8. Given $a_{1}=1, a_{n+1}=\lambda a_{n}^{2}+2\left(n \in \mathbf{N}^{*}\right)$, if the sequence $\left\{a_{n}\right\}$ is bounded above, i.e., there exists a constant $M$, such that $a_{n} \leqslant M$ for all $n \in \mathbf{N}^{*}$, then the maximum value of the real number $\lambda$ is $\qquad$.