DSpace at VNU: Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide

5, Ho Chi Minh City, Vietnam 3 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, USA Received 17 June 2011; accepted 6 July

Temperature-dependent photoluminescence from chemically and thermally reduced

graphene oxide

Tran Viet Cuong, Viet Hung Pham, Eun Woo Shin, Jin Suk Chung, Seung Hyun Hur, Eui Jung Kim, Quang Trung Tran, Hoang Hung Nguyen, and Paul A Kohl

Citation: Applied Physics Letters 99, 041905 (2011); doi: 10.1063/1.3616142

View online: http://dx.doi.org/10.1063/1.3616142

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/4?ver=pdfcov

Published by the AIP Publishing

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Temperature-dependent photoluminescence from chemically and thermally

reduced graphene oxide

Tran Viet Cuong,1Viet Hung Pham,1Eun Woo Shin,1Jin Suk Chung,1Seung Hyun Hur,1

Eui Jung Kim,1,a)Quang Trung Tran,2Hoang Hung Nguyen,2and Paul A Kohl3,b)

1

School of Chemical Engineering and Bioengineering, University of Ulsan, Daehak-ro 102, Nam-gu, Ulsan

680-749, South Korea

2

Department of Solid State Physics, Faculty of Physics, Ho Chi Minh City University of Natural Sciences, 227

Nguyen Van Cu St., Dist 5, Ho Chi Minh City, Vietnam

3

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332-0100, USA

(Received 17 June 2011; accepted 6 July 2011; published online 27 July 2011)

Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine

and heat has been measured to investigate the effect of reduction type on the bandgap of the

reduced GO Nitrogen functionalities formed in the hydrazine-treated GO were responsible for a

strong localization of carriers that caused in a fluctuation in PL peak position with temperature

The intensity of C-OH peak was relatively low in the heat-treated GO, indicating that raising

temperature facilitated the removal of hydroxyl groups, resulting in larger sp2 domain size and

smaller bandgap energy.V C 2011 American Institute of Physics [doi:10.1063/1.3616142]

Although graphene with two-dimensional sheet of

sp2-hybridized carbon becomes promising material,

gra-phene-based applications are still limited since defect-free

graphene exhibits zero bandgap.1 Recently, many attempts

have been made to open the bandgap of graphene For

exam-ple, Wang and co-workers reported photoluminescence (PL)

from graphene fluoride.2 Nourbakhsh et al.3 opened the

bandgap of graphene by using oxygen plasma-treatment, and

Lohet al.4explored PL from chemically modified graphene

such as graphene oxide (GO) or reduced GO whose

elec-tronic structure was modified GO reduction is considered as

an efficient method because this approach is a simple way to

vary the size, shape, fraction of the sp2 domains, its edge

structure, and chemical modification of its interior or edge,

which enables us to modulate the bandgap of graphene In

addition, this method is often based on the solution process

that is inexpensive, facile, scalable, and high yield for

mass-production Despite the above advantages of GO reduction

to open the optical gap of graphene, the effect of reduction

type on the optical bandgap of the reduced GO is not still

clearly understood It is believed that a better understanding

of the reduction mechanism helps open up an opportunity for

tailoring the bandgap of graphene

In this work, temperature-dependent PL was measured

from GO reduced by two different processes: hydrazine and

thermal treatment Temperature-dependent PL spectra can be

effectively used to elucidate the emission origin.5X-ray

pho-toelectron spectroscopy (XPS) study was also performed to

investigate the effect of reduction type on the chemical

struc-ture of the reduced GO

Thermally reduced graphene (TRG) film was prepared

by reducing GO film in a quartz tube at 700C and 106

Torr for 1 h The GO film was obtained by spin-coating GO

dispersion on a quartz substrate at 4000 rpm for 15 s The

GO dispersion was synthesized via a modified Hummers method from expanded graphite which was prepared by microwave-assisted thermal expansion of graphite (Grade

1721, Asbury Carbon).6 Chemically reduced graphene (CRG) film was prepared as follows: the GO dispersion was reduced with excess hydrazine monohydrate at room temper-ature to form graphene dispersion The use of excess hydra-zine makes the dispersion basic to provide a high charge density on the resulting graphene sheets After 4 h reduction, the graphene dispersion was diluted 75 times with water/ ethanol (80:20 v/v) and then homogenized by ultrasonication for 2 min The diluted solution was finally sprayed on a pre-heated quartz substrate to obtain the CRG thin film using an airbrush system with N2as a carrier gas The prepared films were characterized using an XPS Thermal Scientific Instru-ment with a monochromatic AlKa radiation (hk¼ 1486.6 eV) The PL spectra was taken from 10 K to 300 K on a SpectraPro-300i Monochromator (Acton) using He-Cd laser source with a wavelength of 325 nm and an output of 10

MW power

Figure1shows the temperature-dependent PL spectra of CRG and TRG films The PL spectra of the reduced GO thin films are quite similar in shape to what has been observed from nanostructured amorphous carbon and GO suspen-sions,7,8which are attributed to the recombination of elec-tron-hole pairs localized within small sp2 carbon clusters embedded within a sp3matrix and to the presence of oxy-gen-containing functional groups, respectively It is worth-while to note that the peak position of the CRG sample is blue-shifted with fluctuation as the temperature increases from 10 K to 300 K, while that of the TRG sample remains almost unchanged This result indicates that a carrier locali-zation effect in sp2clusters plays a dominant role in emission peak shift over temperature-induced bandgap shrinkage effect that causes a red-shift of the PL peak It appears that the carriers generated in the TRG sample are not

a) Author to whom correspondence should be addressed Electronic mail:

ejkim@ulsan.ac.kr.

b)

Electronic mail: paul.kohl@chbe.gatech.edu.

APPLIED PHYSICS LETTERS 99, 041905 (2011)

thermalized, possibly due to a strong carrier localization

effect Our interesting observation suggests that the optical

emission from the reduced GO may result from the

recombi-nation of electron-hole pairs in localized electronic states

originating from various structure-related defects, rather than

from band-edge transitions as is the case in typical

semicon-ductor materials In addition, at room temperature (300 K),

the peak of the CRG film is located at a lower wavelength

than that of the TRG film A possible explanation would be

that TRG has a larger sp2domain than CRG, resulting in a

smaller bandgap due to a relatively weak carrier

confine-ment The PL results in Fig.1reveal that the shape and shift

of PL emission peak depend largely on reduction type

Accordingly, the bandgap of the reduced GO can be tuned

by varying sp2domain size or by introducing various

func-tional groups

The model structures of GO, CRG, and TRG are

ske-matically illustrated in Fig.2, which is based on the

theoreti-cal work by Gaoet al.9It is known that there exist four kinds

of oxygen functionalities in GO (Lerf-Klinowski model):

epoxide (-O-) and hydroxyl (-OH) located in the basal plane

of GO are the major components and carbonyl (-C¼O) and

carboxyl (-COOH) distributed at the edges of GO are minor

A noticeable difference between hydrazine and thermal reductions are as follows: In the case of hydrazine reduction, nitrogen functionalities are formed such as aminoaziridine and hydrazone, which may be responsible for a strong local-ization of carriers due to the incorporation of nitrogen In the case of thermal reduction at a high temperature of 700C, hydroxyl groups attached to the interior and edge of an aro-matic domain are almost eliminated from GO Thus, the TRG film reduced at high temperature may have larger gra-phitic crystallite size, i.e., larger sp2domain size, than the CRG film The ratio of the D peak intensity to the G peak in-tensity (ID/IG) in the Raman spectra is known to vary inver-sely with in-plane graphitic crystallite size (La) which is given as La¼ 4.4 (IG/ID).10,11 Our previous work revealed that the ID/IG ratio of TRG (1.41) was smaller than that of CRG (1.48),12 indicating that the sp2 domain size of TRG (3.12 nm) is larger than that of CRG (2.97 nm) From the PL spectra taken at room temperature in Fig 1, the optical gap

of the TRG sample (1.99 eV) was found to be lower than that of the CRG sample (2.23 eV) Our results confirm that a larger sp2 domain results in a smaller optical gap An XPS analysis below was performed to verify the PL and Raman interpretations

FIG 1 (Color online) PL spectra taken at tempera-tures from 10 K to 300 K for (a) CRG and (b) TRG.

FIG 2 (Color online) Schematic illustration of the structures of GO, CRG, and TRG.

Figure 3exhibits the C1s and O1s XPS spectra of GO,

TRG, and CRG The C1s XPS spectrum of GO consists of

three separated peaks related to oxygen functionalities such

as hydroxyl, epoxide, and carbonyl After thermal or

hydra-zine reduction, the oxygen functionalities attached to the

inte-rior of an aromatic domain in GO are significantly removed,

which is indicated by a dramatic decrease in peak intensity

A peak at 285.9 eV corresponding to carbon-nitrogen bond

appeared in the C1s spectrum of the CRG sample The

appearance of this peak originates from the conversion of

epoxide into aminoaziridine and of carbonyl into hydrazone

when hydrazine is used as a reducing agent.13 The presence

of these nitrogen functionalities can explain the incorporation

of nitrogen into the reduced GO The incorporated nitrogen is

likely to act as an active defect site to capture carriers at

non-radiative recombination centers, thus resulting in a strong

localization of carriers.14,15This result is well consistent with

the peak fluctuation in the PL spectra of the CRG sample in

Fig.1(a) Combined PL and XPS results suggest that CRG

has a strong carrier localization effect compared to TRG

because of the presence of nitrogen functionalities Fig.3(b)

illustrates the deconvolution of the O1s peaks for GO, TRG,

and CRG The O1s peak is composed of three distinct peaks

assigned to (1): O¼C–OH, (2): C¼O, and (3): C–OH groups

The intensity of the C¼O peak is found to be much greater

than that of the C–OH peak for the TRG sample This

signi-fies that most of the carbonyl groups are not removed with

thermal treatment, while the hydroxyl groups are almost

removed In the case of CRG, the intensity of the C–OH

peak is comparable to that of the C¼O peak, indicating that a

considerable amount of C–OH and C¼O groups still remain

in the CRG sample A shake-up satellite peak observed in the O1s XPS spectra of the CRG sample results from p!p* tran-sition.16 The XPS results favourably support the proposed structures of CRG and TRG in Fig 2 Hydroxyl groups located at the edge of an aromatic domain are readily removed from GO with heat treatment, resulting in lager sp2 domain size, while the hydrazine-mediated reduction forms nitrogen functionalities Our observations are in good agree-ment with previous reports by other groups.9,13

In summary, the temperature-dependent PL from CRG and TRG has been investigated Our results demonstrated that optical emission from the reduced GO results from recombination of electron-hole pairs in localized electronic states, rather than from band-edge transitions The bandgap

of the reduced GO can be modulated by chemical or thermal reduction because the bandgap can be tuned by varying sp2 domain size or by incorporating various functional groups The nitrogen functionalities such as aminoaziridine and hy-drazone formed in hydrazine-treated GO are responsible for

a strong fluctuation in the local band gap In the case of heat-treated GO, hydroxyl groups located at the edge of an aro-matic domain are eliminated from GO, resulting in larger sp2 domain size and smaller bandgap

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FIG 3 (Color online) XPS spectra of GO, TRG, and CRG: (a) C1s peaks

and (b) O1s peaks.