Graphene prospers in the field of electronics-based applications. The electrical properties of graphene plays an effective role in the thin film based Radio Frequency Identification (RFID) tags. Recently, an US company named Vorbeck offers RFID tags containing a small quantity of graphene to advance their high-temperature and high-pressure performance. Vorbeck also uses the same principle to offer specialized antennae for mobile and wireless communications that integrate graphene which afford enhanced performance.
Though, device fabrication is an eminent challenge and involves new fabrication methods to apprehend the multifaceted incorporation of graphene-based devices. Harvard scientists SeungYeon Kang, Christopher C. Evans and their team invented a direct laser writing of reduced graphene oxide (GO) with femtosecond-laser irradiation at k = 795 nm.
They carried out a systematic study about the reduction process of graphene oxide to graphene by attempting both the laser fluence and the pulse repetition rate. Their keen observations show that the reduction has not only thermal features but also non-thermal features and hence it is recommended that one can achieve better results and conductivity phenomena using kHz pulse trains than using MHz pulse trains or a continuous wave laser.
Their reduced graphene oxide lines written at 10-kHz exhibit a 5 order-of-magnitude decrease in resistivity compared to a non-irradiated control sample. At Graphene Laboratories Inc, NY, USA, scientists initiated with the commercial 170-nm-thick graphene oxide (GO) films) deposited on 2-inch diameter glass substrates as the source material. For 4-probe conductivity measurements, they patterned gold contacts via optical lithography and electron-beam evaporation.
This invention leaps into a new perception into the reduction process of graphene oxide and leads to accomplishing a high class of graphene / reduced graphene oxide. It possess high standard flexibility and control in the fabrication of graphene layers.
As we know, the conversion of graphene oxide (GO) into reduced graphene oxide (rGO) is a challenging process for the electronic device applications based on graphene materials. Korean scientists Rowoon Park, Hyesu Kim and his team reported a facile deposition process of GO on flexible polymer substrates to create highly uniform thin films over a large area by a flow-enabled self-assembly approach.
The GO thin films are prepared by selectively reduced and facilitated from the simple laser direct writing process for programmable circuit printing results in less sample damage due to the non-contact mode operation without the use of photolithography, toxic chemistry, or high-temperature reduction methods.
A simple laser setup was used to reduce GO thin film into the patterned rGO on PP substrate as shown in Figure 1. The Nd:YVO4 UV pulsed laser (l = 355 nm) system in TEM00 mode was utilized for GO reduction process (Figure 1a). In their system, the pulse duration, at a repetition rate of 30 kHz, was 20 ns, and the laser beam diameter was set as 1.5 mm. By tuning the laser intensity distribution with the fluence of 0.5–1 J/cm2, the GO reduction was performed between the lowest (0.04 W) to maximum (0.8 W) power gap.
Digital photograph in Figure 1b portrays the university logo (PNU) which was fabricated on laser-scribed GO-coated PP substrate showing the discrete and sophisticated patterns (i.e., GO-rGO-GO) that can be constructed using this simple one-step laser system.
Figure 1: (a) Schematic diagram of UV pulse laser system for the reduction of GO. (b) Digital image of Korean Pusan National University (PNU) logo with sharp contrast (inset) engraved by laser direct writing (LDW) on GO coated PP substrate, showing optical properties with see-through type transparency.
This laser direct writing (LDW) method presented in this work has several advantages over conventional photolithography for the pattern transfer process at the micron scale without the use of complicated processes such as spin-casting photoresist, UV exposure, development, and photoresist removal. Therefore, LDW is easy to carry out and achieves the end results, desired patterning and the conversion GO to rGO simultaneously, in a quick and facile manner.
The pure graphene inks show countless promise for flexible printed electronics. Some of the merits includes high electrical conductivity and strong mechanical, chemical, and environmental stability. While conventional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are mandatory for improved device performance and integration density.
A high-resolution transfer printing method is developed by Minnesota researchers Donghoon Song and his colleagues capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit outstanding electrical and mechanical properties, empowering favourable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%.
The high resolution combined with admirable control over the line edge roughness to below 25 nm allows hostile miniaturization of transistor dimensions, contributing a fascinating route for the scalable process in the fabrication of flexible nanoelectronic devices.
1. Kang, SeungYeon, et al. "Patterning and reduction of graphene oxide using femtosecond-laser irradiation." Optics & Laser Technology 103 (2018): 340-345.
2. Park, Rowoon, et al. "One-Step Laser Patterned Highly Uniform Reduced Graphene Oxide Thin Films for Circuit-Enabled Tattoo and Flexible Humidity Sensor Application." Sensors (Basel, Switzerland) 18.6 (2018).
3. Song, Donghoon, et al. "High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics." ACS nano 11.7 (2017): 7431-7439.