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Optical gain in 2D solution processable
CdSe nanoplatelets
Belgian Physical Society Meeting
18th May 2016
Ghent, Belgium
Presented by
Renu Tomar
Ph.D Student
Physics and Chemistry of Nanostructures
(Supervisor: Prof. Zeger Hens)
Ghent University, Belgium
Colloidal Quantum Dots
Colloidal quantum dots are nanometer
sized (2 to 10nm) semiconductor
crystallites obtained by solution-based
synthesis.
Advantages:
 Cheap and up-scalable synthesis
 Size tunable optical properties
 Shape tunable optical properties
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Energy level Diagram
CdSe Platelets ≈ Quantum well
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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CdSe Platelets Synthesis
N2
N2
Cu
precursor
Se precursor
And Cd(Ac)2
High
Low
Temperature
140-250 °C
Temperature
140-250 °C
Cd
precursor
CdSe
CdSe Platelets
Cadmium precursor: Cadmium Acetate and Cadmium myristate in ODE
Selenium precursor: TOP-Se or ODE-Se
Physics and Chemistry of Nanostructures Group
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Low
Temperat
300
Light hole (LH)
Heavy hole (HH)
350
400 450 500 550
Wavelength (nm)
2MLs
3MLs
4MLs
5MLs
PL Intensity (a.u.)
Absorbance Intensity (a.u.)
Characterization of CdSe Platelets
600
650
2MLs
3MLs
4MLs
5MLs
400
500
600
Wavelength (nm)
700
TEM images of CdSe
5MLs platelets
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Pump-Probe Spectroscopy
ΔA(t,λ)=A(t,λ)–A0(λ)
ΔA<0: Bleach,reduced absorption due to state filling
ΔA>0: Photoinduced absorption,due to intraband absorption
ΔA>0 or <0: Photo-‐induced absorptioon/bleach,due to spectral shifts
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Interaction of light with colloidal QDs
Absorption
Stimulated emission
Intraband Absorption
Intraband relaxation
Auger recombination
Multi-exciton Generation
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Motivation
Physics and Chemistry of Nanostructures Group
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Nanoplatelets Advantages : large optical cross section, slow auger recombination
rates and narrow emission linewidth
Gain in Nanoplatelets: Two different exciton density regiem
Single or biexciton regime
Multiexciton (~ 40 exciton per platelet)
 Lateral Size-Dependent Spontaneous and
Stimulated Emission Properties in Colloidal
CdSe Nanoplatelets (Hilmi Volkan Demir
et.al. ACS Nano, 2015, 9, 5, 5041)
 Carrier Cooling in Colloidal Quantum
Wells (Dmitri V. Talapin et. al. Nano
Letter. 2012, 12, 6158)
 Continuous-wave biexciton lasing at room
temperature using solution-processed
quantum wells (Iwan Moreels et. al. Nature
Nanotechnology, 2014, 9, 891)
 Amplified Spontaneous Emission and Lasing
in Colloidal Nanoplatelets (Hilmi Volkan
Demir et. al. ACS Nano, 2014, 8, 7, 6599)
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Transient Absorption Study on 4MLs CdSe Platelets
Spectra after photo-excitation with 400 nm with Pump power: 32.5 mW
Non-linear absorption
A = A0 + ∆A
∆A
1 ns
1 ns
Time (ps)
Time (ps)
100 ps
10 ps
2.0ps
100 ps
10 ps
1.5
1.0
0.5
1 ps
0.0
420
440
460
480
500
520
540
-0.08
-0.06
-0.04
-0.02
580nm
420
Wavelength (nm)
-0.10OD
560
440
460
480
500
520
540
Wavelength (nm)
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14OD
The white contour line indicates transparency (𝐴 = 0)
Gain is observed inside these contour
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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560
580nm
Characterization of CdSe Platelets
0.1mW
0.2mW
0.4mW
0.5mW
1mW
2mW
3mW
4mW
5mW
7mW
10mW
12mW
15mW
20mW
25mW
30mW
32.5mW
'35mW
A (OD)
0.15OD
0.10
0.05
0.00
420
440
460
480
500
520
540
560
580nm
Wavelength (nm)
𝐴 spectra, taken at 2.5 ps after photo-excitation for different pump fluences.
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Characterization of CdSe Platelets
A (OD)
60mOD
40
15mW
20mW
25mW
30mW
32.5mW
'35mW
20
0
-20
480
485
490
495
500
505
510
515
520
525
530
535 540nm
Wavelength (nm)
𝐴 spectra, taken 2.5 ps after photo-excitation for different pump fluences.
 Retain the excitonic feature
 Gain observed in both Heavy hole (HH) and Light hole (LH)
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Intrinsic absorption and <N> Calculation
 Intrinsic absorption, μi(ω) =δ (1) (ω)/VP ,
…1
Where VP being the platelet volume and δ(1) the linear absorption cross section
…2
…3
μi(400) = 1.466 * 105 cm-1
 Number of excitons per platelet, <N> = Jp* σ pump
Where Absorption crossection of platelets, σ pump = μi(pump) * VP
And Pump fluence, Jp = Pump power* Frequency/ (E pump* Area of pump beam)
Achtstein, A. W., Antanovich, A., Prudnikau, A., Scott, R., Woggon, U., Artemyev, M., J. Phys. Chem. C 2015, 119, 20156−20161
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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The wavelength dependent gain threshold
LH
HH
Threshold
240
220
200
180
160
495
500
505
510
515
520
525
530
Wavelength (nm)
 Gain threshold is 160 excitons for HH at 521 nm.
 Gain threshold is 239 excitons for LH at 500 nm.
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Allowed optical transitions
0.14OD
A (OD)
0.12
0.2mW
0.5mW
1mW
2mW
3mW
4mW
5mW
7mW
10mW
0.10
0.08
0.06
0.04
0.02
0.00
420
440
460
480
500
520
540
560nm
Wavelength (nm)
Se - Sh
Px e - Px h
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Characterization of CdSe Platelets
0.00
At 510 nm
0.5mW
1mW
2mW
3mW
5mW
7mW
10mW
32.5mW
-0.02
OD
-0.04
-0.06
-0.08
-0.10
-0.12OD
0
10
30
40
50ps
Time (ps)
0.00
At 510 nm
10mW
15mW
20mW
25mW
30mW
32.5mW
35mW
-0.02
-0.04
OD
20
-0.06
 Bleach with increase in
pump power
 Similar decay rate for lower
pump power till onset of
gain
 Faster decay with increase
for higher pump fluence
-0.08
-0.10
-0.12OD
0
10
20
30
40
50ps
Time (ps)
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Conclusions
 Four different monolayer thick CdSe platelets has been synthesized.
 Surface area analysis form TEM images.
 Observed gain in 4MLs Platelets in multi exciton regime.
 Need of model other than quantum-dot like models to interpret the
optical properties of CdSe nanoplatelets.
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Thank you for your kind attention
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
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Pump-Probe Spectroscopy
Half Wave
Plate
ND Filter
Pump
110 fs
Chopper
500 Hz
Non-linear
Crystal
Delay Stage
800nm
110 fs
Broadband Probe
Cuvette
Fiber
to CCD
Physics and Chemistry of Nanostructures Group
presented by Renu Tomar
The wavelength dependent gain threshold
80x10
Wavelength (nm)
515.13
516.02
517.35
519.13
521.35
524.01
525.34
526.23
528.01
529.34
-3
40
Wavelength dependence of
bleach around Heavy hole (HH)
bleach wavelength.
20
0
-20
50
100
150
200
<N>
250
Wavelength (nm)
490.27nm
492.05nm
494.27nm
496.04nm
498.26nm
500.04nm
502.26nm
504.03nm
505.37nm
507.59nm
509.36nm
0.12
0.10
Wavelength dependence of
bleach around Light hole (LH)
bleach wavelength.
A(OD)
A (OD)
60
0.08
0.06
0.04
0.02
0.00
50
100
150
<N>
200
250