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The combination of inorganic semiconductor nanocrystals, also called quantum dots (QDs), with biological materials has recently attracted considerable interest since the QDs can be used as superior fluorescent labels. Here, we report on CdSe QD initiated current bursts in lipid bilayer membranes on application of a bias voltage. The current bursts observed resemble those produced by the peptaibol class of antibiotics such as alamethicin and trichorzins. The current fluctuations are dependent on the bias voltage and on the concentration of the QD applied to the membrane. Our data suggest that QDs with intrinsic dipole moments similar to alamethicin can be controlled by an external electric field, which creates a torque resulting in the insertion into the lipid membrane.

The first part of this work evaluates Al-doped ZnO (AZO) as an optically transparent top-gate material for studies on semiconductor quantum dots. In comparison with conventional Ti gates, samples with AZO gates demonstrate a more than three times higher intensity in the quantum dot emission under comparable excitation conditions. On the other hand, charges inside a process-induced oxide layer at the interface to the semiconductor cause artifacts at gate voltages above  U ≈  1 V. The second part describes an optical and simulation study of a vertical electric-field (F)-induced switching from a strong to an asymmetric strong–weak confinement in GaAs cone-shell quantum dots (CSQDs), where the charge carrier probability densities are localized on the surface of a cone. These experiments are performed at low U and show no indications of an influence of interface charges. For a large F, the measured radiative lifetimes are substantially shorter compared with simulation results. We attribute this discrepancy to an F-induced transformation of the shape of the hole probability density. In detail, an increasing F pushes the hole into the wing part of a CSQD, where it forms a quantum ring. Accordingly, the confinement of the hole is changed from strong, which is assumed in the simulations, to weak, where the local radius is larger than the bulk exciton Bohr radius. In contrast to the hole, an increasing F pushes the electron into the CSQD tip, where it remains in a strong confinement. This means the radiative lifetime for large F is given by an asymmetric confinement with a strongly confined electron and a hole in a weak confinement. To our knowledge, this asymmetric strong–weak confinement represents a novel kind of quantum mechanical confinement and has not been observed so far. Furthermore, the observed weak confinement for the hole represents a confirmation of the theoretically predicted transformation of the hole probability density from a quantum dot into a quantum ring. For such quantum rings, application as storage for photo-excited charge carriers is predicted, which can be interesting for future quantum photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Strain-free GaAs cone-shell quantum dots have a unique shape, which allows a wide tunability of the charge-carrier probability densities by external electric and magnetic fields. Here, the influence of a lateral electric field on the optical emission is studied experimentally using simulations. The simulations predict that the electron and hole form a lateral dipole when subjected to a lateral electric field. To evaluate this prediction experimentally, we integrate the dots in a lateral gate geometry and measure the Stark-shift of the exciton energy, the exciton intensity, the radiative lifetime, and the fine-structure splitting (FSS) using single-dot photoluminescence spectroscopy. The respective gate voltage dependencies show nontrivial trends with three pronounced regimes. We assume that the respective dominant processes are charge-carrier deformation at a low gate voltage U, a vertical charge-carrier shift at medium U, and a lateral charge-carrier polarization at high U. The lateral polarization forms a dipole, which can either enhance or compensate the intrinsic FSS induced by the QD shape anisotropy, dependent on the in-plane orientation of the electric field. Furthermore, the data show that the biexciton peak can be suppressed by a lateral gate voltage, and we assume the presence of an additional vertical electric field induced by surface charges. [ABSTRACT FROM AUTHOR]

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone–shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature T x , m a x of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, T x , m a x becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above T x , m a x , the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above T x , m a x , the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes. [ABSTRACT FROM AUTHOR]

We perform microwave spectroscopy on a single quantum dot using a compact pulsed spectrometer. Although DC transport shows strong elastic cotunneling in the Coulomb blockade regime and inelastic tunneling in the single-electron tunneling (SET) regime, these effects are suppressed under picosecond impulses whose energies are commensurate with those of the dot. Instead, under non-zero bias we find excited-state resonances in the induced complex photoconductance as well as strong deviations of the dot capacitance compared to equilibrium values, a signature of the effective relaxation times of SET. [Copyright &y& Elsevier]

Inorganic semiconductor nanocrystals, also called quantum dots, have recently attracted considerable interest as fluorescent labels. We report that CdSe QDs initiate current bursts in lipid bilayer membranes upon application of a bias voltage. The current bursts observed resemble those produced by the peptaibol class of antibiotics such as alamethicin and trichorzins. The current fluctuations are dependent on the bias voltage and on the concentration of the quantum dots applied to the membrane. Our data suggest that quantum dots with dipole moments similar to alamethicin are influenced by an external electric field, which creates a torque resulting in the insertion into the lipid membrane. We predict that at least three quantum dots are required to form a pore due to aggregation that leads to a macroscopic conductance. [ABSTRACT FROM AUTHOR]

We introduce a versatile two-step electron-beam fabrication technique for defining multiport quantum dots in GaAs/AlGaAs heterostructures in close vicinity. Capacitive coupling of the two quantum dots is directly tuned electrostatically via two central gates. Parallel access is realized by patterning source and drain contact regions of both dots with an additional spacer layer. Conductance measurements give evidence of both the continuous wide-range tunability of the dots and parallel access to the circuit. [ABSTRACT FROM AUTHOR]

We present work on single and double quantum dots which have been defined in the two-dimensional electron gas of a freely suspended GaAs/AlGaAs heterostructure. The dots are formed either by pairs of etched constrictions or by gating electrodes and characterized in Coulomb blockade measurements. Because of their small overall dimensions the suspended dots constitute phonon cavities in which electron transport is strongly modified by electron–phonon interaction. Our measurements indicate that the emission of phonons suppresses linear transport in the Coulomb blockade regime. Upon application of a perpendicular magnetic field Coulomb blockade oscillations reemerge, as well as under nonlinear transport conditions. [Copyright &y& Elsevier]

Abstract: A few-electron quantum dot utilizing Schottky, metal top gates in a modulation doped Si/SiGe heterostructure was realized and non-linear transport through the dot was studied. By carefully tuning the capacitively coupled gates, the single quantum dot was transformed into two tunnel-coupled quantum dots in series. The resulting double quantum dot was tuned to the few-electron regime and the charge stability diagram was studied as a function of the gate voltages. Understanding of such a double dot system is essential for the practical implementation of exchange-mediated multi-qubit systems in silicon devices. [Copyright &y& Elsevier]