Oct 3, 2016
Quantum zone explains non-local STM effects
Since the early days of nanotechnology, people have used the scanning tunnelling microscope (STM) to manipulate atoms either mechanically or from the force due to charge injection. More recently, researchers have noted that the charge injected from an STM tip can manipulate molecules some distance away, by “remote control”. Now experiments have visualized the quantum mechanical behaviour of electrons at the surface beneath the tip for the first time, offering a better understanding of charge propagation processes that may also affect device efficiency in solar cells and radiotherapy enhancers.
Take a silicon surface dotted with toluene molecules and shoot electrons at it, and one might reasonably expect some resulting ejection of the surface molecules. Electrons or holes injected into the surface with energy significantly greater than thermal levels would collide with surface molecules, lifting them off the surface. What is less intuitive perhaps is the zone of calm – like the eye of a storm – where molecule ejection is suppressed directly under the tip, just where one might naively expect a higher likelihood of collisions.
“We didn’t expect it and we didn’t understand it,” admits Richard Palmer, head of nanoscale physics at the University of Birmingham, as he describes their initial observations. They were not alone. Several other groups have now observed similar “non-local manipulation” or “remote control” of surface atoms, but without any real understanding of the origin. The mysterious spatial lag between the site of charge injection and surface atom disruption provided the key.
Palmer worked alongside Peter Sloan and colleagues at Birmingham and Bath universities in the UK, and adopted a systematic approach to see what was going on. They were able to visualize a distinct region under the tip where the charge dynamics at the surface behave as an expanding coherent quantum wave function, so that scattering does not take place. Beyond around 10–15 nm from the tip, the wave function collapses and the familiar 2D diffusive transport of charges colliding with surface molecules takes over.
“These electrons are hot,” adds Palmer, referring to their relatively high energy load of a few eV. This is significant because electrons of similar energy feature in a number of devices under development that use charge transport. “If you excite an electron and it propagates a certain distance, it can lead to effects that affect the efficiency of, say, solar cells.” He tells nanotechweb.org that the team are also looking at the potential implications for gold nanoparticles used as radiotherapy enhancers, where the excitation of secondary electrons may be the means of killing targeted cancer cells.
Seek to find
The trail of ejected surface molecules is a useful indicator of how charges propagate across the surface from the tip. However, the usual approaches for STM manipulation – scanning the tip across an area or locating the tip at a particular point and observing the molecule directly beneath – are unlikely to reveal the kinds of insights that Palmer and his team observed.
Instead, the Birmingham and Bath researchers scanned the area, then located the tip at a particular point to inject charge, and then scanned the area (passively) again. This way they could actually visualize the expansion of the wavefunction by the consequences of charge injection. While a lot of STM molecule manipulation experiments are performed in cryogenic conditions, the researchers were able to observe these effects at room temperature, far more practical conditions for exploiting the quantum behaviour in applications.
From observation to control
This year we celebrate the 30th anniversary of the Nobel Prize for STM, and fundamental experiments with the tool are still yielding surprise insights with significant applications for device engineers.
The team are now looking into how they can manipulate the wave function that expands under the tip. Palmer points to an analogy of water ripples from a stone dropped in a pond. When the ripples hit a wall, the reflected ripples interfere with the initial wave and change its shape at the water surface. The team hope to use strategically placed arrays of atoms to modify the wave function by superposition with the reflected wave.
Full details are reported in Nature Communications
For more of the latest developments 30 years on from the Nobel Prize in scanning tunnelling microscopy and the first report of atomic force microscopy, visit the Nanotechnology Focus on Scanning Probe Microscopy
About the author
Anna Demming is editor of nanotechweb.org