Silicon Driven Facet Regulation Enables Tunable Micro-Diamond Architectures in Liquid Ga In

We report an ambient pressure liquid metal assisted CVD strategy that enables shape programmable growth of micro scale diamond by coupling liquid metl Ga In with ferrocene (Fe(C5H5)2) as an carbon precursor, nanodiamond seeds, and nanosilicon. Building on liquid metal diamond synthesis, this approach pushes liquid metal growth toward a low temperature (900 °C, 1 atm) while enabling single crystal diamonds to be scaled from ~10 μm to several tens of micrometers with well developed faceting. Ferrocene decomposition supplies a sustained interfacial carbon flux that is captured and redistributed by the Ga In melt toward seed rich liquid solid interfaces. Defect rich nanodiamond provides the crystallographic template required for reliable sp3 nucleation despite the intrinsically low carbon solubility of Ga In. Nanosilicon plays a distinct, complementary role by tuning interfacial kinetics and facet competition, enabling deliberate control of crystal habit: cubic (~10 μm), truncated tetrahedral, and fully faceted octahedral diamonds are reproducibly obtained by adjusting the nanosilicon:nanodiamond ratio, with octahedral crystals reaching ~50 μm. Importantly, crystal size is further scaled by regulating hydrogen flow: lowering the H2 rate increases net carbon retention at the liquid metal interface, raises effective supersaturation, and accelerates diamond deposition. Together, habit control (via nanosilicon: nanodiamond) and size scaling (via H2 flow) establish a practical route silicon driven facet regulation and size under ambient pressure, offering a pathway to tunable micro sized single crystal diamonds under mild conditions.

💡 Research Summary

The authors present a novel ambient‑pressure, low‑temperature (900 °C, 1 atm) liquid‑metal‑assisted chemical vapor deposition (LM‑CVD) route for synthesizing micro‑scale single‑crystal diamonds (MDDs) with tunable morphology. The process uses a eutectic Ga‑In melt as the solvent, ferrocene (Fe(C₅H₅)₂) as a solid carbon source and Fe catalyst, nanodiamond (NDD) particles as crystallographic seeds, and nanosilicon (NSi) as a facet‑regulating additive. Ferrocene decomposition releases CHₓ radicals and Fe atoms; Fe dramatically increases the otherwise low carbon solubility of Ga‑In, enabling a sustained carbon flux toward the liquid‑solid interface. Defect‑rich NDD provides lattice‑matched nucleation sites that stabilize sp³ nuclei despite the low bulk carbon solubility.

A key innovation is the use of NSi to modulate interfacial kinetics and surface energies of competing crystal facets. By varying the NSi:NDD mass ratio, the authors achieve three distinct diamond habits: (i) cubic (~10 µm) when only NDD is present, (ii) truncated tetrahedral (~10–30 µm) at NSi:NDD = 1:10, and (iii) fully faceted octahedral (~30–50 µm) at NSi:NDD = 1:1. At low Si content, the (100) facets grow faster than (111), leading to cubic or truncated shapes. Increasing Si concentration equalizes the surface energies of (111) planes, promotes isotropic growth, and yields thermodynamically favored octahedra. Raman spectroscopy tracks the sp²→sp³ transition: the G‑band (≈1590 cm⁻¹) diminishes while the diamond line sharpens around 1330 cm⁻¹. X‑ray diffraction confirms the disappearance of Fe‑C intermediate peaks and the emergence of characteristic diamond reflections (≈43°, 75°, 91°, 119° 2θ). X‑ray photoelectron spectroscopy shows a dominant sp³ C 1s component at ~283 eV, with transient Fe‑C and Si‑O signatures that fade as growth proceeds.

Hydrogen flow rate is employed as an independent size‑scaling knob. Reducing the H₂/N₂ flow ratio lowers carbon etching at the melt surface, raises the effective carbon supersaturation, and accelerates diamond deposition. Under identical 3‑hour growth times, decreasing H₂ from 100 sccm to 30 sccm expands the average crystal size from ~10 µm to ~50 µm, demonstrating that supersaturation control can be decoupled from facet regulation.

Mechanistically, the process proceeds through (1) ferrocene decomposition delivering CHₓ and Fe, (2) dissolution of carbon into Ga‑In, (3) migration of carbon to the liquid‑solid interface where NDD seeds capture it, (4) Si‑mediated modification of surface energies that biases attachment on specific crystallographic planes, and (5) hydrogen‑assisted removal of residual sp² carbon, allowing sp³ nuclei to coalesce into well‑defined crystals. The authors provide a comprehensive set of spectroscopic and microscopic data to substantiate each step.

The study establishes liquid‑metal‑mediated CVD as a viable, energy‑efficient pathway for producing high‑quality micro‑diamonds without the need for high pressures or plasma. The demonstrated ability to program crystal habit via nanosilicon and to scale size via hydrogen flow offers a practical toolbox for tailoring diamond properties for quantum photonics, sensing, and micro‑electromechanical applications. Future work could integrate dopants, isotopic enrichment, or engineered defect centers (e.g., NV⁻) within this platform, opening routes to scalable, low‑cost production of functional diamond components.