so this
is a transmission electron microscope
image it's formed by um having beams of
electrons actually transmitted through a
sample and you build up an image from
those beams of electrons
and what it is actually is one of these
a single nanoparticle from this solution
of nanoparticles and the reason i'm
showing it to you is because a couple of
days ago we had
a master class for high school students
and the students
actually made these nanoparticles and
well both they and me got very very
excited when we could see the
nanoparticles they made with quite
simple chemistry and you can see those
right down to the single nanoparticle
limit we take a little droplet of this
solution we put it onto a grid
and
we let that
we let the solvent evaporate and then we
um
put the the sample into the microscope
and we can see the as you can see the
individual particle
it's a gold nanoparticle it's a single
gold nanoparticle tiny tiny chunk of
gold in this case containing about 125
000 atoms
and we can see
the
faces on this we sometimes call it a
chunk or a nanoparticle but it's
actually better described in this case
as a nano crystal you can still see
these ridges these edges and those arise
because when you form these particles
they grow out to a certain size and they
have um
certain certain faces what i'm showing
you here is um it's not quite the same
crystal structure but it doesn't really
matter as gold has and you have
different faces and those faces will
have different reactivities because we
if we look at different atoms on these
faces sometimes they're bonded to three
atoms below sometimes in other cases
they're bonded to two atoms below and
that leaves what are called dangling
bonds sticking out and those dangling
bonds affect the reactivity affect how
the crystal grows
and some faces will grow some crystal
faces will grow faster than others
and we end up with these you can see
these facets on this so this is these
are facets and and
when you grow crystals you go you can go
crystals out to very large structures of
course if you look at diamonds you can
see the facets in the damage but this is
at an unimaginably small level this is
15 nanometers across and yet it still
has 125 and yet it has 125 000 atoms yes
um when you work because an atom is
incredibly small a gold atom diameter of
a gold atom is roughly a little bit less
than three angstroms so three tenths of
a nanometer so yeah we're talking about
roughly 125 000. all i've done there i
must admit before you get your
calculators out and all the experts fill
up the common sheet going how did he
manage to work that out he's out by a
factor of two percent um what i've done
is i've just taken the
the volume of an atom i've taken the
volume of the nanoparticle divided one
by the other and got a very rough
estimate for the number of atoms maybe i
should point out at this point
then
i guess one of the interesting culture
changes when people come that we we
experience when people come from school
from high school from secondary school
to university uh to do a physics degree
is that
we don't care in the vast majority of
cases we don't really care if we've got
an answer to 19 decimal places
we we we make a lot of approximations so
for example you know if we're talking
about uh gold nanoparticles um we're
and we want to work out the number of
atoms in those i will make a lot of
approximations and i'll give you a
guesstimate
okay and it's interesting because it
seems to me
that um many of you or many
first-year students undergraduates and
and
people who aren't scientists find that
quite strange because science is always
portrayed as this
um very very exact we really want the
the the values defined down to the nth
decimal place and that's not really how
physicists work so the the common joke
is that you know a cow to a physicist is
basically just a large sphere sometimes
you consider the tail sometimes you
don't but that's that's really really it
so
about 125 000 atoms i've been doing some
backing literally back where's my
envelope purposely yeah literally back
of the envelope calculation so we do a
lot of this we do a lot of mucking
around in back of an envelopes on backs
of envelopes we actually use the back of
an envelope i didn't age yes because it
was the closest thing to hand and then
when you work out in terms of the volume
of this compared to this you'd get 10 to
the 18 which is uh one quintillion of
these into this volume brady continually
tells me stop talking in terms of 10 to
the 18 and 10 to the 19 and 10 to the
bloody blood means nothing and it
absolutely doesn't mean anything but um
it turns out that uh if you wanted to
have 10 to the 18 of these how much
volume would that take up well again
back in the envelope calculation it
would you'd require a 20 meter deep pile
of
maltesers which spread out over an area
the size of ireland and for those of you
on the other side of the big pond
ireland is about the same area as the
state of indiana as well as sort of
seeing the faces these lines are lattice
planes and they arise from
the wavelength of the
electrons that we're using to make this
image is thus that
they uh diffract from the crystal planes
and those diffraction just the same ways
if you take a cd
made of a cd
queens of the stone edge
the colors that you get
so so the rainbow color that comes from
the fraction that comes because the size
of the of the features on the disc
that's used to encode the information
are roughly the same size as the
wavelength of light now what you
basically have here is an atomic scale
dvd or atomic scale cd and now instead
of bouncing light off it you're bouncing
electron waves and in this case the
electron wave
wavelength is roughly as is comparable
to the spacing of the planes and you end
up seeing the diffraction from those
planes in the in the image now i've not
i've seen that as i say in papers before
i've never seen it in real life before
as awareness it's really neat to see