Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy

Article abstract of DOI:10.1021/jacs.5b09248

Understanding the chemistry of colloidal quantum dots (QDs) is primarily hampered by the lack of analytical methods to selectively and discriminately probe the QD core, QD surface and capping ligands. Here, we present a general concept for studying a broad range of QDs such as CdSe, CdTe, InP, PbSe, PbTe, CsPbBr₃, etc., capped with both organic and inorganic surface capping ligands, through dynamic nuclear polarization (DNP) surface enhanced NMR spectroscopy. DNP can enhance NMR signals by factors of 10-100, thereby reducing the measurement times by 2-4 orders of magnitude. 1D DNP enhanced spectra acquired in this way are shown to clearly distinguish QD surface atoms from those of the QD core, and environmental effects such as oxidation. Furthermore, 2D NMR correlation experiments, which were previously inconceivable for QD surfaces, are demonstrated to be readily performed with DNP and provide the bonding motifs between the QD surfaces and the capping ligands.

Full text of DOI:10.1021/jacs.5b09248

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Article
The Structure of Colloidal Quantum Dots from Dynamic
Nuclear Polarization Surface Enhanced NMR Spectroscopy
Laura Piveteau, Ta-Chung Ong, Aaron J Rossini, Lyndon
Emsley, Christophe Coperet, and Maksym V. Kovalenko
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b09248 • Publication Date (Web): 16 Oct 2015
Downloaded from http://pubs.acs.org on October 19, 2015
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The Structure of Colloidal Quantum Dots from
Dynamic Nuclear Polarization Surface Enhanced
NMR Spectroscopy
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§
Laura Piveteau,† ‡ Ta-Chung Ong,† Aaron J. Rossini,§ Lyndon Emsley,* Christophe Copéret* †
and Maksym V. Kovalenko* † ‡
†
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5,
CH-8093, Switzerland
‡Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf,
Überlandstrasse 129, CH-8600, Switzerland
§Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne
(EPFL), 1015 Lausanne, Switzerland
KEYWORDS: Nanocrystals, Quantum Dots, Semiconductors, Colloids, Dynamic nuclear
polarization, solid-state NMR
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ABSTRACT
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Understanding the chemistry of colloidal quantum dots (QDs) is primarily hampered by the lack
of analytical methods to selectively and discriminately probe the QD core, QD surface and
capping ligands. Here we present a general concept for studying a broad range of QDs such as
CdSe, CdTe, InP, PbSe, PbTe, CsPbBr3, etc., capped with both organic and inorganic surface
capping ligands, through dynamic nuclear polarization (DNP) surface enhanced NMR
spectroscopy. DNP can enhance NMR signals by factors of 10-100, thereby reducing the
measurement times by two-to-four orders of magnitude. 1D DNP enhanced spectra acquired in
this way are shown to clearly distinguish QD surface atoms from those of the QD core, and
environmental effects such as oxidation. Furthermore, 2D NMR correlation experiments, which
were previously inconceivable for QD surfaces, are demonstrated to be readily performed with
DNP and provide the bonding motifs between the QD surfaces and the capping ligands.
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Introduction
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Colloidal semiconductor nanocrystals, commonly referred to as quantum dots (QDs),
exhibit size-tuneable optical and electronic properties and have become important as luminescent
labels in life sciences1, 2 and as building blocks for optoelectronic devices (solid-state lighting,
photovoltaics, photodetectors and electronic circuitry).3, 4 QDs are complex entities comprising
an inorganic crystalline core, surface atoms and surface capping ligands.5 Adjustments of the QD
surface chemistry control the optical properties of individual QDs as well as the charge and the
energy transfer in assemblies of QDs, and allow for their integration into solid-state devices and
versatile bio-functionalization. Importantly, QDs are highly dynamic and reactive objects, in both
their core regions (e.g. with polymorphism, and rapid chemical transformations via oxidation,
reduction, galvanic or ion-exchange, or self-purification from impurity atoms, etc.) and at their
surfaces (e.g. surface reconstruction, ligand adsorption-desorption, etc). It is therefore not
surprising that structural characterization and the atomistic description of QDs remains a
formidable challenge. In principle, nuclear magnetic resonance (NMR) spectroscopy would be
the method of choice because of its ability to characterize molecules, solids, surfaces and
interfaces. Both solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) and
solution NMR investigations of sub-10 nm colloidal QDs (CdSe, CdTe, InP and PbSe, etc.) have
provided information about the internal structure of the QD cores6-9 and about the organic ligand
capping, including the QD-ligand-binding equilibria in the liquid state.9-13 However, both
approaches have intrinsic limitations with respect to the chemistry of QDs. Solution NMR
provides accurate information about the structure and binding equilibria of molecular species
such as QD capping ligands, but is essentially blind to surface-bound ligands due to their slow
and non-uniform tumbling. In the solid state, direct excitation MAS NMR records signal from all
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species present in the sample, with the principle possibility to yield insight specific to the atomic
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structure of the QD surface and capping ligands via surface selective cross-polarization MAS
(CPMAS) experiments.14 However, CPMAS experiments on QDs are highly challenging due to
the low concentrations of surface sites, the residual ligand dynamics that reduce hetero-nuclear
1H-X dipolar couplings,7 and most importantly, the generally poor sensitivity of NMR
spectroscopy. From the QD chemistry point of view, NMR characterization should ideally be
carried out directly on colloidal dispersions of QDs to rule out the effects of the isolation and
purification procedures on the surface composition and surface coverage of the ligands. This
challenge becomes even more daunting when considering that pristine QD colloids contain only
milligram quantities of QDs, and the surface atoms and the stabilizing ligands constitute an even
smaller fraction of the sample. To become practically useful, solid-state NMR of dispersed
colloids of QDs requires orders of magnitude increase in the measured signal.
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In the past twenty years, in situ high-field dynamic nuclear polarization (DNP) enhanced
MAS NMR spectroscopy has become an extremely powerful method to enhance NMR signals
via microwave (µw) induced polarization transfer from unpaired electrons of a radical polarizing
agent to nuclear spins.15, 16 With typical enhancement factors (ɛ) of 10-100, and corresponding
shortening of the measurement times by two-to-four orders of magnitude (∝ ɛ2), DNP enhanced
NMR experiments may open new avenues for studying inorganic materials containing
unreceptive NMR nuclei, such as QDs. Indeed, in the recent years, DNP enhanced NMR studies,
exploiting unreceptive NMR nuclei such as 13C, 15N, 17O, 29Si, 59Co, 119Sn, have focused on
inorganic surfaces and surface-bound molecules, providing enhanced spectra from surface atoms,
including sub-surface atomic layers of the material.17-27 DNP surface enhanced NMR
spectroscopy should thus be uniquely suited for colloidal QDs, which are usually just 3-10 nm
large and have high surface areas, and therefore can be polarized as a whole, allowing
simultaneous and discriminative studies of the QD cores, surfaces and capping ligands; all in
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colloidal solutions, and without the need for invasive isolation and purification procedures.
Virtually all known colloidal QD materials and their organic- and inorganic ligand capping layers
contain NMR active nuclei that potentially can benefit from DNP enhancement, including spin ½
nuclei (31P, 77Se, 111/113Cd, 115In, 117/119Sn, 125Te, 207Pb, etc.) and also quadrupolar nuclei (nuclear
spin > ½,133Cs, 115In, 17O, 67Zn, etc).
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Here we demonstrate how DNP enhanced NMR can provide unprecedented information
about the structure of colloidal QDs, kept in the original colloidal state in the amount of just few
milligrams. We obtain high signal enhancement factors of up to 80, corresponding to gains in
acquisition times up to a factor 6400. Such efficient polarization results from the homogeneous
dispersion of biradical polarizing agents and QDs in mesoporous silica (meso-SiO2) and is readily
obtained for all common QD materials such as CdSe, CdTe, InP, PbSe, PbTe, and CsPbBr3, and
stabilized by all kinds of surface functionalities (inorganic or organic, charge and steric
stabilization) and dispersed in any suitable polar or apolar solvent. New insights specific to the
structure of QDs were obtained with DNP enhanced 1D NMR spectra. For a canonical QD
system – CdSe QDs – we present the first direct evidence for the core-shell CdSe-CdX2
(X=oleate, phosphonate) morphology, by resolving bulk and surface Cd and Se atoms with 113Cd
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and Se DNP enhanced spectra. Similarly, InP QDs are seen to exhibit clearly resolvable core
and surface 31P DNP enhanced signals. 125Te DNP NMR spectra point to massive differences
between CdTe and PbTe QDs. CdTe QDs are seen to remain un-oxidized even upon prolonged
storage at ambient air, whereas PbTe QDs are found to comprise exclusively Te-oxide species at
their surfaces, even with cautious air-free handling. To show new horizons for unreceptive
quadrupolar nuclei, we show DNP enhancements of 133Cs NMR spectra from perovskite CsPbBr3
QDs. Previously inconceivable 2D correlation experiments can also be readily accomplished
owing to strong signal enhancements, as demonstrated here for CdSe QDs with a 2D dipolar-
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HMQC 13C-111Cd correlation spectrum, which reveals the identity of Cd atoms to which
carboxylate ligands bind as a single surface moiety.
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Experimental Section.
Synthesis of QDs. Organic-capped InP, W-CdSe, ZB-CdSe, PbSe, PbTe, CdTe QDs were
synthesized and purified according to previously reported protocols, with slight modifications as
detailed in Supporting Information. Care was taken to remove excess of ligands in the colloidal
solutions used for DNP NMR experiments. Inorganic capped InP-S2- and CdSe-Se2- QDs were
prepared via ligand-exchange reactions, as described by Nag et al.28
Conventional solid-state MAS NMR experiments. Solid-state NMR experiments at ambient
conditions were performed on two Bruker spectrometers (11.7 T and 17.6 T) equipped with 2.5
mm and 4 mm two-channel solid-state probe heads and Avance III consoles, while spinning
between 0.6-20 kHz. Low temperature experiments were conducted on a commercial 14.1 T
Bruker instrument equipped with an Avance III console and a double resonance 3.2 mm low
temperature MAS probe using MAS spinning of 10 kHz. Chemical shifts were referenced to 85%
H3PO4 in H2O (31P), Me2Se (77Se), Me2Cd (113Cd) and 0.1 M aqueous CsCl (133Cs).
DNP surface enhanced experiments. DNP NMR spectra were acquired on two Bruker DNP
NMR spectrometers (9.4 T and 14.1 T) equipped with Avance I and III consoles, and 263 and
395 GHz gyrotron microwave sources, respectively.29 Double and triple 3.2 mm low temperature
MAS probes were used in combination with 3.2 mm sapphire rotors.29 For DNP NMR
experiments, 1-4 mg of QDs were dispersed in 15-20 µL solution of biradical (16 mM TEKPol in
TCE or 8 mM AMUPol in DMSO-d6/H2O/D2O). This QD-biradical solution was either loaded
directly into the rotor for reference experiments without meso-SiO2 or, for one-step filling
method, the meso-SiO2 powder was impregnated with the QD-biradical solution on a watch glass,
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then transferred into a rotor. The mass ratio of meso-SiO2 to QDs has been optimized for each
system, as illustrated for MSU-H meso-SiO2 and InP QDs in Figure S2. In a two-step method, the
QD dispersion was first impregnated into the meso-SiO2 host, and dried, followed by
impregnation of the biradical solution, aiming at the same overall amounts of QDs and biradicals
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as in one-step method. Chemical shifts were referenced to Me4Si (1H, C), 85% H3PO4 in H2O
(31P), Me2Se (77Se), Me2Cd (111/113Cd), Me2Te (125Te), 0.1 M CsCl in H2O (133Cs). See the
Supporting Information for further details on NMR measurements and sample preparation.
Results and Discussion.
DNP enhanced NMR of QDs using meso-SiO2 matrix. The basic principle of DNP enhanced
NMR experiments on colloidal QDs is outlined in Figure 1: the paramagnetic polarizing agent
(here a nitroxide biradical) is dispersed in a suitable solvent (here TCE) and then brought into
contact with the sample. Microwave (µw) irradiation transfers the polarization of the electron
spins to the protons of the solvent and of other proton-containing molecular species residing at
the QD surface. The enhanced proton polarization is then transferred through a CP step30 to the
desired hetero-nuclei. For QDs the range of this transfer from the surface is such that nuclei
residing in the capping ligands, the QD surface or even the QD core are hyper-polarized.
However, in situ DNP NMR experiments are conducted at rather low temperatures, typically
around 100 K to slow nuclear and electron relaxation.15, 16, 31 In addition to the DNP effect,
thermal (Boltzmann) enhancement of the NMR signal potentially contributes a factor 2.8 to the
sensitivity at 100 K as compared to room temperature (RT). The straightforward approach of
mixing solutions of QDs and biradicals has proven to yield little or no enhancement. This is
because colloidal dispersions of QDs usually aggregate and precipitate upon cooling, or even at
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room temperature in some cases, leading to phase separation from the biradical polarizing agent,
and obliteration of the DNP effect. This aggregation can be clearly observed visually by placing
concentrated QD colloids, with or without added biradicals, into a freezer at –40 C. Importantly,
due to the small size of the QDs (<10 nm), the voids within the aggregates are too small to allow
easy access for the relatively large polarizing molecules, and are mainly filled by the 1.5-2.5 nm-
long surface ligands of QDs.
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Figure 1. Schematics of the DNP surface enhanced NMR experiment for colloidal QDs. By
irradiating the sample with monochromatic microwaves (from a gyrotron source, here 263 or 395
GHz, depending upon magnetic field of the NMR spectrometer), the polarization of the unpaired
electron spins of the biradical is transferred to the protons of the solvent and then relayed to the
protons of the ligand by spin diffusion. CP is then used to transfer DNP enhanced 1H polarization
to the nuclei of interest in the capping ligand layer or to the hetero-nuclei of the QD surface or
QD core. Alternative pathways, omitted for clarity, are also possible, such as direct polarization
transfer from the biradical to the protons of the ligand. The depth of excitation in the QD depends
on the nuclei, and the CP contact time (for details of the pulse sequences used see Figure S1),
allowing either signal from preferentially the uppermost surface or excitation into the subsurface
layers, as discussed in the main text. L stands for organic and/or inorganic capping ligands such
as oleate, myristate, oleylamine, phosphonate, sulfide, selenide etc. A 3 nm zinc-blende CdSe QD
is shown as a model QD. 1,1,2,2-tetrachloroethane (TCE) is a known suitable DNP polarizing
medium and TEKPol is a state of the art DNP polarizing agent.32, 33
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To solve this problem, here we introduce meso-SiO2 (3-250 nm pore sizes) as a host matrix.
The meso-SiO2 is impregnated with a solution containing both the QDs and the biradicals,
thereby maintaining homogeneity of the QD-biradical mixtures in an environment similar to the
native chemical environment of colloidal QDs. Inexpensive, commercially available samples of
non-functionalized meso-SiO2 were used throughout this study. Importantly, the meso-SiO2 is
highly porous (71% porous volume for MSU-H with 7-nm pore size), meaning that the dilution
factor is no higher than 1.5 (close to experimentally observed, as discussed below). Furthermore,
sequential filling of the pores of meso-SiO2 first with QDs and then with polarizing agent allows,
for instance, study of highly-charged inorganic-capped QDs with hydrophobic biradicals and
vice-versa.
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Myristate-capped InP QDs
Our initial DNP surface enhanced NMR experiments on QDs focused on myristate-capped InP
QDs since 31P is a highly receptive NMR nucleus, enabling the rapid optimization of sample
preparation protocols. Owing to the presence of organic or inorganic capping ligand on their
surfaces, 3-10 nm QDs form thermodynamically stable colloidal solutions at ambient
temperatures (Figure 2a). For DNP NMR, the sample is usually loaded into a rotor at RT, and
then transferred to the pre-cooled DNP NMR probe where it is quickly frozen upon cooling to
100 K. In numerous attempts to obtain DNP surface enhanced NMR spectra of QDs simply by
adding biradical solutions to concentrated QD colloids we usually observed rather small DNP
enhancement factors (Ɛ < 5) for InP QDs (Figure 2b, d) and no enhancement for all other QD
samples; in all cases due to aggregation and phase separation of QDs. On the contrary, as shown
in Figure 2e, when the same colloidal solution is impregnated into commercial powders of meso-
SiO2, high DNP enhancement factors of up to 80 can be obtained. Overall, with the QD
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colloid/biradical/meso-SiO2 system an absolute QD signal enhancement by a factor of 51 is
obtained by comparison to rotor filled with frozen QD colloid only, clearly outweighing the
diluting effect of the added meso-SiO2 material and possible effects from the faster relaxation due
to proximity of paramagnetic biradical (Figure 2c, e and Figure S2, including detailed
comparison of enhancements for various sample formulations).
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Figure 2. The effect of a meso-SiO2 matrix on the success of DNP surface enhanced NMR
experiments on colloidal myristate-capped 3 nm InP QDs. (a) At RT, the colloidal solution of
organic-ligand capped QDs is fully thermodynamically stable, often up to concentrations of 30
wt% in tetrachloroethane. This solution is then mixed with the biradical polarizing agent TEKPol,
retaining colloidal stability. (b) In the absence of meso-SiO2, cooling to the measurement
temperature of 100 K induces aggregation of QDs and phase separation from biradicals; an effect
that is easily reversed upon warming the sample after withdrawal from the cooled DNP NMR
probe and which thus often goes unnoticed. (c) The pores of meso-SiO2 can fully (smaller pores)
or partially (large pores) limit the aggregation. Consequently, (d) no or very low DNP
enhancements are obtained in the absence of meso-SiO2, while, (e) reproducibly high
enhancement factors of between 60-80 are obtained for surface POx signal with meso-SiO2. In (e)
spinning sidebands are marked by asterisks. Figure (f) shows the DNP enhancements obtained for
InP QD-TEKPol-TCE solutions with varying pore size of the meso-SiO2. All experiments used 1-
4 mg of InP QDs per sample.
31P DNP NMR spectra of 3 nm InP QDs clearly illustrate the high enhancements for signals at
0-10 ppm, assigned to surface POx species.34, 35 Furthermore, a large portion of the InP QD core is
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also enhanced, with a very broad peak centered at –200 ppm. The intensity of the surface and
core QD NMR signals depend on the contact time set in the CP pulse sequence (Figure S3),
consistent with previous conventional solid-state NMR experiments on InP QDs.35 As expected,
conventional single-pulse MAS NMR spectra taken from dried powders of same InP QDs mainly
show signals from the QD core (Figure S4), highlighting the utility of DNP for obtaining surface-
selective information.
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We then evaluated the effect of pore size of meso-SiO2 on the DNP enhancement of the
QDs, as presented for the POx-signal from myristate-capped InP QDs in Figure 2f. First we
should note that each meso-SiO2 sample has a distribution of pore sizes, around the one indicated
as a nominal mean pore size. Thus, the 3.3 nm pore meso-SiO2 with the smallest pore size, has in
fact a relatively broad pore size distribution in the 1-8 nm range, and therefore can uptake large
quantities of 3-4 nm QDs. The average population of QDs in meso-pores can vary from less than
one-per-pore to as high as 20 QDs-per-pore (i.e., by placing a 300 mg/mL dispersion of the
smallest 3 nm InP QDs, with a 2 nm-thick ligand shell, into 50 nm large spherical pores). Even if
aggregation occurs in the latter case, the single aggregate size is still small enough to expose
nearly all QDs to the effects of the biradical polarizing agent. A noticeable decrease in the
enhancement factors can be found for all studied QDs only at extremely large pore sizes, above
100 nm, that allow aggregates sufficiently large to exclude a substantial fraction from the
biradical. In control experiments without meso-SiO2 DNP enhancements of 5 are obtained.
Another beneficial effect is the inhibition of solvent crystallization in meso-SiO2. DNP
experiments typically require glass forming solvents which prevent aggregation of radicals upon
freezing.19 Therefore, the use of meso-SiO2 as a host for DNP experiments has the added benefit
that it could allow for a broader range of solvents to be used.
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CdSe QDs. After two decades of research, CdSe QDs have become a canonical QD material in
academic research, and the first QD product commercialized for consumer electronics (Sony’s
LCD TV displays, 2013). Importantly, CdSe QDs can be deliberately obtained in two
polymorphs - zinc-blende (ZB, cubic) or wurzite (WZ, hexagonal). CdSe QD surface chemistry
has received a great deal of attention in recent years.5, 12, 36, 37 The current consensus is that non-
oxidized carboxylate-capped ZB-CdSe QDs (and likely WZ-CdSe QDs) comprise a CdSe core
and a labile layer of surface Cd-atoms bound to carboxylate or other X-type anionic ligands, e.g.
a CdSecore(CdX2)shell model, and therefore they are Cd-rich as a whole.36, 38 Similar conclusions
were drawn for Pb-chalcogenide QDs.36, 39 Inspired by the ability to differentiate core and surface
31P NMR signals in the InP QDs, we turned to oleate-capped ZB-CdSe QDs, containing two
NMR-accessible elements (111/113Cd and 77Se), seeking to directly probe the prevailing
CdSecore(CdX2)shell model. With DNP surface enhanced NMR we observed a signal at –20 ppm
from the QD core and a strong signal from Cd surface species at -317 ppm (Figure 3a). Both core
and surface Cd signals exhibit an inhomogeneous line broadening of approximately 4000 Hz,
which is considerably larger than that of bulk CdSe,7 and likely arises from positional variations
in electronic environments when moving from the surface to the core.13, 35, 40 The NMR signal of
the surface Cd atoms gives rise to an extensive manifold of spinning sidebands, which reflects a
substantial chemical shift anisotropy of ca. 400 ppm and the lower symmetry about the surface
Cd atoms. On the other hand the Cd core signal shows fewer sidebands, consistent with the more
symmetric environment expected in the core. In agreement with our assignments, control single-
pulse MAS NMR experiments only show signals from the ZB-CdSe QD core at -83 ppm (Figure
S5). Previous direct excitation 113Cd MAS NMR experiments did not study ZB-CdSe QDs, but
found 113Cd chemical shifts of –96 ppm in bulk WZ-CdSe,41 –65 ppm in WZ-CdSe QDs,7 and –
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46 ppm for nearly ZB-like 2 nm CdSe nanoclusters14 (all values are converted to a chemical shift
scale with the chemical shift of Me2Cd set to 0 ppm).
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77Se DNP surface enhanced NMR was also carried out for both ZB- and WZ-polymorphs,
showing close chemical shifts of –579 ppm and –654 ppm (Figure 3b) and –556 ppm (Figure S6),
respectively. Two peaks in Figure 3b may point to the distinct contributions from the QD core
(lower intensity peak) and (near) surface Se atoms, as was previously demonstrated for WZ-CdSe
QDs.14 The 77Se chemical shifts obtained from DNP experiments on WZ-CdSe match the values
previously reported for WZ-CdSe QDs using conventional 1H-77Se CPMAS. 6, 14
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Figure 3. DNP surface enhanced NMR of oleate-capped 4nm ZB-CdSe QDs. Spectra for both
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the QD core (113Cd and Se, a nd b) and ligand (13C, c) were acquired and DNP enhancements
were obtained for all nuclei. The spinning side bands are marked by asterisks. Isotropic peaks and
spinning sidebands were distinguished by varying the MAS frequency, Figure S7. (77Se) is
referenced to Me2Se, (13C) to Me4Si, (113Cd) to Me2Cd (for more discussion of 111Cd/113Cd -
scale in Supp. Information Table S1).
2D DNP surface enhanced hetero-nuclear correlation spectra of CdSe QDs. Figure 3c shows
that using the matrix-assisted approach, DNP enhanced 13C-spectra from the surface-bound oleate
ligands (e.g. 13C-1-oleate; 27% 13C-labeled on the carboxylate), with clearly resolved signals
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from all C-atoms, can be acquired in less than 2 minutes (32 scans). This opens the possibility to
acquire 13C-X hetero-nuclear 2D correlation spectra within reasonable timeframes. Partial 13C-
labeling of the carboxylate carbon was employed in order to help in collecting a DNP enhanced
13C-111Cd 2D dipolar hetero-nuclear multiple-quantum coherence (D-HMQC) correlation
spectrum of oleate-capped ZB-CdSe QDs (Figure 4). Here 111Cd was used rather than the more
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sensitive 113Cd since the triple resonance probe could not be configured in H/13C/113Cd mode.
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Such experiments yield correlations between C and 111Cd hetero-nuclei that are close in space.
The 2D spectrum (obtained overnight) shows clear correlations between carboxylate and Cd
atoms at the QD surface, providing direct evidence for the CdSecore(CdX2)shell model. This model
was previously inferred indirectly by observing the ligand absorption/displacement dynamics
with 1H NMR.36 Based on the signal-to-noise ratios of on/off 13C-spectra, the acquisition time of
non-DNP-enhanced 2D spectra would be prohibitively long, taking more than 1000 days.
Sequential filling method of meso-SiO2 for inorganic-capped and air-sensitive QDs. In the
studies presented thus far, one-step filling of meso-SiO2 with a QD-biradical mixture was
sufficient to prevent phase separation. Furthermore, common long-chain ligand-capped QDs
enjoy full compatibility and miscibility with apolar solvent-biradical mixtures previously
optimized for DNP. Negative consequences were encountered when attempting the same
approach for highly polar, charge-stabilized QDs, such as those capped with S2- and Se2- ions as
inorganic ligands (in the form of K2S and K2Se). Such inorganic capping ligands are of strategic
importance for integration of QDs into (opto)electronic devices due to the high charge carrier
mobility they impart into QD solids.42, 43
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Figure 4. DNP enhanced D-HMQC spectrum correlating C-1-oleate ligands and 111Cd of ZB-
CdSe. Projections of the 2D spectrum are shown in dark blue. A DNP NMR 111Cd CPMAS
spectrum acquired separately is shown in light grey. The isotropic shifts of the surface and core
Cd-species identified before (Figure 3) are highlighted with dotted black curves. The spinning
sidebands are marked by asterisks. It is apparent that the Cd-species with signal at -317 ppm is
directly binding to oleic acid. No correlation between the 111Cd of the QD core (-20 ppm) and 13C
nuclei of the ligand was observed for the recoupling time used (ms). In control experiments, the
DNP enhanced NMR spectrum of free Cd(oleate)2 diluted in TCE is characterized by a single
peak with chemical shift distinctly different from all Cd signals recorded from ZB-CdSe QDs
(Figure S8). Here 111Cd was used rather than the more sensitive 113Cd since the triple resonance
probe could not be configured in 1H/13C/113Cd mode. First order rotary resonance recoupling (R3)
was employed.
There are much fewer polar solvent-biradical mixtures which simultaneously solubilize
charge-stabilized QDs and allow for efficient DNP enhancements. The only working example we
identified so far is AMUPol in DMSO-d6/H2O/D2O, which is suitable for studying chemically
robust materials such as sulfide capped Sn/SnOx nanocrystals.22 Water in this mixture is required
for efficient polarization transfer, but promotes hydrolysis and oxidation of sensitive QD
materials. To circumvent these problems, a facile and versatile two-step method for filling the
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meso-SiO2 matrix is presented in Figure 5. This method eliminates a tedious search for suitable
compatible QD-solvent-radical combinations. First, impregnation of the meso-SiO2 was done
with a QD dispersion in any given type of solvent, polar or apolar, followed by the evaporation of
the solvent to deposit the QDs in the meso-SiO2. Second, the meso-SiO2 filled with QDs was then
impregnated with any other given (polar or apolar) biradical-solvent mixture. Hence apolar
solvent-biradical combinations can be used for enhancing NMR signals of highly polar inorganic-
capped QDs, and vice-versa. In the first case study, highly oxygen and moisture-sensitive
selenide (Se2-) capped CdSe QDs, prepared from oleate-capped QDs via a ligand-exchange
reaction,28 were deposited into meso-SiO2 from formamide solution. Subsequent 77Se DNP NMR
with the TEKPol-TCE polarization mixture yields a spectrum with higher signal-to-noise ratio
(Figure 5a, 3000 scans) than was obtained for carboxylate-capped CdSe NCs (Figure 3b). The
77Se spectrum of CdSe-Se2- QDs is, however, asymmetrically broader (see direct comparison in
Figure S9), presumably due to contribution from the surface-bound Se2- with a chemical shift
only subtly different to the core Se nuclei. This conclusion is supported by the control
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experiments with unbound Se2-, where the Se chemical shifts of frozen solutions and powders
of K2Se were much further separated (by several hundred ppm, Figure S9).
More chemically robust sulfide (S2-) inorganic-capped InP QDs are compatible with an
AMUPol-DMSO-d6/H2O/D2O polarizing mixture, allowing direct comparison between one-step
and two-step filling methods. Similar DNP enhancement factors were obtained in both cases
(Figure 5b and c), yet with a noticeable difference in the obtained 31P NMR spectra. The 31P
CPMAS NMR spectrum from the two-step, water-free deposition features an additional peak
(marked with an arrow in Figure 5c). Although the chemical identity of this signal is not fully
clear, we can speculate that they are due to POxSy surface species, which are otherwise fully
oxidized and/or hydrolyzed in a water-based polarizing mixture.
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Figure 5. DNP surface enhanced NMR on air-sensitive and inorganic-capped QDs using
sequential filling of meso-SiO2 (two-step method). CdSe-Se2- QDs are highly oxygen and
moisture sensitive and could be studied only with two-step deposition method (a). InP-S2- QDs
have been deposited into silica pores by one (b) and two-step (c) procedures. The two-step
procedure does not involve any water during preparation and yields an additional peak in the
spectrum (indicated by an arrow), attributed to POxSy species, absent in (b) due to full conversion
into POx species. The spinning sidebands are marked by asterisks.
Other tested systems, concluding remarks and outlook.
So far precipitation of QDs to form powdered samples had remained the only practical option for
solid-state NMR studies on QDs. Precipitation may change the chemical identity of the surface
QD atoms and/or surface ligands. In this study, we have shown that meso-SiO2-assisted DNP
experiments allow for using colloidal dispersions of QDs directly for solid-state NMR
spectroscopy to study QDs in an environment more similar to a dispersion. Owing to efficient
proton DNP enhancements (ƐH = 40-80), absolute signals obtained with small sample amounts (1-
4 mg of QDs per sample throughout this study) are always much greater than conventional MAS
NMR signals from a rotor densely packed with large quantities of QDs (up to 100-200 mg). In
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fact, there are clear physical and hardware limits to scaling the MAS NMR signal by using larger
sample quantities and larger rotors. Without DNP enhancement, acquisition of 1D CPMAS
spectra from 1-2 mg of QDs would take months or even years. The matrix–assisted method
enables acquisition of 2D spectra between hetero-nuclei in reasonable timeframes (hours instead
of years) and allows a whole new plethora of studies to be carried out, such as directly probing
metal-ligand binding at the QD surface (as shown here for Cd-oleate at the CdSe surface), and
possibly intra-ligand and intra-QD local chemical structure. With a two-step sequential filling
method, problems such as air and moisture sensitivity, solvent incompatibility and low solubility
can be easily circumvented. With the advent of commercial DNP NMR spectrometers, and the
active development of suitable biradical polarization transfer agents,32, 44 DNP enhanced NMR
spectroscopy holds great promise to address various research problems in contemporary
nanomaterials research. Meso-SiO2 of appropriate pore size is also likely to be of great help for
ensuring homogeneous DNP excitation of many other important analytes such as polymers,
biomolecules, colloids, and coordination compounds, yet with minimal dilution effect.
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Specific to the chemistry of QDs, DNP surface enhanced NMR yields signatures from the
QD core, QD surface, and surface-bound ligands. For CdSe QDs as a case study, we were able
for the first time to clearly distinguish Cd atoms from the QD core and QD surface, thanks to the
tunable surface-selectivity of DNP. Hence strong evidence is obtained here in support of a
CdSecore(CdX2)shell (X=oleate ligand) morphology of these QDs. Correspondingly, 77Se spectra
point to the gradient of chemical shifts within the QD core, rather than new surface species.
Furthermore, large DNP enhancements permitted fast acquisition of 111Cd-13C 2D-correlation
spectra within the surface Cd-carboxylate layer, directly determining ligand binding to the CdSe
QD surface. Future studies will expand such analysis to shape and composition engineered CdSe
nanostructures such as highly-luminescent core-shell CdSe/Cd1-xZnxS QDs (x=0-1).45, 46 or
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atomically flat Cd chalcogenide nanoplatelets47 and nanorods.48 For the later, preliminary
experiments shown enhancement factors of up to 35 using 30-50 nm large pores of meso-SiO2.
Besides two detailed case studies – InP and CdSe QDs – we were also able to record DNP
enhanced spectra for a variety of other important QD core materials in a short survey study (DNP
enhanced nuclei in parentheses, see Figure S10): PbSe (77Se), CdTe (125Te), and PbTe (125Te).
For example, the DNP enhanced 125Te solid-state NMR spectra illustrate large difference in
stability with respect to oxidation of CdTe and PbTe QDs (Figure S10). We observe that PbTe
QDs comprise exclusively Te-oxide species at their surfaces, even when freshly prepared and
handled air-free, while CdTe QDs remain un-oxidized upon long storage in air. These
preliminary results pave the way towards new atomistic insights into the chemistry of these QDs
in future studies. In another trial, we deposited nanocrystalline CsPbBr3, perovskite-type highly
luminescent semiconductors,49 into pores of meso-SiO2 and recorded DNP enhanced spectra of
133Cs (Figure S11), for the first time for this nucleus to the best of our knowledge. This result
highlights the utility and future prospects of DNP also for nanomaterials containing quadrupolar
nuclei.
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Acknowledgements M.K. acknowledges financial support from the European Union through the
FP7 (ERC Starting Grant NANOSOLID, GA No. 306733) and L.E. acknowledges support from
ERC Advanced Grant No. 320860. René Verel and Maxence Valla are acknowledged for their
help at the NMR facility at ETH Zurich. Davisil silica matrices with ≥50nm pore sizes were gifts
of GRACE. Georgian Nedelcu is acknowledged for providing InP QDs, Tanja Zünd for CdTe
QDs, Maria Ibáñez Sabate for PbTe QDs, David Trummer for CsPbBr3 nanostructures, and Indre
Thiel for supplying meso-SiO2 with 3.3 nm pore size.
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Competing financial interests: The authors declare no competing financial interest.
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Supporting information. Materials, Methods, Supporting Figures S1-S11 and Table S1 and
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additional explanations are available free of charge via the Internet at http://pubs.acs.org/.
Author information
The manuscript was prepared through the contribution of all authors.
Corresponding author:
mvkovalenko@ethz.ch; ccoperet@ethz.ch; lyndon.emsley@epfl.ch
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