Keywords

4.1 Background

As discussed in Chap. 1, layered transition-metal dichalcogenides (TMDs) with the general formula MX2, where M represents a transition metal from groups 4–10 and X is a chalcogen (S, Se, or Te), received significant attention in the last dozen years due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Depending on the coordination environment and oxidization state of the transition metal, layered TMDs can be metals, semiconductors, and insulators and thus show various physical properties. Recent investigations of MX2 have resulted in discoveries of dramatically different electronic structures at the monolayer limit compared to the bulk materials due to quantum confinement effects. For example, while pushing from bulk to monolayer, MoS2 and MoSe2 show an indirect-to-direct bandgap transition [1,2,3,4]. With these exciting findings, experimental research efforts so far have been mainly focused on prototypical semiconducting MX2 with group VIB transition metals (M = Mo, W). Note, however, that about 40 different MX2 compounds can form stable, 2D single-layer TMDs structures [5, 6], as summarized in Fig. 4.1. In the large family of layered TMDs, many other promising single-layer TMDs and related quantum defined properties remain to be explored experimentally. For example, IrTe2 and 1T-TaS2 exhibit novel low-temperature phenomena including superconductivity and charge density wave [7, 8]; bulk ReS2 shows monolayer behavior due to electronic and vibrational decoupling [9]. These interesting properties motivate considerable interest in exploring other promising TMDs materials, such as group 10 TMDs, which have rarely been reported.

Fig. 4.1
figure 1

Reprinted with permission from Ref. [6], © 2013 Springer Nature

Summary of about 40 different layered MX2 compounds, which are highlighted. Partial highlights for Co, Rh, Ir and Ni indicate that only some of the dichalcogenides form layered structures.

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Reliable preparation of ultra-thin 2D TMDs is the essential step for exploring their properties and applications. Among various production methods, chemical vapor deposition (CVD) and CVD analogs are the most important approaches of bottom-up synthesis. As described in Sect. 1.3.4.2, MX2 growth using CVD-related methods usually involves two components containing M and X as precursors, which increases experimental steps and complexities.

4.2 Growth and Atomic Structure

In this chapter, we report epitaxial growth of monolayer PtSe2 - a heretofore-unexplored member of the single-layer TMDs family—on a Pt substrate by direct “selenization” [10], an analog of direct oxidation. In contrast to conventional fabrication methods of MX2 by exfoliation or chemical vapor deposition, the present route toward a monolayer dichalcogenide is very straightforward: only one element, Se, is deposited on a Pt(111) substrate, and then the sample is annealed to ~200 °C to obtain epitaxial PtSe2 films, as illustrated in Fig. 4.2a.

Fig. 4.2
figure 2

Reprinted with permission from Ref. [10], © 2015 ACS

a Schematic illustration of synthesizing PtSe2 monolayer by a single step of direct selenization of a Pt(111) substrate. The Pt spheres with different colors and sizes are used just to differentiate the Pt atoms in the Pt(111) substrate and in the PtSe2 sublayer. b XPS measurements for the binding energies of Se during PtSe2 growth demonstrating the formation of PtSe2 at 270 °C. The blue arrows indicate the peak positions (55.19 and 54.39 eV) corresponding to the binding energy of Se2−. The Se0 peaks (at 55.68 and 54.80 eV) are dominant at 25 °C, whereas at 200 °C the peaks in the curve indicate the coexistence of Se0 and Se2−.

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The growth of PtSe2 thin films was monitored by in situ X-ray photoelectron spectroscopy (XPS). Figure 4.2b shows the XPS spectra of the Se 3d core level during PtSe2 growth. When Se-deposited Pt(111) substrate is annealed to 200 °C, two new peaks appear at binding energies of 55.19 and 54.39 eV (labeled by the blue arrows), which can be explained by a change in the chemical state of Se from Se0 to Se2−, corresponding to the selenization process of the sample. Further annealing of the sample to 270 °C results in the disappearance of Se0 peaks (at 55.68 and 54.80 eV) and the dominance of Se2− peaks, indicating full crystallization and complete formation of PtSe2 films.

To obtain the structural information on the as-grown epitaxial films, we observed the samples by LEED. Figure 4.3a shows a LEED pattern. Hexagonal diffraction spots from PtSe2 (red circles) are observed to have the same orientation as those from the Pt(111) substrate (blue circles), suggesting a rotational-domain-free growth. A (3 × 3) diffraction pattern of the epitaxial PtSe2 film is clearly identified, which corresponds to a well-defined moiré superstructure arising from the lattice mismatch between the PtSe2 film and Pt(111) substrate. Furthermore, identical LEED patterns were observed on the entire sample surface (4 mm × 4 mm in size), indicating the growth of a large-area, homogeneous, and high-quality film.

Fig. 4.3
figure 3

Adapted with permission from Ref. [10], © 2015 ACS

a LEED pattern of PtSe2 films formed on the Pt substrate. The blue, red, white circles indicate the diffraction spots from the Pt(111) lattice, PtSe2 thin film, and (3 × 3) superstructure with respect to PtSe2, respectively. b Large-scale STM image (U = −1.9 V, I = 0.12 nA) shows the moiré pattern of PtSe2 thin film on Pt(111). The white rectangle marks the size of the close-up image in (c). c Atomic-resolution STM image (U = −1.0 V, I = 0.12 nA) of single-layer PtSe2 showing the hexagonal lattice of Se atoms in the topmost sublayer of the PtSe2 sandwich-type structure. A (3 × 3) moiré superstructure is visible. The red and white rhombi denote the unit cell of the PtSe2 lattice and (3 × 3) superlattice, respectively. The inset displays the FFT pattern corresponding to PtSe2 and the superstructure.

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To investigate the atomic structure of the PtSe2 film, we performed STM studies. Figure 4.3b is a large-scale STM image with a well-ordered moiré pattern of PtSe2 thin film on Pt(111). The periodicity of this moiré pattern is about 11.1 Å, four times the lattice constant of Pt(111). Figure 4.3c shows an atomic-resolution image of the area indicated by the white square in Fig. 4.3b, revealing hexagonally arranged protrusions with an average lattice constant of a1 = 3.7 Å, which agrees perfectly with the interatomic spacing of Se atoms in the (0001) basal plane of bulk PtSe2. Therefore, we interpret the hexagonal protrusions in Fig. 4.3c to be the Se atoms in the topmost Se plane of a PtSe2 film. A regular (3 × 3) moiré superstructure with respect to the PtSe2 lattice is then established, with a periodicity of b1 = 3a1 ≅ 11.1 Å (labeled by the white rhombus). The orientation of the moiré pattern is aligned with that of the PtSe2 lattice. This is in agreement with the LEED observation (Fig. 4.3a), where the diffraction spots of the (3 × 3) superlattice are in line with those of the PtSe2 lattice. Based on LEED and STM measurements, the moiré pattern can be explained as the (3 × 3) PtSe2 supercells located on the (4 × 4) Pt(111) atoms.

To gain further insight into interfacial features of the PtSe2/Pt(111) sample, we performed a cross-section high-angle annular-dark-field (HAADF) STEM study. A Z-contrast image of the PtSe2/Pt(111) interface is shown in Fig. 4.4a. One bright layer combined with two dark layers observed on the topmost surface suggests a Se–Pt–Se sandwich configuration (as indicated by a model diagram superimposed in Fig. 4.4a). The atomically resolved bulk Pt substrate lattice with an experimentally measured interlayer spacing of 2.28 Å served as a reference for calibrating other spacing measurements. After the calibration, the spacing between the Se sublayers in the Se–Pt–Se sandwich is found to be 2.53 Å, which is in agreement with the calculated value in single-layer PtSe2, as displayed in Fig. 4.4b. These atomic-scale cross-section data obtained by STEM further verify that the fabricated structure is indeed a single-layer PtSe2 film on the Pt(111) substrate.

Fig. 4.4
figure 4

Adapted with permission from Ref. [10], © 2015 ACS

a Atomic-resolution STEM Z-contrast image of the PtSe2/Pt(111) interface along the [110] zone axis. A PtSe2 single layer over the Pt(111) substrate is resolved at atomic scale with a model diagram overlaid for clarity. b The relaxed model. The blue and orange spheres represent Pt atoms and Se atoms, respectively. c Simulated STM image (U = −1.0 V) based on the calculated atomic structure in (b) is consistent with the experimental observation in Fig. 4.3c.

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The combination of LEED, STM, and STEM studies indicates a (3 × 3) single-layer PtSe2 on a (4 × 4) Pt(111) structure. We then carried out DFT calculations based on this structure. The simulated STM image is shown in Fig. 4.4c, in which the overall features of the experimental STM image (Fig. 4.3c) are well reproduced. Only the hexagonally arranged Se atoms in the topmost sublayer of monolayer PtSe2 are imaged. The remarkable agreement between the STM simulation and experimental STM observation strongly supports our conclusions and thus demonstrates the successful growth of a highly crystalline PtSe2 monolayer.

4.3 Electronic Structure

Having grown highly crystalline single-layer PtSe2, we investigated its electronic energy band structure by ARPES. Figure 4.5a shows ARPES data measured along the high symmetry direction K-Γ-M-K in the hexagonal Brillouin zone at a photon energy of 21.2 eV. Data taken at other photon energies show the same dispersion, confirming the 2D character of the monolayer PtSe2. Second-derivative spectra of raw experimental band structures (Fig. 4.5a) are depicted in Fig. 4.5b to enhance the visibility of the bands. Here, the top of the valence band is observed to be at −1.2 eV at the Γ point and the conduction band is above the Fermi level, indicating that monolayer PtSe2 is a semiconductor. It is quite different from the bulk PtSe2, which-according to calculations-is a semimetal. A direct comparison between the ARPES spectrum (Fig. 4.5b) and the calculated band structure (green dotted lines in Fig. 4.5b) shows excellent quantitative agreement. Combining the ARPES spectra with DFT calculations, we confirm that we have synthesized a single-layer PtSe2 and that the epitaxial PtSe2 essentially has the same electronic properties as the free-standing single-layer PtSe2. For the first time, the band structure of monolayer PtSe2 films has been determined experimentally.

Fig. 4.5
figure 5

Reproduced with permission from Ref. [10], © 2015 ACS

ARPES spectra and valence bands of single-layer PtSe2. a ARPES spectra obtained on the monolayer PtSe2 on Pt(111). The high symmetry directions are shown in the inset. b Second-derivative spectra of the raw ARPES data in (a). The calculated valence bands, superimposed as green dashed lines, are in excellent agreement with the experimental data. The bands marked by white arrows are from the Pt substrate. c, d Theoretically calculated band structure and density of states of monolayer PtSe2 and bulk PtSe2, respectively.

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The semimetal-to-semiconductor transition was revealed by DFT-LDA calculations. As shown in Fig. 4.5c, the band structure and density of state (DOS) suggest single-layer PtSe2 is a semiconductor with an energy gap of 1.20 eV (2.10 eV bandgap is predicted from GW calculation [11]). As a comparison, the band structure and DOS of bulk PtSe2 are plotted in Fig. 4.5d, which confirm that bulk PtSe2 is semimetallic [12, 13]. Actually, bilayer PtSe2 remains a semiconductor, but the energy gap decreases to 0.21 eV. Starting from a trilayer, PtSe2 becomes semimetallic, as shown in Fig. 4.6. Therefore, only single-layer PtSe2 is a semiconductor with a sizeable bandgap.

Fig. 4.6
figure 6

Reproduced with permission from Ref. [10], © 2015 ACS

Calculated band structures of free-standing bi- and tri-layer PtSe2.

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4.4 Photocatalytic Properties

The opening of a sizable bandgap within the range of visible light makes monolayer PtSe2 potentially suitable for optoelectronics and photocatalysis. We explored the photocatalytic properties of monolayer PtSe2 by the degradation of methylene blue (MB) aqueous solution, which serves as a typical indicator of photocatalytic reactivity [14, 15].

4.4.1 Experimental Setups

Figure 4.7a displays the schematic diagram of photocatalytic experiments. The photocatalytic activity of as-prepared monolayer PtSe2 films was tested by catalytic degradation of methylene blue serving as a standard model dye under visible-light irradiation at room temperature. The ultraviolet/visible-light source was a 150 W Xe lamp located at a distance of 15 cm above the solution. A set of appropriate cut-off filters was applied to determine illumination wavelength and ensured that the photocatalytic reaction took place just under visible light. The as-prepared monolayer PtSe2 films were exfoliated from the Pt(111) substrate by ultra-sonication in aqueous solution. The Pt(111) crystal was picked out from the solution to avoid its possible influence on the photocatalytic activity. Then MB molecules and ethanol (0.01 mL) were added to the solution containing peeled PtSe2 monolayers. Before the photocatalytic reaction, the solution was kept in darkness for one hour in order to achieve an adsorption-desorption equilibrium between the PtSe2 film and MB molecules. After that, the suspension was exposed to visible-light irradiation. At time intervals of 4 min, solution samples were collected and their absorbance was measured by a commercial ultraviolet-visible (UV-vis) spectrophotometer. Accordingly, the intensity changes of characteristic absorbance peaks of the MB molecules were recorded. The photocatalytic performance of PtSe2 monolayers was thus evaluated by the time-dependent degradation rate Ct/C0, as exhibited in Fig. 4.7b.

Fig. 4.7
figure 7

Reproduced with permission from Ref. [10], © 2015 ACS

Photocatalytic activity of a single-layer PtSe2 film. a Schematic diagram of the photocatalytic degradation of methylene blue (MB) molecules. Electrons and holes are excited by visible-light irradiation of epitaxial PtSe2 monolayer films. The MB degradation by photo-induced electrons demonstrates the photocatalytic activity of PtSe2 monolayer films. b Time trace of the normalized concentration (Ct/C0, where Ct and C0 are the MB concentrations at time t min and 0 min, respectively) of the absorbance at a wavelength of 667 nm, the main absorbance peak of MB. The inset shows the UV-vis absorption spectra of MB, recorded at time intervals of 4 min.

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4.4.2 Photocatalytic Characterizations

The MB molecules adsorbed on the PtSe2 films are degraded by electrons that are excited by visible light. The time-dependent degradation of MB with a single layer PtSe2 catalyst was monitored by checking the decrease in the intensities of characteristic absorbance peaks of the MB molecules. As we can see in Fig. 4.7b, the photodegradation portion of MB molecules reached 38% after visible-light irradiation for 24 min. This rate is about four times faster than the rate obtained using PtSe2 nanocrystals [16, 17], putting single-layer PtSe2 in the same class as nitrogen-doped TiO2 nanoparticles for photocatalysis [18, 19].

4.5 Valleytronics

With the existence of an energy gap in the single-layer PtSe2, optical excitations can occur between the Γ point and the valley point along the Γ-M direction. This is reminiscent of the recently discovered valley-selective circular dichroism in MoS2 by circularly polarized light [20,21,22]. To explore this possibility, we calculated the degree of circular polarization of free-standing single-layer PtSe2. The calculated circular polarization due to the direct interband transition between the top of the valence band (VB) to the bottom of the conduction band (CB) is shown in Fig. 4.8a. It shows significant circular dichroism polarization along the M-K direction and near the Γ point. In view of the indirect energy gap, this process can be assisted by lattice vibrations. It is noteworthy that the circular dichroism polarization not only exists in the transition between the top of the VB and the bottom of the CB, it also exists in transitions to higher energy levels. Circular dichroism polarization due to transitions from the vicinity of the VB to the vicinity of the CB can be clearly seen in Figs. 4.8b–d. Due to this significant circular polarization, in the presence of a nonvanishing in-plane electric field, the anomalous charge current driven by the Berry curvature would flow to the opposite edges, leading to a valley polarized current and the resulting quantum valley Hall effect [23, 24].

Fig. 4.8
figure 8

Reproduced with permission from Ref. [10], © 2015 ACS

Momentum dependence of circular polarization of single-layer PtSe2. a–d represent the calculated circular polarization due to the direct interband transition from the top of the valence band (VB1) to the bottom of the conduction band (CB1), from VB1 to the higher conduction band (CB2), from the lower valence band (VB2) to CB1, and from VB2 to CB2, respectively. Significant circular dichroism polarization exists along the M-K direction and near the Γ point.

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4.6 Summary and Outlook

We have successfully fabricated high-quality, single-crystalline, monolayer PtSe2 films, a new member of the TMDs family, through a single-step, direct selenization of a Pt(111) substrate at a relatively low temperature (∼270 °C).

  1. (1)

    Characterizations by LEED, STM, STEM, and DFT calculations elucidated both in-plane and vertical monolayer structures with atomic resolution.

  2. (2)

    The ARPES measurements and their agreement with calculations revealed the semiconducting electronic structure of the single-layer PtSe2.

  3. (3)

    Together with the photocatalytic performance observed experimentally, monolayer PtSe2 shows promise for potential applications in optoelectronics and photocatalysis.

  4. (4)

    The circular polarization of monolayer PtSe2 in momentum space indicates a promising potential for valleytronic devices.

  5. (5)

    Our studies are a significant step forward in expanding the family of single-layer semiconducting TMDs and exploring the application potentials of ultra-thin TMDs in photoelectronic and energy-harvesting devices.