Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders

Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization

Si nanoparticles (NPs) have been actively developed as a hyperpolarized magnetic resonance imaging (MRI) contrast agent with an imaging window close to one hour. However, the progress in the development of NPs has been hampered by the incomplete understanding of their structural properties that correspond to efficient hyperpolarization buildup and long polarization decays. In this work we study dynamic nuclear polarization (DNP) of single crystal porous Si (PSi) NPs with defined doping densities ranging from nominally undoped to highly doped with boron or phosphorus. To develop such PSi NPs we perform low-load metal-assisted catalytic etching for electronic grade Si powder followed by thermal oxidation to form the dangling bonds in the Si/SiO2 interface, the Pb centers. Pb centers are the endogenous source of the unpaired electron spins necessary for DNP. The controlled fabrication and oxidation procedures allow us to thoroughly investigate the impact of the magnetic field, temperature and doping on the DNP process. We argue that the buildup and decay rate constants are independent of size of Si crystals between approximately 10 and 60 nm. Instead, the rates are limited by the polarization transfer across the nuclear spin diffusion barrier determined by the large hyperfine shift of the central 29Si nuclei of the Pb centers. The size-independent rates are then weakly affected by the doping degree for low and moderately doped Si although slight doping is required to achieve the highest polarization. Thus, we find the room temperature relaxation of low boron doped PSi NPs reaching 75 ± 3 minutes and nuclear polarization levels exceeding ∼6% when polarized at 6.7 T and 1.4 K. Our study thus establishes solid grounds for further development of Si NPs as hyperpolarized contrast agents.

Facile Redox Synthesis and Surface Engineering of Porous Silicon from Zintl Compound for High-Performance Lithium Ion Battery Anodes

Porous silicon (pSi) is considered a promising candidate for next-generation high-energy-density lithium-ion battery (LIB) anodes due to its ability to mitigate volume expansion stress. However, the lack of efficient preparation methods and severe side reactions due to its large specific surface area have hindered its commercial development. This study leverages the redox reaction between the Zintl compound Mg2Si and SiO2 at certain temperatures, using intermediate products as templates, and incorporates CVD deposition to create carbon-coated porous silicon (pSi@C) composite anode materials with excellent electrochemical performance. This approach enables pSi to achieve a high specific capacity, high rate performance, and long lifetime. Additionally, a prelithiation process effectively addresses the issue of low initial Coulombic efficiency (ICE) in pSi electrodes. In half-cell tests, the pSi@C electrode delivered a reversible specific capacity as high as 1500 mAh g-1 and outstanding rate performance (over 500 mAh g-1 at a high current density of 5 A g-1). After repeated charge/discharge 1000 times at 1 A g-1, the reversible capacity remained at 555 mAh g-1. Full-battery assembly with NCM811 cathodes also demonstrated the potential of pSi@C as a promising anode candidate. This work aims to expand the preparation methods for pSi materials and provide guidance for their application in high-energy-density LIBs.

Electron Injection in Metal Assisted Chemical Etching as a Fundamental Mechanism for Electroless Electricity Generation

Metal-assisted chemical etching (MACE) is a widely applied process for fabricating Si nanostructures. As an electroless process, it does not require a counter electrode, and it is usually considered that only holes in the Si valence band contribute to the process. In this work, a charge carrier collecting p-n junction structure coated with silver nanoparticles is used to demonstrate that also electrons in the conduction band play a fundamental role in MACE, and enable an electroless chemical energy conversion process that was not previously reported. The studied structures generate electricity at a power density of 0.43 mW/cm² during MACE. This necessitates reformulating the microscopic electrochemical description of the Si-metal-oxidant nanosystems to separately account for electron and hole injections into the conduction and valence band of Si. Our work provides new insight into the fundamentals of MACE and demonstrates a radically new route to chemical energy conversion by solar cell-inspired devices.

Deep Etching of Silicon Based on Metal-Assisted Chemical Etching

A deep etching method for silicon "micro"structures was successfully developed. This wet etching process is based on metal-assisted chemical etching (MACE), which was previously mainly utilized to etch the features that have lateral dimensions of "nanometers." In this novel MACE, the critical improvement was to promote the "out-of-plane" mass transfer at the metal/Si interface with an ultrathin metal film. This enabled us to etch micrometer-wide holes, which was previously challenging due to the mass transport limitation. In addition, it was found that when ethanol was used as a solvent instead of water, the formation of porous defects was suppressed. Under the optimized etch conditions, deep (>200 μm) and vertical (>88°) holes could be carved out at a fast etch rate (>0.4 μm/min). This novel deep MACE will find utility in applications such as microelectromechanical systems (MEMS) devices or biosensors.

Tailoring porous/filament silicon using the two-step Au-assisted chemical etching of p-type silicon for forming an ethanol electro-oxidation layer

In this paper, we are reporting on the fabrication of a porous silicon/Au and silicon filament/Au using the two-step Au-assisted chemical etching of p-type Si with a specific resistivity of 0.01, 1, and 12 Ω·cm when varying the Au deposition times. The structure analysis results show that with an increasing Au deposition time of up to 7 min, the thickness of the porous Si layer increases for the same etching duration (60 min), and the morphology of the layer changes from porous to filamentary. This paper shows that the uniform macro-porous layers with a thickness of 125.5-171.2μm and a specific surface area of the mesopore sidewalls of 142.5-182 m²·g^-1are formed on the Si with a specific resistivity of 0.01 Ω·cm. The gradient macro-porous layers with a thickness of 220-260μm and 210-290μm, the specific surface area of the mesopore sidewalls of 3.7-21.7 m²·g^-1and 17-29 m²·g^-1are formed on the silicon with a specific resistivity of 1 and 12 Ω·cm, respectively. The por-Si/Au has excellent low-temperature electro oxidation performance with ethanol, the activity of ethanol oxidation is mainly due to the synergistic effect of the Au nanoparticles and porous Si. The formation mechanism of the uniform and gradient macro-porous layers and ethanol electro-oxidation on the porous/filament silicon, decorated with Au nanoparticles, was established. The por-Si/Au structures with perpendicularly oriented pores, a high por-Si layer thickness, and a low mono-Si layer thickness (with a specific resistivity of 1 Ω·cm) are optimal for an effective ethanol electro-oxidation, which has been confirmed with chronoamperometry measurements.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.