Tailoring Optical and Vibrational Properties by Bond Confinement: The Case of Metavalent Chalcogenide and Pnictogen Thin Films

This perspective explores how film thickness influences the optical and vibrational properties of crystalline chalcogenide and pnictogen thin films. Measurements of the dielectric function reveal systematic property trends within these families of materials. A distinct class, including crystalline bismuth (Bi), germanium telluride (GeTe), and antimony telluride (Sb 2 Te 3 ), exhibits strong responses to electromagnetic wave excitation. These materials are characterized by large values of the optical dielectric constant ɛ ∞ and high maximum values of ɛ 2 (ω). Crystalline GeTe, Bi and Sb 2 Te 3 furthermore display an unconventional bond rupture observed via atom probe tomography. They utilize a bonding mechanism (metavalent bonding) that balances electron localization typical in iono‐covalent solids with electron delocalization found in metals. In contrast, crystalline Sb 2 Se 3 , In 2 Te 3 , and amorphous GeTe neither exhibit these distinctive characteristics nor the bond rupture pattern. Significant changes in optical and vibrational properties occur with varying film thicknesses for the metavalent solids, i.e., crystalline Bi, GeTe, and Sb 2 Te 3 . For instance, substantial shifts in phonon frequencies are observed in these solids as thickness decreases, reaching frequency shifts up to 30%. This unusual dependence contrasts with covalent counterparts (In 2 Te 3 , amorphous GeTe), which show minimal effects. These findings underscore the potential of metavalent bonding for tailoring materials in optoelectronic applications.