eV Illumination Research Articles

A Faster Path to Solar Water Splitting

Finding new wide-bandgap light absorbers that are stable in aqueous solutions is a long-standing challenge in photoelectrochemical water splitting research. Two papers in this issue describe recent advances in high throughput experimentation that may accelerate the discovery of suitable materials.

Hydrogen evolution with fluorescein-sensitized Pt/SrTiO3 nanocrystal photocatalysts is limited by dye adsorption and regeneration

Photocatalysts are one of several technologies for the conversion of sunlight and water into hydrogen fuel. Here, we show that the photocatalytic activity of fluorescein-sensitized platinum modified SrTiO3 nanocubes for visible light hydrogen evolution from aqueous solutions of methanol, potassium iodide, potassium hexacyanoferrate (II), and triethanolamine is limited by light absorption from the fluorescein and by its self-oxidation, even in the presence of the sacrificial donors. The highest hydrogen evolution rate was observed with Pt/SrTiO3 suspended in 0.5 mM fluorescein at pH 13 with triethanolamine as a sacrificial donor (60 μmol h−1, 0.67 % AQE at 2.64 eV). The other donors (methanol and KI) gave lower H2 rates or no H2 (hexacyanoferrate (II)). This trend can be rationalized with surface photovoltage spectroscopy (SPS) measurements on FL-sensitized SrTiO3 films on indium tin oxide substrates. Electron transfer from the dye to SrTiO3 under 2.75 eV illumination and hole transfer to the sacrificial donors are modulated by the redox potentials of the donors and acceptors. The low activity of hexacyanoferrate appears to be due to e/h recombination, while triethanolamine appears to replace the dye from the SrTiO3 nanocrystals under the conditions of the SPS experiment. Photovoltage values suggest charge transfer is accompanied by irreversible self-oxidation of FL and this is confirmed by transient surface photovoltage measurements. Overall these results suggest that system optimization requires dyes with greater redox stability and stronger binding affinity to the SrTiO3 nanocrystal surface.

Physical mechanism for saturation of persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction

The saturation in persistent photoconductivity (PPC) in a Si-doped GaAs–AlAs/GaAs heterostructure is investigated using magnetotransport technique at 4.2 K. Against the donor density of 1.2×1013 cm−2, the maximum electron concentration including the contribution from the two-dimensional channel after 1.41 eV illumination is found to be 7.9×1011 cm−2. This result is anomalous as the DX states are expected to respond to 1.41 eV photons so as to bring the electron density close to the donor density. In order to investigate this issue a high electron-mobility transistor has been fabricated. The channel of this device contains a Hall-bridge pattern. The capacitance–voltage in conjunction with quantum transport measurements reveals that at saturation of PPC a second channel of electron gas is present in GaAs–AlAs superlattice. Furthermore, this channel is a nonconducting one as the electrons in it do not participate in the transport process. The reason behind it is the presence of potential barriers that separate the electron gas in the superlattice (EGS) from the alloyed regions meant for ohmic contacts to two-dimensional electron gas (2DEG). The formation of these potential barriers is linked to the lateral diffusion of In atoms used to make ohmic contacts to 2DEG. At saturation of PPC the electron density of EGS and 2DEG together is found to be 2.2×1012 cm−2. The remaining donors are believed to form electronically inactive complexes. At saturation of PPC the two-dimensional electron density (n2D) is determined by the tunneling probability through the potential barrier adjacent to the alloyed region. During the course of this investigation n2D at 4.2 K measured in the dark was found to depend on the cooldown rate. This phenomenon is attributed to the formation of a nonconducting channel.

Physical origin of negative persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction

When a Si-doped GaAs–AlAs/GaAs heterostructure is illuminated with 1.98 eV light at 4.2 K, a reduction in the two-dimensional (2D) electron density (n2D) is observed. This reduction is followed at times by a small increase in n2D even after the illumination has been switched off. This change is observed on the time scale of minutes and can be explained based on the band bending that results after a reduction in n2D. The negative persistent photoconductivity (NPPC) effect characterized by a persistent reduction in n2D, a postillumination change in n2D, and a long persistence time for T⩽40 K has been investigated. I have used Shubnikov–de Haas oscillations and time-resolved, as well as temperature-dependent, Hall-effect measurements to investigate the origin of this phenomenon. The illumination generates electron–hole (e–h) pairs in the superlattice, where the electrons are trapped into the shallow donor state (SDS) of Si and the holes drift to the two-dimensional channel to recombine with the 2D electrons. All the trapped electrons can be recovered by heating the sample to 60 K. The temperature dependence of the NPPC effect is determined only by the binding energy of the SDS of Si, which is found to be about 5 meV. The e–h recombination in the 2D channel is caused by negatively charged defects, which temporarily bind the holes. This fact is manifested also in the optical quenching of this effect by photons with 1.41 eV or larger energy. The saturation values of n2D(n2Dsat) obtained for 0.8, 1.41, or 1.98 eV illumination at 4.2 K have been investigated and the results confirm the presence of these fixed negative charges (FNCs) near the 2D channel. The change in n2Dsat for 0.8 eV illumination, caused by 1.98 eV illumination, also confirms the presence of FNCs.

Investigation of vibrational and photoluminescence spectra of nanocrystalline silicon by micro-Raman spectroscopy using various laser powers

The Raman spectra of silicon nanocrystals embedded in silicon oxide and in porous silicon were measured at various laser powers. It was found that the Si–Si stretching Raman peak shifts to lower wavenumbers and broadens when the laser power increases. The effect is significant and reversible, i.e. the peak returns to its former position when the laser power is decreased to the initial level. It was found that this reversible phenomenon is caused by an increase in bond length due to the heating effect of the laser. In addition, the appearance of weak bands on both sides of the main Si–Si stretching mode peak at 155, 332, 620, 940, 2087, 2114 and 2145 cm−1 was observed at high laser power. It was found that the first four bands could be attributed to one- and multi-phonon Raman scattering and the last three bands are the expected SiH2, SiH and SiH3 stretching vibration modes. The photoluminescent spectrum was measured with the use of 1.959 eV illumination from an 11 mW helium–neon laser. It was found that the photoluminescence spectrum consists of a broad line centered at 1.79 eV with a full width at half-maximum of 0.21 eV and a tail located in an energy region higher than the laser incident excitation energy. Based on the analysis of Raman scattering spectra and the assumption that the luminescence spectrum arises from the ensemble of various sized particles distributed in the material, it is suggested that the later band of photoluminescence should be excited by the Raman scattering light in silicon nanocrystals. Copyright © 1999 John Wiley & Sons, Ltd.

Verification of EL2 electronic absorption effect on charge transfer in semi-insulating GaAs.

The electronic absorption of EL2 centers has been clarified to be related to the electron and hole photoionizations, and the transition from its ground state to metastable state, respectively. Under an illumination with a selected photon energy in the near infrared region, these three processes with different optical cross sections will show different kinetics against the illumination time. It has recently been shown that the photosensitivity (measured under 1.25 eV illumination) of the local vibrational mode absorption induced by some deep defect centers in SI-GaAs is a consequence of the electron and hole photoionizations of EL2. This paper directly measures the kinetics of the electronic transition associated with EL2 under 1.25 eV illumination, which implies the expected charge transfer among different charge states of the EL2 center. A calculation based on a simple rate equation model is in good agreement with the experimental results. \textcopyright{} 1996 The American Physical Society.

Positron annihilation studies of defects in molecular beam epitaxy grown III–V layers

A summary of recent positron annihilation experiments on molecular beam epitaxy (MBE) grown III–V layers is presented. Variable energy positron beam measurements on Al 0.32Ga 0.68As undoped and Si-doped have been completed. Positron trapping at an open volume defect in Al 0.32Ga 0.68As:Si for temperatures from 300 to 25 K in the dark was observed. The positron trap was lost after 1.3 eV illumination at 25 K. These results indicate an open volume defect is associated with the local structure of the deep donor state of the DX center. Stability of MBE GaAs to thermal annealing was investigated over the temperature range 230–700°C. Proximity wafer furnace anneals in flowing argon were used. Samples grown above 450°C were shown to be stable but for samples below this temperature an anneal-induced vacancy-related defect was produced for anneals between 400 and 500°C. The nature of the defect was shown to be different for materials grown at 350 and 230°C. Activation energies of 2.5-2.3 eV were obtained from isochronal anneal experiments for samples grown at 350 and 230°C, respectively.

Photobehavior of paramagnetic anion antisites in plastically deformed GaAs

We investigate the behavior of paramagnetic AsGa+-related defects under 1.2 eV illumination at 4.2 K in plastically deformed bulk semi-insulating GaAs. We find them identical to the grown-in antisites with a similar photoquenchability and analogous parameters. Further, there is effective generation of additional AsGa centers after deformation. The photobehavior can be interpreted by a charge transfer model, where the metastability involved with the AsGa+ signal decay is not a defect inherent feature.

Trap detection at A p-p In 0.53Ga 0.47As/InP heterojunction by means of C-V, I-V and photocapacitance measurements

We report our investigations on LPE grown p-p InGaAs/InP heterojunctions by means of C− V, I− V and photocapacitance measurements. At 77 K we calculate a valence band offset of 0.47 eV and a fixed interface charge of 5x10 11cm -2. The acceptor-like trap levels at 0.37, 0.40 and 0.56 eV, and the donor-like trap level at 0.39 eV are located at the InGaAs side of the heterojunction. At the InP side we detect a donor-like trap level at 0.47 eV. Under illumination photogenerated holes create an additional charge in the hole accumulation layer at the heterointerface. We calculate an additional sheet carrier concentration of 4x10 10cm -2 at 0.4 eV illumination level and 8.4x10 10cm -2 sheet carrier concentration at 0.6 eV illumination level. The first and second sheet charges are caused by ionization of the acceptor level at 0.40 and 0.56 eV, respectively. At 300 K, C- V profiles measured under daylight show an increasing hole accumulation layer at the InGaAs side of the heterojunction and an increasing hole depletion layer at the InP side with decreasing measuring frequency.

Photoelectrochemical cells based on amorphous hydrogenated silicon thin film electrodes and the behavior of photoconductor electrode materials

The behavior of intrinsic a-Si:H photoanodes has been investigated. Thin films (approx.3.9 ..mu..m) of intrinsic a-Si:H, on a stainless steel substrate coated with a very thin (approx.200 angstrom) heavily n-type doped a-Si:H layer for ohmic contacting, behave as electrodes only under greater than or equal toE/sub g/ = 1.7 eV illumination, irrespective of the E/sup 0/' of the redox couple investigated in the range -1.78 to +0.41 V vs. SCE in CH/sub 3/CN/0.1 M (n-Bu/sub 4/N)ClO/sub 4/. The nature of the contact, stainless steel/n-type a-Si:H/intrinsic a-Si:H, indicates that redox couples having E/sup 0/' more positive than the bottom of the conduction band, E/sub CB/, can result in a field across the intrinsic a-Si:H that will drive photogenerated holes to the surface in contact with the redox couple while the electrons excited to the conduction band are driven into the bulk. In cyclic voltammetry experiments, the extent to which the oxidation peak is more negative than at Pt is a measure of the photovoltage, E/sub v/, that could be obtained at open circuit in the presence of equal concentrations of the oxidized and reduced forms of the couple. Surface photocorrosion of a-Si:H is a problem at low (approx.1 mM) concentrations ofmore » redox reagents used in cyclic voltammetry, and the largest E/sub v/ found is approx.0.4 V at approx.40 mW/cm/sup 2/ of 632.8-nm light. Derivatization of a-Si:H with (1,1'-Ferrocenediyl)dichlorosilane suppresses photocorrosion, and E/sub v/ of approx.0.75 V has been obtained for this surface-confined ferrocene derivative, exceeding the E/sub v/ on single-crystal n-type Si photoanodes. Durable cells using a Pt cathode, the a-Si:H photoanode, and EtOH/0.1 M (n-Bu/sub 4/N)ClO/sub 4//10/sup -3/ M ferricenium/0.07M ferrocene solution have been studied.« less