Nanostructured Borate Halides for Optical Second Harmonic Generation at Surfaces
Optical second harmonic generation (SHG) at surfaces and interfaces is an exciting new technique. In order to achieve high surface specificity and sensitivity of the SHG technique, smaller particles of SHG materials are highly desired. Borate halides are regarded as a great source of SHG materials because of their outstanding nonlinear optical properties. Phase‐pure nanostructured samples of three borate halides of the type $A_2B_5O_9X$ were synthesized at low temperature in task‐specific ionic liquids (IL). Optimized reactions of acetates A(OAc)2 (A = Sr, Ba) and boric acid B(OH)3 with the mixed IL [P66614]X (X = Cl, Br) and $LiNTf_2$ led to nano‐rods of $Sr_2B5O_9Cl$, nano‐needles of $Sr_2B_5O_9Br$, and nano‐sheets of $Ba_2B_5O_9Cl$. The reaction route presented here might trigger the synthesis of other borate compounds. Efficient second‐order nonlinear conversion by the nanostructured borate halides was confirmed using confocal SHG microscopy.