Distribution Of Surface States Research Articles

Spectroscopic imaging of photoexcited states at a polycrystalline copper metal surface via two-photon photoelectron emission microscopy

The local photoexcited surface states of a polycrystalline-copper (Cu) substrate were observed via two-photon photoelectron emission microscopy (2P-PEEM) using a femtosecond laser. The 2P-PEEM results revealed local distributions of the photoexcited electrons, strongly depending on the crystal domains. With the energy-filtered images, the local 2P-photoemission spectra were selectively obtained for (111)- and (110)/(100)-crystal domains. Furthermore, the 2P-PEEM could visualize propagating Cu surface plasmon polaritons with wavevector of 7.61 × 106 m−1. These results indicate that the 2P-PEEM technique is a promising tool for studying the distribution of nonequivalent photoexcited surface states for various inhomogeneous functional nanomaterials.

Expediting hole transfer via surface states in hematite-based composite photoanodes.

Regarding the indirect hole transfer route in hematite-based photoelectrodes, the widely accepted viewpoint is that the FeIVO states act as a hole transfer medium, while other types of surface states act as recombination centers. Alternatively, it has rarely been reported that the recombining surface states may contribute to the charge transport in modified photoelectrodes. In this study, we employed CoCr layered double hydroxide (LDH)/Fe2O3 and CoCr LDH/Zr:Fe2O3 as research models to investigate the distinct charge transfer pathways in composite photoanodes. Different from the adverse role of surface states at ∼0.7 V versus the reversible hydrogen electrode (r-SS) in the bare hematite photoelectrodes (Fe2O3 or Zr:Fe2O3), the r-SS in the composite photoanodes (CoCr LDH/Fe2O3 or CoCr LDH/Zr:Fe2O3) served as a hole transfer station to induce high-valent Co cations, and the position of r-SS determined the onset potential of the composite photoelectrodes. Moreover, the FeIVO states still acted as active intermediates to transport numerous holes to the cocatalyst, which enhanced the charge utilization efficiency at 1.23 V versus the reversible hydrogen electrode (RHE) to a large extent. Besides, a noteworthy fact is that Zr doping increased the number of active FeIVO states, which significantly contributed to the enhancement in current density. However, it led to a delayed onset potential because of the positively shifted surface states (r-SS and FeIVO). Evidently, the different surface state distributions between Fe2O3 and Zr:Fe2O3 gave rise to anisotropic charge transfer and recombination behavior in the composite photoanodes. This study gives extensive insight into the hole transfer route in composite photoanodes and reveals the surface state-tuning effects of dopants and cocatalysts, which are significant for a deep understanding of the surface states and optimal design of composite photoanodes via surface state modulation.

Surface states modulation of hematite photoanodes for enhancing photoelectrochemical catalysis

Sluggish interfacial carrier transfer and oxygen production kinetics of photoelectrochemical water splitting are related to the Fermi-level pinning of the hematite surface states, thus it requires extra energy input and results in low PEC efficiency. Here we develop a facile secondary calcination strategy for regulating the density and distribution of surface states in hematite photoanode system. The fabricated α-Fe2O3-450 possesses significantly reduced disordered layers and renovated surface oxygen vacancy structure to increase carrier density and effectively passivate surface trapping states. Thus, both bulk and interfacial recombination of electrons and holes are effectively suppressed, and the quasi-hole Fermi level moves to more positive position, leading to a large photovoltage for reducing kinetic overpotential. Therefore, the photocurrent density of α-Fe2O3-450 (1.035 mA cm−2) is 1.8 times higher than that of pristine α-Fe2O3 (0.58 mA cm−2) at 1.23 V versus reversible hydrogen electrode, and α-Fe2O3-450 has more cathodic onset potential than α-Fe2O3.

Surface states regulation of sulfide-based photoanode for photoelectrochemical water splitting

Regulating the surface states distribution on the photoanode/electrolyte interface is crucial to photoelectrochemical (PEC) water splitting. Here we develop a CdIn2S4/InOx/NiFe-LDH adaptive junction to improve the PEC performance and stability of bare CdIn2S4, which achieves a photocurrent density of 5.47 mA cm−2 at 1.23 VRHE as well as good durability under AM 1.5G illumination without sacrificial reagent. The experimental characterizations evidence that both the charge carrier density (Nd) and surface states distribution (Nss) alter along with the interfacial structural regulation in the CdIn2S4/InOx/NiFe-LDH photoanode, and a proper ratio of Nss/Nd guarantee the optimal photocurrent. Meanwhile, a strained surface states distribution and negative shifted Nss center facilitate the charge transfer and interfacial water oxidation kinetics as well. This work highlights the influence of interfacial structural regulation on the entangled surface states distribution and charge carrier density and may inspire more excellent work on designing other efficient photoanodes.

Frequency Response of C–V and G/ω-V Characteristics of Au/(Nanographite-doped PVP)/n-Si Structures

This paper reports that frequency response on profile of C–V–ƒ and G/ω–V–ƒ characteristics of spin-coated nanographite (NG)-doped polyvinylpyrrolidone (PVP)/n-Si structures in a wide frequency (1 kHz–5 MHz) and voltage (± 3 V) ranges at room temperature. Hereby, the basic parameters of the structure such as diffusion potential (VD), doping donor density (ND), Fermi energy level (EF), maximum electric field (Em), depletion layer thickness (Wd), and barrier height (ΦB) are derived by using the intercept and slope of C−2–V–ƒ plot for each frequency. Additionally, the energy density distribution of surface states (Nss) and their relaxation time values (τ) are also attained from the conduction method and their values are found as 4.999 × 1012 eV−1 cm−2 and 2.92 µs at 0.452 eV, and 3.857 × 1012 eV−1 cm−2 and 164 µs at 0.625 eV, respectively. The lower Nss values are the consequence of passivation effect of the used nanographite (NG)-PVP polymer interlayer. As a result, the polymer interlayer based nanographite (NG)-PVP is candidate instead of the widely used oxide/insulator layer for the purpose of decreasing the surface states or dislocations.

Analysis of trap states in AlGaN/GaN self-switching diodes via impedance measurements

The presence of trap states in self-switching diodes (SSD) based on an AlGaN/GaN heterojunction has been identified by means of their AC characterization between 75 kHz to 30 MHz in a wide temperature range, from 80 K to 300 K. Measurements allow us to determine two different characteristic energies of the traps, 12 meV and 61 meV, being associated to a distribution of surface states and one discrete bulk trap, respectively. The impact of the trapping effects on microwave detection at zero-bias has also been analyzed in the same temperature range, the measured responsivity showing an unusual enhancement and a low-frequency roll-off at low temperatures.

Electrical characteristics of organic heterojunction with an alternating benzotriazole and fluorene containing copolymer

The current–voltage ( $$I-V$$ ) and capacitance–voltage ( $$C-V$$ ) characteristics of the organic heterojunction diode were investigated in a wide temperature range from 80 to 320 K and frequency range from 10 kHz to 1 MHz, respectively. Alternative to the copolymer partner poly(3-hexylthiophene) (P3HT) to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(thien-2-yl)2-dodecyl-benzo[1,2,3]triazole)) (named as copolymer in this work) was adapted to the bulk-heterojunction layer in the organic diode. Together with the use of Lif/Al bilayer electrode, the diode was fabricated as in the form of Al/LiF/copolymer:PCBM/PEDOT:PSS/ITO/glass. Under the applied bias voltage, this organic-based diode shows two-orders of magnitude rectifying behavior. According to thermionic emission (TE) model, the diode parameters such as saturation current, barrier height and ideality factor were determined and parasitic resistances were also extracted from the conventional ohmic relation. As to the temperature dependency of the diode parameters and their response to the temperature variation, barrier inhomogeneity, surface state and series resistance effects were found in dominant behavior on the current flow. The conduction mechanism was modeled by assuming low-barrier patches around the main barrier that supports TE at low temperatures and their distribution was expressed by a Gaussian function. In addition, series resistance values were detailed depending on temperature using Cheung’s model. $$C-V$$ analysis was performed to evaluate the distribution of surface states at the interface as a function of frequency. Based on the $$C-V$$ plots, the effects of charges at these traps were observed especially at low frequencies. Additionally, from these results, Fermi level, surface potential and donor concentration values were evaluated in a wide frequency range.

Investigation of effects on dielectric properties of different doping concentrations of Au/Gr-PVA/p-Si structures at 0.1 and 1 MHz at room temperature

In order to improve and detailedly investigate the dielectric properties of polymer interfaces of Metal–Polymer–Semiconductor (MPS) structures, three types of MPS were fabricated by doping 1, 3 and 5% graphene (Gr) into the polyvinyl alcohol (PVA) interface material. Capacitance–Voltage (C-V) and Conductance–Voltage (G/ω-V) measurements were used to analyze the dielectric properties of three types of MPS. Using C-V and G/ω-V data, series resistance (Rs) affecting device performance and interface properties besides basic dielectric parameters of each structure such as both the real and imaginary components of complex dielectric constant (e' and e''), complex electrical modulus (M' and M''), loss tangent (tanδ), and ac electrical conductivity (σac) were also calculated. The effect of graphene doping was examined for each parameter and obtained results were compared at both low (0.1 MHz) and high (1 MHz) frequencies. It was observed that e and e'' decreased with increasing graphene doping at both 0.1 and 1 MHz, while M' and M'' increased under same conditions. Moreover, both the M' and M'' vs V plots have two distinctive peaks between −2.0 V and 0.0 V due to a special density distribution of surface states between (Gr-PVA) and p-Si. The tanδ gradually increased with increasing graphene doping at only 0.1 MHz. As the doping ratio of graphene increases, the charge carriers in the structure generate more dipoles and create an earlier relaxation process. In other words, increasing the doping ratio helps to improve the series resistance effects in MPS structures. As a result, it was seen that the interfacial properties of MPS structures were improved by increasing the rate of graphene doping.

Nanoscale Studies at the Early Stage of Water-Induced Degradation of CH3NH3PbI3 Perovskite Films Used for Photovoltaic Applications

Understanding the surface properties of hybrid perovskite materials is a key aspect to improve not only the interface properties in photovoltaic cells but also the stability against moisture degradation. In this work, we study the local electronic properties of two series of CH 3 NH 3 PbI 3 perovskite films by atomic force microscopybased methods. We correlate nanoscale features such as the local surface potential (as measured by Kelvin probe force microscopy) to the current response (as measured by conductive atomic force microscopy). CH 3 NH 3 PbI 3 perovskites made using lead acetate as a precursor result in films with high purity and crystallinity and also result in heterogeneous local electrical properties, attributed to variations in the density of surface states. In contrast, when using lead iodide as a precursor, the perovskite surface exhibits a uniform distribution of surface states. This work also aims to understand the early stages of water-induced degradation at the surface of those films. Through high-precision exposure to small amounts of water vapor, we observe a higher stability for surfaces prepared with lead iodide precursors. More importantly, each precursor-based fabrication route is associated with either nor p-type behavior of the films. These characteristics are determined by the type of surface states, which also and eventually preside over stability. This work should help discriminate between perovskite synthesis routes and improve their stability in photovoltaic cell applications.

Highly active deficient ternary sulfide photoanode for photoelectrochemical water splitting

The exploration of photoanode materials with high efficiency and stability is the eternal pursuit for the realization of practically solar-driven photoelectrochemical (PEC) water splitting. Here we develop a deficient ternary metal sulfide (CdIn2S4) photoanode, and its PEC performance is significantly enhanced by introducing surface sulfur vacancies, achieving a photocurrent density of 5.73 mA cm−2 at 1.23 V vs. RHE and 1 Sun with an applied bias photon-to-current efficiency of 2.49% at 0.477 V vs. RHE. The experimental characterizations and theoretical calculations highlight the enhanced effect of surface sulfur vacancies on the interfacial charge separation and transfer kinetics, which also demonstrate the restrained surface states distribution and the transformation of active sites after introducing surface sulfur vacancies. This work may inspire more excellent work on developing sulfide-based photoanodes.

Prediction of Potential Barrier at Crystallite Boundaries in Poly- and Nanocrystalline Semiconductors

The distribution of the potential and the parameters of the potential barrier for electrons in semiconductor crystallite is numerically calculated. The calculations are made in a spherical crystallite with uniformly distributed surface states and uniformly distributed donors. It is considered in the calculations that the surface charge is screened both by the ionized donors and on free electrons; the contibution of free electrons should not be neglected in semiconductors with a high concentration of free electrons. It is demonstrated that the height of the potential barrier depends non-monotonically on the concentration of the donors in the crystallite. Moreover, in the curve of the height of the potential barrier as a function of the concentration of the donors, it is possible to highlight two segments corresponding to the cases of the complete and partial depletion of the crystallite. The height of the potential barrier increases with the concentration of the donors in the first segment and decreases in the second segment. It is established that the height of the potential barrier increases with the increase in the concentration of the surface states. The possibility of the existence of surface potential barriers in nano- and polycrystalline metal-oxide semiconductors, which are applied as a sensitive layer of gas sensors, is estimated. It is concluded that if the crystallite radius in metal-oxide semiconductors does not exceed 10 nm the sensor’s sensitivity to gas could hardly be attributed to the usual barrier model. It is demonstrated that shape of crystallite and the contribution of free electrons to screening of surface charge have to be taken into account to calculation of width of potential barrier.

Cathode Surface State and Cathode Temperature Distribution After Current Zero of Different AMF-Contacts

The objective of this paper is to compare the state of cathode surface of different axial magnetic field (AMF) contact systems (CSs) during arcing and after arc extinction. The arcs were ignited by opening of electrodes and were fed by sine half-wave current pulses with a duration of 10 ms. Investigations were performed in the range of arc currents up to the levels of guaranteed failure. The measurements were done by a high-speed Phantom M310 video camera. The cathode surface state during arcing was studied using a set of neutral density filters. It was discovered that melting of a part of the cathode surface during arc burning takes place in high-current vacuum arcs. A changing relief in the form of hillocks and hollows forms on the melted surface. We found out that there are two components in droplet erosion of the cathode, when its surface is partially melted: in addition to droplets formed directly in the cathode spot (which can be called droplets of the first type), the liquid part of the surface also emits droplets (which can be called droplets of the second type) significantly different in their sizes and velocities. The formation mechanisms of droplets of the first and the second type are different. The cathode surface temperature distribution immediately after current zero was obtained by comparing the results of cathode surface filming to the results of calibrated band-lamp filming in identical conditions. Reflection of anode radiation from cathode surface was analyzed by mathematical simulation. The filming was carried out using an interference filter. We found that the distribution of the cathode surface temperature is substantially nonuniform. The axial region is the most heated. We conclude that in all investigated CSs with the same amplitude value of the current, the temperature of the most heated parts of the cathode surface is significantly lower than the temperature of the most heated parts of the anode surface. Analysis of obtained results shows that switching failure can be provoked by the cathode and by the anode processes depending on CS design.

Anode Surface State and Anode Temperature Distribution After Current Zero for Different AMF-Contact Systems

The aim of this paper is to compare the state of anode surface of different AMF contact systems during arcing and after arc extinction. Arcs were ignited by opening of electrodes and were fed by sine half-wave current pulses with durations of 10 ms. CuCr electrodes were used. We performed investigations in the range of arc currents from successfully interrupted ones up to the levels of guaranteed failure (limiting interrupted current) if the transient recovery voltage was applied. The measurements were implemented by a high-speed Phantom M310 video camera equipped with a Carl Zeiss 100/2 macro lens. We studied the anode surface state during the arcing using the set of neutral density filters. We found that at currents close to the limiting interrupted current, most of the anode surface melts. Liquid metal is squeezed out of the axial area and an annular crest is formed. The crest splashes out and breaks into separate jets. The jets are broken up into fragments, from which droplets separate. The anode surface temperature distribution just after current zero was obtained by comparison of the results of anode surface filming and the results of calibrated band-lamp filming in identical conditions. We carried out the filming using an interference filter (775 nm). The effect of reflection from electrodes was evaluated using mathematical simulation. The temperature distribution of the anode surface is highly non-uniform. The temperature of the most heated fragments of the anode surface increases with increasing current and reaches 2000 K with the limiting interrupted current. However, the size of these fragments is small, substantially less than the length of the interelectrode gap. They cannot create the vapor concentration in the entire gap up to the opposite electrode sufficient to provide the Townsend breakdown of the gap when the transient recovery voltage is applied. The analysis of the obtained results suggests that droplets, and hence, the phenomena on the surface that lead to their appearance play an important role in the breakdown process in the investigated contact systems at extreme currents.

Dielectric, modulus and conductivity studies of Au/PVP/n-Si (MPS) structure in the wide range of frequency and voltage at room temperature

Both the real-imaginary components of complex dielectric permittivity and electric modulus, and ac conductivity of Au/PVP/n-Si (MPS) structures were analyzed by admittance spectroscopy technique between 1 and 500 kHz, − 3 and 5 V. The polyvinylpyrrolidone (PVP) polymer layer was deposited on n-Si wafer by spin-coating technique. The e′, e″, M′, M″ and σac values were calculated from the admittance measurements and they are quite function of frequency and voltage due to a special distribution of surface states at PVP/n-Si interface, interfacial/dipole polarizations at low frequencies. While the e′ and e″ values decrease as frequency increases, the σac, M′ and M″ increase. The ln(σac) vs ln(ω) plot for 3V has two linear region between 1 and 20 kHz, 30 and 500 kHz frequencies, respectively. The obtained 0.033 slope value for low-frequencies which corresponding to dc conductivity and it is almost independent of frequency, but it obtained 0.46 for high-frequencies which corresponding ac conductivity and is strong function of frequency due to the increase eddy current. As a result, the prepared MPS structure can be used as charges/energy storage device due to the dielectric property of the PVP polymer layer.

Experimental validation of the surface state distribution model in the Suzuki theory to qualify the thin film surface materials

The relevance of the theoretical Suzuki’s model of surface state density to determine the surface state distribution at the active layer/silicon dioxide interfaces is carried out in polycrystalline silicon thin film transistors (TFTs) issued from two different crystallization technologies of the active layer: furnace solid phase crystallized (FSPC) and laser solid phase crystallized (LSPC) TFTs. The experimental validation of this model is demonstrated using the field effect conductance method. Results show that distribution is higher for the FSPC TFTs in relationship with the process crystallization of the active layer. In addition, it is shown that the theoretical surface state model allows discriminating dangling bond state and tail state distributions and acts as relevant model to qualify the surface of thin film material.

Dielectric properties of Al/PVA/p-Si (MPS) Structure as a function of frequency and voltage

Abstract Metal-Polymer-Semiconductor (MPS) structure Al/PVA/p-Si was fabricated using polyvinyl alcohol (PVA) as an organic interfacial layer on a p-type silicon wafer by spin coating method. Frequency dependent dielectric properties like real and imaginary part of dielectric permittivity (dielectric constant e′ and dielectric loss e″), dissipation factor or loss tangent (tan δ), real and imaginary parts of the electric modulus (M′, M′′) of the fabricated MPS structure were investigated for various frequencies from 50 KHz to 1 MHz. Frequency dispersion has been found for all dielectric parameters. Simultaneously these were found to be a function of voltage. It was found that e′ and e″ decrease with the increase in frequency. M′ increases with frequency while M′′ shows a peak behaviour that shifts towards lower frequency with increasing applied voltage. All these changes in dielectric parameters may be attributed to surface states distribution at PVA/p-Si interface and interfacial polarization.

Role of cobalt–iron (oxy)hydroxide (CoFeOx) as oxygen evolution catalyst on hematite photoanodes

Photocurrent density of a composite hematite photoanode is influenced by electrocatalyst charging and surface states distribution.

Variational study of the 2DEG wave function in InAlN/GaN heterostructures

The variational method has been widely used to study the electronic structures of heterostructure materials in spite of this method being less accurate than the numerical method, because analytical formulas for some electrical parameters can be derived using this method. However, effects of surface states on the two-dimensional electron gas (2DEG) have not been taken into account in the variational studies of GaN-based heterostructures. In the present study, analytical formulas for the electron wave function and ground state energy level of the 2DEG in InAlN/GaN heterostructures are derived using the variational method, and the influences of structural parameters of InAlN/GaN heterostructures on the electrical properties are discussed. In the theoretical model, evenly distributed surface states below the conduction band are assumed to be the origin of the 2DEG, and the polarization charges at the InAlN surface and the InAlN/GaN interface due to spontaneous and piezoelectric polarization effects in InAlN/GaN heterostructures are taken into account. A trial envelope wave function with two variational parameters is used to derive the expectation value of the total energy per electron. The variational parameters are determined by minimizing the expectation value. The model predicts a linear conduction band profile in InAlN barrier layer and an approximately triangular-shaped potential well on the GaN side of the InAlN/GaN interface. Electrons released from the surface states are confined in the potential well, forming the 2DEG. The 2DEG sheet density for the lattice-matched InAlN/GaN heterostructure with a 15 nm InAlN layer is 1.961013 cm-2, and the average distance from the InAlN/GaN interface of electrons is 2.23 nm. The 2DEG sheet density increases rapidly with InAlN thickness increasing when the InAlN layer exceeds the critical thickness, and starts to be saturated above 15 nm. The dependence of the calculated 2DEG sheet density on the InAlN thickness quantitatively agrees with recently reported experimental data. The increasing 2DEG sheet density results in increasing the ground state energy level and Fermi energy, and the energy spacing between the two also increases for containing more electrons. The polarization discontinuity at the InAlN/GaN interface decreases with increasing In mole fraction, causing the 2DEG sheet density to decrease, and thus the ground state energy level and the Fermi energy to decrease. This model is conducive to understanding the electrical behaviors of InAlN/GaN heterostructures and providing readily applicable formulas for studying the electron transport and optical transitions.

Новый механизм реализации омических контактов

AbstractAnalysis of the contact-barrier height taking into account the distribution of surface states along coordinate x perpendicular to the insulator–semiconductor interface is performed for metal–semiconductor contacts with a dielectric gap. It is shown that taking into account the spatial dependence of the density of surface states at rather high semiconductor doping levels leads to a substantial decrease in the barrier height, which promotes the realization of ohmic contacts. It is established that the smaller the metal–semiconductor contact potential difference ϕ_ ms is, the stronger the effect of barrier-height lowering. If ϕ_ ms is negative, this effect can lead to potential sign reversal, i.e., to the realization of an enrichment layer in the space-charge region of the semiconductor even at a high density of surface states. This in turn promotes the manifestation of an anomalous dependence of the contact resistivity on temperature; the resistivity increases with an increase in temperature.

On a New Mechanism for the Realization of Ohmic Contacts

Analysis of the contact-barrier height taking into account the distribution of surface states along coordinate x perpendicular to the insulator–semiconductor interface is performed for metal–semiconductor contacts with a dielectric gap. It is shown that taking into account the spatial dependence of the density of surface states at rather high semiconductor doping levels leads to a substantial decrease in the barrier height, which promotes the realization of ohmic contacts. It is established that the smaller the metal–semiconductor contact potential difference ϕ ms is, the stronger the effect of barrier-height lowering. If ϕ ms is negative, this effect can lead to potential sign reversal, i.e., to the realization of an enrichment layer in the space-charge region of the semiconductor even at a high density of surface states. This in turn promotes the manifestation of an anomalous dependence of the contact resistivity on temperature; the resistivity increases with an increase in temperature.