Surface Trap Sites Research Articles

Blinking photoluminescence properties of single TiO2nanodiscs: interfacial electron transfer dynamics

Blinking photoluminescence was observed in single TiO2 nanodiscs (NDs) by using a laser scanning confocal microscope (LSCM)-coupled steady-state and ps-time-resolved photoluminescence (PL) spectroscopic system, while it was not significantly observed for TiO2 quantum dots (QDs). Analysis of the PL blinking time trajectories revealed single-exponential kinetics with the average lifetimes of on-state (approximately 286 ms) and off-state (approximately 58 ms), implying the existence of inherent surface-trap sites which can be filled by photogenerated electron or hole. The PL spectra of single TiO2 NDs exhibited broad surface emissions with four decay times, which may be due to diffusion of the energies of electron or hole trap states related to surface structural changes by modification of TiO2 QDs. These results and the surface structural analysis (IR and XPS) suggests a simple model for the PL blinking of single TiO2 NDs that is based on repetitive interfacial electron transfer to the inherent surface trap sites (4Ti4+-OH) with Auger-assisted hole trapping in the multiple surface states as modified by the diffusive coordinate model and the surface-trap-filling model. Based on this blinking mechanism and kinetics, the rates of the interfacial electron transfer and the back electron transfer in TiO2 NDs were determined to be 18 ns and 58 ms, respectively, which are slow enough to keep the polarization of e-h pairs at the surface for efficient photocatalysis and photovoltaic activities. The present methodology and results may be applicable to obtain surface exciton dynamics of various photoelectronic semiconductor nanostructures.

The Role of Band Bending in Affecting the Surface Recombination Velocities for Si(111) in Contact with Aqueous Acidic Electrolytes

The role of band bending in affecting surface recombination velocity measurements has been evaluated by combining barrier height data with charge-carrier lifetime measurements for Si(111) surfaces in contact with a variety of acidic aqueous electrolytes. Charge-carrier lifetimes and thus surface recombination velocities have been measured by contactless radio frequency photoconductivity decay techniques for long bulk lifetime n-Si(111) samples in contact with 11 M (40% by weight) NH4F(aq), buffered (pH = 5) HF(aq), 27 M (48% by weight) HF(aq), or concentrated 18 M H2SO4. Regardless of the sample history or surface condition, long charge-carrier lifetimes were observed for n-Si(111) surfaces in contact with 11 M NH4F(aq) or buffered HF(aq). On the basis of previous barrier height measurements, this behavior is consistent with the formation of an electrolyte-induced surface accumulation layer that reduces the rate of steady-state surface recombination even in the presence of a significant density of surface trap sites. A straightforward evaluation of the surface trap state density from the measured surface recombination velocities, S, is thus precluded for such Si/liquid contacts. In contrast, a wide range of S values, depending on the history of the sample and the state of the surface, were observed for n-Si(111) surfaces in contact with 27 M HF(aq). These results in conjunction with previously measured barrier height data indicate that the charge-carrier lifetimes measured for n-Si(111) in contact with 27 M HF(aq) can be directly correlated with the surface condition and the effective surface-state trap density. These conclusions were confirmed by measurements of the apparent S values of n-Si(111) surfaces in contact with various solutions in the presence of the known deep trap, Cu. For Si(111)/HF(aq) contacts, very high (≥920 ± 270 cm s-1) surface recombination velocities were observed when 0.16 mM (10 ppm) Cu2+ was in the solution and/or adsorbed onto the Si(111) surface as Cu0 deposits, whereas low (100 ± 75 or 225 ± 20 cm s-1) apparent surface recombination velocities were measured for Cu-contaminated Si(111) samples in contact with 0.16 mM (10 ppm) Cu2+-containing 11 M NH4F(aq) or BHF(aq) solutions, respectively.

Simple Surface-Trap-Filling Model for Photoluminescence Blinking Spanning Entire CdSe Quantum Wires

The mechanism for synchronous photoluminescence intensity fluctuations, blinking, spanning entire micrometer-long CdSe quantum wires (Glennon, J. J.; Tang, R.; Buhro, W. E.; Loomis, R. A. Nano Lett. 2007, 7, 3290) is likely different than that commonly cited for semiconductor quantum dots. We present a simple model for the intensity blinking in quantum wires that is based on the dynamic, transient filling of surface-trap sites by photogenerated excitons and the emptying of these occupied trap sites. The model implements a kinetic Monte Carlo scheme and a dynamic photoluminescence quantum yield that depends on the fraction of trap sites filled. When a majority of the surface traps are filled, one-dimensional excitons are formed and a high emission efficiency is realized. Autocorrelation analysis of both experiment and simulation reveal the nonergodic kinetics governing the blinking phenomenon.

Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires

Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters approximately 9 nm and lengths >5 microm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites.

Influence of Zn(II) adsorption on the photocatalytic activity and the production of H2O2 over irradiated TiO2

The photocatalytic activity of semiconductor oxides, in particular TiO2 powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti−OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O2 reduction (by conduction band electrons) is strongly influenced by the presence of Zn2+ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of ≡Ti−OH surface sites blocked by inner sphere complexation of Zn2+ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of ≡Ti−OH surface sites by Zn2+ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn2+ is accompanied by an increasing production of H2O2 during formate degradation in the presence of O2. Zn(II) cations are not complexed by peroxide/superoxide species derived from O2 reduction. When ≡Ti−OH sites are blocked by Zn2+, the complexation on the TiO2 surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO2.

Excess Electrons Stabilized on Ionic Oxide Surfaces

Surface excess electrons are remarkable chemical entities that provide great opportunities for the design of new materials with precisely tuned electronic and magnetic properties. In this Account, we describe the structure and electronic properties of excess electron centers generated at the surface of insulating oxides. We also outline the elementary mechanisms that are at the basis of the generation of excess electrons at solid surfaces, setting a comparison to the general problem of excess electron localization in condensed media. Emphasis is given to morphological aspects relative to the surface-trapping sites as deduced from combined electron paramagnetic resonance and accurate quantum chemical calculations. The remarkable reactivity featured by the so formed "electron-rich" surfaces is illustrated, describing the reduction of simple diatomic molecules that form adsorbed radical anions via direct surface to adsorbate electron transfer.

Recombination Pathways in the Degussa P25 Formulation of TiO2: Surface versus Lattice Mechanisms

Charge migration between electron trapping sites within the mixed-phase titania photocatalyst Degussa P25 has been studied. In addition to previously described lattice electron trapping sites on both anatase and rutile phases, surface electron trapping sites and an anatase-rutile interface trapping site specific to Degussa P25 are identified. The relationship between these sites and recombination with surface hole trapping sites is also determined. It is experimentally shown that upon band-gap illumination holes appear at the surface and preferentially recombine with electrons in surface trapping sites. These findings indicate that in mixed-phase TiO2, such as Degussa P25, photogenerated holes are trapped exclusively on the particle surface, while photogenerated electrons are trapped within the nanoparticle lattice. Recombination reactions are dominated by surface reactions that follow charge migration. These findings indicate that, in mixed-phase TiO(2), such as Degussa P25, a random flight mechanism of recombination predominates. Such knowledge simplifies the mechanistic mathematical models used for process design and points the way for improving future oxidative titania catalysts.

Nano-Crystalline Gold Studied by Positron Annihilation

Nano-crystalline Au (n-Au) prepared by gas deposition method has been compared with vapor deposited (v-Au), fine powder and polycrystalline Au by positron lifetime spectroscopy. Dislocations, vacancies, grain boundaries, micro-voids at intersections of grain boundaries, and surface trapping sites have been measured depending on the samples and on the condition of heat treatments. Other techniques, scanning tunneling microscopy and hardness measurement, were also used for comparison. It has been shown that small vacant sites with the size corresponding to micro-voids are left in n-Au and, to a lesser extent, in v-Au at high temperatures at which all the vacancies would have been annealed out in normal polycrystals.

Time-Resolved Radio Frequency Conductivity (TRRFC) Studies of Charge-Carrier Dynamics in Aqueous Semiconductor Suspensions

The time-resolved radio-frequency conductivity (TRRFC) method provides a useful tool for in situ measurements of charge carrier dynamics in aqueous suspensions of semiconductor particles. In this report, the effects of pH on surface states of ZnO and the effects of hole scavengers (2-propanol) are examined. The experimental results are interpreted in terms of surface mediated recombination processes, in which holes are trapped in a fast process by surficial sites on the ZnO. Recombination rates appear to be governed by the reaction rate of electrons with surface-trap sites. At higher pH (i.e., pH 12), electrostatic repulsion due to a negatively charged ZnO surface leads to slower surface recombination rates compared to lower pH (Le., pH 7) conditions. Addition of a hole scavenger significantly decreases the absolute charge-carrier concentration as detected by TRRFC. This decrease is attributed to the loss of trapped surface holes or surface-bound OH due to oxidation of the hole scavenger. When 2-propanol is used as a solvent, the holes react in a fast step with the solvent at the semiconductor interface within the time resolution of the experiment. The observed TRRFC signal is then due to electrons which are thought to be predominantly transferred to dissolved oxygen (O_2) leading to the formation of hydroperoxyl radicals (HO_2) and subsequently hydrogen peroxide (H_2O_2).

Nucleation of helium precipitates in nickel observed by HDS

Thermal Helium Desorption Spectrometry (HDS) has been used to study the room temperature nucleation of helium precipitates at point defects in Ni(110), notably HeV defects at depth ~ 20 nm below the crystal surface. Helium is injected into the crystal by 50 eV He ion-irradiation which causes no atomic displacements. It has been observed that He nV defects with occupation from n = 2 He to n = 4 He bind helium equally strongly,but weaker than for HeV. For n⩾5 He the binding increases rapidly. The observed behaviour is attributed to helium induced trap mutation and agrees qualitatively with results of atomistic calculations in nickel for this case. Helium precipitation at near surface trapping sites is held responsible for the observed increase of helium release temperatures with helium dose when an undamaged crystal is irradiated. Preliminary TEM observations of Ni specimens irradiated with 50 times higher helium doses than the maximum dose used in the HDS experiments indicated planar clustering of the helium.

A Radiation-Induced Instability in Silicon MOS Transistors

Data is presented on a room-temperature instability which has been observed in certain types of MOS systems after bombardment with ionizing radiation. This instability can cause the transfer characteristic of an irradiated MOS transistor to shift rapidly when bias is applied. The shift can be as much as 3 volts for a unit that has been bombarded with 1015 1 MeV electrons/cm2. In order to explain this effect it is postulated that the ionizing radiation introduces a number of surface trapping sites in the oxide which can exchange charge very slowly with silicon. Shifts in transfer characteristic under bombardment, due to immobile charge accumulation in oxide were also observed. The shifts observed were unusual in that they indicate negative charge buildup, which is contrary to the usual finding.