First-principles Study Of Quasi-two-dimensional Semiconductors:Materials Prediction And Strain Modulation

The next generation electronic devices that are bendable/foldable,lighter weight,higher performance have motivated researches exploring new quasi-two-dimensional?quasi-2D?semiconductor materials with high carrier mobility and high flexibility.Using the first-principles calculations,we systematically investigate the biaxial tensile strain effect on the transport properties of the single layer tin selenide.We predict that both monolayer lead iodide and bismuth iodide are quasi two-dimensional semiconductors with wide band gap and also study the strain modulation of the electronic properties of these quasi-2D semiconductors.We also provide a new method to open the band gap of graphene.Firstly,we predict that the single layer tin selenide is a near-direct band gap quasi-2D semiconductor,which has a relative large electron mobility under zero strain.The ideal biaxial tensile strain limit of the single layer tin selenide is about 12%.The band gap value of monolayer tin selenide can be tuned effectively by biaxial tensile strain.At the same time,biaxial tensile strain can also switch the single layer tin selenide to be direct band gap semiconductor within a large scale strain?2% ? 8%?.We also predict that electrons in monolayer SnSe are much more mobile than holes at zero strain and the electron mobilities are as high as the order of 104cm²V^-1s^-1.This electron mobility decreases gradually with increasing biaxial tensile strain but remains above 600cm²V^-1s^-1within 2% ? 8% biaxial strain.Our finding of the strong anisotropy in 2D SnSe makes it a promising candidate for future mechanical and electronic applications.Secondly,we predict the existence of wide band gap semiconductor monolayer metal halogen compound?single lead iodide and bismuth iodide?.Monolayer lead iodide is an indirect band gap semiconductor with a gap value of about 2.63 eV.We demonstrate that quantum confinement effect is responsible for the transformation of lead iodide from the direct band gap semiconductor in bulk form to an indirect one in monolayer form.Here,biaxial strain can also modify gap size of monolayer lead iodide effectively,but it can convert monolayer lead iodide to a direct band gap semiconductor,which can be explained by that biaxial strain induces uniformly change the electric field and an iodine atom layer.We also predict that a single layer bismuth iodide can be obtained by mechanical exfoliation method.According to our calculations,monolayer bismuth iodide is also an indirect band gap semiconductor.With increasing layer thickness,the band gap decreases slightly.Under the biaxial tensile strain,the band gap of single-,double-,and triple-layers bismuth iodide decrease with a similar trend,indicating that the inter-layer coupling in bismuth iodide is rather weak.The hetero-structure that hybridizing the single bismuth iodide or lead iodide with graphene can enhance the absorption of visible light.Our finding highlight a new family of promising quasi-2D semiconductors for potential application in building novel 2D photonics and photovoltaic devices.Finally,we show that a zigzag-GNR like structure formed in continuous graphene can open a band gap.We provide a new method to open gap in continuous graphene utilizing both substrate interaction and quantum confinement effect.Using the first-principles simulation,we confirm that the strong interaction between graphene and the metal substrate is capable of opening the energy gap of about 0.7 eV,which is in well agrement with experimental results.