PMMA direct exfoliation for rapid and organic free transfer of centimeter-scale CVD graphene

Despite the various techniques developed for the transfer of large area graphene grown by chemical vapor deposition (CVD), the conventional PMMA transferring technique has been widely applied in laboratories due to its convenience and economical cost. However, the complete removal of PMMA on graphene surface has become a troublesome, and the PMMA residue could degrade the properties of graphene significantly. We report here a facile water assisted technique to directly peel off the PMMA layer over centimeter-sized CVD graphene film for the first time. No organic solvents are involved in the whole transfer process. The transferred graphene film is clean and intact over large area because of the cooperative effect of the capillary force and the van der Waals force which facilitates the conformal contact between graphene film and the substrate. Various types of graphene samples (i.e. monolayer, multilayer, and incomplete domains) can be easily transferred to diverse substrates including silicon wafer, sapphire, and quartz with good integrity. The transferred graphene film is of high cleanliness, and the graphene transistors show higher carrier mobility and lower level of p-type doping comparing to the conventional wet transfer technique.

Deep-Ultraviolet (DUV)-Induced Doping in Single Channel Graphene for Pn-Junction

The electronic properties of single-layer, CVD-grown graphene were modulated by deep ultraviolet (DUV) light irradiation in different radiation environments. The graphene field-effect transistors (GFETs), exposed to DUV in air and pure O2, exhibited p-type doping behavior, whereas those exposed in vacuum and pure N2 gas showed n-type doping. The degree of doping increased with DUV exposure time. However, n-type doping by DUV in vacuum reached saturation after 60 min of DUV irradiation. The p-type doping by DUV in air was observed to be quite stable over a long period in a laboratory environment and at higher temperatures, with little change in charge carrier mobility. The p-doping in pure O2 showed ~15% de-doping over 4 months. The n-type doping in pure N2 exhibited a high doping effect but was highly unstable over time in a laboratory environment, with very marked de-doping towards a pristine condition. A lateral pn-junction of graphene was successfully implemented by controlling the radiation environment of the DUV. First, graphene was doped to n-type by DUV in vacuum. Then the n-type graphene was converted to p-type by exposure again to DUV in air. The n-type region of the pn-junction was protected from DUV by a thick double-coated PMMA layer. The photocurrent response as a function of Vg was investigated to study possible applications in optoelectronics.

Ultrafast and stable planar photodetector based on SnS2 nanosheets/perovskite structure

Two-dimensional (2D) transition metal dichalcogenides are promising candidates of photodetectors where they are commonly grown parallel to the substrate due to their 2D characteristics in micrometer scales from exfoliation of bulk crystals or through high temperature chemical vapor deposition (CVD) methods. In this study, semi-hexagonal vertical nanosheets of SnS2 layered have been fabricated on FTO substrate without using Sn source through CVD method at relatively low temperature (500 °C). Due to exceptional band alignment of triple cation lead perovskite (TCLP) with semi-hexagonal SnS2 nanosheets, an improved photodetector has been fabricated. This type of photodetectors fabricated through lithography-free and electrodes metallization free approach with remarkable fast response (20.7 µs/31.4 µs as rising /falling times), showed high photoresponsivity, external quantum efficiency and detectivity of 1.84 AW−1, 513% and 1.69 × 1011, respectively under illumination of incident light with wavelength of 445 nm. The stability of the photodetectors has been studied utilizing a protective PMMA layer on the perovskite layer in 100% humidity. The introduced growth and fabrication process of the planar photodetector, including one/two dimensional interface through the edges/basal planes of layered materials with perovskite film, paves a way for the large scale, cost-effective and high-performance optoelectronic devices.

Self-Assembled Ag Nanocomposites into Ultra-Sensitive and Reproducible Large-Area SERS-Active Opaque Substrates

This work describes a novel, one-shot strategy to fabricate ultrasensitive SERS sensors based on silver/poly(methyl methacrylate) (PMMA) nanocomposites. Upon spin coating of a dispersion of PMMA and silver precursor on N-doped silicon substrate, closely separated silver nanoparticles were self-assembled into uniform nanospheres. As a result, a thin hydrophobic PMMA layer embedded with Ag nanoparticles (AgNPs) was obtained on the whole silicon substrate. Consequently, a large-scale, reproducible SERS platform was produced through a rapid, simple, low-cost, and high-throughput technology. In addition, reproducible SERS features and high SERS enhancement factors were determined (SEF ~1015). This finding matches the highest SEF reported in literature to date (1014) for silver aggregates. The potential and novelty of this synthesis is that no reducing agent or copolymer was used, nor was any preliminary functionalization of the surface carried out. In addition, the AgNPs were fabricated directly on the substrate’s surface; consequently, there was no need for polymer etching. Then, the synthetic method was successfully applied to prepare opaque SERS platforms. Opaque surfaces are needed in photonic devices because of the absence of secondary back reflection, which makes optical analysis and applications easier.

Optimizing the PMMA Electron-Blocking Layer of Quantum Dot Light-Emitting Diodes

Quantum dots (QDs) are promising candidates for producing bright, color-pure, cost-efficient, and long-lasting QD-based light-emitting diodes (QDLEDs). However, one of the significant problems in achieving high efficiency of QDLEDs is the imbalance between the rates of charge-carrier injection into the emissive QD layer and their transport through the device components. Here we investigated the effect of the parameters of the deposition of a poly (methyl methacrylate) (PMMA) electron-blocking layer (EBL), such as PMMA solution concentration, on the characteristics of EBL-enhanced QDLEDs. A series of devices was fabricated with the PMMA layer formed from acetone solutions with concentrations ranging from 0.05 to 1.2 mg/mL. The addition of the PMMA layer allowed for an increase of the maximum luminance of QDLED by a factor of four compared to the control device without EBL, that is, to 18,671 cd/m2, with the current efficiency increased by an order of magnitude and the turn-on voltage decreased by ~1 V. At the same time, we have demonstrated that each particular QDLED characteristic has a maximum at a specific PMMA layer thickness; therefore, variation of the EBL deposition conditions could serve as an additional parameter space when other QDLED optimization approaches are being developed or implied in future solid-state lighting and display devices.

Spin-coating of micron-scale thick PMMA films with embedded Au nanoparticles formed by laser ablation in liquid.

Background: A Facile is a scalable approach to fabricating organic thin films with an embedded layer of nanoparticles in the ambient environment. The approach is based on step-by-step spin-coating of polymethylmethacrylate (PMMA) films and a nanoparticle layer. Objective: The goal of the present work is to fabricate a sandwich structure of the PMMA films for the top and bottom layers of a sandwich structure as well as a middle layer of nanoparticles formed in solution by the laser ablation in liquid (LAL) method. Methods: First, a PMMA thin film was fabricated by spin-casting of PMMA solution in ethyl acetate. Secondly, a solution of Au nanoparticles synthesized by laser ablation in ethanol was spin-cast on a prefabricated PMMA film. The distribution of Au nanoparticles and the morphology of the resulting film were analyzed using scanning electron microscopy (SEM), optical microscopy, and atomic microscopy (AFM). Finally, another PMMA layer was spin-cast on the nanoparticle-decorated film. Results: A hybrid organic film with the embedded layer of nanoparticles was fabricated using the spin-casting method for top and bottom layers and the middle layer of Au nanoparticles fabricated by laser ablation in ethanol by a pulsed UV laser. Statistical and fractal analysis shows uniform distribution of nanoparticles on a length scale above ten microns. Conclusion: Spin-cast-based layer-by-layer approach to fabricating sandwich structures of organic films with embedded nanoparticles is a facile and scalable method for hybrid organic – nanoparticle films. This approach can be extended for the fabrication of multi-layered hybrid structures.

Characterization of Inkjet-Printed Digital Microfluidics Devices

Digital microfluidics (DMF) devices enable precise manipulation of small liquid volumes in point-of-care testing. A printed circuit board (PCB) substrate is commonly utilized to build DMF devices. However, inkjet printing can be used to fabricate DMF circuits, providing a less expensive alternative to PCB-based DMF designs while enabling more rapid design iteration cycles. We demonstrate the cleanroom-free fabrication process of a low-cost inkjet-printed DMF circuit. We compare Kapton and polymethyl methacrylate (PMMA) as dielectric coatings by measuring the minimal droplet actuation voltage for a range of actuation frequencies. A minimum actuation voltage of 5.6 V was required for droplet movement with the PMMA layer thickness of 0.2 μm and a hydrophobic layer of 0.17 μm. Significant issues with PMMA dielectric breakdown were observed at actuation voltages above 10 V. In comparison, devices that utilized Kapton were found to be more robust, even at an actuation voltage up to 100 V.

Ultrafast and Stable Planar Photodetector Based on SnS2 Nanosheets/Perovskite Structure

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates of photodetectors where they are commonly grown parallel to the substrate due to their 2D characteristics in micrometer scales from exfoliation of bulk crystals or through high temperature chemical vapor deposition (CVD) methods. In this study, semi-hexagonal vertical nanosheets of SnS2 layered have been fabricated on FTO substrate without using Sn source through CVD method at relatively low temperature (500°C). Due to exceptional band alignment of triple cation lead perovskite with semi- hexagonal SnS2 nanosheets, an improved photodetector has been fabricated. This type of photodetectors fabricated through lithography-free and electrodes metallization free approach with remarkable fast response (20.7µs/31.4µs as rising /falling times), showed high photoresponsivity, external quantum efficiency and detectivity of 1.58 AW-1, 453% and 8.35 ×10 11, respectively under illumination of incident light ith 445nm. The stability of the photodetectors has been studied utilizing a protective PMMA layer on the perovskite layer in 100% humidity. The introduced growth and fabrication process of the planar photodetector, including one/two dimentional interface through the edges/ basal planes of layered materials with perovskite film, paves a way for the large scale, cost-effective and high-performance optoelectronic devices.