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C: Plasmonics, Optical Materials, and Hard Matter

Single Crystal Electrospun Plasmonic Perovskite Nanofibers Ahmed M. Abdellah, Ahmed M Hafez, Sajanlal R Panikkanvalappil, Mostafa A. El-Sayed, and Nageh K. Allam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00788 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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The Journal of Physical Chemistry

Single Crystal Electrospun Plasmonic Perovskite Nanofibers Ahmed M. Abdellah a ‡, Ahmed Hafez a ‡, Sajanlal R. Panikkanvalappil b, Mostafa A. El-Sayed b and Nageh K. Allama*. a

Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in

Cairo, New Cairo 11835, Egypt. b

Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology,

Atlanta, Georgia 30332, United States ‡These authors contributed equally. *E.mail: [email protected]

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ABSTRACT

Hexagonal single crystal perovskite-like (A3B8O21) nanofibers were synthesized via the electrospinning technique. HRTEM imaging showed that each single nanofiber is composed of several interconnected small crystals. The selected area electron diffraction (SAED) analysis reveals a single crystalline nature of the fabricated nanofibers. The fabricated nanofibers showed surface plasmon resonance characteristics. Theoretical and computational calculations using Mie-Gans theory were performed to investigate the plasmonic and optical properties of the obtained nanofibers, which were also confirmed experimentally and compared to gold nanoparticles. The fabricated nanofibers were tested as novel substrate materials for surface enhanced Raman spectroscopy (SERS), showing exceptional performance.

INTRODUCTION Surface plasmons arising from dielectrically confined nanoparticles have attracted a lot of interests nowadays due to their potential in a variety of promising applications, such as energy harvesting,1 optical devices,2 cancer photothermal therapy,3 and biosensing.4 In such fields, the need for tunable plasmonic materials that cover a wide range of the solar spectrum is quiet important. However, the reported plasmonic materials so far are limited to nanoparticles made of noble metals such as silver and gold. Despite their desirable optical capabilities, a plethora of challenges remain for those nanoparticles made of noble metals, including their high cost, lossy nature, poor thermal and chemical stability, diffusion into surrounding structures, and incompatibility with the complementary metaloxide-semiconductor (CMOS) technology, hindering their practical implementation.5 Also, Recent studies showed that conductive metal oxides can be used as an alternative class of plasmonic materials.6 2 ACS Paragon Plus Environment

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Another way to obtain tunable surface plasmonics, that is not quietly investigated, is through the use of semiconductor nanocrystals7-11. Such nanocrystals can exhibit similar size, shape, and tunable surface plasmons as metals.7,

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Moreover, doping and phase transitions of such nanocrystals at different

temperatures give them preferential tunable plasmonic characteristics.12. This allows the dynamic control of these materials, unlike the nanostructured metallic plasmonic materials. Those properties make the materials optically active in the Near-Infrared (NIR) region of the light spectrum, making them attractive in many applications such as CMOS image sensors and energy saving, among others 13 . Among the widely used nanocrystalline materials nowadays are inorganic perovskites due to their exceptional optoelectronic and photocatalytic characteristics as well as their ease of fabrication using conventional thin film technology.14-15 Perovskites constitute a group of isomorphic materials with the unit formula ABX3. While cation A (the larger) is twelve-coordinated with the anions, cation B is octahedrally (six-fold) coordinated with the anions. Nanostructured perovskite materials resulted in more interesting properties, such as controlling the carrier diffusion length, varying the bandgap, and increasing the effective surface area.16-20 However, most of the reported nanostructured perovskites are in the form of nanoparticles,21 resulting in poor carrier transport with high recombination rates of charge carriers. To this end, one-dimensional (1D) nanostructures are investigated as a solution to decrease the high recombination rates and overcome carrier transport problems. Among all studied 1D structures, nanofibers exhibit high surface area and aspect ratio, resulting in better performance upon their use in devices.22 Specifically, electrospun oxide nanofibers can be synthesized via two basic preparation strategies. On one hand, the use of soluble metal precursor as a spinning solution through a sol-gel method results in the formation of oxide nanofibers, followed by calcination to get rid of the polymer.2324

On the other hand, the preformed oxide nanoparticles can be used as building units for electrospun

oxide nanofibers.25 The nanoparticle-based synthetic approach suffers from the presence of a plethora of

grain boundaries that hinder the diffusion of photo-excited charges26.27 More recently, 3 ACS Paragon Plus Environment


electrospinning

of

numerous

complex

oxide

nanofibers

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for

multiferroic, photocatalytic,

ferroelectric, and battery applications have also been investigated,28-34 indicating the validity of this synthetic

technique. Specifically, oxide perovskites such as BaTi3Nb4O17, Ba6Ti14Nb2O39,

Ba3Ti5Nb6O28, BaTiNb4O13 and Ba3Ti4Nb4O21 have been investigated as high dielectric and low loss materials33-34. Also, Ba3Ti4Nb4O21 was synthesized via a conventional solid-state reaction and characterized as a microwave dielectric resonator material 34. Herein, we demonstrate, for the first time, the fabrication of electrospun Ba3Ti4Nb4O21 nanofibers, revealing their interesting surface plasmon properties. Those plasmonic characteristics were also confirmed via theoretical and computational calculations as well as upon their use as substrates for surface enhanced Raman spectroscopy (SERS). EXPERIMENTAL AND COMPUTATIONAL METHODS Preparation of the composite precursor solution: Under constant stirring, barium carbonate (1 g) was dissolved in 3g of acetic acid at 70 °C till obtaining a clear solution of barium acetate. Then, 10 % PVP (Mw = 1300000) solution was prepared in ethanol (99.9%). Titanium isobutoxide (1.1 g), niobium isopropoxide (1.58 g) and barium acetate solutions were slowly added to PVP solution (8 g) under continuous stirring to enhance the electrospinning viscosity. The electrospinning process was performed using an electrospinner (MECC Nanon-01A, Japan) at a working voltage of 26 kV, a working distance of 15 cm, and a feeding rate of 3.9 ml /h. Samples were collected on aluminum foil. The dried perovskite nanofiber was calcined for 4 h at temperatures of 450, 650, and 900 ⁰C in air. SERS Measurements: SERS measurements have been performed when 5 µL of Peovskite solution was added to 2 µL of 1mM methylene blue (MB) solution, then placed on a glass substrate. A thin glass coverslip was used to cover the mixture solution. The previous steps have been done with 5 µL gold nanoparticles (Au NPs) added to 2 µL of 1mM methylene blue (MB) solution as a reference material with strong plasmonic characteristics. Spectra has been measured directly from the suspension at 4 ACS Paragon Plus Environment

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different time intervals at ∼37 °C. Renishaw InVia Raman spectrometer has been used to collect all SERS spectra using 532nm and 785nm laser with 1800 lines/mm and 1200 lines/mm grating, respectively. A 50×0.75 N. A. objective was used to focus the laser beam to ∼1 µm laser spot size. A CCD detector was used to measure the backscattered signals from the samples. Following the same procedure, SERS spectra of 1mM methylene blue (MB) solution sample was also measured. Removal of the spectral background has enhanced the spectra processing. Here, to fit the baseline, cubic spline interpolation was used through selecting the points representative of the background manually. These measurements gave been assessed 4 times at different spots of the sample. Afterwards, Origin 8.0 has been used to average the spectra.

Computational methods: The properties of the fabricated nanofibers were investigated using first principles calculations. The GGA functional is used,35 in the framework of CASTEP package using plane wave basis sets.36 The structure was firstly relaxed before carrying out the calculations. The energy convergence was set to 5.0 x 10-5 eV/atom, the maximum force between the atoms inside the crystal was 0.01 eV/Å, and the stress was set to 0.02 GPa. The self-consistent energy calculations were performed with a cut-off energy of 750 eV and a convergence criteria of 5.0 x 10-6 eV/atom.

Results

and

Discussion

Figure 1a-c depicts field emission scanning electron microscope (FESEM) images of the as-spun and calcinated nanofibers, indicating the formation of homogeneous fibers with lengths exceeding 10 µm. However, annealing the electrospun nanofibers at temperatures as high as 900°C does not result in any morphological collapse of the nanofibers, with the emergence of small crystallites, see Figure S1. The transmission electron microscopy (TEM) images of the nanofibers calcinated at 900 °C (Figure 1d) confirm the integrity of the morphology of the nanofibers at such a high temperature, in agreement with the FESEM analysis. The high5 ACS Paragon Plus Environment


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resolution TEM (HRTEM) images indicated the nanofiber to be composed of inter-connected single crystals, with lattice fringe spacing of 0.391nm, which corresponds to the (200) plane of the Ba3Ti4Nb4O21 nanofibers. The selected area electron diffraction (SAED) pattern (Figure 1 div) confirms the single crystalline nature of the fabricated Ba3Ti4Nb4O21 nanofibers.22

Figure 1. FESEM images of the (a-c) calcined Ba3Ti4Nb4O21 nanofibers at 450, 650 and 900°C, respectively, and (d) HRTEM images of (i-iii) the calcinated Ba3Ti4Nb4O21 nanofibers at 900 °C, and (iv) the SAED pattern.

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To determine the elemental composition as well as the distribution of each element in the fabricated nanofibers, energy-dispersive X-ray spectroscopy (EDX) analysis was performed as shown in Figure S2. The EDX spectrum shows sharp and intense peaks for Ba, Nb, Ti, and O, corresponding to a chemical formula of Ba3Ti4Nb4O21. The EDX mapping analysis shows the homogenous distribution of Ba, Ti, Nb and O in the nanofibers matrix. To investigate the purity of the annealed nanofibers, Fourier transform infrared (FTIR) spectra were measured for the nanofibers calcined for 4 hours at 650 °C and 900 °C, (Figure 2a). The spectra indicate the complete decomposition of polyvinylpyrrolidone (PVP) and most of the organic groups above 450 °C with no bands detected for residual carbon impurities.

Figure 2 . (a) FT-IR of PVP and perovskite nanofibers calcinated at 650 ⁰C and 900 ⁰C and (b) experimental XRD patterns of the Ba3Nb4Ti4O21 nanofibers annealed at 900 ̊C, 650 ̊C and 450 ̊C.

To further confirm the crystallinity and composition of the fabricated nanofibers, the calculated XRD pattern (considering the unit cell shown in Figure S3) was compared to that obtained experimentally (Figure 2b), revealing a strong matching (PDF No. 04-009-2509). Also,


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some unique features can be drawn from the XRD data: (1) the nanofibers have a chemical composition of Ba3Ti4Nb4O21, (2) the obtained nanofibers are phase pure with no impurities, and (3) the Ba3Ti4Nb4O21 nanofibers show high crystallinity at low temperatures (650°C), which is significantly lower than that needed for the powder counterpart (1320 °C ).37 To get an insight into the optical properties of the fabricated perovskite nanofibers, the dielectric constant was calculated using density functional theory (DFT) as implemented in CASTEP code 38. The calculation is based on Fermi’s golden rule, where the imaginary part of the dielectric function was obtained using Eq. 1:39 (ω) =

π

Ω ο

) − Εκν ( ) − ℏω ∑κ,ν, 〈ψκ | |ψκν 〉 δ Εκ (

(1)

where ℏω is the photonic energy, is the perturbation matrix element of the normal Hamiltonian operator between the valence and conduction band states, and δ-function is the conservation energy at the wave vector. Then, the real dielectric part (ω) is obtained using Kramers-Kronig transform. Figure 3a shows the variation of the dielectric function of the Ba3Ti4Nb4O21 with the wavelength. Note that the imaginary part of the dielectric function is positive along the frequency range, which can be related to the absorption properties of the material. However, the real dielectric function has a negative peak at 200 nm with maximum extinction coefficient, indicating a plasmonic property of the material around this wavelength. To further investigate the plasmonic properties of the material theoretically, Mie-Gans theory was used to calculate the extinction spectra (Figure 3b), where the extinction is the sum of the electromagnetic absorption and scattering of the incident waves on the nanofibers. It can be represented by Eq. 2.40 =

"/

!

%$∑/

(/&' ( *,-'

) * +

-'

! .

+

(2)


8

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where N represents the total number of particles exists per unit volume, 0 is the external dielectric constant, V is the average volume of the particle, λ is the wavelength of the incident light, and are the imaginary and real parts of the dielectric functions, respectively, and 1/ is the depolarization factor in the three axes (A, B and C) of the nanofibers. Herein, we assumed that the nanofibers have circular cross section area, thus B=C and the aspect ratio (R) is defined as A/B. The depolarization factor is defined as:40 12 =

3

+

4 ln 73 − 1(9

1: = 1; =

3&<

(3)

(4)

where = = >1 − (?)

(5)

By substituting the dielectric functions obtained from the DFT calculations, Eq. 2 is used to find the extinction spectra for the fabricated nanofibers. While the aspect ratio used was 6.25, the surrounding dielectric medium 0 = 1.5. Note that the experimental absorption spectrum showed two peaks at 225 nm (transverse mode) and 520 nm (longitudinal mode), which are close to those expected via the Mie-Gans theory (at 200 and 720 nm). The well-resolved longitudinal mode reveals that the broadening in the optical absorption of the nanofibers measured experimentally is ascribed to the plasmonic properties of the nanofibers, which agrees with the negative value of the imaginary part of the dielectric function. Figure 3c shows the calculated absorption spectra (Eq. 1) upon varying the aspect ratio (R) of elongated ellipsoids, with fixed medium dielectric constant of 4. Upon changing the aspect ratio from 1.25, 2.5 to 6.25, a significant shift of the λmax was observed at 670, 690 and 720 nm, respectively. Figure 3d shows the corresponding experimental absorption of the samples annealed at different temperatures


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(450, 650, and 900). Increasing the temperature from 650 to 900 resulted in an increase in the particles size of the inter-connected single crystals, and consequently a red shift of the λmax from 470 to 520 nm, which is significantly matching the theoretical data.41 Note also that the longitudinal mode peak observed experimentally is broader than that expected by Mie-Gans theory, which can be related to the sample inhomogeneity and plasmon damping due to the surface scattering electrons.10, 41

Figure 3. (a) real and imaginary dielectric functions, (b) experimentally measured absorption versus the extinction spectra expected from Mie-Gans theory, (c) calculated absorption spectra of elongated ellipsoids with varying aspect ratios (R), and (d) experimental absorption of different annealing temperatures (450, 650, and 900).


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Note also that a carrier density of 4.6 x 1021 cm-3 was reported for BaTi1-xNbxO3, which is similar to our fabricated material42. Furthermore, it was reported that high carrier densities (∼1021 cm−3) can be achieved for doped metal oxide (MO) NCs, which are intermediate between semiconductors and metals, placing their LSPR in the NIR12. Moreover, in doped metal oxides, the role of oxygen vacancies and of other ionic defects crucially influences the electron or hole donor activity. For example, in tungsten oxides (WO3−x) it was reported that oxygen vacancy formation is sufficient to increase the carrier concentration up to the 1021 cm−3 with LSPRs in the NIR10.

Figure 4. Raman spectra of (a) Ba3Ti4Nb4O21 nanofibers and methylene blue (1mM) and (b) SERS of methylene blue adsorbed on the fabricated Ba3Ti4Nb4O21 nanofibers and gold nanoparticles(AuNPs). An additional proof of the plasmonic characteristics of the prepared perovskite nanofiber is obtained via the SERS measurements. Figure 4a shows the Raman spectra of plain perovskite nanofibers as well as methylene blue (MB). While the plain perovskite shows no detectable Raman peak after 1000 cm-1, MB shows a clear peak at 1624 cm-1 with an intensity of 727.95. Upon adsorbing the MB molecules on the perovskite nanofibers and gold NPs (which is a plasmonic reference material in the same absorbance band at 520 nm, Figure 4b), a significant


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enhancement of MB Raman signal is observed on both Ba3Ti4Nb4O21 nanofibers and gold NPs when using 532 nm laser. However, upon the use of 785 nm laser, no enhancement was observed, which demonstrates that the palsmonic band is loacted aroud 520 nm, which can be ascribed to the SPR, leading to a local electromagnetic field enhancement and resulting in the enhancement of the MB Raman signal43-45. It should be mentioned that, for a clear and strong evidence of our findings, several SERS measurements were performed at different spots on the MB adsorbed on the perovskite nanofibers and gold NPs samples (Figure S4), where the presented data in Figure 4b are an average of the obtained results.

CONCLUSIONS We demonstrated the ability to fabricate single crystalline Ba3Ti4Nb4O21 nanofibers. The plasmonic properties of electrospun Ba3Ti4Nb4O21 nanofibers have been explored for the first time through both experimental and computational approaches. Furthermore, surface-enhanced Raman spectroscopy (SERS) of methylene blue adsorbed on Ba3Ti4Nb4O21 nanofibers is strongly supporting the obtained SPR findings. Our results can be extended to other perovskites as alternative plasmonic materials to the conventional plasmonic metals with different optical and electronic properties.

ASSOCIATED CONTENT Supporting Information. S1. FESEM images of the as-electrospun Ba3Ti4Nb4O21 nanofibers. S2. EDX spectrum, S3 Crystal structure, S4. SERS of M.B at different spots on the samples. The Supporting Information is available free of charge on the ACS Publications website


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ACKNOWLEDGMENT N.K.A. acknowledges the financial support by the Egyptian Academy of Scientific Research and Technology (ASRT) under JESOR grant. M.A.E. acknowledges the financial support by the U.S. National Science Foundation (Grant Number OISE-1103827) REFERENCES 1. Atwater, H. A.; Polman, A., Plasmonics for Improved Photovoltaic Devices. Nature Materials 2010, 9, 205-213. 2. Stockman, M. I., Nanoplasmonics: Past, Present, and Glimpse into Future. Optics Express 2011, 19, 22029-22106. 3. West, J. L.; Halas, N. J., Engineered Nanomaterials for Biophotonics Applications: Improving Sensing, Imaging, and Therapeutics. Annual Review of Biomedical Engineering 2003, 5, 285-292. 4. Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A., Review of Some Interesting Surface Plasmon Resonance-Enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems. Plasmonics 2007, 2, 107-118. 5. Mattox, T. M.; Ye, X.; Manthiram, K.; Schuck, P. J.; Alivisatos, A. P.; Urban, J. J., Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures. Advanced Materials 2015, 27, 5830-5837. 6. Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T., Indium Tin Oxide Nanoparticles with Compositionally Tunable Surface Plasmon Resonance Frequencies in the near-Ir Region. Journal of the American Chemical Society 2009, 131, 17736-17737. 7. Agrawal, A.; Cho, S.; Zandi, O.; Ghosh, S.; Johns, R.; Milliron, D., Localized Surface Plasmon Resonance in Semiconductor Nanocrystals. Chemical reviews 2018, DOI: 10.1021/acs.chemrev.7b00613. 8. Zhao, M.; Zhang, J.; Gao, N.; Song, P.; Bosman, M.; Peng, B.; Sun, B.; Qiu, C. W.; Xu, Q. H.; Bao, Q., Actively Tunable Visible Surface Plasmons in Bi2te3 and Their Energy‐ Harvesting Applications. Advanced Materials 2016. 9. Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P., Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nature materials 2011, 10, 361-366. 10. Manthiram, K.; Alivisatos, A. P., Tunable Localized Surface Plasmon Resonances in Tungsten Oxide Nanocrystals. Journal of the American Chemical Society 2012, 134, 3995-3998. 11. Alsaif, M. M.; Latham, K.; Field, M. R.; Yao, D. D.; Medehkar, N. V.; Beane, G. A.; Kaner, R. B.; Russo, S. P.; Ou, J. Z.; Kalantar‐zadeh, K., Tunable Plasmon Resonances in Two‐ Dimensional Molybdenum Oxide Nanoflakes. Advanced Materials 2014, 26, 3931-3937. 12. Kriegel, I.; Scotognella, F.; Manna, L., Plasmonic Doped Semiconductor Nanocrystals: Properties, Fabrication, Applications and Perspectives. Physics Reports 2017, 674, 1-52. 13. Liu, Y.; Njuguna, R.; Matthews, T.; Akers, W. J.; Sudlow, G. P.; Mondal, S. B.; Tang, R.; Gruev, V.; Achilefu, S., Near-Infrared Fluorescence Goggle System with Complementary Metal–Oxide–Semiconductor Imaging Sensor and See-through Display. Journal of biomedical optics 2013, 18, 101303.


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Single-Crystal Electrospun Plasmonic Perovskite Nanofibers - The

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