Strong Confinement Regime Research Articles

Self-induced transparency quadratic solitons

We discover and theoretically explore self-induced transparency quadratic solitons (SIT-QS) supported by the media with quadratic optical nonlinearities, doped with resonant impurities. The fundamental frequency of input pulses is assumed to be close to the impurity resonance. We envision an ensemble of inhomogeneously broadened semiconductor quantum dots (QD) in the strong confinement regime grown on a substrate with a quadratic nonlinearity to be a promising candidate for the laboratory realization of SIT-QS. We also examine the influence of inhomogeneous broadening as well as wave number and group-velocity mismatches on the salient properties of the introduced solitons.

Dimension‐Controlled Synthesis of CdS Nanocrystals: From 0D Quantum Dots to 2D Nanoplates

The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.

Band-gap tuning at the strong quantum confinement regime in magnetic semiconductor EuS thin films

Ultraviolet-visible absorption spectra of nanoscaled EuS thin films reveal a blue shift of the energy between the top-valence and bottom-conduction bands. This band-gap tuning changes smoothly with decreasing film thickness and becomes significant below the exciton Bohr diameter ∼3.5 nm indicating strong quantum confinement effects. The results are reproduced in the framework of the potential morphing method in Hartree Fock approximation. The large values of the effective mass of the holes, due to localization of the EuS f-states, limit the blue shift to about 0.35 eV. This controllable band-gap tuning of magnetic semiconductor EuS renders it useful for merging spintronics and optoelectronics.

Synthesis and characterization studies of ZnSe quantum dots

ZnSe QDs have been synthesized by wet chemical, template free process by zinc acetate and elemental selenium powder in presence of ethylene glycol, hydrazine hydrate and a defined amount of water at 90 °C. The product was in strong quantum confinement regime, having yield as high as 50 %. The transmission electron microscopy image indicated that the particles were well dispersed and spherical in shape. The X-ray diffraction analysis showed that the ZnSe nanoparticles were of the Cubic structure, with average particle diameter of about 3.50 nm. The FTIR characteristic indicates that the N2H4 molecule has intercalated into the complex and formed a molecular precursor.

Size-dependent absorption properties of CdX (X = S, Se, Te) quantum dots

A unified nanothermodynamic model was developed to study the size effects on first absorption peak energy and molar extinction coefficient of semiconductor quantum dots (QDs) based on size-dependent cohesive energy and quantum confinement effect. It is found that: (1) the first absorption peak energy increases as QD size decreases; (2) the molar extinction coefficient decreases with decreasing QD size in strong confinement regime and (3) tunable absorption properties of semiconductor QDs are caused by size-induced cohesive energy variation owing to severe bond dangling. The accuracy of the developed model was verified with experimental data of CdS, CdSe and CdTe QDs.

Simulation of Semiflexible Cyclic and Linear Chains Moderately and Strongly Confined in Nanochannels

The constraints due to the chain closure in combination with the geometrical constraints of a DNA molecule are inevitable for many biological processes. In this work, structural properties of flexible and semiflexible cyclic chains and their linear analogues confined in cylindrical channels were studied using the coarse-grained Metropolis Monte Carlo simulations. The radius of gyration satisfactorily represents the longitudinal stretching of both chain topologies. Transition between the moderate and strong confinement regime of semiflexible cyclic chains is described for the first time. Qualitatively similar response of the chain elongation to the confinement strength variation Rg(D) is obtained in the case of cyclic chains. However, the relative chain extension is stronger, the Odijk strong confinement regime is extended to larger channel diameters D, and under moderate confinement the chain extension declines less steeply for cyclic chains. All three findings are explained in terms of the strong self-av...

Weak-to-strong confinement transition of semi-flexible macromolecules in slit and in channel

We compare confinement of stiff macromolecule in channel and in slit. Whereas in the channel a distinct and established transition exists, we elucidate here an ongoing controversy reported from previous experiment and simulation on existence of such transition in the slit. Our extensive molecular simulations in both geometries show only a very weak conformational crossover between moderate and strong confinements in slit in the same range of confinements where the distinct transition in channel is observed. In contrast to situation in channels relatively stable hairpin-like structures are not indicated around this weak transition in the slit. Observed difference from the prediction on behavior in blob regime under moderate confinement is explained by a crossover between dimensionalities in the slit and the extent of ideal conformation statistics to which the stiffer chains are prone. The strong confinement regime of stiff chain in slit characterized here has not been interpreted yet and it differs from the respective saturation-like Odijk regime in the channel.

Optical and Morphological Properties of ZnO- and TiO2-Derived Nanostructures Synthesized via a Microwave-Assisted Hydrothermal Method

A microwave-assisted hydrothermal method was used to synthesize ZnO and TiO2nanostructures. The experimental results show that the method resulted in crystalline monodispersed ZnO nanorods that have pointed tips with hexagonal crystal phase. TiO2nanotubes were also formed with minimum bundles. The mechanism for the formation of the tubes was validated by HRTEM results. The optical properties of both ZnO and TiO2nanostructures showed characteristics of strong quantum confinement regime. The photoluminescence spectrum of TiO2nanotubes shows good improvement from previously reported data.

Influence of confinement regimes on magnetic property of pristine SnO2 quantum dots

We demonstrate a clear correlation between the quantum confinement effect and magnetization of pristine tin dioxide (SnO2) quantum dots (QDs). We have synthesized single crystalline QDs of SnO2 above and below the exciton Bohr radius (2.7 nm). Such fine control over the size of the QDs is a challenging task. The 2 nm QDs belong to strong confinement regimes and are found to be ferromagnetic in nature, whereas 3 nm QDs are diamagnetic like bulk SnO2. To the best of our knowledge, so far no experimental studies on the influence of confinement effect on the magnetic behaviour of SnO2 QDs have been reported. We propose two possible mechanisms based on the theory of localization of holes due to a strong confinement effect to explain room temperature ferromagnetism in 2 nm QDs. The localization of holes is confirmed from photoluminescence and UV visible spectroscopy.

A Molecule to Detect and Perturb the Confinement of Charge Carriers in Quantum Dots

This paper describes unprecedented bathochromic shifts (up to 970 meV) of the optical band gaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R∼1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices. Facilitating carrier-specific electronic coupling across heterogeneous interfaces is especially important for nanostructured devices, which comprise a high density of such interfaces.

Magnetic confinement of a high-density cylindrical plasma

The stationary structure of a weakly collisional plasma column, confined by an axial magnetic field and a cylindrical vessel, is studied for the high-density case, when the diamagnetic azimuthal current is large enough to demagnetize partially the plasma. The plasma response is characterized mainly by two dimensionless parameters: the ratios of the electron gyroradius and the electron skin-depth to the plasma radius, and each of them measures the independent influence of the applied magnetic field and the plasma density on the plasma response. The strong magnetic confinement regime, characterized by very small wall losses, is limited to the small gyroradius and large skin-depth ranges. In the high-density case, when the electron skin-depth is smaller than the electron gyroradius, the skin-depth turns out to be the magnetic screening length, so that the bulk of the plasma behaves as unmagnetized.

Synthesis of Monodisperse CdSe QDs Using Controlled Growth Temperatures

This paper reports on the effect of growth temperatures on optical properties of monodisperse CdSe QDs synthesized via wet chemical method. The growth temperature was varied from 260 – 310 °C while growth period was fixed at 60 s. The resulting colloidal was in strong quantum confinement regime and having yield as high as 53%. As the growth temperature increased, the monodispersed CdSe QDs with diameter in the range of 3-7 nm were obtained. Both absorption and PL spectra of the QDs revealed a strong red-shift with the increment of growth temperature. Thus, the size of QDs was enlarged with increment of the wavelength from both the spectra. Consequently, the energy gap of resulting QDs can be tuned by tuning the size of QDs. More importantly, the narrow widths of the PL emissions with different colors make these colloidal to be a potential candidate for different optical and optoelectronic devices.

Vortex Fusion and Giant Vortex States in Confined Superconducting Condensates

In a direct scanning tunneling spectroscopy experiment we address the problem of the quantum vortex phases in strongly confined superconductors. The strong confinement regime is achieved in in situ grown ultrathin single nanocrystals of Pb by tuning their lateral size to a few coherence lengths. Upon an external magnetic field, the scanning tunneling spectroscopy revealed novel ultradense arrangements of single Abrikosov vortices characterized by an intervortex distance up to 3 times shorter than the bulk critical one. At yet stronger confinement we discovered the giant vortex phase; the spatial evolution of the excitation tunneling spectra in the cores of these unusual quantum objects was explored. We anticipate the giant vortex phase to be a common feature of other confined quantum condensates such as superfluids, Bose-Einstein condensates of cold atoms, etc.

ZnO-NANOCRYSTALS IN STRONG CONFINEMENT REGIMES: INSIGHT ON RELAXATION DYNAMICS OF DEFECT STATES RESPONSIBLE FOR THE VISIBLE LUMINESCENCE

The broad visible photoluminescence (PL) observed in ZnO nanocrystals (NCs) is widely attributed to multiple low lying surface-defects. We have performed steady state and time-resolved PL measurements on size-selected ZnO NCs in the strong confinement regimes. Our results show that radiative relaxation rates and coupling between excitons and surface defect states vary dramatically for sizes between 2 nm and 3 nm. Energy dependent PL lifetimes reveal that relaxation dynamics of these defect states in the blue- and red-edge of the emission are very different from each other.

Role of surface vibration modes in Si nanocrystals within light emitting porous Si at the strong confinement regime

In this work, we study theoretically the resonant coupling between longitudinal optical surface vibrations of Si-OH and/or Si-O-Si and electron and hole states in the silicon nanocrystals (Si NCs) within light emitting porous Si (PSi) thin films in the framework of the Fröhlich interaction. The results of this analysis are compared with experimental results, which show considerable enhancement and a redshift of the photoluminescence (PL) spectrum of a fresh as-grown PSi thin film after prolonged laser irradiation or after aging in air. These effects coincide with the formation of Si-OH and Si-O-Si bonds on the surface of PSi. The redshift of the PL spectrum is due to the pinning of the bandgap of the light emitting Si NCs, as both oxidation via laser irradiation in air and aging in air introduce energy states in the Si NC band gaps. According to the theoretical analysis, the PL enhancement is assigned to inhibition of nonradiative channels rather than to an enhancement of radiative channels in the Si NCs within the PSi film, due to a strong coupling of the surface Si-OH and/or Si-O-Si vibrational modes to the electronic sublevels in the Si NCs within the PSi layer.

Simultaneous effects of electron-hole correlation, hydrostatic pressure, and temperature on the third harmonic generation in parabolic GaAs quantum dots

The combined effects of electron-hole correlation, hydrostatic pressure, and temperature on the third harmonic generation in disk-shaped para- bolic GaAs quantum dots are studied under the density matrix formalism and the effective mass approximation. Two well-defined regimes are dis- cussed: (1) the strong-confinement regime, where the Coulomb interaction between the electron and hole is neglected and (2) the weak-confinement regime where the parabolic confinement term is neglected and the system reaches the limit of a hydrogenic problem. The results show that the third harmonic- generation coefficient is strongly dependent on the localization of the electron-hole pair. Also, that by using external perturbations like hydrostatic pressure or by considering the temperature effects it is possible to induce a blue-shift and/or red-shift on the resonant peaks of the third harmonic generation coefficient.

New methodology for obtaining CdTe quantum dots by using ultrasound

Luminescent CdTe quantum dots (Qdots) have been produced at few minutes by using a new, simple and fast methodology in an aqueous medium by using ultrasound irradiation to accelerate the process of tellurium reduction. The structural and optical properties were characterized by TEM, XRD, absorption and fluorescence spectrocopy. The produced Qdots are in a strong quantum confinement regime and have only one fluorescence band. Moreover, the nanoparticles seem to be monodispersed, which is in accordance with the fluorescence results. We have developed a simple route for preparing monodispersed CdTe Qdots in an aqueous media. The use of ultrasound allows the morphology to be better controlled and the surfaces defects of Qdots to be reduced.

Nanoengineering the second order susceptibility in semiconductor quantum dot heterostructures

We study second-harmonic generation from single CdTe/CdS core/shell rod-on-dot nanocrystals with different geometrical parameters, which allow to fine tune the nonlinear properties of the nanostructure. These hybrid semiconductor-semiconductor nanoparticles exhibit extremely strong and stable second-harmonic emission, although the size of CdTe core is still within the strong quantum confinement regime. The orientation sensitive polarization response is analyzed by means of a pointwise additive model of the third-order tensors associated to the nanoparticle components. These findings prove that engineering of semiconducting complex heterostructures at the single nanoparticle scale can lead to extremely bright nanometric nonlinear light sources.

Dielectric mismatch and central-cell corrections in doped silicon nanodots

The effect of the central-cell corrections on the shallow donor states in Si spherical quantum dot is studied within the effective mass approximation. Finite step-like spatial confining potential, Coulomb and image charge potentials arising from the dielectric mismatch at the interface of the media are taken into account. We found that it is possible to tune the impurity energies by varying the dot radius and dielectric constant of the barrier material. In the strong confinement regime, due to the enhanced weight of the donor wave functions on the impurity atoms, large values of the chemical shifts for typical donors in Si compared to the ones in bulk are obtained. The calculated size-dependence of the effective Bohr radius in donor doped nanocrystals is in reasonable accord with electron spin resonance measurements on Si quantum dots embedded in insulating glass matrices. We conclude that both the dielectric mismatch and central-cell corrections must be considered in the study of these systems in order to obtain satisfactory agreement with experimental data.

Sized controlled synthesis, purification, and cell studies with silicon quantum dots

This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using (1)H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales.