DSpace at VNU: Study on the fabrication of CdZnSe ZnSeS ternary alloy quantum dots

The influence of the growth temperature on the structural properties crystalline phase and cell, size and elemental compositions of the core and core/shell QDs is presented.. Keywords: t

Study on the fabrication of CdZnSe/ZnSeS ternary alloy quantum dots

Pham Thu Nga*, Nguyen Hai Yen, Dinh Hung Cuong, Nguyen Ngoc Hai and Nguyen Xuan Nghia

Institute of Materials Science, Vietnam Academy of Science and Technology,

18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam Fax: +84 43 83 60 705

Email: phtnga@ims.vast.ac.vn Email: haiyen@ims.vast.ac.vn Email: DINH.Hungcuong@nims.go.jp Email: c3hoangquocviet.quangninh@moet.edu.vn Email: nghianx@ims.vast.ac.vn

*Corresponding author

Vu Thi Hong Hanh Faculty of Physics,

Thai nguyen University of Education, Luong Ngoc Quyen Street,

Thai Nguyen City, Vietnam Email: hanhvth@dhsptn.edu.vn

Le Van Vu Center for Materials Science, University of Natural Science, VNU, Hanoi,

334 Nguyen Trai St, Thanh Xuan Dist Hanoi, Vienam Email: levanvu@hus.edu.vn

Laurent Coolen Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7588, Institut de NanoSciences de Paris (INSP), F-75005, Paris, France

Email: laurent.coolen@insp.jussieu.fr

Abstract: In an effort to search for new compositions and structures of

quantum dots with suppressed blinking photoluminescence (random switching between states of high (on) and low (off) emissivity under continuous photo-excitation) and to serve application purposes in biosensors and in optoelectronic devices, we have studied the fabrication of new CdZnSe/ZnSeS ternary alloy quantum dots (QD) In this work, we present new results on the fabrication of the alloy core/shell quantum dots having a shell thickness of about 1.52 nm and a varied shell composition in Se/S ratio The influence of the growth temperature on the structural properties (crystalline phase and cell), size and elemental compositions of the core and core/shell QDs is presented

Analysis and in-depth comparison of the calculated ternary QDs’ composition are performed based on the energy dispersive X-ray spectroscopy (EDS) data and the first exciton absorption peak position and size Interpretation of the experimental results is also provided

Keywords: ternary quantum dots; structural properties; CdZnSe/ZnSeS;

crystalline phase; alloy composition; size

Reference to this paper should be made as follows: Nga, P.T., Yen, N.H.,

Cuong, D.H., Hai, N.N., Nghia, N.X., Hanh, V.T.H., Vu, L.V and Coolen, L

(2015) ‘Study on the fabrication of CdZnSe/ZnSeS ternary alloy quantum

dots’, Int J Nanotechnol., Vol 12, Nos 5/6/7, pp.525–537

Biographical notes: Pham Thu Nga is an Associate Professor, PhD in the

Physics at the Institute of Materials Science, of Vietnam Academy of Science and Technology (VAST) Her research interests include colloidal semiconductor quantum dots (QDs) from synthesis, optical spectroscopy to device fabrication, aimed for different applications fields such as biosensor, quantum dot-light emitting diode (QD-LED) and quantum dots-sensitised solar cell (QDSSC) She is also interested in the interaction of light in nano-sized solids and opal photonic crystals

Nguyen Hai Yen obtained her Master’s degree in Physics in 2010 Currently, she is a PhD student at Institute of Materials Science-VAST Her research focuses on the structural and optical properties of the alloy quantum dots

Dinh Hung Cuong was a Bachelor of Engineering Physics at University of Engineering and Technology, Vietnam National University, Hanoi (VNUH)

From 2008 to 2013, he was a Master and PhD student at Graduate School, Division of Energy System Research at Ajou University, Korea His interests are synthesis and characterisation of electrode materials, quantum dots and photonic crystals, electrochemistry, nanoparticle morphology and orientation crystal

Nguyen Ngoc Hai graduated from the Faculty of Physics of Thai Nguyen University of Education, in 2001 He obtained his Master’s degree in Physics of Solid States (2008) at Hanoi National University of Education

He is PhD student at Institute of Materials Science, VAST Currently, he is focusing on the fabrication and the optical properties of the II-VI group quantum dots and the application in biosensors for the detection of pesticide residues

Nguyen Xuan Nghia obtained his PhD in Solids Physics in 1994 His research focuses on the field of nanomaterials including the synthesis of differently shaped nanocrystals, their optical and vibrational properties, and the application

of nanostructures in optoelectronics

Vu Thi Hong Hanh obtained her PhD in Solids Physics in 2012 at the Institute

of Physics at Vietnam Academy of Science and Technology Her research focuses on the optical properties of quantum dots and the effect of the shells’

thickness on spectroscopic properties of semiconductor nanocrystals

Le Van Vu is an Associate Professor, PhD and lecturer at the Department of Physics and the Centre of Materials Science, University of Natural Science, VNUH, Hanoi, Vietnam He research focuses on the structural properties of nano materials

Laurent Coolen acquired a PhD in Solids Physics at the Université Denis Diderot, Paris 7 (Laboratoire Kastler-Brossel) in 2006 on nanocrystal photophysics (advisor: J.-P Hermier) He worked as a post-doctoral researcher

at Stanford University in W.E Moerner’s group in 2007 He has been since

2007 a Lecturer at the Université Pierre et Marie Curie, Paris 6 and studies nanophotonics at the Institute de NanoSciences de Paris

This paper is a revised and expanded version of a paper entitled ‘Study on the fabrication of CdZnSe/ZnSeS ternary alloy quantum dots’ presented at

4th International Workshop on Nanotechnology and Application, Vung Tau,

Vietnam, 14–16 November, 2013

1 Introduction

Nanocrystal quantum dots (QDs) have great potential as a unique optical material in a broad range of applications that rely on downshifting light, especially those which rely

on achieving spectral purity at high optical flux [1] High particle uniformity, high photoluminescence quantum yields, narrow and symmetric emission spectral line shapes and minimal single-dot emission intermittency (known as blinking) have been recognised

as universal requirements for the successful use of colloidal quantum dots in nearly all optical applications However, synthesising samples that simultaneously meet all these four criteria has proven challenging [1] The colloidal core/shell ternary QD nanocrystals

of CdZnSe were reported as non-blinking semiconductor nanocrystals [2] Thus, the synthesis of continuously emitting single nanocrystals would have profound influences

on the usage of nanocrystals in applications for biology, quantum optics and optoelectronics [2] On the other hand, research on band gap engineering via control of nanocrystal composition, which is achieved by adjusting the constituent stoichiometry of alloyed semiconductors, is still in its infancy [3] For alloy nanocrystal quantum dots, their optical properties depend on both parameters: their size and composition There have been some reports on alloy QDs like ZnxCd1−xSe [4–7] To better confine the electron-hole pairs in CdZnSe QD core, the shielding with an extra layer of a wider band gap semiconducting material, such as ZnS, ZnSeS, and CdZnS is necessary in order to ensure carrier confinement to the core regions Wang et al [2] described ‘non-blinking’

core/shell CdZnSe/ZnSe QDs that have an alloy composition core By using the high-temperature reaction route, at high-temperatures slightly lower than that used for the core growth, the shell layer is formed In an effort to study the monitoring of CdZnSe alloy QDs’ optical properties and their photoluminescence blinking when shielded with an extra shell, which has changes in band-gap different from a ZnSe binary composition layer, we have studied the fabrication of ZnSeS ternary composition shell In this work,

we have used our familiar synthesis method [8,9] to prepare the CdZnSe/ZnSeS QDs whose structural characterisation and properties will be presented in this paper In order

to create CdZnSe alloy QDs, Zhong et al [6] presented in the process of forming a CdSe seed, then forming alloys, following the mechanisms of embryonic nuclei-induced alloying process In our fabrication process, the ZnSe seeds were pre-prepared then formed alloys with Cd2+ ions at intermediate temperatures, following the mechanisms of embryonic nuclei-induced alloying process to obtain CdZnSe alloy QDs The synthesis method of CdZnSe/ZnSeS ternary alloy QDs along with some new results on the effects

of the growing temperature to structural properties (crystalline phase, shape), size and composition of the CdZnSe ternary QDs are presented in this paper For different synthesis temperatures, we discuss the obtained QD size and alloy composition, as well

as their photo physical properties We demonstrate perfect shape and size distribution of the CdZnSe/ZnSeS QDs and discuss their structural properties

2 Experimental

2.1 Materials

Cadmium acetate (Cd (Ac)2, 99.9%), zinc acetate (Zn (Ac)2, 99.9%), selenium powder (Se, 99.99%), hexamethyl disilthiane (TMS)2S, trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 90%) and hexadecylamine (HDA, 99%) and organic solvents (chloroform, toluene, methanol, hexane, etc.) were purchased from Aldrich The chemicals were used as received unless otherwise specified

2.2 Synthesis of CdZnSe ternary core quantum dots

For a typical synthesis of 0.5 mM of CdZnSe ternary nanocrystal QDs, the molar ratio

of the precursors for the reaction is: Cd/Zn/Se = 0.2/0.8/3.33; TOPO/HAD = 55/45

The core fabrication process is performed through two steps For the synthesis of CdZnSe QDs samples, 3.325 g of TOPO and 1.6625 g of HDA are poured into a three-neck reaction flask Nitrogen gas was used to remove water vapour and oxygen from the reaction flask at room temperature for 30 min, then at 120°C for one hour The TOP-Se precursor is injected into the flask under vigorous stirring and heating at temperatures up

to 100°C in N2 atmosphere After heating the reactor up to 190°C under continuous stirring, the zinc precursor solution is injected into the reaction flask Then the temperature of the flask is increased up to 280°C, at which temperature the cadmium stock solution is injected into the reactor As the temperature of the liquid in the reaction flask drops to ~260°C, the nucleation of CdZnSe alloy quantum dot nanocrystals starts quickly After growing CdZnSe alloy quantum dots for 20 min at different temperatures

from 260°C to 310°C, we then obtain CdZnSe ternary QDs core with the x value of Zn

varied from 0.5 to 0.7 The growing time of 28 min gives the same result as for 20 min

2.3 Method of growing ZnSeS shell on CdZnSe core

The molar ratio of the precursors for growing the ZnSeS shell is: Zn/(Se + S) = 1.37/1

The shells are grown following a modified version of the successive ion layer adsorption

and reaction (SILAR) procedure originally described by Jack Li et al [10] For the ZnSeS shell growth, TOP-Se is mixed with (TMS)2 S (Se/S molar ratio = x/(1 – x); x = 0.2, 0.4,

0.5, 0.6 and 0.8) The zinc stock solution drop-by-drop is injected very slowly (a drop

~50 µl) at a rate of 1–2 drops/second, under vigorous stirring, then the mixture of (TMS)2S and TOP-Se is added to the alloy QD core solution After the end of each turn

of Zn precursors and (TOP-Se & S2–) mixture’s injection, the reaction temperature is kept

at 240°C and stirred strongly under N2 atmosphere for 15 min so that ions can have enough time to stick to the outer layer of the CdZnSe core, thus forming single-layers and giving the best reaction productivity Several 15-min delays between injection steps are necessary to produce multi-shell QDs The time for ternary ZnSexS1–x shell growth is

15 min At the end of this step we obtain the CdZnSe/ZnSeS QDs and the expected shell thickness is approximately 1.52 nm In this experiment, the amount of chemicals for the shell calculated for the estimated cores size is approximately 5 nm After 15 min after the core/shell ternary quantum dots formed, the heater is removed and the reaction mixture is cooled down to stop the reaction When the temperature of the reaction mixture cools to below 70°C, the ternary QDs are dispersed in organic solvent (such as chloroform and toluene etc.) A little amount of the sample is extracted after the growth of each shell for optical measurements

2.4 Methods of characterisation of CdZnSe/ZnSeS ternary quantum dots

Each sample was purified three times to remove excess organic coordinating compounds before powder X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) measurements Methanol is used to precipitate the QDs, which are isolated by centrifugation and decantation The powder X-ray diffraction (Siemens, D5005) is used

to identify the wurtzite (w) or zinc-blende (zb) crystal structure with characteristic peaks

of the QD samples The XRD patterns were compared with the tabulated values of bulk CdSe (JCPDS card No 19-191 (zb) and 8-459 (w)), ZnSe (JCPDS 37-1463 (zb) and 15-105 (w)) and ZnS (JCPDS 5-566 (zb) and 39-1363 (w)) The elemental analysis is carried out using a Nova NanoSEM 450 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) The EDS method has been used

to analyse the composition of the elements Cd, Zn, Se and S present in the ternary

QD samples The size of the core QDs and the shell thickness are determined by the transmission electron microscopy (TEM) method, using a JEOL Jem 1010 microscope operating at 100 kV The absorption characteristics of the QDs were measured by UV-visible absorption spectroscopy (Shimadzu, UV-1800) For optical characterisation

of the sample, all the QD samples were diluted with toluene

3 Result and discussion

For the CdZnSe QD cores, during the synthesis process, we noticed that the growth temperature of the nanocrystals is the most important factor, affecting the composition and size of the CdZnSe ternary quantum dots For the synthesis of different samples,

in this fabrication method, all parameters but the nanocrystals’ growth temperature are kept constant

3.1 XRD studies

The crystalline phase of the ternary QDs were investigated using powder XRD method

The CdSe as well as ZnSe normally show duality in their crystalline structure, proving that they can be formed with either sphalerite (cubic, zinc blende type) or wurtzite (hexagonal type) structure The XRD patterns indicate the CdZnSe ternary QDs samples obtained at low temperature (260°C–280°C) possess a cubic structure (zinc-blend (zb)) (curves 4, 6, 7 and 7a in Figure 1) The peak positions are in between those of bulk zb CdSe (JCPDS 19-191 data) and zb ZnSe (JCPDS 37-1463 data) As a result, these CdZnSe cores show a zinc-blende crystal structure, and the diffraction peaks shifted toward the higher-angle side compared with the pattern of pure zb CdSe Reflections with the highest intensity observed from CdZnSe QDs originated from (111) and (220) planes

The addition of Zn to CdSe has shown to shift (111) and (220) planes to lower ‘d’ value (i.e., higher θ values) as reported on the CdZnSe thin films [11] Peaks corresponding

to hexagonal modification of CdSe were observed in small quantities for the obtained sample at 280°C as observable in Figure 1 (curves 7, 7a) For the ternary QD cores fabricated in higher temperatures from 285°C to 310°C, (curves 11, B, 13, C, D and E in Figure 1), we find experimental XRD peaks between the tabulated peaks corresponding

to the wurtzite phases of CdSe and ZnSe, which would be in agreement with a wurtzite CdZnSe alloy (although, given the width of the experimental spectra, we cannot exclude a polytype of zinc-blende and wurtzite phases) The wurtzite structure would be consistent with the structure reported in Zhong et al [12] with a ZnSe-seeded growth process In ternary QDs samples, the two components CdSe and ZnSe differ only in the cations As the group II cations diffuse much more easily than the group VI anions in II-VI semiconductors, the Cd2+ and Zn2+ can be intermixed to form an alloy As the Zn2+

content increased, the diffraction peaks shifted toward the higher-angle side compared to the pattern of pure CdSe

Broad XRD peaks are attributed to the small particle size The growth time of the QD nanocrystals changes from 20 min to 28 min without changing the properties of the crystallisation phase of the ternary QDs The ternary CdZnSe all crystallise at the zb-alloy CdZnSe phase (curves 7 and 7a- Figure 1) Therefore, we chose the QDs’

growing time to be 20 min

When growing the ternary ZnSeS shell on the zb CdZnSe cores, at the temperature of 240°C, we can observe that CdZnSe cores show a zinc blende crystalline structure

With the shell the diffraction peaks shifted toward the higher-angle side compared to the pattern of zb CdZnSe core The peak positions are in between those of bulk zb ZnS (JCPDS 5-566) and zb ZnSe (JCPDS 37-1463) (Figure 2) The peak positions for the core/shell QDs are shifted to higher angles as compared to the core sample because of the ZnSeS shell contribution with smaller lattice constant Although the samples E1–E4 are synthesised with different Se/S precursor ratios, the XRD patterns look similar, suggesting that Se and S concentrations in the shell did not have clear influence to the structure of the shell

Elemental composition analysis by the EDS method enables us to establish a detailed formula of the CdZnSe ternary QD samples and the core/shell structure presented above

On the other hand, in order to determine the accuracy of the ternary QD composition by

EDS analysis, we have carried out extra calculations based on the TEM images taken from the samples, and the experimental data Eg of alloy QDs obtained from the absorption spectra Composition of the CdZnSe cores calculated from EDS data is normalised to Cd+Zn = 1 and rounded up/down to ~0.1

Figure 1 Powder XRD patterns of CdZnSe ternary QD cores with the same reaction time

of 20 min but different growing temperature, from 260°C (6) to 310°C (D) Bulk diffraction peaks for zinc blende (zb) ZnSe (top blue sticks) and wurtzite (w) ZnSe (top black sticks) and zb CdSe (bottom red sticks) and w CdSe (bottom blue sticks) are indexed for identification purpose (see online version for colours)

Figure 2 Powder XRD patterns of CdZnSe ternary QD cores (sample E) and CdZnSe/ZnSeS

core/shell (samples E1, E2, E3 and E4) The sticks patterns show the standard peak position of bulk wurtzite ZnS (top black sticks) and zb ZnS (top blue sticks), zb ZnSe (middle red sticks) and zb CdSe (bottom red sticks) are shown (see online version for colours)

The Cd/Zn ratios of the CdZnSe (samples B, C, D and E) are established (Table 1)

In pursuit of determining the composition of the shell, with the ternary core/shell QDs,

we suppose that the ratios of Cd/Zn and Cd/Se of the core do not change during the shell creation process From the calculation results of the experimental EDS data, we saw that there is a change in the Se amount in different samples, from 0.2 to 0.5 (with sample series B) from 0.2 to 0.6 with sample series C, etc

Table 1 Cd/Zn ratios from EDS data of the CdZnSe ternary QD samples

We also found that, for the quantum dot samples in powder form, their crystalline phase shows quite clearly that, depending on the fabricating temperature, they either fully crystallise in zb or w crystalline phase with partly crystallise in a zb small amount,

as shown in Figures 1 and 2 However, with the same samples, when analysing the elemental composition with the EDS method, we did not get the results on charge balance between atoms with positive valence (like Cd and Zn) and atoms with negative valence (like Se), for CdZnSe core QDs Calculations showed the same for alloy core/shell QDs To explain this, we managed to deduce the following reasons:

• The size of these quantum dots is small (from 4.1 nm to 6.5 nm), making the number

of atoms on the surface much larger than that inside the QDs, so the ratio of the atoms with positive charge will be other than 1 This is also observed and reported

by the group of Morris-Cohen [13] with CdSe QDs

• Since the only experimental analysis method used is EDS, the results on the elemental composition to deduce the formula of ternary QDs that may have little errors

Moreover, the calculated elemental composition with the ternary core/shell QDs may also have small error because of the assumption that during the shell growth process, the composition of the core and the number of atoms on their surface are not changed

However, since the process of shell crystal growth takes place in the solution, we cannot

be sure about this Thus, the ternary QDs’ formula obtained is actually not balanced in electronic charge

The images from TEM measurement has also been used to clarify the shape, size and uniformity in size of the QDs, as a function of the core crystal growth temperature and the stages of the shell crystal growth with different composition Figure 3 shows the TEM images of four ternary QD samples grown at temperatures 285°C (B), 290°C (E), 300°C (C) and 310°C (D)

From these TEM images, we can see that the QDs of the sample grown at 285°C have

a wide range of size distribution On the other hand, between 290°C and 310°C, the QDs

are grown with size uniformity The average particle diameter of the samples changes accordingly: 5, 5.3, 6.5 and 6.57 nm (the error is approximately ±0.5 ÷ 1 nm)

Figure 3 TEM images of 5 nm (sample B), 6.5 nm (E), 5.3 nm (C) and 6.5 nm (D) CdZnSe core

QDs Scale bars: 20 nm

However, while growing the QD samples at 310°C, the organic molecules such as TOPO and HDA in the flask evaporate strongly during the QD growing process Also, the samples created at this temperature have lower emission intensity than the other samples

Therefore, we choose the temperature condition of 285°C-300°C to synthesise the QD samples Here, we can also see that when ZnxCd1-xSe QDs are prepared from ZnSe nuclei

or ZnSe QD seeds as used in this study, a red-shift in the emission maximum was observed during the alloying stage as a result of the decrease in band gap energy as Cd2+

is progressively incorporated into the ZnSe lattice [6,7] Alloy formation occurred at temperatures lower than the alloying point for the CdSe-seeded growth, implying that cation exchange reaction between ZnSe nanocrystals and Cd2+ is more favoured than that between CdSe and Zn2+. This is attributed to the much lower bond dissociation energy of Zn-Se (136 kJ mol–1) relative to Cd-Se (310 kJ mol–1) as reported in Regulacio and Han [3] From the results on analysing crystalline phase with XRD and all of the analyses above, it can be assumed that the chosen QD synthesis temperature of 2900C or 300°C is suitable

The TEM images of CdZnSe core/shell QDs samples are presented in Figure 4

The ZnSeS shell with a different ratio of Se/S in the precursor was brought into reaction

The Se quantity varied from 2.0 to 0.8 in moles The pre-calculated precursor amount for

the same shell thickness is ~1.52 nm (1 ML is equivalent to 0.38 nm, the a lattice

constant of ZnS) We found that, when more layers of ZnSeS shell are grown, the size of the QDs increases a bit, and the average diameter is ~7 nm

Figure 4 TEM images of CdZnSe QDs and CdZnSe/ZnSex S 1-x core/shell QDs with the average

size of 5.3 nm (C), 5 nm (B) to 6.5 nm (E) for the cores and 6.3 nm (bottom left-B), 5.3 nm (bottom middle-E) and 7 nm (bottom right-C) for the core/shell QDs Scale bars:

20 nm

However, for sample series E, TEM images show that the fabrication of the ZnSeS shell altered the shape of the QDs, from quasi-spherical shape of the core, to an elongated shape, and sometimes include both spherical particles and cubes Owing to the