Cathodoluminescence

Unique insight into the chemical and electronic properties of materials at the microscopic level.

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Overview: 

What is cathodoluminescence?

Cathodoluminescence (CL) is the emission of light when a material is stimulated by an electron beam. 

Advantages     Uses     Workflow

Cathodoluminescence in a scanning or scanning transmission electron microscope (SEM or STEM) is a unique tool to characterize the composition and optical and electronic properties of materials, then correlate them with morphology, microstructure, composition, and chemistry at the micro- and sub-nanoscale.

Cathodoluminescence microscopy is the analysis of luminescence (emitted light or photons) from a material when stimulated by the electron beam of an electron microscope; the luminescence ranges from ultraviolet to infrared wavelength ranges.

Cathodoluminescence occurs because the impingement of a high energy electron beam elevates the sample to an excited state, which can then induce it to emit a (cathodoluminescence) photon when the sample returns to the ground state. In a semiconductor, this excitation process will result in the promotion of electrons from the valence band into the conduction band, which leaves behind a hole. Therefore when the electron and hole recombine, a photon will emit from the semiconductor.

The photon energy (color) and the probability that a photon (not a phonon) will be emitted depends on the material, its purity, and the defects it contains. Therefore to measure cathodoluminescence, you can examine almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way. In metals, the excitation of the specimen may be by surface plasmons being launched, which when they decay can result in the emission of a cathodoluminescence photon.

The electron microscope holds distinct advantages over conventional light microscopy, as the spatial resolution of light microscopy is limited by fundamental physics to an approximate resolution of 200 – 300 nm (or half the wavelength of the illuminating source). However in an electron microscope, you can focus the electron beam to a very small spot that potentially gives sub-nanometer spatial resolution. You can also use the emitted signals from the specimen to reveal morphological information, such as size and shape, as well as composition, chemistry, crystallography, electronic properties, plus much more. Thus, the volume of information from a specimen combined with the ability to correlate it directly with information from luminescence (spectroscopy) is what makes cathodoluminescence such a powerful characterization technique.

Advantages

Capability Advantage
Reveals quality of semiconductor materials Enables optimum manufacture of materials and devices; measure dislocation density
Permits characterization of optical materials and devices Allows investigation of optical properties at a spatial resolution better than the diffraction limit of light 
Exposes texture in minerals Enables geochemical processes to be reconstructed by revealing trace element distributions
Simultaneous measurement of morphology and composition Gives a full description of a sample with direct correlation of a sample’s shape, size, crystallinity or composition with its optical properties
Eliminates the need for complete device manufacture Material properties can be examined in a non-destructive manner without the need for full device processing

 

Uses

The characterization of optical properties at extremely small length scales is critical for many areas of scientific research and technologies. Including:

  • Light emitting diodes (LEDs)
  • Nanoparticles
  • Oil and geology
  • Optoelectronic and photovoltaic materials
  • Phosphors
  • 2D materials
  • Minerals, glasses, ceramics and gemstones
  • Pharmaceuticals
  • Polymers
  • Noble metals (plasmonics) 
  • Organic materials
  • Solar cells

 

Electronics and optoelectronics – In semiconductors, you can measure the local electronic band gap and reveal the defect distribution at the micro- and nano-scale. This technique allows you to easily examine direct bandgap semiconductors with strong cathodoluminescence, such as GaAs or GaN, as well as measure indirect semiconductors that emit weak cathodoluminescence, such as silicon. In particular, you can use differences in luminescence between dislocated and perfect crystalline silicon to map defects in integrated circuits. To add, the high spatial resolution provided by the focused electron beam is particularly well suited for you to examine low-dimensional semiconductor structures, such a quantum wells or quantum dots.
Geosciences – In rocks and minerals, observation of trace element chemistry and geochemical effects enables you to reconstruct geological processes. A SEM fitted with a cathodoluminescence detector, or an optical cathodoluminescence microscope, may be used to reveal internal structures not observable with other techniques to determine the composition, growth and provenance of minerals.
Material science – New sensor and communication technologies are being developed based on the interaction of light with metal nanoparticles. You can determine these properties by surface plasmons and local surface plasmon resonance modes. Recent publications show that researchers use cathodoluminescence performed in electron microscopes to study surface plasmon resonance in metallic nanoparticles at sub-diffraction limit resolutions.
Organic molecules – Many polymers and pharmaceutical active ingredients are shown to be cathodoluminescent. The luminescence signature is dependent on the chemical structure of the molecule rather than sample composition; hence cathodoluminescence can be used to rapidly map the distribution of organic molecules with sub-100 nm spatial resolution. 

 

Electron microscope setup

When the electron beam excites the specimen, this causes luminescence to be emitted from the near-surface region of the specimen. To collect the cathodoluminescence emitted in the upper hemisphere, a mirror is often inserted between the specimen and the pole piece. The mirror is specifically shaped to couple the light out of the microscope vacuum chamber to a spectrograph or photon detector(s). For thin samples, such as electron transparent TEM samples, mirrors above and below the specimen may be employed to collect light emitted in both hemispheres.

When you scan the microscope's focused electron beam in an X,Y pattern then measure the light emitted with the beam at each point, you can obtain an optical activity map of the specimen. The primary advantages of this electron microscope based technique is the ability to resolve features down to 1 nm and correlate the optical properties of an object with structural, compositional and chemical properties measured simultaneously or, at least within the same instrument. 

Typical light levels emitted from a specimen can be extremely low and often require you to collect and detect as many photons as possible. Even in samples intended to efficiently emit light, it is important to optimize experimental conditions to collect and detect photons with the minimum of optical losses so you can attain the highest spatial resolution results

Workflow

To better understand the cathodoluminescence workflow, select the appropriate electron microscope to support you intend to use.

SEM          STEM

Research Spotlight

TEM team & collaborators from left to right: Dayne Swearer, Rowan Leary, Emilie Ringe, and Sadegh Yazdi.

The Ringe Group was established in 2014 in the department of Materials Science and NanoEngineering (MSNE) at Rice University, Houston...

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