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2022 Vol. 4, No. 4

The advanced multi-functional carbon dots in photoelectrochemistry based energy conversion
Yunjie Zhou, Fan Liao, Yang Liu, Zhenhui Kang
2022, 4(4) doi: 10.1088/2631-7990/ac8e3a

Carbon dots (CDs), as a unique zero-dimensional member of carbon materials, have attracted numerous attentions for their potential applications in optoelectronic, biological, and energy related fields. Recently, CDs as catalysts for energy conversion reactions under multi-physical conditions such as light and/or electricity have grown into a research frontier due to their advantages of high visible light utilization, fast migration of charge carriers, efficient surface redox reactions and good electrical conductivity. In this review, we summarize the fabrication methods of CDs and corresponding CD nanocomposites, including the strategies of surface modification and heteroatom doping. The properties of CDs that concerned to the photo- and electro-catalysis are highlighted and detailed corresponding applications are listed. More importantly, as new non-contact detection technologies, transient photo-induced voltage/current have been developed to detect and study the charge transfer kinetics, which can sensitively reflect the complex electron separation and transfer behavior in photo-/electro-catalysts. The development and application of the techniques are reviewed. Finally, we discuss and outline the major challenges and opportunities for future CD-based catalysts, and the needs and expectations for the development of novel characterization technologies.

Advances in selective laser sintering of polymers
Wei Han, Lingbao Kong, Min Xu
2022, 4(4) doi: 10.1088/2631-7990/ac9096

Polymers are widely used materials in aerospace, automotive, construction, medical devices and pharmaceuticals. Polymers are being promoted rapidly due to their ease of manufacturing and improved material properties. Research on polymer processing technology should be paid more attention to due to the increasing demand for polymer applications. Selective laser sintering (SLS) uses a laser to sinter powdered materials (typical polyamide), and it is one of the critical additive manufacturing (AM) techniques of polymer. It irradiates the laser beam on the defined areas by a computer-aided design three-dimensional (3D) model to bind the material together to create a designed 3D solid structure. SLS has many advantages, such as no support structures and excellent mechanical properties resembling injection moulded parts compared with other AM methods. However, the ability of SLS to process polymers is still affected by some defects, such as the porous structure and limited available types of SLS polymers. Therefore, this article reviews the current state-of-the-art SLS of polymers, including the fundamental principles in this technique, the SLS developments of typical polymers, and the essential process parameters in SLS. Furthermore, the applications of SLS are focused, and the conclusions and perspectives are discussed.

Electrostatic atomization minimum quantity lubrication machining: from mechanism to application
Wenhao Xu, Changhe Li, Yanbin Zhang, Hafiz Muhammad Ali, Shubham Sharma, Runze Li, Min Yang, Teng Gao, Mingzheng Liu, Xiaoming Wang, Zafar Said, Xin Liu, Zongming Zhou
2022, 4(4) doi: 10.1088/2631-7990/ac9652

Metal cutting fluids (MCFs) under flood conditions do not meet the urgent needs of reducing carbon emission. Biolubricant-based minimum quantity lubrication (MQL) is an effective alternative to flood lubrication. However, pneumatic atomization MQL has poor atomization properties, which is detrimental to occupational health. Therefore, electrostatic atomization MQL requires preliminary exploratory studies. However, systematic reviews are lacking in terms of capturing the current research status and development direction of this technology. This study aims to provide a comprehensive review and critical assessment of the existing understanding of electrostatic atomization MQL. This research can be used by scientists to gain insights into the action mechanism, theoretical basis, machining performance, and development direction of this technology. First, the critical equipment, eco-friendly atomization media (biolubricants), and empowering mechanisms of electrostatic atomization MQL are presented. Second, the advanced lubrication and heat transfer mechanisms of biolubricants are revealed by quantitatively comparing MQL with MCF-based wet machining. Third, the distinctive wetting and infiltration mechanisms of electrostatic atomization MQL, combined with its unique empowering mechanism and atomization method, are compared with those of pneumatic atomization MQL. Previous experiments have shown that electrostatic atomization MQL can reduce tool wear by 42.4% in metal cutting and improve the machined surface Ra by 47% compared with pneumatic atomization MQL. Finally, future development directions, including the improvement of the coordination parameters and equipment integration aspects, are proposed.

Recent progress in synthesis and surface modification of nickel-rich layered oxide cathode materials for lithium-ion batteries
Jing Li, Wentao Zhong, Qiang Deng, Qimeng Zhang, Chenghao Yang
2022, 4(4) doi: 10.1088/2631-7990/ac92ef

Nickel-rich layered oxides have been identified as the most promising commercial cathode materials for lithium-ion batteries (LIBs) for their high theoretical specific capacity. However, the poor cycling stability of nickel-rich cathode materials is one of the major barriers for the large-scale usage of LIBs. The existing obstructions that suppress the capacity degradation of nickel-rich cathode materials are as a result of phase transition, mechanical instability, intergranular cracks, side reaction, oxygen loss, and thermal instability during cycling. Core–shell structures, oxidating precursors, electrolyte additives, doping/coating and synthesizing single crystals have been identified as effective methods to improve cycling stability of nickel-rich cathode materials. Herein, recent progress of surface modification, e.g. coating and doping, in nickel-rich cathode materials are summarized based on Periodic table to provide a clear understanding. Electrochemical performances and mechanisms of modified structure are discussed in detail. It is hoped that an overview of synthesis and surface modification can be presented and a perspective of nickel-rich materials in LIBs can be given.

Mass transfer techniques for large-scale and high-density microLED arrays
Furong Chen, Jing Bian, Jinlong Hu, Ningning Sun, Biao Yang, Hong Ling, Haiyang Yu, Kaixin Wang, Mengxin Gai, Yuhang Ma, YongAn Huang
2022, 4(4) doi: 10.1088/2631-7990/ac92ee

Inorganic-based micro light-emitting diodes (microLEDs) offer more fascinating properties and unique demands in next-generation displays. However, the small size of the microLED chip (1–100 µm) makes it extremely challenging for high efficiency and low cost to accurately, selectively, integrate millions of microLED chips. Recent impressive technological advances have overcome the drawbacks of traditional pick-and-place techniques when they were utilized in the assembly of microLED display, including the most broadly recognized laser lift-off technique, contact micro-transfer printing (µTP) technique, laser non-contact µTP technique, and self-assembly technique. Herein, we firstly review the key developments in mass transfer technique and highlight their potential value, covering both the state-of-the-art devices and requirements for mass transfer in the assembly of the ultra-large-area display and virtual reality glasses. We begin with the significant challenges and the brief history of mass transfer technique, and expand that mass transfer technique is composed of two major techniques, namely, the epitaxial Lift-off technique and the pick-and-place technique. The basic concept and transfer effects for each representative epitaxial Lift-off and pick-and-place technique in mass transfer are then overviewed separately. Finally, the potential challenges and future research directions of mass transfer are discussed.

Burst mode enabled ultrafast laser inscription inside gallium arsenide
Andong Wang, Pol Sopeña, David Grojo
2022, 4(4) doi: 10.1088/2631-7990/ac8fc3

Ultrafast laser inscription (ULI) inside semiconductors offers new perspectives for 3D monolithic structures to be fabricated and new functionalities to be added in electronic and photonic microdevices. However, important challenges remain because of nonlinear effects such as strong plasma generation that distort the energy delivery at the focal point when exposing these materials to intense infrared light. Up to now, the successful technological demonstrations have primarily concentrated on silicon (Si). In this paper, we target at another important semiconductor: gallium arsenide (GaAs). With nonlinearities higher than those of Si, 3D-machining of GaAs with femtosecond pulses becomes even harder. However, we show that the difficulty can be circumvented by burst-mode irradiation. We generate and apply trains of pulses at terahertz repetition rates for efficient pulse-to-pulse accumulation of laser-induced free carriers in the focal region, while avoiding an overdose of prefocal excitations. The superior performance of burst-mode irradiation is confirmed by a comparative study conducted with infrared luminescence microscopy. The results indicate a successful reduction of the plasma density in the prefocal region so that higher pulse energy reaches the focal spot. The same method is applied to identify optimum irradiation conditions considering particular cases such as asymmetric pulse trains and aberrated beams. With 64-pulse trains, we successfully manage to cross the writing threshold providing a solution for ULI inside GaAs. The application potential is finally illustrated with a stealth dicing demonstration by taking benefit of the burst mode. The irradiation method opens wide possibilities for 3D structuring inside GaAs by ULI.

Compressive mechanical properties and shape memory effect of NiTi gradient lattice structures fabricated by laser powder bed fusion
Wei Chen, Dongdong Gu, Jiankai Yang, Qin Yang, Jie Chen, Xianfeng Shen
2022, 4(4) doi: 10.1088/2631-7990/ac8ef3

Laser additive manufacturing (AM) of lattice structures with light weight, excellent impact resistance, and energy absorption performance is receiving considerable attention in aerospace, transportation, and mechanical equipment application fields. In this study, we designed four gradient lattice structures (GLSs) using the topology optimization method, including the unidirectional GLS, the bi-directional increasing GLS, the bi-directional decreasing GLS and the none-GLS. All GLSs were manufactureed by laser powder bed fusion (LPBF). The uniaxial compression tests and finite element analysis were conducted to investigate the influence of gradient distribution features on deformation modes and energy absorption performance of GLSs. The results showed that, compared with the 45° shear fracture characteristic of the none-GLS, the unidirectional GLS, the bi-directional increasing GLS and the bi-directional decreasing GLS had the characteristics of the layer-by-layer fracture, showing considerably improved energy absorption capacity. The bi-directional increasing GLS showed a unique combination of shear fracture and layer-by-layer fracture, having the optimal energy absorption performance with energy absorption and specific energy absorption of 235.6 J and 9.5 J g-1 at 0.5 strain, respectively. Combined with the shape memory effect of NiTi alloy, multiple compression-heat recovery experiments were carried out to verify the shape memory function of LPBF-processed NiTi GLSs. These findings have potential value for the future design of GLSs and the realization of shape memory function of NiTi components through laser AM.

Mechanical property and biological behaviour of additive manufactured TiNi functionally graded lattice structure
Chaolin Tan, Cheng Deng, Sheng Li, Alessandro Abena, Parastoo Jamshidi, Khamis Essa, Likang Wu, Guohua Xu, Moataz M Attallah, Jia Liu
2022, 4(4) doi: 10.1088/2631-7990/ac94fa

Bio-inspired porous metallic scaffolds have tremendous potential to be used as artificial bone substitutes.In this work,a radially graded lattice structure (RGLS),which mimics the structures of natural human bones,was designed and processed by laser powder bed fusion of martensitic Ti-rich TiNi powder.The asymmetric tension-compression behaviour,where the compressive strength is significantly higher than the tensile strength,is observed in this Ti-rich TiNi material,which echoes the mechanical behaviour of bones.The morphologies,mechanical properties,deformation behaviour,and biological compatibility of RGLS samples were characterised and compared with those in the uniform lattice structure.Both the uniform and RGLS samples achieve a relative density higher than 99%.The graded porosities and pore sizes in the RGLS range from 40%-80% and 330-805 µm,respectively,from the centre to the edge.The chemical etching has significantly removed the harmful partially-melted residual powder particles on the lattice struts.The compressive yield strength of RGLS is 71.5 MPa,much higher than that of the uniform sample (46.5 MPa),despite having a similar relative density of about 46%.The calculated Gibson-Ashby equation and the deformation behaviour simulation by finite element suggest that the dense outer regions with high load-bearing capability could sustain high applied stress,improving the overall strength of RGLS significantly.The cell proliferation study suggests better biological compatibility of the RGLS than the uniform structures.The findings highlight a novel strategy to improve the performance of additively manufactured artificial implants by bio-inspiration.

Plasma-enabled electrochemical jet micromachining of chemically inert and passivating material
Jiajun Lu, Shunda Zhan, Bowen Liu, Yonghua Zhao
2022, 4(4) doi: 10.1088/2631-7990/ac84b3

Electrochemical jet machining (EJM) encounters significant challenges in the microstructuring of chemically inert and passivating materials because an oxide layer is easily formed on the material surface, preventing the progress of electrochemical dissolution. This research demonstrates for the first time a jet-electrolytic plasma micromachining (Jet-EPM) method to overcome this problem. Specifically, an electrolytic plasma is intentionally induced at the jet-material contact area by applying a potential high enough to surmount the surface boundary layer (such as a passive film or gas bubble) and enable material removal. Compared to traditional EJM, introducing plasma in the electrochemical jet system leads to considerable differences in machining performance due to the inclusion of plasma reactions. In this work, the implementation of Jet-EPM for fabricating microstructures in the semiconductor material 4H-SiC is demonstrated, and the machining principle and characteristics of Jet-EPM, including critical parameters and process windows, are comprehensively investigated. Theoretical modeling and experiments have elucidated the mechanisms of plasma ignition/evolution and the corresponding material removal, showing the strong potential of Jet-EPM for micromachining chemically resistant materials. The present study considerably augments the range of materials available for processing by the electrochemical jet technique.

Laser-assisted growth of hierarchically architectured 2D MoS2 crystals on metal substrate for potential energy applications
Parvin Fathi-Hafshejani, Jafar Orangi, Majid Beidaghi, Masoud Mahjouri-Samani
2022, 4(4) doi: 10.1088/2631-7990/ac8f73

Recently, there has been substantial interest in the large-scale synthesis of hierarchically architectured transition metal dichalcogenides and designing electrodes for energy conversion and storage applications such as electrocatalysis, rechargeable batteries, and supercapacitors. Here we report a novel hybrid laser-assisted micro/nanopatterning and sulfurization method for rapid manufacturing of hierarchically architectured molybdenum disulfide (MoS2) layers directly on molybdenum sheets. This laser surface structuring not only provides the ability to design specific micro/nanostructured patterns but also significantly enhances the crystal growth kinetics. Micro and nanoscale characterization methods are employed to study the morphological, structural, and atomistic characteristics of the formed crystals at various laser processing and crystal growth conditions. To compare the performance characteristics of the laser-structured and unstructured samples, Li-ion battery cells are fabricated and their energy storage capacity is measured. The hierarchically architectured MoS2 crystals show higher performance with specific capacities of about 10 mAh cm-2, at a current rate of 0.1 mA cm-2. This rapid laser patterning and growth of 2D materials directly on conductive sheets may enable the future large-scale and roll-to-roll manufacturing of energy and sensing devices.

Fabrication of extreme wettability surface for controllable droplet manipulation over a wide temperature range
Chengsong Shu, Qitong Su, Minghao Li, Zhenbin Wang, Shaohui Yin, Shuai Huang
2022, 4(4) doi: 10.1088/2631-7990/ac94bb

Droplet controllable manipulation over a wide temperature range has promising applications in microelectronic heat dissipation, inkjet printing, and high temperature microfluidic system. However, the fabrication of a platform for controllable droplet manipulation using the methods commonly used in industry remains a tremendously challenge. The popular method of controlling droplets is highly dependent on external energy input and has relatively poor controllability in terms of droplet motion behaviors and manipulation environment, such as distance, velocity, direction and a wide temperature range. Here, we report a facile and industrially applicable method for preparing Al superhydrophobic (S-phobic) surfaces, which enables controlled droplet bouncing, evaporation, and transport over a wide temperature range. Systematic mechanistic studies are also investigated. Extreme wettability surfaces were prepared on Al substrate by a composite process of electrochemical mask etching and micro-milling. To investigate the evaporation process and thermal coupling characteristics, controlled evaporation and controlled bouncing of droplet in a wide temperature range were conducted. Based on the evaporation regulation and bouncing mechanism of droplets on an extreme wettability surface, by using Laplace pressure gradients and temperature gradients, we realized controlled transport of droplets with confluence, split-flow, and gravity-resistant transport over a wide temperature range, offering a potential platform for a series of applications, such as new drug candidates and water collection.

Tailoring mechanical properties of PμSL 3D-printed structures via size effect
Wenqiang Zhang, Haitao Ye, Xiaobin Feng, Wenzhao Zhou, Ke Cao, Maoyuan Li, Sufeng Fan, Yang Lu
2022, 4(4) doi: 10.1088/2631-7990/ac93c2

Projection micro stereolithography (PμSL) has emerged as a powerful three-dimensional (3D) printing technique for manufacturing polymer structures with micron-scale high resolution at high printing speed, which enables the production of customized 3D microlattices with feature sizes down to several microns. However, the mechanical properties of as-printed polymers were not systemically studied at the relevant length scales, especially when the feature sizes step into micron/sub-micron level, limiting its reliable performance prediction in micro/nanolattice and other metamaterial applications. In this work, we demonstrate that PμSL-printed microfibers could become stronger and significantly more ductile with reduced size ranging from 20 μm to 60 μm, showing an obvious size-dependent mechanical behavior, in which the size decreases to 20 μm with a fracture strain up to ~100% and fracture strength up to ~100 MPa. Such size effect enables the tailoring of the material strength and stiffness of PμSL-printed microlattices over a broad range, allowing to fabricate the microlattice metamaterials with desired/tunable mechanical properties for various structural and functional applications.