Imaging with Nanometer Resolution Using Optically Active Defects in Silicon Carbide

Nanostructured and bulk silicon carbide ($\mathrm{Si}\mathrm{C}$) materials are relevant for electronics, nano- and micromechanical systems, and biosensing applications. $\mathrm{Si}\mathrm{C}$ has recently emerged as an alternative platform for nanophotonics and quantum applications due to its intra-band-gap point defects, emitting from the visible to the near-infrared, which are ideal for photoluminescent probes. Here, we use a single-molecule localization microscope to study the photoluminescence (PL) properties of $\mathrm{Si}\mathrm{C}$ point defects in bulk, quantum dots, and nanoparticles of different sizes in the 3C and $4H\text{−}\mathrm{Si}\mathrm{C}$ polytypes using intra-band-gap excitation. We study the PL dynamics by using different excitation wavelengths, and we use the point-defect PL intermittency to achieve superresolved images, with a resolution of 20 nm and a minimum distance between emitters of 40 nm. We observe that, while 561 nm is an ideal excitation wavelength to obtain a sufficient blinking behavior, 638 nm is mostly quenching the PL. We further incubate $4H\text{−}\mathrm{Si}\mathrm{C}$ nanoparticles with MCF10A cells in vitro and observe superresolved images of the nanoparticles in cells by combining 561 and 638 nm excitation. This approach is very promising for the application of $\mathrm{Si}\mathrm{C}$ fluorescent nanocrystals and their quantum dots, hosting a combination of intra-band-gap color centers and surface defects, for quantum nanophotonics, magnetic sensing, and biomedical imaging, paving the way for single-particle tracking combined with spin sensing within a cellular environment using near-infrared emission.

Figure 1

Room-temperature experiments. (a) SMLM at different excitation wavelengths (primarily 561 and 638 nm) with various samples of 3C- and $4H\text{−}\mathrm{Si}\mathrm{C}$ NPs, including those incubated in cells. (b) Image frames of $\mathrm{Si}\mathrm{C}$ NPs with a time of 20 ms, showing photoluminescence blinking behavior of the circled emitter, appearing dark in the second frame. (c) Typical spectra of $4H\text{−}\mathrm{Si}\mathrm{C}$ NPs obtained at 532 nm excitation and the detection spectral range using SMLM. Two curves represent two types of emission in the red and near-infrared present in the studied NPs. (d) Time trace of blinking emitters in bulk $4H\text{−}\mathrm{Si}\mathrm{C}$ excited at 561 nm. (e) Superresolved image of a $4H\text{−}\mathrm{Si}\mathrm{C}$ NP in a cell using dual excitation. (f) Superresolved image of a larger $4H\text{−}\mathrm{Si}\mathrm{C}$ NP using separately 561 nm (green color code) and 638 nm (red color code) excitation, showing coexistence of emitters of possible different origin within the same NP. (g) Atomistic representation (larger atoms are silicon) of one possible intra-band-gap emitter in the 600–700 nm spectral range attributed to CAV center in $4H\text{−}\mathrm{Si}\mathrm{C}$.

Figure 2

Sample F, $4H\text{−}\mathrm{Si}\mathrm{C}$ NPs, excited at 561 nm. (a) Wide-field image and filtering 60 ms time frame. (b) Superresolved image. Magnification of superresolved NPs 1 (c) and 2 (e). (d),(f) Examples of point-spread functions (PSFs), indicating a subdiffraction lateral resolution of about 25 nm.

Figure 3

Composite image of the NPs in sample F. Green emitters excited with 532 nm and red emitters excited with 638 nm. (a) Wide-field image and filtering 60 ms time frame of agglomeration of two nanoparticles at locations 1 and 2. (b) Superresolved image showing two NPs at locations 1 and 2; insets are magnifications of superresolved locations 1 and 2, showing that emission from 638 nm excitation is located at <100 nm from emission excited at 561 nm in the same NP. (c) Superresolved image of a single NP using 561 and 638 nm excitation at the same time. (d) Distance between two emitters.

Figure 4

(a) Wide-field imaging of cells incubated with $4H\text{−}\mathrm{Si}\mathrm{C}$ NPs using 488 and 561 nm excitation at the bottom; in the cell; and on the cell surface, showing the $\mathrm{Si}\mathrm{C}$ NP inside the cell. Z indicates the focal depth. (b) Image of cells and $\mathrm{Si}\mathrm{C}$ NPs using 561 and 638 nm excitation, and the superresolution image (c) of the NP with 18 nm resolution and emitters at about 40 nm distance (d).

Figure 5

Summary of the confocal photoluminescence study of the emission in $3C\text{−}\mathrm{Si}\mathrm{C}$ (a) and $4H\text{−}\mathrm{Si}\mathrm{C}$ (b),(c) using 532 nm excitation. (a) Typical PL observed in thin-film $3C\text{−}\mathrm{Si}\mathrm{C}$ grown on silicon, with an observed SPS spectrum of the untreated material. Subsequent spectra are from the NPs described in Table 1. (b),(c) Typical emission in electron-irradiated intrinsic bulk material attributed to CAV center with low-fluence irradiation (b) and V_Si center with high-fluence irradiation in n-type bulk $4H\text{−}\mathrm{Si}\mathrm{C}$ (c); red line indicates emission in bulk irradiated sample; black line indicates emission after laser fabrication. Observed PL in the NPs is derived from irradiated samples. No annealing is performed on the NP samples. Notably, emission at 650 nm in the laser-ablated NPs only correlates to CAV center emission in $4H\text{−}\mathrm{Si}\mathrm{C}$, but it is not currently identified. Laser ablation could produce local annealing and convert V_Si into CAV [60].

Figure 6

Wide-field 20 × 20 µm² image of $3C\text{−}\mathrm{Si}\mathrm{C}$ NPs with 488 and 561 nm excitation for two spectral collection regions of 580–639 and 650–750 nm, showing higher brightness at 561 nm excitation and photoswitching for spectral red emission.

Figure 7

$3C\text{−}\mathrm{Si}\mathrm{C}$ NPs (∼100 nm, sample A) , with 561 nm excitation and 650–750 nm collection. (a) Wide-field image and filtering 19.3 ms time frame. (b) Superresolved image. (c) Magnification of superresolved NP agglomeration. (d) Example of PSF indicating a subdiffraction lateral resolution of about 70 nm.

Figure 8

$3C\text{−}\mathrm{Si}\mathrm{C}$ NPs (∼100 nm, sample A) with 561 nm excitation (a) NP2 from image in Fig. 7 and background (BG) time trace with 19.3 ms time sampling, showing a relative frequency of counts distribution, (b) indicating the on and off state’s occurrence. (c) On and off state dwell time statistic with fitted exponential decay functions, providing T_on and T_off time estimates.

Figure 9

$3C\text{−}\mathrm{Si}\mathrm{C}$ NPs (45–55 nm, sample B) excited at 561 nm as received dispersed in ethanol. (a) Wide-field image and filtering 15.47 ms time frame. (b) Superresolved image. (c) Magnification of superresolved. (d) Example of PSF, indicating a subdiffraction lateral resolution of about 50 nm.

Figure 10

$3C\text{−}\mathrm{Si}\mathrm{C}$ NPs (45–55 nm, sample B) excited at 561 nm oxidized at 400 °C for 30 min. (a) Wide-field image with time trace image of one blinker and filtering 19.3 ms time frame. (b) Superresolved image. (c) Magnification of superresolved NPs. (d) Example of PSF, indicating a subdiffraction lateral resolution of about 50 nm. Higher PL is observed. (e) Selected time trace of NPs before and after oxidation, with a background that remained unchanged.

Figure 11

$3C\text{−}\mathrm{Si}\mathrm{C}$ NPs (15–20 nm, sample C) excited at 561 nm. (a) Wide-field image and 19.3 ms filtered time frame. (b) Superresolved image. (c) Magnification of superresolved NPs. (d) Example of PSF, indicating subdiffraction lateral resolution of about 40 nm.

Figure 12

QDs (sample D) excited at 638 nm with 650–750 nm collection, after oxidation at 400 °C for 30 min. (a) Wide-field image and 15.82 ms filtered time frame. (b) Superresolved image. (c) Magnification of superresolved NPs. (d) Example of PSF, indicating subdiffraction lateral resolution of about 35 nm for 638 nm excitation of QD1 observed at both excitation wavelengths. (e) Time trace of three QDs excited at 638 nm. (f) Blinking frequency. (g) On-off time statistics.

Figure 13

Intrinsic $4H\text{−}\mathrm{Si}\mathrm{C}$ bulk (sample E) irradiated with 2 MeV at 10¹⁴ cm⁻², with no annealing excited at 561 nm (top panel) and at 638 nm (lower panel). (a) Wide-field image and 15.6 ms filtered time frame. (b) Superresolved image. (c) Magnification of superresolved emitters. (d) Example of PSF, indicating subdiffraction lateral resolution of about 50 nm for 561 nm excitation. (e) Superresolved image of another sample area after annealing at 500 °C for 30 min in argon. (f) Composited dual-color superresolution image of defect before annealing. Green channel, 561 nm laser; red channel, 638 nm laser.

Figure 14

Blinking statistics of sample E using 561 nm excitation for bulk $4H\text{−}\mathrm{Si}\mathrm{C}$ material. (a) Single emitters blinking occurrence showing on and off more states corresponding to higher and lower counts. (b) Single emitters time traces.

Figure 15

Blinking behavior of sample F using 561 nm excitation, which is attributed to emission in the red. (a) On and off state statistics of single NPs blinking with time trace in (b), which is attributed to typical emission of smaller NPs in (c). Time traces show both NPs off states coinciding with the related background. NPs with the emission of V_Si only are photostable, as shown in Ref. [55].

Figure 16

$4H\text{−}\mathrm{Si}\mathrm{C}$ NPs (sample F) excited at 638 nm. (a) Wide-field image and 60 ms filtered time frame. (b) Superresolved image. (c) Magnification of superresolved NPs 1 and 2. (d) Example of PSF, indicating subdiffraction lateral resolution of about 33 nm.

Figure 17

Wide-field images (left panel; scale bars, 200 nm) of a single $4H\text{−}\mathrm{Si}\mathrm{C}$ NP (sample F) excited at 561 and 638 nm and time-trace count rates when subjected to the same power (right panel), showing a considerable reduction of counts at 638 nm, allowing superresolution, even when blinking is not present.