The Effect of Adjacent Materials on the Propagation of Phonon

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The Effect of Adjacent Materials on the Propagation of Phonon Polaritons in Hexagonal Boron Nitride Kris S Kim, Daniel Trajanoski, Kevin Ho, Leonid Gilburd, Aniket Maiti, Luuk van der Velden, Sissi de Beer, and Gilbert C Walker J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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The Journal of Physical Chemistry Letters

The Effect of Adjacent Materials on the

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Propagation of Phonon Polaritons in Hexagonal

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Boron Nitride

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Kris S. Kim1, Daniel Trajanoski1, Kevin Ho1, Leonid Gilburd1, Aniket Maiti1,2, Luuk van der

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Velden1,3, Sissi de Beer1,3, Gilbert C. Walker1*

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Department of Chemistry, University of Toronto, 80 St. George Street Toronto, Ontario M5S

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3H6, Canada 2

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Department of Physics, Indian Institution of Technology, Kanpur, 208016, India

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands

*E-mail: [email protected]

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ABSTRACT

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In order to apply the ability of hexagonal boron nitride (hBN) to confine energy in the form of

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hyperbolic phonon polariton (HPhP) modes in photonic-electronic devices, approaches to finely

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control and leverage the sensitivity of these propagating waves must be investigated. Here, we

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show that by surrounding hBN with materials of lower/higher dielectric responses, such as air

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and silicon, lower/higher surface momenta of HPhPs can be achieved.

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alternative method for preparing thin hBN crystals with minimum contamination is presented,

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which provides opportunities to study the sensitivity of the damping mechanism of HPhPs on

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adsorbed materials. Infrared scanning near field optical microscopy (IR-SNOM) results suggest

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that the reflections at the upper and lower hBN interfaces are primary causes of the damping of

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HPhPs and that the damping coefficients of propagating waves are highly sensitive to adjacent

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layers, suggesting opportunities for sensor applications.

Furthermore, an

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TOC GRAPHICS

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KEYWORDS

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2D materials, hBN, phonon-polaritons, SNOM, near-field

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When electromagnetic waves are exposed to an interface, they can interact and couple with

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elementary excitations at the surface of the underlying material. Among these coupled states are

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surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs), and the extent of each

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coupling is dependent on the intrinsic degrees of freedom of the electrons and lattice vibrations of

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the material, respectively1–3. In contrast to the widely studied SPPs, SPhPs occur when

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electromagnetic waves couple with the lattice vibrations in polar dielectrics, between the

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longitudinal and transverse optical (LO and TO) phonon frequencies, commonly known as the

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Reststrahlen bands2,4,5.

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momentum and field densities, but SPPs commonly rely on interactions in the visible to near-

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infrared frequencies6–8 while SPhPs are found at infrared to terahertz frequencies4,9.

SPPs and SPhPs share similar properties, including high surface

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Heterogeneous and anisotropic 2D materials, such as boron nitride, can exhibit different types

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of vibrational coupling events within and between layers, opening opportunities to volume

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confined photonic applications3,5,10. In particular, hexagonal boron nitride (hBN) has drawn

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interest due its ability to strongly confine hyperbolic phonon-polariton (HPhP) modes within the

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volume of the material.

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hyperbolicity within the Reststrahlen bands2,5,11. Positive and negative (in- and out-of-plane)

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principle components of the dielectric functions within these bands allows for this strong

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confinement of HPhPs5,10,12. Additionally, HPhPs experience low optical losses, which arise

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from the scattering of optical phonons that typically occur on the order of picoseconds4,10,13. Due

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to these properties, hBN has been suggested for various applications, such as control of energy

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transfer14, super-resolution imaging10, construction of superlenses15, and biosensors16–19.

While hBN is optically anisotropic, the material exhibits natural

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Numerous reports have shown that HPhPs can be launched through different mechanisms. For

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instance, studies15,20 have shown that metal-coated atomic force microscopy (AFM) probes can

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launch HPhPs in hBN and form volumetric standing waves within the bulk of the material in the 4 ACS Paragon Plus Environment

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proximity of a reflective edge21. On the other hand, Li et al.10 showed that a gold overlayer edge

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can serve as a platform to launch propagating HPhPs in thin sheets of hBN. Gilburd et al22

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showed the launching of phonon polaritons in hBN using intrinsic folds of the crystal, and

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Caldwell and coworkers5 showed that dots of hBN could admit phonon polaritons. In order to

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exploit these mechanisms of launching and propagating HPhPs in small photonic-electronic

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devices, approaches to finely controlling these waves must be considered. To address this

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limitation and to better understand the subsequent dissipation of these waves as they propagate,

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this paper examines the effect of adjacent materials on HPhP of hBN.

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Several types of adjacent materials are probed in this work. Using near-field scattering optical

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microscopy we compare the momenta of HPhP of hBN placed directly on silicon with the case of

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an airgap between the materials. Because a common method of exfoliation of hBN leaves a

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dispersive, oily layer on the crystal surface, we compare HPhP momenta on oil-covered hBN

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versus those on clean hBN. Finally, we examine the effect of an IR-absorbing overlayer on hBN

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to evaluate the possibility of detecting small amounts of infrared absorbing materials on hBN via

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the HPhPs.

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In order to implement a bottom-up strategy for fabricating photonic-electronic devices, which

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utilize HPhPs of hBN, the first component to build such a device that must be considered is the

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substrate.

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scanning probe microscopy techniques to study these standing waves in boron nitride of different

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structures, such as thin sheets15,20 and nanotubes3,23–25, with high spatial resolution. Silicon with a

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thick oxide layer is commonly employed to prepare boron nitride for many of these studies15,20.

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Dai et al.20 have previously reported the presence of HPhPs in hBN crystals prepared on silicon

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dioxide and developed a method of obtaining the polariton wavelengths and corresponding

Previous studies have taken advantage of recent advances in near-field optical



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momentum at a range of IR frequencies. Below is the analytical expression proposed to describe

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the polariton dispersion relation for hBN crystals of thickness d [Eq. 1] for large momenta.

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+ = − arctan =

+ arctan

+

[Eq. 1]

∥ √!

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where + is the complex momentum as a function of the wavenumber of the

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incident IR light (ω), l represents the mode of a propagating wave, " , # , ∥ , and

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! are the dielectric functions of air, substrate, and hBN (in- and out-of-plane components),

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respectively. As seen from the expression above, HPhPs are sensitive to the dielectric functions

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of materials adjacent above and below an hBN crystal, represented as air (" ) and substrate (# ) in

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this particular expression.

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materials of differing dielectric properties.

Therefore, HPhPs can be controlled by surrounding hBN with

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To investigate the effect of a substrate on HPhPs in hBN, thin crystals of hBN were deposited

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on a silicon grating. Phase-controlled homodyne SNOM26–29 was used to inject and measure the

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SPhPs in hBN microcrystals. A height and representative normalized out-of-phase near-field IR

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response collected at an excitation frequency of 1560 cm-1 on a 250 nm thick hBN crystal is

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shown in Fig.1. The observed patterns in the IR images are evanescent waves detected on the

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surface of hBN from the volume-confined HPhPs. The standing waves which occur between the

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metalized AFM probe and a reflective edge of an hBN crystal (schematic in Fig.1) have half the

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HPhP wavelength, and thus satisfy the following relationship: $%&# =

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hBN on a silicon grating, the effect of air and silicon as the underlying adjacent material on the

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λobs can be studied (Fig.S1).

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simulated and experimentally constructed by measuring the λobs values on top of hBN sitting on

$'()(+ 2. By preparing

Dispersion profiles for an air-hBN-air and air-hBN-Si were


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the alternating regions of the grating (see SI for more details). These results show that by

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supporting hBN crystals on low (air) or high (silicon) dielectric response material, a lower/higher

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HPhP surface momentum can be achieved. The experimental data collected from hBN on top of

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silicon on the grating is fit with a simulated dispersion profile for a Si/SiO2 substrate (see SI).

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Furthermore, the apparent intensity of HPhPs over the region where hBN is on top of air is higher

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than the measured intensity of the phonon polaritons propagating in hBN over silicon across

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various wavenumbers (see Fig.1C and Fig.S4). Previous reports10,15,30 have shown that metallic

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edges, both above and below hBN, are capable of launching HPhPs. Here, silicon edges from the

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grating can also launch HPhPs and stripe-like patterns can be seen in the IR images across

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various wavenumbers (Fig.1C and Fig.S4) which closely resemble the pattern of the silicon

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grating underneath hBN. The intense brightness over the region where hBN is on top of air can

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also be attributed to the interference of phonon polaritons from the region over silicon,

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illustrating a coupling of phonon polariton states over a region of air (150 nm above silicon).

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Figure 1. (A) Side view schematic of air-hBN-Si system showing volumetric standing waves in

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hBN which occur between an AFM probe and reflective edge. Top view images of (B) height

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and (C) IR field magnitude (at 1560 cm-1) of a 250 nm thick hBN crystal on a silicon grating.

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The HPhPs in hBN when the crystal is on top of silicon have higher momenta, as indicated by the

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smaller λobs in (D), than those on top of air. (E) The experimentally constructed dispersion

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profiles (open circles) of 250 nm thick hBN crystal in an air-hBN-air (red) and air-hBN-Si

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(black) system closely agree with the simulated profiles.

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Based on [Eq.1], HPhPs are not only sensitive to dielectric properties of the substrate

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underneath hBN, but also the material adjacent on top of hBN. When preparing samples by a

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widely-used tape exfoliation method31,32 (Fig.2A), we found that residue from dicing tape was

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left behind on the substrate and on top of hBN. This residue was identified to consist of

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dimethylsiloxane-type moieties through surface sensitive analytical techniques and was found to

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be

The Effect of Adjacent Materials on the Propagation of Phonon

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