Surface Photovoltage Spectroscopy Spectra Research Articles

Transformation of amorphous Bi2O3 to crystal Bi2O2CO3 on Bi nanospheres surface for photocatalytic NOx oxidation: Intensified hot-electron transfer and reactive oxygen species generation

• Amorphous Bi 2 O 3 shell on the surface of Bi nanospheres was transformed to crystal Bi 2 O 2 CO 3 . • Bi 2 O 2 CO 3 shell reduced the transmission resistance of hot electrons excited from Bi core. • The generation of reactive oxygen species were enhanced over Bi@crystal Bi 2 O 2 CO 3. • •O 2 − radicals were the major reactive oxidation species contributing to the oxidation of NO x . • Generation mechanisms of ROS were proposed over Bi@crystal Bi 2 O 2 CO 3 photocatalyst. The inevitable amorphous Bi 2 O 3 on the surface of plasmonic Bi nanospheres serving as the recombination center of hot electron-hole pairs, hindered the transfer of photo-generated hot electrons and production of reactive oxygen species (ROS) seriously. In this study, Bi@amorphous Bi 2 O 3 was transformed to Bi@crystal Bi 2 O 2 CO 3 by secondary hydrothermal reaction. Bi@Bi 2 O 2 CO 3 core–shell photocatalyst exhibited higher visible-light catalytic activity (34.1%) than Bi@Bi 2 O 3 did (12.3%) in terms of NO x removal. Photoelectrochemical, surface photovoltage spectroscopy spectra and Kelvin probe force microscopy measurement results indicate that Bi 2 O 2 CO 3 shell reduced the transmission resistance of hot electrons excited from Bi core. O 2 -TPD, trapping experiments and density functional theory further confirmed that the transferred hot electrons were a help for ROS generation and •O 2 − radicals were the major contributor. This study not only addresses the unsettled issue of surface amorphous Bi 2 O 3 , but also provides a facile method to transform the amorphous shell to crystal phase of core–shell photocatalyst and new insights into the hot electrons separation and the formation of main ROS over the surface crystal transition of plasmonic Bi.

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.

Construction of a 2D Co(II) Coordination Polymer with (4,4)-Connected Topology: Synthesis, Crystal Structure, and Surface Photo-electric Property

A 2D grid-like (4, 4)-connected topology coordination polymer, <TEX>$[Co(BTA)_2(H_2O)_2</TEX><TEX>]</TEX><TEX>_n$</TEX> (1), where HBTA = 2-(1H-benzotriazol-1-yl)acetic acid, has been synthesized by hydrothermal method and characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis and surface photovoltage spectroscopy (SPS). X-ray diffraction analyses indicated that 1 displays octahedral metal centers with secondary building units (SBUs) [<TEX>$Co(BTA)_2(H_2O)_2$</TEX>] bridged by the <TEX>$BTA^-$</TEX> ligands. In the crystal, the 2D supramolecular architecture is further supported by <TEX>$O-H{\cdots}O$</TEX>, <TEX>$O-H{\cdots}N$</TEX>, <TEX>$C-H{\cdots}O$</TEX> hydrogen bonds and <TEX>${\pi}{\cdots}{\pi}$</TEX> stacking interactions. The SPS of polymer 1 indicates that there are positive response bands in the range of 300-600 nm showing photo-electric conversion properties. There are good relationships between SPS and UV-Vis spectra.

Cd(II)-btec配聚物的合成、结构及光电性能

Two Cd(II) coordination polymers [Cd_3Na_2(btec)_2 (H_2O)_2 ]_n (1) and {[Cd_3Na_2 (btec)_2∙(H_2O)_2]∙4H_2O}_n(2)(H_4btec = 1,2,4,5-benzenetetracarboxylic acid) were synthesized by hydrothermal method. The structures were determined by single crystal X-ray diffraction. Structural analyses show that the asymmetric units of polymers 1 and 2 contain two crystallographically unique Cd(II) ions with six-and eight- coordinate, respectively, and they possess 3D structures bridged by btec^(4-) groups. The structure differences between them are that the coordination environment around Na(I) ions is different and polymer 2 has lattice water molecules. The photo-electric properties of the two polymers were investigated by surface photovoltage spectroscopy(SPS). The results of SPS show that the two polymers exhibit obvious photovoltage responses in the range of 300-500 nm, which indicates that they all possess certain photo-electric conversion properties. The SPS and UV-Vis spectra of the polymers were interrelated, and XRD patterns and TG curves of the two polymers were measured and analyzed.

Synthesis and Surface Photoelectric Properties of a Trinuclear Chromium(III) Complex

A trinuclear Chromium(III) complex [Cr3(μ3-O)(μ2-PCBA)6(CH3OH)3]NO3∙H2O∙CH3OH (HPCBA = 4-Chlorobenzoic acid) has been synthesized and characterized by single-crystal X-ray diffraction, IR spectra, UV-Vis absorption spectra and elemental analysis, which belongs to monoclinic, space group P21/c with a = 12.443 (3), b = 24.543 (5), c =22.094 (7)Å, α = 90°, β =115.76 (2) °, γ =90°, V = 6077 (3)Å3, Z = 4.The surface electron behavior of the chromium complex has been studied emphatically by surface photovoltage spectroscopy (SPS). Structural analyse indicate that the complex is molecular complex. The result of SPS show that there is positive photovoltage response bands within 300–800 nm, which indicate that it possess obvious photoelectric conversion properties. There was a good correlation between the SPS and UV-Vis absorption spectra.

Control of Li configuration and electrical properties of Li-doped ZnO

Li-doped ZnO after different thermal treatments was characterized by depth-resolved cathodoluminescence spectroscopy (DRCLS), secondary ion mass spectrometry, surface photovoltage spectroscopy (SPS), coupled with other surface science techniques. It is found that the Li configuration and electrical properties of Li-doped ZnO could be controlled by different thermal processes. Within a 500–600 °C annealing temperature range, subsequent quenching of ZnO leaves Li as interstitial donors, resulting in n-type low room temperature resistivity. In contrast, slower cooling in air enables these interstitials to fill Zn vacancies, forming Li acceptors 3.0 eV below the conduction band edge. Emergence of this acceptor and the resultant resistivity increase agree with the calculated diffusion lengths based on published diffusion coefficients. In general, these acceptors are compensated by residual intrinsic and extrinsic donors, resulting in a semi-insulating material. DRCL spectra exhibit a 3.0 eV optical signature of the LiZn acceptor and its depth distribution in slow-cooled ZnO. A 3.0 eV SPS absorption feature corresponding to a conduction band-to-acceptor level transition confirms this acceptor assignment. Nanoscale SPS spectra reveal p-type band bending localized near ZnO surface nano-mounds, where VZn and LiZn acceptor densities increase. The slow-cooled and quenched Li-doped ZnO spectra display an inverse relationship between the optical emission densities of lithium on zinc versus zinc vacancy sites, demonstrating the time dependence of Li interstitial diffusion to reach zinc vacancies and form substitutional Li acceptors.

Optical characterization of a GaAsSb/GaAs/GaAsP strain-compensated quantum well structure grown by metal-organic vapor phase epitaxy

Optical properties of a strained GaAs0.64Sb0.36/GaAs and a strain-compensated GaAs0.64Sb0.36/GaAs/GaAs0.79P0.21 triple quantum well (TQW) structures were investigated by photoluminescence (PL), photoreflectance (PR) and surface photovoltage spectroscopy (SPS). The samples were grown on GaAs(100) substrates by a horizontal metal-organic vapor phase epitaxy (MOVPE) system. For GaAs0.64Sb0.36/GaAs TQW, a large blueshift of the peak position of PL feature at low temperature with increasing excitation power density and only a very weak PR feature observed in the vicinity of fundamental transitions and were attributed to a weakly type-II heterojunction formed between GaAs0.64Sb0.36 and GaAs. On the other hand, the PR and SPS spectra of GaAs0.64Sb0.36/GaAs/GaAs0.79P0.21 TQW showed a series of intersubband originated transition features, which is a typical characteristic of type-I QW structure. The results revealed that the energy band of QW structures is significantly influenced by the inserted tensile GaAsP layers, which changes the weakly type-II to a type-I structure. The strain-compensated GaAs0.64Sb0.36/GaAs/GaAs0.79P0.21 TQW has a larger overlap integral and hence a higher transition probability, providing a possibility for fabricating high efficiency near infrared laser diodes.

Synthesis and Crystal Structure of a New Co(II) Complex With Surface Photoelectric Property

A new cobalt(II) complex [Co(bidc)(H2O)2]n (H2bidc = Benzimidazole-5,6- dicarboxylic acid) has been synthesized by hydrothermal method and characterized by single-crystal X-ray diffraction, IR, UV-Vis, and surface photovoltage spectroscopy (SPS). Structural analysis indicated that the complex is a 1D chain structure and further connected to a 3D supramolecular architectures by hydrogen bonds. The SPS of the complex was analyzed emphatically. The result of SPS indicates that there are obvious response bands in the range of 300∼600 nm showing a certain level of photoelectric properties. There are good corresponded relationships between the SPS and UV-Vis spectra.

Optical Transitions between Confined and Unconfined States in p-Type Asymmetric GaAs/InGaAs/AlGaAs QW Structures

We present contactless surface photovoltage spectroscopy and photoreflectance studies of 10 nm wide, p-type doped asymmetric GaAs/InGaAs/AlGaAs quantum well structures. The MBE grown structures differ in spacer thickness between the quantum well and the reservoir of holes. The doping causes that quantum well is placed in electric field. The surface photovoltage spectroscopy measurements gave us detailed information about the optical transitions between confined states and between confined and unconfined states. The comparison of experimental and numerical analysis allows us to identify all features present in the surface photovoltage spectroscopy and photoreflectance spectra. It has been found that spacer layer thickness has significant influence on surface photovoltage spectroscopy spectra.

A Series of CrIII Coordination Supramolecules: Synthesis, Structure and Study of Surface Electron Behaviors

Three chromium(III) coordination supramolecules, [CrQ2(H2O)Cl]·H2O 1, [Cr(Pic)2(phen)]·NO3·2.5H2O 2, [Cr(ATA)(phen)]·CH3OH·3H2O 3 (HQ = Quinoline 2-acid, HPic = Picolinic 2-acid, H3ATA = Ammonia triacetic acid, phen = 1,10-Phenanthroline), have been synthesized solvothermally and characterized by single-crystal X-ray diffraction, IR spectra, UV-Vis absorption spectra and elemental analysis. The surface electron behavior of chromium complexes have been studied emphatically by surface photovoltage spectroscopy (SPS). Structural analyses indicate that the three complexes are all molecular complexes with the ligands of carboxylate groups; and, they are further connected into supramolecules by hydrogen bonds and π–π interactions. The results of SPS show that there are positive photovoltage response bands within 300–800 nm, which indicate that they all possess obvious photoelectric conversion properties. Energy-band theory for semiconductors and crystal field theory have been combined to analyze and assign the SPS bands. There are three types of band-to-band transitions in the SPS of the complexes; i.e., LMCT bands for Cl → Cr, O → Cr, N → Cr. The similarities and differences between the SPS and UV-Vis absorption spectra for the three complexes were compared and reasonably explained. There was a good correlation between the SPS and UV-Vis absorption spectra.

Optical characterization ofZn0.95−xBexMn0.05Semixed crystals

A systematic study of a series of ${\text{Zn}}_{0.95\ensuremath{-}x}{\text{Be}}_{x}{\text{Mn}}_{0.05}\text{Se}$ mixed crystals with different Be content $(0.05\ensuremath{\le}X\ensuremath{\le}0.20)$ grown by the modified high-pressure Bridgman method were carried out by room-temperature surface photovoltage spectroscopy (SPS), temperature-dependent photoluminescence (PL), and contactless electroreflectance (CER). A typical PL spectrum at low temperature consists of free exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and the ${\text{Mn}}^{2+}$-related intraionic emission. The near band-edge transition energies determined by analyzing the CER and SPS spectra showed a blueshift with the increase in Be content. The peak positions of band-edge exciton features in the PL spectra shifted slightly toward lower energies as compared to the corresponding transition energies obtained from CER and SPS data. The observed increases in the CER-PL shift with the increasing of Be content are explained by the increasing compositional disorder causing the smearing of the band-edge energies. The excitonic line broadening for the samples with larger Be/Zn ratio are attributed in part to the alloy-scattering effects and also to the poorer crystalline quality of the samples with higher content of Be. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band-edge excitonic transitions were evaluated and discussed.

Surface photovoltage spectroscopy analyses of Cd1−xZnxTe

Cd 1 − x Zn x Te alloys have been studied by surface photovoltage spectroscopy (SPS) and energy dispersive spectroscopy (EDS). The analyses of surface photovoltage spectra have been perfomed at near and above band gap energies. Surface recombination effects on the surface photovoltage have been investigated. SPS analyses of Cd1−xZnxTe alloys with different surface conditions have shown that the surface recombination velocity significantly affects the SPS determination of the material band gap. Accounting for this and preparing the surfaces accordingly, SPS spectra of Cd1−xZnxTe samples have allowed an accurate determination of the optical band gap as a function of the Zn concentration, determined via EDS analyses. The local increases in the density of states associated with band structure features in Cd1−xZnxTe alloys have been investigated by SPS spectra in the above band gap energy range.

Effect of surface species on Cu-TiO 2 photocatalytic activity

The Cu-TiO 2 nanoparticles with different Cu dopant content were prepared by sol–gel method. The structure of the as-prepared catalysts and the surface species of Cu-TiO 2 were determined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and diffuse reflection spectroscopy (DRS). The relationship between the photocatalytic activity and the surface species of Cu-TiO 2 was revealed via the measurement of surface photovoltage spectroscopy (SPS) as well as the degradation of the rhodamine B (RhB). The experimental results suggest that the Cu-TiO 2 photocatalysts with appropriate content of Cu (about 0.06 mol%) possess abundant electronic trap, which effectively inhibits the recombination of photoinduced charge carriers, improving the photocatalytic activity of TiO 2. While at high Cu dopant region (>0.06 mol%), the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes. Meanwhile, at heavy Cu doping concentration, excessive P-type Cu 2O can cover the surface of TiO 2, which leads to decrease in the photocatalytic activity of photocatalyst. The photocatalytic experimental results are in good agreement with the conclusions of SPS measurements, indicating that there is a close relationship between the photocatalytic activity and the intensity of SPS spectra.

Surface photovoltage spectroscopy and photoluminescence study of vertically coupled self-assembled InAs∕GaAs quantum dot structures

Multilayer self-assembled InAs∕GaAs quantum dot (QD) structures with varying GaAs spacer layer (SL) thickness are systematically investigated using surface photovoltage spectroscopy (SPS) and photoluminescence (PL). The optical transitions are more clearly visible in the room-temperature SPS spectra and exhibit more features in comparison to PL. The enhanced electronic coupling effect for thinner SL results in the appearance of an additional excited state lying higher than the second excited quantum dot state. A peculiar feature denoted as QDX below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is lower than that of vertically coupled ones. The transition blueshifted with a decrease of the SL thickness indicates that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain. The intensity ratio between the ground state and QDX in the temperature dependent PL spectra is found to be associated with the processes of enhanced thermal carrier transfer between QDs and tunneling of carriers between QDs in adjacent layers for a stack due to a reduction in SL thickness. The results demonstrate considerable diagnostic value of SPS for the nondestructive characterization of vertically coupled self-assembled QD structures.

Enhancement of the photocatalytic activity of TiO 2 nanoparticles by surface-capping DBS groups

TiO 2 nanoparticles capped with sodium dodecylbenzenesulfonate (DBS) are synthesized by a sol–hydrothermal process using tetrabutyl titanate and DBS as raw materials. The effects of surface-capping DBS on the surface photovoltage spectroscopy (SPS), photoluminescence (PL) and photocatalytic performance of TiO 2 nanoparticles are principally investigated together with their relationships. The results show that the surface of TiO 2 nanoparticles can be well capped by DBS groups while the pH value and added DBS amount are controlled at 5.0 and 2% of TiO 2 mass weight, respectively, and the linkage between DBS groups and TiO 2 surfaces is mainly by means of quasi-sulphonate bond. The intensities of SPS and PL spectra of TiO 2 obviously decrease after DBS-capping, while the activity can greatly increase during the photocatalytic degradation of Rhodamine B (RhB) solution, which are mainly attributed to the electron-withdrawing character of the DBS groups. Moreover, the enhancement of photocatalytic activity of DBS-capped TiO 2 is also related to the increase in the capability for adsorbing RhB.

Photoreflectance and surface photovoltage spectroscopy characterisation of an InGaP∕InGaAsN∕GaAs NpN DHBT structure

An InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor (DHBT) structure has been characterised using the techniques of photoreflectance (PR), including the dependence of the signals on the polarisation {[1 1 0] and [1 1 0]} of the incident radiation, and surface photovoltage spectroscopy (SPS). The ordering parameter of the InGaP is deduced from the polarisation dependence of the PR signals from the emitter region. The observed Franz–Keldysh oscillations have been used to evaluate the electric fields in the collector and emitter regions. The field in the collector region agrees well with the theoretical value, while the field in the emitter region is found to be about 25 kV/cm smaller than the theoretical value not taking into account the possible ordering-induced screening effect. The difference is ascribed to the influence of the piezoelectric field related to ordering. In addition, the InGaAsN band gap is determined to be 1.196 eV by analysing the PR and SPS spectra in the base region. The narrower band gap of InGaAsN has led to a lower turn-on voltage, which shows great potential for the application of InGaAsN in low-power electronics.

Surface photovoltage spectroscopy as a valuable nondestructive characterizationtechnique for GaAs/GaAlAs vertical-cavity surface-emitting laser structures

We have investigated an 850 nm GaAs/GaAlAs (001) vertical-cavity surface-emittinglaser (VCSEL) structure using angle- and temperature-dependent surfacephotovoltage spectroscopy (SPS). The SPS measurements were performed asfunctions of angle of incidence (0° ≤ θ ≤ 60°)and temperature (25° C ≤ T ≤ 215° C)for both the metal–insulator–semiconductor (MIS) andwavelength-modulated MIS configurations. Angle-dependent reflectance(R)measurements have also been performed to illustrate the superior features of theSPS technique. The SPS spectra exhibit both the fundamental conduction toheavy-hole excitonic transition of quantum well and cavity mode (CM) plusa rich interference pattern related to the mirror stacks, whereas in theRspectra only the CM and interference features are clearly visible. The variations ofSPS spectra as functions of incident angle and temperature enable exploration oflight emission from the quantum well confined in a microcavity with relation tothe Fabry–Pérot cavity mode. The results demonstrate considerable potential ofSPS for the contactless and nondestructive characterization of VCSEL structures.

Surface photovoltage spectroscopy and normal-incidence reflectivity characterization of a 1.3 μm InGaAlAs/InP vertical-cavity surface-emitting laser structure

We have investigated a 1.3 μm InGaAlAs/InP (001) vertical-cavity surface-emitting laser (VCSEL) structure using surface photovoltage spectroscopy (SPS) and normal-incidence reflectivity. The SPS measurements were performed as a function of angle of incidence relative to the normal (0°–55°) and temperature (300 K&amp;lt;T&amp;lt;420 K). The SPS spectra exhibit both the fundamental conduction to heavy-hole excitonic transition and cavity mode plus a rich interference pattern related to the mirror stack. The advantages of SPS in relation to other methods to study VCSELs, such as photoreflectance and photocurrent spectroscopy, are discussed. This experiment demonstrates the considerable potential of SPS for the characterization of these device structures.

Surface photovoltage spectroscopy of semi-insulating GaAs in the 800–1100 nm range

Surface photovoltage spectroscopy (SPS) studies on thick semi-insulating (SI) GaAs wafers have been done in the range 800–1100 nm using chopped light geometry. SPS peaks at 880 nm, 900 nm, and a broadband in the range 930–1080 nm have been observed. These observations are important as SPS is routinely used to study absorption-related features in InAs self-organized quantum dots grown on SI GaAs. The effect of the ambient and chopping frequencies on SPS spectra is also presented.

Surface photovoltage spectroscopy, photoreflectance, and reflectivity characterization of an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser including temperature dependence

We have investigated an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser using surface photovoltage spectroscopy (SPS) in the temperature range of 15 K&amp;lt;T&amp;lt;400 K. For comparison purposes, we have also (a) measured temperature-dependent photoreflectance (PR) and normal-incidence reflectivity (NIR); and (b) calculated the number of photoexcited carriers (PEC), which is related to the SPS signal. The SPS spectra exhibit both the fundamental conduction to heavy-hole (1C–1H) excitonic transition and cavity mode plus a rich interference pattern related to the properties of the mirror stack. The PR data show only the 1C–1H excitonic transition (plus Franz–Keldysh oscillations) while the cavity mode is detected by NIR. The temperature variation of 1C–1H/cavity mode is the same for SPS and PR/SPS and NIR. The SPS interference pattern from the DBRs is in good agreement with the PEC calculation. This experiment demonstrates the considerable potential of SPS for the characterization of these devices.