Face-centered cubic carbon as a fourth basic carbon allotrope with properties of intrinsic semiconductors and ultra-wide bandgap
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-07-02 |
| Journal | Communications Materials |
| Authors | I. Konyashin, Ruslan Muydinov, Antonio Cammarata, Andrey Bondarev, Marin Rusu |
| Institutions | Technische Universität Berlin, Charles University |
| Citations | 6 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- New Carbon Allotrope: Face-centered cubic (fcc) carbon is confirmed as a fourth basic carbon allotrope, designated as a âquasivalent solidâ due to its unique electronic structure.
- Intrinsic UWBG Semiconductor: The material exhibits intrinsic semiconductor behavior (conductivity increases with temperature) despite possessing an Ultra-Wide Bandgap (UWBG) of approximately 6.1 eV, comparable to diamond.
- Unique Bonding and Structure: The fcc structure (Fm 3 m) is characterized by a large C-C distance (2.51 Angstrom) and noncovalent sharing of p-electrons, resulting in a peculiar valence band structure containing an intraband gap.
- Negative Electron Affinity (NEA): The fcc-carbon film exhibits a strong NEA (EA = -0.96 eV), a highly desirable property for electron emission applications.
- Epitaxial Synthesis: High-quality fcc-carbon films were successfully grown epitaxially on single-crystalline diamond substrates using Plasma-Assisted Chemical Vapor Deposition (PACVD) under highly specific, low-rate conditions.
- Pathway to Carbon Electronics: This material is proposed as the first intrinsic UWBG semiconductor made purely of carbon, opening new avenues for high-power, high-frequency, and deep-UV âcarbon electronics.â
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Crystal Structure | Face-Centered Cubic (fcc) | N/A | Space Group Fm 3 m |
| Lattice Constant (a0) | 3.54 - 3.57 | Angstrom | Determined by Electron Diffraction |
| Optical Bandgap (Eg opt) | ~6.1 | eV | VUV Reflection Spectroscopy (Tauc plot) |
| Calculated Bandgap (Eg calc) | ~7.1 | eV | Ab initio (revPBE DFT) |
| Electrical Conductivity (Ď) | ~0.1 | S¡m-1 | Measured at Room Temperature |
| Activation Energy (Ea) | ~230 | meV | Temperature dependence of conductivity |
| Work Function (ÎŚ) | 4.60 | eV | Kelvin Probe measurement |
| Ionization Energy (Ei) | 5.14 | eV | PYS measurement |
| Electron Affinity (EA) | -0.96 | eV | Negative Electron Affinity (NEA) |
| VBM Position (EF - EVBM) | -0.54 | eV | Relative to Fermi Level |
| Synthesis Temperature | ~600 | °C | PACVD process |
| Deposition Rate | ~0.14 | nm/min | Extremely low rate, critical for fcc phase formation |
| Raman Shift (fcc-C) | None detected | cm-1 | Consistent with fcc structure (no optical phonons) |
Key Methodologies
Section titled âKey Methodologiesâ- Synthesis (PACVD): Thin films were obtained via Plasma-Assisted Chemical Vapor Deposition (PACVD) on single-crystalline diamond (100) substrates.
- Critical Deposition Conditions: The fcc-carbon phase was achieved in a narrow process window characterized by insufficient metastable C-H species and an unusually low deposition rate (~0.14 nm/min), which favors the fcc phase over diamond due to differential etching rates.
- Gas Mixture: Methane (CH4) at 2 vol% and Hydrogen (H2) at 98 vol% were used, with a total pressure of 29.33 kPa and a microwave power of nearly 2.2 kW.
- Structural Analysis:
- Electron Diffraction/HRTEM: Confirmed the face-centered cubic crystal lattice (Fm 3 m space group) and epitaxial growth on diamond.
- XRD: Used grazing incidence and Bragg-Brentano experiments to confirm forbidden diamond reflexes, corroborating the fcc structure.
- Electronic and Spectroscopic Analysis:
- Kelvin Probe (KP) and PYS: Measured work function (ÎŚ) and ionization energy (Ei) in a dry nitrogen atmosphere to determine the Negative Electron Affinity (NEA).
- VUV Reflection Spectroscopy: Used to determine the optical bandgap (Eg opt ~6.1 eV) via Tauc plot reconstruction based on Kubelka-Munk theory.
- XPS: Confirmed the absence of sp2 carbon contamination and revealed an unusually low binding energy C1s peak (283.9 eV), attributed to the unique noncovalent p-electron sharing.
- Electrical Measurement: Four-point measurements in Van der Pauw geometry were used to determine electrical conductivity and activation energy (Ea ~230 meV) across a temperature range (302 K down to 2 K).
- Computational Modeling: Ab initio calculations (DFT, revPBE functional) were performed to model the electronic band structure, explaining the intraband gap and the low degree of s-p hybridization.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique combination of UWBG, intrinsic semiconducting behavior, and NEA makes fcc-carbon highly promising for advanced electronics:
- High Power/High Frequency Electronics: UWBG materials are critical for next-generation power electronics and RF devices, offering superior breakdown fields and electron saturation velocity compared to SiC and GaN.
- Deep-UV Optoelectronics: The 6.1 eV bandgap is ideal for manufacturing deep-UV light sources, detectors, and sensors.
- Field Emitters and Electron Multipliers: The Negative Electron Affinity (NEA) allows for easy electron emission, enabling highly efficient field emission displays and vacuum microelectronic devices.
- Transparent Electronics: Potential use in devices requiring high transparency combined with semiconductor functionality.
- Harsh Environment Applications: As a carbon allotrope, it is expected to maintain stability and performance in high-temperature, high-radiation, and chemically aggressive environments (e.g., automotive, aerospace, energy production).
- Quantum Technology: Potential applications in quantum memory and other quantum devices due to its unique electronic structure.
View Original Abstract
Abstract Carbon is considered to exist in three basic forms: diamond, graphite/graphene/fullerenes, and carbyne, which differ in a type of atomic orbitals hybridization. Since several decades the existence of the fourth basic carbon allotropic form with the face-centered cubic ( fcc ) crystal lattice has been a matter of discussion despite clear evidence for its laboratory synthesis and presence in nature. Here, we obtain this carbon allotrope in form of epitaxial films on diamond in a quantity sufficient to perform their comprehensive studies. The carbon material has an fcc crystal structure, shows a negative electron affinity, and is characterized by a peculiar hybridization of the valence atomic orbitals. Its bandgap (~6 eV) is typical for insulators, whereas the noticeable electrical conductivity (~0.1 S m â1 ) increases with temperature, which is typical for semiconductors. Ab initio calculations explain this apparent contradiction by noncovalent sharing p -electrons present in the uncommon valence band structure comprising an intraband gap. This carbon allotrope can create a new pathway to âcarbon electronicsâ as the first intrinsic semiconductor with an ultra-wide bandgap.