Hexagonal Diamond Research Articles

Transferable Percolated Particle Monolayers Derived from Marangoni Flows for Thermally Conductive Dielectric Coatings.

Thermal management is becoming one of the most significant design and size limitations for high power density electronics, including motherboards, power converters, and phased array antennas for 5G communications. There are few options for conducting heat away with dielectric materials that avoid shortening or distorting the performance of these electronics. Certain highly thermally conductive 2D and 3D materials, including hexagonal boron nitride and diamond, offer ideal material properties to address these issues but are extremely challenging to process. This work studies highly oriented single-particle thick films of hexagonal boron nitride, manufactured through a modified Langmuir-Blodgett process and densified further using the Marangoni effect to attain remarkable thermal conductivity enhancement with minimal coating thickness. High loadings of hexagonal boron nitride (∼60 vol %) in dense, castable films are also produced to compare thermal spreading ability in a comparatively simpler but less-coordinated percolated system to the highly percolated particle monolayers. These procedures were applied to glass fiber reinforced polymers used in aerospace and radome applications as well as to a single-board computer to demonstrate enhanced thermal management.

Pure Hexagonal Diamond with Symmetry-Doping Properties.

The difficulties in asymmetrical doping of wide band gap materials, especially for the n-type diamond challenge, hamper their application in electronic devices. Here, we propose doping diamond polytypes with reasonable band structures to solve this problem. Various elements are doped into six diamond polytypes, and their doping behaviors are investigated. The results show that pure hexagonal (2H) diamond has the band gap with a value of 4.42 eV and the smallest carrier effective masses among six calculated diamond polytypes, exhibiting high electron mobility and hole mobility up to 4250 and 5840 cm2·V-1·s-1, respectively. Compared to cubic (3C) diamond, the impurity formation energy in 2H-diamond is reduced, which is attributed to its C3v symmetry. More importantly, 2H-diamond exhibits favorable doping symmetry with a donor level of 0.14 eV for phosphorus and an acceptor level of 0.19 eV for boron, respectively, which originate from smaller effective masses and stronger delocalization at the band edge in 2H-diamond. These ionization energies are smaller than those in 3C-diamond of 0.32 and 0.58 eV, respectively. This reveals that phosphorus and boron in 2H-diamond are more easily excited at room temperature, producing good n- and p-type conductivities. All of these make 2H-diamond a potential candidate for the replacement of 3C-diamond as a wide band gap material theoretically. These provide a way to realize high-quality n-type diamond, giving significant insights into the realization of diamond-based electronic devices. This also supplies a solution for the difficulties in asymmetric doping of other wide band gap materials.

The Transformation Mechanism of Graphite to Hexagonal Diamond under Shock Conditions.

The formation of a hexagonal diamond represents one of the most intriguing questions in materials science. Under shock conditions, the graphite basal plane tends to slide and pucker to form diamond. However, how the shock strength determines the phase selectivity remains unclear. In this work, using a DFT-trained carbon global neural network model, we studied the shock-induced graphite transition. The poor sliding caused by scarce sliding time under high-strength shock leads to metastable hexagonal diamond with an orientation relationship of (001)G//(100)HD+[010]G//[010]HD, while under low-strength shock due to long sliding distance cubic diamond forms with the orientation (001)G//(111)CD+[100]G//[110]CD, unveiling the strength-dependent graphite transition mechanism. We for the first time provide computational evidence of the strength-dependent graphite transition from first-principles, clarifying the long-term unresolved shock-induced hexagonal diamond formation mechanism and the structural source of the strength-dependent trend, which facilitates the hexagonal diamond synthesis via controlled experiment.

Shock-induced phase transition and damage in nano-polycrystalline graphite affected by grain boundaries

Dynamic structural response of nano-polycrystalline graphite under shock compression is investigated using molecular dynamics (MD) simulations. Hugoniot data shows that the structural transition is activated at shock pressure P∼30 GPa (experimental range, 20–50 GPa), resulting in the formation and extension of hexagonal diamond nuclei along grain boundaries, embedded incoherently among thin-graphite grains. As P increases from 130 GPa, the structure starts to liquefy, accompanied by a decrease in shear stress τ from approximately 5.3 GPa, and completely liquefies at P∼250 GPa (melting pressure of graphite, 180–280 GPa) and τ ∼ 0 GPa. In ultrahigh-pressure region, a two-wave structure is generated consisting of an elastic shock wave and a phase transition wave, and when the piston velocity exceeds 5.2 km/s, the latter wave can catch up with the elastic one, eventually becoming a single over-driven wave. During the relaxation of compressed nano-polycrystalline graphite, void nucleation inside the sample induces the initiation of visible cracks when piston velocity is higher than 1 km/s. At low piston velocities, the cracks propagate gradually along grain boundaries due to shear-slip effects. While at high piston velocities, direct spall of the nano-polycrystalline graphite makes it into multiple fragments by ultrahigh strain rate tensile forces. This study provides a useful guide to the structural transition and dynamic damage evolution of nano-polycrystalline graphite under shock compression.

Atomistic evidence of nucleation mechanism for the direct graphite-to-diamond transformation

The direct graphite-to-diamond transformation mechanism has been a subject of intense study and remains debated concerning the initial stages of the conversion, the intermediate phases, and their transformation pathways. Here, we successfully recover samples at the early conversion stage by tuning high-pressure/high-temperature conditions and reveal direct evidence supporting the nucleation-growth mechanism. Atomistic observations show that intermediate orthorhombic graphite phase mediates the growth of diamond nuclei. Furthermore, we observe that quenchable orthorhombic and rhombohedra graphite are stabilized in buckled graphite at lower temperatures. These intermediate phases are further converted into hexagonal and cubic diamond at higher temperatures following energetically favorable pathways in the order: graphite → orthorhombic graphite → hexagonal diamond, graphite → orthorhombic graphite → cubic diamond, graphite → rhombohedra graphite → cubic diamond. These results significantly improve our understanding of the transformation mechanism, enabling the synthesis of different high-quality forms of diamond from graphite.

Unveiling Novel Direct Bandgap Allotropes of Germanium: A Computational Exploration

Germanium crystallizes in a cubic diamond structure, which restricts its applications in optoelectronics due to its indirect band gap. However, the slight energy difference between the direct and indirect band gaps in germanium presents a promising opportunity to engineer its structure into a direct band gap material, with the hexagonal diamond (lonsdaleite) phase being a notable example. In this study, we conducted an extensive computational search to find germanium allotropes with a direct band gap and low energy using a sensible random structure search approach informed by data-derived interatomic potentials. Among the predicted allotropes, we identified a hexagonal 8H structure with the lowest energy compared to all known germanium allotropes and only 5 meV/atom above the ground state cubic diamond structure. Compared to the cubic diamond phase, which has complete cubicity, the 8H phase consists of 3/4 cubicity and 1/4 hexagonality. This structural motif, coupled with band structure back folding in the hexagonal Brillouin zone, results in a direct band gap of 0.25 eV. Given the prior experimental discovery of 2H and 4H polytypes, the experimental synthesis of the 8H allotrope in germanium is highly feasible. Future research could explore alloying 8H germanium with silicon to optimize the bandgap energy and optical transitions, potentially achieving lifetimes comparable to those of group III-V semiconductors like GaAs.

Colossal Core/Shell CdSe/CdS Quantum Dot Emitters.

Single-photon sources are essential for advancing quantum technologies with scalable integration being a crucial requirement. To date, deterministic positioning of single-photon sources in large-scale photonic structures remains a challenge. In this context, colloidal quantum dots (QDs), particularly core/shell configurations, are attractive due to their solution processability. However, traditional QDs are typically small, about 3 to 6 nm, which restricts their deterministic placement and utility in large-scale photonic devices, particularly within optical cavities. The largest existing core/shell QDs are a family of giant CdSe/CdS QDs, with total diameters ranging from about 20 to 50 nm. Pushing beyond this size limit, we introduce a synthesis strategy for colossal CdSe/CdS QDs, with sizes ranging from 30 to 100 nm, using a stepwise high-temperature continuous injection method. Electron microscopy reveals a consistent hexagonal diamond morphology composed of 12 semipolar {101̅1} facets and one polar (0001) facet. We also identify conditions where shell growth is disrupted, leading to defects, islands, and mechanical instability, which suggest synthetic requirements for growing crystalline particles beyond 100 nm. The stepwise growth of thick CdS shells on CdSe cores enables the synthesis of emissive QDs with long photoluminescence lifetimes of a few microseconds and suppressed blinking at room temperature. Notably, QDs with 80 and 100 CdS monolayers exhibit high single-photon emission purity with second-order photon correlation g(2)(0) values below 0.2.

Research on the formation and evolution mechanism of cracks in laser stealth dicing of silicon carbide crystals

In this study, to enhance the cutting efficiency and precision of the chip while minimizing waste from cutting damage, molecular dynamics simulation is used to investigate the formation mechanism of defects and cracks of silicon carbide crystals during the laser stealth dicing. The results showed that the high thermal stress generated by the laser scanning induced the production and expansion of cracks. Thus, the crack propagates in the direction of [100], and subsequently forms branches in the directions of [101] and [101‾]. It also can be found that the silicon carbide crystals produced dislocation slip, and the dislocation lines moved along the slip surface, which impeded the crack extension in the directions of [101‾] and [1‾01‾]. In addition, atomic phase transformation and loss is occurred under the high-temperature environment of the laser heating process. Cubic diamond crystal structure atoms are partially transformed into amorphous structure, while a small portion transformed into hexagonal diamond structure. The crystal structural arranged orderliness temporarily increased and then rapidly decreased due to prefabrication defects, and new unknown crystal structures are produced.

Interplay between the Formation of Colloidal Clathrate and Cubic Diamond Crystals.

Controlling the valency of directional interactions of patchy particles is insufficient for the selective formation of target crystalline structures due to the competition between phases of similar free energy. Examples of such are stacking hybrids of interwoven hexagonal and cubic diamonds with (i) its liquid phase, (ii) arrested glasses, or (iii) clathrates, all depending on the relative patch size, despite being within the one-bond-per-patch regime. Herein, using molecular dynamics simulations, we demonstrate that although tetrahedral patchy particles with narrow patches can assemble into clathrates or stacking hybrids in the bulk, this behavior can be suppressed by the application of external surface potential. Depending on its strength, the selective growth of either cubic diamond crystals or empty sII clathrate cages can be achieved. The formation of a given ordered network depends on the structure of the first adlayer, which is commensurate with the emerging network.

A novel sp3 carbon allotrope with 4+5+6+7+8 odd-even ring.

In this study, we report a novel monoclinic phase of carbon that contains 4+5+6+7+8 member rings in P21/m symmetry, identified by applying the stochastic surface walking method combined with high dimensional neural network potentials. We demonstrate that this phase possesses lower energy than graphite above 21.5 GPa. The phonon spectra show that this structure is stable under ambient pressure. This phase is a super hard material with a shear hardness as high as 81.9 GPa while it possesses an indirect band gap of 3.16 eV. The energy barrier of graphite to the Y phase is 0.27 eV, slightly higher than that of the hexagonal diamond (0.21 eV) in a similar phase transition mechanism. Two types of thermodynamically stable interfaces can be formed with the hexagonal diamond (HD), namely (001)Y//(100)HD, [100]Y//[010]HD and (001)Y//(001)HD, [010]Y//[001]HD. Although the discrete bulk Y phase is hard to synthesize, a faulted structure between HD is possible because of the well-matched interface between Y and HD. Our work shows that the Y phase may be formed in some special conditions and enhances our understanding of the formation of novel carbon allotropes.

A study on the Si1-xGex gradual buffer layer of III-V/Si multi-junction solar cells based on first-principles calculations.

III-V/Si multi-junction solar cells have been widely studied in recent years due to their excellent theoretical efficiency (∼42%). In order to solve the problem of lattice mismatch between Si and III-V compounds of III-V/Si solar cells, different hexagonal Si1-xGex buffer layer models on the surface of hexagonal diamond Si(001) were built, and the structural, electronic and optical properties of the proposed models were calculated based on first principles calculations. The results showed that all models of the designed buffer layer could effectively reduce the lattice mismatch, and the buffer layer hex-Si1-xGex (x = 0, 0.75, and 1) is the ideal model and has achieved the best lattice-matching improvement with high defect formation energy, as well as direct band gap properties and a larger light adsorption coefficient. These theoretical models, with their analyzed properties, could offer a promising pathway toward realizing high efficiency and low cost III-V/Si multi-junction solar cells.

Hydrogen and oxygen mixed-termination for nitrogen-vacancy quantum sensors in hexagonal diamond

Controlling the chemical termination of hexagonal diamond surface opens up new possibilities for shallow nitrogen-vacancy (NV) centers in quantum sensing applications. In this work, the H/O/OH mixed terminations on hexagonal diamond (100) surfaces to host NV centers are investigated by first-principles calculations. Results indicate that H–O, O–OH, and H–O–OH mixed surfaces produce positive electron affinity with neither spin noise nor surface-related state. It is concluded that the surface-connected ether bonds should be maintained, and unconnected ether bonds should be avoided, preserving the shallow NV centers unaffected. Based on the surface stability analysis, we further confirm that the H–O surface has a higher possibility to be fabricated. Moreover, we make an intensive study by placing an NV center ∼12 Å from the hexagonal (100) surface, and the results elucidate that the H–O surface has no obvious effect on the defect states of the NV center. Therefore, we identify a combination of surface terminators that is a prospective candidate for NV-based quantum sensors.

Crystal structure, electronic structure, and optical properties of Cu4ZnGe2S7 and Cu4CdGe2S7: Polar diamond-like semiconductors exhibiting second harmonic generation

The novel I4-II-IV2-VI7 quaternary diamond-like semiconductors (DLSs), Cu4ZnGe2S7 and Cu4CdGe2S7, were prepared from the constituent elemental powders at elevated temperatures. Cu4ZnGe2S7 crystallizes in the polar, chiral space group C2 and adopts the Cu4NiSi2S7 structure-type, which can be considered a derivative of the cubic diamond structure. The cadmium analog takes on the Cu5Si2S7 structure-type, which is related to the hexagonal diamond structure, and displays polar, Cc space group symmetry. Both compounds contain [Ge2S7]6- anions created by corner-sharing GeS4 tetrahedra. The crystal structures were carefully evaluated using: 1) extended connectivity tables, 2) minimum bounding ellipsoid analysis, and 3) bond valence sum (BVS) and global instability index (G) calculations. Both compounds are thermally stable up to temperatures exceeding 950 °C and melt incongruently. As assessed using diffuse reflectance spectroscopy, Cu4ZnGe2S7 and Cu4CdGe2S7 possess optical bandgaps of 1.65 eV and 1.90 eV, respectively, which are narrower than the analogous I2-II-IV-VI4 DLSs. Electronic structure calculations show that both compounds are direct-bandgap semiconductors with significant contributions of Cu-d and S-p orbitals to the states at the top of the valence band. While Cu4CdGe2S7 displays a weak second harmonic generation (SHG) response, the second-order nonlinear optical susceptibility, χ(2), of Cu4ZnGe2S7 is 14.8 ± 1.5 pm V-1, assessed using the Kurtz-Perry powder method and an optical-grade AgGaSe2 (AGSe) reference. Both compounds exhibit laser-induced damage threshold values similar to AGSe, ∼0.1 GW cm-2, for λ = 1.064 µm using picosecond laser pulses. Most importantly, Cu4ZnGe2S7 exhibits a phase-matching behavior at λ = 3.1 µm. These results suggest that Cu4ZnGe2S7 holds potential for use in low-powered laser applications involving efficient wave mixing in the mid-IR region.

Canyon Diablo lonsdaleite is a nanocomposite containing c/h stacking disordered diamond and diaphite.

In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Nanocrystalline Ag–C Composites Containing a Hexagonal Diamond Structure to Inhibit Tarnishing of Sterling Silver

Nanocrystalline Ag–C Composites Containing a Hexagonal Diamond Structure to Inhibit Tarnishing of Sterling Silver

Fluorine and oxygen terminated hexagonal diamond (100) surfaces for nitrogen-vacancy based quantum sensors

Chemical modification of hexagonal diamond surface is a new and important issue for the negatively charged nitrogen-vacancy (NV) center in quantum sensors. In this work, six types of terminated hexagonal diamond (100) surfaces for shallow NV centers are proposed, and structural and electronic properties are investigated by structure prediction method combined with first-principles calculation. In these structures, only fluorine and oxygen terminated surfaces (F-hex and α-O-hex) have positive electron affinity of 3.00 and 2.85 eV, respectively, no surface-related states and electron spin noise, indicating the F-hex and α-O-hex surfaces could be a promising host of NV center. We make further inspections by placing an NV center ~12 Å below the surfaces, and the results confirm that the surfaces have little effect on the defect states of NV center. Consequently, the F-hex and α-O-hex surfaces are prospective candidates for NV-based quantum sensors.

Clean fabrication strategy of diamond crystals containing nanocomposite films for corrosion and wear protection of AISI-321 alloy

This study focused on the synthesis of Niobium/Hexagonal-Diamond/Amorphous-Carbon (Nb/H-D: a-C) films on AISI-321 alloy. The substrate heating used as a vital deposition parameter to tailor structure of the films in an unbalanced magnetron (UB-M) sputtering system for the first time. The substrate heater temperature (Th) ranged between (RT) to 150 °C. The results indicated that increasing the substrate heater temperature promoted the abundance of hexagonal diamond (H-Diamond) crystals through the solid phase separation. Therefore, the film deposited at the highest temperature (150 °C) demonstrated the highest plenty of H-Diamond. Electrochemical impedance spectroscopy (EIS) measurements (after 2, 8, 24, 72, and 168 h immersion) and polarization tests (after 168 h immersion) disclosed that increasing the substrate heater temperature positively affects the corrosion resistance of (Nb/H-D: a-C) films. The results proved that the film deposited at the highest temperature of 150 °C, demonstrated the exceptional corrosion protection efficiency of 99.69 %. Moreover, this film presented acceptable low-frequency impedance (|Z|10KHz) and total resistance (Rt) values of >108 Ω·cm2 and >117 MΩ·cm2 at all immersion times in 3.5 wt% NaCl solution. The highest abundance of the H-Diamond phase, with remarkable chemical inertness at higher substrate heater temperatures, can be an effective reason for elevating anti-corrosion performance of the films. Additionally, as the substrate heater temperature enhanced from RT to 150 °C, mechanical properties illustrated an remarkable increment trend (increasing hardness from 20 to 32.7 GPa and the elasticity modulus from 407 to 470 GPa). Moreover, the anti-wear performances of the films significantly improved with increasing substrate heater temperature, reducing the wear rate from 9.4 × 10−6 to 9 × 10−7 mm3/N·m. These results are mainly attributed to the highest abundance of the superhard H-Diamond phase in the film deposited at higher substrate heater temperature. In conclusion, the (Nb/H-D: a-C) film synthesized at 150 °C represented a promising candidate for tailoring the electrochemical and anti-wear performances of AISI-321 alloy.

Evolution of Cu-In Catalyst Nanoparticles under Hydrogen Plasma Treatment and Silicon Nanowire Growth Conditions.

We report silicon nanowire (SiNW) growth with a novel Cu-In bimetallic catalyst using a plasma-enhanced chemical vapor deposition (PECVD) method. We study the structure of the catalyst nanoparticles (NPs) throughout a two-step process that includes a hydrogen plasma pre-treatment at 200 °C and the SiNW growth itself in a hydrogen-silane plasma at 420 °C. We show that the H2-plasma induces a coalescence of the Cu-rich cores of as-deposited thermally evaporated NPs that does not occur when the same annealing is applied without plasma. The SiNW growth process at 420 °C induces a phase transformation of the catalyst cores to Cu7In3; while a hydrogen plasma treatment at 420 °C without silane can lead to the formation of the Cu11In9 phase. In situ transmission electron microscopy experiments show that the SiNWs synthesis with Cu-In bimetallic catalyst NPs follows an essentially vapor-solid-solid process. By adjusting the catalyst composition, we manage to obtain small-diameter SiNWs-below 10 nm-among which we observe the metastable hexagonal diamond phase of Si, which is predicted to have a direct bandgap.

Graphite–hexagonal diamond hybrid with diverse properties

The recently discovered graphite–diamond hybrid materials (Gradia) with mixed sp2- and sp3-hybridizations have opened up a new direction in carbon allotropes research. Herein, we reported Gradia-HZ, constituted by interfaced graphite and hexagonal diamond parts in the unit cell, which demonstrates distinct electronic and mechanical properties. With the modulation of graphite width, Gradia-HZ exhibits unexpected topological nodal-line semimetal, semiconductor, and normal metal integrating with a distinctive Quasi-1D electronic transport capability based on first-principles calculations. More interestingly, pressure-induced graphite phase transformation might be an implementable and effective method to regulate the structure and physical properties of Gradia-HZ. The discovery of rich and peculiar physical properties in Gradia-HZ, e.g., high-conductivity metals, semiconductors with variable bandgap, and topological semimetals, will arouse great research interest to graphite–diamond hybrid materials, to promote their development and application in advanced devices.

Driving forces for ultrafast laser-induced sp2 to sp3 structural transformation in graphite

Understanding the microscopic mechanism of photoinduced sp2-to-sp3 structural transformation in graphite is a scientific challenge with great importance. Here, the ultrafast dynamics and characteristics of laser-induced structural transformation in graphite are revealed by non-adiabatic quantum dynamic simulations. Under laser irradiation, graphite undergoes an interlayer compression and sliding stage, followed by a key period of intralayer buckling and interlayer bonding to form an intermediate sp2-sp3 hybrid structure, before completing the full transformation to hexagonal diamond. The process is driven by the cooperation of charge carrier multiplication and selective phonon excitations through electron-phonon interactions, in which photoexcited hot electrons scattered into unoccupied high-energy conduction bands play a key role in the introduction of in-plane instability in graphite. This work identifies a photoinduced non-adiabatic transition pathway from graphite to diamond and shows far-reaching implications for designing optically controlled structural phase transition in materials.