Polycrystalline Ni Films Research Articles

Linear vs. Non-linear Behaviour in Ion Irradiation Nanostructuring of Nickel and Silicon Surfaces

Spontaneous nanometre-scale quasi-periodic ripple-like structures are formed at the surface of polycrystalline Ni films and Si(111) single crystal wafers by irradiation with a broad Ar+ ion beam at room temperature and studied with Atomic Force Microscopy as a function of fluence. The development of these structures can be reproduced by numerical solution of a continuum equation describing the evolution of surface morphology under ion irradiation, using realistic coefficients derived from material properties. In particular, we demonstrate that differences observed in pattern formation on the two surfaces under the conditions studied, such as wavelength stability and exponential growth of interface width for the Ni surfaces compared with wavelength coarsening and interface width saturation on Si(111), can be understood in terms of a cross-over between linear and non-linear behaviours.

Quasi-static strain governing ultrafast spin dynamics

The quasi-static strain (QSS) is the product induced by the lattice thermal expansion after ultrafast photo-excitation. Although the ultrafast spin dynamics driven by the QSS and thermal effects are barely distinguishable in time, they should be treated separately because of their different fundamental actions. By employing ultrafast Sagnac interferometry and the magneto-optical Kerr effect, we demonstrate quantitatively the existence of QSS and the decoupling of two effects counteracting each other in typical polycrystalline Co and Ni films. The Landau-Lifshitz-Gilbert and Kittel equations considering a magnetoelastic energy term showed that QSS, rather than the thermal energy, in ferromagnets plays a governing role in ultrafast spin dynamics. This demonstration provides a way to analyze ultrafast photo-induced phenomena.

Non-volatile voltage control of in-plane and out-of-plane magnetization in polycrystalline Ni films on ferroelectric PMN–PT (001)pc substrates

We identify room-temperature converse magnetoelectric effects (CMEs) that are non-volatile by using a single-crystal substrate of PMN–PT (001)pc (pc denotes pseudocubic) to impart voltage-driven strain to a polycrystalline film of Ni. An appropriate magnetic-field history enhances the magnetoelectric coefficient to a near-record peak of ∼10−6 s m−1 and permits electrically driven magnetization reversal of substantial net magnetization. In zero magnetic field, electrically driven ferroelectric domain switching produces large changes of in-plane magnetization that are non-volatile. Microscopically, these changes are accompanied by the creation and destruction of magnetic stripe domains, implying the electrical control of perpendicular magnetic anisotropy. Moreover, the stripe direction can be rotated by a magnetic field or an electric field, the latter yielding the first example of electrically driven rotatable magnetic anisotropy. The observed CMEs are associated with repeatable ferroelectric domain switching that yields a memory effect. This memory effect is well known for PMN–PT (110)pc but not PMN–PT (001)pc. Given that close control of the applied field is not required as for PMN–PT (110)pc, this memory effect could lead the way to magnetoelectric memories based on PMN–PT (001)pc membranes that switch at low voltage.

Fracture of void-embedded high-entropy-alloy films: A comprehensive atomistic study

Comprehensive molecular dynamics (MD) simulations are performed to study the stress response and deformation mechanism in void-embedded, single-crystal and polycrystalline, high-entropy-alloy (HEA) films under uniaxial tensile loading. Our results reveal that certain void-embedded HEA films can be, by far, superior to pure Ni in terms of tensile ductility and the resistance to crack propagation. The fracture strain of the 10%Co CoCrFeMnNi and the equiatomic CoFeMnNi, respectively, doubles or triples that of the equiatomic CoCrFeMnNi, which still doubles that of pure Ni. Regarding the deformation mechanism, high tensile ductility of HEAs can be attributed to the formation of partial dislocations, nanotwinning and the impediment of the otherwise glissile dislocations due to the lattice distortion effect. The ultimate tensile strength of HEA film shows better resistance against stress deterioration due to elliptical voids. The stress response of the void-embedded, polycrystalline Ni films obeys the reverse Hall–Petch effect, while the void-embedded, polycrystalline HEA films do not.

Quantitative analysis of magnetization reversal in Ni thin films on unpoled and poled (0 1 1) [PbMg1/3Nb2/3O3]0.68–[PbTiO3]0.32 piezoelectric substrates

The field angle dependence of the magnetization reversal in 20 nm thick polycrystalline Ni films grown on piezoelectric (0 1 1) [PbMg1/3Nb2/3O3]0.68–[PbTiO3]0.32 (PMN–PT) substrates is analysed quantitatively to study the magnetic anisotropy induced in the film by poling the piezosubstrate. While the PMN–PT is in the unpoled state, the magnetization reversal is almost isotropic as expected from the polycrystalline nature of the film and corresponding to an orientation ratio (OR) of 1.2. The orientation ratio is obtained by fitting the angular dependence of normalized remanent magnetization to an adapted Stoner–Wohlfarth relation. Upon poling the piezosubstrate, a strong uniaxial anisotropy, whose hard axis is oriented along the [1 0 0] direction of the PMN–PT, is induced, yielding an OR of 3.1. The angular dependence of the coercivity for the poled state is found to consist of a strong increase for increasing field angles away from the easy axis direction and of a sharp decrease for angles close to the hard direction. It is best described by a two-phase model, implying that the magnetization reversal is determined by both, coherent rotation of the magnetic moments, according to the Stoner–Wohlfarth model, and the gradual displacement of the domain walls in obedience to the Kondorsky model.

Modification of magnetic anisotropy in Ni thin films by poling of (011) PMN-PT piezosubstrates

ABSTRACTThis study reports the magnetic and magnetotransport properties of 20 nm thick polycrystalline Ni films deposited by magnetron sputtering on unpoled piezoelectric (011) [PbMg1/3Nb2/3O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The magnetoresistance (MR), as well as the magnetization reversal, is found to depend on the polarization state of the piezosubstrate. Upon poling the PMN-PT substrate, which results in a transfer of strain to the Ni film, the MR value decreases by a factor of 12 at room temperature and a factor of 21 at 50 K for the current direction along the PMN-PT [100] direction, and slightly increases for the [01] current direction. Simultaneously, a strong increase in the room temperature coercive field value is observed, while the ratio between the remnant and saturation magnetization shows a pronounced minimum for the [100] direction, indicating it as a hard axis, induced by poling the piezosubstrate.

Local anodic oxidation of Ni films with (2 0 0) and (1 1 1) texture

Formation of the oxide nanostructures by the local anodic oxidation of textured polycrystalline Ni(111) and Ni(200) films was investigated. It was shown that the local anodic oxidation is significantly affected by the texture of the Ni film: threshold process voltage and time required for the local anodic oxidation initiation is higher for (111) texture than for (200) texture; at the same process parameters (oxidation time, voltage, etc.) the oxide nanostructures formed on the Ni(200) are bigger than for Ni(111).

Electric field modification of magnetotransport in Ni thin films on (011) PMN-PT piezosubstrates

This study reports the magnetotransport and magnetic properties of 20 nm-thick polycrystalline Ni films deposited by magnetron sputtering on unpoled piezoelectric (011) [PbMg1/3Nb2/3O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The longitudinal magnetoresistance (MR) of the Ni films on (011) PMN-PT, measured at room temperature in the magnetic field range of −0.3 T < μ0H < 0.3 T, is found to depend on the crystallographic direction and polarization state of piezosubstrate. Upon poling the PMN-PT substrate, which results in a transfer of strain to the Ni film, the MR value decreases by factor of 20 for the current along [100] of PMN-PT and slightly increases for the [011¯] current direction. Simultaneously, a strong increase (decrease) in the field value, where the MR saturates, is observed for the [011¯] ([100]) current direction. The anisotropic magnetoresistance is also strongly affected by the remanent strain induced by the electric field pulses applied to the PMN-PT in the non-linear regime revealing a large (132 mT) magnetic anisotropy field. Applying a critical electric field of 2.4 kV/cm, the anisotropy field value changes back to the original value, opening a path to voltage-tuned magnetic field sensor or storage devices. This strain mediated voltage control of the MR and its dependence on the crystallographic direction is correlated with the results of magnetization reversal measurements.

Stress engineering using low oxygen background pressures during Volmer–Weber growth of polycrystalline nickel films

A robust strategy for controlling the level of residual stress in polycrystalline films remains elusive, owing to the complex coevolution of the surface, microstructure, and intrinsic stress during Volmer–Weber film growth. Recent improvements in the understanding of stress evolution mechanisms have led to the possibility of engineering the intrinsic stress through the control of thin film growth conditions. Here, the authors demonstrate stress engineering during deposition of polycrystalline Ni films through control of the oxygen partial pressure. The physical mechanisms of stress management during codeposition of nickel and oxygen are investigated using in situ stress measurements and ex situ structural and chemical characterizations. The intrinsic stress in Ni films is affected by grain growth during deposition (which causes a tensile stress) and by Ni adatom trapping at grain boundaries and oxygen incorporation in the Ni lattice (which cause a compressive stress). The authors show direct evidence that a small amount of oxygen suppresses grain growth during deposition. They suggest that the presence of chemisorbed oxygen limits surface diffusion of Ni adatoms, thereby limiting adatom trapping at grain boundaries. The presence of oxygen therefore affects the mechanisms for development of both tensile and compressive stresses, providing a direct method for engineering the residual stress in as-deposited Ni films. Finally, the authors demonstrate a process for evaporative deposition of “zero” stress Ni films by introducing a very low level of background impurities, with the resultant films containing only 1.2 at. % oxygen.

Review of Chemical Vapor Deposition of Graphene and Related Applications

Since its debut in 2004, graphene has attracted enormous interest because of its unique properties. Chemical vapor deposition (CVD) has emerged as an important method for the preparation and production of graphene for various applications since the method was first reported in 2008/2009. In this Account, we review graphene CVD on various metal substrates with an emphasis on Ni and Cu. In addition, we discuss important and representative applications of graphene formed by CVD, including as flexible transparent conductors for organic photovoltaic cells and in field effect transistors. Growth on polycrystalline Ni films leads to both monolayer and few-layer graphene with multiple layers because of the grain boundaries on Ni films. We can greatly increase the percentage of monolayer graphene by using single-crystalline Ni(111) substrates, which have smooth surface and no grain boundaries. Due to the extremely low solubility of carbon in Cu, Cu has emerged as an even better catalyst for the growth of monolayer graphene with a high percentage of single layers. The growth of graphene on Cu is a surface reaction. As a result, only one layer of graphene can form on a Cu surface, in contrast with Ni, where more than one layer can form through carbon segregation and precipitation. We also describe a method for transferring graphene sheets from the metal using polymethyl methacrylate (PMMA). CVD graphene has electronic properties that are potentially valuable in a number of applications. For example, few-layer graphene grown on Ni can function as flexible transparent conductive electrodes for organic photovoltaic cells. In addition, because we can synthesize large-grain graphene on Cu foil, such large-grain graphene has electronic properties suitable for use in field effect transistors.

Different growth behaviors of ambient pressure chemical vapor deposition graphene on Ni(111) and Ni films: A scanning tunneling microscopy study

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO2 using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.

On the Mechanisms of Ni‐Catalysed Graphene Chemical Vapour Deposition

The development of a scalable, economical production technique for monoand few-layer graphene (M-/FLG) is a key requirement to exploit its unique properties for applications. Catalytic chemical vapour deposition (CVD) has emerged as one of the most promising and versatile methods for M-/FLG growth. The generic principle of catalytic, rather than pyrolytic, CVD is to expose a catalyst template to a gaseous precursor at temperatures/conditions for which the precursor preferentially dissociates on the catalyst. Hence, the catalyst is key to M-/FLG formation, in particular its role in precursor dissociation, C dissolution, M-/FLG nucleation and domain growth/merging. Although the structure of as-formed graphitic layers on crystalline transition metal surfaces under ultra-high vacuum conditions has been extensively studied in surface science, a central question remains: what M-/FLG quality can be achieved with CVD, in particular, if for cost effectiveness sacrificial polycrystalline metal films/foils and less stringent vacuum/CVD process conditions are used. There have been numerous recent reports of large area M-/FLG CVD on for instance poly-crystalline Ni and Cu, including integrated roll-to-roll processing. However, there is currently very limited understanding of the detailed growth mechanisms, and the mostly empirical process calibrations provide little fundamental insight in to how the process and M-/FLG quality/domain size can be optimised. Herein, we study M-/FLG CVD by complementary in situ probing under realistic process conditions with the aim of revealing the key growth mechanisms. We focus on poly-crystalline Ni films and simple one-step hydrocarbon exposure conditions. However, as highlighted by Figure 1, even for such seemingly simple CVD conditions, the parameter space is manifold which leads to ambiguity in the interpretation of post-growth process characterisation and motivates our in situ approach. For catalyst metals with a high C solubility, such as Ni, current literature typically assumes C precipitation upon cooling as the main growth process. 8] M-/FLG precipitation has been studied in detail for slow, near thermodynamic equilibrium thermal cycling of C doped crystals. 9] For CVD, however, the conditions are distinctly different (Figure 1): an isothermal C precursor exposure phase, which represents a variation in composition rather than temperature, is followed by a typically fast cooling or thermal quenching. Hence kinetic aspects are important. Additionally, competing processes might influence the growth outcome such as etching of M-/FLG in a reactive atmosphere, for example, hydrogen or water, during the CVD process. By combining in situ, timeand depth-resolved X-ray photoelectron spectroscopy (XPS) and in situ X-ray diffraction (XRD), we can clearly show that M-/FLG growth occurs during isothermal hydrocarbon exposure and is not limited to a precipitation process upon cooling. While the fraction of M-/FLG due to isothermal growth and precipitation upon cooling strongly depends on process conditions, we show that the former is dominant for the low-temperature CVD conditions used. We find that M-/FLG nucleation is preceded by an increase in (subsurface) dissolved C with the formation of a solid solution of C in the Ni film, which indicates that graphene CVD is not a purely surface process. We discuss our data here in the context of simple considerations of C solubility and diffusivity as well as rate equations of the basic contributing processes, in order to establish a framework to guide future improvements in graphene CVD by a more fundamental understanding. We perform in situ XPS during low-pressure CVD of M-/FLG from hydrocarbon precursors on Ni(550 nm) films. Figure 2A Figure 1. Illustrative processing profile for a simple one-step hydrocarbon exposure consisting of four major phases: catalyst pretreatment, C dissolution into the catalyst during initial precursor exposure, isothermal M-/FLG growth with continued precursor exposure, M-/FLG growth by precipitation upon cooling. The key catalyst and M-/FLG properties that may be defined at each phase of growth are also listed.

Anomalous Hall effect in polycrystalline Ni films

We systematically studied the anomalous Hall effect in a series of polycrystalline Ni films with thickness ranging from 4 to 200 nm. It is found that both the longitudinal and anomalous Hall resistivity increased greatly as film thickness decreased. This enhancement should be related to the surface scattering. In the ultrathin films (4–6 nm thick), weak localization corrections to anomalous Hall conductivity were studied. The granular model, taking into account the dominated intergranular tunneling, has been employed to explain this phenomenon, which can explain the weak dependence of anomalous Hall resistivity on longitudinal resistivity as well.

Growth of Few‐Layer Graphene on a Thin Cobalt Film on a Si/SiO2 Substrate

Few‐layered sheets of graphene were synthesized on polycrystalline thin Co and Ni films by low pressure chemical vapor deposition. On average, thinner layers of graphene were synthesized on Co than on the conventional Ni at the same process conditions, but slightly higher defects were detected on Co‐grown graphene layers than on Ni‐grown ones. Possible explanations for this are suggested.

Determination of substrate pinning in epitaxial and supported graphene layers via Raman scattering

The temperature-induced shift of the Raman $G$ line in epitaxial graphene on SiC and Ni surfaces, as well as in graphene supported on SiO${}_{2}$, is investigated with Raman spectroscopy. The thermal shift rate of epitaxial graphene on 6H-SiC(0001) is found to be about three times that of freestanding graphene. This result is explained quantitatively as a consequence of pinning by the substrate. In contrast, graphene grown on polycrystalline Ni films is shown to be unpinned, i.e., to behave elastically as freestanding, despite the relatively strong interaction with the metal substrate. Moreover, it is shown that the transfer of exfoliated graphene layers onto a supporting substrate can result in pinned or unpinned layers, depending on the transfer protocol.

Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition

In this contribution we demonstrate a method of synthesizing a hexagonal boron nitride (h-BN) thin film by ambient pressure chemical vapor deposition on polycrystalline Ni films. Depending on the growth conditions, the thickness of the obtained h-BN film is between ∼5 and 50 nm. The h-BN grows continuously on the entire Ni surface and the region with uniform thickness can be up to 20 μm in lateral size which is only limited by the size of the Ni single crystal grains. The hexagonal structure was confirmed by both electron and X-ray diffraction. X-ray photoelectron spectroscopy shows the B/N atomic ratio to be 1:1.12. A large optical band gap (5.92 eV) was obtained from the photoabsorption spectra which suggest the potential usage of this h-BN film in optoelectronic devices.

On the nature of displacement bursts during nanoindentation of ultrathin Ni films on sapphire

Quasi-static nanoindentation tests were performed on polycrystalline Ni films sputter deposited on basal plane-oriented sapphire substrates. In the majority of tests a combined elasto-plastic response of the film was observed, without detectable displacement bursts during loading. In some of the tests a single large displacement burst was observed slightly below or at the maximal load (75 or 50 μN). The residual plastic depth of the corresponding indents was close to the film thickness. These large displacement bursts were interpreted as manifestations of intergranular brittle fracture in the indented region. The strength of the disclination-type defect at the triple junctions leading to brittle fracture at the grain boundaries was estimated based on the theory of strain gradient plasticity.

Engineering polycrystalline Ni films to improve thickness uniformity of thechemical-vapor-deposition-grown graphene films

It has been shown that few-layer graphene films can be grown byatmospheric chemical vapor deposition using deposited Ni thin films onSiO2/Si substrates. In this paper we report the correlation between the thickness variations of thegraphene film with the grain size of the Ni film. Further investigations were carried outto increase the grain size of a polycrystalline nickel film. It was found that theminimization of the internal stress not only promotes the growth of the grains with (111)orientation in the Ni film, but it also increases their grain size. Different types ofSiO2 substrates also affect the grain size development. Based upon these observations, anannealing method was used to promote large grain growth while maintaining the continuityof the nickel film. Graphene films grown from Ni films with large versus small grains werecompared for confirmation.

Simulation of kinetically limited growth of electrodeposited polycrystalline Ni films

A kinetic Monte Carlo (KMC) technique was adopted to simulate the growth dynamics of electrodeposited polycrystalline Ni thin films under kinetically limited conditions with a two-dimensional triangle lattice mapping the cross-section of the film. Effects of surface diffusion and surface-energy anisotropy on the microstructures of the plated film such as shape and size of grains, density, surface roughness, and textures were examined. This work focuses on the microstructure evolutions for different deposition parameters (for example, deposition rate and electrolyte temperature). We obtained the growth exponent β=0.46, which characterizes the scaling behavior of this growth. Preferential orientation of 〈1 1 1〉 and 〈1 0 0〉 have been observed at elevated temperature and at elevated deposition rate, respectively. To simulate the evolution of texture, the influence of hydrogen absorption on the surface energy of six main surfaces of nickel was investigated.

Structure and magnetic properties of Ni/Cu/Fe/MgO(001) films

The structural and magnetic properties of thin Ni films grown on Cu/Fe/MgO(001)and Cu/MgO(001) buffer layers are investigated and compared to those grownon Cu/Si(001). The use of an Fe seed layer a few monolayers thick leads to theepitaxial growth of high surface quality Cu(001) buffer layers on MgO(001),while Cu growth on the bare MgO(001) substrate results in polycrystallinefilms. Magneto-optic Kerr effect magnetometry shows that Ni films grown onCu/Fe/MgO(001) exhibit dominant perpendicular magnetic anisotropy up to∼90 Å, which is similar to that of Ni films grown on Cu/Si(001). The polycrystalline Ni films alsoexhibit perpendicular magnetic remanence, but with a dominant in-plane magnetizationcomponent.