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October 7, 2013

The emphasis in materials science instruction for my generation was crystalline materials. Most of what we studied was metallurgy, simply because the mathematics to describe materials comprised of orderly arrays of atoms is easy. Glasses, which are random networks of atoms, are harder to analyze mathematically. Today, modern computer techniques, aided by improved Xray and electron diffraction instruments, have made glass an easier object of study.

William Zachariasen (1906-1979) was the first to propose the random network model of glasses. Zachariasen was well qualified in the physics of materials, since he had been a post-doc of Sir Lawrence Bragg and was invited to work at the University of Chicago by Arthur Holly Compton. Zachariasen, who also worked on the Manhattan Project, remained at Chicago, serving twice as chair of the physics department.

Zachariasen introduced his random network model in a 1932 paper, "The Atomic Arrangement in Glass."[1] In this model, atoms are bonded to each other as you would expect from valence bond theory, but their arrangement is not the periodic arrangement of crystals. Silica glass, for example, has short-range order, since a tetrahedral arrangement of oxygen atoms around silicon atoms is energetically favored. These tetrahedra, however, will share oxygen atoms with other tetrahedral in a random arrangement.

Calcium-sodium-alumino-silicate glass atomic structure

A possible arrangement of atoms in a calcium-sodium-alumino-silicate glass.

While crystals are usually particular on what atoms might be incorporated into their structure, glass compositions are generally less constrained.

(Via Wikimedia Commons.)

Before Zachariasen's time, glass was thought to be a collection of nanocrystals, since X-ray diffraction experiments showed a single, broad diffraction peak and not the sharp peaks found for crystals. Based on the width of this peak, the nanocrystal size was calculated to be of the order of a few tens of angstroms; that is, about ten atom spacings. Zachariasen's model had the credible feature that it explained that the strength of glass was of the same order as other solids, since the chemical bonding was the same.

Scientists always try to verify theory by experiment, and in 2012, an international collaboration by scientists at Cornell University (Ithaca, New York), the University of Ulm (Ulm, Germany), the Max Planck Institute for Solid State Research (Stuttgart, Germany), the University of Vienna (Vienna, Austria), the University of Helsinki (Helsinki, Finland) and Aalto University (Aalto, Finland) visualized the individual atoms in an extremely thin glass layer supported on graphene using atomic resolution transmission electron microscopy (see figure).[2] The atomic arrangement was as Zachariasen's random network model predicts. Now, this thin sheet of glass has won acclaim in non-scientific circles by being certified the world's thinnest sheet of glass by the Guinness Book of World Records.

Micrograph of a thin silica glass sheet on graphene.

Micrograph of a thin silica glass sheet on graphene.

(Contrast-enhanced Cornell University image from ref. 2.)[2)]

As often happens in science, the process for formation of such glass sheets was an accidental discovery by scientists at the University of Ulm.[2-5] During studies on graphene formation on copper foil in a quartz glass furnace, it was noticed that a layer had formed on the graphene. Investigations at Cornell University showed that the layer was silica glass formed by an air leak that had caused the copper to react with the quartz glass, which, like silica, is a compound of silicon and oxygen.[4]

Atomic-resolution electron spectroscopy at Cornell University showed that the glass was silica (SiO2) formed from a bilayer of (SiO4)2– tetrahedra. There was no detectable covalent bonding to the graphene.[2-3] The small thickness of the glass layer allowed its structure to be observed. The many atoms in thicker layers would produce a fuzzy image in which the structure couldn't be seen.[5]

Said David A. Muller, professor of applied and engineering physics and director of the Kavli Institute at Cornell for Nanoscale Science,
"This is the work that, when I look back at my career, I will be most proud of... It's the first time that anyone has been able to see the arrangement of atoms in a glass."[5]

<icrograph of a two-atom thickness sheet of glass with a diagram of its structure.

In this image, a micrograph of a two-atom thickness sheet of glass is blended with an artist's conception of its structure.

(Kavli Institute at Cornell for Nanoscale Science image.)[4)]

Muller says that this sheet of glass should be just slightly stronger than a soap bubble.[5] The Guinness Book of World Records featured this thinest glass layer in its 21st Century Science spread.[4] The work at Cornell was funded by the National Science Foundation.[4]


  1. W. H. Zachariasen, "The Atomic Arrangement in Glass," J. Am. Chem. Soc., vol. 54, no. 10 (October 1932), pp 3841-3851.
  2. Pinshane Y. Huang, Simon Kurasch, Anchal Srivastava, Viera Skakalova, Jani Kotakoski, Arkady V. Krasheninnikov, Robert Hovden, Qingyun Mao, Jannik C. Meyer, Jurgen Smet, David A. Muller and Ute Kaiser, "Direct Imaging of a Two-Dimensional Silica Glass on Graphene," Nano Letters, vol. 12, no. 2 (January 23, 2012), pp 1081-1086.
  3. Ref. 2 as a PDF file on the Cornell University Web Site.
  4. Anne Ju, "Shattering records: Thinnest glass in Guinness book," Cornell Chronicle, September 12, 2013.
  5. Deborah Netburn, "Thinnest glass ever is just two atoms thick. Does it shatter?" LA Times, September 13, 2013.

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