Scanning Tunneling Microscopy PDF

SCANNING TUNNELING MICROSCOPY ▶ Gerd Binnig and Heinrich Rohrer – 1986 – Nobel Prize in Physics ▶ Based on quantum tunneling – where an electron can penetrate into regions that would be forbidden to it according to Classical physics ▶ SUBSTANCE UNDER INVESTIGATION SHOULD BE ELECTRICALL...

SCANNING TUNNELING MICROSCOPY ▶ Gerd Binnig and Heinrich Rohrer – 1986 – Nobel Prize in Physics ▶ Based on quantum tunneling – where an electron can penetrate into regions that would be forbidden to it according to Classical physics ▶ SUBSTANCE UNDER INVESTIGATION SHOULD BE ELECTRICALLY CONDUCTING – Eg. Copper ▶ Inner shell electrons – tightly bound ▶ Outer shell electrons (valence electrons) – loosely bound – form ‘electron gas’ ▶ Atomic nuclei + ISE – net positive charge – arrange in lattice ▶ Classical physics – Electrons as particles can never leave the metal unless sufficient energy to surmount barrier is given ▶ Quantum tunneling – Electrons as waves can ‘tunnel’ through the barrier ▶ STM construction - Metal probe with fine tip (single atom), positively charged positioned above electrically conducting surface to be imaged – ▶ Gap between probe tip and surface carefully adjusted – tip is only a few tenths of a nanometer from surface ▶ Flow of tunneling electrons is measured as an electric current, called ‘tunneling current’. ▶ The scanning tunneling microscope (STM) is a powerful tool that allows scientists to visualize and manipulate individual atoms on a surface. It was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich. ▶ Here's how it works: ▶ 1. *Probe tip*: A sharp probe tip, typically made of tungsten or platinum, is brought close to the surface of a material. ▶ 2. *Tunneling current*: When the tip is close enough (about 1 nanometer), electrons can "tunnel" through the vacuum between the tip and the surface, creating a small electric current. ▶ 3. *Scanning*: The probe tip is slowly scanned across the surface, keeping the tunneling current constant. ▶ 4. *Topographic map*: The up-and-down motion of the probe tip creates a topographic map of the surface, revealing individual atoms and molecules. The STM has revolutionized surface science, enabling researchers to: 1. *Visualize* individual atoms and molecules 2. *Manipulate* atoms and molecules (e.g., move them, create patterns) 3. *Study* surface properties (e.g., conductivity, reactivity) 4. *Observe* dynamic processes (e.g., chemical reactions, diffusion) History: 1. *1981*: Gerd Binnig and Heinrich Rohrer invented the STM at IBM Zurich. 2. *1986*: The first commercial STMs were released. 3. *1990s*: STM became a widely used tool in research, with advancements in resolution and speed. 4. *2000s*: New techniques, like non-contact atomic force microscopy (nc-AFM), were developed to complement STM. Applications: 1. *Atomic-scale imaging*: STM allows researchers to visualize individual atoms and molecules on a surface, enabling the study of surface structures and properties. 2. *Surface manipulation*: By moving atoms and molecules on a surface, scientists can create nanostructures, patterns, and even artificial molecules. 3. *Catalysis research*: STM helps researchers understand surface reactions and catalytic processes at the atomic level. 4. *Materials science*: STM is used to study the properties of materials, such as conductivity, magnetism, and superconductivity. 5. *Nanotechnology*: STM is a crucial tool for building and characterizing nanostructures, such as nanotubes, nanowires, and nanoparticles. 6. *Biological research*: STM can image biological samples, like DNA, proteins, and cells, in their natural environment. Some notable achievements with STM include: - Imaging individual atoms on a surface (1982) - Moving individual atoms on a surface (1990) - Creating artificial molecules on a surface (2000) - Observing chemical reactions at the atomic level (2007) STM has also led to the development of related techniques, such as: - Atomic force microscopy (AFM) - Non-contact atomic force microscopy (nc-AFM) - Scanning near-field optical microscopy (s-SNOM) - Scanning tunneling spectroscopy (STS) ▶ Current generated is nanosized, being few tenths of a nanoampere ▶ Probability of tunneling : 1) Atom involved 2 ) Separation of tip and surface – probability falls off rapidly with increasing tip-to-specimen distance ▶ Even 0.01 nm in separation can cause measurable changes in tunneling current ▶ As probe is moved horizontally across the surface (scanning), tunneling current will vary continuously ▶ When tip is over a ‘bump’ of an atom, current is higher and vice versa ▶ By scanning in several directions, possible to build up a ‘map’ showing arrangement of atoms on the surface of specimen, with resolution of 0.1 nm ▶ Constant – current mode: STM operated in CCM by constantly adjusting using computer to maintain a constant value of tunneling current. ▶ If current increases – tip is automatically raised and if decreases - lowered Image formed by STM of a nickel surface convered with a layer of sulfur atoms. ▶ Control done by mounting probe on three tiny posts set at right angles to each other, made of ceramic that expands or contracts on a scale of tenths of nanometers when voltage is applied across it. ▶ Piezo electric – materials that respond to voltage across them by changing their shape, or conversely respond to deformation by generating a voltage across themselves ▶ The system maintains the current at constant value to within a few percent – ie. the distance between the tip and specimen varies only by a few thousandths of a nanometer ▶ Recording of heights at different points across the surface is represented as a topographic map. ▶ Used to image not only nanostructures, but also to make them (bottom-up approach) – POSITIONAL ASSEMBLY – building structures by putting individual atoms into the right places – molecular manufacturing