Objective AlSi10Mg alloy, prepared by selective laser melting, is one of the most widely investigated aluminum alloys recently. At present, most studies focus on the tensile strengths and fatigue properties of as-built or heat-treated alloys. However, fracture toughness is reported rarely. A study on as-built alloys indicated that fracture toughness was anisotropic. The plane-strain fracture toughness (KIC) is the lowest when the crack surface is parallel to the building direction, whereas it is the highest when the crack surface is perpendicular to the building direction. Heat treatment is beneficial to reduce or even eliminate the microstructural and property anisotropy and the residual stress. However, the anisotropy of fracture toughness and intrinsic principle for AlSi10Mg alloy after annealing have not been reported. Therefore, to explore the anisotropy of fracture toughness of annealed AlSi10Mg alloy prepared by selective laser melting, the alloy annealed at an optimum temperature is used to measure fracture toughness for different opening directions and analyze the reasons for fracture toughness anisotropy.
Methods AlSi10Mg alloy is fabricated on XLine 1000R Concept Laser equipment using atomized alloying powders. The selective laser melting process is performed by a checkerboard pattern scanning strategy under an argon atmosphere with the volume fraction of oxygen controlled below 0.1%. Then, as-built blocks are annealed at 275 ℃ for 2 h. The sample blocks, ground on a series of diamond sandpaper, are polished on the LectroPol-5 electrolytic machine. The microstructures, fracture morphologies, and grain boundary distribution are observed via optical microscopy, field emission scanning electron microscopy, and electron backscattered diffraction, respectively. The room temperature tensile properties in the X, Y and Z directions, analyzed via tensile tests according to GB/T 228.1—2010, are used to calculate KIC. The compact tensile specimens with a width (W) of 70 mm in different opening directions are prepared and tested according to GB/T 4161—2007. The load and crack opening displacement during tests are recorded. The conditional values of KIC (KQ) for different compact tensile specimens are calculated, and their validity are evaluated according to GB/T 4161—2007. Finally, the J-integral value and crack tip opening displacements are computed for different opening direction samples.
Results and Discussions Both KQ and KIC are invalid for all compact tensile specimens because the specimen thicknesses (B), pre-crack lengths (a), ligament lengths (b), and values of Fmax/FQ do not meet the requirements of KQ and KIC evaluation criteria stipulated by GB/T 4161—2007. Therefore, the fracture toughness and its anisotropy of annealed alloy are estimated by J-integral values and crack tip opening displacements. Results show that the J-integral value of the X-Y opening direction sample is approximately 430 kJ/m 2 (Table 2), which is the same as the Y-Z opening direction sample. Meanwhile, it is 250 kJ/m 2 for the Z-Y opening direction sample. Similarly, the crack tip opening displacements of the X-Y and Y-Z opening direction samples are also almost equal, approximately 0.8 mm. However, it is significantly lower for the Z-Y opening direction specimen, with a value of 0.47 mm, indicating that the fracture toughness of the annealed AlSi10Mg alloy is also anisotropic and similar to the as-built alloy. The results of microstructural observation indicate that the annealed alloy still exhibits the characteristics of “fish-scale” melt pools stacking layer by layer in parallel to the building direction, whereas it presents an interwoven morphology of melt pools as the structure is perpendicular to the building direction, indicating that the difference in fracture toughness in different directions is related to microstructural anisotropy. Because the structures near molten pool boundaries are relatively coarse and the ratio of low angle grain boundary is high, the cracks of specimens in the Z-Y opening direction tend to propagate along molten pool boundaries, resulting in lower fracture toughness. However, the internal structures of the molten pool, existing with a higher ratio of high angle grain boundary, are relatively fine, inducing good fracture toughness when cracks propagate through the interior of molten pools for the X-Y and Y-Z opening direction samples.
Conclusions The microstructures and fracture toughness of annealed AlSi10Mg alloy are anisotropic. When a crack surface is parallel to the building direction, the fracture toughness is high, J-integral value and crack tip opening displacement are 430 kJ/m 2 and 0.8 mm, respectively. Meanwhile, it is lower when the crack surface is perpendicular to the building direction, and the J-integral value and crack tip opening displacement are just 250 kJ/m 2 and 0.47 mm, respectively. The microstructural anisotropy and diversity between molten pool boundaries and the internal structure of the molten pools are the reasons for fracture toughness anisotropy. Because the fracture toughness of the annealed AlSi10Mg alloy manufactured by selective laser melting is relatively high, it is difficult to obtain effective KIC value according to the general test method, such as GB/T 4161—2007. It may be necessary to use a special linear elastic plane-strain fracture toughness test method, for instance, ASTM B645.
Significance With the development of new electronic devices for miniaturization, flexibility and intelligence, the diversity of nanomaterial properties and limitations of traditional electrical connection methods bring new challenges in new electronic device preparation. Researchers are encouraged to continue to explore ways to break the limit of the device size. The manufacturing technology has gradually developed to the nanoscale level. Nanowelding is one of the key technologies for integrating nanomaterials with micro and macro systems.
Metal nanomaterials (e.g., Ag, Au, and Cu) and some carbon-based nanomaterials (e.g., carbon nanotubes, and graphene) exhibit excellent electrical and thermal properties. Besides, some wide bandgap semiconductor nanomaterials (e.g., ZnO) have shown great potential in future electronic devices. Not only for homogenous connections but also for the study of electrical and mechanical properties of heterogeneous connections, evaluating their mechanical and electrical properties is crucial for predicting the failure modes of electronic devices.
Stable device performance depends on reliable nanointerconnected structures. The size effect and high specific surface area of nanomaterials make them exhibit different welding characteristics from bulk materials during the welding process. The study on the electrical performance of nanowelding consists of single joints and interconnection networks. The study of a single nanojoint is essential to deepen the understanding of the welding mechanism. For interconnection networks, especially with the rapid development of industries, such as smart touch interactive terminals and wearable electronic equipment flexible solar devices, their performance has attracted significant attention.
Progress Currently, the electrical and mechanical characterization of nanoconnection quality consists of two methods. The first method is aimed at electrical testing and characterization of nanoconnected single-welded joints, such as direct in-suit measurement of one-dimensional (1D) nanowire and nanotube-welding points. The second method indirectly characterizes macroscopic devices based on nanointerconnections, especially for some flexible film structures. For the study on the performance of 1D nanowires and single-nanometer connection joints of tubes, some researchers have used molecular dynamics-related simulation software to simulate the mechanical and electrical properties of their interface and perform atomic simulation of the entire welding process. The morphology and influencing factors are analyzed to obtain theoretical electrical performance before and after welding. For experimental measurement, if the electrical and mechanical properties are to be directly characterized at such a small scale, with the development of characterization technology, direct mechanical measurement of solder joints can be achieved. However, there are still many challenges in the actual measurement process.
The current nanowelding methods are low-temperature cold welding, pressure welding, ultrasonic welding, electric field and chemical-assisted welding, high temperature and Joule welding, high-energy beam welding (e.g., electron and ion beam), and laser-induced plasma welding at local low temperature. During the preparation of nanointerconnection devices, especially for the new generation of flexible nanoelectronics, it is necessary to prepare interconnect joints with high electrical performance and a low-temperature and low-stress welding environment, which does not cause damage to other surrounding nanodevices and substrates. The nanojoints obtained using high-temperature melting are often accompanied by a relatively large heat-affected area, which will also have a thermal impact or even damage to the structure of the nonconnected parts, and then reduce the electrical performance of the overall interconnection structure.
Conclusion and Prospect This study summarizes and prospects the electrical and mechanical properties of different materials from the atomic scale to single welded joints, and then to macroscopic multinanoscale welded joints by combining the characteristics of current different nanowelding technologies and their welding interfaces. The discussion of welding structure and deformation mechanism, welding strength, fatigue characteristics, and electrical performance showed that laser-induced plasma welding with characteristics of self-limiting and low-temperature has great potential in fabricating nanodevices and flexible electronic devices.
Although the current study on laser-induced plasma self-limiting low-temperature welding technology has achieved a certain progress, it still faces huge challenges for achieving high-efficiency, high-precision, and high-resolution laser-induced nanocontrollable interconnection manufacturing. The realization of the energy precise control of the nanoscale joints and interconnection mechanism of materials at the nanoscale still needs further study. Besides, for interconnection functional structures with nanoscale line widths, effective manipulation techniques are often required to arrange and assemble them before the connection. It is necessary to achieve subsequent high-precision positioning. This process relies on the integration of high-precision laser nanowelding equipment; however, related technologies still need further study and development. It is believed that the continuous development of laser nanowelding technology will play a significant role in the next generation of electronic device interconnection packaging.
Significance Owing to excellent adaptability to different working conditions, flexible electronics have attracted significant attention in many fields, such as wireless communication, human-machine interaction, and personal healthcare. Functional parts and conductive circuits are the basic components of electronics that respond to external stimulus and conduct signals, respectively.
Nanomaterials with unique physical and chemical properties are widely used for developing flexible electronics. Noble metals, such as silver, gold, and platinum are good candidates for manufacturing conductive parts because of their high conductivity and chemical stability. However, the high price of these metals limits their large scale production. Recently, copper has been considered a good alternative to noble metals for developing conductive component owing to its low-cost and excellent electrical properties. Furthermore, copper oxides (cuprous oxide and cupric oxide) are important transition metal oxides because of their semiconductive properties. They have been widely used as functional parts owing to their high sensitivity for external stimulation, such as humidity, temperature. Efficient manufacturing methods for materials play a major role in developing high-performance devices.
The typical “bottom-up” process, such as hydrothermal and chemical precipitation, provides a low-cost, precise control, and large-scale synthesis route to manufacture the micro/nanostructured copper. However, post-treatment processes, such as printing and sintering, are required to obtain the desired properties in a device. Such step-by-step manufacturing requires the cooperation of various techniques, which increases the process cost and complexity. Thus, developing a low-cost process for manufacturing the micro/nanostructures has attracted significant attention.
Direct laser writing, as an advanced processing technology developed recently, provides a novel approach for micro/nanostructure manufacturing. This technology has been used to process the structure, including noble metals, metal oxides, and carbon-based materials. In this study, the technical characteristics of manufacturing copper-based micro/nanostructures with direct laser writing have been summarized.
Progress The typical laser processing of micro/nanostructures, such as laser assembly, sintering, and synthesis, has been elaborated (
Conclusions and Prospect In summary, direct laser writing has been an efficient manufacturing process for copper micro/nanostructures owing to its noncontact, maskless, and rapid processing characteristics. Direct laser writing based on ionic precursors integrates the synthesis, positioning, assembly, and joining of copper nanomaterials into a one-step, which shows unique advantages in structure and composition control. This process still faces challenges in processing copper structures, such as the accurate control of products, diversification of composite structures, and further expansion of application. Further, in-depth study is needed to explore the writing mechanism and fully understand the processing characteristics for copper-based micro/nanostructures.
Objective There is an increasing demand for die attach materials with the rapid development of SiC devices, which can be bonded at low-temperature and function at high temperature. Nano-Ag sintering has been extensively investigated for application in high-temperature power electronics. However, the electrochemical-migration of Ag ions is the main drawback. Pd is famous for its chemical stability, and various studies have focused on the influence of Pd content on the effectiveness and its mechanism. Recently, researchers have been trying to mix Pd and Ag nanoparticles (NPs) to improve the resistance to electrochemical-migration of the sintered layer. However, Pd has a melting point higher than that of Ag, whereas the alloying process needs high temperature (~850 ℃) to form Ag-Pd alloy. Pulsed laser deposition (PLD) is a physical method feasible for fabricationg Ag-Pd nanoalloy without using organic additives such as polyvinylpyrrolidone, which is required in the chemical method. In this work, Ag-10%Pd nanoalloy was fabricated by the PLD method, which can be used to connect SiC and Ag-coated direct bonding copper (DBC) substrates. The sintered layer enhances resistance to electrochemical-migration with low-temperature bonding characteristics. The microstructure of the bonding, shear properties, and its electrochemical-migration resistance are studied.
Methods Ag-10%Pd NPs were fabricated using PLD with a pressure of 750 Pa of Ar atmosphere. The Ag-Pd target was fabricated by powder sintering with weight ratio of 90∶10. A picosecond laser with a pulse width of 10 ps was employed to ablate the target. Ag-Pd NPs were deposited on the back side of SiC chip (G.P.Tech, Ti/Ni/Ag metallization), then the SiC chip was removed from the substrate and placed on the Ag-coated DBC (HuaSemi Electronics, Ni/Au metallization). The interconnecting process is performed at a temperature range of 200 ℃-350 ℃ assisted with a pressure of 5 MPa for 30 min in air. The shear test is conducted using Dage 4000. The electrochemical-migration test is conducted using a water drop test.
Results and Discussions The microstructure of as-deposited Ag-Pd film comprises various NPs with diameters less than 1 μm (Fig. 3). Element results indicate that these deposited NPs are in alloy state with a uniform composition distribution. The sintered joint comprises SiC chip, bondline and Ag-coated substrates (Fig. 4). The bondline thickness is about 27 μm, which is only 31.6% of the as-deposited state. Thus, the Ag-Pd film had excellent deformability. The bondline exhibited Ag-9.57%Pd alloy microstructure without obvious element segregation. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which is higher than the US military standard MIL-STD-883K(7.8 MPa). Therefore, Ag-Pd nanoalloy film can be used as die attach material for low-temperature bonding. The sintering temperature provides the driving force for sintering process, as a denser bondline is achieved when the temperature is increased to 300 ℃ (Fig. 6). Fracture surface reveals that the failure mainly occurred at the bondline, indicating that high bonding quality interface is realized (Fig. 7). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple resistance to electrochemical-migration during the water drop test (Fig. 8). For pure Ag electrode, the current reached 1 mA with only 81.4 s, while the Ag-Pd electrode required 349.7 s for the short-circuit process. The dissolution of Ag ion was blocked by PdO formation on the anode, which played a paramount role in extending the short-circuit time, whereas the migration product was cloud-like instead of dendritic growth. This work proposed a method for fabricating Ag-Pd nanoalloy films as die attach material without the high alloying temperature. It should be noted that, Pd has a higher melting point (1554 ℃) than Ag (961.7 ℃), and Ag-Pd nanoalloy sintering requires higher sintering temperature than pure Ag NPs. Moreover, adding Pd is costly. Consequently, the sintering temperature, demand of electrochemical-migration resistance and its cost should be balanced when applying Ag-Pd nanoalloy in electronic packaging.
Conclusions Ag-10%Pd nanoalloy was successfully fabricated as die attach material using PLD. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which was higher than the US military standard MIL-STD-883K (7.8 MPa). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple electrochemical-migration resistance. The dissolution of Ag ion was blocked by PdO formation on the anode with obviously extended short-circuit time, whereas the migration product was cloud-like. Compared with conventional direct sintering of Ag and Pb nanoparticles, pulsed laser deposited Ag-Pd nanoalloy sintering avoids high-temperature alloying process (850 ℃), which is promising for Ag-Pd low-temperature bonding and is expected to provide a solution for the high-reliability power electronic packaging.
Significance Micro-holes with a diameter of tens to few hundreds of microns are widely used in different industrial fields, such as injection nozzles in the automotive industry, cooling holes in jet engine components, and interconnecting micro-via in electronic packages. Methods, such as electro discharge, mechanical, electrochemical, and continuous or pulsed laser drilling, are used for micro-hole machining. However, these methods cannot be used in the micro-hole drilling with a diameter smaller than 100 μm. For micro-holes larger than this size, these methods have their limitations, such as poor accuracy, low efficiency, and incapable of drilling in non-conductive materials (e.g., glass). For the past decades, ultrafast laser has been a reliable tool for such processing due to its unique characteristics. Various hard-to-machine and newer materials, such as glass, diamond, biological materials, and superalloy, can be ablated by ultrafast laser through nonlinear absorption of energy. The pulse is too short that only a small amount of heat transfers to the surrounding of the irradiated zone, leading to minimized heat-affected zone and high processing precision.
Although the ultrafast laser has advantages in micro-drilling, some points need to be carefully investigated for practical applications. The quality and processing efficiency of ultrafast laser drilling are affected by processing methods, materials, auxiliary methods and beam characteristics such as pulse energy, frequency, pulse width, polarization, etc. Different process parameters will result in different roundness, taper, defects,and surface quality. The study on process technology parameters is one of the core issues of ultrafast laser micro-hole machining.
Recently, many advances have been achieved in the processing technology of ultrafast laser drilling in various materials. However, due to the differences in materials, processing, and auxiliary methods, different studies have different conclusions about the same process parameters. Thus, it is essential to summarize and compare the results of the same process parameter.
Progress The study of ultrafast laser micro-hole machining began in the 1990s. Early studies focused on the interaction mechanism between the laser and material. A series of models of interaction between ultrafast laser and material were proposed. Many studies have focused on the investigation of aspect ratio and surface morphology of the micro-holes. With the progress of laser technology, the laser power has been increased, and the pulse width is further compressed. Besides, high-speed and high-precision laser processing technologies, such as helical drilling, have appeared. The study focuses on micro-hole processing gradually turn to the acquisition of higher precision, larger aspect ratio, and drilling on various hard-to-machine materials. Recently, due to its super-short time resolution, several research groups have employed the high-speed observation technique, such as pump-probe, to study the mechanism of laser-matter interaction. There has been an increasing interest in parallel processing, high-speed scanning, and other technologies with high efficiency suitable for micro-hole array drilling.
Typical materials and applications of ultrafast laser micro-hole drilling are summarized (
For stable energy transmission in the micro-hole, some researchers have proposed water-guided laser processing technology. The laser beam is coupled into the water jet, and the laser travels forward due to total reflection in the water jet. The water-guided laser has great advantages in high aspect ratio micro-holes processing. Recently, multi-beam parallel processing and high-speed scanning technology have appeared in the field of efficient micro-hole drilling. For example, the diffractive optical element (DOE), spatial light modulator (SLM), and acousto-optic modulator (AOM) are used for beam splitting and energy modulation. By using these devices, parallel processing of 16 laser beams is realized. Polygon laser scanning technology is another approach to enhance the processing speed by increasing the laser scanning speed from several meters per second to a hundred meters per second.
Conclusions and Prospects With the development of laser technology, the pulse width is getting shorter, while the frequency and power are getting higher. The ultrafast laser has gradually become a reliable tool for high aspect ratio and precision micro-hole machining; however, several problems remained. Thus, in-depth and detailed explorations are essential to enhance the development of this micro-hole drilling technology.
Objective With the rapid development of nanotechnology, new devices are gradually developing toward miniaturization, complexity, multimaterial, and multifunction. Selective nanojoining of nanowires is essential for the fabrication and assembly of high-performance functional nanounits. The development of good quality nanojoined structures based on new material systems has attracted considerable attention. Owing to its high peak power and small heat-affected zone, the femtosecond laser has unique advantages in accurate selective nanojoining. It is difficult to choose the parameters of laser processing joint fabrication, and material selection of nanowires, thus far, laser irradiation has only realized the nanojoining of metal-metal nanowires, metal-semiconductor nanowires, and n-n type semiconductor nanowires. The nanojoining system of nanowires under laser irradiation is still imperfect and needs further improvement. Therefore, we propose a method to successfully nanojoin two p-type copper oxide (CuO) nanowires using the local energy field of femtosecond laser with high spatial and temporal accuracy. Simultaneously, we investigate the influence of different femtosecond laser energy inputs on the interconnection joint and fabricate optoelectronic devices based on the nanojoined structure. The results show that the electrical response and photoelectric properties of the nanowire structure fabricated under femtosecond laser irradiation are significantly improved compared with those before nanojoining and can reach the level of the base material on electric properties.
Methods The CuO joints are prepared using the dry transfer method. The nanowires are ultrasonically dispersed into an ethanol solution and spread on the surface of a polydimethylsiloxane(PDMS) film. The suitable target nanowires are obtained using an optical imaging system and aligned with the test electrode of the substrate using light transmittance of the film. The substrate is heated to 120 ℃ and held for 10--20 min to ensure that homogeneous joint of CuO nanowires is formed at the designated position on the substrate. (Fig.1(a)--(f)). The femtosecond laser is focused on the surface of the sample using a focusing microscope, and a CCD camera is used for real-time observation to ensure that the laser spot is focused accurately at the joint (Fig.1(g)). The continuous adjustment of laser power is achieved using a polarizer. The main characterization methods include a scanning electron microscope (SEM, Zeiss Supra 55), transmission electron microscope (TEM,JEM-2100F), and energy dispersive spectrometer (EDS). COMSOL Multiphysics 5.4 is used for simulation software, and Keithley 2636B is used for electrical tests.
Results and Discussions The prepared CuO nanowires are cylindrical with diameters ranging from 100--250 nm (Fig. 2). SEM is used to observe the morphology of CuO nanowire joints under different femtosecond laser parameters. When the single pulse energy density of the laser reaches 22.3 mJ/cm 2, the melting and wetting of the nanowires can be observed at the joint while the two base CuO nanowires remain intact, and almost no damage occurs, indicating that femtosecond laser can nanoweld two CuO nanowires with a minimal heat-affected zone (Fig.3(a)(d)). When the laser energy density is increased to 27 or 30 mJ/cm 2, a partial ablation or fracture of nanowires occurs, respectively, resulting in joint failure (Fig. 3 (e) and Fig. 3 (f)). The light field enhancement caused by geometric factors occurs at the contact area of nanowires by simulating the electric field distribution under laser irradiation using COMSOL, which is conducive to forming joints with a minimal heat-affected zone while nanojoining (Fig.4). The current response of the CuO homojunction device fabricated using this method is more than three orders of magnitude higher than that of the sample without nanojoining at 10 V bias, indicating that the properties of the nanowelded device are restored to the base material level (Fig. 5 and 6). CuO is a common optical sensing material. After femtosecond laser nanojoining, the fabricated CuO homojunction photoelectric sensor reaches the photoelectric performance of the base material, and the current growth ratio under 25.3 mW halogen lamp irradiation is the same as that of the base CuO nanowire (Fig.7).
Conclusions In this paper, we have successfully achieved the nanojoining between two p-type semiconductor CuO nanowires by combining the method of dry transfer and femtosecond laser irradiation. Under the influence of the laser energy field, the cylindrical CuO nanowires generate local energy field enhancement at the contact area due to the geometric factors, promoting the nanowelded joint formation with the minimal heat-affected zone. Under laser irradiation with a single pulse energy density of 22.3 mJ/cm 2, atomic-scale diffusion occurs at the joint of CuO nanowires to form a wetting structure, which transits the contact condition of nanowires from point contact to surface contact, greatly reduces the interface barrier, and widens the carrier transmission channel. This process increases the current level by more than three orders of magnitude compared with samples without nanojoining at 10 V bias, which almost reaches the current level of the base material. The photodetector based on the nanowelded structure obtains the same current growth ratio as that of the base material under a power of 25.3 mW of a halogen lamp. This study broadens the material system of semiconductor nanowires, which can be nanowelded, and provides a basis for the fabrication of miniaturized, high-performance, and multifunctional nanowire networks nanojoining.
Significance Nanomaterials have been researched and developed in the fields of solar cells, biological detection, sensors, and information storage. However, the interconnection between nanomaterials and external units is limited to simple mechanical contact, and many nanoscale features, such as excellent electrical, optical, and magnetic properties, are not exhibited. The rapid development of nanotechnology has high demands on the joining technology of nanomaterial units to realize complex functional systems. The interconnection of nanomaterials is the basis of nanoscale product integration and will immensely enrich its functionality.
Progress According to the size of the joining materials, if the size is at least in the range 1--100 nm, it is called nanojoining. The essence of nanojoining technology is material interconnection, and conventional joining methods via the force/heat strategy are still applicable in nanojoining. Compared with traditional macro-joining, nanomaterials are melted or interdiffused to obtain effective joints. By using the nanosize effect, the sintering temperature of metallic nanoparticles (NPs) will be much lower than the melting point of the bulk metal, they will be interconnected by sintering at a low temperature, and the metallurgical interface will be formed by diffusion. Surface diffusion is the main sintering mechanism of NPs, while the grain boundary diffusion is the sintering mechanism of large particles.
The metallurgical connection between the metal materials is realized via cold welding without external direct energy input. In situ transmission electron microscopy shows that the joining is almost perfect (
Laser irradiation is one of the most common joining methods in nanomaterials. This method can avoid the high requirement for mechanical manipulation in cold welding. Surface plasmon heated local nanomaterials, which could achieve cross-scale, cross-material low-damage joining. Owing to surface excitation, the electromagnetic field occurring in the metal nanostructures and the enhanced plasmon contributes to heat and join nanomaterials. In addition to the strong thermal effect of surface plasmon, the electromagnetic field will promote interconnection. If a femtosecond laser with low power density is irradiated, particles will achieve an orderly arrangement. If the laser power density is high, the ends of the nanorod will be arranged under the action of local heat, and the crystal faces will match to realize interconnection.
Numerous studies have been conducted on the interconnection of various metals and nonmetals with the formation of electrical signal connections in the printed electronic products as the main driving force. The interconnection of heterogeneous and homogeneous nanomaterials has the same diffusion mechanism, but the challenge of heterogeneous material interconnection is the lattice matching at the interface. When an ultrafast laser irradiates Ag and Pt NPs, Ag NPs are first melted and interconnected with the surrounding Pt NPs. Ag NPs act as metal solder, and the interface shows good Ag-Pt lattice matching (
Conclusions and Prospects Nanoscience provides many strategies for building high-performance materials and devices. The bottom-up manufacturing process is conducive to large-scale synthesis, the joining and interconnections, especially heterogeneous nanomaterials, still need further development. The joining between materials should be extended to different systems to ensure the versatility of interconnected nanomaterials and devices and meet the design function requirements. An essential factor in the interconnection of nanomaterials is to precisely control the melting depth to prevent NPs from merging to form a single particle. To avoid excessive damage, space-limited energy input will become necessary. Ultrafast laser-precise irradiation may be an ideal method for joining and interconnection of nanomaterials.
Significance With the continuous exploration of novel materials, especially nanomaterials, in developing advanced flexible and high-performance micro/nano optical and electrical devices, high-quality nanojoint formation within nanomaterials has become a key issue for device nanofabrication. However, with the restriction of the size and microstructures of nanomaterials, conventional macro-and microjoining technologies cannot achieve highly controlled spatial energy input within the structures, and therefore fail to produce low-damage joints. Optical nanojoining (i.e. plasmonic nanojoining) technology, arising from the surface plasmon resonance generated at the metal-dielectric interface, is advantageous for joining nanomaterials. Specifically, the spatial energy input is confined at the locations with geometric discontinuities, owing to the strong localized plasmonic effect. Therefore, material damage is minimized, even when the entire nanostructure is covered by a large laser beam. This noncontact laser nanojoining technique permits precise and low-damage material interconnection at the nanoscale. Compared with other joining methods, such as nanobrazing and focused ion/electron-beam nanojoining, laser nanojoining can greatly simplify the joining process and reduce the demand for high-precision operation of the energy input. In addition, it is known that the introduction of an ultrafast laser with a pulse duration of femtoseconds or picoseconds can further enhance the electromagnetic field intensity generated by the surface plasmonic effect, which can extend the processed materials selection from metal to oxide/semiconductor. Therefore, ultrafast laser nanojoining can enable heterogeneous material integration, which is of vital importance to the implementation of advanced nanomaterials in micro/nanoelectronic applications.
Progress Since the plasmonic nanowelding of silver nanowires was demonstrated in detail by E. C. Garnett in 2012, the nanojoining of metal nanomaterials, including nanoparticles, nanorods, and nanowires, has been widely studied. Self-limited energy inputs within the nanostructures during the nanojoining process have been observed for low-damage nanojoint formation. Initially, most research work only focused on improving the electrical conduction of the joined metal nanowire networks by using lamps in the visible spectrum. However, because of its low efficiency of energy conversion and high dissipation of incident energy, the lamp was replaced with a laser beam. Based on the processed nanomaterials, laser nanojoining has shown high efficiency and low material damage due to thermal accumulation, even for temperature-and environment-sensitive materials. Therefore, the production rate has been improved by several orders of magnitude, as has the electrical performance.
Although laser nanojoining has been used widely in the fabrication of nanowire-based transparent electrodes, the material selection is limited to metals, owing to the low photon-absorption efficiency of dielectric materials (e.g., oxides and semiconductors) under conventional laser-beam irradiation. A. Hu and Y. Zhou proposed applying an ultrafast laser for nanojoining a broad range of materials. Because of the effects of nonlinear photon absorption and intense electromagnetic fields, oxides and semiconductors can be processed accordingly under ultrafast laser irradiation. On this basis, L. C. Lin systematically studied the ultrafast laser nanojoining of materials in different combinations. Specifically, heterogeneous metal and oxide nanowire joining was demonstrated using femtosecond laser irradiation. The as-received heterogeneous nanowire joint shows robust joint strength and improves electrical conduction, further demonstrating the effectiveness of using an ultrafast laser to join metal and oxide materials. Notably, this ultrafast laser nanojoining process is a generic joining technology, in which the joined materials are not limited to metals and oxides, but can also be applied to other metal and dielectric combinations.
With the formation of low-damage homogeneous and heterogeneous nanowire joints, the development of nanowires in nanodevices has become possible. Notably, ultrafast laser nanojoining has shown great advantages over the nanosecond laser in the fabrication of transparent electrodes, as the substrate can be protected well during ultrafast laser irradiation (
Conclusion and Prospect Ultrafast laser nanojoining has been used successfully in low-damage nanowire joining with broad material combinations, including metal-metal and metal-oxide/semiconductor. The spatial energy input within the nanowire structures, arising from the localized plasmonic effects, can be confined precisely at the junction area, which greatly simplifies the operation of the laser beam and thus allows mass production of high-quality nanowire joints. By constructing nanoscale homogeneous and heterogeneous joints, ultrafast laser nanojoining can be used not only in fabricating individual functional nanowire devices, but also in scalable material integration, which shows great potential in applications including small-scale additive manufacturing and integrated nanoelectronics manufacturing.
Significance Continuous miniaturization of traditional silicon electronic devices and photoelectric components increases the integration and performance of devices and introduces some undesirable problems caused by the size and quantum effects, and increased power consumption. Thus, the development of multifunctional next-generation nano-devices with more excellent performance than traditional devices is inevitable and significant in the post-Moore era. Owing to the excellent mechanical, thermal, electrical, and optical properties of nanomaterials, such as nanoparticles, quantum dots, nanowires, nanotubes, and two-dimensional (2D) materials, many studies have suggested that these materials are suitable for channel or electrode of multifunctional and high-performance nano-devices. Thus, the study and development of nano-devices based on nanomaterials are crucial for solving the bottleneck problems of electronics in the future.
Recently, abundant theoretical and experimental results have demonstrated that bending, folding, twisting a single nanomaterial, and arranging, assembling, connecting several nanomaterials can improve properties further or bring extraordinary characteristics of nano-devices. For instance, compared with chemical doping and contact engineering, deformation of 2D materials can solve the Fermi-level pinning and carrier concentration decreasing in nano-devices and may introduce new phenomena, such as piezotronics and piezo-phototronics. Thus, methods and accompanying systems for moving, arranging, deforming nanomaterials, and fabricating nano-structures and nano-devices will be crucial and indispensable in the electronics field in the future. The most “top-down” approaches for fabricating electronic and optoelectronic devices, such as ultraviolet lithography, electron beam lithography, and laser writing, are unfit for the mentioned purpose. Instead, nano-manipulation technology, as a “bottom-up” method, is proposed to move or spin atoms, nanomaterials, and cells in the nanoscale resolution. Based on this, it is promising in moving, deforming, and assembling nanomaterials in high-precision than other methods. For example, some indirect methods for bending 2D materials (e.g., thermal expansion mismatch, deformation of flexible substrates, and substrate surface topography modification) exist some problems, such as slipping of materials, small deformation, and uncontrollability. Nano-manipulation can use probes to push or fold materials in nano-/micro-scale directly and achieve large, complex, and controllable deformation. With electron beam-induced deposition, laser processing, and nano-welding, this technology can also develop nano-structures with excellent properties, weld a single material onto an electrode to fabricate devices, and test properties of a single material and device. Thus, it provides a new idea for the development of new-generation nano-devices with excellent performance.
Among many nano-manipulation techniques, methods and systems based on the microscopes with nano-level imaging accuracy, e.g., scanning probe microscope (SPM) and electron microscope (EM), are widely used. With the microscope monitor, the system controls the motion module to move the probes, tweezers and other manipulation tools in high-precision, and then moves, picks up, and bends nanoparticles (NPs), nanowires (NWs), and 2D materials. Besides, optical tweezers, magnetic tweezers, and acoustic tweezers can apply force to materials and trap or move these further by controlling the optical, magnetic, and sound fields. To develop nano-devices, manipulation methods based on SPM, EM, and optical tweezers are promising and anticipated. Thus, it is necessary and significant to introduce and summarize the recent studies in these nano-manipulation methods and understand their application in nano-devices.
Progress This study introduces the recent research in nano-manipulation based on scanning probe microscope(SPM), electron microscope(EM), and optical tweezers. For SPM manipulation, the principles and typical process demonstrate the capacity for accurately moving particles of tens to a few nanometers in diameter and weakness in real-time imaging, efficient and complex manipulation. For real-time imaging during manipulation, representative improvements include strategy optimization and development of parallel imaging/manipulation system (
Conclusions and Prospects SPM manipulation, EM manipulation, and optical tweezers have their advantages, limitations, and applications (