Atomic-scale resolution is needed to study the arrangement of atoms in materials and advancing their understanding. Since the seventeenth-century optical microscopes using visible light as illumination source have led our quest to observe microscopic species but the resolution attainable reached physical limits due to the much longer wavelength of visible light. After the discovery of wave nature associated with particle bodies, a new channel of thought opened considering much shorter wavelength of particles and their special properties when interacting with the sample under observation.
These particles i.e. electrons, neutrons and ions were developed in different techniques and were used as illumination sources. Herein, the development of scanning tunneling microscopy which used electrons to uncover irregularities in the arrangement of atoms in thin materials via the quantum mechanical phenomenon of electron tunneling became a sensational invention. Atomic Force Microscopy (AFM) is a development over STM which relied on measuring the forces of contact between the sample and a scanning probe which overcame the earlier technique only allowing conductors or pretreated surfaces for conducting to be observed.
Since measuring contact forces between materials is a more fundamental approach that is equally but more sensitive than measuring tunneling current flowing between them, atomic force microscopy has been able to image insulators as well as semiconductors and conductors with atomic resolution by substituting tunneling current with an atomic contact force sensing arrangement, a delicate cantilever, which can image conductors and insulators alike via mechanical "touch" while running over surface atoms of the sample. AFM has seen a massive proliferation in hobbyist’s lab in form of ambient-condition scanning environment as opposed to an ultra-high vacuum of sophisticated labs and self-assembled instrumentations.
The success of ATM as a cost-effective imaging tool with dramatically increased ease of conceptual understanding and use particularly with the assistance of significant computing power in the form of personal computers which offsets the computational difficulty of resolving experimental information which makes up for physical simplicity of instrument design has seen its proliferation to numerous labs in universities and technology companies worldwide.
Table of Contents
- Article:
- Quantum physics
- Tunneling current
- STM general assembly
Objectives and Key Themes
The objective of this text is to provide an overview of Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), focusing on their principles, capabilities, and applications in atomic-scale imaging.
- Principles of Scanning Tunneling Microscopy (STM)
- Quantum mechanical principles underlying STM
- The nature of tunneling current and its dependence on tip-sample separation
- Technical aspects of STM assembly and operation
- Comparison of STM and AFM
Chapter Summaries
Article: This section introduces Scanning Tunneling Microscopy (STM), highlighting its groundbreaking role in achieving atomic-resolution imaging. It discusses the quantum tunneling phenomenon, the dependence of the tunneling current on tip-sample separation, and the application of STM in solving the structure of the Si (111)-(7x7) surface. The invention's significance is emphasized, including the Nobel Prize awarded to Binnig and Rohrer. The section also introduces Atomic Force Microscopy (AFM) as a development over STM, capable of imaging both conductors and insulators with atomic resolution. The importance of a vacuum environment and surface cleanliness for successful AFM imaging is also noted.
Quantum physics: This part delves into the quantum mechanical principles that govern STM. It contrasts classical and quantum physics, explaining how a particle is treated as a quantifiable "blob" in quantum mechanics, represented by a wave function. The act of measurement is described as collapsing this wave function into an observable particle. The explanation extends to the tunneling effect, where an electron can penetrate a potential barrier even if its energy is lower than the barrier height. This quantum phenomenon is crucial to understanding how STM works. The section describes the wave function's exponential decay during barrier penetration, and the tunneling current's dependence on the applied voltage and the local density of states.
Tunneling current: This section focuses on the tunneling current in STM, explaining its dependence on tip-sample separation, voltage difference, and local density of states. It clarifies how the exponential dependence on distance ensures that the current from the foremost atom dominates, making STM feasible even with blunt tips. The section includes a graph illustrating the relationship between current and tip-sample separation, highlighting the strong correlation enabling high-precision imaging. Further discussion centers on the error analysis involved in measuring tip-sample separation based on the tunneling current, demonstrating the remarkable precision achievable with STM.
STM general assembly: This section details the electronic modules and components of an STM, specifically focusing on the experimental measurement and noise aspects of imaging. The power supply requirements are described, along with the crucial role of a high-performance current-to-voltage converter (IVC) in amplifying the very small tunneling current. The section provides a schematic diagram of the STM assembly, illustrating the various components and their connections. This section offers an insightful look into the practical workings of STM beyond just the theoretical concepts presented in earlier parts.
Keywords
Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM), quantum tunneling, tunneling current, tip-sample separation, atomic resolution, local density of states (LDOS), surface imaging, Si (111)-(7x7) reconstruction, quantum mechanics, classical physics, Nobel Prize.
Frequently Asked Questions: A Comprehensive Guide to Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM)
What topics are covered in this text?
This text provides a comprehensive overview of Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). It covers the fundamental principles of each technique, their technical aspects, and their applications in atomic-scale imaging. Specific topics include quantum tunneling, tunneling current, tip-sample separation, the assembly and operation of STM, and a comparison between STM and AFM. The text also delves into the quantum mechanical principles underlying STM and the importance of vacuum environment and surface cleanliness for successful AFM imaging.
What are the key objectives of this text?
The main objective is to offer a clear and concise understanding of STM and AFM, focusing on their capabilities and applications in atomic-resolution imaging. The text aims to explain the underlying quantum mechanical principles, the technical aspects of the instruments, and their significance in scientific research.
What are the key themes explored in the text?
Key themes include the quantum mechanical basis of STM, the nature and behavior of the tunneling current, the crucial role of tip-sample separation in achieving atomic resolution, the technical details of STM assembly and operation, and a comparison of STM and AFM. The significance of the invention of STM and its impact on nanotechnology are also highlighted.
What are the principles of Scanning Tunneling Microscopy (STM)?
STM operates on the principle of quantum tunneling. An extremely sharp tip is brought very close to a conductive sample. A small voltage is applied, and electrons tunnel across the gap between the tip and the sample. The tunneling current is highly sensitive to the distance, allowing for the mapping of the surface at the atomic level.
How does the tunneling current work in STM?
The tunneling current in STM is exponentially dependent on the tip-sample separation, voltage difference, and local density of states. This exponential dependence means that the current is dominated by the foremost atom of the tip, allowing for atomic-resolution imaging even with slightly blunt tips. The text details the relationship between these factors and the resulting current, explaining how this relationship enables high-precision imaging.
What is the significance of the Si (111)-(7x7) surface reconstruction in the context of STM?
The Si (111)-(7x7) surface reconstruction is a significant example used in the text to illustrate the power of STM in solving complex surface structures. It demonstrates STM’s ability to image and analyze atomic structures, showcasing its capabilities in materials science.
What is the difference between STM and AFM?
While both STM and AFM achieve atomic resolution, STM requires a conductive sample, whereas AFM can image both conductors and insulators. AFM uses a cantilever tip to measure forces between the tip and the sample, rather than the tunneling current used in STM. The text discusses the advantages and limitations of both techniques.
What are the technical aspects of STM assembly and operation?
The text details the electronic modules and components of an STM, including the power supply, the crucial role of a high-performance current-to-voltage converter (IVC), and the experimental considerations for noise reduction. A schematic diagram illustrating the assembly is provided.
What quantum mechanical principles underlie STM?
The text explains how the quantum mechanical concept of tunneling, where particles can pass through potential barriers even if their energy is insufficient, is the foundation of STM. It contrasts this with classical physics, highlighting the wave-particle duality of electrons and the concept of wave function collapse upon measurement.
What are the keywords related to this text?
Keywords include: Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM), quantum tunneling, tunneling current, tip-sample separation, atomic resolution, local density of states (LDOS), surface imaging, Si (111)-(7x7) reconstruction, quantum mechanics, classical physics, Nobel Prize.
- Quote paper
- Suchit Sharma (Author), 2015, Scanning Tunneling Microscope and Atomic Force Microscopy, Munich, GRIN Verlag, https://www.grin.com/document/382666
