Silicon nanoparticles (SiNPs) possess unique optical properties that hold immense potential across various fields. However, realizing this potential hinges on developing straightforward and precise fabrication methods. Traditional approaches like chemical methods, plasma synthesis, and laser ablation, while capable of producing SiNPs in a broad size range, often lack the necessary control over size and deposition accuracy. Lithographic techniques, though offering more precision, are complex and typically not suited for creating spherical nanoparticles.
This article introduces an innovative femtosecond Laser Printing technique for both generating and precisely positioning silicon nanoparticles. This method leverages laser-induced forward transfer, where focused laser pulses initiate the transfer of molten nanodroplets. Similar techniques have shown promise in fabricating metal nanoparticles. However, applying this to silicon, especially with the precision and control needed for advanced applications, marks a significant step forward. A related method using ring-shaped femtosecond laser intensity to generate SiNPs from bulk silicon has limitations in nanoparticle size control, typically resulting in particles larger than 400 nm.
Our refined approach utilizes silicon-on-insulator (SOI) wafers and tightly focused Gaussian laser pulses to achieve superior control over nanoparticle size and shape during laser printing. In our experiments, we used SOI wafers featuring a thin 50 nm single crystalline silicon layer atop a 200 nm silicon oxide substrate. This structure is crucial for the precise laser printing of SiNPs. We employed a commercial femtosecond laser system (Spectra Physics Tsunami Femtosecond Oscillator and Spitfire Amplifier) emitting 800 nm wavelength pulses, with energies up to 3 mJ and a 50 fs pulse duration at a 1 kHz repetition rate. For controlled SiNP fabrication, we used single laser pulses with energies around 5 nJ. These pulses were tightly focused using a long-distance microscope objective (Nikon) with ×50 magnification and a numerical aperture (NA) of 0.45. The objective’s 17 mm working distance allows irradiation through the receiver glass substrate, minimizing laser beam distortion. Based on the formula d=λ/(2NA), the estimated laser focus diameter is approximately 890 nm.
Figure 1: Femtosecond Laser Printing Process for Silicon Nanoparticles. (a) Illustration of the femtosecond laser printing process using a silicon-on-insulator (SOI) wafer to transfer spherical Si nanoparticles from a 50 nm crystalline silicon layer onto a transparent glass receiver substrate. (b) SEM images showcasing the SOI substrate target before and after nanoparticle ejection at increasing laser pulse energies. The final image on the right reveals the hole remaining in the silicon layer after nanoparticle ejection (scale bar, 400 nm). (c) Dark-field microscopy image of an array of amorphous Si nanoparticles (diameter ≈160 nm) produced by laser printing (scale bar, 20 μm). The inset shows a SEM image of a single Si nanoparticle within the array (scale bar, 200 nm).
When a tightly focused Gaussian laser pulse illuminates the SOI wafer, the top silicon layer absorbs the laser energy, leading to rapid heating and localized melting. Crucially, at these laser pulse energies, the silicon oxide substrate, with its significantly higher melting point, remains unaffected. Molten silicon is denser than solid silicon, causing a volume reduction upon melting. The outcome varies with laser pulse energy, as illustrated in the scanning electron microscope (SEM) images in Figure 1b. At lower energies, the silicon layer doesn’t fully melt (Figure 1b, left). Surface tension causes the molten silicon to contract towards the center, forming a hemispherical bump (Figure 1b, middle images). At higher energies, the entire 50 nm silicon layer melts, forming a droplet due to surface tension. This droplet formation elevates its center of mass, imparting an upward momentum that propels it towards the receiver substrate positioned above the SOI wafer. The molten silicon then solidifies on the receiver substrate. This process is conducted in air at atmospheric pressure, with a gap of approximately 5 μm between the receiver and donor substrates.
The rightmost image in Figure 1b shows the cavity left in the silicon layer after droplet ejection. The molten area diameter is estimated to be around 250 nm. Considering the 50 nm silicon layer thickness, the liquid disk volume transforms into a spherical droplet with a diameter of roughly 160 nm. This demonstrates that the silicon layer thickness is a key parameter for controlling the size of the generated nanoparticles.
Figure 1c shows a dark-field microscopy image of a printed array of Si nanoparticles with a 160 nm diameter and a 5 μm periodicity. The inset is a SEM image of a single Si nanoparticle from this array, captured in low-vacuum mode without conductive coating. By adjusting the laser pulse energy in increments of approximately 0.1 nJ from a starting point of 5 nJ, we can control the nanoparticle diameter. Figure 2a presents SEM images of nanoparticles fabricated at slightly varying laser pulse energies. The diameters of these nanoparticles can be tuned within the 160–240 nm range, and their corresponding dark-field microscopy images are shown in Figure 2b.
Figure 2: Nanoparticle Diameter Control via Laser Pulse Energy. (a) SEM images of nanoparticles fabricated using slightly varied laser pulse energies, starting at 5 nJ. Each row, from top to bottom, represents an increase of approximately 0.1 nJ in laser pulse energy (scale bar, 300 nm). (b) Dark-field microscopy images illustrating the changes in the optical response of the generated nanoparticles (scale bar, 5 μm). The nanoparticle diameters provided are average values with a ±5 nm deviation.
Optical Characterization of Laser-Printed Nanoparticles
To understand the optical properties of these laser-printed Si nanoparticles, we conducted optical characterization using a dark-field microscope setup connected to an optical fiber spectrometer (HR 2000, Ocean Optics). A dark-field condenser (Mueller Optronic, NA=0.9) illuminated the Si nanoparticles from below, through the glass substrate. Light scattered by individual nanoparticles was collected by a ×50 magnification microscope objective with an NA of 0.55 (Zeiss), and simultaneously directed to a CCD camera and an optical fiber with a 200 μm aperture. This setup allowed us to measure the scattering spectrum of a single nanoparticle.
Figure 3a (blue curve) shows the scattering spectrum of a single Si nanoparticle immediately after laser printing onto a glass substrate. The spectrum exhibits two distinct resonance peaks. Comparing this experimental spectrum with a theoretical scattering spectrum (red curve) calculated using Mie theory for a 161 nm diameter spherical Si nanoparticle (based on experimental measurements), reveals that the laser-printed nanoparticle is amorphous. The observed resonances correspond to magnetic (λ≈720 nm) and electric (λ≈590 nm) dipole scattering. The glass substrate has minimal impact on the spectral positions of optical resonances for spherical Si nanoparticles compared to a homogeneous environment. These spectral positions are primarily determined by the dielectric permittivity of the nanoparticle material and its size. The agreement between experimental and Mie theory spectra also confirms the spherical shape of the laser-printed nanoparticles. For crystalline Si nanoparticles of the same size, the resonance positions would be blue-shifted, and the scattering efficiency would be higher, as shown in Figure 3b,c (red curves). Calculations for nanoparticles with mixed crystallographic phases consider their dielectric functions as a sum of crystalline and amorphous Si dielectric functions, weighted by fitting parameters representing the phase fractions within the nanoparticle volume.
Figure 3: Optical Scattering Properties of Silicon Nanoparticles with Varying Crystallographic Phases. Experimental (blue curves) and Mie theory calculated (red curves) scattering spectra of spherical Si nanoparticles. Insets show dark-field microscopy images of the corresponding Si nanoparticles. (a) Scattering spectrum of a laser-printed Si nanoparticle compared to the theoretical spectrum for a 161 nm amorphous Si nanoparticle. (b) Spectrum of a laser-printed Si nanoparticle after irradiation with a low-energy second laser pulse, compared to the theoretical spectrum for a 70% crystalline, 166 nm Si nanoparticle. (c) Spectrum of a laser-printed Si nanoparticle after irradiation with a high-energy second laser pulse, compared to the theoretical spectrum for a pure crystalline, 163 nm Si nanoparticle. Dielectric functions for amorphous and crystalline silicon are from ref. [44].
Laser-Induced Crystallization for Tunable Properties
Laser-induced crystallization of amorphous semiconductors is a well-studied phenomenon in materials science. To induce a crystallographic phase transition in our amorphous Si nanoparticles, we applied a secondary single-pulse laser irradiation. We used the same femtosecond laser system for this purpose.
To achieve uniform and reproducible crystallization across nanoparticles, we irradiated them with a single femtosecond laser pulse featuring a flat-top intensity distribution and a pulse energy of 58 nJ. This flat-top profile was achieved by imaging a square aperture (250 μm × 250 μm) onto the sample surface, resulting in a 5 μm × 5 μm square intensity distribution after ×50 demagnification. The crystallization of laser-printed Si nanoparticles is immediately evident in their scattering spectra (Figure 3b,c). Direct evidence of crystallinity for a nanoparticle similar to that in Figure 3c is provided by the transmission electron microscope (TEM) image in Figure 4. The image clearly shows parallel lines corresponding to the atomic lattice, confirming the monocrystalline nature of the particle.
Figure 4: Transmission Electron Microscope (TEM) Image of a Monocrystalline Silicon Nanoparticle. Yellow lines highlight the atomic lattice for clarity. The nanoparticle surface is visible in the lower right corner, indicated by a yellow line (scale bar, 10 nm).
Crystallized nanoparticles exhibit a significantly stronger scattering signal and a different color compared to non-irradiated amorphous particles, as seen in Figure 3 (insets). This laser-induced crystallization process provides a route to generate silicon nanoparticles with tunable optical properties (Figure 3b,c). Furthermore, applying laser-induced crystallization to arrays of amorphous Si nanoparticles enables selective crystallization of individual nanoparticles, as demonstrated in Figure 5.
Figure 5: Selective Crystallization of Laser-Printed Silicon Nanoparticles. Dark-field microscopy image showing laser-printed Si nanoparticles. Nanoparticles within the white lines have been crystallized via additional laser pulse irradiation, resulting in a visible color change (scale bar, 10 μm).
In conclusion, femtosecond laser printing offers a powerful and versatile technique for the controlled fabrication of silicon nanoparticles with precise size and position control. Furthermore, laser-induced crystallization provides an additional layer of control, enabling the tuning of their optical properties and opening up new possibilities for applications in nanophotonics, metamaterials, and beyond. This method’s ability to selectively crystallize nanoparticles within arrays also suggests potential for creating complex and functional nanostructured materials.
