1. David R. Myers April 11, 2007 EE 235 Ultimate Strength in Germanium Nanowires

2. Motivation In General –Properties of thin films, nanowires are often different than bulk materials –Need to characterize these properties to be able to design micro and nano systems –Mechanical Strength for electromechanical device construction Nano-Specific –Can we get stronger materials?

3. Stress Intensity Cracks cause local stress increases depending on loading condition Type1Type2Type3 Type IType II Type III Thougless, et. al, 1 found that for brittle plates, the stress intensity factors are:

4. Material Testing So Far… Before Study –Near Theoretical Strengths for composite whiskers have been reported –Cracks & Imperfections typically cause deviation from ultimate strength –Only about 50% theoretical strength for silicon has been reported Germanium –High Mobility –Inferior Oxide –Wafers are brittle

5. Supercritical Fluid-Liquid-Solid Apparatus Monodisperse Gold nanocrystals as catalysts Hexane with tetraethylgermane and diphenylgermane (silicon precursor) at 365C, 5.5x10 4 mbar Very high throughput & 90% in direction Batch Reactor Porous Alumina with Gold Nanocrystals

6. Supercritical Fluid-Liquid-Solid Mechanism Coexistance of Au:Ge Liquid and Germanium Solid at 360C, 28% Ge conc. Germanium most likely to dissolve into gold particle, until super-saturation, in which it will be expelled as nanowire

7. Device Testing Method Sonicate IPA Drop onto Trench Pin with Platinum

8. Testing Method

9. Results & Comparison MaterialUltimate Strength Bulk Silicon~300 MPa - 1 GPa Mild Steel~450 MPa Hard Steel~1.3 GPa Aluminum~260 MPa Ge Nanowire~15 GPa Note Lack of Ductile Behavior Failure –Shows AFM application of force does not give rise to defects –The ultimate theoretical strength of Ge is shown

10. Results for Various Radii Interesting to note max theoretical strength is 17GPa, but data gives 15 +/- 4 GPa Also Displacement at failure is a function of radius

11. Questions? Thanks!

12. Ductile Failure

13. Thin Film Application These Stress Intensity Factors (previous equations) have been derived for films on brittle materials 2 h z I = Moment of Inertia Note: Normally the stress intensity factor are determined by loading conditions. In this special case, the stress intensity factor is determined solely by the geometry.

14. Type II Stress Concentration Type II Stress concentration determines direction of crack propagation K II < 0 K II = 0 K II > 0 Into substrate Crack growth aimed away from surface Crack growth aimed towards the surface Crack growth parallel to surface Results from Drory 2