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Synthesis of Surface Modified ZnO
Quantum Dots
University of West Florida, Pensacola, FL 32514
Michael T. Smith, Lena F. Ibrahim, Samuel M. Bynum,
Dr. Karen S. Molek, Dr. Pamela P. Vaughan, Dr. Alan Schrock
What are Quantum Dots?
• Nanocrystals with a semiconductor structure that fluoresce
at different, tunable emissions in accordance to size
• The smaller the quantum dot, the bigger the band gap
• Band gap is the area in which no electron state can exist
Fig. 1: QD color as
related to size1
ZnO Quantum Dots
Benefits
• Cost effective
• Desired in use for biological applications2
• Wide range in applications3,4
Drawbacks
• Aggregation of particles
• Unstable in aqueous dispersions
Surface Modification
• Silane surface modifiers were used to stabilize particles and inhibit aggregation
Fig. 2: Surface modification of ZnO QDs5
• Modifiers: 3-aminothrimethoxysilane and 3-aminotriethoxysilane
Surface Modification Continued
Fig. 3: Surface modification of ZnO QDs6
• The network created by the attached silanes obstructs
ZnO QDs from aggregating.
Synthesis
• Synthesis was adapted from Shi et al5
• A 0.1M Zn acetate solution was made in absolute ethanol and
refluxed at 80°C for 2 hours
• Various molarities of LiOH solution in absolute ethanol were
made
• The zinc solution and LiOH solution were cooled to 0°C
Synthesis Continued
• Capping agent was added to the zinc solution and stirred for 20
minutes while on ice, giving the Zn(II) precursor
• LiOH solution was added via drip method to the stirring solution
and left on ice for 30 min
• QDs were precipitated out of solution using n-hexane
Our Modified ZnO QDs
Figure 4: Silane modified ZnO QDs at tuned emissions
Characterization
• Infrared Spectroscopy (IR)
- Shows bonding of the ZnO to the modifier
• Powder X-Ray Diffraction (XRD)
- Substantiates particle size and ZnO particles
• Fluorescence Spectroscopy
- Shows tuned emissions
• Scanning Electron Microscopy (SEM)
- Provides particle size and morphology
• Energy Dispersive Spectroscopy (EDS)
- Elemental Analysis
Infrared Spectroscopy
Characteristic Zn-O-Si
bond7,8,9
Figure 5: IR of unmodified9, APTMS, and
APTES modified ZnO QDs from top to bottom,
respectively.
X-Ray Diffraction
Figure 6: XRD of synthesized unmodified ZnO QDs. Each peak is in close
correlation with literature spectra9,10.
X-Ray Diffraction
Figure 7: XRD shows uniform modification with differing modifiers.
0.20M APTMS
0.20M APTES
X-Ray Diffraction
Figure 8: APTES samples exemplifying broadening of peaks as particle size increases.
0.26M
APTES
0.14M APTES
0.20M APTES
0.10M APTES
Fluorescence
Figure 9: Fluorescence spectra
as it relates to size of unmodified
QDs (top right). As consistent
with literature, the smallest
particle size has the smallest
wavelength.5
Fluorescence
Figure 10: Fluorescence
spectra as it relates to size of
APTMS modified QDs (top
right). As expected, the
lowest particle size
corresponds to the smallest
wavelength.
400 600 8000
200
400
600
800
1000
Wav elength (nm)
Inte
nsity
(a.u
.)
0.14 aptms1
47
6.0
6 ,
32
6.8
76
62
3.9
7 ,
62
6.0
60
400 600 8000
10
20
30
Wav elength (nm)
Inte
nsit
y (
a.u
.)
0.14M APTMS
4 7 5 . 2 0
400 600 8000
200
400
600
800
1000
Wav elength (nm)
Inte
nsity
(a.u
.)
0.26 aptms33
96
.00
, 6
1.4
08
62
2.9
4 ,
68
3.9
36
400 600 8000
50
100
150
200
Wav elength (nm)
Inte
ns
ity
(a
.u.)
0.26M APTMS
3 9 1 . 1 8
400 600 8000
200
400
600
800
1000
Wav elength (nm)In
tensity
(a.u
.)
0.10 aptms4
48
8.9
3 ,
27
1.0
67
61
5.0
0 ,
99
6.1
12
400 600 8000
20
40
60
Wav elength (nm)
Inte
ns
ity
(a
.u.)
0.10M APTMS
494. 97
400 600 800
0
200
400
600
Wav elength (nm)
Inte
ns
ity
(a
.u.)
Normalize("0.20 aptms5")
400 600 8000
200
400
600
800
1000
Wav elength (nm)
Inte
ns
ity
(a
.u.)
0.20 aptms5
400 600 800
0
200
400
600
800
1000
Wavelength (nm)
Inte
nsity (
a.u
.)
0.20 aptms6
0.20M APTMS
461
.06 ,
159
.967
615
.00 , 1
000
.000
4 5 4 . 4 4
400 500 600
10
20
30
Wavelength (nm)
Inte
nsity (
a.u
.)
0.10M APTMS
0.14M APTMS
0.26M APTMS
0.20M APTMS
494.97 475.20 391.18 454.44
Fluorescence
Figure 11: Fluorescence
spectra as it relates to size of
APTES modified QDs (top
right). As expected, the
lowest particle size
corresponds to the smallest
wavelength.
Scanning Electron Microscopy
Figures 12 and 13: SEM of unmodified ZnO QDs on a 50 μm and 5 μm scale, left and
right respectively. The particles are large and show aggregation expected from
unmodified ZnO QDs.
50 mm5.0 mm
Scanning Electron Microscopy
Figures 14 and 15: SEM of 0.3M APTMS ZnO QDs on a 5 μm and 1 μm scale, left
and right, respectively. While at the limit for resolution on the SEM, the images
allude to nanometer-sized, spherical particles.
5.0 mm
1.0 mm
Figure 16: Low
resolution SEM of
0.25M APTES QDs.
Marked particles are
believed to be
quantum dots.
Scanning Electron Microscopy
200 nm100 nm
200 nm1.0 mm
Energy Dispersive Spectroscopy
Figure 17: Elemental analysis of 0.2M APTES ZnO QDs. As seen in Figure 3,
the network of silanes would provide for a higher concentration of Si and O in
respect to Zn. The EDS alludes that there is not enough silane present.
Conclusions
• All characterization affirms the synthesis of QDs
- IR shows surface modification
- XRD shows uniform modification between modifiers
- XRD shows controlled particle size
- Fluorescence shows tuned emissions
• QD emissions were tuned by altering particle size
• Uniform modification for each particle size was achieved with
different modifiers
• Aggregation is still occurring as seen in the distribution of sizes
in the SEM and the ratio of atoms observed from the EDS.
Future Directions
• Determine optimal amounts of modifier to eliminate aggregation and
provide for stable QDs
• Affirm stability of capped ZnO QDs by measuring consistent
fluorescence over time
• Differ modifier structure to affirm synthetic route and determine the
effects of structure on capping of ZnO QDs
• Determine method to eliminate larger particle sizes from product
solutions as shown in SEM
• TEM and light scattering techniques to affirm particle size and shape
at a high resolution and provide particle distribution
• Testing cytotoxicity of modified ZnO QDs as they relate to biological
applications
References
1. University of Wisconsin Stout. Science Olympiad: Materials Science.
www.uwstout.edu/chemistry/scienceolympiad.cfm (accessed Nov 12, 2012).
2. Fu, Y.; Du, X.; Kulinich, S. A.; Qiu, J.; Qin, W.; Li, R.; Sun, J.; Liu, J. J. Am. Chem. Soc., 2007, 129, 16029-16033.
3. Sablon, K. A.; Little, J. W.; Mitin, V.; Sergeev, A.; Vagidov, N.; Reinhardt, K. Nano Lett. 2011, 11, 2311-2317.
4. MIT Technology Review. The First Full-Color Display with Quantum Dots. http://www.technologyreview.com/
news/422857/the-first-full-color-display-with-quantum-dots/ (accessed Nov 12, 2012).
5. Shi, H.; Li, W.; Sun, L.; Liu, Y.; Xiao, H.; Fu, S. Chem. Commun., 2011, 47, 11921-11923.
6. Santra, P. K.; Kamat, P. V. J. Am. Chem. Soc., 2012, 134 (5), pp.2508–2511.
7. Sun, L.; Shi, H.; Li, W.; Xiao, H.; Fu, S.; Cao, X.; Li, Z. J. Mater. Chem., 2012,22, 8221-8227
8. Soares, J. W.; Whitten, J. E.; Oblas, D. W., Steeves, D. M. Langmuir 2008, 24, 371-374.
9. Wang, W.; Liu, J.; Yu, X.; Yang, G. J. Nanosci. Nanotechnol. 2010, 10, 5196-5201.
10. Chen, T.; Zhao, T.; Wei, D.; Wei, Y.; Li, Y.; Zhang, H. Carbohydrate Polymers 2013, 92, 1124-1132.
Acknowledgements
We would like to thank the following organizations and people for
making this work possible:
• The University of West Florida Office of Undergraduate
Research
• Mr. Michael Ishee (Pall Corporation) for the generous donation
of SEM instrument time
• Mr. Edward Magowan (Pall Corporation) for his expertise and
time in running SEM characterization of samples