C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

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UV-assisted production of ferromagnetic graphitic quantumdots from graphite

Akshaya Kumar Swain a, Dan Li b, Dhirendra Bahadur c,*

a IITB Monash Research Academy, Department of Metallurgical Engineering and Materials Science, IIT-Bombay, Mumbai 400076, Indiab Department of Materials Engineering, Monash University, VIC 3800, Australiac Department of Metallurgical Engineering and Materials Science, IIT-Bombay, Mumbai 400076, India

A R T I C L E I N F O

Article history:

Received 31 August 2012

Accepted 30 January 2013

Available online 8 February 2013

0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.01.082

* Corresponding author: Fax: +91 22 2572 348E-mail address: dhirenb@iitb.ac.in (D. Bah

A B S T R A C T

Graphitic quantum dots (GQDs) are synthesized from natural graphite powder. This process

involves a few steps such as oxidation, reduction and filtration to obtain the precursor to

prepare GQDs. Finally, a combination of UV irradiation and sonication is used to produce

GQDs. These quantum dots are further investigated by various characterization tech-

niques. They exhibit blue luminescence and ferromagnetic behavior. The ferromagnetic

nature of the GQDs is discussed and explained. Based on the experimental data obtained

and theoretical models available in literature, a possible mechanism for the formation of

GQDs is proposed. Their properties, including the production yield can be tuned by simply

changing the synthesis parameters.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, a 2-dimensional sheet of carbon atoms arranged in a

honeycomb pattern has intrigued the world’s science commu-

nity with its extraordinary properties [1–3]. However, properties

like magnetism, luminescence, band gap etc. are absent in pure

graphene. There have been frequent endeavors to accommo-

date such properties into graphene by making composite

materials and nano sized graphene derivatives. Quantum con-

finement of excitons of a material plays an important role in its

properties. Reducing the size of a material brings quantum ef-

fects into play thereby inducing several interesting properties

in it. Graphitic quantum dots (GQDs) containing a few layers

of graphene manifest itself in such a way that one can demon-

strate the magnetic and optical properties emerging due to its

quantum effects [4,5]. GQDs do exhibit very exciting properties

which are found neither in graphene sheets nor in graphitic

materials. Recently, there have been several reports on photo-

luminescence (PL) [6–11] and magnetism (by defects not impu-

rities) of carbon-based materials [12–17].

er Ltd. All rights reserved

0.adur).

Graphene quantum dots have gained much popularity

mainly due to their ability to be used for bioimaging [8,10].

Several methods using different sources of carbon have been

proposed for its synthesis. For example, Liu et al. [6] have

used hexa-peri-hexabenzocoronene as a substitute for the

carbon source. They produced artificial graphite by pyrolizing

the precursor at very high temperatures (600, 900 and 1200 �C)

followed by oxidation and exfoliation by Hummers method.

The resultant oxide was then reduced by hydrazine. The

advantage of their method lies in producing multicolor quan-

tum dots (QDs) with uniform morphology. Shen et al. [7] have

used graphite oxide (GO) as a starting material to yield lumi-

nescent QDs. They treated GO with nitric acid at 70 �C for 24 h

followed by surface passivation with polyethylene glycol. The

advantage of this one pot hydrothermal process was that they

could achieve up-converted PL properties for the QDs. Pan

et al. [8] have used a mixture of concentrated H2SO4 and

HNO3 to oxidize graphene sheets followed by hydrothermal

deoxidization. The major advantage of this method is in

obtaining a strong green fluorescence for the QDs. Again,

.

C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6 347

Pan et al. [9] reported a novel method to obtain blue-lumines-

cent graphene QDs by a hydrothermal route where a mixture

of concentrated H2SO4 and HNO3 is used to cut the graphene

sheets obtained by thermally reducing at high temperatures.

Peng et al. [10] have used carbon fibers having resin-rich sur-

face as carbon source to derive graphene QDs by treating the

starting precursor in a strong acidic medium (HNO3 + H2SO4)

for prolonged periods. Until now, graphene QDs are synthe-

sized mainly through oxidation cutting methods which re-

quires that the starting precursor is put in a strong acidic

medium (HNO3 + H2SO4) for longer duration followed by a

physical separation involving either dialysis or filtration to

separate the QDs from the unoxidized graphene sheets [18].

Here, we report a simple method that does not require an

acidic medium (HNO3 + H2SO4) for oxidation cutting to produce

GQDs. However, we follow Hummers method to produce GO

which was treated with hydrazine to obtain reduced graph-

ene/graphitic oxide (RGO).This was further modified to obtain

the starting precursor. The advantage of choosing GO or RGO

to prepare GQDs is in its ability for mass production for com-

mercial applications. One of the important techniques applied

so far to produce graphene (or graphitic) QDs is to have an oxi-

dation cutting of the starting material followed by size reduc-

tion. For the present case, we first reduced the size of RGO by

physically separating the large sized sheets from it by vacuum

filtration (using a 0.2 lm filter) to obtain size reduced graphene/

graphitic oxide (SRGO) so as to incorporate quantum effects

within it. SRGO of size less than 200 nm is considered as our

starting material for the synthesis of GQDs. The advantage of

choosing SRGO as precursor is in its smaller size which will

have more dangling bonds and defects making it chemically

reactive compared to large sized RGO sheets. GQDs were ob-

tained simply by exposing SRGO sheets with UV radiation fol-

lowed by a mild sonication. A detailed characterization of

GQDs and SRGO is performed to identify their structural and

microstructural characteristics. Fourier transform infra-red

(FTIR) and X-ray photoelectron spectroscopy (XPS) studies are

adopted to determine the nature and degree of their function-

alization. The optical and magnetic properties are studied

which further facilitate in understanding the system and

explaining the underlying mechanism. Finally, on the basis of

the data discussed and theoretical models from literature, we

propose a mechanism for the production of GQDs from SRGO

by UV irradiation. This method has the potential to tune the

properties and the production yield of the GQDs by varying

the synthesis parameters. For example, we obtained a 60%

yield in producing GQDs by increasing the sonication time to

4 h. For the estimation of the production yield of GQDs, the

Supporting information (SI) may be referred. Also see Fig. S1

in SI for color of dispersions of GO, RGO and GQDs in water.

2. Experimental

2.1. Synthesis of GQDs

Natural graphite powder (trace metals < 100.0 ppm, pur-

ity > 99.99%, average particle size < 45 lm) was purchased

from Sigma–Aldrich (CAS: 7782-42-5) and used as received.

This was oxidized to produce GO by a modified Hummers

Method. To oxidize the graphite powder, we followed the syn-

thesis procedure [19] described elsewhere that uses H2SO4/

H3PO4 (purchased from Sigma–Aldrich) to improve the effi-

ciency of oxidation. GO was washed repeatedly with HCl,

deionized (DI) water and ethanol. It was then vacuum (<10�3 -

mbar) dried at 35 �C before its further use. 75 mg of GO was

added into 150 ml of DI water (resistivity: 18.2 MX cm @

25 �C) and sonicated in a bath sonicator for 30 min to result

in a brown colored dispersion which was subjected to reduc-

tion at �95 �C for 2 h [20]. The flow chart of the synthesis pro-

cedure is given in Fig. 1. DI water was added to the RGO

solution obtained after the reduction so as to make it up to

a 200 ml solution. RGO was subjected to sonication in a soni-

cator bath for �1 h. Then, it was filtered through metricel fil-

ter paper with a pore size of 0.45 lm. The large sized RGO

sheets retained on the filter paper is the byproduct. The fil-

tered solution from the receiver funnel was again sonicated

for �1 h and then filtered though a nylon filter paper with a

pore size of 0.2 lm. Once again the sediment retained on

the filter paper was the byproduct which was discarded. But

the solution obtained from the receiver funnel of the filter

assembly containing smaller sized RGO sheets called as SRGO

were subjected to UV radiation for �24 h. Only 50–75 ml of the

filtrate placed in a petri dish was exposed to UV radiation

(Philips TUV 25W/G25T8, Wavelength �365 nm) at a time.

10–15 ml of DI water was added unto the petri dish containing

SRGO after 12 h of UV irradiation and then again exposed to

UV radiation for another 12 h. This was then sonicated for

30 min so as to realize sonication cutting along the line de-

fects already present in the material. To remove moisture

content, GQDs were vacuum (<10�3 mbar) dried for 4–6 h at

40 �C. The main advantage of this process is that it helps to

produce RGO and GQDs simultaneously.

2.2. Mechanism of the reaction

Recently, Garcia et al. [21] have reported that Bernal graphite

do have narrow band gap of the order of 40 meV. The presence

of various functional groups alters the band gap depending on

its number of rings in it [22]. From FTIR analysis (presented in

Section 3), it is evident that SRGO contains various functional

groups. Thus SRGO (<200 nm) is expected to behave like a

semiconductor to allow photochemical reactions within it.

The UV irradiation of wavelength 365 nm has sufficient en-

ergy (3.4 eV) to generate electron–hole (eh) pairs. With contin-

uous incidence of photons, extra energy (after the creation of

eh pairs) will be consumed to impart kinetic energy to it [23].

SRGOþ hm! SRGOþ ðe� þ hþÞe� þO2 ! O��2hþ þH2O! H–OH�þ ! OH� þHþ

Since the oxidizing power of the holes is greater than that

of reducing power of electrons, h+ will break down the water

molecules to form hydrogen gas and hydroxyl radicals and

the e�will react with oxygen molecules producing superoxide

anions. These superoxide anions are effective oxygenation

agents which will readily attack the surface and any radical

adsorbed on it [23]. The hydroxyl radicals produced in due

process plays an important role in the photochemical mech-

anism thereby initiating interfacial electron transfer. The

UV irradiation would become much more active if the e–h

Fig. 1 – Chart for the synthesis of GQDs starting from graphite powder.

348 C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

recombination life time is substantially increased which is

possible either by trapping the photogenerated electron, hole

or both. A scheme of the reaction mechanism is presented in

Fig. 2. SRGO has delocalized p electrons because of which hole

trapping can easily be achieved. It is well known that COOH,

OH, C@O groups are present at the edges of the graphene

sheets [24], while basal planes are mostly covered by epoxide

groups in a linear fashion [25]. As carbonyl groups are more

stable than epoxy groups, the sites containing epoxy groups

are more prone towards chemical attacks. Density functional

theory (DFT) calculations reveal that a single isolated epoxy

group has higher energy than that of a paired epoxy group

[24]. Thus, a chain of epoxy groups is energetically favored

over isolated epoxy groups. High angular strain is produced

when the epoxy groups get aligned in a line. In turn, the C–

C bond length will increase which will further make the C–C

bond susceptible to attack and rupture. Thus a high density

of edge defects will be produced which will be more prone

to be attacked by the surrounding oxygen functionalities

[26]. After the UV exposure, a mild sonication was performed

to enhance the cutting process along the line defects formed

during the oxidation cutting by UV irradiation. The sonication

cutting is possible because the carbon bonds along these line

defects are in general weaker than the regular C–C bonds [27].

Thus a mild sonication will also start to unzip at these defect

sites. Hence a combination of UV irradiation and sonication

allows one to produce GQDs with inherent PL and magnetic

behavior arising basically from its defect states. An experi-

mental evidence for cutting of smaller sized RGO sheets

(<0.45 lm) is given in SI (Fig. S2).

2.3. Instrumentation

Raman measurements were carried out using a LabRAM HR

800 micro-Raman microscope (514.5 nm line of an Argon la-

ser). FTIR spectra were obtained from a JASCO spectrometer

(6100 type-A). Zeta potential was measured by zeta potential

analyzer, (Delsa Nano C, Beckman Coulter Inc.). The presence

of trace metals was checked using an inductively coupled

plasma-atomic emission spectroscopy (ICP-AES) atomic emis-

sion spectrometer (ARCOS-Germany) and energy dispersive

spectroscopy (EDS) on field emission gun-scanning electron

microscope (FEG-SEM). Selected area electron diffraction

(SAED) patterns, particle size and morphologies of GQDs were

Fig. 2 – Reaction mechanism for the formation of GQDs from SRGO due to UV exposure. e-h pairs are generated by UV photons

which help in forming reactive radicals. Epoxy groups get aligned in due process. Oxidation cutting is initiated along this

chain of epoxy groups producing GQDs.

C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6 349

analyzed by transmission electron microscope (TEM) using a

JEOL JEM-2100 facility. The height profile and the surface mor-

phologies of GQDs were obtained by atomic force microscopy

(AFM). AFM images were obtained by scanning probe micro-

scope (Digital Instruments Multimode Nanoscope IV). XPS

measurements were carried out using an electron spectros-

copy for chemical analysis probe (MULTILAB, Thermo VG

Scientific) with a monochromatic Al Ka radiation (Ener-

gy = 1486.6 eV). PL studies were performed on cary eclipse

fluorescence spectrophotometer (Agilent Technologies). The

magnetic measurements were performed by superconducting

quantum interference device (SQUID) magnetometer (MPMS,

Quantum Design, USA).

3. Results and discussions

Crystalline graphite has only one Raman active peak at

�1575 cm�1, called the G band. However, GO, SRGO and GQDs

do have defects which would break the translational symme-

try present in graphite. Thus to perform a comparative Ra-

man study of GO, SRGO and GQDs, a proper deconvolution

of the spectra using Lorentzian lineshape is required. Fig. 3a

shows the Raman spectra of the GO, SRGO and GQDs.

Fig. 3b gives deconvoluted peaks of GQDs with two Voigt con-

tours (3L + 2V-model). For comparison, deconvoluted peaks of

GQDs, GO and SRGO with single consolidated Voigt contours

(3L + V-model) are presented in Fig. 3b (inset), c and d, respec-

tively. However, the presence of D 0 in GQDs at �1620 cm�1 re-

quires a combination of two Voigt contours (3L + 2V-model)

for proper analysis of GQDs [28]. The first order Raman spec-

trum of GQDs is comprised of 5 peaks (3L + 2V-model) cen-

tered at �1163 (band-1), 1353 (band-2), 1536 (band-3), 1593

(band-4) and 1620 cm�1 (band-5). The band-1 having peak at

�1163 cm�1 is a fingerprint of transpolyacetylene like struc-

tures which are formed at the zigzag edges of graphite nano-

particles. The D peak which is ascribed to the defect states is

at �1353 cm�1 in band-2. The peak at �1536 cm�1 in band-3 is

called as A-band or D00 band which arises due to presence of

amorphous carbon or odd membered ring structure in gra-

phitic compounds [28]. This is a dispersionless broadband

whose relative intensity with respect to D or G band signifies

the amount of amorphous carbon in disordered graphite par-

ticles. For the GQDs, the G band which signifies the sp2

bonded carbon is obtained at �1593 cm�1 in band-4 while

the D 0 band (band-5) is found at �1620 cm�1. The D 0 band is

due to high density of defects in the material. The inactive

phonon modes become active due to non-zero phonon den-

sity of states and D 0 band starts to merge with G-band [29]

thereby producing a wider band in comparison to that of GO

and SRGO. The band-S gives the sum of all the individual

deconvoluted bands. The ID/IG ratio of the GQDs was found

to be 1.42 (by 3L + 2V-model). By using the empirical formula

given by Concado et al. [30], the crystallite size (La) was found

to be 4.7 nm from the relation, La = 560/{E4(AD/AG)}where AD,

AG are their integrated areas and E is the photon energy (for

514 nm laser, E = 2.41 eV). The definition of the deconvoluted

bands (1–5) and band-S remains the same in all of the Raman

spectra presented (Fig. 3b–d).

The D 0 band is absent in SRGO and GO. Thus, these are

analyzed by a single consolidated Voigt contours (3L + V-mod-

el) [28]. Using this model, the ID/IG ratio of GO, SRGO and GQDs

are found to be 1, 1.1 and 1.3, respectively. The D band (band-

2) is obtained at �1355, 1351 and 1353 cm�1 while the G band

(band-4) is found at �1599, 1601 and 1609 cm�1 for GO, SRGO

and GQDs, respectively. ID00/ID for SRGO (using 3L + V-model)

and GQDs (using 3L + 2V-model) were found to be 0.3 and

0.2, respectively. The ID00/ID ratio indicates the relative amor-

phous carbon content in the sample. Thus we note a reduc-

tion in amorphous carbon content and an increase in the

defect concentration after the formation of GQDs due to UV

irradiation.

We suggest that UV irradiation on SRGO creates high de-

fect density in the basal plane of the nano sized graphitic

flakes produced by oxidation cutting of large sized flakes.

These GQDs get functionalized with various functional

groups [31] by virtue of oxidation cutting and creation of sev-

eral edges in this process resulting in a higher ID/IG ratio with

a shift in D and G bands [28,29].

The GQDs were further characterized by FTIR spectroscopy

so as to discern the kind of functional groups attached to the

graphene layers after exposing the SRGO to UV radiation.

Fig. 4 gives the FTIR spectra of RGO, SRGO and GQDs. The

OH stretching vibrations are seen between 3200 and

3600 cm�1. The other bands above 3000 cm�1 can be assigned

Fig. 3 – (a) Raman spectra of GO, SRGO and GQDs. Peak fit analysis of (b) GQDs (using 3L + 2V-model). Inset in 3b is the peak fit

of GQDs in accordance to 3L + V-model. Decomposition of Raman spectra using 3L + V-model of (c) GO and (d) SRGO. The band-

S is the sum of the individual deconvoluted bands (1–5). The band-1 is a fingerprint of transpolyacetylene like structures

formed at the zigzag edges of graphite nanoparticles. The band-2, 3 and 4 are called as D, A (or D00) and G band, respectively.

The band-5 called as D 0 band is absent in GO and SRGO.

350 C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

to various C@C vibrations which may be due to alkenes or

aromatic rings. The intensity of C@C aromatic ring stretching

has decreased in GQDs while these bands were prominent in

SRGO. The presence of aromatic rings is confirmed by the

vibrations in between �1500 and 1600 cm�1. Since there are

several small closely spaced bands in this range, we can infer

that there may be presence of variable aromatic nuclei pro-

duced by the UV exposure.

The band seen below 900 cm�1 can be assigned to the C–H

out of plane bending bands. A C@O absorption could be found

in between 1690 and 1760 cm�1. The prominent band in the

spectrum of GQDs is seen between 1080 and 1300 cm�1 which

is ascribed to the C–O absorption. The intensity of this band

increases due to UV exposure. Thus it is very likely that C–O

groups are still retained after oxidation cutting by UV expo-

sure. There may be presence of C–N groups in this range

too. Thus the Raman and FTIR data suggest that the GQDs

produced do have significant number of defect states due to

presence of several functional groups [32].

Fig. 5a gives the TEM image of SRGO before the UV expo-

sure. The SAED pattern of a sheet is presented in the inset

of Fig. 5a. Fig. 5b is taken after the UV exposure which repre-

sents GQDs. High resolution TEM image of GQDs is given in

the inset of Fig. 5b. From the TEM images, it can be seen that

UV exposure breaks down the larger flakes into smaller GQDs.

Similar information is obtained from FEG-SEM images of

SRGO and GQDs (Fig. S3 in SI). The size distribution of GQDs

is shown in Fig. 5c which gives an average value of �4.2 nm.

Thus the UV exposure gives us a simple and clean way to gen-

erate QDs. The GQDs were further characterized by AFM so as

to get an idea of the height of the QDs and their surface mor-

phologies. A 3-dimensional AFM image of GQDs is presented

in Fig. 5d. Section analysis of GQDs at a scale bar of 2.5 and

1.0 lm is shown in Fig. 5e. Fig. 5f gives the height distribution

of the quantum dots measured by section analysis. From the

height distribution, it is observed that the height of the GQDs

produced vary from 3 to 11 nm, while the average height of

GQDs were measured to be �6.6 nm. Thus we decipher that

the GQDs contain a few layers of graphene. However the

number of graphene layers could be decreased by increasing

the time of UV exposure and sonication. From the microstruc-

ture analysis, it is inferred that the regular sheet structure in

SRGO is reduced to smaller sizes in GQDs due to UV exposure.

Thus it is natural to expect that the generation of edge defects

is due to reduction of size. As edge defects are vulnerable to

be attacked by oxygen functionalities. XPS measurements

Fig. 4 – FTIR spectra of RGO, SRGO and GQDs.

C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6 351

were performed to determine the degree and nature of func-

tionality in GQDs.

The GQDs were investigated using XPS so as to obtain

more insight into composition and functional groups of

GQDs. From the two XPS survey given in Fig. 6a, N1s spectrum

Fig. 5 – (a) TEM image of SRGO (before UV exposure) at a scale b

GQDs at 100 nm scale bar, the inset shows HRTEM image of GQ

TEM images. The average size of GQDs were found to be 4.2 nm

2.5 · 2.5 lm. (e) Section analysis of GQDs are given to the right

GQDs. An average height of 6.6 nm was obtained for GQDs by s

is seen in GQDs probably due to UV exposure which is absent

in SRGO.

Thus it is obvious that UV exposure causes chemical

changes in SRGO thereby functionalizing it. The high resolu-

tion normalized C 1s spectra of both SRGO and GQDs are given

in the Fig. 6b. Fig. 6c gives the best possible peak fit analysis of

the C 1s spectrum of the GQDs using the software XPS Peak41.

Qualitatively, a difference between the XPS of SRGO and GQD

could be seen. The nature of the curve changes significantly

for GQDs indicating the presence of functional groups in it.

The functional groups start to play a major role in GQDs which

are the resultant of the UV exposure to the SRGO. The GQDs

were found to contain C@C (�284.6 eV), C–N (�285.5 eV), C–O

(�286.6 eV), C@O (�287.7 eV), O@C–O (�289.4 eV) bonds which

signify that the quantum dots are functionalized by cyanide,

epoxy, carbonyl, and carboxyl groups. The presence of nitrogen

may come from the residual hydrazine and ammonia present

in RGO after the chemical reduction of GO. The N-doped GQDs

[33] shows a close resemblance to the XPS spectrum of the

GQDs prepared by the present method. O1s XPS spectrum is gi-

ven in SI (Fig. S4).

The UV–VIS absorption spectrum is shown together with

that of the PL emission spectra for GQDs in Fig. 7a. A broad

UV–VIS absorption spectrum can be seen whose edge starts

at �246 nm and ends at �314 nm. The evidence of several

functional groups in the FTIR and XPS spectra bespeaks of a

broad spectra in the UV–VIS spectrum. Thus electronic transi-

tions can occur from their ground state to an excited state

facilitating the PL process. These electronic transitions may

be due to the molecules containing p or n electrons which ab-

sorb UV–VIS light so as to move to an excited state of anti-

bonding molecular orbitals. The broad spectrum in the UV–

VIS spectrum could be due to p to p* and n to p* transitions [9].

ar 0.5 lm, the inset gives the SAED patern. (b)TEM image of

Ds at a scale bar of 5 nm. (c) Size distribution of GQDs from

. (d) 3D view of an AFM image of GQDs at a cross section of

of corresponding AFM images. (f) The height distribution of

ection analysis.

Fig. 6 – (a) XPS spectra of SRGO (blue) and GQDs (red). (b) High resolution XPS C1s spectra of SRGO and GQDs. (c) XPS peak fit

analysis of GQDs. The GQDs are functionalized by epoxy, carbonyl, carboxylic acid and cyanide groups. (For interpretation of

the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7 – (a) UV–VIS and PL spectra of GQDs with different excitation wavelengths. (b) PL spectra of SRGO and RGO with different

excitation wavelengths. Normalized PL spectra of (c) GQDs and (d) SRGO indicating peak shift and no peak shift, respectively

with various excitation wavelengths.

352 C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

It has been reported [34] that graphene oxide can be

tuned so as to exhibit optical properties even in the ab-

sence of a natural band gap. Thus, to investigate the evolu-

tion of quantum dots further, PL studies were performed for

RGO, SRGO and GQDs. It is well known that graphene itself

is not photo-luminescent due to its zero band gap. But this

can be made PL active if there is enough phonon density

of states to make it respond to the PL. We observed a

broader PL response in case of GQDs. The intensity gradu-

ally decreased as the excitation energy is decreased

(Fig. 7a).

It is interesting that the peak shifts to the right. RGO is

non-luminescent while SRGO do show a single peak at

�456 nm which is given in Fig. 7b. The emergence of a broad

PL peak in SRGO may be related to the presence of small sized

few-layered graphene sheets (<0.2 lm) with inherent defects

due to oxidation and reduction. The normalized PL spectra

for GQD and SRGO are shown in Fig. 7c and d, respectively.

While the PL peaks do not shift in case of SRGO, it is notewor-

thy that the peak shifts to longer wavelengths in the PL emis-

sion spectra for GQDs which justifies the presence of

chromophore conjugation thereby energetically favoring the

Fig. 8 – Hc v/s T plot of GQDs. Inset is the Dm(T) v/s T plot of

GQDs at two different applied fields (H = 100 and 500 Oe).

C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6 353

excitations between the highest occupied molecular orbitals

and lowest unoccupied molecular orbitals. Similar PL emis-

sion behavior in graphene QDs has already been reported by

several authors [6–11].

The Raman spectrum suggests that GQDs have consider-

able amount of defects. Emergence of PL spectra is another

evidence to support the presence of defects and dangling

bonds in the material. This may be due to any interaction

between these sp2 bonded clusters present. The emission

spectrum has a direct relation to the energy gap [21] which

causes the various transitions to obtain PL and the energy

gap is dependent on the size of the sp2 clusters. The PL

spectrum of GQDs is dependent on the excitation energy

[35]. Also there is a shift in the PL peak which signifies

that there are various sizes of sp2 clusters in our sample

assisting in radiative transitions. Thus the nature of the

sp2 clusters do have influence on the PL spectra making

it depend on the size, shape, topology and symmetry of

the sp2 clusters. The mechanism claiming the PL emission

due to the presence of zigzag sites in the carbene-like trip-

let ground states [9] is likely to explain the spectra in the

present case too. We suggest that there are multiple struc-

tural defects created in the process of synthesis giving rise

to PL spectra.

Ferromagnetism in carbon-based materials with only s

and p electrons is well documented. There are various

sources for ferromagnetic behavior in nano graphitic struc-

tures. Several kinds of defects and the interaction between

them induce magnetic behavior in the absence of 3d or 4f

electrons [36–40]. Out of all possible origins, one has to be very

cautious in dealing with magnetic measurements of the

materials whose moment may be comparable to that of the

magnetic impurities (ppm level) [41]. As the starting material

(graphite) contained trace metals (<100 ppm), we analyzed

GQDs for any possible Fe contamination. Graphite was found

to contain 0.573 ppm of Fe impurity in it while GQDs had re-

tained �0.01 ppm of Fe in it. The decrease in the Fe impurity

is due to the fact that GO prepared by Hummers method was

washed with HCl, DI water and ethanol repeatedly. As a re-

sult, RGO, SRGO and GQDs should be free (or contain negligi-

ble amount) of trace metal impurities due to rigorous acid

treatment of GO.

1 ng of pure Fe is required to produce a magnetic moment

(m) of �0.22 lemu [41]. This value would be smaller if Fe3O4

clusters are considered instead of Fe. As EDS (Fig. S5 in SI) is

not sensitive to ppm concentrations of impurities, ICP-AES

was performed to obtain the amount of Fe and other trace

metal impurity concentration in the material. The details of

the sample preparation are presented in SI. From ICP analysis,

the amount of Fe impurity in GQDs was found to be 0.01 ppm

and this would bring a maximum of 0.04 ng of Fe in the sam-

ple used for the magnetic measurements. Thus the maximum

magnetic moment contribution from the Fe impurity, if ferro-

magnetic, would be <9 nemu which is at least 3 orders smaller

than the signal obtained from SQUID. No other magnetic (or

metal) impurities were detected. Another evidence for the ab-

sence of ferrimagnetic interaction as in Fe3O4 comes from the

ESR data. From ESR analysis, a line-width of DH � 10 Gauss

with a g-value of 2.00055 were obtained. The small value of

DH and the nearly free spin g-value rule out the presence of

Fe3O4 [40]. It is therefore convincing that the ferromagnetic

signal obtained is intrinsic to the sample.

Another possible source of error comes from the diamag-

netic background (BG). To overcome this problem, we also

measured the moment of the BG and subtracted the same

from that of the material’s moment. We have then plotted

the difference between the magnetic moment in the zero field

cooled (ZFC) and field cooled (FC) states, Dm(T) = mFC

(T) �mZFC(T) [42], which would naturally eliminate the BG.

Recently, Scheike et al. [42] have observed the presence of

superconducting regions in water-treated graphite powder.

The magnetic behavior in the sample was neither found com-

patible with magnetic domain wall pinning nor with ferro-

magnetism of graphite powder and is rather believed to

arise due to granular superconductivity. Hence, we checked

our material for the same through Dm(T) v/s T and M v/s H

plots. The coercivity of the GQDs as a function of temperature

was determined from M v/s H plots at various temperatures

(Fig. S6 in SI). This is presented in Fig. 8 as a plot of coercivity,

Hc as a function of temperature. The inset in Fig. 8 gives the

Dm(T) curves at two different applied fields (H = 100 and

500 Oe). Below a certain temperature, Dm(T) was found to in-

crease with increase in applied field as expected in ferromag-

netic materials. Also the coercivity of GQDs decreases with

increase in temperature as expected generally from ferromag-

netic samples. In addition, Dm(T) Curves of GQDs in the inset

of Fig. 8 do not imply the presence of diamagnetic behavior in

the sample [42]. Hence, we rule out the presence of any

detectable isolated superconducting grains in the material.

Fig. 9a gives the Dm(T) v/s T plots for RGO and GQDs at an

applied field (H) of 50 Oe and the inset shows the ZFC-FC mea-

surement of GQDs after BG subtraction. ZFC-FC plot of RGO at

an applied field of 50 Oe is presented in SI (Fig. S7). RGO exhib-

its diamagnetic behavior throughout the temperature range

except at temperatures below 10 K where a sharp transition

to positive magnetic moment is observed. Similar behavior

in RGO is well established and reported by several authors

[40,43]. However the GQDs are inherently magnetic in nature

all over the temperature range from 5 to 300 K. There is a very

minor hump (change in slope) in the ZFC curve of GQDs at

�120 K (inset of Fig. 9a). Sufficient care has been taken to keep

354 C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

the samples clean and free from any kind of contamination.

This kind of change in slope is also seen in tin oxide quantum

dots [44]. Matte et al. [40] have observed similar discontinu-

ities in graphene prepared by arc evaporation of graphite in

hydrogen followed by adsorption with a 0.01 M solution of tet-

rathiafulvalene. Interestingly, graphene quantum sheets pre-

pared by chemical reduction of GO also show a change in

slope in the ZFC–FC curves between 100 and 125 K [45]. The

origin of this behavior (in the absence of magnetic impurities)

around 120 K which is quite similar to Verwey transition in

magnetite remains unexplained. Further studies will be done

in future to understand this observation which is in general

found at nanoscale in many systems. As stated before,

0.01 ppm level of Fe impurity is not sufficient to produce

any ferro/ferri-magnetic behavior in GQDs. A detailed study

is required to further investigate this behavior.

The emergence of magnetic behavior in the GQDs supports

the formation of defect centers in the material [36–40]. Fig. 9b

gives the M v/s H plot of BG and GQDs (as measured and after

BG subtraction) at 300 K. The inset in Fig. 9b shows the M v/s H

plots (1st quadrant only)of GQDs at various temperatures.

Fig. 9 – (a) ZFC-FC measurement of RGO and GQDs at a field

of 50 Oe. The inset shows the ZFC-FC measurement of GQDs

at H = 50 Oe. (b) m v/s H Loop of background and GQDs (as

measured and after diamagnetic background subtraction) at

300 K. The inset gives the m v/s H of GQDs (only first

quadrant) at various temperatures as indicated.

Esquinazi et al. [16] first reported the emergence of ferro

and ferrimagnetic behavior of highly oriented pyrolitic graph-

ite (HOPG) at room temperature by proton irradiation. The

proton irradiation produces two types of magnetic contribu-

tions. First, a temperature dependent Curie-type paramagne-

tism and second part is assigned to ferro or ferrimagnetism.

The ferromagnetism in HOPG originates from two-dimen-

sional arrays of point defects in it due to the presence of local-

ized electron states at grain boundaries of HOPG. Enoki et al.

[17] have reported that nanopores in the nano graphite do-

mains modify the inter-domain electronic interaction by

accommodating several guest gaseous species thereby con-

tributing to magnetic response. Similar arguments based on

covalent-adsorption induced magnetism in graphene have

been reported by Li et al. [46].

As mentioned earlier, the GQDs prepared by UV irradiation

have defects and are functionalized by various functional

groups. Thus we expect a collective effect in magnetic contri-

butions. Also, the zigzag edges which were partly responsible

for PL properties are not expected to contribute to the ferro-

magnetic order due to its inability to maintain a long range or-

der. This is because the correlation length of the short range

magnetic order present at the zigzag edges was predicted to

be less than 1 nm at 300 K [47]. FTIR and XPS analysis indi-

cates the addition of nitrogen to GQDs in the form of C–N

bonds. It is this nitrogen which may also contribute to the

magnetic moment. From the first principles studies based

on spin polarized DFT by Li et al. [46], C and N adatom adsorp-

tion contributes moment of about 0.5 and 1.0 lB, respectively.

In addition, ferromagnetic signal will also arise from the grain

boundaries in GQDs that propagates along the c axis forming

a 2D plane of defects [36] and an inter-domain electronic

interaction between GQDs domains [17]. There are several re-

ports on nano-graphites, graphene nanoribbons and other

systems exhibiting defect based magnetism which is dis-

cussed in literature in terms of defect induced magnetism

(DIM) [48–52]. We believe that DIM kind of mechanism be-

comes plausible origin of ferromagnetic behavior in GQDs.

4. Summary

A simple method to produce GQDs by exposing SRGO to UV

radiation is reported. The GQDs get functionalized due to irra-

diation which is revealed by XPS and FTIR studies. These

GQDs exhibit blue luminescence and ferromagnetic behavior.

The intriguing magnetism of GQDs is discussed with suitable

mechanism. The underlying mechanism of the photochemi-

cal reaction is also discussed with experimental evidences

on oxidation cutting. It is envisaged that these may be further

used for bioimaging and quantum computing applications.

This method can be used to scale up the production and tun-

ing the properties by varying the synthesis parameters. Also

the byproducts can be used in several other applications.

Acknowledgements

This project was sponsored by DST-Nano Mission of govern-

ment of India and IITB Monash Research Academy. We hum-

bly express our gratitude for their support.

C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6 355

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2013.01.082.

R E F E R E N C E S

[1] Geim AK. Graphene: status and prospects. Science2009;324:1530–4.

[2] Rao CNR, Sood AK, Subrahmanyam AKS, Govindaraj A.Graphene: the new two-dimensional nanomaterial. AngewChem Int Ed 2009;48:7752–77.

[3] Geim AK, Novoselov KS. The rise of graphene. Nat Mater2007;6:183–91.

[4] Zhou ZJ, Liu ZB, Li ZR, Huang XR, Sun CC. Shape effect ofgraphene quantum dots on enhancing second-ordernonlinear optical response and spin multiplicity in NH2–GQD–NO2 systems. J Phys Chem C 2011;115:16282–6.

[5] Potasz P, Guclu AD, Voznyy O, Folk JA, Hawrylak P. Electronicand magnetic properties of triangular graphene quantumrings. Phys Rev B 2011;83:174441–6.

[6] Liu R, Wu D, Feng X, Mullen K. Bottom-up fabrication ofphotoluminescent graphene quantum dots with uniformmorphology. J Am Chem Soc 2011;133:15221–3.

[7] Shen J, Zhu Y, Yang X, Zong J, Zhang J, Li C. One-pothydrothermal synthesis of graphene quantum dots surface-passivated by polyethylene glycol and their photoelectricconversion under near-infrared light. New J Chem2012;36:97–101.

[8] Pan D, Guo L, Zhang J, Xi C, Xue Q, Huang H, et al. Cutting sp2

clusters in graphene sheets into colloidal graphene quantumdots with strong green fluorescence. J Mater Chem2012;22:3314–8.

[9] Pan D, Zhang J, Li Z, Wu M. Hydrothermal route for cuttinggraphene sheets into blue-luminescent graphene quantumdots. Adv Mater 2010;22:734–8.

[10] Peng J, Gao W, Gupta BK, Liu Z, Aburto RR, Ge L, et al.Graphene quantum dots derived from carbon fibers. NanoLett 2012;12:844–9.

[11] Dong Y, Shao J, Chen C, Li H, Wang R, Chi Y, et al. Blueluminescent graphene quantum dots and graphene oxideprepared by tuning the carbonization degree of citric acid.Carbon 2012;50:4738–43.

[12] Nair RR, Sepioni M, Tsai IL, Lehtinen O, Keinonen J,Krasheninnikov AV, et al. Spin-half paramagnetism ingraphene induced by point defects. Nat Phys2012;8:199–202.

[13] Ohldag H, Esquinazi P, Arenholz E, Spemann D, Rothermel M,Setzer A, et al. The role of hydrogen in room-temperatureferromagnetism at graphite surfaces. New J Phys2010;12:123012-1–123012-10.

[14] Xia H, Li W, Song Y, Yang X, Liu X, Zhao M, et al. Tunablemagnetism in carbon-ion-implanted highly orientedpyrolytic graphite. Adv Mater 2008;20:4679–83.

[15] Ohldag H, Tyliszczak T, Hohne R, Spemann D, Esquinazi P,Ungureanu M, et al. p-Electron ferromagnetism in metal-freecarbon probed by soft X-ray dichroism. PRL 2007;98:187204-1–4.

[16] Esquinazi P, Spemann D, Hohne R, Setzer A, Han KH, Butz T.Induced magnetic ordering by proton irradiation in graphite.PRL 2003;91:227201-1–4.

[17] Enoki T, Kobayashi Y. Magnetic nanographite: an approach tomolecular magnetism. J Mater Chem 2005;15:3999–4002.

[18] Zhu S, Tang S, Zhang J, Yang B. Control the size and surfacechemistry of graphene for the rising fluorescent materials.Chem Commun 2012;48:4527–39.

[19] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z,Slesarev A, et al. Improved synthesis of graphene oxide. ACSNano 2010;4:4806–14.

[20] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processableaqueous dispersions of graphene nanosheets. NatNanotechnol 2008;3:101–5.

[21] Garcia N, Esquinazi P, Quiquia JB, Dusari S. Evidence forsemiconducting behavior with a narrow band gap of bernalgraphite. New J Phys 2012;14:053015-1–053015-14.

[22] Chen F, Tao NJ. Electron transport in single molecules: frombenzene to graphene. Acc Chem Res 2008;42:429–38.

[23] Fox MA, Dulay MT. Heterogeneous photocatalysis. Chem Rev1995;83:341–57.

[24] Li Z, Zhang W, Luo Y, Yang J, Hou JG. How graphene is cutupon oxidation. J Am Chem Soc 2009;131:6320–1.

[25] Fujii S, Enoki T. Cutting of oxidized graphene into nanosizedpieces. J Am Chem Soc 2010;132:10034–41.

[26] Kosynkin DV, Higginbotham AL, Sinitskii A, Lomeda JR,Dimiev A, Price BK, et al. Longitudinal unzipping of carbonnanotubes to form graphene nanoribbons. Nature2009;458:872–6.

[27] Wu ZS, Ren W, Gao L, Liu B, Zhao J, Cheng HM. Efficientsynthesis of graphene nanoribbons sonochemically cut fromgraphene sheets. Nano Res 2010;3:16–22.

[28] Osipov YV, Baranov AV, Ermakov VA, Makarova TL, ChungongLF, Shames AI, et al. Raman characterization and UV opticalabsorption studies of surface plasmon resonance inmultishell nanographite. Diamond Relat Mater 2011;20:205–9.

[29] Ferrari AC. Raman spectroscopy of graphene and graphite:disorder, electron–phonon coupling, doping andnonadiabatic effects. Solid State Commun 2007;143:47–57.

[30] Cancado LG, Takai K, Enoki T, Endo M, Kim YA, Mizusaki H,et al. General equation for the determination of thecrystallite size La of nanographite by Raman spectroscopy.Appl Phys Lett 2006;88:163106-1–3.

[31] Yan L, Zheng YB, Zhao F, Li S, Gao X, Xu B, et al. Chemistryand physics of a single atomic layer: strategies andchallenges for functionalization of graphene and graphene-based materials. Chem Soc Rev 2012;41:97–114.

[32] Zhu S, Zhang J, Qiao C, Tang S, Li Y, Yuan W, et al. Stronglygreen-photoluminescent graphene quantum dots forbioimaging applications. Chem Commun 2011;47:6858–60.

[33] Li Y, Zhao Y, Cheng H, Hu Y, Shi G, Dai L, et al. Nitrogen-doped graphene quantum dots with oxygen-rich functionalgroups. J Am Chem Soc 2012;134:15–8.

[34] Loh KP, Bao Q, Eda G, Chhowalla M. Graphene oxide as achemically tunable platform for optical applications. NatChem 2010;2:1015–24.

[35] Zhu S, Zhang J, Liu X, Li B, Wang X, Tang S, et al. Graphenequantum dots with controllable surface oxidation, tunablefluorescence and up-conversion emission. RSC Adv2012;2:2717–20.

[36] Cervenka J, Katsnelson MI, Flipse CFJ. Room-temperatureferromagnetism in graphite driven by two-dimensionalnetworks of point defects. Nat Phys 2009;5:840–4.

[37] Banhart F, Kotakoski J, Krasheninnikov AV. Structural defectsin graphene. ACS Nano 2011;5:26–41.

[38] Volnianska O, Boguslawski P. Magnetism of solids resultingfrom spin polarization of p orbitals. J Phys Condens Matter2010;22:073202-1–073202-19.

[39] Rao CNR, Matte HSSR, Subrahmanyam KS, Maitra U. Unusualmagnetic properties of graphene and related materials.Chem Sci 2012;3:45–52.

[40] Matte HSSR, Subrahmanyam KS, Rao CNR. Novel magneticproperties of graphene: presence of both ferromagnetic and

356 C A R B O N 5 7 ( 2 0 1 3 ) 3 4 6 – 3 5 6

antiferromagnetic features and other aspects. J Phys Chem C2009;113:9982–5.

[41] Esquinazi P, Quiquia JB, Spemann D, Rothermel M, Ohldag H,Garcia N, et al. J Magn Magn Mater 2010;322:1156–61.

[42] Scheike T, Bohlmann W, Esquinazi P, Quiquia JB, Ballestar A,Setzer A. Can doping graphite trigger room temperaturesuperconductivity? Evidence for granularhigh-temperature superconductivity in water-treatedgraphite powder. Adv Mater 2012;24:5826–31.

[43] McIntosh R, Mamo MA, Jamieson B, Roy S, Bhattacharyya S.Improved electronic and magnetic properties of reducedgraphene oxide films. EPL 2012;97:38001-1–5.

[44] Dutta D, Bahadur D. Influence of confinement regimes onmagnetic property of pristine SnO2 quantum dots. J MaterChem 2012;22:24545–51.

[45] Saha SK, Baskey M, Majumdar D. Graphene quantum sheets:a new material for spintronic applications. Adv Mater2010;22:5531–6.

[46] Li W, Zhao M, Xia Y, Zhang R, Mu Y. Covalent-adsorptioninduced magnetism in graphene. J Mater Chem2009;19:9274–82.

[47] Yazyev OV, Katsnelson MI. Magnetic correlations at grapheneedges: basis for novel spintronics devices. Phys Rev Lett2008;100:047209-1–4.

[48] Hohne R, Esquinazi P. Can carbon be ferromagnetic. AdvMater 2002;14:753–6.

[49] Rao SS, Jammalamadaka SN, Stesmans A, Moshchalkov VV.Ferromagnetism in graphene nanoribbons: split versusoxidative unzipped ribbons. Nano Lett 2012;12:1210–7.

[50] Panigrahy B, Aslam M, Misra DS, Ghosh M, Bahadur D.Defect-related emissions and magnetization properties ofZnO nanorods. Adv Funct Mater 2010;20:1161–5.

[51] Andriotis AN, Sheetz RM, Menon M. Defect-induced defect-mediated magnetism in ZnO and carbon-based materials. JPhys Condens Matter 2010;22:334210-1–5.

[52] Andersson OE, Prasad BLV, Sato H, Enoki T, Hishivama Y,Kaburagi Y, et al. Structure and electronic properties ofgraphite nanoparticles. Phys Rev B 1998;58:16387–95.