Networks based on nanoscale resistive switching junctions are considered promising for the fabrication of neuromorphic computing architectures.

Networks of nanoscale objects are the subject of increasing interest as resistive switching systems for the fabrication of neuromorphic computing architectures. Nanostructured films of bare gold clusters produced in gas phase with thickness well beyond the electrical percolation threshold, show a non-ohmic electrical behavior and resistive switching, resulting in groups of current spikes with irregular temporal organization. Here we report the systematic characterization of the temporal correlations between single spikes and spiking rate power spectrum of nanostructured Au two-terminal devices consisting of a cluster-assembled film deposited between two planar electrodes. By varying the nanostructured film thickness we fabricated two different classes of devices with high and low initial resistance respectively. We show that the switching dynamics can be described by a power law distribution in low resistance devices whereas a bi-exponential behavior is observed in the high resistance ones. The measured resistance of cluster-assembled films shows a 1 / f α scaling behavior in the range of analyzed frequencies. Our results suggest the possibility of using cluster-assembled Au films as components for neuromorphic systems where a certain degree of stochasticity is required.

Major efforts to reproduce the brain performances in terms of classification and pattern recognition have been focused on the development of artificial neuromorphic systems based on topdown lithographic technologies typical of highly integrated components of digital computers. Unconventional computing has been proposed as an alternative exploiting the complexity and collective phenomena originating from various classes of physical substrates. Materials composed of a large number of non-linear nanoscale junctions are of particular interest: these systems, obtained by the self-assembling of nano-objects like nanoparticles and nanowires, results in non-linear conduction properties characterized by spatiotemporal correlation in their electrical activity. This appears particularly useful for classification of complex features: nonlinear projection into a high-dimensional space can make data linearly separable, providing classification solutions that are computationally very expensive with digital computers. Recently we reported that nanostructured Au films fabricated from the assembling of gold clusters by supersonic cluster beam deposition show a complex resistive switching behaviour. Their non-linear electric behaviour is remarkably stable and reproducible allowing the facile training of the devices on precise resistive states. Here we report about the fabrication and characterization of a device that allows the binary classification of Boolean functions by exploiting the properties of cluster-assembled Au films interconnecting a generic pattern of electrodes. This device, that constitutes a generalization of the perceptron, can receive inputs from different electrode configurations and generate a complete set of Boolean functions of n variables for classification tasks. We also show that the non-linear and non-local electrical conduction of clusterassembled gold films, working at room temperature, allows the classification of non-linearly separable functions without previous training of the device.

We report the observation of non-metallic electrical conduction, resistive switching, and a negative temperature coefficient of resistance in nanostructured gold films above the electrical percolation and in strong-coupling regime, from room down to cryogenic temperatures (24 K). Nanostructured continuous gold films are assembled by supersonic cluster beam deposition of Au aggregates formed in the gas phase. The structure of the cluster-assembled films is characterized by an extremely high density of randomly oriented crystalline nanodomains, separated by grain boundaries and with a large number of lattice defects. Our data indicates that space charge limited conduction and Coulomb blockade are at the origin of the anomalous electrical behavior. The high density of extended defects and grain boundaries causes the localization of conduction electrons over the entire investigated temperature range.

A film fabricated by the assembling of nanoparticles that retain, at least partially, their individuality is expected to show substantially different structural and functional properties compared to the case where atoms or molecules are used as building blocks. Although films assembled with metallic clusters or nanoparticles have been predicted to have unusual functional properties, it has been tacitly assumed that cluster-assembled metallic films have the same conduction behavior observed for polycrystalline metallic thin films grown from atoms. Unexpectedly, in the last decade, several studies showed that nanogranular metallic films show a non-linear electric behavior, substantially different from their polycrystalline counterparts. Here we review and discuss the electrical transport properties of cluster-assembled films. Our aim is to provide a background and a common language for the systematic investigation and exploitation of nanogranular metallic thin films where the extremely high density of defects and grain boundaries causes the departure from ohmic behavior. We will focus on the non-linear electrical conduction and resistive switching relevant for neuromorphic applications.