Communications of the ACM
Software Supports Explorations Into the Intramolecular Picoworld
Brookhaven physicist Percy Zahl uses scanning microscopy and GXSM software to image and manipulate individual atoms and molecules. These computer screens display simulated non-contact atomic force microscopy images.
Credit: Brookhaven National Laboratory
For physicist Percy Zahl, traversing surfaces is not only a long-time hobby but also the focus of his research. An avid cyclist who is part of the Green Arm Bandits, a sub-team of Long Island's East End Cycling Team, he has pedaled more than 10,000 miles in 2016 alone, competing in races as close to home as the New York tri-state area and touring in places as far away as Chile. Weather permitting, he bikes the 20 miles it takes each day to make the round trip between his home and the U.S. Department of Energy's Brookhaven National Laboratory, where he covers terrain of a different kind — mapping the atomic structure and electronic properties of material surfaces through scanning probe microscopy (SPM).
Since 2005, Zahl has been working as an associate scientist at Brookhaven's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility. He came to Brookhaven from a postdoc position at IBM Zurich, where SPM was founded, with the invention of the scanning tunneling microscope in 1981.
In 1994, Zahl began developing control software for scanning tunneling microscopy. At the time, he was attending the Institute for Solid State Physics at Leibniz Universität Hannover, working on his thesis with a group in Germany that had microscopes but lacked the financial means for a full-scale commercial system. This software later evolved into what is known today as Gnome X Scanning Microscopy, or GXSM, an open-source software that enables scientists to acquire and interactively visualize data from digitally controlled scanning probe microscopes and similar instruments, in real time and offline.
With GXSM, scientists not only can explore and image the atomic structure and electronic properties of material surfaces in multiple dimensions and at atomic resolution but also manipulate individual atoms and molecules.
Now, Zahl has recently completed a major upgrade to GXSM that makes use of the latest computer technologies and incorporates new features.
"This upgrade involved a lot of behind-the-scenes work," says Zahl, who re-coded roughly half of the original 300,000 lines of source code and, in the process, reduced the amount of code by nearly 20 percent. "The software's GUI was totally redesigned not only in its look and feel but also its function. I also added several features for enhancing data acquisition and visualization."
The modernized GUI is based on GNOME-3, a desktop environment for computers using the Linux operating system.
One of the new features is a function to export visualizations in publication-quality vector format (PDF and SVG files). Previously, GXSM users relied on bitmap-based export (PNG files) to save their images, plots, and other graphics.
Another new feature is a remote-control console for automating complex or time-consuming scanning and manipulation tasks. In particular, the automation is useful for mapping electronic properties and force fields on the molecular scale within a multidimensional space above a sample's surface.
A new non-contact atomic force microscopy (NC-AFM) simulation plug-in module helps scientists understand the imaging process and makes data acquisition more efficient. The NC-AFM technique involves measuring the force between atoms and molecules on the surface of a sample and a highly sensitive single-molecule probe that oscillates with a controlled amplitude (at a fraction of an atom's diameter). The resulting "force" images resolve the internal structure of single molecules in great detail — for example, revealing carbon rings and "shadows" of hydrogen bonds.
However, getting the probe molecule in the right position to get these detailed images can take days of preparation work and is largely a trial-and-error process. The new simulation module is a valuable tool for scientists to explore imaging modes before and during an experiment to get an idea of what the molecules should ideally look like. Post-experiment, the module can be used to overlay the simulated molecules onto the actual microscopy images for comparison and optimization in future scans.
"In force imaging, the probe needs to be close enough to the surface that it 'feels' the repulsive force of the surface molecules, but if the probe gets too close, it loses its probing molecule and the probe-forming process must start all over again," says Zahl. "Using this plug-in module has helped me to better understand the imaging process so that I could operate the microscope most efficiently and under the best imaging conditions."
A full list of the new features can be found on the GXSM website, and the software is available for download through SourceForge, an online service where software developers can manage their projects and source code. A project discussion forum allows for interaction with users and provides user support and the latest software information. Since 2000, GXSM has been available under the GNU General Public License, which guarantees that users can freely access, share, and modify the source code. Zahl's collaborator, Thorsten Wagner of the Institute for Experimental Physics in Austria, packages the software and offers support to the more than 50 organizations around the world that use GXSM.
According to Zahl, though the major software overhaul is done, he will continue to make tweaks.
"As the GXSM project administrator and software developer, a GXSM user, and a microscopy instrument developer, I have the best possible combination of perspectives from which I can continuously make improvements," says Zahl. "I am always looking for feedback and ideas from users across the world, and, if a novel experiment presents the need for new functionality, it most likely can be made to work, as long as the instrument/hardware is capable."
In fact, Zahl purposely used a modular approach to design the original software so that it could be adapted to different types of data acquisition hardware and next-generation experimental techniques or data analysis tasks by adding new plug-in modules instead of modifying the large-scale code. Since its inception, GXSM has supported a line of commercially available data acquisition hardware optimized for scanning probe microscopy and instrument control, including hardware to amplify output signals from the control unit.
However, GXSM can be used for any kind of point-by-point scanning multichannel data acquisition on any scale. A laser laboratory at the CFN is currently using GXSM to measure optical signals directly via counting pulses. Zahl imagines GXSM could be used for various X-ray, ultraviolet, and infrared imaging techniques at Brookhaven's National Synchrotron Light Source II, also a DOE Office of Science User Facility. Via a customized control plug-in, users could even manage camera images with GXSM.
"I am very thrilled about the software's new features, particularly the mechanical probe particle simulation module for generating NC-AFM images from a molecular model," Zahl says. "Plus, the new technologies that I have incorporated into the software provide the next level of ease and operational comfort for long experimental immersions into the intramolecular picoscale world."
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