Lowest Conduction Band Research Articles

Interlayer Coupling Induced Phonon-Glass–Electron-Crystal Behavior in van der Waals Heterostructure PtSe2/γ-GeSe

We report the electronic band structure and interlayer coupling induced phonon-glass–electron-crystal behavior in the van der Waals heterostructure PtSe2/γ-GeSe. The heterostructure is dynamically stable and possesses an indirect band gap of 0.63 eV (at the Heyd–Scuseria–Ernzerhof functional level) with type II band alignment. A low conduction band offset of 0.20 eV compared to the valence band offset of 0.92 eV suggests fluent electrons drive from γ-GeSe to PtSe2. Interlayer coupling induced strong phonon coupling and a unique “avoided crossing” feature between longitudinal acoustic and low-lying optical phonon modes between K and Γ points. Significant suppression of acoustic phonon modes and giant phonon scattering rates (a maximum value of 73.76 ps–1) results in a low lattice thermal conductivity of 1.20 W/(m·K) at 300 K after enforcing the mandatory rotational invariance condition. The calculated lattice thermal conductivity is 14-fold smaller than that of monolayer PtSe2 (16.97 W/(m·K)). The lattice thermal conductivity upsurges by 24% as the out-of-plane acoustic phonon dispersion undergoes linear dispersion, indicating the importance of pure quadratic flexural phonon dispersion. Our results disclose a strategy for having the phonon-glass–electron-crystal character in van der Waals heterostructures.

In situ MOCVD growth and band offsets of Al2O3 dielectric on β-Ga2O3 and β-(AlxGa1−x)2O3 thin films

The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al2O3 dielectrics on β-Ga2O3 and β-(AlxGa1−x)2O3 films is investigated as a function of crystal orientations and Al compositions of β-(AlxGa1−x)2O3 films. The interface and film qualities of Al2O3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al2O3 dielectrics with abrupt interfaces on (010), (100), and (2¯01) oriented β-(AlxGa1−x)2O3 films. The surface stoichiometries of Al2O3 deposited on all orientations of β-(AlxGa1−x)2O3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al2O3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al2O3/β-(AlxGa1−x)2O3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(AlxGa1−x)2O3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al2O3/β-(AlxGa1−x)2O3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the (2¯01) orientation. The results from this study on MOCVD growth and band offsets of amorphous Al2O3 deposited on differently oriented β-Ga2O3 and β-(AlxGa1−x)2O3 films will potentially contribute to the design and fabrication of future high-performance β-Ga2O3 and β-(AlxGa1−x)2O3 based transistors using MOCVD in situ deposited Al2O3 as a gate dielectric.

A comprehensive DFT evaluation of catalytic and optoelectronic properties of BaTiO3 polymorphs

BaTiO3 exhibits promising photocatalytic characteristics and has the potential to be employed in photocatalytic water splitting and pollutant degradation. Herein, we have investigated the structural, electronic, and optical properties of BaTiO3 in the framework of density functional theory (DFT). The obtained results are found to be consistent with theoretical results. However, using hybrid HSE06 functional, the calculated bandgap values are 3.254, 3.894, 3.694, 3.519, and 3.388 eV corresponding to cubic, rhombohedral, orthorhombic, tetragonal, and hexagonal phase of BaTiO3 polymorphs. This is the first time a DFT calculation touched over the experimental bandgap values of BaTiO3 polymorphs.Electronic band structure analysis revealed that all the BaTiO3 polymorphs exhibit semiconducting character with indirect bandgap except hexagonal structure which possesses direct bandgap. The density of states (DOS), Mulliken bond analysis and charge density distribution indicates considerable hybridization between O-2p and Ti-3d states caused to the formation of upper valence band (VB) and lower conduction band (CB). The wide bandgap and suitable redox potential of BaTiO3 polymorphs will be effective for water splitting as well as breakdown organic contaminants under UV–vis irradiation. The lower electron effective mass of the polymorphs indicates higher electrical conductivity. The energy dependent optical parameters show close agreement with the underlying electronic band structure. The high absorptivity, lower reflectivity and optical anisotropy (orthorhombic and tetragonal) of the polymorphs suggested that these could be a potential candidate for UV based optical device applications as well as to design wave guides, dielectric condensers and optical display devices.

Thickness-dependent electronic band structure in MBE-grown hexagonal InTe films

Films of the hexagonal InTe with thicknesses from one to three tetralayers (TLs) were synthesized on the bilayer graphene/SiC by molecular beam epitaxy. Valence bands of the one- and two-TL-thick films were found to be flat-like near $\overline{\mathrm{\ensuremath{\Gamma}}}$ point, but become parabolic for the three-TL-thick film and beyond. The band gap of the InTe was found to be equal to 2.1 eV for the single tetralayer and tends to reduce its size with thickness. The band structure calculations revealed a large spin splitting of the InTe single tetralayer lower conduction band with exclusive out-of-plane spin polarization. Bearing in mind inaccessibility of the hexagonal InTe in a bulk form, all above-mentioned findings open up a way for the further study of this perspective material.

Unraveling the strain and inherent onsite-correlation effect on the electronic structure of pure and iso-electronic Ag doped copper nitride

Promising optoelectronic properties of semiconducting copper nitride (Cu3N), have brought the material into limelight for numerous device applications. As band gap (Eg), undoubtedly is a key parameter in determining proficiency of a semiconductor in optoelectronics, hence its modulation needs clear understanding both from the fundamental and application perspectives. The present study discusses the impact of inherent onsite Coulomb-correlation (through Hubbard U) and strain on the electronic properties of pure and iso-electronic element (Ag) doped Cu3N using density functional calculations. A slow rate of increase in Eg with U recognizes Cu3N a weakly correlated system. Alternately, U for the doped system (Cu2AgN) is discovered to have further weaker effect on the Eg, nearly independent for U > 2 eV. However, doping of the 4d electronic element in the 3d transition metal nitride predicts an increased carrier mobility which will subsequently improve electrical conductivity. While examining the effect of strain in pristine Cu3N, the lowest conduction band is found to have both approaching and avoiding feature with respect to the Fermi level at two different high symmetry k-points. Consequently, Eg decreases (increases) for the compressive (tensile) strain which is attributed to the higher (lower) dispersive nature of the valence bands due to stronger (weaker) p-d hybridization. In case of Cu2AgN, though a similar shifting in the lower conduction band is observed, the variation of Eg with strain is quite different to that of Cu3N. While Eg increases (decreases) for the uniaxial compressive (tensile) strain, it decreases both for tensile and compressive strains corresponding to the isotropic deformations.

First-Principles Prediction of Structural, Elastic, Mechanical, and Electronic Properties of Cu2ZnSnS4 under Pressure

Quaternary compounds are high desirable for realizing advanced optoelectronic and spintronic devices. In this paper, we have systematic studied the effects of pressure on the structural, elastic, mechanical, and electronic properties of Cu2ZnSnS4 by means of first-principles calculations. The results indicated that both the lattice constant and cell volume decrease with the increase of pressure, which are matched well with available previous values. The pressure has a more significant influence on the c direction than the a and b direction. The obtained elastic constants reveal the Cu2ZnSnS4 is mechanically stable between 0 GPa and 15 GPa. The bulk modulus, shear modulus, and Young’s modulus are evaluated by Voigt-Reuss-Hill approximation. All these elastic moduli exhibit a monotonic feature as a function of pressure. The Poisson’s ratio, Pugh’s criterion, and Cauchy pressure indicate that the quaternary compound Cu2ZnSnS4 is ductile against pressure. Meanwhile, the analysis of the electronic structures reveals that the states near the valence band top are derived from Cu 3d and S 3p orbitals, and the lowest conduction band is composed of Sn 5s and S 3p orbitals. We expect that the findings predicted the physical properties of this compound will promote future experimental studies on Cu2ZnSnS4.

Photocatalytic behaviour of Ag3PO4, Fe3O4 and Ag3PO4/Fe3O4 heterojunction towards the removal of organic pollutants and Cr(VI) from water: Efficiency and light-corrosion deactivation

This work aims to synthesize and study the photocatalytic behaviour of Ag3PO4, Fe3O4 and Ag3PO4/Fe3O4 to oxidize organic pollutants (diclofenac sodium and acid green 25) and photoreduction of Cr(VI). In terms of organic pollutants oxidation, Ag3PO4/Fe3O4 composite showed higher efficiency than bare Ag3PO4 and Fe3O4. Based on the reactive oxygen species scavenging experiments, it was found that Ag3PO4/Fe3O4 can enhance the photogeneration of −O2 species, solving the low ability of bare Ag3PO4 to reduce O2 into −O2 because of its lower conduction band compared to the redox potential of O2/−O2. As a result, it was claimed that the Z-scheme heterojunction is more plausible than type II heterojunction system for enhancing photoproduced charges separation and photoactivity in Ag3PO4/Fe3O4 platform. In terms of photoreduction of Cr(VI), Fe3O4 and Ag3PO4/Fe3O4 were more effective than bare Ag3PO4 under solar light. The deactivation tests carried out comparatively on Ag3PO4 and Ag3PO4/Fe3O4 for successive oxidation of diclofenac sodium under solar light showed that Ag3PO4/Fe3O4 has better long-term use. At the same time, Ag3PO4 lost its activity significantly along with a total change of its colour from green to black because of the strong light corrosion. On top of the high stability and easy magnetic recovery of Ag3PO4/Fe3O4, the coating of Ag3PO4 on low-cost Fe3O4 can have an economic benefit at large scale.

Degradation of AMARANTH with TiO2 Synthesised by Sol-Gel Process

The photo-degradation of persistent contaminants in aqueous systems such as drugs, pesticides or dyes has been proposed as an alternative for the remediation of aqueous systems. TiO2 is one of the main photocatalysts that have been used for these purposes, it has two phases with photocatalytic properties, anatase and rutile, the latter being the one with the lowest conduction band 3.0 eV. The present work shows the synthesis of TiO2 by sol gel process, observing that the rutile phase is favored with heat treatment at temperatures above 600ºC, the comparative study of the degradation kinetics of AMARANTH with synthesized TiO2 and commercial TiO2 was evaluated, observing a rate constant of 1.38 and 0.345 Lmol-1min-1 for commercial and synthetic TiO2, respectively.

Jacutingaite family: An efficient platform for coexistence of spin valley Hall effects, valley spin-valve realization, and layer spin crossover

Jacutingaite family, a family of naturally occurring exfoliable minerals (${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ and ${\mathrm{Pd}}_{2}{\mathrm{HgSe}}_{3}$) discovered in Brazil, is recently predicted to be the very first large-gap Kane-Mele quantum spin Hall insulators. However, yet to date, the door is locked for realizing versatile spin-valley based phenomena due to inversion symmetry in this family. By exploiting the inversion symmetry using different strategies such as placing the jacutingaite family in the vertical electric field or growing its composites with some suitable well-matched systems, we reveal several promising valley spin based phenomena in the jacutingaite family. Overall, we achieve the coupled spin valley Hall effect, valley spin-valve effect, Rashba spin splitting around the low-symmetric $M$ point, layer crossover accompanied by spin crossover, and selective-excitation of carriers from opposite valleys in this family. More interestingly, swapping of the induced spin splittings, Berry curvatures, and the spin texture between the two valleys occur upon the reversal of electric field from ${E}_{z}>0$ to ${E}_{z}<0$ without destroying the ${\mathcal{Z}}_{2}$ topological hallmarks. Meanwhile, considerable hexagonal trigonal warping effects around the ${K}_{+}$ and ${K}_{\ensuremath{-}}$ valleys are realized. Furthermore, by applying vertical electric field through interfacial coupling with ferroelectric ${\mathrm{III}}_{2}\ensuremath{-}{\mathrm{VI}}_{3}$ film, valley contrasting Berry curvature in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer with a larger spin splitting in the lower conduction band $(\ensuremath{\sim}188\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ is achieved in the $+P$ polarization state. By inverting the polarization state from $+P$ to $\ensuremath{-}P$, the Fermi level shifts down into the valence band of the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer $(\ensuremath{\sim}82\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ revealing a hole doping in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer. This could establish an effective mechanism to control doping levels in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer as desirable for achieving superconductivity at finite doping in the jacutingaite family. Our paper provides a path towards integrating valleytronics and spintronics in multivalley materials with broken inversion symmetry.

Facile synthesis of strontium molybdate coupled g-C3N4 composite for effective tetracycline and dyes degradation under visible light

This work was designed to synthesize SrMoO4/g-C3N4 heterojunction for efficient degradation of tetracycline (TC) hydrochloride via photocatalysis. SrMoO4/g-C3N4 samples were prepared through a grinding and roasting process. The prepared nanocomposite exhibited excellent visible-light-driven photocatalytic activity. The reaction rate of TC photodegradation reaches 0.0171 min−1, which is 5.9 times higher than that of neat g-C3N4. The origin of the high photoactivity of SrMoO4/g-C3N4 was investigated using a variety of characterization techniques including XRD, FT-IR, TG, SEM, TEM, XPS, DRS, Mott-Schottky, PL, PC, and EIS. Result showed that the added SrMoO4 was closely loaded on the g-C3N4 surface, which is conducive to the electron transfer between SrMoO4 and g-C3N4. Mott-Schottky analysis indicated that SrMoO4 has a lower conduction band (CB) position than g-C3N4. As a result, photogenerated electrons in g-C3N4 can move to the CB of SrMoO4 to hinder the recombination of charge carriers, thereby increasing the photocatalytic activity under visible light. The cycling test further suggested that the SrMoO4/g-C3N4 heterojunction has good stability in the photocatalytic degradation of TC. Super oxygen radicals and holes are the main reactive species.

Thiophene insertion and lanthanum molybdate modification of g-C3N4 for enhanced visible-light-driven photoactivity in tetracycline degradation

This work was designed to enhance the photocatalytic activity of g-C3N4 nanosheets via the insertion of thiophene group and modification of La2(MoO4)3 nanoparticles with a size of 200–600 nm. The insertion of thiophene ring into the g-C3N4 matrix introduced an additional absorption at about 500 nm, which greatly extends the optical property of g-C3N4. Additionally, the thiophene group can also increase the separation of photogenerated electron-hole pairs via promoting electron delocalization. La2(MoO4)3 was closely loaded on the surface of thiophene inserted g-C3N4 (TPCN), which is conducive to the migration of electrons between them. Because of the lower conduction band (CB) of La2(MoO4)3 than TPCN, the photoexcited electrons in TPCN can migrate to the La2(MoO4)3 CB to further suppress the recombination of electron-hole pairs, which finally results in the enhanced photoactivity under visible light. The optimal La2(MoO4)3/TPCN composite presents a tetracycline (TC) degradation rate of 0.048 min−1, which is 3.4 and 1.8 times higher than that of g-C3N4 and TPCN, respectively. The cycling tests further demonstrated that the La2(MoO4)3/TPCN hybrid has good stability in photocatalytic degradation of TC. This result once again emphasizes the benefits of promoting the photoactivity of g-C3N4 through the combined action of doping and surface modification.

Oxidative dissolution of Sb2O3 mediated by surface Mn redox cycling in oxic aquatic systems

Antimony trioxide (Sb2O3) is one of the primary forms of Sb in the environment, and its dissolution significantly impacts the migration and bioavailability of Sb. However, the dissolution of Sb2O3 coupled with abiotic redox of Mn processes is unclear. Here, we investigated the kinetics of Sb2O3 dissolution in the presence of the ubiquitous Mn(II) by kinetic experiments, spectroscopies, density functional theory calculations and the chemical kinetic modeling. The oxidative dissolution of Sb2O3 was catalyzed by Mn(II) through the in-situ generated amorphous Mn oxides (MnOx) under oxic conditions, during which the generation of Mn(III) is a critical step in Sb(V) release. The released Sb(V) was partially retained on MnOx through bidentate-binuclear (corner-sharing) complexes as revealed by extended X-ray absorption fine structure analysis. The coexistent morphological forms of Sb2O3, i.e., senarmontite and valentinite exhibited distinct dissolution patterns. Valentinite showed higher activity in catalyzing Mn(II) oxidation and faster oxidative dissolution than senarmontite, due to its higher surface energy and lower conduction band minimum of its exposed facets. These abiotic processes can extrapolate to other metal(loid)s (hydr)oxides, further supplying for the comprehensive understanding of the redox transformation of Mn

A Polyimide-Based Photocatalyst for Continuous Hydrogen Peroxide Production Using Air and Water under Solar Light

A Polyimide-Based Photocatalyst for Continuous Hydrogen Peroxide Production Using Air and Water under Solar Light

Computational design of a nanoconjugate model of pyrene-linked CdTe quantum dot for the detection of trinitrotoluene

This paper demonstrates a computational design of the CdTe QD-based nanoconjugate to detect trinitrotoluene (TNT). The bandgap of QD-pyrene nanoconjugate has been engineered depending on the nitrotoluenes attached. The bandgap is reduced significantly for TNT in comparison to others because the LUMO of TNT appeared at the mid-gap region of CdTe QD. The QD guides the VBT, and the lowest conduction band is the LUMO of TNT; hence CdTe QD-pyrene/TNT constructs a type-II band alignment. The dinitrotoluenes (DNTs) also construct the type-II band alignment. However, toluene and mononitrotoluenes (NTs) do not alter the bandgap of larger CdTe QD and construct the type-I band alignment. Significant reduction of the bandgap along with type-II band alignment may facilitate the photoexcited electron transfer from QD to TNT. One can expect rapid fluorescence quenching of CdTe QDs by TNT. So, the designed pyrene-linked CdTe QD-based nanoconjugate can act as an effective TNT sensor.

Systematic study of phase transformation, wide-to-narrow electronic band transition and optical properties of barium zirconium Oxynitrate: Ab initio calculations

We report, for the first time, the structural phase transformation, wide-to-narrow electronic band transition and impact on optical properties, computed via ab initio methods by employing generalized gradient approximation of Perdew–Burke–Ernzerhof and ultra-soft pseudo-potential, with different nitrogen (N2) doping concentrations at oxygen (O2) lattice sites in BaZrO3. With an increase in N2 concentration, we observe an increase in the population of density of states with both energy bands and conduction band- moves towards the Fermi level. Consequently, a systematic reduction in the electronic band gap (from 3.119 to 1.152 eV) is observed due to prominent shifting of Zr d-states to lower conduction bands along with N2 and O2 p-states in upper valence bands, and also, its nature changes into the direct form. The mystery of decreasing band gap is nicely decoded by the total density of states and elemental partial density of states. Optical properties have also been computed to see the impact of decrement in band gap on them. The static refractive index is increased (from 2.196 to 2.988). The absorption edge is shifted to lower energy (from 1.6 to 0.02 eV, red shift). The optical properties confirm that the doped compound would be appropriate for absorbing electromagnetic radiations in the visible and IR regions of the electromagnetic spectrum as it has vast applications in optoelectronics.

Compact SnO2/Mesoporous TiO2 Bilayer Electron Transport Layer for Perovskite Solar Cells Fabricated at Low Process Temperature.

Charge transport layers have been found to be crucial for high-performance perovskite solar cells (PSCs). SnO2 has been extensively investigated as an alternative material for the traditional TiO2 electron transport layer (ETL). The challenges facing the successful application of SnO2 ETLs are degradation during the high-temperature process and voltage loss due to the lower conduction band. To achieve highly efficient PSCs using a SnO2 ETL, low-temperature-processed mesoporous TiO2 (LT m-TiO2) was combined with compact SnO2 to construct a bilayer ETL. The use of LT m-TiO2 can prevent the degradation of SnO2 as well as enlarge the interfacial contacts between the light-absorbing layer and the ETL. SnO2/TiO2 bilayer-based PSCs showed much higher power conversion efficiency than single SnO2 ETL-based PSCs.

Facet-dependent activity of TiO2/covalent organic framework S-scheme heterostructures for CO2 photoreduction

Covalent organic framework (COF) has shown great potential in constructing S-Scheme junctions with both spatial separation and strong redox ability of photogenerated charge carriers. Engineering the interfacial structure between COF and semiconductor photocatalysts is of vital importance to realize the high-efficiency CO2 photoreduction. Herein, the contribution of material facets to the S-scheme photocatalysis of TiO2/COF heterojunctions is studied. The internal electric field between TiO2 and COF benefits the S-scheme transfer of photoexcited charge carriers, which is highly dependent on the exposed facets of TiO2. 101-TiO2 with electron-rich surface and lower conduction band is proved to be a promising oxidation photocatalyst for the construction of S-scheme junctions. Nanoarchitecture of T-101/COF can effectively convert CO2 into valuable CO fuels, with a reduction rate of 11.6 μmol h−1, which is 14.5 and 4.6 times higher than that of pristine 101-TiO2 and T-001/COF, respectively. This work reveals that facet engineering can provide a versatile approach to improve the efficiency of S-scheme heterostructures for artificial photosynthesis.

Enhancement of visible light-driven hydrogen production over zinc cadmium sulfide nanoparticles anchored on BiVO4 nanorods

Photocatalytic water splitting to produce H 2 is a promising technology for clean energy generation. However, the use of expensive noble metals, toxicity, low charge separation efficiency and wide band gap of semiconductors hampering the widespread commercialization. Herein, we showed the potential of combining BiVO 4 nanorods with ZnCdS forming a hetero-structure which extend the spectral responsive range, separate the charge carriers effectively and enhances photocatalytic activity compared to single-component materials. The two components of hetero-structure forms an interface contact which also mitigate the problems of lower conduction band position of BiVO 4 and fast recombination of charge carriers of ZnCdS. The BiVO 4 –ZnCdS hetero-structure was studied through surface morphology, crystallization properties, elemental analysis and optical properties. Under visible light irradiation, the BiVO 4 –ZnCdS heterostructure produced 152.5 μmol g −1 h −1 hydrogen from water splitting, which was much higher than that of the individual components and stability of the hydrogen production was observed in three consecutive cycles. The as-obtained heterostructure showed improved visible light harvesting ability, prolong life of charges carriers and charge separation efficiency and Z-scheme mechanism features which results in enhanced photocatalytic activity for water splitting. • A novel BiVO 4 –ZnCdS heterostructure is introduced as photocatalyst. • The developed catalyst was used for visible light driven hydrogen production. • Interface among BiVO 4 and ZnCdS improved photocatalytic profile of catalyst. • BiVO 4 –ZnCdS heterostructure produced up to 152.5 μmol g −1 h −1 hydrogen. • Z-scheme mechanism enhanced the performance herein as noble-metal-free catalyst.

Performance-Enhancing Sulfur-Doped TiO2 Photoanodes for Perovskite Solar Cells

High-performance electron transport layer (ETL) anode generally needs to form a uniform dense layer with suitable conduction band position and good electron transport properties. The TiO2 photoanode is primarily applied as the ETL because it is low-cost, has diverse thin-film preparation methods and has good chemical stability. However, pure TiO2 is not an ideal ETL because it lacks several important criteria, such as low conductivity and conduction band mismatch with compositional-tailored perovskite. Thus, TiO2 is an inefficient photo-anode or ETL for high-performance perovskite devices. In this study, sulfur as dopant in the TiO2 photo-anode thin film is used to fabricate solid-state planar perovskite solar cells in relatively high humidity (40–50%). The deposited S-doped thin film improves the power conversion efficiency (PCE) of the device to 6.0%, with the un-doped TiO2 producing a PCE of 5.1% in the best device. Improvement in PCE is due to lower recombination and higher photocurrent density, resulting in 18% increase in PCE (5.1–6.0%).

Negative electron affinity opens quantum well in MgO layers on Ag(100)

Epitaxial MgO films on Ag(100) were studied by photoelectron spectroscopy. From the low-energy part of the spectra we obtain a negative electron affinity of about −0.9 eV. Even though electrons in the lowest conduction band are not confined by a potential barrier at the surface, quantum-well resonances are observed. The dispersion of the conduction band is determined in good agreement with theoretical calculations. Aspects of observing image-potential states predicted by theory on MgO films are discussed.