Exploring the revolutionary tool that transformed our ability to visualize and understand atomic-scale surface processes
Imagine trying to see individual atoms with your own eyes—distinguishing their arrangement, watching how they bond with other atoms, and observing the molecular structures they form. For most of scientific history, this remained an impossible dream, as the atomic world was far too small to be visualized directly.
Understanding how atoms and molecules behave on surfaces is crucial to answering fundamental questions: Why do some materials corrode while others resist? How do catalysts efficiently transform one chemical into another? What processes enable the creation of advanced materials with tailored properties?
The development of the scanning tunneling microscope (STM) in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich—earning them the Nobel Prize in Physics just five years later—transformed this dream into reality 1 . This revolutionary instrument provided scientists with unprecedented atomic-level vision, allowing them not only to see individual atoms but to probe their electronic properties and even manipulate them into desired configurations.
When applied to the study of chemisorbed layers—single layers of atoms or molecules chemically bonded to metal surfaces—STM opened a window into fundamental processes that had previously only been theorized.
This article explores how STM unveiled the hidden landscape of chemisorbed layers on metal surfaces, revealing the intricate atomic architectures that govern surface chemistry and enabling advances in fields ranging from industrial catalysis to materials science.
The scanning tunneling microscope operates on principles that defy classical physics, harnessing the quantum mechanical phenomenon of electron tunneling to achieve atomic-scale resolution.
Unlike conventional microscopes that use lenses and light waves, STM employs an exquisitely sharp metallic tip—often made of tungsten or platinum-iridium—that is brought to within nanometers of the sample surface 1 .
When a small voltage is applied between the tip and the conductive sample, electrons can perform what seems like magic: they "tunnel" through the empty space between tip and sample, creating a measurable electrical current despite never having enough energy to cross this gap according to classical physics 5 . This tunneling current is exponentially sensitive to the tip-sample separation—changing by a factor of about 10 for each angstrom (0.1 nanometer) change in distance 1 . This extreme sensitivity is what enables STM to detect individual atoms.
Quantum tunneling visualization
| Principle | Description | Role in STM |
|---|---|---|
| Quantum Tunneling | Quantum mechanical effect where electrons traverse classically-forbidden regions | Enables measurement of surface features at atomic scale |
| Exponential Distance Dependence | Tunneling current decreases exponentially with increasing tip-sample separation | Provides extreme sensitivity to atomic-scale topography |
| Piezoelectric Effect | Crystals that expand or contract minutely when voltage is applied | Allows precise positioning and scanning of the tip |
| Feedback Control | Electronic system that maintains constant tunneling current during scanning | Enables automatic tracking of surface contours |
The most common approach where the feedback electronics continuously adjust the tip height to maintain a stable tunneling current as the tip scans across the surface. The recorded vertical movements create a topographical map of the surface 1 .
The tip remains at a fixed height while changes in tunneling current are directly recorded, enabling faster scanning on atomically flat surfaces 5 .
Technical Note: The entire system requires exceptional stability—often employing vibration isolation systems and operating in ultra-high vacuum environments—to prevent even the slightest disturbances from disrupting the delicate tip-sample interaction 1 . When all these elements come together, the result is an instrument that can distinguish features smaller than 0.1 nanometers with a depth resolution of 0.01 nanometers—allowing individual atoms to be routinely imaged and manipulated 1 .
The power of STM to transform our understanding of surface chemistry is beautifully illustrated by pioneering research on sulfur chemisorbed on rhenium metal surfaces.
This system, studied by M. Salmeron and colleagues, demonstrated how STM could reveal atomic structures and processes that had eluded characterization by other surface science techniques 7 .
When a clean rhenium surface is exposed to sulfur atoms under controlled ultra-high vacuum conditions, the sulfur atoms form chemical bonds with the rhenium surface atoms, creating what scientists call a chemisorbed layer. Prior to STM, techniques like Low Energy Electron Diffraction (LEED) could provide limited information about the average periodic arrangements of such layers, but they couldn't visualize local structures, defects, or the dynamics of layer formation.
| Sulfur Coverage | Observed Structure | Significance |
|---|---|---|
| Low (<0.25 monolayers) | Isolated sulfur atoms distributed across surface | Individual adatoms are stable and mobile |
| Medium (~0.25 monolayers) | Beginning of organized structures | Transition from individual atoms to aggregates |
| High (>0.25 monolayers) | Trimers and more complex aggregates | Isolated adatoms become unstable and coalesce |
The most striking finding was that above 0.25 monolayers of coverage, isolated sulfur adatoms become unstable and coalesce into structured aggregates, particularly forming trimers (groups of three sulfur atoms) and other more complex arrangements 7 . This was a significant revelation because it demonstrated that adsorbate-adsorbate interactions play a crucial role in determining the geometry of the chemisorbed layer—something that had been difficult to observe directly before STM.
Researchers prepared an atomically clean rhenium surface in an ultra-high vacuum chamber through cycles of sputtering (bombarding with ions) and annealing (heating).
Sulfur was then deposited onto this pristine surface, either from a gas source or by heating a solid sulfide source.
The sample was transferred to the STM stage while maintaining the vacuum to prevent contamination. STM imaging was performed at various sulfur coverages, carefully controlled by the deposition time and temperature.
Another fascinating aspect of this research was the discovery that a single layer of sulfur atoms could passivate the rhenium surface, protecting it from oxidation and contamination when exposed to air 7 . This practical insight has implications for industrial processes where metal surfaces need protection from reactive environments.
Conducting STM studies of chemisorbed layers requires more than just the microscope itself—it demands an array of specialized materials, reagents, and samples that enable precise atomic-scale investigations.
| Material/Reagent | Function | Specific Examples & Notes |
|---|---|---|
| Single Crystal Metal Substrates | Provides atomically flat, well-defined surfaces for adsorption studies | Pt(111), Mo(001), Re(0001) - oriented and polished crystals 7 |
| High-Purity Gaseous Sources | Source of adsorbate atoms or molecules for creating chemisorbed layers | H₂S, O₂, CO - purified to prevent contamination 7 |
| STM Tip Materials | Fabrication of sharp conductive tips for scanning and tunneling | Tungsten, platinum-iridium, gold wires 1 |
| Electrochemical Etching Supplies | Preparing sharp STM tips through controlled etching | NaOH solution, electrochemical cells 9 |
| Ultra-High Vacuum Components | Maintaining pristine, contamination-free environments | Ion pumps, vacuum gauges, leak valves |
The metal single crystals used as substrates—such as platinum (Pt), molybdenum (Mo), and rhenium (Re)—are not ordinary metal pieces. They are carefully grown as single crystals, then cut along specific crystallographic planes (noted by Miller indices like (111) or (001)) and polished to achieve atomically flat surfaces 7 . These pristine surfaces provide the ideal canvas for studying chemisorption without the complications of random grain boundaries and defects.
The adsorbate sources must be of exceptional purity, as even trace contaminants can compete with the desired adsorbates or alter surface properties. For sulfur adsorption studies, researchers often use hydrogen sulfide (H₂S) gas, which decomposes on the metal surface, releasing sulfur atoms that bond with the metal while hydrogen desorbs 7 .
Tip preparation is both an art and a science. Tungsten tips are commonly prepared by electrochemical etching, where a tungsten wire is immersed in a sodium hydroxide solution and a voltage is applied to dissolve the metal, creating a sharp point 1 . Platinum-iridium tips may be sharpened by mechanical cutting. The quality of the tip ultimately determines the resolution of the images.
Critical Requirement: All these materials must be integrated within an ultra-high vacuum system that maintains pressures as low as 10⁻¹¹ torr—equivalent to the vacuum of space—to prevent contamination by airborne molecules that would instantly coat the surface under normal conditions 1 . The meticulous attention to each component in this research toolkit enables the extraordinary precision that makes atomic-scale imaging possible.
The initial STM studies of chemisorbed layers on metals represented just the beginning of a continuing revolution in surface science.
The ability to directly visualize atomic arrangements transformed theoretical models of surface processes, allowing scientists to move beyond averaging techniques and study local phenomena—defects, domain boundaries, and individual impurities—that often determine the overall properties of a material 7 9 .
Understanding how reactant molecules arrange themselves on metal surfaces has guided the design of more efficient and selective catalysts.
The passivating effect of certain adsorbate layers has led to better strategies for protecting metals from degradation.
The principles of self-organization observed in adsorbate layers have been applied to create precisely structured nanomaterials with tailored properties.
Allows scientists to measure the local density of electronic states on a surface, providing information about electronic properties in addition to topography 1 .
Can now capture dynamic processes on surfaces, recording videos of molecular diffusion and reaction processes at frame rates up to 80 Hz 1 .
Advanced techniques combining STM with other methods, such as synchrotron X-ray spectroscopy, enable elemental and magnetic characterization at the nanoscale 9 .
Researchers are exploring laser-induced STM using femtosecond pulses to study electron dynamics on ultrafast timescales 9 .
Looking forward, STM techniques continue to push boundaries. Researchers are exploring laser-induced STM using femtosecond pulses to study electron dynamics on ultrafast timescales 9 . Millikelvin STM systems with high magnetic fields are probing quantum phenomena like superconductivity and magnetic interactions at the atomic scale 9 . Each advancement provides new windows into the atomic world, further illuminating the intricate dance of atoms and electrons that underpins so much of chemistry and materials science.
The development of scanning tunneling microscopy and its application to the study of chemisorbed layers represents one of the most profound achievements in modern experimental science.
By harnessing the quantum mechanical phenomenon of electron tunneling, STM gave researchers what Gerd Binnig called "the eyes to see atoms," transforming surface science from indirect deduction to direct observation.
The investigation of sulfur on rhenium and similar systems demonstrated not only that we can image individual atoms, but that we can understand their interactions, their self-organization, and their dynamic behavior in ways previously unimaginable. This atomic-level perspective has bridged the gap between theoretical predictions and experimental observation, validating some long-held hypotheses while overturning others.
As STM techniques continue to evolve, combining atomic-scale imaging with spectroscopic capabilities and ultrafast time resolution, they promise to reveal even deeper secrets of the atomic world. The once-hidden landscape of chemisorbed layers on metal surfaces now stands as testament to human ingenuity—our relentless drive to see, understand, and ultimately master the invisible forces that shape our material world.
"The journey from wondering about atoms to actually seeing them has transformed not only science but our very conception of what is possible when human curiosity is coupled with technological innovation."