Silicon Nanocrystals Research Articles

(Invited) Silicon Nanocrystals and Electrum Nanoclusters with High Quantum Yield for Light Converting Applications

While QDs are typically made of semiconductors, metals at low dimensions also turn to discrete electronic structure. The characteristic transition size from bulk to nanodot is lower, because Fermi level position is at the band middle, while discretization of levels starts from the band edge. Therefore, only for sizes of a metal entity ~ 1 nm a QD-like behavior manifests.Here we show that metal nanoclusters Au16Ag7 of an electrum alloy (gold-silver) with adamantanethiolate ligands possess PL quantum yield of ~ 70% in a thiol-ene polymer matrix [1]. PL is in NIR range with a large Stokes shift. Due to parity-forbidden transition the PL lifetime is in microseconds. These characteristics are similar to silicon QDs, where optical transition is also partly forbidden. Luminescence from Si QDs (5-10 nm size) may also feature high quantum efficiency when a defect-free core with good surface passivation is prepared. We have demonstrated a new synthesis method, which can reduce precursor cost by an order of magnitude from the established HSQ-based technique [2]. Si QDs prepared in this way have near-unity internal and > 50% external (quantum yield) efficiency with a large Stokes shift. In both nanomaterials the re-absorption is suppressed, which is of value for several applications.One such application is a semi-transparent photovoltaics for glazing in building-integration. It is based on a luminescent solar concentrator concept, where high efficiency and a large Stokes shift are necessary requirements for nanophosphors. In this configuration absorbed solar light is re-emitted and a large fraction of it is guided by total internal reflection to the edges for collection by standard solar cells. As a proof-of-concept we fabricated 20x20 cm2 prototypes, where Si QD-doped polymer layer is sandwiched between glass plates in a triplex geometry. Such “solar windows” feature high transparency (>80%), low haze (<3%), high color rendering index (~ 88) and, at the same time, deliver up to 0.6 W of electrical peak power under one sun [3]. Original measurement techniques were developed to characterize large-area devices [4,5].

Low-Temperature Anodization: An Electrochemical Route to Tailored Nanostructures in Heavily Doped p-Type Silicon for Forming Quantum Dots

This study presents a pioneering electrochemical approach to tailor the anodization outcomes of heavily doped p-type silicon, a subset of low-resistivity semiconductor materials, via low-temperature anodization. Conventionally, anodizing such silicon in hydrofluoric acid at room temperature leads to the formation of coarse porous silicon with negligible photoluminescence (PL) upon exposure to ultraviolet (UV) radiation.In this investigation, we embark on an unconventional path by conducting the anodization process at an exceptionally low temperature of -20°C. This seemingly subtle adjustment, however, triggers a remarkable transformation in the optical characteristics of the silicon substrate. Strikingly, the anodized surface, when illuminated with UV radiation, emits a vibrant and distinct red PL signal (Figure 1), signifying the emergence of quantum confinement effects attributable to nanocrystals, i.e., quantum dots.To substantiate the existence of these nanostructures, we employ advanced characterization techniques, with a focus on transmission electron microscopy (TEM). These rigorous analyses confirm the presence of well-defined silicon nanocrystals, typically measuring within the 3~4 nm range (Figure 2), meeting the prerequisites for quantum confinement effects. Notably, it is worth mentioning that the formation of such nanostructures is unattainable on heavily boron-doped silicon due to its high etching rate.In conclusion, our findings underscore the potential of low-temperature anodization as a versatile tool to tailor the luminescent properties of porous silicon at the nanoscale. This breakthrough not only advances our fundamental comprehension of semiconductor electrochemistry but also holds the promise of driving progress in electrochemical fabricating semiconductor quantum dots and optoelectronics. Figure 1

A model for the lattice thermal conductivity of nanocrystalline materials with arbitrary grain size distribution

Nanocrystalline materials are a popular metamaterial used to control heat transport in semiconductor industrial applications. An accurate and reliable theoretical model is urgently needed but challenging for elucidating detailed microscopic heat transfer phenomena. In this study, we develop a mathematical model to predict the lattice thermal conductivity of nanocrystalline materials with arbitrary grain size distribution. The model first obtains the effective scattering rate of grain boundaries in single-grain-size materials, and then integrates the Landauer’s system transmission concept and series transmission concept to become applicable to multiple grain sizes. It takes into account not only intrinsic and grain boundary scattering effects, but also the effects of finite system size and grain size distribution. Numerical samples of silicon nanocrystals with single grain size, dual size, constant grain size distribution, and logarithmic normal grain size distribution are constructed and full-spectrum Monte Carlo simulations are performed for comparison. The excellent agreement observed between model predictions and simulation results validates the accuracy of the proposed theoretical model. This model will be very helpful in designing micron-scale thermal manipulation systems using nanostructures.

High-efficiency electroluminescence devices containing Si nanocrystals/SiC multilayers via improved carrier injection and recombination process

Realizing high efficiency of all Si-based light-emitting devices is currently one of interesting issues in order to develop monolithic opto-electronic integration on chips. Here, we report an electroluminescence device based on phosphorus (P)-doped silicon nanocrystals (Si NCs)/silicon carbide (SiC) multilayers by modulating carrier injection and recombination process. The p+-Si substrate is used instead of p-Si substrate for facilitating the hole injection into Si NCs. Additionally, the influences of annealing temperature on the device performance have been studied, and the optimized annealing temperature is achieved by balancing the crystallinity, defect state density, and recombination process.

Luminescent Silicon Nanocrystals as Metal Ion Sensors.

At relatively low concentrations in aqueous solution, Fe3+, Fe2+, Cu2+, and Ni2+ quench the photoluminescence (PL) of the undecenoic acid-capped silicon (Si) nanocrystals. The PL could be restored by adding a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), to remove the ions. Fe3+ and Cu2+ also significantly increase the PL lifetime. Other metal ions, including Cd2+, Mn2+, Pb2+, Zn2+, In3+, K+, and Ca2+, had no effect on the Si nanocrystal PL. The limits of detection (LODs) for Fe3+ and Cu2+ of 370 and 150 nM, respectively, are low enough for metal ion sensing applications.

Density of electronic states in silicon nanocrystals embedded in a SiO2 matrix and passivated by hydrogen

Subject of study. This study is devoted to the optical transitions and density of electron and hole states in silicon nanocrystals embedded in a SiO2 dielectric matrix and silicon nanocrystals passivated by hydrogen. Aim of study. The aim is to calculate the probability of optical transitions, density of electron and hole states, and absorption cross-section in silicon nanocrystals surrounded by different environments. Another goal is to adapt the tight-binding method for correct passivation of dangling bonds of silicon. Method. The calculations are performed using the variant of the tight-binding method that accounts for a large number of orbitals: s, p, d, and s∗. Main results. The local density of states of electrons and holes, probabilities of optical transitions, and absorption cross-sections of silicon nanocrystals passivated by hydrogen and nanocrystals embedded in a SiO2 matrix were calculated. It is shown that the electron and hole states in hydrogen-passivated silicon nanocrystals are localized within the crystal, and optical transitions occur at wavelengths in the range of 440–620 nm. The presence of the SiO2 matrix surrounding the nanocrystal increases the probability of optical transitions and leads to the appearance of the local density of states outside the nanocrystal, with radiative emission in the wavelength range of 410–620 nm. Practical significance. The results highlight the feasibility of utilizing silicon nanocrystals in photonics and photovoltaics. They are expected to contribute to the development of technology for creating silicon nanocrystals with specific optical properties.

Optical properties of cerium-doped SiNx thin films containing silicon nanocrystals

Optical properties of cerium-doped SiNx thin films containing silicon nanocrystals

Silicon nanocrystal slab optical waveguide by multi-energy ion implantation: Linear and nonlinear optical properties

Silicon nanocrystals (Si-nc) have gained attention in optics and photonics research areas in recent years due to their advanced optical nonlinear properties. However, a nonlinear integrated optical platform that allows the fabrication of photonic devices with different Si-nc composition is yet under investigation. In this work, we experimentally demonstrated a nonlinear optical waveguide based on ion implantation that offers advantages in terms of material composition, low propagation losses, and enhanced nonlinear optical properties, such as a high nonlinear refractive index. In this work we present the fabrication of Si-nc containing waveguiding structures, and a study of their linear and nonlinear optical properties. Our nonlinear integrated optical platform presents a two-photon absorption coefficient β = 3.17 × 10−6 cm W−1 and a nonlinear refractive index n2 = 1.19 × 10−10 cm2 W−1, which shows potential applications in all-optical signal processing devices, such as single-photon sources.

Ytterbium femtosecond fiber laser ablation synthesis of silicon nanocrystals in water: Laser frequency and pulse energy dependence

The utilization of femtosecond laser pulses in synthesizing spherical silicon nanocrystals (SiNCs) poses great potential in producing stable nanostructures with chemically pure surfaces and easy-to-manipulate optical properties. In this work, we present an approach that enhances the production and optical quality of surfactant-free spherical SiNCs synthesized in water by ablating silicon wafers with ytterbium fiber femtosecond laser operating at different pulse energies and repetition rates. By further post-synthesis laser treatment of SiNCs suspensions, fragmentation of larger aggregates is observed leading to absorption broadening and absorption-band blueshift. Furthermore, the increase in the repetition rate, from 1 to 500 kHz, of laser pulses having energies of 20 and 240 μJ leads to a decrease in the Si wafer ablation efficiency from 35 to 1.15 pg/pulse; and from 440 to 5.2 pg/pulse, respectively. Using laser pulses with energy of 240 μJ and a repetition rate of 50 kHz results in maximum SiNCs productivity of 33 mg/h. The dependence of the nanostructures's size and ablation efficiency on the pulse energy and repetition rate is attributed to thermal effects on the Si/water interface and within the water layer illuminated by the strong laser radiation.

Enhancement of light emission of a low-resistivity silicon nanocrystal thin film: A simulational and experimental study

A silicon nanocrystal (Si NC) white light-emitting (λ = ∼400–900 nm) thin film with a relatively low resistivity of 1.6 × 104 Ω m has been prepared as an active medium for electrically driven Si light sources. The average size of Si NC is 2.4 ± 0.4 nm. To enhance the light emission efficiency of this low-resistivity Si NC thin film, approach of hydrogen passivation suitable for the traditional high-resistivity (1.2 × 107 Ω m in this work for example) red light-emitting Si NC thin film has been tried and found unavailable unfortunately. Our first principles simulation shows that Si NCs bonded to −O, −NH2, −OH, and −H ligands are responsible for red, green, and blue (RGB) primary color emissions in this white light-emitting sample, respectively. Passivation of the sample in NH3 and H2O atmosphere is then conducted, aiming to increase the number of the RGB light emitters. The light emission is significantly enhanced, with photoluminescence intensity, photoluminescence quantum yield, electroluminescence intensity, and net optical gains increased by factors of 4.6, 4.2, 4.0, and ∼3.0, respectively, after 10-day passivation. Further enhancements are expected for longer passivation.

Enhancement of the Silicon Nanocrystals’ Electronic Structure within a Silicon Carbide Matrix

Using plasma-enhanced chemical vapor deposition (PECVD), a mixed gas of silane (SiH4) and methane (CH4) was diluted with hydrogen (H2) to produce thin films of silicon nanocrystals embedded in a silicon carbide (SiC) matrix. This method prevents the co-deposition of SiH and SiC from high-temperature annealing procedures. This study experimentally explores the improvement of the electronic structure by adjusting two processing parameters according to classical nucleation theory (ratio of SiH4 to CH4 and working gas pressure). The deposited films were examined using ellipsometry spectroscopy, X-ray diffraction, scanning electron microscopy, atomic force microscopy, and photoluminescence to determine grain size, crystal volume fraction, topography, and bond configurations. The results show that increasing the working gas pressure can increase the density of SiC, while increasing the ratio of SiH4 to CH4 can only produce larger grain sizes. This is consistent with how SiC works and grows. Without using a high-temperature annealing procedure, this technique can improve the electrical structure of SiC contained in the SiC matrix formed by PECVD.

Photoluminescent porous silicon nanowires as contrast agents for bioimaging

Porous silicon nanowires (pSi NWs) have attracted considerable interest due to their unique structural, optical properties and biocompatibility. The most common method for their top-down synthesis is metal-assisted chemical etching (MACE) of crystalline silicon (c-Si) wafers using silver nanoparticles as a catalyst. However, the replacement of silver with bioinert gold nanoparticles (Au NPs) markedly improves the efficiency of pSi NWs in biomedical applications. The present study demonstrates the fabrication of porous pSi NWs arrays using Au NPs as the catalyst in MACE of c-Si wafers with a resistivity of 1–5 mOhm·cm. Using scanning electron microscopy (SEM), the formation of arrays of porous nanowires with a diameter of 50 nm that consist of small silicon nanocrystals (nc-Si) and pores was observed. Raman spectroscopy analysis determined the size of nc-Si is about 4 nm. The pSi NWs exhibit effective photoluminescence (PL) with a peak in the red spectrum, which is attributed to the quantum confinement effect occurred in small 4 nm nc-Si. In addition, the pSi NWs exhibit low toxicity towards MCF-7 cancer cells, and their PL characteristics allow them to be used as contrast agents for bioimaging

Cluster-doping in silicon nanocrystals.

Creating tin-alloyed silicon nanocrystals with tailored bandgap values is a significant challenge, primarily because a substantial concentration of tin is essential to observe useful changes in the electronic structure. However, high concentration of Sn leads to instability of the silicon-tin nanocrystals. This work introduces a completely new approach to doping and the modification of the electronic structure of nanoparticles by incorporating few-atom clusters in nanocrystals, deviating from isolated atom doping or attempting alloying. This approach is exemplified via a combined theoretical and experimental study on tin (Sn) 'cluster-doping' of silicon (Si) nanocrystals, motivated by the opportunities offered by the Si-Sn system with tailored band energy. First-principles modelling predicts two noteworthy outcomes: a considerably smaller bandgap of these nanocrystals even with a modest concentration of tin compared to an equivalent-sized pure silicon nanocrystal and an unexpected decrease in the bandgap of nanocrystals as the diameter of nanocrystals increases, contrary to the typical quantum confined behaviour. Experimental verification using atmospheric pressure microplasma synthesis confirms the stability of these nanocrystals under ambient conditions. The plasma-synthesised nanocrystals exhibited the predicted atypical size-dependent behaviour of the bandgap, which ranged from 1.6 eV for 1.4 nm mean diameter particles to 2.4 eV for 2.2 nm mean diameter particles.

Flexible transparent and hydrophobic SiNCs/PDMS coatings for anti-counterfeiting applications.

Silicon nanocrystals (SiNCs) have attracted considerable attention in many advanced applications due to silicon's high natural abundance, low toxicity, and impressive optical properties. However, little attention has been paid to fluorescence anti-counterfeiting applications based on lipophilic silicon nanocrystals. Moreover, it is also a challenge to fabricate aging-resistant anti-counterfeiting coatings based on silicon nanocrystals. Herein, this paper presents a demonstration of aging-resistant fluorescent anti-counterfeiting coatings based on red fluorescent silicon nanocrystals. In this work, lipophilic silicon nanocrystals (De-SiNCs) with red fluorescence were prepared first by thermal hydrosilylation between hydrogen-terminated silicon nanocrystals (H-SiNCs) and 1-decene. Subsequently, a new SiNCs/PDMS coating (De-SiNCs/DV) was fabricated by dispersing De-SiNCs into reinforcing PDMS composites with vinyl-capped silicone resin. Interestingly, the De-SiNCs/DV composites exhibit superior transparency (up to 85%) in the visible light range, outstanding fluorescence stabilities with an average lifetime of 20.59 μs under various conditions including acidic/alkaline environments, different organic solvents, high-humidity environments and UV irradiation. Meanwhile, the encapsulation of De-SiNCs is beneficial to enhancing the mechanical properties and thermal stability of De-SiNCs/DV composites. Additionally, the De-SiNCs/DV coating exhibits an excellent anti-counterfeiting effect on cotton fabrics when used as an ink in screen-printing. These findings pave the way for developing innovative flexible multifunctional anti-counterfeiting coatings in the future.

Singlet-Triplet Radiative Transitions in Silicon Nanocrystals with Shallow Donors

Singlet-Triplet Radiative Transitions in Silicon Nanocrystals with Shallow Donors

Controlled synthesis of silver/silicon hybrid nanostructures enables enhanced photocatalytic CO2 reduction.

Localized surface plasmon resonances in plasmonic nanoparticles enable effective photon harvesting and generate energetic electrons and holes. However, fast charge recombination and surface contamination due to reactant-involved synthesis can significantly hinder the activity. Herein, we report a facile, solution-processed synthesis of hybrid uncoated silver nanoparticles combined with silicon nanocrystals to address these issues. The resulting hybrid nanocolloid exhibits enhanced photocatalytic CO2 reduction, achieving 1552 μmol g-1 and 100% selectivity for CO production.

Silicon nanocrystal hybrid photocatalysts as models to understand solar fuels producing assemblies

A rhenium metal complex tethered to a silicon quantum dot allows exploration of the effects of catalyst anchoring and Re–Si energetic alignment on photocatalytic CO2 reduction activity.

Controlling photoluminescence of silicon quantum dots using pristine-nanostates formation

This study investigates the effective control of photoluminescence (PL) of silicon nanocrystals (Si–NCs) embedded in a SiO2 matrix, with a specific focus on tuning the luminescent wavelength profile and intensity for silicon-based optoelectronic applications. By manipulating the composition ratio (x) of SiOx, the well-known simultaneous changes in PL peak position and intensity were observed after annealing. To preserve the desired wavelength characteristics and enhance the luminesce intensity, we propose a novel approach to generate pristine nanostates (PNs) for Si-NC formation using proton irradiation (PI). The application of proton irradiation was found to be successful in significantly increasing PL intensity while strongly suppressing the peak position shifts. Through extensive spectroscopic analysis including PL, X-ray photoelectron spectroscopy, and absorption spectroscopy, we investigate specific defect states in the SiOx matrix, and identified as PNs. We interpret that these PNs consist of self-trapped exciton, E′ center, non-bridging oxygen hole center, and oxygen-deficiency center states. In the as-grown sample, the PNs promote appropriate phase separation and play a significant role in stabilizing the structure during the subsequent annealing processes. This study elucidates the Si-NC formation mechanism by enhancing O-diffusion-induced phase separation, a process accelerated in SiOx with controlled PNs. The proposed PI-induced PNs offer a novel pathway for developing Si-based optoelectronic devices with desired characteristics in the operating wavelength range. These findings open up promising possibilities for advanced silicon-based optoelectronic applications.

Silicon nanocrystals produced by adiabatic expansion of silicon vapor: emergence of body-centered cubic silicon nanocrystals

Silicon (Si) nanocrystals with diameters of 2–5 nm were produced through non-equilibrium condensation of Si vapor. Electron diffraction analysis indicated that the nanocrystals formed a bcc structure with eight atoms per primitive cell (BC-8). The optical bandgap energy of the nanocrystals was about 1.6 eV, and photoluminescence was observed in the region of the bandgap energy. The obtained energy is compared with a theoretically predicted value of the BC-8 Si crystal of the present size.

Hydrogenation of silicon-nanocrystals-embedded silicon oxide passivating contacts

We investigate the effect of hydrogen passivation of dangling bonds in silicon oxide passivating contacts with embedded silicon nanocrystals (NAnocrystalline Transport path in Ultra-thin dielectrics for REinforced passivation contact, NATURE contact). We first investigated the differences in electrical properties of the samples after hydrogen gas annealing and hydrogen plasma treatment (HPT). The results show that the NATURE contact was efficiently passivated by hydrogen after HPT owing to the introduction of hydrogen radicals into the structure. Furthermore, we examined the dependence of process parameters such as HPT temperature, duration, and H2 pressure, on the electrical properties and hydrogen depth profiles. As a result, HPT at 500 °C, 15 min, and 0.5 Torr resulted in a large amount of hydrogen inside the NATURE contact and the highest implied open-circuit voltage of 724 mV. Contact resistivity and surface roughness hardly increased when HPT was performed under the optimized condition, which only improved the passivation performance without deteriorating the electron transport properties of the NATURE contact.