1 SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR By XIAN CHEN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007
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1
SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR
By
XIAN CHEN
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
1.1 Introduction..................................................................................................................11 1.2 General Synthetic Methods for Nanocrystals ..............................................................11
2 SYNTHESIS OF CADMIUN SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR .................................................................................................19
3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR .................................................................................................39
4.2.1 Injection-Synthetic Method for CdSe ................................................................50 4.2.2 Improvement of Other Metal Selenide Nanocrystals.........................................51 4.2.3 Mechanism Study...............................................................................................51
LIST OF REFERENCES...............................................................................................................52
1-5 Representation of the synthetic apparatus employed in the one-pot synthetic method......16
2-1 Sizing curve of CdSe nanocrystals .....................................................................................19
2-2 HWHF and peak sharpness used for size distribution determination.................................20
2-3 Schematic diagram of a UV-Vis microscope. ....................................................................21
2-4 Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer. .........................21
2-5 Molecular structures of organic solvents used. ..................................................................23
2-6 Temporal evolution of the absorption spectra during the CdSe synthesis .........................24
2-7 Characterization of CdSe nanocrystals synthesized in ODE with reaction time of 40 minutes...............................................................................................................................25
2-8 Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with different C16-diol/SeO2 ratios............................................................................................26
2-9 CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios ................27
2-10 CdSe particle size and normalized nuclei number in the synthesis with different C16-diol/SeO2 ratios. .........................................................................................................28
2-11 Temporal evolution of the absorption spectra during the CdSe synthesis with different diols .....................................................................................................................29
2-12 HWHM of CdSe during synthesis with different diols. .....................................................29
2-13 Temporal evolution of CdSe nanocrystal concentration with different diols.....................30
2-14 CdSe particle size in the synthesis with different numbers of carbon atom per diol. ........30
2-15 Temporal evolution of the absorption spectra during the CdSe synthesis with different alcohols. ..............................................................................................................31
8
2-16 Temporal evolution of the absorption spectra during the CdSe synthesis with different Cd precursors ......................................................................................................32
98%), 1-octadecanol (C18-OH, 99%), and phenol were purchased from Sigma-Alrich. Methanol
(99.9%), toluene (99.9%), acetone (99.8%) were purchased from Fisher. Sodium myristate
(CH3(CH2)12COONa), sodium stearate (CH3(CH2)16COONa) were purchased from TCI.
Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O) was purchased from Alfa Aesar.
Tetrabutylammonium hydroxide (1M in methanol) was purchased from Acros. All chemicals
were used without further purification.
2.2.2 Instrumentation
Absorption spectra of aliquots were collected by a Shimadzu UV-1700 UV-Visible
Spectrophotometer (Figure 2-3). The wavelength, absorption and the half width at half maximum
21
(HWHM) of first exciton peak for each aliquot were recorded. This method also was compared
with our previous one-pot synthesis method and other current injection method.
Figure 2-3. Schematic diagram of a UV-Vis microscope.
Photoluminescence (PL) was measured at room temperature from nanocrystals suspended
in toluene using a JOBIN YVON HORIBA Fluorolog-3 Model FL3-12 spectrofluorometer
(Figure 2-4).
Figure 2-4. Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer.
22
High resolution transmission electron microscopy (HR-TEM) images were obtained using
a JEOL 2010F microscope for lattice imaging and crystal size determination. TEM samples were
prepared by dispersing the nanocrystals in toluene and depositing them onto formvar-coated
copper grids.
2.2.3 Preparation of Cd-Precursors
2.2.3.1 Cadmium myristate (CdC14)
1.5g cadmium nitrate was dissolved in 75ml methanol, while sodium myristate solution
was prepared by dissolving 3.8g sodium myristate in 550ml methanol. Then the cadmium
nitrate solution was dropped slowly into sodium myristate solution under magnetic stirring
conditions. The observed white precipitate was washed with methanol 4-5 times to get rid of
impurities and dried under vacuum overnight to remove all solvents.
2.2.3.2 Cadmium stearate (CdC18)
0.3g Cadmium nitrate was dissolved in 40ml methanol, Sodium stearate solution was
prepared by dissolving 0.90g sodium stearate in 600ml methanol. Then the cadmium nitrate
solution was dropped slowly with into sodium stearate solution under magnetic stirring
conditions. The observed white precipitate was washed with methanol 2-3 times get rid of
impurities then put back in flask with 600 ml methanol and ultrasonicated. Wash the product
again and dried under vacuum overnight to remove all solvents.
2.2.3.3 Cadmium docosanate (CdC22)
0.3g Cadmium nitrate was dissolved in 40ml methanol. Sodium docosanate solution was
prepared by dissolving 0.90g docosanoic acid in 600ml methanol and slowly adding 1.1 ml
tetrabutylammonium hydroxide (1M in methanol) dropwise. Then the cadmium nitrate solution
was dropped slowly into sodium docosanoicate solution under magnetic stirring conditions. The
observed white precipitate was washed with methanol 2-3 times get rid of impurities then put
23
back in flask with 600 ml methanol and ultrasonicated. Wash the product again and dried under
vacuum overnight to remove all solvents.
2.2.4 Preparation of CdSe Nanocrystals
Cadmium precursor (0.1mmol), SeO2 (0.05mmol), C16-diol (0.05mmol) and non
coordinating solvent (5g) were mixed in a three-neck flask equipped with condenser, magnetic
stirrer, thermocouple, and heating mantle (as shown in Figure 1-5), degassed before heated to
265 oC with gentle stirring under vacuum to synthesize CdSe nanocrstals. Aliquots of the
solution for each reaction were taken quantitatively with a syringe at different time intervals, and
quickly cooled and diluted in toluene to stop further growth. These aliquots were employed to
monitor the reaction via UV-Vis and photoluminescence measurement.
2.3 Results and Discussion
2.3.1 Diol Effect
C16-diol Effect on the Quality and Size of CdSe Nanocrystals
ODE TDE
Squalene Octyl ether
Figure 2-5. Molecular structures of organic solvents used.
CdSe nanocrystals were formed using SeO2 compound instead of Se element in the solvent
of ODE, which means that SeO2 is active at high temperature. However, the quality is not as
24
good. We found that for CdSe nanocrystals synthesized in ODE, addition of equal molar
amounts of C16-dio land SeO2 has several effects on the growth, including growth rates,
HWHMs, sharpness, optical densities, and final sizes. This phenomenon was also observed in
CdSe nanocrystals synthesized in TDE, squalene, and octyl ether. The molecular structures of
these four solvents are shown in Figure 2-5.
Figure 2-6 shows the absorption spectra of CdSe nanocrystals made of 0.1 mmol CdC14,
0.05 mmol SeO2 and 0.05 mmol C16-diol in different solvents.
Figure 2-6. Temporal evolution of the absorption spectra during the CdSe synthesis in (a) and (e) ODE, (b) and (f) squalene, (c) and (g)octyl ether and (d) and (h)TDE; in syntheses (a), (b), (c) and (d), C16-diol was not added while in syntheses (e), (f), (g) and (h), C16-diol was added.
In Figure 2-7, we show the absorption and photoluminescent (PL) spectra and TEM image
of CdSe nanocrystals which were made in ODE and have a reaction time of 40 minutes. It can be
400 500 600 700
0
2
4
6
8
0
2
4
6
8
e) ODE, C16-diol added
a) ODE, no C16-diol
30min
10min
5 min
240oC
220oC
0 min
Abs
orba
nce
(a.u
.)
wavelength (nm)
220oC
30min
10min
5 min
0 min
240oC
Abs
orba
nce
(a.u
.)
400 500 600 7000
2
4
6
8
0
2
4
6
8
30min
10min
5 min
0 min
wavelength (nm)
h)TDE, C16-diol added
d) TDE, no C16-diol
0 min
30min
10min
5 min
400 500 600 7000
2
4
6
8
10
0
2
4
6
8
10
30min10min5 min0 min
wavelength (nm)
g)octyl ether, C16-diol added
c) octyl ether, no C16-diol
30min
10min5 min
0 min
400 500 600 700
0
2
4
6
8
10
0
2
4
6
8
10
30min
10min
5 min
0 min
250oC
wavelength (nm)
f)squalene, C16-diol added
b) squalene, no C16-diol
240oC220oC
30min
10min
5 min
0 min
220oC
25
seen that the sample is nearly monodispersed. The average diameter is 3.3 nm, which is very
close to the calculated diameter of 3.4 nm.
The CdSe nanocrystals formed with the presence of C16-diol have much better quality than
those formed without adding C16-diol. With the presence of C16-diol, the first peak is narrower
and deeper than that without the presence of C16-diol, which indicates that size distribution is
better. Thus one can conclude that adding C16-diol can improve the quality of CdSe nanocrystals.
It was also observed that the final sizes of CdSe nanocrystals are smaller and the ODs are higher
in the case of adding C16-diol. To better understand the role that C16-diol assumes in the
synthesis, three syntheses with different amounts of C16-diol were performed. Nuclei
concentration, nuclei number, growth rate as well as peak sharpness and HWHM were employed
to analyze the data.
500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
a
Inte
nsity
Wavelength (nm)
b
Figure 2-7. Characterization of CdSe nanocrystals synthesized in ODE with reaction time of 40 minutes. (a) Absorption (in blue) and photoluminescent (PL) (in red) spectra and (b) TEM image.
26
Nuclei concentrations are calculated by using the exciton energy to determine the particle
radius and then the extinction coefficient for each size to determine the particle concentration.52
In the above equation, D (nm) is the diameter of a given nanocrystal sample, and λ (nm) is the
wavelength of the first excitonic absorption peak of the corresponding sample.
The extinction coefficient of CdSe is calculated as
65.2)(5857 D=ε (2-2)
Then using the Lambert-Beer’s law,
CLA ε= (2-3)
The molar concentration C (mol/L) of the nanocrystals of the sample can be calculated. A is the
absorbance at the peak for a given sample and L is the path length (cm). In our experiments, L
was fixed at 1 cm.
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.81
2
3
4
5
6
7
8
9
10
c
abC
dSe
nucl
ei c
once
ntra
tion
(10-6
M)
size of CdSe nanocrystals (nm)
Figure 2-8. Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with
different C16-diol/SeO2 ratios, (a) C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.
27
The calculated temporal evolution of concentrations and growth rates of CdSe nanocrystals
made in ODE with different C16-diol/SeO2 ratio is shown in Figure 2-8 and Figure 2-9,
respectively. Figure 2-8 shows that the more C16-diol in the reaction solution, the higher the
CdSe nuclei concentration and the concentration dramatically increases when the C16-diol to
SeO2 ratio changes from 1 to 2. It can be seen in Figure 2-9 that the higher the ratio of C16-diol
to SeO2, the slower the growth rate.
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0
2
4
6
8
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0
2
4
6
8
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0
2
4
6
8
c
b
CdS
e pa
rtic
le g
rpw
th ra
te (n
m3 /m
in)
CdSe particle size (nm3)
a
Figure 2-9. CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios (a)
C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.
It was found that the C16-diol affects not only the quality and nuclei number of CdSe
nanocrystals, but also the final CdSe particle sizes, as shown in Figure 2-10. The more the
C16-diol in the reaction solution, the smaller the CdSe nanocrystals will be obtained.
Addition of C16-diol can slow the CdSe particle growth remarkably with the nuclei
number increasing at the same time. It can be concluded that nuclei number is related the particle
28
growth rate. The faster the particles growth, the lower the nulclei number. This leads us to a
hypothesis that C16-diol acts as a reducing agent in the reaction, helping reduce the selenium in
SeO2 from Se+4 to Se -2, helping increase the concentration of selenium monomers. This leads to
easier nucleation and a higher nuclei number and thus, a smaller size.
0.0 0.5 1.0 1.5 2.03.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Size
of C
dSe
nano
crys
tals
(nm
)
Ratio of C16-diol/SeO2
Figure 2-10. CdSe particle size and normalized nuclei number in the synthesis with different
C16-diol/SeO2 ratios.
Effect of Numbers of Carbon Atoms per Diol
Effect of different carbon chain length of diols (C8-diol, C10-diol and C16-diol) was
studied. The temporal evolution of the absorption spectra is shown in Figure 2-11. As shown in
Figure 2-12, the HWHM of CdSe nanocrystals made with C16-diol is the best and this is
equivalent to a tighter size distribution. Figure 2-13 shows CdSe nanocrystal concentration with
different diols. The nuclei concentration with C10-diol is very close to but slightly higher than
that with C16-diol, while the concentration with C16-diol is higher than that of C8-diol. Figure
2-14 illustrates that with C10-diol and C16-diol, the final particle sizes are very similar and with
C8-diol slightly larger size particles can be obtained.
29
Figure 2-11. Temporal evolution of the absorption spectra during the CdSe synthesis with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol.
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
14
15
16
17
18
c
b
a
HW
HM
(nm
)
Size of CdSe nanocrystals (nm)
Figure 2-12. HWHM of CdSe during synthesis with different diols: (a) C16-diol, (b) C10-diol
and (c) C8-diol.
400 500 600
0
2
4
6
8
30min
10min
5min
0min
240oC
220oC
a)
Abs
orba
nce
(a.u
.)
wavelength (nm)400 500 600
0
2
4
6
8b)
230oC
250oC
0 min
30min
5 min
10min
wavelength (nm)400 500 600
0
2
4
6
8
30min
10min
5 min
0 min
240oC
220oC
c)
wavelength (nm)
30
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.00
1
2
3
4
b
a
c
CdS
e nu
clei
con
cent
ratio
n (1
0-6 M
)
Size of CdSe nanocrystals (nm)
Figure 2-13. Temporal evolution of CdSe nanocrystal concentration with different diols: (a)
C16-diol, (b) C10-diol and (c) C8-diol.
8 9 10 11 12 13 14 15 16 173.3
3.4
3.5
3.6
3.7
3.8
3.9
Size
of C
dSe
nano
crys
tals
(nm
)
number of carbon atoms per diol
Figure 2-14. CdSe particle size in the synthesis with different numbers of carbon atom per diol.
31
Based on all the information, one can conclude synthesis with C16-diol has the lowest
growth, smallest size. The possible explanation for this is that the longer the carbon chain in the
diol molecule, the hydrogen bonding will be weaker, and thus the more active the diol would be.
So among all three, C16-diol is the most active.
Comparison of Alcohols and Diols
Figure 2-15. Temporal evolution of the absorption spectra during the CdSe synthesis with different alcohols. (a) C16-diol, (b) C18-OH and (c) phenol.
Synthesis with alcohols was performed. The results are compared with diols. C18-OH and
phenol were used. The nanocrystals were formed by cadmium myristate (0.1 mmol) reacted with
SeO2 (0.05 mmol) and C18-OH or phenol (0.05 mmol). The absorption spectra are shown in
Figure 2-15. Compare HWHM and sharpness, one can conclude that the qualities CdSe
nanocrystals with C18-OH are better than that without alcohol, but not are not comparable to
those with C16-diol. The CdSe nanocrystals made with phenol did not get improved.
400 500 600 700
0
2
4
6
8
c)
30min
10min
5 min
0 min
240oC
220oC
wavelength (nm)400 500 600 700
0
2
4
6
8
b)
30min
10min
5 min
0 min
240oC
220oC
wavelength (nm)400 500 600 700
0
2
4
6
8
30min
10min
5min
0min
240oC
220oC
a)
Abs
orba
nce
(a.u
.)
wavelength (nm)
32
The possible reaction happened is proposed below. The SeO2 got reduced by the alcohol
and selenium was formed. The active selenium then reacted with cadmium precursor to form
CdSe nanocrystals.
2.3.2 Precursor Effect
400 500 600 700
0
2
4
6
8
30min
10min
5min
0min
240oC
220oC
a)
Abs
orba
nce
(a.u
.)
wavelength (nm)
400 500 600 700
0
2
4
6
8
Abs
orba
nce
(a.u
.)
c)
30min
10min
5 min
0 min
255oC
240oC
wavelength (nm)
Figure 2-16. Temporal evolution of the absorption spectra during the CdSe synthesis with different Cd precursors. (a) CdC14, (b) CdC18, (c) CdC22 and (d) CdC10.
400 500 600 700
0
2
4
6
8
b)
30min
10min
5 min
0 min
240oC
220oC
wavelength (nm)
400 500 600 700
0
2
4
6
8
10
10min
30min
5min
0min
220oC
210oC
d)
wavelength (nm)
33
Besides the diol effect, it was found that using precursors that have longer carbon chains
can also improve the quality of CdSe nanocrystals. Figure 2-16 shows the absorption spectra of
CdSe nanocrystals made of four different cadmium precursors (0.1 mmol) reacted with SeO2
(0.05 mmol) and C16-diol (0.05 mmol) in ODE. The particles made of CdC22 have the best
quality, followed by those made of CdC18, and particles formed by CdC10 have poor spectra. So
we can conclude that the longer carbon chain in the cadmium precursors, the higher-quality
nanocrystals can be obtained. The absorption spectrum of CdSe nanocrystals made of CdC18 and
CdC22 exhibit multiple exiton peaks (Figure 2-17).
Figure 2-18. Effect of Cd precursor on the nuclei concentration during the CdSe synthesis. (a) CdC14, (b) CdC18 and (c) CdC22.
8 10 12 14 16 18 20 22 242.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Size
(nm
)
carbon atom number in precursor
Figure 2-19. Effect of Cd precursor on the CdSe particle size in the synthesis.
35
The calculated nuclei concentrations of syntheses with different cadmium precursors are
shown in Figure 2-18. The more carbon atoms in the cadmium precursors, the higher nuclei
concentration will be achieved. Figure 2-19 illustrates the relationship between the final particle
size and the number of carbon atoms per cadmium precursor. When a cadmium precursor with
longer carbon chains is used, CdSe nanocrystals with smaller size will be generated. Growth
rates are shown in Figure 2-20. The longer the carbon chains in the precursor, the slower the
growth rate. The result is that cadmium precursor and selenium precursor have more comparable
reactivity and this may cause high-quality nanocrystals.
The relationship between the nuclei number and the particle growth rate is also shown
here. As has been discussed, the ratio of diol to SeO2 effect, the faster the particles growth, the
lower the nuclei number. Precursor with longer carbon chains causes slower growth rate and
higher nuclei number probably because its molecular size is larger and thus it is harder to get
activated. It takes a longer time to transfer to monomers and results in higher nuclei number.
4 6 8 10 12 14 16 18 20 22
0
2
4
6
8
10
12
14
4 6 8 10 12 14 16 18 20 22
0
2
4
6
8
10
12
14
4 6 8 10 12 14 16 18 20 22
0
2
4
6
8
10
12
14
c
b
CdS
e pa
rtic
le g
row
th ra
te (n
m3 /m
in)
CdSe particle size (nm3)
a
Figure 2-20. CdSe particle growth rate in the synthesis with different Cd precursors: a) CdC14, (b) CdC18 and (c) CdC22.
36
2.3.3 Multiple-Addition Method
With the above method, the acceptable nanocrystals with the largest size we can get is 3.4
nm. It was found that with the multiple-addition method CdSe nanocrystals with a size of 4.5 nm
were generated, which has an absorption peak at 582 nm. In the experiment, cadmium myristate
(0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added into a three-neck flask with
5 g ODE. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room
temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to
265°C (25°C /min) with gentle stirring and let to react for 1 hour. The reaction solution was
cooled down to room temperature, and then cadmium myristate (0.025 mmol), SeO2 (0.0125
mmol), and C16-diol (0.0125 mmol) were added. After degassing, the solution was heated to
265°C again and let react for 1 hour. Lastly, the second addition step and condition was repeated
twice. Aliquots were taken out for UV and photoluminescence (PL) measurement.
400 500 600 700
0
2
4
6
8
10
12 b
Abs
orba
nce
(a.u
.)
wavelength (nm)
a
Figure 2-21. Characterization of CdSe nanocrystals during the multiple-addtion synthesis (a)
Temporal evolution of the absorption spectra multiple-addition synthesis. (b) TEM image.
37
The temporal evolution of the absorption spectra during the CdSe synthesis is shown in
Figure 2-21(a). The TEM image of the final CdSe nanocrystals made by this method is shown in
Figure 2-21(b). It can be seen that the nanocrystals are uniformly spherical, and the size
distribution is good. The average size is around 4.5 nm.
2.4 Conclusion
In this part of work, it is showed that SeO2 can be used to replace selenium powder to
synthesize CdSe nanocrystals. With the presence of C16-diol, the quality of CdSe nanocrystals
can be improved. The effects of the C16-diol to SeO2 ratio were studied. It was found that the
more diol added, the higher-quality and smaller-size CdSe nanocrystals will be obtained. The
quality of our product is comparable to the best results published and nanoparticles smaller than
3 nm are even better. C16-diol can make the CdSe particle growth slower with the nuclei number
increasing at the same time. Compared the quality of CdSe nanocrystals made with C8-diol,
C10-diol and C16-diol, these diols have similar effect. Results with C18-OH and phenol were
worse than those with diols. Cadmium precursor effect was studied. The results show that the
longer the carbon chains in cadmium precursor, the smaller size particles will be synthesized.
Cadmium precursors with longer carbon chains can retard the growth rate and increase the nuclei
number. Multiple-addition method was performed and obtained CdSe nanocrystals with size of
around 4.5 nm. The best spectrum of different peak position were chosen from several
experiments and shown in Figure 2-22.
38
400 500 600
0
5
10
15
20
25
Abs
orba
nce
(a.u
.)
wavelength (nm)
Figure 2-22. Temporal evolution of the absorption spectra of the as-prepared CdSe nanocrystals. Black: made by CdC22+SeO2+C16-diol (0.1:0.05:0.05); Red:: made by CdC18 +SeO2+C16-diol (0.1:0.05:0.05); Blue: made by CdC14 +SeO2+C16-diol (0.1:0.1:0.05); Purple: CdC14 +SeO2+C16-diol (0.1:0.05:0.05); Green: motile-addtion reaction, CdC14 +SeO2+C16-diol (0.1:0.05:0.05).
39
CHAPTER 3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM
DIOXIDE AS PRECURSOR
3.1 Introduction
In Chapter 2 it was that using SeO2 to replace selenium element, CdSe nanocrystals can be
obtained. Experiments where SeO2 was employed with gallium, lead, silver, copper and nickel
precursors were performed to synthesize metal selenide nanocrystals.
The Kelly group has done a lot of research on the GaSe nanoparticles, from synthesis to
physical properties.53-56 GaSe has a hexagonal layered structure57 consisting of Se-Ga-Ga-Se
sheets. GaSe is a semiconductor with indirect band gap58,59 having a 2.11 eV direct band gap.
Their GaSe synthesis is based on the reaction of an organometallic (GaMe3) with TOPSe in a
high-temperature solution of TOP and TOPO. The absorption spectra of GaSe nanoparticles have
an onset in the 400-500 nm region. The nanoparticle diameters range from 2 to 6 nm, with an
average size of about 4 nm. After chromatographic purification the average size is about 2.5
nm.53
Lead selenide nanocrystals have been widely studied. The popular method is colloidal
synthesis method. In the Murray group, PbSe nanocrystals is synthesized by rapidly injecting a
lead oleate and TOPSe dissolved in trioctylphosphine into a well-stirred solution of dioctylether
at 150oC.60 Temperature is tuned to control the size of the nanocrystals.
Ag2Se has two phases. The low-temperature phase (α-Ag2Se) is a narrow band-gap
semiconductor, and has been widely used as a photosensitizer in photographic flims to
thermochromic materials. β-Ag2Se is the high-temperature phase, and it is a superionic conductor
that is used a solid electrolyte in photochargable seconndary batteries.61 These two phases are
reversible. There are only a few reports on the preparation of Ag2Se nanocrystals. Yi Xie et .al
synthesize Ag2Se nanocrystals at room temperature through the reaction of AgNO3, Se, and
40
KBH4 in pyridine.62 The Vittal group synthesized Ag2Se nanoparticles by thermolysis of silver
selenocarboxlyate in TOPO/TOP.61
CuSe is used in solar cells.63 The methods to synthesize CuSe nanocrystals include
thermolysis of Cu and Se powder mixtures64, the mechanical alloying of Se and Cu in a
high-energy ball mill, and the reaction of Se with Cu element in liquid ammonia65. The Vittal
group synthesized CuSe nanoparticles by thermolysis of copper selenocarboxlyate in
TOPO/TOP.66
NiSe is one of the typical Pauli paramagnets with metallic conductivity.67 These
stoichiometric compounds and the solid solutions between them now have been regarded as
typical materials for studies of the physical characteristics associated with a narrow band
electron system.68-70 Meanwhile, transition metal dichalcogenides have extensive applications in
energy areas such as electrochemistry and catalysis.71,72 The large surface areas and high activity
of nanomaterials will enhance their applications in these fields.73 Several methods have been
used to prepare NiSe nanocrystals, including elemental reactions74, organnometallic precursor
(36) Tran, P.T.; Goldman, E.R.; Anderson, G.P.; Mauro, J.M.; Mattoussi, H. Physca Status Solidi B 2002, 229, 427-432.
(37) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B. 2001, 105, 8861-8871.
(38) Parak, W. J.; Gerion, D.; Zanchet, D.; Woerz, A. S.; Pellegrino, T.; Micheel, C.; Williams, S. C.; Seitz, M.; Bruehl, R. E.; Bryant, Z.; Bustamante, C.; Bertozzi, C. R.; Alivisatos, A. P. Chem. Mater. 2002, 14, 2113-2119.
(39) Wang, S.; Mamedova, N.; Kotov, N. A.; Chen, W.; Studer, J. Nano Lett. 2002, 2, 817-822.
(40) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 3125-3133.
(62) Wang, W.; Geng, Y.; Qian, Y.T; Ji, M.; Xie, Y. Materials Research Bulletin 1999, 34, 877-882. (63) Ohtani, T.; Motoki, M. Mater. Res. Bull. 1995, 30, 1495-1504. (64) Henshaw, G.; Parkin, I. P.; Shaw, G. Chem. Commun. 1996, 1095-1096. (65) Henshaw, G.; Parkin, I. P.; Shaw, G. J. Chem. Soc., Dalton Trans. 1997, 231-236. (66) Lu, Z.; Huang, W.; Vittal, J.J. New J. Chem. 2002, 26, 1122-1129. (67) Noue, I.; Yasuoka, H.; Ogawa, S. J. Phys. Soc. Jpn. 1980, 48, 850-856. (68) Matsuura, A. Y.; Watamable, H.; Kim, C.; Doniach, S.; Shen, Z. X.; Thio, T.; Bennett, J. W.
Phys. Rev. B. 1998, 58, 3690-3696. (69) Otoro, R.; De Vidales, J. L. M.; De Las Heras, C. J. Phys.: Condens. Matter. 1998, 10,
6919-6930. (70) Miyadai, T.; Saitoh, M.; Tazuke, Y. J. Magn. Magn. Mater. 1992, 104-107, 1953-1954. (71) Jacobson, A. J.; Chianelli, R. R.; Whittingham, M. S. J. Electrochem. Soc. 1979, 126, 2277-2278. (72) Dines, M. B. J. Chem. Educ. 1974, 51, 221-223. (73) Yang, J.; Cheng, G.H.; Zeng, J.H.; Yu, S.H.; Liu, X.M.; Qian,Y.T. Chem. Mater. 2001, 13,
848-853. (74) Voorhoeve, R. J. H.; Stuiver, J. C. J. Catal. 1971, 23, 243-252. (75) Brennan, J.G.; Siegrist, T.; Kwon, Y.U.; Stuczynski, S. M.; Steigerwald, M. L. J. Am. Chem.
Soc. 1992, 114, 10334-10338.
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BIOGRAPHICAL SKETCH
Xian Chen was born in Xiamen, a beautiful city on the southeast coast of China. In 1999,
she started her college life at the University of Science and Technology of China (USTC). After
5 years of study in the Department of Polymer Science and Engineering, she received her
bachelor’s degree in engineering in 2004. Then, she joined the Department of Chemistry at the
University of Florida. She would like to pursue a Ph.D. degree after graduation.