Experimental demonstration and transformation mechanism of quenchable two-dimensional diamond

Domain: chemistry
Published: 2026-01-08
Editorial outcome: major_revision

Abstract
Two-dimensional (2D) diamond has aroused remarkable interest in nanoelectronics and optoelectronics, owing to its superior properties and flexible characteristics compared to bulk diamond. Despite significant efforts, great challenges lie in the experimental synthesis and transformation conditions of 2D diamond. Herein, we have demonstrated the experimental preparation of high quality 2D diamond with controlled thickness and distinguished properties, realized by laser-heating few-layer graphene in a diamond anvil cell. The quenched 2D diamond exhibited a narrow T2g Raman peak (linewidth ~3.6 cm−1) and intense photoluminescence of SiV- (linewidth ~6.1 nm) and NV0 centers. In terms of transformation mechanism, atomic structures of hybrid phase interfaces suggested that the intermediate rhombohedral phase subtly mediate hexagonal graphite to cubic diamond transition. Furthermore, the tunable optical bandgap and thermal stability of 2D diamond sensitively depend on its sp3 concentration. We believe our results can shed light on the structural design and preparation of many carbon allotropes and further uncover the underlying transition mechanism.

Introduction
Two-dimensional (2D) diamond has attracted considerable interest due to its atomically thin structure in sp3-hybridization and unique quantum confinement effect1–3. Bulk diamond is well known for its exceptional hardness, high thermal conductivity, and chemical inertness. However, it exhibits relatively low fracture toughness and poor electrical conductivity compared to many metallic and ceramic materials4–7. Various strategies have been developed to optimize its physical properties, such as structural modification and chemical doping8–15. The synthesized amorphous carbon, which exhibited a short-range ordered diamond-like structure, demonstrated its tunable electronic bandgaps although its thermal conductivity is significantly reduced compared to crystalline counterpart9. Nanotwinned diamond was developed with their simultaneous enhancement of hardness and toughness following the Hall-Petch relation11,12. Alternatively, thickness, as an exotic degree of freedom, is rarely considered to regulate the diamond structure and further extend its applications, and the related research remains unmatured16.

It is predicted that 2D diamond can inherit the superior properties of bulk diamond and exhibit unique physicochemical properties, raised from its 2D characteristic17. Chernozatonskii et al. proposed the C H structures as “diamane”, by absorbing hydrogen atoms on bilayer graphene18. Theoretical simulations underline that chemical functionalization and stacking sequence effectively modulate bandgap and thermal conductivity of 2D diamond19,20. Extensive efforts have been paid to synthesize 2D diamond in experiments, such as the direct phase transformation from few-layer graphene (FLG) to diamondol21, diamane22–24, diamene25–27, and diamondene28–30. Hydrogenated fluoride 2D diamond have been achieved by chemical adsorption of FLG under moderate conditions, while this approach usually produces the incomplete internal transformation22–24. Moreover, bilayer graphene on SiC reversibly transformed into diamond-like film upon tip compression, which was severely hindered by the stacking sequence of FLG films26. Thin hexagonal diamond (HD) synthesized from FLG and its layer-dependent transition pressure was elucidated, and this metastable phase can be unfortunately preserved to ~1.0 GPa during decompression27. Biaxial strain approach was explored to play an important role in the reversible phase transition from multilayer graphene (13–94 nm thick) to diamondene on different substrates30. In theory, high pressure and high temperature (HPHT) are supposed to irreversibly transform FLG into the quenchable 2D diamond31, which remains to be further confirmed experimentally.

To understand the daunting transition mechanism, numerous pathways have been proposed to illustrate the transformation at the atomic scale, such as the concerted mechanism and nucleation assumption32–34. Theoretical simulation predicts that hexagonal graphite (HG) lattice undergoes slipping, followed by puckering to form cubic diamond (CD) and HD34. Rhombohedral graphite (RG), as an intermediate phase, may play a key role for HG to CD transformation35. Meanwhile, considerable effort has been paid to study the heterophase structures between diamond and graphite, including the van der Waals interaction-governed and covalently bonded diaphite with the fancy interfacial motifs36–38. Compared to bulk diamond, significant gap still remains to elaborate the transition mechanisms in 2D diamond, both experimentally and theoretically. Theoretical results indicate that nucleation-and-growth mechanism, structural defects, and interlayer sliding may be essential to 2D diamond formation30,39,40. Despite considerable effort invested, the controlled synthesis and phase transition mechanism of 2D diamond remain persistently challenging. Here, we report an effective strategy for 2D diamond synthesis with controlled thickness extending from bilayer graphene to several hundred nanometers, by laser-heating metal-supported graphene precursor under high pressure. Lattice structures of recovered samples together with the hybrid interfaces evidently confirm the mediation of RG in HG to CD transition and crystallographic orientational relationships following (002)HG ∥ (003)RG ∥ (111)CD and (002)HG ∥ (100)HD. Moreover, our results reveal that the prepared ultrathin diamond exhibit prominent thermal stability, and its sizable bandgaps vary from 1.4 to 1.9 eV, originated from its obvious sp3 content-dependence. This work can provide critical insights into the structural control and long-stand puzzle in transition mechanism of 2D diamond.

Results
First, we performed HPHT preparations of 2D diamond from graphene precursors with various thicknesses, ranging from bilayer to several hundred nanometers (Supplementary Figs. 1–8 and Supplementary Table 1). Regarding to its low absorbance of graphene (2.3% for monolayer limit)41, we established a strategic approach to load FLG into diamond anvil cell (DAC), followed by laser heating to achieve HPHT conditions. The polished metallic Re foil was utilized as both graphene substrate and laser absorber to endure high temperature (Fig. 1 and Supplementary Fig. 1), due to its high melting point (3450 K at 1 atm), unique mechanical modulus and carbide-free characteristic42,43. The prepared graphene/Re was sealed in laser-heated DAC using KBr as the pressure transmission medium (PTM) and thermal insulator, as illustrated in Fig. 1a. Both HD and CD phases are expected after HPHT treatment, and the latter one usually undergoes a lower-barrier pathway35 in Fig. 1b. Figure 1c–f show the representative optical evolutions of a trilayer (3L) graphene during the entire experimental process. The FLG on Re substrate exhibited weak optical contrast resulting from its low absorbance. The morphology of FLG can be precisely traced from precursor preparation through HPHT treatment, even after being quenched to ambient condition from 32.3 GPa and 2924 K, as indicated in Fig. 1c–f.

Raman spectrum before and after HPHT treatment were measured to verify the transition from FLG to 2D diamond, as shown in Fig. 1g. Raman spectrum of pristine 3L graphene before compression displays the prominent G peak at 1580 cm−1 and a complete absence of the D band, confirming that our optimized transfer protocol preserves the defect-free nature of graphene precursor. The quenched 3L sample from 32.3 GPa and 2924 K exhibits an evident Raman T2g peak around 1332 cm−1, which is uniquely originated from the lattice structure of sp3 carbon, i.e., diamond structure (the detailed Raman characterization will be discussed later).

The uniform and ultrathin nature of the quenched 2D diamond was confirmed by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) characterizations. HRTEM results (Fig. 1h) reveal a continuous carbon layer with a uniform thickness of ~1.0 nm, sandwiched between the gold coating and the Re substrate as confirmed by the EDS mapping results in Fig. 1i. The thickness of the quenched 2D diamond derived from 3L graphene is comparable to the theoretical thickness (9 Å) of a double-sided hydrogenated 3L diamane44.

Raman characterization serves as a robust tool for distinguishing diamond phase from other metastable structures. Photoluminescence (PL) analysis and microstructural characterization also provide complementary criteria for diamond identification, as discussed below. Besides, we further investigated the 2D diamond synthesis from graphene with varied thicknesses from several layers to several hundred nanometers (the details of synthesis conditions summarized in Supplementary Table 1). Our studies revealed the thickness-dependent transition conditions, that is, higher pressure and temperature are demanded for thinner precursors (Atomic structures and physical properties of the obtained 2D diamond will be discussed later).

The representative Raman and PL results of 2D diamond samples recovered from different synthesis conditions are shown in Fig. 2. Laser heating method allows us to selectively treat the partial area of graphene precursor, while the rest remains opaque without direct laser irradiation (Fig. 2a). After high pressure and room temperature (HPRT), the primary G and 2D peaks appeared at 1587 and 2774 cm−1, arising from the doubly degenerate E2g mode and second-order scattering mode, respectively. As temperature increased, the enhanced D band at 1386 cm−1 indicated that the structural distortion or transition occurred in graphene lattice, coinciding with the appearance of characteristic diamond Raman signal. Significantly, Raman intensity of diamond dramatically increased along the temperature gradient, while G band of graphene broadened and eventually disappeared. It is clear that sp3 concentration in 2D diamond principally depends on the temperature under a certain pressure. The transparent flatten nanoflake recovered from 20.0 GPa and 1435 K exhibits rather low roughness (2.2 nm) and only a sharp Raman peak at 1332 cm−1, and its fitted full width at half maximum (FWHM) of 3.6 cm−1 approximates to commercial diamond crystal (3.3 cm−1), suggesting the superior crystalline quality of our formed 2D diamond (more results in Supplementary Figs. 9, 10).

Furthermore, the quenched 2D diamond shows strong PL from the unique color centers, such as silicon-vacancy (SiV-) and nitrogen-vacancy (NV0). In Fig. 2b, the intense and sharp peaks located at ~738 and ~575 nm correspond to the zero-phonon lines (ZPL) of SiV- and NV0 centers, respectively, well consistent with literature results45,46. In addition, the temperature- and pressure-dependence of ZPL peak positions can principally prove the color center as well (Supplementary Fig. 11). These intense PL characters illustrate that the obtained 2D diamond has promising application potential in quantum computing and sensor devices. Raman and PL mapping measurements enable us to effectively identify the diamond section, and the results demonstrate a non-uniform distribution of color centers in Fig. 2c–e. The Si impurities originate from polydimethylsiloxane (PDMS) used during sample exfoliation process, and the N impurities may be attributed to the nitrogen-rich ambient air. The introduction of Si and N impurities serve as diamond nucleation sites and further lower the nucleation barriers32,47,48.

Atomic structure of 2D diamond and transition mechanism were investigated by HRTEM and selected area electron diffraction (SAED) (Fig. 3 and Supplementary Figs. 12–15). HRTEM results reveal the coexistence of HD (Fig. 3a–c and Supplementary Fig. 12e–f) and cubic diamond (Fig. 3d–f and Supplementary Fig. 12g–i) when the samples were recovered from HPHT. A compact three-dimensional network structure displayed in Fig. 3b corresponds to HD lattice observed from [001] zone axis. Carbon atoms are assembled as interconnected hexagonal rings in HD and the adjacent (100) layers with a d spacing of 2.23 Å, consistent with the strong diffraction pattern. Alternatively, atomic arrangement in Fig. 3c was observed from HD structure along [110] zone axis as well. Figure 3e and f show the HRTEM images and SAED patterns of CD structures from [011] and [101] zone axes, respectively (more results in Supplementary Fig. 12). The measured d spacing of ~2.12 Å is referred to the typical (111) plane.

It is worth noting that we observed RG structure next to the CD grain in Fig. 3g, h, suggesting that RG may mediate HG to CD transition. The lattice planes with ~2.14 and 3.29 Å spacings are recognized as (101)RG and (003)RG, respectively, and the interfacial angle between (003)RG is measured as ~78°. It is well known that the carbon layers in RG are stacked with ABC order, as highlighted in Fig. 3h. The CD nuclei can be formed by sliding HG planes into ABC stacking, and then buckling RG basal planes into chair configuration35. The atomic arrangement indicates the orientational relations of (002)HG ∥ (003)RG ∥ (111)CD in our case. Besides, atomic structure of HG-HD interface was acquired as shown in Fig. 3i. HG with interlayer distance of ~3.35 Å is adjacent to (100)HD plane of HD lattice with d spacing of ~2.19 Å, which clearly suggests the orientation relation of (002)HG ∥ (100)HD. The HG/RG/CD and HG/HD hybrid interfaces can be recovered from the transitional region with moderate temperature attributed to the energy distribution during laser heating and thermal diffusion effect. These atomic configurations of RG/CD and HG/HD interfaces agree well with the simulation results32,38.

Atomic nanostructures of the existent defects in 2D diamond can offer more insights into the transformation mechanism, including the apparent twin boundaries (TBs), stacking faults (SFs) and other diamond polytypes in Fig. 4. The nanotwins in CD are found with sharing the typical {111} planes due to relatively lower defect energy49,50 and further offsetting by step-like Σ3{211} TBs (Fig. 4b, g). The Σ3{211} incoherent TBs are more inclined to maintain asymmetric structures with lower excess energy, and its length is determined by the spacing between neighboring {111} coherent TBs13. Fig. 4d, f reveal the detailed fine structures of 2D diamond crystal along [011], [011] zone axes, and CD nanotwin, respectively. The adjacent CD crystals share common (111) plane, as indicated by the yellow arrow in Fig. 4f, and adopt the same atomic arrangement with mirror symmetry. Owing to the constraints imposed by coherent {111} TBs, the asymmetric Σ3(211) TBs are less mobile and may contribute to the potential strengthening of 2D diamond13.

Furthermore, diamond polytypes and the coherent graphene–2D diamond interfaces (termed “Gradia”) shed light on the phase transition behavior under high pressure. The diamond polytypes surrounded by CD exhibit different lattice structures from HD and CD (Fig. 4g). These diamond polytypes are coherently connected to CD lattice, and the rectangular pattern in FFT image illustrates a triple periodicity compared to CD, as shown in Fig. 4h. These structural features can be potentially regarded as “M-diamond”, reported with a range of non-CD polytypes previously11,12. All d-spacings of diamond polytypes extracted from FFT image in Fig. 4h are summarized in Supplementary Table 2, in order to compare with the simulated d-spacings of diamond polytypes (such as 2H, 4H, 9R and 15R)11,51. Besides, Fig. 4i presents the identified interfacial structures between graphene precursor and the formed 2D diamond, primarily corresponding to the reported gradia-CA, gradia-CO and gradia-HC37.

Electron energy loss spectroscopy (EELS) and absorption measurement were performed to explore intrinsic properties of ultrathin diamond. In Fig. 5a, the strong 1s-σ* peak at 292.5 eV confirms the dominant sp3 concentration, and the absence of 1s-π* peak at 283.2 eV represents the nearly complete phase transition from graphene precursor to 2D diamond. Our results indicate that sp3 concentrations of 2D diamond vary from 71.3% to 89.9%. The optical bandgap of ultrathin diamond, determined with Tauc plot approach, sensitively depends on the sp3 component of the recovered samples. The tunable bandgap energy of ultrathin diamond ranges from 1.4 to 1.9 eV as the sp3 percentage increases (Fig. 5b). The optical, Raman and PL results of the samples can be found in Supplementary Fig. 16. We further derived the Raman intensity ratio of diamond T2g (IDia) to graphene G band (IG). As expected, the bandgap of 2D diamond exhibits a positive correlation with the calculated IDia/IG, resulted from the increased sp3 concentration, as shown in Fig. 5c.

Moreover, thermal stability of ultrathin diamond was probed by thermal annealing treatment in inert argon atmosphere. Two recovered samples, labeled as #S1 and #S2 with different IDia/IG ratios, were progressively annealed up to 1000 °C for an hour. The evolution of IDia/IG is shown in Fig. 5d, e (more details in Supplementary Figs. 17–20). Obviously, the #S1 nanoflake with higher sp3 percentage shows superior thermal stability up to 1000 °C, with the IDia/IG value (the average ratio ~11.8) almost unchanged. In contrast, the #S2 nanoflake with lower sp3 percentage tends to graphitize at 900 °C as observed in Fig. 5e and Supplementary Fig. 19. Raman intensity of sp3-bonded phase at ~1340 cm−1 drops gradually, accompanied by the enhanced G band of graphene. The obtained IDia/IG significantly decreased from 0.45 at 25 °C to 0.13 at 900 °C, and eventually vanished at 1000 °C (Fig. 5f). Thermal stability of our prepared 2D diamond far surpasses that of many other diamond-like counterparts, such as diamond-like nanocomposite (DLN)52, diamond-like carbon (DLC)53, nano-diamond powder (Nano-dia.)54, chemical vapor deposited diamond (CVD-dia.)55, and Co-polycrystalline diamond (Co-PCD)56, as summarized in Fig. 5g.

Methods
Two-dimensional diamond synthesis
The graphene precursor with different thickness was mechanically exfoliated by PDMS. The exact layer number of FLG was determined by Raman spectroscopy together with atomic force microscopy. HPHT conditions can be realized via a homemade double-sided laser-heated DAC. The layout and photograph of our laser-heating system are illustrated in Supplementary Fig. 21. In short, two near-infrared lasers with tunable power were utilized to heat the sample, and the temperature can be determined by fitting the acuqired black-body radiation curve with Planck’s theory. Importantly, the whole sytem should be calibrated with the standard lamp, such as the quartz tungsten halogen lamp (50 W, Newport Corp.) measured by National Institute of Metrology of China in our case. The high pressure was applied via a DAC with ~300 μm anvil culet. The sample chamber (~100 μm diameter) was prepared by drilling the pre-indented T301 stainless steel or Re gasket to form a through hole at the center. NaCl or KBr was served as both PTM and thermal insulator during HPHT experiments. Pressure was measured by the red shift of R1 fluorescence line of ruby ball. Considering the low absorbance of FLG, a Re foil with flat surface was utilized to play the role of graphene substrate and thermal absorber. The Re foil can be thinned to ~10 μm thick via a polishing machine and then shaped into small chips by laser drilling system. For the thick graphene precursor (normally >50 nm), it can be sandwiched with two NaCl or KBr chips for the direct heating treatment. Once decompressed, the hydrophilic PTM can be conveniently removed by deionized water, and the recovered 2D diamond samples were transferred onto other desirable substrates for further investigations.

Raman, PL, and absorption spectroscopy
Raman and PL spectra of the recovered 2D diamond samples were measured using Horiba iHR550 spectrometer equipped with a CCD detector. The 405 nm laser was used as exciation source, and the beam size of focused laser approached ~1 μm with a 100× objective. Raman signal of 2D diamond and PL spectrum of its color center were detected with 1800 and 150 g mm−1 grating, respectively, while the pressure or temperature dependence of SiV- PL was investigated by 1800 g mm–1 grating to ensure high spectral resolution. The prepared 2D diamond samples can be loaded into the homemade cryostat setup for in situ Raman and PL measurements, and the minimum temperature ~77 K realized via liquid nitrogen. Morevoer, the recovered 2D diamond samples were transferred onto the fused silica surface for transmission measurement. The absorption spectrum was collected by Horiba iHR550 spectrometer combined with halogen tungsten lamp with broad spectral range.

HRTEM and EELS measurements
In order to prepare HRTEM sample, the obtained 2D diamond was precoated with Pt layers, and then shapped into ultrathin slice with a thickness of 60–70 nm by focus ion beam (FEI Helios NanoLab 600i). HRTEM and SAED measurements were conducted by FEI Titan Cube 80–300 and JEM 2100F with an acceleration voltage of 300 kV. Meanwhile, EELS spectra around the carbon K-edge were collected to investigate the bonding states in the recovered samples.

Thermal stability characterizations
The recovered 2D diamond samples were transferred onto the silica substrate for thermal annealing. Samples were annealed in tube furnace with flowing argon gas for 1 h, and the annealing temperature varied from 200 to 1000 °C. Optical images and Raman mapping results were collected after annealing treatment under desired temperature.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article ‘Experimental Demonstration and Transformation Mechanism of Quenchable Two-Dimensional Diamane’ by Li et al. presents a nice study on the phase transition of a few-layer graphene phase into a diamond phase called diamane. The authors demonstrate the stability of quenched diamane after the high-pressure, high-temperature treatment by measuring the diamond Raman mode, which has comparable FWHM with the natural gemstone. Additionally, the photoluminescence measurements showed strong peaks corresponding to SiV- and NV0 centers. The paper is original, self-contained and well-written. The results can be important for future phase engineering of carbon-based materials. I recommend publication in Nature Communications. Please see a few points to the article. 1. Line 97-99: It states that the structure of FLG can be clearly seen on the optical images (Fig 1 c-f). However, I must say that for me, it is difficult to distinguish between FLG and PTM in these images. A movie showing the evolution of FLG with changing pressure in the SI would increase the credibility of this article. 2. The temperature is measured with a spectrometer (Fig S16) ranging from 600nm to 750nm (Fig S2d). Have you calibrated the spectrometer? What emissivity value do you use for the FLG when applying Plank’s law to determine the temperature? What about signal losses on the optical elements, do you consider them?", "reviewer_2": "Dear editor and authors, please, find below my review for the manuscript entitled \"Experimental Demonstration and Transformation Mechanism of Quenchable Two-Dimensional Diamane\" submitted for publication in Nature Communications by Jiayin Li, and co-workers. The authors investigated the synthesis of 2D diamond via laser-heating graphene samples with thicknesses ranging from 3-5 layers to hundreds of nanometers. The properties of the recovered samples were analyzed via Raman spectroscopy, photoluminescence, high-resolution transmission electron microscopy and electron energy loss spectroscopy. The results are interesting from a material’s science perspective. However, my main concern is that vast majority of the recovered “diamane” samples have nanometer thickness, (Table S1), therefore could hardly be considered 2D diamond. It seems that all structural, elemental and optical characterizations described in Figs. 3-5 are from flakes with nanometer thickness. Thus, the authors should reframe their work in terms of synthesis of nanodiamonds, which is certainly an interesting topic, but not as novel in my opinion, to grant publication in Nature Communication. For this main issue, and others described below, I cannot recommend this manuscript for publication. 1- Introduction I believe the authors should do a better job in describing previous efforts in 2D diamond research, especially because there aren’t many experimental papers available. For instance, in “Extensive efforts have been paid to synthesize 2D diamond in experiments, such as the direct few-layer graphene (FLG) to diamane conversion. [21-27]”. The references cited here include different proposed 2D diamond structures. Diamane is only one of those proposed structures, where functionalization occurs at both top and bottom surfaces. However, there are other proposed structures such as diamene where there is no functionalization at either top or bottom surfaces [Nature nanotechnology 13.2 (2018): 133-138, Nano letters 20.8 (2020): 5916-5921], and diamondene [Nature communications 8.1 (2017): 96, Carbon 173 (2021): 744-757] where functionalization occurs only at the top surface. Thus, it is not only a matter of semantics: whether there is functionalization or not can drastically change the properties of the final structure. For instance, diamondene is supposed to be a 2D ferromagnetic semiconductor while diamane should be an insulator. For a review on the topic please check Two-dimensional diamonds from sp2-to-sp 3 phase transitions. Nature Reviews Materials 7.10 (2022): 814-832. Also, I believe important references are missing. For synthesis via tip compression: Advanced Materials 23.27 (2011): 3014-301 - this was the first experimental evidence of 2D diamond’s existence). For synthesis via hydrostatic compression: Carbon 173 (2021): 744-757 and Rajaji, V., et al. \"Biaxial strain effect in the room temperature formation of 2D diamond from graphene stacks.\" Carbon (2025): 120267 - those experimental works investigate the mechanism of 2D formation. Also, in lines 66-73, the authors mostly describe works that investigated the mechanism of bulk diamond formation from compressed graphite, while the mechanism of 2D diamond formation – one of the main claims of the paper- is left out. 2- Results and discussion Figure 1b is confusing. I understand it was supposed to represent the energetic barriers to form cubic versus hexagonal diamond, but it mixes physical concepts and experimental schematics in the same figure. The authors state “Obviously, the structural morphology of FLG on Re surface remained observable, even quenched to ambient condition from 30.3 GPa and 2452 K as highlighted by the dashed profiles.”. But the few-layer samples cannot be distinguished even before compression, because of the use of the Re substrate. For Fig. 1h, what was the criteria used to claim the formation of diamond and metastable phases? This is never mentioned in the text. Furthermore, the figure is confusing since it groups different symbols representing flakes of random thicknesses such as hundreds of nm, 20 ~ 40 nm, ~ 12.1 nm, which can hardly be considered as 2D, and it groups flakes 2-5 layers with a single symbol. In my understanding, the latter should be the focus, since they are on the atomically thin side. In Fig.2a, what was the region that was laser heated, and most importantly, what was the initial flake thickness? From the main text and from Figs. S4 and S5, it seems that the typical recovered flakes have nanometer thickness. Once again, this can hardly be considered as 2D. For the discussion on color centers, the authors state “We speculate that the Si and N impurities maybe induced during the sample preparation process.” Could the authors elaborate on how these Si and N impurities could arise in their process? For the follow up discussions in Figs. 3-5, it seems that the authors used recovered samples with nanometer thickness. Please, specify in the main text, what was the thickness of the samples for each measurement. This changes the whole narrative of the paper. If the recovered samples have nanometer thickness, those should be considered nanodiamonds, not diamane. To me, the only possible evidence of 2D diamond formation is the detection of the T2g peak in 3L and 5L graphene. But I have several follow up questions for these measurements. How did the authors make sure they were measuring regions of 3L and 5L, since in those experiments, it is common for pieces of the sample to break and fall on top of the other once you open the DAC. Since they have no optical contrast, it would be hard to tell them apart. Also, the authors should provide the same structural, elemental and optical characterizations described in Figs. 3-5, for the atomically thin samples, if they want to claim 2D diamond formation. 3- Smaller issues The paper has some confusing sentences that overall diminishes its quality. Below are a few examples. I would recommend revision by a native English speaker or specialist to better communicate the science of their paper. In “It is well known that bulk diamond has exceptional hardness, thermal conductivity and chemical inertness, but its toughness and electrical conductivity are inferior to that of many counterparts”, what do the authors mean by “but its toughness and electrical conductivity are inferior to that of many counterparts”? “The synthesized amorphous carbon, consisted of diamond-like and short-range structural order, was found to exhibit the tunable bandgaps, accompanied by the sacrificial thermal conductivity.” “Despite the tremendous efforts have been invested” “Notably, the absence of D band implies the graphene precursor free of any defects, suggesting our sophisticated preparation process”", "reviewer_3": "The paper demonstrates the experimental synthesis of two-dimensional (2D) diamane by laser-heating few-layer graphene under high-pressure in a diamond anvil cell. The study highlights the importance of silicon-vacancy (SiV⁻) and nitrogen-vacancy (NV⁰) color centers in the quenched diamane for its stability, which exhibit strong photoluminescence properties. Additionally, the study reports an experimental observation of an intermediate rhombohedral phase mediating the transition from hexagonal graphite to cubic diamond using high-resolution STEM. The optical bandgap and thermal stability of diamane are shown to be tunable and dependent on its sp³ structure concentration. The findings certainly contribute to improving the experimental synthesis of novel carbon allotropes and provide insights into the mechanisms governing structural transitions. It is especially commendable that the authors successfully demonstrate the HG – RG – CD and HG – HD correlations from the recovered diamane sample as experimental evidence to the theoretically argued RG-mediated phase transition pathways. However, I should raise some concerns regarding the interpretation of the phase stability and the potentially misleading arguments describing the “metastable” diamond-like carbon. First, I would like to point out that, if there are any short- or long-term stable DLC phases quenched from HPHT conditions, they are all metastable. Remember bulk diamond is metastable at ambient pressure and temperature, so is any other carbon allotrope except graphite. Therefore, identifying “m-DLC” separately from “DLC” seems obviously a mistake. It should be described as transient instead. Secondly, when the authors describe what happened during the aging process, they reasoned “incomplete growth” of the stable diamane at HPHT conditions as to be the cause for the observed back-conversion, which is likely the case. However, if more rigorously adopting the established concept of nucleation and growth process in solid-state phase transition, the aging process should be described in terms of the nucleation and growth of the graphite/graphene/sp2-structure as the thermodynamically stable phase. The stable phase grows because it is thermodynamically favored and kinetically feasible, not because the HPHT phase is incomplete. Again, the “m-DLC” should be described as transient due to the “incomplete” process or “insufficient” condition to complete the HPHT process. Finally, then, why and how the metastable diamane/DLC phases could retain in the quenched sample must be reasoned. In that context, a question of what are the conditions or criteria for the “complete growth” of stable diamane at HPHT condition should be addressed. The authors’ observation across the temperature gradient and the characteristic SiV- and NV0 signatures for “complete” diamane formation apparently suggest that not only the HPHT condition but also the actual process that resulted in the formation of characteristic defects should be critically important. The authors only emphasize the importance of SiV- and NV0 color centers for the potential applications but lacked an insight into the potential importance of those defects to stabilize the sp3-structure, i.e., the role of defects to set the necessary high kinetic-energy barrier for the metastability at ambient conditions. Unfortunately, the current study hasn’t been done in such a way that explicitly controls the defects, but the authors implicitly address that there could be a stage during the sample preparation that allows introduction of those defect-forming substances. This topic may be the authors’ next challenge, but the potential role of defects in stabilizing the sp3-structure needs to be discussed. Other than that, the paper is concise yet delivers valuable experimental results, making it worth considering for publication in Nature Communications. I recommend accepting the manuscript after minor revisions to address the above concerns. The followings are minor wording suggestions and questions: • In Abstract, line 24, “to bulk diamond” should be “compared to bulk diamond”. • In Abstract, words like “sharp” and “strong” are qualitative and relative expressions. Make them quantitative and specific as possible. • Line 104-105, “suggesting our sophisticated preparation process” would be better clarified with “suggesting sophistication of our preparation process”. • Line 165, “along the phase boundary” should be “across the phase boundary”? • Line 182, on the temperature gradient as expressed in Fig. 2a, would it be possible to explicitly point out where the HG/RG/CD and HG/HD hybrid interfaces were observed? • In Fig. 2, Line 194, the description needs more elaboration for why the boundaries between recovered sample and PTM are different in d (diamond Raman) and e (SiV- emission) from the same measurement area in c. • Line 209, is it specifically Σ3(211) or can it be expressed as family Σ3{211}? • In SI, line 50, the sample transfer procedures need more elaboration. It points to previous publications, in which the detailed processes are again referenced to other publications. In the SI document, the essential details can be repeated while citing the previous publications."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "All my comments have been addressed, and I am satisfied with the edits to the main text. I think the manuscript can be accepted.", "reviewer_2": "Dear editor and authors, please, find below my review for the manuscript entitled \"Experimental Demonstration and Transformation Mechanism of Quenchable Two-Dimensional Diamond\" re-submitted for publication in Nature Communications by Jiayin Li, and co-workers. I would like to thank the authors for putting effort into addressing some of the points I raised, such as (i) a more thorough literature review on experimental and theoretical efforts on 2D diamond synthesis and (ii) clarifications on the precise criteria used to claim the formation of diamond and metastable phases. However, my main concern has not been addressed, which is the fact that most of the recovered samples have several nanometer thickness, therefore could hardly be considered 2D diamond. More importantly, for the few samples that are indeed on the 2D scale, such as those obtained from 3- and 5- layer graphene, the only evidence the authors provide is from Raman spectroscopy, which is typically not enough to claim 2D diamond synthesis. This is certainly not a minor issue, and I will digress why. Previously, I mentioned that the authors should reframe their work in terms of synthesis of nanodiamonds instead of 2D diamonds. They argued that nanodiamonds are defined as diamond grain or particles with diameters ranging from ~1 to 100 nanometers, and since their samples have lateral dimensions reaching several tens of micrometers, significantly exceeding their thickness, they do not meet the criteria for nanodiamonds. Sure, they may not fit the conventional criteria for nanodiamonds, but most of their samples certainly do not meet the criteria for 2D materials either. Lateral dimensions exceeding thickness is typically the definition of thin films, which is what most of their recovered samples are. In that regard, the authors stated that “Based on these literature data, we can see that the precursors thicknesses range from bilayer to 94 nm and the synthesized 2D diamond can hardly be quenched to ambient conditions (Table R2).” to claim that they can refer to their samples as 2D diamonds. However, from the 7 papers listed in Table R2, the only one that focused on graphene samples with several nm-scale thickness is the one from V. Rajaji, et al., Carbon, 239 (2025), 120267, all the others focused on few-layer graphene samples with thickness up to 5 layers. The authors then argued that they “have supplemented the manuscript with additional data from FLG precursors with thickness within 10 layers and systematically reorganized the results in Figure R5 and Table R1”. By comparing their old Table S1 with their new Table R1, it is hard to tell which are the new data because they did not highlight them. It seems they have new data from 3-,4- and 10-L precursors but they only reported the PT conditions that those samples were synthesized without showing actual Raman, photoluminescence, high-resolution transmission electron microscopy and electron energy loss spectroscopy to support this claim. Furthermore, the authors did not even include this extra data in the new SI - check their Supplementary Table 1. In short, the criteria used by the authors to assess the formation of diamond and metastable phases, which is according to them ”through characteristic Raman spectra, intense photoluminescence from unique color centers (silicon-vacancy and nitrogen-vacancy), and the sp3-bonded microstructure”, was only applied to samples with several nanometer thickness. The only evidence for 2D diamond formation (which would be from the 3 and 5- layer precursors) comes from Raman spectroscopy, shown in Fig. 1h. In their own words the authors stated that “the initial flake in Figure 2a has thickness on the order of hundreds of nanometers” and that “Samples for microstructural characterization (Figures 3 and 4) and property measurements (Figure 5) exhibit thickness on the hundred-nanometer scale, which have been annotated in the revised manuscript.” They also state that “In this study, we primarily employed Raman spectroscopy to identify the formation of atomically thin samples. The structure and property investigations of atomically thin diamond remain challenging, and we are actively addressing these issues.” Thus, I believe the authors recognize that (1) most of their data and evidence is from samples with several nanometer thickness and that (2) structural characterization for atomically thin samples remain challenging. I could not agree more with the latter statement and that is precisely why I cannot agree that the authors successfully synthesized 2D diamond. It is extremely challenging to synthesize and characterize atomically thin 2D diamond samples, that is why there are only a few reports on this topic (see their Table R2). For instance, let us consider the work of Bakharev, Pavel V., et al. Nature nanotechnology 15.1 (2020): 59-66. The authors convincingly showed the synthesis of atomically thin 2D diamond via fluorination of bilayer graphene by employing several techniques such UV photoelectron spectroscopy, Raman, UV-Vis, High- resolution cross- sectional transmission electron microscopy micrograph and EELS. I want to stress that this is not merely a matter of semantics: the physics, kinetics, and chemistry differ entirely when comparing the synthesis of an atomically thin diamond to that of a nanometer-thick one. For example, the thinner the sample, the greater the influence of surface effects and the stronger the need for chemical groups to stabilize the entire structure. In contrast, thicker samples are generally easier to stabilize and synthesize, as they typically do not require functionalization. Overall, I do believe the authors have successfully shown the synthesis of diamond thin films, which is an interesting topic on its own, but not enough to grant publication in Nat Communications in my opinion. I still believe the article falls short on the claim of 2D diamond synthesis, and I cannot recommend it for publication.", "reviewer_3": "The authors addressed all major and minor concerns raised in the first review; the qualtity of manuscript has been substantially improved as the result. I recommend the revised manuscript to be published in Nature Communications. Significant amount of information and discussions which will be useful for other researchers as well as for general readers are placed in the Supplementary Information. I wish those are well exposed to the public together with the main text. The section title \"RESULTS\" would better be \"RESULTS AND DISCUSSIONS\", and \"DISCUSSION\" should be either \"CONCLUSION\" or \"SUMMARY\"."}, "author_response": "We sincerely thank the three reviewers and the editor for giving us the opportunity to revise our manuscript and the constructive comments on our work. We are pleased that the previous revision of our manuscript has addressed most of the reviewers’ concerns, and the quality of our manuscript has been significantly improved. We are delighted that reviewers #1 and #3 fully support the publication of our work at present. We also thank reviewer# 3 for raising the formatting concern and we have addressed it accordingly. Regarding the additional concerns raised by reviewer #2, we have performed new significant experiments and analyses as requested, including Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). We believe these new data can comprehensively resolve the reviewer’s main concerns and further significantly strengthen the quality of our work. Please check the point-by-point responses to the comments below. For clarity, reviewers’ comments are marked in black and our responses in blue. Revisions in the improved manuscript are marked in red in the main text and Supplementary Information as required by editors, but not included in this response letter for brevity. R1 Reviewer #1 (Remarks to the Author): All my comments have been addressed, and I am satisfied with the edits to the main text. I think the manuscript can be accepted. Reply: We sincerely appreciate that the Reviewer #1 have agreed the acceptance of our manuscript for the final publication. Thank the Reviewer #1 very much! R2 Reviewer #2 (Remarks to the Author): Dear editor and authors, please, find below my review for the manuscript entitled \"Experimental Demonstration and Transformation Mechanism of Quenchable Two-Dimensional Diamond\" re-submitted for publication in Nature Communications by Jiayin Li, and co-workers. I would like to thank the authors for putting effort into addressing some of the points I raised, such as (i) a more thorough literature review on experimental and theoretical efforts on 2D diamond synthesis and (ii) clarifications on the precise criteria used to claim the formation of diamond and metastable phases. However, my main concern has not been addressed, which is the fact that most of the recovered samples have several nanometer thickness, therefore could hardly be considered 2D diamond. More importantly, for the few samples that are indeed on the 2D scale, such as those obtained from 3- and 5- layer graphene, the only evidence the authors provide is from Raman spectroscopy, which is typically not enough to claim 2D diamond synthesis. This is certainly not a minor issue, and I will digress why. Reply: We thank Reviewer #2 for the careful review of our manuscript. We appreciate that Reviewer #2 recognized the improvements made in our previous revision, particularly regarding the thorough literature review on two-dimensional (2D) diamond synthesis and the well-defined criteria for diamond and metastable phases. We greatly appreciate Reviewer #2’s acknowledgment that our 3- and 5-layer samples qualify as 2D samples, as the comment “…for the few samples that are indeed on the 2D scale, such as those obtained from 3- and 5- layer graphene…”. We also supplemented the manuscript with additional data from few-layer graphene (FLG) precursors with thickness within 10 layers in the previous revision. In this revision, we have supplemented our work with many new experiments and data to further confirm the successful synthesis of 2D diamond. The synthesis conditions and characterization methods for the 2D diamond are summarized in Table R1, and the new experiments and data related to the original submission are highlighted in bold font. R3 Table R1. Synthesis conditions and characterization methods for the quenched 2D diamond/metastable phase. Error bars indicate the inevitably fluctuated temperature during lase-heating process for each sample. No. Precursor P/T conditions Characterization method #1 2L 31.4 GPa/1556-2020 K OM; Raman; PL #2 2L 32.8 GPa/3005-3481 K OM; Raman; PL; SEM; AFM; XPS; AES #3 2L 32.8 GPa/1772-2216 K OM; Raman; PL; SEM; AFM; XPS; AES #4 3L 29.7 GPa/2301-2604 K OM; Raman; PL #5 3L 32.3 GPa/2706-3142 K OM; Raman; PL; EDS; HRTEM #6 3L 30.2 GPa/2090-2474 K OM; Raman; PL; SEM #7 3L 30.1 GPa/2126-2229 K OM; Raman; PL; SEM; XPS #8 4L 27.5 GPa/2488-2712 K OM; Raman; PL #9 5L 29.7 GPa/2301-2604 K OM; Raman; PL OM; Raman; PL; SEM; AFM; EDS; #10 6L 30.5 GPa/2370-2440 K HRTEM #11 7L 30.5 GPa/1923-2010 K OM; Raman; PL; SEM; AFM #12 7L 30.3 GPa/2145-2501 K OM; Raman; PL; SEM #13 8L 30.3 GPa/2064-2355 K OM; Raman; PL; SEM #14 9L 30.2 GPa/1907-2266 K OM; Raman; PL; SEM #15 10L 27.5 GPa/2310-2996 K OM; Raman; PL #16 10L 30.3 GPa/2292-2375 K OM; Raman; PL; SEM #17 11L 30.1 GPa/1992-2231 K OM; Raman; PL; SEM; XPS #18 13L 30.2 GPa/1937-2337 K OM; Raman; PL; SEM #19 ~ 18L 32.8 GPa/2862-3156 K OM; Raman; PL; SEM; AFM; XPS; AES #20 ~ 25L 30.2 GPa/2482-2698 K OM; Raman; PL; SEM #21 12.1 nm 23.8 GPa/1775-2040 K OM; Raman; PL #22 12.1 nm 23.8 GPa/2016-2317 K OM; Raman; PL #23 15.4 nm 30.5 GPa/1934-2309 K OM; Raman; PL; SEM; AFM; XPS #24 15.4 nm 30.5 GPa/3153-3346 K OM; Raman; PL; SEM; AFM; XPS OM; Raman; PL; SEM; XPS; AES; EDS; #25 23.0 nm 29.3 GPa/2990-3228 K HRTEM #26 20.2 nm 28.5 GPa/2864-3026 K OM; Raman; PL; SEM R4 During the last three months, we have devoted substantial efforts to supplement our work with new graphene precursors (especially focusing on few-layer graphene), laser-heating treatments, and structural and properties measurements of 2D diamond products. More importantly, as pointed out by the reviewer, we have explored multiple characterization techniques to conclusively demonstrate the synthesis of 2D diamond, including Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Raman spectroscopy revealed the characteristic diamond Raman peak after high pressure and high temperature (HPHT), thus providing preliminary confirmation of the 2D diamond synthesis. AFM and SEM characterizations were used to resolve the sample’s morphology, with AFM additionally providing measurement of its thickness. Furthermore, EDS and HRTEM characterizations determined the thickness and uniformity of the quenched 2D diamond precisely. Moreover, the transition from sp2 to sp3 hybridization was verified by XPS and AES analysis. Below, we provide a detailed response to Reviewer #2’s comments, along with the revisions made to the manuscript, and we hope these would meet your expectations. Previously, I mentioned that the authors should reframe their work in terms of synthesis of nanodiamonds instead of 2D diamonds. They argued that nanodiamonds are defined as diamond grain or particles with diameters ranging from ~1 to 100 nanometers, and since their samples have lateral dimensions reaching several tens of micrometers, significantly exceeding their thickness, they do not meet the criteria for nanodiamonds. Sure, they may not fit the conventional criteria for nanodiamonds, but most of their samples certainly do not meet the criteria for 2D materials either. Lateral dimensions exceeding thickness is typically the definition of thin films, which is what most of their recovered samples are. In that regard, the authors stated that “Based on these literature data, we can see that the precursors thicknesses range from bilayer to 94 nm and the synthesized 2D diamond can hardly be quenched to ambient conditions (Table R2).” to claim that they can refer to their R5 samples as 2D diamonds. However, from the 7 papers listed in Table R2, the only one that focused on graphene samples with several nm-scale thickness is the one from V. Rajaji, et al., Carbon, 239 (2025), 120267, all the others focused on few-layer graphene samples with thickness up to 5 layers. Reply: We thank Reviewer #2 for the careful review of our manuscript. We appreciate Reviewer #2 for acknowledging that our samples do not conform to the conventional definition of nanodiamonds. Accordingly, we have also reframed the “2D diamane” and “diamane” in terms of “2D diamond” in our previous revision to provide a more accurate description. Currently, in the literature and research community of diamond (-like) materials, there is no clear distinction between 2D materials and thin films for diamond structures. For example, the investigation of unique electrical properties in monolayer graphene by K. S. Novoselov et al. represents a seminal milestone in the field of 2D materials [Ref.: K. S. Novoselov, et al., Electric Field Effect in Atomically Thin Carbon Films, Science, 306 (2004), 666-669]. In their work, they refer to both monolayer graphene and FLG up to 10 nm as “carbon film” and “2D material”. As they stated, […We report the observation of the electric field effect in a naturally occurring two-dimensional (2D) material referred to as few-layer graphene (FLG)…We have studied more than 60 devices with d < 10 nm]. Additionally, Y. Gao et al. also refer to the 2L graphene precursor and the diamond-like product under compression as “film”, as in their work entitled “Ultrahard carbon film from epitaxial two-layer graphene” [Y. Gao, et al., Nature Nanotechnol., 13 (2018), 133-138]. In our study, while all our synthesized samples are characterized by lateral dimensions that significantly exceed their thickness, it is crucial to note that the final thickness of prepared diamond products quantitatively depends on the layer number of graphene precursors. As shown in Table R1 (Sample No. #1-#9) and Figure 1c-i in our revised manuscript, we can successfully obtain 2D diamond by using graphene precursors within five layers, which can fully satisfy the definition of 2D diamond. More importantly, the novelty of our work lies not only in the thickness of few-layer precursors, but also in the successful quenching of the ultrathin diamond/metastable R6 phase to ambient conditions for further studies, as exemplified by the 3-layer sample in the revised Figure 1c-i, and the 2- to 25-layer (even thicker) samples presented in Supplementary Figure 1a-c, Supplementary Table 1 and Table R1. We believe this represents a critical breakthrough in the synthesis of 2D diamond and paves the way for future in-depth investigations into its structure and properties. As shown in Figure R1, many previous studies primarily report reversible phase transitions between FLG and 2D diamond under pressure, while we have achieved successful quenching of the 2D diamond phase to ambient pressure and temperature. V. Rajaji et al. identified the phase transition to diamond in FLG with thickness of 15 ~ 100 nm by observing slope changes in the pressure-dependent Raman G-band [V. Rajaji, et al., Carbon, 239 (2025), 120267]. Their work revealed that under the combined effects of chemical functionalization and biaxial strain, FLG undergoes a reversible transition to diamond at ~7 GPa, as shown in Figure R1a. A. P. M. Barboza et al. used electric force microscopy (EFM) to investigate the reversible phase transition from FLG (bilayer to 7.45 nm) to 2D diamondol under compression [A. P. M. Barboza, et al., Adv. Mater., 23 (2011), 3014-3017] (Figure R1b). F. Ke et al. employed optical absorption, electrical transport measurements, and simulations to investigate the thickness-dependent reversible phase transition conditions from 3- to 12-layer FLG to 2D diamond [F. Ke, et al., Nano Lett., 20 (2020), 5916-5921], as shown in Figure R1c. Notably, all the studies mentioned above have been confirmed to undergo reversible phase transitions from FLG to 2D diamond. In contrast, we have irreversibly converted FLG of varying layer numbers into 2D diamond with corresponding thickness under HPHT. As shown in Figure R1d, the 2D diamond quenched from 3L graphene exhibits an intense diamond Raman signature. R7 a b V. Rajaji, et al.,Carbon, 2025, A. Barboza, et al., Adv. Mater., 2011 Reversible transition Reversible transition 1640 h = 94 nm max 1 ) 50 -m hift ( c1620 ( H ) z 40 n s | Δ ω m a 1600 | 30 a R h = 7.45 nm max 20 1580 0 5 10 15 20 0 200 400 600 Pressure (GPa) F/nN c F. Ke, et al.,Nano Lett.,2020 d J. Li, et al., this work Reversible transition Irreversible and Quenchable T 2g Diamond Quenched ) u. a. T ( Quenched g y -l o n sit G e Graphene I nt 3L/Re 3L/SiO x 0 10 20 30 40 50 1300 1480 1660 1840 Pressure (GPa) Raman shift (cm-1) Figure R1. Comparison of our 2D diamond with previous studies. a, Pressure evolution of the G band Raman spectra of FLG samples (15 ~ 100 nm) for the unprotected and fully covered with hBN regions deposited on SiO /Si and diamond substrates. b, Graph of the 2 EFM response frequency shift, Δω, as a function of the applied force on the few-layer graphene (h = 7.45 nm). c, Pressure dependence of the absorbance of tri-, hexa-, 12-, max and multilayer graphene at a photon energy of 2.0 eV. d, Raman spectra showing the phase evolution from initial 3L graphene (exfoliation and after transferring) to the quenched 2D diamond synthesized after HPHT, identified by the characteristic T band. 2g R8 Moreover, in response to the Reviewer #2’s comments on sample thickness, we have taken many further steps to supplement our work with additional and detailed data for the phase-transformation products derived from 2-, 3-, 4-, 6-, 7-, 8-, 9-, 10-, 11-, 13-, 18- and 25-layer graphene under high pressure and high temperature, as shown in Figures R2-R10. We believe the new data can satisfy the specific concerns raised by Reviewer #2: 1) We have successfully synthesized 2D diamond by using graphene precursors within five layers, which can fully satisfy the definition of 2D diamond (as shown in sample No. #1-#9 in Table R1 and Figure 1c-i in the revised manuscript). 2) We have explored multiple characterization techniques to conclusively demonstrate the synthesis of 2D diamond, including Raman spectroscopy, PL, AFM, SEM, EDS, HRTEM, XPS and AES. Our results indicate that 2D diamond with three carbon layers (~1.0 nm in thickness) has been synthesized with characteristic diamond Raman peak and intense PL from silicon-vacancy (SiV-) color centers. Additionally, AFM characterization provided approximate thickness measurements of ~ 5.6 nm for the 6-layer and ~ 1.5 nm for the 2-layer 2D diamond. Furthermore, AES results serve as direct evidence for the sp2-to-sp3 transition, confirming successful 2D diamond synthesis under HPHT. The authors then argued that they “have supplemented the manuscript with additional data from FLG precursors with thickness within 10 layers and systematically reorganized the results in Figure R5 and Table R1”. By comparing their old Table S1 with their new Table R1, it is hard to tell which are the new data because they did not highlight them. It seems they have new data from 3-,4- and 10-L precursors but they only reported the PT conditions that those samples were synthesized without showing actual Raman, photoluminescence, high-resolution transmission electron microscopy and electron energy loss spectroscopy to support this claim. Furthermore, the authors did not even include this extra data in the new SI - check their Supplementary Table 1. Reply: Thank Reviewer #2 very much for the constructive comments. We appreciate R9 Reviewer #2 for raising concern regarding the thickness and characterization of recovered samples in our work. We recognize the oversight in the previous version of Table R1, where the newly added data points were not highlighted from the original ones, as expected by the reviewer. We have now rectified this in the updated Table R2 provided in this revision, where new data points added relative to the original submission are marked in bold font. Furthermore, we have updated Supplementary Table 1 to include all new data points. Obviously, we have provided additional data points for quenched 2D diamond and metastable phase with thicknesses within 5 layers, which fully addresses the reviewer’s definition of 2D diamond. Table R2. Ultrathin diamond synthesized under varied pressure and temperature conditions in this study. Error bars indicate the inevitably fluctuated temperature during lase-heating process for each sample. No. Precursor thickness P/T conditions Recovered sample #1 2L 32.8 GPa/3005-3481 K #2 3L 29.7 GPa/2301-2604 K #3 3L 32.3 GPa/2706-3142 K #4 5L 29.7 GPa/2301-2604 K #5 ~ 18L 32.8 GPa/2862-3156 K #6 ~ 25L 30.2 GPa/2482-2698 K #7 15.4 nm 30.5 GPa/3153-3346 K #8 23.0 nm 29.3 GPa/2990-3228 K Diamond #9 20.2 nm 28.5 GPa/2864-3026 K #10 16.3 GPa/3511-3765 K #11 20.3 GPa/1506-2069 K #12 20.4 GPa/3149-3351 K #13 ~ 24-40 nm 21.9 GPa/1568-2140 K #14 21.9 GPa/1795-2295 K #15 24.0 GPa/2025-2251 K #16 25.8 GPa/1958-2219 K R10 No. Precursor thickness P/T conditions Recovered sample #17 16.5 GPa/1420-1846 K #18 16.7 GPa/1275-1431 K #19 16.9 GPa/1383-1473 K #20 18.4 GPa/1632-1829 K #21 19.6 GPa/1540-1650 K ~ several hundred nanometers #22 20.0 GPa/1320-1550 K #23 20.6 GPa/1252-1595 K #24 20.7 GPa/1389-1696 K #25 22.9 GPa/1434-1660 K #26 30.7 GPa/1699-1952 K #27 2L 31.4 GPa/1556-2020 K #28 2L 32.8 GPa/1772-2216 K #29 3L 30.2 GPa/2090-2474 K #30 3L 30.1 GPa/2126-2229 K #31 4L 27.5 GPa/2488-2712 K #32 6L 30.5 GPa/2370-2440 K #33 7L 30.5 GPa/1923-2010 K #34 7L 30.3 GPa/2145-2501 K #35 8L 30.3 GPa/2064-2355 K #36 9L 30.2 GPa/1907-2266 K #37 10L 27.5 GPa/2310-2996 K Metastable phase #38 10L 30.3 GPa/2292-2375 K #39 11L 30.1 GPa/1992-2231 K #40 13L 30.2 GPa/1937-2337 K #41 12.1 nm 23.8 GPa/1775-2040 K #42 12.1 nm 23.8 GPa/2016-2317 K #43 15.4 nm 30.5 GPa/1934-2309 K #44 20.2 GPa/1466-1830 K ~ 20-40 nm #45 21.9 GPa/1577-2076 K #46 18.7 GPa/1572-1682 K ~ several hundred #47 20.7 GPa/1271-1497 K nanometers #48 30.4 GPa/1335-1482 K R11 We thank Reviewer #2 for the valuable comments regarding the characterization of the quenched samples. Despite the significant challenges in characterizing the structure and properties of atomically thin 2D diamond, we are actively devoted to addressing these issues and achieved some new advancements. We have explored multiple characterization techniques to conclusively demonstrate the synthesis of 2-, 3-, 6-, and 18-layer 2D diamond, including Raman spectroscopy, PL, AFM, SEM, EDS, HRTEM, XPS and AES: 1) Raman and PL results demonstrate the characteristic diamond Raman peak and intense SiV- photoluminescence of the quenched 2D diamond. 2) SEM, EDS, HRTEM, and AFM reveal the morphology and ultrathin thickness of the 2-, 3-, 6- and 18-layer 2D diamond. EDS and HRTEM results confirm a uniform thickness of ~ 1.0 nm for the 3L diamond. 3) AFM characterization provides approximate thickness measurements of ~ 5.6 nm for the 6-layer and ~ 1.5 nm for the 2-layer 2D diamond. 4) AES results serve as direct evidence for the sp2-to-sp3 transition, confirming successful 2D diamond synthesis under HPHT. New data from 2-, 3-, 6-, and 18-layer graphene transformed into 2D diamond are provided below. We have synthesized 2D diamond from trilayer graphene under HPHT, and further investigated the morphology and properties of the quenched sample. In Figure R2, the new FLG sample with contaminant-free and uniform surface was mechanically exfoliated onto Si substrate and then transferred to polished Re substrate via a propylene carbonate (PPC)-assisted wet transfer process mentioned in Supplementary Information. The optical images of FLG in the transfer process are shown in Figure R2a-c. The layer number of this sample was accurately determined to be 3-layer on Si substrate, as identified by the Raman Si-peak intensities from graphene domains and Si substrate, and the derived ratio is correlated with the corresponding layer number [the layer numbers identification method is described in Y. S. No, et al., Sci. Rep., 8 (2018), 571; X. L. Li, et al., Nanoscale, 7 (2015), 8135], as shown in Figure R2d-e. Raman spectra of the 3L graphene, which were acquired following its exfoliation on a Si substrate and transfer onto Re surface, are shown in Figure R2f. The R12 absence of D band indicates that no defect was introduced during the sample preparation. a b c FLG/SiO FLG/PPC FLG/Re x 3L 10 μm 10 μm 20 μm d e f 2400 1.0 3L gra. EXP. X. Li et al. G 2D on Si Fitting This work 1200 sit y ( a. u.) 2400 0 Bare Si EXP. I ( S S i i ) ) / 0 0.8 3L n sit y ( a. u.) 3L gra./Re I nt e n 1200 Fitting I (G0.6 I nt e 3L gra./SiO x 0 0.4 480 500 520 540 560 0 3 6 9 1350 1800 2250 2700 Raman shift (cm-1) Layer number (N) Raman shift (cm-1) Figure R2. Layer-number identification and Raman measurements of 3L graphene for HPHT treatment. a-c, Optical images of the FLG during transfer process. The FLG exfoliated onto Si substrate (a) followed by PPC-mediated (b) and transferred to the Re substrate (c). d, Raman spectra of the Si-peak (~520 cm-1) depending on the layer number of FLG and Raman spectra of silicon substrate. The layer number is determined to be 3 layers. e, The I (Si)/I (Si) value as a function of layer number. The objective NA is 0.50 G 0 and the excitation wavelengths is 532 nm. The black square data are taken from the literature [X. L. Li, et al., Nanoscale, 7 (2015), 8135]. f, Raman spectra of 3L precursor tracked after exfoliating on Si substrate and then transferring onto Re surface. No detectable D band confirms the defect-free feature of the sample. The trilayer graphene sample was subsequently loaded into diamond anvil cell (DAC), compressed to 32.3 GPa, and laser-heated to approximately 2924 K, as shown in Figure R3a-c. The trilayer graphene exhibits poor optical contrast in DAC due to its intrinsic low R13 optical absorption (~2.3% for monolayer graphene) [R. R. Nair, et al., Science, 320 (2008), 1308], making morphological identification challenging. Nevertheless, since the 3-layer graphene can be optically distinguished from the Re substrates before HPHT treatment, the position of 3-layer graphene on the Re substrate can be precisely determined by carefully comparing the relative position of FLG precursor on the Re substrate before and after HPHT. As demonstrated in our previous response letter, the few-layer graphene maintains its structural integrity without tearing or overlapping after decompression, and its position relative to the Re substrate is preserved post-HPHT. Therefore, comparing optical images allows for the precise determination of the quenched sample’s location (Figure R3d). We then performed a systematic Raman and PL characterization on the quenched atomically thin sample. The sample quenched from 32.3 GPa and 2924 K exhibits the evident Raman T peak (Figure R3e), which is uniquely originated from the sp3-bonded 2g lattice structure (i.e. diamond structure). In addition, the nearly vanished G and D bands, in contrast to the sharp G peak of the initial sample (Figure R2f), signify an almost complete transformation into 2D diamond under HPHT. The Raman spectra in Figure R3e correspond to three different positions marked as point 01 (red), point 02 (pink) and point 03 (violet) in Figure R3d. Moreover, the quenched 2D diamond shows strong PL from the unique SiV- color centers. The PL spectra in Figure R3f were acquired from the same locations as the Raman spectra, which are marked in Figure R3d. The intense peak located at ~737 nm is attributed to the zero-phonon lines of SiV-, and the other broad spectral features may correspond to the defect-related emissions in the sample. The Raman and PL results provide strong evidence for the successful synthesis of 2D diamond. R14 a b c 0.5 GPa, 300 K 32.0 GPa, 300 K 32.3 GPa, 2924 K 30 μm 30 μm 30 μm f e d Quenched -V T Quenched Ambient (quenched) u.) iS u.) D 2 ia g mond 03 03 01 a. a. y ( 03 y ( sit sit 02 02 n n e e nt 02 nt I I 01 01 30 μm 450 600 750 900 1200 1400 1600 1800 PL wavelength (nm) Raman shift (cm-1) Figure R3. Experimental synthesis of 2D diamond from 3L graphene under HPHT. a-d, Optical evolution of 3L graphene on Re substrate under HPHT. The 3L graphene was compressed to 0.5 GPa at 300 K (a); Graphene/Re compressed to 32.0 GPa at 300 K (b); Graphene/Re heated to about 2924 K under 32.3 GPa (c); The sample quenched to ambient condition finally (d). Scale bars in a-d are 30 μm. The yellow dashed line highlights the morphology of 3L graphene. e, Raman spectra of the quenched 2D diamond after HPHT with the significant T band. f, Typical PL spectra of 2D diamond with the remarkable 2g SiV- (737 nm) color centers. The spectra in e, f correspond to three different positions marked as point 01 (red), point 02 (pink) and point 03 (violet) in d. The excitation wavelength for Raman and PL measurements is 405 nm. Furthermore, we have devoted considerable efforts to investigate the structure of 2D diamond. First, the sample location for HRTEM characterization was determined by SEM. To protect the sample from electron beam damage, a gold layer was deposited onto the sample surface. Subsequently, the sample was shaped into ultrathin slice with a thickness of 60-70 nm by focused ion beam (FIB), with an additional amorphous carbon coating R15 applied to protect the sample from ion beam damage. The HRTEM and EDS results are shown in Figure R4. We have successfully demonstrated the ultrathin nature of the quenched 2D diamond through HRTEM and EDS. As shown in Figure R4a-b, the HRTEM images of the 2D diamond reveal a uniform and relatively thin thickness of approximately 1.0 nm, which corresponds closely to the theoretical thickness (9 Å) of a double-sided hydrogenated trilayer diamane [Samir El Masaoudi, et al., J. Phys.: Condens. Matter, 37(2015), 445701]. The slight discrepancy is likely due to the inherent uncertainty in thickness measurement from HRTEM images. Most theoretical studies on 2D diamond have primarily focused on interlayer/intralayer C-C bond length, formation energy, and electrical bandgap, whereas the relationship between its thickness and the number of layers remains elusive. Qiu et al. focused specifically on 2D (110) diamond and established relationships among bandgap, layer number, and thickness for hydrogenated 2D diamond [D. C. Qiu, et al., Diam. Relat. Mat., 84(2018), 55-61]. Most theoretical studies on the phase transition mechanism demonstrate that (001) graphite under HPHT results in the formation of either (111) cubic diamond or (100) hexagonal diamond [Khaliullin R., et al., Nature Mater., 10 (2011), 693-697; Yao-Ping Xie, et al., J. Am. Chem. Soc., 139(2017), 2545-2548]. Consequently, we consider that the conclusions from Qiu et al. are not applicable to our case. While Li et al. calculated the layer-number dependence of interplanar spacing and buckling parameter for 6- to 14-layer 2D (111)-oriented diamond, their study did not extend to systems below 5 layers [H. D. Li, et al., Chem. Phys. Lett., 550(2012), 130-133]. Although P. V. Bakharev et al. reported the interlayer spacing and thickness of F-diamane, these values are strongly affected by fluorination, making their conclusions less relevant and not applicable to our work [P. V. Bakharev, et al., Nature Nanotechnol., 15 (2020), 59-66]. Samir El Masaoudi, et al. calculated the overall thickness of diamane formed via the hydrogenation of trilayer graphene. They reported a distance of 9 Å between the outer hydrogen atoms on the top and bottom surfaces in the -H terminated structure [Samir El Masaoudi, et al., J. Phys.: Condens. Matter, 37(2015), 445701]. Therefore, our work is primarily compared with that of Samir El Masaoudi et al., and the thickness of 3L 2D diamond closely matches their computational results. R16 a c Au Amorphous carbon Au 10 nm C d Re C 20 nm b 10 nm e Au Re C 1.0 nm Re 5 nm 10 nm Figure R4. The spatial distribution carbon layers and HRTEM of 2D diamond transformed from 3L graphene. a-b, The cross-sectional HRTEM images of 2D diamond. Green dashed lines in b highlight the extent of the 2D diamond, indicating a thickness of 1.0 nm. c-e, EDS elemental mapping images of the 2D diamond cross-section for Au (c), C (d) and Re (e) elements. The EDS elemental mapping was performed over the sample region as shown in Figure R4c-e. The EDS results reveal the spatial distribution of carbon, showing a continuous and uniform carbon layer sandwiched between the gold coating and the Re substrate. We then attempted to perform high-resolution structural characterization on the 2D diamond. However, the Kikuchi patterns generated by the atomically thin 2D diamond are too weak to be used for crystallographic alignment. This makes it impossible to tilt the sample to a precise zone axis. As a result, achieving high resolution atomic-scale imaging of the 2D diamond lattice remains challenging. R17 a b c 7L 10 μm 20 μm 20 μm FLG/SiO FLG/PPC FLG/Re x d e f 4000 6L EXP. 4000 7L EXP. 4000 7L EXP. Fitting Fitting Fitting u.) 2000 u.) 2000 u.) 2000 sit y ( a. 6000 0 Si EXP. sit y ( a. 6000 0 Si EXP. sit y ( a. 6000 0 Si EXP. n Fitting n Fitting n Fitting e e e nt 3000 nt 3000 nt 3000 I I I 0 0 0 480 500 520 540 560 480 500 520 540 560 480 500 520 540 560 Raman shift (cm-1) Raman shift (cm-1) Raman shift (cm-1) g h i 2400 0.8 > 15L Exp. X. Li et al. G 2D Fitting > 15L/Re sit y ( a. u.) 1 7 2 0 0 0 0 0 0 Si Exp. I ( S S i i ) ) / 0 0.6 6L 7L This work n sit y ( a. u.) > 1 7 7 6 5 L L L L / / / / R R R Si e e e O x nt e n 3500 Fitting I (G0.4 I nt e 7 7 L L / / S S i i O O x I x > 15L 6L/SiO 0 x 0.2 480 500 520 540 560 3 6 9 12 15 b1u8lk 1300 1800 2300 2800 Raman shift (cm-1) Layer number (N) Raman shift (cm-1) Figure R5. Layer-number identification and Raman measurements of 6L, 7L and multilayer graphene. a-c, Optical images of the FLG during transfer process. The FLG exfoliated onto Si substrate (a) followed by PPC-mediated (b) and transferred to the Re substrate (c). d-g, Raman spectra of the Si-peak (~520 cm-1) depending on the layer numbers of FLG and Raman spectra of silicon substrate. The layer numbers are determined to be 6 (d), 7 (e), 7 (f), > 15 (g) layers. The layer-number identification of 7L graphene in f corresponds to the region marked by the blue dashed line in a-c. h, The I (Si)/I (Si) value G 0 as a function of layer number. The objective NA is 0.50 and the excitation wavelengths is 532 nm. The black square data are taken from the literature [X. L. Li, et al., Nanoscale, 7 (2015), 8135]. i, Raman spectra of FLG precursor tracked after exfoliating on Si substrate and then transferring onto Re surface. No detectable D band confirms the defect-free feature of the sample. R18 Moreover, we have carried out additional SEM and AFM characterizations to investigate the morphology and thickness of 2D diamond. The new FLG sample with contaminant-free and uniform surface was first mechanically exfoliated onto Si substrate and then transferred to polished Re substrate following the same procedure mentioned in Supplementary Information. The optical images of FLG in the transfer process are shown in Figure R5a-c. The layer numbers of this sample were accurately determined to be 6-, 7-, >15-layer (approx. 18-layer) on Si substrate, as identified by the Raman Si-peak intensities from graphene domains and Si substrate, and the derived ratio is correlated with the corresponding layer number [the layer numbers identification method is described in Y. S. No, et al., Sci. Rep., 8 (2018), 571; X. L. Li, et al., Nanoscale, 7 (2015), 8135], as shown in Figure R5d-h. Raman spectra of the FLG graphene, which were acquired following its exfoliation on a Si substrate and transfer onto Re surface, are shown in Figure R5i. The absence of D band indicates that no defect was introduced during the sample preparation. The FLG sample was subsequently loaded into DAC, compressed to 30.5 GPa, and laser-heated to approximately 2405 K (Figure R6a-c). Likewise, the position of FLG on the Re substrate can be precisely determined by carefully comparing the relative position of FLG precursor on the Re substrate before and after HPHT. Therefore, the location of the quenched sample on the Re substrate can be precisely determined, as indicated by the yellow and blue dashed lines in Figure R6d. The region outlined by yellow dashed line quenched from HPHT, while the region outlined by blue dashed line recovered from high pressure and room temperature (HPRT). R19 a b 1.1 GPa, 300 K 30.0 GPa, 300 K 7L Re 30 μm d c Quenched 30.5 GPa, 2405K Figure R6. Experimental synthesis of 2D diamond from 6L, 7L and multilayer graphene under HPHT. a-d, Optical evolution of FLG on Re substrate under HPHT. The 6L, 7L, and >15-layer (approx. 18-layer) graphene were compressed to 1.1 GPa at 300 K (a); Graphene/Re compressed to 30.0 GPa at 300 K (b); Graphene/Re heated to about 2405 K under 30.5 GPa (c); The sample quenched to ambient condition finally (d). Scale bars in a-d are 30 μm. The yellow and blue dashed lines highlight the morphology of FLG sample. To confirm the 2D character of the quenched sample, we employed AFM to determine its thickness. The SEM images of the quenched sample are shown in Figure R7a-c. The profile of the quenched sample on the Re substrate remains visible, as indicated by the yellow arrows in Figure R7b-c. AFM characterization of the magenta-marked region in Figure R7a reveals a well-defined profile (indicated by yellow arrows), and a thickness of approximately 5.6 nm for the quenched 6L diamond (as shown in Figure R7d-e). While the measured thickness is larger than expected due to the significant roughness of the Re substrate, it still can be concluded that the quenched sample persists as an ultrathin 2D film and exhibits a distinct step on the Re substrate. R20 a b c Re 7 L > 6 15L 7 L Sample L d Sample 20 μm 10 μm 10 μm Re d e Recovered sample Sample 5.6 nm Re 3 μm 1μm Figure R7. Morphology and thickness characterizations from 6L, 7L and multilayer graphene of 2D diamond. a, A low-magnification SEM image of quenched sample on Re substrate. The yellow dashed line highlights the morphology of the sample. b-c, High-magnification SEM images of the area selected from red (b) and green (c) solid box in a. d, AFM image of the quenched sample from the marked magenta box in a. e, AFM image of the sample from the marked blue box in d. Height profile along the cyan dashed line in inset indicates a thickness of ~5.6 nm. The yellow arrows in b-e imply the profile of the quenched sample. Additionally, we have performed SEM, AFM and AES characterizations to investigate morphology, thickness and sp2-to-sp3 hybridization transformation of an additional 2D diamond sample. The new FLG sample was mechanically exfoliated onto Si substrate and then transferred to polished Re substrate following the same procedure mentioned in Supplementary Information (as shown in Figure R8a-b). The layer numbers of the sample were accurately determined to be 2-, >15-layer (approx. 18-layer) on Si substrate, as identified by the Raman Si-peak intensities from graphene domains and Si substrate, and the derived ratio is correlated with the corresponding layer number [the layer R21 numbers identification method is described in Y. S. No, et al., Sci. Rep., 8 (2018), 571; X. L. Li, et al., Nanoscale, 7 (2015), 8135], as shown in Figure R8c-e. Besides, the absence of Raman D band indicates that no defect was introduced during the sample preparation (as shown in Figure R8f). a b FLG/SiO FLG/Re x ＞15L 2L 30 μm 30 μm c d 4000 2000 2L EXP. > 15L EXP. Fitting Fitting 2000 1000 u. ) u. ) sit y ( a. 400 0 0 Si EXP. sit y ( a. 6000 0 Si EXP. n Fitting n Fitting e e nt 2000 nt 3000 I I 0 0 480 500 520 540 560 480 500 520 540 560 Raman shift (cm-1) Raman shift (cm-1) e 1.0 f X. Li et al. G 2L 2D This work 0.8 ) > 15L/Re ) u. Si a. I ( Si )/ 0 0.6 n sit y ( 6L/Re I ( G 0.4 nt e > 15L/SiO x I > 15L 6L/SiO x 0.2 0 5 10 15 b1u8lk 1300 1800 2300 2800 Layer number (N) Raman shift (cm-1) Figure R8. Layer-number identification and Raman measurements of 2L and multilayer (>15L) graphene. a, Optical image of the FLG exfoliated on Si substrate with 300 nm oxide layer. b, Optical image of FLG transferred onto the polished Re substrate. The black dashed lines mark the transfer-induced tear locations in the sample. c-d, Raman spectra of the Si-peak (~520 cm-1) depending on the layer numbers of FLG and Raman R22 spectra of silicon substrate. The layer numbers are determined to be 2 (c), > 15 (d) layers. e, The I (Si)/I (Si) value as a function of layer number. The objective NA is 0.50 and the G 0 excitation wavelengths is 532 nm. The black square data are taken from the literature [X. L. Li, et al., Nanoscale, 7 (2015), 8135]. f, Raman spectra of FLG precursor tracked after exfoliating on Si substrate and then transferring onto Re surface. No detectable D band confirms the defect-free feature of the sample. The FLG sample was subsequently loaded into DAC, compressed to 32.8 GPa, and laser-heated to approximately 3243 K, as shown in Figure R9a-c. Likewise, the location of the quenched sample on the Re substrate can be precisely determined by carefully comparing the relative position of FLG precursor on the Re substrate before and after HPHT, as indicated by the yellow dashed line in Figure R9d. Under high-magnification optical microscopy, the quenched 2L sample exhibits optical contrast sufficient to identify its profile on the Re substrate. This contrast is further enhanced under SEM characterization, where the sample profile becomes distinctly clear, as indicated by the yellow arrows in Figure R9e-f. To confirm the two-dimensional character and measure the thickness of the quenched sample, we conducted AFM characterization on the region of the 2L sample that shows a well-defined profile. AFM characterization of the orange-marked region in Figure R9f reveals a well-defined profile, and a thickness of approximately 1.5 nm for the quenched 2L sample (as shown in Figure R9g-h). It can be concluded that the quenched sample persists as a relatively uniform ultrathin film and exhibits a distinct step on the Re substrate, even though the measured thickness is larger than expected due to the rough Re substrate. We then performed Raman characterization on the quenched atomically thin sample. While the sample quenched from HPHT exhibits an intense characteristic diamond Raman peak at approximately 1335 cm-1, the sample recovered from HPRT shows a broadened G band (as shown in Figure R9i). This further demonstrates that high pressure and high temperature conditions are demanded for FLG precursors to completely transform into 2D diamond. R23 a b c 13.3 GPa, 300 K 32.5 GPa, 300 K 32.8 GPa, 3243K ＞15L 2L 30 μm 30 μm 30 μm f e d SEM OM Boundary Quenched Recovered Recovered sample sample Re Re 10 μm 10 μm g h i T Quenched 2g D G u.) HPHT Recovered a. sample 1.5 nm y ( HPHT Re sit n e I nt HPRT 4 μm 500 nm 1260 1440 1620 1800 Raman shift (cm-1) Figure R9. Experimental synthesis of ultrathin 2D diamond from 2L and multilayer (>15L) graphene under HPHT. a-d, Optical evolution of FLG on Re substrate under HPHT. The 2L and >15-layer (approx. 18-layer) graphene were compressed to 13.3 GPa at 300 K (a); Graphene/Re compressed to 32.5 GPa at 300 K (b); Graphene/Re heated to about 3243 K under 32.8 GPa (c); The sample quenched to ambient condition finally (d). Scale bars in a-d are 30 μm. The yellow dashed lines highlight the morphology of FLG sample. e, The magnified optical image of the area selected from blue solid box in d. f, Magnified SEM image of the area selected from blue solid box in d. The yellow arrows in e-f imply the profile of the quenched sample. g, AFM image of the quenched sample from the marked orange box in f. h, AFM image of the sample from the marked red box in g. R24 Height profile along the cyan dashed line (inset) indicates a thickness of 1.5 nm. i, Raman spectra of the 2D diamond. The spectra in i correspond to the sample recovered from HPRT (black line) and quenched from HPHT (blue and red lines), respectively. The excitation wavelength for Raman measurements is 405 nm. As a powerful technique for characterizing material structure and atomic chemical environment, the electron beam excited C KLL AES was employed to confirm the transformation from FLG to 2D diamond. For graphite and diamond standard samples, the Auger peak of diamond with 100% sp3 concentration shifts toward higher kinetic energy compared to that of graphite with 100% sp2 content (as shown in Figure R10a-b). In our work, the Auger peak of the quenched 2D diamond with distinct T Raman peak shows a 2g shift toward higher kinetic energy relative to the sample recovered from HPRT (as shown in Figure R10c-d). This indicates an increase of the sp3 concentration within the 2D diamond. However, since Auger electron emission involves a three-electron process, the Auger line shape and peak position are highly sensitive to the structure, making accurate quantitative analysis of sp3 content challenging via AES. In summary, we have provided plenty of new data from Raman, PL, SEM, AFM, EDS, HRTEM, XPS and AES that collectively confirm the successful synthesis of ultrathin 2D diamond with a thickness down to three atomic carbon layers. We believe the additional data can resolve the concerns raised by Reviewer #2. R25 a b C KLL T 2g 1334 cm-1 u.) Standard a. u.) y ( a. diamond d E ( sit G E)/ nt e n 1580 cm-1 d N( I Standard E graphite Standard diamond Standard graphite 1200 1350 1500 1650 1800 240 255 270 285 Raman shift (cm-1) Kinetic energy (eV) c d 1335 cm-1 Recovered C KLL T 2g D u.) G HPHT a. u.) sit y ( a. G E)/ d E ( e n D N( nt d I HPRT E Recovered from HPHT Recovered from HPRT 1200 1350 1500 1650 1800 240 255 270 285 Raman shift (cm-1) Kinetic energy (eV) Figure R10. AES characterizations of the quenched 2D diamond. a, Raman spectra of diamond and graphite standard samples. b, C KLL Auger spectra of diamond and graphite standard samples. c, Raman spectra of samples quenched from HPRT and HPHT. d, C KLL Auger spectra of the recovered samples. The black and red dashed lines in b and d indicate the shift of Auger peaks. The spatial resolution of AES is on the order of nanometers. In short, the criteria used by the authors to assess the formation of diamond and metastable phases, which is according to them ”through characteristic Raman spectra, intense photoluminescence from unique color centers (silicon-vacancy and nitrogen-vacancy), and the sp3-bonded microstructure”, was only applied to samples with several nanometer thickness. The only evidence for 2D diamond formation (which would be from the 3 and 5- layer precursors) comes from Raman spectroscopy, shown in Fig. 1h. R26 Reply: We thank Reviewer #2 for raising an important concern regarding the criteria for diamond structure identification in our work. To address Reviewer #2’s concern regarding the use of Raman spectra as the only identification criterion, we have made exhaustive efforts to expand our experimental analysis. In the revised Manuscript and Supporting Information, we have performed new PL, AFM, EDS, HRTEM and AES results to confirm the synthesis of 2D diamond. We have investigated the synthesis of 2-, 3-, 6-, and 18-layer 2D diamond with multiple characterization techniques, including Raman spectroscopy, PL, AFM, SEM, EDS, HRTEM, XPS and AES. Raman and PL results indicate the diamond Raman peak and intense SiV- photoluminescence characters of the 2D diamond. In addition, EDS and HRTEM results confirm a uniform thickness of ~ 1.0 nm for the 3L diamond and AFM characterizations provide approximate thickness measurements of ~ 5.6 nm for the 6-layer and ~ 1.5 nm for the 2-layer diamond. Furthermore, AES results verify the sp2-to-sp3 transition, confirming successful 2D diamond synthesis under HPHT. A trilayer 2D diamond was successfully synthesized at 32.3 GPa and 2924 K, and further confirmed by Raman, PL and HRTEM as shown in Figures R2-R4. Our results reveal that the 3L 2D diamond exhibits a characteristic diamond Raman peak and intense PL from SiV- color centers. EDS and HRTEM results further reveal the uniform and relatively thin thickness of approximately 1.0 nm for 3L diamond, which corresponds closely to the theoretical thickness (9 Å) of a double-sided hydrogenated trilayer diamane [Samir El Masaoudi, et al., J. Phys.: Condens. Matter, 37(2015), 445701]. Additionally, AES results indicate that the Auger peak of the quenched 2D diamond shows a shift toward higher kinetic energy relative to the sample recovered from HPRT (as shown in Figure R10c-d). This shift confirms an increase of the sp3 concentration within the 2D diamond. Moreover, our comparative analysis of multiple characterization techniques confirms that Raman spectroscopy provides the most definitive identification criterion for the 2D diamond formation. The characteristic Raman peak at approximately 1333 cm-1 serves as a reliable indicator for confirming the formation of diamond crystalline lattice. The formation of color centers requires the introduction of specific impurities into the diamond lattice and occurs probabilistically. We understand R27 that the 2D diamond and diamond-like samples in our study proved challenging for resolving the atomic lattice by HRTEM, owing to the atomically thin character. The extremely weak Kikuchi patterns from the atomically thin 2D diamond prevent precise crystallographic alignment, thereby making it impossible to tilt the 2D diamond to a specific zone axis and achieve atomic arrangements observation. Collectively, our comparative analysis of multiple characterization techniques leads us to propose that Raman spectroscopy remains the most robust criterion for confirming the synthesis of 2D diamond, as we mentioned in the manuscript “Raman characterization serves as a robust tool for distinguishing diamond phase from other metastable structures”. In their own words the authors stated that “the initial flake in Figure 2a has thickness on the order of hundreds of nanometers” and that “Samples for microstructural characterization (Figures 3 and 4) and property measurements (Figure 5) exhibit thickness on the hundred-nanometer scale, which have been annotated in the revised manuscript.” They also state that “In this study, we primarily employed Raman spectroscopy to identify the formation of atomically thin samples. The structure and property investigations of atomically thin diamond remain challenging, and we are actively addressing these issues.” Thus, I believe the authors recognize that (1) most of their data and evidence is from samples with several nanometer thickness and that (2) structural characterization for atomically thin samples remain challenging. I could not agree more with the latter statement and that is precisely why I cannot agree that the authors successfully synthesized 2D diamond. Reply: We thank Reviewer #2 for the careful review of our manuscript. We appreciate Reviewer #2 for acknowledging that the structural characterization for atomically thin samples remain challenging. In response to Reviewer #2’s comments, we have made substantial efforts to perform characterization of the 2D diamond, as shown in Figures R2-R10. The revised manuscript and Supporting Information now contain substantial additional experimental data that provide strong evidence for 2D diamond synthesis. We R28 have investigated the synthesis of 2-, 3-, 6-, and 18-layer 2D diamond with multiple characterization techniques, including Raman spectroscopy, PL, AFM, SEM, EDS, HRTEM, XPS and AES. The quenched 2D diamond exhibits characteristic diamond Raman peak and intense SiV- photoluminescence according to the Raman and PL results. EDS and HRTEM measurements confirm a uniform thickness of approximately 1.0 nm for the 3L diamond, while AFM characterization provides thickness values of ~ 5.6 nm and ~ 1.5 nm for the 6-layer and 2-layer diamond respectively. Additionally, AES results verify the sp2-to-sp3 transition, thereby confirming the successful synthesis of 2D diamond. Meanwhile, we attempted to obtain atomic-resolution HRTEM images of the 2D diamond. However, due to their atomically thin character, the 2D diamond samples produced extremely weak Kikuchi patterns, preventing precise zone-axis alignment and thus high-resolution imaging of the atomic lattice. Furthermore, due to the absence of crystallographic relationship between the 2D diamond and the metal substrate, it is also challenging to determine the crystal orientation of the 2D diamond by aligning the zone axis of the metal substrate. Although atomic-resolution image of the 2D diamond was not obtained, we have successfully demonstrated its synthesis through multiple techniques including Raman, SEM, AFM, PL, EDS, HRTEM, XPS and AES. Our study establishes the synthesis of 2D diamond with a thickness as low as three atomic carbon layers. The quenched 2D diamond exhibits a strong characteristic Raman peak, intense SiV- photoluminescence, and a uniform 1.0 nm thickness of a 3L 2D diamond as measured by HRTEM (Figures R2-R4). Besides, the shift of the C KLL Auger peak position (Figure R10), though not enabling quantitative sp3 determination, also supports the phase transition to 2D diamond in our work. This study presents a preliminary exploration on irreversible synthesis conditions of quenchable 2D diamond, achieving the preparation of 2D diamond under HPHT for the first time, which establishes a foundation for further studies of its structure and properties. It is extremely challenging to synthesize and characterize atomically thin 2D diamond R29 samples, that is why there are only a few reports on this topic (see their Table R2). For instance, let us consider the work of Bakharev, Pavel V., et al. Nature nanotechnology 15.1 (2020): 59-66. The authors convincingly showed the synthesis of atomically thin 2D diamond via fluorination of bilayer graphene by employing several techniques such UV photoelectron spectroscopy, Raman, UV-Vis, High- resolution cross- sectional transmission electron microscopy micrograph and EELS. Reply: We thank Reviewer #2 for the constructive suggestions regarding characterization of the 2D diamond. In response to Reviewer #2’s comments, we have made considerable efforts to verify the synthesis of 2D diamond by employing a variety of characterization techniques. The results from Raman spectroscopy, PL, SEM, AFM, EDS, HRTEM, XPS, and AES conclusively confirm the successful synthesis of 2D diamond, as shown in Figures R2-R10. Additionally, we have summarized the synthesis conditions and characterization methods employed for the ultrathin 2D diamond, as presented in Table R1. Our work demonstrates the successful synthesis of 2D diamond with thickness down to three atomic carbon layers. A 3L graphene precursor prepared by mechanical exfoliation, with its layer number accurately determined by Raman spectroscopy, was transferred onto a polished Re substrate via a wet transfer method for subsequent HPHT treatment. The quenched 2D diamond exhibits a characteristic Raman signature, as well as intense PL character from SiV- and a uniform thickness of 1.0 nm as confirmed by HRTEM (as shown in Figures R2-R4). Furthermore, we also performed AFM characterization to provide thickness values of ~ 5.6 nm and ~ 1.5 nm for the 6-layer and 2-layer diamond respectively. Additionally, our AES analysis supports the transition toward the diamond phase, as evidenced by the shift of the C KLL Auger peak position (as shown in Figure R10). P. V. Bakharev et al. synthesized F-diamane from bilayer graphene via fluorination [P. V. Bakharev, et al., Nature Nanotechnol., 15 (2020), 59-66]. However, their work was confined to the two-carbon-layer structure and did not investigate the synthesis of 2D diamond with other thicknesses. The chemical functionalization method for synthesizing 2D diamond is limited by the thickness and the electronegativity of functional groups. It is challenging to obtain structurally stable 2D diamond when the precursor thickness R30 increases or the terminal group exhibits less electronegativity. In contrast, we have investigated the synthesis conditions of 2D diamond with varying layer numbers under HPHT, wherein the final thickness is determined by the initial layer number of the FLG precursor. This work establishes a foundation for synthesizing 2D diamond with controlled thickness, paving the way for future studies of the structure and properties. I want to stress that this is not merely a matter of semantics: the physics, kinetics, and chemistry differ entirely when comparing the synthesis of an atomically thin diamond to that of a nanometer-thick one. For example, the thinner the sample, the greater the influence of surface effects and the stronger the need for chemical groups to stabilize the entire structure. In contrast, thicker samples are generally easier to stabilize and synthesize, as they typically do not require functionalization. Overall, I do believe the authors have successfully shown the synthesis of diamond thin films, which is an interesting topic on its own, but not enough to grant publication in Nat Communications in my opinion. I still believe the article falls short on the claim of 2D diamond synthesis, and I cannot recommend it for publication. Reply: We thank Reviewer #2 for the thoughtful feedback. To address Reviewer #2’s concerns regarding sample thickness, we have provided additional data points for the investigation of 2D diamond synthesis with thickness within 5 layers, as shown in Table R1 (sample No. #1-#9) and Figure 1c-i in our revised manuscript. We have explored multiple characterization techniques to conclusively demonstrate the successful synthesis of 2D diamond, including Raman spectroscopy, PL, AFM, SEM, EDS, HRTEM, XPS and AES (Figures R2-R10). The new obtained results together with the previous data indicate that 2D diamond with thicknesses down to three carbon layers (~1.0 nm in thickness) was successfully synthesized. The quenched 2D diamond exhibits a characteristic Raman peak, as well as intense photoluminescence from SiV-, as shown in Figures R2-R4. In addition, AFM characterization provided thickness estimates of ~ 5.6 nm for the 6-layer and ~ 1.5 nm for the 2-layer 2D diamond. Moreover, AES results also serve as direct evidence for the sp2-to-sp3 transition, confirming the successful 2D diamond synthesis under HPHT. R31 As previously indicated, the quenched sample in our work is distinct from both conventional nanodiamond and the diamane structure proposed by L. A. Chernozatonskiia et al. in 2009 [L. A. Chernozatonskiia, et al., JETP Lett., 90 (2009), 134-138]. Therefore, we propose the term “2D diamond” for the quenched sample in our work, which has a thickness as low as three carbon layers. Atomically thin 2D diamond is expected to exhibit numerous superior physical and chemical properties arising from its ultrathin character. This small characteristic length is normally comparable or smaller than the mean free path of electron/hole carriers, leading to their 2D quantum confinement and unconventional transport. This work presents a preliminary exploration of synthesis conditions for 2D diamond with varying thicknesses, achieving the synthesis of a 3L 2D diamond under HPHT for the first time, which paves the way for intrinsic properties investigation in the future. We are grateful for your valuable suggestions regarding our work and sincerely hope that our responses have fully addressed your concerns. Changes made: 1) We have updated Figure 1c-i and the corresponding discussion in the revised Manuscript. 2) Page 5, Lines 101-119 in the Manuscript: We have updated the description of Figure 1c-i: “Figure 1c-f show the representative optical evolutions of a trilayer (3L) graphene during the entire experimental process. The FLG on Re substrate exhibited weak optical contrast resulting from its low absorbance. The morphology of FLG can be precisely traced from precursor preparation through HPHT treatment, even after being quenched to ambient condition from 32.3 GPa and 2924 K, as indicated in Figure 1c-f. Raman spectrum before and after HPHT treatment were measured to verify the transition from FLG to 2D diamond, as shown in Figure 1g. Raman spectrum of pristine 3L graphene before compression displays the prominent G peak at 1580 cm- 1 and a complete absence of the D band, confirming that our optimized transfer R32 protocol preserves the defect-free nature of graphene precursor. The quenched 3L sample from 32.3 GPa and 2924 K exhibits an evident Raman T peak around 1332 2g cm-1, which is uniquely originated from the lattice structure of sp3 carbon, i.e., diamond structure (the detailed Raman characterization will be discussed later). The uniform and ultrathin nature of the quenched 2D diamond was confirmed by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) characterization. HRTEM results (Figure 1h) reveal a continuous carbon layer with a uniform thickness of approximately 1.0 nm, sandwiched between the gold coating and the Re substrate as confirmed by the EDS mapping results in Figure 1i. The thickness of the quenched 2D diamond derived from 3L graphene is comparable to the theoretical thickness (9 Å) of a double-sided hydrogenated 3L diamane”. 3) We have updated the caption of Figure 3 in the revised Manuscript: Atomistic observations and transformation mechanism of the 2D diamond quenched from HPHT conditions. 4) We have updated Supplementary Figure 3 and the corresponding legend in the revised Supplementary Information. 5) Page S7, Lines 98-121 in the Supplementary Information: We have added the description of Supplementary Figure 3: “The 2L and >15-layer (approx. 18-layer) graphene samples were subsequently loaded into DAC, compressed to 32.8 GPa, and laser-heated to approximately 3243 K (Figure S3a-c). The location of the quenched sample on the Re substrate can be precisely determined by carefully comparing the relative positions of FLG precursor on the Re substrate before and after HPHT, as indicated by the yellow dashed line in Figure S3a-d. Under high-magnification optical microscopy, the quenched 2L sample exhibits the significant optical contrast sufficient to identify its profile on the Re substrate. This contrast is further confirmed under SEM characterization, where the sample profile becomes distinctly clear, as indicated by the yellow arrows in Figure S3e. To confirm the two-dimensional character and further measure the thickness of the quenched sample, we conducted AFM characterization on the region of the 2L R33 sample that shows a well-defined profile. AFM characterization of the orange-marked region in Figure S3e reveals a well-defined profile, and a thickness of approximately 1.5 nm for the quenched 2L sample, as shown in Figure S3f-g. It can be concluded that the quenched sample persists as a relatively uniform ultrathin film and exhibits a distinct step on the Re substrate, even though the measured thickness is larger than expected due to the rough Re substrate. As a powerful technique for characterizing material structure and atomic chemical environment, the electron beam excited C KLL Auger electron spectrum (AES) was employed to confirm the transformation from FLG to 2D diamond. In our work, the Auger peak of the quenched 2D diamond with distinct T Raman peak 2g shows a remarkable shift toward higher kinetic energy relative to the sample recovered graphene phase from HPRT (as shown in Figure S3h-i). This indicates an great increase of the sp3 concentration within the 2D diamond. However, since Auger electron emission involves a three-electron process, the Auger line shape and peak position are highly sensitive to the structure, making the accurate quantitative analysis of sp3 content challenging”. 6) We have updated Supplementary Table 1 in the revised Supplementary Information with the additional new data. R34 Reviewer #3 (Remarks to the Author): The authors addressed all major and minor concerns raised in the first review; the qualtity of manuscript has been substantially improved as the result. I recommend the revised manuscript to be published in Nature Communications. Significant amount of information and discussions which will be useful for other researchers as well as for general readers are placed in the Supplementary Information. I wish those are well exposed to the public together with the main text. The section title \"RESULTS\" would better be \"RESULTS AND DISCUSSIONS\", and \"DISCUSSION\" should be either \"CONCLUSION\" or \"SUMMARY\". Reply: We thank Reviewer #3 for the careful review of our manuscript. We sincerely appreciate the positive comments on our work and the useful suggestions on how to improve our manuscript. As suggested by Reviewer #3, we have included the extensive data and discussions in Supplementary Information to support our work. Moreover, all experimental data supporting the findings of this study will be available to the public within the paper and Supplementary Information files. The Source Data files generated over the course of this study will be provided as well. Regarding your question on the section titles, our current formatting strictly follows the Guide to Formatting Articles of Nature Communications (the details listed in https://www.nature.com/documents/ncomms-formatting-instructions.pdf). We are willing to accommodate any further modifications, according to your and the editor’s recommendations. Once again, we sincerely appreciate your support and encouragement of our research. R35 --- In all, we greatly appreciate having highly constructive remarks and suggestions from all reviewers. With all their comments addressed in the revised manuscript, we are looking forward to the publication of our manuscript in Nature Communications. Thank you! Sincerely yours, Prof. Yabin Chen (on behalf of all authors) E-mail: chyb0422@bit.edu.cn R36"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have successfully addressed all my concerns. I acknowledge their effort in carrying out several additional experiments to confirm the formation of 2D diamond at the atomic-thin limit, therefore I recommend this work for publication at Nature Communications."}, "author_response": ""}]

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