Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

Mesoscopic nanomaterials generated by interfering femtosecond laser processing

Mesoscopic nanomaterials with a size ranging from a few nanometers to hundreds of nanometers were generated on a metallic thin film by interfering femtosecond (fs) laser processing. Because the nanomaterials seemed to demonstrate liquid behavior, we named their structure nano water drop. The structure is considerably similar to that of a real water drop observed with a high-speed camera. Using the phase shift between interfering beams, a duplicate structure was generated. This is a new surface modification technique, using a top-down approach.     

Nickel catalyzed Grob fragmentation: Omega-dienyl aldehydes synthesis

With a proper choice of phosphane ligands, a Ni(cod)2-phosphane catalyst promotes decarboxylative ring-opening reaction of a wide structural variety of cyclic carbonates to give omega-dienyl aldehydes in good yields.     

Enantioselective synthesis mediated by chiral crystal of achiral hippuric acid in conjunction with asymmetric autocatalysis

Enantiomorphous crystals composed of achiral hippuric acid, i.e., naturally occurring N-benzoylglycine, have been used successfully as chiral inducers in enantioselective synthesis in combination with asymmetric autocatalysis to afford the product with extremely high enantiomeric excess.     

Structural study of NO adsorbed on the reconstructed Pt(110)-(1 x 2) surface with X-ray photoelectron diffraction and near-edge X-ray absorption fine structure spectroscopy

The adsorption structure of NO on the reconstructed Pt(110)-(1 x 2) surface was studied with X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low-energy scanned-angle photoelectron diffraction (LESA-PD), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experiments were performed at 180 K, where no surface lifting from (1 x 2) to (1 x 1) takes place after NO adsorption. XPS indicates that the (1 x 2) unit cell of the Pt(110) surface contains 1.5 NO molecules at the saturated coverage. XPD and LESA-PD analyses allow us to propose a structural model for the NO adlayer, where two-thirds of the NO molecules in the (1 x 2) unit cell are adsorbed on the atop site of the close-packed Pt rows (ridges) along the [10] direction with an inclined geometry and one-third of the NO molecules adsorb on the bridge site between the Pt ridges with an upright configuration. This model is supported by the N K-edge NEXAFS experiments and is consistent with the recently reported model based on the density functional theory (Orita, H.; Nakamura, I.; Fujitani, T. J. Phys. Chem. B 2005, 109, 10312).     

Recent topics in the syntheses of β-keto carboxylic acids and the derivatives

β-Keto carboxylic acids are key intermediates in organic syntheses, used for the development of fine chemicals, natural products, and various biologically relevant molecules. Their utilities stem from the structural features and facile bond formations, e.g., asymmetric reduction of carbonyl groups for the synthesis of β-hydroxy carboxylic acids and conjugated addition reactions through decarboxylative enolate nucleophiles, which utilize the amphiphilic reactivity of β-keto carboxylic acids. Despite their versatility and utilities, development of efficient and straightforward synthetic methods for β-keto carboxylic acids has not attracted considerable attention owing to their instability. As efficient synthetic strategies for β-keto carboxylic acids and their derivatives, reactions of α-diazoesters, acylation of malonate anions, cross-coupling reactions, and CO₂ insertion reactions are summarized in this review.     

N=8 Armchair Graphene Nanoribbons: Solution Synthesis and High Charge Carrier Mobility**

Structurally defined graphene nanoribbons (GNRs) have emerged as promising candidates for nanoelectronic devices. Low band gap (<1 eV) GNRs are particularly important when considering the Schottky barrier in device performance. Here, we demonstrate the first solution synthesis of 8-AGNRs through a carefully designed arylated polynaphthalene precursor. The efficiency of the oxidative cyclodehydrogenation of the tailor-made polymer precursor into 8-AGNRs was validated by FT-IR, Raman, and UV/Vis-near-infrared (NIR) absorption spectroscopy, and further supported by the synthesis of naphtho[1,2,3,4-ghi]perylene derivatives ( 1 and 2 ) as subunits of 8-AGNR , with a width of 0.86 nm as suggested by the X-ray single crystal analysis. Low-temperature scanning tunneling microscopy (STM) and solid-state NMR analyses provided further structural support for 8-AGNR . The resulting 8-AGNR exhibited a remarkable NIR absorption extending up to ∼2400 nm, corresponding to an optical band gap as low as ∼0.52 eV. Moreover, optical-pump TeraHertz-probe spectroscopy revealed charge-carrier mobility in the dc limit of ∼270 cm² V⁻¹ s⁻¹ for the 8-AGNR .     

Chameleon reactivity of the allene bond of 4-vinylidene-2-oxazolidinone: novel through-space conjugative nucleophilic addition of electron-rich alkenes and hetero-nucleophiles

The Calpha==Cbeta double bond of allene carbamates 1 serves as an electron acceptor similar to the double bond of conjugated enones by means of a through-space interaction with the N--SO2 bond; the carbamate double bond is thus subject to nucleophilic addition for a wide variety of nucleophiles, which proceeds under mild conditions by heating at 70-100 degrees C. Depending on the kind of nucleophiles, 1 displays three different reaction modes: 1) Typically enol ethers and allylsilanes promote 1,3-sulfonyl migration of 1 and undergo the inverse electron demand Diels-Alder reaction with the 1-aza-1,3-butadiene intermediates II thus formed to furnish bicyclic 2-alkoxy-5-sulfonyltetrahydropyridines 2 and 2-silylmethyl-5-sulfonyltetrahydropyridines 3, respectively, with high regio- and stereoselectivity and retention of configuration of the double bonds of these electron-rich alkenes; 2) silanes (RnSiH4-n, n=1-3) and thiols deliver the hydride and the thiolate at the Cbeta carbon and promote the 1,3-sulfonyl migration, followed by protonation of the thus-formed carbamate anion (Z)-III to provide, for example, (Z)-4 a and (Z)-4 j, respectively; 3) alcohols simply add to the Calpha==Cbeta double bond and provide (E)-6. Usually, the reaction with alcohols is accompanied by the second pathway, giving rise to, for example, (Z)-4 b in addition to (E)-6 b. Phenol engages in the third pathway and provides (E)-6 g exclusively. Heteroaromatics, such as furans and benzofurans follow the first pathway, however, in a different regioselectivity from enol ethers and allylsilanes, delivering the oxygen atom at the 3-position of 5-sulfonyltetrahydropyridines (2 g and 2 h). Indoles, on the other hand, show a dichotomy, equally enjoying the first and the third pathways and provide mixtures of (E)-7 and (E)-8, respectively.