Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype

Hexagonal-Ge Nanostructures with Direct-Bandgap Emissions in a Si-Based Light-Emitting Metasurface

Si-based emitters have been of great interest as an ideal light source for monolithic optical-electronic integrated circuits (MOEICs) on Si substrates. However, the general Si-based material is a diamond structure of cubic lattice with an indirect band gap, which cannot emit light efficiently. Here, hexagonal-Ge (H-Ge) nanostructures within a light-emitting metasurface consisting of a cubic-SiGe nanodisk array are reported. The H-Ge nanostructure is naturally formed within the cubic-Ge epitaxially grown on Si (001) substrates due to the strain-induced nanoscale crystal structure transformation assisted by far-from-equilibrium growth conditions. The direct-bandgap features of H-Ge nanostructures are observed and discussed, including a rather strong and linearly power-dependent photoluminescence (PL) peak around 1562 nm at room temperature and temperature-insensitive PL spectrum near room temperature. Given the direct-bandgap nature, the heterostructure of H-Ge/C-Ge, and the compatibility with the sophisticated Si technology, the H-Ge nanostructure has great potential for innovative light sources and other functional devices, particularly in Si-based MOEICs.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Recrystallization of Si Nanoparticles in Presence of Chalcogens: Improved Electrical and Optical Properties

Nanocrystals of Si doped with S, Se and Te were synthesized by annealing them in chalcogen vapors in a vacuum at a high temperature range from 800 to 850 °C. The influence of the dopant on the structure and morphology of the particles and their optical and electrical properties was studied. In the case of all three chalcogens, the recrystallization of Si was observed, and XRD peaks characteristic of noncubic Si phases were found by means of electronic diffraction for Si doped with S and Se. Moreover, in presence of S and Te, crystalline rods with six-sided and four-sided cross-sections, respectively, were formed, their length reaching hundreds of μm. Samples with sulfur and selenium showed high conductivity compared to the undoped material.

Atomistic deformation mechanism of silicon under laser-driven shock compression

Silicon (Si) is one of the most abundant elements on Earth, and it is the most widely used semiconductor. Despite extensive study, some properties of Si, such as its behaviour under dynamic compression, remain elusive. A detailed understanding of Si deformation is crucial for various fields, ranging from planetary science to materials design. Simulations suggest that in Si the shear stress generated during shock compression is released via a high-pressure phase transition, challenging the classical picture of relaxation via defect-mediated plasticity. However, direct evidence supporting either deformation mechanism remains elusive. Here, we use sub-picosecond, highly-monochromatic x-ray diffraction to study (100)-oriented single-crystal Si under laser-driven shock compression. We provide the first unambiguous, time-resolved picture of Si deformation at ultra-high strain rates, demonstrating the predicted shear release via phase transition. Our results resolve the longstanding controversy on silicon deformation and provide direct proof of strain rate-dependent deformation mechanisms in a non-metallic system.

Understanding the how silicon deforms under pressure is important for several fields, including planetary science and materials design. Laser-driven shock compression experiments now confirm that shear stress generated during compression is released via a high-pressure phase transition.

Phase stability and the interface structure of a nanoscale Si crystallite in Al-based alloys

An atomic-scale understanding of the role of strain on the microstructural properties of nanoscale precipitates will be helpful to explore the precipitation behavior as well as the structure-property relationships in crystalline multi-phase systems. Nanoscale Si precipitates are formed in Al-based alloys prepared by selective laser melting. The phase structure and the nature of heterointerface have been characterized using advanced electron microscopy. The nanocrystalline Si mainly contains two polymorphs, diamond-cubic Si (DC-Si) and 4H hexagonal Si (4H-Si). Heteroepitaxy occurs at the DC-Si(111)/Al(100) and 4H-Si(0001)/Al(100) interfaces in terms of a coincidence-site lattice model. The nanocrystalline Si undertakes tensile strain superposed by the matrix through heterointerfaces, facilitating the formation of 4H-Si in the nanoscale crystallite, which provides a strategy for designing Si polymorphic materials by strain engineering.

The amorphous-crystalline transition in SinH2m nanoclusters

Silicon nanocrystals (NCs) have great potential for applications in optoelectronics, photovoltaics and biomedicine. The photo-physical characteristics of these particles strongly depend on whether they are crystalline or amorphous. This structural order is sensitive to the synthesis details. To understand the morphology of hydrogen-passivated silicon clusters and find how it depends on the passivation degree, we calculated the optimal structures of Si_nH_2m clusters with n ≤ 21 and 2m ≤ 30. We found that as the hydrogen amount increases, clusters run through three structural types: (i) amorphous clusters with dangling bonds (DBs), (ii) amorphous clusters without DBs at intermediate passivation, and (iii) crystalline clusters. We describe a mechanism which removes dangling bonds in the amorphous clusters of the second type and shows its key importance for cluster structure formation. The crystalline lattice (diamond or lonsdaleite) is found to emerge when all broken bonds at the NC surface are passivated. We constructed the phase P-T diagram of Si-H clusters, compared it with the available experimental data and discussed the transfer of our results to large Si nanoparticles.

A straightforward approach to high purity sodium silicide Na4Si4

Sodium silicide Na₄Si₄ is a reductive and reactive source of silicon highly relevant to designing non-oxidic silicon materials, including clathrates, various silicon allotropes, and metal silicides. Despite the importance of this compound, its production in high amounts and high purity is still a bottleneck with reported methods. In this work, we demonstrate that readily available silicon nanoparticles react with sodium hydride with a stoichiometry close to the theoretical one and at a temperature of 395 °C for shorter duration than previously reported. This enhanced reactivity of silicon nanoparticles makes the procedure robust and less dependent on experimental parameters, such as gas flow. As a result, we deliver a procedure to achieve Na₄Si₄ with purity of ca. 98 mol% at the gram scale. We show that this compound is an efficient precursor to deliver selectively type I and type II sodium silicon clathrates depending on the conditions of thermal decomposition.

Hexagonal Si-Ge Class of Semiconducting Alloys Prepared by Using Pressure and Temperature

Multi-anvil and laser-heated diamond anvil methods have been used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500-1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery at ambient pressure of hexagonal Ge-Si solid solutions (P6₃ /mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be prepared for all Ge-Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge-Si solid solution family, and is of interest for optoelectronic applications.

In Situ High-Pressure Synthesis of New Outstanding Light-Element Materials under Industrial P-T Range

High-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the "large volume" presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under "traditional" industrial pressure range (3-8 GPa). We illustrated the power of such methodology by (i) two classical examples of "reference" superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years.

Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition

We report the synthesis of bulk, highly oriented, crystalline 4H hexagonal silicon (4H-Si), through a metastable phase transformation upon heating the single-crystalline Si_{24} allotrope. Remarkably, the resulting 4H-Si crystallites exhibit an orientation relationship with the Si_{24} crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that 4H-Si exhibits an indirect band gap near 1.2 eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline 4H-Si in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.

Single-crystal synthesis and properties of the open-framework allotrope Si24

Si₂₄ is a new, open-framework silicon allotrope that is metastable at ambient conditions. Unlike diamond cubic silicon, which is an indirect-gap semiconductor, Si₂₄ has a quasidirect gap near 1.4 eV, presenting new opportunities for optoelectronic and solar energy conversion devices. Previous studies indicate that Na can diffuse from micron-sized grains of a high-pressure Na₄Si₂₄ precursor to create Si₂₄ powders at ambient conditions. Remarkably, we demonstrate here that Na remains highly mobile within large (~100 µm) Na₄Si₂₄ single crystals. Na readily diffuses out of Na₄Si₂₄ crystals under vacuum with gentle heating (10^-4 mbar at 125 °C) and can be further reacted with iodine to produce large Si₂₄ crystals that are 99.9985 at% silicon, as measured by wavelength-dispersive x-ray spectroscopy. Si₂₄ crystals display a sharp, direct optical absorption edge at 1.51(1) eV with an absorption coefficient near the band edge that is demonstrably greater than diamond cubic silicon. Temperature-dependent electrical transport measurements confirm the removal of Na from metallic Na₄Si₂₄ to render single-crystalline semiconducting samples of Si₂₄. These optical and electrical measurements provide insights into key parameters such as the electron donor impurity level from residual Na, reduced electron mass, and electron relaxation time. Effective Na removal on bulk length scales and the high absorption coefficient of single-crystal Si₂₄ indicate promise for use of this material in bulk and thin film forms with potential applications in optoelectronic technologies.

Direct-bandgap emission from hexagonal Ge and SiGe alloys

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal¹ of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades^2-6. Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III-V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.

Quasi-hydrostatic equation of state of silicon up to 1 megabar at ambient temperature

The isothermal equation of state of silicon has been determined by synchrotron x-ray diffraction experiments up to 105.2 GPa at room temperature using diamond anvil cells. A He-pressure medium was used to minimize the effect of uniaxial stress on the sample volume and ruby, gold and tungsten pressure gauges were used. Seven different phases of silicon have been observed along the experimental conditions covered in the present study.

Advances in the experimental exploration of water's phase diagram

Water's phase diagram displays enormous complexity with currently 17 experimentally confirmed polymorphs of ice and several more predicted computationally. For almost 120 years, it has been a stomping ground for scientific discovery, and ice research has often been a trailblazer for investigations into a wide range of materials-related phenomena. Here, the experimental progress of the last couple of years is reviewed, and open questions as well as future challenges are discussed. The specific topics include (i) the polytypism and stacking disorder of ice I, (ii) the mechanism of the pressure amorphization of ice I, (iii) the emptying of gas-filled clathrate hydrates to give new low-density ice polymorphs, (iv) the effects of acid/base doping on hydrogen-ordering phase transitions as well as (v) the formation of solid solutions between salts and the ice polymorphs, and the effect this has on the appearance of the phase diagram. In addition to continuing efforts to push the boundaries in terms of the extremes of pressure and temperature, the exploration of the "chemical" dimensions of ice research appears to now be a newly emerging trend. It is without question that ice research has entered a very exciting era.