The ionization energy of α-sexithiophene and p-sexiphenyl in 2D and 3D thin films grown on silicon oxide surfaces.

The ionization energy (IE) of molecular thin films is sensitive to the molecular environments surrounding ionized molecules. In this study, we investigate the evolution of the IE on dimension-controlled 2D and 3D thin films of rod-like molecules, namely α-sexithiophene (α-6T) and p-sexiphenyl (p-6P), grown on SiO2 surfaces via photoelectron yield spectroscopy. In 2D thin films, the IE of α-6T and p-6P showed nearly constant values of 4.93 eV and 5.50 eV, respectively. In 3D thin films, however, with increasing coverage, the IE of the α-6T thin films gradually decreased to 4.84 eV at 15 ML (ML: monolayer) and the IE of the p-6P thin films gradually increased to 5.65 eV at 7 ML. These experimental findings underline the less significant impact of the domain size on the IE in 2D thin films and the significant impact of the neighboring layers on the IE in 3D thin films.

Evolution of the Ionization Energy in Two- and Three-Dimensional Thin Films of Pentacene Grown on Silicon Oxide Surfaces.

The ionization energy (IE) of pentacene in two- and three-dimensional (2D and 3D) thin films and its evolution with coverage were studied via photoelectron yield spectroscopy in ambient conditions. In the 2D thin films, the IE of pentacene was found to be nearly constant at 4.91 eV, irrespective of its island size, for an average island size exceeding 1.6 × 104 nm2. In the 3D thin films, however, a reduction in IE by 0.04 eV was clearly observed upon stacking an additional molecular layer on top of the monolayer film, and the IE decreased to 4.73 eV at 20 monolayers. These experimental findings demonstrate the IE evolution in the buried layers of the 3D thin films and the significant impact of the neighboring molecular layers on the IE in layered systems with molecular aggregation.

Rotation in an Enantiospecific Self-Assembled Array of Molecular Raffle Wheels.

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system-a rigid kagome network with each pore occupied by a guest molecule-employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.

Structural, Electronic, and Magnetic Properties of Cobalt Tetrakis (Thiadiazole) Porphyrazine Molecule Films on Au(111).

We investigated the structural and electronic/spin configurations of a film of the Co tetrakis(1,2,5-thiadiazole) porphyrazine (CoTTDPz) molecule adsorbed on the Au(111) surface by a scanning tunneling microscope (STM). CoTTDPz has a structure similar to that of the Co phthalocyanine molecule, but the benzene ring of the isoindole of the phthalocyanine molecule is replaced by the pentagon moiety of 1,2,5-thiadiazoles that has an S atom at the apex. We find an ordered molecular lattice with a threefold symmetry where a nearest-neighbor distance of 1.30 nm was measured, which is significantly smaller than that observed for the metal Pc molecule. The unit cell of the lattice contains two molecules that are rotated by 60° relative to each other. With the configuration achieved by this rotation, the neighboring molecules can form a stronger interaction through bonding between the S atom at the apex of one molecule and the N atom of the other (the N atom that is bridging the thiadiazoles). The strong interaction between the molecule and the substrate appears in the spin state examined by the detection of the Kondo resonance, which is formed by the screening of an isolated spin by the conduction electron. Even though the existence of the spin was confirmed for the bulk and thick films of this molecule, no Kondo features are detected for the molecules in the first, second, and third layers of the films. However, the isolated molecule in the third layer showed an intriguing combination of the Kondo feature and an inelastic excitation feature caused by a spin-flip process.

Enhanced magnetic spin-spin interactions observed between porphyrazine derivatives on Au(111).

Magnetic molecules are of interest for application in spintronic and quantum-information processing devices. Therein, control of the interaction between the spins of neighboring molecules is the critical issue. Substitution of outer moieties of the molecule can tune the molecule-molecule interaction. Here we show a novel spin behavior for a magnetic molecule of vanadyl tetrakis (thiadiazole) porphyrazine (abbreviated as VOTTDPz) adsorbed on Au(111), which is modified from vanadyl phthalocyanine (VOPc) by replacing the inert phthalocyanine ligand with a reactive thiadiazole moiety. The magnetic properties of the molecules are examined by observing the Kondo resonance caused by the screening of an isolated spin by conduction electrons using scanning tunneling spectroscopy. The Kondo features are detected at the molecule whose shape and intensity show site-dependent variation, revealing complex spin-spin interactions due to the enhanced interaction between molecules, originating from the functionalization of the ligand with a more reactive moiety.

Ionic liquid thin layer-induced memory effects in organic field-effect transistors.

We examined the morphologies and structures of pentacene and C60 thin films grown on thin layers of an ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), and found that the characteristics of the films depended significantly on the thickness of DEME-TFSI. In addition, we fabricated organic field-effect transistors (OFETs) of pentacene and C60 in which a thin layer of DEME-TFSI was inserted between the organic semiconductor (pentacene or C60) and the gate insulating layer, and measured their performance in situ. We found that 1.5-2 ML (ML: monolayer) DEME-TFSI produced a large hysteresis loop in the transfer characteristics in these OFETs, but 5 ML DEME-TFSI resulted in the formation of normally-on states with far smaller memory effects. The curvatures of the hysteresis loops were caused by the formation of trap states induced by the DEME-TFSI layers. This novel technique provides a simple tool for creating hysteresis behavior and could potentially be applied to transistor memory devices.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1'-yl-dithiadiazolyl.

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C16H9CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m2 at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

Highly-oriented molecular arrangements and enhanced magnetic interactions in thin films of CoTTDPz using PTCDA templates.

In the present work, the templating effect of thin layers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on the growth of cobalt tetrakis(thiadiazole)porphyrazine (CoTTDPz) thin films was examined. X-ray diffraction and optical absorption spectra indicate that while CoTTDPz forms amorphous thin films on the bare substrates, it forms crystalline thin films on the PTCDA templates, in which the molecular planes of CoTTDPz are considered to be parallel to the substrates. Magnetic measurements reveal a significantly enhanced antiferromagnetic interaction of CoTTDPz in the templated thin films, with values reaching over 13 K. The ability to generate crystalline films and to control their orientation using molecular templates is an important strategy in the fields of organic electronics and spintronics in order to tailor the physical properties of organic thin films to suit their intended application.

Anisotropic thermal expansion and cooperative Invar and anti-Invar effects in mn alloys.

We have investigated thermal expansion of a tetragonal Mn(88)Ni(12) alloy by x-ray diffraction, Mn and Ni K-edge extended x-ray-absorption fine-structure spectroscopy, and the computational simulations based on the path-integral effective-classical-potential theory. It is found from the x-ray diffraction that the tetragonal lattice constant c exhibits almost no thermal expansion like an Invar alloy, while the lattice constant a shows even larger thermal expansion than usually expected from anharmonicity, implying significant anisotropy in thermal expansion. The extended x-ray-absorption fine-structure reveals that the Mn local environment is actually tetragonally distorted, while the Ni one retains its inherent cubiclike symmetry. Combined with the computational simulations, it is concluded that large thermal expansion along the a axis originates from the anti-Invar effect, while negligibly small thermal expansion along the c axis originates from the cooperative Invar effect. Namely, the tetragonally distorted more stable antiferromagnetic Mn state gives a significantly smaller (slightly longer) atomic radius along the a (c) axis than the radius of the spherical paramagnetic state.

Anharmonicity and quantum effects in thermal expansion of an Invar alloy.

We have investigated the anharmonicity and quantum effects in the Invar alloy Fe(64.6)Ni(35.4) that shows anomalously small thermal expansion. We have performed Fe and Ni K-edge extended x-ray-absorption fine-structure spectroscopic measurements and the computational simulations based on the path-integral effective-classical-potential theory. The first nearest-neighbor (NN) shells around Fe show almost no thermal expansion, while those around Ni exhibit meaningful but smaller expansion than that of fcc Ni. At low temperature, the quantum effect is found to play an essentially important role, which is confirmed by comparing the quantum-mechanical simulations to the classical ones. The anharmonicity (asymmetric distribution) clearly exists for all the first NN shells as in normal thermal expansion systems, implying the breakdown of the direct correspondence between thermal expansion and anharmonicity.