Two-Dimensional Platinum Diselenide: Synthesis, Emerging Applications, and Future Challenges

Highlights A comprehensive review of the recent development of two-dimensional (2D) PtSe2 synthesis strategies has been extensively surveyed. The applications of 2D PtSe2 materials in areas, including opto/electric devices, photocatalysis, hydrogen evolution reaction, and sensors, have been reviewed. Current challenges in the development of 2D PtSe2 materials are identified, and outlooks toward unexplored research areas are suggested.

As an emerging 2D material, PtSe 2 possesses not only the merits of previously discussed 2D materials, but also many unique advantages. For examples, PtSe 2 exhibits a strong layer-dependent band structure. Bulk PtSe 2 exhibits semimetallic character, while monolayer and fewlayer PtSe 2 are semiconductors [25,[41][42][43]. Moreover, PtSe 2 exhibits anisotropic carrier mobility along different directions. The theoretically calculated carrier mobility of PtSe 2 is larger than 3250 cm 2 V −1 S −1 (x direction) and 16,300 cm 2 V −1 S −1 (y direction) at room temperature, respectively [28]. The theoretically predicted carrier mobility is at least 8 times larger than that of MoS 2 (about 410 cm 2 V −1 S −1 for x direction and 430 cm 2 V −1 S −1 for y direction) [28]. The outstanding inherent properties (including tunable band gap and carrier mobility) of PtSe 2 are comparable to black phosphorus (BP), but the stability of PtSe 2 is much better than BP [44][45][46]. Besides, experimental and theoretical studies have proven the intriguing transport properties and interesting spin physics of PtSe 2 . Overall, these outstanding properties motivating further studies of the electrical transport properties, optoelectronic properties, and piezo-resistivity of 2D PtSe 2 .
Herein, we divulge a comprehensive review based on experimental and theoretical research evolution on 2D layered PtSe 2 , covering the progress, challenges, and prospects in future 2D material. The crystal structure, electronic band structure, and properties of few-layer PtSe 2 are introduced to give an overview of this material. Next, some recent progress on the various methods to synthesis monolayer and few-layer PtSe 2 , including mechanical exfoliation, chemical vapor deposition (CVD), thermally assisted conversion (TAC), molecular beam epitaxy (MBE), and chemical vapor transport (CVT), are discussed in detail. Furthermore, the applications of 2D PtSe 2 in many areas, including photodetector, field effect transistors (FETs), mode-locked laser, photocatalytic, hydrogen evolution reaction (HER), and sensors, are highlighted. At last, the perspectives and outlooks for the 2D PtSe 2 materials are concluded.

Crystal Structure
Generally, there are two common structural phases for monolayer TMDCs, which are characterized by either octahedral trigonal prismatic (2H or D 3h ) or (1T or D 3d ). Unlike group-6 TMDCs, group-10 TMDCs tend to form d2sp3 hybridization due to group-10 metal atoms hold rich d-electrons and less d orbitals are involved. As a result, group-10 TMDCs lead to the generation of the thermodynamically favored 1T-phase. The 2D layered structure of TMDCs (such as PtS 2 /PtSe 2 / PtTe 2 and PdS 2 /PdSe 2 /PdTe 2 ) has been proposed in 1950s since the pioneering work of Kjekus et al. and Grønvold et al. [47][48][49]. As a rising star of group-10 TMDCs, PtSe 2 has a thermodynamically favored 1T-phase structure and the atoms stack in the AA arrangement [28,50]. PtSe 2 crystal belongs to the D 3d 3 (P3m1) space group of the trigonal system [34,51,52]. The crystal structure of PtSe 2 from different view is shown in Fig. 1a. Many techniques have been employed to characterize the atomic structure of monolayer PtSe 2 , such as high-resolution scanning transmission electron microscope (HR-STEM), scanning tunneling microscope (STM), low energy electron diffraction (LEED), and density functional theory (DFT) calculation. As shown in Fig. 1b-f, the HR-STEM image, LEED patterns, STM images, and simulated STM images are presented, respectively. Figure 1b shows the representative HR-STEM image of PtSe 2 . The fast Fourier transform of the image (inset of Fig. 1b) shows hexagonal structure and confirms the single-crystalline feature of the few-layer PtSe 2 samples. The HR-STEM image of PtSe 2 clearly shows that each Pt atom is in a tilted octahedral site and surrounded by six Se atoms, which is consistent with the octahedral structure of 1T phase TMDCs [28,53,54]. As shown in Fig. 1c, hexagonal diffraction spots from monolayer PtSe 2 film are observed in a LEED pattern. The STM image and enlarged atomic resolution image of monolayer PtSe 2 are shown in Fig. 1d, e. By employing LEED, STEM, and STM methods, the atomic structure of PtSe 2 and lattice constant (a 1 = 3.7 Å, shown in Fig. 1e) are experimentally defined. Moreover, Wang et al. [25] conducted the DFT simulation based on the structure parameters obtained from the experimental characterizations. The simulated STM image is shown in Fig. 1f, and the results are well consistent with the STM observation results, which strongly demonstrated the highly crystalline structure of the 2D layered PtSe 2 .

Electronic Band Structure
The electronic structure of 2D layered TMDCs materials strongly depends on the coordination environment of  [23]. PtSe 2 presents a layer-dependent band structure with dimensional reduction from bulk to monolayer. Zhao et al. [28] found that the monolayer PtSe 2 is an indirect semiconductor and the band gap of monolayer PtSe 2 is about 1.17 eV. Figure 2a shows the band structure of monolayer PtSe 2 . The valence band maximum (VBM) of monolayer PtSe 2 situated at the Г point, which comprised of the p x and p y orbitals of Se atoms (p Se x and y ). The conduction band minimum (CBM) of monolayer PtSe 2 is situated between the Г and M points, which is dominated by d states of Pt and p states of Se. The band gap of PtSe 2 abruptly decreased with the increased number of layers (NL), due to the exceptionally strong interlayer electronic hybridization of p z orbital of Se atom (p Se z ). Figure 2b presents the band gap evolution of PtSe 2 as function of NL. The band gap of PtSe 2 shows a sharp decrease as the NL increased. As the NL larger than four the PtSe 2 shows a semiconductor-to-metallic transition. With increase in stacked layers, the energy level of VBM exceeds that of CBM between Г and M because of the increase in  interlayer electronic hybridization [23,28]. As a result, a semiconductor-to-semimetal evolution occurred. It has been proved that thicker PtSe 2 (layer numbers large than four or five) becomes semimetallic without a band gap [25,28,55]. As shown in Fig. 2c, the band gap structure of bulk PtSe 2 explicitly shows the semimetallic characteristics, and the CBM moves from a point between the Г and M to the K point due to the strong interlayer interaction of PtSe 2 [28]. In 2015, Wang et al. [25] experimentally measured the band structure of monolayer PtSe 2 for the first time by using angle-resolved photoemission spectroscopy (ARPES). Figure 2d shows the ARPES spectra data measured along the high symmetry direction K-Γ-M-K in the hexagonal Brillouin zone at photon energy of 21.2 eV. As shown in Fig. 2e, the location of VBM and CBM in the derivative spectra indicates that monolayer PtSe 2 is a semiconductor. The ARPES results show excellent quantitative agreement with the DFT simulation results. With this pioneer work, ARPES has become one of the most important techniques to investigate the electronic structure of PtSe 2 [43,56,57]. In order to study the layer-dependent electronic structure, Yan et al. measured ARPES data along the Γ-K direction [43]. As indicated by gray arrow in Fig. 2f, an M shape band was observed in thicker PtSe 2 (NL ≥ 2). Moreover, the M-shaped band moves toward the Fermi energy as the atomic layers increased, indicating a reduction of the band gap. Therefore, the ARPES results provide direct evidences for the layer-dependent band gap of PtSe 2 as theoretically predicated [27,58,59].

Properties of 2D PtSe 2
This section highlights the unique properties of 2D PtSe 2 such as band gap tenability, phase transition, and vibration spectroscopic and optical properties. The band gap tuned by various kinds of external parameters has been introduced at first. Then, the phase transition of 1T phase, 1H phase, and non-layered PtSe 2 are reviewed. At last, the vibration spectroscopic and optical properties are introduced in details.

Band Gap Tunability
It has been widely proved that the band structure of 2D TMDCs can be tuned by doping, defect engineering, strain, and external electric field [60][61][62][63][64]. Besides the inherent thickness-dependent band gap, band gap of PtSe 2 also can be tuned by applying external parameters. For example, band gap of few-layer PtSe 2 can be tuned over a wide range by applying strain. The band structure of monolayer PtSe 2 with symmetrical biaxial compressive strains and symmetrical tensile strains reveals the band structure evolution, as shown in Fig. 3a, b [65]. Du et al. [66] have also demonstrated that the band gap decreases approximately linearly with the increased tensile strain, but it is different for the band gaps evolution under compressive strain. As shown in Fig. 3a, monolayer PtSe 2 exhibits a direct gap semiconductor characteristic as the compressive strain reaches − 8%. The same transformation has also been reported by other published papers [52,[66][67][68]. Moreover, due to the chemical interaction (p orbital coupling) between Se atoms of the two layers, a reversible semiconducting metallic transition bilayer PtSe 2 under critical vertical strain, as shown in Fig. 3c [52]. Besides strain engineering, doping also has significant effect on the band structure of 2D PtSe 2 [36,39,69]. As shown in Fig. 3d, the band structures of the halogen elements (including F, Cl, Br, and I)-doped monolayer PtSe 2 have been calculated by DFT [36]. As compared with pristine monolayer PtSe 2 , the localized impurity states located close to the CBM are identified. The band structure of transition metal-doped PtSe 2 has been simulated by Kar et al. by using DFT [69]. And they found that group IIIB, VB, VIII8, VIII9, and IB transition metal-doped monolayer PtSe 2 exhibits half-metallic properties together with spin gap. Besides, the other transition metal-doped PtSe 2 exhibits tunable semiconducting or tunable magnetic semiconducting properties.

Phase Transition
Due to the strong covalent bond strength and weak interlayer interaction, the structure of 2D materials strongly depends on varying external effects (pressure, strain, irradiation, annealing, or lithiation) [23]. Phase transition can be induced by ionic intercalation, high pressure, strain, thermal treatment, and external electrical and magnetic field. Since 1T-PtSe 2 is a very stable structure, it is difficult to expect a continuous phase transition unless inducing additional electron beam irradiation and annealing treatment.
As show in Fig. 4a, b, Lin et al. studied the reversible phase transition of 1T phase and 1T/1H patterned PtSe 2 by using in situ STM [70]. The homogeneous 1T-PtSe 2 shown in Fig. 4a was directly synthesized on a Pt (111) substrate via a TAC process at 270 °C. As the 1T-PtSe 2 film annealed at 400 °C, periodic triangular pattern structure of alternating 1H/1T patterned phases formed. The STM images of the 1H/1T patterned phases are shown in Fig. 4b. Moreover, the triangular 1H/1T pattern reverts to a homogeneous 1T phase PtSe 2 by annealing the periodic triangular 1H/1T patterned PtSe 2 at 270 °C in Se steam atmosphere. However, Lin et al. found that the 1H/1T triangular pattern can be directly     [73,74]. In addition, Ryu et al. [75] demonstrated that 1T phase PtSe 2 can transform into non-layered 2D PtSe 2 ultrathin film. The phase transformation from 1T PtSe 2 into nonlayered PtSe 2 crystals induced due to the Se loss during the additional heating process at high temperature (550 °C). As shown in Fig. 4c, d, the rearrangement and restacking of the atoms have been in situ observed by taken successive annular dark field scanning transmission electron microscope (ADF-STEM) images. It can be found that the phase transition occurred only in the bilayer region. Further characterization of the phase transition process has been observed by constructed AA stacking and AB stacking bilayer PtSe 2 . As shown in Fig. 4e-g, the phase transition occurs only in the AA stacking PtSe 2 region. As the PtSe 2 film was heated, the non-layered PtSe 2 structure continued to expand and blocked at the grain boundary.
Besides the annealing and heating process, plasma treatment process has also been proved as an efficient method Reproduced with permission [70]. Copyright 2017, Springer Nature. c PtSe 2 at the edge of bilayer PtSe 2 . d, e Successive annular dark field STEM images at a position indicated by a cyan arrow in (c). f Time between images ~ 60 s. g-j Successive annular dark field STEM images at the position indicated by cyan arrows in (j). White dashed line indicated the grain boundary. c-j Reproduced with permission [75]. Copyright 2019, American Chemical Society to induced phase transition in 2D PtSe 2 . Yang et al. [76] reported an inductively coupled plasma treatment method to selectively controlling the thickness of PtSe 2 flakes. With the decrease in thickness, PtSe 2 flake transforms from a semimetal to semiconductor. This is well consistence with the prediction concerning their intrinsic thickness-dependent band structure. However, Shawkat et al. discovered a reversed transition of semiconducting to metallic as the PtSe 2 film irradiated by plasma. Shawkat et al. [77] realized a semiconductor-to-metallic transition in wafer-scale PtSe 2 film by controlled plasma irradiation. Extensive structural and chemical characterization has proven that large concentration of near atomic defects and selenium vacancies introduced by the plasma irradiations induced the transition of semiconductor to metallic. The phase transition driven by thermal heating makes it possible for fabricating lateral heterojunctions composed of 1T-PtSe 2 , 1H-PtSe 2 , and PtSe. The electronic properties of 2D PtSe 2 materials can be modulated by the induced phase transition, which offers new opportunities in both fundamental research and (opto-) electronic devices applications.

Vibration Spectroscopic Modes
Raman spectroscopy is a powerful and nondestructive optical characterization technique to study the lattice vibrations as well as electron-phonon coupling of 2D materials. Due to the strong interlayer coupling and hybridization, the Raman spectra of PtSe 2 exhibit interesting anomalous changes.
The schematic diagram of four Raman active vibrational modes in PtSe 2 is shown in Fig. 5a [43]. The A 1g mode and E g mode are originated from the out-of-plane vibration and the in-plane vibration of Se atoms, respectively. Figure 5b exhibits the Raman spectra of PtSe 2 with different thickness. As shown in Fig. 5b, the Raman spectra of 2D PtSe 2 with different thickness, laser wavelength, and laser polarization were systematically studied. Three primary Raman peaks which allocated to E g (~ 180 cm −1 ), A 1g (208.5 cm −1 ), and LO (~ 240 cm −1 ) modes are observed. As the Raman spectra are normalized to the E g peak, the relative intensity of the A 1g peak exhibits an obvious decreasing as the thickness decreased. As shown in Fig. 5c, the peak positions of the E g and A 1g mode are extracted and plotted as a function of number of layers. The position of E g mode exhibits a clear red shift with increase in thickness. However, the position of A 1g mode is almost unchanged for few-layer PtSe 2 and exhibits an obvious red shift as the number of layers larger than 22 layers. The layer dependence of Raman spectra properties may be attributed to the strong long-range interlayer interactions [78,79]. To further study the relationship between the thickness and Raman spectra, the intensity ratio of the A 1g peak to E g peak is extracted and plotted in Fig. 5d. The extracted thickness\intensity ratio is well consistent with the enhanced van der Waals interactions between the layers in thicker 2D materials [68,78,80]. Besides the distinct E g and A 1g modes, Raman peaks ascribed to less prominent LO mode, which attributed to a combination of the in-plane E u and out-of-plane A 2u vibrations, are also observed [43,55]. The enlarge spectra in Fig. 5e show the thickness-dependent position of the LO peaks. With increase in number of layers, the LO peaks change into a broader hump and the intensity decreased. Moreover, the vibration modes of PtSe 2 were further characterized by polarization-dependent Raman spectra [28,78]. As shown in Fig. 5f, the variation of the polarization of the incident light has no effects on the intensity of E g peak (around 180 cm −1 ), which confirming the in-plane nature of this mode. However, an obvious intensity decrease in the A 1g peak (around 208 cm −1 ) and LO peak (around 240 cm −1 ) depending on light polarization is observed. The decrease in the intensity of A 1g peak and LO peak confirms the outof-plane vibration nature of these two modes. It has been proved that these Raman peaks have been observed in the mechanic exfoliated PtSe 2 single crystal as well as the PtSe 2 film grown via TAC process [28,78].

Layer-Dependent Optical Absorption Spectra
The refractive index and extinction coefficient are fundamental properties of a material that not only determines its optical responses, but also directly connects to its complex permittivity and dielectric constant. Wang et al. measured the refractive index and extinction coefficient of the PtSe 2 (~ 3 nm) in the wavelength range from 200 to 900 nm by using spectroscopic ellipsometry [31]. The refractive index of the ultrathin PtSe 2 film increased from 1.5 to 4.5 as the wavelength increased from 200 to 900 nm. However, the extinction coefficient of the ultrathin PtSe 2 film is almost unchanged and maintained around 2.4. Xie et al. [81] also measured the refractive index and extinction coefficient of the PtSe 2 film in the wavelength range from 360 to 1700 nm. The refractive index and extinction coefficient values are strongly dependent on the thickness of PtSe 2 film. By analyzing the spectroscopic ellipsometry results, the values of refractive index and extinction coefficient increased as the film thickness increasing.
As shown in Fig. 6, the thickness-dependent optical absorption spectra of PtSe 2 films were measured in the wavelength range of 200-3300 nm [82]. As shown in Fig. 6a, PtSe 2 exhibits a broadband absorption response with a smooth absorption band in the wavelength range of 400-800 nm. However, in the wavelength range of 800-2200 nm, the absorption intensity decreased as the wavelength increased. In the range of 2200-3300 nm, PtSe 2 still exhibits a broadband absorption and the intensity almost unchanged in this wavelength range. Besides, the absorption spectra of thicker PtSe 2 film exhibit an obvious red shift. Usually, semiconductors cannot absorb light with energy much smaller than the band gap, but PtSe 2 film exhibits strong light absorption in a broadband wavelength range from deep ultraviolet to mid-infrared (mid-IR) [28,66,[81][82][83]. The strong IR light absorption of PtSe 2 mainly attributes to the semimetallic components of the films [81,82].
The band gap of semiconductors can be easily experimentally measured by using optical absorption spectra. The layer-dependent Tauc plots of PtSe 2 are presented in Fig. 6b. With increase in thickness, the absorption edge of Tauc plot shows an obvious red shift. The band gaps of monolayer PtSe 2 are well consistence with the DFT calculation results. Meanwhile, the transition from semiconductor to semimetal of PtSe 2 has been verified by the layer-dependent Tauc plots, which is also well agreement with DFT calculation.

Isotropic Optical Properties
Xie et al. [81] studied the optical isotropy properties by using polarized optical imaging method and polarizationdependent optical absorption measurement, and they ascertained the optical isotropy in the 2D PtSe 2 . As shown in Fig. 6c, the intensity of the red, green, blue (RGB) channels and the total intensity at different rotation angles were extracted from the polarized optical images of a PtSe 2 film (~ 5.3 nm). As the rotation angle changed, the intensity of RGB channels and total intensity are almost unchanged, which indicate the optical isotropy of PtSe 2 . The absorption spectra in the range of 400-800 nm under polarization directions of 0° (horizontally), 90° (vertically), and nonpolarized light for PtSe 2 film (~ 5.3 nm) were measured. These absorption spectra are well consistent with each other, indicating the in-plane isotropic optical absorption in PtSe 2 film.

Nonlinear Optical Properties
Nonlinear optical (NLO) properties of 2D materials have been taken as the forefront of the research, which are crucial for developing high-performance ultrafast laser and optoelectronic devices [85][86][87][88][89][90][91][92][93]. PtSe 2 has nonlinear effects in a wide wavelength range due to its narrow band gap. Tao et al. [94] investigated the NLO properties of TAC-synthesized PtSe 2 films. A modulation depth of 12.6% and saturation fluence of 17.1 μJ cm −2 were obtained based on the NLO transmittance curve. The saturable absorption (SA) characteristics of the transverse-electric and transverse-magnetic modes of PtSe 2 are studied by Zhang et al. [95] Modulation depth of 4.90% (transverse-electric modes) and 1.11% (transverse-magnetic modes) are obtained based on the NLO transmittance curves.
The NLO properties of few-layer PtSe 2 have been systematic studied by using the Z-scan method and pump-probe-technique [84]. As shown in Fig. 6d, two small peaks near the symmetrical valley are observed in the open aperture (OA) signals of 4L and 7L PtSe 2 films, which suggest the OA signals consist of both SA and two-photon absorption (2PA) response at 1030 nm. However, there are no peaks observed near the symmetrical valley in the OA signals of 17L and 55L PtSe 2 (Fig. 6e), which indicate the pure SA response. The evolution of the saturation (IS at ) and irradiance nonlinear refractive index (n 2 ) are extracted and plotted in Fig. 6f, g. The large IS at at 515 nm indicates that PtSe 2 -based saturable absorber possesses higher saturation intensity in visible range than that in near-IR range. Besides, the large value n 2 of PtSe 2 suggests the huge potential to developing self-defocusing materials in NLO devices.

Synthesis Methods
Generally, the crystal structure, crystallinity, and properties of 2D PtSe 2 are strongly related to the synthesis process. It is still challenging to prepare 2D PtSe 2 with the desirable thickness, lateral size, and microstructure for specific applications. Indeed, various strategies have been proposed to prepare 2D PtSe 2 , specially aiming to materials with high quality and large lateral size. Up to now, great achievements have been made to prepare 2D PtSe 2 with controllable thickness, morphology, and lateral size. In this section, different synthesis methods Reproduced with permission [28]. Copyright 2018, John Wiley and Sons. b Schematic diagram of the three zones CVD system for the synthesis of 2D PtSe 2 . c SEM morphology of the PtSe 2 nanosheets synthesized by CVD process. d AFM image of the PtSe 2 nanosheets synthesized by CVD process. c, d Reproduced with permission [106]. Copyright 2016, John Wiley and Sons. The AFM image of the e bilayer and f four layer PtSe 2 film and corresponding height profiles [107]. g AFM image of the bilayer PtSe 2 synthesized via rapid cool down process. e-g Reproduced with permission [107]. Copyright 2019, John Wiley and Sons for preparing 2D PtSe 2 are systematically discussed. CVD and TAC are the most widely studied methods to synthesis 2D PtSe 2 . Moreover, the other methods such as mechanical exfoliation and CVT have also been studied.

Mechanical Exfoliation
Mechanical exfoliation (ME) is one of the most commonly used methods to prepare high quality 2D materials [96][97][98][99]. Monolayer or few-layer 2D materials obtained by this method can maintain their intrinsic structure and are suitable for fundamental research. Mechanical exfoliation has been extensively exploited to prepare monolayer or few-layer 2D materials, such as graphene, BP, nitride, TMDCs, and MXene. The mechanical exfoliation process is a relatively simple and fast process by repeating adhesion and splitting. As the monolayer or few-layer 2D materials attached on the surface of scotch tape, the as-prepared 2D materials can be easily transferred to selected substrate [1,99]. In 2017, Zhao et al. [28] prepared monolayer PtSe 2 by using mechanical exfoliation. The high quality PtSe 2 bulk crystal was grown by CVT method. Ultrathin PtSe 2 was peeled from bulk PtSe 2 using a scotch tape, as shown in Fig. 7a. Huang et al. [100] prepared PtSe 2 nanoflakes with the thickness of ~ 70 nm by mechanical exfoliation its single crystal. However, mechanical exfoliated 2D materials can only fulfill the using demands of fundamental studies due to the limited yield and relatively small lateral size.

Chemical Vapor Deposition
CVD is an important synthesis method to prepare high quality 2D materials with scalable size, controllable thickness, and perfect crystal structure for both fundamental research and practical applications [101][102][103][104]. To date, various materials with controllable layer number, lateral size, and microstructure have been successfully prepared via CVD methods, such as graphene, TMDC, Xene, boron nitride, and MXene. Recently, the CVD growth of monolayer or few-layer 2D PtSe 2 has also attracted extensively attention and has been taken as a promising method to realize the large-scale growth of 2D PtSe 2 . PtSe 2 with controlled morphology can be synthesized by CVD process via precise tuning of the growth temperature, pressure, and precursors [83,[105][106][107][108]. Figure 7b shows the typical schematic illustration of a 3-zone CVD growth setup, wherein the precursors are placed in different zone of the quartz tube. Typically, Se powder and PtCl 4 or H 2 PtCl 6 powder are chosen as the precursors; the obtained PtSe 2 is found to be nearly hexagonal with the thickness ranging from 3.5 to 10 nm [106]. However, by tuning the growth temperature of zone 3 from 900 to 500 °C, Xu et al. successfully prepared polycrystalline PtSe 2 film with controlled thickness by tuning the growth time [107]. The morphology of the single-crystalline and polycrystalline PtSe 2 is shown in Fig. 7c-g, respectively. As shown in Fig. 7e-g, large area continuous PtSe 2 films with controlled thickness have been successfully synthesized via a one-step CVD process. Furthermore, the cooling down rate also has great effect to the surface morphology. The rapid cooling rate may suppress the diffusion of reactive atoms, leading to the formation of the multilayer island on the surface [107]. The multilayer islands on the surface of PtSe 2 thin film are shown in Fig. 7g.
The morphology, thickness, microstructure, and lateral size of 2D materials can be well controlled by precise controlling CVD growth parameters. The quality of the as-grown PtSe 2 can be determined by many factors including but not limited to the precursors, pressure, temperature, heating rate, and substrate. Thus, in-depth understanding of the CVD growth mechanism is beneficial to the improvement of scalability and controllability for PtSe 2 synthesis.

Thermally Assisted Conversion
TAC of pre-deposited metal on substrate is also an effective strategy to grow wafer-scale 2D materials [109][110][111]. PtSe 2 prepared by this method is a just simple chemical reaction, Pt + 2Se = PtSe 2 . Direct selenization of the Pt film provides a simple and fast approach to obtain wafer-scale 2D PtSe 2 film.
The TAC process is a straightforward and simple route for synthesizing large-scale PtSe 2 with controlled thickness. Pt film with different thickness is initially deposited on a given substrate via a magnetron sputtering process or electron beam evaporate process. Then, the PtSe 2 film is prepared via the directly selenization process. As shown in Fig. 8a, the Se powder is placed at the upstream side in the tube furnace, and the Pt coated substrates are placed in the heating zone. During the selenization process, the growth temperature is usually set to about 270-500 °C, while the pressure remains at about 80 mTorr with argon gas protection [25,112,113]. In 2015, Wang et al. [25] firstly fabricated a single crystal monolayer PtSe 2 by direct selenization of Pt (111). Han et al. [114] prepared large-scale 2D PtSe 2 with different thickness on SiO 2 /Si substrate. Figure 8b shows the photograph of the PtSe 2 film with different thickness on SiO 2 /Si substrate. The lateral size and thickness of PtSe 2 film can be controlled by modulating thickness of the pre-deposited Pt film [84,115].

Other Methods (Molecular Beam Epitaxial, CVT)
In addition to the aforementioned methods, some other approaches also have been reported to synthesize 2D PtSe 2 . For example, some pioneer works have been reported that PtSe 2 can be prepared via a Sol-Gel solution process [130][131][132][133][134]. Umar et al. [135] reported the successful synthesis of scalable 2D PtSe 2 nanosheets via an aqueous-phase synthetic strategy for the first time. PtSe complexes precursors are initially prepared via surfactant-template self-assembly process. Then, the mesoporous 2D PtSe 2 nanosheets are prepared by thermal annealing the PtSe complexes precursors. As shown in Fig. 9a, the 2D PtSe 2 nanosheets with a thickness about 11-25 nm are synthesized, indicating that scalable PtSe 2 can be produced by a straightforward process to scalable produce PtSe 2 . Pawar et al. [136] also prepared 2D PtSe 2 nanosheets by using the almost same method that Umar reported.
As a widely studied traditional crystal growth method, CVT has also been employed to direct synthesize 2D semiconductor materials, such as TiSe 2 , MoS 2 , WS 2 , and ReS 2 [137][138][139][140][141]. Benefitting from the good controllability of the growth parameters, the properties, structure, and composition of 2D materials can be well regulated. In 2016, Yu et al. [33] successfully synthesized single crystal of PtSe 2  [117]. Copyright 2018, American Chemical Society by using CVT method. This achievement makes it possible for us to grown 2D PtSe 2 by precise controlling the growth condition. Hu et al. [142] successfully synthesizes 2D PtSe 2 nanosheets with controlled thickness by using CVT. As shown in Fig. 9b, the schematic diagram of the CVT process is presented. The raw materials are put in a sealed the quart tube, while the substrate is placed in the other side of the quart tube. By carefully adjusting the amount of precursors and transport agent, triangular-shaped single-crystalline PtSe 2 flakes were obtained on the mica substrate. The optical morphology of the triangular-shaped single-crystalline PtSe 2 flakes is shown in Fig. 9c, and the relationship of the thickness with temperature and reactants is exhibited in Fig. 9d. However, only few papers have reported the synthesis of 2D  PtSe 2 by using CVT due to the complex growth condition. Since growth of bulk semiconductor crystal by CVT is much easier than direct growth 2D semiconductor materials, CVT is generally employed to grow high quality single-crystalline bulk materials, ultrathin 2D flakes are then peeled from bulk crystal by mechanical exfoliation [142][143][144]. For example, Zhao et al. [28] grow PtSe 2 single crystal by using CVT method and the air stable 2D PtSe 2 are peeled from the bulk PtSe 2 crystal. Molecular beam epitaxy (MBE) has been playing an important role in the growth of high quality 2D materials film with controlled thickness [145,146]. Yan et al. [43] successfully prepared high quality PtSe 2 films on bilayer graphene/6H(0001) substrate by using MBE method for the first time. The surface morphology of the as-prepared PtSe 2 on the surface of bilayer graphene is shown in Fig. 9e. The obtained PtSe 2 film is single crystalline and the thickness ranges from 1 to 22 layers.
The reliable production of 2D PtSe 2 with controlled structure is a prerequisite in exploring their properties and possible applications. As mentioned above, 2D PtSe 2 has been prepared by various approaches including mechanical exfoliation, CVD, CVT, TAC, and other methods. A comprehensive summary and comparison with these methods is presented in Table 1. The aforementioned methods have inherent disadvantages which make it difficult to achieve the large area and highly crystalline structure. And the synthesis of large lateral size and uniform monolayer or few-layer 2D PtSe 2 is still challenge. Moreover, the growth mechanism has yet to be clarified. Therefore, extended works need to be done to achieve the controllable synthesis of 2D PtSe 2 .

Photodetectors
Photodetectors can directly convert optical signals to electrical signals. It has been widely applied in many fields such as optical communication, industrial automatic control, and military [147][148][149]. 2D materials, including graphene, BP, and TMDCs, are considered to be promising candidates for high-performance photodetectors due to their excellent properties and complementary metal oxide semiconductor compatible [147,[150][151][152][153][154][155]. However, it is still challenge to fabricate high responsivity 2D material-based photodetectors along with ultrafast response. Although group-6 TMDCs (such as MoS 2 and WS 2 ) have exhibited impressive optoelectronic properties [156][157][158], their photodetection performance is severely limited due to their relatively large band gap and low carrier mobility, especially in the IR range.
As newly emerged 2D materials, group-10 TMDCs have been widely studied as high-performance photodetectors [29,117,121,159]. Among these group-10 TMDCs materials, PtSe 2 has been demonstrated to have excellent photoelectric and electrical properties. As introduced above, the band gap of monolayer and bilayer PtSe 2 is 1.2 and 0.21 eV, respectively [25]. Simulation results have revealed that only monolayer PtSe 2 has a sizeable band gap and PtSe 2 become semimetallic as the number of layers larger than three or four. Thus, 2D PtSe 2 is proposed as an excellent candidate for broadband photodetectors in the visible to mid-IR range [32, 33, 116-118, 120, 121, 123, 129, 160]. As shown in Table 2, the performance of PtSe 2 -based photodetectors is summarized for comparison. Yu et al. investigated the photoresponse of FETs based on bilayer PtSe 2 in the wavelength range from 632 nm to 10 μm, as shown in Fig. 10a [33]. The photoresponsivity of 6.25 A W −1 and a rise time of about 1.2 ± 0.1 ms were achieved for 640 nm laser illumination. Moreover, the photoresponsivity in the near-IR (~ 1.47 μm) wavelength range and mid-IR (~ 10 μm) wavelength range is about 5.5 and 4.5 A W −1 , respectively. The fitted rise and fall time for the bilayer PtSe 2 -based photodetector are much better than those 2D materials (such as BP, MoS 2 , and MoSe 2 )-based photodetectors [15,147,149,150,[161][162][163][164][165][166]. These results indicate that 2D PtSe 2 is highly promising platforms for high sensitive and broadband optoelectronic application in the range of visible light to mid-IR wavelengths.
Su et al. investigated the performance of PtSe 2 film-based photodetector on SiO 2 /Si [55]. The schematic structure of the photodetector device and the corresponding optical image are shown in Fig. 10b. As shown in Fig. 10c, the broadband photoresponse is demonstrated in the wavelengths range from 408 to 640 nm. When the photodetector was irradiated by 408 nm laser, the device exhibited the highest photoresponse with the photocurrent reaches 9 μA, while the photocurrent was about ~ 6 and ~ 1 μA as irradiated by 640 and 510 nm laser, respectively. The corresponding photoresponsivity with incident power density of 12.73 mW cm −2 is 0.1A W −1 (at 640 nm), 0.25 A W −1 (at 515 nm), and 0.4 A W −1 (at 408 nm). Moreover, the PtSe 2 can be directly grown on a flexible polyimide substrate owing to the advantage of the low-temperature growth process. Su et al. [55] also fabricated a flexible photodetector based on PtSe 2 film on the polyimide substrate by using the same conditions of photodetectors fabricated on the SiO 2 /Si substrate. The photodetector exhibits great stability under different bending radius with almost no degradation in the photocurrent even after 1000 bending cycles.
Yim et al. studied the photoresponse of the layered PtSe 2 -based Schottky barrier diodes on n-type Si [32,112]. The diode was fabricated by transferring PtSe 2 thin films onto the pre-patterned n-type Si substrate. The PtSe 2 film exhibits strong photoresponse over a broadband wavelength range of 360-2000 nm. The maximum photoresponsivity of 0.49 A W −1 and minimum photoresponsivity of 0.0001 A W −1 were measured at photon energies above and below the band gap of Si. In the visible region, the large part of   [33,116]. The PtSe 2 films were grown directly on Si substrates, which can effectively avoid the interface contamination, structural continuity deterioration, and materials surface tear. This heterojunction-based photodetector is highly sensitive in a broad wavelength region from deep ultraviolet (200 nm) to near-IR (1550 nm). As shown in Fig. 10d [33]. The broad band gap range and high carrier mobility of PtSe 2 make it be an excellent candidate for developing high-performance photodetectors. However, the ultrathin thickness of 2D PtSe 2 materials result in a low absorption to incident light, leading to a small photocurrent, large dark current and low specific detectivity. In order to develop broadband, high sensitive, low power, and high photoresponsivity photodetector, PtSe 2 -based heterostructure for optoelectronic applications has been studied [117,118,120,121,129]. Wu et al. [121] designed a vertical PtSe 2 /CdTe heterojunction-based photodetector and this photodetector exhibited a broad detection wavelength range of 200-2000 nm. This heterojunction structure can enhance the absorption to near-IR light, as well as the improvement of response speed due to the formation of a built-in electric field. Zeng et al. [121] fabricated a PtSe 2 / GaAs heterojunction on SiO 2 /Si substrate via a deposition process and wet transfer process. The PtSe 2 /GaAs heterojunction-based photodetector exhibited high sensitivity to broad wavelength range from 200 to 1200 nm. As shown in Fig. 10e  absorber. c Single pulse profile indicates the pulse duration. d Radio-frequency spectrum of the mode-locked pulses and inset shows the corresponding wideband (0-300 MHz) radio-frequency spectrum. Reproduced with permission [177]. Copyright 2018, American Chemical Society the range from 650 to 810 nm, which exclusively originates from the PtSe 2 layer. The rise/fall time for the photodetector is 5.5/6.5 μs (shown in Fig. 10f), which are faster than other state-of the art 2D materials (such as BP, MoS 2 , WS 2 , and graphene/Si heterojunction) photodetectors [167][168][169][170]. Wang et al. [118] fabricated a PtSe 2 / Ge heterojunction-based photodetector, which is highly sensitive to the near-IR light. The photodetector device can operate without an external power supply due to the photovoltaic effect under the near-IR light illumination [112,118,120].

Mode-Locked Laser
The mode locking based on SA has been taken as the most important and efficient optical technique to generate ultrafast pulse laser from a continuous wave laser [171][172][173][174][175][176].
The mode-locked laser systems have been widely applied in areas including ultrafast pump sources, high-accuracy measurement, ultrafine laser micromachining, and laser surgery [171]. This technique exhibits many advantages such as low cost, high power scalability, high reliability, good mechanical stability, and excellent beam quality. 2D materials with saturable absorber properties have been widely utilized as saturable absorber in the laser cavity for ultrafast pulse generation. In 2018, Yuan et al. [177] reported the SA properties of 2D PtSe 2 film for the first time. Figure 11a shows the schematic diagram of the experimental setup of the ytterbiumdoped fiber (YDF) laser ring cavity. As shown in Fig. 11b, the NLO measurements results show that PtSe 2 film (about 10 nm) exhibits a large modulation depth up to 26% at the wavelength of 1064 nm with a lower saturable intensity, while the saturable intensity is as low as 0.316 GW cm −2 . The modulation depth refers to the maximum change of transmission or total amount of light loss by saturable absorption of the absorber. The relatively large modulation depth at the wavelength of 1064 nm indicates the potential of PtSe 2 to be an excellent nonlinear absorption material. As shown in Fig. 11c, d, the mode-locking performance of PtSe 2 film is investigated by transferring a PtSe 2 film onto the fiber tip. The mode-locked pulse centered at 1064.47 nm has the pulse duration of 470 ps.
Tao et al. [94] also reported the properties of the passively mode-locked solid state laser by using a 24-nm-thick PtSe 2 film as the saturable absorber. A pulse duration of 15.8 ps is obtained in the mode-locked fiber laser based on a PtSe 2 film coated fiber. Zhang et al. [95] fabricated and studied 1563 nm Er-doped fiber laser based on PtSe 2 film, with pulse duration of 1.02 ps and maximum single pulse energy of 0.53 nJ. Huang et al. [100] fabricated a femtosecond fiber mode locking by transferring thicker PtSe 2 (~ 73 nm) onto a D-shaped fiber. Due to the nonlinear modulation from the PtSe 2 , the pulse duration of 861 fs and single-to-noise ratio of 61.1 dB were achieved for the 1567 nm mode-locking laser. The recent progresses on the PtSe 2 -based mode-locking laser make PtSe 2 a promising 2D material for on-chip integration of GHz laser sources toward higher repetition rates and shorter pulse duration [31,82,84,125].

Field Effect Transistors
One of the important applications of 2D PtSe 2 materials is the field effect transistors (FETs). The very first report on 2D layered PtSe 2 material for FETs was reported by Zhao et al. in 2017 [28]. The room temperature electron mobility of the few-layer PtSe 2 FETs device is 210 cm 2 V −1 s −1 , which is much smaller than the theoretically predicted value [28,178]. Zhao et al. further studied the temperature-dependent mobility of PtSe 2 FETs and the mobility of few-layer PtSe 2 FETs (~ 11 nm). The field effect mobility and the gate-dependent mobility of the 11 nm-thick-PtSe 2 FETs are shown in Fig. 12a, b. The mobility of the PtSe 2 FETs in a back-gated configuration on SiO 2 /Si increased from 210 to 414 cm 2 V −1 s −1 , as the temperature decreased from 300 to 100 K. Moreover, as the temperature continues to decrease to 25 K, the mobility of the PtSe 2 FETs decreased from 414 to 353 cm 2 V −1 s −1 . For comparison, the temperaturedependent mobility of a thinner few-layer PtSe 2 (~ 8 nm) FETs is measured. When the temperature increased from 25 to 300 K, the mobility increased from 149 cm 2 V −1 s −1 (at 25 K) to 233 cm 2 V −1 s −1 (at 125 K) and then decreased to 140 cm 2 V −1 s −1 (at 300 K). The variation of carrier mobility mainly ascribed to the layer-dependent band gap of PtSe 2 . The reduced band gap of thicker PtSe 2 leads to the increased carrier density, which improves the screening of charge impurities by the bottom layer. The conductivity and carrier mobility are thus significantly improved due to the carriers in the bottom layer can effectively suppress the Coulomb potential of the charge impurities at the interface [28,179].
Previous theoretical and experimental results have demonstrated that the thinner PtSe 2 exhibits a semiconducting behavior, while the thicker PtSe 2 exhibits a metallic behavior [42,55]. As shown in Fig. 12c full PtSe 2 FETs wherein the thicker PtSe 2 (~ 50 nm) is used as the electrodes and the thinner PtSe 2 (~ 3 nm) is used as the channel materials. To further confirm the existence of the PtSe 2 in both the channel and electrode, Raman spectra and TEM images of the channel and electrode materials are presented in Fig. 12c, d. The measured electrical properties of the full PtSe 2 FETs are shown in Fig. 12e. The mobility of the full PtSe 2 FETs ranges from 0.007 to 0.021 cm 2 V −1 s −1 , which is lower than the device using pure Pt electrodes [55]. Yim et al. studied the effect of contact metals and edge contact at the metal/PtSe 2 interface to the transport characteristics of the FETs devices [113]. They found that by increasing the edge contact length, the contact resistivity was improved by up to 70% compared to devices with conventional top contacts, which provide a quick insight into the realization of high-performance opto/electronic devices. Ansari et al. fabricated a back-gated FETs device with different channel thickness [119]. The on/off ratio and carrier mobility are measured at room temperature. The I on /I off ratio of thinner PtSe 2 film (2.5-3 nm) FETs exceeds 230, while the I on /I off ratio of thicker PtSe 2 film (5-6.5 nm) FETs is sharply decreased to about 1.4. These variations are mainly due to the quantum confinement effect in the thin 2D PtSe 2 film. Xu et al. systematically studied the electrical properties of n-doping and p-doping PtSe 2 film by fabricating top-gated FET [107]. The optical microscopic image of an as-fabricated FETs array is shown in Fig. 12f. The I on /I off ratio of the PtSe 2 FETs is about 25 (n-type) and 40 (p-type). The channel length-dependent electrical properties of the PtSe 2 FETs have been studied, and the effective field effect mobility of different configurations is presented in Fig. 12g. The four-terminal field effect mobility is nearly three times higher than two terminal field effect mobility for the p-type PtSe 2 , and two times higher than the n-type PtSe 2 , respectively. Han et al. [114] further identified the interrelation of structural morphology and electrical transport in 2D PtSe 2 thin film by applying corroborating HR-TEM and FETs characterization. The highest mobility measured in this FETs device reached 625 cm 2 V −1 s −1 , which is among the highest experimentally measured mobility value reported for PtSe 2 FETs. Besides the FETs devices on conventional rigid substrate, Okogbue et al. [180] fabricated a kirigami FETs on flexible polyimide substrate. By taking advantage of the low-temperature synthesis process, they fabricated integrated 2D PtSe 2 film on flexible. These 2D PtSe 2 /polyimide kirigami patterns exhibit an extremely large stretchability of 2000% without compromising their intrinsic electrical conductance. The corresponding I ds -V g transfer characteristics from the kirigami FETs of varying stretch level (0%, 100%, and 200%) are measured, and these plots clearly reveal that p-type semiconducting transports are well retained with slightly decreasing I ds during the increasing mechanical stretch.
Recently, impressive advances have been achieved for the fabrication of PtSe 2 FETs devices. The experimentally measured carrier mobility of PtSe 2 is much higher than the carrier mobility of group-6 2D TMDC materials, yet it is still much lower than the theoretically predicated value. For 2D materials, there are several extrinsic factors mainly dominating the charge transport, including structurally defects, charge impurity, surface optical phonon scattering, and surface traps [181][182][183][184]. These critical issues also existed in 2D PtSe 2 -based device, the negative effects induced by the unexpected impurity (Se dopant), heterojunction interface (electrode/PtSe 2 ), and contact resistance still need to be overcome. Besides, due to the low-temperature synthesis process of large-scale 2D PtSe 2 film, it is would be interesting to develop high-performance flexible devices.

Photocatalysis
2D materials have been widely studied as high-performance photocatalyst due to its large specific area and excellent electronic properties [185][186][187][188]. In 2013, Zhuang et al. [40] performed a systematic theoretical study on the photocatalytic performance of monolayer TMDCs by using first principles calculation. As shown in Fig. 13a, the position of CBM and VBM of some monolayer 2D TMDCs at pH = 0 and 7 is summarized. According to the calculation results, PtSe 2 , PtS 2 , MoS 2 , and WS 2 show potential for photocatalysis. Moreover, the solvation enthalpies (ΔHS olv ) of monolayer PtSe 2 , PtS 2 , MoS 2 , and WS 2 are calculated, as shown in Fig. 13b. For both case of isolated and associated ions, the ΔHS olv are significantly large than the value of HgS, which set as a reference. These calculation results indicate that these monolayer 2D TMDCs are insoluble and stable in aqueous solution, which make them ideal candidate for highperformance photocatalyst. The calculation results show that few-layer PtSe 2 exhibit great potential for high-performance photocatalysis due to the sizable band gap within the visible wavelength range.
Wang et al. [25] carried out a methylene blue photocatalytic degradation experiment to evaluate the photocatalytic property of PtSe 2 film. As shown in Fig. 13c, d, the schematic diagram of the photocatalytic degradation of methylene blue process and the time dependent photocatalytic degradation of methylene blue were evaluated by test the concentration of the methylene blue aqueous solution. It can be seen that almost 38% of methylene blue molecules are degraded in 24 min. As the PtSe 2 catalyst absorbed a photon, an active electron-hole pair generated and the absorbed methylene blue are degraded by the high energy photon excited electrons. For comparison, the monolayer PtSe 2 exhibits high photocatalytic degradation rate comparable with the nitrogen doped TiO 2 nanoparticles [189].
Sun et al. [127] fabricated a PtSe 2 film onto FTO substrate via TAC process and studied the solar-driven water splitting performance of the PtSe 2 film. The highest photocatalytic H 2 production rate can reach 506 mmol hm −1 . The photocatalytic activity of the PtSe 2 /FTO thin film has no obvious decrease in ambient and acidic/alkaline solution even after aging for 1 year. Moreover, the PtSe 2 -based composite also shows high photocatalytic performance, such as PtSe 2 /graphene [130,131] and PtSe 2 /TiO 2 /graphene [131][132][133].

Hydrogen Evolution Reaction
Hydrogen has been recognized as the future energy carrier due to its ultrahigh energy density as a sustainable clean energy source [190,191]. Experimental and theoretical efforts have indicated that 2D TMDCs materials can serve as ultrathin electrocatalysts for the hydrogen evolution reaction (HER) [38,186,[192][193][194][195].
Chia et al. [196] studied the HER electro-catalytic properties of Pt dichalcogenides by performing DFT calculations. As shown in Fig. 14a, b, the PtSe 2 has over-potential of 0.63 eV and Tafel slop of 132 mV dec −1 . However, the HER performance of PtSe 2 can be further enhanced by both  [25]. Copyright 2015, American Chemical Society reduction and oxidation process. For example, the oxidized PtSe 2 has over-potential of 0.36 eV and Tafel slop of 93 mV dec −1 . The HER performance for PtSe 2 is activated by both oxidation and reduction, and the oxidized and reduced PtSe 2 exhibited better HER efficiency by a 46% and 9% decline in over-potential, respectively. Wang et al. investigated the HER performance of CVD synthesized 2H-PtSe 2 and 1T-PtSe 2 single crystal nanosheets. The 2H-PtSe 2 shows the Tafel slope of 78 mV dec −1 , which is much higher than that of 1T-PtSe 2 (48 mV dec −1 ) [106]. Due to the semimetallic structure, the 1T-PtSe 2 exhibits relatively higher electrochemical activity (lower Tafel slop and higher over-potential). Shi et al. also found that the monolayer or few-layer 1T-PtSe 2 can serve as high-performance HER catalyst, and a record high HER efficiency [197]. As shown in Fig. 14c, d, the catalytic activity of monolayer 1T-PtSe 2 was calculated by DFT to identify the electrocatalytically active sites. The calculated Gibbs free energy (ΔG H* ) values of H adsorption at the 50-edge, 100-edge, and basal planes of the monolayer 1T-PtSe 2 are 0.07, 0.50, and 1.07 eV, respectively.
The relatively low ΔG H* values for H adsorption at the edges indicate that the catalytically active sites mainly sit at the domain edges of 1T-PtSe 2 . Besides, the lower Gibbs free energy values endow the monolayer 1T-PtSe 2 with excellent HER activity.
In 2017, Lin et al. proposed a facile strategy to synthesize edge rich PtSe 2 film with controlled edge density and make it possible to systematic study the relationship between the edge density and the HER performance [128]. A linear relationship between the edge density and the current density on the top surface of PtSe 2 film is established, as shown in Fig. 14e. As shown in Fig. 14f, the Tafel slope of PtSe 2 with different thickness ranging from 32 to 63 mV dec −1 can be found. The current density increases with the edge density increases, which suggested that the edge density plays a key role in enhancing the HER activity of PtSe 2 .
The HER performance of PtSe 2 has been experimentally and theoretically studied in the past few years. It has been revealed that the number of layers, edge density, and defect engineering play a key role in enhancing the HER activity   [197]. Copyright 2019, American Chemical Society. e Relationship between the current density and edge sites density on the top surface of PtSe 2 film and the corresponding f Tafel plots. e-f Reproduced with permission [128]. Copyright 2017, Elsevier 1 3 of PtSe 2 [38,142,[196][197][198]. However, the relationship between the structure, electronic structure, and HER activity of 2D PtSe 2 still is not elucidated, and the batch production of 2D metallic PtSe 2 is still not controllable enough in experimental.

Sensors
Sensors are a kind of integrated circuit devices that detect a specific physical parameter (gas, pressure, motion, moisture, etc.) and convert it to an electrical signal. Theoretical simulation is a very effective approach to analyze and predict gas sensing properties of 2D PtSe 2 materials. In 2017, Sajjad et al. [199] conducted a systematically theoretical study on the absorption of various gases molecules on monolayer PtSe 2 by using first principles calculations. The adsorption energy, relaxed height, charge density differences, and electronic structure of monolayer PtSe 2 with absorbed CO, CO 2 , H 2 O, NH 3 , NO, and NO 2 molecules were calculated, and the results indicate that sensors based on 2D PtSe 2 posse superior gas detection sensitivity. Chen et al. [200] investigated the response of a simulated monolayer PtSe 2 -based gas sensor to the five types of SF6 decompositions (HF, H 2 S, SO 2 , So 2 F 2 , and SOF 2 ) by using the first principles study. The sensor shows rapid and intense response to the SF6 decomposition molecular, and it could be controlled by regulating the bias voltage. Moreover, theoretical simulation suggested that the gas sensitivity of PtSe 2 can be further enhanced by the p-type dopants of Ge and As [201].
Besides the theoretical simulation, also some experimental achievements have been reported. Figure 15a shows the PtSe 2 film-based gas sensors and its response to periodic NO 2 gas [112]. As the PtSe 2 film exposed to a 100 sccm flow of NO 2 mixture with N 2 carrier gas, an immediately response time upon to 10 s was measured. The resistance change, transient response/recovery time as a function of NO 2 concentration at a certain exposure time was further tested. The sensors exhibit ultrafast response/ recover speed at room temperature. Moreover, 100 ppb of NO 2 can be detected at room temperature and the theoretical limit of detection is estimated to be a few parts per billion. The detection limit, sensitivity, responses/recovery time of 2D PtSe 2 gas sensors is much better than other 2D materials, such as graphene, MoS 2 , MoSe 2 , and MoTe 2 [202][203][204][205].
The unique structural and electronic properties of 2D PtSe 2 also make it a promising material for pressure sensors. As shown in Fig. 15b, c, centimeter-scale PtSe 2 films with thickness of 4.5 and 9 nm were synthesized and used to fabricate pressure sensors [122]. The sensitivity of the PtSe 2 film-based sensors can reach 1.05 × 10 −1 mbar −1 , which is much better than other low-dimensional materialsbased pressure sensors [206][207][208][209][210]. As shown in Fig. 15d, the piezo-resistive gauge factor of PtSe 2 film was measured by using a bending beam setup, and a negative gauge factor of − 84.8 was obtained for the PtSe 2 film. According to the DFT calculation in Fig. 15e, an increase in DOS at Fermi level is observed for the in-plane stretching and out-of-plane compression, leading to a decrease in resistance under the applied stains and ascribe to the negative gauge factor. Moreover, Boland et al. [41] further demonstrated that the growth temperature and thickness of the PtSe 2 film have a great effect to the performance of the PtSe 2 -based strain gauges. They found that the PtSe 2 -based pressure sensors show strong response to high frequency mechanical vibrations. By attaching a film to a speaker, a strong resistance changes of PtSe 2 /Polyimide film, with high signal-to-noise, is seen for to vibrations with frequencies of 95,190, and 380 Hz were observed. These achievements suggest PtSe 2 as a very promising candidate for future micro-and nanoelectromechanical systems applications.

Conclusions and Perspectives
During the last decades, the newly emerged 2D PtSe 2 has exhibited noticeable intrinsic nature and has experienced a remarkable development in theoretical and experimental. The most recent advances of 2D PtSe 2 including structure (crystal structure and electronic structure), properties (phase transition, vibration spectroscopic modes, and optical properties), synthesis methods (CVD, CVT, TAC, MBE, CVT, and sol-gel solution process), and potential applications (photodetectors, mode-locked laser, field effect transistors, photocatalytic, hydrogen evolution reaction, and sensors) are reviewed in this review. Although a tremendous progress has been achieved in the past few years, there are still some remaining especially for their practical application. Here, some major perspectives on the key challenges and the potential research directions are suggested to address these issues.
1. In order to fulfill the using demands for both fundamental studies and practical applications, more efficient and controllable synthesis methods should be developed. Previous study of graphene and TMDCs has inspired us that CVD is one of the most promising methods to grow 2D materials. However, the CVD growth of 2D PtSe 2 is still in its infancy.
More compressive works about CVD should be developed to grow high quality single crystal 2D PtSe 2 with controlled thickness, lateral size, and defects, which is prerequisite for further understanding the optoelectronic properties of PtSe 2 . Besides, in order to fulfill the demand of industrialization, highly efficient synthetic approaches should be proposed to synthesize a mass of high quality 2D PtSe 2 . 2. 2D PtSe 2 has been theoretically predicted to be a promising candidate to fabricate high-performance electronic and optoelectronic devices [211][212][213][214][215][216][217][218][219][220][221][222][223][224]. Although some pioneer works have been reported, the performance of 2D PtSe 2 -based devices are stills much lower than theoretical prediction. Due to the layer-dependent band gap of 2D PtSe 2 , photodetector based on 2D PtSe 2 may have excellent performance in a broadband from visible light to mid-IR. Vertical or lateral heterostructure based on 2D PtSe 2 may bring some novel properties, which have been proved in graphene and TMDCs. 3. Theoretical studies have demonstrated that the 2D PtSe 2 possesses excellent thermoelectric properties [67,225,226]. However, related experimental works are still lacking. The development of 2D PtSe 2 -based flexible film or nanostructured thermoelectric materials may provide great opportunities for fabricating highly efficient thermoelectric devices. 4. 2D Janus materials have attracted extensive attentions due to their unique structure, electronic, and optoelectronic properties [227][228][229]. The formation of Janus crystal structure broke the inversion and mirror symmetry, leading to an intrinsic built-in electric field. Janus monolayer 2D materials with sandwiched structure may induce remarkable influence on their carrier mobility, band gap, and optical properties. Theoretical simulations reveal that Janus monolayer PtSSe may have great potential in optoelectronics and thermal management communities. However, related experimental studies are still lacking. It is still challenging to synthesize Janus monolayer PtSSe materials and study their fundamental properties.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.