Structural and chemical study of complex silver patterns additively manufactured by multi-photon reduction.

Multi-photon reduction (MPR) based on femtosecond laser makes rapid prototyping and molding in micro-nano scale feasible, but is limited in material selectivity due to lack of the understanding of the reaction mechanism in MPR process. In this paper, additively manufacturing of complex silver-based patterns through MPR is demonstrated. The effects of laser parameters, including laser pulse energies and scanning speeds, on the structural and chemical characteristics of the printed structures are systematically investigated. The results show that the geometric size of printed cubes deviates from the designed size further by increasing laser pulse energy or decreasing scanning speed. The reaction mechanism of MPR is revealed by studying the elemental composition and chemical structures of printed cubes. The evolution of Raman spectra upon the laser processing parameters suggests that the MPR process mainly includes two processes: reduction and decomposition. In the MPR process, silver ions are reduced and grow into particles by accepting the electrons from ethonal molecules; meanwhile carboxyl groups in polyvinylpyrrolidone are decomposed and form amorphous carbon that is attached on the surface of silver particles. The conductivity of silver wires fabricated by MPR reaches 2 × 105S m-1and stays relatively constant as varying their cross section area, suggesting excellent electrical conduction. The understanding of the MPR process would accelerate the development of MPR technology and the implementation of MPR in micro-electromechanical systems could therefore be envisioned.

Spontaneous formation of multilayer refractory carbide coatings in a molten salt media.

Refractory materials hold great promise to develop functional multilayer coating for extreme environments and temperature applications but require high temperature and complex synthesis to overcome their strong atomic bonding and form a multilayer structure. Here, a spontaneous reaction producing sophisticated multilayer refractory carbide coatings on carbon fiber (CF) is reported. This approach utilizes a relatively low-temperature (950 °C) molten-salt process for forming refractory carbides. The reaction of titanium (Ti), chromium (Cr), and CF yields a complex, high-quality multilayer carbide coating composed of 1) Cr carbide (Cr3C2), 2) Ti carbide, and 3) Cr3C2 layers. The layered sequence arises from a difference in metal dissolutions, reactions, and diffusion rates in the salt media. The multilayer-coated CFs act as a permeable oxidation barrier with no crystalline degradation of the CFs after extreme temperature (1,200 °C) and environment (oxyacetylene flame) exposure. The synthesis of high-quality multilayer refractory coating in a fast, efficient, easy, and clean manner may answer the need for industrial applications that develop cheap and reliable extreme environment protection barriers.

Optical-field-induced surface nanobumps in near-infrared laser direct cleaning of nanoparticles on silicon.

The evolution of surface damage in laser direct cleaning (LDC) of nanoparticulate contamination on silicon (Si) was theoretically and experimentally investigated. Nanobumps with a volcano-like shape were found in near-infrared laser cleaning of polystyrene latex nanoparticles on Si wafers. According to the finite-difference time-domain simulation and the high-resolution surface characterization, unusual particle-induced optical field enhancement in the vicinity of the interface between Si and nanoparticles is mainly responsible for the generation of volcano-like nanobumps. This work is of fundamental significance for the understanding of the laser-particle interaction during LDC and will promote the development of nanofabrication and nanoparticle cleaning applications in optics, microelectromechanical systems, and semiconductors.

Blind-zone formation in laser shockwave nano-cleaning.

Laser shockwave cleaning (LSC) has attracted growing attention due to its advantages in non-contact, site-selective nanoparticle removal on microelectronic/optical devices. However, an uncleaned blind-zone formed directly under the laser-induced plasma kernel severely affects the cleaning effect. Laser shockwave cleaning of 300 nm polystyrene latex nanoparticles on silicon wafers is fully explored to understand the blind-zone formation mechanism. The size of the uncleaned blind-zone quickly increases from 0.84 to 19.50 mm2 associated with a growing fraction of the uncleaned blind-zone area within the whole cleaned area from 0.05 to 0.93 as the plasma-substrate gap distance is increased from 0.5 to 2 mm and the laser fluence is increased from 75 to 150 J/cm2. Besides, the variation of the blind-zone size is more strongly dependent on the plasma-substrate gap distance than the laser fluence. A time-resolved analysis of the laser-induced plasma evolution shows an inseparable relationship between the blind-zone and the geometric location of the plasma kernel. Theoretical analysis of the removal force in LSC based on the rolling mode reveals that the lack of dragging force acting on the nanoparticles in the region right under the plasma kernel impedes their removal and causes the uncleaned blind-zone formation.

Laser-induced breakdown spectroscopy of ammonia gas with resonant vibrational excitation.

In this work, laser-induced breakdown spectroscopy (LIBS) of gaseous ammonia (NH3) molecules on- and off-resonant vibrational excitation was studied in open air. A wavelength-tunable, continuous wave (CW), carbon dioxide (CO2) laser tuned at a resonant absorption peak (9.219 µm) within the infrared radiation (IR) range was used to resonantly excite the vibration of the N-H wagging mode of ammonia molecules. A pulsed Nd:YAG laser (1064 nm, 15 ns) was used to break down the ammonia gas for plasma imaging and spectral measurements. In this study, plasmas generated with the ammonia molecules without additional CO2 laser beam irradiation and with additional CO2 laser beam irradiation with the wavelengths on- and off-resonant vibrational excitation of ammonia molecules were investigated and referred as LIBS, LIBS-RE-ON and LIBS-RE-OFF, respectively. The experimental results showed that the temporal and spatial evolution as well as electron temperature and density of plasmas induced with LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas in LIBS-RE-ON showed larger spatial expansion and enhanced emission after a delay time of 1 µs in this study, as well as significantly enhanced electron temperature by ∼ 64%. Time-resolved electron temperatures and densities showed that the emission signal enhancement in LIBS-RE-ON can be primarily attributed to the electron temperature enhancement. Signal enhancement in LIBS indicated improved detection sensitivity. This study could inspire future works on LIBS for gas detection with improved sensitivity and selectivity probably by using ultrafast/intense laser-induced molecular breakdown/ionization with resonant vibrational excitation of molecules.

Time-resolved resonance fluorescence spectroscopy for study of chemical reactions in laser-induced plasmas.

Identification of chemical intermediates and study of chemical reaction pathways and mechanisms in laser-induced plasmas are important for laser-ablated applications. Laser-induced breakdown spectroscopy (LIBS), as a promising spectroscopic technique, is efficient for elemental analyses but can only provide limited information about chemical products in laser-induced plasmas. In this work, time-resolved resonance fluorescence spectroscopy was studied as a promising tool for the study of chemical reactions in laser-induced plasmas. Resonance fluorescence excitation of diatomic aluminum monoxide (AlO) and triatomic dialuminum monoxide (Al2O) was used to identify these chemical intermediates. Time-resolved fluorescence spectra of AlO and Al2O were used to observe the temporal evolution in laser-induced Al plasmas and to study their formation in the Al-O2 chemistry in air.

Laser-Induced Fast Assembly of Wettability-Finely-Tunable Superhydrophobic Surfaces for Lossless Droplet Transfer.

Rose-petal-like superhydrophobic surfaces with strong water adhesion are promising for microdroplet manipulation and lossless droplet transfer. Assembly of self-grown micropillars on shape-memory polymer sheets with their surface adhesion finely tunable was enabled using a picosecond laser microprocessing system in a simple, fast, and large-scale manner. The processing speed of the wettability-finely-tunable superhydrophobic surfaces is up to 0.5 cm2/min, around 50-100 times faster than the conventional lithography methods. By adjusting the micropillar height, diameter, and bending angle, as well as superhydrophobic chemical treatment, the contact angle and adhesive force of water droplets on the micropillar-textured surfaces can be tuned from 117.1° up to 165° and 15.4 up to 200.6 µN, respectively. Theoretical analysis suggests a well-defined wetting-state transition with respect to the micropillar size and provides a clear guideline for microstructure design for achieving a stabilized superhydrophobic region. Droplet handling devices, including liquid handling tweezers and gloves, were fabricated from the micropillar-textured surfaces, and lossless liquid transfer of various liquids among various surfaces was demonstrated using these devices. The superhydrophobic surfaces serve as a microreactor platform to perform and reveal the chemical reaction process under a space-constrained condition. The superhydrophobic surfaces with self-assembled micropillars promise great potential in the fields of lossless droplet transfer, biomedical detection, chemical engineering, and microfluidics.

The Analysis of the Innovation Consciousness of College Student Entrepreneurs Under the Teaching Concept of Chinese Excellent Traditional Culture.

The purpose of this study was to analyze the current situation of the entrepreneurial consciousness of college student entrepreneurs and to explore the role of innovative and entrepreneurial talents in social and economic development. Based on the teaching concept of Chinese excellent traditional culture, first, the relevant theories of innovation and entrepreneurship, as well as the characteristics of entrepreneurial talents in colleges and entrepreneurs, are analyzed and elaborated; moreover, the definition of college student entrepreneur is explained; then, from the perspective of entrepreneurial teaching management, entrepreneurial education, and place support, the questionnaire method is selected to show the understanding of the entrepreneurship of college students; finally, based on the Cobb-Douglas function, the model before and after the introduction of innovative and entrepreneurial talents is tested and analyzed. Investigation and analysis suggest that most college students have entrepreneurial intention, and 61.5% of them choose to start their own business after having working experience; the relative freedom of time and space is the main factor to attract college students to start their own businesses, accounting for 42.3%; 69.3% of college students think that capital is a restricting factor for entrepreneurship, while 76.2% think that lack of experience is a major restricting factor for entrepreneurship; college students have a certain demand for entrepreneurship training and guidance from the school, especially in the setting of entrepreneurship incubation park and resource pool; the characteristics of entrepreneurship, professional skills, and interpersonal resources are more crucial for college students; most college students have a positive cognition of the excellent traditional Chinese teaching concepts; the analysis based on the Cobb-Douglas function reveals that the introduction of innovative and entrepreneurial talents can promote economic development. This exploration has a positive effect on the cultivation of awareness of college students of entrepreneurship and innovation, as well as the relationship discussion between the introduction of innovative and entrepreneurial talents and social economy.

Vertically Aligned Single-Crystalline CoFe2O4 Nanobrush Architectures with High Magnetization and Tailored Magnetic Anisotropy.

Micrometer-tall vertically aligned single-crystalline CoFe2O4 nanobrush architectures with extraordinarily large aspect ratio have been achieved by the precise control of a kinetic and thermodynamic non-equilibrium pulsed laser epitaxy process. Direct observations by scanning transmission electron microscopy reveal that the nanobrush crystal is mostly defect-free by nature, and epitaxially connected to the substrate through a continuous 2D interface layer. In contrast, periodic dislocations and lattice defects such as anti-phase boundaries and twin boundaries are frequently observed in the 2D interface layer, suggesting that interface misfit strain relaxation under a non-equilibrium growth condition plays a critical role in the self-assembly of such artificial architectures. Magnetic property measurements have found that the nanobrushes exhibit a saturation magnetization value of 6.16 B/f.u., which is much higher than the bulk value. The discovery not only enables insights into an effective route for fabricating unconventional high-quality nanostructures, but also demonstrates a novel magnetic architecture with potential applications in nanomagnetic devices.

Colossal oxygen vacancy formation at a fluorite-bixbyite interface.

Oxygen vacancies in complex oxides are indispensable for information and energy technologies. There are several means to create oxygen vacancies in bulk materials. However, the use of ionic interfaces to create oxygen vacancies has not been fully explored. Herein, we report an oxide nanobrush architecture designed to create high-density interfacial oxygen vacancies. An atomically well-defined (111) heterointerface between the fluorite CeO2 and the bixbyite Y2O3 is found to induce a charge modulation between Y3+ and Ce4+ ions enabled by the chemical valence mismatch between the two elements. Local structure and chemical analyses, along with theoretical calculations, suggest that more than 10% of oxygen atoms are spontaneously removed without deteriorating the lattice structure. Our fluorite-bixbyite nanobrush provides an excellent platform for the rational design of interfacial oxide architectures to precisely create, control, and transport oxygen vacancies critical for developing ionotronic and memristive devices for advanced energy and neuromorphic computing technologies.

Ultraviolet laser photolysis of hydrocarbons for nondiamond carbon suppression in chemical vapor deposition of diamond films.

In this work, we demonstrate that ultraviolet (UV) laser photolysis of hydrocarbon species alters the flame chemistry such that it promotes the diamond growth rate and film quality. Optical emission spectroscopy and laser-induced fluorescence demonstrate that direct UV laser irradiation of a diamond-forming combustion flame produces a large amount of reactive species that play critical roles in diamond growth, thereby leading to enhanced diamond growth. The diamond growth rate is more than doubled, and diamond quality is improved by 4.2%. Investigation of the diamond nucleation process suggests that the diamond nucleation time is significantly shortened and nondiamond carbon accumulation is greatly suppressed with UV laser irradiation of the combustion flame in a laser-parallel-to-substrate geometry. A narrow amorphous carbon transition zone, averaging 4 nm in thickness, is identified at the film-substrate interface area using transmission electron microscopy, confirming the suppression effect of UV laser irradiation on nondiamond carbon formation. The discovery of the advantages of UV photochemistry in diamond growth is of great significance for vastly improving the synthesis of a broad range of technically important materials.

Kinetically Controlled Fabrication of Single-Crystalline TiO2 Nanobrush Architectures with High Energy {001} Facets.

This study demonstrates that precise control of nonequilibrium growth conditions during pulsed laser deposition (PLD) can be exploited to produce single-crystalline anatase TiO2 nanobrush architectures with large surface areas terminated with high energy {001} facets. The data indicate that the key to nanobrush formation is controlling the atomic surface transport processes to balance defect aggregation and surface-smoothing processes. High-resolution scanning transmission electron microscopy data reveal that defect-mediated aggregation is the key to TiO2 nanobrush formation. The large concentration of defects present at the intersection of domain boundaries promotes aggregation of PLD growth species, resulting in the growth of the single-crystalline nanobrush architecture. This study proposes a model for the relationship between defect creation and growth mode in nonequilibrium environments, which enables application of this growth method to novel nanostructure design in a broad range of materials.

Epitaxial Growth of Intermetallic MnPt Films on Oxides and Large Exchange Bias.

High-quality epitaxial growth of inter-metallic MnPt films on oxides is achieved, with potential for multiferroic heterostructure applications. Antisite-stabilized spin-flipping induces ferromagnetism in MnPt films, although it is robustly antiferromagnetic in bulk. Moreover, highly ordered antiferromagnetic MnPt films exhibit superiorly large exchange coupling with a ferromagnetic layer.

Thermal characterization of diamond films through modulated photothermal radiometry.

Diamond (Dia) films are promising heat-dissipative materials for electronic packages because they combine high thermal conductivity with high electrical resistivity. However, precise knowledge of the thermal properties of the diamond films is crucial to their potential application as passive thermal management substrates in electronics. In this study, modulated photothermal radiometry in a front-face configuration was employed to thermally characterize polycrystalline diamond films deposited onto silicon (Si) substrates through laser-assisted combustion synthesis. The intrinsic thermal conductivity of diamond films and the thermal boundary resistance at the interface between the diamond film and the Si substrate were investigated. The results enlighten the correlation between the deposition process, film purity, film transverse thermal conductivity, and interface thermal resistance.

Highly efficient and recyclable carbon soot sponge for oil cleanup.

Carbon soot (CS) has the advantages of cost-effectiveness and production scalability over other carbons (i.e., graphene, CNTs) in their synthesis. However, little research has been conducted to explore the potential applications of CS. In this study, we demonstrated that a common daily waste-CS-can be used for developing a cost-effective absorbent (CS-sponge) to remove oil contaminants from water. The CS was synthesized by an ethylene-oxygen combustion flame. The CS-sponge was prepared via a dip-coating method. Without further surface modification and pretreatments, the CS-sponge demonstrates high absorption capacities (up to 80 times its own weight) for a broad spectrum of oils and organic solvents with a recyclability of more than 10 times. These research results show evidence that the CS-sponge is promising in environmental remediation for large-scale, low-cost removal of oils from water.