Tunable Multipolar Surface Plasmons in 2D Ti3C2Tx MXene Flakes

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Tunable Multipolar Surface Plasmons in 2D TiCT MXene Flakes Jehad K. El-Demellawi, Sergei Lopatin, Jun Yin, Omar F. Mohammed, and Husam N. Alshareef ACS Nano, Just Accepted Manuscript • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Tunable Multipolar Surface Plasmons in 2D Ti3C2Tx MXene Flakes Jehad K. El-Demellawi,†,‡ Sergei Lopatin,§ Jun Yin,‡ Omar F. Mohammed,‡ and Husam N. Alshareef*,† †

Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia

‡

KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia §

Core Labs, King Abdullah University of Science and Technology (KAUST), Thuwal 239556900, Kingdom of Saudi Arabia

KEYWORDS: low-loss/core-loss EELS; independent polarizability; in-situ heating STEM; temperature-dependent STEM-EELS; surface plasmon blue-shift

ABSTRACT

2D Ti3C2Tx MXene was recently shown to exhibit intense surface plasmon (SP) excitations, however, their spatial variation over individual Ti3C2Tx flakes remains undiscovered. Here, we use scanning transmission electron microscopy (STEM) combined with ultra-high resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both

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optically-active and -forbidden modes) in mono- and multi-layered Ti3C2Tx flakes. With STEMEELS mapping, the inherent inter-band transition in addition to a variety of transversal and longitudinal SP modes (ranging from visible down to 0.1 eV in MIR) are directly visualized and correlated with the shape, size and thickness of Ti3C2Tx flakes. The independent polarizability of Ti3C2Tx monolayers is unambiguously demonstrated, and attributed to their unusual weak interlayer coupling. This characteristic allows for engineering a class of nanoscale systems, where each monolayer in the multi-layered structure of Ti3C2Tx has its own set of SPs with distinctive multipolar characters. Moreover, the tunability of the SP energies is highlighted by conducting insitu heating STEM to monitor the change of the surface-functionalization of Ti3C2Tx through annealing at temperatures up to 900 oC. At temperatures above 500 oC, the observed fluorine (F) desorption multiplies the metal-like free electron density of Ti3C2Tx flakes resulting in a monotonic blue-shift in the SP energy of all modes. These results underline the great potential for the development of Ti3C2Tx-based applications, spanning the visible-MIR spectrum, relying on the excitation and detection of single SPs.

Light-matter interactions at the nanoscale have been greatly enriched by the collective oscillations of free charge carriers, commonly known as surface plasmons (SPs).1-10 The ability to manipulate light at nanometer scales through coupling with resonating SPs, is known to enhance the performance of numerous applications ranging from biological, chemical and optical sensors to photovoltaic devices, and surface-enhanced Raman spectroscopy (SERS).1-5,11-14 Despite the predominance of metallic nanostructures e.g., gold, silver, aluminum and copper, in plasmonic applications,3,5,7 SP excitations are not necessarily limited to metals and are already evidenced in other materials with substantial free charge carrier density.3,5 In this regard, owing to their reduced dimensionality, 2D materials standout for their electronic and photonic


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properties.14-16 The outstanding light-confinement at their surfaces makes them excellent candidates for the plasmonic platform.17-20 Thus far, several 2D materials such as graphene,17,19,21 black phosphorous,22 hexagonal boron nitride,20 transition metal chalcogenides e.g., MoS218,23 and TiS2,5 and topological insulators,24 have been already exploited in plasmonic applications. MXenes, another class of transition metal-based 2D materials, in particular Ti3C2Tx (Tx denotes surface-terminated moieties such as −O or −F), has also garnered a lot of interest among the plasmonic community. Ti3C2Tx, the most studied MXene by far, has demonstrated an outstanding performance in a plethora of applications including energy storage,26-30 sensing31 and electronics.32,33 Owing to its optoelectronic properties and metal-like free electron density,5,14,32,34,35 Ti3C2Tx has also shown a great potential for electromagnetic shielding,36 photonic diodes,14 photo-thermal therapy,37-39 and SERS.40,41 Recently, Mauchamp et al. had revealed that Ti3C2Tx is a plasmonic material.42 However, unlike other 2D materials, they showed that Ti3C2Tx exhibits a thickness-independent bulk plasmon energy due to the weak coupling between the layers. Whereas the SP band, featured at infrared (IR) frequencies, has clearly demonstrated a thickness-dependent behavior. Furthermore, the dependence of such SP band on the functional groups, tethered to the MXene sheets, has been computed. Nevertheless, the specific modes of the SP were not investigated. In this context, EELS in conjunction with STEM stands out for its technical capability to investigate local SP modes with high spatial resolution,7,12,44,46 where the incident fast electrons couples to the SPs, resulting in inelastic kinetic energy losses.1,6,8,47 In contrast to light-based techniques with very reduced spatial resolution due to light diffraction-limit,8,48 electron beams allow the excitation of both optically-active and -forbidden SP modes.2,4,13,49,50 Those modes have been extensively analyzed in metal nanostructures e.g., gold and silver, by the use of


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STEM-EELS.3,4,8,51 However, to our knowledge, no progress has been made on specific plasmon modes sustained by Ti3C2Tx neither their thickness-dependence nor morphological-dependence; i.e., shape and size. Not to mention their SPs localization and spatial distribution over the flakes. Herein, we employ STEM and ultra-high resolution EELS,45,46 particularly low-loss, to conduct a comprehensive study of SPs sustained by mono- and multi-layered Ti3C2Tx flakes. We address a variety of SP modes spanning the near-infrared (NIR) and mid-infrared (MIR) wavelengths, which is a crucial spectral range for optical communication, biological imaging, chemical sensing, and heat management.11,34,42 We visualize a set of edge and central longitudinal modes with distinctive multipolar characters, in addition to a transversal mode and the inherent inter-band transition (IBT) supported by Ti3C2Tx flakes. We investigate the independent nature of monolayers constituting multi-layered Ti3C2Tx flakes and demonstrate the correlation of the longitudinal SP modes (both optically-active and -forbidden) with the shape, size and thickness of the flakes. Eventually, we use in-situ heating (from room temperature to 900 oC) combined with STEM-EELS to follow the evolution of the longitudinal and transversal modes as well as the inherent IBT sustained by the Ti3C2Tx flakes. Consequently, we demonstrate that, by means of annealing, SP energy can be tuned by controlling the population of Tx, hence, the free electron density of Ti3C2Tx.

RESULTS AND DISCUSSION In the current study, mono- and multi-layered flakes of Ti3C2Tx were synthesized following a previously reported method50 as described in supporting information (SI). Afterwards, we conducted X-ray diffraction (XRD) and Raman spectroscopy to confirm the structural characteristic peaks of Ti3C2Tx as shown in Figure S1a-b. In parallel, density functional theory


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(DFT) calculations were used to obtain the dielectric function of Ti3C2Tx (see computational methods in SI). Figure S2 shows the real and imaginary components of the dielectric function of both bulk and monolayer Ti3C2Tx. For mono-layered flakes, the real component of the dielectric function became negative from 615 to 740 nm and at wavelengths larger than 885 nm, indicating the onsets of free charge carrier oscillations. To perform STEM imaging in combination with EELS (both core-loss and low-loss regions), a large cluster of Ti3C2Tx flakes was randomly deposited on ultrathin 5 nm-thick silicon nitride (Si3N4) TEM grids. Core-loss EELS revealed the C-K, Ti-L and O-K edges which are typical features for Ti3C2Tx (see Figure S1c).53,54 It has also allowed us to trace the change of relative oxygen/fluorine concentration vs. temperature during in-situ heating experiments. Additionally, the low-loss EELS spectra was used to estimate the thickness of various Ti3C2Tx flakes (see experimental methods for more details). In a typical Ti3C2Tx sample, a large assembly of nano- and micro-sized 2D flakes with regular and complex geometries was obtained. The inset of Figure 1a shows the high-angle annular dark field (HAADF) STEM micrograph of a 7.5 nm-thick truncated triangular Ti3C2Tx flake. The ZLP-subtracted EEL spectra (Figure 1a) considerably varied as the probe was scanned from one position to another over the Ti3C2Tx flake. Six distinct features were identified at 0.44, 0.66, 0.8, 0.95, 1.7 and 3.5 eV, respectively. By tracing the spatial distribution of the intensity maxima of these features, we obtained the EELS intensity maps (Figure 1b-g) corresponding to each peak.


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Figure 1. (a) ZLP-subtracted EELS acquired on a truncated triangular flake of Ti3C2Tx (7.5 nm ± 0.04). The spectra are normalized to the peak at 1.7 eV (yellow shading) and magnified by a factor of 10 above 2.5 eV. Inset: STEM-HAADF micrograph of the Ti3C2Tx flake on a Si3N4 supporting membrane (black area). Red circle and green ellipse mark the most intense excitation positions for the dipole and quadrupole modes, respectively. Blue and purple regions denote the strongest excitation areas for the central modes. White dotted contour defines the perimeter within which the transversal mode (1.7 eV) and IBT (3.5 eV) could be excited. (b-g) EELS fitted intensity maps of the corresponding SP modes and IBT sustained by the flake at the energy losses plotted in (a). The dotted lines are added to define the boundaries of the flake. Considering the wavelike nature demonstrated by the images in Figure 1b-e, together with their highly dissipative EELS signals (full width at half maximum (FWHM) is ~ few hundreds of meV), we interpret the excitations at 0.44, 0.66, 0.8 and 0.95 eV as the multipolar longitudinal modes of the stationary SP wave sustained by the Ti3C2Tx flake.4,57 The first two modes will be referred to as edge modes as their EEL probability is localized at the lateral edges of the


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triangular flake. Each mode has a distinctive multipolar character defined by the number of nodes along the edge. The first mode at 0.44 eV, with no nodes, corresponds to the fundamental dipole mode with its ubiquitous spatial distribution at the corners (red circle in the inset of Figure 1a).4,7,50,57,58 This mode is strongly coupled to the electromagnetic field of an incident light and is thus considered as the best optically-active SP mode. It is often referred to as the super-radiant mode.51,57,59 The second peak at 0.66 eV, with one node at the middle of the lateral edges (green ellipse in the inset), represents the so-called quadrupole mode.57,60,61 In the same manner, more nodes at the lateral edges will correspond to higher-order multipolar edge modes as will be discussed later in this work. High-order SP modes, which are weakly coupled to incident light, are referred to as subradiant modes.7,13,49,50 Nonetheless, these modes are of particular interest for many applications. Owing to their vanishing dipole moments, they can store considerably electromagnetic energy more efficiently than super-radiant modes due to their impediment of radiative losses.13,59,50 Analogous multipolar modes have been observed in other metallic nanostructures.44,48,51,55,57 As the number of nodes increases, another set of multipolar modes (sub-radiant) starts to display strong EEL probability in the central region of the flake, where the free electron density oscillates radially.64-66 These modes will be referred to as central modes. The first prominent peak localized at the center of the flake at 0.8 eV (blue region in the inset), is assigned to the first-order central mode or so-called breathing mode. A higher-order central mode was also detected at 0.95 eV (purple region in the inset), which arises from a combination of dipole and breathing modes.7,65 For simplicity, this higher-order central mode will be referred to as ring mode. These central modes are typically excited on both top and bottom surfaces of the


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flakes.2,12 Thus far, similar modes were only reported for metals with various 2D nanostructures including nanotriangles, 1,12,57,63,64,66 nanodisks,2,65 and nanosquares.7 Furthermore, an additional mode was detected at 1.7 eV (yellow shading in Figure 1a), which is relatively intense for excitation at any position on the flake. The corresponding EELS intensity map of this mode (Figure 1f) supports its attribution to the transversal SP mode owing to the uniform distribution over the entire area of the flake (dashed white contour in the inset). It follows the general pattern where a transversal mode is higher in energy and is size-independent contrary to longitudinal modes, which are much stronger, lower in energy and highly sizedependent.6,8,45,51,57,59,62,68 Transversal modes in the same energy range were reported for other 2D materials such as graphene.21 Apart from the aforementioned longitudinal and transversal SP modes, the EELS peak with an onset at ~3.5 eV and extending up to ~6 eV (grey spectrum in Figure 1a) was attributed to the IBT of Ti3C2Tx.42,43 For better visualization, the ZLP-subtracted spectrum above 2.5 eV in Figure 1a was magnified by a factor of 10 as the SP modes are much more intense than IBTs.42 The EELS map of the IBT (Figure 1g) showed a homogenous intensity distribution all over the flake. To investigate the geometrical dependence of SPs sustained by Ti3C2Tx, we studied various flakes with different shapes, sizes and thicknesses. The lateral length of the studied flakes ranged from 80 to 1080 nm with thickness between 1.5 (equivalent to mono-layered Ti3C2Tx)69,70 and 55 nm. The STEM-HAADF micrographs and EELS intensity maps obtained for various shapes e.g., triangular, rectangular, slabs and polygon-like, with different sizes and thicknesses are depicted below and in the SI. In all flakes, multipolar edge and central modes were clearly evidenced with similar spatial distributions as imaged in Figure 1 but different energy maxima. The lowest energy edge modes; i.e., dipole and quadrupole modes, are distinguishable as they occupy a


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special place as illustrated in Figure 1. The central modes are also conspicuous with their spatial localization on the surface of the flakes. Conversely, the size-independent behavior of the transversal SP mode and IBT was ubiquitous in all flakes with no change in either their spatial distribution or energy position. To this point, we have shown that both energy and spatial distribution of the abovementioned longitudinal modes arise from their confinement at the lateral edges and surfaces of the flakes. Thus, they are primarily linked to the flakes’ morphology. Table S1 in the SI summarizes the energy dependence of the longitudinal SPs (dipole and quadrupole modes) on the geometrical features of the studied flakes. For flakes with similar shapes, the energy of a given mode is clearly red-shifting with increasing the length and/or reducing the thickness. The SP energies do separately depend on the length and the thickness as well as the shape. This phenomena is related to the so-called retardation effect.57,67,71 However, in metallic nanostructures, with sizes less than 200-400 nm; i.e., half the typical wavelength of light, the energy of longitudinal modes merely depends on the aspect ratio (length/thickness) rather than the actual shape or size.49,57,67 It is worth mentioning that, owing to retardation, coupling of light to sub-radiant modes may become possible.4 In terms of the biggest red-shift, we address the ultimate case (energy resolution limit, see experimental methods) by studying a large (~1µm) triangular flake (Figure 2a). The thickness of this flake (everywhere except for a small part at the bottom-right corner) was estimated to be around 1.5 nm, on par with previously reported thickness for Ti3C2Tx monolayers.69,70 Note that other mono-layered flakes are shown in Figure S4 and S9 in the supporting information. The large lateral length of the monolayer Ti3C2Tx flake (Figure 2a) not only red-shifted the energy of the multipolar longitudinal modes but also allowed for the appearance of a higher-order edge


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mode; i.e., hexapole mode, with two nodes at the edges.50,58 Similar hexapole modes were also detected in other flakes (see Figure S5-S8 and S10). The corresponding EELS intensity maps of the dipole, quadrupole, hexapole, breathing and ring modes at 0.11, 0.19, 0.24, 0.28 and 0.47 eV are displayed in images (1-5) in Figure 2a. Figure S3 illustrates the spatial distribution of those modes with respect to the boundaries of the large triangular flake. Noticeably, we were able to resolve and map spectral features with energy losses between 11300 to 2600 nm in the MIR domain, with energy difference of just 40 meV. The distinctive size-independent transversal mode as well as the IBT at 1.7 and 3.5 eV are depicted respectively in images (6-7) in Figure 2a.


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Figure 2. (a) STEM-HAADF micrograph of a monolayer (1.54 nm ± 0.05 thick) triangular Ti3C2Tx flake. (1-7) EELS fitted intensity maps of SPs and IBT sustained by the flake. Images (1-5) visualize the dipole, quadrupole, hexapole, breathing and ring modes, respectively. Images (6-7) correspond to the transversal mode (1.7 eV) and IBT (3.5 eV), respectively. The red-dashed rectangle in (a) denotes the region where a second monolayer triangular Ti3C2Tx flake is formed on the large flake. (b) Magnified image for the small triangular flake shown inside the red rectangle in (a). Images (8-10) show the corresponding independent dipole, quadrupole and breathing modes supported by the small monolayer triangular flake in (b). What makes the flake in Figure 2a special is the evident presence of a second much smaller (~200 nm) triangular Ti3C2Tx flake (denoted by the red-dashed rectangle) formed on the surface of the large triangular flake. The thickness of the specimen in that area was estimated to be 3.08 nm ± 0.04, which indicates an increment of just a monolayer in transition to the small triangular sub-part. The doubled thickness explains the augmentation of EELS intensity at the bottom right corner of images (6-7) in Figure 2. Most importantly, specific to this area only, distinct low-loss features, attributed as dipole, quadrupole and breathing modes, were detected respectively along the corners, edges and the center of the small triangular flake. The corresponding EELS intensity maps are shown in Figure 2b (8-10) at 0.34, 0.44 and 0.62 eV, respectively. Such results are considered as a visual experimental proof that Ti3C2Tx layers act as independent polarizable sheets owing to their weak interlayer coupling.42 Another example is depicted in Figure S8, where two distinct multipolar modes were recorded by EELS mapping at 0.6 eV. The first one was attributed to the hexapole mode of the rectangular slab, while the other was assigned to the dipole mode sustained by the rectangular sub-part on the right.


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As reported for metals, semi-metals or semi-conductors, the SPs in Ti3C2Tx would strongly depend on the free charge carrier (electron) density on the surface Ti3C2Tx.3-5,42,51,72 In this context, Ti3C2Tx is known to exhibit a metal-like free electron density, which is partially related to the chemically active surface-terminated moieties (Tx).34,36 Hence, in addition to geometrical dependence, it is also possible to tune the SP resonance in Ti3C2Tx by controlling the population of Tx e.g., F, O.42,43,73 Wherefore, Ti3C2Tx flakes were annealed at different temperatures under ultra-high vacuum inside the TEM chamber, to alter the population of Tx. In-situ heating combined with STEM-EELS was performed to investigate the effect of annealing on the energy of longitudinal and transversal SP modes supported by Ti3C2Tx. It is worth mentioning that no phase transformation has taken place during the in-situ heating process, which might well happen at such elevated temperatures under continuous electron-beam irradiation.74 In our experiments, the flakes were first annealed for 30 minutes at each temperature point (300, 500, 700, 900 oC) then cooled down to room temperature (Troom) before collecting the EELS spectra and maps; i.e., all the measurements were performed at the same Troom. Figure 3a shows the STEM-HAADF micrograph of a triangular Ti3C2Tx flake along with the corresponding EELS intensity maps of the dipole and transversal modes as well as the inherent IBT. The ZLP-subtracted low-loss EEL signals for the annealed samples, ranging from Troom to 900 oC, for the dipole and transversal SP modes are, respectively, displayed in Figure 3b-c. The SP modes showed a significant monotonic shift in the peak maximum after annealing at temperatures above 500 oC. The shift was also observed in other SP modes e.g., quadrupole, sustained by different Ti3C2Tx flakes regardless of the shape, size or thickness (Figure S10b). Conversely, there was no detectable change in the IBT (see Figure 3d).


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Figure 3. In-situ heating STEM-EELS of Ti3C2Tx at 300, 500, 700 and 900 oC. (a) STEMHAADF micrograph of a triangular flake (55.4 nm ± 0.04 thick in the center), and its corresponding EELS fitted intensity maps of SPs and IBT supported by the flake (before annealing). Dotted lines are added to define the boundaries of the flake. ZLP-subtracted temperature-dependent EEL signals (acquired after cooling down to Troom) of the (b) dipole and (c) transversal SP modes as well as (d) IBT. (e) Core-loss temperature-dependent EELS of Ti3C2Tx averaged over a large area. The dashed lines are guides for the characteristic O-K, Ti-L1 and F-K edges at 535, 567 and 685 eV, respectively. Notice the gradual desorption of F after annealing at temperatures >500 oC. (f) The estimated [O]/[F] ratio at each temperature. Inset shows the decrease in the amount of F relative to O. To probe the effect of in-situ heating on the population of Tx, we used the core-loss EELS to estimate the relative amounts of [O] and [F] on the MXene surfaces. Figure 3e presents the characteristic O-K, Ti-L1 and F-K core-loss spectra of Ti3C2Tx,53,54,73 acquired under the same parameters after each annealing cycle. Similarly, for samples annealed at temperatures higher than 500 oC, there was also a change in the core-loss EELS, where a gradual desorption in F was observed as indicated by the waning F-K peaks in Figure 3e. This correlates very well with the recently reported F desorption via temperature-dependent XPS at temperatures higher than 550 o

C.75 In contrast, the integrated intensity of the O-K spectrum remained unaltered throughout the

in-situ heating process. Figure 3f shows the evolution in the estimated [O]/[F] ratio as a result of F desorption during the in-situ heating. The ratio ~ 1.75 for the specimen before annealing (i.e. kept at Troom) is in good agreement with previous reports.54 The manifested fluorine desorption in virtue of in-situ heating increased the metal-like free electron density (𝑁𝑒 ) at the surface of the conductive Ti3C2Tx. Consequently, the SP modes were


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blue-shifted to higher energies as displayed in Figure 3 and S10. Nonetheless, such an increase in 𝑁𝑒 induced an additional screening by the flake’s charged surface limiting the mean free path of the fast electrons.2,5,9,76 Hence, a strong damping in the SPs, lessening and broadening the corresponding EELS signals, is induced as shown in Figure 3 and S10.5,17,76 In principle, SPs are highly dissipative excitations,4 where the peak linewidth (𝛾) of their EELS signals is directly proportional to the induced damping that affects the quality factor of all SP modes.4,5,75 The quality factor of a SP mode is defined by the ratio between the resonance energy at peak maximum and 𝛾.3 For the specimen (the flake displayed in Figure 3) before annealing, the quality factors for the dipole and transversal modes were estimated to be ~2.5 and 3.3, respectively. These values are on par with other plasmonic materials e.g., gold nanoparticles and copper sulfide quantum dots.3 However, the upsurge in the damping effect after in-situ heating induced an omnipresent drop in the SP quality factors of both modes as shown in Figure 4a-b. The concurrent increase in 𝑁𝑒 instigating the additional screening, was estimated using the bulk-plasmon energy, which comes after the IBT. At resonance, the SP energy is described as 𝑁 𝑒2

ω𝑝

ω𝑠𝑝 = √1+2𝜀 − 𝛾 2 , where ω𝑝 = √𝜀 𝑒𝑚 denotes the bulk-plasmon energy which directly 𝑚

𝑜

𝑒

depends on 𝑁𝑒 ,3,51,72 𝜀𝑚 is the dielectric constant of the medium, 𝛾 (peak linewidth) represents the energy dissipation, 𝜀𝑜 is the free space permittivity, 𝑒 is the electron charge, and 𝑚𝑒 is the electron effective mass. For the triangular Ti3C2Tx flake shown in Figure 3a, we used the energy of the dipole mode (Figure 3b) as the SP energy (sp) to calculate the bulk-plasma energy (p). From the above equations, 𝑁𝑒 of the same flake was estimated to be 1.6 x 1021 cm-3 at Troom before annealing, which accords well with previous reports for Ti3C2Tx.34,35 Correspondingly, 𝑁𝑒 was also estimated after each annealing cycle. Noteworthy, the effect of thermal expansion was neglected because the low-loss and core-loss EEL signals were recorded at Troom after cooling


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down from each annealing temperature. Figure 4c shows the increase in 𝑁𝑒 as the used temperature increases, both with (𝛾 ≠ 0) and without (𝛾 = 0) the presence of energy dissipation. Figure 4d (right axis) displays the dependence of 𝑁𝑒 on the population of surface moieties (Tx). The energy variation of the dipole and transversal modes as a function of F desorption, which proliferated the free electron density of Ti3C2Tx, is shown in Figure 4d (red and blue left axes).

Figure 4. SP energy as a function of the annealing temperature for the (a) dipole and (b) transversal modes. All are obtained after cooling down to Troom. The estimated quality factors of the (a) dipole and (b) transversal modes are plotted on the right (red-axis). Insets: Variation in 𝛾 of the (a) dipole and (b) transversal modes with the annealing temperature. (c) The variation in Ne with the annealing temperature in the presence (blue columns) and absence (red circles) of


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dissipation. (d) Effect of F desorption on Ne (black triangles), dipole (red circles) and transversal (blue squares) modes.

CONCLUSIONS In summary, we have used spatially resolved ultra-high resolution EELS to show the existence of edge and central longitudinal multipolar modes accompanied by a transversal mode in addition to the inherent inter-band transition sustained by Ti3C2Tx flakes. As expected, the energy and spatial distribution of both the transversal SP mode and the IBT were shown to be invariant across the flakes. On the other hand, the longitudinal multipolar modes (both superradiant and sub-radiant) demonstrated a substantial energy and spatial variation over mono- and multi-layered Ti3C2Tx flakes. They are strongly related to the morphological aspects of the flake e.g., shape, size and thickness, and thus allowing for tunable SP energies. Besides, sub-radiant multipolar modes have never been demonstrated for any other 2D materials. Most importantly, we have shown unambiguously that each monolayer in a multi-layered Ti3C2Tx flake behaves like an isolated sheet, sustaining discerned set of longitudinal SP modes. This precisely exposes the true 2D nature of Ti3C2Tx and the fundamental difference from the “classical” plasmonic metals. Ultimately, we have performed the in-situ heating of Ti3C2Tx flakes up to 900 oC to reveal the tunability of their longitudinal and transversal SP energy by altering the population of Tx on the surface of the sheets. The induced desorption of surface-terminated F has noticeably multiplied the free electron density of Ti3C2Tx. Our results show that Ti3C2Tx holds a great potential for the development of MXene-based sensing and photonic applications such as biological sensors, broadband photodetectors and plasmonic waveguides.

EXPERIMENTAL METHODS


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Detailed experimental information on the synthesis of 2D mono- and multi-layer flakes of Ti3C2Tx can be found in the Supporting Information. XRD and Raman Characterization: X-ray diffraction (XRD) analysis was conducted using a powder X-ray diffractometer (Bruker D8 Advance, Cu Kα radiation) with scanning rate of 0.02/step and 0.5 s/step in the range 2 angles of 5- 60. Corresponding Raman spectra were measured by micro-Raman spectrometer (LabRAM Aramis, Horiba, Japan) equipped with a 473nm wavelength cobalt blue laser and an Olympus ×50 objective lens. STEM-EELS Characterization: To obtain low-loss and core-loss EELS data in combination with STEM imaging (so-called STEM spectrum imaging or SI) Ti3C2Tx flakes were drop-casted on Si3N4 TEM grids (SiMPore Inc.) with 5 nm thick windows. Measurements were performed at 80 kV with a ThermoFisher USA (former FEI Co) Titan Themis Z (40-300kV) TEM equipped with a double Cs (spherical aberration) corrector, a high brightness electron gun (x-FEG), an electron beam monochromator, and a Gatan Quantum 966 imaging filter (GIF). Both core-loss and low-loss spectra were acquired in so-called microprobe STEM mode with about 1 mrad semi convergence angle (4nm probe size). For low-loss EELS the monochromator operation was optimized by method first implemented in45 and described in details in.46 As a result, mapping of spatial distribution of the SP modes was obtained with the energy resolution of about 45-50 meV (defined as the full width at half maximum of the zero energy loss peak or ZLP), allowing to resolve spectral features (SPs resonant maxima) at energies down to ~110 meV (i.e. MIR range) with no need for further deconvolution of the data. The background subtraction was performed by fitting of ZLP measured on pristine Si3N4 (without Ti3C2Tx flakes). In-situ heating of the specimen was performed using the double tilt heating TEM holder (Gatan, model 652) for different temperatures (300, 500, 700 and 900 oC) under ultra-high vacuum


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inside the microscope. At each elevated temperature point, the specimen was kept for 30 mins. Then it was cooled down to room temperature to obtain low-loss plasmon maps and core-loss averaged spectra. To deduce the relative amounts of oxygen and fluorine in the specimen after annealing at each temperature, we performed standard EELS quantification routine provided by Digital Micrograph software (Gatan). This routine is based on finding the ratio between integrated intensities under the O-K and F-K edges. Although it might result in relatively high error (10-20%) in the determined absolute concentration, this method is accurate enough to find the trend of relative oxygen/fluorine concentration ([O]/[F]) vs. temperature (see Figure 4f) as long as it is applied consistently throughout all the measurements for different temperature points. To improve the accuracy, O-K and F-K core-loss spectra were averaged (obtained) while scanning over large areas (~300 x 100 nm2) of MXene sheets with almost no impurities. Thickness Determination: To derive the thickness of Ti3C2Tx flakes, the log-ratio method was used.55,56 Following this method, the absolute thicknesses (t) can be calculated by knowing the inelastic mean free path () of the incident 80 keV electrons in the material. However, in our measurements, we used the effective inelastic mean free path (eff). Here “effective” implies that it is only valid for our fixed EELS experimental conditions (beam convergence angle, spectrometer collection angle, HT and the selection of energy window for ZLP integration). To estimate eff in Ti3C2Tx first we evaluated eff in Si3N4 supporting membrane, since its absolute thickness is known to be 5 nm. For this we compared the high resolution ZLP signal (an integral in -0.1 to 0.1eV energy window) from the Si3N4 to the ZLP signal from a nearby hole in the substrate; i.e., vacuum. We took the ZLP from vacuum as the total number of electrons in EEL spectrum. The natural log of the ratio between the two signals was taken to be the relative


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thickness (t/eff) of the material.55,56 From our measurements, eff in Si3N4 membrane was estimated to be ~ 33 nm. Likewise, we reliably assumed the eff in Ti3C2Tx to be ≤ 33 nm owing to the higher average atomic number Z of MXene compared to Si3N4 (Z-0.28).77 Table S1 shows the estimated thickness and mean lateral length of all flakes studied in the work.

ASSOCIATED CONTENT The authors declare no financial interest. Supporting Information. Further details related to synthesis of mono- and multi-layered flakes of Ti3C2Tx; Interpretation of their XRD and Raman measurements, Figure S1 displaying the corresponding XRD, Raman and core-loss EELS spectra; Table S1 presenting the geometrical dependence of the dipole and quadrupole modes for all flakes studied in this work; Figure S2 showing the optical response of Ti3C2Tx as obtained using DFT calculations; Figures S3-S9 demonstrating the STEM-HAADF micrographs of various flakes along with their corresponding EELS fitted intensity maps of SPs and IBT; Figure S10 showing the in-situ heating STEM-EELS of a rectangular slab as well as the corresponding STEM-HAADF micrograph and EELS fitted intensity maps of SPs and IBT (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Research reported in this publication has been supported by King Abdullah University of Science and Technology (KAUST). The authors would like to thank Ahmed El-Zohry, Hyunho Kim, Abeer Ahmed, Andrey Chuvilin and Alexander Govyadinov for their fruitful discussions. REFERENCES 1.

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TABLE OF CONTENTS GRAPHIC Keywords: low-loss/core-loss EELS; independent polarizability; in-situ heating STEM; temperature-dependent STEM-EELS; surface plasmon blue-shift

Tunable Multipolar Surface Plasmons in 2D Ti3C2Tx MXene Flakes Jehad K. El-Demellawi,†,‡ Sergei Lopatin,§ Jun Yin,‡ Omar F. Mohammed,‡ and Husam N. Alshareef*,†

Spatial and energy distribution of electronic excitations; i.e., surface plasmons and interband transitions, on 2D Ti3C2Tx MXene flakes, characterized by a combination of STEM and high-resolution EELS. The geometrical dependence of surface plasmons is explicitly demonstrated. The independent polarizability of the Ti3C2Tx monolayers is revealed. In-situ heating STEM experiments are used to effectively tune the energy of surface plasmons.


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Tunable Multipolar Surface Plasmons in 2D Ti3C2Tx MXene Flakes

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