What you see there looks a bit like high-resolution transmission
electron microscopy (HRTEM) of a metal. You might recognize single
crystal grains separated by grain boundaries. You might recognize a lot
of features from a solid state physics textbook: a dislocation line
moving around, vacancies jumping from one lattice site to the next. But
this is not electron microscopy - it is not even a metal sample. What
you see here are small silica (glass) beads in a fluorescent
suspension, imaged by a confocal microscope. Although there is no
attractive interaction between the particles (hard sphere), driven by
entropy, they form closed packed crystals. These crystals behave very
much like metals on the atomic scale. For the physicist, however, they
offer the advantage of convenient time and length scales. As the clock
at the bottom right tells you, the things you see in the movie happened
over the course of one hour. The individual particles have a diameter
of 450nm and can be seen with a modern confocal microscope like ours.
Larger particles can be used of course. They move even more slowely.
There are many techniques to study metals on the atomic scale, but none
of them allow you really to watch what you see in the video above.
HRTEM doesn't show individual atoms and only works for very thin
samples, not for the bulk. X-ray diffraction (XRD) averages over
the whole sample volume and gives you reciprocal space information
only. Scanning electron, tunnel or atomic force microscopy (SEM, STM,
AFM) are surface techniques and not bulk sensitive. Tomographic atom
probe (TAP) is destructive and allows post-mortem analysis only.
Computer simulations have become a very important tool during the last
years, but the number of particles that can be simulated and the time
period over which they can be studied will always be limited by
computer power.
Most of the time we prefer to work not with polycrystals like the
sample shown in the video above, but with a single crystal. Single
crystals of colloids can be grown by preparing a template [1], using
photolithography. The templates we fabricate ourselves at the
CNS-Facility here at Harvard are just hole patterns in PMMA films on a
microscopy slide:
Both pictures show the same slide held at slighty different angles to
the light source and the observer. Depending on the orientation,
constructive interference causes the pattern to shine brightly in
different colors. As shown in the cartoon inset, the sedimenting
particles in the first layer settle into the hole pattern and act as a
seed to direct the formation of a single crystal in the subsequent
layers. In this way, we can grow large crystals in various
crystallographic orientations:
The left image shows a crystal grown on top of a (100) template, the
right one a crystal grown on a (110) template. The lattice is face
centered cubic (FCC), although the free energy of a stacking fault is
negligibly small. The particles used here have a diameter of about 1.5
µm.
With crystals like these we can now do all kind of fun stuff [2,3] (see
also Peter
Schall and Claudia
Friedsam's work). An example is shown
below, where we induced the collapse of a stacking fault by locally
disordering the crystal. The video has been generated from real data by
identifying all particle positions in (x,y,z) and rendering images
using only select particles. All white beads belong to a stacking
fault, all yellow beads have a disordered neighborhood and are not part
of the crystal. The particles of the perfect crystal are are not shown.
There are more than 20,000 particles in the imaged volume, as many as
in state-of-the-art numerical simulations. However, unlike in
simulations, we don't have to worry about boundary effects, since our
image volume is part of a crystal that is an order of magnitude larger
in both x and y.
Literature
[1] A.van Blaaderen, R.Ruel and
P.Wiltzius: Template directed colloidal
crystallization. Nature 385 (1997)
321-324
[2] P.Schall, I.Cohen, D.Weitz and F.Spaepen: Visualization of
Dislocation Dynamics in Colloidal Crystals. Science 305 (2004) 1944-1948
[3] P.Schall,
I.Cohen, D.Weitz and F.Spaepen: Visualizing dislocation nucleation by
indenting colloidal crystals. Nature 440
(2006) 319-323
