TMDs (transition metal dichalcogenides) are materials that can have a layered structure. When moving from bulk to monolayers, the bandgap changes from indirect to direct according to the image (DFT to MoSe$_2$). From what I understand, this may be due to the effect of quantum confinement, but I didn't understand what this phenomenon consists of for the specific case of TMDs. What is quantum confinement for TMDs?
(Kumar, A. e Ahluwalia, P., Electronic structure of transition metal dichalcogenides monolayers 1H-MX$_2$ (M = Mo, W; X = S, Se, Te) from ab-initio theory: New direct band gap semiconductors. European Physical Journal B, 85:1–7, 2012.)