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.
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.
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.
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.
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.
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.
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.
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.
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.