Excitonic Properties of Chemically Synthesized 2D Organic–Inorganic Hybrid Perovskite Nanosheets

Article abstract of DOI:10.1002/adma.201704055

2D organic–inorganic hybrid perovskites (OIHPs) represent a unique class of materials with a natural quantum-well structure and quasi-2D electronic properties. Here, a versatile direct solution-based synthesis of mono- and few-layer OIHP nanosheets and a systematic study of their electronic structure as a function of the number of monolayers by photoluminescence and absorption spectroscopy are reported. The monolayers of various OIHPs are found to exhibit high electronic quality as evidenced by high quantum yield and negligible Stokes shift. It is shown that the ground exciton peak blueshifts by ≈40 meV when the layer thickness reduces from bulk to monolayer. It is also shown that the exciton binding energy remains effectively unchanged for (C₆H₅(CH₂)₂NH₃)₂PbI₄ with the number of layers. Similar trends are observed for (C₄H₉NH₃)₂PbI₄ in contrast to the previous report. Further, the photoluminescence lifetime is found to decrease with the number of monolayers, indicating the dominant role of surface trap states in nonradiative recombination of the electron–hole pairs.

Full text of DOI:10.1002/adma.201704055

CommuniCation
Photoluminescence
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Excitonic Properties of Chemically Synthesized
2D Organic–Inorganic Hybrid Perovskite Nanosheets
Qi Zhang, Leiqiang Chu, Feng Zhou, Wei Ji, and Goki Eda*
exhibit unique properties that are poten-
tially useful for photonics applications.[7,8]
OIHPs having a chemical structure of
(R-NH3)2PbI4 where R represents a large
hydrocarbon or phenethyl group have
been studied extensively for their ultra-
fast nonlinear optical response and strong
excitonic character arising from dielectric
confinement effect.[9–16]
Similar to graphene and its inorganic
analogues such as transition metal dichal-
cogenides (TMDs),[17–20] nanosheets (NSs)
of 2D OIHPs can be isolated on a substrate
by various methods and studied by optical
spectroscopy. Yaffe et al.[21] recently studied
the optical properties of mechanically exfo-
liated 2D (C4H9NH3)2PbI4 (C4PI) crystals
and found that the exciton binding energy
of bilayer OIHP is substantially larger than
that of the corresponding layered bulk
material. The reported binding energy of
2D organic–inorganic hybrid perovskites (OIHPs) represent a unique class
of materials with a natural quantum-well structure and quasi-2D electronic
properties. Here, a versatile direct solution-based synthesis of mono- and
few-layer OIHP nanosheets and a systematic study of their electronic
structure as a function of the number of monolayers by photoluminescence
and absorption spectroscopy are reported. The monolayers of various
OIHPs are found to exhibit high electronic quality as evidenced by high
quantum yield and negligible Stokes shift. It is shown that the ground
exciton peak blueshifts by ≈40 meV when the layer thickness reduces from
bulk to monolayer. It is also shown that the exciton binding energy remains
effectively unchanged for (C6H5(CH2)2NH3)2PbI4 with the number of layers.
Similar trends are observed for (C4H9NH3)2PbI4 in contrast to the previous
report. Further, the photoluminescence lifetime is found to decrease with the
number of monolayers, indicating the dominant role of surface trap states in
nonradiative recombination of the electron–hole pairs.
Organic–inorganic hybrid perovskites (OIHPs) exhibit unique
anisotropic crystal structure that depends on their chemical
composition.[1] Those with a chemical formula of ABX3 where
A is an organic cation, B is a bivalent metal cation, and X
is a halogen anion, exhibit cubic structure and have been
intensely studied as a light absorber for solar cells in recent
years.[2–4] Precursors of OIHPs can also self-organize into a
van der Waals layered structure with A2MX4 2D layers when
the organic moieties are sufficiently large to prevent forma-
490 meV for bilayer C4PI is comparable to the values for mon-
olayer TMDs, such as WS2 and WSe2, highlighting the strong
excitonic nature of exfoliated C4PI at room temperature.[22,23]
This finding suggests that the electronic properties of OIHPs
are strongly influenced by surrounding dielectric environment
despite the insulating organic layers encapsulating each inor-
ganic layer. The large exciton binding energy of OIHP NSs makes
them an excellent candidate for the investigation of exciton-polar-
itons[24] and development of novel photonic devices.[25]
4−
tion of 3D network of corner-sharing BX6 octahedra.[5,6] Such
Due to the low formation energy, OIHP crystals can be syn-
thesized in solution phase. Dou et al.[26] recently demonstrated
synthesis of ultrathin OIHP crystals with C4 alkyl chain ter-
mination using ternary solvent coevaporation technique. The
process involves self-assembly and precipitation of precursors
from a ternary mixture of good and poor solvents upon mild
heating. The precipitation is accompanied by self-organiza-
tion and formation of OIHP crystals yielding NSs with lateral
sizes of a few micrometers. Following the same recipe, Chen
et al. systematically investigated the controllable growth of
(C4H9NH3)2PbBr4.[27] Using a similar approach, synthesis of
sub-micrometer-sized (C6H5(CH2)2NH3)2PbI4 (PEPI) NSs was
recently reported by Yang et al.[28] However, controllable syn-
thesis of high quality ultrathin NSs that are sufficiently large for
systematic microspectroscopic studies has remained a challenge.
Here, we report versatile solution-based synthesis of C4PI
and PEPI NSs (see Figure 1a for their crystal structure) having
large lateral sizes (>10 µm) from a binary cosolvent at room
temperature. We show that they exhibit high emission quantum
yield and negligible Stokes shift. We further demonstrate that
OIHPs consist of alternating layers of insulating organic moie-
ties and semiconducting inorganic component, resembling the
multiple quantum well structure. Due to the naturally occur-
ring quantum well structure and direct band gap, 2D OIHPs
Q. Zhang, Dr. L. Q. Chu, F. Zhou, Prof. W. Ji, Prof. G. Eda
Physics Department
National University of Singapore
2 Science Drive 3, Singapore 117542, Singapore
E-mail: g.eda@nus.edu.sg
Dr. L. Q. Chu, Prof. G. Eda
Centre for Advanced 2D Materials and Graphene Research Centre
National University of Singapore
6 Science Drive 2, Singapore 117546, Singapore
Prof. G. Eda
Chemistry Department
National University of Singapore
3 Science Drive 3, Singapore 117543, Singapore
DOI: 10.1002/adma.201704055
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Figure 1. a) Chemical structures of PEPI and C4PI (hydrogen atoms are not shown for clarity). b) Synthesis schematics of PEPI and C4PI NSs. Si/Si3N4
substrate was selected to give high optical contrast. Substrate can also be Si/SiO2 or fused quartz.
the optical band gap of both C4PI and PEPI scales weakly with
the number of layers. Using photoluminescence spectroscopy,
we reveal that the exciton binding energy and electronic band
gap of PEPI are effectively independent on the number of layers
in contrast to the previously reported findings on C4PI.[21]
this precursor solution until the solvent volume ratio reached
1:3 (acetonitrile:toluene). A small volume of this binary solu-
tion (7 µL) was drop-cast onto a substrate and the solvent was
allowed to evaporate completely under ambient condition. NSs
were formed during solvent evaporation and deposited on the
substrate. Similar procedures were followed for the synthesis
of C4PI. The weight ratio of the ammonium salt and PbI2 was
varied to optimize the morphology of the final product (Figures
S1–S4, Supporting Information).
After complete evaporation of the solvents, micrometer-sized
flakes with distinct shapes and various thicknesses were found
across the substrate as shown in Figure 2a–d. The flakes exhib-
ited photoluminescence (PL) peaked at around 516–528 nm
(will be discussed in next section), which is characteristic band
gap excitonic emission of PEPI and C4PI.[29] Figure 2a–d
shows changes in flake morphologies for two different pre-
cursor concentrations, namely 0.5 mg mL−1 (2a and 2c) and
2 mg mL−1 (2b and 2d), revealing its effect on the average
flake thickness. It is worth noting that monolayer flakes of
both C4PI and PEPI were obtained with nearly identical syn-
thesis parameters. The thickness and lateral size distributions
of C4PI and PEPI lakes are shown in Figure S5 (Supporting
Information). We found this synthesis protocol can be applied
to other 2D OIHPs having similar chemistry. Figure 2e–g
shows the bright field images of ultrathin C6PI, C12PI, and
We used a binary mixture of good and poor solvents for
the perovskite precursors instead of a ternary solvent reported
previously.[26] We tested various combinations of solvents and
found that a mixture of acetonitrile and toluene yields larger
and superior quality crystals with a higher yield. In par-
ticular, ternary solvents containing N,N-dimethylformamide,
which was previously used for the synthesis of bromide-based
2D perovskite, yielded NSs with limited size and poor thickness
homogeneity. Acetonitrile is a good solvent for the precursors
while toluene is a poor solvent and induces the precipitation
of the perovskite crystals. The two solvents are perfectly mis-
cible and slowly coevaporate at room temperature, allowing
controlled synthesis. Figure 1b displays the synthesis proce-
dure. We synthesized OIHP for a range of precursor concentra-
tions between 0.5 and 2 mg mL−1 (Figures S1–S4, Supporting
Information). For the synthesis of PEPI NSs, C6H5(CH2)2NH3I
(PEI) and lead(II) iodide (PbI2) powders in weight ratio of 60:40
were dissolved in acetonitrile to achieve a precursor solution
of a desired concentration (see Experimental Section for the
synthesis of PEI and C4I). Then toluene was added slowly into
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Figure 2. Optical images of a) thin PEPI NSs, b) thick PEPI NSs, c) thin C4PI NSs, d) thick C4PI NSs. e–g) Optical images of C6PI, C12PI, and C16PI
thin NSs. h) AFM image and height profile of monolayer PEPI. i) Bright field and j) PL images of C4PI NSs suspending in precursor solution. k) Bright
field and l) PL images of PEPI NSs suspending in precursor solution. Scale bars for (a)–(g) are 20 µm, for (i)–(l) are 50 µm.
C16PI NSs, respectively. We verified their crystal structure
and high crystallinity by X-ray diffraction (XRD) measurement
(Figure S6, Supporting Information). Most flakes were either
rectangular or square in shape, occasionally with truncated
edges, reflecting the orthorhombic crystal structure of these
materials. Atomic force microscopy (AFM) analysis reveals
that the thinnest flakes show a step height of ≈1.6 nm, which
corresponds to the thickness of a monolayer PEPI (Figure 2h;
Figure S7, Supporting Information).[30] AFM images of thicker
flakes are presented in the Supporting Information (Figures S8
and S9, Supporting Information). Thick flakes (>30 nm) were
typically more stable in ambient conditions than mono- to few-
layer flakes which tended to degrade over several minutes to
hours as evidenced by reduction in PL intensity and changes in
surface morphology most likely due to dissociation of organic
ligands (Figure S10, Supporting Information). PEPI flakes
exhibited superior stability over C4PI probably due to their
highly hydrophobic organic moieties.
As-deposited flakes of different orientations were overlapped
in a random manner, suggesting that nucleation occurred in
the solution phase and deposition took place after complete
growth of these crystallites. In order to verify this, we moni-
tored the nucleation and growth of the crystals in situ in liquid
under optical microscope. Figures 2i–l show the bright field
and fluorescence microscopy images of C4PI and PEPI flakes
in liquid phase during precipitation of crystal. The flakes with
distinct shape with characteristic PL can be clearly seen, further
evidencing liquid phase growth of these crystals. In the following
discussions, thin and thick flakes refer to crystals containing
less than three monolayers (≤3 L) and those consisting of more
than 20 monolayers (≥20 L), respectively.
We investigated the electronic properties of C4PI and PEPI
flakes having different thicknesses using differential reflectance
(DR) and PL spectroscopy. For a thin sample on a transparent
substrate, DR is proportional to the absorption coefficient of
the sample and can be used to obtain its approximate absorp-
tion spectrum.[31] All optical measurements were performed in
vacuum except transmittance measurements which were con-
ducted in ambient condition.
Figure 3a shows the normalized DR and PL spectra of a thin
(≤3 L) and thick (≥20 L) PEPI flakes measured at room temper-
ature. The spectra are nearly identical for the two samples and
are dominated by the characteristic ground (1s) exciton feature.
The Stokes shift, which is the energy difference between exci-
tonic peak in DR and PL, is negligible (<10 meV), indicating
the high quality of PEPI samples. Figure 3b shows that C4PI
flakes exhibit similar ground exciton features in the DR and PL
spectra. In contrast to PEPI, the Stokes shift is more evident
(≈11 meV) for both thin and thick C4PI samples. Further, the
asymmetric PL peak of C4PI suggests that multiple emission
species such as defect-bound excitons are responsible for the
emission,[10,29] while PEPI flakes typically exhibit comparatively
sharper (full width at half maximum (FWHM) ≈ 59 meV) and
more symmetric emission peak. Figure 3c shows reflectance,
transmittance, and absorption spectra of a thick PEPI and C4PI
flakes measured at room temperature. Both samples exhibit
strong excitonic absorption (≈55%) with an extracted absorption
coefficient of ≈1.87 × 105 cm−1 at the ground exciton resonance
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Figure 3. DR and PL spectra of: a) PEPI and b) C4PI for thin and thick NSs, respectively. Insets show optical images of corresponding perovskite flakes.
c) Reflectance, transmittance, and absorption spectra of thick PEPI and C4PI NSs. d) Dielectric functions of thick PEPI and C4PI NSs.
a
b
PEPI
C4PI
Bulk
Bulk
60L
20L
10L
60L
20L
10L
6L
6L
3L
3L
1L
1L
450
500
550
600
450
500
550
600
Wavelength (nm)
Wavelength (nm)
2.40
c
d
2.39
2.38
2.37
2.36
2.35
2.40
C4PI
PEPI
2.39
2.38
2.37
0
20
40
60
800
820
Bulk
0
20
40
60
800
820
Bulk
Number of layers
Number of layers
Figure 4. Thickness-dependent PL spectra of: a) PEPI and b) C4PI NSs. The dashed lines are guides for the eye. Optical band gap of: c) PEPI and
d) C4PI NSs as a function of layer numbers.
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(see Supporting Information for calculation method), which is
in accordance with the previously reported values.[10,21] Similar
absorption values of the PEPI and C4PI samples suggest that
organic moiety plays a minor role in the ground exciton absorp-
tion.[32] From the reflectance and transmittance spectra, we
obtained the dielectric function of these two samples as shown
in Figure 3d. The high-frequency dielectric constant of both
samples is ≈4.0 in agreement with the reported values for the
bulk materials.[10]
liquid nitrogen temperature. Here we use photoluminescence
excitation (PLE) spectroscopy to investigate the transitions
above the optical band gap.[35]
Figure 5a shows the PL and PLE spectra of PEPI flakes with
different thicknesses at 78K. In contrast to the DR spectra, the
excitation spectra reveal a region of almost fully suppressed PL
followed by a distinct onset of absorption around 2.56 eV. This
step-like increase in absorption, which is a signature of free
carrier absorption in 2D electronic systems, matches very well
with the previously reported band edge absorption.[10] Thus we
conclude this energy to be the electronic gap of these materials.
This electronic band gap is almost identical for thick and thin
samples. These observations also indicate that exciton binding
energy of PEPI remains nearly constant (≈200 meV) and its
electronic structure depends only weakly on the flake thickness.
The PLE spectra of thin and thick C4PI also show similar abrupt
onset near 2.8 eV. Note that here we monitored the emission
intensity of the low temperature (LT) phase peak (≈2.53 eV) that
emerges below ≈250 K.[11,21] Assuming that there is no energy
transfer between different phases, we estimate the binding
energy of the LT phase to be ≈260 meV for both thin and thick
C4PI flakes. Figure 5c summarizes the exciton binding energies
of PEPI and C4PI measured in this work and those reported
in literature for PEPI and C4PI with different thicknesses. We
believe that the discrepancies arise from the uncertainty of dif-
ferent estimation methods employed by different groups.
We measured room-temperature exciton lifetime of isolated
individual PEPI flakes in vacuum (≈10−5 mbar) using the time-
correlated single photon counting technique. For both PEPI and
C4PI, we found the exciton average lifetime to decrease gradu-
ally with decreasing thickness of the flake as shown in Figure 6a.
The decay curves were fitted with a biexponential function and
fast (τ1) and slow (τ2) decay lifetimes were extracted. These decay
times were found to be τ1 = 166 ps (16.8%) and τ2 = 1037 ps
Figure 4a,b displays the normalized PL spectra of PEPI and
C4PI flakes with different thicknesses. It can be seen that the
optical gap is weakly dependent on the thickness of the flakes
and the emission peak energy difference between monolayer and
bulk samples is about 35 meV for both materials. Figure 4c,d
shows that this thickness dependent shift is prominent for
thicknesses below ≈20 monolayers. We conducted PL measure-
ments of PEPI and C4PI flakes on various substrates with dif-
ferent dielectric constants but did not observe significant peak
shift suggesting that substrate dielectric screening[33] plays a
minor role (Figure S11, Supporting Information). Similar thick-
ness-dependent trends were observed for C6PI, C12PI, and
C16PI as shown in Figure S12 (Supporting Information).
Ground exciton peak energy is determined by quasiparticle
band gap and exciton binding energy. It has been shown that
both these quantities scale with the number of layers for TMDs
due to strong interlayer screening.[34] The absence of prominent
PL energy shift with flake thickness therefore does not rule out
the possibility of interlayer coupling in C4PI and PEPI sam-
ples. Thus, exciton binding energy needs to be independently
measured in order to understand the thickness scaling effect.
Yaffe et al.[21] extracted the exciton binding energy of mechani-
cally exfoliated bilayer C4PI from the energy of higher order
exciton peaks that appear in DR spectra at 5K. These higher
order exciton peaks were not observable in our sample at
Figure 5. a) PL and PLE spectra of thin and thick PEPI NSs at 78 K. b) PL and PLE spectra of thin and thick C4PI NSs at 78 K. Band gap positions are
extracted by Tauc plot and they are indicated in the plots. c) Summary of exciton binding energy for PEPI and C4PI with different thickness.
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Figure 6. a) Room temperature PL decay curves of PEPI and C4PI NSs with thickness from bulk to thin. b) Evolution of fast decay time (τ1) for PEPI
and C4PI NSs as a function of layer numbers. c) QY of PEPI and C4PI NSs with different thickness.
(83.2%) for the bulk PEPI crystal and τ1 = 103 ps (36.0%) and
τ2 = 530 ps (64.0%) for the bulk C4PI crystal (see Table S1 in
the Supporting Information). These decay times are compa-
rable to the lifetimes reported for PEPB and C4PI crystals that
are considered to be of high quality due to slow precipitation
growth.[36,37] On the other hand, they are an order of magni-
tude longer compared to the lifetimes reported for polycrystal-
line PEPI thin film samples prepared by spin-coating.[38] The
PL lifetimes were consistently lower for flakes with smaller
thicknesses as summarized in Figure 6b. Furthermore, we
performed thickness scaling study of intrinsic radiative rate (kr)
C4PI was calculated to be ≈0.4 ns−1 for both samples. We thus
attribute the shorter lifetime and lower QY of the thin samples
to higher degree of structural defects that give rise to the fast
nonradiative recombination of excitons.
In summary, we have developed a facile liquid-phase syn-
thesis for 2D OIHP NSs with thicknesses ranging from a single
monolayer to tens of monolayers. The synthesis technique is
highly versatile and allows preparation of NSs with a variety of
organic moieties. The PEPI NSs exhibit high PL quantum yield
up to 31% with exciton lifetimes comparable to that of high
quality bulk samples. We found the exciton binding energy
and quasiparticle band gap of PEPI to be independent on the
number of layers, which is distinctly different from the recently
reported case of C4PI. Our work highlights the versatility of 2D
OIHP NSs synthesis and opens up avenues for exploring their
fundamental excitonic properties in solid state.
which can be expressed in terms of PL quantum yield (QY) and
QY
exciton lifetime (τ) as kr =
. We estimated the QY by meas-
τ
uring the integrated PL intensity of the samples relative to that of
a reference dye sample with a known QY (see Experimental Sec-
tion for details). Figure 6c summarizes the QY of thick and thin
PEPI and C4PI samples. The PEPI flakes exhibited significantly
higher QY compared with the C4PI flakes under identical meas-
urement conditions. The estimated QY for thick and thin PEPI
flakes was 31% and 13%, respectively. Combined with exciton
lifetimes of these PEPI samples (54 ps for ≈3 L and 105 ps
for ≈20 L), kr was found to be of the order of 1 ns−1 for both
PEPI samples. QY of C4PI was estimated to be 5% and 2%,
for thick and thin samples, respectively. The radiative rate of
Experimental Section
Chemicals and Reagents: Phenethylamine (≥99%), butylamine (≥99%),
hexylamine (99%), dodecylamine (98%), hexadecylamine (98%), lead(II)
iodide (PbI2) (99%), hydriodic acid (HI) (57 wt%), acetonitrile (99.7%,
analytical reagent (AR)), and toluene (99.99%, high-performance liquid
chromatography (HPLC)) were purchased from Sigma Aldrich. All
chemicals were used as received without any further purification.
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Synthesis of Ammonium Salts: Synthesis of C6H5(CH2)2NH3I. 10 mL
ethanol and 2.52 mL phenethylamine (C6H5(CH2)2NH2) were added to
a beaker to form a mixture which was kept at 0 °C using an ice bath.
Stoichiometric amount of HI (2.62 mL) was added to the mixture
dropwise and it was stirred for 2.5 h to ensure fully reaction. Heating
the solvent at 60 °C with a rotary evaporator until whitish ammonium
salt powder appeared and the solvent residue evaporated completely.
The ammonium salt powder was collected and washed with diethyl ether
for three times. Then the powder was transferred to a vacuum oven and
dried at 60 °C for 24 h for later use. Synthesis processes of C4I, C6I,
C12I, and C16I are similar to PEI, except that replacing C6H5(CH2)2NH2
with corresponding amines.
and reflectance spectra measurement was performed to achieve accurate
absorption values.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Synthesis of 2D OIHP Nanosheets: (I) PEPI: 12 mg PEI and 8 mg
PbI2 were dissolved in 10 mL acetonitrile to prepare 2 mg mL−1 stock
solution. When synthesizing relatively thick (≥10 L) PEPI NSs, dropped
2 mL stock solution to a vial, then added toluene dropwise to the vial
until the total volume reached 8 mL. 7 µL of this cosolvent was drop-
casted onto a substrate and let it evaporate naturally under ambient
condition. The stock solution was diluted with acetonitrile to 0.5 mg mL−1
for synthesizing relatively thin (≤6 L) PEPI NSs. 2 mL of the diluted stock
solution was dropped to a vial and followed by adding 6 mL toluene.
The same drop-casting process was then conducted. (II) C4PI: 10 mg
C4I and 10 mg PbI2 were dissolved in 10 mL acetonitrile and 2 mg mL−1
stock solution was obtained. When synthesizing relatively thick (≥10 L)
C4PI NSs, 2 mL stock solution was put to a vial and toluene was added
dropwise until the total volume reached 10 mL. The stock solution was
diluted with acetonitrile to 0.5 mg mL−1 for synthesizing relatively thin
(≤6 L) C4PI NSs. 2 mL of the diluted stock solution was dropped to
a vial and followed by adding 6 mL toluene. Drop-casting processes
for thin and thick C4PI samples are similar to the PEPI counterparts.
(III) For C6PI, C12PI, and C16PI, similar processes were performed to
synthesize crystalline NSs with different thickness.
This work was supported by MOE under AcRF Tier
2
(MOE2015-T2-2-123). The authors acknowledge the support from the
National Research Foundation, Prime Minister’s Office, Singapore,
under its NRF research fellowship (NRF-NRFF2011-02) and Medium
Sized Centre Program.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
2D perovskite nanosheets, dielectric screening effect, photoluminescence
excitation spectroscopy, thickness scaling study
Received: July 20, 2017
Revised: January 16, 2018
Published online:
AFM Measurement: Atomic force microscope (Bruker Dimension FastScan,
Tapping mode) was used to measure height profile of perovskite NSs.
X-ray Diffraction Measurement: XRD studies were performed on 2D
perovskite flakes and wide-angle XRD patterns were collected on Bruker
D8 Focus Powder X-ray diffractometer using Cu Kα radiation (40 kV,
40 mA) at room temperature.
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Optical Measurements: Micro-PL spectra were obtained with a laser
confocal microscope (NT-MDT, NTEGR Spectra) in back scattering
geometry with 473 nm (continuous wave (cw)) excitation laser. The
differential reflectance of individual flakes was performed with a focused
white light from a quartz tungsten light source. The DR is defined as
(RS – Rq)/Rq, where RS is the reflective light intensity from sample and Rq
is the counterpart of quartz substrate. Incident light intensity attenuation
function is described by Beer–Lambert law as I(t) = I0exp (−αt), where
α(λ) is absorption coefficient and t is penetration depth. Thus, α(λ) can
be extracted from absorption spectra when sample thickness is known.
PLE spectra were obtained by a monochromator and a super-continuum
light as the excitation source which is coupled to a tunable laser filter
in back scattering geometry. The PLE experiment was conducted in
vacuum (≈10−5 mbar) at liquid nitrogen temperature since the emission
will become more prominent and discernable at low temperature. The
excitation intensity was kept below 50 W cm−2 in order to avoid any
nonlinear effect. Exciton lifetime measurement was also performed
under vacuum (≈10−5 mbar) using a picosecond pulse-laser (480 nm)
with the repetition rate of 6.0 MHz and the average power of ≈20 W cm−2
.
Time-correlated single photon counting (PicoHarp 300) setup was
used as the detection system. To estimate the PL quantum yield of
perovskite NSs, standard dye sample (5,5″′-Bis(N,N-diphenylamino)-4′-
dimesitylboryl-2,2′:5′,2″:5″,2″′-quaterthiophene) with known QY (≈36%)
was used as reference. Absorbance of thin perovskite NSs was calculated
Rs(λ)− Rq(λ)
4
from DR spectra using
=
A(λ), where n is substrate
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n2 − 1
Rq( λ)
refractive index and here it is assumed to be wavelength independent.
Absorption of thin NSs was then obtained from 1 − 10−A(λ). The QY
I0AperovQYperovηperov PL
I0AdyeQYdyeηdye
absorption and η is collection efficiency of CCD spectrometer. Extinction
of thin NS was obtained using
perov , where A is
=
PLdye
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