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Transmission solid line and reflection dotted line spectra at 300 K of a Cd 1x Zn x Se layer with x53% left axis and the squared absorption coefficient 2 dashed line, right axis. An extrapolation to 2 0 leads to an estimation of the energy gap E g , which is indicated by the arrow. 

Transmission solid line and reflection dotted line spectra at 300 K of a Cd 1x Zn x Se layer with x53% left axis and the squared absorption coefficient 2 dashed line, right axis. An extrapolation to 2 0 leads to an estimation of the energy gap E g , which is indicated by the arrow. 

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The temperature dependence of the energy gap of zinc-blende CdSe and Cd 1x 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 fun...

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... T is the transmission. The refractive index of glass s and the refractive index of the layer n were assumed to be constant in the region of strong absorption. The refractive index n and the thickness d of the layer were estimated from the reflection spectra. Assuming parabolic band structure, the absorption coefficient is proportional to EE g 0.5 and an extrapolation to 2 0 yields a good approximation of the energy gap E g . Figure 1 shows the transmission and reflec- tion spectra and the squared absorption coefficient of a Cd 0.47 Zn 0.53 Se layer at 300 K. The energy gap is indicated by an arrow. The variation of the band-gap energy with compo- sition at temperature T is conventionally described by the quadratic ...

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... When comparing temperature dependence of maximum emission energy for NBP:CdSe/ZnS(0.3) and NBP:CdTe composites with bandgap energy of bulk CdSe [19] and CdTe [20] (Figure 5), it can be noticed that in the case of both QD-doped materials it changes with increasing temperature faster than bandgap energy of a bulk semiconductor. It indicates that the emission originates from a quantum structure rather than from a solid material, that would be created if the QDs had melted into the glass matrix. ...
... Combining QDs with metallic nanoparticles exhibiting plasmonic properties could lead to strong PL enhancement [25,26] or lasing. [27,28] Additionally, as the NPDD method can allow the use of particles of different shapes, asymmetric nanoparticles exhibiting multiple [19] and CdTe. [20] The slopes of calculated linear regression are more steep for QD-doped composite materials. ...
Article
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.
... As reported by others [18,27], the bandgap shows a nonlinear dependence on composition x, which was fitted by Eg(x) = Eg(CdSe) + (Eg (ZnSe)-Eg(CdSe)-b)x + bx 2 , where b is the bowing parameter. The least square fit yields b = 0.55 eV, which is slightly larger than that of bulk (0.41~0.48 eV), but lower than that obtained by Yoon (0.79 eV) [18,31,35]. As we know, bowing parameter b in a mixed crystal of A x B 1-x C reflects their miscibility between AC and BC [30,36,37]. ...
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ZnO/Zn x Cd1-x Se coaxial nanowires (NWs) have been successfully synthesized by combining chemical vapor deposition with a facile alternant physical deposition method. The shell composition x can be precisely tuned in the whole region (0 ≤ x ≤ 1) by adjusting growth time ratio of ZnSe to CdSe. As a result, the effective bandgaps of coaxial nanowires were conveniently modified from 1.85 eV to 2.58 eV, almost covering the entire visible spectrum. It was also found that annealing treatment was in favor of forming the mixed crystal and improving crystal quality. An optimal temperature of 350°C was obtained according to our experimental results. Additionally, time resolved photo-luminescence spectra revealed the longest carrier lifetime in ZnO/CdSe coaxial nanowires. As a result, the ZnO/CdSe nanowire cell acquired the maximal conversion efficiency of 2.01%. This work shall pave a way towards facile synthesis of ternary alloys for photovoltaic applications.
... It has been investigated experimentally that the band gap reduces with the increase of temperature for bulk semiconductors [28][29][30]. On the other hand, the temperature dependence of band gap has been experimentally determined as [31][32][33] ...
... For our simulation purposes, we have calculated the band gap energy vs. temperature, E T g( ) , for CdSe and CdS QDs using Eq. (7) and values of E g 0 K°( ) , α, β reported in [28,29] as shown in table 1. Also, for CdTe QD and TiO 2 , ZnO, SnO 2 MOs, we have calculated E T g( ) using the relations reported in [30,[34][35][36][37]. Table 2 shows values of the effective masses of electron and hole, the relative permittivity, and the conduction band edge vs. vacuum, for three QDs and three MOs. ...
Article
Electron transfer rate from quantum dot (QD) to metal oxide (MO) in quantum dot sensitized solar cells (QDSSCs) has an important role in the efficiency. In this work, we analyse the electron transfer rate from CdSe, CdS and CdTe QDs to TiO2, ZnO and SnO2 MOs by extending the related equations with considering various effects, based on the Marcus theory. In this regard, the effects of QD diameter, QD-MO spacing, the crystalline defects, temperature, and the reorganizational energy, on the electron transfer rate are investigated. The results show that, the maximum electron transfer rate is achieved for CdTe QD with the mentioned three MOs. Moreover, in order to direct the designer to reach the appropriate QDs-MOs combinations for obtaining the maximum electron transfer rate, the average electron transfer rate for various combinations is calculated. For the verification of simulation method, a part of work has been compared with the previous experimental and theoretical results, which indicates the correctness of our simulation algorithm.
... Da in dieser Arbeit sehr kleine Strukturen (< 4 nm) betrachtet werden, ist davon auszugehen das sowohl CdSe als auch ZnS in Zinkblendestruktur vorliegen ([36], [37])[39]. betragen E g, ZnS = 3.68 eV [40] und E g, CdSe = 1.66 eV [41]. Werden die Strukturen kleiner und die Materialien entsprechend kombiniert, wie im Falle eines CdSe-oder CdSe/ZnS-Quantenpunktes, kann die Bandlücke signifikant abgesenkt oder angehoben werden, sowie die Exzitonenbindungsenergie durch den Ladungsträgereinschluss erhöht werden. ...
Article
The knowledge of the electronic and optical coupling as well as electron transfer across the internal interfaces of hybrid nanostructures opens possibilities to specifically tailor their microscopic transport and luminescence processes by combining specific properties of different organic and inorganic material systems. In this thesis, two different hybrid systems are studied, ZnO nanowire/CdSe quantum dot structures and nanostructures coated with p-type polymers. First, CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study luminescence dynamics and electron tunneling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. After linking the QDs to the ZnO NW surface, photo-induced electron tunneling from an excited state of the QD into the conduction band of the nanowire becomes visible by a clear decrease of the PL decay time of the QDs. By comparing the PL transients of QDs in solution with those of QDs linked to ZnO NWs, the photo-induced electron transfer (PET) process between excited states of the QD and the nanowire is demonstrated and discussed in the frame of a rate equation model. Efficient electron tunneling is confirmed by a strong enhancement of the photocurrent through the functionalized nanowires. The tunneling rate can be controlled by using different organic linker molecules. Surface functionalization of ZnO nanostructures by QD systems with different QD sizes and surface modifications will lead to hybrid solar cells with high absorption over a wide spectral range, and a high energy conversion efficiency. Secondly, the coating of ZnO nanowires and GaN microrods with p-conductive polymers (polypyrrole, poly(3,4-ethylenedioxythiophene)) is analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and photoluminescence spectroscopy. For the fabrication of hybrid ZnO/polymer and GaN/polymer core-shell nanostructures, oxidative chemical vapor deposition (oCVD) is used. oCVD, compared to wet-chemical processes, is a completely solventless, dry process where both, the oxidizing agent and the monomer are provided in the gaseous phase. The thickness and homogeneity of the polymer coating depend on the amount of the oxidizing agent (here FeCl3), the substrate and the substrate temperature. With oCVD deposition a 15nm thin and homogeneous polypyrrole layer on the ZnO nanowire surface is demonstrated, whereas, the primary optical properties of ZnO are not affected. A controlled deposition of the polymer shell with a thickness control in the nanometer range is required to tailor the electronic and optical properties. This offers a huge potential for the realization of efficient light-emitting devices
... The shift magnitude is of about 100 meV for the I X1 band, 57 meV for the I QD band and 50 meV for the I X2 band. The red-shift of the I QD band agrees with CdSe band gap shrinkage [15,16], while the I X1 band shifts to the red much stronger and similarly to the band gap change of bulk CdS [17]. The temperature-dependent increase of the FWHM of the I QD band is apparently determined by the exciton scattering with acoustic and LO phonons [15,18,19]. ...
... The PL spectra in the range of the I X band (solid curves) and their fitting with two Gaussian lines corresponding to the I X1 and I X2 bands (dashed lines) at 83 and 298 K (a), temperature dependences of the PL intensity (b), blue-shift of PL peak position (c) and FWHM (d) for the I QD band (circles), I X1 (open triangles) and I X2 (filled triangles) components of the I X band; λ exc ¼325 nm. The dashed lines in (c) show change of band gap energy with temperature for bulk CdSe[16] and CdS[17]. Room-temperature micro-Raman spectra of bio-conjugated QDs under excitation with different lines of Ar-Kr laser: 514.5 nm (curve 1), 488.0 nm (curve 2) and 457.9 nm (curve 3). ...
... The experimentally observed E g values for the shift indicated an 8 alloying between nanocrystalline PbS. Such increase has been observed by other authors [19,20]. The E g for doped samples in the 2.4-3.8 ...
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The growth of nanocrystalline PbS films by chemical bath deposition (CBD) onto glass at temperature 𝑇=20±2∘C is presented in this research. We report on the modification of structural, optical, and electrical nanostructures due to in situ Ni-doping. The morphological changes of the layers were analyzed using SEM, AFM, and TEM. XRD spectra displayed peaks at 2θ = [26.00, 30.07, 43.10, 51.00, 53.48], indicating growth on the zinc blende face. The grain size determined by X-rays diffraction of the undoped samples was ∼36 nm, whereas with the doped sample was 3.2–5 nm. By TEM, the doped PbS was found crystalline films in the range 3.5–5 nm. Optical absorption (OA), and forbidden bandgap energy (𝐸𝑔) shift disclose a shift in the range 2.1–3.8 eV. Likewise, the dependence of 𝐸𝑔 with the radius size and interplanar distance of the lattice is discussed. Raman spectroscopy (RS) exhibited an absorption band ∼135 cm−1 displaying only a PbS ZB structure. The thermal energy for the films was determined from the slope of dark conductivity (DC) and the energy was estimated to be 0.15 to 0.5 eV.
... The technological interest in polycrystalline-based devices is mainly caused by their very low production costs. Different researchers [8][9][10] prepared Cd 1x Zn x Se films by different techniques and studied their structural, optical and photoelectrochemical properties. In the present study we have prepared Cd 1x Zn x Se films by electron beam evaporation technique at 100 o C with various zinc content incorporated with cadmium as x 0.2, 0.4, 0.6 and 0.8 and their structural, optical and surface morphological properties were studied. ...
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Cd 1 – x Zn x Se films with different zinc content were deposited by electron beam evaporation technique onto glass substrates for the application of solid-state photovoltaic devices. The structural, surface morphological and optical properties of Cd 1 – x Zn x Se films have been studied in the present work. The host material, Cd 1 – x Zn x Se, have been prepared by the physical vapor deposition method of electron beam evaporation technique (PVD: EBE) under a pressure of 1 10 – 5 mbar. The X-ray diffractogram indicates that these alloy films are polycrystalline in nature, hexagonal structure with strong preferential orientation of the crystallites along (002) direction. Linear variation of lattice constant with composition (x) is observed. The optical properties shows that the band gap (E g) values varies from 2.08 to 2.64 eV as zinc content varies from 0.2 to 0.8. The surface morphological studies show the very small, fine and hardly distinguishable grains smeared all over the surface. It is observed that the grain size is decreasing with increasing zinc content.
... La différence de structure induit des différences de propriétés optiques et électroniques. En effet, les écartements de bandes ne sont pas les mêmes pour les deux structures et on peut noter, entre autres, que les largeurs de bandes interdites diffèrent : 1.74 eV [16] en wurtzite et 1.66 eV [16][17][18] en zinc blende. Dans la structure wurtzite, il y a une dégénérescence en k=0 due au champ cristallin, lui-même induit par l'élongation suivant l'axe c. ...
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We have determined the local pressure in CdS/ZnS nanocrystals, thanks to the manganese phosphorescence signal. A few dopant atoms per nanoparticle were placed at controlled radial positions in a ZnS shell formed layer by layer. The experimental pressure measurements are in good agreement with a simple spherically symmetric elastic continuum model. Using spherically symmetric elastic continuum model could be used to better understand some structural phenomena observed in these nanocrystals, such as changes in crystalline phases, or cracking of some shells and could be used to design better core/shell nanoparticles. In a second step, we developed the colloidal synthesis of CdSe, CdS and CdTe quantum wells. The thicknesses of these nanoparticles are tuned at the atomic level and they present some new physical properties. We cite, in particular, their emission with a full width half maximum of the order of kT at room temperature. Finally, we show that it is possible to laterally extend these nanoparticles and we used the k.p model applied to quantum wells to determine the real values of nanoplatelets thicknesses and to verify the physical parameters for the three materials.
... The technological interest in polycrystalline-based devices is mainly caused by their very low production costs. Different researchers [16][17][18][19] prepared Cd 1−x Zn x Se films by different techniques and studied their structural, optical and photoelectrochemical properties. In the present study we have prepared Cd 1−x Zn x Se films by electron beam evaporation technique at 100 • C with various zinc content incorporated with cadmium as x = 0.2, 0.4, 0.6 and 0.8 and their structural, optical and surface morphological properties were studied. ...
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Cd1−xZnxSe films with different zinc content were deposited by electron beam evaporation technique onto glass substrates for the application of solid-state photovoltaic devices. The structural, surface morphological and optical properties of Cd1−xZnxSe films have been studied in the present work. The host material, Cd1−xZnxSe, has been prepared by the physical vapor deposition method of electron beam evaporation technique under the pressure of 1 × 10 −5 mbar. The X-ray diffractogram indicates that these alloy films are polycrystalline in nature, of hexagonal structure with strong preferential orientation of the crystallites along (0 0 2) direction. Linear variation of lattice constant with composition (x) is observed. Surface roughness measured by atomic force microscopy is used to estimate the interface roughness. The optical properties show that the band gap (Eg) values vary from 2.08 to 2.64 eV as zinc content varies from 0.2 to 0.8. The surface morphological studies show the very small, fine and hardly distinguishable grains smeared all over the surface. The material properties would be altered and excellently controlled by adiusting the system composition x.
... The reported room-temperature band gap of wurtzite CdSe is 1.738 eV, and that of zinc-blende CdSe is 1.66 eV. 18,19 This energy difference between wurtzite and the zinc-blende structures is only a few millivolts per atom, so switching between the two growth modes can occur during the growth. Factors that affect the preference for cubic or hexagonal phase include temperature, concentration of precursors, and the presence of coordinating ligands. ...
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We exploit the polytypism of group II-VI semiconductors and the long-range dipolar interactions typical of CdSe nanoparticle formation to modulate the geometrical structure and the optical emission properties of novel branched CdSe nanocrystals through shape-dependent quantum confinement effects. X-ray diffraction confirms that these materials incorporate crystalline domains of cubic zinc-blende and hexagonal wurtzite within a polycrystalline growth form whose geometry can be controlled by varying thermodynamic conditions. In particular, labyrinthine-shaped nanoparticles of tunable dimensions are reproducibly synthesized based on a heterogeneous reaction between cadmium acetate in a solution in hexadecylamine and trioctylphosphine with Se as a solid precursor at a relatively low temperature (110 degrees C). The resulting highly branched CdSe structures resemble labyrinthine patterns observed in magnetic fluids and superconductors films in magnetic fields, and in lipid films and other materials where strong dipolar interactions "direct" large-scale pattern formation. Surprisingly, these novel maze-like structures emit light within a narrow bandwidth (full-width at half-maximum approximately equal to 33-42 nm) of the visible spectrum (508 nm < lambda < 563 nm), so the regular dimensions of the core regions of these branched structures govern their emission characteristics rather than overall nanoparticle size. This property should make these materials attractive for applications where luminescent materials having tunable emission characteristics and a narrow emission frequency range are required, along with the insensitivity of the particles' luminescent properties to environmental conditions.