Supplementary Information
Electrical effects of metal nanoparticles embedded in ultra-thin colloidal
quantum dot films
F. J. Beck, F. P. Garcia de Arquer, M. Bernechea Navarro, G. Konstantatos
ICFO-The Institute of Photonic Sciences, 08860, Castelldefels (Barcelona), Spain
Synthesis of PbS with excitonic peak at ~950 nm: 2 mmol of PbO, 4 mmol of OA (Oleic acid) and 8
mmol of ODE (1-Octadecene) were pumped overnight at 95ºC. After this time, 15 ml of ODE were
added and the temperature of the reaction was raised to 120ºC. When this point was reached, 1 mmol
of TMS (Hexamethyldisilathiane) mixed with 10 ml of ODE were quickly injected. The heating was
stopped (without removing the heating mantel) and the reaction was allowed to cool down slowly (~
60 min). The nanocrystals were isolated by addition of acetone and centrifugation, purified by
dispersion/precipitation with toluene/acetone and finally dispersed in anhydrous toluene. All the
reactions were carried out using standard Schlenk techniques. Reagents were purchased from Sigma
Aldrich.
A partial ligand exchange of OA to Oleylamine was carried out before film deposition: 1.5 ml of
Oleyamine was added to 3 ml of nanocrystal solution (~ 150 mg/ml), mixed well and left in an inert
N2 atmosphere inside a glovebox for 2 days. The solution is then purified by dispersion/precipitation
with toluene/methanol and finally redispersed in anhydrous toluene.
Oleylamine capped PbS CQDs are spin-cast from solution in toluene, with a concentration of
20mg/ml. The long oleylamine ligands are exchanged for shorter ethanedithiol (EDT) ligands to
cross-link the QDs by spin casting one drop of EDT (2% vol. in acetonitrile). The film is then cleaned
with acetonitrile and toluene. This results in a PbS CQD film of estimated thickness 20 nm. The
process is repeated multiple times to build up the film thickness.
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The samples were illuminated using collimated light from a 100 W halogen-tungsten lamp.
Transmitted light was collected using a microscope objective with NA=0.95 and fed into an imaging
spectrometer. After passing through a 100 µm slit, a diffraction grating with 150 lines mm-1 dispersed
the light onto a Peltier-cooled EMCCD camera. Signal and reference were measured simultaneously
in different pixels of the camera, allowing any spectral features arising from either the substrate or
optical elements in the set-up to be normalized.
To measure the photocurrent, the samples were illuminated with collimated light from a 100 W
halogen-tungsten source which was spectrally split by a Newport monochromator and chopped at
20Hz. A bias of 20V was applied with a Keithly power source and the photocurrent signal was
collected by a lock-in amplifier. IV characteristics were measured under illumination by a diode laser
at a wavelength of 635 nm and at a bias of 20V, also using a Keithly power source. The responsivity
at λ = 635 nm and V = 20V was calculated as the difference between the measured currents under
illumination and in the dark, divided by the incident power of the laser. The photocurrent spectra
were normalised to the responsivity values at λ = 635 nm. In this way we were able to avoid
inaccuracies arising from the internal resistance of the lock-in amplifier when calculating the
responsivity spectra.
Figure 1SI. Responsivity spectra for photoconductors with Au (a), Ag (b) and Al (c)
embedded MNP arrays (MNP, solid lines), for ~60 nm thick devices (3LBL). For
comparison, spectra are also shown for devices without embedded MNPs (ref,
dashed lines).
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Figure 2SI. Current-Voltage curves for photoconductors with Au (a,b, red lines), Ag
(c,d, blue lines) and Al (e,f, green lines) embedded MNP arrays, for ~60 nm (3LBL,
a,c,e) and ~200 nm thick devices (10LBL, b,d,f). Data is shown for devices under
illumination at a wavelength of 635 nm (solid lines) and in the dark (dashed lines).
For reference, data is also given for a photoconductor of similar thickness without
embedded MNPs (black lines).
The nanoparticle arrays were simulated on semi-infinite glass substrates for comparison with
experiment, illuminated by a plane wave source incident from air, as shown in Figure 3SI. The
particles are disk shaped with a rounded top, with a height, h = 50 nm and radii given by the measured
radii in Fig.4(a). The simulation was terminated with perfectly matched layer boundary conditions
perpendicular to the plane of incidence and periodic boundary conditions in the other two dimensions.
The thickness of the PbS was varied from no PbS (Fig.2SI (a,d,g)), to compare with transmission
measurements in Fig.2(b); to t = 60 nm (Fig.4SI (b,e,h)) and t = 200 nm (Fig.4SI (c,f,i)) to compare
with responsivity measurements (Fig.1SI). The optical constants of the metals were calculated using
a Drude model fitted to optical data from Johnson and Christy1 for Ag, and Palik2 for Au and Al.
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Figure 3SI. Schematic of the simulation set up showing the position of the
simulation boundaries (orange, solid lines) and the monitors (black, dashed lines).
The optical data for the glass was taken from Palik. For the PbS CQD film, the optical properties
were determined from experimental absorption measurements using the Kramers-Krönig relation, as
described in ref3. The absorption in the MNP (A MNP, green line) was estimated by calculating the
net optical power flow into a box surrounding the nanoparticle.
Figure 4SI. (a,d,g) Simulated reflection (R) and transmission (T) from arrays of
MNPs on semi-infinite glass slides and calculated absorption in the MNP (A MNP)
for different metals. (b,e,h) Calculated absorption in the total structure (R) and
transmission (T) from arrays of MNPs on semi-infinite glass slides and calculated
absorption in the MNP (A MNP) for different metals
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This slightly overestimates the absorption in the nanoparticle due to the discrepancy between the
cubic monitor box and the shape of the nanoparticle. However, this simplification avoids including
unphysical hotspots due to the sharp edges of the particle. The power transmitted (T, blue line,
squares) and reflected (R, red dashed line) from the entire structure was monitored and the total
absorption (A Total, dashed blue line, circles) was calculated from 1-reflection-transmission. The
absorbed power in the PbS layer (A PbS CQD, red line, crosses) was then calculated as the total
absorption – absorption in the box around the nanoparticle. The calculated absorption in the PbS
CQD layer was normalized to the absorption in to a reference case with no MNP present (A ref, black
dashed line) to calculated absorption enhancement and compare with responsivity enhancement in
Fig.3.
Figure 5SI. Percentage integrated enhancement (IEnh) from arrays of Ag MNPs
embedded in PbS CQD films of thickness (a) t = 60 nm and (b) t = 200 nm, for
varying periods (L) and radii (r).
To estimate obtain the optimum parameters for light trapping with Ag MNPs in PbS CQD thin films,
parameter sweeps were performed. The simulations were simplified somewhat: the glass substrate
was removed and hemi-ellipsoidal nanoparticles were simulated, embedded in PbS CQD films in air.
The height of the particles was kept constant at 50 nm and the period (L) and the radius (r) were
varied systematically for two different film thicknesses: t = 60 nm (Fig. 5SI(a)) and t = 200 nm (Fig.
5SI(b)). The percentage integrated enhancement (IEnh) was calculated by integrating the total
absorption in the nanoparticle sensitised film over the wavelength region simulated and then
normalising to the integrated total enhancement from a planar film of the same thickness. From these
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simulations periods of L = 550 nm and radii of r = 200 nm were taken as close to optimum for light
trapping purposes, for both thicknesses.
For the planar IDEs with Ag contacts: inter-digitated Ag contacts, of length 3 mm and separation 20
um, were defined by standard photolithography techniques on quartz glass substrates. PbS-CQD
films were deposited using a ‘layer by layer’ (LBL) process to build up the films to the desired
thickness between the Au electrodes. IV characteristics were measured under illumination by a diode
laser at a wavelength of 635 nm and at a bias of 20V using a Kiethly power source as described
above. Figure SI6 below shows a typical IV curve measured for a 60 nm thick device.
Figure 6SI. IV characteristics of a 3LBL PbS CQD photoconductor of length 20um
and width 3mm, with Ag contacts forming back-to-back Schottky junctions.
References
1
P.B. Johnson and R.W. Christy, Phys. Rev. B 6, 4370 (1972).
2
E.D. Palik, Handbook of Optical Constants of Solids (Academic Press, New York, 1998).
3
F.P. García de Arquer, F.J. Beck, and G. Konstantatos, Opt. Express 19, 21038-49 (2011).
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