Silicon Nanoclusters Research Articles

Photoinduced relaxation dynamics of nitrogen-capped silicon nanoclusters: a TD-DFT study

ABSTRACTHerein we have developed and implemented a TD-DFT-based surface-hopping dynamics simulation method with a recently proposed numerical algorithm capable of efficiently computing nonadiabatic couplings, a semiclassical spectrum simulation method, and an excited-state character analysis method based on one-electron transition density matrix. With the use of these developed methods, we have studied the spectroscopic properties, excited-state characters, and photoinduced relaxation dynamics of three silicon nanoclusters capped with different chromophores (Cl@SiQD, Car@SiQD, Azo@SiQD). Spectroscopically, the main absorption peak is visibly red-shifted from Cl@SiQD via Car@SiQD to Azo@SiQD. In contrast to Cl@SiQD and Car@SiQD, there are two peaks observed in Azo@SiQD. Mechanistically, the excited-state relaxation to the lowest S1 excited singlet state is ultrafast in Cl@SiQD, which is less than 190 fs and without involving excited-state trapping. In comparison, there are clear excited-state trappings in Car@SiQD and Azo@SiQD. In the former, the S2 state is trapped more than 300 fs; in the latter, the S3 excited-state trapping is more than 615 fs. These results demonstrate that the interfacial interaction has significant influences on the spectroscopic properties and excited-state relaxation dynamics. The knowledge gained in this work could be helpful for the design of silicon nanoclusters with better photoluminescence performance.

Treatment of Silicon Dioxide Films Resulting in Luminescence from Silicon Nanoclusters

A large conglomerate of silicon atoms is known to group together forming a silicon nanocluster. Through quantum confinement effects, these silicon nanoclusters are known to luminesce when they are grown to a critical radius. Luminescence may have applications ranging from data transmission over wireless networks, to building low cost biomedical lasers, and to effectively eliminating the bandwidth of a signal inside of a silicon chip, hence it is a worthwhile research interest. This critical radius was modeled as a function of the concentration of excess silicon originally embedded in a silicon dioxide film. Using a simplified diffusional approach, the theoretical size, growth and pacification of nanoclusters were compared to experimentally confirmed results. Processing techniques that may allow for the growth of such a film were examined, and an American company called Veeco was found that could accommodate such a process.

Nanopores creation in boron and nitrogen doped polycrystalline graphene: A molecular dynamics study

In the present paper, molecular dynamic simulations have been conducted to investigate the nanopores creation on 10% of boron and nitrogen doped polycrystalline graphene by silicon and diamond nanoclusters. Two types of nanoclusters based on silicon and diamond are used to investigate their effect for the fabrication of nanopores. Therefore, three different diameter sizes of the clusters with five kinetic energies of 10, 50, 100, 300 and 500 eV/atom at four different locations in boron or nitrogen doped polycrystalline graphene nanosheets have been perused. We also study the effect of 3% and 6% of boron doped polycrystalline graphene with the best outcome from 10% of doping. Our results reveal that the diamond cluster with diameter of 2 and 2.5 nm fabricates the largest nanopore areas on boron and nitrogen doped polycrystalline graphene, respectively. Furthermore, the kinetic energies of 10 and 50 eV/atom can not fabricate nanopores in some cases for silicon and diamond clusters on boron doped polycrystalline graphene nanosheets. On the other hand, silicon and diamond clusters fabricate nanopores for all locations and all tested energies on nitrogen doped polycrystalline graphene. The area sizes of nanopores fabricated by silicon and diamond clusters with diameter of 2 and 2.5 nm are close to the actual area size of the related clusters for the kinetic energy of 300 eV/atom in all locations on boron doped polycrystalline graphene. The maximum area and the average maximum area of nanopores are fabricated by the kinetic energy of 500 eV/atom inside the grain boundary at the center of the nanosheet and in the corner of nanosheet with diameters of 2 and 3 nm for silicon and diamond clusters on boron and nitrogen doped polycrystalline graphene.

Изучение влияния временных характеристик модулированной DC-плазмы с (SiH-=SUB=-4-=/SUB=--Ar-O-=SUB=-2-=/SUB=-)-газовой фазой на рост ncl-Si в матрице a-SiO-=SUB=-x-=/SUB=-:H (C-=SUB=-O-=SUB=-2-=/SUB=--=/SUB=-=15.5 мол%)

AbstractThe effect of various operating conditions of time-modulated DC (direct current) plasma on the formation of an amorphous a -SiO_ x :H matrix and silicon nanoclusters is studied using IR (infrared) and photoluminescence spectra. DC-plasma modulation consists in repeated ( n = 180) switching off and on of the magnetron magnet coil with various time combinations, t _off = 1, 2, 5, 10, 15 s and t _on = 5, 10, 15 s, respectively, at a fixed oxygen concentration ( $${{C}_{{{{{\text{O}}}_{{\text{2}}}}}}}$$ = 15.5 mol %) in a (SiH_4 + Ar + О_2) gas mixture. The positive effect of self-induction on the formation of both the amorphous matrix and silicon nanoclusters is confirmed. The largest values of x in a -SiO_ x :H and photoluminescence intensity are observed in the case of the combination of prolonged plasma residence in the working state ( t _on = 10–15 s) and the maximum magnetic-field strength. The effect of t _off on the processes of the formation of both the a -SiO_ x :H matrix and silicon nanoclusters is also noted.

Interaction of solid and polymeric lithium electrolytes with composites based on carbon fibers and silicon nanoclusters: Quantum-chemical modeling

In the framework of the search for promising electrodes and membranes for lithium-ion batteries, quantum-chemical modeling of the contact area of the solid (Li10GeP2S12) and polymeric LiNafion · nDMSO-based electrolytes with an anode as carbon fibers coated with silicon nanoclusters (Si n C m ) has been performed by the density functional theory method with inclusion of gradient correction and periodic conditions (PBE/PAW). It has been found that the polymeric electrolytes form a better contact with the electrode surface than the solid electrolytes. The barriers to lithium transport in the polymeric LiNafion · nDMSO-based electrolyte have been estimated at 0.3 eV, and those to Li migration from the electrolyte into the electrode have been estimated at 0.4 eV.

Metal nanoparticle-loaded porous carbon hollow spheres by twin polymerization

The formation of silver and gold nanoparticle-encapsulated hollow carbon spheres (HCS) by twin polymerization is reported. Therefore, silica spheres with different diameters (Aerosil® AS90, d = 20 nm; Aerosil® OX50, d = 40 nm; Stober particles, d = 200 nm) were coated with the metal carboxylates [AgO2C(CH2OCH2)3H] (1) or [(PPh3)AuO2C(CH2OCH2)3H] (2). Thermal treatment of the as-produced templates gave the respective metal nanoparticle-functionalized systems, which were characterized by powder X-ray diffraction (PXRD) and transmission electron microscopy. The plasmon resonance of the surface-bonded particles was determined by using UV–Vis spectroscopy showing absorptions at 412 nm for silver and 524 nm for gold. The metal nanoparticle-modified templates were then coated with a twin polymer layer as result of the acid-catalyzed twin polymerization of 2,2′-spiro-bi[4H-1,3,2-benzodioxasiline] (3). This coating consists of a phenolic resin and in situ formed silicon dioxide nanoclusters. After carbonization and removal of the SiO2 phase by refluxing with a NaOH solution, the appropriate metal-loaded HCS were obtained. The thus prepared carbon materials were characterized by PXRD, electron microscopy and nitrogen adsorption–desorption isotherms. Mainly micro-porous materials (IUPAC type I) were obtained with a surface area between 910 and 1110 m2/g. These HCS materials were used for the catalytic reduction of methylene blue and 4-nitrophenol proving the accessibility of the incorporated M-NPs. A size-dependent influence of the used carbon hull was found.

Lithiation of Silicon Nanoclusters

Engineering better batteries for mobile electronics is a major endeavor in energy research. Silicon is potentially a great anode material for lithium-ion systems, but unfortunately it undergoes dramatic volume changes during lithiation and delithiation, leading to rapidly fading ability to hold a charge. The authors show that lithiation of amorphous silicon nanoclusters is a two-stage process governed by Coulomb repulsion, and that at high loading the Li atoms switch where they like to reside in the clusters, causing expansion. Knowing the physics of this threshold allows for optimized battery design.

Photoluminescence of amorphous and crystalline silicon nanoclusters in silicon nitride and oxide superlattices

The photoluminescence properties of silicon nitride and oxide superlattices fabricated by plasmaenhanced chemical vapor deposition are studied. In the structures annealed at a temperature of 1150°C, photoluminescence peaks at about 1.45 eV are recorded. The peaks are defined by exciton recombination in silicon nanocrystals formed upon annealing. Along with the 1.45-eV peaks, a number of peaks defined by recombination at defects at the interface between the nanocrystals and silicon-nitride matrix are detected. The structures annealed at 900°C exhibit a number of photoluminescence peaks in the range 1.3–2.0 eV. These peaks are defined by both the recombination at defects and exciton recombination in amorphous silicon nanoclusters formed at an annealing temperature of 900°C. The observed features of all of the photoluminescence spectra are confirmed by the nature of the photoluminescence kinetics.

Spectrum-splitting approach for Fermi-operator expansion in all-electron Kohn-Sham DFT calculations

We present a spectrum-splitting approach to conduct all-electron Kohn-Sham density functional theory (DFT) calculations by employing Fermi-operator expansion of the Kohn-Sham Hamiltonian. The proposed approach splits the subspace containing the occupied eigenspace into a core-subspace, spanned by the core eigenfunctions, and its complement, the valence-subspace, and thereby enables an efficient computation of the Fermi-operator expansion by reducing the expansion to the valence-subspace projected Kohn-Sham Hamiltonian. The key ideas used in our approach are: (i) employ Chebyshev filtering to compute a subspace containing the occupied states followed by a localization procedure to generate non-orthogonal localized functions spanning the Chebyshev-filtered subspace; (ii) compute the Kohn-Sham Hamiltonian projected onto the valence-subspace; (iii) employ Fermi-operator expansion in terms of the valence-subspace projected Hamiltonian to compute the density matrix, electron-density and band energy. We demonstrate the accuracy and performance of the method on benchmark materials systems involving silicon nano-clusters up to 1330 electrons, a single gold atom and a six-atom gold nano-cluster. The benchmark studies on silicon nano-clusters revealed a staggering five-fold reduction in the Fermi-operator expansion polynomial degree by using the spectrum-splitting approach for accuracies in the ground-state energies of $\sim 10^{-4} Ha/atom$ with respect to reference calculations. Further, numerical investigations on gold suggest that spectrum splitting is indispensable to achieve meaningful accuracies, while employing Fermi-operator expansion.

Large-scale all-electron density functional theory calculations using an enriched finite-element basis

We present a computationally efficient approach to perform large-scale all-electron density functional theory calculations by enriching the classical finite element basis with compactly supported atom-centered numerical basis functions that are constructed from the solution of the Kohn-Sham (KS) problem for single atoms. We term these numerical basis functions as enrichment functions, and the resultant basis as the enriched finite element basis. The enrichment functions are compactly supported through the use of smooth cutoff functions, which enhances the conditioning and maintains the locality of the basis. The integrals involved in the evaluation of the discrete KS Hamiltonian and overlap matrix in the enriched finite element basis are computed using an adaptive quadrature grid based on the characteristics of enrichment functions. Further, we propose an efficient scheme to invert the overlap matrix by using a block-wise matrix inversion in conjunction with special reduced-order quadrature rules to transform the discrete Kohn-Sham problem to a standard eigenvalue problem. Finally, we solve the resulting standard eigenvalue problem by using a Chebyshev polynomial based filtering technique to compute the relevant eigenspectrum. We demonstrate the accuracy, efficiency and parallel scalability of the proposed method on semiconducting and heavy-metallic systems of various sizes, with the largest system containing 8694 electrons. We obtain accuracies in the ground-state energies that are within $\sim 1$mHa with ground-state energies obtained from classical finite element as well as gaussian basis sets. We observe a $50-300$ and $\sim 8$ fold reduction in the overall computational time when compared to classical finite element and gaussian basis, respectively. We also observe good parallel scalability up to 384 processors for a benchmark system comprising of 280-atom silicon nano-cluster.

The temperature behavior and mechanism of exciton luminescence in quantum dots.

The processes of direct and indirect optical excitation of spatially confined excitons in quantum dots (QDs) embedded in a silica thin-film matrix have been reported and discussed. A generalized scheme for the electronic transitions is employed for a detailed description of luminescence temperature behavior using various excitation methods. This scheme considers three different models of exciton relaxation and substantiates the occupation of the triplet radiative states as a result of singlet-triplet intersystem crossing or excitation energy transfer from free excitons of the matrix. Analytical expressions describing five types of different temperature curves were derived. It is established that their shapes are exactly defined by the excitation mechanism and the parameters involved in the numerical model. The conditions allowing the estimation of the energy and kinetic characteristics of QD photoluminescence are formulated. We have shown that the confinement effect causes a decrease in the thermal activation barriers and frequency characteristics for non-radiative transitions. An application of the developed concepts allows predicting and estimating the temperature dependences for direct and indirect optically excited QD luminescence employing silicon nanoclusters in a silica thin-film matrix as an example.

Melting Properties of Medium-Sized Silicon Nanoclusters: A Molecular Dynamics Study

The structures and melting properties of the medium-sized silicon nanoclusters have been comparatively studied using the molecular dynamics method. Structural and thermodynamic parameters are used to characterize the melting properties of the clusters. The size dependence of the melting temperature of silicon nanoclusters is determined using the computation results. Different from the homogeneous melting of bulk silicon, melting of silicon nanoparticles proceeds over a finite temperature range due to surface effects, which shows the heterogeneous melting of nanoclusters. We found that the melting starts at the cluster surface and progressively shifts into the core region. This study provides a fundamental perspective on the melting behaviors of semiconductor silicon nanoclusters at the atomistic level.

Transformation of point defects in silicon dioxide during annealing

In our previous studies, we have demonstrated that annealing of silicon dioxide in the absence of oxygen leads to the formation of silicon clusters near the surface. The mechanism of the formation of silicon clusters by this technique has not been sufficiently investigated. However, it has been found that the rate of the formation of nanoclusters and their sizes depend on the concentration of point defects in the silicon dioxide and on the concentration of impurities, for example, hydroxyl groups. As a continuation of these studies, in the present work we have investigated changes in the concentration of point defects in silicon dioxide films during high-temperature annealing. A new method has been proposed for the evaluation of changes in the concentration of point defects in silicon dioxide films before and after annealing. A model of the transformation of point defects in silicon dioxide into silicon nanoclusters due to the high-temperature annealing has been developed.

The impact of electron correlations on the energetics and stability of silicon nanoclusters.

The first-principles prediction of stable nanocluster structure is often hampered by the existence of many isomer configurations with energies close to the ground state. This fact attaches additional importance to many-electron effects beyond density functional theory (DFT), because their contributions can change a subtle energy order of competitive structures. To analyze this problem, we consider, as an example, the energetics of silicon nanoclusters passivated by hydrogen Si10H2n (0 ≤ n ≤ 11), where passivation changes the structure from compact to loosely packed and branched. Our calculations performed with DFT, hybrid functionals, and Hartree-Fock methods, as well as by the GW approximation, confirm a considerable sensitivity of isomer energy ordering to many-electron effects.

Size-, electric-field-, and frequency-dependent third-order nonlinear optical properties of hydrogenated silicon nanoclusters.

We investigated the electronic properties and second hyperpolarizabilities of hydrogenated silicon nanoclusters (H-SiNCs) by using the density functional theory method. The effects of cluster size, external electric field and incident frequency on the second hyperpolarizability were also examined, respectively. We found that small H-SiNCs exhibit large second hyperpolarizability. With the increase of the number of silicon atoms in H-SiNCs, the frontier molecular orbital energy gap decreases, attributed to the enhancement of the second hyperpolarizability. Interestingly, we also found the electric-field-induced gigantic enhancement of the second hyperpolarizability for H-SiNCs due to the change of electron density distributions. In addition, our results demonstrate a significant dependence on the frequency of incident light.

Growth of silicon nanoclusters in thermal silicon dioxide under annealing in an atmosphere of nitrogen

It is found for the first time that silicon nanoclusters are formed in the surface layer of thermal silicon dioxide under high-temperature annealing (T = 1150°C) in dried nitrogen. Analysis of the cathodoluminescence spectra shows that an imperfect surface layer appears upon such annealing of silicon dioxide, with silicon nanoclusters formed in this layer upon prolonged annealing. Transmission electron microscopy demonstrated that the silicon clusters are 3–5.5 nm in size and lie at a depth of about 10 nm from the surface. Silicon from the thermal film of silicon dioxide serves as the material from which the silicon nanoclusters are formed. This method of silicon-nanocluster formation is suggested for the first time.

On the formation of silicon nanoclusters ncl-Si in a hydrogenated amorphous silicon suboxide matrix a-SiO x :H (0 < x < 2) with time-modulated dc magnetron plasma

Activation of the process of amorphous-silicon-nanocluster formation in a hydrogenated amorphous- silicon suboxide matrix with time-modulated dc discharge plasma is investigated. The plasma is modulated by repeatedly switching on and off a dc magnetron magnetic coil. It is demonstrated that the resulting self-induction effect leads eventually to an increase in the probability of collisions between discharge electrons and gaseous components. The infrared (IR) spectra of the films showed that plasma modulation enhances predominantly the content of bridging oxygen in the a-SiOx:H matrix by strengthening the oxygenionization process. It is assumed that this also increases the concentration of silicon nanoclusters ncl-Si with an oxidized outer surface in the plasma and, thus, enhances the ncl-Si flux toward the electrodes of the dc magnetron. The photoluminescence spectra include two broad overlapping bands characteristic of amorphous ncl-Si with maxima in the range of 600–1000 nm.

Composition and optical properties of amorphous a-SiO x :H films with silicon nanoclusters

The phase composition and optical properties of hydrogenated amorphous films of silicon suboxide (a-SiOx:H) with silicon nanoclusters are studied. Ultrasoft X-ray emission spectroscopy show that silicon- suboxide films with various oxidation states and various amorphous silicon-cluster contents can be grown using dc discharge modulation. In films with an ncl-Si content of ∼50%, the optical-absorption edge is observed, whose extrapolation yields an optical band gap estimate of ∼3.2–3.3 eV. In the visible region, rather intense photoluminescence bands are observed, whose peak positions indicate the formation of silicon nanoclusters 2.5–4.7 nm in size in these films, depending on the film composition.

How Big Does a Si Nanocluster Favor Bulk Bonding Geometry?

Low-energy structures of silicon nanoclusters (NCs) at the size about 220 atoms were studied using tight-binding and first-principles calculations. The structures including bulk-like, icosahedral, bucky–diamond, and onion-like structures have been extensively investigated. The results showed that the bulk-like structures are energetically favorable for the Si220 NCs. Our calculations also showed that although the icosahedral structure is energetically competitive with the bulk-like structures at the special size of 220 atoms, the bulk-like structures are more robust as the number of atoms varies. We also studied the structure-energy trend for Si NCs as the function of cluster size and the structural transition to bulk-like motif occur at the size range of 173–215 atoms.

Super-oxidation of silicon nanoclusters: magnetism and reactive oxygen species at the surface

Oxidation of silicon nanoclusters depending on the temperature and oxygen pressure is explored from first principles using the evolutionary algorithm, and structural and thermodynamic analysis. From our calculations of 90 SinOm clusters we found that under normal conditions oxidation does not stop at the stoichiometric SiO2 composition, as it does in bulk silicon, but goes further placing extra oxygen atoms on the cluster surface. These extra atoms are responsible for light emission, relevant to reactive oxygen species and many of them are magnetic. We argue that the super-oxidation effect is size-independent and discuss its relevance to nanotechnology and miscellaneous applications, including biomedical ones.