Film Exfoliation Research Articles

Cycle Performance of Si Film Negative Electrodes with Lithium Fluoride Coating As an Artificial SEI

Introduction There is a growing need for high-capacity lithium-ion batteries as a power source for electric vehicles and a backup power source for renewable energy. Si negative electrodes, which have about ten times the capacity of conventional graphite electrodes, are expected to be used practically to increase the capacity of lithium-ion batteries. However, there are still issues that need to be addressed before Si electrodes can be used practically. One of them is the decomposition of electrolytes on the negative electrode surface. In conventional lithium-ion batteries, a small amount of additive such as FEC is added to the electrolyte to suppress electrolyte decomposition. However, suppression of the electrolyte decomposition is still difficult for Si electrodes. On the other hand, it has been found that lithium fluoride (LiF) in the SEI film formed from the decomposition products of the electrolyte improves charge and discharge properties of Si electrode.1-3 In this study, we aimed to suppress electrolyte decomposition and improve cycle life by forming a LiF layer as an artificial SEI on the Si electrode using a thin-film technique. Experimental Amorphous Si thin films, 100 nm in thickness, were prepared on copper foil by RF magnetron sputtering. Subsequently, LiF layers were deposited on the Si surfaces by sputtering with varying thicknesses. The thickness of the LiF layers was measured by a stylus surface profiler. The LiF-coated Si films were transferred to an Ar-filled glovebox without exposure to air and punched into disks 13 mm in diameter. A coin-type half-cell (CR2032) was assembled with the LiF-coated Si film as a working electrode and a Li disk of 14 mm in diameter as a counter electrode. A solution of 1 M LiPF6 dissolved in a mixture of ethylene carbonate and diethyl carbonate was used as an electrolyte. Charge and discharge characteristics were measured with a C/3 rate in a thermostatic chamber at 30°C. After cycling, the coin cells were disassembled in the glovebox, the Si electrodes were rinsed with dimethyl carbonate, and the Si electrode surfaces were observed with a scanning electron microscope (SEM). Result and Discussion Figure 1 shows variations in discharge capacity of the Si film electrodes with and without LiF coating. The discharge capacity of the Si electrode without LiF rapidly dropped after 10 cycles, but LiF coating significantly improved cyclability. Capacity retention at the 100th cycle, compared to that at the 5th cycle, increased from 4% without LiF to 52% with LiF coating. LiF coating also improved initial Coulombic efficiency from 89.9% to 92.5%, suggesting that LiF coating suppresses electrolyte decomposition. Si electrode surfaces after 30 cycles were observed by SEM as shown in Fig. 2. Exfoliation of the Si film without LiF was observed in many areas after 30 cycles, but it was suppressed by LiF coating. LiF coating is effective in improving the cycle performance of Si negative electrodes as an artificial SEI. Acknowledgments This work was supported by GteX Program Japan Grant Number JPMJGX23S3 and JSPS KAKENHI Grant Number 2201967.

Friction and Wear Mechanisms of Ti3SiC2/Cu Composites under the Synergistic Effect of Velocity–Load Field at 800 °C

Ti3SiC2/Cu composites were prepared using spark plasma sintering technology, and the effect of the velocity–load bivariate on the tribological behaviors of the Ti3SiC2/Cu-45# steel tribo-pair at 800 °C was investigated. The physical change and frictional chemical reaction during the friction process were analyzed based on the morphology characterization and frictional interface phases. The related friction and wear mechanism model was established. The results showed that the influence of velocity and load on the tribological performance of the Ti3SiC2/Cu-45# steel tribo-pair was not monotonically linear. When both the velocity and load were large, the coordinated effect of the two led to a low friction coefficient (0.52). At 800 °C, the velocity mainly affected the exfoliation and re-formation of the oxide film on the wear surface of the Ti3SiC2/Cu-45# steel tribo-pair, while the load affected the extrusion and fragmentation of the oxide film on the wear surface of the tribo-pair. In the friction process, frictional oxidation was the main influencing factor for the formation of the oxide film. When the velocity and load were small, the main frictional oxide consisted of SiO2−x and a small amount of CuO. When the velocity reached 1 m/s and the load reached 3 N, the oxide film was partially broken down and flaked off, and the matrix of the Ti3SiC2/Cu composite was exposed and oxidized, at which time the oxide film was composed of SiO2−x, TiO2, CuO, and Fe2O3. Under the synergistic effect of the velocity–load–temperature field, the friction and wear mechanism of the Ti3SiC2/Cu-45# steel tribo-pair changed from abrasive wear to frictional oxidation wear with the increase in velocity and load.

Controlled exfoliation of wafer-scale single-crystalline AlN film on MOCVD-grown layered h-BN

In this work, we present a stress-free AlN film with improved crystal quality assisted by h-BN and demonstrate the mechanical exfoliation of wafer-scale single-crystal AlN freestanding membrane and reveal the controllable exfoliation mechanism of AlN. Uniform and continuous wafer-scale h-BN is directly grown on c-plane sapphire using a flow modulation epitaxy mode by metal-organic chemical vapor deposition. The nucleation and evolution processes of quasi-van der Waals epitaxy (QvdWE) of AlN on h-BN are revealed. It is found that O2-plasma-treated h-BN can effectively promote the nucleation islands of AlN and contribute to the release of biaxial stress and the reduction of dislocation density in the epilayers. Eventually, the QvdWE growth of a stress-free AlN film (0.08 GPa) is achieved, and wafer-scale mechanical exfoliation of the AlN membrane has been realized. This work provides an effective strategy for the quality improvement of III-nitride films and paves the way for the vertical structure and flexible deep-ultraviolet optoelectronic devices.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Failure mechanisms of nitrogen-doped diamond-like carbon films: Exfoliation or localized attack behaviour of films under hydrochemistry evolution

Nitrogen-doped diamond-like carbon (N-DLC) films with different nitrogen content were prepared on the surface of 2024 aluminium alloy using plasma-enhanced chemical vapour deposition technology. The failure mechanisms of the N-DLC films during seawater exposure have been investigated. The results demonstrated that nitrogen doping significantly affected the failure behaviour of the DLC films in seawater. Extensive exfoliation of the DLC film was found without the addition of nitrogen. However, when the nitrogen flow rate is 10 sccm and 15 sccm, significant localized corrosion of the DLC film and the aluminium alloy substrate occurred. The difference in the thermodynamic process of corrosion of the N-DLC films was mainly due to the changes in pH value on the local solution caused by the formation and hydrolysis of NH4+. In the DLC film without nitrogen additive, large oxide particles were formed at the film-substrate interface due to the acidification of the solution. Meanwhile, the internal stress of the DLC film was high under this condition. The film was easily exfoliated and ultimately lost its protective effect on the substrate. The corrosion products could not grow sustainably due to the variable pH value when excess nitrogen was added to the DLC film. No extensive exfoliation of the DLC films with 10 sccm and 15 sccm nitrogen addition has occurred, but obvious corrosion channels were formed in the substrate because of the localized corrosion. The evolution mechanism of oxidation behaviour in the presence of variations in local hydrochemistry is also discussed.

Sub-micro organic light emitting diode arrays defined by tip-induced resist hollow structures

Fabrication of micro-nanoscaled organic light-emitting diodes (OLEDs) with uniformity and scalability has emerged as a promising technology in next-generation displays and light sources. Patterning polymer resist into well-defined structures is a crucial step in the fabrication of optoelectronic device arrays, especially for cost-effectively fabricating lithographical sub-micro resist structures. Atomic force microscopy (AFM) ploughing is promising for patterning high-resolution polymer resist structures owing to nanoscaled resolution, high controllability, with relatively low cost under flexible working conditions. However, the defects of structures resulting from polymer debris contamination or spin-coated film exfoliation in ploughing procedure limit their further application in fabrication of device arrays. Herein, lithographical poly(methyl methacrylate) (PMMA) resist patterns in micro-nanoscale with excellent reproducibility and uniformity are achieved via AFM mechanical ploughing at the optimized lithography conditions. Taking use of the isolated PMMA cavities as current channels, we fabricate prototype OLED arrays with pixels of 500 nm and 1 μm. This work paves the way for developing a tip-based mechanical lithography approach to fabricate micro-nanoscaled optoelectronic devices in a simple and low-cost way.

Enhanced surface blistering efficiency of H+ implanted lithium tantalate by chemical reduction modification

The crystal ion slicing (CIS) fabricated heterogeneously integrated lithium tantalate (LiTaO3, LT) thin film is prospective in optical modulators, infrared sensors, and acoustic filters, therefore improving the transferring efficiency of the LT thin film will definitely promote its development and application. In the present work, the chemical reduction method was found effective to enhance the forming ability of H+ implantation-induced defects and surface blistering efficiency of lithium tantalate crystal during the CIS process. According to the investigations by XPS, XRD, RBS, Micro-Raman, and TEM on the reduction-induced defects, H+ implantation-induced defects, and the surface blistering behavior, the treated black lithium tantalate (BLT) present a higher blistering efficiency, confirming the contribution of reduction-induced defects in implantation and blister formation process than the referenced congruent lithium tantalate (CLT). In detail, the improvement is in ascribing to the existing intrinsic oxygen vacancy defects in BLT. These defects increase the forming efficiency of Ta defects in the implantation process and enhance the diffusion of H+ in the blistering formation process, thus decreasing the activation energy for the blistering process and increasing the film exfoliation area. These results highlight an avenue for enhancing ion slicing efficiency of piezoelectric and pyroelectric single crystal oxide.

Radiation effects on He+- and H+-implantation for ion slicing of rutile titanium dioxide thin film

The thin film exfoliation mechanisms of He+- and H+- implanted rutile-phase titanium dioxide (r-TiO2), as well as the surface morphology and lattice damage in the implanted r-TiO2 samples, are investigated and compared. Layer splitting in the He+-implanted r-TiO2 samples after annealing at 350 °C was observed. Due to the formation of He bubbles accumulated by nano-defects during annealing, the peak damage ratio in the damage layer is enhanced. The microcracks induced by He+-implantation found in the damage layer correspond two types of defects, which were analyzed. In contrast, H+-implantation in the r-TiO2 samples induced only little lattice damage, and with low annealing temperature the damage is almost fully removed, suggesting thermal activity of the implanted H+-ions in the r-TiO2.

Degradation process of Ag/AgCl chloride-sensing electrode in cement extract with low chloride concentration

The accelerated degradation process of Ag/AgCl chloride-sensing electrode was investigated in cement extract by electrochemical behaviors monitoring and surface analysis. The results showed that the potential of Ag/AgCl chloride-sensing electrode was negatively shifted in cement extract, accompanied by the continuous dissolution and exfoliation of AgCl film; however, no Ag2O formed on the electrode surface. The adhesive strength of AgCl to Ag substrate maintained the equilibrium potential of Ag/AgCl chloride-sensing electrode during early stage, but the negative potential shift was pronounced during late stage, which was related to the adsorption of OH- at Ag/AgCl interface, suppressing cathodic reaction on the electrode.

The structural evolution of light-ion implanted into GaAs single crystal after annealing

Four groups of He+ and H+ with different dose combinations are implanted into GaAs crystal. After annealing, different phenomena of blisters and exfoliation appear on the surface. High resolution X-ray diffraction (HRXRD) is used to irradiate the surfaces of the four samples, and the diffraction satellite peaks are observed to move to the lower angles. The surface morphology of the four samples after annealing is observed by optical microscope. The depth of craters and the height of blisters are measured by atomic force microscope (AFM), and the critical pressure and internal stress of blisters are calculated. The damage microstructure induced by ion implantation after annealing is observed by transmission electron microscope (TEM), and the stress intensity factor at the crack tip is calculated. The different mechanisms of He+ and H+ in GaAs single crystal are compared. The dose combination of He+ and H+ is optimized to explore a more cost-effective way for GaAs film exfoliation.

Stable two-dimensional pentagonal tellurene: A high ZT thermoelectric material with a negative Poisson’s ratio

Motivated by the recent experimental exfoliation of tellurium thin film, and the amazing physical and chemical properties of various theoretically tellurene allotropes, we design a novel and stable pentagonal structure of tellurene (SP-type Te), and make a detailed comparison with other possible Te allotropes, related to their mechanical, electronic and thermoelectric properties. SP-type Te shows the higher expandability than other Te allotropes, due to its lower layer modulus and Young’s modulus. More interestingly its Poisson’s ratio along x (y) direction is unusually negative (~ −0.01) which can be further decreased by applying coaxial strain. Three-layers SP-type Te can turn into a topological insulator (TI) with a nontrivial band gap ~ 0.03 eV when uniaxial compressional strain σx = −7%. Moreover the higher the number of layers in SP-type Te, the lower σx inducing its topological transition requires. At room temperature, lattice thermal conductivity kL of SP-type Te is the lowest in all Te allotropes, and meanwhile its peak ZT for n-type doping even can reach up to 2.84. Thus if SP-type Te can be synthesized in future, these extraordinary properties would promote it with great potential in designing low-dimensional mechanical, spintronic and thermoelectric devices.

Free-standing membranes from the chemical exfoliation of mesoporous amorphous titania thin film.

Thin films are typically bound to their substrate, limiting their integration on rough, porous, curved or chemically/thermally sensitive surfaces. Instead of employing tedious and expensive back-etching processes, specific chemical routes can enable the exfoliation of such thin structures. Herein, we demonstrate that an alkaline treatment can exfoliate a hybrid thin film comprising amorphous titania embedded in well-ordered block-copolymer micelles, which can be redeposited elsewhere. We provide sufficient evidence of the preservation of pore ordering and the importance of neutralizing the solution to spare the system from the redissolution of the titania species.

Microstructural effects of the substrate on adhesion strength and mechanical properties of TiN Thin Films

Microstructure of base material plays an important role in adhesion strength and mechanical properties of Titanium Nitride (TiN) coating especially to increase the lifecycle of parts when in practical use. Present study covers the influence of grain size/microstructure of plain carbon steel (tailored via heat treatment) on TiN thin films deposited by physical vapor deposition (PVD) technique. Strong effect of grain size (microstructure) on adhesion strength of TiN thin films on annealed, normalized and quenched substrates have been observed. Mechanical characterization of TiN films e.g. Elastic modulus (E), Hardness (Hv), Stiffness (S) etc. have been studied via nano-indentation technique. TiN thin films failure investigation has been performed with Micro scratch testing under progressive load. Film exfoliation under critical loads has been corroborated via scanning electron microscopy (SEM). The results showed that TiN films deposited on fine microstructure substrate possess excellent mechanical properties and good adhesion strength as compared to coarser microstructure substrate. Insights of this study might be helpful in designing engineered thin films on optimized microstructures.

Controlled Cracking for Large-Area Thin Film Exfoliation: Working Principles, Status, and Prospects

The production of flexible monocrystalline semiconductor thin films less than a few tens of micrometers in thickness is currently receiving huge interest in various emerging applications such as mo...

Raman Studies of Graphene Films Grown on 4H-SiC Subjected to Deposition of Ni

Raman spectroscopy is used to evaluate the structural perfection of epitaxial graphene films before and after deposition of a Ni layer to their surface by magnetron sputtering. Two deposition modes with different gas pressures and deposition times are investigated. It is found that Ni deposition under low pressure combined with long deposition time does not lead to the separation of graphene/Ni film. On the other hand, higher pressure and shorter deposition time results in successful but uncontrollable exfoliation of graphene together with the Ni film. The results obtained will serve as the basis for the optimization of Ni deposition modes, in order to achieve complete exfoliation of the graphene film from the SiC substrate without damaging the graphene layer.

Phase Transformation in Germanium-on-Insulator (GeOI) Films Using Raman Spectroscopy and Nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the SMART CUT wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the published SOI literature and bulk germanium results obtained by Liu et al., 2020 [1] and SOI films results obtained by Miller et al., 2007 [2]. From our measured hardness results of 8 to 10 GPa for very thin single crystal Ge(100) films compared to 13.1 GPa for Bulk Ge (100), one can conclude that the thinning of crystalline Ge films leads to a considerable softening of the crystalline Ge resulting in less brittle robust films. Similarly, our measured modulus of 130 to 140 GPa for thin Ge (100) films compared to the reported bulk Ge (100) value of 157 GPa corroborates these research findings. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicate bands of metastable Ge phases @ 220 cm-1, 195 cm-1 and 184 cm-1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm-1, 399 cm-1 and 430 cm-1.Apart from the well documented electrical advantages of GeOI, the reported research on the nanomechanical properties reveals considerable softening and flexibility of the very thin exfoliated single crystal Ge films that promise added benefits and further implications for enhancing potential device application of GeOI wafers for process integration.1.] N. Liu, X-J Yang, Z. Yu, and L. Zhao, Trans. Nonferrous Met. Soc. China, 30, 181 (2020).2.] N. Miller, K. Tapily, H. Baumgart, G.K. Celler, F. Brunier, and A.A. Elmustafa, Mater. Res. Soc. Symp. Proc. Vol. 1021 © 2007 Materials Research Society, 1021-HH05-24. Boston, Massachusetts. (2007).

Realization of wafer-scale single-crystalline GaN film on CMOS-compatible Si(100) substrate by ion-cutting technique

Heterogeneous integration of gallium nitride (GaN) film on complementary metal-oxide-semiconductor (CMOS)-compatible Si(100) substrate provides a material platform for future high-performance chips with multiple functions. In this work, a 2 inch wafer-scale single-crystalline GaN film is transferred from commercialized bulk GaN wafer onto Si(100) substrate by combining ion-slicing and modified surface-activated bonding with a sputtering-deposited Si nanolayer. The H+ implantation fluence for the exfoliation of GaN film is as low as 2.5 × 1017 cm−2 and the full width at half maximum of the (0002) x-ray rocking curve of GaN film is 203 arcsec. The sliced bulk GaN wafer is recycled, which is beneficial to reduce the cost and to enhance the mass application of the ion-cutting technique to GaN. The exfoliation mechanism of H-implanted GaN is investigated. The activation energy for slicing GaN is only 2.08 eV owing to the high quality of the GaN wafer, while the wide residual damage band is still an obstacle to improving the quality of the GaN film. The successful demonstration of wafer-scale single-crystalline GaN film on Si(100) substrate will be of great benefit to the integration of high-performance GaN devices and Si CMOS integrated circuits with mature processing technology.

Water-assisted exfoliation of epitaxial CdTe film from mica analyzed with azimuthal RHEED

Semiconductor thin films grown on weakly interacting substrates have recently attracted much attention due to the fact that one can peel/release the film from its substrate for flexible electronic and optoelectronic applications. In this work we report separable CdTe(111) epitaxial films grown on mica substrate via vapor transport deposition. The CdTe films were separated from the mica substrate using double-sided Cu tape, exposing the CdTe interface-surface. Azimuthal reflection high-energy electron diffraction (ARHEED) was used to characterize the fresh CdTe interface-surface and the original CdTe surface. Dramatic differences between these two surfaces were observed. (1) Diffraction streaks were observed from the interface-surface, indicating a smooth surface. Diffraction spots were observed from the surface, indicating a rough surface. (2) The rough CdTe surface has twin and secondary twin structures, which are difficult to be observed with conventional X-ray diffraction. (3) The full-width-at-half maximum (FWHM) analysis of the streaks and spots in RHEED patterns reveals that despite the smoothness of the interface-surface, there is a lower degree of lateral long-range order as compared with the rough surface. This etchant-free method to create smooth interface-surface and ARHEED characterization may be applicable to other films that are detachable from weakly interacting substrates.

Evaluation of the mechanical properties of germanium-on-insulator (GeOI) films by Raman spectroscopy and nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the Smart Cut™ wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the literature published Silicon-on-Insulator and bulk germanium results. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicates bands of metastable Ge phases @ 220 cm−1, 195 cm−1, and 184 cm−1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm−1, 399 cm−1, and 430 cm−1.

An Effect of Water Presence on Surface Exfoliation of Polypropylene Film Initiated by Photodegradation

A polypropylene (PP) film photodegradation test was performed in water at 60 °C with a specific photocatalyst under visible light irradiation to identify its fragmentation mechanism. The photodegradation degree reached a maximum at the 144 h photodegradation time and showed the decreases at the 288 h. The surface being uneven was observed at the 288 h in the SEM photograph. These results showed that the surface exfoliation was provoked by the test. The spherulite structure contributed to the surface microcrack location and growth. The microcrack allowed entrance of water into the inner amorphous part. The water caused an internal stress by expanding itself due to radiant heat from the visible light irradiation and induced a microcraze in the interlamellar amorphous region. The microcraze developed into an internal microcrack, and surface microcracks finally coalesced together, producing a planar exfoliation. The major long side length of the exfoliation part was below 100 μm at both the 144 h and 288 h. The weight change ratio and rate of the photodegraded PP film showed multi-stages with the increase of photodegradation time. The increment of positive rate was small, showing that the exfoliation caused suppression of the autoxidation rate. The negative rate was due to the exfoliation behavior and was nonlinear to the photodegradation time.