Transcript (8 of 16) SW1 - Nano Charge Writing

EPSRC Portfolio Partnership in
Complex Fluids and Complex Flows
PRIFYSGOL CYMRU ABERTAWE
UNIVERSITY OF WALES SWANSEA
Nanoscale Charge Writing on SnO2
(a)
Introduction
(b)
The ability to selectively position nanoscale objects on a surface is critical to the
success of nanotechnology.
20 nm
• Therefore, the ability to selectively pattern surfaces either chemically, structurally
or electronically on a nanometer scale is a technological challenge in establishing
routes for controlled assembly of systems in this spatial regime.
(c)
(d)
•This challenge could be met by patterning a surface at the nanometer scale with
localised charge to act as nucleation or reaction sites, allowing molecules to bond
specifically to the surface in selected geometric designs.
Writing
•The samples used for writing consist of
a thick layer of 8nm SnO2 particles
deposited on silicon from a suspension
of nanopowder.
Fig. 5: Erasing of written dots. (a) has been acquired 192 hours after writing, while (b), (c) and
(d) have been acquired at +3V tip voltage 38, 47 and 71 min. after switching the tip voltage to
+3V.
nm
8
4
0
•
200
nm
•The writing was performed using the tip
of a scanning tunnelling microscope
in ultra high vacuum (UHV).
100
•During imaging with the STM (typically
at –3V tip voltage), the tip is sent to the
desired location and a voltage pulse of
–6V is applied to the tip for 100s to
write a single dot.
200
100
0
nm
•
Fig. 1: STM image of written pattern
acquired at –3V tip voltage.
•Fig. 1 shows an example of a written pattern. The written dots are 15 to 20 nm in
size, and are likely to be made up of 1 to 4 charged nanoparticles.
• The written features remain stable for up to 2 weeks in UHV.
a) The height of the written dots depends
on the imaging tip voltage (Fig. 2).
b) Leakage is observed after a few days
(Fig. 3)
c) The written features are strongly
related to the surface topography
before writing (Fig. 4).
d) Scanning with a positive tip voltage
erases the written features in less than
2 hours (Fig. 5).
18
16
14
12
10
8
6
4
2
0
(b)
(a)
-2.1V
-2.5V
-3V
0
Charged
nanoparticles
After injection, the depletion
layer should become less
deep, which would make
tunnelling
into
the
nanoparticles
easier
and
therefore account for the
enhanced
height
of
the
charged nanoparticles (Fig. 6).
Fig. 6: possible interpretation of the charge
writing mechanism. The light blue denotes
depletion regions.
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•To test the potential of charge patterning for molecular docking applications, a
written pattern was exposed to a small O2 partial pressure (1.7x10-9 mb) while
scanning at -3V tip voltage.
•Fig. 7 shows that the written pattern is clearly disturbed by the O2 and is eventually
almost entirely removed in less than 1 hour, even though these features are normally
very stable over time.
50
75 100
Distance (nm)
125
•Additionally, exposing un-charged areas of the sample surface to O2 did not result
in any changes, indicating a strong interaction between charged nanoparticles and
electronegative oxygen.
150
Fig. 2: (a) Tip height profiles acquired at 3
different tip biases across 3 written dots (b).
Before O2
24 min. of O2
35 min. of O2
50 min. of O2
The –6V voltage pulse could inject electrons into the nanoparticles where they
can remain confined by the potential barrier at particles boundaries. Normal STM
imaging at –3V tip voltage could occur via surface states.
Before
+46 hours
+74 hours
+42 min
Tip height (nm)
•
Tip height (nm)
Deposition of material from the tip can
be ruled out for a number of reasons:
Tip
O2 exposure
Discussion
•
A depletion layer caused by
chemisorbed oxygen species
exists below the surface of the
nanoparticles. This depletion
layer enhances the barrier
between
neighbouring
nanoparticles and could help
confine the injected charge.
+74 hours
6
4
2
0
6
4
2
0
6
4
2
0
+46 hours
+42 min
2
0
Fig 7: STM images acquired before, and after 24, 35, 50 min. of O2 exposure at 1.7x10-9 mb.
The “clouds” of small dots (~3nm) are believed to be areas where O2 has been adsorbed.
0 10 20 30 40 50 60 70 80 90
Lateraldistance
Distance (nm)
Lateral
(nm)
Fig. 3: STM images and tip height profiles, before, 42 min. 48 hours and 74 hours after writing
three dots.
Before
After
Conclusions
We have shown that the STM tip can be used to write features 10 to 20nm in size on
nanocrystalline SnO2 surfaces.
•The written features were produced by applying negative pulses of –6V to the tip
and are stable for more than a week in UHV but can be erased in an hour by
scanning at a positive tip bias.
•The writing mechanism is believed to be associated with charge confinement within
the 8nm SnO2 particles rather than being topographical in origin.
20nm
20nm
Fig. 4: STM images of 8nm SnO2 particles, before and after applying two voltage pulses to the
tip (shown by yellow crosses)
•Charged areas of the sample surface reacted strongly with O2 in UHV, highlighting
the potential of nanoscale charge writing for molecular docking applications.