Guijun Li

A Polyferroplatinyne Precursor for the Rapid Fabrication of L1 0 -FePt-type Bit Patterned Media by Nanoimprint Lithography

A polyferroplatinyne polymer can be patterned on the surface of Si wafer in ordered nanoline or nanodot shapes with PDMS molds through nanoimprint lithography (NIL), and subsequent thermal treatment gives rise to the nanopatterned arrays of L1(0) -FePt nanoparticles with the same periodicities. The method offers excellent potential to be utilized in the simple and rapid fabrication of bit patterned media for magnetic data recording.

Liver cancer immunoassay with magnetic nanoparticles and MgO-based magnetic tunnel junction sensors

We have demonstrated the detection of alpha-fetoprotein (AFP) labeled with magnetic nanoparticles (MNPs) using MgO-based magnetic tunnel junction (MTJ) sensors. AFP is an important hepatic tumor biomarker and the detection of AFP has significant applications for clinical diagnostics and immunoassay for early-stage liver cancer indications. In this work, MgO-based MTJ sensors and 20-nm iron-oxide magnetic nanoparticles (MNPs) were used for detecting AFP antigens by a sandwich-assay configuration. The MTJ sensors with a sensing area of 4 × 2 μm2 possess tunneling magnetoresistance (TMR) of 122% and sensitivity of 0.95%/Oe at room temperature. The target AFP antigens of three concentrations were successfully detected, and the experimental data indicate that the resistance variations of the MTJ sensor increased with the AFP concentration ratios proportionally. These results demonstrate that MgO-based MTJ sensors together with MNPs are a promising biosensing platform for liver cancer immunoassay.

Review of Noise Sources in Magnetic Tunnel Junction Sensors

Noise problem limits the sensitivity of magnetic tunnel junction (MTJ) sensors for ultra-low magnetic field applications. Noise analysis not only helps in finding ways to eliminate noise disturbances but also essential for understanding the electronic and magnetic properties of MTJs. These approaches provide insight for optimizing the design of MTJ sensors before fabrication. This paper reviews the noise sources in MTJ sensors reported in recent years. Both the origins and mathematical derivations of the noise sources are presented, illustrating how different factors affecting the performance of MTJ sensors. A brief outlook of challenges in the future is also given.

Investigation of pyrolysis temperature in the one-step synthesis of L10 FePt nanoparticles from a FePt-containing metallopolymer

Ferromagnetic (L10 phase) FePt alloy nanoparticles (NPs) with extremely high magnetocrystalline anisotropy are considered to be very promising candidates for the next generation of ultrahigh-density data storage systems. The question of how to generate L10 FePt NPs with high coercivity, controllable size, and a narrow size distribution is a challenge. We report here a single-step fabrication of L10 FePt NPs by employing one of the two new polyferroplatinyne bimetallic polymers as precursors. The influence of the pyrolysis temperature on the size and magnetic properties of the resulting FePt alloy NPs has been investigated in detail.

Nanopatterned L10-FePt nanoparticles from single-source metallopolymer precursors for potential application in ferromagnetic bit-patterned media magnetic recording

Bit-patterned media (BPM) with a precise stoichiometry ratio of Fe and Pt atoms are promising for future high areal density magnetic recording. Here, we report a new FePt-containing metallopolymer P as the single-source precursor for the synthesis of magnetic metal alloy nanoparticles. This polymer was synthesized from a random copolymer poly(styrene-4-ethynylstyrene) PES-PS and the bimetallic precursor TPy-FePt ([Pt(4′-ferrocenyl-(N^N^N))Cl]Cl) by the CuI-catalyzed dehydrohalogenation. After pyrolysis of P, the stoichiometry of Fe and Pt atoms in the synthesized nanoparticles is nearly close to 1 : 1, which is more precise than that by using TPy-FePt as the precursor. Also, polymer P is more suitable for patterning by high-throughput nanoimprint lithography (NIL) compared to TPy-FePt. Ferromagnetic nanolines, potentially useful for fabricating bit-patterned magnetic recording media, were successfully obtained from P and fully characterized.

Eye-friendly reduced graphene oxide circuits with nonlinear optical transparency on flexible poly(ethylene terephthalate) substrates

The selective reduction of graphene oxide (GO) with direct laser writing is a rapid and efficient process to pattern conductive tracks for flexible electronic circuit applications. Here we report novel eye-friendly reduced graphene oxide (rGO) conductive tracks on transparent and flexible poly(ethylene terephthalate) (PET) substrates synthesized by one-step laser reduction of graphene oxide in an ambient environment. Resistivity as low as 1.07 × 10−4 μm has been achieved for a 20 nm thick rGO film after the industrial-grade 1064 nm Nd:YAG laser treatment. Fingerprints of the synthesized rGO were verified by Raman spectroscopy with an increased intensity ratio of the 2D band over the G band, and the deoxygenation results were examined using both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) characterization. The rGO synthesized by this infrared laser reduction showed increased absorption toward a shorter wavelength of up to 96% in UV regions, which can significantly protect human eyes from high energy light hazards. Electronic circuit tracks with rGO synthesized with this 1064 nm laser patterning process can provide novel methods for the fabrication of future eye-friendly flexible electronics with nonlinear optical transparency.

Magnetic Tunnel Junction Sensors With Conetic Alloy

Al2O3 magnetic tunneling junction (MTJ) sensors were fabricated with Conetic alloy Ni77Fe14Cu5Mo4 deposited as the free layer and pinned layer for its soft magnetic properties. It was observed that the Al2O3 MTJ sensors with Conetic exhibited relatively small easy-axis coercivity. Tunneling magnetoresistance (TMR) and noise measurements were carried out to characterize the sensors. TMR of 9.5% and Hooge parameter of 3.825 × 10-7 μm2 were achieved without any hard-axis field. Hard-axis bias field was applied to eliminate the hysteresis and improve the linear field response of the MTJ sensor. The hysteresis was removed by applying an external magnetic field along the hard axis at 8 Oe and the sensor sensitivity was 0.4 %/Oe within a linear region at room temperature. The relationship between the Hooge parameter and hard-axis field was also investigated and the result demonstrated that the 1/f noise can be suppressed by an optimized hard-axis bias field. This work shows that it is feasible to use Conetic alloy as the soft magnetic layers in MTJ sensors for its small coercivity, and a hard-axis bias field can be used to linearize the sensor response and suppress the 1/f noise.

Additive and Photochemical Manufacturing of Copper

In recent years, 3D printing technologies have been extensively developed, enabling rapid prototyping from a conceptual design to an actual product. However, additive manufacturing of metals in the existing technologies is still cost-intensive and time-consuming. Herein a novel platform for low-cost additive manufacturing is introduced by simultaneously combining the laser-induced forward transfer (LIFT) method with photochemical reaction. Using acrylonitrile butadiene styrene (ABS) polymer as the sacrificial layer, sufficient ejection momentum can be generated in the LIFT method. A low-cost continuous wave (CW) laser diode at 405 nm was utilized and proved to be able to transfer the photochemically synthesized copper onto the target substrate. The wavelength-dependent photochemical behaviour in the LIFT method was verified and characterized by both theoretical and experimental studies compared to 1064 nm fiber laser. The conductivity of the synthesized copper patterns could be enhanced using post electroless plating while retaining the designed pattern shapes. Prototypes of electronic circuits were accordingly built and demonstrated for powering up LEDs. Apart from pristine PDMS materials with low surface energies, the proposed method can simultaneously perform laser-induced forward transfer and photochemical synthesis of metals, starting from their metal oxide forms, onto various target substrates such as polyimide, glass and thermoplastics.

High-Performance, Self-Powered Photodetectors Based on Perovskite and Graphene

An ideal photodetector must exhibit a fast and wide tunable spectral response, be highly responsive, have low power consumption, and have a facile fabrication process. In this work, a self-powered photodetector with a graphene electrode and a perovskite photoactive layer is assembled for the first time. The graphene electrode is prepared using a solution transfer process, and the perovskite layer is prepared using a solution coating process, which makes the device low cost. Graphene can form a Schottky junction with TiO2 to efficiently separate/transport photogenerated excitons at the graphene/perovskite interface. Unlike the conventional photovoltaic structure, in this photodetector, both photogenerated electrons and holes are transported along the same direction to graphene, and electrons tunneled into TiO2 are collected by the cathode and holes transported by graphene are collected by the anode; therefore, the photodetector is self-powered. The photodetector has a broad range of detection, from 260 to 900 nm, an ultrahigh on-off ratio of 4 × 106, rapid response to light on-off (<5 ms), and a high level of detection of ∼1011 Jones. The high performance is primarily due to the unique charge-transport property of graphene and strong light absorption properties of perovskite. This work suggests a new method for the production of self-powered photodetectors with high performance and low power consumption on a large scale.

Photochemical Copper Coating on 3D Printed Thermoplastics

3D printing using thermoplastics has become very popular in recent years, however, it is challenging to provide a metal coating on 3D objects without using specialized and expensive tools. Herein, a novel acrylic paint containing malachite for coating on 3D printed objects is introduced, which can be transformed to copper via one-step laser treatment. The malachite containing pigment can be used as a commercial acrylic paint, which can be brushed onto 3D printed objects. The material properties and photochemical transformation processes have been comprehensively studied. The underlying physics of the photochemical synthesis of copper was characterized using density functional theory calculations. After laser treatment, the surface coating of the 3D printed objects was transformed to copper, which was experimentally characterized by XRD. 3D printed prototypes, including model of the Statue of Liberty covered with a copper surface coating and a robotic hand with copper interconnections, are demonstrated using this painting method. This composite material can provide a novel solution for coating metals on 3D printed objects. The photochemical reduction analysis indicates that the copper rust in malachite form can be remotely and photo-chemically reduced to pure copper with sufficient photon energy.