Mingjun Hu

High-yield synthesis of graphene quantum dots with strong green photoluminescence

A facile and high-yielding hydrothermal method for synthesizing graphene quantum dots (GQDs) from glucose is presented. The GQDs, with fluorescence quantum yield (FL QY) of 44.3%, demonstrate strong green photoluminescence (PL) and excitation-independent PL emission characteristics.

Direct Pen Writing of Adhesive Particle-Free Ultrahigh Silver Salt-Loaded Composite Ink for Stretchable Circuits

In this article, we describe a writable particle-free ink for fast fabrication of highly conductive stretchable circuits. The composite ink mainly consists of soluble silver salt and adhesive rubber. Low toxic ketone was employed as the main solvent. Attributed to ultrahigh solubility of silver salt in short-chain ketone and salt-assisted dissolution of rubber, the ink can be prepared into particle-free transparent solution. As-prepared ink has a good chemical stability and can be directly filled into ballpoint pens and use to write on different substrates to form well adhesive silver salt-based composite written traces as needed. As a result of high silver salt loading, the trace can be converted into highly conductive silver nanoparticle-based composites after in situ reduction. Because of the introduction of adhesive elastomeric rubber, the as-formed conductive composite written trace can not only maintain good adhesion to various substrates but also show good conductivity under various deformations. The conductivity of written traces can be enhanced by repeated writing-reduction cycles. Different patterns can be fabricated by either direct handwriting or hand-copying. As proof-of-concept demonstrations, a typical handwriting heart-like circuit was fabricated to show its capability to work under different deformations, and a pressure-sensitive switch was also manufactured to present pressure-dependent change of resistance.

A facile way of fabricating a flexible and conductive cotton fabric

There is an increasing interest in the area of conductive textiles, also called e-textiles, in both the scientific and industrial fields. Herein, we have successfully prepared a conductive textile via electroless deposition onto cotton textiles by using a three-step treatment process. The cotton textiles are first dipped in P4VP–SU8 solution to form a uniform layer for the subsequent absorption of silver ions. Then, the cotton textiles are immersed in silver nitrate solution in preparation for the next step electroless deposition. The parameters like P4VP concentration and ELD conditions are studied to optimize the whole process. Compared to other methods, the whole fabrication process is efficient, facile and time-saving. The as-made cotton fabric sample showed sheet resistance as low as 0.05 Ω sq−1, which makes this method a promising candidate for applications in the fabrication of functional textile-based wearable devices.

SU-8-Induced Strong Bonding of Polymer Ligands to Flexible Substrates via in Situ Cross-Linked Reaction for Improved Surface Metallization and Fast Fabrication of High-Quality Flexible Circuits.

On account of in situ cross-linked reaction of epoxy SU-8 with poly(4-vinylpyridine) (P4VP) and its strong reactive bonding ability with different pretreated substrates, we developed a simple universal one-step solution-based coating method for fast surface modification of various objects. Through this method, a layer of P4VP molecules with controllable thickness can be tethered tightly onto substrates with the assistance of SU-8. P4VP molecules possess a lot of pyridine ligands to immobilize transitional metal ions that can behave as the catalyst of electroless copper plating for surface metallization while functioning as the adhesion-promoting layer between the substrate and deposited metal. Attributed to interpenetrated entanglement of P4VP molecules and as-deposited metal, ultrathick (>7 μm) strongly adhesive high-quality copper layer can be formed on flexible substrates without any delamination. Then through laser printer to print toner mask, a variety of designed circuits can be easily fabricated on modified flexible PET substrate.

A laser printing based approach for printed electronics

Here we report a study of printing of electronics using an office use laser printer. The proposed method eliminates those critical disadvantages of solvent-based printing techniques by taking the advantages of electroless deposition and laser printing. The synthesized toner acts as a catalyst for the electroless copper deposition as well as an adhesion-promoting buffer layer between the substrate and deposited copper. The easy metallization of printed patterns and strong metal-substrate adhesion make it an especially effective method for massive production of flexible printed circuits. The proposed process is a high throughput, low cost, efficient, and environmentally benign method for flexible electronics manufacturing.

Localization of Printed Chipless RFID in 3-D Space

A novel localization technique of printed chipless RFID in 3-D space is presented by using a frequency domain chipless tag. The chipless tag is interrogated by an ultra-wideband (UWB) signal, and the response from tag is analyzed for extracting the resonance information. The range information is used for accurate localization of chipless tag. The validity and accuracy of the localization method has been verified by moving the tag to different points in a fixed interrogation zone. The measured angular deviations achieved based on the proposed approach are less than 5° when localization radius is from 10 cm to 40 cm.

Application of 3D Printed Porous Copper Anode in Microbial Fuel Cells

In this study, 3D printing technique was utilized to fabricate three-dimensional porous electrodes for microbial fuel cells with UV curable resin, followed by copper electroless plating. A maximum voltage of 62.9 ± 2.5 mV and a power density of 6.45 ± 0.5 mWm-2 were achieved for MFCs with 3D printed porous copper (3D-PPC) anode, which were 8.3 and 12.3-fold higher than copper mesh electrode, respectively. This illustrated the great advantage of 3D porous anodes in MFCs compared to flat anode structures. Besides, the biocompatibility of the copper anode with Shewanella oneidensis MR-1 was examined by comparing with carbon cloth, which produced a 3-fold larger maximum voltage and a ~10-fold higher power density vs. 3D-PPC anode and thus indicated the possible copper corrosion during MFC operation. ICP-MS analysis of MFC solution revealed the high concentration of 732µg/L copper ions detected in the MFC effluent. This result, coupled with EDX showing the lower copper content on the 3D-PPC anode surface after >15 days of MFC operation, confirmed the copper dissolving behavior in MFC. MR-1 biofilm formation under copper suppression was finally characterized by SEM and less biofilm was observed on copper anodes, illustrating their poor biocompatibility, even though 3D printing technology and porous structures were quite promising for future scale-up.

Water based fluidic radio frequency metamaterials

Electromagnetic metamaterials offer great flexibility for wave manipulation and enable exceptional functionality design, ranging from negative refraction, anomalous reflection, super-resolution imaging, transformation optics to cloaking, etc. However, demonstration of metamaterials with unprecedented functionalities is still challenging and costly due to the structural complexity or special material properties. Here, we demonstrate for the first time the versatile fluidic radio frequency metamaterials with negative refraction using a water-embedded and metal-coated 3D architecture. Effective medium analysis confirms that metallic frames create an evanescent environment while simultaneously water cylinders produce negative permeability under Mie resonance. The water-metal coupled 3D architectures and the accessory devices for measurement are fabricated by 3D printing with post electroless deposition. Our study also reveals the great potential of fluidic metamaterials and versatility of the 3D printing proc...