  Colloidal semiconductor quantum dots are robust emitters implemented in
numerous prototype and commercial optoelectronic devices. However, active
fluorescence color tuning, achieved so far by electric-field induced Stark
effect, has been limited to a small spectral range, and accompanied by
intensity reduction due to the electron-hole charge separation effect.
Utilizing quantum dot molecules that manifest two coupled emission centers, we
present a novel electric-field induced instantaneous color switching effect.
Reversible emission color switching without intensity loss is achieved on a
single particle level, as corroborated by correlated electron microscopy
imaging. Simulations establish that this is due to the electron wavefunction
toggling between the two centers dictated by the electric-field and affected by
the coupling strength. The quantum dot molecules manifesting two coupled
emission centers may be tailored to emit distinct colors, opening the path for
sensitive field sensing and color switchable devices such as a novel pixel
design for displays or an electric field color tunable single photon source.
