Sub-nanometer Roughness Research Articles

Hard transparent nanogradient coating for ultradurable omniphobic liquid-like surface

Omniphobic liquid-like surface (LLS) offers excellent dynamic liquid repellency owing to the highly mobile polymer chains mediating at the solid–liquid interface. These polymer chains, often achieved by surface grafting, are not durable against abrasion or oxidation, which seriously hinders their practical application. Here we report a new vapor-based approach to create transparent nanogradient coating for LLSs with unprecedented mechanical strength and durability. The nanogradient coating is composed of a sandwich structure with an ultrathin primer, a rigid bulk, and a nanocomposite surface with abundant polydimethylsiloxane-like chains, all of which achieved via sequential plasma polymerization of a linear and a cyclic siloxane. The plasma-polymerized cyclic siloxane enables coating good mechanical strength while the plasma-polymerized linear siloxane confers coating sub-nanometer roughness and excellent dynamic liquid repellency. The resultant coating exhibits contact angle hysteresis < 5° towards many low-surface-energy liquids, as well as good mechanical strength with the hardness exceeding 1 GPa and superior durability enduring 100,000 cycles of abrasion. The coating also survived industrial standard thermal and accelerated UV-aging tests without losing its liquid repellency and transparency. The reported method is facile, scalable, and substrate-independent, offering exciting new opportunities towards industrial fabrication and real-world application of LLS.

Wide-Range and Multistate Work Functions of Organometallic Halide Perovskite Films Regulated by Ferroelectric Substrates.

Work function of organometallic halide perovskite (OHP) films is one of the most crucial photoelectric properties, which dominates the carrier dynamics in OHP-based devices. Despite surface treatments by additives being widely used to promote crystallization and passivate defects in OHP films, these chemical strategies for modulation of work functions face two trade-offs: homogeneity on the surface versus along the thickness; the range versus the accuracy of modulation. Herein, by using ferroelectric substrates of uniform polarization and subnanometer roughness, homogeneous CH3NH3PbI3 films are fabricated with five states of work functions with large spanning (∼0.8 eV) and high precision (sd ∼ 0.01 eV). We reveal that the ferroelectric polarizations and the smooth surfaces regulate CH3NH3+ orientations and suppress distortions of PbI6 octahedrons. The wide-range and multistate work functions originate from the ordered CH3NH3+ orientations and PbI6 octahedrons, which result in intensity enhancements and wavelength shifts in photoluminescence with a 30-fold increase of photoexcited carrier lifetime.

A Review of Emerging Technologies in Ultra-Smooth Surface Processing for Optical Components.

Advancements in astronomical telescopes and cutting-edge technologies, including deep ultraviolet (DUV) and extreme ultraviolet (EUV) lithography, have escalated demands and imposed stringent surface quality requirements on optical system components. Achieving near-ideal optical components requires ultra-smooth surfaces with sub-nanometer roughness, no sub-surface damage, minimal surface defects, low residual stresses, and intact lattice integrity. This necessity has driven the rapid development and diversification of ultra-smooth surface fabrication technologies. This paper summarizes recent advances in ultra-smooth surface processing technologies, categorized by their material removal mechanisms. A subsequent comparative analysis evaluates the roughness and polishing characteristics of ultra-smooth surfaces processed on various materials, including fused silica, monocrystalline silicon, silicon carbide, and sapphire. To maximize each process's advantages and achieve higher-quality surfaces, the paper discusses tailored processing methods and iterations for different materials. Finally, the paper anticipates future development trends in response to current challenges in ultra-smooth surface processing technology, providing a systematic reference for the study of the production of large-sized freeform surfaces.

Fabrication of a large computer-generated hologram with high diffraction efficiency and high accuracy by scanning homogenization etching.

The diffraction efficiency, defined as the ratio of diffracted power to incident power, is one of the key working indicators for a computer-generated hologram (CGH). The CGH with high diffraction efficiency could suppress stray light and eliminate ghost images, thus improving interferometric performance in aspherical testing of low-reflectivity or large off-axis distance surfaces. However, the high-efficiency CGH is hard to precisely fabricate by traditional reactive ion etching and focusing ion beam, because it requires high etching depth with a high uniformity and sub-nanometric roughness in the glass, especially in the fabrication of a large CGH with an aperture of up to 300 mm. In this study, fabrication of the above-mentioned CGH was demonstrated via what we believe to be a new method called scanning homogenization etching (SHE), in which the ion source with a Gaussian energy distribution accurately scans the glass surface to realize homogenization etching. Different from controlling dwell time at each etching point, this paper proposes to control the scanning rate to achieve not only uniform but also quantitative depth removal in a single scan. Moreover, the depth errors in deep etching across the whole glass surface can be remarkably reduced due to homogenization effects introduced by multiple scanning etching. Finally, the target etching depth of 692.3 nm with an etching uniformity of 2.2% in the etching of a 300 mm CGH was achieved. The roughness of the etched and unetched area both have Ra values of 0.3 nm. The diffraction efficiency of working order is 39.998%, achieving 98.6% of the theoretical diffraction efficiency. In addition, the SHE is not limited by the aperture of the ion source, so it can achieve even larger diffractive optical elements with high diffraction efficiency and high accuracy.

Ultra-fast 3D printing of assembly—free complex optics with sub-nanometer surface quality at mesoscale

Complex-shaped optical lenses are of great interest in the areas of laser processing, machine vision, and optical communications. Traditionally, the processing of complex optical lenses is usually achieved by precision machining combined with post-grinding or polishing, which is expensive, labor-intensive and difficult in the processing of ultra-complex optical lenses. Additive manufacturing is an emerging technology that provides significant advantages in producing highly intricate optical devices. However, the layer-by-layer method employed in such manufacturing processes has resulted in low printing speeds, as well as limitations in surface quality. To address these challenges, we apply tomographic volumetric printing (TVP) in this work, which can realize the integrated printing of complex structural models without layering. By coordinating the TVP and the meniscus equilibrium post-curing methods, ultra-fast fabrication of complex-shaped lenses with sub-nanometric roughness has been achieved. A 2.5 mm high, outer diameter 9 mm spherical lens with a roughness value of RMS = 0.3340 nm is printed at a speed of 3.1 × 104 mm3 h−1. As a further demonstration, a complex-shaped fly-eye lens is fabricated without any part assembly. The designed spherical lens is mounted on a smartphone’s camera, and the precise alignments above the circuit board are captured. Upon further optimization, this new technology demonstrates the potential for rapid fabrication of ultra-smooth complex optical devices or systems.

Mold-free self-assembled scalable microlens arrays with ultrasmooth surface and record-high resolution

Microlens arrays (MLAs) based on the selective wetting have opened new avenues for developing compact and miniaturized imaging and display techniques with ultrahigh resolution beyond the traditional bulky and volumetric optics. However, the selective wetting lenses explored so far have been constrained by the lack of precisely defined pattern for highly controllable wettability contrast, thus limiting the available droplet curvature and numerical aperture, which is a major challenge towards the practical high-performance MLAs. Here we report a mold-free and self-assembly approach of mass-production of scalable MLAs, which can also have ultrasmooth surface, ultrahigh resolution, and the large tuning range of the curvatures. The selective surface modification based on tunable oxygen plasma can facilitate the precise pattern with adjusted chemical contrast, thus creating large-scale microdroplets array with controlled curvature. The numerical aperture of the MLAs can be up to 0.26 and precisely tuned by adjusting the modification intensity or the droplet dose. The fabricated MLAs have high-quality surface with subnanometer roughness and allow for record-high resolution imaging up to equivalently 10,328 ppi, as we demonstrated. This study shows a cost-effective roadmap for mass-production of high-performance MLAs, which may find applications in the rapid proliferating integral imaging industry and high-resolution display.

Self-Assembled Monolayers As a Tool to Investigate the Effect of Surface Chemistry on Protein Nucleation.

Modified surfaces like siliconized glass are commonly used to support protein crystallization and facilitate obtaining crystals. Over the years, various surfaces have been proposed to decrease the energetic penalty required for consistent protein clustering, but scarce attention has been paid to the underlying mechanisms of interactions. Here, we propose self-assembled monolayers that are surfaces exposing fine-tuned moieties with a very regular topography and subnanometer roughness, as a tool to unveil the interaction between proteins and functionalized surfaces. We studied the crystallization of three model proteins having progressively narrower metastable zones, i.e., lysozyme, catalase, and proteinase K, on monolayers exposing thiol, methacrylate, and glycidyloxy groups. Thanks to comparable surface wettability, the induction or the inhibition of nucleation was readily attributed to the surface chemistry. For example, thiol groups strongly induced the nucleation of lysozyme thanks to electrostatic pairing, whereas methacrylate and glycidyloxy groups had an effect comparable to unfunctionalized glass. Overall, the action of surfaces led to differences in nucleation kinetics, crystal habit, and even crystal form. This approach can support the fundamental understanding of the interaction between protein macromolecules and specific chemical groups, which is crucial for many technological applications in the pharmaceutical and food industry.

Dopant Selective Photoelectrochemical Etching of SiC

Single crystalline 4H-SiC is a wide-gap semiconductor with optical properties that are poised to enable new applications in MEMS and quantum devices. A number of key hurdles remain with respect to the micro and nano-fabrication of SiC to prepare precise photonic structures with nanometer-scale precision. These challenges include development of a fast, scalable etching process for SiC capable of producing a sub-nanometer roughness semiconductor surface while simultaneously reducing the total thickness variation across a wafer. Our investigation into UV photoelectrochemical processing of SiC reveals high dopant-type selectivity and the advantage of multiple etch stops to reduce layer thickness variation. We demonstrate dopant-type selectivities >20:1 using a single step and a >100x reduction in surface variation by combining two etch stops. Moreover, the etch rate is fast (>4 μm h−1) and the etched surface is smooth (∼1 nm RMS). These results demonstrate a scalable path to the fabrication of precise nanoscale SiC structures and electronic devices that will enable the next generation of MEMS and photonic quantum devices.

Spin Selectivity in Chiral Hybrid Cobalt Halide Films with Ultrasmooth Surface.

Introducing chirality into low-dimensional hybrid organic-inorganic halides (HOIHs) creates brand-new opportunities for HOIHs in spintronics and spin-related optoelectronics owing to chirality-induced spin selectivity (CISS). However, preparing smooth films of low-dimensional HOIHs with small roughness is still a great challenge due to the hybrid and complex crystal structure, which severely inhibits their applications in spintronic devices. Exploring new lead-free chiral HOIHs with both efficient spin selectivity and excellent film quality is urgently desired. Here, cobalt-based chiral metal halide crystals (R/S-NEA)2 CoCl4 constructed by 0D [CoCl4 ] tetrahedrons and 1-(1-naphtyl)ethylamine (NEA) are synthesized. The orderly configuration of NEAmolecules stabilized by noncovalent CH···π interaction endows (NEA)2 CoCl4 with good film-forming ability. (NEA)2 CoCl4 films exhibit strong chiroptical activity (gCD ≈ 0.05) and significant spin-polarized transport (CISS efficiency up to 90%). Furthermore, ultrasmooth films (roughness ∼ 0.3nm) with enhanced crystallinity can be achieved by incorporating tiny amount tris(8-oxoquinoline)aluminum that has analogous conjugated structure to NEA. The realization of highly efficient spin selectivity and sub-nanometer roughness in lead-free chiral halides can boost the practical process of low-dimensional HOIHs in spintronics and other fields.

Improvement in the polishing characteristics of titanium-based materials using electrochemical mechanical polishing

Titanium (Ti) and its alloys are widely used in aerospace components and biomaterials. The smoothing of Ti materials is essential for realizing high-quality Ti-based devices and components to enhance their physical and chemical properties, such as fatigue strength and biocompatibility. However, with conventional polishing methods, obtaining sub-nanometer roughness in a large measurement area is difficult because of the generation of crystal grain steps on the polished surface. This paper presents an electrochemical mechanical polishing (ECMP) method that involves electrochemical oxidation and subsequent oxide removal using SiO2 particles to achieve a high material removal rate (MRR) and an ultra-smooth surface of pure Ti and Ti-6Al-4V. Under suitable electrolytic current and mechanical conditions, ECMP provides a higher MRR and smoother pure Ti surface than electroless polishing. X-ray and Raman analyses indicate that the mechanical condition affects the chemical states of polished surfaces; higher mechanical conditions retain less oxide film on the polished surface, resulting in a smoother surface. The highly efficient ECMP provides a defect-free and sub-nanometer roughness for pure Ti and Ti-6Al-4V surfaces without using chemicals. Furthermore, it is an environment-friendly and cost-effective approach to prepare Ti-based components.

Identification of the seeding mechanism in the spinodal instability of dewetting liquids

HypothesisSpinodal dewetting is one of the basic processes inducing a spontaneous withdrawal of a liquid from a substrate surface. In the accepted theory, thickness fluctuations generated by thermally activated capillary waves are amplified by the competing actions of surface tension and disjoining pressure. Ubiquitous sub-nanometric substrate roughness also produces thickness fluctuations and may play a role analogous but even more efficient in seeding the process. ModellingAnalytic calculations valid at the early linear stage of the process and simulations extending the study to its whole non-linear development have been performed to compare features and the relative relevance of the two seeding mechanisms. FindingsCalculations and simulations have shown that substrate roughness can replace capillary waves in seeding spinodal dewetting. A typically larger amplitude and a steady nature compared to the transitory one of capillary waves allow us to conclude that, contrary to the common view, substrate roughness is the prevailing seed of the spinodal instability. The consequence of our statement is that spinodal dewetting loses most of its stochastic nature and becomes, in principle, a process that can be tuned by engineering substrate roughness.

Low‐Cost Volumetric 3D Printing of High‐Precision Miniature Lenses in Seconds (Advanced Optical Materials 17/2022)

Targeting the fast prototyping and massive production of complicated lenses, Zhiwei Zhu and co-workers report a low-cost volumetric 3D printing method for the ultra-fast and high-precision fabrication of miniature lenses with sub-nanometric roughness (see article number 2200488). The complex-shaped lenses are generated by the volumetric projection stereolithography and corrected by introducing an iterative learning scheme. Super-smooth surfaces are guaranteed by the meniscus equilibrium effect.

Ultra-smooth surface with 0.4 Å roughness on fused silica

Ultra-smooth surface with sub-nanometer roughness is of great significance in various fields, including Ring Laser Gyro (RLG), high-power laser apparatus, ultraviolet optical systems, and the semiconductor industry. However, the mechanism of the ultra-smooth polishing process remains to be studied, which is of great value for understanding and optimizing optical-fabrication methods. This paper establishes a relationship between surface densification characteristics and surface roughness in conventional polishing. Moreover, we conclude that the densification process created by the longitudinal pressure, which avoids non-uniformity, is beneficial to forming super-smooth surfaces in the conventional polishing methods. Under this guidance, we can stably fabricate ultra-smooth surfaces on fused silica with 0.4 Å roughness. This result overturns the mainstream view that the longitudinal pressure in polishing should be minimized. This conclusion is meaningful for the understanding of the ultra-smooth surface formation not only in conventional polishing but also in other optical-fabrication technologies.

Achieving sub-nanometer roughness on aspheric optical mold by non-contact polishing using damping-clothed tool.

The surface quality of optical lenses is highly required in imaging functions. Normally, ultra-precision turning is employed to fabricate the optical lenses. However, ultra-precision turning cannot meet the surface quality demands due to the tool marks. In this study, a new damping-clothed (DC) tool and chemical enhanced non-Newtonian ultrafine (CNNU) slurry for non-contact polishing are proposed to achieve sub-nanometer roughness on aspherical optical molds. A material removal model based on the hydrodynamic pressure and velocity simulation was established to calculate the dwell time in curved surface machining. The formation mechanism of sub-nanometer roughness is clarified. The proposed method and slurry were verified by the experiments in processing NiP alloy aspheric optical mold. After the process, surface roughness Sa achieved 0.54 nm and the form accuracy is less than PV 600 nm.

100km wear-free sliding achieved by microscale superlubric graphite/DLC heterojunctions under ambient conditions.

Wear-free sliding between two contacted solid surfaces is the ultimate goal in the effort to extend the lifetime of mechanical devices, especially when it comes to inventing new types of micro-electromechanical systems where wear is often a major obstacle. Here we report experimental observations of wear-free sliding for a micrometer-sized graphite flake on a diamond-like-carbon (DLC) surface under ambient conditions with speeds up to 2.5 m/s, and over a distance of 100 km. The coefficient of friction (COF) between the microscale graphite flake, a van der Waals (vdW) layered material and DLC, a non-vdW-layered material, is measured to be of the order of n}{}{10^{ - 3}}, which belongs to the superlubric regime. Such ultra-low COFs are also demonstrated for a microscale graphite flake sliding on six other kinds of non-vdW-layered materials with sub-nanometer roughness. With a synergistic analysis approach, we reveal the underlying mechanism to be the combination of interfacial vdW interaction, atomic-smooth interfaces and the low normal stiffness of the graphite flake. These features guarantee a persistent full contact of the interface with weak interaction, which contributes to the ultra-low COFs. Together with the extremely high in-plane strength of graphene, wear-free sliding is achieved. Our results broaden the scope of superlubricity and promote its wider application in the future.

Low-loss GeO2thin films deposited by ion-assisted alternating current reactive sputtering for waveguide applications

We report low optical loss germanium oxide (GeO2) thin films which have been deposited by low pressure ion-assisted alternating current dual magnetron sputtering of germanium targets in an oxygen plasma environment. The germanium oxide films ranging from 0.7- to 1.0-µm-thick were fabricated at low temperature and high deposition rates of 6–38 nm/min on silicon and thermally oxidized silicon substrates. The refractive index of the films was determined through variable angle spectroscopic ellipsometry and found to be 1.6051 on average at a wavelength of 638 nm. The films were shown to be near stoichiometric through Rutherford backscattering spectroscopy analysis and displayed sub-nanometer roughness when measured with atomic force microscopy. High peak-to-valley uniformity on a 3-inch substrate with relative deviation of 0.8% was achieved. We characterized the attenuation of the GeO2 thin films from visible to near-infrared wavelengths and observed it to be as low as 0.1 dB/cm at 638 nm for deposition rates of 8 nm/min. This deposition technique provides complementary metal-oxide-semiconductor compatible GeO2 thin films suitable for integrated photonics applications and is promising for the fabrication of other dielectric waveguide materials.

Amorphous cellulose thin films

Amorphous cellulose thin films with thicknesses of 8–100 nm and sub-nanometer roughness have been fabricated by spin-casting from molecular solutions of cellulose in mixtures of ionic liquids and organic solvents. Combining X-ray, optical and scanning probe measurements, cellulose films have been found to be fully amorphous, and their mass densities are comparable to that of the bulk amorphous cellulose. Full amorphization of cellulose films is achieved through sequential removal of solvents, successive diffusion of ionic liquid molecules out of cellulose films, and simultaneous formation of hydrogen bonding among cellulose chains.

Ti doping enhanced alumina dielectrics for low-voltage thin-film transistors via low-temperature lightwave irradiation

Herein, we report a high-k Al1 − xTixO dielectric films and device applications via a facile low-temperature lightwave irradiation process. The Al1 − xTixO films possess the smooth surface with subnanometer roughness. The successful doping of Ti in Al2O3 films was confirmed via several characterizations. The properties of Al1 − xTixO films are highly dependent on the ratio of Ti. With the introduction of 20% Ti, Al1 − xTixO films obtain much better properties than single TiO2 and Al2O3. The leakage current density reached 6.4 × 10− 8 A/cm2, and its capacitance is as high as 168 nF/cm2 (at 1000 Hz). For the device applications, InZnO/Al1 − xTixO TFTs show enhanced performance, such as a great field-effect mobility of 12.5 cm2/Vs, while InZnO/Al2O3 TFTs have a mobility of 5.7 cm2/Vs. Moreover, the operating voltage is as low as 3 V. The low-temperature lightwave irradiated Al1 − xTixO films, without high temperature or vacuum process, reveal great potential for the low-cost- but high-performance electronics.

Removal characteristics and performance of brick-shape permanent magnet–assisted magnetorheological lapping

A brick-shape magnet-assisted magnetorheological (MR) lapping was proposed for ultra-smooth flat surface finish, where a straight ribbon was produced to enhance polishing efficiency. Finite element analysis was performed to understand the magnetic property of the brick-shape magnet. Polishing marks and surface topographies were compared with the removal functions and distributions to reveal effects of translational oscillation, period ratio, work gap, eccentricity and trough speed on surface finish. Trajectories were calculated for kinematical analysis. Polishing performances on K9, sapphire and GaN showed that the polishing area was expanded to 1100 mm(2) by the brick-shape magnet. Surface planarity as 1 mu m in PV with sub-nanometric roughness were obtained on sapphire, GaN and K9 substrates from removal rates of 0.019, 0.037 and 0.437 mu m/min respectively.

Roughness and amorphization impact on thermal conductivity of nanofilms and nanowires: Making atomistic modeling more realistic

In this work, the impact of roughness and amorphization on the effective thermal conductivity of silicon nanofilms and nanowires is studied with atomistic simulations. The discrepancy between simulations and experimental measurements shows that it is necessary to consider realistic roughness and amorphization to reach an agreement. We show that subnanometric roughness and specific correlation length can reduce thermal transport by a factor of two in both nanofilms and nanowires; in addition, this reduction is even more pronounced than the one related to the existence of native oxides or amorphous phases on nanostructure edges. Furthermore, an interfacial thermal resistance parallel to the heat flux is observed. This thermal resistance is increasing upon the increase of the amorphous shell thickness, reaching a maximum value for thickness of 6 nm. Our findings could improve the strategy to elaborate nanomaterials with enhanced thermoelectric efficiency by tuning thermal conductivity through the engineering of surfaces.