nwnsnt- dr m ramrakhiani
TRANSCRIPT
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
1/100
1
ELECTROLUMINESCENCEIN
NANOCRYSTALSAND
NANOCOMPOSITS
Meera Ramrakhiani
Department of Post-Graduate Studies and Researchin Physics and Electronics
Rani Durgavati University , Jabalpur
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
2/100
2
IntroductionNanometer sized semiconductor clusters are
representative of a state of matter intermediatebetween molecules and bulk matter.
This new class of material shows a number of striking effects such as surface effect, sizequantization, lattice contraction, unusualfluorescence and enhanced oscillator strength,which are potentially useful for technologicalapplications.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
3/100
3
Semiconductor nanocrystals exhibit manyunique properties, which are promising for theimprovement of electroluminescence devices.
1st
: the color of emission can be tuned by varyingthe size of the particle, while their chemicalproperties remain nearly the same.
2nd
: high fluorescence quantum yield andphotochemical stability can be achieved by carefulmodification of nanocrystal surface and this mayimprove the efficiency of the device.
3rd: oscillator strength is enhanced in nanocrystalsdue to modified density of states.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
4/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
5/100
5
The study electroluminescence of nanocrystalsas well as their nanocomposites with polymersis described here.
Undoped and doped II-VI semiconductornanocrystals and their nanocomposites inpolymers have been prepared by chemicalroute.
The samples have been characterized by SEM,
TEM, AFM, XRD and/or optical absorptionand photoluminescence and theirelectroluminescence has been investigated.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
6/100
6
Preparation of samplesIn our laboratory, following methods has
been used for preparation of nanocrystals:a. Chemical precipitation with capping
agentb. Organometallic precursor (at low
temperature)
c. Chemical capping method
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
7/100
7
Nanocrystals of CdS and ZnS doped with Mn, Ag orCu are synthesized by chemical precipitation method
using mercaptoethenol as capping agent. In 0.01M solutions of CdCl2 or ZnCl2 and Na2S was
added in presence of different concentrations of
metrcaptoethenol.CuCl, MnCl2 or AgCl was mixed in starting solutionfor doping.
Chemical reaction gave CdS or ZnS with properdoping.
The resulting precipitate was washed, centrifuged and
then air dried to obtain nanocrystalline powder.
Chemical Precipitation With Capping Agent
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
8/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
9/100
9
.
.
.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
10/100
10
Organometallic Precursor (at Low Temperature)
The CdS nanocrystals of various sizes were prepared by singlesource organometallic precursor.
Solution A was prepared by dissolving 36 mg cadmium chloride
(CdCl2) and 12 mg thiourea into a 30 ml ethanol in a flask undermagnetic stirring at 60C in an oil bath.Solution B was prepared by dissolving 10 mg Sodium Hydroxide
(NaOH) in 10 ml ethanol.
Solution A and B were mixed and stirred at 60C. In the beginning a white solution was obtained which gradually
become transparent and the colour changed from white to green-yellow.
Nanocrystalline CdS samples were extracted from the flask atdifferent reaction times of 20 - 90 minutes.
These samples were then centrifuged and washed with acetone and
dried to obtain nanocrystalline CdS powder.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
11/100
11
Chemical Capping Method
CdSe nanocrystals have been prepared by this method.Aqueous solutions of cadmium acetate andmercaptoacitic acid were prepared with adjusting proper
pH as 10.The solutions were deaeraced with N2 bubbling for 30minutes and then sodium selenosulfite solution was
mixed.2 propanol was added drop-wise while stirring themixture till it becomes turbid.
The precipitate was separated by centrifuge and driedto get powder.Different concentration of mercaptoacitic acid gave
nanoparticles of various sizes.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
12/100
12
Nanocrystal-polymer composites of ZnS-PVK, CdS-
PVK, ZnS/PVA and CdS-PVA have been preparedwith different loading of nanocrystals.
The polymer granules of PVK or PVA were
dissolved in dimethyl farmamide (DMF). Thenproper amount of zinc or cadmium acetate was addedto it and stirred for 30 minutes and concentrated to
reduce the solution volume to half of the initialvolume.
To this clear solution, H2S was passed for a few
seconds and then degassed with nitrogen.Then the dense liquid was transferred to glass slides
and allowed to dry for 24 hours. This gave optically
clear films of nanocomposites.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
13/100
13
CdSe-PVA polymer nanocomposite films were
prepared by reacting cadmium chloride with sodiumselenosulfite (Na2SeSO3) in polyvinyl alcohol (PVA)solution at proper pH.
CdCl2 was dissolved in distilled water to obtained 0.10
M solution. 20 ml PVA solution was taken and 1.05 ml of cadmiumchloride solution was added with constant stirring.
Ammonia solution was used to adjust pH value toabout 10 and then 1ml of diluted sodium selenosulfite(0.1M) was introduced.
The mixture was stirred for 3 hours to obtain a red
solution.The solution was cast on a glass substrate; upon solventevaporation, nanocomposite films were obtained.
Various samples were prepared by changing Cd:Se
ratio.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
14/100
14
Characterization
The samples were characterized by ScanningElectron Microscope (SEM), Atomic Force
Microscope (AFM), transmission electronmicroscope (TEM) and x-ray diffraction (XRD).
UV-VIS absorption spectra of the samples have
been recorded by Perkin Elmer Lembda-12spectrometer.
PL Studies have been carried out by excitingwith monochromatic light from a UV lamp ormercury lamp and also using a filter. PL spectra
were obtained by using a monochromator.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
15/100
15
The change in colour of samples has been observed
by changing the nanoparticle size by varyingpreparation conditions.
Small particles Largerer particles Large particles
CdSe nanoparticles prepared at different pH
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
16/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
17/100
17
SEM of ZnS Nanoparticles(By chemical bath deposition - particle size: 23nm)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
18/100
18
SEM image of clusters of CdS nanoparticles onto
plastic substrate (particle size: 26 nm)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
19/100
19
TEM image of nanocrystalline ZnS
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
20/100
20
It is difficult to determine individual particle sizesince the particles are clustered together.The average particle sizes were in the range of 10-12
nm.
0
5
10
15
20
25
30
35
40
45
8 10 11 12 14
(%)
()
TEM micrograph of
CdSe/PVA nanocompositeParticle size distribution
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
21/100
21
XRD of ZnS nanoparticles prepared by
chemical precipitation method with differentca in a ent concentrations
The XRD studiesindicate that thenanocrystalline powder
specimens of ZnS arecubic in nature havingzinc-blende structure.
For all the samplesthree peaks areobservedcorresponding todiffraction from (111),(220) and (311) planes.
b) X-ray diffraction studies
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
22/100
22
The broadening of peaks is indicative ofsmall particle size. The sizes have beencomputed by using Dubey-Scherrer
formula -D=k/Cos.
where k is instrumental constant, iswavelength of x-rays, is width of the
peak and is Bragg angle. The sizes have been obtained in the
range of 2 to 10 nm.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
23/100
23
XRD for Mn doped and undoped nanocrystalsMn doping causes broadening of the XRDpeaks; but other methods do not show smaller
particle size.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
24/100
24
The XRD pattern of CdS can
be consistently indexed on thebasis of the hexagonal, wurtzitestructure with lattice constant a= 4.121, c = 6.682, in which the
six prominent peaks at 2 valuesof 24.4, 26.7, 28.4, 44, 47.8and 51.9 angles corresponds to
the reflections at (100), (002),(101), (220) (103) and (112)planes. The weak peak due to (102)plane was also observed.In case of CdS nanoparticles,size increases with increasing
reaction time.
XRD of CdS nanocrystals prepared byorganometallic precursor method with
diff. reaction time
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
25/100
25
XRD of CdSe nanocrystals prepared by different capping agent
concentrationsn (XRD indicate hexagonal phase)
20 25 30 35 40 45 50 55 60
2 Theta
6 mmol
7 mmol
8 mmol
9 mmol
10 mmol
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
26/100
26
It is observed that in case of powderspecimens, smaller particles are
obtained by increasing capping agentconcentration.
In case of samples prepared byorganometallic precursor technique,the crystal size increases with
increasing reaction time period.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
27/100
27
In nanocrystal /polymer composites, XRDshows halo due to amorphous polymer andpeaks superimposed on it due to the
nanocrystallites. CdSe as well as its polymer composite and
CdS/PVA composite have found to containhexagonal crystals.
The lattice constants have been found in
close agreement with the standard ones.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
28/100
28XRD Pattern of CdS/PVA
10 20 30 40 50 60
( )
(.
)
5 %
20 %
40%
311220
200
111
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
29/100
29
Structural parameters of CdS/PVA
Sample 2 theta
(degree)
hkl d Standard d
(JCPDS-80-0019)
Lattice
constanta (in )
Average a
(in )
Diameter
D (innm)
CdS 5% 29.2644.0852.6
200220311
3.04892.0511.738
2.902.051.75
6.09785.805.76
5.78 11.2 nm
CdS 20% 26.5444.0652.34
111220311
3.3552.0521.745
3.352.051.75
5.8115.805.79
5.79 6.4 nm
CdS 40% 26.5443.952.1
111220311
3.352.051.75
3.352.051.75
5.805.795.80
5.80 3.8 nm
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
30/100
30
XRD pattern of ZnS/ PVA nanocomposite film
(A- 2 % ZnS, B- 5%, D- 20 % & F- 40 %)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
31/100
31
Analysis of X-Ray diffraction pattern of ZnS/PVA
nanocomposite film
Samplename
Loading ofZnS
2
(degree)hkl Inter
plannerspacingd
Standardd
Latticeconstanta ()
Latticeconstant c()
Crystalsize D(nm)
B 5 % 28.3 002 3.14 3.12 3.85 6.29 5.8 nm
39.7 102 2.26 2.27 3.82 6.24
D 20 % 28.5 002 3.12 3.12 3.82 6.24 4.7 nm
47.7 110 1.90 1.91 3.81 6.22
F 40 % 28.9 002 3.08 3.12 3.78 6.16 3.2 nm39.9 102 2.25 2.27 3.80 6.21
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
32/100
32
XRD patterns of CdSe/PVA Nanocomposite film
indicating hexagonal structure
10 20 30 40 50 60
(
.)
( )
2:1
3:1
4:1
100
002 102
202
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
33/100
33
c) Absorption studies
Absorption spectra of powder and filmspecimens have shown blue shift in
absorption edge or first absorption peakas compared to their bulk counterpart
indicating increased band gap energydue to quantum confinement effect.
No effect of doping has been observedon the absorption spectra.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
34/100
34
Fig. shows the UV/VISoptical absorption spectrafor ZnSI, ZnSII, ZnSIII, ZnSIV and ZnSV
samples in the range 400nm-200 nm prepared withcapping agentconcentration of 0M.0.005M, 0.01M, 0.015Mand 0.02M respectively.
Absorption edge shifts
towards lower wavelengths by increasingcapping agent
concentration.
200 250 300 350 400
Wavelength (in nm)
AB
S
(inarb.units)
ZnS-I
ZnS-II
ZnS-III
ZnS-IV
ZnS-V
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
35/100
35
The effective band gap energy has been
determined from the absorption spectra andparticle size is computed from the effective
mass approximation (EMA) model usingthe formula-
+
2
2
22 +1
*
1*
The particle sizes obtained by this methodare in agreement with those from XRD.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
36/100
36
The particle size obtained for various
concentration of capping agent
Sample Cappingagentconcentration
AbsorptionEdgeWavelength
Bandgap(in eV)
Size byEMA(in nm)
ZnS-IZnS-IIZnS-IIIZnS-IVZnS-V
0.000M0.005M0.01M0.015M0.02M
260 nm250 nm240 nm230 nm220 nm
4.764.965.165.395.63
2.42.22.081.91.78
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
37/100
37
400 450 500 550 600
WAVELENGT (in nm)
CdSIII
CdSV
ABSOR
BANCE(arb.unit)
In case of CdS
nanoparticles, theabsorption edge for allthe samples is blue
shifted as compared tothat of bulk CdS.For CdS V sample, the
absorption edge is nearlyequal to that of the bulkCdS.
The blue-shift in theabsorption edge indicatesincrease in effective band
of the samples.
Optical absorption spectra ofCdS nanoparticles.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
38/100
38
Size of CdS nanoparticles by various techniques(Prepared by organometallic technique)
S.No.
Reactiontime
(in min )
Size byTEM
(in nm )
Size byXRD
(in nm )
Absorption Edge( in nm)
Size byabs. Edge
(in nm)
I 20 5 7 470 5.7
II 30 - - 480 6.6
III 60 7 9 490 8
IV 80 9 - - -
V 90 - 12 500 11
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
39/100
39
Absorption spectra of CdSe nanocrystals prepared by
different capping agent concentrations show a smallabsorption peak and then sudden increase in absorption.
0 .5
0 .7
0 .9
1 .1
1 .3
1 .5
1 .7
1 .9
3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0
W a v e le n g t h ( in n m )
Absorption(inArb.
Unit)
5m Mo l
6m Mo l
7m Mo l
8m Mo l
9m Mo l
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
40/100
40
CdSe nanocrystal size for various cappingagent concentration
Sample Concentration
(in mmol)
Absorption
peak(in nm)
Band gap
Eg(in eV)
Diameter
by EMA(in nm)
C1 6 350 3.54 2.86
C2 7 345 3.60 2.82
C3 8 340 3.64 2.79
C4 9 330 3.75 2.71
C5 10 310 4 2.53
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
41/100
41
Particle size of CdSe Nanocrystals
Capping agent
Concentration
Particle size by
absorption usingEMA (in nm)
Particle size
by XRD(in nm)
6 mmol 2.86 2.97
7 mmol 2.82 2.948 mmol 2.79 2.77
9 mmol 2.71 2.76
10 mmol 2.53 2.58
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
42/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
43/100
43
Absorption spectra of CdS/PVA nanocomposite film
(CdS loading in polymer is 2, 5, 10, 20, 30 and 40 %by weight for a, b, c, d, and f samples )
400 500 600 700 800
(.
.
)
()
Si f CdS/PVA i fil
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
44/100
44
Size of CdS/PVA composite filmsSample
nameLoading
ofCdS
Absorptionwavelength
(nm)
Estimated sizeby absorptionedge (nm)
a 2 % 500 12.2
b 5 % 490 9
c 10 % 470 6.4d 20 % 450 5.1
e 30 % 440 4.75
f 40 % 430 4.3
Particle size decreases with increasing CdS loading
Absorption Spectra of ZnS/PVA Composite
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
45/100
45
Absorption Spectra of ZnS/PVA Composite
Film It is noticed that the
absorption edge is blueshifted with increasing theloading of ZnS.
The energy band gap isincreased with higher loadingof ZnS in PVA matrix due toformation of smaller
crystallite, which isconfirmed by the XRDstudies.
The absorbance is also
increased with increasing theloading of ZnS in PVAmatrix, because of thetransparency of the film is
reduced with higher loadingof ZnS.
Absorption spectra of ZnS/PVA
nanocomposite films (A- ZnS 2%, B- 5%,
C- 10%, D- 20 %, E- 30%, and F- 40%)
The particle size is estimated by the effective mass
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
46/100
46
The particle size is estimated by the effective massapproximation model.By increasing ZnS loading (% at wt), the particle size isdecreased.
Samplename
% (atwt)ZnS
Absorptionedge
wavelength (nm)
Energyband gap
Eg (eV)
Radius ofcrystal
r (nm)
Crystalsize
D (nm
A 2% 327 3.79 4.9 9.8
B 5% 316 3.92 3.3 6.7
C 10% 310 4.0 2.91 5.8D 20% 305 4.06 2.6 5.3
E 30% 300 4.13 2.4 4.8
F 40% 290 4.27 2.1 4.2
Estimated particle size from absorption edge of ZnS/PVA
nanocomposite film
Absorption Spectra Of CdSe/PVA Composite
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
47/100
47
p p p
Films
It is seen that the band edgeis shifted to shorterwavelengths compared to thebulk CdSe.This may be attributed tothe quantum confinementeffect on the electron bandstructure of CdSe samples.The shoulder present in the
spectra is assigned to theoptical transition of the firstexcitonic state.
0.35
0.45
0.55
0.65
0.75
0.85
0.95
400 500 600 700 800
(.
)
( )
Optical absorption spectra of
CdSe/PVA composite film
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
48/100
48
The crystal size of CdSe in CdSe/PVA composite film is
estimated by the change in the absorption edge using EMAmodel.
Samplename Precursor ratioCd:Se
Absorptionedge Energyband gapEg
Size byEMA
CdSe I 1:1 660 nm 1.87 eV 10.6 nmCdSe II 2:1 590 nm 2.06eV 6.39 nm
CdSe III 3:1 620 nm 1.93 eV 7.6 nm
CdSe IV 4:1 670 nm 1.85 eV 11.5 nm
d) Ph t l i t di
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
49/100
49
PL spectra for ZnS nanoparticles at various concentration of cappingagent (excited by 250nm UV light).
PL intensity increases and the peak shifts towards lower
wavelength for smaller particles.
d) Photoluminescence studies
0
50
100
150
200
250
300 400 500 600 700 800Wavelength (in nm)
PL
Intensity(inarb.u
nits)
0.000M
0.005M
0.01M
0.015M
0.02M
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
50/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
51/100
51
0
20
40
60
80
100
120
140
300 400 500 600 700 800Wavelength (in nm)
PLIntensity(inarb.un
it
0%0.10%
0.50%
1%
5%
PL intensity Of ZnS:Mn at various Mn concentration
PL intensity increases by increasing Mn%
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
52/100
52
0
20
40
60
80
100
120140
160
180
200
300 400 500 600 700
Wavelength ( in nm)
Intensity(ina.m.u
.)
1mmol
2mm3mm4.8mm
6mm7mm8mm
9mm10mm
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
53/100
53
It is seen that as the particle size isreduced, the intensity of
luminescence increases and the peakbecomes wider and shifts towardsshorter wavelength.This indicates that surface states are
better passivated by increasing
capping agent.
Photoluminescence of CdS nanoparticles of different sizes
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
54/100
54
0
3
6
9
12
15
500 520 540 560 580 600
WAVELENGTH (in nm)
PLINTENSITY(ar
b.unit
CdS II
CdS III
CdS V
prepared by orgenometallic precursor
In the PL spectra of CdS nanocrystals, band edge luminescence wasnot detected. One PL peaks has been observed, at 528 nm. The peak
intensity increases by decreasing particle size. No shift is observed.
Photoluminescence spectra of CdS/PVA
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
55/100
55
p
nanocomposite films excited by 450nm light(a -2%, b 5%, d -20% and e -30%)
A broad peak at 535
nm and a long tailtowards higher wavelengths.
Peak due torecombination ofelectrons in sulphurvacancy with hole invalence bandIntensity increaseswith CdS loading, i.e.
for smaller particles.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
56/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
57/100
57
In case of CdSe/PVAnanocomposite films, PL was
excited by 475 nm light and PLspectrum shows two peaks.
First peak at 525 nm do not shift,but the second peak at 575-585 nmshifts to shorter wave length for
smaller particles.
4
CdSe IV
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
58/100
58
0
0.5
1
1.5
2
2.5
3
3.5
4
450 500 550 600 650 700
Wavelength (in nm)
PL
Intensity(arb
.unit)
CdSe I
CdSe II
CdSe III
/
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
59/100
59
Sample Absorption
Edge(nm)
Absorptio
n Maxima(nm)
I st PL
Peak(nm)
II nd PL
peak(nm)
CdSe I 660 590 525.5 590
CdSe II 590 530 525 576
CdSe III 620 540 525 580
CdSe IV 670 580 525.5 586
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
60/100
60
This excitonic emission peak is
Stoke shifted with respect toband edge emission.
Nanocrystalline CdSe powdershow intense PL as compared tonanocomposite.
Electroluminescence Studies
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
61/100
61
For EL investigations, the emission material layer isplaced between two electrodes.
The transparent electrode has been prepared bydepositing a layer of SnO2 on heated glass substrateby chemical vapor deposition technique. ITO coated
glass is better for this purpose.For study of electroluminescence in nanocrystalline
powder samples, a piece of mica sheet having a
window of 2x2 mm is placed over the conductingglass and the sample powder is placed within thiswindow and fixed with adhesive.
In case of nanocrystal/polymer composites mica
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
62/100
62
y p y p
sheet with window was placed over the compositelayer on the conducting glass plate.
An aluminum strip is fixed over the sample along
with conducting gel in order to obtain good contact.Voltage is applied at the conducting glass and the
aluminum strip, and light emitted from the sample is
viewed through the conducting glass side.The EL brightness is measured with the help of
photomultiplier tube. The EL was studied at differentvoltages and frequencies.
EL spectra was recorded with the help of HM 104
monochromator.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
63/100
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
64/100
EL of CdS nanoparticles prepared by organometallicprecursor method (30, 60 & 90 minutes)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
65/100
65
precursor method (30, 60 & 90 minutes)
Higher brightness is obtained for smaller nanocrystals
EL starts at a threshold voltage (200 V)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
66/100
66
EL starts at a threshold voltage (200 V)and then increases with increasingvoltage.
As voltage is increased, more electronsand holes are injected into the emissionlayer. Due to high field, bending of bandstakes place and release the electrons from
traps. Their subsequent recombinationwith holes give rise to the light emission.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
67/100
67
I-V Characteristics of CdS nanoparticles prepared byorganometallic precursor method
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
68/100
68
The linear relation between currentand voltage indicates the ohmic
nature.The slope of the lines increases
with increasing particle size,indicating decrease in impedance ofthe cell.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
69/100
69
EL intensity increases exponentially with increasing voltage.Higher intensity is obtained for higher Mn doping.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
70/100
Electroluminescence Spectra of CdS nanoparticles
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
71/100
71
2
3
4
5
6
7
8
9
10
11
12
450 500 550 600
WAVELENGTH (in nm)
INT
ENSITY
(arb
.unit)
EL and PL Spectra of CdS 30 minute sample
PL
EL
The room temperature EL Spectrum is nearly
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
72/100
72
identical to the PL with same peak position.The EL peak is broader than the PL peak.This is probably due to local joule heating
from the large current flux and poor thermalconductivity. The correspondence between ELand PL spectra indicates that EL and PL
originate from the same states.In the PL and EL spectra of CdSnanocrystal, band edge luminescence was
not detected.One PL peaks has been observed, at 528nm.
EL of ZnS Nanocrystals
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
73/100
73
0
5
10
15
20
25
30
550 600 650
VOLTAGE (in volts)
BRIGHTNESS(
arb.unit
1 Khz
900 hz
800 hz
The threshold voltage is quite high.EL brightness increases with frequency
In case of ZnS:Mn also the EL brightness increases with voltage.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
74/100
74
0
2040
60
80
100
120
140
250 300 350 400 450 500
VOLTAGE (in volts)
BRIG
HTNESS
(arb.u
nit
Frq.1Khz
frq800Hz
frq600hz
EL starts at lower voltage for higher frequencies.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
75/100
t)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
76/100
76
For ZnS:Cu maximum brightness is obtained at0.01%.At higher concentration intensity decreases due to
concentration quenching.
0 0.02 0.04 0.06 0.08 0.1
Cu Dopent Concentration (in %
ELB
righ
tness(arb.un
it
Dependence of EL brightness on dopent concentration
400
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
77/100
77
Electroluminescence Brightness for CdSe nanocrystals
0
100
200
300
280 330 380 430 480
Voltage (in volts)
In
tensity(in
a.u.) 5mM
6mM
7mM
8mM9mM
400.)
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
78/100
78
EL brightness increases with frequency of applied voltage.
The figure reveals higher EL brightness for smaller CdSe
nanocrystals.
0
100
200
300
400
500 600 700 800 900 1000
Frequency (in Hz)
ELInten
sity(ina.u
5 mMol
6 mMol
7 mMol
8 mMol
9 mMol
Frequency dependence of EL intensity of CdSe nanocrystals
Similar results are obtained for nanocrystal/polymercomposites
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
79/100
79
By increasing nanocrystalline loading, EL starts atlower threshold voltage and higher intensity is obtained.
Electroluminescence Brightness for CdS/PVK Composites
EL Brightness of CdS/PVK Nanocomposites
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
80/100
80
EL Brightness vs Nanocrystal ConcentrationCharacteristics of CdS/PVK Nanocomposites
0
50
100
150
200
250
0 5 10 15 20 25
E
LBright(in
a.u.)
CdS Concentration (in %)
375 V
325 V
280 V265 V
250 V
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
81/100
Electroluminescence spectra of CdS/PVA nanocompositefilms show single peak from recombination at defect states.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
82/100
82Electroluminescence spectra of CdS/PVA nanocomposite films.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
83/100
83
The emission peak is observed at 530 nm for30% loading. For the same samplephotoluminescence peak has been obtained
at 535 nm.It has been speculated that the luminescent is
due to recombination emission from electron
trapped in the shallow defects and holetrapped in the deep defects, and shallowlytrapped electron still posses small effective
masses and therefore exhibit the quantumsize effect.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
84/100
84
The EL peak shifts towards lowerwavelengths by increasing
nanocrystalline loading.This suggests a multimodal particle size
distribution with higher loading of CdSin polymer, which indicate smallerparticle size with wider size distribution
and could be full of defects.
Similar results are obtained for ZnS/PVA.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
85/100
85
Voltage Brightness Characteristics of ZnS/PVA (A - 2% ZnS, B
- 5 %, D 20 %& F 40 %) at freq. 1 Khz
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
86/100
86
Voltage Current Characteristics of ZnS/PVA at diff. Freq,
The impedance decreases with increasing frequency.
12
F
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
87/100
87
0
2
4
6
8
10
350 400 450 500 550 600
Wavelength (nm)
Brightness(a.u.)
F
D
Electroluminescence spectra of ZnS/ PVA composite film
(voltage 312V ; D 20 %& F 40 %))
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
88/100
88
From the spectra two peaks at about 425nm and other at about 480 nm were
observed.The peak becomes sharper with higher
loading of ZnS.
It is speculated that the emission is fromthe some deep trap luminescence.
250
300
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
89/100
89
Brightness- Voltage curve of CdSe/PVA nanocomposite film
with different precursor ratio of Cd/Se
0
50
100
150
200
200 400 600 800
(.
.
)
( )
12
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
90/100
90
0
2
4
6
8
10
470 520 570 620 670
E
LI
ntensity(arb.
unit)
Wavelength (in nm)
EL Spectra of CdSe/PVA II sample (Cd:Se 2:1) at 1 Khz
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
91/100
91
There is only one emission peak inelectroluminescence of CdSe/PVAnanocomposite film at 580 nm.
Electroluminescence peak position issame as photoluminescence secondpeak position.
This indicates that charge carriesinjected from the metal electrodesrecombine solely at the CdSenanoparticles and not in PVA.
Electroluminescence spectra of all the Three
nanocomposites
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
92/100
92
nanocomposites
0
2
4
6
8
10
12
350 400 450 500 550 600 650 700
(.
.)
()
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
93/100
93
It can be seen from the EL spectra ofCdS/PVA, ZnS/PVA and CdSe/PVA that CdS/PVA gives emission peak at 520 nm in
green region,
ZnS/PVA gives two peaks at 425 and 480 nmwith violet-blue color and
CdSe/PVA gives only one peak at 580 nm inorange region.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
94/100
I f d d l ti
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
95/100
95
In case of doped samples, optimumEL is obtained for a particular dopantconcentration.
Usually Low energy states are alsopopulated by electrical excitation that
can not be populated by opticalprocess. Therefore EL emission isobtained at photon energies much less
than the band gap of the material.
Th l i i t it i d i ll
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
96/100
96
The luminescence intensity increased in allthe samples with increasingnanocrystalline loading in the composite.
This indicate that the donor states involvedin the light emission is mostly related tothe surface states of the particle, whichincrease as size of particle is reduced sincethe surface to volume ratio is increased.
The surface states need to be passivated inorder to get efficient luminescence.
CONCLUSION
Th i ti ti h l d th t
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
97/100
97
The investigations have revealed that The EL efficiency can be increased by reducing
the size of semiconductor crystals to nanometerrange.
Nanocrystallites show better photoluminescence
but for electroluminescence, nanocomposites arebeneficial.
Different materials or various sizes of samematerial may be used for different color lightemission in polymer matrix.
The EL properties of composite film consistingof polymer (PVA) and semiconductor
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
98/100
98
of polymer (PVA) and semiconductornanocrystals (CdS, ZnS, CdSe) are reported tobe interesting and promising, even though
these studies are still in the early stage, anddemand improvements in terms of luminescentpower efficiency and device life time.
The presented material system of II-VIsemiconductor nanocrystals embedded in apolymer could be promising for luminescence
devices.
Acknowledgement
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
99/100
99
The work has been carried out with the help of: Ms Rubi Tamrakar
Ms Vikas Nogriya Ms Preeti Gupta Mr. Piyush Vishwakarma Ms Amrita Diwedi Ms Sakshi Sahare Ms Mamta Tiwari Ms Reena Teckchandani
The financial support by M.P.Council of Scienceand Technology is gratefully acknowledged.
-
8/10/2019 NWNSNT- Dr M Ramrakhiani
100/100
100