Scanning Tunneling Microscopy (STM) is a powerful tool used to image and analyze the surface of conductive materials at the atomic level. The STM operates by scanning a sharp conductive probe very close to the surface of a conductive specimen and forcing electrons to traverse the gap between them, allowing for the mapping of the sample’s electronic density of states.
Understanding the Principles of Scanning Tunneling Microscopy
The fundamental principle behind STM is the quantum mechanical phenomenon of tunneling. When a conductive probe is brought within a few angstroms of a conductive sample surface, a small bias voltage applied between the two can cause electrons to tunnel through the vacuum gap, resulting in a tunneling current. The magnitude of this tunneling current is exponentially dependent on the distance between the probe and the sample, as described by the following equation:
$I_t = V_b \cdot \frac{4\pi e}{h} \cdot \Phi \cdot \exp(-\frac{4\pi\sqrt{2m\Phi}d}{\hbar})$
Where:
– $I_t$ is the tunneling current
– $V_b$ is the bias voltage
– $e$ is the elementary charge
– $h$ is Planck’s constant
– $\Phi$ is the work function of the sample material
– $m$ is the mass of the electron
– $d$ is the distance between the probe and the sample
This exponential dependence on the distance allows the STM to achieve extremely high spatial resolution, with the ability to resolve individual atoms on the surface of a conductive material.
Modes of Operation
The STM can operate in two distinct modes: constant height mode and constant current mode.
Constant Height Mode
In constant height mode, the probe tip is maintained at a fixed height above the sample surface as it raster scans across the sample. The tunneling current is then measured and used to construct a topographic image of the sample surface. This mode is typically used for samples with a relatively flat surface, as the fixed height of the probe can lead to the tip crashing into surface features if the sample is too rough.
Constant Current Mode
In constant current mode, the tunneling current is held constant by a feedback loop system that adjusts the distance between the tip and the surface. As the probe scans across the sample, the feedback loop adjusts the height of the tip to maintain a constant tunneling current. The height adjustments are then used to construct a topographic image of the sample surface. This mode is more suitable for samples with significant surface roughness, as the feedback loop can accommodate changes in the sample topography.
Enhancing Resolving Power
The STM has a high inherent resolving power, which can be further improved through the use of automated distortion correction and multi-frame averaging (MFA) techniques.
Automated Distortion Correction
Distortions in STM images can arise from various sources, such as thermal drift, piezoelectric creep, and mechanical vibrations. Automated distortion correction algorithms can be used to identify and correct these distortions, resulting in more accurate and precise images.
Multi-Frame Averaging (MFA)
MFA is a technique that involves acquiring multiple frames of the same sample area and then averaging them together. This approach has been shown to achieve a sixfold enhancement of the signal-to-noise ratio (SNR) of the Si(111)-(7 × 7) reconstruction, and allow for images with sub-picometre height precision to be routinely obtained. For example, images of a monolayer of Ti2O3 on Au(111) with sub-picometre height precision have been demonstrated using this approach.
Measuring Surface Characteristics
The STM can be used to measure a wide array of surface characteristics, including:
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Surface Roughness: The STM can provide detailed information about the surface topography, including the root-mean-square (RMS) roughness and the power spectral density (PSD) of the surface.
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Defects: The STM can be used to identify and characterize various types of surface defects, such as vacancies, adatoms, and dislocations.
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Molecular Conformation: The STM can be used to study the size and conformation of molecules adsorbed on the surface, providing insights into their interactions with the substrate.
Energy Resolution and Voltage Modulation
The energy resolution of the STM is limited by the amplitude of the voltage modulation used. Ideally, a voltage modulation smaller than 0.36 mV should be used for the best energy resolution. However, in practice, a 2 mV RMS modulation is often used to balance the signal-to-noise ratio and measurement time.
For example, in a study of the electronic structure of the (4 × 4) reconstructed SrTiO3(111) surface, a voltage modulation of 2 mV RMS was used to achieve a good balance between energy resolution and measurement time. This allowed for the automated classification of the two chiral variants of the surface unit cells.
Applications and Limitations
The STM has a wide range of applications in materials science, surface science, and nanotechnology, including the study of:
- Semiconductor surfaces and interfaces
- Superconducting materials
- Catalytic surfaces
- Molecular self-assembly
- Graphene and other 2D materials
However, the STM is limited to the study of conductive or semiconductive materials, as the tunneling current required for imaging cannot be established on insulating surfaces. Additionally, the STM requires a stable and vibration-free environment to achieve the best possible resolution and image quality.
Conclusion
Scanning Tunneling Microscopy is a powerful tool for imaging and analyzing the surface of conductive materials at the atomic level. By leveraging the principles of quantum tunneling, the STM can achieve extremely high spatial resolution and provide detailed information about surface characteristics, such as roughness, defects, and molecular conformation. Through the use of advanced techniques like automated distortion correction and multi-frame averaging, the resolving power of the STM can be further enhanced, allowing for the routine acquisition of images with sub-picometre height precision. As a versatile and powerful tool, the STM continues to play a crucial role in the advancement of materials science, surface science, and nanotechnology.
References:
– LAB UNIT 5: Scanning Tunneling Microscopy, University of Washington, https://depts.washington.edu/nanolab/NUE_UNIQUE/Lab_Units/5_Lab_Unit_STM.pdf
– Maximising the resolving power of the scanning tunneling microscope, NCBI, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5992247/
– Scanning Tunneling Microscopy (STM): An Overview, Oxford Instruments, https://afm.oxinst.com/modes/scanning-tunneling-microscopy-stm/
– Automated classification of surface unit cells in scanning tunneling microscopy images, PNAS, https://www.pnas.org/doi/10.1073/pnas.1707745114
– Scanning Tunneling Microscopy of Graphene, Graphene-Based Electrochemical Sensors, and 2D Materials, Nanomaterials, https://www.mdpi.com/2079-4991/8/2/92
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