Miniature Machines: "Pulling" Nanowires

c. Michelle M. Francl, 2004

Full module available as PDF from the author at mfrancl@brynmawr.edu

keywords: thermodynamics - quantum chemistry - nanotechnology

Primary Literature Reference: This module is based on Angew. Chem. Int. Ed. 2003, 42, 2251-2253. "Towards 'Mechanochemistry': Mechanically Induced Isomerizations of Thiolate-Gold Clusters" by Daniel Krueger, Roger Rousseau, Harald Fuchs, and Dominik Marx.

Background:

Nanotechnology opens a window between engineering, physics and chemistry. Broadly defined, it is an area of research where the critical dimensions are on the order of 103 nm or less. Chemists might define nanotechnology more tightly, as the design and construction of devices which are well defined on the molecular level. The ability to see and manipulate matter at the atomic level has changed dramatically over the last 25 years. My physical chemistry text made the point that you could not "see" an atom; ten years later, in 1989, scientists at IBM used atomic force microscopy to position Xe atoms on a Ni surface to spell "IBM" in what likely remains the world's smallest advertisement. Now researchers can assemble machines using structures such as quantum dots, nanotubes and nanowires. For example, CdSe-ZnS quantum dots have been used as in vivo labels to image blood vessels in mice [Science 2003, 300, 1434]. Carbon nanotubes have been used as building blocks in molecular electronics [Nature 1998, 393, 49].

Researchers in nanotechnology are interested in learning how to control the size and shape of nanomaterials and in characterizing their unique properties. Nanoscale materials can have very different optical and electronic properties relative to the bulk state. Gold nanorods, for example, have 10 million times the fluorescence intensity of bulk gold [Chem. Phys. Lett. 2000, 317, 517]. The authors of the paper we are discussing use a theoretical approach to understand how mechanical forces can result in the growth of single atom gold nanowires on a surface.

Sample Critical Reading Questions:

• The authors constrained one of the gold atoms to remain in the left-hand plane (see Figure 1 of the paper) as they pull the two planes apart. Identify that gold atom in structure A-I in Figure 2. Could another atom have been reasonably selected? In that case would the results of the authors' simulation be different? In what way?
• In the description of their model in the text of the paper, the authors state that z_i=0 �, yet Figure 1 seems to imply that z_i>0 �. Can you reconcile this apparent notational inconsistency? How long is the wire in the end (that is in structure E-II)?

Sample Problem:

• Using the data in the table below, estimate the work done in pulling the 5 atom long gold wire.

  z/ �	Force/nN	z/ �	Force/nN
  0.0	0.0	4.5	-0.75
  0.35	-0.25	5.0	+0.25
  0.67	-0.50	5.5	-0.3
  1.0	-1.0	6.0	-0.8
  1.33	-1.25	6.5	-1.8
  1.67	-1.4	7.0	-2.25
  2.0	-1.4	7.5	0.0
  2.33	-1.5	8.0	-0.25
  2.67	-1.5	8.5	-0.75
  3.0	-1.6	9.0	-1.0
  3.5	-2.0	9.5	-1.5
  4.0	-1.25	10.0	-1.5
  4.25	-1.40

Culture of Chemistry Feature:

Biographical sketch of Prof. Mildred Dresselhaus of MIT.