Computationally Derived Rules for Persistence of C₆₀ Nanowires on Recumbent Pentacene Bilayers

Abstract

The tendency for C(60) nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 55° to 90°, motivating simulation studies of essentially the entire range of tilt angles (10°-90°) to predict the optimum surface tilt angle for C(60) nanowire formation. The persistence of a 1D nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C(60) nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for ϕ ≤ ∼60°. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range ϕ(1) = 30°-50°. Flatter pentacene surfaces, that is, tilt angles above about 60°, are subject to disorder caused by C(60) molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C(60) nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C(60) molecules to step edges.

Intermixing at the Pentacene‐Fullerene Bilayer Interface: A Molecular Dynamics Study

“…We note that the formation of terraced/stepped surfaces is determined by the surface energy, roughness, and temperature of the substrate on which the pentacene film is deposited along with deposition temperature and rates 29, 30. Our findings, consistent with the “burrowing” picture of Cantrell and Clancy,25 indicate that, when the pentacene (010) face is open to interact with C 60 , the interface can become significantly disordered and lead to intermixing between the two components. Hence, the heterojunction takes on a more graded or mixed composition8, 9, 31, 32 vs. the strict planar bilayer structure often depicted 15…”

“…Due to the relatively small size and rigid structural characteristics of these molecules, the pentacene‐C 60 interface has also undergone a number of theoretical investigations, ranging from electronic‐structure calculations on molecular complexes to atomistic molecular dynamics (MD) simulations 13–15, 23, 24–27. Regarding the latter, different configurations of pentacene and C 60 have been considered.…”

“…Regarding the latter, different configurations of pentacene and C 60 have been considered. Cantrell and Clancy23, 25 performed a series of studies of C 60 deposited on pentacene layers to examine the (anisotropic) diffusion of a single fullerene in the valleys of a corrugated pentacene landscape and to investigate the formation of C 60 nanowires along pentacene step edges; importantly, Cantrell and Clancy have shown that the thin‐film phase of pentacene has a more energetically isotropic surface than the bulk phase, which results in more ordered fullerene film growth. Muccioli et al have modeled the vapor‐phase deposition and growth of the first layers of pentacene on the C 60 (001) surface 27.…”

Deposition Growth and Morphologies of C60 on DTDCTB Surfaces: An Atomistic Insight into the Integrated Impact of Surface Stability, Landscape, and Molecular Orientation

“…As the surface coverage grows, these C 60 molecules will go deeper into the holes, and then push the DTDCTB molecules upward into the C 60 layers (see Figures S4c, S6c, and S12c,e, Supporting Information). This “burrowing” picture has been also observed recently for C 60 on pentacene(010) or DPSQ(1‐11) by Clancy and co‐workers and Brédas co‐workers We notice that while the surface energy of the most stable (100)B surface is very close to that of C 60 (001), the DTDCTB molecules do not intermix with C 60 at the (100)B‐C 60 interface, which seems to be controversial to the previous finding . On the other hand, although the surface energy of (010)B is similar to those of (100)A, (001)A, and (001)B, the morphology of C 60 on the (010)B surface is quite ordered as well.…”

Structure and Disorder in Squaraine–C60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor–Acceptor Interface

“…[ 8 ] Much of this development has been due to improvements in terms of materials packing confi gurations in these regions, [ 29,30,41 ] in particular in the presence of disorder. It is the reason why molecular dynamics (MD) simulations are increasingly used as a tool to gain further understanding of the bulk and interfacial morphologies [ 35,[42][43][44][45] with recent results highlighting the potential for intermixing and/or disorder at bilayer interfaces. Here, our goal is to employ MD simulations to investigate the bilayer squaraine-C 60 interface morphology and couple them with electronic-structure calculations to analyze how the electronic properties evolve as a function of interfacial confi gurations (see Computational Methods for full details).…”