| Metadata | Details |
|---|
| Publication Date | 2024-12-12 |
| Journal | Nanomaterials |
| Authors | Artūrs Medvids, Artūrs Plūdons, A. Vaitkevičius, S. Miasojedovas, Patrik Ščajev |
| Institutions | Riga Technical University, Vilnius University |
| Analysis | Full AI Review Included |
This research elaborates a novel, simple optical method for visualizing and estimating the top diameter (sharpness) of 0D to 1D nanostructures, such as Atomic Force Microscope (AFM) tips and nanocones, leveraging the quantum confinement effect.
- Core Value Proposition: Provides a non-destructive, in situ optical procedure using Photoluminescence (PL) spectroscopy to determine the sharpness of nanocone structures, eliminating the need for high-vacuum Scanning Electron Microscopy (SEM) or complex sample preparation.
- AFM Tip Sizing: The method successfully estimated the top diameter of a sharp Silicon (Si) AFM probe (Nanosensors SSS-NCL) to be 1.5 nm (based on the PL blue cutoff), which correlates well with expected dimensions after accounting for the native oxide layer.
- DLC Nanocone Sizing: The technique confirmed the top dimensions of Diamond-like Carbon (DLC) nanocones to be 2.0 nm, based on the strongly blue-shifted PL spectrum (3.3 eV bandgap).
- Visualization: Confocal microscopy confirmed green PL emission localized specifically at the AFM probe top, demonstrating the feasibility of optical visualization despite diffraction limits.
- Recombination Kinetics: Time-resolved PL measurements revealed a stretched-exponent decay (dispersion parameter β = 0.58) with an average lifetime (τ0) of 1.0 ns, indicating exciton drift and gradual lifetime decrease due to surface and radiative recombination along the cone height.
- Mechanism: Size estimation relies on modeling the PL spectrum shift caused by quantum confinement (EQc) and exciton binding energy (Eex), which are highly dependent on the local nanostructure diameter (d).
| Parameter | Value | Unit | Context |
|---|
| Calculated Si AFM Tip Diameter (Top) | 1.5 | nm | Determined from PL blue cutoff (Egmax = 2.8 eV) |
| Calculated Si AFM Tip Diameter (Peak PL) | 1.72 | nm | Determined from peak PL emission (2.45 eV) |
| DLC Nanocone Top Diameter | 2.0 | nm | Calculated from blue-shifted bandgap (Eg = 3.3 eV) |
| Average Exciton Lifetime (τ0) | 1.0 | ns | Fitted using stretched-exponent decay (AFM tip) |
| Dispersion Parameter (β) | 0.58 | - | Non-exponential decay fitting (AFM tip) |
| Si Bandgap (Eg0) | 1.16 | eV | Bulk Silicon |
| Si Transverse Optical Phonon Energy (ETO) | 58 | meV | Dominates PL emission in nanowires |
| Excitation Wavelength (Confocal PL) | 405 | nm | Used for tip analysis |
| Excitation Power (Confocal PL) | 0.25 | mW | Low excitation regime |
| Confocal Microscope Resolution (FWHM, x-section) | 640 | nm | Diffraction limited image size |
| Estimated Radiative Efficiency (AFM Tip) | ~15 | % | Based on lifetime analysis |
| Estimated Electron Density (AFM Tip) | ~1019 | cm-3 | Based on probe resistivity (0.01-0.025 Ω·cm) |
| DLC Annealing Temperature | 1060 | °C | Thermal treatment in N atmosphere |
| DLC Nanocone Height | 80 | nm | Formed via Stranski-Krastanow model |
- Nanostructure Fabrication (DLC): DLC films (400 nm thick) were deposited on Si substrates via magnetron sputtering, followed by thermal annealing at 1060 °C in N2 atmosphere to form 80 nm-high nanocones.
- Probe Selection (AFM): Commercial SuperSharpSilicon™ (SSS-NCL) AFM probes (Nanosensors) made of doped silicon were used, featuring a half cone angle of less than 0° and a nominal top radius of less than 2 nm.
- Confocal Photoluminescence (PL) Microscopy: Performed using a WITec Alpha 300S microscope. Excitation was achieved with a 405 nm laser (0.25 mW), and light was collected using a 100× objective (NA = 0.9).
- Time-Resolved PL (TRPL) Spectroscopy: Measurements utilized a Hamamatsu streak camera system. Excitation was provided by 350 nm, 200 fs laser pulses generated by an ORPHEUS parametric generator operating at 10 kHz.
- Size Estimation via Quantum Confinement (QC): The nanostructure diameter (d) was calculated by fitting the blue cutoff of the PL spectrum (Egmax) using a modified Si nanowire bandgap formula:
- Eg(z) = Eg0 + EQc(z) - Eex(z).
- This formula relates the bandgap energy (Eg) to the bulk bandgap (Eg0), quantum confinement energy (EQc), and exciton binding energy (Eex), all dependent on the local diameter d(z).
- Spectral Modeling: The broad PL spectra were modeled by integrating the emission intensity (IPL) along the height (z) of the cone, incorporating thermal broadening and a strongly diameter-dependent radiative lifetime (τrad(d)).
| Industry/Sector | Application/Product Relevance |
|---|
| Metrology & Microscopy | AFM Probe Quality Control: Provides a fast, non-destructive, in situ method to verify the sharpness (tip radius, 1.5 nm resolution achieved) of commercial AFM probes, replacing time-consuming SEM analysis in manufacturing and pre-use checks. |
| Nanophotonics & Display Technology | Localized Light Sources: Enables the development and characterization of highly localized, small-dimension light sources (nanocones/quantum dots) for microelectronics, biological imaging, and high-resolution displays. |
| Semiconductor Manufacturing | Graded Bandgap Devices: The Si nanocone structure exhibits a graded bandgap, which is valuable for designing specialized optoelectronic devices, sensors, and potentially improving exciton transport efficiency in solar cells. |
| Materials Science Research | Quantum Structure Characterization: Offers a reliable optical technique for estimating the size and size distribution of 0D and 1D semiconductor nanostructures (quantum dots, quantum wires) based on their PL spectral shift. |
| Surface Engineering | Passivation Layer Optimization: The TRPL analysis allows for the determination of surface recombination velocity (S), which is critical for optimizing surface passivation layers (e.g., SiOx) on silicon nanostructures to improve device efficiency. |
View Original Abstract
We elaborate a method for determining the 0D-1D nanostructure size by photoluminescence (PL) emission spectrum dependence on the nanostructure dimensions. As observed, the high number of diamond-like carbon nanocones shows a strongly blue-shifted PL spectrum compared to the bulk material, allowing for the calculation of their top dimensions of 2.0 nm. For the second structure model, we used a sharp atomic force microscope (AFM) tip, which showed green emission localized on its top, as determined by confocal microscopy. Using the PL spectrum, the calculation allowed us to determine the tip size of 1.5 nm, which correlated well with the SEM measurements. The time-resolved PL measurements shed light on the recombination process, providing stretched-exponent decay with a τ0 = 1 ns lifetime, indicating a gradual decrease in exciton lifetime along the height of the cone from the base to the top due to surface and radiative recombination. Therefore, the proposed method provides a simple optical procedure for determining an AFM tip or other nanocone structure sharpness without the need for sample preparation and special expensive equipment.
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