The system In-[perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)] on MoS2, prepared by the sequential evaporation of PTCDA and In on a cleaved MoS2 surface, was studied by low energy electron diffraction. The result indicates that reaction products form an ordered structure on the MoS2 surface. From the analysis of the diffraction pattern, the presence of six symmetry-equivalent domains of an oblique unit cell of In-PTCDA species results with the dimensions of 9.5 Å, 16.3 Å, and an enclosed angle of 80.2°. In addition, splitting in two domains by a mirror plane exists with the rotation angle R=±10.8° with respect to each of the three equivalent surface crystal axes of the MoS2 substrate. The new structure is explained by assuming that four In atoms are chemically bonded to the four carbonyl groups of the PTCDA molecules. Furthermore, it is concluded that the In4PTCDA species become tilted after a chemical reaction between the PTCDA molecules and the In atoms, which is in agreement with results previously obtained by angle-resolved ultraviolet photoemission experiments.

We demonstrate that both in-plane molecular rotational and tilting angles of the molecular orientation can be determined using the σ* resonance of sulfur (S) K-edge near-edge X-ray absorption fine structure. We examined dinaphtho­[2,3-b:2′,3′-f]­thieno­[3,2-b]­thiophene (DNTT) thin films on silicon dioxide (SiO2) and copper oxide (CuO x ) substrates which are relevant to the cost-effective organic field-effect transistors. In-plane directed transition moments were attributed to the electron excitation from S 1s to the largely overlapped σ* orbital related to the C–S bonding at the thiophene site. Under coexistence with a minor component of the amorphous region, it turned out that the DNTT domain with the single-crystal structure has the c-axis normal to the substrates; the DNTT molecules on SiO2 orient at a tilting angle (β) of 85° and an in-plane rotational angle (ΦM) of 77 and 103°. In the case of the film on CuO x , values of 84° for β and 73 and 107° for ΦM are acquired. The amount of the amorphous region in the film on CuO x was larger than that on SiO2. Our approach is applicable to other sulfur-containing molecules with a small population of the lowest unoccupied molecular orbital on the S atoms.

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The molecular orientation of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) thin films on modified silicon dioxide (SiO2) and copper oxide (CuOx) substrates whose surfaces were treated by 6-[(3-trisilanol)propylamino]-1,3,5-triazine-2,4-bis(2-aminoethylamine) (TAS) was investigated using near-edge X-ray absorption fine structure and ultraviolet/X-ray photoelectron spectroscopy. The standing orientation of DNTT was obtained in DNTT/TAS/SiO2 and DNTT/TAS/CuOx. The amount of amorphous region in DNTT/TAS/SiO2 was larger than that in DNTT/TAS/CuOx. The TAS treatment on the substrates formed interfacial dipoles at TAS/SiO2 and TAS/CuOx interfaces by chemical interaction, resulting in the vacuum level shift by −0.9 and −1.2 eV, respectively. Functional groups at the outermost TAS surface were amino groups in TAS/SiO2 and silanol groups in TAS/CuOx. The intermolecular distance of DNTT was widened in DNTT/TAS/SiO2 because of an electrostatic repulsion with nitrogen. The interfacial dipole at the modification layer/substrate interface and functional groups at the modified surface affect the molecular orientation of DNTT thin films on the modification layer.