Quenching of liquid carbon under intensive heat transfer to the cold diamond substrate: Molecular-dynamic simulation

ACS Appl. Mater. Interfaces

2019

Q-carbon is a densely packed metastable phase of carbon formed by ultrafast quenching of carbon melt in a super-undercooled state. After quenching, diamond tetrahedra are randomly packed with >80% packing efficiency. This discovery has opened a pathway to fabricate various interesting heterostructures following the highly nonequilibrium route of nanosecond pulsed laser annealing. In the present work, we demonstrate the evolution of Q-carbon/α-carbon and Q-carbon/diamond heterostructures with atomically sharp interfaces, controlled via varying solidification rates of the undercooled C melt. This structure consists of ultrahard Q-carbon (∼80% sp3 and rest sp2) with an overlayer of soft α-carbon (∼40% sp3) on the inert c-Al2O3 substrate. Using high-resolution scanning transmission electron microscopy and Raman spectroscopy analysis, we present the formation of the highly dense Q-carbon/α-carbon bilayer structure with distinctly different atomic and electronic structures. The laser-solid interaction simulations coupled with atomistic ab initio modeling further confirm the conversion of C melt into Q-carbon by achieving maximum undercooling near the substrate and further into α-carbon with a decrease in regrowth velocity (<6 m/s) away from the substrate. We present details of the evolution of heterointerfaces formed from carbon melt for designing heterostructures far from equilibrium for various functional applications by using pulsed laser processing.

Section: Resultsmentioning

confidence: 99%

Nonequilibrium Structural Evolution of Q-Carbon and Interfaces

Sachan

ACS Appl. Mater. Interfaces

2019

“…For the quench (relaxation) of randomized coordinates, we use 10 –4 Ry/Bohr as the criterion for the convergence of the force. This method of randomization and formation of disordered solids is known as the liquid quench method. , …”

Section: Resultsmentioning

confidence: 99%

Reduced Graphene Oxide/Amorphous Carbon P–N Junctions: Nanosecond Laser Patterning

ACS Appl. Mater. Interfaces

2019

The device integration of graphene and reduced graphene oxide (rGO) is impeded by scalability and high temperature (>2000 K) treatment required for effective reduction into high-quality rGO. In this article, we present a novel approach for direct laser writing of heavily reduced graphene oxide films by nanosecond laser melting of amorphous carbon on silicon (001) substrates under ambient conditions. Ultrafast quenching from the undercooled melt state above the melting threshold energy density (E d) of 0.4 J/cm2 leads to the formation of large-area rGO films. The first-order phase transformation of liquid carbon into graphene is triggered by low undercooling at the C melt/silicon interface. The laser-irradiated rGO films exhibit electron mobility of 12.56 cm2/V s and charge carrier concentration of −1.2 × 1021/cm3 at 300 K. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest low disorder and charge transport via 2D Mott variable range hopping between the graphene islands for rGO films. The localization length corresponding to the size of these graphitic domains is 3 nm. The ultrafast regrowth of rGO creates an atomically sharp interface between n-type rGO and p-type amorphous carbon, resulting in p–n junction heterojunction diodes with a turn-on voltage of 0.3 V, rectification ratio of 110@±1.5 V, and activation energy of 0.13 eV under reverse bias. This unique laser processing method solves the problems of traps and defects associated with equilibrium-based rGO fabrication methods, enabling high conductivity and mobility, providing insights into the fundamental mechanism driving laser writing of graphene-based materials on silicon.

“…The structure of melt regrown Q-carbon and α-carbon phases is simulated using the LAMMPS code. The simulations are based on “liquid quench method” , generally used for randomization and formation of disordered solids, where amorphous carbon is melted and subsequently quenched under isotropic pressure. In this case, to equilibrate the structures formed under HPHT condition, the pressure is first released at 2000 K to 760 Torr (see Section for details), and then subsequent quenching to room temperature is performed.…”

Section: Results and Discussionmentioning

confidence: 99%

Characteristics of Diamond Deposition on Al₂O₃, Diamond-like Carbon, and Q-Carbon

Haque

ACS Appl. Electron. Mater.

2020

We have conducted a comparative study on the deposition of diamond thin film on uncoated Al 2 O 3 , diamond-like carbon (DLC, grown by pulsed laser deposition) film-coated Al 2 O 3 , and Q-carbon (fabricated by nanosecond pulsed laser annealing)-coated Al 2 O 3 substrates by hot filament chemical vapor deposition (HFCVD). Scanning electron microscopy shows that the continuous large-area diamond thin film can be grown on the Q-carbon/Al 2 O 3 substrate, whereas the deposition of diamond on DLC/Al 2 O 3 or uncoated Al 2 O 3 substrates gives clusterlike/patches of discontinuous diamond thin film formation. Raman spectroscopy of the diamond on Q-carbon/Al 2 O 3 shows a considerably small residual stress (1.7 GPa) compared to the diamond on DLC/Al 2 O 3 (3.1 GPa) or diamond on the uncoated Al 2 O 3 (3.9 GPa) substrate. The nondiamond content in the diamond crystals on Q-carbon-coated Al 2 O 3 is found to be only 0.12%, which is about 2.5 and 3.5 times less than that of diamond on DLC-coated Al 2 O 3 and uncoated Al 2 O 3 , respectively. Consistent with the Raman analysis, the X-ray diffraction analysis shows the crystalline growth of the HFCVD diamond on Q-carbon/Al 2 O 3 with relatively small in-plane stress compared to the diamond film on the other two systems. We propose that the high density of diamond tetrahedron in the Q-carbon structure provides a good platform for diamond growth with a very high nucleation density over 10 9 cm −2 and can act as a seed layer for diamond growth without cracking or delamination over the surface of the substrate. The optically transparent high-quality diamond film on Q-carbon-coated Al 2 O 3 showed better adhesion compared to that on DLC-coated Al 2 O 3 and uncoated Al 2 O 3 , as measured by a simple scotch tape test. These results open a path toward the fabrication of large-area high-quality diamond films on an optically transparent substrate for commercial applications.