Auger photoelectron coincidence spectroscopy (APECS) has emerged as a powerful tool for probing electron-electron correlation in solids. When a photoexcited core hole decays via a core-valence-valence Auger transition, two valence holes are created in the final state. Consequently, the line shape of the Auger electron spectrum is influenced by both Coulomb and exchange interactions between the two holes. By simultaneously detecting the photoelectron and Auger electron emitted from a single photoionization event, APECS measurements place constraints on the two final state holes, enabling one to independently measure Coulomb and exchange correlation energies. This review highlights the key advantages of APECS over other electron spectroscopic techniques including separating overlapping spectral features, removing the background signal from inelastically scattered electrons, enhanced surface sensitivity and site specificity. It also cites examples of how these attributes can be used to investigate the properties of solids in an unprecedented manner. To aid in understanding APECS data and assist in experimental design, a phenomenological model for the probability of electron pair emission as a function of the photo- and Auger electron kinetic energies and emission angles ispresented. The model is applied to a hypotheticalsolid, demonstrating how the contribution to APECS spectrum of final states with different spin configurations depends on these experimental parameters. A summary of results obtained by performing one-dimensional (as a function of Auger [EA]orphotoelectron [EP] kinetic energy), two-dimensional (parallel detection ofEAandEP) and angle-resolved (AR) (as a function ofEAat specific photo- and Auger electron emission angles) APECS measurements is presented. In particular, measurements of ferromagnetic metals and antiferromagnetic transition metal oxides demonstrate how AR-APECS is a powerful tool for independently measuring Coulomb and exchange correlation energies. Finally, potential future applications of APECS and further developments of this experimental technique are discussed.

Antiferromagnetic states with no stray magnetic fields can enable high-density ultra-fast spintronic technologies. However, the detection and control of antiferromagnetic Néel vectors remain challenging. Linear magnetoelectric antiferromagnets (LMAs) may provide new pathways, but applying simultaneous electric and magnetic fields, necessary to control Néel vectors in LMAs, is cumbersome and impractical for most applications. Herein, we show that Cr 2 O 3 , a prototypical room-temperature LMA, carries a topologically-protected surface magnetism in all surfaces, which stems from intrinsic surface electric fields due to band bending, combined with the bulk linear magnetoelectricity. Consequently, bulk Néel vectors with zero bulk magnetization can be simply tuned by magnetic fields through controlling the magnetizations associated with the surface magnetism. Our results imply that the surface magnetizations discovered in Cr 2 O 3 should be also present in all LMAs.

Directing covalent on-surface polymerization of molecular precursors through rational synthetic design has been impaired by the lack of control over reaction sites and by limited chemical routes tocovalent bond formation between precursors, resulting in limited control of the outcomes and parasitic surface reaction by products. Here we investigate the mechanism of a recently reported chemicalroute to C-C bond formation: dehydrofluorination on metal surfaces .We demonstrate that this chemical route, involving specifically C-H/F-C pairs, favors the elimination of HF in the gas phase as a single step, therefore eliminating reaction byproducts at the surface ofmetals. Unlike dehalogenation, wefind that the remarkable selectivity of the dehydrofluorination reaction renders this C-C bondformation strategy chemoselective as well as potentially regioselective, if employed with a properly designed molecular precursor. Additionally, we demonstrate that the catalytic role of the metal substrate can be used to steer reaction pathways and select between dehydrofluorination and dehydrogenation reaction. For these reasons, the dehydrofluorination reaction, largely unexplored on metal surfaces, could become a valuable tool for on-surface synthesis.

The first experimental realization of the intrinsic (not dominated by defects) charge conduction regime in lead‐halide perovskite field‐effect transistors (FETs) is reported. The advance is enabled by: i) a new vapor‐phase epitaxy technique that results in large‐area single‐crystalline cesium lead bromide (CsPbBr 3 ) films with excellent structural and surface properties, including atomically flat surface morphology, essentially free from defects and traps at the level relevant to device operation; ii) an extensive materials analysis of these films using a variety of thin‐film and surface probes certifying the chemical and structural quality of the material; and iii) the fabrication of nearly ideal (trap‐free) FETs with characteristics superior to any reported to date. These devices allow the investigation of the intrinsic FET and (gated) Hall‐effect carrier mobilities as functions of temperature. The intrinsic mobility is found to increase on cooling from ≈30 cm 2 V −1 s −1 at room temperature to ≈250 cm 2 V −1 s −1 at 50 K, revealing a band transport limited by phonon scattering. Establishing the intrinsic (phonon‐limited) mobility provides a solid test for theoretical descriptions of carrier transport in perovskites, reveals basic limits to the technology, and points to a path for future high‐performance perovskite electronic devices.

Modifying the energy alignment between the frontier levels of a chromophore and the band edges of a semiconductor substrate using a "blanket layer" of molecular dipoles is explored. The electronic structure, measured with ultraviolet and x-ray photoemission spectroscopy, of C-60 deposited directly onto the TiO2(110) surface is compared with that of C-60 deposited onto the surface covered by a blanket layer of the custom-designed helical peptide Z-(Aib)(6)-Ipa, which has a large dipole moment of 18.6 D oriented along its molecular axis and pointing toward the substrate. The C-60 HOMO level is shifted 600 meV to deeper binding energy in the presence of the blanket layer, in good agreement with what is expected from a simple parallel plate capacitor model. These results demonstrate a simple and versatile strategy for controlling energy level alignment at organic/inorganic interfaces.

Five fluorinated zinc tetraphenyl porphyrins derivatives, fluorinated either on all ortho positions of the meso phenyl rings (Zn(II)-5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin) or in [Formula: see text]-positions with two, four, six or eight fluorine groups (2,3-difluoro-5,10,15,20-tetraphenylporphyrin, 7, 8,17,18-tetrafluoro-5,10,15,20-tetraphenylporphyrin, 7,8,12,13,17,18-hexafluoro-5,10,15,20-tetraphenyl- porphyrin and 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, respectively), were studied to probe the effect of fluorine in on-surface synthesis approaches. Additionally, bulkier substituents have been used in 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin and 7,8,17,18-tetramethyl-5,10,15,20-tetraphenylporphyrin, for a direct comparison with the corresponding fluorinated compounds. Reported are the synthesis and electronic structure characterization of the compounds, using UV-vis absorption and fluorescence spectroscopies as well as electronic structure calculations of the ground state molecular properties. Steric and electronic effects of fluorination are explored. For all molecules studied, substitution with fluorine maintains a planar tetrapyrrole macrocycle and shifts in the absorption spectra were consistent with calculated changes in the HOMOs and LUMOs energies of the molecules. In contrast, the bromo and methyl-substituted derivatives exhibit chemical instability and spectral shifts, compared to parent compound ZnTPP, consistent with structural distortions and accounted for calculations. The site-specific effects of fluorination on the electronic structure are discussed in detail.

Lead-halide perovskites emerged as novel semiconducting materials suitable for a variety of optoelectronic applications. However, fabrication of reliable perovskite field-effect transistors (FETs), the devices necessary for the fundamental and applied research on charge transport properties of this class of materials, has proven challenging. Here we demonstrate high-performance perovskite FETs based on epitaxial, single crystalline thin films of cesium lead bromide (CsPbBr3). An improved vapor-phase epitaxy has allowed growing truly large-area, atomically flat films of this perovskite with excellent structural and surface properties. FETs based on these CsPbBr3 films exhibit textbook transistor characteristics, with a very low hysteresis and high intrinsic charge carrier mobility. Availability of such high-performance devices has allowed the study of Hall effect in perovskite FETs for the first time. Our magneto-transport measurements show that the charge carrier mobility of CsPbBr3 FETs increases on cooling, from ~ 30 cm2V-1s-1 at room temperature, to ~ 250 cm2V-1s-1 at 50 K, exhibiting a band transport mostly limited by phonon scattering. The epitaxial growth and FET fabrication methodologies described here can be naturally extended to other perovskites, including the hybrid ones, thus representing a technological leap forward, overcoming the performance bottleneck in research on perovskite FETs.

The use of helical hexapeptides to establish a surface dipole layer on a TiO2 substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)6-COOH or Z-(Aib)6-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO2 surfaces (either nanostructured or single-crystal TiO2(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)6-Ipa on TiO2(110) was exposed for different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group (ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)6-Ipa, forming a coadsorbed monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to lower energy by ∼300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-layer approach, involving coadsorption of a strong molecular dipole compound with a chromophore, is a versatile method to shift the energy levels of such chromophores with respect to the band edges of the substrate.

The valence band spectra of three cyano-ionic liquids based on 1-ethyl-3-methylimidazolium (Im2,1+) paired with thiocyanate (SCN-), dicyanamide (N(CN)2-), and tricyanomethanide (C(CN)3-) have been measured using ultraviolet and X-ray photoemission spectroscopy. Experimental spectra are compared to their corresponding density of states, weighted by photoemission cross sections, calculated for clusters of ions pairs of increasing size. Thus, this study bridges single ion approaches to 3D periodical DFT studies and enables the exploration of the different aspects of electronic structure establishment in ILs. Even for a relatively small cluster size, the relative energy of cation and anions states shifts by an amount that corresponds closely to that expected from the Madelung energy of a bulk IL, and the photoemission cross section-weighted DOS spectra are in good agreement with the measured valence bands. Trends in the relative energy and ionic character of the frontier orbitals across this series of cyano-ILs are discussed.

The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

Inverse photoemission remains the main spectroscopic technique to explore the unoccupied electronic states of materials for both solids and adsorbed molecules. It also allows collecting information regarding the k-dependence of the electronic bands above the Fermi level. For spectrometers that consider the use of a diffraction grating as a way of analyzing the photon energy, the energy calibration of the collected spectra is a problem that needs to be addressed. In this paper we present results related to the application of an energy calibration procedure applied to a recently completed inverse photoemission spectrometer.

With the development of novel semiconductors for optoelectronic applications, new device functionalities utilizing unique characteristics of emerging materials can be particularly appealing. Here, we demonstrate a reversible control of photoluminescence (PL) emission from lead-halide perovskites achieved in perovskite electric-double-layer transistors. PL in several prototypical lead-halide perovskite compounds is shown to be reversibly tuned by a small gate voltage in the range +/- 1.2 V applied to the ionic-liquid gel on the perovskite surface, with the intensity modulation that can reach one to two orders of magnitude. This effect may be mediated by a reversible migration of oxygen ions affecting the crystal region near the interface with the ion gel. The resulting passivation (or activation) of non-radiative recombination centers (traps) by oxygen ions would then modulate the population of mobile photogenerated electrons and holes that give rise to PL, which is thus tuned with an electric "knob" (the gate) in these devices.

It is often assumed that the self-assembly of organic molecules on a noble-metal surface results in a structure that is in thermodynamic equilibrium. Here, using scanning tunnel microscopy, we observe instead a highly ordered metastable striped monolayer phase of zinc tetraphenylporphyrin, self-assembled at 300 K on Ag(100). The usually reported stable square phase is found only after higher-temperature annealing. We use a statistical mechanical model to reveal a possible molecular mechanism for this process, in which the competition of molecule and substrate interactions at the growth front leads to the growth of a kinetically trapped inhomogeneous ordered structure. Our proposed mechanism rests only on simple features of molecular geometry and interactions, and the resulting principles could be used to promote particular outcomes of growth in other examples of molecular assembly at surfaces.

We report the first measurement of the full energy spectrum (down to 0 eV) of electrons emitted from the surface of Ag(100) as a result of the N3VV Auger transition. A comparison with a Monte-Carlo model was used to obtain an estimate of the contribution of multi-electron Auger processes to the low energy part of the Auger spectrum. The experimental Auger spectrum was obtained in time coincidence with photo-emitted 4p(3/2) core electrons from Ag using Auger-Photoelectron coincidence spectroscopy (APECS). The coincidence condition in APECS can remove most, but not all, spectral contributions from events unrelated to the Auger transition. The remaining contributions to the APECS spectrum from Auger-unrelated events, which are significant, were determined from a series of coincidence measurements of inelastically scattered valence band photoelectrons. The contribution from Auger-unrelated events was subtracted from the APECS Auger spectrum of Ag(100) to obtain the full Auger spectrum down to 0 eV. The resulting spectrum includes, in addition to Auger electrons emitted without energy loss, inelastically scattered Auger electrons. We observe that the spectral weight of the low energy tail (LET) in the APECS spectrum is reduced by more than a factor of two after subtracting the Auger-unrelated contributions. However, the intensity of the LET after this subtraction is still two times the integrated intensity in the Auger peak. A Monte Carlo model that includes the inelastic transport of the N3VV Auger electrons through the surface showed that LET produced by the inelastic scattering of the Auger electrons has the same intensity as the integrated intensity in the Auger peak. The excess intensity in the experimental LET provides strong evidence that the Auger related spectrum also includes contribution from multiple electron emissions that are intrinsic to the creation and/or the Auger decay of the core hole. Our estimate shows that as low as 20% to as high as 60% of the core hole decay events result in the emission of more than one Auger electron from Ag (100). This result demonstrates the need for a better understanding of the contribution of valence-electron correlations to the entire CVV Auger spectrum down to 0 eV.

The geometric and electronic structures of a monolayer of rubrene and of a fluorinated rubrene derivative FM-rubrene), adsorbed on a Ag(100) surface, were determined using scanning tunneling microscopy (STM), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPS) to study the influence of fluorine-functionalization on self-assembly, molecular energy levels, and energy-level alignment. STM images at room temperature reveal that the molecules form different molecular assemblies at the monolayer coverage and have the tetracene backbones nearly parallel to the surface, with strongly splayed phenyls. While the assemblies are highly ordered, the molecular orientation and intermolecular spacings differ from those of the respective molecular crystals and thus do not act as templates for the epitaxial growth of ordered multilayer molecular films. UPS and IPS measurements indicate that the frontier orbitals of an adsorbed FM-rubrene molecular layer are shifted downward with respect to the Ag(100) Fermi level by 0.2 eV as compared to those of a rubrene monolayer. Moreover, the intrinsic molecular dipole of FM-rubrene contained in the first layer leads to an interface dipole of 0.2 eV, further shifting added molecular layers to higher energies. Comparison of the adsorption of rubrene to that of FM-rubrene on Ag(100) provides valuable insights into the nature of the molecule surface and intermolecular interactions that drive self-assembly and energy-level alignment, as well as their effects on the potential growth of ordered molecular multilayers.

The influence of fluorine-functionalization of the rubrene molecule is explored by comparing rubrene and a fluorinated rubrene (FM-rubrene), adsorbed on a Ag(100) surface. The self-assembly, studied using scanning tunneling microscopy, reveals highly ordered molecular assemblies at the monolayer level, that are incompatible with epitaxial growth of rubrene or FM-rubrene ordered crystals. Moreover, the molecular order has a direct influence on the resulting electronic structure and energy alignment of the molecular levels with the band edges of Ag(100), measured using UV and inverse photoemission spectroscopies. A careful comparison of the adsorption of rubrene and FM-rubrene on Ag(100) enables a fundamental understanding of the molecular-surface and intermolecular interactions, as well as their effects on molecular crystal growth and energy alignment with supporting surfaces.