# In Situ Growth of Suspended Zirconene Islets Inside Graphene Pores

Rafael Mendes, Huy Quang Ta, Thomas Gemming, Heleen van Gog, Marijn van Huis, Alicja Bachmatiuk, Mark Rümmeli

► To cite this version:

Rafael Mendes, Huy Quang Ta, Thomas Gemming, Heleen van Gog, Marijn van Huis, et al. In Situ Growth of Suspended Zirconene Islets Inside Graphene Pores. Advanced Functional Materials, 2025, 35 (2), pp.2412889. 10.1002/adfm.202412889. hal-05409071

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https://hal.science/hal-05409071v1

Submitted on 11 Dec 2025

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# In Situ Growth of Suspended Zirconene Islets Inside Graphene Pores

Rafael G. Mendes, Huy Quang Ta, Thomas Gemming, Heleen van Gog, Marijn A. van Huis, Alicja Bachmatiuk, and Mark H. Rümmeli*

Experiments using a transmission electron microscope decomposed zirconium acetylacetonate with an electron beam, forming zirconium nanoparticles on graphene. Continued electron irradiation transformed these nanoparticles into atomically thick zirconium islets (zirconene islets) within the graphene lattice. The electron beam caused zirconium atom dislocations and vacancies that are rapidly refilled, a process repeating until the vacancies evolved into zirconium nanoribbons before breaking. This study offers insights into the electron-driven growth and degradation of zirconene islets, showcasing a method to fabricate freestanding zirconenes for use as atomically thin coatings in extreme environments.

## 1. Introduction

The exploration of two dimensional (2D) materials has revolutionized various fields of nanotechnology, offering exceptional properties for applications ranging from electronics to drug delivery. Among these materials, 2D metals, also known as metallenes, exhibit atomically thin planar arrangements of elemental metals.[1] This elusive class of layered materials has attracted considerable interest in recent years owing to its exceptional application potential in nanoelectronics,[2,3] sensors,[4] catalysis,[5-7] coatings,[1] and energy storage and conversion.[8] However, the isolation of metallenes remains challenging as these materials

cannot be simply exfoliated and transferred as other layered structures, such as graphene, hexagonal boron nitride, black phosphorous, transition metal dichalcogenides (TMDs), metal carbides and nitrides (MXenes), and easily investigated free-standing. Moreover, the nature of metallic bonds favors the formation of 3D nanostructures (nanoparticles) instead of 2D structures. However, reports on the complexities of isolated metallenes are limited. Only a few metallenes have been synthesized epitaxially on other specific metallic substrates, with even fewer being observed and investigated for freestanding applications.[9] Although

many metallenes have been theoretically predicted to exhibit stable structures,[10] the practical fabrication of 2D metals remains a fundamental knowledge gap.[9]

To address this challenge, advanced characterization and synthesis techniques are essential. Transmission electron microscopy (TEM) has immense potential for the characterization and synthesis of 2D nanomaterials with ultrahigh spatial and temporal resolutions.[11] TEM enables the visualization of atomic-scale structures and defects in 2D materials, thus facilitating the investigation of the crystallographic arrangement, stacking order, and edge structures, providing critical insights into the properties and performance of these materials. Moreover, TEM

R. G. Mendes, H. Q. Ta, T. Gemming, M. H. Rümmeli
Leibniz Institute for Solid State and Materials Research Dresden
Helmholtzstr. 20, 01069 Dresden, Germany
E-mail: mhr1@ifw-dresden.de

R. G. Mendes, M. A. van Huis
Soft Condensed Matter
Utrecht University
Princetonplein 5, Utrecht 3584CC, The Netherlands

R. G. Mendes
Interfaces, Confinement, Matériaux et Nanostructures
CNRS-Orléans
UMR7374, 1b rue de la Férollerie - CS40059, Orléans 45071, France

H. van Gog
Nanostructured Materials and Interfaces
Zernike Institute for Advanced Materials
University of Groningen
Nijenborgh 4, Groningen 9747 AG, The Netherlands

A. Bachmatiuk
Lukasiewicz Research Network
PORT Polish Center for Technology Development
Stablowicka 147, Wroclaw 54-066, Poland

M. H. Rümmeli
EBEAM Centre
Institute for Environmental Technology
Center for Energy and Environmental Technologies
VŠB-Technical University of Ostrava
17 Listopadu 15, Ostrava 70800, Czech Republic

M. H. Rümmeli
Soochow Institute for Energy and Materials Innovations
College of Energy

Collaborative Innovation Center of Suzhou Nano Science and Technology
Key Laboratory of Advanced Carbon Materials and Wearable Energy
Technologies of Jiangsu Province

Soochow University
Suzhou 215006, China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202412889

© 2024 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202412889
can be used to investigate the growth and synthesis of 2D nano-
materials by studying their nucleation, growth dynamics, and
evolution during real-time fabrication. A key method used in
TEM is scanning transmission electron microscopy-high-angle
annular dark-field (STEM-HAADF) imaging, which provides en-
hanced contrast and sensitivity to heavy elements. In addition to
imaging, TEM uses various spectroscopic techniques for mate-
rial characterization. Energy-dispersive X-ray spectroscopy (EDS)
is commonly used to determine the elemental composition of a
sample by detecting the characteristic X-rays emitted from the
interaction between the sample and electron beam, allowing the
identification and mapping of different elements. Another valu-
able TEM technique is electron energy loss spectroscopy (EELS),
which analyzes the energy loss of the primary electron beam as
it interacts with the atoms of the specimen, and provides infor-
mation on the chemical bonding, elemental oxidation states, and
electronic structure of the sample under investigation. Another
emerging technique in TEM is integrated differential phase con-
trast (iDPC), which combines phase-contrast imaging and TEM
by quantitatively measuring the differences in the phases of the
transmitted electrons after interacting with the atoms of the sam-
ple.

In this study, TEM plays a pivotal role in overcoming the chal-
lenges of metallene synthesis by offering advanced capabilities
for in situ analysis. Using TEM, it is possible to directly observe
the nucleation and growth processes of metallenes in real-time,
enabling a deeper understanding of their formation and stability,
particularly under different electron-irradiation conditions. This
technique not only bridges the gap between theoretical predic-
tions and practical synthesis, but also indicates new possibilities
for the controlled fabrication and applications of these promising
materials.

Among the various elements predicted to form stable 2D struc-
tures, zirconium (Zr), a group-IV metal in the period table with
an oxidation state of +4, stands out. Zr is primarily used as a
refractory and opacifier material.[12] Owing to its exceptional cor-
rosion resistance,[13] Zr is used as an alloying agent and coating
in materials exposed to extreme conditions such as aggressive
chemical and radioactive environments, high temperatures, and
surgical appliances.[12] Given the various industrial applications
of Zr, 2D Zr (zirconene) can potentially be used as a thin coat-
ing of materials exposed to extreme environments; for example,
it may be used as a radiation- and thermal-stress-resistant coat-
ing of materials and electronic components in the fast-developing
multi-billion-dollar space exploration industries.[14,15] Zr is also
widely employed in producing ceramics and refractory materials
owing to its high melting point and low thermal neutron absorp-
tion cross section[16] and is a key component in manufacturing
zirconia-based ceramics applicable in dental prosthetics, thermal
barrier coatings, and high-performance ceramics.[17,18] In addi-
tion, Zr compounds are used as catalysts[19] in various chemical
reactions and additives in the production of alloys, including su-
peralloys, for aerospace applications.[20]

The primary objective of this study is to advance the under-
standing and practical realization of metallenes by achieving the
in situ growth and analysis of zirconene. For this purpose, we
use graphene as a substrate to investigate the real-time growth
of suspended zirconene islets inside a TEM column at an atomic
resolution. This study not only demonstrates a method for fab-

ricating free-standing zirconene islets, but also provides insights
into their stability, phase behavior, and degradation under elec-
tron beam irradiation by monitoring the entire process using
high-resolution imaging, EDS, EELS, and iDPC. The ability to
observe these processes in real time is significant for overcom-
ing the challenges associated with the synthesis and application
of 2D metals. Ultimately, this study forms the basis for the use of
zirconene in extreme environments, such as in the aerospace and
nuclear industries, where lightweight and resilient materials are
in high demand. By elucidating the phase behavior and structural
integrity of zirconene at the atomic scale, this study contributes
to broader efforts to harness the unique properties of metallenes
for next-generation technologies.

## 2. Results and Discussion

Graphene has been successfully used as an electron-transparent
substrate to grow 2D nanomaterials inside TEM systems.[21-23]
As a single layer of carbon atoms arranged in a hexagonal lat-
tice, graphene provides a stable and atomically flat surface for
the deposition and growth of various nanomaterials. The nu-
cleation of heteroatoms in other 2D nanomaterials can be con-
trolled by utilizing graphene as a substrate. Lattice matching
and van der Waals interactions between graphene and nano-
materials promote epitaxial growth, facilitating the synthesis of
heterostructures and stacked layers, thus enabling the creation
of new materials with tailored properties and novel functionali-
ties, as well as the growth of lateral heterostructures inserted in-
side the graphene lattice. Using graphene as a substrate for the
growth of 2D nanomaterials offers considerable potential for ad-
vancing nanoelectronics, optoelectronics, and energy storage de-
vices.

The standard steps to transfer the graphene to the TEM grid and deposit Zr on the graphene are described in the Experimental Section and Figures S1 and S2 (Supporting Information). Typically, large amounts of graphene are found in amorphous materials after the deposition of Zr acetylacetone (Zracac), as illustrated in Figure 1a. The decomposition of Zracac under electron irradiation involves several steps. As reported in previous studies, high-energy electrons interact with acetylacetonate ligands, causing the cleavage of metal-ligand bonds. This leads to the reduction of Zr⁺ ions and the formation of free Zr atoms, which aggregate into metallic Zr nanoparticles. Simultaneously, the acetylacetonate ligands fragment into smaller volatile molecules such as carbon monoxide (CO), methane (CH₄), and other hydrocarbons, some of which may continue to be trapped within the substrate or around the Zr particles.[24-26] To reduce the amount of amorphous coating, electron beam irradiation (beam shower) was used to remove the excess amorphous carbon (C) and induce the formation of Zr clusters. The beam shower was performed at 130 kx magnification (approximately field of view of 80 × 80 nm²) and a screen current in the range of 15 nA (≈ 4715 e Å⁻²s⁻¹) for 25 min. Although lower electron doses can be used, longer irradiation periods are required. For example, when a 10 nA screen current (≈ 3140 e Å⁻²s⁻¹) is used, the irradiation time increases to 30–35 min. The irradiation period also depends on the amount of amorphous material on graphene (a lower amount of residue requires a shorter beam shower period). Screen currents higher than 15 nA (≈ 4715 e Å⁻²s⁻¹) tend to
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Removal of amorphous contaminations with electron irradiation. a–c) Illustrations of the beam shower depicting the reduction of amorphous carbon and the formation of Zr clusters. d,e) Typical initial state of the beam shower. f) EDS spectrum highlighting the large intensity of carbon. After 25 min of electron beam shower, there is a reduction of amorphous material, exposing the graphene and the formation of Zr clusters (depicted by the blue arrows), as presented in g,h). i) Bright field TEM-EDS spectrum after the beam shower depicting the reduction of carbon and oxygen intensities. j,k) STEM-HAADF image depicting the irradiated and non-irradiated area. l) Annular dark-field (ADF) image showing a higher magnification of the irradiated and non-irradiated areas. This same region was used to acquire elemental STEM-EELS mapping, shown in m–r).</figcaption></figure>

cause faster degradation of graphene, leading to the formation of various lattice defects, which result in instabilities that prevent the proper growth of the 2D nanostructures and the imaging conditions. The beam shower causes the removal of amorphous residues and the clustering of Zr atoms, as illustrated in Figure 1b,c. As electron irradiation continues, carbon contamination is gradually removed, leading to the isolation of pure metallic Zr nanoparticles. The initial state of a typical beam-shower experiment is shown in Figure 1d,e, where relatively large amounts of amorphous residue (amorphous C) can be observed. Video S1 (Supporting Information), shows a typical beam shower process with amorphous carbon removal, and Zr cluster formation is clearly visible. The quantification of the EDS spectrum collected during an acquisition time of 1 min is presented in Figure 1f, which shows that ≈87% of the atomic content was carbon. The remaining 13% was distributed between oxygen (11%) and Zr (2%). Figure 1g,h present the same region of Figure 1d,e after 25 min of beam shower at a 15 nA screen (≈ 4715 e Å⁻²s⁻¹) current readout. Figure 1h shows a clear visual reduction in the amount of amorphous C, as well as the formation of high-contrast regions depicted by the blue arrows, attributed to the formation of Zr clusters directly in contact with the graphene monolayer. The EDS spectrum presented in Figure 1i, collected under the same conditions as the initial state, shows a 9% reduction in C atomic content, whereas the oxygen (O) and Zr atomic contents increase to 17% and 5%, respectively. The overall O:C, Zr:C, and

Zr:O atomic content ratios before the beam shower were 0.13, 0.18, and 0.02, respectively, while those after the beam shower were 0.21, 0.29, and 0.06. The atomic ratios ultimately confirmed that the overall C content was reduced, whereas the O and Zr contents were higher than those before beam showering, implying that, while C was removed, Zr atoms clustered and formed distinguishable nanoparticles that were easier to detect (higher signal intensity) than those sparsely separated and coated by individual atoms with no order (amorphous). Considering that the sample was in an ultrahigh vacuum, the data also suggest that the initial O content most likely remained in the sample after the beam shower. Various other EDS measurements were performed using a bright-field TEM before and after the beam shower. The results show all observations, with the data shown in Figure S3 (Supporting Information).

To better evaluate the elemental distribution and quantify the C, O, and Zr contents before and after the beam shower, STEM-EDS elemental mappings were acquired, as shown in Figure S4 (Supporting Information). The atomic content of C, O, and Zr before the beam shower averages 82.28% (±3.17), 14.87% (±2.59), and 2.85% (±0.37), respectively. The atomic content of C, O, and Zr for the same area after the beam shower is 76.22% (±3.05), 19.27% (±3.16), and 4.50% (±0.58), respectively. These values are consistent with the quantifications obtained by EDS in the bright-field TEM mode, as shown in Figure 1. The STEM-EDS elemental maps presented in Figure S4 (Supporting Information)
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Typical formation and degradation of zirconene islet. a–d) Initial state of growth process when the Zr is introduced as small nanoparticles. e,f) Simulated and experimental images excluding the Zr nanoparticle signal. g–j) Complete formation of the zirconene islet inserted inside a graphene pore. k,l) Simulated and experimental images excluding the zirconene islet signal. m–p) Ruptured zirconene islet exposing the vacuum. q,r) Simulated and experimental images, excluding the ruptured zirconene islet signal.</figcaption></figure>

visually illustrate the signal intensities of the various mapped elements.

Figure 1j,k shows STEM-HAADF images with clearly distinguishable irradiated and non-irradiated areas, respectively. Higher magnification images of the interface between the irradiated and non-irradiated areas are presented in Figure 1l, clearly indicating that the non-irradiated area had a large amount of amorphous carbon with small Zr species (brighter spots) embedded within the amorphous phase, whereas the irradiated area clearly had larger Zr clusters. A comparison between the irradiated and non-irradiated bright-field TEM and STEM-HAADF images is presented in Figure S5 (Supporting Information).

STEM-EELS elemental mappings and overlays were obtained, as presented in Figure 1m–r. As shown in Figure 1m, the irradiated area had a lower C signal than the non-irradiated area, whereas the O and Zr signals were stronger in the irradiated area, as shown in Figure 1o,q, respectively. The direct comparison between atomic contents determined by STEM-EDS and STEM-EELS is summarized in Figure S6 (Supporting Information). The elemental mappings of both analytical techniques converged and corroborated each other, providing a systematic analytical study of the electron beam shower as part of the growth process of zirconene.

A detailed analysis of the STEM-EELS mappings indicated that the O signal often overlapped with larger clusters of Zr particles embedded in amorphous C (Figure 1r), suggesting that O atoms tended to be trapped inside these regions, possibly forming a mixture of oxidized C and Zr in these agglomerations. High-magnification STEM-EELS mapping and quantification of a different region (Figure S5f–j, Supporting Information) led to simi-

lar conclusions regarding the elemental distribution of C, O, and Zr, resulting in the initial hypothesis that all Zr clusters may contain C or O; however, later in this manuscript, the high-resolution analytical characterization of Zr clusters and zirconene islets provides evidence against this hypothesis.

Once the beam shower was complete and sufficient Zr clusters could be seen directly in contact with the graphene, the formation and degradation of zirconene islets were investigated, and the typical results are summarized in Figure 2. Immediately after the beam shower, the intensity of the electron beam is rapidly reduced to 1 nA (≈ 314 e Å⁻²s⁻¹) by spreading the electron beam intensity to avoid damage and the regions with isolated Zr clusters on the graphene are selected as shown in Figure 2a. The growth of 2D Zr was investigated using an electron beam with a screen current readout of 5 nA (≈ 1570 e Å⁻²s⁻¹). Typically, Zr clusters are present as small nanoparticles (1–3 nm), as shown in Figure 2b–d. Continuous electron irradiation under standard imaging conditions resulted in previously induced lattice transformations in Zr nanoparticles. Bulk Zr exhibits three primary phases: alpha (α), beta (β), and omega (ω), each with distinct structural properties and stability conditions. Among these, the α-phase is the most thermodynamically stable at room temperature and ambient pressure, characterized by a hexagonal close-packed (hcp) structure. An uninterrupted video showing the complete growth of the zirconene islets is presented in Video S2 (Supporting Information). The first observation is that the Zr nanoparticles are mobile on the graphene, which is the initial evidence that the Zr nanoparticles start on top of the graphene monolayer, as presented in the image simulation and experimental results with the excluded signal of the Zr nanoparticles
in Figure 2e,f. Further dynamic evolution of the system shows that during electron irradiation, individual Zr atoms escape from the nanoparticle, deposit in the vicinity of the nanoparticle, and come into direct contact with the graphene. Immediately after the Zr atoms move back into the main Zr nanoparticle, they leave a defect on the graphene lattice, and the Zr nanoparticles continue to move on top of the graphene, as shown in Figure S7 (Supporting Information). As the number of defects increased, the Zr nanoparticles stopped moving and appeared anchored on graphene, presumably stabilizing the C vacancy/defect. Based on the evidence that individual Zr atoms create defects in graphene, we hypothesize that continuous irradiation, in combination with the etching capability of Zr atoms, continues to slowly consume the graphene directly underneath the Zr nanoparticles until the graphene pores are sufficiently large to accommodate all the Zr atoms expanded as a confined single-crystalline atomic Zr layer.

Based on the hypothesis that the graphene under the Zr nanoparticles is degraded, the data demonstrate that the Zr nanoparticles continue to spread and are confined inside the graphene pores. Under electron-beam irradiation, the Zr atoms in the nanoparticles are not completely stable and continuously change phase until a 2D crystalline islet (namely, a zirconene islet) is formed, as shown in Figure 2g-j. According to our investigations, the only and most stable phase observed is most compatible with the α-phase (hcp phase) described for bulk zirconium.[27–29] Other bright-field TEM and STEM-HAADF images of the zirconene islets, as well as typical fast Fourier transform (FFT) and graphene, are shown in Figure S8 (Supporting Information). The lateral diameter of the islets (3.5 nm) was almost twice that at the initial nanoparticle stage (2 nm). Typically, zirconene islets remain stable for ≈1–2 min (particularly under low-dose exposure to an electron beam; < 150 e Å⁻²s⁻¹) until they rupture at the interface with graphene, exposing the vacuum, as shown in Figure 2m-p.

The results presented to date clearly demonstrate that the experimental results and image simulations agree with the initial hypothesis of suspended zirconene islets in graphene pores. To acquire experimental evidence for the graphene etching process, we analyzed the FFT of different stages of the zirconene islet growth process. Separating the signals of the different Zr nanostructures from the graphene signal in the FFT makes it possible to obtain further experimental evidence that no graphene was found under the zirconene. The methodological steps used to remove the individual FFT signals from the different structures are presented in Figure S9a-f (Supporting Information). Despite the difficulty in obtaining experimental information on the graphene under the Zr nanoparticles and zirconene islets, the FFT signal filtering allowed the observation of the initial Zr nanoparticle phase, which can help reveal the existence of graphene, unlike the case with the growth of zirconene islets. The results of FFT signal filtering for different growth phases are shown in Figure S9g-r (Supporting Information). The FFT signal filtering and image simulations of different zirconene islets are presented in Figure S10 (Supporting Information), ensuring the reproducibility of the method and the suspended nature of the zirconene islets.

The exclusion of the intact zirconene islet signal revealed the absence of graphene underneath, confirming its free-standing nature, as shown in the image simulation and experimental re-

sults in Figure 2k,l, respectively. These observations remained unchanged until the zirconene islets ruptured, as shown in the image simulation and experimental data in Figure 2q,r. To ensure the reproducibility of the data, other examples of zirconene islet formation are shown in Videos S3–S5 (Supporting Information).

The formation and chemical analysis of another zirconene islet are shown in Figure 3. Snapshots of the formation stages are presented in the high-resolution TEM images in Figure 3a-h. As described previously, the process starts with a Zr nanoparticle on graphene, which keeps evolving under electron irradiation (screen current readout of 6 nA or ≈ 1880 e Å⁻²s⁻¹) at maximum magnification (1.1 Mx). After ≈25 s, the Zr nanoparticles were fully crystallized as zirconene islets, as shown in Figure 3e. Continuous electron irradiation caused the islet to change phase multiple times, losing its characteristic hexagonal crystal symmetry, until it ruptured after 75 s. Figure 3i shows the initial state of the Zr nanoparticle, and Figure 3j shows a typical point EELS spectrum collected at the center of the nanoparticle to avoid any C signal from the surrounding graphene. The EELS spectrum of the Zr nanoparticles shows Zr edges (Zr-M₄.5:170 eV and Zr-M₃:330 eV) and a C edge (C-K: 284 eV). The C signal confirms the previous observation that the Zr nanoparticles were initially supported on graphene. Figure 3k shows the fully formed zirconene islets, and Figure 3l shows the EELS spectrum, which was also collected from the center of the nanostructure, where only the Zr edges are present (Zr-M₄.5:170 eV, Zr-M₂:344 eV, Zr-M₃:330 eV), with no C edges observed, indicating that the zirconene islets were suspended inside the graphene pores and that no C atoms were incorporated into the islets.

Notably, no O signal (O-K edge: 532 eV) was detected in the initial Zr nanoparticles or zirconene islets, contradicting the initial EDS and EELS elemental mapping results presented previously, where a higher relative O content after the beam shower was measured and the O and Zr signals overlapped. In all investigated cases, O was not detected in any phase of zirconene formation, suggesting that either the initial Zr nanoparticles are rapidly reduced under electron beam irradiation or that O is located somewhere else in the sample. To address this issue, various parts of the sample were investigated in detail using EELS. The results summarized in Figure S11 (Supporting Information) show that O is found only in large Zr clusters (> 10 nm) embedded in large amounts of amorphous carbon. Smaller Zr nanoparticles (< 5 nm) did not contain O even when nearby C species were observed. These results indicate that O remains trapped in the remaining amorphous carbon, which coincides with the location of the larger Zr cluster, which can act as a more stable reservoir of O (and carbon) in the form of an amorphous metal oxide or carbide. Moreover, Zr is known for its high oxidation resistance, and under ultrahigh vacuum and electron irradiation of the microscope, small Zr nanoparticles might be even more prone to be easily reduced to pure Zr during beam showering. Importantly, these results suggest that to the best of our knowledge, O is not present and does not play a role in the growth of zirconene islets (typically 1–4 nm in size).

After the complete formation of zirconene islets during electron irradiation, as illustrated in Figure 4a, the lattice parameters were investigated in detail. Figure 4b-d shows a ball-and-stick model, high-resolution bright-field TEM image, and image simulation of a fully crystallized freestanding zirconene islet. The
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Formation and chemical analysis of zirconene islet. a–h) Snapshots depicting each stage of zirconene formation. i) Initial stage of a Zr nanoparticle and j) a typical local EELS spectrum. k) Fully crystalline zirconene islet grown inside a graphene pore and l) a typical EELS spectrum.</figcaption></figure>

simulation results agree well with the experimental observations. Figure 4e depicts the region of the graphene and zirconene islets used to acquire the intensity line profiles of Zr and C. Figure 4f shows the intensity line profile of the Zr atoms; an average lattice spacing of 0.282 nm was measured. Density functional theory (DFT) calculations showed that an infinitely large zirconene sheet has a lattice spacing of 0.293 nm, consistent with experimental observations within a deviation of < 4% (see Section S5,

Supporting Information for DFT calculation details). A certain degree of variation in the lattice parameters is expected when comparing nanosized and confined heterostructure-like systems such as our zirconene islets in graphene pores with large, isolated sheets. Density functional theory (DFT) calculations also predicted that zirconene would retain metallic properties similar to those in the bulk state. Figure 4g shows the line profile of graphene with a lattice spacing of 0.247 nm, which is
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Lattice investigation of zirconene islets. a) Illustration highlighting the fully crystalline 2D Zr inserted inside the graphene lattice. b) Top view model of the 2D Zr inside a graphene pore. c) Experimental TEM image of a typical zirconene islet and d) TEM simulation of the zirconene inside the graphene pore. e) Regions from where the intensity line profiles of the Zr and C atoms were extracted and presented in f,g), respectively. h) HAADF image of the Zr membrane in graphene pore. i) iDPC image of zirconene islet and graphene. j) iDPC image in a larger magnification of the interface between Zr membrane and graphene. k) Same iDPC image shown in panel (j), depicting Zr atoms on top left side and graphene on bottom right. The dotted red line delineates the boundary between graphene and Zr membrane.</figcaption></figure>

consistent with theoretical and experimental values reported in the literature (0.246 nm). Here, graphene can also be considered as a built-in default scale bar, ensuring that the characterization of the islets is performed without the influence of lattice strain. However, this phenomenon can also indirectly reflect the structural variations of the islets.

Notably, the lattice parameters of the zirconene islets were not completely homogeneous across the entire structure, even under the normal relaxed conditions of the graphene lattice surrounding the islet. By measuring all the interatomic distances in the different islets, we observed that the Zr atoms closer to the edge of the graphene pore had smaller interatomic distances than those measured at the center of the islet. The measurement results for all directions are shown in Figure S12 (Supporting Information). Such inhomogeneities in the interatomic distances have already been reported for other metallenes and may be caused by the na-

ture of the element, the stability of the structure as a whole (particularly under electron irradiation), and the interaction with the C atoms on the edge of the graphene.

The HAADF detector is sensitive to the atomic number (× Z²), making it easier to observe heavier elements like Zr. However, the difference in atomic number between C (Z = 6) and Zr (Z = 40) is very large (ΔZ = 34), which does not enable the imaging of both elements simultaneously with the HAADF detector, as shown in Figure 4h. To overcome this challenge, integral differential phase contrast (iDPC) was used to image both C and Zr, as shown in Figure 4i, where the zirconene islets and honeycomb lattices of graphene can be observed. Figure 4j,k shows higher-magnification images of the interface between the zirconene and graphene regions. Because of the dynamic and unstable nature of the zirconene islets under electron irradiation, the atoms were never steady in the lattice, resulting in an iDPC image of the Zr
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Intensity line profiles for experimental and simulated images. a–d) Experimental high-resolution TEM images of zirconene and simulated images for mono-, bi-, and trilayer Zr. e) Intensity line profiles for all the experimental and simulated images depicting the Zr and C intensity line profiles.</figcaption></figure>

atoms that were not completely round. Nevertheless, the iDPC results represent the direct imaging of the system, indicating the absence of heteroatoms (C or O) integrated or underneath the zirconene islets, confirming that the zirconene islets were free-standing in the graphene pores. Figure S13 (Supporting Information) shows another area of zirconene with iDPC, further confirming that the islets were suspended in the graphene pores.

To investigate whether the zirconene islets were composed of a single atomic layer of Zr, the intensity profile signal method was used to reveal the presence of multiple Zr layers. This was achieved by normalizing the intensity signals of the C atoms and analyzing those of the Zr atoms. Because the C atoms are lighter, their signal in the image is higher than the intensity of the signal for heavier atoms such as Zr. Figure 5a shows a typical experimental image of zirconene, and Figure 5b–d show images simulating the presence of one, two, and three Zr layers stacked epitaxially on top of each other, respectively. The red, black, green, and blue areas indicate the directions in which the intensity line profiles of Zr and C were measured. The results show that the monolayer Zr intensity profile differs from that of other multilayered Zr systems and significantly differs from the intensity profiles of Cu and Fe. Whereas the intensity of the trilayer Zr was almost zero, the intensities of the bilayer and monolayer Zr were 86% and 91% higher, respectively. This analysis clearly indicates the presence of a single atomic layer of Zr. Figure S14 (Supporting Information) shows two other examples of line profile analyses in different zirconene islets, all of which present identical intensity line profile behaviors. Therefore, by combining the analytical EELS data presented in Figure 3 with the systematic intensity line profile analysis presented in Figure 5, the zirconene area was determined to be composed of an elemental Zr monolayer.

Until this point in the study, we described zirconene islets without defects. However, as highlighted previously and observed in the videos available in the supplementary information, these islets are very dynamic and, even when crystallized, can present constant atom displacement under electron irradiation imaging

conditions, ultimately leading to the rather constant formation and subsequent healing of Zr vacancies. Figure 6a illustrates that the continuous electron irradiation of the zirconene islets caused the dislocation of Zr atoms from the zirconene lattice, leaving Zr vacancies. The Zr atoms were sometimes dislocated, even in the surrounding graphene. Continuous irradiation caused the dislocated atoms to return to the Zr lattice, thereby reconstructing the initial 2D crystalline structure. A typical example of the healing process is shown in the high-resolution TEM images in Figure 6b–f. The exact moment at which the Zr atom was ejected is shown in Figure 6c,d, whereas Figure 6e depicts the Zr atom dislocated back to the lattice, reforming the intact zirconene islets, as shown in Figure 6f. Short Video S6 (Supporting Information), which is presented in the Supplementary Information, illustrates this process.

To investigate the formation of vacancies and healing in more detail, HAADF-STEM was used to image the Zr clusters on graphene. However, Zr vacancy formation and healing were not observed in the STEM mode. During convergent beam scanning, the islet was quickly destroyed, making any measurement in the STEM mode extremely challenging. This most likely occurred because once the convergent beam scans over the islet and creates a vacancy, the displaced Zr atom moves farther away in the subsequent scanning, which causes more Zr atom displacements and instabilities that lead to a quick rupture of the islet.

The direction of the scan may also play a role in the direction in which the Zr atoms are displaced in the microscope operating in scanning mode. To verify this observation, a small cluster of Zr attached to the edge of a graphene pore was observed with a screen current readout of 0.124 nA (40 e Å⁻²s⁻¹) and an acquisition time of 0.5 s. Figure 6g,h shows the Zr cluster (Zr atoms in purple) and depicts the region of a graphene pore with the white-dashed circular area. Continuous electron scanning dislocates the Zr atoms around the graphene edge, as shown in Figure 5i,j. After ≈30 s, the graphene pores are completely filled with Zr atoms, forming a small zirconene islet with 17 Zr atoms, as shown in Figure 6k.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Formation and healing of Zr vacancies on zirconene islets. a) Formation of Zr vacancies and lattice reconstruction during electron irradiation. b–f) Bright-field TEM images of the electron beam irradiation, depicting the formation and healing of Zr vacancies in zirconene islets. g–j) HAADF-STEM snapshot images depicting dislocation of Zr atoms over the edge of a graphene pore, creating a k) 17-atom zirconene islet.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Formation of zirconium nanoribbon. a) Formation of a Zr nanoribbon. b) Top view model of the Zr nanoribbon attached to opposing graphene edges. c) Experimental TEM image of a typical Zr nanoribbon and d) TEM simulation of the Zr nanoribbon inside the graphene pore. e) Regions from where the line profiles of the f) Zr nanoribbon and g) graphene were extracted.</figcaption></figure>

Video S7 (Supporting Information) available in the supplementary information shows the formation of the zirconene islet from the Zr cluster on the graphene edge using the scanning of the electron beam. These results provide preliminary evidence that electron beams can be used to manipulate Zr atoms on graphene. A more systematic study should be conducted to develop strategies that enable greater control of Zr atom displacement, which may even be extended to other elements.

In most cases, the Zr vacancies in the zirconene islets lead to complete rupture of the structure, leaving either a partially filled graphene pore or an amorphous Zr cluster on the edge of the graphene pore. However, continuous electron-beam irradiation sometimes leads to the formation of Zr nanoribbons, as shown in Figure 7a. Figure 7b–d compares the ball-and-stick model, experimental observations, and image simulations. Figure 7e shows the regions of the graphene and Zr nanoribbons from where the line profiles were measured, as previously performed for intact zirconene islets. The line profile of the Zr nanoribbon is presented in Figure 7f, which shows that the average interatomic distance was 13% larger (0.318 nm) than that of the pristine zirconene islet (0.282 nm), attributed to the lattice strains arising from the opposite pulling forces from both attachment extremities of the nanoribbon. Detailed measurements of all the interatomic distances in the nanoribbon were also performed (Figure S15, Supporting Information), and the results indicated that, as in the zirconene islets, the interatomic distances in the Zr nanoribbon were larger at the center of the nanoribbon than closer to the anchoring extremities with the graphene. Figure 7g shows the line profile of the graphene region with a lattice spacing of 0.250 nm, which is also larger than the same lattice parameter of the nondefective graphene neighboring the zirconene islets presented in Figure 4g. Video S8 (Supporting Information) shows the typical formation of Zr nanoribbons, and Video S9 (Supporting Information) shows the rupture of Zr nanoribbons.

### 3. Conclusion

This study presents a systematic step-by-step method for growing zirconene islets inside graphene pores. The growth of the new metallene islets began with the formation of Zr nanoparticles in the range of 1–4 nm on top of the monolayer graphene, resulting from the electron beam shower. The continuous electron irradiation of the Zr nanoparticles induced the formation of defects in the graphene lattice, which served as anchoring points for the Zr nanoparticles. We provided evidence that the Zr atoms created defects in graphene. The etching of graphene progressed under continuous electron irradiation until the Zr nanoparticles spread inside the graphene pores and finally crystallized into zirconene islets.

The purity and suspended nature of the zirconene islets were investigated using EELS and FFT signal filtering, which demonstrated that the islets were composed of pure Zr, with no graphene detected underneath the islets. Direct imaging of the interface between the islets and graphene was performed using iDPC, which revealed that no heteroatoms were embedded in the islets.

The results also showed that the interatomic distances of the zirconene islets varied across the lattice, where the Zr atoms closer to the graphene pore edge interface had smaller interatomic spacings than those in the center of the islet. The islets are electron beam-sensitive and continuously alternate between crystalline and amorphous phases. The electron beam also causes Zr vacancies, which can heal or grow larger, eventually evolving into an islet rupture. Occasionally, the destruction of the zirconene islets leads to the formation of Zr nanoribbons.

We also present preliminary results regarding the possibility of scanning a convergent electron beam to move Zr atoms to the graphene edge of a pore, leading to the formation of a zirconene islet. The manipulation of atoms on a support using an electron beam is a flourishing field in materials science and engineering,
potentially culminating in the complete bottom-up development of a new generation of high-performance materials.

Finally, the study was reproducible using different microscopes, manufacturers, and electron guns. The experiments were repeated using the most used equipment in the field.

## 4. Experimental Section

**Graphene Transfer:** Large-area graphene was grown on copper foil using chemical vapor deposition (CVD) and transferred onto gold TEM grids with a Lacey carbon film. Briefly, a piece of the graphene on the copper foil (1 × 1 cm²) was spin-coated in two steps (10 s @ 600 rpm and 60 s @ 2000 rpm) with 50 µL of poly-(methyl methacrylate) (PMMA) 950 k purchased from Alfa Aesar diluted to 4% in acetone. The PMMA-graphene-copper foil was then placed in a copper etchant solution (CE-200) purchased from Merck to etch the copper substrate for 30 min. The floating PMMA-graphene pieces were then rinsed five times in different containers with distilled water and placed onto the TEM grid by immersing the TEM grid in the distilled water and lifting the PMMA-graphene piece on the grid. The PMMA was removed with the vapor of acetone that gently condensates on the tweezer and rolls down the TEM grid, gently dissolving and removing the PMMA layer but leaving the graphene on the TEM grid. Annealing the graphene grids in dynamic high vacuum (in the order of 10⁻⁵ mbar) at 120 °C for 12 h was also recommended to remove excess remainders of PMMA on graphene. The transfer details are described in more detail and summarized in Figure S1 (Supporting Information).

**Zirconium Deposition on Graphene:** Zr was deposited on graphene by sublimating zirconium (IV) acetylacetonate ($Zr_{acac}$) powder in a sealed vial under high vacuum. A quartz tube containing the graphene grid and a few fine grains of the $Zr_{acac}$ powder was sealed under high vacuum (in the order of 10⁻⁷ mbar) and annealed at 200 °C for 12 h. Annealing causes the $Zr_{acac}$ powder to sublimate and deposit onto graphene. The details of zirconium deposition are described in more detail and summarized in Figure S2 (Supporting Information).

**Electron Beam Shower:** Typically, after $Zr_{acac}$ deposition, excess graphene is found in the amorphous materials. To reduce the amount of amorphous coating, electron-beam irradiation (beam shower) was used to expose the graphene monolayer and cluster the Zr atoms into small nanoparticles. The beam shower was operated at a magnification of 130 × with a screen current of 15 nA for 25 min.

**S/TEM Imaging:** Experiments were conducted using double aberration-corrected microscopes (FEI Titan, Thermo Fisher Scientific Spectra 300, and JEOL ARM200F) operating at 80 kV to reduce sample damage. Imaging in conventional mode was conducted with a screen current readout range of 4–6 nA. To avoid the rapid degradation of the grown zirconene islets, the beam intensity was reduced to a screen current readout of 1 nA (or less). The bright field TEM-EDS elemental analyses were performed with the same conditions of the beam shower irradiation (130 kx magnification, 15 nA screen current readout (≈ 4715 e Å⁻²s⁻¹), and 0° tilt in a SuperX EDS detector and 15° tilt in a single EDS detector), and the acquisition time was 60 s. The EELS investigation was performed in scanning mode with a camera length of 71 mm and convergence angle of 21.4 mrad. The EELS spectra were obtained with low current (typically 10 pA or ≈40 e Å⁻²s⁻¹) and an acquisition time of 1 s and then integrated ten times to improve the signal intensity. Notably, zirconene islets are extremely sensitive and can be rapidly destroyed by electron-beam irradiation. Hence, a low dose was crucial for imaging and spectroscopic measurements of zirconene islets. The ball-and-stick models were constructed using Materials Studio software, with multislice image simulations performed using JEMS software, as described in the Experimental Section.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

To access the TFS Spectra300 microscope at the Electron Microscopy Center, Utrecht, the authors acknowledge the Netherlands Electron Microscopy Infrastructure (NEMI), project number 184.034.014, part of the National Roadmap, financed by the Dutch Research Council (NWO). This project benefited from the expertise and facilities of the MACLE-CVL platform, which is co-funded by the European Union and Central Val de Loire Region (FEDER). A.B. thanks The National Science Center, project 2021/41/B/ST5/04328. M.H.R. thanks the National Natural Science Foundation of China (Grant No. 52071225), the European Union's Horizon Europe Research and Innovation Program under grant agreement No. 101087143 (Electron Beam Emergent Additive Manufacturing (EBEAM)), REFRESH Research Excellence for Region Sustainability, and High-tech Industries project No. CZ.10.03.01/00/22_003/0000048 via an operational program transition.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

graphene, metallene, transmission electron microscopy, zirconene

Received: July 19, 2024

Revised: August 22, 2024

Published online: September 10, 2024

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