  Insulator to metal (IMT) transition (T$_t$ $\sim$ 341 K) in the VO2
accompanies transition from an infrared (IR) transparent to IR opaque phase.
Tailoring of the IMT and associated IR switching behavior can offer potential
thermochromic applications. Here we report on effects of the W and the Tb
doping on the IMT and associated structural, electronic structure and optical
properties of the VO2 thin film. Our results show that the W doping
significantly lowers IMT temperature ($\sim$ 292 K to $\sim$ 247 K for 1.3\% W
to 3.7\% W) by stabilizing the metallic rutile, $\it{R}$, phase while Tb doping
does not alter the IMT temperature much and retains the insulating monoclinic,
$\it{M1}$, phase at room temperature. It is observed that the W doping albeit
significantly reduces the IR switching temperature but is detrimental to the
solar modulation ability, contrary to the Tb doping effects where higher IR
switching temperature and solar modulation ability is observed. The IMT
behavior, electrical conductivity and IR switching behavior in the W and the Tb
doped thin films are found to be directly associated with the spectral changes
in the V 3$\it{d_{\|}}$ states.
