Using ligands to control reactivity, size and phase in the colloidal synthesis of WSe2 nanocrystals

Article abstract of DOI:10.1039/c9cc03326b

Colloidal chemistry is leveraged for size- and phase-tuning of transition metal dichalcogenide nanomaterials. Specifically, nucleation and growth of colloidal WSe₂ nanocrystals are controlled via mixtures of oleic acid (OA) and trioctylphosphine oxide. Increased OA yields slower nucleation, larger nanocrystals and a shift from the 2H to 1T′ phase.

Full text of DOI:10.1039/c9cc03326b

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Reactivity differences of Sc
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DOI: 10.1039/C9CC03326B
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Using ligands to control reactivity, size and phase in the colloidal
synthesis of WSe nanocrystals
2
Jessica Q. Geisenhoff, Ashley K. Tamura and Alina M. Schimpf*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Colloidal chemistry is leveraged for size- and phase-tuning of which has proven particularly challenging to access via traditional
methods. Herein, we show that coordinating ligands greatly
influence the precursor reactivity and resultant nanocrystal size and
phase in colloidally synthesized WSe . Specifically, when synthesized
2
in mixtures of oleic acid (OA) and trioctylphosphine oxide, inclusion
of increasing amounts of OA hinders the reactivity of the precursor,
leading to fewer nucleation events and larger nanocrystals. This
trend is accompanied by a change in the phase composition, from 2H
when minimal OA is present, to 1T′ when OA is introduced in
higher concentrations. These results demonstrate the promise of
transition metal dichalcogenide nanomaterials. Specifically,
nucleation and growth of colloidal WSe nanocrystals are
controlled via mixtures of oleic acid (OA) and trioctylphosphine
oxide. Increased OA yields slower nucleation, larger nanocrystals
and a shift from the 2H to 1T′ phase.
2
The discovery of extraordinary properties in graphene has catalyzed
an intense interest in two-dimensional (2D) nanomaterials, including
a
wide variety of elemental, binary and ternary inorganic
using colloidal chemistry to tune the size and phase of WSe
2
and
1-4
nanostructures.
Among these 2D materials, transition metal
other TMD nanocrystals.
dichalcogenides (TMDs) are extensively studied because of their
unique electronic, optical, catalytic and mechanical properties.5-7
TMDs can host a variety of phases that each have a unique electronic
WSe
2
nanocrystals (Figure 1) were synthesized via hot
Se ) into a solution
injection of diphenyl diselenide (Ph
2
2
structure, allowing access to a compositionally and electronically
8
diverse set of 2D materials. Semiconducting TMDs (ME
2
, M= Mo,
W; E = S, Se) have a thermodynamically favoured 2H phase,
known for its unique optical and optoelectronic properties,
9
particularly at the monolayer. Additionally, this class of
materials can adopt the metallic 1T or small-bandgap 1T
′
1
0-13
phases, which demonstrate promise for use in catalysis
or
1
4-15
as topological insulators,
respectively. Due its metastability,
however, the 1T′ phase of these group VI TMDs remains relatively
unexplored. Only recently have these materials been synthesized
using direct chemical methods that do not require charge
1
6-20
intercalation or other post-synthetic modification.
Colloidal chemistry offers a promising route to obtain metastable
TMD phases through strategic choice of precursor or ligand identity,
concentration, temperature or other reaction conditions to access
2
1-24
kinetic growth regimes.
exploited to access the metastable wurtzite phase of
CuInSe nanocrystals by choosing precursors with strong C–Se bonds
that favour slow nucleation.
with varying reactivity have been used to achieve highly selective and
Indeed, colloidal chemistry has been
2
2
5-26
Similarly, organosulfur precursors
2
7
tunable phases of Fe
x y
S nanocrystals. Recently, colloidal synthesis
was used to obtain 1T′ phases of WS
1
6, 19
2
and WSe2,
the latter of
Figure 1. TEM images of WSe
2
nanocrystals synthesized via
injection of 1ml Ph Se (0.04 M in TOPO or OA) into a solution of
2
2
University of California, San Diego, La Jolla, CA 92093, USA
aschimpf@ucsd.edu
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
W(CO) (0.02 mmol) in OA/TOPO. The final reaction mixtures
6
*
contained (a) 2, (b) 10, (c) 100 or (d) 1000 eq OA/W. Exact ligand
amounts are provided in Table S1.
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containing W(CO)
nanocrystals were synthesized with OA/W molar equivalents both the 2H (Figure 2a) and 1T′ (Figure 2b
eq) of 2, 10, 100, and 1000. OA/TOPO amounts were varied the role of ligands, precursors and reaction conditions in
while keeping the overall ligand/W ratio consistent. Exact ligand promoting various phases has not been explored. Figures 2b
amounts for each OA/W ratio are provided in Table S1. In a and 2c show high-resolution TEM (HRTEM) images of the WSe
typical synthesis, a degassed solution of 20 mg (0.057 mmol) nanocrystals synthesized with OA/W mol ratios of 2 and 1000,
W(CO) in OA/TOPO was heated to 330 °C, at which point a respectively (corresponding to nanocrystals imaged in Figures
solution containing 64 mg (0.2 mmol) Ph Se in 1 ml OA or TOPO 1a and 1d). The HRTEM for the nanocrystals synthesized with 2
6 2
in a mixture of TOPO and OA. Specifically, Previous studies have reported colloidal syntheses of WSe in
View Article Online
9, 28, 30-33
1
D
)
O
p
I:
h
1
a
0
s
.1
e
0
s
3
,
9/C9CC033
but
26
B
(
2
6
2
2
was rapidly injected. Following injection, a color change from eq OA/W show a hexagonal arrangement of the tungsten atoms
yellow to black was observed within a few minutes. The solution (Figure 2c), characteristic of the hexagonal 2H lattice. The
was heated for 30 min, after which it was cooled to room- reflections present in the FFT (Figure 2e) correspond to
temperature and nanocrystals were collected via interplanar spacings of 풅ퟏퟎퟎ = 2.8 Å and
centrifugation. The resulting nanocrystals could be the expected lattice parameter for 2H WSe
풅
ퟐퟏퟎ
̅
= 1.6 Å, yielding
(a2H = 3.3 Å). In
2
resuspended in toluene to form colloidal suspensions that were contrast, nanocrystals synthesized in 1000 eq OA/W adopt a zig-
stable for several hours (Figure S1). zag arrangement of W atoms (Figure 2d), consistent with the 1T′
Figure 1 shows transmission electron microscope (TEM) phase. This distorted octahedral coordination results in an
images for nanocrystals synthesized with OA/W molar ratios of elongation of the unit cell along the x-axis, while the y-axis
(
a) 2, (b) 10, (c) 100, and (d) 1000. In each case, the nanocrystals remains unchanged (a1T
are disk-like, with lateral dimensions (xy) larger than the heights To probe the bulk phase of the nanocrystals synthesized
z direction). Notably, as the relative amount of OA increases, with varying amounts of OA, Raman and X-ray photoelectron
′ ′
= 5.9 Å, b1T = 3.3 Å, Figure 2f).
(
so does the nanocrystal size. The powder X-ray diffraction spectroscopies and powder X-ray diffraction were used. Figure
patterns show a sharpening of the (002) reflection with 3a shows the Raman spectra of nanocrystals synthesized with
increasing OA (Figure S2a), indicating that larger nanocrystals (bottom to top) 2, 10, 100, and 1000 eq OA/W. The Raman
also have more layers. For even the smallest sample (2 eq spectrum of nanocrystals synthesized with 2 eq OA/W (Figure
OA/W), the Raman peaks (vide infra) match those of bulk WSe
indicating multiple layers.
2
,
3, bottom) is dominated by an intense peak centered at 251
cm , formed by the unresolved combination of the A1g and E2g
−
1
It has previously been reported that OA can be used to favor
large lateral growth due to relatively weak binding to edge sites
28
2
in 2H WSe . Although TOPO is not expected to bind strongly
to the nanocrystal surfaces, small particles are observed when
very little OA is present. This trend may suggest that OA
interacts strongly with the W precursor, and the absence of OA
allows for rapid nucleation with more nucleation sites,
ultimately yielding smaller nanocrystals. A similar trend is
observed in the synthesis of CdSe nanocrystals in the presence
of OA, where strong coordination to the Cd precursor reduces
the active monomer concentration and leads to larger
29
nanocrystals.
Further evidence that OA hinders nucleation is observed
qualitatively in the timescale of the color change following
injection. Under high OA concentration conditions (1000 eq),
the yellow precursor solution persists for minutes before
turning black to indicate nanoparticle nucleation. Low OA
concentration (2 eq) results in immediate color change from
yellow to black, indicating fast onset of nucleation. These results
together indicate that interaction with OA is inhibiting the
reactivity of the tungsten precursor, despite allowing fast WSe
2
growth. The relatively weak size-dependence from 2–100 eq
OA/W may reflect the fact that TOPO is still the dominant ligand
in those reactions (Table S1).
It is important to note that reactions in all TOPO (0 eq OA)
showed a nucleation event without the injection of Ph
2 2
Se .
Specifically, a solution of W(CO) in TOPO becomes turbid and
6
black (Figure S3a) when heated above 260 °C and amorphous
nanoparticles could be isolated (Figure S3b). Due to this
additional nucleation event, WSe nanocrystals synthesized in
2
all TOPO (Figure S3c) may be impure and are not included in
further analysis. Inclusion of just 2 eq OA/W is enough to
prevent this nucleation and the reaction mixture stays clear
Figure 2. Crystal structures showing the xy-plane of (a) 2H and
prior to injection of Ph
Interestingly, the OA/W molar ratio has a significant effect
on the phase of the WSe nanocrystals synthesized here.
2 2
Se (Figure S4).
(
2 2
b) 1T′ WSe . HRTEM images for WSe synthesized with (c) 2 eq
OA/W and (d) 1000 eq OA/W and (e,f) corresponding FFTs. The
electron beam is coincident with the (001) vector.
2
2
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DOI: 10.1039/C9CC03326B
Figure 3. Raman spectra of WSe
2
nanocrystals synthesized with
Figure 4. Raman spectra of WSe
2 eq OA/W. Aliquots were taken at (bottom to top) 5, 10 and 20
2
nanocrystals synthesized with
(bottom to top) 2, 10, 100, and 1000 eq OA/W.
min following Ph Se injection.
2
2
modes of multilayer 2H WSe
2
.34-35 When the OA/W ratio is
increased to 10, this peak broadens and the peak at 219 cm
assignment of this phase to the monoclinic P2
consistent with the 1T′ phase observed for other TMDs.
The gradual change in phase is corroborated by X-ray
photoelectron spectroscopy (Figure S5). Nanocrystals
synthesized with 2 eq OA/W show W 4f binding energies
1
/m space group,
−
1
3
6-37
increases in relative intensity. When the OA/W ratio is
increased to 100, the spectrum displays 3 distinct peaks
−
1
−1
−1
centered at 219 cm , 240 cm , and 260 cm , which have been
1
9
recently been assigned to the 1T′ phase of WSe
there is little Raman data on 1T′ WSe , the emergence of 3 peaks
is consistent with a break in the degeneracy of the 2g mode, as no
2
.
Although
1
2, 28
matching that of 2H WSe
2
(Figure S5a).
As the OA/W ratio is
2
increased, the W 4f peaks shift to lower binding energies,
consistent with an increase in the W–Se bond distances
E
allowed point groups in the monoclinic space groups contain
doubly degenerate representations. In addition, an underlying
broad peak is observed, likely due to residual contributions
from the A1g and E2g modes. At 1000 eq OA/W, the spectrum is
composed only of the three distinct peaks with no underlying
broadening. These Raman spectra suggest a gradual change
from the 2H phase to the 1T′ phase with increasing amount of
OA.
15, 19
expected for the 1T′ phase.
A similar trend is also observed
in the Se 3d binding energies with increasing OA/W ratios
Figure S5b).
(
To gain further insight into the role of OA during nucleation and
growth, WSe
the phase monitored over time. Figure 4 shows the Raman spectra
of aliquots taken at 5, 10 and 20 min following Ph Se injection.
2
nanocrystals were synthesized with 2 eq OA/W, and
2
2
Interestingly, the spectrum of the 5 min aliquot (Figure 4, bottom)
shows the 3 peaks assigned to the 1T′ phase. In the spectrum of the
The change in phase with increasing OA content is also seen
in the powder X-ray diffraction patterns (Figure S2a). The
powder patterns observed herein are similar to those reported
1
0 min aliquot (Figure 4, middle), a broad peak centered around 250
−
1
−1
cm is observed in addition to a peak at 220 cm , indicating the
formation of a mixed phase. Finally, at 20 min (Figure 4, top) the
19, 28, 30-33
previously for both 2H and 1T′ WSe
2
.
Increasing OA/W
leads to a distinct loss of the (013) reflection (2H, 2 = 37.9 deg) and
an increase in the reflection at 2 = 34.8 deg. These characteristic
−
1
signal is dominated by the peak at 251 cm , diagnostic of the 2H
phase. This time-dependence of the phase suggests that, under these
differences have been seen previously for 2H vs 1T′ WSe
the reflections for 1T′ WSe have not been rigorously assigned. An
initial estimate of the 1T′ WSe powder pattern was simulated using
the known Wyckoff positions for 1T′ MoTe
with a and b obtained from TEM (Figure 2f), and c floated to match
the (002) reflection (2 = 13.92 deg, c = 12.74 ). Although this
method for predicting the 1T′ WSe powder pattern has been used
previously, the resulting pattern shows some discrepancies with the
2
, although
conditions, WSe
2
nucleates in the metastable 1T′ phase and
2
subsequently converts to the thermodynamically favourable 2H
phase. This conversion may be hindered by large amounts of OA,
leading to the trend observed at 30 min for increasing amounts of OA
2
3
6
2
1
(P2 /m,  = 93.917 deg),
(
Figure 3). Importantly, these results demonstrate that it is possible
Å
to obtain the 1T′ phase in small nanocrystals (Figure S6) as well as in
large ones (Figure 1d), suggesting that nanocrystal size and phase can
be tuned independently.
The data presented herein show that ligand mixtures can be
used to tune the precursor reactivity and resultant size of
2
1
9
experimental pattern (Figure S2a, top). Le Bail fitting (Figure S2b)
of the 1000 eq pattern yielded similar unit cell parameters (a =
5
.8543 Å, b = 3.2675 Å, c = 12.7884 Å,  = 93.9107 deg) and good
2
colloidally synthesized WSe nanocrystals. Although neither
ligand is expected to bind strongly to the nanocrystal surfaces, they
agreement with the experimental powder pattern, indicating proper
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HRTEM and XPS were collected at the UC Irvine Materials
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National Science Foundation Major Research Instrumentation
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