At nano level band structure ceases to exist. Hence material starts behaving as insulator. I hope it helps.

The surface properties of the semiconductor, surface defects and Fermi level pinning, can create a natural state of inversion near the surface.  If the sample is of small size then the whole sample may be under inversion which means there can be no electrical control of the charge.  This can be independent of the quantum size effects mentioned by Chauhan.

Thank you Aditya Chauhan and Thomas A Detemple for yours' replies .

In my opinion, nano-scale may have two effects that both contributing to the fact of losing semiconducting properties:

(1) silicon in nano-scale has a different density of states profile, couple with Fermi selection law, which give you different conductive properties. A extreme example is a silicon quantum dot (very small piece of silicon). If you inject one electron into this dot, then it would be very difficult for you to inject a second one due to the electric repulsion. This is called coulomb blockade of semiconductor in nano-scale.

(2) Si in nanoscale will also change its energy band structure from continuous band in K-space to discontinued bands. The band gap will also increase as you decrease the size of silicon. This is called quantum confinement of semiconductor in nano-scale.

Hope this can help you.

Thank you Yixuan Yu 

Another Explanation  is how the continous bands are deduced theoretically: The discrete energy Levels of neighbouring Atoms are superpositioned. In a nanoparticle the number of Atoms are limited, resulting in different energy bands. 

I agree with Thomas and Aditya:  semiconductor properties come from band structures, and the formation of bands is a 'collective' behavior from many atoms; you loose this behavior when you just have a few atoms.

При размере наночастиц меньше 70 нм структура кристалла теряет дальний порядок- в результате кристаллическая структура переходит в аморфную и материал ведет себя как изолятор.

Thank you all for your comments