Selenium

Selenium (Se, atomic number 34) is a Group 16 chalcogen nonmetal with semiconductor properties and the most pronounced photoconductive behavior of any element — its electrical conductivity increases by up to five orders of magnitude under illumination, a discovery made in 1873 that led directly to the first photoelectric cells and later to xerography. Selenium exists in several allotropic forms: red amorphous Se (disordered chains/rings, bandgap ~2.0 eV), black vitreous Se (amorphous, metastable), and gray trigonal Se (thermodynamically stable, helical Se chain structure, density 4.81 g/cm³, bandgap ~1.74 eV) — the form relevant to virtually all functional applications. It is a moderately rare element (~0.05 ppm crustal abundance) recovered almost entirely as a byproduct of copper refining, with global production of ~3,000 tonnes/year. Selenium is an essential biological trace element (incorporated into ~25 human selenoproteins as selenocysteine) but toxic in modest excess — the gap between the nutritional RDA (~55 µg/day) and toxic intake (~400 µg/day) is less than one order of magnitude.

The dominant commercial applications of selenium are in glass manufacturing and steel, which together consume roughly 60% of global production, and in amorphous Se flat-panel X-ray detectors, which represent the largest high-technology use of elemental Se. In glass, Se functions both as a decolorizer (oxidizing Fe²⁺ and reacting with residual sulfide to neutralize the green tint in container glass) and as a red/orange colorant via cadmium sulfoselenide (CdSₓSe₁₋ₓ) pigments stable above 1,200 °C. In free-machining stainless steel (AISI 303Se), Se additions of 0.15–0.35 wt% improve chip breakage with less mechanical property anisotropy than sulfur. Amorphous Se flat-panel detectors achieve direct X-ray-to-charge conversion without an intermediate scintillator, giving superior spatial resolution (DQE >0.7 at 5 lp/mm) and are the gold standard for full-field digital mammography systems from Hologic and GE.

Selenium's most rapidly growing application is as the chalcogenide component of Cu(In,Ga)Se₂ (CIGS) thin-film solar cells — the highest-efficiency thin-film photovoltaic technology — and in CdSe quantum dots for QLED displays and biological imaging. CIGS cells achieve record efficiency of 23.35% (small area) using a ~2 µm Se-containing absorber with a bandgap tunable from 1.0 to 1.7 eV by varying the Ga/(Ga+In) ratio. Colloidal CdSe quantum dots (2–10 nm, bandgap tunable 490–620 nm by size) are synthesized by hot-injection methods and used in Samsung QD-OLED displays, quantum dot color filters in LCD televisions, and as fluorescent biological labels with far superior photostability to organic dyes. Selenium is also the defining chalcogenide in Bi₂Se₃ topological insulators — the model system for topologically protected surface states and proximity-induced topological superconductivity research.