Si NCs Research Articles

Room-temperature solution processed silicon nanocluster: Theory prediction and experiment achievement

We have presented in this work that silicon nanoclusters (Si NCs) embedded in silicon nitride (▪) can be synthesized by vacuum ultra-violet irradiation of perhydropolysilazane (PHPS) at room temperature. The formation of Si NCs is first predicted by reactive molecular dynamics (ReaxFF-MD) simulations based on a self-developed Si/N/H force field. Then the existence of Si NCs is further verified by photoluminescence (PL) spectroscopy and Raman spectroscopy characterization. Additionally, the photochemical conversion mechanism of PHPS is examined. Moreover, it has been found based on Fourier transform infrared (FT-IR) measurements that ▪ samples prepared with an intermediate irradiation time are the most readily to be oxidized, which can be attributed to the high concentration of the active group -▪, being consistent with the reaction mechanism. We have demonstrated that ReaxFF-MD simulation can be a powerful complement to experimental investigations in revealing the conversion mechanism of polymer precursor to ceramics at the atomic level.

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.

Effect of Hybridization on the Photoluminescence Properties of Atomically Precise Silver Nanoclusters

AbstractThe Förster Resonance Energy Transfer (FRET) is a widely used mechanism harnessed for sensing applications. For temporal stability of the energy transfer between donor and acceptor molecules their photostability and maintenance of the coupling geometry are important. A donor‐acceptor system was created from silicon nanoclusters (Si NCs) and silver nanoclusters (dihydrolopic acid (DHLA)‐protected Ag29 NCs). Optical spectroscopy and lifetime measurements helped to understand the energy transfer process between the Si NCs and Ag29 NCs. Compared with donor‐acceptor pairs based on gold nanocluster and organic dyes, the FRET pair formed between Si NCs and Ag29 (DHLA)12 NCs reaches a coequal efficiency of 60 %. A fine‐tuning of the system‘s parameters opens the door to investigating the sensitivity of the two nanocluster FRET system to various species and incorporating highly photostable moieties in a process relying on photoluminescence.

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.

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.

First principles study on phosphorus or boron doping in si nanocrystals with various sizes embedded in SiO2 matrix

Doping in Si nanocrystals (Si NCs) is an interesting topic since the doping behaviors in the nanoscale are quite complicated compared with the case in bulk Si. In our present work, we use the first-principles calculation to study Phosphorus (P) or Boron (B) doping in Si NCs with the size of 2–8 nm embedded in SiO2 matrix by taking into account the existence of dangling bonds on the interfacial region. It is found that both P and B impurities tend to stay at the interfacial region to passivate the dangling bonds when the dot size is as small as 2 nm. However, P impurities exhibit the possibility to occupy the inner sites of Si NCs while B impurities are more difficult to be introduced into Si NCs due to the large formation energy. Our detailed study suggests that P or B impurities preferentially stay at the intermediate sites between Si and oxygen to form stable bonding configurations. With increasing the dot size from 2 nm to 8 nm, both P and B impurities can enter into the Si NCs more easily due to the relaxation of stress in the larger-sized Si NCs. Our theoretical results are in good agreement with the experimental observations.

Investigation on boron diffusion effects in buried oxide of Si nanocrystals (Si NCs) embedded Silicon-on-Insulator

Si nanocrystals (Si NCs) embedded in an oxide matrix can introduce additional defects and stress, which enhances the dopant diffusion and trapping in buried oxide (BOX) layer that can be detrimental for device applications. In this paper, we have investigated the boron gettering effect of Si NCs in Silicon-on-Insulator (SOI) using Secondary Ion Mass Spectrometry (SIMS), Transmission Electron Microscopy (TEM). And image identification technique is used to analysis the Si NCs concentration in BOX. By analyzing the decline of threshold voltage and vanishing of kink effect of nMOSFET with different Si NCs concentration, it is found that the variation of boron concentration is induced by Si NCs. The higher annealing temperature and boron concentration in top Si can accelerate the boron diffusion velocity. The boron concentration rises almost tenfold in BOX layer when Si implantation dose increases from 5 × 1015 cm−2 to 5 × 1016 cm−2. The results indicate that the boron gettering effect is caused by Si NCs and highly related to the Si NCs induced defects and stress. This paper may offer a guidance for compensating the boron loss in SOI MOSFET fabricating process.

Experimental observations on metal-like carrier transport and Mott hopping conduction behaviours in boron-doped Si nanocrystal multilayers

Studies on the carrier transport characteristics of semiconductor nanomaterials are the important and interesting issues which are helpful for developing the next generation of optoelectronic devices. In this work, we fabricate B-doped Si nanocrystals/SiO2 multilayers by plasma enhanced chemical vapor deposition with subsequent high temperature annealing. The electronic transport behaviors are studied via Hall measurements within a wide temperature range (30–660 K). It is found that when the temperature is above 300 K, all the B-doped Si nanocrystals with the size near 4.0 nm exhibit the semiconductor-like conduction characteristics, while the conduction of Si nanocrystals with large size near 7.0 nm transforms from semiconductor-like to metal-like at high B-doping ratios. The critical carrier concentration of conduction transition can reach as high as 2.2 × 1020 cm−3, which is significantly higher than that of bulk counterpart and may be even higher for the smaller Si nanocrystals. Meanwhile, the Mott variable-range hopping dominates the carrier transport when the temperature is below 100 K. The localization radius of carriers can be regulated by the B-doping ratios and Si NCs size, which is contributed to the metallic insulator transition.

Energy transfer between Si nanocrystals and protoporphyrin molecules as a function of distance, orientation and size.

The interaction between protoporphyrin molecules (donor) and Si nanocrystals (acceptor) up to 2.5nm for varying distances and orientations is studied by DFT, semi-empirical and TDDFT methods. Simulations show an effect on electronic structure, indicative of electron charge transfer in parallel orientation and small distances and nonelectron energy transfer for different orientations and larger distances. An absorption-emission spectral overlap is observed. The calculations of coupling and energy transfer rates show a distance dependence typical of fluorescence resonance energy transfer (FRET) in the long range, while in the short and ultra-short range the distance dependence indicates electron transfer in addition to FRET. The Si NCs with the smallest size yield larger couplings than the larger nanocrystals. The PPIX/Si NC coupling was enhanced by adding a plasmon nanoparticle as a bridge in the donor-acceptor system. Results using Au nanoparticles show increased energy transfer rates up to 104 and lower distance dependence.

Single-atom infrared emission in doped silicon nanocrystals.

Silicon luminescence, due to silicon being abundant, non-toxic and harmless, is a topic of pivotal importance in optoelectronics and biological imaging. However, a major challenge in developing high-efficiency silicon light sources is the relatively weak allowable transitions. This study focuses on single atom-doped silicon nanocrystals (Si NCs) and theoretically investigates the emission behavior of single atoms within a tetrahedral coordination field. Doping a single atom in Si NCs can result in a ∼102 times improvement at least in the squared transition dipole moment (TDM2), and induce a spectral shift towards near- and mid-infrared wavelengths. These findings offer a strong foundation for designing Si NCs for on-chip optical communication and single photon emitters.

Electron spin resonance in P-doped Si nanocrystals/SiC stacked structures with various dot sizes

Doping in semiconductor nanocrystals have a significant impact on electronic structures and the relative studies are needed in order to deeply understand the doping effect in nanoscale. Here, we fabricate un-doped and P-doped Si nanocrystals (Si NCs)/Silicon carbide (SiC) multilayers with various dot sizes (8, 4 and 2 nm). Low temperature (77 K) electron spin resonance spectra are measured to study the electronic structures before and after P doping. It is found that the g value is kept at 2.005 for un-doped Si NCs samples, which is independent of the dots size, indicating the existence of dangling bonds at the surface of Si NCs. After P doping, for samples with dot size of 8 nm, the g value is 1.998, which is assumed to the conduction electrons. It suggests that P impurities are introduced into the inner of Si NCs substitutionally to provide the conduction electrons. However, with the reduction of dot size, the signals with g = 2.004, which is associated with the Si vacancies, are identified in P-doped samples, while the peak-to-peak linewidth ΔBpp is narrowed with the dots size decreasing. It shows the possibility to apply P-doped Si NCs/SiC multilayers with ultra-small dot sizes into Si-based spintronics.

Temperature-dependent carrier transport behaviors in phosphorus-doped silicon nanocrystals/silicon carbide multilayers

Size-controllable silicon nanocrystals/silicon carbide multilayers with various phosphorus-doping ratios are fabricated by plasma enhanced chemical vapor deposition system. The carrier concentration of Si NCs with suitable doping levels can be enhanced by six orders of magnitude than that of the un-doped sample and the room temperature conductivity can reach as high as 111 S/cm. The temperature-dependent carrier transport behaviors of the P-doped samples are systematically studied. It is revealed that the carrier transport behaviors are dominated by three different conduction mechanisms in the various temperature range, i.e. Mott variable-range hopping (within 20–80 K), multiple phonon hopping (within 80–300 K) and thermally activation conduction (above 300 K). Meanwhile, the carrier transport mechanism depends on the doping levels. At high doping levels, the localization length is about seven times of the size of dot diameters which may induce the metal-insulator transition.

Enhancement of intrinsic optical transitions in silicon nanocrystals by state localization

Silicon photoluminescence and lasing have been critical issues to breakthrough bottlenecks in the understanding of luminescence mechanisms. Unfortunately, long-standing disputes about the exciton recombination mechanism and fluorescence lifetime remain unresolved, especially about whether silicon nanocrystals (Si NCs) can realize fast direct-bandgap-like optical transitions. Here, using ground-state and excited-state density functional theory (DFT), we obtained intrinsic phonon-free optical transitions at sizes from Si22 to Si705, showing that very small Si NCs can realize a strong direct optical transition. Orbital labeling results show that this rapid transition does not come from the {\Gamma}-{\Gamma} -like transition, contrary to the conclusions from the effective mass approximation (EMA) and that {\Gamma}-X mixing leads to a quasi-direct bandgap. This anomalous transition is particularly intense with decreasing size (or enhancement of quantum confinement). By investigating electron and hole distributions generated in the optical transition, localized state-induced enhanced emission (LIEE) in Si NCs was proposed. Quantum confinement (QC) distorts the excited-state electron spatial distribution by localizing Bloch waves into the NC core, resulting in increased hole and electron overlap, thus inducing a fast optical process. This work resolves important debates and proposes LIEE to explain the anomalous luminescence--a phase transition from weak (or none) luminescent state to strong optical transition, which will aid attempts at realizing high-radiative-rate NCs materials and application-level Si lasers.

Enhancement of sensitized photoluminescence of erbium chloride silicate through regulating annealing

Erbium chloride silicate (ECS) nanocrystals and Si nanocrystals (Si NCs) co-embedded in silica films were prepared. And the sensitized luminescence of ECS was realized through interparticle energy transfer (IPET) in solid matrix. We focus on the effect of annealing temperature on the film microstructure and sensitized luminescence. The samples annealed at 1100 °C have a moderate level of energy transfer efficiency and total Er3+ concentration capable of radiative recombination. At the same time, they also have high luminescence intensity of Si NCs. Therefore, the samples annealed at 1100 °C have good sensitizing luminescence performance of ECS. The strong luminescence intensity of sensitizers Si NCs and adjacent crystalline ECS nanocrystals are the keys to achieve excellent IPET in the solid matrix. The results provide a basis for optimizing sensitized luminescence of erbium compounds by regulating annealing.

(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.

Relationship Between Periodontal Disease and Glycemic Control in Pediatric Patients with Type 2 Diabetes Mellitus

IntroductionType 2 Diabetes Mellitus (T2DM) is a disease of heterogeneous etiology, in which the patient presents alterations in metabolism, which are associated with a total or relative deficiency in insulin production and variable degrees of resistance to it1,2.T2DM and periodontal disease are chronic diseases, the evidence suggests an association between the two. Studies report that T2DM worsens the periodontal disease and glycemic control plays a fundamental role in this3.4.ObjectiveTo determine the relationship between periodontal disease and glycemic control in pediatric patients with T2DM.MethodologyAn observational, descriptive, cross‐sectional, prospective, homodemic and single‐center study was carried out in the pediatric stomatology service of the “Hospital para el Niño Poblano” (Puebla city, Mexico) in the period January‐July 2019 with 20 patients (between 6 to 18 years old) diagnosed with T2DM and were under treatment for their glycemic control.ResultsThe population was classified into patients with glycemic control based on glycated hemoglobin (HbA1c ≤ 7.5%) and patients without glycemic control (HbA1c> 7.5%).The prevalence of periodontal disease for patients diagnosed with T2DM was higher in uncontrolled patients who were classified within high Community Periodontal Index (CPI) codes between 3 (71.4%) and 4 (28.6%)5(Table 1).Similarly, it was observed that in the group of patients without glycemic control presented averages of Simplified Oral Hygiene Index (OHI‐S) cataloged between fair (57.1%) and poor (42.9%) (Table 2).DiscussionSimilar results were obtained by Janket (2005)6and Carneiro (2015)7, who relate a higher rate of periodontal disease in patients with higher HbA1c values.ConclusionIt was determined a relationship between glycemic control and periodontal disease in children, because patients with poor glycemic control (HbA1c> 7.5%) presented CPI with degrees of inflammation and presence of periodontal pockets and poor OHI‐S.References Islas S, Revilla MC. Diabetes Mellitus: Actualizaciones. IMSS. México, D.F.: Editorial Alfil. 2013. Menon S, Francis J, Zdraveska N, Dusabimana A & Bhattacharyya S. La epidemiología de la hiperglucemia inducida por TB intransitoria en individuos con diabetes mellitus 2 no diagnosticada previamente: un protocolo para una revisión sistemática y un metanálisis. Revisiones sistemáticas. 2019; 8 (1): 1‐5. Britos MR, Sin CS, & Ortega SM. Enfermedad periodontal y su implicancia en la diabetes mellitus: revisión de la literatura. Rev. Ateneo Argent. Odontol. 2019: 33‐40. Pérez I. Diabetes Mellitus. Gac Med Mex. 2016;152(1):50‐55. IPC. OMS (online, July 2019). URL available at: https://es.scribd.com/doc/84956710/Indice‐Periodontal‐de‐Russell‐forma‐OMS Janket S, Wightman A, Baird AE, Van Dyke T, Jones J. Does periodontal treatment improve glycemic control in diabetic patients? A meta‐analysis of intervention studies. J Dent Res. 2005;84(12):1154‐1159. Carneiro V, Calixto F, Morais F, Pegoretti T, Borges AC, da Silva M. The influence of glycemic control on the oral health of children and adolescents with diabetes mellitus type 1. Arch Endocrinol Metab. 2015;59(6):535‐540.

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.

Effect of carbon concentration on optical and structural properties in the transition from Silicon Rich Oxide to SiCxOy films formation

The role of carbon atomic concentration on the transition from Silicon Rich Oxide (SRO) to Silicon Oxicarbide films (SiCxOy) is reported. For this, optical and structural properties were analyzed by Photoluminescence (PL), Micro-Raman, Fourier Transform Infrared (FTIR) and X-Ray Photoelectron Spectroscopy (XPS) measurements. An SRO film with 16.6 at.% of Si-excess and SiCxOy films with carbon concentration from 1.3 to 7.2 at.% were grown by Hot Filament Chemical Vapor Deposition Technique (HFCVD). The SRO film (C = 0 at.%) and SiCxOy films with low carbon atomic concentration (C = 1.3 at.%) exhibited a very low PL emission in the red-infrared region (1.5–1.7 eV) manly attributed to SiO defects which are reduced or annihilated by incorporation of carbon. An increase in carbon atomic concentration greater than 1.3 produces a strong PL emission at white region (3.35–1.72 eV) characteristic of SiCxOy films attributed to radiative defects such as defects associated to Si-related oxygen vacancies (Si-NOVs), Si-related oxygen deficiency centers (Si-ODC) and C-related oxygen vacancies (C-NOVs). In this work it was found that a carbon atomic concentration greater that 1.3 is required to have a high density of these radiative defects. Also, due to the high growth temperature (900 °C) and Si-excess (from 16.6 to 3.9 at.%), we propose the presence of Si nanocrystals (Si–NCs) embedded in both SRO and SiCxOy films with low carbon concentration (SiO2-Like characteristics). Whereas an increase in carbon concentration of 7.2 at. %, Si–NCs precipitation was inhibited in the SiCxOy films.

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.