Evidence of a two-dimensional nitrogen crystalline structure on silver surfaces

Domain: physics
Published: 2025-12-13
Editorial outcome: major_revision

Abstract
Nitrogen, the most abundant element in Earth’s atmosphere, exists as a diatomic gas under standard temperature and pressure. In the two-dimensional (2D) limit, atomically thin nitrogen, termed nitrogene, has been theoretically predicted to form crystalline materials with various polymorphic configurations, exhibiting diverse chemical and physical properties. However, the synthesis of nitrogene has remained elusive due to the strong nitrogen-nitrogen triple bonds. Here, we report experimental evidence of the formation of nitrogen-based crystalline structures compatible with nitrogene on silver surfaces via ion-beam-assisted epitaxy. Through a combination of scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and first-principles calculations, we demonstrate that the nitrogene-like structure adopts a puckered honeycomb lattice. Notably, our calculations predict a nitrogene band gap of up to 7.5 eV, positioning it as a promising candidate for ultraviolet optoelectronic devices and high-κ dielectric applications.

Introduction
Synthetic two-dimensional (2D) materials without layered parent allotropes exhibit a remarkable diversity of physical and chemical properties due to their structural variability. A prominent class of such materials is the Xenes, which include borophene1,2, silicene3,4, stanene5,6, tellurene7,8, molybdenene9, and goldene10. Most Xenes identified to date are semimetals or metals, which limits their potential applications in logic electronics and optoelectronics. To address this limitation, pnictogen-based Xenes, such as phosphorene11–13, antimonene14,15, and bismuthene16–18, have garnered significant attention due to their semiconducting properties and promising device applications19,20, as well as their range of exotic properties. For instance, black phosphorene field-effect transistors exhibit excellent drain current modulation and high carrier mobility12, while honeycomb bismuthene is a quantum spin Hall insulator with a substantial inverted band gap16. Additionally, black phosphorus-like bismuthene has been identified as an elemental ferroelectric material18.

As the lightest pnictogen, nitrogen predominantly exists as diatomic molecules in nature, which are gaseous under standard temperature and pressure. The strong N≡N triple bonds impart high stability and chemical inertness to molecular nitrogen. In contrast, crystalline nitrogen can usually be realized under extreme conditions of low temperature and high pressure, including cubic-gauche nitrogen (cg-N)21, black phosphorus-like nitrogen (bp-N)22,23, and hexagonal layered polymeric nitrogen (HLP-N)24. However, the pure phases of these crystalline structures decompose upon pressure release and cannot exist under ambient conditions, although cg-PN can be stabilized in a composite environment under near-ambient conditions25.

The 2D crystalline monolayer of nitrogen, termed nitrogene, has been theoretically proposed to adopt various polymorphic structures, including the honeycomb26–28, black phosphorus-like29,30, square-octagon31–33, and zigzag sheet structures34,35. Its strong covalent bonding and high chemical reactivity make nitrogene a promising candidate for applications in field-effect transistors, catalysis, gas storage, and solar cells. However, the synthesis of nitrogene remains challenging due to the significant energy required to break the strong N≡N triple bonds and the need to select an appropriate substrate for epitaxial growth.

Here, we report the synthesis of a nitrogene-like structure on a Ag(100) surface via ion-beam-assisted epitaxy (IBAE). Scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) measurements reveal that the nitrogene-like structure forms a (√2×√2)R45° superstructure relative to the Ag(100) substrate. In contrast to other Xenes, it exhibits a wide electronic band gap (~7.5 eV), as confirmed by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. Based on these results, we propose a puckered honeycomb structure for the 2D nitrogen sheets, which is consistent with all experimental observations. The successful synthesis of the nitrogene-like structure not only offers insights into the bonding behavior of crystalline nitrogen but also paves the way for the development of nitrogene-based optoelectronic and plasmonic devices.

Results
Synthesis of nitrogene-like structures on Ag(100)
The growth mechanism of nitrogen on Ag(100) via IBAE is illustrated in Fig. 1a. Molecular N2 is chemically inert due to its strong N≡N triple bond (~10 eV), which prevents it from adsorbing on substrates. To address this challenge, we employed an ion source operating with pure N2 gas to generate reactive nitrogen species, primarily N+ ions, which also provides sufficient kinetic energy for the ions to reach the Ag(100) surface. During growth, the kinetic energy of the N+ ions is crucial for breaking the N≡N triple bonds through collisions with Ag atoms. Meanwhile, the remaining energy of the cracked N atoms can increase atomic mobility on the surface and improve the quality of the sample. However, excessive kinetic energy can damage the Ag(100) surface, leading to surface corrugation. Through optimization experiments, we determined that the ideal kinetic energy required for the synthesis of the nitrogene-like structure on Ag(100) is ~30 eV.

The substrate temperature was maintained within a narrow range of 400 ± 10 K during the growth process. At lower temperatures, disordered nitrogen clusters formed on the surface, while higher temperatures led to nitrogen desorption. For comparison, we performed growth under the same N2 pressure but with the ion source turned off. In this case, no nitrogen remained on the surface, indicating that the generation of N+ ions is essential for the successful growth of the nitrogene-like structure.

Figure 1b presents a large-scale STM image of the nitrogene-like structure on the Ag(100) surface (see also Supplementary Fig. 1). The apparent height of the islands is slightly lower than that of the adjacent Ag(100) surface, likely due to the removal of the topmost Ag layer during growth by the chemically assisted reorganization process. This phenomenon is commonly observed in systems involving foreign atoms on metal surfaces, such as the Si/Ag interface36,37. Another reason for the recessed feature is the insulating nature of nitrogene, which results in a lower apparent height in constant-current STM imaging.

The islands exhibit a well-ordered atomic structure, as shown in the high-resolution STM image in Fig. 1c and the line profile in Fig. 1d. These images reveal that the nitrogene-like structure adopts a square lattice with a lattice constant of 4.2 Å. The primitive lattice vectors of the nitrogene-like structure are rotated by 45° relative to those of the Ag(100) substrate, resulting in a (√2×√2)R45° superstructure. This superstructure is further confirmed by the LEED measurements shown in Fig. 1e. Moreover, systematic LEED measurements confirm the formation of a single structural phase on Ag(100), as shown in Supplementary Fig. 2.

ARPES measurements
To investigate the electronic structure of the nitrogene-like structure, we performed ARPES measurements. Figure 2a–e display the constant energy contours (CECs) at various binding energies, while Fig. 2f–j show the corresponding CECs for pristine Ag(100) for comparison. No electronic states associated with the nitrogene-like structure were observed within 1.3 eV below the Fermi level, indicating a semiconducting or insulating nature of the overlayer. At a binding energy of ~1.4 eV, a prominent circular feature appears at the Γ point. As the binding energy increases, this feature gradually shrinks into a dot-like shape at 2.6 eV, then expands back into a circular feature at 2.8 eV, indicating the presence of both electron-like and hole-like bands. Notably, similar features are also observed at higher-order Γ points, consistent with the periodicity of the nitrogene-like structure.

ARPES intensity plots along Cuts 1-3 and 1’-3’, as indicated in Fig. 2a, f, are shown in Fig. 2k–p. Along the Γ-M direction of the nitrogene-like structure (Cut 1, Fig. 2k), both an electron-like band and a hole-like band are observed at the Γ point, nearly touching each other at ~2.5 eV below the Fermi level. A parabolic fit of the electron-like bands yields an effective mass of ~0.5 m0 (see Supplementary Fig. 3). Cuts 2 and 3 are both along the Γ-X direction, but with parallel and perpendicular orientations relative to the slit of the electron energy analyzer, respectively. In Cut 2 (Fig. 2m), the lower hole-like band is more pronounced, while in Cut 3 (Fig. 2o), the upper electron-like band is more intense. These variations in spectral intensity are likely due to photoemission matrix element effects. Importantly, no similar bands are observed in pristine Ag(100), regardless of momentum or energy shifts, as shown in Fig. 2l, n, and p. This excludes the possibility of band-folding effects or rigid shifts of the bulk bands.

First-principles calculations
We performed first-principles calculations to explore potential structural models of the nitrogene-like structure on Ag(100). The most likely structural model is shown in Fig. 3a, b. In this model, the N atoms in the nitrogene layer exhibit buckling at different heights, forming a black-phosphorus-like structure. The slight difference from the black phosphorus structure might be attributed to the substrate-induced relaxation. An alloyed buffer layer, composed of AgN in a stoichiometric ratio, exists between the nitrogene layer and the Ag(100) substrate. This buffer layer serves to passivate the metal surface, thereby facilitating the formation of the nitrogene overlayer. Such phenomena are commonly observed in the growth of other 2D pnictogens38–40. Notably, the buffer layer is unstable on its own and requires the presence of the nitrogene layer for stabilization, as demonstrated in Supplementary Fig. 4.

Figure 3c presents the top view of the structural model, along with experimental and simulated STM images, in agreement with each other. The bright protrusions in the STM images correspond to the topmost N atoms in the model, as indicated by the red balls. The existence of a buffer layer is consistent with our XPS measurement results, as shown in Fig. 3d, e. We observe two N 1s peaks, which correspond to the N atoms in the buffer layer and the nitrogene layer, respectively.

We further calculated the formation energies per atom of freestanding nitrogene and N2 molecules, obtaining values of −8.047 eV and −8.304 eV, respectively. Although nitrogene is thermodynamically less stable than molecular nitrogen in the freestanding form, it can be stabilized on the Ag(100) substrate through interaction with the buffer layer, enabling its experimental realization under our growth conditions.

Figure 4a, b display the ARPES spectra and calculated band structures along the M-Γ-M direction, respectively. There is good agreement between the experimental and calculated band structures, as highlighted by the orange rectangles and red dashed lines in Fig. 4a, b. Figure 4c shows the calculated density of states (DOS) for the system. A peak is observed at 2–3 eV below the Fermi level, corresponding to the touching points of the electron and hole pockets in the ARPES spectra. Above the Fermi level, a prominent peak at ~1.1 eV is visible in the DOS, which aligns with our dI/dV spectra shown in Fig. 4d. These results confirm the proposed structural model for the nitrogene-like structure on Ag(100).

The calculated band structures of freestanding nitrogene are shown in Supplementary Fig. 5. Notably, the conduction bands of freestanding nitrogene are identical to those calculated for nitrogene on the substrate, indicating weak hybridization between nitrogene and the buffer layer. It should be noted that the interaction between nitrogene and the buffer layer is still strong enough to prevent lateral displacement or tip-induced manipulation of the nitrogene islands. The calculated band gap of freestanding nitrogene is ~7.5 eV, the largest among experimentally realized 2D wide-gap materials, such as h-BN. The valence bands are located around 5 eV below the Fermi level, which are masked by the strong spectral weight of the bulk bands from Ag(100), as shown in Supplementary Fig. 6.

Methods
Experiments
Both sample preparation and measurements were conducted in an ultrahigh vacuum system with a base pressure of approximately 1 × 10−8 Pa. The Ag(100) surfaces were prepared through repeated cycles of Ar+ ion sputtering and annealing to ensure cleanliness. Prior to nitrogen deposition, the Ag(100) substrate was maintained at 400 K for 30 min. During the IBAE growth process, low-energy nitrogen ions (~30 eV) were generated using an ion source41,42 and bombarded onto the clean Ag(100) substrate for 15 min. The pressure of N2 during growth was approximately 3 × 10−3 Pa.

Following the growth process, the samples were characterized in situ using LEED, STM, and ARPES. STM imaging was performed in constant-current mode, while STS was carried out with an ac modulation voltage of 30 mV at 667 Hz. All STM images and STS spectra were acquired at 78 K, with bias voltages defined as the sample bias relative to the tip. ARPES measurements were performed using a SPECS PHOIBOS 150 electron energy analyzer and a helium discharge lamp (He Iα, 21.2 eV) at 70 K. XPS measurements were performed at room temperature using an Al Kα X-ray source (SPECS XR50).

Calculations
First-principles calculations were conducted based on density functional theory (DFT) using the projector augmented wave method as implemented in the Vienna Ab initio Simulation Package (VASP)43,44. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional45,46 was employed. A kinetic energy cutoff of 500 eV was applied to the plane-wave basis set, and a Γ-centered 12 × 12 × 1 k-point mesh was used for sampling the first Brillouin zone (BZ). The slab model consisted of three layers of the Ag substrate, a monolayer of AgN, a nitrogene layer, and a vacuum layer exceeding 15 Å in thickness. Convergence criteria were set to 0.01 eV/Å for ionic relaxation and 10−6 eV for the electronic self-consistent loop. The band structure of the slab system was unfolded into the first BZ of the Ag substrate using the orbital-selective band unfolding technique47.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I am generally satisfied that the authors have addressed my comments to a reasonable level. Their responses to my specific points are mostly technically sound, although a few minor points remain unclear. These points, however, are not critical in my opinion, and I have no additional questions beyond what was initially asked for their submission to Nature Communications.\nHowever, I note that several of the more substantial questions posed by other reviewers concerning chemical speciation, reaction mechanism, and structure identification appear to be only partially resolved in the rebuttal. Whether the authors’ replies and revisions adequately address these issues is critical to assessing the suitability of this work for Nature Communications.", "reviewer_3": "In their revision, the authors have included additional information regarding the suggested nitrogen structures. Importantly, they have added XPS spectra of the structures on the silver substrate and of the substrate itself. The difference between these two spectra revealed two peaks associated with nitrogen, indicating that it exists in these structures with two different bonding environments. They have also included a large-scale STM image that reveals a regular pattern of nitrogen-associated islands, suggesting perhaps that there is a strain-associated limit to their size and separation. Finally, they have also included new calculation results that suggest that the proposed buffer layer of nitrogen in the top layer of silver is not stable without an additional nitrogen structure on top of it. The authors have also clarified that the interaction between the nitrogen structure and the buffer layer is weaker than a covalent bond but stronger than a van der Waals interaction.\nOverall, the added information does make the claim somewhat stronger, especially since the structures have been confirmed to consist of nitrogen. However, they do not sufficiently demonstrate that the presented interpretation is the only possible explanation for the experimental data, thereby justifying the unambiguous discovery of nitrogene. One of the main concerns I still have is the unclear interaction between the suggested nitrogen structure and the buffer layer. If the interaction is stronger than van der Waals, this implies that there is a hybridization of the surface states and the states of nitrogene, which in turn would mean that the structure is actually not nitrogene, but rather a nitrogen-consisting surface structure that may or may not resemble the theoretically predicted 2D material. Indeed, it remains a possibility that the observed structures arise from nitrogen-associated surface reconstruction or correspond to a nitrogen-terminated silver surface with two differently bound nitrogen atoms, possibly one below and one above the surface.\nAll this being said, the presented experimental and computational results are interesting and should be made available to the community. However, the way they are presented now does not reflect the strength of the evidence supporting the hypothesis. My recommendation is to revise the manuscript to tone down the claims and to introduce a careful discussion about the remaining open questions and alternative interpretations.", "reviewer_4": "In this article, Hu et al. synthesized a novel two-dimensional (2D) material, nitrogene, by ion-beam-assisted epitaxy on an Ag(100) substrate. The 2D nitrogene was characterized using STM, ARPES, XPS, and theoretical calculations. Overall, I believe this work is highly significant, not only because it expands the family of 2D materials, but also because its large band gap could potentially enhance applications in ultraviolet optoelectronics. From my perspective, the data are convincing and well-supported. For these reasons, I recommend that the paper be accepted for publication in Nature Communications after minor revision.\n1. The growth experimental conditions are extremely important for the successful fabrication of 2D nitrogene. However, the description of these conditions is not sufficient. For example, key parameters such as the growth time and the duration for which the substrate was kept at 400 K during growth should be provided.\n2. The experimental conditions for the XPS measurements should be included in the Methods section. In addition, I suggest that the authors move the XPS data shown in Fig. S3 to the main text, as it is fundamentally important for confirming the presence of nitrogen.\n3. I suggest that the authors extract key parameters from the band dispersion shown in Fig. 2k, such as the effective mass, since these are important for discussing potential device applications.\n4. It appears that 2D nitrogene is quite stable at room temperature, as some of the LEED and STM data were obtained under these conditions. I recommend that the authors discuss potential applications in the manuscript to better highlight the importance of this work."}, "author_response": "Response to Reviewer #1\n-------------------------------------------------------------------------------------------------------\nC1: I am generally satisfied that the authors have addressed my comments to a\nreasonable level. Their responses to my specific points are mostly technically sound,\nalthough a few minor points remain unclear. These points, however, are not critical in\nmy opinion, and I have no additional questions beyond what was initially asked for\ntheir submission to Nature Communications.\nHowever, I note that several of the more substantial questions posed by other\nreviewers concerning chemical speciation, reaction mechanism, and structure\nidentification appear to be only partially resolved in the rebuttal. Whether the authors’\nreplies and revisions adequately address these issues is critical to assessing the\nsuitability of this work for Nature Communications.\nResponse:\nWe sincerely appreciate the reviewer’s positive assessment and are glad that our\nrevisions and clarifications satisfactorily addressed the earlier concerns. We also thank\nthe reviewer for noting the importance of the broader issues raised by other referees,\nwhich have also been satisfactorily addressed.\nResponse to Reviewer #3\n-------------------------------------------------------------------------------------------------------\nC1: In their revision, the authors have included additional information regarding the\nsuggested nitrogen structures. Importantly, they have added XPS spectra of the\nstructures on the silver substrate and of the substrate itself. The difference between\nthese two spectra revealed two peaks associated with nitrogen, indicating that it exists\nin these structures with two different bonding environments. They have also included a\nlarge-scale STM image that reveals a regular pattern of nitrogen-associated islands,\nsuggesting perhaps that there is a strain-associated limit to their size and separation.\nFinally, they have also included new calculation results that suggest that the proposed\nbuffer layer of nitrogen in the top layer of silver is not stable without an additional\nnitrogen structure on top of it. The authors have also clarified that the interaction\nbetween the nitrogen structure and the buffer layer is weaker than a covalent bond but\nstronger than a van der Waals interaction.\nOverall, the added information does make the claim somewhat stronger, especially\nsince the structures have been confirmed to consist of nitrogen. However, they do not\nsufficiently demonstrate that the presented interpretation is the only possible\nexplanation for the experimental data, thereby justifying the unambiguous discovery of\nnitrogene. One of the main concerns I still have is the unclear interaction between the\nsuggested nitrogen structure and the buffer layer. If the interaction is stronger than van\nder Waals, this implies that there is a hybridization of the surface states and the states\nof nitrogene, which in turn would mean that the structure is actually not nitrogene, but\nrather a nitrogen-consisting surface structure that may or may not resemble the\ntheoretically predicted 2D material. Indeed, it remains a possibility that the observed\nstructures arise from nitrogen-associated surface reconstruction or correspond to a\nnitrogen-terminated silver surface with two differently bound nitrogen atoms, possibly\none below and one above the surface.\nAll this being said, the presented experimental and computational results are\ninteresting and should be made available to the community. However, the way they are\npresented now does not reflect the strength of the evidence supporting the hypothesis.\nMy recommendation is to revise the manuscript to tone down the claims and to\nintroduce a careful discussion about the remaining open questions and alternative\ninterpretations.\nResponse:\nWe thank the reviewer for this constructive suggestion. In the second-to-last\nparagraph, where alternative structural models are discussed, we have removed the\nphrase “confirming the validity of the proposed model” and added the following\nsentence: “However, unambiguous confirmation of the precise structural model of the\nN/Ag(100) system will require further experimental and theoretical investigation.” In\naddition, we followed the editorial requests and replaced “nitrogene” with “nitrogene-\nlike structure” throughout the Main Text and Supplementary Information.\nResponse to Reviewer #4\n-------------------------------------------------------------------------------------------------------\nC1: In this article, Hu et al. synthesized a novel two-dimensional (2D) material,\nnitrogene, by ion-beam-assisted epitaxy on an Ag(100) substrate. The 2D nitrogene was\ncharacterized using STM, ARPES, XPS, and theoretical calculations. Overall, I believe\nthis work is highly significant, not only because it expands the family of 2D materials,\nbut also because its large band gap could potentially enhance applications in\nultraviolet optoelectronics. From my perspective, the data are convincing and well-\nsupported. For these reasons, I recommend that the paper be accepted for publication\nin Nature Communications after minor revision.\nResponse:\nWe thank the reviewer for the positive assessments of our manuscript. In the\nfollowing, we will address all these comments in detail.\nC2: The growth experimental conditions are extremely important for the successful\nfabrication of 2D nitrogene. However, the description of these conditions is not\nsufficient. For example, key parameters such as the growth time and the duration for\nwhich the substrate was kept at 400 K during growth should be provided.\nResponse:\nWe thank the reviewer for highlighting the importance of providing detailed growth\nparameters. In the revised manuscript, we have expanded the description in the Methods\nsection to clearly specify these parameters. The revised text now reads: Prior to\nnitrogen deposition, the Ag(100) substrate was maintained at 400 K for 30 min. During\nthe IBAE growth process, low-energy nitrogen ions (~30 eV) generated by an ion\nsource41,42 were bombarded onto the clean Ag(100) substrate for 15 min.\nC3: The experimental conditions for the XPS measurements should be included in the\nMethods section. In addition, I suggest that the authors move the XPS data shown in\nFig. S3 to the main text, as it is fundamentally important for confirming the presence of\nnitrogen.\nResponse:\nWe thank the reviewer for these valuable suggestions. We have updated the Methods\nsection to include the experimental conditions for the XPS measurements. The revised\ntext now reads: XPS measurements were performed at room temperature using an Al\nKα X-ray source (SPECS XR50).\nIn addition, as recommended, the XPS data originally shown in Fig. S3 has been\nmoved to Fig. 3 of the main text. The relevant discussion in the main text has been\nadjusted to describe the XPS data (the first paragraph in Page 7).\nC4: I suggest that the authors extract key parameters from the band dispersion shown\nin Fig. 2k, such as the effective mass, since these are important for discussing potential\ndevice applications.\nResponse:\nWe thank the reviewer for this helpful suggestion. In the revised manuscript, we\nhave extracted the effective mass of the electron-like band located near the Fermi level.\nThe band dispersion was fitted using a parabolic function, yielding an effective mass of\napproximately 0.5 mₑ. The corresponding analysis and fitting results are now provided\nin the Supplementary Information as Supplementary Fig. 3 (formerly Fig. R1). In the\nrevised manuscript, Fig. R1 has been included in the Supplementary Information as\nSupplementary Fig. 3, and a corresponding discussion has been added to the main text.\nSupplementary R.1: Effective mass fitting of the electron-like bands. a, ARPES intensity plot\nreproduced from Fig. 2k of the main text. The red markers denote the band extrema extracted from\nmomentum-distribution-curve analysis and used for fitting. b, Parabolic fit of the extracted band\ndispersion to determine the effective mass.\nC5: It appears that 2D nitrogene is quite stable at room temperature, as some of the\nLEED and STM data were obtained under these conditions. I recommend that the\nauthors discuss potential applications in the manuscript to better highlight the\nimportance of this work.\nResponse:\nWe thank the reviewer for this constructive suggestion. The potential applications\nof 2D nitrogene have been comprehensively discussed in the final paragraph of the\nmanuscript. Following the reviewer’s recommendation, we have additionally\nemphasized its room-temperature stability by adding the following sentence: “Notably,\nthe nitrogene-like structure is stable at room temperature during both synthesis and\ncharacterization, which facilitates device integration and broadens the range of\npractical environments where nitrogene-based materials may operate.”"}]

Decision letter