Quasi-two-dimensional perovskites have emerged as a new material platform for optoelectronics on account of its intrinsic stability. A major bottleneck to device performance is the high charge injection barrier caused by organic molecular layers on its basal plane, thus the best performing device currently relies on edge contact. Herein, by leveraging on van der Waals coupling and energy level matching between two-dimensional Ruddlesden-Popper perovskite and graphene, we show that the plane-contacted perovskite and graphene interface presents a lower barrier than gold for charge injection. Electron tunneling across the interface occurs via a gate-tunable, direct tunneling-to-field emission mechanism with increasing bias, and photoinduced charge transfer occurs at femtosecond timescale (~50 fs). Field effect transistors fabricated on molecularly thin Ruddlesden-Popper perovskite using graphene contact exhibit electron mobilities ranging from 0.1 to 0.018 cm²V⁻¹s⁻¹ between 1.7 to 200 K. Scanning tunneling spectroscopy studies reveal layer-dependent tunneling barrier and domain size on few-layered Ruddlesden-Popper perovskite.

Semimetal MoTe ₂ can be a type II Weyl semimetal in the bulk, but monolayer of this material is predicted to be quantum spin hall insulators. This dramatic change in electronic properties with number of layers is an excellent example of the dimensional effects of quantum transport. However, a detailed experimental study of the carrier transport and band structure of ultrathin semimetal MoTe ₂ is lacking so far. We performed magneto-transport measurements to study the conduction behavior and quantum phase coherence of 1T′ MoTe ₂ as a function of its thickness. We show that due to a unique two-band transport mechanism (synergetic contribution from electron conduction and hole conduction), the conduction behavior of 1T′ MoTe ₂ changes from metallic to p-type unipolar, and finally to ambipolar as the thickness decreases, suggesting that this effect can be used in devices by effectively controlling the thickness. Our transport studies, optical measurements and first-principles electronic structure calculations reveal that 1T′ MoTe ₂ remains gapless down to a few (~2–3) layers. Despite being gapless, 1T′ MoTe ₂ exhibits metal-insulator transition at 3-layer thickness, due to enhanced carrier localization effect.