We describe the formation of hydrogen sensors by deposition of Pd clusters onto silicon dioxide coated silicon substrates
with electrical contacts defined by a simple shadow masking technique. The clusters are prepared by sputtering in a gas
aggregation source. The sensors are characterized by exposure to hydrogen in a simple flow chamber and by measuring
the temperature dependence of the sensor resistance. Sensors with cluster coverage greater than the percolation threshold
form "thin film" type sensors which exhibit a small increase in resistance on exposure to hydrogen, consistent with the
increase in resistivity of bulk Pd on absorption of hydrogen. Sensors with coverage smaller than the percolation threshold
form sensors which exhibit a much larger decrease in resistance on exposure to hydrogen. The response of these
"percolating-tunneling" sensors is due to the absorption of hydrogen by the Pd clusters, which causes the tunnel gaps in
the film to decrease in size, leading to an increase in conductance. Finally we describe tunneling sensors, where gold
islands are grown on the substrate prior to cluster deposition, and which exhibit similar characteristics to the percolating-tunneling
sensors.
Random deposition of conducting nanoparticles on a flat two dimensional (2D) substrate leads to the formation of a conducting path at the percolation threshold. In sufficiently small systems significant finite size effects are expected. However, in the 2D square systems that are usually studied, the random deposition means that the main effect of small system sizes is that stochastic fluctuations become increasingly large.
We have performed experiments and simulations on rectangular 2D nanoparticle films with nanoscale overall dimensions. The sample geometry is chosen to limit stochastic fluctuations in the film’s properties. In the experiments bismuth nanoparticles with mean diameters in the range 20-60nm are deposited between contacts with separations down to 300nm. At small contact separations there is a significant shift in the percolation threshold (pc) and the conducting
path formed close to pc resembles a nanowire. Percolation theory describes the experimental onset of conduction well: there is good agreement between predicted and measured values of the power law exponent for the correlation length.
Atomic clusters can be produced in a size range (100nm to 0.5nm) that bridges the gap between the limits of current lithographic fabrication technologies for integrated circuits and the atomic/molecular regime. The work presented here aims to combine established top-down device processing with bottom-up engineered cluster assembly. Conducting cluster deposition and standard optical fabrication techniques have been used to produce wires on a textured (V-grooved) substrate. The lengths of the wires (ranging from 2μm to 1mm) are defined simply by the separation of NiCr/Au contacts. The deposited nanoparticles range in size from 20-100nm and in principle define the width of the nanowire. In-situ conductance measurement allows precise control of the deposition process and the onset of conduction in the wire is readily monitored as a function of deposition time. The effectiveness of the surface templating technique is demonstrated by SEM and AFM imaging carried out after deposition. The surface coverage is seen to vary from <20% on the unpatterned (normal-to-beam) surface (which is required to be non-conducting) to >100% at the apexes of the V-grooves used to promote growth of the wire. Self assembly of the nanoparticles leads to completion of a wire between the pre-formed contacts with no possibility of a parasitic conduction path. Wires formed through this technique currently have minimum widths of ~1μm but straightforward extensions of the technique should soon allow nanowire formation.
Conference Committee Involvement (6)
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems IV
10 December 2008 | Melbourne, Australia
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems III
11 December 2006 | Adelaide, Australia
Device and Process Technologies for Microelectronics, MEMS, and Photonics IV
12 December 2005 | Brisbane, Australia
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems II
13 December 2004 | Sydney, Australia
Device and Process Technologies for Microelectronics, MEMS, and Photonics
10 December 2003 | Perth, Australia
Smart Electronics, MEMS, BioMEMS, and Nanotechnology
3 March 2003 | San Diego, California, United States
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.