Magnetic enhancement of photoluminescence from blue-luminescent graphenequantum dotsQi Chen, Chentian Shi, Chunfeng Zhang, Songyang Pu, Rui Wang, Xuewei Wu, Xiaoyong Wang, Fei Xue,Dengyu Pan, and Min Xiao Citation: Applied Physics Letters 108, 061904 (2016); doi: 10.1063/1.4941818 View online: http://dx.doi.org/10.1063/1.4941818 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The emission wavelength dependent photoluminescence lifetime of the N-doped graphene quantum dots Appl. Phys. Lett. 107, 241905 (2015); 10.1063/1.4937923 Facile synthesis and photoluminescence mechanism of graphene quantum dots J. Appl. Phys. 116, 244306 (2014); 10.1063/1.4904958 Tuning photoluminescence of reduced graphene oxide quantum dots from blue to purple J. Appl. Phys. 115, 164307 (2014); 10.1063/1.4874180 Synthesis and photoluminescence of fluorinated graphene quantum dots Appl. Phys. Lett. 102, 013111 (2013); 10.1063/1.4774264 Synthesis and upconversion luminescence of N-doped graphene quantum dots Appl. Phys. Lett. 101, 103107 (2012); 10.1063/1.4750065

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Magnetic enhancement of photoluminescence from blue-luminescentgraphene quantum dots

Qi Chen,1,a) Chentian Shi,1,a) Chunfeng Zhang,1,b) Songyang Pu,1 Rui Wang,1 Xuewei Wu,1

Xiaoyong Wang,1 Fei Xue,2 Dengyu Pan,3 and Min Xiao1,4,5,c)

1National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Centerof Advanced Microstructures, Nanjing University, Nanjing 210093, China2Research Institute, Baosteel Group Corporation, 655 Fujin Road, Shanghai 201900, China3Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 201800, China4Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Scienceand Technology of China, Hefei, Anhui 230026, China5Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA

(Received 21 December 2015; accepted 1 February 2016; published online 10 February 2016)

Graphene quantum-dots (GQDs) have been predicted and demonstrated with fascinating optical and

magnetic properties. However, the magnetic effect on the optical properties remains experimentally

unexplored. Here, we conduct a magneto-photoluminescence study on the blue-luminescence GQDs

at cryogenic temperatures with magnetic field up to 10 T. When the magnetic field is applied, a

remarkable enhancement of photoluminescence emission has been observed together with an

insignificant change in circular polarization. The results have been well explained by the scenario of

magnetic-field-controlled singlet-triplet mixing in GQDs owing to the Zeeman splitting of triplet

states, which is further verified by temperature-dependent experiments. This work uncovers the pivotal

role of intersystem crossing in GQDs, which is instrumental for their potential applications such as

light-emitting diodes, photodynamic therapy, and spintronic devices. VC 2016 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4941818]

By breaking the symmetry of two-dimensional hexagonal

lattice of graphene, extraordinary optical and magnetic prop-

erties of technical significances can be introduced to a variety

of graphene-based nanostructures.1–5 One prototypical exam-

ple is the material of graphene quantum dots (GQDs) synthe-

sized by cutting one- or few-layer graphene with nanoscale

boundaries.2,6–13 In the last few years, GQDs have proven to

be efficient fluorophores with a broadband spectral cover-

age6,12,14–19 that are excitable under electrical20–22 as well as

linear/nonlinear optical pumps.6,12,14–16,23,24 A variety of

emissive centers relevant to the intrinsic band structures and

extrinsic surface/edge states have been found in GQDs, allow-

ing photoluminescence (PL) manipulation with multiple fac-

tors including morphology, dopings, surface passivation,

excitation wavelength, and synthesis environment.2,24–27 The

unique emitters have shown biocompatibility, photostability,

and temperature insensitivity, enabling many optoelectronic

applications including light-emitting diodes (LEDs),20–22 solar

energy conversion,28,29 and biomedical photonics.15,23,24,30

Of equal importance is the unique magnetic property

known as a novel type of magnetism with s-p elec-

trons.1–3,31–36 The atom-scale structure imperfections in gra-

phene nanostructures can host unpaired spins,31,33 affording

the magnetic order in GQDs due to the reduced dimensional-

ity,34 the point defects,31,33 and the important “zigzag” edge

structures.32,36 Spin multiplicity is also well defined in

GQDs with excitonic levels having singlet and triplet charac-

teristics.2,16 The process of intersystem crossing (ISC) due to

the mixing of singlet and triplet states plays a key role to the

electronic relaxation dynamics in some GQDs.16,35,37 These

spin-relevant features are susceptible to magnetic field,

bringing about the exotic magneto-optical properties in

GQDs in analogue to other carbon nanomaterials,38,39 which,

however, remain experimentally unexplored.

In this work, we conduct a magneto-PL study on

blue-luminescent GQD films at cryogenic temperatures with

magnetic field (B) up to 10 T. We report an observation of

PL enhancement of �10% for PL emission while the ratio of

the two circular polarizations is insignificantly changed at

4 K with B raising up to 10 T. The abnormal magneto-PL

properties can be well explained by considering the magneti-

cally controlled ISC in GQDs originating from the variation

of singlet-triplet energy levels caused by Zeeman splitting

of the triplet excitonic level, which is further verified by

additional temperature-dependent experiments. Our work

suggests the essential role played by ISC in determining

the magneto-optical properties in GQDs, which is instrumen-

tal for potential applications of GQDs in areas such as

LEDs,20–22 photodynamic therapy,30 and spintronic

devices.1,4,31,32,40–42

The samples of GQDs were prepared by the hydrother-

mal cutting method as reported previously.6,43 The GQDs

exhibit uniform thickness of �1 nm with an average diame-

ter of �20 nm as characterized by atomic force microscopy

(AFM) and transmission electron microscopy (Figures

1(a)–1(d)), respectively. The basic units of sp2-hybridized

carbon atomic clusters dictate the energy alignments of the

highly occupied molecular orbitals (HOMOs) and the lowly

unoccupied molecular orbitals (LUMOs) in GQDs while the

carboxyl-bonded sp3 CO matrices are relevant to the surface/

a)Q. Chen and C. Shi contributed equally to this work.b)cfzhang@nju.edu.cn.c)mxiao@uark.edu.

0003-6951/2016/108(6)/061904/5/$30.00 VC 2016 AIP Publishing LLC108, 061904-1

APPLIED PHYSICS LETTERS 108, 061904 (2016)

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edge states (see supplementary Figure S144).2,45 For

magneto-optical measurements, films of GQDs were pre-

pared by drop-casting GQD solution onto glass substrates,

which exhibit good optical quality with an average height of

<5 nm as shown in the AFM image (see supplementary

Figure S144). Because of the quantum confinement effect,

many aspects of the optical properties of such GQDs have

been discussed in analogy to semiconductor QDs.7,45 Upon

excitation at 400 nm, PL from the film sample in this work

exhibits a broad emission band centered at �450 nm (Figure

1(e)). The insignificant redshift in the emission wavelength

with increasing temperature up to 150 K (Figure 1(e)) is far

below that expected for the excitonic emission in conven-

tional semiconductor QDs. Moreover, the correlation

between the size and the emission wavelength of GQDs

remains highly controversial,25,43 which is also divergent

when using samples prepared under different conditions.7

These unusual spectral features suggest the involvement of

complicated surface states or subdomains in the PL mecha-

nism which have been a subject under debate.24,27,46 The

strong blue emission has been experimentally observed as

dominant emission from minimally oxidized or low-defect

GQDs, which has been argued as the intrinsic emission rele-

vant to the isolated sp2 carbon hexagons.22,47,48

To investigate the magneto-optical properties in GQDs,

we acquire the PL spectra by placing the film of GQDs with

a Faraday configuration in the sample chamber of a super-

conducting magnet (SpectroMag 4000) as described with

details in supplementary material. Briefly, PL emission col-

lected from the sample under backside excitation is split into

two channels for analyzing PL spectra of right (rþ) and left

(r�) circularly polarizations, respectively (Figure 2(a)).

When the magnetic field is turned on, PL emissions increase

for both rþ and r� polarizations (Figures 2(b) and 2(c)), in

particular, under high-field condition (B> 3 T) (Figure 2(d)).

No saturation has been observed with field up to 10 T (the

maximum value achievable in the system). For comparison,

we quantify the PL increment with an enhancement factor

defined as

gðBÞ ¼ ðIPLðBÞ � IPLð0ÞÞ=IPLð0Þ; (1)

where IPL(B) is the field-dependent PL intensity. The field-

dependent change in circular polarization has been frequently

used to quantify the magneto-optical effect in semiconduc-

tors,49–53 which is quantified as

P ¼ ðIrþPL � Ir�

PLÞ=ðIrþPL þ Ir�

PLÞ; (2)

where IrþPL and Ir–

PL are the PL intensities of rþ and r� polar-

izations, respectively. With increasing magnetic field, the

change in P is insignificant in a scale close to the noise level,

suggesting that the theoretically predicted magnetic order is

less important for the magneto-optical properties of GQDs

studied here. This is consistent with the nearly identical

spectra for rþ and r� polarized PL (Figure 2(e)). The slight

difference of PL intensities recorded with rþ and r� polar-

izations may be relevant to the magnetic orders at the edge

FIG. 1. Sample characterizations. (a) TEM image of GQDs. Inset shows a

high resolution TEM image of an individual GQD. The scale bars in the

main panel and inset panel are 100 and 5 nm, respectively. (b) Lateral size

distributions of GQDs obtained from the TEM image. (c) AFM image of

GQDs dispersed on a mica substrate. (d) A height profile measured along

the solid line labelled on the AFM image (c). (e) Normalized PL spectra of

the film of GQDs recorded at different temperatures.

FIG. 2. Magneto-PL study of GQDs at 4 K. (a) A schematic diagram of the

magneto-PL setup used in this study. (b) and (c) plot the field-induced

changes in PL spectra (DPL) at various fields by subtracting off the spectra

at 0 T, for right (rþ) and left (r�) circular polarizations, respectively. (d)

The factor (g) of PL enhancement for rþ and r� circular polarizations and

the ratio of circular polarizations (P) are plotted versus the magnitude of

magnetic field. (e) Normalized spectra of rþ and r� polarized PL curves

recorded at 10 T. All experimental data are acquired at 4 K.

061904-2 Chen et al. Appl. Phys. Lett. 108, 061904 (2016)

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of isolated sp2 carbon hexagons, which deserves in-depth

study in the future. The minor change is due to the possible

effect of partially random orientations of GQDs in the film.

It has been characterized that the typical energy scale of

magnetic orders at the edges of graphene nanostructures is in

the order of 100–102 meV,4,32 which is much lower than the

photon energy of PL emission concerned here. The magnetic

orders and emissive centers are likely relevant to different

domains in GQDs, resulting in the insignificant dependence

of P on the magnetic field.

Next, let us try to understand the mechanism of the

unexpected enhancement of PL emission at a high-field limit

(Figures 2(b)–2(d) and 3(a)). At 10 T, the enhancement fac-

tor of g approaches �10% while the spectral profiles remain

unchanged, indicating no detectable modification of the

energy distribution of emissive states. In a previous work on

carbon nanotubes, the magnetic enhancement of PL emission

has been ascribed due to the symmetry-breaking-induced

change in excitonic band structures,38 which is unlikely to be

the major factor for the results observed here with no spectral

shift. In principle, PL increment can be caused by increasing

the occupation probability at the emissive states and/or

enhancing the radiative efficiency of the emissive states. The

radiative efficiency enhancement is always accompanied by

the variation in PL decay lifetime since the emissive effi-

ciency is enhanced by promoted radiative recombination or

suppressed nonradiative recombination. The normalized

time-resolved PL (TRPL) traces as measured by time-

correlated single-photon counting54,55 in our experiment

remain nearly identical at 0 T and 10 T (Figure 3(b)). Both

curves can be well reproduced by the same biexponential

decay function with two components having lifetimes

(amplitudes) of �2.6 ns (60%) and �8.9 ns (40%), respec-

tively, suggesting that the radiative efficiency enhancement

can be safely excluded from being the major factor for the

observed PL enhancement.

To understand the enhanced probability occupied at the

emissive states, we propose a model by considering the

effect of magnetic field on the process of ISC to explain the

PL enhancement, as illustrated in Figure 4. The spin multi-

plicity in GQDs results in the singlet and triplet excitonic

states.2,16,37 In the presence of spin-orbital coupling (SOC),

the process of ISC with phonon activation can be evidenced

by the observation of phosphorescence in GQDs.16 The rate

of ISC is dictated by the energy splitting between the singlet

and triplet states (EST ¼ ES � ET).16,37 The value of EST, in

scaling versus the lateral diameter (d) as EST / d�2,2,16 is

estimated to be in the order of 0.1–1.0 meV for the GQDs

with relatively large sizes studied here. This small energy

gap can be easily compensated by phonons at room tempera-

ture. After cooling the sample down to 4 K, the process of

phonon activation is inhibited, so that the ISC process favors

the relaxation from singlet to triplet states, reducing the pop-

ulation at the singlet states. The population at the excited

triplets relaxes to ground states through some nonradiative

processes. When the magnetic field is turned on, different

Zeeman interactions for the singlet and triplet states can

cause a variation of EST that modifies the rate of ISC.56 Such

a scenario of magnetic-field controlled ISC, as previously

established in some biradicals,57 is likely to be responsible

for the PL enhancement observed in our experiment at the

high-field regime.

Qualitatively, let us consider a simplified picture with

the lowest singlet (S1) and triplet (T1) excitonic states to

explain the magnetic enhancement of PL emission in GQDs

(Figure 4). The Zeeman interaction splits the triplet level

(T1) into three sub-levels (T1þ, T1

0, and T1�). To the first

order, the energy of T10 is unchanged with increasing B,

while the level of T1þ (T1

�) shifts to upper (down) side with

an offset of DET ¼ glBB (�1.2 meV at 10 T) (Figure 4).

Since DET is in the same energy scale as EST , the rate of ISC

could be significantly modified when the field is applied.

Specifically, when increasing B, the ISC from S1 to T10

remains unchanged; The ISC from S1 to T1� declines with an

increased EST ; However, the ISC rate from S1 to T1þ state

gets more complicated. It increases in the weak-field regime

with a decrease in EST, but should be suppressed with a fur-

ther increase in the field since the reverse process from T1þ

to S1 becomes energy favorable in the high-field regime with

T1þ higher than S1. Consequently, the net population occu-

pied at the singlet state S1 increases with increasing B under

the high-field condition, which is responsible for the

observed PL enhancement (B> 3 T) (Figure 2(d)). In the

low-field regime, the net population at the lowest singlet

state is relevant to the tradeoff between the ISC rates rele-

vant to S1 ! T1þ and S1 ! T1

� processes. In principle, a

drop in PL emission is anticipated when the levels S1 and

T1þ approach to degeneracy.57 Such a resonant effect is not

distinct here, which is probably averaged out by the broad

size distribution of GQDs (i.e., the broad distribution of

EST / d�2), resulting in a less significant variation in PL

with B< 3 T (Figure 2(d)). The overall probability for transi-

tion S1! T1 (i.e., a sum of the probabilities for S1! T1þ, S1

FIG. 3. (a) Total PL spectra recorded at different magnitudes of magnetic

field. (b) Normalized TRPL curves recorded at 0 T and 10 T. The inset shows

the rising components of the TRPL curves together with the curve of instru-

mental response (IRF). The solid lines are curves fitting to an exponential

growth function.

FIG. 4. Diagram of the scenario of magnetic control of ISC that explains the

observed magneto-PL properties of GQDs.

061904-3 Chen et al. Appl. Phys. Lett. 108, 061904 (2016)

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! T10, and S1 ! T1

�) shows different dependences on the

magnetic field in the relatively weak and intense regimes,

which is plausibly the reason for the observed superlinear de-

pendence of PL enhancement on the magnetic field. From

the above discussions, it is safe to ascribe the PL enhance-

ment as due to the magnetically controlled ISC in GQDs.

Moreover, it is worth noting here that the simultaneous

emergence of two spin sub-levels (T1þ and T1

�) when the

field is turned on makes the net effect of magnetic field to be

independent of the field polarity, which agrees well with the

experimental data (Figure 2(d)).

As mentioned above, the PL mechanism is not well

determined yet for the blue-luminescent GQDs. The major

debate has been on the question of whether the LUMO-

HOMO transition (i.e., the singlet and triplet states) or the

recombination relevant to other emissive states (i.e., defect

and surface states) is the primary radiative chan-

nel.24,25,27,43,46 The magneto-PL study presented here may

shed light on resolving this controversy. We have shown

that the scenario of magnetic-field controlled ISC well

explains the magneto-PL properties observed in this study

and the modified ISC changes the population dynamics of

the singlet states. Since the decay dynamics is independent

of the field (Figure 3(b)), the radiative recombination from

the photo-excited singlet states is unlikely to be the primary

origin of PL emission (at least not the origin for the field-

dependent PL). The radiative recombination of emissive

states (i.e., the defect or surface states) occupied after elec-

tron relaxation from the singlet states plays an important

role as illustrated in Figure 4, which is consistent with previ-

ous spectroscopic experiments on single GQDs or assemble

samples.25,43 The magnetic field induced changes in emis-

sive states (�meV) are orders of magnitude smaller than the

broad energy distribution (�0.5 eV) as revealed in PL emis-

sion, resulting in the nearly identical spectra for rþ and r�

polarizations (Figure 2(e)).

We note here that the onset of PL kinetics also sup-

ports the above explanation (Figure 3(b)), where the maxi-

mum of TRPL trace appears at a time delay of �0.5 ns

post-excitation. This rising component can be assigned to

the buildup process of emissive states due to relaxation

from the singlet states. Roughly speaking, the relaxation

rate has been quantified by fitting the rising component

with an exponential growth function. The estimated life-

time at 10 T (�0.173 ns) becomes slightly longer than that

at 0 T (�0. 158 ns) (inset, Figure 3(b)), indicating the decay

of S1 state to be slower at 10 T. This change in PL kinetics

is consistent with the scenario of magnetically controlled

ISC discussed above (Figure 4) where the suppressed ISC

at 10 T leads to a relatively slow relaxation of S1 state. The

difference in relaxation rate (�0.54 ns�1) is �9% variation

of total decay, which is comparable to the ratio of PL

change. Assuming the variation of ISC is the primary ori-

gin of the kinetic change, the rate of ISC can be estimated

in the scale of ns�1 which is comparable to that observed

in the organic photovoltaic materials.58 Such an efficient

ISC is reasonable since the hydrothermally cut GQDs are

rich in the bonds and impurities that can dramatically

enhance the SOC.59,60

We have carried out a temperature-dependent experi-

ment to further confirm the above arguments. In principle,

the change in EST in the scale of �1 meV can be cancelled

out by thermal effect due to phonon activation with an

increased temperature in GQDs.2 This effect is manifested in

the temperature dependence of g, as shown in Figure 5. With

increasing temperature, the PL enhancement at 10 T becomes

less important and finally vanishes at temperatures above

�40 K (Figure 5) except with a slight anomaly at 10 K that

may be caused by the interplay between low-energy mag-

netic orders. The thermal energy at 40 K becomes larger than

DET , which wipes out the magnetic effect on ISC where the

field-induced change in EST is no longer a dominant factor.

Such experimental results clearly confirm the key role played

by the magnetic control of ISC for PL enhancement at low

temperature, which has also been observed in a recent study

on the carbon-based nanostructures of nitrogen-vacancy cen-

ters in diamond.61

In summary, a remarkable enhancement of PL emission

induced by magnetic field is observed in the film of blue-

fluorescent GQDs, which is ascribed to the effect of mag-

netic-field-controlled ISC on the net population of singlet

states. The essential role of spin multiplicity suggests the

similarity between the magneto-optical behaviors of GQDs

and that of organic semiconductors. Considering the diverse

forms of GQDs and their colorful optical properties, our

work opens a door to search for the fascinating magneto-

optical properties in GQDs, which can be very essential for

practical applications including LEDs,20–22 photodynamic

therapy,30 and spintronic devices.40–42 It is worth noting that

the physical process studied here can even play an important

role for practical devices working under magnetic-free con-

ditions. For instance, in LEDs, the current-injected carriers

are in form with either singlet (25%) or triplet (75%) spin

characteristics.62,63 The crossover from triplet to singlet

states can significantly enhance the LED output as well

established in organic LEDs.63,64 For photodynamic therapy,

the process of ISC is pivotal for generating singlet oxygen in

GQDs—the functional chemical curing the tumors.30

This work was supported by the National Basic

Research Program of China (2013CB932903 and

2012CB921801, MOST), the National Science Foundation

of China (91233103, 11574140, 11227406, and 11321063),

and the Priority Academic Program Development of Jiangsu

Higher Education Institutions (PAPD).

FIG. 5. The factor (g) of PL enhancement is plotted as a function of

temperature.

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