Scanning tunneling microscopy ͑STM͒, atomic force microscopy ͑AFM͒, and near-edge x-ray absorption fine structure ͑NEXAFS͒ have been used to study the structure of tetracene films on hydrogen-passivated Si͑001͒. STM imaging of the films with nominal thickness of three monolayers ͑3 ML͒ exhibits the characteristic "herringbone" molecular packing known from the bulk crystalline tetracene, showing standing molecules on the ab plane. The dimensions and orientation of the herringbone lattice indicate a commensurate structural relationship between the lattice and the crystalline substrate. The corresponding AFM images illustrate that at and above the third layer of the films, the islands are anisotropic, in contrast with the submonolayer fractals, with two preferred growth directions appearing orthogonal to each other. The polarization dependent NEXAFS measurements indicate that the average molecular tilting angle with respect to the surface first increases with the film thickness up to 3 ML, then stabilizes at a value close to the bulk tetracene case afterwards. The combined results indicate a distinct growth morphological change that occurs around a few monolayers of thickness.

We have measured in-plane single-crystal domain changes in size and shape for a tetracene film on H/Si(001)-2×1 as a function of film thickness and substrate step density using grazing-incidence x-ray diffraction (GIXD). The x-ray results reveal that the film is commensurate with the substrate along the a axis of the tetracene lattice (i.e., the longer base vector of the a-b plane unit cell). The preferred crystalline domain growth direction is found to be along the b axis (the shorter base vector of the unit cell). The single-crystal domain size is layer dependent and orders of magnitude smaller than a typical surface island observed using atomic-force microscopy (AFM). The domain size and shape are sensitive to the substrate step density and step anisotropy. The mechanism of substrate step influence on the domain properties has been discussed.

Visualizing molecular crystalline domains and influence of substrate defects are important in understanding the charge transport in organic thin film devices. Vacuum evaporated tetracene films of four monolayers on hydrogen‐terminated Si(001)‐2 × 1 substrate, as a prototypical system, have been studied with ex situ atomic force microscopy (AFM), transverse shear microscopy (TSM), friction force microscopy (FFM), and low‐energy electron microscopy (LEEM). Two differently oriented in‐plane lattice domains are found due to the symmetry of the substrate lattice, with no visible azimuthal twist between adjacent molecular layers in surface islands, indicating significant bulk‐like crystallization in the film. Meanwhile, two types of subdomains are observed inside of each in‐plane lattice domain. The subdomains are anisotropic in shape, and their sizes and distribution are highly influenced by the substrate atomic steps. TSM and FFM measurements indicate that these subdomains result from molecule‐tilt orderings within the bulk‐like lattice domains. TSM evidently shows a sensitivity to probe vertical molecule‐tilt anisotropy for the molecular crystals, in addition to its known ability to map the lateral lattice orientations.

Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). A distinct growth morphology change that occurs around a few monolayers of film thickness was characterized. This coverage-dependent film structural phase transition leads to a molecularly ordered film structure commensurate with the crystalline substrate.

Atomic force microscopy, transverse shear microscopy, and friction force microscopy have been used to study coverage-dependent crystalline domain structures of a tetracene film on a hydrogen-passivated Si (001)-2 × 1 surface. Though submonolayer fractals present some nonepitaxial domains, the coalesced first monolayer, which possesses a partial commensurate registration with the substrate lattice, shows two lattice domains (major domains) orthogonally oriented with each other. The second-layer lattice exhibits 90° azimuthal rotation from the first-layer lattice, and the third and subsequent layers show a commensurate registration with their respective underlayers. The major-domain boundaries are not the preferable nucleation sites, indicating a potential energy barrier at the boundary. Meanwhile, the domain structure rigidity increases with the layer height until it saturates on the fourth layer, where the bulklike structure emerges with the formation of two (molecular tilting) subdomains in a major domain. The authors conclude that the structural-phase transition can take place locally wherever the fourth molecular layer emerges, and significant bulklike crystallization occurs at a nominal coverage of ∼3–4 ML.

Visualizing molecular crystalline domains and influence of substrate defects are important in understanding the charge transport in organic thin‐film devices. As a prototypical system, vacuum evaporated tetracene films of four molecular layers on hydrogen‐terminated Si(001)‐2 × 1 substrate have been studied by Tersigni et al. (article no. http://doi.wiley.com/10.1002/pssb.201600777). The three microscopic images on the cover page present the same film surface location with atomic force microscopy (AFM), transverse shear microscopy (TSM), and friction force microscopy (FFM), respectively; clearly different structural information contents are revealed. What can be retrieved from these measurements includes two differently oriented in‐plane molecular packing domains due to the substrate lattice symmetry; and two types of subdomain inside each packing domain reflecting molecular tilting orderings and the influence by the substrate atomic steps. TSM evidently shows a sensitivity to probe vertical molecule‐tilt anisotropy for the materials, in addition to its known ability to map the lateral lattice orientations.