Si Nanoclusters Research Articles

Towards ZnO-Based Near-Infra-Red Radiation Detectors: Performance Improvement via Si Nanoclusters Embedment

Si nanoparticles embedded in a ZnO matrix were produced by a sequential deposition of ZnO/Si/ZnO layers, by radio frequency sputtering. Sample growth temperatures of 25 °C, 300 °C, and 500 °C were used to deposit ZnO/Si/ZnO layers on soda lime glass and p-type silicon substrates; ZnO layers were deposited by reactive radio-frequency sputtering employing a mixture of Ar/O2, with a ratio of 66/33, as working atmosphere. The type of substrate and the growth temperature affect the first ZnO layer roughness, promoting the formation of silicon nanoparticles, matrix characteristics, and as consequence, spectral response. The roughness of the initial ZnO layer is transferred to the top layer of ZnO, and it can be tailored between 65 and 370 Å, depending on the sample growth temperature. Transmission electron microscopy show that substrate temperature mainly affects the density of silicon nanoparticles rather than their size. ZnO/Si/ZnO films deposited on p-type silicon substrate were processed and photosensors were obtained, showing a selective response in the 950 to 1150 nm wavelength range, making them suitable candidates for near infrared detectors.

Negative Capacitance and Dielectric Constant of Nanocomposite SiAlzOxNy(Si) Films with Semiconductor Nanoparticles.

The electrical properties of nanocomposite SiAlzOxNy(Si) films containing Si nanoclusters embedded into amorphous SiAlzOxNy matrix have been studied by measurements of DC current-voltage and AC capacitance-voltage characteristics. Analysis of the results allowed us to conclude the existence of a negative dielectric constant. The temperature dependence of the negative dielectric constant has been obtained and analyzed. The negative capacitance has been revealed during measurements of capacitance-voltage characteristics at testing signal frequency of 2 kHz. The negative capacitance also points out the appearance of a negative dielectric constant effect. The qualitative model for explanation of negative dielectric constant based on peculiarities of SiAlzOxNy(Si) films polarization due to electron capture at Si nanoparticles-amorphous SiAlzOxNy matrix interface traps near cathode region has been proposed. In the case of AC C-U measurements, a negative capacitance is observed if conductivity current through the nanocomposite film is relatively high.

Dual high-Q resonance sensing for refractive index and temperature based on all-dielectric asymmetric metasurface

In this work, we present a high-quality (Q) dual-resonance sensor based on all-dielectric metasurface for refractive index and temperature sensing simultaneously. The all-dielectric metasurface was composed of a periodic array of Si nanocluster placed on the commercially available glass substrate. Two distinct optical resonance modes with extremely high Q factors of 13500 and 14900, a magnetic quadrupole mode (MQ), and toroidal dipole mode (TD), are observed at 1614.12 nm, and 1639.02 nm in the reflection spectra, and confirmed by the electromagnetic field distribution of each resonance. To further reveal the underlying coupling effects of the two resonances, we also perform a systematic analysis of the influence of the differences in radius (ΔR) and gap (Δx and Δy). At last, the refractive index and temperature sensing performance were investigated. More specifically, the proposed sensor exhibits a maximum refractive index sensitivity of 418 nm/RIU for the analyte refractive index range between 1.33 and 1.37, and maximum temperature sensitivity of 86.4 pm/°C in the range of 0 °C–40 °C, which demonstrates great linear relationship and is much better than the previous sensors based on all-dielectric structure. The proposed all-dielectric metasurface is an effective way to achieve refractive index and temperature dual-parameter sensing, which will facilitate wide applications in unstable environment, such as the measurement of temperature and salinity in seawater.

Composition and optical properties of amorphous plasma-chemical silicon oxynitride of variable composition a-SiO-=SUB=-x-=/SUB=-N-=SUB=-y-=/SUB=- : H

The a-SiOxNy : H films of various compositions were obtained by plasma chemical deposition from a gas mixture of 10% monosilane (diluted with argon) and nitrogen in the presence of residual oxygen in the working gas mixture. The nitrogen flow rate varied in the range from 4 to 6 cm3/min, the power of the high-frequency generator (13.56 MHz) varied in the range of 50-150 Watts. The electronic structure and optical properties of the films were studied using X-ray photoelectron spectroscopy, vibrational spectroscopy, transmission and reflection spectroscopy, and spectral ellipsometry. It is shown that, as the generator power decreases, the content of excess silicon in the films increases and amorphous silicon nanoclusters appear. As the generator power increases, the oxygen concentration in the films decreases. Apparently, this is due to the greater dissociation of molecular nitrogen with an increase in the power of the plasma discharge and an increase in the concentration of active nitrogen. Thus, it is possible to control the composition of aa-SiOxNy : H films not only by changing the nitrogen flow, but also by varying the generator power. Keywords: silicon oxynitride, plasma-chemical deposition, stoichiometric composition, Si nanoclusters.

Determination of the phonon sidebands in the photoluminescence spectrum of semiconductor nanoclusters from ab initio calculations

We propose a theoretical approach based on (constrained) density functional theory and the Franck-Condon approximation for the calculation of the temperature dependent photoluminescence of nanostructures. The method is computationally advantageous and only slightly more demanding than a standard density functional theory calculation and includes transitions into multiphonon final states (higher class transitions). We use the approach for Si and CdSe colloidal nanoclusters with up to 693 atoms and obtain very good agreement with experiment which allows us to identify specific peaks and explain their origin. Generally, breathing type modes are shown to dominate the phonon replicas, while optical modes have significant contributions for CdSe nanoclusters (NCs) and play a lesser role in Si NCs. We obtain significant anti-Stokes peak starting at 140 K for Si NC explaining the broadening observed in the corresponding experiment. We also apply the method to small molecular-like carbon structures (diamondoids), where electron-phonon coupling is typically large, and find that multiphonon processes (up to class 4) are very relevant and necessary to compare favorably with experiment. While it is crucial to include these multiphonon states in the small diamondoids with few tens of atoms, neglecting them in only marginally larger ${\mathrm{Si}}_{87}{\mathrm{H}}_{76}$ and ${\mathrm{Cd}}_{43}{\mathrm{Se}}_{44}{\mathrm{H}}_{76}^{*}$ (and larger) quantum dots represents a good approximation.

Diluted silicon promoting Pd/Pt catalysts for oxygen reduction reaction with strong anti-poisoning effect

Poisoning species, such as methanol and air pollutants, degrade the electrocatalytic activity for oxygen reduction reaction (ORR). Here, via taking advantage of the varied electronic property of nano-silicon, a versatile strategy is reported for enhancing the catalytic activity and anti-poisoning ability toward ORR by decorating noble metal catalysts with the size-controlled silicon nanoclusters (DS). Consequently, DS-Pd exhibits the enhanced ORR performance with a large halfwave potential of 0.892 V and an onset potential of 1.010 V. More importantly, it also shows enhanced anti-poisoning performance in methanol, NaNO2 and NaHSO3 electrolytes, compared to those of commercial Pt/C and Pd/C catalysts. Mechanism studies reveal that the synergistic effect between Pd and more oxygen-philic and less poison-philic Si nanoclusters largely abbreviate the absorption of poisoning species on the active sites. This strategy also improves the methanol-tolerant performance of Pt based catalysts, providing a versatile strategy for the fabrication of methanol-tolerant ORR catalysts.

Silicon Cluster Arrays on the Monolayer of Hexagonal Boron Nitride on Ir(111)

Periodic structures of silicon are of interest in quantum-dot-based applications because of their unique optical and electronic properties. We report on the fabrication of stable quasi-ordered Si nanocluster arrays on the moiré of a hexagonal boron nitride (h-BN) monolayer on Ir(111). The h-BN monolayer promotes the growth of regular Si nanoclusters at 130 K and electronically decouples the clusters from the underlying metallic substrate. Using scanning tunneling microscopy and spectroscopy, we have investigated the cluster binding sites, their electronic structure, and their thermal stability. We find that the clusters display a size-dependent bandgap and that they are stable up to 577 K, after which cluster coalescence degrades the arrays.

Electron transport through composite SiO2(Si)&FexOy(Fe) thin films containing Si and Fe nanoclusters

In this study, electron transport mechanisms at a direct current in SiO2(Si)&FexOy(Fe) granular composite films containing Si and Fe nanoinclusions in the temperature range of 95–340 K were determined. The composite films were obtained by co-sputtering of Si and Fe targets in oxygen containing atmosphere (Ar+O2) followed by temperature annealing. It was found out that hopping conductivity with a variable-range hopping was realized at low electric fields. In the temperature range of 115<T<180 K, the electron transport could be reasonably described by the Efros-Shklovsky model taking into account the Coulomb interaction. At higher temperatures (180<T<340 K), the Mott model have been shown to be applicable. Using this model, a number of the characteristics of traps and transport process, namely the density of electron traps near the Fermi level, the activation energy of hopping, and the hopping length were determined. In the range of intermediate and high electric fields, the field enhanced thermal activation of electrons in the conduction band (Poole-Frenkel mechanism) was concluded. At this, the temperature independent current was obtained at the highest values of the electric field. This effect was explained by the influence of the energy position of electron traps taking part in the conductivity as well as the temperature dependence of a dielectric constant.

Nonadiabatic Dynamics in Si and CdSe Nanoclusters: Many-Body vs Single-Particle Treatment of Excited States.

In this work, we report a new nonadiabatic molecular dynamics methodology that incorporates many-body (MB) effects in the treatment of electronic excited states in extended atomistic systems via linear-response time-dependent density functional theory (TD-DFT). The nonradiative dynamics of excited states in Si75H64 and Cd33Se33 nanocrystals is studied at the MB (TD-DFT) and single-particle (SP) levels to reveal the role of MB effects. We find that a MB description of the excited states qualitatively changes the structure of coupling between the excited states, leading to larger nonadiabatic couplings and accelerating the dynamics by a factor of 2-4. The dependence of excited state dynamics in these systems on the surface hopping/decoherence methodology and the choice of the dynamical basis is investigated and analyzed. We demonstrated that the use of special "electron-only" or "hole-only" excitation bases may be advantageous over using the full "electron-hole" basis of SP states, making the computed dynamics more consistent with the one obtained at the MB level.

Correlation between Photoluminescence Properties of Surface Defects and Laser-Induced Damage Threshold of Fused Silica

Laser-induced damage threshold is the main limitation for fused silica optics in high-power laser applications. The existence of various defects near the surface is the key factor for the degradation of the threshold. In this work, the photoluminescence spectra at different regions of the damaged and recovered fused silica samples are recorded to analyze the correlation between photoluminescence of surface defects and laser-induced damage threshold. The experimental data concluded the inverse proportional correlation between fluorescence and laser-induced damage threshold value. The weak photoluminescence is the guarantee of the high laser-induced damage threshold, and then the higher local Si nanocluster concentration corresponds with the higher laser-induced damage threshold value for the fused silica optics after CO2 laser treatment. The investigation reveals that photoluminescence measurement can be employed to check the quality of pristine fused silica and evaluate the tendency of the laser-induced damage threshold value. The current results are helpful for understanding the evolution of interaction from CO2 laser treatment and fused silica optics and can provide the guide of process technology for the high quality of fused silica optics.

Chemical-sensing of Amphetamine drug by inorganic AlN nano-cage: A DFT/TDDFT study

The electronic sensitivity and reactivity of a pristine Si, Ga and Ge-doped Al24N24 nanocluster with AM drug were investigated using density functional theory (DFT). With adsorption energy of approximately −35.5 kcal/mol, AM drug was found to be adsorbed chemically on pristine Al24N24 nanocluster through its N-head and to exert no effects on the electrical conductivity of this cluster. Substituting Si, Ga and Ge atoms for Al atoms in Al24N24 nanocluster significantly elevates the reactivity of Al24N24 nanocluster respectively at predicted adsorption energies of approximately −24.05, −11.75 and −34.38 kcal/mol. Significant HOMO destabilization in Si-AlN nanocluster through AM drug adsorption increases the electrical conductivity of Si-AlN nanocluster while generating electrical signals and reduces its Eg from 2.17 to 1.35 eV. These signals are associated with the presence of AM drug in the environment. Therefore Si-doped Al24N24 nanocluster is found to constitute a promising electronic AM drug sensor. AM drug adsorption increases electron emission from the surface of this sensor and significantly reduces its work function. Significant effects of AM drug adsorption on the Fermi levels and work function of Si-AlN nanocluster make it an Φ–type candidate for AM drug sensors.

Europium‐linked structures and electronic properties of nanosize semiconductor EuSin0/− (n = 11‐18) clusters

AbstractThe ground‐state structures of europium‐doped Si nanocluster and their anions EuSin0/− (n = 11‐18) are verified by using the ABCluster unbiased global search technique married with density functional theory methods and matching of simulated and experimental photoelectron spectroscopy. The results revealed that structural growth pattern for neutral and anionic nanoclusters is from Eu‐substitutional to Eu‐linked structure starting from n = 13 and 12, respectively. The natural population analysis showed that the 4f electrons of Eu atom in EuSin0/− (n = 11‐18) hardly participate in bonding, resulting the total magnetic moments are 7 and 8 μB, respectively. The highest occupied molecular orbital‐lowest unoccupied molecular orbital gap and average bond energies of EuSin0/− (n = 11‐18) have also been calculated respectively to examine their thermodynamic and chemical stability. The result showed that EuSi15 nanocluster possesses good thermodynamic and chemical stability, which may make it as the most suitable building blocks for novel functional nanomaterials.

Selective photosensitivity of metal–oxide–semiconductor structures with SiOx layer annealed at high temperature

SiOx layers with x = 1.15 and 1.3 and thicknesses of 50 and 100 nm were deposited on crystalline silicon wafers by thermal evaporation in vacuum. Part of the samples were annealed at 250 °C (control samples), while the rest were annealed at temperatures of 700, 800, and 1000 °C. The high-temperature annealing leads to phase separation and formation of Si nanoclusters in a dielectric matrix with higher x than the initial one. Metal–Oxide–Semiconductor structures were fabricated and characterized by current–voltage and capacitance–voltage measurements. The relative permittivity (er) and refractive index (n) of the layers depend on the annealing temperature and vary in the 5.4–6.9 (er) and 1.72–1.90 (n) ranges for x = 1.15. For x = 1.3, er and n vary in the 5.1–6.5 and 1.67–1.78 ranges, respectively. All annealed samples showed selective photosensitivity as the responsivity increases with the photon energy. The observed effect is explained by assuming that the blue and UV lights provide sufficient energy to photocarriers generated in the Si wafer to be injected into the SiOx layer at relatively low electric fields, ~ 0.3 MV/cm for layers annealed at 700 and 800 °C and ~ 1 MV/cm for those annealed at 1000 °C (x = 1.15). It was found that the current through the annealed layers under UV and blue light illumination is controlled by Fowler–Nordheim tunneling. The structures with x = 1.3 show higher selectivity to UV light but lower responsivity compared to structures with x = 1.15. No selective photosensitivity was observed in the control samples.

Enhanced Photodetection with Crystalline Si Nanoclusters.

SiOx nanodots were fabricated on a TiO₂ thin film using glancing angle deposition technique. The fabricated samples were annealed at 950 °C in open air configuration to obtain Si nanoclusters resulting from phase separation of SiOx nanodots. Field Emission Gun Scanning electron microscopy and atomic force microscopy were used to examine the topography of the samples. The elemental composition of the samples was analyzed using energy dispersive X-ray mapping and their crystallinity was confirmed by analyzing the bandgap determined from the Tauc plots. The annealed samples show a broadband absorption which is about two folds in magnitude as compared to the as deposited (unannealed) samples. The photoluminescence spectra confirms the quantum confinement effect in the annealed samples. A photodetector was fabricated from an annealed sample by depositing gold contacts on top of it. This photodetector showed a two-fold increase in dark current and a 1.5-fold increase in light current compared to a photodetector made from the as-deposited SiOx samples-which is due to the increased crystallinity in Si nanoclusters. Finally, the rise and fall times of the device were measured through a switching experiment.

Electronic structure and nanoscale potential fluctuations in strongly nonstoichiometric PECVD SiOx

The electronic structures of SiOx:H films synthesized by plasma-enhanced chemical vapor deposition on silicon and glass substrates were studied. The film stoichiometric parameter x varied in the range from 0.57 to 2 depending on the oxygen supply to the reactor conditions. According to XPS, Raman scattering and high-resolution electron microscopy, a core-shell model of the a-SiOx:H structure was applied. According to this model, the films consist of silicon suboxide SiOy, in which clusters of different compositions are introduced — from amorphous silicon to silicon oxide — with a characteristic size of 2–10 nm. The SiOx:H energy diagram was constructed. The nanoscale potential fluctuations model, according to which Si nanoclusters are potential wells for electrons and holes in SiOx, is proposed. The data obtained are important for the correct interpretation of the charge transport in a-SiOx:H films, and they are important for the development of nonvolatile memory elements on their basis.

2D island nucleation controlled by nanocluster diffusion during Si and Ge epitaxy on Si(1 1 1)-(7 × 7) surface at elevated temperatures

We have studied two-dimensional (2D) island nucleation at the initial stages of Si and Ge epitaxial growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(1 1 1)-(7 × 7) surface in 500–750 °C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). Concentration of 2D islands nucleated on the terraces and depletion zone width near the steps have been measured as functions of deposition rate and substrate temperature. We have shown that GeSi and Si nanoclusters are dominating diffusion species providing surface mass transport on the Si(1 1 1)-(7 × 7) surface during Ge and Si epitaxial growth at temperatures above 500 °C and 600 °C, respectively; the activation energy of GeSi nanocluster diffusion has been first estimated at 1.3–1.4 eV. The kinetics of 2D island nucleation and growth near the steps (both for Si and Ge epitaxy) and on the wide terraces (for Ge epitaxy) is limited by surface diffusion only, and critical nucleus consists of i ≫ 1 nanoclusters.

Computer modeling of single-layer nanocluster formation in a thin SiO2 layer buried in Si by ion mixing and thermal phase decomposition

A single sheet of Si nanoclusters with an average diameter of about 2 nm has been formed in a 30 nm Si/7 nm SiO2/Si layer stack by 50 and 60 keV Si+ ion-beam mixing at room temperature and fluences between 8.5 ⋅ 1015 and 2.6 ⋅ 1016 ions/cm2 and by subsequent thermal annealing at a temperature above 1000 °C. Computer modeling of the process is accomplished by TRIDYN dynamic ballistic simulation of ion mixing and subsequent lattice kinetic Monte Carlo simulation of the phase decomposition of substoichiometric silicon oxide into Si nanoclusters in a SiO2 matrix. The simulation algorithms are briefly described with special emphasis on the choice of governing parameters for the present system. In comparison to the experimental results, it is concluded that the predicted ion mixing profiles overestimate the interface broadening. This discrepancy is attributed to the neglect of chemical driving forces in connection with thermal-spike induced diffusion, which tends to reconstitute the Si/SiO2 interfaces. With a corresponding correction and a suitable number of Monte Carlo steps, the experimentally obtained areal densities and average diameters of the nanoclusters are successfully reproduced.

UV Sensitivity of MOS Structures with Silicon Nanoclusters.

Selective UV sensitivity was observed in Metal-Oxide-Semiconductor structures with Si nanoclusters. Si nanocrystals and amorphous Si nanoparticles (a-Si NPs) were obtained by furnace annealing of SiOx films with x = 1.15 for 60 min in N2 at 1000 and 700 °C, respectively. XPS and TEM analysis prove phase separation and formation of Si nanocrystals in SiO2, while the a-Si NPs are formed in SiO1.7 matrix. Both types of structures show selective sensitivity to UV light; the effect is more pronounced in the structure with nanocrystals. The responsivity of the nanocrystal structure to 365 nm UV light is ~ 4 times higher than that to green light at 4 V applied to the top contact. The observed effect is explained by assuming that only short wavelength radiation generates photocarriers in the amorphous and crystalline nanoclusters.

Cross‐Linking Tin‐Based Metal‐Organic Frameworks with Encapsulated Silicon Nanoparticles: High‐Performance Anodes for Lithium‐Ion Batteries

AbstractA simple one‐pot strategy is proposed to attach encapsulated Si nanoparticles to rod‐like Sn‐based metal‐organic frameworks (MOFs) through an in situ self‐assembly method. The active Sn‐MOFs amalgamating with Si nanoclusters perform their part by coordinating their individual functionalities in a complementary way. 1) As a superior capacity contributor, the rod‐like Sn‐MOFs, externally, function as a structural reinforcement that maintains the electrode integrity and mitigates the agglomeration of materials; internally, they can serve as a host material to provide inherent open channels and sufficient reserved expansion space, thus alleviating the capacity fading and further increasing the cyclability of electrodes. 2) The encapsulated Si nanoparticles not only facilitate access to a high specific capacity, but also enhance the connection between the electrode and the substrate to enable efficient electron transport upon cycling. These remarkable synergies, combined with the hybridization design, ensure that the Si@Sn‐MOF composite delivers a superior capacity as high as 1360.4 mAh g−1 at 200 mA g−1 after 250 cycles and an exceptional rate capability. When utilized as an anode material for the full cell, it exhibits a discharge capacity up to about 117.7 mAh g−1 after 150 cycles. These results reveal that such a Si@Sn‐MOF composite could be an ideal alternative anode material for high‐energy lithium‐ion batteries.

Selective growth of metal-free silicon nanowire mat by chemical vapor deposition on hexagonal boron nitride film

The one-dimensional form of silicon (Si) has been attracting significant scientific and industrial interest again in the field of Li-ion batteries as well as electronic devices, offering higher reversible capacity than carbon materials in an anode. In this work, a new method to grow a uniform Si nanowire mat without metallic nanoparticles was developed, and the growth conditions and conformational properties were also characterized. It is suggested that Si nanowire could be grown by the charge transfer through the hexagonal boron nitride (h-BN) film from the Cu foil as the previously reported graphene growth on the h-BN film on Cu. Web-like bundles of Si nanowires or nanoclusters could be selectively grown on this h-BN/Cu substrate by controlling the substrate temperature during Si deposition. The morphology and chemical composition of the Si nanowire mats, therefore, were systematically characterized using scanning tunneling microscopy, atomic force microscopy, and X-ray photoemission spectroscopy. When decoupled from the metal by a thin h-BN film during the growth, this pristine Si nanowire mat can provide promising technical breakthroughs for anode applications in Li-ion batteries.