Tag Archives: John Randall

How to prevent your scanning tunneling microscope probe’s ‘tip crashes’

The microscopes used for nanoscale research were invented roughly 35 years ago and as fabulous as they’ve been, there is a problem (from a February 12, 2018 news item on Nanowerk),

A University of Texas at Dallas graduate student, his advisor and industry collaborators believe they have addressed a long-standing problem troubling scientists and engineers for more than 35 years: How to prevent the tip of a scanning tunneling microscope from crashing into the surface of a material during imaging or lithography.

The researchers have prepared this video describing their work,

For those who like text, there’s more in this February 12, 2018 University of Texas at Dallas news release,

Scanning tunneling microscopes (STMs) operate in an ultra-high vacuum, bringing a fine-tipped probe with a single atom at its apex very close to the surface of a sample. When voltage is applied to the surface, electrons can jump or tunnel across the gap between the tip and sample.

“Think of it as a needle that is very sharp, atomically sharp,” said Farid Tajaddodianfar, a mechanical engineering graduate student in the Erik Jonsson School of Engineering and Computer Science. “The microscope is like a robotic arm, able to reach atoms on the sample surface and manipulate them.”

The problem is, sometimes the tungsten tip crashes into the sample. If it physically touches the sample surface, it may inadvertently rearrange the atoms or create a “crater,” which could damage the sample. Such a “tip crash” often forces operators to replace the tip many times, forfeiting valuable time.

Dr. John Randall is an adjunct professor at UT Dallas and president of Zyvex Labs, a Richardson, Texas-based nanotechnology company specializing in developing tools and products that fabricate structures atom by atom. Zyvex reached out to Dr. Reza Moheimani, a professor of mechanical engineering, to help address STMs’ tip crash problem. Moheimani’s endowed chair was a gift from Zyvex founder James Von Ehr MS’81, who was honored as a distinguished UTD alumnus in 2004.

“What they’re trying to do is help bring atomically precise manufacturing into reality,” said Randall, who co-authored the article with Tajaddodianfar, Moheimani and Zyvex Labs’ James Owen. “This is considered the future of nanotechnology, and it is extremely important work.”

Randall said such precise manufacturing will lead to a host of innovations.

“By building structures atom by atom, you’re able to create new, extraordinary materials,” said Randall, who is co-chair of the Jonsson School’s Industry Engagement Committee. “We can remove impurities and make materials stronger and more heat resistant. We can build quantum computers. It could radically lower costs and expand capabilities in medicine and other areas. For example, if we can better understand DNA at an atomic and molecular level, that will help us fine-tune and tailor health care according to patients’ needs. The possibilities are endless.”

In addition, Moheimani, a control engineer and expert in nanotechnology, said scientists are attempting to build transistors and quantum computers from a single atom using this technology.

“There’s an international race to build machines, devices and 3-D equipment from the atom up,” said Moheimani, the James Von Ehr Distinguished Chair in Science and Technology.

‘It’s a Big, Big Problem’

Randall said Zyvex Labs has spent a lot of time and money trying to understand what happens to the tips when they crash.

“It’s a big, big problem,” Randall said. “If you can’t protect the tip, you’re not going to build anything. You’re wasting your time.”

Tajaddodianfar and Moheimani said the issue is the controller.

“There’s a feedback controller in the STM that measures the current and moves the needle up and down,” Moheimani said. “You’re moving from one atom to another, across an uneven surface. It is not flat. Because of that, the distance between the sample and tip changes, as does the current between them. While the controller tries to move the tip up and down to maintain the current, it does not always respond well, nor does it regulate the tip correctly. The resulting movement of the tip is often unstable.”

It’s the feedback controller that fails to protect the tip from crashing into the surface, Tajaddodianfar said.

“When the electronic properties are variable across the sample surface, the tip is more prone to crash under conventional control systems,” he said. “It’s meant to be really, really sharp. But when the tip crashes into the sample, it breaks, curls backward and flattens.

“Once the tip crashes into the surface, forget it. Everything changes.”

The Solution

According to Randall, Tajaddodianfar took logical steps for creating the solution.

“The brilliance of Tajaddodianfar is that he looked at the problem and understood the physics of the tunneling between the tip and the surface, that there is a small electronic barrier that controls the rate of tunneling,” Randall said. “He figured out a way of measuring that local barrier height and adjusting the gain on the control system that demonstrably keeps the tip out of trouble. Without it, the tip just bumps along, crashing into the surface. Now, it adjusts to the control parameters on the fly.”

Moheimani said the group hopes to change their trajectory when it comes to building new devices.

“That’s the next thing for us. We set out to find the source of this problem, and we did that. And, we’ve come up with a solution. It’s like everything else in science: Time will tell how impactful our work will be,” Moheimani said. “But I think we have solved the big problem.”

Randall said Tajaddodianfar’s algorithm has been integrated with its system’s software but is not yet available to customers. The research was made possible by funding from the Army Research Office and the Defense Advanced Research Projects Agency.

Here’s a link to and a citation for the paper,

On the effect of local barrier height in scanning tunneling microscopy: Measurement methods and control implications by Farid Tajaddodianfar, S. O. Reza Moheimani, James Owen, and John N. Randall. Review of Scientific Instruments 89, 013701 (2018); https://doi.org/10.1063/1.5003851 Published Online: January 2018

This paper is behind a paywall.

Toggling atomic switches and other talks at the Foresight Institute’s 2013 technical conference

The correct title for the conference, which took place almost one year ago (Jan. 11-13, 2013 in Palo Alto, California, US, is the 2013 Foresight Technical Conference: Illuminating Atomic Precision, and the organizers, the Foresight Institute in a Dec. 2, 2013 posting by James Lewis have announced a number of conference videos have been made available and have provided a transcript of sorts for one of the videos,

A select set of videos from the 2013 Foresight Technical Conference: Illuminating Atomic Precision, held January 11-13, 2013 in Palo Alto, have been made available on vimeo. Videos have been posted of those presentations for which the speakers have consented. Other presentations contained confidential information and will not be posted.

Here’s a listing of the 2013 conference presentations made available (click to access the videos),

  • Larry Millstein: Introductory comments at Foresight Technical Conference 2013
  • J. Fraser Stoddart: Introductory comments at Foresight Technical Conference 2013
  • Leonhard Grill: “Assembly and Manipulation of Molecules at the Atomic Scale”
  • John Randall: “Atomically Precise Manufacturing”
  • Philip Moriarty: “Mechanical Atom Manipulation: Towards a Matter Compiler?”
  • David Soloveichik: “DNA Displacement Cascades”
  • Alex Wissner-Gross: “Bringing Computational Programmability to Nanostructured Surfaces”
  • Joseph Puglisi: “Deciphering the Molecular Choreography of Translation”
  • Feynman Awards Banquet at Foresight Technical Conference 2013
  • Gerhard Klimeck: “Multi-Million Atom Simulations for Single Atom Transistor Structures”
  • William Goddard: “Nanoscale Materials, Devices, and Processing Predicted from First Principals” [Note: He’s a wearing a jaunty beret adding a note of style not usually found at technical conferences.]
  • Gerhard Klimeck: “Mythbusting Knowledge Transfer Mechanisms through Science Gateways”
  • Art Olson: “New Methods of Exploring, Analyzing, and Predicting Molecular Interactions”
  • George Church: “Regenesis: Bionano”
  • Dean Astumian: “Microscopic Reversibility: The Organizing Principle for Molecular Machines”
  • Larry Millstein: Closing comments at Foresight Technical Conference 2013

In his Foresight Institute blog posting  Lewis goes on to offer a description of Philip Moriarty’s presentation “Mechanical Atom Manipulation: Towards a Matter Compiler?,”

Prof. Moriarty presented his work with the qPlus technique of non-contact AFM of semiconductors, using chemical forces to mechanically move atoms around to structure matter, focusing on the tip of the probe—specifically how to optimize the tip structure, and how to return the tip to a previously known state. He begins with a brief review of how non-contact AFM uses a damped, driven oscillator to measure and manipulate what is happening at the level of single chemical bonds. The tip at the end of the oscillating cantilever measures the frequency shift of the cantilever as it approaches and interacts with the surface, and it maintains a constant amplitude of oscillation by pumping energy into the system. The frequency shift provides information about conservative forces acting on the tip, and the amount of energy pumped in gives a handle on non-conservative, or dissipative, forces. Before diving into the experimental details of his own work, Prof. Moriarty noted that various experimental accomplishments have vindicated Eric Drexler’s assertion that single atom chemistry could be done using purely mechanical force.

I found this description to be a beautiful piece of technical writing although I do have to admit to being distracted by thoughts of Sherlock Holmes on reading “Prof. Moriarty.” One final note, I noted the reference to Eric Drexler in the last sentence of my excerpt as Drexler was a Foresight Institute founder amongst his many other accomplishments.