Download "Excitation Enhancement of CdSe Quantum Dots by Single Metal

"Excitation Enhancement of CdSe Quantum Dots by Single Metal Nanoparticles"
Yeechi Chen, Keiko Munechika, Ilan Jen-La Plante, Andrea M. Munro, Sara E. Skrabalak, Younan
Xia, David S. Ginger*
Department of Chemistry, University of Washington, Seattle, Washington 98195-1700
* Corresponding Author: [email protected]
Supporting Information
Synthesis of CdSe/CdS/CdZnS/ZnS core/shell quantum dots
CdSe quantum dots were synthesized by the CdO/amine-route of Peng et al.1 0.077 g of CdO
was heated to 220 °C with 0.68 g of oleic acid to form Cd-oleate; when the solution turned clear, it
was removed from heating and allowed to cool to room temperature. Subsequently, 1.5 g of ODA,
0.5 g of TOPO, and 2 g of ODE were added to the mixture and heated. When the solution reached
270 °C, 3 g of Se-TBP solution (1.4 g Se, 3.84 g TBP, 12.33 g ODE previously prepared in a
glovebox) was swiftly injected. The temperature was lowered to 250 °C and the quantum dots were
grown at this temperature. Upon reaching the desired size, the solution was removed from heating.
All reactions were performed under N2(g) on a Schlenk line. After cooling to <70 °C the quantum
dots were extracted twice with hexanes and methanol. The final hexane solution was centrifuged to
remove unsuspended quantum dots and stored in the dark.
A CdS/CdZnS/ZnS multi-layer inorganic shell was grown using a multi-shell2-4 method. All
solutions used to grow the inorganic shells were prepared on a Schlenk line. In a flask, 2 mL of
~5.8 × 10-5 M CdSe core quantum dot solution in hexanes was added to 1.5 g of ODA and 5 g of
ODE. The core quantum dot solution concentration was determined using the empirical formula for
the extinction coefficient of CdSe quantum dots determined by Yu et al.5 with the first absorbance
peak at 582 nm. The hexanes were then removed by heating the flask to ~100 °C under vacuum until
all bubbling ceased. The solution was then heated to 245 °C, and the first shell monolayer (CdS) was
grown by adding a small aliquot (~0.7 mL) of a 0.04 M Cd-oleate solution (0.0615 g of CdO, 1.083
g of oleic acid, 10.8 mL of ODE) dropwise to the CdSe cores, followed by an equal volume of a 0.04
M S solution (0.0128 g of S, 10 mL of ODE), which was also added dropwise. The second shell
monolayer (Cd0.5Zn0.5S) was grown by adding a small aliquot (~1 mL) of a 0.02 M Cd-oleate and
0.02 M Zn-oleate solution (0.0512 g of CdO, 0.0325 g of ZnO, 0.9038 g of oleic acid, 19 mL of
ODE). Subsequently, an equal volume of 0.04 M S solution was added to complete the shell
monolayer. For the ZnS shell growth, ~1.45 mL of a 0.04 M Zn-oleate was added dropwise to the
reaction, followed by an equal volume of 0.04 M S solution. Both the Cd0.5Zn0.5 and Zn precursor
solutions were heated to 100 °C before use in the shell synthesis. Core/shell quantum dots were
precipitated with acetone and resuspended in chloroform three times before use. The quantum
efficiency of the CdSe/CdS/CdZnS/ZnS quantum dots was 0.28 after the initial washing.
Synthesis of Metal Nanoparticles
Silver nanocubes were prepared by a modified polyol process.6 6.0 mL of ethylene glycol
(EG, J.T. Baker 9300-01 Lot C42B27) were added to a glass vial and heated at 152 °C for 1 hour
while being stirred with a Teflon-coated magnetic stir bar at 260 rpm. 90 μL of a 3 mM sodium
sulfide (J. T. Baker 3910)-EG solution was then injected into the vial. After 8.5 minutes, 1.5 mL of a
PVP (20 mg/mL, J. T. Baker, cat. no. 3910, MW = 29,000) solution in EG was injected into the vial.
Immediately thereafter, 0.50 mL of a AgNO3 (48 mg/mL, Sigma-Aldrich 209139) solution in EG
was injected into the vial. A series of color changes were observed over the next 12 minutes, with
the reaction being stopped by cooling the vial to room temperature after the reaction media appeared
opaque, green-ochre when viewed head-on and ruddy-red when viewed from the top. Plating was
observed on the vial walls. The quenched reaction media was then diluted to twice its volume with
acetone and the silver nanocubes were collected by centrifugation. The silver nanocubes were
resuspended in water and washed an additional 3 times then stored in 4 mL of deionized water.
Silver nanoprisms were photochemically converted from spherical silver nanoparticles.7
Silver nanospheres were synthesized by borohydride reduction8,
9
of AgClO4 at 0 °C by rapidly
injecting 1 mL of 0.01 M AgClO4 into 99 mL of an ice cold solution of 1 mM NaBH4 and 0.30 mM
sodium citrate in water. The solution immediately turned light yellow and deepened to a bright
yellow after twenty minutes of stirring. The extinction spectrum (Fig. S1) (UV-Vis
spectrophotometer system, Agilent) showed a strong peak at 400 nm, confirming the presence of
spherical silver particles. Next, the spherical silver nanoparticles were photochemically converted
into flat prism shapes by placing the colloidal solution ~10 cm from a white fluorescent tube light
for ~100-120 hours.7 During this time, the solution changed from bright yellow to green; the new
extinction spectrum exhibited two broad plasmon peaks at 470 nm and 640 nm.
Gold spheres were obtained from Ted Pella and used as received.
Silanization of glass coverslips
Glass coverslips (VWR, Vista Vision, No. 1) were sonicated for 10 minutes in isopropyl
alcohol and dried in a nitrogen stream.
The coverslips were then plasma cleaned in air for
~2 minutes (Harrick plasma cleaner PDC-32G), and then immersed in 0.54% (v/v) 3aminopropyltrimethoxysilane (APTMS) (Aldrich) in a mixture of 95% ethanol 5% water for 3
minutes. After thoroughly rinsing with ethanol, they were dried in a nitrogen stream. The coverslips
were then cured for 4-16 hours at ~90 °C under nitrogen and thereafter stored at room temperature.
Sample Assembly and Characterization
Diluted nanoparticle solutions (< 0.2 OD at peak absorption) were placed onto silanized
coverslips for a few minutes. Unbound nanoparticles were rinsed off with Millipore water and the
coverslip dried in a nitrogen stream. The film was spin-coated at 6100 rpm for 1 minute (WS-400B6NPP/LITE, Laurell Technologies Corporation) from 150 μL of 0.21 μM quantum dots in a solution
of 1 mg/mL PMMA in chloroform to produce a 5-nm layer as measured by AFM (Asylum MFP-3D,
Asylum Research, Santa Barbara, CA). The quantum dots form clumps that are partially visible in
an AFM topography scan in intermittent contact mode.
Figure S1. AFM topography scan of a triangular silver nanoprism with an overlayer of quantum
dot-doped PMMA. The small features are clumps of quantum dots that protrude from the 5 nm
PMMA layer thickness.
Figure S2. SEM image of quantum dots clusters embedded in a PMMA matrix.
Optical microscopy and spectroscopy
Optical microscopy and spectroscopy were performed using a Nikon TE-2000 inverted
microscope fitted with a transmitted darkfield condenser and a 50X objective (Nikon Plan RT, NA
0.7, CC 0-1.2) with an intermediate 1.5X lens (total magnification 75X). The microscope output
was either directed to a thermoelectrically-cooled color CCD camera (Diagnostic Instruments,
FlexRT, FX1520) or a fiber optic cable (diameter = 100 m, UV-vis transmission, Ocean Optics,
Dunedin, FL) coupled to a portable charge coupled device spectrometer (USB2000, Ocean Optics).
A standard tungsten halogen lamp was used for transmitted light darkfield illumination, and metal
halide lamp (EXFO X-Cite 120) was used for epi-fluorescence illumination.
Scattering efficiencies and local surface plasmon resonance spectra
A lamp reference spectrum was obtained by measuring scattered white light from a KimWipe
placed on the sample coverslip. The fiber optic cable isolated the scattered light from each
nanoparticle; this particle spectrum divided by the lamp reference spectrum gave the scattering
efficiency (and LSPR peak).
Photoluminescence excitation (PLE) spectra measurements
We obtained fluorescence intensities for each particle by imaging the sample while swapping
in a series of 21 excitation filters with the same dichroic mirror and emission bandpass filter.
Excitation: center wavelengths 400, 410, 420, … 600 nm, 10 nm fwhm (Comar)
Dichroic: cutoff wavelength 615 nm (Chroma)
Emission: center wavelength 645 nm; 60 nm fwhm (Chroma)
Figure S3. Relevant spectra of bandpasses and quantum dots. Colored solid traces are the
normalized spectra of excitation light through each of the 10 nm fwhm bandpass filters used
to excite the quantum dots, peaking at 400-600 nm. The shaded trace is the absorption
spectrum of the quantum dots. Solid trace centered at 625 is the quantum dot emission
spectrum.
The 8-bit grayscale fluorescence images were aligned to the corresponding darkfield images.
Exposure times for each image were adjusted to avoid saturation, and reported fluorescence
intensities are all normalized by exposure time and excitation light intensities. The excitation light
intensity output from the microscope was measured by photodiode response (UDT Sensors Inc.,
Model 10DP/SB) and converted to photon counts.
Image Analysis
The photoluminescence values of both background and near-nanoparticle quantum dots were
extracted from 8-bit greyscale photoluminescence images and corrected for exposure time and lamp
intensity. For the background continuum, we take the average intensity value per pixel, which
accounts for approximately a 200 nm square region. Near the nanoparticle, we estimate the nearfield enhancement extends approximately 50 nm beyond the physical bounds of a 100 nm wide
nanoparticle,10 so we assume the enhanced photoluminescence must originate only from quantum
dots within a 200 nm square region (the area represented by one CCD pixel).
We consider the diffraction-limited spot size of the enhanced fluorescence of a single 200 nm
square area on the sample to occupy a 3x3 pixel area. This minimizes contributions from an uneven
distribution of quantum dots in the PMMA film, though it underestimates the total fluorescence
enhancement. We subtract the average background intensity per pixel (values from a plane-fit to the
11x11 pixel area centered on the bright spot), and sum the remaining enhanced emission intensity for
the fraction of enhanced fluorescence. The total photoluminescence intensity from the nanoparticlemodified quantum dots in a 200 nm square area is then the sum of the enhanced emission and the
average background intensity per pixel:
PLnanoparticle  sum of excess emission  avg bkg intensity
This calculation returns the average background value when there is no enhancement.
Excitation Enhancement Variation
We provide estimates of the distribution of excitation enhancement factors between
nanoparticles of the same type. To do this, we selected an area on the substrate and extracted the
total fluorescence enhancement factor at each excitation wavelength for every nanoparticle in the
area, as described above and shown in Figure 3. We then calculated an excitation enhancement
factor for each particle by ratioing the highest total enhancement factor to the lowest total
enhancement factor. We assume this to be the on- to off-resonance ratio, though no LSPR data were
taken. The distribution of the excitation enhancement factors are given here, though we believe
them to be more representative of the heterogeneity of the nanoparticle solutions than of the
individual nanoparticles.
Figure S4.
Silver Nanoprism Excitation Enhancement Distribution. Histogrammed
distribution of the excitation enhancement factors for (A) all silver nanoprisms, (B) only red
nanoprisms (with scattering peak wavelength ≥ 600 nm), and (C) all non-red particles.
Figure S5.
Gold Sphere Excitation Enhancement Distribution.
Histogrammed
distribution of Au spheres of (A) 80 nm diameter and (B) 100 nm diameter.
Figure S6. Silver Cube Excitation Enhancement Distribution. Histogrammed distribution
of excitation enhancement of silver cubes (side ~ 50 nm).
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