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Unique Electronic Response of a Nanoscale Al/Alumina System Hansoo Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07774 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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The Journal of Physical Chemistry

Unique Electronic Response of a Nanoscale Al/Alumina System Hansoo Kim Microscopy and Imaging Center, Texas A&M University, College Station, Texas, U.S.A. 77843

Abstract Research is conducted to understand relatively unknown electronic behavior of a nanoscale Al/alumina system. This is acquired by a two-step process where characterization by electron microscopy is first performed for crystallography, followed by electron energy loss spectroscopy for electronic response from spatially inequivalent spots of Al nanoparticles with a protective surface oxide. A novel and unique plasmon from the surface oxide is found in the energy loss spectra between 5 and 6 eV other than the previously-reported transitions for individual Al and Al2O3. In the current study, various properties of the new transition are sought by evaluating its dependence on crystal structure and particle size along with related physical functions. The results indicate that the novel transition has collective properties, preceded by a single-electron excitation at a slightly lower energy. Application of the Bethe f-sum rule shows that the concentration of charge carriers reaches around the Mott density by the transition. The new excitation is theorized to be involved with a defect energy band which is formed by diffusion in the surface oxide. Another distinctive result is that the excitonic transition typical of an insulator is weak in the surface oxide on Al. This investigation into properties of the Al/alumina system will benefit the applications of Al for plasmonics as well as the study on the nanometer-scale metal/metal oxide interface.

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Introduction Recently, many scientists have started to pay much attention to plasmonics due to its promising applications in diverse fields such as plasmonic circuits, photovoltaic devices, and bioassay.1-3 This is especially obvious after the introduction of nanometer-scale materials. Research has so far been focused on noble metals such as gold and silver. However, their high price poses a hurdle for easy access and mass production. Aluminum presents an alternative research area as an abundant cost-effective element.4 The metal is also renowned for its Drudelike behavior along with its capability to confine plasmons in a small space. Furthermore, its localized surface plasmon shows superior properties including a long lifetime, a high adaptability in a wide range of energy (NIR to UV), and a large optical cross-section.5 Electronic response from aluminum including plasmonic behavior has been analyzed in detail in several previous studies.6-8 It has the bulk plasmons at 15 and 30 eV by single and double scattering, respectively. The surface plasmon is seen at 10 eV when an Al plane is in vacuum. The cut-off momentum transfer (qc) above which a collective excitation is damped to a singleparticle transition is 1.3 Å-1. In the energy-momentum space, the dispersion curve of the Al plasmon shows a biquadratic relation rather than a typical quadratic dependence. The anisotropy of the dispersion is relatively small and is only up to ~ 0.1 eV along two major crystallographic directions – [100] and [110]. Al is oxidized quickly at ambient atmosphere. The oxidation occurs only at the thin surface region, affording high stability to the remaining Al core. It also modifies plasmonic properties of Al since surface plasmon of a conductor is affected by its neighboring dielectric. In fact, the surface plasmon of an Al plane shifts the resonance energy to 7 eV when the surface is oxidized.6 Surface-oxidized Al is used in plasmonics as well. It was employed to study localized and propagating surface plasmon,5,9 surface-plasmon-coupled emission,10 surface-enhanced Raman spectroscopy,11 and plasmonic nano-antennas.12 A nonstoichiometric aluminum oxide is, on the


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other hand, regarded as a new plasmonic material due to its low loss in the range of near-infrared and visible wavelengths.13 Aluminum oxide (alumina) has one stable and many metastable polymorphs at room temperature.14 The stable phase is α-Al2O3 with a trigonal crystal system and a space group of R3c, otherwise known as a corundum structure. Metastable phases of alumina (transition aluminas) have the defect spinel structure with Fd-3m (γ-Al2O3), the orthorhombic structure with Pna21 (κAl2O3), the monoclinic structure with C2/m (θ-Al2O3), and so forth. Different classes of alumina have different uses. Alpha alumina is used for a laser and electronic packaging of integrated circuits.15 Gamma alumina is known as an important catalyst.16 Alumina can also have amorphous and nonstoichiometric phases.17 Amorphous alumina is one of the high dielectricconstant materials useful as a gate insulator in complementary metal oxide semiconductor (CMOS).18 It has superb properties such as thermodynamic stability on silicon and a wide band gap. It is also used in silicon-oxide–nitride-oxide–silicon (SONOS) structure for flash memory devices. In the SONOS structure, amorphous alumina can go through heat treatment to transform to γ-Al2O3 for a better charge retention.19 Electron energy loss spectroscopy (EELS) measurement for alumina has been performed as well.20 In the result an EELS spectrum for α-Al2O3 shows an exciton peak at 9 eV, an interband transition at ~ 15 eV, and a bulk plasmon at ~ 26 eV. Gamma and delta aluminas also present similar electronic response in the energy region for valence-electron excitation. The Al/alumina system has not been thoroughly investigated for electronic response as much as pure aluminum and pure alumina. In the case of nanoscale Al particles, the thickness of the native surface oxide (typically 2 – 5 nm) becomes comparable to the size of the unoxidized core. Thus, the oxide layer is expected to play a crucial role in defining overall properties of Al nanomaterials. It indicates the importance in carefully collecting and analyzing electronic response from Al nanoparticles with surface oxide and understanding their plasmonic behavior.



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In this study, surface-oxidized Al nanoparticles are characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Then, energy loss spectra from various local points of the nanoparticles with different sizes are acquired by a scanning TEM (STEM) – EELS combinatorial analytical system. Physical functions such as the energy loss function are calculated from the results. The electronic response around the center of the Al nanoparticles is found to differ little from the sum of the responses for pure Al and pure alumina. EELS spectra of Al particles show the surface and the bulk plasmons of Al and the bulk plasmon of alumina together with interband transitions. Interestingly, a unique feature is found at around the edge of the particle: the surface oxide of the Al nanoparticles shows a novel peak between 5 and 6 eV. This excitation is located close in energy and in position to the surface plasmon at the Al/Al2O3 interface. Weakening of the excitonic transition increases the uniqueness of electronic response from the Al/alumina system. Properties of the new excitation and the excitonic transition are analyzed here for further studies and applications of Al nanomaterials. Experiments Sample Preparation: Aluminum nanospheres with a surface oxide were prepared by electrical explosion. This method is used for fabrication of various nanopowders and well developed to commercial scale. A detailed description about the procedures was reported previously (ref. 21 for a review paper). Briefly, a metal wire with a diameter typically smaller than 1 mm is quickly heated up above the boiling temperature by a high current density (104 -106 ampere/mm2) in an inert gas. Then, the wire is exploded forming metal vapor and droplets. The resultant product for this study consists of spherical Al particles with a small amount of pure γAl2O3. The diameter of the Al nanospheres, when the surface oxide is included, is ~ 71 nm on average. Alpha alumina nanoparticles of 30 – 60 nm in size were purchased from Aldrich. These particles were sonicated in ethanol for homogeneous dispersion. Then, a few drops of the dispersion were deposited onto TEM grids with a lacey carbon film and dried on filter paper.


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Electron Microscopy and Spectroscopy: The nanoparticles were characterized by JEOL JEM 2010F with a field emission gun (FEG: ZrO2/W (100) Schottky field emitter) operating at 200 keV. It is equipped with a Gatan imaging filter (GIF: Tridiem 865P with a 2k × 2k chargecoupled

device

(CCD))

and

a

high

angle

annular

dark

field

(HAADF) detector. Energy dispersive X-ray spectroscopy (EDS) experiments were carried out in FEI TECNAI G2 F20 Super Twin fitted with the same type of FEG, an Orius SC200 camera (model 830) with a 2k × 2k CCD, an HAADF detector, and an ultrathin-window Si (Li) detector of an EDAX RTEM EDS system. Bright and dark field images and high resolution images were recorded in both TEMs. Energy-filtered TEM images and EELS were taken in JEOL 2010F. Preliminary EDS spectra (not shown) were taken in STEM mode to figure out proper conditions for spectrum acquisition. Then, the EDS spectrum profiles were obtained and analyzed by the TIA/ES Vision software. For high spatial resolution and sufficient EDS signal the electron beam was converged by the lens system to provide approximately a 1 nA beam current and the sample was tilted toward the detector by ~ 10 degree. The 50 to 80 pixel spectral profiles were acquired with a pitch of 1 to 4 nm and a dwell time of 1 to 3 sec. The spatial drift was measured and corrected during data collection through the cross-correlation filter, which is a band-pass filter allowing only medium-range spatial frequencies from HAADF-STEM images. The EDS spectra were quantified to display the atomic fraction of each element along a probed line. Component elements were first identified before the background counts typically from the bremsstrahlung X-rays were modeled by polynomial functions and subtracted. Then, after fitting the characteristic X-ray peaks with Gaussians or standard spectra, their intensity was integrated and converted to the atomic fraction by the Cliff-Lorimer factor. For both O and Al the transitions to the K shell (Kα transitions) were used and no matrix corrections were applied during quantification. The conditions for EELS vary depending on its applications. For VEELS a convergence semi-angle of 2.0 mrad and an energy dispersion of 0.05 eV per channel were adopted. The



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collection semi-angle was 4.2 mrad. An energy resolution of 0.80 eV was measured at the full width at half-maximum (FWHM) of the zero loss peak. A typical acquisition time was 0.2 sec. For Al L23 edges in ELNES these conditions were maintained. The total acquisition time was 1 sec. For the other core-loss spectra a convergence semi-angle of 2.0 mrad and an energy dispersion of 0.3 eV per channel were used. Also, a collection semi-angle of 9.8 mrad was employed. The total acquisition time was ~ 5 sec. These conditions are chosen in order for the angular spread of the incident beam to be small enough in comparison with the collection semiangle. In this way the angular correction for Kramers-Kronig analysis or quantification can be simplified or avoided. For high energy resolution or high beam intensity the collection aperture of the GIF system can be adjusted to a small or large size. Some core-loss spectra were collected at multiple equivalent spots and accumulated for a better signal-to-noise ratio and a less beam damage. All EELS spectra were obtained after dark-current removal and gain correction with an approximately 0.5-nm electron probe in STEM mode. Deconvolution was performed by the Fourier-log method for low energy loss spectra and by the Fourier-ratio method for core-loss spectra. The power law was used in an appropriate energy range to fit and remove the background signal of core-loss spectra. The Hartree-Slater model was exploited to calculate the cross sections of the ionization edges for quantification. Elemental mapping by the three-window method was carried out for energy-filtered TEM images at around a transition energy with an energy window of 1-eV width. Results Images of typical Al nanoparticles in the batch are presented in Fig. 1 (a) and (b). Nanoparticles have an Al core and a surface oxide. Two types of surface oxide, the shell and the tail, are seen from the Al particles (marked in Fig. 1 (b)). It is found after measurement of more Al particles that the thickness of the bright and thin crust (shell) is ~ 3 nm on average. The long and protruding oxide (tail) has a length of more than 10 nm and a diameter of normally 10 to 40 nm at the widest part. The shell consists of two layers as in the high resolution TEM images of


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Fig. 1 (c). One layer with an amorphous structure is found right on the Al core and the other layer with a crystalline structure is in the outer region. The phase contrast image in the inset shows the shell composed of the two parts in greater detail and the Al core oriented in the zone axis of the face-centered-cubic structure. Between the red lines the crystalline part of the shell is seen. This high resolution image supports a previous report that amorphous phase of alumina is more stable on the Al surface.22 A high resolution image of the tail (Fig. 1 (d)) magnified from Fig. 1 (b) displays several sets of atomic fringes from different grains. The fast Fourier transforms (FFTs) in Fig. 1 (e) to (i) are collected from each grain in Fig. 1 (d) and show the arrangement of the atomic fringes clearly. These high resolution image and FFTs indicate that the tail is polycrystalline. In fact, tails of the Al particles are normally polycrystalline. When an FFT (Fig. 1 (j)) is taken from the entire tail, multiple spots are shown randomly oriented. It also contains a broad ring (yellow lines) coming from the thin amorphous layer on the surface of the tail. In Fig. 1 (k) interplanar spacings (d-spacings) for shells and tails are displayed after measurements from high resolution TEM images. For tails, the d-spacings are well indexed with the γ-Al2O3 phase. For example, the grain 1 in Fig. 1 (d) and (e) is indexed to align along the zone axis of the γ-Al2O3 phase. Spots in the FFTs of Fig. 1 (e) to (i) are assigned by Miller indices of crystallographic planes for γ-Al2O3. Some crystallographic planes such as the {002} planes with a d-spacing of 3.95 Å are not kinematically allowed in an ideal spinel structure to show fringes but still observed due to disorder in tetrahedral interstitial sites23 or dynamical scattering. The d-spacings for the crystalline part of shells are not clearly analyzed with any phase of Al2O3. The closest phase of alumina can be δ-Al2O3. Even though crystallographic planes of δAl2O3 roughly match with most planes of the crystalline part in shells, the largest d-spacings for the phase such as 7.6 and 6.4 Å are not observed. The exact structure of δ-Al2O3 is not well defined yet and the phase given in Fig. 1 (k) is one of the suggested structures with a tetragonal system.14,23 Consequently, the aluminum nanoparticles used here have two different kinds of



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surface oxide: a tail with the γ-Al2O3 phase and a shell with partially amorphous and partially crystalline alumina. In this study, only the thin surface oxide on Al except the alumina tail is called a shell. In Fig. 2, two different polymorphs of alumina containing no metallic aluminum are presented. The alumina particle in Fig. 2 (a) is a byproduct of the fabrication process for the Al nanoparticles used in this study. In the magnified image (Fig. 2 (b)), the particle shows atomic fringes and a porous surface. One set of the fringes are arranged with a spacing of 0.45 nm along the yellow arrow corresponding to the crystallographic direction of γ-Al2O3. In its FFT (inset) the spots represent crystallographic planes aligned in the zone axis. The FFT also shows a ring pattern coming from an amorphous structure of the surface. The diffraction pattern (Fig. 2 (c)) collected from a large area of the particle confirms that the particle is γ-Al2O3 and aligned along the zone axis of the spinel structure. In addition to the diffraction spots in the zone, there are additional spots (for example, the spots pointed by the yellow arrow in Fig. 2 (c)) indicating that it is polycrystalline. The other byproduct particles measured for the study are all indexed to be γ-Al2O3. They can be either single-crystalline or polycrystalline. The γ-Al2O3 particles are often in the shape of an isosceles trapezoid like the particle in Fig. 2 (a) and very large compared to the Al nanoparticles. In case of polycrystalline γ-Al2O3 particles their grains are generally bigger than those of the tail in an Al nanoparticle. Note that the tail of an Al particle is not porous as can be seen in Fig. 1 (a), (b), and (d) even though the tail and the γ-Al2O3 particle have the same crystal structure. Gamma alumina is known for its porous structure.14 Another particle in a rod shape is presented in Fig. 2 (d). The diffraction pattern in Fig. 2 (e) indicates that the particle is α-Al2O3 oriented in the zone axis. The reflections from the {0003} planes are also kinematically forbidden but dynamically allowed. These alpha- and gamma-alumina particles are used to support analysis of Al particles. When measured by EDS (Fig. 2 (f)), the alumina particles show no other elements than Al and O while their composition


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is about the same (36.6 and 35.4 at. % in Al for α-Al2O3 and γ-Al2O3, respectively) within the error range (3.3 – 3.7 %p). Representative valence electron energy loss spectra (VEELS) are given in Fig. 3, which are collected from various local spots of Al nanoparticles including the surface oxide. For collection of the space-resolved VEELS, the parts of Al particles overlapping with the supporting carbon film in the TEM grid are excluded. Also, the incident beam is away from a zone axis of tails. The diameter of the Al nanoparticles used for the VEELS in Fig. 3 (a) – (d) is 91, 50, 110, and 36 nm. The 91-nm and 50-nm particles are used to analyze the oxide shell while the other two are used to characterize the tail area. The thickness of the shell is 7 and 5 nm for the 50-nm and 91-nm particles, respectively. The tail of the 36-nm (or 110-nm) particle has a length of 11 (or 20) nm and a diameter of 13 (or 31) nm at the widest part. A colored spectrum is collected around the same-colored spot in the inset of Fig. 3. VEELS obtained at the center of the nanoparticles show a typical electronic response of Al. Thus, the transition at 15 eV is from excitation of the bulk plasmon for Al while the one at ~ 7 eV stands for the surface plasmon at the interface of aluminum and alumina. As the electron probe moves to the edge, another peak emerges at around 22 - 24 eV, which is the bulk plasmon of alumina. More excitations are observed at 9 and 13 eV from the alumina region. These transitions for the surface oxide are seen as well in VEELS of pure alumina particles (Fig. 4). Then, the position of the surface plasmon for aluminum shifts slightly to a lower energy around the Al/alumina interface. The energy shift of electronic excitations at the edge of materials was reported previously.24 In Fig. 4, VEELS from γ-Al2O3 and α-Al2O3 particles are presented for comparison. They are collected along an imaginary line perpendicular to an edge of each particle and away from a zone axis of each crystal. For α-Al2O3 the electron beam is incident neither parallel nor perpendicular to the c axis of the corundum structure. The electronic response from both alumina particles is similar to each other. Note that the peak at ~ 9 eV is seen in both aluminas. It indicates the



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exciton of γ-Al2O3 is also formed at an energy close to the one (9 eV) for the exciton of α-Al2O315 and the band gap of both aluminas is similar to each other. It is also noteworthy that the exciton peak is not as clear in the VEELS of the surface oxide. One difference between the VEELS for the two alumina particles is the position of the bulk plasmon, which is 23.1 and 23.6 eV for the γAl2O3 and α-Al2O3 particles, respectively. Therefore, most excitations of the Al/alumina system are similar to those of pure Al and pure Al2O3. A distinct feature is found, however, between 5 and 6 eV in the VEELS collected around the surface oxide. In Fig. 5 (a), several VEELS spectra for the shell of the 91-nm particle are selected to display the low energy region in detail. A novel peak is clearly seen adjacent to the surface plasmon. It appears as a shoulder at the edge of the Al core and then grows strong in the vicinity of the Al/alumina interface. The peak is primarily arranged along the dashed white line at 5.2 eV and then shifts to lower energies at around the alumina/vacuum interface. The surface plasmon changes its position along the dashed yellow curve. Energy-filtered TEM images (Fig. 5 (b) and (c)) are acquired from an Al particle at 5 and 7 eV to see the spatial distribution of the novel excitation and the surface plasmon. Clearly, both excitations are localized around the Al/alumina interface and the surface oxide. The excitation in 5 - 6 eV is not found in previous reports. An analogous transition from partially-oxidized Al particles was reported in ref. 25. However, in the report the VEELS were collected from the center of Al particles and the transition was regarded as the shift of the surface plasmon. When collected from the center of Al particles, the novel excitation is too weak and embedded in the zero loss peak and the surface plasmon (Fig. 5). The same transition is not observed from pure Al.6-8 In current study, it is not seen either from the pure Al2O3 particles, which mean its properties need to be known for a clear understanding of the electronic response from Al nanoparticles. The reason that the excitonic transition is undermined in the surface oxide is discussed together. Discussion


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a. Properties of the novel transition After additional measurement with more Al particles, positions of the bulk and the surface plasmons are located and displayed in Fig. 6 (a) and (b) with respect to the diameter of Al particles. The positions of the novel electronic transitions in the shell and the tail are presented together (Fig. 6 (c)). VEELS acquired around the spots with the maximum penetration depth (tmax) of e-beam in shells, tails, and cores are analyzed for the transition energies. As an Al particle is bigger, tmax in its shell and core is larger. This relation between the size of an Al particle and tmax generally applies to the tail of an Al particle as well. A few unique features stand out from the results. First, the average position of the novel transition is higher in energy for the tail than for the shell. Second, the position of the new excitation is not affected by the size of Al particles as that of the bulk plasmon. It is a sharp contrast with the behavior of the surface plasmon, which shows almost a linear relationship (dark cyan line in Fig. 6 (b)) with the particle size. The excitation positions in Fig. 6 (b) are located from VEELS of the central region in Al particles and thus shells and tails cannot be differentiated regarding the surface plasmon. Lastly, the new peak and the bulk plasmon show large deviation in position for the shell. On the other hand, the tails and α-Al2O3 particles show the bulk plasmon at higher energies than the shells and γ-Al2O3 particles, respectively. The bulk plasmon in the Drude model is closely related with the density of valence electrons (nv) through Eq. 1.26 Ep is the bulk plasmon energy and m0 is the mass of an electron. The other symbols are known physical constants. Thus, a denser alumina is supposed to have the bulk 1

E

p

 n e2 2  = h  v  ε 0m 0 

Eq. 1

plasmon at a higher energy. In Fig. 6 (a) this relation is clearly seen. The volume density of αAl2O3 and γ-Al2O3 is respectively 3.99 and ~ 3.66 g/cm3.14 Those of amorphous alumina is 2.45 – 3.2 g/cm3.27 Accordingly, the bulk plasmon for α-Al2O3 is located at a higher energy than that for γ-Al2O3 as in Fig. 6 (a). Note that the tail requires a high energy, similar to Ep for α-Al2O3, to



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excite the bulk plasmon indicating its density is about as high as that of pure α-Al2O3. Thus, the tail of an Al particle shows unique structural features, that is, non-porous morphology and a high volume density. The shell and pure γ-Al2O3 have the bulk plasmon at similar energies suggesting a high density of the partially-amorphous shell as well. The peak in 5 - 6 eV (Fig. 6 (c)) exhibits a similar trend (Eq. 1) between the position and the density. The novel peak is seen at a lower energy in the shell (5.3 eV) than in the tail (5.7 eV). Each shell has a different proportion of amorphous to crystalline components. It may lead to the large deviation in the positions of the bulk plasmon and the new excitation for shells. The surface plasmon energy (Es) for a spherical particle shows the relation Es = Ep/[1 + εr(Es)(h+ 1)/h]0.5 (Eq. 2) with Ep where εr(Es) is the relative dielectric constant at around Es and h is an integer.26 The resonance energy is affected by the size of a particle in a manner that a higher-order mode (a larger h) is excited for a larger particle. For the dipole mode (h = 1) of an Al/α-Al2O3 sphere, the resonance energy is calculated to be 5.4 eV using εr(5.4 eV) of 3.36.28 The surface plasmon energy for a 44-nm particle is 5.9 eV as seen in Fig. 6 (b). Then, it shifts up to around 7.2 eV for large (≥ ~ 100 nm) particles, which is similar to the position for the planar Al/Al2O3 interface.6 The surface plasmon energies for various modes of an Al/α-Al2O3 sphere are displayed as red dots in Fig. 6 (b). In Fig. 6 (d) – (h) a few VEELS collected from the tail of the 110-nm Al particle are fitted by Gaussians after deconvolution with the Fourier-log method. In the inset, the positions of an electron probe for the spectra are modeled by a schematic. The surface plasmon (wine-colored spectra) shows the largest intensity at the Al/alumina interface because the differential probability of surface-plasmon scattering has the maximum value at the glancing incidence angle. The novel transition represented by the red spectra is seen from around the edge of the Al core and has a larger intensity when the probe moves to the Al/alumina interface. Immediately outside of the Al core, the oscillation strength of the novel transition has the largest value before slightly reducing


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at the tip area. The residual surface plasmon away from the interface (Fig. 6 (g) and (h)) is primarily the background from the bulk plasmon. Note that the penetration depth and the interaction volume of the incident electrons become smaller as the probe moves to the tip of the tail. Thus, the net change in the intensity of the peak between 5 eV and 6 eV should be smaller than it shows in Fig. 6. Knowing furthermore that the peak in 5 - 6 eV is not affected by the particle size, this semi-constant intensity along the tail indicates that the new transition is not a surface plasmon. Since the peak in 5 – 6 eV is still clearly observed far away from the Al/alumina interface it is not from coupling of the surface plasmon for the interface either. The peak at 3.7 eV in Fig. 6 (g) and (h) may be an artifact of the deconvolution since it is not observed in the original VEELS spectra. The energy loss function (ELF: Im(-1/ε(E)) contains a wealth of information about optical and electronic properties of a material. It can be calculated from VEELS. After the single scattering spectrum (S(E)) is extracted by deconvolution of VEELS, an angular correction is

(

)

S ( E ) / ∫ q −2 dΩ ∝ Im(−1 / ε ( E ))

Eq. 3

made to deduce the ELF through Eq. 3. Further normalization is performed with a known value of a refractive index. One of the characteristics for current STEM-EELS system is the spread (0 – 1.1 Å-1) of the momentum transfer (q). The ELF obtained after the angular correction is not a function of q and can still resemble the one calculated from optical measurement when the range of q is small.29 This is since a considerable number of electrons are incident close to q = 0 and thus the average momentum transfer ( ∫ q (∂ 2σ / ∂Ω∂E )dΩ / ∫ (∂ 2σ / ∂Ω∂E )dΩ ) is small. In current system the average momentum transfer is smaller than 0.22 Å-1 in 0 - 10 eV. The ELF of ZnO nanoparticles acquired under an analogous apparatus system shows close similarities with that of bulk ZnO deduced from optical reflectivity.24



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The ELFs for the surface oxide of Al nanoparticles and pure alumina particles are displayed in Fig. 7. VEELS used for the ELFs of Al nanoparticles are collected from spots inside the surface oxide and a few nanometers away from the Al/alumina interface. These spots are selected since scattering events from the Al core are greatly reduced there. The refractive indices used for normalization are 1.6, 1.7, and 1.8 for amorphous alumina,30-32 γ-Al2O3, and α-Al2O3,20 respectively. These indices were obtained in the visible-light region. The difference between the pure alumina particles and the surface alumina is readily noticed. Below 9 eV the ELFs for the pure alumina particles monotonously decrease to zero. The ELFs for the surface oxide, on the other hand, present large peaks at energies (5.1 – 5.8 eV) similar with the ones for the novel transitions in VEELS. An excitation property can be determined by comparing the value of ELF with π.26 If the value at a peak is much smaller than π (or Im(-1/ε) « π), it is considered to be a single-electron excitation. Otherwise, it is plasmonic. For example, the values of the ELFs at the bulk-plasmon energy are 0.65 to 1.19 in Fig. 7. Since these values are not too small compared to π, the transition can be a plasmon. On the contrary, the values for the transitions at 13 and 15 eV are very small (< 0.1) when the background is removed. The values of the peak in 5 – 6 eV for the surface oxide range from 0.36 to 0.64. They are smaller than but comparable to those values for the bulk plasmon and about 11 – 20 % of the number π. Therefore, the ELFs imply that the excitation in 5 - 6 eV can be plasmonic. The double differential scattering cross-section for the inelastic scattering can be expressed by Fermi’s Golden Rule (Eq. 4) which includes the transition matrix element (M:

r r ∑ j < ψ f | exp(iq ⋅ r j ) | ψ i > ) and the joint density of states (ρ: JDOS). The inelastic-form-factorweighted JDOS (M2ρ: I-JDOS) can be deduced from VEELS since the integration of the 2 ∂ 2σ 1 r r ∝ 4 ∑ j < ψ f | exp(iq ⋅ rj ) | ψ i > ρ ( E ) ∂Ω∂E q


Eq. 4

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differential scattering cross-section (∫(∂2σ/∂E∂Ω)dΩ) is proportional to deconvoluted VEELS of a thin material or ELF. Then, the band gap and its property can be estimated from I-JDOS.29,33 From the ELFs, the tail and the shell are derived to have a band gap of ~ 9.2 and ~ 8.6 eV, respectively. It needs to be mentioned that these values are only rough approximation due to the exciton peak around the band gap and the difficulty in finding the appropriate energy region. Gamma and amorphous aluminas are known to have the energy band gap of 8.7 and 6.95 eV, respectively.34,35 The deduced value for the tail is only slightly larger than the band gap of gamma alumina. However, the value for the shell is much larger than that of amorphous alumina. The dielectric function (ε = ε1 + iε2) of a material can be obtained by the Kramers-Kronig transformation (Eq. 5) of ELF. P is the Cauchy principal part of the integral. In Fig. 8 (a) and (b) ∞

Re( −1 / ε ( E )) = 1 − (2 P / π ) ∫ Im(−1 / ε ( E )) E ' dE ' /( E ' 2 − E 2 ) 0

Eq. 5

the dielectric functions of the surface oxide, α-Al2O3, and γ-Al2O3 are presented together with that of pure Al.6 Around 5.5 eV, both the real and the imaginary parts of the dielectric functions for the surface oxide significantly diminish. This is different from the case of the isolated alumina particles for which only ε2 becomes zero below 5 eV. When ε1 and ε2 are close to zero at a specific energy, a transition around it becomes plasmonic. It is because when ε is small the screening electric field diverges. For example, ε1 and ε2 for the alumina particles and the surface alumina have small values in the range of 22 to 24 eV where they have the bulk plasmon. On the other hand, a large peak below 5 eV is seen in ε2 of the surface oxide. Another important physical function, the real part of the optical conductivity (σ1), is derived from the relation σ1 = ε0ε2E/ћ and presented in Fig. 8 (c). Peaks in σ1 for the surface oxide are seen at ~ 8.9, 13, 15, 16.7 – 19.6 eV. The isolated alumina particles also show peaks at similar energies and one more at around 11 eV. A significant difference is that σ1 for the surface oxide contains a large peak below 5 eV originating from the peak around the same energy in ε2. It is seen at 3.8 and 4.2 eV for the shells of the 50-nm and 91-nm Al particles while at 4.4 and 4.7 eV



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for the tails of the 36-nm and 110-nm Al particles. Since σ1 is proportional to JDOS these peaks are considered to be from a single-electron transition. For pure Al and the pure Al2O3 particles σ1 does not show any peaks around the energy. Therefore, it is noticed from σ1 that there is an additional single-electron excitation for the surface oxide at an energy lower than the novel transition seen in the VEELS. One more remarkable feature is seen in the range of 8 – 12 eV. The values of σ1 are very large around the energy region for both pure alumina particles but noticeably small for the surface oxide of Al particles. The transitions in the range are related with an exciton and an interband excitation. The exciton peak (~ 9 eV) in σ1 is embedded by another peak (~ 11 eV) ftom γ-Al2O3 but seen explicitly for α-Al2O3. It is larger than the peak around 9 eV for the surface oxide. In VEELS (Fig. 3 and 4) the excitonic transition is observed clearly for both pure aluminas but only ambiguously for the surface oxide. The large suppression is also found around the same energy region in ε2. Considering the band gap for the tail is close to the known value for gamma alumina, the reduction in the values of σ1 around 9 and 11 eV may imply modification in the electronic band structure. Collective resonance and single-electron transition are the two representative modes for inelastic scattering. The excitation between 5 and 6 eV in VEELS for the surface oxide shows properties indicative of a collective excitation. First, the ELF for the surface oxide has large values in 5 - 6 eV. Also, both parts (ε1 and ε2) of the dielectric function have small values around the novel peak in VEELS and ELF. On the contrary, σ1 indicates that there is a single-electron excitation below 5 eV. Criteria for a plasmonic oscillation are less restrictive than the known conditions of ε1 being zero and ε2 being small.36 Instead, around the energy showing a peak in ELF, ε1 and ε2 should have small values and change linearly with ∂ε1/∂E > 0 and ∂ε2/∂E < 0 while ∂(lnε1)/∂(lnE) and ∂(lnε2)/∂(lnE) should have large values. These criteria are broadly satisfied for the novel peak of the surface oxide. Furthermore, the novel transition is seen at energies higher by 1.0 to 1.8 eV in


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ELF than the new peak below 5 eV in σ1, which indicates it is not just a single-electron transition. Therefore, the transition in 5 - 6 eV is considered to be collective and preceded by a singleelectron transition. In other words, the novel transition has properties of both single-electron and plasmon excitations simultaneously. Although some previous works37,38 suggest that γ-Al2O3 has the band gap of ~ 4 eV, the isolated γ-Al2O3 particle does not show a transition at a similar energy and the band gap for the surface oxide is still very large as deduced above. The dimension of the tail is large enough to exclude the quantum effect. The peak in 5 - 6 eV is not caused by the metal-induced gap states since the gap states are extremely localized around the interface39. The crystal phase of γ-Al2O3 is continuously observed up close to the Al/alumina boundary in the tail of an Al particle. It indicates that the effect of a new chemical bonding between Al and alumina, if any, is very limited. Possibilities of surface plasmon and coupling of the surface plasmon at the Al/alumina interface are ruled out above. It is not considered to be the surface exciton either since the acceleration voltage (200 keV) is too high to be surface-sensitive and the response of pure alumina at a grazing incidence of e-beam does not show a similar transition in Fig. 4. Therefore, the discussion about the origin of the novel peak should begin with the fact that one sharp difference between the surface oxide and the pure alumina particles is the presence of Al right next to the surface oxide. The Al neighbor appears to affect the electronic response of the surface oxide. b. Origin of the novel transition Al and alumina can interact with each other through electronic and atomic flow when they are in intimate contact. Both types of flow can cause alterations in the electronic structure. First, electronic flow or charge transfer stems from the difference in the work function of two contacting materials. When the work functions of Al and alumina are different from each other, electrons flow between Al and alumina to align the Fermi level (EF) in both sides. Transferred



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charge carriers can be excited collectively by external stimulus at small q’s. Since current system uses a range of momentum transfer, the plasmon may reduce its oscillation strength and become a single-electron transition at large q’s. However, the intraband plasmon induced by the charge transfer is normally located at low energies (for example, see ref. 40 and 41) due to the small number of the transferred electrons and the continuum of the conduction and the valence bands. Following the Drude model for quasifree electrons (Eq. 1), Ep should be small when nv is small. In addition, a single-electron transition above the cut-off momentum transfer should have a small value in ε2 since the contribution at and around q = 0 is excluded. It is not true as seen in Fig. 8 (b) where the value of the single-electron excitation below 5 eV is much larger than those of the other interband transitions. Also, transferred electrons are more likely to populate around the Al/alumina interface but the oscillation strength of the novel peak does not change much along the tail of an Al particle. Therefore, the novel peak in VEELS is not considered to be an intraband plasmon caused by charge transfer. An alternative way of interaction between Al and alumina in contact with each other is through atomic flow or diffusion derived from the abrupt change in Al and O concentration across the Al/alumina interface. Gamma alumina has a defect spinel structure with a poor crystalline periodicity.14 In this structure, there are many interstitial sites, only less than a quarter of which are occupied by Al cations. Moreover, a polycrystalline tail of an Al particle has grain boundaries. Therefore, Al atoms can be diffused into the alumina tail of an Al particle. Diffusion of Al into the alumina shell is also supposed to be substantial due to the vacancies and voids in amorphous alumina.42 Diffusion of O into Al can occur as well. Al may also diffuse into the surface oxide from the Al vapor when the oxide is formed by residual oxygen during the synthesis process. The high temperature attained during the synthesis can facilitate the atomic diffusion.


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Al cations have tetrahedral or octahedral coordination with oxygen anions in the spinel structure (Fig. 9 (a)). Occupation of both sites can be analyzed by Al L23 edges in electron energy loss near edge structure (ELNES).43 Figure 9 (b) presents Al L23 ELNES for the tail and the shell of an Al particle and for a pure gamma-alumina particle after the background removal and Fourier-ratio deconvolution. The two peaks located at 78.3 and 79.5 eV represent transitions from Al cations in tetrahedral and octahedral interstitial sites, respectively. The relative intensities of the two peaks for the surface oxide in the Al particles are different from the one for the isolated gamma alumina. That is, the intensity of the peak at 78.3 eV is higher in the case of the surface oxide indicating Al cations occupy relatively more in the tetrahedral sites for the tail and the shell of the Al particles than for pure γ-Al2O3. The relatively higher intensity of the peak at 78.3 eV continues along the tail (the red and the yellow Al L23 ELNES in Fig. 9 (b)). These results show that the relatively higher occupation of the tetrahedral sites maintains all over the surface oxide. They may also indicate introduction of Al atoms into the tetrahedral sites of the surface oxide through diffusion. A tetrahedral interstice is similar to or smaller than an octahedral interstice in the spinel structure.44 Meanwhile, in the unit cell of the defect spinel structure for γ-Al2O3 ⅛ of the 64 tetrahedral interstices and ½ of the 32 octahedral interstices are occupied by 21⅓ Al cations with more vacancies (by 2⅔ sites) randomly distributed in the tetrahedral interstices than a normal spinel structure.23 A large number of vacancies in the tetrahedral sites may still render the sites favorable for atomic flow since diffused Al atoms have a higher chance to meet a tetrahedral interstice. O vacancies can also change the coordination number (CN) of Al ions. The shell of an Al particle shows another peak located at 76.9 eV. This peak corresponds to a transition from Al cations in the amorphous alumina layer. Amorphous alumina can have a CN smaller than 4 for Al interstitials.14 Diffusion in the surface oxide can be also measured by analysis of the composition. EDS spectra are carefully acquired from an Al particle and γ-Al2O3 (Fig. 9 (c) and (d)). Both the γ-



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Al2O3 particle in Fig. 9 (d) and the tail of an Al particle have the same configuration, so that they have metallic Al nearby and taper to their tips. Representative spectra from the paths 1 and 2 of the tail and the carbon film are given in Fig. 9 (e). The EDS spectra of the tail acquired along the path 1 exhibit relatively higher Al content (inset of Fig. 9 (e)) than those of pure alumina (Fig. 2). The spectra collected along the path 2 of the carbon film do not show the Al transition. The paths 1 to 3 are all equidistant from metallic Al. It indicates that the effects of the electrons and x-rays scattered by the microscopy system are ignorable. However, fast secondary electrons generated by the sample can affect X-ray florescence severely depending on its relative geometry. Along the path 3 in Fig. 9 (d) a drastic surge in the Al content from 41 to 55 at. % is observed due to the geometric change around the yellow arrowhead. The thickness of the particle has about the maximum value right before the probe moves to the yellow arrowhead and it reduces dramatically thereafter. Thus, the EDS experiment indicates an increase in the Al content of the surface oxide at least qualitatively since the tail shows only a moderate geometrical change. The core-loss spectra in Fig. 9 (f) demonstrate more clearly that the Al content in the surface oxide is higher than in pure γ-Al2O3. The spectra are normalized with the amplitude of the O K near-edge peak after the background removal and Fourier-ratio deconvolution. The spectra of the surface oxide have higher intensities for Al L23 edges than that of γ-Al2O3. Their unprocessed spectra are provided in the inset of Fig. 9 (f). Quantification of these and additional core-loss spectra shows enrichment in the Al content of the surface oxide by about 4 to 7 %p increase in the atomic percent (Fig. 9 (g) and Tab. 1). Diffusion can induce modification in the energy band structure of Al2O3 by introducing defects. Other than Schottky and Frenkel types of defects in a stoichiometric oxide, some defects will become more abundant under the Al-excess condition of the fabrication. Diffused Al atoms can fill interstitial sites (Al → Ali0 (Eq. 6)). Subsequently, they can be ionized into several oxidation states (Ali0 → Alil+ + le-, l = 1, 2, 3 (Eq. 7)). Since equivalently, it can be regarded as an oxygen-deficient condition, O vacancies can form and be ionized (OO → VO0 + 1/2O2 (g) (Eq. 8)


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and VO0 → VOm+ + me-, m = 1, 2 (Eq. 9)). These Al interstitials (Ali+, Ali2+, and Ali3+) or O vacancies (VO+1 and VO+2) are considered to be the dominant defects in alumina on the Al nanoparticles and to form energy states and bands in the band gap. Electronic structure for alumina is known to be similar between phases.45-48 It was reported that defect states are located in the band gap of amorphous alumina,49 α-Al2O3,49-51, θ-Al2O3,51 and κ-Al2O3.52 For example, oxygen vacancies are known to form an optical absorption band near 6 eV in particle-irradiated α-Al2O3.53,54 Defect states for γ-Al2O3 were theoretically calculated or empirically observed in the band-gap region.55-60 These reports confirm that there are various kinds of defect states available in the band gap of alumina. Diffused Al atoms can also form new Al and O sites in the surface oxide through the nonstoichiometric defect reaction 2Ali0 + 3/2O2 (g) → 2AlAl + 3OO (Eq. 10). New atomic sites at the pores of γ-Al2O3 can reduce the surface area. This may explain aforementioned morphological differences in the porosity of an isolated gamma alumina and the tail of an Al nanoparticle. In addition to diffusion into interstitial sites, the formation of new sites can also account for the high volume density of the tail seen in Fig. 6. In this study, electrons trapped in one of the dominant defect bands below EF are hypothesized to be responsible for the novel excitation (Fig. 10 (a)). When electrons in the band are stimulated below 5 eV, single-electron transitions occur to the conduction band. These excited electrons respond collectively by slightly further excitation in 5 – 6 eV when they are well spread in a large quantity in the conduction band. Involved in an excitation to the bottom of the conduction band the transition may look similar with the one at ~ 9 eV for pure alumina. A transition to the vicinity of the conduction band minimum, in fact, can induce collective excitations like excitons, plasmons, or both simultaneously.61-63 However, the excitonic transition for pure alumina does not show the typical properties of plasmon. Thus, around 9 eV ε1 and ε2 of pure alumina have large values and their slopes do not satisfy the condition of ∂ε1/∂E > 0 and ∂ε2/∂E < 0. The difference between the two transitions is, then, that the transition at 9 eV for pure



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alumina is only excitonic while the one in 5 – 6 eV for the surface oxide takes on plasmonic properties. The Coulomb interaction between the electron and the hole of an exciton can be screened by the conduction electrons.63,64 Previous results show that excitons can be dissociated to free electron-hole pairs by doping64-66 or by external stimulation67 above the Mott density (NM). It is shown below that the surface oxide has a high density of electrons participating in the transition between 5 and 6 eV. Then, the electron density is compared with NM. The plasmonic properties of the novel peak can, therefore, originate from the large number of conduction electrons due to the transition from the defect band. The NM for alumina can be estimated approximately. Excitons of alpha alumina have the exciton binding energy (Eb) of 0.13 eV.68 Considering NM is the density where the wave functions of excitons overlap with each other, NM can be thought of as inversion of the exciton volume, that is, NM ≈ (4πrB3/3)-1 ≈ 0.239 rB-3.69 In the equation rB is the exciton Bohr radius, which can be known from Eb. Since Eb = µRH/m0εr2 and rB = m0εraH/µ where RH is 13.6 (eV), aH 5.29 x 10-11 (m), and µ the reduced mass, rB = RHaH/εrEb (Eq. 11). Using Eb = 0.13 eV and εr = 9.5 or 11.6 for αAl2O3,27 rB is then calculated to be about 0.58 or 0.47 nm, respectively. Here for εr the static relative dielectric constant (εs), larger than the high-frequency one (ε∞), is used to assume a more severe condition since a larger εr means a smaller rB and a higher NM. The Haken correction needs to be used to obtain the right εr after interpolation between εs and ε∞.70 With rB of 0.47 – 0.58 nm, NM for α-Al2O3 is computed at about 1.23 – 2.25 x 1021/cm3. It is easily assumed that excitons of γ-Al2O3 have properties similar with those of α-Al2O3 based on their analogous electronic structure45-48 and dielectric constant (Fig. 8 and ref. 71). Thus, Eb and NM for the two aluminas may have similar values. When electrons of a material participate in inelastic scattering their number (Neff: effective number of electrons) can be estimated through Eq. 12 which is derived from the Bethe f-sum rule.26 In the equation na is the number of atoms or molecules per unit volume of the material. The


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N eff ( E ) =

2ε 0 m 0 πh 2e 2na

E

∫Eε '

2

( E ' ) dE '

Eq. 12

0

results (Fig. 10 (b)) show that Neff per unit cell involved in energy loss up to 75 eV is 177 and 128 for pure γ-Al2O3 and the tail of the 110-nm Al particle, respectively. Since the defect spinel structure contains 32 O2- anions and 21⅓ Al3+ cations in a unit cell, 177 and 128 electrons correspond to 16.6 and 12 electrons per Al2O3, which is smaller than the number of the valence electrons (= 24 electrons) for Al2O3. Slightly above 75 eV core-shell transitions (Al L23) for Al already start to occur (see also Fig. 9) and the Neff graph does not show saturation around 75 eV. It can be the reason that Neff does not achieve the total number of the valence electrons at 75 eV.26 Another feature is that Neff for the tail is noticeably reduced from around 9 eV (yellow arrow) through Al L23 absorption edges indicating a large decrease in the number of electrons related with excitonic and interband transitions. In the inset of Fig. 10 (b) the energy region around the novel transition is seen in detail. The tail of the 110-nm Al particle has about 5 electrons per unit cell contributing to the energy losses up to 5.8 eV. This electron density should be close to the real value since the novel transition is well separated from the other transitions. Pure γ-Al2O3 has no electron participating around the energy. The density of ~ 5 electrons per unit cell equals ~ 1 x 1022 electrons/cm3. It is analogous to the charge-carrier concentration of heavily-doped semiconductors. This high concentration of electrons can even produce a surface plasmon around the visible-light region.72 Thus, the oscillator strength sum rule shows that the surface oxide has a high density of charge carriers engaged in the novel transition. Then, the value of NM (~ 1.23 – 2.25 x 1021/cm3) for γ-Al2O3 is close to the Neff (1 x 1022/cm3) for the novel transition of the tail. Therefore, by the transition to around the conduction band minimum the surface oxide on Al nanoparticles can form free electron-hole pairs instead of excitons. These calculations also suggest that an insulator on metal can attain the Mott density through diffusion (or self-doping) and a subsequent excitation.



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In pure alpha and gamma aluminas a high density of charge carriers is not generated to reach the Mott transition by excitation around 5 – 6 eV. Though formed together with the Al nanoparticles during the fabrication process, the large γ-Al2O3 particles may still have only an insufficient number of Al interstitials or O vacancies, which are not enough to form a defect band and contain numerous charge carriers. The valence electrons can be excited in the two representative modes depending on their localization. One example can be found from the π electrons in sp2-hybridized carbons. Localized as in the case of the phenyl group of polystyrene they show the π – π* single-electron transition.29 Delocalized as in the case of periodically-arrayed carbons of graphite73 or irregularly-arranged carbons of graphitic amorphous carbon74 the π electrons respond as the π plasmon. Likewise, when electrons trapped in a defect band of the surface oxide are delocalized in a high density, their transition can become plasmonic. The fact that a high concentration of electrons participates in the transition between 5 – 6 eV implies that the related defect band contains dense charge carriers. Consequently, the numerous electrons available in a defect band of the surface oxide can differentiate the novel transition behave from an exciton and a single-electron transition. It is well known that a single-electron transition can give rise to a plasmon in the mean time. For example, graphite has the π and the π + σ bulk plasmons at 7.2 and 28 eV.73 The π plasmon originates from the π – π* interband transition below 5 eV while the π + σ plasmon mainly from the σ – σ* interband transition below 20 eV. Kociak et al suggested that every allowed interband transition can induce a surface or bulk plasmon in carbon onions.75 Therefore, the single-electron transition in the surface oxide below 5 eV can trigger the plasmonic excitation between 5 – 6 eV leading to an interband plasmon. The defect-band theory can elucidate other experimental results. Diffusion can occur both in amorphous and gamma aluminas when they are in contact with Al. It is the reason that the novel transition is seen both in the shell and in the tail. The energy band gap of amorphous alumina is smaller (6.95 eV) than that of gamma alumina (8.7 eV).34,35 Thus, the single-electron transition


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below 5 eV is likely to occur at a lower energy in the shell. On the other hand, the electrons in the defect band can fill the exciton states and the conduction band minimum by the single-electron transition below 5 eV. It may suppress the transition at around 9 eV from the valence band. The defect-band theory can also account for the reason that the novel excitation in VEELS is affected by crystal structure and larger than a normal intraband plasmon. It can be known from the extended Drude model for an interband plasmon, that is, (Epb)2 = Ep2 + Et2 (Eq. 13).26 According to the model the plasmon energy (Epb) of bound electrons is affected by a singleelectron transition of which the transition energy (Et) is comparable to the free-electron plasmon energy (Ep). Thus, the lower excitation energy of the novel peak from the shell than the tail can be caused by the smaller single-electron-excitation energy below 5 eV. Due to the term Et2 in Eq. 13, an interband plasmon can be easily found at high energies. Meanwhile, diffusion takes place everywhere in the surface oxide as seen above (Fig. 9). All the regions in the shell and the tail show the high Al content which implies many Al interstitials or O vacancies are expected to be well distributed in the surface oxide of the Al nanoparticles. This is the reason the intensity of the novel peak does not change much along the tail. Therefore, a defect band in the band gap formed by diffusion of Al atoms can well describe the empirical results. It would be informative to know whether the surface oxide reaches a local dynamic equilibrium under the electron irradiation for EELS. From the spot 1 (Fig. 11 (a)) in the tail of an Al particle, a series of VEELS (Fig. 11 (b)) are collected after continuous irradiation to address the equilibrium issue. The electronic response does not show any significant difference for up to 8 sec of pre-exposure maintaining the features at 5.6 and 13 eV as seen in the VEELS. The bulk plasmon is seen at 23.9 eV. When the spot is exposed to the electron beam much longer, noticeable differences emerge for the three transitions. After the 104-sec pre-exposure the bulk plasmon at 23.9 eV and the interband plasmon at 5.6 eV shift while the peak at 13 eV is embedded. These are primarily from the accumulation of hydrocarbons around the exposed spot, which can be easily seen during spectrum collection (spots 1 to 4 in Fig. 11 (a)). The spot 1 in the



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image taken after all the irradiation shows a small loss of mass and accumulation of the hydrocarbons. A slight increase in intensity of the bulk plasmon for the first 8 sec also originates from the hydrocarbons. The same pattern is seen from the transitions of pure γ-Al2O3 (Fig. 11 (c)) as well. Normally the beam damage in γ-Al2O3 particles is, however, serious and seen as loss of specimen mass after a long exposure (Fig. 11 (d)). This result may offer an insight into the local dynamic equilibrium. No change in intensity of the transitions with a few seconds of pre-exposure indicates the transitions seen in VEELS occur at dynamic equilibrium. Previously, it was reported that an insulator subject to irradiation of electrons under an acceleration voltage from 5 kV to 1.5 MV can attain dynamic equilibrium after incidence of a threshold dose on the order of 1014 electrons/cm2.76,77 It is a few orders smaller than the typical dose used in this study to acquire VEELS.28,32 The transition from the valence band to the conduction band called intrinsic ionization is expressed by the electronic defect reaction 0 → e- + h+ (Eq. 14), where e- and h+ are an electron and a hole, respectively. This ionization is directly related with the excitonic and single-electron transitions for alumina. When the ionization is balanced by recombination (dynamic equilibrium), concentration of the electronic defects is regulated by the equilibrium constant K = [e-][h+] or np where the parenthesis indicates concentration and n = [e-] and p = [h+]. Thus, the product of electron and hole concentrations (np) should still maintain constant at a given temperature. Diffused Al in the surface oxide can take part in both defect reactions of Eq. 6 and Eq. 10. Meanwhile, the reactions of Eq. 6 and 7 or Eq. 8 and 9 can increase the electron concentration (n). Therefore, n increased through the defect reactions can spur recombination and reduce the overall intrinsic ionization. It may be another way of explaining the suppression in the excitonic transition from the surface oxide of Al particles. It can be also the reason that the surface oxide shows a weakened interband transition around 11 eV and the smaller Neff above the novel transition. Furthermore, it implies that at dynamic equilibrium the transition from the valence


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band to a defect band above EF may be responsible for the excitation in 5 – 6 eV as well. In that case holes in the valence band can also oscillate collectively in response to the external stimulation. Conclusions A nanoscale Al/alumina system is analyzed for electronic response. VEELS are collected at various local positions of surface-oxidized Al particles together with TEM data for crystallography. The results show the shell of the Al nanoparticles has both amorphous and crystalline alumina structures while their tail has normally a polycrystalline γ-Al2O3 structure. The tail is observed to have a high volume density and a non-porous structure, which is different from bulk γ-Al2O3. Interestingly, a novel peak is observed between 5 and 6 eV in the VEELS from both shells and tails. It exhibits spatial and energy proximity to the surface plasmon for the Al/alumina interface. The study monitors the effect of size of the particles and crystallinity of the surface oxide on the transition. Furthermore, ELF, the dielectric function, and the real part of the optical conductivity from Al nanoparticles and pure alumina particles are calculated and compared. The analysis shows that the new excitation has properties of an interband plasmon. A new single-electron transition is also found in ε2 below 5 eV, which is believed to provoke the plasmon between 5 and 6 eV. It is demonstrated by the oscillator strength sum rule that in the surface oxide a high concentration of free charge carriers can be excited up to around the Mott density by the transition in 5 – 6 eV. The charge carriers participating in the new transitions are theorized to be induced by a defect energy band. The defect band is considered to be generated by diffusion, which is substantiated by analysis of core-level excitations and EDS. This theory can well explain unique features in electronic response of the Al/alumina system while the Al diffusion accounts for the distinct structural properties of the tail in density and porosity additionally. It is also found that the excitonic transition and an interband transition seen in pure alumina around 9 and 11 eV are weakened in the surface oxide on Al particles. Together with the attenuation of some transitions the presence of the new excitations in the surface oxide of Al or a



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nonstoichiometric alumina will be a crucial factor in utilization of Al and alumina for plasmonics and a valuable reference for the study of the metal/metal oxide interface.


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References 1. Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions Science 2006, 311, 189-193. 2. Clavero, C. Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices Nature Photon. 2014, 8, 95-103. 3. Petryayeva, E.; Krull, U. J. Localized Surface Plasmon Resonance: Nanostructures, Bioassays and Biosensing—A Review Analytica Chimica Acta 2011, 706, 8–24. 4. Moscatelli, A. The Aluminium Rush Nature Nanotechnol. 2012, 7, 778. 5. Langhammer, C.; Schwind, M.; Kasemo, B.; Zoric, I. Localized Surface Plasmon Resonances in Aluminum Nanodisks Nano Lett. 2008, 8, 1461-1471. 6. Raether, H. Solid State Excitations by Electrons Springer Tr. Mod. Phys. 1965, 38, 84157. 7. Batson, P. E.; Silcox, J. Experimental Energy-Loss Function, Im[—1/e(q, ω)], for Aluminum Phys. Rev. B 1983, 27, 5224-5239. 8. Sprosser-Prou, J.; Felde, A.; Fink, J. Aluminum Bulk-Plasmon Dispersion and its Anisotropy Phys. Rev. B 1989, 40, 5799-5801. 9. Quail, J. C.; Rako, J. G.; Simon, H. J. Long-Range Surface-Plasmon Modes in Silver and Aluminum Films Opt. Lett. 1983, 8, 377-379. 10. Malicka, J.; Gryczynski, I.; Gryczynski, Z.; Lakowicz, J. R. Surface Plasmon-Coupled Ultraviolet Emission of 2,5-Diphenyl-1,3,4-oxadiazole J. Phys. Chem. B 2004, 108, 19114-19118. 11. Dorfer, T.; Schmitt, M.; Popp, J. Deep-UV Surface-Enhanced Raman Scattering J. Raman Spectrosc. 2007, 38, 1379-1382. 12. Knight, M. W.; Liu, L.; Wang, Y.; Brown, L.; Mukherjee, S.; King, N. S.; Everitt, H. O.; Nordlander, P.; Halas N. J. Aluminum Plasmonic Nanoantennas, Nano Lett. 2012, 12, 6000−6004.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13. Boltasseva, A.; Atwater H. A. Low-Loss Plasmonic Metamaterials Science 2011, 331, 290-291. 14. Levin, I.; Brandon, D. Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences J. Am. Ceram. Soc. 1998, 81, 1995–2012. 15. French, R. H. Electronic Band Structure of Al2O3, with Comparison to AlON and AlN J. Am. Ceram. Soc. 1990, 13, 471-489. 16. Trueba, A. M.; Trasatti, S. P. γ-Alumina as a Support for Catalysts: A Review of Fundamental Eur. J. Inorg. Chem. 2005, no volume, 3393–3403. 17. Wriedt, H. A. The Al-O (Aluminum-Oxygen) System Bull. Alloy Phase Diagrams 1985, 6, 548-553. 18. Wilk, G. D.; Wallace, R. M.; Anthony, J. M. High-κ Gate Dielectrics: Current Status and Materials Properties Considerations J. Appl. Phys. 2001, 89, 5243-5275. 19. Lisiansky, M.; Heiman, A.; Kovler, M.; Fenigstein, A.; Roizin, Y.; Levin, I.; Gladkikh, A.; Oksman, M.; Edrei, R.; Hoffman, A. et al SiO2/Si3N4/Al2O3 Stacks for Scaled-down Memory Devices: Effects of Interfaces and Thermal Annealing Appl. Phys. Lett. 2006, 89, 153506. 20. French, R. H.; Mullejans, H.; Jones, D. J. Optical Properties of Aluminum Oxide: Determined from Vacuum Ultraviolet and Electron Energy-Loss Spectroscopies J. Am. Ceram. Soc., 1998, 81, 2549–57. 21. Kotov, Y. A. Electric Explosion of Wires as a Method for Preparation of Nanopowders J. Nanopart. Res. 2003, 5, 539–550. 22. Jeurgens, L. P. H.; Sloof, W. G.; Tichelaar, F. D.; Mittemeijer, E. J. Thermodynamic Stability of Amorphous Oxide Films on Metals: Application to Aluminum Oxide Films Phys. Rev. B 2000, 62, 4707-4719.


Page 30 of 53

Page 31 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Lippens, B. C.; De Boer, J. H. Study of Phase Transformations During Calcination of Aluminum Hydroxides by Selected Area Electron Diffraction Acta Cryst. 1964, 17, 13121321. 24. Kim, H. Non-quantum Electronic Responses of Zinc Oxide Nanomaterials Nanotechnol. 2013, 24, 115701. 25. Stöckli, T.; Stadelmann, P.; Châtelain, A. Low-Loss EELS Study of Oxide-Covered Aluminum Nanospheres Microsc. Microanal. Microstruct. 1997, 8, 145-155. 26. Eagerton, R. F. Electron Energy Loss Spectroscopy 2nd Ed.; Plenum: New York, 1996. 27. Begemann, B.; Dorschner, J.; Henning, T.; Mutschke, H.; Gurtler, J.; Kompe, C.; Nass, R. Aluminum Oxide and the Opacity of Oxygen-Rich Circumstellar Dust in the 12-17 Micron Range Astrophys. J. 1997, 476, 199-208. 28. Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: San Diego, 1985. 29. Kim, H. Controlled Modifications in Electronic and Chemical Structures of a Nanoscale Region of Polystyrene by Fast Electrons J. Phys. Chem. B 2008, 112, 12579–12584. 30. Eriksson, T. S.; Hjortsberg, A.; Niklasson, G. A.; Granqvist, C. G. Infrared Optical Properties of Evaporated Alumina Films Appl. Opt. 1981, 20, 2742–2746. 31. Ortiz, A.; Alonso, J. C.; Pankov, V.; Huanosta, A.; Andrade, E. Characterization of Amorphous Aluminum Oxide Films Prepared by the Pyrosol Process Thin Solid Films 2000, 368, 74-79. 32. Catherine, Y.; Talebian, A. Plasma Deposition of Aluminum Oxide Films J. Electron. Mater. 1988, 17, 127–134. 33. Kim, H. Modifications in Electronic Properties of Polystyrene Methacrylic Acid by Neutralization and Fast Electrons J. Phys. Chem. B 2009, 113, 9359–9363. 34. Miyazaki, S. Photoemission Study of Energy-Band Alignments and Gap-State Density Distributions for High-k Gate Dielectrics J. Vac. Sci. Technol. B 2001, 19, 2212– 216.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35. Ealet, B.; Elyakhloufi, M. H.; Gillet, E.; Ricci, M. Electronic and Crystallographic Structure of γ-alumina Thin Films Thin Solid Films 1994, 250, 92-100. 36. Ehrenreich, H.; Philipp, H. R. Optical Properties of Semiconductors in the Ultra-violet in ‘Proc. Int. Conf. Phys. Semiconductors’ (Edited by A. C. Strickland), pp 367-374. Bartholomew Press Dorking. UK 1962 37. Pinto, H. P.; Nieminen, R. M.; Elliott, S. D. Ab initio Study of γ-Al2O3 Surfaces. Phys. Rev. B 2004, 70, 125402. 38. Menéndez-Proupin, E.; Gutiérrez, G. Electronic Properties of Bulk γ-Al2O3 Phys. Rev. B 2005, 72, 035116. 39. Monch, W. Semiconductor Surfaces and Interfaces 3rd Ed.; Springer: Berlin 2002. 40. Liu, X.; Pichler, T.; Knupfer, M.; Fink, J.; Kataura, H. Electronic Properties of Potassium-intercalated C60 Peapods Phys. Rev. B 2004, 69, 075417. 41. Ritsko, J. J.; Mele, E. J.; Gates, I. P. Excitations of Back-Folded Graphite Bands in KC8 Phys. Rev. B 1981, 24, 6114–6120. 42. Verwey, E. J. W. Incomplete Atomic Arrangement in Crystals J. Chem. Phys. 1975, 3, 592. 43. Hansen, P. L.; Brydson, R.; McComb, D. W.; Richardson, I. EELS Fingerprint of AlCoordination in Silicates Microsc. Microanal. Microstruct. 1994, 5, 173-182. 44. Bowles, J. F. W.; Howie, R. A.; Vaughan, D. J.; Zussman, J. Rock-Forming Minerals Vol. 5A: Non-Silicates:Oxides, Hydroxides and Sulphides 2nd Ed.; Geological Society of London: London, 2011. 45. Balzarotti, A.; Bianconi, A. Electronic Structure of Aluminium Oxide as Determined by X-Ray Photoemission Phys. Status Solidi B 1976, 76, 689-694. 46. Britov, I. A.; Romashenko, Y. N. X-ray Spectroscopic Investigation of the Electronic Structure of Silicon and Aluminum Oxide Fizika Tverdogo Tela 1978, 20, 664 [or Sov. Phys.—Solid State 1978, 20, 384-389].


Page 32 of 53

Page 33 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47. Perevalov, T. V.; Gritsenko, V. A.; Kaichev, V. V. Electronic Structure of Aluminum Oxide: Ab initio Simulations of α and γ Phases and Comparison with Experiment for Amorphous Films Eur. Phys. J. Appl. Phys. 2010, 52, 30501. 48. Menéndez-Proupin E.; Gutiérrez, G. Electronic Properties of Bulk γ-Al2O3 Phys. Rev. B 2005, 72, 035116. 49. Ciraci, S.; Batra, I. P. Electronic Structure of α-alumina and its Defect States Phys. Rev. B 1983, 28, 982–992. 50. Matsunaga, K.; Tanaka, T.; Yamamoto, T.; Ikuhara, Y. First-Principles Calculations of Intrinsic Defects in Al2O3 Phys. Rev. B 2003, 68, 085110. 51. Liu, D.; Clark, S. J.; Robertson, J. Oxygen Vacancy Levels and Electron Transport in Al2O3 Appl. Phys. Lett. 2010, 96, 032905. 52. Weber, J. R.; Janotti, A.; Van de Walle, C. G. Native Defects in Al2O3 and their Impact on III-V/Al2O3 Metal-oxide Semiconductor-Based Devices J. Appl. Phys. 2011, 109, 033715. 53. Evans, B. D.; Hendricks, H. D.; Bazzarre, F. D.; Bunch, J. M. Ion Implantation in Semiconductors (Edited by F. Chernpw, J. Borders, and D. Brice); Plenum: New York, 1977. 54. Lee, K. H.; Crawford, J. H. Electron Centers in Single-Crystal A12O3 Phys. Rev. B 1977, 15, 4065-4070. 55. Pustovarov, V. A.; Perevalov, T. V.; Gritsenko, V. A.; Smirnova, T. P.; Yelisseyev, A. P. Oxygen Vacancy in Al2O3: Photoluminescence Study and First-principle Simulation Thin Solid Films 2011, 519, 6319–6322. 56. Zahid, M. B.; Arreghini, A.; Degraeve, R.; Govoreanu, B.; Suhane, A.; Van Houdt, J. Electron Trap Profiling Near Al2O3/Gate Interface in TANOS Stack Using Gate-Side Trap Spectroscopy by Charge Injection and Sensing IEEE Electron Device Lett. 2010, 31, 1158-1160.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

57. Govoreanu, B.; Degraeve, R.; Van Houdt, J.; Jurczak, M. Statistical Investigation of the Floating Gate Memory Cell Leakage through High-k Interpoly Dielectrics and Its Impact on Scalability and Reliability Tech. Dig. - Int. Electron Devices Meet. 2008, no volume, 775–778. 58. Evans, B. D.; Stapelbroek, M. Optical Properties of the F+ Center in Crystalline Al2O3 Phys. Rev. B 1978, 18, 7089-7098. 59. Stashans, A.; Kotomin, E.; Calais, J. L. Calculations of the Ground and Excited States of F-type Centers in Corundum Crystals Phys. Rev. B 1994, 49, 14854–14858. 60. Sankaran, K.; Pourtois, G.; Degraeve, R.; Zahid, M. B.; Rignanese, G. M.; Van Houdt, J. First-principles Modeling of Iintrinsic and Extrinsic Defects in γ-Al2O3 Appl. Phys. Lett. 2010, 97, 212906 . 61. Horie, C. Exciton and Plasmon in Insulating Crystals Progress of Theoretical Physics 1959, 21, 113-134. 62. Giaquinta, P. V.; Parrinello, M.; Tosatti, E.; Tosi, M. P. Plasmons and Excitons in Insulators: Dielectric Treatment J. Phys. C: Solid State Phys. 1976, 9, 2031-2048. 63. Egri, I. Excitons and Plasmons in Metals, Semiconductors and Iinsulators: A Unified Approach Physics Reports (Review Section of Physics Letters) 1985, 119, 363-402. 64. Aspnes, D. E.; Studna, A. A.; Kinsbron, E. Dielectric Properties of Heavily Doped Crystalline and Amorphous Silicon from 1.5 to 6.0 eV Phys. Rev. B 1984, 29, 768-779. 65. Asnin, V. M.; Rogachev, A. A. Exciton Absorption in Doped Germanium Phys. Stat. Sol. 1967, 20, 755-757. 66. Brehmea, S.; Fenskea, F.; Fuhsa, W.; Nebauerb, E.; Poschenriedera, M.; Sellea, B.; Sieber, I. Free-Carrier Plasma Resonance Effects and Electron Transport in Reactively Sputtered Degenerate ZnO:Al Films Thin Solid Films 1999, 342, 167-173.


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67. Amo, A.; Martín, M. D.; Viña, L.; Toropov, A. I.; Zhuravlev, K. S. Photoluminescence Dynamics in GaAs along an Optically Induced Mott Transition J. Appl. Phys. 2007, 101, 081717. 68. French, R. H.; Jones, D. J.; Loughin, S. lnterband Electronic Structure of α-Alumina up to 2167 K J. Am. Ceram. Soc. 1994, 77, 412-422. 69. Fox, M. Optical Properties of Solids; Oxford: New York, 2001. 70. Pelant, I.; Valenta, J. Luminescence Spectroscopy of Semicoductors; Oxford: New York, 2012. 71. Ahuja, R.; Osorio-Guillen, J. M.; Souza de Almeida, J.; Holm, B.; Ching, W. Y.; Johansson, B. Electronic and Optical Properties of γ-Al2O3 from Ab initio Theory J. Phys.: Condens. Matter 2004, 16, 2891–2900. 72. Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots Nature Mater. 2011, 10, 361-366. 73. Marinopoulos, A. G.; Reining, L.; Olevano, V.; Rubio, A.; Pichler, T.; Liu, X.; Knupfer, M.; Fink, J. Anisotropy and Interplane Interactions in the Dielectric Response of Graphite Phys. Rev. Lett. 2002, 86, 076402. 74. Fink, J. Recent Developments in Energy-Loss Spectroscopy Advances in Electronics and Electron Physics 1989, 75, 121-232. 75. Kociak, M.; Henrard, L.; Stephan, O.; Suenaga, K.; Colliex, C. Plasmons in Layered Nanospheres and Nanotubes Investigated by Spatially Resolved Electron Energy-Loss Spectroscopy Phys. Rev. B 2000, 61, 13936-13944. 76. Speth, A. J.; Fang, F. F. Effects of Low-Energy Electron Irradiation on Si-insulated Gate FETs Appl. Phys. Lett. 1965, 7, 145–146.



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77. Stanley, A. G. Model for Shifts in the Gate Turn-Qn Voltage of Insulated-Gate FieldEffect Devices Induced by Ionizing Radiation IEEE Trans. Electron Dev., 1967, 14, 134138.


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Table of Contents

IP

IP

SP

Intensity

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Al IP

Alumina SP

4

6

Energy (eV)

8



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Table 1

Tail Shell γ-Al2O3 α-Al2O3

EELS Al (at. %) {SD (%p)} O (at. %) {SD (%p)} 44.2 ± 5.5 {3.0} 55.8 ± 6.1 {3.0} 47.4 ± 5.9 {5.7} 52.6 ± 6.3 {5.7} 39.6 ± 5.0 {1.6} 60.4 ± 7.5 {1.6} -

EDS Al (at. %) {SD (%p)} O (at. %) {SD (%p)} 35.4 ± 3.6 {3.3} 64.6 ± 5.1 {3.3} 36.6 ± 3.3 {3.7} 63.4 ± 4.8 {3.7}

Table Caption Table 1. Atomic fraction of Al and O in different aluminas with an average quantification error and a standard deviation (SD).


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Figure Captions Figure 1. Morphology and crystallography of surface-oxidized Al nanoparticles. (a), (b) Al particles with their tail or shell sitting on an amorphous carbon film. The surface oxide and the Al core are seen in different contrast with the tail or the shell sticking out in empty space. Scale bars: 50 nm. (c) High resolution image magnified from the yellow dotted box of the Al particle in (a). The inset shows an image magnified further to show the amorphous and the crystalline layers in the shell. (d) High resolution image of the tail in (b). Different grains are separated by the yellow lines. Scale bars in (c) and (d): 5 nm. (e) – (i) FFT from each grain of the tail in (d). The image (e) is an FFT of the grain 1 (G1) and so on. (j) FFT from the entire area of the tail in (d). Crystalline spots and an amorphous ring are seen. Scale bars in (e) – (j): 10 nm-1. (k) Lattice spacings measured from high resolution images of tails and crystalline parts of shells in Al particles. Miller indices are assigned to the lattice spacings for γ-Al2O3. Figure 2. Morphology, crystallography, and composition of two alumina phases. (a) Image of a γ-Al2O3 particle. Scale bar: 100 nm. (b) High resolution image magnified from part of the alumina particle in (a). The inset is an FFT of the image showing crystalline spots and an amorphous ring. Scale bars: 5 nm and 9.12 nm-1 (inset). (c) Diffraction pattern from the particle in (a). This pattern and the FFT in (b) are all aligned in the zone axis of γ-Al2O3. Scale bar: 9.12 nm-1 (d) Image of an α-Al2O3 particle (scale bar: 50 nm) and (e) its diffraction pattern (scale bar: 4.20 nm-1) aligned in the zone axis of α-Al2O3. (f) Typical EDS spectra from both alumina particles showing characteristic Al and O Kα X-ray fluorescence lines at 1.5 and 0.52 keV, respectively. In the inset EDS line profiles for the Al and O contents of both aluminas are displayed, which are collected along the yellow dotted lines in (a) and (d). C and Cu peaks are from the TEM grids. Figure 3. Electronic response from Al particles. VEELS collected along an imaginary line (yellow arrow in the inset) from the center to slightly beyond the shell of an Al particle with a diameter of (a) 91 nm or (b) 50 nm. VEELS collected along an imaginary line (green arrow in the inset) from the center to beyond the tail of an Al particle with diameter of (c) 110 nm or (d) 36



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nm. The inset shows approximate positions of probed spots with colors, which are used for the same-colored spectra. Figure 4. Electronic response from pure alumina particles. VEELS collected along an imaginary line from the center to slightly beyond an edge of (a) a γ-Al2O3 particle and (b) an α-Al2O3 particle. Figure 5. Spectroscopic and microscopic representation of the surface plasmon (SP) and the novel transition (IP). (a) VEELS from the 91-nm Al particle in a low energy region. Energyfiltered TEM images of an Al particle collected at (b) 5 ± 0.5 eV and (c) 7 ± 0.5 eV. The color scale is given for intensity level. An upper color indicates a higher intensity. Scale bars are 50 nm. Figure 6. Variation of excitation energies in VEELS from Al nanoparticles. Positions of (a) bulk plasmon (BP) and (b) surface plasmon (SP) peaks for the surface oxide depending on the diameter of Al particles together with average BP positions of pure α- and γ-Al2O3. The red dots indicate the resonance energy of surface plasmon for a spherical Al/α-Al2O3 particle in diverse modes corresponding to h = 1 (bottommost) to 50 (topmost) in Eq. 2. (c) Positions of the novel excitation (IP) for tails and shells in Al particles of various diameters. (d) – (h) Changes in oscillation strength of the novel transition and surface plasmon with movement of the electron probe. VEELS from the 110-nm Al particle are used after deconvolution. The intensity axes in (d) – (h) have the same scale. The inset shows a schematic for an Al particle with colored spots around which the same-colored VEELS in (d) – (h) are collected. Figure 7. ELF for shells and tails of Al nanoparitcles and pure γ-Al2O3 and α-Al2O3 particles. The ELF for pure Al is from ref. 6. The figure legend shows materials used for the ELFs. When the material is the surface oxide the diameter of the corresponding Al particle is displayed together. Figure 8. Physical functions for shells and tails of Al nanoparitcles, pure γ-Al2O3, and α-Al2O3 particles calculated from the ELFs in Fig. 7. The ones for pure Al are from ref. 6. (a) Real part and (b) imaginary part of the dielectric function. (c) Real part of the optical conductivity.


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Figure 9. Interstitial positions available for atomic diffusion and their occupation together with composition. (a) Tetrahedral and octahedral interstitial sites (yellow and red balls, respectively) in the face-centered cubic structure (bluish gray balls), which is also an octant of the spinel structure. (b) Al L23 ELNES from the tail and the shell of an Al particle and from a pure γ-Al2O3 particle. The spectra are multiplied properly to show similar intensity at 79.5 eV. The inset shows approximate positions of the probed spots with colors, which are used for the same-colored spectra. The color code and the probe positions in (b) apply to spectra in (f) as well. The CN of each Al interstitial site is displayed together. CN 4 and 6 correspond to tetrahedral and octahedral sites. HAADF-STEM images of (c) an Al particle with a long tail and (d) a trapezoidal γ-Al2O3 particle with Al neighbors. Scale bars: 80 and 100 nm, respectively. (e) EDS from local spots of the paths 1 and 2 in (c). The EDS line profiles for the Al content are collected by an electron probe moving across the paths 1 and 3 in (c) and (d) (inset). (f) Normalized core-loss spectra containing Al and O absorption edges with their raw spectra (inset). (g) Atomic percent of Al in the surface oxide and γ-Al2O3 acquired by quantification of core-loss spectra. Figure 10. Electronic band structures and Neff. (a) Anticipated energy band diagrams for the Al/alumina system and pure γ-Al2O3. σ1 for pure γ-Al2O3 (green) and the tail of the 110-nm Al particle (red) are from Fig. 8. In contact with Al, alumina can have a defect band through diffusion, which is asserted to be responsible for the novel peak in VEELS. (b) Neff for the tail of the 110-nm Al particle and pure γ-Al2O3. Figure 11. Modifications in an Al nanoparticle and pure γ-Al2O3 by continuous exposure to electron beam. (a) Morphological changes in the tail of an Al particle recorded by STEM. The inset shows the entire Al particle. Regions around long-exposed spots (spots 1 to 4 indicated by the yellow arrows) look brighter (yellow) due to the accumulation of hydrocarbons. Scale bars: 10 and 50 (inset) nm. (b) VEELS collected from the spot 1 in (a) after 0 to 104 sec of preexposure by the electron probe used for the spectra. There is no interruption of irradiation between the pre-exposure and the VEELS collection. (c) VEELS from a pure γ-Al2O3 particle



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after pre-exposure for a given time (figure legend). The bulk plasmon shifts and the intensity around 5.3 eV is also increased by the pre-exposure. The VEELS in (b) and (c) are all normalized with respect to the zero loss peaks and equally displaced along the intensity axis. (d) Morphological changes in pure γ-Al2O3 recorded by STEM. The arrow points at the spot exposed for 78 sec and used for VEELS. Scale bar: 40 nm.


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Figure 1

(a)

(b) Shell

Tail

Al Core

(c)

(d)

G5 G1

G4 G2

G3

(e) 1-13

220

G1 (j)

(f)

220

(g)

(i)

(h) 220

220

G2

G4

G3 (k)

113

113

G5

Tails γ-Al2O3

30

Shells δ-Al2O3

004

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 10

133 024

111

022 222

002

0 2.0

2.5

3.0

3.5

d Spacing (A)


4.0

4.5


Figure 2

(a)

(c)

11-1 2-20 ZA 112 (d)

(b)

-220 11-1 ZA 112 (e)

ZA 1-100 0003 11-20

0.45 nm

α -Al 2O 3 γ -Al 2O 3

C K α Al K α O Kα

0

2

Al, O (at. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 53

70 60 50 40 30 20

(f)

Al /O: α-Alumina Al /O: γ-Alumina 40

60

80 100 120 Cu K α Cu Kβ

Distance (nm)

4

6

E nergy (keV )

8


Page 45 of 53

Figure 3

(a)

Intensity

Intensity

(c)

5

10

15

20

25

5

10

15

20

25

Energy (eV)

Energy (eV)

(d)

Intensity

(b)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

10

15

20

25

5

10

15

Energy (eV)

Energy (eV)


20

25


Figure 4

Intensity

(a)

5

10

15

20

25

Energy (eV) (b)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5

10

15

20

25

Energy (eV)


Page 46 of 53

Page 47 of 53

Figure 5

(a)

SP

IP Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

6

8

Energy (eV) (b)

IP

(c)


SP


Figure 6

BP

(a)

24

Tail

SP

(d)

α

IP γ

22

SP

(b)

(e)

Shell

Intensity

Peak Position (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 53

7 6

(f)

(g)

IP

(c)

5.6

Tail

5.2

Shell

(h)

40

60

80

100

Diameter (nm)

120

2

4

6

8

Energy (eV)


10

Page 49 of 53

Figure 7

Im(-1/ε(E))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tail110nm Tail36nm Shell91nm Shell50nm γ -Al2O 3

0.8

α-Al2 O3

x 0.5

Al

0.4

x 0.5 x 0.02

0.0

5

10

15

20

Energy (eV)


25


Figure 8

(a)

3

Tail110nm Tail36nm Shell91nm Shell50nm γ-Al2O3

2

α-Al2O3

5

ε1

4

Al

1 0

x 0.02

-1 4

8

12

16

20

24

Energy (eV) (b)

4

ε2

3 2 1 x 0.25 0

4

8

12

16

20

24

20

24

Energy (eV) (c)

5

5 4

-1

σ1(Ωcm) /10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3 2 1 x 0.25 0

4

8

12

16

Energy (eV)


Page 50 of 53

Page 51 of 53

Figure 9

(f)

Path 2

Log(Intensity)

(c)

(a)

Intensity

Al - L23 Path 1

(d)

Al - L

O-K

200

400

Energy (eV)

600

O - K (x10) Path 3

(e)

C Kα

CN < 4

O Kα Tail 1 Tail 2 Shell γ-Al2O3

76

78

100

Along Path 3

Energy (eV)

82

0

Along Path 1

2

Shell Tail

55

0

10

20

30

Distance (nm) Cu K α Tail Cu K β

C Film 80

γ-Al2O3

60

40

Al K α

600

(g)

50

30

550

Energy (eV)

60

CN = 6

CN = 4

90

4

50 45 40

6

Energy (keV)


Al (at. %)

(b) Al L23 ELNES

Al (at. %)

80

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

0

20

40

60

Measurements


Figure 10

Al

Energy (eV)

σ1 CB

CB

10

EF

5 Defect Band

0 VB

VB

γ-Al2O3

Alumina

Alumina

Tail

Al

< Tail of an Al particle>

(b)

Tail 110 nm γ-Al2O3

160

Neff per unit cell

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 53

120 4

80

E (eV) 6

7

8

8 6 4 2 0

40 0

5

0

10

20

30

40

50

60

70

Energy (eV)


80

Page 53 of 53

Figure 11

(a)

2 3

1

0 s 56 s 4 s 76 s 8 s 104 s

4

1

4

(b)

Intensity

2

8

12

16

8

12

16

20

Energy (eV)

24 (d)

0 s 70 s 4 s 74 s 8 s 78 s

4

20

Energy (eV)

(c)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24


Alumina System - The

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