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