We have a MoS2 thin film (~ 50 nm) obtained by magnetron sputtering. Inside this layer, crystallites of approximately 4-5 nm are formed (calculated from XRD: Scherrer equation). The energy gap of this thin film suggests the occurrence of a quantum size effect (we have Eg = 1.9 eV). Why does the quantum effect occur in crystallites of the same material? The quantum effect occurs when the electron is trapped. We suspect that the electron is trapped in crystallite, but why can not it "jump" to another crystallite? Is it a barrier of potential that the grain boundaries confers on it? Is there an article that discusses this problem?
Dear Lukasz,
grain boundaries can connected with a potential barrier. Typical values are between 10meV and 50 meV.
See: Seto, J. Y. W. The electrical properties of polycrystalline silicon films.
J. Appl. Phys. 46, 5247–5254 (1975)
Such thermal barriers can be overcome at sufficient high temperature. But at lower T the charge carriers are "trapped" within the crystallite. Due to the confinement and due to the low crystallite size, the gap energy is enhanced and the energy of electrons in the ground state is increased. A value of 4 nm gives certainly an remarkable size effect.
With Regard
R. Mitdank
Crystallite are extremely small comparable to wavelength of electron gives rise to quantum confinement.
Kindly read the following article
Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide