Quantum dots for wastewater treatment for the removal of heavy metals

Photoluminescent and transparent Nylon-6 nanofiber mat composited by CdSe@ZnS quantum dots and poly (methyl methacrylate)

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Colloidal quantum dots (QDs) of group II-VI are key ingredients of next-generation QD light-emitting diodes technology for display and lighting, yet the understanding of their luminescent characteristics are far from being mature. Using a hybrid time-dependent density functional theory, we have studied the electronic and excitonic properties of blue-emitting colloidal QDs within group II-VI containing a thousand atoms or more, including CdSe, CdS, ZnSe, and ZnS QDs, considering both quantum confinement and surface ligand effects. It is found that the calculated optical gaps are in excellent quantitative agreement with experiment, irrespective of the QD nature. Scaling laws of size-dependent energy gaps governed solely by quantum confinement effects have further been explored at both single-particle level and correlated excitonic level for all QDs. With concurrently stoichiometric control and enhancing quantum confinement effects, we have predicted an unusual switching of symmetry character of the highest occupied molecular orbital state from a ${\mathrm{\ensuremath{\Gamma}}}_{3}$ to a ${\mathrm{\ensuremath{\Gamma}}}_{1}$ symmetry at ultrasmall size ($\ensuremath{\sim}1$ nm) for all QDs. After the switching, pronounced linearly polarized band-edge excitonic emission is activated. The radiative exciton decay lifetime is found to increase monotonically with increasing the QD size and tends to saturate at larger sizes. Finally, we have explored the surface passivation mechanism of inorganic chloride ligand, and identified various favorable Cd-Cl bonding configurations which enable an effective surface passivation resembling the commonly applied pseudohydrogen passivation scheme. We find that chloride ligand serves as a hole delocalization ligand and tends to redshift the absorption spectra, reduce the absorption intensity, and significantly enhance the exciton decay lifetime. Our results provide a guideline for spectroscopic studies of excitonic characteristics of colloidal QDs within group II-VI.

InGaN quantum dot (QD) formation in a wide pressure range MOCVD reactor was studied. The existence of QDs and their lateral size (2–5 nm) were demonstrated using transmission electron microscopy and high spatial resolution (~ 100 nm) near-field magneto-photoluminescence spectroscopy. We found that an increase of the reactor pressure from 400 to 1000 mbar leads to an order of magnitude increase in light emission efficiency of the InGaN / GaN QDs accompanied by ~ 100 meV redshift of the emission wavelength. We explored stimulated phase separation (SPS) to control carrier localization and emission wavelength. The SPS was achieved by adding In in the matrix material. This leads to formation of extremely deep InGaN / InGaN QDs having energy localization up to ~ 0.8 eV, which was revealed from selectively excited far-field photoluminescence (PL) spectra. Without SPS the QD activation energy is found to be below 0.2 eV. A nonequilibrium carrier population strongly suppresses the temperature-induced shift of the PL emission in deep InGaN QDs.

Incorporation of quantum dots into the lipid bilayer of giant unilamellar vesicles and its stability

Ultrafast intraband Auger process in self-doped colloidal quantum dots

The quantum dots derived from the 2D material are finding their applications in sustainable and emerging technologies due to their tunable properties by quantum confinement and scalable synthesis. Elemental doping in these quantum dots can enhance the performance favourably for the desired application. It can further tune the properties of parent counterparts leading to novel and interesting properties and applications. This review demonstrates the excellence of 2D materials-based quantum dots as a material platform. We critically analyzed and present a summary of the top-down and bottom-up synthesis of 2D material-derived quantum dots. Further, the doping of quantum dots and prominent characterization techniques to identify the successful incorporation of dopants in them are presented. In the end, we comprehensively analyzed the applications of these two-dimensional derived quantum dots in energy, optoelectronic, and quantum technological applications.

We discuss fabrication of GaAs quantum wires and dots using both selective growth and spontaneous growth technique in MOCVD, including the optical properties of those nano-structures. In the selective growth, triangular-shaped GaAs quantum wires as narrow as 7nm and quantum dots with 25nm lateral width were obtained. The quantum confinement effect is evidenced by photoluminescence (PL) and magneto-PL. On the other hand, self-organizing growth achieved InGaAs quantum dots with a diameter of 15nm. For characterization of the nanostructures, observation of photoluminescence from a single quantum dot was achieved. Finally, as the first step to the ultimate quantum lasers, a vertical microcavity quantum wire lasers was demonstrated.

On the basis of the density functional theory (DFT) within local density approximations (LDA) approach, we calculate the band gaps for different size SnO2 quantum wires (QWs) and quantum dots (QDs). A model is proposed to passivate the surface atoms of SnO2 QWs and QDs. We find that the band gap increases between QWs and bulk evolve as ΔEgwire = 1.74/d1.20 as the effective diameter d decreases, while being ΔEgdot = 2.84/d1.26 for the QDs. Though the ∼d1.2 scale is significantly different from ∼d2 of the effective mass result, the ratio of band gap increases between SnO2 QWs and QDs is 0.609, very close to the effective mass prediction. We also confirm, although the LDA calculations underestimate the band gap, that they give the trend of band gap shift as much as that obtained by the hybrid functional (PBE0) with a rational mixing of 25% Fock exchange and 75% of the conventional Perdew−Burke−Ernzerhof (PBE) exchange functional for the SnO2 QWs and QDs. The relative deviation of the LDA calculated band gap ...

Self-assembled ZnSe quantum dots (QDs) embedded in ZnS were grown by metal-organic chemical vapour deposition and investigated with atomic-force microscopy (AFM) and photoluminescence (PL) measurements. Lens-shaped ZnSe QDs with the height of 1–2.4 nm and the radius of 25–37 nm are observed through the AFM measurements. The size and areal density of QDs increases as ZnSe deposition is increased. PL spectra of ZnSe QDs show consistent blueshifts due to quantum confinement effects. A simple model taking into account the lens shape of ZnSe QDs is introduced to calculate the quantum confinement effects in ZnSe QDs. Obtained excitonic ground state transition energies are in good agreement with the experimental PL peaks, which proposes that the model is adequate to describe the quantum confinement effect in lens-shaped ZnSe QDs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystal phase defined heterostructures, or polytype heterostructures, are atomically sharp with no intermixing, which makes them ideal contenders for a wide range of applications. Although polytype quantum dots have shown promising results as single photon sources, a high degree of control on the dimensions and the number of polytype quantum dots is necessary before any application can be developed. In this work, we show results from optical characterization of highly controlled wurtzite (wz)–zinc blende (zb) GaAs quantum dots with sharp photoluminescence signal and a strong indication of 0D density of states. One band effective mass calculations show good agreement with the measured data. Radially confined nanowires with a single wz–zb GaAs interface also show sharp photoluminescence signal and 0D density of states. This indicates the existence of quantum dot like states in triangular wells formed at the wz–zb GaAs interface. These results show the potential of polytype quantum dots for physics and optics applications.

In this communication, we report on Cu2SnS3 quantum dots synthesized by the solvothermal process using different solvents. The optical properties of the quantum dots are analyzed by UV-Vis-NIR and photoluminescence spectroscopy. The results suggest that Cu2SnS3 material has tunable energy bandgap and appropriate wavelength for fabrication of light emitting diodes and laser diodes as sources for fiber optic communication. They exhibit wide absorption in the near infrared range. Further morphological studies with the use of atomic force microscope confirm the surface topography and the existence of quantum dots. The observed characteristics prove the efficiency of Cu2SnS3 quantum dots for O-band wavelength detection used in fiber optic communication and solar cell applications.

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

The evidence of a parabolic potential well in quantum wires and dots was reported in the literature, and a parabolic potential is often considered to be a good representation of the “barrier” potential in semiconductor quantum dots. In the present work, the variational and fractionaldimensional space approaches are used in a thorough study of the binding energy of on-center shallow donors in spherical GaAs-Ga1-xAlxAs quantum dots with potential barriers taken either as rectangular [Vb (eV) ??1.247 x for r >] or parabolic [Vb (r) ??β2?r2] isotropic barriers. We define the parabolic potential with a β?parameter chosen so that it results in the same E0 groundstate energy as for the spherical quantum dot of radius R and rectangular potential in the absence of the impurity. Calculations using either the variational or fractional-dimensional approaches both for rectangular and parabolic potential result in essentially the same on-center binding energies provided the dot radius is not too small. This indicates that both potentials are alike representations of the quantum-dot barrier potential for a radius R quantum dot provided the parabolic potential is defined with?β?chosen as mentioned above.

Self-organized GaN/AlN stacked quantum dots (QDs) have been studied by means ofcathodoluminescence (CL), near field scanning optical microscopy (NSOM), photoluminescence,μ-Raman, and transmission electron microscopy. Assignment of the optical emissions wasmade on the basis of the structural parameters, power-dependent optical studies anddepth-resolved CL.Power-dependent studies allowed us to distinguish between quantum confined and bufferemissions. On increasing the power injection conditions, a QD-size-dependent blueshift due to the screening of the internal electric fields was found together with atrend to saturation observed in the high injection limit. The possible evidenceof excited states has also been shown by power-dependent photoluminescenceand CL. Different blue shifts in specimens with different numbers of stackedlayers suggested possible different residual strain values as confirmed byμ-Raman studies.Depth-resolved CL investigations performed at constant power injection per unit volumeallowed us to distinguish between QD layers with different nominal GaN coveragesand a linear dependence of peak energy versus GaN monolayer number has alsobeen found. Adding 1 ML of GaN resulted in an average shift of about 150 meV.The existence of QDs with different size distributions along the growth axis was also found.The observations were confirmed by NSOM spectroscopy.