AA- and ABA-stacked carbon nitride (C3N4): novel photocatalytic water splitting solar-to-hydrogen energy conversion.

We report the development of the C3N4 structure by integrating two different structures: (i) two identical layers as AA-stacked C3N4 and (ii) intercalating one different layer between two identical layers as ABA-stacked C3N4. This in turn endows C3N4 with significantly promoted charge migration, up-shifted conduction-band (CB) level, enhanced CB potential from -0.89 eV (AA-stacked C3N4) to -1.03 eV (ABA-stacked C3N4), broadened band gap as well as enhanced surface area, all of which favor the enhancement of the photocatalytic performance. The optical absorption level exhibited significant enhancement in the visible light region when shifting from AA-stacked C3N4 to ABA-stacked C3N4, where the absorption edge moves from λ = 508.1 → λ = 454.1 nm. This corresponds to the direct optical band gap of 2.44 eV → 2.73 eV, which is well matched with the solar spectrum and the sufficient negative CB potential for H+/H2 reduction. Based on these results, we can conclude that AA-stacked and ABA-stacked C3N4 satisfies all the requirements to be efficient photocatalysts. This study will significantly improve the search efficiency and considerably aid the experimentalists in the exploration of novel photocatalysts.

Novel photocatalytic water splitting solar-to-hydrogen energy conversion: CdLa2S4 and CdLa2Se4 ternary semiconductor compounds.

Comprehensive ab initio calculations from first- to second-principles methods are performed to investigate the suitability of non-centro-symmetric CdLa2S4 and CdLa2Se4 to be used as active photocatalysts under visible light illumination. The calculations reveal the direct band gap nature of both compounds with large absorption coefficients (104-105 cm-1). The absorption edges of CdLa2S4 and CdLa2Se4 occur at λ = 579.3 nm and λ = 670.1 nm, and the optical band gaps are estimated to be 2.14 eV and 1.85 eV for CdLa2S4 and CdLa2Se4, respectively. These gaps are larger than 1.23 eV the required optical band gap for photocatalytic performance to split water under visible light illumination. The calculated potentials of the conduction band and the valence band edges indicate that CdLa2S4 and CdLa2Se4 have strong reducing powers for H2 production. The obtained results reveal that the high photogenerated carrier mobility favors enhancement of the photocatalytic performance. It has been found that there is a large mobility difference between the electrons (e-) and the holes (h+), which is useful for the separation of e- and h+, reduction of e- and h+ recombination rate, and improvement of the photocatalytic activity. Based on these findings, one can conclude that CdLa2S4 and CdLa2Se4 satisfied all requirements to be efficient photocatalysts. This will greatly improve the search efficiency and greatly help experiments to save resources in the exploration of new photocatalysts with good photocatalytic performances.

Lithium borate Li3B5O8(OH)2 with large second harmonic generation and a high damage threshold in the deep-ultraviolet spectral range.

The electronic structure and linear and nonlinear optical susceptibility dispersions of lithium borate Li3B5O8(OH)2 are comprehensively investigated. The investigation is achieved on Li3B5O8(OH)2 in the form of single crystals, taking into account the influence of the packing of the structural units on the linear and nonlinear optical susceptibility dispersion. The calculations highlight that the BO3 structural unit packing is the main source of the large birefringence in Li3B5O8(OH)2 due to the high anisotropic electron distribution, and, hence, it affects the macroscopic second harmonic generation (SHG) coefficients. This work provides a new path for the design of UV-NLO materials with high SHG efficiencies and short cutoff edges by introducing an alkali metal into borates. The large SHG is due to hyperpolarizability formed by co-parallel BO3 triangle groups. The absorption edge of Li3B5O8(OH)2 occurs at λ = 190 nm and the optical band gap is estimated to be 6.52 eV, which is in good agreement with the experimental data (6.526 eV). The energy gap value confirms that Li3B5O8(OH)2 exhibits an exceptional laser damage threshold and is expected to produce coherent radiation in the deep-ultraviolet (DUV) region. The obtained value of SHG at λ = 1064 nm is about 1.5 times that of the well-known NLO crystal KH2PO4 (KDP) at λ = 1064 nm and 3.5 times that of KDP at λ = 190 nm, which is transparent down to the DUV region. Thus, one can conclude that the combination of an alkali metal with borates leads to the generation of promising DUV-NLO crystals. This work is aimed at qualitative and quantitative investigation to report a reliable SHG value and provide details of the NLO tensor for bulk Li3B5O8(OH)2 single crystals.

Quantum dots in photocatalytic applications: efficiently enhancing visible light photocatalytic activity by integrating CdO quantum dots as sensitizers.

The amalgamation of a wide optical band gap photocatalyst with visible-light-active CdO quantum dots (QDs) as sensitizers is one of the most efficient ways to improve photocatalytic performance under visible light irradiation. The photocatalytic performance of cadmium benzoate ((Cd(C7H5O2)2)3(CH3CN)1) is comprehensively investigated. The estimated optical band gap of cadmium benzoate is 2.64 eV and the EPc and EPv are about -0.09 V (vs. NHE) and +2.55 V (vs. NHE), respectively, which implies that cadmium benzoate possesses a high negative reduction potential of excited electrons due to its higher conduction band position, and hence, the locations of the conduction band minimum and the valence band maximum meet the redox capacity. Thus, this composite photocatalyst exhibits superior activity in visible-light-driven photocatalytic H2 evolution. We found that introducing the QDs enhance the photocatalytic performance towards the visible light region. The electronic band structure shows high k-dispersion bands around the Fermi level, which implies low effective masses, and hence, the high mobility carriers favor the enhancement of the charge transfer process. The mobility of the photogenerated carriers significantly influences the photocatalytic efficiency and the higher photogenerated carriers' mobility enhances the photocatalytic performance. Moreover, the result shows a great effective mass difference between electrons (e-) and holes (h+), which can facilitate the e- and h+ migration and separation, and finally improve the photocatalytic performance. The large mobility difference is useful for the separation of e- and h+, the reduction of the e- and h+ recombination rate, and the improvement of the photocatalytic activity. Thus, cadmium benzoate exhibits rapid generation of e--h+ pairs with photoexcitation and a high negative reduction potential of excited electrons due to its higher CB position. Based on these results one can conclude that cadmium benzoate satisfied all requirements to be an efficient photocatalyst. This will greatly improve the search efficiency and greatly help experimentalists in saving resources in the exploration of new photocatalysts with good photocatalytic performance.

Two haloid borate crystals with large nonlinear optical response.

The photophysical properties of the noncentrosymmetric haloid borates K3B6O10X (X = Cl or Br) are calculated using density functional theory within the recently modified Becke-Johnson potential. The calculated electronic band structure reveals that the theoretical direct band gaps, 5.21 eV (K3B6O10Cl) and 4.85 eV (K3B6O10Br), are in good agreement with the previous calculation for K3B6O10Cl (5.16 eV) and experimental data for K3B6O10Br (4.86 eV). The calculated absorption coefficients, refractive indices, and birefringence are also in good agreement with the experimental data. The calculated nonzero second harmonic generation (SHG) coefficients, d33, d22 (= -d21), and d15(= d32 = d31), show good agreement with the experimental values. Furthermore, we have obtained the microscopic first hyperpolarizability for the dominant tensor component of the SHG.

Spin-polarized Second Harmonic Generation from the Antiferromagnetic CaCoSO Single Crystal.

The spin-polarized second harmonic generation (SHG) of the recently synthesized CaCoSO single crystal is performed based on the calculated electronic band structure. The calculation reveals that the spin-up (↑) channel of CaCoSO possesses a direct energy gap (Γv-Γc) of about 2.187 eV, 1.187 eV (Kv-Kc) for the spin-down (↓) channel and an indirect gap (Γv-Kc) of about 0.4 eV for the spin-polarized CaCoSO single crystal. The linear optical properties obtained reveal that the recently synthesized crystal exhibits considerable anisotropy with negative uniaxial anisotropy and birefringence favor to enhance the SHG. We have calculated the three non-zero tensor components of the SHG and found the is the dominat component, one with a large SHG of about (d33 = 6.936 pm/V at λ = 1064 nm), the half value of KTiOPO4 (KTP). As the values of (↑) < (↓) < spin-polarized are related to the values of the energy gap of (↑) 2.187 eV> (↓) 1.187 eV> spin-polarized gap 0.4 eV; therefore, a smaller energy gap gives better SHG performance. Furthermore, the microscopic first hyperpolarizability, ßijk, is calculated.

Engineering oxygen vacancies towards self-activated BaLuAl(x)Zn(4-x)O(7-(1-x)/2) photoluminescent materials: an experimental and theoretical analysis.

Novel self-activated yellow-emitting BaLuAlxZn4-xO7-(1-x)/2 photoluminescent materials were investigated by a combined experimental and theoretical analysis. The effects of Al/Zn composition modulation, calcination atmosphere and temperature on the crystal structure and photoluminescence properties have been studied via engineering oxygen vacancies. Accordingly, BaLuAl0.91Zn3.09O7 prepared in an air atmosphere was found to be the stable crystalline phase with optimal oxygen content and gave a broad yellow emission band with a maximum at 528 nm. The self-activated luminescence mechanism is ascribed to the O-vacancies based on the density functional theory (DFT) calculation. A theoretical model originating from the designed oxygen vacancies has been proposed in order to determine the influence of O-vacancies on the band structure and self-activated luminescence. Therefore, the appearance of a new local energy level in the band gap will cause the wide-band optical transitions in the studied BaLuAlxZn4-xO7-(1-x)/2 materials.

Specific features of electronic structures and optical susceptibilities of g-BC3 and t-BC3 phases.

Details of comparison for some specific features of electronic structures and optical susceptibilities of g-BC3 and t-BC3 phases are provided. Calculations show that the g-BC3 phase is a narrow band gap semiconductor constructed from the ABAB stacking sequence. Whereas t-BC3 is a metallic phase constructed by a sandwich-like metal-insulator lattice from an alternately stacking sequence of metallic CBC and insulating CCC blocks. The two phases possess only two types of bonds (B-C and C-C). The density of states at the Fermi level N(EF) of the t-BC3 phase is determined by the overlapping of B-2p and C-2p empty orbitals of the CBC block with C-2p empty orbitals of the CCC block, and the shape of the Fermi surface originated from these empty orbitals. The B atoms cause a small perturbation on the C-ring's structure and hence to the charge density distribution. The linear optical properties of the two phases confirm the existence of the lossless regions and the considerable anisotropy. The second harmonic generation of the t-BC3 phase shows that χ(ω) is the dominant component of about 3.9 pm V(-1) at the static limit and 5.8 pm V(-1) at λ = 1064 nm, which suggests that the t-BC3 phase could be considered as a promising nonlinear optical material in comparison with the well known KTiOPO4 nonlinear optical single crystal.

Ab initio study of TaON, an active photocatalyst under visible light irradiation.

Tantalum oxynitride has been studied as an active photocatalyst under visible light, using a full potential linearized augmented plane wave method within the framework of density functional theory. The electronic and optical properties of TaON are calculated using local density approximation (LDA), generalized gradient approximation (GGA), Engel-Vosko generalized gradient approximation (EVGGA) and the modified Becke-Johnson (mBJ) potential approximation to describe the exchange-correlation potential. The calculated band gap value obtained by the mBJ approximation approach (2.5 eV) is very close to the experimental result (2.5 eV). We found that hybridization among the Ta-d, O-p and N-p states results in the formation of a covalent bond between Ta-N and Ta-O. The calculated optical properties confirm that the TaON is an active photocatalyst under visible light irradiation. TaON has a high dielectric constant and the components show anisotropy in the energy range between 3.0 eV and 10.0 eV. A high refractive index of 2.47 at 632.8 nm is obtained which shows better agreement with the experimental value (2.5 at 632.8 nm) than previous results.

Optical spectra and band structure of Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals: experiment and theory.

Theoretical and experimental studies of the Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals are performed. These crystals possess a lot of intrinsic defects which are responsible for their optoelectronic features. The theoretical investigations were performed by means of DFT calculations using different exchange-correlation potentials. The experimental studies were carried out using the modulated VUV ellipsometry for dielectric constants and birefringence studies. The comparison of the structure obtained from X-ray with the theoretically optimized structure is presented. The crucial role of the intrinsic defect states is manifested in the choice of the exchange correlation potential used. The data may be applicable for a large number of the ternary chalcogenides which are sensitive to the presence of the local disordered states near the band edges.

Linear and nonlinear optical susceptibilities and the hyperpolarizability of borate LiBaB9O15 single-crystal: theory and experiment.

The single-crystal borate LiBaB9O15 was synthesized by a high-temperature solution reaction and structurally determined by the single-crystal X-ray diffraction technique. It crystallizes in the noncentrosymmetric space group R3c and features a three-dimensional ∞3[B9O15]3­ anionic framework, with infinite channels in which the Li+ and Ba2+ cations are located. The linear optical properties were investigated experimentally in terms of the absorption spectrum, which reveals an optical gap of 5.17 eV. In addition we have calculated the linear optical properties using state-of-the-art all-electron full potential linearized augmented plane wave method. The nonlinear optical susceptibilities, namely, the second harmonic generation and the hyperpolarizability of the single-crystal borate LiBaB9O15 are calculated and evaluated at a static limit and at λ = 1064 nm. The calculation shows there exists three second-order nonlinear optical susceptiblities tensors components. We present measurements of the IR spectra in the range 500­2000 cm­1, and the second harmonic generation was performed using a Quantel 15 ns Nd:YAG laser operating at 1064 nm.

Surface modification via wet chemical etching of single-crystalline silicon for photovoltaic application.

The potential of solar cells have not been fully tapped due to the lack of energy conversion efficiency. There are three important mechanisms in producing high efficiency cells to harvest solar energy; reduction of light reflectance, enhancement of light trapping in the cell and increment of light absorption. The current work represent studies conducted in surface modification of single-crystalline silicon solar cells using wet chemical etching techniques. Two etching types are applied; alkaline etching (KOH:IPA:DI) and acidic etching (HF:HNO3:DI). The alkaline solution resulted in anisotropic profile that leads to the formation of inverted pyramids. While acidic solution formed circular craters along the front surface of silicon wafer. This surface modification will leads to the reduction of light reflectance via texturizing the surface and thereby increases the short circuit current and conversion rate of the solar cells.

Electrical behaviour of MEH-PPV based diode and transistor.

The potential of organic semiconductor based devices for light generation is demonstrated by the commercialisation of display technologies using organic light emitting diode (OLED). In OLED, organic materials plays an important role of emitting light once the current is passed through. However OLED have drawbacks whereby it suffers from photon loss and exciton quenching. Organic light emitting transistor (OLET) emerged as a new technology to compensate the efficiency and brightness loss encountered in OLED. The structure has combinational capability to switch the electronic signal such as the field effect transistor (FET) as well as to generate light. Different colours of light could be generated by using different types of organic material. The light emission could also be tuned and scanned in OLET. The studies carried out in this paper focuses on investigation of fabricated MEH-PPV based OLED and also OLET via current voltage characteristics. These studies will continue with a view to develop an optimised MEH-PPV based OLET.

Comparative study on human and bovine AT-SC isolation methods.

Mammalian adipose tissue derived stem cells (AT-SC) have a tremendous potential in regenerative medicine for tissue engineering and somatic nuclear transfer (SNT). The isolation methods of human and bovine adipose tissue derived stem cells are compared in this paper to determine the feasibility and optimum method of isolation. The optimum isolation method will reduce the processing time, efforts and money as isolation is the first crucial and important step in stem cells research. Human abdominal subcutaneous adipose tissue and bovine abdominal subcutaneous adipose tissue are digested in three collagenase type 1 concentration 0.075%, 0.3% and 0.6% agitated at 1 h and 2 h under 37 °C in 5% CO2 incubator. The cultures are then morphologically characterised. Human adipose tissue stem cells are found to be best isolated using abdominal subcutaneous depot, using 0.075% collagenase type 1 agitated at 1 h under 37 °C in CO2 incubator. While bovine adipose tissue derived stem cells are best isolated using abdominal subcutaneous depot, using 0.6% collagenase type 1 agitated at 2 h under 37 °C in CO2 incubator.

Linear, non-linear optical susceptibilities and the hyperpolarizability of the mixed crystals Ag(0.5)Pb(1.75)Ge(S(1-x)Se(x))4: experiment and theory.

As the starting point for a comprehensive theoretical investigation of the linear and nonlinear optical susceptibilities, we have used our experimental crystallographic data for Ag0.5Pb1.75GeS3Se (Ag2Pb7Ge4S12Se4) reported. The experimental crystallographic positions were optimized by minimizing the forces acting on each atom to get meaningful theoretical predictions of the optical properties. The linear optical susceptibilities are calculated. We find that the optical band gap shows very good agreement with our measured gap. The second-order nonlinear optical (NLO) susceptibilities dispersion namely the optical second harmonic generation (SHG) is calculated and compared with our experimental measurements. The microscopic first order hyperpolarizability, ß123, vector component along the principal dipole moment directions for the χ((2))(123)(ω) component was obtained theoretically and compared with our measured values at different temperatures. The dependence of the two-photon absorption (TPA) for the pump-probing at SHG of the microsecond CO2 laser was measured. In addition we explored the linear electro-optical effect in these crystals. This effect is described by the third rank polar tensors similarly to the SHG. However, for the Pockels effect besides the electronic contribution, the phonon subsystem also begins to play a principal role. As a consequence we study the dispersion of the linear electro-optical effects in the mentioned crystals.

Copper ion-exchanged channel waveguides optimization for optical trapping.

Optical trapping of particles has become a powerful non-mechanical and non-destructive technique for precise particle positioning. The manipulation of particles in the evanescent field of a channel waveguide potentially allows for sorting and trapping of several particles and cells simultaneously. Channel waveguide designs can be further optimized to increase evanescent field prior to the fabrication process. This is crucial in order to make sure that the surface intensity is sufficient for optical trapping. Simulation configurations are explained in detail with specific simulation flow. Discussion on parameters optimization; physical geometry, optical polarization and wavelength is included in this paper. The effect of physical, optical parameters and beam spot size on evanescent field has been thoroughly discussed. These studies will continue toward the development of a novel copper ion-exchanged waveguide as a method of particle sorting, with biological cell propulsion studies presently underway.

Photoelectrical properties and the electronic structure of Tl(1-x)In(1-x)Sn(x)Se2 (x = 0, 0.1, 0.2, 0.25) single crystalline alloys.

Photoelectrical properties of Tl1-xIn1-xSnxSe2 single crystalline alloys (x = 0, 0.1, 0.2, 0.25) grown using the Bridgman-Stockbarger method were studied. The temperature dependence of electrical and photoconductivity for the Tl1-xIn1-xSnxSe2 single crystals was explored. It has been established that photosensitivity of the Tl1-xIn1-xSnxSe2 single crystals increases with x. The spectral distribution of photocurrent in the wavelength spectral range 400-1000 nm has been investigated at various temperatures. Photoconductivity increases in all the studied crystals with temperature. Therefore, thermal activation of photoconductivity is caused by re-charging of the photoactive centers as the samples are heated. Based on our investigations, a model of center re-charging is proposed that explains the observed phenomena. X-ray photoelectron valence-band spectra for pristine and Ar(+)-ion irradiated surfaces of the Tl1-xIn1-xSnxSe2 single crystals have been measured. These results reveal that the Tl1-xIn1-xSnxSe2 single-crystal surface is sensitive to the Ar(+) ion irradiation that induced structural modification in the top surface layers. Comparison on a common energy scale of the X-ray emission Se Kß2 bands representing energy distribution of the Se 4p-like states and the X-ray photoelectron valence-band spectra was done.

Influence of replacing Si by Ge in the chalcogenide quaternary sulfides Ag2In2Si(Ge)S6 on the chemical bonding, linear and nonlinear optical susceptibilities, and hyperpolarizability.

The linear and nonlinear optical properties of Ag2In2SiS6 and Ag2In2GeS6 are calculated so as to obtain further insight into the electronic properties. The influence of using different exchange correlation potentials and the effect of replacing Si by Ge on the geometry, chemical bonding, and on the optical properties are presented. There is notable increasing in the energy band gap when moving from LDA to GGA, EVGGA then to mBJ. The effect of replacing Si by Ge atom causes a geometric change, which leads to large changes in the linear as well as the nonlinear optical susceptibilities. For the linear optical properties, it causes to increase the amplitude of the left-hand hump of ε(2)(average)(ω) as well as a small shift of the main peak to lower energies. We have evaluated ε(1)(average)(0) and find that a smaller energy gap yields a larger ε1(0) value. From the calculated refractive indices we obtained the birefringence, which is important for second harmonic generation (SHG) and optical parametric oscillation (OPO) as it is defined by the phase-matching condition. The second-order nonlinear optical susceptibilities, namely, the SHG are investigated for χ(111)(2)(ω), χ(122)(2)(ω), χ(133)(2)(ω), χ(221)(2)(ω), and χ(331)(2)(ω). We find that χ(111)(2)(ω) is the dominant component. The microscopic second order hyperpolarizability, ß111, for the dominant component χ(111)(2)(ω) was obtained. We should emphasize that replacing Si by Ge enhances the linear and nonlinear optical susceptibilities so that Ag2In2GeS6 shows higher values of the linear and nonlinear optical susceptibilities and ß111 in comparison to Ag2In2SiS6.

Second harmonic generation imaging of the deep shade plant Selaginella erythropus using multifunctional two-photon laser scanning microscopy.

BACKGROUND: Multifunctional two-photon laser scanning microscopy provides attractive advantages over conventional two-photon laser scanning microscopy. For the first time, simultaneous measurement of the second harmonic generation (SHG) signals in the forward and backward directions and two photon excitation fluorescence were achieved from the deep shade plant Selaginella erythropus. RESULTS: These measurements show that the S. erythropus leaves produce high SHG signals in both directions and the SHG signals strongly depend on the laser's status of polarization and the orientation of the dipole moment in the molecules that interact with the laser light. The novelty of this work is (1) uncovering the unusual structure of S. erythropus leaves, including diverse chloroplasts, various cell types and micromophology, which are consistent with observations from general electron microscopy; and (2) using the multifunctional two-photon laser scanning microscopy by combining three platforms of laser scanning microscopy, fluorescence microscopy, harmonic generation microscopy and polarizing microscopy for detecting the SHG signals in the forward and backward directions, as well as two photon excitation fluorescence. CONCLUSIONS: With the multifunctional two-photon laser scanning microscopy, one can use noninvasive SHG imaging to reveal the true architecture of the sample, without photodamage or photobleaching, by utilizing the fact that the SHG is known to leave no energy deposition on the interacting matter because of the SHG virtual energy conservation characteristic.

Acentric nonlinear optical 2,4-dihydroxyl hydrazone isomorphic crystals with large linear, nonlinear optical susceptibilities and hyperpolarizability.

A systematic ab initio study of the linear, nonlinear optical susceptibilities, and hyperpolarizability of noncentrosymmetric-monoclinic 2,4-dihydroxyl hydrazone isomorphic crystals (DHNPH) within density functional theory in the local density approximation (LDA), general gradient approximation (GGA), the Engel-Vosko generalized gradient approximation (EV-GGA) and modified Becke-Johnson potential (mBJ) has been performed. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence are studied. Using scissors' corrected mBJ we find a large uniaxial dielectric anisotropy (-0.56) resulting in a significant birefringence (0.61). We also find that 2,4- DHNPH possess large second harmonic generation. The calculated second order susceptibility tensor components for the static limit |χ(111)(2)(0)| and |χ(111)(2)(ω)| at λ=1.9 µm (0.651 eV) and at λ = 1.064 µm (1.165 eV) are 53, 91, and 209 pm/V, respectively. A remarkable finding, applying the scissors' correction has a profound effect on value, magnitude and sign of χ(ijk)(2)(ω). In additional we have calculated the microscopic hyperpolarizability, ß(111), vector component along the principal dipole moment directions for the dominant component. We find that the value of ß(111) equal to 47× 10(-30) esu, in good agreement with the measured value (48.2× 10(-30) esu).