Silicon-based Nanomaterials

Lithium-ion batteries are ubiquitous in our modern society, powering everything from cell phones, laptops, and power tools.They are also powering emerging applications such as electric vehicles and used for on-grid power stabilization. Lithium-ion batteries are a significant and growing part of this market due to their high specific energy. The worldwide market for lithium-ion batteries is projected to reach more than USD 9 billion by 2015. While lithium-ion batteries are often selected for their high specific energy, the market is demanding yet higher performance, usually in terms of energy stored per unit mass of battery. Many groups have recently turned their attention toward developing a silicon-based anode material to increase lithium-ion battery density. Silicon continues to draw great interest as an anode for lithium-ion batteries due to its large specific capacity as compared to the conventional graphite. Despite this exciting property, its practical use has been limited due to a large volume change associated with the insertion and extraction of lithium, which oftentimes leads to cracking and pulverization of the anode, limiting its cycle life. To overcome this problem, significant research has been focused toward developing various silicon nanostructures to accommodate the severe volume expansion and contraction. The structuring of the silicon often involves costly processing steps, limiting its application in price sensitive commercial lithium-ion batteries. To achieve commercial viability, work is being pursued on silicon battery anode structures and processes with a special emphasis on the cost and environment. In this review book chapter, we will summarize recent development of a cost-effective electrochemically etched porous silicon as an anode material for lithium-ion batteries. Briefly, the new approach involves creating hierarchical micron-and nanometer-sized pores on the surface of micron-sized silicon particulates, which are combined with an excellent conductor binder.

Silicon and germanium are among the most promising anode materials for high performance lithium ion batteries, due to their unprecedented high capacities. In recent few years, increasingly enormous efforts have been dedicated to these two important anodes, leading to significant improvement in their cycling life, practical capacity, rate capability, and coulombic efficiency. Nanostructuring is playing a crucial role in enabling the improvement and will lead to their widespread use in various battery markets. Nanoscale particles can better tolerate the wild volume change upon cycling and maintain their integrity than micron-sized particles. They can also shorten the diffusion distance of lithium ions and electrons and thus have high capacity. Further, one-dimensional nanowires exhibit superior stress behavior and electron transport. Porous and hierarchical nanostructures can provide extra space to accommodate the volume change. Wisely manipulating these handles have produced impressively better-performing systems. Porous single-crystal silicon nanowires have shown more stable capacity than solid nanowires. Hierarchical porous amorphous \(\mathrm{{GeO}}_\mathrm{x}\) is another system with very long cycle life and high capacity.

Light absorption is investigated in coaxial nanowires (NWs) of crystalline silicon (c-Si) cores and amorphous silicon (a-Si) shells, including both cases of single coaxial NWs and coaxial NW arrays, for an incident light spectrum of 1.0–4.0 eV covering the major solar band for photovoltaic cells. Based on the Lorenz-Mie light scattering theory for the single coaxial NWs and the rigorous coupled-wave analysis method for the coaxial NW arrays, it is found that the incident light is effectively trapped in the coaxial NWs through absorption resonances so that the light absorption of the coaxial NWs can be significantly enhanced compared to that of c-Si NWs. In the coaxial NWs, the absorption resonances occur due to their subwavelength dimensions, as in the c-Si NWs, whereas the absorption enhancement originates from the a-Si shells. By tuning their structural parameters, the light absorption in the coaxial NWs can be readily optimized for photovoltaic applications. At the optimal absorption conditions, the photocurrent in the coaxial NWs can be enhanced up to 560 % (single case) and 14 % (array case) compared to that in the c-Si NWs. The underlying physics of the light absorption in the coaxial NWs is discussed in terms of the excitation of leaky-mode resonances. The practical use of the coaxial NWs for photovoltaic cells is also addressed.

Nanostructured silicon thin-film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top–down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of \({\sim }{99}\,\%\) (upper bound) over wavelengths 400–1100 nm by a thickness of only \(5\,\upmu \mathrm{{m}}\), is realized by our recently reported technique self-powered parallel electron lithography that has high throughput and high resolution. High-efficiency (up to 10.8 %) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. Moreover, these ordered nano-structures have been successfully integrated into nano-electro-mechanical system (NEMS), enabling high-efficiency and broad-band optical actuation for NEMS devices. The first-ever nanopillar membrane acoustic speaker, using nano-scale photonic crystal optical absorbers for thermo-mechanical excitation of speaker membrane, is demonstrated.

A comprehensive method to study semiconductor nanoparticles in thin film SiO\(_{2}\) matrices has been developed. Selective and high-intensity synchrotron excitation allows the investigation of the nanoparticles energy structure. It is shown that the interference fringes affecting the optical excitation spectra of thin films may be neutralized by means of a special numerical technique. The spectral and kinetic properties of the Si, C, and SiC quantum dots (QD) formed by ion implantation in thin silica films were studied in details. Photoluminescence thermal quenching is shown to contain two stages and is dominated by Street law at low temperatures. Several indirect QD excitation mechanisms are realized, involving point defects, free, and self-trapped SiO\(_{2}\) matrix excitons. An exciton-assisted mechanism is dominating at helium temperatures. A resonant energy transfer mechanism taking place in the silica matrix reveals average defect-QD distance of 6–9 nm. A direct excitation channel is found only for carbon nanoclusters. An overall scheme of energy levels and optical transitions in the “matrix-cluster” system is proposed.

This chapter is focused on the study of the microstructural, optical, electrical, and electro-optical properties of Si-nanoparticles (Si-nps) embedded in a silica matrix for light emitting devices applications. Si-nps were created from silicon-rich oxide [SRO, (\(\mathrm{{SiO}}_\mathrm{x}\), \(\mathrm{x}<2\))] films which are deposited by low pressure chemical vapor deposition and followed by a thermal annealing at high temperature. The composition, microstructure, and optical properties of SRO films are analyzed as a function of the silicon excess and thermal annealing temperature. Once the properties of these materials are known, SRO films which exhibited the best photoluminescent (strongest PL) properties were chosen in order to analyze their electrical and electroluminescent (EL) properties. Simple Metal–Oxide–Semiconductor structures using the SRO films as the dielectric layer were fabricated for these studies. Blue and red EL was observed by changing the Si-np size from 1.5 to 2.7 nm embedded in the silica matrix, respectively. EL is ascribed to the charge injection into the Si-nps embedded in the SRO films through a balanced transport network. The EL emission is observed with the naked eye and in daylight conditions on the whole area of devices. Therefore, these results prove the feasibility to obtain LECs by using simple capacitors with SRO films as the active layer.

The electronic and optical properties of quasi-one-dimensional single-walled zigzag/armchair silicon-carbide nantotubes (SiC-NTs) as well as a two-dimensional SiC monolayer are investigated by ab initio methods. In order to elucidate many-electron effects on SiC nanosystems, we apply the ab initio many-body Green’s function approach to calculate the quasiparticle and optical properties of SiC nanostructures. The significant band gap correction, more than 1 eV, to the Kohn-Sham gap of density functional theory within the local density approximation of semiconducting SiC-NTs and a SiC monolayer is mainly due to the many-electron interaction effect, which is included in the GW approximation for the electron self energy. Furthermore, taking into account electron-hole interaction, the optical spectra of SiC-NTs are calculated by solving the Bethe-Salpeter equation (BSE) for the electron-hole amplitudes. Our GW+BSE calculations reveal the presence of excitons with a large binding energy as well as strong anisotropy in the optical properties in the low-dimensional SiC systems. The characteristics of the strongly bound electron-hole pairs or excitons in SiC nanostructures are also discussed in terms of the corresponding excitonic wavefunctions.

Silicon carbide is one of the promising materials for the fabrication of various one- and two-dimensional nanostructures. In this chapter, we discuss experimental and theoretical studies of the plasma-enabled fabrication of silicon carbide quantum dots, nanowires, and nanorods. The discussed fabrication methods include plasma-assisted growth with and without anodic aluminium oxide membranes and with or without silane as a source of silicon. In the silane-free experiments, quartz was used as a source of silicon to synthesize the silicon carbide nanostructures in an environmentally friendly process. The mechanism of the formation of nanowires and nanorods is also discussed.

Chemical vapor deposition hydrogen reduction of methyltrichlorosilane (MTS) is most prominent method for production of silicon carbide (SiC) nanowires with controlled morphology. In a typical SiC nanowire synthesis process the cracking of MTS is carried out in reducing atmosphere of hydrogen using chemical vapor deposition technique at high temperature and normal atmospheric pressure. Taguchi method is very useful to design experiments specially in the cases where large numbers of variables are to be considered. This statistical method has been used to design the experiments to get the optimum parameters for bulk production of silicon carbide wires of uniform diameter in nanometer range. Further the effect of different parameters on the morphology of SiC deposit has been discussed. XRD and SEM analysis showed the growth of crystalline \(\upbeta \)-SiC wires having different morphology. From the analysis of variance (ANOVA) of data it has been observed that growth temperature and hydrogen to MTS ratio in carrier gas are the two important parameters which decide the final growth morphology of SiC deposition. At higher temperature (\(\ge \)1400 \(^{\circ }\)C), the SiC nuclei prefer to grow as SiC grains rather than wires. The optimum deposition conditions have been obtained by analyzing Signal to Noise (S/N) ratio corresponding to lowest deposition rate and minimum growth diameter of SiC wires. The optimum deposition conditions have been used for uniform diameter growth of SiC nanowires, smoothness of the surface, and homogeneous growth of SiC on the surface. It has been observed that the hydrogen to MTS flow rate ratio value should be above 20 for the growth of SiC wires of nanometer diameter. The deposition temperature for the growth of crystalline SiC wires should be 1100–1300 \(^{\circ }\)C. The total flow rate of carrier gas comprising of argon and hydrogen should be in moderate range for particular hydrogen to MTS ratio. The effect of H\(_{2}\)/MTS mole ratio on morphology of SiC deposition by varying H\(_{2}\)/MTS mole ratio from 0 to \(\sim \)80 has been discussed in detail. This detail process study has given a new perspective to produce SiC nanowires of high purity and homogeneous diameter by a simple atmospheric pressure CVD method without using a metallic catalyst. Even manipulation of growth parameters can be done to get desired morphology of SiC deposit.

The ternary nanocomposite material Si–C–N was first introduced to the scientific community as a high temperature oxidation resistant polymer-derived ceramic (PDCs).

The effects of vacancy defects and boron-nitrogen dopants on electron transport properties of SiCNT are investigated. The results of geometry optimization of vacancy defects show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Analysis of frontier molecular orbitals (FMO) and transmission spectrum show that the introduction of vacancy reduces the bandgap of (5,5) semiconducting SiCNTs and increases the bandgap of (4,0) conducting SiCNTs (converts them to semi-metallic nanotubes). Bias voltage-dependent current characteristics show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT which is due to introduction of new electronic states around the Fermi level followed by conduction through the defect sites. The results of the study on the effect of BN co-doping in SiCNTs suggests that co-doping BN impurities suppresses the important negative differential resistance (NDR) property. NDR suppression is attributed to the introduction of new electronic states near the Fermi level followed by weak orbital localization. BN co-doping results in exponential current-voltage (I–V) characteristics which is in contrast to linear I–V characteristics for individual boron and nitrogen doped SiCNTs. HOMO has no contribution from B impurity, whereas LUMO has contribution from N impurity at low and high bias.

One-dimensional (1D) nanostructures of transition metal silicide (TMS) have attracted more attention due to their unique properties and potential applications in microelectronics. A variety of synthetic approaches were developed to fabricate 1D TMS nanostructures. Chemical vapor deposition (CVD) is the most widely used method to synthesize 1D TMS nanostructures. Various precursors and growth mechanisms are involved in the CVD processes. Other methods such as chemical vapor transport (CVT) method, silicidation method, reactive epitaxial method, hydrothermal method, were also successfully employed for the formation of 1D TMS nanostructures. Electrical transport measurements reveal that many TMS nanowires exhibit metallic behavior with extremely high conductivity. At the same time, semiconducting behaviors were observed in some situations. Some silicide nanowires (silicides of Fe, Co, Ni, Cr, Mn, etc.) have ferromagnetic properties, even at room temperature. Unique properties such as field-emission, optical, thermoelectric, mechanical were also investigated in the 1D TMS nanostructures. Based on these properties, 1D TMS nanostructures were utilized in microelectronic devices, lithium ion batteries, memory device and capacitor, etc.

The great interest in the implementation of GaAs quantum nanostructures (QNs) on silicon substrates is mainly due to the possibility of integrating specialized high efficiency optoelectronic and photonic devices on the existing complementary metal-oxide semiconductor technology developed on Si. This would allow the realization of specialized III–V devices such as nanoemitters and intersubband detectors directly embedded with a large number of existing Si devices. Of particular, technological interest is the possibility of carrying out the III–V device fabrication after the integrated circuit has been already realized, i.e., as a back-end process. In this case, the compatibility with the underlying integrated circuit is possible only imposing strict constraints on thermal budget for growth and processing of the epilayer.

The integration of III–V photonics materials and devices with Si microelectronics will enable the fabrication of complex optoelectronic circuits, which will permit the creation of the long-dreamed chip-to-chip and system-to-system optical communications. Direct epitaxial growth of semiconductor III–V compounds on Si substrates is one of the most promising candidates for the fabrication of photonics devices on the Si platform. III–V quantum dots (QDs) offer an attractive alternative to conventional quantum wells(QWs) for building III–V lasing devices on a Si platform due to their unique advantages. We developed the long-wavelength InAs/GaAs QD materials and devices monolithically grown on Si, Ge, and Ge-on-Si substrates by the use of Molecular Beam Epitaxy. Room-temperature(RT) lasing at a wavelength of around 1.3 \(\upmu \)m has been achieved with threshold current densities of 64.3 A/cm\(^{2}\) and lasing operation up to 83\(\,^{\circ }\mathrm{{C}}\) for Si-based ridge-waveguide InAs/GaAs QD lasers with as-cleaved facets. The optical and electrical properties of InAs/GaAs QDs grown on Si substrates were further investigated to evaluate the potential for Si-based photodiodes. A peak responsivity of 5 mA/W was observed at 1.28 \(\upmu \)m, while the dark current was two orders of magnitude lower than those reported for Ge-on-Si photodiodes. These studies ultimately form the basis for the monolithic integration of 1.3-\(\upmu \)m InAs/GaAs QD lasers and detectors on the Si platform.

In this chapter we demonstrate the growth and characterization of nonpolar relaxed cubic GaN by plasma-assisted molecular beam epitaxy on prepatterned 3C-SiC/Si (001) substrates. Nanopatterning of 3C-SiC/Si (001) was achieved by two different fabrication techniques: nanosphere lithography (NSL) to generate large-area pattern, and conventional electron beam lithography (EBL) for tailoring particular surface morphologies. Both methods were followed by a lift-off and a reactive ion etching (RIE) process. We analyze the influence of the substrate on the GaN growth and show that it is possible to grow single phase and defect-reduced cubic GaN crystals on 3C-SiC nanostructures. Furthermore cubic GaN/AlN multiquantum wells were grown on 3C-SiC nanostructures, which is a further step toward nanoscaled device applications.