Bulk Electronic Structure Research Articles

Dirac nodal surfaces and nodal lines in ZrSiS.

Topological semimetals are characterized by symmetry-protected band crossings, which can be preserved in different dimensions in momentum space, forming zero-dimensional nodal points, one-dimensional nodal lines, or even two-dimensional nodal surfaces. Materials harboring nodal points and nodal lines have been experimentally verified, whereas experimental evidence of nodal surfaces is still lacking. Here, using angle-resolved photoemission spectroscopy (ARPES), we reveal the coexistence of Dirac nodal surfaces and nodal lines in the bulk electronic structures of ZrSiS. As compared with previous ARPES studies on ZrSiS, we obtained pure bulk states, which enable us to extract unambiguously intrinsic information of the bulk nodal surfaces and nodal lines. Our results show that the nodal lines are the only feature near the Fermi level and constitute the whole Fermi surfaces. We not only prove that the low-energy quasiparticles in ZrSiS are contributed entirely by Dirac fermions but also experimentally realize the nodal surface in topological semimetals.

(Invited) The Role of Electronic Bulk and Surface Structures for Efficient Photo-Electro-Synthesis

For an effective conversion of solar energy to a chemical fuel a lot of different materials as well as device structures have been suggested but only very few provide technological competitive conversion efficiencies. Limitations and loss processes can be deduced from a detailed consideration of the involved photovoltaic and electrochemical elementary steps. Optimized performance can only be reached when the photovoltaic and electrolytic boundary conditions of integrated systems are comparable to those of separated devices with no extra loss due to the coupling process. Feasible solutions seem to be possible for water splitting and H2-generation, as we will show with a number of investigations performed recently combining electrochemical investigations with surface science studies. Especially photoemission results on the demands on the bulk and surface electronic structure provide clear boundary conditions on the material’s and surface properties: i) the semiconductors must provide a wide splitting of quasi Fermi levels which will be provided by multli-junctions but hardly can be expected for low band gap oxides based on localized d electron states as e. g. hematite due to polaron formation. ii) the interaction of H2O and their intermediates directly onto the light energy converting junction must be avoided because of possibly formed surface states. As a consequence a buried junction is needed. iii) Photocatalytic converters must be prepared which uses different junction properties in opposing directions (Janus type surfaces) as proven for facet engineering or hetero structure formation. In summary, we do believe that effective photoelectrochemical devices need the input of different disciplines and expertise ranging from solid state physics, surface science to electrochemistry. Based on an improved understanding of the correlation of material’s properties with the involved elementary steps promising advanced technological solutions seem possible.

Quantum oscillations in iron-doped single crystals of the topological insulatorSb2Te3

We investigated the magnetotransport properties of Fe-doped topological insulator Sb1.96 Fe0.04Te3 single crystals. With doping, the band structure changes significantly and multiple Fermi pockets become evident in the Shubnikov-de Haas oscillations, in contrast to the single frequency detected for pure Sb2Te3. Using complementary density functional theory calculations, we identify an additional bulk hole pocket introduced at the {\Gamma} point which originates from the chemical distortion associated with the Fe-dopant. Experimentally, both doped and undoped samples are hole-carrier dominated, however, Fe doping also reduces the carrier density and mobility. The angle dependent quantum oscillations were analyzed to characterize the complex Fermi surface and isolate the dimensionality of each SdH feature. Among the components, at least two pockets possess 2D-like behavior according to their rotational dependence. These results indicate a complex interplay of hybridized dopant bands with the bulk and surface topological electronic structure.

A versatile multicomponent database for the surface tension of liquid metals

Experimental surface tension data for many binary, ternary and higher order metallic systems is unfortunately currently unavailable in the literature. This could be detrimental in several practical and industrial applications. Consequently, having a theoretical model with a good predictive capability is highly desirable. In general, the surface tension of metallic alloys is predicted via the well-established Butler model. This model assumes a linear relationship between the excess Gibbs energy in the Bulk and on the surface. For many systems, this assumption is not valid, especially for systems based on elements with different bulk electronic structures, for which the Butler model fails to predict the composition dependence of the surface tension (As demonstrated in this work, the Butler model is accurate only for about 65% of metallic system for which experimental data is available). The aim of this paper is to propose an alternative to the Butler model to represent accurately the surface tension multi component liquid metals. The proposed model is an extension of the Guggenheim model of ideal solutions to take into account the difference of electronic structures between elements of a solution. The model is compared with the 36 binary and 7 ternary metallic alloys for which experimental data is available. It is shown that the accuracy of the model is higher than 95%. On that basis, the composition dependence of the surface tension of many other binary alloys for which experimental data is not available is predicted. The extension of the model to ternary and higher order systems is proposed without introducing any new parameters, i.e. by considering only the binary parameters. It is shown that the extended model provides also an accurate prediction of the surface tension for ternary metal.

Well-Dispersed Nickel- and Zinc-Tailored Electronic Structure of a Transition Metal Oxide for Highly Active Alkaline Hydrogen Evolution Reaction.

The practical scale-up of renewable energy technologies will require catalysts that are more efficient and durable than present ones. This is, however, a formidable challenge that will demand a new capability to tailor the electronic structure. Here, an original electronic structure tailoring of CoO by Ni and Zn dual doping is reported. This changes it from an inert material into one that is highly active for the hydrogen evolution reaction (HER). Based on combined density functional theory calculations and cutting-edge characterizations, it is shown that dual Ni and Zn doping is responsible for a highly significant increase in HER activity of the host oxide. That is, the Ni dopants cluster around surface oxygen vacancy of the host oxide and provide an ideal electronic surface structure for hydrogen intermediate binding, while the Zn dopants distribute inside the host oxide and modulate the bulk electronic structure to boost electrical conduction. As a result, the dual-doped Ni, Zn CoO nanorods achieve current densities of 10 and 20 mA cm-2 at overpotentials of, respectively, 53 and 79 mV. This outperforms reported state-of-the-art metal oxide, metal oxide/metal, metal sulfide, and metal phosphide catalysts.

Non-Hermitian nodal-line semimetals with an anomalous bulk-boundary correspondence

Recently, topological quantum states of non-Hermitian systems, exhibiting rich new exotic states, have attracted great attention in condensed-matter physics. As for the demonstration, most of non-Hermitian topological phenomena previously focused on are in one- and two-dimensional systems. Here, we investigate three-dimensional non-Hermitian nodal-line semimetals in the presence of a particle gain-and-loss perturbation. It is found that this perturbation will split the original nodal ring into two exceptional rings (ERs). The topological nature of the bulk electronic structure is characterized by two different topological invariants, namely, the vorticity and the winding number defined for a one-dimensional loop in momentum space, both of which are shown to take half-integer (integer) values when an odd (even) number of ERs thread through the loop. The conventional bulk-surface correspondence in non-Hermitian nodal-line semimetals is found to break down, where the surface zero-energy flat bands are no longer bounded by projections of bulk ERs. Alternatively, a macroscopic fraction of the bulk eigenstates can be localized near the surface, thus leading to the so-called non-Hermitian skin effect.

Surface Chemistry-Mediated Near-Infrared Emission of Small Coinage Metal Nanoparticles.

From size-dependent luminescence to localized surface plasmon resonances, the optical properties that emerge from common materials with nanoscale dimensions have been revolutionary. As nanomaterials get smaller, they approach molecular electronic structures, and this transition from bulk to molecular electronic propertiesis a subject of far-reaching impact. One class of nanomaterials that exhibit particularly interesting optoelectronic features at this size transition are coinage metal (i.e., group 11 elements copper, silver, and gold) nanoparticles with core diameters between approximately 1 to 3 nm (∼25-200 atoms). Coinage metal nanoparticles can exhibit red or near-infrared photoluminescence features that are not seen in either their molecular or larger nanoscale counterparts. This emission has been exploited both as a probe of electronic behavior at the nanoscale as well as in critical applications such as biological imaging and chemical sensing. Interestingly, it has been demonstrated that theirphotoluminescence figures of merit such as emission quantum yield, energy, and lifetimeare largely independent of particle diameter. Instead, emission from particles at this size range depends heavily on the particle surface chemistry, which includes both its metallic composition and the capping ligand architecture. The strong influence of surface chemistry on these emergent optoelectronic phenomena has powerful implications for both the study and use of these particles, primarily due to the theoretically limitlesspossible surface ligand architectures and metallic compositions. In this Account, we highlight recent work that studies and uses surface chemistry-mediated photoluminescence from coinage metal nanoparticles. Specifically, we emphasize the distinct, as well as synergistic, roles of the nanoparticle capping ligand and the nanoparticle corefor controlling and/or enhancing theirnear-infrared photoluminescence. We then discuss the implications of surface chemistry-mediated photoluminescence as it relates to downstream applications such as energy transfer, sensing, and biological imaging. We conclude by discussing current challenges that remain in the field, including opportunities to develop new particle synthetic routes, analytical tools, and physical frameworks with which to understand small nanoparticle emission. Taken together, we anticipate that these materials will be foundational both in understanding the unique transition from molecular to bulk electronic structures and in the development of nanomaterials that leverage this transition.

NMR probe of bulk electronic structures in [formula omitted] beam irradiated Bi2Te3 topological insulator

We report a nuclear magnetic resonance (NMR) study on H+ beam irradiated Bi2Te3 powdered single crystals. In this work, we demonstrate that the beam creates defects within its penetration range giving rise to delocalized charge carriers, thereby making further 125Te NMR Knight shift and line broadening. Upon increasing temperature, the NMR line narrowing manifests the activated motions of thermally excited charge carriers in the irradiated sample. In contrast, it reveals that in the unirradiated sample the free-charge carriers at the Fermi level dominantly contribute to the Knight shift. Our results show that the orbital contribution to the Knight shift in the bulk state of Bi2Te3 becomes predominant in the system with the higher density of defects, as evidenced by modified electronic structures induced by the beam irradiation.

First-Principles Study on the Electronic Structure of Bulk and Single-Layer Boehmite

Two-dimensional (2D) or layered materials have a great potential for applications in energy storage, catalysis, optoelectronics and gas separation. Fabricating novel 2D or quasi-2D layered materials composed of relatively abundant and inexpensive atomic species is an important issue for practical usage in industry. Here, we suggest the layer-structured AlOOH (Boehmite) as a promising candidate for such applications. Boehmite is a well-known layer-structured material and a single-layer can be exfoliated from the bulk boehmite by breaking the interlayer hydrogen bonding. We study atomic and electronic band structures of both bulk and single-layer boehmite, and also obtain the single-layer exfoliation energy using first-principles calculations.

The Vacancy‐Induced Electronic Structure of the SrTiO3−δ Surface

AbstractThe emergence of a 2D electron gas (2DEG) on the (001) surface of oxygen‐deficient strontium titanate (SrTiO3−δ) is investigated. Using in situ soft X‐ray spectroscopy and effective mass modeling, a series of quantitative band diagrams are developed to describe the evolution of near‐surface and bulk carrier concentrations, downward band bending, and Fermi level along a lateral gradient of oxygen vacancies formed on SrTiO3−δ by direct‐current resistive heating under ultrahigh vacuum conditions. Electrons are accumulated over a 3 nm region near the surface, confined within a potential well with saturated 300 meV downward band bending. The relation between Fermi levels and carrier concentrations near the surface suggests the density of states near the surface is much lower than the bulk density of states, which is consistent with the quantum‐confined subbands of a 2DEG. The quantitative relationship between the surface and bulk electronic structures developed in this work provides a guide for precise engineering of the oxygen‐vacancy‐induced 2DEG in SrTiO3.

Temperature-Driven Topological Phase Transition and Intermediate Dirac Semimetal Phase in ZrTe_{5}.

We present an infrared spectroscopy study of ZrTe_{5}, which confirms a recent theoretical proposal that this material exhibits a temperature-driven topological quantum phase transition from a weak to a strong topological insulating state with an intermediate Dirac semimetal state around T_{p}≃138 K. Our study details the temperature evolution of the energy gap in the bulk electronic structure. We found that the energy gap closes around T_{p}, where the optical response exhibits characteristic signatures of a Dirac semimetal state, i.e., a linear frequency-dependent optical conductivity extrapolating to the origin (after subtracting a weak Drude response). This finding allows us to reconcile previous diverging reports about the topological nature of ZrTe_{5} in terms of a variation of T_{p} that depends on the crystal growth condition.

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO_{2}.

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO_{2}. Using a combination of insitu angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO_{2} have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO_{2} in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure.

New Bonding Model of Radical Adsorbate on Lattice Oxygen of Perovskites.

A new model of bonding between radical adsorbates and lattice oxygens is proposed that considers both the adsorbate-oxygen bonding and the weakening of the metal-oxygen bonds. Density functional calculations of SrMO3 perovskites for M being 3d, 4d, and 5d transition metals are used to correlate the bulk electronic structure with the surface-oxygen reactivity. Occupation of the metal-oxygen antibonding states, examined via the crystal orbital Hamilton population (COHP), is found to be a useful bulk descriptor that correlates with the vacancy formation energy of the lattice oxygen and its hydrogen adsorption energy. Analysis of density-of-states and COHP indicates that H adsorption energy is a combined result of formation of the O-H bond and the weakening of the surface metal-oxygen bond due to occupation of the metal-oxygen antibonding states by the electron from H. This insight will be useful in understanding the trends in surface reactivity of perovskites and transition-metal oxides in general.

Evidence of a Coulomb-Interaction-Induced Lifshitz Transition and Robust Hybrid Weyl Semimetal in Td−MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of T_{d}-MoTe_{2}. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe_{2}, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.

Bulk transport properties of bismuth selenide thin films grown by magnetron sputtering approaching the two-dimensional limit

Transport properties of topological insulator Bi2Se3 thin films are investigated with an emphasis on understanding finite-size effects as the two-dimensional limit is approached. Ultra-smooth, (000l)-orientated Bi2Se3 thin-films fabricated using radio-frequency magnetron sputtering reveal disproportionately large changes in bulk resistivity and Hall mobility below six quintuple layers (QL). We correlate such changes to the bulk (other than surface) electronic structure where a bandgap enlargement is observed below six QL in optical absorption spectroscopy measurements. This effect is strongest at the three and two quintuple layers that show the largest changes in bandgap. Temperature dependent transport measurements reveal the effect of disorder from substrate and high carrier concentration. Films on sapphire substrate demonstrate better transport properties compared to amorphous quartz. While temperature dependence of bulk resistivity is both thickness and substrate sensitive, the temperature dependence of Hall coefficient is determined only by the carrier concentration in films. Our work highlights the influence of the bulk electronic structure on the transport properties of few-layer Bi2Se3 which is of interest in the applied areas of optoelectronics, nanoelectronics, and spintronics.

Edge States Caused by Shift of Dirac Points at the Armchair Edge of Distorted Nanographene

We present a theoretical study of non-bonding edge modes at the mono-hydrogenated armchair edge (A1 edge) of a graphene ribbon by modifying its bulk electronic structure. Structural deformation of ...

(Keynote) Clean and Sustainable Energy from Seawater

The evolution from hydrocarbon-based energy sources to renewable and sustainable clean ones is a top priority worldwide due to the severe effects of pollution endangering public health and the environment and the exponential demand from emerging countries. A hydrogen-based energy source [1,2] is ideal as it produces zero carbon emission (only water), and consumer hydrogen fuel cell-powered cars and trucks as well as buses are already available in our societies. Unfortunately, most of the hydrogen industrially produced nowadays still comes from nonrenewable sources, made by steam reforming of methane which produces significant amount of carbon mono/dioxide. The most natural and cleanest way to sustainably produce hydrogen at large scale is by splitting seawater photocatalytically. Consequently, a great increase in research during the past decade, with dedicated studies on material design, surface and electronic structure engineering have been conducted to identify ideal materials and systems [1,3,4]. Our approach consists of fabricating heteronanostructures featuring oriented arrays of quantum rods and dots of high purity synthesized by low cost aqueous chemical growth at low temperature without surfactant and with controlled dimensionalities and surface chemistry [5,6] with intermediate bands for high visible-light conversion, bandgap and band edges optimized for stability against photocorrosion and operation conditions at neutral pH and low or no bias without sacrificial agent [7]. Such unique characteristics, combined with the in-depth investigation of their size-dependent [8], surface [9], doped [10] and bulk electronic structure [11], and electrical conductivity [12] effects do provide better fundamental understanding and structure-efficiency relationships for a cost-effective and sustainable generation of clean hydrogen from the two most abundant and geographically-balanced free resources on earth’s surface, the sun and seawater. An overview of the past decades progress along with the latest advances in controlled fabrication of highly ordered hybrids consisting of visible-light active semiconductors and molecular co-catalysts [13], the atomic-scale origin of performance and stability of gallium nitrides [14] for overall unassisted water splitting in pure water and seawater [15] as well as the latest development in highly efficient single junctions for solar hydrogen generation without transparent substrates will be presented. References J. Z. Su and L. Vayssieres, ACS Energy Lett. 1, 121 (2016)Y. Tachibana, L. Vayssieres, J. R. Durrant, Nat. Photonics 6, 511 (2012)J. Z. Su, Y. K. Wei, L. Vayssieres, J. Phys. Chem. Lett. 8, 5228 (2017)C. X. Kronawitter et al. Energy Environ. Sci. 4, 3889 (2011)L. Vayssieres, J. Phys. Chem. C 113, 4733 (2009); Int. J. Nanotechnol. 2, 411 (2005)L. Vayssieres et al. Appl. Phys. A 89, 1 (2007); Angew. Chem. Int. Ed. 43, 3666 (2004); Adv. Mater. 15, 464 (2003); J. Phys. Chem. B 107, 2623 (2003); Chem. Mater. 13, 4395 (2001); Chem. Mater. 13, 233 (2001); J. Phys. Chem. B 105, 3350 (2001); Pure Appl. Chem. 72, 47 (2000) On Solar Hydrogen & Nanotechnology, L. Vayssieres ed. (Wiley, 2010), Ch. 17, pp. 523-558L. Vayssieres et al. Appl. Phys. Lett. 99, 183101 (2011); Adv. Mater. 17, 2320 (2005)C. X. Kronawitter et al. Nano Lett. 11, 3855 (2011); Phys. Rev. B 85, 125109 (2012); J. Phys. Chem. C 116, 22780 (2012)C. X. Kronawitter et al. Energy Environ. Sci. 7, 3100 (2014)C. X. Kronawitter et al. PhysChemChemPhys 15, 13483 (2013); C. L. Dong et al. Phys. Rev. B 70, 195325 (2004); J. H. Guo et al. J. Phys.: Condens. Matter 14, 6969 (2002)J. Engel, S. R. Bishop, L. Vayssieres, H. L. Tuller, Adv. Funct. Mater. 24, 4952 (2014)Y. K. Wei et al. Nano Res. 9, 1561 (2016)M. G. Kibria et al. Adv. Mater. 28, 8388 (2016)X. G. Guan et al. J. Phys. Chem. C (2018) Article ASAP DOI: 10.1021/acs.jpcc.8b00875

Spin-orbit coupling induced band splitting in bulk ferroelectric BiAlO3 with tetragonal phase

The bulk ferroelectric materials lacking spacing inversion symmetry were proposed to exhibit Rashba-Dresselhaus spin-splitting effects, which may allow for a nonvolatile electric control of spin degrees of freedom. In the present work, we investigated the structural and ferroelectric properties of tetragonal phase of ferroelectric BiAlO3 by means of density functional calculations. Two metastable tetragonal phases of BiAlO3 were predicted. Both metastable tetragonal phases demonstrated Rashba-Dresselhaus effects, giving rise to spin-splitting due to spin-orbit coupling in their bulk electronic structure. The calculated metastable confirmed the hypothesis that BiAlO3 possessed a large variety of metastable structural phases as BiFeO3, which was expected to show some extraordinary behavior similar with BiFeO3. Particularly, BiAlO3 was considered as the first known oxide to exhibit a coexistence of ferroelectricity and Rashba-Dresselhaus effects.

Topological states induced by local structural modification of the polar BiTeI(0001) surface

The layered polar semiconductor BiTeI exhibits large Rashba-type spin–orbit splittings in its bulk and surface electronic structure. Here we report an artificial structural modification near the surface of BiTeI(0001) induced by annealing in vacuum. Using scanning tunneling microscopy we show that the annealing-induced change in the near-surface stoichiometry results in a structural change from a non-centrosymmetric triple-layered to a quintuple-layered structure. The structural change gives rise to the emergence of topological surface states with helical spin texture as demonstrated by angle-resolved photoemission experiments and relativistic first-principles calculations. The results provide a way to modify the electronic structure of layered materials by a controlled manipulation of the atomic stacking sequences.

Accurate and efficient band-offset calculations from density functional theory

We perform a systematic study of the factors that determine the accuracy of heterostructure band alignments in density functional calculations. Band alignments are calculated for a number of representative test structures using either the generalized gradient approximation (GGA) or the more accurate hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). The alignment of the bulk band structures is achieved through a potential-alignment term from an explicit superlattice calculation, with full inclusion of atomic relaxations. The potential alignments calculated within the GGA are in agreement with those calculated using HSE to within 50 meV, despite the GGA calculations being 10–100 times less computationally expensive. The GGA potential alignments are remarkably accurate, even for heterostructures between materials with a large lattice mismatch, and even when the GGA lattice parameters are markedly different from HSE. We also find that the errors in the potential alignment are small even when the optimum HSE mixing parameter (α) (to bring the band gap in agreement with experiment) is markedly different for the two materials at the interface; the error associated with using a fixed α for a superlattice calculation is small. To obtain band offsets, the potential-alignment values are combined with bulk band structure calculations. GGA exhibits large errors in the positioning of the eigenvalues at the band extrema with respect to the average electrostatic potential, and the resulting all-GGA band alignments are unreliable. HSE produces much more accurate band structures than GGA. Given the accuracy of potential alignments calculated within the GGA, they can be combined with HSE calculations of the bulk electronic band structure to achieve accurate band alignments with a high computational efficiency.