Nano, by Saad

I needed a couple weeks to figure out how I want to present my “journal club” posts.  Each post will have three elements:

1. Technical background to help approach the topic
2. Description of the key findings reported in the article
3. Assessment of how the findings can be applied to the real world

On to this week’s article…

From Germanium Nanowires to Germanium-Silicon Oxide Nanotubes: Influence of Germanium Tetraiodide Precursor.  Nano Letters 9, 583 (2009).

Background.  This week’s article is on the synthesis of germanium (Ge) nanowires and germanium-silicon oxide nanotubes.  Ge is a semiconductor material with similarities to silicon (see periodic table) but one reason that silicon has been the integrated circuits workhorse is that its oxide is extremely stable and permits the fabrication of reliable field effect transistors.

Creating nanostructures of Ge allows access to certain properties that may be superior to silicon’s.  Ge has higher electron and hole mobilities—all things being equal, current travels more efficiently in Ge.  Nanowires are small enough that they can be made free of defects and grain boundaries that offer resistance to current flow.  Ge also has a larger exciton Bohr radius than Si (24.3 nm vs. 4.9 nm).  When a structure is smaller than the Bohr radius, quantum confinement effects may be harnessed to tune the energy levels and distributions of electrons and holes in the material.  Given the difference in Bohr radius, these effects may be accessed more readily in Ge since Ge nanostructures do not have to be as small as those made of silicon.

Key Findings.  The Ge nanostructures are grown in a furnace from a Ge powder precursor.  Gold nano-dots on a silicon substrate serve as growth sites.  When the Ge powder is used, nanowires are produced.  When a GeI4 precursor is added to the system, nanotubes are produced.  The nanotubes contain some silicon, likely originating from the substrate and entering by diffusion.  GeI4 as the sole precursor yields nothing.  Absence of the gold nano-dots also yields nothing.

Conclusion: Ge powder is requisite to the formation of any Ge-containing nanostructure.  Addition of GeI4 to the system results in the formation of nanotubes instead of nanowires.  The article suggests that GeI4 passivates (covers) the gold nano-dot, preventing wire growth.  Instead, growth occurs from a rim-like shape around the gold dot, resulting in a hollow nanotube vs. a filled-in nanowire.

Real World Assessment.  Germanium is an exciting material and its nanostructures (wires, tubes) give access to advantageous properties that are not available in the bulk form.  The vapor-phase synthesis technique described in this article could be scaled up to a larger furnace and still yield material of good crystalline quality.  It would be ideal to get this material into colloidal form to facilitate bottom-up processing.  Otherwise, the locations of Ge nanowires in a device have to be pre-patterned to permit deposition directly from the vapor synthesis.

The major obstacle to widespread use of Ge is its high cost, due to its natural abundance scarcity.