Effect of water layer at the SiO2/graphene interface on pentacene morphology

Nanoscale electronic transport at graphene/pentacene van der Waals interfaces

We report a study on the relationship between the structure and electron transport properties of nanoscale graphene/pentacene interfaces. We fabricated graphene/pentacene interfaces from 10 to 30 nm thick needle-like pentacene nanostructures down to two-three layer (2L-3L) dendritic pentacene islands, and we measured their electron transport properties by conductive atomic force microscopy (C-AFM). The energy barrier at the interfaces, i.e., the energy position of the pentacene highest occupied molecular orbital (HOMO) with respect to the Fermi energy of graphene and the C-AFM metal tip was determined and discussed with an appropriate electron transport model (a double Schottky diode model and a Landauer-Buttiker model, respectively) taking into account the voltage-dependent charge doping of graphene. In both types of samples, the energy barrier at the graphene/pentacene interface is slightly larger than that at the pentacene/metal tip interface, resulting in 0.47-0.55 eV and 0.21-0.34 eV, respectively, for the 10-30 nm thick needle-like pentacene islands, and 0.92-1.44 eV and 0.67-1.05 eV, respectively, for the 2L-3L thick dendritic pentacene nanostructures. We attribute this difference to the molecular organization details of the pentacene/graphene heterostructures, with pentacene molecules lying flat on graphene in the needle-like pentacene nanostructures, while standing upright in the 2L-3L dendritic islands, as observed from Raman spectroscopy.

The Effect of C60 and Pentacene Adsorbates on the Electrical Properties of CVD Graphene on SiO2

Graphene is an excellent 2D material for vertical organic transistors electrodes due to its weak electrostatic screening and field-tunable work function, in addition to its high conductivity, flexibility and optical transparency. Nevertheless, the interaction between graphene and other carbon-based materials, including small organic molecules, can affect the graphene electrical properties and therefore, the device performances. This work investigates the effects of thermally evaporated C60 (n-type) and Pentacene (p-type) thin films on the in-plane charge transport properties of large area CVD graphene under vacuum. This study was performed on a population of 300 graphene field effect transistors. The output characteristic of the transistors revealed that a C60 thin film adsorbate increased the graphene hole density by (1.65 ± 0.36) × 10¹² cm^-2, whereas a Pentacene thin film increased the graphene electron density by (0.55 ± 0.54) × 10¹² cm^-2. Hence, C60 induced a graphene Fermi energy downshift of about 100 meV, while Pentacene induced a Fermi energy upshift of about 120 meV. In both cases, the increase in charge carriers was accompanied by a reduced charge mobility, which resulted in a larger graphene sheet resistance of about 3 kΩ at the Dirac point. Interestingly, the contact resistance, which varied in the range 200 Ω-1 kΩ, was not significantly affected by the deposition of the organic molecules.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Thin film growth of aromatic rod-like molecules on graphene

Research on graphene (Gr) is a vastly expanding field due to its potential for technological applications. Its close structural and chemical relationship to conjugated organic molecules makes it a superior candidate as a transparent electrode material in organic electronics and optoelectronics. The growth of organic thin films-intensively investigated in the past few decades-has demonstrated the complexity in growth and nucleation processes arising from the anisotropy and spatial extension of the molecular building blocks. Choosing the small, conjugated rod-like molecules para-hexaphenyl and pentacene as model representatives for small organic molecules, we review recent findings in organic thin film growth on a variety of Gr substrates. Special attention is paid to the differences in the resulting growth arising from the various methods of Gr fabrication and support that affect both the Gr-molecule interfacing and the involved molecular diffusion processes.

A pentacene monolayer trapped between graphene and a substrate

A self-assembled pentacene monolayer can be fabricated between the solid-solid interface of few-layered graphene (FLG) and the mica substrate, through a diffusion-spreading method. By utilizing a transfer method that allows us to sandwich pentacene between graphene and mica, followed by controlled annealing, we enabled the diffused pentacene to be trapped in the interfaces and led to the formation of a stable monolayer. We found that the formation of a monolayer is kinetically favored by using a 2D Ising lattice gas model for pentacene trapped between the graphene-substrate interfaces. This kinetic Monte Carlo simulation results indicate that, due to the graphene substrate enclosure, the spreading of the first layer proceeds faster than the second layer, as the kinetics favors the filling of voids by molecules from the second layer. This graphene assisted monolayer assembly method provides a new avenue for the fabrication of two-dimensional monolayer structures.