STM Tutorial
STM is not just an imaging tool—it's a versatile platform for probing the atomic and electronic structure of materials. By mastering topographic imaging, spectroscopy, and conductance mapping, we can uncover the hidden quantum behaviors that drive novel material properties.
1. Topographic Imaging (Constant Current Mode)
Objective: Map the surface structure of the material.
How it works:
The STM tip scans laterally across the surface while a feedback loop adjusts its vertical (z-axis) position to maintain a constant tunneling current.
As the tip height changes to accommodate bumps and valleys, these adjustments are recorded and translated into a detailed image of the surface.
Key Principle:
The tunneling current depends on both the tip-sample distance and the integrated density of states (DOS) between the Fermi level and the applied bias voltage.
For samples with uniform DOS, the recorded height profile closely reflects the real physical corrugation of the surface.
What you can see: Atomic lattice patterns, step edges, vacancies, and other defects.
2. Differential Conductance Spectroscopy (dI/dV)
Objective: Measure the local electronic properties of the material.
How it works:
Position the STM tip at a fixed point on the surface.
Turn off the feedback loop (so the tip height remains constant).
Sweep the bias voltage while recording the tunneling current.
Apply a small bias modulation and use a lock-in amplifier to measure dI/dV, which is proportional to the local density of states (LDOS) at a given energy.
Key Principle:
The dI/dV curve shows how the LDOS varies with energy, revealing features like energy gaps, peaks (resonances), and localized states.
What you can learn: Electronic band gaps, superconducting gaps, impurity states, and many other material-specific properties.
3. Differential Conductance Mapping (dI/dV Maps)
Objective: Create spatial maps of the electronic structure at different energies.
How it works:
Record dI/dV spectra across a grid of points over a region of the surface.
Compile these into a "3D" data set: X and Y represent surface position, while the third axis is energy (bias voltage).
You can extract slices (dI/dV maps) at specific energies to view how electronic states are distributed across the surface.
Bonus Technique:
Perform a Fourier Transform (FT) on the dI/dV maps.
This translates the spatial patterns into frequency (momentum or q-space), revealing periodic modulations and scattering processes in the material.
What you can explore: Charge density waves, quasiparticle interference patterns, and hidden periodicities.