AVS 70 Session SE-MoM: Plasma-Assisted Surface Modification and Deposition Processes/Nanostructured and Multifunctional Coatings

MAX phases are a family of atomically laminated ceramics where M is a transition metal, A is a group A element, and X is C or N. These materials are a playground for design of both three- and two-dimensional (3D/2D) phases, for diverse applications. MAX phases are to date primarily synthesized in powder form, but we present epitaxial thin films of Ti₃AlC₂ and Ti₃SiC₂ on sapphire through magnetron sputtering from three elemental targets. We show that Ti₃AlC₂ can be converted to 2D Ti₃C₂ MXene through selective etching of Al in hydrofloric acid (HF), while Ti₃SiC₂ can be transformed into 3D Ti₃AuC₂, Ti₃Au₂C₂ and Ti₃IrC₂ by noble metal substitution reaction, the latter forming high-temperature-stable Ohmic contacts to SiC. Evidence is also presented for synthesis of single-atom thick layers of gold by selective removal of Ti₃C₂ from Ti₃AuC₂ by Murakami’s reagent. Insight into these 3D and 2D materials and the methods by which they are formed is given through a combination of first principles simulations and electron microscopy, which suggest additional pathways for design of new phases.

Thin crystalline films are commonly deposited on bulk solids and the development of texture (preferred orientation) in such thin films is reasonably well understood.¹ Efforts to grow highly crystalline thin films have included, for example, the use of low-energy ion irradiation² and van der Waals (vdW) epitaxy.³ Exciting developments based on vdW epitaxy include the use of vdW buffer layers (e.g., graphene, and hBN) on crystalline substrates to grow highly oriented MoS₂ and (VNbTaMoW)S₂ thin films.^{4, 5} Recently, Koichi et al.⁶ reported that Ta₂C thin films sputter-deposited on hBN/Ta₂C surfaces are more highly oriented than those grown using the same deposition parameters on bare Ta₂C. These results led us to question the need for a bulk substrate and if crystalline thin films can be directly deposited on free-standing vdW layers instead.

Here, we present results obtained from sputter-deposition of Ta₂C films on monolayer-thick graphene substrates and on relatively thicker (~8 nm) amorphous silicon nitride (a-SiN_x) membranes supported by transmission electron microscopy (TEM) grids. Using plan-view TEM and selected area electron diffraction, we compare and contrast the microstructures of the Ta₂C films on graphene and a-SiN_x. We find that the Ta₂C layers deposited on a-SiN_x are composed of a mixture of nanoscale crystallites and non-crystalline phases, while the Ta₂C film on graphene is polycrystalline with grains that are oriented in-plane as [2-1-10]_film|| [10-10]_graphene. These results indicate that even a single-atom-thick crystal can promote crystalline and oriented growth.

References

1. J. E. Greene, Critical Reviews in Solid State and Materials Sciences 11 (3), 189-227 (1983).
2. T. Lee, H. Seo, H. Hwang, B. Howe, S. Kodambaka, J. E. Greene and I. Petrov, Thin Solid Films 518 (18), 5169-5172 (2010).
3. A. Koma, K. Sunouchi and T. Miyajima, Microelectronic Engineering 2 (1), 129-136 (1984).
4. A. Deshpande, K. Hojo, K. Tanaka, P. Arias, H. Zaid, M. Liao, M. Goorsky and S. K. Kodambaka, ACS Applied Nano Materials 6 (4), 2908-2916 (2023).
5. K. Tanaka, H. Zaid, T. Aoki, A. Deshpande, K. Hojo, C. V. Ciobanu and S. Kodambaka, Nano Lett. 24 (1), 493-500 (2024).
6. K. Tanaka, P. Arias, K. Hojo, T. Watanabe, M. E. Liao, A. Aleman, H. Zaid, M. S. Goorsky and S. K. Kodambaka, Nano Lett. 23 (10), 4304-4310 (2023).

Plasma surface interaction has been a critical area of research for many applications such as Plasma-Enhanced Atomic Layer Etching (PEALE). To meet the demanding needs of more advanced atomically controlled microfabrication methods, the physics of PEALE needs to be better understood to enable high quality, repeatable and controllable deposition process. Several challenges that need to be addressed regarding PEALE include damage to the substrate from highly energetic species and UV radiation, need for precise amorphous/crystalline modulated selective layer deposition, conformality in coating non-uniform substrates, achieving an aspect ratio of >100, repeatability and controllability of the finish. To address these challenges, we are developing laser diagnostics methods to measure species over substrates by advanced laser diagnostics such as femtosecond- Two-Photon Absorption Laser Induced Fluorescence (fs-TALIF) to image atomic species over substrates. Here we present measurements of N, O atom densities over a substrate with high spatial (< 10 microns) and temporal resolution (<1 ns) using fs-TALIF at pressures of 5-150 mTorr. Temperature was measured over a substrate surface using NO 2-line LIF using femtosecond laser excitation.

While several studies around the usage of nitrogen-incorporated tetrahedral amorphous carbon (ta-C:N) in electroanalysis have been published, they mainly focus on ta-C:N films deposited on conductive substrates. This is due to the relatively high resistivity of ta-C:N compared to other carbon and metal-based electrodes: without the electrically conducting substrate, there are high ohmic losses in the ta-C:N when subjected to an electrical current. This limits the use of ta-C:N in optically transparent electrodes (OTEs), which must be deposited on optically transparent (typically electrically insulating) substrates such as quartz. In this study we deposited 50 nm of ta-C:N by laser controlled pulsed cathodic vacuum arc (Laser-Arc) onto insulating quartz substrates to investigate the electrochemical response compared to the same film deposited on conductive silicon. To test the responses of the films, we performed electrochemical oxidation/reduction of potassium ferrocyanide during cyclic voltammetry (CV). Here we find that no oxidation or reduction during CV could be observed at the ta-C:N electrode deposited on quartz. To address this and to maintain optical transparency over the visible wavelength range, we then introduced a 5 nm chromium (Cr) interlayer deposited by magnetron sputtering between the ta-C:N and quartz. While this electrode configuration led to clear cathodic and anodic CV peaks of potassium ferrocyanide, the peak separation compared to the ta-C:N deposited on conductive silicon was increased. That finding indicates that the electrode has a higher resistance. However, we further improved ta-C:N’s electrode functionality on quartz by optimizing the Cr sputtering conditions and introducing a plasma pretreatment by a single-beam ion source. Atomic force microscopy revealed that these changes caused an improved Cr growth homogeneity, which led to the enhanced electrical conductivity. These results show that ta-C:N’s potential as an OTE is not precluded by its high ohmic losses on insulating substrates. In fact, the promise of mechanically stable and electrochemically active ta-C:N requires only that a conductive interlayer be used, and these films could impact the realms of optical materials, flexible electronics, sensors, and more.

Bismuth molybdates, a family of Bi-Mo-O materials with diverse elemental compositions and crystalline structures, have been extensively investigated for their excellent catalytic properties in the oxidation of lower olefins. These properties facilitate the synthesis of organic chemicals widely used in the plastic industry. Their photocatalytic activity under visible light has recently been demonstrated in micro and nano powder samples. However, there has been limited research on the deposition and characterization of Bi-Mo-O materials as thin films. Since 2016, Matova et al. have shown that sputtering deposition using Bi and Mo targets in an Ar+O₂ atmosphere is feasible, demonstrating the visible-light photocatalytic degradation of dyes in water. Despite these promising results, the broader application of this bismuth-based photocatalyst has seen limited advancement. To address this gap, our research group has renewed the investigation of Bi-Mo-O semiconductors for photocatalytic degradation of recalcitrant pollutants, leveraging their active response to visible light. In this study, we report on the thin-film deposition of Bi-Mo-O samples using a co-sputtering system with Bi₂O₃ and Mo targets. The direct current power applied to the Mo target varied from 20 to 60 W, while the RF power applied to the Bi₂O₃ target was fixed at 30 W, allowing us to achieve films with different Bi/Mo ratios. The substrate was pre-heated to 420 K and rotated at approximately 10 RPM to ensure film uniformity. Our results indicated that the films transitioned from crystalline to amorphous as the Mo content increased. Annealing experiments using rapid thermal processing equipment, introducing air into the chamber, performed at 773.15, 873.15, and 973.15 K for 20 minutes for each sample, aiming to obtain various Bi-Mo-O crystalline structures. The structure variations, optical band gap, and bonding-composition analysis are presented. Mo-rich samples presented high optical absorption with band gaps below 0.5 eV, but most samples presented band gaps in the visible-range. Pure phase Bi-Mo-O films and heterostructures containing MoO_x phases were obtained and tested for the degradation of the indigo carmine dye under visible light.

Ion irradiation is a key tool for controlling epitaxy-to-nanostructure, phase content, and physical properties of refractory ceramic thin films grown by magnetron sputtering. Until recently, thin film growth relied on enhancing adatom mobility in the surface region by inert and/or reactive gas ion irradiation to obtain dense layers at low deposition temperatures. Development of high-power pulsed magnetron sputtering (HiPIMS), which provides metal-ion plasmas with tunable degree of ionization, enabled systematic studies of the effects of metal-ion irradiation on properties of refractory ceramic thin films. A motivation for the use of metal-ions stems is that they are film constituents, hence they can provide the benefits of ion-mixing without causing the high compressive stresses associated with trapping of gas ions at interstitial sites.

This presentation reviews growth experiments of transition metal nitride model systems including TiAlN, TiSiN, VAlN, TiTaN, TiAlTaN, and TiAlWN. Film synthesis is carried out in a hybrid configuration with one target powered by HiPIMS and other operated in direct current magnetron sputtering (DCMS) mode.A substrate bias potential V_s is synchronized with the metal-ion-rich portion of the HiPIMS pulses to control the metal-ion energy. The time-resolved mass spectrometry analyses performed at the substrate position enables us to suppress the role of gas ion irradiation and select intense

Irradiation with lower-mass metal-ions (Al⁺ or Si⁺) results in near-surface implantation with the depth controlled by V_s amplitude. This enables synthesis of metastable ternary cubic Me₁Me₂N solid solutions far above the Me₁N concentration range achieved by DCMS. At the other end, bombardment of the growing film surface with pulsed high-mass metal ion fluxes (W⁺ or Ta⁺)during hybrid HiPIMS/DCMS high-rate deposition of dilute Ti_1–xTa_xN, Ti_1–x–yAl_xTa_yN, and Ti_1–x–yAl_xW_yN alloys provides high fluxes of low energy recoils and results in fully-dense, low-stress, hard and superhard coatings without external substrate heating (temperature ≤130 ^oC).

In this work, heterojunction materials of cadmium sulfide (CdS) with carbon nitride (g-C₃N₄) were prepared and the photocatalytic activity in hydrogen (H₂) production was studied using an ethanol-water solution. The influence of ammonia (NH₃) on the physicochemical and photocatalytic properties of g-C₃N₄ was investigated. It was evaluated in the photocatalytic H₂ production, obtaining a null response, but the g-C₃N₄ exhibit activity in the photodegradation of the indigo carmine dye (IC) solution using blue LED light. From the analysis of the results, a parameter defined as SA/(WCA*gap) (surface area (SA); water contact angle (WCA) and photon absorption (band gap)) is proposed to show how the different surface parameters in the photocatalytic response. Subsequently, the effect of the amount of g-C₃N₄ on the heterojunction formed with the CdS nanofibers, which were synthesized in the solvent mixture of ethylenediamine and butanol, was studied. The heterojunction of CdS/g-C₃N₄ was carried out using the g-C₃N₄ synthesized by polymerization with 1 mL of hydrazine (UH1, the g-C₃N₄ with the maximum value of SA/(WCA*gap) and with the g-C₃N₄ synthesized with 2 mL of hydrazine (UH2, the g-C₃N₄ that presented the greatest physical-chemical and optoelectronic modification). The CdS heterojunctions with the modified g-C₃N₄ exhibited a high H₂ production rate of (5258 μmol h⁻¹g⁻¹) ~2.0 times higher than the unmodified CdS nanofibers. The increase in H₂ production rates of the heterojunctions was related to the coupling of the CdS nanofibers on the surface of g-C₃N₄ lamellar plate: (1) result of a better capacity to absorb visible light; (2) the lower resistance to charge transfer, decreasing the recombination of the e^-/h⁺ pairs. For the heterojunctions, the increase in photocatalytic activity suggests that the coupling of the CdS materials with g-C₃N₄ was satisfactorily achieved, observing a synergy of CdS with g-C₃N₄. Physical mixtures equivalent to heterounions were made, and they presented a low rate in the evolution of H₂, the low activity is due to the fact that there is no coupling between the CdS nanofibers and g-C₃N₄.

Nowadays, Thin Dense Chromium (TDC) coatings are being industrially used in rolling bearings applications due to their claimed advantages such as high hardness, low wear, and good corrosion resistance. However, despite their broad commercial use, very little has been published in the open scientific literature regarding their microstructure, nanomechanical and tribological properties.

In this presentation, TDC coatings with a thickness of about 5 µm were deposited by a customized electrochemical process on 100Cr6 bearing steel substrates. Surface microstructure and chemical composition analysis of the TDC coatings was carried out by X-ray Diffraction, Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy, and Atomic Force Microscopy. The results revealed a coating with a dense, nodular, and polycrystalline microstructure. Unlike standard electrodeposited “Hard Chromium” coatings, TDC coatings show no presence of micro/nano-cracks, likely contributing to their superior corrosion resistance. The nanomechanical behavior, studied by nanoindentation as a function of penetration depths exhibits a pronounced size effect near the coating surface, that can be linked to the nodular microstructure. A hard surface with hardness H_IT ~ 25 GPa and reduced Young’s modulus E_IT ~ 300 GPa at 150 nm depth from the surface was observed. Tribological characterization was performed by two single-contact tribometers using coated and uncoated steel balls against flat steel substrates. An in-house fretting wear rig was used to measure the lubricated friction coefficient in pure sliding conditions, whilst the friction performance in rolling/sliding lubricated conditions was evaluated using a WAM test rig. In pure sliding, both steel/steel and TDC/TDC show similar friction coefficients. However, under rolling/sliding conditions with 5% sliding, the traction coefficient of TDC/TDC coating contact was 22% lower than that for steel/steel contact. The tribological results obtained in various contact conditions demonstrate the benefits of applying TDC coatings to reduce bearing friction.