Nanotech
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Development Of Ultra-Sensitive Nanoparticle-Based Sensors

The Nanotech team is developing sensors based on compact assemblies of nanoparticles produced by convective/capillary deposition, such as strain gauges, humidity sensors and temperature sensors.

These sensors can exploit both the resistive and capacitive parts of the nanoparticle assemblies.

1 – Development of resistive nanoparticle-based strain sensors

These sensors are extremely sensitive because of the exponential variation of their electrical resistance as function of induced strain. As a comparison, conventional metallic strain gauges exhibit a linear answer.

Figure 1: Scheme (left) and photograph (right) of a typical nanoparticle-based resistive strain gauge.
Figure 2: Relative resistance variation as a function of induced strain for four 18nm gold nanoparticle-based strain gauges with different thicknesses: M1, four layers; M2, three layers, M3, one monolayer. The dashed green line represents the response of a conventional metal foil gauge with a gauge factor G = 2 [Réf : C. Farcau, H. Moreira, B. Viallet, J. Grisolia, D. Ciuculescu-Pradines, C. Amiens et L. Ressier J. Phys. Chem. C 115 (30), 14494-14499 (2011)]
The total resistance of the nanoparticle assemblies can then be roughly written as :where β is the decay tunneling constant which essentially depends on the barrier height, l is the distance between nanoparticles, EC=e²/2C is the Coulomb charging energy which is linked to the total capacitance of the NP assemblies, k the Boltzmann constant and T is the temperature. The electron transport on such NP assemblies is hence a function of the tunneling decay and also more interestingly of the charging energy. An electron flowing through these granular assemblies must then overcome the tunnel barrier length l and also the Coulomb charging energy: All experimental ∆R/R0 versus strain ε curves can be well fitted with the equation ∆R/R0= A(egε -1) where A and g are constants characterizing the sensitivity of the NP-based strain gauges.
2 – Development of capacitive nanoparticle-based strain sensors

Building on its experience with nanoparticle resistive strain gauges, the Nanotech team investigated the possibility of developing nanoparticle capacitive strain gauges.

We then demonstrated that these resistive sensors could also be used as capacitive sensors by taking advantage of the insulating properties of the ligands surrounding the nanoparticles.

They can then be used to deploy large-scale communicating sensor networks in embedded systems where wireless, low-power and highly sensitive sensors are required.

Owing to an analytic model, the sensor design has been optimized in order to maximize the capacitor of the active area (most of the time neglected because of its low value) which is composed of a monolayered assembly of gold nanoparticles realized by CSA. This study revealed that an active area of 1mm² addressed by interpenetrating electrodes was capable of reaching reliable capacitors > 1pF.

The electromechanical study of these sensors using 14 nm gold nanoparticles demonstrated sensitivity 5 times higher compared to conventional capacitive gauges (Figure 3). The proof that a simple low cost circuit coupled with a microcontroller is capable of realizing the measurements on these gauges opens new promising paths on the feasibility of a new entirely wireless set-up.

Figure 3: (a) Optical images by SEM of a capacitive nanoparticle-based strain sensors, (b) Typical electromechanical results as a function of the nanoparticle size used (7nm, 12nm et 14nm)
Our modeling is based on the measurement of the Cij capacitance between two NPs and embedded in a dielectric matrix of permittivity εr. Several models were proposed in the literature to calculate Cij.  A good starting model is given by Quinn’s et al based on approximations where the junction capacitance between two NPs can be simplified by the following equation:      where l is the distance between two nanoparticles, r and d are the radius and diameter of the nanoparticle respectively, ɛ0 is the permittivity of the vacuum and ɛr is the relative permittivity of ligands. The total capacitance C of this geometry with given dimensions can be calculated using:      where k is the geometrical factor of the interdigited electrodes taking into account the capacitances in series and parallel. The derivative of the total capacitance leads to the relative capacitance variation as a function of strain, diameter of NPs and the inter-nanoparticle distance:     where e is the strain applied to the sensor, d is the diameter of the particle, and C is the gauge capacitance. This gives an analytical expression for the capacitive gauge factor GFC of such sensors defined as the following equation below: This modeling shows that the capacitance gauge factor of such capacitive strain gauges is a function of the NP and ligand sizes.