Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector

1. Introduction

Photodetectors, which can capture, identify and visualize the optical signals, have been indispensable devices in modern integrated electronics and communication technology [1, 2, 3, 4, 5]. Nowadays, various photosensitive materials have been investigated as the functional materials in photodetectors. For example, gallium nitride (GaN) is commercially for ultraviolet light detection (UV, <400 nm), Si for visible–near-infra (NIR, 400–1100 nm), InGaAs for NIR–mid-IR (MIR, 1–5 μm), and HgCdTe for MIR–far-IR detection (FIR, >5 μm) [2]. However, the ultra-miniaturization and integration of photodetectors with multi-materials are challenging, which require complex nanomanufacturing process and exorbitant production costs. In addition, there are some inherent disadvantages. For example, poor flexibility is a common problem in these conventional semiconductor materials, which restricts their application potential in flexible and wearable electronics. Some specific materials (e.g., HgCdTe) are environment toxic and cannot operated at room temperature [1]. The development trend for high-performance detection and different application scenarios prompts scientists to continue to pursue new materials with novel physical properties.

Two-dimensional (2D) materials have attracted tremendous attention in the past few decades [6, 7, 8, 9, 10, 11, 12]. Among them, 2D Transition-Metal Dichalcogenides (TMDs) are considered to be promising for next-generation optoelectronics due to the strong light-matter interaction, weak interlayer van der Waals (vdW) interaction, flexible characteristics and the ease of integration with current silicon-based optical electronics [13, 14, 15, 16, 17]. Group-10 noble TMDs (NTMDs) are outstanding representatives in the TMDs family [18, 19, 20]. The reintroduced new materials are generalized formulated by Group-10 noble elements (Pt, Pd, and so on.) and chalcogens (S, Se, or Te). Unlike traditional TMDs, the d-electrons in NTMDs are fully occupied their d-orbitals resulting in the highly hybridized P_z orbits and strong interlayer interactions [21, 22]. Therefore, NTMDs exhibit relatively small and widely tunable bandgaps compared with traditional TMDs (such as MoS₂ and WS₂). For example, PtS₂ shows a layer-dependent bandgap from 1.6 to 0.25 eV [21], while PtSe₂ changes from a typical semiconductor state (1.2 eV in 1 L PtSe₂) to semi-metal state when the thickness increases to over 5 layers [23]. Combining with the high mobility (>1000 cm²V⁻¹S⁻¹, larger than most other TMDs and comparable for that of BP) and environmental stability, NTMDs has great potential in photodetectors applications [21, 23, 24, 25]. Moreover, the unique puckered pentagonal structure of PdS₂ and PdSe₂ inherently provides them with anisotropic properties [26, 27, 28] and may promote the development of polarized photodetectors.

In this chapter, we first discuss the structural, electronic and optical properties of NTMDs. Then we focus on the NTMDs based photodetectors. Wafer-scale NTMDs films with high-quality and large-scale monocrystalline NTMDs nanosheets have been fabricated, which are appropriate for optoelectronic applications. NTMDs and their heterostructure based photodetectors show many advantages such as high-performance, ultrawide spectra detection, long-term environment stability, and anisotropic characteristics. NTMDs have great potential for large-scale imaging and flexible devices, which could be the next-generation optoelectronic core materials.

2. Structural and electronic properties of 2D NTMDs

The atomic coordination of monolayer TMD usually is either trigonal prismatic phase (2H or D_3h) or octahedral phase (1 T or D_3d), as shown in schematics in Figure 1a and b [22, 29, 30]. In 2H phase, the d orbitals in transition metal centers split into three degenerated d orbitals (dz2, dx2−y2,xyand dxz,yz) and there is usually an energy bandgap (~1 eV in TMDs) between the first two degenerated d orbitals. While in 1 T phase, the centers of transition metal have two degenerated d orbitals (dz2,x2−y2and dxz,yz,xy) [22, 29, 30]. Therefore, the thermodynamically favored phase is highly influenced by d electrons count in the transition metals. For NTMDs, the noble metal atoms have abundant d electronics and the d²sp³ hybridization is preferred, which lead to the full-filled d-bands. Most NTMDs have thermo-dynamically favored 1 T phase, such as PtSe₂, PtS₂, PtTe₂ and PdTe₂ (See Figure 1c) [31]. The strong interlayer hybridization of adjacent chalcogen atoms makes the widely tunable electronic energy band structure with the layer numbers. Here we use PtS₂ and PtSe₂ as examples. Both of them are 1 T favored phase, where the bandgap is about 1.17 and 1.6 eV in monolayer PtSe₂ and PtS₂, respectively (Figure 1e and f) [21]. With the increase of stacked layers, the interlayer hybridization would be stronger, with lead to the rapidly decrease of energy gap. According to theoretical calculations, the energy gap in bi-layer PtSe₂ is only 0.3 eV, while the stacked layered increase beyond 4 layers, the energy level of valence band maximum (VBM) will exceed that of conduction band minimum (CBM) and PtSe₂ undergoes a transition from semiconductor to metallic state (Figure 1h) [32]. Similarly, as shown in Figure 1i, the energy bandgap in bulk PtS₂ decreases to 0.25 eV from 1.6 eV.

Apart from conventional TMDs materials with hexagonal structures, PdS₂ and PdSe₂ consist of pentagonal rings with the puckered vertical conformation (Figure 1d) [26]. In each layer, a Pd atom binds to four chalcogen atoms other than six chalcogen atoms, while every two neighbor chalcogen atoms bind each other with a covalent bond. The unique pentagonal structure not only provides the materials with anisotropic properties, but also can realize the transition of topological quantum phase and the spin-orbit coupling enhancement. In 2017, Akinola O., et al. experimentally and theoretically prove that monolayer PtSe₂ has 1.3 eV indirect band gap and semi-metal state in the bulk (Figure 1g and j) [26].

3. Optical properties of 2D NTMDs

The widely tunable electronic energy gap of NTMDs make them layer-dependent optical absorption [32]. As shown in Figure 2a, PtSe₂ samples with thickness from 2.2 nm to 7.8 nm show broadband light absorption from 450 nm to over 3000 nm. The absorption peaks have significant red-shift with the increase of thickness, which is originates from the narrower energy gap in thicker samples. In particular, the semi-metal nature in thick PtSe₂ samples allows them absorb mid-NIR and even far-NIR light. In Figure 2b, all of these samples have broadband absorption in the range from 2 to 5 μm, which is different from tradition TMDs materials. The optical polarization properties of PtSe₂ were studied by polarized light imaging experiments. The optical responses of 2D PtSe₂ film almost unchanged under the incident channel with different rotation angle, which indicate the in-plane isotropic absorption of PtSe₂ (Figure 2c).

On contrary, due to the unique orthorhombic pentagonal structure, PdSe₂ shows anisotropic optical response in the van der Waals plane [33]. From the calculated optical conductivity spectra in Figure 2d and e, the cut-off energy in bulk PdSe₂ is lower than that in 1 L PdSe₂, and the conductivity curves in xx and yy direction in both bulk and 2D PdSe₂ perform very different characteristics. The anisotropic phenomenon appears at ~1.5 and 1.25 eV in bulk and 2D structure, respectively. The large anisotropy also be predicted at ~2 eV in monolayer PdSe₂. Second harmonic generation (SHG) polarization diagram is also performed for observing the anisotropic properties [34]. When the polarization direction of incident light and the crystal orientation are parallel (position of 0° and 180° in Figure 2f), the intensity achieves the maximum, while at the position of 0° and 180°, the SHG signal shows significantly decrease.

4. Synthesize of 2D NTMDs

In order to realizing the practical applications of the new kind of TMDs materials, the effective synthesis methods are essential to prepare particular samples with high crystallinity quality, desirable thickness and large lateral size. Up to now, various of synthesis strategies have been conducted to a variety of high-quality NTMDs. Here we do a general review on the different fabrication methods for NTMDs. Chemical vapor transport (CVT) and chemical vapor deposition (CVD) techniques are most two important methods for NTMDs which are applied to the following photodetector applications.

CVT method is a traditional crystal growth method, which is recently reintroduced for the direct synthesis of TMDs with high crystal quality [36, 37, 38]. The synthesis setup is as shown in Figure 3a [39]. Pt and Se powders with strict ratio are loaded in the quartz ampoule. After the vapor reactions with the help of a gaseous reactant under high temperature and vacuum, PtSe₂ crystals are formed and deposited elsewhere. By carefully adjusting the amounts of reactants and transport, Hu et al. successfully obtained triangular-shaped PtSe₂ flakes with 10–50 μm and good controllability [40]. From Figure 3d, the optical images exhibit that PtSe₂ nanoflakes have controlled layer numbers from 1-layer (1 L) to 20 layers (~20 L) and the atomic force microscope (AFM) images in insets provide the thickness information. The as-grown nanoplates with monocrystalline structure, controllable thickness and large lateral size are very suitable for electronic and photonic devices. Due to the ease of growing bulk crystals by CVT, people also use this method to grow high quality single-crystalline bulk NTMDs and obtain one- to few-layer 2D flakes by peeled from the bulk NTMDs crystal.

CVD is a very common synthesis method in which a large number of 2D materials with scalable size, controllable thickness and high-quality crystal structure have been prepared such as graphene, TMDs, Xene, MXene, boron nitride and so on [41, 42, 43]. Recently, the CVD method also be adopted for large-scale NTMDs fabrication. Figure 3b shows a CVD selenization method for scalable PtSe₂ films. The Pt film as seed were deposited on the substrate (usually the SiO₂ or Si wafer) at first and placed in the center of CVD furnace. The Se powder is at the upstream side. Then the direct selenization of the Pt film happens under high temperature, low pressure and argon gas protection. In 2015, Wang et al. firstly synthesized monolayer PtSe₂ nanosheets [44]. Then Han et al. obtained large area PtSe₂ film (> a few cm²) with controllable thickness [45]. Figure 3e shows the photographs of as-grown 2D PtSe₂ polycrystal films from 0.75 to 10 nm (corresponding to the layer numbers from 1 L to ~15 L). In 2018, Yuan et al. successfully fabricated PtSe₂-PtS₂ heterostructure film with wafer-scale and successfully achieved the wafer-scale photodetector application [46]. Besides, CVD method can also synthesize high-quality 2D NTMDs nanocrystals. Figure 3c exhibits a schematic of growing 2D nanosheets and through the method, Ma et al. successfully fabricated 2D PtTe₂ nanoplates with tunable thickness and a large lateral size up to 80 μm [47]. From Figure 3f, the high-angle annular darkfield scanning-TEM (HAADF-STEM) image as well as the EDS mapping analysis shows the well-faceted triangular geometry and the uniformly spatial distribution of Pt and Te elements. The Raman spectrum and High-resolution TEM (HRTEM) furtherly show the high quality of nanosheets and the 6-fold symmetry SEAD pattern shows the hexagonal crystal structure. Type-II Dirac fermions are observed in the high-quality nanocrystal platform. Another advantage of the grown method is that 2D materials can be grown on arbitrary substrates, because both the pre-deposition and post-selenization process do not have strict requirements to the substrate. Till now, 2D NTMDs have been fabricated on different substrates including Si, SiO₂, Sapphire, gallium nitride (GaN), fused quartz, and flexible polyimide.

There are some other synthesize ways for atomic TMDs. Mechanical exfoliation (ME) is one of the most extensively adopted approaches for 2D nanoflakes from their bulk counterparts [13]. Therefore, the as-prepared 2D flakes can maintain the intrinsic structure. Nowadays, most of mechanically exfoliated NTMDs thin flakes are from bulk crystals grown by CVT [48] and self-flux method [26, 49]. These typical nanosheets show the extraordinary electronic properties, but their small lateral size and uncontrollability during the fabrication process limit their application potential in practical devices. Molecular beam epitaxy (MBE) has also been applied for 2D NTMDs, including PtSe₂ [50], PdTe₂ [51] and PdSe₂ [52], which shows the merits of large-size monocrystalline, and controllable thickness on different substrates. For example, the high-quality PtSe₂ atomic film was epitaxial grown on bi-layer graphene/6H-SiC substrate through MBE method [50]. The as-grown film had controllable thickness from single-layer to over 22 layers, which shows extraordinary thickness-dependent electronic properties and tunable bandgaps.

5. 2D NTMDs for photodetection

So far, various NTMDs based photodetectors with diverse constructions and high-performance have been reported [53]. Table 1 summarizes their characteristic parameters. The strong optical absorption capability and large carrier mobility of NTMDs provide high responsivity (R) and detectivity (D*) for these photodetectors, while the narrow bandgaps of atomic layered PtS₂, PtSe₂ and PdSe₂ make them inherently suitable for NIR detection. For multi-layer NTMDs (over 5 L for PtSe₂), which can be regarded as semimetal materials, they can be combined with other semiconductor materials and construct Schottky heterostructures. By choosing a suitable semiconductor functional layer with a particular bandgap (such as n-Si, III − V, 2D perovskite, 2D MoS₂, and so on), the photodetector can work efficiently at a specifical wavelength. In addition, owing to the majority-carrier-dominant current-flow mechanism, photodetectors based on NTMDs heterostructures have advantages in high-speed applications. Combined with other electronic characteristics, different photodetectors with wide-spectral, fast-speed, self-powered and anisotropic have been realized. NTMDs based wafer-scalable and flexible photodetectors arrays could be the future development trend. We will comprehensively discuss them in this section.

5.1 2D NTMDs photodetectors

Due to the great electronic transport and optical properties of NTMDs, various of NTMDs based phototransistors have been studied [23, 39, 48, 54]. Here we illustrate a typical PtS₂ phototransistor as an instance [48]. The device schematic is as shown in Figure 4a, in which few-layer PtS₂ as the channel material on h-BN substrate. The device shows a high field-effect mobility of ~13 cm²V⁻¹S⁻¹ and the high on/off ratio of 10⁵. Then the photo-response ability under light illumination at visible wavelength (500 nm) is studied. Both photogenerated conductive and photo-gating effects are observed in the device. Figure 4b is the 3D diagram which shows the combined photocurrent with incident light intensity and gate voltage (V_g). By calculation, the detectivity and responsivity are obtained with the function of V_g (Figure 4c). when V_g is zero, the responsivity is highly at 1560 AW⁻¹, which shows 10⁶ times higher than that of graphene and 10³ times higher than that of BP detectors (~0.5 and 657 mAW⁻¹, respectively). Similarly, the detectivity (D*), as the inverse of the noise-equivalent power and the key parameter related to the signal-to-noise rate of the device, reaches 2.9 × 10¹¹ Jones, which is also higher than that of other 2D-based devices (Figure 4c). The photo-gain is about 2 × 10⁶ at 30 V of V_g, which could be the highest gain in 2D-based photodetectors. The few-Layered PtS₂ phototransistor shows that NTMDs is outstanding candidate in photodetection area at visible wavelength range.

Mid-IR optoelectronics is fantastic and important because there is an optical transparent window at Mid-IR in the atmosphere. However, in traditional TMDs based photodetectors, it is very difficult to realize the effective detection at Mid-IR. NTMDs can overcome the difficulty due to the narrow bandgap. Yu et al. fabricated a bi-layer PtSe₂ based phototransistor, which can realize wide-spectral and sensitive detection from 632 nm to 10 μm [39]. As shown in Figure 4d and e, the time-resolved photo-response results are obtained at 632 nm, 1.47 μm and 10 μm, with photo- responsivity of 6.25 AW⁻¹, 5.5 AW⁻¹ and 4.5 AW⁻¹, respectively. The achieved photocurrent responsivity at 10 μm is 3 orders of magnitude higher than that of graphene and comparable to commercial mid-IR detectors. The rise and fall time are also better than other TMDs based photodetectors owing to the high mobility of PtSe₂. Overall, bilayer PtSe₂ shows promising potential in mid-IR optoelectronic applications.

For anisotropic detector applications, Liang et al. adopted PdSe₂ as the photosensitive material [54]. The photodetector shows effective photo-response covering from 532 nm to 4.05 μm. The responsivity is 708 AW⁻¹, which is five orders larger than graphene and two orders larger than commercial InGaAs near-IR photodetectors. Furthermore, with the unique pentagonal structure of PdSe₂, the detector shows anisotropic photo-response for the linear-polarized light with varying polarization angle. In Figure 4f, when the increase of rotation angle with the step of 15° from 0° to 360°, the photocurrent clearly shows periodical variation and reaches the maximum value at 120° and 300°, which is coincident with the angle-resolved polarized Raman response results, furtherly showing the lattice effects. The anisotropic detectors as linear dichroism media have potential in optical communication and structural chemistry analysis.

Overall, with the first realization of PtSe₂ photodetectors in 2016 [23], Various of NTMDs and their photodetection abilities are studied, which show great performance. Till now, NTMDs based photodetectors exhibit higher responsivity and photo-gain than that of graphene, conventional TMDs and other 2D photodetectors. The work wavelength has been extended to 10 μm and the anisotropic detection has also been realized. With the development of NTMDs synthesis technique, the optimization of device structure, and the study of NTMDs photo-current mechanism, the narrow bandgap material will be the excellent candidate in the field of photodetection.

5.2 2D NTMDs heterostructures for photodetection

NTMDs with widely tunable energy gaps and high carrier mobility have broad prospects in developing high-performance photodetectors. However, the ultrathin thickness nature makes 2D NTMDs relatively low light absorption. Constructing NTMDs based heterostructures can not only enhance the light absorption, but accelerate the separation and transmission of carriers, and invent the high-speed photodetectors. Therefore, different NTMDs heterostructures have been studied for fast, broadband, self-powered and polarization-sensitive photodetectors.

Due to the atomic thickness, NTMDs is very convenient to from heterostructures with conventional semiconductors such as Si [55], n-Si [56, 57], Ge [58, 59], GaAs [60], GaN [61] and CdTe [62]. Few-layer PtSe₂ is semimetal state. By choosing p-doped bulk semiconductors with appropriate work function and bandgaps and contacting them with Few-layer PtSe₂ layer, the Schottky junction will be formed. The detection wavelength is determined by the bulk semiconductor. Zeng et al. fabricated the PtSe₂-GaAs vertical heterostructure detector [60]. The device schematic and the photocurrent generation mechanism are depicted in Figure 5a. Under the light illumination, the electron–hole pairs forms at the interface of the heterojunction, then separates with the function of in-built electric field. The photocurrent generates and gathered by two electrodes. The device shows the broadband work wavelength from200 to 1200 nm and a large photo-response at visible wavelength (Figure 5b). The responsivity and specific detectivity reach to 708 mAW⁻¹ and 2.9 × 10¹² Jones at 808 nm, respectively. Moreover, the device achieves the fast response speed, in which the rise and fall time are only 5.5 and 6.5 μs (Figure 5c). By choosing the semiconductor layer with relatively large energy gap, e.g., GaN, the deep-UV photodetectors can be realized [61]. The self-powered PtSe₂-GaN phototransistor has the responsivity of 193 mAW⁻¹, an ultra-high specific detectivity of 3.8 × 10¹⁴ Jones and a fast response time of 45.2/102.3 μs at zero gate voltage. In particular, the calculated linear dynamic range (LDR) exceeds 155 dB, which much higher than all reported 2D based detectors and commercial photodetectors. For infra-wavelength application, Wang et al. designed a near-infrared light photovoltaic detector by constructing few-layer PtSe₂-Ge heterostructure [59]. Since the device works at photovoltaic region, the self-start operation can be realized without any external power supply. The device also has high responsivity (602 mAW⁻¹ at 1550 nm, closed to that of commercial device) and long environment stability. Then Wu and the co-workers designed the improved graphene-PdSe₂-Ge heterostructure (Figure 5d) [58]. With graphene as a transport and protector layer, the device has great stability and can realize the imaging application. In particular, with continuedly working over 5000 cycles, the photo-response still remains stable, showing the practical application potential (Figure 5e). Due to the particularity of PdSe₂, the device can achieve the dipole anisotropic operation (see Figure 5f).

Perovskite is also an emerging material with a large absorption coefficient, long diffusion length and low trapping density, which has aroused extensive research interest in optoelectronics. Zhang et al. reported a new type of detector based on few-layer PtSe₂ and FACsPbI₃ perovskite heterostructure [63]. The device has broad spectra response from 300 to 1200 nm, with the responsivity of 117.7 mAW⁻¹, high I_light/I_dark ratio of 5.7 × 10³ and considerable specific detectivity of 2.6 × 10¹² Jones. Especially, due to the extraordinary electronic properties of PtSe₂ and the perovskite and the well-designed built-in electric field at Schottky junction interface, the response time is only 78/60 ns, which is one of the fastest reported values in mixed-dimensional 2D-3D van der Waals heterostructures. Zeng and the co-workers chose PtSe₂ to construct heterojunction with FA_1 − xCs_xPbI₃ perovskite film, which can realize the self-powered detection operation from 200 to 1550 nm [64]. The device demonstrates high responsivity, large on/off ratio, a good polarization sensitivity over 10⁴, and reliable imaging application at 808 nm.

The heterostructure between NTMDs with other 2D materials is also fantastic. Here we use PdSe₂-MoS₂ heterostructure as an example [65]. Both of PdSe₂ and MoS₂ are multilayer flakes with thickness of ~10 nm. The ultra-thin device can not only a ultrawide spectra working range from 532 nm to 10.6 μm, but contributes an ultrahigh responsivity of 42.1 AW⁻¹ at 10.6 μm.

5.3 Perspective of 2D NTMDs in photodetectors

Due to the industrial demand and the inherent advantages of 2D materials, the development trend of 2D optoelectronics is scalability and flexibility. Yuan et al. has realized the fabrication of wafer-scale PtS₂- PtSe₂ heterojunctions and devices [46]. They pre-deposited 0.8 nm Pt films as arrays of periodic square, then directly grew PtS₂ and PtSe₂ 2D thin films by CVD method. Figure 6a is the photograph and Figure 6b shows the schematic illustration of one single device. The photodetector array can work from 405 nm to 2200 nm. The ultrathin device has a large external quantum efficiency (EQE) (1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm). The response time is several milliseconds. If the quality of thin film is improved, the response time could be faster. The scalable devices can be adopted for high-resolution imaging, as shown in Figure 6c.

For the study of NTMDs flexible devices, Su and the co-workers did the pioneer work [66]. PtSe₂ thin films with 2.5 nm thickness (~3 L) on flexible polyimide substrate were directly grown by plasma-assisted selenization process, which show p-doped semiconductor behaviors and the average field effect mobility of 0.7 cm²V⁻¹S⁻¹. Figure 6d shows the array of devices with the finger-type electrode structure. The flexible photodetector shows good photoresponse with responsivity of 0.4, 0.25 and 0.1AW⁻¹ at 408, 515 and 640 nm, respectively (Figure 6e). Moreover, the great mechanic stability is exhibited. Under large bending with different radius over 1000 cycles, the device can still generate stable photocurrent with almost no degradation, which is depicted in Figure 6f.

6. Conclusions

In this chapter, we focus on 2D NTMDs and their applications in the field of photodetectors. 2D NTMDs exhibit extraordinary structural, electronic and optical properties. Unlike conventional TMD materials, the emerging NTMDs with abundant d-electrons and strong interlayer electronic hybridization have broadband optical absorption and ultra-high mobility, which are promising in optoelectronics. Then we have discussed efficient and controllable synthesis methods for 2D NTMDs with high crystal quality and large scalability. Various NTMDs based photodetectors have been developed till now. We have witnessed their outstanding performance, including wide-spectral range, ultrafast response, self-power and anisotropy. With the development of the materials technology and device manufacturing technology, NTMDs will have great potential in practical optoelectronic applications.

Acknowledgments

This work is supported by the Starting Research Funds from Songshan Lake Materials Laboratory, China (No. Y0D1051F211). Dr. Jian yuan acknowledges the support from Natural Science Research Project for Anhui Universities (grant no. KJ2019A0596), and Youth Project of Provincial Natural Science Foundation of Anhui (grant no. 2008085QF319).

Conflict of interest

The authors declare no conflict of interest.