Shape control of zincblende CdSe nanoplatelets

Article abstract of DOI:10.1039/c6cc05705e

The lateral dimensions of CdSe nanoplatelets have a strong and unique influence on their opto-electronic properties, with sizes that can be tuned from the weak to the strong exciton confinement regime. There are state-of-the-art reports on several nanoplatelet syntheses; however, at present only the thickness is well-controlled. We demonstrate here that we can achieve a control over the aspect ratio and overall nanoplate area by carefully adjusting the reagents that induce the in-plane growth. A variation of the fraction of hydrated Cd(OAc)₂ in a Cd(OAc)₂/Cd(OAc)₂·2H₂O mixture tailors the nanoplatelet aspect ratio. This occurs independently of the reaction time, which can be used to fine-tune the overall length and width. An interpretation is given by the in situ formation of a small amount of hydroxide anions that alter the surface energy of specific planes.

Full text of DOI:10.1039/c6cc05705e

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Shape Control of Zincblende CdSe Nanoplatelets
Guillaume H.V. Bertrand,a Anatolii Polovitsyn,a,b Sotirios Christodoulou,a,b Ali Hossain Khan,a Iwan
Moreelsa,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The lateral dimensions of CdSe nanoplatelets have a strong and the NPL thickness, while at the same time the exciton lifetime
unique influence on its opto-electronic properties, with sizes that is governed by the giant oscillator strength effect5–7 that leads
can be tuned from the weak to the strong exciton confinement to an exciton recombination rate scaling with the NPL area.
regime. The state-of-the-art reports several nanoplatelet Furthermore, the 2D shape enhances the linear8 and
syntheses, however, at present only the thickness is well- nonlinear9 absorption coefficient, and it enables an efficient
controlled. We demonstrate here that we can achieved a control suppression of nonradiative Auger recombination.10 Recent
over the aspect ratio and overall nanoplate area by carefully experimental data have also shown that residual in-plane
adjusting the reagents that induce the in-plane growth.
variation of the fraction of hydrated Cd(OAc)2 in
A
a
confinement effects in NPLs have a strong impact on exciton-
phonon interactions and intraband carrier dynamics.11 Next to
Cd(OAc)2/Cd(OAc)2.2H2O mixture tailors the nanoplatelet aspect the opto-electronic properties, shape and shape dispersion
ratio. This occurs independently of the reaction time, which can be also plays an important role in self-assembly12–14 and the
used to fine-tune the overall length and width. An interpretation associated interparticle charge or energy transfer.
is given by the in situ formation of a small amount of hydroxide These results highlight the importance of shape and area-
anions that alter the surface energy of specific planes.
controlled NPLs, yet literature does not provide a general
synthesis protocol that yields NPLs with tunable width and
length. Published CdSe NPL shapes vary from elongated
rectangles15 to squares16 and polygonal shapes,17 and
interestingly, protocols are all similar. However, previous work
on the synthesis of shape-controlled nanocrystals has shown
that small impurities can have profound effects. For instance,
trace amounts of phosphonic acid in TOPO promoted rod-
shaped NCs18 and nanowires19, the addition of CdCl2 to the
CdSe synthesis can produce octapods20, or lead acetate
impurities in lead oleate caused PbSe NCs to grow as
hexagonal stars21.
Understanding how size and shape influence the opto-
electronic properties is an important topic in the field of
colloidal nanocrystals (NCs). Control over the band structure is
obtained most effectively by dedicated material design, for
instance via growth of nanocrystals with spherical,1 rod-,2 or
plate-like3 geometry. The latter materials class, labeled
colloidal 2D nanoplatelets (NPLs), consists of NCs with a
thickness of only a few atomic layers, typically around 4.5
monolayers (or about 1.4 nm).3,4 This implies that they are
substantially thinner than the bulk exciton Bohr diameter of 11
nm, which brings the NPLs into the strong confinement regime
along this direction. A comparatively longer length and width
typically yields only weak in-plane confinement for the 2D
exciton.
In this manuscript we focus our synthetic efforts on controlling
the aspect ratio (AR) of CdSe NPLs by careful examination of
the different short-chained reactants that induce the lateral
growth22,23. We will show that the AR of CdSe NPLs depends on
the presence of water in the Cd-acetate precursor, with
further experiments pointing toward the in-situ formation of
hydroxide anions. This implies that carefully adjusted mixtures
of short-chained Cd-precursors allow for a tuning of the AR in a
range from square NPLS (AR 1:1) to highly elongated
rectangles of AR 8:1. In combination with a size-controlled NPL
synthesis by adjusting the reaction time, the NPL width can be
tuned from 15 nm down to 2.4 nm, leading to a shift in band
edge absorption (emission) from 513 nm (515 nm) to 498 nm
(502 nm).
The peculiar shape of NPLs implies specific advantages to
engineer the opto-electronic properties. For example, it is well
know that the band gap can be tuned –in discrete steps– by
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It is well known that injection of cadmium acetate (Cd(OAc)2), hydroxide anions have led to a hindered growth of InP
cadmium acetate dihydrate (Cd(OAc)2.2H2O) or cadmium quantum dots,24 and hydroxide anions are also known to bind
proprionate (Cd(OPr)2) triggers the lateral growth of NPLs. to and passivate specific facets of PbS NCs.25 We therefore
Depending on the protocol, the resulting platelet shape varied postulate that also here the active species is a hydroxide anion
from rectangular15 to square16 or polygonal.17 Here, we that can adsorb and alter the surface energy balance, and
performed a typical CdSe NPL synthesis with the injection of therefore lead to a different shape by modifying the growth
dry Cd(OAC)2. In a 50 mL three neck round bottom flask 150 rate of specific facets according to the kinetic model recently
mg of dry cadmium myristate (Cd(Myr)2, 0.26 mmol, absence proposed by Riedinger et al.26. To validate the presence of OH-
of water was verified with thermo-gravitational analysis, TGA, and its effect on the growth, we performed a synthesis where
see ESI), 24 mg of selenium (Se) powder (0.30 mmol) and 15 a small fraction of dry Cd(OAc)2 is replaced by dry Cd(OH)2 (the
mL of octadecene (ODE) were degassed under vacuum at absence of water was verified by TGA, see ESI). A clear
110°C for 20 min. The mixture was placed under argon modification of the aspect ratio was observed, even with small
atmosphere and heated to 140 °C for 10 to 20 minutes to inclusions of OH- (Figure 1d). We were able to tune the NPL AR
achieve total Se dissolution. The temperature was then set to from rectangles with an AR of 7.7:1 to almost square NPLs with
240 °C with a ramp of 50 °C.min-1. Between 205 °C and 215 °C AR 1.3:1 by controlling the fraction of Cd(OH)2 between 0 and
the solution changes color from yellow to deep orange, at 7mol% (ESI Table S5). Note that X-ray diffraction patterns
which point the reaction vessel was opened and 80 mg of dry confirm that the use of either dry or hydrated Cd(OAc)2, as
Cd(OAc)2 (0.35 mmol) was added. When the solution reached well as inclusion of Cd(OH)2 in the synthesis, all lead to NPLs
240°C, the NPLs were grown for 8 min, after which the heating with a zinc blend lattice structure (ESI Section 4).
mantle was removed, the reaction mixture was slowly cooled
to 160 °C, and 2 mL of oleic acid was added. When reaching The results thus suggest that upon injection of Cd(OAc)2.2H2O
room temperature, 20 mL of hexane was added and the at 210 °C in ODE, the majority of water may evaporate, but a
solution was centrifuged for 10 min at 3000 rpm. The small part reacts with the acetate ligands to create hydroxide
supernatant was discarded and the precipitate was dispersed anions and acetic acid, which subsequently leads to a reduced
in 40 mL of hexane. The resulting solution was centrifuged AR during NPL growth. However, considering that only a small
again at 6000 rpm. The desired NPLs remained in the amount (ca. 7 mol%) of Cd(OH)2 was needed to achieve square
supernatant and the precipitate was discarded.
NPLs, the use of Cd(OAc)2.2H2O actually represents a more
convenient route to tune the AR, as it can rely on the in situ
To record bright-field transmission electron microscope (TEM) generation of hydroxide anions. To verify this approach,
images, a few drops of a diluted NPL suspension in hexane different ratios of dry and hydrated cadmium acetate were
were dropped on a TEM grid (E.M. Sciences, Carbon film 300 used while maintaining a fixed total amount of cadmium
mesh on copper) and inserted into a JEM 1011 electron
(Table 1). The AR could be tuned continuously from 7.7:1 for
microscope. Figure 1a shows the result of a typical synthesis dry Cd(OAC)2 to square NPLs for a mixture of 30 mol% dry
using dry Cd(OAc)2. We obtained NPLs with an aspect ratio of Cd(OAc)2 and 70 mol% Cd(OAc)2.2H2O (Figure 2). A similar
7.5:1 and an average width of 6.8 nm (ESI Table S1). We also curve is obtained when reducing the growth to 4 minutes
evaluated the influence of the different reagents (Cd(Myr)2,
(Figure 2c, ESI Table S6), which attests that the AR is
Cd(OAc)2, Se) for this reaction, however, the use of different independent of the reaction time. Note that above 70 mol% of
concentrations of each component has no significant influence Cd(OAc)2.2H2O, the AR increases again, however, this is in the
on the final aspect ratio (ESI Tables S2-S4). In strong contrast, range where we obtained polygon-shaped particles instead of
when replacing dry Cd(OAc)2 by an equimolar amount of rectangular NPLs (note that the lower AR limit is 1:1 by
Cd(OAc)2.2H2O and applying the same synthesis procedure as definition as we place the shorter dimension in the
above, a distinctively different shape was observed (Figure 1b, denominator).
ESI Table S1). While the first precursor leads to highly
Name
Cd(OAc)2
(mol %)
Cd(OAc)2. length
2(H2O)
width
(nm)
ratio
rectangular NPLs, laterally terminated by (100) and (010)
facets,22 the latter yields more irregular shapes with a lower
aspect ratio of 2.3:1, with the appearance of (110) facets. For
obtaining the latter NPLs, the importance to have traces of
water present was confirmed by a control experiment where
(mol %)
(nm)
NPL-1
NPL-2
NPL-3
NPL-4
NPL-5
NPL-6
NPL-7
NPL-8
NPL-9
NPL-10
100
95
90
80
75
70
60
50
40
30
0
5
40.38
33.16
52.98
38.66
33.86
37.88
16.77
17.32
13.88
17.62
5.34
5.11
9.44
8.94
8.58
10.97
8.17
9.97
10.36
15.22
7.67
6.58
5.66
4.39
4
3.51
2.10
1.77
1.34
1.17
10
20
25
30
40
50
60
70
we co-injected 20µL of water together with dry Cd(OAc)2 in an
ODE solution containing the CdSe seeds at 210 °C. Despite a
rapid vapor development that suggested some loss of water,
enough remained present to again afford polygonal platelets
with an AR of 1.2:1 and average width and length of 13.7 nm
and 16.3 nm, resp. (Figure 1c).
20.09
21.04
34.20
13.64
15.14
14.90
1.47
1.40
2.30
NPL-11
NPL-12
NPL-13
20
10
0
80
90
100
It is clear that water plays an important role in the AR-control
of CdSE NPLs. In literature, trace amounts of water or
Table 1: Synthesis of NPLs with different wet/dry Cd(OAc)2 ratios.
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Notes and references
The tuning can be expanded to other combinations of short-
chained precursors as well. We found that for reactions using
dry Cd(OPr)2 (see ESI for TGA data), the addition of small
amounts of cadmium formate (Cd(OOCH)2) yielded a similar AR
control (ESI Table S7). This suggests that OOCH- ions afford an
alternative to the AR-tuning via OH- ions. We achieved NPLs
varying from an aspect ratio of 6.5:1 with dry Cd(OPr)2, to
3.8:1 when the fraction of Cd(OOCH)2 was increased to 15
mol%. However, when using amounts of Cd(OOCH)2 superior
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