scanning probe microscopy

scanning probe microscopy
Name	Scanning probe microscopy
Acronym	SPM
Classification	Surface science
Inventor	Gerd Binnig, Heinrich Rohrer, Calvin Quate
Related	Scanning tunneling microscope, Atomic force microscope

scanning probe microscopy is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. The foundational instrument, the scanning tunneling microscope, earned its inventors, Gerd Binnig and Heinrich Rohrer, the Nobel Prize in Physics in 1986. This family of techniques enables visualization and manipulation of matter at the nanoscale, providing critical insights into fields ranging from materials science to biophysics.

Overview

The development of scanning probe microscopy revolutionized surface science by providing the first real-space images of atomic arrangements on surfaces. Pioneered at the IBM Zurich Research Laboratory, the invention of the scanning tunneling microscope by Gerd Binnig and Heinrich Rohrer overcame the resolution limits imposed by the wavelength of light or electrons in other techniques. Subsequent innovations, like the atomic force microscope developed by Binnig, Calvin Quate, and Christoph Gerber, extended the methodology to non-conductive samples. These instruments are now cornerstone tools in nanotechnology research at institutions like the National Institute of Standards and Technology and are central to the work of organizations such as the International Union of Pure and Applied Physics.

Operating principles

All scanning probe microscopy techniques operate by raster-scanning a sharp tip in close proximity to a sample surface while monitoring a specific interaction. In the scanning tunneling microscope, a voltage is applied between the tip and a conductive sample, and the resulting tunneling current is measured, as described by the theories of quantum mechanics. For the atomic force microscope, the interaction is a mechanical force, such as van der Waals force, which causes deflection of a cantilever measured via a laser beam and photodiode system. Feedback mechanisms, often employing a piezoelectric actuator, maintain a constant interaction parameter, translating its variation into a topographical map. This general principle has been adapted to sense diverse properties including magnetic and electrostatic forces.

Major techniques

The field encompasses numerous specialized techniques derived from core principles. The scanning tunneling microscope provides atomic-resolution imaging and spectroscopy of electronic states. The atomic force microscope has several primary modes, including contact mode, tapping mode, and non-contact mode. Other significant methods include magnetic force microscopy for mapping magnetic domains, electrostatic force microscopy for studying charge distributions, and scanning thermal microscopy for mapping temperature variations. Techniques like scanning capacitance microscopy and Kelvin probe force microscopy are vital for analyzing semiconductor devices, while chemical force microscopy uses modified tips to probe specific molecular recognition events.

Instrumentation and components

A typical scanning probe microscope consists of several key subsystems. The probe itself is a sharp tip, often made of silicon or silicon nitride for atomic force microscopes or tungsten for scanning tunneling microscopes, mounted on a flexible cantilever. Precise positioning in three dimensions is achieved using piezoelectric ceramics, such as lead zirconate titanate. Detection systems vary: scanning tunneling microscopes measure current with a preamplifier, while atomic force microscopes often use an optical lever system involving a laser from a source like a helium–neon laser and a position sensitive detector. The entire apparatus is isolated from vibration using systems like active vibration cancellation or spring-based platforms, and is controlled by sophisticated electronics and software from companies like Bruker Corporation or Oxford Instruments.

Applications

Scanning probe microscopy has become indispensable across scientific and industrial disciplines. In materials science, it is used to characterize thin films, carbon nanotubes, and quantum dots. Within the semiconductor industry, tools like scanning capacitance microscopy are critical for failure analysis and metrology of devices from companies like Intel or TSMC. In molecular biology, researchers use it to image DNA, proteins, and cell membranes. It also enables nanomanipulation, such as the famous arrangement of xenon atoms by Donald Eigler at IBM Almaden Research Center to spell "IBM". Furthermore, it aids in developing novel technologies like molecular electronics and high-density data storage systems.

Limitations and challenges

Despite its power, scanning probe microscopy faces several inherent constraints. The imaging process is relatively slow compared to techniques like scanning electron microscopy, making it unsuitable for observing fast dynamic processes. The finite radius of the probe tip can cause imaging artifacts, a phenomenon described by the convolution of tip and sample geometries. Maintaining a stable tip and interpreting contrast mechanisms, especially in complex environments like electrolyte solutions, remains challenging. Furthermore, most techniques require careful control of the environment, such as ultra-high vacuum for atomic-resolution scanning tunneling microscope work, and are sensitive to external disturbances like acoustic noise or thermal drift.

Category:Microscopy Category:Surface science Category:Nanotechnology