Variable Energy Photoelectron Spectroscopy Research Articles

Probing Chemical-Composition-Induced Heterostructures and Interfaces in Lead Halide Perovskites

Lead halide perovskites (LHP) are of great interest for their optoelectronic properties and photovoltaic applications. Various heterostructures are created in these materials to achieve favorable optical properties and improved stability at the interfaces during the fabrication of devices. Such heterostructures are often assumed to be formed based on the reactivity of precursors and are not directly probed. In this Feature Article, we report how various strategies have been employed in LHP thin films and nanocrystals (NCs) that generate heterostructures to boost their stability and photovoltaic (PV) efficiencies and how variable energy photoelectron spectroscopy (VEPES) can probe the chemical composition variation in heterostructured materials and interfaces. We specifically discussed the internal heterostructures of LHP NCs generated due to the surface chemistry and postsynthesis anion exchange followed by a detailed discussion of the heterostructures induced by the chemical composition (anion, cation, and degradation) of LHP thin films. The difficulties in determining heterostructures as well as the potential scope of the application of VEPES in unwrapping heterostructures in diverse materials are also discussed.

The study of the electronic structure of some N-heterocyclic carbenes (NHCs) by variable energy photoelectron spectroscopy.

The photoionization of three N-heterocyclic carbenes (NHCs) has been studied in the valence and core regions using synchrotron radiation. We observed different variations in the relative band intensities with photon energy for the NHCs in the valence ionization region. This is due to the intra-ring interactions between the C=C bond, nitrogen and carbene lone pairs in the heterocyclic ring of NHCs. In the core ionization region we observed chemical shifts which are consistent with the relative electron affinities of atoms and intramolecular electron density shifts. The core electron binding energies calculated via the unrestricted ΔDFT (B3LYP and M06-2X) approach are in very good agreement with the experiment. The shake-up portion of the core photoionization spectra is adequately described by the time-dependent DFT calculations relying on the CAM-B3LYP functional.

Characterisation of the electronic structure of some stable nitroxyl radicals using variable energy photoelectron spectroscopy.

The photoionization of three stable nitroxyl radicals has been studied in the valence and core regions using synchrotron radiation. We observed different variations of the relative band intensities with the photon energy for two pyrrolidine nitroxyls (nitroxyl8 and nitroxyl9) in the valence ionization region. This is due to strong intramolecular interactions between the amide substituent and the ring π-orbital when present. In the core ionization region we observed chemical shifts which were consistent with the relative electron affinities of different atoms. We also observed the multiplet splitting of core level binding energies in the final ionic states. The core electron binding energies calculated via the restricted open shell Hartree-Fock based ΔSCF method exhibit good agreement with the experimental core ionization bands and with the assignment of the spectra by empirical analysis.

Photoelectron spectroscopic and electronic structure studies of CH(2)O bonding and reactivity on ZnO surfaces: steps in the methanol synthesis reaction.

Adsorption of CH(2)O on ZnO(0001) has been investigated using XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and density functional theory (DFT) calculations. CH(2)O is chemisorbed on the (0001) surface at 130 K. Its C1s XPS peak position at 292.7 eV and NEXAFS sigma shape resonance at 302.6 eV are consistent with an eta(1) bound surface geometry. Geometry optimized DFT calculations also indicate that CH(2)O is bound to the Zn(II) site in an eta(1) configuration through its oxygen atom. The variable-energy PES of the eta(1) bound CH(2)O/ZnO(0001) complex exhibits four valence band features at 21.2, 16.4, 13.8, and 10.7 eV below the vacuum level providing an experimental and theoretical description of this surface interaction. Annealing the ZnO(0001)/CH(2)O surface complex to 220 K decomposes the chemisorbed CH(2)O, producing formyl (291.5 eV), methoxide (290.2 eV), and formate (293.6 eV) intermediates. Thus this reaction coordinate involves the conversion of an oxygen bound formaldehyde to a carbon bound formyl species on ZnO(0001). Only formate is formed on the ZnO(100) surface. DFT is used to explore surface intermediates and the transition state in the methanol synthesis reaction (MSR). The bonding interactions of H(2), CO, CH(3)O(-), HCO(-), and trans-HCOH to the ZnO(0001) surface are elucidated using geometry optimization. H(2) was found to be heterolytically cleaved on the ZnO(0001) surface, and carbon monoxide, formyl, and methoxide are calculated to be eta(1) bound. These results are consistent with observed metal oxide surface reactivity where heterolytic bond cleavage is dominant. The oxygen atom in the bound formyl was found to be activated for attack by a proton. This results in the planar eta(1) bound trans-HCOH surface species. The transition state in the gas phase rearrangement of trans-HCOH to formaldehyde was calculated to have a barrier of 31 kcal/mol. The correlation diagram for this rearrangement in the gas phase indicates that configuration interaction at the crossing of two levels helps to lower the barrier. A transition state calculation was also performed for this rearrangement on the ZnO(0001) surface. The surface transition state geometry is significantly different than the gas phase. The surface geometry is no longer planar (23 degrees dihedral angle) and is displaced parallel to the surface. Interaction with the Zn(II) site at the crossing of surface bound CH(2)O and trans-HCOH levels further lowers the barrier to rearrangement relative to gas phase by 9 kcal/mol. The rearrangement of trans-HCOH (carbon bound) to CH(2)O (oxygen bound) on ZnO(0001) was calculated to be the overall barrier of the MSR reaction.

UV-Photoelectron Spectra of [M(η3-C3H5)2] (M = Ni, Pd, Pt) Revisited: A Quasi-Relativistic Density Functional Study

The molecular and electronic structure of [M(η3-C3H5)2] (M = Ni, Pd, Pt) has been investigated by means of quasi-relativistic gradient-corrected density functional calculations. Geometries have been fully optimized by considering both trans and cis arrangements of the bis(η3-allyl) moiety. Binding energy differences between isomers are always smaller than 0.2 kcal/mol; in particular, cis-[Ni(η3-C3H5)2] is computed to be more stable than trans-[Ni(η3-C3H5)2], while a reversed order is obtained for Pd and Pt analogues. Computed geometrical parameters of trans-[Ni(η3-C3H5)2] compare very well with available structural data. Moreover, a new assignment of variable energy photoelectron spectroscopy measurements [Li, X.; Bancroft, G. M.; Puddephatt, R. J.; Liu, Z. F.; Hu, Y. F.; Tan, K. H. J. Am. Chem. Soc. 1994, 116, 9543−9554] is proposed by assuming that the trans:cis ratio in the gas phase is close to one.

Variable-energy photoelectron spectroscopy of substituted rhenium and manganese pentacarbonyls: molecular orbital assignments and the interatomic resonant effect.

High-resolution variable-energy photoelectron spectra of M(CO)5X [M = Re, X = Re(CO)5, Cl, Br, and I; and M = Mn, X = Mn(CO)5 and Br] are reported. Tunable synchrotron radiation is used to distinguish the Re 5d and Br 4p orbital based peaks for the controversial Re(CO)5Br. Our results provide firm molecular orbital assignments for all of these molecules. The valence orbital in the ordering of ionization energies for M(CO)5Cl (M = Mn and Re) and Mn(CO)5Br is a 1(M-X) > e(X) > b2(M) > e(M); but for M(CO)5I (M = Mn and Re) and Re(CO)5Br the ordering is a1(M-X) > e(M) > b2(M) > e(X). The crossover of the HOMO in the Re molecules due to the change in the halogen electronegativities occurs at Re(CO)5Br. The metal np-->nd resonance is observed for all of these molecules. For molecules like M2(CO)10 (M = Re and Mn) and Mn(CO)5Br, the observation of this np-->nd resonance is useful in assigning the metal nd based orbitals in their valence level spectra. However, for molecules like Re(CO)5X (X = Br and Cl), a np-->nd type resonance is observed on bands arising from both Re 5d and halogen mp based orbitals. This new resonant effect on the ligand-based orbitals is shown to be mainly due to the interatomic resonant effect. The core and valence level chemical shifts of these compounds are treated using Jolly's approach to confirm the assignments for the valence level spectra of some of these molecules. The high-resolution inner valence and core level spectra of these compounds are reported. Broadening of Re 4f, Br 3d, and I 4d core level spectra is discussed. The Auger peaks are observed in the high-resolution, high-intensity Br 3d of Re(CO)5Br and I 4d of Re(CO)5I spectra.

Electron Spectroscopic Studies of CH3OH Chemisorption on Cu2O and ZnO Single-Crystal Surfaces: Methoxide Bonding and Reactivity Related to Methanol Synthesis

Adsorption of CH3OH on Cu2O(111), ZnO(0001), and ZnO(1010) has been investigated with XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and SCF-Xα Scattered Wave (SW) molecular orbital calculations. At high coverage (≥25.0L), CH3OH is adsorbed as molecular multilayers on all three surfaces. At low temperatures (140 K) and coverage (0−0.6L), CH3OH is deprotonated to form chemisorbed methoxide on all of the surfaces investigated. Under these conditions the C1s XPS peak positions are 289.5, 290.2, and 290.2 eV below the vacuum level, respectively. Annealing the CH3O-/Cu2O(111) surface complex to 523 K produces no other surface intermediate. Alternatively, at temperatures above 220 K on the ZnO(0001) surface methoxide decomposes to produce a formate intermediate that is stable at the methanol synthesis reaction temperature (523 K). No formate surface intermediate is observed on the ZnO(1010) surface. The NEXAFS spectrum of chemisorbed methoxide on the Cu2O(111) surface exhibits a σ* shape resonan...

Electronic structure contributions to function in bioorganic chemistry: The blue copper active site

Abstract

Investigation of the bonding in methyl titanium trichloride by variable-energy photoelectron spectroscopy and density functional calculations

Preparation of an authentically pure sample of [TiMeCl 3 ] has enabled measurement of its photoelectron spectrum with photon energies varying between 20 and 50 eV. Study of the relative partial photoionisation cross-sections of the valence photoelectron bands has enabled a full assignment of the related ion states. The character of the ionising orbitals was also deduced from the cross-section variations. The titanium–carbon bond was shown to have a significant Ti 3d character. Density functional calculations at the non-local level, including estimates of the ionisation energies using Slater’s transition-state method, are in good agreement as to the ion state ordering. Fragment analysis of the one-electron eigenfunctions is fully consistent with the experimentally deduced orbital character.

Electronic Structure of Os(CO)4L (L = CO, PMe3) Studied by Variable-Energy Photoelectron Spectroscopy

High-resolution variable-energy photoelectron spectra of Os(CO)5 and Os(CO)4PMe3 are reported. Using variable-energy synchrotron radiation, both the metal 5p → 5d resonance and the Cooper minimum effect are observed on the osmium 5d orbitals, and they have been successfully used to assign the osmium 5d-based bands. The Xα-SW calculation for Os(CO)5 is used to assign the valence level molecular orbitals. Experimental branching ratios of Os(CO)5 agree well with the theoretical results from both Xα-SW and Gelius methods. The electronic structure of Os(CO)5 from the present study is shown to be similar to that of Fe(CO)5, both of which have a trigonal-bipyramidal structure. The binding energies of the Os 5p core level have been determined, which are shown to be important in the observation of 5p → 5d resonances. Both vibrational and ligand field splittings are shown to be important in the line broadening in the high-resolution Os 4f core level spectra. The chemical shifts of valence and core levels for Os(CO)5 and of Os(CO)4PMe3 are compared.

Variable-Energy Photoelectron Spectroscopy of CpM(CO)2 (M = Co, Rh, Ir): Molecular Orbital Assignments and an Evaluation of the Difference in Ground-State Orbital Characters

Photoelectron (PE) spectra with variable photon energy have been recorded between 21 and 70 eV using He I, He II, and synchrotron radiation for the cobalt, rhodium, and iridium complexes CpM(CO)2, where Cp is η5-cyclopentadienyl. The spectra have been assigned on the basis of Xα-SW molecular orbital (MO) calculations for the ground-state CpM(CO)2 molecules and by consideration of trends in photoelectron band intensities as a function of the energy of the radiation used. Photoionization cross sections (σ) have been calculated for the valence ionizations of the three molecules, using both the Xα-SW and Gelius methods. The theoretical branching ratios (σi/∑σ) from both theoretical methods have been compared with the observed branching ratios (Ai/∑A) between 21 and 70 eV. Our assignment (from the He I and He II spectra and the agreement between theoretical and observed branching ratios between 21 and 70 eV) gives a similar ordering of ionization potentials (IP's) for CpCo(CO)2 and CpRh(CO)2 with those reporte...

Electronic structure of the oxidized and reduced blue copper sites: contributions to the electron transfer pathway, reduction potential, and geometry

This is a review of our group's studies on the electronic structure of the blue copper site and its contribution to function. It starts with the known crystallographic results and demonstrates how spectroscopy allows these results to be extended to obtain fundamental insight into reactivity. These studies: (1) demonstrate that the unique spectral features associated with the oxidized blue copper site reflect a novel ground state wavefunction with high anisotropic covalency which plays a key role in activating specific electron transfer pathways; (2) provide the first description of the electronic structure of the reduced blue copper site obtained using variable energy photoelectron spectroscopy as a new method of bioinorganic spectroscopy and (3) determine the change in electronic structure on oxidation which allows for an evaluation of the Jahn-Teller distorting forces present in the oxidized blue copper site. The last result provides insight into whether the reduced geometry is in fact imposed on the oxidized site by the protein (i.e. the ‘entatic’ or ‘rack’ state).

The electronic nature of the metal-metal quadruple bond: variable photon energy photoelectron spectroscopy of Mo2(O2CCH3)4

Variable-energy photoelectron spectroscopy of thin film Mo[sub 2](O[sub 2]CCH[sub 3])[sub 4] on a GaAs substrate in ultrahigh vacuum is used to examine the valence ionizations of the quadruple metal-metal bond. The changes in photoionization cross sections with photon energy over a range of 40 to 90 eV are examined. The metal-based, [sigma], [pi], and [delta] ionizations of the quadruple bond are strongly enhanced relative to the acetate-based ionizations in the region of photon energies from 40 to 50 eV. This is consistent with a molybdenum 4p to 4d resonance and super-Coster-Kronig Auger enhancement of the metal-based ionizations. The extent of resonance enhancement is related to the amount of Mo 4d character associated with the ionization. The [pi] ionization of the Mo-Mo quadruple bond has the largest contribution from the Mo 4d orbitals. The [delta] and [sigma] ionizations of the metal-metal bond contain smaller amounts of Mo 4d character, and some Mo 4d character is observed in the acetate-based ionizations. This is explained in terms of significant overlap and mixing of the [delta] and [sigma] components with the ligand orbitals. Also, in the case of the [sigma] component, there is a possible filled/filled interaction between the Mo 4d[sub z[sup 2]] orbitalmore » on one metal and the Mo 4p[sub z] orbital on the adjoining metal. These interactions have significant impact on the properties of these complexes. 26 refs., 10 figs.« less

Variable photon energy photoelectron spectroscopy on FeCl4-. An unusual electronic structure for high-spin d5 complexes

Variable energy photoelectron spectroscopy (PES) is used to elucidate the valence band electronic structure and bonding in tetrahedral d{sup 5} FeCl{sub 4}{sup {minus}}. PES spectra obtained over the photon energy range 25-150 eV show intensity changes in the valence band features which indicate that more metal character is present in the deepest bonding levels. This is inverted from the normal electronic structure description of transition-metal complexes. The lack of off-resonance intensity in the deep binding energy satellite, which corresponds to a two-electron transition involving metal ionization plus ligand-to-metal charge transfer, indicates that little relaxation occurs on ionization. This result is confirmed by analysis of the satellite structure in the core level XPS Fe 2p spectra. PES spectra taken at the Fe 3p absorption edge, which provide insight into the bonding description of the ionized final state, show dramatic resonance intensity enhancement of the main band peaks as well as the satellite.

Variation of the Pb and Bi 5d binding energies and branching ratios for thin Pb and Bi overlayers on Ni(110).

Thin Pb and Bi overlayers on Ni(110) have been studied by use of variable-energy photoelectron spectroscopy. The overlayers are characterized by use of the Pb and Bi 5d core-level intensities and binding-energy shifts. The Pb 5d cross section (\ensuremath{\sigma}) and 5${d}_{5/2}$:5${d}_{3/2}$ branching-ratio (BR) dependence as a function of photon energy are quite sensitive to the thickness of the overlayer: as the thickness increases, the cross-section maximum shifts to lower photon energy. This shift is reflected by the point at which the BR crosses the statistical 1.5 value: 57.5 eV for 0.5 monolayer to 42.5 eV for bulk Pb. We postulate that this behavior is due to the sensitive nature of the outgoing f-wave potential, as shown recently by theoretical calculations of heavy-metal d cross sections.