Ilan Gur

EE290B/E298A

Spring 2004

# 

# Increasing the Complexity of Capillary-based Nanostructure Assemblies

## Introduction

![](media/image1.pct){width="3.0555555555555554in"
height="1.6805555555555556in"}Recent studies have delineated a facile
and powerful approach for the hierarchical assembly of isotropic and
anisotropic colloidal nanostructures over large areas, representing a
significant step towards integration of bottom-up chemically prepared
nanostructures with lithographically patterned nanostructures^1^.

This approach to the controlled assembly of nanostructures builds on
previous work in assembly of micron or millimeter size objects,
exploiting capillary interactions at the three-phase
vapor-suspension-substrate contact line during solvent evaporation ^2-5^
. These interactions are capable of overcoming thermal fluctuations,
leading to highly efficient assembly. Flat substrates are patterned with
hole and trench templates using electron beam lithography, and
subsequently inserted vertically into a solution containing the
nanostructures. Evaporation of the solvent leads to the three phase
contact line moving slowly across the substrate. When the solution film
thickness on the substrate is less than the height of the nanostructure,
the solution-vapor interface deforms, and the resulting capillary force
slides the nanostructure towards the thicker part of the solution and
pushes the particle towards the substrate. The net result is that
particles are selectively forced into the lithographically defined
features as the evaporation zone passes over them, but no particles are
deposited on the surrounding areas.

![](media/image2.png){width="1.8069444444444445in"
height="1.5020833333333334in"}
![](media/image3.png){width="1.0833333333333333in"
height="0.8722222222222222in"}
![](media/image4.png){width="1.7361111111111112in"
height="1.5104166666666667in"}
![](media/image5.png){width="1.1395833333333334in"
height="1.0416666666666667in"}

The research described above aimed to provide a proof of concept, with
the hopes that such assembled nanostructures may one-day lead to highly
integrated functional nanosystems. However, only very simple devices can
be envisioned based on the existing work. Perhaps the greatest potential
of this assembly method is that it can be applied to nanostructures of
any material, and the designed combination of chemically distinct
nanostructures through multi-step organization could increase both
structure and functional complexity for highly integrated, multifunction
applications. The present work aims to provide a means to explore the
limitations of this approach in integrating disparate nanostructures
into a single system. The simplest scheme for selective integration of
disparate nanostructures is one based on size-selection. With proper
lithographic design, multifaceted structures will allow for sequential
deposition of nanostructures of various sizes (filling appropriate
trenches first with the largest structures and sequentially with smaller
ones), thus resulting in controlled assembly with higher complexity than
has been demonstrated.

## 

## Experimental

The prototype structure consists of an asymmetric dumbbell trench
pattern exposed and developed in resist and replicated over a 4" wafer.
The structure consists of two 55 nm diameter circular pits connected by
a 500nm long, 20 nm wide trench. This will accommodate assembly of two
distinct particle types as selected by size, with the smallest particles
filling a trench connecting two single particles of disparate size
and/or composition.

9x9 arrays of this feature were patterned via electron beam lithography
onto a \<111\> silicon wafer with native oxide, previously coated with a
35 nm film of standard poly(methyl methacrylate) (PMMA) resist, as
determined by profilometry. The PMMA resist film was spincast from a 1%
solution at 6000 rpm for 45 seconds, followed by baking for 15 minutes
at 175 C. Exposure of the 9x9 arrays of this feature was repeated 25
times in a 600 sq micron field, with a base dose of 800 uC/cm^2^ and an
incremental dose increase of 1.03x for subsequent arrays. 16 such
patterns were exposed in all on a 4 inch wafer. Patterns were developed
in a 1:3 MIBK:IPA solution for 45 seconds, and wafers were subsequently
rinsed in fresh IPA and stored in ambient conditions.

Lithographically defined patterns on the nanometer scale in PMMA resist
are difficult to image directly by scanning electron microscopy due to
charging of the insulating resist layer. A 10 nm charge-dissipation
layer of gold was deposited over the developed pattern to assist in
imaging; however, gold deposition via thermal evaporation yielded gold
grains of \> 100 nm, thus obscuring any lithographically defined
features.

Two methods were attempted in assembly of nanocrystalline particles into
nanolithographically-defined features. The first followed the approach
of Cui et. al., whereby the wafer surface is exposed to a short oxygen
plasma to render the surface hydrophilic and allow for capillary-based
assembly of particles. Unfortunately, the PMMA resist film was almost
completely etched by the once-standard oxygen plasma treatment, thus
dictating a requirement for thicker template films in future iterations.

A disparate route to nanocrystal assembly was then attempted, exploiting
amphiphilic organic molecules in binding nanocrystalline particles to
the exposed silicon oxide within the pattern holes and trenches. The
bifunctional molecule 11-mercaptoundecoic acid was employed for
attaching gold and cadmium telluride (CdTe) nanocrystals to exposed
SiO~2~ at the feature sites. This molecule consists of a carboxylic acid
head group, which preferentially binds to the SiO~2~ substrate, and a
thiol terminated head group, which has been shown to preferentially bind
to both gold and CdTe nanocrystals, separated by a 11-carbon alkyl
chain. Patterned substrates were immersed in a 5 mM solution of
11-mercaptoundecoic acid in ethanol, in dry atmosphere, for 6 hours.
Substrates were subsequently rinsed in pure ethanol, dried with N~2~
gas, and immersed in a dilute solution of 50 nm gold colloids in water
for 8 hours. Acetone was added to the gold solution to develop the
remaining PMMA resist and assist in increasing the wetability of the
substrate surface. Substrates were then rinsed in water/acetone and
finally immersed in a dilute solution of 10 nm CdTe quantum dots in
toluene for 8 hours, then rinsed in pure toluene and blow dried.

## Results

Following the attempted deposition scheme, specimens were imaged by
scanning electron microscopy. Light scattering from 5x5 arrays of
features was visible via optical microscopy; however, the nature of the
features was unclear. Observation in the SEM revealed etch pits randomly
distributed on the silicon surface, with significantly larger etch pits
formed at the location of previously exposed lithographic features.

![](media/image6.png){width="5.175694444444445in"
height="3.892361111111111in"}

## 

Higher magnification clearly illustrates the differentiable large-scale
etch pits from those randomly distributed on the wafer. Gold and CdTe
particles are present, both on the silicon surface and within the etch
pits. Large etch pits were found to be spaced in a 5x5 array, with 100
microns spacing, thus validating their hypothesized nucleation at
predefined feature sites. Pits were found to be approximately 1 micron
in depth by profilometry.

![](media/image7.png){width="2.6944444444444446in"
height="2.013888888888889in"}

It appears as though lithographically defined features exposing the
silicon surface provide preferred sites for silicon etching by the
acidic linker molecule. The PMMA resist film thus acts as a sacrificial
protective layer, such that etching eventually occurs throughout the
film, but with much lower efficiency and severity.

**Conclusions**

Further work will be required to assess the potential of the proposed
approach to assembly of heterogeneous nanoparticle systems. Traditional
capillary assembly will require thicker template resist films, and novel
systems must be investigated for assembly via molecular binding of
nanocrystals into predefined lithographic patterns. Still, a technique
has been demonstrated to use lithographic features in defining etch
sites in silicon \<111\>.

Controlled large-scale assembly of disparate nanostructures will in
itself constitute a significant advance in integration of colloidal
nanostructures with process lithography. Moreover, arrays of metal
nanoparticles are of great interest for utilization in optical devices
such as photonic waveguides. The assembly of chemically heterogeneous
systems is especially interesting for such applications, allowing the
introduction of semiconductor-based excitation and detection to a
traditional plasmonic waveguide.

## References

1\. Cui, Y., Bjork, M., Liddle, J.A., Sonnichson, C., Boussert, B. & A.
P. Alivisatos. Integration of colloidal nanocrystals into
lithographically patterned devices. Submitted.

2\. Aizenberg, J., Braun, P. V. & Wiltzius, P. Patterned colloidal
deposition

controlled by electrostatic and capillary forces. *Phys. Rev. Lett.*
**84**, 2997-3000

(2000).

3\. Yin, Y., Lu, Y., Gates, B. & Xia, Y. Template-assisted assembly: a
practical

route to complex aggregates of monodisperse colloids with well-defined
sizes,

shapes and structures. *J. Am. Chem. Soc.* **123**, 8718-8729 (2001).

4\. Nikolaides, M. G. *et al*. Electric-field-induced capillary
attraction between likecharged

particles at liquid interfaces. *Nature* **420**, 299-301 (2002).

5\. Bowden, N. B., Weck, M., Choi, I. S. & Whitesides, G. M.
Molecule-mimetic

chemistry and mesoscale self-assembly. *Acc. Chem. Res.* **34**, 231-238
(2001).

6\. 6. Maier, S. A. *et al.* Plasmonics -- a route to nanoscale optical
devices. *Adv. Mater.* **13,** 1501--1505 (2001).