Here we report and explain such a scenario. Using Raman spectroscopy we show a remarkable change of vibrational modes above the critical thickness that is attributed to a change in the nature of the bonds: While ordinary covalent bonding is found in ultrathin films, resonant bonding can prevail only once a critical thickness is reached. Resonant bonding in chalcogenides is a unique bonding mechanism, which differs significantly from metallic or ordinary covalent bonding. The atoms in crystalline GeTe have 6 nearest neighbors, but only 3 p-electrons per atom to form saturated bonds, sometimes called 2 center – 2 electron bonds. Therefore, there are too many nearest neighbors for the electrons to form ordinary covalent bonds to each neighbor. Instead, the system employs 3 center – 2 electron bonds, also denoted as resonant bonds. In this situation, neighboring atoms are held together just by a single electron, not an electron pair. Unlike in metals, however, these electrons are still rather localized between two atoms, leading to a non-vanishing band gap. GeTe ultra-thin films try to develop resonant bonds in the direction of the film normal. Yet, electron delocalization in z-direction is impossible, if the substrate provides an electronic barrier for the corresponding p-electrons of GeTe. This scenario is further supported by density functional theory calculations showing that ultrathin films do not utilize resonant bonding in contrast to the bulk phase.
|1||Autor||R. N. Wang , W. Zhang , J. Momand , I. Ronneberger , J. E. Boschker , R. Mazzarello , B. J. Kooi , H. Riechert , M. Wuttig , R. Calarco|
Formation of Resonant Bonding during Growth of Ultrathin GeTe Films
|Source||NPG Asia Mater. , 9 , e369 ( 2017 )|
: 10.1038/am.2017.95 |