ICMCTF2019 Session F3-TuA: 2D Materials: Synthesis, Characterization, and Applications

The design and fabrication of an inexpensive and highly efficient electrocatalyst for the hydrogen evolution reaction (HER), were performed by the route of magnetron sputtering. Molybdenum disulfide (MoS₂) was coated directly onto the nanocarbon (C) powder support. Sputtering time was explored as a function of physiochemical composition of MoS₂/C nanocomposites, and its performance in HER. Increased sputtering time gave rise to materials with different compositions and oxidation states of Mo ions, Mo⁴⁺ and Mo⁶⁺, associated with sulfur anions (sulfide, elemental and sulfate) and improved HER outputs. The physiochemical characterisation of the MoS₂/C nancomposites as a function of sputtering time was evaluated using scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman Spectroscopy. An optimised sputtering time of 45 minutes was used to fabricate MoS₂/C nanocomposites. This gave rise to an optimal HER performance in regards to its onset potential (-0.44 mV vs saturated calomel electrode (SCE)), achievable current (-1.45 mVs^-1) and Tafel value (43 mVdec^-1) for the compositions rich in Mo⁴⁺ and sulfide (MoS₂). This bespoke fabricated MoS₂/C nanocomposites were incorporated into the bulk ink utilised in the fabrication of screen-printed electrodes (SPEs) to allow improved electrical wiring to the MoS₂/C and to produce scalable and reproducible electrocatalytic platforms. The MoS₂/C-SPEs displayed far greater HER catalysis with a 450 mV reduction in the HER onset potential and a 1.70 mA cm^{− 2} increase in the achievable current density (recorded at − 0.75 V vs SCE), compared to a bare/unmodified graphitic SPE. The approach of using magnetron sputtering to modify carbon with MoS₂ facilitates the mass production of stable and effective electrode materials for possible use in electrolysers, which are cost competitive to platinum (Pt) and mitigate the need to use time consuming and low-yield exfoliation techniques, typically used to fabricate pristine MoS₂.

The heat generated from electronic devices such as light emitting diodes, batteries, and highly integrated transistors is one of the major causes limiting their performance and reliability. The extraordinarily high thermal conductivity of graphene has let intensive studies for use as a heat spreader. A strategy of further enhancing the thermal conductivity by growing graphene layer on copper will thus proposed in this study. Based on our previous study, it is able to grow graphene layer on copper foil at relative low temperature by using high power impulse magnetron sputtering (HIPIMS) equipped with synchronized substrate bias. The thermal conductivity of the graphene-on-copper (GOC) layer structure was measured, here in this study, based on Angstrom’s method. The thermal conductivity of GOC was significantly enhanced as compared to bare copper foil alone. This is due to epitaxial graphene on copper generated low interfacial thermal resistance, and intrinsic high thermal conductivity of graphene. Finally, a strong correlation between graphene layer thickness and thermal conductivity was reported.

The introduction of dopants, such as nitrogen, into the graphene network, is paramount for many applications such as nanoelectronics, nanophotonics, sensor devices and green energy technology. One way consists in thermal heating of a doped solid carbon source, such as an amorphous a-C:N film, in the presence of a metal catalyst, to obtain nitrogenated graphene (NG) layers. The control of such a process requires to investigate diffusion and segregation mechanisms of the graphene precursor through the metal catalyst.

In the present study, the mechanism of atomic diffusion and NG film growth through a nickel catalyst thin film was investigated using in situ X-ray photoelectron spectroscopy (XPS) performed during thermal heating responsible for NG synthesis. Amorphous a-C:N films, containing 16%at. nitrogen, 10 nm thick, were synthetized by femtosecond pulsed laser ablation on fused silica substrates. A 150 nm thick nickel film was subsequently deposited by thermal evaporation on the a-C:N films. Thermal annealing at various temperatures (200, 300, 500 and 650°C), with different time durations, were performed in ultra-high vacuum during in situ XPS analysis, to carry out the top surface genesis of the NG film onto the nickel catalyst. FEG-SEM, Raman and X-ray absorption (XAS) spectroscopies were also performed to elucidate the nature and chemical composition of NG films. The diffusion of carbon and nitrogen through the nickel film towards the surface from 300°C was observed, without any graphene signature. Graphene films are formed at the highest temperatures, with a final 3%at. nitrogen content, in both pyrrolic and pyridinic configurations. In addition, the kinetics of carbon surface enrichment observed using in-situ XPS is discussed in the frame of the interface segregation theory and modelled using the Du Plessis approach. The solid-state transformation mechanism responsible for the formation of few-layer NG films is thus investigated.

Two-dimensional (2D) materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have recently received lots of attention due to their unique material properties and numerous potential applications. The 2D atomic structure can also facilitate distinct defect formation mechanisms and offer new possibilities for defect engineering.

In my talk, I will present the results from layered molybdenum dichalcogenides (MoS₂, MoSe₂, and MoTe₂), where vacancy, substitutional, interstitial, and grain boundary defects are introduced by electron irradiation or by various chemical treatments. Due to the 2D nature, transmission electron microscopy and scanning tunneling microscopy imaging allows direct monitoring of formation and agglomeration of defects as well as of larger structural changes. First-principles calculations are used to provide microscopic insight into the energetics and kinetics of these processes. The gained understanding together with the computationally predicted defect properties can be used to guide future efforts in tailoring the 2D material properties via defect engineering.

In the following work, we describe studies on the fabrication and the physicochemical performance of thin and free-standing heavy boron-doped diamond (BDD) nanosheets coated by thin nylon 6.6. First, the diamond nanosheets with less than 400 nm of thickness were grown and doped by boron on Ta substrate by using microwave plasma-enhanced chemical vapor deposition technique (MPECVD) [1]. Then, the BDD/Ta samples where covered by 6.6 nylon to improve their stability in harsh environments.

The plasma polymer films, the thickness in the range 500-1000 nm, with different surface energies were obtained by magnetron sputtering of a bulk target. The hydrophilic nitrogen-rich C:H:N :O were prepared by sputtering of nylon 6.6. C:H:N :O as films with high surface energy improves adhesion at ambient condition. However, their disadvantage lays in a natural swelling increasing its volume about of 15% after immersion into an aqueous liquid. This behavior influences diamond-C:H:N :O structure in a wet environment.

The C:H:N :O coated diamond nanosheets were delaminated from Ta substrate creating free-standing nanostructures (Diamond-on-Nylon). The C:H:N :O film fixtures the thin polycrystalline diamond sheets enhancing its mechanical stability and enabling transfer and integration with microelectronic systems.

We have manifested that investigated Diamond-on-Nylon nanostructures possess altered morphology and physicochemical properties, revealed by electron microscopy and Raman spectroscopy. Moreover, the electrical response of investigated nanostructures as conductive electrodes is time-stable and indicates the high activity of the sheets with higher dopant concentrations.

Moreover, the Diamond-on-Nylon is characterized with altered mechanical properties like Young modulus or internal stress. These properties varied strongly with the thickness and density of nylon coverage.

In summary, the Diamond-on-Nylon nanostructures show excellent electrical and thermal conductivity along with high mechanical strength. Composite diamond-on-polymer structures could be further developed for flexible and robust electronic devices or thermal heat spreaders.

Acknowledgments

The authors gratefully acknowledge the financial support of the National Centre for Science and Development Grant Techmatstrateg No. 347324. This work was partially supported by the Science for Peace Programme of NATO (Grant no. G5147). The DS funds of the Faculty of Electronics, Telecommunications, and Informatics are also acknowledged.

References

[1]. Bogdanowicz, Robert, et al. Advanced Functional Materials (2018): 1805242.

[2]. Ryl, Jacek, et al. Carbon 96 (2016): 1093-1105.