Abstract
X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials.
Highlights
Two-dimensional (2D) layered materials attract increasing attention, especially with respect to their manifold incorporation into van der Waals heterostructures.[1,2] Despite scalable synthesis methods being increasingly put forward,[3,4] mechanical exfoliation from bulk crystals remains the prime route to fabricate such 2D materials with the highest quality.[5]
We have demonstrated the feasibility of measuring X-ray diffraction (XRD) on individual micron-sized singlecrystals of bi- and multilayered graphene flakes supported on a substrate
Characteristic in-plane Bragg peaks can be observed by grazing incidence X-ray diffraction (GIXD), whereas the c-axis lattice spacing can be probed by X-ray reflectivity (XRR)
Summary
Lift-off techniques are used to create a set of markers out of 5 nm Ti and 30 nm Au near the center of the substrate. From the calculated XRR of the bare substrate, we obtain dSiO2 = 10.81 nm, given by the spacing between extrema of the Kiessig fringes Their amplitude yields ρSiO2 = 2.196 g/cm[3] that is in close agreement with the literature value of 2.202 g/ cm3.25 The surface roughness of this layer σSiO2 = 0.37 nm (comparable to the findings of ref 26) is largely responsible for the q⊥-dependent drop in signal intensity beyond qc. From the calculated XRR (red solid lines), a distance between graphene sheets c of 0.335 ± 0.005 nm for the pristine flake and c′ = 0.390 ± 0.005 nm for the lithiated one is extracted The latter value exceeds the typical range of interlayer spacings 0.335 nm ≤ c ≤ 0.37 nm achieved in bulk LixC6 with 0 ≤ x ≤ 1.33,34 Quantitatively, c′ might be overestimated due to the difficulty in precisely reproducing the apparent shift between both data sets in Figure 4a at q⊥ > 8 nm−1. Note that variations in FWHMG and b among samples are most probably due to variations in SiO2 surface conditions during device fabrication; see ref 38
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