Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials

Surface Structure of Lecithin-Capped Cesium Lead Halide Perovskite Nanocrystals Using Solid-State and Dynamic Nuclear Polarization NMR Spectroscopy

Inorganic colloidal cesium lead halide perovskite nanocrystals (NCs) encapsulated by surface capping ligands exhibit tremendous potential in optoelectronic applications, with their surface structure playing a pivotal role in enhancing their photophysical properties. Soy lecithin, a tightly binding zwitterionic surface-capping ligand, has recently facilitated the high-yield synthesis of stable ultraconcentrated and ultradilute colloids of CsPbX3 NCs, unlocking a myriad of potential device applications. However, the atomic-level understanding of the ligand-terminated surface structure remains uncertain. Herein, we use a versatile solid-state nuclear magnetic resonance (NMR) spectroscopic approach, in combination with dynamic nuclear polarization (DNP) and atomistic molecular dynamics (MD) simulations, to explore the effect of lecithin on the core-to-surface structures of CsPbX3 (X = Cl or Br) perovskites, sized from micron to nanoscale. Surface-selective (cross-polarization, CP) solid-state and DNP NMR (133Cs and 207Pb) methods were used to differentiate the unique surface and core chemical environments, while the head-groups {trimethylammonium [-N(CH3)3+] and phosphate (-PO4-)} of lecithin were assigned via 1H, 13C, and 31P NMR spectroscopy. A direct approach to determining the surface structure by capitalizing on the unique heteronuclear dipolar couplings between the lecithin ligand (1H and 31P) and the surface of the CsPbCl3 NCs (133Cs and 207Pb) is demonstrated. The 1H-133Cs heteronuclear correlation (HETCOR) DNP NMR indicates an abundance of Cs on the NC surface and an intimate proximity of the -N(CH3)3+ groups to the surface and subsurface 133Cs atoms, supported by 1H{133Cs} rotational-echo double-resonance (REDOR) NMR spectroscopy. Moreover, the 1H-31P{207Pb} CP REDOR dephasing curve provides average internuclear distance information that allows assessment of -PO4- groups binding to the subsurface Pb atoms. Atomistic MD simulations of ligand-capped CsPbCl3 surfaces aid in the interpretation of this information and suggest that ligand -N(CH3)3+ and -PO4- head-groups substitute Cs+ and Cl- ions, respectively, at the CsCl-terminated surface of the NCs. These detailed atomistic insights into surface structures can further guide the engineering of various relevant surface-capping zwitterionic ligands for diverse metal halide perovskite NCs.

Characterization of dielectric properties and their impact on MAS-DNP NMR applications

The dielectric properties of materials play a crucial role in the propagation and absorption of microwave beams employed in Magic Angle Spinning - Dynamic Nuclear Polarization (MAS-DNP) NMR experiments. Despite ongoing optimization efforts in sample preparation, routine MAS-DNP NMR applications often fall short of theoretical sensitivity limits. Offering a different perspective, we report the refractive indices and extinction coefficients of diverse materials used in MAS-DNP NMR experiments, spanning a frequency range from 70 to 960 GHz. Knowledge of their dielectric properties enables the accurate simulation of electron nutation frequencies, thereby guiding the design of more efficient hardware and sample preparation of biological or material samples. This is illustrated experimentally for four different rotor materials (sapphire, yttria-stabilized zirconia (YSZ), aluminum nitride (AlN), and SiAlON ceramics) used for DNP at 395 GHz/1H 600 MHz. Finally, electromagnetic simulations and state-of-the-art MAS-DNP numerical simulations provide a rational explanation for the observed magnetic field dependence of the enhancement when using nitroxide biradicals, offering insights that will improve MAS-DNP NMR at high magnetic fields.

Role of Protons in and around Strongly Coupled Nitroxide Biradicals for Cross-Effect Dynamic Nuclear Polarization

In magic angle spinning dynamic nuclear polarization (DNP), biradicals such as bis-nitroxides are used to hyperpolarize protons under microwave irradiation through the cross-effect mechanism. This mechanism relies on electron-electron spin interactions (dipolar coupling and exchange interaction) and electron-nuclear spin interactions (hyperfine coupling) to hyperpolarize the protons surrounding the biradical. This hyperpolarization is then transferred to the bulk sample via nuclear spin diffusion. However, the involvement of the protons in the biradical in the cross-effect DNP process has been under debate. In this work, we address this question by exploring the hyperpolarization pathways in and around bis-nitroxides. We demonstrate that for biradicals with strong electron-electron interactions, as in the case of the AsymPols, the protons on the biradical may not be necessary to quickly generate hyperpolarization. Instead, such biradicals can efficiently, and directly, polarize the surrounding protons of the solvent. The findings should impact the design of the next generation of biradicals.

Rational Design of Dinitroxide Polarizing Agents for Dynamic Nuclear Polarization to Enhance Overall NMR Sensitivity

We evaluate the overall sensitivity gains provided by a series of eighteen nitroxide biradicals for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and 100 K, including eight new biradicals. We find that in the best performing group the factors contributing to the overall sensitivity gains, namely the DNP enhancement, the build-up time, and the contribution factor, often compete with each other leading to very similar overall sensitivity across a range of biradicals. NaphPol and HydroPol are found to provide the best overall sensitivity factors, in organic and aqueous solvents respectively. One of the new biradicals, AMUPolCbm, provides high sensitivity for all three solvent formulations measured here, and can be considered to be a "universal" polarizing agent.

AsymPol-TEKs as efficient polarizing agents for MAS-DNP in glass matrices of non-aqueous solvents

Two polarizing agents from the AsymPol family, AsymPol-TEK and cAsymPol-TEK (methyl-free version) are introduced for MAS-DNP applications in non-aqueous solvents. The performance of these new biradicals is rationalized in detail using a combination of electron paramagnetic resonance spectroscopy, density functional theory, molecular dynamics and quantitative MAS-DNP spin dynamics simulations. By slightly modifying the experimental protocol to keep the sample temperature low at insertion, we are able to obtain reproducable DNP-NMR data with 1,1,2,2-tetrachloroethane (TCE) at 100 K, which facilitates optimization and comparison of different polarizing agents. At intermediate magnetic fields, AsymPol-TEK and cAsymPol-TEK provide 1.5 to 3-fold improvement in sensitivity compared to TEKPol, one of the most widely used polarizing agents for organic solvents, with significantly shorter DNP build-up times of ∼1 s and ∼2 s at 9.4 and 14.1 T respectively. In the course of the work, we also isolated and characterized two diastereoisomers that can form during the synthesis of AsymPol-TEK; their difference in performance is described and discussed. Finally, the advantages of the AsymPol-TEKs are demonstrated by recording 2D 13C-13C correlation experiments at natural 13C-abundance of proton-dense microcrystals and by polarizing the surface of ZnO nanocrystals (NCs) coated with diphenyl phosphate ligands. For those experiments, cAsymPol-TEK yielded a three-fold increase in sensitivity compared to TEKPol, corresponding to a nine-fold time saving.

The H-D-isotope effect of heavy water affecting ligand-mediated nanoparticle formation in SANS and NMR experiments

An isotopic effect of normal (H2O) vs. heavy water (D2O) is well known to fundamentally affect the structure and chemical properties of proteins, for instance. Here, we correlate the results from small angle X-ray and neutron scattering (SAXS, SANS) with high-resolution scanning transmission electron microscopy to track the evolution of CdS nanoparticle size and crystallinity from aqueous solution in the presence of the organic ligand ethylenediaminetetraacetate (EDTA) at room temperature in both H2O and D2O. We provide evidence via SANS experiments that exchanging H2O with D2O impacts nanoparticle formation by changing the equilibria and dynamics of EDTA clusters in solution as investigated by nuclear magnetic resonance analysis. The colloidal stability of the CdS nanoparticles, covered by a layer of [Cd(EDTA)]2- complexes, is significantly reduced in D2O despite the strong stabilizing effect of EDTA in suspensions of normal water. Hence, conclusions about nanoparticle formation mechanisms from D2O solutions reveal limited transferability to reactions in normal water due to isotopic effects, which thus need to be discussed for contrast match experiments.

Dynamic Nuclear Polarization of Inorganic Halide Perovskites

The intrinsic low sensitivity of nuclear magnetic resonance (NMR) experiments limits their utility for structure determination of materials. Dynamic nuclear polarization (DNP) under magic angle spinning (MAS) has shown tremendous potential to overcome this key limitation, enabling the acquisition of highly selective and sensitive NMR spectra. However, so far, DNP methods have not been explored in the context of inorganic lead halide perovskites, which are a leading class of semiconductor materials for optoelectronic applications. In this work, we study cesium lead chloride and quantitatively compare DNP methods based on impregnation with a solution of organic biradicals with doping of high-spin metal ions (Mn²⁺) into the perovskite structure. We find that metal-ion DNP provides the highest bulk sensitivity in this case, while highly surface-selective NMR spectra can be acquired using impregnation DNP. The performance of both methods is explained in terms of the relaxation times, particle size, dopant concentration, and surface wettability. We envisage the future use of DNP NMR approaches in establishing structure-activity relationships in inorganic perovskites, especially for mass-limited samples such as thin films.

Polarizing agents for efficient high field DNP solid-state NMR spectroscopy under magic-angle spinning: from design principles to formulation strategies

Dynamic Nuclear Polarization (DNP) has recently emerged as a cornerstone approach to enhance the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS), opening unprecedented analytical opportunities in chemistry and biology. DNP relies on a polarization transfer from unpaired electrons (present in endogenous or exogenous polarizing agents) to nearby nuclei. Developing and designing new polarizing sources for DNP solid-state NMR spectroscopy is currently an extremely active research field per se, that has recently led to significant breakthroughs and key achievements, in particular at high magnetic fields. This review describes recent developments in this area, highlighting key design principles that have been established over time and led to the introduction of increasingly more efficient polarizing sources. After a short introduction, Section 2 presents a brief history of solid-state DNP, highlighting the main polarization transfer schemes. The third section is devoted to the development of dinitroxide radicals, discussing the guidelines that were progressively established to design the fine-tuned molecular structures in use today. In Section 4, we describe recent efforts in developing hybrid radicals composed of a narrow EPR line radical covalently linked to a nitroxide, highlighting the parameters that modulate the DNP efficiency of these mixed structures. Section 5 reviews recent advances in the design of metal complexes suitable for DNP MAS NMR as exogenous electron sources. In parallel, current strategies that exploit metal ions as endogenous polarization sources are discussed. Section 6 briefly describes the recent introduction of mixed-valence radicals. In the last part, experimental aspects regarding sample formulation are reviewed to make best use of these polarizing agents in a broad panel of application fields.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li₂OCaO·2SiO₂ and Li₂CaSiO₄, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both ⁶Li and ²⁹Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Binding Between Antibiotics and Polystyrene Nanoparticles Examined by NMR

Elucidating the interactions between plastic nanoparticles and small molecules is important to understanding these interactions as they occur in polluted waterways. For example, plastic that breaks down into micro- and nanoscale particles will interact with small molecule pollutants that are also present in contaminated waters. Other components of natural water, such as dissolved organic matter, will also influence these interactions. Here we use a collection of complementary NMR techniques to examine the binding between polystyrene nanoparticles and three common antibiotics, belonging to a class of molecules that are expected to be common in polluted water. Through examination of proton NMR signal intensity, relaxation times, saturation-transfer difference (STD) NMR, and competition STD-NMR, we find that the antibiotics have binding strengths in the order amoxicillin < metronidazole ≪ levofloxacin. Levofloxacin is able to compete for binding sites, preventing the other two antibiotics from binding. The presence of tannic acid disrupts the binding between levofloxacin and the polystyrene nanoparticles, but does not influence the binding between metronidazole and these nanoparticles.

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si-OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Colloidal-ALD-Grown Hybrid Shells Nucleate via a Ligand-Precursor Complex

Colloidal atomic layer deposition (c-ALD) enables the growth of hybrid organic-inorganic oxide shells with tunable thickness at the nanometer scale around ligand-functionalized inorganic nanoparticles (NPs). This recently developed method has demonstrated improved stability of NPs and of their dispersions, a key requirement for their application. Nevertheless, the mechanism by which the inorganic shells form is still unknown, as is the nature of multiple complex interfaces between the NPs, the organic ligands functionalizing the surface, and the shell. Here, we demonstrate that carboxylate ligands are the key element that enables the synthesis of these core-shell structures. Dynamic nuclear polarization surface-enhanced nuclear magnetic resonance spectroscopy (DNP SENS) in combination with density functional theory (DFT) structure calculations shows that the addition of the aluminum organometallic precursor forms a ligand-precursor complex that interacts with the NP surface. This ligand-precursor complex is the first step for the nucleation of the shell and enables its further growth.

Determining the Three-Dimensional Structures of Silica-Supported Metal Complexes from the Ground Up

The immobilization of molecularly precise metal complexes to substrates, such as silica, provides an attractive platform for the design of active sites in heterogeneous catalysts. Specific steric and electronic variations of the ligand environment enable the development of structure-activity relationships and the knowledge-driven design of catalysts. At present, however, the three-dimensional environment of the precatalyst, much less the active site, is generally not known for heterogeneous single-site catalysts. We explored the degree to which NMR-based surface-to-complex interatomic distances could be used to solve the three-dimensional structures of three silica-supported metal complexes. The structure solution revealed unexpected features related to the environment around the metal that would be difficult to discern otherwise. This approach appears to be highly robust and, due to its simplicity, is readily applied to most single-site catalysts with little extra effort.

Recent Advances and Prospects in Colloidal Nanomaterials

Colloidal nanomaterials of metals, metal oxides, and metal chalcogenides have attracted great attention in the past decade owing to their potential applications in optoelectronics, catalysis, and energy conversion. Introduction of various synthetic routes has resulted in diverse colloidal nanostructured materials with well-controlled size, shape, and composition, enabling the systematic study of their intriguing physicochemical, optoelectronic, and chemical properties. Furthermore, developments in the instrumentation have offered valuable insights into the nucleation and growth mechanism of these nanomaterials, which are crucial in designing prospective materials with desired properties. In this perspective, recent advances in the colloidal synthesis and mechanism studies of nanomaterials of metal chalcogenides, metals, and metal oxides are discussed. In addition, challenges in the characterization and future direction of the colloidal nanomaterials are provided.

Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Core-Shell Nanowires

We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C₃N₅ is a low-band-gap (E_g = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C₃N₅ nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C₃N₅ heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm² V^-1 s^-1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm² V^-1 s^-1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J-V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C₃N₅ in CdS-MHP. C₃N₅, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core-shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.

Ternary ACd4P3 (A = Na, K) Nanostructures via a Hydride Solution-Phase Route

Complex pnictides such as I-II₄-V₃ compounds (I = alkali metal; II = divalent transition metal; V = pnictide element) display rich structural chemistry and interesting optoelectronic properties, but can be challenging to synthesize using traditional high-temperature solid-state synthesis. Soft chemistry methods can offer control over particle size, morphology, and properties. However, the synthesis of multinary pnictides from solution remains underdeveloped. Here, we report the colloidal hot-injection synthesis of ACd₄P₃ (A = Na, K) nanostructures from their alkali metal hydrides (AH). Control studies indicate that NaCd₄P₃ forms from monometallic Cd⁰ seeds and not from binary Cd₃P₂ nanocrystals. IR and ssNMR spectroscopy reveal tri-n-octylphosphine oxide (TOPO) and related ligands are coordinated to the ternary surface. Computational studies show that competing phases with space group symmetries R3̅m and Cm differ by only 30 meV/formula unit, indicating that synthetic access to either of these polymorphs is possible. Our synthesis unlocks a new family of nanoscale multinary pnictide materials that could find use in optoelectronic and energy conversion devices.

Crystal and electronic facet analysis of ultrafine Ni2P particles by solid-state NMR nanocrystallography

Structural and morphological control of crystalline nanoparticles is crucial in the field of heterogeneous catalysis and the development of “reaction specific” catalysts. To achieve this, colloidal chemistry methods are combined with ab initio calculations in order to define the reaction parameters, which drive chemical reactions to the desired crystal nucleation and growth path. Key in this procedure is the experimental verification of the predicted crystal facets and their corresponding electronic structure, which in case of nanostructured materials becomes extremely difficult. Here, by employing ³¹P solid-state nuclear magnetic resonance aided by advanced density functional theory calculations to obtain and assign the Knight shifts, we succeed in determining the crystal and electronic structure of the terminating surfaces of ultrafine Ni₂P nanoparticles at atomic scale resolution. Our work highlights the potential of ssNMR nanocrystallography as a unique tool in the emerging field of facet-engineered nanocatalysts.

Structural and morphological control of crystalline nanoparticles is crucial in heterogeneous catalysis. Applying DFT-assisted solid-state NMR spectroscopy, we determine the surface crystal and electronic structure of Ni₂P nanoparticles, unveiling NMR nanocrystallography as an emerging tool in facet-engineered nanocatalysts.

Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy

Dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) spectroscopy was used to obtain detailed surface structures of zinc blende CdSe nanocrystals (NCs) with plate or spheroidal morphologies which are capped by carboxylic acid ligands. 1D ¹¹³Cd and ⁷⁷Se cross-polarization magic angle spinning (CPMAS) NMR spectra revealed distinct signals from Cd and Se atoms on the surface of the NCs, and those residing in bulk-like environments, below the surface. ¹¹³Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe₃O, CdSe₂O₂, and CdSeO₃ Cd coordination environments on the surface of the NCs, where the oxygen atoms are presumably from coordinated carboxylate ligands. The sensitivity gain from DNP enabled natural isotopic abundance 2D homonuclear ¹¹³Cd-¹¹³Cd and ⁷⁷Se-⁷⁷Se and heteronuclear ¹¹³Cd-⁷⁷Se scalar correlation solid-state NMR experiments which revealed the connectivity of the Cd and Se atoms. Importantly, ⁷⁷Se{¹¹³Cd} scalar heteronuclear multiple quantum coherence (J-HMQC) experiments were used to selectively measure one-bond ⁷⁷Se-¹¹³Cd scalar coupling constants (¹J(⁷⁷Se, ¹¹³Cd)). With knowledge of ¹J(⁷⁷Se, ¹¹³Cd), heteronuclear ⁷⁷Se{¹¹³Cd} spin echo (J-resolved) NMR experiments were used to determine the number of Cd atoms bonded to Se atoms and vice versa. The J-resolved experiments directly confirmed that major Cd and Se surface species have CdSe₂O₂ and SeCd₄ stoichiometries, respectively. Considering the crystal structure of zinc blende CdSe and the similarity of the solid-state NMR data for the platelets and spheroids, we conclude that the surface of the spheroidal CdSe NCs is primarily composed of {100} facets. The methods outlined here will generally be applicable to obtain detailed surface structures of various main group semiconductor nanoparticles.

Hyperpolarization transfer pathways in inorganic materials

Dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free inorganic materials in magic angle spinning NMR experiments. The hyperpolarization is generated on the surface of the material with incipient wetness impregnation and from there it is propagated towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. This method can provide significant gains in sensitivity for MAS NMR spectra of bulk inorganic compounds, but the pathways of the magnetization transfer into the material have not previously been elucidated. Here we show how two-dimensional experiments can be used to study spin diffusion from the surface of a material towards the bulk. We find that hyperpolarization can be efficiently relayed from surface sites to multiple bulk sites simultaneously, and that the bulk sites also engage in rapid polarization exchange between themselves. We also show evidence that the surface peaks can exchange polarization between different sites in cases of disorder.

Colloidal Synthesis and Optical Properties of Perovskite-Inspired Cesium Zirconium Halide Nanocrystals

Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs₂ZrX₆ (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs₂ZrCl₆ and Cs₂ZrBr₆, respectively, consistent with experimental values. Finally, we demonstrate that the Cs₂ZrCl₆ and Cs₂ZrBr₆ nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs₂ZrX₆ nanocrystal thin films into next-generation light-emitting applications.

Prominence of the Instability of a Stabilizing Agent in the Changes in Physical State of a Hybrid Nanomaterial

Shaping ability of hybrid nanomaterials is a key point for their further use in devices. It is therefore crucial to control it. To this end, it is necessary that the macroscopic properties of the material remain constant over time. Here, we evidence by multinuclear Magic-Angle Spinning Nuclear Magnetic Resonance spectroscopic study including ¹⁷ O isotope exchange that for a ZnO-alkylamine hybrid material, the partial carbonation of amine into ammonium carbamate molecules is behind the conversion from highly viscous liquid to a powdery solid when exposed to air. This carbonation induces modification and reorganization of the organic shell around the nanocrystals and affects significantly the macroscopic properties of the material such as it physical state, its solubility and colloidal stability. This study, straightforwardly extendable, highlights that the nature of the functional chemical group allowing connecting the stabilizing agent (SA) to the surface of the nanoparticles is of tremendous importance especially if the SA is reactive with molecules present in the environment.

Alloyed Crystalline CdSe1-xSx Semiconductive Nanomaterials - A Solid State 113Cd NMR Study

Solid‐state NMR analysis on wurtzite alloyed CdSe_1−xS_x crystalline nanoparticles and nanobelts provides evidence that the ¹¹³Cd NMR chemical shift is not affected by the varying sizes of nanoparticles, but is sensitive to the S/Se anion molar ratios. A linear correlation is observed between ¹¹³Cd NMR chemical shifts and the sulfur component for the alloyed CdSe_1−xS_x (0<x<1) system both in nanoparticles and nanobelts (δ_Cd=169.71⋅X_S+529.21). Based on this correlation, a rapid and applied approach has been developed to determine the composition of the alloyed nanoscalar materials utilizing ¹¹³Cd NMR spectroscopy. The observed results from this system confirm that one can use ¹¹³Cd NMR spectroscopy not only to determine the composition but also the phase separation of nanomaterial semiconductors without destruction of the sample structures. In addition, some observed correlations are discussed in detail.

Combining fast magic angle spinning dynamic nuclear polarization with indirect detection to further enhance the sensitivity of solid-state NMR spectroscopy

Dynamic nuclear polarization (DNP) and indirect detection are two commonly applied approaches for enhancing the sensitivity of solid-state NMR spectroscopy. However, their use in tandem has not yet been investigated. With the advent of low-temperature fast magic angle spinning (MAS) probes with 1.3-mm diameter rotors capable of MAS at 40 ​kHz it becomes feasible to combine these two techniques. In this study, we performed DNP-enhanced 2D indirectly detected heteronuclear correlation (idHETCOR) experiments on ¹³C, ¹⁵N, ¹¹³Cd and ⁸⁹Y nuclei in functionalized mesoporous silica, CdS nanoparticles, and Y₂O₃ nanoparticles. The sensitivity of the 2D idHETCOR experiments was compared with those of DNP-enhanced directly-detected 1D cross polarization (CP) and 2D HETCOR experiments performed with a standard 3.2-mm rotor. Due to low CP polarization transfer efficiencies and large proton linewidth, the sensitivity gains achieved by indirect detection alone were lower than in conventional (non-DNP) experiments. Nevertheless, despite the smaller sample volume the 2D idHETCOR experiments showed better absolute sensitivities than 2D HETCOR experiments for nuclei with the lowest gyromagnetic ratios. For ⁸⁹Y, 2D idHETCOR provided 8.2 times better sensitivity than the 1 D⁸⁹Y-detected CP experiment performed with a 3.2-mm rotor.

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono- and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N-O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin-spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.

Stable Hexylphosphonate-Capped Blue-Emitting Quantum-Confined CsPbBr3 Nanoplatelets

Quantum-confined CsPbBr₃ nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C₆H₁₅O₃P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr₃ NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm². ¹³C, ¹³³Cs, and ³¹P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand.

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate

Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H₂O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO₂(OH)₄]^5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced ²⁷Al and ²⁹Si solid-state NMR experiments. We notably assign a narrow ²⁷Al NMR signal at 5 ppm to the silicate-bridging [AlO₂(OH)₄]^5- sites and show that this signal correlates to ²⁹Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by ²⁷Al NMR in C-A-S-H samples.

Solid-state nuclear magnetic resonance studies of nanoparticles

In this trends article, we review seminal and recent studies using static and magic-angle spinning solid-state NMR to study the structure of nanoparticles and ligands attached to nanoparticles. Solid-state NMR techniques including one-dimensional multinuclear NMR, cross-polarization, techniques for measuring dipolar coupling and internuclear distances, and multidimensional NMR have provided insight into the core-shell structure of nanoparticles as well as the structure of ligands on the nanoparticle surface. Hyperpolarization techniques, in particular solid-state dynamic nuclear polarization (DNP), have enabled detailed studies of nanoparticle core-shell structure and surface chemistry, by allowing unprecedented levels of sensitivity to be achieved. The high signal-to-noise afforded by DNP has allowed homonuclear and heteronuclear correlation experiments involving nuclei with low natural abundance to be performed in reasonable experimental times, which previously would not have been possible. The use of DNP to study nanoparticles and their applications will be a fruitful area of study in the coming years as well.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the ^111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.