Spectroscopic Observation and Modeling of Photonic Modes in CeO 2 Nanocubes Yifan Wang Shize Yang and Peter A. Crozier Abstract

2025-05-03 0 0 864.22KB 14 页 10玖币

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Spectroscopic Observation and Modeling of Photonic Modes in CeO2 Nanocubes

Yifan Wang, Shize Yang, and Peter A. Crozier*

Abstract

Photonic modes in dielectric nanostructures, e.g., wide gap semiconductor like CeO2 (ceria),

has potential for various applications such as light harvesting and information transmission.

To fully understand the properties of such phenomenon in nanoscale, we applied electron

energy-loss spectroscopy (EELS) in scanning transmission electron microscope (STEM) to

detect such modes in a well-defined ceria nanocube. Through spectra and mapping, we

demonstrated a geometrical difference of mode excitation. By comparing various spectra

taken at different location relative to the cube, we also showed the transmission properties of

the mode. To confirm our observation, we performed EELS simulation with finite-element

dielectric calculations in COMSOL Multiphysics. We also revealed the origin of the modes

through the calculation. We purposed a simple analytical model to estimate the energy of

photonic modes as well. In all, this work gave a fine description of the photonic modes’

properties in nanostructures, while demonstrating the advantage of EELS in characterizing

optical phenomena in nanoscale.

Introduction

Photonic modes, sometimes referred as cavity modes or waveguide modes, is the optical

response from dielectric material with specific sizes and shapes. Photons travelling through

the structure with specific wavelength will be trapped in the form of photonic modes. Such

modes have been widely applied in fields such as telecommunication [1,2], laser

generation [3,4] and sensors [5,6]. When the size of the structure has reached to nanoscale,

i.e., the wavelengths of the modes are in the visible light region, it becomes possible to

capture solar energy through this mechanism. Comparing to other visible light harvesting

approaches, e.g., photovoltaics or plasmonic harvester, a significant advantage of photon

capturing with photonic modes is that there is no loss from electronic excitation if the energy

of the mode is below the bandgap. Moreover, the modes are straightforward to engineer since

the energy of the modes are solely dependent on the geometry of the structure and the

dielectric properties of the material. By coupling with metal nanoparticles, energy from

photonic modes can be transferred to the catalytic active sites on metal. [7] In other words,

such dielectric nanoparticle-metal system can act as a photocatalyst for reactions like water

splitting [8].

To fully tap the potential of this mechanism for catalytic application, a deeper understanding

of these modes’ properties is needed. However, despite high energy resolution, traditional

optical measurement methods, e.g., Raman spectroscopy [9] and infrared spectroscopy [10],

usually suffers from low spatial resolution to around 1 micrometer. A way to achieve high

spatial resolution is using high energy electron beam as the light source, which has a much

shorter wavelength than photons. For a free electron beam, the electric field in frequency

space are in the form of plane waves, giving rise to virtual photons. [11] Therefore, it can

excite optical responses in materials nearby. An example of this is Smith-Purcell effect, in

which the swift electron travels parallel by a diffraction grating and forms visible light with

certain wavelength [12]. In the case of scanning transmission electron microscopy (STEM),

high energy electron probe can act as a continuum light source with atomic resolution.

With the recent development on electron optics, especially aberration corrector [13,14],

monochromator [15,16], and detectors [17,18], studying low energy features including

plasmonic [19–21] and phononic [22–31] response at atomic resolution with adequate energy

resolution (~10meV) through electron energy-loss spectroscopy (EELS) in STEM becomes

feasible. Also, development of aloof beam technique and theory makes detection on thick

objects that scatter electron beyond detection limit possible. [32] The high spatial resolution

of STEM imaging, especially high angle annular dark-field (HAADF) imaging also grants the

capability to visualize the spatial distribution of energy-loss events. [33]

Previous studies on photonic modes in silicon-based materials with different shapes supported

by thin films has proved STEM EELS as a powerful tool to explore this phenomenon [34–36].

Also, since some of the system have good cylindrical symmetries, e.g., disks and ribbons,

analytical calculations are performed. Numerical calculations on more mathematically

complicated system, e.g., ellipses and triangle pillars, also gives a deeper understanding of the

modes. Also, silicon-based materials have a bandgap in visible light region, therefore not

suitable for light harvesting. Studies on stand-alone dielectric nanoparticles with wider

bandgap, e.g., CeO2, and TiO2 has also demonstrate the existence of photonic modes. [37]

However, due to the rather irregular geometry of the nanoparticles, there are limitations on

interpretation of the spatial related properties of photonic modes.

In this paper, we performed monochromated STEM EELS on well-defined cerium oxide

nano-cubes. Through EELS point spectra and mapping, we demonstrate different mode

excitation due to the geometry difference, i.e., some modes be more preferentially excited

when the beam is placed at specific positions. Besides, we also discuss the transmission

properties of the photonic modes. To further understand such spatial difference in mode

excitation, numerical finite element calculation was performed in the commercial software

COMSOL Multiphysics to visualize the modes. A simple analytical model was used to

calculate the energy of photonic modes in cubic nanoparticles with given dielectric functions.

Methods

Ceria nanocubes with predominantly (100) surfaces were synthesized using hydrothermal

method [38]. Cerium nitrate hexahydrate (Ce(NO3)3·6H2O) and sodium hydroxide (NaOH)

were separately dissolved in deionized water. The solution was mixed and stirred on a

magnetic stirrer for 30 min. The final molar concentration of NaOH was calculated to be 8 M.

In order to get cubes with ideal size for photonic modes, the mixture was heat at 220°C for 24

hours and cooled down naturally. The precipitates, in form of powder, were dispersed in

deionized water by sonicating for 20 min. The top layer of the suspension is used for TEM

observation by drop-casting and baked on a copper grid with lacey carbon film.

Monochromated-EELS was performed with NION UltraSTEM 100 microscope equipped

with aberration corrector and the state-of-the-art Dectris Ela detector [18]. The microscope

was operated at 100 kV. The energy dispersion of the spectrometer was set to 5 meV per

channel. Measured full width half maximum is 18 meV. The convergence and collection semi

angle of the experiment were set to 19 mrad and 21.8 mrad, with a 2 mm EELS entrance

aperture. To increase the signal to noise ratio (SNR), 100 spectra taken at the same position

with 500 ms exposure time are integrated to form a point-and-shoot spectra. EELS mappings

were also done with a dwell time of 100 ms per pixel to minimize the effect of sample drifting

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SpectroscopicObservationandModelingofPhotonicModesinCeO2NanocubesYifanWang,ShizeYang,andPeterA.Crozier*AbstractPhotonicmodesindielectricnanostructures,e.g.,widegapsemiconductorlikeCeO2(ceria),haspotentialforvariousapplicationssuchaslightharvestingandinformationtransmission.Tofullyunderstandtheproper...

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Spectroscopic Observation and Modeling of Photonic Modes in CeO 2 Nanocubes Yifan Wang Shize Yang and Peter A. Crozier Abstract

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