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This study proposed an unsupervised machine-learning approach for analyzing spatially-resolved ARPES. A combination of non-negative matrix factorization (NMF) and k-means clustering was applied to spatially-resolved ARPES spectra of the graphene epitaxially grown on a SiC substrate. The Dirac cones of graphene were decomposed and reproduced fairly well using NMF. The base and activation matrices obtained from the NMF results reflected the detailed spectral features derived from the number of graphene layers and growth directions. The spatial distribution of graphene thickness on the substrate was clearly visualized by the clustering using the activation matrices acquired via NMF. Integration with k-means clustering enables clear visualization of spatial variations. Our method efficiently handles large datasets, extracting spectral features without manual inspection. It offers broad applicability beyond graphene studies to analyze ARPES spectra in various materials. [ABSTRACT FROM AUTHOR]

Despite the striking increase in the power conversion efficiency (PCE) of lead‐based perovskite solar cells (PSCs), their poor operational stability impedes their commercialization. Among the various factors that influence device stability, ion migration has been identified as a key driver of degradation. In this work, the focus is on studying ion migration‐induced degradation in inverted architecture PSCs, which employ either a thin polymer layer or a self‐assembled monolayer (SAM) for hole extraction. It is demonstrated that the difference in texture imposed by the use of these hole transport layers (HTL) is an important and thus far inconspicuous factor that impacts ion migration, and consequently device stability. By investigating the buried interface in detail, it is revealed that its texture has a strong impact on the vertical compositional stratification in the perovskite active layer. By monitoring bias‐induced ion migration in devices with different hole extraction layers, it is demonstrated that the smooth polymer‐based HTL results in a higher degree of ion migration than the rough SAM HTL, corresponding to a stronger degradation in the former. These results further indicate that the use of SAMs for hole extraction is a promising strategy to suppress ion migration and improve device efficiency. [ABSTRACT FROM AUTHOR]

Abstract This study proposed an unsupervised machine-learning approach for analyzing spatially-resolved ARPES. A combination of non-negative matrix factorization (NMF) and k-means clustering was applied to spatially-resolved ARPES spectra of the graphene epitaxially grown on a SiC substrate. The Dirac cones of graphene were decomposed and reproduced fairly well using NMF. The base and activation matrices obtained from the NMF results reflected the detailed spectral features derived from the number of graphene layers and growth directions. The spatial distribution of graphene thickness on the substrate was clearly visualized by the clustering using the activation matrices acquired via NMF. Integration with k-means clustering enables clear visualization of spatial variations. Our method efficiently handles large datasets, extracting spectral features without manual inspection. It offers broad applicability beyond graphene studies to analyze ARPES spectra in various materials.

The soft X-ray photoelectron momentum microscopy (PMM) experimental station at the UVSOR Synchrotron Facility has been recently upgraded by additionally guiding vacuum ultraviolet (VUV) light in a normal-incidence configuration. PMM offers a very powerful tool for comprehensive electronic structure analyses in real and momentum spaces. In this work, a VUV beam with variable polarization in the normal-incidence geometry was obtained at the same sample position as the soft X-ray beam from BL6U by branching the VUV beamline BL7U. The valence electronic structure of the Au(111) surface was measured using horizontal and vertical linearly polarized (s-polarized) light excitations from BL7U in addition to horizontal linearly polarized (p-polarized) light excitations from BL6U. Such highly symmetric photoemission geometry with normal incidence offers direct access to atomic orbital information via photon polarization-dependent transition-matrix-element analysis.

Photoemission spectroscopy, an experimental method based on the photoelectric effect, is now an indispensable technique used in various fields such as materials science, life science, medicine and nanotechnology. However, part of the experimental process of photoemission spectroscopy relies on experience and intuition, which is obviously a problem for novice users. In particular, photoemission spectroscopy experiments using high-brilliance synchrotron radiation as a light source are not easy for novice users because measurements must be performed quickly and accurately as scheduled within a limited experimental period. In addition, research on the application of information science methods to quantum data measurement, such as photoemission spectroscopy, is mainly aimed at the development of analysis methods, and few attempts have been made to clarify the problems faced by users who lack experience. In this study, the problems faced by novice users of photoemission spectroscopy are identified, and a native application named synapse with functions to solve these problems is implemented and evaluated qualitatively and quantitatively. This paper describes the contents of an interview survey, the functional design and the implementation of the application synapse based on the interview survey, and results and discussion of the evaluation experiment.

A flat band structure in momentum space is considered key for the realization of novel phenomena. A topological flat band, also known as a drumhead state, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized surface state in a topological semimetal BaAl4 and provide its full energy and momentum space topology. We find that the observed surface state is localized in momentum, inside a square-shaped bulk Dirac nodal loop, and in energy, leading to a flat band and a peak in the density of state. These results imply this class of materials as an experimental realization of drumhead surface states and provide an important reference for future studies of the fundamental physics of correlated quantum effects in topological materials.

Zinc oxide (ZnO) nanomaterials are promising components for chemical and biological sensors and photocatalytic conversion and operate as electron collectors in photovoltaic technologies. Many of these applications involve nanostructures in contact with liquids or exposed to ambient atmosphere. Under these conditions, single-crystal ZnO surfaces are known to form narrow electron accumulation layers with few nanometer spatial penetration into the bulk. A key question is to what extent such pronounced surface potential gradients can develop in the nanophases of ZnO, where they would dominate the catalytic activity by modulating charge-carrier mobility and lifetimes. Here, we follow the temperature-dependent surface electronic structure of nanoporous ZnO with photoemission spectroscopy to reveal a sizable, spatially averaged downward band bending for the hydroxylated state and a conservative upper bound of

The electrical and mechanical properties of graphene‐based materials can be tuned by the introduction of nanopores, which are sensitively related to the size, morphology, density, and location of nanopores. The synthesis of low‐dimensional graphene nanostructures containing well‐defined nonplanar nanopores has been challenging due to the intrinsic steric hindrance. Herein, we report the selective synthesis of one‐dimensional (1D) graphene nanoribbons (GNRs) containing periodic nonplanar [14]annulene pores on Ag(111) and two‐dimensional (2D) porous graphene nanosheet containing periodic nonplanar [30]annulene pores on Au(111), starting from a same precursor. The formation of distinct products on the two substrates originates from the different thermodynamics and kinetics of coupling reactions. The reaction mechanisms were confirmed by a series of control experiments, and the appropriate thermodynamic and kinetic parameters for optimizing the reaction pathways were proposed. In addition, the combined scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations revealed the electronic structures of porous graphene structures, demonstrating the impact of nonplanar pores on the π‐conjugation of molecules. [ABSTRACT FROM AUTHOR]

The extensive use of perovskites as light absorbers calls for a deeper understanding of the interaction of these materials with light. Here, the evolution of the chemical and optoelectronic properties of formamidinium lead tri‐bromide (FAPbBr3) films is tracked under the soft X‐ray beam of a high‐brilliance synchrotron source by photoemission spectroscopy and micro‐photoluminescence. Two contrasting processes are at play during the irradiation. The degradation of the material manifests with the formation of Pb0 metallic clusters, loss of gaseous Br2, decrease and shift of the photoluminescence emission. The recovery of the photoluminescence signal for prolonged beam exposure times is ascribed to self‐healing of FAPbBr3, thanks to the re‐oxidation of Pb0 and migration of FA+ and Br− ions. This scenario is validated on FAPbBr3 films treated by Ar+ ion sputtering. The degradation/self‐healing effect, which is previously reported for irradiation up to the ultraviolet regime, has the potential of extending the lifetime of X‐ray detectors based on perovskites. [ABSTRACT FROM AUTHOR]

Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.

The performance of five hole‐transporting layers (HTLs) is investigated in both single‐junction perovskite and Cu(In, Ga)Se2 (CIGSe)‐perovskite tandem solar cells: nickel oxide (NiOx,), copper‐doped nickel oxide (NiOx:Cu), NiOx+SAM, NiOx:Cu+SAM, and SAM, where SAM is the [2‐(3,‐6Dimethoxy‐9H‐carbazol‐9yl)ethyl]phosphonic acid (MeO‐2PACz) self‐assembled monolayer. The performance of the devices is correlated to the charge‐carrier dynamics at the HTL/perovskite interface and the limiting factors of these HTLs are analyzed by performing time‐resolved and absolute photoluminescence ((Tr)PL), transient surface photovoltage (tr‐SPV), and X‐ray/UV photoemission spectroscopy (XPS/UPS) measurements on indium tin oxide (ITO)/HTL/perovskite and CIGSe/HTL/perovskite stacks. A high quasi‐Fermi level splitting to open‐circuit (QFLS‐Voc) deficit is detected for the NiOx‐based devices, attributed to electron trapping and poor hole extraction at the NiOx‐perovskite interface and a low carrier effective lifetime in the bulk of the perovskite. Simultaneously, doping the NiOx with 2% Cu and passivating its surface with MeO‐2PACz suppresses the electron trapping, enhances the holes extraction, reduces the non‐radiative interfacial recombination, and improves the band alignment. Due to this superior interfacial charge‐carrier dynamics, NiOx:Cu+SAM is found to be the most suitable HTL for the monolithic CIGSe‐perovskite tandem devices, enabling a power‐conversion efficiency (PCE) of 23.4%, Voc of 1.72V, and a fill factor (FF) of 71%, while the remaining four HTLs suffer from prominent Voc and FF losses. [ABSTRACT FROM AUTHOR]

Determining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub‐bandgap defect states in organic semiconductors. Herein, ER‐EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER‐EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER‐EIS agree with sub‐bandgap features detected by PDS. Surprisingly, ER‐EIS measurements of regiorandom and regioregular poly(3‐hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT. [ABSTRACT FROM AUTHOR]

Here, I fill large 1.7 nm diameter single-walled carbon nanotubes (SWCNTs) with silver chloride (AgCl). I present photoemission insights into the filling of SWCNTs. C1s X-ray photoelectron spectroscopy (XPS) reveals the p-doping of SWCNTs. The Raman spectroscopy data are complementary to the XPS data, and they confirm the strong doping effect of encapsulated silver chloride on SWCNTs. [ABSTRACT FROM AUTHOR]

α‐SnWO4 is an emerging photoelectrode material for photoelectrochemical water splitting, with several promising properties such as the favorable bandgap of 1.9 eV and suitable positions of the valence and conduction band. However, a major challenge remains: unprotected α‐SnWO4 undergoes surface passivation that blocks further charge transfer, and α‐SnWO4 electrodes that are covered with a protection/catalytic overlayer (e.g., NiOx, CoOx) show limited photovoltage. Earlier studies reveal that interfacial oxidation occurs due to the deposition of the overlayer. This negatively impacts the photovoltage that can be extracted, which is attributed to Fermi‐level pinning at the interface. The exact origin of this Fermi‐level pinning mechanism, however, remains unclear. In the present study, a combination of surface photovoltage analysis and hard X‐ray photoelectron spectroscopy is used to elucidate the electronic structure of the α‐SnWO4/oxide interface. Both techniques offer compelling and consistent evidence for the presence of a defect state that is energetically located within the bandgap energy of α‐SnWO4 and is likely responsible for the Fermi‐level pinning. [ABSTRACT FROM AUTHOR]

The electron response of nanoplasmon and nanoplasma in the laser field is greatly important for improving the functionality and efficiency of many potential applications. However, how and under what conditions the nanoplasmon‐nanoplasma transition works remains poorly understood due to radiation damage and charge buildup. Utilizing the combined aerodynamic lens and velocity map imaging spectrometer, this transition mediated by different mechanisms are demonstrated, as verified by the distinct photoelectron momentum distributions from Cu nanoparticles. Initially the polarization‐dependent distributions emphasize the domination of surface emission driven by the plasmonic field. Subsequently the transition starts and the competition of volume‐multiphoton and thermionic emission plays the intermediate state dominating the transition process. Finally, a complete plasma is generated with the leading role of thermionic emission, which is confirmed by first observing the correlated electronic decay in metal nanoparticles. These findings bridge the gap between electron emission in nanoplasmonic and nanoplasma states and identify the mechanism‐specific contributions, which will offer general principles for designing nanostructure and laser fields to control electron excitation and emission in practice. [ABSTRACT FROM AUTHOR]

Abstract Determining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub‐bandgap defect states in organic semiconductors. Herein, ER‐EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER‐EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER‐EIS agree with sub‐bandgap features detected by PDS. Surprisingly, ER‐EIS measurements of regiorandom and regioregular poly(3‐hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT.

The chemical and electronic structures in the near-surface region of Cu(In,Ga)Se2 thin-film solar cell absorbers are investigated using nondestructive soft and hard X-ray photoelectron spectroscopy. In addition to a pronounced surface Cu-depletion, the [Ga]/([In]+[Ga]) composition indicates that the topmost surface is Ga-poor (or In-rich). For the studied depth region, common depth profiling techniques generally fail to provide reliable information and, thus, the near-surface chemical and electronic structure profiles are often overlooked. The relation between the observed near-surface elemental compositions and the derived electronic properties of the absorber material is discussed. It is found that the surface band gap energy crucially depends on the Cu-deficiency of the absorber surface and suggests that it is, in this region, only secondarily determined by the [Ga]/([In]+[Ga]) ratio.

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.

Abstract Ambipolar transistor operation in SnO thin‐film transistors (TFTs) is a promising character for future practical application, such as in integrated logic devices based on oxide semiconductors, because of its ability to develop them using a single material. However, there are only a few reports that demonstrate the apparent ambipolar operation for SnO TFTs owing to the insufficient knowledge on the reasons for deterioration of device performance. Although a previous study controls the operation mode of SnO TFTs [A. W. Lee et al. Adv. Electron. Mater. 6, 200742 (2020)], an additional passivating layer is required; however, it hinders the benefits of SnO usage. In this study, we provide the mechanism of deterioration of the ambipolar character of bare SnO TFTs, that is, the origin of defect states near the valence band maximum (near‐VB defect). Comprehensive spectroscopic approaches including photoemission, X‐ray emission, and X‐ray absorption spectroscopy, reveal that near‐VB defect states originate from oxygen vacancies, existing at the surface, and also in the entire SnO film. This finding is useful to improve device performance for obtaining the ambipolar operation of SnO TFTs.

Photoemission spectroscopy (PES) is an underrepresented part of current and past studies of compositionally complex alloys (CCA) such as high-entropy alloys (HEA) and their derivatives. PES studies are very important for understanding the electronic structure of materials, and are therefore essential in some cases for a correct description of the intrinsic properties of CCAs. Here, we present several examples showing the importance of PES. First, we show how the difference between the split-band structure and the common-band structure of the valence band (VB), observed by PES, can explain a range of properties of CCAs and alloys in general. A simple description of the band crossing in CCAs composed from the early and late transition metals showing a split band is discussed. We also demonstrate how a high-accuracy PES study can determine the variation in the density of states at the Fermi level as a function of Cu content in Ti-Zr-Nb-Ni-Cu metallic glasses. Finally, the first results of an attempt to single out the contributions of particular constituents in Cantor-type alloys to their VBs are presented. The basic principles of PES, the techniques employed in studies presented, and some issues associated with PES measurements are also described. [ABSTRACT FROM AUTHOR]

In this paper, the growth mechanism, structure, growth processes, growth kinetics, and optical, vibronic and electronic properties of metallocene-filled single-walled carbon nanotubes (SWCNTs) are considered. A description of the procedures used to fill the nanotubes is provided. An investigation of doping effects on metallicity-mixed SWCNTs filled with metallocenes by Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy, photoemission spectroscopy, and optical absorption spectroscopy is described. The studies of doping effects on metallicity-sorted SWCNTs filled with metallocenes are discussed. Doping effects in metallicity-mixed and sorted SWCNTs upon the chemical transformation of encapsulated molecules are analyzed. A discussion of the modification of the electronic properties of filled SWCNTs is presented. Applications of metallocene-filled SWCNTs in electrochemistry, thermoelectric power generation, chemical sensors, and magnetic recording are discussed. [ABSTRACT FROM AUTHOR]

This review discusses the phemenology of filling, the investigation of kinetics, and the electronic properties for applications of filled single-walled carbon nanotubes (SWCNTs), and summarizes five main achievements that were obtained in processing the spectroscopic data of SWCNTs filled with metal halogenide, metal chalcogenide, metal and metallocenes. First, the methods of processing kinetic data were developed to reveal precise trends in growth rates and activation energies of the growth of SWCNTs. Second, the metal-dependence of kinetics was revealed. Third, metallicity-sorted (metallic and semiconducting) SWCNTs were filled with a range of substances and the electronic properties were investigated. Fourth, new approaches to processing the data of spectroscopic investigations of filled SWCNTs were developed, which allowed more reliable and precise analysis of the experimental results. Fifth, the correlation between the physical and chemical properties of encapsulated substances and the electronic properties of SWCNTs were elucidated. These points are highlighted in the review. [ABSTRACT FROM AUTHOR]

The paper is dedicated to the discussion of kinetics of growth, and electronic properties of filled carbon nanotubes investigated by spectroscopy for applications. The paper starts with discussion of growth of carbon nanotubes inside metallocene-filled carbon nanotubes. Nickelocene, cobaltocene are considered for growth of carbon nanotubes. Then, the investigations of filled carbon nanotubes by four spectroscopic techniques are discussed. Among them are Raman spectroscopy, near edge X-ray absorption fine-structure spectroscopy, photoemission spectroscopy, optical absorption spectroscopy. It is discussed that metal halogenides, metal chalcogenides, metals lead to changes in electronic structure of nanotubes with n- or p-doping. The filling of carbon nanotubes with different organic and inorganic substances results in many promising applications. This review adds significant contribution to understanding of the kinetics and electronic properties of filled SWCNTs with considering new results of recent investigations. Challenges in various fields are analyzed and summarized, which shows the author's viewpoint of progress in the spectroscopy of filled SWCNTs. This is a valuable step toward applications of filled SWCNTs and transfer of existing ideas from lab to industrial scale. [ABSTRACT FROM AUTHOR]

• The review aims to introduce the researches of salt solvation in water. • Different environmental conditions play a pivotal role in microsolvation. • Understanding the mechanisms of solvation for alkali metal salts is a key focus. • Both theoretical and experimental techniques are used to study microsolvation. • The review analyzes the applications of microsolvation knowledge. Microsolvation is of paramount importance in various phenomena of physical chemistry and biological systems. Structural diversity of chemical clusters in different environmental conditions like in the aqueous phase, gas-phase, and the interior of a host plays pivotal roles in various systems. For mechanisms involved in the solvation of alkali metal salts at the microscopic level along with measurements of their thermodynamic equilibria, equilibrium constants, and water-cation bond energies. Many attempts in the field of theoretical as well as experimental chemistry have been reported. Here, we aim to review the literature based on experimental approaches such as photoelectron spectroscopy, photoemission spectroscopy, Fourier transform emission spectroscopy, FT-IR, Raman spectroscopy, and theoretical studies i.e., density functional theory studies or molecular dynamics simulations studies for microsolvation. The current review provides detailed information about the phenomenon of salt solvation, including its applications and the roles of in vitro and in silico approaches in various aspects of this research field. The comprehensive approach presented in this study offers a deeper understanding of the microsolvation of salts in water, which has implications in various fields such as chemistry, materials science, biology, and environmental science. The findings can contribute to the development of improved models for understanding the behavior of electrolyte solutions, the design of new materials, the optimization of chemical processes, and the development of more efficient water treatment methods. [ABSTRACT FROM AUTHOR]

α‐RuCl3 is a candidate material for the realization of a Kitaev spin liquid with envisioned applications for quantum computing. It is a van‐der‐Waals material with in‐plane honeycomb lattice equivalent to the CrX3 (X= Cl, Br, I) type 2D magnets. Here, possibilities of defect engineering and surface modification of thin crystals are explored by Ar+ dosing and vacuum annealing. Chlorine is easily removed from the surface, which reduces the Ru valence and eventually leaves a Ru rich surface layer behind. A peculiar thickness dependence of the work function emerges after Ar+ sputtering, which is ascribed to the remaining chlorine concentration. This work elucidates material properties of thin α‐RuCl3 and introduces concepts of property engineering to create homojunctions and control level alignment by standard in situ methods. [ABSTRACT FROM AUTHOR]

We investigated Heusler-type Ru2VAl, a candidate material for next-generation thermoelectric conversion, by first-principle calculations of its thermoelectric conversion properties and direct experimental observations of its electronic structures, employing photoemission and infrared spectroscopy. Our results show that Ru2VAl has a wider pseudogap near the Fermi level compared to Fe2VAl. Accordingly, a higher thermoelectric conversion performance can be expected in Ru2VAl at higher temperatures. [ABSTRACT FROM AUTHOR]

Two-dimensional halide perovskites are promising for advanced photonic, optoelectronic, and photovoltaic applications. However, their long-term stability is still a critical factor limiting their implementation into further commercial applications. Here, we present an environmental stability analysis of BA 2 (MA) n- 1 Pb n I 3 n +1 (BA = C 4 H 12 N + , MA = CH 6 N + ) two-dimensional perovskites with the lowest quantum well thicknesses of n = 1 and n = 2, after 1 year of aging under ambient humidity, oxygen content, and light conditions. We observed that both crystal phases ( n = 1 and 2) degraded similarly, resulting in the removal of organic components and crystal decomposition into PbI 2 , Pb oxides, and Pb hydroxides. However, we have found a significant difference between their aging under ambient light and dark conditions, affecting their degraded morphology and photoactivity. Both crystal phases exposed to ambient light aged into a morphology characterized by the formation of several pinholes and voids, accompanied by photoluminescence degradation. Samples stored under dark conditions surprisingly preserved their photoluminescence activity, which morphologically aged into microrod structures. We conclude that the observed loss of photoactivity of 2D perovskites aged under ambient light is attributed to photoaccelerated degradation processes causing faster crystal surface photo-oxidation accompanied by a creation of multiple I vacancies and hydration of the inner crystal. The retainment of photoactivity in 2D perovskites aged under dark conditions is attributed to slower surface oxidation processes into Pb salts, as confirmed by X-ray photoemission spectroscopy. The formed surface layer even allows for a layer-by-layer degradation and acts as a protection barrier against further additional loss of I atoms and the consequent hydration of the inner part of samples. We demonstrate that light is the most critical external factor accelerating 2D perovskite degradation processes in ambient air and thus affecting their long-term stability. We conclude in this work that perovskite material structural engineering together with their surface passivation or encapsulation strategical techniques applied is an essential step for their further application into long-term stable commercial devices.

The Ullmann coupling of aryl halides is a powerful method in the on-surface synthesis of functional materials. Understanding its basic aspects and influencing factors can aid in the use of this tool for the fabrication of intriguing structures. In this study, we unveil (1) the origin of the shift in the elemental binding energy (BE) and (2) the functions of atomic hydrogen (AH) in a typical Ullmann coupling system using combined spectroscopy and microscopy techniques. During debromination of the aryl halide precursor, the work function (WF) alteration is correlated with the surface Br amount. The WF change instead of C-Ag formation is proposed to play a dominant role in the shift of the molecular C 1s BE. AH dosing onto organometallic chains leads to chain decomposition and surface Br removal. In contrast, AH dosing onto covalent poly( para- phenylene) (PPP) chains results in superhydrogenation in addition to Br removal. The C 1s BE shift is attributed to both WF change and superhydrogenation effects. Thermal annealing restores the PPP chains by eliminating superhydrogenation, which causes the C 1s BE to shift to a high BE. This study provides deep insights into the mechanisms of Ullmann coupling on surfaces, highlighting the significant role of WF alterations and AH treatments in these processes.

The kagome metal FeSn consists of alternating layers of kagome-lattice Fe 3 Sn and honeycomb Sn 2 and exhibits great potential for applications in future low-energy electronics and spintronics because of an ideal combination of topological phases and high-temperature magnetic ordering. Robust synthesis methods for ultrathin FeSn films, as well as an understanding of their air stability, are crucial for its development and long-term operation in future devices. In this work, we realize large-area, <10 nm thick, epitaxial FeSn thin films and explore the oxidation process via synchrotron-based photoelectron spectroscopy using in situ oxygen and water dosing, as well as ex situ air exposure. Upon exposure to the atmosphere, the FeSn films are shown to be highly reactive, with a stable ∼3 nm thick oxide layer forming at the surface within 10 min. Notably, the surface Fe remains largely unoxidized when compared with Sn, which undergoes near-complete oxidation. Additionally, the band structure remains metallic under oxygen exposure. These are further confirmed with controlled in situ dosing of O 2 and H 2 O, where only the Sn 2 (stanene) interlayers within the FeSn lattice oxidize, suggesting the Fe 3 Sn kagome layers remain almost pristine. These results are in excellent agreement with first-principles calculations, which show that Fe-O bonds to the Fe 3 Sn layer are energetically unfavorable and a large formation energy preference of 1.37 eV for Sn-O bonds in the stanene Sn 2 layer over Sn-O bonds in the kagome Fe 3 Sn layer. The demonstration that oxidation only occurs within the stanene layers and the preservation of the Dirac bands may provide additional avenues in how to engineer, handle, and prepare future kagome metal devices.

The fullerene C60 is commonly applied as the electron transport layer in high‐efficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra‐sensitive near‐UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5–10 nm, the perovskite/C60 interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement‐induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C60 HOMO‐edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C60 interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C60. [ABSTRACT FROM AUTHOR]

Lead halide perovskites are semiconductor materials which are employed as nonintentionally doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p‐i‐n or n‐i‐p heterojunction. In our study, we have developed and investigated a lateral device, based on methylammonium lead iodide (MAPbI3) in which the p‐i‐n heterojunction develops in the horizontal direction. Our research suggests that the effective doping level in the MAPbI3 film should be very low, below 1012 cm−3. Along the vertical direction, this doping level is not enough to screen the electric field of the buried heterojunction with the SCTL. The perovskite work function is therefore affected by the work function of the SCTL underneath. From drift‐diffusion simulations, we show that intrinsic perovskite‐SCTL structures develop mV range surface photovoltages (SPVs) under continuous illumination. However, perovskite‐SCTL structures can develop SPVs of hundreds of mV, as confirmed by our measurements. We therefore analyzed the compatibility between low doping and low defect densities in the perovskite layer and such high SPV values using numerical modeling. It is shown that these high SPV values could originate from electronic processes due to large band offsets in the buried perovskite‐SCTL heterojunctions, or at the SCTL‐transparent conductive oxide (TCO) buried heterojunction. However, such electronic processes can hardly explain the long SPV persistence after switching off the illumination. [ABSTRACT FROM AUTHOR]

Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl. [ABSTRACT FROM AUTHOR]

Ambipolar transistor operation in SnO thin‐film transistors (TFTs) is a promising character for future practical application, such as in integrated logic devices based on oxide semiconductors, because of its ability to develop them using a single material. However, there are only a few reports that demonstrate the apparent ambipolar operation for SnO TFTs owing to the insufficient knowledge on the reasons for deterioration of device performance. Although a previous study controls the operation mode of SnO TFTs [A. W. Lee et al. Adv. Electron. Mater. 6, 200742 (2020)], an additional passivating layer is required; however, it hinders the benefits of SnO usage. In this study, we provide the mechanism of deterioration of the ambipolar character of bare SnO TFTs, that is, the origin of defect states near the valence band maximum (near‐VB defect). Comprehensive spectroscopic approaches including photoemission, X‐ray emission, and X‐ray absorption spectroscopy, reveal that near‐VB defect states originate from oxygen vacancies, existing at the surface, and also in the entire SnO film. This finding is useful to improve device performance for obtaining the ambipolar operation of SnO TFTs. [ABSTRACT FROM AUTHOR]

The electronic structure of single-crystalline EuO (100) ultrathin films, together with their exchange splitting energy (ΔEEX) and ferromagnetic phase transition temperature (TC), was investigated by temperature- and thickness-dependent angle-resolved photoemission spectroscopy. Both ΔEEX and TC decreased monotonically with decreasing film thickness. The band shift showed an opposite thickness dependence at the Γ and X points, reflecting the balance of the hybridization between the Eu 4f and O 2p states (super-exchange interaction) and between the Eu 4f, O 2p, and Eu 5d states (indirect-exchange interaction). The observed transition from an indirect energy gap in the bulk to a direct gap in the ultrathin films of the ferromagnetic semiconductor EuO could be potential in future spintronic devices.

Here, I fill large 1.7 nm diameter single-walled carbon nanotubes (SWCNTs) with silver chloride (AgCl). I present photoemission insights into the filling of SWCNTs. C1s X-ray photoelectron spectroscopy (XPS) reveals the p-doping of SWCNTs. The Raman spectroscopy data are complementary to the XPS data, and they confirm the strong doping effect of encapsulated silver chloride on SWCNTs.

[Display omitted] • The impact of Fermi level pinning on energy level alignment in PbS quantum dot is examined. • UPS analyses indicate that energetic position of PbS QD's gap-state to be c.a. 4.0 eV. • The pinning factors (S) for PbS QD-EDT and PbS-TBAI are empirically determined by UPS and LEIPES. • Higher S for PbS-EDT indicate the TBAI more effectively passivates the gap-state of PbS QD. PbS quantum dots capped by ethanedithiol (PbS QD-EDT) and tetrabutylammonium iodide (PbS QD-TBAI) and supported by different substrates were examined in terms of Fermi level pinning (FLP), gap states, and electron and hole barriers (Φ e and Φ h , respectively) using ultraviolet and low-energy inverse photoemission spectroscopy. The former analysis showed that TBAI and EDT differed in their ability to induce gap-state passivation, with the corresponding energy difference determined as 4.0 eV. Two FLP regimes were identified: at substrate work function (Ф sub) < 4.0 eV, both ligands showed perfect FLP (S ≈ 0 for holes and electrons), whereas at Ф sub > 4.0 eV, the pinning strength of PbS QD-EDT (S of Ф b,h and Ф b,e = 0.19 and 0.24, respectively) exceeded that of PbS QD-TBAI (S of Ф b,h and Ф b,e = 0.53 and 0.57, respectively). Thus, the gap states were more effectively passivated in the case of PbS QD-TBAI. Our results indicate the importance of considering FLP strength when working with high-work-function substrates for the design of optimized QD-based devices. [ABSTRACT FROM AUTHOR]

When Si atoms are intercalated under the graphene-like zero layer (ZL) generated on the SiC(0001) substrate, the resulting quasi-free-standing monolayer graphene (QFMLG) has lots of crater-like defects due to a strong Si-C reaction. Here, a method for creating crater-free QFMLG above a two-dimensional Si film is investigated using scanning tunneling microscopy and photoemission spectroscopy. The method involves forming a SiSn interfacial film as a precursor via cointercalation performed by sequential Sn and Si deposition on the ZL at room temperature and annealing at 650–700 ° C. The Sn atoms prevent reactive Si atoms from directly contacting the ZL, which would otherwise cause SiC defects. The sample is then annealed at 750 ° C to selectively remove the Sn atoms from the SiSn interfacial film, leaving a well-ordered Si film. The SiSn film composed of a bottom Si layer and top Sn atoms has a short-range-ordered "2 × 2" or a long-range-ordered 2 × 7 superstructure depending on the annealing temperature and the Sn coverage. Both the superstructures are semiconducting and induce less n -doped QFMLG than the pure Si film. [Display omitted] • A method for creating crater-free graphene above a 2D Si film on SiC is investigated. • SiSn cointercalation and selective Sn intercalation form a well-ordered 2D Si film. • The Sn atoms prevent reactive Si atoms from directly contacting C atoms. • The SiSn film is composed of a bottom Si layer and top Sn atoms. • The semiconducting SiSn film induces less n doping to graphene than the pure Si film. [ABSTRACT FROM AUTHOR]

In spite of great application potential as transparent n-type oxides with high electrical mobility at room temperature, threading dislocations (TDs) often found in the (Ba,La)SnO3 (BLSO) films can limit their intrinsic properties so that their role in the physical properties of BLSO films need to be properly understood. The electrical properties and electronic structure of BLSO films grown on SrTiO3 (001) (STO) and BaSnO3 (001) (BSO) substrates are comparatively studied to investigate the effect of the TDs. In the BLSO/STO films with TD density of ~1.32 × 1011 cm−2, n-type carrier density ne and electron mobility are significantly reduced, as compared with the BLSO/BSO films with nearly no TDs. This indicates that TDs play the role of scattering-centers as well as acceptor-centers to reduce n-type carriers. Moreover, in the BLSO/STO films, both binding energies of an Sn 3d core level and a valence band maximum are reduced, being qualitatively consistent with the Fermi level shift with the reduced n-type carriers. However, the reduced binding energies of the Sn 3d core level and the valence band maximum are clearly different as 0.39 and 0.19 eV, respectively, suggesting that the band gap renormalization preexisting in proportion to ne is further suppressed to restore the band gap in the BLSO/STO films with the TDs. [ABSTRACT FROM AUTHOR]

Oxide semiconductors are key materials in many technologies from flat‐panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p‐type oxide semiconductors due to the localized O‐2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba2BiMO6 (M = Bi, Nb, Ta) via the Bi 6s2 lone pair state to achieve p‐type oxide semiconductors with high hole mobility up to 21 cm2 V−1 s−1, and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x‐ray photoemission, x‐ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba2BiMO6. The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4–0.7 me). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P–N junction diode constructed with p‐type Ba2BiTaO6 and n‐type Nb doped SrTiO3 exhibits high rectifying ratio of 1.3 × 104 at ±3 V, showing great potential in fabricating high‐quality devices. This work provides deep insight into the electronic structure of Bi3+ based perovskites and guides the development of new p‐type oxide semiconductors. [ABSTRACT FROM AUTHOR]