A new organic-inorganic bismuth halide crystal structure and quantum dot bearing long-chain alkylammonium cations

Article abstract of DOI:10.1016/j.orgel.2019.04.024

It is report here a new family of organic-inorganic bismuth halides bearing a formula of A₂BiX₅, in which A is monovalent long-chain alkylammonium and X is halide. Two compounds, (BA)₂BiBr₅ (BA: C₄H₉NH₃ ⁺) and (OA)₂BiBr₅ (OA: C₈H₁₇NH₃ ⁺), have been synthesized and investigated by single crystal and powder X-ray diffractions, UV–vis absorption and fluorescence spectroscopies, and density functional theoretical calculations. An orthorhombic crystalline structure with a P2₁2₁2₁ space group, which had not been reported for organobismuth halides before, was found existing in (OA)₂BiBr₅ single crystals. Besides bulk materials, the quantum dots (QDs) of (BA)₂BiBr₅ and (OA)₂BiBr₅ were prepared and demonstrated as blue emitters with photoluminescent quantum yields of 1.26% and 0.50%, respectively, after capping with oleic acid. Finally, mixed halides with various I/Br ratios were prepared and found to form certain solid solutions with homogeneous distributed I^? and Br^?. Upon tuning I/Br ratio, the absorption and emission bands of their QDs can be easily modulated.

Full text of DOI:10.1016/j.orgel.2019.04.024

Accepted Manuscript
A new organic-inorganic bismuth halide crystal structure and quantum dot bearing
long-chain alkylammonium cations
Jiandong Wang, Jia Li, Yong Wang, Wen-Jing Xiao, Xiang Yao, Zi-Wen Xu, Jianhua
Yao, Jian Lin, Wei-Shi Li
PII:
S1566-1199(19)30189-2
DOI:
https://doi.org/10.1016/j.orgel.2019.04.024
ORGELE 5218
Reference:
To appear in: Organic Electronics
Received Date: 15 January 2019
Revised Date: 28 March 2019
Accepted Date: 15 April 2019
Please cite this article as: J. Wang, J. Li, Y. Wang, W.-J. Xiao, X. Yao, Z.-W. Xu, J. Yao, J. Lin, W.-
S. Li, A new organic-inorganic bismuth halide crystal structure and quantum dot bearing long-chain
alkylammonium cations, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.04.024.
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Graphical Abstract

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A new organic-inorganic bismuth halide crystal
structure and quantum dot bearing long-chain
alkylammonium cations
Jiandong Wang,a,b Jia Li,b Yong Wang,b,c Wen-Jing Xiao,b Xiang Yao,b Zi-Wen Xu,b Jianhua
Yao,b,c Jian Lin*a and Wei-Shi Li*b,c
a Key Laboratory of Advanced Civil Engineering of Ministry of Education, School of Materials
Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China.
b Key Laboratory of Synthetic and Self-Assembly Chemistry for Functional Organic Molecules,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, China.
c Engineering Research Centre of Zhengzhou for High Performance Organic Functional
Materials, Zhengzhou Institute of Technology , 6 Yingcai Street, Huiji District, Zhengzhou
450044, China.
Corresponding Author
*E-mail: lin_jian@tongji.edu.cn (J.L.).
*E-mail: liws@mail.sioc.ac.cn (W.-S. L.).
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ABSTRACT
It is report here a new family of organic-inorganic bismuth halides bearing a formula of
A2BiX5, in which A is monovalent long-chain alkylammonium and X is halide. Two compounds,
+
+
(BA)2BiBr5 (BA: C4H9NH3 ) and (OA)2BiBr5 (OA: C8H17NH3 ), have been synthesized and
investigated by single crystal and powder X-ray diffractions, UV-vis absorption and fluorescence
spectroscopies, and density functional theoretical calculations. An orthorhombic crystalline
structure with a P212121 space group, which had not been reported for organobismuth halides
before, was found existing in (OA)2BiBr5 single crystals. Besides bulk materials, the quantum
dots (QDs) of (BA)2BiBr5 and (OA)2BiBr5 were prepared and demonstrated as blue emitters with
photoluminescent quantum yields of 1.26% and 0.50%, respectively, after capping with oleic
acid. Finally, mixed halides with various I/Br ratios were prepared and found to form certain
solid solutions with homogeneous distributed I- and Br-. Upon tuning I/Br ratio, the absorption
and emission bands of their QDs can be easily modulated.
KEYWORDS: organic-inorganic hybrid materials; organobismuth halides; quantum dots; single
crystal structure; solid solutions
1. Introduction
In the past decade, organolead halide materials have rapidly emerged as a new kind of star
materials due to their outstanding optoelectronic properties and a wide range of potential
applications in solar cells, light-emitting diodes, field-effect transistors, photodetectors and
lasers.[1-10] Besides their bulk materials and thin films, organolead halide quantum dots (QDs)
have also attracted broad attention.[11-14] Owing to the existence of specific nanoscale size
effect, such QDs generally possess outstanding light-emitting properties with high
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photoluminescence quantum yield (PLQY) and tunable wavelength by controlling over their
shape, composition and size. These features make them particularly attractive in the field of
light-emitting diodes.
However, lead is a toxic element and its use should be strictly controlled. Therefore, search
for lead-free organic-inorganic hybrid materials have become an important mission in this
field.[15,16] In the past, organotin halides have been extensively studied as one of lead-free
perovskite optoelectronic materials. With continuous effort, tin-based solar cells have achieved a
power conversion efficiency of ~8%,[15-17] and a number of highly photo-luminescent
nanocrystals have been prepared.[18,19] But, tin-based materials have a stability issue because
Sn2+ is easily oxidized into Sn4+.[20,21]
Besides tin, bismuth is another element that can be considered. Compared with Sn2+, Bi3+ is
less toxic, but more stable. So far, bismuth halides with a composition of A3Bi2X9 [A =
+
monovalent cation, such as CH3NH3 (MA) or Cs+] have been mainly studied.[22-26] In this
family, A3Bi2I9 is a zero-dimensional crystal having a space group of P63/mmc,[27,28] while
A3Bi2Br9 adopts a two-dimensional structure.[29,30] These materials have been tested as the
active layer for solar cells, but poor performances were observed.[15-17] Recently, Leng et
al.[29] and Yang et al.[31] prepared QDs of (MA)3Bi2Br9 and Cs3Bi2Br9 and found good blue
emitting properties with a PLQY of 12% and 4.5%, respectively. In addition to A3Bi2X9,
+
CsBi3I10[32] and (HDA)BiI5 (HDA: [H3NC6H12NH3]2 )[33,34] having a layered and one-
dimensional structure respectively have also been studied. A very low power conversion
efficiency (0.027%) was found for (HDA)BiI5-based solar cells, while no application was
reported with CsBi3I10.
From these reported works, it is noticeable that long-chain
+
+
alkylammonium cations, such as C4H9NH3 (BA) and C8H17NH3 (OA) that are often seen in
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organolead halides, have not yet been used in bismuth-based materials. In this work, BA and
OA have been firstly used in the preparation of organobismuth halides, affording new Bi-based
organic-inorganic hybrid materials having a composition of A2BiX5. More interestingly,
(OA)2BiBr5 single crystal was found to hold a new crystalline structure with a space group of
P212121. Moreover, QDs of this kind of organic-inorganic bismuth halides were further prepared
and exhibited blue light-emitting properties with tunable wavelength by varying their halide
composition.
2. Material and methods
2.1 Materials
BiBr3 (99 %), BiI3 (99.999%) and hydrobromic acid (48 wt%) were purchased from Alfa
Aesar (USA). Hydroiodic acid (55-58 wt%) was purchased from Aladdin (China). Octylamine
(99%), butylamine (99%) and oleic acid (99 %) were purchased from TCI (Japan). Quinine
sulfate (99%) and ethanol (super dry, 99.5%) were purchased from J&K (China). Toluene was
purchased from Sinopharm (China) and dried before use.
2.2 Syntheses of (BA)Br, (BA)I and (OA)Br
For synthesis of butylammonium bromide ((BA)Br), butylamine (10 mL, 0.1 mol) was fisrt
dissolved in EtOH (15 mL), and then added with HBr (10.3 mL, 0.091 mol) under Ar protection.
After the reaction was carried out under an ice bath for 2 h, the solvent was evaporated off. The
resulted solid residue was washed with anhydrous diethyl ether three times and dried in a
o
vacuum chamber at 60 C for 24 h, affording 14 g (BA)Br. Elemental analysis calcd. for
C4H12BrN: C 31.2 %, H 7.9 %, N 9.1 %; found: C 31.2 %, H 7.8 %, N 9.2 %.
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The syntheses of (BA)I and (OA)Br followed the same method as that for (BA)Br but using
HI (11.8 mL, 0.091 mol) in place of HBr, and octylamine (16.7 mL, 0.1 mol) in place of
butylamine, respectively. Elemental analysis calcd. for C4H12IN: C 23.9 %, H 6.0 %, N 7.0 %;
found: C 23.8 %, H 6.0 %, N 7.1 %. Elemental analysis calcd. for C8H20BrN: C 45.7 %, H 9.6 %,
N 6.7 %; found: C 45.7 %, H 9.7 %, N 6.8 %.
2.3 Syntheses of (OA)2BiBr5, (BA)2BiBr5 and (BA)2BiI5
For the synthesis of (OA)2BiBr5, (OA)Br (418.2 mg, 2 mmol) and BiBr3 (448.7 mg, 1 mmol)
were dissolved 10 ml EtOH. After evaporation off the solvent, the residue was dried in a vacuum
o
chamber at 80 C overnight, affording 832.6 mg (OA)2BiBr5 powder. Elemental analysis calcd.
for C16H40BiBr5N2: C 22.1 %, H 4.7 %, N 3.2 %; found: C 22.8 %, H 4.8 %, N 3.4 %.
Following the same method, (BA)2BiBr5 powder was prepared by using (BA)Br (308.1 mg, 2
mmol) in place of (OA)Br, and (BA)2BiI5 was prepared by using (BA)I (402.1 mg, 2 mmol) and
BiI3 (589.7 mg, 1 mmol) in place of (OA)Br and BiBr3, respectively. Elemental analysis calcd.
for C8H24BiBr5N2: C 12.7 %, H 3.2 %, N 3.7 %; found: C 12.7 %, H 3.2 %, N 3.7 %; Elemental
analysis calcd. for C8H24BiI5N2: C 9.7 %, H 2.4 %, N 2.8 %; found: C 9.5 %, H 2.3 %, N 3.0 %.
For the synthesis of mixed halide composites, different amounts of (BA)2BiBr5 and (BA)2BiI5
powders with desired ratio was dissolved in EtOH and stirred for a while. After evaporation off
o
the solvent and further dry in a vacuum chamber at 80 C overnight, mixed halide compositions
were obtained.
2.4 Preparation of single crystals
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The powder sample of (OA)2BiBr5 or (BA)2BiBr5 (50 mg) was dissolved in 2 ml EtOH. Then,
the solvent was slowly evaporated off at room temperature. The crystals grew slowly at the
bottom of the bottle.
2.5 Synthesis of quantum dots
For synthesis of (BA)2BiBr5 QDs, (BA)2BiBr5 (151.4 mg, 0.2 mmol) was dissolved in 2 ml
EtOH to form a precursor solution with a concentration of 0.1 M. Then, 1 mL of this precursor
solution was rapidly injected into 10 mL dried toluene, which was pre-heated to 100 oC under Ar
atmosphere, and then subjected to vigorous stirring. Immediately, a large amount of yellow
green particles were precipitated. After centrifugation at 8000 rpm for 10 min, the precipitate at
tube bottom was discarded and the supernatant was collected, affording a colloidal solution of
(BA)2BiBr5 QDs. Similarly, (OA)2BiBr5 QDs were prepared by using (OA)2BiBr5 (173.8 mg,
0.2 mmol) in place of (BA)2BiBr5 for precursor solution.
The synthesis of oleic acid (OlAc)-capped (BA)2BiBr5 (or (OA)2BiBr5) QDs followed the
same method as described above, but adding 1 mL OlAc into 10 mL toluene and stirring several
minutes before injecting (BA)2BiBr5 (or (OA)2BiBr5) precursor solution.
For the synthesis of halide-mixed QDs, the precursor solution was prepared by mixing
(BA)2BiBr5 and (BA)2BiI5 in a desired ratio in EtOH. Typically, for the synthesis of mixed
halide QDs with a I/Br molar ratio of 2/3, (BA)2BiBr5 (60.5 mg, 0.08 mmol) and (BA)2BiI5
(119.0 mg, 0.12 mmol) were dissolved 2 mL EtOH to form a 0.1 M mixed halide precursor
solution. Then 1 mL of this precursor was injected to 10 mL toluene containing 1 mL OlAc at
100 oC and stirred for 10 min. After centrifugation at 8000 rpm for 10 min, a colloidal solution of
mixed halide QDs with a I/Br molar ratio of 2/3 was obtained.
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2.6 Characterization methods
Single crystal X-ray diffraction data were collected on a Bruker AMART APEX CCD
diffractometer (Germany). The crystal structure was solved using SHELXS program and refined
using a full-matrix least squares technique of SHELXS. The elemental analysis on C, H, and N
contents was performed on a Vario Micro Cube elemental analyser (Germany). Powder X-ray
diffraction (XRD) was performed on a Rigaku Highflux Homelab model X-ray diffractometer
o
(Japan) using Cu Kα radiation (λ = 1.5406 Å) with a scanning step of 0.026 . The simulated
XRD pattern was obtained using Diamond program based on the CIF files from single crystal X-
ray diffraction. Transmission electron microscopy (TEM) was recorded on a JEM-2100
instrument (Japan). High resolution TEM image was recorded on a FEI TECNAI G2 S-TWIN
instrument (USA). Size distribution was obtained by measuring the diameters of approximated
150 quantum dots in the TEM image. UV-vis absorption spectroscopy was performed on a
Hitachi U-3310 spectrophotometer (Japan), while photoluminescent (PL) spectra were measured
using a Hitachi F-4600 fluorescent spectrophotometer (Japan) with a 150 W Xe lamp as an
excitation source. Time-resolved PL decay spectra were obtained using a Horiba PTI QM-40
fluorescence spectrometer (Japan) with a 337 nm nitrogen laser. The photoluminescence
quantum yield (PLQY) was measured by a relative method reported before.[29,35] Quinine
sulfate was used as standard sample, which has a PLQY of 57.7 % in 0.1 M H2SO4 under 350 nm
excitation. PLQY could be calculated by the equation as following:
ꢁ
Yꢀ Fꢀ Aꢁ
Yꢁ Fꢁ Aꢀ
nꢀ
∙ ꢂ ꢃ
nꢁ
=
∙
where Y is PLQY, A is absorbance, F is the integrated emission intensity, n is solvent average
refractive index, the subscript of 1 represents the checked QDs, and the subscript of 2 stands for
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the standard sample. In this case, average refractive indexes of 0.1 M H2SO4 and toluene are
1.3443 and 1.4967, respectively.
2.7 DFT calculation
DFT calculations were performed by DMol3 in Material Studio. The general gradient
approximation (GGA)/Perdew-Burke-Ernzerh (PBE) of function without spin-orbit coupling
were employed to calculate the electronic bands and the density of states of (OA)2BiBr5.
3. Results and discussion
The preparations of (BA)2BiBr5 and (OA)2BiBr5 are simple, just by mixing (BA)Br and
(OA)Br with BiBr3 in a molar ratio of 2:1 in ethanol. After evaporation off the solvent, the
products as powder were obtained. In order to get their single crystals, the so-prepared powders
were re-dissolved in ethanol. During the slow solvent evaporation, crystals were gradually
produced in glass bottom. Both (BA)2BiBr5 and (OA)2BiBr5 crystals were subjected to single
crystal X-ray diffraction measurement, but only the structure of (OA)2BiBr5 was successfully
resolved. It was found that (OA)2BiBr5 single crystal at 173 K adopts an orthorhombic phase
with a space group of P212121 and lattice parameters (a, b, c) of 7.9357, 18.7268, and 37.7406 Å,
respectively (Fig. 1a and Table S1). Similar to organolead halides having long-chain
alkylammonium moieties, (OA)2BiBr5 crystal has a layered structure. In each layer, [BiBr6]3-
octahedron sub units are interconnected each other by sharing two Br atoms at the same side and
form a one-dimensional folded chain in space (Fig. 1b, S1 and S2). The cation heads of
alkylammoniums insect into the chain intervals and connect to [BiBr6]3- octahedrons by N-H Br
+
hydrogen-bonding between RNH3 and Br- (Fig 1c and 1d, and Table S2) and electrostatic
interactions between cations and anions. Such connections bundle the [BiBr6]3- octahedron
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folded chains together and stabilize the structure within the layer. Meanwhile, the long tails of
alkyl ammoniums fill in the space between the layers, interdigitate one another, and contribute
van der Waals interactions to stabilize layered structure. Therefore, in addition to Bi-Br ionic
bonding, the above hydrogen-bonding, electrostatic and van der Waals interactions are three
critical factors to stabilize the whole crystal structure. Since it is different from the all previously
reported, this is a new kind of crystal structure for organic-inorganic bismuth halide materials.
The powder X-ray diffraction (XRD) profile of (OA)2BiBr5 is displayed in Fig. 1e. It is
identical to the simulated profile from its single crystal structure. In the case of (BA)2BiBr5,
single crystal X-ray diffraction found that the arrangement of the [BiBr6]3- octahedrons is the
same as the (OA)2BiBr5 crystal (Fig. S3). However, the positons of BA groups were difficult to
be indexed since they were always rotating under measurement conditions. Based on this
partially resolved single crystal structure, the simulated powder X-ray diffraction pattern was
obtained and found to coincide with the actually measured (Fig. 1f).
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Fig. 1. (a) Unit cell of (OA)2BiBr5 crystal. (b) Arrangement illustration of [BiBr6]3- octahedrons
and (c, d) hydrogen bonds (dash line) in (OA)2BiBr5 crystal. (e, f) Measured and simulated
powder XRD patterns of (e) (OA)2BiBr5 and (f) (BA)2BiBr5.
Fig. 2a displays UV-vis absorption and photoluminescent (PL) spectra of (OA)2BiBr5 and
(BA)2BiBr5 films. Obviously, (OA)2BiBr5 film exhibited a strong absorption peak at 417 nm and
a PL peak at 470 nm with a full width at half-maximum (FWHM) of 49 nm. From its Tauc’s
plot,[35,36] the band gap of (OA)2BiBr5 was estimated to be 2.84 eV. In the case of (BA)2BiBr5,
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its film showed an absorption band at 410 nm and a PL peak at 465 nm with a FWHM of 55 nm,
both slightly blue shifted as compared to those of (OA)2BiBr5. Similarly, the band gap of
(BA)2BiBr5 was found to be 2.88 eV, slightly larger than that of (OA)2BiBr5.
Fig. 2. (a) Absorption and PL spectra upon excitation at 350 nm and (b) Tauc’s plot of
(OA)2BiBr5 and (BA)2BIBr5 films. (c) Band structural diagram and (d) density of states (DOS)
of (OA)2BiBr5.
In order to gain insight on molecular electronic band structure, density functional theory (DFT)
calculation was performed on a 2 × 2 × 2 super cell of (OA)2BiBr5. As shown in Fig. 2c and d,
the obtained band structural diagram and its density of states (DOS) indicate that valance band
maximum (VBM) is mainly contributed by Bi s and Br p states, while conduction band minimum
(CBM) forms by p states of both Bi and Br atoms. However, the OA moiety has little
contribution to both VBM and CBM. This suggests that the alkylammonium component would
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not influence electron’s excitation and recombination processes, similar to the previous reported
lead-based perovskite materials.[37,38]
Fig. 3. Photographs of a (BA)2BiBr5 colloidal solution (a) without and (b) with shining with a
UV lamp. (c) TEM and (d) high resolution TEM images of (BA)2BiBr5 QDs without capping of
OlAc. (e) TEM image of (BA)2BiBr5 QDs with capping of OlAc. (f) Size distributions of
(BA)2BiBr5 QDs without and with capping of OlAc. (g) XRD patterns of (BA)2BiBr5 QDs and
bulk powder. (h) Absorption and PL spectra of (BA)2BiBr5 film and QDs without and with
capping of OlAc in colloidal solution state. The excitation wavelengths in PL measurement were
350 nm for film sample, while 325 nm for colloidal solutions. (i) Time-resolved PL spectra and
fitting curves of the colloidal solutions of (BA)2BiBr5 QDs without and with capping of OlAc
upon excitation with 337 nm laser.
After understanding basic properties of bulk materials, we further synthesized and studied
QDs of (OA)2BiBr5 and (BA)2BiBr5. The synthesis followed a general dispersion strategy
[39,40] using EtOH as good solvent while toluene as antisolvent. For example, a precursor
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solution of (BA)2BiBr5 in EtOH was injected into toluene, affording a large amount of
precipitated yellow-green particles. After centrifugation at 8000 rpm for 10 min and the discard
of large particles at tube bottom, a colloidal solution with slight yellow-green color was obtained
(Fig. 3a). When shining with a UV lamp, the colloid emitted bright blue light (Fig. 3b).
Transmission electron microscopy (TEM) revealed that the colloid contained a large amount of
spherical nanoparticles (Fig. 3c) with a size of 2.8 ± 0.7 nm (Fig. 3f). The high resolution TEM
image (Fig. 3d) indicated the high crystallinity nature of the quantum with a lattice space of 0.20
nm. After evaporation off solvent, the afforded QD powder was subjected to XRD analysis (Fig.
3g). The obtained diffraction pattern was found almost identical to that of bulk powder, again
indicating (BA)2BiBr5 QDs had a well-crystallized nature with the same structure of the bulk
state. Moreover, this (BA)2BiBr5 colloidal solution displayed a light absorption peak at 383 nm
and a PL peak at 415 nm with a FWHM of 57 nm (Fig. 3h). Compared with the film spectra,
both the absorption and luminescent peaks were blue-shifted with shifting amounts of 27 and 50
nm, respectively. Such hypochromic phenomena may be ascribed to the quantum confinement
effect taking place in the nanoparticles. Similarly, (OA)2BiBr5 QDs with a size of 3.2 ± 0.8 nm
were prepared (Fig. S4). Compared with bulk state, the obtained (OA)2BiBr5 QDs in colloidal
solution displayed an absorption peak at 387 nm and a PL peak at 429 nm (FWHM: 50 nm) with
blue-shifting of 30 and 41 nm, respectively.
In order to further characterize light-emitting properties of synthesized QDs, their
PLQYs were measured and time-resolved PL spectroscopy was carried out. By reference
to quinine sulfate, a standard sample having a known PLQY (57.7%) upon excitation at
350 nm, the PLQY was estimated to be 0.13 % for (BA)2BiBr5 QDs and 0.15 % for
(OA)2BiBr5 QDs. Obviously, these PLQY values are extremely low, as compared with
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the reported (MA)3Bi2Br9 QDs.[29] In the time-resolved PL spectrum of (BA)2BiBr5
colloidal solution (Fig. 3i), the fluorescence decay can be fitted by a biexponential
function having lifetimes of 1.73 (τ1) and 19.52 ns (τ2) and fractions of 72.6% and 27.4%,
respectively. The τ1 component originates from exciton recombination occurring within
perfect crystalline domains, while that of τ2 is associated with slow recombination at
surface defects.[41] The existence of slow recombination process with considerable
fraction indicates (BA)2BiBr5 QDs had a large amount of defects in their surface. It was
reported oleic acid (OlAc) can passivate surface defects of perovskite QDs.[42-44]
Therefore, certain amount OlAc was added into toluene before the injection of bismuth
halide precursor ethanol solution. The so-afforded OlAc-capped (BA)2BiBr5 QDs had a
size of 2.3 ± 0.6 nm, slightly smaller than aforementioned (BA)2BiBr5 QDs without
adding OlAc (Fig. 3e). It was further found that OlAc modification did not alter light-
absorption (382 nm) and emission properties (416 nm with a FWHM of 69 nm), but did
influence the PL decay components. As shown in Fig. 3i, the PL decay of the colloidal
solution of OlAc-capped (BA)2BiBr5 QDs can be fitted with two components: τ1 = 2.20 ns
(85.0%), τ2 = 21.37 ns (15.0%). It is clear that the OlAc capping significantly reduced the
fraction of τ2 component. This implies that OlAc molecules could occupy the defect sites
on the surface of (BA)2BiBr5 QDs and passivate non-radiative recombination. As a result,
the PLQY of OlAc-capped (BA)2BiBr5 QDs was significantly improved to be 1.26 %.
Similarly, the addition of OlAc into the preparation of (OA)2BiBr5 QDs reduced the QD
size (from 3.2 ± 0.8 nm to 3.0 ± 0.7 nm) and improved PLQY to 0.50%, but did not
change absorption and emission peaks (Fig. S5).
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It is well known that the use of mixed halide anions and control over their ratio is a valid way
to tune crystal structures and optical properties of organic-inorganic hybrid materials. In this
work, a series of mixed halide powders were prepared by using different amounts of (BA)2BiBr5
and (BA)2BiI5 precursors in Br/I molar ratios (m/n) of 4/1, 3/2, 2/3, and 1/4. When the
composition was changed from (BA)2BiBr5 to mixed halides (BA)2BiBrmIn and finally to
(BA)2BiI5, powder XRD (Fig. 4a) found that the diffraction peak at 7.8 ° gradually downshifted
to 7.3 °, while that at 11.3 ° changed into 10.7 ° and finally became weak. Meanwhile, a new
intense peak appeared at 8.5 °. These XRD changes indicate that the mixed halides are not the
physical blends of (BA)2BiBr5 and (BA)2BiI5, but like certain kinds of solid solutions. In order
to learn the detail distribution of halides, TEM and energy dispersive spectroscopy (EDS) and
mapping were performed on a crystal particle of (BA)2BiBr2I3 and the results are displayed in
Fig. 4b-4f. From these images, it is clear that Br- and I- coexisted and distributed
homogeneously in the particle along with Bi2+. Moreover, the I/Br ratio at the crisscross spot in
the particle (Fig. 4a) was determined to be 1.4, which is very close to the feeding ratio. This
further confirms the formation of solid solutions for the mixed halides.
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Fig. 4. (a) XRD profiles of (BA)2BiBr5, (BA)2BiI5 and (BA)2BiBrmIn (m/n = 4/1, 3/2, 2/3, and
1/4) powders. (b) TEM, (c-e) EDS mapping and (f) spectrum of the as shown crisscross spot of a
(BA)2BiBr2I3 crystal particle.
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Fig. 5. Absorption and PL spectra of (BA)2BiBr5, (BA)2BiI5 and (BA)2BiBrmIn (m/n = 4/1, 3/2,
2/3, and 1/4) QDs in colloidal solutions. The excitation wavelengths in PL measurement were
325 nm for (BA)2BiBr5, 355 nm for (BA)2BiBr4I, 385 nm for (BA)2BiBr3I2, 400 nm for
(BA)2BiBr2I3, 420 nm for (BA)2BiBrI4, and 440 nm for (BA)2BiI5.
Following the same method to those of (BA)2BiBr5, the QDs of (BA)2BiBrmIn mixed halides
having m/n of 4/1, 3/2, 2/3, and 1/4 and pure (BA)2BiI5 were prepared. They displayed identical
XRD profiles to their bulk powders (Fig. S6), suggesting the same solid solution existing in both
QD and powder forms. Of expected, as shown in Fig. 5, gradually red shifts were observed in
their absorption and emission bands from pure (BA)2BiBr5 to mixed halide composites with m/n
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of 4/1, 3/2, 2/3, and 1/4, and then to pure (BA)2BiI5 in colloidal solutions. Therefore, different
from that of (BA)2BiBr5 QDs, the appearance of (BA)2BiI5 colloidal solution was dark-yellow
under ambient conditions, and changed to green luminescent under UV light (Fig. S7). It was
found that (BA)2BiI5 QDs had a size of 4.6 ± 0.7 nm, and displayed a light-absorption band at
490 nm and a PL peak at 547 nm with a FWHM of 55 nm in colloidal solution state. These
results indicate that the emission band can be easily tuned from 415 nm to 547 nm by simply
varying the halide composition.
4. Conclusion
In conclusion, long-chain alkylammonium cations, BA and OA, have been firstly used for
preparation of organic-inorganic bismuth halides, and afforded two new organic-inorganic Bi-
based hybrid materials having compositions of (BA)2BiBr5 and (OA)2BiBr5. A new crystal
structure having an orthorhombic phase with a P212121 space group for bismuth halide
perovskites was found existing in (OA)2BiBr5 single crystals. DFT study revealed that the
material VBM and CBM are mainly contributed from Bi3+ and Br-, while alkylammonium has
little contribution. Besides bulk materials, organobismuth halide QDs with these long-chain
alkylammonium moieties were further prepared and demonstrated to be blue emitters. After
capped with OlAc, (BA)2BiBr5 and (OA)2BiBr5 QDs displayed a PLQY of 1.26% and 0.50%,
respectively. Using mixed halide anions and varying their ratio in the composition, the afforded
QDs can tune their photoluminescent peak from 415 nm to 547 nm.
ACKNOWLEDGMENT
The work was financially supported by the National Natural Science Foundation of China
(Nos. 21474129, 21674125, 21672251 and 51761145043), the Strategic Priority Research
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Program of Chinese Academy of Sciences (No. XDB20020000), the Science and Technology
Open Cooperation Projects of Henan Province (Nos. 162106000018 and 172106000067) and
Zhengzhou Institute of Technology.
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Highlights
 A new family of organic-inorganic bismuth halides bearing long-chain
+
alkylammonium, (BA)2BiBr5 (BA: C4H9NH3 ) and (OA)2BiBr5 (OA:
+
C8H17NH3 ), have been synthesized and investigated.
 A new crystal structure for organobismuth halides was found existing in
(OA)2BiBr5 single crystals, which have an orthorhombic structure with a P212121
space group.
 Quantum dots of organobismuth halides bearing long-chain alkylammonium have
been firstly prepared and demonstrated with blue photoluminescence.
 Upon tuning I/Br ratio, the absorption and emission bands of (BA)2BiBrnIm can
be easily modulated.