so today
if you'll excuse the exaggeration we're
going to listen to the sound of atoms
and molecules bonding so in this screen
what we have is a sample on which we've
got some molecules and atoms and we have
a tip
the important thing here is that we've
got a very reflective sample and you can
see the reflection of the tip in that
sample it's a piece of silicon let me
show you what it looks like so that's
the piece of silicon very much like this
so maybe it'll be just a heck of a lot
easier if i if i bring out this this
demo so what we have is we have a tuning
fork right so it's a quartz tuning fork
actually you've very
lightly come across these before
multiple times but you didn't realize it
in every quartz watch in every clock
quartz clock inside there the timing
element is one of these things it's a
tuning fork that vibrates up and down at
32 000 times a second or roughly 32 000
times a second and that acts as the
clock that acts as the the electronic
component or the electro mechanical
component that gives you the the timing
what you can do is you can take the
tuning fork
out of the can and you can glue a tip
onto it
or blue oh you can yeah perhaps no in
the real case we don't bluetack it on um
we actually do glue it on but actually
the glueing is probably just about that
accurate
so what you have is a resonant frequency
there there's a particular frequency
natural frequency this wants to vibrate
at which depends on the density and
depends on the length of the the
tuning fork and all its sort of material
properties and its structural properties
and its geometry but the really
important thing then is if you glue a
tip onto the end of it okay so let's get
it resonating again
so you can see tips resonating tuning
forks resonating
we bring it in on top of an atom and the
interaction between the atom at the end
of the tip and this rather oversized
atom or molecule on the surface
changes the force between the tip and
the sample and that changes the resonant
frequency so phil when when the tip
approaches the atom
what is it about the atom that is
affecting the tip and therefore
affecting brilliant question it's
forming a chemical bond basically at the
level we're looking at here there are a
range of different forces electrostatic
charges for example you can even have
magnetic forces but what we're really
interested in what this experiment is
all about and the experiments we've been
doing off later all about is getting
right down to that regime where it's the
overlap of electron orbitals it's
chemical bond formation at the single
bond level so the um the state of the
art with these techniques with these
scanning probe techniques is we've gone
well beyond atomic resolution we're now
with what people call subatomic
resolution or single bond resolution so
all of this is happening so we did a
video
it's actually almost three years ago
isn't it so we did the atomic switch
video i think certainly two years ago so
it's happening in the same system which
we did the atomic switch video on um and
i'm sure brady will kindly put a link to
that video somewhere it needs all the
publicity you can get
so
it's happening actually in here in this
system so this is an ultra high vacuum
system the pressure as you can see is
1.6 by 10 to the minus 11 millibars 14
orders of magnitude below that you have
below atmospheric pressure well this is
what the data looks like in real time so
what we're looking at here is the
underlying silicon surface it's actually
just to be to clarify it's actually a
mixture of silicon and silver but this
is our substrate this is a the surface
on which we've put our molecules that
looks very ordered it is extremely
ordered that's um and you can see the
the the individual atoms over here these
sort of grain like structures are
assemblies islands a single molecule
thick they look like puffed white they
do look like puffy indeed they do look
like puff and you can see their
differences sort of in the orientation
and the molecule we're looking at now is
this pretty simple molecule where
physicists we like simple molecules once
it gets above you know 20 or 30 atoms i
get very confused each one of these
black things is a carbon but for ntcdi
on both ends of the molecule this atom
is replaced by nitrogen and we have
oxygens hanging off here and what we're
very interested is because you've got
the nitrogen and the oxygens and the
hydrogens on this we're really
interested in probing hydrogen bonding
and that's really what the focus of this
experiment is hydrogen bonding is the
type of bond that holds our dna together
what we do is we take this frequency
this resonant frequency which if this
tuning fork here on which we've got our
tip was bigger we could actually hear
this in fact it turns out the resonant
frequency of the tuning fork here the
actual real thing is about 25 kilohertz
which is not so far off you know the
audible range the limit of human hearing
which is about 20 kilohertz for me it's
much much worse my hearing is scrambled
from playing too much guitar so i can't
really hear above much above 15
kilohertz
what we do is we take that resonant
frequency
um and we look at how that changes as we
move the tip across the surface so we
measure that resonant frequency directly
see how it changes and the way in which
it changes tells us about how the tip is
interacting with the atom at the surface
as you move it across the fact that
you're moving back and forth between
atoms means that at some places you're
directly over an atom or more
importantly directly over a bond you get
a strong interaction you get a bigger
change in resonant frequency and then
you move to a place between atoms or a
place where there isn't a bond a
dangling bond a bond sticking out from
the molecule and you don't you get a
weaker interaction so you don't see a
bigger frequency shift what force is
acting on the tip the chemical bonding
force yeah it's a force of a chemical
bond tip's just being yanked towards it
or pushed away it's now it's very
interesting this is the great question
because if you use for example silicon
for your um for your tuning fork or your
cantilever then what happens is that the
tip will get yanked in and that's not
what you want but you go smack into the
surface and you ruin your surface in
your tip the great thing about these
quartz tuning forks
is that they're very very stiff they're
about 100 times stiffer so that means
you can get these very very small
oscillation amplitudes and bring it in
very very close indeed without it
snapping in and without it ruining your
surface and i should also point out
again for the experts i'm always
conscious that there are people out
there who will fill the comments border
way didn't say this and he didn't say
that and why did he miss this like
important thing here is that what we
actually do in the experiment is we tie
that time down we tie one time down and
we're only relying on this one let's get
to the sound so what we are doing is we
are taking that
signal from the tuning fork which in
principle we can either map out like
this or we can listen to
that's real time as we go across every
scan line this is the change in the
frequency as we scan across each line of
the image and what we do is you can
convert this change in frequency or what
you can do is just say well this lowest
point is going to be black or dark red
this highest point is going to be white
and then scale it or we can do what
we're doing here and we can listen to it
what this little box is doing is
actually from our first year lab and we
spent a lot of time myself and a phd
student called julian sterling thinking
about how we're going to do this how
we're going to convert this thing the
last time you were here brady you might
remember you sat over there for an hour
while i tried to write a piece of code
it didn't work
then we thought we might build our own
electronics to do it and then suddenly i
thought well usually when i'm stuck what
do i do i go and ask george so i went
and asked george he says yeah you want
one of these and so that what this box
is doing is it's converting this value
of the the frequency shift
into a
signal that we can hear it's basically
transposing it from 25 kilohertz down to
a signal that we can hear we can change
that center frequency as you like so in
that sense it's artificial
but
to a certain extent it's no more
artificial than what we're actually
we've got here so what's the title of
the video
i would love to call it the sound of
atoms bonding but um that might be a
little bit misleading are they are they
bonding or have they already bonded the
but no other so when i talk about the
sound of atoms bonding we'll do another
experiment on this i mean the the atom
at the end of the tip bonding with the
atoms at the surface that's that's
really the non-bonding is it yeah that's
exactly it so this is like a really
promiscuous bond now that's exactly it
so it's going bond unbond bond unbond
bond unbond and it depends on the asset
you so depends on the oscillation
amplitude so in some cases you could
have this as much as 10 nanometers so
you're you know here's your bond and
you're swinging all the way up and then
coming back down making the bond
swinging all the way what we tend to do
and really if you want to measure those
hydrogen bonds and see them directly is
that you keep this very very oscillation
amplitude very very small so you are
you're always sort of within that bond
making regime quite a few scanning pro
people would look at this and go that's
a bit of a gimmick but in fact um
it was one of my heroes a guy called don
eichler and his group in ibm and madden
who were responsible for pioneering you
know single atom manipulation this is
what they do when they're doing
manipulation of atoms pushing atoms
around they listen to the signal that's
coming back as well as looking out on
the screen because your ear sometimes is
more sensitive to changes and also this
is at a slightly higher rate than these
lines appearing on the screen so you're
more sensitive and that's what science
is all about is trying to invoke as many
of the senses as possible to make sense
of the data
that's my little martial arm
unfortunately it's um i've now used this
thing called guitar rig to to play
guitar so the martial amp has sort of
fallen into disuse a little so i think
it was feeling quite lonely so i decided
to bring it down and plus it's marshall