Fluorescence spectroscopy of electrochemically self-assembled ZnSe and Mn:ZnSe nanowires

We report room temperature fluorescence spectroscopy (FL) studies of ZnSe and Mn-doped ZnSe nanowires of different diameters (10, 25, 50?nm) produced by an electrochemical self-assembly technique. All samples exhibit increasing blue-shift in the band edge fluorescence with decreasing wire diameter because of quantum confinement. The 10?nm ZnSe nanowires show four distinct emission peaks due to band-to-band recombination, exciton recombination, recombination via surface states and via band gap (trap) states. The exciton binding energy in these nanowires exhibits a giant increase (～10-fold) over the bulk value due to quantum confinement, since the effective wire radius (taking into account side depletion) is smaller than the exciton Bohr radius in bulk ZnSe. The 25 and 50?nm diameter wires show only a single FL peak due to band-to-band electron-hole recombination. In the case of Mn-doped ZnSe nanowires, the band edge luminescence in 10?nm samples is significantly quenched by Mn doping but not the exciton luminescence, which remains relatively unaffected. We observe additional features due to Mn(2+) ions. The spectra also reveal that the emission from Mn(2+) states increases in intensity and is progressively red-shifted with increasing Mn concentration.     

Photoluminescence of ZnO nanocrystals embedded in BaF2 matrices by magnetron sputtering

This paper describes ZnO nanocrystals embedded in BaF2 matrices by the magnetron sputtering method in an attempt to use fluoride as a shell layer to embed ZnO nanocrystals core. BaF2 is a wide-band gap material, and can confine carriers in the ZnO films. As a result, the exciton emission intensity should be enhanced. The sample was annealed at 773 K, and X-ray diffraction (XRD) results showed that ZnO nanocrystals with wurtzite structure were embedded in BaF2 matrices. Raman-scattering spectra also confirmed the formation of ZnO nanoparticles. Abnormal longitudinal-optical (LO) phonon-dominant multiphonon Raman scattering was observed in the sample. Room-temperature photoluminescence (PL) spectra showed an ultraviolet emission peak at 374 nm. The origin of the ultraviolet emission is discussed here with the help of temperature-dependent PL spectra. The ultraviolet emission band was a mixture of free exciton and bound exciton recombination observed in the low temperature PL spectra (at 77 K). Abnormal temperature dependence of ultraviolet near-band-edge emission-integrated intensity of the sample was observed. The band tail state was observed in the absorption spectra, illustrating that the impurity-related defects were caused by the shell of the BaF2 grain layer. For comparison, ZnO films on BaF2 substrates were also fabricated by the magnetron sputtering method, and the same measurement methods were used.     

CdTe/CdMnTe多量子阱的生长和激子复合动力学性质的研究

用分子束外延在GaAs衬底上生长了CdTe/Cd0.8Mn0.2Te多量子 结构,利用X射线衍射（XRD）、低激发密度下的PL光谱和变密度激发的ps时间分辨光谱研究了CdTe/CdMnTe多量子阱的结构和激子复合特性。在变密度激发的ps时间分辨光谱中,发现不同激发密度下发光衰减时间不同,认为它的机理可能是无辐射复合引起的。     

The effect of geometrical misfit dislocation on formation of microstructure and photoluminescence of Wurtzite GaN/Al2O3 (0001) films grown by low pressure metal-organic chemical vapor deposition

The incoherent GaN/sapphire interface and microstructure of GaN were observed by high resolution transmission electron microscopy. The most mobile 60° mixed-type dislocation is related to a structural metastability of the Wurtzite GaN film. In spite of the same feature of interband absorption, the photoluminescence mechanism is sensitive to deep level. A strong light emission from the bound exciton of Wurtzite GaN at 358 nm was observed in an n-type GaN sample with the GaN buffer layer. The donor–acceptor pair recombination at 380 nm with LO phonon replicas at 390 and 403 nm and the deep level at 559 nm were observed in both an undoped GaN sample with GaN buffer layer and an n-type GaN sample with AlN buffer layer. This optical behavior is sensitive to the Si doping and the type of buffer layer materials. The deep level emission along the dislocation line is suggested by the local band bending model providing the potential barrier of 0.63 eV by the space charge.     

Visible luminescence mechanism of ZnO nanoparticles synthesized by sol-gel method

We presented our investigations on the absorption and emission properties of the nanocrystalline ZnO particles of different particle sizes (2 nm-5 nm) by sol-gel method. In the room temperature PL spectra, three emission bands, ultraviolet (UV), blue and green were observed. With increasing the particle sizes, both the UV and the visible emission bands shifted to lower energies progressively. From the size-dependency, there was a linear relationship between the energetic maxima of the UV and the green emission bands with a slope of about 0.26, which indicated that the green luminescence of ZnO was produced by the transitions of electrons from deep level to the valence band (or shallow acceptor level). A linear dependence was also found between the energetic maxima of the UV and the blue emissions with a slope of 0.15, the origin this blue emission band is not clear at present. While in van Dijken et al.'s paper, however, they identified only two emission bands in the emission spectra, an UV and a broad visible emission band, and the linear fit between the energetic maxima of these two bands in particles of different sizes has a slope of 0.6, so they proposed that the visible emission in ZnO was originated from the recombination of a shallowly trapped electron with a deeply trapped hole. We attributed this divergence to the fact that the broad visible band is actually composed of two separate emission bands originated from two different recombination processes, and should not had been treated as one emission band.     

Role of the deposition parameters and aging on the optical and photoluminescence properties of C70 films

We have investigated the influence of the growth parameters (substrate temperature and deposition rate) and the aging process on the optical properties of C₇₀ thin films, by means of absorption and photoluminescence spectra. The Urbach energy, obtained from the absorption spectra, indicates that the substrate temperature influences the film optical properties more than the deposition rate. The luminescence spectra suggest the important role of the disorder in the radiative efficiency. The main structures of the emission spectra have been assigned to an intramolecular polaron-exciton. The analysis of the temperature dependence of the photoluminescence spectra of the as-deposited samples shows that the vibronic transitions are dominant at low temperature, whereas the singlet purely electronic recombination (due to Frenkel-type exciton) is visible at a sufficiently high temperature. On the contrary, in the aged samples this purely electronic transition is well resolved from low to high temperature. This anomalous behaviour is discussed and attributed to the disorder introduced in the film.     

Fabrication and optical properties of mesoporous SnO2 nanowire arrays

Large-scale SnO2 mesoporous nanowires have been successfully synthesized by an improved sol-gel method within the nanochannels of porous anodic alumina templates. In this method, chloride of stannic and urea are used as precursors, chloride of stannic is acting as source of tin ions, and urea offers a basic medium through its hydrolysis. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction are used to characterize the SnO2 mesoporous nanowires. It is found that the as-prepared nanowires consist of SnO2 nanoparticles and pores. They can be indexed as rutile structures and diameters are about 50-70 nm. The growth mechanism of the mesoporous nanowires is also been discussed. The band gap of the as-prepared mesoporous nanowires is 3.735 eV, determined by UV/visible absorption spectral results. The SnO2 mesoporous nanowires show strong and stable photoluminescence with emission peak centered at 3.730 eV, which has never been reported in nanowires. It could be attributed to the exciton recombination.     

Confirmation of K-momentum dark exciton vibronic sidebands using 13C-labeled, highly enriched (6,5) single-walled carbon nanotubes

A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton-phonon coupling opens up additional absorption and emission channels, some of which may "brighten" the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare (12)C and (13)C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright (1)E(11) singlet exciton, one absorptive sideband ~200 meV above, and one emissive sideband ~140 meV below, the bright singlet exciton. Both sidebands demonstrate a ~50 cm(-1) isotope-induced shift, which is commensurate with exciton-phonon coupling involving phonons of A[Formula: see text] symmetry (D band, ω ~ 1330 cm(-1)). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ~25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of (13)C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton.     

Ultrafast carrier dynamics of near-band-edge emission in single-crystal ZnO nanorods

We report on rational synthesis and optical characteristics of highly crystallined ZnO nanorods which were grown by a facile chemical vapor transport method. Temperature-dependent photoluminescence spectra of as-fabricated ZnO nanorods are dominated by near-band-edge emission with a characteristic fine structure due to high crystallinity. Furthermore, the recombination emission involving carrier dynamics of near-band-edge emission in ZnO nanorods was systematically investigated by temperature-dependent time-resolved photoluminescence spectroscopy. Recombination peaks pertaining to the exciton emissions are monitored and resolved in both temporal and spatial regimes.     

The effect of neutron irradiation on the exciton absorption in gallium arsenide

We have studied the effect of neutron irradiation on the exciton absorption in n-GaAs crystals. It is shown that the observed decrease in the absorption coefficient, broadening of the exciton peak, and its shift toward higher energies are caused by the electric and strain (compression) fields generated by the radiation-induced defects.     

Random conjugated polybenzazole copolymers: Synthesis, characterization, and exciton confinement effects in photophysical properties

New conjugated random copolymers of poly(p-phenylene benzobisoxazole) (PBO) and poly(2,5-benzoxazole) (ABPBO) with various compositions have been synthesized and characterized by Fourier transform infrared (FTIR), wide-angle X-ray diffraction (WAXD), UV-absorption, and photoluminescence spectra. The side-to-side distance between two neighboring PBO-ABPBO chains can be modulated by the composition ratio. The absorption, excitation, and emission peaks for the copolymers in methanesulfonic acid can also be regulated by the copolymer composition from the homopolymer ABPBO solution to homopolymer PBO solution. However, the emission peaks of copolymers in solid state are quite different from those of their homopolymers. The copolymers showed blue shifted, structured emission spectra centered with higher quantum efficiency at 470 nm compared with the parent homopolymer PBO, indicating the effects of exciton confinement in the random copolymers. The relationship between the structure and properties for the random copolymers can be interpreted roughly by the sequence length distribution of the different segments estimated from the copolymerization statistics.     

Optical spectroscopy of single Cd0.6Zn0.4Te/ZnTe quantum dots on Si substrate

Microphotoluminescence (μ-PL) measurements were carried out to investigate the optical properties of single Cd_0.6Zn_0.4Te/ZnTe quantum dots (QDs) grown on Si (001) substrate by using molecular beam epitaxy. The high quality of single Cd_0.6Zn_0.4Te/ZnTe QDs is witnessed by resolution-limited emission, negligible background and absence of measurable spectral jitter or blinking. Polarization-dependent and power-dependent μ-PL spectroscopy measurements were performed to identify the exciton, the biexciton, and the charged exciton in the emission spectra of single QDs. Furthermore a weak linearly polarized line is observed on the low energy side of the neutral exciton and is ascribed to dark exciton recombination.     

Layer-structured ZnO nanowire arrays with dominant surface- and acceptor-related emissions

We report the growth of uncommon layer-structured ZnO nanowire arrays via metal-organic chemical vapor deposition (MOCVD). The morphology, microstructure, and photoluminescence (PL) of the nanowires are investigated. The nanowires grow along the [0001] direction, with periodic zig-zag edges formed by the {101?1}-type surfaces. The nanowires exhibit unique PL features. The PL spectra at low temperature are dominated by the surface exciton recombination at 3.366 eV and the controversial 3.32 eV emission. For the 3.32 eV emission, transformation from donor-acceptor pair recombination to free electron-to-acceptor transition is observed with increasing temperature. The stacking faults formed in the interface region between the layers are likely responsible for the strong emission around 3.32 eV.     

Minority and majority carrier traps associated with oxidation induced stacking faults in silicon

Abstract

Majority and minority traps associated with oxidation induced stacking faults (OISFs) have been investigated by deep level transient spectroscopy and minority carrier transient spectroscopy. Electron and hole traps have been characterised in nand p type Si, and the activation energies of all extended defect related traps are found to be dependent on the occupancy of the state associated with the extended defect. Majority and minority carrier traps in n type Si exhibit non-exponential trap filling, which indicates the presence of a significant electrostatic barrier around the OISF. The electrical properties of hole (minority) traps measured by minority carrier transient spectroscopy in n type Si are found to be different from the deep level transient spectroscopy signature of hole (majority) traps in p type Si, and this is explained by examining differences between conditions during the measurements. By examining separately the electron and hole capture properties of OISF related traps, one particular trap can be identified as a recombination centre. The capture cross-section of the OISF related hole trap in n type Si has been measured and it was found that, at low occupancy, the trap capture cross-section is 7 × 10^-14 cm².     

Optical emission and absorption spectra of Zn–ZnO core-shell nanostructures

The core-shell Zn–ZnO nanostructures were fabricated from Zn-powder embedded in graphite (i.e. carbon matrix) in a thin-films form by an inexpensive vacuum arc technique followed by laser ablation. The grazing incidence X-ray diffraction pattern shows that intensity of Zn-peak decreases, and subtle ZnO-peak increasing with the increase in laser power. The high resolution transmission electron microscopic study clearly exhibits the formation of a core-shell nanostructure as fabricated by laser ablation. The emission characteristics of laser ablated (with different powers) samples show a strong exciton peak at 388 nm, and a few more weak peaks (due to weak defect states in the visible range). The optical absorption spectra were obtained from the excitonic peaks (from 344 nm to 317 nm) on decreasing laser power. These peaks occur due to the coupling of exciton absorption (from ZnO shell layer) and core metal interband absorption. The Zn–ZnO core-shell nanostructure is useful for nanophotonic applications.     

Synthesis and photoluminescence of ZnO nanowires/nanorods

We report growth of ZnO nanowires on various substrates using a vapour phase transport method and show that the growth mechanism is vapour-liquid-solid growth. We present photoluminescence data for samples grown on a-plane sapphire at room and low temperatures indicating that the optical quality of these structures is potentially excellent, with intense emission and narrow bound exciton linewidths. The intensity decays rapidly with increasing temperature, indicating a strong temperature-activated non-radiative mechanism whose origin is unclear. We observe a high energy excitonic emission close to the band edge which we assign to the “surface” exciton in ZnO at ∼3.368 eV. This assignment is consistent with the large surface to volume ratio of the nanowire systems under consideration and also indicates that this large ratio has a significant effect on the luminescence even at low temperatures. These surface effects may also be responsible for the rapid decay of the luminescence with increasing temperature via a temperature-activated surface recombination. The nanowire systems appear to offer the prospect of extremely efficient excitonic emission for device applications, and we note that one of the important aspects of achieving this potential will be control of the surface effects via passivation or other means.     

Efficient biexciton emission in elongated CdSe/ZnS nanocrystals

We use a combination of low-temperature magneto-optical and lifetime spectroscopies to study the band-edge exciton fine structure of highly photostable single CdSe/ZnS nanocrystals (NCs). Neutral NCs displaying multiline emission spectra and multiexponential photoluminescence (PL) decays are studied as a function of temperature and external magnetic fields. Three different fine structure regimes are identified as a function of the NC aspect ratio. In particular, we identify an optically inactive ground exciton state, whose oscillator strength is tuned up under magnetic field coupling to bright exciton states, and attribute it to the zero angular momentum ground exciton state of elongated NCs. We also show evidence for highly efficient biexciton emission in these NCs, with radiative yields approaching unity in some cases.     

Nonradiative electron–hole recombination in ZnSSe epitaxial layers examined by piezoelectric photothermal spectroscopy

Piezoelectric photothermal (PPT) and photoluminescence (PL) spectra could be successfully observed at low temperatures for nondoped ZnSe and ZnS_0.08Se_0.92 epitaxial layers grown by molecular beam epitaxy. The donor bound exciton (I₂) and free-to-acceptor (FA) emission bands were dominant in the PL spectra of both samples. A dominant acceptor-type defect on the FA emission might not be due to S-related defects because the PL emission from this defect was observed in both the ZnSe and ZnSSe samples. Furthermore, this acceptor-type impurity behaved as both electron–hole nonradiative and radiative recombination centers because the distinct signals appeared in the PPT and PL spectra of the ZnSSe sample.     

Acceptor and surface states of ZnO nanocrystals: a unified model

Semiconductor nanocrystals have the potential for a range of applications in optoelectronics and nonlinear optics. As the surface-to-volume ratio increases, surface emission processes become more important. Using infrared (IR) and photoluminescence (PL) spectroscopy, we have developed a unified model for the acceptor and intragap surface states of ZnO nanocrystals. A PL peak was observed at 2.97?eV, in agreement with an acceptor level previously observed in the IR (Teklemichael et al 2011 Appl. Phys. Lett. 98 232112). The temperature dependence of the IR absorption peaks, which correspond to a hole binding energy of 0.46?eV, showed an ionization activation energy of only 0.08?eV. This activation energy is attributed to thermal excitation of the hole to surface states 0.38?eV above the valence band maximum. Therefore, while the acceptor is deep with respect to the bulk valence band, it is shallow with respect to surface states. A strong red PL emission centered at 1.84?eV, with an excitation onset of 3.0?eV, is attributed to surface recombination.     

Electric-field effect of near-band-edge photoluminescence in bulk ZnO

We have studied the effect of electric fields on the near-band-edge (NBE) emissions in bulk zinc oxide (ZnO) by using photoluminescence and photocurrent (PC) spectroscopy simultaneously. The intensity-quenching and peak-shift effects of the free exciton spectra were observed with increasing electric field. From the PC result, we find out that the free excitons are disturbed by the PC carriers of the photo-created electrons and holes. This disturbance reduces the recombination ratio and the lifetime of free excitons. Therefore, the intensity-quenching effect was attributed to the decrease in the recombination of free excitons. Thus, the shift of the free exciton peaks was related to Stark effect induced by electric field. As a result, we have found that these phenomena are caused to the exciton–electron scattering due to a strong interaction between the excitons in the conduction band and the photo-generated electron carriers with increasing the applied electric field.