One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface.Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono-to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to band-like in subsequent layers.Such abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ~3nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. 3Organic field-effect transistors (OFETs) offer unique advantages of low cost, lightweight and flexibility and are widely used in electronics and display industry.While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7].Ultrathin organic semiconductors down to few-nanometre thickness are often dominated by traps and disorders and far away from intrinsic transport regime [8][9][10].Another challenge in organic electronics is the development of layer-by-layer epitaxy with the precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13].Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16] and covalent organic frameworks [17] have already shown great promises along this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene epitaxially crystallized on BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our results suggest the possibility of band-like transport...

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400-700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron-hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics.

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/opticalproperties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C 8 -BTBT) and n-type MoS 2 . We find that