Two-dimensional structures of pyrimido[5,4-d]pyrimidine derivatives at solid/liquid interface

Two-dimensional structures of pyrimido[5,4-d]pyrimidine derivatives (PD) were studied by scanning tunneling microscopy (STM) at solid/liquid interface. In order to tune the intervals of functional unit (hydrogen bonding site) in the molecule, the PD with different numbers and length of alkyl chain were designed and synthesized. STM observation at highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface revealed that the PD with four alkyl chains formed a columnar structure, and the alkyl chains were not interdigitated. By contrast, the PD with two alkyl chains formed similar columnar structure, whereas the alkyl chains were interdigitated. These structural features of the PD indicates that the intervals of the functional unit, i.e., hydrogen bonding sites in the PD can be controlled by changing not only the length but also the number of alkyl chains.     

Radial breathing modes of single-walled carbon nanotubes in resonance Raman spectra at high temperature and their chiral index assignment

Radial breathing modes (RBMs) in resonance Raman spectra from single-walled carbon nanotubes (SWCNTs) on a SiO₂/Si (0 0 1) substrate are studied between 25 and 720 °C. A change in the relative intensity of each RBM peak with temperature is observed, which originates from the temperature dependence of the resonance condition of nanotubes. For 25 °C, each RBM peak is reasonably assigned on the basis of data in the literature [J. Maultzsch, H. Telg, S. Reich, F. Hennrich, C. Thomsen, Phys. Rev. B 72 (2005) 205438]. By taking into account the temperature-dependent behavior of the relative intensity of the RBM peaks, each RBM peak is successfully assigned even for 720 °C. It is found that most of the observed RBM peaks for a laser excitation energy of E_exc = 1.96 eV are from chiral SWCNTs. These results make it possible to discuss further details of the chirality-dependent growth behavior observed for in situ Raman spectroscopy.     

Modeling tip performance for combined STM-luminescence and aperture-SNOM scanning probe: Spatial resolution and collection efficiency

Finite-element simulations of the performance of the tip intended for use in combined aperture-SNOM and the scanning tunneling microscope (STM)-luminescence microscopy are presented. Tip geometry and the role of the opening in the protective metal coating were addressed. It is shown that the tip shape can affect transmittance for the excitation SNOM mode by nearly two orders of magnitude and the metal coating can enhance collection efficiency for the STM-luminescence mode. Desired tip configuration can be chosen based on the interplay between the improving collection efficiency and the deteriorating spatial resolution with increasing opening size.     

Characteristics of ZnO epilayer on the post-annealed buffer layer on GaN/sapphire substrate by pulsed laser deposition

We employed epi-GaN substrates for ZnO film growth, and studied the deposition and post-annealing effects. ZnO films were grown by pulsed laser deposition (PLD) method. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were investigated and the post-annealed ZnO films grown at T _g =700^oC have very smooth surfaces and the rms with roughness is about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epilayer and GaN/sapphire substrates. It is confirmed by AFM that growth temperature of 700^oC helps the films grow in step-flow growth mode. It is observed by cathode luminescence spectrum that the ZnO film grown at 700^oC has very low visible luminescence, indicating the decrease of the deep level defects. It is also revealed by Hall measurements that carrier concentration is decreased by increasing the growth temperatures. It is suggested that low temperature buffer layer growth and post-annealing technique can be used to fabricate ZnO hetero-epitaxy.     

Nickel‐Promoted Highly Regioselective Carboxylation of Aryl Ynol Ether and Its Application to the Synthesis of Chiral β‐Aryloxypropionic Acid Derivatives

Nickel(0)‐promoted carboxylation of aryl ynol ether proceeded in a highly regioselective manner to produce α‐substituted‐β‐aryloxyacrylic acid derivatives. The α‐substituted‐β‐aryloxyacrylic acids were transformed into the corresponding β‐aryloxypropionic acid derivative as an optically active form via rhodium‐catalyzed asymmetric hydrogenation.     

Investigation of Mizoroki‐Heck coupling polymerization as a catalyst‐transfer condensation polymerization for synthesis of poly(p‐phenylenevinylene)

Mizoroki‐Heck coupling polymerization of 1,4‐bis[(2‐ethylhexyl)oxy]‐2‐iodo‐5‐vinylbenzene ( 1 ) and its bromo counterpart 2 with a Pd initiator for the synthesis of poly(phenylenevinylene) (PPV) was investigated to see whether the polymerization proceeds in a chain‐growth polymerization manner. The polymerization of 1 with ^tBu₃PPd(Tolyl)Br ( 10 ) proceeded even at room temperature when 5.5 equiv of Cy₂NMe (Cy = cyclohexyl) was used as a base, but the molecular weight distribution of PPV was broad. The polymerization of 2 hardly proceeded at room temperature under the same conditions. In the polymerization of 1 , PPV with H at one end and I at the other was formed until the middle stage, and the polymer end groups were converted into tolyl and H in the final stage. The number‐average molecular weight (M_n) did not increase until about 90% monomer conversion and then sharply increased after that, indicating conventional step‐growth polymerization. The occurrence of step‐growth polymerization, not catalyst‐transfer chain‐growth polymerization, may be interpreted in terms of low coordination ability of H‐Pd(II)‐X(^tBu₃P) (X = Br or I), formed in the catalytic cycle of the Mizoroki‐Heck coupling reaction, to π‐electrons of the PPV backbone; reductive elimination of H‐X from this Pd species with base would take place after diffusion into the reaction mixture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 543–551