Synthesis and Characterization of Single-Crystal Strontium Hexaboride Nanowires

November 22, 2008

Jash, Panchatapa; Nicholls, Alan W.; Ruoff, Rodney S.; Trenary, Michael. Nano Lett. 8 No. 11 (2008) 3794-3798.

Purpose of the study

To synthesize single-crystal strontium hexaboride nanowires and investigate their electrical properties for use in thermoelectric materials. Also, these nanowires may have the potential to be used in hydrogen storage.

Methods

The nanowires were grown in a LPCVD reactor, where SrB₆ was deposited on Si substrates covered with a 1 micron thick thermally grown silica film. First, SrO powders were deposited on the Si substrate. Next, a thin Ni layer (5-6 nm) was evaporated on the SrO surface by a low-pressure thermal evaporator. The substrate was then placed in a 1 in diameter quartz tube reaction chamber and heated to 925 ­­­^oC with continuous flow of argon. Next, a gas mixture of diborane in argon (1.08%) was introduced to the chamber for 75 minutes. After incubation and subsequent cooling, the resulting product was characterized with Raman spectroscopy, SEM, and TEM with SAED and EELS.

Key findings

1.      SEM analysis determined that the SrB₆ nanowires have diameters between 10-40 nm and lengths of several micrometers. Also, most of the wires have bulbous tips caused by the Ni catalyst particles. In addition, wires that were grown in the cooler part of the furnace (~700 ^oC) are shorter (> 1 micron) and thicker (50-60 nm).

2.      Raman spectroscopy determined that the nanowires were in fact SrB₆ and not some unexpected side product.

3.      EELS was used in conjunction with a control sample of SrB₆ powder to confirm that the characteristic peaks are consistant with the nanowires. In addition, it was determined that an amorphous oxide layer (1-2 nm) is present surrounding the nanowires.

4.      TEM analysis with SAED confirmed that the nanowires were crystalline and not amorphous. In addition, growth of the wires is mainly in the [001] direction.

Glossary

- TEM: Transmission Electron Microscopy

- SEM: Scanning Electron Microscopy

- SAED: Selected Area Electron Diffraction

- EELS: Electron Energy Loss Spectroscopy

Ordered Arrays of ZnO Nanorods Grown on Perodically Polarity-Inverted Surfaces

November 4, 2008

Sang Hyun Lee, Tsutomu Minegishi, Jin Sub Park, Seung Hwan Park, Jun-Seok Ha, Hyo-Jong Lee, Hyun-Jae Lee, Sungmo Ahn, Jaehoon Kim, Heonsu Jeon, and Takafumi Yao

Nano Letters  2008, 8, 2419-2422

Purpose

The authors present a method to create periodically polarity inverted (PPI) ZnO templates using molecular beam epitaxy (MBE), which can lead to the synthesis of ZnO nanorods. The goal is to study and understand how to control the size and shape of these 1-D nanostructures for use in electronic devices.  

Methods

Polar surfaces were produced by utilizing ionic crystals that consist of alternating layers of oppositely charged ions stacked perpendicular to the polar surfaces. In short, a Zn-polar ZnO film was produced on a sapphire substrate with a MgO buffer layer using MBE. After using lithography and etching with piranha, MBE is used again to create Zn-polar and O-polar regions. After deposition of an Au film, ZnO nanorods are grown on the Zn-polar surface.

 

Key Findings

·The controlled polar surfaces allow for the fabrication of highly ordered ZnO nanorods with submicron spacing

·The polarity of the template determines the position, density, and diameter of the ZnO nanorods

·Each of the nanorods have a unique piezoelectric response to an applied voltage

 

Definitions

MBE – Molecular beam epitaxy

 

Fabrication of nanopatterns on H-passivated Si surface by AFM local anodic oxidation

Fabrication of nanopatterns on H-passivated Si surface by AFM local anodic oxidation

Mo, Yufei, Wang, Ying, Bai, Mingwu. Fabrication of nanopatterns on H-passivated Si surface by AFM local anodic oxidation. Physica E, Vol. 41. 2008. pp. 146-149.

Joshua D. Swartz

Purpose of Study

To develop nano-sized patterns from localized electrochemical oxidation (local anodic oxidation, LAO) of a H-passivated Si surface with atomic force microscopy (AFM).

Methods

Atomic force microscopy (AFM) was used in contact mode with a silicon cantilever with an electrically conductive tip coated by platinum. The tip is conic with a radius smaller than 25 nm. With the humidity (15-80%) and the temperature (10^oC) controlled, oxides were allowed to grow on a chemically reactive substrate (H-passivated Si) by the application of a pulse bias voltage between the conductive tip and the sample surface, which serves as an anode. Water is present between the tip and surface, providing the oxygen needed for the reaction to occur. This method is called local anodic oxidation (LAO) because oxidation only occurs where the anode makes contact with the surface.

Key Findings

1. The LAO process is controlled by pulsed bias voltage, pulsewidth, and humidity.

2. The tallest silica pillars (1.3 nm) were developed under the highest tip-sample pulse voltage (10 V) and longest pulsewidth (100 ms).

3. There is a linear dependence between the sample height and the tip-sample pulse bias voltage and pulsewidth.

4. An increase in humidity (from 10-85%) showed a linear increase in silica height.

Definitions

Local anodic oxidation (LAO): Oxidation caused by a voltage differential between a small tip and a surface.

H-passivated surface: To clean a surface of silica by introducing H to the surface, preventing non-specific oxidation of the surface.

Au Stabilization and Coverage of Sawtooth Facets on Si Nanowires Grown by Vapor-Liquid-Solid Epitaxy

Christian Wiethoff, Frances M. Ross, Matthew Copel, Michael Horn-von Hoegen and Frank-J. Meyer zu Heringdorf

Nano Letters, Vol. 8, No. 9, 3065-3068, 2008

Purpose of the study

To understand the origin of faceting for a gold (Au) catalyzed vapor-liquid-solid process that grows Si nanowires with micrometer lengths and nanometer widths for microelectronic devices.

Methods

Experiments were carried out in an ultra-high vacuum system, with spot-profile-analyzing low-energy electron diffraction (SPA-LEED). Si(112) wafers were prepared by cutting the wafers at 600^oC and flash heating them to 1250^oC to desorb any native oxide present. An electron beam Au evaporator (at 750^oC) was used to deposit controlled amounts of Au onto the wafer. Two types of information were collected during the experiment. First, one-dimensional SPA-LEED data was recorded to follow the structural and morphological changes during Au deposition. Second, surface morphology of the wafers (primarily facet orientations at different Au coverages) was determined through reciprocal maps derived from SPA-LEED.

Key Findings

1. Eight unique surface phases are observed, which correspond to a distinct surface morphology, with different stepped facets and surfaces. They note that the details of each of these phases would be published in a future article.
2. The angle between the facets of each of the eight surface phases (A-G) increases with Au coverage until the final phase (G), which shows a slight decrease in angle size.
3. The ratio of the (113) [Au depleted] and (111) [Au rich] facet widths for phase G, which is the relevant phase for sawtooth faceting, must be 2:1 to preserve the macroscopic [112] orientation of the sample.
4. Phase G also has the highest root-mean-square roughness (~4 nm) and the highest facets (30 nm for (111) and 60 nm for (113)), which compares well with nanowire sidewalls, which also exhibit large facets.

Key Terms

SPA-LEED – spot-profile-analyzing low-energy electron diffraction

Facets – flat faces on geometric shapes