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Direct Imaging of Band Structure for Powdered Rhombohedral Boron Monosulfide by Microfocused ARPES
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  • Katsuaki Sugawara*
    Katsuaki Sugawara
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
    Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-2926-9436
  • Haruki Kusaka
    Haruki Kusaka
    Department of Materials Science and Tsukuba Research Center for Energy Materials Science (TREMS), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
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  • Tappei Kawakami
    Tappei Kawakami
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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  • Koki Yanagizawa
    Koki Yanagizawa
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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  • Asuka Honma
    Asuka Honma
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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  • Seigo Souma
    Seigo Souma
    Advanced Institute for Materials Research (WPI-AIMR)  and  Center for Science and Innovation in Spintronics, Tohoku University, Sendai 980-8577, Japan
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  • Kosuke Nakayama
    Kosuke Nakayama
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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    Orcidhttps://orcid.org/0000-0003-2462-2253
  • Masashi Miyakawa
    Masashi Miyakawa
    Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan
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    Orcidhttps://orcid.org/0000-0002-0838-8156
  • Takashi Taniguchi
    Takashi Taniguchi
    International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan
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    Orcidhttps://orcid.org/0000-0002-1467-3105
  • Miho Kitamura
    Miho Kitamura
    Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
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  • Koji Horiba
    Koji Horiba
    National Institutes for Quantum Science and Technology (QST), Sendai 980-8579, Japan
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  • Hiroshi Kumigashira
    Hiroshi Kumigashira
    Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
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    Orcidhttps://orcid.org/0000-0003-4668-2695
  • Takashi Takahashi
    Takashi Takahashi
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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  • Shin-ichi Orimo
    Shin-ichi Orimo
    Advanced Institute for Materials Research (WPI-AIMR)  and  Institute for Material Research, Tohoku University, Sendai 980-8577, Japan
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  • Masayuki Toyoda
    Masayuki Toyoda
    Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
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    Orcidhttps://orcid.org/0000-0002-5669-262X
  • Susumu Saito
    Susumu Saito
    Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
    Advanced Research Center for Quantum Physics and Nanoscience, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
    Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan
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  • Takahiro Kondo
    Takahiro Kondo
    Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
    Department of Materials Science and Tsukuba Research Center for Energy Materials Science (TREMS), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
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    Orcidhttps://orcid.org/0000-0001-8457-9387
  • Takafumi Sato*
    Takafumi Sato
    Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    Advanced Institute for Materials Research (WPI-AIMR),  Center for Science and Innovation in Spintronics  and  International Center for Synchrotron Radiation Innovation Smart (SRIS), Tohoku University, Sendai 980-8577, Japan
    *Email: [email protected]
    More by Takafumi Sato
    Orcidhttps://orcid.org/0000-0002-4544-5463
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Nano Letters

Cite this: Nano Lett. 2023, 23, 5, 1673–1679
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https://doi.org/10.1021/acs.nanolett.2c04048
Published February 27, 2023

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Abstract

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Boron-based two-dimensional (2D) materials are an excellent platform for nanoelectronics applications. Rhombohedral boron monosulfide (r-BS) is attracting particular attention because of its unique layered crystal structure suitable for exploring various functional properties originating in the 2D nature. However, studies to elucidate its fundamental electronic states have been largely limited because only tiny powdered crystals were available, hindering a precise investigation by spectroscopy such as angle-resolved photoemission spectroscopy (ARPES). Here we report the direct mapping of the band structure with a tiny (∼20 × 20 μm2) r-BS powder crystal by utilizing microfocused ARPES. We found that r-BS is a p-type semiconductor with a band gap of >0.5 eV characterized by the anisotropic in-plane effective mass. The present results demonstrate the high applicability of micro-ARPES to tiny powder crystals and widen an opportunity to access the yet-unexplored electronic states of various novel materials.

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Keywords

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Developing functional two-dimensional (2D) materials is a current central challenge in materials science, stimulated by the discovery of the quantum Hall effect in graphene. (1) Two-dimensional materials consisting of boron (B) are now attracting particular attention since boron-based materials are expected to exhibit novel electronic properties distinct from those of carbon-based materials. A typical example of B-based 2D materials is hexagonal boron nitride (h-BN), (2) which has a flat honeycomb lattice similar to graphene but a large insulating gap in contrast to the zero-gap semiconducting behavior of graphene. The insulating gap in h-BN is regarded as a great advantage in realizing exotic quantum phenomena when in contact with other 2D materials such as graphene and transition-metal dichalcogenides. (3−8) Furthermore, B-based 2D sheets themselves also serve as a useful platform to realize exotic quantum states, as exemplified by (i) the high-temperature superconductivity in MgB2 (9) with honeycomb B sheets, (ii) Dirac fermions in a 2D borophene sheet, (10,11) (iii) potential high-temperature superconductivity (Tc ≈ 22 K) in borophene/Ag(111), (12) and (iv) efficient H2 storage in hydrogen borides (HB). (13) It is thus of great importance to explore the electronic states of various B-based 2D materials to realize the novel functionalities in the 2D sheets.

Rhombohedral boron monosulfide (r-BS) is attracting great attention as a new type of layered boride. As shown in Figure 1a, r-BS has a unique crystal structure with R3̅m symmetry, characterized by the periodic stacking of 2D sheets consisting of B and S atoms. The unit BS layer can be regarded as a transition-metal disulfide layer where a transition-metal atom is replaced with a covalently bonded B–B pair, and adjacent BS sheets are weakly coupled via the van der Waals force. (14,15) Due to the rhombohedral nature, r-BS layers are stacked in the A–B–C stacking manner so that the bulk crystal contains three layers in the unit cell, leading to the long c-axis length of 20 Å. Due to this long c-axis length, the bulk Brillouin zone (BZ) is short in the direction of the c* axis. One can adopt hexagonal BZ (Figure 1b) by taking into account the convenience to compare the band structure between the bulk and thin films. It has been theoretically predicted that the 2D BS sheet exhibits various functional properties such as high hydrogen storage, (16) high thermal conduction, (17) and efficient photocatalysis. (18) Furthermore, theoretical predictions of spin current control in the MoS2/r-BS junction and high-Tc superconductivity over 20 K in δ-BS (orthorhombic structure with Pmma symmetry) make the BS system even more attractive. (19) Recent successful fabrication of the bulk r-BS crystal (14,15) as well as isolation of a monolayer BS sheet from bulk crystal (20,21) opened a door to examining such intriguing predictions. On the other hand, despite these theoretical studies that suggest the sensitivity of electronic states to the configuration, stacking sequence, and number of BS layers, (17,19−21) no experimental outputs on the basic electronic states have been obtained even for bulk r-BS. This is primarily because obtained bulk single crystals are very small and powder-like (a photograph is shown in Figure 1c; the typical powder size is less than 30 μm), hindering access by spectroscopy techniques such as photoemission spectroscopy (PES).

Figure 1

Figure 1. (a), (b) Schematics of the crystal structure and the first BZ of r-BS, respectively. (c) Photograph of r-BS powders. (d) Schematics of (1) sample mounting on an Au substrate, (2), (3) cleaving by Kapton tape, and (4) micro-ARPES measurements.

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In this Letter, we report the band structure of the r-BS powder crystal studied by microfocused angle-resolved photoemission spectroscopy (μ-ARPES). Although the determination of band structure for such a tiny powder crystal with conventional ARPES is almost impossible, we have overcome this difficulty by utilizing a microfocused photon beam (22) and carefully selecting a tiny single-crystal piece among many islands of powders stamped on a gold (Au) substrate by spatial photoelectron imaging. As a result, we have succeeded in mapping out the 3D band structure of r-BS and have compared it with calculations. We emphasize that the experimental scheme established in this study can be widely applicable to elucidate the electronic band structure of many other powder crystals.

To perform ARPES measurements with a tiny r-BS crystal, we at first dispersed r-BS powders on a polycrystalline Au substrate with a thickness of ∼0.3 mm attached to the sample holder in the atmosphere (Figure 1d, ①), and installed it into an ultrahigh vacuum (UHV) chamber connected to the ARPES chamber. Next, we pasted a piece of Kapton tape onto the Au substrate (Figure 1d, ②) and removed it to cleave the powder samples under UHV (Figure 1d, ③). For details of sample fabrication and characterization, see Supporting Information S1. An optical microscope image after cleavage is shown in Figure 2a, where several r-BS clusters are seen as white areas enclosed by purple line. To distinguish r-BS crystals from the Au substrate, we performed scanning μ-PES measurements (Figure 1d, ④) with a 100 μm step and mapped out the spatial distribution of photoelectron intensity at a binding energy (EB) of 185.5–192.5 eV where the B 1s core-level peak is expected to appear (blue curve in Figure 2b). The result shown in Figure 2c signifies that the weak intensity area with blue color shows good matching with the area where high-density r-BS clusters are seen in the optical microscope image in Figure 2a. This is opposite to an intuitive expectation that the higher-density area of r-BS powders should show a stronger B 1s intensity. However, this unexpected result can be well explained in terms of the strong charging effect of the sample caused by the poor electrical contact due to the high density and/or thick nature of r-BS clusters. Actually, the charging behavior is recognized in the PES spectrum measured in the blue color region as a strong suppression of the spectral weight (red curve in Figure 2b). From these experimental results, we concluded that the sample area where white-colored r-BS clusters are clearly visible (Figure 2a) is not suitable for observing the intrinsic electronic states of r-BS because the PES spectrum suffers from the strong charging effect due to the insulating nature of r-BS. (14,15,20,21) We then looked for smaller pieces of r-BS powder with lower density to obtain reliable PES signals. We chose a sample area (green dashed box in Figure 2c) where the photoelectron intensity is relatively strong and carried out the spatial mapping of photoelectron intensity with a finer step of 20 μm (Figure 2d). The strong intensity region (red color) shows a good correspondence with the area of the exposed Au substrate observed in the magnified optical microscopy image (Figure 2e). This is also confirmed by the experimental result that the ARPES intensity at point 1 in Figure 2d is relatively strong at EB ≈ 2.5, 4, and 6 to 7 eV (Figure 2f) where the Au 5d bands are expected to appear. (23) The nondispersive feature of PES intensity indicates that the Au substrate is polycrystalline. The reason that the B 1s signal is not visible in Figure 2d is that it is covered by the strong-intensity background from the Au substrate as seen in Figure 2b. To enhance the signal from the B 1s peak, we integrated the photoelectron intensity within a narrower window covering only the B 1s peak (188.0 ≤ EB ≤ 189.5 eV; see vertical dashed lines in Figure 2b), and the result is shown in Figure 2g. Now, one can identify the strongest intensity appearing in a single pixel of 20 × 20 μm2 marked as point 2, which was overlooked in the PES-intensity mapping in a wider EB integration window shown in Figure 2d. We found from the laser microscope image in Figure 2h that such a small area is characterized by a flat and mirror-like surface suitable for ARPES measurements. In fact, we were able to obtain high-quality ARPES data at point 2 as shown in Figure 2i (see also Supporting Information S2 and S3; note that an overall reproducibility of experimental band structure was confirmed by measuring a few different flakes as detailed in Supporting Information S4). These bands originate from the r-BS crystal because the PES spectrum of a wider energy range of EB = 0–200 eV (Figure 2j) signifies not only the B 1s peak but also the spin–orbit satellites of S 2p1/2 and S 2p3/2 core-level peaks, besides weaker Au 4f peaks from the Au substrate. Moreover, the composition ratio of B and S is estimated from the spectral weight of the B 1s and S 2p peaks by taking into account that the photoionization cross-section (24) is 1.1 ± 0.1, consistent with the expected value of 1.0. It is emphasized here that, to efficiently find an appropriate sample position for ARPES measurements, it is essential to carry out high-spatial-resolution mapping with a pixel size comparable to that of powder crystals.

Figure 2

Figure 2. (a) Optical microscope image of r-BS powder crystals on a Au substrate after exfoliation with Kapton tape. (b) Representative PES spectra around the B 1s core level at three representative sample positions where the B 1s core-level peak is clearly recognized (blue curve), the strong signal from the Au substrate dominates (green curve), and the strong charging effect is seen (on white-colored r-BS clusters) (red curve). (c) Spatial mapping of photoemission intensity integrated over the EB range of 185.5–192.5 eV, measured with hv = 250 eV in the same spatial region as the microscopy image of (a). (d) Same as (c) but for a smaller spatial region enclosed by a green dashed box (0.8 × 0.8 mm2) in (c) measured with a finer step of 20 μm. (e) Optical microscope image corresponding to (d). (f) ARPES intensity in the valence-band region as a function of EB and the wave vector measured with hv = 100 eV at point 1 in (d). (g) Same as (d) but with a narrower EB window of 188.0 ≤ EB ≤ 189.5 eV which covers only the B 1s peak as indicated by vertical dashed lines in (b). Points on the sample where ARPES measurements were carried out are indicated as squares in (d) and (g) (points 1 and 2). (h) Enlarged laser microscope image around point 2. (i) Same as (f) but measured at point 2. (j) PES spectrum in a wider EB range (EB = 0–200 eV) measured with hv = 250 eV. The inset shows the magnified view in the B 1s and S 2p core-level region.

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Now that the scheme to access the band structure of r-BS powder crystal is established, we present the 3D band structure obtained at point 2 in more detail. To clarify the three-dimensionality of the electronic states, we investigated the band structure along the wave vector perpendicular to the surface (kz) by varying the photon energy in ARPES measurements. We carried out the in-plane band-dispersion mapping at selected kz planes. Figure 3a shows EDCs measured along the ΓKM cut (ky cut) at kz ≈ 0 plane with hν = 105 eV at T = 300 K. (Note that the direction of high-symmetry lines was determined by the symmetry and periodicity of the FS mapping shown in Figure 4e.) One can recognize several dispersive bands. For example, sharp peaks located at 2 and 2.5 eV seen in the normal-emission EDCs have a hole-like dispersion centered at the Γ point and rapidly disperse toward higher EB on moving away from the Γ point. The band dispersion is better visualized in the corresponding ARPES intensity plot in Figure 3b, which signifies that the hole-like band rapidly disperses up to 7 eV midway between the Γ and K points and then disperses back again toward EF on further approaching the K point. After passing the K point, this band moves further upward and becomes nearly flat at ∼2.5 eV around the M point. One can see a weak intensity above 2 eV midway between the Γ and K points as indicated by a black arrow. This band is likely connected to the 2 eV peak at the Γ point to form an M-shaped band centered at the Γ point as predicted in the calculation (details in Figure 4). Besides such a highly dispersive band, there exist less dispersive features, such as a weakly dispersive electron-like band that bottoms out at 9 eV at the Γ point and a band located at 4 eV with small wiggling along the EB axis. (Note that weak features located at 2.5 eV and 6 to 7 eV are assigned to the background from the Au substrate.) When the photon energy is increased to hν = 125 eV to probe the kz ≈ π plane (Figure 3c,d), the uppermost valence band moves upward and shows a holelike dispersion with its top at 0.5 eV at the A point, which corresponds to the valence band maximum. (Note that the A point coincides between triple-layer- and single-layer-based BZs.) The absence of the EF crossing of bands signifies the semiconducting nature of r-BS. As shown in Figure 3d, besides the topmost valence band, there exists another band that bottoms out at 6 eV at the Γ point which does not have a corresponding feature in the kz ≈ 0 plane (Figure 3b). Except for these bands, most of the bands (e.g., band tops out at 2.5 eV at the M point and bottoms out at 9 eV at the Γ point) show a similar overall dispersive feature between kz ≈ 0 and π, supporting the quasi-2D nature of r-BS, consistent with the existence of a van der Waals gap between adjacent BS sheets in the crystal.

Figure 3

Figure 3. (a), (b) EDCs and corresponding ARPES intensity plot measured along the ΓKM cut (kz = 0 plane) with hv = 105 eV. Gray arrows show a weak spectral signature from the topmost valence band. (c), (d) Same as (a) and (b), but measured along the AHL cut (kz = π plane) with hv = 125 eV. There exist notable differences in the ARPES intensity plot between the kz = 0 and π planes, such as the energy position of the hole-band top at the Γ/A point and the absence/appearance of a U-shaped band that bottoms out at 6 eV at the Γ/A point.

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Figure 4

Figure 4. (a), (b) Comparison of ARPES-intensity plot (same as Figure 2e, but in gray scale) with calculated band structure (red curves) along the ΓKM cut. (c), (d) Same as (a) and (b) but along the AHL cut. Bands are labeled A–G. (e) ARPES-intensity plot as a function of in-plane wave vectors, kx and ky, for representative EB slices measured with hv = 125 eV corresponding to the AHL plane, overlaid with the calculated equi-energy contours (red curves). (f), (g) ARPES intensity plots along the AL and AH cuts, respectively, together with the experimental band dispersion (purple circles) and the result of numerical fittings with a parabolic function (blue curve).

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To obtain further insights into the characteristics of observed bands, we directly compare the ARPES intensity along the ΓKM cut (the same as Figure 3b but plotted with gray scale) and the calculated band structure in Figure 4a,b. We also compare the experimental and calculated band structures for the AHL cut in Figure 4c,d. Calculated bands are shifted as a whole so as to align the energy position of the topmost valence band at the Γ/A point. One can recognize very good agreement between the experiment and calculation. For example, (A) a highly dispersive holelike band at the Γ point in the EB range of 2–7 eV, (B) a nearly flat band that tops out at ∼2.5 eV around the M point, and (C) an electronlike band that bottoms out at 8 to 9 eV at the Γ point are commonly recognized in both ΓKM and AHL cuts. In addition, (D) a weak M-shaped band for the topmost valence band which exists only along the ΓKM cut corresponding to the ARPES feature indicated by black arrows in Figure 3a,b, (E) a weakly dispersive band at 4 eV along the ΓKM cut, and (F) an electron-like band that bottoms out at 6 eV and (G) a topmost valence band that tops out at 0.5 eV along the AHL cut are nicely reproduced in the calculation. According to the calculation, all of these bands are assigned to the hybridized B 2p and S 3p orbitals. Among them, the topmost valence band originates from the hybridized B 2pz and S 3pz orbitals in which the electronic wave function extends along the c axis. Since the bandwidth along the kz axis is sensitive to the magnitude of interlayer hopping, exfoliation of the r-BS crystal down to a few layers may cause a dramatic influence on the size of the band gap. (20,21) To discuss the energy dispersion of the topmost valence band in more detail, we show in Figure 4e the energy counter plot as a function of the in-plane wave vectors, kx and ky, obtained at the kz ≈ π plane at some representative EB values. One can recognize from the intensity plot at EB = EF the absence of ARPES intensity, which confirms the semiconducting nature of r-BS. At EB = 0.5 eV, a small spot, corresponding to the valence band maximum, starts to appear at the A point. Upon increasing EB, the small spot gradually enlarges and evolves into a large circular-shaped contour and eventually becomes a large hexagon at EB = 1.75 and 2.25 eV. These features well reflect the symmetry of BZ and show good agreement with the calculated equi-energy contours overlaid by red curves. (Note that the energy position of the topmost valence band is aligned between the experiment and calculation.) It is noted that the band gap value in the occupied region estimated from the present ARPES experiment (0.5 eV) is less than half of the band gap estimated by other experiments such photoluminescence (1.8–2.5 eV) and ultraviolet diffuse reflectance (3.4 eV), as well as the calculated band gap (2.7 eV). (14,20,21) This suggests that r-BS is a hole-doped p-type semiconductor.

The observed hexagonal (noncircular) contour in Figure 4e suggests an anisotropic in-plane effective mass (m*). To estimate the m*, we at first extracted the experimental band dispersion for the topmost valence band from the peak position of EDCs measured along the AL and AH cuts, as shown by red circles in Figure 4f,g. Then, we performed numerical fittings to the experimental band dispersion with a parabolic function and estimated the m*/m0 (m0: free electron mass) value along the AL and AH cuts to be −1.30 ± 0.02 and −0.93 ± 0.02, respectively, which indicates an anisotropy of ∼40%. It is remarked that the average m*/m0 value of r-BS is twice as large as that of bulk h-BN (−0.49), (25) and there is also a sizable difference in the band gap magnitude (the band gap of h-BN is ∼6 eV). (2,14,20,21) This would point to a different usability as a semiconductor device in the application. It is noted that the m* value of monolayer r-BS is predicted to be enhanced by 10 times compared to that of its bulk counterpart due to the possible formation of a relatively flat valence band associated with the band quantization. (16,20) As a next step, it would be challenging to carry out the band-structure investigation of monolayer r-BS and its band-structure engineering with electrical gating and/or chemical doping to search for exotic quantum states predicted for flat-band 2D materials. (5,6,26,27)

Finally, we briefly discuss the implications of the present results in both material and spectroscopy aspects. Two-dimensional semiconductors are useful in a broad range of applications such as optoelectronics and spintronics. A majority of so-far-identified 2D semiconductors are n-type due to the electron doping from intrinsic impurities and/or defects (e.g., electron-doped MoS2 and WS2 with chalcogen vacancies), whereas p-type 2D semiconductors are still limited (e.g., black phosphorus, (28) tellurium, (29) and α-MnS). To develop functional devices based on 2D semiconductors, it would be indispensable to find a new p-type 2D semiconductor in both bulk and monolayer forms to make a p–n junction. Our finding of the p-type semiconducting nature of bulk r-BS would be useful in this respect. In the spectroscopy aspect, we have established concrete methodology to elucidate the band structure of tiny powder crystals by μ-ARPES. This technique can be widely applicable to a variety of materials whose large single crystal has been difficult to synthesize.

In conclusion, we reported a direct visualization of the 3D band structure of the r-BS power crystal by utilizing scanning micro-ARPES. Our key findings are (i) the existence of a band gap exceeding 0.5 eV, supportive of the p-type semiconductor nature of r-BS, (ii) the large effective mass of valence electrons twice as large as that of h-BN, and (iii) the anisotropic nature of the in-plane effective mass. The present results open a pathway toward investigating the band structure and fermiology of a wide variety of powder crystals for which the application of electron spectroscopy techniques has been limited.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c04048.

  • Synthesis of r-BS, micro-ARPES measurements, choice of substrates, microscopic measurements, first-principles band calculations, reproducibility, and photon-energy dependence of ARPES data (PDF)

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Author Information

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  • Corresponding Authors
    • Katsuaki Sugawara - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;  Advanced Institute for Materials Research (WPI-AIMR), , , , and , Tohoku University, Sendai 980-8577, Japan;  Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan;  Orcidhttps://orcid.org/0000-0003-2926-9436;  Email: [email protected]
    • Takafumi Sato - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;  Advanced Institute for Materials Research (WPI-AIMR),  Center for Science and Innovation in Spintronics  and  International Center for Synchrotron Radiation Innovation Smart (SRIS), , , , and , Tohoku University, Sendai 980-8577, Japan;  Orcidhttps://orcid.org/0000-0002-4544-5463;  Email: [email protected]
  • Authors
    • Haruki Kusaka - Department of Materials Science and Tsukuba Research Center for Energy Materials Science (TREMS), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
    • Tappei Kawakami - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    • Koki Yanagizawa - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    • Asuka Honma - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    • Seigo Souma - Advanced Institute for Materials Research (WPI-AIMR)  and  Center for Science and Innovation in Spintronics, , , , and , Tohoku University, Sendai 980-8577, Japan
    • Kosuke Nakayama - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;  Orcidhttps://orcid.org/0000-0003-2462-2253
    • Masashi Miyakawa - Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan;  Orcidhttps://orcid.org/0000-0002-0838-8156
    • Takashi Taniguchi - International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan;  Orcidhttps://orcid.org/0000-0002-1467-3105
    • Miho Kitamura - Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
    • Koji Horiba - National Institutes for Quantum Science and Technology (QST), Sendai 980-8579, Japan
    • Hiroshi Kumigashira - Institute of Multidisciplinary Research for Advanced Materials (IMRAM), , , , and , Tohoku University, Sendai 980-8577, Japan;  Orcidhttps://orcid.org/0000-0003-4668-2695
    • Takashi Takahashi - Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
    • Shin-ichi Orimo - Advanced Institute for Materials Research (WPI-AIMR)  and  Institute for Material Research, , , , and , Tohoku University, Sendai 980-8577, Japan
    • Masayuki Toyoda - Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan;  Orcidhttps://orcid.org/0000-0002-5669-262X
    • Susumu Saito - Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan;  Advanced Research Center for Quantum Physics and Nanoscience, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan;  Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan
    • Takahiro Kondo - Advanced Institute for Materials Research (WPI-AIMR), , , , and , Tohoku University, Sendai 980-8577, Japan;  Department of Materials Science and Tsukuba Research Center for Energy Materials Science (TREMS), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan;  Orcidhttps://orcid.org/0000-0001-8457-9387
  • Author Contributions

    This work was planned and proceeded by discussion among K.S., S.-i.O., T. Kondo, and T.S. H.K., M.M., T. Taniguchi, and T. Kondo carried out the fabrications of samples and their characterization. K.S., A.H., T. Kawakami, K.Y., S. Souma, K.N., M.K., K.H., and H.K. performed micro-ARPES measurements. M.T. and S. Saito carried out the band-structure calculations. K.S., T. Takahashi, and T.S. finalized the manuscript with input from all of the authors.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by JST-CREST (no. JPMJCR18T1), JST-PRESTO (no. JPMJPR20A8), a grant-in-aid for scientific research (JSPS KAKENHI grant numbers JP18H01821, JP19H01823, JP19H02551, JP20H01847, JP21H01757, JP21H04435, JP22K18964, JP22J13724, JP21H00015:B01, and JP18H05513 (hydrogenomics)), KEK-PF (proposal nos. 2020G669, 2021S2-001, and 2022G007), the Foundation for Promotion of Material Science and Technology of Japan, and the World Premier International Research Center, Advanced Institute for Materials Research. T.K. acknowledges support from GP-Spin at Tohoku University and JSPS.

Abbreviations

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2D

two-dimensional

3D

three-dimensional

BZ

Brillouin zone

EF

Fermi level

EB

binding energy

DFT

density functional theory

UHV

ultrahigh vacuum

PES

photoemission spectroscopy

ARPES

angle-resolved photoemission spectroscopy

EDC

energy distribution curve

References

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This article references 29 other publications.

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    Crystalline boron monosulfide nanosheets with tunable bandgaps
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    Two-dimensional (2D) boron monosulfide (BS) nanosheets are predicted to have several stable phases and unique electronic structures, endowing them with interesting attributes, including superconducting, thermoelec., and hydrogen storage properties. In this paper, we report the exptl. realization of 2D BS nanosheets by the phys. exfoliation of rhombohedral boron monosulfide (r-BS). Moreover, we demonstrate the facile sepn. of a mixt. of 2D BS nanosheets and the r-BS powder in acetonitrile; the former were selectively sepd. as a dispersion in the supernatant, whereas the latter remained in the ppt. In addn., d. functional theory calcns. reveal a clear dependence of the bandgap energy (Eg) on the no. of layers of stacked BS nanosheets, where Eg for BS nanosheets is approx. 1.0 eV higher than that for r-BS. Atomic force microscopy, cathode luminescence, UV-visible absorption spectroscopy, and excitation emission matrix expts. revealed a consistent bandgap difference of approx. 1.0 eV between the BS nanosheets and r-BS. We also demonstrate the applications based on the properties that originated from the difference in the bandgap between r-BS and BS nanosheets using photoelectrochem. current switching. These results indicate that the nanosheet bandgap can be tuned to a desired value by controlling the no. of stacked 2D BS nanosheets. Therefore, BS nanosheets are promising non-metal 2D materials for applications requiring bandgap control, such as electronics and photocatalysis.
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    Angle-resolved photoemission spectroscopy using a micro-focused beam spot [micro-angle-resolved photoemission spectroscopy (ARPES)] is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on the synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axis motion, and are vibrationally sepd. from the ARPES measurement system. Spatial mapping of the Au photolithog. pattern on Si signifies the beam spot size of 10μm (horizontal) x 12μm (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro-beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly assocd. with tiny sample bending near the edge of a cleaved topol. insulator single crystal. (c) 2022 American Institute of Physics.
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  2. Satoshi Hagiwara, Fumiaki Kuroda, Takahiro Kondo, Minoru Otani. Electrocatalytic Mechanisms for an Oxygen Evolution Reaction at a Rhombohedral Boron Monosulfide Electrode/Alkaline Medium Interface. ACS Applied Materials & Interfaces 2023, 15 (43) , 50174-50184. https://doi.org/10.1021/acsami.3c10548
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  • Abstract

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    Figure 1

    Figure 1. (a), (b) Schematics of the crystal structure and the first BZ of r-BS, respectively. (c) Photograph of r-BS powders. (d) Schematics of (1) sample mounting on an Au substrate, (2), (3) cleaving by Kapton tape, and (4) micro-ARPES measurements.

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    Figure 2

    Figure 2. (a) Optical microscope image of r-BS powder crystals on a Au substrate after exfoliation with Kapton tape. (b) Representative PES spectra around the B 1s core level at three representative sample positions where the B 1s core-level peak is clearly recognized (blue curve), the strong signal from the Au substrate dominates (green curve), and the strong charging effect is seen (on white-colored r-BS clusters) (red curve). (c) Spatial mapping of photoemission intensity integrated over the EB range of 185.5–192.5 eV, measured with hv = 250 eV in the same spatial region as the microscopy image of (a). (d) Same as (c) but for a smaller spatial region enclosed by a green dashed box (0.8 × 0.8 mm2) in (c) measured with a finer step of 20 μm. (e) Optical microscope image corresponding to (d). (f) ARPES intensity in the valence-band region as a function of EB and the wave vector measured with hv = 100 eV at point 1 in (d). (g) Same as (d) but with a narrower EB window of 188.0 ≤ EB ≤ 189.5 eV which covers only the B 1s peak as indicated by vertical dashed lines in (b). Points on the sample where ARPES measurements were carried out are indicated as squares in (d) and (g) (points 1 and 2). (h) Enlarged laser microscope image around point 2. (i) Same as (f) but measured at point 2. (j) PES spectrum in a wider EB range (EB = 0–200 eV) measured with hv = 250 eV. The inset shows the magnified view in the B 1s and S 2p core-level region.

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    Figure 3

    Figure 3. (a), (b) EDCs and corresponding ARPES intensity plot measured along the ΓKM cut (kz = 0 plane) with hv = 105 eV. Gray arrows show a weak spectral signature from the topmost valence band. (c), (d) Same as (a) and (b), but measured along the AHL cut (kz = π plane) with hv = 125 eV. There exist notable differences in the ARPES intensity plot between the kz = 0 and π planes, such as the energy position of the hole-band top at the Γ/A point and the absence/appearance of a U-shaped band that bottoms out at 6 eV at the Γ/A point.

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    Figure 4

    Figure 4. (a), (b) Comparison of ARPES-intensity plot (same as Figure 2e, but in gray scale) with calculated band structure (red curves) along the ΓKM cut. (c), (d) Same as (a) and (b) but along the AHL cut. Bands are labeled A–G. (e) ARPES-intensity plot as a function of in-plane wave vectors, kx and ky, for representative EB slices measured with hv = 125 eV corresponding to the AHL plane, overlaid with the calculated equi-energy contours (red curves). (f), (g) ARPES intensity plots along the AL and AH cuts, respectively, together with the experimental band dispersion (purple circles) and the result of numerical fittings with a parabolic function (blue curve).

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  • References

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      The demand for compact UV laser devices is increasing, as they are essential in applications such as optical storage, photocatalysis, sterilization, ophthalmic surgery and nanosurgery. Many researchers are devoting considerable effort to finding materials with larger bandgaps than that of GaN. Here we show that hexagonal boron nitride (hBN) is a promising material for such laser devices because it has a direct bandgap in the UV region. We obtained a pure hBN single crystal under high-pressure and high-temp. conditions, which shows a dominant luminescence peak and a series of s-like exciton absorption bands around 215 nm, proving it to be a direct-bandgap material. Evidence for room-temp. UV lasing at 215 nm by accelerated electron excitation is provided by the enhancement and narrowing of the longitudinal mode, threshold behavior of the excitation current dependence of the emission intensity, and a far-field pattern of the transverse mode.
    3. 3
      Dean, C.; Young, A.; Meric, I.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol. 2010, 5, 722– 726,  DOI: 10.1038/nnano.2010.172
      3
      Boron nitride substrates for high-quality graphene electronics
      Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J.
      Nature Nanotechnology (2010), 5 (10), 722-726CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Graphene devices on std. SiO2 substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielecs. that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal BN (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice const. similar to that of graphite, and has large optical phonon modes and a large elec. bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by a mech. transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO2. These devices also show reduced roughness, intrinsic doping and chem. reactivity. The ability to assemble cryst. layered materials in a controlled way permits the fabrication of graphene devices on other promising dielecs. and allows for the realization of more complex graphene heterostructures.
    4. 4
      Shimazaki, Y.; Yamamoto, M.; Borzenets, I.; Watanave, K.; Taniguchi, T.; Tarucha, S. Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene. Nat. Phys. 2015, 11, 1032– 1036,  DOI: 10.1038/nphys3551
      4
      Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene
      Shimazaki, Y.; Yamamoto, M.; Borzenets, I. V.; Watanabe, K.; Taniguchi, T.; Tarucha, S.
      Nature Physics (2015), 11 (12), 1032-1036CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
      The field of 'valleytronics' has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents. Valley is a quantum no. defined in an electronic system whose energy bands contain energetically degenerate but non-equiv. local min. (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical, magnetic and elec. control of the valley degree of freedom. Here we use dual-gated bilayer graphene to elec. induce and control broken inversion symmetry (or Berry curvature) as well as the carrier d. for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.
    5. 5
      Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional superconductivity in magic-angle graphene superlattices. Nature 2018, 556, 43– 50,  DOI: 10.1038/nature26160
      5
      Unconventional superconductivity in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
    6. 6
      Sharpe, A. L.; Fox, E. J.; Barnard, W. W.; Finney, J.; Watanabe, K.; Taniguchi, T.; Kastner, M. A.; Goldhaber-Gordon, D. Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene. Science 2019, 365, 605– 608,  DOI: 10.1126/science.aaw3780
      6
      Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
      Sharpe, Aaron L.; Fox, Eli J.; Barnard, Arthur W.; Finney, Joe; Watanabe, Kenji; Taniguchi, Takashi; Kastner, M. A.; Goldhaber-Gordon, David
      Science (Washington, DC, United States) (2019), 365 (6453), 605-608CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interaction. Here, we present evidence that near three-quarters (3/4) filling of the conduction miniband, these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. In a narrow d. range around an apparent insulating state at 3/4, we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.4 kilohms and indications of chiral edge states. Notably, the magnetization of the sample can be reversed by applying a small d.c. Although the AH resistance is not quantized, and dissipation is present, our measurements suggest that the system may be an incipient Chern insulator.
    7. 7
      Wang, L.; Shih, E. M.; Ghiotto, A.; Xian, L.; Rhodes, D. A.; Tan, C.; Claassen, M.; Kennes, D. M.; Bai, Y.; Kim, B.; Watanabe, K.; Taniguchi, T.; Zhu, X.; Hone, J.; Rubio, A.; Pasupathy, A. N.; Dean, C. R. Correlated electronic phases in twisted bilayer transition metal dichalcogenides. Nat. Mater. 2020, 19, 861– 866,  DOI: 10.1038/s41563-020-0708-6
      7
      Correlated electronic phases in twisted bilayer transition metal dichalcogenides
      Wang, Lei; Shih, En-Min; Ghiotto, Augusto; Xian, Lede; Rhodes, Daniel A.; Tan, Cheng; Claassen, Martin; Kennes, Dante M.; Bai, Yusong; Kim, Bumho; Watanabe, Kenji; Taniguchi, Takashi; Zhu, Xiaoyang; Hone, James; Rubio, Angel; Pasupathy, Abhay N.; Dean, Cory R.
      Nature Materials (2020), 19 (8), 861-866CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Abstr.: In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. Such flat bands in twisted graphene-based systems result in correlated insulator, superconducting and topol. states. Here we report evidence of low-energy flat bands in twisted bilayer WSe2, with signatures of collective phases obsd. over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. At a 5.1° twist, zero-resistance pockets were obsd. on doping away from half filling at temps. below 3 K, which indicates a possible transition to a superconducting state. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.
    8. 8
      Jin, C.; Regan, E. C.; Yan, A.; Iqbal Bakti Utama, M.; Wang, D.; Zhao, S.; Qin, Y.; Yang, S.; Zheng, Z.; Shi, Sh.; Watanabe, K.; Taniguchi, T.; Tongay, S.; Zettl, A.; Wang, F. Observation of moiré excitons in WSe2/WS2heterostructure superlattices. Nature 2019, 567, 76– 80,  DOI: 10.1038/s41586-019-0976-y
      8
      Observation of Moire´ excitons in WSe2/WS2 heterostructure superlattices
      Jin, Chenhao; Regan, Emma C.; Yan, Aiming; Iqbal Bakti Utama, M.; Wang, Danqing; Zhao, Sihan; Qin, Ying; Yang, Sijie; Zheng, Zhiren; Shi, Shenyang; Watanabe, Kenji; Taniguchi, Takashi; Tongay, Sefaattin; Zettl, Alex; Wang, Feng
      Nature (London, United Kingdom) (2019), 567 (7746), 76-80CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Moire´ superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qual. change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moire´ superlattices, whereas correlated insulating states and supercond. have been reported in twisted bilayer graphene moire´ superlattices1-12. In addn. to their pronounced effects on single-particle states, moire´ superlattices have recently been predicted to host excited states such as moire´ exciton bands13-15. Here we report the observation of moire´ superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moire´ exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theor. model in which the periodic moire´ potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moire´ exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topol. excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.
    9. 9
      Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu, J. Superconductivity at 39 K in magnesium diboride. Nature 2001, 410, 63– 64,  DOI: 10.1038/35065039
      9
      Superconductivity at 39 K in magnesium diboride
      Nagamatsu, Jun; Nakagawa, Norimasa; Muranaka, Takahiro; Zenitani, Yuji; Akimitsu, Jun
      Nature (London, United Kingdom) (2001), 410 (6824), 63-64CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      In the light of the tremendous progress that has been made in raising the transition temp. of the copper oxide superconductors it is natural to wonder how high the transition temp., Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk supercond. are 33 K in electron-doped CsxRbyC60 and 30 K in Ba1-xKxBiO3. (Hole-doped C60 was recently found to be superconducting with a Tc as high as 52 K, although the nature of the expt. meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk supercond. in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temp. of 39 K, which we believe to be the highest yet detd. for a non-copper-oxide bulk superconductor.
    10. 10
      Kaneti, Y. V.; Benu, D. P.; Xu, X.; Yuliarto, B.; Yamauchi, Y.; Golberg, D. Borophene: Two-dimensional Boron Monolayer: Synthesis, Properties, and Potential Applications. Chem. Rev. 2022, 122, 1000– 1051,  DOI: 10.1021/acs.chemrev.1c00233
      10
      Borophene: Two-dimensional Boron Monolayer: Synthesis, Properties, and Potential Applications
      Kaneti, Yusuf Valentino; Benu, Didi Prasetyo; Xu, Xingtao; Yuliarto, Brian; Yamauchi, Yusuke; Golberg, Dmitri
      Chemical Reviews (Washington, DC, United States) (2022), 122 (1), 1000-1051CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Borophene, a monolayer of boron, has risen as a new exciting two-dimensional (2D) material having extraordinary properties, including anisotropic metallic behavior and flexible (orientation-dependent) mech. and optical properties. This summarizes the current progress in the synthesis of borophene on various metal substrates, including Ag(110), Ag(100), Au(111), Ir(111), Al(111), and Cu(111), as well as heterostructuring of borophene. In addn., it discusses the mech., thermal, magnetic, electronic, optical, and superconducting properties of borophene and the effects of elemental doping, defects, and applied mech. strains on these properties. Furthermore, the promising potential applications of borophene for gas sensing, energy storage and conversion, gas capture and storage applications, and possible tuning of the material performance in these applications through doping, formation of defects, and heterostructures are illustrated based on available theor. studies. Finally, research and application challenges and the outlook of the whole borophene's field are given.
    11. 11
      Feng, B.; Sugino, O.; Liu, R.-Y.; Zhang, J.; Yukawa, R.; Kawamura, M.; Iimori, T.; Kim, H.; Hasegawa, Y.; Li, H.; Chen, L.; Wu, K.; Kumigashira, H.; Komori, F.; Chiang, T.-C.; Meng, S.; Matsuda, I. Dirac Fermions in Borophene. Phys. Rev. Lett. 2017, 118, 096401,  DOI: 10.1103/PhysRevLett.118.096401
      11
      Dirac fermions in borophene
      Feng, Baojie; Sugino, Osamu; Liu, Ro-Ya; Zhang, Jin; Yukawa, Ryu; Kawamura, Mitsuaki; Iimori, Takushi; Kim, Howon; Hasegawa, Yukio; Li, Hui; Chen, Lan; Wu, Kehui; Kumigashira, Hiroshi; Komori, Fumio; Chiang, Tai-Chang; Meng, Sheng; Matsuda, Iwao
      Physical Review Letters (2017), 118 (9), 096401/1-096401/6CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials esp. in monolayer structures. We present a detailed investigation of the β12 sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding anal. revealed that the lattice of the β12 sheet could be decompd. into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calcns. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.
    12. 12
      Penev, E. S.; Kutana, A.; Yakobson, B. I. Can Two-Dimensional Boron Superconduct?. Nano Lett. 2016, 16, 2522– 2526,  DOI: 10.1021/acs.nanolett.6b00070
      12
      Can Two-Dimensional Boron Superconduct?
      Penev, Evgeni S.; Kutana, Alex; Yakobson, Boris I.
      Nano Letters (2016), 16 (4), 2522-2526CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Addnl., its small at. mass suggests strong electron-phonon coupling, which in turn can enable superconducting behavior. Here the authors perform 1st-principles anal. of electronic structure, phonon spectra, and electron-phonon coupling of selected 2-dimensional boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated supercond., with estd. crit. temp. in the range of Tc ≈ 10-20 K.
    13. 13
      Nishino, H.; Fujita, T.; Cuong, N. T.; Tominaka, S.; Miyauchi, M.; Iimura, S.; Hirata, A.; Umezawa, N.; Okada, S.; Nishibori, E.; Fujino, A.; Fujimori, T.; Ito, S.; Nakamura, J.; Hosono, H.; Kondo, T. Formation and Characterization of Hydrogen Boride Sheets Derived from MgB2 by Cation Exchange. J. Am. Chem. Soc. 2017, 139, 13761– 13769,  DOI: 10.1021/jacs.7b06153
      13
      Formation and characterization of hydrogen boride sheets derived from MgB2 by cation exchange
      Nishino, Hiroaki; Fujita, Takeshi; Cuong, Nguyen Thanh; Tominaka, Satoshi; Miyauchi, Masahiro; Iimura, Soshi; Hirata, Akihiko; Umezawa, Naoto; Okada, Susumu; Nishibori, Eiji; Fujino, Asahi; Fujimori, Tomohiro; Ito, Shin-ichi; Nakamura, Junji; Hosono, Hideo; Kondo, Takahiro
      Journal of the American Chemical Society (2017), 139 (39), 13761-13769CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mech. properties as well as hydrogen storage capacity. Here, we report the exptl. realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an av. yield of 42.3% at room temp. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray at. pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, IR absorption, electron energy loss spectroscopy, and d. functional theory calcns. The established cation-exchange method for metal diboride opens new avenues for the mass prodn. of several types of boron-based 2D materials by countercation selection and functionalization.
    14. 14
      Sasaki, T.; Takizawa, H.; Uheda, K.; Endo, T. High Pressure Synthesis of Binary B-S Compounds. Phys. Status Solidi B 2001, 223, 29– 33,  DOI: 10.1002/1521-3951(200101)223:1<29::AID-PSSB29>3.0.CO;2-O
      14
      High pressure synthesis of binary B-S compounds
      Sasaki, T.; Takizawa, H.; Uheda, K.; Endo, T.
      Physica Status Solidi B: Basic Research (2001), 223 (1), 29-33CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag Berlin GmbH)
      R-BS was synthesized under high pressure/temp. conditions. Rietveld anal. of the powder x-ray diffraction data was performed by applying the structure model of GaS 3R (space group: R3m) with the lattice parameters of ahex = 3.05223(7) Å and chex = 20.4119(5) Å. The crystal structure of r-BS was revealed to be the three-layer structure comprised of the units built up from B-B pairs which are inside the anti-prism of S atoms. The measurement of UV-visible diffused reflectance spectrum showed that the estd. band gap of r-BS was ∼3.4 eV, and the coloring of r-BS was obsd. as broad absorption at 510 nm.
    15. 15
      Cherednichenko, K. A.; Sokolov, P. S.; Kalinko, A.; Le Godec, Y.; Polian, A.; Itíe, J. P.; Solozhenko, V. L. Optical phonon modes in rhombohedral boron monosulfide under high pressure. J. Appl. Phys. 2015, 117, 185904,  DOI: 10.1063/1.4921099
      15
      Optical phonon modes in rhombohedral boron monosulfide under high pressure
      Cherednichenko, Kirill A.; Sokolov, Petr S.; Kalinko, Aleksandr; Le Godec, Yann; Polian, Alain; Itie, Jean-Paul; Solozhenko, Vladimir L.
      Journal of Applied Physics (Melville, NY, United States) (2015), 117 (18), 185904/1-185904/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Raman spectra of rhombohedral BS (r-BS) were measured under pressures ≤34 GPa at room temp. No pressure-induced structural phase transition was obsd., while strong pressure shift of Raman bands towards higher wave nos. was revealed. IR spectroscopy as a complementary technique was used to completely describe the phonon modes of r-BS. All exptl. obsd. bands were compared with theor. calcd. ones and modes assignment was performed. r-BS enriched by 10B isotope was synthesized, and the effect of B isotopic substitution on Raman spectra was obsd. and analyzed. (c) 2015 American Institute of Physics.
    16. 16
      Mishra, P.; Singh, D.; Sonvane, Y.; Ahuja, R. Metal-functionalized 2D boron sulfide monolayer material enhancing hydrogen storage capacities. J. Appl. Phys. 2020, 127, 184305,  DOI: 10.1063/5.0008980
      16
      Metal-functionalized 2D boron sulfide monolayer material enhancing hydrogen storage capacities
      Mishra, Pushkar; Singh, Deobrat; Sonvane, Yogesh; Ahuja, Rajeev
      Journal of Applied Physics (Melville, NY, United States) (2020), 127 (18), 184305CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      In the present work, we have systematically investigated the structural, electronic, vibrational, and H2 storage properties of a layered 2H boron sulfide (2H-BS) monolayer using spin-polarized d. functional theory (DFT). The pristine BS monolayer shows semiconducting behavior with an indirect bandgap of 2.83 eV. Spin-polarized DFT with van der Waals correction suggests that the pristine BS monolayer has weak binding strength with H2 mols., but the binding energy can be significantly improved by alkali metal functionalization. A system energy study indicates the strong bonding of alkali metals with the BS monolayer. The Bader charge anal. also concludes that a considerable charge is transferred from the metal to the BS monolayer surface, which was further confirmed by the iso-surface charge d. profile. All functionalized alkali metals form cations that can bond multiple H2 mols. with sufficient binding energies, which are excellent for H2 storage applications. An ideal range of adsorption energy and practicable desorption temp. promises the ability of the alkali metal functionalized BS monolayer as an efficient material for hydrogen storage. (c) 2020 American Institute of Physics.
    17. 17
      Mishra, P.; Singh, D.; Sonvane, Y.; Ahuja, R. Two-dimensional boron monochalcogenide monolayer for thermoelectric material. Sustain. Energy & Fuels 2020, 4, 2363– 2369,  DOI: 10.1039/D0SE00004C
      There is no corresponding record for this reference.
    18. 18
      Mishra, P.; Singh, D.; Sonvane, Y.; Ahuja, R. Bifunctional catalytic activity of 2D boron monochalcogenides BX (X = S, Se, Te). Mater. Today Ene. 2022, 27, 101026,  DOI: 10.1016/j.mtener.2022.101026
      There is no corresponding record for this reference.
    19. 19
      Fan, D.; Lu, S.; Chen, C.; Jiang, M.; Li, X.; Hu, X. Versatile two-dimensional boron monosulfide polymorphs with tunable bandgaps and superconducting properties. Appl. Phys. Lett. 2020, 117, 013103,  DOI: 10.1063/5.0006059
      19
      Versatile two-dimensional boron monosulfide polymorphs with tunable bandgaps and superconducting properties
      Fan, Dong; Lu, Shaohua; Chen, Chengke; Jiang, Meiyan; Li, Xiao; Hu, Xiaojun
      Applied Physics Letters (2020), 117 (1), 013103CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The typical two-dimensional semiconductors, group IIIA chalcogenides, have garnered tremendous interest for their outstanding electronic, mech., and chem. properties. However, so far, there have been rare reports on boron monosulfides (BS) binary material. Here, four two-dimensional BS sheets, namely, the α-, β-, γ-, and δ-BS sheets, are proposed and discussed from first principles calcns. State-of-the-art calcns. reveal all these structures are thermally and dynamically stable, indicating the potential for exptl. synthesis. Specifically, for α-BS, it has a calcd. exfoliation energy of 0.96 J m-2, suggesting that the prepn. of α-BS is feasible by the exfoliation of bulk rhombohedral-BS. Our results show that α-, β-, and γ-BS are semiconductors, whereas δ-BS is a metallic system. Remarkably, our calcns. indicate that δ-BS is a superconductor with a large electron-phonon coupling (λ ≈ 1.51), leading to a high superconducting crit. temp. (Tc ≈ 21.56 K), which is the interesting property with intrinsic superconducting among all two-dimensional group IIIA chalcogenides. The potential of semiconducting BS monolayers as the gas-sensor or thermoelec. materials is also demonstrated. (c) 2020 American Institute of Physics.
    20. 20
      Kusaka, H. Crystalline boron monosulfide nanosheets with tunable bandgaps. J. Mater. Chem. A 2021, 9, 24631– 24640,  DOI: 10.1039/D1TA03307G
      20
      Crystalline boron monosulfide nanosheets with tunable bandgaps
      Kusaka, Haruki; Ishibiki, Ryota; Toyoda, Masayuki; Fujita, Takeshi; Tokunaga, Tomoharu; Yamamoto, Akiyasu; Miyakawa, Masashi; Matsushita, Kyosuke; Miyazaki, Keisuke; Li, Linghui; Shinde, Satish Laxman; S. L. Lima, Mariana; Sakurai, Takeaki; Nishibori, Eiji; Masuda, Takuya; Horiba, Koji; Watanabe, Kenji; Saito, Susumu; Miyauchi, Masahiro; Taniguchi, Takashi; Hosono, Hideo; Kondo, Takahiro
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (43), 24631-24640CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Two-dimensional (2D) boron monosulfide (BS) nanosheets are predicted to have several stable phases and unique electronic structures, endowing them with interesting attributes, including superconducting, thermoelec., and hydrogen storage properties. In this paper, we report the exptl. realization of 2D BS nanosheets by the phys. exfoliation of rhombohedral boron monosulfide (r-BS). Moreover, we demonstrate the facile sepn. of a mixt. of 2D BS nanosheets and the r-BS powder in acetonitrile; the former were selectively sepd. as a dispersion in the supernatant, whereas the latter remained in the ppt. In addn., d. functional theory calcns. reveal a clear dependence of the bandgap energy (Eg) on the no. of layers of stacked BS nanosheets, where Eg for BS nanosheets is approx. 1.0 eV higher than that for r-BS. Atomic force microscopy, cathode luminescence, UV-visible absorption spectroscopy, and excitation emission matrix expts. revealed a consistent bandgap difference of approx. 1.0 eV between the BS nanosheets and r-BS. We also demonstrate the applications based on the properties that originated from the difference in the bandgap between r-BS and BS nanosheets using photoelectrochem. current switching. These results indicate that the nanosheet bandgap can be tuned to a desired value by controlling the no. of stacked 2D BS nanosheets. Therefore, BS nanosheets are promising non-metal 2D materials for applications requiring bandgap control, such as electronics and photocatalysis.
    21. 21
      Zhang, Y.; Zhou, M.; Yang, M.; Yu, J.; Li, W.; Li, X.; Feng, S. Experimental Realization and Computational Investigations of B2S2 as a New 2D Material with Potential Applications. ACS Appl. Mater. Int. 2022, 14, 32330– 32340,  DOI: 10.1021/acsami.2c03762
      There is no corresponding record for this reference.
    22. 22
      Kitamura, M.; Souma, S.; Honma, A.; Wakabayashi, D.; Tanaka, H.; Toyoshima, A.; Amemiya, K.; Kawakami, T.; Sugawara, K.; Nakayama, K.; Yoshimatsu, K.; Kumigashira, H.; Sato, T.; Horiba, K. Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics. Rev. Sci. Instrum. 2022, 93, 033906,  DOI: 10.1063/5.0074393
      22
      Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics
      Kitamura, Miho; Souma, Seigo; Honma, Asuka; Wakabayashi, Daisuke; Tanaka, Hirokazu; Toyoshima, Akio; Amemiya, Kenta; Kawakami, Tappei; Sugawara, Katsuaki; Nakayama, Kosuke; Yoshimatsu, Kohei; Kumigashira, Hiroshi; Sato, Takafumi; Horiba, Koji
      Review of Scientific Instruments (2022), 93 (3), 033906CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      Angle-resolved photoemission spectroscopy using a micro-focused beam spot [micro-angle-resolved photoemission spectroscopy (ARPES)] is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on the synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axis motion, and are vibrationally sepd. from the ARPES measurement system. Spatial mapping of the Au photolithog. pattern on Si signifies the beam spot size of 10μm (horizontal) x 12μm (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro-beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly assocd. with tiny sample bending near the edge of a cleaved topol. insulator single crystal. (c) 2022 American Institute of Physics.
    23. 23
      Takahiro, K.; Oizumi, S.; Terai, A.; Kawatsura, K.; Tsuchiya, B.; Nagata, S.; Yamamoto, S.; Naramoto, H.; Narumi, K.; Sasase, M. Core level and valence band photoemission spectra of Au clusters embedded in carbon. J. Appl. Phys. 2006, 100, 084325,  DOI: 10.1063/1.2359688
      23
      Core level and valence band photoemission spectra of Au clusters embedded in carbon
      Takahiro, K.; Oizumi, S.; Terai, A.; Kawatsura, K.; Tsuchiya, B.; Nagata, S.; Yamamoto, S.; Naramoto, H.; Narumi, K.; Sasase, M.
      Journal of Applied Physics (2006), 100 (8), 084325/1-084325/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      XPS was applied for size estn. of Au clusters formed by ion implantation into glassy C. The 4f and 5d XPS spectra reveal the cluster 0.7-2.5 nm in diam., depending on the Au concn. The relation between XPS 4f-binding energy shift and 5d splitting is detd. for the Au clusters embedded in the C and is significantly different from the previous data for the ones supported on a C substrate. The authors suppose that this difference results from the effect of the environment around a cluster on Coulomb charging during photoemission at the final state.
    24. 24
      Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross-Sections and Asymmetry Parameters. At. Data Nucl. Data Tables. 1985, 32, 1– 155,  DOI: 10.1016/0092-640X(85)90016-6
      24
      Atomic subshell photoionization cross sections and asymmetry parameters: 1 ≤ Z ≤ 103
      Yeh, J. J.; Lindau, I.
      Atomic Data and Nuclear Data Tables (1985), 32 (1), 1-155CODEN: ADNDAT; ISSN:0092-640X.
      At. subshell photoionization cross sections and asymmetry parameters are calcd. with the Hartree-Fock-Slater one-electron central potential model (dipole approxn.) for all elements Z = 1-103. The cross-section results are plotted for all subshells in the energy region 0-1500 eV, and cross sections and asymmetry parameters are tabulated for selected energies in the region 10.2-8047.8 eV. In addn., more detailed graphs are given for the 4d (Z = 39-71) and 5d (Z = 64-100) subshell cross sections in the vicinity of the Cooper min. These data should be particularly useful for work based on spectroscopic investigations of at. subshells using synchrotron radiation and/or discrete line sources.
    25. 25
      Henck, H.; Pierucci, D.; Fugallo, G.; Avila, J.; Cassabois, G.; Dappe, Y. J.; Silly, M. G.; Chen, C.; Gil, B.; Gatti, M.; Sottile, F.; Sirotti, F.; Asensio, M. C.; Ouerghi, A. Direct observation of the band structure in bulk hexagonal boron nitride. Phys. Rev. B 2017, 95, 085410,  DOI: 10.1103/PhysRevB.95.085410
      25
      Direct observation of the band structure in bulk hexagonal boron nitride
      Henck, Hugo; Pierucci, Debora; Fugallo, Giorgia; Avila, Jose; Cassabois, Guillaume; Dappe, Yannick J.; Silly, Mathieu G.; Chen, Chaoyu; Gil, Bernard; Gatti, Matteo; Sottile, Francesco; Sirotti, Fausto; Asensio, Maria C.; Ouerghi, Abdelkarim
      Physical Review B (2017), 95 (8), 085410/1-085410/6CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      A promising route towards nanodevice applications relies on the assocn. of graphene and transition metal dichalcogenides with hexagonal boron nitride (h-BN). Due to its insulating nature, h-BN has emerged as a natural substrate and gate dielec. for graphene-based electronic devices. However, some fundamental properties of bulk h-BN remain obscure. For example, the band structure and the position of the Fermi level have not been exptl. resolved. Here, we report a direct observation of parabolic dispersions of h-BN crystals using high-resoln. angle-resolved photoemission spectroscopy (ARPES).We find that h-BN exfoliation on epitaxial graphene enables overcoming the tech. difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at const. energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual p-type character of typical h-BN.
    26. 26
      Lee, P. A.; Nagaosa, N.; Wen, X.-G. Doping a Mott insulator: physics of high-temperature superconductivity. Rev. Mod. Phys. 2006, 78, 17– 85,  DOI: 10.1103/RevModPhys.78.17
      26
      Doping a Mott insulator: physics of high-temperature superconductivity
      Lee, Patrick A.; Nagaosa, Naoto; Wen, Xiao-Gang
      Reviews of Modern Physics (2006), 78 (1), 17-85CODEN: RMPHAT; ISSN:0034-6861. (American Physical Society)
      A review, with numerous refs. This article reviews the physics of high-temp. superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of expts. are discussed, focusing on the region of the phase diagram close to the Mott insulator (the under-doped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. But the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qual. account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temp., which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenol. over a limited temp. range, and some addnl. physics is needed to explain the onset of singlet formation at very high temps. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a no. of predictions which are in agreement with expts. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint against double occupation and the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by 1st examg. the U(1) formulation of the gauge theory. Some inadequacies of this formulation for under-doping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liq. phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem vs. the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge sepn. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liq. phase. Inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that d-wave supercond. can be considered as evolving from a stable U(1) spin liq. These ideas are applied to the high-Tc cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topol. structure of the pseudogap phase is described.
    27. 27
      Mielke, A. Ferromagnetism in Single-Band Hubbard Models with a Partially Flat Band. Phys. Rev. Lett. 1999, 82, 4312,  DOI: 10.1103/PhysRevLett.82.4312
      27
      Ferromagnetism in Single-Band Hubbard Models with a Partially Flat Band
      Mielke, Andreas
      Physical Review Letters (1999), 82 (21), 4312-4315CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      A Hubbard model with a single, partially flat band has ferromagnetic ground states. It is shown that local stability of ferromagnetism implies its global stability in such a model: The model has only ferromagnetic ground states if there are no single spin-flip ground states. Since a single-band Hubbard model away from half filling describes a metal, this result may open a route to metallic ferromagnetism in single-band Hubbard models.
    28. 28
      Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H. Black phosphorus field-effect transistors. Nat. Nanotechnol. 2014, 9, 372– 377,  DOI: 10.1038/nnano.2014.35
      28
      Black phosphorus field-effect transistors
      Li, Likai; Yu, Yijun; Ye, Guo Jun; Ge, Qingqin; Ou, Xuedong; Wu, Hua; Feng, Donglai; Chen, Xian Hui; Zhang, Yuanbo
      Nature Nanotechnology (2014), 9 (5), 372-377CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Exptl. identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, the authors fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometers. Reliable transistor performance is achieved at room temp. in samples thinner than 7.5 nm, with drain current modulation ∼105 and well-developed current satn. in the I-V characteristics. The charge-carrier mobility is thickness-dependent, with the highest values up to ∼1,000 cm2 V-1 s-1 obtained for a thickness of ∼10 nm. The authors' results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.
    29. 29
      Wang, Y.; Qiu, G.; Wang, R.; Huang, S.; Wang, Q.; Liu, Y.; Du, Y.; Goddard III, W. A.; Kim, M. J.; Xu, X.; Ye, P. D.; Wu, W. Field-effect transistors made from solution-grown two-dimensional tellurene. Nat. Electron. 2018, 1, 228– 236,  DOI: 10.1038/s41928-018-0058-4
      There is no corresponding record for this reference.

  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c04048.

    • Synthesis of r-BS, micro-ARPES measurements, choice of substrates, microscopic measurements, first-principles band calculations, reproducibility, and photon-energy dependence of ARPES data (PDF)


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