Laser-Driven "Bubble Pen" Patterns Nanoparticles

Researchers create vapor bubbles to pull particles of solution and down to a substrate

2 min read
Laser-Driven "Bubble Pen" Patterns Nanoparticles
Illustration: The University of Texas at Austin/American Chemical Society

Lasers have been used inside microscopes for many years to trap and move objects floating around in solution. 

Now Linhan Lin, a postdoctoral researcher in Yuebing Zheng’s group at the University of Texas in Austin and colleagues have developed a new strategy for drawing those particles down to the surface, where they can be arranged at will.

The technique, which team members have dubbed bubble-pen lithography, uses a laser to excite a special layer of material applied to a glass slide. The resulting hot spot vaporizes solution above it, creating a temporary microbubble. Convective flow around this bubble draws nearby particles to the surface, where heat and short-range attraction can then help the particles stick. 

The process, which was described in the January issue of Nano Letters, was used to pattern microscopic colloidal particles as well as 6-nanometer-wide quantum dots. And it seems surprisingly quick. Here’s a real-time video of the patterning of the requisite Texas Longhorn from 300-micrometer-wide polystyrene spheres in solution. 

This is not the first time that light has been used to pattern particles suspended in solution. In 2009, for example, a group based in California demonstrated a “NanoPen” that used electrodes and low-intensity light to pattern nanoparticles placed in a conductive solution.

Since the bubble-pen approach does not require a conductive solution, Zheng says it should be more flexible. The technique does still need to have a specialized substrate to work, though. In order to create vapor bubbles with a laser intensity that won’t damage particles, the team enhances the laser’s effect through the use of a plasmonic substance—in this case, gold nanoislands. Other materials can be placed on top of this layer; among the demonstrations in the paper is the patterning of particles on a layer of molybdenum disulfide

The team hopes to use the bubble-pen technique to pattern both inorganic particles, such as quantum dots, as well as organic materials for biological applications, such as sensor arrays. They’re investigating whether the technology could be used in a speedy, low-cost roll-to-roll process.

Particle size—particularly when it comes to nanometer-scale particles like quantum dots—could be an obstacle to high-throughput work, says Lih Lin, a professor of electrical engineering at the University of Washington in Seattle who has also done prior solution-based optical nanolithography work.

The smaller the particle, the less of it there will be to fall under the influence of the trapping mechanism. The odds of trapping a particle can be increased by upping the concentration of particles, Lin says, but then the probability of trapping more than one particle at a time will increase as well, which could limit applications.

Zheng agrees it’s an issue. He says the team aims to mitigate the problem by using multiple beams (manipulated with a digital device) to boost throughput. The team says it should be possible for more than 100 beams to pattern in parallel.

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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