Yuhan Zhang,1 Jingsi Qiao,2 Si Gao,3 Fengrui Hu,4 Daowei He,1 Bing Wu,1 Ziyi Yang,1 Bingchen Xu,1 Yun Li,1 Yi Shi,1,* Wei Ji,2,* Peng Wang,3 Xiaoyong Wang,4 Min Xiao,4, 5 Hangxun Xu,6 Jian-Bin Xu,7,1* & Xinran Wang1,*
1National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
2Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China.
3College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
4School of Physics, Nanjing University, Nanjing 210093, China
5Department of Physics, University of Arkansas, Fayetteville, AR72701, USA
6CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China
7Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, China
DOI: Publication Date:
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.