Electron-hole Charge Separation Research Articles

Doped TiO<sub>2</sub> Nanotubes: Synthesis and Application

TiO2 nanotubes have recently been studied extensively due to their high surface areas and finite length scales, allowing highly efficiency electron-hole charge separation and reduced recombination. Owing to these properties they can be used for instance in photocatalysis and lithium-ion batteries. The preparation of TiO2 nanotubes through hydrothermal process has received much attention. However, the practical application of TiO2 nanotubes is significantly hindered by their intrinsic energy band gap > 3 eV, which ensures that these materials cannot be excited by visible light. Visible-lightactivated TiO2 can be prepared by doping through incorporation or decoration with other metal ions, non-metal ions, and semiconductors. This review discusses various types of dopants, doping methods, and applications of doped TiO2 nanotubes. Keywords: Doped TiO2, nanotubes, visible-light adsorption, hydrothermal treatment, photocatalysis.

Coupled Electron–Hole Quantum Dynamics on D−π–A Dye-Sensitized TiO2 Semiconductors

A quantum-mechanical description of the electron–hole charge separation and interfacial electron transfer, including the underlying nuclear motion and solvation dynamics effects, is presented for a prototypical class of D−π–A (electron donor−π conjugated bridge–electron acceptor) organic dyes sensitizing the TiO2 (101) surface. The dyes are comprised of a triphenylamine donor group and a cyanoacrylic acceptor anchoring group, connected by varying thiophene unit lengths (TPAn, n = 1, 2, 3). Simulations show that the electron–hole Coulomb coupling considerably delays charge separation and interfacial electron transfer. A comparative analysis demonstrates that the relevance of thermal nuclear motion, in this case, is secondary relative to electron–hole coupling during the ultrafast transfer from the D−π–A dyes. A dynamic atomistic method is used to describe the solvation effects and the optical properties of the dye-sensitized interface. The solvation dynamics screens the dye molecule, decreasing its complex...

Physico-chemical, photo-catalytic and O 2-adsorption properties of TiO 2 nanotubes coated with gold nanoparticles

Gold-containing titania nanotubes (Au/NT) were found to display higher activity for photooxidation of acetaldehyde, as compared to corresponding gold-free nanotubes and also a Degussa P-25 catalyst. Besides CO 2 as a major reaction product, small amounts of H 2, CH 4, CO, H 2O and CH 3COOH were also formed, irrespective of the catalyst employed. High-resolution TEM examination showed that most of the gold particles in Au/NT were of 1.5–5 nm size, distributed both within and at outside surface of the nanotubes. Some larger size (10–70 nm) clusters were also seen at the external surfaces, particularly in the samples calcined at an elevated temperature. The temperature-programmed desorption measurements revealed that, compared to P-25 TiO 2, a significant entrapment of O 2 occurred at two distinct tubular sites of NT samples, corresponding activation energy of desorption ( E a) being around 36 and 41 kcal mol −1. On the other hand, gold nanoparticles in Au/NT served as independent low-energy ( E a = 26 kcal mol −1) sites for adsorption/activation of O 2. These adsorptive properties of TiO 2 and Au were lost completely on calcination, thus revealing a crucial role played by the particle size. In situ IR spectroscopy results showed that room-temperature exposure to acetaldehyde + air gave rise to a molecularly bound state, i.e. CH 3CHO ad, over both NT and Au/NT samples, which in turn transformed quickly to yield certain acetate (CH 3COO − ad) and formate (HCOO − ad) type transient species with the involvement of the surface OH groups. The decomposition and oxidation of these surface species with the help of O 2 −, O ad and hydroxyl ion radicals (OH −) formed at photo-excited Au/NT interfaces led to the reaction products mentioned above. We conclude that, besides electron–hole charge separation, the adsorptive properties of host matrix and nanosize gold may together play a significant role in deciding the photo-catalytic properties of Au/TiO 2.

Strain-induced charge separation in the photocatalytic single crystalline anatase TiO 2 film

A single crystalline anatase TiO 2 film was grown on a LaAlO 3 substrate. Photocatalytic reduction of Ag ions resulted in Ag precipitation on the surface of the film. The density and the morphology of the Ag precipitation varied at the twin boundaries of the substrate. The anatase film should suffer from the strain caused by this twinning and the strain distribution should be different in each twin domains. Hence, this twin-domain dependent Ag precipitation is a possible clue for the electron–hole charge separation centers induced by the stress concentration in the anatase film which act as active sites for photocatalytic reaction.

Study of optical properties in GaAs1-xSbx/GaAssingle quant um wells

GaAsSb/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy are studied by selectively-excited photoluminescence (SEPL) measurement. For the first time, we have simultaneously observed the PL from both type Ⅰ and type Ⅱ transitions in GaAsSb/GaAs heterostructure in the SEPL. The two transitions exhibit different PL behaviours under different excitation energy. As expected, the peak energy of type I emission remains constant in the whole excitation energy range we used, while type Ⅱ transition shows a significant blue shift with increasing excitation energy.The observed blue shift is well explained in terms of electron-hole charge separation model at the interface. Time-resolved(TR) PL exhibits more type Ⅱ characteristic of GaAsSb/GaAs QW. Moreover, the results of the excitation-power-dependent PL and TRPL provide more direct information on the type-Ⅱ nature of the band alignment in GaAsSb/GaAs quantum-well structures. By combining the experimental results with some simple calculations, we have obtained the strained and unstrained valence band offsets of Qv＝1.145 and Q0v＝0.76 in our samples, respectively.

Selectively excited photoluminescence of GaAs1−xSbx/GaAs single quantum wells

GaAsSb/GaAs single quantum wells grown by molecular-beam epitaxy are studied by selectively excited photoluminescence measurements. We have simultaneously observed the photoluminescence (PL) from both type-I and type-II transitions in the samples. The two transitions exhibit different PL behavior under different excitation energies. As expected, the peak energy of the type-I emission remains constant in the entire excitation energy range we used, while the type-II transition shows a significant blueshift with increasing excitation energy. The observed blueshift can be well explained by an electron–hole charge separation model at interface. This result, along with the excitation-power-dependent PL and the measured longer carrier decay time, provides more direct information on the type-II nature of the band alignment in GaAsSb/GaAs quantum well structures.

Electro-optical properties of semiconductor quantum dots: Application to quantum information processing

A detailed analysis of the electro-optical response of single as well as coupled semiconductor quantum dots is presented. This is based on a realistic ---i.e., fully tridimensional--- description of Coulomb-correlated few-electron states, obtained via a direct-diagonalization approach. More specifically, we investigate the combined effect of static electric fields and ultrafast sequences of multicolor laser pulses in the few-carrier, i.e., low-excitation, regime. In particular, we show how the presence of a properly tailored static field may give rise to significant electron-hole charge separation; these field-induced dipoles, in turn, may introduce relevant exciton-exciton couplings, which are found to induce significant ---both intra- and inter-dot--- biexcitonic splittings. We finally show that such few-exciton systems constitute an ideal semiconductor-based hardware for an all optical implementation of quantum information processing.

Charge-separation effects in 1.3 μm GaAsSb type-II quantum-well laser gain

A microscopic laser theory is used to investigate gain and threshold properties in a GaAsSb quantum-well laser. Depending on the geometry of the type-II quantum-well gain region, there may be appreciable band distortions due to electron–hole charge separation. The charge separation and accompanying band distortions lead to interesting optical behaviors, such as excitation-dependent oscillator strength and band edge energies. Implications to laser operation include significant blueshift of the gain peak with increasing injection current, and inhibition of spontaneous emission, which may result in threshold current reduction.

Electronic Structure of “Sequence Mutations” in OrderedGaInP2Alloys

The electronic consequences of layer thickness fluctuations in CuPt-ordered ${\mathrm{GaInP}}_{2}$ (layer sequence Ga-In-Ga-In. . .) are investigated. We show that the formation of a ``sequence mutated'' Ga-In-In-Ga. . . region creates a hole state $h1$ localized in the In-In double layer, while the electron state $e1$ is localized in the CuPt-ordered region. Thus, the system exhibits electron-hole charge separation in addition to spatial localization. This physical picture is preserved when the dimension of the mutated segment is reduced from 2D to 0D, resulting in disklike dot structures. Our theory explains the long-standing puzzle of the origin of the peculiar luminescence properties of ordered ${\mathrm{GaInP}}_{2}$.

フェムト秒拡散反射分光法の開発と酸化チタン光触媒粉末への応用

Femtosecond diffuse reflectance spectroscopy for studying ultrafast photophysical and photochemical processes of optically scattering materials has been developed and applied to photocatalytic TiO2 powders. It is demonstrated that the spectroscopy has the similar spectroscopic and temporal resolutions to those of transmittance mode transient absorption one. Temporal behavior of diffuse reflected light is analysed by extended Kubelka-Munk model and simulated numerically. Excitation energy relaxation processes of TiO2 powders are measured directly in femtosecond to nanosecond time domains, and electron-hole charge separa-tion and recombination dynamics, and charge transfer to adsorbed molecules on TiO2 surface are made clear. The results are discussed in terms of physical and structural properties of TiO2 and environmental conditions.

Optical Nonlinearities of Hybrid Direct–Indirect Wannier Excitons in Polytype Double Quantum Well Structures

We have calculated the nonlinear optical properties of a double QW structure, where it is possible to achieve a resonance between a real space direct exciton |dir〉 (electron and hole both confined in the same well) and a real space indirect exciton |ind〉 (electron and hole confined in separated wells). |dir〉 and |ind〉 are coupled by electron tunneling between the two QWs, so that the true excitonic eigenstates are the hybrid direct–indirect excitons. Because of the reduced electron–hole overlap, the |ind〉 oscillator strength is negligible compared to |dir〉, but owing to the electron–hole charge separation, it does have large nonlinear properties. Apart from the above electro-optic effect, many-body nonlinearities (due to large two-dimensional Bohr radii) are significant. We have applied our model to the realistic polytype AlSb/InAs/AlSb/GaSb/AlSb structure.

Photoinduced electron-hole charge separation, intrinsic self-doping and novel device applications of Q1D heterojunction materials: mixed-halide MX solids

We have extensively studied, both experimentally and theoretically, the following three mixed-halide systems: (i) PtCl x Br 1− x , (ii) PtCl x I 1− x and (iii) PtBr x I 1− x for a variety of concentrations x. We have obtained direct spectroscopic evidence (vibrational modes) for charge separation from resonance Raman (RR) experiments on doped and photoexcited single crystals of PtCl x Br 1− x . In particular, electron polarons preferentially locate on the PtBr segment while hole polarons are trapped within the PtCl segments. This polaron charge selectivity, which is potentially very useful for photovoltaic device applications, is demonstrated within theoretical calculations based on a discrete, 3 4 - filled , two-band, tight-binding, extended Peierls-Hubbard model, showing strong hybridization of PtCl and PtBr electronic bands as the driving force for separation. In contrast to PtCl x Br 1− x , we find that both the PtCl x I 1− x and the PtBr x I 1− x systems exhibit qualitatively different behavior, such as intrinsic ‘self-doping’. In other words, our simulations predict that electronic charge is spontaneously removed from the PtI segment and injected into the PtCl (or PtBR) segment, thus creating hole defects in PtI and electron defects (polarons/bipolarons) in PtCl (or PtBr).

Nonlinear optical properties of type-II quantum wells.

Experimental and theoretical investigations of the near-band-gap optical nonlinearities in type-II GaAs/AlAs quantum wells are reported. The temporal evolution of the optical nonlinearities of different samples grown at our two different laboratories is analyzed by comparing femtosecond results at different pump-probe time delays with nanosecond measurements at different sample temperatures. The quasi-cw measurements at low temperatures show exciton bleaching and a pronounced heavy-hole exciton blueshift, which is absent at elevated plasma temperatures (\ensuremath{\simeq}100 K). Femtosecond spectra show a delayed onset of the exciton blueshift consistent with hole-plasma cooling in the absence of electrons. The results are analyzed using many-body theory assuming complete electron-hole charge separation. We show that the presence of a one-component plasma modifies the nonlinear optical behavior of the GaAs layer and results in dramatically different properties compared to type-I quantum-well structures.

Electronic properties of micro-n-i-p-istructures in silicon

The electronic properties of ultrathin, heavily doped n-i-p-i structures in silicon have been studied with use of the ab initio pseudopotential method. Three doping configurations are used to model n-i-p-i structures by replacing silicon atoms with aluminum and arsenic atoms. The doped silicon samples are found to retain an indirect energy band gap. The heavy doping levels result in the formation of impurity bands. The top of the valence band and the bottom of the conduction band are impurity-related bands. The electron-hole charge separation even occurs when the n- and p-type impurities are nearest neighbors. The heavy doping significantly increases the electron-hole recombination matrix elements as compared to the light-doping case in which the bulk silicon band structure can be used to determine the matrix elements. However, the matrix elements for the three n-i-p-i structures in Si which were investigated are still 5 orders of magnitude smaller than those for the direct-band-gap transition in GaAs.