Tailoring the Surface Chemical Reactivity of Transition‐Metal Dichalcogenide PtTe2 Crystals

PtTe₂ is a novel transition‐metal dichalcogenide hosting type‐II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe₂ is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium‐oxide phases upon exposing defected PtTe₂ surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe₂‐based devices. On the contrary, in PtTe₂ surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water–gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis.     

Toward the Effective Exploitation of Topological Phases of Matter in Catalysis: Chemical Reactions at the Surfaces of NbAs and TaAs Weyl Semimetals

By means of density functional theory and experiments, surface chemical reactivity of single crystals of NbAs and TaAs Weyl semimetals is studied. Weyl semimetals exhibit outstanding reactivity toward simple molecules (oxygen, carbon monoxide, and water), with several active sites available for surface chemical reactions (adsorption, decomposition, formation of reaction products, recombination of decomposition fragments). When different chemical species are adsorbed on Weyl semimetals, strong lateral interactions between coadsorbed species occur, evidenced by CO‐promoted water decomposition at room temperature. The resulting ? OH groups react with CO to form HCOO, which is an intermediate species in water–gas shift reaction. These findings unambiguously demonstrate that Weyl semimetals could be effectively used in catalysis, whereas their employment in nanoelectronics or plasmonics is complicated by the poor ambient stability, due to the rapid surface oxidation, inevitably occurring unless protective capping layers are used.     

Surface Reconstruction,Oxidation Mechanism,and Stability of Cd3As2

Cadmium arsenide (Cd₃As₂) has recently attracted considerable interest for the presence of 3D massless Dirac fermions with ultrahigh mobility and magnetoresistance. However, its surface properties are currently largely unexplored both theoretically and experimentally, due to the very large unit cell and the challenging growth of single‐crystal samples, respectively. Here, by combining ab initio calculations with surface‐science spectroscopic experiments, the presence of a surface reconstruction is unveiled in centimeter‐scale (112)‐oriented Cd₃As₂ single‐crystal foils produced by the self‐selecting vapor growth. Outermost Cd atoms descend into the As‐sublayer with a subsequent self‐passivation of the dangling bonds with As atoms, forming the triangle lattice previously imaged by scanning tunneling microscopy. Moreover, the oxidation mechanism of this reconstructed surface, dominated by the formation of As? O? Cd bonds, is revealed. Interestingly, it is found that the band structure of the reconstructed surface of Cd₃As₂ is quite robust against surface oxidation. Both computational and experimental findings point to a successful exploitation in technology of Cd₃As₂ single crystals.     

Efficient Field Emission from Vertically Aligned Cu2O1‐δ(111) Nanostructure Influenced by Oxygen Vacancy

In the architecture described, cuprous oxide (Cu₂O) is tamed to be highly (111) plane oriented nanostructure through adjusting the deposition postulate by glancing angle deposition technique. In the controlled atmosphere oxygen vacancy is introduced into the Cu₂O crystal subsequently fostering an impurity energy state (E_im) close to the conduction band. Our model of Cu₂O electronic structure using density functional theory suggests that oxygen vacancies enhance the electron donating ability because of unshared d‐electrons of Cu atoms (nearest to the vacancy site), allowing to pin the work function energy level around 0.28 eV compared to the bulk. This result is also complemented by Kelvin probe force microscopy analysis and X‐ray photoelectron spectroscopy method. Oxygen vacancy in Cu₂O (Cu₂O_1‐δ) exhibits promising field emission properties with interesting field electron tunneling behavior at different applied fields. The films show very low turn‐on and threshold voltages of value 0.8 and 2.4 V μm^?1 respectively which is influenced by the oxygen vacancy. Here, a correlation between the work function modulation due to the oxygen vacancy and enhancement of field emission of Cu₂O_1–δ nanostructure is demonstrated. This work reveals a promising new vision for Cu₂O as a low power field emitter device.     

Modification of the Gallium‐Doped Zinc Oxide Surface with Self‐Assembled Monolayers of Phosphonic Acids: A Joint Theoretical and Experimental Study

Gallium‐doped zinc oxide (GZO) surfaces, both bare and modified with chemisorbed phosphonic acid (PA) molecules, are studied using a combination of density functional theory calculations and ultraviolet and X‐ray photoelectron spectroscopy measurements. Excellent agreement between theory and experiment is obtained, which leads to an understanding of: i) the core‐level binding energy shifts of the various oxygen atoms belonging to different surface sites and to the phosphonic acid molecules; ii) the GZO work‐function change upon surface modification, and; iii) the energy level alignments of the frontier molecular orbitals of the PA molecules with respect to the valence band edge and Fermi level of the GZO surface. Importantly, both density of states calculations and experimental measurements of the valence band features demonstrate an increase in the density of states and changes in the characteristics of the valence band edge of GZO upon deposition of the phosphonic acid molecules. The new valence band features are associated with contributions from surface oxygen atoms near a defect site on the oxide surface and from the highest occupied molecular orbitals of the phosphonic acid molecules.     

Revealing the Pseudo‐Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte

Since the discovery of Ti₃C₂T_x in 2011, the family of two‐dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non‐rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X‐ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X‐ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti₃C₂T_x in 1 m H₂SO₄. Results reveal that a 0.5 Å expansion and shrinkage of the c‐lattice parameter of Ti₃C₂T_x occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.     

Accelerating CO2 Electroreduction to CO Over Pd Single‐Atom Catalyst

The electrochemical conversion of carbon dioxide (CO₂) into value‐added chemicals is regarded as one of the promising routes to mitigate CO₂ emission. A nitrogen‐doped carbon‐supported palladium (Pd) single‐atom catalyst that can catalyze CO₂ into CO with far higher mass activity than its Pd nanoparticle counterpart, for example, 373.0 and 28.5 mA mg^?1_Pd, respectively, at ?0.8 V versus reversible hydrogen electrode, is reported. A combination of in situ X‐ray characterization and density functional theory (DFT) calculation reveals that the Pd? N₄ site is the most likely active center for CO production without the formation of palladium hydride (PdH), which is essential for typical Pd nanoparticle catalysts. Furthermore, the well‐dispersed Pd? N₄ single‐atom site facilitates the stabilization of the adsorbed CO₂ intermediate, thereby enhancing electrocatalytic CO₂ reduction capability at low overpotentials. This work provides important insights into the structure‐activity relationship for single‐atom based electrocatalysts.     

X‐Ray Spectroscopic Investigation of Chlorinated Graphene: Surface Structure and Electronic Effects

Chemical doping of graphene represents a powerful means of tailoring its electronic properties. Synchrotron‐based X‐ray spectroscopy offers an effective route to investigate the surface electronic and chemical states of functionalizing dopants. In this work, a suite of X‐ray techniques is used, including near edge X‐ray absorption fine structure spectroscopy, X‐ray photoemission spectroscopy, and photoemission threshold measurements, to systematically study plasma‐based chlorinated graphene on different substrates, with special focus on its dopant concentration, surface binding energy, bonding configuration, and work function shift. Detailed spectroscopic evidence of C–Cl bond formation at the surface of single layer graphene and correlation of the magnitude of p‐type doping with the surface coverage of adsorbed chlorine is demonstrated for the first time. It is shown that the chlorination process is a highly nonintrusive doping technology, which can effectively produce strongly p‐doped graphene with the 2D nature and long‐range periodicity of the electronic structure of graphene intact. The measurements also reveal that the interaction between graphene and chlorine atoms shows strong substrate effects in terms of both surface coverage and work function shift.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

Customized Electronic Coupling in Self‐Assembled Donor–Acceptor Nanostructures

Charge transfer processes between donor–acceptor complexes and metallic electrodes are at the heart of novel organic optoelectronic devices such as solar cells. Here, a combined approach of surface‐sensitive microscopy, synchrotron radiation spectroscopy, and state‐of‐the‐art ab initio calculations is used to demonstrate the delicate balance that exists between intermolecular and molecule–substrate interactions, hybridization, and charge transfer in model donor–acceptor assemblies at metal‐organic interfaces. It is shown that charge transfer and chemical properties of interfaces based on single component layers cannot be naively extrapolated to binary donor–acceptor assemblies. In particular, studying the self‐assembly of supramolecular nanostructures on Cu(111), composed of fluorinated copper‐phthalocyanines (F₁₆CuPc) and diindenoperylene (DIP), it is found that, in reference to the associated single component layers, the donor (DIP) decouples electronically from the metal surface, while the acceptor (F₁₆CuPc) suffers strong hybridization with the substrate.     

Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ～ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.     

Reversible Amorphous‐to‐Amorphous Transitions in Chalcogenide Films: Correlating Changes in Structure and Optical Properties

Structural transitions in materials are accompanied by appreciable and exploitable changes in physical‐chemical properties. Whereas reversible optically‐driven atomistic changes in crystal‐to‐amorphous transitions are generally known and exploited in applications, the nature of the corresponding polyamorphic transitions between two structurally distinct meta‐stable amorphous phases is an unexplored theme. Direct experimental evidence is reported for the nature of the atomistic changes during fully reversible amorphous‐to‐amorphous switching between two individual states in the non‐crystalline As₅₀Se₅₀ films prepared by pulsed‐laser deposition and consequent changes in optical properties. Combination of surface sensitive X‐ray photoelectron spectroscopy and spectroscopic ellipsometry show that the near‐bandgap energy illumination and annealing induce reversible switching in the material's structure by local bonding rearrangements. This is accompanied by switching in refractive index between two well‐defined states. Exploiting the pluralism of distinct structural states in a disordered solid can provide new insights into the data storage in emerging optical memory and photonic applications.     

Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study

Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar‐to‐chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO₃/TiO₂ heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO₃ layer of vertically aligned nanorods that is then coated with a conformal layer of TiO₂. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO₃ layer into TiO₂. State‐of‐the‐art hybrid density functional theory and hard X‐ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO₃/TiO₂ interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.     

Transition‐Metal Dichalcogenide NiTe2: An Ambient‐Stable Material for Catalysis and Nanoelectronics

By means of theory and experiments, the application capability of nickel ditelluride (NiTe₂) transition‐metal dichalcogenide in catalysis and nanoelectronics is assessed. The Te surface termination forms a TeO₂ skin in an oxygen environment. In ambient atmosphere, passivation is achieved in less than 30 min with the TeO₂ skin having a thickness of about 7 Å. NiTe₂ shows outstanding tolerance to CO exposure and stability in water environment, with subsequent good performance in both hydrogen and oxygen evolution reactions. NiTe₂‐based devices consistently demonstrate superb ambient stability over a timescale as long as one month. Specifically, NiTe₂ has been implemented in a device that exhibits both superior performance and environmental stability at frequencies above 40 GHz, with possible applications as a receiver beyond the cutoff frequency of a nanotransistor.     

Kitkaite NiTeSe,an Ambient-Stable Layered Dirac Semimetal with Low-Energy Type-II Fermions with Application Capabilities in Spintronics and Optoelectronics

The emergence of Dirac semimetals has stimulated growing attention, owing to the considerable technological potential arising from their peculiar exotic quantum transport related to their nontrivial topological states. Especially, materials showing type-II Dirac fermions afford novel device functionalities enabled by anisotropic optical and magnetotransport properties. Nevertheless, real technological implementation has remained elusive so far. Definitely, in most Dirac semimetals, the Dirac point lies deep below the Fermi level, limiting technological exploitation. Here, it is shown that kitkaite (NiTeSe) represents an ideal platform for type-II Dirac fermiology based on spin-resolved angle-resolved photoemission spectroscopy and density functional theory. Precisely, the existence of type-II bulk Dirac fermions is discovered in NiTeSe around the Fermi level and the presence of topological surface states with strong (≈50%) spin polarization. By means of surface-science experiments in near-ambient pressure conditions, chemical inertness towards ambient gases (oxygen and water) is also demonstrated. Correspondingly, NiTeSe-based devices without encapsulation afford long-term efficiency, as demonstrated by the direct implementation of a NiTeSe-based microwave receiver with a room-temperature photocurrent of 2.8 µA at 28 GHz and more than two orders of magnitude linear dynamic range. The findings are essential to bringing to fruition type-II Dirac fermions in photonics, spintronics, and optoelectronics.     

Electrochemically Triggered Metal–Insulator Transition between VO2 and V2O5

Distinct properties of multiple phases of vanadium oxide (VO_x) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VO_x are crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VO_x thin films under a wide range of temperatures. Reversible phase transitions between two adjacent VO_x phases, VO₂ and V₂O₅, are obtained. Cathodic biases trigger the phase transition from V₂O₅ to VO₂, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time‐dependent response of the X‐ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO₂ and V₂O₅ demonstrates the ability to induce major changes in the electronic properties of VO_x by spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.     

Dichotomy in the Lithiation Pathway of Ellipsoidal and Platelet LiFePO4 Particles Revealed through Nanoscale Operando State‐of‐Charge Imaging

LiFePO₄ is a promising phase‐separating battery electrode and a model system for studying lithiation. The role of particle synthesis and the corresponding particle morphology on the nanoscale insertion and migration of Li is not well understood, and elucidating the intercalation pathway is crucial toward improving battery performance. A synchrotron operando liquid X‐ray imaging platform is developed to track the migration of Li in LiFePO₄ electrodes with single‐particle sensitivity. Lithiation is tracked in two particle types—ellipsoidal and platelet—while the particles cycle in an organic liquid electrolyte, and the results show a clear dichotomy in the intercalation pathway. The ellipsoidal particles intercalate sequentially, concentrating the current in a small number of actively intercalating particles. At the same cycling rate, platelet particles intercalate simultaneously, leading to a significantly more uniform current distribution. Assuming that the particles intercalate through a single‐phase pathway, it is proposed that the two particle types exhibit different surface properties, a result of different synthesis procedures, which affect the surface reactivity of LiFePO₄. Alternatively, if the particles intercalate through nucleation and growth, the larger size of platelet particles may account for the dichotomy. Beyond providing particle engineering insights, the operando microscopy platform enables new opportunities for nanoscale chemical imaging of liquid‐based electrochemical systems.     

(Pb1–xCdx)S Nanoparticles Embedded in a Conjugated Organic Matrix,as Studied by Photoluminescence and Light‐Induced X‐ray Photoelectron Spectroscopy

Elliptically shaped (Pb_1–xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p‐phenylene‐ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi‐exponential behavior, is detected in time‐resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co‐existence of Cd and Pb within the same particle is provided by light‐induced X‐ray photoelectron spectroscopy.     

Revisiting the Chemical Stability of Germanium Selenide (GeSe) and the Origin of its Photocatalytic Efficiency

Recently, germanium selenide (GeSe) has emerged as a promising van der Waals semiconductor for photovoltaics, solar light harvesting, and water photoelectrolysis cells. Contrary to previous reports claiming perfect ambient stability based on experiments with techniques without surface sensitivity, here, by means of surface-science investigations and density functional theory, it is demonstrated that actually both: i) the surface of bulk crystals; and ii) atomically thin flakes of GeSe are prone to oxidation, with the formation of self-assembled germanium-oxide skin with sub-nanometric thickness. Surface oxidation leads to the decrease of the bandgap of stoichiometric GeSe and GeSe_1−x, while bandgap energy increases upon surface oxidation of Ge_1−xSe. Remarkably, the formation of a surface oxide skin on GeSe crystals plays a key role in the physicochemical mechanisms ruling photoelectrocatalysis: the underlying van der Waals semiconductor provides electron–hole pairs, while the germanium-oxide skin formed upon oxidation affords the active sites for catalytic reactions. The self-assembled germanium-oxide/germanium-selenide heterostructure with different bandgaps enables the activation of photocatalytic processes by absorption of light of different wavelengths, with inherently superior activity. Finally, it is discovered that, depending on the specific solvent-GeSe interaction, the liquid phase exfoliation of bulk crystals can induce the formation of Se nanowires.