Tunable Colloidal Synthesis Enabling μ‐ARPES on Individual Bismuth Nanocrystals

TL;DR

Researchers developed a scalable method to make high-quality bismuth nanosheets with tunable sizes and shapes, suitable for quantum and energy technologies.

Contribution

A tunable colloidal synthesis method for bismuth nanosheets enabling direct μ-ARPES characterization of individual nanosheets.

Findings

Bismuth nanosheets with tunable sizes (0.6–4.1 µm) and single-crystalline structure were synthesized.

μ-ARPES measurements on individual nanosheets matched DFT calculations, confirming high crystal quality.

The synthesis method is scalable, cost-effective, and produces stable nanosheets with excellent ambient stability.

Abstract

Bismuth (Bi) nanomaterials are a promising platform for quantum and energy technologies due to strong spin–orbit coupling, high thermoelectric efficiency, and magnetoresistance. However, scalable and flexible synthesis of high‐quality Bi nanostructures with fast research turnaround remains challenging. We report a controlled colloidal synthesis of Bi nanosheets with tunable lateral sizes (0.6–4.1 µm), hexagonal shape, and a layered single‐crystalline structure along the {00l} planes. The Bi nanosheets exhibit excellent long‐term structural stability and these colloidal nanostructures offer several key advantages: single‐crystalline structure; tunability in size, shape, dimensionality, and doping; stability in solution, enabling solution‐based processing; protection against oxidation by surface ligands; and cost‐effective and scalable production. Here, we show that colloidal Bi nanosheets can even be directly investigated using techniques such as angle‐resolved photoemission spectroscopy (ARPES). ARPES measurements on individual Bi nanosheets reveal a band structure in excellent agreement with DFT calculations, confirming high crystal quality and uniformity. Our findings demonstrate that colloidal‐synthesized Bi nanosheets provide a scalable platform for producing and characterizing high‐quality nanosheets, offering potential for integrations in advanced quantum, spintronic, and energy technologies.

We report a highly controlled colloidal synthesis of Bi nanosheets with tunable lateral sizes (0.6–4.1 µm), hexagonal shape, and a layered single‐crystalline structure along the {00l} planes. The nanosheets feature high phase purity and excellent ambient stability. Remarkably, O2 promotes larger lateral growth with ultra‐smooth surfaces. This scalable, cost‐effective synthesis method yields high‐quality nanosheets, and μ‐ARPES measurements on individual nanosheets reveal a band structure in excellent agreement with DFT calculations, confirming high crystal quality and uniformity.

Figures

Controlled synthesis and structural characterization of Bi NSs. a, Schematic illustration of the morphological evolution of Bi NSs during colloidal synthesis, along with HAADF‐STEM images showing tunable lateral dimensions. Scale bar = 500 nm. b, Schematic representation of the crystallographic relationship (left: side view; right: top view) between the rhombohedral (red) and hexagonal (blue) unit cells of Bi. c, AFM profile of Bi4.1µm NSs. Scale bar = 2 µm.

Elemental distribution and EELS analysis of Bi NSs. a, Low magnification HAADF‐STEM image of Bi4.1µm NSs with the t/λ value calculated from EELS spectra. EDS mapping of the Bi4.1µm sample (b) Bi, (c) O, and (d) overlay. EELS spectra acquired at selected positions at the center (e) and edge (f) of Bi4.1µm NSs.

XPS, structural models of Bi nanosheets. Bi 5d XPS spectra of Bi4.1µm NSs recorded before (a) and after (b) sputtering and annealing, prior to ARPES measurements in the same area. (c) Schematic of the Bi bilayer structure: top view of the layers and side view of the four layers. (d) The bulk (red) and surface (blue) Brillouin zone of Bi(111). (e) Sample sequence (on top of a SiO2/Si substrate) and photon beam geometry with the polarization direction E in the plane of incidence. (f) The energy‐filtered image of a single Bi4.1µm NSs, recorded using a Hg lamp at a kinetic energy of 4.9 eV on a SiO2/Si substrate; Scale bar = 4 µm.

μ‐ARPES analysis of Bi nanosheets. a, Constant binding energy (∼0.8 eV) momentum image of Bi(111) along M→Γ→M’ directions, collected at a photon energy of 80 eV using p‐polarized light. (b to d) Energy‐momentum cuts along M→Γ→M’ directions of the surface Brillouin zone of Bi(111): (b) raw data; (c) first derivative (d) and second derivative; the band structure is overlaid as a guide to the eye (blue curve), based on the ab initio band structure of a 3‐monolayer Bi(111) film, highlighting the band dispersions along different directions of the surface Brillouin zone of Bi(111).

Full text

Introduction

1

Bismuth (Bi), the heaviest non‐radioactive element [1], is widely recognized for its low toxicity and remarkable physical and chemical properties. Its unique electronic characteristics make Bi an ideal platform for exploring emergent quantum phenomena, including topologically protected surface states and low‐dimensional electronic transport [2, 3, 4]. Notably, its strong spin–orbit coupling, high thermoelectric efficiency, and pronounced magnetoresistance position Bi as a promising candidate for next‐generation quantum and energy‐related technologies [3, 5, 6]. Despite advances in nanomaterial synthesis, achieving high‐quality 2D Bi nanosheets with well‐defined morphology and crystallinity remains a formidable challenge. Existing techniques, including hydrothermal synthesis [7, 8], electrochemical methods [9], and solvothermal methods [10, 11], have produced various Bi nanostructures, including nanowires, nanoplates, and nanodots. However, the controlled fabrication of 2D Bi nanosheets with atomic‐scale smoothness – critical parameters influencing their electronic structure and surface‐related quantum effects [12] – remains largely unexplored. This difficulty arises from the need to finely balance between nucleation and growth to achieve tight control over the nanosheet morphology.

Colloidal synthesis offers precise control over shape, size, and dimensionality in a cost‐effective and scalable manner. However, its application to Bi nanosheets has been hindered by challenges such as surface contamination, disorder, and ligand‐induced reconstruction [13, 14]. Overcoming these barriers is essential for accessing clean, high‐quality crystalline surfaces necessary for advanced electronic characterization. Here, we report a colloidal synthesis strategy enabling scalable production of high‐quality Bi nanosheets with tunable lateral size and thickness. Using bismuth acetate as the precursor and oleic acid as a surface ligand, our method offers unprecedented control over the crystallinity and surface quality. Remarkably, the structural and electronic quality of these colloidally synthesized nanosheets enables angle‐resolved photoemission spectroscopy (ARPES) measurements on individual Bi nanosheets. The observed band structure shows excellent agreement with density functional theory (DFT) calculations, validating both structural order and electronic uniformity of the samples.

ARPES is a powerful technique to probe band structure, symmetry, surface states, and topological features in crystalline and low‐dimensional materials. However, its application has traditionally been limited to Bi samples fabricated via high‐vacuum techniques such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) [15, 16, 17]. While these methods provide high‐purity samples, they are costly, and show limited scalability, substrate constraints, and limited tunability. Our work establishes a solution‐based alternative that overcomes these limitations while preserving a high degree of crystallinity and ultra‐clean surfaces required for ARPES, opening new possibilities for studying topological phases in colloidally synthesized materials. This expands the family of quantum materials accessible via scalable colloidal synthesis and enables μ‐ARPES studies on individual nanocrystals. Complementary characterizations using electron energy loss spectroscopy (EELS), X‐ray photoelectron spectroscopy (XPS), and μ‐ARPES measurements performed on individual nanosheets confirm the structural order and surface cleanliness of the nanosheets. Notably, the μ‐ARPES data reveal a distinct three‐fold symmetry, confirming the preservation of the rhombohedral crystal structure of the synthesized Bi nanosheets.

Results and Discussion

2

Figure 1a illustrates the morphological evolution of the Bi nanosheets (NSs) and their tunable lateral dimensions during synthesis, which exhibits pronounced time‐dependency. Aliquots taken at various reaction stages (Figure S1) reveal a clear progression from Bi nanoparticles (100–200 nm, t = 15 s) to hexagonal nanoplatelets (∼2 µm, t = 65 s), and finally to uniform nanosheets (∼2.8 µm, t = 120 s). This morphological transformation is accompanied by changes in the XRD patterns, which evolve from a prominent (012) peak—indicative of bulk‐like Bi—to an increasing intensity of (003) and (006) reflections, suggesting recrystallization and growth of Bi nanosheets normal to the [001] direction [18]. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) images (Figure 1a) reveal that the lateral size of the Bi NSs can be effectively tuned by adjusting the injection temperature. At 120°C and 140°C, Bi nanosheets of 0.6 µm (Bi_0.6µm_), 1.4 µm (Bi_1.4µm_), respectively, are obtained. At 170°C under inert conditions, larger nanosheets of 2.8 µm (Bi_2.8µm_) are formed. Notably, introducing O_2_ into the reaction mixture at 170°C results in even larger nanosheets of 4.1 µm (Bi_4.1µm_), highlighting the critical role of oxygen in promoting lateral growth. This effect can be attributed to oxygen‐induced partial oxidative dissolution of small Bi nuclei at early stages, reducing nucleation density and favoring the growth of fewer but larger nanosheets. A similar phenomenon has been observed in the synthesis of Ag nanoparticles under air, where oxygen modulates cluster formation and growth [19, 20].

Controlled synthesis and structural characterization of Bi NSs. a, Schematic illustration of the morphological evolution of Bi NSs during colloidal synthesis, along with HAADF‐STEM images showing tunable lateral dimensions. Scale bar = 500 nm. b, Schematic representation of the crystallographic relationship (left: side view; right: top view) between the rhombohedral (red) and hexagonal (blue) unit cells of Bi. c, AFM profile of Bi4.1µm NSs. Scale bar = 2 µm.

The injection of TOP partially reduces Bi^3+^ ions, forming small Bi nanoparticles. These nanoparticles are initially stabilized by oleate‐ligand adsorption, similar to citrate‐stabilized silver nanoparticles [19, 20, 21]. Immediately thereafter, trace amounts of O_2_ are injected and act as a selective etchant, preferentially dissolving the less stable Bi nanoparticles. Therefore, the major role of oxygen is to chemically convert unstable Bi particles into Bi^3+^. At the same time, oleic acid preferentially adsorbs on the (111) facets [22], making sheet‐like Bi nuclei more stable than other morphologies and promoting the anisotropic growth of Bi nanosheets [21]. Therefore, the net effect of O_2_ in this reaction, in synergy with oleate ion, is to promote the nucleation of Bi sheets by removing less stable Bi nanoparticles of other structures. As reported in the literature [20], the dissolution of Bi nanoparticles under O_2_ is not strictly necessary as long as an additional source of Bi^3+^ is provided. In principle, larger Bi nanosheets can be obtained by adding extra Bi^3+^ or through other methods. However, as noted in previous studies [20], the resulting nanosheets tend to be of lower quality, and the reaction becomes more difficult to control due to the continuously changing Bi^3+^ concentration. In contrast, the unstable Bi nanoparticles and oleate act as a buffer, helping to maintain a nearly constant Bi^3+^ concentration throughout the chemical transformation. This allows each surviving nanosheet to grow more slowly and develop a more uniform lattice with fewer defects, and ultimately form 4.1 µm Bi nanosheets with highly ordered, smooth surfaces (Figure 1c). Conversely, strictly inert conditions promote a high nucleation density, limiting lateral growth and leading to aggregation at longer reaction times. These results underscore the role of oxygen in modulating the nucleation–growth dynamics of Bi nanostructures through a dissolution–reprecipitation mechanism, as well as the critical influence of facet energy and crystallographic reorientation on anisotropic growth.

Transmission electron microscopy (TEM) image depicting their hexagonal shape and quasi‐2D morphology with an average lateral dimension of approximately 4.1 µm (Figures S2a and S3). High‐resolution HAADF‐STEM imaging, combined with the corresponding fast Fourier transform (FFT) pattern and selected area electron diffraction (SAED), revealed that the Bi_4.1µm_ nanosheets are single crystalline with a preferential orientation along the [001] zone axis. Distinct lattice fringes with an interplanar spacing of 0.22 nm were observed, corresponding to the (110) plane of Bi_4.1µm_ NSs (Figure S2). Although Bi crystallizes in a rhombohedral A7‐type structure (space group R3¯m), the resulting nanosheets exhibit a regular in‐plane hexagonal shape [23]. This rhombohedral structure can be described using a hexagonal unit cell containing six atoms; the relationship between these unit cells is illustrated in Figure 1b. Further structural insights were obtained through powder X‐ray diffraction (PXRD) analysis (Figure S2d). The diffraction pattern matches well with the bulk Bi reference (COD 96‐231‐0890), confirming the phase purity and crystallographic integrity of the synthesized material. Notably, the (003) and (006) reflections exhibit significantly higher intensity compared to the (012) peak in the reference pattern, suggesting a strong preferred orientation of our sample along the {00l} planes. This preferential alignment likely results from crystal structure anisotropy and thus anisotropic growth mechanisms that favor stacking along the c‐axis, leading to the formation of Bi_4.1µm_ nanosheets with a layered structure. Such texture evolution differs from bulk Bi, where the (012) peak typically dominates, indicating that our synthesis conditions have effectively modulated the crystallographic orientation to promote out‐of‐plane alignment along {00l}. Additionally, atomic force microscopy (AFM) analysis, shown in Figure 1c, provides insights into the thickness distribution of the Bi_4.1µm_ nanosheets. The AFM height profile reveals a uniform thickness of 80 nm, confirming the synthesis of Bi_4.1µm_ nanosheets with well‐defined structural characteristics and a high‐quality flat surface.

Energy‐dispersive X‐ray spectroscopy (EDS) analysis of the Bi nanosheets reveals a high Bi content accompanied by a small amount of oxygen. All Bi nanosheets exhibit a homogeneous distribution of Bi (Figure 2; Figure S4). Interestingly, while oxygen is observed across the surface of Bi_0.6µm_ nanosheets with a higher concentration near the edges, in Bi_1.4µm_, Bi_2.8µm_, and Bi_4.1µm_ nanosheets, oxygen is confined almost exclusively to the edges. EDS mapping further confirmed a uniform distribution of Bi, whereas oxygen was primarily localized as a thin layer surrounding the edges of Bi_0.6µm_ (Figure S5), Bi_1.4µm_, and Bi_2.8µm_ (Figure S6a,b). The formation of this oxide layer could result from surfactants such as oleic acid and/or acetate, or from mild oxidation of the bismuth. The oxidation process, described by the reaction 4Bi(s) + 3O_2_(g) → 2Bi_2_O_3_(s), is thermodynamically favorable due to its negative Gibbs free energy [24]. Consequently, Bi easily leads to the formation of Bi_2_O_3_ nanoparticles [25, 26] or atomically thin Bi_2_O_3_ sheets [27] when exposed to an oxygen‐rich environment [25, 26, 27, 28]. To further investigate the composition of Bi nanosheets and their possible formation mechanism, we collected EELS data from the isolated single Bi nanosheets. The EELS spectra acquired from the central region of Bi nanosheets (Figure 2e; Figure S7), exhibit a distinct peak at 14.4 eV, which is identical to the volume plasmon energy (E p) for bulk Bi [8, 29]. When the energy loss spectrum is obtained from a thick sample, there is a reasonable probability that a transmitted electron will be inelastically scattered more than once, resulting in a total energy loss which is the sum of the individual losses. Consequently, the distinct high‐energy loss peak observed at 27 eV in the EELS spectra is identified as a multiple scattering plasmon peak [29]. It is important to note that when EELS data were collected at the edges of nanosheets Bi_0.6µm_, Bi_1.4µm_, and Bi_2.8µm_ (Figure S7b,c), three distinct features are labeled (Figure S7b): (a) the low energy edge at 10.7 eV, resulting from plasmon excitations. (b) The three spectra exhibit a broad peak located around 21 eV, which can be referred to the bulk plasmon of Bi_2_O_3_. (c) The small edge visible is attributable to Bi and appears around 29 eV. These features are in good agreement with the EELS spectrum of Bi_2_O_3_ [30, 31, 32]. indicating the formation of a Bi_2_O_3_ shell at the edge of the nanosheets Bi_0.6µm_, Bi_1.4µm_, and Bi_2.8µm_. Remarkably, EELS spectra collected from the edges of Bi_4.1µm_ nanosheets are identical to those from their centers, exhibiting no characteristic Bi_2_O_3_ features, indicating the absence of a significant Bi_2_O_3_ oxide layer at the edges (Figure 2e,f). Although EDS detects a minimal presence of oxygen at the edges of Bi_4.1µm_ (Figure 2c; Figure S6c), EELS analysis reveals no distinct Bi_2_O_3_ oxide layer formation. This suggests that Bi_4.1µm_ nanosheets possess significantly enhanced resistance to oxidation compared to their smaller counterparts. This finding implies that, similar to the Cu(111) surface [33], the ultra‐flat Bi(111) surface possesses inherent oxidation resistance properties, contributing to the enhanced stability of Bi_4.1µm_ nanosheets [33].

Elemental distribution and EELS analysis of Bi NSs. a, Low magnification HAADF‐STEM image of Bi4.1µm NSs with the t/λ value calculated from EELS spectra. EDS mapping of the Bi4.1µm sample (b) Bi, (c) O, and (d) overlay. EELS spectra acquired at selected positions at the center (e) and edge (f) of Bi4.1µm NSs.

In addition, through the EELS spectra, we can calculate the relative thickness at different positions of the Bi nanosheets. In Bi_0.6µm_ NSs, the relative thickness (t/λ value) increases from 0.74 at the center to 1.66 at the edges, indicating that the edges are thinner than the center. With increasing nanosheet size, the central region becomes progressively thinner while the edges are thicker, forming a well‐defined Bi_2_O_3_ shell in Bi_1.4µm_ and Bi_2.8µm_ NSs. AFM measurements on Bi_2.8µm_ NSs (Figure S8) confirm the non‐uniform thickness distribution observed in EELS, with nanosheet edges measuring ∼67 nm, significantly thicker than the central regions (∼55 nm). Although XRD patterns of Bi_1.4µm_ and Bi_2.8µm_ NSs show no detectable Bi_2_O_3_ signal (Figure S9), EELS analysis confirms the presence of an oxide layer at their edges. This discrepancy arises because the Bi_2_O_3_ layer is too thin to generate a distinct XRD peak, while EELS, being highly sensitive to the surface and localized chemical compositions, successfully detects it.

Oxidation progresses gradually after the near‐instantaneous formation of an initial monolayer of bismuth oxide. The outer surface of the Bi nanosheets undergoes native oxidation, forming a Bi_2_O_3_ layer with poor oxygen vacancies due to full contact with oxygen in the air [34]. As a result, the probability of the Bi in a deeper layer coming into contact with oxygen is sharply reduced, which prevents the further oxidation of Bi [34]. In this case, the Bi_2_O_3_ shell with a lateral width of ∼30 nm (Figure S10) formed at the edges of Bi_2.8µm_ nanosheets protects the Bi_2.8µm_ NSs from being further oxidized, making Bi/Bi_2_O_3_ stable in air. Long‐term structural stability is further confirmed by XRD analysis, which shows no significant changes in diffraction patterns after 9 months of ambient storage (Figure S9). On the contrary, Bi_4.1µm_ NSs exhibit intrinsic oxidation resistance, maintaining their structural and chemical stability over extended periods (Figure S11). EELS analysis confirms the absence of an oxide shell at their edges, unlike in Bi_2.8µm_ NSs. AFM analysis reveals that Bi_4.1µm_ NSs possess a flatter surface, which closely resembles the atomically flat Cu(111) surface discussed above [33]. Such flat surfaces lack “multi‐atomic step edges” —key reactive sites for oxidation — and thus exhibit strong oxidation resistance, attributed to high oxygen penetration barriers, significant lattice expansion upon oxidation, inhibited O_2_ dissociation, and self‐limiting oxygen adsorption at elevated coverage [33].

As discussed above, the flat surfaces result from the growth mechanism of Bi_4.1µm_ NSs: during the synthesis, a small and controlled amount of oxygen selectively etched the unstable Bi nanoparticles and chemically converted them into Bi^3+^. Together with oleate, these unstable nanoparticles buffer the Bi^3+^ concentration, allowing each surviving nanosheet to grow slowly, form a uniform lattice with fewer defects, and reach large lateral dimensions. In contrast, strictly inert conditions produce higher nucleation density, limited lateral growth, and aggregation. Motivated by the unique smoothness of the Bi_4.1µm_ surface, we performed ARPES measurements on the synthesized Bi_4.1µm_ NSs, to explore their crystallinity and surface morphology on the photo‐electron spectroscopy level.

Photoelectron emission microscopy (PEEM) was employed to investigate XPS and ARPES of individual Bi_4.1µm_ nanosheets. Prior to ARPES measurements, the sample quality was evaluated by XPS. Figure 3a presents the Bi 5d core‐level spectra of the as‐prepared Bi nanosheet. The multiple‐peak structure in the Bi 5d region indicates partial oxidation of the Bi surface, likely resulting from both air exposure and surface coordination of residual oleate ligands used during synthesis. To assess the extent of the surface oxidation, the XPS peaks were deconvoluted. The spin‐orbit‐doublet of metallic Bi, consisting of 5d_5/2_ and 5d_3/2_ peaks, appears at binding energies (BE) of 24.64 and 27.5 eV, respectively. Additional peaks at 26.06 and 29.04 eV correspond to Bi‐O bonding, and are consistent with literature reports that Bi readily oxidizes under high‐humidity conditions [35], indicating that the observed surface species predominantly arise from oxidation of the Bi surface. To remove the surface oxides, the sample was cleaned in ultra‐high vacuum via standard low‐energy Ar^+^ sputtering at 500 eV (p Ar = 2×10^−6^ mbar) followed by annealing at ∼373 K. After this treatment, the XPS spectrum (Figure 3b) exhibited sharper peaks at 24.22 and 27.32 eV, corresponding to the 5d_5/2_ and 5d_3/2_ components of metallic Bi. The significant reduction in the Bi–O peak intensity bonding at 24.84 and 29.17 eV suggests effective removal of surface oxides. This cleaning protocol is critical to restore the native work function and surface stoichiometry, ensuring reliable ARPES measurements of the intrinsic band structure. This suggests that the Bi surface has been largely restored to a metallic state with a single, well‐defined chemical environment, as expected for clean elemental Bi.

XPS, structural models of Bi nanosheets. Bi 5d XPS spectra of Bi4.1µm NSs recorded before (a) and after (b) sputtering and annealing, prior to ARPES measurements in the same area. (c) Schematic of the Bi bilayer structure: top view of the layers and side view of the four layers. (d) The bulk (red) and surface (blue) Brillouin zone of Bi(111). (e) Sample sequence (on top of a SiO2/Si substrate) and photon beam geometry with the polarization direction E in the plane of incidence. (f) The energy‐filtered image of a single Bi4.1µm NSs, recorded using a Hg lamp at a kinetic energy of 4.9 eV on a SiO2/Si substrate; Scale bar = 4 µm.

Following the confirmation of surface cleanness via XPS, μ‐ARPES was performed to probe the intrinsic electronic structure of individual Bi nanosheets. Figure 3f presents the energy‐filtered image of a single Bi nanosheet on a silicon substrate, recorded using a Hg lamp at a kinetic energy of 4.9 eV. The μ‐ARPES measurements were carried out using p‐polarized light with a photon energy of 80 eV, enabling spatially resolved spectroscopy on a single nanosheet. The momentum‐resolved photoemission intensity map recorded at a binding energy of 0.8 eV (Figure 4a; Figures S12–S14) exhibits a pronounced three‐fold rotational symmetry centered at the Γ point. This feature is consistent with the rhombohedral A7 crystal structure of Bi, in which atoms form puckered bilayers stacked along the [111] direction (Figure 3c). Each bilayer possesses a three‐fold rotational axis and vertical mirror planes, characteristic of the C 3 * v

• point group symmetry [23, 36]. The observed three‐fold symmetry in the bulk‐derived bands indicates that the nanosheets retain the native stacking order without signs of twinning or rotational disorder. The structural origin of this symmetry is further illustrated in the bilayer schematic (Figure 3c), where the dark red and light red circles denote atomic positions offset above and below the basal plane. An energy–momentum cut along the high‐symmetry M→Γ direction reveals a nearly parabolic valence band with its maximum located approximately 0.75 eV below the Fermi level at the Γ point (Figure 4; Figures S12–S14) dispersing downward toward M. This dispersion matches well with valence band features of bulk Bi [36, 37] and epitaxial Bi(111) films [38]. Further, it can be found as a split E band in band structure calculations on a Bi triple layer (Figure 4d). Its Γ point wavefunction exhibits a surface state with three‐fold symmetry (Figure S15). Due to smoother transitions, other features of the band structure are more clearly resolved in the first (bands F to I) and second derivative (bands A and B) of the energy–momentum cut image (Gaussian blurred to enable the derivatives). The broken symmetry can be clearly identified in Figure 4 and in comparing the M→Γ→M with the M’→Γ→M’ cut (Figures S13 and S14). The resolution of sharp valence band dispersions and symmetry‐related momentum features directly correlates with the chemical purity established by XPS. Only after thorough in‐vacuum Ar^+^ sputtering and annealing – evidenced by the elimination of Bi–O related peaks and the sharpening of the Bi 5d core‐level doublet – did the μ‐ARPES spectra exhibit the expected characteristics of clean, metallic Bi. These observations underscore the essential role of surface preparation in facilitating accurate electronic structure measurements on colloidal 2D materials.

μ‐ARPES analysis of Bi nanosheets. a, Constant binding energy (∼0.8 eV) momentum image of Bi(111) along M→Γ→M’ directions, collected at a photon energy of 80 eV using p‐polarized light. (b to d) Energy‐momentum cuts along M→Γ→M’ directions of the surface Brillouin zone of Bi(111): (b) raw data; (c) first derivative (d) and second derivative; the band structure is overlaid as a guide to the eye (blue curve), based on the ab initio band structure of a 3‐monolayer Bi(111) film, highlighting the band dispersions along different directions of the surface Brillouin zone of Bi(111).

Conclusion

3

In summary, we established a controlled colloidal synthesis approach for producing high‐quality Bi nanosheets with tunable lateral dimensions and excellent crystallinity. The synthesized Bi nanosheets exhibit flat and ordered surfaces, a single‐crystalline structure, and a preferred orientation along the {00l} planes. Notably, the Bi_4.1µm_ displays exceptional intrinsic oxidation resistance and long‐term ambient stability. Comprehensive structural and spectroscopic characterizations confirm their high purity, crystallographic integrity, and ultra‐clean surfaces. Colloidal nanostructures offer numerous advantages that make them particularly suitable for such studies. They are inherently single crystalline, and their size, shape, dimensionality, and doping can be easily tuned. Their stability in solution allows for solution‐based processing, while surface ligands provide protection against oxidation. Moreover, colloidal synthesis is generally cost‐effective and, in principle, scalable. Crucially, in this work, Bi_4.1µm_ nanosheets enable μ‐ARPES measurements on individual colloidal nanocrystals, revealing bulk‐like electronic band dispersions and distinct three‐fold symmetric momentum maps, consistent with the rhombohedral A7 crystal structure. The experimentally measured band structure shows excellent agreement with band structure calculations, validating both electronic quality and structural uniformity of the nanosheets. These findings demonstrate that solution‐synthesized Bi nanosheets provide a scalable and structurally precise platform to investigate symmetry‐driven electronic phenomena in low‐dimensional systems. Properly cleaned, solution‐processed Bi nanosheets can meet the stringent requirements for advanced photoemission studies, thereby expanding the repertoire of quantum materials accessible via wet‐chemical synthesis. Although translating these advances into practical applications remains challenging and will require further development and optimization, this work highlights the potential for scalable synthesis of high‐quality colloidal Bi nanosheets and their future integration into advanced quantum, spintronic, and energy‐related technologies.

Experimental Section

4

Chemicals and Materials

4.1

Bismuth (III) acetate (Bi (CH_3_CO_2_)3, anhydrous, ≥99.99%, stored in a nitrogen‐filled glovebox) and oleic acid (90%), were purchased from Sigma–Aldrich. Trio‐n‐octylphosphine (TOP; 97%; stored in a nitrogen‐filled glovebox) was from ABCR, and diphenyl ether (≥99%) was purchased from Thermo Fisher Scientific. Ethanol, chloroform, and toluene were purchased from Honeywell. Pure O_2_ (99.998 mol%) was purchased from Air Liquide Medical Germany.

All the chemicals were used as‐received without additional purification. All the syntheses were carried out applying standard air‐free Schlenk‐line techniques.

Synthesis of Bi Nanosheets

4.2

In a three‐necked flask equipped with a condenser, a septum, and a thermocouple in a glass mantle, 96.5 mg of Bi (CH_3_CO_2_)3 (0.25 mmol), and 2 mL of oleic acid (6.25 mmol) were mixed in 9 mL of diphenyl ether. The mixture was stirred at 110°C for 10 min and then degassed under vacuum at 80°C for 1.5 h. Subsequently, the reaction mixture was heated to 170°C under a nitrogen atmosphere. Upon reaching 170°C, 2 mL of tri‐n‐octylphosphine (4.4 mmol) was injected, causing the solution to change from colorless to grey. The reaction was quenched after 2 min by removing the heating mantle. The resultant nanostructures were purified by precipitation with toluene, followed by centrifugation at 4000 rpm for 3 min, repeated three times. The supernatant was removed, and the nanostructures were resuspended in toluene for further characterization or storage. To synthesize Bi_4.1µm_ nanosheets, the same procedure was followed; however, immediately after removing the heating mantle, 0.1 mL of pure O_2_ was injected into the reaction mixture. The solution was then allowed to cool slowly to 80°C at room temperature.

Structural Characterization

4.3

The Transmission Electron Microscopy (TEM) images were performed on a Talos‐L120C microscope with a thermal emitter operated at an acceleration voltage of 120 kV. The TEM samples were prepared by diluting the nanosheet suspension with toluene, followed by drop casting 10 µL of the suspension on a TEM copper grid coated with a carbon film. The crystal structure of the Bi nanosheets was determined by X‐ray Diffraction (XRD) measurements, which were performed on a Panalytical Aeris System with a Bragg–Brentano geometry and a copper anode with an X‐ray wavelength of 0.154 nm from the Cu‐kα1 line. The samples were prepared by drop‐casting 10 µL of the suspended Bi NS solution on a 〈911〉 or 〈711〉 cut silicon substrate. Atomic force microscopy (AFM) measurements were performed with an AFM from Park Systems XE‐100 in non‐contact mode.

Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) measurements were performed using a probe‐corrected JEOL JEM‐ARM200F NeoARM with a cold field emission gun and a Gatan Continuum EELS. All measurements were done at 200 kV acceleration voltage. The full width at half maximum of the zero‐loss peak is 0.27 eV with the dispersion set to 0.015 eV/channel. To improve reliability, the first Dual EELS channel was used to track the position of the ZLP while the second channel was shifted to increase the signal‐to‐noise ratio due to higher acquisition times. During the experimental process, we searched many regions on the TEM grid to identify areas where the nanosheets were well‐separated, not stacking or touching each other. We selected these kinds of regions to ensure that our analysis was performed on individual nanosheets.

Angle‐Resolved Photoemission Spectroscopy (ARPES) and X‐ray Photoelectron Spectroscopy (XPS) Measurements

4.4

For preparing the Bi, we drop‐cast a thin film onto a SiO_2_/Si wafer. Flakes of Bi nanosheets with a clean surface, as verified byXPS, are investigated by µ‐ARPES at the NanoESCA beamline of Elettra, the Italian synchrotron radiation facility, using a FOCUS NanoESCA PEEM in k‐space mapping mode. The PEEM is operated at a background pressure p < 1x10^−10^ mbar. XPS is performed with the same instrument. The photoelectron signal at 80 eV is collected from a spot size with a diameter of 5−10 µm. The total energy resolution of the beamline and analyzer is 50 meV. The geometry of incident light (p‐polarized) is sketched in Figure 3E, main text. The measurement was performed at room temperature. Prior to µ‐ARPES, the samples are annealed at T = 373 K for 10 min. The Fermi level (E F) was calibrated using a clean Au(111) single crystal measured under identical conditions. The Brillouin zone (BZ) orientation is adapted by the repetitive features in the first and second BZs, while the scale of wave vectors parallel to the surface k_∥_ is deduced from the angular dependence of the onset of secondary photoelectron emission.

Density Functional Theory (DFT) Calculations

4.5

We performed simulations of the electronic band structure of a triple‐layer Bi in a hexagonal configuration under periodic boundary conditions. The lattice parameters were fixed to the experimental values of a = b = 4.65 Angstrom and c = 24.35 Angstrom (alpha = beta = 90°, gamma = 120°). We applied 19.0 Angstrom of vacuum in the c direction on top of the layer, infinite in a and b directions.

The band structure has been calculated using the DFT code “EXCTING” (version Neon‐21) (https://exciting‐code.org) [39]. The DFT calculations used the generalized gradient approximation (GGA), an exchange correlation functional of type Perdew–Burke—Ernzerhof [40], and a k‐point grid of 4×4×4. Spin–orbit coupling was included in the calculations. The convergence tolerance of the difference of the total energy was < 1.0 × 10^−5^ Hartree.

Funding

Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, Project ID 525993990). European Regional Development Fund of the European Union (GHS‐20‐0036/P000379642). DFG (INST 264/161‐1 FUGG) and (INST 264/188‐1 FUGG). DFG (Project ID 513136560).

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll72446‐sup‐0001‐SuppMat.pdf.