Ripple Structure Research Articles

Nano-Physical Characterization of Chemical Vapor Deposition-Grown Monolayer Graphene for High Performance Electrode: Raman, Surface-Enhanced Raman Spectroscopy, and Electrostatic Force Microscopy Studies.

To achieve high-quality chemical vapor deposition of monolayer graphene electrodes (CVD-MG), appropriate characterization at each fabrication step is essential. In this article, (1) Raman spectroscopy/microscopy are employed to unravel the contact effect between the CVD-MG and Cu foil in suspended/supported formation. (2) The Surface-Enhanced Raman spectroscopy (SERS) system is described, unveiling the presence of a z-directional radial breathing-like mode (RBLM) around 150 cm−1, which matches the Raman shift of the radial breathing mode (RBM) from single-walled carbon nanotubes (SWCNTs) around 150 cm−1. This result indicates the CVD-MG located between the Au NPs and Au film is not flat but comprises heterogeneous protrusions of some domains along the z-axis. Consequently, the degree of carrier mobility can be influenced, as the protruding domains result in lower carrier mobility due to flexural phonon–electron scattering. A strongly enhanced G-peak domain, ascribed to the presence of scrolled graphene nanoribbons (sGNRs), was observed, and there remains the possibility for the fabrication of sGNRs as sources of open bandgap devices. (3) Electrostatic force microscopy (EFM) is used for the measurement of surface charge distribution of graphene at the nanoscale and is crucial in substantiating the electrical performance of CVD-MG, which was influenced by the surface structure of the Cu foil. The ripple (RP) structures were determined using EFM correlated with Raman spectroscopy, exhibiting a higher tapping amplitude which was observed with structurally stable and hydrophobic RPs with a threading type than surrounding RPs. (4) To reduce the RP density and height, a plausible fabrication could be developed that controls the electrical properties of the CVD-MG by tuning the cooling rate.

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Friction and Mechanical Properties of AFM-Scan-Induced Ripples in Polymer Films

When compliant samples such as polymer films are scanned with an atomic force microscope (AFM) in contact mode, a periodic ripple pattern can be induced on the sample. In the present paper, friction and mechanical properties of such ripple structures on films of polystyrene (PS) and poly-n-(butyl methacrylate) (PnBMA) are investigated. Force volume measurements allow a quantitative analysis of the elastic moduli with nanometer resolution, showing a contrast in mechanical response between bundles and troughs. Additionally, analysis of the lateral cantilever deflection when scanning on pre-machined ripples shows a clear correlation between friction and the sample topography. Those results support the theory of crack propagation and the formation of voids as a mechanism responsible for the formation of ripples. This paper also shows the limits of the presented measuring methods for soft, compliant, and small structures. Special care must be taken to ensure that the analysis is not affected by artefacts.

High Gain DC-DC Converter Integrating Dickson Charge Pump with Coupled Inductor Technique

In this paper, a novel high voltage gain DC-DC converter based on coupled inductor and voltage multiplier technique is proposed. The benefits of the proposed converter are ultra-high voltage gain, low voltage stress across the power switch and very low input current ripple by employing a low current ripple structure (LCR) at the input side. A low on state resistance (RDS(on)) of the power switch can be employed since the voltage stress is a maximum of 25% of the output voltage and the conduction losses of the switch is also reduced. Design of a 1.9kW, 48V at the low voltage side and 430V at the high voltage side is done and verified by simulation. Simulation results show an efficiency of over 93% when operating in continuous conduction mode (CCM).

Engineering electronic structure of topological insulator Bi2Te3 thin films by highly uniform ripple arrays

‘Wrinkle engineering’, as an effective approach to modify the properties of materials without introducing additional chemical doping, has been widely used in a variety of two-dimensional materials, including graphene, phosphorene and transitional metal dichalcogenides, etc. One-dimensional wrinkle or ripple arrays, which breaks the crystalline symmetry of materials, may reformate the nontrivial band structure and greatly affect the electronic properties of topological materials. Until now, such method has been rarely applied experimentally in tuning topological materials. Here, we fabricate highly uniform ripple arrays directly in few-quintuple-layers (QL) Bi2Te3 thin films on SrTiO3 substrate realized by in-situ strained epitaxial growth, investigated by detailed low temperature scanning tunneling microscope/spectroscopy. Well-defined one-dimensional (1D) ripple structures induced by the interfacial strain from the lattice mismatch between the Bi2Te3 films and the substrate has been observed. As the thickness of Bi2Te3 thin films increases, the release of the strain leads to the reduction of the averaged corrugation height and the periodicity of ripple array. The dI/dV spectra and spatially resolved spectroscopic survey acquired on the epitaxial layers with different thicknesses, reveal that both the energy gap and the Fermi velocity of the Dirac surface states are greatly enhanced on thinner films. Dirac fermion velocity vF is estimated about 7.8 × 105 m s−1 for 3 QL, which is greatly enhanced by compared with the bulk values (4.8 × 105 m s−1). An asymmetric dispersion behavior along different − directions is observed, highly related with the modulation of 1D corrugated ripples on the electronic structure. Moreover, an unconventional Bi (S4) termination mainly observed on thinner films shows a stress-related behavior, agreed with previous theoretical calculations. Our work proposes a promising route to fabricate a highly uniform ripple structure on nontrivial topological materials, which can effectively tune the electronic structure. Specially, it may have an immediate application for realizing a high-performance topological-based device with higher operating temperature and speed.

Laser-induced periodic structure formation in AlN ceramic

In this work, we show that nanosecond laser processing of aluminium nitride (AlN) ceramic by a Nd:YAG laser operated at the fundamental wavelength (1064 nm) leads to the formation of a conductive layer decorated by a ripples structure. The characteristic orientation of these structures is perpendicular to the incident laser polarization. The period of the ripples is lower than the laser wavelength, namely, about 830 nm. Such structuring is observed after processing both in air and in vacuum. We discuss the mechanism of periodic structures formation based on the dependence of the surface morphology on the laser pulse number and fluence applied and the results of a simulation using the finite-difference time-domain approach. These results can form the basis of fabricating aluminium micro- and nanostructures with potential applications in electronics and photonics as systems for UV plasmonics.

Transition of nano-ripple to nano-hillock pattern on ion bombarded Si with an enhanced hydrophobicity

Surface patterning in nano-scale dimension and the effect of surface heterogeneity to control surface properties like wettability of solid surfaces have been studied here. This work focuses primarily on the generation of Si nano-patterns at oblique incidence via ion beam assisted impurity co-deposition technique and its impact on the wetting property of liquid. When oblique ion irradiation is performed on a Si surface without impurity, ripple structures evolve that show an increase in its hydrophilic behavior with a rise in ion fluence. On the other hand, the inclusion of stainless steel as seeding material during bombardment brings about a noteworthy change in the surface topography with the emergence of hillocks structures over the ripple morphology. For a higher bombardment time, the Si rippled surface is seen to vanish and a transition from nano-ripple to nano-hillock pattern is witnessed. Such an unusual pattern evolution is aided by FeSi2 bond which further promotes an increase in surface hydrophobicity. Contact angle measurements show directional dependence behavior having a direct influence of the anisotropic patterns synthesized here.

Femtosecond Laser Fabrication of Micro and Nano-Structures on CIGS/ITO Bilayer Films for Thin-Film Solar Cells.

Cu(In, Ga)Se2 (CIGS) thin films have attracted considerable interest as potential photovoltaic solar cells. Moreover, several current studies are focusing on improving their conversion efficiency. This study proposes a method to process micro- and nanostructures onto the surface of CIGS/ITO bilayer films to broaden the field of solar cell application. The bilayer films exhibited optical characteristics different from those of a single-film during processing. Field intensities at different layer positions of the CIGS/ITO bilayer films were analyzed, and different structures were fabricated by varying a set of parameters. Ripples were obtained using a pulse energy of 0.15 μJ and scanning speeds in the range of 0.1–1 mm/s, but after increasing speed to 3–5 mm/s, ripple structures were produced that had a large period of several microns and spatial porous nanostructures. This pattern exhibited low reflectivity. Optimal structures were obtained at a scanning speed of 3.5 mm/s a pulse energy of 0.15 μJ, and a reflectivity lower than 5%. Large areas characterized by micron-sized ripple structures and accompanied by nanoscale porous structures presented high optical performance and efficiency, which can be used to broaden the application of thin film-based solar cells.

Scattering resonances observed in the lasing emission spectrum of large dye-doped droplets

High-resolution spectrum analysis of the lasing emission of mm3-sized pendant droplets that contain aqueous solutions of the Rhodamine 6G (Rh6G) dye is reported. Pumping was performed with a 532 nm pulsed laser beam. The lasing emission demonstrates a complex spectral structure that contains mode-cluster peaks spaced 2 nm apart and a ripple structure formed by periodic resonances, which are superimposed on the peak closest to the maximum of the dye gain curve. The periodic resonances are associated with the light-scattering resonances on a large sphere when light absorption in the sphere occurs.

Enhanced corrosion, tribocorrosion resistance and controllable osteogenic potential of stem cells on micro-rippled Ti6Al4V surfaces produced by pulsed laser remelting

• Pulsed laser remelting was used to generate ripple-like features on Ti6Al4V surfaces. • Laser remelting improved wear, corrosion and tribocorrosion resistance of the surfaces. • Viability and differentiation of hAMSCs were dependent upon the ripple topographies. • Topographies of the rippled structures also influenced the orientation of the cells. Laser-based surface modification methods are employed to augment the surface properties of engineering components. In the presented work, pulsed laser remelting of Ti6Al4V surfaces using a long pulsed laser was used to generate ripple-like micro-features with different feature sizes. The resultant surfaces exhibited improved corrosion and tribocorrosion resistance. Anisotropic wetting characteristics of the laser remelted surfaces led to elongation of water droplets on the surfaces. The osteogenic potential of the surfaces as well as the orientations of human mesenchymal stem cells on the surfaces were dependent on the surface features. On surfaces having 20 μm ripples, differentiation of stem cells towards osteoblastic lineage was higher than that on acid etched surfaces, and the cells were prominently oriented across the ripples, parallel to the laser scanning track. On the other hand, surfaces having 100 μm ripples suppressed the cellular activities on them. While the resistance to corrosion and tribocorrosion of laser remelted surfaces can be attributed to the evolved microstructure and the formation of stable oxide layers, the physical characteristics like topography and wettability dictated cellular behavior on the surfaces. Taken together, the work demonstrates the use of pulsed laser remelting as a simple yet potentially effective route towards manufacturing Ti6Al4V orthopedic implants with improved biofunctionalities.

Adjustment of Subwavelength Rippled Structures on Titanium by Two-Step Fabrication Using Femtosecond Laser Pulses

An effective approach is proposed to adjust the surface morphology induced by using a femtosecond laser, including the area and period of rippled structures. The effect of the processing steps and laser polarization on the surface morphology of rippled structures on a titanium surface was experimentally investigated in this study. A processing sequence was designed for two series of femtosecond laser pulses that irradiate a titanium surface, for example, N = 50(0°) + 50(90°). The experimental results show that the area and period of rippled structures can be simultaneously adjusted by following a two-step method. Due to the enhancement of energy absorption and SP-laser coupling of the initial rippled structures, large area surface structures with small periods are fabricated using two series of femtosecond laser pulses with the same polarization direction. By changing the polarization direction of the two series of femtosecond laser pulses, the recording, erasing, and rewriting of subwavelength ripples is achieved. During the rewriting process, material removal and the formation of new ripples simultaneously occur.

Stacking Relations and Substrate Interaction of Graphene on Copper Foil

AbstractThe crystallinity of graphene flakes and their orientation with respect to the Cu(111) substrate are investigated by means of low‐energy electron microscopy (LEEM). The interplay between graphene and the metal substrate during chemical vapor deposition (CVD) introduces a restructuring of the metal surface into surface facets, which undergo a step bunching process during the growth of additional layers. Moreover, the surface facets introduce strain between the successively nucleated layers that follow the topography in a carpet‐like fashion. The strain leads to dislocations in between domains of relaxed Bernal stacking. After the transfer onto an epitaxial buffer layer, the imprinted rippled structure of even monolayer graphene as well as the stacking dislocations are preserved. A similar behavior might also be expected for other CVD grown 2D materials such as hexagonal boron nitride or transition metal dichalcogenides, where stacking relations after transfer on a target substrate or heterostructure could become important in future experiments.

Quantitative estimation of atom-scaled ripple structure using transmission electron microscopy images

Atom-scaled ripple structure can be intrinsically formed because of thermal instability or induced stress in graphene or two-dimensional (2D) materials. However, it is difficult to estimate the period, amplitude, and shape of such a ripple structure. In this study, by applying the geometrical phase analysis method to atomically resolved transmission electron microscopy images, we demonstrate that the atom-scaled ripple structure of MoS2 nanosheet can be quantitatively analyzed at the subnanometer scale. Furthermore, by analyzing the observed ripple structure of the MoS2 nanosheet, we established that it is inclined by approximately 7.1° from the plane perpendicular to the incident electron beam; it had 5.5 and 0.3 nm in period and amplitude, respectively. For quantitative estimation of ripple structure, our results provide an effective method that contributes to a better understanding of 2D materials in the sub-nanometre scale.

Fabrication of printable nanograting using solution-based laser-induced periodic surface structure process

Laser-induced periodic surface structure (LIPSS) is of significant interest because it can be used to produce sub-wavelength structures through a simple laser patterning process. One of the main applications of LIPSS is color marking, which is due to diffraction from periodic grating structures. Such structural coloration is produced mainly by irradiating metal plates with femtosecond laser beams. However, when used with bulk materials, the unpatterned areas of the metal plate have undesirable properties, such as low transmittance, making it difficult to apply the LIPSS process. To overcome this problem, we propose herein a solution-based LIPSS process for fabricating printable nanogratings. The unique growth process of organometallic compounds allows gratings to form only in the desired area, leaving the surrounding unpatterned area to be easily cleaned using organic solvents. To form a periodic ripple structure, we apply specific processing parameters that enable the simultaneous growth and sintering of Ag nanoseeds. Given that the selective ripple patterns can be used in various applications, the proposed LIPSS process should be of significant practical use.

Femtosecond laser modification of silicon carbide substrates and its influence on CMP process

Silicon carbide (SiC) is one of the most promising third-generation semiconductor materials owing to its superior comprehensive performances. However, high-efficient polishing of the hard-to-process SiC substrate remains a challenge due to its high hardness and high chemical stability. In this paper, the SiC substrate was irradiated by femtosecond laser to enhance the subsequent chemical mechanical polishing (CMP) efficiency. The effects of different laser processing parameters such as laser fluence, focus position, scanning velocity and scanning interval on the surface morphology of SiC were discussed. The influences of the surrounding conditions on the morphology of laser-processed surfaces were also studied. Results show that the Si–O compound was formed on the laser-irradiated SiC surfaces, which is beneficial for the subsequent CMP process due to their lower hardness. Furthermore, the uniform periodic ripple structures formed on the laser-irradiated SiC surface also help to improve the surface flatness. Furthermore, fewer surface defects and lower surface roughness can be achieved in an oxygen atmosphere, which was due to the oxygen participating in the photochemical reaction.

Quality Improvement of Laser-Induced Periodic Ripple Structures on Silicon Using a Bismuth-Indium Alloy Film

In this work, a new buffer layer material, a bismuth-indium (Bi-In) alloy, was utilized to improve the quality of large-area, laser-induced periodic ripple structures on silicon. Better-defined ripple structures and larger modification areas were obtained at different scanning speeds by pre-depositing a Bi-In film. The single-spot investigations indicated that ripple structures were much easier to form on silicon coated with the Bi-In film under laser fluences of 2.04 and 2.55 J/cm2 at a fixed pulse number of 200 in comparison with on bare silicon. A physical model in terms of the excellent thermal conductivity contributed by the free electrons in the Bi-In film homogenizing the thermal distribution caused by the laser irradiation in the early stage of the formation of laser-induced periodic surface structures was proposed to explain the above phenomena. The results show that the Bi-In film enabled a wider range of laser fluences to generate periodic structures and helped to form regular ripple structures on the silicon. In addition, the modulation effects of the laser fluence and pulse number on surface structures were studied experimentally and are discussed in detail.

Formation mechanism of surface morphology in the process of CO2 pulsed laser processing of fused silica optics

In order to comprehensively reveal the surface morphology formation mechanism during CO2 pulsed laser processing of fused silica optics, the material temperature distribution, the surface morphology evolution law, as well as the relationship between laser parameters and surface quality were analyzed through three-dimensional numerical simulation and experimental research. Results showed that the residual temperature between laser pulses was lower than material’s structural transition temperature. The calculated morphologies along laser moving direction and width direction were consistent with the experimental during laser linear scanning. Besides, the superposition of spot energy during multi-pass laser scanning made the processed surface present specific three-dimensional morphology features. Laser processing lines were distributed perpendicular to the scanning direction, and the interval was equal to the track pitch. It was concluded that material’s removal depth was proportional to pulse width, and inversely proportional to the track pitch and spot moving speed. The periodic ripple structures on the surface processed by 10 μs pulsed laser were formed by the superposition of materials’ residual height during laser scanning. The conical repaired site with diameter of 2 mm and angle of 12° obtained by CO2 laser processing could completely eliminate surface damage, and laser-induced periodic surface structures were formed on the repaired surface. These studies could provide guidance for the optimization of CO2 laser processing technology and the improvement of processed surface quality.

Quasi-static 3D structure of graphene ripple measured using aberration-corrected TEM.

Free-standing graphene has a three-dimensional (3D) structure, called a ripple, rather than a perfect two-dimensional (2D) crystal. Since theoretical calculations suggest that a ripple strongly influences various fundamental physicochemical properties of graphene, it is important to clarify the ripple structure quantitatively in experiments. This paper proposes a new method of measuring the 3D atomic structure of a ripple by using aberration-corrected transmission electron microscopy (TEM). The method utilizes the fact that the 2D contrast of a TEM image is sensitive to the height of a six-membered ring. The proposed method is experimentally applied to a monolayer graphene, and the 3D atomic arrangements of consecutively acquired TEM images are reconstructed. In that experiment, the specimen is found to be moving upward. Furthermore, the atomic arrangement can be approximated as a composite of two structures consisting of a 3D ripple and a 2D plane. The ripple is represented as a superposition of sinusoidal waves, with their wave vectors coinciding with the specific direction of the six-membered ring. The time dependences of the height and lateral size of the ripple are also measured.

蓝宝石表面纳米结构随杂质靶距离的演化

Electron cyclotron resonance ion source has been employed to etch the surface of sapphire (C-cut), introducing metallic stainless steel impurities to investigate the evolution law and optical properties of the self-organized nanostructure on the sapphire surface at different target distances. The atomic force microscope was used to observe the morphological changes of the sample surface, the Taylor Surf CCI 2000 white light interference surface measuring instrument was used to measure the surface roughness; X-ray photoelectron spectroscopy was selected to characterize the chemical composition. The experimental results indicate that, with the ion beam energy of 1000 eV, the beam current density of 487 μA/cm2, the oblique incident angle of 65°, and the erosion duration of 60 min, the distance between the sapphire sample and the impurity target increases from 1 cm to 4 cm, island-like structures appear on the sample surface and gradually evolve into continuous ripple structures. At the same time, as the target distance increases, the orderliness of the self-organized nanostructures enhances, the longitudinal height gradually decreases, while the spatial frequency is unchanged. There are very few metal impurities on the etched sample surface. The appearance of microstructures has antireflection effect on sapphire. During the ion beam sputtering process, island-like structures promotes the growth of ripple nanostructures but destroys orderliness.

Fabrication of large‐area linear and nonlinear surface ripple structures on polystyrene films

AbstractRipple structures have been used in electronics, optics, magnetics, chemistry, and structural coloration. Commonly used methods for manufacturing large‐area polymeric thin films with ripple structures include mechanical buckling, fracture‐induced structures (FISs), and laser induced periodic surface structures (LIPSS). In this study, we further proposed two FIS methods to fabricate easily and quickly large area self‐generated nonlinear and linear micro‐ripple structures. The initially featureless polystyrene film was effectively bonded between two rigid JIS SKD 61 (SKD61) steel cylinders and two soft PET films and then peeled off, the latter was cryogenically treated in liquid nitrogen before peeling off, the polystyrene film fractured into two complementary sets of nonlinear and linear micro‐ripple structures of the same period. The SKD61/ polystyrene (PS)/SKD61 sandwiched film used a slower peeling velocity and a thinner polystyrene film, which could fabricate a more complete and shorter period nonlinear wave structure. As the PET/PS/PET sandwiched film was cryogenically processed, it could produce a steeper waveform and flat valley linear ripple structures without cracks and ripple dislocations, with a period about 37 μm and an area about 300 cm2.