  In this paper, we study the asymptotic decay properties for defocusing
semilinear wave equations in $\mathbb{R}^{1+2}$ with pure power nonlinearity.
By applying new vector fields to null hyperplane, we derive improved time decay
of the potential energy, with a consequence that the solution scatters both in
the critical Sobolev space and energy space for all $p>1+\sqrt{8}$. Moreover
combined with Br\'{e}zis-Gallouet-Wainger type of logarithmic Sobolev
embedding, we show that the solution decays pointwise with sharp rate
$t^{-\frac{1}{2}}$ when $p>\frac{11}{3}$ and with rate $t^{
-\frac{p-1}{8}+\epsilon }$ for all $1<p\leq \frac{11}{3}$. This in particular
implies that the solution scatters in energy space when $p>2\sqrt{5}-1$.
