Electrodeposition of patterned CdSe nanocrystal films using thermally
charged nanocrystals
Mohammad A. Islam and Irving P. Herman
a)
Department of Applied Physics and Applied Mathematics, and the Materials Research Science
and Engineering Center, Columbia University, New York, New York10027
Received 14 December 2001; accepted for publication 1 April 2002
A dc electric field is used to attract charged CdSe nanocrystals in hexane to rapidly form very
smooth, robust, large-area, several micron-thick films of equal thickness on both electrodes. This
deposition on both electrodes implies there are both positively and negatively thermally charged
dots, unlike conventional electrophoretic deposition. With patterned electrodes, controllable and
locally selective assembly is achieved. © 2002 American Institute of Physics.
DOI: 10.1063/1.1480878
Use of nanocrystals as the building blocks of complex
structures requires improved ways of forming films and com-
plex patterned structures from these quantum dots. While
there are ways of forming small ordered monolayers
1
and
crystals
2
of dots, there is no satisfactory method to deposit
large areas of uniform multimonolayer films. Dry casting,
i.e., the evaporation of dots in solution, and spin coating
result in films that are typically not uniform and by nature
unpatterned; patterned films may be needed in device appli-
cations. We report a way to rapidly form large areas of very
smooth, robust, several micron-thick films of dots that can be
either unpatterned or selectively patterned.
In solution, CdSe dots appear to have a permanent dipole
moment and a fraction of them are thermally charged.
3,4
A
uniform dc electric field is used to attract those charged CdSe
nanoparticles to form films of controllable and equal thick-
ness on both electrodes, suggesting equal densities of posi-
tively and negatively charged dots. These charged particles
can be locally and selectively transported to the surface for
spatially controllable assembly using patterned electrodes.
This method differs from the electrophoretic deposition of an
ordered monolayer of micron-size latex spheres and smaller
particles,
5–7
direct electrochemical formation of
nanoparticles,
8
and more conventional electrophoretic depo-
sition of particles, which is usually in polar solvents with
particles without organic ligand capping and where films are
formed on one electrode with bulklike density.
9,10
CdSe nanocrystals were prepared according to the meth-
ods of Ref. 11, with trioctylphosphine TOPO capping
ligands. Solutions of these dots usually 3.2 nm diameter
with densities 10
15
– 10
16
/cc ( 4 10
-5
–4 10
-4
volume
fractions were prepared with hexane. Two electrodes, usu-
ally rectangular sections of Si wafers coated over 0.8
1.4 cm by 10 nm Ti and then 150 nm Au, separated by
1.4 mm, were submerged in a beaker with this solution. dc
voltages up to 1000 V were applied across the electrodes at
room temperature in the dark, with solvent added as needed
to counter any evaporation. dc current was monitored during
runs, and the deposits on the electrodes were examined af-
terward.
The initial current between the electrodes was 60–70 nA
54 – 63 nA/cm
2
for 318 V (2.210
5
V/m), 1.010
15
dots/
cc, and linearly proportional to both voltage V and dot den-
sity n ; it decreased from 70 to 25 nA in 45 min. Without
dots, the current was 100 smaller with the hexane sol-
vent only and 20 smaller with TOPO dissolved in hexane
with TOPO mass half that of the usual dot mass.
Uniform, apparently identical films formed on both un-
patterned electrodes. No deposit was formed without the
voltage. Visible microscopy, scanning electron microscopy
SEM, profilometry, and atomic force microscopy showed
that both films were very smooth, with 2 – 4 dot roughness
for a 1000 dot thick film. The initial deposition rate was
(0.010 nm/s) V in volts n in 1.010
15
dots/cc, per
electrode. After long runs 45 min, 318 V, 1.010
15
dots/cc,
3.4 m thick films were deposited as shown in Fig. 1—
most of which formed within 20 min. After drying, these
films did not dissolve in hexane as do those formed by dry
casting, even when voltage of either polarity was applied
across it to a bare Au electrode. Transmission electron mi-
croscopy of these quickly grown thick films did not show
ordered dots.
Photoluminescence PL of these films Fig. 2 reveals
one sharp peak near 567 nm exciton emission and two
smaller peaks at 654 and 745 nm possibly due to defects. In
dot solutions, there is a 541 nm absorption and 560 nm PL
peak. The strong 567 nm PL peak stable for at least two
a
Author to whom all correspondence should be addressed; electronic mail:
iphl@columbia.edu FIG. 1. SEM of 3.4 m thick electrodeposited film.
APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 20 20 MAY 2002
3823 0003-6951/2002/80(20)/3823/3/$19.00 © 2002 American Institute of Physics
Downloaded 06 Aug 2002 to 128.59.86.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp