Samuel L. Mensah

Formation of single crystalline ZnO nanotubes without catalysts and templates

Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures.

Heteroepitaxial growth of GaSb nanotrees with an ultra-low reflectivity in a broad spectral range.

We report on the growth of GaSb nanotrees on InAs { ̅1 ̅1 ̅1}(B) substrates by chemical beam epitaxy. GaSb nanotrees form by the nucleation of Ga droplets on the surface of < ̅1 ̅1 ̅1>(B) oriented GaSb nanowires followed by the epitaxial growth of branches catalyzed by these Ga droplets. In the tip region, the trunks of the GaSb nanotrees are periodically twinned, which is attributed to a change of the effective V/III ratio at the later stage of growth as a consequence of the change in surface structure. The reflectivity of a forest of nanotrees was measured for a broad spectral range and compared to the reflectivity of a GaSb ( ̅1 ̅1 ̅1)(B) wafer and of GaSb nanowires. At wavelengths from 500 to 1700 nm, the presence of GaSb nanotrees decreased the reflection by three orders of magnitude compared to a blank GaSb substrate.

Investigating the role of hydrogen in silicon deposition using an energy-resolved mass spectrometer and a Langmuir probe in an Ar/H2 radio frequency magnetron discharge

The plasma parameters and ion energy distributions (IED) of the dominant species in an Ar-H2 discharge are investigated with an energy resolved mass spectrometer and a Langmuir probe. The plasmas are generated in a conventional magnetron chamber powered at 150 W, 13.56 MHz at hydrogen flow rates ranging from 0 to 25 sccm with a fixed argon gas flow rate of 15 sccm. Various Hn+, SiHn+, SiHn fragments (with n = 1, 2, 3) together with Ar+ and ArH+ species are detected in the discharge. The most important species for the film deposition is SiHn (with n = 0, 1, 2). H fragments affect the hydrogen content in the material. The flux of Ar+ decreases and the flux of ArH+ increases when the hydrogen flow rate is increased; however, both fluxes saturate at hydrogen flow rates above 15 sccm. Electron density, ne, electron energy, Te, and ion density, ni, are estimated from the Langmuir probe data. Te is below 1.2 eV at hydrogen flow rates below 8 sccm, and about 2 eV at flow rates above 8 sccm. ne and ni decrease wit...

Controlled Growth of Carbon, Boron Nitride, and Zinc Oxide Nanotubes

Nanotubes represent a unique class of materials in which all atoms are located near the surface. Since electrons flowing through nanotubes are confined near the surface, nanotubes are attractive for sensing biological and chemical molecules. In addition, their tubular structures enable nanofluidic devices that are useful for novel sensing applications. In this paper, we will discuss current applications and the latest advancements on the growth of carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs), and ZnO nanotubes (ZnONTs). First, CNT growth is highly controlled by regulating the effective catalysts and the dissociative adsorption of the hydrocarbon molecules during chemical-vapor deposition growth. Second, we have achieved low temperature growth of vertically aligned BNNTs at 600 degC , the first success of growing pure BNNTs directly on substrates at temperatures about half of those reported so far. Finally, we have developed an original approach for growing ZnONTs without catalyst or template. Robust, controllable growth techniques for nanotubes are necessary in order to fully realize their sensing potential.

Growth of Carbon, Boron Nitride and ZnO Nanotubes for Biosensors

Summary It was shown that the progress in growing nanostructures affects our ability to use them for applications. In CNTs, growth is easy and controllable, leading to a wealth of study on biological applications. BNNTs, being similar to CNTs but more robust, are a promising material. Until recently, the difficulty of their growth has limited their use, but we have found easier, low-temperature growth methods that should help expand the scope of their application. Finally, ZnO materials are desired for their hydrophilic natures, their tubular structures and wide energy band gaps. These nanotubes can now be grown single-crystal by conventional thermal CVD methods, and, as continuing refinements of the growth techniques take place, they will find more and more use in biological applications. Acknowledgments This work is supported by Michigan Tech Research Excellent Funds, the US Department of Army (Grant No. W911NF-04-1-0029 through the City College of New York), National Science Foundation CAREER Award (Award number 0447555, Division of Materials Research), the U.S. Army Research Laboratory and the Defense Advanced Research Projects Agency (Contract number DAAD17-03-C-0115), and the Center for Nanophase Materials Science (CNMS) sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.

Growth of Single Crystalline ZnO Nanotubes and Nanosquids

The growth of ZnO nanotubes and nanosquids is obtained by conventional thermal chemical vapor deposition (CVD) without the use of catalysts or templates. Characterization of these ZnO nanostructures was conducted by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and photoluminescence (PL). Results indicate that these ZnO nanostructures maintain the crystalline structures of the bulk wurtzite ZnO crystals. Our results show that rapid cooling can be used to induce the formation of ZnO nanotubes and ZnO nanosquids. The self-assembly of these novel ZnO nanostructures are guided by the theory of nucleation and the vapor-solid crystal growth mechanism.

Investigating the plasma parameters of an Ar/O2 discharge during the sputtering of Al targets in an inverted cylindrical magnetron

The plasma parameters and reaction kinetics in an inverted cylindrical magnetron chamber have been studied with an energy resolved mass spectrometer during the sputtering of aluminum targets in an Ar/O2 discharge. Mixtures of argon and oxygen were studied as a function of oxygen percentage (0%–90%) in the discharge. The plasma was powered at 4 kW and 40 kHz at a process pressure of 5 mTorr. Al+, Al, AlO, AlO+, O2+, O+, Al2O+, and Ar+ were among the species detected in the discharge. The deposition rate of the deposited thin film decreased with increasing oxygen percentage in the discharge and results indicated that the pure gamma-alumina was obtained when the percentage of oxygen was approximately 70%. The linear plot of energy distributions of the positively charged film forming species changed from a single peak to a bimodal distribution as the percentage of oxygen exceeds 65%. In a log plot, however, the distributions showed multiple peaks ranging from 2 eV to 78 eV. Fluctuations of about 1 eV in peak ...