Owing to particular physico-chemical properties and high biocompatibility, nanostructured silicon (Si) and germanium (Ge) present very promising materials for biomedical applications, but the fabrication of luminescent Si and Ge nanoparticles (NPs) in pure, uncontaminated, water-dispersible state is almost impossible by using conventional methods of wet-chemical synthesis. We recently showed that such a task can be solved by the elaboration of a technique of pulsed laser deposition (PLD) in gaseous medium under reduced gas pressures (0.5-10 Torr). In particular, PLD-prepared Si-based nanocrystalline layers and NPs could exhibit a photoluminescence (PL) band centered in the red- near infrared (maximum at 760 nm) spectral region (when ablated in pure He) or an intense “green-yellow” PL band centered at 580 nm (when ablated in He and N2 mixture), which were attributed to quantum-confined excitonic states in small Si nanocrystals and a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals, respectively. While as-prepared Ge nanocrystals exhibited a dominating photoluminescence (PL) band around 450 nm, which was attributed to defects in germanium oxide shell, a size-selected portion of relatively small (5-10 nm) Ge NPs exhibited a red-shifted PL band around 725 nm under 633 nm excitation, which could be attributed to the quantum confinement effect in small Ge nanocrystals. After milling by ultrasound and dispersing in water, all such nanocrystals and NPs can be used as efficient non-toxic markers for bioimaging. Here, we give a comparative analysis of the structural and optical properties of Si and Ge nanostructures produced by methods of PLD in He-N2 gaseous mixtures and discuss their potential applications in bioimaging.
The engineering of efficient photodetectors with a nonlinear response to light is an important task for modern optoelectronics because this allows the fabrication of fully light-controlled devices. The two-photon absorption (TPA) phenomenon allows 3D confinement of the excitation and thus, hence, the fabrication of 3D arrays of light-sensitive elements. Moreover, this type of excitation ensures the control of the response efficiency by tuning the intensity of the light. Current technologies mainly use photodiodes based on bulk semiconductors, but the use of colloidal semiconductor quantum dots (QDs) has more advantages due to ease of processing, precisely-controlled optoelectronic properties, quick response, and compatibility with flexible substrates. Moreover, QDs have very high two-photon absorption cross-sections in a wide spectral range in the infrared region. Nevertheless, to activate such photodetectors in the nonlinear optical mode, peak excitation intensities higher than GW/cm^2 are still required. To reduce the excitation intensity it is needed to further increase the TPA. To achieve it we designed plasmon–exciton hybrid material using plasmon nanoparticles (PNPs) with an absorption peak overlapped with excitation wavelength. We found that the effective TPA in QDs near PNPs increased by the order of magnitude due to the nonlinear energy transfer from plasmons in PNPs to excitons of QDs. Finally, we designed a near-infrared plasmon–exciton photodetector based on the designed plasmon-exciton hybrid material. We investigated the effect of plasmons on the photoresponse of the photodetector under near-infrared two-photon excitation of QDs. As a result, we have found that the photocurrent was up to 38 times higher in the detector filled with QD–PNP plasmon–exciton hybrid material than in the detector filled with QDs alone.
This study was supported by the Russian Science Foundation, grant no. 18-72-10143-П. V.K. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, grant agreement no. 101025664 (QESPEM).
Semiconductor quantum dots (QDs) are known for their high capacity to nonlinear interaction with light via two-photon absorption (TPA). This allows them to absorb efficiently the infrared photons with energies lower than the bandgap energy. In addition, the TPA can be further enhanced due to interaction of QD excitons with plasmons of metal nanoparticles making it possible to design highly efficient optoelectronic devices with a nonlinear response to irradiation. To achieve this goal, we have fabricated the nonlinear photodetectors based on the QDs and silver nanoplates (SNPs) which combine both mentioned effects and demonstrate a highly efficient nonlinear photocurrent response at the excitation in the nearinfrared region of optical spectrum. In this study, we compared the photodetectors efficiency enhancement in hybrid devices based on the CdSe QDs and SNPs designed by the different ways. In one case, the SNPs were deposited on the top of 10 layers of QDs, and in the other, they were placed between these layers. We have demonstrated that both types of hybrid photodetectors operate in the two-photon regime. At the same time, we have found that the two-photon absorption efficiency was significantly higher in the sample where the SNPs were located between the QD-layers.
Nanocrystalline silicon (Si) films were synthesized by nanosecond laser ablation of crystalline Si targets in low-pressure helium (He) and nitrogen (N2) gas mixtures. Photoluminescence (PL) spectra of the prepared samples were found to depend on the He/N2 ratio in the gas mixture. The ablation pure He atmosphere allowed us to prepare Si nanocrystals (NCs) exhibiting a PL band in red-near-IR range, while samples prepared in the presence of N2 exhibited a strong PL band with maximum in the green-yellow region. Such a modification of PL properties can be explained by the presence of amorphous Si oxynitride (a-SiNxOy) on the surface of Si-NCs. Structural studies of the prepared samples by means of the scanning electron microscopy revealed different morphology for Si-NCs produced under different gas mixtures. After treating of the films by ultrasound and dispersing in water, Si-NCs can be used as novel biodegradable markers for bioimaging, while the observed spectral tailoring effect makes possible an adjustment of the PL emission of such markers to a concrete bioimaging task.
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