FIELD OF INVENTION

The present invention relates broadly to a method of fabricating 2D blue phosphorus, in particular a non-reconstructed l x l blue phosphorus, to 2D blue phosphorus formed by the method, and a structure comprising 2D blue phosphorus.

BACKGROUND

Any mention and/or discussion of prior art throughout the specification should not be considered, in any way, as an admission that this prior art is well known or forms part of common general knowledge in the field.

The growth of entirely synthetic two-dimensional, 2D, materials could expand the limited library of naturally occurring layered solids and provide opportunities to design materials with finely tunable properties. In 2D materials, sometimes also referred to as two-dimensional nanomaterials, two dimensions are outside the nanoscale and one dimension is only a single or few atomic layers thick. Among them, the synthesis of elemental 2D materials is of particular interest as they represent the chemically simplest case and serve as model system for exploring the on-surface synthesis mechanism.

While the experimental synthesis of 4 x 4 blue phosphorus (BlueP) related superlattice on Au(l 11) surface has been reported, it was recently revealed to be a 2D porous gold-phosphorus network, with BlueP subunits linked by gold atoms.

That is, 2D phosphorus, an elemental 2D semiconducting material derived from group- VA family has emerged with increasing interest recently. However, only black phosphorus are known to occur in layered structure formed in bulk. Various 2D phosphorus allotropes have been theoretically proposed, but very few of them have been experimentally synthesized. Moreover, up to now, all the few layer black phosphorene are prepared by mechanical exfoliation. The size of the cleaved black phosphorene is usually limited to micrometers and the productivity of this method is very low.

Embodiments of the present invention seek to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention, there is provided a method of fabricating two-dimensional, 2D, blue phosphorus, BlueP, comprising the steps of: providing a gold(l 11), Au(l 11), substrate; providing an interlayer on a surface of the Au(l 11) substrate, the interlayer comprising silicon, Si; and forming the 2D BlueP on the interlayer.

In accordance with a second aspect of the present invention, there is provided a 2D BlueP formed by the method of the first aspect.

In accordance with a third aspect of the present invention, there is provided a structure comprising: a gold(ll l), Au(lll), substrate; an interlayer on a surface of the Au(l 11) substrate, the interlayer comprising Si; and 2D BlueP on the interlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure l(a-c) shows scanning tunneling microscopy (STM) images of 0.1 ML P deposited on Au(lll) with the substrate held at room temperature, showing porous network, (a) Vs = -1 V, 100 x 100 nm ² ; (b) Vs = -1 V, 40 x 40 nm ² ; (c) Vs = -1 V, 20 x 20 nm ² .

Figure l(d-f) shows STM images of 0.1 ML P deposited on Au(l 11) with the substrate held at room temperature, showing clusters and short chains on the surface, the herringbone reconstruction of Au(lll) is not disturbed, d) Vs = - IV, 40 x 40 nm ² ; (e) Vs = -1 V, 20 x 20 nm ² ; (f) Vs = -1 V, 10 x 10 nm ² .

Figure 2(a) shows an STM image of 0.2 ML BlueP-Au alloy grown on Au(l 11) held at room temperature and subsequent annealing at 170 °C, large-scale, Vs = -1 V, 40 x 40 nm ² .

Figure 2(b) shows an STM image of 0.2 ML BlueP-Au alloy grown on Au(l 11) held at room temperature and subsequent annealing at 170 °C, close-up, Vs = -30 mV, 10 x 10 nm ² .

Figure 2(c) shows an atomic model of BlueP-Au alloy grown on Au(l 11).

Figure 2(d) shows a simulated STM image of the BlueP-Au alloy embedded in the Au(l l l) terrace.

Figure 2(e) shows a large-scale STM image of the BlueP-Au alloy grown along the step edge, Vs = -1 V, 40 x 40 nm ² . Figure 2(f) shows a close-up STM image of the BlueP-Au alloy grown along the step edge, Vs = -60 mV, 10 x 10 nm ² .

Figure 2(g) shows the atomic model of the BlueP-Au alloy grown along the step edge.

Figure 2(h) shows a simulated STM image of the BlueP-Au alloy grown along the step edge.

Figure 3(a) shows an STM image showing the clean Au(lll) surface with herringbone reconstruction, Vs = 500 mV, 200 x 200 nm ² .

Figures 3(b-c) shows STM images of two different regions of Au(lll) upon phosphorus deposition, where the exposed Au(l 11) surface forms a network of trigons, (b) Vs = -1 V, 100 x 100 nm ² , (c) Vs = -1 V, 100 x 100 nm ² .

Figure 3(d) shows a large scale STM image showing the presence of dark holes caused by the missing of Au atoms at the elbow site of the Au(l 11) surface in trigon reconstruction, Vs = -1 V, 18 x 18 nm ² .

Figures 3(e-f) shows close-up STM images showing the presence of dark holes caused by the missing of Au atoms at the elbow site of the Au(l 11) surface in trigon reconstruction, (e) Vs = -30 mV, 10 x 10 nm ² ; (f) Vs = -10 mV, 5 x 5 nm ² .

Figure 4(a) shows atomically resolved qPlus atomic force microscopy (AFM) images showing the lattice structure of BlueP-Au alloy. (DZ = 0 pm, Vs = 50 mV, It = 1 nA, 3.6 x 3.6 nm ² )

Figures 4(b-d) shows tip height dependent constant height qPlus AFM images. At relatively large tip height, only P atoms can be observed (b) DZ = 30 pm, Vs = 15 mV, It = 1 nA, 5 x 5 nm ² ; (c) DZ = 45 pm, Vs = 30 mV, It = 1 nA, 5 x 5 nm ² ; (d) DZ = 60 pm, Vs = 30 mV, It = 1 nA, 5 x 5 nm ² )

Figure 4(e) shows the atomic model used for the nc-AFM image simulation, DZ = 7.3 A.

Figures 4(f-h) shows simulated nc-AFM images using probe-particle model with different tip- sample distances (DZ), (f) DZ = 7.9 A; (g) DZ = 8.3 A.

Figure 5(a) shows a large scale STM image of monolayer BlueP-Au alloy grown on Au(l 11), Vs = -1 V, 50 x 50 nm ² .

Figure 5(b) shows a large scale STM image after Si deposition on 280 °C BlueP-Au alloy /Au(lll). Three regions coexist on the surface: BlueP-Au alloy, Si intercalated l x l BlueP according to example embodiments, and AuSIL buffer layer, Vs = -1 V, 50 x 50 nm ² .

Figure 5(c) shows a lateral profile along the gray line in Figure 5(b).

Figure 5(d) shows a close up STM image of the partially intercalated BlueP, Vs = -1 V, 30 x 30 nm ² Figure 5(e) shows an atomically resolved STM image of the boundary between the intercalated lxl BlueP according to example embodiment and BlueP-Au alloy, Vs = -10 mV, 3 x 3 nm ² .

Figure 5(f) shows a high resolution STM image of the AuSIL layer, Vs = -1 V, 15 x 15 nm ² .

Figure 6(a-c) shows large scale STM images of fully intercalated BlueP on Au(l 11) according to example embodiments, (a) Vs = -1 V, 100 x 100 nm ² ; (b) Vs = -1 V, 100 x 100 nm ² ; (c) Vs = -1 V, 50 x 50 nm ² ; (d) Vs = -20 mV, 40 x 40 nm ² .

Figure 6(d) shows a close-up STM image showing the completely intercalated BlueP on Au(l 11) according to example embodiments, Vs = -20 mV, 4 x 4 nm ² .

Figure 7(a-c) shows DFT calculations of the adsorption energy of Si on BlueP- Au alloy, specifically top and side views of the several typical adsorbed structures of Si atom on BlueP- Au alloy covered surface are displayed.

Figure 7(d-e) shows DFT calculations illustrating the most stable adsorption configuration of Si and P atom, respectively, on Au(l 11) surface.

Figure 8(a-c) shows LEED patterns for the clean Au(lll), BlueP- Au alloy and silicon intercalated l x l BlueP according to example embodiments, respectively. All these patterns are collected at an electron beam energy of 54.4 eV.

Figure 9(a), AuSIL layer formed by depositing Si on Au(l 11), shows a large scale STM image of the AuSIL layer on Au(lll), Vs = -1 V, 100 x 100 nm ² .

Figures 9(b-c) shows high resolution STM images of the AuSIL structure at two different locations. The bright dots in the images are aligned along the two white arrows, (b) Vs = -IV, 15 x 15 nm ² ; (c) Vs = -1 V, 15 x 15 nm ² .

Figure 10(a-b), growth of 1 x 1 BlueP on top of AuSIL layer, shows large scale STM images of 1 x 1 BlueP grown on top of the AuSIL covered Au(l 11). Three different phases of BlueP coexist on the surface, which are labeled as 1 x 1 BlueP 1, l x l BlueP2 according to example embodiments and Trans BlueP, Vs = -1 V, 40 x 40 nm ² ; (b) Vs = -1 V, 20 x 20 nm ² .

Figures 10(c-d) shows atomically resolved STM images showing the structure of 1 x 1 BluePl, (c) Vs = -1 V, 10 x 10 nm ² ; (d) Vs = -1 V, 4 x 4 nm ² .

Figures 10(e-h) shows large scale and close-up STM images showing structure of 1 x 1 BlueP2 according to example embodiments. The inset shows the lateral profile measured along the line in Figure 10(h), (e) Vs = -1 V, 50 x 50 nm ² ; (f) Vs = -1 V, 50 x 50 nm ² ; (g) Vs = -10 mV, 10 x 10 nm ² ; (h) Vs = -10 mV, 5 x 5 nm ² .

Figure ll(a-b), atomic model for 1 x 1 BluePl phase, shows the atomic model and simulated STM image of (5 x 5)-BlueP on (6 x 6)-Au(lll).

Figure 11 (c-d) shows the atomic model and simulated STM image of (5 x 5)-BlueP on Si- insert (6 x 6)-Au(lll). Figure 12(a), the orientation relationship between l x l BluePl and l x l BlueP2 phase according to example embodiments, shows an atomically resolved STM image of 1 x 1 BluePl phase with its crystallographic orientation highlighted by lines A and B, Vs = -l V, 10 x 10 nm ² .

Figure 12(b) shows an atomically resolved STM image of 1 x 1 BlueP2 phase according to example embodiments, the lines A and B in Figure 12(b) are in the same orientation as in Figure 12(a), while line C indicates the crystallographic orientation of 1 x 1 BlueP2 phase. Careful inspection reveals that lines A and B are orientated along the 7 direction of 1 x 1 BlueP2 phase, which results in an inclusion angle of 19.1° between these two different phases, Vs = -10 mV, 10 x 10 nm ² .

Figures 13(a-b), atomic model for l x l BlueP2 phase according to example embodiments, shows the atomic model and simulated STM image of 7 x V7-BlueP on (3 x 3)-Au(lll).

Figures 13(c-d) shows the atomic model and simulated STM image of 7 x V7-BlueP on Si insert (3 x 3)-Au(lll).

Figure 14(a, b), 1 x 1 BlueP2 phase according to example embodiments in higher coverage, show large scale STM images showing the l x l BlueP2 phase cover almost the entire surface, Vs = -1 V, 80 x 80 nm ² ; (b) Vs = -1 V, 50 x 50 nm ² .

Figure 15(a-c) shows X-ray photoelectron spectroscopy (XPS) investigation of the Si intercalation process of BlueP according to example embodiments, showing XPS core level spectra of Au 4f (a), P 2p (b) and Si 2p (c) for clean Au(l 11), BlueP-Au alloy, Si intercalated BlueP and Si/Au(l 11).

Figure 16(a-c), angle-resolved photoemission spectroscopy (ARPES) investigation of the Si intercalation process of BlueP according to example embodiments, shows second derivative plots of the ARPES spectra for (a) clean Au(l 11) along the GK cut.

Figures 16(b) shows second derivative plots of the ARPES spectra for BlueP- Au alloy /Au(l 11) along the GK cut.

Figure 16(c) shows second derivative plots of the ARPES spectra for Si intercalated l x l BlueP along the GK cut.

Figure 16(d) shows second derivative plots of the ARPES spectra for AuSIL along the GK cut.

Figure 16(e) shows theoretically calculated effective band structures of the BlueP- Au layer on (5 x 5)-Au(lll). High- symmetric k-point paths are sampled along K(l/3, 1/3, 0) G(0, 0, 0) K(l/3, 1/3, 0). All of the band structures are unfolded to the primitive cell of the Au(l 11) surface, and the black solid line corresponds to the band structure of the Au(l 11) primitive cell.

Figures 16(f-g) show the atomic configuration (left part) and effective band structures (right part) of (f) free-standing monolayer l x l BlueP and (g) Si intercalated l x l BlueP according to example embodiments, respectively. The band structure of Si intercalated BlueP according to example embodiments are unfolded to the primitive cell of BlueP.

Figure 17(a), ARPES investigation of the silicon intercalation process of BlueP according to example embodiments, shows second derivative plots of the ARPES spectra for clean Au(l 11) along the GM cut.

Figure 17(b) shows second derivative plots of the ARPES spectra for BlueP- Au alloy/ Au(l 11), along the GM cut.

Figure 17(c) shows second derivative plots of the ARPES spectra for Si intercalated l x l BlueP according to example embodiments along the GM cut.

Figure 17(d) shows second derivative plots of the ARPES spectra for AuSIL along the GM cut.

Figure 18 shows a schematic illustrating the non-reconstructed l x l BlueP layer according to example embodiments directly on top of an S1O2 layer on a Au(lll) substrate, enabling a device fabrication process according to example embodiments without complicated transfer process.

Figure 19 shows a flowchart illustrating a method of fabricating a non-reconstructed l x l BlueP layer, according to an example embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention provide a pure atomically layered l x l blue phosphorus (BlueP) synthesized on Au(lll) through silicon intercalation, an in situ insertion of an atomic layer between the surface and the supporting substrate. The intercalation process is characterized at the atomic scale by low-temperature scanning tunneling microscope, and further corroborated by synchrotron radiation-based x-ray photoelectron spectroscopy measurements. The band structure evolutions from BlueP-Au alloy to Si intercalated l x l BlueP according to example embodiments are clearly revealed by angle -resolved photoemission spectroscopy and further verified by DFT calculations. The successful synthesis of 1 x 1 BlueP monolayer according to example embodiments opens opportunities for exploring its exotic electronic properties and device performances.

It has to be noted that, previously, 2D BlueP has only been theoretically predicted and to the inventors’ knowledge no experimental realization of 2D BlueP has been reported. That is, to the inventors’ knowledge, in embodiments of the present invention, the growth of 2D BlueP has been demonstrated for the first time, i.e. synthesizing BlueP on a supporting substrate.

In example embodiments, the formation of gold silicide (AuSIL) buffer layer lifts the BlueP- substrate coupling, and restores the band structures of pristine BlueP monolayer. The intercalation and the subsequent formation of 1 x 1 BlueP according to example embodiments are characterized specifically by LT-STM at the atomic scale and further corroborated by synchrotron radiation-based X-ray photoelectron spectroscopy (XPS) measurements, low energy electron diffraction (LEED) and DFT calculations. Angle-resolved photoemission spectroscopy (ARPES) was utilized to identify the electronic structures of the intercalated 1 x 1 BlueP according to example embodiments.

Scalable growth method like chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are highly desired for the growth of 2D phosphorus. Embodiments of the present invention provide a scalable growth method of single layer BlueP. Moreover, the intercalated Si layer has the possibility to be oxidized to SiCh layer, which leads to the synthesis of a semiconductor directly on top of the oxide surface, and enables the following device fabrication process without complicated transfer process.

Previous STM studies of phosphorus growth on Au(lll) were mostly carried out at the monolayer coverage, which hinders the direct observation of the growth process at the initial stage. Xu et al. investigated the growth at low coverage; however, more detailed investigation on the atomic structure evolution of the underlying substrate at the initial growth stage is still needed. As part of the analysis of embodiments of the present invention, the growth of phosphorus in a low coverage regime was investigated, where the structure of the phosphorus and the Au(lll) surface are resolved at the atomic level. The P was evaporated from an effusion cell using bulk black phosphorus as the precursor, in a CVD deposition process. When the Au(l 11) substrate was kept at room temperature during the growth, P atoms were found to aggregate into porous networks, clusters and short chains on top of the Au(lll) surface (see Figure la-f). Postannealing of this sample results in the formation of ordered structures. Figure 2a shows a representative STM topographic image collected after the deposition of 0.2 ML P onto a clean Au(lll) surface at room temperature and subsequent annealing at 170 °C. It reveals the incorporation of some honeycomb-like small patches with variable sizes into the topmost Au layer. Further surface inspection via STM imaging reveals that postannealing also induced a structural transformation of the Au(l 11) surface from the herringbone reconstruction to a network of “trigons” (see Figure 3a-f). At the elbow sites of the herringbone reconstruction, where the brighter discommensuration lines bend, missing atoms can be observed (Figure 3d- f). Based on these observations, it is proposed that the embedded honeycomb islands are associated with the formation of a 2D P-Au alloy, where P atoms are incorporated into the topmost layer of Au(l 11) by replacing the surface Au atoms. During this substitution process, some Au atoms are displaced out of the surface and then diffuse and coalesce into Au layers at other regions. The observed alternations of the herringbone reconstruction could be ascribed to the modified surface atomic density upon P atoms incorporation, as has also been observed by exposure Au(l 11) to alkali metals, aluminum and rare earth metals.

Figure 2b is a magnified STM image of the embedded hexagonal superstructures. It reveals the presence of the hexagonally arranged bright trimers. The unit cell is outlined in the image with a periodicity of 1.46 nm, which corresponds to approximately 5 times the Au(lll) lattice constant (0.288 nm). The edges and diagonal of the unit cell are all occupied with three darker atoms. The lattice structure revealed by this high-resolution STM image is consistent with the atomically resolved nc-AFM image, collected with a CO-functionalized tip (see Figure 4a-h). At relatively large tip-sample distances, only the bright trimer-like features can be observed (see Figure 4d). The contrast observed between the brighter and darker atoms suggests that they may represent different elements, here, P and Au. Figure 2c displays the proposed structure model based on this assumption, where the superstructure is composed of zigzag- edged BlueP nanoclusters interconnected by Au atoms. Based on this structural model, STM images were simulated using Tersoff-Hamann approximation and nc-AFM images using a probe-particle model, as is understood by a person skilled in the art. It was found that both the STM and nc-AFM simulations perfectly reproduce the experimental findings (compare Figure 2d and Figure 4).

In addition to the patches confined within the first layer, a large number of the honeycomb structures is found to form preferentially along the step edges, as shown in Figure 2e,f. The atom exchanging process is further confirmed by the existence of the irregular- shaped step edges, which is in stark contrast to the straight edges presented for clean Au(lll). DFT calculations were also performed to reveal the atomic structure along the step edges (Figure 2g). Again, the experimental results are successfully reproduced (Figure 2h). Hereafter, we refer to this structure as the BlueP-Au alloy.

Elimination of the BlueP-gold interaction in order to release BlueP to form 2D BlueP according to example embodiments, in particular a non-reconstructed P layer, involves the passivation of the Au(l 11) surface. A complete monolayer of BlueP- Au alloy on Au(l 11) was first prepared, as shown in Figure 5a. Then Si was evaporated from a directly heated silicon rod in a MBE process onto this BlueP- Au alloy covered surface with the substrate held at 280 °C. After Si deposition, a strong modification of the surface structure is observed, notably with a sharp transition from the honeycomb network to a brighter planar sheet (Figure 5b-d). The brighter island is elevated at about 50 pm in height compared with the BlueP- Au alloy, as shown by the line profile in Figure 5c. Intriguingly, there is a seamless connection between the newly emerged brighter area and the BlueP- Au alloy, as revealed by the atomically resolved STM image in Fig. 5e. The unit cell length of 0.33 nm is exactly the lattice constant predicted for the free standing BlueP. This is a clear indication that Si intercalation and the subsequent formation of 2D Blue P, in particular non-reconstructed l x l BlueP, have taken place on the surface, according to example embodiments.

On the regions free of BlueP, the structure of the exposed AuSIL buffer layer can be revealed. However, there is no long-range order within this AuSIL layer, and the distance between the two nearest neighbor dots is varying between 0.4 and 0.9 nm (Figure 5f). The AuSIL system has attracted significant attention over the past decades, and there have been a number of research works to reveal the crystallographic structure of this gold-silicon mixture. However, an ambiguous determination of the exact structure at the atomic level have not been reported to date. This is due to the significant diffusion of Si into the Au(l 11) subsurface region, which results in the formation of numerous metastable crystalline structures with different compositions and lattice parameters. The formation mechanism of 1 x 1 BlueP according to example embodiments is similar to the graphene decoupling process. It generally involves the breaking of the interfacial P-Au coupling, saturation of P-Au interaction by a passivated AuSIL interfacial layer and subsequent physical decoupling of BlueP. In Figure 5b, the BlueP-Au alloy is only partially intercalated in order to clearly reveal the intercalation process. Complete intercalation can be achieved by subsequent repeats of this procedure or by performing this procedure once but with longer time.

Figure 6a-d shows the completely intercalated l x l BlueP according to example embodiments by preparing one monolayer BlueP- Au alloy on Au(l 11) first, and then exposing the sample to heated Si source with the substrate held at -280 °C. Figure 6d shows an atomically resolved STM image for the intercalated l x l BlueP according to example embodiments.

DFT calculation of the adsorption energy of Si on BlueP-Au alloy on Au(lll) surface are shown in Figure 7(a-c), specifically top and side views of the several typical adsorbed structures of Si atom 701, 702, 703 on BlueP-Au 704 alloy covered Au(lll) surface 706 are displayed. As shown in Figure 7(d-e), E _a ds of Si 708 on Au(lll) 706 is about 0.87 eV larger than that of P atom 710, which suggests that the interaction between Si and Au is much stronger than that between P and Au. This rationalizes the experimentally observed Si intercalation through BlueP- Au alloy to the underlying Au(l 11) surface.

Figure 8a-c display the LEED patterns for the clean Au(l 11), BlueP- Au alloy on Au(l 11) and Si intercalated l x l BlueP according to example embodiments. For the clean Au(lll), the LEED pattern shows intense spots corresponding to the l x l gold lattice (Figure 8a). The diffraction pattern recorded for BlueP- Au alloy reveals a well-defined 5 x 5 superlattice with respect to the Au(lll) surface (Figure 8b). The spots indicated by the darker circles are from the underlying gold substrate, while the spots in the lighter circle are attributed to the diffractions from l x l BlueP unit cell. After Si intercalation, the intensity of the 5 x 5 superstructure spots is strongly suppressed, as displayed in Figure 8c. Meanwhile, the diffraction spots corresponding to the l x l BlueP become more intense. This is consistent with the STM observations and further confirms the formation of non-reconstructed l x l BlueP by Si intercalation, according to example embodiments.

Following the successful Si intercalation of 2D BlueP, in particular non-reconstructed l x l BlueP, on Au(lll according to example embodiments, the direct growth of phosphorus onto AuSIL layer was studied according to other example embodiments. Figure lOa-c shows the large scale and close-up STM images of AuSIL layer formed by depositing Si onto the clean Au(lll) surface. Similar to the result shown in Figure If, no long-range ordered structure is formed on the surface. However, some common features can be extracted from these structures, that is the bright spots presented on the surface always orient along two directions as indicated by the white arrows, with an inclusion angle close to 48°. The LEED pattern was also measured for this AuSIL layer, and detailed analysis illustrates that these two directions should be the 3 and L/7 directions with respect to the Au(l 11) surface.

Figure 10a and lOd show the large scale STM images by depositing black phosphorus precursor onto the AuSIL surface held at 280 °C. Coexistance of several different phases of BlueP on top of AuSIL were observed, which are labeled as 1 x 1 BlueP 1, l x l BlueP2 and Trans BlueP, respectively. Here, Trans BlueP represents the BlueP nanoclusters, which are in the transition state to 2D BlueP, in particular a non-reconstructed BlueP island. They adopt well defined geometries that are predominantly in triangular shape, accompanied by the occasionally presented parallelogram-, trapezoid- and hexagonal- shaped nanoclusters (Figure 10b). There is a smooth transition across the boundary of the Trans BlueP and l x l BlueP 1, and the darker lines along the edges should be attributed to the non-intercalated P-Au coupling. Atomically resolved STM image reveals a lattice constant of 0.33 nm for non-reconstructed l x l BluePl according to example embodiments, which is highly consistent with the theoretically predicted value of the pristine BlueP (Figure 10c and lOd). The lattice mismatch between BlueP and the substrate gives rise to the presence of a moire pattern in 1 x 1 BluePl.

Further DFT calculations reveals that this moire superstructure can be explained by the superposition of a 5 x 5 BlueP supercell in the l x l BluePl phase over a 6 x 6 Au(l 11) lattice. As shown in Figure 11, the (5 x 5)-BlueP (16.40 A) is employed to match with the (6 x 6)- Au(lll) (17.39 A) supercell, which leads to a tensile strain of BlueP close to 6%. Figure 11a is the atomic geometry of the l x l BluePl phase, where the high-buckling P atoms appear as bright dots in the simulated STM image (Figure lib). The observed moire superstructure is caused by the variation of the local binding sites of the buckled-up P atoms within one supercell. Due to lack of precise determination of the coverage of Si atoms on Au(l 11), one Si atom was simply added in the supercell to model the Si-intercalated system. At this coverage, the most stable adsorption site is the octahedral site in the subsurface Au layer (Figure 1 lc, see also Figure 7d), and the experimentally observed features are successfully reproduced by using this atomic model (Figure lid).

It is noted that the buckling structure is an intrinsic property of 2D BlueP, i.e. even for the theoretically predicted free standing blue phosphorus, it has the P atoms buckled into two different heights. This buckling is not understood as “reconstructed”. Reconstruction usually means the newly formed structure has a larger periodicity compared with the pristine structure, for example, the Au(lll) 22 x 3 reconstruction. As the STM and LEED patterns can clearly resolve the lxl periodicity of the BlueP according to example embodiments, the example embodiments are referred to as non-reconstructed.

Returning to Figure 10, Figure lOe-h show the large scale and atomically resolved STM images for the l x l BlueP2 phase. A remarkable difference of this phase compared with the l x l BluePl phase is that no moire pattern is observed. As shown in the profile measured along the blue line (Figure lOh), these P atoms are identical in height with a unit cell length of 0.33 nm. It is noted that here the P atoms being identical in height is referring to the P atoms observed in the STM images. In the STM images, only the buckled-up P atoms can be detected, while the buckled-down P atoms are not visible from the STM images. The STM results clearly indicate that a perfect non-reconstructed l x l BlueP is formed on AuSIL surface according to example embodiments, indicated as 1 x 1 BlueP2 phase in Figure 10. The dilute amounts of P atoms that have darker appearance are attributed to the presence of intercalated Si atoms below BlueP, as confirmed by the following DFT simulations (Figure 13). The existence of two different phases of 2D Blue P according to example embodiments is due to the growth of BlueP in different orientations with respect to the underlying surface. Figure 12a and 12b show the atomically resolved STM images for 1 x 1 BluePl and l x l BlueP2 phase, respectively. The arrows A and B indicate the crystallographic orientation of 1 x 1 BluePl phase. The single rhombus in Figure 12b highlights a 7 x 7 supercell of 1 ^x 1 BlueP2 phase. Careful inspection reveals that the 7 direction of the BlueP2 phase is parallel with the crystallographic orientation of 1 x 1 BluePl phase, which is also the [110] direction of the underlying Au(l l l). Moreover, the 7am _ueP (8.62 A) is very close to the length of 3a _Au (8.62 A). So the observation here suggests that the non-reconstructed l x l BlueP2 phase according to example embodiments is originated from a 7 x V7-BlueP supercell matching with a 3 x 3- Au(lll) supercell.

Figure 13a shows the atomic geometry for the non-reconstructed l x l BlueP2 phase according to example embodiments. Based on the experimental results, a 7 x V7-BlueP on (3 x 3)- Au(l 11) model was built with a lattice mismatch less than 0.2%. Again, one Si atom is put into the supercell to model the Si-intercalated system (Figure 13c, see also Figure 7d), the simulated STM image is in good agreement with the experimental results (Figure 13d). The experimentally observed P atoms with darker appearance can be attributed to the presence of Si atoms below BlueP.

By increasing the phosphorus coverage, the l x l BlueP2 phase can grow larger in lateral size, and finally they can spread over almost the entire surface (Figure 14). That is, the l x l BluePl and Trans BlueP can transform to 1 x 1 BlueP2 at higher phosphorus coverage.

XPS study of Si intercalated l l BlueP according to example embodiments

To ascertain the formation of intercalated non-reconstructed l x l BlueP according to example embodiments, core level XPS measurement using synchrotron radiation was carried out. Au 4f, P 2p and Si 2p spectra were measured for BlueP- Au alloy/ Au, Si intercalated l x l BlueP and also for Si on clean gold for comparison. As shown in Figure 15a, the Au 4f peak for BlueP- Au alloy is located at a binding energy of 83.89 eV. Si interaction can induce the appearance of a second component located at 84.70 eV. By comparison with the Au 4f spectra collected for Si directly deposited on clean gold, the peak at 84.70 eV can be unambiguously assigned to Au-Si bonds within the AuSIL layer. This is a further evidence that Si atoms are indeed intercalated into the gold surface and produces an AuSIL buffer layer. Figure 15b displays the P 2p spectra for BlueP- Au alloy and Si intercalated l x l BlueP, respectively. The P 2p signals are made up of spin-orbit split doublets, and the binding energies are labeled for the P 2p3/2 position. The P 2p3/2 peak measured for BlueP- Au alloy is located at 128.89 eV. After interaction, the P 2p spectra becomes broader and can be accurately fitted by two sets spin-orbit split doublets. One is still located around 128.89 eV and can be attributed to the residual non-intercalated BlueP- Au alloy remaining on the surface. The emergence of another dominant peak located at 128.17 eV is due to the formation of the intercalated l x l BlueP. Similar to P 2p signal, the Si 2p spectra (Figure 15c) also consist of spin-orbit split doublets. For Si directly deposited onto clean gold, a single and narrow Si 2p3/2 peak at 99.23 eV is observed. However, the spectra measured for the intercalated sample according to example embodiments is broadened. This is expected, as the Si atoms in the intercalated sample have two different chemical environments: those underlying the l x l BlueP and those within the exposed AuSIL layer.

ARPES measurement of the band structure of Si intercalated l x l BlueP according to example embodiments

The successful synthesis of the non-reconstructed l x l BlueP according to example embodiments allows to explore the electronic structure of this pristine phosphorus layer. A uniform surface solely covered by non-reconstructed l x l BlueP according to an example embodiment is prepared through complete Si intercalation, which enables the subsequent ARPES measurement. In order to differentiate the BlueP related bands from the intense spectral weight of Au(lll), second derivate plots are used to reveal the band dispersions. The focus was on the bands close to the Fermi level, the dispersion of which is crucial in determining the electronic properties of a material. Below 2 eV, it is difficult to distinguish the electronic features derived from phosphorus, due to the intense contribution from the Au d-bands. Figure 16a shows the ARPES spectra measured along the GK cut for the clean Au(l 11) surface, where the Shockley surface state is clearly observed, as indicated by the white arrows in Figures 16a- c. Upon phosphorus deposition, additional fine features belonging to BlueP- Au alloy emerge in the region between Fermi level and -2 eV, as indicated by the darker arrows in Figure 16b.

After Si intercalation, the band structures related to BlueP- Au alloy vanish and a new feature corresponding to the intercalated l x l BlueP according to example embodiments is established, which is marked by the darker arrow in Figure 16c. The maximum of this band is found approximately at -1.2 eV and adopts a parabola shape, in good agreement with the theoretically predicted valence band maximum at the G point for free-standing monolayer BlueP (Figure 16f).

To corroborate these findings, the band structure for the BlueP- Au alloy/ Au(l 11) system was also calculated. As shown in Figure 16e, the dots denote the effective band structures for the BlueP- Au alloy/ Au(l 11), with the dispersion of a clean Au(l 11) surface superimposed on top (black lines). Below -2 eV, the band dispersions of the BlueP- Au alloy/ Au(l 11) almost coincide with the clean Au(l 11) bands, suggesting that these bands are contributions from the underlying Au(l l l) surface. Between -1 and -2 eV, two newly emerged band dispersions deviating from the Au(l ll) dispersions can be observed, as highlighted by the darker dots. These two bands are in good agreement with the experimentally observed band features in both the band dispersion and energy position, strongly supporting that these states originate from the BlueP- Au alloy layer.

A modification of the shape of the Shockley surface state and a shift toward higher binding energy is also observed and labeled as M-SS (see Figure 16a-d). By comparison with the spectra measured for Si directly deposited on clean gold (Figure 16d), the dispersion toward the K point indicated by the black arrow in Figure 16c can be assigned to the AuSIL-related band structure, also indicated by the black arrow in Figure 16d.

The band structures for Si intercalated non-reconstructed l x l BlueP on Au(l 11) according to example embodiments was also calculated. When taking the substrate and the intercalated Si into account, the bands from the gold substrate and the AuSIL become distinguishable, whereas the contributions from the l x l BlueP remain predominantly unaffected, as highlighted by the arrow in Figure 16g.

Figure 17a-d show the ARPES spectra measured along the GM cut for clean Au(l l l), BlueP- Au alloy/ Au(l 11), Si intercalated l x l BlueP according to an example embodiment, and AuSIL, respectively. The general features are similar to the spectra measured along the GK cut. Figure 17a shows the ARPES spectra measured along the GM cut for the clean Au(l 11) surface, where the Shockley surface state is clearly observed, as indicated by the white arrow in Figures 17a-c. Upon phosphorus deposition, additional fine features belonging to BlueP- Au alloy emerge in the region between Fermi level and -2 eV, as indicated by the darker arrows in Figure 17b. The general features of these newly emerged dispersive bands are consistent with the previously reported results. These states are originated from BlueP-Au alloy layer. A modification of the shape of the Shockley surface state and a shift toward higher binding energy is also observed and labeled as M-SS, which is expected for thin epitaxial layers on noble metals. After Si intercalation, the band structures related to BlueP- Au alloy vanish and a new feature corresponding to the intercalated non-reconstructed l x l BlueP according to example embodiments is established, which is marked by the darker arrow in Figure 17c. The maximum of this band is found approximately at -1.2 eV and adopts a parabola shape, in good agreement with the theoretically predicted valence band maximum at the G point for free-standing monolayer BlueP.

As mentioned above, the intercalated Si layer, i.e. the AuSIL, has the possibility to be oxidized to a silicon oxide, S1O2, layer, which leads to the synthesis of a semiconductor, i.e. the non- reconstructed l x l BlueP layer directly on top of the S1O2 layer on an Au(lll) substrate as shown in Figure 18, enabling a device fabrication process according to an example embodiments without complicated transfer process. For example, the Si interlayer can be oxidized by exposing the sample to pure oxygen at 5xl0 ⁵ mbar for 60mins.

As described above, 2D BlueP. in particular non-reconstructed l x l BlueP, is synthesized on Au(lll) according to example embodiments by tuning the interfacial interaction through Si intercalation. The atomic structure, chemical state and band structures of the fabricated l x l BlueP layer according to example embodiments have been clearly revealed through the combination of LT-STM, nc-AFM, XPS, LEED, ARPES and further corroborated by DFT calculations. Based on the results, the following growth process according to an example embodiment is as follows: direct deposition of black phosphorus precursor on clean Au(lll) leads to the formation of an atomic layer of BlueP- Au alloy; Si intercalation triggers the breaking of the P-Au coupling and subsequent formation of 1 x 1 BlueP layer supported on an AuSIL buffer layer. Two different phases of 1 x 1 BlueP with different orientations with respect to the Au(l 11) surface can be achieved by reversing the growth sequence, i.e. depositing Si on Au(lll) first to form the AuSIL, and then depositing phosphorus onto AuSIL. Embodiments of the present invention can provide a flexible means to tune the interfacial interaction of BlueP with the substrate and enabling exploration of the novel electronic properties of the surface functionalized BlueP, such as quantum spin hall insulators, Dirac cones, tunable quantum phase transitions and novel emergent fermions.

Methods according to example embodiments

LT-STM Characterization. Phosphorus atoms were evaporated from a Knudsen cell containing bulk crystal black phosphorus onto the substrate. Si atoms were deposited onto the substrate by direct current heating of a Si rod. LT-STM measurement was performed in an Unisoku system with a RHK controller at liquid nitrogen temperature. A Pt-Ir tip was used with the bias voltage applied to the sample. The base pressure of the preparation chamber and analysis chamber were better than 1.0 x KG ¹⁰ mbar. qPlus nc-AFM Characterization. A tungsten tip was used for the qPlus nc-AFM measurements, and the tip was functionalized with a CO molecule for high-resolution nc-AFM imaging. The qPlus nc- AFM was performed in a multichamber ultrahigh vacuum (UHV) Omicron system at liquid helium temperature. The resonant frequency, quality factor, and oscillation amplitude of the qPlus sensor were 28.2 kHz, 10500, and 120-150 pm, respectively. The frequency shift of the qPlus sensor and the tunneling current were recorded during the scanning process.

XPS and ARPES Characterization. Photoemission spectroscopy experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. This system comprises four interconnected UHV chambers, including an analysis chamber, a preparation chamber, a molecular beam epitaxy chamber, and a radial distribution chamber, with a base pressure of 7 x KG ¹¹ , 1 x KG ¹⁰ , 5 x KG ¹⁰ , and 2 x KG ¹¹ mbar, respectively. The analysis chamber was equipped with a VG Scienta R4000 analyzer and a manipulator with high precision and five degrees of freedom. Photon energies of 180 and 75 eV were used for the XPS and ARPES measurements, respectively. Each ARPES intensity plot was smoothed to reduce the noise before second derivative processing.

DFT Calculations. First-principles calculations were performed using Vienna ab initio simulation package (VASP) within the framework of the DFT. The projector- augmented wave (PAW) method was adopted to describe the electron-ion interaction, and the plane-wave cutoff energy was set to 350 eV. The generalized gradient approximation (GGA) in Perdew-Burke-Emzerhof (PBE) functional was used to describe the exchange-correlation interaction. To account the long-range van der Waals interaction, the DFT-D3 correction was employed. The convergence criteria of total energy for electron self-consistency and force for geometry optimizations were set to KG ⁵ eV and 0.01 eV/A, respectively. The bulk lattice constant of Au was determined to be 4.10 A by using a Monkhorst-Pack k-point mesh of (21 x 21 x 21), whereas that of single-layer BlueP was found to be 3.28 A. The Au(lll) surface slab was modeled with four layers of metal atoms, and the bottom-layer atoms were kept fixed in their bulk positions during the geometry relaxation. A vacuum layer with a thickness of at least 20 A was included in the simulation cell.

Figure 19 shows a flowchart 1900 illustrating a method of fabricating two-dimensional, 2D, blue phosphorus, BlueP. At step 1902, a gold(ll l), Au(lll), substrate is provided. At step 1904, an interlayer is provided on a surface of the Au(l 11) substrate, the interlayer comprising silicon, Si. At step 1906, the 2D BlueP is formed on the interlayer.

The method may comprise depositing a BlueP- Au alloy layer on the Au(l l l) substrate, and providing the interlayer may comprise silicon, Si, intercalation between the reconstructed BlueP- Au alloy layer and the Au(l l l) substrate to form an AuSIL layer. The Si may be evaporated from a solid Si material.

The reconstructed BlueP- Au alloy layer may be deposited on the Au(lll) substrate by chemical vapor deposition, CVD.

Forming the 2D BlueP may comprise physically decoupling the BlueP- Au alloy layer from the Au(l 11) substrate by the interlayer during the Si intercalation.

Providing the interlayer may comprise depositing Si on the Au(l 11) substrate to form an AuSIL layer. The Si may be deposited using molecular beam epitaxy, MBE.

The method may comprise forming the 2D BlueP on the AuSIL interlayer. Forming the 2D BlueP on the AuSIL interlayer may comprise CVD.

The method may further comprise an oxidization processing to form a silicon oxide, S1O2, layer from the interlayer between the Au(l 11) substrate and the 2D BlueP.

The 2D Blue P may comprise non-reconstructed l x l BlueP. The 2D Blue P may comprises a monolayer of the non-reconstructed l x l BlueP.

In one embodiment, a 2D BlueP formed by the method of embodiments described above with reference to Figure 19 is provided.

In one embodiment a structure is provided comprising a gold(lll), Au(lll), substrate; an interlayer on a surface of the Au(l 11) substrate, the interlayer comprising Si; and 2D BlueP on the interlayer.

The interlayer may comprise AuSIL.

The interlayer may comprise silicon oxide, S1O2. The 2D BlueP may comprise a non-reconstmcted l x l BlueP. The 2D Blue P may comprise a monolayer of the non-reconstructed l x l BlueP.

The above description of illustrated embodiments of the systems and methods is not intended to be exhaustive or to limit the systems and methods to the precise forms disclosed. While specific embodiments of, and examples for, the systems components and methods are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the systems, components and methods, as those skilled in the relevant art will recognize. The teachings of the systems and methods provided herein can be applied to other processing systems and methods, not only for the systems and methods described above.

The elements and acts of the various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the systems and methods in light of the above detailed description.

In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and the claims, but should be construed to include all processing systems that operate under the claims. Accordingly, the systems and methods are not limited by the disclosure, but instead the scope of the systems and methods is to be determined entirely by the claims.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of "including, but not limited to." Words using the singular or plural number also include the plural or singular number respectively. Additionally, the words "herein," "hereunder," "above," "below," and words of similar import refer to this application as a whole and not to any particular portions of this application. When the word "or" is used in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list and any combination of the items in the list.