SCANNING TUNNELING MICROSCOPY (STM)

 SCANNING TUNNELING MICROSCOPY (STM)

·       Scanning Tunneling Microscopy (STM) is a real-space imaging technique, that can produce topographic images of a surface with atomic resolution in all three dimensions.

·       Scanning Tunneling Microscopy (STM) belongs to an expanding family of instruments commonly called Scanning Probe Microscope (SPM).

·       Resolution of Scanning Tunneling Microscopy (STM) is ~0.01 nm.

·       The STM is based on several principles.

ü  One is the quantum mechanical effect of Tunneling. It is this effect that allows us to “see” the surface.

ü  Another principle is the Piezoelectric effect. It is this effect that allows us to precisely scan the tip with angstrom-level control.

ü  Lastly, a Feedback loop is required, which monitors the tunneling current and coordinates the current and the positioning of the tip.

·       Scanning Tunneling Microscopy (STM) is a powerful instrument that allows one to image the sample surface at the atomic level. As the first generation of Scanning Probe Microscopy (SPM), STM paves the way for the study of nano-science and nano-materials. For the first time, researchers could obtain atom-resolution images of electrically conductive surfaces as well as their local electric structures.

History of Scanning Tunneling Microscopy

·       Gerd Binnig and Heinrich Rohrer developed the first working STM in 1981 while working at IBM Zurich Research Laboratories, Switzerland.

·       Binnig and Rohrer chose the surface of gold for their first image. When the image was displayed on the screen of a television monitor, they saw rows of precisely spaced atoms and observed broad terraces separated by steps one atom in height.

·       Binnig and Rohrer had discovered in the STM a simple method for creating a direct image of the atomic structure of surfaces.

·       The discovery of Binnig and Rohrer opened a new era for surface science, and their impressive achievement was recognized with the award of the Nobel Prize for Physics in 1986.

Mode of action of Scanning Tunneling Microscopy

·       The STM can be operated in two modes. They are

i)      Constant current mode: In this mode, the current is made constant during scanning by changing the distance between the tip and surface.

ii)    Constant height mode: In this mode, tip height is made constant and tunneling current at every step of scanning is measured.

Constant current mode

Constant height mode

Working Principle of Scanning Tunneling Microscopy

·       The main component of a Scanning Tunneling Microscope is a rigid metallic probe tip, typically composed of Tungsten, connected to a Piezodrive containing three perpendicular piezoelectric transducers.

·       The tip is brought within a fraction of a nanometer of an electrically conducting sample. At close distances, the electron clouds of the metal tip overlap with the electron clouds of the surface atoms. If a small voltage is applied between the tip and the sample a tunneling current is generated.

·       The magnitude of this tunneling current is dependent on the bias voltage applied and the distance between the tip and the surface.

·       A current Amplifier can covert the generated tunneling current into a voltage, passed to the Distance control and scanning unit.

·       Finally, the information is  gathered  by  monitoring  the  current  according  to  the  tip  position  which  scans  across  the surface, and this information is usually displayed in form of image.

Applications of STM

·       Scanning Tunneling Microscopy (STM) help scientists get a picture of how the atoms are arranged on a surface, by looking at the electron density of the surface atoms.

·       STM is used to image topography, measure surface properties, manipulate surface structures, surface roughness and to initiate surface reactions.

·       Shape, size, and organizations of individual particles or molecules along with the topographical information can be determined by STM.

·       Electronic information of various conducting surfaces can be determined by STM.

·       In Materials science, it provides new insights into the nanoscale properties of known materials and also enables the study of new nanoscale materials such as graphene and carbon nanotubes, as well as assembling structures composed of individual atoms.

·       In Chemistry, STM allows how the surface roughness and electronic properties of catalysts govern their performance to be understood.

·       Many biological samples are not electrically conducting, it has been shown that they can be coated with thin metal films, deposited on conducting substrates or scanned under humid conditions so that they can be studied using STM.

Advantages of STM

·       Capable of capturing much more detail than lesser microscopes.

·       Helps researchers better understand the subject of their research on a molecular level.

·       STMs are also versatile. They can be used in ultrahigh vacuum, air, water and other liquids and gasses.

·       STM will be operated in temperatures as low as Zero Kelvin up to a few 100 °C.

·       High Resolution Images (Atomic scale).

·       Low power application.

·       No damage to the sample.

Disadvantages of STM

·       STM can be difficult to use effectively.

·       STM is a very specific technique that requires a lot of skill and precision.

·       STM require very stable and clean surfaces, excellent vibration control and sharp tips.

·       STM use highly specialized equipment that is fragile and expensive.

·       Need for Vacuum & Vibration isolation.

·       Samples limited to conductors and semiconductors.

·       High Equipment cost.

·       Surface Preparation.

Maintaining the tool sharpness.