Laser gives control over crystal growth

Crystal growth is critically important for industry, not least for the production of medicines and composites. Although we can produce crystals on a large scale, the controlled growth of individual crystals in a solution has until now remained the preserve of natural processes.

Until now, that is — because Marloes Bistervels, during her doctoral research at AMOLF in Amsterdam, developed a method that uses lasers to produce any desired shape of nanocomposite. By combining this method with an ion-exchange process — a collaboration with fellow doctoral candidate Arno van der Weijden — BaCO₃/SiO₂ can be converted into any desired composition. To demonstrate the power of the method, they produced a luminescent lead methylammonium bromide (PbMABr₃) perovskite crystal and integrated it with a BaCO₃/SiO₂ waveguide. This material is of interest for applications in displays or for communications purposes.

Bottlenecks

On paper, using lasers to initiate and guide crystal growth seems straightforward enough, since the position of a laser focus can be controlled to micrometre precision. In practice, however, things proved rather different. Together with her supervisor Wim Noorduin, Bistervels found herself entering an entirely new field of research, as the literature indicated that it is not actually possible to control the crystal growth of biorelevant crystal minerals locally and dynamically using lasers. ‘But we wanted to investigate it anyway, to properly map out exactly where the bottlenecks lie,’ says Bistervels.

In doing so, they came across several methods for which the laser principle did work, even though getting it to function presented considerable challenges. ‘Crystallisation depends on so many factors — if you’re even slightly off with your experiment, nothing happens. But the moment you find that sweet spot, you can produce the most beautiful shapes,’ she explains.

Nucleation

‘Whether you can achieve the desired crystal shape depends on how well you control nucleation [the formation of a crystal, ed.] and growth: these two phases depend on a substantial number of parameters and determine whether a crystal forms at all and what shape it takes,’ says Bistervels. Her aim was to produce biorelevant minerals such as CaCO₃, SrCO₃, BaCO₃, Ca₃(PO₄)₂, and SrSO₄. These make it possible to create nanocomposites — such as those of barium carbonate (BaCO₃) and silicon dioxide (SiO₂) — via the following process:

Ba²⁺ + CO₂ + H₂O → BaCO₃ + 2H⁺

Si(OH)₃O^– → SiO₂ + 2H₂O

‘In the first method I investigated, the concentration of CO₂ is controlled locally, so that BaCO₃ and then SiO₂ together form a composite that resembles crystal-like flowers. In this process, CO₂ is cleaved from a molecule called ketoprofen by focusing a 275 nm LED into the solution at the position where the crystal is to be formed,’ says Bistervels. (see diagram below)

According to Bistervels, the light creates the right conditions for nucleation and growth, with crystal growth following the position of the light. This demonstrated that the shape of the crystal can be brought under precise control. The light acts as the initiator for nucleation and growth: switch off the light source, and crystallisation stops accordingly.

Supersaturation

In the second method within her research, the water is heated locally using a laser. BaCO₃, along with other biorelevant crystal minerals, is a so-called reverse solubility component, meaning that its solubility decreases at higher temperatures. This gives rise to what is known as supersaturation: fewer building blocks can remain dissolved in the liquid, causing them to form solid crystals. Infrared laser light at 1435 nm can raise the temperature of water within a microsecond. ‘And with this local supersaturation, the shape of the nanocomposite can be controlled,’ says Bistervels.

‘The two methods described above control crystal growth in two dimensions. The third dimension is trickier, because a laser beam also carries a fair amount of energy in the section before the focus is reached — which can trigger the reaction there too, causing you to lose precision.’ Bistervels explains that for effective crystallisation in 3D, it is necessary to control the process at an even more localised level. This can be achieved by using a powerful 532 nm laser with an ultrashort pulse of just a few picoseconds (trillionths of a second), initiating a so-called nonlinear process of two-photon absorption. ‘This ensures, for example, that CO₂ only forms at the true focal point of the laser and nowhere else — meaning that any desired 3D crystal shape could in principle be produced.’

Chirality

According to Bistervels, interest from industry is beginning to emerge, but for now the research remains purely fundamental and small-scale. Even so, she believes the potential is enormous. ‘We now have complete control over the crystallisation process, and my setup is already being used to grow the next generation of crystals. Think functional biocrystals, or crystals in which we can direct chirality. The possibilities are endless.’