March_AMP_Digital

A D V A N C E D

M A T E R I A L S

P R O C E S S E S |

M A R C H

2 0 1 6

2 5

Diamond can exist in the interiors

of the outer planets (Uranus and Nep-

tune) and Earth’s mantle, consistent

with the phase diagram, where pres-

sure/temperature are 600 GPa/7000K

and 135 GPa/3500K, respectively.

Currently, diamond powders are syn-

thesized by graphite to diamond con-

version at high pressures and tem-

peratures. Graphite is transformed into

diamond at temperatures above about

2000K at 6-10 GPa using a liquid metal

(iron) catalyst, a process used for com-

mercial synthesis of diamond

[1]

(Fig. 1).

Due to the high binding and activation

energies of transformation, carbon

polymorphs exist metastably well into

a pressure-temperature region where

a different phase is thermodynamical-

ly stable. For example, diamond sur-

vives indefinitely at room temperature,

where graphite is the stable form.

This article discusses the discov-

ery of the direct conversion of carbon

into diamond at atmospheric pressure

and ambient temperature in air

[3-4]

. The

transformative attributes of the discov-

ery enhance diamond yield and reduce

manufacturing costs. Thermodynam-

ic limits are bypassed with the help of

kinetics to create novel carbon-based

structures with exciting new proper-

ties. Rapid melting using nanosecond

laser pulses enables modification

of the equilibrium thermodynamic

(P vs. T) phase diagram. Super-under-

cooled states of molten carbon and

BN are created, which are quenched to

form novel phases (Q-carbon and Q-bo-

ron nitride, or Q-BN) from which differ-

ent diamond and cubic boron nitride

(c-BN) structures are formed. The entire

process is completed in less than a mil-

lionth of a second, and the process can

be repeated 10 to 200 times per second.

The critical breakthrough is that

synthesis and processing are done in

a liquid phase, where diffusivities are

on the order of 10

-4

-1

, some eight

orders of magnitude faster than the

highest solid state diffusivities. This

research focuses on synthesis and

scale-up processing of nanodiamonds

(1-100 nm of uniform size), microdia-

monds (100-500 nm), and microneedles

(>2000 nm), as well as large-area sin-

gle-crystal thin films on practical sap-

phire and glass substrates, from which

diamond and related c-BN are harvest-

ed and substrates recycled. The dis-

covery provides an inexpensive way to

convert carbon into diamond and hex-

agonal boron nitride (h-BN) into c-BN

and harvest them conveniently for a

variety of applications.

RESULTS AND DISCUSSION

The atomic structure of carbon at-

oms consists of six electrons (1s

)

whose charge is balanced by six pro-

tons in the nucleus, which also has six

neutrons. Mixing wave functions of

the outer four electrons (2s

) deter-

mines bonding characteristics in the

crystalline phase. The sp

hybridization

leads to formation of graphite bond-

ing within the sheets, and the extra 2s

electron provides for the delocaliza-

tion of electrons and metallic behavior.

The sp

hybridization, by comparison,

leads to the formation of diamond

semiconductor with a bandgap of

5.52 eV. Thus, transformation of graph-

ite to diamond requires controlled mix-

ing of the outer four electrons, which

has been attempted electronically us-

ing high-power photon (laser) beams

during laser ablation of graphite. Re-

sulting diamond-like carbon films have

a very high fraction of local sp

bonding

in the amorphous phase, but diamond

phase formation with long-range sp

bonding occurs only occasionally. How-

ever, providing diamond seeds enables

growing the diamond phase on the

seeds. In this transformative approach,

the 2s

and 2p

electrons are mixed

thermally. Melting rapidly creates me-

tallic super-undercooled carbon with

delocalized electrons, and quenching

achieves short-range sp

bonding in

Q-carbon and long-range sp

bonding in

diamond, which can be nucleated from

the Q-carbon, or grown on a template

directly from the super-undercooled

carbon.

Amorphous

metastable

dia-

mond-like carbon with some sp

con-

tent and super-undercooled liquid

carbon are introduced into the ther-

modynamically stable forms of carbon,

graphite, diamond, liquid, and vapor

[1]

(Fig. 1). This is accomplished by nano-

second laser melting of diamond-like

carbon, where the undercooled state is

at a temperature around 4000K, about

1000K lower than the melting point of

graphite. Quenching the super-under-

cooled liquid forms Q-carbon, nano-

diamond, and microdiamond by con-

trolling nucleation and growth times.

Providing a growth template enables

growing large-area single-crystal thin

films through the paradigm of domain

matching epitaxy

[7]

. The new state of

carbon (Q-carbon) has a very high frac-

tion of sp

bonded carbon and the rest

, and is expected to exhibit novel

physical, chemical, mechanical, and

catalytic properties. There is more than

a 10% reduction in volume when as-

deposited amorphous carbon is melted

in the undercooled state and quenched

as Q-carbon. The Q-carbon exhibits

unique properties including ferromag-

netismat room temperature and above.

Formation of the cubic diamond phase

can occur if sufficient time is allowed

for homogeneous nucleation, or when

substrates that are lattice and planar

matched with cubic diamond are pro-

vided during nucleation

[2-4]

The primary focus of this work is

on synthesis and processing of nano-

diamonds, microdiamonds, and mi-

croneedles, as well as large-area single-

crystal thin films at ambient pressure

and atmospheric pressure in air. This

is achieved by nanosecond laser melt-

ing of diamond-like amorphous carbon

films on practical sapphire and glass

substrates. Irradiation with ArF Exci-

mer laser pulses (193 nmwavelength or

photon energy of 6 eV and pulse dura-

tion of 20 ns) confines laser energy and

selectively melts diamond-like carbon

films. Undercooling values for amor-

phous diamond-like carbon are con-

siderably higher than those achieved

during melting of crystalline carbon,

such as highly oriented pyrolytic graph-

ite (HOPG) samples, which do not yield

diamond

[8-9]

. Nanosecond pulsed laser

melting of amorphous carbon leads to a

highly undercooled state, which can be