Alkali metal bilayer intercalation in graphene

Alkali metal (AM) intercalation between graphene layers holds promise for electronic manipulation and energy storage, yet the underlying mechanism remains challenging to fully comprehend despite extensive research. In this study, we employ low-voltage scanning transmission electron microscopy (LV-STEM) to visualize the atomic structure of intercalated AMs (potassium, rubidium, and cesium) in bilayer graphene (BLG). Our findings reveal that the intercalated AMs adopt bilayer structures with hcp stacking, and specifically a C6M2C6 composition. These structures closely resemble the bilayer form of fcc (111) structure observed in AMs under high-pressure conditions. A negative charge transferred from bilayer AMs to graphene layers of approximately 1~1.5×1014 e−/cm−2 was determined by electron energy loss spectroscopy (EELS), Raman, and electrical transport. The bilayer AM is stable in BLG and graphite superficial layers but absent in the graphite interior, primarily dominated by single-layer AM intercalation. This hints at enhancing AM intercalation capacity by thinning the graphite material.

The sample was prepared by the vapor phase intercalafion.However, the ions intercalafion into graphene/graphite for energy storage are usually governed by a different driven force.The aim to understand the akali interacfion as the authors introduced is mainly for energy storage and ionic applicafion.Therefore, the intercalated graphene prepared by different driven force need to be invesfigated.
Why does the intercalafion of second layer Cs cause much larger interlayer distance increase as compare with the first layer intercalafion?
As claimed by the author that there is charge transfer between the bilayer graphene and intercalated AMs.This need to be characterized and verified by Raman as well.
For the intercalafion into graphite as compare with the bilayer graphene, the author claimed that the full intercalafion was not observed.Could the AMs absorbed on the top surface of sample affect the intercalafion of AMs into the bilayer graphene or graphite, and thus the intercalated structure?
For the potassium ion baftery, it has been demonstrated that the potassium ions can be reversibly intercalated into graphite with the formafion of uniform intercalated structure.Please discuss more about why the monolayer intercalafion was formed in baftery but bilayer intercalafion was formed in this work.

Reviewer #2 (Remarks to the Author):
In the manuscript "Alkali Metal Bilayer Intercalafion in Graphene", the author use STEM to directly visualize the atomic structure of intercalated alkali metals in bilayer and few-layer graphene, adopt EELS to measure the charge transfer between alkali metals and graphene layers, the reveal the intercalafion mechanism.This is a novel fundamental work.It could be considered further for publicafion in Nature Communicafions after major revisions.1.In the abstract, the author declared that "this research provides the first visualizafion of the precise atomic structure involved in the over-doped alkali metal intercalafion."Is it contradict with Ref.12 that ufilizes TEM to image mulfilayer stacking Li within bilayer graphene.2. The author declared that no single-layer, three-layer, or four-layer arrangements of AMs layer were observed.What the image of 1-4 layers should be?How to disfinguish between 2 layers and 4 layers from the image?If 2 layers is confirmed by EELS quanfificafion, it is befter to present the result of K and Rb.
3. In the manuscript, the author calculates the amount of charge transfer, it is befter to present more details about the esfimafion.4. How to confirm that the honeycomb structure in Figure 3a is bilayer of K.As the author declared that the cross-secfional image of intercalated HOPG may contain single-layer AM intercalafion area, is it possible that the honeycomb image resulted from the overlap of single K layers within different graphene sheets.5. What does the void in the upper part of Figure 3b?Does it result from the intercalafion of Cs and deintercalafion during specimen preparafion?6.Some errors in the manuscripts.It seems that no author is employed by affiliafion 6; "4.3 Gpa" should be "4.3GPa"; "1~1.5x1014"should be "1~1.5×1014".

Reviewer #3 (Remarks to the Author):
Lin et al. report a scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) study of alkali-metal intercalated bi-and few-layer graphene samples.Their major new finding is the observafion of stable alkali metal (AM) bilayer crystals that occur in these very thin forms of graphific carbon but not in bulk graphite.Furthermore, the authors report stable bilayers of several AMs: cesium, potassium, and rubidium.The work is of high quality and the experimental findings consfitute an important advance over previous work in the field, poinfing to possible knowledge gaps in the understanding of intercalafion chemistry of graphific samples at the limit of ultrathin crystals.The phenomenology could potenfially further be generalizable to the intercalafion of thin specimen of other 2D materials and may have implicafions for the design of ion inserfion electrodes.Therefore, it is my opinion that the manuscript presented by Lin et al. is an excellent fit for Nature Communicafions and may aftract aftenfion from a broad audience, including but not limited to the transmission electron microscopy, 2d materials, and electrochemical energy storage communifies.
Overall, I find the manuscript well wriften and the presentafion of the results well suited for publicafion in Nature Communicafions.I was nonetheless confused about a few things I encourage the authors to address to make the manuscript more accessible and comprehensible in parficular for non-experts in scanning transmission electron microscopy.1) How did the authors determine the bilayer nature of the graphene samples studied based on their data?Specifically, how did the authors rule out the samples studied were trilayer graphene where AM single layers could sit in neighboring interlayer spaces?
2) How did the authors determine the AM bilayers studied were confined in the interlayer gap between two layers of graphene, rather than located on top of or below these based on their data?
3) I am not an expert in ADF imaging, and was confused by the presentafion of the data in Fig. 1.I expected to first of all see imaging/diffracfion evidence for bilayer graphene without Cs, only then to be introduced to C6Cs2C6 in a second step.Instead, Figure 1 is all about C6Cs2C6 and the reader is left to deduce that for some reason the carbon atoms are not visible, instead the contrast mainly stems from Cs, the diffracfion paftern itself would be different for bilayer graphene without Cs, etc.I recommend the authors to introduce what is known first, which could include merging Extended Data Fig. 3 with Fig. 1 for example.4) Regarding Extended Data Fig. 3: Did the authors determine the 2.7 degree twist angle between graphene layers from the diffracfion paftern?If so, is there also a 2.7 degree twist in the C6Cs2C6 region?Doesn't the (sqrt(3) x sqrt(3) R 30 deg) structure necessitate AA stacking of the graphene layers or does that supercell refer to only one of the two graphene layers?5) Could the authors expand their discussion by commenfing on what they think is special about the 4 nm thickness beyond which the authors assert no more bilayer AMs would occur in graphite?Couldn't bilayer AMs sfill occur near the surface of an even thicker graphite flake, but maybe not near its center?6) Could the authors expand their discussion by commenfing on why they think only bilayers stable?What do the authors think is the mechanism that rules out the formafion of trilayers or thicker AM layers in their experiment once the two graphene sheets have been separated?Minor comments: 1) Line 192: "charge carrier plasmon from the bilayer AMs to BLG" may have to read "charge carrier transport from the bilayer AMs to BLG" instead.2) Legends in Fig. 2 g-I all read "C6K2C6" which needs correcfion.