Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications

Based on the arrangement of two-dimensional 'melon', we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C3N4 can only account for minor phases within the investigated synthesis products

Photophysics of polymeric carbon nitride: An optical quasimonomer

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer

An optical quasimonomer

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer

Bulk photovoltaic effect of LiNbO3 Fe and its small polaron based microscopic interpretation

Based on recent experimental evidence on the electronic and optical properties of Fe Li 2 and Nb Nb 4 in LiNbO3 Fe, both strongly determined by their small polaron character, a microscopic model is presented accounting for the main features of the bulk photovoltaic effect BPVE in this material. The relative sizes of the components of the photovoltaic tensor are explained on an atomic basis. Optical small polaron transfer from Fe Li 2 to Nb Nb 5 conduction band states and the subsequent coherent bandlike electron transport, terminated by the formation of Nb Nb 4 free small polarons within about 10 13 s, characterize the first steps of the BPVE. These free polarons, transported by thermally activated incoherent hopping, are then trapped by deeper defects such as Nb Li 5 and Fe Li 3 impurities. The model allows us to explain the strong increase of the ionization probability of Fe Li 2 and the coherent transport length with photon energy. The low mobility of the Nb Nb 4 conduction polarons appears to be the reason for the high open circuit photovoltaic fields attainable in LiNbO3. year 2011 volume 83 issue 16 page 165106 doi 10.1103 PhysRevB.83.16510

The effect of surface roughness on the determination of optical constants of CuInSe2 and CuGaSe2 thin films

To investigate the effect of surface roughness on the calculation of optical constants, e.g., the complex refractive index n ik or n; k of CuIn1 xGaxSe2 CIGSe thin films, we took CuInSe2 CISe and CuGaSe2 CGSe as examples and applied the Modified Transfer Matrix MTM method to calculate optical constants with considering the effect of scattering due to surface roughness. Compared to the Transfer Matrix TM method without considering surface roughness, it was revealed that the MTM method could improve the accuracy of calculation. The calculated refractive index values from the MTM method increase by 6.89 for CISe and 2.59 for CGSe in contrast to those from the TM method. In addition, bromine solution was confirmed via Scanning Electron Microscopy and Atomic Force Microscopy to be able to reduce the surface roughness. Calculated results from smoothened samples showed that the accuracy of calculated optical constants was further improved. Finally, optical constants calculated by the MTM method were compared to those from smoothened samples, validating that the MTM method could eliminate the influence of surface roughness on the calculation of optical constants more effectively for CGSe with low surface roughness than for CISe with high surface roughnes

Unprecedented Alkylzinc Magnesium Alkoxide Clusters as Suitable Organometallic Precursors for Magnesium Containing ZnO Nanoparticles

The synthesis and characterization of the first heterobimetallic methylzinc magnesium alkoxide clusters [Me6MgZn6 OR 8 ] [R Et 1a , nPr 1b , nBu 1c ] with a bis cubane shaped MgZn6O8 core is described. The thermal degradation of 1a c and mixtures of 1a and the homometallic MeZn alkoxide cubane [ MeZnOtBu 4 ] 2 in dry synthetic air led to wide band gap semiconducting MgxZn1 xO nanoparticles at temperatures below 500 degrees C. The final materials were characterized by different analytical techniques such as PXRD, REM, TEM, EDX, and IR spectroscopy. The morphology of the as prepared magnesium containing ZnO samples is influenced by the different organo groups alkoxo in the precursors. EDX mapping showed that magnesium is uniformly distributed in the ZnO matrix. The incorporation of magnesium led to a distortion of the ZnO lattice with increased a axis and decreased c axis parameters. Room temperature photoluminescence PL spectra reveal that the near band edge NBE emission of Mg2 doped ZnO is shifted to higher energies relative to that of pure ZnO. year 2011 volume 17 issue 14 pages 3904 3910 doi 10.1002 chem.20100274

Reply to Comment on Methylmagnesium Alkoxide Clusters with Mg4O4 Cubane and Mg7O8 Biscubane Like Cores Organometallic Precursors for Low Temperature Formation of MgO Nanoparticles with Variable Surface Defects

Reply to Comment on Methylmagnesium Alkoxide Clusters with Mg4O4 Cubane and Mg7O8 Biscubane Like Cores Organometallic Precursors for Low Temperature Formation of MgO Nanoparticles with Variable Surface Defects CHEMISTRY OF MATERIALS year 2010 volume 22 issue 15 page 4513 doi 10.1021 cm101691

Ultrafast dynamics in polymeric carbon nitride thin films probed by time resolved EUV photoemission and UV Vis transient absorption spectroscopy

The ground and excited state electronic structures of four polymeric carbon nitride PCN materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water splitting reactions, the ground state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water splitting devices. The ultrafast charge carrier dynamics of the same PCN materials were also investigated using two femtosecond time resolved pump probe techniques extreme ultraviolet EUV photoemission and ultraviolet visible UV Vis transient absorption spectroscopy. The complementary combination of these surface and bulk sensitive methods facilitated photoinduced kinetic measurements spanning the sub picosecond to few nanosecond time range. The results show that 400 nm 3.1 eV excitation sequentially populates a pair of short lived transient species, which subsequently produce two different long lived excited states on a sub picosecond time scale. Based on the spectro temporal characteristics of the long lived signals, they are assigned to singlet exciton and charge transfer states. The associated charge separation efficiency was inferred to be between 65 and 78 for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump fluence dependence of the initial excited state decay kinetics and characteristic carrier concentration effects that emerge beyond few picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice constituting motif, while the long time behaviour is dominated by charge transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic device