Isolated Quantum Dots Research Articles

Nonlinear optical response of a 2D quantum dot supercrystal

We investigate theoretically the nonlinear optical response of a two-dimensional supercrystal comprized of semiconductor quantum dots. An isolated quantum dot is modeled as a three-level ladder-like system with ground, one-exciton, and biexciton states. It is shown that the optical response of supercrystal demonstrate a rich nonlinear dynamics, including bistability, self-oscillations, and dynamical chaos.

Dynamical thermalization in isolated quantum dots and black holes

We study numerically a model of quantum dot with interacting fermions. At strong interactions with small conductance the model is reduced to the Sachdev-Ye-Kitaev black-hole model while at weak interactions and large conductance it describes a Landau-Fermi liquid in a regime of quantum chaos. We show that above the Åberg threshold for interactions there is an onset of dynamical themalization with the Fermi-Dirac distribution describing the eigenstates of an isolated dot. At strong interactions in the isolated black-hole regime there is also the onset of dynamical thermalization with the entropy described by the quantum Gibbs distribution. This dynamical thermalization takes place in an isolated system without any contact with a thermostat. We discuss the possible realization of these regimes with quantum dots of 2D electrons and cold ions in optical lattices.

Enhanced Emission from Single Isolated Gold Quantum Dots Investigated Using Two-Photon-Excited Fluorescence Near-Field Scanning Optical Microscopy.

New approaches in molecular nanoscopy are greatly desired for interrogation of biological, organic, and inorganic objects with sizes below the diffraction limit. Our current work investigates emergent monolayer-protected gold quantum dots (nanoclusters, NCs) composed of 25 Au atoms by utilizing two-photon-excited fluorescence (TPEF) near-field scanning optical microscopy (NSOM) at single NC concentrations. Here, we demonstrate an approach to synthesize and isolate single NCs on solid glass substrates. Subsequent investigation of the NCs using TPEF NSOM reveals that, even when they are separated by distances of several tens of nanometers, we can excite and interrogate single NCs individually. Interestingly, we observe an enhanced two-photon absorption (TPA) cross section for single Au25 NCs that can be attributed to few-atom local field effects and to local field-induced microscopic cascading, indicating their potential for use in ultrasensitive sensing, disease diagnostics, cancer cell therapy, and molecular computers. Finally, we report room-temperature aperture-based TPEF NSOM imaging of these NCs for the first time at 30 nm point resolution, which is a ∼5-fold improvement compared to the previous best result for the same technique. This report unveils the unique combination of an unusually large TPA cross section and the high photostability of Au NCs to (non-destructively) investigate stable isolated single NCs using TPEF NSOM. This is the first reported optical study of monolayer-protected single quantum clusters, opening some very promising opportunities in spectroscopy of nanosized objects, bioimaging, ultrasensitive sensing, molecular computers, and high-density data storage.

Plasmonic Effect on Exciton and Multiexciton Emission of Single Quantum Dots.

Quantum dots are nanoscale quantum emitters with high quantum yield and size-dependent emission wavelength, holding promises in many optical and electronic applications. When quantum dots are situated close to noble metal nanoparticles, their emitting behavior can be conveniently tuned because of the interaction between the excitons of the quantum dots and the plasmons of the metal nanoparticles. This interaction at the single quantum dot level gives rise to reduced or suppressed photoluminescence blinking and enhanced multiexciton emission, which is difficult to achieve in isolated quantum dots. However, the mechanism of how plasmonic structures cause the changes in the quantum dot emission remains unclear. Because of the complexity of the system, the interfaces between metal, semiconductor, and ligands must be considered, in addition to factors such as geometry, interparticle distance, and spectral overlap. The challenges in the design and fabrication of the hybrid nanostructures as well as in understanding the exciton-plasmon coupling mechanism can be overcome by a cooperative effort in synthesis, optical spectroscopy, and theoretical modeling.

Quantum Manipulation of Two-Electron Spin States in Isolated Double Quantum Dots.

We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in an isolated double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows us to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo [Phys. Rev. A 57, 120 (1998)]. The present study demonstrates that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots.

Spin initialization of a p-doped quantum dot coupled to a bowtie nanoantenna

The spin initialization of a hybrid system consisting of a p-doped semiconductor quantum dot coupled to a gold bowtie nanoantenna is analyzed. The quantum dot is described as a four-level atom-like system using the density matrix formalism. The two lower levels are Zeeman-split hole spin states and the two upper levels correspond to positively charged excitons with spin-up, spin-down hole pair and opposite spin electron. The gold bowtie nanoantenna is placed in close proximity to the quantum dot. A linearly polarized laser field drives two of the optical transitions of the quantum dot and produces localized surface charge oscillations in the nanoantenna which act back upon the quantum dot thus changing the effective field felt by it. The angular frequencies of those charge oscillations are very different along its two principal axes, resulting in an anisotropic modification of the spontaneous emission rates of the allowed optical transitions of the quantum dot. These changes are accounted for by using the Green tensor method, and result in a faster spin state initialization than that of the isolated quantum dot. We also show that the presence of the nanoantenna dramatically modifies the optical properties of the fluorescent photons, either in the spectral or in the time domain.

Single QD emission from arrays of QD chains obtained by patterning-free method

The selective growth of InAs quantum dot chains self-ordered at mesoscopic distances on a rippled GaAs(001) substrate is obtained by tuning the direction of the As flux at high temperature and high As/In flux ratio. Microphotoluminescence experiments demonstrate that such isolated quantum dot chains show single quantum dot emission properties.

Measuring the complex admittance of a nearly isolated graphene quantum dot

We measured the radio-frequency reflection spectrum of an on-chip reflection line resonator coupled to a graphene double quantum dot (DQD), which was etched almost isolated from the reservoir and reached the low tunnel rate region. The charge stability diagram of DQD was investigated via dispersive phase and magnitude shift of the resonator with a high quality factor. Its complex admittance and low tunnel rate to the reservoir was also determined from the reflected signal of the on-chip resonator. Our method may provide a non-invasive and sensitive way of charge state readout in isolated quantum dots.

Size Quantization Stark Effect in Quantum Dots

Advantages of semiconductor quantum dots for study and diagnostics of biological nanosystems are discussed. A new method for amino acid diagnostics used semiconductor quantum dots is posed. Interaction of isolated quantum dots with charged amino acids is studied in detail. It is shown that such interaction results in a shift of the quantum dots luminescence spectra by several dozens of meV. This effect provides new possibilities for identification of biological nanoobjects using quantum dots.

Investigation of quantum and classical correlations in a quantum dot system under decoherence

In this paper, we investigate quantitatively the thermal classical and quantum correlations in an isolated quantum dot system (QDS) including the effects of different parameters. We show that the quantum discord (QD) is more resistant against the temperature effect and might be finite even for higher temperatures in the asymptotic limit. Decoherence in a QDS caused by interaction with its environment is another interesting issue in the quantum information field. Assuming Markovian dynamics for the time evolution, we present noise models for the QDS by using Kraus operators for several noisy channels; in particular bit flip, bit-phase flip, phase flip, and depolarizing channels. By analytical and numerical analyses, we investigate the dynamics of different kinds of correlations, namely, the mutual information, the classical correlation, the entanglement of formation (EOF), and the QD in different channels. The sudden change in behavior in the decay rates of correlations and their immunity against certain decoherences are shown. We explore a symmetry among these channels and provide the decoherence areas for which both classical and quantum correlations remain affected in the QDS.

Numerical computation of pyramidal quantum dots with band non-parabolicity

This paper presents an effective and feasible eigen-energy scanning method to solve polynomial matrix eigenvalues introduced by 3D quantum dots problem with band non-parabolicity. The pyramid-shaped quantum dot is placed in a computational box with uniform mesh in Cartesian coordinates. Its corresponding Schrödinger equation is discretized by the finite difference method. The interface conditions are incorporated into the discretization scheme without explicitly enforcing them. By comparing the eigenvalues from isolated quantum dots and a vertically aligned regular array of them, we investigate the coupling effect for variable distances between the quantum dots and different size.

Detecting quantum and classical correlations using quantum dot system

We investigate the thermal classical and quantum correlations in an isolated quantum dot system (QDS) including the effects of different parameters. The thermal density operator is generated by simplifying the Hamiltonian of the quantum dot to the nature Hamiltonian by integrating and finding the unitary matrix. We find that the quantum discord (QD) is more resistant against temperature effect and might be finite even for higher temperatures in the asymptotic limit. Furthermore, we show that there is an optimal value of temperature such that the different kinds of correlations are maximal. Our results show that QDS is a useful resource and may open new perspectives in different quantum information tasks.

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO2 is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO2 QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO2 QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO2 QDs.

Transition from silicon nanowires to isolated quantum dots: Optical and structural evolution

The evolution of the structural and optical properties of a silicon core in oxidized nanowalls is investigated as a function of oxidation time. The same individual nanostructures are characterized ...

Improved performance of nanowire–quantum-dot–polymer solar cells by chemical treatment of the quantum dot with ligand and solvent materials

We report a nanowire–quantum-dot–polymer solar cell consisting of a chemically treated CdSe quantum dot film deposited on n-type ZnO nanowires. The electron and hole collecting contacts are a fluorine-doped tin-oxide/zinc oxide layer and a P3HT/Au layer. This device architecture allows for enhanced light absorption and an efficient collection of photogenerated carriers. A detailed analysis of the chemical treatment of the quantum dots, their deposition, and the necessary annealing processes are discussed. We find that the surface treatment of CdSe quantum dots with pyridine, and the use of 1,2-ethanedithiol (EDT) ligands, critically improves the device performance. Annealing at 380 °C for 2 h is found to cause a structural conversion of the CdSe from its initial isolated quantum dot arrangement into a polycrystalline film with excellent surface conformality, thereby resulting in a further enhancement of device performance. Moreover, long-term annealing of 24 h leads to additional increases in device efficiency. Our best conversion efficiency reached for this type of cell is 3.4% under 85 mW cm−2 illumination.

Combining nanoscale manipulation with macroscale relocation of single quantum dots

We have controllably positioned, with nanometre precision, single CdSe quantum dots referenced to a registration template such that the location of a given nanoparticle on a macroscopic (≈1 cm2) sample surface can be repeatedly revisited. The atomically flat sapphire substrate we use is particularly suited to optical measurements of the isolated quantum dots, enabling combined manipulation–spectroscopy experiments on a single particle. Automated nanoparticle manipulation and imaging routines have been developed so as to facilitate the rapid assembly of specific nanoparticle arrangements.

Finite element modelling of semi and nonpolar GaN/AlN quantum dots

This paper describes results of a finite element analysis of the elastic and electric field distribution in a semipolar and a nonpolar isolated quantum dot based on previously obtained measurements from transmission electron microscopy. The two quantum dot orientations are each investigated and compared in terms of the resultant piezoelectric fields and their redistribution due to growth orientation and quantum dot geometry/surface effects. Alongside that, a standard polar quantum dot is investigated as a reference-state system. It is found that the geometry of quantum dots grown in alternative orientations affect the elastic strain and, along with orientation dependent spontaneous polarisation, modify the electrostatic potential and the built-in electric fields. A theoretical verification of a reduction in the quantum confined Stark effect by determining the band edge splitting energies for electron and hole states is given.

Blinking Dynamics: Photoluminenscence Blinking Dynamics of Colloidal Quantum Dots in the Presence of Controlled External Electron Traps (Small 2/2012)

A single-molecule spectroscopy study on colloidal quantum dots reveals the effect of the external charge trap on the photoluminescence blinking dynamics. The study uses a series of colloidal quantum-dot-bridge-fullerene dimers with varying bridge lengths where the fullerene moiety acts as a well-defined, well-positioned external electron trap. A single external electron trap is shown to be able to switch a quantum dot from having “telegraph-like”, two-state on/off blinking behavior into exhibiting quasi- continuous, multistate blinking. Additionally, charge transfer followed by charge recombination is found to be an important mechanism in determining the blinking behavior of isolated quantum dots.

Coherent Longitudinal-Optical Ground-State Phonon in CdSe Quantum Dots Triggered by Ultrafast Charge Migration

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35meV.

Growth of low density InP/GaInP quantum dots

We demonstrate low density InP quantum dot growth by metalorganic vapour phase epitaxy on (100) GaAs substrates with a 3 ° misorientation towards ⟨111⟩. Using micro-photoluminescence characterisation, we identify the transitional region from single to bimodal quantum dot size distributions where InP deposition thicknesses, over a range of ≈ 1.1 Å, provide a region of spatially and spectrally isolated quantum dots suitable for single dot studies.