Lorenzo Mangolini

Synthesis, properties, and applications of silicon nanocrystals

Silicon nanocrystals have been widely investigated for several years because of their many interesting properties and their potential use in several applications. This field has grown enormously after the observation of quantum confinement in porous silicon and remains an area of great interest for different reasons. Most importantly, silicon is already widely used in the semiconductor industry, is nontoxic at least in its bulk form, is the second most earth-abundant element in the crust, and is relatively cheap to process. A large number of groups have investigated silicon in the form of nanocrystals, and the authors intend to provide a comprehensive review of their contribution to the field. The author has decided to address first the synthesis and properties of silicon nanocrystals. Several different techniques, such as nucleation in substoichiometric thin films or gas-phase nucleation and growth in silane-containing nonthermal plasmas, have been proposed for the controlled synthesis of silicon nanoparticles. The author outlines the strengths and weaknesses of each approach and identify the research groups that have advanced each particular synthesis technique. The understanding of the properties of silicon nanocrystals has evolved as new synthetic approaches were developed, and for that reason the material properties are discussed together with its production approach. The use of silicon nanocrystals for the development of novel electronic devices, light emitting devices, photovoltaic cells, and for biorelated applications will be discussed. Waste heat recovery and energy storage applications are also discussed.

Silicon nanocrystal production through non-thermal plasma synthesis: a comparative study between silicon tetrachloride and silane precursors

Silicon nanocrystals with sizes between 5 and 10xa0nm have been produced in a non-thermal plasma reactor using silicon tetrachloride as precursor. We demonstrate that high-quality material can be produced with this method and that production rates as high as 140xa0mgxa0h(-1) can be obtained, with a maximum precursor utilization rate of roughly 50%. Compared to the case in which particles are produced using silane as the main precursor, the gas composition needs to be modified and hydrogen needs to be added to the mixture to enable the nucleation and growth of the powder. The presence of chlorine in the system leads to the production of nanoparticles with a chlorine terminated surface which is significantly less robust against oxidation in air compared to the case of a hydrogen terminated surface. We also observe that significantly higher power input is needed to guarantee the formation of crystalline particles, which is a consequence not only of the different gas-phase composition, but also of the influence of chlorine on the stability of the crystalline structure.

Colloidal Synthesis of Silicon–Carbon Composite Material for Lithium-Ion Batteries

We report colloidal routes to synthesize silicon@carbon composites for the first time. Surface-functionalized Si nanoparticles (SiNPs) dissolved in styrene and hexadecane are used as the dispersed phase in oil-in-water emulsions, from which yolk-shell and dual-shell hollow SiNPs@C composites are produced via polymerization and subsequent carbonization. As anode materials for Li-ion batteries, the SiNPs@C composites demonstrate excellent cycling stability and rate performance, which is ascribed to the uniform distribution of SiNPs within the carbon hosts. The Li-ion anodes composed of 46u2005wtu2009% of dual-shell SiNPs@C, 46u2005wtu2009% of graphite, 5u2005wtu2009% of acetylene black, and 3u2005wtu2009% of carboxymethyl cellulose with an areal loading higher than 3u2005mgu2009cm-2 achieve an overall specific capacity higher than 600u2005mAhu2009g-1 , which is an improvement of more than 100u2009% compared to the pure graphite anode. These new colloidal routes present a promising general method to produce viable Si-C composites for Li-ion batteries.

Hollow silicon carbide nanoparticles from a non-thermal plasma process

We demonstrate the synthesis of hollow silicon carbide nanoparticles via a two-step process involving the non-thermal plasma synthesis of silicon nanoparticles, followed by their in-flight carbonization, also initiated by a non-thermal plasma. Simple geometric considerations associated with the expansion of the silicon lattice upon carbonization, in combination of the spherical geometry of the system, explain the formation of hollow nanostructures. This is in contrast with previous reports that justify the formation of hollow particles by means of out-diffusion of the core element, i.e., by the Kirkendall nanoscale effect. A theoretical analysis of the diffusion kinetics indicates that interaction with the ionized gas induces significant nanoparticle heating, allowing for the fast transport of carbon into the silicon particle and for the subsequent nucleation of the beta-silicon carbide phase. This work confirms the potential of non-thermal plasma processes for the synthesis of nanostructures composed of ...

Core/shell silicon/polyaniline particles via in-flight plasma-induced polymerization

Although silicon nanoparticles have potential applications in many relevant fields, there is often the need for post-processing steps to tune the property of the nanomaterial and to optimize it for targeted applications. In particular surface modification is generally necessary to both tune dispersibility of the particles in desired solvents to achieve optimal coating conditions, and to interface the particles with other materials to realize functional heterostructures. In this contribution we discuss the realization of core/shell silicon/polymer nanoparticles realized using a plasma-initiated in-flight polymerization process. Silicon particles are produced in a non-thermal plasma reactor using silane as a precursor. After synthesis they are aerodynamically injected into a second plasma reactor into which aniline vapor is introduced. The second plasma initiates the polymerization reactor leading to the formation of a 3–4 nm thick polymer shell surrounding the silicon core. The role of processing conditions on the properties of the polymeric shell is discussed. Preliminary results on the testing of this material as an anode for lithium ion batteries are presented.

Grain-to-Grain Compositional Variations and Phase Segregation in Copper–Zinc–Tin–Sulfide Films

We have performed a rigorous investigation of the structure and composition of individual grains in copper-zinc-tin-sulfide (CZTS) films realized by sulfurization of a sputtered metal stack. Although on average close to the ideal CZTS stoichiometry, elemental analysis shows significant grain-to-grain variations in composition. High-resolution Raman spectroscopy indicates that this is accompanied by grain-to-grain structural variations as well. The intensity from the 337 cm(-1) Raman peak, generally assigned to the kesterite phase of CZTS, remains constant over a large area of the sample. On the other hand, signals from secondary phases at 376 cm(-1) (copper-tin-sulfide) and 351 cm(-1) (zinc-sulfide) show significant variation over the same area. These results confirm the great complexity inherent to this material system. Moreover, structural and compositional variations are recognized in the literature as a factor limiting the efficiency of CZTS photovoltaic devices. This study demonstrates how a seemingly homogeneous CZTS thin film can actually have considerable structural and compositional variations at the microscale, and highlights the need for routine microscale characterization in this material system.

Tin nanoparticles as an effective conductive additive in silicon anodes.

We have found that the addition of tin nanoparticles to a silicon-based anode provides dramatic improvements in performance in terms of both charge capacity and cycling stability. Using a simple procedure and off-the-shelf additives and precursors, we developed a structure in which the tin nanoparticles are segregated at the interface between the silicon-containing active layer and the solid electrolyte interface. Even a minor addition of tin, as small as ∼2% by weight, results in a significant decrease in the anode resistance, as confirmed by electrochemical impedance spectroscopy. This leads to a decrease in charge transfer resistance, which prevents the formation of electrically inactive “dead spots” in the anode structure and enables the effective participation of silicon in the lithiation reaction.

Graphitization of Carbon Particles in a Non-thermal Plasma Reactor

The production of nanomaterials using non-thermal plasmas remains the focus of ongoing investigations due to advantageous properties of this class of processes, most notably the intense plasma-induced heating arising from energetic recombination events occurring at the surface of nanoparticles, which allows for the tailored synthesis of crystalline nanoparticles. In this work, the authors discuss an in situ, in-flight Fourier Transform Infrared absorption spectroscopy technique to investigate the temperature variation of carbon nanoparticles during their synthesis in an acetylene–argon–hydrogen non-thermal RF plasma. Based on the FTIR measurements, decreasing hydrogenation levels and the progressive onset of an incandescence signal were observed at increasing RF input power. In the high RF power region, the carbon particle temperature, derived by fitting the corresponding FTIR spectra with a modified Planck’s law, shows values above 2000xa0K. The corresponding ex situ characterization of the synthesized materials by Transmission Electron Microscopy and Raman Spectroscopy displays the production of highly graphitic particles and loss of bonded hydrogen from the material, hence supporting the substantial nanoparticle heating derived from the FTIR measurements.

Thermoelectric performance of silicon with oxide nanoinclusions

ABSTRACT Silicon nanoparticles produced via a plasma-based technique have been sintered into bulk nanostructured samples. These samples have micron-sized crystalline domains and contain well-dispersed oxide nanoinclusions. We have compared the thermoelectric performance of such structure to that of a control sample produced by sintering ball-milled silicon powders. The control sample has lower precipitate density and is composed of nanograins. Despite the stark difference in nanostructure, both samples have comparable thermal conductivity, and the sample with nanoinclusions has higher power factor and ZT. This result confirms that grain size engineering is not the only promising route to achieving improved thermoelectric performance. GRAPHICAL ABSTRACT IMPACT STATEMENT By controlling the feedstock powder processing technique, it is possible to obtain well-dispersed nanoinclusions in sintered bulk samples. These are as effective at reducing thermal transport properties as grain boundaries.

Silicon-carbon composites for lithium-ion batteries: A comparative study of different carbon deposition approaches

Silicon-carbon composites, usually in the form of core–shell silicon-carbon nanostructures, have been widely investigated as potential candidates for the replacement of graphite in anodes for lithium ion batteries. Due to the availability of a broad range of precursors and protocols for the realization of a carbon shell, research groups active in this area have typically developed their own strategy to manufacture the desired structure. This is problematic since it does not allow for a direct comparison of the performance of similar structures during electrochemical cycling, and it does not provide a mechanistic insight into the factors affecting battery performance. In this work, the authors address this issue by directly comparing core–shell silicon-carbon nanostructures in which the carbon shell is achieved by carbonization of common polymers or by chemical vapor deposition (CVD) using acetylene as precursor. The samples have been prepared using exactly the same type of silicon particles as the active ...