Materials Science

Cations Minding the Gap

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Science  26 Feb 2010:
Vol. 327, Issue 5969, pp. 1061
DOI: 10.1126/science.327.5969.1061-b

The band gap of a material—the energy threshold for excitation of electrons and thus the major determinant of properties ranging from color to conductivity—is usually set by the crystal structure of the constituent atoms or ions. It could prove useful for a variety of applications to be able to vary band gaps associated with a particular type of bulk framework in a systematic way, much as it is currently possible to do at the nanometer scale by tuning the size of dispersed individual particles. Qian et al. studied the assembly of the arsenic-based Zintl cluster As73− with various cations and examined the effects of change transfer between cation and anion. The cations included Cs+, cryptand-sequestered K+ and Rb+, and Au+. The band gap, as determined from optical measurements, could be varied from 1.1 to 2.1 electron volts. Density functional calculations revealed that the changes mainly arose from variation in the positions of the lowest unoccupied orbitals, principally affected by the cations, although some changes were associated with charge transfer induced by the cations that led to covalent links between the clusters.

ACS Nano 4, 235 (2010).

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