Substantially Enhanced Properties of 2D WS2 by High Concentration of Erbium Doping against Tungsten Vacancy Formation.

Doping in 2D materials is an important method for tuning of band structures. For this purpose, it is important to develop controllable doping techniques. Here, we demonstrate a substitutional doping strategy by erbium (Er) ions in the synthesis of monolayer WS2 by chemical vapor deposition. Substantial enhancements in photoluminescent and photoresponsive properties are achieved, which indicate a tungsten vacancy suppression mechanism by Er filling. Er ion doping in the monolayer WS2 is proved by X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS), fluorescence, absorption, excitation, and Raman spectra. 11.5 at% of the maximum Er concentration is examined by energy dispersive X-ray spectroscopy (EDX). Over 6 times enhancement of intensities with 7.9 nm redshift in peaks are observed from the fluorescent spectra of Er-doped WS2 monolayers compared with their counterparts of the pristine WS2 monolayers, which agrees well with the density functional theory calculations. In addition, over 11 times of dark current, 469 times of photocurrents, photoresponsivity, and external quantum efficiency, and two orders of photoresponse speed are demonstrated from the Er-doped WS2 photodetector compared with those of the pristine WS2 device. Our findings prove rare-earth doping in 2D materials, the exciting and ideal technique for substantially enhanced photoluminescent and photoresponsive properties.

1. Introduction

The study of photoluminescent and photoresponsive properties in two-dimensional (2D) semiconductors has attracted remarkable scientific and technological interests [1-5]. Among which, transition metal dichalcogenides (TMDs) with over 40 types of metal and chalcogen combinations is one of the mostly focused family due to their thickness-dependent band gaps from visible to near-infrared [6, 7], theoretically high switching ratios [8, 9], carrier mobilities [10, 11], and photoresponsivities for applications in photoelectric and electric fields [12-16]. As the first representative TMDs, 2D MoS2 is the mostly characterized material. Besides MoS2, WS2 is possibly more attractive due to its 20 times higher of photoluminescent efficiency, better thermal stability, larger valence band splitting, and theoretically reduced effective carrier masses [17-19]. However, a huge gap exists between the theoretical predictions and the experimental performances [20]. For instance, the measured mobilities and photoresponse time of monolayer WS2-based field-effect transistors (FETs) are typically in the range of 1-50 cm2/V∙s and 5-2000 ms, respectively [21-25], which are far less than the predicted mobility of >500 cm2/V∙s and photoresponse time of <100 ns, respectively [17, 26–29]. The reduced performances are partly attributed to the unoptimized device fabrications and structural designs [30]. The essential reason may due to the defect-induced trap states that produced in the synthesizing process [31], since electronic states at energy band edges and defects in 2D WS2 are significantly more active and especially fatal to its photoluminescent and photoresponsive performances compared with their bulk counterparts. 2D WS2 prepared by top-down approaches usually have better crystalline quality but with uncontrollable thicknesses and small sizes [32]. For practical applications, wafer-scale synthesis of homogeneous monolayer WS2 is required. In this regard, chemical vapor deposition (CVD) is extensively employed [22, 23, 33, 34]. Nevertheless, from the perspective of the second law of thermodynamics, a variety of defects are inevitably generated during CVD synthesis. The typical type of intrinsic defects comes from the missing atoms, including transition metal vacancies and chalcogen vacancies, which has been observed in many kinds of 2D TMDs [35-38]. The photoluminescence quantum yields of CVD-grown TMDs are only 0.01–6% of the mechanically exfoliated samples due to the high density of vacancy defects [39]. As the most defective CVD grown 2D TMDs, WSe2 shows 1.48 at% of Se vacancy, but the density of transition metal vacancies in TMDs and the corresponding influences on their photoelectric performances are still unknown [40]. Many methods, including high-temperature treatment, chemical reduction, and ultraviolet irradiation, have been proposed to reduce the chalcogen defects, while approach to lower the transition metal defects is still absent [41, 42]. Hence, it is striking to make clear of the impact caused by transition metal defects and find a solution to effectively reduce the metal defects in 2D semiconductors.

Chemical doping is usually employed to tune the functionality of semiconductors through doping-type and band structure modulations [43-46]. Rare-earth (RE) ions are known to have complicated energy levels due to their unique intra-4f electronic transitions [47], which is commonly doped in traditional insulator or semiconductor hosts to provide abundant excited energy levels, and enable the extension of their absorption and excitation spectrum width, enhancement of their quantum yields, photostabilities, and Stokes shifts [48, 49]. Recently, substitutional doping of TMDCs via CVD has been reported, and ferromagnetism characteristics has achieved [50, 51]. However, RE in situ doping strategies in 2D materials by CVD method are still in the early stage. The mechanism and impact of the RE substitution on the structural, electronic, and photonic properties of 2D monolayers are far from being answered [52].

Herein, a substitutional Er ion doping strategy in synthesis of large scale of 2D WS2 by CVD is proposed. By adding excessive Er2O3 into the tungstic acid (H2WO4) as the tungsten source, erbium ions are successfully in situ doped into the WS2 monolayer with a high concentration of 11.5 at% examined by energy dispersive X-ray spectroscopy (EDX). The high doping concentration may relate to the high vacancy density of tungsten in WS2 synthesized by CVD, and similar ion radii between Er and W ions in the WS2 matrix, which make it convenient for substitutional doping. Strong characteristic peaks contributed from erbium ions are observed both in the X-ray diffraction spectra, X-ray photoelectron spectra, and Raman spectra. The photoluminescent and photoresponsive performances are comprehensively and substantially enhanced by erbium doping compared with the counterparts of the pristine WS2 monolayers fabricated by the same method. Over 6 times of fluorescent intensities are observed in Er-doped spectra from 20 samples. Eleven excitation peaks from 761 nm to 1011 nm are found from the excitation spectra of Er-doped WS2 monolayers; those are nonexistent in the pristine WS2 monolayers. The fluorescence and absorption enhancement are well agreed with the calculation results of density functional theory (DFT). In addition, the Er-doped WS2 photodetectors show over 11 times of dark current, 469 times of photocurrent, photoresponsivity, and external quantum efficiency, and two orders of photoresponse speed compared with those of the pristine WS2 photodetectors fabricated and measured under the same course of processing. The combined multiple characterizations and theoretical calculations indicate that extra lattice distortion is not introduced by erbium doping, but the density of tungsten vacancy is effectively reduced leading to a reduced density of surface trapping states. In the meantime, significant higher density of carriers is provided by the introduction of substitutional erbium doping in the monolayer WS2 matrix.

2. Methods

2.1. Synthesis of WS2 via CVD Process

The preparation of 2D WS2 is carried out in quartz tubes with 2-inch diameter in double temperature furnaces, as is schematically shown in Figure 1(a). 3 × 3 cm2 of SiO2/Si wafers with 300 nm thickness of SiO2 is used as the substrates. 150 mg sulfur powder (>99.5%) is weighed as the precursor, tungstic acid (H2WO4) powder (>99.5%) with the dosage of 65 mg is used as the tungsten source, and 4-8 mg of sodium chloride (NaCl) powder (>99.99%) is used as the growth promoter. 13 mg of Er2O3 powder (>99.99%) is used as the RE dopant for the preparation of Er-doped WS2 membranes (WS2(Er)). The H2WO4, Er2O3, and NaCl powders are mixed sufficiently before being placed in the quartz boats. The SiO2/Si substrates are put directly face down above the quartz boat with a distance of 10 mm between the substrate and the powder.

Figure 1

WS2(Er) and WS2 membranes fabricated by chemical vapor deposition (CVD) method. (a) Schematic illustration of the experimental setup for synthesis of WS2 and WS2(Er) films. (b) Temperature curve and gas flow control in the double temperature zones. (c) Typically synthesized triangular WS2(Er) membrane with the side length over 1 mm. (d) Thickness measured by AFM of the sample in (c). (e) Centimeter scale of WS2(Er) film with indefinite morphology. (f) Thickness measured by AFM of the sample in (e).

In the beginning, the CVD chamber is vacuumed (<5 Pa), and argon gas (UHP, >99.999%) is fed subsequently at a rate of 300 sccm to reach a standard atmospheric pressure in the quartz tube. The temperature curve of the synthesis process is shown in Figure 1(b). In the meantime, the second temperature zone is heated to 150°C at a rate of 5°C/min, and then held for 5 min. The CVD chamber is vacuumed again during the heating process (<5 Pa), and then, the argon gas is fed at a rate of 300 sccm to reach a standard atmospheric pressure in the tube. The purpose of this step is to remove the impurity gases and desorbed dusts generated by heating of the quartz chamber. Subsequently, the first and second temperature zones are heated to 200°C and 850°C (Tg) in 40 minutes, respectively. The synthesis of WS2 is finished in 10 min in an argon-hydrogen mixture gas (UHP, Ar 90%, H2 10%) flow at a rate of 60 sccm. Finally, the samples are naturally cooled to room temperature in a 60 sccm of argon flow.

2.2. Characterization of the Pristine and Er-Doped WS2 Membranes

The surface morphologies of the pristine and Er-doped WS2 samples are characterized by optical microscope (50i POL, Nikon) and energy dispersive spectrum (EDS) in SEM (JSM-7800F, JEOL), and the roughness and thickness of WS2 samples are characterized by atomic force microscope (AFM, Dimension EDGE, Bruker). The Raman spectra of WS2 and WS2(Er) samples are measured by a laser confocal Raman spectrometer (InVia Reflex, Renishaw) with a continue wave (cw) laser wavelength of 532 nm. X-ray diffraction spectra (X'Pert3 Powder) of the samples are performed to confirm the diffracted characteristic peaks of the doped erbium ions. Photoluminescence (PL) spectra are measured by a laser scanning confocal microscope system excited at 532 nm. A spherical aberration correction TEM (Titan Cubed Themis G2 300) is used for the measurement of HAADF-STEM and energy dispersive X-ray spectroscopy (EDX) to characterize the crystalline structures and elemental compositions of the WS2(Er) samples.

3. Results and Discussion

By slightly adjusting the weight of NaCl, up to 1 mm of triangular shape and even centimeter-scale of Er-doped WS2 membranes can be synthesized by the CVD method. Typical optical micrographs of the WS2 samples are shown in Figures 1(c) and 1(e), and their corresponding thicknesses are measured to be nearly 1 nm by AFM, which is consistent with the reported thickness of monolayer WS2, as shown in Figures 1(d) and 1(f) [53, 54]. The pristine WS2 are synthesized under the same conditions without Er2O3 as a dopant. Relevant experimental details can be found in S1 (Supporting Information).

Compared with surface chemical doping approaches, substitutional doping involves covalent bonding and is a more stable doping technique. Caption substitutional doping in TMDs, including niobium, rhenium, erbium, and holmium, has been demonstrated to tune their electronic properties in recent report [45, 52, 54, 55]. However, the doping concentrations are usually low (typically ≦1 at%), and the PL intensities even decreased. To get the maximum Er doping concentration, excessive dosage of Er2O3 is added into the tungsten source with a weight ratio of 1 : 5 to the H2WO4 powder, but the exact concentration of Er ion in the WS2 film is still unknown. SEM and EDS are firstly used to evaluate the [Er] concentration. Figure 2(a) and Figure 2(b) show the SEM image and selected area elemental analysis by EDS of the Er-doped WS2 film. The insert chart in Figure 2(b) shows the elemental type, weight, and atomic ratio of the WS2 film on SiO2/Si substrates measured by EDS in SEM. Besides high contents of [O] and [Si] from SiO2 substrates, there are still tiny amount of [S], [W], and [Er] which are examined from the EDS spectrum. Compared with the atomic ratio of [W], the atomic percentage of [Er] to [W] is close to 10%. The high content of [C] may come from the remnant of organic cleaning. To further confirm the concentration of erbium ions in the WS2 film, the WS2(Er) sample is characterized by high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) with the correction of spherical aberration, as shown in Figure 2(c). A crystalline plane spacing of 0.267 nm can be measured between two immediate lines along the (100) direction, which is consistent with the results of monolayer WS2 [22]. Because the atomic number of [Er] (No. 68) is close to but a little smaller than [W] (No. 74), and their ion radius in WS2 crystalline is similar, it is difficult to distinguish [Er] locations from the HAADF-STEM image of WS2 film by brightness or ion size. But for the same reason, it is also reasonable to infer that minimal crystalline distortions are induced in the WS2 monolayer by Er doping. Substitutional doping of caption ions with large differences in ion radius usually induces large lattice distortions, which in principle leads to additional surface defects. Surface trapping states caused by these defects will further degenerate material properties, including quantum yields, carrier mobilities, photoresponse speed, and especially fatal for 2D materials. Figure 2(d) shows the EDX image of the WS2(Er) sample on TEM Cu grid; a weak Er-M peak implies the existence of doped [Er] atoms with the Er atomic ratio of about 11.5 at% in WS2 analyzed by the system, as shown in Figure 2(i). This atomic ratio is close to the result of about 10 at% measured by the EDS-SEM for the sample on SiO2/Si substrate. A strong [O] peak is also found in the EDX image, which may come from the residue of PMMA or organic contaminations from organic cleaning of the substrates, since PMMA is used to transfer the WS2(Er) membrane onto the TEM Cu grid. The selective area electron diffraction (SAED) diagram of the Er-doped WS2 film shows six diffraction points on the diffraction pattern, which corresponds to tungsten and sulfur sublattices, respectively, indicating the intact hexagonal symmetry of the present WS2 film after high concentration of Er doping (Figure 2(e)). Additionally, elemental mapping images (Figures 2(f)–2(h)) show the homogeneous distribution of [S], [W], and [Er] in the sample, which also suggests that Er atoms are successfully doped into the WS2 matrix.

Figure 2

Microscopic analysis of WS2(Er) monolayer by SEM and TEM. (a) SEM and (b) EDS image of a WS2(Er) membrane synthesized on a SiO2/Si substrate. The inserted table in (b) shows the atomic ratio measured by EDS in a selected rectangular region in (a). (c) WS2(Er) membrane on Cu grid characterized by HAADF-STEM with the correction of spherical aberration. (d) EDX image of the WS2(Er) membrane measured in (c). (e) SAED pattern of the same WS2(Er) membrane. (f), (g), and (h) are the elemental mapping image of [S], [W], and [Er] in the WS2(Er) membrane measured by the HAADF-STEM system, respectively. (i) Atomic fraction of [S], [W], and [Er] elements in the WS2(Er) matrix analyzed by the HAADF-STEM system.

Subsequently, Raman, X-ray diffraction, X-ray photoelectron, photoluminescence, and absorption spectra of the prepared WS2(Er) samples are performed, and the results are compared with the counterparts of the pristine WS2 samples. Figure 3(a) presents Raman spectra of the prepared WS2 and WS2(Er) films under 532 nm laser excitation. The WS2(Er) sample shows almost equal peak intensity in the out-of-plane vibration mode (A1g(Г)), but two times intensity of the in-plane vibration mode (E2g(Г)) compared with that of the pristine WS2 sample. Besides, 1.82 cm−1 and 1.2 cm−1 of redshifts are observed in the E2g(Г) and A1g(Г) from the WS2(Er) sample, respectively. Both of these phenomena indicate the possible effects of Er doping in WS2 matrix. Because intensity of the A1g(Г) mode strongly depends on the layer thickness, the nearly equal intensities of the A1g(Г) mode indicate little thickness difference between these two samples. On this basis, higher defect density usually leads to the lower crystalline symmetry and thus to the lower of E2g(Г) intensity. Two times intensity enhancement of the E2g(Г) mode possibly indicates the lower defect concentration in the WS2(Er) sample. In addition, a Raman peak located at 384.9 cm−1 is obviously seen from the WS2(Er) sample but nonexistent in the WS2 sample. This peak is very likely contributed from the Tg mode of Er ion [56].

Figure 3

Comparison of different spectra between WS2 and WS2(Er) monolayer. (a) Raman spectra, (b) X-ray diffraction spectra, and (c) PL spectra of the CVD synthesized WS2 and WS2(Er) membrane measured in the same conditions for comparison. 20 PL spectra of WS2 and WS2(Er) triangles on SiO2/Si substrates are measured under the same conditions and shown in (c). (d) Distribution of the intensity and wavelength of the PL peak in (c) for statistical analysis. (e) Absorption spectra of WS2 and WS2(Er) membranes measured from 1300 nm to 2000 nm. (d) High-resolution XPS spectra of Er 4d for WS2(Er) and pure WS2 membranes for comparison.

X-ray diffraction spectra are measured on WS2 and WS2(Er) films to qualitatively confirm the successful doping of Er ions. Because Bragg diffraction effect is weak on a monolayer sample, thick films with large sizes are required in this measurement. In the meantime, concentration of Er ions in the WS2 matrix is not dependent on the thickness and size of the films. Thus, WS2 and WS2(Er) samples with about 10 nanometer in thicknesses and several millimeter in sizes are used in this measurement. Figure 3(b) shows the X-ray diffraction spectra of the WS2 and WS2(Er) samples. The WS2 (101) and WS2 (104) diffraction peaks are observed in both of the two type of samples. Meanwhile, a strong extra peak is discovered in the WS2(Er) sample located at the position of 2θ = 14.4°, which is highly consistent with the X-ray diffraction peak of Er2S3 (201). The strength ratio between this peak and WS2(101) peaks is 0.51, further indicating the high concentration of Er doping in the WS2(Er) sample.

PL spectra of the two types of monolayers are performed under 532 nm laser excitation. Interestingly, over 6 times enhancement of the PL intensity and 7.9 nm redshift of the peak wavelength are achieved in the WS2(Er) monolayer. No obvious broadening is observed by comparing FWHM of the PL peaks from WS2(Er) and WS2 monolayers, indicating that very slight energy transfer occurs after doping. To better identify the spectral change, the same experiments are carried out on 20 WS2 and WS2(Er) triangles, respectively, and similar results are obtained as shown in Figure 3(c). Their statistical variations of the PL intensities and peak positions are shown in Figure 3(d). The small variations in the intensities and peak positions of the same type of samples may due to the fluctuation of laser power densities and slight inhomogeneous thicknesses. However, the significantly enhanced PL intensities and redshifts of the peak positions between the WS2 and WS2(Er) samples should originate from the Er doping. The possible reasons may lie in three factors: Firstly, the formation of tungsten vacancy in the CVD synthesis is probably suppressed by the doped Er ions and thus the suppression of nonradiative recombinations. It is well known that tungsten transition metal sulfides usually have much higher of vacancy density compared with that of the molybdenum transition metal sulfides. The quantum efficiency increases substantially as the tungsten vacancy is effectively reduced by the Er ion filling. Secondly, additional defects caused from crystalline distortion by the introduction of Er ions into WS2 matrix is negligible, which requires similar ion radii between Er and W in the WS2 crystalline. Thirdly, the energy bands become more rich and active by the introduction of Er ions, since complex and rich electronic levels exist in Er ions. The 7.9 nm of photoluminescent redshift between WS2 and WS2(Er) samples implies the occurrence of shallow impurity level transitions in the PL process. Figure 3(e) shows the absorption spectra from 1300 nm to 2000 nm of the WS2(Er) and pure WS2 films on SiO2/Si substrates measured by an infrared spectrum imager (Spotlight, Platinum Elmer). It is obvious that the WS2(Er) film shows peak absorption coefficiency at 1314 nm and with much higher of absorption coefficency compared with that of the WS2 film from 1300 nm to 1800 nm. This result is highly consistent with the calculated absorption characteristics based on DFT as shown in Figure 4(d). Furthermore, X-ray photoelectron spectra (Thermo scientific K-Alpha+) are performed on both of the two films. A strong Er 4d peak is seen to be located at 169.34 eV as shown in Figure 3(f), which is assigned to Er3+ [57].

Figure 4

First principle calculations on the S-vacancy, W-vacancy, and W-vacancy filled by Er of a 3 × 3 × 1 WS2 supercell. (a) Schematic illustration of the S-vacancy, W-vacancy, and W-vacancy filled by Er of WS2 matrix. (b) Energy band structures corresponding to the S-vacancy, W-vacancy, and Er-doped conditions of WS2 supercell described in (a). Weighted bubble chart is used and displayed by the red dotted lines representing the band structures of vacancy and Er-doped WS2 monolayers, while the black solid curves represent band structure of the protocell WS2. (c) PDOS of monolayer WS2 under the S-vacancy, W-vacancy, and Er filled in W-vacancy conditions, respectively. (d)–(i) Dependences of absorption index, refractivity index, extinction index, reflectivity, energy loss, and dielectric constants, respectively, on the incident photon energy (from 0 to 1.8 eV) of the pristine WS2 and WS2(Er) monolayer.

To further confirm our deduction, the first principle calculations based on the density functional theory (DFT) are performed on the S-vacancy, W-vacancy, and W-vacancy filled by Er of WS2 monolayers, respectively. Electronic band structures and partial density of states (PDOS) of these WS2 monolayers are calculated by generalized gradient approximation (GGA) method. Because 11.5 at% of Er is measured in the EDX spectra, 3 × 3 × 1 supercell is used in the calculation corresponding to 11 at% of [Er] concentration to [W] atoms. Band unfolded technique and weighted bubble chart are applied for the vacancy and Er-filled band structures. The calculation details are described in the supporting information S2. Figure 4(a) shows the schematic images of the S-vacancy, W-vacancy, and Er filled in W-vacancy of monolayer WS2, respectively. Figure 4(b) shows the energy band structures corresponding to Figure 4(a). The weighted red bubble charts in Figure 4(b) represent the energy bands of the corresponding vacancy and Er-filled WS2 monolayers, while the black curves represent the energy bands of the WS2 protocell for comparison. For S-vacancy WS2 monolayer, the energy bands change slightly compared with the protocell WS2, except two deep defect energy levels with low weight in the forbidden band. High weight of valance band maximum (VBM) and conduction band minimum (CBM) exist in the high symmetric K point in Brillouin zone, which indicates a relatively small impact of the S-vacancy on the direct transition. For W-vacancy WS2 monolayer, the energy bands change to more complicated. Multiple valance levels extend to the center of the forbidden band with uniform of low weight, and the VBM shows very small weight in theKpoint; thus, a very low possibility of carrier exists in theKpoint, implying that a very low possibility of direct transition occurs in theKpoint. Furthermore, when W-vacancy is filled by Er ion, the VBM moves upward in the K point with much higher of weight, indicating much higher of possibility for direct transition, and thus the enhancement of the PL efficiency with a redshift. In comparison of these three conditions, S-vacancy in the WS2 monolayer leads to relatively small impact on the PL efficiency and minute peak shift. W-vacancy degenerates the PL efficiency more seriously than S-vacancy condition. When Er filled in W-vacancy, the PL intensity is enhanced greatly together with a redshift in the PL peak, which well interprets the experimental results as mentioned above. Figure 4(c) shows the PDOS of the S-vacancy, W-vacancy, and Er-filled WS2 monolayer. Figures 4(d)–4(i) show the dependences of indexes, including refractivity, extinction, reflectivity, energy loss, and dielectric constants, respectively, on the incident photon energy of the pristine WS2 and WS2(Er) monolayer. Notably, the WS2(Er) sample shows much higher of absorption, extinction, energy loss, and imaginary part of dielectric indexes in the infrared region centered at 0.85 eV, while its refractivity, reflectivity, and real part of dielectric indexes show amplitude exchanges in the infrared region centered at 0.85 eV compared with the pristine WS2 monolayer. To understand the effects of Er doping with lower concentrations, 4 × 4 × 1 and 5 × 5 × 1 supercells of the S-vacancy, W-vacancy, and Er filled in W-vacancy of WS2 monolayers are also calculated and discussed in detail in supporting information S2.

To understand the effect of Er doping in photoluminescence under different excitations from ultraviolet to infrared, the excitation spectra excited under 250 nm and 850 nm on the WS2 and WS2(Er) samples are performed at 77 K, as shown in Figures 5(a) and 5(b), respectively. When the samples are excited at 250 nm, the WS2(Er) sample exhibits higher fluorescence intensity from 500 nm to 1100 nm. In particular, 11 strong and sharp PL peaks are apparently observed from 761 nm to 1011 nm in the WS2(Er) sample, but nonexistent in the WS2 sample. The appearance of these peaks is undoubtedly due to the impurity levels in the forbidden band introduced by the doped Er ions, especially the Er intra-4f levels, which has strong localization characteristics, stable optical excitation, and recombination process. The 11 sharp peaks are probably originated from the transitions from 4F9/2 to the ground state of 4I15/2 (736 nm), from 4I9/2 to 4I15/2 (804 nm), and from 4I11/2 to 4I15/2 (980 nm), respectively.55 Because of the orbital interactions between the Er-4f and W-4d orbits, the original energy levels split into multiple sublevels, which leads to the present 11 sharp spectra terms at 77 K. This result proves the rich optical characteristics and new features of 2D materials by Er doping. When the excitation wavelength changes to 850 nm, similar result is obtained, and 9 sharp peaks from 881 nm to 1011 nm are observed (Figure 5(b)). The excitation spectra of the WS2 and WS2(Er) monolayers by scanning the excitation wavelength and under the monitor wavelength of 1175 nm and 1475 nm for photoluminescence are shown in Figures 5(c) and 5(d), respectively. When the excitation wavelength scans from 600 nm to 900 nm and the photoluminescent intensity is monitored at 1175 nm, both the WS2 and WS2(Er) samples show increasing fluorescent efficiency from 600 to 815 nm, but the WS2(Er) sample shows a higher fluorescent efficiency compared with WS2 sample. The maximum peak intensities are seen when excited at 849 nm for both of the samples. Similarly, when the excitation wavelength is scanned from 300 nm to 900 nm, and the PL intensity is monitored at 1475 nm, the PL intensities increase in the excitation region from 600 nm to 815 nm for both of the samples, and the WS2(Er) sample shows over two times of efficiency compared with the counterpart of WS2 sample. The maximum peak intensity of the two samples also appears at the excitation wavelength of 849 nm. Combined with the PL measurements by a 532 nm laser excitation, the photoluminescent results prove the enhancement of quantum efficiency from ultraviolet to infrared region of the Er-doped WS2.

Figure 5

Comparison of the excitation spectra between WS2 and WS2(Er) monolayer. The excitation spectrum is measured at 77 K under the excitation wavelength of (a) 250 nm and (b) 850 nm. Excitation spectra by scanning of the excitation wavelength are also measured and the photoluminescent intensities are monitored at (c) 1175 nm and (d) 1475 nm, respectively.

Researches concerning the doping effects of WS2 monolayers on the photoresponse characteristics are of interests, and photodetectors based on monolayer WS2 and WS2(Er) with similar thickness, morphologies, and sizes are fabricated under the same conditions with Au electrodes for ohmic contacts. Triangular side length of the WS2 and WS2(Er) samples are 73 μm and 71 μm, respectively. The length of the channels between two electrodes of these two types of devices is both 40 μm. Their optical micrographs are shown in Figures 6(a) and 6(b). I–V and photoresponse characteristics of the monolayer devices are measured on a shielded probe station connected to a Keithley semiconductor analyzer (SCS4200) by a standard DC technique. Details of the I–V curves in dark condition of the WS2 and WS2(Er) devices are discussed in the supporting information (S3). The I–V curves of the WS2 and WS2(Er) devices illuminated by a 635 nm diode laser at different incident intensities from dark to 180 mW/cm2 are shown in Figures 6(c) and 6(d), respectively. Compared with the pristine WS2 monolayer device, the WS2(Er) device shows 11 times of dark currents at Vds = 1 V (S3), and 469 times of photocurrents at Vds = 1 V, and under 180 mW/cm2 of illumination. Photocurrent values of both samples under different illumination power densities are extracted and shown in Figure 6(e) for comparison. Moreover, critical parameters including photoresponsivity (R) and external quantum efficiency (EQE) are evaluated by the following equations [58, 59]: where Iphoto and I are photocurrent and dark current, respectively; P is the illumination laser power density; SS is the effective area under illumination; e is the electronic charge; h is the Planck constant; and c is the light velocity. Based on the experimental data in Figure 6(e), R and EQE at different illumination power densities from 2.4 to 180 mW/cm2 are calculated for WS2 and WS2(Er) devices, and shown in Figures 6(f) and 6(g), respectively. The WS2 and WS2(Er) devices show the maximum R of 7.8 mA/W and 994.5 mA/W at the Vds = 1 V and 2.4 mW/cm2 of illumination, respectively. Similarly, the WS2 and WS2(Er) devices show the maximum EQE of 1.5% and 194.6% at the Vds = 1 V and 2.4 mW/cm2 of illumination, respectively. According to Equations (1) and (2), theRandEQEonly depend on the photocurrents; as a result, the maximum ratio ofRandEQEbetween the two type of devices is also 469 times atVds = 1 Vand 180 mW/cm2 of illumination. This result provides a powerful proof that the doped Er ion contributes to the photoelectric efficiency in a large scale due to its active and rich electronic energy levels.

Figure 6

Photoresponsive characteristics of the WS2 and WS2(Er) devices. (a) and (b) are optical micrographs of the photodetectors based on WS2 and WS2(Er) monolayers, respectively. I-V and photocurrents of the (c) pristine WS2 and (d) WS2(Er) photodetectors under a 635 nm cw laser illumination at different laser power densities are demonstrated. Photocurrents of both samples in the condition of Vds = 1 V and illuminated at different power densities are extracted from (c) and (d) and shown in (e). (f) Photoresponsivities and (g) external quantum efficiencies of both samples are evaluated based on (e). Photoresponse time of the (h) pristine WS2 and (i) WS2(Er) photodetectors under the condition of Vds = 1 V and 180 mW/cm2 of illumination for on states are also demonstrated. Lines to connect the points in (e), (f), and (g) are guides for eyes.

WS2(Er) monolayer is proved to be photosensitive, and its potential in fast photodetector applications is also demonstrated from the photoswitching characteristics, as shown in Figures 6(h) and 6(i). A high-performance graphic sampling multimeter (Keithley DMM7510) accompanied with a chopper-modulated 635 nm cw laser and a DC voltage supplier is used to measure the photoswitching behavior of the WS2 and WS2(Er) devices at Vds = 1 V and 180 mW/cm2 of illumination for on states. 25 ms of rising time and 35 ms of falling time are measured from the WS2 device. In comparison, the WS2(Er) device shows over two orders enhancement of the photoresponse speed with the rising time of 190 μs (1/132 of 25 ms) and falling time of 282 μs (1/124 of 35 ms) from one period of illumination. Notably, this speed is also over 200 times faster than the monolayer MoS2 device, and 20 times faster than the WS2 device with 6 nm thickness of WS2 [60, 61]. More comparisons are provided in the supporting information S4. The photoresponse speed is strongly depended on the density of surface trapping states, since surface states lead to the parasitic capacitance. Two orders enhancements of the photoresponse speed indicate the drastic decrease of the density of surface states in monolayer WS2(Er). Although multitype of sources including intrinsic defects, impurities, surface dangling bonds, and oxidations will lead to surface states, the main cause is still the intrinsic vacancy defects due to the inert surface feature of 2D materials. Therefore, the present two orders enhancement in the photoresponse speed provides further evidence for the vacancy filling mechanism by Er against the formation of tungsten vacancies in the CVD synthesis process.

4. Conclusion

In summary, high concentration of substitutional Er doping strategy in the synthesis of large scale of 2D WS2 by CVD is achieved experimentally. 11.5 at% of Er concentration in the WS2(Er) monolayer is examined by EDX. The high doping concentration may relate to the nature of high tungsten vacancy density formed during the CVD synthesis process. In addition, strong characteristic peaks contributed from the erbium ions are observed both in the X-ray diffraction spectra, X-ray photoelectron spectra, and the Raman spectra. The photoluminescent, absorptive, and photoresponsive performances are comprehensively and substantially enhanced by the erbium doping compared with the counterparts of the pristine WS2 monolayers synthesized by the same method. Over 6 times of fluorescent intensities are observed from the WS2(Er) spectra. The fluorescent enhancements and redshifts are well agreed with the calculation results by density functional theory (DFT). Eleven excitation peaks from 761 nm to 1011 nm are found from the excitation spectra of WS2(Er) monolayer, and those are nonexistent in the pristine WS2 monolayer. In addition, the WS2(Er) photodetectors show over 11 times of dark current, 469 times of photocurrent, photoresponsivity, and external quantum efficiency, and over two orders of photoresponse speed compared with those of the pristine WS2 photodetectors fabricated and measured under the same conditions. The combined characterizations and theoretical calculations indicate that extra lattice distortions are not obviously introduced by the substitutional erbium doping, but the defect density is effectively reduced, which leads to a reduced density of surface trapping states, and the significantly higher density of carriers are provided by the introduction of substitutional Er ions in the monolayer WS2 matrix.