Transistor level design of flip flops - Is the complementary clock necessary?
Yes actually, there are a plethora of different designs of FF's at the transistor level.

One of my favorites are known as TSPC (True Single Phase Clock) FF's and I can recommend an excellent paper 

" Yuan, J., &amp; Svensson, C. (1997). New single-clock CMOS latches and flipflops with improved speed and power savings. Solid-State Circuits, IEEE Journal of, 32(1), 62–69. http://doi.org/10.1109/4.553179"






The terminology comes from the paper (so do the pictures), there are differential, static, dynamic, semi-static and all with minimal transistor count.