Poking tips at surfaces –optical
properties of molecules, electronic
properties of quantum dots and the
formation of synapses
Peter Grutter, FRSC
Physics Department, McGill
Assoc. Chemistry Department, McGill
Assoc. CRULRG, U. Laval
RQMP & CIFAR
P. Grutter
The effect of concept-driven revolution is to explain old
things in new ways. The effect of tool-driven revolution is
to discover new things that have to be explained.
—Freeman Dyson, Imagined Worlds, p. 50
P. Grutter, McGill University
AFM – ‘seeing’ atoms and molecules
C60
PTCDA on KBr,
unpublished
Burke et al,
PRL 94, 096102 (2005)
PRL 100, 186104 (2008)
KBr
P. Grutter, McGill University J. Mativetsky, S. Burke, S. Fostner, R. Hoffmann, P. Grutter
3D Force Field Imaging
Ag
Cl
CO
Pentacene
Pentacene on Cu(111)
al., Science
P.Gross
Grutter,et
McGill
University
325, 1110 (2009)
Outline:
Function determination by AFM
• Background: force spectroscopy
• Optical spectra and Kelvin probe of PTCDA
• Energy level spectroscopy of Qdots
• How does a neuron form a synapse?
P. Grutter, McGill University
W(111) tip
on Au(111)
Field ion microscope manipulation
of atomic structure of AFM tip
Cross et al.
PRL 80, 4685 (1998)
Nature Materials 5, 370 (2006)
P. Grutter, McGill University
Site specific chemical interaction
potential: Si(111) 7x7
Lantz, Hug, Hoffmann, van Schendel, Kappenberg, Martin,
Baratoff, and Guentherodt , Science 291, 2580 (2001)
P. Grutter, McGill University
DNA “Unwinding”
Anselmetti, Smith et. al. Single Mol. 1 (2000) 1, 53-58
AFM probe
Au surface
Nature - DNA replication,
polymerization
Experiment - AFM force
spectroscopy
DNA Intercalant Ethidium Bromide
A,G
T,C
Fo rce [400 pN / div.]
Duplex poly(dG-dC) with EB
b = 0.8 nm
L = 462 nm
200
Duplex poly(dG-dC)
A,G
Fo rce [400 pN / div.]
EB
T,C
300
500
b = 0.8 nm
L = 778 nm
Melting Transition ~ 300 pN
B-S Transition ~ 70 pN
300
Anselmetti et. al. Single Mol. 1, 58 (2000)
400
450
600
750
Molecula r Extension [nm]
Typical forces and length scales
Gaub Research Group, Munchen
Outline:
Function determination by AFM
• Background: force spectroscopy
• Optical spectra and Kelvin probe of PTCDA
• Energy level spectroscopy of Qdots
• How does a neuron form a synapse?
P. Grutter, McGill University
Why study molecules on surfaces?
Molecular Electronics uses molecules as
the building blocks of organic electronic and
optoelectronic circuits.
“Proof of concept” single molecule device
Other applications: OFETS, OLEDS, sensors, rectifying
junctions, non-linear optics, photovoltaics. Also interesting
surface science.
Structure of PTCDA on NaCl
+
-
+
1.44nm
PTCDA (3,4,9,10-perylene
tetracarboxylic dianhydride)
•non-uniform charge distribution
•Herringbone crystal structure
•Interest for thin-film transistors, OLEDs
P. Grutter,
NaCl and KBr are well known within NCAFM community: “easy” to achieve atomic
resolution
•Cubic structure cleaves along (100)
planes
•“Checkerboard” surface
•Can get large atomically flat terraces
Can be prepared UHV start to finish
Crystals cleaved in situ
Molecules thermally deposited in situ
McGill University
nanostructured NaCl
 nanoscale pits created by
charge damage


size controlled by charge
dose
density controlled by
temperature
 pits used ~15-20nm
average edge length, 9% of
the surface removed
 we use our e-beam evaporator as a
charge source
3 types of structures


herringbone crystallite
-PTCDA
p3x3 free monolayer

p2x3 trapped
monolayer
PTCDA Multilayer Calculation
Wei Ji, Hong-Jun Gao, Hong Guo
 MM calculation shows p2x3 monolayer
more favorable than p3x3


p2x3 intermolecular interactions dominate
p3x3 intermolecular and molecule-substrate
interactions comparable
 Multilayer calculations show ordered
2nd layer only for p2x3 interface

2nd layer structure close to -PTCDA
 Supports idea that interface is
altered to accommodate additional
layers  leads to dewetting
Influence of structure?
 imaging provides structural information, but how
does structure influence functional properties?
 light can be a control knob

photogating
 light can be a power source

photovoltaics
 light can be a measurement tool

optical spectroscopies
Measure properties by force
spectroscopy
∆f
VCPDtip-sample
vacuum level
Vt-s
Experimental set-up
3 source molecule
evaporator (behind)
4 source metal
evaporator (behind)
Quartz crystal
LEED/Auger
deposition monitor
cryostat
Imaging chamber
Cleaving
station
Ion sputtering gun
 Imaging: JEOL-JSPM 4500a with NanoSurf PLL
SEM
illuminating!
3 wavelengths:



473nm (DPSS) / spot size 0.6mm
488nm (Ar-ion) / spot size ~2mm
514nm (Ar-ion) / spot size ~2mm
Example CPD measurement
 ∆f vs. bias voltage measurements
taken at each site for each
wavelength and dark
 each curve fit to determine CPD
 more reliable measurement than
KPFM feedback
 eg: PTCDA crystallite, 488nm


dark: typical dfv with parabolic
shape
488nm illumination: shifted
curve (-0.9V), asymmetry at +ve
bias
Photoinduced shifts
 Notable shift for all wavelengths of
PTCDA crystallite

bulk-like structure should show thinfilm characteristic
 Decreased shift for PTCDA-pit



similar structure to bulk
small size of pits limits accuracy of
CPD measurement
variety of defects in pit structures
may lead to variability in CPD
 No shift in PTCDA ML except 514

extended nature of p3x3 epitaxy
reduces intermolecular interactions
 monomer-like states
structure  function
 nc-AFM used to characterize an opto-electronically active
molecule on an insulator
 high resolution imaging combined with
optical/electrostatic characterization allows link between
molecular scale structure and functional properties
 selection of substrate and templating strategies give
control over structure: could be used to tune properties
S.A. Burke, J.M. LeDue, J.M. Topple, S. Fostner, P. Grutter
Advanced Materials 21, 2029 (2009)
Outline:
Function determination by AFM
• Background: force spectroscopy
• Optical spectra and Kelvin probe of PTCDA
• Energy level spectroscopy of Qdots
• How does a neuron form a synapse?
P. Grutter, McGill University
Why study Qdots?
• Engineerable atom
• Fundamental physics of strongly coupled quantum
cavity system (mechanical oscillator with electron)
• Potential scalable solid state cellular automata
• Potential solid state, scalable Qbits
• Single photon sources for quantum cryptography
P. Grutter, McGill University
Electrifying results at low T
– Qdot spectroscopy by AFM
– Single electron back action on cantilever
– Electrostatic potential variations on surfaces
– Spatially localizing and optically controlling a
1/f fluctuator
Electrostatic Force Microscopy
101
Type of AFM where electrostatic forces are probed by using a
conductive force sensor and back electrode (either the sample or
underneath the sample).
1 C
2
Fes 
(Vtip sample  VCPD )
2 z
C: Tip-sample Capacitance
Applied Bias
Z: Tip-sample gap
Voltage
Contact
Potential
Difference
AFM as electrometer with single electron
sensitivity
C. Schonenberger and S.F. Alvarado,
Phys. Rev. Lett. 65, 3162 (1990)
L. J. Klein and C.C. Williams
Appl. Pys. Lett. 70, 1828 (2001)
4 K, 8T AFM
M. Roseman and P. Grutter, Rev. Sci. Instr. 71, 3782 (2000)
Atomic Resolution at 250mK, 16T
James Hedberg
InAs quantum dots on InP
substrate
Spectroscopy on quantum dots
Shell structure in SAQD
Theory: S. Bennett & A. Clerk
Band diagram of system
Band diagram of system
Band diagram of system
Note in particular  = f(x,y,z)
Bennett et al, Phys. Rev. Lett. 104, 017203 (2010),
We understand the dissipation
peak shape in great detail
Constant height dissipation
images
20 nm
tip-sample distance 18 nm, 4.2K
We can also quantitatively determine
coupling between dots
Multiple quantum dots
coupled via Coulomb
and tunneling
interaction. Can be
quantified.
What do we learn?
•Energy level spectroscopy by EFM
by detecting the back-action of single
electrons
•AFM tip used as gate as well as a
sensitive electrometer
•Requires no electrode! No It
•Isolated and coupled QDs can be
studied
•Excited state spectroscopy possible
•Manipulation & spatial mapping of
charge traps possible
DNA as a scaffold for Au NP quantum
cellular automata
Collaboration with Prof. Hanadi Sleiman,
(Chemistry Department, McGill)
F. Aldaye, H. F. Sleiman;
J. Am. Chem. Soc. 129, 4130 (2007)
DNA as a scaffold for Au NP cellular
automata
Collaboration with Prof. Hanadi Sleiman,
(Chemistry Department, McGill)
F. Aldaye, H. F. Sleiman;
J. Am. Chem. Soc. 129, 4130 (2007)
Outline:
Function determination by AFM
• Background: force spectroscopy
• Optical spectra and Kelvin probe of PTCDA
• Energy level spectroscopy of Qdots
• How does a neuron form a synapse?
P. Grutter, McGill University
Neurons
Cells: primary hippocampal
neurons in culture derived
from E17/18 rat embryos
Transfected on 7th day in
vitro (Lipofectamine)
Collaboration with:
Montreal Neurological
Inst.
(D. Colman)
U. Laval (Y. de Koninck)
F. Suarez, Dr. .Thostrup,
B. Smith, Dr. H. Bourque
McGill Centre for NeuroEngineering Project
Overview
Formation of an artificial synapse
Observation:
Rapid assembly of functional
presynaptic boutons triggered by
adhesive contacts with Poly-D-Lysin
coated beads.
Control: uncoated beads
J. Neuroscience 29, 12449 (2009)
Modified AFM tip
• Beads: polystyrene latex beads, 7m diameter
– Sulfonated at cell surface
– Coated with poly-D-lysine (positively charged)
• Bead-on-a-tip: Bead mounted on AFM cantilever
30μm
AFM for the Life Sciences
AFM/SNOM on inverted
optical microscope with
•Heated profusion cell
•CCD with single photon
sensitivity
•5 color TIRF (allows e.g.
FRET)
•Patch clamp
axon
spine
Relevant presynaptic proteins
Proteins of the active zone:
synaptophysin
Reference: Ziv and Garner, Nature Rev. Neurosci. 5, 385 (2004)
Axon length
Forming a synapse:
GFP labeled bassoon
time
Picolo-Bassoon Transport Vesicles: 23±10 min
Synaptophysin Transport Vesicles: 43±9 min
Formation of strings upon pulling
on adhesive contact
The strings were shown to contain synaptophysin,
bassoon, actin and tubulin.
Although not explored in detail, the presence of actin
(an important scaffold protein) and tubulin (a protein
important for vesicle transport) could indicate that the
strings are functional. In a few cases vesicles
transport as observed.
(images: actin GFP labeled)
Development of liquid SNOM
Lopez-Ayon, LeDue, Miyahara, Bourque and Grutter
Aim: develop reliable SNOM probe
Methods:
Etching, bending & evaporating on a fiber,
Focused Ion Beam nanomachining
Understanding
system noise:
layering of OMCTS
on mica in water
seen on inverted
optical microscope!
SNOM tip; high Q liquid operation;
First application: Mechanotransduction
in live osteoblasts and osteoclasts
Nanotechnology 20, 264018 (2009)
AFM and biology
• Powerful tool, in particular in combination with optical
techniques, not just for imaging, but also spectroscopy
and positioning.
• Challenging to make impact in the life sciences
The effect of concept-driven revolution is to explain
old things in new ways. The effect of tool-driven
revolution is to discover new things that have to be
explained.
—Freeman Dyson, Imagined Worlds
-> close collaborations necessary !!!
Conclusion
• Force is an important concept!
• Can be measured by AFM
• Can be used to determine structure at sub-nm
length scales
• Is being developed as a tool enabling the
measurement of properties
• Structure-property relationships at nm scale in
many different systems in the near future! Very
exciting for Material Science as well as the Life
Sciences!!
Development and application of
AFM new techniques:
the Grutter Research Group
• Magnetic reversal
MFM with in-situ field
• Molecular electronics UHV AFM/STM/FIM, AFM/STM/SEM
4K, 8T and 50mK, 16T AFM
• Quantum dots
AFM + patch clamp + single photon
• Interfacing to living
fluorescence + TIRFM
neurons
• Biochemical sensors Cantilevers and electrochemical cells
www.physics.mcgill.ca/~peter
The Team:
Supported by
NSERC, CIHR , FQRNT, CFI, CIfAR,
GenomeQuebec, James McGill Chair
16 graduate students,
6 post doctoral fellows