Gold colloidal nanoparticle electrodeposition on a silicon surface in a uniform electric field
© Buttard et al; licensee Springer. 2011
Received: 17 June 2011
Accepted: 4 November 2011
Published: 4 November 2011
The electrodeposition of gold colloidal nanoparticles on a silicon wafer in a uniform electric field is investigated using scanning electron microscopy and homemade electrochemical cells. Dense and uniform distributions of particles are obtained with no aggregation. The evolution of surface particle density is analyzed in relation to several parameters: applied voltage, electric field, exchanged charge. Electrical, chemical, and electrohydrodynamical parameters are taken into account in describing the electromigration process.
The emerging fields of nanoscience and nanoengineering are helping us to better understand and control the fundamental building blocks in the physics of materials [1, 2]. The manipulation of nano-objects is also essential and requires expertise in several domains (mechanics, electro-chemistry, optics...) [3–5]. The traditional top-down approach is by far the most widespread within the micro-electronics industry, but it relies on a complex lithography technique that results in very high production costs. Alternative approaches are therefore being investigated with a view to achieving a spontaneous self-assembly of nano-components. Among these approaches, the so-called bottom-up method is attracting increasing attention. Based on this method, the self-organization of gold nanoparticles on a planar surface is providing new solutions for electrical or catalytic systems [6, 7]. However, the deposition of particles on a substrate [8, 9] must conform to several criteria such as irreversibility of the deposition process , stability, and high density. Deposition of gold colloidal nanoparticles can be achieved with different methods. For instance, the electrophoretic deposition method (EPD) [11, 12] uses a uniform external electric field to drive the suspended particles from the solution toward the substrate surface. The advantage of the EPD method is that it requires no special surface passivation on the colloidal particles and it can be controlled conveniently by the applied field [13, 14]. The deposition process, however, is complex  and many questions remain unanswered, despite the extensive use of EPD.
In this article, we describe the uniform electric field-assisted deposition of gold colloidal nanoparticles from an aqueous solution onto a planar silicon surface. The adsorption of nanoparticles onto silicon is described and the surface density obtained is investigated in function of the usual experimental parameters: applied voltage, electric field, and initial nanoparticle density existing in the solution.
2. Material and methods
3. Results and discussion
This is corroborated by Figure 2e, showing a typical two-dimensional self-correlation function g(r), calculated from the SEM image at t = 10 min. This radial distribution corresponds to the probability of finding a particle at a center-to-center distance r from another particle . This statistical result, based on an evaluation of all particles observed on the image, confirms the uniform distribution of the nanoparticles. A profile from a g(r) cross section (Figure 2f(1)) shows several oscillations, despite the lack of periodic ordering. This cross section was normalized by r 0 which corresponds to the average distance between nearest neighbors. Here, we measure r 0 = 46.9 nm (abscissa of first peak of g(r)) which indicates that the 20 nm diameter nanoparticles are only separated by a surface-to-surface distance of 26.9 nm on average. We note that other peaks are clearly visible on g(r). This is evidence that, although there is no periodic distribution in the observation plane, the nanoparticles are uniformly scattered over all the substrate with a measurable nearest neighbor distance . Self-correlation functions were also computed for other SEM images (Figure 2a-c). An example is shown in Figure 2f(2).
Figure 2g shows the corresponding r 0 for each deposition time. As expected, r 0 is long for short deposition times (low density) and saturates around 40 nm at longer deposition times. This value (at saturation) corresponds to a surface-to-surface distance l · 20 nm between nearest particles, which is close to the nominal particle diameter. This distance corresponds to an electrical equilibrium between charged particles. Gold colloidal nanoparticles are embedded by citrate ions leading to a negative charge at the surface of the colloids. This negative charge is balanced by the adsorption of positive ions present in the electrolyte. The electrical atmosphere around the particles is therefore very complex [18, 19] and there are a lot of charge interactions between the particles. In the well-known double layer model based on the Gouy-Chapmann theory [20, 21] and Stern's model , the particle is embedded both by a compact layer, adsorbed at the surface, and by a diffuse layer. Usually in an electrolyte, the Debye length λ D is taken as the thickness of both the compact and the diffuse layers. The Debye length is an important factor in determining the stability of gold colloid. Under appropriate conditions, particles do not coalesce. This stability is due to the repulse potential of the diffuse Debye layer when two particles come close to each other. This is greater than the attractive Van der Waals potential/force of the gold particle, which would lead to coalescence of the particles. In other words, the homogenous lateral distribution of colloids is interpreted as the repulsion between two neighbors on account of the negative shell from citrate ions.
where j is the current density and dt is the experimental time increment between two experimental points (0.5 s). Figure 6b shows the nanoparticle density versus the integrated charge Q (normalized by the sample surface). We observe a clear charge threshold above which density increases by two orders of magnitude. For low Q values (Q < 1 mC/cm2), the density is low (δ ≈ 4 × 104 cm-2), whereas for high Q values (Q > 2 mC/cm2) the density is high (δ ≈ 1 × 107 cm-2). Between these two regimes a clear transition charge threshold is observed at Q ≈ 1.5 mC/cm2. We explain this behavior by the anodic oxidation of the silicon substrate, whereas the platinum is chemically inert at these voltages.
Under these conditions, a hydrodynamical flow of charged ionic species is set up in the direction of the positive electrode and this helps drive the nanoparticles toward the silicon surface. Consequently, both electrical (E > 80 V/m) and electrochemical parameters (Q > 1 mC/cm2) are essential to the electromigration of gold colloidal nanoparticles onto the silicon surface.
In this study, we have investigated the electrodeposition of gold colloidal nanoparticles on p-type-doped Si surfaces. Uniform distribution was obtained and adsorption was irreversible. The density of a gold nanoparticle assembly was investigated and analyzed in relation to several parameters such as voltage, the electric field, and the charge exchanged. Deposition was found to be associated with a minimum electric field (E trans ≈ 80 V/m) combined with an electrochemical process (Q > 1 mC/cm2) that oxidises the surface of the Si anode.
scanning electron microscopy.
We would like to thank E. André for help with platinum deposition and P. Gentile for numerous fruitful discussions.
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