Cathodoluminescence

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

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Overview: 

What is cathodoluminescence?

While luminescence is the emission of photons from a solid when excited by an energy source, be it light, electrons, current flow, etc., cathodoluminescence (CL) is the emission of photons, as light, when a material is specifically stimulated by high-energy electrons.

A meeting of optical emission spectroscopies and electron microscopy.

Cathodoluminescence microscopy is the analysis of the 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 (200 – 2,300 nm or 6 – 0.5 eV). The ability to focus the electron beam of an electron microscope to sub-nanometer length scales allows us to investigate optical properties of materials down to the nanoscale, far below the diffraction limit constraints of optical microscopes. Furthermore, the cathodoluminescence signal may be correlated directly with information gained from other signals.

Advantages     Modes     Uses      Setup      Workflow

Why Use Cathodoluminescence?

CL 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, while simultaneously correlating them with morphology, microstructure, composition, and chemistry at the micro- and nano-scale.

Although other techniques such as optical emission spectroscopies and photo- and electro-luminescence enable the characterization of optical properties, CL in the electron microscope holds distinct advantages due to the vastly increased spatial resolution available and the ability to correlate directly with structural information. Light microscopes are limited by fundamental physics to an approximate spatial resolution of 200 – 300 nm (approximately half the wavelength of the illuminating source). However, in an electron microscope, the electron beam can be focused to a very small spot potentially giving optical information with sub-nanometer spatial resolution. Furthermore, other emitted signals 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 CL such a powerful characterization technique.

How is CL Generated?

CL 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 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. 

In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way with luminescence resulting from energy transitions across the band gap or from mid-gap energy states. 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.

Advantages

Capability Advantage
Optical characterization far below the diffraction limit Study the optical properties of individual nanostructures and assemblies
Reveals quality of semiconductor materials Validates semiconductor growth (materials and devices); composition, quantification of point and extended defect distributions 
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 a 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

 

Modes of Operations

The CL light emission may be defined by its distributions in wavelength (energy), angle (momentum), and polarization and may be measured from a single location (a point or small region of a specimen), or from an array of locations to form an image or spectrum image (depending on the distributions recorded).  CL detectors permit analysis of the spatial distribution of one or more of the distributions; in some advanced forms of analysis, more than one distribution may be measured simultaneously.  Analysis modes that select a single (or small range of) wavelength, angle or polarization distributions may be described as “-filtered” whereas the recording of an entire distribution may be described as “-resolved” e.g., distribution is dispersed (by some optical method) and the entire distribution is then measured in parallel.

Distribution Analyzed Selection Method Measurement from a Single Point Measurement from an Array of Locations
Integrated None Unfiltered CL signal Unfiltered CL map
Wavelength Filtered Wavelength-filtered CL signal Wavelength-filtered CL map
Wavelength Resolved Wavelength-resolved CL spectrum Wavelength-resolved CL spectrum image (3D)
Angle Filtered Angle-filtered CL signal Angle-filtered CL map
Angle Resolved Angle-resolved CL pattern Angle-resolved CL spectrum image (4D)
Polarization Filtered Polarization-filtered CL signal Polarization-filtered CL map
Wavelength and Angle Resolved Wavelength- and angle-resolved CL patterns Wavelength- and angle-resolved CL spectrum image (5D)

 

Depending on the exact operating mode, the result may be an intensity (a value), a line plot (a spectrum), an image or map, or a spectrum image (3-, 4- or even 5D).  A description of the commonly employed operating modes are described below:

Unfiltered signal: Where all wavelengths are present, e.g., no filtering or dispersion applied to the emitted light. The signal intensity is measured by a light-sensitive detector such as a photomultiplier tube (PMT) or solid-state diode (SSD).

Unfiltered map: The electron beam is scanned across the specimen surface and the emitted light intensity is recorded at each location and displayed as a (2D) map.
Sometimes referred to as panchromatic, polychromatic, or integrated intensity imaging.

Uses: Reveal texture in mineralogy, e.g., zonation, overgrowths, and micro-cracks and, reveal extended defects in semiconductor materials and devices 

Wavelength-filtered signal: Where a single - or small range of - wavelengths are present. The wavelength selection may be performed by an optical filter or by the exit slit of a wavelength dispersive spectrometer before the signal intensity is measured by a light-sensitive detector.

Wavelength-filtered map: The electron beam is scanned across the specimen surface and the emitted light intensity within the defined wavelength range is recorded at each location and displayed as a (2D) map.
Sometimes referred to as monochromatic imaging.

Uses: Discriminate materials by composition and/or crystal structure, reveal the purity of semiconductor materials, determine resonance modes in nanophotonic materials 

Wavelength-resolved CL spectrum: Spectroscopic analysis comes from collecting the emitted light and dispersing it so that intensity can be determined as a function of wavelength. Dispersion is typically performed by a diffraction grating-based spectrometer or spectrograph and detected by an array detector (e.g., a CCD).  In some cases a wavelength-resolved CL spectrum may be captured by recording the signal intensity as the dispersed light is scanned over a wavelength-selecting exit slit of the spectrometer by (e.g.) a PMT.

Wavelength-resolved CL spectrum image: The electron beam is scanned across the specimen surface and a wavelength spectrum is recorded at each location and displayed as a (3D) data cube.  An array detector may be used to collect a complete wavelength-resolved CL spectrum pixel-by-pixel.

Sometimes referred to as hyperspectral CL analysis or wavelength imaging.

Wavelength-filtered CL spectrum image:  The electron beam is scanned across the specimen surface and a wavelength spectrum is recorded at each location and displayed as a (3D) data cube.  A series of wavelength-filtered CL images may be recorded by a PMT or SSD with the wavelength measured stepped after each pass of the electron beam.

Uses:  Identify compounds (and phases), determine the electronic structure, map composition, stress or overlapping spectral features 

Angle-resolved (AR) CL pattern: The emitted light is captured as a 2D image where the information pertaining tot he a photon's emission direction is maintained; each pixel in the image within the image corresponds to a unique emission direction of the emitted light. Often, the captured image is transformed into an emission pattern taking account of the optical system employed in detection and displayed in the polar coordinate system with the emission resolved in the zenith and azimuthal angles.

Sometimes referred to as momentum spectroscopy.

Angle-resolved (AR) CL spectrum image: A 2D emission pattern is collected over an array of spatial locations generating a 4D data set.

Uses: To understand the emission pattern of light emitting devices and how light and matter interact far below the optical diffraction limit.

Wavelength- and angle-resolved CL (WARCL): The angle-resolved emission pattern is dispersed by wavelength using a diffraction-grating based spectrograph.  Commonly, for higher spatial and spectral resolutions, a subset of the angular range is sequentially scanned collecting a full emission spectrum in each emission direction.

Sometimes referred to as energy-momentum spectroscopy. 

Uses: Understand the wavelength distribution (color) of light emitting devices as a function of viewing angle and gain deep insight into how light and matter interact.

 

 

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
  • 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 cathodoluminescences, 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 as quantum wells or quantum dots.
Geosciences – In rocks and minerals, observation of trace element chemistry and geochemical effects enables you to reconstruct geological processes. An 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 are 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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