  We study the nonlocal scalar field equation with a vanishing parameter \[
\left\{\begin{array}{lll}
  (-\Delta)^s u+\epsilon u &=|u|^{p-2}u -|u|^{q-2}u
\quad\text{in}\quad\mathbb{R}^N \\ u >0, & u \in H^s(\mathbb{R}^N),
  \end{array}
  \right. \] where $s\in(0,1)$, $N>2s$, $q>p>2$ are fixed parameters and
$\epsilon>0$ is a vanishing parameter. For $\epsilon>0$ small, we prove the
existence of a ground state solution and show that any positive solution of
above problem is a classical solution and radially symmetric and symmetric
decreasing. We also obtain the decay rate of solution at infinity. Next, we
study the asymptotic behavior of ground state solutions when $p$ is
subcritical, supercritical or critical Sobolev exponent $2^*=\frac{2N}{N-2s}$.
For $p<2^*$, the solution asymptotically coincides with unique positive ground
state solution of $(-\Delta)^s u+u=u^p$. On the other hand, for $p=2^*$ the
asymptotic behaviour of the solutions is given by the unique positive solution
of the nonlocal critical Emden-Fowler type equation. For $p>2^*$, the solution
asymptotically coincides with a ground-state solution of $(-\Delta)^s
u=u^p-u^q$. Furthermore, using these asymptotic profile of solutions, we prove
the \textit{local uniqueness} of solution in the case $p\leq 2^*$.

Analysis of PDEs