High‐Performance Langmuir–Blodgett Monolayer Transistors with High Responsivity

Abstract

Shrinking the dimensions of organic field-effect transistors (OFETs) down to the nanometer scale offers new, defect-free charge transport regimes. This may lead to the improvement of the device performance, such as larger carrier mobilities, increased device speed, lower power dissipation, and enhanced on/off ratios. Therefore, much effort has recently been made to scale down both the thickness of the OFET devices (the semiconductor and/or insulator layers) [1] and their lateral dimension (source-drain distance). [2] With size reduction, however, the device performance is mainly hampered by the parasitic contact resistances with a high injection barriers and the poor long-range order of organic semiconductors. Most commonly, gold source-drain (S/D) electrodes are used as the charge-injecting metal in organic electronic devices. Gold is used because of its chemical stability and its work function that matches the energy level of organic semiconductors in most cases, thus lowering the Schottky barriers. To reduce the contact resistance, several alternative materials, including carbon nanotubes (CNTs), [3] carbon nanotube/polymer nanocomposites, [4] graphene multilayers, [5] and conductive polymers, [6] have been utilized as potential substitutes for the expensive gold S/D electrodes. On the other hand, molecular organization can be improved by forming dense and well-ordered self-assembled monolay-ers through bottom-up approaches, [7] as illustrated by the work of Smits et al. [7d] To optimize the performance of OFETs, device fabrication should be considered as a holistic process. The electrode materials, the contact surface, and device fabrication are so closely interrelated that they cannot be optimized independently. To date, only few examples of OFETs have been demonstrated to achieve high-performance by holistic consideration of all these parameters. [8] With this in mind, herein we present a new class of highperformance photoresponsive molecular field-effect transistors formed from Langmuir-Blodgett (LB) monolayers of copper phthalocyanine (CuPc), using two-dimensional (2D) ballistically-conductive single-layer graphene as planar contacts. The unique feature detailed herein is the integration of LB techniques with the fabrication of nanogap electrodes to build functional molecular electronic devices. LB techniques offer a promising and reliable method to prepare large-area ordered ultrathin films with well-defined architectures. In previous work, we and others [9] have demonstrated the successful applications of the LB technique to CuPc and conjugated polymers in producing ultrathin film OFETs. However, the charge carrier mobilities m in these devices were low, namely about 10 À7 -10 À3 cm 2 V À1 s À1 . This might be mainly ascribed to the high contact resistance when gold was used for S/D electrodes and to the defects in the micrometer-long channels. For this study, we employed single-layer graphene as S/D nanoelectrodes to overcome these difficulties. This choice is because graphene, a new class of 2...

Thin‐Film‐Based Nanoarchitectures for Soft Matter: Controlled Assemblies into Two‐Dimensional Worlds

“…The LB technique has long been known to offer a promising and reliable method for preparing large‐area ordered ultrathin films with well‐defined molecular orientation. Recently, high‐performance organic field‐effect transistors have been successfully prepared by this technique 241–243. In addition, the LB technique is capable of controlling and concentrating not only conventional amphiphiles, but also carbon nanotubes,244–247 inorganic nanomaterials,248–251 and organic nanocrystals 252…”

Crystal-Domain Orientation and Boundary in Highly Ordered Organic Semiconductor Thin Film

“…17 Although CuPc thin films are usually polycrystalline with small grain sizes and many DBs that limit the performance of CuPc-based devices, [17][18][19] weak epitaxy growth (WEG) has been developed and succeeded in fabricating high-quality CuPc thin films by introducing an ultrathin template molecular layer and elevating the growth temperature. 20 The sizes of CuPc crystalline domains can be dramatically increased to an average diameters of over 10 µm, leading to a significant improvement in carrier transport.…”

Ultra-flat coplanar electrodes for controlled electrical contact of molecular films

“…4 Smit et al studied monolayer organic field-effect transistors (FET) fabricated by under-etching gold electrodes separating a SiO 2 dielectric covered by self-assembled monolayer (SAM) of silanes that extended underneath the electrodes. 5 In another study, Cao et al 6,7 used graphene to establish electrical contact with the organic layers. An alternative method to create non-perturbative contacts is to embed metal electrodes in the surface of an insulator to create a flat, co-planar metal-insulator-metal substrate over which an organic film can be deposited to form the channel material of an FET.…”