Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Color Centers

  • Richard J. D. Tilley
Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_223

Definition

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”).]

Color Centers

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.

Since these early studies, many different color center types have been characterized (Table 1). All are electronic defects that possess similar characteristics in that they form in colorless, insulating, often relatively ionic, solids and incorporate either trapped electrons, to produce electron excess centers, or trapped holes, to produce hole excess centers. Solids may contain several different types of color center, including populations of both electron and hole excess centers in the same host matrix. Color centers are usually labeled with a letter symbol.
Color Centers, Table 1

Some color centers

Host crystal

Symbol

Description

Alkali halide: MX

F

Electron trapped an anion vacancy

M, F2

A pair of adjacent interacting F centers

FA

F center next to an alkaline metal substitutional impurity

F′, F

F center with 2 trapped electrons

R, M+, F2+

Three adjacent interacting F centers on (111)

VK

Two adjacent anion vacancies with 1 trapped electron

Alkaline earth halide: MX2

F

Electron trapped an anion vacancy

M

A pair of adjacent F centers aligned along [100]

F3

Three adjacent F centers aligned along [100]

Alkaline earth oxide: MO

F

Oxygen vacancy with two trapped electrons

F′, F

Oxygen vacancy with three trapped electrons

F+

Oxygen vacancy with one trapped electron

Quartz: SiO2

E′, E

Oxygen vacancy with one trapped electron

Diamond: C

C, P1

Isolated N atom substituted for a C atom

A

Two C atoms substituted by N, forming an N–N pair

N3

Three N atoms on C sites surrounding a C vacancy

N2

Two N atoms on C sites adjacent to a C vacancy

NV

One N atom on a carbon site adjacent to a C vacancy

NV

Negatively charged NV center

As the alkali halide studies demonstrated, color centers can be created in a host matrix in a number of ways. For example, strong ultraviolet light can transform clear glass into purple-colored “desert glass” and intense radiation from nuclear weapons or accidents may color ceramics such as porcelain a blue color, both changes being due to the formation of color centers. Controlled irradiation in nuclear reactors or similar is used to produce artificial gemstones from colorless and less valuable starting materials. For example, irradiation of colorless topaz, Al2SiO4(F, OH)2, induces a beautiful blue color due to color center formation (Fig. 1).
Color Centers, Fig. 1

Blue topaz, produced by irradiation of clear crystals; the color arises from a population of color centers (Photograph R J D Tilley)

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

The first color center to be characterized was the F center found in alkali halides, and these remain the best-known electron excess centers. Anion vacancies, which occur in low concentrations in alkali halides, MX, have an effective positive charge, and an F center is an anion vacancy (VX) plus a trapped electron to form an effectively neutral defect (VX e) (Fig. 2a). The F center behaves rather like an isolated hydrogen atom in the structure, and the electron is able to absorb electromagnetic radiation, jumping from one energy level to another, just as an electron absorbs radiation in the Bohr model of the H atom. The peak of the absorption curve, lmax, corresponds to a total removal of the electron from the F center, and usually falls in the visible, so coloring the originally transparent crystals (Table 2). In terms of band theory, alkali halides are insulators with a considerable energy gap between a filled valence band and an empty conduction band. The F center in its ground state creates a new energy level or narrow energy band in the band gap (Fig. 2b). The color-producing optical absorption peak corresponds to electron promotion into the conduction band. The variation in the color observed depends upon the host crystal band gap and the energy level of the F center, both of which are linked to the lattice parameter of the host matrix (Table 2).
Color Centers, Fig. 2

(a) An F center in an alkali halide MX crystal (schematic); large circles, anions X; small circles, cations, M+; (b) schematic energy level diagram for an F center in an alkali halide crystal

Color Centers, Table 2

Alkali metal halide F centers

Compound

Absorption peak lmax/nm

Colora

Lattice parameter/nm

LiF

235, ultraviolet

Colorless

0.4073

NaF

345, ultraviolet

Colorless

0.4620

KF

460, blue

Yellow brown

0.5347

RbF

510, green

Magenta

0.5640

LiCl

390, ultraviolet (just)

Yellow green

0.5130

NaCl

460, blue

Yellow brown

0.5641

KCl

565, green

Violet

0.6293

RbCl

620, orange

Blue green

0.6581

LiBr

460, blue

Yellow brown

0.5501

NaBr

540, green

Purple

0.5973

KBr

620, orange

Blue green

0.6600

RbBr

690, red

Blue green

0.6854

aThe appearance of the crystal is the complementary color to that removed by the absorption band.

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.

The situation in amethyst, which is a form of silica containing Fe3+ and H+ impurities, is similar. The impurity Fe3+ forms [FeO4] groups. These crystals are known as the pale yellow semiprecious gemstone citrine and also in a pale green form, the color arising from the impurity Fe3+ ions (see “Cross-References”). On irradiation, (FeO4h) color centers form by interaction with H+ ions. The color centers impart the purple amethyst coloration to the crystals (Fig. 3).
Color Centers, Fig. 3

Amethyst crystals; the purple color arises from a population of color centers. The pale green stone is unirradiated (Photograph R J D Tilley)

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 mm. 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.

Persistent Luminescence

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

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.

Color Centers in Diamond

Colored Diamonds

The diamond structure is built up of carbon atoms each coordinated to four carbon atom neighbors, the linking being via tetrahedral sp3 hybrid bonds (Fig. 4a). Diamond has a band gap of about 5.5 eV which is too large to absorb visible light, and perfect diamonds are clear. The commonest impurity in natural diamonds is nitrogen, which mostly substitutes for carbon on normal tetrahedral sites in the crystal. Natural diamonds are often subjected to temperatures of 1,000–1,200 °C, over geological timescales, allowing these nitrogen atoms diffuse through the structure, leading to populations of defect clusters as well as isolated point defects.
Color Centers, Fig. 4

(a) The diamond structure as a linkage of tetrahedrally coordinated carbon atoms (small circles); (b) structure of a C center in diamond, consisting of a nitrogen atom (large circle) occupying a carbon position, schematic; (c) structure of a boron impurity center (large circle) in diamond (schematic)

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.

Nitrogen-Vacancy Centers

The diamond color center that has been studied in most detail is that consisting of a single nitrogen substitutional impurity located next to a carbon vacancy, together with a trapped electron to form a negatively charged nitrogen-vacancy center, NV (often just called an NV center). These defect centers are readily created by the irradiation of nitrogen-containing natural or synthetic diamonds, diamond thin films, or diamond nanoparticles with high-energy protons. The proton irradiation results in the formation of carbon vacancies, and if the crystals are then annealed at above 600 °C, the temperature at which the vacancies become mobile, they diffuse through the structure until they encounter a nitrogen impurity. The strain around the nitrogen atom effectively traps the vacancy, preventing further migration. In the resultant NV centers, the tetrahedron surrounding the carbon vacancy is composed of three carbon atoms and one nitrogen atom (Fig. 5a).
Color Centers, Fig. 5

(a) An NV center in diamond, schematic; (b) approximate energy level diagram of an NV center

These centers are being investigated for applications, including room temperature quantum computing, nanoscale magnetometers, and fluorescent markers. 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  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 [6] 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.

Postscript

The electronic structure 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 similarly 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.

Cross-References

References

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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Queen’s BuildingsCardiff UniversityCardiffUK