Chia-Chi Tuan

Ultrafast Molecular Stitching of Graphene Films at the Ethanol/Water Interface for High Volumetric Capacitance

Compact graphene film electrodes with a high ion-accessible surface area have the promising potential to realize high-density electrochemical energy storage (or high volumetric capacitance), which is vital for the development of flexible, portable, and wearable energy storage devices. Here, a novel, ultrafast strategy for stitching graphene sheets into films, in which p-phenylenediamine (PPD) molecules are uniformly intercalated between the graphene sheets, is simply constructed at the ethanol/water interface. Due to uniformly interlayer spacing (∼1.1 nm), good wettability, and an interconnected ion transport channel, the binder-free PPD-graphene film with a high packing density (1.55 g cm-3) delivers an ultrahigh volumetric capacitance (711 F cm-3 at a current density of 0.5 A g-1), high rate performance, high power and energy densities, and excellent cycling stability in aqueous electrolytes. This interfacial stitching strategy holds new promise for the future design of enhanced electrochemical energy-storage devices.

High Refractive Index and Transparent Nanocomposites as Encapsulant for High Brightness LED Packaging

A high refractive index (RI) and transparent encapsulant material is in great demand for light emitting diode (LED) packaging to lower the RI contrasts between a LED chip and an encapsulant, and therefore improve the light extraction efficiency. In this paper, we prepared TiO2/silicone nanocomposites and studied the effects of the crystalline phases of TiO2, and the TiO2 surface modifications on their optical properties. The rutile TiO2 was found to be more effective to increase the RI of the nanocomposite than the anatase phase TiO2. At a 5 wt.% loading of TiO2, the RI was as high as 1.62 at the wavelength of 589 nm, which represents a significant improvement from 1.54 for silicone resin. In addition, surface modification was carried out using vinyl-terminated silane to improve the dispersion of nanoparticles in a silicone matrix, leading to a high relative transmittance of 84%. We also demonstrated that the optical property degradation of the nanocomposites in this paper was negligible after the accelerated reliability test.

Controlling Kink Geometry in Nanowires Fabricated by Alternating Metal-Assisted Chemical Etching

Kinked silicon (Si) nanowires (NWs) have many special properties that make them attractive for a number of applications, such as microfluidics devices, microelectronic devices, and biosensors. However, fabricating NWs with controlled three-dimensional (3D) geometry has been challenging. In this work, a novel method called alternating metal-assisted chemical etching is reported for the fabrication of kinked Si NWs with controlled 3D geometry. By the use of multiple etchants with carefully selected composition, one can control the number of kinks, their locations, and their angles by controlling the number of etchant alternations and the time in each etchant. The resulting number of kinks equals the number times the etchant is alternated, the length of each segment separated by kinks has a linear relationship with the etching time, and the kinking angle is related to the surface tension and viscosity of the etchants. This facile method may provide a feasible and economical way to fabricate novel silicon nanowires, nanostructures, and devices for broad applications.

A novel, facile, layer-by-layer substrate surface modification for the fabrication of all-inkjet-printed flexible electronic devices on Kapton

A facile, environmentally-friendly, low-cost, and scalable deposition process has been developed and automated to apply polyelectrolyte multilayers (PEMs) on flexible Kapton HN substrates. Two weak polyelectrolytes, poly(acrylic acid) and polyethylenimine, were deposited in an alternating, layer-by-layer fashion under controlled pH and ionic strength. Compared to strong polyelectrolytes, weak electrolytes can control the properties of the PEMs more systmatically and simply. To our knowledge, this work on surface modification of Kapton is not only the first to use only weak polyelectrolytes, but is also the first to take advantage of the surface properties of calcium-bearing additive particles present in Kapton HN. The resulting surface-modified Kapton HN substrate allowed for the inkjet printing of water-based graphene oxide (GO) inks and organic solvent-based inks with good adhesion and with desired printability. While the deposition of a single PEM layer on a Kapton substrate significantly reduced the water contact angle and allowed for the inkjet-printing of GO inks, the deposition of additional PEM layers was required to maintain the adhesion during post-printing chemical treatments. As a conceptual demonstration of the general applicability of this PEM surface modification approach, a flexible, robust, single-layered gas sensor prototype was fully inkjet printed with both water- and ethanol-based inks and tested for its sensitivity to diethyl ethylphosphonate (DEEP), a simulant for G-series nerve agents. The electrical conductivity and morphology of the sensor were found to be insensitive to repeated bending around a 1 cm radius.

Fabricating and Controlling Silicon Zigzag Nanowires by Diffusion-Controlled Metal-Assisted Chemical Etching Method

Silicon (Si) zigzag nanowires (NWs) have a great potential in many applications because of its high surface/volume ratio. However, fabricating Si zigzag NWs has been challenging. In this work, a diffusion-controlled metal-assisted chemical etching method is developed to fabricate Si zigzag NWs. By tailoring the composition of etchant to change its diffusivity, etching direction, and etching time, various zigzag NWs can be easily fabricated. In addition, it is also found that a critical length of NW (>1 μm) is needed to form zigzag nanowires. Also, the amplitude of zigzag increases as the location approaches the center of the substrate and the length of zigzag nanowire increases. It is also demonstrated that such zigzag NWs can help the silicon substrate for self-cleaning and antireflection. This method may provide a feasible and economical way to fabricate zigzag NWs and novel structures for broad applications.

Capacitance enhancement by electrochemically active benzene derivatives for graphene-based supercapacitors

Various aromatic molecules have been reported to improve the performance of reduced graphene oxide (rGO)-based supercapacitors. However, the mechanism for this improvement remains unclear. Here we design a facile approach that clearly identifies the main reason for the enhancement. Benzene derivatives, namely p-phenylenediamine (PPD), m-phenylenediamine (MPD), o-phenylenediamine (OPD), hydroquinone (HQ), phenol, aniline and p-aminophenol (PAP) are incorporated into graphene oxide (GO) layers during their reduction and assembly at room temperature. We find that the capacitance increase mainly arises from the pseudocapacitance of specific benzene derivative molecules rather than their spacing effect. Moreover, the para and ortho substituted benzene derivatives contribute much more than the meta substituted ones do. With a small amount of PPD (∼11 wt%), the specific capacitance reaches 273 F g−1, much higher than that of pure rGO electrodes (113 F g−1). The hybrid electrode also shows great stability with a capacitance retention of up to 86% after 10 000 charge/discharge cycles.

A facile and low-cost route to high-aspect-ratio microstructures on silicon via a judicious combination of flow-enabled self-assembly and metal-assisted chemical etching

A viable and low-cost strategy for fabricating high-aspect-ratio microstructures on silicon (Si) based on a judicious combination of flow-enabled self-assembly (FESA) and metal-assisted chemical etching (MaCE) is reported. First, polymer patterns were directly formed on a bare Si substrate in one step by FESA of polymers in a two-parallel-plate setup consisting of a fixed upper plate and a movable lower Si substrate that was placed on a programmable translation stage. The implementation of FESA to yield polymer patterns eliminates the complicated manipulation of polymer resist in conventional lithography methods. Subsequently, these polymer patterns were utilized as a stable etching mask in the MaCE step and exhibited a remarkable selectivity of 467 : 1 over Si during etching. Notably, such a combined FESA and MaCE strategy (i.e., a FESA–MaCE route) avoids the use of a hard mask which is necessary for the conventional plasma etching method. During MaCE, a layer of Au thin film was used as a catalyst and hydrogen peroxide–hydrofluoric acid solution was employed as the etching solution. Consequently, trenches and gratings on Si at a micrometer scale with uniform controllable geometry were successfully produced. The aspect ratio of microstructures was up to 16 : 1 and the lateral edge roughness was below 0.5 μm. The influence of processing conditions on the geometry of the etched structures was scrutinized through a comparative study. The geometry of the etched structures was found to effectively adjust their surface wettability in a continuous manner. Clearly, due to the ease of implementation and the batch processing capability, the FESA–MaCE strategy is promising in manufacturing a broad range of high-quality Si-based devices at low cost.

Ultra-high refractive index LED encapsulant

In this paper, we report a method for preparing a silicone-based nanocomposite material for high brightness light emitting diode (LED) packaging. High refractive index (RI) encapsulant is desired in order to reduce the RI contrast between the LED chip and air, thus increasing the light extraction efficiency (LEE). In our method, TiO2 nanoparticles were added to the silicone resin to prepare the nanocomposite, and the RI was improved from 1.56 of neat silicone to 1.73 at the GaN-emitting wavelength of 460 nm. The nanocomposite also exhibits high relative transparency in the visible light wavelength of 300-800 nm. At 460 nm, the material containing 10 wt% filler loading shows above 90% relative transparency. Accelerated reliability test was conducted on the high-performance nanocomposites, and less than 2% degradation was measured in both RI and relative transparency.

Self-patterning, pre-applied underfilling technology for stack-die packaging

Die stacking is one of the next-generation 3D IC packaging methods, but its stringent material requirements are unlikely to be met by traditional underfills. Moreover, filler trapping is becoming an increasingly serious issue in no-flow and wafer-level underfills. In this report, we demonstrate a novel underfilling technology for the reduction of filler trapping in fine-pitch interconnects. In our method, we fabricate superhydrophobic bond pads, and control the flow of the underfill material by the surface energy difference between the bond pads and the Si3N4 substrate. The superhydrophobic bond pads are shown to have no effect on the bonding of soldering materials to the pads.

Effects of Defects on the Mechanical Properties of Kinked Silicon Nanowires

Kinked silicon nanowires (KSiNWs) have many special properties that make them attractive for a number of applications. The mechanical properties of KSiNWs play important roles in the performance of sensors. In this work, the effects of defects on the mechanical properties of KSiNWs are studied using molecular dynamics simulations and indirectly validated by experiments. It is found that kinks are weak points in the nanowire (NW) because of inharmonious deformation, resulting in a smaller elastic modulus than that of straight NWs. In addition, surface defects have more significant effects on the mechanical properties of KSiNWs than internal defects. The effects of the width or the diameter of the defects are larger than those of the length of the defects. Overall, the elastic modulus of KSiNWs is not sensitive to defects; therefore, KSiNWs have a great potential as strain or stress sensors in special applications.