Processing and characterization of monodisperse phosphine-free CdSe colloidal quantum dots

Processing and Characterization of Monodisperse Phosphine-Free CdSe Colloidal Quantum Dots Hadi Maghsoudi1, 2, Melika Mahboub1, 2 and Sirous Asgari2*

1 Department of Materials Science and Engineering, University of California, Riverside, USA. 2 Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran

ABSTRACT

In this research, mono-dispersed quantum dots of CdSe were produced using a phosphine-free approach to synthesis of

colloidal quantum dots. Selenium precursor was selected as the main precursor. It was found that the initial concentra-

tion ratio of monomers critically controlled the size distribution of the nanoparticles through its influence on the growth

kinetics of these particles. The best result was obtained using an initial Se/Cd ratio of 5 where CdSe quantum dots of a

uniform size were synthesized. This was manifested in the absorption spectra of these particles by occurrence of sharp

peaks at a wavelength of about 615 nm.

Colloidal Quantum Dots, Growth Kinetics, Size Distribution

1. INTRODUCTION

Colloidal quantum dots (CQD) have shown impressive potentials in a wide range of modern applications such

as photovoltaic cells, biomedical imaging, sensors, lasers and electronic devices (1-11). This mainly stems from their

unique combination of desirable physical and chemical properties due to the quantum size effect and their capability of

size-tuning synthesis method. The latter involves rapid injection of a precursor into a coordinating solution of another

precursor at a particular temperature to start nucleation and growth. In most opto-electronic applications, absorption at a

limited range of optical wavelengths is of great interest. Consequently, processing of nano size CQDs with a sharp size

distribution in the desired range has been a major task in processing of these materials.

Nanoengineering: Fabrication, Properties, Optics, and Devices XI, edited by Eva M. Campo, Elizabeth A. Dobisz, Louay A. Eldada, Proc. of SPIE Vol. 9170,

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A common practice in processing of many Se-based CQDs such as CdSe and PbSe for opto-electronic devices

is to use alkylphosphine ligands (e.g. TOP or TBP) to avoid aggregation (12-18). One problem with this approach is the

high cost of the ligand used in the process which makes the large-scale synthesis unfavorable. Also, this particular lig-

and is very toxic and environmentally unfriendly. On the other hand, this ligand in Se-based QD/polymer composites

for electro-optic device applications is used to avoid phase separation. This is due to the fact that the surface passivation

of CQDs by organic ligands enables a uniform QDs dispersion in various solvents and polymers. The main problem,

however, is that the long alkyl chains of alkylphosphine ligands form an interfacial layer between the nanoparticles and

the polymer matrix and act as an electrically insulating layer (of 1-2 nm in thickness) which deteriorates the electro-

optical properties of the device (19-22). For instance, Greenham et al. (19) demonstrated that the presence of

alkylphosphine ligands in QD/polymer composites impeded the charge transfer between QDs and polymer molecules, as

well as the electron transport between QDs themselves. These undesirable aspects of alkylphosphine ligands in

QD/polymer composites are usually alleviated by exchanging the native ligands with small volatile molecules such as

pyridine, butylamine, and hexanoic acid (23-28). Although ligand exchange is mostly successful in surface modification

of QDs, it causes disordering of the surface atoms of QDs and deteriorates their functionality. Alternatively, it has been

recently reported that the control over QD aggregation may be achieved by surface modification of the CQDs (29, 30) or

by using a strongly adsorbing polymer31.

To avoid undesirable effects of alkylphosphine ligands on the properties of opto-electronic devices, there has

been a phosphine-free approach to processing of CQDs in recent years (32-35). In these studies, a viable “phosphine-

free” selenium precursor has developed by dissolving selenium directly in noncoordinating solvents such as 1-

octadecene (ODE) followed by heating at elevated temperatures. The common practice is to inject a large volume of

selenium into another precursor since without using organic phosphine ligands a large volume of ODE is needed to dis-

solve selenium. This has the disadvantage of sudden drop in temperature of the solution which results in a random size

distribution of CQDs. Our main goal in this investigation has been to synthesize mono-dispersed phosphine-free CdSe

quantum dots through a better control of the process temperature and the particle growth kinetics during the colloidal

synthesis.



2. MATERIAL AND METHODS

Materials used in this investigation include Chemicals; Cadmium oxide (CdO, Merck, 99%), selenium powder

(Merck, 99%), oleic acid (Merck, 90%), 1-octadecene (ODE, Merck, 92%), and were used in the preparations described

below. Ethanol, chloroform, and toluene were all of analytical grade and purchased from Merck. All chemicals and sol-

vents were used without further purification.

Two precursor solutions, CdO in ODE+OA and Se in ODE were prepared. The cadmium source was made by

first degassing of CdO (0.128 g, 1 mmol) and OA (1.6 ml, 5 mmol) in 10 ml of ODE at 100 °C for 30 min to remove

residual water formed during the reaction. The solution was stirred during process using a magnetic bar stirrer. The red-

dish CdO mixture gradually changed to a light yellowish homogeneous solution. The Se stock solution was made by

degassing of Se (0.0789 g, 1 mmol) in 30 ml of ODE at 180 °C for 1 h and subsequently heating the mixture under argon

at 200 °C for 30 min. During this stage, the ODE/Se mixture changed from colorless to yellow while some molten Se

formed at the bottom of the glass container. The molten Se was dissolved gradually as the process continued and the

solution color changed to orange-red and finally to orange. In the next step, the Cd precursor was injected to the Se so-

lution at the reaction temperature of 240 °C with three molar ratios of 0.1, 0.2, and 1. It should be noted that the com-

mon practice is to inject Se solution into Cd. Since the volume of Se injected is large (due to low solubility of Se in

ODE) it usually causes a sudden drop in temperature with harmful effects on final size distribution of CQDs. This is

why we have added Cd to Se in this study for a better control of reaction temperature. At different time intervals, sam-

ples were taken from the mixture for further evaluation by UV-vis and TEM/HREM techniques. To obtain CdSe parti-

cles for TEM studies, ethanol with 2:1 volume ratio was first added to the samples taken from each solution followed by

particle separation in a centrifuge unit. CdSe samples for UV-vis testing were diluted by chloroform.



Figure 1. Fluorescence photograph of differently sized CdSe QDs excited with a UV lamp.

Figure 1 shows a fluorescence photograph obtained from CdSe particles with different sizes under UV illumina-

tion. TEM studies on selected colored samples showed that CdSe particle size obtained in this investigation were in the

range of 2 to 6 nm corresponding to a change in color from violet to red. Figures 2(a) and (b) show TEM/HREM images

of orange color CdSe QDs with an average size of 5.8 nm. From HREM image (Fig. 2(b)) the interplanar spacing of

(111) planes and the lattice constant of CdSe particles are measured to be about 0.359 nm and 6.218 nm, respectively.

This is in agreement with the values reported in literature (36).

3. RESULTS



a4.

nm

41

1.4

0.4

o400 500 600 700

Wavelength(nm)

a-3 sec

-10 see

-30sc-1 inn-3 mkt

-5 inn-15 mn

30 mn

-45 mn

1.4-

1.2

N1.0

.O

Q 0.8

0.4

400 500 600

Wavelength(nm)

b

-3 sec-10 sec

-30 sec

-1 min-3 min-5 min-15 mit_30mn-40 mn

_50 mit

700

1.4

1.2

4)

A1.

Ao

Q 0.8

OA

-3 ses-10 see

-30 nee

-3 min-5 min-15 min

30min

-45min

Wavelength(nm)7 00

length(nm)

Figure 2. (a) TEM image and (b) HREM lattice image taken from [110] axis of CdSe nanocrystals.

Figure 3. Temporal evolution of UV-vis of a growth reaction of CdSe nanocrystals with different initial Se:Cd molar ratio: (a) 1:1 (b)

5:1 (c) 10:1.

Figure 4 shows the UV-vis absorption spectra of samples of different colors obtained at different process times and ini-

tial concentration ratios. In each figure, a peak in the spectrum corresponds to an optical wavelength of maximum ab-



sorption. Fig 4a shows the absorbance spectrum obtained for a Se/Cd molar ratio of 1 at different time intervals during

the growth process which is assumed to start immediately after injection (37). It is seen that after some process time, a

rather sharp absorbance peak appears in the spectrum and is shifted to longer wavelengths at longer times. With further

increase in the process time, the absorbance peak first broadens and then disappears, i.e. the optical absorbance of the

material falls continuously with increasing wavelength. Figures 4b and 4c also show similar trends to that observed in

Fig 3(a). It is important to note that, at a given time, the wavelength at which the absorbance peak occurs is rather fixed.

Also, note that in Figure 3(b), the absorbance peak becomes sharper even at long process times, in contrast to that seen

in Figures 2(a) and 2(c). For comparison, in each figure, the location of the sharpest absorbance peak is marked by an

arrow. While a sharp peak is observed in the spectrum in Figure 3(b) no such a peak is observed in the corresponding

spectra shown in Figures 2(a) and (c). In fact, the absorbance peaks in these figures have already been broadened after

30 min of synthesis.

4. DISCUSSION

Data presented in the previous section reveal two interesting aspects of processing of CdSe nonocrystals. First-

ly, it was shown that at a particular Se/Cd ratio, with increasing the process time an absorption peak appears in the spec-

trum and is shifted to longer wavelength and eventually broadens and disappears. This has been previously reported (38,

39) and is attributed to the size distribution of CQDs. It is suggested that, after the nucleation is completed, the resulting

nonuniform distribution of nuclei becomes initially more uniform in size during the subsequent growth process. This is

manifested by the appearance of a peak in the absorption spectrum which is shifted to longer wavelengths with increas-

ing the process time. Beyond a certain time, however, the size distribution of the particles becomes increasingly non-

uniform, leading to the degradation of the absorbance properties (37, 40-45). This is due to the Ostwald ripening phe-

nomenon, and is discussed in more details in the following text.

Secondly, with changing the initial Se/Cd ratio, the optical absorbance properties of the product QDs changed

significantly. With increasing the Se/Cd ratio from 1 to 5, the time after which the absorbance peak broadened was de-

layed from 10 min (for the Se/Cd of 1) to 30 min (for the Se/Cd ratio of 5). With increasing the ratio from 5 to 10, how-

ever, the peak broadening occurred much earlier, i.e. only after 3 minute of processing. This result is important in that it

provides a simple means for the synthesis of uniform CQDs with higher absorption wavelengths. We explain this behav-



2r'

Focussing size regime

Particle radius (r)

Broadening regime

ior based on the growth process of CQDs which consists of two steps (i) diffusion of monomers (the atomic components

of CQDs) towards the surface of the nuclei and (ii) the surface reaction at monomer/nucleus interface. Initially, because

of the high concentration of free monomers available, the rate of the growth process is determined by the interface reac-

tion, i.e. the growth of CQDs is reaction-controlled. In this regime, the growth rate is proportional to the surface area of

the CQDs and is independent of particle size. This implies that the width of the absorbance peak does not vary with

time. As processing continues, the concentration of free monomers is decreased and the growth rate is limited by the

diffusion rate of monomers towards the particle/solution interface. In this diffusion-controlled regime, growth rate is

significantly affected by the particle size, as shown in the Figure 4 plotted based on Equation 1.

= ( ∗ − ) (1)

Figure 4. Variation of the growth rate versus particle size, according to model of Sugimoto37.

Here, k is a constant defined by ( ), D is the diffusion constant of the monomer, denotes the density of monomers

in the crystal, σ is specific surface energy, r is the radius of the nanocrystal, kB is the gas constant, T is the absolute tem-

perature and r* is the critical value of radius. We can infer that when the diffusion-control regime starts, the size distri-

bution of CQDs becomes wider with time if their initial average radius ( ̅) is in the range of r* to 2r*. This is because



tek,

St sisgsp:::::,zztIzzzz:zfz.-tz-_IV, ,::,:,.

1 #

Holitic Itisatis110fitt(lc,"

Particle radius (r)

the growth rate is directly proportional to the particle size in this region (see Figure 4), i.e. the finer particles grow slow-

er than the coarser ones. For the initial ̅ of CQDs larger than 2r*, however, the size of the particles become more uni-

form with increasing time since the growth rate is inversely proportional to the particle size. This regime is thus called

the focusing size regime. As the time overs, critical size increases described by the equation (3) due to the decreasing of

the concentration of monomers.

C = C e ∗ ≈ C 1 + ∗ (2)

r∗ = ( ) (3)

( ) = (− ∗ + ) (4)

Figure 5. Sketch of growth rate (dr/dt) of CQDs for different radii and concentration of monomers (or r*).

Where Cb is the monomer concentration in the solution and C∞ is the monomer concentration in equilibrium with a bulk

flat solid (r=∞). With increasing the process time, the value of 2r* exceeds the minimum particle size in the solution

leading to the onset of a size broadening regime for those particles with r < 2r*. Finally, when r* passes the ̅, particles

larger than r* continue to grow at the expense of smaller particles, i.e. Ostwald ripening stage sets in causing significant

non-uniformity in particle size distribution. The effect of focusing size distribution is directly related to the slope of the



growth rate versus particle size which can be calculated from Equation 4. It shows that the slope of growth rate versus

particle size is inversely related to r*. As a result, the effect of size focusing regime decreases dramatically with increas-

ing the process time. Figure 5 shows the growth rate versus radius of CQDs for different monomer concentrations (or

r*).

Based on the above discussion, we may now explain the origin of the observed changes in the optical absorb-

ance spectra presented in Figure 4 for different initial Se/Cd concentration ratios. We propose that this ratio dramatically

affects the diffusion rate of monomers and therefore determines the time at which the reaction control regime ends.

With increasing Se/Cd ratio, this time becomes shorter and the diffusion controlled growth starts with a smaller r* and ̅. Now, if ̅ < 2r*, defocusing of size distribution occurs, similar to what seen for Se/Cd ratio of 10 in Fig. 4(c). For val-

ues of ̅ much larger than 2r*, Oswald ripening occurs rapidly and the influence of focusing step is reduced, leading to

the effects seen in Fig. 4(a). These two effects may be justified since the rate of increase in r* with concentration

(dr*/dc) is inversely proportional to the monomer concentration while the slope of dr/dt at a particular r is directly relat-

ed to the monomer concentration. Therefore, the focusing step is most effective when diffusion controlled growth starts

at an average radius which is slightly larger than 2r*. This, we believe, may explain the observed sharpening of the ab-

sorbance peaks at large wavelengths seen in Fig. 4b for a Se/Cd ratio of 5.

5. CONCLUSIONS

An improved phosphine-free approach for synthesis of colloidal stable CdSe quantum dots was presented. It

was shown that the use of Se precursor as the main precursor generated mono-dispersed quantum dots of CdSe the quali-

ty of which was controlled by the initial concentration ratio of the monomers. The Se/Cd ratio is suggested to affect the

particle growth kinetics which in turn has a decisive effect on the size distribution of the final products characterized by

the occurrence of sharp peaks in the absorption spectra of the synthesized quantum dots. This new route provides a sim-

ple alternative to TOPSe, TBPSe, and (TMS)2Se precursors and may be used for the synthesis of high quality quantum

dots based on selenium such as CdSe and PbSe.

6. ACKNOWLEDGMENT



Help from Prof. Hormozi Nezhad of Dept. of Chemistry, Sharif University of Technology is gratefully acknowledged.

The authors wish to thank Prof. A. Heuer, director of SCSAM, Case Western Reserve University for providing access to

TEM/HREM facilities.



Processing and characterization of monodisperse phosphine-free CdSe colloidal quantum dots

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