Editing Cathodoluminescence (section)

== Microscopy ==

[[File:Side by side adaption of Hydr-Qz-tb.jpg|thumb|Thin section of quartz from a hydrothermal vein - left in CL and right in transmitted light]]

In [[geology]], [[mineralogy]], [[Materials science and engineering|materials science]] and [[semiconductor]] engineering, a [[scanning electron microscope|scanning electron microscope (SEM)]] fitted with a cathodoluminescence detector, or an optical [[cathodoluminescence microscope]], may be used to examine internal structures of semiconductors, rocks, [[ceramic]]s, [[glass]], etc. in order to get information on the composition, growth and quality of the material.

=== Optical cathodoluminescence microscope ===

[[Image:HC3-CL-microscope.gif|thumb|Hot cathode CL microscope]]

A '''cathodoluminescence''' ('''CL''') '''microscope''' combines a regular (light optical) [[microscope]] with a [[cathode-ray tube]]. It is designed to image the [[luminescence]] characteristics of polished thin sections of solids irradiated by an [[electron beam]].

Using a cathodoluminescence microscope, structures within [[crystal]]s or fabrics can be made visible which cannot be seen in normal light conditions. Thus, for example, valuable information on the growth of minerals can be obtained. CL-microscopy is used in [[geology]], [[mineralogy]] and [[materials science]] for the investigation of [[rock (geology)|rock]]s, [[mineral]]s, [[volcanic ash]], [[glass]], [[ceramic]], [[concrete]], [[fly ash]], etc.

CL color and intensity are dependent on the characteristics of the sample and on the working conditions of the [[electron gun]]. Here, [[acceleration voltage]] and beam current of the [[electron beam]] are of major importance. Today, two types of CL microscopes are in use. One is working with a "[[cold cathode]]" generating an electron beam by a [[corona discharge]] tube, the other one produces a beam using a "[[hot cathode]]". Cold-cathode CL microscopes are the simplest and most economical type. Unlike other electron bombardment techniques like [[electron microscopy]], cold cathodoluminescence microscopy provides positive ions along with the electrons which neutralize surface charge buildup and eliminate the need for conductive coatings to be applied to the specimens. The "hot cathode" type generates an electron beam by an electron gun with tungsten filament. The advantage of a hot cathode is the precisely controllable high beam intensity allowing to stimulate the emission of light even on weakly luminescing materials (e.g. [[quartz]] – see picture). To prevent charging of the sample, the surface must be coated with a conductive layer of [[gold]] or [[carbon]]. This is usually done by a [[sputter deposition]] device or a carbon coater.

=== Cathodoluminescence from a scanning electron microscope ===

[[File:Cl-scheme.svg|thumb|Sketch of a cathodoluminescence system: The electron beam passes through a small aperture in the parabolic mirror which collects the light and reflects it into the [[spectrometer]]. A [[charge-coupled device]] (CCD) or [[photomultiplier]] (PMT) can be used for parallel or monochromatic detection, respectively. An [[electron beam-induced current]] (EBIC) signal may be recorded simultaneously.]]

[[File:AllalinDesign.png|thumb|Sketch of a cathodoluminescence objective inserted in a SEM column]]

In [[scanning electron microscope]]s a focused beam of electrons impinges on a sample and induces it to emit light that is collected by an optical system, such as an elliptical mirror.  From there, a [[optical fiber|fiber optic]] will transfer the light out of the microscope where it is separated into its component wavelengths by a [[monochromator]] and is then detected with a [[photomultiplier]] tube.  By scanning the microscope's beam in an X-Y pattern and measuring the light emitted with the beam at each point, a map of the optical activity of the specimen can be obtained (cathodoluminescence imaging). Instead, by measuring the wavelength dependence for a fixed point or a certain area, the spectral characteristics can be recorded (cathodoluminescence spectroscopy).  Furthermore, if the photomultiplier tube is replaced with a [[CCD camera]], an entire [[spectrum]] can be measured at each point of a map ([[hyperspectral imaging]]). Moreover, the optical properties of an object can be correlated to structural properties observed with the electron microscope.

The primary advantages to the electron microscope based technique is its spatial resolution. In a scanning electron microscope, the attainable resolution is on the order of a few ten nanometers,<ref>{{cite journal|doi=10.1088/0022-3727/47/39/394010|arxiv=1405.1507|bibcode=2014JPhD...47M4010L|title= Localization and defects in axial (In,Ga)N/GaN nanowire heterostructures investigated by spatially resolved luminescence spectroscopy
|journal=J. Phys. D: Appl. Phys.|volume=47|issue=39|pages=394010|year=2014|last1=Lähnemann|first1=J.|last2=Hauswald|first2=C.|last3=Wölz|first3=M.|last4=Jahn|first4=U.|last5=Hanke|first5=M.|last6=Geelhaar|first6=L.|last7=Brandt|first7=O.|s2cid=118314773 }}</ref> while in a (scanning) [[transmission electron microscope]] (TEM), nanometer-sized features can be resolved.<ref>{{cite journal|last1=Zagonel|title=Nanometer Scale Spectral Imaging of Quantum Emitters in Nanowires and Its Correlation to Their Atomically Resolved Structure|journal=Nano Letters|volume=11|issue=2|pages=568–73|year=2011|doi=10.1021/nl103549t|display-authors=etal|pmid=21182283|arxiv = 1209.0953 |bibcode = 2011NanoL..11..568Z |s2cid=18003378 }}</ref> Additionally, it is possible to perform nanosecond- to picosecond-level time-resolved measurements if the electron beam can be "chopped" into nano- or pico-second pulses by a beam-blanker or with a pulsed electron source. These advanced techniques are useful for examining low-dimensional semiconductor structures, such a [[quantum well]]s or [[quantum dots]].

While an electron microscope with a cathodoluminescence detector provides high magnification, an optical cathodoluminescence microscope benefits from its ability to show actual visible color features directly through the eyepiece. More recently developed systems try to combine both an optical and an electron microscope to take advantage of both these techniques.<ref>{{cite web
 |title        = What is Quantitative Cathodoluminescence?
 |url          = http://www.attolight.com/technology/cathodoluminescence-principles/
 |date         = 2023-08-23
 }}</ref>