Si Core Research Articles

Trap-Assisted Switching in Silicon Nanocrystal Based p-i-n Device

An all Si, p-i-n device, consisting of a nanostructured porous Si layer sandwiched between a p-type crystalline Si and an n-type amorphous Si, exhibits current controlled switching. Temperature-dependent charge transport characteristics of the device reveal that trapping and detrapping of injected carriers at the interface of nanocrystalline Si core and the oxide shell, through impact excitation, are responsible for the observed switching. The abundance of surface inhomogeneities in the form of sub-oxides and dangling bonds creates multiple charge trapping centers which, in turn, endow the device with switching characteristics. The density of trap states ( $ \sim 10^{11}$ /cm $^{2}$ ) estimated from the detrapping potential is in very good agreement with the trap density obtained directly from C-V measurements.

Synthesis of Al-25 wt% Si@Al2O3@Cu microcapsules as phase change materials for high temperature thermal energy storage

High temperature phase change materials (PCMs) are drawing increasing attentions globally due to their high latent heat storage ability for solar thermal energy. Encapsulation of PCMs is an effective method for preventing the leakage of liquid PCM during heat storage, which makes PCMs easier to handle. In this paper, the synthesis of microencapsulated PCM (MEPCM) with Al-25 wt% Si core (spheres, average diameter 36 µm) and Al2O3@Cu multilayered shell is reported, which works at temperature up to 500 °C. The homogeneous and effective composite shell is synthesized in multi-steps: First, a boehmite (AlOOH) shell is formed by boiling treatment in water, which is then transferred to Al2O3 shell by heating at 500 °C. Second, the Al2O3 covered particles are etched with HCl solution. Third, electroless plating of Cu layer is carried out to form the multilayered MEPCM. The surface morphology, cross-sectional structure, and thermal durability and stability of the MEPCM are investigated. The core/shell Al-25 wt%Si@Al2O3@Cu particles can maintain their integrity even after 100 melting–solidification cycles with a low breakage ratio of about 1.7%. The results indicate that as-prepared MEPCM can be used for high temperature thermal energy storage such as solar thermal power generation. The success of Cu plating on spherical particles can also be applied to fabricate other Cu coated PCMs due to expected high thermal conductivity of Cu, especially for the low temperature ones such as organic PCMs.

Formation of silicon nanocrystal chains induced via Rayleigh instability in ultrathin Si/SiO2 core/shell nanowires synthesized by an inductively coupled plasma torch process

A new class of silicon (Si) nanohybrids is formed exploiting the Rayleigh instability experienced by Si nanowires (SiNWs) synthesized via inductively coupled plasma. These nanohybrids consist of three main categories of Si nanostructures: (i) core/shell silicon/silicon oxide nanowires (where the inner Si cores have diameters as small as 2–3 nm), (ii) sequences of almond-shaped silicon nanocrystals (SiNCs), having diameters in the range of 4–5 nm, connected by an ultrathin silicon nanowire (diameter of 1–2 nm) and embedded in a silica nanowire, and (iii) sequences of isolated spherical SiNCs (having diameters in the range of 4–7 nm) embedded in an otherwise continuous silica nanowire. The predominance of one morphology over the others can be tuned via post-synthesis thermal treatments, with a clear predominance of the spherical SiNC chain configuration after high-temperature annealing (1200 °C). It is demonstrated that the Rayleigh model describes very well the morphological transformations undergone by the Si core of the nanostructures when subject to the capillarity-related instability induced by the thermal annealing. More interestingly, we have been able, for the first time to our knowledge, to follow in situ the occurrence of Rayleigh instability in SiNWs leading to the breakage of their inner core and formation of smaller SiNCs. Finally, the optoelectronic properties of the Si nanohybrids, studied via photoluminescence measurements, have confirmed the occurrence of quantum confinement effects in the ultra-small SiNCs present in these new nanohybrids.

Characterisation of silicon fibre Bragg grating in near‐infrared band for strain and temperature sensing

Fibre Bragg grating inscription in the crystalline silicon (Si)-core fibre is described using visible light femtosecond laser pulses. The femtosecond pulses at 517 nm propagate through the transparent silica glass cladding and are fully absorbed by the Si core generating locally high temperatures and stress fields in forming the grating periods. The Si FBGs were characterised in reflection, calibrated for a first time as strain and temperature sensors and compared with that of FBGs inscribed in standard silica fibres.

Structural properties of silicon–germanium and germanium–silicon core–shell nanowires

Core–shell nanowires made of Si and Ge can be grown experimentally with excellent control for different sizes of both core and shell. We have studied the structural properties of Si/Ge and Ge/Si core–shell nanowires aligned along the direction, with diameters up to 10.2 nm and varying core to shell ratios, using linear scaling density functional theory. We show that Vegard’s law, which is often used to predict the axial lattice constant, can lead to an error of up to 1%, underlining the need for a detailed ab initio atomistic treatment of the nanowire structure. We analyse the character of the intrinsic strain distribution and show that, regardless of the composition or bond direction, the Si core or shell always expands. In contrast, the strain patterns in the Ge shell or core are highly sensitive to the location, composition and bond direction. The highest strains are found at heterojunction interfaces and the surfaces of the nanowires. This detailed understanding of the atomistic structure and strain paves the way for studies of the electronic properties of core–shell nanowires and investigations of doping and structure defects.

In Situ Transmission Electron Microsopy of Oxide Shell-Induced Pore Formation in (De)lithiated Silicon Nanowires

Silicon (Si) nanowires with a silicon oxide (SiOx) shell undergoing lithiation and delithiation were examined by in situ transmission electron microscopy (TEM). Large pores formed in the nanowires during the delithiation cycle. We found that the oxide shell constrains the expansion of the Si nanowires during lithitation and then induces pore formation in the nanowires. We propose that the SiOx shell prevents the vacancies that result from the loss of lithium from escaping the Si core, leading to pore nucleation and growth. It is also possible that the difference in mechanical properties of the expanding and contracting Si nanowire and SiOx shell contribute to the observed pore formation. This in situ study reaffirms the need to directly observe structural changes that occur during cycling in battery materials, especially when modified by coatings.

Double core-shell of Si@PANI@TiO2 nanocomposite as anode for lithium-ion batteries with enhanced electrochemical performance

A unique double core-shell structure of Si@PANI@TiO2 nanocomposite is synthesized by a simple in-situ growth method. The two shells of polyaniline (PANI) and TiO2, hand in hand, play a key role to improve the electrochemical performance: First, the flexible properties of polyaniline (PANI) effectively accommodate the volume change of Si during the cycling. Second, the good mechanical feature of TiO2 can maintain the structural integrity and attenuate the volume expansion of Si cores. Finally, both of polyaniline and the lithiated TiO2 enhance the conductivity of Si, which promotes the electrons transport. Resulting in the Si@PANI@TiO2 double core-shell nanocomposite exhibits remarkable synergy in large, reversible lithium storage, delivering a reversible capacity as high as 1027 mAh g−1 after 500 cycles and a superior rate capacity of 640 mAh g−1, at a current of 500 and 4000 mA g−1, respectively. This excellent cycling and high-rate capability can be ascribed to the unique and well-designed double core-shell structure with the synergistic effect between polyaniline (PANI) and TiO2.

Optical properties of ZnO deposited by atomic layer deposition (ALD) on Si nanowires

In this work, we report proof-of-concept results on the synthesis of Si core/ ZnO shell nanowires (SiNWs/ZnO) by combining nanosphere lithography (NSL), metal assisted chemical etching (MACE) and atomic layer deposition (ALD). The structural properties of the SiNWs/ZnO nanostructures prepared were investigated by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies. The X-ray diffraction analysis revealed that all samples have a hexagonal wurtzite structure. The grain sizes are found to be in the range of 7–14 nm. The optical properties of the samples were investigated using reflectance and photoluminescence spectroscopy. The study of photoluminescence (PL) spectra of SiNWs/ZnO samples showed the domination of defect emission bands, pointing to deviations of the stoichiometry of the prepared 3D ZnO nanostructures. Reduction of the PL intensity of the SiNWs/ZnO with the increase of SiNWs etching time was observed, depicting an advanced light scattering with the increase of the nanowire length. These results open up new prospects for the design of electronic and sensing devices.

Formation of n- and p-type regions in individual Si/SiO2 core/shell nanowires by ion beam doping

A method for cross-sectional doping of individual Si/SiO2 core/shell nanowires (NWs) is presented. P and B atoms are laterally implanted at different depths in the Si core. The healing of the implantation-related damage together with the electrical activation of the dopants takes place via solid phase epitaxy driven by millisecond-range flash lamp annealing. Electrical measurements through a bevel formed along the NW enabled us to demonstrate the concurrent formation of n- and p-type regions in individual Si/SiO2 core/shell NWs. These results might pave the way for ion beam doping of nanostructured semiconductors produced by using either top-down or bottom-up approaches.

Spin-Orbit Interaction and Induced Superconductivity in a One-Dimensional Hole Gas.

Low dimensional semiconducting structures with strong spin–orbit interaction (SOI) and induced superconductivity attracted great interest in the search for topological superconductors. Both the strong SOI and hard superconducting gap are directly related to the topological protection of the predicted Majorana bound states. Here we explore the one-dimensional hole gas in germanium silicon (Ge–Si) core–shell nanowires (NWs) as a new material candidate for creating a topological superconductor. Fitting multiple Andreev reflection measurements shows that the NW has two transport channels only, underlining its one-dimensionality. Furthermore, we find anisotropy of the Landé g-factor that, combined with band structure calculations, provides us qualitative evidence for the direct Rashba SOI and a strong orbital effect of the magnetic field. Finally, a hard superconducting gap is found in the tunneling regime and the open regime, where we use the Kondo peak as a new tool to gauge the quality of the superconducting gap.

The hollow mesoporous silicon nanobox dually encapsulated by SnO2/C as anode material of lithium ion battery

The hollow mesoporous silicon nanobox dually encapsulated by tin oxide and carbon layers is successfully fabricated and used for anode active material of lithium ion battery. The effects of the double coating layers on the structure, morphology and features of nanocomposite are systematically investigated by transmission electron microscopy, field emission scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and galvanostatic charge-discharge tests. The results reveal that hollow mesoporous silicon/tin oxide/carbon nanobox has the adjustable void space and a rationally designed hierarchical structure, which permits the free expansion of Si core without breaking double coating layers and preserves the structural integrity of the electrode during the (de)lithiation processes, as well as significantly enhances the electronic conductivity of anode. As expected, the resultant hollow mesoporous silicon/tin oxide/carbon nanobox exhibits a high reversible capacity up to 1535 mAh g−1 at 0.5 A g−1 after 200 cycles of (dis)charging due to the unique nanostructure. Especially, the as-synthesized nanocomposite exhibits an excellent rate capacity as high as 858 mAh g−1 at 6 A g−1. This rational structure design and innovative approach can be of visible significance for the development of new anode active materials of next-generation lithium-ion battery.

Critical SiO2 nanolayers for improving corrosion resistance and lithium storage performances of core-shell nano-Si/C composites

Core-shell nano-Si/C composites are promising anodes for Li-ion batteries due to the unique structures of encapsulating Si nanoparticles in highly conducting carbon matrixes. However, such single carbon shells coated Si nanostructures still suffer from LiPF6-based electrolyte corrosions on the Si cores, limiting the lithium storage capability. To improve corrosion resistance, core dual-shell nano-Si@SiO2/C composites are synthesized by sequential coatings with ∼2 nm of SiO2 interlayers followed by ∼6 nm of outermost carbon shells. The effects of such SiO2 interlayers on electrolyte corrosion resistance and lithium storage performances of the achieved nano-Si@SiO2/C composites are investigated for the first time. The extra SiO2 nanolayers effectively protect nano-Si against LiPF6-based battery electrolyte corrosions, i.e., the formation of layers containing the insulating Li2SiF6 on Si cores, as confirmed by the techniques of XRD and TEM. Compared to the core-shell nano-Si/C, the unique core dual-shell nano-Si@SiO2/C electrodes (with ∼2 nm of SiO2 interlayers) exhibit remarkably improved lithium storage performances (∼1369 mAh g−1 after 50 cycles at 50 mA g−1), showing ∼45% higher reversible capacities. This can be ascribed to the unique core dual-shell nanostructures of the SiO2 interlayers and outermost carbon nanolayers, better managing volume variations and protecting nano-Si cores against electrolyte corrosions. This study helps us further understand the critical roles of SiO2 nanolayers on corrosion resistance and lithium storage performances of core-shell Si/C nanostructures.

Improving electrical performance in Ge–Si core–shell nanowire transistor with a new stripped structure

A new device structure based on Ge–Si core–shell nanowire is proposed. Owing to the fact that the Si shell is stripped at the source/drain end, the density and electrostatic potential of the holes is gradually distributed in the source/drain region of the new structure, which results in a higher gradient of quasi-Fermi potential in the channel underneath the gate. We observe a higher current ratio for the on-off state and improved output characteristics in the new structure via TCAD simulation. Besides, in the proposed structure we can adjust the on-state current by changing the source/drain length for the same gate length. This provides a feasible approach to optimizing a junctionless nanowire transistor device.

Controlled scalable synthesis of yolk-shell structured large-size industrial silicon with interconnected carbon network for lithium storage

Yolk-shell structured nano-Si@void@C can greatly improve the electrical conductivity and buffer the large volume change of silicon nanoparticles, whereas inter poor electrical connection between Si cores and carbon shells still limits the lithium storage performance. Besides, it usually requires complex synthesis conditions and expensive nano-level silicon sources. Herein, we succeed in making the large-size industrial silicon (ca. 400–500 nm) to a yolk-shell structure with a controllable space buffer layer. Most importantly, an interconnected porous carbon network deriving from the thermal decomposition of carbon chains in silicon sources is formed within the carbon shell simultaneously, which can improve the electrical contact between Si cores and hollow carbon shells and prevent the active particles exposure to the electrolyte. With the help of static outer thin carbon shell, optimized inner void and the flexible electrically conductive carbon network, such yolk-shell structured Si@void@C submicron particles synthesizing by one-step carbon-coating procedure exhibit high initial coulomb efficiency of 62%, good rate performance, and excellent long cycling stability with 950.7 mAh g−1 at 100 mA g−1 after 100 cycles. The low consumption and stable synthesis method make it feasible for large-scale fabrication of the yolk-shell structured Si/C composites as commercial anodes for energy storage.

Carbon-coated Si particles with self-supported Si nanowires as anode material for lithium-ion batteries

Although extremely high specific capacity has been envisioned for Si anode materials, the application of Si in lithium-ion batteries (LIBs) is still hampered due to the rapid capacity fading mainly caused by the huge volume changes during the cycling. In this study, carbon-coated Si particles with self-supported Si nanowires were prepared using low-cost raw materials by a versatile and scalable method and investigated as anode materials in LIBs. The single Si particle with size of about 30 ∼ 40 μm is composed of a solid Si core and numerous porous Si nanowires radially distributed on the Si core. This unique architecture can effectively accommodate the volume expansion of Si, release the mechanical stress, prevent the aggregation of Si nanowires and keep the structural integrity. Utilized as anode materials for LIBs, the as-prepared carbon-coated Si particles show improved cycle stability and high initial coulombic efficiency than the raw metallurgical Si particles.

Dual stabilized architecture of hollow Si@TiO2@C nanospheres as anode of high-performance Li-ion battery

The hollow Si nanospheres modified by the mechanically robust titanium dioxide (TiO2) shell and the uniform carbon layer are intentionally designed and successfully prepared as the anode active material of high performance lithium-ion batteries. The effects of the robust TiO2 shell and the uniform carbon layer on the structure and electrochemical performances for the Si@TiO2@C nanospheres are studied in detail by X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction and charge/discharge tests. The results show that the hollow structure of the Si core can spontaneously absorb the huge volume expansion stress, the robust TiO2 shell is used as a compact fence to promote the expansion towards the interior of the Si cavity instead of the exterior in the processes of charge/discharge, and the uniform carbon layer can effectively enhance the electrical conductivity and further control the integrity and stability of the well-wrapped core-shell-shell framework. Typically, the resultant hollow Si@TiO2@C nanospheres exhibit a high initial discharge capacity of 2557.1 mAh g−1 with coulombic efficiency of 86.06% as well as a large recuperative discharge capacity of 1270.3 mAh g−1 after 250 cycles at 1 A g−1 with a mean coulombic efficiency of 99.53%. Therefore, the hollow Si@TiO2@C nanospheres prepared by one-step sol-gel coating process show outstanding electrochemical properties and are considered as a prospective candidate to the adhibitions of the anode material for new generation power LIBs.

Direct Rashba spin-orbit interaction in Si and Ge nanowires with different growth directions

We study theoretically the low-energy hole states in Si, Ge, and Ge/Si core/shell nanowires (NWs). The NW core in our model has a rectangular cross section, the results for a square cross section are presented in detail. In the case of Ge and Ge/Si core/shell NWs, we obtain very good agreement with previous theoretical results for cylindrically symmetric NWs. In particular, the NWs allow for an unusually strong and electrically controllable spin-orbit interaction (SOI) of Rashba type. We find that the dominant contribution to the SOI is the "direct Rashba spin-orbit interaction" (DRSOI), which is an important mechanism for systems with heavy-hole-light-hole mixing. Our results for Si NWs depend significantly on the orientation of the crystallographic axes. The numerically observed dependence on the growth direction is consistent with analytical results from a simple model, and we identify a setup where the DRSOI enables spin-orbit energies of the order of millielectronvolts in Si NWs. Furthermore, we analyze the dependence of the SOI on the electric field and the cross section of the Ge or Si core. A helical gap in the spectrum can be opened with a magnetic field. We obtain the largest g factors with magnetic fields applied perpendicularly to the NWs.

Effect of phonon confinement on photoluminescence from colloidal silicon nanostructures

Room temperature time-resolved photoluminescence (TRPL) of colloidal silicon nanostructures prepared by repetitive oxidation-etching-oxidation of mechanically milled silicon, indicate towards phonon confinement in Si nanostructures. The Si nanocrystals have a well-defined Raman peak, which is asymmetrically broadened and red shifted, compared to bulk silicon. The PL intensities exhibit tri-exponential decay characteristics with decay times ranging between pico to nanosecond. For a fixed emission wavelength, the decay time is found to be strongly dependent on excitation wavelength. Steady state luminescence spectra reveal that PL peak positions and intensities are dependent of excitation energy. On the basis of these observations we show that phonon confinement plays a significant role in the non-radiative relaxation of excited carriers at the band edges of nanocrystalline Si core.

Formation of SiOx shell on Si nanoparticles and its effects on electrochemical properties as a Li-ion battery's anode

Crystalline Si core–amorphous SiOx shell nanoparticles were synthesized from SiCl4 using an atmospheric microwave plasma process and their microstructures were investigated. It was found that the amorphous SiOx shell thickness was determined by varying the applied microwave power. The increase of applied power directly increased the length of the flame at the end of the plasma. Through observation of the microstructures with varying lengths of the flame, it was found that a highly crystalline Si phase was formed in the plasma and amorphous SiOx phase was formed in the flame. The longer flame resulted in the thicker SiOx shell around the Si core. The electrochemical properties of the varying SiOx shell thicknesses were also investigated in that the synthesized Si-SiOx nanoparticles were used as anode materials in a lithium-ion battery (LIB). Thin shells such as native oxide did not influence the electrochemical behaviour of the Si core, whilst some shells were too thick to allow Li-ions to react with the Si core. Using Si-SiOx nanoparticles with the optimum SiOx:Si ratio of 18:82, the following electrochemical properties were obtained: a first reversible capacity of 932 mAh/g, an initial columbic efficiency (ICE) of 52.5%, and a capacity retention of 83.7% at the 100th cycle.

Fabrication of ternary silicon-carbon nanotubes-graphene composites by Co-assembly in evaporating droplets for enhanced electrochemical energy storage

Silicon (Si) particles are considered as promising anode electrode materials in lithium ion batteries (LIBs) because of high theoretical energy capacity of 3579 mA h/g. However, the large volume expansion of Si during the cycling process cause structural degradation and pulverization of the electrode, and this leads to solid electrolyte interphase (SEI) film reformation. In order to solve the problem, wrapping of Si particles with carbon materials is considered as an effective solution. In this study, we report ternary silicon core-carbon nanotubes winding-graphene shell (Si-CNTs-GR) nano-structured composites, which can cushion volume change of Si core particles by carbon nanotubes (CNTs) winding and protect SEI layer by graphene (GR) shell. The composites were fabricated using a facile aerosol process followed by heat-treament for anode materials of LIBs. The average particle size of ternary Si-CNTs-GR increased with respect to concentrations of CNTs at fixed other conditions. The as-prepared composites exhibited higher performance as LIB anodes such as capacity and coulombic efficiency than commercial Si particles or Si-GR composites. The ternary Si-CNTs-GR composites showed high capacity of 1700 mA h/g at 50 cycles.