A group of investigators led by Assistant Professor Kamil Ekinci (AME) have developed an inventive technique that will allow researchers to collect nanoscale microscopy images roughly 100 times faster than current state-of-the-art scanning tunneling microscopes. The findings, published in the Nov. 1st issue of the journal Nature, are a modification of the 25-year-old traditional microscopy technique and will remedy the slow pace and limited high-frequency response of current microscopy circuitry.
To achieve atomic-scale spatial resolution at the nanoscale level, traditional scanning tunneling microscopy (STM) relies on localized electron tunneling between a sharp probe tip and a conducting sample. In this technique, the tip is brought to within a few angstroms (one ten-millionth of a millimeter) of the sample surface and an electrical current is drawn and measured between the tip and the surface.
While traditional STM has uncovered a wealth of discoveries in many diverse physical systems, the technique has visual limitations.
“In traditional STM, the electrical current between the tip and the sample needs to be boosted up by a large amount because it is very tiny,” Ekinci said. “But when you boost up, there is trade off. You lose temporal resolution and bandwidth, which means you cannot detect fast signals.”
Ekinci’s group, which includes Boston University graduate student Utku Kemiktarak (CAS ’09), overcame the shortcoming by increasing the high-frequency response of the tunnel current’s readout circuitry. The new higher-frequency results demonstrated electronic bandwidths as high as 10 megahertz – a 100-fold improvement over traditional STM.
“In our approach, we were able to eliminate the problems introduced by the boosting process by attaching an impedance matching circuit to the probe tip,” Ekinci said. “This opened up a usable bandwidth which resulted in good temporal resolution. While the technique itself is quite simple, this is the first time it is applied to STM.”
The new technique provides the ability for fast surface imaging, which was previously difficult because of the resolution loss and the decrease in bandwidth.
“With regular STM, it was hard to do fast surface scans because you would not be able to see the images,” Ekinci said. “If you went too fast with scanning, everything would be blurred out. Our technique has allowed the circuit to become responsive to fast changes.”
In addition to collecting rapid-paced imagery, the new technique will act as a motion detector at high frequencies and measure the noise of the tunneling electrons, the latter of which is very important for the detection of temperature on the sample surface.
“We’ll be able to use our tip on the surface to measure the temperature,” Ekinci said. “This is a very important feature for high-tech integrated circuits. By extracting temperature for the surface, it will allow us to see which areas in a circuit may be hotter than others.”
The full text of the Nature article is available from a BU-based Internet connection, or to Nature subscribers, by clicking here.