bandgap

selenium ruby

Cadmium sulfoselenide crystals give glass a razor-sharp absorption edge — the purest reds and oranges glass can make, and 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. The red of every traffic light, railway signal, and automotive tail light of the pre-LED century came from this chemistry. Colors so pure that international signal standards were effectively written around it.

The mechanism that makes these colors exceptional is different from both dissolved ions and metal colloids. A semiconductor crystal doesn’t absorb at a specific wavelength — it absorbs everything above a cutoff. Below the cutoff, the glass transmits freely; above it, nearly everything is blocked. The result is a razor-sharp absorption edge, not a broad hump, which is what makes the colors look lit from within rather than merely 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, which places its absorption edge in the green, near 515 nm. It absorbs violet through green and transmits a brilliant yellow. Cadmium selenide (CdSe) has a smaller gap near 1.7 eV — its edge sits in the deep red, at the edge of the near-infrared (~730 nm), meaning it absorbs nearly the entire visible spectrum, producing deep red to near-black.

The two compounds form a continuous solid solution, CdS₁₋ₓSeₓ: by adjusting the ratio of sulfur to selenium, you slide the band gap — and therefore the absorption edge — smoothly across the warm side 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 — typically 600–700 °C — 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 discipline used in colloidal color.

Artifact stories

Traffic-light red

The dominant red in traffic signals and railway semaphores for most of the 20th century was selenium ruby glass — specifically, a composition tuned to transmit a narrow red band above roughly 620–630 nm while blocking everything below. The reason signal engineers favored this chemistry over ionic reds or other alternatives was the sharpness of the edge: a steep cutoff maximizes chromatic purity at maximum brightness. 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.

The signal industry’s specifications were written to match what this chemistry could deliver, which meant that when LEDs eventually displaced glass filters in traffic lights, the LED specifications had to match the CdS-CdSe optical performance — the glass standard persisted in the standards documents long after the glass itself was gone.

Sharp-cut filters

Precision optical filter glass — used in photography, scientific instruments, microscopy, and industrial inspection — exploits the band-gap edge with even more discipline. 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 classic 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 is the physical signature of band-gap absorption — you cannot achieve it with any other mechanism available in glass.

The quantum dot postscript

Make the CdS-CdSe crystals small enough — below roughly 10 nm — and the electrons in the crystal begin to feel 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 Vavilov State Optical Institute in Leningrad was studying color-filter glass and recognized this size-dependent shift for what it was: quantum confinement of charge carriers in a semiconductor nanocrystal. His first demonstration used copper-chloride nanocrystals in glass; 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 happened in glass, in the same material system that colored traffic lights for a century, recognized by glassmakers who had been unknowingly tuning quantum confinement with their striking schedules for decades before the physics had a name.

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 remains the standard — there is currently nothing else that draws the same edge.

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