HIGH-TEMPERATURE CADMIUM-FREE NANOPHOSPHORS FOR

DAYLIGHT-QUALITY WHITE LEDS

REU Student: Nathaniel C. Cook

Graduate Student Mentor: Brian A. Akins

Faculty Mentor: Dr. Marek Osiński

FINDINGS

Fig. 1. PL spectra of ZnSe:Mn/ZnS NC’s bright 497 nm emission excited by 453 nm light. Followed by

PL excitation spectrum of ZnSe:Mn/ZnS NC’s taken at 497 nm, showing peak excitations at 453 nm &

483 nm.

497

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ZnSe_Mn_3

BAGA_080811_1

In Toluene

PL BA_081011

FF Detection

Exc. 1nm Em. 1nm

Excitation 453 nm

22 CInte

nsity

(CP

S)

Wavelength (nm)

453

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S1 / R

1

ZnSe_Mn_3

BAGA_080811_1

In Toluene

PL BA_081011

FF Detection

Exc. 1nm Em. 2nm

Emission 497 nm

22 C

Photoluminescence (PL) and quantum efficiency (QE) were measured using a Horiba Jobin

Yvon Fluorolog-3 spectrofluorometer, using Spectrosil® quartz cuvettes. ZnSe:Mn/ZnS NCs

have two emission peaks: a shorter wavelength emission peak at 497 nm, which is excited by

453 nm light and has a shoulder at 525 nm (Fig. 1), and the longer wavelength peak at 587 nm

that was excited by 418 nm light (Fig. 2).

Fig. 2. PL spectra of ZnSe:Mn/ZnS NC’s bright 587 nm emission excited by 418 nm light. Followed by

PL excitation spectrum of ZnSe:Mn/ZnS NC’s taken at 587 nm, showing peak excitations at 418 nm.

496

587

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ZnSe_Mn_3

BAGA_080811_1

In Toluene

PL BA_081011

FF Detection

Exc. 1nm Em. 2nm

Excitation 418 nm

22 C

Inte

nsi

ty (

CP

S)

Wavelength (nm)

418

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16000000ZnSe_Mn_3

BAGA_080811_1

In Toluene

PL BA_081011

FF Detection

Exc. 1nm Em. 2nm

Emission 589 nm

22 C

S1 / R

1

QE measurements of ZnSe:Mn/ZnS NCs were performed using the Fluorolog-3 integrating

sphere attachment and a liquid sample holder. QE measurements at the 497 nm emission with

453 nm excitation resulted in a quantum yield of 91.26% (Fig. 3). At 587 nm emission with 418

nm excitation, QE was 39% (Fig. 4).

Fig. 3. PL QE measurement of ZnSe:Mn/ZnS NCs at 497 nm using Horiba Jobin Yvon Fluorolog 3

system with integrating sphere attachment.

Fig. 4. PL QE measurement of ZnSe:Mn/ZnS NCs at 587 nm using Horiba Jobin Yvon Fluorolog 3

system with integrating sphere attachment.

The high temperature emission from ZnSe:Mn/ZnS NCs was observed in contrast to

CdSe/ZnS NCs (Fig. 5), whose emission drops sharply with increasing temperature, and

practically disappears above 60 °C. High PL efficiency at elevated temperatures makes the

ZnSe:Mn/ZnS NCs particularly attractive for applications as nanophosphors in high-power white

LEDs. Abnormal thermal behavior comes from competing thermal factors, such as thermal

release of trapped charges and phonon-electron interaction [1].

Fig. 5. Temperature dependence of PL from ZnSe:Mn/ZnS NCs (blue) compared to CdSe/ZnS NCs

(black).

TEM images of the ZnSe:Mn/ZnS NCs sample were completed on the JEOL 2010 high-

resolution transmission electron microscope located in the labs of the Earth and Planetary

Science Department at UNM. This device uses the following systems to perform extremely

important forms of characterization: Gatan Orius digital camera system, Digital Micrograph system, and Oxford - Inca energy dispersive x-ray spectroscopy system. Unfortunately, at the

time these images were recorded the Oxford – Inca energy dispersive x-ray spectroscopy system

was under repair. TEM images are produced from the interaction between the electron beam and

the sample. The portion of the image that is bright, is where the electron beam is just passing thru

and not interacting with any material, but the dark areas are where the beam encounters material

that absorbs some of the electrons. These images were used to determine the size and the shape

of this NC material. The first image at 80k magnification (Fig.6) shows that these nanocrystals

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PL

In

ten

sity [C

PS

]

Temperature [C]

ZnSe:Mn/ZnS

CdSe/ZnS

are evenly dispersed. Depending on the orientation, some individual crystals may appear to have

different sizes and shapes, but most of these NCs are identical in size and shape.

Fig. 6. TEM image of ZnSe:Mn/ZnS NCs at 80k magnification. Scale bar 50 nm.

The next image in Fig. 7 gives a closer look at the individual crystals in this sample. At higher

magnification, it is possible to view fringes, which are the planes of atoms for the individual

crystals. The image in Fig. 7 is a good representation of this phenomenon. In this image at 1000k

magnification, fringes of single nanocrystals can be seen. These fringes can be measured using

fast Fourier transform (FFT) to determine the crystal structure for this sample.

Fig. 7. TEM image of ZnSe:Mn/ZnS NCs at 1000k magnification scale bar 2 nm.

The image in Fig. 8 at 300k magnification also shows some fringing on individual crystals. A

few larger particles are also visible in this image. These larger particles are not consistent with

the other particles in this image, because they may have a different orientation. These larger

particles are more likely multiple particles being either completely or partially on top of each

other. This can result in fringes that are cross hatched, depending on the orientation of the

individual crystals.

Fig. 8. TEM image of ZnSe:Mn/ZnS NCs at 300k magnification scale bar 10 nm.

In order to obtain images like these that show the fringing, it is necessary to use higher

magnification and to be slightly out of focus. Depending on the type of material that is being

imaged fringes can become visible around 250k to 300k magnification. The image in Fig. 9 is

another bright field image, but at a magnification of 100k. This image shows a smaller group of

the nanocrystals to provide another good representation of the size and shape of this material.

From this image, it is easier to see that the nanocrystals range from about 10 to 20 nm in size and

appear in this orientation to be rod shaped.

Fig. 9. TEM image of ZnSe:Mn/ZnS NCs at 100k magnification scale bar 20 nm.

The next type of characterization that will be shown also has to do with the size of

ZnSe:Mn/ZnS quantum dots. Dynamic light scattering (DLS) is another tool that is used in

characterizing nanomaterials. As the laser light hits the small particles in solution, light is

scattered and that scatter is collected by the detector and analyzed. The data is then returned in

the form of intervals of time and relative particle size that was detected. That data can then be

plotted in a histogram for graphical representation of the analysis. This process requires that the

particles in the sample to be less than 250 nm in size. It is also important that the sample is not

too concentrated, because this can overload the detector as more of the light will be scattered.

Once the sample was properly prepared, it was placed in a special DLS cuvette that was

unaffected by the toluene solution that the sample is suspended in. The graph below, which

resulted from the collected data, confirms that the sample contains nanoparticles that are

approximately 15 nm in diameter (Fig. 10).

Fig. 10. DLS histogram of ZnSe:Mn/ZnS NCs.

The final characterization method used on this material was X-ray Diffraction (XRD). This is

a process that uses different types of x-rays depending on the type of anode that is selected. The

anode is typically a copper anode with a wavelength of 1.54Å. However, depending on the type

of sample this can be changed to a number of different types of anodes to improve the collection

of the diffraction pattern. The x-rays from the anode interact with the sample as it passes thru a

series of 2θ angles, which, with a randomly orientated powder sample, will result in all possible

diffractions from the lattice. The peaks in the pattern at the given angles can be compared to

known material, therefore providing a relatively quick confirmation of the phases of materials

that are present in a given sample. The ZnSe:Mn/ZnS nanocrystals were analyzed and the

following data was recorded in the graph below (Fig. 11). This XRD pattern of ZnSe:Mn/ZnS

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

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Co

un

ts

Diameter [nm]

BAGA_080811_1

ZnSe:Mn/ZnS

NCs shows the phase match to Zn0.95Mn0.05Se, which confirms the material that was expected to

be present in the sample.

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Zin

c M

anganese S

ele

nid

e, (1

1 1

)

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c M

anganese S

ele

nid

e, (2

0 0

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c M

anganese S

ele

nid

e, (2

2 0

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c M

anganese S

ele

nid

e, (3

1 1

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c M

anganese S

ele

nid

e, (2

2 2

)

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c M

anganese S

ele

nid

e, (4

0 0

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c M

anganese S

ele

nid

e, (3

3 1

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c M

anganese S

ele

nid

e, (4

2 0

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c M

anganese S

ele

nid

e, (4

2 2

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c M

anganese S

ele

nid

e, (5

1 1

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c M

anganese S

ele

nid

e, (4

4 0

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c M

anganese S

ele

nid

e, (5

3 1

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c M

anganese S

ele

nid

e, (6

0 0

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c M

anganese S

ele

nid

e, (6

2 0

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c M

anganese S

ele

nid

e, (5

3 3

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c M

anganese S

ele

nid

e, (6

2 2

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Zin

c M

anganese S

ele

nid

e, (4

4 4

)

Zin

c M

anganese S

ele

nid

e, (1

1 1

)

Zin

c M

anganese S

ele

nid

e, (2

0 0

)

Zin

c M

anganese S

ele

nid

e, (2

2 0

)

Zin

c M

anganese S

ele

nid

e, (3

1 1

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Zin

c M

anganese S

ele

nid

e, (2

2 2

)

Zin

c M

anganese S

ele

nid

e, (4

0 0

)

Zin

c M

anganese S

ele

nid

e, (3

3 1

)

Zin

c M

anganese S

ele

nid

e, (4

2 0

)

Zin

c M

anganese S

ele

nid

e, (4

2 2

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Zin

c M

anganese S

ele

nid

e, (5

1 1

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Zin

c M

anganese S

ele

nid

e, (4

4 0

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Zin

c M

anganese S

ele

nid

e, (5

3 1

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c M

anganese S

ele

nid

e, (6

0 0

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c M

anganese S

ele

nid

e, (6

2 0

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Zin

c M

anganese S

ele

nid

e, (5

3 3

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c M

anganese S

ele

nid

e, (6

2 2

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c M

anganese S

ele

nid

e, (4

4 4

)

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Inte

gra

ted Inte

nsity (

cps d

eg)

2-theta (deg)

Meas. data:BAGA_080811_1 ZnSe_Mn_T

heta_2-Theta/Data 1

Zinc Manganese Selenide,Zn0.950 Mn0.0

5 Se,01-079-0014

Inte

nsity (

cps)

Zinc Manganese Selenide, Zn0.950 Mn0.05 Se, 01-079-0014

Fig. 11. XRD pattern of ZnSe:Mn/ZnS NCs showing phase match to Zn0.95Mn0.05Se.

The polymerization of the ZnSe:Mn/ZnS NCs in poly methyl methacrylate (PMMA) first

required the material to be dried in the oven at 110 ˚C, after the sample was synthesized and

dispersed in toluene. A 7 mL vial was weighed and then 1 mL of the ZnSe:Mn/ZnS NCs were

added, and the vial was placed in the oven to dry. Once the sample was completely dried, the vial

was weighed again and compared to the first weight to determine the weight of the nanocrystals

that were dispersed in the polymer. Then the vial was transferred into the glove box for the NCs

to be dispersed into the distilled monomer solution. The monomer was distilled in the glove box

by passing methyl methacrylate (MMA) dropwise thru the inhibitor removal tube (Hydroquinone

(HQ) and Monomethyl Ether Hydroquinone (MEHQ) Removal Column) and into a beaker.

0.0025g of initiator was added to the dried sample and then a 10mL syringe was used to add 5

mL of distilled monomer into the vial. The vial was capped and removed from glove box and

placed in sonication until the sample was fully dispersed in the monomer. Once the sample was

fully dispersed in the monomer the vial was placed in the oven at 60 ˚C. The sample was

checked regularly and sonicated in order to prevent bubbles from forming. After the sample

remained in the oven for 24 hrs, the sample had not completely polymerized, so the temperature

was increased to 70 ˚C. The sample was checked regularly, and after 24 hrs at 70 ˚C the sample

was completely polymerized.

Once the sample was polymerized, it was cut and polished to enable front face PL

measurements to compare to previous PL measurements of the sample in solution. These

measurements were also done using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. This

comparison was to provide information on whether or not polymer encapsulation enhanced PL

for this sample. The polymerized sample was analyzed on the PL setup using a solid sample

holder and using the same excitation and emission slit widths as it was done for the sample in

solution (Fig. 12).

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nsity (

CP

S)

Wavelength (nm)

498 nm

523 nm

ZnSe:Mn_3

BANC_040912_1

In PMMAFF Detection

Exc. 1 nm Em. 1 nm

Excitation 453 nm

Fig. 12. PL spectra of ZnSe:Mn/ZnS NCs encapsulated in PMMA at 453 nm excitation.

Under these same settings, the sample in PMMA did not provide as strong of a signal as the

sample in solution. Although the peak locations are very much the same with only a small

variation, the peak intensity has dropped from approximately 700,000 counts per second (CPS)

to below 100,000 CPS. The graph in Fig. 13 shows that with the 418 nm excitation the results

were also not ideal in comparison.

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nsity (

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S)

Wavelength (nm)

593 nm

459 nm

ZnSe:Mn_3

BANC_040912_1

In PMMAFF Detection

Exc. 1 nm Em. 1 nm

Excitation 418 nm

Fig. 13. PL spectra of ZnSe:Mn/ZnS NCs encapsulated in PMMA at 418 nm excitation.

This graph obtained under the same settings as for the sample in solution shows a greater shift

in the peaks location and intensity. The original peaks locations were 496 nm and 587 nm. The

original peak intensity for the 496 nm peak was approximately 100,000 CPS, while the 587 nm

peak was at 550,000 CPS. However, in the PMMA the first peak is a broader peak with a

maximum at 459 nm and intensity at 150,000 CPS. The second peak being at 593 nm is still

close to the original 587 nm mark, but the intensity dropped to 275,000 CPS. With the results

that have been obtained during this project, other polymers that may be able to enhance PL of

ZnSe:Mn/ZnS NCs will be investigated.

As this project continues, the next polymer that will be investigated will be PVT (polyvinyl-

toluene), while other types are being researched. Although the PMMA did not prove to be a

beneficial choice, the sample itself turned out to be a great success. With quantum efficiencies as

high as 91% and multiple peaks with a single excitation, ZnSe:Mn/ZnS NCs may in the near

future provide us with a single material that can produce day-light quality LEDs. Work will be

continued on this project to improve the efficiency of these materials.

References:

[1] J.S. Park, S.W. Mho, and J.C. Choi, “Abnormal thermal properties of ZnS:Mn2+

Nanophosphor”, J. Korean Phys. Soc. 50 (#3), pp. 571-574 (March 2007).