Vapor growth of V-doped MoS₂ monolayers with enhanced B-exciton emission and broad spectral response

Abstract

Dynamically engineering the optical and electrical properties in two-dimensional (2D) materials is of great significance for designing the related functions and applications. The introduction of foreign-atoms has previously been proven to be a feasible way to tune the band structure and related properties of 3D materials; however, this approach still remains to be explored in 2D materials. Here, we systematically demonstrate the growth of vanadium-doped molybdenum disulfide (V-doped MoS₂) monolayers via an alkali metal-assisted chemical vapor deposition method. Scanning transmission electron microscopy demonstrated that V atoms substituted the Mo atoms and became uniformly distributed in the MoS₂ monolayers. This was also confirmed by Raman and X-ray photoelectron spectroscopy. Power-dependent photoluminescence spectra clearly revealed the enhanced B-exciton emission characteristics in the V-doped MoS₂ monolayers (with low doping concentration). Most importantly, through temperature-dependent study, we observed efficient valley scattering of the B-exciton, greatly enhancing its emission intensity. Carrier transport experiments indicated that typical p-type conduction gradually arisen and was enhanced with increasing V composition in the V-doped MoS₂, where a clear n-type behavior transited first to ambipolar and then to lightly p-type charge carrier transport. In addition, visible to infrared wide-band photodetectors based on V-doped MoS₂ monolayers (with low doping concentration) were demonstrated. The V-doped MoS₂ monolayers with distinct B-exciton emission, enhanced p-type conduction and broad spectral response can provide new platforms for probing new physics and offer novel materials for optoelectronic applications.

Graphical abstract

Keywords

Atomic substitution / V-doped MoS₂ / Distinct B-exciton / Broad spectral response

1 Introduction

Two-dimensional (2D) semiconductors have attracted attention due to their future application in field effect transistors [1–6], photodetectors [7–10], laser [11, 12], photoelectrochemical [13], and other devices [14, 15]. Molybdenum disulfide (MoS₂), as a typical 2D semiconductor, has direct band gap (~ 1.8 eV) [16–18] and high mobility [1, 5], and shows great potential in optoelectronic devices [1–3, 5, 8, 14]. Due to the broken inversion symmetry, inducing the spin–orbit coupling, there is a large splitting of ~ 160 meV over the valence band in monolayer MoS₂, giving rise to A- and B-excitons [16, 18–20]. Generally, A-exciton behavior dominates the emission of the MoS₂, while observing B-exciton behavior is difficult due to the forbiddance of its transition [17, 21]. Meanwhile, achieving p-type conductivity in chemical vapor deposition (CVD) grown MoS₂ monolayer is challenging because of its insurmountable sulfur vacancy defect [22, 23]. Furthermore, due to the large band gap, photodetectors based on MoS₂ monolayers show a limited detection range in visible light [8]. Consequently, effectively manipulating the photoelectric properties of MoS₂ and further enriching its properties are important for both fundamental research and applications.

Foreign substitutional doping is efficacious, stable, and exhibits long-range ordering, making it a preferred method for altering the electronic band structure, conduction type, and carrier concentration of pristine materials. Previous reports suggested that vanadium (element V) is an attractive dopant for room-temperature ferromagnetism and has found widespread use in many magneto-optic devices [24–27]. The introduction of V element in MoS₂ offers an effective means to modulate both the optical and the electrical properties of the samples, potentially leading to new applications and functionalities. Very recently, CVD growth of V-doped MoS₂ for use in synaptic transistors and quasi-continuously tunable carrier polarity transistors have been reported [28–30]. However, to the best of our knowledge, the A- and B-excitons properties and the photoelectric applications of V-doped MoS₂ have not been systematically studied.

In this work, V-doped MoS₂ monolayers were grown using an alkali metal-assisted CVD approach. The achievement of V doping in high-quality crystalline monolayers were confirmed by scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and Raman spectra. The emergence of p-type conduction in the achieved samples further demonstrated the successful introduction of V atoms into the crystal structure. What's even more interesting is, the distinct B-exciton emission, enhanced p-type conduction and broad spectral response observed in the V-doped MoS₂. Our results could offer a novel approach to modulating exciton properties in 2D semiconductors and potentially trigger numerous applications in spintronics.

2 Experimental

2.1 Materials synthesis

A one-zone furnace was used for growth of V-doped MoS₂ monolayers. Firstly, one quartz boat (boat 1), loaded with sublimate sulfur powder (S), was placed at the upstream, while another (boat 2), containing a mixture of molybdenum trioxide (MoO₃, Alfa Aesar, 99%), vanadium pentoxide (V₂O₅, Alfa Aesar, 99%), and potassium iodide (KI, Alfa Aesar, 99%), was positioned at the center of a quartz tube (25 cm in diameter and 100 cm long). A long piece of SiO₂/Si (270 nm SiO₂, 1 cm × 3 cm) substrate was placed above the quartz boat 2. The quartz tube containing the reaction sources and the deposited substrate was placed in the furnace, and the quartz boat 2 was ensured to be located in the center of the furnace. High purity Ar was introduced from the upstream as carrier gas and discharged from the downstream of the quartz tube. Prior to heating, high-rate Ar gas flow (1000 SCCM) was used to remove any active gases. The furnace was then heated to 750–800 ℃ in 30 min and held at that temperature for 5−10 min to grow the V-doped MoS₂ monolayers. Throughout the growth process, the Ar gas flow was maintained at 50 SCCM. Subsequent to growth, the furnace was then allowed to cool naturally to room temperature.

2.2 Characterizations

Raman and photoluminescence (PL). The Raman/PL was performed using a μ-PL system (WITec, alpha-300) equipped with a 532 nm argon ion laser.

Atomic force microscope (AFM). The AFM observations were carried out using a Bruker Dimension ICON instrument.

Scanning transmission electron microscopy. The STEM observations were acquired using a JEOL ARM200F microscope.

2.3 Device fabrication and measurements

Source-drain electrodes of the back-gated V-doped MoS₂ devices were fabricated using a transfer method. Au with a thickness of 50 nm was prepared on a bare SiO₂/Si substrate using photolithography and thermal evaporation deposition methods and subsequently transferred onto the top of the V-doped MoS₂ monolayers as source/drain. The optoelectrical properties of the V-doped MoS₂ devices were measured using Lake Shore Probe Station and Agilent B1500A semiconductor analyzer.

3 Results and discussion

V-doped MoS₂ monolayers were synthesized using an alkali metal-assisted CVD method as depicted in Fig. 1a. As is the case in previous reports, the chemical reaction between MoO₃, V₂O₅ and the KI results in formation of volatile oxyhalide species [31, 32]. In brief, solid-phase MoO₃ reacts with solid-phase KI, producing the gaseous-species MoO₂I₂. Similarly, the solid-phases V₂O₅ and KI react with each other to form gaseous-phase VOI₃. The gas–gas phase reactions among MoO₂I₂, VOI₃ and S could occur easily and efficiently. The possible reaction routes in the system are listed as follows:

\begin{array}{}\text{(1)}& 2{\text{MoO}}_3(\text{s})+2\text{KI}(\text{s})\to {\text{K}}_2{\text{MoO}}_4(\text{s})+{\text{MoO}}_2{\text{I}}_2(\text{g})\end{array}

\begin{array}{}\text{(2)}& 2{\text{V}}_2{\text{O}}_5(\text{s})+3\text{KI}(\text{s})\to 3{\text{KVO}}_3(\text{s})+{\text{VOI}}_3(\text{g})\end{array}

\begin{array}{}\text{(3)}& \begin{array}{l}2(1-x){\text{MoO}}_2{\text{I}}_2(\text{g})+2x{\text{VOI}}_3(\text{g})+3\text{S}(\text{g})\\ \to 2{\text{V}}_x{\text{Mo}}_{1-x}{\text{S}}_2(\text{S})+(2-x){\text{SO}}_2(\text{g})\\ +(2+x){\text{I}}_2(\text{g})\end{array}\end{array}

Figure 1b displays the side- and top-views of the atomic structure of the V-doped MoS₂ monolayer, wherein the yellow, blue and gray spheroids represent the S, Mo and V atoms, respectively. V atoms have substituted and occupied the sites of Mo atoms. Figure 1c shows the optical image of the V-doped MoS₂ monolayers, indicating the uniform distribution on a large scale (> 100 μm). Additionally, though in a small proportion, V-doped MoS₂ bilayers with deeper contrast than monolayers were observed and are marked in Fig. 1c. AFM characterization was used to confirm the thickness of the V-doped MoS₂ nanosheet (Fig. 1d), which was measured to be about 1.0 nm (inset in Fig. 1d), indicating a single layer nature.

The atomic structure and chemical composition of the V-doped MoS₂ monolayers were investigated by STEM, energy dispersive X-ray spectroscopy (EDS), high-resolution STEM (HRSTEM) and selected-area electron diffraction (SAED). Figure 2a displays a typical low-magnification annular dark-field (ADF) STEM image of a single-domain V-doped MoS₂ sample transferred onto a Cu grid. In Fig. 2b, the EDS spectrum obtained from the white dot in Fig. 2a revealed elemental peaks corresponding to S, Mo, and V, confirming the composition of the doped nanosheets. Thus, we could deduce the V mole fraction of the V-doped MoS₂ sample, indicating a composition of V_0.11Mo_0.89S₂. Figure S1 shows the EDS spectra taken from more samples, these samples were used for Raman, PL and electrical characterizations.

Figure 2c presents the SAED pattern of the sample, displaying a single set of diffraction spots with sixfold symmetry, confirming the single-crystal nature of V-doped MoS₂. The lattice spacing of the V-doped MoS₂ was obtained using the diffraction peaks yields (100) and was measured to be 0.272 nm, which is similar with that of MoS₂ [5]. HRSTEM was employed to characterize the atomic structure of the achieved V-doped MoS₂ (Fig. 2d), in which the different atoms could be identified using Z-contrast (Z²_Mo = 1764, Z²_V = 529 and Z²_S + Z²_S = 512). Figure 2e and f provide enlarged experimental and simulation HRSTEM images, in which V atoms are marked with yellow dotted circles. As shown in Fig. 2g, the ADF intensity profile acquired along the blue arrow in Fig. 2e and f also confirms similar results.

XPS was adopted to characterize the chemical states of elements in another V-doped MoS₂ sample. From the XPS survey spectra as shown in Fig. S2, we determined that the V mole fraction (x) of the tested samples was 9%, indicating a composition of V_0.09Mo_0.91S₂. Figure 3a shows the element binding energy comparison between V-doped MoS₂ (upper half) and pristine MoS₂ (lower half). For MoS₂, five peaks were observed at 233.2, 230.0, 227.2, 164.0 and 162.9 eV. The first two peaks are attributed to the Mo element (Mo⁴⁺ 3d_3/2 and Mo⁴⁺ 3d_5/2), while the remaining peaks belong to S element (S²⁻ 2s, S²⁻ 2p_3/2 and S²⁻ 2p_3/2) [33]. In the case of V-doped MoS₂, a slight down-shift of the above peaks was observed. Such down-shift indicated that the Fermi level of the achieved V-doped MoS₂ was closer to the valence band compared with that of the pristine MoS₂ monolayer, which demonstrated an increase in p-type doping concentration [34, 35]. Moreover, two additional peaks located at 516.7 and 524.3 eV were observed, which was attributed to the presence of V atoms. These two peaks were indexed as V⁴⁺ 2p_3/2 (516.7 eV) and V⁴⁺ 2p_1/2 (524.3 eV) [36].

Raman spectra were used to investigate the vibration modes of the V-doped MoS₂ monolayers (V_0.11Mo_0.89S₂), while pristine MoS₂ was chosen as a representative sample. Pristine MoS₂ exhibited two Raman peaks centered at 384 and 404 cm⁻¹, corresponding to the E_2g(S-Mo) mode and the A_1g(S-Mo) mode, respectively (bottom panel in Fig. 3b). However, in the Raman spectrum collected from the V-doped MoS₂ monolayer (top panel in Fig. 3b), four additional peaks located at 323, 349, 389 and 406 cm⁻¹ were observed, indicating the introduction of V atoms [37, 38]. It is worth noting that, compared with pristine MoS₂, the E_2g(S-Mo) and A_1g(S-Mo) peaks taken from V-doped sample exhibited a slight down-shift of 2.82 and 2.96 cm⁻¹, respectively, further implying hole doping induced by V atoms [34]. Raman mapping images of the V-doped MoS₂ monolayer (as shown in Fig. 3c) demonstrated the uniform chemical composition of the tested sample.

The optical properties of the V-doped MoS₂ monolayers were investigated with steady-state PL spectroscopy. Figure 4a shows the optical image of a selected V-doped MoS₂ monolayer with low doping concentration (V_0.05Mo_0.95S₂). The typical PL spectrum collected from pristine MoS₂ (depicted by a black line in Fig. 4d) exhibited two peaks centered at 1.85 and 2.00 eV, corresponding to A- and B-excitons, respectively. Clearly, the A-exciton emission dominated the whole emission behavior. The ratio of the emission integrated intensities between the B- and A-excitons was calculated as η_P = I_B/I_A = 0.083, where I_A and I_B are the integrated intensities of A- and B-excitons, respectively. In contrast, the spectrum of the V-doped MoS₂ (illustrated by a red line in Fig. 4d) showed distinct B-exciton emission that did not occur in the case of pristine MoS₂. The _ηP of the V-doped MoS₂ was calculated to be 3.13, which was 38 times higher than that occurring in the case of pristine MoS₂ monolayer. In addition, noticeable red-shifts of both the emission peaks of A- (~ 46 meV) and B-excitons (~ 99 meV) were observed in the V-doped MoS₂ monolayer, primarily resulting from doping-induced bandgap narrowing [39]. The split of A- and B-exciton bands (Δ_BA = E_B−E_A) was determined to be ~ 110 meV for V-doped MoS₂ monolayer, smaller than that of pristine MoS₂ monolayer of ~ 150 meV, indicating that the doped V atoms effectively reduced the spin–orbit splitting between A- and B-excitons behavior in MoS₂ monolayer (Fig. S3). As shown in Fig. S4, for the V_0.02Mo_0.98S₂ and V_0.05Mo_0.95S₂ monolayers (with low doping concentration), the η_P increased with the increase of V composition. But the sample with high doping concentration (V_0.11Mo_0.89S₂) did not have PL emission. Such PL quenching is contributed to the localized exciton trapped states caused by doped V atoms (Fig. S3) [39].

Figure 4b and c show the energy-selected PL intensity mapping images of the V-doped MoS₂ at 1.80 and 1.91 eV, respectively. The slightly non-uniform PL mapping is mainly caused by strain effects [40]. To further explore the spectral features, we studied the excitation power dependence of A- and B-excitonic transitions. Figure 4e presents the 2D pseudo-color plots of PL spectra under illumination by different laser excitation power densities. The log–log plots of integrated PL intensities of the A- (black dots in Fig. 4f) and B-excitons (red dots in Fig. 4f), as functions of excitation power, are presented in Fig. 4f. The relationship between integrated PL intensity I and excitation power P can be written as: I = P^β, where β ≈ 1 implies an exciton-like transition [41]. Temperature-dependent experiments were conducted to explore the distinct B-exciton emission. Figure 4g exhibits the PL spectra measured in the V-doped MoS₂ monolayer at different temperatures, while the corresponding 2D pseudocolor results are shown in Fig. 4h. The inset in Fig. 4g presents the integrated intensities ratio (η_P) of A- and B-excitons at different temperatures. As can be seen, η_P increased gradually with rising temperature, indicating the activation of B-exciton emission was at high temperature. This can be attributed to the enhancement of efficient valley scattering with the assistance of phonons (Fig. S3) [42]. On the contrary, the abnormal behavior here was attributed to the efficient valley scattering induced by the V atoms doping, which should be largely suppressed at low temperature, subsequently quenching the B-exciton emission.

In addition, we also studied the exciton-phonon interactions of the V-doped MoS₂ monolayer. As can be seen, the two emission peaks broadened and shifted towards lower energy with increasing the temperature (Fig. 4i). Such variation can be well interpreted in terms of by Varshni's semi-empirical formula:

\begin{array}{}\text{(4)}& E_{\text{g}}(T)=E_{\text{g}}(0)-\frac{\alpha T^2}{T+\beta }\mathrm{,}\end{array}

where α and β are the characteristics of the given material. We found that the energies shift of A- and B-excitons of the V-doped MoS₂ matched well Eq. (4), and the results are shown in Table 1. Such observed unequal shifts of A- and B-excitons were quite similar to those of other TMDCs [42].

Full width variation at half-maximum (FWHM) of these spectra is shown in Fig. 4i, which can be described by Bose−Einstein type expression:

\begin{array}{}\text{(5)}& \text{Γ}(T)={\text{Γ}}_0+\frac{{\text{Γ}}_{\text{LO}}}{{\text{e}}^{{\omega }_{\text{LO}}/(kT)}-1}\mathrm{,}\end{array}

where Γ₀ represents the inhomogeneous broadening term and the line width at 0 K, Γ_LO represents the exciton-longitudinal optical (LO) phonon coupling, and the ω_LO is the dominant phonon or an average phonon energy. The values of the parameters are summarized in Table 2. The temperature-dependent shifts of the energies and FWHM of the excitonic transitions are mainly due to the lattice constant variations and interactions with relevant acoustic and optical phonons [43].

To explore the electronic properties and applications of our achieved V-doped MoS₂ monolayers, we fabricated back-gated field-effect transistors. Figure 5a provides the schematic diagram of the device. Figure 5b shows the transport properties of the samples with different V composition (MoS₂, V_0.02Mo_0.98S₂, V_0.05Mo_0.95S₂ and V_0.11Mo_0.89S₂). It is evident that both the MoS₂ and V_0.02Mo_0.98S₂ exhibited typical n-type unipolar transfer behavior. With the introduction of V atoms, transitions from n-type first to ambipolar and then to p-type conduction occurred, revealing that the V atom doped in MoS₂ monolayer could effectively implement p-type doping. Ambipolar conduction behavior was observed in V_0.05Mo_0.95S₂ based device, while p-type unipolar conduction behavior was obtained with further increases in the V doping concentration (V_0.11Mo_0.89S₂), which was consistent with the Raman and XPS results. The above results demonstrated that with the increase of V component, the V-doped MoS₂ exhibited a transition from non-degenerate (MoS₂ and low doping concentration V-doped MoS₂) to degenerate (high doping concentration V-doped MoS₂) semiconductor.

The photoelectric behavior of the V-doped MoS₂ was then examined and V_0.02Mo_0.98S₂ monolayer based device was selected. Figure 5c shows the output characteristic curves, exhibiting liner results. Figure S5 displays the photoconductive properties of the V-doped MoS₂ monolayer device under illumination by 450 and 633 nm lasers with different power densities. It is clear that, the channel current increased gradually with increases in the power densities (P_in), which demonstrated that the V-doped MoS₂ monolayer has good visible light response. It is worth noting that our V-doped MoS₂ monolayer device exhibited significant light response under illumination by near infrared light at a wavelength of 980 nm; the I_ds–V_ds curves are shown in Fig. 5d. The obtained extracted photocurrents (I_ph = I_light − I_dark) under illumination by different laser power are shown in Fig. 5e, fitted with an equation of I_ph = aP^α. The fitted parameter of a were 3.36 × 10^–10, 9.51 × 10⁻¹⁰ and 1.57 × 10⁻¹¹; and α were 0.65, 0.32 and 0.50 under illumination by 450, 633 and 980 nm lasers, respectively. The responsivity (R) was calculated by the equation of R = I_ph/(P × A), where I_ph is the photocurrent, P is the incident light power density, and A is the effective area of the device channel (A = 1207.44 μm). As shown in Fig. 5f, the responsivity of the device based on V-doped MoS₂ monolayer reached up to 0.23, 0.25 and 1.41 × 10⁻⁴ A/W under illumination by 450, 633 and 980 nm lasers, respectively. Detectivity (D*) is used to characterize the sensitivity of a photodetector, which can be estimated by D^* = RA^1/2/(2e × I_dark)¹/2. The maximum estimated D* values were 6.59 × 10⁸, 1.31 × 10⁹ and 1.07 × 10⁶ Jones under illumination by 450, 633 and 980 nm lasers, respectively (Fig. 5g). The summarized result of the responsivity and detectivity is shown in Table 3. The performance comparison between the photodetectors based on V-doped MoS₂ monolayer and other reported photodetectors is shown in Table S1, which demonstrates that our photodetectors have moderate performance [44–52]. Figure 5h shows the switching behavior of the photodetector measured under light irradiation with visible light 450 (green line), 633 (red line) and near-infrared light 980 nm (violet line) lasers, indicating that the device could be switched effectively between high resistance and low resistance states. As shown in Fig. S3, in V-doped MoS₂ monolayers, an accept band lever caused by V doping between the conduction band and valence band can be introduced, which can facilitate the absorption of short-wavelength light, enabling near-infrared light detection. The above results indicate that the V-doped MoS₂ device exhibits significant broad spectral response.

4 Conclusion

In summary, a series of substitutional V-doped MoS₂ monolayers were synthesized using an alkali metal-assisted CVD growth method. The samples were systematically characterized by XPS, Raman, STEM and electrical transport characterizations, confirming that V atoms were uniformly doped into the MoS₂ samples. Interestingly, enhanced B-exciton emission was observed in the doped samples and the emission behaviors were systematically studied using steady temperature-dependent PL experiments. Electrical transport measurements indicated that enhanced p-type conduction occurred in the achieved V-doped MoS₂ monolayers. Moreover, the photodetector based on V-doped MoS₂ monolayer showed broad spectral response from visible to near-infrared light. The synthesized V-doped MoS₂ nanosheets can provide new material platform for spintronics related fundamental research and device applications.

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