New research reveals how hydrogenation can remove contamination from graphene

April 25, 2021 - Graphene is as a two-dimensional material prone to hydrocarbon contaminations, which can significantly alter its intrinsic electrical properties. A team of scientists from the Universities of Leiden and Eindhoven in the Netherlands as well as the University of Ulm and the Technical University of Dresden in Germany have presented new research on how to clean graphene post-growth to improve its lattice integrity and fabricate reproducible transistors.

High cleanness is a prerequisite for many applications of graphene [1]. Scientists already understood that surface contaminations are generally introduced during, e.g., post-growth transfer [2]. They can alter the intrinsic properties and performances of graphene by, e.g., large resistance fluctuations and batch-to-batch variations [2,3]. Such hydrophobic contaminants drastically alter the wettability and the electrochemistry [4] of graphene. Quite a few post-growth methods had been explored to eliminate these surface contaminants, such as current annealing, mechanical removal [5], chemical etching, and plasma cleaning [6]. Of all these techniques, the hydrogen plasma technique has the advantage of large- scale applicability and uniformity. The introduced hydrogenated defects (H-sp₃ sites) could be completely removed by annealing, thus leading to a re-covered high carrier mobility of the dehydrogenated graphene.

The new study used a mild hydrogen plasma followed by dehydrogenation. During the mild hydrogen plasma conditions, surface hydrocarbons shall be removed. And during dehydrogenation, sp₃ defect sites are restored to sp₂ graphene sites. Aberration-corrected high-resolution transmission electron microscopic images (AC-HRTEM) was used to quantify which part of the surface was restored. By statistical analysis of the HRTEM images, the degree of removal of adsorbed hydrocarbons from the surface could be quantified. Furthermore, in situ spectroscopic techniques including temperature programmed desorption-infrared spectroscopy (TPD-IR) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) demonstrated that a layer of water adsorbed on the surface of hydrogenated graphene preserves the cleanness of the basal planes [7].

Hydrocarbons on graphene in the TEM
For the new study, hydrogenated graphene (H-G) was obtained from untreated graphene upon 60 s of hydrogen plasma treatment. Figure 1(a) shows the AC-HRTEM images of untreated G with the lattice fully covered with amorphous contaminations [8]. The first-order reflection in the fast Fourier transform (FFT) patterns (in the inset) indicates the preserved crystallinity of monolayer graphene. By contrast, H-G in Figure 1(b) shows larger visible areas of the graphene lattice surrounded by less amorphous patterns. The preserved crystallinity of H-G is confirmed by the six reflections FFT patterns in the inset. Particularly, a magnified HRTEM of H-G in Figure 1(c) confirms the clean and well- preserved lattice integrity in H-G.

The cleanness of H-G is estimated to be as ~3-fold as that for Ar-G at similar magnifications, see Figure 2. Ar-G: graphene treated by 13 s of argon plasma. The surface of H-G exhibits a clear contrast between the clean lattice and amorphous patterns, which allows for further statistical analysis of lattice cleanness. Such cleaning contrasts between hydrogen and argon plasma should mainly be related to the plasma conditions employed in this work. In fact, the energies of plasma particles (i.e., ions, radicals) determine the degree of functionalization, defect generation or etching of graphene lattices, and/or cleaning effect by removing hydrocarbons adsorbed on the surface of graphene [9].

In view of the cleanness contrast between H-G and G in Figure 1, it is concluded that the cleaned H-G is, to a certain degree, resistant to the re-contamination of hydrocarbons. When the ambient exposure time extends to longer than two weeks, no contrast in surface morphology between H-G and untreated G was observed (Figure 3).

In the new study, the scientists now performed a statistical analysis of the TEM images. To do so, the TEM images were converted into a black-white binary image. Specifically, the cleanness of the image is calculated as the sum of the number of white pixels (clean graphene area) normalized by the total number of pixels. The quantitative evaluation of the cleanness summarized in Figure 1(e) shows that contaminations on the surface of H-G are about 50% while that of untreated G were only 20%. The cleanness of H-G was ranging from 40%–72% which is higher than for the dry-cleaning method that enabled only up to 40% cleanness[8]. Moreover, the amorphous contaminations exhibit a thinner and more homogeneous morphology after hydrogenation than that of untreated G (Figure 1(a)).

Key findings from the study
The p-doped H-G was previously reported to show n-doping behavior after heating the sample in vacuum to reduce water adsorption at the surface [7]. In situ temperature-programmed desorption-infrared spectroscopy (TPD-IR) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) were used to understand the cleaning effect introduced by the hydrogenation of graphene. For in situ TPD-IR, a piece of CVD graphene is transferred via a biphasic polymer-free transfer method [10] Prior to the TPD-IR experiment, the monolayer quality and the lattice integrity of the transferred untreated G and H-G have been confirmed, see the inset of Figure 4(a,b). Specifically, the D peak (~1,350 cm^-1) and D’ peak (~1,620 cm^-1) as indicators of the presence of defects in graphene lattices are confirmed to be more intensive for H-G after 60 s of hydrogenation.

During cooling down from 324 K to 180 K, the scientists observed almost no change in the IR spectra, indicating that the graphene surface is already saturated by water after the hydrogenation. After water deposition, the sample is heated up at a linear ramp rate of 3 K/min to 324 K to desorb the water from the graphene. Typically, physiosorbed water desorbs below 180 K, and chemisorbed water desorbs above this temperature [11]. Therefore, IR spectra collected during the heating process records mainly the relative changes induced by the desorption of chemisorbed water molecules at each temperature.

To compare the difference of water adsorption for un- treated G and H-G, the scientists calculated the ratios of the component areas of H2O versus Cu and versus the peak area of C spectra (Figure 5(e)). The results suggest that the cleaned surface for H-G exhibits a higher water affinity. Despite of more water adsorption on the graphene surface after hydrogenation (Figure 5), the hydrophilicity of hydrogenated graphene measured by water contact angle measurements is similar [14]. The underlying mechanism of the observed cleaning effect on H-G is ascribed to water adsorption. It is well-known that hydrocarbon contaminants interact with graphene via van der Waals interactions to minimize the intrinsically high surface energy of graphene [15, 16]. As H-G adsorbs more water molecules than untreated graphene, the van der Waals interaction distance between hydrocarbons and graphene is extended, thus leading to less hydrocarbon adsorption, or slowing down this process for H-G [16]. Additionally, the possibility for hydrocarbons to re-contaminate graphene is lowered when water molecules occupy most of the adsorption sites. Therefore, the water adsorption on the clean lattice of H-G, which is surrounded by hydrocarbons, prevents, or at least effectively slows down the process of hydrocarbon contaminations. Therefore, the recovered graphene after dehydrogenation is expected to present a clean and high quality sp₂ hybridized carbon lattice with few carrier scatterings from defects or charged contaminations than that of untreated graphene [3].

Dehydrogenated graphene for high quality electronics
Figure 6(a) shows the optical microscopy images of un- treated G and DH-G on a SiO₂/Si wafer. DH-G shows a clean surface with less polymer residues than that of untreated G. Furthermore, Raman spectroscopy further confirmed the lattice integrity of DH-G (Figure 6(b)). After annealing, DH-G is restored to pristine-like status by recovering the sp₃ C–H bonds to sp₂ C–C bonds, reflected by the dramatically decreased D peak in comparison with that of H-G. Similar Raman spectra for DH-G and untreated G also corroborate the restored carbon lattices.

Electrical characterizations of untreated G and DH-G were performed with field-effect transistors architectures to verify it upon dehydrogenation. Indeed, the charge carrier mobilities in graphene could be restored to the values higher than that of untreated and contaminated graphene. The transport properties of as-prepared graphene field effect transistors (GFETs) were measured in a liquid gating configuration (0.1 M KCl solution containing 10 mM pH 8 Tris as the buffer) at room temperature (Figure 6(c)). Figure 6(d) plots the typical conductance (G) of an untreated G, H-G (60 s) and a DH-G as a function of the gate voltage (Vg). As H-sp₃ defects introduce short-range scattering into the lattice, H-G shows degraded conductivity and mobilities (~160–180 cm² V^-1 s^-1) compared to untreated G (~500–840 cm² V^-1 s^-1). In contrast to untreated G, DH-G exhibits a higher minimum conductance (G_min) value at the charge neutrality point (CNP) and p-doping behavior. Such a contrast can be attributed to the reduction of scattering events induced by hydrocarbons in DH-G, as G_min is dominated by the density of charged impurities ac- cording to the Boltzmann theory. On the other hand, the observed contrast in G_min and p-doping effect are also related to the residue water molecules on DH-G as the physisorption of water is reported to slightly increase the minimum conductance while induce p-doping effect in graphene [7]. Considering short duration in the annealing step (~1 h) to dehydrogenate H-G, the pre-existing water molecules on H-G are likely to retain on the surface of DH-G after annealing. From the G(Vg) curves of six different GFETs, the mobilities for DH-G are (high) standard for CVD graphene, i.e., from ~1,150 to ~1,630 cm² V^-1 s^-1 (Figure 6(e)), which are about two to three-fold higher than that of the untreated contaminated G (~500–840 cm² V^-1 s^-1).

The new research provides evidence that hydrogenated graphene generally presents and keeps a cleaner surface than the untreated, contaminated samples. The mechanistic investigation shows that hydrogen radicals first remove surface-adsorbed hydrocarbons and then chemically functionalize the underlying graphene lattice from sp₂ to sp₃, resulting in an increased water adsorption of H-G to prevent further hydrocarbon contaminations. The low level of contaminations from hydrocarbons allows a better control of the surface chemistry of graphene, facilitating more studies of the properties of pristine Graphene and new applications including new environmental and highly sensitive sensors.

Resource: Jiang Lin, Deursen Pauline, Arjmandi-Tash Hadi, Belyaeva Liubov, Qi Haoyuan, He Jiao, Kofman Vincent, Wu Longfei, Muravev Valery, Kaiser Ute, Linnartz Harold, Hensen Emiel, Hofmann Jan Philipp, Schneider Grégory. Reversible hydrogenation restores defected graphene to graphene. (2021) Science China Chemistry 64, 1047-1056, doi: 10.1007/s11426-020-9959-5, [PDF], see also the supporting information.