SetDirectory[NotebookDirectory[]];

tightBinding3D=Plot3D[-2(Cos[kx] + Cos[ky]),{kx,-π,π},{ky,-π,π},Ticks->{{-π,-π/2,0,π/2,π},{-π,-π/2,0,π/2,π}},AxesLabel->{"(Subscript[k, x])","(Subscript[k, y])","ϵ(k)"}]

tightBindingContour=ContourPlot[-2(Cos[kx] + Cos[ky]),{kx,-π,π},{ky,-π,π},FrameTicks->{{-π,-π/2,0,π/2,π},{-π,-π/2,0,π/2,π}},FrameLabel->{"(Subscript[k, x])","(Subscript[k, y])"}]

Export["tightBinding3D.pdf",tightBinding3D];
Export["tightBindingContour.pdf",tightBindingContour];