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the application of WLEDs due to their
high fl uorescent quantum yield, narrow
emission bandwidth, and resistance to
the photobleaching.
[ 3,7–9 ]
Integration of
quantum dots into WLEDs can overcome
the problem of whole-organic-material
WLEDs. However, the external quantum
effi ciency (EQE) of WLEDs based on
semiconductor quantum dots is still not
ideal. For example, EQE of WLED with
CdSe nanocrystal as light-emitting layer is
only 0.0013%.
[ 10 ]
Furthermore, semicon-
ductor quantum dots usually contain toxic
heavy metals, which hamper their further
development.
[ 8,11–13 ]
Graphene quantum dots (GQDs), one
kind of carbon dots derived from graphite,
are superior to semiconductor quantum dots due to their low
toxicity, high chemical stability, and excellent carrier transport
mobility.
[ 14,15 ]
Furthermore, the abundant functional groups
such as carboxyl, hydroxyl, and epoxy groups on GQDs endow
them with good dispersion in water and some organic solvents,
and thus make them easy for the further functionalization with
polymers or organic molecules in the process of constructing
light-emitting diodes (LEDs) devices.
[ 16,17 ]
Although yellow fl uo-
rescent GQDs
[ 17 ]
or carbon dots
[ 18 ]
have been used as emitting
layers to fabricate WLEDs, the luminance and current density
are too low. Moreover, white light emission of previous reports
results from combination of separate dopants or multiple
components, which often causes phase separation and color
variation.
[ 19,20 ]
Single-phase white-light-emitting phosphors
are attracting great interests because of their high color sta-
bility and luminous effi ciency for WLEDs.
[ 21–28 ]
Therefore, it is
highly urgent to develop a novel kind of carbon-based single-
phase white-light-emitting phosphor for WLEDs with enhanced
performance.
In this work, white-light-emitting graphene quantum dots
(WGQDs) were prepared by a facile two-step microwave-
assisted hydrothermal method ( Scheme 1 ). Yellow-green fl uo-
rescent GQDs were synthesized beforehand through exfoliation
of oxidized graphite under the ultrasonication and microwave
irradiation. After that, the prepared GQDs were further treated
through microwave reaction under alkaline condition (pH
13.0), giving rise to GQDs with white-light emission (WGQDs).
WGQDs were subsequently used as a phosphor to fabricate
WLED device by a solution-processing method. WLED based
Microwave-Assisted Preparation of White Fluorescent
Graphene Quantum Dots as a Novel Phosphor for
Enhanced White-Light-Emitting Diodes
Zhimin Luo , Guangqin Qi , Keyu Chen , Min Zou , Lihui Yuwen , Xinwen Zhang , *
Wei Huang , * and Lianhui Wang *
Graphene quantum dots (GQDs) with white fl uorescence are synthesized by
a microwave-assisted hydrothermal method using graphite as the precursor.
A solution-processed white-light-emitting diode (WLED) is fabricated using
the as-prepared white fl uorescent GQDs (white-light-emitting graphene
quantum dots, WGQDs) doped 4,4-bis(carbazol-9-yl)biphenyl as the emissive
layer. White-light emission is obtained from the WLED with 10 wt% doping
concentration of WGQDs, which shows a luminance of 200 cd m
−2
at the
applied voltage of 11–14 V. Importantly, an external quantum effi ciency of
0.2% is achieved, which is the highest among all the reported WLED based
on GQDs or carbon dots. The results demonstrate that WGQDs as a novel
phosphor may open up a new avenue to develop the environmentally friendly
WLEDs for practical application in solid-state lighting.
DOI: 10.1002/adfm.201505044
Dr. Z. Luo, G. Qi, K. Chen, M. Zou, Dr. L. Yuwen,
Dr. X. Zhang, Prof. W. Huang, Prof. L. Wang
Key Laboratory for Organic Electronics and Information
Displays & Institute of Advanced Materials (IAM)
National Jiangsu Synergistic Innovation Center
for Advanced Materials (SICAM)
Nanjing University of Posts & Telecommunications
9 Wenyuan Road , Nanjing 210023 , P. R.China
E-mail: iamwhuang@njupt.edu.cn ; xwzhang@njupt.edu.cn ;
iamlhwang@njupt.edu.cn
Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE)
& Institute of Advanced Materials (IAM)
National Jiangsu Synergistic Innovation Center
for Advanced Materials (SICAM)
Nanjing Tech University (NanjingTech)
30 South Puzhu Road , Nanjing 211816 , P. R.China
1. Introduction
White-light-emitting diodes (WLEDs) are considered the most
promising light source in the future due to their advantages
such as low power consumption, high luminous effi ciency,
and long lifetime.
[ 1–3 ]
WLEDs based on whole organic mate-
rials are relatively low-cost and easy for large-scale production,
but their environmental stability is low and device lifespan is
limited.
[ 4–6 ]
Semiconductor nanocrystals, especially semicon-
ductor quantum dots, have attracted increasing attention in
Adv. Funct. Mater. 2016,
DOI: 10.1002/adfm.201505044
www.afm-journal.de
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