Thermally-Induced Evolution of 'Ge(OH)2': Controlling the Formation of

Jul 6, 2018 - Germanium nanocrystals (GeNCs) hold great promise as active materials in many applications including solar cells, biological imaging, Br...
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Cite This: J. Phys. Chem. C 2018, 122, 17518−17525

Thermally Induced Evolution of “Ge(OH)2”: Controlling the Formation of Oxide-Embedded Ge Nanocrystals Morteza Javadi, Vladimir K. Michaelis, and Jonathan G. C. Veinot* Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta T6G 2G2, Canada

J. Phys. Chem. C 2018.122:17518-17525. Downloaded from pubs.acs.org by UNIV OF TOLEDO on 09/29/18. For personal use only.

S Supporting Information *

ABSTRACT: Germanium nanocrystals (GeNCs) hold great promise as active materials in many applications including solar cells, biological imaging, Bragg reflectors, light-emitting diodes, and nonvolatile memory devices. However, because of the size-, shape-, and composition-dependent nature of their properties, it is essential that methods affording high-purity, well-defined GeNCs be developed. Herein, we report a systematic investigation of a series of “Ge(OH)2” precursors using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and solid-state 73Ge NMR and correlate their properties with the GeNCs obtained from their thermal processing. This study provides important insights into the role of the Ge(OH)2 internal structure and will allow for future rational design and practical application of GeNCs.

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INTRODUCTION As silicon’s heavier cousin, germanium is often overlooked. Although some similarities exist between these two periodic congeners (e.g., both are indirect-band gap semiconductors with similar chemical reactivity, crystal structure, etc.), key differences make germanium an attractive functional material for numerous applications. In light of its unique (bulk) properties including a small band gap (0.67 eV at 300 K),1 large Bohr exciton radius (24.3 nm),2,3 and high electron and hole mobilities (∼3900 cm2/V·s)4 as well as large capacity for and diffusivity of ions (e.g., Li+), germanium has the potential to impact many far-reaching applications in semiconductor industries.5 Preparing Ge on the nanoscale adds an additional degree of freedom that offers further property tailoring because of the impacts of surface area and quantum confinement.6,7 To date, numerous prototype applications of germanium quantum dots (or GeNCs) have appeared, including solar cells,8 biological imaging,9,10 Bragg reflectors,8 light-emitting diodes, and nonvolatile memory devices.11−15 If these and heretofore unknown applications of GeNCs are to be realized, methods for preparing well-defined NCs must be established and their material properties must be understood. To date, GeNCs have been prepared using a variety of methods including top-down procedures such as laser ablation16 and ball milling17 as well as bottom-up techniques such as plasma pyrolysis of GeH4,18 metathesis of Ge Zintl salts,6,7,19,20 solution-phase reduction of Ge(II) and Ge(IV) compounds,21−23 and thermal decomposition of organogermane precursors.24−26 Although these methods provide access to GeNCs, each method brings with it its own unique challenges, which may include limited product yield, size polydispersity, ill-defined shape, material purity, and complex surface chemistry. © 2018 American Chemical Society

A series of sub-stoichiometric metal oxides (RMOx, 0 < x < 2, M: Si or Ge, and R: phenyl, ethyl, n-butyl, allyl, benzyl, carboxyethyl, and t-butyl) have been prepared via sol−gel processing of appropriate precursors. Thermal processing of these sol−gel derived products provides a convenient method for preparing oxide-embedded NCs that can be used directly as a NC/metal oxide composite or liberated and employed as freestanding NCs.27−31 This general approach is particularly appealing because the metal oxide precursor often provides a high purity source of the constituents of the target nanomaterial (e.g., Si, Ge, etc.) as well as a matrix (“solvent”) in which the NCs form and growthis combination provides a system that allows effective separation of NC nucleation and growth leading to narrow size distributions as well as controlled particle size. We and others have exploited metal suboxide thermal processing extensively to prepare size-tunable silicon NCs with narrow size distributions.27,28,32,33 While thermal processing of organic functionalized germanium-rich oxides [(RGeO1.5)n] has provided a means of obtaining oxide- and oxycarbide-embedded GeNCs,28,30,31 these materials were not ideal because of the possibility of contamination as well as issues with NC liberation. Until recently, a suitable precursor containing only germanium, oxygen, and hydrogen had not been reported. However, independently, our group and the Ozin team published procedures in which oxide-embedded GeNCs were prepared via thermal processing of “Ge(OH)2”.34,35 Despite the promising results, the NCs reported in these early reports are not idealthey often exhibited ill-defined shapes, wide Received: May 15, 2018 Revised: July 5, 2018 Published: July 6, 2018 17518

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stored under ambient atmosphere for characterization. Typical yields were >0.4 g. Isolation of Hydride-Terminated GeNCs. Hydrideterminated GeNCs (H-GeNCs) were liberated from the composites noted above upon etching in alcoholic HF solutions using a variant of a procedure developed in our laboratories.35 Briefly, the finely ground thermally processed product of choice (∼50 mg) was magnetically stirred in 1.0 mL of a 1:1 solution of ethanol and water for ca. 10 min in a PET beaker. Aqueous HF (49%; 0.5 mL) was added dropwise. (Caution: HF must be handled with care and in accordance with local guidelines.) The mixture was stirred for 15 min. Subsequently, toluene (3 × 5 mL) was added to extract the GeNCs. The cloudy toluene layer was removed via a pipette, and a black precipitate was isolated upon centrifuging at 3000 rpm. This precipitate was dispersed in toluene containing activated molecular sieves (ca. 1 g, 4 Å) and agitated for approximately 1 min. The suspension was transferred to centrifuge tubes, and the black precipitate (i.e., H-GeNCs) was recovered upon centrifuging at 3000 rpm and immediately functionalized as outlined below. The typical yield of HGeNCs was ca. 10 mg. (Note: this yield is dependent upon the nature of the Ge(OH)2 precursor used to prepare the GeNC/ GeO2 composite.) Preparation of Dodecyl-GeNC. The black product (i.e., H-GeNCs) obtained from the etching procedure was dispersed in dodecene (5.0 mL) and deoxygenated using an argon charged double manifold via three freeze−pump−thaw cycles. Subsequently, the mixture was heated to 190 °C with stirring in an argon atmosphere for 12 h. The resulting transparent orange-brown suspensions of crude dodecyl-terminated GeNCs (dodecyl-GeNCs) were purified via precipitation/ resuspension cycles as described previously.35 The resulting purified dodecyl-GeNCs were dispersed in ca. 5 mL of dry toluene and filtered through a 0.45 μm PTFE syringe filter to yield a clear brown suspension. A typical yield for dodecylfunctionalized GeNCs was ca. 15 mg from 50 mg of GeNC/ GeO2 composite. The dodecyl-GeNCs were stored as a toluene suspension until needed for material characterization. Material Characterization and Instrumentation. Fourier-transform infrared (FTIR) spectroscopy was performed using a Nicolet Magna 750 IR spectrophotometer. Samples were drop-cast from a toluene suspension containing GeNCs. X-ray powder diffraction was performed using an INEL XRG 3000 X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.54 Å). The crystallinity of all samples was evaluated for finely ground powders mounted on a lowintensity background Si(100) holder. Bright-field transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analyses were performed using a JEOL-2010 (LaB6 filament) electron microscope with an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) imaging was performed on a JEOL-2200FS TEM instrument with an accelerating voltage of 200 kV. TEM samples of GeNCs were prepared by drop-coating 1−3 drops of the toluene suspension (ca. 1 mg/mL) containing NCs of choice onto a holey carboncoated copper grid (300 mesh, Electron Microscopy Science), and the solvent was removed under vacuum. TEM and HRTEM images were processed using ImageJ software (version 1.48 v). Particle size distributions (PSDs) were determined by measuring ca. 300 NCs. Raman spectroscopy was performed using a Renishaw inVia Raman microscope equipped with a 514 nm diode laser operated at a power of

polydispersities, and were even sintered. In this context, we have turned our attention to investigating the nature of Ge(OH)2 because its reported structure and composition remain controversial, to gain greater insights into the processes leading to GeNC formation.35 Herein, we report a systematic study of three Ge(OH)2 variants [i.e., yellow, orange, and brown Ge(OH)2] obtained from established literature procedures that employed X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrational spectroscopy (infrared and Raman), and 73Ge solid-state nuclear magnetic resonance (NMR) spectroscopy to elucidate the Ge(OH)2 composition and internal structure. Once identified, the Ge(OH)2 variants were each evaluated as GeNC precursors, and a mechanism for NC formation was proposed that will facilitate further advancement of GeNC design and application.

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EXPERIMENTAL SECTION Reagents and Materials. Germanium dioxide powder (GeO2, 99.9%) was purchased from Gelest. Hypophosphorous acid solution (50 wt % in H2O), sodium hydroxide pellets, 1dodecene (97%), and reagent-grade methanol, toluene, and ethanol were purchased from Sigma-Aldrich. Electronics-grade hydrofluoric acid (HF, 49% aqueous solution) was purchased from J. T. Baker. Hydrochloric acid (36.5−38%) and ammonium hydroxide (28−30%) were purchased from Caledon Laboratories. Ultrapure H2O (18.2 MΩ/cm) purified in a Barnstead Nanopure Diamond purification system was used in all reactions. Synthesis of Germanium(II) Hydroxide (Ge(OH)2). Ge(OH)2 was prepared using a modified literature procedure.35−41 Commercial germanium dioxide powder was dissolved in freshly prepared aqueous NaOH (∼17 M, 7.0 mL). Subsequently, aqueous HCl (6 M; 24 mL) was added slowly to cause a white precipitate to form and then redissolved to provide a solution of Ge(IV) ions to which an aqueous solution of H3PO2 (50%, 7.5 mL) was added. This colorless solution was refluxed under static Ar for 5.5 h and then cooled to room temperature in an ice-salt (NaCl) bath. After cooling to below 5 °C, concentrated aqueous NH4OH solution (10 mL) was added dropwise to yield a yellow precipitate of Ge(OH)2. (Caution: this reaction is extremely exothermic; the temperature must be maintained at ca. 5 °C.) The yellow precipitate was recovered by vacuum filtration, washed with deionized/deoxygenated water (3 × 100 mL), dried in vacuo for 12 h, and stored in an argon-filled glovebox for future use and analysis. Orange and brown versions of Ge(OH)2 (referred to as “orange Ge(OH)2” and “brown Ge(OH)2”, respectively) were prepared by maintaining the temperature of the reaction mixture at 50 and 100 °C, respectively, during the addition of aqueous NH4OH. The orange and brown Ge(OH)2 were isolated, purified, and stored as described above for yellow Ge(OH)2. Thermal Processing of Ge(OH)2. Thermal processing of four varieties of Ge(OH)2 was performed using a Lindberg/ Blue tube furnace in a flowing argon atmosphere (15 mL/ min). The Ge(OH)2 (ca. 0.5 g) of choice was placed in a quartz boat housed in a furnace and heated to the desired processing temperature (i.e., 300, 350, 400, 450, or 500 °C) at 20 °C/min, where it remained for 1 h. The product (i.e., a germanium oxide composite of GeNCs) obtained from the thermal processing was cooled to room temperature, mechanically ground using an agate mortar and pestle, and 17519

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The Journal of Physical Chemistry C 3.98 mW on the sample. Samples were prepared by mounting the suspension on the gold-coated glass. X-ray photoelectron spectra were acquired in the energy spectrum mode at 210 W, using a Kratos AXIS Ultra X-ray photoelectron spectrometer. The X-ray source was Al (mono) Kα line (1486.6 eV) with a probing area of 1 × 2 mm2. Samples were prepared as films drop-cast from a toluene solution/suspension onto a copper foil substrate. Binding energies were calibrated using the C 1s emission as an internal reference (284.8 eV). The CasaXPS version 2.3.5 software was used to accomplish the Shirley-type background subtraction. The high-resolution Ge 3d region of the spectra was collected for all samples and was fit to Ge 3d3/2/Ge 3d5/2 partner lines, with spin−orbit splitting fixed at 0.6 eV, and the Ge 3d3/2/Ge 3d5/2 intensity ratio was set to 0.67.31 Solid-State NMR Spectroscopy. Germanium-73 NMR spectra were acquired at 21.1 T on a Bruker AVANCE II 900 NMR spectrometer. Nonspinning 73Ge NMR spectra were collected using a home-built 7 mm H/X probe with a two-coil design. Magic-angle spinning (MAS) spectra were acquired using a specially designed low-ringing 7 mm single resonance ultralow-gamma Bruker probe. The magic angle was set using the Hall sensor method42 with a spinning frequency (ωr/2π) of 3 kHz. The air-sensitive samples were handled in an Ar-filled glovebox and packed into thin-walled 7 mm (o.d., 400 μL fill volume) ZrO2 Bruker rotors and Vespel caps equipped with Oring seals. All spectra were acquired using either a Bloch pulse (MAS) or solid echo (nonspinning) with short interpulse delays of 20 μs and 73Ge (γB1/2π) of 10 kHz (solution, I = 9/ 2; solid π/2 = 4 μs). All spectra were acquired with a recycle delay of 5 s and between 512 and 5632 coadded transients. All 73 Ge excitation pulse spectra were referenced externally with respect to GeCl4 (73Ge, δiso = 0.0 ppm)43 by setting the 73Ge peak of GeCl4 to 30.9 ppm All experiments were acquired at room temperature and under dry nitrogen purge gas. All spectra were processed using TopSpin 3.5 Bruker software with between 25 and 100 Hz exponential apodization. Sensitivity enhancing experimental methods including CPMG and WURST-CPMG and longer recycle delays were also attempted during the characterization of these solids. We note that the 73Ge NMR nutation behavior of microcrystalline Ge metal (Fm3̅m), δiso = −73.8 ppm, is nearly identical to that of the liquid reference (GeCl4, δiso = 30.9 ppm).

Figure 1. Photographs of (a) yellow, (b) orange, and (c) brown Ge(OH)2. (d) XRD patterns obtained from (i) yellow, (ii) orange, and (iii) brown Ge(OH)2. Standard reflections of bulk Ge (PDF#040545) and rutile GeO2 are provided for comparison. (e) Raman spectra obtained from (i) yellow, (ii) orange, and (iii) brown Ge(OH)2.

orange, and brown Ge(OH)2. In agreement with literature reports, none of the Ge(OH)2 variants show well-defined reflections expected for crystalline materials.37,41 In all cases, the XRD patterns show broad features at ca. 2θ = 25−35° and 45−50° (Figure 1d). While similar features have been attributed to the presence of amorphous Ge,22 the complexity of the present systems makes any definitive assignment premature. Sharp (albeit weak) reflections are also noted at 2θ = 28.7, 37.4, 56.7, 59.3, and 72.3°, which are confidently assigned to rutile GeO2 [P42/mnm (136)].44 These rutile GeO2 features are more intense for the orange and brown Ge(OH)2 and may be related to these materials being precipitated from hot Ge(II) reaction mixtures. Raman spectroscopy allows detection/identification of Ge− Ge bonds within the Ge(OH)2 variants. Ge−Ge bonding could exist in Ge inclusions (e.g., crystalline, noncrystalline, and/or clusters), which could reasonably form as a result of thermally induced disproportionation reactions occurring during precipitation from Ge(II) reaction mixtures. If present, these Ge inclusions could influence GeNC formation and growth by acting as heterogeneous nucleation sites and/or sources of elemental Ge and impact the shape, as well as size uniformity, of the resulting NCs (vide infra). Raman data (Figure 1e) acquired for yellow, orange, and brown Ge(OH)2 show a general increase in the intensity of the Ge−Ge optical phonon feature at ca. 280 cm−1 with the temperature of the Ge(OH)2 precipitation. The lack of features associated with Ge−Ge in the Raman spectrum of yellow Ge(OH)2 indicates no detectable Ge−Ge bonding and suggests that broad reflections in the XRD pattern obtained for this compound arise from an amorphous Ge(OH)x structure. In contrast, the appearance of Ge−Ge bonding in orange and brown Ge(OH)2 qualitatively supports the proposal that amorphous Ge nanodomains are present.

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RESULTS AND DISCUSSION Germanium(II) hydroxide, commonly referred to as hydrous germanium(II) oxide, has an uncertain stoichiometry; it is typically represented by formulae such as Ge(OH)2, Ge(OH)2· xH2O, and GeO·xH2O.37 The existing literature is conflicted regarding the nature of Ge(OH)2, and this solid product can vary widely in physical appearance; it can be yellow, orange, or brown (Figure 1a−c) depending upon seemingly minor variations in the conditions under which it was prepared.37−39,41 For example, a yellow precipitate forms if ammonium hydroxide is added when the reaction mixture is chilled to below 5 °C, whereas orange and brown precipitates form if the temperature of the reaction mixture is 50 and 100 °C, respectively. Not surprisingly, the differences exhibited by these materials reach beyond straightforward physical appearance and are evident when using more detailed characterization techniques. XRD, Raman spectroscopy, and XPS provide complementary information regarding the underlying structure of yellow, 17520

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Figure 2. (a) XRD patterns, (b) high-resolution XP spectra of the Ge 3d region, and (c) Raman spectra obtained for brown Ge(OH)2 before and after thermal processing at indicated temperatures (for 1 h under Ar). Reflections of Ge (PDF#04-0545) and hexagonal GeO2 are provided for comparison.

has been reported in 73Ge NMR studies of oxides, halides, and organogermanium solids.43,48−52 For example, quadrupole coupling constants for hexagonal, rutile, and amorphous GeO2 vary from 9 to 19.3 MHz (900−4000 ppm broad at 21.1 T).49 Combining the complementary data obtained from XRD, Raman, XPS, and solid-state 73Ge NMR analyses, we conclude that none of the presented Ge(OH)2 variants contain nanocrystalline domains; however, all possess low-oxidation state germanium [i.e., Ge(II) and Ge(III)]. Furthermore, the evidence of Ge−Ge bonding in orange and brown Ge(OH)2 is consistent with the presence of noncrystalline Ge inclusions. To gain insights into the evolution of the Ge(OH)2 precursors upon thermal processing, the discussion will now shift to the structural interrogation of the products obtained from solidphase heating of yellow, orange, and brown Ge(OH)2 at predefined temperatures in Ar. In all cases, thermal processing produces dark brown/black powders. XRD, XPS, and Raman analyses show that these dark brown/black powders contain elemental germanium [i.e., Ge(0)] as well as germanium dioxide [i.e., Ge(IV)], regardless of the processing temperature. General trends in material properties evaluated using these spectroscopic techniques are consistent across the series of yellow, orange, and brown Ge(OH)2 [see Figure S3 and Table S1 for yellow Ge(OH)2 and Figure S4 for orange Ge(OH)2]. For convenience, the following discussion will focus on brown Ge(OH)2; because of the present Ge(OH)2 species, it provided the most well-defined GeNCs (vide infra). Figure 2a shows the evolution of the XRD pattern obtained for brown Ge(OH)2 upon solid-state thermal processing for 1 h in an argon atmosphere at the indicated temperatures (i.e., 300, 350, 400, 450, and 500 °C). As noted above, brown Ge(OH)2 shows broad, ill-defined reflections, which we attribute to an amorphous GeOx species. Raman and XPS data indicate that Ge−Ge bonding is present, which may provide heterogeneous nucleation sites for GeNC formation and a source of elemental Ge that could supply NC growth. After heating, the ill-defined features of the XRD pattern evolve into broadened reflections centered at 27.3, 45.3, 53.7, 66.1, 72.9, and 83.8°, characteristic of cubic Ge (PDF#040545) and consistent with the formation of crystalline nanodomains (i.e., GeNCs).53 The breadth of the XRD reflections narrows with increasing processing temperature, suggesting the growth of these domains. Debye−Scherrer analysis of XRD peak broadening

Survey XP spectra of yellow, orange, and brown Ge(OH)2 (Figure S1) indicate that they contain only germanium and oxygen at the sensitivity of the method. Previous reports indicate that orange and brown Ge(OH)2 possess the empirical formula Ge(OH)2.37−39,41 On the basis of the experimental XPS atomic percentages (Figure S1), yellow, orange, and brown Ge(OH)2 possess empirical formulae of GeO1.80, GeO1.64, and GeO1.65, respectively. (Note: XPS cannot detect hydrogen.) Clearly, all materials are “germanium-rich” oxides. Ge LMM signals are also observed at binding energies of 300−600 eV.45,46 Comparing the deconvolution of the high-resolution XP spectra of the Ge 3d region, it becomes clear that yellow Ge(OH)2 (Figure S2a) is dominated by a Ge(II) emission centered at ca. 31 eV. More intense features related to intermediate oxides as well as Ge(0) and Ge(IV) centered at ca. 29 and 33 eV, respectively, appear in the spectra of orange and brown Ge(OH)2 (Figure S2a). These observations are consistent with Raman and XRD data (vide supra) and suggest that disproportionation reactions are occurring during the synthesis of orange and brown Ge(OH)2 and that these reactions are inducing the formation of lowvalent Ge and rutile GeO2. 73 Ge NMR can provide valuable insights into the internal structure of the present Ge(OH)2 species; however, it is extremely challenging as a result of the characteristically unfavorable NMR properties of 73Ge, which include low natural abundance (7.7%), low gyromagnetic ratio (γ = −0.936 × 107 rad s−1 T−1) characterizing it as an ultralow-gamma NMR nucleus with a low resonance frequency (νL = 31.4 MHz at 21.1 T), and large quadrupolar moment (−19.6 fm2).47 Unfortunately, evaluation of yellow, orange, and brown Ge(OH)2 using solid-state 73Ge NMR did not reveal any signals even after exhaustive attempts using a variety of specialized pulse sequences including WURST-CPMG, CPMG, and echo. Complementary 73Ge NMR spectra of Ge metal, composite, and hexagonal GeO2 can be found in Figure S2b. Considering that the Ge(OH)2 precursors have not been heat-treated (vide infra), it is reasonable that they contain a highly disordered Ge chemical environment that suffers from a sizable quadrupolar coupling constant. This phenomenon is illustrated well when considering that progressing from cubic Ge metal to rutile GeO2 provides nearly a 13 000-fold increase in line width; in this context, even minor deviations from the cubic symmetry can produce substantial and significant inhomogeneous broadening of the central transition, which 17521

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The Journal of Physical Chemistry C (assuming that the dimensionless shape factor is 0.94, calculations are provided in Table S2) indicates that raising the processing temperature through the series of 400, 450, and 500 °C induces nucleation and growth of the NC Ge domains with estimated dimensions of ca. 6.1, 8.0, and 9.4 nm, respectively. In addition, higher thermal processing temperatures (i.e., 500 °C) are sufficient to also induce crystallization of the GeOx matrix to produce hexagonal GeO2, P3121 (152).44,54 These observations support the proposal that disproportionation is occurring and brown Ge(OH)2 is transforming into two thermodynamically favored crystalline materials (i.e., GeNCs and GeO2 matrix). Survey XP spectra of the GeNC/GeO2 composites obtained from thermal processing of brown Ge(OH)2 for 1 h under Ar are shown in Figure S5. In all cases, the composites comprise 56 at. % germanium and 43 at. % oxygen. Evolution of the Ge 3d spectral region of the high-resolution XP spectrum (Figure 2b) is consistent with a general disproportionation reaction mechanism. Brown Ge(OH)2 shows a broad range of binding energies from ca. 34 to 28 eV. These emissions broaden, and the maximum initially shifts to a higher binding energy upon solid-state heating to as high as 300 °C. Consistent with XRD analyses (vide supra) that show the appearance of nanodomains of Ge upon thermal processing at 350 °C, two emissions arising from Ge(0) (ca. 29 eV) and Ge(IV) (ca. 33 eV) remain upon heating to higher temperatures.30 Raman spectroscopy further supports the thermally induced disproportionation proposal. The Raman spectrum of the parent brown Ge(OH)2 shows some evidence of Ge−Ge bonding. Upon thermal processing at 300−350 °C, the spectrum of the resulting composite shows an asymmetric peak at ca. 298 cm−1 associated with the Ge−Ge optical phonon (i.e., Figure 2c). In addition to confirming the presence of Ge−Ge bonds, the shape and position of the peak are useful for understanding the domain size and crystallinity.31,55,56 The Ge−Ge optical phonon at 298 cm−1 intensifies and sharpens with increased processing temperature (i.e., from 350 to 500 °C; Figure 2c), qualitatively indicating the growth and increasing crystallinity of the nanodomains, which supports the present XRD and XPS observations. Ex situ thermal processing of Ge(OH)2 annealing followed by evaluation using 73Ge NMR further illuminates the formation mechanism of crystalline Ge nanodomains (i.e., GeNCs). Brown Ge(OH)2 yields the most well-defined GeNC morphology (vide infra); hence, it was chosen for the present study. Figure 3 shows the 73Ge NMR spectra of brown Ge(OH)2 before and after thermal processing at the indicated temperatures (for 1 h under Ar); the 73Ge NMR spectrum of bulk Ge metal is provided for comparison. There is no detectable 73Ge NMR signal of the brown Ge(OH)2 precursor, suggesting the absence of crystalline cubic Ge [i.e., the electric field gradient (EFG) of Ge is sizeable, inhibiting the ability to observe a 73Ge NMR signal]. After thermal processing at 400 °C, signals arising from GeNCs begin to emerge; these findings agree with XRD, XPS, and Raman spectroscopy data (vide supra), further supporting our analysis. As the processing temperature is increased from 400 to 600 °C, the particles increase in size, from 4 to 10 nm, according to our diffraction results (see above); as a result, the resonance (ca. −78 ppm) associated with the Ge atoms within the NCs sharpens. The broad resonance observed within the nanocrystalline Ge solids is tentatively assigned to the high surface area of the Ge nanoparticles (i.e., disordered Ge on the surface with a

Figure 3. 73Ge MAS NMR spectra of brown Ge(OH)2 before and after ex situ thermal processing treatment indicated at specific temperatures (for 1 h under Ar) compared to Ge metal. Asterisks (*) indicate spinning sidebands.

nonzero EFG). As the particles grow and the core size increases (i.e., surface area will shrink), the line width will shrink as the interruption of the periodic crystal potential decreases. On the micron scale (Ge metal), the isotropic peak for Ge is 10 Hz or 0.3 ppm broad. This effect in NMR has been reported in the past for many materials including 23Na or 207 Pb NMR of Na or Pb metal confined in porous glass, 195Pt NMR of small particles, or 63Cu NMR of silica-supported colloidal copper particles, among others.57−62 Further analysis of this broader 73Ge component to lower frequency (400 °C, attributed to a noncubic Ge chemical environment) is limited because of the long 73Ge NMR experimental times (∼48 h). Direct evaluation of the morphology of GeNCs within oxide matrices formed during the thermal processing of the presented Ge(OH)2 variants using electron microscopy (i.e., TEM) is nontrivial. It is necessary to liberate freestanding GeNCs from the GeNC/GeO2 composites; this is achieved via alcoholic HF etching. This procedure provides freestanding hydride-terminated GeNCs. Although the NCs are freed from the oxide matrix that occludes imaging, the H-GeNCs are also challenging to image because of limitations associated with effective sample preparation. Hence, H-GeNCs were surfacemodified with dodecyl moieties and rendered solution compatible using thermally induced hydrogermylation.35 Spectroscopic characterization (e.g., FTIR, XPS, and EDX) of these surface-functionalized GeNCs is shown in Figure S6. A detailed discussion of these analyses is beyond the scope of the present contribution and has been reported elsewhere.35 Insights into the formation and growth of oxide-embedded GeNCs can be inferred from the bright-field TEM imaging of the functionalized NCs. While the surface functionalization and limited purification procedures used herein may impact the final size distributions of the NCs slightly because the NCs obtained from the different precursors were treated identically, it is reasonable to expect the present analysis to be representative. Bright-field electron micrographs of dodecyl17522

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Figure 4. Bright-field TEM images and associated PSDs of dodecyl-GeNCs obtained from thermal processing of (a) yellow, (b) orange, and (c) brown Ge(OH)2. Insets: photographs showing colors and physical appearances for each Ge(OH)2 material.

GeNCs (Figure 4) qualitatively show that the NC morphology depends upon the precursor employed. While all NCs are pseudospherical, the uniformity of particle shape qualitatively improves through the series of yellow to orange to brown Ge(OH)2. This observation may result from the impact of seeded growth that is possible in orange to brown Ge(OH)2, which contain Ge inclusions (Raman and XPS analyses, vide supra). Consistent with our proposal of seeded growth occurring in orange and brown Ge(OH)2, we note that the NC size distribution narrows dramatically through yellow to orange to brown Ge(OH)2 and the average particle size decreases (see the histograms in Figure 4). Bright-field TEM of dodecyl-GeNCs liberated from the Ge/ GeO2 composites obtained from the thermal processing of brown Ge(OH)2 at predefined temperatures provides additional information regarding the particle nucleation and growth processes. Figure 5 shows the representative bright-field TEM images of dodecyl-functionalized GeNCs obtained from thermal processing of brown Ge(OH)2 at 350, 400, 450, and 500 °C; consistent with XRD and 73Ge NMR analyses of oxide-embedded GeNCs (see above), the average particle sizes of the GeNCs are 5.9 ± 1.2, 7.2 ± 0.9, 8.3 ± 1.1, and 10.1 ± 4.3 nm, respectively. This trend in average particle size combined with broadening of the size distribution at higher processing temperatures is consistent with an Ostwald ripening model in which Ge atom diffusion is occurring at higher temperatures.63

Figure 5. Brown Ge(OH)2 and its thermal processing. (a−d) TEM images and particle size distributions of dodecyl-GeNCs synthesized from thermal processing of brown Ge(OH)2 for 1 h under Ar at 350 (a), 400 (b), 450 (c), and 500 °C (d).

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CONCLUSIONS The present investigation provides important insights into the nature of Ge(OH)2 and its yellow, orange, and brown variants. Despite maintaining the empirical formula of Ge(OH)2, it is clear that the internal structure depends upon the preparation conditions and more specifically the temperature at which Ge(OH)2 is prepared. The products precipitated at higher temperatures contain higher concentrations of the Ge−Ge bond. In addition, the present data are consistent with disproportionation reactions occurring upon thermal processing of Ge(OH)2 and that the low valent inclusions of Ge clearly impact the formation and growth of oxide-embedded GeNCs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b04640. Survey XP spectra of various Ge(OH)2 precursors before and after thermal annealing; full characterization of yellow and orange Ge(OH)2; and FTIR, Raman, and EDX analyses of dodecyl-GeNCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-780-492-7206. Fax: +1-780-492-8231. 17523

DOI: 10.1021/acs.jpcc.8b04640 J. Phys. Chem. C 2018, 122, 17518−17525

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The Journal of Physical Chemistry C ORCID

(12) Kanoun, M.; Busseret, C.; Poncet, A.; Souifi, A.; Baron, T.; Gautier, E. Electronic Properties of Ge Nanocrystals for Non Volatile Memory Applications. Solid-State Electron. 2006, 50, 1310−1314. (13) Batra, Y.; Kabiraj, D.; Kanjilal, D. Charge Retention and Optical Properties of Ge Nanocrystals Embedded in GeO2 Matrix. Solid State Commun. 2007, 143, 213−216. (14) Das, S.; Manna, S.; Singha, R.; Anopchenko, A.; Daldosso, N.; Pavesi, L.; Dhar, A.; Ray, S. K. Light Emission and Floating Gate Memory Characteristics of Germanium Nanocrystals. Phys. Status Solidi A 2011, 208, 635−638. (15) Ray, S. K.; Das, K. Luminescence Characteristics of Ge Nanocrystals Embedded in SiO2 Matrix. Opt. Mater. 2005, 27, 948− 952. (16) Riabinina, D.; Durand, C.; Chaker, M.; Rowell, N.; Rosei, F. A Novel Approach to the Synthesis of Photoluminescent Germanium Nanoparticles by Reactive Laser Ablation. Nanotechnology 2006, 17, 2152−2155. (17) Giri, P. K. Strain Analysis on Freestanding Germanium Nanocrystals. J. Phys. D: Appl. Phys. 2009, 42, 245402. (18) Gorla, C. R.; Liang, S.; Tompa, G. S.; Mayo, W. E.; Lu, Y. Silicon and Germanium Nanoparticle Formation in an Inductively Coupled Plasma Reactor. J. Vac. Sci. Technol., A 1997, 15, 860−864. (19) Ma, X.; Wu, F.; Kauzlarich, S. M. Alkyl-Terminated Crystalline Ge Nanoparticles Prepared from NaGe: Synthesis, Functionalization and Optical Properties. J. Solid State Chem. 2008, 181, 1628−1633. (20) Tanke, R. S.; Kauzlarich, S. M.; Patten, T. E.; Pettigrew, K. A.; Murphy, D. L.; Thompson, M. E.; Lee, H. W. H. Synthesis of Germanium Nanoclusters with Irreversibly Attached Functional Groups: Acetals, Alcohols, Esters, and Polymers. Chem. Mater. 2003, 15, 1682−1689. (21) Kornowski, A.; Giersig, M.; Vogel, R.; Chemseddine, A.; Weller, H. Nanometer-sized Colloidal Germanium Particles: Wetchemical Synthesis, Laser-induced Crystallization and Particle Growth. Adv. Mater. 1993, 5, 634−636. (22) Chiu, H. W.; Chervin, C. N.; Kauzlarich, S. M. Phase Changes in Ge Nanoparticles. Chem. Mater. 2005, 17, 4858−4864. (23) Fok, E.; Shih, M.; Meldrum, A.; Veinot, J. G. C. Preparation of Alkyl-surface Functionalized Germanium Quantum Dots via Thermally Initiated Hydrogermylation. Chem. Commun. 2004, 386−387. (24) Gerion, D.; Zaitseva, N.; Saw, C.; Casula, M. F.; Fakra, S.; Van Buuren, T.; Galli, G. Solution Synthesis of Germanium Nanocrystals: Success and Open Challenges. Nano Lett. 2004, 4, 597−602. (25) Lu, X.; Ziegler, K. J.; Ghezelbash, A.; Johnston, K. P.; Korgel, B. A. Synthesis of Germanium Nanocrystals in High Temperature Supercritical Fluid Solvents. Nano Lett. 2004, 4, 969−974. (26) Zaitseva, N.; Dai, Z. R.; Grant, C. D.; Harper, J.; Saw, C. Germanium Nanocrystals Synthesized in High-Boiling-Point Organic Solvents. Chem. Mater. 2007, 19, 5174−5178. (27) Hessel, C. M.; Henderson, E. J.; Veinot, J. G. C. Hydrogen Silsesquioxane: A Molecular Precursor for Nanocrystalline Si− SiO2Composites and Freestanding Hydride-Surface-Terminated Silicon Nanoparticles. Chem. Mater. 2006, 18, 6139−6146. (28) Hessel, C. M.; Henderson, E. J.; Veinot, J. G. C. An Investigation of the Formation and Growth of Oxide-Embedded Silicon Nanocrystals in Hydrogen Silsesquioxane-Derived Nanocomposites. J. Phys. Chem. C 2007, 111, 6956−6961. (29) Kelly, J. A.; Henderson, E. J.; Veinot, J. G. C. Sol-Gel Precursors for Group 14 Nanocrystals. Chem. Commun. 2010, 46, 8704−8718. (30) Hoffman, M.; Veinot, J. G. C. Understanding the Formation of Elemental Germanium by Thermolysis of Sol-Gel Derived Organogermanium Oxide Polymers. Chem. Mater. 2012, 24, 1283−1291. (31) Henderson, E. J.; Hessel, C. M.; Veinot, J. G. C. Synthesis and Photoluminescent Properties of Size-Controlled Germanium Nanocrystals from Phenyl Trichlorogermane-Derived Polymers. J. Am. Chem. Soc. 2008, 130, 3624−3632. (32) Hessel, C. M.; Reid, D.; Panthani, M. G.; Rasch, M. R.; Goodfellow, B. W.; Wei, J.; Fujii, H.; Akhavan, V.; Korgel, B. A. Synthesis of Ligand-Stabilized Silicon Nanocrystals with Size-

Morteza Javadi: 0000-0002-2249-326X Vladimir K. Michaelis: 0000-0002-6708-7660 Jonathan G. C. Veinot: 0000-0001-7511-510X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grants Program and the University of Alberta are acknowledged for generous research support. The ATUMS training program supported by NSERC CREATE is thanked for the continued generous financial support. The authors would like to thank Dr. Victor Terskikh for scientific discussions and assistance at the National Ultrahigh-Field NMR Facility for Solids. Access to the 21.1 T NMR spectrometer was provided by the National UltrahighField NMR Facility for Solids (Ottawa, Canada), a national research facility funded by a consortium of Canadian Universities and by an NSERC RTI grant, supported by the National Research Council of Canada and Bruker BioSpin, and managed by the University of Ottawa (http://nmr900.ca). G. Popowich and W. Moffat are thanked for assistance. Michelle Ha, Dr. Y. Khaniani, and the members of the Veinot research team are also thanked for useful discussions.

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REFERENCES

(1) Collings, P. J. Simple Measurement of the Band Gap in Silicon and Germanium. Am. J. Phys. 1980, 48, 197−199. (2) Maeda, Y.; Tsukamoto, N.; Yazawa, Y.; Kanemitsu, Y.; Masumoto, Y. Visible Photoluminescence of Ge Microcrystals Embedded in SiO2 Glassy Matrices. Appl. Phys. Lett. 1991, 59, 3168−3170. (3) Maeda, Y. Visible Photoluminescence from Nanocrystallite Ge embedded in a Glassy SiO2 Matrix: Evidence in Support of the Quantum-confinement Mechanism. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 1658−1670. (4) Cho, S.; Kang, I. M.; Kim, K. R.; Park, B.-G., Jr; Harris, J. S. Silicon-Compatible High-Hole-Mobility Transistor with an Undoped Germanium Channel for Low-Power Application. Appl. Phys. Lett. 2013, 103, 222102. (5) Seng, K. H.; Park, M.-h.; Guo, Z. P.; Liu, H. K.; Cho, J. Catalytic Role of Ge in Highly Reversible GeO2/Ge/C Nanocomposite Anode Material for Lithium Batteries. Nano Lett. 2013, 13, 1230. (6) Taylor, B. R.; Kauzlarich, S. M.; Lee, H. W. H.; Delgado, G. R. Solution Synthesis of Germanium Nanocrystals Demonstrating Quantum Confinement. Chem. Mater. 1998, 10, 22−24. (7) Taylor, B. R.; Kauzlarich, S. M.; Delgado, G. R.; Lee, H. W. H. Solution Synthesis and Characterization of Quantum Confined Ge Nanoparticles. Chem. Mater. 1999, 11, 2493−2500. (8) Lee, D. C.; Pietryga, J. M.; Robel, I.; Werder, D. J.; Schaller, R. D.; Klimov, V. I. Colloidal Synthesis of Infrared-Emitting Germanium Nanocrystals. J. Am. Chem. Soc. 2009, 131, 3436−3437. (9) Prabakar, S.; Shiohara, A.; Hanada, S.; Fujioka, K.; Yamamoto, K.; Tilley, R. D. Size Controlled Synthesis of Germanium Nanocrystals by Hydride Reducing Agents and Their Biological Applications. Chem. Mater. 2010, 22, 482−486. (10) Lambert, T. N.; Andrews, N. L.; Gerung, H.; Boyle, T. J.; Oliver, J. M.; Wilson, B. S.; Han, S. M. Water-Soluble Germanium(0) Nanocrystals: Cell Recognition and near-Infrared Photothermal Conversion Properties. Small 2007, 3, 691−699. (11) Choi, W. K.; Chim, W. K.; Heng, C. L.; Teo, L. W.; Ho, V.; Ng, V.; Antoniadis, D. A.; Fitzgerald, E. A. Observation of Memory Effect in Germanium Nanocrystals Embedded in an Amorphous Silicon Oxide Matrix of a Metal-insulator-semiconductor Structure. Appl. Phys. Lett. 2002, 80, 2014−2016. 17524

DOI: 10.1021/acs.jpcc.8b04640 J. Phys. Chem. C 2018, 122, 17518−17525

Article

The Journal of Physical Chemistry C

(53) Downs, R. T.; Bartelmehs, K. L.; Gibbs, G. V.; Boisen, M. B.; Grnns, G. V.; Borsnn, M. B. Interactive Software for Calculating and Displaying X-Ray or Neutron Powder Diffractometer Patterns of Crystalline Materials. Am. Mineral. 1993, 78, 1104−1107. (54) Javadi, M.; Yang, Z.; Veinot, J. G. C. Surfactant-free synthesis of GeO2 nanocrystals with controlled morphologies. Chem. Commun. 2014, 50, 6101−6104. (55) Heath, J. R.; Shiang, J. J.; Alivisatos, A. P. Germanium Quantum Dots: Optical Properties and Synthesis. J. Chem. Phys. 1994, 101, 1607−1615. (56) Fujii, M.; Hayashi, S.; Yamamoto, K. Growth of Ge Microcrystals in SiO2Thin Film Matrices: A Raman and Electron Microscopic Study. Jpn. J. Appl. Phys., Part 1 1991, 30, 687−694. (57) Bucher, J. P.; Buttet, J.; Van Der Klink, J. J.; Graetzel, M. Electronic Properties and Local Densities of States in Clean and Hydrogen Covered Pt Particles. Surf. Sci. 1989, 214, 347−357. (58) Borisov, B. F.; Charnaya, E. V.; Plotnikov, P. G.; Hoffmann, W.D.; Michel, D.; Kumzerov, Y. A.; Tien, C.; Wur, C.-S. Solidification and Melting of Mercury in a Porous Glass as Studied by NMR and Acoustic Techniques. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 5329−5335. (59) Bercier, J. J.; Jirousek, M.; Graetzel, M.; van der Klink, J. J. Evidence from NMR for Temperature-Dependent Bardeen-Friedel Oscillations in Nanometre-Sized Silver Particles. J. Phys.: Condens. Matter 1993, 5, L571−L576. (60) Williams, M. J.; Edwards, P. P.; Tunstall, D. P. Probing the Electronic Structure of Small Copper Particles: 63Cu NMR at 1.5 K. Faraday Discuss. 1991, 92, 199−215. (61) Terskikh, V. V.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A.; Reinhold, C. J.; Anderson, P. A.; Edwards, P. P. Size Effects on the Nuclear Magnetic Resonance of Sodium Metal Confined in Controlled Pore Glasses. Magn. Reson. Colloid Interface Sci. 2002, 76, 469−475. (62) Charles, R. J.; Harrison, W. A. Size Effect in Nuclear Magnetic Resonance. Phys. Rev. Lett. 1963, 11, 75−77. (63) Ostwald, W. Lehrbuch Der Allgemeinen Chemie; W. Engelmann: Leipzig, 1896.

Dependent Photoluminescence Spanning Visible to Near-Infrared Wavelengths. Chem. Mater. 2011, 24, 393−401. (33) Puzzo, D. P.; Henderson, E. J.; Helander, M. G.; Wang, Z.; Ozin, G. A.; Lu, Z. Visible Colloidal Nanocrystal Silicon LightEmitting Diode. Nano Lett. 2011, 11, 1585−1590. (34) Sun, W.; Zhong, G.; Kübel, C.; Jelle, A. A.; Qian, C.; Wang, L.; Ebrahimi, M.; Reyes, L. M.; Helmy, A. S.; Ozin, G. A. Size-Tunable Photothermal Germanium Nanocrystals. Angew. Chem. 2017, 129, 6426−6431. (35) Javadi, M.; Picard, D.; Sinelnikov, R.; Narreto, M. A.; Hegmann, F. A.; Veinot, J. G. C. Synthesis and Surface Functionalization of Hydride-Terminated Ge Nanocrystals Obtained from the Thermal Treatment of Ge(OH)2. Langmuir 2017, 33, 8757−8765. (36) Jolly, W. L.; Latimer, W. M. The Solubility of Hydrous Germanous Oxide and the Potential of the Germanous OxideGermanic Oxide Couple. J. Am. Chem. Soc. 1952, 74, 5751−5752. (37) Yang, D. J.; Jolly, W. L.; O’Keefe, A. Conversion of Hydrous Germanium(II) Oxide to Germynyl Sesquioxide (HGe)2O3. Inorg. Chem. 1977, 16, 2980−2982. (38) Dennis, L. M.; Hulse, R. E. Germanium XXXV. Germanium Monoxide. Germanium Mono Sulfide. J. Am. Chem. Soc. 1930, 52, 3553−3556. (39) Gayer, K. H.; Zajicek, O. T. The Solubility of Germanium (II) Hydroxide in Perchloric Acid and Sodium Hydroxide at 25 °C. J. Inorg. Nucl. Chem. 1964, 26, 2123−2125. (40) Babich, O. A.; Ghosh, M. C.; Gould, E. S. Preparation of Aqueous Solutions of Hypovalent Germanium; Reactions Involving Germanium-(II) and -(III). Chem. Commun. 2000, 907−908. (41) Terrey, H.; Everest, D. A.; Terrey, H. Germanous Oxide and Sulphide. J. Chem. Soc. 1950, 2282−2285. (42) Mamone, S.; Dorsch, A.; Johannessen, O. G.; Naik, M. V.; Madhu, P. K.; Levitt, M. H. A Hall Effect Angle Detector for SolidState NMR. J. Magn. Reson. 2008, 190, 135−141. (43) Michaelis, V. K.; Kroeker, S. 73Ge Solid-State NMR of Germanium Oxide Materials: Experimental and Theoretical Studies. J. Phys. Chem. C 2010, 114, 21736−21744. (44) Atuchin, V. V.; Gavrilova, T. A.; Gromilov, S. A.; Kostrovsky, V. G.; Pokrovsky, L. D.; Troitskaia, I. B.; Vemuri, R. S.; Carbajal-Franco, G.; Ramana, C. V. Low-Temperature Chemical Synthesis and Microstructure Analysis of GeO2 Crystals with α-Quartz Structure. Cryst. Growth Des. 2009, 9, 1829−1832. (45) Kuhn, W. K. Zinc Germanium Phosphide by XPS. Surf. Sci. Spectra 1994, 3, 93−99. (46) Purkait, T. K.; Swarnakar, A. K.; De Los Reyes, G. B.; Hegmann, F. A.; Rivard, E.; Veinot, J. G. C. One-Pot Synthesis of Functionalized Germanium Nanocrystals from a Single Source Precursor. Nanoscale 2015, 7, 2241−2244. (47) Harris, R. K.; Becker, E. D. NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts-IUPAC Recommendations. J. Magn. Reson. 2002, 156, 323−326. (48) Greer, B. J.; Michaelis, V. K.; Terskikh, V. V.; Kroeker, S. Reconnaissance of Diverse Structural and Electronic Environments in Germanium Halides by Solid-state 73Ge NMR and Quantum Chemical Calculations. Can. J. Chem. 2011, 89, 1118−1129. (49) Michaelis, V. K.; Aguiar, P. M.; Terskikh, V. V.; Kroeker, S. Germanium-73 NMR of Amorphous and Crystalline GeO2. Chem. Commun. 2009, 4660−4662. (50) Stebbins, J. F.; Du, L.-S.; Kroeker, S.; Neuhoff, P.; Rice, D.; Frye, J.; Jakobsen, H. J. New Opportunities for High-Resolution Solid-State NMR Spectroscopy of Oxide Materials at 21.1- and 18.8-T Fields. Solid State Nucl. Magn. Reson. 2002, 21, 105−115. (51) Sutrisno, A.; Hanson, M. A.; Rupar, P. A.; Terskikh, V. V.; Baines, K. M.; Huang, Y. Exploring the Limits of 73Ge Solid-state NMR Spectroscopy at Ultrahigh Magnetic Field. Chem. Commun. 2010, 46, 2817−2819. (52) Hanson, M. A.; Sutrisno, A.; Terskikh, V. V.; Baines, K. M.; Huang, Y. Solid-State 73Ge NMR Spectroscopy of Simple Organogermanes. Chem.Eur. J. 2012, 18, 13770−13779. 17525

DOI: 10.1021/acs.jpcc.8b04640 J. Phys. Chem. C 2018, 122, 17518−17525