Article pubs.acs.org/JPCC
Evolution of Vibrational Properties in Lanthanum Hexaboride Nanocrystals Tracy M. Mattox,* Shruthi Chockkalingam, Inwhan Roh, and Jeffrey J. Urban* The Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Lanthanum hexaboride (LaB6) is known for its hardness, mechanical strength, thermionic emission, and strong plasmonic properties. However, given the lack of colloidal synthetic methods to access this material, very little is understood about its physical properties on the nanoscale. Recently, a new moderate-temperature synthetic technique was developed to directly synthesize LaB 6 nanoparticles [Mattox et al. Chem. Mater. 2015, 27, 6620]. We report the influence of nanoparticle size on the structural and vibrational properties of LaB6 using a combination of Raman and Fourier transform infrared spectroscopies. Our studies indicate that the size of the lanthanum salt anion has a larger influence on LaB6 vibrational energies than particle size. Surprisingly, our work finds that the LaB6 lattice readily expands to accommodate larger ions and contracts with their removal, while ligand incorporation significantly amplifies and shifts the Raman stretching modes.
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INTRODUCTION Raman spectroscopy provides powerful insights into the fundamental vibrational characteristics of nanomaterials. When nanoparticle size decreases, the distribution of vibrational modes broadens as surface modes become more prominent due to the increase in the surface-to-volume ratio. As a result, Raman bands shift and new plasmon bands may appear, as observed in several nanoparticle systems: TiO2,2,3 SnO2,4 and MgO, ZnO, and CdSe.5−7 The Raman-active modes of bulk rare earth hexaborides (MB6) are well-established. MB6 crystallizes in a CsCl-type cubic structure with Pm3m symmetry, and octahedron boron clusters are primarily responsible for the Raman-active modes. The covalent bonds in the boron cluster are thought to give MB6 its exceptional hardness,8 and the plasmonic properties,9,10 ferromagnetic properties,11 and high mechanical strength12 of these materials remain active topics of investigation. However, the evolution of Raman modes in MB6 with respect to particle size remains unexplored, largely due to limitations in the synthesis which have prevented access to size-controlled, uniform, singlecrystalline MB6 nanocrystals. The origins of this synthetic difficulty are manifold. Bulk MB6 compounds are typically made at temperatures above 1500 °C or under high pressure,13−16 or by using chemical vapor deposition.17−19 These methods make it difficult to create uniform nanosized particles unless some extrinsic, mechanical process such as ball milling is used. However, such techniques may damage the crystal and introduce contaminants, which further complicate the study of vibrational properties. Until recently, the synthesis of MB6 has been © XXXX American Chemical Society
inaccessible via traditional colloidal methods due to the high temperatures required to drive the reaction, and to date no solution-based reaction has been realized. Recently, we reported a low-temperature synthesis of lanthanum hexaboride (LaB6) at 360 °C by reacting lanthanum chloride with sodium borohydride under a flow of argon.1 This method allows for some synthetic control over particle diameter without requiring postprocessing techniques to reduce the size. Given the new ability to synthesize phase-pure nanoscale LaB6, this study focuses on the influence of LaB6 particle size on Raman-active modes.
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EXPERIMENTAL METHODS Synthesis. Synthesis of LaB6 was based on a recently reported procedure.1 All chemicals were purchased from SigmaAldrich and used without further purification. To a 25 mL round-bottom flask equipped with an air-cooled condenser was added 0.53 g (14.0 mmol) of NaBH4 and 0.49 g (2.0 mmol) of LaCl3. In order to study the influence of the anion of the lanthanum metal, 1.04 g (2.0 mmol) of LaI3 was used in place of LaCl3. To study vibrational changes due to ligand incorporation, 0.05 g (0.3 mmol) of isophthalic acid was added to the reaction prior to heating. Reactants were stirred under vacuum at 140 °C for 1 h to remove water, and then stirred at 360 °C under argon. After 1−2.5 h (depending on the desired particle size), the black product was cooled completely Received: December 29, 2015 Revised: February 11, 2016
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DOI: 10.1021/acs.jpcc.5b12680 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. (A) One unit cell of LaB6. (B) La surrounded by a cage of octahedral boron clusters (also called the “rattling mode”). (C) Raman-active stretching modes (one A1g and two degenerately allowed Eg) and one bending mode (T2g) of the octahedron boron cluster (La atoms removed for clarity). (D) IR-active T1u acoustic modes (one unit cell).
surrounding La (Figure 1B). We adopt this formalism to facilitate vibrational mode analysis. The octahedron boron cluster has A1g and Eg stretching modes and a T2g bending mode (Figure 1C) that are Raman active. The T1u acoustic mode (Figure 1D), which is the movement of La with respect to the boron cluster, is also observed in the Raman spectra.20
to room temperature. Methanol was added to remove excess NaBH4 and the solid black product was collected. Hydrochloric acid (0.5 M) converted traces of Na metal to NaCl, and the NaCl was removed with deionized water. Analysis. The crystal structure of LaB6 was analyzed by powder X-ray diffraction (XRD) analysis on a Bruker D8Discover operated at 40 kV/20 mA using Cu Kα radiation. The particles were also analyzed using Fourier transform infrared spectroscopy (FTIR) on a PerkinElmer SpectrumOne FTIR equipped with HATR assembly. Elemental analysis was performed by energy dispersive X-ray spectroscopy (EDX) on a Zeiss Gemini Ultra-55 analytical field emission scanning electron microscope. All Raman spectra were collected on a Horiba Jobin Yvon LabRAM ARAMIS automated scanning confocal Raman microscope exciting at a laser wavelength of 532 nm. Particles were imaged on a JEOL 2100 transmission electron microscope (TEM) operated at 200 kV. Thermal gravimetric analysis and mass spectrometry (TGA−MS) were performed on a TA Instruments Q5000IR TGA attached to a Pfeiffer MS, operating from 45 to 600 °C with a 25 °C/min ramp rate.
Γ = A1g + Eg + T1g + T2g + 3T1u + T2u
(1)
Analysis of the Size Dependence of LaB6 Vibrational Modes. In order to study size effects on vibrational properties, we compare the vibrational modes of small (2.5 nm) and large (5.4 nm) LaB6 nanoparticles to that of bulk LaB6. Control over the size of nanocrytalline LaB6 was exerted by varying the reaction time: a 1 h synthesis produces 2.3 ± 0.3 nm particles and a 2.5 h synthesis creates larger 5.4 ± 3.8 nm particles.1 The crystal structure of LaB6 was confirmed by XRD (Figure 2), and the bulk sample used for comparison was an NIST LaB6 standard. Figure 3A shows the Raman spectroscopy results of LaB6 samples of varying particle size. In bulk LaB6, the Raman-active modes T2g, Eg, and A1g appear respectively at 650 cm−1, 1074 cm−1 (with a 1125 cm−1 shoulder), and 1200 cm−1 (Figure 3A). This is in agreement with published results.21,22 The acoustic T1u mode at 206 cm−1 is typical of MB6 materials,23−27 while the vibration of La within the boron cage (the “rattling mode”)21,24,28 is observed at 103 cm−1. The broad peak at 1346 cm−1 is also indicative of trivalent MB6 materials, and is not a
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RESULTS AND DISCUSSION A unit cell of bulk LaB6 is comprised of octahedral boron clusters surrounding a central La ion as shown in Figure 1A. The Pm3m symmetry of LaB6 yields a set of allowed vibrational modes shown in eq 1, and the system may be described as a series of boron cages with octahedron boron clusters B
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We hypothesize that, as crystal size shrinks, we enter a regime where rattling modes are moved to higher energy due to volumetric constriction. In principle, this data may be extracted from XRD data. In practice, attempts at refining XRD data to determine the exact unit cell size were unsuccessful due to uncertainties regarding the asymmetric peak shape. We focused our study on how the very clear rattling and acoustic bands changed with particle size. In bulk LaB6, the vibration of the La atom is reported to exhibit Einstein oscillation because the La vibration is independent of oscillations from the surrounding B atoms.28 The rattling mode is the same for bulk and 5.4 nm particles. Interestingly, the 2.3 nm particles have a slight shift to lower energy compared to the larger particles (103 cm−1 vs 94 cm−1), and the intensity significantly increases (Figure 3C). To our knowledge, this is the first time such a shift has been observed in the rattling mode of any MB6 structure and suggests that the cage space becomes small enough to influence La vibrations. The T1u mode only shifts to lower energy with the 2.3 nm particles (from 206 to 200 cm−1), remaining unchanged for larger particles (Figure 3C). To our knowledge, this is also the first time a shift of T1u has been reported for any MB6, and is another result of the cage space becoming small enough to impact the La vibrations. In addition to increasing the vibrational energy of M in the rattling mode, when the particle size of LaB6 decreases to 2.3 nm an extra Raman peak appears at 371 cm−1. Given the position of this new peak, we hypothesize that it is due to crystal-electric-field (CEF) excitations, which are the breathing and deforming modes of the octahedron B6. CEF vibrations have been observed in NdB6,30 PrB6,24 and CeB6,31,24 but never before in LaB6. It is possible that the larger cage size of the Lacontaining MB6 typically prevents CEF vibrations when the particles are large, while decreasing the particle size decreases the cage size and forces more interaction between La and the B6 cluster. It is also possible that the increase of boron vacancies with decreasing particle size in LaB61 changes the vibrations of the B6 network enough to allow CEF to be observed. By far the largest peak shift in the Raman spectra with respect to particle size is above 1300 nm (Figure 3D). This peak is common in trivalent MB6 systems and exceeds the energy of A1g, so it is thought to result from an electronic state and is labeled a “nonphononic” peak.32,33 Slight shifts of this peak have been reported for MB6 with varying M sizes,24 where decreasing the size of M results in a shift to higher energy. This is the first example of the peak position changing with particle size in MB6, where decreasing the particle size shifts the Raman-active modes to higher energy (a size change from bulk to 2.3 nm shifts the non-phononic peak from 1346 to 1582 cm−1). Though the vibrations of the hexaboride cluster are wellestablished and size dependence is observable, the type of bonding within LaB6 remains unclear. La−B bonding in LaB6 has been described as both covalent34 and ionic,35 and there is speculation as to whether La−La bonds are significant to overall bonding34 or if La ions are independently immersed
Figure 2. XRD patterns of 2.5 nm (red), 5.4 nm (black), and bulk (blue; NIST standard) LaB6.
Figure 3. Raman spectra of (A) varying LaB6 particle sizes, including 2.5 nm (red), 5.4 nm (black), and bulk (blue; NIST standard), with magnified images of (B) Eg and A1g modes, (C) rattling and T1u modes, and (D) the nonphonon peak. (Lines added to aid in observing peak shifts.)
result of single-phonon excitation as it exceeds the highest energy of A1g.24 When small nanoparticles of LaB6 are studied, there are distinct changes observed in the Raman spectra (Table 1). For instance, when the particle size decreases from bulk to 2.3 nm, all three Raman-active modes shift to higher energies; A1g shifts from 1200 to 1225 cm−1, respectively (Figure 3B). Decreasing particle size also results in the broadening of diffraction peaks (Figure 2). As particle size decreases the strain and spring constants within the structure increase, causing the Raman bands to shift to higher energies. This is in agreement with results reported for nanoparticles of TiO23 and CuAlO2.29 The intensities of T2g and A1g also decrease significantly with decreasing particle size.
Table 1. Comparison of Raman Peak Positions for All Measured LaB6 Particle Sizes
bulk 5.4 nm 2.5 nm
rattling (cm−1)
T1u (cm−1)
103 103 94
206 206 200
CEF (cm−1)
T2g (cm−1)
Eg (cm−1)
A1g (cm−1)
nonphonon (cm−1)
371
650 653 706
1074 1078 1090
1200 1208 1225
1346 1564 1582
C
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The Journal of Physical Chemistry C within a rigid boron framework.36 In both classical covalent and ionic nanoparticles, a decrease in particle size causes all Ramanactive modes to broaden and shift to lower energy, as observed for silicon37−39 and CdSe.40−42 When the particle size decreases the vibrational energies “soften” due to the increasing number of uncoordinated bonds on the surface, and in nanoparticles less than 10 nm this is described by the electrostatic sphere model.43 In traditional models, Raman-active modes systematically blue shift with decreasing particle size. Our LaB6 particles follow the opposite trend, where the Raman modes red shift with decreasing particle size. This is most likely due to boron vacancies within the system. As previously reported, reducing the particle size results in a slight decrease in the amount of B observed by EDX in LaB6.1 This same red-shifting trend is also observed as a result of vacancies in α-GeTe,44 and is due to optically active phonons being confined by the presence of increasing concentrations of atomic vacancies with decreasing particle size. Halogen Ion Dependence. Given the notable shift of vibrations and appearance of CEF excitations in the smallest LaB6 nanoparticles, we wanted to determine whether increasing the concentration of anions would influence the vibrational modes. An excess of Cl− was introduced by adding NaCl prior to heating. The excess Cl− caused the reaction to darken faster (∼10 min compared to ∼20 min) and in a 1 h reaction increased the particle size from approximately 5.4 nm to 5.9 ± 7.4 nm. The size distribution increased and caused a broadening of all three Raman-active peaks compared to the 5.4 nm particles. Increasing the amount of Cl− caused all three Raman-active peaks (650, 1074, and 1200 cm−1) to shift to higher energy levels (670, 1100, and 1230 cm−1, respectively), suggesting that the anion remains within the structure. To further investigate the influence of the anion, the LaB6 reaction was performed using I−, which is nearly 30% larger than Cl−, by replacing LaCl3 with LaI3. The resulting particles are similar in size and XRD confirms the LaB6 crystal structure with a slight shift to lower 2θ values. Interestingly, when I− is used only the Eg and A1g Raman stretching modes have a notable shift to higher energy (Table 2) while all other
mostly due to water, though our focus was solely on observing the ions in MS. In Cl− containing LaB6 the Cl ion was observed by MS at 350 °C, while Cl appears throughout the entire heating range from 45 to 600 °C with an excess of Cl− from NaCl. These results confirm our suspicion that Cl− stays within the structure, and suggests that excess Cl− is less tightly held in the structure. Unfortunately, multiple attempts to observe I in the I− containing sample were unsuccessful, though a weight loss of 8.6% between 100 and 200 °C did occur. We analyzed XRD and Raman spectra postheating, operating under the assumption that I− was removed. The Eg and A1g stretching modes of the I− containing sample shifted to lower energy after heating, to the same positions of the Cl− containing sample. This further illustrates the ability of LaB6 to retain ions. The XRD peaks postheating also shifted to higher 2θ, aligning with the same peak positions of the Cl− containing sample (Figure 5B). This suggests that LaB6 expands to incorporate the larger I− anion and contracts when the anion is removed. No changes were noted in the Cl− containing sample after ion removal, so Cl− must be small enough to reside in the structure without much influence on cage size or vibrations. In order to more fully understand the evolution of the vibrational properties of these nanoscale particles, we used IR spectroscopy to provide complementary information as Raman inactive modes tend to be IR active. According to the symmetry of this system, two IR-active modes should be observed in LaB6: one in the 1000 cm−1 region and another at very low frequencies. Previous studies have shown that there are three IR features in the regions of 1450, 1000, and 750 cm−1, and the features of our nanosized LaB6 samples fall within these regions. It was previously reported that LaB6 contains vacancies within the crystal lattice.48 Cell parameter and density measurements determined that the vacancies occur at the boron octahedron sites,49 which was confirmed by studying the effect excess boron has on displacing the phonon branch toward the Brillouin zone center.48 These vacancies result in a reduction of symmetry from Oh to C4v,48,50 and the defect-induced scattering induces vibration modes normally forbidden by selection rules. The bending T2u mode becomes active and is observed in the 750 cm−1 region in IR spectra while the bands near 1450 cm−1 become active due to a combination of the T2u bending mode and translatory mode, which is the movement of the La+ with respect to the octahedron boron cluster.48,50,51 When decreasing particle size to the nanoscale, the only notable difference in the IR spectra is an increase of the intensity of the shoulder at 1475 cm−1 (Figure 6A). This defectinduced feature is interpreted as a result of La+ movement with respect to the cage. Interestingly, the anion size has a much bigger influence on Raman spectra than particle size. Increasing the amount of Cl− in the system by adding NaCl to the reaction causes all three IR features to shift to lower wavenumbers, with no notable change to the XRD pattern. Replacing Cl− with the larger I− causes an even further shift to lower wavenumbers, and the shoulder of the defect-induced feature at 1475 cm−1 is indistinguishable (Figure 6B). This again suggests that the size of the cage expands to accommodate the larger anion, lowering the energy of the boron cluster vibrations. Influence of Ligand Interaction on LaB6 Vibrational Modes. Our most recent report is the only example, to our knowledge, of a ligand interacting with a metal hexaboride system, where isophthalic acid converts to 1,3-phenylenedimethanol in situ and results in organic-soluble LaB6 nanoparticles.1
Table 2. Raman Peak Positions of Various LaB6 Particle Sizes −
Cl I−
T2g (cm−1)
Eg (cm−1)
A1g (cm−1)
653 654
1078 1088
1208 1215
vibrational modes remain unchanged (Figure 4A). This supports the postulate that the anion of the lanthanum salt remains within the material. The bending T2g mode is not influenced by this change, so the anion likely sits within the boron vacancies and between La atoms (Figure 4B). It is possible that the anions behave like bridging atoms, as reported for bridging chloride in lanthanum−lithium45 and organolanthanide complexes46 as well as bridging iodine in transition metal complexes.47 This data suggests that a bridging ion may be present and implies that the La atoms exist within a boron network rather than bonding with one another. LaB6 samples were analyzed by TGA−MS to determine the presence of anions within the structure (Figure 5A). One distinct weight loss was observed for all samples, with weight losses between 45 and 250 °C of 10.7 and 11.6% for Cl− and NaCl containing samples, respectively. This weight loss is D
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Figure 4. (A) Raman-active modes of 5.4 nm LaB6 synthesized using I− (red) and Cl− (black) anions on the lanthanum salt and (B) cartoon of the hypothesized positions of anion (yellow) positions within boron vacancies and between La atoms in LaB6.
Figure 5. (A) Raman spectra of LaB6 samples made using I− and Cl−, and LaB6 with I− removed by TGA (lines added to aid in observing peak shift). (B) XRD of the same.
Figure 6. FTIR spectra of LaB6 of (A) varying nanoparticle size and (B) varying anion size (in a 5.4 nm LaB6 nanocrystal).
ligand causes the defect-induced mode to shift to lower wavenumbers and the T1u mode to become indistinguishable.
The FTIR spectra of ligand-bound LaB6 nanoparticles differ slightly from those of ligand-free LaB6 (Figure 7A), where the E
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Figure 7. (A) FTIR of 5.4 nm LaB6 and ligand-bound LaB6, and illustrations of (B) side-on view of the 1,3-phenylenedimethanol ligand (some H atoms omitted for clarity), (C) ligand fitting between La atoms to interact with B6 cluster, and (D) ligands interacting with the boron terminated surface.
As observed with the lanthanum salt ions, we hypothesized that the ligand incorporates on the particle surface, which explains why the intensity of the already-weak T1u vibration is too low to be seen. In contrast, the T 2u mode shifts to higher wavenumbers when the ligand is introduced. These results suggest that the ligand interacts with the surface of the nanoparticles. Surface studies have been reported for LaB6 involving the clean surface52,53 as well as demonstrating the adsorption of oxygen54−56 and carbon monoxide.57 It is agreed that the topmost atomic layer of the (100) surface consists of La atoms, and that triangular groupings of boron atoms extend above the La atoms on the (111) surface.57 As a result, there are two surfaces on LaB6 where a ligand may interact. Rotating the view of the ligand such that it is observed in the side view (Figure 7B), it is possible to see how the ring portion of the ligand may extend between La atoms and interfere with B6 vibrations while having minimal impact on La vibrations (Figure 7C). On the other surface of LaB6, the ligand may interact with the boron that extends beyond the plane of La atoms, thus changing the vibrations of boron without impacting La (Figure 7D). Ligand interaction with the boron cluster is also apparent in the Raman spectra (Figure 8) with two broad features present (at 1100 and 1590 cm−1). No −OH bending or −OH stretching are observed for the ligand due to the intensity of the LaB6 peaks. The reaction was performed in the absence of a lanthanum salt and the residue collected and analyzed, as described previously,1 so we are confident that these peaks are a result of the ligand interacting with the LaB6 and are not signals from the reacted ligand itself. The two prominent Raman
Figure 8. Raman spectra of LaB6 (black) and ligand-bound LaB6 (pink).
vibrations are most likely the Eg and A1g stretching modes of the boron cluster. These Raman peaks are shifted to much higher energy compared to the ligand-free LaB6, supporting the hypothesis that the ligand impacts the vibrations of the boron cluster. The bending T2g vibration is unobservable. Given that the triangular boron-terminated surface is so accessible to the ligand it is logical for the stretching vibrations to be very intense, making the T2g vibration indistinguishable from the baseline.
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CONCLUSION Using Raman and FTIR analysis, we report that decreasing the particle size of LaB6 increases the stretching and bending vibration energies of the hexaboride cluster. Decreasing the F
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(4) Zuo, J.; Xu, C.; Liu, X.; Wang, C.; Wang, C.; Hu, Y.; Qian, Y. Study of the Raman Spectrum of Nanometer SnO2. J. Appl. Phys. 1994, 75, 1835. (5) Schlecht, R. G.; Bockelmann, H. K. Raman Scattering from Microcrystals of MgO. Phys. Rev. Lett. 1973, 31, 930. (6) Schreder, B.; Dem, C.; Schmitt, M.; Materny, A.; Kiefer, W.; Winkler, U.; Umbach, E. Raman Specroscopy of II-IV Semiconductor Nanostructures: CdS Quantum Dots. J. Raman Spectrosc. 2003, 34, 100. (7) Hayashi, S.; Kanamori, H. Raman Scattering from the Surface Phonon Mode in GaP Microcrystals. Phys. Rev. B: Condens. Matter Mater. Phys. 1982, 26, 7079. (8) Perkins, P. G.; Armstrong, D. R.; Breeze, A. On the Electronic Structure of Some Metal Hexaborides. J. Phys. C: Solid State Phys. 1975, 8, 3558. (9) Schelm, S.; Smith, G. B.; Garrett, P. D.; Fisher, W. K. Tuning the Surface-Plasmon Resonance in Nanoparticles for Glazing Applications. J. Appl. Phys. 2005, 97, 124314. (10) Takeda, H.; Kuno, H.; Adachi, K. Solar Control Dispersions and Coatings With Rare-Earth Hexaboride Nanoparticles. J. Am. Ceram. Soc. 2008, 91, 2897−2902. (11) Young, D. P.; Hall, D.; Torelli, M. E.; Fisk, Z.; Sarrao, J. L.; et al. High-Temperature Weak Ferromagnetism in a Low-Density FreeElectron Gas. Nature 1999, 397, 412−414. (12) Oettinger, P. E. Measured Brightness of Electron Beams Photoemitted from Multicrystalline LaB6. Appl. Phys. Lett. 1990, 56, 333. (13) Lafferty, J. M. Boride Cathodes. J. Appl. Phys. 1951, 22, 299− 309. (14) Rea, J. R.; Kostiner, E. The Formation of Calcium and Certain Rare-Earth Hexaboride Single Crystals. J. Cryst. Growth 1971, 11, 110−112. (15) Niemyski, T.; Pracka, I.; Jun, J.; Paderno, J. On Zone Melting of Alkaline and Rare-earth Metal Hexaboride rods. J. Less-Common Met. 1968, 15, 97−99. (16) Zhang, M.; Yuan, L.; Wang, X.; Fan, H.; Wang, X.; Wu, X.; Wang, H.; Qian, Y. A Low-Temperature Route for the Synthesis of Nanocrystalline LaB6. J. Solid State Chem. 2008, 181, 294−297. (17) Brewer, J. R.; Deo, N.; Morris Wang, Y.; Cheung, C. L. Lanthanum Hexaboride Nanoobelisks. Chem. Mater. 2007, 19, 6379− 6381. (18) Kher, S. S.; Spencer, J. T. Chemical Vapor Deposition of Metal Borides: The Relatively Low Temperature Formation of Crystalline Lanthanum Hexaboide Thin Films from Boron Hydride Cluster Compounds by Chemical Vapor Deposition. J. Phys. Chem. Solids 1998, 59, 1343−1351. (19) Zhang, H.; Zhang, Q.; Tang, J.; Qin, L.-C. Single-Crystalline LaB6 Nanowires. J. Am. Chem. Soc. 2005, 127, 2862−2863. (20) Zirngiebl, E.; Blumenroder, S.; Mock, R.; Guntherodt, G. Relation of Phonon Anomalies to Charge Fluctuation Rates in Intermediate Valence Compounds. J. Magn. Magn. Mater. 1986, 54− 57, 359−360. (21) Ogita, N.; Nagai, S.; Okamoto, N.; Udagawa, M.; et al. Raman Scattering Investigation of RB6 (R = Ca, La, Ce, Pr, Sm, Gd, Dy, and Yb). Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 224305. (22) Selvan, R. K.; Genish, I.; Perelshtein, I.; Calderon Moreno, J. M.; Gedanken, A. Single Step, Low-Temperature Synthesis of Submicron-Sized Rare Earth Hexaborides. J. Phys. Chem. C 2008, 112, 1795−1802. (23) Ogita, N.; Nagai, S.; Okamoto, N.; Iga, F.; Kunii, S.; Akamtsu, T.; Akimitsu, J.; Udagawa, M. Raman Scattering Study of CaB6 and YbB6. J. Solid State Chem. 2004, 177, 461−165. (24) Ogita, N.; Nagai, S.; Okamoto, N.; Iga, F.; Kunii, S.; Akimitsu, J.; Udagawa, M. Raman Scattering Study of Hexaboride Crystals. Phys. B 2003, 328, 131. (25) Xu, J.; Hou, G.; Li, H.; ZHai, T.; Dong, B.; Yan, H.; Wang, Y.; Yu, B.; Bando, Y.; Golberg, D. Fabrication of Vertically Aligned SingleCrystalline Lanthanum Hexaboride Nanowire Arrays and Investigation of Their Field Emission. NPG Asia Mater. 2013, 5, e53.
particle size below 3 nm also results in a decrease in the rattling vibration of the La atom within the boron network in addition to revealing a new vibrational mode, likely due to CEF vibrations. Interestingly, we also report the influence of the anion of the lanthanum salt on the vibrations of LaB6. This is the first suggestion that the anion plays a role in the reaction and has a larger influence on LaB6 vibrational energies than particle size. Our study suggests that the LaB6 lattice readily expands to accommodate larger ions and contracts upon their removal, leading us to suspect LaB6 may be a potential material for ion storage. We also report that ligand incorporation in LaB6 significantly amplifies and shifts the Raman stretching modes of the boron cluster. Adsorption studies have theorized that the surface of LaB6 contains triangle-shaped B3 ends extending beyond the La atoms of the structure. Our ability to incorporate a ligand on LaB6 and our findings that doing so drastically change the vibrational properties complement previous surface studies. This work may lead to further insights on surface coordination and tuning the optical properties of LaB6. Furthermore, this work may be of significance to the broader scientific community as lanthanide nanocrystals are touted as possible future technologies ranging from biological imaging to solid state devices, as implementation of nanocrystal materials involves an understanding of how interfaces and variations in size may be used to tune physical properties.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12680. Full details of individual reactions, Raman inactive modes, additional Raman spectra, XRD data, and TEM images (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was completed at The Molecular Foundry, Lawrence Berkeley National Laboratory, a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DOE) under Contract No. DE-AC0205CH11231. We would like to give a special thanks to Shaul Aloni for discussions on ion analysis and to Wendy Queen for her discussions on X-ray diffraction.
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REFERENCES
(1) Mattox, T. M.; Agrawal, A.; Milliron, D. J. Low Temperature Synthesis and Surface Plasmon Resonance of Colloidal Lanthanum Hexaboride (LaB6) Nanocrystals. Chem. Mater. 2015, 27, 6620−6624. (2) Pottier, A.; Cassaignon, S.; Chaneac, C.; Villain, F.; Tronc, E.; Jolivet, J.-P. Size Tailoring of TiO2 Anatase Nanoparticles in Aqueous Medium and Synthesis of Nanocomposites. Characterization by Raman spectroscopy. J. Mater. Chem. 2003, 13, 877−822. (3) Choi, H. C.; Jung, Y. M.; Kim, S. B. Size Effects in the Raman Spectra of TiO2 Nanoparticles. Vib. Spectrosc. 2005, 37, 33−38. G
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The Journal of Physical Chemistry C
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DOI: 10.1021/acs.jpcc.5b12680 J. Phys. Chem. C XXXX, XXX, XXX−XXX
