a Christmas winter is usually pretty
cold in here it's absolutely roasting
I'm sweatin Brady stripped down to his
underpants not quite um it's really
really hot and the reason it's hot in
here
is though inside here we are baking one
of our ultra high vacuum systems we
heated up 250 degrees and you'll see if
you look at this other system which
isn't being bit you'll see there's tin
foil everywhere and if the reason for
the tin foil is is to keep everything
nice and toasty and the reason we bake
it is because we want to drive off all
the water that's on the surfaces and
then when we cool it down it reaches a
really really fantastic pressure but
enough of that what we're actually here
to do is to try and make the world's
smallest Christmas tree so over here
what's the system really because
actually what's happening is that this
is the the system in which everything is
happening Morten who's the student who's
been doing most of this work alongside
all of his colleagues has put a very
touching Christmas message torch and you
will be killed on the system and that's
because this experiment is quite tricky
what's happening what you've seen on the
screen now this image that's building up
is happening right in here which means
we don't want to touch that machine we
don't wanna walk into what we don't want
to bounce into it now our surface is not
great or surfaces as you can see all
these little specks well first of all
this is 20 nanometers so these little
specks are actually individual molecules
individual atoms but there are
individual atoms that have come out of
the vacuum the residual gases in the
vacuum and they're corrupting our sample
so that samples been in there a long
time so the underlying surface is
silicon but it's a special type of
silicon surface it's a silicon surface
where we've taken all the bonds of the
surface that stick out of the vacuum and
covered them with hydrogen so what the
surface what happens is that the silicon
atoms pair up each one of these gray
think of that each one of these great
things as a silicon atom and that's
silicon atom is bonded to three others
so one two can you see that pretty sorry
maybe if I tilt it like this bonded to
three others responded to this one it's
bonded to that one and it's bonded to
that one which leaves this one sticking
up out into the vacuum and that we call
a dangling bond and normally that's
incredibly reactive so what we do is we
expose the surface to hydrogen this is
one of our hydrogens and we come along
and we cap
those silicon bonds so this is
incredibly unreactive it's a very inert
very passive surface exactly that's
exactly it yes so we put a little happen
we put a little hydrogen hat on each one
of the dangling bonds and so here we've
got a bare silicon and here we've got
the hydrogen cap selling and now we'd
love when we do the hydrogen exposure
we'd love if it was as neat as this that
every single silicon bond got tied up
but we don't we have some natural bonds
that are left over but the great thing
and what we're going to talk about in
this video is that you can actually take
one of these hydrogen's off we have a
sharp tip we bring it in close to a
surface and when I say close I mean
within a couple of atomic diameters and
we move it back and forth across the
surface and what we're doing all the
time is quantum mechanics in action is
we're measuring a tiny tiny current of
electricity tiny flow of electrons
between the tip and the sample even
though they're not in contact we still
have electrons for and that's an effect
called quantum mechanical tunneling you
use it as a measuring tape but you can
also use it as a manipulation device as
well so you can do while you're up here
you can measure that current and you
don't really disturb the surface too
much but what you can do is you can do a
number of things you can increase the
voltage and shoot in a pulse of
electrons which shakes up this atom and
causes it to come off and you can do it
that precisely or indeed you can come on
in
cool very much closer and actually bond
to it and tick it off that way but what
we're going to do is we're going to hold
the tip above it inject a flow of
electrons increase the flow of electrons
and it heat it up heat it up locally and
it's exactly what we're doing were
causing it to vibrate and to come off
that's the idea anyway need to point out
of course that we didn't payoneer this
the the person who developed this or the
group that developed this was a
University of Illinois back actually in
the mid 90s somebody called Joe lighting
fantastic work they introduced this and
then from your home country pretty
somebody who's really taking this or
none with it is Michele Symons group in
university New South Wales they've
pushed this all the way down to making
single atom transistors so what I'm
going to do is I'll try to show you one
remover we've done over the past few
weeks a couple of students been working
really really hard on this technique and
they've really pushed it too well
certainly for us to get in very high
levels of precision they've done a
really really nice Christmas tree so
we'll show you that later on but for now
let's just look at the process in action
for a single bond so what we see here is
a fairly cruddy silicon hydrogen
passivated silicon surfaces a lot of
because we've had this in the system for
a long long time there's a lot of stuff
has ended up landing on there on the
surface and sticking their atoms but I'd
love to be able to tell you one of the
disadvantages of scanning tunneling
microscopy as you see blobs you know the
stuff they're trying to work at the
chemical nature of that is very very
tricky in the system we've got water
there's residual water even though we
bake it it's really hot in here
even though we bake the system we still
got some residual water the co2 co2
those those are the key contaminants in
the system what we're going to do is
we're going to let it build up an image
and what it's basically doing is you can
see the green line going back and forth
it's moving the tip back and forth
across the surface and measuring that
current and from that current generating
an image of the atoms and now what we're
going to do once it's acquired this
image we're going to hold the tip at
above a certain point inject some
electrons and hopefully dissolve a
hydrogen you might expect that when you
desorb the hydrogen the image might look
darker because there's no longer a bump
there but the important thing is by
desorbing the hydrogen you change the
ability for electrons to flow through
this bond and in fact the electrons find
it easier to flow through this exposed
bond so you get a big bright patch in
the image I'm gonna train in where this
cursor is that's right let's try here
yeah and it worked so that little block
that little white splodge brilliant I
didn't think it'll work first time that
little bit bright splodge is where we've
desorbed our hydrogen atom yeah exactly
and that we can do a few more let's get
that whole scan this is where it gets
boring pretty because now we've gotta
wait for it to come down and go back up
again so we get the full scan
absolutely yeah absolutely no it's it's
fantastic and the problem is it can
become a din you sit in a lab really hot
you're sweltering and if you can't come
belittlement in but you've got to stand
back every now and again and go Christ
yes why I've actually desorbed an atom
we move atoms around we see bonds it's
it's just staggering absolutely
staggering it though that this is
possible
exactly exactly you just click the point
and click and off it comes it's most of
the time not all the time let's in true
blue Peter fashion show you one that we
did earlier so this is the work of
Morton who's a PhD student here
unfortunately had to catch a flight back
to Sweden yesterday but he did a
Christmas tree and I don't think it's a
bad Christmas tree each one of these is
where those hydrogen atom has been
dissolved each blob is where we've
stripped off one of the hydrogen atoms
so it's like that is like a crowd of
people at a football game and we can see
all the people who yeah that's exactly
what it's like
yes it's basically as if you're looking
top-down and people have taken a lot of
the hats off each one is where we've
removed the hydrogen how many this is
good 42 well certainly let's put it this
way the algorithm that we counted up the
algorithm that uses the XY coordinates
there were forty two different sets of
XY coordinates in there I don't know if
he's a Douglas Adams fan or not but I
found that a nice coincidence it's a
nice Christmas present for me Saturn and
Jupiter very close to each other in the
sky and this is something that the
astrologers kind of get very hung up
about they think that it actually means
something of great significance when you
have one of these conjunctions
especially between the major planets