Intrinsic Surface States Research Articles

First-principles study of electron trapping by intrinsic surface states on β-Si3N4 (0001)

Charge trapping by intrinsic surface states on β-Si3N4 surfaces is a relatively unexplored topic in the study of the electronic properties of this material. In the present work, ab-initio density functional theory is used to observe the transfer of an electron from a substitutional-oxygen donor impurity to a coordinatively-unsaturated surface site on the (0001) surface. The process occurs spontaneously, with little or no activation energy, and has a profound effect on surface electronic structure that leads to a range of different states distributed throughout the band gap. The results are discussed in relation to experimentally-observed surface charging that depends on O2 exposure. The related issue of the β-Si3N4 band gap, which has been somewhat controversial, is also discussed in light of recent experimental and theoretical results.

Ultrathin-layer α-Fe2O3 deposited under hematite for solar water splitting

This work proposes a new strategy to prepare a hematite (α-Fe2O3) bilayer photoanode by hydrothermally depositing α-Fe2O3 (B) on the α-Fe2O3 (A) films prepared by electrochemical deposition. Compact smooth surfaced α-Fe2O3 (A) films were electrochemically deposited on FTO (SnO2:F) substrates from an aqueous bath. The α-Fe2O3 (A), α-Fe2O3 (B), and α-Fe2O3/α-Fe2O3 bilayer films’ characteristics were defined by X-ray diffraction (XRD) measurements, field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray (EDX) spectroscopy. Pure crystalline α-Fe2O3 (B) films with a typical anisotropic-like nanoparticle formation, which exhibited nanostructured rods covering the substrate and formed the characteristic mesoporous film morphology, were hydrothermally deposited on α-Fe2O3 (A) films prepared by electrochemical depositing in a solution bath at 25 °C and a potential of − 0.15 V. The photocurrent measurements exhibited increased intrinsic surface states (or defects) at the α-Fe2O3 (A)/α-Fe2O3 (B) interface. The photoelectrochemical performance of the α-Fe2O3 (A)/α-Fe2O3 (B) structure was examined by chronoamperometry, which found that the α-Fe2O3 (A)/α-Fe2O3 (B) structure exhibited greater photoelectrochemical activity than the α-Fe2O3 (A) and α-Fe2O3 (B) thin films. The highest photocurrent density was obtained for the bilayer α-Fe2O3 (A)/α-Fe2O3 (B) films in 1 M NaOH electrolyte. This great photoactivity was ascribed to the highly active surface area, and to the externally applied bias that favored the transfer and separation of photogenerated charge carriers in α-Fe2O3 (A)/α-Fe2O3 (B). The improved photocurrent density was attributed to an appropriate band edge alignment of semiconductors and to enhanced light absorption by both semiconductors. The best performing samples were α-Fe2O3 (A)/α-Fe2O3 (B), which reached the maximum incident photon conversion efficiencies (IPCE) of 400 nm at the potential of 0.1 V. In this case, the IPCE values were 3-fold higher than those of the α-Fe2O3 (A) and α-Fe2O3 (B) films.

Aggregated Molecular Fluorophores in the Ammonothermal Synthesis of Carbon Dots

Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic blue-green double emission. The high energy...

Synchrotron-based photoemission study of electronic structure of the Cs/GaN ultrathin interface

Electronic structure of the Cs/n-GaN nano-interface has been studied in situ via synchrotron-based photoelectron spectroscopy by excitation in the energy range of 70–400 eV. The GaN sample was grown by an original method of epitaxy of low-defect unstressed nanoscaled films on AlGaN/SiC/Si substrate. Changes in the surface state spectra and in the Ga 3d, Cs 4d, Cs 5p, N 1s core level spectra have been revealed under different cesium coverages. The intrinsic surface states for the clean GaN surface at binding energies of ∼5.0 eV and ∼7.0 eV are attenuated during Cs adsorption. Simultaneously three Cs induced surface states are found to arise. Drastic changes in the surface state spectrum were ascertained and shown to be originated from the local interacting Ga dangling bonds and adsorbed Cs atoms initiating the electron redistribution effect with formation of the semiconductor-like Cs/n-GaN interface.

Morphological, structural and optical characterization of SnO2 nanotube arrays fabricated using anodic alumina (AAO) template-assisted atomic layer deposition

Self-aligned and equal-space SnO2 nanotubes (NTs) with external diameters ranging from ca. 124 to ca. 325nm and wall thickness of around 30nm were synthesized by AAO template-assisted atomic layer deposition. The bulk and surface structure of the SnO2 nanostructures were studied in detail by XRD and XPS techniques, respectively. The SnO2 NTs were polycrystalline with an average crystallite size of ca. 3nm. The structure analysis has revealed that the SnO2 NTs are composed of the rutile-type SnO2. The SnO2 nanotube arrays with the smaller external diameters emitted green light centered at around 520nm. The emission was ascribed to the radiative recombination between electrons trapped in shallow donor levels and holes at the intrinsic surface states, associated with the oxygen deficient sites near surface region. The PL intensity diminished with the nanotube diameter and spacing. With increasing external diameter of the SnO2 nanotubes and decreasing the distance between the neighboring nanotubes, the emission become progressively weaker. The results demonstrate that the structure, morphology and arrangement of SnO2 nanotubes play an important role in their luminescent properties.

Thermoelectric properties of 3D topological insulator: Direct observation of topological surface and its gap opened states

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from the bulk by employing single crystal Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ films epitaxially grown in the ultrathin limit. Two intrinsic nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters (electrical conductivity (${\sigma}$), thermal conductivity (${\kappa}$), and thermopower ($S$)) are accurately determined. Pure m-TSDS gives $S$=-44 {\mu}VK$^{-1}$, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to -212 {\mu}VK$^{-1}$ for g-TSDS. It is clearly shown that the semi-classical Boltzmann transport equation (SBTE) in the framework of constant relaxation time (${\tau}$) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time ${\tau}(E)$ beyond the Born approximation is essential for making intrinsic interpretations. Although ${\sigma}$ is protected on the m-TSDS, ${\kappa}$ is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport.

Defect-related optical bandgap narrowing and visible photoluminescence of hydrothermal-derived SnO2 nanoparticles

Tin oxide (SnO2) nanoparticles are fabricated by a novel hydrothermal route using H2O2 as oxidizer without adding any corrosive acids or bases. The morphology, composition, and microstructure are characterized by transmission electron microscopy, X-ray diffraction (XRD) and Raman spectra. XRD analysis reveals the formation of single phase rutile type tetragonal structure of all samples which is further supported by Raman studies. The average crystallite size is observed to vary from 2.8 to 4.2 nm as the reacting temperature increases from 120 to 180 °C, suggesting the promotion of crystal growth with increasing temperature. Raman spectroscopy measurement presents the existence of the defect modes, of which the mode at about 565 cm−1 is identified to relate to oxygen vacancies as this mode becomes more prominent in the as-grown sample. Unusual optical band gap narrowing and the excitation wavelength dependent visible luminescence are observed, and both are supposed to be induced by the intrinsic defects. The photoluminescence (PL) spectra of all samples consist of two defect-related subbands, i.e., orange and blue luminescence bands. The two subbands are attributed to optical transitions in oxygen vacancies and some intrinsic surface states, respectively. Spectral analysis suggests that the excitation wavelength dependences of the two subbands are due to a distribution of the band gap of the nanoparticles induced by the internal strain, defect concentration as well as the quantum confinement effect. This work provides a feasible route to modulate the band gap and defect emissions and a good understanding of the PL behavior in the present SnO2 nanoparticles.

Photo-assisted Kelvin probe force microscopy investigation of three dimensional GaN structures with various crystal facets, doping types, and wavelengths of illumination

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron–hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid micro-structures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mg-doped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light–emitting diode micro-structures, the m-plane is more chemically stable.

Soft X-ray photoelectron spectroscopy of the ultrathin Ba/InGaN interface

Electronic structure of the clean In0.22Ga0.78N surface and the ultrathin Ba/In0.22Ga0.78N interface has been studied in situ by the synchrotron-based photoelectron spectroscopy during excitation in the photon energy range of 100–650eV. Changes in the valence band and surface states spectra and in the In 3d, In 4d, N 1s, Ga 3d, Ba 4d, Ba 5p core levels spectra have been investigated under the step-by-step Ba submonolayer deposition. Changes in the surface electronic structure of the InGaN caused by Ba adsorption are found to originate predominantly from the local interaction of the Ga and In dangling bonds and Ba adatoms that results in effect of the suppression of the two intrinsic surface states and appearance of a new induced state. The Ba atomic layer deposition is revealed to induce the charge transfer between the Ba adatoms and the N surface atoms with increasing N-ionicity.

Nature of radiative recombination processes in layered semiconductor PbCdI2 nanostructural scintillation material

We report on the efficient photoluminescence (PL) and radioluminescence (RL) of the PbI2 nanoclusters (NCLs), which are naturally formed in the nanostructured Pb1-XCdxI2 alloys (X=0.70). Here, we carried out the studies of the nature of radiative recombination processes in the NCLs of various sizes by measuring PL temperature evolution. Our results indicate that at low temperatures the PL is mainly caused by exciton emission and recombination of donor-acceptor pairs, generated in volume of large NCLs. The broad bands, which are associated with the deep intrinsic surface states, including self-trapped excitons (STEs), are dominant in the PL spectra at higher temperature (>100 K). Our work shows that the nature of emission, associated with RL bands is analogous to that for PL bands. It was shown that the investigated nanostructured material is strongly radiation-resistant. Thus, the Pb1-XCdXI2 alloys can be considered as new effective layered semiconductor nanostructured materials which can be suitable for the elaboration of perspective semiconductor scintillators. These nanomaterials have promising prospects for applications in new generations of devices for biomedical diagnostics and industrial imaging applications.

Electrochemical Fabrication and Characterization of p-CuSCN/n-Fe2O3 Heterojunction Devices for Hydrogen Production

p-CuSCN/n-Fe2O3 heterojunctions were electrochemically prepared by sequentially depositing α-Fe2O3 and CuSCN films on FTO (SnO2:F) substrates. Both α-Fe2O3 and CuSCN films and α-Fe2O3/CuSCN heterojunctions were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Pure crystalline CuSCN films were electrochemically deposited on α-Fe2O3 films by fixing the SCN/Cu molar ratio in an electrolytic bath to 1:1.5 at 60°C, and at a potential of −0.4 V. The photocurrent measurements showed increased intrinsic surface states or defects at the α-Fe2O3/CuSCN interface. The photoelectrochemical performance of the α-Fe2O3/CuSCN heterojunction was examined by chronoamperometry and linear sweep voltammetry techniques. The α-Fe2O3/CuSCN structure exhibited greater photoelectrochemical activity compared to the α-Fe2O3 thin films. The highest photocurrent density was obtained for the α-Fe2O3/CuSCN films in 1 M NaOH electrolyte. This strong photoactivity was attributed to both the large active surface area and the external applied bias, which favored the transfer and separation of the photogenerated charge carriers in the α-Fe2O3/CuSCN heterojunction devices. The flatband potential and donor density were maximal for the heterojunction. These results suggest a substantial potential to achieve heterojunction thin films in photoelectrochemical water splitting applications.

(Invited) Fundamental Differences Between the Traditional III-V Compounds and Nitride Semiconductors

In this talk I will address the fundamental question as to why the performance of emitters based on nitride semiconductors, grown heteroepitaxially on foreign substrates, is insensitive to high concentration of extended defects, which is not the case for traditional III-V compounds. I will discuss the following important differences between these two families of materials, which contribute to this finding: (a) the growth of Nitride semiconductors occurs at temperatures much lower than the half of their melting temperature, where the brittle to ductile transition occurs. In traditional III-V compounds the growth temperature is generally higher than ½ TM; (b) the chemical bonding in nitrides is strongly ionic while is mostly covalent in traditional III-V compounds. This leads to the bunching of the intrinsic surface states as well as the states associated with dangling bonds in edge dislocations near the band edges where they act as traps rather than recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (c) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs by only a few meV. Thus, conversion between the two phases occurs easily by creation of stacking faults along the close-packed (0001) and (111) planes, which due to the smaller energy gap of the cubic phase leads to strong band structure potential fluctuations in the wurtzite matrix; (d) additional band structure potential fluctuations exist in the nitride alloys due to various forms of partial alloy ordering or to compositional inhomogeneities introduced by phase separation or the growth mode, These potential fluctuations contribute to exciton localization and thus efficient radiative recombination.

Temperature dependence of the partially localized state in a 2D molecular nanoporous network

Two-dimensional organic and metal-organic nanoporous networks can scatter surface electrons, leading to their partial localization. Such quantum states are related to intrinsic surface states of the substrate material. We further corroborate this relation by studying the thermally induced energy shifts of the electronic band stemming from coupled quantum states hosted in a metal-organic array formed by a perylene derivative on Cu(111). We observe by angle-resolved photoemission spectroscopy (ARPES), that both, the Shockley and the partially localized states, shift by the same amount to higher binding energies upon decreasing the sample temperature, providing evidence of their common origin. Our experimental approach and results further support the use of surface states for modelling these systems, which are expected to provide new insight into the physics concerning partially confined electronic states: scattering processes, potential barrier strengths, excited state lifetimes or the influence of guest molecules.

Rutherford Backscattering Spectrometry analysis of iron-containing Bi2Se3 topological insulator thin films

Fe-containing Bi2Se3 topological insulators (TI) thin films have been grown to investigate the intricate interplay between topological order and the incorporation of ferromagnetic atoms. Here we present the quantitative characterisation of the Bi2Se3 thin films with up to 16at% Fe incorporated during the growth process on GaAs (111) substrate by Molecular Beam Epitaxy. We report the elemental composition and depth profiles of the Bi2Se3:Fe films obtained using Rutherford Backscattering Spectrometry (RBS) and their formed crystalline phase obtained by X-ray diffraction (XRD). Resistance of the TI to beam-induced damage was investigated by channelling RBS. Using the elemental composition from RBS and the thickness from XRD measurements the Fe-free film density was deduced. For Fe-containing samples, the diffraction reveals the formation of two distinct crystalline phases, as well as their intergrowth pattern, in which the basal planes of Bi2Se3 coexist with an additional Fe–Se phase. This intergrown composite, with chemical compatibility of the Fe–Se phase with the crystalline Bi2Se3 structure, preserves the intrinsic topological surface states of the TI component despite the inhomogeneous distribution of the constituent phases. RBS analysis gives the stoichiometry of the Bi2Se3, and Bi2Se3:Fe samples (estimated between 0 and 16at% Fe) and gives insights into the composition of FeSex phases present.

Synchrotron radiation photoemission study of the ultrathin Cs/InN interface

Abstract Electronic structure of the ultrathin Cs/n-InN interface has been studied in situ via synchrotron-based photoemission spectroscopy by excitation in the energy range of 70–400 eV. Changes in the In 4d, Cs 4d, Cs 5p, N 2s core level spectra and in the surface state spectra have been revealed under different cesium coverages. The intrinsic surface state for the clean InN surface at binding energy of 2.5 eV (SS1) is found to attenuate during the Cs adsorption. Simultaneously the Cs induced surface state at binding energy of 0.9 eV (SS2) arises. For the Cs/InN interface, the In 4d peak displays the strong core level shift and the appearance of an additional In 4d peak originated from In–Cs interface bonding. Change in the surface electronic structure of the InN caused by Cs adsorption is found to originate predominantly from suppression of the intrinsic surface state concerned with the local interaction of In dangling bonds and Cs adatoms.

Polarity-dependent pinning of a surface state

We illustrate a polarity-dependent Fermi level pinning at semiconductor surfaces with chargeable surface states within the fundamental band gap. Scanning tunneling spectroscopy of the $\mathrm{GaN}\left(10\overline{1}0\right)$ surface shows that the intrinsic surface state within the band gap pins the Fermi energy only at positive voltages, but not at negative ones. This polarity dependence is attributed to arise from limited electron transfer from the conduction band to the surface state due to quantum mechanically prohibited direct transitions. Thus, a chargeable intrinsic surface state in the band gap may not pin the Fermi level or only at one polarity, depending on the band to surface state transition rates.

Lattice-Matched InGaAs-InAlAs Core-Shell Nanowires with Improved Luminescence and Photoresponse Properties.

Core–shell nanowires (NW) have become very prominent systems for band engineered NW heterostructures that effectively suppress detrimental surface states and improve performance of related devices. This concept is particularly attractive for material systems with high intrinsic surface state densities, such as the low-bandgap In-containing group-III arsenides, however selection of inappropriate, lattice-mismatched shell materials have frequently caused undesired strain accumulation, defect formation, and modifications of the electronic band structure. Here, we demonstrate the realization of closely lattice-matched radial InGaAs–InAlAs core–shell NWs tunable over large compositional ranges [x(Ga)∼y(Al) = 0.2–0.65] via completely catalyst-free selective-area molecular beam epitaxy. On the basis of high-resolution X-ray reciprocal space maps the strain in the NW core is found to be insignificant (ε < 0.1%), which is further reflected by the absence of strain-induced spectral shifts in luminescence spectra and nearly unmodified band structure. Remarkably, the lattice-matched InAlAs shell strongly enhances the optical efficiency by up to 2 orders of magnitude, where the efficiency enhancement scales directly with increasing band offset as both Ga- and Al-contents increase. Ultimately, we fabricated vertical InGaAs−InAlAs NW/Si photovoltaic cells and show that the enhanced internal quantum efficiency is directly translated to an energy conversion efficiency that is ∼3–4 times larger as compared to an unpassivated cell. These results highlight the promising performance of lattice-matched III–V core–shell NW heterostructures with significant impact on future development of related nanophotonic and electronic devices.

Highly fluorescent, near-infrared-emitting Cd²⁺-tuned HgS nanocrystals with optical applications.

Bulk HgS itself has proven to be a technologically important material; however, the poor stability and weak emission of HgS nanocrystals have greatly hindered their promising applications. Presently, a critical problem is the uncontrollable growth of HgS NCs and their intrinsic surface states which are susceptible to the local environment. Here, we address the issue by an ion-tuning approach to fabricating stable, highly fluorescent Cd:HgS/CdS NCs for the first time, which efficiently tuned the band-gap level of HgS NCs, pushing their intrinsic states far away from the surface, reducing the strong interaction of the environment with surface states and hence drastically boosting the exciton transition. As compared to bare HgS NCs, the obtained Cd:HgS/CdS NCs exhibited tunable luminescence peaks from 724 to 825 nm with an unprecedentedly high quantum yield up to 40% at room temperature and excellent thermal and photostability. Characterized by TEM, XRD, XPS, and AAS, the resultant Cd:HgS/CdS NCs possessed a zinc-blende structure and was composed of a homogeneous alloyed HgCdS structure coated with a thin-layer CdS shell. The formation mechanism of Cd:HgS/CdS NCs was proposed. These bright, stable HgS-based NCs presented promising applications as fluorescent inks for anticounterfeiting and as excellent light converters when coated onto a blue-light-emitting diode.

Transport of massless Dirac fermions in non-topological type edge states.

There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of “topological” states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions.

Ionization of high-density deep donor defect states explains the low photovoltage of iron pyrite single crystals.

Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.