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Color centers are point defects or point defect clusters associated with trapped electrons or holes in normally transparent materials. These centers cause the solid to become colored when the electronic ground state of the defect is excited to higher energy states by the absorption of visible light [1, 2, 3, 4, 5]. [Note that transition metal and lanthanoid ion dopants that engender color in an otherwise colorless matrix are frequently called color centers. These are dealt with elsewhere (see “Cross-References”).]
The concept of color arising from point defects was initially developed in the first half of the twentieth century, principally by Pohl, in Germany. It was discovered that clear alkali halide crystals could be made intensely colored by diverse methods, including irradiation by X-rays, heating crystals in the vapor of any alkali metal, and electrolysis. Crystals with induced color were found to have a lower density than the crystals before treatment and appeared to contain a population of anion vacancies. The absorption spectrum was always a simple bell shape. It was notable that the color engendered in the crystal was always the same and was not dependent upon the method of color production. That is, if a crystal of KCl was heated in an atmosphere of any alkali metal, or irradiated by X-rays, or electrolyzed using any cathode material, the crystal took on a violet color. Similarly, crystals of NaCl always took on an orange brown hue under all preparation methods. The fact that the color was a unique property of the host structure implied that it was a property of the crystal itself. The color was ultimately attributed to the formation of defects called Farbzentren (= color centers), which were equated with mistakes in the crystal structure.
Some color centers
Alkali halide: MX
Electron trapped an anion vacancy
A pair of adjacent interacting F centers
F center next to an alkaline metal substitutional impurity
F center with two trapped electrons
R, M+, F2+
Three adjacent interacting F centers on (111)
Two adjacent anion vacancies with one trapped electron
Alkaline earth halide: MX2
Electron trapped an anion vacancy
A pair of adjacent F centers aligned along 
Three adjacent F centers aligned along 
Alkaline earth oxide: MO
Oxygen vacancy with two trapped electrons
Oxygen vacancy with three trapped electrons
Oxygen vacancy with one trapped electron
Oxygen vacancy with one trapped electron
Isolated N atom substituted for a C atom
Two C atoms substituted by N, forming an N-N pair
Three N atoms on C sites surrounding a C vacancy
Two N atoms on C sites adjacent to a C vacancy
One N atom on a carbon site adjacent to a C vacancy
Negatively charged NV center
From an optical viewpoint, color centers behave something like isolated atoms dispersed throughout the host matrix. As such, they make ideal probes of the interaction of light with the matrix and are currently being explored for wide ranging electro-optic applications.
F Centers: Electron Excess Centers
Alkali metal halide F centers
Absorption peak λmax/nm
390, ultraviolet (just)
The detailed mechanism for the formation of F centers depends to some extent upon the manner in which they are generated. In the case of irradiation by X-rays, for example, the energetic radiation is able to displace an electron from a normal anion, and some of these become trapped at existing anion vacancies. The corresponding anion that has lost an electron creates a hole energy level in the valence band.
F centers occur in many alkaline earth halides and oxides (Table 1). For example, the mineral Blue John is a rare naturally occurring purple blue form of fluorite, CaF2. The coloration is caused by F centers believed to have formed when the fluorite crystals were fortuitously located near to uranium compounds in the rock strata. Radioactive decay of the uranium produced the energetic radiation necessary to form the color centers.
Hole Excess Centers
One of the best understood hole excess centers gives rise to the color in smoky quartz, a naturally occurring form of silica (SiO2). This material contains small amounts of Al3+ substitutional impurities. These replace Si4+ions in [SiO4] tetrahedral units which form the building units of the crystal. Overall charge neutrality is preserved by the incorporation of one H+ for each Al3+. These H+ ions sit in interstitial positions in the rather open SiO2 structure. The color center giving rise to the smoky color in quartz is formed when an electron is liberated from an [AlO4] unit by ionizing radiation and is trapped on one of the H+ ions present, leaving a hole (h) behind. The color center is a (AlO4 h) group. The color arises when the trapped hole absorbs radiation exactly as the electron in an F center.
Color Centers in Diamond
The color of the highly prized natural yellow diamonds called Canaries is due to isolated nitrogen atoms located on carbon sites, which form color centers called C centers. The color arises in the following way. Nitrogen, with an electron configuration 1s2 2s2 2p3, has five bonding electrons, one more than carbon, with a configuration 1s2 2s2 2p2. Four of the electrons around each impurity nitrogen atom are used to fulfill the local sp3 bonding requirements of the crystal structure and one electron remains unused. Substitution of nitrogen for carbon on a normal carbon atom site in the crystal thus creates an electron excess color center (Fig. 4b). On an energy level diagram, this gives rise to a donor level in the band gap, which, because of lattice vibrations and other interactions, consists of a narrow band of energies centered at 2.2 eV and extending to 1.7 eV, the ionization energy of the N atom in diamond. The electron can be excited into the conduction band by absorption of incident visible light of wavelengths longer than about 564 nm, giving the stones a faint yellow aspect. As the number of C centers increases, the color intensifies.
The N3 center, which consists of three nitrogen atoms on neighboring carbon sites adjacent to a carbon vacancy, seems to be responsible (at least in part) for the pale straw color of expensive Cape Yellow diamonds. The N3 center has a complex electronic structure which absorbs just in the blue end of the visible, at 415 nm, resulting in yellowish stones. The N3 centers are often accompanied by neutral N2 centers consisting of two nitrogen atoms on neighboring carbon sites adjacent to a carbon vacancy. These absorb at approximately 475 nm, giving a yellow color to the stones and adding to that contributed by the N3 clusters. When crystals are irradiated, either naturally or artificially, the N2 cluster can trap an electron to form a negatively charged N2- center that has an absorption peak at approximately 989 nm in the infrared. This absorption band can spill over into the red part of the visible spectrum, leading to stones with a blue tone and producing blue diamonds. When all these nitrogen-based color centers are present in roughly equal quantities, the stones take on a green hue.
Although nitrogen-linked color centers give rise to the highly valued yellow hued diamonds, many prized blue diamonds are the result of boron impurities on normally occupied carbon atom sites (Fig. 4c). Boron, with an electron configuration 1s2 2s2 2p1, has only three outer bonding electrons instead of the four found on carbon. These three are used in fulfilling the bonding requirements of the structure, but one bond of the four is incomplete and lacks an electron, making the defect a hole excess center. In semiconductor physics terms, the center introduces a narrow band of energy levels approximately 0.4 eV above the valence band. The transition of an electron from the valence band to this band gives rise to an absorption peak in the infrared with a high-energy tail encroaching into the red at 700 nm. The boron-doped diamonds therefore absorb some red light and leave the gemstone with an overall blue color.
These centers are being investigated for applications, including room temperature quantum computing, nanoscale magnetometers, fluorescent markers, and biological imaging [6, 7]. The applications follow from the unique features of the energy levels of the (NV-) center. The ground state term of the electronic structure is 3A and the first excited state term is 3E (Fig. 5b) [see “Cross-References”: Transition-Metal Ion Colors, for a description of 3A and 3E terminology.] (Note that both the ground state and excited state terms are split into several levels. This splitting is of prime importance for many applications but does not dominate the overall color aspects of the centers and can be ignored in the present context.) Excitation from the 3A ground state to the 3E excited state is by absorption of light over the range of approximately 514–560 nm, giving stones a pink hue. Emission falls in the range 630–800 nm, but the observed color is dominated by a particularly strong red fluorescence at 637 nm. Under suitable observing conditions, single bright red fluorescent (NV-) centers can be observed, making nanodiamonds that incorporate these defects ideal probes to track vital processes in living cells .
Tenebrescence (reversible photochromism) is the property of darkening, especially reversibly, in response to incident radiation. It is associated particularly with minerals which change color when exposed to sunlight. The color fades when the sunlight is shaded, and the effect can be repeated indefinitely. The best known minerals which exhibit tenebrescence are scapolite (Na,Ca)4(Al,Si)3Si6O24(Cl,CO3, SO4); tugtupite, Na4AlBeSi4O12Cl; and hackmanite, Na8Al6Si6O24(Cl,S)2, also given the formula (Na8(Al,SiO4)6(Cl,S)2, reflecting that variable Al to Si concentrations may be present
Hackmanite is a derivative form of sodalite, Na8Al6Si6O24Cl2, in which some of the Cl has been replaced by a sulfur containing group S22-, SO32-, or SO42-. The tenebrescent colors observed in originally colorless hackmanite crystals range over pink, red, purple, and blue. These colors appear rapidly when the crystals are exposed to strong sunlight or ultraviolet light and then fade over the course of several minutes when in shadow. This coloring and fading process can be repeated indefinitely. For tenebrescence to occur it is found that only a low level of S is required, amounting to just a few percent. At greater concentrations, the effect is lost.
The structure of hackmanite is cubic and is built of rings of four apex-linked SiO4 and AlO4 tetrahedra centered on each cube face and rings of six similar apex-linked tetrahedra around each cell corner. These form cubo-octahedral cavities containing the Na+ cations and the Cl- anions. In order to maintain charge balance in the tenebrescent crystals, the interpolation of, for example, an S22- anion into a cavity requires the loss of 2Cl- anions, which thereby introduces an anion vacancy into a nearby cavity where it is associated with surrounding Na+ cations in a (Na4V)4+ cluster. Although the mechanism for the tenebrescence has not been fully elucidated, it is believed that an electron is released from S22- groups by ultraviolet irradiation and this migrates to the nearby (Na4V)4+ cluster where it becomes trapped. This trapping center acts as a color center and so colors the normally clear crystals. The color observed is proportional to the lattice parameter of the hackmanite, which itself is a reflection of the size of the F center. The trap energy is quite low, however, and the electrons are liberated by thermal energy and diffuse back to the sulfur groups, where they are again incorporated. This leads to the diminishing color after the exciting radiation is removed.
Some Applications of Color Centers
F Center Lasers
F centers do not exhibit laser action but F centers that have a dopant cation next to the anion vacancy are used in this way. These are typified by FLi centers, which consist of an F center with a lithium ion neighbor. Crystals of KCl or RbCl doped with LiCl, containing FLi centers, have been found to be good laser materials, yielding emission lines with wavelengths between 2.45 and 3.45 μm. A unique property of these crystals is that in the excited state an anion adjacent to the FLi center moves into an interstitial position. This is type II laser behavior, and the active centers are called FLi (II) centers. These centers are stable if the crystal is kept at −10 °C.
Color centers are active in materials that show persistent luminescence. In these compounds, irradiation by ultraviolet light present in normal daylight gives rise to luminescence for many hours after dark. Such materials are making their way into applications as diverse as road signs that do not need a power supply at night and bicycle frames that glow in the dark.
Although the color centers responsible for persistent luminescence vary from material to material, the principle can be illustrated with the oxide SrAl2O4 doped with B3+, Eu2+, and Dy3+, which gives a green luminescence. The SrAl2O4 structure consists of a framework of corner-linked [AlO4] tetrahedra enclosing Sr2+ ions in the cavities so formed. The B3+ substitutes for Al3+ to create [BO4] tetrahedra and [BO3] triangular groups, which can be thought of as [BO4] tetrahedra in which one of the oxygen ions is absent to form an oxygen vacancy (VO), creating a (BO3 VO) center. The substitution of Dy3+ for Sr2+ results in a charge imbalance that is compensated for by the creation of an equal number of vacancies on Sr2+ positions (VSr) to form defect clusters (Dy BO4 VSr). Following irradiation with ultraviolet light, an electron is transferred from a (BO3 VO) cluster to a (Dy BO4 VSr) unit, resulting in the formation of two complex color centers: (Dy BO4 VSr h) which are hole excess centers and (BO3 VO e), which are electron excess centers. The origin of the luminescence lies in this pair of color centers. Under normal conditions, the electron and hole centers are metastable, and over the course of several hours, the holes and electrons gradually recombine. The energy liberated is transferred to the Eu2+ ions, which lose energy by the emission of photons, thus producing a long-lasting green fluorescence.
Photostimulable phosphors that make use of color centers are widely used in X-ray imaging, particularly by dentists, where they have largely replaced X-ray film recording. The first commercial material to fulfill these requirements, introduced in 1983, was BaFBr doped with Eu2+. Although the detailed mechanism by which these phosphors work is still not entirely clear, it is established that an important component of the process is the formation of F centers, produced as a result of the X-ray irradiation. In dental X-ray imaging, a plate covered with a thin layer of phosphor is placed into the mouth and exposed to X-rays. The X-rays initially displace an electron from an anion to form an electron–hole pair. The electron is subsequently trapped at an anion vacancy to form an F center. This fairly stable pair of electronic defects constitutes a latent image in the phosphor. Subsequently, the exposed plate is irradiated with 633 nm light from a helium–neon laser. The electrons trapped in the F centers are initially promoted to the conduction band, after which they are free to recombine with the holes. The energy liberated is transferred to Eu2+ ions which decay from the subsequent excited state by the emission of visible light at 420 nm, subsequently recorded as a digital image. The number of F centers and holes, and therefore the amount of light emission, is proportional to the X-ray intensity in the phosphor. The optical image thus records accurately the degree to which the X-rays have penetrated the subject.
The electronic structures of the neutral NV and negative NV- color centers in diamond have been intensively studied, and it has taken some 35 years to reach the current understanding of energy levels of this defect. It would seem reasonable to suspect that similar detailed investigations of the other color centers described will also lead to significant revisions in their electronic structures and consequently a more precise description of their color-engendering abilities.
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