Monday 10 Dec 2012 , Prof Phil Bartlett, Southampton uni : Presentation on
SERS, Surface Enhanced Raman Spectroscopy
19 people, 3/4 hour talk, 3/4 hr Q&A
I decided to talk about how to create metal surfaces have particular colours ,
leads on to SERS then used to discriminate DNA . It would be a mistake to leave this talk,
thinking people set out to make some device or sensor , for diagnostic application then
work through - that is not how its done. Its more stumbling along from one thing to another.
The only thing I would claim to is when getting to a certain point then recognising
that something may be useful in one direction and then take it off in another direction.
About 4 topic areas
- which is how I got into this. I was interested in trying
to make nano-scale structures , using electrochemistry and the things you find at home
in the kitchen rather than via clean-rooms and expensive equipment. I was doing this 10
years ago using detergent and electroplating and making oredered structures in metal
, a couple of nonometres in diameter in regular arrays. Seemed interesting and I started talikng
to people in physics and they said that if the holes you were making were 100 times
bigger it would be more interesting to them.
So how could you make the holes 100 times bigger and then see what you could do with that.
Interesting things happen when you match the size of the physical phenomenon,
in this case the wavelength of light. Originally the person I talked to it was
in relation to magnetic domain wall width or the superconducting coherence length.
When you match a physical property to the scale of your structure. You are then in the
meso-scopic regime where the simple physical methods and models we may have, don't
work. If I make a mirror , I can use ray optics for how light bounces off the surface
, if I make lots of tiny holes in the surface as I was doing with the surfactant and the plating
, then the holes are too small compared to the light wavelength and the light just bounces off.
If I make the holes of the order of the wavelength of light then you're into a different realm
with funny and interesting effects.
How do we do this deposition. I wanted to make these structures in a regular way
, withiout using all the cleanroom kit, electron beam lithography and the rest.
So we took some coloidal particles of polystyrene. You can make them quite
easily or buy them , made by emulsion polymerisation , sold as size
standards for miroscopy. And assemble them on a surface.
We assembled them on the surface, just be controlling the wetting chemistry
so they packed themselves together
A hexagonal array on the surface
You can do that easily in a beaker with some solution and a few bits to assemble
them , on a gold coated slide . Then we did some electroplating , going back to
Faraday and Faraday's law. Pass a current through a solution of metal ions and we
can deposit metal on the surface. Plenty of electroplating around from 1950s
chroming on a Cadillac through to interconnects on a microprocessor chip.
So we use plating through those templates . So an conducting metal flat surface ,
with insulating spheres on it and we can grow the metal through the template and
out into the structure .
human hair compared to the surface and have to zoom in a bit to make out the holes.
So a few hundred nm in diameter for the pores in this structure. So you are assembling these
things and you may question that you could not produce the regular arrays as you would
using the fancy techniques. But as you've arranged the chemistry and the forces between
the particles as the wetting goes ahead you can get reasonably good patterns.
pic of such a structure with a few defects , some missing spheres , some defects
caused by some spheres being a bit smaller but still a reasonable structure even zooming in.
When we experiment with light etc we are using a laser spot size in a spectromete r
of size taking in 50 or 100 of these openings and sampling the average of those .
You get good reproducability without using any expensive patterning technology,
just using the spheres and the self-assembly.
view of part grown metal with some of the spheres removed
Start with the flat surface , packing the spheres on the surface , doing that with the
de-wetting and as it evaporates , they pack onto the surface and then start growing the
metal through. So starts at the flat and grows up through the polystyrene spheres.
These are around the wavelength of light. So as it grows, I can choose when
to stop because I measure the amount of charge that has been passed and work out how
thick the film will be at any particular point. Now I have spheres embedded in the metal
this is gold but you can use other things. Use an organic solvent to dissolve away the
spheres . The remaining structure is called a sphere segment void structure , hexagonal
array of cavities in the metal on the wavelength of light scale. You get nice
packing . The metal that you put down is very smooth. Veiwing down through the
spheres, there is a rough patch in each , a disc where deposition is blocked off
right at the bottom of each sphere. The rough patch is the evaporated gold on the
glass slide. That you would normally call a gold mirror but at this scale it looks
rough . The made surface of the electroplated surface of the gold is quite smooth.
These structues are quite pretty. Nice bright colours as you look at them like jewels on the
Water droplet over a patch of this made metal and the colour depends on the viewing
angle, when lit with white light. There is a story about the angle and wetting of these structures,
concerning lotus leaves and butterflies. The standard physics did not seem to explain
the colours we were getting. With Jeremy Bamburg? , now at Cambridge, looked at
the optics of these kinds of structures/surfaces and several post-docs and PhDs
later we had the physics. With these chameleon metals , the colour was changing
as the structure of the surface was changing. We had samples with different thicknesses
and depths of cavities and the colour is changing . The spectra are changing through 24
different sample thicknesses. Changing in a consistent way that you would like to
understand. How does the shape and surface affect the reflection spectra.
Silver surfaces , again different thicknesses , starting silver and going through intense colours
and back to silver
Paladium surfaces, my favourite, some nice colours in there . Same size spheres but differnt
thicknesses of metal.
The colour of my chameleon metal depends on th eshape of the surface and how I structure the
surface, and because I'm doing this on the scale of the wavelength of light. The
meso-scale effect coming in. Spheres about 600nm , wavelength of red light.
Repeated with spheres of several microns , ray optics works . You can make an experiment
showing how the light is reflected between cross-polarizers, out of these surfaces. There
is a nice matching with theory of ray optics. All we've done using micron scales is
to make very small spherical mirrors . Light bouncing around in them agrees with ray optics
but does not apply when taking to light wavelength scale.
We have to look at the reflection spectra , a light source that can bounce off the surface
and vary the angles , rotate the sample and plot out lots of spectra. There is a lot of
information in these spectra. Wavelength or energy in one direction against different angles
of incidence and versus different thicknesses. Different rotation angles and different ways of
polarisation for another set of plots. An enormous about of data that we had to try
and pick out what it is , and what the features are that cause the strong colours on these
To cut short a long story and a number of PhDs in physics. We were able to show there were
2 kinds of interactions and they are to do with plasmons.
A plasmon is effectively an interaction between a photon of light and electrons
in a metal at the surface. So they sort of get mixed up and stick to the surface for
a while and it generates a thing called a a plasmon that exists at the metal surface.
You can think of it like ripples on the surface of a pond. Wher ethe ripples are ripples in the
electric fields , the electrical properties of the surface and its partly inside the metal and
partly spreading out over a distance less than the wavelength of light , out into the
air above the sample.
Animation graphic of an electric field that is oscillating perpendicular to the surface
and associated with the surface and is often used in a technique called surface
plasmon resonance . The plasmons propogate across the surface trapped in the interface
between the metal surface and the dielectric, the air.
We were able to show on these structured surfaces that we had these waves propogating
across the surface of the structure , they get scattered off the cavities, tghe openings, . In one direction
they keep bumping into the openings and in another direction they can propogate
quite freely. But there is a second type of plasmon that is inside the cavity and its like
a standing wave. Inside the cavity I can have an integral number of wavelengths with
discrete energies. So 2 kinds of plasmon at the surface, one inside the cavity and one
moving around on the top surface . The interplay between these 2 types varies .
For very thin layers grown through the template I've got a large amount of flat topped
surfaces with little dimples in it. So a lot of Bragg? scattering , a lot of propogating
plasmons on this 2D array.
As I grow upwards, cavities start to appear and the amount of top surface area is
decreasing. At the depth where the spheres are touching each othe rin the array ,
there is no growth of metal between the touching spheres, just like there was no growth
at the base of each sphere where they were touching the original surface. A notch
starts to appear and the top surface is longer continuous but is made up
of sculpted triangles. So at this range at half height , 45 percent of the diameter the top surface becomes
broken up and no longer have waves propogating across the surface.
As you carry on growing you return to where the top surface reforms and you have the cavities
again. By changing the height I can change the balance between these 2 sorts of plasmons.
One that moves around on the top surface , if there is a continuous top surface , which is
very angle dependent because of the way its being scattered. And the one that is in the
cavity which is angle independent, because its just living in the cavity going round and
round. The solution of the wave equation inside a cavity. The solutions work out
the different kinds of modes and standing waves that occur inside a cavity.
If you had a perfectly spherical cavity , the solutions would be the same as the hydrogen
atom , same wave functions. When you chop the top off , like your boild egg, you
change the degeneracy , some go up in energy and some go down in energy and they
get localised in different places. The 2 sorts of plasmons can mix together. That explains
the whole richness that you see on this top surface. They mix together to form coupling
between plasmons and therefore can change their ?.
You can do some neat things. If we deposit a bit of silver to begin with , then gold on
top of it , take the spheres away , I can chemically dissolve the silver but not the gold
to make a free standing film. Looking like tripe , it is a free standing nano-structured gold
film. You can fold it up . It has holes of one size , viewed from one side and another
size from the oither. One way you loook at it, its red and the other way its green.
We have a lot of control over how we do that , can choose the size of the spheres, how thick
the deposit of the silver layer and chosse how thick for the deposited gold layer.
Its like a finely structured bit of gold leaf. So some electroplating and solvents to disolve the
template away and some etching to disolve the metal.
We also made an interesting structure that was 100% absorbing of light falling on it
from all angles, at certain wavelengths. By growing this up with almost a complete sphere and
a small neck on the way in. The light falls on the top surface, excites plasmons
and the plasmons couple into the cavity and never come out . For a limited range
of wavelengths absorbs 100 % of the light that falls on it , no matter from what angle
it comes. Useful for making solar cells .
We know quite a lot on how to make these structures , we know we excite surface
plasmons and at this stage as a chemist you realise we can do surface enhanced
C V Raman , you could do this scattering effect , shining light on a molecule,
or in this case a ruby. Most of the light would be elastically scattered , Raleigh ? scattered
but a small amount of the light was shifted in energy because it had given up or
picked up photons and a small amount of energy to the sample, the raman effect.
These days typically laser light, on to a molecule. Most is Raleigh scarttered off ,
with the same energy. For about 1 in 10^7 photons the light comes out with a slightly shifted energy
, shifted by 1 vibrational quantum of the molecule. So doing vibrational spectroscopy on
the molecule but doing it with light in the visible spectrum. Normally vibrational
spectroscopy would be i nthe infra red . The problem with IR and trying to look at
molecules in water , you can't get any signal in because the water absorbs the IR.
So a chance to do vib spectroscopy in solutions , with visible light in
and slightly shifted visible light out. The vibrational modes and the individual energies of those
vibrations are characteristic of the molecule and can tell us the structure of the molecule.
Example of Benzine-thiol ? and the band in the spectrum is the stretching of the
S-H ? bond , anothe rband is C-H bond stretching on the aromatic ring . So a fingerprint
of this molecule. We can calculate and work out the structure. a lot of information
is in there. The problem is that the effect is very week, only a small number of photons
undergo the Raman scattering. Raman did his experiments in Indi with an inverted
telescope, to collect the Sun's light , making an intense beam of light. Someone provided
him with a large ruby to do the experiment. The side bands on each side are the
Raman spectra. Some giving up energy and some gathered energy from the sample
and are characteristic of the vibrational modes of the species in his ruby.
The crucial part that was played in Southampton University , in 1974, discovered that you could
get a very large enhancement in the Raman signal if you carried out the measurement
over a roughened metal/silver surface. Experiment domne by Martin Fleichman and Patrick Hendra
and Jim Mcquillan . Could they get some spectra of molecules on metal surfaces.
Did not seem sensible as the Raman effect is very weak and now you want to try
it with a monolayer of molecules , just on the surface. So few molecules there,
you would expect to get such a small dsignal that you could not measure.
Surprisingly whan they did this , they were able to see very clear spectra.
They tried to up their chances by making the silver surface very rough , for a high
surface area and up the signal . The increase is something like 6 orders of
magnitude , much more than area increas. They had discovered surface enhancement
of raman Spectroscopy. Since that time this has developed into a very active area
of research. A lot of research has been done on how the enhancement comes about.
If you get it right you can take this form of spectroscopy almost down to the
single molecule level. Some papers claim to detect spectra from single molecules using SERS.
Coming along later with these structured surfaces , with these intense colours , where we
can excite surface plasmons, was to use them to try the raman experiment.
Our structured surface, excite the plasmons sitting on the surface with laser light and
look for the Raman scattered photons coming off.
Red laser through the Raman microscope , light coming back into the microscope to
be collected. We want to couple the light in to the structured surface and couple the
light coming out. The light coming out is shifted in energy scale . With 633nm
laser we excite plasmons at one energy and we have to look for light coming out
in the suitable range , so choosing the right geometries is critical, in the size of the
pores and the way the structured surfaces are made.
Comparison of flat surface, roughened surface and structured surface.
7 orders of magnitude of enhancement. Using benzene-thiol molecule as it sticks to the
gold surface with a mono-layer. Mentioned before the S-H vibration , its not here as the
S-H isn;t there as the molecules are sitting on the gold surface and no S-H
vibration in the spectrum. You still see the C-H , as before. The spot size covers a number
of cavities and when you move it around you are looking at a number of the cavities
and it averages out so you get very reproducible spectra , despite different places on the
surface. This isa significant improvement. In comparison to the roughened surface
situation, and moving the spot around, the intensity goes up and down enormously
by a factor of about 100. With our ctured surface then about 10 percent variation over the
surface. So not only a bigger enhancement but easier to rreproduce that enhancement
from place to place.
Piridene on the surface , bonded to the surface with nitrogen , and change the potential at
the metal surface . Either pushing electrons at the molecule or pulling them
away, this changes the vibrational energies of the molecules , so the spectra change and
the bands move around. This is what they did in 1973 to confirm they were dealing with
molecules on the surface and we did the same thing also. If you change the potential
on this surface mono-layer you change the spectra.
The big thing with htis spectroscopy is that its coming just from the surface and its a
We had a PhD student who wanted to try some analytic applications and persuaded
us to look at DNA as a molecule on these surfaces and see if we could discrinate DNA.
So moving from physics to biology. In DNA you have 2 strands held together
by H bonds , ATGC pairs in Watson-Crick pairing . On the backbone you have negative
charges from the phosphate groups on the outside. All assembled in the double helix.
Have we ot 2 strands that perfectly match each other , for every A there is a T and for a G there is a C
or is there a mutation. One being wrong and can we detect those mutations.
Those changes are important for characterising ddiseases or bacteria and other bio-medic
applications. Done conventionally you have to see how easy it is to take apart the
2 strands and detecting the difference between where there is the perfect match .
Melting, by heating the sample the 2 strands come apart . If there is a mismatch
, a SNIP a single nucleotide polymorphism ,just 1 base is changed.
In this case a C is now a T , it does not match up with a G , one little defect in the
middle of the strand . This will be easier to melt and take apart than the perfectr
match. So the normal way is to take a solution of the DNA in double strand form
and see wht temp the 2 strands come apart and so discriminate. Maybe 40 or 50
degrees and 1 or 2 deg at most between the 2 forms. Usually that is followed using
flourescence or flourescent energy transfer ?, in order to see when the 2 strands come apart.
We take our surface , attach on a strand of DNA which we call the probe which
is the one we used for testing , allow that to hybridise on the surface with the target and then
we want to see how easy it is to separate the 2 for the discrimination.
So a piece the cystic fibrosis gene. So a perfect complementary match for the probe
which we stick on the surface and then a Delta-F508 mutation , the miost common
CF mutation. It has 3 Ts that are missing, a triplet deletion. Those 3 Ts code
for a phenyl-aniline in the protein so the pprotein is missing 1 amino acid so
it folds wrong and don't have the Cl transporter expressed properly in the lung
and then the CF symptoms.
We also used the rarer mutation 1653CT , a C has changed to a T , 3 residues along, which again
changes the mino acid for a different one , again a misfolding .
With one subtle change it would be more difficult for us to detect than the triplet deletion, so
a good challennge for our method. We put our probe attached to the surface and bring
along these little bits of DNA, the wild type or the mutations which are labelled on the ends for
SERS and see how easy it is to make them come off the surface using our sensitive
SERS technique to do that. A bit of DNA is attached to the surface and the bit that is
complementary and wound onto it , separate out the DNA on the suface and we put that
in our spectrometer in a little cell. What can we see in the spectra.
Spectra at different temperatures , 2 sharp bands come from the label molecule
, there at 25 degrees, incrrease the temperature and by the time we get to 60 its gone.
The probe is still on the surface but the double strand of DNA which has the label on has come off the
surface. When its there attached to the surface , in this case with this dye label , there
is about 7 orders of magnitude of signal enhancement. When it comes off the surface
and just floiating around you don't see anything, its gone down in 10 million
of signal. So its using this high sensitivity to see when this is attached to the surface.
Not only can we do this with temperature , can also do it with the potential.
Make the potential of the electrode go negative and as you go negative , the DNA disappears
off the surface, hust like as with temp. You can break the 2 strands apart by simply
applying a potential and scanning the potential.
Looking at the intensity of the spectrum as we scan the potential, plotted out.
For the wild type we have to go to the most negative to break the 2 strands apart
, for the single mismatch I don't have to go so negative and the triplet
deletion is the easiest one to separate on the surface. So discrimination by
scanning the potential across.
I can reuse this surface. By taking the surface after doing one experiment and hybridising
something else onto it and rescanning. The probe just stays there ready to pick up
another DNA molecule. This is a quite sensitive method because , if you work out
how many molecules are in the light spot for the experiment, its a few thousand
for the spectra. You don't need large amounts of DNA to make these experiments
work, just a few thousand in the light spoot , on the surface.
So one possibility is DNA fingerprinting. One of the students I had in the lab is
from Taiwan , a scene of crime officer and came to Southampton to do a PhD
as he'd seen what we'd been doing. He wanted to see if he could use this method to
do DNA fingerprinting. DNA fingerprinting relies on Short Tandem Repeats STRs regions
in the DNA. We all have parts of the genes where there are these STRs.
A phrase of DNA is repeated like GTAGTAGTA and the difference between my DNA
and your DNA , I may have 16 repeats and you have 13. A number of these loci have
been found by forensic scientists and can use for DNA fingerprinting .
Different places on the gene where these repeats happen and can be recognised with
a high degree of reliability.
So in this experiment we take this GATA region and put on our surface the
complementary strand and now we want to discriminate how many of these repeats
there are when we try the melting.
When there are 14 repeats in this example, we go to the most negative potential and can
discrimate for 13,12,11,10 repeats as well/.
I wanted to show you that we can template these surfaces and can use them to make these
plasmonic structures . Using simple electro-chemistry and electro-plating
in order to make these nano-structures which have interesting optical properties.
That led us into thinking how light interacted with those surfaces and to understand
how electrons were excited into interacting with photons on those surfaces
and the resulting plasmonics have become a very trendy area in physics at the moment.
From that we can then do SERS and then maybe able to use that for something , for example
taking it to DNA. I don't want you to go away with thinking that we set out thinking
of a way to do DNA fingerprinting because I knew nothing about DNA fingerprinting
back when I was doing the early electro-deposistion. Just at each developemental
stage we went on to do things that we thought were interesting and then Ah! leading
to something else that may be interesting. Following through it led us to developing things that
have relevance to medical diagnostics , detecting bacteria etc , not that we started out intending that from
Q: If you made your voids a little bit bigger , would you get light reflecting around them
for a little while, in other words store light in there.?
We can make a sort of cavity mirror , yes. In the bit I showed you briefly with the larger
cavities , we actually went on to make a laser out of that structure , by bringng
fibre-optic down to it and make it lase.
Q: For some years and where the developement of computers is going and using light
rather than electricity and one of the problems is storing it for a while, can you
do that sort of thing with bigger nearly complete spheres?
You would still have some losses from reflection . I worked for a while on a
different project in the ORC where we were assembling spheres on surfaces , glass spheres
, about 10 microns, using the whispering gallery mode and use them as a way
of guiding light. So excite the whispering gallery mode on the outside of the sphere
, put the sphere between 2 waveguides you can use it to make an adhoc?
multiplexer . Light coming along one waveguide , sphere and another waveguide ,
using the refractive index of the sphere you can pick a signal off this waveguide
, excite the whispering gallery mode and drop it onto the other one.
A light addressable multiplexer. The trick there was trying to find a way of
positioning the spheres on the surface, not in a dense pattern , but in a rather
Q: I would have thought your prepared surfaces were highly absorbtive and be
impossible to clean for reuse?
We can clean them using ultrasonic baths , they are quite robust
Q: Dropping out of these miniscule holes ? I would have though such a surface grabbed everything in
sight and would not release it?
No. One of the interesting things about these surfaces is their wetting properties.
Because of the tstructured surface , the contact angle depends on the structure of the surface
and vary the contact angle wioth the geometry in a meta-stable way. A drop would tend
to sit on the surface and once wetted, they stay wet. Compared to the earlier roughened surfaces
with a finer nanostructure , then the properties of such surfaces change quite a lot when you try
to clean them. The original roughened surfaces were made by taking the potential
backwards and forwards and disolving and replacing the material, disolving and replacing
repeatedly. Starting with a flat surface and ending with a very rough surface in
a totally random fashion and then look around the surface for some hot spots for
good spectra. The hot spots came about where you have small particles that
come close together and a small gap between them. As opposed to
these structured surfaces with larger cavities.
Q: Do you always have one size voids, is there any advantage in having various sizes.? I
used to do soil mechanics at one time and we found that with very compacted soil you have spheres of
sand and if you graded it properly you got smaller ones in between?
We played around with some of those things . The first thing that happened is that when
you mix 2 types of spheres is that they tend to segregate - large ones
clumped together in one area and small ones somewhere else.
You can get coloidal crystal structures if you get the ratio of the sizes right. There is an
AB12? structure you can form over a small range of sizes that will work. Those
refer to the 3D structure , not the 2D . We thought we could put small spheres
between the bigger ones but then they are much much smaller and when you're growing
through you completely bury them too quickly. What we have done is start out withthe
larger spheres , grow some metal, form part of cavities, dissolve away, so a dimpled
surface. Come back with some smaller spheres and put them on the surface.
They end up , one sphere in each cavity , you may think that a bit surprising. But if
I have a sphere on surface and there is some water , the water is between the
sphere and the surface , if its not in the centre then surface tension , pulls it to
the centre - its a self-centring system. If it was off to one side then the surface tension
is a lot higher on one side than the other. There is a mechanism that operates
during evaporation that pulls alll the spheres straight to the middle of the cavities.
Then you grow more metal around them and take them out and you have small
cavities on a bigger pitch.
If you have a small sphere in a big void , you can tune the cavity in between?
We played around with a lot of this. We tried the spheres o nthe surface, growing
a polymer conduscting around the spheres , destroying the conductivity
of the polymer so it becomes an insulator , dissolve the sphere out and plate metal
in , so have metal spheres and then burn away the polymer. We were interested in the
magnetic properties of materials like Nickel in small circular structures approaching
each other. There is an interesting magnetic property as to which way the magnetic
moment goes in those small spherical cavities it curls around and around and then
in the middle it has to point up or down so a 1 or a 0. That would be a great way of
doing data storage except that you'd have to make the spheres 10 to 100 times
smaller than the ones we could make, so scuppered that idea.
Q: Have you made an invisibility cloak yet ?
Have you seen my friend here.
Q: Even at one wavelength only?
No, not yet, black at all wavelengths ,but no
One material , you said one side looked red with the gold, is that to do with
the nano-particle with gold being red?
As in red glass in mediaeval churches, that is the gold nano-particle and the plasmonics
of the gold nano-particles. Its the same kind of effect. The "red" particles in ruby glass
are about 20nm in diameter . The mediaeval glassmakers discovered that by throwiong
in some gold leaf into the glass melt it would spontaneously form these small particles.
Thats why such red glass never fades, its actually trapped gold , 20nm in size
dispersed throughout the glass. With our red/green nano-tripe ( TM?) looking from one side you
have a wider opening coming to a narrow and the other side a narrow opening , opening
out. So from one side it looks red and the other side looks green.
These films are over glass?
This one is free floating . You make them on glass but in future for hadling ,w e intend to make a stamp of PDMS?
a polymer stamp on a bit of silicon and we can pick the film up on the stamp and then
transfer it onto something else, printing it on, stamping it on, like potatoe
printing. Here its floating around in solution, connected up to the grid on the electron
microscope. We can make it roll up like a Swiss Roll. Put some polymer or some stress on
top of the film , as it disolves away the silver it rolls itself up. It looks
like a brandy snap. It has some interesting optical effects.
How thick would that be in comparison to graphene which is thin material?
This is a couple of hundred nm thick. A gold atom is about 0.1nm ,
So much thicker. If I only have one atom of gold thick layer it won't develop the optical
properties of gold or the electrical conductivity of gold. Its needs several layers
fo rthe overlap of wave-functions to make the ? and the band structure.
When nature made iridescent butterfly wings , what does that surface look like
in comparison to your surfaces?
Pic of a blue butterfly , a morpho? butterfly, with iridescent blue wings. They have structures in the
wing, made of dielectric material. A christmas tree like structure that modulates the refractive
index of the material in the wing. The light is reflecting off these, giving the blue colour.
Also the structure of the wing means water does not wet the wing, it just rolls off.
Q: why is it so consistently the same wavelength of blue across all the different
generations of the same species. ?
You have to control the genetics that makes this structure. This is one type a morpho-retinous ?
type of butterfly, there is another type the morpho-vidius? they have different structures and different
a cross-section of this active part of a wing
a wing made of what look like tiles on a roof and then a section through a tile
with the structure and 2 different refractive indeces of the 2 different materials.
The scale dimensions are irrelevant to the colour. If you take a butterfly wing
like this and a drop of acetone onto it , the colour changes because the refractive
index changes and then when the acetone evaporates, the original colour comes back again.
The colour depends on the refractive index of the over layer as well as the substrate.
Its the difference in the refractive indeces. If you go to the Isle of Wight you
may find a similar species to this , but the same blue effect. More a 3D structure
rather than my more 2D structures. The verticality of the stack is having the effect
on the light. Like a stack of layers , called structural colours . That is why
butterfly wings never fade in colour. No dye , so just as bright 50 years later .
When they try to focus Xrays they have to do it by little bits, can't focus on a glass
like ordinary light. A series of mirrors so the light is almost parallel to the mirror all
In stacks, in a way its similar , in a different wavelength regime. You see the same
iridescence in some kinds of beetles. The colours depend on the spacings and the
Going back to Mr Raman and his ruby, it always showed a cut ruby but I assume the
effect is irrelevant to the faceting and a rough ruby would be exactly the same.?
Yes. He had an intense source of light , the Sun in India , inverted telescope ,
some kind of filter for 1 wavelength . He needed the ruby surface to be reasonably
flat as he didn't want it scattered off . Then looking at the shift in energy of the
light coming back out.
He didn't use Mercury discharge lamps , which I assume were available then?
No just withthe sun. In the early days and the need for one defined wavelength
because you were looking for small shifts then you would take a long toime to
acquire the spectrum . As an undergrad I remember doing the Raman spectrum of
carbon tetrachloride . Set it up with a piece of film and leave the lamp on for
12 hours . Come back and develope the film and some blurry lines of the
Raman spectrum. Now we have lasers and they give you much more intensity
and at a single wavelength. So Raman spectroscopy came much easier to do in a
reasonable amount of time. Well colimated light source and good filters allows
us to look close to the Raman line. Our spectrometer would look
up to about 400 wavenumbers from the main line but can get better than that with better
filters. 400 is not that much in terms of energy shifted.
I'm surprised you get a regular thickness when builing up your structures , as you
go up, even a mirror top surface?
Thats the skill of the molecular chemist being able to control the nucleation
growth . To get a mirror surface you need high nucleation density , a lot of
nuclei at the metal surface where it starts growing from. You add in additives
that are called brighteners which lead to the surface growing very smooth.
So molecules that absorb on the growing metal surface and slow down
the growth of aspherities? and things that grow rough and tend to make
it out flat. There are a lot of tricks that people have developed with electroplating to
enable this . Under some circumstances you can get very rough surfaces , its all
about the conditions under which you do it , dendritic or not, depends on the driving
potential , how hard you're driving the deposition , how fas tthe mass transport
is . If transport limited , it would tend to grow into dendrites and also the additives
that you put there. The art of this plating is in the surface chemistry
and how you control that.
When you showed some of the faults of the cavities in some of this material ,
one of them , unless it was an artefact of the projection, was very linear, over 20 cells or more ,
was that a fracture?
A slight slip. You're only seeing the meatal there , but it was a slight mis alignment in the
template . The spheres we use have about 1.5 percent coefficient of variation
in diameter, very uniform. All one single crystal domain , just a defect from where the
spheres have slipped a bit.
There are 2 such defects exactly parallel and I would have expected more random
wormy-like defect paths. ?
The way these were packed , I don;t know from this micrograph, in how it was organised .
But basically a vertical surface , where you're going to do the packing , put a solution of
the spheres into the cell . Make the cell thin because we don't want the miniscus to
be oscillating around. 50 to 100 micron separation between the 2 sides of the cell and fill
the solution in there. The solution is dragged out to a miniscus in this thin
cell . We make sure the gold surface is extremely clean , that we want to pack on to.
Sometimes we add chemicals on the surface to control the contact angle to about
20 degrees. The spheres and particles in there and we put it into an incubator
at a fixed temperature and allow the water to evaporate slowly. With the evaporation
the miniscus is going down over a few hours. This evaporation sets up convection of
the particles in the solution which brings particles up to where the miniscus is
and when the solution is thinner than the particles is a large force called the
capilliary immersion force, that pulls the particles together, much bigger than kT (?),
that is what is doing the packing and overall organised to keep feeding in
new particles as the miniscus sweeps down the surface as it evaporates.
If you get it right you get a well packed region over your template . Get it wrong and the
miniscus sticks and jumps, sticks and jumps , then you get cracks and breaks in the structure.
The fact that all those defects were aligned , the first I'd have to ask is how they are
aligned to the moving miniscus but I don;t have that info. I guess the student or whover
put it in the microscope lost that info somewhere. Even which part was up and which was down.
Over the years we've got much better at the packing . A pic of an earlier version
with more of those fefects .
Do you have to be hypercritical of the purity of your chemicals, not exactly household
materials or even BP pharmaceutical purity.?
There is nothing fancy here that you couldn't do in the kitchen, you need to spend time
cleaning things up. We use water that comes from a reagent grade water purification
system, for example. We clean things with surfactantsa that make the glass really clean,
but not in a clean room. Electrochemistry is very sensitive to things absorbing on the
surface . A monolayer of molecules on the dsurface can affect the properties .
How consistent are the spheres? like the person who makes the balls for biros .
Thats a neat bit of chemistry. They are made by emilsion polymerisation.
Yo u make an emulsion of little droplets of monomer and you polymerise
each droplet. So as long as you can make droplets in a liquid, that are all
of the same size then you get spheres all of the same size. The people who make them have
got very good at making them, about 1.5% coefficient of variation. So if I buy
spheres trhat are 610nm in diameter then 10nm variation. Its not so easy making the
spheres if 5,10 or 20nm diameter . We have bottles of different sizes kept in a fridge.
Do sieves exist for those sorts of dimensions if you wanted to sieve anything?
I think that is what we've made , sort of. You would separate them by sedimentation
and fractionate. They are about 100 quid for a bottle , but that will do a lot
You said you could control the colours by how big the holes were , how does the colour
change so much by that relationship?
Its not as simple as that. When its a thin thickness , its not much different to the
silvery metal, but just with little dimples in th esurface. The most intense colours
come about in the middle, at half height. Where all the interactions occur and gets all
mixed up. You get the standing waves in the cavities .
Why does the roughness increase the Raman effect in the original raman experiments?
Is it expressing the direction or somehow enhancing ???
The SERS enhncement , the predominant effect is the electromagnetic effect . When you have
aplasmon , you have the light at the surface and you get an enhancement of the electric
field at the surface. The SERS effect goes as the coupling of the laser in as the
square of the electric field and the coupling of the light out goes as the square
of the electric field. If I increase the surface by making it rough or by my
nanostructure , by a factor of 100 then I'll have 8 orders of magnitude in the SERS
spectrum . Because there is a square on the way in and the square on the way out.
On this kind of surface I can enhance the local electric field by a factor between
10 and 100 . Thats why its such a big effect.
When you start moving from the relatively simple molecules like the benzine-thiol
or piridine , how do you ensure that you get consistent alignment, ie not get random
There's a whole bit I didn't tell you. If you think about this electromagnetic
enhancement at the surface, its actually a field perpendicular to the surface.
the orientation of the molecule , in respect to this field has a big effect on the
enhancement. The benzine-thiol is sitting on the surface with the benzine ring
up from the surface , you get the enhancement. If the benzine was sitting
in the plane of the surface , you would not see those modes enhanced and you'd
see a different spectrum . It does matter what the orientation is. In the dye systems
that we use on our DNA experiments, the DNA is aligned perpendicular
to the surface and the dye system is sticking to the side of the DNA so its in the
right orientation to give us a largge enhancement.
How do you specifically set it up to ensure that orientation?
We do that by the chemistry. We attach the DNA to the surface tby the 6 thiol residues,
that is the footprint. Then a flexible ethylene oxide linker DNA to DNA strand , we passivate the rest of the surface
with a thiol molecule with an O-H molecule on the outside to stop the DNA
lying down and sticking to the surface. This approach has been demonstrated
by neutron reflection and other experiments to basicallly make the DNA stand up.
It can pitch around on the surface , but broadly stands upright. The reason our
spectra, you don't see the DNA bases . With aromatic rings you see nice
Raman spectra but they are in the plane of the DNA . When onthe surface they are in a different plane
and not significantly enhanced. Our dye molecule, in this case, wants to stick to the side
and will be enhanced. That is what we see in that orientation. One experiment that
we've been doing in the last few months is to make the DNA attach to the surface
in 2 places so it lies down . When we achieve that then we can see the Raman spectra of the dNA bases
because now we've flipped it over. Playing around with the orientation is an interesting thing .
In order to calculate the spectra as a function of orientation , you have to calculate
the polarisiblity tensor of the molecule , know how its varying and then get some idea
of what the spectra will be and get some orientation information out.
We are doing that just at the moment. People don't often do that in SERS , but now
that is becoming interesting because one of the interesting things about DNA
is molecules that react and bind to DNA . Some are called intercolators
that slide in between the bases , intercolate into the chain. THey are aromatic systems and if they
intercolate into our DNA this way we don't see them in the Raman spectrum but
if they are groove-binders , running around the outside , then in the other orientation we can
see a spectrum for them. Some are carcinogins and a whole interesting area
of stuff we are working on at the moment.
Q: You're interseted in a single scan/span? looking at the intercolators
and the groove-binders at the same time or 2 separate stages?
The first thing is we found it tedious having to attach the dye label to the molecule in
order to see that , so if we could get rid of that and put i na groove-binder , just by adding
to the solution, then we have a label-free way of doing our experiments. That works and we've
just done that. But then when we did that, we could measure how easy it was
to oxidise and reduce the molecule in the groove of the dNA and compare it to water
and that may be related to genetic damage. Then we want to flip the molecule around to
explore the intercolators or if we have methylation of the DNA - Epigenetics .
If we put it this way round we could discover how much methylation we have in the DNA.
Lots of interesting chemistry, biochemistry and things to do by manipulating the
DNA on the surface and the SERS now we have this high sensitivity. That is all
stuff we are playing around with at the moment.
Despite the fact these spheres are polystyrene , are available and the right size and
your clever techniique is an efficient way to lay down a monolayer do you have any
intentions to play with other shapes other than spheres , cylindrical or hexagonal holes ?
People have made cylindrical holes using e-beam lithography . the structures are much
rougher , electrochemistry is uniquely good at making very smooth surfaces
which is good for the plasmonics because plasmon propogation is better on a smooth
surface. They don't have such an interesting property . If you think about confinement
of the plasmon , the fact the thing curves over like a segment of a sphere , keeps it in there
and wandering out . A cylinder can move up and down and leak out.
I'm stil inclined to not go with brute force e-beam lithography and stick with trying
to do these tricks. It would be nice to get ohter shapes other than spheres and pack
them as templates but its not easy t get polystyrene cubes or tetrahedra or something.
Some people have thought of using diatoms , structures from sea creatures and growing
through them , then disolving away . Its quite easy for people to use the spheres technique,
you don't need some special bit of equipment. A colleague wants to put a supported
lipid bi-layer on the top and look at molecules that interact with the lipid bilayer
via SERS, getting SERS enhancement via the lipid bi-layer and look at proteins etc.
Presumably the electrochemical environment is far too unfriendly to start using it on
the likes of globular ptoteins to create that structure ?
People have done things with protein templates in fact, some in the USA. They were down
in the 10s of nm scales and growing metal through those. Some of the protein structures
that come from the outside of organisms , covering the shell membrane .
With these spheres of light wavelength dimendsion , if you go up to microwave and bigger spheres ,
is there any possibility ?
You can certainly buy spheres that go up to 10 or 20 micron diameter . I said earlier
about making mirrors out of cavities, but thats all we did. Just to show that bigger
spheres behave like ray optics so I've not thought about what you can do in
terms of microwaves. When you go down to smalle rsizes the problem becomes
that you can't get the spheres reproducible in size. So at 20 nm they tend to be +/-
a few nm v so they don't pack so well. The packing is also more difficult because of the
effects of Brownian motion are much greater. That is one thing that disuaded me from
trying to make high density magnetic memory from 10nm spheres. You cannot even pack neatly
Is there a practical use , say backed onto plastic ?
If you would like to make some jewellery . Jeremy Baumberg ? who I colaborated on
with this. He took the spheres and built another polymer around them and 2 dielectrics
and he was able to make a T-shirt that when you flexed your muscles it changed colour
with no batteries at all. You might be able to buy them soon. You can make strips of this
stuff, stretch it and it will change colour. Making this kind of metal structure but in a polymer
, stretchy polymer.
You could make good strain gauges for attaching to test materials?
Has anyone looked it as a potential for using it for catalysis, you could get finely tuned?
Another interesting area, getting plasmonic enhancement of catalysis, yes. I think the
answer is that no one has looked at this kind of structure . I know of one projected
experiment for trying this, a very hot topic.
What we have done is use this sort of structure to make a plasmonically
enhanced solar cell. Silver in that case, light hits the structure , excites the plasmon
and generates electron-hole pairs in semiconductor or polymer overlayer , they'r e
separated and you get a photocurrent.
Enhancing by how much?
The idea is that if i make a junction in a polymer solar cell , 2 polymers with a junction ,
I have the light coming in and gets absorbed in only the junction region , makes holes and electrons
that I can separate. This other way of doing it is to make a sculpted surface , have the light
excite a plasmon on the surface , that plasmon extends about 100nm out from the
metal surface and you arrange your polymer junction in hat 100nm . So shining the light
just along the interface that you want. In this case about a factor of 5 or 6 times improvement.
The fill factor was not as good , and it depends on how good you are at making the cell.
the idea is an interesting one and bears more investigation.
It strikes me that making your surface with larger spheres , perhaps not requiring
such tight constraint on the miniscus forming and regular packing , would produce a metal
with a high surface area that could be useful in battery technology.?
This doesn't make a very high surface area, for each layer of spheres that you grow up,
the area increase is by aboutr a factor of 4 increase. Not bad, but where I started from
and the surfactants , making 2nm diameter holes in the metal and about 1000 fold
increase in surface area. 2nm across and cylindrical pores so a very big enlargement.
Some of my colleagues in France used this kind aof structure , made several
layers, put enzymes on it and used it to makle biosensors for glucose and stuff, with this
surface enhancement. There is too much metal in the wall, if you like, if thinking
about it as a catalysis material. I prefer the spheres as a way of controlling the
geometry . We started in the area of magnetism , making magnetic
materials and looked at magnetic properties and superconductivity.
Would a wall of this act like an anarchoic chamber does for sound and absorb all
the light and give you completely black ?
The completely black material with near tortal spheres and a small neck out
at the surface and lobster-cages the plasmon and never comes out. You calculate
that for all incident angles and for a range of wavelengths it will all be absorbed.
The absorbtion goes up to 1 , so is completely absorbing. Over the range 1.4 to 1.7eV
or so . My thinking was a bunch of these with lots of different sizes and
then I'd have your ideal black. Thats been shelved for a while. But it comes back to
mixing up the spheres of 3 or 4 sizes , getting 3 or 4 features in the right places
and ending up with something that was black over a reasonable range of wavelengths.
But as I said the spheres tend to segregate out.
How much of this bouncing around of ideas comes out of a formal process and how
much is random coffee-machine type dicussion or interdisciplinary stuff ?
This is science that is done in groups of 3 or 4 , not enormous teams . Its a
question of challenging people saying , I bet you can't do X. Then people
taking up the challenge to see if they can. So you need people who are prepared to
try stuff. PhDs or post-docs who are prepared to be challenged and have a go.
You have to couple that with people who have a reasonable knowledge of the
literature and what might be interesting things to do. Its about an attitude
of being flexible and wanting to try new things and not keep turning the handle.
It requires the right attitude amongst the people in your lab . And sometimes
just prepared to bodge something up to see if it may work, rather than engineer
something and find its never going to work. Then do it properly afterwards.
Not having a narrow vision, this is what I'm working on and not interested in a seminar
by someone on a topic that doesn't seem to be related. You may find that you pick
up strat bits of info , via that process and store away for a year or two.
When does the commercial application side come in , I'm thinking of those
hand-held analysis SERS machines for identifying drug samples or whatever?
Does an outside company say we've got this idea can you develop it?
So I may get some funding to do sonething on DNA say , often a mixture of
both as far as where the ideas come from . Or like the Taiwan student and his DNA
fingerpring idea , that was just him. He is sponsored by the Taiwan government but
he read a paper we published and decided it was interesting and he wanted to come
and do something. Its about who you know , and who knows what you're doing.
So is the ideal challenge environment cross-dicipline discussions in the pub?
We still have one challenge which is the Marylin Munroe structure which we haven't
made yet which is one cavity, close it up, make a sphere on top , a double structure
like that may have some interseting properties.
There is something that Jeremy wanted, and I think I've worked out how to
do now. Put something in the hollow sphere that comes up from the bottom
, inthe middle of the cavity. Electron micrograph of spheres sitting on a flat
surface , that can be a different metal and I can selectively put
things on one metal or the other. Say nickel and gold I can do things that stick
to nickel but not to gold and then build other structures within .
If you disolved that base metal away , turn the layer the other way up , you could
stand the spheres on those little holes and make a Marylin Munroe?
a man after my own heart, exactly yes. Exactly how you think about what to try.
Some public domain images and graphics in the thesis of one of the team, Tim Kelf
fig 4.1 shows the template production method
one of the images out there
2007 MRS SPRING MEETING "SCIENCE AS ART" IMAGES
2nd Place Winner
Water on a Nanostructured Gold Surface
The image is a photograph of a droplet of water sitting on a nanostructured gold surface
prepared by templated electrodeposition. The colours are produced by the reflection of white
light and excitation of surface plasmons on the structured surface.
Surfaces of this type show strong surface enhancement for SERS of molecules adsorbed at
Credit: P. N. Bartlett, University of Southampton
Photographer: Steve Shrimpton, University of Southampton
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