Decreasing Quantum Dots Size Research Articles

Structural and Optical Characterisation of Size-Selected Glutathione-Capped Colloidal Cu–In–S Quantum Dots

Size-selected series of copper-deficient colloidal Cu–In–S quantum dots (QDs) stabilized with glutathione are obtained by the exchange reaction in aqueous solutions under mild synthesis conditions. The optical bandgap and photoluminescence maximum position shift toward higher energies with decreasing QD size. Based on X-ray diffraction data, the QDs are assigned to a tetragonal chalcopyrite-type structure. The average size of QDs, estimated from the Scherrer formula and from the comparison with the absorption edge-based sizing curves, exhibits a fair agreement, being in the interval of 1.2–2.9 nm. The Raman spectra of Cu–In–S QDs are analyzed with the account for the QD structure, confinement-related effects, non-stoichiometry, and possible existence of secondary phases.

Synthesis and Optical Properties of Ag–Ga–S Quantum Dots

Ag–Ga–S quantum dots (QDs) are obtained by colloidal synthesis in aqueous solutions with glutathione ligands at mild conditions. The QDs can be assigned to metastable orthorhombic, rhombohedral, or rocksalt‐type phases, assuming a significant internal pressure within crystallites, while the conventional tetragonal AgGaS2 phase is less likely formed. Similarly to these metastable phases, the QDs reveal a relatively narrow indirect bandgap contrary to the wide and direct bandgap of tetragonal AgGaS2. Size‐selected fractions of the colloidal solutions are separated by repeated centrifuging with addition of 2‐propanol. The QD size is evaluated from atomic force microscopy, while the crystallite size determined using the Scherrer formula is underestimated. X‐ray photoemission spectroscopy reveals that the QD composition changes from nearly stoichiometric for the fraction with the largest QDs toward Ag deficient and S rich for smaller QDs. The synthesized QDs are characterized by the absorption edge blueshift with decreasing QD size. A blueshift of the photoluminescence (PL) peak with decreasing QD size is observed as well as a nonmonotonous size dependence of the PL intensity. Raman spectra of the Ag–Ga–S QDs are discussed compared to those of bulk crystals and a strong contribution of surface phonons is revealed.

Influence of Size and Shape Anisotropy on Optical Properties of CdSe Quantum Dots.

We used low-temperature reactions to synthesize different-sized CdSe quantum dots (QDs) capped with fatty-acid and phosphine ligands. From the correlation of high-resolution transmission electron microscopy and X-ray diffraction (XRD) analyses of the synthesized QDs, we observed size-dependent shape anisotropy. In addition, the recorded XRD patterns revealed mixed crystal facets with zinc blende and wurtzite structures in small-sized QDs. Furthermore, from differential absorption (DA) spectra, we extracted the electronic transition energies for different-sized QDs, which were found to be similar to the calculated values of the quantum size levels associated with band mixing of CdSe QDs with a moderate bandgap. We found that the excitonic absorption peaks are increasingly “hidden” with decreasing QD size because of the crystal structure and crystalline quality. The results show good agreement with the obtained diffraction patterns and the estimation errors obtained from the DA spectra.

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ∼0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes.

Ultrafast carrier dynamics in CuInS2 quantum dots

The ultrafast carrier dynamics in CuInS2 (CIS) quantum dots (QDs) was studied by means of femtosecond transient absorption (TA) spectroscopy. The size-dependent 1S transition energy determined from bleaching spectra is in agreement with that calculated on the finite-depth-well model in the effective mass approximation. The TA bleaching comes from filling of electron quantized levels, allowing us to know the dynamics of the 1S electron in CIS QDs. The sub-100-ps electron trapping at surface defects in bare QDs accelerates with decreasing QD size, while is effectively suppressed in well-passivated CIS/ZnS core/shell QDs.

In-situ fabrication and characterization of ordered Ge QDs in Si3N4 matrix without barrier layers by rf-magnetron sputtering

In this work, in-situ fabrication and characterization of ordered arrays of germanium (Ge) quantum dots (QDs) embedded in silicon nitride matrix were studied. A single layer of Ge and Si3N4 composite was deposited by rf-magnetron sputtering with substrate heating. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to investigate structural properties of the Ge QDs. TEM images showed that uniformly sized Ge QDs were formed in diagonal QD arrays grown from the substrate. The average size estimated from XRD was in agreement with the QD size observed in TEM images. Chemical composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS), confirming the Ge0 state of the Ge–Ge bond in the QDs. Phonon confinement in the QDs was demonstrated by phonon peak broadening in Raman spectra. The confinement and stress effect resulted in phonon peak shifting to 297cm−1. With decreasing QD size, E1 and E2 transitions estimated from spectroscopic ellipsometry were blue-shifted, indicating the quantum confinement effect. This closely spaced Ge QD structure would be potentially advantageous for third generation photovoltaic applications as this structure could improve electrical conductivity while confining the QD growth.

Size-dependent optical absorption of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix

Optical constants of silicon quantum dots (Si QDs) in SiO2/Si3N4 hybrid matrix on quartz substrates have been determined in the wavelength range of 350–2500nm. Forouhi–Bloomer (F-B) dispersion model is applied to describe the optical properties of silicon nanocrystals and the mismatch due to global fitting is fixed point by point. To extract the specific optical bandgap of silicon nanocrystal samples, Tauc-plots with γ=1/2 and γ=1/3 are used and discussed for the spectral range of 2eV–3eV. The effect of quantum dot size on the optical bandgap is explored. The results from both Tauc-plot and photoluminescence show the trend that the optical bandgap increases with decreasing quantum dot size, which agrees with the prediction of quantum confinement.

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.

Insertion of CdSe quantum dots in ZnSe nanowires: Correlation of structural and chemical characterization with photoluminescence

ZnSe nanowires with CdSe quantum dot insertions were grown by molecular beam epitaxy using gold as a catalyst. Structural, chemical, and optical properties of the wires and quantum dots were characterized using electron microscopy and photoluminescence spectroscopy. We determined the crystalline structure, the chemical composition, and the size of the quantum dot and established a correlation between quantum dot size and luminescence. As expected, a blueshift of the luminescence was observed for decreasing quantum dot size. The comparison of calculated photoluminescence energy and experimental data seems to indicate that the quantum dots consist of a ZnxCd1-xSe ternary alloy rather than pure CdSe.

Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots

Strain and piezoelectric potential effects on optical properties in CdSe/CdS core/shell quantum dots (QDs) were investigated theoretically using an eight-band strain-dependent k·p Hamiltonian. The strain effect on the shift of the subband energies is found to be larger than the piezoelectric field effect. As a result, interband transition energies are blueshifted with the inclusion of strain and piezoelectric field effects. We know that the theoretical interband transition energy shows a reasonable agreement with the experimental result. The absolute value of the hydrostatic strain in the QD increases with decreasing QD size, whereas that in the barrier decreases with decreasing QD size.

Size dependence of carrier dynamics and carrier multiplication in PbS quantum dots

The time dynamics of the photoexcited carriers and carrier-multiplication efficiencies in PbS quantum dots (QDs) are investigated. In particular, we report on the carrier dynamics, including carrier multiplication, as a function of QD size and compare them to the bulk value. We show that the intraband 1P→1S decay becomes faster for smaller QDs, in agreement with the absence of a phonon bottleneck. Furthermore, as the size of the QDs decreases, the energy threshold for carrier multiplication shifts from the bulk value to higher energies. However, the energy threshold shift is smaller than the band-gap shift and, therefore, for the smallest QDs, the threshold approaches 2.35 Eg, which is close to the theoretical energy conservation limit of twice the band gap. We also show that the carrier-multiplication energy efficiency increases with decreasing QD size. By comparing to theoretical models, our results suggest that impact ionization is sufficient to explain carrier multiplication in QDs.

Exciton-exciton and exciton-phonon interactions in an interfacial GaAs quantum dot ensemble

Using optical two-dimensional Fourier transform spectroscopy, we report temperature- and excitation-density-dependent measurements of the homogeneous linewidth of the exciton ground-state transition in a single layer of interfacial GaAs quantum dots (QDs). We show that the homogeneous linewidth increases nonlinearly with temperature from 6 to 50 K and that the thermal broadening is well described by an activation term and offset. The absence of a phonon-activation peak in the two-dimensional spectra reveals that elastic scattering of excitons with acoustic phonons via virtual transitions between the ground and excited states significantly contributes to the thermal broadening. We find that the combination of increasing virtual activation energy and exciton-phonon coupling strength with decreasing QD size results in greater thermal broadening for excitons localized in smaller QDs. The homogeneous linewidth also exhibits a strong excitation-density dependence and is shown to increase linearly as the photon density increases from $2\ifmmode\times\else\texttimes\fi{}{10}^{11}$ to $1\ifmmode\times\else\texttimes\fi{}{10}^{12}$ photons pulse${}^{\ensuremath{-}1}$ cm${}^{\ensuremath{-}2}$ at 6 K. This trend is attributed to strong coupling of excitons within the same QD and is independent of the quantum-well exciton population density.

III–V semiconductor quantum dots with a magnetic impurity

AbstractWe investigate the hole states in GaAs and InAs quantum dots (QDs) containing a single substitutional Mn impurity, which is a shallow acceptor in the configuration d5+h. For an on‐center Mn, in spherical nanocrystals the hole spectrum is found to retain the bulk level scheme, while the binding energy and the sp –d exchange contribution increase rapidly with decreasing QD size. In contrast, in lens‐shaped self‐assembled QDs the four‐fold ground state is split into two well‐separated doublets, the confinement‐induced enhancement is slower and the effective exchange strength an order‐of‐magnitude smaller than in the bulk. A preliminary study of the twohole states indicates that the ground state in InAs/GaAs QDs is a singlet, almost uncoupled to the Mn spin. We deduce the zero‐field fine structure of the excitonic transitions and compare our results with recently reported photoluminescence spectra (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure of Mn-doped III-V semiconductor quantum dots

The electronic structure of GaAs and InAs quantum dots (QDs) containing a single substitutional Mn impurity is investigated in the envelope-function formalism. The Mn impurity in these compounds is known to be a shallow acceptor in the configuration ${d}^{5}+h$ and characterized by a strong antiferromagnetic $sp\text{\ensuremath{-}}d$ exchange interaction between the hole $(h)$ and the Mn ion. Our model for the hole states is based on the Luttinger Hamiltonian and the Coulomb potential with a central-cell correction that accounts for the observed binding energy and the effective $g$ factor in the bulk. The binding energy as well as the exchange contribution is found to increase with decreasing QD size. However, in contrast with the case of spherical nanocrystals (NCs), the binding energy in lens-shaped self-assembled QDs in the low-confinement limit is lower than that in the bulk because of their highly anisotropic shape. With an on-center impurity, NCs retain the bulk ${T}_{d}$ symmetry and the ground state is a $j=3/2$-like ${\ensuremath{\Gamma}}_{8}({T}_{d})$ level. In self-assembled QDs it splits into two doublets: ${\ensuremath{\Gamma}}_{6}$ $(|{j}_{z}|=1/2)$ and ${\ensuremath{\Gamma}}_{7}$ $(|{j}_{z}|=3/2)$ of ${D}_{2d}$, which mix in the presence of in-plane asymmetry, both belonging to ${\ensuremath{\Gamma}}_{5}$ of the reduced symmetry ${C}_{2v}$. The order and the splitting between the doublets depend on the degree of confinement and the strain-induced separation between the light- and heavy-hole valence bands. In lattice-matched GaAs/(Ga,Al)As QDs the ground-state doublet is $|{j}_{z}|=3/2$-like in the low-confinement limit. As the lateral size decreases there is a rapid crossover to a $|{j}_{z}|=1/2$-like ground state in QDs of typical sizes. On the other hand, in strained InAs/GaAs QDs the ground state is always $|{j}_{z}|=3/2$-like and the splitting relatively large. The $sp\text{\ensuremath{-}}d$ coupling with the Mn spin $S=5/2$ finally leads to a splitting of the ground-state doublet into six doubly-degenerate levels. The components are close to one another as the effective exchange parameters are an order of magnitude smaller than in the bulk. Our results thoroughly contradict the previously adopted picture based on treating the confinement potential as a small perturbation to the bulk impurity levels. We also consider the lowest two-hole states: the ground state in InAs/GaAs QDs is a singlet almost uncoupled to the Mn spin. We deduce the zero-field fine structure of the excitonic transitions and compare the results with the recently reported photoluminescence spectra.

Size Dependence of Carrier Recombination Efficiency in GaN Quantum Dots

The dependence of radiative recombination rate and efficiency on GaN quantum-dot (QD) size and temperature is studied by time-resolved photoluminescence (PL) spectroscopy. The emission is dominated by radiative recombination at low temperatures (<125 K) and exhibits high PL efficiency at room temperature. The radiative lifetime and the relative quantum efficiency decrease with the decreasing QD size.

Role of Orbital Dynamics in Spin Relaxation and Weak Antilocalization in Quantum Dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.

Ge/Si quantum dots in external electric and magnetic fields

Electric field-induced splitting of the lines of exciton optical transitions into two peaks is observed for Ge/Si structures with quantum dots (QDs). With increasing field, one of the peaks is displaced to higher optical transition energies (blue shift), whereas the other peack is shifted to lower energies (red shift). The results are explained in terms of the formation of electron-hole dipoles of two types differing in the direction of the dipole moment; these dipoles arise due to the localization of one electron at the apex of the Ge pyramid and of the other electron under the base of the pyramid. By using the tight-binding method, the principal values of the g factor for the hole states in Ge/Si quantum dots are determined. It is shown that the g factor is strongly anisotropic, with the anisotropy becoming smaller with decreasing QD size. The physical reason for the dependence of the g factor on quantum-dot size is the fact that the contributions from the states with different angular-momentum projections to the total wave function change with the QD size. Calculations show that, with decreasing QD size, the contribution from heavy-hole states with the angular-momentum projections ±3/2 decreases, while the contributions from light-hole states and from states of the spin-split-off band with the angular-momentum projections ±1/2 increase.

Influence of phonons on the electronic energy spectrum of small semiconductor quantum dots in a dielectric matrix

A diagrammatic technique developed for Green’s functions with inclusion of multiphonon processes is used to investigate the electronic energy levels and the phonon replicas corresponding to them in a semiconductor quantum dot (QD) embedded in a dielectric matrix. It is shown, with reference to GaAs, CdSe, and CuCl quantum dots embedded in glass, that in the case of QD potential wells of a finite depth the shifts of the electronic energy levels decrease with decreasing QD size, irrespective of the strength of electron-phonon coupling in the nanoheterostructure. Theoretically calculated positions of the phonon replicas for CdSe in glass agree with the experimental data on Raman scattering.

Size-dependent Raman study of InP quantum dots

Raman spectrum of a quantum dot (QD) is characterized by transverse (TO) and longitudinal (LO) optical modes as well as surface optical modes, occurring between the TO and LO modes. We have studied in detail the size-dependence of the Raman spectrum of InP QD of diameter larger than 35 Å. The LO phonon frequency decreases while the TO phonon frequency increases with decreasing QD size. The linewidth of the LO phonon broadens and the broadening becomes increasingly asymmetrical towards the low frequency side as the QD size decreases. By analyzing the Raman intensity ratio of the LO phonon to its overtone, we find that the electron-phonon coupling decreases with decreasing QD size.

Optical properties of GaN quantum dots

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on AlxGa1−xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the AlxGa1−xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant.