Synthesis of diamond nanostructures from carbon nanotube and formation of diamond-CNT hybrid structures
Graphical abstract
Introduction
Carbon nanotubes, the tubular form of graphite sheets (100% sp2 bonded carbon), are one of the most promising forms of carbon which is expected to be implemented in many nano-scale device applications owing to its exceptional properties such as large thermal conductivity, good electron mobility, excellent electrical conductivity, and dual band-gap properties (metallic and semiconductor as a function of chirality) [1,2]. On the other hand, diamond (consisting of 100% sp3-bonded carbon) has tremendous chemical inertness, extreme hardness, biocompatibility, unique electrochemical properties, large bandgap, and negative electron affinity (observed in H-terminated diamond) [[3], [4], [5]]. It is one of the most studied and most promising materials for cold cathode electron source, heat sink substance for electronic devices, hermetic corrosion resistant coating for bio-devices, protective coating in machining tools, photon-enhanced thermionic emission (PETE) solar cells, and the structural material for micro- and nano-electromechanical systems (MEMS/NEMS) [[6], [7], [8], [9], [10]]. In fact, the diamond-based PETE solar cells fabricated by the defect engineering of diamond, obtained by the laser treatment, has shown immense potential [[11], [12], [13]]. Diamond is also widely used to fabricate radiation detectors to detect electromagnetic radiation with energy of greater than 5.5 eV. These includes UV, X-rays, γ rays, high energy particle radiation such as α-particles, electrons, neutrons, pions and other exotic particles [14]. However, the diamond cannot be directly integrated in most of these applications due to different processing limitations [15,16]. It is understood that a combination of carbon nanotubes and nanocrystalline diamond provides hybrids with unprecedented properties that can be advantageously used in electronics, field emission, and load transfer applications. This hybrid material can have excellent electrical and thermal conductivities, and field emission characteristics comparable to or better than pure diamond (as pure diamond without hydrogen termination has very limited electrical conductivities). The diamond/CNT hybrid structures may thus find applications in various fields that require a combination of excellent mechanical, thermal, and electrical properties. As an example, diamond tipped carbon nanotube will provide an ideal high-efficiency field emitter, where electrons can be carried via nanotube for enhanced field emission by diamond tips.
Since the discovery of carbon nanotubes in 1991 [1] different routes have been tried to transform CNTs into diamond to form CNT/diamond hybrid structure [[17], [18], [19]]. Synthesis of diamond from CNTs was achieved by shock wave synthesis [20], chemical vapor (CVD) deposition by nanotube coating [21], high-pressure-high-temperature (HPHT) treatments [22], and hydrogen plasma post-treatment [23]. Most of these processes lead to diamond structures with limited yield and defects which make them unsuitable for electronic applications. Generally, pressures above several GPa and high-temperature conditions are required and catalysts such as Ni, Co, and other metals or alloys are necessary for the conversion process. The catalyst induced formation of diamond often yields contaminated diamond crystals. Researchers also reported direct synthesis of diamond from CNTs by microwave plasma enhanced chemical vapor deposition (MPCVD) [24]. During the transformation of CNTs to diamond in MPCVD a solid-gas-solid transformation mechanism was involved with limited yield and control over the resulting microstructure. The transformation of CNTs into diamond has been reported by spark plasma sintering at 1500 °C temperature and 80 MPa pressure [25]. In this process, the diamond structures are sheathed with an amorphous carbon layer which may hinder its electronic device based applications. Moreover, the stringent requirement of high pressure, high-vacuum, and complicated formation procedure of the spark plasma at a tremendously large current (>1000 A) are some major drawbacks of this synthesis process. It is important to note that none of these diamond formation methods invoke melting as the mechanism for conversion of CNTs into diamond. Moreover, most of these studies do not provide evidence of diamond structure by TEM and SEM (EBSD) techniques and bonding characteristics by EELS for σ* peak, Raman spectrum with the characteristic peak at 1332 cm−1, and the phonon confinement effect in the Raman spectrum in the case of nanostructured diamond, as the synthesis routes are driven by the solid-to-solid or solid-gas-solid equilibrium phase conversions.
Apart from the above growth and structural quality related limitations, little evidence of direct bonding between the CNTs and diamond was observed in most of the diamond-CNT hybrid structures owing to the limitation of the solid-to-solid phase transformation. This could lead to poor emission efficiency owing to the presence of trap states at the interface between the CNT and diamond and high interfacial resistance as well. Furthermore, the lack of direct bonding weakens the hybrid structure at the interface, leading to an inefficient mechanical load transfer between these two structures (matrix and reinforcement). To overcome these problems, we followed an ultrafast pulsed laser annealing technique to directly convert the tips and bends of the CNTs into diamond at ambient temperature and pressure in air. This undercooling driven conversion of CNTs into diamond involves melting in a super undercooled state using nanosecond laser pulses and quenching rapidly to transform into phase-pure diamond. Subsequent laser pulses can be used to enhance and expand the diamond regions. So far diamond has been doped successfully with p-type dopants. These diamond structures can be also fabricated with n- and p-type dopants with concentrations far higher than equilibrium thermodynamic solubility limits as a result of rapid quenching from liquid and solute trapping phenomenon. Single crystal diamond with such high level of doping concentration has several exciting applications, i.e. high-temperature superconductivity, power electronics, and efficient field emission devices. Attaining such high doping concentration (otherwise impossible to realize) in single crystal diamond is only possible by this novel discovery of the controlled, direct, ultrafast, and non-equilibrium transformation of carbon into diamond. Thus, this novel process of fabricating single crystal diamond structures and diamond-CNT hybrids using the highly non-equilibrium technique points towards a new promising direction for fabricating next-generation diamond-CNT based devices.
Section snippets
Experimental details
The double-walled and multi-walled CNTs were grown using a thermal CVD system. The substrate, a polished Si wafer, was placed at the center of the horizontal quartz tube inside the furnace (30 cm in length and 4 cm in diameter). The FeCl2 powder (∼99.9% pure, Kojundokagaku Laboratory) in a quartz boat was also placed inside the quartz tube which acts as a catalyst during the synthesis of the CNTs. The chamber pressure was maintained at 10−3 Torr during the heating cycle, and the chamber was
SEM
We performed field emission SEM analysis to examine changes in the CNTs before and after PLA and to investigate the size distributions and growth characteristics of the nanodiamonds formed by the PLA process. Fig. 2 (a), (b), and (c) show the SEM image of the CVD grown CNTs (1-D structure) before PLA. The diameters of these nanowires are in the range of 10–50 nm and their lengths are up to several micrometers. At a higher magnification, in Fig. 2 (b) and (c), no evidence of diamond or any other
Conclusions
In summary, diamond is directly formed from CNTs as a result of nanosecond laser melting of the sp2-bonded carbon of CNTs in a super-undercooled state and subsequent ultrafast quenching. The process takes place at room temperature and pressure in air and can be scaled up easily for industrial applications by translating the sample or scanning the 100–200 Hz pulses of the laser beam. The EBSD and Raman spectroscopy of the laser irradiated CNTs illustrate the structural and bonding
Acknowledgments
This research was supported by the Army Research Office (ARO) grant number W911NF-17-1-0596. The authors would like to acknowledge the use of the analytical instrument facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation. RS also acknowledges the support of faculty start-up funding at Oklahoma State University. The authors are very pleased to thank Lew Reynolds and John Prater for their useful feedback. We wish
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