straineffectsonthebandgapanddiameterofcdsecoreand cdse...

Research ArticleStrain Effects on the Band Gap and Diameter of CdSe Core andCdSeZnS CoreShell Quantum Dots at Any Temperature

Md Rezaul Karim12 Mesut Balaban 3 and Hilmi Unlu 1

1Istanbul Technical University Faculty of Science and Letters Department of Physics Engineering 34485 MaslakIstanbul Turkey2Department of Physics Dhaka University of Engineering amp Technology Gazipur-1700 Bangladesh3Yıldız Technical University Faculty of Science Department of Physics 34220 Esenler Istanbul Turkey

Correspondence should be addressed to Hilmi Unlu hunluituedutr

Received 3 May 2019 Revised 29 July 2019 Accepted 9 August 2019 Published 22 September 2019

Academic Editor Luca De Stefano

Copyright copy 2019 Md Rezaul Karim et al -is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

We present the results of an experimental study about strain effects on the core band gap and diameter of spherical bare CdSe coreand CdSeZnS coreshell quantum dots (QDs) synthesized by using a colloidal technique at varying temperatures Structuralcharacterizations were made by using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM)techniques Optical characterizations were made by using UV-Vis absorption and fluorescence emission spectroscopies-e XRDanalysis suggests that the synthesized bare CdSe core and CdSeZnS coreshell QDs have zinc blende crystal structure HRTEMresults indicate that the CdSe core and CdSeZnS QDs have average particle sizes about 350 nm and 484 nm respectivelyFurthermore compressive strain causes an increase (decrease) in the core band gap (diameter) of spherical CdSeZnS coreshellQDs at any temperature An elastic strain-modified effective mass approximation (EMA) predicts that there is a parabolic decrease(increase) in the core band gap (diameter) of QDs with temperature-e diameter of spherical bare CdSe core and CdSeZnS coreshell QDs calculated by using the strain-modified EMA with core band gap extracted from absorption spectra are in excellentagreement with the HRTEM data

1 Introduction

Group II-VI compound semiconductor-based spherical bareCdSe core and CdSeZnS coreshell quantum dots (QDs)have generated much interest among device scientists andengineers because of their potential use in fabrication of newgeneration solar cells light-emitting diodes (LEDs) fluo-rescent biosensors etc [1ndash6] When CdSe and ZnS arebrought in contact to form a heterostructure CdSeZnS coreshell QD by using a crystal growth technique conductionand valence band edges of core and shell regions are alignedin a way that an electron-hole pair excited near the heter-ointerface (Type I structure) tend to localize in the coreregion -e exciton energy in Type I coreshell QDs is theresult of a direct exciton transition inside the core Figure 1shows the schematic cross-sectional view formation andband diagram of a spherical heterostructure -e difference

in the band gaps of CdSe and ZnS is accommodated by thespike ΔEc in conduction band and the potential step ΔEv invalence band at the heterointerface causing the electron andhole confinement in the CdSe core creating an electron-holepair there

As a nanoscale heterostructure coreshell QD is formedbetween two semiconductors with different lattice constantsand thermal expansion coefficients an elastic strain developsacross the heterointerface -e elastic strain can modify thestructural electronic optical and dielectric properties ofconstituent semiconductors in a way that is not seen in two-dimensional quantum wells and superlattices If the corelattice constant is greater than the shell lattice constant thecore (shell) region will be under compressive (tensile) elasticstrain resulting in an increase (decrease) in the core (shell)band gap energy and a decrease (increase) in the diameterReliable and precise determination of the magnitude of

HindawiAdvances in Materials Science and EngineeringVolume 2019 Article ID 3764395 10 pageshttpsdoiorg10115520193764395

interface strain effects on structural and electronic propertiesof coreshell QDs become important in determining theirtunable properties such as the core band gap and diameterwhich are essential for predicting their potential as electronicand optical devices

-e aim of this work is to prove that elastic strain hasparabolic effects on the band gap and diameter of bare CdSecore and CdSeZnS coreshell QDs as a function of tem-perature which were synthesized by using a colloidaltechnique and characterized by using X-ray diffraction(XRD) high-resolution transmission electron microscopy(HRTEM) UV-Vis absorption and fluorescence emissiontechniques respectively Colloidal synthesis of CdSe coreand CdSeZnS coreshell QDs is summarized in Section 2-e results of HRTEM and XRD characterizations of the sizeof QDs will be discussed in Sections 31 and 32 respectively-e optical UV-Vis absorption and fluorescence emissionspectral analysis of QDs will be presented in Sections 33 and34 respectively -e core diameter of QDs calculated byusing strainndashmodified two-band effective mass approxi-mation with the core band gap extracted from UV-Visabsorption spectra will be compared with the results ofHRTEM analysis

2 Synthesis of CdSe Core and CdSeZnS CoreShell QDs

-e bare CdSe core quantum dots were synthesized by usingmodifications of the method of He and Gu [7] During thesynthesis reaction 069 g cadmium acetate and 25mL oleicacid were dissolved with 10mL phenyl ether in a three-neckflask -e reaction mixture was heated to 140degC understirring and continuous nitrogen flow and then the mixturewas cooled to room temperature -en 3mL 1M TOPSe wasadded to the mixture rapidly and heated to high tempera-tures (eg 160degC and 170degC) for 1 to 20minutes of reactiontimes

-e CdSeZnS coreshell QDs were synthesized by usingmodification of the technique by Zhu et al [8] In brief theresulting core solution (reaction for 1 to 20minutes)Zn(OAc)2-2H2O (0085mmol) and S powder (0085mmol)

were mixed together in a reaction vessel -e reactionvolume was adjusted to 15mL by adding paraffin liquidNext with stirring the mixture was degassed at 80degC forabout 20minutes Afterwards temperature was set to 160degCto 170degC for the capping of CdSe core with ZnS under N2atmosphere-e reaction mixture was cooled down to 300Kafter 50minutes and then 1mL aliquot crude solution waswashed with methanol and isolated by centrifugation toremove excess insoluble organics and salts that may haveformed during the reaction After the fine isolation of growthCdSeZnS coreshell QDs the precipitation was dissolvedwith different volume of hexane -e reaction was moni-tored with Shimadzu UV-Vis NIR absorption spectrometerwith aliquots taken at different time and temperature

3 Results and Discussion

31 HRTEM Characterization Results -e synthesized bareCdSe core and CdSeZnS coreshell nanocrystals werecharacterized by using high-resolution transmission elec-tron macroscopy (HRTEM) with JEOL JEM-ARM200CFEGUHR-TEM at an acceleration voltage of 200 kV A drop ofdispersed nanocrystals diluted in n-hexane dropped over anamorphous carbon substrate supported on a copper grid of400 mesh for taking images Figures 2(a) and 2(b) show theHRTEM images of bare CdSe core NCs synthesized at 160degCfor 1 and 20minutes of reaction time respectivelyFigures 2(c) and 2(d) show the HRTEM images of CdSeZnScoreshell NCs synthesized at 160degC and 170degC for20minutes of reaction time respectively HRTEM imagesshown in Figure 2 indicate that both the bare CdSe core andCdSeZnS coreShell NCs are uniform in size and shape-enanocrystal size becomes larger as reaction time is increasedWe estimate the average diameters of bare CdSe core andCdSeZnS coreshell NCs are 350 nm and 484 nmrespectively

One can estimate the thickness of ZnS shell by sub-tracting the size of the bare CdSe core from that of the CdSeZnS coreshell QD However since changes in the core sizeduring the shell deposition is inevitable due to strain acrossthe coreshell interface TEM images after the shell

ab

(a)

Tens

ile st

rain ZnS

CdSe

Com

preh

ensiv

e str

ain

(b)

ZnS

E CA

E CB

E VB

E VA

CdSe

ΔEC

ΔEV

endash e+

(c)

Figure 1 Schematic cross-sectional view of coreshell (a) interface strain (b) and band diagram (c) of pseudomorphic CdSeZnSheterostructure coreshell quantum dot

2 Advances in Materials Science and Engineering

deposition may not be a reliable reference to estimate theexact core size

32 XRD Characterization Results X-rays diffraction tech-nique is used for determining the mean size of single crystalnanoparticles and nanocrystallites in bulk materials XrsquoPert3MRD (XL) X-ray diffractometer operating at 45 kV40mAusing copper Kα line (λ 015406 nm) was used in thestructural characterization of the bare CdSe core and CdSeZnS coreshell nanocrystals Figure 3 shows the comparisonof the XRD patterns of bare CdSe core and coreshell CdSeZnS NCs both prepared at 170degC -e XRD spectra forsamples were fitted by a Gaussian profile for each peak and aquadratic function for the background

-e XRD pattern of bare CdSe core NCs exhibits broadpeaks at 2θ values of 25deg related to (111) 42deg to (220) and 48degto (311) crystalline plane for low-temperature-synthesizedzinc-blende CdSe JCPDS data base (Joint Committee onPowder Diffraction Standards file No 77-2100) -ere is aslight shift in the XRD pattern after capping the CdSe corewith ZnS shell to form a CdSeZnS coreshell NC-e broadnature of the peaks suggests nanocrystalline particles Herewe point out that there is no drastic change in the diffraction

patterns of CdSeZnS coreshell NCs relative to that of bareCdSe core NCs -is is due to the fact that the thickness ofZnS shell layer is very small (about one monolayer)

5nm

35nm

(a)

5nm

36nm

(b)

10nm

484nm

(c)

5nm

487nm

(d)

Figure 2 HRTEM images of bare CdSe core QD synthesized at 160degC for 1min (a) and 20min (b) reaction times and of CdSeZnS coreshell QD synthesized at 160degC (c) and 170degC (d) for 20min reaction time respectively

30 402θ

50

CdSe

CdSe QD at 170degC

Inte

nsity

(au

)

ZnS (111) (220) (311)

CdSeZnS QD at 170degC

(111) (220) (311)

6020

Figure 3 -e XRD patterns of (a) bare CdSe core QD prepared at170degC for 2min and at 190degC for 15min respectively and (b) coreshell CdSeZnS QDs and corresponding bare CdSe core QDsprepared at 170degC for 2min reaction time

Advances in Materials Science and Engineering 3

Although during purification almost all of paraffin wasextracted from aliquots paraffin as an amorphous elementsuppresses peaks in the XRD pattern -e XRD patterns inFigure 3 provide strong evidence that bare CdSe core andcoreshell CdSeZnS NCs have a zinc-blende (ZB) crystalstructure No reflection pattern from wurtzite latticestructure (102) at 2θ asymp 35deg and (103) at 2θ asymp 46deg is found inthe XRD pattern which is considered as further evidencethat bare CdSe core and CdSeZnS coreshell NCs have a ZBcrystal structure

33 UV-Vis Absorption and PL Emission CharacterizationResults Figures 4(a) and 4(b) compare the growth tem-perature effects on the absorption and emission spectra ofbare CdSe core QDs synthesized at 155degC 165degC and 175degCfor 5 and 10minutes of reaction times

-e position of the maximum wavelength at whichabsorption and emission coefficients of bare CdSe core NCsare maximum tends to slightly shift to higher wavelengths astemperature is increased Emission peaks vary between507 nm to 542 nm for different reaction times

Figures 5(a) and 5(b) compare the effect of growthtemperature on the absorption and emission coefficients ofCdSeZnS coreshell QDs synthesized at 160degC and 170degCfor 1 5 10 and 20minutes of reaction times -e emissionpeaks of coreshell NCs range from 550 to 570 nm corre-sponding to the full-width at half maximum (FWHM) ofband-edge luminescence between 32 nm and 36 nm

One can observe from the PL emission spectra that thePL output wavelengths corresponding to the emission peaksof CdSeZnS coreshell NCs are in the range between 507 nmand 542 nm for different reaction times -e PL emissionpeaks increase with increasing temperatures

-e first exciton transition energies in UV-Vis opticalabsorption spectra are known as the core band gaps of bothspherical bare CdSe core and CdSeZnS coreshell QDs andthey are determined from UV-Vis absorption spectral datashown in Figures 4 and 5 at maximumwavelength accordingto conventional expression [9]

Encg (d)

hc

λmax (1)

where Encg (d) is the core band gap measured at the maxi-

mum wavelength λmax at which absorption of nanoparticlesis maximum and c and h are respectively the speed of lightand Planckrsquos constant

-e quantum yield of bare CdSe core and CdSeZnScoreshell QDs were calculated by comparing with a stan-dard (Rhodamine-101 in ethanol) with an assumption of itsQYs as 95 and using the data from the fluorescence andabsorbance spectra of QDs estimated using the followingexpression [10]

φx φsIxIs

1113888 1113889As

Ax1113888 1113889

n2x

n2s

1113888 1113889 (2)

where Ix (sample) and Is (standard) are integrated emissionpeaks upon 480 nm excitation Ax (sample) and As (stan-dard) are absorption areas at 480 nm nx (sample) and ns

(standard) are refractive indices of solvents and φx and φsare FL QYs for measured and standard samples -emaximum wavelengths of UV-Vis absorption and PLemission and corresponding Stokes shifts and full width athalf maximum (FWHM) for bare CdSe QDs synthesizedfrom 150degC to 175degC are given in Table 1 for 5 and10minutes of reaction times

-e peak intensity values of UV-Vis absorption and PLemission spectra Stokes shift full width at half maximum(FWHM) and quantum yield (QY) of CdSeZnS coreshell QDs synthesized at 160degC and 170degC are listed inTable 2 for reaction times between 1 and 20min re-spectively -e fluorescence quantum yield (QY) of CdSeZnS coreshell QDs synthesized at 160degC and 170degC in-creases monotonically from 27 to 45 with reactiontime increase and decreases as reaction time is increasedfurther -is result suggests that one can optimize thereaction time to get maximum quantum yield Further-more the FWHM of CdSeZnS coreshell QDs synthe-sized at 170degC (160degC) decreases (increases) as reactiontime is increased

-e difference between the first exciton energy of ab-sorption spectra and the PL peak energy in emission spectraat the maximum wavelength is known as the Stokes shiftgiven as ΔE 2SZωp where Zωp is the energy of the photoncoupled to the electron (25meV for CdSe) and S is calledthe Huang-Rhys factor [11] which is the measure of thestrength of electron-phonon coupling Figure 6(a) showsthe temperature dependence of the maximum wavelengthof absorption and emission spectra and Stokes shift of thebare CdSe core QD as a function of temperature for severalreaction times -e maximum wavelength of emission andthe absorption spectral data given in Table 2 are used tostudy the effect of the growth temperature and reactiontime on the Stokes shifts in CdSeZnS coreshell QDFigure 6(b) shows the reaction-time dependence of theStokes shift of CdSeZnS coreshell QD synthesized at160degC and 170degC

Stokes shift in bare CdSe core NCs is observed to have aparabolic decrease with temperature increase and can befitted to the following expression

ΔE(T) minus 115 times 10minus 2T2

+ 475T minus 43644 (3)

which indicates that increasing temperature decreases themagnitude of the Stokes shift and the corresponding Huang-Rhys factor due to the increasing effect of the electron-phonon interaction with the temperature increase Fur-thermore the reaction time dependence of the Stokes shift inthe spherical CdSeZnS coreshell QD synthesized at 160degCand 170degC shown in Figure 6(b) indicates that increasingreaction time has a varying effect on the magnitude of theStokes shift and the corresponding Huang-Rhys factor

34 Discussion -e conventional two-band effective massapproximation [12] gives a qualitative understanding of theconfinement effects on core band gap and diameter ofquantum dots In this model the solution of the Schrodingerequation for a particle in a spherical box gives the first


Table 1 Maximumwavelengths of UV-Vis absorption and PL emission spectra and the corresponding Stokes shift and the full width at halfmaximum (FWHM) of bare CdSe core QDs at different temperatures for 5 and 10minutes of reaction times

Reactiontime (min)

Growthtemperature (degC)

λmax of absorptionspectra (nm)

λmax of emissionspectra (nm)

Stokesshift (nm) FWHM (nm)

5

150 452 521 69 31155 476 525 49 33160 484 527 43 42165 496 532 36 65170 508 538 30 43175 512 541 29 48

10

150 453 525 72 30155 481 527 46 32160 488 530 42 33165 500 535 35 32170 512 542 30 44175 516 545 29 62

0

05

1

15

2

25

3

35

ndash50

0

50

100

150

200

300 350 400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 5 min

175degC

155degC

155degC

165degC

165degC

175degC

Abso

rban

ce (a

u)

(a)

ndash1

0

1

2

3

4

5

0

20

40

60

80

100

120

140

300 350 400 450 500 550 600 650 700

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 10 min

155degC

165degC155degC

165degC

175degC

175degC

Abso

rban

ce (a

u)

(b)

Figure 4 Absorbance and emission spectra of bare CdSe core QDs synthesized at 155degC 165degC and 175degC for 5min (a) and 10min (b)plotted as a function of photon wavelength

0

05

1

15

0

50

100

150

200

250

300

400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSeZnS QDsat 160degC

20 min

5 min

1 min

10 min

1 min

5 min

10 min

20 min

Abso

rban

ce (a

u)

(a)

0

01

02

03

04

05

06

07

08

0

50

100

150

200

250

300

350

350 400 450 500 550 600 650 700

450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

1 min

10 min 1 min

5 min10 min20 min

5 min

Abso

rban

ce (a

u)

(b)

Figure 5 Absorbance and emission spectra of CdSeZnS coreshell QDs synthesized at 160degC (a) and 170degC (b) for different reaction timesplotted as a function of photon wavelength


excited state (1s-1s) energy (core band gap) of nanoparticlegiven by the following expression

Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4

Z2mlowastcvε2infin (4)

where Ebg is the band gap mlowastcv mlowaste mlowasth(mlowaste + mlowasth) is the

reduced effective mass of the electron-hole pair with effectivemasses of electrons and holes mlowaste and mlowasth respectively andεinfin is the optical dielectric constant of CdSe in bulk form-e second and third terms respectively represent theconfinement energy and the Coulomb interaction energywith a 1d2 and 1d dependence on QD diameter Finally thelast term is the Rydberg correlation energy which is neg-ligibly small for when εinfin of the semiconductor componentis large

Since spherical coreshell QDs are grown from twosemiconductors with different lattice constants and thermalexpansion coefficients at temperatures above 300K therewill be an elastic strain developed across heterointerface-eelastic strain can modify the structural and electronicproperties of the core and shell constituents in a way that isnot seen in two-dimensional quantum wells and

superlattices In order to understand the strain effects on thestructural and electronic properties of bare CdSe core andCdSeZnS coreshell QDs we will use the recent extension ofthe universal Eshelbyrsquos elastic strain model [13] to study thestrain effects in nanoscale spherical coreshell hetero-structures at any temperature [14ndash16] According to thismodel one assumes that the core radius is much smallerthan that of the shell constituent (altlt b) of the sphericalcoreshell heterostructure and writes the interface strain inthe core region as [16]

εi ai εi( 1113857 minus ai

ai αi(T)T minus

2Em 1 minus 2]i( 1113857 εim + αi minus αm( 1113857T1113858 1113859

Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)

where εim (ai minus am)am and (αi minus αm)T are lattice andthermal mismatches respectively Here ai and am are latticeconstants and αi and αm are the linear expansion coefficientsof the core and shell at 300K Ei (Em) is Youngrsquos modulusand ]i (vm) is Poissonrsquos ratio of the core (shell) semi-conductor in bulk form defined as E (C11 minus C12)(C11 +

2C12)(C11 + C12) and ] C12(C11 + C12) where C11 andC12 are elastic stiffness constants

Table 2 Absorption and emission peak intensities full width at half maximum (FWHM) and quantum yield (QY) of CdSeZnS QDssynthesized at 160degC and 170degC and measured at different reaction times

Temperature(degC)

Time(min)

Peak absorbance wavelength(nm)

Peak emission wavelength(nm)

Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)

Figure 6 Stokes shift in bare CdSe core QDs (a) as a function of temperature and that in CdSeZnS coreshell QDs (b) as a function ofreaction time respectively


Using the elastic constants C11 102GPa and 667GPaand C12 646GPa and 463GPa linear thermal expansioncoefficients αi 730 times 10minus 6 Kminus 1 αm 69 times 10minus 6 Kminus 1 andlattice constants ai 0607 nm and am 041 nm for CdSeand ZnS [17] equation (5) gives εi minus 370 compressivestrain at interface of spherical CdSeZnS coreshell QDwhich has positive lattice mismatch Δaa (ai minus am)am

123 at room temperature -erefore we can conclude thatreliable modelling and precise determination of the mag-nitude of the strain effects on the electronic structure ofnanoscale heterostructures is extremely important for pre-dicting their potential and the simulation of the performanceof nanoscale electronic and optical devices In doing so onemust thenmodify equation (4) to the following expression inorder to take into account the strain effects on the core bandgap for a reliable understanding of charge confinement inCdSeZnS coreshell QDs [14]

Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4


where Ebcg (T εi) is the strain-dependent band gap of the

core semiconductor in bulk which is obtained by using astatistical thermodynamic model of semiconductors [18]as

Ebcg T εi( 1113857 E

bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)

where P is the hydrostatic pressure acting on the bandstructure of the core region given by

P minus 3Biεi(T) minus 3Biαi(T)ΔT

+ 3Bi2Em 1 minus 2]i( 1113857 εim + αi minus αm( 1113857T1113858 1113859

Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)

Logarithmic term ΔC0PT(1 minus ln T) in equation (7)

represents lattice vibration contribution to the band gapchange ΔC0

P is the heat capacity of the reaction for for-mation of the electron-hole pair obtained by fitting the bulkband gap calculated from equation (7) at constant pressureto the measured band gap [18] fitted to

Ebg(T) E

bg(0) +

αT2

β + T (9)

which is known as Varshni equation [19] Here α 409 times

10minus 4 and β 187 are constants for bulk CdSe We can nowuse equation (7) to get a qualitative understanding of thevariation of the core band gap with its diameter at anytemperature

Using the material parameters for CdSe and ZnS bulksemiconductors [17] we calculated the effects of the in-terface strain electron-phonon interactions and chargeconfinement on the shift in core band gap of bare CdSe coreQD and CdSeZnS coreshell QD as a function of tem-perature Figures 7(a) and 7(b) compare the contribution ofthe interface strain and electron-phonon interaction(Figure 7(a)) and charge confinement (Figure 7(b)) on bandgap decrease in band gaps of CdSeZnS coreshell QDFigure 7(a) indicates that electron-phonon interaction(lattice vibration) contribution to core band gap decrease isalways less than zero and interface strain and lattice vi-bration tends to decrease core band gap with temperatureincrease However Figure 7(b) indicates that charge

Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)

Effect of lattice vibration

CdSeZnS QD

Effect of interface strain and lattice vibration

ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12

3 4 5 62Core diameter (nm)

(b)

Figure 7 Effects of interface strain and electron-phonon interactions (a) and charge confinement effects (b) on band gap decrease of CdSeZnS coreshell QDs respectively


confinement has parabolic dependence on the core diameterand becomes nearly flat as the core diameter is increasedabove 6 nm

Once first exciton energy (core bad gap) is de-termined from the UV-Vis absorption data usingequation (1) core diameter quantum dot can also becalculated by converting equation (6) to a quadraticequation as d is variable Ad2 + B d + C 0 where A Band C are material parameter-dependent coefficients-e positive root of such quadratic equation will give thediameter of QDs -is will then allow one to calculate thecore diameter of the bare core and core shell QDs fromthe first excited state energy extracted from UV-Visabsorption spectra Figures 8(a) and 8(b) show themeasured core band gaps and calculated diameter of bareCdSe core QDs synthesized at temperatures between155degC and 180degC and processed at 1 5 10 15 and20minutes respectively

A polynomial fit to the core band gap of bare CdSe QDsprocessed at 5min suggests a parabolic temperature of thecore band gap given as

Encg (T) aT

2+ bT + c (10)

where a b and c are coefficients which can be strain- andtemperature-dependent -e parabolic fit in Figure 8(a)yields a 443 times 10minus 4 Kminus 2 b minus 016Kminus 1 and c 1626 for10minutes of reaction time -e parabolic decrease of thecore band gap of bare CdSe QDs with temperature increaseshown in Figures 8(a) and 8(b) is consistent with the fact thatthe band gap of most of the group IV elemental and groupsIIIndashV and IIndashVI compound semiconductors tends to de-crease with increasing temperature due to electron-phononinteraction and free expansion of the lattice constant withincreasing temperature Similar polynomial fit to the corediameter of CdSe QD is given by

dnc

(T) alowastT2

+ blowastT + clowast (11)

where alowast blowast and clowast d e and f are coefficients which can betemperature dependent alowast minus 504 times 10minus 4K2 blowast 018Kand clowast minus 1296 for 10minutes of reaction time Since there isonly free thermal expansion of lattice constant in bare coreQDs shift in their band gap and diameter is mainly due toelectron-phonon interaction with temperature increase

Since the lattice constant and thermal expansioncoefficient of the CdSe core are greater than those of theZnS shell of CdSeZnS coreshell QDs the CdSe coreregion will be under compressive strain resulting in anincrease (decrease) in its band gap energy (diameter) astemperature is increased Table 3 gives list of the UV-Vismaximum wavelength of absorption coefficient andcorresponding first excited state energies (core bandgaps) of CdSeZnS QDs coreshell QDS synthesized at160degC and 170degC and processed for 1 5 10 and20minutes respectively

Using the results given in Table 3 the unstrained andstrained values of core diameters are compared in Fig-ure 9 as a function of the core band gap of CdSeZnScoreshell QDs synthesized at 160degC and 170degC for

23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min

Temperature (degC) 155 160 165 170 175 180

(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)

Figure 8 Temperature variation of core band gap energy (a) and diameter (b) of bare CdSe core QDs processed at 1 5 10 and 20minutes ofreaction times respectively

Table 3 UV-Vis maximum wavelength and correspondingmeasured band gap and calculated unstrained and strained corediameter of CdSeZnS QDs synthesized at 160degC and 170degC fordifferent reaction times (min)

Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474


various reaction times respectively Calculated un-strained and strained diameters show a parabolic de-crease (increase) as core band gap increase (decreases) atboth temperatures

e strained core diameters are in excellent agreementwith HRTEM results which indicates that the bare CdSecore and coreshell CdSeZnS QDs synthesized at 160degC and170degC for 20minutes of reaction time have average particlesizes about 350 nm and 484 nm respectively

4 Conclusion

We presented the results of a comprehensive study ofelastic strain eects on the core band gap and diameter ofspherical bare CdSe core and CdSeZnS heterostructurecoreshell quantum dots that are synthesized by a colloidaltechnique at various temperatures e XRD analysissuggests that the synthesized CdSe core and CdSeZnScoreshell QDs have a zinc-blende crystal structureHRTEM measurements indicate that the bare CdSe coreand CdSeZnS coreshell QDs have average particle sizesabout 350 nm and 484 nm respectively UV-Vis absorp-tion spectra measurements show that the increase ofgrowth temperature leads to a decrease (increase) in theband gap (diameter) of CdSeZnS coreshell QDs Fur-thermore we also show that the eect of elastic strain to theband gap change is greater than that of lattice vibration inCdSeZnS coreshell QDs at any temperature e com-pressive strain causes an increase (decrease) in the coreband gap (diameter) of spherical CdSeZnS coreshell QDsElastic strainndashmodied eective mass approximationpredicts that there is a parabolic decrease (increase) in thecore band gap (diameter) of CdSeZnS coreshell QDs withtemperature e calculated core diameter of bare CdSecore and CdSeZnS coreshell QDs are in excellentagreement with HRTEM measurements

Data Availability

e data used to support the ndings of this study are in-cluded within the article

Conflicts of Interest

e authors declare that they have no conicts of interest

Acknowledgments

e authors greatly acknowledge the nancial support by theResearch Foundation of Istanbul Technical University (ITUBAP Project No 34537) Preliminary part of this work waspresented at the Turkish Physical Society 33rd InternationalPhysics Congress (TPS-33) AIP Conference Proceedings1935 050004 (2018)

References

[1] H Unlu N J M Horing and J Dabrowski Low Dimensionaland Nanostructured Materials and Devices Springer BerlinGermany 2015

[2] H Zhu A Prakash D Benoit C Jones and V Colvin ldquoLowtemperature synthesis of ZnS and CdZnS shells on CdSequantum dotsrdquo Nanotechnology vol 21 no 25 Article ID255604 2010

[3] A Nadarajah T Smith and R Konenkamp ldquoImprovedperformance of nanowire-quantum-dot-polymer solar cellsby chemical treatment of the quantum dot with ligand andsolvent materialsrdquo Nanotechnology vol 23 no 48 p 4854032012

[4] C-Y Huang Y-K Su T-C Wen T-F Guo and M-L TuldquoSingle-layered hybrid DBPPV-CdSe-ZnS quantum-dot light-emitting diodesrdquo IEEE Photonics Technology Letters vol 20no 4 pp 282ndash284 2008

[5] L Deng L Han Y Xi X Li and W P Huang ldquoDesignoptimization for high-performance self-assembled quantumdot lasers with fabryndashperot cavityrdquo IEEE Photonics Journalvol 4 no 5 pp 1600ndash1609 2012

[6] W Luan H Yang N Fan and S-T Tu ldquoSynthesis of edeg-ciently green luminescent CdSeZnS nanocrystals viamicrouidic reactionrdquo Nanoscale Research Letters vol 3no 4 pp 134ndash139 2008

[7] R He and H Gu ldquoSynthesis and characterization of mon-dispersed CdSe nanocrystals at lower temperaturerdquo Colloidsand Surfaces A Physicochemical and Engineering Aspectsvol 272 no 1-2 pp 111ndash116 2006

[8] C-Q Zhu P Wang X Wang and Y Li ldquoFacile phosphine-free synthesis of CdSeZnS coreshell nanocrystals withoutprecursor injectionrdquo Nanoscale Research Letters vol 3 no 6pp 213ndash220 2008

[9] S Suresh ldquoStudies on the dielectric properties of CdSnanoparticlesrdquo Applied Nanoscience vol 4 no 3 pp 325ndash329 2013

[10] M Grabolle M Spieles V Lesnyak N GaponikA Eychmuller and U Resch-Genger ldquoDetermination of theuorescence quantum yield of quantum dots suitable pro-cedures and achievable uncertaintiesrdquo Analytical Chemistryvol 81 no 15 pp 6285ndash6294 2009

[11] A Joshi K Y Narsingi M O Manasreh E A Davis andB D Weaver ldquoTemperature dependence of the band gap ofcolloidal coreshell nanocrystals embedded into an ultraviolet

Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48

225 23 23522Core band gap (eV)

Figure 9 Core diameter of CdSeZnS coreshell QDs plotted andas a function of core band gap with and without strain at 160degC and170degC respectively


curable resinrdquo Appl Phys Lettvol 89 no 13 Article ID131907 2006

[12] L E Brus ldquoElectron-electron and electron-hole interactionsin small semiconductor crystallites the size dependence of thelowest excited electronic staterdquo De Journal of ChemicalPhysics vol 80 no 9 pp 4403ndash4409 1984

[13] J Eshelby ldquo-e determination of the elastic field of an el-lipsoidal inclusion and related problemsrdquo Proceedings of theRoyal Society of London Series A Mathematical and PhysicalSciences vol 241 no 1226 pp 376ndash396 1957

[14] H Unlu ldquoModelling of CdSeCdZnS and ZnSeCdZnS bi-naryternary heterostructure coreshell quantum dotsrdquo AIPConference Proceedings vol 1935 Article ID 050003 2018

[15] M R Karim and H Unlu ldquoTemperature and strain effects oncore bandgap and diameter of bare CdSe core and CdSeZnSheterostructure coreshell quantum dotsrdquo AIP ConferenceProceedings vol 1935 Article ID 050004 2018

[16] H Unlu ldquoA thermoelastic model for strain effects onbandgaps and band offsets in heterostructurecoreshellquantum dotsrdquo De European Physical Journal AppliedPhysics vol 86 no 3 Article ID 30401 2019

[17] O Madelung Ed Numerical Data and Functional Re-lationships in Science and Technology Part d of Vol 17 (1984)Springer Berlin Germany

[18] H Unlu ldquoA thermodynamic model for determining pressureand temperature effects on the bandgap energies and otherproperties of some semiconductorsrdquo Solid-State Electronicsvol 35 no 9 pp 1343ndash1352 1992

[19] Y P Varshni ldquoTemperature dependence of the energy gap insemiconductorsrdquo Physica vol 34 no 1 pp 149ndash154 1967


CorrosionInternational Journal of

Hindawiwwwhindawicom Volume 2018

Advances in

Materials Science and EngineeringHindawiwwwhindawicom Volume 2018


Journal of

Chemistry

Analytical ChemistryInternational Journal of


ScienticaHindawiwwwhindawicom Volume 2018

Polymer ScienceInternational Journal of



Advances in Condensed Matter Physics


International Journal of

BiomaterialsHindawiwwwhindawicom

Journal ofEngineeringVolume 2018

Applied ChemistryJournal of


NanotechnologyHindawiwwwhindawicom Volume 2018

Journal of


High Energy PhysicsAdvances in

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

TribologyAdvances in



ChemistryAdvances in


Advances inPhysical Chemistry


BioMed Research InternationalMaterials

Journal of


Na

nom

ate

ria

ls


Journal ofNanomaterials

Submit your manuscripts atwwwhindawicom

interface strain effects on structural and electronic propertiesof coreshell QDs become important in determining theirtunable properties such as the core band gap and diameterwhich are essential for predicting their potential as electronicand optical devices

-e aim of this work is to prove that elastic strain hasparabolic effects on the band gap and diameter of bare CdSecore and CdSeZnS coreshell QDs as a function of tem-perature which were synthesized by using a colloidaltechnique and characterized by using X-ray diffraction(XRD) high-resolution transmission electron microscopy(HRTEM) UV-Vis absorption and fluorescence emissiontechniques respectively Colloidal synthesis of CdSe coreand CdSeZnS coreshell QDs is summarized in Section 2-e results of HRTEM and XRD characterizations of the sizeof QDs will be discussed in Sections 31 and 32 respectively-e optical UV-Vis absorption and fluorescence emissionspectral analysis of QDs will be presented in Sections 33 and34 respectively -e core diameter of QDs calculated byusing strainndashmodified two-band effective mass approxi-mation with the core band gap extracted from UV-Visabsorption spectra will be compared with the results ofHRTEM analysis

2 Synthesis of CdSe Core and CdSeZnS CoreShell QDs

-e bare CdSe core quantum dots were synthesized by usingmodifications of the method of He and Gu [7] During thesynthesis reaction 069 g cadmium acetate and 25mL oleicacid were dissolved with 10mL phenyl ether in a three-neckflask -e reaction mixture was heated to 140degC understirring and continuous nitrogen flow and then the mixturewas cooled to room temperature -en 3mL 1M TOPSe wasadded to the mixture rapidly and heated to high tempera-tures (eg 160degC and 170degC) for 1 to 20minutes of reactiontimes

-e CdSeZnS coreshell QDs were synthesized by usingmodification of the technique by Zhu et al [8] In brief theresulting core solution (reaction for 1 to 20minutes)Zn(OAc)2-2H2O (0085mmol) and S powder (0085mmol)

were mixed together in a reaction vessel -e reactionvolume was adjusted to 15mL by adding paraffin liquidNext with stirring the mixture was degassed at 80degC forabout 20minutes Afterwards temperature was set to 160degCto 170degC for the capping of CdSe core with ZnS under N2atmosphere-e reaction mixture was cooled down to 300Kafter 50minutes and then 1mL aliquot crude solution waswashed with methanol and isolated by centrifugation toremove excess insoluble organics and salts that may haveformed during the reaction After the fine isolation of growthCdSeZnS coreshell QDs the precipitation was dissolvedwith different volume of hexane -e reaction was moni-tored with Shimadzu UV-Vis NIR absorption spectrometerwith aliquots taken at different time and temperature

3 Results and Discussion

31 HRTEM Characterization Results -e synthesized bareCdSe core and CdSeZnS coreshell nanocrystals werecharacterized by using high-resolution transmission elec-tron macroscopy (HRTEM) with JEOL JEM-ARM200CFEGUHR-TEM at an acceleration voltage of 200 kV A drop ofdispersed nanocrystals diluted in n-hexane dropped over anamorphous carbon substrate supported on a copper grid of400 mesh for taking images Figures 2(a) and 2(b) show theHRTEM images of bare CdSe core NCs synthesized at 160degCfor 1 and 20minutes of reaction time respectivelyFigures 2(c) and 2(d) show the HRTEM images of CdSeZnScoreshell NCs synthesized at 160degC and 170degC for20minutes of reaction time respectively HRTEM imagesshown in Figure 2 indicate that both the bare CdSe core andCdSeZnS coreShell NCs are uniform in size and shape-enanocrystal size becomes larger as reaction time is increasedWe estimate the average diameters of bare CdSe core andCdSeZnS coreshell NCs are 350 nm and 484 nmrespectively

One can estimate the thickness of ZnS shell by sub-tracting the size of the bare CdSe core from that of the CdSeZnS coreshell QD However since changes in the core sizeduring the shell deposition is inevitable due to strain acrossthe coreshell interface TEM images after the shell

ab

(a)

Tens

ile st

rain ZnS

CdSe

Com

preh

ensiv

e str

ain

(b)

ZnS

E CA

E CB

E VB

E VA

CdSe

ΔEC

ΔEV

endash e+

(c)

Figure 1 Schematic cross-sectional view of coreshell (a) interface strain (b) and band diagram (c) of pseudomorphic CdSeZnSheterostructure coreshell quantum dot






5nm

35nm

(a)

5nm

36nm

(b)

10nm

484nm

(c)

5nm

487nm

(d)


30 402θ

50

CdSe

CdSe QD at 170degC

Inte

nsity

(au

)

ZnS (111) (220) (311)


(111) (220) (311)

6020









Encg (d)

hc

λmax (1)




φx φsIxIs

1113888 1113889As

Ax1113888 1113889

n2x

n2s

1113888 1113889 (2)







+ 475T minus 43644 (3)





Reactiontime (min)





5

150 452 521 69 31155 476 525 49 33160 484 527 43 42165 496 532 36 65170 508 538 30 43175 512 541 29 48

10

150 453 525 72 30155 481 527 46 32160 488 530 42 33165 500 535 35 32170 512 542 30 44175 516 545 29 62

0

05

1

15

2

25

3

35

ndash50

0

50

100

150

200

300 350 400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 5 min

175degC

155degC

155degC

165degC

165degC

175degC

Abso

rban

ce (a

u)

(a)

ndash1

0

1

2

3

4

5

0

20

40

60

80

100

120

140

300 350 400 450 500 550 600 650 700

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 10 min

155degC

165degC155degC

165degC

175degC

175degC

Abso

rban

ce (a

u)

(b)


0

05

1

15

0

50

100

150

200

250

300

400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

5 min

1 min

10 min

1 min

5 min

10 min

20 min

Abso

rban

ce (a

u)

(a)

0

01

02

03

04

05

06

07

08

0

50

100

150

200

250

300

350

350 400 450 500 550 600 650 700

450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

1 min

10 min 1 min

5 min10 min20 min

5 min

Abso

rban

ce (a

u)

(b)




Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4







ai αi(T)T minus


Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)




Temperature(degC)

Time(min)



Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)





Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls








5nm

35nm

(a)

5nm

36nm

(b)

10nm

484nm

(c)

5nm

487nm

(d)


30 402θ

50

CdSe

CdSe QD at 170degC

Inte

nsity

(au

)

ZnS (111) (220) (311)


(111) (220) (311)

6020









Encg (d)

hc

λmax (1)




φx φsIxIs

1113888 1113889As

Ax1113888 1113889

n2x

n2s

1113888 1113889 (2)







+ 475T minus 43644 (3)





Reactiontime (min)





5

150 452 521 69 31155 476 525 49 33160 484 527 43 42165 496 532 36 65170 508 538 30 43175 512 541 29 48

10

150 453 525 72 30155 481 527 46 32160 488 530 42 33165 500 535 35 32170 512 542 30 44175 516 545 29 62

0

05

1

15

2

25

3

35

ndash50

0

50

100

150

200

300 350 400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 5 min

175degC

155degC

155degC

165degC

165degC

175degC

Abso

rban

ce (a

u)

(a)

ndash1

0

1

2

3

4

5

0

20

40

60

80

100

120

140

300 350 400 450 500 550 600 650 700

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 10 min

155degC

165degC155degC

165degC

175degC

175degC

Abso

rban

ce (a

u)

(b)


0

05

1

15

0

50

100

150

200

250

300

400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

5 min

1 min

10 min

1 min

5 min

10 min

20 min

Abso

rban

ce (a

u)

(a)

0

01

02

03

04

05

06

07

08

0

50

100

150

200

250

300

350

350 400 450 500 550 600 650 700

450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

1 min

10 min 1 min

5 min10 min20 min

5 min

Abso

rban

ce (a

u)

(b)




Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4







ai αi(T)T minus


Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)




Temperature(degC)

Time(min)



Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)





Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls










Encg (d)

hc

λmax (1)




φx φsIxIs

1113888 1113889As

Ax1113888 1113889

n2x

n2s

1113888 1113889 (2)







+ 475T minus 43644 (3)





Reactiontime (min)





5

150 452 521 69 31155 476 525 49 33160 484 527 43 42165 496 532 36 65170 508 538 30 43175 512 541 29 48

10

150 453 525 72 30155 481 527 46 32160 488 530 42 33165 500 535 35 32170 512 542 30 44175 516 545 29 62

0

05

1

15

2

25

3

35

ndash50

0

50

100

150

200

300 350 400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 5 min

175degC

155degC

155degC

165degC

165degC

175degC

Abso

rban

ce (a

u)

(a)

ndash1

0

1

2

3

4

5

0

20

40

60

80

100

120

140

300 350 400 450 500 550 600 650 700

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 10 min

155degC

165degC155degC

165degC

175degC

175degC

Abso

rban

ce (a

u)

(b)


0

05

1

15

0

50

100

150

200

250

300

400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

5 min

1 min

10 min

1 min

5 min

10 min

20 min

Abso

rban

ce (a

u)

(a)

0

01

02

03

04

05

06

07

08

0

50

100

150

200

250

300

350

350 400 450 500 550 600 650 700

450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

1 min

10 min 1 min

5 min10 min20 min

5 min

Abso

rban

ce (a

u)

(b)




Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4







ai αi(T)T minus


Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)




Temperature(degC)

Time(min)



Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)





Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





Reactiontime (min)





5

150 452 521 69 31155 476 525 49 33160 484 527 43 42165 496 532 36 65170 508 538 30 43175 512 541 29 48

10

150 453 525 72 30155 481 527 46 32160 488 530 42 33165 500 535 35 32170 512 542 30 44175 516 545 29 62

0

05

1

15

2

25

3

35

ndash50

0

50

100

150

200

300 350 400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 5 min

175degC

155degC

155degC

165degC

165degC

175degC

Abso

rban

ce (a

u)

(a)

ndash1

0

1

2

3

4

5

0

20

40

60

80

100

120

140

300 350 400 450 500 550 600 650 700

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)

CdSe QDsat 10 min

155degC

165degC155degC

165degC

175degC

175degC

Abso

rban

ce (a

u)

(b)


0

05

1

15

0

50

100

150

200

250

300

400 450 500 550 600 650

400 450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

5 min

1 min

10 min

1 min

5 min

10 min

20 min

Abso

rban

ce (a

u)

(a)

0

01

02

03

04

05

06

07

08

0

50

100

150

200

250

300

350

350 400 450 500 550 600 650 700

450 500 550 600 650

PL in

tens

ity (a

u)

Wavelength (nm)


20 min

1 min

10 min 1 min

5 min10 min20 min

5 min

Abso

rban

ce (a

u)

(b)




Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4







ai αi(T)T minus


Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)




Temperature(degC)

Time(min)



Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)





Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





Encg (d) E

bg +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4







ai αi(T)T minus


Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(5)




Temperature(degC)

Time(min)



Stokes shift(nm)

FWHM(nm)

QY()

160

1 532 550 18 32 275 535 553 18 33 4110 538 554 16 32 4520 539 556 17 36 36

170

1 549 560 11 35 285 552 563 11 34 4010 555 565 10 33 4420 557 570 13 33 42

30

40

50

60

70

150 155 160 165 170 175

CdSe QDs

Temperature (degC)

10 min

5 min

Poly fit

Stok

e shi

ft (n

m)

(a)

10

12

14

16

18

20

0 5 10 15 20

CdSeZnS QDs

Reaction time (min)

160degC

170degC

Stok

e shi

ft (n

m)

(b)





Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






Encg εi( 1113857 E

bcg T εi( 1113857 +

2Z2π2

mlowastcvd2 minus

3572e2

εinfind+

0124e4





bcg (0) + ΔC0

iPT(1 minus ln T)

minusagi

BiP minus

P2

2Biminus

1 + Brsquoi1113872 1113873

3B2i

P3⎛⎝ ⎞⎠

(7)




Ei 1 + ]m( 1113857 + 2Em 1 minus 2]i( 1113857

(8)




Ebg(T) E

bg(0) +

αT2

β + T (9)




Stra

in an

d la

ttice

vib

ratio

n sh

ift (e

V)


CdSeZnS QD


ndash02

ndash01

00

01

02

03

100 200 300 400 500 6000Temperature (K)

(a)Ch

arge

conf

inem

ent s

hift

(eV

)

CdSeZnS QD

00

02

04

06

08

10

12


(b)






Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







Encg (T) aT

2+ bT + c (10)


dnc

(T) alowastT2





23

235

24

245

25

255

26

265Co

re b

and

gap

ener

gy (e

V)

CdSe QDs

15 min

10 min

20 min

1 min

5 min


(a)

Core

dia

met

er (n

m)

Temperature (degC)

38

4

42

44

46

48

5

CdSe QDs

15 min

1 min

20 min

10 min

5 min

155 160 165 170 175 180

(b)



Temperature(degC)

Time(min)

λmax(nm)

Encg

(eV)d

(nm)d(εi)

(nm)

160

1 532 234 389 4355 535 232 392 43910 538 231 395 44320 539 230 396 445

170

1 549 226 402 4615 552 225 405 46610 555 223 408 47120 557 223 411 474




4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






4 Conclusion


Data Availability




Acknowledgments


References












Core

dia

met

er (n

m)

170degC

170degC

160degC

160degC

Unstrained

Strained

CdSeZnS QDs

38

4

42

44

46

48
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls
















Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




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