We have measured the 75As signals arising from the interface region of single-crystal semi-insulating GaAs that has been coated and passivated with an aluminum oxide film deposited by atomic layer deposition (ALD) with optically pumped NMR (OPNMR). Using wavelength-selective optical pumping, the laser restricts the volume from which OPNMR signals are collected. Here, OPNMR signals were obtained from the interface region and distinguished from signals arising from the bulk. The interface region is highlighted by interactions that disrupt the cubic symmetry of the GaAs lattice, resulting in quadrupolar satellites for nuclear isotopes, whereas NMR of the “bulk” lattice is nominally unsplit. Quadrupolar splitting at the interface arises from strain based on lattice mismatch between the GaAs and ALD-deposited aluminum oxide due to their different coefficients of thermal expansion. Such spectroscopic evidence of strain can be useful for measuring lattice distortions at heterojunction boundaries and interfaces.

•NMR apparatus capable of elevated-temperature and -pressure under flow.•Detailed descriptions of components are given for ease of reproducibility.•In-situ data is shown of a reaction of forsterite under flow of CO2 (aq).We report an apparatus for in-situ nuclear magnetic resonance (NMR) studies of chemical reactions of dissolved 13CO2 with minerals (rock or powder) under continuous flow. The operating range of the apparatus is 18–150 °C and 1–140 bar. A flow pump is used to circulate a CO2-water solution, with a heated mixing vessel where CO2 gas equilibrates with a water solution. The NMR probe is built around a strong zirconia ceramic vessel, with o-ring sealed connections; the mineral is contained inside. The horizontal orientation of the zirconia vessels allows use of a radio frequency (rf) solenoid for improved spin sensitivity.Download high-res image (73KB)Download full-size image

•Experimental apparatus for optically-pumped NMR (OPNMR) and optically-detected NMR (ODNMR).•NMR probes for horizontal bore magnets; easier optical alignment onto the sample.•First evidence of a He recirculating cryostat used horizontally, maintaining <6 K.•We demonstrate 2-channel OPNMR on CdTe and ODNMR on AlGaAs/GaAs quantum wells.•Magneto-optical capability: photoluminescence (PL), PLE, polarized PL detection.Instrumentation for optically-pumped and optically-detected nuclear magnetic resonance (OPNMR and ODNMR) has been developed and implemented as a single experimental apparatus to study semiconductors such as GaAs and CdTe. These two measurement schemes use many of the same components for experiments. Here we describe, in two parts, the apparatus that can record such measurements and give examples of representative data. In Part 1, the radio-frequency probe and low-temperature cryostat are described, including single-channel and two-channel static cryogenic probes that both incorporate a modified solenoid coil that permits better optical access. In Part 2, the optical bench is described in detail, which uses a set of experiments (magneto-photoluminescence, photoluminescence excitation, detection of polarized photoluminescence) as important input for ODNMR. We are able to portray a robust design that encompasses multiple measurement modalities, along with the ability to change many experimental parameters with ease.Download high-res image (133KB)Download full-size image

•We have modeled OPNMR phenomena in two single-crystal semiconductors: GaAs, InP.•The model adds a revised expression for 〈Sz〉 to our earlier “penetration depth” model.•The photon energy-dependent 〈Sz〉 and penetration depth models both InP and GaAs well.•Theoretical optical transitions are shown with utility for future OPNMR studies.We have modified the model for optically-pumped NMR (OPNMR) to incorporate a revised expression for the expectation value of the z  -projection of the electron spin, 〈Sz〉〈Sz〉 and apply this model to both bulk GaAs and a new material, InP. This expression includes the photon energy dependence of the electron polarization when optically pumping direct-gap semiconductors in excess of the bandgap energy, EgEg. Rather than using a fixed value arising from coefficients (the matrix elements) for the optical transitions at the k=0k=0 bandedge, we define a new parameter, Sopt(Eph)Sopt(Eph). Incorporating this revised element into the expression for 〈Sz〉〈Sz〉, we have simulated the photon energy dependence of the OPNMR signals from bulk semi-insulating GaAs and semi-insulating InP. In earlier work, we matched calculations of electron spin polarization (alone) to features in a plot of OPNMR signal intensity versus photon energy for optical pumping (Ramaswamy et al., 2010). By incorporating an electron spin polarization which varies with pump wavelength into the penetration depth model of OPNMR signal, we are able to model features in both III-V semiconductors. The agreement between the OPNMR data and the corresponding model demonstrates that fluctuations in the OPNMR intensity have particular sensitivity to light hole-to-conduction band transitions in bulk systems. We provide detailed plots of the theoretical predictions for optical pumping transition probabilities with circularly-polarized light for both helicities of light, broken down into illustrative plots of optical magnetoabsorption and spin polarization, shown separately for heavy-hole and light-hole transitions. These plots serve as an effective roadmap of transitions, which are helpful to other researchers investigating optical pumping effects.

Highlights•Solid-state 71Ga NMR of hydroxo-aquo metal clusters and the impurities present.•High-field NMR capability allows for quadrupolar species, such as 71Ga, to be routinely studied.•Efficient and environmentally friendly synthetic routes have been developed to prepare hydroxo-aquo metal clusters.Solid-state 71Ga NMR was used to characterize a series of [Ga13(μ3-OH)6(μ2-OH)18(H2O)](NO3)15 “Ga13” molecular clusters synthesized by multiple methods. These molecular clusters are precursors to thin film electronics and may be employed in energy applications. The synthetic routes provide varying levels of impurities in the solid phase, and these impurities often elude traditional characterization techniques such as powder X-ray diffraction and Raman spectroscopy. Solid-state NMR can provide a window into the gallium species even in amorphous phases. This information is vital in order to prevent the impurities from causing defect sites in the corresponding thin films upon gelation and condensation (polymerization) of the Ga13 clusters. This work demonstrates the resolving power of solid-state NMR to evaluate structure and synthetic quality in the solid state, and the application of high-field NMR to study quadrupolar species, such as 71Ga.Graphical abstractThe various synthetic routes and 71Ga solid-state NMR spectra of the nanoscale inorganic cluster [Ga13(μ3-OH)6(μ2-OH)18(H2O)](NO3)15.

In the conversion of CO2 to mineral carbonates for the permanent geosequestration of CO2, there are multiple magnesium carbonate phases that are potential reaction products. Solid-state 13C NMR is demonstrated as an effective tool for distinguishing magnesium carbonate phases and quantitatively characterizing magnesium carbonate mixtures. Several of these mineral phases include magnesite, hydromagnesite, dypingite, and nesquehonite, which differ in composition by the number of waters of hydration or the number of crystallographic hydroxyl groups. These carbonates often form in mixtures with nearly overlapping 13C NMR resonances which makes their identification and analysis difficult. In this study, these phases have been investigated with solid-state 13C NMR spectroscopy, including both static and magic-angle spinning (MAS) experiments. Static spectra yield chemical shift anisotropy (CSA) lineshapes that are indicative of the site-symmetry variations of the carbon environments. MAS spectra yield isotropic chemical shifts for each crystallographically inequivalent carbon and spin–lattice relaxation times, T1, yield characteristic information that assist in species discrimination. These detailed parameters, and the combination of static and MAS analyses, can aid investigations of mixed carbonates by 13C NMR.

Hyperbranched amine polymers (HAS) grown from the mesoporous silica SBA-15 (hereafter “SBA-15–HAS”) exhibit large capacities for CO2 adsorption. We have used static in situ and magic-angle spinning (MAS) ex situ 13C nuclear magnetic resonance (NMR) to examine the adsorption of CO2 by SBA-15–HAS. 13C NMR distinguishes the signal of gas-phase 13CO2 from that of the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines, leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO–), carbamic acid (RnNCOOH), and bicarbonate (HCO3–) moieties. Carbamates and bicarbonate fall within a small 13C chemical shift range (162–166 ppm), and a mixture was observed including carbamic acid and carbamate, the former disappearing upon evacuation of the sample. By examining the 13C–14N dipolar coupling through low-field (B0 = 3 T) 13C{1H} cross-polarization MAS NMR, carbamate is confirmed through splitting of the 13C resonance. A third species that is either bicarbonate or a second carbamate is evident from bimodal T2 decay times of the ∼163 ppm peak, indicating the presence of two species comprising that single resonance. The mixture of products suggests that (1) the presence of amines and water leads to bicarbonate being present and/or (2) the multiple types of amine sites in HAS permit formation of chemically distinct carbamates.

Solid-state 71Ga NMR was used to investigate the structures of several heterometallic Group 13 hydroxo-aquo clusters, [Ga13−xInx (μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 which are envisioned for thin film transistors. The characterization of these clusters in the solid state provides additional information in understanding the synthesis, structure and speciation of these precursors for high-quality, ultrasmooth thin films. Yet important structural information regarding these clusters – including the exact composition, isomeric structure, and coordination environments – were unknown prior to this precise NMR spectroscopy study. These molecular species, termed “Ga13−xInx”, contain three types of six-coordinate metal sites, with bridging OH− groups and H2O as capping ligands, and we report results on Ga7In6, Ga8In5, Ga10In3, Ga11In2, Ga12In1, and Ga13. Utilizing two magnetic fields (13.9 T and 21.1 T), the solid-state NMR spectra were interpreted in conjunction with computational modeling (using CASTEP) and simulation of spectral lineshapes (using Dmfit). The metal sites are best represented as distorted octahedra, and they exhibit a range of quadrupolar couplings and asymmetry parameters, which can be addressed using longitudinal strain analysis. Until now, there has been speculation about the sites for transmetallation within the synthetic cluster community. Here, we show that Ga NMR is a powerful technique to monitor the transmetallation of In for Ga in the Ga13−xInx clusters, specifically substituting in the “outer ring” sites, and not the “core” or “middle ring”.

This Perspective article highlights some of the traditional and non-traditional analytical tools that are presently used to characterize aqueous inorganic nanoscale clusters and polyoxometalate ions. The techniques discussed in this article include nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), dynamic and phase analysis light scattering (DLS and PALS), Raman spectroscopy, and quantum mechanical computations (QMC). For each method we briefly describe how it functions and illustrate how these techniques are used to study cluster species in the solid state and in solution through several representative case studies. In addition to highlighting the utility of these techniques, we also discuss limitations of each approach and measures that can be applied to circumvent such limits as it pertains to aqueous inorganic cluster characterization.

Solid-state 71Ga and 69Ga NMR was used to probe the structure of the hydroxo-aquo cluster, [Ga13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15, envisioned as a solution-processable material for thin film electronics. This species, termed Ga13, is made up of three types of 6-coordinate gallium sites, with bridging OH groups and H2O species decorating the outer edges. Solid-state NMR at two magnetic fields (13.9 and 21.1 T) of these quadrupolar nuclei, in conjunction with modeling, demonstrates that these sites are best represented as distorted octahedra, exhibiting a wide range of distinct quadrupolar couplings and asymmetry parameters. This information is critical for analyses of the local coordination environment for gallium in related gallium-oxide films, and this work adds to the growing body of evidence that gallium solid-state NMR is a useful tool for structural analyses.

•Light-hole-to-conduction band transition is uncovered by OPNMR and Hanle curves.•Phase inversion of signals indicates spin orientation of conduction electrons.•B0 field dependence of quantum well transitions shows Zeeman splitting of states.•Zero-field value for a GaAs/AlGaAs light-hole state can be extracted.•Spin splitting term for the transition energy yields the excitonic g-factor.Optically-pumped 69Ga NMR (OPNMR) and optically-detected measurements of polarized photoluminescence (Hanle curves) show a characteristic feature at the light hole-to-conduction band transition in a GaAs/AlxGa1−xAs multiple quantum well sample. OPNMR data are often depicted as a “profile” of the OPNMR integrated signal intensity plotted versus optical pumping photon energy. What is notable is the inversion of the sign of the measured 69Ga OPNMR signals when optically pumping this light hole-to-conduction band energy in OPNMR profiles at multiple external magnetic fields (B0 = 4.7 T and 3 T) for both σ+ and σ− irradiation. Measurements of Hanle curves at B0 = 0.5 T of the same sample exhibit similar phase inversion behavior of the Hanle curves at the photon energy for light hole excitation. The zero-field value of the light-hole state in the quantum well can be predicted for the quantum well structure using the positions of each of these signal-inversion features, and the spin splitting term in the equation for the transition energy yields consistent values at 3 magnetic fields for the excitonic g-factor (gex). This study demonstrates the application of OPNMR and optical measurements of the photoluminescence to detect the light hole transition in semiconductors.

We explore a new in situ NMR spectroscopy method that possesses the ability to monitor the chemical evolution of supercritical CO2 in relevant conditions for geological CO2 sequestration. As a model, we use the fast reaction of the mineral brucite, Mg(OH)2, with supercritical CO2 (88 bar) in aqueous conditions at 80 °C. The in situ conversion of CO2 into metastable and stable carbonates is observed throughout the reaction. After more than 58 h of reaction, the sample was depressurized and analyzed using in situ Raman spectroscopy, where the laser was focused on the undisturbed products through the glass reaction tube. Postreaction, ex situ analysis was performed on the extracted and dried products using Raman spectroscopy, powder X-ray diffraction, and magic-angle spinning 1H-decoupled 13C NMR. These separate methods of analysis confirmed a spatial dependence of products, possibly caused by a gradient of reactant availability, pH, and/or a reaction mechanism that involves first forming hydroxy-hydrated (basic, hydrated) carbonates that convert to the end-product, anhydrous magnesite. This carbonation reaction illustrates the importance of static (unmixed) reaction systems at sequestration-like conditions.

A single crystal of α-trans-cinnamic acid was synthesized with a 13C-label at the β-carbon position and photoreacted to yield the [2 + 2] cycloaddition product, α-truxillic acid. 13C{1H} cross-polarization (CP) single-crystal NMR experiments were performed on the unreacted and sequentially photoreacted samples for different goniometer orientations, and the spectra were simulated using the SIMMOL and SIMPSON software packages. Atomic coordinates from single-crystal X-ray diffraction data were used as inputs in the simulations, which allowed the chemical shift tensor to be precisely measured and related to the unit cell (or molecular) reference frame of cinnamic acid. The line widths of the 13C resonances observed at different goniometer rotations were utilized to estimate the orientational dispersion of the cinnamic acid species, which ultimately provides a measure of disorder in the single crystal. The photoreacted sample, a solid solution of cinnamic and truxillic acids, maintained its single-crystal nature, even up to 44% conversion to truxillic acid, keeping its P21/n symmetry. Upon photoirradiation, however, a slight loss of order was observed in the cinnamic acid species as evidenced by an increase in the 13C NMR line widths, demonstrating that NMR can be used to monitor subtle orientational imperfections in single crystal to single crystal photoreactions.

We have detected a phase transition during the progress of the solid-state [2 + 2] photocycloaddition reaction of α-trans-cinnamic acid. The reaction was monitored using 13C CPMAS experiments as a function of irradiation time of the parent α-trans-cinnamic acid, which forms the product dimer, α-truxillic acid. UV light centered at 350 nm was used for photoirradiation, which is in the “tail” of the absorption band of cinnamic acid. Two different crystal polymorphs of α-truxillic acid are observed (P21/n and C2/c) at different stages of conversion of the parent crystal, assigned through 13C NMR and powder X-ray diffraction. The two polymorphs showed clear, distinguishable patterns in the 13C NMR spectra: a 2-peak versus 3-peak pattern corresponding to sites on the 4-membered sp3 hybridized ring in the photoproduct. A phase transition is observed midway through the reaction, which we have assigned to the conversion of the P21/n polymorph to the C2/c polymorph of α-truxillic acid. The packing energy of the resultant mixed crystal of cinnamic acid and truxillic acid changes during the course of the photoreaction, which allows for the C2/c polymorph of truxillic acid to appear. Both phases have been confirmed via X-ray powder diffraction. Two techniques—differential scanning calorimetry and solid-state CPMAS NMR using increasingly fast rotational frequencies—demonstrate that the P21/n phase is metastable.

Formation of three Cd(II)–ethylenediamine (en) complexes ([Cd(en)n]2+, n = 1–3) in aqueous solution and in DMSO solvent has been established by means of 113Cd NMR spectroscopy. It is clearly shown that Cd(II)–en complexes form primarily in basic solutions. A correlation between the 113Cd NMR chemical shifts and the ethylenediamine (en) coordination number has been observed and discussed. Two single crystals with the composition [Cd2(en)5](ClO4)4 (1) and [Cd(en)3](ClO4)2 (2) were prepared from aqueous solution, and their structures were determined by single crystal X-ray diffraction. Cd(II) ions are coordinated by six atoms in both compounds, 1 and 2: via five N-donor atoms and one O-donor atom forming a bimetallic complex 1, and via six N-donor atoms forming a distorted octahedral monometallic complex 2. Raman spectra of complexes 1 and 2 also provide additional evidence that the cis-form of the bridging en is present in complex 1.The formation of the above Cd(II)–ethylenediamine (en) species in aqueous solution has been determined by 113Cd NMR spectroscopy. In addition, two single crystal structures of [Cd2(en)5](ClO4)4 and [Cd(en)3](ClO4)2 were determined by single crystal X-ray diffraction. Based on both single crystal X-ray diffraction and Raman spectra, we observed that the cis-conformation of the bridging en is present in the complex, [Cd2(en)5](ClO4)4.

An in-depth study on the etching process for producing carbide-derived carbons from Al4C3 has been performed. These materials were investigated at a range of etching temperatures from 300 to 900 °C and a range of times from 15 min to 6 h. By altering the etching time and temperature, the surface area, residual aluminum content, and pore size distribution can be tuned. A maximum surface area of 1126 m2 g−1 was observed for materials etched at 500 °C for 1 h. The pore size has shown to be tunable from ≤0.7 to 8 nm. Interestingly, aluminum-based nanoparticles were observed via TEM and SEM for partially etched samples, with evidence of tunable metal species on the surface of the Al4C3-CDC samples at different etching temperatures between 300 and 700 °C. Characterization of the aluminum species present over this temperature range took place using solid-state 27Al NMR. The formation of crystalline α-Al2O3 was observed at etching temperatures of 700 °C. The results of this work provide detailed synthesis strategies for controlling not only the porosity and surface area of a carbide-derived carbon, but also the extent and type of residual metal nanoparticles embedded in the final structure.

This Perspective article highlights some of the traditional and non-traditional analytical tools that are presently used to characterize aqueous inorganic nanoscale clusters and polyoxometalate ions. The techniques discussed in this article include nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), dynamic and phase analysis light scattering (DLS and PALS), Raman spectroscopy, and quantum mechanical computations (QMC). For each method we briefly describe how it functions and illustrate how these techniques are used to study cluster species in the solid state and in solution through several representative case studies. In addition to highlighting the utility of these techniques, we also discuss limitations of each approach and measures that can be applied to circumvent such limits as it pertains to aqueous inorganic cluster characterization.

Solid-state 71Ga NMR was used to investigate the structures of several heterometallic Group 13 hydroxo-aquo clusters, [Ga13−xInx (μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 which are envisioned for thin film transistors. The characterization of these clusters in the solid state provides additional information in understanding the synthesis, structure and speciation of these precursors for high-quality, ultrasmooth thin films. Yet important structural information regarding these clusters – including the exact composition, isomeric structure, and coordination environments – were unknown prior to this precise NMR spectroscopy study. These molecular species, termed “Ga13−xInx”, contain three types of six-coordinate metal sites, with bridging OH− groups and H2O as capping ligands, and we report results on Ga7In6, Ga8In5, Ga10In3, Ga11In2, Ga12In1, and Ga13. Utilizing two magnetic fields (13.9 T and 21.1 T), the solid-state NMR spectra were interpreted in conjunction with computational modeling (using CASTEP) and simulation of spectral lineshapes (using Dmfit). The metal sites are best represented as distorted octahedra, and they exhibit a range of quadrupolar couplings and asymmetry parameters, which can be addressed using longitudinal strain analysis. Until now, there has been speculation about the sites for transmetallation within the synthetic cluster community. Here, we show that Ga NMR is a powerful technique to monitor the transmetallation of In for Ga in the Ga13−xInx clusters, specifically substituting in the “outer ring” sites, and not the “core” or “middle ring”.