Optoelectronic properties of graphene thin films prepared by thermal reduction of graphene oxide Jae Seong Kima, Eui Jung Kima,⁎ a Department of Chemical Engineering, University of Ulsan,
Trang 1Optoelectronic properties of graphene thin films prepared by thermal reduction of graphene oxide
Jae Seong Kima, Eui Jung Kima,⁎
a
Department of Chemical Engineering, University of Ulsan, Ulsan 680-749, South Korea
b
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
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 20 August 2009
Accepted 6 January 2010
Available online 13 January 2010
Keywords:
Graphene oxide
Thin films
Luminescence
Optical materials and properties
Graphene thinfilms have been prepared by thermal reduction of graphene oxide Raising the reduction temperature results in a red-shift of the G peak in Raman spectra The reduction temperature turns out to strongly affect the morphology of the prepared graphenefilm Photoluminescence (PL) results show that the band gap of graphene can be tuned by varying the reduction temperature The thermal reduction process has been optimized in an effort to minimize the formation of wrinkles/folds on the graphene surface leading to enhanced PL and Raman peak intensities and reduced electrical sheet resistance
© 2010 Elsevier B.V All rights reserved
1 Introduction
Graphene has increasingly attracted attention owing to its
fascinating physical properties including quantum electronic
trans-port, tunable band gap, extremely high mobility, high elasticity, and
electromechanical modulation[1] These unique properties hold great
promise for potential applications in many technologicalfields such as
nanoelectronics, sensors, nanocomposites, batteries, supercapacitors
and hydrogen storage[2] In order to turn graphene applications into
reality, one must fabricate the material in large-scale In the
beginning, isolated graphene was prepared by the micromechanical
cleavage of graphite crystals However, the low productivity of this
method makes it unsuitable for large-scale use Recently, an
alternative method for creating single graphene sheets starting from
graphene oxide (GO) which demonstrated the possibility of low-cost
synthesis and the fabrication of large-scale transparentfilms has been
suggested Graphite can be oxidized to produce GO and then
exfoliated to create stable aqueous dispersions of individual sheets
After deposition, GO may be reduced to graphene either chemically or
thermally[3] Although there are a number of reports on the synthesis
of graphenefilms, very few studies have been done to systematically
explore the effect of fabrication condition on the electrical and optical
properties of graphene To the best of our knowledge, the
photo-luminescence (PL) spectra of graphenefilms have not been reported
yet In this letter, we investigate the optoelectronic properties of
graphene thinfilms prepared by thermal reduction of GO Especially,
PL measurements were carried out to analyze the optical property of thefilms
2 Experimental The graphene thin films investigated in this work have been obtained by thermal reduction of GO in a quartz tube To study the optoelectrical properties of the graphenefilm, the quartz tube was heated to temperatures ranging from 600 °C to 800 °C at 10− 6Torr for
1 h The GO films were prepared by spin-coating GO paste-like dispersion on quartz substrate at 4000 rpm for 15 s The GO paste-like dispersion was produced via a modified Hummers method from expanded graphite which was prepared by the microwave-assisted thermal expansion of graphite (Grade 1721, Asbury Carbon) The surface morphology of thefilms was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) The structure and bonding configurations in the films were examined using a Laser Raman Spectrometer SPEX 1403 with a He–Ne laser at an excitation wavelength of 632.8 nm PL measurement was performed employing
a SpectraPro-300i monochromater (Acton) The samples were excited
by using a 325 nm He–Cd laser with an output of 10 mW power at room temperature The sheet resistance was determined byfitting current–voltage (I–V) curves obtained from two probes station using Keithley 4200 semiconductor character system (MS Tech)
3 Results and discussion The SEM and AFM images of the graphene thinfilms prepared by thermal reduction of GO at three different temperatures are shown in
⁎ Corresponding author Tel.: +82 52 259 2832; fax: +82 52 259 1689.
E-mail address: ejkim@ulsan.ac.kr (E.J Kim).
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Trang 2Fig 1 The SEM images exhibit that the prepared graphenefilms are
uniform with slight wrinkles/folds on their surface The thermal
reduction process may reduce the contact area of GO with the quartz
substrate Consequently, the graphene sheets tend to aggregate due to
the attractive force between layers and an overall decrease in
hydrophilicity, thus resulting in the formation of wrinkles/folds One
can also clearly see graphene wrinkles/folds in AFM images The
surface roughness is found to depend largely on the reduction
temperature and the smooth surface is achieved for the sample
reduced at 700 °C These results imply that the reduction temperature
has a strong effect on the surface features Accordingly, the thermal
reduction process has been optimized to obtain graphene thinfilms
with a mirror-like surface
Fig 2(a) shows the Raman spectra of GO and graphene thinfilms
prepared by thermal reduction of GO at three different temperatures
Two major peaks are observed referred to as D and G Two minor
peaks named 2D located at∼2655 cm− 1and S3 near 2906 cm− 1are
also observed in the as-prepared GO sample and vanish in the
post-reduced samples This implies that the graphenefilms prepared by
thermal reduction of GO have considerable defects In addition, the G
band of the samples is red-shifted from 1594 cm− 1to 1572 cm− 1
when the reduction temperature increases from 600 °C to 800 °C,
which is consistent with the results reported by other groups[4,5]
These results indicate that a red-shift of the G peak results from heat
treatment during the thermal reduction process Raising the reduction temperature results in a red-shift of the G peak Initially, there are small sp2clusters in the GO separated by an amorphous and highly disordered sp3matrix, which forms a high tunnel barrier between the clusters During the heat treatment, the thermal energy facilitates clustering of sp2phase forming connection between ordered rings and phase transition from amorphous to the two-dimensional nanocrystalline graphene This transformation results in a red-shift and broadening of the G band Note inFig 2(a) that the sample reduced at 700 °C exhibits the maximum peak intensity The Raman peak intensity is found to correlate well with the surface roughness Until now, very little is known about the PL of graphene thinfilm, which is an important property for application in optoelectronic devices.Fig 2(b) represents the PL spectra of graphene thin films prepared by thermal reduction of GO at three different temperatures
It is somewhat surprising to note that the PL spectra for the thermally reduced GO films are quite similar to what is observed from nanostructured amorphous carbon For instance, inFig 2(b), three broad visible peaks located at 512, 572, and 627 nm are bluer than previous reports[6,7] It is well known that the PL in amorphous carbon occurs due to the radiative recombination of electrons and holes in the band-tail states created by sp2rich clusters However, the
sp2 clusters in the thermally reduced GO films are small in size compared with highly disordered sp3matrix, which creates a very
Fig 1 SEM and AFM images of graphene thin films (2 µm × 2 µm) prepared by thermal reduction of GO at three different temperatures: (a)–(b) 600 °C, (c)–(d) 700 °C, and (e)–(f) 800 °C.
Trang 3strongfluctuation in the local band gap Thus, sp2clusters with a
narrower gap are embedded in a sp3matrix which acts as a tunnel
barrier between them and causes a blue-shift in the PL spectra The
sample reduced at 700 °C shows a stronger PL peak intensity with a
broader full width at half-maximum (FWHM) than the samples
reduced at 600 °C and 800 °C This broad and strong intensity at
700 °C may be related to the mirror-like surface morphology In fact,
thefilms reduced at 600 °C and 800 °C have rougher surface and
larger clustering than thefilm reduced at 700 °C as confirmed in SEM,
AFM images, and Raman spectra Although the mechanism of light
emission observed in the graphenefilm remains to be explored, a
significant finding in this study indicates that thermal reduction of GO
might result in strained, disordered graphenefilm, which enables us
to potentially tune the band gap of graphene
I–V curves of graphene thin films prepared by thermal reduction of
GO at different reduction temperatures are depicted inFig 3 The
sheet resistances of our graphenefilms prepared by thermal reduction
of GO at 600 °C, 700 °C and 800 °C are found to be 14 kΩ, 5.7 kΩ and 7.8 kΩ, respectively, which are determined by a linear fitting of I–V curves Therefore, the reduction temperature has also a significant effect on the electrical property of the graphene film The sheet resistance becomes a minimum at 700 °C where the graphenefilm has smooth surface morphology A relatively high sheet resistance of our samples may result from extensive modification of the 2-dimensional crystal lattice in the preparation of the solution-based graphenefilm, which usually degrades electrical and thermal conductivities[8]
4 Conclusions
In summary, GO thinfilm was thermally reduced to form graphene thinfilm Under heat treatment, the G peak in Raman spectra was red-shifted and higher reduction temperature caused a more red-shift of the G peak The SEM and AFM results indicated that the reduction temperature has a strong effect on the surface morphology via the formation of wrinkles/folds The maximum PL and Raman peak intensities and the minimum electrical sheet resistance were achieved for the sample reduced at 700 °C The thermal reduction process was optimized to reduce the formation of wrinkles/folds which enables us
to tune the band gap and improve the optoelectronic properties of graphene thinfilms
Acknowledgement This work was supported by the EXCEED program of the University
of Ulsan
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Fig 3 Current–voltage curves of graphene thin films prepared by thermal reduction of GO.
Fig 2 (a) Raman spectra of GO and graphene thin films prepared by thermal reduction
of GO at different temperatures, and the inset on the top right corner indicating the
variation of G peak position with reduction temperature, (b) room temperature PL
spectra of grapheme thin films prepared by thermal reduction of GO.