Search PubMed⌕ Search

Biomedical subjects

Daoben Zhu

Publications and source records attributed to Daoben Zhu.

At least 19 recordsLinked to original sources
PubMed2005

n-Type field-effect transistors made of an individual nitrogen-doped multiwalled carbon nanotube.

We report on the fabrication and characterization of field-effect transistor based on an individual multiwalled nitrogen-doped carbon nanotube. Our measurements show that the N-doped carbon nanotubes have n-type properties. The contact properties of the tube and Pt electrodes are also studied in detail. Temperature dependence of two-terminal transport experiments suggests that transport is dominated by thermionic emission and tunneling through a 0.2 eV Schottky contact barrier.

Journal Article↗
PubMed2005

High-performance field-effect transistors based on Langmuir-Blodgett films of cyclo[8]pyrrole.

We demonstrate the field-effect transistors (FETs) made of cyclo[8]pyrrole thin films prepared by the Langmuir-Blodgett (LB) method. The cyclo[8]pyrrole molecule possesses a 30-pi-electron system and narrower highest-occupied molecular orbital-lowest-unoccupied molecular orbital energy gap (0.63 eV), forms a stable, reproducible monolayer at the air-water interface, and transfers onto a substrate with a nearly unity transfer ratio and face-to-face configuration due to its strong pi-pi interaction. The LB films are uniform characterized by atomic force microscopy and in ordered form confirmed by X-ray diffraction. The FET exhibited high performances with one of the highest hole mobilities (0.68 cm2 V(-1) s(-1)) for thin-film transistors and a high on/off ratio, implying a promising material in the FET family.

Journal Article↗
PubMed2005

Structures, electronic states, photoluminescence, and carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles.

The excellent electroluminescent (EL) properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), and 1,1,2,3,4,5-hexaphenylsilole (HPS) have been found. Despite some studies devoted to these materials, very little is known about the real origin of their unique EL properties. Therefore, we investigated the structures, photoluminescence (PL), and charge carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles as well as the effect of substituents on these characteristics. The single crystals of the three siloles involving 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), MPPS, and HPS were grown and their crystal structures were determined by X-ray diffraction. Three siloles have nonplanar molecular structures. The substituents at 1,1-positions enhance the steric hindrance and have predominant influence on the twisted degree of phenyl groups at ring carbons. This nonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation, and increases PL efficiency in the solid state. The silole films show high fluorescence quantum yields (75-85%), whereas their dilute solutions exhibit a faint emission. The electronic structures of the three siloles were investigated using quantum chemical calculations. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the silole ring and two phenyl groups at 2,5-positions in all cases, while the LUMOs have a significant orbital density at two exocyclic Si-C bonds. The extremely theoretical studies of luminescent properties were carried out. We calculated the nonradiative decay rate of the first excited state as well as the radiative one. It is found that the faint emission of DMTPS in solutions mainly results from the huge nonradiative decay rate. In solid states, molecular packing can remarkably restrict the intramolecular rotation of the peripheral side phenyl ring, which has a large contribution to the nonradiative transition process. This explains why the 1,1-disubstituted 2,3,4,5-tetraphenylsiloles in the thin films exhibit high fluorescence quantum yields. The charge carrier mobilities of the MPPS and HPS films were measured using a transient EL technique. We obtained a mobility of 2.1 x 10(-)(6) cm(2)/V.s in the MPPS film at an electric field of 1.2 x 10(6) V/cm. This mobility is comparable to that of Alq(3), which is one of the most extensively used electron transport materials in organic light-emitting diodes (LEDs), at the same electric field. The electron mobility of the HPS film is about approximately 1.5 times higher than that of the MPPS film. To the best of our knowledge, this kind of material is one of the most excellent emissive materials that possess both high charge carrier mobility and high PL efficiency in the solid states simultaneously. The excellent EL performances of MPPS and HPS are presumably ascribed to these characteristics.

Journal Article↗
PubMed2005

A self-assembled nano optical switch and transistor based on a rigid conjugated polymer, thioacetyl-end-functionalized poly(para-phenylene ethynylene).

A nanometer-scale optical switch and transistor were fabricated with thioacetyl-end-functionalized poly(para-phenylene ethynylene)s and Au nanogap electrodes by self-assembly. With photoirradiation, the switch can be switched on/off quickly with a switching ratio as high as 1000. Moreover, the device works well as a p-type transistor. With an increase in gate bias, strong conductance oscillation was observed in this self-assembled transistor (under low temperature 147 K), which is very likely due to single-electron charging oscillations arising from electron tunneling through the nanometer-scale transistor.

Journal Article↗
PubMed2005

Field emission properties of large-area nanowires of organic charge-transfer complexes.

In this contribution, large-area organic charge-transfer complex (AgTCNQ and CuTCNQ) nanowires were synthesized by organic vapor-solid-phase reaction at mild experimental conditions. These nanowires were facilitated on the surface of Cu and Ag foils or different kinds of substrates coated with a layer of silver and copper on a large scale. The excellent field emission properties were observed in the as-grown AgTCNQ and CuTCNQ nanowires. They should have great potential in vacuum device applications.

Journal Article↗
PubMed2005

Fabrication of ultrathin films with large third-order nonlinear optical properties.

An ultrathin composite film containing anionic Ag-His complexes (His: L-Histidine) and oppositely charged BH-PPV was fabricated by means of the electrostatic layer-by-layer self-assembly technique. UV/Vis spectra showed a continuous deposition process of Ag-His complexes and BH-PPV. The film structure was characterized by using small-angle X-ray diffraction, AFM, and SEM. The nonlinear optical properties of the ultrathin film were studied by using the Z-scan technique with a laser duration of 8 ns at a wavelength of 532 nm. The film sample exhibited a strong nonlinear saturated absorption, with an alpha2 value of -3.9 x 10(-5) mW(-1) and a self-defocusing effect with an n2 value of -4.78 x 10 (-12) m2W(-1).

Journal Article↗
PubMed2004

Assembly and characterization of novel hydrogen-bond-induced nanoscale rods.

A class of bis-urea compounds with perylene bisimide was synthesized and characterized successfully. (1)H NMR and fluorescence spectra confirmed that strong hydrogen-bonding interactions between neighboring urea groups were formed. Interestingly, the photocurrent measurement showed that the self-assembled films of bis-urea compounds could produce steady and rapid anodic photocurrent responses. The TEM images indicated that well-defined nanoscale rods with uniform diameter distribution could be fabricated by self-assembly of hydrogen-bonding interactions and pi-pi stacking interactions of perylene rings.

Journal Article↗
PubMed2004

Molecular and nanoscale materials and devices in electronics.

Over the past several years, there have been many significant advances toward the realization of electronic computers integrated on the molecular scale and a much greater understanding of the types of materials that will be useful in molecular devices and their properties. It was demonstrated that individual molecules could serve as incomprehensibly tiny switch and wire one million times smaller than those on conventional silicon microchip. This has resulted very recently in the assembly and demonstration of tiny computer logic circuits built from such molecular scale devices. The purpose of this review is to provide a general introduction to molecular and nanoscale materials and devices in electronics.

Computers, Molecular↗
PubMed2004

7-trifluoromethylquinoline-functionalized luminescent photochromic spiropyran with the stable merocyanine species both in solution and in the solid state.

A new spiropyran (SP2) with the stable merocyanine form (MC2) both in solution and in the solid state at room temperature was designed and synthesized. The stability of MC2 is believed to be due to the electron-withdrawing effect of both the quinoline and the trifluoromethyl groups. (1)H NMR spectra indicate that the ratio of the open form vs the closed form of SP2 is dependent on the polarity of solvents. Single crystals composed of only the open form (MC2) were successfully obtained. X-ray structural analysis indicates that except trifluoromethyl and two methyl groups MC2 is completely planar with an s-trans,s-cis conformation. It should be noted that this is the first report of the X-ray crystal structure of the pure open form of spiropyran. MC2 can be slowly transformed into SP2 at -30 degrees C or lower temperature, and the process is accelerated by visible light irradiation. This special photochromic behavior can be explained by the calculated thermodynamic data. The spectral properties of SP2/MC2 in the presence of different metal ions are also studied, and the results show the potential application of SP2/MC2 in sensing metal ions.

Journal Article↗
PubMed2004

New types of heterospin complexes from trans-oxamido-bridged copper(II) binuclear units and nitronyl nitroxide radicals: crystal structure and magnetic characterization.

Three new heterospin complexes derived from trans-oxamido-bridged copper(II) binuclear units [Cu(2)(oxen), oxen = N,N-bis(2-aminoethyl)oxamide] and pyridine-substituted nitronyl nitroxides (o-, m-, and p-PYNN) were synthesized and characterized structurally and magnetically. Complexes 1 and 2 are four-spin complexes. Interestingly, it is found that in complex 3, the Cu(II) ions and m-PYNN units are arranged to form 1D double-stranded helical chains, which to the best of our knowledge is the first example of a metal nitronyl nitroxide complex with such a 1D helical structure. The temperature dependencies of the magnetic susceptibilities of 1 and 2 were fitted to the four-spin model with the Hamiltonian H = -2Js(Cu1)s(Cu2) - 2j(s(Cu1)s(rad1) - s(Cu2)s(rad2)), leading to J = -150.5 cm(-)(1) and j = 47.2 cm(-)(1) for complex 1 and J = -191.7 cm(-)(1) and j = -18.9 cm(-)(1) for complex 2. The temperature dependence of the magnetic susceptibility of complex 3 was approximately simulated with a simple model composed of a dimer of Cu(II) ions and two m-PYNN molecules. The best fitting leads to the values of J = -183.0 cm(-)(1) and zJ' = -0.55 cm(-)(1) for the magnetic exchange of two Cu(II) ions through the oxamide bridge and that between the dimer of Cu(II) ions and two m-PYNN molecules, respectively. The antiferromagnetic exchange of oxamido-bridged Cu(II) ions in complexes 1-3 is strong. The strength of such antiferromagnetic interactions is also similar for the three complexes.

Journal Article↗