Synthesis of Two-Dimensional Perovskite by Inverse Temperature Crystallization and Studies of Exciton States by Two-Photon Excitation Spectroscopy

Article abstract of DOI:10.1002/adfm.202002661

Two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs), a natural multiple-quantum-well structure with quasi-2D electronic properties, have recently emerged as a promising class of semiconducting materials for photovoltaic and optoelectronic ap

Full text of DOI:10.1002/adfm.202002661

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Synthesis of Two-Dimensional Perovskite by Inverse
Temperature Crystallization and Studies of Exciton States
by Two-Photon Excitation Spectroscopy
Zhihui Chen, Qi Zhang, Menglong Zhu, Xinyun Wang, Qixing Wang,
Andrew Thye Shen Wee, Kian Ping Loh, Goki Eda,* and Qing-Hua Xu*
OIHPs have ABX three-dimensional (3D)
3
Two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs), a natural
multiple-quantum-well structure with quasi-2D electronic properties, have
recently emerged as a promising class of semiconducting materials for photo-
voltaic and optoelectronic applications. However, facile synthesis of high-quality
lattice frameworks, where A represents
+
small-sized organic cation (CH3NH3 ,
+
+
+
CH(NH ) , Cs , CH (NH ) ), B repre-
2 2
3
2 2
2
+
2+
sents bivalent metal cation (Pb , Sn ,
Ge ), and X is a halogen anion (Cl , Br ,
I ). In addition to the cubic 3D structure,
2+
−
−
2D OIHPs single crystals is still lacking. The layer dependence of the exciton
−
binding energy of (C H NH ) PbI (C4PI), a widely studied 2D OIHP, is still
4
9
3 2
4
precursors of OIHPs can assemble into
a van der Waals layered A BX structure
debated. Herein, a novel synthesis technique based on inverse temperature
crystallization in a binary-solvent system is used to prepare 2D OIHPs and a
systematic study of excitonic states of the synthesized 2D OIHPs by two-photon
excitation (TPE) spectroscopy is conducted. The obtained TPE spectra indicate
that the exciton binding energies are similar for C4PI nanosheets and bulk
crystals with different number of layers, most likely due to the intrinsically weak
interlayer coupling. Further, the dark excitonic 2p states of (C H (CH ) NH ) PbI
4
2
4
where the large-sized organic moieties
separate the corner-shared BX64 octa-
−
[
7–10]
hedra inorganic layers (see Figure 1a).
As the band gap of the organic layers is
much larger than that of the inorganic
[
11]
layers, the organic molecules electroni-
cally isolate the inorganic layers to form
6
5
2 2
3 2
[
12–19]
multiple-quantum-well
structure,
(PEPI) and C4PI are also observed by TPE spectroscopy. The results provide a
which is responsible for the observation
of thickness independent band gap of 2D
perovskites in some studies.
novel synthesis protocol and insight into exciton properties of 2D OIHPs.
Similar to graphene and its inorganic
1
. Introduction
analogues such as transition metal dichalcogenides (TMDs),
electronic and optical properties of 2D OIHPs are very different
[
14,20,21]
Organic–inorganic hybrid perovskites (OIHPs) have emerged
as a promising platform for photovoltaic and optoelectronic
device applications owing to their long carrier diffusion
from their 3D counterparts.
Due to strong Coulombic
interactions in the electron-hole pair, 2D OIHPs generally
exhibit strong excitonic absorption and emission. The exciton
[
1–6]
lengths, large absorption coefficients, and defects tolerance.
binding energy (E ) can be determined by measuring the
b
energy difference between the ground excitonic state (E ) and
1
s
[
22–25]
Dr. Z. H. Chen, Dr. M. L. Zhu, Prof. K. P. Loh, Prof. Q.-H. Xu
Department of Chemistry
electronic band gap (Eg).
The exciton binding energy is
generally sensitive to dielectric environment, ranging from tens
National University of Singapore
[26–28]
to hundreds of meV.
In order to study the excitonic proper-
3
Science Drive 3, Singapore 117543, Singapore
ties of the 2D OIHPs as well as integration with other 2D mate-
rials to achieve novel functionalities, it is important to under-
stand layer-dependent excitonic properties, especially exciton
E-mail: chmxqh@nus.edu.sg
Dr. Q. Zhang. X. Y. Wang, Dr. Q. X. Wang, Prof. A. T. S. Wee, Prof. G. Eda
Department of Physics
National University of Singapore
[
29–31]
binding energy, which is critical for device performance.
There are several reports on investigation of exciton binding
2
Science Drive 3, Singapore 117542, Singapore
E-mail: g.eda@nus.edu.sg
[
12,13,16,17,32]
energy of 2D OIHPs.
The understanding of the
Dr. M. L. Zhu, Prof. A. T. S. Wee, Prof. K. P. Loh, Prof. Q.-H. Xu
SZU-NUS Collaborative Innovation Center and International
Collaborative Laboratory of 2D Materials for Optoelectronics Science
and Technology of Ministry of Education
Institute of Microscale Optoelectronics
Shenzhen University
Shenzhen 518060, China
exciton binding energy in these 2D systems still remains
controversial. It was previously reported that exciton binding
energy of mechanically exfoliated bilayer (C H NH ) PbI
4
9
3 2
4
(
C4PI) crystals (≈490 meV) was significantly larger than that
[13]
of the corresponding C4PI bulk material (≈360 meV). This
result suggests that their electronic properties are strongly influ-
enced not only by the adjoining insulating organic ligands but
also by their surrounding dielectric medium. However, a recent
work by Zhang et al. showed that exciton binding energies of
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202002661.
DOI: 10.1002/adfm.202002661
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in Figure 1c. On the other hand, in excitonic systems, TPE
spectrum can distinguish discrete dark states and continuum
states. TPE spectroscopy has been utilized to study excitonic
[
43]
states in bulk materials such as GaAs, 2D materials such
[
22,44]
as TMDs,
and 1D materials such as single-walled carbon
[
45]
nanotube. Here, TPE spectroscopy was utilized to probe the
excitonic states of chemically synthesized C4PI single crystals.
2
. Results and Discussion
Bulk single crystals of C4PI were synthesized in a binary solvent
system through an inverse temperature crystallization method.
Figure 2a shows the schematic illustration of the synthetic
process (see detailed synthetic conditions in the Experimental
Section). Precursors (lead iodide and ammonium salt) with stoi-
chiometric ratio were first dissolved in N,N-dimethylformamide
(DMF) at room temperature. Dropwise addition of toluene, as
anti-solvent, into this solution resulted in precipitation of yel-
lowish solids, which were ascribed to semi-amorphous crystal-
lites of 2D perovskite (Figure S1a, Supporting Information).
These crystallites showed retrograde solubility behavior and
could be redissolved in the binary solution at temperatures
below ≈−20 °C (Figure S1b, Supporting Information). Single
crystals of 2D OIHPs were then formed as the temperature of
precursor solution increased (see Videos S1 and S2, Supporting
Information). With this method, single crystals were prepared at
a growth temperature ranging from 40 to 120 °C. The average
size of the prepared crystal is inversely correlated to the growth
temperature, indicating that the nucleation rate is greater than
crystal growth rate at higher temperatures. Optical images of
the obtained crystals at a growth temperature of 40 °C (Figure
S2, Supporting Information) show that the crystal size can reach
to millimeter scale, which is comparable to that of slow-growth
Figure 1. a) The atomic structure of C4PI. Black, purple, brown, light
grey, and pink balls represent Pb, I, C, N, and H atoms. Schematic of the
optical band structure, showing the two-photon absorption (red arrows)
and fluorescence emission (green arrows) processes in an excitonic and a
non-excitonic system, respectively. b) In the exciton picture, the excitonic
1
s state is not accessible under two-photon excitation, while the 2p and
continuum states are excited. The fluorescence emissions still generate
from 1s state following the Kasha’s rule. c) In the band picture, the onset
for two-photon absorption occurs at the band edge, where the fluores-
cence emission also happens.
both (C H (CH ) NH ) PbI (PEPI) and C4PI of different thick-
6
5
2 2
3 2
4
nesses, ≈3 layers and ≥20 layers, were both ≈200 and ≈250 meV,
[33]
that is, weakly sensitive to the number of layers.
Single crystal is a preferred form for investigating intrinsic
physical properties of OIHPs because grain boundaries in
polycrystalline systems obscure optical features, making
detailed analysis difficult. Various protocols have been proposed
[35,36,38]
methods.
The stoichiometry of the crystals obtained from
to synthesize single crystals of OIHPs, including hot solution
electron dispersive spectroscopy (EDS) measurements agrees
well with the theoretical value for C4PI and PEPI (Figure S3, Sup-
porting Information). Powder X-ray diffraction spectrum shows
sharp peaks, suggesting their high crystallinity (Figure S4, Sup-
porting Information). Atomic force microscopy (AFM) results
show that the typical thickness of bulk C4PI crystals was around
2.4 µm (Figure 2c,d). Bulk C4PI crystal exhibits a broad absorp-
tion band with a maximum at ≈510 nm and a tail stretching up
to 700 nm. Photoluminescence (PL) spectrum of the crystal at
room temperature shows an intense excitonic peak centered at
cooling-induced crystallization,[
34]
seed-assisted crystalliza-
tion,[
35,36]
and anti-solvent-induced crystallization,
[37,38]
whereas
these three methods are time-consuming. A novel method
based on retrograde solubility (i.e., lower precursor solubility at
higher temperature) was recently employed to synthesize high-
quality 3D OIHPs single crystals within minutes, which is cost-
effective and convenient.[ This synthesis technique, known
as the inverse temperature crystallization, can be potentially
extended to the preparation of 2D OIHPs. However, this has
not been demonstrated to date.
39]
[12]
≈523 nm, a common feature of 2D OIHPs (Figure 2h).
In this work, single crystals of C4PI were prepared through
the inverse temperature crystallization method, and their
excitonic states were probed by two-photon excitation (TPE)
spectroscopy. Complementary to the conventional one-photon
excitation process that allows access to the final states with even
parity (1s, 2s, 3s), two-photon excitation allows access to final
C4PI nanosheets were prepared by mechanical exfoliation
of C4PI bulk crystal onto the surface of a quartz substrate.
Chemically inert hexagonal boron nitride (h-BN) was used as
a protective capping layer of C4PI nanosheets to prevent them
from degradation in ambient condition. A multilayer h-BN was
[
40]
first exfoliated onto a polydimethylsiloxane (PDMS) film and
then transferred onto the C4PI nanosheets, forming a C4PI/h-
BN heterostructure (Figure 2e). AFM measurements indicate
that the thickness of the C4PI nanosheets was ≈7.68 nm, cor-
[
40–42]
states with odd parity such as 2p, 3p states (see Figure 1b).
These states with odd parity are generally known as “dark”
states and do not appear in the linear absorption spectrum. TPE
spectroscopy is a powerful tool for studying excitonic effects
because of enhanced resonance features and weak sensitivity
to disorder effects.[ In the system where the excitonic effects
are negligible, the onset of two-photon absorption is expected to
be consistent with that of one-photon absorption as illustrated
[
46,47]
responding to 5 layers of C4PI (Figure 2f,g).
Compared
to the bulk crystals, the absorption peak of C4PI nanosheets
(≈513 nm) is significantly narrower. The corresponding PL
peak occurs at ≈517 nm. The Stokes shift of nanosheet is
≈20 meV, which is distinctly smaller than that of bulk sample
22]
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Figure 2. a) Schematic procedures for synthesis of C4PI bulk crystal. b) Optical image for C4PI bulk crystal. c,d) AFM topology and height profile of
C4PI bulk crystal. e) Optical image for C4PI nanosheet. The region encircled by blue dash line represents multilayer h-BN, while orange region outlines
C4PI nanosheets. f,g) AFM topology and height profile of C4PI nanosheet on quartz substrate. The height profiles were tracked along the red lines.
h) Absorption and PL spectrum of C4PI bulk crystal at room temperature (λEXC = 473 nm). i) Differential reflectance (DR) and PL spectrum of C4PI
nanosheets at room temperature (λEXC = 473 nm). The scale bars of optical images are shown in panels.
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(
a)
(b)
1
0
0
.0
.5
.0
C4PI Bulk crystal
C4PI Nanosheets
1
0
0
0
0
0
.0
.8
.6
.4
.2
4
3
S=1.98
S=2.01
1
1
0
0
0.1
1
10
Excitation Power (mW)
.0
4
50
500
550
600
650
700
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Figure 3. a) PL spectrum of C4PI bulk crystal (dark cyan line) and C4PI nanosheets (orange line) (λEXC = 473 nm) under 78 K. b) Main panel: TPPL
emission spectrum of C4PI bulk crystal (dark cyan line) and C4PI nanosheets (orange line) (λEXC = 890 nm) under 78 K. Inset: the power dependence
TPPL at 490 nm. PL intensities exhibit quadratic power dependence on the excitation power range, confirming the two-photon excitation nature of the
fluorescence.
(
≈60 meV; Figure 2h,i). The narrow bandwidth of absorption
and nanosheets at 78 K are found at 2.9 eV. The resonance is
attributed to the secondary absorption above the absorption
edge region, which is believed associated with long-lived exci-
tonic state trapped in the strong electrostatic field induced by
and PL peak is comparable to those reported for single crystals
prepared by other methods, suggesting that the crystal quality
of our synthesized material is comparably high.
In order to reduce the thermal broadening effect,
[13]
[
22,48]
+
−
[7,52]
PL
the localized (C H NH ) ions around the PbI4 layers.
At
4
9
3
spectrum of the 2D perovskites was also measured at 78 K. PL
spectra of both bulk crystals and nanosheets at 78 K show three
emission bands (Figure 3a). The resonance peak at 515 nm can
be ascribed to the room temperature (RT) phase, while the peak
at 490 nm is related to the low temperature (LT) phase. These
two phases are structurally different with different degrees
the lower energy side of the resonance peak, TPPL intensities
of all three samples increase linearly with the excitation photon
energy, which is the signature of two-photon absorption above
the band edge in 2D systems excited with in-plane polariza-
[
44]
tion. TPE spectra for PEPI and C4PI bulk crystals were also
measured at 295 K for comparison (Figure S7, Supporting Infor-
mation). However, the feature of linearly increased TPPL inten-
sities with the excitation energy at 295 K is not very distinctive
as a result of the broadening effects that smear the band edge
[
26,27]
of interface strain between the alkylammonium chains.
The additional broadband emission is due to mid-gap defect
states.[
11]
[
48]
TPE and two-photon PL (TPPL) spectra were both measured
by using femtosecond laser pulses as the excitation source.
The experimental setup is shown in the supporting informa-
tion (Figure S5, Supporting Information). TPPL spectra of
C4PI nanosheets and bulk crystals at 78 K (Figure 3b) show
similar excitonic emissions to their corresponding one-
photon-excitation PL spectrum. However, the broadband emis-
sion contribution to one-photon excitation PL spectra of C4PI
nanosheets decreases significantly under two-photon excita-
tion (Figure 3). The broadband emission in 2D OIHPs was
attributed to the emission of the defect states,[13] which is sensi-
tive to the surface of the sample. Under two-photon excitation,
the focal point may not be the same with that of one-photon
excitation, resulting in different contributions of broadband
emission under two situations. Two-photon excitation nature
of the observed emission is confirmed by its quadratic power
dependences on the excitation power (Figure 3b inset). Within
the excitation power range, no saturation effects in TPPL were
observed, which precludes the effects of ground state deple-
at room temperature. The energy of the band edge, which
was determined by extrapolation of the linear absorption edge
(Figure 4a), is: 2.57 eV for PEPI bulk crystal, 2.77 eV for C4PI
bulk crystal, and 2.74 eV for C4PI nanosheets. Moreover, the
energy of 1s state (E ) was determined from the peak of TPPL
1
s
spectra at 78 K with the addition of Stokes shift. For C4PI and
PEPI, the Stokes shift is temperature independent due to the
[
13,53]
polaron origin of the emission line.
was 2.41 eV for PEPI, 2.59 eV for C4PI bulk crystal, and 2.55 eV
for C4PI nanosheets. The exciton binding energies (Eb
E − E ) are therefore determined to be 160 meV for PEPI bulk
As a result, E1s energy
=
g
1s
crystal, 180 meV for C4PI bulk crystal, and 190 meV for C4PI
nanosheets, respectively. The measured values of the electronic
band gap and the exciton binding energy of the PEPI bulk
[
4,10]
crystal agree with the previously reported results,
which
verifies the reliability of the TPE spectroscopy method and the
measurement system as an effective tool to probe the excitonic
states of these materials. Note that the TPE spectrum of room
temperature phase yields similar results (Figure S8, Supporting
[
49]
[50,51]
tion or exciton–exciton annihilation.
Information). Meanwhile, E values were also obtained by fit-
g
TPE spectrum of C4PI at 78 K was measured by monitoring
the emission intensity of the dominant low temperature phase
peak at 490 nm under excitation at different wavelengths and
normalizing the TPPL intensities with the corresponding exci-
tation power (Figure 4a). TPE spectrum of PEPI bulk crystal
ting the linear absorption spectrum near the band edge using
Elliott’s model (Figure S9, Supporting Information). For C4PI
[
54]
nanosheet, the obtained E value according to Elliott’s model
g
matches well with that of TPE spectroscopy (Table S1, Supporting
Information). The deviations of E values for C4PI and PEPI
g
was measured under the same experimental condition as a con-
bulk crystals may be due to the ensemble absorption measure-
ments (UV–vis spectroscopy). The presence of surface and edge
trol sample.[
12,17]
TPE resonance peaks of both C4PI bulk crystal
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Figure 4. a) TPE spectrum of C4PI nanosheets, C4PI bulk crystal and PEPI bulk crystal at 78 K. The intercepts of the red dash lines indicate the electronic
band edge of the samples. b) Summary of exciton binding energies of C4PI with different thicknesses.
3
states of the bulk crystals may alter and broaden the absorption
spectrum, which induces the deviation from the intrinsic exci-
tonic and continuum absorption of bulk crystals.
oscillator strength is nonzero only for s states and falls like n−
and becomes continuum as the absorption edge is approached.
The oscillator strength per atom of 1s state is generally on
3
Figure 4b summarizes the exciton binding energies of C4PI
measured in this work and those of previous reports. The
exciton binding energy is determined by the difference between
absorption edge and 1s state. As the individual excitonic layers
are electronically isolated in 2D OIHPs due to the insulating
organic layers, the band gap of C4PI is expected to be insensi-
the order of (atomic radius/exciton radius) per atom. For 2D
OIHPs, atomic radius is ≈1.4 Å for iodine and ≈1.8 Å for Pb
atom. The exciton radius is comparable to the unit cell size of
≈20–50 Å. Under two-photon excitation, excitonic resonance
will leads to an enhancement near the band edge, analogous to
the results for one-photon excitation. The relative contribu-
tion of 2p state relative to that of the continuum of the band
edge is expected to be one order of magnitude smaller than that
of one-photon absorption spectrum. The weak 2p resonance
[57]
[45]
[
55]
tive to the number of layers, unlike the case of TMDs. How-
ever, E1s could be affected by environmental dielectric effects,
reorganization of the organic ligands and strain in the thin
layer limit.[
9,13,14,31]
Here, the exciton binding energies obtained
was also observed in (C H NH ) PbBr 2D perovskite.
[16]
4
9
3 2
4
from nanosheets and bulk crystal are both around 185 meV,
indicating that these effects do not play a dominant role for
crystals thicker than five layers.
In 2D hydrogenic model, exciton binding energy is described
µ
by Eb(n) = R
y
, where n is principle quantum number,
2
2
ε (n −0.5)
R is Rydberg constant, μ is exciton effective mass, and ϵ is
y
Dark excitonic states such as 2p states can be probed using
TPE spectroscopy by scanning the excitation wavelength within
dielectric constant. Binding energies of 1s and 2p excitons are
4
µ
4µ
9ε
the sub-band-gap energy range.[ The 2p state is the lowest
lying symmetry-allowed state for the two-photon excitation pro-
cess and a corresponding resonance peak appears in the TPE
22]
Eb(1s) = Ry
and Eb(2p) = Ry
.
The energy difference between 1s
2
2
ε
and 2p excitons could be utilized to estimate exciton binding
9
9
spectrum.[22] Figure 5a shows high resolution TPE spectra of the
energy: E
b
= E
g
−E1s = Eb(1s)
=
(Eb(1s) −Eb(2p) ) = (E2p −E1s ). Energy
8
8
OIHP crystals near the band edge region. We observe a weak
resonance feature on top of the band edge absorption tail in this
spectral range. Their energy values relative to that of the ground
exciton state and electronic band gap suggest that they arise
from 2p exciton resonance. Optical absorption close to the edge
levels of 1s, 2p resonance and electronic band gap of PEPI and
C4PI are summarized in Figure 5b. The exciton binding ener-
gies of PEPI and C4PI (bulk crystal/nanosheets) are estimated
to be 146 and 158/152 meV from energy difference between
1s and 2p resonance. The estimated exciton binding energies
according to the 2D hydrogenic model are a little bit smaller
than experimentally measured results for PEPI and C4PI
(bulk crystal/nanosheets). These deviations of exciton binding
(e.g., 1s and 2p excitons) in semiconductors can be examined
by band theory together with the effective-mass approximation
for the excitons.[ In direct band gap semiconductors, the
56]
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Figure 5. a) TPE spectrum of C4PI nanosheets, C4PI bulk crystal and PEPI bulk crystal at 78 K in sub-band-gap energy range. b) Exciton excitation
spectrum of PEPI, C4PI bulk crystal and C4PI nanosheets determined experimentally in this work compared with the 2D hydrogenic model. Orange
lines denote the one-photon active 1s states and two-photon active 2p states. Green rectangles represent the continuum states. Dark green dash lines
denote the estimated band edges according to the 2D hydrogenic model.
energies could arise from the existence of image charge effect
Inverse Temperature Crystallization of PEPI: Stoichiometric amount of
in 2D OIHPs.[
58]
PEI and PbI was dissolved in DMF to prepare 0.1 ꢀ precursor solution.
2
Seven milliliters of toluene was added dropwise to 0.2 mL the precursor
solution in a vessel and PEPI semi-amorphous crystallites would
precipitate out from the mixture. The mixture was stored in a freezer
at ≈−20 °C until the crystallites redissolve in the solution and then PEPI
single crystal will grow upon heating the vessel at 80 °C.
3
. Conclusion
Inverse Temperature Crystallization of C4PI: Stoichiometric amount of
C4I and PbI2 was dissolved in DMF to prepare 0.2 ꢀ precursor solution.
Eight milliliters of toluene was added dropwise to 0.2 mL precursor
solution in a vessel and C4PI crystallites would precipitate out from the
mixture. The mixture was put in a freezer at ≈−20 °C until the crystallites
redissolve in the solution and then C4PI single crystal will grow upon
heating the vessel at 80 °C.
Energy Dispersive Spectroscopy (EDS) and Scanning Electron
Microscope (SEM) Characterizations: EDS results were obtained by using
the JEOL JSM6700-F SEM mode, with the Oxford Instruments X-MaxN
In summary, we have demonstrated a novel synthesis method
of 2D OIHPs using the inverse temperature crystallization
and investigated their excitonic properties. At 78K, same TPPL
emission peak wavelengths and similar Stokes shifts were
obtained for C4PI nanosheests and bulk crystal, suggesting
similar E1s energies for them. Furthermore, TPE spectroscopy
was utilized to determine the electronic band edge of the 2D
OIHPs. Interestingly, we found that the exciton binding ener-
gies of C4PI are similar for 5-layer and bulk crystals, despite
their different number of layers. This is most likely due to the
isolated electronic structure of the organic–inorganic quantum-
well structure and weak interlayer coupling in both 2D OIHPs
bulk crystals and nanosheets. Moreover, we observed the 2p
states of 2D OIHPs, from which we verified the consistency in
the exciton binding energy of these materials.
1
50 EDS detector. Silicon substrate was made to stick to the sample
holder with a carbon tape to improve conduction. Measurements were
conducted in high vacuum at ≈7 × 10 Torr with beam voltage 5 and
1
−
7
5 kV for SEM and EDS, respectively. Beam current was 10 µA for both
characterizations.
Powder X-Ray Diffraction (PXRD) Measurement: The PXRD
experiments were performed in transmission mode with a Bruker D8
Advance A25 diffractometer equipped with graphite monochromated Cu
Kα radiation (λ = 1.54073 Å) at room temperature.
Atomic Force Microscopy (AFM) Measurement: AFM (Bruker
Dimension FastScan, Tapping mode) was used to measure height profile
of the exfoliated C4PI bulk crystal and nanosheets.
Linear Optical Measurements: Bright field and fluorescence images
were taken using optical microscope (Olympus, BX51TR). Absorbance
spectra were obtained by UV–vis-NIR spectrophotometer (Shimadzu
UV-2600). Micro-PL spectra were obtained with a laser confocal
microscope (NT-MDT, NTEGR Spectra) in back scattering geometry with
a 473 nm (continuous wave) excitation.
TPE Spectroscopy: The laser beam with a pulse duration of ≈140 fs
and a repetition rate of 80 MHz from a femtosecond mode-locked Ti:
sapphire laser (Chameleon Ultra II, Coherent) acted as the excitation
source. The laser beam was first cleaned by passing through a long
pass filter (with a cutoff wavelength of 715 nm) and then reflected by a
50/50 beam splitter into the objective lens (10×, NA = 0.3) to focus onto
the sample. The samples were loaded in to a Linkam stage (THMS600)
for temperature control. The same objective lens was utilized to collect
the TPPL signals. Their spectra were measured by a monochromator
(Acton, Spectra Pro 2300i) coupled CCD (Princeton Instruments, Pixis
100B).
4
. Experimental Section
Chemicals and Reagents: Phenethylamine (≥99%), n-butylamine
(
≥99%), toluene (ACS reagent, ≥99.5%), ethanol (ACS reagent, ≥99.5%),
N,N-dimethylformamide (ACS reagent, ≥99.8%), lead (II) iodide (99%),
hydroiodic acid (57 wt%) were purchased from Sigma Aldrich. All
chemicals were used as received without any further purification.
Synthesis of Ammonium Salts C H (C H )NH I (PEI) and C H NH I
6
5
2
4
3
4
9
3
(C4I): Ethanol (10 mL) and phenethylamine (2.52 mL) (C H (C H )NH )
6 5 2 4 2
were mixed in a beaker and kept at 0 °C under an ice bath. Stoichiometric
amount of HI (a.q. 2.62 mL) was added to the mixture dropwise under
stirring. The stirring continued for 2 h to ensure complete reaction. The
solution was heated up to 50 °C with a rotary evaporator to remove the
solvent until whitish ammonium salt powder appeared. The ammonium
salt powder was collected and washed with diethyl ether three times and
then transferred to a vacuum oven to dry at 60 °C for 24 h for later use. A
similar process was utilized to synthesis of C4I by using n-butylamine as
the precursor solvent instead.
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