Method prints nanostructures using hard, sharp 'pen' tips floating on soft polymer springs

Method prints nanostructures using hard, sharp 'pen' tips floating on soft polymer springs

Northwestern researchers have developed an innovative method for printing nanostructures using hard, sharp ?pen? tips that float on soft polymer springs. The technique quickly and inexpensively produces patterns of high quality and with high resolution and density.

(PhysOrg.com) -- Northwestern University researchers have developed a new technique for rapidly prototyping nanoscale devices and structures that is so inexpensive the "print head" can be thrown away when done.

Hard-tip, soft-spring lithography (HSL) rolls into one method the best of scanning-probe lithography -- high resolution -- and the best of polymer pen lithography -- low cost and easy implementation.

HSL could be used in the areas of electronics (electronic circuits), medical diagnostics (gene chips and arrays of biomolecules) and pharmaceuticals (arrays for screening drug candidates), among others.

To demonstrate the method's capabilities, the researchers duplicated the pyramid on the U.S. one-dollar bill and the surrounding words approximately 19,000 times at 855 million dots per square inch. Each image consists of 6,982 dots. (They reproduced a bitmap representation of the pyramid, including the "Eye of Providence.") This exercise highlights the sub-50-nanometer resolution and the scalability of the method.

The results will be published Jan. 27 by the journal Nature.

"Hard-tip, soft-spring lithography is to scanning-probe lithography what the disposable razor is to the razor industry," said Chad A. Mirkin, the paper's senior author. "This is a major step forward in the realization of desktop fabrication that will allow researchers in academia and industry to create and study nanostructure prototypes on the fly."

Mirkin is the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences and professor of medicine, chemical and biological engineering, biomedical engineering and materials science and engineering and director of Northwestern's International Institute for Nanotechnology.

Micro- and nanolithographic techniques are used to create patterns and build surface architectures of materials on a small scale.

Scanning probe lithography, with its high resolution and registration accuracy, currently is a popular method for building nanostructures. The method is, however, difficult to scale up and produce multiple copies of a device or structure at low cost.

Scanning probe lithographies typically rely on the use of cantilevers as the printing device components. Cantilevers are microscopic levers with tips, typically used to deposit materials on surfaces in a printing experiment. They are fragile, expensive, cumbersome and difficult to implement in an array-based experiment.

"Scaling cantilever-based architectures at low cost is not trivial and often leads to devices that are difficult to operate and limited with respect to the scope of application," Mirkin said.

Hard-tip, soft-spring lithography uses a soft polymer backing that supports sharp silicon tips as its "print head." The spring polymer backing allows all of the tips to come in contact with the surface in a uniform manner and eliminates the need to use cantilevers. Essentially, hard tips are floating on soft polymeric springs, allowing either materials or energy to be delivered to a surface.

HSL offers a method that quickly and inexpensively produces patterns of high quality and with high resolution and density. The prototype arrays containing 4,750 tips can be fabricated for the cost of a single cantilever-based tip and made in mass, Mirkin said.

Mirkin and his team demonstrated an array of 4,750 ultra-sharp silicon tips aligned over an area of one square centimeter, with larger arrays possible. Patterns of features with sub-50-nanometer resolution can be made with feature size controlled by tip contact time with the substrate.

They produced patterns "writing" with molecules and showed that as the tips push against the substrate the flexible backing compresses, indicating the tips are in contact with the surface and writing is occurring. (The silicon tips do not deform under pressure.)

"Eventually we should be able to build arrays with millions of pens, where each pen is independently actuated," Mirkin said.

The researchers also demonstrated the ability to use hard-tip, soft-spring lithography to transfer mechanical and electrical energy to a surface.

Scientists create highly ordered artificial spin ice using nanotechnology

Scientists create highly ordered artificial spin ice using nanotechnology

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(PhysOrg.com) -- An international team of researchers has succeeded in creating artificial spin ice in a state of thermal equilibrium for the first time, allowing them to examine the precise configuration of this important nanomaterial.

Scientists from the University of Leeds, the US Department of Energy's Brookhaven National Laboratory and the UK Science and Technology Facilities Council's Rutherford Appleton Laboratory say the breakthrough will allow them to study in much greater detail a scientific phenomenon known as 'magnetic monopoles', which are thought to exist in such structures. Their findings are published today in the journal Nature Physics.

Artificial spin ice is built using nanotechnology and is made up of millions of tiny magnets, each thousands of times smaller than a grain of sand. The magnets exist in a lattice in what is known as a 'frustrated' structure. Like water ice, the geometry of the structure means that all of the interactions between the atoms cannot be satisfied at the same time.

"It's like trying to seat alternating male and female diners around a table with an odd number of seats – however much you re-arrange them you will never succeed," said Dr Christopher Marrows from the University of Leeds, co-author of the paper.

In spin ice, magnetic dipoles with a north and south pole are arranged in tetrahedron structures. Each dipole has magnetic moments, similar to the protons on H2O molecules in water ice, which attract and repel each other. Consequently, the dipoles arrange themselves into the lowest possible energy state, which is two poles pointing in and two pointing out.

Dr Marrows said: "Spin ices have created a lot of excitement in recent years as it has been realised that they are a playground for physicists studying magnetic monopole excitations and Dirac string physics in the solid state. However, until now all of the samples of these artificial structures created in the lab have been what we call 'jammed'.

"What we have done is find a way to un-jam spin ice and get it into a well-ordered ground state known as thermal equilibrium. We can then freeze a sample into this state, and use a microscope to see which way all the little magnets are pointing. It's the equivalent of being able take a picture of every atom in a room as it allows us to inspect exactly how the structure is configured."

Jason Morgan, PhD student at the University of Leeds and lead author of the paper, was the first member of the team to observe the sample in equilibrium. He said: "Getting the sample to self-order in such a way has never been achieved experimentally before and for a while had been considered impossible. But when we looked at the sample using magnetic force microscopy and saw this beautiful periodic structure we knew instantly that we had achieved an ordered ground state."

The researchers have also been able to observe individual excitations out of this ground state within their sample, which they say is evidence for monopole dynamics within the lattice.

Magnetic monopoles – magnets with only a single north or south pole ¬¬– are former hypothetical particles that are now thought to exist in spin ice. There is hope among scientists that understanding these monopoles in more detail could lead to advances in a novel technology field known as 'magnetricity' – a magnetic equivalent to electricity.

Co-author Sean Langridge, a Science and Technology Facilities Council (STFC) Fellow and visiting Professor at the University of Leeds, added: "In the naturally occurring spin-ice systems this ground state is predicted but has not been experimentally observed.

"Now that is has been observed in an artificial system the next step is to observe dynamically the excitations from this ground state. We can only do this by controlling the interactions with state of the art lithographic techniques. This level of control will provide an even greater level of understanding in this fascinating system."

The team created "artificial" spin ice samples at Brookhaven using a state-of-the-art nanotechnology tool called an electron beam writer. A similar £4 million facility is shortly to be opened at the University of Leeds which will be unique to the UK and will allow continued collaboration with the researchers at Brookhaven.