Cathodoluminescence

Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is the generation of light by an electron beam scanning the phosphor-coated inner surface of the screen of a television that uses a cathode ray tube. Cathodoluminescence is the inverse of the photoelectric effect, in which electron emission is induced by irradiation with photons.

Cl-scheme
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.

Origin

Luminescence in a semiconductor results when an electron in the conduction band recombines with a hole in the valence band. The excess energy of this transition can be emitted in form of a photon. The energy (color) of the photon, and the probability that a photon and not a phonon will be emitted, depends on the material, its purity, and the presence of defects. However, first the electron has to be excited from the valence band into the conduction band. In cathodoluminescence, this occurs as the result of an impinging high energy electron beam onto a semiconductor. However, these primary electrons carry far too much energy to directly excite electrons. Instead, the inelastic scattering of the primary electrons in the crystal leads to the emission of secondary electrons, Auger electrons and X-rays, which in turn can scatter as well. Such a cascade of scattering events leads to up to 103 secondary electrons per incident electron.[1] These secondary electrons can excite valence electrons into the conduction band when they have a kinetic energy about three times the band gap energy of the material .[2] The excess energy is transferred to phonons and thus heats the lattice. One of the advantages of excitation with an electron beam is that the band gap energy of materials that are investigated is not limited by the energy of the incident light as in the case of photoluminescence. Therefore, in cathodoluminescence, the "semiconductor" examined can, in fact, be almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way.

Microscopy

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

In a scanning electron microscope

InGaN crystal SEM+CL
Color cathodoluminescence overlay on SEM image of an InGaN polycrystal. The blue and green channels represent real colors, the red channel corresponds to UV emission.

In these instruments 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 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,[3] while in a (scanning) transmission electron microscope, nanometer-sized features can be resolved.[4] 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 wells 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. [5]

Extended applications

Although direct bandgap semiconductors such as GaAs or GaN are most easily examined by these techniques, indirect semiconductors such as silicon also emit weak cathodoluminescence, and can be examined as well. In particular, the luminescence of dislocated silicon is different from intrinsic silicon, and can be used to map defects in integrated circuits.

Recently, cathodoluminescence performed in electron microscopes is also being used to study surface plasmon resonances in metallic nanoparticles.[6] Surface plasmons in metal nanoparticles can absorb and emit light, though the process is different from that in semiconductors. Similarly, cathodoluminescence has been exploited as a probe to map the local density of states of planar dielectric photonic crystals and nanostructured photonic materials.[7]

See also

References

  1. ^ Mitsui, T; Sekiguchi, T; Fujita, D; Koguchi, N. (2005). "Comparison between electron beam and near-field light on the luminescence excitation of GaAs/AlGaAs semiconductor quantum dots". Jpn. J. Appl. Phys. 44 (4A): 1820–1824. Bibcode:2005JaJAP..44.1820M. doi:10.1143/JJAP.44.1820.CS1 maint: Uses authors parameter (link)
  2. ^ Klein, C. A. (1968). "Bandgap dependence and related features of radiation ionization energies in semiconductors". J. Appl. Phys. 39 (4): 2029–2038. Bibcode:1968JAP....39.2029K. doi:10.1063/1.1656484.
  3. ^ Lähnemann, J.; Hauswald, C.; Wölz, M.; Jahn, U.; Hanke, M.; Geelhaar, L.; Brandt, O. (2014). "Localization and defects in axial (In,Ga)N/GaN nanowire heterostructures investigated by spatially resolved luminescence spectroscopy". J. Phys. D: Appl. Phys. 47 (39): 394010. arXiv:1405.1507. Bibcode:2014JPhD...47M4010L. doi:10.1088/0022-3727/47/39/394010.
  4. ^ Zagonel; et al. (2011). "Nanometer Scale Spectral Imaging of Quantum Emitters in Nanowires and Its Correlation to Their Atomically Resolved Structure". Nano Letters. 11 (2): 568–73. arXiv:1209.0953. Bibcode:2011NanoL..11..568Z. doi:10.1021/nl103549t. PMID 21182283.
  5. ^ "What is Quantitative Cathodoluminescence?". 2013-10-21. Archived from the original on 2016-10-29. Retrieved 2013-10-21.
  6. ^ García de Abajo, F. J. (2010). "Optical excitations in electron microscopy" (PDF). Reviews of Modern Physics. 82 (1): 209–275. arXiv:0903.1669. Bibcode:2010RvMP...82..209G. doi:10.1103/RevModPhys.82.209. hdl:10261/79235.
  7. ^ Sapienza, R.;Coenen, R.; Renger, J.; Kuttge, M.; van Hulst, N. F.; Polman, A (2012). "Deep-subwavelength imaging of the modal dispersion of light". Nature Materials. 11 (9): 781–787. Bibcode:2012NatMa..11..781S. doi:10.1038/nmat3402. PMID 22902895.CS1 maint: Uses authors parameter (link)

Further reading

External links

Albert Polman

Albert Polman (born 21 April 1961, Groningen) is a Dutch physicist and former director of the AMOLF research laboratory in Amsterdam.

Polman received his master's degree in physics (1985) and his Ph.D. degree in materials science and engineering (1989) from the University of Utrecht. From 1989 to 1991 he was a post-doctoral staff researcher at AT&T Bell Laboratories (Murray Hill, New Jersey). Since 1991 he has been associated with AMOLF, first as a group leader, since 1999 also as a department head. In 2005 he initiated the Center for Nanophotonics at AMOLF; in 2006 he was appointed as director of AMOLF. Polman was one of the initiators of the Amsterdam nanoCenter, a regional facility for nanofabrication founded in 2003. From March 2003 to February 2004 he was on sabbatical leave at Caltech, where he was a research associate in the group of Prof. H.A. Atwater.Polman is one of the pioneers of the research field of nanophotonics: the control, understanding, and application of light at the nanoscale. He is best known for inventing optical doping, i.e., the incorporation and optical activation of optically active ions in thin-film materials by ion implantation. Polman's research group at AMOLF specializes in fundamental studies at the interface between optical physics and materials science.

In 2009, Albert Polman was appointed as a member of the Royal Dutch Academy of Sciences.Polman's group invented angle-resolved cathodoluminescence imaging spectroscopy, a super-resolution method that can create images with a resolution of up to 10 nanometers. As of 2011, this technology has become commercially available.

Carbonado

Carbonado, commonly known as the "black diamond", is the toughest form of natural diamond. It is an impure form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon. It is found primarily in alluvial deposits in the Central African Republic and in Brazil. Its natural colour is black or dark grey, and it is more porous than other diamonds.

Cathodoluminescence microscope

A cathodoluminescence (CL) microscope combines methods from electron and regular (light optical) microscopes. It is designed to study the luminescence characteristics of polished thin sections of solids irradiated by an electron beam.

Using a cathodoluminescence microscope, structures within crystals 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 (rocks, minerals, volcanic ash, glass, ceramic, concrete, fly ash, etc.). More recently, scientists have begun to investigate its application for studying biological samples, using rare earth element-doped inorganic nanocrystals as imaging probes. Correlative Cathodoluminescence Electron Microscopy (CCLEM) can also be performed on focus ion beam (FIB) sectioned samples, hence potentially enabling 3D CCLEM.

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.

CL systems can also be attached to a scanning electron microscope. These devices are traditionally used for special applications like e.g. investigations in materials science, geoscience, optics research, or quality determination of ceramics. New SEM CL systems can be used for research in nanophotonics. The most prominent advantage is their higher magnifications. However, CL colour information can only be obtained by a spectroscopic analysis of the luminescence emission.

Direct viewing of emission colors is only provided by optical CL microscopes, both "cold" and "hot" cathode types.

More recently, an angle-resolved cathodoluminescence microscopy system has been developed at the FOM Institute AMOLF. This is a super-resolution technique that can create images with a resolution of up to 10 nm. As of 2011, this technology has become commercially available.

Dinosaur egg

Dinosaur eggs are the organic vessels in which a dinosaur embryo develops. When the first scientifically documented remains of dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs because they were reptiles. In 1859, the first scientifically documented dinosaur egg fossils were discovered in France by Jean-Jacques Poech, although they were mistaken for giant bird eggs. The first scientifically recognized dinosaur egg fossils were discovered in 1923 by an American Museum of Natural History crew in Mongolia. Since then many new nesting sites have been found all over the world and a system of classification based on the structure of eggshell was developed in China before gradually diffusing into the West. Dinosaur eggshell can be studied in thin section and viewed under a microscope. The interior of a dinosaur egg can be studied using CAT scans or by gradually dissolving away the shell with acid. Sometimes the egg preserves the remains of the developing embryo inside. The oldest known dinosaur eggs and embryos are from Massospondylus, which lived during the Early Jurassic, about 190 million years ago.

Egg paleopathology

Egg paleopathology is the study of evidence for illness, injury, and deformity in fossilized eggs. A variety of pathological conditions afflicting eggs have been documented in the fossil record. Examples include eggshell of abnormal thickness and fossil eggs with multiple layers of eggshell. The identification of egg paleopathologies is complicated by the fact that even healthy eggs can be modified during or after fossilization. Paleontologists can use techniques like cathodoluminescence or thin sectioning to identify true paleopathologies in fossil eggs. Despite the diversity of paleopathologies known from fossil eggs, the vast majority of conditions known to afflict modern eggs have not yet been seen among fossils.

Electron-stimulated luminescence

Electron-stimulated luminescence (ESL) was a claimed method of producing light by cathodoluminescence, i.e. by a beam of electrons made to hit a fluorescent phosphor surface. This is also the method used to produce light in a cathode ray tube (CRT), but, unlike CRTs, ESL lamps do not include magnetic or electrostatic means to deflect the electron beam.A cathodoluminescent light has a transparent glass envelope coated on the inside with a light-emitting phosphor layer. Electrons emitted from a cathode strike the phosphor; the current returns through a transparent conductive coating on the envelope. The phosphor layer emits light through the transparent face of the envelope. The system has a power supply providing at least 5kVDC to the light emitting device, and the electrons transiting from cathode to anode are essentially unfocused. Additional circuits allow triac-type dimmers to control the light level. Lights produced so far have a color rendering index of 90. The energy consumption can be 70% less than that of a standard incandescent light bulb. Claimed lifetime can be as long as 10,000 hours which is more than ten times that of a standard incandescent light bulb.Unlike fluorescent lamps, which produce light through the electrical excitation of mercury vapor, ESL lamps do not use mercury. The first commercially available ESL product was a reflector bulb.

Drawbacks include high weight, a slightly larger-than-normal base and – as with compact fluorescent lamps – when switched on, a slight delay before illumination begins and a static charge which attracts dust to the bulb face. As of 2016 the cost is higher and claimed efficiency is less than half that of commercially available LED bulbs, although it is considerably better than that of traditional incandescent lamps.

Electron beam-induced current

Electron-beam-induced current (EBIC) is a semiconductor analysis technique performed in a scanning electron microscope (SEM) or scanning transmission electron microscope (STEM). It is used to identify buried junctions or defects in semiconductors, or to examine minority carrier properties. EBIC is similar to cathodoluminescence in that it depends on the creation of electron–hole pairs in the semiconductor sample by the microscope's electron beam. This technique is used in semiconductor failure analysis and solid-state physics.

Environmental scanning electron microscope

The environmental scanning electron microscope or ESEM is a scanning electron microscope (SEM) that allows for the option of collecting electron micrographs of specimens that are "wet," uncoated, or both by allowing for a gaseous environment in the specimen chamber. Although there were earlier successes at viewing wet specimens in internal chambers in modified SEMs, the ESEM with its specialized electron detectors (rather than the standard Everhart-Thornley detector) and its differential pumping systems, to allow for the transfer of the electron beam from the high vacuums in the gun area to the high pressures attainable in its specimen chamber, make it a complete and unique instrument designed for the purpose of imaging specimens in their natural state. The instrument was designed originally by Gerasimos Danilatos while working at the University of New South Wales.

Frankdicksonite

Frankdicksonite is a halide mineral with the chemical formula BaF2 which corresponds to the chemical compound barium fluoride. It occurs in the Carlin gold deposit of Eureka County, Nevada as cubic crystals sized between 0.1 and 4 mm, and is of hydrothermal origin. Its only associated mineral is quartz and the frankdicksonite crystals are always completely encapsulated in it. Frankdicksonite has fluorite crystal structure with a cubic symmetry and the lattice constant a = 619.64 pm. Its Vickers hardness on the {111} cleavage crystal faces varies between 88 and 94 kg/mm2 and is close to that of the synthetic barium fluoride (95 kg/mm2). Its refractive index (1.475) is almost identical to that of BaF2 (1.474). Under electron irradiation, it emits strong blue cathodoluminescence. The major impurity in frankdicksonite is strontium with concentrations up to 0.5% by weight. Also present are silicon (0.02%) and magnesium (0.0015%); other impurities have concentrations below 0.0015%.Although synthetic barium fluoride has been commonly known from at least 1846, it had not been found in nature. The natural existence of barium fluoride was predicted by Michael Fleischer in 1970. Later in the same year, the mineral was discovered by Arthur S. Radtke and named after Frank W. Dickson (born 1922), professor of Geochemistry at Stanford University in recognition of his contributions to geology and geochemistry of low-temperature ore deposits. Frankdicksonite was officially recognized by the Commission of New Minerals and Mineral Names in 1974.

Indium gallium nitride

Indium gallium nitride (InGaN, InxGa1−xN) is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN). It is a ternary group III/group V direct bandgap semiconductor. Its bandgap can be tuned by varying the amount of indium in the alloy.

InxGa1−xN has a direct bandgap span from the infrared (0.69 eV) for InN to the ultraviolet (3.4 eV) of GaN.

The ratio of In/Ga is usually between 0.02/0.98 and 0.3/0.7.

List of light sources

This is a list of sources of light, including both natural and artificial processes that emit light. This article focuses on sources that produce wavelengths from about 390 to 700 nanometers, called visible light.

Luminescence

Luminescence is spontaneous emission of light by a substance not resulting from heat; it is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a substance as a result of heating. Historically, radioactivity was thought of as a form of "radio-luminescence", although it is today considered to be separate since it involves more than electromagnetic radiation.

The dials, hands, scales, and signs of aviation and navigational instruments and markings are often coated with luminescent materials in a process known as "luminising".

MountainsMap

Mountains is an image analysis and surface metrology software platform published by the company Digital Surf. Its core is micro-topography, the science of studying surface texture and form in 3D at the microscopic scale. The software is dedicated to profilometers, 3D light microscopes ("MountainsMap"), scanning electron microscopes ("MountainsSEM") and scanning probe microscopes ("MountainsSPIP").

Optical properties of carbon nanotubes

Within materials science, the optical properties of carbon nanotubes refer specifically to the absorption, photoluminescence (fluorescence), and Raman spectroscopy of carbon nanotubes. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the nanotube synthesis can be changed, intentionally or unintentionally, to alter the nanotube quality. As shown below, optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this "nanotube quality" in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. Those features determine nearly any other property, such as optical, mechanical, and electrical.

Carbon nanotubes are unique "one-dimensional systems" which can be envisioned as rolled single sheets of graphite (or more precisely graphene). This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter typically varies in the range 0.4–40 nm (i.e. "only" ~100 times), but the length can vary ~10,000 times, reaching 55.5 cm. The nanotube aspect ratio, or the length-to-diameter ratio, can be as high as 132,000,000:1, which is unequalled by any other material. Consequently, all the properties of the carbon nanotubes relative to those of typical semiconductors are extremely anisotropic (directionally dependent) and tunable.

Whereas mechanical, electrical and electrochemical (supercapacitor) properties of the carbon nanotubes are well established and have immediate applications, the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in optics and photonics. In particular, light-emitting diodes (LEDs) and photo-detectors based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotubes.

Scanning electron microscope

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the detected signal to produce an image. SEM can achieve resolution better than 1 nanometer. Specimens are observed in high vacuum in conventional SEM, or in low vacuum or wet conditions in variable pressure or environmental SEM, and at a wide range of cryogenic or elevated temperatures with specialized instruments.The most common SEM mode is the detection of secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons that can be detected depends, among other things, on specimen topography. By scanning the sample and collecting the secondary electrons that are emitted using a special detector, an image displaying the topography of the surface is created.

Sodalite

Sodalite is a rich royal blue tectosilicate mineral widely used as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group with hauyne, nosean, lazurite and tugtupite.

First discovered by Europeans in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.

Vacuum fluorescent display

A vacuum fluorescent display (VFD) is a display device used commonly on consumer electronics equipment such as video cassette recorders, car radios, and microwave ovens.

A VFD operates on the principle of cathodoluminescence, roughly similar to a cathode ray tube, but operating at much lower voltages. Each tube in a VFD has a phosphor coated anode that is bombarded by electrons emitted from the cathode filament. In fact, each tube in a VFD is a triode vacuum tube because it also has a mesh control grid.Unlike liquid crystal displays, a VFD emits a very bright light with high contrast and can support display elements of various colors. Standard illumination figures for VFDs are around 640 cd/m2 with high-brightness VFDs operating at 4,000 cd/m2, and experimental units as high as 35,000 cd/m2 depending on the drive voltage and its timing. The choice of color (which determines the nature of the phosphor) and display brightness significantly affect the lifetime of the tubes, which can range from as low as 1,500 hours for a vivid red VFD to 30,000 hours for the more common green ones. Cadmium was commonly used in VFDs in the past, but the current RoHS-compliant VFDs have eliminated this metal from their construction.

VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s).

The first VFD was the single indication DM160 by Philips in 1959. The first multi-segment VFD was the 1962 Japanese single-digit, seven-segment device. The displays became common on calculators and other consumer electronics devices. In the late 1980s hundreds of millions of units were made yearly.

Zircon

Zircon ( or ) is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate, and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is (Zr1–y, REEy)(SiO4)1–x(OH)4x–y. Zircon forms in silicate melts with large proportions of high field strength incompatible elements. For example, hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, red, brown, blue and green. Colorless specimens that show gem quality are a popular substitute for diamond and are also known as "Matura diamond".

The name derives from the Persian zargun, meaning "gold-hued". This word is corrupted into "jargoon", a term applied to light-colored zircons. The English word "zircon" is derived from Zirkon, which is the German adaptation of this word. Yellow, orange and red zircon is also known as "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.