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We fabricated a bright and thin Zn2SiO4 luminescent film to serve as a nanometric light source for high-spatial-resolution optical microscopy based on electron beam excitation. The Zn2SiO4 luminescent thin film was fabricated by annealing a ZnO film on a Si3N4 substrate at 1000 °C in N2. The annealed film emitted bright cathodoluminescence compared with the as-deposited film. The film is promising for nano-imaging with electron beam excitation-assisted optical microscopy. We evaluated the spatial resolution of a microscope developed using this Zn2SiO4 luminescent thin film. This is the first report of the investigation and application of ZnO/Si3N4 annealed at a high temperature (1000 °C). The fabricated Zn2SiO4 film is expected to enable high-frame-rate dynamic observation with ultra-high resolution using our electron beam excitation-assisted optical microscopy.
© 2015 Optical Society of America
1. Introduction
The observation of cellular structures and organelles is important for revealing cellular functions through intercellular dynamics. Optical microscopy is the primary visualization technique for understanding cellular functions in vivo. However, the spatial resolution of conventional optical microscopy is restricted by the diffraction limit of light (about 200 nm). This limit hampers detailed observation of biological samples, especially the ultrafine structures of cellular organelles. To overcome the limitation, superresolution optical microscopy techniques have been developed, such as stimulated emission depletion (STED) microscopy [1], photo-activated localization microscopy (PALM) [2, 3], stochastic optical reconstruction microscopy (STORM) [4], structured illumination microscopy (SIM) [5, 6], scanning near-field optical microscopy (SNOM) [7], and saturated excitation (SAX) microscopy [8], and these techniques have been gaining in importance in the biological fields. Although these microscopy techniques have resolutions beyond the diffraction limit of light, they require a labeling procedure to visualize cellular structures.
From the viewpoint of spatial resolution, transmission electron microscopy (TEM) is a powerful tool for observing ultra-cellular structures compared with optical microscopy, and it is typically used in the fields of molecular biology, histology and so on. In this technique, the specimens must be placed in a vacuum. Thus, cellular dynamics in wet conditions or in vivo cannot be observed using conventional TEM. Recently, to overcome this problem, de Jonge et al. proposed liquid scanning transmission electron microscopy (STEM) using a vacuum chamber sealed with two Si3N4 thin films [9, 10]. However, in the observation of living cells using this liquid STEM, they showed that excess electron irradiation induces damage and cell death [10].
To observe the ultrafine structures and physiological functions of living cells at high spatial resolution without labeling, we have proposed an electron-beam excitation assisted (EXA) optical microscope, in which an electron beam focused on a luminescent thin film excites a nanometric light source near the specimen [11–13]. EXA microscopy is very similar to SNOM in that a nanometric light source is produced with a small aperture, except that in EXA microscopy, the light source is produced by the electron beam focused on the luminescent film. The light source can reach nanoscale size because the electron beam can be focused in a region with a size of a few nanometers. EXA microscopy is also capable of fast image acquisition because the electron beam can be scanned rapidly, and this is advantageous for the dynamic observation of living biological specimens. In addition, the specimen is not directly irradiated by an electron beam because the electron beam is scattered and stopped by the luminescent thin film under the specimen. Taking advantage of this indirect excitation of specimens, both organic materials and inorganic materials can be observed by EXA microscopy without serious damage to the samples caused by electron beam irradiation. EXA microscopy can also observe non-fluorescent materials because a nanometric light source is used for illuminating samples. These are the unique advantages of EXA microscopy in comparison with other superresolution microscopy techniques, such as STED, PALM, and SIM.
In our previous study, a Si3N4 membrane was used as the emitter for generating the nanometric light source in the EXA microscope [11]. This material emits cathodoluminescence (CL) in the visible region [11, 14] and can be used to seal a vacuum due to its high mechanical strength. With this technique, a 50 nm-diameter polystyrene latex sphere was observed with sufficiently high spatial resolution. A ZnO (100 nm)/Si3N4 (50 nm) membrane has also been used as a luminescent thin film to improve the signal-to-noise ratio (SNR) of EXA microscopic images [14].
The development of a thin and bright luminescent film is a key technology in EXA microscopy. A thinner film prevents broadening of the CL light spot size due to the scattering of electrons, and a bright film offers a high signal-to-noise ratio and fast image acquisition. In this report, we fabricated a Zn2SiO4 film to serve as a thin and bright luminescent film for EXA microscopy and demonstrated high-spatial-resolution imaging of 100 nm gold nano-particles.
2. Principle of EXA microscopy
Figure 1 shows a prototype and the configuration of the EXA microscope we developed. The EXA microscope is a combination of a scanning electron microscope (SEM) and an optical microscope. An inverted-type SEM equipped with a field emission gun (APCO Ltd.) is used for excitation of cathodoluminescence. A biological specimen is cultured on the luminescent film directly. The luminescent thin film is also employed as a seal to separate the atmosphere and the vacuum. CL from the luminescent thin film excited by the focused electron beam is used as a nanometric light source for illuminating the specimen, and the light transmitted or scattered by the specimen is detected by a photomultiplier tube (PMT). The nanometric light spot is scanned on the specimen to acquire an image. In this microscope, a light spot with a diameter as small as a few tens of nanometers can be produced because of the excitation by the focused electron beam.
Fig. 1 (a) Photograph of the prototype EXA microscope we developed. The EXA microscope is a combination of an electron microscope for excitation and an optical microscope for detection. (b) Schematic diagram of the EXA microscope. The inset shows an enlarged schematic diagram of a specimen irradiated with a nanometric light spot from a luminescent thin film excited by an electron beam. The nanometric light spot excited by the electron beam illuminates the specimen, and the transmitted light is detected with the optical microscope.
EXA microscopy has the potential to observe the dynamic activity of living biological specimens at video rates with high spatial resolution, because the electron beam can be scanned with electrical or magnetic field modulation without any mechanical components. Another advantage of EXA microscopy is the ability to observe specimens in various environments, such as air, gases, liquids and vacuum conditions.
3. Luminescent thin film for producing nanometric light spot
3.1 Fabrication of luminescent thin film
A ZnO thin film was deposited by radio frequency (RF) magnetron sputtering (ANELVA, SPF-332H) using a ZnO target (99.99%) in oxygen and argon mixed reactive plasma. An amorphous Si3N4 film (50 nm) deposited on a Si substrate (Silson Ltd.) was used as the substrate [Fig. 2(a)]. The 50 nm Si3N4 thin film with a size of 50 μm × 50 μm has enough strength to sustain the pressure between vacuum and atmosphere. Sputter deposition was carried out at a pressure of 0.8 Pa in pure Ar and oxygen mixed gas (Ar/O2 = 25 sccm: 5 sccm) with an RF power of 100 W. The substrate temperature was 500 °C, and the deposition time was 15 min. The thickness of the ZnO layer was about 50 nm. After the deposition, the ZnO film was annealed at 1000 °C for 1 hour in N2. The heating rate and cooling rate were set to 10 °C/min, and the flow rate of N2 was set to 500 mL/min. After annealing of the film, the thickness of the annealed film had decreased to 20 nm due to evaporation of the ZnO layer [Fig. 2(b)].
Fig. 2 Configurations of luminescent thin films on Si3N4 substrate. (a) Before annealing. (b) After annealing. After annealing at 1000 °C in N2, the film thickness of the ZnO had decreased from 50 nm to 20 nm due to evaporation of the ZnO.
3.2 Characterization of annealed Zn2SiO4 luminescent thin films
The structural properties of the luminescent thin film were investigated by X-ray diffraction (XRD) with Cu Kα radiation (Rigaku, RINT-Ultima III). The film surface morphology was characterized by an atomic force microscope (AFM; SII, SPA300/SPI3800N). The luminescence properties of the film were characterized by an FE-SEM-CL microscope (JEOL equipped with HORIBA LS-100-EM-TYPE2). In the FE-SEM-CL measurement, a photomultiplier tube (PMT; Hamamatu, H11461-P11) was used for detecting the CL detection after passing through the spectrometer (Princeton Instrument, SP-2300i).
Figure 3 shows the XRD spectra of the Si3N4 substrate, the as-deposited ZnO film, and the film after annealing at 1000 °C in N2. A diffraction peak around 34°, corresponding to the diffraction from the (002) plane of wurtzite ZnO, was observed in the as-deposited film [Fig. 3(a)]. This result indicates that the as-deposited ZnO film was highly c-axis oriented. In the annealed film, the intensity of the ZnO (002) peak decreased because of evaporation of the ZnO layer in the reductive atmosphere at the high temperature.
Fig. 3 (a) XRD spectra of Si3N4 / Si substrate, as-deposited ZnO film, and ZnO film annealed at 1000 °C in N2. (b) Magnified view of XRD spectra in low-intensity region. In the annealed film, diffraction peaks of Zn2SiO4 were found due to the diffusion of Si ions from the Si3N4/Si substrate.
In the annealed film, weak diffraction peaks appeared around 25.5°, 31.5°, and 34°, corresponding to diffraction from the (220), (113), and (410) planes of Zn2SiO4, respectively [15, 16] [Fig. 3(b)]. The peaks were also assigned using powder diffraction data (00-001-1076). This indicates that Zn or/and Si diffused around the interface between the ZnO and Si3N4 films [15, 17], and that the Zn2SiO4 film was produced during the annealing process. The annealed film was deduced to be an a-axis or b-axis oriented film. In the XRD measurements, the diffraction peak at around 33° was derived from the Si3N4/Si substrate [circle in Fig. 3(b)]. This peak was observed from the as-deposited ZnO film but disappeared in the annealed ZnO film. The device-dependent peaks marked with “#” in Fig. 3(b) were observed in all experiments.
Figure 4 shows the surface morphology of the annealed film and the as-deposited film, which were measured by an AFM in a 500 nm × 500 nm area. Grains were observed in both films. The size of the grains in the annealed film was from about 20 to 50 nm, and the size of the grains in the as-deposited film was from about 20 to 30 nm. The surface roughnesses of the annealed and as-deposited films were 2.3 nm and 2.0 nm root mean square, respectively. After annealing, the grain size increased; however, the surface roughness was almost the same as that of the as-deposited film.
Fig. 4 AFM images of (a) ZnO film annealed at 1000 °C and (b) as-deposited film. Grain size of annealed film was about 20 nm to 50 nm. Surface roughness of annealed film was almost the same as that of as-deposited film (2.3 nm root mean square).
The crystallite size of the luminescent thin film was calculated from peak broadening of the XRD spectrum using Scherrer’s formula. The crystallite size D is given by
where K is a numerical constant, β is the diffraction broadening in radians (full width at half maximum), λ is the wavelength of the X-rays, which was 1.5406 A°, and θ is the Bragg angle in radians. K was taken as 0.9 for the calculations. The crystallite sizes calculated from the XRD spectra were 33 nm, 28 nm, and 26 nm on the (220), (113), and (410) planes, respectively. These sizes were comparable with the sizes observed in AFM. The crystallite sizes calculated from Scherrer’s formula mentioned above became smaller at high diffraction angles. This indicates that the Zn2SiO4 crystals have lattice strains such as point defects and dislocations, because lattice strain is not involved in Scherrer's formula.Figure 5 shows the luminescence properties of the films. The films were excited by an electron beam using the FE-SEM-CL system. The acceleration voltage was 5 kV, and the probe current was 1 nA. CL from the films was collected with an ellipsoidal mirror and was sent to a spectrometer through an optical fiber bundle. The CL spectra were measured with a grating spectrometer. The acquisition time of the spectrometer was set to 1 s/nm.
Fig. 5 CL spectra of as-deposited ZnO film and film annealed at 1000 °C. After annealing at 1000 °C, the total intensity of the annealed film was 15 times larger than that of the as-deposited film, and a new peak was observed at around 300 nm, which originated from Zn2SiO4 (not band gap emission).
Figure 5 shows CL spectra of the as-deposited ZnO film and the annealed film, measured at an acceleration voltage of 5 kV. Narrow UV and broad visible emissions were observed. The UV emission at around 380 nm corresponds to the band-gap emission of ZnO, and the visible emission is considered to be luminescence that originated from oxygen defects in the ZnO film [18].
In the annealed film, narrow deep UV emission at around 300 nm, broad UV emission at around 380 nm, and visible emission at around 580 nm were observed. The narrow deep UV emission at 300 nm is assumed to be indirect emission from the Zn2SiO4 [19–21] because the band gap of Zn2SiO4 is 5.4 eV (230 nm). It is unclear whether the emissions at around 380 nm and 580 nm are derived from ZnO, Zn2SiO4, or SiOx [19].
The CL intensity of the annealed film was very high despite the film being thinner than the as-deposited film. The spectrally integrated CL intensity of the annealed film was 15 times larger than that of the as-deposited film. This annealed film is an ideal candidate for EXA microscopes because it is thin and emits bright CL. Furthermore, the deep UV emission at around 300 nm is useful for observing absorption images of biological samples because it has high absorbance in the UV and deep UV regions. This Zn2SiO4 film fabrication procedure is a new and promising method for various applications, such as light emitting devices and photonic devices, because it is a simple process involving annealing ZnO/Si3N4 at high temperature (1000 °C).
4. Evaluation of spatial resolution of EXA microscope with Zn2SiO4 film
In order to evaluate the spatial resolution of our EXA microscope with the annealed Zn2SiO4 luminescent thin film, 100 nm gold nanoparticles under atmospheric pressure were observed. An Si3N4 substrate of 30 nm thickness was used to create a vacuum seal for SEM.
Figure 6(a) shows EXA microscopic images of gold nanoparticles dispersed on the annealed film in air. The images were acquired with an acceleration voltage of 5 kV and a current of 1 nA. The time required to acquire each EXA microscope image of 1024 × 1024 pixels was 200 seconds. Each gold particle was clearly observed as the black region in the image. The particle locations and sizes matched those in the SEM image of the same area shown in Fig. 6(b). Figures 6(c) and 6(d) show line-profiles of a single gold particle, shown as the regions between the arrows in Figs. 6(a) and 6(b), respectively. A single gold nanoparticle was imaged at a size of 121 nm FWHM with the EXA microscope. As a result, we may say that the EXA microscope has a resolution beyond the diffraction limit of light. The spatial resolution of the EXA image was comparable to that of the SEM image. The signal-to-noise ratio of the annealed film was 15, which is higher than that of the as-deposited film due to enhancement of the CL intensity by high-temperature annealing. The CL intensity of the Zn2SiO4 (annealed film) was high enough to obtain a clear EXA image, and the intensity inhomogeneity of the film did not affect the EXA image.
Fig. 6 (a) (b) EXA microscopy and SEM images of 100 nm gold nanoparticles in air. (c) (d) Line profiles of single gold nanoparticle at same position. High-SNR imaging was achieved using the Zn2SiO4 film, and the spatial resolution of the EXA image was comparable to that of the SEM image.
5. Summary
We developed a bright and thin Zn2SiO4 film for an EXA microscope by high-temperature annealing of ZnO/Si3N4 at 1000 °C in N2. The CL intensity was enhanced significantly, and the spectrally integrated CL intensity of the Zn2SiO4 film was 15 times higher than that of the as-deposited film. The Zn2SiO4 film can serve as a nanometric light source with high emission intensity even in a film about 20 nm thick. The film provided clear EXA microscopic image with high signal-to-noise ratio and also allowed high-spatial-resolution imaging of specimens such as gold nanoparticles.
This is the first report of a method of fabricating a Zn2SiO4 film by annealing ZnO/Si3N4 at high temperature (1000 °C), and its CL properties. Zn2SiO4 films are promising for high-frame-rate dynamic observation with ultra-high resolution and will be attractive candidates for EXA microscopes. This film is a new and promising material for various applications, such as light emitting devices and photonic devices. We believe that this method of fabricating a thin and bright luminescent film will be a useful technique that may lead to the discovery of new cellular functions with EXA microscopy.
For cellular imaging, we should investigate electron-induced damage of the cells because some electrons can penetrate the luminescent thin film even though most of them are stopped at the luminescent thin film. Optimization of the acceleration voltage and thickness of the membrane is required for cellular imaging in vivo.
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