Photoluminescence study of ZnCdSe/ZnSe quantum dot systems

The optical properties of the populated ZnCdSe/ZnSe quantum dots have been studied by photoluminescence spectra measured with different laser excitation apertures at temperatures from 22 to 300 K. The differences of spectral features between small and large excitation spot suggest the existence of quantum dot size fluctuation in the system. The temperature evolution of photoluminescence spectral features revealed that two types of quantum dots with different densities and sizes coexist in ZnCdSe/ZnSe system. The energy spacings of the two kinds of quantum dot emissions are about 50 meV at various temperatures. The thermally activated lateral transfer processes of carriers populated in the two sorts of quantum dots are investigated by temperature dependences of spectral intensities.     

Aqueous synthesis of Cu-doped ZnSe quantum dots

In this paper, we described a simple growth-doping approach for aqueous synthesis of Cu-doped ZnSe quantum dots (Cu:ZnSe d-dots) with mercaptopropionic acid as stabilizer. The influences of the ratios of precursors and the concentration of Cu dopant ions on Cu:ZnSe d-dots synthesis were studied in detail in this study. The Cu dopant ions had significant influence on the optical properties of ZnSe d-dots. The bandgap emission of ZnSe was effectively restrained through Cu doping. The prolonged reflux facilitates the doping of Cu, which led to the red-shift of the emission of Cu:ZnSe d-dots from 465 to 495 nm. The stable Cu:ZnSe d-dots with high quality can be obtained under optimal conditions. As compared with cadmium-based nanocrystals synthesized in aqueous solution, Cu:ZnSe d-dots have much lower toxicity, indicating that they can be applied as outstanding fluorescent labels for biological assays, imaging of cells and tissues, even in vivo investigations.     

Electronic states in the valence band of a cylindrical ZnSe quantum dot

Electronic states in the valence band of a cylindrical ZnSe quantum dot are studied with allowance for the valence band mixing. Dependences of the wave functions and energy levels on the quantum dot radius are calculated. The influence of a magnetic field parallel to the quantum dot radius on the energy levels is also considered.     

Facile and controllable synthesis of molybdenum disulfide quantum dots for highly sensitive and selective sensing of copper ions

Molybdenum disulfide quantum dots(MoS_2QDs) were synthesized via a hydrothermal method using sodium molybdate and cysteine as molybdenum and sulfur sources, respectively. The optimal hydrothermal time was studied. Furthermore, the as synthesized water-soluble MoS_2QDs were used as a fluorescence probe for the sensitive and selective detection of copper ions. The fluorescence of the MoS_2QDs was quenched after the addition of copper ions; the reason may be that the transfer of the excited electron from QDs to copper ions leads to the reduction of the radiative recombination.The fluorescence quenching of MoS_2QDs is linearly dependent on the copper ions concentration ranging from 0.1 μM to600 μM, the limit of detection is 0.098 μM, which is much lower than that of existing methods. Moreover, the MoS_2QDs show highly selectivity towards the detection of copper ions.     

Sol-gel synthesis of highly luminescent magnesium oxide nanocrystallites

An attempt has been made to prepare MgO nano-crystallites through decomposition of sol-gel derived magnesium oxalate dihydrate in air, oxygen and nitrogen ambient at 500, 600, 800, and 1000 °C for 2 h each and study them with regard to their phase, average crystallite size, morphology, and photoluminescence (PL) behaviour. They are shown to possess f.c.c. (NaCl-type) structure with lattice parameter a∼4.211 Å, average crystallite size in the range of 3.0-73.5 nm, 〈1 1 1〉 preferred orientation at decomposition temperature of 500 °C (in nitrogen and oxygen ambient), and a distorted rod-like morphology. The PL peaks observed around 395 and 440 nm have been assigned to ²T_1u→²A_1g and ³B_1u→¹A_g transitions associated with the relaxation of excited states of F⁺- and F₂²⁺-centres, respectively. Further, the emission band intensity is found to depend on decomposition temperature and gas ambient, crystallite size, and their morphology. However, in nitrogen ambient above 800 °C, several other PL peaks observed at 491.8, 501.8, 503.5, 509.3, 561.5, and 563.0 nm arise due to aggregates of F centres and/or extra energy levels created in the energy band gap by nitrogen incorporation. A mechanism for nitrogen trapping in MgO has been suggested. Further, emission intensity depends on both colour centres and surface states.     

Effect of a magnetic impurity on the optical properties of a spherical ZnSe quantum dot

We consider the electron and hole states in a semiconductor ZnSe spherical quantum dot, in the center of which a magnetic impurity atom of manganese is located. In calculations the quantum dot is approximated by a spherical rectangular well with a finite depth. Within the framework of perturbation theory, the effect of exchange spin interaction of an electron and a hole with a magnetic impurity on the band structure of the system is considered. The optical spectrum of the system for different polarizations of the incident light is studied also.     

Photocurrent oscillations in luminescent epitaxial thin films of ZnSe (Cu,Ga)

Photoluminescent epitaxial thin films of ZnSe (Cu, Ga), n-type, vacuum-deposited onto CaF₂ crystals, or air cleaved mica substrates, have been studied. At temperatures between 80 K and 220 K, slow photocurrent oscillations can be observed if ZnSe (Cu, Ga) thin films are illuminated with light (200 nm ? λ ? 530 nm) and an electric field is simultaneously applied, whose value is above a definite threshold. These oscillations, with amplitude and frequency varying as a function of temperature, applied electric field and light intensity, are attributed to moving high-field domains similar to those found by Boer and co-workers on CdS single crystals. Field quenching of luminescence is observed, which visualizes moving high-field domains. Anode-adjacent high-field domains occur at applied voltages above the range at which moving domains are observed and a simultaneous electroluminescence is initiated near the anode.     

Facile synthesis of three-dimensional porous carbon networks for highly stable sodium storage

Novel three-dimensional porous carbon network (3D-PC) anode was developed by a facile in situ NaCl-template method utilizing citric acid as carbon source. The synthesis process involves the dissolution of NaCl and citric acid in deionized water, citric acid coated on NaCl template during freeze-drying process, carbonization of the composites, and removal of the template with water. The resultant 3D-PC presents high electrical conductivity, large specific surface area, sufficient active sites, large interlayer distance, and high mechanical flexibility, which are contributed to the efficient Na-storage. Therefore, the 3D-PC anode displays enhanced rate performance of 101 mAh g^?1 at 1000 mA g^?1 and extremely long cycle life of 138 mAh g^?1 after 2000 cycles at 200 mA g^?1. The unique synthesis strategy coupled with the excellent Na-storage performance ensures 3D-PC a promising anode material for low-cost sodium-ion batteries.     

Fabrication of highly luminescent InP/Cd and InP/CdS quantum dots

Highly luminescent InP/Cd and InP/CdS core-shell QDs were fabricated by sequential addition of cadmium acetylacetonate and dodecanethiol to InP core solutions, which showed a red-shift in absorption and emission. ICP measurement revealed the existence of cadmium and TEM images showed the increased size of InP/CdS QDs. PXRD data identified zinc blend structures of InP and InP/CdS QDs, which indexed to the (1 1 1), (2 2 0) and (3 1 1) planes. The slight shift of peaks between InP and InP/CdS QDs can demonstrate the existence of CdS shell structures.     

Morphology response to strain field interferences in stacks of highly ordered quantum dot arrays

Twofold stacked InGaAs/GaAs quantum dot (QD) layers are grown on GaAs(001) substrates patterned with square arrays of shallow holes. We study the surface morphology of the second InGaAs QD layer as a function of pattern periodicity. Comparing our experimental results with a realistic simulation of the strain energy density E(str) distribution, we find that the second InGaAs QD layer sensitively responds to the lateral strain-field interferences generated by the buried periodic QD array. This response includes the well-known formation of vertically aligned QDs but also the occurrence of QDs on satellite strain energy density minima. Our calculations show that base size and shape as well as lateral orientation of both QD types are predefined by the E(str) distribution on the underlying surface.     

Bloch-wave engineering of quantum dot micropillars for cavity quantum electrodynamics experiments

We have employed Bloch-wave engineering to realize submicron diameter high quality factor GaAs/AlAs micropillars (MPs). The design features a tapered cavity in which the fundamental Bloch mode is subject to an adiabatic transition to match the Bragg mirror Bloch mode. The resulting reduced scattering loss leads to record-high vacuum Rabi splitting of the strong coupling in MPs with modest oscillator strength quantum dots. A quality factor of 13,?600 and a splitting of 85 μeV with an estimated visibility v of 0.41 are observed for a small mode volume MP with a diameter d{c} of 850 nm.     

Modeling of confinement potential for cylindrical quantum dot

Quantum states and energy levels of an electron in a cylindrical quantum dot with different models of confinement potentials are studied. Two models of confinement potentials, Morse potential and modified Pöschl-Teller potential, are considered. It is shown that due to distinction between symmetric and asymmetric nature of potentials, there is a fundamental difference in behavior of the ground levels of charge carriers in these potentials. At small values of the width of Morse potential, quantum emission of electron occurs which is not observed in case of the modified Pöschl-Teller potential.     

Facile synthesis of red- to near-infrared-emitting CdTexSe1−x alloyed quantum dots via a noninjection one-pot route

High-quality CdTeSe colloidal nanocrystals with gradient distribution of components, consisting of Te-rich inner cores and Se-rich outer shells, were synthesized in a “green” solvent paraffin via a noninjection one-pot approach with the use of cadmium oxide (CdO), elemental tellurium, and elemental selenium as Cd, Te, and Se sources, respectively. All of these reactants were loaded at room temperature. This features synthetic reproducibility and large-scale capability. The bandgap engineering of the obtained CdTeSe QDs can be conveniently realized through the variation of growth temperature. Red- to near-infrared-emitting (620-780 nm) QDs with nearly identical particle sizes can be obtained when the reaction temperature was changed from 180 to 280 °C with the fixation of precursor feed ratio at 5Cd-0.5Te-0.5Se. The as-prepared CdTeSe QDs exhibit PL QY as high as 53%. The resulting CdTeSe QDs were characterized by UV-vis and photoluminescence spectroscopy, powder X-ray diffraction, transmission electron microscopy, and inductively coupled plasma atomic emission spectroscopy.     

Facile synthesis of near-infrared CuInS<Subscript>2</Subscript>/ZnS quantum dots and glycol-chitosan coating for in vivo imaging

This study describes the synthesis method of water-soluble, low-toxicity, photostable highly luminescent probes based on I–III–VI₂ type semiconductor quantum dots (QDs) and the possibility of tumor targeting in living animals. Cd-free high-quality CuInS₂/ZnS core/shell QDs were synthesized, and their surfaces were reacted with mercaptoundecanoic acid for aqueous phase transfer followed by reaction with glycol-chitosan; lastly, Arg-Gly-Asp (RGD) integrin-binding peptide was covalently attached for in vivo tumor targeting. Dowtherm A, a highly viscous heat-transfer organic fluid, was used to control semiconductor crystal growth at high temperature (>230 °C) during organic synthesis. The structural and optical properties of the resulting CuInS₂/ZnS QDs were investigated. The average diameters of CuInS₂ and CuInS₂/ZnS QDs were 3.0 and 3.7 nm, respectively. Cell toxicity and in vivo tumor targetability in RR1022 cancer cell-xenografted mice were further evaluated using cRGDyk-tagged glycol-chitosan-coated CuInS₂/ZnS QDs. Glycol-chitosan-coated MUA-QDs displayed a Z-average diameter of 203.8 ± 7.67 nm in water by dynamic light scattering.

Open image in new window

Graphical abstract In vivo tumor targeting using cRGDyk-tagged glycol-chitosan-coated MUA-CuInS₂/ZnS QDs nanoparticles

     

Structural imaging of a Si quantum dot: Towards combined PL and TEM characterization

Individual silicon quantum dots were fabricated by electron-beam lithography, plasma etching and a two-step oxidation process. This enables photoluminescence (PL) from individual dots at various temperatures to be detected and spectrally resolved using a sensitive charge-coupled device camera-imaging system, as reported previously. The regular array-like arrangement of oxidized pillars containing individual nanocrystals, in principle, enables combined transmission electron microscopy (TEM) and low-temperature PL characterization of the same Si quantum dot. To this end, a technique employing focused ion beam was developed for preparation of the pillar/nanocrystal of interest for TEM. It is shown that silicon quantum dots of several nanometers in size can be characterized using such a method.     

A facile one-pot synthesis of highly luminescent CdS nanoparticles using thioglycerol as capping agent

Absence of emission concentration quenching accompanied by high emission efficiency in a solid state is highly challenging though very attractive, for example, for fabrication of solid state light emitters or fluorescent organic nanoparticles (FONs). Here, formation of FONs based on novel p-phenylenediacetonitrile derivatives by re-precipitation method in aqueous solutions is demonstrated. The exceptionality of the derivatives employed is manifested by nitrile groups-induced steric hindrance effects inhibiting concentration quenching of emission. Consisting of different size and polarity end-groups, phenyl groups in one compound and hexyl-carbazolyl in another, the derivatives were examined and compared in regard to nanoparticle formation morphology, size tunability, spectral signatures, and fluorescence turn-on efficiency. The variation of solvent/non-solvent ratio allowed to achieve tuning of the FON sizes from 55?nm up to 360?nm and resulted in maximal fluorescence on/off ratio of 38. Spectrally resolved confocal fluorescence microscopy revealed somewhat different molecule arrangement in different FONs suggesting dominant amorphous-like phase, which was confirmed by small angle X-ray scattering measurements. The FONs were verified to be stable against degradation or conglomeration into larger clusters at least over a couple of months thus implying their feasibility for practical applications. Finally, potential application of the fluorescent p-phenylenediacetonitrile nanoparticles for organic vapor sensing via fluorescence on/off switching was demonstrated.     

Growth of three-dimensional quantum dot crystals on patterned GaAs (0 0 1) substrates

The growth of a three-dimensional (3D) InAs quantum dot (QD) crystal on a patterned GaAs (0 0 1) substrate is demonstrated. The morphology of QDs grown on a surface patterned with shallow holes is studied as a function of the amount of deposited InAs. We observe that the QDs form in the patterned holes close to each other forming lateral QD bimolecules for InAs coverages below the commonly observed critical thickness of 1.6 monolayers. When the coverage increases, the QD bimolecules coalesce to form larger single QDs. The QDs in the holes are then capped with a Ga(Al)As spacer. The buried QD array serves as a strain template for controlling the formation site of the QDs in the second layer. By tuning the growth conditions for the second and subsequent layers, we achieve a 3D InAs QD crystal with a high degree of perfection. A detail investigation of the growth on hole patterns with different periodicities is presented.     

Thermodynamic properties of two-dimensional few-electrons quantum dot using the static fluctuation approximation (SFA)

The two-dimensional GaAs parabolic quantum dot (QD) is investigated in the static fluctuation approximation (SFA) where both the magnetic field and the electron-electron interaction are fully taken into account. The thermodynamic properties of the system are computed as functions of the temperature and magnetic field for different numbers of electrons (N). It is noted that the mean internal energy increases with increasing number of electrons and magnetic field. This increase is not smooth, and a sudden change in slope is observed at a certain value of magnetic field for N≥4. It is found that the magnetization has sharp jumps for N≥4; for N≤3 a smooth monotonic diamagnetism is exhibited. A sharp peak is observed in the specific heat when the magnetic field is increased. It is found that the lowest-state transitions occur at a critical value of magnetic field. These transitions appear in the mean internal energy and magnetization.     

The dynamic field effect in Mn delta-doped quantum wells and In(Ga)As/GaAs quantum dot heteronanostructures

The dynamic field effect in Mn delta-doped epitaxial layers and quantum dimensional heteronanostructures of the p type with quantum wells and quantum dots was investigated. It was shown that an embedded Mn delta layer leads to the considerable capture of injected carriers on traps associated with the delta layer.