Novel plasmon resonances of nonstoichiometric alumina

Applied Surface Science 488 (2019) 648–655

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Novel plasmon resonances of nonstoichiometric alumina Hansoo Kim

T

Microscopy and Imaging Center, Texas A&M University, College Station, TX 77843, USA

ARTICLE INFO

ABSTRACT

Keywords: Nonstoichiometric alumina Surface plasmon resonance Plasmonics

Aluminum monoxide (AlO) and a nonstoichiometric alumina (n-alumina) with its chemical composition similar to that of AlO were proposed theoretically for a promising low-loss plasmonic material. However, they were rarely studied due to the high thermodynamic instability. Recently, n-aluminas with their O/Al atomic ratios close to 1 were found to form on Al nanospheres synthesized by electrical explosion. Although a new bulk plasmon resonance was observed below the excitonic transition energy from the n-aluminas, no investigation into surface plasmon resonance – the key component for plasmonics - was conducted yet. Here I report on surface plasmons of the n-aluminas discovered by spatially-resolved electron energy loss spectroscopy. Energy loss spectra collected from local spots of the n-aluminas are analyzed to reveal that there are two types of naluminas; one with two surface plasmons (n-alumina A) and the other with one surface plasmon (n-alumina B). Remarkably, the oscillator strengths of the novel surface plasmons are as high as that of surface plasmon of Al. The electronic band structure proposed for the n-alumina A shows a lossless energy zone. Consequently, current study demonstrates that the n-aluminas have the potential as a new low-loss plasmonic material.

1. Introduction Plasmonics is now growing fast in a revolutionary way along with the progress of nanoscience and nanotechnology. Noble metals such as gold and silver play an important role in the growth due to their freeelectron-like conduction electrons. However, a few drawbacks of the noble metals - for example, huge loss at optical frequencies - require development of new plasmonic materials [1,2]. Nonmetallic materials can form surface plasmon (SP) as well after doping, but they also have various sources of loss like interband transitions [3–5]. An ideal plasmonic material should show zero loss and a negative permittivity. However, materials in nature cannot fulfill the requirements. Thus, it was suggested that metamaterials, materials with artificially designed compositions and structures [6], should be built to have the desired properties. One strategy is to incorporate a nonmetallic element into a metal in order to reduce its exceedingly high free-carrier concentration and cut down on the loss [1,2]. It can also increase the interatomic spacing of the metal by a factor of 2 through adjustment of the composition, which can introduce an energy gap in the conduction band due to the reduced interatomic interaction. The gap provides a lossless energy regime, while the compound may still allow the critical property - negative permittivity. For example, when aluminum (Al) is oxidized to AlO and the AleAl distance is increased, theoretically AlO can still be metallic by retaining

one of the three Al valence electrons in the conduction band and show the energy gap [7]. Therefore, aluminum oxide with its chemical composition close to that of AlO is proposed as one of the promising low-loss plasmonic materials [1,2,7]. However, an actual solid-phase alumina with a similar composition, readily applicable for solid-state devices, has not been investigated empirically for the presence of SP, not to mention its plasmonic properties. Al is abundant on earth and shows a few good plasmonic properties [8]. Thus, Al is a cost-effective and efficient plasmonic material, even though it shares some disadvantages with noble metals, such as loss by the high carrier concentration [2]. The Drude model can well describe the electronic response of Al except about 1.5 eV, where a strong interband transition - another source of loss - is excited [9]. On the other hand, the most common stoichiometric alumina, Al2O3, has many polymorphs which can be used for a wide variety of applications [10]. Some forms of alumina with nonstoichiometric or different stoichiometric compositions were reported but rarely confirmed in a reliable way, indicating the high thermodynamic stability of Al2O3 [11]. However, low dimensional aluminas with different chemical compositions are often found, for example, at the surface of Al2O3 [12–14] and in thin layers of alumina and Al [15,16]. Recently, n-aluminas with atomic ratios of O to Al smaller than 1.5 for Al2O3 and close to 1 for AlO were observed from the surface of Al nanoparticles [8]. The n-aluminas were found by electron energy loss spectroscopy to have a novel interband bulk plasmon (BP) resonance

E-mail address: [email protected]. https://doi.org/10.1016/j.apsusc.2019.05.178 Received 8 March 2019; Received in revised form 7 May 2019; Accepted 15 May 2019 Available online 17 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 488 (2019) 648–655

H. Kim

(a)

S

C

Cloth Filter

Explosion Chamber

PS

The product on the filter has Al nanospheres with a small number of pure γ-Al2O3 particles. The surface of the Al nanospheres is oxidized. The average diameter of the Al particles is ~71 nm and the standard deviation ~28.8 nm when calculated from the direct measurement of individual Al particles. The histogram for the size distribution is given in Fig. 1(b). The lognormal distribution can approximate the histogram with parameters of μ = 4.3 and σ = 0.48 (red solid curve). The parameters are the mean (μ) and the standard deviation (σ) of the natural logarithm of the variable. Here the variable is the data set in the histogram. The surface alumina of an Al nanoparticle consists of a shell and normally one or two tails. Analysis of the chemical composition shows that the surface alumina has only Al and O with no other elements [8]. The average thickness of a shell is ~3 nm and the length of a tail is larger than 10 nm. The average atomic ratio of O to Al is measured by electron energy loss spectroscopy to be 1.11 for shells and 1.27 for tails. The γ-Al2O3 particles are observed to have different shapes like isosceles trapezoid and bigger sizes of about one to a few hundred nanometers. Some of the dimension and the composition data are from the reference [8] and the rest of them are measured or calculated in current experiments. The nanoparticles were dispersed in ethanol by sonication. Then, a few drops of the dispersion were deposited onto TEM grids with a lacey carbon film and dried on a paper filter. The nanoparticles were characterized by JEOL JEM 2010F operating at 200 keV, which is equipped with a Gatan imaging filter (Tridiem 863P with a 2 k × 2 k chargecoupled device (CCD)) and a Fischione high angle annular dark field (HAADF) detector. Bright field images were taken in transmission electron microscopy (TEM) mode and dark field images in scanning transmission electron microscopy (STEM) mode. An electron probe of about 0.5-nm diameter was used in STEM mode for collection of electron energy loss spectra after dark-current removal and gain correction. An energy dispersion was 0.05 eV per channel and an energy resolution 0.80 eV at the full width at half-maximum (FWHM) of the zero loss peak. A typical acquisition time for valence electron energy loss spectra (VEELS) was 0.2 s. Deconvolution of low energy loss spectra was performed by the Fourier-log method. More details about measurements are available in the reference [8].

L

Al wire

Ar Trap

Number of Particles

(b) 30 20 10 0 0

50

100

150

Diameter (nm) Fig. 1. Synthesis system for the Al nanospheres and their size distribution. (a) Circuit diagram together with the fabrication facility for the electrical-explosion method. (b) Histogram of the diameter of Al nanospheres and its lognormal fit (solid curve). The total number of particles used for the histogram is 226.

below the band-gap energy (Eg) [8]. The BP shifts to slightly lower energies around the surface. The peak does not vanish immediately after the electron probe moves out of the alumina. It was found in the reference [8] that the n-aluminas have high concentrations of electrons involved in excitation of the interband BP. The concentrated electrons suggest that a related SP may exist at an energy lower than the resonance energy of the new BP. Here the n-aluminas are studied further for SP resonance and its properties. It is found that the n-aluminas can show multiple localized SP resonances together with an additional BP resonance. These results are crucial for exploitation in plasmonics of the n-alumina with an O/Al atomic ratio close to 1. They may also provide insight into the electronic structure and response of AlO, and help the design of new plasmonic metamaterials.

3. Results and discussion An Al nanosphere with a shell and a tail is displayed in Fig. 2(a). The high resolution TEM (HRTEM) image from the tail and its fast Fourier transform (FFT) (Fig. 2(b) and (c)) confirm that the tail is polycrystalline with the defect spinel structure. One of the stoichiometric transition aluminas, γ-Al2O3, is known to have the defect spinel structure [10]. In the crystal structure 32 O2– anions form a cubic unit cell composed of 8 component subcells, each of which is similar to the facecentered-cubic (FCC) structure (Fig. 2(d)). Out of the 64 tetrahedral and the 32 octahedral interstices in the unit cell 21⅓ interstices are occupied by Al3+ cations in γ-Al2O3. The n-alumina tail, however, has more Al cations or O vacancies than γ-Al2O3 [8]. The lattice constant of the tail is the same with that (0.79 nm) of γ-Al2O3 when measured by HRTEM images. VEELS acquired from around the center of the tail (orange penetrating probe in Fig. 2(b)) shows two clear peaks after deconvolution (Fig. 2(e)). The peak at 5.8 eV was reported to be an interband BP (IBP2 in the figure) [8]. Remarkably, another novel peak (IBP1) is seen at a lower energy (4.5 eV). VEELS collected in an aloof geometry (green non-penetrating probe ~5 nm away from the surface of the tail in Fig. 2(b)) still contains the two peaks at lower energies - 4.3 (ISP1) and 5.5 (ISP2) eV (Fig. 2(f)). The redshift of an electronic excitation around the surface of a material, whether it is an interband or a plasmonic transition, can be caused by the increase in surface-to-volume ratio [17]. Change in the relative strength of the two oscillators with the redshift is notable. VEELS from γ-Al2O3 are displayed together in Fig. 2(e) and (f) (black and olive-

2. Experiment Nanoscale and spherical Al particles with surface alumina were synthesized by electrical explosion – a method developed to commercial scale. The facility for the fabrication (Fig. 1(a)) is composed of an explosion chamber, a trap, and a cloth filter. The circuit for the electrical explosion comprises a high-voltage power source (PS), a switch (S), an inductor (L), and a capacitor bank (C). Current pulses at high current density (~105 A/mm2) are applied to an Al wire with a diameter of ~ 0.45 mm in an Ar atmosphere after purging the explosion chamber with the same inert gas. Typical capacitance and charging voltage are 3 μF and 25 kV, respectively. The Ar gas circulating in the synthesis facility contains a small but finite amount of residual oxygen. The Ar pressure of the chamber is maintained at ~0.2 MPa. Then, the wire experiences sublimation, which looks like explosion, forming superheated Al vapor and droplets. The product is carried along the facility by the circulating Ar gas after the reaction and collected by the filter after large particles are removed by the trap. 649

Applied Surface Science 488 (2019) 648–655

H. Kim

FFT of Tail

(f)

(c) FFT

ISP1

ISP2

VEELS {111}

{220} Zone Axis: <112> (b)

e’ ~ 5 nm (a)

2

4

6

Energy (eV)

(e)

8

IBP2

VEELS

IBP1 (d) ⅛ Spinel Structure

: O site : Octahedral Interstice : Tetrahedral Interstice

Crystal Structure 2

4

6

Energy (eV)

8

Fig. 2. Crystal structure and electronic response of one type of n-alumina (n-alumina A) on Al. (a) TEM image of an Al nanoparticle with surface alumina. Scale bar: 25 nm. (b) High resolution TEM image of the surface alumina (tail) magnified from the blue dotted box in (a). The orange and the green arrows represent electron probes used for collection of the VEELS. Scale bar: 5 nm. (c) FFT of the image in (b). One set of spots in the yellow circles are aligned in the zone axis < 112 > of the defect spinel structure and additional spots in the colored triangles and squares are indexed by the same crystal structure. It shows the tail is polycrystalline. (d) Octant of the spinel structure. (e) VEELS collected by the orange electron probe in (b). (f) VEELS collected by the green probe in (b). Black and olive-colored VEELS in (e) and (f) are acquired from the center and the outside of a γ-Al2O3 particle, respectively. The scale of the vertical axis in (e) and (f) is not the same but adjusted to show high intensity of component peaks. The vertical axis of all the plots for VEELS represents the intensity of the inelastic scattering in arbitrary units throughout the paper.

colored spectra, respectively). They show no transition below Eg around 9 eV. The peak positions of a few n-alumina tails are displayed in Fig. 3(a)

Plasmon Positions (eV)

(a) 6.0

IBP2

5.5

with the averages and the standard deviations. The novel transition located at lower energies exhibits a clear redshift from 4.5 to 4.2 eV (IBP1 → ISP1) around the surface of the n-alumina tails as the other transition from 5.8 to 5.4 eV (IBP2 → ISP2). The transition energies of the IBP1 and the ISP1 show larger deviations than those of the IBP2 and the ISP2. The energy loss function (ELF: Im(−1/ε)) can be calculated from the double-differential cross-section for the inelastic scattering (Eq. (1)). In the equation q is the momentum transfer, E energy, and ε the dielectric function.

(b)

ELF SLF

0.4

ISP2 IBP1

5.0

2

4.5 4.0

30

E

0.2

90

120

Diameter (nm)

0.0

2

4

6

Energy (eV)

(1)

The ELF is calculated from the VEELS of the tail in Fig. 2, normalized with the index of refraction (1.7) for γ-Al2O3 at visible-light wavelengths, and displayed in Fig. 3(b). Analogous to the VEELS, the ELF shows transitions around 4.5 and 5.8 eV. Inelastic scattering at the surface of a material is expressed by the differential probability of surface scattering (Eq. (2)), where qs is the surface component of q. The equation describes the dependence of the differential probability on q and the incidence angle (θi). It includes the term (Im[(εa-εb)2/εaεb(εa + εb)]), known as the surface loss function (SLF). Proportional to the excitation probability of SP [18,19], the SLF is a function of εa and εb – the dielectric functions of two neighboring materials. It is presented in Fig. 3(b). The n-alumina tail has clear peaks

ISP1 60

1 Im( 1/ ( q , E ) ) q2

8

Fig. 3. Excitation energies and loss functions of n-alumina tails (n-alumina A). (a) Location of the new transitions of n-alumina tails (tails A) below Eg plotted against the diameter of Al nanospheres. Yellow lines are for the average values and the other horizontal lines for the standard deviations. (b) ELF (navy) and SLF (red) of the n-alumina tail in Fig. 2. The ELF is fitted by the two blue Gaussians representing the IBP1 and the IBP2 components. 650

Applied Surface Science 488 (2019) 648–655

H. Kim

FFT of Tail

(c) FFT

(b)

IBP2

(d) {220} {113} Zone Axis: <332> (a)

0.6 (e)

VEELS

~ 3 nm

0.4

ELF SLF

e’ ISP2

2

4

6

Energy (eV)

0.2

8

0.0

2

4

6

8

Energy (eV)

Fig. 4. Crystal structure and electronic response of the other type of n-alumina (n-alumina B) on Al. (a) TEM image of an Al nanoparticle. Scale bar: 25 nm. (b) High resolution TEM image of the tail magnified from the blue dotted box in (a). Scale bar: 5 nm. (c) FFT of the image in (b). One set of spots in the yellow circles are aligned in the zone axis < 332 > of the defect spinel structure and additional spots in the colored triangles are indexed by the same crystal structure. It shows the tail is polycrystalline. (d) Orange and green VEELS obtained respectively by the orange and the green probes in (b). (e) ELF (orange) and SLF (green) of the tail.

at 4.2 and 5.1 eV in the SLF. 2P s

E

|qs | q 4 cos

Im i

(

a

a b( a

b)

+

the IBP1 (or ISP1) resonance energy reveals that the IBP1 (ISP1) is not just a single electron transition. The IBP1 and the ISP1 do not show a single-electron character even around the surface of the tail. The IBP1 is not an exciton induced from the IT2 since in that case it should be observed whenever the IT2 (or IBP2) is present. Then, there should be no demarcation between n-aluminas A and B. However, the IBP1 is not seen even from an n-alumina B showing sharp and strong IBP2 and IT2 transitions (Figs. 4 and 5). Also, the exciton binding energy (EexB) of γ-Al2O3 is expected to be about 0.1 eV based on the fact that the electronic structure and the dielectric constant of γ-Al2O3 are analogous to those of α-Al2O3 (reference [8] and more references therein). It was reported before that EexB of α-Al2O3 is 0.13 eV [20,21]. It may be assumed that the n-alumina tail has a similar EexB as well since it shares the same crystal structure with γ-Al2O3 (this assumption is revisited in the reference [22]). The difference between the excitonic transition energy (Eex) and the interband transition energy (EIT) of the tail should be then about the small EexB or even smaller since Eex = EIT – EexB/n2 where n is the principal quantum number [23]. For example, γ-Al2O3 shows the excitonic transition at an E very close to Eg due to the small EexB [8]. However, the IBP1 is located at an energy even higher by the large gap (~1 eV) than the IT1 transition energy. The IBP2 also resonates at an energy higher by about 1 to 2 eV than the IT2 transition energy [8]. It indicates that the IBP1 is not an exciton from the IT1 either just like the IBP2 is not one from the IT2. Moreover, the effective numbers of electrons participating in the IBP1 and the IBP2 are larger than the Mott density (~1–2 × 1021/cm3) of γ-Al2O3 [8,22]. It suggests that the transitions corresponding to the IBP1 and the IBP2 should form free electron-hole pairs rather than excitons. Above all, the ISP1 in the VEELS of the n-alumina A is seen evidently in an aloof mode (Fig. 2) and pronounced in the SLF at an

2 b)

(2)

Another Al nanoparticle with a different type of n-alumina is presented in Fig. 4(a). The HRTEM image and its FFT (Fig. 4(b) and (c)) show that the tail of the Al particle also has the defect spinel structure. The orange VEELS in Fig. 4(d) collected from the center of the tail by the orange probe in Fig. 4(b) has only the IBP2 at 5.8 eV below Eg as reported in the reference [8]. It is shifted to the ISP2 (green spectrum in Fig. 4(d)) at 5.5 eV with the non-penetrating probe (green probe ~3 nm away from the surface of the tail in Fig. 4(b)). The ELF and the SLF of the tail (Fig. 4(e)) show a peak at 5.8 and 5.1 eV, respectively. In fact, the n-aluminas used for this study are found to have either the IBP2/ISP2 only (Fig. 4) or the IBP1/ISP1 additionally (Figs. 2 and 3). Thus, the n-aluminas in the same batch of the product show the two types of electronic response below Eg. In this paper the n-alumina with both the IBP1/ISP1 and the IBP2/ ISP2 is called n-alumina A (tail A and shell A) and the one with only the IBP2/ISP2 is n-alumina B (tail B and shell B) (see Table 1). The dielectric functions (ε = ε1 + iε2) of the n-alumina tails are acquired through the Kramers-Kronig transformation of the ELFs (Fig. 5(a) and (b)). It is clear that two single-electron transitions occur at 3.5 (IT1) and 4.9 (IT2) eV for the tail A, while only one singleelectron transition at 4.4 eV (IT2) for the tail B below Eg. These singleelectron transitions are considered to be interband transitions (ITs). The IT2 of the n-alumina B is seen clearly in ε2 (Fig. 5(b)) but not in the corresponding VEELS (Fig. 4(d)). It indicates that the IBP1 and the ISP1 in the VEELS of the n-alumina A are not the IT2 of the n-alumina B even though the IT2 and the IBP1 (or ISP1) are observed at similar energies. In addition, the large gap (~1 eV) between the IT1 energy and Table 1 List of abbreviations used in the text for new transitions and materials. IT

Interband transition

ISP (IBP)

Interband surface (bulk) plasmon

IT1 IT2 n-Alumina A (tail A, shell A)

IT excited below ISP1 IT excited between IT1 and ISP2 n-Alumina (tail and shell) showing ISP1/IBP1 and ISP2/IBP2

ISP1 (IBP1) ISP2 (IBP2) n-Alumina B (tail B, shell B)

ISP (IBP) excited below 5 eV ISP (IBP) excited between 5 eV and Eg n-Alumina (tail and shell) showing only ISP2/IBP2

651

Applied Surface Science 488 (2019) 648–655

H. Kim

n-alumina A (a)

ε1, ε2

4

4

(b)

ε1, ε2

IT2

IT1

Empirical ε

3

3 IT2

2

2

1

1

0

2

4

6

8

Energy (eV)

0

2

4

2.8 3.1

Parameter (eV) Eb Ep

1.6 2.2

4.4

ε1, ε2

8

3.8

2.0

Calculated ε

3.5 4.9

2

6

Energy (eV)

Parameter (eV) 2 (d) Eb Ep

(c)

ε1, ε2

1

1

0

0 2

4

quite similar to each other, especially in graph shape and peak position above 3 eV. The close similarities indicate that the IBP1 and the IBP2 are interband BPs since they are employed in the calculation as BPs triggered by the IT1 and the IT2. For both types of tails A and B, ε1 and ε2 show concave curvatures around the plasmon resonance energies. In Fig. 5 the empirical and the calculated ε2's have values of 2.0 and 1.4, respectively, at 4.3 eV (ISP1). They are 1.2–1.5 and 0.7 at 5.5 eV (ISP2). Thus, some degree of damping is expected for the ISP1 and the ISP2. For comparison, ε2 values for Al, Ag, Au, and Cu are about 0.1, 0.6, 5.3, and 4.3 at their SP resonance energies (10.6, 3.8, 2.7, and 2.2 eV, respectively, when these metal plates neighbor air) [24–27]. However, the n-alumina B has very small ε2 values at optical frequencies and so does the n-alumina A except around the IT1 energy. One thing to note is that ε1's for the n-aluminas are positive even around the ISP resonance energies, which does not satisfy the condition for a localized SP resonance [28,29]. The dielectric properties of the nalumina A around the IBP1 resonance energy do not appear to meet the criteria for a bulk plasmon resonance either. A couple of reasons that the plasmon resonances can still occur below Eg of the n-aluminas are suggested in the reference [22]. The ELF in Fig. 3(b) has a value of ~0.3 at the IBP1 excitation energy (4.5 eV) when it is measured after the background from the IBP2 is removed (blue Gaussians in the figure). It is comparable to the value (~0.5) of the ELF at the IBP2 resonance energy. Also, the intrinsic bulk plasmon resonance of the n-aluminas and γ-Al2O3, the collective excitation of valence electrons, shows a value of ~0.6 to 1.3 in ELF between 21 and 25 eV under the same experimental conditions. When the value of a transition is not too small in ELF compared to π, the transition is regarded as a collective oscillation. Thus, at low-energy (0–50 eV) losses where inelastic scattering by valence electrons dominates, major peaks in ELF of most materials except materials like transition metals can be assigned to collective excitations [29]. From the equation for ELF, Im(−1/ε) = ε2/(ε12 + ε22), the reason can be deduced that strong peaks in ε2 (single-electron transitions) cannot be seen as major peaks in ELF at the same energy. In fact, the values of single-electron transitions for the n-alumina A are smaller than 0.1 in the ELF (Fig. 3). Therefore, the value (0.3) of the clear peak for the IBP1 excitation in the ELF is considered to be not too small. It confirms that the IBP1 excitation is collective. SP has high intensity at the glancing incidence of the probing electrons due to the cosθi term in Eq. (2). However, no noticeable increase in intensity around the n-alumina/vacuum interface is observed for the IBP1/ISP1 in the VEELS. Second, the Drude–Lorentz model can describe the dielectric properties of the n-alumina reasonably well with one or two Lorentz oscillators related with the IBP1 and the IBP2. Third, VEELS collected around the center of the tail clearly show the intrinsic BP beyond 20 eV [8], which is observed with various forms of Al2O3. These results suggest that the IBP1 takes rather on properties of a bulk plasmon resonance. Since it accompanies the IT1, the IBP1 excited around the center of an n-alumina tail A is another interband BP as aforementioned. The IT1 gradually changes the excitation energy depending on the probe position but consistently presents itself. Thus, the IT1 provokes a BP around the center of an n-alumina tail A and an SP around its surface. Therefore, the ISP1 seen at and near the surface of an n-alumina tail A is a localized interband SP. In a similar fashion the IT2 located between the IT1 and the ISP2 excitation energies activates the interband BP (IBP2) [8] and another localized interband SP (ISP2) between 5 and 6 eV. As the electron probe moves from the center to the surface of an n-alumina tail, the BP below 5 eV and the BP between 5 and 6 eV in the VEELS partake more of the nature of SP. Differently from a localized intraband SP of noble metals, the ISPs shows no clear size dependence (Fig. 3(a)). One reason can be the high IT energies required to initiate the ISPs. They are larger than the difference between the IT and the ISP excitation energies, which may render the possible size dependence ambiguous. The VEELS of an n-alumina shell can also show the IBP1/ISP1 and

n-alumina B

6

Energy (eV)

8

2

4

6

Energy (eV)

8

Fig. 5. Dielectric function (ε) of the n-aluminas. Experimental ε of (a) a tail A (b) a tail B. Calculated ε of (c) a tail A (d) a tail B.

analogous energy (Fig. 3). These results indicate that the ISP1 is an SP resonance. Since it is confined in the tail, the ISP1 is a localized SP resonance. As the IT2 is involved with the IBP2 [8], the IT1 is considered to take part in the IBP1 and the ISP1 resonances of the n-alumina A. Both types of tails A and B show a large peak between 5 eV and Eg (~9 eV) in the SLF (Figs. 3 and 4). The IBP2 redshifts to the ISP2 in the VEELS around the surface of the tails (Figs. 2 and 4). The ISP2 is similar in properties to the ISP1. Thus, the ISP2 is still present in an aloof configuration, and observed at similar energies in the VEELS and the SLF. No single-electron transition is found in ε2 of the n-alumina at the ISP2 excitation energy. Because of these results the ISP2 is also considered as a localized SP resonance. The relatively large difference between the ISP2 excitation energies in the VEELS and in the SLF (for example, the 0.4-eV gap between the peaks at 5.5 and 5.1 eV in the green spectra of Fig. 4(d) and (e)) may be traced back to the fact that Eq. (2) is derived for the conduction electrons in metal. The dielectric function of the bound electrons can be calculated by Eq. (3). In the equation Ep would be the plasmon resonance energy if the bound electrons were free. Also, Eb is the resonant energy of a Lorentz oscillator, Γ the damping parameter and χL the electronic susceptibility calculated by the Drude–Lorentz model.

(E ) = 1 +

L

=1

Ep2 E2

Eb2 + iE

(3)

In Fig. 5(c) and (d) calculated ε's for the n-aluminas A and B are given in the low energy region. Here contribution from free electrons is excluded. The parameters used for the calculation are obtained from the ELFs and ε's of the n-alumina tails. Generally, the calculated ε shows smaller values than the empirical ε and rather gradual decrease below 3 eV. However, overall they are 652

Applied Surface Science 488 (2019) 648–655

H. Kim

analogous Eg's (~8.6 and 9.2 eV for shells and tails, respectively) when measured from the ELF [8,31]. In case of γ-Al2O3 it does not show any transition below Eg (Fig. 2). However, the IBP1 and the IT1 are seen in the ELF and ε2 of the n-alumina A below Eg (Figs. 3 and 5). It indicates that the transitions are related with defect states in the band gap. In Fig. 6(a) and (b) the optical joint density of states (OJDOS: J(E)) calculated by Eq. (4) for a tail A shows two ITs in the band-gap region while the OJDOS for a tail B has only one IT below 5 eV as corresponding ε2's.

Table 2 Mean resonance energies of new plasmons in eV (with the standard deviations in parentheses). n-Alumina A

IBP1 ISP1 IBP2 ISP2

n-Alumina B

Tail A

Shell A

Tail B

Shell B

4.5 4.1 5.8 5.4

4.6 4.4 5.5 5.3

5.7 (0.12) 5.5 (0.18)

5.3 (0.15) 5.1 (0.22)

(0.28) (0.22) (0.11) (0.11)

(0.28) (0.25) (0.16) (0.20)

J (E ) =

n-alumina A (a)

OJDOS

Energy (eV)

0 DB2

IT1

IT2

IT1

4

EF DB1

IT2 8

0

5

10 -3

-1

OJDOS (x 10 (eV ))

VB n-alumina A

Al

n-alumina B (b)

OJDOS

(d)

CB

Energy (eV)

0

IT2

4

EF

IT2

DB

8

0

5

10

15 -3

-1

OJDOS (x 10 (eV ))

VB n-alumina B

(4)

In this study the IT1 and the IT2 from the n-alumina A are considered to be induced by defect bands in the large forbidden gap of its energy-band structure (Fig. 6(c)). In the figure VB, DB, and CB stand for the valence band, the defect band, and the conduction band. The band structure for the n-alumina A is similar to the one suggested for the nalumina B (Fig. 6(d)) [8] except the additional defect band (DB2). The DB2 is involved in the IT1 and thus the IBP1/ISP1. In the proposed band structure the DB1 is located below the Fermi level (EF) and have numerous electrons available after EF lines up in both the n-alumina and Al. Electrons in the lower defect band (DB1) excited to the final energy bands (DB2 and CB) by the IT1 and the IT2 can oscillate collectively by slightly further stimulation. Diverse defect bands can be introduced by the nonstoichiometry of the n-aluminas. Al interstitials (Ali+, Ali2+, and Ali3+) or O vacancies (VO+1 and VO+2) can be reasonable candidates for the defect bands considering the Al-rich or O-deficient synthetic atmosphere. The band structure of the n-alumina A resembles that of the theoretical plasmonic metamaterial of which the metal interatomic spacing is elongated by a nonmetallic element [7]. Thus, according to the band structure in Fig. 6(c) the energy gap between the higher defect (DB2) and the conduction bands can afford a lossless regime (red lines). In other words, the transition from the valence and the lower defect (DB1) bands to the energy gap is prohibited. The ISP1 and the IBP1 resonances occur at the energies required for transitions from the DB1 to the energy gap. The defect bands also allow the n-aluminas to start the initial excitations from low energies below Eg. It will cause the n-aluminas to behave differently in many aspects from the wide band-gap insulator γAl2O3 in spite of the same crystal structure [22]. In a previous report an analogous localized SP mode was found between 2 and 5 eV from surface-oxidized Al nanospheres [32]. However, in the report the SP was only excited around the smaller Al particle of an asymmetric AleAl dimer but neither around the larger Al particle of the dimer nor around an isolated singular Al particle. Thus, the SP was regarded as coupling of SP fields in the system of two Al spheres. Also, current study reports novel plasmons from n-aluminas, which can be excited even far away from the Al core. Moreover, the ISP1 (or ISP2) of the n-aluminas and the SP of Al are seen together when the electron probe is near both the n-alumina and the Al core (Fig. 7). Thus, the ISP1 and the ISP2 are not the localized SP of Al shifted by oxidation [33]. The ISP1 and the ISP2 of the n-aluminas A and B are compared for the differential probability of surface scattering (Eq. (2)) with the SP of Al in Fig. 7. In Fig. 7(a) an Al particle with the n-alumina A is shown. In the magnified image of the tail area (Fig. 7(b)) the red and the orange circled crosses are equally away by about 4 nm from the Al surface and from the n-alumina surface, respectively. When VEELS are collected from the two circled crosses, the ISP1 and the ISP2 are observed together with the localized SP of Al (Fig. 7(c) and (d)). Another Al nanoparticle shows the ISP2 of the n-alumina B and the SP of Al in the VEELS collected at two spots equally away by about 4 nm from the Al surface and from the n-alumina surface (Fig. 7(e)–(h)). The two Al particles in Fig. 7(a) and (e) have similar diameters of 82 and 86 nm, respectively.

(c)

CB

2 2E Ep2

Al

Fig. 6. OJDOS and energy band structure. OJDOS of (a) a tail A (b) a tail B. Energy band structure of (c) a tail A and (d) a tail B ((d) from ref. [8]).

the IBP2/ISP2. The BPs excited from shells at the center of an Al particle are considered to possess more SP properties than from the center of tails due to the small thickness (~3 nm) of shells and thus the large surface-to-volume ratio. It can be another reason that the IBP2 is easily located at a lower energy in shells than in tails [8]. On the contrary, the IBP1 and the ISP1 resonance energies are higher for shells than tails as seen in Table 2. The mean resonance energies of the new plasmons of the n-aluminas are given with the standard deviations in the table. The dissimilar trend in the IBP1 and the ISP1 resonance energies between shells and tails and the factors differentiating the n-aluminas A and B require further studies. Gamma alumina has Eg of 8.7 eV [30]. The n-aluminas have 653

Applied Surface Science 488 (2019) 648–655

H. Kim

n-alumina B

n-alumina A

HAADF-STEM image

(a)

(b) 4 nm 4 nm

Shell

Al

(e)

Al

Al

4 nm (f)

4 nm

Al

Tail

Tail Tail

Tail

From

(c)

ISP2

(d)

From

From

(g)

ISP2

(h)

From

ISP2

VEELS

ISP1 ISP1

ISP2

SP

SP

SP 2

4

6

Energy (eV)

8

2

4

6

8

Energy (eV)

2

4

6

Energy (eV)

8 2

4

6

8

Energy (eV)

Fig. 7. Comparison of the SPs of the n-aluminas and Al. HAADF-STEM image of (a) an Al nanosphere with the n-alumina A and (b) its tail area. Scale bars: 20 and 10 nm. VEELS collected from (c) the red (d) the orange circled crosses in (b). HAADF-STEM image of (e) another Al nanosphere with the n-alumina B and (f) its tail area. Scale bars: 20 and 10 nm. VEELS collected from (g) the red (h) the orange circled crosses in (f). The VEELS in (c), (d) and (g), (h) are displayed after normalization (navy dotted line - see the text for more details). When excited by aloof probes equidistant from the Al and the n-alumina surfaces, the ISP1 and the ISP2 of the n-aluminas and the SP of Al show similar intensity (red dotted lines).

reasonably with one (for n-alumina B) or two (for n-alumina A) Lorentz oscillators. The differential probabilities of surface scattering of the nalumina ISP1 and ISP2 are high enough to be comparable to that of the Al SP. The ε2 values of the n-aluminas are small at the visible-light wavelengths indicating only a small amount of damping in the range. The band structure proposed for the n-alumina A bears similarities to the one suggested for AlO. Especially, it has a lossless energy zone between two empty energy bands where the ISP1 is excited.

The VEELS for the n-aluminas A and B are normalized with the ISP2 in Fig. 7(c) and (g) (navy dotted line). The scale of the vertical axis for Fig. 7(c) and (d) is the same and so is the one for Fig. 7(g) and (h). The SP of Al in Fig. 7(c) and (g) shows almost the same intensity. Moreover, it is clear that the intensities of the n-alumina ISP1 and ISP2 in Fig. 7(d) and (h) are similar to that of the Al SP in Fig. 7(c) and (g) when corresponding VEELS are obtained at the spots equidistant from the nalumina and the Al surfaces. The intensities of the Al SPs and the nalumina ISP1 and ISP2 are well compared along the three red dotted lines in Fig. 7(c), (d) and (g), (h). Therefore, the differential probabilities of surface scattering of the n-alumina ISP1 and ISP2 are about as high as that of the Al SP.

References [1] G.V. Naik, V.M. Shalaev, A. Boltasseva, Alternative plasmonic materials: beyond gold and silver, Adv. Mater. 25 (2013) 3264–3294. [2] A. Boltasseva, H.A. Atwater, Low-loss plasmonic metamaterials, Science 331 (2011) 290–291. [3] J.M. Luther, et al., Localized surface plasmon resonances arising from free carriers in doped quantum dots, Nat. Mater. 10 (2011) 361–366. [4] X. Liu, M.T. Swihart, Heavily-doped colloidal semiconductor and metal oxide nanocrystals: an emerging new class of plasmonic nanomaterials, Chem. Soc. Rev. 43 (2014) 3908–3920. [5] A. Agrawal, R.W. Johns, D.J. Milliron, Control of localized surface plasmon resonances in metal oxide nanocrystals, Annu. Rev. Mater. Res. 47 (2017) 1–31. [6] D.R. Smith, J.B. Pendry, M.C.K. Wiltshire, Metamaterials and negative refractive index, Science 305 (2004) 788–792. [7] J.B. Khurgin, G. Sun, In search of the elusive lossless metal, Appl. Phys. Lett. 96 (2010) 181102. [8] H. Kim, Unique electronic response of a nanoscale Al/alumina system, Phys. Chem. C 119 (2015) 26064–26078. [9] H. Ehrenreich, H.R. Philipp, B. Segall, Optical properties of aluminum, Phys. Rev. 132 (1963) 1918–1928. [10] I. Levin, D. Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, J. Am. Ceram. Soc. 81 (1998) 1995–2012. [11] H.A. Wriedt, The Al-O (aluminum-oxygen) system, Bull. Alloy Phase Diagr. 6 (1985) 548–553. [12] S.W. Weller, A.A. Montagna, Studies of alumina I. reaction with hydrogen at elevated temperatures, J. Catal. 21 (1971) 303–311. [13] M. Vermeersch, et al., The Al/Al2O3 interface formation as studied by electron spectroscopies, Surf. Sci. 235 (1990) 5–14. [14] M. Vermeersch, et al., The aluminium/sapphire interface formation at high temperature: an AES and LEED study, Surf. Sci. 323 (1995) 175–187. [15] M.F. Gillies, et al., The optimum oxidation state of AlOx magnetic tunnel junctions,

4. Conclusions Nonstoichiometric aluminas (n-aluminas) with excess Al are examined in this study for new plasmons and their properties. They are the surface alumina (shells and tails) of Al nanoparticles produced by electrical explosion. The shell and the tail have average O/Al atomic ratios of 1.11 and 1.27, respectively. The n-aluminas (both shell and tail) can show two different types of plasmonic properties. The n-aluminas with each type of properties are named here n-alumina A and nalumina B. The VEELS from the n-alumina A show two novel localized interband SP resonances (ISP1 and ISP2) excited around the surface. Further investigation with physical functions reveals that the ISP1 is related with a new interband BP resonance (IBP1) occurring around the center of the n-alumina. The ISP1 and the IBP1 are preceded by an interband transition below 4 eV (IT1). In a similar manner, another localized interband SP resonance (ISP2) is excited between 5 eV and the IBP2 resonance energy. The n-alumina B, on the other hand, has only one interband BP and one localized interband SP resonances (IBP2 and ISP2). The novel transitions are theorized to be associated with defect bands stemming from the nonstoichiometry of the n-aluminas in the band gap. The Drude–Lorentz model can explain these new excitations 654

Applied Surface Science 488 (2019) 648–655

H. Kim IEEE Trans. Magn. 35 (1999) 2991–2993. [16] A.A. Smits, Tunnel Junctions Noise and Barrier Characterization, Eindhoven University of Technology, 2001. [17] H. Kim, Non-quantum electronic responses of zinc oxide nanomaterials, Nanotechnol 24 (2013) 115701. [18] R.H. Ritchie, Plasma losses by fast electrons in thin films, Phys. Rev. 106 (1957) 874–881. [19] H. Raether, Surface plasma oscillations as a tool for surface examination, Surf. Sci. 8 (1967) 233–246. [20] R.H. French, Electronic band structure of Al2O3, with comparison to AION and AIN, J. Am. Ceram. Soc. 73 (1990) 471–489. [21] R.H. French, D.J. Jones, S. Loughin, Interband electronic structure of α-alumina up to 2167 K, J. Am. Ceram. Soc. 77 (1994) 412−422. [22] H. Kim, Unique Properties of a Degenerately Self-Doped Aluminum Oxide, (2019) (Manuscript in Preparation). [23] M. Fox, Optical Properties of Solids, New York, Oxford, 2001. [24] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, San Diego, 1985. [25] H. Raether, Excitations of Plasmons and Interband Transitions by Electrons

(Springer Tracts in Modern Physics, Vol. 88), Springer-Verlag, Berlin, 1980. [26] J. Daniels, C.V. Festenberg, H. Raether, K. Zeppenfeld, Optical constants of solids by electron spectroscopy, Springer Tracts in Modern Physics, vol. 54, Springer, New York, 1970. [27] P.F. Robusto, R. Braunstein, Optical measurements of the surface plasmon of copper, Phys. Status Solidi B 107 (1981) 443–449. [28] J.M. Pitarke, V.M. Silkin, E.V. Chulkov, P.M. Echenique, Theory of surface plasmons and surface-plasmon polaritons, Rep. Prog. Phys. 70 (2007) 1–87. [29] R.F. Egerton, Electron Energy Loss Spectroscopy, 2nd ed., Plenum, New York, 1996. [30] S. Miyazaki, Photoemission study of energy-band alignments and gap-state density distributions for high-k gate dielectrics, J. Vac. Sci. Technol. B 19 (2001) 2212–2216. [31] H. Kim, Controlled modifications in electronic and chemical structures of a nanosized area of polystyrene by fast electrons, J. Phys. Chem. B 112 (2008) 12579–12584. [32] P.E. Batson, Surface plasmon coupling in clusters of small spheres, Phys. Rev. Lett. 49 (1982) 936–940. [33] M.W. Knight, et al., Aluminum for plasmonics, ACS Nano 8 (2014) 834–840.

655