Download Nanoscience Experiments Utilizing Scanning Tunneling Microscopy STM Imaging Applications

Nanoscience Experiments Utilizing Scanning Tunneling Microscopy
Corey Slavonic, Chaitra Rai, Kevin F. Kelly
Graduate Research
Department of Electrical and Computer Engineering
STM Imaging
Applications
Scanning Tunneling Microscopy (STM) is a local scanning probe
experimental technique in which an atomically sharp metallic tip is
brought to within a nanometer of, and then raster scanned over, a
surface. An applied bias voltage between the surface and the tip causes
electrons to tunnel through the nanometer gap, which is read as a
current, and a feedback loops adjusts the tip-surface distance to
maintain a set current. This technique can resolve the atomic structure
of flat, conducting surfaces and, because of the dependence of the
tunneling current on tip height, can be used to perform electronic
spectroscopy.
An energy diagram showing
the filled states in the sample
and tip, the work function Φ,
the applied bias V, and the
decaying wavefunction (blue
curve) tunneling through the
gap
The STM provides researchers a tool to explore
the topographical and electrical properties of
surfaces, and molecules adsorbed on surfaces,
with atomic resolution. For example, the atomic
lattice of graphite (HOPG) is seen (left-top),
along with a self assembled monolayer (SAM) of
octanethiol chains on gold steps (left-bottom).
The STM tip can also be used to manipulate
individual atoms and molecules as shown with 35
Xenon atoms on Nickel (right-top). Another
novel application is the imaging of standing
electron waves(right-bottom) in a quantum well,
a demonstration of STM as a quantum probe.
D.M. Eigler, E.K. Schweizer. Nature 344, 524526 (1990).
1. M. F. Crommie, C. P. Lutz, D. M. Eigler,
Science 262, 218–220 (1993).
Manipulating Individual
Molecular-scale Machines
We explored a class of molecules called Nanocars. The chemistry behind
these various molecules has created a kind of molecular tinkertoy set,
with varying arrangements of oligo (phenylene ethynylene) (OPE)
constituents. Molecules of various sizes and configurations have been
synthesized and characterized, exhibiting a unique collection of surface
properties and behaviors including significant flexibility, atomic step
crossing, and rolling surface mobility.
C10H21O
H
C10H21O
OC10H21
C10H21O
C10H21O
C10H21O
H
H
OC10H21
OC10H21
OC10H21
C10H21O
OC10H21
H
OC10H21
The STM tip can be used to move
individual molecules over the
surface by adjusting the tip
position. The image above shows
the before and after of a nanocar
manipulation perpendicular to the
axles, which successfully moved
the molecule. On the left is an
unsuccessful attempt parallel to
the axles. Substrate annealing up
to 200 ºC (seen on the right) has
also shown that the Nanocar
moves much more easily in a
direction perpendicular to the
axles.
Imaging CVD graphene
over polycrystalline Cu foil
Using traditional ambient pressure chemical vapor deposition (CVD)
method, Cu foils were used as a substrate for graphene growth with
large grain sizes, controllable thicknesses and high in-plane continuity.
With atomically resolved STM, we find
that monolayer graphene grows heavily
influenced with the symmetries of the
different underlying Cu facets.
The Cu foil was subjected to low
temperature annealing, revealing striped
Moiré patterns. These are caused due to the
lattice mismatch of graphene and
crystalline domains of Cu foil. The
hexagonal lattices connect well at the facet
boundaries and over defects on the surface.
Probing Topological States
Topological insulators (TI) are a new class of materials with properties
related to the quantum hall effect. Due to their strong spin-orbit coupling
electrons in the bulk localize leaving only the edge (or surface if the
material is 3D) available to conduct, with the electron’s spin locked to its
momentum. With STM we measure the convolution of local density of
states and topography, and so can probe both electronic and structural
information. Here we image select bias voltages and look at how
electrons scatter between available states at a particular energy (eV).
Real space
Rectangular
depressions on
the Cu surface
A
B
C
IV-curves and differential conductance
curve of single layer graphene.
D
Gr/Cu Moiré
Pattern at two
different Cu
facets.
Here we can see two types of patterns on the surface: at the small scale,
due to the atomic periodicity; and slightly larger, due to electron
scattering on the surface. Taking the Fourier transform shows the
wavenumbers of the scattered electrons, which we can map as a function
of bias voltage, as shown below.
E
-15 mV
The nanocar project aims to explore how scientists can control chemically
synthesized bottom-up systems and merge them with the ongoing research
in biological machines. The STM is the only instrument that can provide
direct control through tip manipulation, though other methods, such as
switching motion due to light irradiation, are currently being researched.
Through the options given by synthetic chemistry we can continue to
explore different molecular wheels and gauge their surface interaction on
a variety of materials.
?
?
Fourier transform
?
In collaboration with the Tour Group, Department of Chemistry
-25 mV
-35 mV
-45 mV
scattering peaks
Gr on Cu (111)
Gr on Cu (100)
Graphene growth on the facets Cu (111) and Cu (100) with
corresponding Fast Fourier Transforms (FFT) .
Graphene growth is found to be preferential on these two low index facets.
Regions without graphene have an amorphous appearance, while atomic
steps and crystalline features of the Cu foil are preserved in the regions with
a graphene overlayer.
The electronic properties and
quality
of
graphene
are
dependent upon the microscopic
growth over various substrates.
The aim of the graphene project
is to shed light on the control and
quality of CVD grown monoand bilayer graphene over
defects, grain boundaries, step
Continuous monolayer
Continuous bilayer
graphene suspended over a edges and vacancies of the
graphene over Cu steps.
large vacancy in the Cu.
underlying substrate.
In collaboration with the Kono Group and Ajayan Group
Stacking the FFTs shows the
evolution of the scattering peak
maximum with energy (bias
voltage) vs. wavenumber, from
which we can map out the
dispersion relation, E(k) vs. k to
compliment
other
common
techniques, such as angle resolved
photoemission spectroscopy.
Preliminary data also suggests
that we can remove the
topological state by application of
a magnetic field, breaking time
reversal symmetry.
Here we
apply spin polarized STM with a
Cr tip and notice the lack of
scattering signature.
In collaboration with the Morosan Group, Department of Physics
-55 mV
atomic
peaks