DSpace at VNU: Luminescent ZnS:Mn thioglycerol and ZnS:Mn ZnS core shell nanocrystals: Synthesis and characterization

The photoluminescence spectra of bare ZnS:Mn nanocrystals exhibited a dominant ultraviolet–violet emission peaked at the wavelength range of 395–450 nm and an weak orange emission peaked

Luminescent ZnS:Mn/thioglycerol and ZnS:Mn/ZnS core/shell nanocrystals:

Synthesis and characterization

Tran Thi Quynh Hoaa, Le Thi Thanh Binha, Le Van Vua, Nguyen Ngoc Longa,⇑, Vu Thi Hong Hanhb,

a

Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam

b Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received 15 February 2012

Received in revised form 2 July 2012

Accepted 14 July 2012

Available online 11 September 2012

Keywords:

ZnS:Mn/thioglycerol

ZnS:Mn/ZnS

Core/shell nanocrystals

Absorption

Photoluminescence

Photoluminescence excition

a b s t r a c t

The synthesis and photoluminescent properties of Mn2+-doped ZnS nanocrystals coated with an organic shell of thioglycerol and an inorganic shell of ZnS are reported in this paper The photoluminescence spectra of bare ZnS:Mn nanocrystals exhibited a dominant ultraviolet–violet emission peaked at the wavelength range of 395–450 nm and an weak orange emission peaked at the wavelength range of 580–600 nm The ultraviolet–violet emission was attributed to the surface defect states The orange emission was assigned to the4T1–6A1transition of Mn2+ions These two channels of radiative recombi-nation compete with each other The coating ZnS:Mn nanocrystals with the thioglycerol shells or the ZnS shells reduced the surface defects and led to the enhancement of the emission of Mn2+ions On the other hand, the overcoating ZnS:Mn nanocrystals by thioglycerol shell restricted the growth of the nanocrys-tals, while the overcoating ZnS:Mn nanocrystals by ZnS shells made the band edge of the ZnS:Mn/ZnS core/shell nanocrystals shift to the lower energy side (the red shift) compared with the bare ZnS:Mn nanocrystals as observed in both the absorption and the photoluminescence excitation spectra

Ó 2012 Elsevier B.V All rights reserved

1 Introduction

It is well known that there are two techniques for fabrication of

strong photoluminescent nanocrystalline materials First

tech-nique is the introducing some impurities into nanocrystals (NCs)

In the existing literature much effort has been devoted to the

dop-ing nanocrystals, for example, CdS:Mn[1], CdS:Eu [2], ZnO:Co,Ni

[3], ZnSe:Mn[4], ZnS:Cu [5], ZnS:Pb,Cu[6], and ZnS:Mn[7–13]

Second technique is the coating semiconductor NCs with organic

or inorganic materials It has been shown that the coating NCs

im-proves the photoluminescence quantum yields by passivating

sur-face nonradiative recombination sites Since the 1990s up to now

many articles have been devoted to core/shell and core/multishell

NCs, for example, CdS/hexadecylamine (HDA)[14],

ZnS/thioglyc-erol (TG) [15], ZnSe:Mn/TG[16], CdSe/ZnS [17], CdSe/ZnSe [18],

CdSe/CdS [19], CdSe/CdS/ZnS, CdSe/ZnSe/ZnS [20], CdSe/CdZnS/

ZnS[21], and InP/ZnS[22] Currently, few reports on synthesis of

ZnS:Mn/ZnS core/shell NCs appeared An enhanced luminescence

was observed in Mn:ZnS/ZnS quantum dots synthesized by a

re-verse micelle route[23], a high-boiling solvent process[24], and

a nucleation-doping strategy [25] However, the effects of the

ZnS shell on the absorption (ABS), photoluminescence (PL), and

photoluminescence excitation (PLE) spectra of core/shell nanocrys-tals have not been studied or incompletely studied

In this paper, we coated the ZnS:Mn NCs with organic (TG) and inorganic (ZnS) shells The study was focused on the effect of the shell upon ABS, PL and PLE spectra The results showed that the surface states in our ZnS:Mn/TG and ZnS:Mn/ZnS core/shell NCs acted as radiative recombination centers competing with the

Mn2+ optical centers Our experiment results indicated that the

TG or ZnS shells could reduce the surface defects and led to the enhancement of luminescence of Mn2+ions In addition, the TG shell acted as a stabilizer, decreasing the growth speed of the nano-crystals, while the ZnS shells caused the red shift of the band edge

of the ZnS:Mn/ZnS NCs due to the increase of NC size

2 Experimental 2.1 Synthesis of ZnS:Mn/TG core/shell NCs Typically, in order to prepare ZnS nanocrystals doped with

1 at.% Mn, 2.634 g (0.012 mol) of Zn(CH3COO)22H2O, 29.785 g (0.12 mol) of Na2S2O35H2O, and 0.029 g (0.12 mmol) of Mn(CH3COO)24H2O were mixed in 160 mL deionized water The mixture solution was magnetically stirred for 30 min at room temperature to produce an optically clear solution The mixture solution temperature was then raised to 96 °C and was kept 0925-3467/$ - see front matter Ó 2012 Elsevier B.V All rights reserved.

⇑Corresponding author.

E-mail address: longnn@vnu.edu.vn (N.N Long).

Contents lists available atSciVerse ScienceDirect

Optical Materials

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / o p t m a t

constant at that temperature for 55 min At that time 20 mL of this

mixture solution was taken to obtain bare ZnS:Mn nanocrystalline

sample (denoted by 55 min no TG sample) In order to synthesize

thioglycerol (TG)-coated ZnS:Mn NCs, 7 mL of 11 M TG (C3H8O2S)

solution was quickly injected into the above mixture solution In

this case TG could be used as good ligands and a stabilizer This last

solution was under steady stirring at constant temperature of

96 °C After 5, 10, 15, 30, 60, 180, and 360 min since the time of

injecting TG, every 20 mL of solution was taken to obtain

ZnS:Mn/TG core/shell nanocrystalline samples (denoted by

55 min + TG 05 min sample, etc.) The produced solutions were

centrifuged and washed many times with double distilled water

The final powder products were dried in air at 60 °C for 12 h

2.2 Synthesis of ZnS:Mn/ZnS core/shell NCs

The TOPZn (0.4 M) and TOPMn (0.4 M) stock solutions were

prepared by adding Zn(CH3COO)22H2O and MnCl24H2O into

trioc-tylphosphine (TOP), respectively For preparation of a basic

trioctylphosphine oxide (TOPO) were put into a three neck reaction

flask closed and filled with N2gas The mixture solution was

mag-netically stirred at 220 °C for 15 min Then the appropriate

amounts of TOPMn and TOPZn stock solutions and 0.21 mL of

hex-amethyldisilathiane, also known as bis(trimethylsilyl)sulfide

((TMS)2S), one after another 10 min were quickly injected into

the basic mixture solution During this process, the mixture

solu-tion was being stirred at 220 °C After that, the temperature was

re-duced to 120 °C under stirring for 15 min The desired amount of

received solution was taken and dispersed into the same quantity

of toluene to obtain ZnS:Mn NCs The doping concentration of Mn2+

was 0.5, 1.0, 5.0, and 10.0 at.% of Zn2+in ZnS

In order to prepare ZnS:Mn/ZnS core/shell structure, it is

neces-sary to calculate the required amount of shell precursors to obtain

the desired shell thickness Our calculation is as follows[27]: The

ZnS NCs have a face-centered-cubic structure with a lattice

con-stant a There are 4 Zn atoms and 4 S atoms in a unit cell Suppose

that the radius of the ZnS:Mn core NCs is r, the number of Zn and/

or S atoms in one ZnS NC will be:

nat

ZnðSÞcore¼

4pr3

a3

ZnS

If the amount of Zn and/or S precursors used for preparation of the

core NCs is m (mol), the mole amount of ZnS core NCs synthesized

will be (assume all reactants have been consumed):

nmol

ZnðSÞcore¼ m

nat

ZnðSÞcore

ð2Þ When the thickness of one monolayer (ML) is d, the number of Zn

and/or S atoms needed to coat one ZnS core NC with a shell of p

ML is:

nat

ZnðSÞshell¼

4p½ðr þ pdÞ3 r3

a3

ZnS

Therefore, the mole amount of Zn and/or S precursors needed for

coating the as-prepared ZnS core NCs is:

nmol

Putting formulae(1)–(3)into(4), we obtain:

nmol

ZnðSÞshell¼ m½ðr þ pdÞ

3

 r3

In our experiment m = 1 mmol, r = 2.47 nm, d is the spacing

be-tween the adjacent (1 1 1) lattice planes of cubic phase ZnS equal

to 0.31 nm[12], p is the ML number of 2, 4, 6, and 8

For fabrication of ZnS:Mn/ZnS core/shell structure, first, ZnS:Mn core NCs were synthesized by the mentioned above method The temperature of ZnS:Mn core containing solution was kept at

200 °C In typical procedure for preparation of a ZnS shell with a thickness of 2 ML, 2.45 mL of TOPZn stock solution and 0.2 mL of (TMS)2S were added drop wise into the colloidal solution contain-ing ZnS:Mn core NCs After 15 min of stirrcontain-ing, the suitable amount

of the solution was taken and dispersed into the same quantity of toluene to obtain ZnS:Mn/ZnS core/shell NCs To receive ZnS:Mn/ ZnS NCs with more ZnS shell thickness, the similar processes were further repeated with different amounts of the stock solutions for shell growth The result of PL measurements indicated that the intensity of Mn2+emission reached to maximum for the samples doped with 1 at.% Mn At Mn2+ concentration of 5 and 10 at.%, the Mn2+ emission disappeared because of the concentration quenching Therefore, in the current work, we performed coating only for the ZnS:Mn NCs with Mn concentration of 1.0 at.% and the shell thickness varied from 0 to 8 ML

2.3 Characterization of the samples Crystal structure of the NCs was analysed by using an X-ray dif-fractometer (XRD, SIEMENS D5005, Bruker, Germany) with Cu Ka1

(k = 1.54056 Å) irradiation The ultraviolet–visible (UV–vis) absorption spectra were obtained by a Shimadzu UV 2450 PC spec-trometer The PL and the PLE spectra were measured at room tem-perature on a spectrofluorometer (Fluorolog FL 3-22 Jobin Yvon Spex, USA) with a 450 W xenon lamp as an excitation source

3 Results and discussion 3.1 Structural characterization

Fig 1shows typical powder XRD patterns for the ZnS:Mn/TG core/shell NCs with different TG-reaction times of 5, 10, 15, 30,

60, 180, and 360 min (Fig 1a), and the ZnS:Mn/ZnS core/shell NCs with the ZnS shell thickness of 0, 2, 4, 6, and 8 ML (Fig 1b) The diffraction peaks at 2h values of 28.7, 47.7 and 56.6 Å corre-spond to the (1 1 1), (2 2 0) and (3 1 1) diffraction planes, respec-tively It is obvious that all the peaks in the XRD patterns can be indexed to the cubic zinc blend phase of ZnS No other diffraction peaks are detected except for the ZnS related peaks The lattice constant calculated from XRD patterns is a = 0.540 ± 0.003 nm, in good agreement with the value in the standard card (JCPDS No 05-0566, a = 0.5406 nm)

FromFig 1it can be seen that the diffraction lines are rather wide, indicating that the size of the NCs is small The average sizes

of the ZnS nanocrystals were estimated by Scherrer’s formula[26]:

L ¼ 0:9k

where b is the full width at half maximum (FWHM) in radians of the diffraction peaks, h is the Bragg’s diffraction angle and k is the wave-length for the Ka1 component of the employed copper radiation (1.54056 Å)

The average sizes of the nanocrystals were estimated using (1 1 1) reflection peak to be 1.54, 2.09, 1.86, 2.03, 2.13, 2.16, 2.54

nm for the ZnS:Mn/TG with the TG-reaction times of 5, 10, 15,

30, 60, 180, and 360 min, respectively and 2.07, 2.16, 2.49, 2.30, 2.62 nm for the ZnS:Mn/ZnS NCs with the ZnS shell thickness of

0, 2, 4, 6, and 8 ML, respectively

Fig 1a also shows the XRD pattern of the ZnS:Mn NCs without

TG coating prepared at 96 °C for 415 min the same as the condition for preparing the (55 min + TG 360 min) ZnS:Mn/TG NCs It can be seen that the diffraction lines of the ZnS:Mn NCs without TG

coating are much narrower than that of the ZnS:Mn/TG NCs The

average size of the ZnS:Mn no TG NCs is 11.9 nm larger than that

of the ZnS:Mn/TG NCs This fact indicated that the TG coating

pre-vents the growth of ZnS:Mn NCs

3.2 Optical characterization

3.2.1 Absorption

Temporal evolution of UV–vis absorption spectra at room

tem-perature of the ZnS:Mn/TG NCs during the growth of NCs at 96 °C

and the evolution of UV–vis spectra of the ZnS:Mn/ZnS NCs with

different number of monolayers were investigated in detail It

was found that all the spectra exhibit a sharp absorption edge

and the absorption onset of the ZnS:Mn/TG and ZnS:Mn/ZnS NCs

is shifted to the long wavelength side (the red shift) with

increas-ing the TG-reaction time or the ZnS shell thickness, as usually

observed in the core/shell NCs[27]

It is well known that cubic ZnS is a direct-gap semiconductor

[28] The relation between the absorption coefficients (a) and the

incident photon energy (hm) for the case of allowed direct

transi-tion is written as follows[29]:

where A is a constant and Egis the bandgap of the material

The plots of (ahm)2versus hmfor the two kinds of core/shell NCs

are represented inFig 2 By extrapolating the straight portion of

the graph on hm axis at a= 0 we found the bandgap Eg of the

ZnS:Mn/TG NCs with the TG-reaction times of 0, 5, 10, 15, 30, 60,

180, and 360 min to be 4.06, 4.06, 4.03, 4.01, 3.99, 3.95, 3.94, and 3.75 eV, respectively The Egof the ZnS:Mn/ZnS NCs with the shell thicknesses of 0, 2, 4, 6, and 8 ML was found to be 3.94, 3.93, 3.89, 3.85, and 3.83 eV, respectively

It noticed that the values Egof all the ZnS:Mn/ZnS NCs are higher than the bandgap value of 3.61 eV at room temperature for the cubic bulk ZnS[27] This fact is a clear evidence of the quantum con-finement effect: In the case of ZnS:Mn/TG NCs, the band edge shift toward the lower energy side originated from increasing of the NCs size during their growth[14] In the case of ZnS:Mn/ZnS NCs, the red shift of band edge, in general, was due to a partial leakage of the exciton into the shell material[27] But in our case, the shell material was ZnS as same as the ZnS core, therefore, the observed red shift of band edge was assigned to the increase of NC size From the blue shift of the band edge (DEg), the nanocrystalline radius r could be determined using the Brus relation given as fol-lows[30]:

DEg¼ EgðNCÞ EgðbulkÞ¼h

2

p2

2r2

1

mþ 1

m h

1:8e

2

where Eg(NC)and Eg(bulk)are the bandgaps of NC and bulk material, respectively; h is the reduced Plank’s constant, e is the electron charge, m and m are the effective masses of electron and hole, respectively, e is the dielectric constant of the material

In the SI units and the energy is calculated in eV, this formula is written as follows:

DEg¼ EgðNCÞ EgðbulkÞ¼h

2

p2

2er2

1

mþ 1

m h

 1

4pe0

1:8e

Fig 1 Typical powder XRD patterns for (a) the ZnS:Mn/TG NCs with different

TG-reaction time and (b) the ZnS:Mn/ZnS NCs with the ZnS shell thickness of 0, 2, 4, 6,

and 8 ML.

Fig 2 The plots of (am) 2

versus hmfor (a) the ZnS:Mn/TG NCs with different TG-reaction times and (b) the ZnS:Mn/ZnS NCs with different shell thicknesses The concentration of Mn in all the core ZnS:Mn NCs is 1 at.%.

wheree0¼ 8:854  1012Fm1is the permitivity of free space For

the ZnS m¼ 0:34m0, m¼ 0:23m0, ande¼ 8:76, m0is the free

elec-tron mass[31] Using the mentioned Eq.(9), the nanoparticle radius

was determined to be 2.16, 2.16, 2.23, 2.28, 2.33, 2.44, 2.47, and 3.50

nm for the ZnS:Mn/TG with the TG-reaction times of 0, 5, 10, 15, 30,

60, 180, and 360 min, respectively The nanocrystal radius was

esti-mated to be 2.47, 2.50, 2.65, 2.82and 2.92nm for the ZnS:Mn/ZnS

NCs with the shell thicknesses of 0, 2, 4, 6 and 8 ML, respectively

It can be seen that the size values obtained from the band gap

change are not in good agreement with those obtained from the

XRD data This is because both Scherrer formula and Brus relation

used for the size calculation are of approximation However, we

chose the bigger value of 2.47 nm for Eq.(5)in anticipation of the

fact that the reactants for the shell growth have not been consumed

It is worth noting from the above results that the size increase

ob-served from the band gap change was smaller than that expected

from calculation The reason for this most likely is that the reactants

for the shell growth, in fact, were not consumed

3.2.2 Photoluminescence

Fig 3shows the room temperature PL spectra excited with a

wavelength of 336 nm for the ZnS:Mn/TG NCs with different

TG-reaction times and the ZnS:Mn/ZnS NCs with various numbers of

shell monolayers It is found that in the PL spectra excited by the

wavelength of 336 nm, for all the kinds of NCs, two emission peaks

are observed One is a UV–violet emission peak (located at 432 or

401 nm) and another is an orange emission peak (located at 594 or

580 nm) These two radiative recombination channels compete with each other The UV–violet emission can be ascribed to the radiative recombination via surface defect states of NCs[24] The orange emission peak was attributed to the4T1(4G) ?6A1(6S) tran-sitions within the 3d shell of Mn2+ion[12]

It is interestingly noticed that for ZnS:Mn NCs without coating with TG or ZnS shell, the UV–violet emission peak is dominant (lines a inFig 3) As the TG-reaction time increases, the intensity

of 432 nm emission peak significantly reduces, while the 594 nm emission peak becomes dominant and its intensity remarkably in-creases with increasing the TG-reaction time (lines b–h,Fig 3a) The change of the relative intensity of the two mentioned above emission peaks is clearly observed inFig 3b as well For the bare ZnS:Mn NCs the 401 nm peak is dominant so that intensity ratio

of the orange and the UV–violet peaks IO/IUVis 0.23 When an addi-tional ZnS shell with the thickness of 2, 4 and 6 ML is grown on the core ZnS:Mn NCs, the intensity of the 401 nm emission peak de-creases, whereas the intensity of the 580 nm emission peak in-creases so that the ratio IO/IUVraises up to 1.69, 2.74, and 3.81, respectively Thus, optimal thickness of ZnS coating to obtain a good PL is 6 ML The further increasing the shell thickness up to

8 ML causes a little decrease of the ratio IO/IUV(3.53) This is be-cause when the shell is too thick, inside the thick ZnS shell again appeared the structural defects (dislocations, gain boundaries), which play the role of nonradiative recombination sites reducing the PL intensity[14,20]

Thus, by formation of an additional TG or ZnS shell with an appropriate thickness on the core ZnS:Mn NCs, the orange emis-sion intensity of the ZnS:Mn/TG or the ZnS:Mn/ZnS core/shell NCs will be enhanced because of the passivating the surface defects due to a good ligand or a good match between core and shell lattice constants, which is in good agreement with previous reports

[23,25]

3.2.3 Photoluminescence excitation The room temperature PLE spectra monitored at the 592 nm emission peak for the ZnS:Mn/TG NCs are illustrated in Fig 4a The PLE spectra exhibit both the excitation band due to the near-band-edge absorption (bands lower than 375 nm) and three excita-tion bands peaked at 430 nm (2.88 eV), 466 nm (2.66 eV), and

496 nm (2.50 eV) The three excitation bands at 430, 466, and

496 nm are attributed to the absorption transitions from basic state 6A1(6S) to excited states 4T2(4D); 4E(4G), 4A1(4G); and

4T2(4G) within Mn2+ion, respectively[12] The PLE spectra monitored at the 580 nm emission peak for the ZnS:Mn/ZnS NCs are depicted inFig 4b For the bare ZnS:Mn NCs, the PLE spectrum exhibits an weak excitation band peaked at

381 nm (line a,Fig 4b) When the core ZnS:Mn NCs are overcoated with ZnS shells, the 381 nm excitation band becomes weaker (lines b–e,Fig 4b) From this fact, it is possible to infer that the 381 nm excitation band relates to the band-to-defect state absorption tran-sitions It is worth noted that when the 381 nm excitation band be-comes weaker, four weak excitation peaks located in the wavelength region of 380–500 nm can be clearly observed (inset

of Fig 5) These four excitation peaks at 389 nm (3.10 eV),

425 nm (2.92 eV), 463 nm (2.68 eV), and 493 nm (2.51 eV) are assigned to the absorption transitions from basic state6A1(6S) to excited states4E(4D);4T2(4D);4E(4G),4A1(4G); and4T2(4G) within

Mn2+ion, respectively[12] The other excitation peaks appeared at the wavelengths lower than 360 nm are assigned to the band-to-band absorption transi-tions in the ZnS:Mn/ZnS NCs It should be noted that in addition

to the weakening of the 381 nm excitation peak, the red shift of the absorption onset with increasing the shell thickness is observed as well, which is in good agreement with our results Fig 3 The room temperature PL spectra excited by the wavelengths of 336 nm for

(a) the ZnS:Mn/TG NCs with different TG- reaction times and (b) the ZnS:Mn/ZnS

obtained from the direct absorption measurements and the results

of previous report for CdSe/CdS core/shell NCs[32]

4 Conclusions

The ZnS:Mn/TG and ZnS:Mn/ZnS core/shell NCs have been

synthesized Both the synthesized ZnS:Mn/TG and ZnS:Mn/ZnS

NCs possess a cubic zinc blend structure with lattice constant

a = 0.540 ± 0.003 nm The PL spectra of the two core/shell NCs exhibited a UV–violet emission located at 395–450 nm, and an orange emission peaked at 580–600 nm These two radiative recombination channels compete with each other The UV–violet emission was attributed to the surface defect states The orange emission was assigned to the4T1–6A1transition of Mn2+ions The coating ZnS:Mn NCs with a shell of organic (TG) or inorganic (ZnS) material enhanced the emission of Mn2+ions due to passiv-ating the surface defects In addition, the absorption and the PLE measurements indicated that the overcoating ZnS:Mn NCs by TG shell limited the growth of the nanocrystals, while the overcoating ZnS:Mn nanocrystals by ZnS shells made the band edge of the ZnS:Mn/ZnS core/shell nanocrystals shift to the lower energy side (the red shift) in comparison with the bare ZnS:Mn nanocrystals; the reason for this is the increase of NC size

Acknowledgement This work is financially supported by Vietnam National Univer-sity, Hanoi (TRIG A project No QGTD 10.24)

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Fig 4 (a) Normalized room temperature PLE spectra monitored at the 592 nm

emission peaks of the ZnS:Mn/TG NCs with various TG-reaction times and (b) PLE

spectra monitored at the 580 nm emission peaks of the ZnS:Mn/ZnS NCs with

various numbers of shell ML.

Fig 5 Normalized room temperature PLE spectra monitored at the 580 nm

emission peaks of the ZnS:Mn/ZnS NCs with 4 and 6 ML ZnS shells The inset shows

four weak excitation peaks related to the absorption transitions within Mn 2+

ion.