Zhikun Zhang

Formal carbon insertion of N-tosylhydrazone into B-B and B-Si bonds: gem-diborylation and gem-silylborylation of sp3 carbon.

A convenient method is developed to synthesize 1,1-diboronates from the corresponding N-tosylhydrazones. This method is also applicable to synthesize 1-silyl-1-boron compounds. Meanwhile, derivatization and consecutive Pd-catalyzed cross-coupling reactions with 1,1-boronates were explored, demonstrating the synthetic potential of 1,1-diboronates.

Cu(I)-Catalyzed Cross-Coupling of Terminal Alkynes with Trifluoromethyl Ketone N-Tosylhydrazones: Access to 1,1-Difluoro-1,3-enynes

C-C Bond formation and β-F elimination have been achieved in a Cu(I)-catalyzed cross-coupling reaction of terminal alkynes and trifluoromethyl ketone N-tosylhydrazones. The reaction represents an efficient synthesis of 1,1-difluoro-1,3-enyne derivatives. Mechanistically, the migratory insertion of the copper carbene intermediate leads to the C-C bond formation, which is followed by C-F bond cleavage.

High-performance n-type carbon nanotube field-effect transistors with estimated sub-10-ps gate delay

High-performance top-gated n-type single-walled carbon nanotube (CNT) field-effect transistors (FETs) have been fabricated using scandium contacts and HfO2 gate oxide and are benchmarked against the state-of-the-art n-type Si metal-oxide semiconductor FETs. Two key device metrics, the intrinsic gate-delay (CV∕I) and energy-delay product (CV∕I⋅CV2) per unit width, of the n-type CNT FETs are found to show significant improvement over the Si devices. In particular, the gate-delay time is estimated to be 2.1ps for an n-type CNT FET which is based on a CNT with a diameter of 1.1nm and a channel length of 220nm.

Reaction of Diazo Compounds with Difluorocarbene: An Efficient Approach towards 1,1‐Difluoroolefins

A transition-metal-free difluoromethylenation of diazo compounds that proceeds under mild conditions has been developed and is based on the use of TMSCF2 Br as the difluoromethylene source and tetrabutylammonium bromide (TBAB) as the promoter. The chemoselective formal carbene dimerization reaction is achieved owing to the electronic properties and the relative stability of the difluorocarbene intermediate.

Palladium(0)-Catalyzed Cross-Coupling of 1,1-Diboronates with Vinyl Bromides and 1,1-Dibromoalkenes†

Palladium-catalyzed cross-coupling reactions of 1,1-diboronates with vinyl bromides and dibromoalkenes were found to afford 1,4-dienes and allenes, respectively. These reactions utilize the high reactivities of both 1,1-diboronates and allylboron intermediates generated in the initial coupling.

Catalytic asymmetric trifluoromethylthiolation via enantioselective [2,3]-sigmatropic rearrangement of sulfonium ylides

The trifluoromethylthio (SCF3) functional group has been of increasing importance in drug design and development as a consequence of its unique electronic properties and high stability coupled with its high lipophilicity. As a result, methods to introduce this highly electronegative functional group have attracted considerable attention in recent years. Although significant progress has been made in the introduction of SCF3 functionality into a variety of molecules, there remain significant challenges regarding the enantioselective synthesis of SCF3-containing compounds. Here, an asymmetric trifluoromethylthiolation that proceeds through the enantioselective [2,3]-sigmatropic rearrangement of a sulfonium ylide generated from a metal carbene and sulfide (Doyle–Kirmse reaction) has been developed using chiral Rh(II) and Cu(I) catalysts. This transformation features mild reaction conditions and excellent enantioselectivities (up to 98% yield and 98% e.e.), thus providing a unique, highly efficient and enantioselective method for the construction of C(sp3)–SCF3 bonds bearing chiral centres. The asymmetric Doyle–Kirmse reaction using chiral Rh(II)- or Cu(I)-catalysts provides SCF3-containing compounds in a highly efficient and enantioselective manner. The reaction proceeds through enantioselective formation of sulfonium ylide from a diazoester and allyl- or propargyl trifluoromethyl sulfide, followed by concerted [2,3]-sigmatropic rearrangement with the transfer of chirality from sulfur to carbon.

Fabrication of high performance top-gate complementary inverter using a single carbon nanotube and via a simple process

High performance complementary inverters have been fabricated using single-walled carbon nanotubes. The Al2O3 top-gate dielectric is grown via first depositing an Al film followed by complete oxidation of the film. It is shown that the quality of the Al2O3 film can be significantly improved by annealing at 400°C, and stable p-type and n-type carbon nanotube field-effect transistors (CNTFETs) may be fabricated using either Pd (p-type) or Al (n-type) electrodes. High performance complementary inverter is demonstrated by integrating the p-type and n-type CNTFETs on the same carbon nanotube, and a gain of about 3.5 is achieved.

Analytical analysis of heat conduction in a suspended one-dimensional object

An analytical solution is given for the self-heating conduction equation of a suspended one-dimensional (1D) object. The conductivity of the 1D object is given by combining Umklapp and second-order three-phonon processes. Using this analytical solution, several relations among some important parameters are discussed and are shown to be consistent with existing experimental results. A method to retrieve the coefficients for thermal conductivity is proposed for a general thermal conductor without knowing the detailed temperature profile along the 1D object.

Switching electron current in a semiconductor nanowire via controlling the carrier injection from the electrode

Well characterized in situ electron field-emission measurements have been made on individual ZnO nanowires (NWs) inside a transmission electron microscope. It is found that the electron field-emission current from a semiconductor NW is determined not only by the NW/vacuum interface, but also by the Schottky barrier formed at the electrode/NW interface. It is demonstrated that the electron injection efficiency through the Schottky barrier and therefore the final electron emission current can be modulated by electronic excitations in the metal electrode, and it is proposed that this phenomenon could be used to design Schottky barrier switches for nanoelectronics.

Measuring the electrical characteristics of individual junctions in the SnO2 capped ZnO nanowire arrays on Zn substrate

Direct measurements on electrical characteristics have been carried out in situ inside a scanning electron microscope using a multiple nanoprobe system on individual SnO2 capped ZnO nanowires (NWs) within a NW film on a Zn substrate. It is shown that while good Ohmic contacts can be made at Zn–ZnO NW and ZnO NW–SnO2 cap (when heavily doped with Zn) junctions, the overall I-V characteristics of the Zn–ZnO–SnO2 junction system differ significantly among different NWs, suggesting doping inhomogeneity in the NW film.