Journal of the American Chemical Society
Article
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2
information from prior studies, we conclude that the surfaces
of both types of NCs primarily consist of {100} Se surfaces
chloroform, acetone, methanol, oleic acid, and oleylamine (volume
ratio = 5:5:5:3:1), and the precipitate was isolated by centrifugation at
1
0 000 rpm for 2 min. Decantation was performed to remove the
[
Se(Cd ) (Cd
) units], where the surface cadmium
atoms consist of Cd(Sesurface)2(O CR) units. The Cd CP-
MAT spectra suggest that in the nanospheroids, secondary Cd
surface species with CdSe O and CdSeO coordinations are
core 2
surface 2
113
supernatant. The last precipitation procedure was repeated twice. The
CdSe nanoplatelets were then vacuum-dried at room temperature for
2
1
h.
66
3
3
Cadmium Myristate. Cadmium nitrate tetrahydrate (5.0 mmol,
also present and likely associated with the {111} family of
facets. Unfortunately, NMR signals from the secondary {111}
surfaces could not be observed in the homonuclear and
heteronuclear correlation NMR experiments due to the
combination of the breadth of their isotropic 1 Cd NMR
signals, their large chemical shift anisotropies, their lower signal
intensities, and their overlap with intense signals from the
primary CdSe O surface species.
1
.542 g) was dissolved in MeOH (50 mL). A sodium myristate
solution was prepared by dissolving NaOH (15 mmol, 0.599 g) and
myristic acid (15 mmol, 3.425 g) in MeOH (500 mL). The
Cd(NO ) solution was added dropwise (1 drop/s) into the sodium
myristate solution with vigorous stirring. The resulting white
precipitate was washed with methanol three times and then dried at
3 2
13
60 °C under vacuum overnight.
66,67
CdSe Nanospheroids.
SeO (0.1 mmol, 0.011 g) and cadmium
2
2
2
myristate (0.1 mmol, 0.056 g) were added to a three-neck flask with
ODE (5.0 g, 6.34 mL). The mixture was degassed for 10 min under
vacuum at room temperature. Under argon flow, the solution was
stirred and heated to 240 °C at a rate of 25 °C/min. After reaching
240 °C, the sample was held at this temperature for 2 min, and then 1
mL of oleic acid was added dropwise into the reaction solution to
stabilize the nanocrystals’ growth. The reaction was maintained at 240
On the basis of the results of this study, there are many
interesting future directions of research. First, it should be
possible to detect the minor surface facets and defects in 2D
correlation NMR experiments by further enhancing NMR
sensitivity. Sensitivity could be increased by combining fast
9
6
1
MAS and DNP and potentially using H detection and/or
performing DNP at lower temperatures with helium gas for
°
C for an additional 30 min, and then the reaction mixture was cooled
97,98
to room temperature by removing from the heat source. The resulting
particles were precipitated by adding acetone, centrifuged (2 min at
sample cooling and spinning.
would simplify the
Faster MAS frequencies
Cd NMR spectra by reducing the
113
14 000 rpm), and then redispersed in toluene. The particles were
intensity and number of spinning sidebands. Work along these
lines is underway in our laboratories. Second, CdSe NCs with
different morphologies, exposed surface facets, surface ligands,
etc. can be studied to unravel the influence of surface structure
on chemical and optoelectronic properties. Lastly, the solid-
state NMR experiments demonstrated here should be
applicable to other inorganic semiconductors NCs consisting
further purified by precipitation−redispersion three more times.
Dynamic Nuclear Polarization Solid-State NMR Spectros-
copy. DNP Sample Preparation. In general, 25 mg of powdered
CdSe NCs (nanoplatelets or nanospheroids) and 25 mg of h-BN were
weighed out and placed into a mortar. The two materials were gently
ground for ∼1 min with a mortar and pestle to mix the CdSe NCs and
h-BN support material. A ∼30 mg portion of the powder mixture was
then placed in a watch glass, where 15 μL of a 16 mM TEKPol TCE
solution was added and thoroughly mixed for the nanospheriods and
nanoplatelets, respectively. The sample was then packed into a 3.2
mm (outer diameter) DNP sapphire rotor with a Teflon insert and
zirconia drive cap.
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of elements with spin-1/2 isotopes such as Si, P, Se,
Cd, Sn, Te, and Pb.
1
13
119
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EXPERIMENTAL SECTION
■
General Experimental Details. All DNP experiments were
performed on a Bruker 400 MHz/263 GHz NMR magnet/gyrotron
equipped with a Bruker AVANCE III spectrometer console and a
Bruker 3.2 mm HXY MAS DNP-NMR probe configured in double
resonance mode. The sample temperature was approximately 100−
Synthesis. Materials for Synthesis. Cadmium acetate dihydrate
Cd(OAc) ·2H O, 98%), cadmium nitrate tetrahydrate (Cd(NO ) ·
(
2
2
3 2
4
7
H O, 98%), 1-octadecene (ODE, 90%), and oleylamine (OleyNH ,
2
2
0%) were purchased from Sigma-Aldrich. Sodium hydroxide
(
(
NaOH, >97%), methanol (MeOH, HPLC grade), stearic acid
1
13
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1
10 K for all experiments. H, C, Se, and Cd radio frequency
laboratory grade), hexanes (99.9%), chloroform (CHCl , certified
3
1
(RF) pulses were directly calibrated on each sample through a ( H)
ACS), toluene (certified ACS), and acetone (certified ACS) were
from Fisher. Tetramethylammonium hydroxide (98%), myristic acid
1
3
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1
13
77
single pulse or ( C, Se, and Cd) H → X (X = C, Se, or
1
13
1
13
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Cd) CP-90 nutation experiment. H → X (X = C, Se, or Cd)
(
(
98%), and oleic acid (90%) were from Alfa Aesar. Selenium powder
CP was performed at the beginning of all experiments to enhance
sensitivity and selectively probe the surface of the NCs. All H CP
spin-lock RF fields were linearly ramped from 90% to 100%
amplitude
given in Table S1. 100 kHz H RF field SPINAL-64 heteronuclear
decoupling was applied during all evolution periods and during the
acquisition of C, Se, or Cd. H chemical shifts were referenced
Se, 99.5%) and selenium dioxide (SeO , 99.8%) were from Strem. All
2
1
chemicals were used as received.
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9
Cadmium Stearate. Stearic acid (20 mmol, 5.689 g) and
tetramethylammonium hydroxide (20 mmol, 1.823 g) were dissolved
in MeOH (100 mL) by stirring for 20 min. Cd(OAc) ·2H O (10
100
to broaden to Hartman−Hahn match condition. The
2
2
1
mmol, 2.665 g) was dissolved in MeOH (20 mL) and added into the
mixture dropwise. A white precipitate formed while stirring for 20
min. The precipitate was separated by filtration and washed three
times with MeOH. The precipitate was dried under vacuum for 6 h.
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1
3
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1
1
to neat tetramethylsilane using frozen TCE (δiso( H) = 6.2 ppm) as a
secondary chemical shift standard. Previously published relative NMR
frequencies were used to indirectly reference the C, Cd, and Se
3
2
CdSe Nanoplatelets. Cadmium stearate (0.30 mmol, 0.203 g)
and ODE (9.468g, 12 mL) were degassed under a dynamic vacuum at
room temperature for 10 min inside a Schlenk flask. The flask was
refilled with argon gas and heated to 240 °C. Se-ODE (2 mL, 0.1 M)
was quickly injected. After 1 min, Cd(OAc) ·2H O (0.45 mmol,
1
3
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77
102
103
chemical shifts7. DNP-enhanced CP spin echo, CP-CPMG, CP
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79
pulse cooling, CP spin diffusion, refocused-INADEQUATE-
6
4,82
84,104
CPMG,
heteronuclear J-resolved,
and J-HMQC experi-
2
2
8
6
0
.120 g) was suspended in ODE (0.789 g, 1 mL) and injected into the
ments were performed using previously described pulse sequences.
A schematic illustration of all pulse sequences is shown in Figure S6.
mixture. The reaction was kept at 240 °C for 30 min. The mixture was
cooled to 160 °C, and oleic acid (0.48 mmol, 0.15 mL) was swiftly
injected to stop the reaction. After cooling to 80 °C, the final products
were first isolated by centrifugation at 14 000 rpm for 2 min. The
supernatant was removed, and the precipitate was then dissolved in
toluene to form a clear solution and was isolated by centrifugation at
1
13
Cd CP pulse cooling experiments were acquired with CPMG signal
detection, a 7 s delay (τ ) after each CP block, and 15 CP cycles (L)
to amplify bulk polarization.
SSNMR spectra of the nanospheroids and nanoplatelets are compared
z
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Cd CP-CPMG and CP pulse cooling
1
13
Cd CP-MAT experiments were
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1
0 000 rpm for 2 min. The supernatant was removed for the second
acquired using the five-π pulse MAT pulse sequence. The Cd
time. The dark red precipitate was dissolved in a mixture of
chemical shift tensor parameters (δ , Ω, κ, using the Herzfeld−
iso
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J. Am. Chem. Soc. 2021, 143, 8747−8760