A new generation of printed devices. Recent developments in nanochemistry have led to the creation of functional nanoparticle-based inks acting as a base for a new generation of printed devices especially in the bio- and optoelectronics fields of application.
However, to take the full benefit of nanoscale properties, at least one dimension of the system must be below the characteristic length associated with the property that is being considered, e.g. thermal diffusion length, the thickness of the diffusion layer, a wavelength of electromagnetic radiation. An example of this principle concerns the realization of enzymatic cascade reactions, which requires the enzymes to be co-localized within a distance comparable to the diffusion layer thickness, i.e from 100 nm to 1 μm. Another example is represented by plasmonic structures such as Localized Surface Plasmon Resonance (LSPR) devices require achieving nanostructures separated by a gap smaller than the light wavelength in order to produce enhanced spectroscopic signatures of elements arranged in between. Such an application needs the pattern’s characteristic dimensions to be below 100 nm.
All the high-added value applications considered here underline the need to control materials deposition and organization on a large scale which is sometimes up to three orders of magnitude, i.e. from 100 nm to 100 μm. The current limitations of printing techniques do not allow spatial resolution below the micrometer scale. Nevertheless, the field is extremely dynamic, and new methods of improving printing resolution have recently been explored.
Two main approaches can be identified, namely the top-down and the bottom-up. In the top-down approach, the resolution issue is overcome by implementing physical or chemical nano- or micro structuration on the substrate to be printed, thus allowing control of the ink-wetting processes. In parallel, continuous advances made in the field of nano- and supramolecular chemistry are used directly to improve printing resolution through a bottom-up approach. Here, ink constituents are designed to organize themselves in multiscale patterns, including nanostructures.
PRINTUP INSTITUTE Research activities
PRINTUP INSTITUTE develops research activities at the intersection between printing technologies and
deep-tech applications such as optoelectronics and (bio)detection based on its expertise in surface
chemistry, nanochemistry, and supramolecular chemistry.
We formulate functional inks showing high stability in physiological media but also smart formulations allowing to form structures having characteristic dimensions far below the conventional resolution limit of inkjet printing. One approach consists in tailoring the nanoparticles surface functionalization in order to induce their self-organization upon ink drying. Recently, we synthesized 160 nm SiO2 nanoparticles functionalized by ydrophobic groups (see Fig. a).
SiO2 nanoparticles functionalization
When a formulation containing 0,5 g/L of nanoparticles in 90:10 water: ethanol is used, the pattern obtained is composed of separate, regular ellipses showing an accumulation of NP on their perimeter and a zero quantity of NP in their center (see Fig. b).
Printing of functionalized silica 0.5 g/L in a 90% water 10% ethanol mixture with a drop spacing of 5 μm,
Each ellipse has an average size of 140 μm (main ellipse axe), and they are spaced about 10 μm apart. Atomic Force Microscopy (AFM) characterization indicates that the nanoparticles layer has a thickness of 200 nm (see Fig c). This structure is due to the Rayleigh-Plateau instability. When a line is printed on a surface, it may experience slight oscillations. These oscillations induce differences in the radii of curvature on the printed line. These differences in radii of curvature lead to differences in pressure (according to Laplace's law) creating a flow of material towards lower-pressure areas. Laplace’s law is expressed as ΔP= 2γ/R, with P the pressure, R the radius of curvature, and γ the surface tension. In the case of the formulations used, the associated oscillations and material flow lead to the breaking of the line and the formation of periodic ellipses.
AFM characterization of the printed pattern
The aforementioned elliptical periodic structure can be used as a template for the subsequent deposition of silver lines, printed perpendicularly to the ellipse's major axes. The interaction between the hydrophilic silver ink and the hydrophobic underlying SiO2 structures allows the creation of gaps between adjacent silver lines much smaller than those obtained in the absence of the SiO2 ellipses (see Figure d).
Printing of the semiconductor in the gap and the corresponding profilometry measurements
Subsequently, a semi-conducting, organic ink made of DPP-DTT (thiophene derivative) is printed. DPP-DTT makes it possible to locally cover the interline distance (see Figure d). Profilometry measurements were carried out to determine the topography of the gap. Two profiles were measured. The first one permits obtaining topographic information of the lines of Ag (cf. blue curve Fig. d) in the absence of the SiO2 underlying template. The distance between two lines is about 100 μm. When the silver is printed on the top of the SiO2 ellipses, the pattern has a pronounced coffee ring effect, which reduces the distance between the two parallel lines by one order of magnitude. The Ag deposit has a thickness of 1 μm in the center of the line and 2 μm on its two edges. When the semiconductor is printed (cf. red curve Fig. d) the coffee ring structure is maintained with a thickness of 1.25 μm in the center and about 3 μm on the edges.
The electrical resistance of the semiconductor was determined using a semiconductor parameter analyzer; two values (resistance measured between the lines in the presence and in the absence of the micro structuring SiO2 pattern) were obtained and compared. Resistances of 750 Ω and 10 kΩ are obtained respectively with and without the micro structuring pattern. The resistance of this type of semiconductor linearly depends on the size of the gap. By considering a gap of 10 μm in the presence of microstructure and of 100 μm for an unmodified surface, the resistance ratio is close to the ratio of the gap lengths.
Perspectives
Several perspectives are opened by the results of this work. On the one hand, it is possible to modify a posteriori the chemical functions of the silica nanoparticles to adapt the surface energy contrast to ink other than Ag (silane chemistry). Similarly, the nanoparticle's surface chemical functions can be tuned to adapt to any substrates. The proposed micro structuring method, therefore, has significant potential in terms of versatility.
On the other hand, it is possible to target other microstructures than periodic beads, in particular the formation of twin lines of micrometric thickness. By adopting the drop spacing, the formulation of NP inks, periodic structures of silica lines could be obtained and used as a template for printing Ag ink, this time in the direction parallel to the ellipse's major axes.
By adopting the NP surface chemical functions, the Ag ink is expected to divide on either side of the silica lines to form periodic conductive lines separated by micrometric gaps and with a length that can reach several cm.
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