If you are just inferring from the term 'microcontroller' as some thing that is at micro level then 'nanocontrollers' are also there. Actually the technology being used for fabriacting many microcontrollers is 'nano-technology'.
Actually microcontroller is just a term to describe a-computer-on-a-chip. In that sense nanocontroller isn't any term.
In practice some people are using this term for the microcontrollers/ICs specifically designed for some bio applications.
Xilinx offers a controller called PicoBlaze. It is just a basic 8-bit controller able to address 1kB of RAM and its ROM is 1k 18-bit words. Actually it's a soft core and is mainly used in simple control applications, or in heavily massive parallel processing arrays. They can run up to some 150MHz and occupy very little space.
Designers can stuff dozens of them in a single FPGA chip.
Perhaps there are simpler and smaller uCs but this one belongs to the PICO class by its name
PicoBlaze is good for those who like to program in Assembly. To my knowledge, there is no C compiler for PicoBlaze. On the other hand, the IP for the PicoBlaze is free.
PicoBlaze is good for those who like to program in Assembly. To my knowledge, there is no C compiler for PicoBlaze. On the other hand, the IP for the PicoBlaze is free.
The smallest microcontrollers that I know of are approx. 2-4 sqmm in size. So the die between 1x2mm or 2mm x 2mm.
Does that fit your definition of nano-controllers?. If so, they do exist today, if you are looking for "invisible" devices, you need to wait a little longer.
The smallest packaged devices I know are 3mm x 3mm for example from Silicon Labs and Philips.