This is a guest post by Predrag Djuranovic, currently a graduate student at the MIT Department of Materials Science and Engineering.
In 2005, I was a graduate student in Francesco Stellacci’s lab at MIT. My project was investigating a potential phase separation in the ligand shells on semiconductor nanoparticles. I explain below how, after months of strenuous STM imaging, I came to the conclusion that the “ripples” and “hexagonal packing”, were nothing but common scanning artifacts, called feedback oscillations or ”ringing”.
When I started to have doubts, I performed simple control experiments, i.e. STM imaging on bare conducting substrates (clean substrates without any ligands). I selected two conductive substrates: gold foil (surface roughness comparable to the size of gold nanoparticles) and ITO glass (relatively flat surface islands in the 20-50 nm range). STM scans of those surfaces using the same instrument and similar settings as the ones used by Jackson et al. led to the images shown in Fig. 1. They are remarkably similar to the STM images that had just been reported in Nature Materials.
Obviously, these features are neither ligands nor any kind of nanostructure on the surface, since those surfaces were not functionalized. What are they?
The images are collected in constant tunneling current mode: the tunneling current and voltage across the tunneling junction is set to a particular value. The tunneling junction is connected to a feedback loop, which controls the movement of the STM tip – if the tunneling current is above the set value, the feedback loop retracts the tip away from the surface, thus decreasing the tunneling current. And vice versa.
It is normal to deviate from the set point by a few percents, but the scans reported in the Nature Materials 2004 article, as well as those presented in the images above, show deviation of one order of magnitude from the set point.
There are three parameters fundamental to feedback circuits in STM:
1. Proportional gain (the error signal gets multiplied by a gain and is sent to the piezo)
2. Integral gain (multiplies the integrated error while scanning)
3. Differential gain (multiplies the difference between the current tunneling current and the previous tunneling current reading)
These parameters are extremely important to consider, because they directly control the STM tip movement. Setting the gains, scan rate and scan size is crucial to obtain a good STM image. Scanning larger areas (> 50 nm) immediately implies dropping the scan rate so the feedback loop has enough response time to track the set current. Scanning too fast over large areas will lead the feedback system into an unstable mode of operation and will generate artificial features.
To better understand the tip response to sudden changes of topography, I decided to use elementary control theory. I implemented in matlab a simple second order feedback control system commonly found in STM electronics . The image generated (Fig 2, left) is remarkably similar to those obtained on ITO and those published in the Nature Materials 2004 article.To conclude, I have shown in 2005, while in Francesco Stellacci’s group, that the features in the 2004 paper STM images result from scanning probe artifacts. They are not representative of any surface ligands.
To obtain STM images that are less prone to scanning probe artifacts, I suggest imaging of smaller areas, for example, 20×20 nm and zooming in on several nanoparticles. Scanning large surface areas (100x100nm) with 512×512 resolution – an approach commonly used by Francesco Stellacci — introduces an intrinsic linear lateral uncertainty of 0.2 nm and leads the STM feedback circuit into an unstable mode of operation. To prove the validity of STM images in constant current mode, it would also be useful to share the error signal and show that it is not oscillating and significantly overshooting the set current value.
Seven years after these events, I am still wondering how so many high-impact publications based on immediately apparent scanning probe artifacts have been published. And consequently, how did reporting scanning artifacts consistently propagate through multiple peer review processes?
Comments are more than welcome.
 Predrag Djuranovic. https://dl.dropbox.com/u/8856561/ring.pdf ; Feedback Oscillation in STM Imaging, 2005
 Predrag Djuranovic, matlab script (https://dl.dropbox.com/u/8856561/sphere_PID_3D.m) used to generate Fig 2 left panel. Note that this is not a simulation of real STM dynamics, but a demonstration of the kind of topography that could be rendered under improper integral/proportional gain settings.
Jackson, A., Myerson, J., & Stellacci, F. (2004). Spontaneous assembly of subnanometre-ordered domains in the ligand shell of monolayer-protected nanoparticles Nature Materials, 3 (5), 330-336 DOI: 10.1038/nmat1116