bandgap

selenium ruby

Cadmium sulfoselenide crystals give glass a sharp absorption edge, which produces the purest reds and oranges glass can make. It is also the system in which quantum dots were discovered.

Selenium ruby, glass colored by cadmium sulfoselenide crystals, produces the most saturated reds, oranges, and yellows glass can achieve. Much of the signal red of traffic lights and railway signals in the pre-LED century came from this chemistry (a general account, unverified here).

The mechanism differs from both dissolved ions and metal colloids. Instead of absorbing at a specific wavelength, a semiconductor crystal absorbs everything above a cutoff. Below the cutoff, the glass transmits freely; above it, nearly everything is blocked. The result is a sharp absorption edge rather than a broad absorption band, which is why the colors look saturated rather than tinted.

How it works

Band-gap absorption. A semiconductor has a characteristic energy threshold, the band gap, below which photons pass through and above which they are absorbed (promoting an electron from the lower valence band to the upper conduction band). Cadmium sulfide (CdS) has a band gap of roughly 2.4 electron volts (a handbook figure, unverified here), which by λ = 1240 / E places its absorption edge in the green, near 515 nm. It absorbs violet through green and transmits yellow. Cadmium selenide (CdSe) has a smaller gap near 1.7 eV, which by the same rule puts its edge at the long-wavelength end of the visible, bordering the near-infrared (about 730 nm); it absorbs nearly the entire visible spectrum and looks dark red to near-black.

The two compounds form a continuous solid solution, CdS₁₋ₓSeₓ: by adjusting the ratio of sulfur to selenium, you move the band gap, and therefore the absorption edge, continuously across the longer-wavelength part of the visible spectrum. A single chemical system covers the full range from yellow through orange to signal red.

Striking, again. Like gold ruby glass, selenium ruby comes out of the melt nearly colorless: the cadmium, sulfur, and selenium are dissolved in the glass. A controlled reheat (600–700 °C is the commonly quoted range; unverified) nucleates and grows CdS-CdSe microcrystals, and the color “strikes” as the crystals grow. Under-struck glass is pale; over-striking coarsens the crystals and dulls the color. The process is the same controlled nucleation and growth used in colloidal color.

Artifact stories

Traffic-light red

A common red in traffic signals and railway semaphores for much of the 20th century was selenium ruby glass, a composition tuned to pass the red above roughly 620–630 nm and block everything shorter (figures unverified). The physics favors it: a steep cutoff gives the purest red for the most light let through. Dissolved-ion reds, which have broad, overlapping absorption humps rather than a vertical edge, let through enough orange and yellow to dilute the perceived red and reduce visibility at distance.

Sharp-cut filters

Precision optical filter glass, used in photography, scientific instruments, microscopy, and industrial inspection, relies on the band-gap edge under tighter control. These are long-pass filters: they transmit wavelengths above a defined cutoff and block everything below, with the steepness of the transition itself being the specification. The standard product lines from major optical glass manufacturers include families of orange-glass and red-glass filters designated by their cutoff wavelength, produced by carefully controlled CdS-CdSe striking schedules. The “sharp-cut” in the name refers to the steep edge of band-gap absorption, which no other mechanism available in glass can produce.

The quantum dot postscript

When the CdS-CdSe crystals are small enough (a few nanometers, comparable to the size of an exciton in the crystal), the electrons in the crystal are confined by the boundaries of the particle. This is quantum confinement: the allowed energy levels shift as the crystal shrinks, widening the effective band gap and blue-shifting the absorption edge. Size becomes an additional color control, independent of composition.

In 1981, physicist Alexei Ekimov at the S. I. Vavilov State Optical Institute in Leningrad was studying colored glass and identified this size-dependent shift as quantum confinement of charge carriers in a semiconductor nanocrystal (Ekimov and Onushchenko, JETP Letters 34, 345). His first demonstration used copper-chloride nanocrystals in glass, heated for between one and 96 hours at 500–700 °C to grow them to different sizes (Nobel Committee); cadmium-based systems followed. Glass was the medium where quantum dots were first experimentally observed and characterized.

This work, along with parallel contributions from Louis Brus studying colloidal solutions and later Moungi Bawendi’s synthesis methods, earned a share of the 2023 Nobel Prize in Chemistry. The discovery was made in colored glass, the same kind of material as the traffic-light reds, whose makers may well have been tuning crystal size, and with it confinement, through their striking schedules long before the physics had a name (a plausible inference, not a documented one).

Cadmium is toxic, and modern consumer and art-glass formulations have largely moved away from this chemistry where substitutes exist. For precision sharp-cut optical filters, however, the chemistry is still in use (unverified for current catalogs).

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