Filling
the lowest quantum state of the conduction band of colloidal
nanocrystals with a single electron, which is analogous to the filling
the lowest unoccupied molecular orbital in a molecule with a single
electron, has attracted much attention due to the possibility of harnessing
the electron spin for potential spin-based applications. The quantized
energy levels of the artificial atom, in principle, make it possible
for a nanocrystal to be filled with an electron if the Fermi-energy
level is optimally tuned during the nanocrystal growth. Here, we report
the singly occupied quantum state (SOQS) and doubly occupied quantum
state (DOQS) of a colloidal nanocrystal in steady state under ambient
conditions. The number of electrons occupying the lowest quantum state
can be controlled to be zero, one (unpaired), and two (paired) depending
on the nanocrystal growth time via changing the stoichiometry of the
nanocrystal. Electron paramagnetic resonance spectroscopy proved the
nanocrystals with single electron to show superparamagnetic behavior,
which is a direct evidence of the SOQS, whereas the DOQS of the two-
or zero-electron occupied nanocrystals in the 1S<sub>e</sub> exhibit
diamagnetic behavior. In combination with the superconducting quantum
interference device measurement, it turns out that the SOQS of the
HgSe colloidal quantum dots has superparamagnetic property. The appearance
and change of the steady-state mid-IR intraband absorption spectrum
reflect the sequential occupation of the 1S<sub>e</sub> state with
electrons. The magnetic property of the colloidal quantum dot, initially
determined by the chemical synthesis, can be tuned from diamagnetic
to superparamagnetic and vice versa by varying the number of electrons
through postchemical treatment. The switchable magnetic property will
be very useful for further applications such as colloidal nanocrystal
based spintronics, nonvolatile memory, infrared optoelectronics, catalyst,
imaging, and quantum computing