Introduction

Two-dimensional (2D) diamond has attracted considerable interest due to its atomically thin structure in sp3-hybridization and unique quantum confinement effect1,2,3. 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 materials4,5,6,7. Various strategies have been developed to optimize its physical properties, such as structural modification and chemical doping8,9,10,11,12,13,14,15. 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 C2H 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, diamane22,23,24, diamene25,26,27, and diamondene28,29,30. Hydrogenated and fluoride 2D diamond have been achieved by chemical adsorption of FLG under moderate conditions, while this approach usually produces the incomplete internal transformation22,23,24. 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 assumption32,33,34. 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 efforts have 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 motifs36,37,38. 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 exhibits 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

2D diamond synthesis under HPHT

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.

Fig. 1: Experimental synthesis of 2D diamond under high pressure and high temperature (HPHT).
Fig. 1: Experimental synthesis of 2D diamond under high pressure and high temperature (HPHT).
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a Schematic strategy to prepare 2D diamond from graphene in laser-heated diamond anvil cell (DAC). Re foil (light blue), polished down to ~10 μm with surface roughness around 3.9 nm, was used as graphene (pink) substrate and thermal absorber for near-infrared (NIR) laser heating. b Illustration of phase transformation from bilayer graphene/graphite to hexagonal diamond (HD) and cubic diamond (CD) under HPHT. The dashed arrows denote the energy profile of phase transition, with CD having lower free energy. The transition pressure and temperature gradually decrease with increasing graphene thickness, as illustrated by the red-to-blue gradient arrow. c–f Optical evolution of few-layer graphene (FLG) on Re substrate under HPHT. The 3L graphene (inset, optical image before transfer) was transferred onto Re surface and then compressed to 0.5 GPa at 300 K (c); Graphene/Re compressed to 32.0 GPa at 300 K (d); Graphene/Re heated to ~2924 K under 32.3 GPa (e); The sample quenched to ambient condition finally (f). Scale bars in (c–f) are 30 μm. The yellow dashed line highlights the morphology of graphene before laser heating. g Raman spectra showing the phase evolution from the initial 3 L graphene (exfoliated and after being transferred) to the quenched 2D diamond synthesized after HPHT, identified by the characteristic T2g band. The spectra of quenched 2D diamond correspond to the positions marked as pink and violet in (f). Arb. u. means arbitrary unit. h Cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization of 2D diamond. Low-magnification TEM image reveals the uniform morphology (top). HRTEM image shows the 2D diamond extent highlighted by green dashed lines, indicating a thickness of ~1.0 nm (bottom). i Energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of the 2D diamond cross-section for Au (top), C (middle) and Re (bottom) elements.

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 and PL characterizations

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).

Fig. 2: Raman and photoluminescence (PL) measurements (405 nm excitation) of 2D diamond quenched from HPHT.
Fig. 2: Raman and photoluminescence (PL) measurements (405 nm excitation) of 2D diamond quenched from HPHT.
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a Raman spectra acquired from different positions along the blue dashed arrow in the optical image (inset). The partial graphene flake became transparent after 20.0 GPa and 1435 K, while the rest section remained opaque after high pressure and room temperature (HPRT) treatment. The red box in inset indicated the preset laser heated region. Dia. means diamond. b Typical PL spectra of recovered samples (blue and red) from HPHT with the remarkable SiV- (738 nm) and NV0 (575 nm) color centers. The black line without any significant PL signal was taken from the opaque part in (a). c Optical image of quenched sample from 20.6 GPa and 1424 K to ambient condition for Raman and PL mapping measurements. d 2D mapping result of diamond Raman intensity from the marked yellow box in (c). e Contour map of SiV- emission intensity of the recovered sample from the marked yellow box in (c). The white dashed lines in (d, e) imply the boundaries between recovered sample and PTM. The slight variation of boundaries arises from positional displacement between two measurements, with the sample in (d) exhibiting an upward shift relative to (e).

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.

Lattice structure and transition mechanism

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 [\(1\bar{1}0\)] zone axis as well. Figure 3e and f show the HRTEM images and SAED patterns of CD structures from [\(01\bar{1}\)] and [\(10\bar{1}\)] zone axes, respectively (more results in Supplementary Fig. 12). The measured d spacing of ~2.12 Å is referred to the typical (\(111\)) plane.

Fig. 3: Atomistic observations and transformation mechanism of the 2D diamond quenched from HPHT conditions.
Fig. 3: Atomistic observations and transformation mechanism of the 2D diamond quenched from HPHT conditions.
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a Schematic of crystalline structure of HD. Atomic arrangements were observed along different zone axes (blue arrows). b HRTEM image of HD from [\(001\)] zone axis. c HRTEM image of HD from [\(1\bar{1}0\)] zone axis. The inset shows the fast Fourier transform (FFT) pattern. d Schematic of lattice structure of CD. Atomic structures viewed along several zone axes (red arrows). e HRTEM image of CD from [\(01\bar{1}\)] zone axis. f HRTEM image of CD from [\(10\bar{1}\)] zone axis. The insets in (b) and (e, f) show the corresponding selected area electron diffraction (SAED) patterns. g Atomic arrangement of RG/CD hybrid interface. RG represents rhombohedral graphite. h Details of ABC stacking orders selected from the marked orange box in (g). The orange dashed lines highlight the RG/CD boundaries. i Atomic arrangement of HG/HD hybrid interfaces. HG represents hexagonal graphite. The interfacial boundaries are highlighted by blue dashed lines. All labeled numbers (unit: Å) mean the interplanar spacings measured from each HRTEM image. The 2D diamond samples for microstructural characterization were prepared with hundred-nanometer scale thickness.

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) 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 [\(01\bar{1}\)], [\(0\bar{1}1\)] 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.

Fig. 4: Atomic characterizations of structural defects in the recovered 2D diamond.
Fig. 4: Atomic characterizations of structural defects in the recovered 2D diamond.
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a A low-magnification TEM image of ultrathin diamond. b High-magnification TEM image of the area selected from black solid box in (a). The twin boundaries (TBs) (yellow dotted lines), stacking faults (SFs) (arrowheads) and Σ3(\(211\)) TBs (pale green arrows) are well marked. c High-magnification TEM image of the area selected from dotted box in (a). HRTEM images of CD crystalline structures along [\(01\bar{1}\)] (d), [\(0\bar{1}1\)] (e) zone axes and TB (f, highlighted with yellow double arrow) from the regions marked in (c). The insets show the corresponding fast Fourier transform (FFT) results. g TEM image of diamond polytype coherently embedded in CD domains with TBs and SF. h Magnified TEM image from the violet-boxed region in (g). The FFT patterns of diamond polytype and CD are shown in insets as well. i The identified interfacial structure between graphene and 2D diamond. The 2D diamond samples for atomic characterization were prepared with hundred-nanometer scale thickness.

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.

Tunable bandgap and thermal stability of 2D diamond

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.

Fig. 5: The tunable bandgap energy and thermal stability of 2D diamond quenched from various synthesis conditions.
Fig. 5: The tunable bandgap energy and thermal stability of 2D diamond quenched from various synthesis conditions.
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a Carbon K-edge electron energy loss spectroscopy (EELS) results of the samples recovered from HPHT. The different curves indicate various temperature and pressure conditions. The features at 283.2 (π* peak) and 292.5 eV (σ* peak) correspond to sp2 and sp3 hybridizations, respectively. A progressive increase in sp3 fraction from 71.3 to 89.9% correlates with enhanced σ* peak intensity, as shown by the gradient blue shaded region. b Absorption spectra of ultrathin diamond with different sp3 percentages. c The IDia/IG-dependent optical bandgaps of quenched 2D diamond. The dashed line is used to guide the eyes. d Raman spectra of ultrathin diamond (#S1) taken after different annealing temperatures. The two dashed lines correspond to the characteristic Raman peaks of diamond (T2g) and graphite (G), respectively. e Annealing temperature-dependent Raman spectra of ultrathin diamond (#S2) from 25 to 1000 °C. The dashed lines indicate the characteristic Raman peaks of diamond (T2g) and graphite (D and G). f The evolution of IDia/IG of #S1 and #S2 with different annealing temperatures (TA). g Comparison of thermal stability of our 2D diamond with the reported carbon materials, including diamond-like nanocomposite (DLN)52, diamond-like carbon (DLC)53, nano-dia.54, chemical vapor deposited (CVD)-dia.55, Co-polycrystalline diamond (Co-PCD)56 and natural diamond57. The 2D diamond samples for property investigations were prepared with hundred-nanometer scale thickness.

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.

Discussion

In conclusion, the quenchable 2D diamond with thicknesses down to two carbon layers has been successfully synthesized by the direct conversion of FLG under HPHT. Our observations confirm that 2D diamond exhibited well-crystalline sp3 structures and strong PL peaks from SiV- and NV0 color centers. Optical absorption and thermal annealing process indicated that the optical bandgaps and thermal stability of 2D diamond show sp3 content-dependence. Atomic arrangements of 2D diamond revealed that intermediate RG mediate phase transition from HG to CD and the pathways along HG→RG→CD and HG→HD. This work can lay the foundation for the design and synthesis of exotic carbon allotropes and further broaden their potential applications.

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 approcached ~1 μm with a 100× objective. Raman signal of 2D diamond and PL spectrum of its color center were detected with 1800 and 150 gm m−1 grating, respectively, while the pressure or temperature dependence of SiV- PL was investigated by 1800 gm m–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 JEM2100F 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.