Graphene etching methods and their applications. Source: “NA” is “not applicable”.
Recently, graphene nanomaterial has drawn great interest due to its excellent electrical and optoelectrical properties. The etching of graphene based on plasma engineering to achieve atomically thin layer and extremely clean surface is a hot issue, which is highly desirable for industrial applications. The resided contaminants with high intrinsic roughness create the degradation of performance. The impurities are removed via surface cleaning method and layer-by-layer plasma etching via top-down lithography. Recently, new plasma technology-based etching causes no damage and secures its π-binding, which plays a key role in conductivity and other characteristics. Thus, this chapter presents the recent advances in new etching technologies for nanomaterials (e.g., graphene) as well as emerging applications based on these technologies.
- graphene etching
- ion beam
- neutral beam
- inductively coupled plasma (ICP)
- atomic layer etching (ALE)
- reactive ion etching (RIE)
- chemical vapor deposition (CVD)
An atomical graphene layer, which was invented in 2004 [1, 2], is considered as one of the best candidates for a broad application range with novel electronic and optoelectronic behaviors [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. Unfortunately, the conductive graphene with no bandgap prevented its outstanding physical and chemical potentials. Thus, its bandgap tuning via various approaches is highly desired for extreme performance devices (Figure 1).
Recently, etching technologies are emerging as one of the best efficient tools to tune a device’s performance, thereby extending to many different fields in broadband [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. The new approaches include the following: (i) inductively coupled plasma (ICP), neutral beam-based atomic layer etching (ALE), ion beam and reactive ion etching (RIE) [22, 23, 24, 25, 26, 27, 28, 29], (ii) chemical vapor deposition (CVD) [30, 31] and (iii) thermally activated nanoparticles . Plasma has used Si-integrated circuits for etching . Among breakthroughs, plasma etching represents an important role in Si and non-Si (metal)-assisted devices. This chapter will present recent advances in new graphene etch technologies and their related applications.
2. Emerging etch technologies of graphene surfaces
Graphene layers have a number of independent bandgaps, e.g., single layer has no bandgap, but a bilayer has bandgap, and could be utilized to make transistor a superior performance. The layer-by-layer graphene etching would form (i) a cleaner surface with removed residues and (ii) thinner graphene film leading to smaller bandgap value until there is no bandgap at a single layer. Depending on the types of defects such as disorder , doping , external field  and mechanic strain [36, 37, 38, 39, 40], the etching can make the host material (e.g., graphene) very useful (high conductivity, high mobility, high work function, etc.) . As a result, bandgap can be higher (or lower) depending on the types of vacancy defects and etching rates .
2.1 O2 plasma etching
Plasma etch technology presents many advantages such as easy scale-up, manipulation and mass production. Under O2 plasma exposure, graphene multilayers were well-etched on SiO2 [23, 25] or SiC . In 2014, the etching of host bilayer graphene was carried out by O2 using ICP and RIE apparatuses on the vertical and horizontal etch directions (Figure 2a,b) . However, this approach formed defects during the use of RIE, but the defects were very few in the ICP case because of the high damage energy of RIE. Raman data provided the proof through disorder characteristics based on ID/IG ratio (0.94 and 1.18) when utilizing RIE and ICP, respectively . Treating another substrate, SiC, the contact angle changed from 92.7° (multilayer), 91.9° (bilayer) and 92.5° (single layer) down to 70° when one layer epitaxial graphene etched away at 10 W and 2 min (Figure 2c,d) . In 2011, through nanosphere lithography with low-power O2 plasma, Liu et al. found out the etched ordering of graphene nanoribbons (GNRs) on SiO2, which performed well in various shapes such as branches, chains, connected rings and circular rings (Figure 2e) .
2.2 N2 plasma etching and postannealing
Yang et al. utilized N2 plasma and postannealing (Ar/O2, 900°C), another technology in integration of layer-by-layer thinned plasma and post-annealing. As a result, this dry-etching thinned regarding layer-by-layer easily from intrinsic multilayer graphene (Figure 3a–c) . In another innovative etch technology by Lim et al.  and Kim et al. , Lim et al. utilized a neutral beam ALE via two-step process of O2 radical absorption and Ar neutral beam desorption, and multilayer graphene was well-etched for each layer (Figure 3d). Although this etching was much more effective than the previous study [24, 25, 26, 27], defects formed slightly on graphene lattice as high Raman D-peak (Figure 3c) .
2.3 Cyclic etching (O2 adsorption and Ar desorption by ion beam)
In 2017, Kim et al. newly innovated by adding two mesh grids between the plasma source and the substrate holder in the ICP chamber (Figure 3e–j) . Consequently, the damage on graphene surface disappeared after the two-step plasma etching process of chemical absorption of O2 radical and physical desorption of Ar ion beam at optimized plasma energy (11.2 eV).
2.4 Others (RIE, H2, CH4/H2 and Fe NPs)
In addition, there are still strategies for graphene surface etching such as Ar/H2 mixture in reactive ion etching (RIE) (Figure 4a) , H2 etching during CVD graphene growth (Figure 4b–e) , CH4/H2 etching during CVD graphene growth (Figure 4f)  or thermally activated Fe nanoparticles (NPs) (Figure 4g,h) . However, the demonstrated results showed high defects through very high D-peak intensity in Raman spectra  or the random and nonuniform nanoribbon-etched graphene  and nanotrench-etched graphene based on Fe NPs . Compared with the developed etch technologies above, the etching method by Kim et al.  revealed to be the best to date because of perfectly no damage and layer-by-layer etching from an innovative ion beam ICP.
3. Applications based on etched graphene
In Table 1, applications associated with the above etched-graphene investigations are briefly summarized. A chip utilized nanosphere-etched GNRs by O2 plasma at low power  and revealed the high-performance electronic device with the exotic GNR architectures (chain, branch and circle ring). In another application, a metal oxide semiconductor (MOS)-like transistor was made; although the etched effect was formed, it simultaneously produced a high-energy plasma damage that induced poor electrical characteristics . One more application related to the monolayer deep patterning was fabricated by etching (N2) and annealing (Ar/O2), and this pattern presented good quality for slight defects . In 2015, Papon et al. fabricated the Y- and Z-shaped GNRs during CVD graphene growth, and the etching effect vehemently happened at high H2 concentrations . But these shapes were random and not well-controlled mechanically.
Generally, there are many unexploited huge potentials from the etched-graphene products, but the perspectives are bright. If these etching technologies are extended to other nanomaterials such as transition metal dichalcogenides (TMDs) or transition metal carbides, nitrides, and carbonitrides (MXenes), and black phosphorous , it will definitely achieve high-quality electronics and optoelectronics. Bandgap tuning for nanomaterials will significantly improve the on/off current ratio, photoresponsivity, quantum efficiency, conductivity and others. Layer-by-layer etching for multilayer materials by low-energy plasma technology (double mesh grids inserted for plasma apparatus such as chlorine-radical ICP, neutral-beam ICP and ion beam ICP) with no physical damage would be the next research direction and can be applied to the other low-dimensional materials [28, 43] to achieve ultrahigh performance of electronic and optoelectronic devices [28, 43]. For instance, in the latest report in 2019, Kim et al. utilized a chlorine ICP innovative plasma apparatus that has no physical damage effect by inserting double mesh grids for cyclic ALE process on intrinsic multilayer MoS2 and successfully fabricated heterostructured photodetector with ultra-responsivity (~106 A/W) in the visible range .
Conflict of interest
There are no conflicts of interest to declare.