Uniqueness of positive solutions for a class of nonlinear elliptic equations with Robin boundary conditions

We prove the uniqueness of positive solutions to the BVP$ \begin{align} \left\{ \begin{array}{c} -\Delta u = \lambda f(u)\ \ \ \text{in }\Omega , \\ \frac{\partial u}{\partial n}+bu = 0\ \ \ \text{on }\partial \Omega , \end{array} \right. \end{align} $when the parameter $ \lambda $ is large independent of $ b\in \mathbb{(} 0, \infty ) $. Here, $ \Omega $ is a bounded domain in $ \mathbb{R}^{n} $ with smooth boundary $ \partial \Omega , \ f:[0, \infty )\rightarrow [ 0, \infty ) $ is continuous, sublinear at $ \infty $, and satisfies a concavity-like condition for $ u $ large.