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Temperature dependence of the energy gap of several alloys. The lines represent fits of the experimental data using the Varshni formula.
The temperature dependence of the energy gap of zinc‐blende CdSe and Cd 1-x Zn x Se has been determined over the entire range of composition from optical transmission and reflection measurements at temperatures between 5 and 300 K. The experimental results can be expressed by the following modified empirical Varshni formula, whose parameters are fu...
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... J Fundam Appl Sci. 2016, 8(3S), ...
... The narrow peaks result from free exciton emission and are close to the absorption edge of the obtained CdSe. For hexagonal CdSe, the near-band-edge (NBE) emission at room temperature is about 1.738 eV i.e., ∼716 nm . Therefore, the emission peak at 702 nm in samples is attributed to the NBE emission of hexagonal CdSe and sample CdSe 8 -400 is a typical hexagonal CdSe thin film. ...
... The experimentally observed values for the shift indicated an alloying in nanocrystalline PbS. Such increase has been observed [17,18]. is not observed in these doped films, presumably because of complete mixing of PbS with Hg 2+ affording a unique ternary intermetallic compound of the Pb Hg 1− S type . It is observed that the size effect on the optical band gap is stronger in nanoparticle films than in PbS nanoparticle of 24-10 nm (average crystallite size) with from 2.22 to 2.65 eV . ...
Nanocrystalline PbS thin films were prepared by Chemical Bath Deposition (CBD) at 40 ± 2°C onto glass substrates and their structural and optical properties modified by in-situ doping with Hg. The morphological changes of the layers were analyzed using SEM and the X-rays spectra showing growth on the zinc blende (ZB) face. The grain size determined by using X-rays spectra for undoped samples was found to be ~36 nm, whereas with the doped sample was 32–20 nm. Optical absorption spectra were used to calculate the , showing a shift in the range 1.4–2.4 eV. Raman spectroscopy exhibited an absorption band ~135 cm−1 displaying only a PbS ZB structure.
... The band diagram along the 40 nm NW, which is shown below the TEM image in , and an effective mass of 0.13m e for electrons, and 0.45m e for holes were used . ...
... From the edges of the spectra the band-gaps for WZ and ZB can be estimated to be E ZB = 1.72eV and E WZ = 1.84 eV. They are labeled in the figure and agree with low temperature band-gaps for WZ and ZB in Ref.. ...
Using high-spatial-resolution time-resolved and temperature-dependent spectroscopy, this dissertation investigates emission properties of CdSe nanowires (NWs) synthesized using solid-liquid-solid (colloidal) growth. Temperature-dependent near-field scanning optical microscopy (NSOM) achieved spatial resolution of ˜50 nm and imaged single NWs having wurtzite(WZ)-zincblende(ZB) polytype structure. Temperature-dependent NSOM and micro-photoluminescence (microPL) spectra produce s-shape peak emission energy dependence on temperature, which reveal the formation of band-tail states in the NWs due to the variation of the polytype structure along the length of the NW. Time-resolved photoluminescence (TRPL) measures emission decay data that shows type-I band alignment between the WZ-ZB heterojunction due to the spectral dependence of the emission decay: the decay times decrease as the energy decreases. Finally, a robust rate equation model was completed for a single NW that produces theoretical calculations for the measurements.
Most high-quality quantum dots (QDs) are synthesized in the organic phase, and are often coated with polymers for use in aqueous biological environments. QDs can exhibit fluorescence losses during phase transfer, but evaluating underlying mechanisms (e.g., oxidation, surface etching, loss of colloidal stability) can be challenging because of variation in synthesis methods. Here, fluorescence stability of QDs encapsulated in block co-polymer (BCP) micelles was investigated as a function of BCP terminal functionalization (i.e., -OH, -COOH, and -NH2 groups) and synthesis method (i.e., electrohydrodynamic emulsification-mediated selfassembly (EE-SA), sonication, and manual shaking). Fluorescence losses, fluorescence intensity, energy spectra, and surface composition were assessed using spectrofluorometry and cathodoluminescence spectroscopy (CL) with integrated X-ray photoemission spectroscopy (XPS). QDs passivated using charged BCPs exhibited 50-80% lower fluorescence intensity than those displaying neutral groups (e.g., -OH), which CL/XPS revealed to result from oxidation of surface Cd to CdO. Fluorescence losses were higher for processes with slow formation speed, but minimized in the presence of poly(vinyl alcohol) (PVA) surfactant. These data suggest slower BCP aggregation kinetics rather than electrostatic chain repulsion facilitated QD oxidation. Thus, polymer coating method and BCP structure influence QD oxidation during phase transfer and should be selected to maximize fast aggregation kinetics.
Temperature-dependent infrared transmission spectroscopy has been applied to study the optical properties of Hg0.815Cd0.185Se thin film grown by molecular beam epitaxy on GaSb (2 1 1)B substrate. Discrepancies were observed between the measured optical bandgaps and the calculated values based on the previously reported empirical expression for the energy bandgap Egx,T of Hg1-xCdxSe. A new empirical expression has been established based on ab initio calculations, and the results are shown to be in a good agreement with the experimental data of this study. By comparing both expressions with all of known experimental data for x ≤ 0.354 from other studies over the temperature range of 0 < T < 310 K, it is found that for the new expression presented in this study, the standard error between calculations and experimental data is 0.010 eV which is comparable to that of the previously reported expression for Hg1-xCdxSe with 0.194 ≤ x ≤ 0.354 and is approximately 50% more accurate for 0 ≤ x ≤ 0.19.
Quantum dot (QD)‐based light‐emitting materials are gaining increased attention because of their easily tunable optical properties desired for various applications in biology, optoelectronics, and photonics. However, few methods can be used to manufacture volumetric materials doped with more than one type of QD other than QD‐polymer hybrids, and they often require complicated preparation processes and are prone to luminescence quenching by QD aggregation and separation from the matrix. Here, simultaneous doping of a volumetric glass‐based nanocomposite with two types of QDs is demonstrated for the first time in a single‐step process using the nanoparticle direct doping method. Glass rods doped with CdTe, CdSe/ZnS, or co‐doped with both QDs, are obtained. Photoluminescence and lifetime experiments confirm temperature‐dependent double emission with maxima at 596 and 720 nm with mean lifetimes up to 16 ns, as well as radiative energy transfer from the short wavelength–emitting QDs to the long wavelength–emitting QDs. This approach may enable the simple and cost‐efficient manufacturing of bulk materials that produce multicolor luminescence with cascade excitation pumping. Applications that could benefit from this include broadband optical fiber amplifiers, backlight systems in LCD screens, high‐power LEDs, or down‐converting solar concentrators used to increase the efficiency of solar panels.
Much interest has recently been focused on Znx Cd1-x Se alloy because of its potential applications in optoelectronic devices in the blue-green spectral region , Bulk Znx Cd1-x Se alloy crystallizes either in the cubic zinc-blende structure (x>0.7), in the hexagonal wurtzite structure (x>0.5), or in mixture of these two structures for 0.5≤x≤0.7 . It has, however, been reported that the heteroepitaxy of Znx Cd1-xSe on GaAs(100) by MBE results in single-phase zinc-blende crystals over the entire composition range from ZnSe to CdSe . High-quality Znx Cd1-x Se (X=0.53) epilayer can also be grown on (lOO)InP substrate lattice-matched by MBE .
Cadmium selenide (CdSe) is a member of the II–VI semiconductor compounds. At normal temperature and pressure it crystallizes in the hexagonal, wurtzite lattice. CdSe can also crystallize in the cubic, zinc-blende modification (c-CdSe), although it is me-tastable under normal conditions. By using the technique of MBE and suitable substrates it is now possible to grow cubic CdSe relatively easily . Although the blue-green lasing action has been observed in II–VI alloy system Zn1-x Cdx Se/ZnSe [2,3], there is relatively little work on the electronic and optical properties of the end-pint binary c-CdSe itself.
In this paper we investigate the characteristics of ZnSe-based cw-laser diodes under high current injection in cw- and pulsed mode. The maximum output power is 75 mW in cw-mode and 294 mW in pulsed mode. From the shift of the emission wavelength between the two modes a temperature change of the active region of 9 K is calculated. An increase of the operating current up to twice the threshold current raises the temperature only about 20 K. Still, when operated with moderate output powers, the devices degrade via the accumulation of heat. However, at high output powers we found that a catastrophic optical damage process is responsible for device failure. This has not been reported in literature before for ZnSe-based laser diodes.