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Ballistic Excitons and Surface Functionalization in a Superatomic Semiconductor

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and  lossy  transport  that  curtails  all  semiconductor  technologies.  Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, IRG2 PIs have demonstrated  the  formation  of  acoustic  exciton-polarons,  an  electronic quasiparticle  shielded  from  phonon  scattering  (Fig.  1a).  Polaron  transport  in Re6Se8Cl2  is  directly  image  at  room  temperature  (Fig.  1a),  revealing  quasi-ballistic, wavelike propagation sustained for nanoseconds and several microns. Shielded  polaron  transport  leads  to  electronic  energy  propagation  orders  of magnitude  greater  than  in  other  vdW  semiconductors  (Fig.  1b,c),  exceeding even  silicon  over  nanoseconds.  We  propose  that,  counterintuitively,  quasi-flat electronic  bands  and  strong  exciton–acoustic  phonon  coupling   are  together responsible for the remarkable transport properties of Re6Se8Cl2, establishing a new  path  to  ballistic  room-temperature  semiconductors.  This  work,  recently accepted for publication in Science, is a major step in the long and active search for  room-temperature  ballistic  semiconductors,  which  could  power  next-generation technologies such as lossless transistors operating over three orders of magnitude faster than current electronics.

Using Re6Se8Cl2 as a superatomic pegboard, the IRG2 team have also developed robust and general method to functionalize 2D superatomic sheets with  high  precision  and  controllable  densities.  Fig.  1d  shows  the  covalent attachment  of  2,2’-bipyridine  (bipy)  ligands  to  exfoliated  Re6Se8Cl2  sheets through  radical-based  Cl  substitution.  These  ligands  can  bind  molecular  Co catalysts,  and  with  no  optimization,  the  resulting  surface-tethered  complexes catalyze  the  oxidation  of  water  with  no  release  or  decomposition  of  the  Co complex.