Additional Surface States Research Articles

Unraveling the impact of tetravalent and pentavalent ions on the charge dynamics of hematite photoelectrodes for solar water splitting

This study focuses on understanding the impact of tetravalent (Hf4+) and pentavalent (Nb5+) ions in hematite photoelectrodes using a two-step synthesis approach. Structural, morphological, and compositional analyses confirms that Nb5+ is segregated over the hematite crystal surface while Hf4+ is allocated over the crystal surface and between the fluorine-doped tin oxide substrate (FTO)-hematite interface. Our results indicate that the modifier insertion led to substantial improvements in overall efficiency and photoelectrochemical (PEC) performance. However, it creates additional surface states, as evidenced by shifts in the anodic onset potential (Vonset) observed in the linear sweep voltammetry curves. For Nb-modified photoelectrodes, Vonset shift of ∼20 mV with respect to hematite was observed and confirmed by intensity modulated photocurrent spectroscopy (IMPS). For Hf-modified hematite, this Vonset shift is more pronounced (∼30 mV). When combined with NiFeOx, Nb5+ acted synergistically showing a 4-fold efficiency increase compared to hematite and ∼70 mV cathodic shift in the Vonset. Otherwise, NiFeOx showed no significant impact in the Hf4+ modified hematite photoelectrodes, suggesting that Hf4+ role lies in enhancing electron collection at the FTO-hematite interface. This study not only sheds light on the charge dynamics of modified hematite photoelectrodes but also provides strategies for boosting the PEC devices efficiency.

Photoluminescence enhancement of aluminum ion intercalated MoS2 quantum dots

Low photoluminescence (PL) quantum yield of molybdenum disulfide (MoS2) quantum dots (QDs) has limited practical application as potential fluorescent materials. Here, we report the intercalation of aluminum ion (Al3+) to enhance the PL of MoS2 QDs and the underlying mechanism. With detailed characterization and exciton dynamics study, we suggest that additional surface states including new emission centers have been effectively introduced to MoS2 QDs by the Al3+ intercalation. The synergy of new radiative pathway for exciton recombination and the passivation of non-radiative surface traps is responsible for the enhanced fluorescence of MoS2 QDs. Our findings demonstrate an efficient strategy to improve the optical properties of MoS2 QDs and are important for understanding the regulation effect of surface states on the emission of two dimensional sulfide QDs.

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes.

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO2. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na2SO3), the hole transfer in Au18-TiO2 occurs directly through the highest occupied molecular orbital (HOMO) of Au18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au18-TiO2 does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au18-TiO2. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.

Thickness-dependent topological phase transition and Rashba-like preformed topological surface states of α-Sn(001) thin films on InSb(001)

Topological materials, possessing spin-momentum locked topological surface states (TSS), have attracted much interest due to their potential applications in spintronics. \ensuremath{\alpha}-phase Sn (\ensuremath{\alpha}-Sn), being one of them, displays enriched topological phases via band-gap engineering through a strain or confinement effect. In this work, we investigated the band evolution of in-plane compressively strained \ensuremath{\alpha}-Sn(001) thin films on InSb(001) in a wide range of thickness from 3 bilayers (BL) to 370 BL by combining angle-resolved photoemission spectra and first-principles calculations. Gapped surface states evolved to a linearly dispersive TSS at a critical thickness of 6 BL, indicating that the system undergoes a phase transition from topologically trivial to nontrivial. For films thicker than 30 BL, additional Rashba-like surface states (RSS) were identified. These RSS served as preformed TSS in another strain-induced topological phase transition. In thick films, 370-BL \ensuremath{\alpha}-Sn(001), so as to preclude the confinement effect in thin films, our results were consistent with a Dirac semimetal phase with Dirac nodes located along $\mathrm{\ensuremath{\Gamma}}--Z$. This thickness-dependent band-structure study deepens our understanding of topological phase transitions and the evolution of Dirac states. Furthermore, the coexistence of TSS and RSS in a Dirac semimetal \ensuremath{\alpha}-Sn might significantly enhance the potential for spintronic applications.

Surface reconstruction of tetragonal methylammonium lead triiodide

We present a detailed first-principles analysis of the (001) surface of methylammonium lead triiodide (MAPbI3). With density functional theory, we investigate the atomic and electronic structure of the tetragonal (I4cm) phase of MAPbI3. We analyzed surface models with MAI-termination (MAI-T) and PbI2-termination (PbI2-T). For both terminations, we studied the clean surface and a series of surface reconstructions. We find that the clean MAI-T model is more stable than its counterpart, PbI2-T. For the MAI-T, reconstructions with added or removed units of nonpolar MAI and PbI2 are most stable. The corresponding band structures reveal surface states originating from the conduction band. Despite the presence of such additional surface states, our stable reconstructed surface models do not introduce new states within the bandgap.

Atomic and electronic structure of cesium lead triiodide surfaces.

The (001) surface of the emerging photovoltaic material cesium lead triiodide (CsPbI3) is studied. Using first-principles methods, we investigate the atomic and electronic structure of cubic (α) and orthorhombic (γ) CsPbI3. For both phases, we find that CsI-termination is more stable than PbI2-termination. For the CsI-terminated surface, we then compute and analyze the surface phase diagram. We observe that surfaces with added or removed units of nonpolar CsI and PbI2 are most stable. The corresponding band structures reveal that the α phase exhibits surface states that derive from the conduction band. The surface reconstructions do not introduce new states in the bandgap of CsPbI3, but for the α phase, we find additional surface states at the conduction band edge.

Effect of Microimpurities on the Electronic Structure and Emission Properties of Microwave Device Cathode Materials

The effect of microimpurities on the electronic structure of barium oxide has been studied using optical and electron spectroscopy. As was shown previously, oxygen vacancies play a decisive role in the formation of the emission properties of barium oxide crystallites; however, the possible role of microimpurities in barium oxide crystallites was not considered. It has been established that some microimpurities, including nickel, scandium, and combination of calcium and strontium, order oxygen vacancies in barium oxide crystallites, which directly affects oxygen diffusion in them. Nickel and scandium impurities form additional surface states near the edge of the valence band, which is accompanied by a decrease in the electronic work function of a cathode material.

Influence of fluorine and oxygen atom adsorption on electronic structure of InSb(111) surface

The projector augmented-wave method was applied for investigation of oxygen and fluorine atom adsorption on the InSb(111) surface depending on its termination. It was shown that oxygen adsorption leads to appearance of additional surface states in the fundamental gap in case of the In-terminated surface while the density of surface states decreases for the Sb-terminated surface. Fluorine co-adsorption results in partial or complete removal of the surface state and an unpinning of the Fermi level. The increase of fluorine concentration leads to considerable changes of the near-surface-layer structure due to the penetration of both electronegative adsorbates (O and F) into the substrate.

Distinct multiple fermionic states in a single topological metal

Among the quantum materials that have recently gained interest are the topological insulators, wherein symmetry-protected surface states cross in reciprocal space, and the Dirac nodal-line semimetals, where bulk bands touch along a line in k-space. However, the existence of multiple fermion phases in a single material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic material Hf2Te2P and discover properties, which are unique in a single topological quantum material. We experimentally observe weak topological insulator surface states and our calculations suggest additional strong topological insulator surface states. Our first-principles calculations reveal a one-dimensional Dirac crossing—the surface Dirac-node arc—along a high-symmetry direction which is confirmed by our ARPES measurements. This novel state originates from the surface bands of a weak topological insulator and is therefore distinct from the well-known Fermi arcs in semimetals.

Structures and Surface States of Polymer Brushes in Good Solvents: Effects of Surface Interactions

The influence of the surface interaction on the mesoscopic structure of grafted polymers in good solvents has been examined. At high surface coverage, tethered polymers are in the brush state and the parabolic segment density profile is confirmed by self-consistent field theory (SCFT) calculations. It is found that this is a universal behavior for a whole range of surface interactions from complete repulsion to strong attraction. More interestingly, finite surface repulsion may lead to the maximum in the proximal layer of its segment density profile, which is significantly different from both the depletion layer of pure repulsion and the adsorbing layer of attraction. In addition to the brush state on both repulsive and attractive surfaces, three additional surface states were identified by analyzing the scaling behavior of the layer thickness of polymer brushes: the mushroom state on repulsive substrates, the dilute and the semidilute surface states on attractive substrates.

Fluorine and Oxygen Adsorption and Their Coadsorption on the (111) Surface of InAs and GaAs

Oxygen and fluorine adsorption and their coadsorption on the (111) unreconstructed surface of semiconductors InAs and GaAs were studied using the projector augmented-wave method with the generalized gradient approximation for the exchange–correlation functional and hybrid functional approach. The energetically preferable adsorbate sites on the surface were determined. It is shown that fluorine adsorption above surface cations on the AIIIBV(111)A-(1 × 1) unreconstructed surface leads to a removal of the surface state formed by cation pz-orbitals and to an unpinning of the Fermi level, whereas oxygen adsorption induces additional surface states in the band gap. The influence of fluorine and oxygen coadsorption and also fluorine concentration on the surface states in the band gap is discussed. It is shown that oxygen-induced surface states are completely or partially removed from the band gap by fluorine coadsorption if it forms bonds with cation surface atoms involved in an interaction with oxygen. The incr...

Hole trap distribution on 2 MeV electron irradiated high-k dielectrics

The effects of 2 MeV electron irradiation on the electrical properties of high-k dielectric based metal–insulator–semiconductor capacitors have been studied. Samples consist of 5.9 nm-thick films of aluminum oxide and hafnium oxide deposited by atomic layer deposition on silicon substrates. Deep-level transient spectroscopy (DLTS) and admittance measurements reveal that electron irradiation modifies the defect density of both surface states at the dielectric–semiconductor interface and border traps existing inside the dielectric. The experimental results indicate that irradiation has a double effect. The incident electrons transfer their energy and generate additional surface states, leading to a degradation of the interface. On the other hand, irradiation generates electron–hole pairs inside the dielectric. Some of the holes are trapped by border traps located inside the dielectric at locations close to the interface. As a result, border traps capturing holes are neutralized and become inactive after irradiation. Moreover, interface state profiles, as measured by DLTS, are affected by the presence of border traps and yield overestimated interface state densities. Admittance spectroscopy is used to distinguish among border traps and interface traps. A detailed study of the conductance signal as a function of voltage, temperature and frequency for samples with different irradiation doses (nonirradiated, 2.5, 25, and 250 Mrad) is presented here. The influence of the irradiation dose has been analyzed in order to compare the defect distribution before and after irradiation.

Synthesis of Guanosine 5′‐Monophosphate (GMP)‐Mediated Ag/CdS Nanohybrids: Their Self‐Assembly and Optoelectronic Properties

AbstractGuanosine 5′‐monophosphate (GMP)‐mediated Ag/CdS nanohybrids have been synthesized in the quantum‐confined region. The presence of Ag in the nanohybrid and its binding with GMP has been ascertained by XRD, IR spectroscopy, and X‐ray photoelectron spectroscopy (XPS) studies. GMP is attached to the surface of Ag/CdS through its various functionalities, namely, –NH2, –C=O, –NH–, –OH, N‐7–C‐8 + C‐8–H, PO32–, P–O–5′‐sugar, and sugar ring, through supramolecular interactions and brings a change in their optical and electronic properties. The presence of Ag at the interface of GMP‐mediated CdS introduces several additional surface states at the interface of the two colloids, which enhances the radiative transition to cause an increase in the quantum efficiency of fluorescence from 0.005 in the absence of Ag to 0.039 in its presence. It is also evidenced by an increase in the fluorescence lifetime from 40 to 160 ns. In this time domain, different intermediates are understood to be produced by the relaxation of charge within the surface states of varied energies from 220–367 eV. The nature of the surface states could be controlled kinetically by optimizing the amount of Ag in the nanohybrid. Aging of the Ag/CdS nanohybrids induces self‐assembly, which changes its morphology from quantum dots to nanowires over a period of one month unlike GMP‐mediated CdS, which forms nanorods. These changes in morphology are also reflected by a significant increase in the rotational correlation time from 42.99 to 167 ns.

Zinc Oxide as a Model Transparent Conducting Oxide: A Theoretical and Experimental Study of the Impact of Hydroxylation, Vacancies, Interstitials, and Extrinsic Doping on the Electronic Properties of the Polar ZnO (0002) Surface

The technology-relevant zinc-terminated zinc oxide (0002) polar surface has been studied at the density-functional theory level using both Perdew–Burke–Ernzerhof (PBE) and hybrid Heyd–Scuseria–Ernzerhof (HSE06) functionals. We have considered a number of surface conditions to better understand the impact of surface hydroxylation and intrinsic and extrinsic surface defects, including zinc vacancies, oxygen vacancies, zinc interstitials, and aluminum dopants on the surface electronic properties. Our calculations point to large variations in surface work function and energy band gap as a function of the surface model; these variations can be attributed to changes in surface charge carrier density and to additional surface states induced by the defects. The calculated shifts in O(1s) core-level binding energy of the surface oxygens in different bonding configurations are in good agreement with experimental X-ray photoelectron spectroscopy data and point to the presence of two distinct OH-species on the ZnO su...

Electrochemical and optical characterizations of anodic porous n-InP(1 0 0) layers

In this paper, electrochemical and optical characterizations of anodic porous n-InP(1 0 0) are reported. The direct relation between the observed pore morphology and the physical properties is demonstrated using electrochemical methods such as cyclic voltammetry and impedance spectroscopy as well as optical techniques like photocurrent spectroscopy and photoluminescence measurements. An enhancement of the interfacial capacitance, proportional to the anodic charge, is revealed by voltammetry and Mott-Schottky analysis. It is related to the drastic increase of the area of the porous electrode. However, when the porous samples are sufficiently reverse-biased, the capacitance enlargement disappears because the nanosized pore walls are fully depleted and the electroactive area recovers its initial value. Photocurrent spectroscopy and photoluminescence measurements show the porous film behaves like an absorbent layer. This effect is also ascribed to the specific geometry of the space charge layer within the pore walls. A model based on the absorption coefficient and the effective optical path length is thus used to describe the phenomenon. However the model is not sufficient to depict the phenomenon and the charge recombination in the additional surface states created during the pore formation and the long transit time of electrons in the porous matrix are also significant. Additional effects such as the initial enhancement of the photocurrent response and the redshift of the absorption edge of the photocurrent spectra are observed. Inversely, no shift of the photoluminescence peak is detected. However an exponential quenching of the photoluminescence is also attributed to an absorbent behavior of the porous layer.

Morphology and electronic states of chemically treated CdZnTe surfaces

The morphology and electronic structure of chemically treated CdZnTe surfaces has been studied using atomic force microscopy and scanning tunneling microscopy/spectroscopy. Studies have been performed on wet chemically etched and passivated surfaces. Etching in bromine methanol solutions effectively removes surface damage after polishing and produces a hillock like structure. A number of surface electronic states are introduced, both donor-like and acceptor-like, and are attributed to various surface defects. Because of the amorphous nature of the surface, these states have tails that extend into the band gap and produces a very narrow surface band. Charge hopping across the narrow band gap is proposed to explain the high surface conductivity. Surfaces passivated in hydrogen peroxide show increased roughness and using atomic force microscopy, the evolution of the formation of an oxide layer has been observed. Additional acceptor-like surface states of higher energy are introduced and the surface band gap is found to increase, reducing the surface conductivity.

Electronic structure of bimetallic Ni–Rh nanowires

The electronic structure of a bare Rh(553) surface and of a Ni-decorated Rh(553) surface has been investigated by angle-resolved UV photoelectron spectroscopy and density functional theory calculations. The self-assembly of Ni adatoms leads to the decoration of the steps of the Rh(553) surface with monoatomic Ni rows under suitable kinetic conditions, thus forming a regular array of pseudomorphic bimetallic Ni–Rh nanowires. The electronic structure of the clean Rh(553) surface has been compared to the one of the flat Rh(111) surface, and additional surface states localized at the step edges due to the lower coordination of the step atoms have been detected. The Ni wires are weakly hybridized with the Rh substrate states and are characterized by only weakly dispersing states. This leads to a strong narrowing of the d-band, which is argued to be the origin of the observed high chemical reactivity of the Ni–Rh nanowires.

Changes of Electrical Characteristics for AlGaN/GaN HEMTs Under Uniaxial Tensile Strain

This letter investigates the characteristics of unpassivated AlGaN/GaN high-electron mobility transistors (HEMTs) under uniaxial tensile strain. Mechanical stress can produce additional charges that change the HEMT channel current. This phenomenon is dependent upon gate orientation and may be the result of the piezoelectric effect and changes in electron mobility due to the applied uniaxial stress. In addition, results show that tensile strain reduces the transient current, which is likely due to the additional donorlike surface states created through the piezoelectric effect.

Ordering and segregation of aCu75Pt25(111)surface: A first-principles cluster expansion study

Configurational thermodynamics for ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}\phantom{\rule{0.3em}{0ex}}(111)$ surfaces is examined by a first-principles calculation in conjunction with the cluster-expansion technique. The calculated surface segregation profile just below the bulk order-disorder transition temperature exhibits Pt segregation to the top and the third layers and Cu segregation to the second layer. No long-range ordered structure is found at the top layer. However, the simulated short-range-order parameter shows a small negative value, indicating a weak ordering tendency at the ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}$ alloy surface. This fact can be interpreted by the competition between the layer-confined spontaneous $p(2\ifmmode\times\else\texttimes\fi{}2)$ ordering tendency and the Pt segregation and interlayer surface ordering, which certainly disrupts the $p(2\ifmmode\times\else\texttimes\fi{}2)$ ordering. Five possible surface ground-state structures are found for the bulk $L{1}_{2}$ structure. The surface ground-state structures exhibit strongly localized electronic states composed of Pt and Cu $d$ states at the topmost layer around $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi energy, which is consistent with the previous ultraviolet photoelectron spectroscopy measurement. One of the surface ground-state structures possesses additional surface states just below and above the Fermi energy, which are composed of Pt $d$ states at the topmost layer. A significant ordering effect on the surface electronic states is confirmed for the ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}$ alloy.

Spectroscopic and microscopic study of vanadium oxide nanotubes

V 2 O 5 nanotubes synthesized via the sol-gel route has been studied by electron energy loss spectroscopy (EELS), x-ray absorption spectroscopy (XAS), and energy dispersive x-ray analysis, in order to understand the local structure of vanadium in the nanotubes. Contrary to our expectation, all the features of the XAS and EELS spectra of the V2O5 nanotubes are in line with that of bulk layered vanadium oxide revealing that vanadium is present in the 5+ oxidation state in the nanotubes. However, V2O5 nanotubes exhibit additional surface states in their electronic structure in comparison with bulk V2O5. A comparison of measured and calculated spectra allows us to distinguish single-wall from multiwall V2O5 nanotubes.