NANOSOURCE · METALLIC AND SEMICONDUCTING NANOPARTICLE SOURCE FOR ELECTRONIC AND OPTOELECTRONIC APPLICATIONS

METALLIC AND SEMICONDUCTING NANOPARTICLE SOURCE FOR ELECTRONIC AND OPTOELECTRONIC APPLICATIONS

The project objectives were to demonstrate the fabrication of different types of nanoparticles (metallic and semiconducting) using a vacuum based technique developed by Mantis. The nanoparticles are generated by a magnetron which attracts the ionized argon gas and sputters the platinum target material into the aggregation zone. The pressure (approximately 1 mbar) causes the sputtered material to aggregate to form particles that increase in size until they reach the expansion zone. The nanoparticles then enter a second chamber with lower pressure (3 x 10-3 mbar) where they deposit onto the substrate. The argon gas flow rate and distance traveled through the aggregation zone can be adjusted to produce nanoparticles of a controllable size onto the substrate. In a first time period the consortium has worked on the growth of various nanoparticles focusing on the control of both size and density. This was achieved mainly by Transmission Electron and AFM imaging of nanoparticle samples. We have demonstrated the fabrication of nanoparticles in the range of 2-10 nm with surface densities varying up to 3.1012 cm-2. At this point we should clarify that this represents a high limit of aerial density for non-touching between each other nanoparticles. The density can then be further increased to obtain a continuous monolayer of touching nanoparticles or multiple nanoparticle layers. Nanoparticles formed using the above described technique are mostly charged because of their interaction with plasma. We have taken advantage of this property to study electrostatic driven self-assembly of these nanoparticles. We have thus demonstrated that we can selectively deposit nanoparticles in lines defined by conventional lithography using a self-focusing effect created when charged nanoparticles are deposited on an insulating material like a photoresist. Another phenomenon of electrostatic nature observed relates with nanoparticle preferential deposition on top of sharp edges. We have studied how nanoparticles are deposited along sharp lines made either from photoresist or silicon oxide material demonstrating the formation of nanowire arrays with width dimensions less than 30 nm made out of closed packed nanoparticles. This interesting feature was used to demonstrate increased sensitivity of Surface Enhanced Raman Spectroscopy of rhodamine molecules as compared with the sensitivity obtained by isolated nanoparticles on a planar surface. The effect is due to the creation of hot spots of increased electromagnetic intensity in a close packed nanoparticle assembly. The nanoparticle applications we have investigated are in the field of sensors and electronic memories. Our sensor oriented research was based on the formation of a 2-D layer of metallic nanoparticles over interdigitated (IDE) metallic electrodes. We have explored and demonstrated then that such a device can be used either as a strain sensor or chemical sensor. The aerial density of nanoparticles measured using TEM is correlated with the resistance value measured between the electrodes. Interestingly we have found that for higher resistance values, a regime where the nanoparticles exhibit an inter-particle distance, the sensitivity of that device to applied strain increases over an order of magnitude compared with the low resistance configuration. It is then the modification of inter-particle distance by straining that influences the current transport in the nanoparticle film changing its resistance. A similar effect was observed in chemical sensing applications. In that case the nanoparticle layer is covered with a polymer film deposited by ink-jet printing. When an analyte is absorbed by the polymer swelling takes place that also results in modification of the interparticle distance and change of the nanoparticle film resistance. There is then direct relation of the quantity of the analyte with measured resistance value. These devices have been used for Volatile Organic Compound identification and humidity. Our research in electronic memories has focused on nanoparticle flash-like memories using metallic nanoparticles within an insulating matrix as well as in resistive memories (memristors) made from nanoparticle assemblies. In nanoparticle flash-like memories the charge is stored in metallic nanoparticles. We have shown that by reducing the size of metallic nanoparticles we can achieve high density and large memory windows at moderated programming voltages. The devices exhibit 10 years retention and high endurance properties. The demonstrated memristor is made from titanium oxide nanoparticle film that is obtained by oxidizing titanium nanoparticles while these are deposited over a substrate. In that case to make a dense film of nanoparticles we have used acceleration voltage to land the nanoparticles with high kinetic energy on the substrate allowing for nanoparticle intermixing. Large high to low resistance ratio and no need for electroforming have been observed in these devices. Our research has resulted in 10 publications in international journals and numerous Conference presentations of the partners. Work on sensors was especially promoted by nanotechweb (http://nanotechweb.org/cws/article/lab/50231)

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