Writing with nanoparticle ink

Scientists from Yamagata University, Japan, used a Menlo Systems C-Fiber 780 High Power femtosecond fiber laser to write micropatterns with nanoparticles in a solution onto glass. This novel versatile laser writing technique is not dependent on photosensitivity of the substrate material or on the optical properties of the nanoparticles. It widens the range of materials for direct laser writing (DLW) to produce functionalized features.

Fabricating functional micro- and nanodevices using lithography is scalable but involves complex apparatus and a high degree of parameter control since it relies on techniques such as vacuum deposition and plasma etching. Directly writing functional micro- and nanofeatures onto photosensitive substrates using focused low-power laser light significantly reduces fabrication costs and increases flexibility, but is limited to materials exhibiting nonlinear response to the specific laser wavelength applied.

DLW with C-Fiber 780

Figure: Scanning electron microscope (SEM) image of micro-characters written using SiO2, TiO2, and Fe2O2 nanoparticles, correspondingly. (Figure adapted from original publication.)

The team around Dr. Hiroaki Nishiyama at the Graduate School of Science and Engineering, Yamagata University, Japan, report a novel method of DLW which can be applied to a variety of materials, irrespective of their optical properties. They used Menlo Systems frequency-doubled erbium-doped femtosecond fiber laser, which they purchased in 2007, and which corresponds to today's Menlo laser model C-Fiber 780 High Power. This fabrication tool is comparatively inexpensive, easy to operate, small size, and reliable even after more than a decade of use.

The proposed method for micro- and nanopatterning is based on multi-photon induced assembly of nanoparticles dispersed in a dilute silver nitrate (AgNO3) solution. In order to demonstrate the versatility of the method, the researchers used the three types of nanoparticles: silicon dioxide (SiO2), titanium dioxide (TiO2), and ferric oxide (Fe2O3). They focused the laser light onto the surface of a glass substrate, which was in contact with the nanoparticle dispersion. When translating the laser focal spot, the nanoparticles form a clad along the trace resulting in a highly stable pattern, which is firmly adhering the substrate surface. The transmission electron microscopy (TEM) image of the structure's semi-circular cross section reveals that the clad layer is organized around a core consisting of an intermediate layer and a void at its center, with an additional slight dimple on the substrate surface beneath the void. After a closer analysis of the materials present in the distinct layers and the dynamics of the energy flow during the formation process, the scientists suggest that the formation mechanism of such hierarchical structures involves several stages. At first, silver microdots in the size of the laser focal spot form by multi-photon and additional thermal reduction of the Ag+ ions from the silver nitrate solution on top of the substrate surface. The microdots accumulate a significant amount of heat, which is brought by the femtosecond laser pulses at a 100 MHz rate. In the center of the silver microdots, the heating leads to the formation of a void and the dimple in the substrate material. At its outer border, which is in contact with the solution microbubbles form, rapidly drawing the dispersed nanoparticles from a large space around the bubble to the bubble surface by convection, thus forming a clad layer consisting of the densely packed nanoparticles.

The novel method for versatile DLW presented by the Japanese team can be applied to a wide range of materials such as glass or ceramics since it does not require photosensitive substrates. It is independent of the nanomaterial optical properties and therefore enables tailored functionalization. Moreover, it does not require high laser power – the clad layers formed at an average laser power in the range of 10 to 30 mW (corresponding to a laser pulse energy of 0.3 nJ). And finally, the results are continuous, uninterrupted features. With an appropriate choice of the nanomaterial they may serve as paths of engineered electrical properties.

Under the aspect, that the laser source put into operation nearly 15 years ago is still showing excellent performance, the results of this work give another example for Menlo Systems' long-term reliable technology.

Author: Patrizia Krok


Original publication:

H. Nishiyama, K. Umetsu, and K. Kimura:
Versatile direct laser writing of non-photosensitive materials using multi-photon reduction-based assembly of nanoparticles;
Scientific Reports 9, no. 14310 (2019);
DOI: https://doi.org/10.1038/s41598-019-50630-1