Silicon Nanocrystals Research Articles

(Invited, Digital Presentation) Exploring New Applications of Luminescent Silicon Nanocrystals

The luminescence of silicon nanocrystals (Si NCs) has been traditionally studied for various applications such as light-emitting diodes, solar cells and bioimaging[1]. In the past few years we have started exploring the use of Si NCs for emerging optoelectronic synaptic devices[2,3]. As an important type of optoelectronic synaptic devices, electroluminescent synaptic devices have been fabricated by using Si NCs[4,5]. Under the stimulation of electrical spikes these devices give out electroluminescence that decays in the timeframe of tens of microseconds, enabling a series of synaptic functionalities and logic functions. In addition, we have synthesized Er-hyperdoped Si NCs, which emit light simultaneously at the wavelengths of 830 and 1540 nm. It is found that Er-hyperdoped Si NCs are a powerful ratiometric near-infrared fluorescent platform. The rationmetric near-infrared fluorescence may be well used for temperature sensing, which further enables the construction of logic gates[6].[1] Z. Y. Ni, et al., Materials Science & Engineering R 138, 85-117 (2019).[2] L. Yin, et al., Nano Letters 20, 5, 3378–3387 (2020).[3] Y. Wang, et al., Advanced Functional Materials 2107973 (2021).[4] S. Y. Zhao, et al., Nano Energy 54, 383-389 (2018)[5] S. Y. Zhao, et al., Science China Materials 62, 1470-1478 (2019)[6] K. Wang, et al., to be submitted.

Infrared nanoplasmonic properties of hyperdoped embedded Si nanocrystals in the few electrons regime

Abstract Using localized surface plasmon resonance (LSPR) as an optical probe we demonstrate the presence of free carriers in phosphorus doped silicon nanocrystals (SiNCs) embedded in a silica matrix. In small SiNCs, with radius ranging from 2.6 to 5.5 nm, the infrared spectroscopy study coupled to numerical simulations allows us to determine the number of electrically active phosphorus atoms with a precision of a few atoms. We demonstrate that LSP resonances can be supported with only about 10 free electrons per nanocrystal, confirming theoretical predictions and probing the limit of the collective nature of plasmons. We reveal the appearance of an avoided crossing behavior linked to the hybridization between the localized surface plasmon in the doped nanocrystals and the silica matrix phonon modes. Finally, a careful analysis of the scattering time dependence versus carrier density in the small size regime allows us to detect the appearance of a new scattering process at high dopant concentration, which can be explained by P clustering inside the SiNCs.

Silicon nanoparticles as active materials for LED: laser synthesis and functionalization for tuning luminescence

Silicon nanocrystals (SiNPs) are an extensively studied light-emitting material due to their inherent biocompatibility and easy integration with silicon-based technology. Although low luminescence quantum yields of as-prepared SiNPs, the exploitation of the surface chemistry of SiNPs plays a key role in tuning their luminescence. In this work, the development of SiNPs as active materials for innovative LEDs was investigated. SiNPs were synthesized by using laser pyrolysis, a very flexible tool for lab-scale production in developmental quantities. The synthesized SiNPs have dimensions from 5 to 15 nm and high purity grade but show weak luminescence. For this reason, chemical treatments were performed on SiNPs: surface modifications lead to an improvement of their luminescence properties. Chemical and optical characterization were performed by conventional and advanced techniques, such as FTIR, BET, UV-VIS absorption and emission, XPS, TEM and confocal Raman. A Life Cycle Assessment (LCA) study of laser synthesis of SiNPs was conducted with the aim to identify the potential environmental impacts coming from this production method.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids.

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Site‐Selective Functionalization of Sila‐Adamantane and Its Ensuing Optical Effects

AbstractThe first syntheses of functionalized sila‐adamantanes via site‐selective reactions are described. Mechanistic inquiry into the isomerization of sila‐adamantane revealed new approaches for installing halides at the 2‐position of the cluster. Meanwhile, isomerization via Lewis acid catalysts with non‐nucleophilic counteranions provided access to sila‐adamantane on the gram‐scale, enabling us to discover strategies for substituting its 1‐, 3‐, 5‐, and 7‐positions with identical or distinct functional groups. Optical absorbance and density functional theory studies show that σ‐withdrawing substituents at the 1‐position strongly perturb optical absorbance in sila‐adamantane, whereas substituents at the exocyclic and 2‐position are optically inert. As silicon diamondoids are atomically precise models for silicon nanocrystals, our findings suggest that passivation at tertiary surface sites carries an outsized impact on the optical properties of surface‐functionalized Si nanocrystals.

Site‐Selective Functionalization of Sila‐Adamantane and Its Ensuing Optical Effects

The first syntheses of functionalized sila-adamantanes via site-selective reactions are described. Mechanistic inquiry into the isomerization of sila-adamantane revealed new approaches for installing halides at the 2-position of the cluster. Meanwhile, isomerization via Lewis acid catalysts with non-nucleophilic counteranions provided access to sila-adamantane on the gram-scale, enabling us to discover strategies for substituting its 1-, 3-, 5-, and 7-positions with identical or distinct functional groups. Optical absorbance and density functional theory studies show that σ-withdrawing substituents at the 1-position strongly perturb optical absorbance in sila-adamantane, whereas substituents at the exocyclic and 2-position are optically inert. As silicon diamondoids are atomically precise models for silicon nanocrystals, our findings suggest that passivation at tertiary surface sites carries an outsized impact on the optical properties of surface-functionalized Si nanocrystals.

Observation of distributed feedback lasing in silicon nanocrystals under electrical pumping

Electrically pumped distributed feedback (DFB) lasing in silicon nanocrystals (SiNCs) has been observed. The DFB SiNC laser was made based on a SiNC light-emitting diode (LED) by incorporating a second-order DFB grating. The light emission covered the visible regime. Optical gains in the order of 102 cm−1 were found in the light emitting regime. The DFB grating was so designed and fabricated that lasing wavelength in the red regime was selected. Signals emitting from the surface of the DFB SiNC laser were recorded. The laser was pumped by a pulsed direct-current power supply at room temperature. Four typical criteria of lasing were identified, i.e., 1) threshold behavior of the electroluminescence (EL) intensity versus pumping power density, 2) evident spectral narrowing of the EL spectrum, 3) well-recognized degree of polarization, and 4) small spread of light emitting angle. The latter three were for the pumping power density beyond the threshold one.

Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Raman mediated solitonic pulse compression

A comprehensive analysis of Raman mediated solitonic compression of pulses inside a silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW) is presented. The ultrahigh Raman gain coefficient of the silicon nanocrystal material, being fortified further by the intense optical confinement inside the SPCW, essentially outweighs the contribution of the Kerr in the self-phase modulation phenomenon that is responsible for the solitonic propagation of a pulse. This work particularly emphasizes the contribution of Raman nonlinearity in the soliton dynamics, which however has been neglected conventionally, leading to an unprecedented miniaturization in both the operating power and footprint. Spatiotemporal evolution of pulses inside the SPCW has been calculated using the nonlinear Schrödinger equation considering the slow-light regime. A compression factor as high as 44.3 is obtained using a 30 µm long SPCW with a peak input power of 8 mW at 1550 nm.

Simplified GW/BSE Approach for Charged and Neutral Excitation Energies of Large Molecules and Nanomaterials.

Inspired by Grimme's simplified Tamm-Dancoff density functional theory approach [Grimme, S. J. Chem. Phys. 2013, 138, 244104], we describe a simplified approach to excited-state calculations within the GW approximation to the self-energy and the Bethe-Salpeter equation (BSE), which we call sGW/sBSE. The primary simplification to the electron repulsion integrals yields the same structure as with tensor hypercontraction, such that our method has a storage requirement that grows quadratically with system size and computational timing that grows cubically with system size. The performance of sGW is tested on the ionization potential of the molecules in the GW100 test set, for which it differs from ab initio GW calculations by only 0.2 eV. The performance of sBSE (based on the sGW input) is tested on the excitation energies of molecules in Thiel's set, for which it differs from ab initio GW/BSE calculations by about 0.5 eV. As examples of the systems that can be routinely studied with sGW/sBSE, we calculate the band gap and excitation energy of hydrogen-passivated silicon nanocrystals with up to 2650 electrons in 4678 spatial orbitals and the absorption spectra of two large organic dye molecules with hundreds of atoms.

Annealing effect on SiNx/SiO2 superlattice with ultrathin sublayer fabricated using plasma-enhanced atomic layer deposition

Because of the precise control of film thickness, atomic layer deposition (ALD) offers significant advantages in fabricating high-quality superlattice films. For the first time, a SiNx/SiO2 superlattice with an ultrathin SiNx sublayer layer (1.5 nm) and a SiO2 barrier layer (3 nm) was successfully fabricated using plasma-enhanced ALD. Transmission electron microscopy (TEM) measurements demonstrate the maintenance of multilayer structures with smooth interfaces. Next, the annealing effect on the microstructural, optical, and chemical properties of these SiNx/SiO2 superlattice structures was thoroughly investigated. With increasing annealing temperature, X-ray reflection (XRR) and TEM measurements revealed consistent changes in interface, morphology, roughness, density, and thickness. After high-temperature annealing, Fourier-transform infrared spectroscopy (FTIR) measurements combined with X-ray photoelectron spectroscopy (XPS) measurements revealed significant reductions in Si–H-related defects and absorption bands. Photoluminescence (PL) was used to characterize the light emission property, and it was discovered that increasing the annealing temperature decreased the PL peak intensity at ∼550 and 900 nm. The silicon nanocrystals (Si–NCs)-related PL peak (∼900 nm) became dominant as the annealing temperature increased. The findings in this paper shed light on the annealing effect on the new superlattice material prepared via ALD. Improved PL properties of this type of superlattice are expected and can be obtained in future research by optimizing the ALD deposition process and annealing parameters.

Distance-dependent resonance energy transfer in alkyl-terminated Si nanocrystal solids.

Understanding and controlling the energy transfer between silicon nanocrystals is of significant importance for the design of efficient optoelectronic devices. However, previous studies on silicon nanocrystal energy transfer were limited because of the strict requirements to precisely control the inter-dot distance and to perform all measurements in air-free environments to preclude the effect of ambient oxygen. Here, we systematically investigate the distance-dependent resonance energy transfer in alkyl-terminated silicon nanocrystals for the first time. Silicon nanocrystal solids with inter-dot distances varying from 3 to 5nm are fabricated by varying the length and surface coverage of alkyl ligands in solution-phase and gas-phase functionalized silicon nanocrystals. The inter-dot energy transfer rates are extracted from steady-state and time-resolved photoluminescence measurements, enabling a direct comparison to theoretical predictions. Our results reveal that the distance-dependent energy transfer rates in Si NCs decay faster than predicted by the Förster mechanism, suggesting higher-order multipole interactions.

Absorption and Photoluminescence of Silicon Nanocrystals Investigated by Excited State DFT: Role of Embedding Dielectric and Defects

Absorption and photoluminescence (PL) properties of silicon (Si) nanocrystals (NCs) covered with silicon dioxide () are fairly well unraveled; corresponding information for silicon nitride () coverage is scarce. We elucidate important optical and electronic features depending on the embedding dielectric and interface defect (dangling bond, DB) properties. Using density functional theory (DFT) and time‐dependent (TD‐) DFT for ground state (GS) and excited state (ES) properties, respectively, we compute fully ‐ and OH‐covered NCs of 11–26 Å size, enabling comparisons with experimental data. Our non‐radiative Shockley‐Read‐Hall (SRH) recombination model of DBs at NC/dielectric interfaces demonstrates that SRH recombination is substantially higher for ‐covered NCs. An ensemble TD‐DFT calculation of the eight lowest fundamental transitions accurately describes the absorption edge. Exciton binding energies are significantly smaller in ‐ versus ‐covered NCs due to the delocalizing versus self‐localizing impact of the dielectric onto the exciton. We find higher optical absorption rates for ‐embedded NCs versus ‐embedded NCs. However, SRH interface recombination renders the PL of ‐embedded NCs inferior to their ‐embedded counterparts. Finally, we explain a discrepancy in PL gaps of free‐standing oxidized versus ‐embedded NCs by considering adequate phononic boundary conditions.

Nanostructural crystallization of silumins

The crystallization of silumins has been shown to be a nanostructured process. Liquid silumin consists of elementary nanocrystals of aluminum and silicon, free atoms of aluminum and silicon, aluminum-silicon complexes. Primary microcrystals are formed from them. Eutectic microcrystals are formed from elementary nanocrystals of silicon and aluminum, aluminumsilicon complexes.

Individual silicon nanocrystals photo-current response confirming the quantum single charge residence on single electron nanopixel devices

In this original work we report the photocurrent (PC) properties of nanopixel photodetectors containing a few number of silicon nanocrystals (Si-ncs) (4 Si-nc per pixel for our case). PC results performed on the nanopixels shows the individual contribution of each nanocrystal (a single response per charge for each Si-nc). The influence of quantum dots (QDs) size on transition energies has been calculated using 2D symmetrical self-consistent Poisson–Schroïdinger simulations. These quantum simulations have been used to propose a QD filling model based on strong confinement process. Finally, our results confirm that the nanopixel device acts as single quantum level photodetectors, and it can be added to high sensitive quantum tunneling (QT) devices. • Impressive performance of nanopixel photodetectors containing a few number of silicon nanocrystals. • Individual contribution of each QD. • The nanopixel device acts as single quantum level photodetectors.

Efficient Sensitized Photoluminescence from Erbium Chloride Silicate via Interparticle Energy Transfer.

In this study, we prepare Erbium compound nanocrystals and Si nanocrystal (Si NC) co-embedded silica film by the sol-gel method. Dual phases of Si and Er chloride silicate (ECS) nanocrystals were coprecipitated within amorphous silica. Effective sensitized emission of Er chloride silicate nanocrystals was realized via interparticle energy transfer between silicon nanocrystal and Er chloride silicate nanocrystals. The influence of density and the distribution of sensitizers and Er compounds on interparticle energy transfer efficiency was discussed. The interparticle energy transfer between the semiconductor and erbium compound nanocrystals offers some important insights into the realization of efficient light emission for silicon-based integrated photonics.

The degree of polarization of Raman scattering of light in silicon nanocrystals

Raman scattering of light (RS) on an array of oriented silicon nanocrystals was studied experimentally. The angular dependence of the intensity of the polarized components of the RS was measured and the parameter of the degree of polarization of scattered light was determined. It was found that the degree of polarization of the RS is related to the size of the nanocrystals in the samples. An array of nanocrystals with the same crystallographic orientation was obtained by thermal annealing of a silicon single crystal damaged by ion implantation. During thermal annealing of the sample, the crystallinity of the layer is restored not simultaneously in the entire damaged volume, but in the form of nanocrystals separated by amorphous area. All the clusters formed have the same crystallographic orientation of the original single crystal. The features of the degree of polarization of the RS in nanocrystals are associated with the quantum-mechanical uncertainty of the phonon wave vector and the uncertainty of the phonon direction in a limited volume of the nanocrystal. The relations linking the degree of polarization of the RS with the size of nanocrystals are obtained. A technique for determining the size of nanocrystals by measuring the degree of polarization of the RS is discussed. Keywords: Raman scattering of light, silicon, nanocrystals, ion implantation, annealing, amorphous phase.

A novel sustainable and green mechanochemical route from a (HSiO1.5)n polymer to emissive silicon nanocrystals

A novel, sustainable and green strategy to transform the (HSiO1.5)n polymer into functional Si NCs via a room-temperature mechanochemical process without H2 supply.

Micro‐ and Nanostructure of Layered Si\Sn\Si Films, Formed by Vapor Deposition

The peculiarities of surface relief of the Si‐Sn‐Si layered films, used to produce the silicon nanocrystals with properties of quantum dots, were studied by the methods of electron and atomic force microscopy with the involvement of X‐ray fluorescent microanalysis. It was shown that the quasispherical structuring of the relief at the scale 20‐20000 nm is typical for the surface of silicon‐tin layered films. The role of layer thicknesses is experimentally analyzed under formation of the surface relief (roughness, shapes, and lateral dimensions) during their vacuum deposition from a vapor‐gas phase. The peculiarities of the relief of amorphous silicon films deposited on the surface of molten tin are shown. The fractal‐like type nanostructuring of amorphous silicon films deposited on the surface of liquid tin was discovered.

Taming non-radiative recombination in Si nanocrystals interlinked in a porous network.

A range of the distinctive physical properties, comprising high surface-to-volume ratio, possibility to achieve mechanical and chemical stability after a tailored treatment, controlled quantum confinement and the room-temperature photoluminescence, combined with mass production capabilities offer porous silicon unmatched capabilities required for the development of electro-optical devices. Yet, the mechanism of the charge carrier dynamics remains poorly controlled and understood. In particular, non-radiative recombination, often the main process of the excited carrier's decay, has not been adequately comprehended to this day. Here we show, that the recombination mechanism critically depends on the composition of surface passivation. That is, hydrogen passivated material exhibits Shockley-Read-Hall type of decay, while for oxidised surfaces, it proceeds by two orders of magnitude faster and exclusively through the Auger process. Moreover, it is possible to control the source of recombination in the same sample by applying a cyclic sequence of hydrogenation-oxidation-hydrogenation processes, and, consequently switching on-demand between Shockley-Read-Hall and Auger recombinations. Remarkably, irregardless of the recombination mechanism, the rate constant scales inversely with the average volume of individual silicon nanocrystals contained in the material. Thus, the type of the non-radiative recombination is established by the composition of the passivation, while its rate depends on the degree of the charge carriers' quantum confinement.