Amorphous Carbon Thin Films Research Articles

TEMAS: A Flexible Non‐AI Algorithm for Metrology of Single‐Core and Core‐Shell Nanoparticles from TEM Images

AbstractAn essential application of electron microscopy is to provide feedback to tune the fabrication of nanoparticles (NPs). Real samples tend to follow a size distribution commonly linked to the synthesis process used and in turn to their functional properties. This study presents an algorithm for measuring particle size distributions in electron microscopy images. State‐of‐the‐art methods based on Artificial Intelligence (e.g., Deep Learning) require extensive datasets of labeled images similar to those expected to be analyzed, and extensive supervised re‐training is often required for cross‐domain application. In contrast, the non‐AI algorithm described in this study is accurate and can be quickly set up for measuring new experimental images in different domains. The accuracy of the method is validated quantitatively and comparing graphical and descriptive statistics. Different size distributions are measured on images of platinum and gold nanocatalysts supported on carbon black, amorphous carbon, and titanium dioxide crystals. Also, images of platinum‐iron core‐shell NPs supported on thin amorphous carbon film are successfully analyzed. The limitation of evaluating different algorithms for NPs metrology is the lack of standards that different researchers can use as ground truth. In order to overcome this limitation, the images and the ground truth measurements presented here are shared as an open dataset.

A review of plasma-assisted deposition methods for amorphous carbon thin and ultrathin films with a focus on the cathodic vacuum arc technique

A review of plasma-assisted deposition methods for amorphous carbon thin and ultrathin films with a focus on the cathodic vacuum arc technique

Electron-Beam-Induced Carbon Contamination in STEM-in-SEM: Quantification and Mitigation

Abstract Contamination is an undesired side effect in many electron microscopy studies that covers structures of interest and degrades resolution. Although contamination has been studied for decades, open questions remain regarding favorable imaging conditions for contamination minimization and the efficiency of contamination-mitigation strategies. This work focuses on electron-beam-induced carbon contamination in scanning transmission electron microscopy at electron energies of 30 keV and below. A reliable method to measure contamination thicknesses was developed in this work and enables the identification of imaging conditions that minimize contamination. Thin amorphous carbon films were used as test samples. The variation of important imaging parameters shows that the contamination thickness increases with the reduction of the electron energy to about 1 keV but decreases below 1 keV. Contamination increases with the beam current but saturates at high currents. Applying a given dose with a high dose rate reduces contamination. Among the tested contamination-mitigation methods, plasma cleaning and beam showering are most effective. Most experiments in this work were performed with focused scanning illumination. Experiments were also carried out with a stationary defocused beam for comparison with a theoretical contamination model with good agreement between measured and calculated contamination thickness.

Amorphous Carbon Films for Electronic Applications.

While various crystalline carbon allotropes, including graphene, have been actively investigated, amorphous carbon (a-C) thin films have received relatively little attention. The a-C is a disordered form of carbon bonding with a broad range of the CC bond length and bond angle. Although accurate structural analysis and theoretical approaches are still insufficient, reproducible structure-property relationships have been accumulated. As the a-C thin film is now adapted as a hardmask in the semiconductor industry and new properties are reported continuously, expectations are growing that it can be practically used as active materials beyond as a simple sacrificial layer. In this perspective review article, after a brief introduction to the synthesis and properties of the a-C thin films, their potential practical applications are proposed, including hardmasks, extreme ultraviolet (EUV) pellicles, diffusion barriers, deformable electrodes and interconnects, sensors, active layers, electrodes for energy, micro-supercapacitors, batteries, nanogenerators, electromagnetic interference (EMI) shielding, and nanomembranes. The article ends with a discussion on the technological challenges in a-C thin films.

Single-beam plasma source deposition of carbon thin films.

A single-beam plasma source was developed and used to deposit hydrogenated amorphous carbon (a-C:H) thin films at room temperature. The plasma source was excited by a combined radio frequency and direct current power, which resulted in tunable ion energy over a wide range. The plasma source could effectively dissociate the source hydrocarbon gas and simultaneously emit an ion beam to interact with the deposited film. Using this plasma source and a mixture of argon and C2H2 gas, a-C:H films were deposited at a rate of ∼26 nm/min. The resulting a-C:H film of 1.2 µm thick was still highly transparent with a transmittance of over 90% in the infrared range and an optical bandgap of 2.04eV. Young's modulus of the a-C:H film was ∼80GPa. The combination of the low-temperature high-rate deposition of transparent a-C:H films with moderately high Young's modulus makes the single-beam plasma source attractive for many coatings applications, especially in which heat-sensitive and soft materials are involved. The single-beam plasma source can be configured into a linear structure, which could be used for large-area coatings.

Influence of Si atoms on the structure and electronic properties of amorphous DLC films

The paper considers the effect of silicon on the formation of the structure and properties of amorphous diamond-like carbon (DLC) films. Thin films of silicon-modified amorphous diamond-like carbon (DLC:Si) were synthesized by magnetron ion-plasma co-sputtering of a combined target. Studies of the structure and properties of amorphous DLC:Si films were carried out in the range of silicon concentrations up to 6.75 at.%. A change in the formation of the film structure is shown using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS) revealed a significant change in the local structure of amorphous DLC:Si films. Which, in turn, leads to a change in the density of states of π-electrons in the band and, as a rule, to a change in the electronic properties.

Investigation of Vacuum Arc-Deposited ta-C and ta-C:N Thin Films on Silicon and Stainless-Steel Foil Substrates Using Raman Spectroscopy

In this paper, we investigated the vacuum arc-deposited tetrahedral amorphous carbon (ta-C) and nitrogen-doped ta-C (ta-C:N) thin films grown on silicon and stainless-steel foil substrates by the visible Raman Spectroscopy. We found that carbon films grown on silicon surfaces prefer more sp3 hybridized carbon compared to the stainless-steel foil, which prefer more sp2 hybridized carbon even though the films are grown concurrently under the same conditions. The impact energy of plasma ions modifies the interfacial layer formation, giving a strong dependence of the sp2/sp3 ratio with the substrate bias, behaving differently for different substrates. However, nitrogen doping in amorphous carbon thin films helps to increase sp2 contents uniformly on both the surfaces for a fixed substrate bias. Understanding such interfaces are of interest for many electronic, optoelectronic, and energy storage devices as these interfaces get buried and can influence the properties significantly.

Electrochemical Generation of Hot Electrons in Fully Aqueous Solutions at Tetrahedral Amorphous Carbon Thin Film Electrodes and Electrochemiluminescence Immunoassay of Serum Amyloid A

AbstractElectrochemistry of hot electrons in fully aqueous solutions at tetrahedral amorphous carbon thin film electrodes is discussed. The generation of these highly reducing chemical species was confirmed by normal pulse voltammetry and several electrochemiluminescent systems. Electron transfer into pre‐existing solvent cavities was observed at approximately −2.65 V vs. Ag/AgCl (sat.). Electrogenerated hot electrons were utilized as chemiluminescent mediators in heterogeneous sandwich immunoassay of Serum Amyloid A. The calibration curve was linear over four orders of magnitude and the detection limit was 85 ng L−1 that demonstrates the efficiency of hot electron generation at this electrode material.

Thermal stability of the magnetic moment in amorphous carbon thin film - An experimental and ab-initio study

The effect of vacuum annealing on the microstructure and its consequence on the thermal stability of magnetic moment in amorphous carbon (a-C) thin film is studied. The magnetic a-C thin film of thickness 8 nm is vacuum annealed for 2 h at 523 K and 923 K. Microstructural and chemical characterization using Raman spectroscopy and x-ray photoelectron spectroscopy, respectively, reveals that with increasing annealing temperature, the sp2 fraction in the film increases. The magnetic measurement carried out at room temperature (RT) using SQUID-VSM reveals a weak ferromagnetic signal in the film due to the interaction between the unpaired sp2 sites, which prevails till 923 K. In the as-deposited sample, an isolated sp2 atom surrounded by sp3 atom results in the paramagnetic contribution, and an sp2-sp2 atom with twisted π-orbitals results in weak ferromagnetic coupling. Compared with the as-deposited case, initially, the saturation magnetization increases when the sample is annealed at 523 K and later decreases when annealed at 923 K. Subsequently, the ab-initio molecular dynamics calculation predicts that due to the increase in sp2 fraction, the net magnetization of the system increases from 300 K to 523 K. π-Orbital Axis Vector (POAV) analysis shows that further annealing to 923 K enhances π-orbital overlapping, which leads to the annihilation of moments sites. The present study differentiates between the two structures that contribute to the magnetism in a-C in terms of their thermal stability. Our simulation indicates that the twisted π-orbitals structure is thermodynamically more stable than the structure where sp3 surrounds sp2 hybridized atom.

Carbon-film-based Zernike phase plates with smooth thickness gradient for phase-contrast transmission electron microscopy with reduced fringing artefacts.

Phase plates (PPs) in transmission electron microscopy (TEM) improve the contrast of weakly scattering objects under in-focus imaging conditions. A well-established PP type is the Zernike (Z)PP, which consists of a thin amorphous carbon (aC) film with a microscaled hole in the centre. The mean inner potential of the aC film is exploited to shift the phase of the scattered electrons while the unscattered electrons in the zero-order beam propagate through the hole and remain unaffected. However, the abrupt thickness increase at the hole edge induces an abrupt change of the phase-shift distribution and leads to fringing, that is, intensity oscillations around imaged objects, in TEM images. In this work, we have used focused-ion-beam milling to fabricate ZPPs with abrupt and graded thickness profiles around the centre hole. Depending on the thickness gradient and inner hole radius, graded-ZPP-TEM images of an aC/vacuum interface and bundles of carbon nanotubes (CNTs) show strongly reduced fringing. Image simulations were performed with ZPP-phase-shift distributions derived from measured thickness profiles of graded ZPPs, which show good agreement with the experimental images. Fringing artefacts, that is, intensity oscillations around imaged objects, are strongly reduced for Zernike phase plates with a graded thickness profile around the centre hole. Focused-ion-beam milling is used to fabricate graded Zernike phase plates with specific inner hole radius and thickness gradients. The phase-shift distribution is obtained from measured thickness profiles around the centre hole. Image simulations based on experimentally measured thickness/phase-shift distributions show good agreement with experimental Zernike phase-plate TEM images.

Tribo-functional PVD thin films deposited onto additively manufactured Ti6Al7Nb for biomedical applications

For biomedical applications, the additive manufacturing of titanium-based alloys in combination with a subsequent physical vapor deposition (PVD) of tribo-functional thin films enables producing complex-shaped implants and devices with improved tribological behavior. Titanium nitride (TiN), titanium carbonitride (TiCN), amorphous carbon (a-C), and Ag-containing amorphous carbon (a–C:Ag) thin films were coated on laser powder bed fused (L-PBF) Ti6Al7Nb substrates by magnetron sputtering. TiN exhibits a high adhesion on Ti6Al7Nb, whereas TiCN, a–C, and a–C:Ag have a lower adhesion strength. In lubricated tribometer tests against Al2O3, the PVD thin films are highly effective in improving the tribological properties of additively manufactured Ti6Al7Nb. TiCN, a–C, and a–C:Ag show lower friction than uncoated Ti6Al7Nb and TiN, with a–C and a–C:Ag having the lowest coefficients of friction. Compared to uncoated Ti6Al7Nb, the PVD films also considerably reduce both the wear and counterpart wear.

Friction wear characteristics of a femtosecond laser-patterned amorphous carbon thin film

In this study, an amorphous carbon thin film on a cemented carbide substrate was prepared with surface micropatterns using femtosecond laser machining. The micropatterns that were considered were made of lines, dotted lines, and zigzag dotted lines. We performed a tribology test using a ball-on-disk friction wear test and investigated the coefficient of friction (COF) and wear amount as a function of pattern size and shape. The micropattern made using femtosecond laser patterning exhibited better friction coefficient properties than the bare amorphous carbon samples. Moreover, the pattern size and shape influenced the friction coefficient value of the patterned samples. The dotted line pattern shape showed superior results in friction coefficient characteristics compared to those of other micropatterns.

Investigating the effect of thickness on the structural and magnetic properties of carbon thin film

The effect of thickness on the magnetic property of amorphous carbon thin film is reported. Three different carbon thin films are deposited by varying the deposition time keeping all the parameters fixed. Microstructural characterization carried out using Grazing Incidence X-ray Diffraction, and Raman spectroscopy indicates the films are amorphous with the presence of molecular units whose edges are bonded with sp3 hybridized atoms. The chemical characterization performed using X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge spectroscopy (XANES) indicates that the sp3 fraction and the conjugation between the sp2 hybridized atoms increases with increasing film thickness. The magnetic property studied using SQUID–VSM reveals the existence of weak ferromagnetic coupling between the unpaired spins present at sp2 sites. The saturation magnetization of the film decreases with increasing thickness. However, estimated magnetic moment/area reveals that there lies a threshold of thickness beyond which the insignificant enhancement in the magnetic moment of the film occurs.

Well-adhered hydrogenated amorphous carbon thin films on ferrous alloy using silicon-containing interlayers at low temperatures

Si-containing interlayers are commonly used to overcome the low adhesion of DLC films to steel. However, a relatively high deposition temperature of 300 °C is necessary to deposit interlayers that ensure adhesion when tetramethylsilane is used as precursor. Moreover, oxygen impairs the adhesion between thin films and ferrous alloys. This work aims to verify the influence of low background pressure in the deposition of a-SiCx:H interlayers using tetramethylsilane at low deposition temperatures. Also, this letter contains a systematic study of the physicochemical properties of the interlayer and its influence on the film's structure. The results show that a background pressure of 0.8 Pa suits the formation of well-adhered a-C:H thin films even at temperatures as low as 85 °C. Chemical analysis associates this outstanding increase in adhesion with fewer oxygen found within the interlayer and at both interfaces when compared to previous results at high background pressures.

Optimizing mechanical properties in single-layered and multi-layered amorphous carbon coatings

Hydrogenated amorphous carbon coatings improve mechanical performance characteristics on a variety of substrates, resulting in widespread industry applications requiring a combination of high hardness, low coefficient of friction, wear resistance, elasticity, and adhesion. However, such a combination of properties is difficult to achieve. We investigated the structure, mechanical properties and scratch behavior in single-layered and multilayered amorphous carbon thin films made by plasma immersion ion implantation-based plasma enhanced chemical vapor deposition. The elastic modulus and hardness in single-layered films were improved up to 149 GPa and 17 GPa respectively by varying the applied bias voltage and pulse width. We observed a non-equivalent impact on mechanical properties and coating structure based on whether pulse width or frequency were varied in the duty cycle range of 1–12.5%. Mechanical properties were independent of pulse frequency within experimental uncertainty. All the single-layered films showed exceptional elastic recovery of ~70%. Film microstructures as measured by Raman microscopy showed that the maxima in modulus/hardness as a function of bias voltage correlated to films with the highest sp3 bonding fraction, highest sp2 disorder, and overall lowest disorder. The maxima in hardness and elastic modulus as a function of pulse width correlated with intermediate sp3 bonding fraction and highest overall disorder, but the sp2 disorder was nearly unchanged. Based on single-layer results, multilayers were developed. Nanoindentation and scratch testing showed that while multilayering preserved high hardness and improved friction behavior irrespective of the recipe, the scratch volume was strongly correlated to the hardness.

Anti-Stokes Raman scattering in amorphous carbon thin films

Anti-Stokes Raman scattering in amorphous carbon thin films

Application of a microwave synthesized ultra-smooth a-C thin film for the reduction of dielectric/semiconductor interface trap states of an oxide thin film transistor

Improvement in TFT performance has been achieved by introducing a solution-processed ultra-smooth nitrogen-doped amorphous carbon (a-C) thin film in between gate dielectric and semiconductor.

Influence of amorphous carbon interlayers on nucleation and early growth of lithium metal at the current collector-solid electrolyte interface

Nucleation and early growth of Li metal is critical to the performance of anode-free solid-state batteries. We report the use of amorphous carbon deposited by direct current magnetron sputtering as an intermediate layer between the Cu current collector and the Lipon solid electrolyte. The density, conductivity, and microstructure of the carbon interlayer are varied and their influence on the reversible formation and removal of the Li metal anode is investigated. It is shown that thin films of amorphous carbon act as seed layers, reducing the overpotential for Li plating and increasing the critical current density for Li plating and stripping from 2 up to 8 mA cm−2. It is further demonstrated that the ionic conductivity of the Li ions in the carbon interlayers determines their optimum thickness to be 100 nm or less, and that the initial Li loss due to interphase formation can be reduced to a few tens of nm by decreasing the density of the carbon films.

Laterally controlled ultra-low energy ion implantation using electrostatic masking

We describe the ongoing development of an ultra-low energy (ULE) ion implantation system for the implementation of a laterally controlled ion incorporation into 2D materials like graphene or transition metal dichalcogenides (TMDs). For this purpose, we use ion optics simulations to demonstrate how the generation of an electrostatic potential gradient laterally along the sample surface can be used to control the size of the implanted sample area. The simulations are complemented by experiments on the incorporation of 25eV Mn ions into thin tetrahedral amorphous carbon (ta-C) films on Si substrates. With the help of RBS measurements, it is shown how potential gradients of different strengths influence the amount of introduced Mn.

Effect of atomic hydrogen exposure on hydrogenated amorphous carbon thin films

The effect of atomic hydrogen exposure on hydrogenated amorphous carbon (a-C:H) films was investigated by X-ray photoelectron spectroscopy (XPS). From the dependence of the wide-scan XPS spectra of an a-C:H film on atomic hydrogen exposure, it was shown that the film was etched with an etching rate of 0.2 nm min−1. In addition, by analyzing the C 1s XPS spectra, the coordination of C atoms in the a-C:H film was investigated as a function of the atomic hydrogen exposure and photoelectron emission angle. This indicated that the coordination of C atoms at the surface of the a-C:H film was not influenced by atomic hydrogen exposure. Therefore, we propose that the depth profile of a-C:H films can be measured with no damage using atomic hydrogen etching.