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    ELECTROLUMINESCENCEIN

    NANOCRYSTALSAND

    NANOCOMPOSITS

    Meera Ramrakhiani

    Department of Post-Graduate Studies and Researchin Physics and Electronics

    Rani Durgavati University , Jabalpur

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    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.

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    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.

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    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.

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    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

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    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

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    .

    .

    .

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    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.

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    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.

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    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.

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    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.

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    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.

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    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

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    SEM of ZnS Nanoparticles(By chemical bath deposition - particle size: 23nm)

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    SEM image of clusters of CdS nanoparticles onto

    plastic substrate (particle size: 26 nm)

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    TEM image of nanocrystalline ZnS

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    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

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    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

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    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.

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    XRD for Mn doped and undoped nanocrystalsMn doping causes broadening of the XRDpeaks; but other methods do not show smaller

    particle size.

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    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

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    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

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    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.

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    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.

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    28XRD Pattern of CdS/PVA

    10 20 30 40 50 60

    ( )

    (.

    )

    5 %

    20 %

    40%

    311220

    200

    111

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    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

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    XRD pattern of ZnS/ PVA nanocomposite film

    (A- 2 % ZnS, B- 5%, D- 20 % & F- 40 %)

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    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

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    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

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    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.

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    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

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    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.

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    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

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    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.

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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%

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    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

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    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

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    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

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    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.

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    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

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    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

    /

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    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

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    This excitonic emission peak is

    Stoke shifted with respect toband edge emission.

    Nanocrystalline CdSe powdershow intense PL as compared tonanocomposite.

    Electroluminescence Studies

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    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

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    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.

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    EL of CdS nanoparticles prepared by organometallicprecursor method (30, 60 & 90 minutes)

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    precursor method (30, 60 & 90 minutes)

    Higher brightness is obtained for smaller nanocrystals

    EL starts at a threshold voltage (200 V)

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    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.

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    I-V Characteristics of CdS nanoparticles prepared byorganometallic precursor method

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    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.

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    EL intensity increases exponentially with increasing voltage.Higher intensity is obtained for higher Mn doping.

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    Electroluminescence Spectra of CdS nanoparticles

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    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

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    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

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    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.

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    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.

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    t)

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    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

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    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.)

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    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

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    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

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    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

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    Electroluminescence spectra of CdS/PVA nanocompositefilms show single peak from recombination at defect states.

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    82Electroluminescence spectra of CdS/PVA nanocomposite films.

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    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.

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    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.

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    Voltage Brightness Characteristics of ZnS/PVA (A - 2% ZnS, B

    - 5 %, D 20 %& F 40 %) at freq. 1 Khz

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    Voltage Current Characteristics of ZnS/PVA at diff. Freq,

    The impedance decreases with increasing frequency.

    12

    F

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    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 %))

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    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

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    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

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    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

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    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

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    nanocomposites

    0

    2

    4

    6

    8

    10

    12

    350 400 450 500 550 600 650 700

    (.

    .)

    ()

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    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.

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    I f d d l ti

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    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

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    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

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    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

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    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

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    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.

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