Thickness-dependent Change Research Articles

Investigation of the hydrophilic nature and surface energy changes of HfO2 thin films prepared by atomic layer deposition

The wettability of metal oxides, which is directly related to the surface free energy (SFE), is used in a wide range of robust functional coatings, from hydrophilic to hydrophobic. Because wettability is significantly affected by solid surface properties, several studies on the determinant factors and their correlations have been conducted through the thin-film coating of metal oxides. Herein, we found that HfO2 thin films deposited via atomic layer deposition (ALD) are intrinsically hydrophilic. Simultaneously, a thickness-dependent wettability change is observed. These observations were investigated by measuring water contact angles (WCAs), film surface compositions, roughness, morphologies, and microcrystalline structures of the ALD-grown HfO2 thin films. It was deduced that the surface oxygen species significantly affected the intrinsic hydrophilicity of HfO2 thin films. In addition, the crystalline orientations evolved with film thickness, and thermal annealing was used to determine the thickness-dependent WCA trend. In particular, the transformation of the preferred m(-111) crystal orientation with a low SFE had a dominant impact on the SFE change in the HfO2 thin films. Considering the lack of fundamental studies on the SFE of ALD-grown HfO2 thin films, we believe that this study will help understand the wettability of HfO2 and its overall surface science.

FEM ANALYSIS OF ENERGY LOSS DUE TO WAVE PROPAGATION FOR MICRO-PARTICLE IMPACTING SUBSTRATE WITH HIGH VELOCITIES

The impact between particles and material surface is a micro-scaled physical phenomenon found in various technological processes and in the study of the mechanical properties of materials. Design of materials with desired properties is a challenging issue for most industries. And especially in aviation one of the most important factors is mass. Recently with the innovations in 3D printing technologies, the importance of this phenomenon has increased. Numerical simulation of multi-particle systems is based on considering binary interactions; therefore, a simplified but as much accurate as possible particle interaction model is required for simulations. Particular cases of axisymmetric particle to substrate contact is modelled at select impact velocities and using different layer thicknesses. When modelling the particle impact at high contact velocity, a substrate thickness dependent change in the restitution coefficient was observed. This change happens is due to elastic waves and is important both to coating and 3D printing technologies when building layers of different properties materials.

Sign reversal of unidirectional magnetoresistance in monocrystalline Fe/Pt bilayers

The discovery of unidirectional magnetoresistance (UMR) in ferromagnet/heavy metal heterostructures provides one possibility to detect the magnetization direction with a simple two-terminal geometry. We investigated the temperature and thickness dependence of UMR in single-crystalline Fe/Pt bilayers and discovered an obvious sign reversal when increasing the Fe thickness at low temperature. Meanwhile, the same phenomenon is observed when increasing temperature for thick Fe samples. All the UMR mechanisms are quantitatively analyzed and the UMR contribution induced by the thermal effect shows a thickness-dependent sign change, thus the sign reversal of UMR is attributed to the competition between the contributions on UMR from the thermal effect and the spin accumulation induced by the spin Hall effect in the Pt layer. Our results emphasize the thermal contribution to the UMR in ferromagnet/heavy metal heterostructures and suggest a promising way to tune the UMR, which will promote its application in information storage devices.

Thickness dependence of magnetotransport properties of tungsten ditelluride

We investigate the electronic structure of tungsten ditelluride (WTe$_2$) flakes with different thicknesses in magneto-transport studies. The temperature-dependent resistance and magnetoresistance (MR) measurements both confirm the breaking of carrier balance induced by thickness reduction, which suppresses the `turn-on' behavior and large positive MR. The Shubnikov-de-Haas oscillation studies further confirm the thickness-dependent change of electronic structure of WTe$_2$ and reveal a possible temperature-sensitive electronic structure change. Finally, we report the thickness-dependent anisotropy of Fermi surface, which reveals that multi-layer WTe$_2$ is an electronic 3D material and the anisotropy decreases as thickness decreases.

Optical microscopy–based thickness estimation in thin GaSe flakes

We have implemented three different optical methods to quantitatively assess the thickness of thin GaSe flakes transferred on both transparent substrates, like Gel-Film, and SiO2/Si substrates. We show how their apparent color can be an efficient way to make a quick, rough estimation of the thickness of the flakes. This method is more effective for SiO2/Si substrates as the thickness-dependent color change is more pronounced on these substrates than on transparent substrates. On the other hand, for transparent substrates, the transmittance of the flakes in the blue region of the visible spectrum can be used to estimate the thickness. We find that the transmittance of flakes in the blue part of the spectrum decreases at a rate of 1.2%/nm. On SiO2/Si, the thickness of the flakes can be accurately determined by fitting optical contrast spectra to a Fresnel law-based model. Finally, we also show how the quantitative analysis of transmission mode optical microscopy images can be a powerful method to quickly probe the environmental degradation of GaSe flakes exposed to aging conditions.

Refinement of Sustainable Polybutylene Adipate Terephthalate (PBAT) with Amorphous Hydrogenated Carbon Films (a-C:H) Revealing Film Instabilities Influenced by a Thickness-Dependent Change of sp2/sp3 Ratio

The increasing use of polymers is related to a growing disposal problem. Switching to biodegradable polymers such as polybutylene adipate terephthalate (PBAT) is a feasible possibility, but after industrial production of commercially available material PBAT is not suitable for every application. Therefore, surface refinements with amorphous hydrogenated carbon films (a-C:H) produced by plasma-assisted chemical vapor deposition (PE-CVD) changing the top layer characteristics are used. Here, 50 µm-thick PBAT films are coated with a-C:H layers up to 500 nm in 50 nm steps. The top surface sp2/sp3 bonding ratios are analyzed by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) both synchrotron-based. In addition, measurements using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were performed for detailed chemical composition. Surface topography was analyzed by scanning electron microscopy (SEM) and the surface wettability by contact angle measurements. With increasing a-C:H layer thickness not only does the topography change but also the sp2 to sp3 ratio, which in combination indicates internal stress-induced phenomena. The results obtained provide a more detailed understanding of the mostly inorganic a-C:H coatings on the biodegradable organic polymer PBAT via in situ growth and stepwise height-dependent analysis.

Thickness-Dependent Phase Change and Morphological and Electric Characteristic of Devices with Pentacene

Thickness-Dependent Phase Change and Morphological and Electric Characteristic of Devices with Pentacene

Thickness-modulated temperature dependent optical properties of VO2 thin films

The optical properties of VO2 have a sharp change during metal–insulator transition, which indicates wide applications in manipulating electromagnetic waves. Here, VO2 thin films with different thicknesses were synthesized by polymer-assisted deposition. The transmittance change trend of the VO2 film with temperature is related with the thickness of the film, which is caused by a dramatic change of complex refractive index during phase transition. However, the transient reflectivity of 150-nm-thick VO2 films presented abnormal change trend with temperature compared with other thinner VO2 films, which may be related with the coexistence of multiphase VO2. The thickness dependent change trend of transmittance and transient reflectivity at specific light may open a new method to fabricate thickness periodic distribution metasurfaces by VO2 film and promote the applications of VO2 film as a metamaterial.

Thickness dependent surface plasmon of silver film detected by nitrogen vacancy centers in diamond.

Precise detection of surface plasmons is crucial for the research of nanophotonics and quantum optics. In this Letter, we used a single nitrogen vacancy center in diamond as a probe to detect the surface plasmon that was tuned by the thickness of a metallic film. The fluorescence intensity and lifetime of the nitrogen vacancy (NV) center were measured to obtain the information of local light-matter interaction. A nonlinear thickness dependent change of the surface plasmon was observed, with the maximum at the thickness of approximately 30nm. With optimized thickness of silver film, the fluorescence intensity of a single NV center was enhanced 2.6 times, and the lifetime was reduced by a factor of 3, without affecting the coherence time of the NV spin state. The results proved that this system can quantitatively detect the light-matter interaction at nanoscale, and it provides an approach to enhance the fluorescence intensity of a quantum emitter.

Influence of thickness on crystallinity in wafer-scale GaTe nanolayers grown by molecular beam epitaxy

We grew wafer-scale, uniform nanolayers of gallium telluride (GaTe) on gallium arsenide (GaAs) substrates using molecular beam epitaxy. These films initially formed in a hexagonal close-packed structure (h-GaTe), but monoclinic (m-GaTe) crystalline elements began to form as the film thicknesses increased to more than approximately 90 nm. We confirmed the coexistence of these two crystalline forms using x-ray diffraction and Raman spectroscopy, and we attribute the thickness-dependent structural change to internal stress induced by lattice mismatch with the substrate and to natural lattice relaxation at the growth conditions.

Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe2

Monoclinic group 6 transition metal dichalcogenides (TMDs) have been extensively studied for their intriguing 2D physics (e.g. spin Hall insulator) as well as for ohmic homojunction contacts in 2D device applications. A critical prerequisite for those applications is thickness control of the monoclinic 2D materials, which allows subtle engineering of the topological states or electronic bandgaps. Local thickness control enables the realization of clean homojunctions between different electronic states, and novel device operation in a single material. However, conventional fabrication processes, including chemical methods, typically produce non-homogeneous and relatively thick monoclinic TMDs, due to their distorted octahedral structures. Here, we report on a post-patterning technique using laser-irradiation to fabricate homojunctions between two different thickness areas in monoclinic MoTe2. A thickness-dependent electronic change from a metallic to semiconducting state, resulting in an electronic homojunction, was realized by the optical patterning of pristine MoTe2 flakes, and a pre-patterned device channel of monoclinic MoTe2 with a thickness-resolution of 5 nm. Our work provides insight on an optical post-process method for controlling thickness, as a promising approach for fabricating impurity-free 2D TMDs homojunction devices.

Controlled growth of ultrathin Mo2C superconducting crystals on liquid Cu surface

Exhibiting thickness-dependent change in the critical temperature (Tc) for the onset of superconductivity, Mo2C has emerged as an important new member in the family of two-dimensional atomic crystals. Controllable growth in terms of morphology and thickness is necessary to elucidate its intrinsic properties at the 2D limit. Here we demonstrate the chemical vapor deposition of ultrathin Mo2C crystals on liquid Cu surface where the morphology of the crystals can be controlled by tuning the carbon supersaturation. A unique staggered carbon vacancy ordering is discovered in Mo2C crystals having particular geometries. Thickness engineering of the crystal can be achieved by controlling the thickness of the Cu catalyst layer, which affords a facile route to grow ultrathin 2D samples. Ultrathin Mo2C crystals so obtained, have been characterized using aberration corrected scanning transmission electron microscopy annular dark field imaging, where the co-existence of both AA and AB stacking modes is observed. The high crystallinity of the Mo2C crystals synthesized in this work is attested by its characteristic sharp superconducting transition.

Thickness-dependent change in the valence band offset of the SiO2/Si interface studied using synchrotron-radiation photoemission spectroscopy

We have studied the thickness-dependent change in the valence band offset (VBO) of the SiO2/Si(001) interface using synchrotron-radiation photoemission spectroscopy with soft and hard X-rays. The SiO2-film thickness (Tox) and X-ray irradiation time (tirrad) were systematically parameterized to distinguish between the “intrinsic” Tox effects in the VBOs and the “extrinsic” differential charging phenomena in SiO2 films on Si substrates. The results revealed that at a spontaneous time (tirrad ≈ 5 s) that suppresses the differential charging phenomena as much as possible, the experimental VBO abruptly increases as a function of Tox and gradually saturates to the traditional VBO value range determined by the internal photoemission and photoconduction measurements. This effect is not attributed to the differential charging phenomena, but rather it is attributed to the “intrinsic” Tox-dependent change in the VBO. The two possible physical behaviors include electronic polarization and image charge. We have derived the electronic polarization contribution from experimental data by carefully describing the effects of the long-range image charges based on the classical dielectric-screening model.

In Situ Polymerization and Characterization of Highly Conducting Polypyrrole Fish Scales for High-Frequency Applications

Polypyrrole (Ppy) thin films on alumina were synthesized by an in situ chemical oxidative polymerization method at 300 K with equal monomer-to-oxidant ratio. Fourier transform infrared spectroscopy (FTIR) and FT-Raman spectroscopy confirmed the formation of Ppy. A thickness-dependent change from cauliflower to fish-scale morphology was observed. Microwave properties such as transmission, reflection, shielding effectiveness, permittivity, and microwave conductivity are reported in the frequency range from 8 GHz to 12 GHz. The direct-current (DC) conductivity varied from 9.45 × 10−3 S/cm to 17.29 × 10−3 S/cm, whereas the microwave conductivity varied from 63.07 S/cm to 349.08 S/cm. The shielding effectiveness varied between 6.18 dB and 10.39 dB.

Atomic layer deposition, characterization, and growth mechanistic studies of TiO2 thin films.

Two heteroleptic titanium precursors were investigated for the atomic layer deposition (ALD) of titanium dioxide using ozone as the oxygen source. The precursors, titanium (N,N'-diisopropylacetamidinate)tris(isopropoxide) (Ti(O(i)Pr)3(N(i)Pr-Me-amd)) and titanium bis(dimethylamide)bis(isopropoxide) (Ti(NMe2)2(O(i)Pr)2), exhibit self-limiting growth behavior up to a maximum temperature of 325 °C. Ti(NMe2)2(O(i)Pr)2 displays an excellent growth rate of 0.9 Å/cycle at 325 °C while the growth rate of Ti(O(i)Pr)3(N(i)Pr-Me-amd) is 0.3 Å/cycle at the same temperature. In the temperature range of 275-325 °C, both precursors deposit titanium dioxide in the anatase phase. In the case of Ti(NMe2)2(O(i)Pr)2, high-temperature X-ray diffraction (HTXRD) studies reveal a thickness-dependent phase change from anatase to rutile at 875-975 °C. X-ray photoelectron spectroscopy (XPS) indicates that the films have high purity and are close to the stoichiometric composition. Reaction mechanisms taking place during the ALD process were studied in situ with quadrupole mass spectrometry (QMS) and quartz crystal microbalance (QCM).

Understanding the influence of grain boundary thickness variation on the mechanical strength of a nickel-doped tungsten grain boundary

Grain boundaries (GBs) in nickel (Ni)-doped tungsten (W) are found to have thickness as a function of the level of saturation of W atoms with respect to Ni atoms in the GBs. While the unsaturated Ni-doped W GBs have average thickness of approximately 0.3nm, the saturated Ni-doped W GBs have twice the average thickness (∼0.6nm). The present work examines (110)–(210) W GB mechanical strength as a function of thickness using an ab initio calculation framework based on Car–Parrinello molecular dynamics (CPMD) simulations. The atomic fraction of Ni atoms is varied to understand the influence of Ni addition and its correlation with thickness variation on the GB mechanical strength. In the case of GBs with 0.3nm thickness, the variation of peak tensile strength as a function of Ni atomic fraction variation from 5% to 50% is negligible. However, in the case of 0.6nm GB, the changes in the peak tensile strength are significant with the maximum peak tensile strength observed in the case of 58% Ni atomic fraction. Analyses examine electron density of states and phonon dispersion relations to delineate the role of atomic level bond strength in thickness dependent GB mechanical strength. The thickness dependent GB strength variation is found to be strongly correlated to the thickness dependent change in lower acoustic mode phonon frequencies. At the same time Ni atomic fraction dependent change in GB strength is found to be strongly correlated to the corresponding changes in electron density of states. Based on the analyses performed, an analytical relation to predict GB peak tensile strength as a function of atomic cohesive energy, GB thickness (level of saturation), and the Ni atomic fraction is proposed.

Atomic-level study of a thickness-dependent phase change in gold thin films heated by an ultrafast laser

An ultrafast laser-induced phase change in gold thin films with different thicknesses has been simulated by the method of coupling the two-temperature model and the molecular dynamics, including transient optical properties. Numerical results show that the decrease of film thickness leads to faster melting in the early nonequilibrium time and a larger melting depth. Moreover, earlier occurrence and a higher rate of resolidification are observed for the thicker film. Further analysis reveals that the mechanism for the thickness-dependent phase change in the films is the fast electron thermal conduction in the nonequilibrium state.

A systematic spectroscopic study of the FePc–Si interfaces

A systematic spectroscopic study on the interfaces between the organic molecular semiconductor and the (110) phase of the doped polysilicon has been carried out using valence and core-level photoemission spectroscopy. All photoemission spectra (UPS/XPS) exhibited shifts in the positions of the peaks following the deposition of the organic films. A set of clearly resolved valence band features is found associated to the dispersion of the molecular orbitals. The nonexistence of Fermi edge in the FePc films is thought to account for its semiconducting nature. The thickness dependent change in the work function of the substrate is noticed and a minute amount of charge transfer is found. The asymmetric C1s peak is resolved into three components, reflecting photoemission from multiple sites within the organic molecule. A larger shift in the benzene carbon (Cb) as compared to the pyrrole carbon (Cp) peak suggests that more charge transfers from the former carbon to the substrate. The observed negative sign of the sizable interface dipole potential indicates that the guest molecule and the hosting substrate have donor and acceptor character, respectively. Furthermore, the dipole width, i.e. d=1.35Å, is determined on the basis of the measured quantities and basic physics.

The Influence of Specific Anion Adsorption on the Surface Roughness of Electrodeposited Polycrystalline Cu Films

We studied the effect of submillimolar quantities of specifically adsorbing anions on the surface roughness evolution of Cu films electrodeposited from acidic perchlorate electrolytes. Using scaling analysis, we found that the roughness evolution of the thin films galvanostatically deposited at constant overpotential in the presence of different anions is correlated with the anions’ adsorption energy. The roughness of the Cu films deposited in perchlorate electrolyte and the ones with the addition of nonadsorbing and weakly adsorbing anions follow almost the same anomalous scaling behavior. However, in the presence of specifically adsorbing halides, the roughness significantly changes, exhibiting near-normal scaling. This change is illustrated by a decrease in the local growth exponent, , with an increase in anion adsorption strength . The X-ray diffraction analysis showed the thickness-dependent change of the preferred microstructure orientation from {111} to {220} occurring at lower thicknesses in the presence of specifically adsorbing halides. Focused ion beam microstructure analysis also reveals columnar growth and larger grain size for films grown in the presence of specifically adsorbing halides. In this paper we show how the effects of specific adsorption on the mechanism of deposition and surface transport processes influence the changes in both roughness and microstructure evolution.

Growth of atomically flat ultra-thin Ag films on Si surfaces

A two-step growth with deposition at low temperature and subsequent slow annealing to room temperature (RT) has been widely used to obtain atomically flat metal thin films on semiconductor substrates. In the case of Ag films, almost atomically flat films were obtained at a thickness of six monolayers (ML) on Si and GaAs. The flatness then gradually degraded with an increase in thickness. The existence of the critical thickness has been well explained by the “electronic growth theory”, which considers thickness-dependent change of vertically quantized electrons and their spilling out at the interfaces. However, several questions remain unanswered. How does the electronically grown Ag flat film accommodate the large misfit energy between the film and substrate? Up to what deposition temperature is the two-step growth effective to obtain atomically flat films? In this article, we review previous studies on the electronic growth of Ag films on Si(1 1 1) substrates, paying special attention to these two points. In addition, we also discuss the possibility of engineering electronic growth for artificial control of the critical thickness.