Synthesis of Ti₄Au₃C₃ and its derivative trilayer goldene through chemical exfoliation

Abstract

Achieving large two-dimensional (2D) sheets of any metal is challenging due to their tendency to coalescence or cluster into 3D shapes. Recently, single-atom-thick gold sheets, termed goldene, was reported. Here, we ask if goldene can be extended to include multiple layers. The answer is yes, and trilayer goldene is the magic number, for reasons of electronegativity. Experiments are made to synthesize the atomically laminated phase Ti₄Au₃C₃ through substitutional intercalation of Si layers in Ti₄SiC₃ for Au. Density functional theory calculations suggest that it is energetically favorable to insert three layers of Au into Ti₄SiC₃, compared to inserting a monolayer, a bilayer, or more than three layers. Isolated trilayer goldene sheets, ~100 nanometers wide and 6.7 angstroms thick, were obtained by chemically etching the Ti₄C₃ layers from Ti₄Au₃C₃ templates. Furthermore, trilayer goldene is found in both hcp and fcc forms, where the hcp is ~50 milli–electron volts per atom more stable at room temperature from ab initio molecular dynamic simulations.

Goldene takes on more forms, where trilayer is the magic number.

INTRODUCTION

Single-atom-thick sheets of gold, termed goldene, was recently discovered (1). Beyond the well-explored realm of graphene, a rich landscape of two-dimensional (2D) materials emerges, each with distinctive characteristics and applications (2, 3). Among these materials, gold—renowned for its noble nature and catalytic prowess—has demonstrated notable potential in the form of nanoparticles for practical applications in biosensing, catalysis, and gene therapy (4–6). When confined to an atomically thin layer, gold exhibits fascinating properties, further broadening its potential in various fields. Bandgap opening and spin-orbit splitting have been reported in substrate-supported monolayer Au (7, 8), qualifying it for optoelectronic and spintronic devices. Because of the high surface area–to–volume ratio, 2D configurations offer more active sites that can participate in electrochemical reactions, thereby enhancing the catalytic performance (9). The interaction of 2D Au with light, owing to the quantum confinement effect, results in intensified surface plasmon resonance, which plays a key role in maximizing solar light absorption and scattering (10). Density functional theory (DFT) calculations predict that 1D Au nanorods and 2D Au nanoprisms with thiolate-ligand passivation could improve near-infrared absorption properties (11).

Nevertheless, 2D materials are often susceptible to structural instabilities due to their high surface area. Gold typically forms 3D metallic bonds, and forcing it into a 2D configuration can lead to structural instability and reconstruction. Therefore, achieving precise control over the size, shape, thickness, and quality of 2D materials is crucial for progress. Efforts have been made to fabricate atomically thin Au membranes through physical or chemical methods. [001]-Oriented Au nanosheets with a thickness of 0.2 to 0.4 nm (1 to 2 atomic layers) were synthesized and stabilized within the confined space provided by layered double hydroxides (12). Wang et al. (13) fabricated alleged free-standing monoatomic thick Au by dealloying bulk Au-Ag crystals through electron beam irradiation. Free-standing two-atomic-thick Au nanosheets were synthesized via a wet-chemical route using methyl orange as a surfactant, which exhibited very high catalytic activity (9, 14). It has been reported that free-standing monolayer Au with nanoribbon structures can suspend in graphene pores (15). Monolayer gold quantum dots were coated onto hexagonal boron nitride (BN) surfaces using pulsed laser deposition, demonstrating tunable bandgaps depending on their size and shape (7). Au was thermally intercalated and stabilized as 2D configuration with a single atomic thickness between silicon carbide and monolayer graphene (8). In addition, single-layer Au can be intercalated between graphene layers to form a graphene-goldene-graphene structure with weak interlayer interactions (16). Microwave synthesis of atomically thin Au crystals and their 2D hybrids with graphene, BN, and molybdenum disulfide (MoS₂) has also been demonstrated (17). Theoretically, DFT and ab initio molecular dynamics (AIMD) have predicted stable free-standing 2D monolayer of Au (1, 18, 19). First-principles calculations suggested that goldene has excellent conductivity at room temperature, potentially reaching the same order of magnitudes as carrier-doped graphene (20). The protocol for preparing single-atom-layer goldene (1) involves etching away the M and X layers (T₃C₂ sheets) from the Ti₃AuC₂ MAX phase using an alkaline potassium ferricyanide solution (Murakami’s reagent) together with surfactants, leaving free-standing single-atom-thick Au layers. M_n+1AX_n phases are a class of materials that combines metallic and ceramic properties (21). The acronym “MAX” stands for the three main constituents of these compounds: M represents an early transition metal, A is an element from group IIIA or IVA of the periodic table, and X denotes carbon or nitrogen. The layers consist of transition metal carbide or nitride slabs interleaved with A-group elements. Selective etching of the A layers produces 2D M_n+1X_n sheets, known as MXenes, with the most well-known example being Ti₃C₂ (22, 23).

The question now arises: Can goldene be generalized to a varying number of atomic layers in a series 1, 2, 3, …? The study of multilayer goldene would help understand how additional layers influence physical and chemical properties of 2D layered materials and provide a test platform for studying quantum confinement and interlayer coupling effects. Trilayer goldene, depending on how its atomic layers are stacked, could support the exploration of exotic electronic states of noble metals. A recent study (24) indicated the possibility to make percolation-free quasi-2D Au films (thickness down to 3.5 nm) using graphene-inspired technology, providing inspiration for goldene exfoliation.

Here, we report isolated 2D trilayer-thick Au sheets (termed trilayer goldene), with a thickness of 6.7 Å, achieved by etching away Ti₄C₃ from Ti₄Au₃C₃. The template, Ti₄Au₃C₃, is a rising MAX phase formed by thermal annealing of Au-capped Ti₄SiC₃ thin films, where Si is completely replaced by Au. The trilayer goldene can adopt both ABA and ABC stackings (hcp and fcc structures) due to similar thermodynamic free energy and dynamic stability, as confirmed by DFT calculations and molecular dynamic simulations. The Ti₄AuC₃ phase is also found to be attainable, but it is energetically less stable than Ti₄Au₃C₃ with respect to competing compound formation. Furthermore, the electronic properties of the trilayer goldene were studied by x-ray photoelectron spectroscopy (XPS). A family of goldene allotropes is thus discovered.

RESULTS

Synthesis of Ti₄Au₃C₃

Figure 1A shows a high-resolution scanning transmission electron microscopy (HRSTEM) image of a sputter-deposited epitaxial Ti₄SiC₃ film on Al₂O₃(0001) substrates with a TiC(111) seed layer, taken along the [$11\overline{2}0$] direction. The image reveals a characteristic laminated structure where Ti₄C₃ sheets are sandwiched between Si (c.f. silicene) layers. The film was sputter-coated with a 200-nm-thick Au layer, and the Au-covered Ti₄SiC₃ sample was then annealed at 670°C for 30 hours in a N₂ atmosphere to obtain a high-quality Ti-Au-C phase. The HRSTEM image and correspoding energy-dispersive x-ray (EDX) map of the annealed sample in Fig. 1 (B and C) show a characteristic nanolaminated structure corresponding to the M₄A₃X₃ phase, where Ti₄C₃ sheets are now sandwiched between three-atom-thick Au layers. The c-lattice parameter increased from 22.8 Å for Ti₄SiC₃ to 32.5 Å for Ti₄Au₃C₃, reflecting a 42.5% increase. Despite this conspicuous lattice expansion, the coherent laminate structure is retained. The expansion is attributed to the insertion of Au, where each Au layer consists of atoms larger than those of the original Si in the A layer. The Ti and Au signals in the EDX map align well with the alternating laminated Ti and Au layers, confirming the insertion and location of Au. The corresponding EDX spectrum shows the presence of Ti, Au, and Si (Fig. 1D). The relative atomic ratio of Ti:(Au + Si) is about 4:3, consistent with the stoichiometry of a hybrid 433 MAX phase, as shown in the inset table of Fig. 1D. The low content of Si (at the 1.5 atomic % detection level) indicates that the Au intercalation was nearly complete. Figure 1E shows the x-ray diffraction (XRD) patterns of the as-grown Ti₄SiC₃ film and the annealed Au-covered Ti₄SiC₃ sample. The corresponding (000l) peaks shifted to lower angles after annealing, reflecting the lattice expansion along the c axis. The c parameter is calculated to be ~32.3 Å from the (0004) peak, which is in good agreement with the value obtained from direct observation in the scanning transmission electron microscopy (STEM) image (Fig. 1B).

Fig. 1. Synthesis of Ti₄Au₃C₃.

(A) HRSTEM image of sputter-deposited Ti₄SiC₃. (B) HRSTEM image of the Ti₄Au₃C₃ phase obtained after 30 hours of annealing at 670°C. Both STEM images were recorded along the [$11\overline{2}0$] direction. Ti and Au atoms are circled in light blue and yellow, respectively. (C) Corresponding EDX elemental mapping of (B). (D) EDX spectrum of Ti₄Au₃C₃ phase showing atomic ratio of Ti, Au, and C elements. (E) XRD patterns of Ti₄SiC₃ and Au-covered Ti₄SiC₃ after 30 hours of annealing at 670°C. at %, atomic %; cps, counts per second; a.u., arbitrary units.

The mechanism of the substitution intercalation reaction can be explained by the loosely bonded Si atoms that are provided with a reduced chemical potential path to diffuse out into the Au capping layer, leaving behind vacancies that are subsequently backfilled with Au. This process is facilitated by the interdiffusion abilities of the two elements, in conjunction with their eutectic phase diagram (25, 26). Detailed mechanism can be found in section S1. Similar transformations of Ti₃Au_xC₂ (x = 1 and 2) from Ti₃SiC₂ and Ti_n+1Au₂C_n from Ti_n+1AlC_n (n = 1 and 2) have been reported (25, 27). An attractive aspect of this work is the demonstration that three atomic layers of Au can substitute each Si layer in Ti₄SiC₃.

In large-scale STEM images (section S1), peculiar examples of intercalation frontlines for the trilayer Au were observed, where the second and third gold layers closely follow the first layer, effectively pushing apart two adjacent Ti₄C₃ sheets. The deformation of the Ti₄C₃ sheets is conspicuous during the insertion of three atomic layers of Au, as indicated by the blue lines. Ti₃C₂T_x sheets have shown a comparable Young’s modulus and exhibit high ductility, tensile strength, as well as toughness (22, 28). Similarly, high elasticity is expected for Ti₄C₃ sheets, which explains why the laminated structure was retained despite undergoing notable deformation and lattice expansion.

The intercalation of monolayer Au in Ti₄SiC₃ was also explored at lower annealing temperatures with shorter durations to synthesize Ti₄AuC₃ (section S2); however, this phase is not prevalent. To investigate the impact of inserting additional layers of Au into Ti₄AuC₃ from a theoretical perspective, multiple structures were considered. These are illustrated in section S3 and include different stackings of the Au layers as well as different stackings of the Ti₄C₃ subunits relative to the Au layers. Their energies have been calculed using DFT, with a focus on the configurations found to be lowest in energy. These are shown in Fig. 2A with calculated lattice parameters and space group symmetries provided in table S1. It is important to note that for Ti₄Au₃C₃, Ti₄Au₄C₃, and Ti₄Au₅C₃, there are additional structures with alternative stacking of Au that are close in energy to those illustrated in Fig. 2A. The common feature of all these low-energy structures is that the Au layers are stacked in an fcc, hcp, or mixed configuration. Using eq. S1, we calculated the energy gain or cost associated with inserting additional layers into Ti₄AuC₃. Figure 2B shows that it is energetically favorable to insert additional layers of Au into Ti₄AuC₃, as indicated by the negative energy change of −0.285 eV when transitioning from Ti₄AuC₃ to Ti₄Au₂C₃ and −0.078 eV when transitioning from Ti₄Au₂C₃ to Ti₄Au₃C₃. At three layers of Au, i.e., Ti₄Au₃C₃, a minimum energy of −0.363 eV per inserted Au layer is achieved relative to Ti₄AuC₃. The addition of a fourth or fifth Au layer, however, will cost energy by +0.061 eV and +0.032 eV, respectively, when transitioning from Ti₄Au₃C₃ to Ti₄Au₄C₃ and Ti₄Au₅C₃. Additional phase stability data for Ti₄Au_1+xC₃ compared to its set of most competing phases can be found in table S2.

Fig. 2. DFT calculations for Ti₄Au_1+xC₃ phases.

(A) Schematic illustration of increasing number of Au layers in Ti₄Au_1+xC₃ with Ti, Au, and C atoms colored in blue, gold, and black, respectively. (B) Change in energy for Ti₄Au_1+xC₃ upon inserting layers of Au to Ti₄AuC₃. Bonding analysis in terms of integrated partial crystal orbital Hamiltonian population (IpCOHP) given (C) per formula unit (fu) and (D) per interaction. (E) Bond lengths for structures depicted in (A).

A reason why three layers of gold (Ti₄Au₃C₃) are energetically most favorable, as shown in Fig. 2B, is revealed through the bonding analysis in Fig. 2 (C and D) and fig. S11, combined with the evaluation of bond lengths shown in Fig. 2E. For Ti₄AuC₃, Au-Au interactions are only found in-plane. However, when additional layers of Au are inserted into the structure, out-of-plane Au-Au interactions appear. Beyond the trilayer Au configuration, for the Au layer in the middle, out-of-plane Au-Au interactions occur without direct bonding to Ti. This positively affects bonding, as seen in Fig. 2C, where the integrated partial crystal orbital Hamiltonian population increases with additional Au layers. Analysis of the individual contributions in Fig. 2D shows that Ti-Au interactions also benefit from the presence of multiple Au layers. Figure 2D further demonstrates that three layers of Au in Ti₄Au₃C₃ provide the largest total individual contribution. This is corroborated by the bond lengths shown in Fig. 2E, where Ti-Au and Au-Au bonds (adjacent to the Ti layer) are shortest for Ti₄Au₃C₃, indicating improved bonding strengths compared to configurations with fewer—or more—than three Au layers. The experimentally calibrated in-plane Au-Au distances, which are slightly shorter than the calculated values for Ti₄Au₃C₃ embedded in Fig. 2E, are found in section S4.

Another perspective on the relative stability of the trilayer goldene is from the difference in electronegativity between Au (2.54), Ti (1.54), and C (2.55) compared to Si (1.90) or Al (1.61). As Au is more electronegative than Ti, there will be electronic charge transfer from Ti to Au. The stability of goldene stacks would then be a delicate balance of charge transfer for the metal bonding between the Au atoms and a trilayer of Au happens to cause the “right” amount of charge transfer from Ti to Au for its highest stability, compared to less or more gold layers (see section S4, including test DFT calculations).

Atomic stackings in trilayers Au in Ti₄Au₃C₃

In this study, we found that trilayers of Au in Ti₄Au₃C₃ contain both ABA and ABC stacking configurations (Fig. 1B and section S4), corresponding to 2H hcp and fcc structures, respectively. The coexistence of hcp and fcc stacking does not lead to a spread in the c parameter. For the fcc phase of Au, the calculated a parameter is 3.091 Å and the c parameter is 32.529 Å. These values are very similar to those of the hcp phase of Au (table S1), which aligns with the experimental observations (section S4). The Ti₄Au₃C₃ phase with ABA stacking of Au has an energy of −0.363 eV, whereas the ABC stacking has an energy of −0.354 eV (fig. S8). Both trilayer structures are therefore feasible as stable Au allotropes. From simulated ABA structures (fig. S8), Ti₄C₃ sheets are mirrored with Au layers. In contrast, for the ABC stacking, Ti₄C₃ sheets stack in a zig-zag pattern along [$11\overline{2}0$] with respect to the Au layers. Since the Ti₄C₃ sheets in Ti₄SiC₃ and Ti₄AuC₃ are mirrored with A layers (Si or Au layers), as observations in STEM images and simulated structures (Fig. 1A and figs. S2 and S6), the position of the Ti₄C₃ sheets did not shift laterally when the monolayer Au was introduced. This also applies to the Ti₄Au₃C₃ with ABA-stacked Au, where the Ti₄C₃ sheets did not shift. To achieve the formation of ABC-stacked Au layers, displacement of two adjacent Ti₄C₃ sheets is required. Such a displacement was observed in STEM images (Fig. 1B), where Ti atoms move laterally by approximately 0.5 Å relative to their original mirrored positions. Similar phenomena have also been observed when inserting bilayer Au into Ti₃SiC₂ to form Ti₃Au₂C₂ (25). However, gliding of M_n+1X_n slabs in the MAX family has not previously been reported. A possible mechanism for Ti₄C₃ gliding is discussed in section S5.

The coexistence of hcp and fcc quasi-goldene sheets can be understood by considering the reduced dimensionality of the metal, which increases the proportion of surface energy relative to the total system energy. Consequently, material properties can be tuned through crystal phase engineering. For instance, hcp metals often exhibit anisotropic properties due to the directional arrangement of atoms, while fcc metals typically have isotropic properties. These anisotropic materials are of interest across various disciplines, because of unique properties. Compared to their fcc counterparts, hcp Au nanoparticles have shown plasmon and interband transitions (29), and a 130-fold increase in in-plane resistivity and more pronounced plasmon absorption have been demonstrated in 4H Ag (30). Zhang’s group (31, 32) synthesized ~2.4-nm-thick 2H hcp Au nanosquare sheets and 4H Au nanoribbons using wet-chemical methods. Ye et al. (9) reported an hcp Au phase existing at the edge of two-atomic-thick fcc Au nanosheets. Kondo and Takayanagi (33) synthesized fcc Au nanowires encapsuled by an hcp Au outer shell. Such non-fcc crystalline phase is known to stabilize ultrathin Au nanostructures (34, 35). An hcp/fcc alternating structure was observed in a square-like Au plate due to the hcp to fcc transformation as the plate grows thicker (36). To the best of our knowledge, the finding of large-scale, isolated, subnanometer-thick Au sheets with hcp structures is also original.

Preparation of trilayer goldene

To obtain isolated trilayer goldene, Ti₄C₃ sheets were selectively etched away using 0.5% Murakami’s reagent with 5 mM cetrimonium bromide (CTAB) for 168 hours. A schematic illustration is shown in Fig. 3A. Ti₄C₃ sheets are stepwise oxidized by radical nascent oxygen [O] generated in an alkaline solution of potassium ferricyanide (K₃[Fe(CN)₆] in KOH) (1, 37). CTAB surfactant was used to permeate the gaps once Ti₄C₃ slabs between freed and to impede the agglomeration of 2D gold layers into multilayers or nanoparticles, as applied for gold nanoparticles (38, 39). We obtained an average in-plane Au-Au spacing of 2.86 Å in trilayer goldene (section S6), which is close to the equilibrium interatomic distance in fcc bulk Au (2.884 Å) and slightly larger than the DFT-calculated values (section S7). The Au-Au spacing in trilayer goldene is approximately 6.5% smaller than that in Ti₄Au₃C₃ due to lattice contraction after exfoliation. Figure 3B shows an etching frontline observed at the edge of a Ti₄Au₃C₃ film, where trilayer goldene is being exfoliated from the right side. The goldene sheets remain separate after etching but begin to ripple from the edges once losing the support of the Ti₄C₃ sheets. The gap opening observed at the edges can be attributed to the fact that a CTAB chain, with a length of up to 20 Å, is larger than the distance between goldene sheets. CTAB molecules, which bind vertically to Au surfaces in a bilayer formation (40), can expand the gaps between adjacent goldene sheets. The goldene sheets in the more deeply etched regions (the central area of the image) appear to maintain relatively better flatness, likely due to a slower infiltration of CTAB molecules parallel to them. Initial results indicate that an etched-free trilayer goldene is more stable than the corresponding monolayers (1) and can maintain its structure with ripple features extending over a hundred nanometers. Meanwhile, blob formation of Au at the edges and their lateral diffusion through the sheets occur (fig. S19). A magnified image reveals some four- and five-layer goldene sheets near the edges, as shown in Fig. 3C. The formation of blobs and thicker layers can be attributed to the rapid interactions between the released goldene sheets and excess spurious Au atoms during etching processes (1). In addition, this phenomenon is related to the coalescence of goldene sheets during ion milling in the transmission electron microscopy sample preparation processes (fig. S20). The black regions in Fig. 3C clearly indicate the complete removal of Ti₄C₃ sheets and confirm that the thickness of the three-atomic-layer Au is approximately 6.7 Å. The thickness of trilayer goldene inside of Ti₄Au₃C₃ is measured to be 6.7 Å (see Fig. 3B). The corresponding EDX map of Au exhibits a distinct lamellar signal, while Ti is evenly dispersed (Fig. 3, D and E), further confirming the complete etching. Solutions with higher concentration of Murakami’s reagent would be more aggressive toward Ti₄Au₃C₃, leading to faster etching of Ti₄C₃ sheets and the formation of Au particles through sheet clustering and curling up of sheets. Conversely, etching with a lower concentration of 0.2% requires a much longer time to achieve complete etching (fig. S21). Results about etching without CTAB and the stability of trilayer goldene can be found in figs. S22 and S23. In addition, AIMD simulations have shown that two adjacent goldene layers that contain Si impurities coalesce in few picoseconds if their interlayer spacing is within 7 Å (1). The goldene-goldene interaction weakens substantially as the interlayer spacing increases from 10 to 12 Å and 14 Å. In this work, the distance between Au sheets is about 12 Å in Ti₄Au₃C₃, which is larger than the ~9.3-Å spacing in Ti₃AuC₂. In contrast, the coalescence of two adjacent trilayer goldene after etching was less observed, as wider channels are provided for surfactants to stabilize the free-standing goldene sheets. Furthermore, the as-synthesized Ti₄Au₃C₃ film is very close to ideal stoichiometry with negligible Si content, as confirmed by STEM-EDX and XRD (Fig. 1), implying that any disturbance of goldene layers by Si impurities is minimal.

Fig. 3. Preparation of trilayer goldene.

(A) Schematic illustration of the trilayer goldene preparation. (B) Cross-sectional HRSTEM image of an etching frontline, where the original Ti₄Au₃C₃ structure remains at the left side and isolated trilayer goldene sheets appear at the right side. (C) A magnified HRSTEM image of trilayer goldene extracted from fig. S19 and its corresponding EDX elemental mapping of Au (D) and Ti (E).

Pure Au hcp square sheets have been found to become stable under ambient conditions when they are less than ~6 nm thick (31), and ultrathin nanowires have been stabilized by hcp surface structures (33). Therefore, it is not unexpected that the present trilayer goldene contains hcp-stacked regions. These factors collectively suggest promising potential for the production of larger quasi-2D Au sheets. The process yield of trilayer goldene is discussed in section S6.

DFT and AIMD investigations of free-standing trilayer goldene

To investigate the dynamic and energetic stability of fcc-like and hcp-like trilayer goldene sheets, DFT calcuations were carried out at 0 K and AIMD simulations at 300 K (section S7). DFT calculations show that ABA and ABC trilayer Au slabs have nearly equal energy. For both Perdew-Burke-Ernzerhof and local density approximation results, the energy difference between fcc and hcp slabs is less than 1 meV/atom. It has been experimentaly demonstrated that the hcp phase exists in Au nanostructures (41), as the stacking fault energy in fcc metals is quite low.

On-the-fly machine learning–assisted AIMD simulations at 300 K were conducted to verify the dynamical stability of fcc- and hcp-stacked trilayer goldene. The dynamics of the trilayers were monitored for 0.18 ns (fcc) and 0.38 ns (hcp). The simulations indicate that both fcc and hcp stackings are dynamically stable, as shown by time-averaged Au positions in fig. S25. After a brief transient period, during which the Au layers shift to their equilibrium separation distance, the atoms continue vibrating around their respective fcc (or hcp) lattice positions for the entire simulation. The time-averaged potential energies suggest that, at room temperature, the hcp stacking is approximately 45 meV/atom more stable than the fcc stacking. An additional contribution arises from the vibrational free energy (F_vib), with the hcp-structured sheet being further stabilized by 5 meV/atom more than the fcc (fig. S24). Accordingly, the Helmholtz free energy (F) of the two allotropes, obtained by adding F_vib to the time-averaged potential energies, shows a difference of ∆F_hcp-fcc ≈ 50 meV/atom, indicating that hcp trilayer Au is substantially more stable than fcc trilayer Au at 300 K. Therefore, we predict that both structures would be retained after etching, with the hcp structure likely being more prevalent, as inferenced from the dominant ABA-Au in Ti₄Au₃C₃ (section S4). hcp-structured trilayer goldene is observed (fig. S18). The prevalence of trilayer goldene may thus be attributed to the relative stability of the hcp stacking.

Electronic properties of trilayer goldene

We performed XPS measurements on Ti₄Au₃C₃ before and after etching with 0.5% Murakami’s reagent as well as on a reference sputter-etched Au foil. Figure 4 shows the corresponding Ti 2p, C 1s, and Au 4f core-level XPS spectra. The Au 4f_7/2 peak of the pure reference Au film (Fig. 4, right) is located at 84.0 eV with a 4f spin-orbit splitting of 3.7 eV, consistent with reference values (42). The Au 4f_7/2 and 4f_5/2 peaks of the unetched Ti₄Au₃C₃ film are asymmetric, revealing the presence of a second low-intensity doublet located at binding energies (E_b) of 84.9 eV and 88.6 eV, respectively, i.e., shifted by 0.9 eV to higher E_b with respect to the E_b of the reference Au metal (84.0 and 87.7 eV). The occurrence of the high E_b doublet is attributed to electronic charge transfer primarily from the Au 5d states to the Ti₄C₃ sheets in Ti₄Au₃C₃. Similar effects were observed in the Ti₃AlC₂ and Ti₂AlC MAX phases (1, 43, 44), where charge transfer takes place from Al to the Ti_n+1C_n (n = 1 or 2) sheets. The stronger 4f doublet in the spectrum from the unetched Ti₄Au₃C₃ film appears at nearly the same E_b as for the reference Au. This can be attributed to the partially remaining capping Au layer on top of Ti₄Au₃C₃ after chemical-mechanical polishing and/or to Au-Au interactions in the trilayer Au. In contrast to Ti₃AuC₂, which has in-plane Au-Au interactions within each Au monolayer and Au-Ti interactions on both sides, in the case of Ti₄Au₃C₃ Au-Au interactions are stronger relative to Ti-Au resulting in reduced charge transfer and a higher intensity of the main Au 4f doublet with respect to the high E_b pair.

Fig. 4. X-ray photoelectron spectra of the trilayer goldene, Ti₄Au₃C₃, monolayer goldene, and reference Au.

Ti 2p (left), C 1s (middle), and Au 4f (right) core level spectra measured on the reference sputter-cleaned Au foil (yellow), Ti₄Au₃C₃ (green), etched Ti₄Au₃C₃ (red), and etched Ti₃AuC₂ (blue) (1). Black solid and colored dash-dot lines represent experimental data and the sum of fitted peaks, respectively. Au-Ti in Ti₄Au₃C₃, Au atoms with fewer ligancy in trilayer goldene, monolayer goldene, and Au-Au interactions are shown in gray, blue, pink, and orange, respectively.

The etched Ti₃AuC₂ film exhibits high E_b shoulders, shifted by ~0.9 eV with respect to the main Au 4f doublets. These shoulders are attributed to final state screening and charge transfer effects (1). After etching, the Au 4f spectrum of the trilayer goldene, shown at the top of Fig. 4, reveals that the 4f doublet peaks are nearly at the same energy positions as those of the reference Au. In addition, a less intense high-energy tail is observed, which can be fitted with a second 4f_7/2-4f_5/2 doublet shifted to higher E_b values of 84.8 and 88.5 eV, respecitvely. Such E_b shifts in the high-energy tails of Au 4f spectra from monolayer goldene have been attributed to the final state effects (45, 46). It has been demonstrated that both the E_b and the peak width of the Au 4f lines increase as the size of Au nano particles and clusters decreases, due to limited screening of the core hole left after photoionization (47). In monolayer goldene produced from Ti₃AuC₂ (Fig. 4), the final state effects are enhanced because of the smaller coordination number of Au atoms (six or fewer) compared to bulk fcc Au and Ti₃AuC₂ (1). In contrast, in trilayer goldene, the coordination number of the middle layer is 12, while the outerlayers have a coordination number of 9, likely decreasing further at the sheet edges. Therefore, the final state effects are weaker in trilayer goldene compared to the monolayer, resulting in a less intense tail and a slightly smaller E_b shift. The dominant Au 4f doublet in the spectrum from etched Ti₄Au₃C₃ may originate from residuals of the capping Au layer or from sheets clustering and their curling-up. The tendency of goldene sheets to transform to 3D shapes can be expected from the surface morphology on the Ti₄Au₃C₃ after etching (section S9).

The C 1s XPS spectrum of the etched sample (Fig. 4, middle) reveals that the intensity of the carbide peak at ~281.7 eV has markedly decreased as compared to the unetched sample. This indicates that almost all Ti₄C₃ has been removed during the etching of Ti₄Au₃C₃. The corresponding Ti 2p spectra (Fig. 4, left) further confirm this conclusion: The Ti 2p_3/2-Ti 2p_1/2 peaks due to the carbide (at 454.8 and 460.8 eV, respectively) are very weak in the spectrum from the etched sample. The latter is dominated by a doublet peaks located at 458.6 and 464.5 eV, respectively, i.e., indicative of TiO₂ formation on the Au layers. Minor impurity peaks from Fe 2p, Br 3d, and K 2p photoemissions (48) imply that a small amount of iron and potassium residue from the etchant occurs at the surface after etching (section S10).

DISCUSSION

We propose that, in addition to monolayer (1) and trilayer goldene (present work), isolated bilayer goldene can also be obtained by etching Ti₃Au₂C₂, a compound reported in 2017 (25). Combined with the recently reported goldene (1, 25), this work demonstrates that the thickness of isolated Au sheets can be tuned into one-, two-, and three-atomic layers using our devised etching scheme, which includes the use of surfactants. This approach provides a foundation for future fundamental and applied studies on ultrathin Au with varying thicknesses. When using a MAX phase precursor with a larger interlayer distance, such as Ti₄SiC₃, there is a greater likelihood that surfactants will penetrate in-between the A layers and stabilize them during exfoliation. Furthermore, by selecting appropriate exchangeable elements, the A layers can be substituted with other noble metals such as silver, platinum, or iridium, leading to the formation of both rising MAX phases and their corresponding metallenes through selective removal of the Ti_n+1C_n slabs.

In conclusion, Ti₄Au₃C₃ MAX phase forms through a solid-state substitution reaction during annealing of Au-covered Ti₄SiC₃ thin films. In this process, each Si layer is replaced by three atomic layers of Au, resulting in a 32.5% c-axis lattice expansion compared to the original Ti₄SiC₃. The insertion of tripple Au layers into Ti₄SiC₃ is thermodynamically more stable than the insertion of one or two layers, whereas four and five layers are less favorable. Trilayer goldene has the magic number for reasons of electronegativity and bonding analysis, which identifies Ti₄Au₃C₃ as the optimum structure. Isolated three-atomic-layer Au sheets—trilayer goldene—were obtained by selectively removing the Ti₄C₃ sheets from Ti₄Au₃C₃ through wet-chemical etching. Benefiting from the coexistence of fcc and hcp structures with out-of-plane Au-Au interactions, the trilayer goldene, with a thickness of 6.7 Å, is sufficiently stable to reach a lateral size of up to 100 nm and potentially up to several micrometers, corresponding to the single-crystal domain size in present-day Ti₄Au₃C₃ material. The Au 4f binding energy increases by approximately 0.80 eV compared to bulk Au, likely due to the reduced coordination numbers of the surface layers, resulting in a pronounced final state effect. Along with previously reported monolayer goldene, the isolation of gold trilayers in this work suggests that tuning the thickness of quasi-2D Au is feasible, offering research opportunities for exploring physical phenomena and applications based on diverse planar Au nanostructures.

MATERIALS AND METHODS

Ti₄SiC₃ thin films were deposited onto Al₂O₃(0001) substrates in a high-vacuum chamber using direct current magnetron sputtering (DCMS) with Ti (99.9%), Si (99.9%), and graphite (99.999%) targets. Before depositions, the substrates underwent a thorough cleaning process, including ultrasonic cleaning in acetone and isopropyl alcohol, followed by rinsing in deionized water. Substrates were then dried under a stream of nitrogen gas. The chamber was maintained at a base pressure of 1 × 10⁻¹⁰ torr and filled with Ar of 5 × 10⁻⁵ torr during all depositions. The substrates were preheated to 900°C at a rate of 25°C/min and kept at constant temperature during the depositions. To assist the Ti-Si-C MAX phase nucleation, a 20-nm-thick TiC seed layer was deposited for 8 min at 900°C with the applied power of 90 and 200 W for Ti and C targets, respectively. The TiC growth was then interrupted by a shutter; meanwhile, the Si magnetron was switched on with an applied power of 30 W. After 1 min, when the targets were stable, the shutter was removed, and Ti-Si-C deposition was initiated. The as-grown Ti₄SiC₃ films were around ~100 nm thick with 30-min deposition.

For the Au intercalation, a 200-nm-thick Au layer was sputter-deposited onto the MAX phase film by a separate DCMS system from an Au target (99.99%) under a base pressure of ~5 × 10⁻⁶ torr. The Au-capped Ti₄SiC₃ samples were subsequently annealed in a vacuum annealing furnace at 670°C for 30 hours and 600°C for 8 hours. The quartz tube in the furnace was outgassed a few times before the annealing. The ramping rate was ~5°C/min, and a nitrogen gas flow was introduced during the annealing to avoid oxidation.

Before etching away Ti₄C₃ sheets, the residual Au capping layer with a thickness of up to 200 nm was removed by chemical-mechanical polishing to expose the Ti₄Au₃C₃ film. The chemical-mechanical polishing slurry was prepared by mixing fumed silica (2 g, Sigma-Aldrich), I₂ (1.2 g, Sigma-Aldrich), KI (12 g, Sigma-Aldrich), citric acid (16 g, Sigma-Aldrich), and trisodium citrate (3.7 g, Sigma-Aldrich) in 200 ml of deionized water. Trilayer goldene was produced by etching the Ti₄Au₃C₃ films for 168 hours using 0.5% Murakami’s reagent with 5 mM CTAB (Merck) as a surfactant under complete darkness (1). The 0.5% Murakami’s reagent solution was prepared by adding 5 mg of KOH, 5 mg of K₃[Fe(CN)₆], and 36 mg CTAB into 10 ml of H₂O under highly weak ambient light. CTAB used here can hinder the coalescence of the isolated gold sheets into thicker layers and nanoparticles.

HRSTEM measurements were implemented using the Linköping monochromated double-spherical aberration-corrected FEI Titan³ 60-300 microscope operating at 300 kV, equipped with an EDX analysis module. The phase composition, crystal structure, and orientation of the samples were analyzed using XRD in a Philips PW 1820 diffractometer with Cu K_α radiation. Surface morphology and chemical composition were examined by scanning electron microscopy and EDX analysis with an LEO 1550 Gemini instrument.

XPS analysis was carried out in Axis Ultra DLD instrument from Kratos Analytical (UK) equipped with a monochromatic Al Kα radiation (1486.6 eV) operating at 150 W under the base pressure below 1.5 × 10⁻⁷ Pa. High-resolution spectra were recorded at a normal emission angle with a pass energy of 20 eV. The spectrometer was calibrated by testing positions of Au 4f_7/2, Ag 3d_5/2, and Cu 2p_3/2 peaks from sputter-etched Au, Ag, and Cu samples in comparison with the recommended ISO (International Organization for Standardization) standards for monochromatic Al Kα sources (42). Samples were stored in a glovebox filled with Ar and transferred to the XPS chamber with an Ar-filled sample box before analysis. No Ar etching was carried out before spectra acquisitions. All spectra were charge-corrected to the Fermi edge (section S10).

Supplementary Materials

This PDF file includes:

Sections S1 to S11

Figs. S1 to S28

Tables S1 to S6

References