New stamping technique creates functional features at nanoscale dimensions.
The next time you place your coffee order, imagine slapping onto your to-go cup a sticker that acts as an electronic decal, letting you know the precise temperature of your triple-venti no-foam latte. Someday, the high-tech stamping that produces such a sticker might also bring us food packaging that displays a digital countdown to warn of spoiling produce, or even a window pane that shows the day’s forecast, based on measurements of the weather conditions outside.
Engineers at MIT have invented a fast, precise printing process that may make such electronic surfaces an inexpensive reality. In a paper published today in Science Advances, the researchers report that they have fabricated a stamp made from forests of carbon nanotubes that is able to print electronic inks onto rigid and flexible surfaces.
A. John Hart, the Mitsui Career Development Associate Professor in Contemporary Technology and Mechanical Engineering at MIT, says the team’s stamping process should be able to print transistors small enough to control individual pixels in high-resolution displays and touchscreens. The new printing technique may also offer a relatively cheap, fast way to manufacture electronic surfaces for as-yet-unknown applications.
“There is a huge need for printing of electronic devices that are extremely inexpensive but provide simple computations and interactive functions,” Hart says. “Our new printing process is an enabling technology for high-performance, fully printed electronics, including transistors, optically functional surfaces, and ubiquitous sensors.”
Sanha Kim, a postdoc in MIT’s department of Mechanical Engineering, is the lead author, and Hart is the senior author. Their co-authors are Hossein Sojoudi, a postdoc in mechanical engineering and chemical engineering; Hangbo Zhao and Dhanushkodi Mariappan, graduate students in mechanical engineering; Gareth McKinley, the School of Engineering Professor of Teaching Innovation; and Karen Gleason, professor of chemical engineering and MIT’s associate provost.
A stamp from tiny pen quills
There have been other attempts in recent years to print electronic surfaces using inkjet printing and rubber stamping techniques, but with fuzzy results. Because such techniques are difficult to control at very small scales, they tend to produce “coffee ring” patterns where ink spills over the borders, or uneven prints that can lead to incomplete circuits.
“There are critical limitations to existing printing processes in the control they have over the feature size and thickness of the layer that’s printed,” Hart says. “For something like a transistor or thin film with particular electrical or optical properties, those characteristics are very important.”
Hart and his team sought to print electronics much more precisely, by designing “nanoporous” stamps. (Imagine a stamp that’s more spongy than rubber and shrunk to the size of a pinky fingernail, with patterned features that are much smaller than the width of a human hair.) They reasoned that the stamp should be porous, to allow a solution of nanoparticles, or “ink,” to flow uniformly through the stamp and onto whatever surface is to be printed. Designed in this way, the stamp should achieve much higher resolution than conventional rubber stamp printing, referred to as flexography.
Kim and Hart hit upon the perfect material to create their highly detailed stamp: carbon nanotubes — strong, microscopic sheets of carbon atoms, arranged in cylinders. Hart’s group has specialized in growing forests of vertically aligned nanotubes in carefully controlled patterns that can be engineered into highly detailed stamps.
“It’s somewhat serendipitous that the solution to high-resolution printing of electronics leverages our background in making carbon nanotubes for many years,” Hart says. “The forests of carbon nanotubes can transfer ink onto a surface like massive numbers of tiny pen quills.”
Printing circuits, roll by roll
To make their stamps, the researchers used the group’s previously developed techniques to grow the carbon nanotubes on a surface of silicon in various patterns, including honeycomb-like hexagons and flower-shaped designs. They coated the nanotubes with a thin polymer layer (developed by Gleason’s group) to ensure the ink would penetrate throughout the nanotube forest and the nanotubes would not shrink after the ink was stamped. Then they infused the stamp with a small volume of electronic ink containing nanoparticles such as silver, zinc oxide, or semiconductor quantum dots.
The key to printing tiny, precise, high-resolution patterns is in the amount of pressure applied to stamp the ink. The team developed a model to predict the amount of force necessary to stamp an even layer of ink onto a substrate, given the roughness of both the stamp and the substrate, and the concentration of nanoparticles in the ink.
To scale up the process, Mariappan built a printing machine, including a motorized roller, and attached to it various flexible substrates. The researchers fixed each stamp onto a platform attached to a spring, which they used to control the force used to press the stamp against the substrate.
“This would be a continuous industrial process, where you would have a stamp, and a roller on which you’d have a substrate you want to print on, like a spool of plastic film or specialized paper for electronics,” Hart says. “We found, limited by the motor we used in the printing system, we could print at 200 millimeters per second, continuously, which is already competitive with the rates of industrial printing technologies. This, combined with a tenfold improvement in the printing resolution that we demonstrated, is encouraging.”
After stamping ink patterns of various designs, the team tested the printed patterns’ electrical conductivity. After annealing, or heating, the designs after stamping — a common step in activating electronic features — the printed patterns were indeed highly conductive, and could serve, for example, as high-performance transparent electrodes.
Going forward, Hart and his team plan to pursue the possibility of fully printed electronics.
“Another exciting next step is the integration of our printing technologies with 2-D materials, such as graphene, which together could enable new, ultrathin electronic and energy conversion devices,” Hart says.
Learn more: Printable electronics
It is the Philosopher’s Stone of Nanotechnology: using a technological trick, scientists at TU Wien (Vienna) have succeeded in creating nanostructures made of pure gold.
This work is a giant leap forward for 3D nano-printing of gold structures which will be the core part of nanoplasmonics and bioelectronics devices
The idea is reminiscent of the ancient alchemists’ attempts to create gold from worthless substances: Researchers from TU Wien (Vienna) have discovered a novel way to fabricate pure gold nanostructures using an additive direct-write lithography technique. An electron beam is used to turn an auriferous organic compound into pure gold. This new technique can now be used to create nanostructures, which are needed for many applications in electronics and sensor technology. Just like with a 3D-printer on the nanoscale, almost arbitrary shapes can be created.
The long search for the right production process
“Gold is not only a noble metal of exceptional beauty, but also a highly desired material for functional nanostructures”, says Professor Heinz Wanzenböck from TU Wien. Especially patterned gold nanostructures are key enabling structures in plasmonic devices, for biosensors with immobilized antibodies and as electrical contacts. For decades the fabrication of pure gold nanostructures on non-planar surfaces as well as of 3-dimensional gold nanostructures has been the bottleneck. Up to now, only 2-dimensional gold nanostructures on planar surfaces were achievable by resist based lithography.
The new technology, developed at TU Wien, can now solve this problem. The principle is the local decomposition of a metalorganic precursor by the focused electron beam of an electron microscope. With extremely high precision, the electron beam can decompose the organic compound at exactly the right position, leaving behind a 3D-trail of solid gold.
The final obstacle was getting the material purity right, as the electron-induced decomposition of metalorganic precursors has typically yielded metals with high carbon contaminations. This last bottleneck on the road to custom-designed, pure gold nanostructures has now been overcome as described in the work on “Highly conductive and pure gold nanostructures grown by electron beam induced deposition” published in Scientific Reports.
While conventional gold deposition usually contains about 70 atomic % carbon and only 30 atomic % gold, the new approach developed by a research group lead by Dr. Heinz Wanzenboeck at TU Wien has allowed to fabricate pure gold structures by in-situ addition of an oxidizing agent during the gold deposition. “The whole community has been working hard for the last 10 years to directly deposit pure gold nanostructures”, says Heinz Wanzenböck. At last, the group’s expertise in engineering and chemical reactions paid off and direct deposition of pure gold was successful. “It’s a bit like discovering the legendary philosopher’s stone that turns common, ignoble material into gold” joked Wanzenboeck.
This deposited pure gold structure exhibits extremely low resistivity near that of bulk gold. Generally, a FEBID gold structure has a resistivity around 1-Ohm-cm which is about 1 million times worse than the resistivity of purest bulk gold. However, this specially enhanced FEBID process produces a resistivity of 8.8 micro-Ohm-cm which is only a factor 4 away from the bulk resistivity of purest gold (2.4 micro-Ohm-cm).
The authors of the paper Dr. Mostafa Moonir Shawrav and Dipl.Ing. Philipp Taus stated, “This highly conductive and pure gold structure will open a new door for novel nanoelectronic devices. For example, it will be easier to produce pure gold structures for nanoantennas and biomolecule immobilization which will change our everyday life”. Dr. Shawrav added “it is remarkable how a regular SEM (Scanning Electron Microscope) nowadays can deposit nanostructures compared to 20 years back when it was only a characterization device”. And with pure gold direct deposition available now, he expects nanodevices to be deposited directly and utilized in many different applications for technological revolution. Concluding, this work is a giant leap forward for 3D nano-printing of gold structures which will be the core part of nanoplasmonics and bioelectronics devices.
Learn more: Nanostructures Made of Pure Gold
Scientists at the Department of Energy’s Oak Ridge National Laboratory are the first to harness a scanning transmission electron microscope (STEM) to directly write tiny patterns in metallic “ink,” forming features in liquid that are finer than half the width of a human hair.
The automated process is controlled by weaving a STEM instrument’s electron beam through a liquid-filled cell to spur deposition of metal onto a silicon microchip. The patterns created are “nanoscale,” or on the size scale of atoms or molecules.
Usually fabrication of nanoscale patterns requires lithography, which employs masks to prevent material from accumulating on protected areas. ORNL’s new direct-write technology is like lithography without the mask.
Details of this unique capability are published online in Nanoscale, a journal of the Royal Society of Chemistry, and researchers are applying for a patent. The technique may provide a new way to tailor devices for electronics and other applications.
“We can now deposit high-purity metals at specific sites to build structures, with tailored material properties for a specific application,” said lead author Raymond Unocic of the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science User Facility at ORNL. “We can customize architectures and chemistries. We’re only limited by systems that are dissolvable in the liquid and can undergo chemical reactions.”
The experimenters used grayscale images to create nanoscale templates. Then they beamed electrons into a cell filled with a solution containing palladium chloride. Pure palladium separated out and deposited wherever the electron beam passed.
Liquid environments are a must for chemistry. Researchers first needed a way to encapsulate the liquid so the extreme dryness of the vacuum inside the microscope would not evaporate the liquid. The researchers started with a cell made of microchips with a silicon nitride membrane to serve as a window through which the electron beam could pass.
Then they needed to elicit a new capability from a STEM instrument. “It’s one thing to utilize a microscope for imaging and spectroscopy. It’s another to take control of that microscope to perform controlled and site-specific nanoscale chemical reactions,” Unocic said. “With other techniques for electron-beam lithography, there are ways to interface that microscope where you can control the beam. But this isn’t the way that aberration-corrected scanning transmission electron microscopes are set up.”
Enter Stephen Jesse, leader of CNMS’s Directed Nanoscale Transformations theme. This group looks at tools that scientists use to see and understand matter and its nanoscale properties in a new light, and explores whether those tools can also transform matter one atom at a time and build structures with specified functions. “Think of what we are doing as working in nanoscale laboratories,” Jesse said. “This means being able to induce and stop reactions at will, as well as monitor them while they are happening.”
Jesse had recently developed a system that serves as an interface between a nanolithography pattern and a STEM’s scan coils, and ORNL researchers had already used it to selectively transform solids. The microscope focuses the electron beam to a fine point, which microscopists could move just by taking control of the scan coils. Unocic with Andrew Lupini, Albina Borisevich and Sergei Kalinin integrated Jesse’s scan control/nanolithography system within the microscope so that they could control the beam entering the liquid cell. David Cullen performed subsequent chemical analysis.
“This beam-induced nanolithography relies critically on controlling chemical reactions in nanoscale volumes with a beam of energetic electrons,” said Jesse. The system controls electron-beam position, speed and dose. The dose—how many electrons are being pumped into the system—governs how fast chemicals are transformed.
This nanoscale technology is similar to larger-scale activities, such as using electron beams to transform materials for 3D printing at ORNL’s Manufacturing Demonstration Facility. In that case, an electron beam melts powder so that it solidifies, layer by layer, to create an object.
“We’re essentially doing the same thing, but within a liquid,” Unocic said. “Now we can create structures from a liquid-phase precursor solution in the shape that we want and the chemistry that we want, tuning the physiochemical properties for a given application.”
Precise control of the beam position and the electron dose produces tailored architectures. Encapsulating different liquids and sequentially flowing them during patterning customizes the chemistry too.
The current resolution of metallic “pixels” the liquid ink can direct-write is 40 nanometers, or twice the width of an influenza virus. In future work, Unocic and colleagues would like to push the resolution down to approach the state of the art of conventional nanolithography, 10 nanometers. They would also like to fabricate multi-component structures.