Si QDs Research Articles

Silicon quantum dots delivered phthalocyanine for fluorescence guided photodynamic therapy of tumor**Project supported by the National Natural Science Foundation of China (Grant Nos. 21374074, 21422404, and U1532108) and the Undergraduate Training Program for Innovation and Entrepreneurship of Soochow University, China (Grant No. 2016xj010).

Imaging-guided cancer therapy provides a simultaneous tumor imaging and treatment, which helps to eliminate the excessive toxicity to the healthy tissues. For this purpose, multifunctional probes capable of both imaging and curing are needed. In this work, we synthesize water-soluble silicon quantum dots (Si QDs) smaller than 5 nm. Such Si QDs are used for delivering the hydrophobic drug phthalocyanine (Pc). The as-prepared Si/Pc nanocomposite particles show efficient transmembrane delivery into cells and feasible biocompatibility. Moreover, these composite particles emit dual-channel fluorescence signals even after cellular internalization and demonstrate robust photostability in the Si channel. More interestingly, the Si/Pc composite particles show efficient photodynamic therapy effects against tumors both in vitro and in vivo.

Effect of Thermal Cross-Linking of 4-Ethynylstyryl Capping Groups on Electronic Coupling between Silicon Quantum Dots in Silicon Quantum Dot Solids

Colloidal 4-ethynylstyryl and octyl cocapping silicon quantum dot (4-Es/Oct Si QD) and its spin-coated film were synthesized and fabricated at three different curing temperatures, 150, 250, and 350 °C under argon for 4 h. Thermal cross-linking of 4-ethynylstyryl-terminated 4-Es/Oct Si QD during the curing process was confirmed by differential scanning calorimetry for the 4-Es/Oct Si QD powder and by Fourier transform infrared spectroscopy for the Si QD thin films. The effect of thermal cross-linking of 4-ethynylstyryl capping groups on the electronic coupling between Si QDs in Si QD solids of thin film states was investigated by monitoring optical and electrical properties of the Si QD thin films at different curing temperatures. The valence-band spectra of 4-Es/Oct Si QD thin films obtain from high-resolution photoemission spectroscopy at 130 eV showed the shift to lower binding energy as curing temperature increases from 150 to 350 °C. The optical bandgap values estimated from the extinction-coefficient...

Emitting modification in Si-rich-SiNx films versus silicon nitride compositions

Abstract

Silicon Quantum Dots in Dielectric Scattering Media: Broadband Enhancement of Effective Absorption Cross Section by Light Trapping.

We report strong enhancements of the effective absorption cross section and photoluminescence (PL) intensity of silicon quantum dots (Si QDs) with 2.8-6.8 nm in diameter in a highly scattering dielectric medium. The scattering medium is a polymer thin film with submicrometer size pores inside, supporting the resonant cavity modes in the visible range. By the scattering associated with the cavity modes, efficient light trapping into a polymer film with ∼1 μm in thickness is achieved, which leads to 30-40 times enhancement of the effective absorption cross section of embedded Si QDs in a green-red wavelength range. The scattering medium can also enhance up to 40 times the PL of QDs. Detailed analysis reveals that the enhancements of the extraction efficiency as well as the excitation efficiency contribute to the PL enhancement.

In situ-grown silicon quantum dots in SiCx:H/a-C:H hetero-multilayer films prepared by plasma enhanced chemical vapor deposition method

Silicon quantum dots (Si QDs) were grown in situ in SiCx:H/a-C:H hetero-multilayer films at a low substrate temperature using a radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD) method. The component and bonding configuration of the SiCx:H/a-C:H hetero-multilayer films have been systematically investigated by Raman, FTIR and HRTEM testing techniques. The results show that there are amorphous Si QDs in SiCx:H/a-C:H hetero-multilayer films. Without high temperature post-deposition annealing treatments, as increasing CH4 flow rate, the coexistence of amorphous Si QDs and silicon nanocrystals in SiCx:H matrix layers has been confirmed by Raman, HRTEM and FTIR measurements. X-ray photoelectron spectroscopy and UV–Vis spectrometry measurements show that the a-C:H layer is mainly consisted of the sp2-bonded and sp3-bonded carbon atoms and its optical band gap is 1.63 eV. The smaller optical band gap suggested that SiCx:H/a-C:H hetero-multilayer films can reduce the barrier height between Si QDs and barrier layer in vertical direction as compared with SiCx/SiC multilayer films.

Silicon-Quantum-Dot Light-Emitting Diodes With Interlayer-Enhanced Hole Transport

Despite the technological importance of silicon quantum dots (Si QDs) which are solely made of abundant and nontoxic Si, Si-QD light-emitting diodes (LEDs) clearly lag behind those based on other QDs, especially Cd- or Pb-containing QDs. It is imperative that novel measures should be taken to boost the performance of Si-QD LEDs. Here, we demonstrate that Si-QD LEDs can work much more efficiently after the use of interlayers between indium tin oxide (ITO) and poly(ethylene-dioxythiophene):polystyrene sulphonate (PEDOT:PSS) to enhance the hole transport of the devices. The interlayer of dipyrazino (2, 3-f:2 ′, 3 ′-h) quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) or MoO3 increases the work function of ITO and improves the band alignment, leading to better hole injection from ITO to PEDOT:PSS. The resulting mitigated charge unbalance causes both the external quantum efficiency (EQE) and stability of Si-QD LEDs to significantly increase (up to ∼170% for EQE and ∼240% for device half-lifetime). The highest EQE of ∼2.4% obtained in the current work is among the best values that have been reported for Si-QD LEDs. Even without encapsulation, the device half-lifetime is up to ∼8.5 h. The enhancement of the hole transport induced by MoO3 is more significant than that induced by HAT-CN. Therefore, MoO3 more significantly enhances the performance of Si-QD LEDs than HAT-CN.

Fabrication of double barrier structures in single layer c-Si–QDs/a-SiOx films for realization of energy selective contacts for hot carrier solar cells

Thin films of c-Si–QDs embedded in an a-SiOx dielectric matrix forming arrays of double barrier structures have been fabricated by reactive rf-magnetron sputtering at ∼400 °C, without post-deposition annealing. The formation of larger size c-Si–QDs of reduced number density in homogeneous distribution within a less oxygenated a-SiOx matrix at higher plasma pressure introduces systematic widening of the average periodic distance between the adjacent ‘c-Si–QDs in a-SiOx’, as obtained by X-ray reflectivity and transmission electron microscopy studies. A wave-like pattern in the J-E characteristics identifies the formation of periodic double-barrier structures along the path of the movement of charge carriers across the QDs and that those are originated by the a-SiOx dielectric matrix around the c-Si–QDs. A finite distribution of the size of c-Si–QDs introduces a broadening of the current density peak and simultaneously originates the negative differential resistance-like characteristics, which have suitable applications in the energy selective contacts that act as energy filters for hot carrier solar cells. A simple yet effective process technology has been demonstrated. Further initiative on tuning the energy selectivity by reducing the size and narrowing the size-distribution of Si–QDs can emerge superior energy selective contacts for hot carrier solar cells, paving ground for accomplishing all-Si solar cells.

Ratiometric Fluorescent Silicon Quantum Dots-Ce6 Complex Probe for the Live Cell Imaging of Highly Reactive Oxygen Species.

The monitoring of reactive oxygen species (ROS) in living cells remains challenging because of the complexity, short half-life, and autofluorescence of biological samples. In this work, we designed a ratiometric fluorescent probe for the detection and imaging of ROS, which was constructed from silicon quantum dots (Si QDs) with chlorin e6 (Ce6) through electrostatic attraction and showed well-resolved dual fluorescence emission signals (490 and 660 nm). Sensitive and selective biosensing of hydroxyl radical (•OH) was demonstrated on the basis of fluorescence quenching of the Si QDs and Ce6 as an internal reference to avoid environmental interference, with a detection limit of ∼0.97 μM. The endogenous release of •OH was also monitored and imaged in living cells.

Fluorescent Si QD decoration onto a flexible polymeric electrospun nanofibrous mat for the colorimetric sensing of TNT

UV range light was used for the facile synthesis of visible light emitting, surface-protected silicon quantum dots (Si QDs) and electrospun Nylon 6,6 nanofibrous mats were decorated with these Si QD's for selective TNT sensing application.

Surface chemistry and density distribution influence on visible luminescence of silicon quantum dots: an experimental and theoretical approach.

The impact of the surface reconstruction of the density distribution and photoluminescence of silicon quantum dots (QDs) embedded in a silicon oxide matrix (SiOx) has been studied. Annealing treatments carried out on the as-deposited samples provoked the effusion of hydrogen species. Moreover, depending on the surrounding density and coalescence of QDs, they resulted in a change in the average size of the particles depending on the initial local environment. The shift in the luminescence spectra all over the visible region (blue, green and red) shows a strong dependence on the resultant change in the size and/or the passivation environment of QDs. Density functional theoretical (DFT) calculations support this fact and explain the possible electronic transitions (HOMO-LUMO gap) involved. Passivation in the presence of oxygen species lowers the band gap of Si29 and Si35 nanoclusters up to 1.7 eV, whereas, surface passivation in the environment of hydrogen species increases the band gap up to 4.4 eV. These results show a good agreement with the quantum confinement model described in this work and explain the shift in the luminescence all over the visible region. The results reported here offer vital insight into the mechanism of emission from silicon quantum dots which has been one of the most debated topics in the last two decades. QDs with multiple size distribution in different local environments (band gap) observed in this work could be used for the fabrication of light emission diodes (LEDs) or shift-conversion thin films in third generation efficient tandem solar cells for the maximum absorption of the solar spectrum in different wavelength regions.

Effect of spin-coated i-layer properties on electrical and optical characteristics of p-i-n structured all-Silicon Quantum Dot Solar Cell

Spin coating is a well-developed and established method for the application of thin films on the substrates. Key benefits of spin coating method are repeatability, ease of process, and the control over film parameters. Hydrogen Silsesquioxane (HSQ) materials are useful as spin-on-glass (SOG) dielectrics for fabrication of quantum confined Nano-size semiconductors. The paper reports some preliminary results of the electrical and optical characterization of all-Si Quantum Dot Solar Cell (QDSC), having a p-i-n structure and fabricated using ‘top-down’ approach. Spin coated intrinsic layer of the cell accommodates Si QDs synthesized on sonication of freestanding porous silicon film prepared using anodization technique. After spin coating, the device is cured on furnace treatment at 400°C for 1 hour in the presence of N2. Curing decreases, the number of Si-H bonds present in HSQ turning HSQ into porous hydrogenated silicon oxide having a network structure. To achieve higher intrinsic layer thickness, multiple coats of the ‘i-(QDs),’ layers are applied. All the variants of the QDSC fabricated, are tested for photovoltaic effect using I-V characterization. Effect of variation in bandgap due to change in the size of quantum dot and effect of variation in intrinsic layer thickness on quantum efficiency is observed using external quantum efficiency analysis.

Single-dot spectroscopy of boron and phosphorus codoped silicon quantum dots

Boron (B) and phosphorous (P) codoped silicon quantum dots (Si QDs) are dispersible in polar solvents without organic ligands, and exhibit size controllable photoluminescence (PL) from 0.85 to 1.85 eV due to the electronic transitions between the donor and the acceptor states. We study the PL spectra of the codoped Si QDs at room temperature and at 77 K. We show that the broad PL band of codoped colloidal Si QDs (full width at half maximum is over 400 meV) is composed of narrower PL bands of individual QDs with different PL energies. We also show that the PL linewidth of individual codoped Si QDs is almost twice as large as those of undoped Si QDs. In contrast to the significant narrowing of the PL linewidth of undoped Si QDs at low temperatures, that of codoped Si QDs is almost independent of the temperature except for a few very small QDs. These results suggest that a large number of B and P are doped in a QD and there are a number of non-identical luminescence centers in each QD.

Influence of hydrogen passivation on the luminescence of Si quantum dots embedded in Si3Nx

Silicon quantum dots (Si-QDs) of various diameters were formed by annealing of Si-rich silicon nitride (Si3Nx) films synthesized using plasma-enhanced chemical vapor deposition. To investigate the effect of annealing temperature on the luminescence of this system, samples were annealed at temperatures from 400 to 1000 °C. Photoluminescence (PL), x-ray absorption near edge spectroscopy, elastic recoil detection, and Fourier-transform infrared spectroscopy measurements were used for characterization. The authors found that luminescence originated from both quantum confinement effects (QCE) and defects, and that hydrogen passivation affects the PL intensity. For lower annealing temperatures, radiative recombination due to the QCE of the Si-QDs films was observed. For higher annealing temperatures (above 600 °C), desorption of hydrogen from the sample caused the PL intensity to decrease significantly. Si3Nx films with a lower Si content were less sensitive to this reduction in PL intensity after annealing at high temperatures (above 600 °C). Our results emphasize the importance of hydrogenation of the silicon nitride matrix if Si QDs are to be used in optoelectronic devices.

Probing Purcell enhancement in plasmonic nanoantennas by broadband luminescent Si quantum dots

Colloidal silicon quantum dots (Si QDs) with a very broad photoluminescence (PL) band are proposed as a probe to monitor the Purcell enhancement in a plasmonic nanostructure. Si QDs placed on an arbitrary plasmonic nanostructure enable us to determine the Purcell enhancement factors in a broad spectral range (600–900 nm). As a proof-of-concept experiment, a layer of Si QDs is spin-coated on gold film-over nanosphere structures, and the Purcell enhancement is quantitatively determined from the analyses of the PL spectra and the decay rates. The method proposed in this work provides a facile approach to quantitatively measure the performance of plasmonic substrates for PL and Raman enhancements.

Insights into the morphology of human serum albumin and sodium dodecyl sulfate complex: A spectroscopic and microscopic approach

Exploring and understanding the fundamental interaction between protein and surfactant is utmost important for various pharmaceutical and biomedical applications. However, very less is known about the arrangement of individual negatively charged sodium dodecyl sulfate (SDS) molecules on the human serum albumin (HSA). Here, we have investigated the morphology and mechanistic insights of complexation between HSA and SDS by means of photoluminescence (PL) spectroscopy, circular dichroism (CD) and PL microscopy using amine-functionalized silicon quantum dot (Si QD) as an external luminescent marker. The present study is based on a unique and dynamic SDS-Si QD system. The synthesized allylamine–functionalized Si QDs show a distinct PL band centered at 455nm upon excitation at 375nm. At neutral pH, these Si QDs form ordered aggregates in the presence of 1mM SDS due to the hydrogen bonding interaction with the sulfate head groups of surfactants, which is manifested in the appearance of a large Stokes shifted luminescence band centered at 610nm. It has been observed that the PL intensity of SDS-Si QD aggregates at 610nm decreases gradually with concomitant increase in the PL intensity of monomeric Si QDs at 455nm upon increasing the concentration of HSA from 1 to 10μM. These observations combined with PL lifetime, PL microscopy and CD results reveal that SDS forms micelle-like aggregates on the partially unfolded HSA mainly via electrostatic interaction between negatively charged sulfate head groups and positively charged residues of partially unfolded HSA. For the present HSA-SDS system, our results fit a model with type I “necklace and bead”-like structures, where micelle-like SDS aggregates wrap around by the partially unfolded HSA backbone.

Effect of Water Adsorption on the Photoluminescence of Silicon Quantum Dots.

The optical properties of silicon quantum dots (Si QDs) are strongly influenced by circumjacent surface-adsorbed molecules, which would highly affect their applications; however, water, as the ubiquitous environment, has not received enough attention yet. We employed the time-dependent density functional calculations to investigate the water effect of photoluminescence (PL) spectra for Si QDs. In contrast with the absorption spectra, PL spectra exhibit distinct characteristics. For Si32H38, PL presents the single maximum in the dry and humid environment, while the emission spectrum displays a dual-band fluorescence spectroscopy in the low-humidity environment. This phenomenon is also observed in the larger Si QDs. The distinct character in spectroscopy is dominated by the stretching of the Si-Si bond, which could be explained by the self-trapped exciton model. Our results shed light on the Si-water interaction that is important for the development of optical devices based on Si-coated surfaces.

Graphene Coupled with Silicon Quantum Dots for High‐Performance Bulk‐Silicon‐Based Schottky‐Junction Photodetectors

Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.

(Invited) Silicon Nanostructures: A Versatile Material for Photonics

The increased control over nanoscale materials structuring is reshaping a number of applications and technologies at a fast pace. Semiconducting materials are playing an increasingly important role and Silicon is so far the most investigated one thanks to its broad availability and its fundamental role in microelectronics. Despite all its favorable properties, a major drawback of silicon is the lack of an efficient light emission mechanism that prevents its use in active photonics. Quantum size effects has been demonstrated a viable strategy to overcome this limit and to achieve light emission from silicon nanostructures. Si nanostructures are well controlled and can be tuned to achieve the desired properties, often exploiting the mature CMOS technological platform. Simultaneously several synthetic strategies have been developed in order to fabricate silicon quantum dots from chemical synthesis. Silicon chemistry is (rather) well known and permits to synthesize nanoparticles and to functionalize their surface using a broad number of reactions. Such flexibility allows for the grafting of both organic and inorganic species onto the Si QDs surface, to add functionalities and to protect the nanoparticle from their natural oxidations that, in turn, quenches the photoluminescence properties. Thus, despite the initial research was focused on the possibility to fabricate inorganic light sources that might rival with III-V or II-VI semiconductors. More recently a large research effort has been devoted to the use of Silicon nanostructures in biological-related applications. In such research topics Silicon has several properties take renders it an ideal candidate: it is considered a biocompatible material; it is readily dissolved in biological environment without releasing toxic byproducts; its red and tunable luminescence make it an interesting fluorescent tag able to exploit the tissue transparency window. During the presentation I will make a brief review of the fabrications methods proposed so far and I will discuss in details some of the recent research activities performed in our group in ICT- as well as in bio-related topics, all sharing the common denominator of Si QDs as active optical element. About the ICT-related topics I will discuss two applications of Si QDS. The first is about the use of Si QDs in quantum technologies. Data encryption and cryptography are core technologies hidden in our daily life and their importance is constantly growing. We have developed a robust physical random number generator that exploits Si QDs as a random bit source. The preliminary results are promising and the random sequences pass the standard NIST test suite. Furthermore the system is fully compatible with CMOS technology and can be scaled up to achieve large bit-rate generation. The second case in point is about the large optical nonlinearities possessed by Si QDs and their exploitation to develop integrated photonic devices. As a bio-related application I will discuss how nanostructured silicon microparticles can be used as fluorescent tags and are successfully up-taken by human dendritic cells without stimulating apoptosis up to a very large concentration. Porous silicon micro-particles (micro-pSi) with size in the range of 1-10 um are functionalized obtaining an interface that exposes either positive or negative charges at neutral pH. Their porous structure enables their use as drug carriers. Moreover, the surface modification stabilizes their intense photoluminescence and suggests their use for simultaneous imaging and drug delivery. The few cases discussed underline the great potentialities of Silicon nanostructures in different technologies and suggest its possible implementation as a functional interface between organic and inorganic systems.

Surface Passivation Effect of Hydrogen and Methyl on the Structural and Electronic Properties of Silicon Quantum Dots: Density Functional Calculation

We have carried out a series of DFT calculations to investigate changes on the structural and electronic properties of Silicon (Si) quantum dots as a function of surface passivation. In particular, we have study non-polar passivation effect of hydrogen (H) and methyl (CH3) at the surface of quantum dots. From geometry optimization result, we find that clusters with reconstructed surfaces a complete methyl passivation is possible and steric repulsion prevents full passivation of Si dots with unreconstructed surfaces. On the electronic properties point of view, it is noticed for small nanocrystals, the presence of mini-gaps are more pronounced which can limit the non-radiative relaxation of excitons. Obviously, methyl passivation weakly affects the band gap values of silicon quantum dots, while it substantially decreases the band gap and reduce mini-gap appearance compared to hydrogen passivation Si QDs. On the basis of our results we propose that methyl terminated quantum dots may be size selected taking advantage of the reduction on mini-gap and the localization of electron as a function of the cluster size.

Improved electrical properties of silicon quantum dot layers for photovoltaic applications

The low density of silicon quantum dots (Si QDs) in a dielectric matrix is one of the plausible reasons for the low power conversion efficiency (PCE) of Si QD solar cells due to large spacing between QD and substrate. In this study, the electrical properties of the Si QD layers were improved by increasing the density of Si QDs and reducing the barrier width. In the previous studies, Si QD layers were fabricated by alternating deposition of SiO2/SiOx layers and subsequent high temperature (1100°C) annealing, where the diffusion of Si species in the SiOx layer into Si substrate was observed. In order to prevent the diffusion, we propose the substitution of the SiOx layer with an amorphous Si (a-Si) layer. In [SiO2 (2nm)/SiOx (2nm)]×34 multilayer structures, the first few SiOx layers were substituted with a-Si layers. As a result, Si QDs could be well formed in the a-Si layer with high density and survived near the interfaces between the first Si QD layer and Si substrate. The distance of the nearest Si QDs from the Si substrates was also reduced from 7.2nm to 3.6nm, which results in the effective carrier transfer by tunneling between the Si QDs. The PCE of heterojunction B-doped Si QDs/crystalline Si solar cells was enhanced by using the Si QD structures with the improved electrical properties. Since the variations of open-circuit voltage and short-circuit current are not so large, the enhancement of PCE is mainly related with the improvement of fill factor due to the decrease of series resistance.