EPSRC Portfolio Partnership in Complex Fluids and Complex Flows

Nanoscale Charge Writing on SnO2

The ability to selectively position nanoscale objects on a surface is critical to the success of nanotechnology.

• 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.

•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.

•The samples used for writing consist of a thick layer of 8nm SnO2 particles deposited on silicon from a suspension of nanopowder.

•The writing was performed using the tip of a scanning tunnelling microscope in ultra high vacuum (UHV).

•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 100s to write a single dot.

Fig. 1: STM image of written pattern acquired at –3V tip voltage.

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• Deposition of material from the tip can be ruled out for a number of reasons:

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).

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Fig. 2: (a) Tip height profiles acquired at 3 different tip biases across 3 written dots (b).

Fig. 6: possible interpretation of the charge writing mechanism. The light blue denotes depletion regions.

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Fig. 3: STM images and tip height profiles, before, 42 min. 48 hours and 74 hours after writing three dots.

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Fig. 4: STM images of 8nm SnO2 particles, before and after applying two voltage pulses to the tip (shown by yellow crosses)

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.

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Writing

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.

• 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. 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.

Introduction

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• 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.

•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.

•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.

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.

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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.

•Charged areas of the sample surface reacted strongly with O2 in UHV, highlighting the potential of nanoscale charge writing for molecular docking applications.

O2 exposure

Conclusions

PRIFYSGOL CYMRU ABERTAWE

UNIVERSITY OF WALES SWANSEA