Does the photoelectric effet occurs only with metals? As the title said, does the photoelectric effect occurs only with metals?
I searched on Google about it and on the first result's webpage I read that : 

In metal the electrons are in a loose connection to the metal's atoms

How's that ? What does it means ? I mean, what's the difference between atoms of a plate of Nickel and a piece of wood ?
How can we explain this using the terms "conduction band" and "valence band" ?
Thanks a lot in advance!
 A: The photoelectric effect is a photon absorption with energy kept in balance by emission (from an atomic or molecular orbital) of a bound electron.
If electrons are ejected from an insulator, the remaining charges (positive)
will attract any subsequent electrons: there will be, in observed emitted
electrons, a diminution of current, until no electron has enough kinetic energy to
escape the insulator and its positive (attractive) charge.   So, for
experimental purposes, a metal or semiconductor target and grounding wire are advantageous.   That allows observation of electrons freed by the
photoelectric effect.
Photoelectric rates of absorption are highest in materials with broad energy bands, because there are lots of electrons at a range of energies, rather
than only at discrete (sharp) energy levels, as in a gas.   Metals all have a
broad energy band (the 'conduction band') which causes metallic (covalent) bonding.   When a photon energy is within the band energy, it has an
absorption-probability advantage over photons that, when absorbed, leave large amounts of leftover energy.
Photoelectric effect is seen in all material substances that contain electrons.
