($(h) = 6.63 \times 10^{-34}\, Js$,$e = 1.6 \times 10^{-19}\, C$)
\(v_{0}=\frac{h v}{e}-\frac{\phi}{e}\)
\(\mathrm{v}_{0}\) is zero for \(v=4 \times 10^{14} \mathrm{Hz}\)
\(0=\frac{h v}{e}-\frac{\phi}{e}\)
\(\Rightarrow \phi=\mathrm{hv}\)
\( = \frac{{6.63 \times {{10}^{ - 34}} \times 4 \times {{10}^{14}}}}{{1.6 \times {{10}^{ - 19}}}}\) \( = 1.66\,\,{\text{eV}}\)