Home Built STM

2016-08-03 | By Daniel Berard

License: General Public License Teensy

There’s no need to buy costly purpose-built STMs (scanning tunneling microscope).  You can now achieve atomic resolution imaging in air with the Teensy 3.1 based home-built scanning tunneling microscope. This device also utilizes an inexpensive piezo buzzer element with one of the electrodes cut into quadrants to enable XYZ motion. Although it is a given that buzzers are less rigid than purpose-built scanners, image for a highly-oriented pyrolytic graphite (HOPG) is still achieved with the STM in atomic resolution!

STM is a tool capable of imaging surfaces with atomic resolution. In a STM, a sharp metallic needle is brought within a few angstroms on the surface of a conductive sample and a small bias voltage is applied across the gap. If the gap is small enough (<1 nm), electrons can cross the gap via quantum tunneling. This “tunneling current” is typically in the pA – nA range, and can be measured with a trans impedance amplifier. The STM tip is mounted on a piezoelectric scanner, which is capable of sub-angstrom motion in all directions. The tunneling current, measured by the trans impedance amplifier, is fed into a feedback loop which controls the voltage applied to the Z-axis electrode of the piezo scanner and acts to maintain a constant tunneling current, and therefore a constant tip-sample distance. The X and Y axes of the scanner are used to raster scan the tip across the sample. By measuring the Z-axis voltage as a function of the scan position, an image of the sample topography is constructed. If the tip moves closer to the sample surface, the tunneling current increases exponentially. This exponential relationship is what makes the STM sensitive enough to resolve individual atoms, even under ambient conditions. If the STM tip is atomically sharp (not as hard to achieve as you might think!), nearly all of the tunneling current will flow through the single atom on the tip which is closest to the sample surface, resulting in images with atomic resolution.

Scan Head

The buzzer used has a 19 mm diameter and a 6.3 kHz resonant frequency. The STM tip mounts on a standoff glued to the buzzer. The STM scan head is made from two 2″ x 2″ x 1/2″ aluminum blocks connected by three 1/4″-80 precision adjustment screws.

Atomic-sharp needle

The STM tip needs to be atomically sharp so that most of the tunneling current flows through only one atom. You can cut tips from a 30 AWG tungsten wire with wire cutters to achieve this.

Vibration Isolation

The STM head is mounted on a stack of three 5″ diameter x 1/2″ thick steel plates each separated by 3 small pieces of viton perfluoroelastomer cut from an O-ring. This stack is mounted on an MDF and aluminum plate, which is suspended by three 2′ long springs. Three hard drive magnets at the base provide some eddy-current damping.

Electronics

Tunneling current is measured by a preamplifier which outputs a voltage proportionate to the tunneling current. This voltage is compared to a setpoint voltage by an instrumentation amplifier, whose output is equal to the difference between the two. The error signal is fed into an integrator, whose output ramps up or down at a rate which depends on the error signal. This generates the Z-axis signal for the scanner. The X and Y scanning signals, as well as the sample bias and setpoint voltages, are generated by a 4-channel 16-bit DAC. The tunneling current or Z-axis piezo signals can be read by a 16-bit ADC and are used to construct the image. The Teensy 3.1 controls the scanning and data acquisition and feeds it to a computer via USB.

So far I have only imaged gold and highly oriented pyrolytic graphite (HOPG) surfaces. The surface must be conductive and free of an oxide layer, which actually leaves out most metals for imaging in air. This is why STM is often performed in an ultra-high vacuum.

More details about the project.

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