Chemical substitution of Cd ions by Hg in CdSe nanorods and nanodots: Spectroscopic and structural examination

The chemical substitution of cadmium by mercury in colloidal CdSe quantum dots (QDs) and nanorods has been examined by absorption, photoluminescence and Raman spectroscopy. The crystalline structure of original CdSe QDs used for Cd/Hg substitution (zinc blende versus wurtzite) shows a strong impact on the optical and structural properties of resultant Cd_xHg_1−xSe nanocrystals. Substitution of Cd by Hg in isostructural zinc blende CdSe QDs converts them to ternary Cd_xHg_1−xSe zinc blende nanocrystals with significant NIR emission. Whereas, the wurtzite CdSe QDs transformed first to ternary nanocrystals with almost no emission followed by slow structural reorganization to a NIR-emitting zinc blende Cd_xHg_1−xSe QDs. CdSe nanorods with intrinsic wurtzite structure show unexpectedly intense NIR emission even at early Cd/Hg substitution stage with PL active zinc blende Cd_xHg_1−xSe regions.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd1?xMnxSe

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Dark-red-emitting CdTe0.5Se0.5/Cd0.5Zn0.5S quantum dots: Effect of chemicals on properties

CdTe_0.5Se_0.5/Cd_0.5Zn_0.5S core/shell quantum dots (QDs) with a tunable photoluminescence (PL) range from yellow to dark red (up to a PL peak wavelength of 683 nm) were fabricated using various reaction systems. The core/shell QDs created in the reaction solution of trioctylamine (TOA) and oleic acid (OA) at 300 °C exhibited narrow PL spectra and a related low PL efficiency (38%). In contrast, the core/shell QDs prepared in the solution of 1-octadecene (ODE) and hexadecylamine (HDA) at 200 °C revealed a high PL efficiency (70%) and broad PL spectra. This phenomenon is ascribed that the precursor of Cd, reaction temperature, solvents, and ligands affected the formation process of the shell. The slow growth rate of the shell in the solution of ODE and HDA made QDs with a high PL efficiency. Metal acetate salts without reaction with HDA led to the core/shell QDs with a broad size distribution.     

CdSe/Cd1−x Zn x S core/shell quantum dots with tunable emission: growth and morphology evolution

We demonstrate an organic synthesis to fabricate hydrophobic core/shell CdSe/Cd_1?x Zn _x S quantum dots (QDs) with tunable photoluminescence (PL) between green and red at relatively low temperature using trioctylphosphine S reacted directly with cadmium and zinc acetate. A seeded growth strategy was used for preparing large CdSe cores. Large CdSe cores revealed a rod-like morphology while small one exhibited a spherical shape. Being coated with a Cd_1?x Zn _x S shell on spherical CdSe cores with an average size of 3.9 nm in diameter, core/shell QDs exhibited a cubic morphology (a length of 5 nm). In contrast, the core/shell QDs created using a small core (3.3 nm in diameter) show a spherical morphology. Namely, the anisotropic aggregation behavior of CdS monomers on CdSe cores occurs when the rod-like core is coated with a Cd_1?x Zn _x S shell. CdS interlayer plays an important role for such morphology evolution because all CdSe cores with a pure ZnS shell exhibited a spherical morphology. The PL properties of CdSe/Cd_1?x Zn _x S core/shell QDs depended strongly on the size and morphology of the cores. The QDs revealed a narrow and tunable PL spectrum. It is believed that this facile strategy can be extended to synthesize other core–shell QDs at low temperature.     

Photoluminescence of CdHgTe based nanoheterostructures

We have studied the photoluminescence (PL) spectra of Cd _x Hg_{1 − x} Te/Cd _y Hg_{1 − y} Te nanohetero-structures grown by molecular beam epitaxy on CdTe/ZnTe/GaAs substrates. The width of potential wells in the structures was varied within d = 12−200 nm and the material composition was changed within x ∼ 0.25–0.40 in the well and y ∼ 0.68–0.82 in the barrier layers. The PL spectra of samples with d ≤ 33 nm exhibit transitions between quantum confinement levels. The samples with d > 50 nm display the PL due to excitons localized on composition fluctuations, which is characteristic of Cd _x Hg_{1 − x} Te epilayers with thicknesses above 3 μm. It is established that the exciton PL band in Cd _x Hg_{1 − x} Te exhibit broadening that is determined both by stochastic fluctuations of the composition and by its macroscopic inhomogeneities.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Electrical properties of CdxHg1−x Te and ZnxCdyHg1−x−y Te modified by low-energy ion bombardment

The phenomenon of conversion of the conductivity type in p-type samples of the Cd_xHg_1−x Te (0.28≤x≤0.55) and Zn_xCd_yHg_1−x−y Te solid solutions bombarded by low-energy argon ions was studied. It is shown that a necessary condition for the conversion effect in Cd_xHg_1−x Te with 0.28≤x≤0.39 is ion neutralization in the bombarding beam. The dependence of the conversion layer thickness in Cd_xHg_1−x Te on the solid solution composition agrees with that predicted by the diffusion model of ion-bombardment-induced conversion of the conductivity type.     

Electrical properties and microstructure of CdxHg1 ? x ? yCrySe crystals

Crystals of the Cd _x Hg_{1 − x − y} Cr _y Se (x = 0.4, y = 0.1) quaternary solid solution have been grown by the Bridgman method, and their microstructure and electrical properties have been studied. The crystals are shown to contain various types of inclusions in the form of filaments and triangles.     

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II–VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_xS_1−x)₄-(CdS)₂ core/shell QDs with a proper S_surface/Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm⁻² under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.     

Band gap modified Al-doped Zn1?xMgxO and Zn1?yCdyO transparent conducting thin films

Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg²⁺ and Cd²⁺ for Zn²⁺ in their solid solution. The effects of narrowing and widening band gap (E _g) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.     

Absorption Enhancement in “Giant” Core/Alloyed‐Shell Quantum Dots for Luminescent Solar Concentrator

Luminescent solar concentrators (LSCs) can potentially reduce the cost of solar cells by decreasing the photoactive area of the device and boosting the photoconversion efficiency (PCE). This study demonstrates the application of “giant” CdSe/Cd_xPb_1–xS core/shell quantum dots (QDs) as light harvesters in high performance LSCs with over 1.15% PCE. Pb addition is critical to maximize PCE. First, this study synthesizes “giant” CdSe/Cd_xPb_1–xS QDs with high quantum yield (40%), narrow size distribution (<10%), and stable photoluminescence in a wide temperature range (100–300 K). Subsequently these thick alloyed‐shell QDs are embedded in a polymer matrix, resulting in a highly transparent composite with absorption spectrum covering the range 300–600 nm, and are applied as active material for prototype LSCs. The latter exhibits a 15% enhancement in efficiency with respect to 1% PCE of the pure‐CdS‐shelled QDs. This study attributes this increase to the contribution of Pb doping. The results demonstrate a straightforward approach to enhance light absorption in “giant” QDs by metal doping, indicating a promising route to broaden the absorption spectrum and increase the efficiency of LSCs.     

Composition and predominant electron scattering mechanisms in Hg1 − x − y Cd x Eu y Se crystals

The surface of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals has been examined by electron microscopy using backscattered and secondary electron imaging, and the composition of the crystals has been determined. Using transport and optical measurements, we have identified the predominant electron scattering mechanisms in the Hg_{1 ? x ? y} Cd _x Eu _y Se crystals.     

Laser induced photoluminescence and morphological characterization of Cd(1?x)?yZnxMnyS nanocrystals

Doped II–VI chalcogenide semiconductor nanostructures have recently attracted a lot of attention due to the possibility of their application in various modern era devices. In the present study, Cd_(1−x)−y Zn _x Mn _y S {(0 ≤ x ≤ 0.5); (0.0001 ≤ y ≤ 0.1)} nanocrystals (NC) have been synthesized by facile wet chemical technique. Morphological and structural analyses of these synthesized quaternary NC have been done using X-ray diffraction (XRD) and transmission electron microscope (TEM) studies. Room temperature photoluminescence (PL) has been investigated using high peak power pulsed N₂-laser. Important optical parameter; excited state lifetime values have been calculated from the recorded multi-exponential decay curves. These fast and efficient nanophosphors have wide applications in opto-electronic industry as futuristic displays, lasers, nanoelectronics and nanosensors.     

Study on the optical properties of Zn1−xMgxS (0 ≤ x ≤ 0.55) quantum dots prepared by precipitation method

Zn_1−xMg_xS (0 ≤ x ≤ 0.55) quantum dots (QDs) were successfully synthesized by precipitation method. The crystal structures, microstructures, and optical properties of the Zn_1−xMg_xS QDs were investigated using X-ray diffraction, scanning electron microscopy, and ultraviolet-visible and photoluminescence (PL) spectroscopy. The Zn_1−xMg_xS QDs were found to have a cubic crystal structure and an average crystallite size of 6.40-7.96 nm. It has been shown that an increase in doping Mg²⁺ concentration in Zn_1−xMg_xS QDs led to a gradual widening of the band gap and a weakening in the PL intensity of the Zn_1−xMg_xS QDs.     

Photoelectrochemical properties of brush plated Cd x Zn1−x Se films

Cd _x Zn_1−x Se films(0 ≤ x ≤ 1) were deposited for the first time by the brushplating technique at room temperature from an aqueous bath containing zincsulphate, cadmium sulphate and selenium oxide. The deposition current densitywas varied in the range of 50–250 mA cm⁻². The as deposited films exhibitedcubic structure. Composition of the films was estimated by EDAX studies. XPSstudies indicated the binding energies corresponding to Zn(2p_3/2), Cd(3d_5/2 and 3d_3/2) and Se(3d_5/2 and 3d_3/2). Optical band gap of the films varied from 1.72 to 2.70 eV as the composition varied from CdSe to ZnSe side. Atomic forcestudies indicated grain size in the range of 20–150 nm. Photoelectrochemical cells were made with polysulphide as the redox electrolyte. The output was maximum for the photoelectrodes of composition Cd_0.9Zn_0.1Se.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Physical properties of Hg1 − x − y Cd x EuySe crystals

We have studied the magnetic, optical, and kinetic properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals. The behavior of their magnetic susceptibility can be accounted for by the presence of clusters of various sizes. The Hg_{1 ? x ? y} Cd _x Eu _y Se crystals are shown to be n-type. Their Hall coefficient (R _H) is temperature-independent, which points to electron gas degeneracy. The optical band gap of the crystals has been determined.     

Two-step growth of Cd0.96Zn0.04Te/Si(111) epilayers by HWE

High quality Cd_1−xZn_xTe, x = 0.04 epilayers are successfully grown directly on hydrogen-terminated Si(111) substrates by hot wall epitaxy method. Growth conditions are optimized in order to grow single crystal films with desired composition. It is found that surface morphology of the epilayers is dramatically affected by the growth temperature and the growth rate at the early stage of the crystal growth. Applying limited high substrate temperature of T_sub = 440 °C and low growth rate of 0.04 μm/h, the crystallinity is significantly improved and for the first time a pseudomorphic 2D growth is observed notwithstanding of the large lattice mismatch. Designing a suitable two-step growth process makes it possible that Cd_1−xZn_xTe/Si(111), x = 0.04 epilayers are fabricated with good surface morphology, which could be used as lattice matched substrates for MCT and MCZT epitaxy.     

Relationship between photoluminescence and structural properties of the sputtered Zn1−xCdxO films on Si substrates

Zn_1−xCd_xO (x=0.2, 0.4) alloyed crystal thin films have been deposited on Si(1 1 1) substrates at different temperatures by using dc reactive magnetron sputtering technique. The Zn_1−xCd_xO films are of highly (0 0 2)-preferred orientation possessing the hexagonal wurtzite structure of pure ZnO. At 450 °C, the films have better crystal quality and photoluminescent characteristics. For the films with x=0.2 and 0.4, the corresponding near-band-edge (NBE) energies are 3.10 and 3.03 eV, respectively, both have red-shifts compared with that of ZnO (3.30 eV). For the substrate temperatures lower or higher than 450 °C, the other NBE emission peak appears, the X-ray diffraction intensity of (0 0 2) peak decreases and the related FWHM increases. With the Cd addition up to x=0.4 both the XRD and PL intensity of the Zn_1−xCd_xO films decrease sharply in comparison with x=0.2.     

Optical absorption of II–VI semiconductor-doped glasses exposed to 7 MeV electron irradiation

Optical absorption spectra of 7 MeV electron irradiated borosilicate glasses doped with CdS_1−xSe_x, CdSe_1−xTe_x, and Cd_1−yZn_yS semiconductor nanocrystals were investigated. The observed transformation of the confinement-related features in the spectra is related to irradiation-induced negative ionization of the nanocrystals with charge transfer across the nanocrystal/matrix interface. The dependence of the radiation-induced changes on the nanocrystal size and composition is discussed.