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We calculate, within the density-functional theory, the atomic and electronic structure of the clean Pt(111) and Au(111) surfaces and the nML-Au/Pt(111) systems with n varying from one to three. The effect of the spin–orbital interaction was taken into account. Several new electronic states with strong localization in the surface region were found and discussed in the case of clean surfaces. The Au adlayers introduce numerous quantum well states in the energy regions corresponding to the projected bulk band continuum of Au(111). Moreover, the presence of states resembling the true Au(111) surface states can be detected at n = 2 and 3. The Au/Pd interface states are found as well. In nML-Au/Pt(111), the calculated work function presents a small variation with a variation of the number of the Au atomic layer. Nevertheless, the effect is significantly smaller in comparison to the s-p metals. [ABSTRACT FROM AUTHOR]

Quantum well states (QWSs) can form at the surface or interfaces of materials with confinement potential. They have broad applications in electronic and optical devices such as high mobility electron transistor, photodetector, and quantum well laser. The properties of the QWSs are usually the key factors for the performance of the devices. However, direct visualization and manipulation of such states are, in general, challenging. In this work, by using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we directly probe the QWSs generated on the vacuum interface of a narrow band gap semiconductor Nb2SiTe4. Interestingly, the position and splitting of QWSs could be easily manipulated via potassium (K) dosage onto the sample surface. Our results suggest Nb2SiTe4 to be an intriguing semiconductor system to study and engineer the QWSs, which has great potential in device applications.

We calculate, within the density-functional theory, the atomic and electronic structure of the clean Pt(111) and Au(111) surfaces and the nML-Au/Pt(111) systems with n varying from one to three. The effect of the spin–orbital interaction was taken into account. Several new electronic states with strong localization in the surface region were found and discussed in the case of clean surfaces. The Au adlayers introduce numerous quantum well states in the energy regions corresponding to the projected bulk band continuum of Au(111). Moreover, the presence of states resembling the true Au(111) surface states can be detected at n = 2 and 3. The Au/Pd interface states are found as well. In nML-Au/Pt(111), the calculated work function presents a small variation with a variation of the number of the Au atomic layer. Nevertheless, the effect is significantly smaller in comparison to the s-p metals.

Interlayer exchange coupling in polycrystalline Co2FeAl/Cr/Co2FeAl multilayers with Ta and MgO capping layers is investigated. Compared with Ta‐capped samples, the MgO capping layer induces the oscillatory coupling periods, ≈2.1 nm for Si substrate and ≈4.0 and ≈1.9 nm for SiO2 substrate. The oscillatory coupling period originates from the formation of quantum well states at the Cr/Co2FeAl interface due to the MgO capping layer. Meanwhile, Δ1 Bloch state in the majority spin and Δ2' Bloch state in the minority spin contribute to the short (≈2.1 and ≈1.9 nm) and long oscillatory period (≈4.0 nm) in the disordered B2 Co2FeAl multilayers, respectively. [ABSTRACT FROM AUTHOR]

Exploring emerging two-dimensional (2D) van der Waals (vdW) semiconducting materials and precisely tuning their electronic properties at the atomic level have long been recognized as crucial issues for developing their high-end electronic and optoelectronic applications. As a III-VI semiconductor, ultrathin layered hexagonal GaTe ( h -GaTe) remains unexplored in terms of its intrinsic electronic properties and band engineering strategies. Herein, we report the successful synthesis of ultrathin h -GaTe layers on a selected graphene/SiC(0001) substrate, via molecular beam epitaxy (MBE). The widely tunable quasiparticle band gaps (∼2.60-1.55 eV), as well as the vdW quantum well states (QWSs) that can be strictly counted by the layer numbers, are well characterized by onsite scanning tunneling microscopy/spectroscopy (STM/STS), and their origins are clearly addressed by density functional theory (DFT) calculations. More intriguingly, distinctive 8|8E and 4|4P (Ga) mirror twin boundaries (MTBs) are identified in the ultrathin h -GaTe flakes, which can induce decreased band gaps and prominent p-doping effects. This work should deepen our understanding on the electronic tunability of 2D III-VI semiconductors by thickness control and line defect engineering, which may hold promise for fabricating atomic-scale vertical and lateral homojunctions toward ultrascaled electronics and optoelectronics.

We calculate, within the density-functional theory, the atomic and electronic structure of the clean Pt(111) and Au(111) surfaces and the n ML-Au/Pt(111) systems with n varying from one to three. The effect of the spin-orbital interaction was taken into account. Several new electronic states with strong localization in the surface region were found and discussed in the case of clean surfaces. The Au adlayers introduce numerous quantum well states in the energy regions corresponding to the projected bulk band continuum of Au(111). Moreover, the presence of states resembling the true Au(111) surface states can be detected at n = 2 and 3. The Au/Pd interface states are found as well. In n ML-Au/Pt(111), the calculated work function presents a small variation with a variation of the number of the Au atomic layer. Nevertheless, the effect is significantly smaller in comparison to the s - p metals.

Highlights • Thin silver films have been created on a Si(111) substrate at room temperature. • A Ga-induced surface reconstruction allows for Ag to create a special interface. • Atoms in the interface form line structures oriented in the Si(111) 1 × 1 directions. • Scanning tunneling spectroscopy shows filled state peaks moving toward Fermi level. • Photoelectron spectra on 2 monolayer Ag film confirmed quantum well states. Graphical abstract Abstract Silver thin films have been formed by room temperature deposition of Ag on a Ga/Si(111) (3 × 3)R30° surface. Scanning tunneling microscopy and spectroscopy (STM/STS) have been used to study both the clean Ga/Si(111) (3 × 3)R30° surface and Ag films with different coverages. For the film formation, Ag first grows into 2D islands on Ga/Si(111) (3 × 3)R30°. The first layer of the islands forms atomic lines with three-fold symmetry in the 1 × 1 direction of the underlying Si(111) substrate. From a 2 ML coverage, the growth becomes layer-by-layer mode. STS measurements have been performed on Ag films with different coverages in order to investigate their electronic structures. Between 2–4 ML, the STS spectra reveal 2 and 3 peaks below the Fermi level. These peaks move towards the Fermi level with increased film thicknesses and thus share the same behavior as those of quantum well states (QWSs). The energy positions of the peaks have been compared with valence band photoelectron spectra in order to assign them to various parts of the QWSs. In addition, the photoemission results also confirm the existence of QWSs for a 2 ML Ag film on Ga/Si(111) (3 × 3)R30°. [ABSTRACT FROM AUTHOR]

Highlights • We investigated the electronic properties and the morphology of Ag nano-structures on c(2 × 2)-N/Cu(001) surface with STM and ARPES. • This system is an example of epitaxial growth on nanoscale areas due to an adsorbate induced reconstruction. • At low coverage, Ag is adsorbed on the bare Cu areas, forming 1nanowires, while at higher coverage flat Ag islands are formed. • Correspondingly to island formation, ARPES reveals the occurrence of Ag quantum well (QW) states with parabolic dispersion. • These QW states have been assigned to 2D confinement of electrons in wide islands, each of them behaving like a film. Abstract There has been a strong interest in methods of creating nanometer scale structures and in particular forming one- and two-dimensional electron confinement structures. Self-organisation has been recognised as a promising way for growing large nanostructure domains with sufficiently regular size and spacing as required for the observation of quantum well states. We investigated the electronic properties and the morphology of Ag nano-structures on c(2 × 2)-N/Cu(001) surface. This system is an example of epitaxial growth confined on nanoscale regions due to the occurrence of an adsorbate induced reconstruction. Using a combination of Scanning Tunneling Microscopy and Angle Resolved Photoemission Spectroscopy techniques we were able to determine the morphology and the growth mode of Ag on N-modified Cu(001) surface and the occurrence of quantum size effects in the electron properties of Ag nanostripes and nanoislands, evidenced in the observation of quantum well states. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Conventional two-dimensional electron gas (2DEG) typically occurs at the interface of semiconductor heterostructures and noble metal surfaces, but it is scarcely observed in individual 2D semiconductors. In this study, few-layer gallium selenide (GaSe) grown on highly ordered pyrolytic graphite (HOPG) is demonstrated using scanning tunneling microscopy and spectroscopy (STM/STS), revealing that the coexistence of quantum well states (QWS) and 2DEG. The QWS are located in the valence bands and exhibit a peak feature, with the number of quantum wells being equal to the number of atomic layers. Meanwhile, the 2DEG is located in the conduction bands and exhibits a standing-wave feature. Additionally, monolayer GaSe/HOPG heterostructures with different stacking angles (0°, 33°, 8°) form distinct moiré patterns that arise from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials, which effectively modulate the electron effective mass, charge redistribution, and band gap of GaSe. Overall, this work reveals a paradigm of band engineering based on layer numbers and moiré patterns that can modulate the electronic properties of 2D materials., (© 2023 Wiley-VCH GmbH.)

Bismuth has been the key element in the discovery and development of topological insulator materials. Previous theoretical studies indicated that Bi is topologically trivial and it can transform into the topological phase by alloying with Sb. However, recent high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements strongly suggested a topological band structure in pure Bi, conflicting with the theoretical results. To address this issue, we studied the band structure of Bi and Sb films by ARPES and first-principles calculations. The quantum confinement effectively enlarges the energy gap in the band structure of Bi films and enables a direct visualization of the Z 2 topological invariant of Bi. We find that Bi quantum films in topologically trivial and nontrivial phases respond differently to surface perturbations. This way, we establish experimental criteria for detecting the band topology of Bi by spectroscopic methods.

Temperature gradient effects on spin torque (ST) are studied in MgO-based double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer. Using non-equilibrium Green's function (NEGF) formalism, it is found that the thermal ST (TST) show oscillatory behavior with an increase of theNM spacer thickness. The results also show a magnitude enhancement of the TST in a DBMTJ compared with a single-barrier MTJ (SBMTJ). Such behavior is related to the existence of the quantum well states inside the NM spacer and resonant tunneling effect. Therefore, a faster magnetization switching is possible by TST in a DBMTJ without any bias voltage. [ABSTRACT FROM AUTHOR]

We report an angle-resolved photoemission study of the electronic structure of Au layers grown epitaxially on Pt(100) in the coverage range 1–10 monolayers (ML). Our results include an analysis of the electronic band structure and the Fermi surface, combined with structural information from low-energy electron diffraction. The Au films grow epitaxially with a (1 × 1) pattern up to 4–5 ML. We monitor the electronic band structure near the surface X ¯ -point vs. Au coverage. In the 1–3 ML range we observe interface electronic states related to the formation of a Au–Pt alloy in this coverage range. Starting at 2–3 ML coverage, we identify quantum well states from the incipient Au sp band, which converge into a bulk like Au sp band near 6 ML. After 5–6 ML, a (1 × 7) pattern is observed, due to the formation of a surface reconstruction in the epitaxial Au film with a topmost hexagonal layer, as in the reconstruction of bulk Au(100). We identify specific electronic states of quasi-one-dimensional character coming from the corrugated hexagonal layer. We obtain a complete picture and understanding of the electronic structure of Au/Pt(100), including sp Au band formation, hybridization and electronic confinement, and with implications in the understanding of the distinct electronic behavior of Au layers and particles in the nm size range. [ABSTRACT FROM AUTHOR]

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) study and first-principles calculations to investigate the atomic structure and electronic properties of silicon nanoribbons (SiNRs) grown on Ag(110). Despite the extensive research on SiNRs in the last decades, its atomic structure is still not fully understood so far. In this report we determine that the structure of SiNRs/Ag(110) is armchair silicene nanoribbon with reconstructed edges. Meanwhile, pronounced quantum well states (QWS) in SiNRs were observed and their energy spectrum was systematically measured. The QWS are due to the confinement of quasiparticles perpendicular to the nanoribbon and can be well explained by the theory of one-dimensional (1D) “particle-in-a-box” model in quantum mechanics. [ABSTRACT FROM AUTHOR]

We investigate Ag(100) thin films epitaxially grown on a Fe(100) substrate using a low-temperature scanning tunneling microscope. Fabrication of a wedge structure by evaporating Ag through a shadow mask allows us to observe a systematic evolution of quantum well (QW) states for layer thicknesses varying from 3 to 16 monolayers (ML). Close inspection of differential conductance spectra and images reveals significant modulations of QW states in the lateral directions, presumably due to the local defects at the Ag/Fe interface. The area where QW states are modulated extends over ~ 5 nm. In clear contrast, near a surface atomic step, QW states exhibit negligible changes at least up to 1 nm away from the step, leaving unmixed the two sets of neighboring QW states belonging to different thicknesses. The results are discussed in terms of a simple electron wave diffraction model. [ABSTRACT FROM AUTHOR]

The confinement effects of electrons in ultrathin films and nanowires grown on metallic and semiconducting substrates investigated using band mapping of their electronic structures using angle-resolved photoemission spectroscopy is discussed here. It has been shown that finite electron reflectivity at the interface is sufficient to sustain the formation of quantum well states and weak quantum well resonance states even in closely matched metals. The expected parabolic dispersion of sp-derived quantum well states for free-standing layers undergoes deviations from parabolic behaviour and modifications due to the underlying substrate bands, suggesting the effects of strong hybridization between the quantum well states and the substrate bands. Electron confinement effects in low dimensions as observed from the dispersionless features in the band structures are also discussed. [ABSTRACT FROM AUTHOR]

We investigate the unoccupied Rashba-type spin-orbit split band structure of the commensurate and incommensurate Bi monolayer on Cu(1 1 1) with spin- and angle-resolved two-photon-photoemission spectroscopy. Because of the unique geometrical structure of these Bi monolayers on Cu(1 1 1), it can be expected that both in-plane and out-of-plane potential gradients play an important role for the Rashba-type spin-structure in these systems. Our spin-resolved data of spin-split states in Bi/Cu(1 1 1) confirm the expected Rashba behavior of the in-plane spin-components that is caused by the out-of-plane potential gradient. But in addition, we indeed find out-of-plane spin components with different magnitudes in both monolayer Bi/Cu(1 1 1) systems, which we therefore attribute to the structurally induced in-plane potential gradients. [ABSTRACT FROM AUTHOR]

The thickness dependence of Au–Pb alloy formation on Pb quantum films is studied via in-situ low-temperature scanning tunneling microscopy/spectroscopy (STM/S). 2D islands with moiré pattern are observed on top of Pb films when sub-monolayer Au is deposited onto Pb films at a substrate temperature of 90 K. After annealing at up to 250 K, various types of 2D islands, including Au–Pb alloys, are formed and show very different electronic structures. A detailed STS study suggests that the formation of these islands is directly related to the quantum well states (QWS) of Pb films, indicating quantum size effects (QSE) on the formation of Au–Pb alloys. [ABSTRACT FROM AUTHOR]

• High-quality Mg(0001) film grows on Si(111) 3 × 3 -B reconstruction. • The lattice mismatch is reduced by rotational epitaxy. • The film displays a honeycomb close-packed structure. Rotational epitaxy of Mg(0001) film on the Si(111) surface has been demonstrated at room temperature using a Si(111) 3 × 3 -B surface reconstruction as a template. The 3 × 3 -B reconstruction, which spontaneously forms on the surface of a heavily-doped Si(111) substrate upon annealing, prevents the formation of magnesium silicide and provides the proper orientation of the film at which the lattice mismatch is lowered down. The film has excellent crystal quality as revealed by diffraction, scanning tunneling microscopy at low temperature (60 K) and photoelectron spectroscopy methods. Density functional theory calculations prove the hexagonal close-packed structure typical for the bulk Mg. The film demonstrates a pronounced quantum-well states effect. The energy position of the surface state is close to the relaxed bulk-like position at early stages of growth, which indicates the smaller interaction of the film with the substrate compared to growth on the bare Si(111). [ABSTRACT FROM AUTHOR]

The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the Γ ¯ → M ¯ direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6 p z states with the Si substrate. The experimentally obtained dispersion E ( k || ) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E ( k || ) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6 p z states with the Si band. However, the momentum range at which mixing of 6 p z and 6 p x , y derived subbands is found to occur in the presence of the Si substrate is closer to Γ ¯ than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a p x , y mediated interaction of the p z states with the Si conduction band. [ABSTRACT FROM AUTHOR]

Highlights: [•] Au grows epitaxially on MoS2(0001) surface even at room temperature. [•] Photoemission spectra reveal Au quantum well states along with a surface state. [•] Low effective masses of Au quantum wells are in semblance with free standing films. [ABSTRACT FROM AUTHOR]

Orientational dependence of catalytic activity for water splitting reaction on a two-dimensional gold cluster supported on MgO/Ag(001) has been identified using first-principles calculations. Strong oscillations are found in water adsorption energy, the dissociation barrier, and the binding energy of the dissociated H atom, with two different orientational patterns. These two patterns correlate with the wavefunction symmetry of frontier orbitals of selected quantum well states (QWSs). This finding reveals a new aspect of orbital symmetry in catalytic reactions without involving changes in the shape or size of the atomic cluster, and is promising for potential applications in chemical reactions using the orbital degree of freedom of QWSs.

Abstract: We have performed synchrotron-radiation angle-resolved photoemission and angle-resolved two-photon photoemission (2PPE) study of Bi(111) films with the thickness of 12 bilayers grown on Si(111) – 7×7. From the detailed analysis of the excitation energy dependence of angle-resolved 2PPE spectra, we have determined the electronic structures in the occupied and unoccupied regions. The observed electronic structures in the unoccupied states above the Fermi-energy were attributed to the quantum-well states originate from the quantization of the 6p-derived bulk bands. [Copyright &y& Elsevier]

The magnetic anisotropy of Fe film grown on vicinal Ag(1,1,10) surfaces studied with the in situ magneto-optic Kerr effect shows oscillatory behavior. We found that both the in-plane step-induced uniaxial anisotropy and the perpendicular anisotropy measured at low temperature show strong oscillations as a function of Fe thickness with a period of \sim5.7 monolayers. Such novel oscillation of the anisotropy is attributed to the quantum well states (QWS) of d-band electrons at the Fermi level in the Fe film. These QWS have been observed by angle-resolved photoemission spectroscopy from a Fe wedge films grown on Ag(001) surface. [ABSTRACT FROM AUTHOR]

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existenceof quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant ic, resulting in a very rich phenomenon of "quantum oscillations" in ic as a function of thickness, L, and bias voltage. V5. More specifically. we find that the phase of the quantum oscillations in ic vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand. at very low sample bias (1V51 <0.03 V) the measurement of ic vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of ic vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function. [ABSTRACT FROM AUTHOR]

We present high-resolution electron energy loss spectroscopy (HREELS) measurements on surface plasmon (SP) dispersion in systems exhibiting quantum well states (QWS), i.e. Na/Cu(111), Ag/Cu(111), and Ag/Ni(111). Our results demonstrate that the dominant coefficient of SP dispersion for thin and layer-by-layer Ag films presenting QWS is quadratic even at small q||, in contrast with previous measurements on Ag semi-infinite media and Ag thin films deposited on Si(111). We suggest that this behavior is due to screening effects enhanced by the presence of QWS shifting the position of the centroid of the induced charge less inside the geometrical surface compared with Ag surfaces and Ag/Si(111). For ultrathin Ag films, i.e. two layers, the dispersion was found to be not positive, as theoretically predicted. Annealing of the Ag film caused an enhancement of the free-electron character of the QWS, thus inducing a negative linear term of the dispersion curve of the SP. Moreover, we report the first experimental evidence of chemical interface damping in thin films for K/Ag/Ni(111). As regards Na/Cu(111), we found a different dispersion curve compared with thick Na films, thus confirming the enhanced screening by Na QWS. Results reported here should shed light on the influence of QWS on dynamical screening phenomena in thin films. [ABSTRACT FROM AUTHOR]

Abstract: The modification of dynamic screening in the electron–electron interaction in systems with reduced dimensionality and tunable one-particle electronic structure is studied. Two examples of such systems are considered, namely, the adsorbate-induced quantum well states at the Na adlayer covered Cu(111) surface, and metal clusters of sizes up to few nanometers. The dependence of the electron–electron decay rates on the Na coverage in the former case and on the cluster size in the latter is investigated. The role played by the dynamical screened interaction in such processes is addressed as well. [Copyright &y& Elsevier]

Abstract: The decay rates of electron and hole excitations at metal surfaces as determined by a scanning tunnelling microscope are presented and discussed. Surface-localised electron states as diverse as Shockley-type surface states and quantum well states confined to ultrathin alkali metal adsorption layers are covered. Recent developments in the analysis of the experimental procedures that are used to determine decay rates with the scanning tunnelling microscope, namely the analysis of line shapes and the spatial decay of standing wave patterns, are discussed. [Copyright &y& Elsevier]

Abstract: Here we report on high-resolution photoemission of iron layers grown on a W(110) substrate. The evolution of the substrate states upon sub-monolayer adsorption of Fe atoms leads to a shift in surface state binding energy. For thicker (110) films, sharp metallic surface states are obtained. Their dispersion displays the signature of quasiparticle renormalization due to dressing with excitations. The energy scale is characteristic for the spin wave spectrum in iron, thereby giving evidence of electron–magnon coupling. Furthermore, it is found that quantum well states occur as a function of layer thickness. These modify the spin density of states at the Fermi level in the ferromagnetic film. [Copyright &y& Elsevier]

Pursuing the confinement of linearly dispersive relativistic Fermions is of interest in both fundamental physics and potential applications. Here, we report strong STM evidence for the equally spaced, strikingly sharp, and densely distributed quantum well states (QWSs) near Fermi energy in Pb(111) nanoislands, van der Waals epitaxially grown on graphitized 6 H -SiC(0001). The observations can be explained as the quantized energies of confined linearly dispersive [111] electrons, which essentially "simulate" the out-of-plane relativistic quasiparticles. The equally spaced QWSs with an origin of confined relativistic electrons are supported by phenomenological simulations and Fabry-Pérot fittings based on the relativistic Fermions. First-principles calculations further reveal that the spin-orbit coupling strengthens the relativistic nature of electrons near Fermi energy. Our finding uncovers the unique equally spaced quantum states in electronic systems beyond Landau levels and may inspire future studies on confined relativistic quasiparticles in flourishing topological materials and applications in structurally simpler quantum cascade laser.