Photoluminescence Spectra Of Quantum Dots Research Articles

Влияние давления мышьяка при заращивании квантовых точек InAs тонким низкотемпературным слоем GaAs на их оптические свойства

This paper presents the results of the experimental studies of InAs quantum dot overgrowth by a low-temperature GaAs layer at different arsenic vapor pressures. It is revealed that a threefold decrease in the arsenic pressure at a fixed deposition rate of the capping layer leads to a change in the shape of the photoluminescence spectrum of quantum dots with one maximum at the level of 1.19 eV to the shape of the spectrum with two low-energy contributions at the levels of 1.08 and 1.15 eV. Based on the analysis of the power dependences of the photoluminescence spectra, it is found that the low-energy contributions of the photoluminescence of quantum dots overgrown at a low arsenic pressure correspond to the ground-state emission two groups of quantum dots with different average sizes formed during mass transfer in the “quantum dot – wetting layer – matrix” system.

Photoluminescence Properties of InAs Quantum Dots Overgrown by a Low-Temperature GaAs Layer under Different Arsenic Pressures

We studied the influence of the arsenic pressure during low-temperature GaAs overgrowth of InAs quantum dots on their optical properties. In the photoluminescence spectrum of quantum dots overgrown at a high arsenic pressure, we observed a single broad line corresponding to unimodal size distribution of quantum dots. Meanwhile, two distinct peaks (~1080 and ~1150 nm) at larger wavelengths are found in the spectra of samples with quantum dots overgrown at a low arsenic pressure. We attributed this phenomenon to the high-pressure suppression of atom diffusion between InAs islands at the overgrowth stage, which makes it possible to preserve the initial unimodal size distribution of quantum dots. The same overgrowth of quantum dots at the low arsenic pressure induces intensive mass transfer, which leads to the formation of arrays of quantum dots with larger sizes. Integrated photoluminescence intensity at 300 K is found to be lower for quantum dots overgrown at the higher arsenic pressure. However, a difference in the photoluminescence intensity for the high- and low-pressure overgrowths is not so significant for a temperature of 77 K. This indicates that excess arsenic incorporates into the capping layer at high arsenic pressures and creates numerous nonradiative recombination centers, diminishing the photoluminescence intensity.

Epitaxial InGaAs Quantum Dots in Al0.29Ga0.71As Matrix: Intensity and Kinetics of Luminescence in the Near Field of Silver Nanoparticles

Quantum dots of indium gallium arsenide buried in a thin layer of aluminum gallium arsenide were grown by means of molecular-beam epitaxy. The influence of silver nanoparticles grown on the surface of the semiconductor structure by vacuum thermal evaporation on photoluminescence of quantum dots was investigated. Photoluminescence spectra of quantum dots were obtained under stationary and pulsed excitation. The influence of silver nanoparticles exhibiting plasmon resonances on spectral distribution and kinetics of luminescence of the epitaxial quantum dots was studied.

Эпитаксиальные квантовые точки InGaAs в матрице Al-=SUB=-0.29-=/SUB=-Ga-=SUB=-0.71-=/SUB=-As: интенсивность и кинетика люминесценции в ближнем поле серебряных наночастиц

AbstractQuantum dots of indium gallium arsenide buried in a thin layer of aluminum gallium arsenide were grown by means of molecular-beam epitaxy. The influence of silver nanoparticles grown on the surface of the semiconductor structure by vacuum thermal evaporation on photoluminescence of quantum dots was investigated. Photoluminescence spectra of quantum dots were obtained under stationary and pulsed excitation. The influence of silver nanoparticles exhibiting plasmon resonances on spectral distribution and kinetics of luminescence of the epitaxial quantum dots was studied.

Nature of luminescence of PbS quantum dots synthesized in a Langmuir–Blodgett matrix

The luminescence and structural properties of PbS quantum dots in a Langmuir–Blodgett matrix and after the removal of this matrix have been studied. It has been found that the photoluminescence peak of quantum dots in the matrix is near 1.6 eV, and luminescence disappears after the removal of the matrix. Beyond the matrix, quantum dots begin to luminesce at an energy of 0.8 eV only after aging in air lasting many days, which can be attributed to the passivation of surface defects because of oxidation. The size distribution of quantum dots in the initial Langmuir–Blodgett matrix and after its removal has been determined by transmission microscopy. Using the size distribution of quantum dots, the photoluminescence spectra have been calculated for quantum dots with different shells—Langmuir–Blodgett matrix/alkanes or oxide. It has been established that the shift of the photoluminescence spectrum of quantum dots toward lower energies is due to a decrease in the height of barriers and to an increase in the sizes of quantum dots.

Aspects of emission variation in CdSeTe/ZnS quantum dots conjugated to antibodies

CdSeTe/ZnS quantum dots (QDs) with the emission peak at 705 nm have been studied comparatively in the non-conjugated state and after bioconjugation to anti-pseudo rabies virus antibodies (ABs) by means of photoluminescence (PL) and Raman scattering methods. It is revealed that PL spectra of QDs vary significantly after conjugating to ABs. In PL spectra of non-conjugated QDs only one PL band of Gaussian shape peaked at 1.76–1.78 eV and related to exciton emission in the CdSeTe core has been detected. The PL spectra of bio-conjugated QDs demonstrate the high energy spectral shift and asymmetric shape of PL bands. The study of Raman scattering spectra permits to estimate the CdSeTe alloy composition and to detect the surface enhanced Raman scattering (SERS) effect for bioconjugated QDs. The last fact testifies on the interaction of excitation light electromagnetic field with the electric dipoles excited in ABs. The optical band gap in CdSeTe core has been calculated numerically versus core radius on the base of the effective mass approximation model. Then the energy band diagrams for non-conjugated and bio-conjugated states of CdSeTe/ZnS QDs have been designed. It is revealed the type II quantum well in CdSeTe core that explains the optical transition at 705 nm in the wide band gap CdSeTe alloy. The analysis has shown that AB dipoles excited in bio-conjugated QDs stimulate changing the profile of QD energy band diagram that manifests itself in the mentioned PL spectrum transformations. Actually, the study of PL spectrum varying in CdSeTe/ZnS QDs conjugated to specific antibodies can be an informative tool in biology and medicine for early medical diagnostics.

Shaping Photoluminescence Spectra with Magnetoelectric Resonances in All-Dielectric Nanoparticles

We measure the near-infrared photoluminescence spectra of colloidal quantum dots coupled to the localized electric and magnetic resonances of subwavelength silicon nanodisks. The spectral position of the resonances with respect to each other is controlled via the nanodisk geometry. We observe a strong influence of the nanodisk resonance positions on the quantum dot photoluminescence spectra. For separate resonances, the spectral density observed in transmittance measurements correlates with the spectral range covered by a broad emission spectrum. For the case of spectral overlap of the electric and magnetic dipolar resonances we enter a new regime for coupling, where the characteristic transparency effect evident in the transmittance spectra is accompanied by a pronounced single emission maximum. Our experimental observations are in good qualitative agreement with numerical calculations.

Luminescence Properties of InAlAs/AlGaAs Quantum Dots Grown by Modified Molecular Beam Epitaxy

Self-assembled InAlAs/AlGaAs quantum dots (QDs) on GaAs substrates were grown by using modified molecular epitaxy beam in Stranski-Krastanov method. In order to study the structural and optical properties of InAlAs/AlGaAs QDs, atomic force microscopy (AFM) and photoluminescence (PL) measurements are conducted. The size and uniformity of QDs have been observed from the AFM images. The average widths and heights of QDs are increased as the deposition time increases. The PL spectra of QDs are composed of two peaks. The PL spectra of QDs were analyzed by the excitation laser power- and temperature-dependent PL, in which two PL peaks are attributed to two predominant sizes of QDs.

Gold nanoarray deposited using alternating current for emission rate-manipulating nanoantenna

We have proposed an easy and controllable method to prepare highly ordered Au nanoarray by pulse alternating current deposition in anodic aluminum oxide template. Using the ultraviolet–visible-near-infrared region spectrophotometer, finite difference time domain, and Green function method, we experimentally and theoretically investigated the surface plasmon resonance, electric field distribution, and local density of states enhancement of the uniform Au nanoarray system. The time-resolved photoluminescence spectra of quantum dots show that the emission rate increased from 0.0429 to 0.5 ns−1 (10.7 times larger) by the existence of the Au nanoarray. Our findings not only suggest a convenient method for ordered nanoarray growth but also prove the utilization of the Au nanoarray for light emission-manipulating antennas, which can help build various functional plasmonic nanodevices.

Organic Molecules as Tools To Control the Growth, Surface Structure, and Redox Activity of Colloidal Quantum Dots

In order to achieve efficient and reliable technology that can harness solar energy, the behavior of electrons and energy at interfaces between different types or phases of materials must be understood. Conversion of light to chemical or electrical potential in condensed phase systems requires gradients in free energy that allow the movement of energy or charge carriers and facilitate redox reactions and dissociation of photoexcited states (excitons) into free charge carriers. Such free energy gradients are present at interfaces between solid and liquid phases or between inorganic and organic materials. Nanostructured materials have a higher density of these interfaces than bulk materials. Nanostructured materials, however, have a structural and chemical complexity that does not exist in bulk materials, which presents a difficult challenge: to lower or eliminate energy barriers to electron and energy flux that inevitably result from forcing different materials to meet in a spatial region of atomic dimensions. Chemical functionalization of nanostructured materials is perhaps the most versatile and powerful strategy for controlling the potential energy landscape of their interfaces and for minimizing losses in energy conversion efficiency due to interfacial structural and electronic defects. Colloidal quantum dots are semiconductor nanocrystals synthesized with wet-chemical methods and coated in organic molecules. Chemists can use these model systems to study the effects of chemical functionalization of nanoscale organic/inorganic interfaces on the optical and electronic properties of a nanostructured material, and the behavior of electrons and energy at interfaces. The optical and electronic properties of colloidal quantum dots have an intense sensitivity to their surface chemistry, and their organic adlayers make them dispersible in solvent. This allows researchers to use high signal-to-noise solution-phase spectroscopy to study processes at interfaces. In this Account, I describe the varied roles of organic molecules in controlling the structure and properties of colloidal quantum dots. Molecules serve as surfactant that determines the mechanism and rate of nucleation and growth and the final size and surface structure of a quantum dot. Anionic surfactant in the reaction mixture allows precise control over the size of the quantum dot core but also drives cation enrichment and structural disordering of the quantum dot surface. Molecules serve as chemisorbed ligands that dictate the energetic distribution of surface states. These states can then serve as thermodynamic traps for excitonic charge carriers or couple to delocalized states of the quantum dot core to change the confinement energy of excitonic carriers. Ligands, therefore, in some cases, dramatically shift the ground state absorption and photoluminescence spectra of quantum dots. Molecules also act as protective layers that determine the probability of redox processes between quantum dots and other molecules. How much the ligand shell insulates the quantum dot from electron exchange with a molecular redox partner depends less on the length or degree of conjugation of the native ligand and more on the density and packing structure of the adlayer and the size and adsorption mode of the molecular redox partner. Control of quantum dot properties in these examples demonstrates that nanoscale interfaces, while complex, can be rationally designed to enhance or specify the functionality of a nanostructured system.

Correlation between the size and photoluminescence spectrum of quantum dots in InAs-QD/GaAs

A computational method for determining the real sizes of quantum dots based on the data of atomic force microscopy (AFM) is suggested. It is shown that the experimentally observed measurement error (for the AFM method) of the sizes of the pyramidal quantum dots with a base edge of a ∼ 30–46 nm, stemming from convolution, is approximately equal to a quarter of the curvature radius of the AFM probe. Calibration curves of the dependence of the photoluminescence-peak positions of InAs-QD/GaAs heterostructures (in the range 900–1800 nm) on the sizes of the quantum dots measured with the probes whose curvature radii were from from 10 to 100 nm are established.

Electronic excitation energy transfer between CdS quantum dots and carbon nanotubes

Stationary and transient photoluminescence of CdS quantum dots deposited on silicon substrates and carbon nanotubes is investigated. The photoluminescence spectrum of quantum dots on a silicon substrate is dominated by a band originating from electron transitions between the quantum-confinement levels in the dots. When the quantum dots are deposited on carbon nanotubes, the intensity of this band decreases significantly. Furthermore, the kinetics of the photoluminescence decay becomes faster, which brings evidence of an additional channel for the quantum-dot deexcitation. The analysis of the experimental data demonstrates that the Forster energy transfer from CdS quantum dots to carbon nanotubes is most probably responsible for this channel. The efficiency of this process exceeds 60%.

Effect of Doping on the Structural and Optical Properties of Microwave-Assisted Synthesis of ZnSe@ZnS Core-Shell Quantum Dots

AbstractPure and Cu-doped quantum dots of ZnSe@ZnS were synthesized in aqueous phase using microwave irradiation at 140 °C. X-ray diffraction analyses suggested the development of a ZnSe-ZnS structure. UV-vis measurements evidenced that the presence of Cu species in quantum dots caused the blue shift of exciton peaks with respect to pure, i.e. non doped ones. Photoluminescence spectra of quantum dots synthesized at Zn/Cu mole ratios of 1/0.001 and 1/0.005 exhibited a very strong emission peak centered on ˜ 515 nm. On the contrary, a weak emission peak was observed at 412 nm in pure ZnSe@ZnS quantum dots. The observed emission at 515 nm was attributed to the internal doping of Cu species, which should have induced d-d transitions in the host lattice. Quenching of the luminescence at 515 nm was observed for nominal Cu concentrations above 0.005 mM.

Excitonic band edges and optical anisotropy of InAs∕InP quantum dot structures

Optical transitions in InAs∕InP quantum dot structures are studied and interpreted in terms of heavy- (hh) and light-hole (lh) excitons originating in InAs valence band. Appearance of a step-like absorption and two activation energies in photoluminescence spectra of quantum dots under temperature elevation is described as a transition from the excitonic states to the two-dimensional-like states realized in the InAs∕InP quantum dots. A mixture of hh and lh states determines the anisotropy of the transmission spectra and may result in rotation of the polarization plane for light transmitted through the InAs∕InP quantum dot system.

Photoluminescence spectroscopy of bioconjugated CdSe∕ZnS quantum dots

The authors performed scanning photoluminescence (PL) spectroscopy on CdSe∕ZnS core/shell quantum dots (QDs) bioconjugated to Interleukin 10 (IL10) antibody. The influence of IL10 on the QD photoluminescence spectra was explored on samples dried on solid substrates at various temperatures. A “blue” up to 15nm spectral shift of the PL maximum was observed on the bioconjugated QDs. The spectral shift is strongly increased after samples annealing above room temperature. A mechanism of the observed effect is attributed to changes in the QD electronic energy levels caused by local electric or stress field or chemical reactions which occurred on the QD surface.

High-Density ZnSe Self-Assembled Quantum Dots Grown on ZnS ∕ GaAs with Special ZnS Buffer Layer

The fabrication of high-density self-assembled quantum dots is important for high-efficiency blue-green laser diodes. self-assembled quantum dots cannot be obtained on the high-strain with the thickness of , which is higher than its critical thickness . self-assembled quantum dots on 15, 57, 103, 130, and were studied. For thick , the density of quantum dots can reach for a fixed growth. All photoluminescence spectra of quantum dots show blue shifts but relative red shifts with the thickness of 57, 103, and . The mechanisms were studied.

Estimation of built-in dipole moment in InAs quantum dots

We have fabricated a Schottky diode embedding InAs self-assembled quantum dots (QDs) grown by alternately supplying In and As sources. As a function of the electric field, we have investigated the photoluminescence (PL) for the InAs QDs in the Schottky diode at 300 K. We controlled the electric field in order that the QD layer was located in the depletion region of Schottky diode. The relationship between the electric field and the depletion width of the Schottky diode was deduced through the capacitance–voltage measurement. The Stark shift was observed in PL spectra for QDs; the energy of the PL line shifted to the lower energy as the electric field increased. It was also observed that the PL emission intensity gradually decreased. By the fitting to the experimental data, we determined a built-in dipole moment, corresponding to an electron–hole separation.

Electron–phonon interaction in absorption and photoluminescence spectra of quantum dots

Abstract Optical absorption of quantum dots is investigated using the adiabatic approach, in which a Frohlich-type interaction between electron–hole pair and longitudinal optical phonons has been assumed. The role of the finite barrier potentials in predicting the magnitude of the Huang–Rhys factors is considered. For the small GaAs/AlAs quantum dots, the Huang–Rhys factors are found to be larger than those in the bulk phase by one to two orders of magnitude. The absorption and photoluminescence spectra of such nanostructures are analyzed.

Intraband carrier relaxation in quantum dots mediated by surface plasmon-phonon excitations

A new mechanism of the intraband carrier relaxation in quantum dots embedded into a heterostructure at a relatively large distance from its doped elements is proposed. The relaxation process is related to the coupling between the electronic subsystem of a quantum dot and surface plasmon-phonon excitations of the doped components of the heterostructure via the electric potential produced by these excitations. It is found that, in layered heterostructures, the dispersion relations of the surface plasmon-LO-phonon modes display critical points giving rise to pronounced singularities in the relaxation rate spectra. The estimates of the relaxation rates for InAs quantum dots embedded into a GaAs heterostructure have shown high efficiency of the proposed mechanism even when the quantum dots are located at a distance of up to 100 nm from the doped regions of the heterostructure. When this distance lies in the range of a few tens of nanometers, this mechanism appears to be predominant. Possible manifestations of the relaxation mechanism under consideration in the photoluminescence spectra of quantum dots are discussed.

Optical spectra of quantum dots: effects of non‐adiabaticity

It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton–phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck–Condon progression, prescribed by the adiabatic approach.