-principles calculations have so far proven prohibitively expensive. Here, we circumvent this difficulty by using an efficient machine-learning model trained on density-functional theory energies and forces. We compute nucleation rates at atmospheric pressure, over a broad range of supercoolings, using the seeding technique and systems of up to hundreds of thousands of atoms simulated with ab initio accuracy. The key quantity provided by the seeding technique is the size of the critical cluster (i.e., a size such that the cluster has equal probabilities of growing or melting at the given supersaturation), which is used together with the equations of classical nucleation theory to compute nucleation rates. We find that nucleation rates for our model at moderate supercoolings are in good agreement with experimental measurements within the error of our calculation. We also study the impact of properties such as the thermodynamic driving force, interfacial free energy, and stacking disorder on the calculated rates.

Charge Redistribution in Mechanochemical Reactions for Solid Interfaces.

Mechanochemical strategies are widely used in various fields, ranging from friction and wear to mechanosynthesis, yet how the mechanical stress activates the chemical reactions at the electronic level is still open. We used first-principles density functional theory to study the rule of the stress-modified electronic states in transmitting mechanical energy to trigger chemical responses for different mechanochemical systems. The electron density redistribution among initial, transition, and final configurations is defined to correlate the energy evolution during reactions. We found that stress-induced changes in electron density redistribution are linearly related to activation energy and reaction energy, indicating the transition from mechanical work to chemical reactivity. The correlation coefficient is defined as the term "interface reactivity coefficient" to evaluate the susceptibility of chemical reactivity to mechanical action for material interfaces. The study may shed light on the electronic mechanism of the mechanochemical reactions behind the fundamental model as well as the mechanochemical phenomena.

Understanding Trap States in InP and GaP Quantum Dots through Density Functional Theory.

The widespread application of III-V colloidal quantum dots (QDs) as nontoxic, highly tunable emitters is stymied by their high density of trap states. Here, we utilize density functional theory (DFT) to investigate trap state formation in a diverse set of realistically passivated core-only InP and GaP QDs. Through orbital localization techniques, we deconvolute the dense manifold of trap states to allow for detailed assignment of surface defects. We find that the three-coordinate species dominate trapping in III-V QDs and identify features in the geometry and charge environment of trap centers capable of deepening, or sometimes passivating, traps. Furthermore, we observe stark differences in surface reconstruction between InP and GaP, where the more labile InP reconstructs to passivate three-coordinate indium at the cost of distortion elsewhere. These results offer explanations for experimentally observed trapping behavior and suggest new avenues for controlling trap states in III-V QDs.

Regulating Second Coordination Shell of Ce Atom Site and Reshaping of Carrier Enable Single-Atom Nanozyme to Efficiently Express Oxidase-like Activity.

Single-atom nanozymes (SANs) are considered to be ideal substitutes for natural enzymes due to their high atom utilization. This work reported a strategy to manipulate the second coordination shell of the Ce atom and reshape the carbon carrier to improve the oxidase-like activity of SANs. Internally, S atoms were symmetrically embedded into the second coordination layer to form a Ce-N4S2-C structure, which reduced the energy barrier for O2 reduction, promoted the electron transfer from the Ce atom to O atoms, and enhanced the interaction between the d orbital of the Ce atom and p orbital of O atoms. Externally, in situ polymerization of mussel-inspired polydopamine on the precursor helps capture metal sources and protects the 3D structure of the carrier during pyrolysis. On the other hand, polyethylene glycol (PEG) modulated the interface of the material to enhance water dispersion and mass transfer efficiency. As a proof of concept, the constructed PEG@P@Ce-N/S-C was applied to the multimodal assay of butyrylcholinesterase activity.

Electron Localization and Mobility in Monolayer Fullerene Networks.

The novel 2D quasi-hexagonal phase of covalently bonded fullerene molecules (qHP C60), the so-called graphullerene, has displayed far superior electron mobilities, if compared to the parent van der Waals three-dimensional crystal (vdW C60). Herein, we present a comparative study of the electronic properties of vdW and qHP C60 using state-of-the-art electronic-structure calculations and a full quantum-mechanical treatment of electron transfer. We show that both materials entail polaronic localization of electrons with similar binding energies (≈0.1 eV) and, therefore, they share the same charge transport via polaron hopping. In fact, we quantitatively reproduce the sizable increment of the electron mobility measured for qHP C60 and identify its origin in the increased electronic coupling between C60 units.

Synthesis and Polymorph Manipulation of FeSe2 Monolayers.

Polymorph engineering involves the manipulation of material properties through controlled structural modification and is a candidate technique for creating unique two-dimensional transition metal dichalcogenide (TMDC) nanodevices. Despite its promise, polymorph engineering of magnetic TMDC monolayers has not yet been demonstrated. Here we grow FeSe2 monolayers via molecular beam epitaxy and find that they have great promise for magnetic polymorph engineering. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we find that FeSe2 monolayers predominantly display a 1T' structural polymorph at 5 K. Application of voltage pulses from an STM tip causes a local, reversible transition from the 1T' phase to the 1T phase. Density functional theory calculations suggest that this single-layer structural phase transition is accompanied by a magnetic transition from an antiferromagnetic to a ferromagnetic configuration. These results open new possibilities for creating functional magnetic devices with TMDC monolayers via polymorph engineering.

Atomically Dispersed Palladium Catalyst for Chemoselective Hydrogenation of Quinolines.

Chemoselective hydrogenation of quinoline and its derivatives is a significant strategy to achieve the corresponding 1,2,3,4-tetrahydroquinolines (py-THQ) for various potential applications. Here, we precisely constructed a titanium carbide supported atomically dispersed Pd catalyst (PdSA+NC/TiC) for quinoline hydrogenation, delivering above 99% py-THQ selectivity at complete conversion with an outstanding turnover frequency (TOF) of 463 h-1. AC-HAADF-STEM and XAFS demonstrate that the atomic dispersion of Pd includes Pd-Ti2C2 single atoms and Pd clusters with atomic-layer thickness. Theoretical calculation and experimental results revealed that H2 dissociation and subsequent hydrogenation rates were greatly promoted over Pd clusters. Although the adsorption of quinolines and intermediates are easier on Pd clusters than on Pd single atoms, the desorption of py-THQ is more favored over Pd single atoms than over Pd clusters. The desorption step may be the main reason for 5,6,7,8-tetrahydroquinoline (bz-THQ) and decahydroquinoline (DHQ) formation. Thus, a low reaction activity and py-THQ selectivity were received over PdSA/TiC and PdNP/TiC, respectively.

Purcell-Induced Bright Single Photon Emitters in Hexagonal Boron Nitride.

Single photon emitters (SPEs) in hexagonal boron nitride (hBN) are elementary building blocks for room-temperature on-chip quantum photonic technologies. However, fundamental challenges, such as slow radiative decay and nondeterministic placement of the emitters, limit their full potential. Here, we demonstrate large-area arrays of plasmonic nanoresonators (PNRs) for Purcell-induced room-temperature SPEs by engineering emitter-cavity coupling and enhancing radiative emission. Gold-coated silicon pillars with an alumina spacer enable a 10-fold local-field enhancement in the emission band of native hBN defects. We observe bright SPEs with an average saturated emission rate surpassing 5 million counts per second, an average lifetime of <0.5 ns, and 29% yield. Density functional theory reveals the beneficial role of an alumina spacer between hBN and gold, mitigating the electronic broadening of emission from defects proximal to the metal. Our results offer arrays of bright, heterogeneously integrated single-photon sources, paving the way for robust and scalable quantum information systems.

Light-Induced Polaronic Crystals in Single-Layer Transition Metal Dichalcogenides.

Light-induced ordered states can emerge in materials after irradiation with ultrafast laser pulses. However, their prediction is challenging because the inverted band occupation confounds our chemical intuition. Hence, we use a recently developed constrained density functional perturbation theory approach to systematically screen single-layer transition metal dichalcogenides (TMDs) for light-induced ordered states. We demonstrate that all examined single-layer TMDs reveal similar light-induced charge orderings. The corresponding reconstructions are periodic arrangements of polarons (polaronic crystals), characterized by triangular metal clusters and having no equivalent at equilibrium conditions. The polarons are accompanied by localized midgap states in the electronic band structure, detectable by experimental methods. We assess the selenides as the most promising candidates for potential photoexcitation experiments because they transition at low critical fluences, have low transition barriers, and maintain an open band gap under photoexcitation. Our work paves the way for innovative material design approaches targeting light-induced phases.

Modulation of Electrostatic Potential in 2D Crystal Engineered by an Array of Alternating Polar Molecules.

The moiré potential in rotationally misfit two-dimensional (2D) heterostructures has been used to build artificial exciton and electron lattices, which have become platforms for realizing exotic electronic phases. Here, we demonstrate a different approach to create a superlattice potential in 2D crystals by using the near field of an array of polar molecules. A bilayer of titanyl phthalocyanine (TiOPc), consisting of alternating out-of-plane dipoles, is deposited on monolayer MoS2. Time-resolved two-photon photoemission spectroscopy reveals a pair of interlayer exciton states with an energy difference of ∼0.1 eV, which is consistent with the electrostatic potential modulation induced by the TiOPc bilayer as determined by density functional theory calculations. Because the symmetry and the period of this potential superlattice can be changed readily by using molecules of different shapes and sizes, molecule/2D heterostructures can be promising platforms for designing artificial exciton and electron lattices.

Unlocking the Potential: Atomically Thin 2D Fluoritene from Exfoliated Fluorite Ore and Its Electrochemical Activity.

Fluorite mineral holds significant importance because of its optoelectronic properties and wide range of applications. Here, we report the successful exfoliation of bulk fluorite ore (calcium fluoride, CaF2) crystals into atomically thin two-dimensional fluoritene (2D CaF2) using a highly scalable liquid-phase exfoliation method. The microscopic and spectroscopy characterizations show the formation of (111) plane-oriented 2D CaF2 sheets with exfoliation-induced material strain due to bond breaking, leading to the changes in lattice parameter. Its potential role in electrocatalysis is further explored for deeper insight, and a probable mechanism is also discussed. The 2D CaF2 with long-term stability shows overpotential values of 670 and 770 mV vs RHE for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at 10 mA cm-2. Computational simulations demonstrate the unique "direct-indirect" band gap switching with odd and even numbers of layers. Current work offers new avenues for exploring the structural and electrochemical properties of 2D CaF2 and its potential applicability.

Proposed Quantum Twisting Scanning Probe Microscope over Twisted Bilayer Graphene.

Twisted bilayer graphene (TBG) has the natural merits of tunable flat bands and localized states distributed as a triangular lattice. However, the application of this state remains obscure. By density functional theory (DFT) and pz orbital tight-binding model calculations, we investigate the tip-shaped electrostatic potential of top valence electrons of TBG at half filling. Adsorption energy scanning of molecules above the TBG reveals that this tip efficiently attracts molecules selectively to AA-stacked or AB-stacked regions. Tip shapes can be controlled by their underlying electronic structure, with electrons of low bandwidth exhibiting a more localized feature. Our results indicate that TBG tips offer applications in noninvasive and nonpolluting measurements in scanning probe microscopy and theoretical guidance for 2D material-based probes.

Rational Design of Tetrahedral Derivatives as Efficient Light-Emitting Materials Based on "Super Atom" Perspective.

Traditional semiconductor quantum dots of groups II-VI are key ingredients of next-generation display technology. Yet, the majority of them contain toxic heavy-metal elements, thus calling for alternative light-emitting materials. Herein, we have explored three novel categories of multicomponent compounds, namely, tetragonal II-III2-VI4 porous ternary compounds, cubic I2-II3-VI4 ternary compounds, and cubic I-II-III3-V4 quaternary compounds. This is achieved by judicious introduction of a "super atom" perspective and concurrently varying the solid-state lattice packing of involved super atoms or the population of surrounding counter cations. Based on first-principles calculations of 392 candidate materials with designed crystal structures, 53 highly stable materials have been screened. Strikingly, 34 of them are direct-bandgap semiconductors with emitting wavelengths covering the near-infrared and visible-light regions. This work provides a comprehensive database of highly efficient light-emitting materials, which may be of interest for a broad field of optoelectronic applications.

Atomically Thin Kagome-Structured Co9Te16 Achieved through Self-Intercalation and Its Flat Band Visualization.

Kagome materials have recently garnered substantial attention due to the intrinsic flat band feature and the stimulated magnetic and spin-related many-body physics. In contrast to their bulk counterparts, two-dimensional (2D) kagome materials feature more distinct kagome bands, beneficial for exploring novel quantum phenomena. Herein, we report the direct synthesis of an ultrathin kagome-structured Co-telluride (Co9Te16) via a molecular beam epitaxy (MBE) route and clarify its formation mechanism from the Co-intercalation in the 1T-CoTe2 layers. More significantly, we unveil the flat band states in the ultrathin Co9Te16 and identify the real-space localization of the flat band states by in situ scanning tunneling microscopy/spectroscopy (STM/STS) combined with first-principles calculations. A ferrimagnetic order is also predicted in kagome-Co9Te16. This work should provide a novel route for the direct synthesis of ultrathin kagome materials via a metal self-intercalation route, which should shed light on the exploration of the intriguing flat band physics in the related systems.

Impact of Single-Melamine Tautomerization on the Excitation of Molecular Vibrations in Inelastic Electron Tunneling Spectroscopy.

Vibrational quanta of melamine and its tautomer are analyzed at the single-molecule level on Cu(100) with inelastic electron tunneling spectroscopy. The on-surface tautomerization gives rise to markedly different low-energy vibrational spectra of the isomers, as evidenced by a shift in mode energies and a variation in inelastic cross sections. Spatially resolved spectroscopy reveals the maximum signal strength on an orbital nodal plane, excluding resonant inelastic tunneling as the mechanism underlying the quantum excitations. Decreasing the probe-molecule separation down to the formation of a chemical bond between the melamine amino group and the Cu apex atom of the tip leads to a quenched vibrational spectrum with different excitation energies. Density functional and electron transport calculations reproduce the experimental findings and show that the shift in the quantum energies applies to internal molecular bending modes. The simulations moreover suggest that the bond formation represents an efficient manner of tautomerizing the molecule.