Photoluminescence of ZnS Mn quantum dot by hydrothermal method(水热法制备ZnSMn量子点及其光学性能)

Photoluminescence of ZnS: Mn quantum dot by hydrothermal method Yun Hu, Zhaorong Wei, Bo Wu, Bing Shen, Qun Dai, and Pengxian Feng

Citation: AIP Advances8, 015014 (2018);

View online: https://https://www.360docs.net/doc/7513746250.html,/10.1063/1.5010833

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Published by the American Institute of Physics

Photoluminescence of ZnS:Mn quantum

dot by hydrothermal method

Yun Hu,a Zhaorong Wei,Bo Wu,Bing Shen,Qun Dai,and Pengxian Feng

College of Optoelectronic Technology,Chengdu University of Information Technology,

Chengdu,P.R.China,610225

(Received27October2017;accepted4January2018;published online12January2018)

ZnS:Mn quantum dots(QDs)with the average grain size from4.2to7.2nm were

synthesized by a hydrothermal method.All samples were cubic zinc blende structure

(β-ZnS)measured using X-ray diffraction(XRD).And the main diffraction peaks

of ZnS:Mn shifted slightly towards higher angle in comparison with the intrin-

sic ZnS because of the substitution of Mn2+for Zn2+.Due to the small grain size

(4-7nm)effect,the poor dispersion and serious reunion phenomenon for the sam-

ples were observed from transmission electron microscopy(TEM).ZnS:Mn QDs

had four peaks centered at466,495,522,and554nm,respectively,in the pho-

toluminescence(PL)spectra,in which the band at554nm absent in the intrinsic

ZnS:Mn is attributed to the doping of Mn2+in the lattice sites.As the concen-

tration of Mn2+increasing from0%to0.6at%,the intensity of the PL emission

also increased.But the concentration reached0.9at%,quenching of PL emission

occurred.The peak in ZnS:Mn QDs observed at490cm-1was originated from the

stretching vibration of the Mn–O bonds in the Fourier transform infrared(FTIR)

spectra.And the small changes about this peak compared with the previous reports

at500cm-1can be attributed to the formation of quantum dots.This method we uti-

lized to synthesize ZnS:Mn QDs is very simple,low cost,and applicable for other

semiconductor QD materials.?2018Author(s).All article content,except where

otherwise noted,is licensed under a Creative Commons Attribution(CC BY)license

(https://www.360docs.net/doc/7513746250.html,/licenses/by/4.0/).https://https://www.360docs.net/doc/7513746250.html,/10.1063/1.5010833

I.INTRODUCTION

II–VI compound semiconductor has attracted a great deal of attention because of its potential candidates for electronic and optoelectronic applications.1–6Zinc sul?de as a direct wide band gap binary II–VI semiconductor,with two stable crystal structures,the hexagonal wurtzite(3.77eV) at high temperature and the cubic zinc blende(3.72eV)at low temperature,3has high absorption coef?cient,high refractive index,and high transmittance in the visible region.4Compared with bulk counterparts,ZnS nanostructure has a lot of unique nonlinear and electro-optic characteristics due to its quantum size effect.4–6Fang et al.4,5have made a very meticulous and profound research on ZnS nanostructure,including various preparation methods for different types of nano-ZnS,the in?uence of different synthesis conditions on its grain size,surface morphology,structure of three kinds(0D,1D,and2D)of ZnS,and its potential applications in?elds emitters,?eld effect tran-sistors,photodetectors,catalyzators,UV-light sensors,chemical sensors,biosensors and nanogen-erators.3

As a new type of functional nanoscale materials,ZnS QDs have also attracted much attention due to the quantum con?nement effect.6,7Because of its wide band gap,ZnS is suitable as a matrix material for various types of doping atoms very well.8Especially ZnS:Mn QDs not only has high luminous ef?ciency,but also has no toxicity.Therefore,ZnS:Mn QDs prepared by different techniques have been investigated.Kumar et al.9prepared ZnS:Mn QDs by chemical precipitation method using

a Corresponding author.huyun@https://www.360docs.net/doc/7513746250.html,

tetraethyl orthosilicate as a capping agent and concluded that silica capped ZnS:Mn QDs were suitable materials for speci?c kind of tunable optoelectronic devices.Chen et al.10found doping concentration of Mn2+,reaction temperature and time,pH value of mixture were key factors for controlling the dopant emission intensity in ZnS:Mn QDs by growth doping https://www.360docs.net/doc/7513746250.html,ing colloidal synthesis method,Subha et al.11obtained ef?cient PL ZnS:Mn QDs excited by two-photon absorption in the second near-infrared window.Qian et al.12observed a much stronger emission centered at590nm due to the4T1-6A1transition of Mn2+in the ZnS/CdS:Mn/ZnS QDs by the reverse micelle process, and this yellow emission intensity increased from9%to45%compared to CdS:Mn/ZnS core/shell QDs.As mentioned above,the luminescence properties of ZnS:Mn QDs synthesized by different methods have a good correlation with the synthetic conditions.13

So far,few work on the hydrothermal method for high quality ZnS:Mn QDs has been reported. In this paper,we prepared ZnS:Mn QDs by the hydrothermal method,investigated the effects of dopant concentrations on the structural and optical properties of QDs,and found a new luminescent center that has not been reported yet in the ZnS:Mn QDs.

II.EXPERIMENTAL

The ZnS:Mn nanoparticles was carried out by the hydrothermal method.The principle is the reaction of metal ions and sulfur ions in the hydrothermal solution.First,dehydrate zinc acetate, sodium sul?de,sixteen alkyl three methyl ammonium bromide,manganese sulfate according to a certain amount were dissolved in the mixed medium of deionized water and stirring for0.5h.Then pour the mixture into autoclave with?lling degree of75%.It was then heated to150?C and maintained the temperature for8h under thermostatic drying chamber.After repeatedly washed with deionized water and methanol6times and then dried under thermostatic drying chamber.At last,the system was cooled down to room temperature.In contrast,we performed4group experiments.The concentrations of manganese in the samples are shown in Table I.

The structure of the samples was measured by XRD(DX2700)with Cu Ka radiation (k=0.154nm)at35kV and25mA.Morphologies of the sample were examined by S-4800SEM operated at15kV.The sizes and microstructure of the QDs were observed by a G2F20S-TWIN transmission electron microscopy operated at200kV.The PL measurements were performed at room temperature under the325nm UV?uorescent light excitation using RF-5301(Shimadzu)?u-orescence spectrophotometer with a Xe lamp.The vibrational properties of the sample was detected by FTIR using IR Prestige-21(Shimadzu).

III.RESULTS AND DISCUSSION

Figure1shows the XRD patterns of all the samples.We found three apparent peaks of all samples corresponding to(111),(220),and(311)planes ofβ-ZnS,which well matched with the standard card (PDF#99-0097).In addition,the diffraction peak of manganese impurity was not detected.Obviously, the broadened peaks of the samples suggest that the size of samples is on the nanoscale.The average crystal size of the sample was calculated from the half maximum width of the XRD lines with2θvalues being of the three lattice planes(111),(220),and(311)by means of the Debye–Scherer formula,

D=kλ/βcosθ(1)

TABLE I.Average grain size of ZnS:Mn(0at%-0.9at%)QDs.

doping concentration of Mn2+average grain size 0at% 4.2nm

0.3at% 4.8nm

0.6at% 5.9nm

0.9at%7.2nm

FIG.1.X-ray diffraction patterns of the ZnS:Mn with different concentrations of Mn2+.

Where D represents crystalline size,k the geometric factor(～0.94),βthe full-width at half maximum at the double Bragg angleθ.Table I shows the calculated grain size of the undoped ZnS and Mn-doped ZnS QDs.From Table I,one can see the similar average grain size for the samples with the Mn2+ concentrations from0.0-0.6at%,but as the Mn2+concentrations increased at0.9at%,the average grain size increased from～4to～7nm.

The diffraction peaks of(111),(220),and(311)planes of ZnS:Mn(0.3%,0.6%,0.9%)showed a slight red shift compared with the undoped ZnS.β-ZnS belongs to ionic coordination polyhedron [ZnS4],in which the radius of Mn ion(0.067nm)is smaller than that of Zn ion(0.074nm).Once Mn ions replace Zn ions,crystal lattice constant will decrease,thus,the diffraction peak shifts towards a

high angle.

The morphology of the synthesized ZnS:Mn QDs was examined by SEM.In Fig.2,it reveals that all the particle size distribution is very uniform and the average particle size is about20nm.The particle sizes shown from SEM images are different from the results of XRD.In general,the primary particle size is not equal to the grain size that calculated by Debye–Scherer formula,large particle size observed from SEM should be the aggregate of many grains.

Fig.3shows the TEM images of ZnS:Mn QDs.From Fig.3a,we can see that the undoped ZnS QDs are spherical with an average diameter of about5nm.In Fig.3b,ZnS:Mn(0.3at%)QDs are almost unchanged in average diameter compared with undoped ZnS QDs.Fig.3c shows the distant image of ZnS:Mn(0.6at%)QDs,the diameter of which is around6nm.The ZnS:Mn(0.9at%) QDs have the same nano features with an average diameter of about7nm as shown in Fig.3d.All the TEM images indicate that the nanospheres have a uniform distribution of particle size,and the particle size is in good agreement with the XRD results.We also found that the samples were poor dispersion and serious reunion phenomenon occurred.While the smaller grain size is,the larger speci?c surface area is,and thus it is easier to agglomerate.All these show that all the grains are round in shape with a nearly uniform size distribution.This is also consistent with the polymers conglomerate by much more grains observed from SEM.

PL spectrum of ZnS QDs is intricate because it is sensitive to the synthesis conditions,crystal sizes and shapes.Broad and asymmetric emission spectra indicate that they may be superposition of more than one peak.The PL spectra for ZnS:Mn QDs are made up of four peaks,which were located at about466,495,522,and554nm respectively(Fig.4(a),Table II).The blue emission located at～466nm(2.66eV)is attributed to defect-related emission of the ZnS host.14The second band(blue emission)which centered at495nm(2.50eV)may be caused by the transition from the conduction band to the zinc vacancies V Zn level.15As shown in Fig.4(a),the peak position of this blue emission(495nm)does not change with the increase of the Mn2+concentration,which indicates that the energy level of zinc vacancy relative to the conduction band nearly keeps constant.

FIG.4.(a)PL emission spectra of ZnS with different Mn concentrations,and(b)dopant position of Mn ion for emission peak centered(I)at600nm,(II)at578nm,and(III)at554nm.

TABLE II.Average particle size of ZnS:Mn(0at%-0.9at%)nanoparticles sample.

The position of the emission spectra

Doping concentration of Mn2+I II III IV 0at%466nm495nm522nm-0.3at%465nm496nm521nm554nm 0.6at%465nm497nm522nm553nm 0.9at%466nm495nm522nm554nm

Generally,the reported PL spectra for Mn-doped ZnS are totally different by individual authors16–20 due to the different quantum particle sizes,surfaces or impurity defect energy levels by the differ-ent synthetic techniques.18,19The emission centered at522nm is not clear.20But Liu et al.21also observed this green emission in sphalerite ZnS and attributed it to some self-activated centers.Dif-ferent locations of Mn2+in the ZnS host lattice resulted in different luminescence properties.22–24 It is well-known that the yellow emission located at around586nm is assigned to the4T1-6A1 transition due to the doping of Mn2+into ZnS nanostructure.11The formation of the different lumi-nescent centers(557,578,600nm)implies the different doping mechanisms for Mn2+in the ZnS lattice.Bacherikov et al.25reported that the emission centered at600nm is because of Mn2+as the interstitial ions in ZnS.The emission located at578nm is due to Mn2+doping in the point defects or dislocations.The peak centered at557nm is ascribed to the substitution of Mn ions for zinc ions in the lattice sites ofβ ZnS.In this work,we observe the band at554nm,which is also attributed to Mn Zn.It is consistent with our previous XRD results that the diffraction peaks of ZnS:Mn generated a red shift because of the substitution doping.The dopant position diagram is shown in Fig.4(b).And evidently,the emission located at554nm is absent in the ZnS QDs,indicating that the Mn2+ions have been successfully incorporated into the ZnS host lattice.These emission ?rst increases and then decreases with the doping amount of the Mn2+.When the concentration of Mn2+is up to0.6at%,the emission intensity reach the maximum,and it quench when the doping concentration of Mn2+increased to0.9at%.This is because when the intensity of luminescence reached the maximum,further increasing the concentration of Mn2+led to accumulation of impuri-ties,and the energy transferred between the center of Mn2+,resulting in the decrease of the emission intensity.

Figure5shows FTIR spectra of ZnS:Mn(0-0.9%)QDs in the range of3,000–500cm-1.Bands positioned at1,617cm-1and1,403cm-1may be formed by the microstructure of the sample.The band at1,105cm-1demonstrate that the doped Mn2+impurity affected partial structure of ZnS.24 A very strong band at622cm-1is attributed to Zn–S stretching modes.9The band positioned at 490cm-1is due to the Mn–O stretch vibration.26The possible reason is that some of the Mn2+

combining with O2-to form MnO in the thermal solution.Band at490cm-1is slightly smaller than

FIG.5.FTIR spectra of ZnS with different Mn concentrations(a)0%,(b)0.3at%(c)0.6at%,(d)0.9at%.

The band located at490cm-1is also absent in the ZnS QDs.In summary,the FTIR results con?rm the Mn ions were successfully doped in the lattice of the ZnS host.

IV.CONCLUSIONS

ZnS:Mn QDs were deposited by a hydrothermal technique.The samples are of cubic zinc blend structure with the average crystalline size from4.2to7.2nm.The diffraction peaks of(111),(220), and(311)planes of ZnS:Mn(0.3%,0.6%,0.9%)shift slightly towards a high angle due to the radius of Mn2+smaller than that of Zn2+,resulting in the decrease of crystal lattice constant.All the samples are poor dispersion and reunion phenomenon were serious,because the grain size of the sample is smaller than7nm,leading to the agglomeration.The PL spectra for the ZnS:Mn nanoparticles consist of four peaks,which are centered at466,495,522and554nm respectively.The peaks at 466nm is attributed to defect-related emission of the ZnS host,the peaks at495nm is caused by the zinc vacancies V Zn level,and the emission band at554nm is attributed to the Mn2+in the lattice sites,which is not present in the undoped ZnS nanometer quantum dot.These emission?rst increases and then decreases with the doping amount of the Mn2+.As the concentration of Mn2+is at0.6at%, the emission intensity reach the maximum,and it quench once the doping concentration of Mn2+ increased to0.9at%.The FTIR spectra observed at490cm-1is assigned to the Mn–O stretching modes.And the small changes about this peak compared with the previous reports at500cm-1can be attributed to the formation of nano phase.This study demonstrates that the method we adopt is practical and easy to prepare semiconductor QDs.

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