1. A mechanistic study towards biexciton emission enhancement of single QDs near gold nanoparticles
Swayandipta Dey, Xiangdong Tian, Julie Jenkins and Jing Zhao , Department of Chemistry,University of Connecticut,55 North Eagleville Road,Storrs,CT 06269,USA
Electric
field
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- - -
- - -
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Electron
cloud
Metal sphere
When metal nanoparticles are excited by electromagnetic radiation, they exhibit
collective oscillations of their conduction electrons known as localized surface
plasmon resonance (LSPR).The LSPR maximum of the nanoparticles(NPs) are highly
sensitive to the size,shape and local dielectric environment which makes them
highly functional for applications in photovoltaics,biosensors and even plasmon
based waveguides.
Localized Surface Plasmon Resonance (LSPR)
of Metallic Nanostructures
LSPR Substrate Preparation
AuAu
Au nanoparticles with silica shell silica shell 5 nm silica shell 10 nm
Au Au Au Au
Au@SiO2 immobilized on glass
Au nanoparticle of 120 nm diameter were synthesized by a two-step seeded growth ap-
proach. [1] The 120 nm Au nanoparticles can be coated with silica shells with thickness
ranged from 4 to 10 nm based on the method described in ref [2].The TEM images show the
120 nm Au nanoparticle with a 5 nm and 10 nm silica shell,respectively.
Au
Au nanoparticles of 120 nm diameter with silica
shells were immoblized on APTES silanized glass.
The SEM images show the distribution of Au@SiO2
nanoparticles on glass. The LSPR peak of the Au@-
SiO2 nanoparticles on glass exhibited a red shift
with increase in the silica shell thickness due to an
effective increase in the local dielectric constant
around the Au NPs.
LSPR peaks of Au@SiO2
nanoparticles
SEM image of Au@SiO2
Au@SiO2
-5nm
Au@SiO2
-10nm
Time resolved fluorescence decay of QDs
Au
QD
Au Au Au
QDQD
NormalizedPLIntensity
Time (ns)
(a) , QDs on Glass
(b) , QDs on Au@SiO2
-5nm
(c) , QDs on Au@SiO2
-10nm
ab c
g(2)
measurements of single QDs on various substrates
100 200 300 400 500 600
0
10
20
30
40
Time (s)
Intensity(kcps)
0 20000 10-1
100
101
102
10-2
10-1
100
101
102
103
Time (s)
NumberofEvent
−1 −0.5 0 0.5 1
0
0.2
0.4
0.6
0.8
1
t ( s)
Normalizedg(2)
on
off
100 200 300 400 500 600
0
10
20
30
Time (s)
Intensity(kcps)
0 500
Time (s)
NumberofEvent
100 200 300 400 500 600
0
50
100
Time (s)
Intensity(kcps)
0 1000
Time (s)
NumberofEvent
on
off
on
off
Normalizedg(2)
t ( s)
Normalizedg(2)
t ( s)
0
0.2
0.4
0.6
0.8
1
−1 −0.5 0 0.5 1
−1 −0.5 0 0.5 1
0
0.2
0.4
0.6
0.8
1
10-1
100
101
102
103
100
101
102
103
10-1
10-1
100
101
102
10-1
100
101
102
(A)
(D)
(G)
(B) (C)
(E) (F)
(H) (I)
Relative distribution of g(2)
minimum data of single QDs
1
0 0.2 0.4 0.6 0.8 1
0
10
20
30
0
4
8
12
16
0 0.2 0.4 0.6 0.8 1
NumberofQDs
NumberofQDs
g(2)
minimum g(2)
minimum
glass Au@SiO2
-5nm
0 0.2 0.4 0.6 0.8
g(2)
minimum
0
4
8
12
16
NumberofQDs
Au@SiO2
-10nm
(A) (B) (C)
From the distribution of the g(2)
data,it can be observed that majority of
the single QDs on Au NP substrates show a higher g(2)
value which indi-
cates a relatively higher biexciton(BX) quantum yield(QY).
Electrodynamics modeling
Theoretical explanation of BX emission enhancement :
Photon emission statistics and g(2)
Ideal single photon source : g
(2)
( 0 ) = 0
This“antibunching”is the signature of a single quantum emitter.
filter
filter BSP dichroic
APD1
APD2Pulse counters
and
t2-t1 correlator
objective
fluorescent
emitter
Laser
Pulse counters: average intensities I1
( t ) & I2
( t )
Correlator: histogram of photon time separations g(2)
( t )
2 1
−500 0 500
0
0.2
0.4
0.6
0.8
1
τ = t - t (ns)
Schematic of g(2)
of a single photon
emitter under continuous wave
excitation.Complete antibunching
is shown here.
Ratio of the“dip”to the“plateau” is related to the statistics of the number
of photons emitted after excitation,n.[3-4]
1.For a QD with a QY of η1
the emission intensity of the QD when placed
near a metal NP relative to that without a NP can be calculated using
= magnitude of electric field enhancement
The model can be further extended to any multiexcitonic processes.For
biexciton generation the above equation can be modified as
Average Silica shell thickness = 5 nm Silica shell thickness = 10 nm
|E|2
1.96 1.90
Rel. X PL Intensity 0.70 0.96
Rel. BX PL Intensity 5.64 4.67
Rel. X PL lifetime 0.20 0.38
Rel. BX PL lifetime 0.68 0.83
X PL QY 0.33 0.48
BX PL QY 0.14 0.13
Ratio (BX QY/ X QY) 0.39 0.26
2.Additional non-radiative processes(kNP,X
) arising due to the energy transfer
from the QD to the Au NP has a bigger impact on the X QY than on the
BX QY resulting in an increased BX QY but decreased X QY.
Theoretical results :
The plasmonic effect due to metal NPs results in the changes of X, BX life-
times and QYs in single QDs.We hope that these findings will open up new
routes to investigate and manipulate the multiexcitonic processes of QDs,
and modify their properties for desired applications.
References
Acknowledgement
[1] P.Fang,J.F.Li,Z.L.Yang,L.M.Li,B.Ren,Z.Q.Tian,J.Raman.Spectrosc.2008,39,1679.
[2] J.F.Li,X.D.Tian,S.B.Li,J.R.Anema,Z.L.Yang,Y.Ding,Y.F.Wu,Y.M.Zeng,Q.Z.Chen,B.Zen,Z.L.
Wang,Z.Q.Tian,Nat.Protoc.2013,8,52.
[3] G.Nair,J.Zhao and M.G.Bawendi,Nano Lett,2012,11,1136.
[4] J.Zhao,O.Chen,D.B.Strasfeld and M.G.Bawendi,Nano Lett.2012,12,4477.
[1]University of Connecticut Startup Grant and Faculty Large Grant
[2]Yadong Zhou,Dr.Shengli Zou,University of Central Florida
[3]Dr.Ou Chen,Massachusetts Institute of Technology