title: Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit.
abstract: 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 single-crystalline mono- to tetralayer 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 bandlike in subsequent layers. Such an 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 ∼3  nm. 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.
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Yuhan
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China.
authors.forename: Jingsi
authors.initials: J
authors.lastname: Qiao
authors.affiliation: College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China.
authors.forename: Si
authors.initials: S
authors.lastname: Gao
authors.affiliation: School of Physics, Nanjing University, Nanjing 210093, China.
authors.forename: Fengrui
authors.initials: F
authors.lastname: Hu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Daowei
authors.initials: D
authors.lastname: He
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Bing
authors.initials: B
authors.lastname: Wu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Ziyi
authors.initials: Z
authors.lastname: Yang
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Bingchen
authors.initials: B
authors.lastname: Xu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Yun
authors.initials: Y
authors.lastname: Li
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Shi
authors.affiliation: Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China.|Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China and Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Ji
authors.affiliation: College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China.
authors.forename: Peng
authors.initials: P
authors.lastname: Wang
authors.affiliation: School of Physics, Nanjing University, Nanjing 210093, China.
authors.forename: Xiaoyong
authors.initials: X
authors.lastname: Wang
authors.affiliation: School of Physics, Nanjing University, Nanjing 210093, China.|Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Xiao
authors.affiliation: CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China.
authors.forename: Hangxun
authors.initials: H
authors.lastname: Xu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.|Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, China.
authors.forename: Jian-Bin
authors.initials: JB
authors.lastname: Xu
authors.affiliation: National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
authors.forename: Xinran
authors.initials: X
authors.lastname: Wang