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We measured the long term spontaneous electrical activity of neuronal networks with different sizes, grown on lithographically prepared substrates and recorded with multi-electrode-array technology. The time sequences of synchronized bursting events were used to characterize network dynamics. All networks exhibit scale-invariant Lévy distributions and long-range correlations. These observations suggest that different-size networks self-organize to adjust their activities over many time scales. As predictions of current models differ from our observations, this calls for revised models.

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photo-depletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QDimpurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons. 73.20.Dx, 78.66.Fd, 78.55.Cr Quantum confinement in low-dimensional semiconductors has been shown to profoundly affect Coulomb correlations among charge carriers. In two-dimensional (2D) quantum wells (QWs), enhanced electron-hole correlations yield stable excitons that dominate the optical absorption and emission spectra near the band edge [1]. In 1D quantum wires (QWRs), excitons play an even more important role due to the reduced Sommerfeld factor [2], and their dominance in optical spectra was observed across many interband transitions [3]. In quantum dot (QD) systems, the importance of Coulomb correlations varies considerably as a function of the dot size L due to the difference between the 1/L dependence of the Coulomb potential versus the ∼ 1/L 2 dependence of the confinement energy. Many-particle states can dramatically change the electronic spectra of QDs compared to the simple-minded single particle picture of these fully confined states.Experimentally, the role of Coulomb correlation and many-body effects in quantum nanostructures has been extensively studied using different techniques. Evidence for formation of few-electron states in QDs was provided by capacitance and by far-infrared spectroscopies [4]. Multi-exciton states were observed in the luminescence spectra of QDs formed in QWs and QWRs due to interface disorder [5] and of QDs produced by Stranski-Krastanow island growth [6]. The formation of charged excitons in doped QWs was also reported [7]. In the present Letter, we report the observation of multicharged exciton states in the photoluminescence (PL) spectra of QDs with controlled structure and composition. The binding energies and PL-fine-structure of the multi-particle states incorporating up to six electrons are found to be in good agreement with a theoretical model. Our QDs are fabricated by epitaxial growth on (111)Boriented GaAs substrates patterned with an array of micron-sized tetrahedral recesses [8,9]. Deposition of AlGaAs/GaAs/AlGaAs QW-layers results in the selfformation of a GaAs QD exactly at the sharp tip of each tetrahedral recess. Thus, each QD's position is precisely controlled by the placement of the recess-patterns while its size is controlled by the growth parameters [10]. Based on atomic force microscopy studies [11], we estimate the thickness and di...

Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, we present a new and easy approach to obtain high purity gold lines: the use of inorganic PF3AuCl as a precursor. Electrical resistivities as low as 22 μΩ cm at 295 K (ten times the bulk Au value) were obtained. This is to our knowledge the best value for focused electron beam deposition obtained from the vapor phase so far. No special care was taken to prevent hydrocarbon contamination. The deposited nanostructure consists of gold grains varying in size and percolation with beam parameters.