Knowing enough about the way light is scattered through materials would allow physicists to see through opaque substances, such as the sugar cube on the right. In addition, physicists could use information characterizing an opaque material to put it to work as a high quality optical component, comparable to the glass lens shown on the left.

In order to demonstrate their approach to characterize opaque substances, the researchers first passed light through a layer of zinc oxide, which is a common component of white paints. By studying the way the light beam changed as it encountered the material, they were able to produce a numerical model called a transmission matrix, which included over 65 000 numbers describing the way that the zinc oxide layer affected light. They could then use the matrix to tailor a beam of light specifically to pass through the layer and focus on the other side. Alternatively, they could measure light emerging from the opaque material, and use the matrix to assemble of an image of an object behind it.

In effect, the experiment shows that an opaque material could serve as a high quality optical element comparable to a conventional lens, once a sufficiently detailed transmission matrix is constructed. In addition to allowing us to peer through paper or paint, and into cells, the technique opens up the possibility that opaque materials might be good optical elements in nano-scale devices, at levels where the construction of transparent lenses and other components is particularly challenging.

Source: original article

Nanotechnology and nanoscience have certainly been one of the most popular fields of research in the last decade. Manufacturing processes are now able to carry out the deterministic synthesis of nanostructures with properties radically different from their macroscopic forms, enabling the realization of previously unthinkable devices. Despite these advances, few nanofabrication techniques feature the required characteristics for the commercial manufacturing of these new products in an effective fashion since they are either too slow, or too expensive and complex.

This page is a summary of my doctoral thesis project, accomplished at the Laboratory for multiscale mechanics at the École Polytechnique de Montréal (LM2, mechanical engineering). The main objective of this project was to develop a new manufacturing technique allowing the local synthesis of nanostructures on a surface for their eventual integration into nanodevices. The desired process has to be selective, reproducible, versatile, simple, fast and inexpensive for potential industrial utilization. Moreover, the manufacturing process must have a minimal environmental impact for sustainable development.

To implement the required specifications, a laser process combining the characteristics of laser-induced chemical liquid deposition (LCLD) and of sol-gel synthesis was proposed. The technique is very simple and consists of three steps. A precursor solution is first prepared. Next, a droplet of a controlled volume is transferred on a substrate by means of a micropipette. The droplet is then irradiated using a laser emitting in the infrared to induce the fast synthesis of nanostructures.

ZnO nanostructure synthesis by LCLD

In order to prove the feasibility of the manufacturing concept for the synthesis of nanostructures, the new process was used for the synthesis of zinc oxide (ZnO) nanostructures. ZnO has become one of most studied nanomaterials in the five last years as it presents very interesting properties for optoelectronics and sensing applications, while being synthesizable in a plethora of nanoscale morphologies.

Using laser processing, the deposition of coatings of several square millimetres of various nanostructures (nanorods, nanowires, porous films of nanoparticles) was carried out. In particular, nanorods with an average width of 300 nm and a length of two micrometers with hexagonal cross-sections and almost atomically flat surfaces were synthesized (see figure 1). Nanowires with diameters of approximately 50 nm and lengths exceeding four micrometers were also grown. This constituted an innovation among the laser processing techniques, as only laser-induced chemical vapour deposition (LCVD) and pulsed laser deposition (PLD) had been used to produce ZnO, and just in the form of thin films and nanoparticles.

Figure 1: Example of nanorods grown by LCLD

Caracterization and optimization of the process

One of the secondary objectives of this thesis was to improve the properties of the deposits for one of the target applications of ZnO, photoluminescent devices. For this reason, a parametric study was carried out during which the influence of the laser-related parameters (irradiation time, intensity) and of the solution-related parameters (precursor, additives, concentration) on morphology and crystallinity was studied. The use of scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that an increase in laser intensity and irradiation time increased nanostructure length and crystallite size (see figure 2A). Transmission electron microscopy (TEM) demonstrated that the nanorods grew along the c axis of the crystal lattice, at the apex of randomly oriented ZnO crystals forming a seed layer on the substrate. Additionally, an increase in precursor concentration was found to increase the thickness of this seed layer and the introduction of additives in the solution had the effect of promoting the vertically aligned growth of nanorods. Raman and photoluminescence (PL) spectroscopy also showed that the deposits were of high quality with few crystalline defects. In particular, the PL spectroscopy results gave evidence that the ZnO nanostructured deposits produced by the laser process were good for ultraviolet emission applications with the presence of an intense peak at 390 nm (see figure 2B). The extensive characterization of the samples also allowed the development of a qualitative growth model for the laser-grown ZnO nanostructures inspired by the growth and nucleation models used for conventional chemical synthesis in a liquid medium.

Figure 2: Microstructure of samples A)XRD, B)PL

This project led to the development of a new technique allowing the synthesis of high quality ZnO nanostructures in a few seconds whereas the traditional techniques of chemical synthesis need several hours. Once the reproducibility and selectivity problems are resolved, this very promising technique could easily be upgraded for the production of nanodevices such as UV nanolasers or light emitting diodes. Three scientific articles were published during this project:

[1] Fauteux, C., Smirani, R., Pegna, J., El Khakani, M. A., Therriault, D., Fast synthesis of ZnO nanostructures by laser-induced chemical liquid deposition, Applied Surface Science, vol. 225, 2009, pp 5359-5362;

[2] Fauteux, C., El Khakani, M. A. , Pegna, J., Therriault, D., Influence of solution parameters for the fast growth of ZnO nanostructures by laser-induced chemical liquid deposition, Applied physics A: Materials Science and Processing, vol.94, 2009, pp. 819-829;

[3] Fauteux, C., Longtin, R, Pegna, J., Therriault, D., Fast Synthesis of ZnO Nanostructures by Laser-Induced Decomposition of Zinc Acetylacetonate, Inorganic Chemistry, v. 46, 2007, pp. 11036-11047;

Author: Christian Fauteux