Charge-Transfer-Controlled Growth of Organic Semiconductor Crystals on Graphene

Self-Assembly of Organic Semiconductors on Strained Graphene under Strain-Induced Pseudo-Electric Fields

Graphene is used as a growth template for van der Waals epitaxy of organic semiconductor (OSC) thin films. During the synthesis and transfer of chemical-vapor-deposited graphene on a target substrate, local inhomogeneities in the graphene-in particular, a nonuniform strain field in the graphene template-can easily form, causing poor morphology and crystallinity of the OSC thin films. Moreover, a strain field in graphene introduces a pseudo-electric field in the graphene. Here, the study investigates how the strain and strain-induced pseudo-electric field of a graphene template affect the self-assembly of π-conjugated organic molecules on it. Periodically strained graphene templates are fabricated by transferring graphene onto an array of nanospheres and then analyzed the growth and nucleation behavior of C60 thin films on the strained graphene templates. Both experiments and a numerical simulation demonstrated that strained graphene reduced the desorption energy between the graphene and the C60 molecules and thereby suppressed both nucleation and growth of the C60. A mechanism is proposed in which the strain-induced pseudo-electric field in graphene modulates the binding energy of organic molecules on the graphene.

Intermolecular and Electrode-Molecule Bonding in a Single Dimer Junction of Naphthalenethiol as Revealed by Surface-Enhanced Raman Scattering Combined with Transport Measurements

Electron transport through noncovalent interaction is of fundamental and practical importance in nanomaterials and nanodevices. Recent single-molecule studies employing single-molecule junctions have revealed unique electron transport properties through noncovalent interactions, especially those through a π-π interaction. However, the relationship between the junction structure and electron transport remains elusive due to the insufficient knowledge of geometric structures. In this article, we employ surface-enhanced Raman scattering (SERS) synchronized with current-voltage (I-V) measurements to characterize the junction structure, together with the transport properties, of a single dimer and monomer junction of naphthalenethiol, the former of which was formed by the intermolecular π-π interaction. The correlation analysis of the vibrational energy and electrical conductance enables identifying the intermolecular and molecule-electrode interactions in these molecular junctions and, consequently, addressing the transport properties exclusively associated with the π-π interaction. In addition, the analysis achieved discrimination of the interaction between the NT molecule and the Au electrode of the junction, i.e., Au-π interactions through-π coupling and though-space coupling. The power density spectra support the noncovalent character at the interfaces in the molecular junctions. These results demonstrate that the simultaneous SERS and I-V technique provides a unique means for the structural and electrical investigation of noncovalent interactions.

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C₆₀ molecular crystals. Transmission electron microscopy and theoretical analysis of the C₆₀ /BP vdW heterostructure clearly confirm that the BP template results in oriented C₆₀ assembly with higher-order commensurism. Lateral and vertical devices with C₆₀ /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C₆₀ /BP junctions. Effective tuning of the C₆₀ /BP junction barrier from 0.2 to 0.5 eV and maximum on-current density higher than 10⁴  mA cm^-2 are achieved with graphite/C₆₀ /BP vertical vdW transistors. Due to the formation of high-quality C₆₀ film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W^-1 as well as a fast response time under visible light illumination.

van der Waals Epitaxy of Organic Semiconductor Thin Films on Atomically Thin Graphene Templates for Optoelectronic Applications

ConspectusOrganic semiconductors (OSCs) offer unique advantages with respect to mechanical flexibility, low-cost processing, and tunable properties. The optical and electrical properties of devices based on OSCs can be greatly improved when an OSC is coupled with graphene in a certain manner. Our research group has focused on using graphene as a growth template for OSCs and incorporating such high-quality heterostructures into optoelectronic devices. The idea is that graphene's atomically flat surface with a uniform sp² carbon network can serve as a perfect quasi-epitaxial template for the growth of OSCs. In addition, OSC-graphene heterostructures benefit from graphene's unique characteristics, such as its high charge-carrier mobility, excellent optical transparency, and fascinating mechanical durability and flexibility.However, we have often found that OSC molecules assemble on graphene in unpredictable manners that vary from batch to batch. From observations of numerous research systems, we elucidated the mechanism underlying such poor repeatability and set out a framework to actually control the template effect of graphene on OSCs. In this Account, we not only present our scientific findings in this spectrum of areas but also convey our research scheme to the readers so that similar heterostructure complexes can be systematically studied.We began with experiments showing that the growth of OSCs on a graphene surface was driven by van der Waals interactions and is therefore sensitive to the cleanliness of the graphene surface. Nonetheless, we noted that, even on similarly clean graphene surfaces, the OSC thin film still varied with the underlying substrate. Thanks to the graphene-transfer method and in situ gating methods that we developed, we discovered that the decisive parameter for molecule-graphene interaction (and, hence, for the growth of OSCs on graphene) is the charge density in the graphene. Thus, to prepare a graphene template for high-quality graphene-OSC heterostructures, we controlled the charge density in the graphene to minimize the molecule-graphene interaction. Moreover, the possible charge transfer between OSC molecules and graphene, which induces additional molecule-graphene interactions, should also be taken into account. Eventually, we demonstrated a wide range of optoelectronic applications that benefitted from high-quality OSC-graphene heterostructures fabricated using our proof-of-concept systems.

Significant Performance Improvement in n-Channel Organic Field-Effect Transistors with C60 :C70 Co-Crystals Induced by Poly(2-ethyl-2-oxazoline) Nanodots

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C₆₀ -I_h )[5,6]fullerene (C₆₀ ) and (C₇₀ -D_5h(6) )[5,6]fullerene (C₇₀ ) blend films, with an increase from 0.1 to 2.1 cm² V^-1 s^-1 . The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C₆₀ :C₇₀ co-crystal structure within the blend films.

Multiscale modeling of structure formation of C60 on insulating CaF2 substrates

Morphologies of adsorbed molecular films are of interest in a wide range of applications. To study the epitaxial growth of these systems in computer simulations requires access to long time and length scales, and one typically resorts to kinetic Monte Carlo (KMC) simulations. However, KMC simulations require as input transition rates and their dependence on external parameters (such as temperature). Experimental data allow only limited and indirect access to these rates, and models are often oversimplified. Here, we follow a bottom-up approach and aim at systematically constructing all relevant rates for an example system that has shown interesting properties in experiments, buckminsterfullerene on a calcium fluoride substrate. We develop classical force fields (both atomistic and coarse-grained) and perform molecular dynamics simulations of the elementary transitions in order to derive explicit expressions for the transition rates with a minimal number of free parameters.

Lowering the Contact Barriers of 2D Organic F16 CuPc Field-Effect Transistors by Introducing Van der Waals Contacts

2D organic crystals exhibit efficient charge transport and field-effect characteristics, making them promising candidates for high-performance nanoelectronics. However, the strong Fermi level pinning (FLP) effect and large Schottky barrier between organic semiconductors and metals largely limit device performance. Herein, by carrying out temperature-dependent transport and Kelvin probe force microscopy measurements, it is demonstrated that the introducing of 2D metallic 1T-TaSe₂ with matched band-alignment as electrodes for F₁₆ CuPc nanoflake filed-effect transistors leads to enhanced field-effect characteristics, especially lowered Schottky barrier height and contact resistance at the contact and highly efficient charge transport within the channel, which are attributed to the significantly suppressed FLP effect and appropriate band alignment at the nonbonding van der Waals (vdW) hetero-interface. Moreover, by taking advantage of the improved contact behavior with 1T-TaSe₂ contact, the optoelectronic performance of F₁₆ CuPc nanoflake-based phototransistor is drastically improved, with a maximum photoresponsivity of 387 A W^-1 and detectivity of 3.7 × 10¹⁴ Jones at quite a low V_ds of 1 V, which is more competitive than those of the reported organic photodetectors and phototransistors. The work provides an avenue to improve the electrical and optoelectronic properties of 2D organic devices by introducing 2D metals with appropriate work function for vdW contacts.

Solution-Processed, Large-Area, Two-Dimensional Crystals of Organic Semiconductors for Field-Effect Transistors and Phototransistors

Organic electronics with π-conjugated organic semiconductors are promising candidates for the next electronics revolution. For the conductive channel, the large-area two-dimensional (2D) crystals of organic semiconductors (2DCOS) serve as useful scaffolds for modern organic electronics, benefiting not only from long-range order and low defect density nature but also from unique charge transport characteristic and photoelectrical properties. Meanwhile, the solution process with advantages of cost-effectiveness and room temperature compatibility is the foundation of high-throughput print electrical devices. Herein, we will give an insightful overview to witness the huge advances in 2DCOS over the past decade. First, the typical influencing factors and state-of-the-art assembly strategies of the solution-process for large-area 2DCOS over sub-millimeter even to wafer size are discussed accompanying rational evaluation. Then, the charge transport characteristics and contact resistance of 2DCOS-based transistors are explored. Following this, beyond single transistors, the p-n junction devices and planar integrated circuits based on 2DCOS are also emphasized. Furthermore, the burgeoning phototransistors (OPTs) based on crystals in the 2D limits are elaborated. Next, we emphasized the unique and enhanced photoelectrical properties based on a hybrid system with other 2D van der Waals solids. Finally, frontier insights and opportunities are proposed, promoting further research in this field.