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Graphene Quantum Dots Embedded in a Polymer Film S. Ushiba 1,2 , X. He 2 , E. Haroz 2 , J. Peng 3 , P. M. Ajayan 3 and J. Kono 2 1 NanoJapan Program and Department of Applied Physics, Osaka University 2 Department of Electrical and Computer Engineering, Rice University 3 Department of Mechanical Engineering and Materials Science, Rice University Graphene quantum dots (GQDs) are a few nanometer sized pieces of graphene sheet that exhibit photoluminescence (PL) in the visible region, as well as unique electron transport and superior mechanical properties. The PL originates from large edge effects and exhibits strong quantum confinement effects. The intrinsic characteristics of GQD can be applied to various devices, e.g., light emitters, optoelectronic devices and photovoltaics. The technique to embed GQDs in a solid material is indispensable for realizing GQD-based devices. Here we report a simple method that produces a GQD-polymer composite film by drying a mixture solution of GQD and poly-vinyl alcohol (PVA). GQDs were homogeneously dispersed in the film, which was measured by absorption and Raman spectroscopy. The PL of GQDs in the film was also investigated. The simple technique enables the development of GQD-based composites for numerous applications of GQDs.

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Page 1: Graphene Quantum Dots Embedded in a Polymer Filmterananopire.rice.edu/Downloads/2011 RQI/2011_Ushiba...Graphene Quantum Dots Embedded in a Polymer Film S. Ushiba1,2, X.He2, E. Haroz2,

Graphene Quantum Dots Embedded in a Polymer Film

S. Ushiba1,2, X. He2, E. Haroz2, J. Peng3, P. M. Ajayan3 and J. Kono2

1NanoJapan Program and Department of Applied Physics, Osaka University 2Department of Electrical and Computer Engineering, Rice University

3Department of Mechanical Engineering and Materials Science, Rice University

Graphene quantum dots (GQDs) are a few nanometer sized pieces of graphene sheet

that exhibit photoluminescence (PL) in the visible region, as well as unique electron

transport and superior mechanical properties. The PL originates from large edge effects

and exhibits strong quantum confinement effects.  The intrinsic characteristics of GQD can

be applied to various devices, e.g., light emitters, optoelectronic devices and photovoltaics.  

The technique to embed GQDs in a solid material is indispensable for realizing GQD-based

devices.  Here we report a simple method that produces a GQD-polymer composite film by

drying a mixture solution of GQD and poly-vinyl alcohol (PVA).   GQDs were

homogeneously dispersed in the film, which was measured by absorption and Raman

spectroscopy.  The PL of GQDs in the film was also investigated.  The simple technique

enables the development of GQD-based composites for numerous applications of GQDs.

Page 2: Graphene Quantum Dots Embedded in a Polymer Filmterananopire.rice.edu/Downloads/2011 RQI/2011_Ushiba...Graphene Quantum Dots Embedded in a Polymer Film S. Ushiba1,2, X.He2, E. Haroz2,

Graphene Quantum Dots Embedded in a Polymer Film 1 NanoJapan Program and Department of Applied Physics, Osaka University

2 Department of Electrical and Computer Engineering, Rice University3 Department of Mechanical Engineering and Materials Science, Rice University

S. Ushiba1,2, X. He2, E. Haroz2, J. Peng3, P. M. Ajayan3 and J. Kono2

We aim to develop a simple method that produces a GQD-polymer composite. Here we demonstrate the composite film by drying a mixture solution of GQD and poly-vinyl alcohol (PVA). The PL of GQD/PVA composite film and the temperature-dependence of PL intensity are investigated.

Graphene Quantum Dots (GQDs) are a few nanometer s ized pieces of graphene sheet that exhibi t photoluminescence (PL) in the visible region due to size effects and quantum confinements, as well as unique electron transport and superior mechanical properties.

Purpose: To develop a GQD/Polymer composite

Introduction: Graphene Quantum Dots

Absorption and Photoluminescence of GQDs in waterAbsorption of GQDs Photoluminescence of GQDs

Daylight UV light

GQDs in water

The absorbance peak at 310 nm is a feature of GQDs due to quantum confinement effects and edge effects.

GQDs exhibit PL in the visible region, which depends on the excitation wavelength. The dependence of PL could be attributed to the size effect of GQDs.

Excitation wavelength

PL of the GQDs at 595 nm was suppressed when the GQDs were embedded in the film. The intensity decrease is probably related to the decrease of the concentration, the aggregation and in particular energy transfer to the polymer.

PL characterization of GQD/PVA composite film

Fabrication Procedure: GQD/PVA composite film

Stir

PVA solutionPVA 1.5 g

DI water10 ml

2. Water-dissolved PVA1. Chemical Synthesis of GQDsGQDsGraphite

Cut down Neutralization

3. GQD/PVA composite film

Stir GQD/PVA filmGQD/PVA

Dry&PeelGQDs PVA

CastGQD/PVA

Substrate

3hH2SO4, HNO3 NaOH

1h 2 days Diameter ~1 cmThickness ~1 mm

Density Gradient Ultracentrifugation (DGU) for the size-separation

Emission Wavelength [nm]

Exci

tatio

n W

avel

engt

h [n

m]

Emission Wavelength [nm]

Exci

tatio

n W

avel

engt

h [n

m]

Before DGU After DGU

Before

After

519

508300x10

3

200

100PL intensity

700650600550500450Wavelength [nm]

PL spectra at 380 nm

PL peaks were significantly blue-shifted by 10 nm after DGU. DGU could remove large-diameter GQDs and/or aggregated GQDs.

GQDs in density gradient medium

AcknowledgementThis research was sponsored by Rice University and the NSF Grant (OISE-0968405). I would like to thank Saunab Ghosh for the PL measurements and the Kono, Ajayan, and Kawata groups for their kind advice. In addition, I would like to express gratitude to all of the NanoJapan students.

The overall PL intensity increased with decrease temperature. The enhancement originated from the suppression of thermal quenching.

PVA: poly vinyl alcohol

208,000 g18 h

GQDs in 30 % iodixanol

40%30%

27.5%

25%22.5%20%Iodixanol

E. Haroz et.al., ACS Nano, 4, 1955 (2010)

310 nm: a feature of GQDs

200 nm:

Wavelength [nm]

Abso

rban

ce

π-π* transition ofgraphene

0.10

0.08

0.06

0.04

0.02

800600400200

Intensity

400x103

300

200

100

Emission Wavelength [nm]

Exci

tatio

n W

avel

engt

h [n

m]

440 nm400 nm360 nm

390

380

370

360

350

700650600550500450

390

380

370

360

350

700650600550500450

440

400

360

320

280700650600550500

2

www.advmat.dewww.MaterialsViews.com

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, XX, 1–5

COM

MUN

ICATI

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wileyonlinelibrary.com

of high quality few-layer graphene nanoribbons, [ 22 , 23 ] which not only indicates the high quality of as-prepared GQDs, but also the uniqueness of the electrochemical method developed here for GQD preparation.

XPS measurements were carried out to probe the composi-tion of as-prepared GQDs. As can be seen in Figure 2c , the XPS results show a predominant graphitic C 1s peak at 284.8 eV and an O 1s peak at ca. 532 eV for both the initial graphene fi lm and the as-prepared GQDs (Figure 2c ). The higher O 1s peak rela-tive to the corresponding C 1s peak seen for the GQDs (com-pared to the graphene fi lm) suggested that oxidation reaction took place during electrochemical cycling for GQD preparation. In comparison with graphene fi lm, Figure 2d clearly revealed the wider C 1s peak of GQDs to be associated with the oxygen-containing groups. A high-resolution spectrum of C 1s (Inset in Figure 2d ) confi rmed the presence of C = C (284.8 eV), C–O (286.8 eV), C = O (287.8 eV) and COOH (289 eV) bonds, [ 24 ] indi-cating the as-prepared GQDs were rich in hydroxyl, carbonyl and carboxylic acid groups on the surfaces.

Figure 1 . a,b) TEM images of as-prepared GQDs with different magnifi cation. c) Size distribution of GQDs. d) AFM image of the GQDs on Si substrate. e) The height profi le along the line in (d). f,g) UV–vis absorption and photoluminescence spectra of GQDs in water, respectively; inset in (f) is a photo of GQD aqueous solution under UV irradiation (365 nm).

350 400 450 500 550 600 650 700 7500

50000

100000

150000

200000

250000

300000

340 nm 360 nm 380 nm 400 nm 420 nm 440 nm 460 nm 480 nm 500 nm 520 nm

PL

Inte

nsity

(a.u

.)

Wavelength (nm)300 400 500 600

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

320 nm

(f) (g)

of GQDs is awaiting discovery, the distinctiveness of GQDs has emerged.

Figure 2a shows typical XRD profi les for the original graphene fi lm and the as-prepared GQDs. Compared with the graphene fi lm, the GQDs have a broader (002) peak cen-tered at around 25 o , indicating the electrochemical process has introduced more active sites along the surfaces of GQDs, as confi rmed by XPS (Figure 2d ) and FT-IR spectroscopy inves-tigations (Figure S3 in the Supporting Information). The XRD peak for as-prepared GQDs shifted to a higher degree com-pared with the graphene fi lm, indicating that the GQDs had a more compact interlayer spacing than the original graphene fi lm did. Raman spectroscopy (Figure 2b ) confi rms the quality of the as-prepared GQDs. The major Raman features are the D band at around 1365 cm ! 1 and the G band at around 1596 cm ! 1 . Unlike the GQDs synthesized hydrothermally with a much more intense D band, [ 18 ] the relative intensity of the “disorder” D-band and the crystalline G-band ( I D / I G ) for as-produced GQDs in this work is only around 0.5, similar to that

20 nm

(nm)

Frac

tion

(%)

0

10

20

30

2 4 6

TEM image of GQDs

Email: [email protected]

PL spectra at 514.5 nm excitation Temperature-dependence of the PL intensity

PL intensity (a.u.)

640620600580560540520

Wavelength [nm]

595

605

GQD/PVA film

PVA

GQD in water

GQD/PVA solution

14x103

12

10

8

6

4

2

PL intensity

640620600580560540520

Wavelength [nm]

-60 ℃-45 ℃-30 ℃-15 ℃0 ℃25 ℃

GQD/Polymer composites were developed by the simple method. The PL from GQD/PVA composite film was suppressed, but enhanced with decrease of temperature.

Summary

Y. Li et.al., Adv.Mater.,23, 776 (2011)

D. Valerini et.al., Phys. Rev. B, 71, 235409 (2005)Y. Sun et.al., Acc. Chem.Res., 35, 1096 (2002)