Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot

Employing the Anderson impurity model, we study tunneling properties through an ideal quantum dot near the conductance minima. Considering the Coulomb blockade and the quantum confinement on an equal footing, we have obtained current contributions from various types of tunneling processes; inelastic cotunneling, elastic cotunneling, and resonant tunneling of thermally activated electrons. We have found that the inelastic cotunneling is suppressed in the quantum confinement limit, and thus the conductance near its minima is determined by the elastic cotunneling at low temperature ($k_BT \ll \Gamma$, $\Gamma$: dot-reservoir coupling constant), or by the resonant tunneling of single electrons at high temperature ($k_BT \gg \Gamma$).Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.

Gate-Voltage Studies of Discrete Electronic States in Al Nanoparticles

We have investigated the spectrum of discrete electronic states in single, nm-scale Al particles incorporated into new tunneling transistors, complete with a gate electrode. The addition of the gate has allowed (a) measurements of the electronic spectra for different numbers of electrons in the same particle, (b) greatly improved resolution and qualitatively new results for spectra within superconducting particles, and (c) detailed studies of the gate-voltage dependence of the resonance level widths, which have directly demonstrated the effects of non-equilibrium excitations.Comment: 4 pages, 7 figure

Spin transitions in a small Si quantum dot

We have studied the magnetic field dependence of the ground state energies in a small Si quantum dot. At low fields the first five electrons are added in a spin-up -- spin-down sequence minimizing the total spin. This sequence does not hold for larger number of electrons in the dot. At high fields the dot undergoes transitions between states with different spins driven entirely by Zeeman energy. We identify some features that can be attributed to transitions between different spin configurations preserving the total spin of the dot. For a few peaks we observed large linear shifts that correspond to the change of the spin of the dot by 3/2. Such a change requires that an electron in the dot flips its spin during every tunneling event.Comment: Revtex, 5 pages, 3 figure

Fabrication of single-electron tunneling transistors with an electrically formed Coulomb island in a silicon-on-insulator nanowire

For the purpose of controllable characteristics, silicon single-electron tunneling transistors with an electrically formed Coulomb island are proposed and fabricated on the basis of the sidewall process technique. The fabricated devices are based on a silicon-on-insulator (SOI) metal-oxide-semiconductor (MOS) field effect transistor with them depletion gate. The key fabrication technique consists of two sidewall process techniques. One is the patterning of a uniform SOI nanowire, and the other is the formation of n-doped polysilicon sidewall depletion gates. While the width of a Coulomb island is determined by the width of a SOI nanowire, its length is defined by the separation between two sidewall depletion gates which are formed by a conventional lithographic process combined with the second-sidewall process. These sidewall techniques combine the conventional lithography and process technology, and guaran tee the compatibility with complementary MOS process technology. Moreover, critical dimension depends not on the lithographical limit but on the controllability of chemical vapor deposition and reactive-ion etching. Very uniform weakly p-doped SOI nanowire defined by the sidewall technique effectively suppresses unintentional tunnel junctions formed by the fluctuation of the geometry or dopant in SOI nanowire, and the Coulomb island size dependence of the device characteristics confirms the good controllability. A voltage gain larger than one and the controllability of Coulomb oscillation peak position are also successfully demonstrated, which are essential conditions for the integration of a single-electron tunneling transistor circuit. Further miniaturization and optimization of the proposed device will make room temperature designable single-electron tunneling transistors possible in the foreseeable future.open101

Fluorescent nanoparticles for sensing

Nanoparticle-based fluorescent sensors have emerged as a competitive alternative to small molecule sensors, due to their excellent fluorescence-based sensing capabilities. The tailorability of design, architecture, and photophysical properties has attracted the attention of many research groups, resulting in numerous reports related to novel nanosensors applied in sensing a vast variety of biological analytes. Although semiconducting quantum dots have been the best-known representative of fluorescent nanoparticles for a long time, the increasing popularity of new classes of organic nanoparticle-based sensors, such as carbon dots and polymeric nanoparticles, is due to their biocompatibility, ease of synthesis, and biofunctionalization capabilities. For instance, fluorescent gold and silver nanoclusters have emerged as a less cytotoxic replacement for semiconducting quantum dot sensors. This chapter provides an overview of recent developments in nanoparticle-based sensors for chemical and biological sensing and includes a discussion on unique properties of nanoparticles of different composition, along with their basic mechanism of fluorescence, route of synthesis, and their advantages and limitations

A novel chronopharmacological drug delivery systems based on PEG-containing nanoparticles for protein delivery

In this work, novel chrono-pharmacological drug delivery systems based on a multilayer microcapsule were developed. The multilayer microcapsules consisted of drug containing hydrogel layers and seal coatings. The hydrogel layer was prepared from poly(ethylene-glycol)-containing nanoparticles and the seal coating was prepared from ethyl cellulose dispersions. The systems were designed such that upon contact with water, water molecule started to diffuse through the seal coat and swell the hydrogel nanoparticles. This build-up pressure from the osmotic pressure and the swelling of the nanoparticles forced the seal coat to rupture and exposed the crosslinked hydrogel nanoparticles, which dispersed and released the drug loaded inside them. The PEG-nanoparticle systems were prepared using a thermal initiated dispersion polymerization. Temperature sensitive PNIPAAm was included into the systems along with PEGDMA as the crosslinker. The nanoparticle systems were designed to have a high loading efficiency and to be able to protect highly sensitive therapeutic agents from the high temperature, high shear stress, and high air/liquid interface encountered during the Wurster bed coating process. The size of the nanoparticles changed by one order of magnitude when the temperature of the system was changed from 5°C to 35°C as measured using the photon correlation spectroscopy. The nanoparticulate systems were able to protect insulin from the simulated harsh environment encountered during the coating process. In order to elucidate the structure and performance of the multilayer microcapsules, several microscopic techniques were employed. Polarizing microscopy was employed to observe the uniformity of the layers created and also to observe the redispersion of the PEG-nanoparticles. Fluorescent microscopy was utilized to observe the release behavior of FD4 from the multilayer microcapsules. Confocal laser scanning microscopy was used to dissect the microcapsule and observe the uniformity of each layer throughout. Factors affecting the coating process, such as the feed rate of the dispersions, the flow rate of the air, and the temperature of the inlet air were investigated and optimized. These intensive studies of the multilayer microcapsules, the PEG-containing nanoparticles, and the seal coat materials showed that these systems had the potential to be applied as a chronopharmacological drug delivery system