Print atom by atom
(Nanowerk News) It took the chemist Liaisan Khasanova less than a minute to turn an ordinary silica glass tube into a printing nozzle for a very special 3D printer. The chemist inserted the capillary tube – only one millimeter thick – into the blue apparatus, closed the lid and pressed a button. After a few seconds there was a loud bang and the nozzle was ready for use.
“The laser beam inside the device heats the tube and separates it. Then we suddenly increase the force of attraction so that the glass breaks in the middle and a very sharp edge forms,” explains Khasanova, who is working on her PhD in chemistry. in the Electrochemical Nanotechnology Group.
Khasanova and his colleagues need very small nozzles to print very small three-dimensional metal structures. This means that the nozzle opening must be equally small – in some cases so small that only one molecule can fit. “We are trying to take 3D printing to its technological limits,” said Dr Dmitry Momotenko, who leads a junior research group at the Institute of Chemistry. The goal: “We want to assemble objects atom by atom.”
“Metal is the perfect solution”
Nanoscale 3D printing – in other words, 3D printing of objects that are only a billionth of a meter in size – opens up tremendous opportunities, chemists explain. For metallic objects in particular, he can envision many applications in fields such as microelectronics, nanorobots, sensors and battery technology: “Electroconductive materials are required for all kinds of applications in these fields, so metals are the perfect solution.”
While 3D printing plastics has advanced to this nano dimension, fabricating small metal objects using 3D technology is proving more difficult. With some techniques, the mold structure is still a thousand times too large for many advanced applications, while with others it is impossible to manufacture objects of the required degree of purity.
Momotenko specializes in electroplating, a branch of electrochemistry in which metal ions suspended in a salt solution are brought into contact with a negatively charged electrode. Positively charged ions combine with electrons to form neutral metal atoms which are deposited on the electrodes, forming a solid layer.
“The molten salt solution becomes solid metal – a process that we electrochemists can control very effectively,” says Momotenko. This same process is used for chrome plated car parts and gold plated jewelery on a larger scale.
Slightly smaller than usual
However, transferring them to the nanoscopic scale requires considerable ingenuity, effort and care, as confirmed by a visit to the group’s small laboratory on the Wechloy campus. The lab contains three printers – all built and programmed by the team itself, as Momotenko points out. Like other 3D printers, it consists of a print nozzle, a tube to feed the printed material, a control mechanism and a mechanical part to drive the nozzle – but in this printer everything is a bit smaller than usual.
The colored salt solution flows through a fine tube into a thin capillary tube, which in turn contains a hair-thin piece of wire – the anode. This closes the circuit with a negatively polarized cathode, a flake of gold-plated silicon smaller than a fingernail, which is also the surface on which the printing takes place. Special micro-motors and crystals that change instantly when an electric voltage is applied rapidly move the nozzle by a fraction of a millimeter in all three spatial directions.
Because even the slightest vibration can interfere with the printing process, the two printers are housed in a box covered with a thick layer of dark acoustic foam. Next, they rest on granite slabs, each of which weighs 150 kilograms. Both actions are aimed at preventing unwanted vibrations. The light in the lab is also battery-powered because the electromagnetic fields generated by the alternating current from a power outlet will interrupt the small electrical currents and voltages needed to control the nanoprinting process.
Meanwhile, Liaisan Khasanova had prepared everything for the print test: the print nozzle was in the initial position, the box was closed, a vial containing light blue copper solution was connected to the tube. He starts a program that starts the printing process. Measurement data appears on the screen as curves and points. It shows variations in current flow and registers the nozzle touching the media briefly and then withdrawing again and again. What is a printing press? “Just a few columns,” he replied.
Explore deep into the nano world
Columns are the simplest geometric shapes produced in 3D printing, but the Oldenburg researchers can also print spirals, rings and all kinds of overhanging structures. This technique can now be used for printing with copper, silver and nickel alloys, as well as nickel-manganese and nickel-cobalt alloys.
In some of their experiments, they have ventured deep into the nanoworld. Momotenko and an international team of researchers report in a study published in the journal Nano Letters in 2021 (“Bringing Electrochemical Three-dimensional Printing to the Nanoscale”) that they have produced a copper column with a diameter of just 25 nanometers – 3D printing metal under the 100 nanometer limit for the first time.
One of the cornerstones of this success is the feedback mechanism which allows precise control of the movement of the print nozzle. It was developed by Momotenko together with Julian Hengsteller, a PhD student he supervised at his previous workplace, ETH Zurich in Switzerland. “Continuous removal of the print nozzle is very important, otherwise it quickly becomes clogged,” explains the chemist.
How to control the invisible
The team printed tiny objects layer by layer at a rate of several nanometers per second. Momotenko still marvels that objects too small for the human eye to see were created here. “You start with objects that you can touch. Then certain transformations take place and you can control these invisible things on a very small scale – it’s almost unbelievable,” said the chemist.
Momotenko’s plans for his nanoprinting technique are also quite astonishing: the goal is to lay the foundation for a battery that can be charged a thousand times faster than current models. “If that can be achieved, you can charge the e-car in seconds,” he explained. The basic idea he is pursuing is about 20 years old.
The principle is to drastically shorten the ion paths inside the battery during the charging process. To do this, the flat current electrode must have a three-dimensional surface structure. “With current battery designs, charging takes a long time because the electrodes are relatively thick and far apart,” explained Momotenko.
The solution, he says, is to lock the anode and cathode like a finger at the nanoscale and reduce the distance between them to just a few nanometers. This will allow ions to move between the anode and cathode at lightning speed. The problem: so far it has not been possible to produce battery structures with the required nano dimensions.
Battery material fabrication with the feature of ultra-small structure
Momotenko has now taken up the challenge. In his NANO-3D-LION project, funded by the European Research Council Starting Grant awarded in March 2021, the aim is to develop and apply advanced nanoscale 3D printing techniques to fabricate active battery materials with ultra-small structural features.
After successfully collaborating with a research group led by Prof. Dr Gunther Wittstock at the Institute of Chemistry on an earlier project, Momotenko then decided to base the project at the University of Oldenburg. “The Research and Transfers Department was very helpful with my grant application, so I’m moving here from Zurich in early 2021,” he explains.
The research group now has four members: apart from Khasanova, PhD student Karuna Kanes and Masters student Simon Sprengel have joined the team. Kanes focused on new methods aimed at optimizing print nozzle precision, while Sprengel investigated the possibility of printing combinations from two different metals – a process required to simultaneously produce cathode and anode materials in a single step.
Liaisan Khasanova will soon focus on lithium compounds. His mission was to find out how the electrode materials currently used in lithium batteries could be assembled using 3D printing. The team plans to investigate compounds such as lithium-iron or lithium-tin, and then to test how big the nano’s “finger” is on the electrode surface, what spacing is feasible, and how the electrodes should be aligned.
Research in the “glove box”
One of the main obstacles here is that lithium compounds are highly reactive and can only be handled under controlled conditions. For this reason, the team recently acquired an extra-large version of the laboratory glove box, a gas-tight chamber that can be filled with an inert gas such as argon. It has a handling glove built into one side with which the researchers can manipulate the objects inside.
The room, which is about three meters long and weighs half a ton, is not operational yet, but the team plans to install another printer inside. “Chemical conversion and all other tests must also be carried out indoors,” explains Momotenko.
The team will be faced with several big questions throughout the project: How do tiny impurities in the argon atmosphere affect the printed lithium nanostructures? How to get rid of the heat that must be generated when the battery is charged in seconds? How do you score not just a small battery cell but a large battery to power a cell phone or even a car – in a reasonable amount of time?
“On the one hand, we are working on the chemistry necessary to produce active electrode materials at the nanoscale; on the other hand, we are trying to adapt printing technologies to these materials,” said Momotenko, outlining the current challenges.
The problem of energy storage is very complex
The energy storage problem is extremely complex, and his team can play only a minor role in solving it, the researcher stressed. Nonetheless, he sees his group in a good starting position: according to him, electrochemical 3D metal printing is currently the only viable option for fabricating nanostructured electrodes and testing the concept.
In addition to battery technology, the chemist is also working on other bold concepts. He wanted to use his printing technique to produce metal structures that would allow more directional control of chemical reactions than had been possible so far. Such a plan plays a role in the relatively young research field known as spintronics, which focuses on the manipulation of “spin” – the quantum mechanical property of electrons.
Sensors that detect individual molecules
Another idea he wanted to put into practice was creating sensors capable of detecting individual molecules. “That would be very helpful in medicine, for detecting tumor markers or biomarkers for Alzheimer’s at very low concentrations, for example,” said Momotenko.
All these ideas are still very new approaches to chemistry. “It’s not yet clear how everything will work out,” he admits. But that’s how it is in science: “Any meaningful research project takes a lot of thought and planning, and most ideas fail in the end,” he concludes with a smile. But sometimes they don’t – and he and his team have taken the first successful step in their journey.