Lawrence Berkeley National Laboratory
Recent Work
Title
Essentially Trap-Free CsPbBr
3
Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface
Treatment.
Permalink
https://escholarship.org/uc/item/3vn9q0vp
Journal
Journal of the American Chemical Society, 139(19)
ISSN
0002-7863
Authors
Koscher, Brent A
Swabeck, Joseph K
Bronstein, Noah D
et al.
Publication Date
2017-05-01
DOI
10.1021/jacs.7b02817
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
Essentially Trap-Free CsPbBr
3
Colloidal Nanocrystals by
Postsynthetic Thiocyanate Surface Treatment
Brent A. Koscher,
†,§,∥
Joseph K. Swabeck,
†,§,∥
Noah D. Bronstein,
†,§,∥
and A. Paul Alivisatos*
,†,‡,§,∥
†
Department of Chemistry, University of California Berkeley, Berkeley, California 94720, United States
‡
Department of Materials Science and Engineering, University of California Berkeley, Berkeley, California 94720, United States
§
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥
Kavli Energy NanoScience Institute, University of California Berkeley and Lawrence Berkeley National Lab, Berkeley, California
94720, United States
*
S
Supporting Information
ABSTRACT: We demonstrate postsynthetic modification
of CsPbBr
3
nanocrystals by a thiocyanate salt treatment.
This treatment improves the quantum yield of both freshly
synthesized (PLQY ≈ 90%) and aged nanocrystals (PLQY
≈ 70%) to within measurement error (2−3%) of unity,
while simultaneously maintaining the shape, size, and
colloidal stability. Additionally, the luminescence decay
kinetics transform from multiexponential decays typical of
nanocrystalline semiconductors with a distribution of trap
sites, to a monoexponential decay, typical of single energy
level emitters. Thiocyanate only needs to access a limited
number of CsPbBr
3
nanocrystal surface sites, likely
representing under-coordinated lead atoms on the surface,
in order to have this effect.
I
n recent years, lead halide perovskites have attracted
considerable attention as promising optoelectronic materials
for photovoltaics,
1a,b
photodetectors,
1c,d
and light-emitting
diodes,
1e,f
among other applications. Thin film lead halide
perovskites are already in early stages of potential commercial
development for photovoltaic devices. Research on their
nanocrystalline counterparts lags a few years behind, but are
also beginnin g to show promise.
2a−c
Facile syntheses and
excellent optoelectronic properties have led t o the rapid
emergence of cesium lead halide (CsPbX
3
; X = Cl, Br, I)
nanocrystals (NCs). Using solution-based procedures, CsPbX
3
NCs have been shown to present high photoluminescence
quantum yields (PLQYs) of up to 90%
3a−c
and narrow emission
line widths without the need for a passivating higher band gap
semiconductor shell, which is required for highly luminescent
metal-chalcogenide NCs.
4a−c
Following this initial success, there is ongoing debate in the
literature on the origin of the excellent optical performance in
CsPbX
3
nanocrystals,
5a−e
but it is generally accepted that the lead
halide perovskites somewhat uniquely have a high defect
tolerance. This tolerance is frequently attributed to the ionic
nature of the material or the orbital composition of the energy
bands that are responsible for the optical transition.
5a−e
Although
the lead halide perovskites are defect tolerant, they are not defect
impervious. A number of theoretical
5a−c
and experimental
6a−d
studies have suggested the potential presence of non-negligible
defects in lead halide perovskites. The potential contribution of
surface defects becomes increasingly important in nanocrystalline
semiconductors due to the increased surface-to-volume
ratio.
4c,7a−e
The impact of these defects is most readily evident
in the subunity PLQYs and extended multicomponent excited
state photoluminescence (PL) decay kinetics of the CsPbX
3
nanocrystals.
2c,3a,8a,b
These observations are consistent with the
theorized shallow surface traps that would arise from a lead-rich
surface.
5a−d,6c,d
This lead-rich surface is likely due to a
combination of lead-rich synthetic conditions and the lability of
the oleylammonium halide surface species.
3a,6c,8a,b
Postsynthetic
processing, such as puri fication withantisolvents or even aging the
NCs in solution, causes the PLQY to drop from 90% to 70% or
lower. The PLQY deterioration is indicative of inadequate NC
surface passivation, each scenario representing an opportunity for
the labile surface oleylammonium halide ligands to be removed
and lost.
7c,9a
The unusually rapid success in producing nanocrystals with
90% PLQY is encouraging; however, this begs the question: what
prevents the PLQY from being unity? The presence and role of
surface states in CsPbX
3
NCs has been relatively unexplored
experimentally thus far. In this study, we demonstrate a surface
treatment with thiocyanate that improves the PLQY of CsPbBr
3
to near unity while maintaining colloidal stability, NC shape, and
crystal structure. We investigate the chemical effect of the
treatment and find that no more than 10−15% of the surface
ligands are replaced with thiocyanate while the stoichiometry of
the NC surface changes from about 10% lead-rich to a
stoichiometric 1:3 ratio of Pb to Br. We believe this treatment
is an effective way of removing excess lead from the surface,
consequently removing shallow traps and making the nanocryst-
als into near-unity green emitters.
For this investigation, colloidal CsPbBr
3
NC cubes were
synthesized following the procedures developed by Protesescu et
al.
3a
with minor modifications (Supporting Information (SI) for
details). The isolated as-synthesized CsPbBr
3
NCs have the
desired cube-shaped morphology with typical size dispersions
around ±10% (in edge length, determined by TEM; SI). The
typical sample presents line widths comparable to single-particle
line widths
8a,b
and PLQYs between 85% and 93%, determined
Received: March 21, 2017
Published: April 27, 2017
Communication
pubs.acs.org/JACS
© 2017 American Chemical Society 6566 DOI: 10.1021/jacs.7b02817
J. Am. Chem. Soc. 2017, 139, 6566−6569
optically using an integrating sphere (SI). Following the synthesis,
samples were dispersed in anhydrous hexanes or toluene, fresh
samples were used right away, and aged samples were stored for a
few months first. For the thiocyanate treatment, the salt powders,
either ammonium (NH
4
SCN) or sodium thiocyanate (NaSCN),
were added directly into the solution. The heterogeneous mixture
was stirred at rt, with most optical changes occurring within the
first few minutes and with little change after 20 min. Although the
thiocyanate salts are added in excess, the amount of thiocyanate
available in the solution at any given time is controlled by the
limited solubility of the ionic salts into the nonpolar solvents.
After the thiocyanate treatment, the remaining thiocyanate salt
powder was removed either by using a PTFE syringe filter or by
centrifugation followed by decantation. The thiocyanate salts are
deliquescent and must be used dry; otherwise, the salt treatment is
inco nsistently effective. Powders wer e purchased new and
maintained under dry nitrogen atmosphere. While this report
focuses on NH
4
SCN and NaSCN, other thiocyanate salts have
not been conclusively ruled out and may also produce similar
results.
Both the freshly synthesized and aged samples present uniform
size distributions and regular morphologies (SI), which remains
unchanged following the thiocyanate treatment, Figure 1A. The
spacing between neighboring packed NCs on a TEM grid is
dictated by the length and number of ligands on the surface,
Figure 1A. On average, the spacing between NCs is unchanged
(2.5 ± 0.1 nm before and 2.5 ± 0.2 nm after) following the
thiocyanate treatment, a result that agrees with the small-angle X-
ray scattering data collected on the sample (SI), indicating
minimal, if any, change to the NC and ligand shell as a result of the
treatment. By HR-TEM, we find that the lattice spacing is 0.58 nm
before and after treatment (SI). The peak positions of the powder
X-ray diffraction pattern of the NC sample remain unchanged as a
result of the treatment, Figure 1B, with peaks consistent with
either a Pm3m or Pnma structure.
9a,10
Taken together, we find that
the treatment does not result in macroscopic structural changes to
the NC ensemble.
While we observe essentially no macroscopic structural
changes to the NCs as a result of the treatment, we find much
more significant changes in the optical properties of the NCs,
changes that are more pronounced in the aged than in the freshly
synthesized samples. Both the aged and freshly synthesized
samples exhibit symmetric and narrow PL spectra, with line
widths of ∼80 meV at fwhm, Figure 1C. However, there is a
significant difference in the pretreatment PLQY of the freshly
synthesized sample at 92 ± 2% and the aged sample at 63 ± 2%
(SI). Following the thiocyanate treatment, the optical perform-
ance of both samples improved, boosting the PLQY to within
error of unity, 99 ± 2% for the fresh sample and 100 ± 3% for the
aged sample (SI). Accompanying the rise in PLQY, we find that
the PL emission of the sample blue shifts by ∼10 meV following
thiocyanate treatment, Figure 1C, a small but consistently
observed shift in the PL. However, we do not observe an
equivalent change in the absorption spectrum following the
treatment, Figure 1C, suggesting the nature of the emitting states
themselves have changed slightly as a result of the thiocyanate
treatment rather than being a consequence of the NCs becoming
slightly smaller.
Considering the substantial improvem ent in the PLQY,
particularly for the aged samples, the photoluminescence lifetime
should show an accompanying change as a result of the treatment.
Prior to thiocyanate exposure, even the freshly synthesized NCs,
with a PLQY in excess of 90% following the synthesis, present PL
lifetimes that are multiexponential in nature, Figure 2A. The
deviation from single exponential behavior is more pronounced in
the aged sample, a sample which presents a much lower PLQY,
Figure 1. (A) Representative TEM images of the CsPbBr
3
NC samples before and after treatment for both fresh and aged samples, scale bar represents 25
nm. (B) X-ray diffraction patterns of the untreated (blue line) and treated (red line) aged NC sample, along with the cubic reference. (C) Absorption and
photoluminescence of the fresh and aged samples both before (blue line) and after (red line) treatment.
Figure 2. Time resolved photoluminescence lifetimes (Picoquant FluoTime 300)under pulsed407.1nmexcitation (5 MHz) at room temperature for the
fresh (A) and aged (B) CsPbBr
3
, both before and after ammonium thiocyanate treatment. Inset figures (A and B) highlight the differences in the untreated
and treated samples at early decay times. (C) Tables displaying relevant values from the PL lifetimes and PLQY, including: the PL lifetime (τ), the radiative
rate (k
r
), and the nonradiative rate (k
nr
). The lifetime values of the untreated samples are amplitude weighted averages of a biexponential fit, and values for
the treated sample are from a single-exponential fit.
Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b02817
J. Am. Chem. Soc. 2017, 139, 6566−6569
6567
Figure 2B. The observed multiexponential excited state decays are
not unique to this study; indeed, it is a common feature of
semiconductor NCs,
4b,7a−e
including CsPbBr
3
NCs.
2c,4a,6a,8a,b
When presented with a multiexponential lifetime, the inter-
pretation is often difficult without a well-defined kinetic model of
the PL decay process, and even then interpretations may be
tenuous. In stark contrast, following the thiocyanate treatment we
find that the PL lifetimes are highly monoexponential in character,
deviating after over three decades of intensity decay (X
reduced
2
=
1.2). In contrast, prior to treatment, the PL decay curves cannotbe
explained by a monoexponential model (X
reduced
2
= 9.2). This
indicates that after treatment there is a single rate-limiting step in
the luminescence process, but not before, Figure 2A,2B. For the
fresh sample, the improvement in PL decay kinetics is more subtle
than the improvement in the aged system, consistent with the
relatively high starting PLQY of the fresh sample (92 ± 2%),
compared to the relatively lower PLQY in the aged sample (63 ±
2%). In both cases, the thiocyanate treatment is capable of
minimizing the nonradiative pathways present in untreated
samples, Figure 2C. Since the nonradiative rate is proportional to
the fraction of excited states that do not luminesce, we are unable
to measure it for the treated samples; their PLQY is within
measurement error (2−3%) of unity. Due to the uncertainty of
the PLQY measurement, we place an upper bound of 0.005 ns
−1
on the nonradiative rate of the treated samples.
While the thiocyanate salt treatments are very effective in
boosting the optical performance of the CsPbBr
3
NCs, it is of
interest to examine whether sodium and ammonium thiocyanate
are unique or just a member of a class of performance-enhancing
species. There are two particularly notable examples in the
literature, one in which the optical performance of CsPbBr
3
nanowires was improved to ∼50% by treating with a solution of
lead-oleate and oleylammonium bromide
6e
and another where
the performance of CsPbBr
3
NCs was improved using didodecyl-
dimethylammonium bromide (DDAB) reaching PLQYs of
∼70%.
6a
When we conduct the same treatments, we find
comparable results (SI) to the previous literature reports
2b,6a,b,e
but we do not observe the same level of improvement seen using a
thiocyanate tre atment. However, it is not unreasonable to
consider that the larger sizes of these chemical species limit
access to the nanocrystal surface. With this in mind, we also
investigated a number of smaller molecules, particularly a number
of small ammonium bromide salts. Perhaps the most promising
treatment was using ammonium bromide (NH
4
Br), with which
we were able to observe a modest improvement in the optical
performance, the PLQY going from ∼65% to ∼80% following
treatment (SI). We investigated other potential treatments with
other thiocyanate species, lead nitrate,
2b
and other small ionic
salts (a more complete list is in the SI). From both the literature
and other species we investigated, we were unable to find one that
improves the optical performance to the same level as the
thiocyanate, suggesting that ammonium and sodium thiocyanate
are particularly advantageous, although other thiocyanate salts
may result in similar improvements.
Both IR and XPS studies were performed to reveal what role
thiocyanate plays on the NC surface. An easily accessible
technique to identify ligand bonds on the surface of colloidal
NCs, particularly the bonds of small molecules such as
thiocyanate, is by using an FTIR equipped with a liquid cell
sample holder to maintain colloids in the vacuum of the FTIR
beam path. The bonds of greatest interest are those related to the
oleate/oleic acid and thiocyanate bonds which are present in the
window between 1550 and 2300 cm
−1
. Following the thiocyanate
treatment, we observe the presence of a broad peak at 2060 cm
−1
,
Figure 3A, consistent with the CN bond of a thiocyanate bound
to lead with a Pb−S bond, the position of this peak has distinctive
shifts depending on the identity of the atom the thiocyanate is
bound to.
11a
When we attempted to solubilize thiocyanate salts in
pure hexanes, we were not able to find IR signatures related to free
thiocyanate in solution, Figure 3A, so any IR signatures of
thiocyanate are related to thiocyanate species that are interacting
with the nanoparticle in some way. The treatment with either
NaSCN or NH
4
SCN results in the presence of a broad
thiocyanate peak at 2060 cm
−1
, but there are additional distinctive
peaks appearing depending on the counterion. Treating with
NaSCN results in a peak at 1560 cm
−1
attributed to CO
stretching of sodium oleate, Figure 3A, while treating with
NH
4
SCN shows the presence of a peak at 1712 cm
−1
attributed to
the CO stretching of oleic acid. It seems that the counterion
(Na, NH
4
) interacts with oleate species present in the sample,
while the thiocyanate interacts with lead in the nanoparticle.
A previous report showed that by treating CsPbBr
3
NCs with
ammonium thiocyanate dissolved in isopropanol, all of the native
ligands are exchanged with thiocyanate.
11b
In our study, we
observe a very different behavior. Rather than replacing all of the
ligands, we find that there are a limited number of sites
thiocyanates can access, with the CN IR peak quickly growing
and leveling off, representing a fraction of the total number of NC
surface ligands, Figure 3B. In fact, if the NCs remain with excess
anhydrous thiocyanate salts we find that there is no further
increase in the number of lead−thiocyanate bonds per NC, even
after days of exposure. This is consistent withthe colloidal stability
and interparticle packing of the NCs observed by TEM following
the treatment. However, the situation becomes more complicated
when we begin considering the surface of the nanocrystals by
probing with X-ray photoelectron spectroscopy (XPS). Prior to
Figure 3. (A) FTIR transmission spectra for the hexanes matrix (black line), untreated (violet line), NH
4
SCN treated (orange line), and NaSCN treated
aged particles (green line) over the IR region of interest, between 1550 cm
−1
to 2300 cm
−1
. (B) Time dependent change in the number of lead-thiocyanate
bonds per nanocrystal determined by focusing on the broadened peak at 2060 cm
−1
for both the NH
4
SCN and NaSCN treated particles over the course of
30 min. (C) XPS spectra of the Pb 4f 7/2 and Br 3d regions for both the untreated and treated aged sample.
Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b02817
J. Am. Chem. Soc. 2017, 139, 6566−6569
6568
treatment, the aged sample has a Pb/Br ratio of 1:2.7, Figure 3C,
showing that the NCs present a lead-rich surface. Following the
treatment we find that the sample’s Pb/Br ratio is 1:3.0,Figure 3 C,
the ideal ratio for CsPbBr
3
perovskites. This is very consistent
with previous results, in which these lead-rich surfaces have been
shown to be deleterious to optical performance
5d,6c,9a
due to the
orbital composition of the conduction band.
5c,d,6c
However,
thiocyanate treatment produces lead−thiocyanate bonds, and
therefore we expect to find the appearance of a sulfur peak by XPS,
but we do not (SI). The XPS results seem to suggest that the
thiocyanate treatment removes excess lead atoms from the
surface, removing the shallow electron traps that are harming the
optical performance of the CsPbBr
3
NCs, possibly the underlying
mechanism of action. However, the treatment is changing a small
fraction of the lead atoms in the nanoparticle, representing at most
15% of the surface lead atoms, something that is difficult to detect,
limiting the potential to understand this effect in more depth.
The findings of this communication highlight the importance
and relevance of surface defects to the optical performance of the
lead halide perovskites. To summarize, we have presented a
thiocyanate salt treatment of CsPbBr
3
NCs that is able to very
effectively decrease the nonradiative pathways of PL decay,
leading to near-unity PLQYs. We find a lack of structural change
accompanied by a recovery of the ap propriate surface
stoichiometry. We find the thiocyanate treatment is unique
compared to other surface treatments in literature and similar
chemical species. This treatment is able to work very effectively on
both freshly synthesized and aged samples. Our data suggest that
thiocyanate is able to repair a lead-rich surface, accessing a limited
number of surface sites without leading to the destruction of the
entire nanoparticle. While we have found success with
thiocyanate treatments on CsPbBr
3
NCs, attempts to extend to
other halide compositions are much less successful, with minor
improvements for CsPbBr
x
Cl
3−x
compositions but virtually no
change from CsPbBr
x
I
3−x
compositions. We hope that future
work will extend this surface-repair strategy to other lead halide
perovskites, enabling unity emission across the visible spectrum.
■
ASSOCIATED CONTENT
*
S
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.7b02817.
Experimental section; additional figures (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.alivisatos@berkeley.edu.
ORCID
Brent A. Koscher: 0000-0001-8233-0852
Joseph K. Swabeck: 0000-0003-2235-2472
A. Paul Alivisatos: 0000-0001-6895-9048
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy,
Office of Science, Offi ce of Basic Energy Sciences, Materials
Sciences and Engineering Division, under Contract No. DE-
AC02-05-CH11231 within the Physical Chemistry of Inorganic
Nanostructures Program (KC3103).
■
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Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b02817
J. Am. Chem. Soc. 2017, 139, 6566−6569
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