Article
pubs.acs.org/cm
Hybrid ZnO-Based Nanoconjugate for Efficient and Sustainable
White Light Generation
Arunasish Layek, Paul C. Stanish, Vadim Chirmanov, and Pavle V. Radovanovic*
Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
S Supporting Information
*
ABSTRACT: Developing new ways of generating white light
is of paramount importance for the design of the next
generation of smart, energy-efficient lighting sources. Here we
report tunable white light emission of hybrid organic−
inorganic nanostructures based on colloidal ZnO nanocrystals
conjugated with organic fluorophores. These materials act as
single nanophosphors owing to the distance-dependent energy
transfer between the two components. The defect-based sizetunable ZnO nanocrystal blue-green emission coupled with
complementary color emission from different fluorophores
allows for the generation of white light with targeted
chromaticity, color temperature, and color rendering index. We further show that silane layer-protected nanoconjugates result
in increased stability of white light emission over a long period of time. The results of this work demonstrate an inexpensive,
green, and sustainable approach to general solid-state lighting, without the use of rare earth or heavy metals. Colloidal form of the
reported hybrid nanoconjugates allows for their further functionalization or incorporation into light-emitting devices. More
broadly, size dependence of the electronic structure of native defects in transparent metal oxide nanocrystals and their electronic
coupling with conjugated organic species could also represent a vehicle for introducing and manipulating new properties in these
hybrid nanostructures.
■
(Ce 3+:YAG).10,14−16 These GaN/Ce3+:YAG and similar
phosphor-converted WLEDs generally suffer from the
deficiency of the red emission component, resulting in low
color rendering index (CRI < 80%) and high correlated color
temperature (CCT ≈ 7000 K).16,17 The past decade has seen
an ongoing effort to develop rare earth element-based
converters that could produce optimal quality white light in
conjunction with blue LEDs.18−20 However, increasing
deficiency and strategic importance of rare earth elements call
for investigating other approaches to WLEDs.21,22 The slow
adoption of WLED technologies is exacerbated by complex
design and inconsistency of key figures of merit, ultimately
leading to a high device manufacturing cost. In order to fully
realize the potential of solid state lighting, new materials and
technologies must be developed to generate desired quality of
white light in an energy-efficient and economically viable way.
Although organic semiconductor light emitting diodes
(OLEDs) have been used to fabricate low-cost large-area
white light illuminating panels, these devices are typically still
plagued with short lifespan, low efficiency, and instability
problems.23−25 An attractive alternative is to use highly
luminescent colloidal semiconductor nanocrystals (NCs),
which combine higher resistance to thermal and photo-
INTRODUCTION
The ever increasing energy demands along with alarming
concerns over global warming and climate change place an
enormous importance on the investigation of new highefficiency lighting sources to reduce the world’s electricity
usage.1−4 Because of the poor energy conversion efficiency of
incandescent light bulbs (<5%) and the total amount of
electricity used for lighting (∼20%), switching from these
conventional to more energy-efficient lighting sources is critical
for reducing the global energy consumption.2,5−7 Solid-state
lighting, and in particular light emitting diodes (LEDs), have
emerged as the most promising long-term solution that could
meet the stringent efficiency, safety, durability, and reliability
standards.2−4,8−10 The market for LEDs is predicted to
experience a multifold expansion in the next decade, with the
majority of the growth coming from bright and efficient white
light-emitting diodes (WLEDs).8,10−12 Currently, WLEDs are
found in a wide variety of applications, including general
lighting, automotive industry, architectural lighting, and displays.9,10
White LEDs are traditionally based on a multicolor (RGB)
approach, in which white light is generated by controlled
mixing of the emission from blue, green, and red LED chips or
electroluminescent fluorophores.13 More recently, WLEDs
have primarily been fabricated by combining GaN blue LED
chip with one or more remote phosphors, most notably yellowemitting yttrium aluminum garnet doped with cerium
© 2015 American Chemical Society
Received: November 25, 2014
Revised: January 3, 2015
Published: January 20, 2015
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DOI: 10.1021/cm504330k
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Chemistry of Materials
and size-dependent optical properties. Although the nature of
the defect-based emission in ZnO NCs continues to be
debated, it also shows size-dependent behavior, allowing for the
manipulation of visible PL via NC size.42−46 Recent reports
have suggested the use of defect states in transparent metal
oxide NCs as energy donor species, coupled with conjugated
organic fluorophores as acceptors.47−49 Broad-band blue-green
PL of ZnO NCs synthesized in this work allows for partial
resonance energy transfer to orange-red organic fluorophores
(i.e., ATTO 565 and 590) conjugated on NC surfaces, leading
to an efficient generation of white light by this new quasi-single
fluorophore. Owing to the size-tunability of ZnO NC PL, its
spectral overlap with the absorption band of conjugated
fluorophores can be controlled, resulting in a wide tunabity
of the emission chromaticity, including the generation of white
light having targeted characteristics. The hybrid complex
(nanoconjugate) formed in this way allows for seamless
integration with LEDs, paving the way for inexpensive, rare
earth element-free, broadly tunable single white light converter
for new solid-state lighting technologies.
degradation with the advantage of low-cost solution phase
modification and processing characteristic for organic fluorophores. Furthermore, the emission color of colloidal NCs is
readily tunable by changing their size or composition, as well as
via postsynthetic modification by surface functionalization.
Alivisatos and co-workers have pioneered CdSe NC-based
electroluminescent LEDs,26 while Coe et al. further modified
electroluminescent quantum dot (QD) devices and demonstrated high-efficiency (1.6 cd A −1 at 2000 cd m−2 )
monochromatic LEDs by employing phase segregation between
CdSe NCs and an organic matrix.27 In another example of
achieving WLEDs, Gigli and co-workers proposed mixing of
blue, green, and red emitting colloidal II−VI NCs in an
appropriate ratio.28 Similarly, WLED having stable electroluminescence has been realized by employing yellow-emitting
CdSe/CdS core/shell NCs and blue-emitting poly(N,N′-bis(4butylphenyl)-N,N′-bis(phenyl)benzidine) (or poly-TPD) molecules.29 Huang et al. incorporated green- and red-emitting
CdSe/ZnS core/shell NCs into blue-emitting polyfluorene
polymer to achieve white light emission.30 While these
intuitively appealing approaches eliminate the necessity of
using rare earth elements, they still rely on multiple
independent emitters, making it difficult to control the
proportion of the individual constituents, maintain the
uniformity of the emission, and reproduce the same quality
of white light in different devices.
Recently, there has been much interest in the development of
NC-based white light emitters that can generate white light by
photoluminescence (PL) from only one type of NCs upon
single wavelength excitation by an ultraviolet (UV) LED. Chen
and co-workers have developed white light-emitting ZnSe NCs
where white light generation is a result of a combination of
band edge (blue) and broad surface trap (green to red)
emissions.31 Similarly, white light has been achieved in ∼1.5 nm
“magic-sized” CdSe NCs by the coexistence of band edge and
surface defect emissions.32 Transition metal ion-doped NCs,
such as Pb2+-doped ZnS33 and Mn2+-doped CdS,34 have also
shown a potential as a new class of white light-emitting
materials. In Mn2+-doped CdS NCs the generation of white
light is suggested to involve the combination of broad surface
state emission of NC host lattice and ligand field emission of
Mn2+ dopant centers.34 The widespread use of CdSe NCs is a
testament to their excellent size-tunable PL in the visible range,
high quantum yield, and well-understood synthesis and
modification methods.35−37 However, the toxicity of cadmium-containing compounds and nanomaterials is well documented, limiting their potential applications.38 Furthermore,
photo-oxidation, relatively low abundance of Cd and Se in
earth’s crust, and the difficulties in controlling dopant
incorporation and surface states remain as some of the
formidable challenges.39−41 The development of nontoxic,
environmentally benign, and inexpensive single white lightemitting colloidal nanomaterial made of earth-abundant
elements can make significant impact on the solid-state lighting.
In response to these requirements, we demonstrate the white
light-emitting hybrid nanostructures constructed using colloidal
ZnO NCs. ZnO is a naturally occurring wide band gap n-type
semiconductor (band gap energy ≈ 3.37 eV), which has
significant industrial importance because of its low cost,
nontoxicity, chemical stability, optical transparency, conductivity, and robust room-temperature visible light emission.
Importantly, colloidal ZnO NCs are readily synthesized at
room temperature and exhibit strong quantum confinement
■
EXPERIMENTAL SECTION
Chemicals. All reagents and solvents are commercially available
and were used as received. Zinc acetate dihydrate (Zn(OAc)2·2H2O),
lithium hydroxide monohydrate (LiOH·H2O), (3-glycidyloxipropyl)
trimethoxy-silane (≥98%), ATTO 590 (≥90%), and ATTO 565
(≥75%) were all purchased from Sigma-Aldrich Corporation. Ethanol
(EtOH, ≥98%) was purchased from Fischer Scientific.
Synthesis of ZnO NCs. Zn(OAc)2·2H2O (2 mmol) was added to
20 mL of EtOH in a round-bottom flask and refluxed for 1 h at 70 °C
under continuous stirring. The resulting Zn(II) precursor was cooled
in an ice−water bath. In another reaction vessel, LiOH·H2O
corresponding to the LiOH/Zn molar concentration ratio of 3.5 was
sonicated in 20 mL of EtOH for 30 min to achieve complete
dispersion, and then cooled down to 0 °C using an ice−water bath.
The LiOH ethanolic solution was mixed with the Zn(II) precursor at 0
°C and stirred vigorously for about an hour. The average NC size was
adjusted by controlling the starting amount of LiOH in EtOH. In
order to obtain ZnO NCs of the largest average size, 10 mL of the
Zn(II) precursor was reacted with 100 μL of 10 M aqueous NaOH
solution at 60 °C under vigorous stirring for 60 min. The unreacted
ions do not have a significant influence on the phenomena relevant for
this work, and the obtained NCs were used as synthesized without
further purification.
Preparation of Dye-Conjugated ZnO NCs. Stock suspension of
ZnO NCs and solutions of ATTO 590 and ATTO 565 having
proportional concentrations were prepared in EtOH. The initial
sample was prepared by the addition of 20 μL of stock solution of an
ATTO dye to 3 mL of ZnO NC suspension, followed by sonication
for 1 h. A series of samples were subsequently prepared by changing
the starting volume of the ATTO stock solution added to the NC
suspension in increments of 20 μL, up to the total of 200 μL.
For the preparation of NC surface-protected white light emitters, 25
μL of diluted (3-glycidyloxipropyl) trimethoxy-silane (hereinafter
referred as silane) in ethanol (4-fold dilution) was added to dyeconjugated ZnO NC suspension and further sonicated for 1 h in an ice
bath. The sample was then left undisturbed overnight to form silane
overlayer.
ZnO Nanocrystal and Nanoconjugate Characterization.
Transmission electron microscopy (TEM) imaging was performed
with JEOL 2010F microscope operating at 200 kV. The NC
suspensions were diluted, sonicated, and subsequently deposited on
copper grids with lacey Formvar/carbon support films purchased from
Ted Pella, Inc. Fourier-transform IR (FTIR) spectra were recorded
with a Bruker Tensor 27 spectrometer. For FTIR measurements the
samples were deposited on NaCl plates by drop casting the NC and
nanoconjugate suspensions, or dye molecule solutions.
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molecular fluorophores, as a potential approach to tunable
white light generation. Figure 1a shows PL spectra of a series of
Absorption and Steady-State PL Spectroscopy. All absorption
measurements were carried out at room temperature in a double-beam
mode with a Varian Cary 5000 UV−vis−NIR spectrophotometer
using a standard quartz cuvette (1 cm path length). The steady-state
PL spectra were recorded with a Varian Cary Eclipse fluorescence
spectrometer, using standard PL quartz cuvettes. The data were
recorded over the wavelength range 320−800 nm, and the slit widths
for both excitation and emission were set to 5 nm. The quantum yields
of the NC samples were determined relative to quinine bisulfate
(QBS) as the reference (the quantum yield of QBS in 1 N H2SO4 is
reported to be 0.546).50
Time-Resolved PL Measurements. Time-resolved PL measurements were performed by time correlated single photon counting
(TCSPC) method using Horiba Jobin Yvon IBH Ltd. system.
NanoLEDs (IBH Ltd.) with the output wavelengths of 340 and 563
nm were used as the excitation source for ZnO NCs and dye
molecules, respectively, and the emission was collected at the right
angle geometry. The signals were monitored at the emission maxima,
ca. 600 nm for ATTO 590 and 480 nm for ZnO NCs. Both excitation
and emission slit widths were set to 16 nm. All data were acquired over
1024 channels using 1 MHz repetition rate and 0.059 ns/channel. The
maximum intensity for a channel was 20,000 counts. The instrument
response function was recorded using a Ludox solution (SigmaAldrich) by detecting the scattered excitation. An iterative
reconvolution method was employed to analyze all decays using
IBH DAS 6.2 software. An experimental curve was fit with a program
generated curve, which was convoluted numerically with the
instrument response function. The decays were then fit to a
multiexponential function (up to three exponentials) using the IBH
DAS 6.2 program:
⎛
t⎞
I(t ) = I(0) ∑ Ai exp⎜ − ⎟
⎝ τi ⎠
1
Figure 1. (a) PL spectra of ZnO NCs in ethanol having different
average sizes. The samples were excited at the wavelength
corresponding to the maximum of the lowest lying band gap
absorption transition. Inset: Absorption spectra of the same samples.
(b) Absorption (dashed lines) and emission (solid lines) spectra of
ZnO NCs (2.4 ± 0.2 nm) and ATTO 590 dye molecules. Excitation
wavelengths are 300 nm for ZnO NCs and 590 nm for ATTO 590.
Shaded area indicates overlap of the emission spectrum of ZnO NCs
and the absorption spectrum of ATTO 590, as an essential
requirement for FRET process.
i
(1)
where I(t) and I(0) are the respective luminescence intensities at time
t and zero (immediately after the excitation by the laser pulse). Ai
values are the relative contributions to the amplitude of the decay by
the ith species involved, where
i
ΣAi = 1
1
(2)
The goodness of the fit to the experimental data was evaluated by
considering the reduced χ2 values, which should be between 0.9 and
1.2, and analyzing the randomness of the weighted residuals.
Development of the Prototype White LEDs. Monochromatic
UV LEDs (300 ± 5 nm) with flat top window, purchased from Sensor
Electronic Technology, were used to prepare prototype white LEDs.
The window of an LED was first cleaned with hexane and ethanol. A
suitable white light-emitting sample prepared using the approach
outlined above was deposited on the flat top surface of an LED in 10
μL aliquotes, followed by air drying at room temperature. The
deposition continued until sufficient thickness was achieved, resulting
in the emission of bright white light upon applying 7.0 V to the LED.
CRI and CCT of the fabricated LEDs were measured with illuminace
spectrophotometer CL-500A purchased from Konica Minolta.
colloidal ZnO NCs with varying average sizes, upon excitation
into NC band gap. These NCs exhibit intense broad-band
visible emission, which red-shifts with increasing NC size.
Although typical semiconductor NCs display significant exciton
emission, the spectra in Figure 1a are dominated by lower
energy defect emission, with no evidence of the radiative
exciton recombination. The absorption spectra of the same
NCs are shown in the inset. The lowest-lying absorption
transition also shifts to lower energy with increasing NC size
due to a decrease in quantum confinement effect. The relatively
small bandwidth and the sharp onset of absorption transition
reflect narrow size distributions of ZnO NCs.54 TEM images of
typical NCs synthesized using varying base concentrations
confirm different average NC sizes and relatively narrow size
distribution, as shown in Figure S1 (Supporting Information).
Significant shifts of the PL bands relative to the corresponding
band gap absorption transitions are consistent with the
presence of sub-band gap states associated with structural
defects. The red shift of the emission band indicates that the
corresponding defect states shift in conjunction with the
electronic band structure of NCs with increasing particle size.
Broadening of the PL spectra is mostly associated with
inhomogeneous distribution and speciation of defects in
individual NCs.43 The quantum efficiency of this luminescence
was determined to be in the range 15−20%, without any further
modification or optimization.
■
RESULTS AND DISCUSSION
Design of White Light-Emitting Nanoconjugate.
Visible emission of ZnO is an extrinsic property related to
discrete electronic states in the band gap associated with
structural defects or dopants.51−53 The luminescence of ZnO
NCs has attracted particular attention owing to its size and
shape-dependent behavior. Different origins of the observed PL
have been proposed involving donor and/or acceptor states,
often associated with specific surface sites.42,46,53 While the
mechanism of this emission is not a subject of current
investigation, we were intrigued by the possibility of coupling
broadly emitting defect states in ZnO NCs with surface-bound
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Figure 2. Schematic representation of (a) ATTO 590-conjugated ZnO NCs, indicating possible energy transfer processes from NC defect-based
excited states to the attached dye molecules upon excitation of ZnO NCs into the band gap (gray balls in the molecular model represent carbon, red
balls oxygen, and blue balls nitrogen atoms). (b) Encapsulation of ZnO-ATTO 590 nanoconjugate in a protecting layer of silane to obtain stable
single white light-emitting nanophosphor under UV-excitation (NC is shown in blue, ATTO 590 in red, and silane layer in purple).
Figure 3. (a) Absorption spectra of 2.4 nm ZnO-ATTO 590 nanoconjugates prepared with different starting concentrations of ATTO 590, as
indicated in panel b. Inset: Zoomed-in spectra of the nanoconjugates in ATTO 590 absorption region. (b) PL spectra of the same samples (λexc =
300 nm), showing the starting ATTO 590 concentrations. Thick dashed blue line indicates the spectrum corresponding to pure white light. (c,d)
PLE spectra of the nanoconjugates collected by monitoring the emission at (c) 600 nm (ATTO 590 PL maximum) and (d) 480 nm (ZnO PL
maximum). Arrows in panels a−d indicate the trend in intensity of the corresponding transitions with increasing ATTO 590 concentration. (e) CIE1931 diagram showing the color coordinates (dots) corresponding to the spectra in panel b. The encircled green dot indicates nearly pure white light
(0.330, 0.325). (f) Photographs of a colloidal suspension of white light-emitting ZnO-ATTO 590 nanoconjugates (left), an LED prepared from the
same sample (top right), and a quartz substrate coated with the same sample without exposure to UV light (middle right) and when exposed to UV
light (bottom right).
One of the attractive features of colloidal NCs is the
possibility of their manipulation in the solution phase, including
the conjugation with molecular species to form new structures
with enhanced functionality. Given the large bandwidth and
energy range of ZnO NC PL, we propose that immobilization
of a fluorophore having a complementary PL spectrum on the
surface of appropriately sized NCs can generate white light of
desired chromaticity by Förster resonance energy transfer
(FRET) mechanism. The primary requirements for FRET are
an overlap between a donor emission and an acceptor
absorption spectra, and a short separation between the donor
and acceptor (<10 nm). The ATTO 590 dye has a maximum
absorbance at 585 nm, whereas the emission spectrum is
Stokes-shifted by ca. 20 nm, which is complementary to the
broad blue emission of ZnO NCs having an average diameter of
2.4 ± 0.2 nm (Figure 1b). It is also evident from Figure 1b that
the ZnO NC emission and the ATTO 590 absorption spectra
have significant overlap, potentially enabling their electronic
coupling via FRET. The dye absorbance around 300 nm is
negligible, allowing for the selective excitation of ZnO NCs.
Figure 2a illustrates the concept of the white light generation
using dye-conjugated ZnO NCs. Broad-band defect-based blue
PL of ZnO NCs allows for a partial resonance energy transfer
to orange-red chromophores (i.e., ATTO 590) conjugated on
NC surfaces, resulting in the formation of a new quasi-single
fluorophore, which simultaneously emits from both centers by
single energy excitation. Although the origin of defect-based
ZnO NC emission has not yet been fully understood, there are
strong indications that it involves surface states and possibly
multiple emissive centers.42,53,55 Van Dijken et al. have
proposed that the visible emission is due to the recombination
of an electron at the conduction band edge, or a level close to
the band edge, with deeply trapped hole localized at an oxygen
42,56
It has been further
vacancy site in a NC (V••
o center).
suggested that this process is enabled by the transfer of the
photogenerated hole from the valence band to the V•o site,
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which is mediated by initial trapping of the hole at an O2− or
OH− surface site.42,55,56 Lower energy contribution to the
visible ZnO NC emission has been associated with the
recombination of deeply trapped electrons with valence band
holes (or holes in shallow trap states).43,44,46 It is likely that free
carboxylic acid groups in ATTO 590 facilitate binding of the
molecule to colloidal ZnO NC surfaces allowing for the
formation of a FRET pair, where trapped charge carriers on
ZnO defects transfer energy to the adsorbed dye molecules
upon excitation into the ZnO band gap (Figure 2a). This
nanoconjugate can be further modified to optimize and
enhance its properties, e.g., increase its stability and efficiency
(Figure 2b). Specifically, encapsulation of the conjugated NCs
in a protecting layer of a stabilizing agent could enhance its
stability as a single white light-emitting phosphor (vide inf ra).
Spectroscopic Properties of ATTO 590-Conjugated
ZnO NCs. The absorption spectra of typical ZnO-ATTO 590
nanoconjugate suspensions prepared using increasing concentrations of ATTO 590 (from 0 to 170 nM) are displayed in
Figure 3a. The broad shoulder centered at ca. 300 nm is due to
the NC band gap absorption, while weaker asymmetric band
peaking at ca. 590 nm is assigned to S0 → S1 transition of
ATTO 590 (inset). The molar extinction coefficient of ATTO
590 in ethanol at the maximum of S0 → S1 transition is ca.
120,000 cm−1 M−1. The PL spectra of the same samples upon
excitation at 300 nm are shown in Figure 3b. The increasing
concentration of the dye leads to a decrease in the intensity of
the defect emission concurrently with an increase in the dye
emission. This observation suggests electronic coupling
between NCs and dye molecules, consistent with the
nanoconjugate formation. The increase in the ATTO 590
emission cannot be associated with the direct excitation of the
dye because free ATTO 590 emission is more than an order of
magnitude weaker than the observed emission for this
excitation wavelength (Figure S2 in Supporting Information).
The possibility of coupling via electron transfer is also ruled out
since the absorption spectra of the nanoconjugates are
qualitatively similar to the spectrum of a simple mixture,
showing no evidence of characteristic charge transfer
transitions. Therefore, the simultaneous quenching of ZnO
NC and an increase in ATTO 590 emission can be ascribed to
FRET, similarly to the previously reported examples.47,48
Furthermore, the presence of an isosbestic point in the
emission spectra at ca. 580 nm (Figure 3b) indicates that
both quenching of the NC trap emission and the enhancement
of ATTO 590 emission occurs directly and is due to the same
process. The corresponding PL excitation (PLE) spectra are
shown in Figure 3c,d. The excitation spectra of ATTO 590 in
the molecular form (dashed line) and bound to ZnO NCs
(solid lines) are compared in Figure 3c. In the absence of ZnO,
the excitation spectrum (PL monitored at 600 nm) shows an
intense maximum at ca. 585 nm and a set of weak features at
250−400 nm, characteristic for ATTO 590. After conjugation
with ZnO NCs the excitation spectra for the emission at the
same wavelength are dominated by a broad structured band
peaking at ca. 300 nm, characteristic for the ZnO exciton state.
These results indicate that the ATTO 590 emission at 600 nm
occurs upon the excitation of ZnO NCs and affirm that energy
transfer takes place from the NCs to the dye adsorbates. We
also recorded the excitation spectra of ZnO NCs (PL
monitored at 480 nm) in the presence of different
concentrations of ATTO 590 (Figure 3d). As expected, the
excitation spectra display the features characteristic of the ZnO
band gap transition, similarly to Figure 3c. Importantly, with
increasing concentration of dye the maximum PLE intensity
around 300 nm decreases even though ZnO NC concentration
remains unchanged (Figure 3a). This observation further
confirms the energy transfer from ZnO NCs to conjugated
ATTO 590.
The ATTO 590 contains carboxylic groups, which can allow
for binding of the molecule to ZnO NCs. Shift of the
characteristic peaks in the FTIR spectrum of ATTO 590 was
observed after the addition of the dye to the ZnO NC stock
suspension, suggesting that the majority of dye molecules bind
to NC surfaces (Figure S3 in Supporting Information). Upon
conjugation, ATTO 590 exhibits a change in the electronic
structure, evidenced by a distinct red shift of both absorption
and PL transitions relative to those in free molecules in solution
(Figure S4 in Supporting Information). The red shift of S0 →
S1 transition is accompanied by an extension of its average
emission lifetime from 3.5 to 4.2 ns upon direct excitation due
to reduced nonradiative decay rate associated with restricted
molecular motion of NC-conjugated ATTO 590 (Figure S5 in
Supporting Information). To establish whether nonbound dye
could also quench ZnO PL, we first prepared silane-coated
ZnO NCs having emission maximum at ca. 480 nm and then
titrated different concentrations of ATTO 590. In this
configuration silane coating prevents direct conjugation of
dye molecules on the NC surface, which in turn prevents
energy transfer from ZnO to ATTO 590 (Figure S6 in
Supporting Information).
Figure 3e shows the CIE-1931 chromaticity diagram
indicating the color coordinates corresponding to the spectra
in Figure 3b. With increasing amount of ATTO 590 per ZnO
NC, the perceived emission color gradually transforms from
blue to orange-red. This color transformation can be controlled
with high precision and reproducibility over a wide range. We
accentuate this point by demonstrating the generation of nearly
pure white light, represented by color coordinates (0.330,
0.325), using 32 nM ATTO 590 solution and 2.4 nm ZnO NCs
(indicated with a dotted circle in Figure 3e). The white light
point in the CIE-1931 diagram corresponds to the spectrum
shown with a thick dashed line in Figure 3b. The colloidal
suspensions of ATTO 590-conjugated ZnO NCs are
completely transparent, potentially allowing for their incorporation into transparent films and optical windows. Photographs
of this white-emitting sample in the colloidal form, incorporated into an LED, and as a transparent film prepared on a
quartz substrate are shown in Figure 3f. A thin transparent layer
of NCs generates bright white illumination that is sufficiently
strong to be visible even in daylight. Furthermore, CRI, as a
quantitative measure of the ability of a light source to reproduce
an object color, was measured to be up to 95 for the prepared
LEDs. This value is better than CRI of a typical cathode
fluorescent lamp (∼80) measured under the same conditions
with the same instrument. The correlated color temperature of
this particular sample is ∼5300 K, which is very similar to the
typical values obtained using complex rare earth or heavy metalbased phosphors.57−59
Characterization of FRET by Steady-State and TimeResolved Measurements. According to the Förster theory,
the rate of energy transfer for an isolated single donor−
acceptor pair separated by a distance r is given by
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k T(r ) =
6
1 ⎛ R0 ⎞
⎜
⎟
τD ⎝ r ⎠
(3)
where τD is the lifetime of the donor in the absence of the
acceptor, and R0 is known as the Förster distance, or the
distance at which the energy transfer efficiency is 50%. At the
Förster distance, r = R0, the energy transfer rate, kT(r), is equal
to the spontaneous decay rate of the donor. The Förster
distance (R0) is defined as
R 06 =
9000(ln 10)κ 2ϕD
128π 5Nn 4
J (λ )
(4)
where ϕD is the quantum yield of the donor in the absence of
acceptor, n is the refractive index of the surrounding medium,
N is Avogadro’s number, and κ2 describes the relative
orientation of the interacting donor and acceptor transition
dipoles in space. This κ2 factor usually reflects random motion
of the donor and acceptor and is assumed to have the value of
2/3. J(λ) is spectral overlap integral, which is defined as
J (λ ) =
∫0
∞
FD(λ)εA (λ)λ 4 dλ
(5)
where FD(λ) is the emission intensity of the donor in the range
λ to λ + Δλ, with the total integrated intensity normalized to 1,
and εA(λ) is the extinction coefficient of the acceptor at
wavelength λ.
The energy transfer efficiency (η), which represents the
fraction of photons absorbed by the donor that are transferred
to multiple acceptors surrounding the donor, is related to the
energy transfer and donor decay rates, and as such can be
expressed via R0
η=
Figure 4. (a) Normalized ZnO NC PL intensity (red dots) and FRET
efficiency (black squares) as a function of dye molecule concentration,
upon excitation at 300 nm. Solid black line represents best fit to the
experimental data (eq 7). (b) Time-resolved PL decay of ZnO NCs assynthesized (black) and conjugated with ATTO 590 using 52 (blue),
98 (green), and 147 nM (gray) ATTO 590 solutions. The excitation
wavelength was 340 nm, and emission was detected at 480 nm.
Dashed red lines are exponential fits to the decay curves.
11 to 72% as the ATTO 590 concentration increases from 9 to
170 nM. The Poisson-corrected FRET efficiency (eq 7) shows
a good fit to the sixth power dependence on the donor−
acceptor separation distance (Figure 4a, solid line). The NC PL
intensity decreases symmetrically to the increase in FRET
efficiency with dye NC-surface coverage, affirming the energy
transfer mechanism based on the Förster theory. To further
study the mechanism and dynamics of the energy transfer in
hybrid ZnO-ATTO 590 nanoconjugates, we carried out timeresolved PL studies of ZnO NCs in the presence of the
increasing amounts of ATTO 590 (Figure 4b). The colloidal
samples were excited at 340 nm (ZnO NC band edge) using a
pulsed laser, while the PL signal was detected at the maximum
intensity of the NC emission band (480 nm). The temporal
decays in the absence and presence of ATTO 590 molecules
are roughly triexponential. Notably, the decay rate of ZnO NC
emission becomes higher with increasing concentration of
ATTO 590 attached to NCs, consistent with the FRET
mechanism. ATTO 590 quenches both the fast and slow
components of the defect luminescence of ZnO. The average
lifetime ⟨τ⟩ of ZnO emission is found to decrease from ca. 45 to
34 ns in the studied concentration range. The temporal
parameters obtained from the time-resolved measurements are
provided in the Supporting Information (Table S1). Taken
together, steady-state and time-resolved PL results confirm the
resonance energy transfer from ZnO NC defect states to
conjugated ATTO 590 fluorophore molecules.
Stability of White Light-Emitting Nanoconjugate. In
spite of the demonstrated promise of ZnO-ATTO 590
nanoconjugate for generating and tuning white light emission
by controlling the FRET process, PL of colloidal ZnO NCs
tends to degrade over time.47,61,62 This phenomenon has
nR 0 6
nR 0 6 + R6
(6)
where n is the acceptor/donor ratio or discrete number of
acceptors attached to a donor, and R is the average donor−
acceptor distance for the donor−multiacceptors conjugate.
Recent report has demonstrated that incorporation of Poisson
correction is necessary to account for the distribution of
integer-number dye/NC ratio. The modified Poisson-corrected
FRET efficiency can be expressed as
∞
η (ξ , R ) =
∑
n=0
6
⎤
ξ ne−ξ ⎡ nR 0
⎢ 6
⎥
6
n! ⎣ R + nR 0 ⎦
(7)
where n is a whole-number acceptor to donor ratio and ξ is the
average number of dyes per NC.60 From the steady-state PL
spectra in Figure 3b, the FRET efficiency can also be calculated
using the equation
η=1−
FDA
FD
(8)
where FD and FDA are the relative emission intensities of the
donor ZnO NCs in the absence and presence of dye molecule
acceptors, respectively. Figure 4a shows the FRET efficiencies
obtained from the steady-state PL spectra (black squares) and
the normalized ZnO NC PL intensity (red dots) as a function
of the concentration of ATTO 590 molecule added to the
suspensions of the fixed concentration of ca. 2.4 nm NCs. The
Förster distance and the spectral overlap integral calculated
using eqs 4 and 5 are determined to be 38.6 Å and 1.97 × 1015
M−1 cm−1 nm4, respectively. The FRET efficiency varies from
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demonstrated in Figure 5b. Contrary to unprotected ZnOATTO 590 nanoconjugate, the PL intensities at 480 and 600
nm (peak maxima) for silane-capped ATTO 590-conjugated
ZnO NCs do not change significantly over a long period of
time under ambient conditions. Similarly, the silane-protected
nanoconjugates experience no change in the spectral density
distribution and color coordinates (Figure 5b, inset). These
results clearly demonstrate that silane-protected ZnO-based
hybrid nanophosphor samples not only display pure white light
emission but also show enhanced stability over a long period of
time.
Generality of ZnO-Based Nanophosphor. Resonance
energy transfer described by the Förster theory is a universal
phenomenon, which makes the demonstrated mechanism for
generating white light broadly applicable for a variety of
different fluorophores. To corroborate the generality of the
FRET-based inorganic−organic hybrid nanoconjugate approach for the realization of single-phased white light emitting
phosphor, we demonstrated the ZnO-ATTO 565 system,
which also exhibits near white light emission. This versatility
provides an opportunity to tune the chromaticity of white light
by simply selecting a molecular conjugate emitting the
complementary spectral density to a given NC size. Similarly
to ATTO 590, ATTO 565 dye also has a free carboxylic
functional group, which can anchor to the ZnO NC surfaces
(Figure S8 in Supporting Information), but the S0 → S1
transition is blue-shifted relative to that for ATTO 590. This
blue shift induces larger spectral overlap with the PL band of
ZnO NCs, requiring somewhat smaller average NC size to
produce light of the same chromaticity, based on the color
coordinates of the two coupled fluorophores. Figure 6a shows
the absorption spectra of ca. 2.2 nm ZnO-ATTO 565 hybrid
nanoconjugates for different starting concentrations of ATTO
565 added to NC stock suspension. The PL spectra of the same
samples excited into the NC band gap are shown in Figure 6b.
A decrease in the ZnO NC emission intensity accompanied by
an increase in ATTO 565 intensity with increasing number of
molecules per NC is consistent with the excitation of ATTO
565 by FRET, similarly to the ZnO-ATTO 590 nanoconjugates
(vide supra). The corresponding excitation spectra are shown in
Figure 6c. The excitation spectra recorded for the emission at
575 nm are dominated by ZnO exciton transition, clearly
indicating FRET-mediated excitation of conjugated ATTO 565
molecules. The apparent colors associated with the spectra in
Figure 6b are indicated in the CIE-1931 diagram (Figure 6d).
The sample emitting pure white light is designated with a
dotted circle and corresponds to the ATTO 565 starting
concentration of 21 nM. The photographs of this white lightemitting sample in colloidal form and coated on a commercially
available UV LED are shown in the insets of Figure 6d. The
CRI and CCT of a typical LED were measured to be ca. 92 and
5100 K, respectively.
We also calculated the FRET efficiency for ZnO-ATTO 565
nanoconjugates. Figure 7 shows the FRET efficiencies and
normalized ZnO NC emission intensities, obtained from the
steady-state PL spectra, as a function of the concentration of
ATTO 565 added to NC stock suspensions. The FRET
efficiency varies from 11 to 76% as the concentration of ATTO
565 increases from 75 to 147 nM, following the same functional
form and correlation with donor PL intensity as in Figure 4a.
We also performed time-resolved PL measurements for ATTO
565-conjugated ZnO NCs (Figure S9 in Supporting
Information). Time-resolved data show that in the presence
generally been associated with dynamic nature of NC surfaces
and their structure, which are suggested to play a key role in PL
of ZnO NCs (vide supra).55,63 The inset of Figure 5a shows the
Figure 5. Maximum intensity of the PL emission band of ZnO (black
squares) and ATTO 590 (red dots) for typical white light-emitting
ZnO-ATTO 590 nanoconjugate upon excitation into ZnO band gap
(300 nm). (a) As-prepared nanoconjugate and (b) nanoconjugate
encapsulated in a protective layer of silane. Insets show the PL spectra
from which the data points were obtained, indicating the number of
days upon synthesis.
evolution of the PL spectra of as-synthesized 2.4 nm ZnO NCs
conjugated with ATTO 590 over time. The measured PL
intensities at the ZnO (black squares) and ATTO 590 (red
dots) peak maxima are shown in Figure 5a. The emission
intensities of this sample show dramatic change over a period of
40 days. The ZnO PL decreases in intensity by an order of
magnitude and red shifts from ca. 480 to 510 nm 40 days after
the sample preparation. Decrease in the ZnO PL intensity and
the spectral shift to lower energies indicate gradual increase in
NC size, likely due to aggregation and Ostwald ripening, and
accompanying change in the structure and concentration of NC
surface defects. The decrease in the ZnO PL intensity reduces
the amount of energy that can be transferred to the acceptor
molecules, leading to a simultaneous decrease in ATTO 590 PL
intensity and the overall light emission efficiency. In addition to
a decrease in the efficiency, the red shift of ZnO NC emission
also impacts the chromaticity of the generated white light. To
enhance the stability of this hybrid nanophosphor we
encapsulated ATTO 590-conjugated ZnO NCs in a silane
layer to protect the NC surface sites responsible for ZnO NC
emission. The encapsulation of the nanoconjugate is verified by
the comparison of the FTIR absorption spectra of ATTO 590conjugated ZnO NCs before and after the addition of silane
(Figure S7 in Supporting Information). The stability of the
emission of the silane-protected colloidal nanophosphor is
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Figure 6. (a) Absorption spectra of 2.2 nm ZnO-ATTO 565 nanoconjugates prepared with different starting concentrations of ATTO 565, as
indicated in panel b. Inset: Zoomed-in spectra of the nanoconjugates in ATTO 565 absorption region. (b) PL spectra of the same samples (λexc =
300 nm), showing the starting ATTO 565 concentrations. Dashed pink line indicates the spectrum corresponding to pure white light. (c) PLE
spectra of the nanoconjugates collected by monitoring the emission at 575 nm (ATTO 565 PL maximum). Inset: PLE spectra collected by
monitoring the emission at 480 nm (ZnO PL maximum). Arrows in panels a−c indicate the trend in intensity of the corresponding transitions with
increasing ATTO 565 concentration. (d) CIE-1931 diagram indicating the color coordinates corresponding to the spectra in panel b. The encircled
dot indicates nearly pure white light (0.332, 0.336). Insets: photographs of the colloidal suspension of white light-emitting ZnO-ATTO 565
nanoconjugates and an LED prepared from the same sample.
and native defect formation65 allows for further optimization of
the chromaticity and efficiency of these hybrid nanoconjugates.
■
CONCLUSIONS
In summary, we reported tunable white light generation by dyeconjugated colloidal ZnO NCs, prepared by a simple and
flexible method. This hybrid organic−inorganic fluorophore
acts as a single nanophosphor owing to the electronic coupling
between ZnO NCs and the organic dye by FRET. The energy
transfer is enabled by the spectral overlap between a broad
native defect-induced emission band of ZnO NCs and S0 → S1
absorption of conjugated dye molecules. The ability to
modulate blue ZnO emission by NC size and the orange-red
dye emission by the type and concentration of molecular
conjugates allows for a precise tuning of the white light
characteristics, including chromaticity, correlated color temperature, and color rendering index. Additionally, the emission
tunability implies that even though different hybrid nanoconjugates have different absorption spectra, they can have the
same or similar emission color coordinates. This phenomenon
can help avoid discrete color mixing from multiple phosphors,
regardless of the number and type of hybrid nanoconjugates
used. Unlike traditional solid state phosphors the colloidal
nanoconjugates described in this article can be modified using
solution chemistries to optimize their properties or introduce
new functionalities. We demonstrated that silane-protected
ZnO-based hybrid nanophosphor samples show enhanced
stability of white light emission. Furthermore, the colloidal
form of these NCs allows for their easy manipulation using
chemical means, including their incorporation into lightemitting devices. The results of this work demonstrate
Figure 7. (a) Normalized ZnO NC PL intensity (red circles) and
FRET efficiency (black squares) ZnO-ATTO 565 nanoconjugate,
determined from steady-state PL data in Figure 6b, as a function of dye
molecule concentration. Solid black line is the best fit of eq 7 to the
experimental data.
of NC-bound ATTO 565 the average lifetime of ZnO emission
decreases from 45 to 35 ns. From these experiments we can
infer that ZnO-ATTO 565 hybrid nanoconjugates behave as
FRET-based single white light emitting nanophosphor,
similarly to ZnO-ATTO 590 pair. Furthermore, a comparison
of the results for ATTO 565 and 590 conjugated on ZnO NCs
demonstrate the accuracy and precision with which the
chromaticity can be controlled for different dyes or by changing
the average NC size by as little as 0.2 nm. Taken together, the
results of this work enable tunability of white light emission
using inexpensive facile methodology and earth abundant
materials. The ability to manipulate visible emission of colloidal
transparent metal oxide NCs by controlling their composition64
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inexpensive, environmentally benign, and rare earth or heavy
metal-free approach to generating white light having improved
properties relative to currently used WLEDs. We therefore
believe that the ZnO-based hybrid nanophosphors described in
this article have potential for fabrication of cost-effective,
greener, and more sustainable light emitting devices.
■
ASSOCIATED CONTENT
S Supporting Information
*
TEM images of ZnO NCs, control FRET experiments, FTIR
spectra of ZnO-ATTO 590 nanoconjugate, spectroscopic data
comparing ATTO 590 in bound and free form, effect of
distance on FRET efficiency, spectral overlap of ZnO PL and
ATTO 565 absorption, time-resolved ZnO PL decay for ATTO
565-conjugated ZnO NCs, PL decay parameters for ZnO NCs
and ZnO-dye nanoconjugates. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by NSERC (Idea to Innovation
(I2IPJ 419645-11 052902) and Discovery grants) and Ontario
Centres of Excellence (Project No. 20074). P.V.R. acknowledges generous support by the Canada Research Chairs
program.
■
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