Photoluminescence properties of cadmium-selenide quantum dots embedded in a liquid-crystal polymer matrix

The photoluminescence properties of cadmium-selenide (CdSe) quantum dots with an average size of ～3 nm, embedded in a liquid-crystal polymer matrix are studied. It was found that an increase in the quantum-dot concentration results in modification of the intrinsic (exciton) photoluminescence spectrum in the range 500–600 nm and a nonmonotonic change in its intensity. Time-resolved measurements show the biexponential decay of the photoluminescence intensity with various ratios of fast and slow components depending on the quantum-dot concentration. In this case, the characteristic lifetimes of exciton photoluminescence are 5–10 and 35–50 ns for the fast and slow components, respectively, which is much shorter than the times for colloidal CdSe quantum dots of the same size. The observed features of the photoluminescence spectra and kinetics are explained by the effects of light reabsorption, energy transfer from quantum dots to the liquid-crystal polymer matrix, and the effect of the electronic states at the CdSe/(liquid crystal) interface.     

Effect of cadmium-selenide quantum dots on the conductivity and photoconductivity of nanocrystalline indium oxide

The effect of cadmium-selenide quantum dots addition on the electrical and photoelectric properties of nanocrystalline indium oxide with nanocrystal dimensions in the range from 7 to 40 nm is studied. By impedance spectroscopy, it is shown that the addition of quantum dots substantially influences the resistance of interfaces between In₂O₃ crystals. A change in the character of the photoconductivity spectrum of In₂O₃ upon the addition of CdSe quantum dots is detected, and it is established that this change depends on the In₂O₃-nanocrystal dimensions. An energy band diagram is proposed to explain the observed change in the photoconductivity spectrum of In₂O₃ upon the addition of CdSe quantum dots.     

Dependence of the optical gap of Si quantum dots on the dot size

The dependence of the optical gap of Si quantum dots, embedded in a SiO₂ insulator host, on the dot size was calculated in terms of an envelope-function approximation. It is shown that consideration of the finiteness of the SiO₂ band gap and an abrupt change in the effective mass at the Si-SiO₂ interface significantly decreases the optical gap of quantum dots in comparison with a model in which the potential barriers for electrons and holes are assumed to be infinitely high. The obtained results are in good agreement with the experimental data.     

Stacking number dependence of size distribution of vertically stacked InAs/GaAs quantum dots

Vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The dependence of the size distribution of quantum dots on the stacking numbers is theoretically and experimentally investigated. We show that the size distribution of quantum dots decreases with increasing the stacking number, and it occurs drastically when the stacking number is changed from 1 to 2. The quantitative analysis on in-plane strain energy distribution is also performed for the explanation.     

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass μ = μ(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E _ex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a _ex, the exciton binding energy E _ex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.     

Size dependence saturation and absorption of PbS quantum dots

The saturation intensity for lead sulfide quantum dots in a titanium dioxide-glycerol matrix (PbS/TiO₂-glycerol) on a glass substrate has been studied as a function of quantum cluster concentration and cluster size. The saturation intensity in these materials is strongly dependent on the size of the semiconductor nanocrystals, their concentration, or the sample thickness. The samples are reflective at a certain range of the incoming intensity of the optical field and become transparent over a threshold intensity beyond which the output and input intensities are linearly related. The system studied involves very dilute distribution of PbS QD of dimension∼10 nm embedded in a matrix of TiO₂-glycerol. Since the distribution is relatively constant in each deposited layer, the total number of QDs in a given area is proportional to the thickness. We found that the threshold of power separating absorption-bleaching, Pth is linearly related to the thickness, with values of Pth ∼few mW/cm², more than 2-3 orders of magnitude below that of QDs with dimension∼1 μm, representing a typical solid.     

Geometric dependence of the dielectric properties of quantum dots arrays

The experimental techniques advances in the formation of artificial crystals based on quantum dots (QD) allow us to think about the possibility of generating novel materials. However, there is a lack of theoretical calculations which permits pre-designing the properties of the new materials. Taking advantage on a theoretical derivation of the electronic dielectric response of semiconductor nano-crystals using a tight binding framework [F. Trani, D. Ninno, G. Iadonisi, Phys. Rev. B 76 (2007) 085326], we calculate the dielectric function of QD arrays of finite size introducing the screening of surface polarization by means of a tuning static electric field. In previous calculations we report drastic geometric effects in the electronic structure of QD super-crystals, as the shape changes in 3D semiconductors, while in 2D the dielectric function peaks correspond to the predicted dipole transitions energies [J.F. Nossa, A.S. Camacho, J.L. Carrillo, Rev. Mex. Fis. 53 (7) (2007) 123]. With the aim of explaining the role of the dimensionality in nanosystems we apply the above method to III–V and II–VI QD 3D supercrystals of several geometries. In this report we study the dielectric behavior of finite 3D supercrystals built up of spherical, cylindrical and conical QD and particularly we discuss the shape dependence of the QD constituents on the response function and therefore on the surface polarization fields of finite arrays. Finally, we compare the results in 3D systems with the 2D systems.     

Shape stabilization and size equalization of InGaAs self-organized quantum dots

We report a simultaneous shape stabilization and size equalization after shape transformation of InGaAs self-organized quantum dots (QDs) formed via a fractional monolayer (ML) deposition technique. The density of QD increases rapidly from an initial value of 110±10/μm² (at a total deposition of 4 ML) to 270±30/μm² (at 5 ML) and saturates at a level of 240±20/μm² (at 10 ML). At an intermediate stage of 7 ML deposition, bimodal QD height (peaked at 8.5 nm and 14.5 nm) and aspect ratio (peaked at 0.18 and 0.26) distributions occur, confirming the QD shape transformation from a shallower to a steeper shape. The eventual convergence in lateral size, height and aspect ratio is the direct result of the simultaneous QD size equalization and shape stabilization. The QD size and shape evolution is also substantiated by the low temperature (4 K) photoluminescence (PL) data taken from samples with QDs capped by GaAs.     

Charge accumulation of quantum dots at room temperature

The accumulation of charge in InGaAs quantum dots has been measured at room temperature by the photoelectrochemical capacitance-voltage (CV) technique for the first time. A carrier per quantum dot ratio greater than four has been observed. The use of atomic force microscopy and low temperature and room temperature photoluminescence (PL) confirm the existence of quantum dots. Also, a possible excited state is indicated by room temperature PL in a sample with small quantum dots.     

Correlation between the temperature dependence of the band gap and the temperature dependence of the enthalpy of semiconductor crystals

The first-principles pseudopotential method of density-functional theory is used to study electron-phonon interactions in silicon. The temperature shift of the indirect band gap, the phonon spectrum, and the enthalpy are calculated consistently within the density-functional theory. The relationship between the temperature dependence of the energy gap ΔE _g(T) and the temperature dependence of the enthalpy ΔH(T) is ΔH(T)=K|ΔE _g(T)|. The physical origin of this correlation is discussed. Fiz. Tekh. Poluprovodn. 32, 1025–1028 (September 1998)     

Size dependence of the magnetic properties of electrochemically self-assembled Fe quantum dots

A quasi-periodic array of Fe quantum dots of cylindrical shape has been synthesized by electrodeposition of Fe in porous alumina. By controlling the fabrication parameters, we have controlled the length, diameter, and spacing of the dots. The magnetic properties are shown to depend on these parameters. It has been found that at room temperature, there exists a critical diameter of the dots for which the coercivity is a maximum. The largest value of coercivity obtained at room temperature is 2640 Oe which rises to 2900 Oe on annealing. At a low temperature of 5K, an increase in coercivity is observed for most of the samples. The largest value is 3800 Oe which rises to a value of 4100 Oe in the corresponding annealed counterpart. At 5K, no maximum is seen in the coercivity as a function of diameter. Instead, the coercivity is found to decrease with increasing diameter. This dependence of the coercivity on diameter is discussed in terms of localized reversal effects.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

On the temperature dependence of the dc conductivity of a semiconductor quantum wire in an insulator

The dc resistivity of a semiconductor quantum wire in an insulator host matrix, caused by the interaction between charge carriers and longitudinal acoustic phonons of the host, is considered. Based on an approximation of the relaxation time, simple analytical expressions for calculating the conductivity were derived for the case of a nondegenerate gas of carriers in a quantum wire. If the carrier concentration is independent of temperature, the wire resistivity increases with temperature as T^5/2, i.e., more steeply than in a bulk covalent semiconductor.     

Growth and characterizations of InP self-assembled quantum dots embedded in InAIP grown on GaAs substrates

We report the characteristics of InP self-assembled quantum dots embedded in In_0.5Al_0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 10⁸ mm⁻². These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.     

Temperature dependence of Si-GaAs energy gap using photoconductivity spectra

In the present work, we study the Energy gap (E_g) of the Si-GaAs at various temperatures in the range from 250 up to 350 K, using the photoconductivity method, in order to find the relation between the E_g and the temperature. We have measured the photocurrent as a function of photon energy from 1.36 up to 1.55 eV for each temperature. From the analysis of the spectra for each temperature we have found three peaks corresponding to inter-band transitions and we plotted the energy that corresponds to the peaks as a function of temperature.     

The energy structure of quantum dots induced in quantum wells by a nonuniform electric field

The study is focused on a theoretical treatment of electrons in quantum dots produced by a nonuniform electric field in a heterostructure with a single quantum well. A nonuniform electric field can be formed with the use of a mosaic electrode with a regular system of nanometer-sized apertures. A structure involving a one-aperture electrode deposited onto the surface is considered. The potential in the entire region between the electrode plates is calculated numerically. The analytical expression for the depth of the potential well is derived, and the potential profile in the vicinity of the bottom of the well is determined. The energy level structure in the vicinity of the bottom of the potential well is calculated. The geometric characteristics of the mosaic electrode most favorable for conservation of electron spin in the quantum dots of this type are determined. It is shown that in the quantum dots of this type the spin lifetime can be expected to lie within the microsecond region at a liquid-helium temperature.     

On the shape formation of the droplet epitaxial quantum dots

In this paper, the shape evolution kinetics of droplet epitaxially grown QDs is investigated. Here, the growth parameter dependent of two distinct QD shape regimes is discussed. We show that the QD shape is determined by the size and the contact angle of the initial droplet. Furthermore, the surface tension dependence on the droplet size is also discussed. Finally, the temporal course of the crystallization process is investigated.     

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

Semiconductors - The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence...     

Renormalization of energy spectrum of quantum dots under vibrational resonance conditions

The problem of modification of the energy spectrum of electronic and phonon excitations in semiconductor quantum dots (QDs) under conditions of vibrational resonance is considered. Analytical expressions for the energy of polaron-like states in QDs in the form of a sphere or rectangular parallelepiped, taking into account the size dependence of the electron-phonon interaction, are derived. Experimental data on renormalization of the lowest exciton states in QDs based on CuCl in a NaCl matrix have been obtained with the use of two-photon secondary emission resonance spectroscopy. Comparison with the calculated results demonstrates quantitative agreement indicating the adequacy of the employed theoretical model. It is established that the vibrational resonance strongly modifies the energy spectrum of small QDs.     

Exciton spin dynamics in zinc-blende GaN/AlN quantum dots: Temperature dependence

The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dots. By picosecond time-resolved photoluminescence experiments, we show that the exciton linear polarization does not decay with time from 20 to 300 K.