Reversible ON/OFF switching of photoluminescence from CsPbX3 quantum dots coated with silica using photochromic diarylethene

Article abstract of DOI:10.1039/c9cc03797g

Highly luminescent silica-coated CsPbX₃ quantum dots (QDs) with good photostability were synthesized and coupled with photochromic diarylethene to modulate the QDs' photoluminescence (PL). Upon successive UV and visible light irradiation, the P

Full text of DOI:10.1039/c9cc03797g

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Reversible ON/OFF switching of photoluminescence from CsPbX3
quantum dots coated with silica using photochromic diarylethene
Yuji Akaishi,a Azzah Dyah Pramata,a Shuhei Tominaga,a Shimpei Kawashima,a Tsuyoshi Fukaminato*b
Received 00th January 20xx,
Accepted 00th January 20xx
and Tetsuya Kida*b
DOI: 10.1039/x0xx00000x
Highly luminescent silica-coated CsPbX3 quantum dots (QDs) with a
Here, we demonstrate a strategy that can be used to reversibly
good photostability were synthesized and coupled with modulate PL from colloidal CsPbBr3 QDs using photochromic
photochromic diarylethene to modulate QDs’ photoluminescence diarylethene. There have been pioneering reports in modulating
(PL). Upon successive UV and visible light irradiation, the PL PL of semiconductor QDs such as CdSe/CdS/ZnS using
emission from silica-coated CsPbX3 QDs was repeatedly quenched diarylethene derivatives.31-34 However, the PL quenching
and restored, demonstrating the promising feasibility of the efficiency remains low, e.g. 40% and thus the substantial
QD/diarylethene-based photoswiches.
improvement is desired. Fig.1 shows the molecular structure of
diarylethene,
cyclopentene
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluoro-
, which was used in this study, together with its
Lead halide perovskite, with novel electrical and optical
properties,1-6 have drawn considerable attention as emerging
photonic materials in recent years. They are applied in various
fields such as light-emitting diodes (LEDs),5-9 displays,10,11 and
solar cells.12 Particularly, all inorganic CsPbX3 (X = Cl, Br, I)
perovskite quantum dots (QDs) possess many advantages
including tunable photoluminescence (PL) emission color, sharp
emission peak with a small FWHM (full width at half-maximum)
and high PL quantum yields. In addition, CsPbX3 QDs are easily
synthesized by simple liquid phase routes, which produce cube-
shaped nanocrystals (nanocubes).13 These characteristics make
the QDs optimal candidates for several optoelectronic
applications.
1
UV-vis absorption spectra under UV and visible light irradiation.
The molecular structure can be switched from open- to closed-
ring forms via cyclization upon UV light irradiation and returned
from closed- to open-ring forms via cycloreversion upon visible
light irradiation. Accordingly, the absorbance and the color of
the molecule reversibly change. Scheme 1 shows the PL
switching mechanism in the CsPbBr3 QD/diarylethene system.
Irradiating diarylethene with UV light produces a closed-ring
isomer having absorption in the visible region, thus quenching
The reversible PL modulation of semiconductor QDs is
attractive because of the expectation that their emission control
would lead to many applications including imaging probes,16
chemo and biosensing,17 smart windows,18 information
processing19 and optical memories.20 The functionalization of
QDs with organic materials has been extensively studied to
create novel functional materials by synergetic combination of
inorganic and organic components.21-23 In particular,
photochromic molecules would be promising counterparts to
control the PL emission from QDs because of their remote
accessibility, robust reversibility, and no waste during switching
processes.24-30 Among many kinds of photochromic molecules,
diarylethene and its derivatives are ideal photoswitches due to
their high fatigue resistance and excellent thermal stability.28,30
Fig. 1 (a) Molecular structures of open-ring and closed-ring
isomers of diaylethene 1. (b) Photographs of a solution containing
diarylethene before and after UV (365 nm) and visible light
irradiation (λ > 530 nm). (c) Absorption spectra of open-ring
(black line) and closed-ring forms (red line) of diarylethene in a
toluene solution.
a. Department of Applied Chemistry and Biochemistry, Graduate School of Science
and Technology, Kumamoto University, Kumamoto 860-8555, Japan
b. Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto
860-8555, Japan
E-mail: tetsuya@kumamoto-u.ac.jp (T.K.), tuyoshi@kumamoto-u.ac.jp (T.F.)
† Electronic Supplementary Information (ESI) available: Experimental details and
additional figures. See DOI: 10.1039/x0xx00000x
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Scheme 1 Schematic illustration of the PL switching mechanism.
The energy transfer from CsPbBr3 QD to diarylethene occurs after
UV light irradiation (Off state), but it does not occur after visible
light irradiation (On state).
PL from QDs by Förster Resonance Energy Transfer (FRET).
The QD and diarylethene with the closed-ring form act as a
fluorescence donor and acceptor, respectively. When irradiated
with visible light, the closed-ring isomer is converted to the
open-ring one having no absorbance in the visible region,
resulting in the restoration of PL. This quenching and restoration
F
ig. 2 TEM image of (a) native CsPbBr3 and (c) silica-coated
CsPbBr3 QDs. UV-vis absorption and PL emission spectra of (b)
native CsPbBr3 and (d) silica-coated CsPbBr3 QDs. The excitation
wavelength is 390 nm for both QDs.
process can be repeated because of the excellent photochromic hydrolysis of methoxy groups in APTMS, followed by a
properties of diarylethene.
condensation reaction that produced -Si-O-Si- networks,
The synthesis route of diarylethene
1
and its detailed amorphous silica layers should form on the QD’s surface. The
procedure are depicted in Electronic Supplementary Information progressive extension of -Si-O-Si- networks involving
(ESI†). CsPbBr3 QDs were synthesized from CsBr and PbBr2 in neighbouring NCs finally produced rather aggregated
toluene by a supersaturated-recrystallization method according nanoparticles. The XRD results (Fig. S2, ESI†) confirmed that
to the literature.5 However, the chemical stability of CsPbX3 QDs the silica coating also changed the shape of individual QDs,
is poor; they immediately decompose when in contact with water producing particles without oriented growth direction. The
and polar solvents.14,15 Their stability improvement is one major crystal structure changed to orthorhombic. TEM and high angle
challenge for practical applications. In this study, we coated the annular dark-field scanning transmission electron microscopy
QDs with silica shells to improve the stability and ensure (HAADF) images (Fig. S4, ESI†) show that the particles were
reversible PL modulation. Coating QDs with silica shells was covered by shells with a less contrast. The presence of SiO2
carried out just by adding (3-aminopropyl)trimethoxysilane shells was confirmed by energy dispersive X-ray spectroscopy
(APTMS) in a precursor solution and crystallizing QDs that were (EDS) point analysis. A high-resolution TEM (HR-TEM) image
coated with amorphous silica. The detailed procedure is
(Fig. S4) clearly indicates that the core of CsPbBr3 nanocrystals
described in ESI†. Fig. 2a shows a representative transmittance remained intact even after amorphous SiO2 coating. Fig. 2d
electron microscopy (TEM) image of native CsPbBr3 QDs. A shows UV-vis absorption and PL emission spectrum of the
lower magnification image is also shown in Fig.S1 (ESI†). The CsPbBr3 QDs coated with silica. The emission peak and the
image clearly show the formation of cube-shaped nanocrystals excitonic peak were red-shifted after silica coating, which is
(NCs) of 12 nm in size. The crystal structure of the QDs was probably due to the delocalization of the electron wave function
identified to be cubic by XRD analysis, as shown in Fig. S2 from CsPbBr3 core into silica shell. This would lower the
(ESI†). The highly oriented XRD peaks in (100) direction also electron confinement energy, as often seen for CdSe/CdS
supports the formation of well-defined cube-shaped NCs with core/shell nanocrystals.36 The results suggest that the silica
{100} plane. Fig. 2b shows UV-vis absorption and PL emission coating did not significantly change the optical properties of
spectrum of the CsPbBr3 QDs in toluene. There is a clear CsPbBr3.
excitonic peak at 501 nm in the absorption spectrum. The QDs
First, the native CsPbBr3 QDs was mixed with diarylethene 1
exhibited a sharp emission peak centered at 515 nm with a and changes in optical properties of the system were monitored.
FWHM of 19 nm. The results suggest the good crystal quality of Fig. 3a and 3b show the emission spectra changes and
the QDs, which are comparable with high quality II–VI photographs of a toluene solution containing the native CsPbBr3
semiconductor QDs such as CdSe.35 TEM images of silica- QDs coupled with diarylethene
1 upon photoirradiation,
coated CsPbBr3 QDs are displayed in Fig. 2c and Fig. S3 (ESI†). respectively. After irradiation with 365 nm UV light (0.5 mW),
The images show that the aggregation of NCs occurred by the color of the solution changed to blue. As a result, the intensity
introduction of aminosilane to produce nanoparticles of ca. of PL emission from the QDs significantly decreased (“OFF”
30~40 nm in size. It is probable that the surface of QDs were state). Very weak PL emission was observed even under UV
capped with APTMS in the course of synthesis. By the
excitation. Upon irradiation with visible light (λ > 530 nm, 150
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Fig. 4 PL emission spectra of (a) silica-coated CsPbBr2Cl and (b)
silica-coated CsPbBrI2 QDs in toluene containing diarylethene
(1) upon alternate irradiation with UV and visible light.
number of diarylethene relative to that of QDs is sufficient to
completely cover the whole surface of all of QDs.
Despite the successful PL modulation at the first cycle, a 40%
decrease in PL intensity at the “ON” state was observed after the
2nd cycle. This should be due to the degradation of CsPbBr3 QDs.
A significant decrease in PL intensity of CsPbBr3 QDs by
continuous light irradiation, oxygen and moisture exposure, and
annealing has been reported.14,15 To avoid the PL loss due to the
degradation of the QDs, we performed the reversible PL
modulation using the silica-coated CsPbBr3 QDs. Fig. 3c shows
the emission spectra of the silica-coated CsPbBr3 QDs in the
Fig. 3 PL emission spectra of (a) native CsPbBr3 QDs and (c)
silica-coated CsPbBr3 QDs in toluene containing diarylethene
upon alternate irradiation with UV (365 nm) and visible light (λ >
530 nm). Photographs depicting the PL modulation cycles of (b)
native CsPbBr3 QDs and (d) silica-coated CsPbBr3 QDs in toluene
containing diarylethene under room light and UV light.
presence of diarylethene
1 upon alternate irradiation with UV
mW), the solution color returned to the original and the green and visible light. The PL intensity reversibly decreased and
emission from the QDs became visible again under UV recovered upon alternate irradiation with UV and visible light,
irradiation (“ON” state). The results prove the validity of our idea respectively. It should be noted that the gradual decrease in PL
to control the emission from perovskite QDs using photochromic intensity at the “ON” state was successfully suppressed, enabling
molecules, as shown in Scheme 1. Inset of Fig. 3a shows the reversible PL modulation. In a separate experiment, we
repeating cycles of the PL ON/OFF switching. The PL intensity examined the photostability of the QDs. The silica-coated
reversibly changed upon alternating the irradiation with UV and CsPbBr3 QDs showed better stability than the uncoated QDs
visible light. The cycle was repeated three times. In each cycle, against alternate irradiation with UV and visible light in air, as
the “ON” and “OFF” states can be clearly distinguished. The shown in Fig. S6 (ESI†). It is possible that some of
response time of the photoswitching from the “ON” to “OFF” photogenerated electrons and holes in native QDs can reach the
states was 5 min when irradiated with 365 nm UV light (0.5 surface and react with oxygen in solution to produce reactive
mW). On the other hand, a change in PL intensity was saturated radical species that attack the QDs themselves under UV
after 60 sec when using high intensity UV light (300 mW), as irradiation. Direct hole attack to the QDs is also possible.
shown in Fig.S5 (ESI†), suggesting that the response time was Contrary to that, coating the surface with an insulating silica
strongly dependent on the power of light sources. Thus, the layer prevented such photocatalytic reactions, which resulted in
control of light intensity and also the concentrations of QDs and the stable ON/OFF switching of PL. The long-term
diarylethene in solution is important to achieve a quick ON/OFF photostability of the QDs was also tested by irradiating them
switching.
A noticeable finding is that the reversible PL modulation was
with strong UV (196 mW) and visible light (160 mW) for 120 h
Fig. S7). The silica-coated CsPbBr3 QDs showed much better
(
realized just by mixing the fluorescence donor and the acceptor photostability than the native QDs although the PL intensity
in solution without chemically binding the two together. The decreased to almost half of the original value after 120 h
quenching efficiency was more than 90%, which is higher than irradiation. In addition, we revealed that the silica-coated QDs
that of the previous reports that chemically attached diarylethene were much more stable in ethanol and acetone than the native
onto QDs.31-34 The sharp PL at 515 nm did not efficiently drive QDs, as shown in Fig. S8 (ESI†).
pre-existing closed-form diarylethene back to its open form that
has no quenching capability. In this experiment, QDs and switching using blue emitting silica-coated CsPbBr2Cl QDs and
diarylethene were mixed in toluene at 2 : 1 in molar ratio, which red emitting silica-coated CsPbBrI2 QDs. The optical properties
corresponds to ca. 3400 molecules of diarylethene per one of the above QDs are shown in Fig. S9 (ESI†). As seen in Fig.
nanocube of CsPbBr3. Taking into consideration the molecular , reversible ON/OFF switching of blue and red PL was attained
(ca. 0.5 nm2 at the maximum) and using the two different QDs. The decrease in PL at the “ON”
Furthermore, we tried to achieve multi-color ON/OFF
1
4
cross section of diarylethene
1
the surface area of a single 12 nm nanocube (864 nm2), the
state was also suppressed by coating the QDs’ surface with silica.
The success of this reversible modulation of different colours
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would allow for the modulation of white light upon alternate
irradiation with UV and visible light. However, the complete
quenching of PL was not achieved in all the cases. One probable
reason is the mismatch of the absorption peak of the closed ring-
isomer and the emission peaks of the QDs, i.e. a small FRET
overlap integral particularly for the blue and green emitting QDs.
Note that the emission from the red emitting QDs was more
efficiently quenched although a red shift in emission wavelength
was seen after ON/OFF switching. It is probable that the
emission peak of the CsPbBrI2 QDs can be deconvoluted into
two peaks peaking at 630 and 640 nm that occurred via different
electron-hole recombination pathways. The results suggest that
the peak at 630 nm was dominantly quenched because of the
larger FRET overlap integral, leading to the apparent shift in
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emission wavelength from 630 to 640 nm, as shown in Fig.4b
Currently, tuning the molecular structure of diarylethene and
.
1
the composition of the QDs are under way to perfectly quench
the PL emission.
In conclusion, we demonstrate a simple, convenient, and
efficient way to modulate photoluminescence using colloidal
CsPbX3 QDs coupled with diarylethene. The photogenerated
closed-ring isomer of diarylethene
photoluminescence by energy transfer after UV irradiation. This
has been achieved by just simply mixing the QDs and
1 efficiently quenched the
diarylethene in
a solution. The photoluminescence was
recovered after visible light irradiation. Silica coating was very
effective in sustaining the reversible ON/OFF switching. The
ON/OFF switching of the very sharp photoluminescence with a
FWHM of 19 nm from perovskite QDs would open up new
opportunities for several new optical applications.
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Acknowledgement
This work was supported by a Grant-in-Aid for Exploratory
Research (16K13630) from Japan Society for the Promotion of
Science (JSPS) and an Adaptable and Seamless Technology
transfer Program through Target-driven R&D (A-STEP)
(VP29217944242 and VP30218087399) from Japan Science and
Technology Agency (JST), Japan.
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Conflicts of interest
There are no conflicts to declare.
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