Surface Photovoltage Technique Research Articles

Role of Ligand Exchange at CdSe Quantum Dot Layers for Charge Separation

Layers of CdSe quantum dots (QDs) with fixed diameter (4.5 nm) and various ligands were deposited on ITO substrates and investigated by photoluminescence (PL) and surface photovoltage (SPV) techniques. The distance between QDs decreased after washing and ligand exchange. The PL signals in the first excitonic peak as well as in the defect range strongly decreased with successive ligand exchange. In contrast to multilayer deposition, no PL signal related to CdSe QDs has been observed for a monolayer of QDs with pyridine surfactants on ITO. Neither modulated nor transient SPV signals were found for pristine CdSe QD layers. The modulated SPV signals strongly increased with successive ligand exchange. SPV transients became longer with successive ligand exchange. The experiments gave evidence for strong generation of surface defect states with ongoing ligand exchange and for fast initial charge separation of electrons from the first monolayer of CdSe QDs to the ITO surface. SPV transients could be described within a single QD approximation model, in which the number of surface defect states per QD, the energy distributions of defect states, hopping and distant dependent recombination were considered.

Electronic interface properties of silicon substrates after ozone based wet-chemical oxidation studied by SPV measurements

The preparation of ultra-thin oxide layers on mono-crystalline silicon substrate surfaces with ozone dissolved in ultra pure water at ambient temperature was investigated as a low cost alternative to current wet-chemical cleaning and passivation processes in solar cell manufacturing. Surface photovoltage technique was applied as fast, nondestructive, and surface sensitive method, to provide detailed information about the influence of oxidation rate and substrate surface morphology on electronic properties of the oxidised silicon interfaces and subsequently prepared hydrogen terminated surfaces. Sequences of wet-chemical oxidation in ozone containing ultra pure water and subsequent oxide removal in diluted hydrofluoric acid solution could be utilised to prepare hydrophobic substrates, which are predominantly required as starting point for layer deposition and contact formation. On so prepared hydrogen-terminated substrates values of interface state densities Dit,min≈5×1011eV−1cm−2 could be achieved, comparable to values obtained on the same substrates by the standard RCA process followed by HF dip.

Effects of low temperature anneals on the photovoltage in Si nanocrystals

We report on the time decays of surface photovoltage (SPV) and SPV spectra for Si nanocrystals (nc-Si) embedded into a SiO2 matrix. After precipitation at 1150 °C anneal in Ar the SPV increases by a factor of ≈30 compared with the value observed in an oxidized Si substrate. An increase in the signal is accompanied by longer time decays in the SPV transients (roughly from tens to hundreds of microseconds). The separation of photoexcited electrons and holes at the nc-Si/SiO2 interface is expected to play a major role in increasing the SPV signal. We emphasize that annealing of nc-Si at 450 °C in either N2 + O2 or H2 results in a remarkable increase (up to 10-fold) in photoluminescence intensity, which is accompanied by a concomitant decrease in the SPV signal and modification of the SPV decay transients. Anneal in N2 + O2 ambient slightly accelerates the SPV decay, whereas anneal in H2 dramatically speeds it up. Employment of Fourier transform infrared absorption and x-ray photoelectron spectroscopy techniques allows us to gain insight about the nature of chemical bonds into the oxidized matrix. A hypothesis is suggested that the observed effects are attributable to different passivating abilities of H2 and N2 + O2 ambients. A simple model, which takes into account the capture of photoexcited carriers at the nc-Si/SiO2 interface and the varying passivation ability of the interface traps, is capable of explaining the observed changes in the SPV transients, even quantitatively. The results may be of interest for studying the passivation of dangling bonds at the nc-Si/SiO2 interface by SPV techniques and can be used in advancing the development of silicon based photovoltaic materials with high efficiency.

Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity

A series of different crystalline phases BiVO4 photocatalysts (tetragonal, monoclinic, and monoclinic/tetragonal heterophase) have been prepared by a coprecipitation and molten salt method. High-resolution transmission electron microscopy (HRTEM) results show that an interface of intimate contact is formed in monoclinic/tetragonal heterophase and monoclinic phase is mainly on the surface of nanoparticles. Surface photovoltage (SPV) and transient photovoltage (TPV) techniques are used to further investigate the transfer process of photoinduced charge carriers. The results show that the behavior of photoinduced charges markedly depend on the crystalline phases of BiVO4 samples, and the presence of interface in monoclinic/tetragonal heterophase provides a spatial condition for charge transfer, promotes the separation of photoinduced electron-hole pairs, and changes the migration direction of photoinduced carriers. The relationship between behavior of photoinduced charge carriers and photocatalytic activity was discussed in detail, which would provide a greater insight into the intrinsic reasons of the enhancement in photocatalytic activity.

Formation of Au/n-Si(100) Schottky Barrier Using Au Aqueous Solution

Behavior of gold (Au) on n-type Si(100) surface dipped in Au aqueous solution and thermally oxidized have been studied using by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), alternating current surface photovoltage (AC SPV) techniques and ellipsometer. AFM and XPS analyses identified Au to exist at both the top of SiO2 and SiO2/Si interface as a cluster. Au/n-type Si Schottky-barrier causes an occurrence of frequency-dependent AC SPV. The frequency-dependent AC SPV still appeared in Au-contaminated (∼2×1015 atoms/cm2) and thermally oxidized Si surfaces at between 550 and 700oC, demonstrating that Si surface was weakly inverted. This result indicates that Au/n-type Si Schottky-barrier is formed at SiO2/Si interface even after thermal oxidation. The Au at SiO2/Si interface diffused into bulk Si as temperature increased higher than 750oC, resulting in drastic reduction of AC SPV. On oxidation kinetics at between 750 and 900oC, Au is thought to act as catalyst to promote SiO2 growth at the Si surface, resulting in the enhanced oxidation.

半导体硫化银纳米球的控制合成及室温气敏性

Silver sulfide (Ag2S) nanospheres (NSs) were fabricated by a modified hydrothermal strategy. The obtained products were characterized by powder X-ray diffraction, photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and UV-vis absorption techniques. The highly pure Ag2S NSs possessed a monoclinic structure, smooth surface, and their optical absorption spectra exhibited a blue shift of 20 nm compared with that of bulk Ag2S. The gas sensing properties of Ag2S NSs and conventional powders (CPs) at room temperature (RT) were investigated using the surface photovoltage (SPV) technique. The Ag2S NSs exhibited novel photoelectric gas sensitivity to ethanol gas and a RT gas sensor containing the as-fabricated NSs operated below 10 mL/m3, while that containing CPs had a limit of about 50 mL/m3 under the same conditions. These results reveal a new potential application for Ag2S nanodevices at room temperature.

Surface photovoltage phase spectroscopy study of the photo-induced charge carrier properties of TiO2 nanotube arrays

By using the surface photovoltage (SPV) technique based on a lock-in amplifier, surface states located 3.1 eV below the conduction band of TiO2 have been detected in TiO2 nanotube arrays prepared by anodization of titanium foil in fluoride-based ethylene glycol solution. The photo-induced charge transportation behavior of TiO2 nanotube arrays was also studied by qualitatively analyzing their SPV phase spectra measured under different external bias. When a negative bias was applied, carriers excited from surface states have the same transportation properties as those excited from the valence band; in contrast, when a positive bias was applied, these two kinds of photo-excited carriers exhibit different transportation behavior.

Effect of the Residual Template on Surface Photoelectric Characteristics of Structurally-Ordered Macroporous Nanocrystalline TiO<sub>2</sub>

The Macroporous Nanocrystalline TiO2 Thin Film Was Prepared by Dipping-Coating Method. Polystyrene (PS) Microspheres Template Was Used as Pore-Forming Assistant and TiO2 Sol Was Used as Precursor. the Photoelectric Characteristics of the Samples Were Studied Using Surface Photovoltage (SPV) and Photoacoustic (PA) Techniques. the Presence of a Small Amount of Residual Template Damages the Integrity of the Porous TiO2 Film with Inverse Opal Structure, and TiO2 May Be Restricted in Certain Application Fields as Photonic Crystals. a Small Amount of Residual Template Changes the Surface Photovoltaic Properties of the Porous TiO2 Film, Resulting Mainly in the Broadened Scope and the Increased Intensity of SPV Response.

Electro-optic investigation of the surface trapping efficiency inn-alkanethiol SAM passivated GaAs(001)

The electro-optic characteristics of the semi-insulating andn + -type GaAs(001) surfacespassivated with n-alkanethiol self-assembled monolayers were investigated using Kelvin probe surface photovoltage(SPV) and photoluminescence (PL) techniques. Referencing the equilibrium surface barrierheight established in an earlier report, SPV measurements demonstrated a significant (>100 mV) increase in the non-equilibrium band-bending potential observed under low-levelphoto-injection. Modeling of the SPV accounts for these observations in terms of a large (>104) decreasein the hole/electron ratio of surface carrier capture cross-sections, which is suggested to result from theelectrostatic potential of the interfacial dipole layer formed upon thiol chemisorption. Thecross-section effects are verified in the high-injection regime based on carrier transportmodeling of the PL enhancement manifested as a reduction of the surface recombinationvelocity.

Interface junction at anatase/rutile in mixed-phase TiO2: Formation and photo-generated charge carriers properties

Nanosized TiO2 photocatalysts of anatase, rutile and mixed-phase were synthesized through hydrolysis method. The influence of the interface junction between anatase and rutile TiO2 on the photo-generated charge carriers properties were studied by Kelvin probe (KP), surface photovoltage (SPV) and transient photovoltage (TPV) techniques. The KP results revealed that the difference of the surface work function between anatase and rutile might cause the formation of built-in field at the interface in mixed-phase TiO2. The role of the interface junction was further testified through the characteristics of photo-generated charge carriers by SPV and TPV.

Photovoltaic properties of the copper-phthalocyanine and ZnO nanorod array system affected by ethanol

Well-aligned ZnO nanorod array, fabricated on conductive indium-tin-oxide (ITO) substrate by wet chemical bath deposition (CBD) method, was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Surface photovoltage (SPV) technique was employed to study the photovoltaic properties of the copper-phthalocyanine (CuPc) and ZnO nanorod array system affected by ethanol. Prior to ethanol adsorption, two pronounced SPV response bands were exhibited for this system in the range 300–410 and 540–760 nm, respectively. Post-adsorption measurements reveal that the SPV intensity of the former band is enhanced, while that of the latter band is suppressed if ethanol was used to modify CuPc surface. Moreover, both of the SPV intensity of two response bands is enhanced if ethanol was used to modify ZnO and CuPc interface. Mechanisms of these phenomena were suggested.

Characterization of optoelectronic properties of the ZnO nanorod array using surface photovoltage technique

Well-aligned ZnO nanorod array, synthesized by wet chemical bath deposition (CBD) method on conductive indium–in-oxide (ITO) substrate, was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Surface photovoltage (SPV) technique based on a scanning Kelvin Probe system was employed to investigate the optoelectronic behavior of ZnO nanorod array. The surface photovoltage and its time-resolved evolution process are used to determine the energy level structure of the ZnO nanorod array.

A study on photo-generated charges property in highly ordered TiO 2 nanotube arrays

In this study TiO 2 nanotube arrays were fabricated by potentiostatic anodization of titanium sheet. The X-ray diffraction (XRD) pattern and field emission scanning electron microscopy (FE-SEM) image indicated the TiO 2 nanotube arrays were of pure anatase form and highly ordered. The properties of the photo-generated charges in the nanotube arrays were investigated by transient photovoltage (TPV) technique and surface photovoltage (SPV) technique based on lock-in amplifier with dc bias, in comparison with the commercial powder derived film. The separation processes of the photo-induced charges in the system of TiO 2 nanotubes on Ti have been demonstrated to be correlated with the incident light intensity, surface trapping states, and the interfacial electric field between Ti and TiO 2. The results also show that the highly ordered nanotube film could generate much stronger SPV responses under external electric field than the commercial powder derived film.

Fe Ions Diffusion and Oxygen Complexes Evolution during Thermodonors Annealing in B-Doped Czochralsky Silicon Crystal

The great enhancement of the minority carrier recombination rate after optical activation of FeB pairs was used for investigation of interstitial Fe ions diffusion in p-type Czochralsky silicon by Surface Photo-Voltage Technique (SPV). Highly reproducible profiles of the Fe ions distribution were obtained after one-time thermodonors annealing process (650 °C, 30 min.) with Fe film evaporated on one side of silicon slab and successive layer by layer silicon slab etching. Evolution of Fe concentration distribution between both sides of initially inhomogeneously contaminated silicon slab in durable annealing processes was investigated. Gradual Fe concentration reduction near preliminary contaminated slab surface and Fe concentration increase near opposite surface were observed. Fe concentrations near both surfaces become equal after approximately 10-times annealing in agreement with diffusion rate obtained by the first method. Chaotic release and hiding of latent Fe were observed after 5-7 hours of annealing process. Two-exponential increase of other than Fe recombination centers concentration was observed during 650 °C annealing. Both exponential factors are found to be functions of interstitial oxygen concentration in the silicon ingot.

Photoluminescence and surface photovoltage of ethynyl derivative‐terminated Si(111) surfaces

AbstractThe electrochemical grafting of ethynyl derivatives on Si(111) surfaces has been investigated by means of pulsed photoluminescence (PL) and surface photovoltage (SPV) techniques. Ethynyl derivatives (ethynyl‐MgCl, ethynyl‐MgBr, propynyl‐MgBr, and phenylethynyl‐MgBr) from Grignard compounds were used. By SPV, a high surface photovoltage signal, UPh, is observed. The halogene in the Grignard reagents influences strongly the electronic properties and the polymerisation process, i.e. Br in the Grignard compound leads to better electronic properties than Cl. The propynyl‐terminated Si surface shows the highest PL intensity and at the same time the weakest UPh signal, i.e. this polymeric layer induces the smallest amount of recombination active defects at the interface but also the smallest change in surface band bending. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoinduced charge transfer in ZnO/Cu2O heterostructure films studied by surface photovoltage technique

ZnO/Cu(2)O heterostructure films were prepared by a two-step electrodeposition method in aqueous solution on fluorine-doped tin oxide (FTO) substrates. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and UV-vis transmission measurements were utilized to characterize the films. Surface photovoltage (SPV) technique was used to investigate the process of photoinduced charge transfer. The results show that there is an electric field located at the interface between ZnO and Cu(2)O film and the photoinduced electrons in Cu(2)O film inject into ZnO under the effect of interfacial electric field with visible light irradiation. While under ultraviolet light illumination, the photoinduced electrons in Cu(2)O film accumulate at the surface of Cu(2)O film instead of injecting into ZnO under the action of surface built-in electric field of Cu(2)O film. The work function measurements confirm that the direction of interfacial electric field is from ZnO to Cu(2)O. These results are help to future design of high performance heterostructure photovoltaic devices.

Synthesis, photoelectric properties and photocatalytic activity of the Fe2O3/TiO2 heterogeneous photocatalysts

Fe(2)O(3)/TiO(2) heterogeneous photocatalysts with different mass ratios of Fe(2)O(3)vs. TiO(2) were synthesized by impregnation of Fe(3+) on the surface of TiO(2) microrods and calcination at 300 degrees C. Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), photoluminescence spectra and X-ray diffraction (XRD) have been used to characterize the samples. The photocatalytic activities of Fe(2)O(3)/TiO(2) heterocomposites, pure Fe(2)O(3) and pure TiO(2) were evaluated by the photodegrading efficiency of Orange II under visible light (lambda > 420 nm). The experiments demonstrated that Orange II in aqueous solution was more efficiently photodegraded using Fe(2)O(3)/TiO(2) heterogeneous photocatalysts than either pure Fe(2)O(3) or TiO(2) under visible light irradiation. With an optimal mass ratio of 7:3 in Fe(2)O(3)/TiO(2) the highest rate of Orange II photodegradation was achieved under the experimental conditions. We have also compared the photoelectric properties of Fe(2)O(3)/TiO(2) heterogeneous photocatalysts with that of pure Fe(2)O(3) by surface photovoltage (SPV) and transient photovoltage (TPV) techniques. Based on the photovoltage responses, we discussed the influence of the hetero-interface between Fe(2)O(3) and TiO(2) on transfer characteristics of photogenerated charge carriers. We demonstrated that the formation of heterojunctions between Fe(2)O(3) and TiO(2) for Fe(2)O(3)/TiO(2) composites was pivotal for improving the separation and thus restraining the recombination of photogenerated electrons and holes, which accounts for the enhancement of photocatalytic activity.

Synthesis of nanocrystalline anatase TiO 2 by one-pot two-phase separated hydrolysis-solvothermal processes and its high activity for photocatalytic degradation of rhodamine B

Si-doped and un-doped nanocrystalline TiO 2 samples have been synthesized by simple one-pot water-organic two-phase separated hydrolysis-solvothermal (HST) processes, and characterized by XRD, BET, TEM, FT-IR, DRS and surface photovoltage techniques. The effects of the solvothermal temperature and Si doping on the anatase thermal stability, and on the photocatalytic activity for degrading rhodamine B were investigated in detail. The results show that, as the solvothermal temperature rises, the crystallinity and thermal stability of the resulting nano-sized anatase TiO 2 gradually increase. Noticeably, the as-prepared TiO 2 obtained at appropriate solvothermal temperature (160 °C) exhibits high photocatalytic activity. Moreover, although Si doping does not improve the photocatalytic activity of the as-prepared anatase TiO 2, it greatly enhances the anatase thermal stability and inhibits crystallite growth during the process of post-thermal treatment. Interestingly, the Si-doped TiO 2 post-treated at high temperature displays much higher photocatalytic activity than commercial P25 TiO 2. It is clearly demonstrated that the joint effects of high anatase crystallinity and large surface area lead to high photocatalytic activity. This work provides a simple and effective strategy for the synthesis of high-performance TiO 2-based functional nanomaterials.

Highly sensitive room-temperature gas sensors based on hydrothermal synthesis of Cr2O3 hollow nanospheres

This paper reports that Cr2O3 hollow nanospheres (HNs) were synthesized via a hydrothermal approach and characterized by scanning electron microscopy, x-ray powder diffraction, transmission electron microscopy (TEM), selective area electron diffraction and high resolution TEM, respectively. In addition, the room-temperature (RT) gas sensing properties of Cr2O3 HNs and conventional powders (CPs) were investigated by means of the surface photovoltage technique. The experimental data demonstrate that the RT gas sensor of the as-fabricated HNs reaches below 5 ppm whereas that of the CPs is about 40 ppm, which results from there being much more adsorbed and desorbed oxygen in HNs than in CPs at RT. The as-prepared Cr2O3 HNs could have potential applications as RT nanosensors.

Investigation of Carrier Recombination Processes in GaAs/AlAs Multiple Quantum Wells Using Piezoelectric Photothermal and Surface Photovoltage Techniques

We investigate the confinement, radiative, and nonradiative recombinations of photoexcited carriers in each subband of GaAs/AlAs multiple quantum wells (MQWs) using two nondestructive techniques, namely, the surface photovoltage (SPV) and the piezoelectric photothermal (PPT) techniques. In the temperature range from 160 to 300 K, there is no difference between the temperature dependences of carrier confinement in 1HH, 2HH, and 3HH subband transitions. As temperature decreased, increases in the value of the nonradiative recombination component in 2HH and 3HH were observed. These are attributed to the phonon-emitting intersubband relaxation. It was found that the number of emitted phonons by this relaxation increased with increasing order of the subband and with decreasing sample temperature. We conclude that this phonon-emitting intersubband relaxation is the most important process for electron transition in the higher-order subband in MQWs and for PPT signal generation. This is the first time that phonon-emitting intersubband relaxation was directly detected from the nonradiative recombination viewpoint.