Key role of molecular kinetic energy in early stages of pentacene island growth

Abstract

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO x /Si. We show that the kinetic energy of the impinging molecule (E k ) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing E k , the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam de- position, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E k ≥ 5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule-surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.PACS 72.80.Le · 81.15-z · 79.20Rf · 68.37.Ps · 68.55.J · 34.25.+a

Single‐Layer Pentacene Field‐Effect Transistors Using Electrodes Modified With Self‐assembled Monolayers

“…The molecular islands formed in each layer mimic exactly the geometrical shape of the larger islands underneath. [31] Pentacene islands in Figure 1a signify the typical crystal facets with regular geometric shape with very sharp and straight island edges. This implies a homoepitaxial-like growth among the intra-pentacene layers.…”

“…High crystallinity in the pentacene was achieved by carrying out the SuMBD at a kinetic energy of 6.4 eV. [31] Reference morphology analysis were performed by examination of monolayers deposited on bare silicon oxide using ex situ tapping mode atomic force microscopy (AFM) and X-ray diffraction (XRD). Prior to the deposition of pentacene on the FET substrates, the lithographically patterned gold source and drain electrodes on a standard heavily doped Si substrate with a 250 nm SiO 2 gate insulator, were covered with SAMs of thiols with opposing dipole moments.…”

Nucleation and growth of thin films of rod-like conjugated molecules

“…Wu et al have achieved five layers in the desired LbL growth for the important case of 5A on SiO x by using SuMBD [149]. Using conventional OMBD, Zhang et al showed the strain relaxation driven transition from LbL to rapid roughening after 5 layers for the plate-like molecule DIP [150].…”

Formation of intra-island grain boundaries in pentacene monolayers

“…14,28 Under our experimental conditions, the kinetic energy of 6.4 eV was achieved by properly varying the degree of seeding through changing the He carrier gas pressure.…”

“…24 The kinetic energy of the impinging molecules can thus be tuned from less than 1 eV up to several tens of eV, i.e., within a range of energies of surface processes such as elastic-inelastic scattering, physisorption, chemisorption, and activation of surface chemical reactions. Hyperthermal energies reached using supersonic beam methods have given access to controlling growth, 25 reactivity, 26 morphology and structure 27,28 of molecular thin films beyond the level achievable by OMBD.…”