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Near-Field Infrared Pump-Probe Imaging of Surface Phonon Coupling in Boron Nitride Nanotubes Leonid Gilburd, Xiaoji G. Xu, Yoshio Bando, Dmitri Golberg, and Gilbert C Walker J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02438 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 6, 2016

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The Journal of Physical Chemistry Letters

Near-Field Infrared Pump-Probe Imaging of Surface Phonon Coupling in Boron Nitride Nanotubes Leonid Gilburd,1 Xiaoji G. Xu,1,2 Yoshio Bando,3 Dmitri Golberg,3 and Gilbert C. Walker1*

1

Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada

2

Department of Chemistry, Lehigh University, 6 E Packer Avenue Bethlehem, PA, 18015, USA

3

National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan

Email: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT

Surface phonon modes are lattice vibrational modes of a solid surface. Two common surface modes, called longitudinal and transverse optical modes, exhibit lattice vibration along or perpendicular to the direction of the wave. We report a two-color, infrared pump-infrared probe technique based on scattering type near-field optical microscopy (s-SNOM) to spatially resolve coupling between surface phonon modes. Spatially varying couplings between the longitudinal optical and surface phonon polariton modes of boron nitride nanotubes are observed, and a simple model is proposed.

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Couplings between phonons or vibrational modes within materials govern the ability to store energy in a given mode1 and illustrate the mechanisms of energy transduction. While there has been a long history of such studies on molecules in an ensemble and crystals of micrometer dimensions and larger, spectroscopic studies of vibrational or phononic energy transduction within individual nanoscale samples are rare. On the other hand, studies of individual nanostructures are of interest because the heterogeneous materials are likely to have couplings between different lattice vibrations that vary spatially at the nanoscale. The coupling information obtained from the existing multidimensional phonon-resonant methods2 is ensemble-averaged and the details on the couplings within individual nanostructures cannot be recovered. As examples, the abilities of material to serve as a mid-IR waveguide,3,4 as a thermal conductor,5 sub-diffractional gain media6 or to couple to chemical reactions,7,8 depend on these couplings. More exotic developments, such as polaritonics9 or topolaritonics,10 which envision low-loss phenomena and eventually devices that could function in the frequency region between photonics and sub-microwave electronics, depend on reliable methods to detect (and guide) phonon-polariton transduction in individual structures. This report introduces a novel method of pump-probe IR spectroscopy on nanoscale using a continuous wave light source, which can locally place energy in one mode and observe the migration of the energy into another mode. It is a surface sensitive method. The method is applied to boron nitride nanotubes (BNNTs),11,12,13,14 which are representative of a large class of polar15 covalent compounds with a two-particle basis commonly found in crystalline forms.16,17 Such compounds typically show two characteristic optical phonon bands, known as longitudinal optical (LO) and transverse optical (TO) phonon modes.18 In addition, they can exhibit a third surface mode, the surface phonon polariton (SPhP)19,20,21,22,23,24 In this report the experiment involves 3



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pumping the LO-mode, and then probing induced changes in the TO and SPhP modes. We provide a simple theoretical model for the coupling via localized nonlinear optics. The coupling can also be viewed as coupling through the lattice vibrations. The SPhPs are a hybridization of bound electromagnetic field with a lattice vibration. The lattice vibration at phonon-frequencies allows coupling through the aharmonicity of the vibrational modes. This is more obvious at the lattice defects, which is observed by our measurements.

Figure 1. (a) Simplified scheme of the two-color CW infrared pump-probe scattering type nearfield microscopy apparatus. (b) Illustration of the image dipole model for the sample response.

Figure 1 shows the instrument and summarizes the interacting infrared fields and sample. To obtain the pump-induced probe signal response of the sample (panel a), the phase-sensitive detected probe field was demodulated at harmonics of the sum of photoelastic modulator (PEM) and atomic force microscopy (AFM) tip tapping frequencies, respectively. In panel b, E and E are the pump and probe field respectively. ENF is the scattered near field that carries the sample response. The experimental setup was adapted from a previously reported homodyne detection, scattering type infrared (IR) near-field microscopy apparatus.25,26 In the setup, two quantum 4


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cascade IR lasers (QCL, Daylight Solutions) in the continuous wave (CW) regime simultaneously provide infrared radiation. One of the lasers (noted as ‘IR Pump’) was used to excite the BNNT at 1532cm-1. The polarization state of the pump infrared was modulated by a Photo-Elastic Modulator27 (PEM 90, Hinds Instruments) at the ‘half wavelength’ mode, in other words, the light beam is modulated between two orthogonal, linearly polarized states (parallel and perpendicular to the AFM tip) at twice the PEM’s frequency (2Ω’). More information about the polarization rotation can be found in the Supporting Information. Both lasers were aligned collinearly and focused on the apex of the AFM tip by an off-axis parabolic mirror with a numerical aperture NA=0.25. Because the AFM probe tip is more polarizable along the vertical direction, the coupling between the tip near field and the far field is polarization dependent. By modulating the polarization of the incident light the induced near-field intensity is being modulated while keeping the photon flux on the probe constant. We have found that it does not introduce imaging artifacts in the AFM modes. Either mechanically chopping the beam intensity or using high power IR laser in the pulse mode can affect the normal AFM tapping mode due to varying local heating or/and optical pressure. The collection of the near-field scattered infrared signal that carries the material response signal was made with the same parabolic mirror. The infrared signal was then interferometrically homodyned with the reference field with the same input frequencies as the pump and probe. Two long wave pass IR filter (LP-6715, Spectrogon) were placed in front of a MCT detector (J15D12, Teledyne-Judson) to allow the detection of the probe frequency only. A lock-in amplifier (HF2Li Zurich instrument) is used to demodulate the voltage signal from the MCT detector at the 3rd harmonic of the AFM tip tapping frequency Ω. To obtain the coupled response of the studied material between the pump and the probe fields (the pump-induced probe signal), the detected 5



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probe field was demodulated at the sideband frequency f=2Ω+2Ω’, where Ω’ was the PEM frequency (~50 kHz). A lock-in integration times of 20ms was chosen for the measurement. In the probe-only experiment, it is customary to employ an image dipole representation for the polarization of the tip-sample interface.28,29 In the pump-probe experiment, the polarization of the coupled image dipole including response up to third order in field is

where

,

1

16

1 ,

1

,

and

2 2

:

are the pump and probe field respectively, see Figure 1b.

is the polarizability of the tip dipole. As is shown in Supporting Information, if the third order response is relatively small it therefore can be treated as a perturbation of the first order response, and one can obtain an approximation for the measured difference signal.

∆

2

,

1

1 ,

,

2 2

2

:

2

carries the mode coupling information, and bridge 2D spectroscopy2 with scattering type near

field microscopy. b is a scaling factor representing the overall signal collection and detection coefficient. For detailed derivation, please see the supplementary information. Figure 2 shows the far field (bulk) IR absorption spectrum of the BNNT sample studied. The sample consists of BNNTs placed on top of rough gold coated Si substrate (r.m.s. roughness of ~4 nm). Gold layer of about 100 nm in thickness covers the Si/SiO2 substrate. The sample was synthesized and prepared in a similar way to that described in our earlier work.23 Two primary resonances are centered at ~1375cm-1 and ~1532cm-1, which have been previously reported.30, 31 6


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The resonance at ~1375cm-1 is a TO mode (in-plane, longitudinal stretch). The resonance at ~1530cm-1 is an LO mode (in-plane, transverse stretch). A peak at 1532cm-1 has been previously reported and assigned to the tangential vibration of h-BN network.

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The in-plane normal

hexagonal boron nitride (h-BN) vibrational mode at 1376cm-1 is the brightest IR mode of this material. 32 An orthogonal mode has been reported at 1610cm-1 for 2D h-BN and within the range of 1530-1545cm-1 for BNNTs.33,34,35

Figure 2. Far-field IR absorption spectra for BNNTs. Pump and probe regions of the experiment are shown.

Atomic force microscopy (AFM) images of a BNNT are shown on Figure 3. The tapping mode AFM topography (Figure 3.a) shows a tapered BNNT of about 40nm in diameter placed on a rough gold substrate. The tube has tapered shape along about 2/3 of its length. The rest of the tube (the right part) is not homogeneous.36

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Figure 3. Images of a tapered BNNT. (a) AFM topography image.

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/2 near-field images at 1378(b),

1396(c), 1400(d), 1404(e) and 1408(f) cm-1 probe frequencies (collected without presence of a pump field). g) /2 near-field image collected at 1532cm-1, which illustrates the spatial distribution of absorption of the pump-field used to collect (h-l), which are 1532cm-1 pump-induced / 1378(h), 1396(i), 1400(j), 1404(k) and 1408(l) cm-1 probe images. No pump-induced response was observed at wavenumbers less than ~1388cm1

. The scale bar is 200 nm.

In linear measurements, it is useful to first characterize the spatial distribution of the IR probe field absorbance for both the pump and probe fields. Figure 3.b-f data were obtained in the regions where the probe field had been absorbed ( /2 (out-of-phase) homodyne for the probe field) in the absence of the pump. This response is above the peak frequency of the TO mode of BNNT and instead corresponds to the spectral region where an associated surface phonon polariton has

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been previously observed.19 Figure 3.g shows the regions of the tube where the pump field is absorbed. This is in the frequency region where excitation of the nominal LO mode is expected. Figure 3.h-l show images where the sample is pumped with an IR laser field operating at 1532cm-1, with a probe field in the range of 1386-1408cm-1. The polarization of the pump field is modulated at ~50 kHz, and the probe field is detected with a lock-in amplifier at the sum of twice the tip oscillation frequency and twice the modulation frequency. The images in Figure 3.h-l show signatures of the coupled excitation. A region where the pump is strongly absorbed is recapitulated in many of the probe field images that are pump modulated. The pump-induced response is detectable at /2 homodyne phase only. At this phase, the absorptive profile is dominant in the detected signal. (see Supporting Information for more details and experimental data). Figure 3 illustrates that it is necessary, but not sufficient, to have a signal at /2 for the probe field to demodulate a pump-induced probe response. Experimentally this finding can be seen in Figure 3.d and j and Figure 3.e and k. The observation illustrates that there is no simple spatial correlation between the probe-only signal and the pump-induced signal. In the current work, more than 50 different tubes have been studied (for more details, please refer to the supplementary materials). The LO and TO modes are found in the region of the pump and probe fields, respectively, and we find coupling through the material response. However, the pump-induced probe signal is not strongest at the peak of the TO spectrum, which is ~1376cm-1. Instead, the induced signal is strong where the material exhibits a highly negative real part of the dielectric function37. This region of the spectrum is known to support surface phonon polaritons, 23 and is also in the general range of the phonon of the surface layer of BN layers. The presence of the surface phonon polariton

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mode increases the density of states,38 and therefore more efficiently collects the energy that is down-converted through relaxations of the LO mode. The direction of the electric force from the LO field is appropriate for momentum coupling to the phonon polariton. Energy conservation may be provided by coupling of a low frequency ~125cm-1 mode39, though we have no direct evidence for this. At low frequency (100-200 cm-1) there are a number of modes and mode types active in hBN and BNNT; these include acoustic modes, including Z-acoustic (ZA), longitudinal acoustic (LA) and transverse acoustic modes.40 While small diameter BNNTs exhibit a ZA mode frequency that falls within the necessary region, the mode frequency falls rapidly with increasing diameter for large cylindrical tubes, and is unlikely to provide the necessary coupling. The LA and transverse mode frequencies are also too low near the gamma point. On the other hand, the radial breathing mode is a promising candidate; in BN layered structures it has been showed that a suitable frequency can arise from the counter-phase coupled motions of adjacent sheets.41 This mode has an oscillating electric vector perpendicular to the tube long axis. Stronger exchange coupling between closer layers gives rise to a higher frequency of the out-of-phase coupled motions. While a detailed understanding of the origin of the coupling requires further study, we note several points. The ring breathing mode is one of the few known with frequency appropriate to couple the excitation at 1532 cm-1 with the observed induced change around 1400 cm-1. The spatial variation of the coupling could naturally result from variations in the separation between outer sheets of the BNNT, as well as from sheet regions of different lengths. Our observation here of localized surface phonon polaritons, and of different spatial extents is consistent with the existence of varying sheet lengths within the tubes. Finding such regions is also consistent with earlier work 10


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involving tube electron microscopy12 and shear modulus.13 Variations in a tube’s curvature affect the frequency of the out of-phase mode that results from coupling two adjacent sheets41. The orientation of different layers within the same tube together with tip and substrate field enhancement could cause interlayer coupling of the two orthogonal modes. Golberg et al.30 describes multi-walled BNNT with zig-zag oriented inner layers, arm-chair outer layers and other chirality ranging in between. Localized structural defects such as edges are often accompanied by increased sp3 hybridization which can cause mode shifting42 and may affect the coupling. Whether thermal heating is the primary mechanisms behind the coupling is now considered. To address this question, we collected and compared two probe spectra – with and without the presence of the pump field, (see Supporting Information for details). Due to spectral similarity between the two, one can conclude that heating does not play the main role in the coupling between the modes, though it may play some role. An additional example of a larger diameter and longer BNNT is shown on Figure 4. The pi/2 out-of-phase (left column) and the corresponding pump-induced probe images (middle

column) of the tubes are shown. A pump-induced response was observed within a similar probe field range. Representative wavenumbers are shown. To study the effect of which phonon is initially excited, the pump frequency was varied also. Figure 4 (right column) shows several representative wavenumbers of the pump field while the probe field is kept at 1400cm-1. The pump-induced probe images show two obvious peaks – at 1532cm-1 and 1600cm-1. The two peaks correspond to the LO modes of BNNTs and h-BN, correspondingly. As was reported earlier,42 large-diameter tubes within the same cross-section may have polygonal internal and cylindrical outer layers. The polygonal structure was accompanied

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with sp3-like defects. The 1600cm-1 mode appears more strongly coupled to the 1400cm-1 mode than the 1532cm-1 mode. This appears consistent with a significant role for mechanical coupling between walls of the multi-walled tubes.

Figure 4. (left column) Near-field probe-only and (middle) corresponding pump-induced probe images. (right) Pump-induced probe images, employing a 1400cm-1 probing field while changing the pumping field from 1532-1650 cm-1. Scale bar is 500nm.

The features from the near-field measurements suggest the presence of local and long range SPhP. Figure 4 shows the periodic structure characteristic of the SPhP standing wave on the left

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hand tube, for example. Other features come and go with probe wavelength, consistent with localized surface phonon polaritons. The tapered tube in Figure 3 illustrates this well. Previous work suggests a simple way to describe the photonics observed here: hybridization of surface electric fields of similar frequencies, such as from surface plasmons and surface phonon polaritons results from an admixture of the electric fields according to their relative amplitudes at a given frequency.43 In the case of this nonlinear polarization, three fields of different frequencies are mixed. The radial mode can couple pumped and probed IR surface waves even if their electric vectors are not parallel. If this way, it is possible to see the generation of the lower surface frequency phonon polariton as difference frequency mixing between the high frequency mode and the low frequency radial mode. This is naturally enhanced by the asymmetry of the tip-sample interface. The high density of optical states of SPhPs is also a favorable factor for the coupling. In summary, this work describes a novel pump-probe CW s-SNOM apparatus and shows the first experimental data on the cross-talk between two IR optical surface phonons in BNNTs. It reports the coupling between phonon modes and a phonon polariton mode. The experimental data, as an example, illustrates the coupling between the pump-excited and the probed modes in hexagonal multiwall BNNTs. The energy transfer happens from a high-energy LO phonon mode (1532cm-1) to a lower energy surface phonon polariton mode where the real part of dielectric function is negative (1386-1408cm-1). This method should enable a wide range of in-situ mode coupling studies with nanometer scale resolution. A logical extension of this method may involve the use of ultrafast infrared lasers44 in a photo-echo type of detection mechanism.2 ASSOCIATED CONTENT

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Supporting information Spectral analysis, system response testing, TEM images, PEM operation diagram, and theory of the signal. AUTHOR INFORMATION Corresponding Author Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by NSERC. D. G. acknowledges the WPI-MANA Centre of NIMS (Tsukuba, Japan) for financial support.

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(30) André, C.; Guillaume, Y. C. Boron Nitride Nanotubes and Their Functionalization via Quinuclidine-3-thiol with Gold Nanoparticles for the Development and Enhancement of the HPLC Performance of HPLC Monolithic Columns. Talanta. 2012, 93, 274–278. (31) Rokuta E.; Hasegawa Y.; Suzuki K.; Gamou Y.; and Oshima C. Phonon Dispersion of an Epitaxial Monolayer Film of Hexagonal Boron Nitride on Ni (111). Phys. Rev. Lett., 1997, 79, 4609. (32) Rokuta E.; Hasegawa Y.; Suzuki K.; Gamou Y.; and Oshima C. Phonon dispersion of an epitaxial monolayer film of hexagonal boron nitride on Ni (111). Phys. Rev. Lett., 1997, 79, 4609. (33) Lee C. H.; Xie M.; Kayastha V.; Wang J.; Yap Y. K. Patterned growth of boron nitride nanotubes by catalytic chemical vapor deposition. Chem. Mater. 2010, 22, 1782–1787. (34) Yap Y. K. B-C-N Nanotubes and Related Nanostructures, Springer, 2009, p34. (35) Lee, C. H.; Wang, J.; Kayatsha, V. K.; Huang, J. Y.; Yap, Y. K. Effective growth of boron

nitride nanotubes by thermal chemical vapor deposition. Nanotechnology. 2008, 19, 455605. (36) Nai, C. T.; Lu, J.; Zhang, K.; Loh, K. P. Studying Edge Defects of Hexagonal Boron Nitride using High-resolution Electron Energy Loss Spectroscopy. J. Phys. Chem. Lett., 2015, 6, 4189– 4193. (37) Geick R.; Perry C. H. Normal Modes in Hexagonal Boron Nitride. Phys. Rev. 1966 146, 543. (38) Chen, D. A.; Narayanaswamy, A.; Chen, G. Surface Phonon-polariton Mediated Thermal Conductivity Enhancement of Amorphous Thin Films. Phys. Rev. B. 2005, 72, 155435. (39) Xiao, Y.; Yan, X. H.; Xiang, J.; Mao, Y. L.; Zhang, Y.; Cao, J. X.; Ding, J. W. Specific Heat of Single-walled Boron Nitride Nanotubes. Appl. Phys. Lett. 2004, 84, 4626. 18


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Figure 1. (a) Simplified scheme of the two-color CW infrared pump-probe scattering type near-field microscopy apparatus. (b) Illustration of the image dipole model for the sample response. 82x44mm (299 x 299 DPI)


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Figure 2. Far-field IR absorption spectra for BNNTs. Pump and probe regions of the experiment are shown. 82x65mm (299 x 299 DPI)



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Figure 3. Images of a tapered BNNT. (a) AFM topography image. ߨ/2 near-field images at 1378(b), 1396(c), 1400(d), 1404(e) and 1408(f) cm-1 probe frequencies (collected without presence of a pump field). g) ߨ/2 near-field image collected at 1532cm-1, which illustrates the spatial distribution of absorption of the pumpfield used to collect (h-l), which are 1532cm-1 pump-induced / 1378(h), 1396(i), 1400(j), 1404(k) and 1408(l) cm-1 probe images. No pump-induced response was observed at wavenumbers less than ~1388cm1. The scale bar is 200 nm. 82x84mm (299 x 299 DPI)


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Figure 4. (left column) Near-field probe-only and (middle) corresponding pump-induced probe images. (right) Pump-induced probe images, employing a 1400cm-1 probing field while changing the pumping field from 1532-1650 cm-1. Scale bar is 500nm. 178x123mm (299 x 299 DPI)



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82x44mm (300 x 300 DPI)


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