Phonons in Hybrid Lamellar Supercrystals - ACS Publications

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Phonons in Hybrid Lamellar Supercrystals Lucien Saviot, Gianvito Caputo, and Nicola Pinna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11574 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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The Journal of Physical Chemistry

Phonons in Hybrid Lamellar Supercrystals L. Saviot,∗,† G. Caputo,‡ and N. Pinna¶ †Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université Bourgogne Franche-Comté, 9 av. A. Savary, BP 47870, F-21078 Dijon Cedex, France ‡Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ¶Institut f¨ ur Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany E-mail: [email protected]

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Abstract Phonons in hybrid lamellar supercrystals are investigated by Raman scattering. A special attention is paid to low-frequency vibrations as a mean to shed some light into the structure at the nanometer scale. In particular little is known about the structure of the organic capping agents in supercrystals made of colloidal nanopoarticles. This is due to the fact that characterization tools such as electron microscopy and X-ray diffraction are mostly sensitive to the inorganic phase due to their larger electronic density. Raman scattering does not suffer from this limitation. In this class of materials, synthesized following the “benzyl alcohol route”, the inorganic layers are kept together by π–π conjugated interactions. Low-frequency Raman spectra have been measured for yttrium and neodymium oxides-based lamellar organic-inorganic supercrystals synthesized with three benzyl alcohol derivatives. The spectra strongly depend on both the nature of the inorganic and organic phase despite the resulting lamellar structure being similar in terms of layer thickness and crystalline structure for all the samples. This makes low-frequency Raman spectroscopy the tool of choice for the study of the assembling process driving the formation of colloidal supercrystals.

Introduction In the last decades, different methodologies for the synthesis of colloidal nanoparticles have been developed, allowing to precisely tune their size, shape and composition. 1 At the same time, many groups have reported the use of nanoparticles as building-blocks for the fabrication of supercrystals, which are ordered nanoparticle assemblies. 2–9 Capping agents (i.e., organic ligands) injected or formed in-situ during the synthesis can act as structuring agents for the nanoparticles growth. At the same time they can lead to nanoparticle assembly through the interaction of their functional groups. 9 Despite the large amount of reported superstructures, the studies devoted to the elucidation of the supercrystal growth are focused on the use of electron microscopy (SEM, TEM) and in- and ex-situ X-ray diffraction tech2


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niques (SAXS, GISAXS), 10,11 which provide poor information concerning the interactions of the ligand between the nanoparticles. 12,13 Moreover, only few articles have been devoted to the study of the interaction of the organic ligands in supercrystals. 14,15 Self-assembly resulting from π–π conjugated interactions has been shown to be a powerful tool to synthesize a variety of periodic hybrid organic-inorganic nanostructures. 16 Various experimental techniques have been used to characterize their composition and structure. From a chemical point of view, information on the inner structure of the organic layers is of prime interest since it is strongly correlated to the formation mechanism of the superstructures. In most of the cases, assessing the structure of the organic species is difficult. This is because many experimental techniques such as X-ray diffraction and electron microscopy are mostly sensitive to the heavy atoms in the inorganic compound. The organic species can be analyzed using vibrational spectroscopies such as infra-red absorption and Raman scattering. These techniques provide valuable information about the nature of the chemical bonds through the observation of the internal vibration of the molecules. However, little is known about the arrangement of the organic species at the molecular level such as those expected for π–π conjugated interactions. Measuring low-frequency Raman spectra has become in recent years rather straightforward thanks to the availability of notch filters made of volume Bragg gratings. 17 Moreover, low-frequency vibrations are well-known for their sensitivity to the structure at the nanometer scale. This is because acoustic phonons whose wavelength is about the size of the molecules or nanostructures fall in the low-wavenumber range (typically between 1 and 100 cm−1 ). Low frequency vibrations in molecular crystals such as those observed by lowfrequency Raman scattering originate from the frustrated translations and rotations of the molecules possibly mixed with low-frequency internal vibrations of the molecules. 18 The high sensitivity of low-frequency Raman scattering to the molecular arrangement has already been pointed out by various authors. 19,20 In a previous work, 21 we investigated self-assembled ZrO2 nanoparticles. A broad low-

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frequency Raman band was assigned to the organic species. The arrangement of the organic species around small and almost spherical nanocrystals lacks periodicity which results in broad Raman peaks as expected for a disordered material. In this work we focus on yttrium and neodymium oxides hybrid lamellar supercrystals synthesized using the “benzyl alcohol route”. 22,23 The planar interfaces between the organic and inorganic layers favor an in-plane translation periodicity in the organic layers. This may result in narrow peaks. The nature of the organic and inorganic precursors has been varied to help assigning the Raman bands.

Samples preparation and characterization Syntheses Yttrium(III) isopropoxide (Strem; Y5 O(OC3 H7 )13 ), neodymium(III) isopropoxide synthesized following a known procedure, 23 benzyl alcohol, 4-methyl benzylalcohol, and 4-methoxy benzylalcohol were purchased from Aldrich. All the precursors were stored in a glovebox and used without any further purification. All the manipulations were carried out under inert atmosphere (O2 and H2 O < 0.1 ppm). Sample preparation in benzyl alcohol has been previously described. 22,23 Typically 500 mg of yttrium(III) isopropoxide or neodymium(III) isopropoxide were mixed with 20 mL of benzyl alcohol in a glass vial. 4-methyl benzylalcohol and 4-methoxy benzylalcohol replaced benzyl alcohol in the reaction mixture using the same amount. After stirring, the content was transferred into a vessel slid into a steel autoclave which was sealed, removed from the glovebox and then heated in a furnace at 250 or 300 ◦C for 48 hours. The resulting powders were washed with acetone and dried at 60 ◦C for further characterization. Six samples have been considered in this work and their parameters (synthesis and average inter-lamellar spacing) are presented in Tab. 1.

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Table 1: Reactants and temperature used for the synthesis of the samples. The duration of the reaction is 48 hours for all the samples. The last column shows the spatial periodicity of the layered structures as determined by X-ray diffraction. sample Y300 Y250 Yx250 Ym250 N250 Nm250

reactants i Y(O Pr)3 benzyl alcohol Y(Oi Pr)3 benzyl alcohol Y(Oi Pr)3 4-methoxy benzyl alcohol Y(Oi Pr)3 4-methyl benzyl alcohol Nd(Oi Pr)3 benzyl alcohol Nd(Oi Pr)3 4-methyl benzyl alcohol

T (◦C) 300 250 250 250 250 250

d (nm) 1.78 1.78 2.02 1.98 1.73 1.93

Transmission electron microscopy Transmission electron microscopy (TEM) characterization was carried out using a Philips CM200 microscope equipped with a LaB6 cathode and operated at 200 kV. A few drops of a suspension of the material dispersed in ethanol were deposited onto a copper grid coated with amorphous carbon. Fig. 1 shows overview TEM images of the samples considered. Although the particle size can differ from sample to sample, the peculiar lamellar structure with periodicity varying between ≃ 1.8 to 2.0 nm are clearly visible. The darker contrast is due to the inorganic and the brighter to the organic phases, respectively. The images are similar to the ones previously reported for synthesis carried out in benzyl alcohol and in biphenylmethanol. 22–24

X-ray powder diffraction X-ray powder diffraction (XRD) was performed on a Siefert diffractometer (CuKα radiation at 40 kV and 40 mA) in refection mode from 3◦ to 70◦ 2θ with a 0.2◦ step size (data not shown). The interplanar distances were determined using the Bragg equation and are given in Tab. 1. From previous studies the thickness of the inorganic layer was evaluated to only 0.6 nm and the crystal phase corresponded to the monoclinic modification of rare earth sesquioxides by combining XRD and selected area diffraction experiments under a TEM. 22,23,25

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Figure 1: Overview transmission electron micrographs of a) Y300, b) Yx250, c) Ym250, d) N250, e) Nm250 and f) Y250. Scales bars: 20 nm 6 ACS Paragon Plus Environment

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Raman setup and results A Renishaw inVia setup with 532 nm excitation was used for all the Raman measurements. Low-frequency measurements used a BragGrate notch filter from OptiGrate to record the Stokes and anti-Stokes Raman spectra simultaneously down to 10 cm−1 with high throughput. Fourier Transform Infrared (FTIR) spectra were acquired using a Nicolet iS5 spectrometer in Attenuated Total Reflectance (ATR) mode in the range 4000–400 cm−1 with a resolution of 4 cm−1 for 64 scans.

Raman intensitiy (arb. units) / IR transmission

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0

500

1000

1500

2000

Wavenumber

2500

3000

3500

(cm-1)

Figure 2: Full Raman intensity (bottom) and IR transmission (top) spectra of Y250.

In order to have a broad overview of the Raman and IR bands, we first present the full Raman and IR spectra of sample Y250 in Fig. 2. Several Raman and IR absorption bands 7



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are observed between 20 and 3660 cm−1 . Similarly to a previous work on supercrystals of ZrO2 synthesized in a similar way, 21 it is worth noting that the lowest frequency Raman peaks between 20 and 200 cm−1 are among the most intense features. The Raman spectra related to all samples are shown in Fig. 3. Each plot represents a different part of the Raman spectra: the low-frequency range, the carboxyl group stretching frequency range and the typical alkyl and hydroxyl stretching frequency range, respectively. Low-frequency Raman features (. 200 cm−1 ) will be discussed later. Most of the features above 200 cm−1 can be assigned to vibrations related to the organic species as already discussed in a previous publication. 22 The weak peak at 476 cm−1 is tentatively assigned to the yttrium oxide layers. Indeed, the aforementioned peak is observed for all the yttrium samples independent of the organic precursor (Y300, Y250, Yx250 and Ym250). Similarly, the peak at 416 cm−1 is assigned to the neodymium oxide layer (N250 and Nm250). In both cases, the positions do not exactly match those of known bulk lattice structures of Y2 O3 and Nd2 O3 , respectively. This behavior is most probably related to the intimate proximity of the yttrium and neodymium atoms to the organic-inorganic interface in such very thin inorganic layers. Regarding the intermediate frequency range, spectra similar as those in Fig 3b have been already discussed. 22 The features in the range from 1400 to 1600 cm−1 are assigned to the carboxylate groups and can be used to determine the nature of the coordination between the carboxylic group and the metal center by calculating the difference between the frequencies of the asymmetric (νas ) and symmetric (νs ) stretching of the –COO- group. 26 Using the same approach we calculated ∆ = 159 and 148 cm−1 from the IR and Raman spectra of Y300, respectively. Similar or slightly larger values were obtained for the other samples. As noted before 22 these values are fairly consistent with a bridged coordination of the carboxylate moieties to the metal atoms (i.e., two metal ions bound to one carboxylate group). Regarding the high frequency range (Fig.3c), the peaks can be assigned to the C–Hx bond stretching at about 3000 cm−1 and they depend only on the nature of the organic precursor used during

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Normalized intensitiy (arb. units)

Y300

Y250

Yx250

(a)

Ym250

N250

Nm250 -400

-200

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Nm250 1300

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Y300 Normalized intensitiy (arb. units)

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Y250

Yx250

(c)

Ym250

N250 Nm250

3000

3100

3200

3300

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3500

3600

3700

Raman shift (cm-1)

Figure 3: Room-temperature Raman spectra in the (a) -400–1300, (b) 1300–1700 and (c) 3000–3700 cm−1 ranges. Each plot contains the spectra of Y300, Y250, Yx250, Ym250 and N250 from top to bottom.

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the synthesis. On the other hand, the position of the free O–H stretching band in the Raman spectra at about 3650 cm−1 depends only on the composition of the oxide layer. In order to explain this experimental result, we propose that the corresponding O–H groups are located at the surface of the inorganic layers. Indeed, the position of the narrow peak assigned to the free O–H stretching vibration is a typical signature of the surface hydroxyl groups bound to either yttrium or neodymium, respectively. 27 The Raman shifts observed in our case are shifted by about 30 cm−1 compared to the corresponding bulk inorganic material. ν OH = 3598 cm−1 for Nd2 O3 28 and we measured ν OH =3628 cm−1 for Y300, Y250, Yx250 and YM250. ν OH = 3694 cm−1 for Y2 O3 29 and we measured ν OH = 3660 cm−1 for N250 and Nm250.

Low-frequency Raman measurements We now focus on the low-frequency dynamics. Spectra in this frequency range were measured with a better spectral resolution (using a grating with a larger number of grooves per millimeter) for better shape and position resolutions in this relatively narrow frequency range. The resulting spectra for all the samples are presented in Fig. 4. The position of the Raman peaks observed in this figure are reported in Tab. 2. We first note that none of them correspond to those of known benzoate crystals such as Na-benzoate and K-benzoate (see Electronic Supplementary Information of Ref. 21). This is an indication that this part of the spectra depends on the the lamellar structure. Table 2: Position of the low-frequency Raman peaks observed in Fig. 4. sample Y300 Y250 Yx250 Ym250 N250 Nm250

wavenumbers (cm−1 ) 26, 41, 47, 53, 63, 77, 94, 109, 122, 131, 161 32, 42, 47, 83, 108, 162 27, 37, 45, 59, 80, 91, 104 20, 40, 80, 98 30, 45, 57, 72, 85, 89, 95, 115, 148, 171 33, 52, 66, 89, 165

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Y300 Normalized intensitiy (arb. units)

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Y250

Yx250

Ym250

N250

Nm250 -150

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Figure 4: Room-temperature low-frequency Raman spectra with a “high” spectral resolution. The spectrum of Y300 brings out many narrow Raman peaks. The features observed for the other samples are less resolved. Polarized spectra were also measured by selecting scattered photons whose polarization is either parallel or perpendicular to the polarization of the laser. The features are identical in both configurations for all samples except for Y300. See Fig. 5. The weighted difference between both spectra is also shown in this figure as a simple way to show polarized features. Their origin are not known at this point but the temperature during synthesis seem to play a significant role since they do not appear for Y250. For this reason, we will focus on the samples synthesized at 250 ◦C in the following to focus only on the composition related variations.

Composition variations In order to assign the low-frequency peaks to vibrations of either the organic or inorganic layers, we varied the composition of the layers by changing the precursors. Assuming that

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Figure 5: Polarized Raman spectra of Y300. From top to bottom, the spectra were measured in the parallel and crossed configurations. The bottom spectrum is the difference between the parallel and crossed configuration spectra.

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the (in)organic layer composition and structure are determined only by the corresponding (in)organic precursor, and that the Raman spectra consist of the superposition of both, we expect to observe common features in the spectra. The same method is commonly used to described Raman features at larger frequencies (see above). The composition of the inorganic layers is the same in Y250, Yx250 and Ym250. (As explained before, we don’t consider Y300 now.) Two peaks at about 80 and 100 cm−1 are observed in the 3 spectra. Their positions decrease when going from Y250 to Yx250 and then Ym250. This effect is more pronounced for the second peak which varies from 108 to 98 cm−1 while the first peak shifts from 83 to 80 cm−1 only. The changes below 50 cm−1 are more significant and there is no clear peak with a weak dependence on the composition of the organic layer. We identified a similar behavior for N250 and Nm250 with two peaks near 70 and 90 cm−1 . A few additional peaks are also observed but they vary too much between the two samples to assign their origin to vibrations of the inorganic layer. We can also compare the spectra with identical organic precursors in order to identify peaks related to the organic layers. But there are no convincing common features between Y250 and N250. The same is true for Ym250 and Nm250. In both cases, the peaks between 70 and 108 cm−1 which we have already pointed out vary much more than when the inorganic precursor is unchanged. At this point we have therefore identified two peaks which are candidates for vibrations concerning primarily the inorganic layers.

Origin of the peaks Acoustic modes Acoustic vibrations whose wavelength is about the thickness of the layers are one possible explanation for the low-frequency Raman peaks. We tried to reproduce the observed frequencies using a model for acoustic phonons in multilayered structures starting from models 13



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developed to describe the elastic properties of superlattices. At first order, with such a model, 30 the Raman spectra consist of frequency combs related to the eigenvibrations of the organic and inorganic layers respectively because the acoustic impedance mismatch is not small. However, it is not reasonable to expect an organic medium to sustain the propagation of acoustic waves in the THz frequency range. In addition, as discussed before, the observed Raman peaks of interest are primarily governed by the composition of the inorganic layers. For these reasons, we turn to a simpler approach. We consider only the eigenvibrations of free 2D plates made of the inorganic material. 31 In this simple approach, their breathing frequency ν0 is given by: ν0 =

v 2e

(1)

where v is the longitudinal sound speed and e the thickness of the layer. For Y2 O3 layers, v ≃7 × 103 m/s. 32 With e =0.6 nm, the calculated Raman shift is about 190 cm−1 . This is about two times the observed values. The applicability of the previous approach is questionable in particular because it relies on the continuum approximation which can not be taken for granted for such thin layers. Yet, in a recent work, this approximation was shown to be valid for CdS and CdSe nanoplatelets with e smaller than 1 nm. However, for such thin nanoplatelets, the mass of the ligands bound to the two surfaces was also shown to significantly decrease the breathing mode frequencies. 31 Modeling the ligands as an additional surface mass density, the frequencies are obtained by searching the roots of the following equation: m ξ sin ξ M

cos ξ =

(2)

with m the mass of the ligands on both sides, M the mass of the inorganic layer and ξ the reduced frequency given by: ξ=

π ν 2 ν0

(3)

In order for this model to match our experimental data, we need ν/ν0 ≃ 2 which is obtained 14


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for m/M ≃ 1.3. As a comparison, we now try to calculate m/M from what we know about the lamellar structures. The organic layers are about twice thicker than the inorganic ones. The typical mass density of organic solids is about 2 g/cm3 while the one of Y2 O3 is about 5 g/cm3 . Using these numbers, we obtain m/M ≃ 1. This coarse value is in rather good qualitative agreement with the one obtained above. It confirms that the mass of the ligand can not be neglected in our model. At this point, we have a reasonable model which predicts a Raman peak at the breathing mode frequency of the inorganic layers mass loaded by the inorganic ones. The remaining issue is that we previously identified two peaks in our spectra instead of one. However, the highest frequency peak we observed shifts significantly when varying the organic layer composition. While such a shift is predicted by our simple model, the variation of m/M when changing the precursor is rather small and can not result in a significant shift. As a consequence, we propose that the peaks at about 80 cm−1 for Y250, Ym250 and Yx250 and the peaks at about 70 cm−1 for N250 and Nm250 are breathing vibrations of the inorganic layers. Optical phonons of the organic layers The previous discussion has shown that at most one peak might be assigned to confined acoustic vibration (breathing mode of the inorganic layer). However, the origin of the other features is to be found elsewhere. They are assigned to optical vibrations in the organic layers. Low-frequency optical phonons originates from heavy atoms or weak interatomic forces. No reported lattice structure of yttrium or neodymium oxides can result in many low-frequency Raman peaks such as those we observed. But optical phonons in the organic layers are good candidates. Low-frequency optical phonons in organic materials originate from the constrained rotations and translations of “molecular entities” such as the phenyl rings present in our samples. In the gas or liquid phases, organic molecules move and rotate

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(almost) freely. In the solid state, neighbor molecules hinder these free motions which results in restoring forces leading to new vibrations. These manifest as narrow Raman peaks if the organic groups are arranged in an ordered structure (organic crystal) and as broader peaks for disordered structures. The number of observable Raman peaks (Raman-active phonons) is related to the lattice structure. Such low-frequency Raman peaks have been reported before in numerous organic crystals. 19,20 Y300 presents a larger number of narrower peaks compared the other samples and in particular Y250 (Fig. 4). Higher temperature during synthesis probably result in more ordered organic layer structures. This is also in agreement with the rather low quasielastic scattering intensity for this sample (tail of the elastic peak below 10 cm−1 , see Fig. 4). The arrangement of the organic molecules in the organic layers is also more disordered for samples with para-substituted benzoate molecules (Ym250 and Yx250). This difference is less striking for neodymium-based samples as the spectra of N250 and Nm250 are qualitatively more similar.

Conclusion We have synthesized various yttrium and neodymium-oxides hybrid lamellar supercrystals. Structural and morphological characterization together with vibrational spectroscopic investigation down to ∼ 10 cm−1 were carried out. The Raman spectra in the high frequency domain match previous observations and have been interpreted in terms of the vibrations of the organic layers with one additional peak assigned to optical vibrations in the inorganic one. The low-frequency spectra revealed an unexpected complexity preventing a detailed assignment of the various features at this stage. Numerous peaks are observed which depend on the organic and inorganic layers composition as well as the synthesis temperature. Most low-frequency Raman peaks probably originate from optical vibrations in the organic lay-

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ers. They are therefore sensitive to the arrangement of the ligands surrounding the inorganic layers. This makes low-frequency Raman scattering a promising tool to investigate the structure of the organic layers. In addition, we have tentatively pointed out one peak which could have an acoustic origin. Its frequency matches the breathing vibration of the inorganic layers mass loaded with the organic ones.

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Phonons in Hybrid Lamellar Supercrystals - ACS Publications

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