Lanthanide Nd ion-doped two-dimensional In₂Se₃ nanosheets with near-infrared luminescence property

1 Introduction

Two-dimensional (2D) sheet-liked materials have attracted great attention in optoelectronic devices, solar cells, photocatalysis, and biomedicine over the last few years [1], [2], [3], [4], [5], [6], [7], [8]. Generally, nanomaterials with a thickness less than 5 nm are defined as 2D materials [9]. Thanks to its unique physical and chemical features, 2D materials possess lots of unexpected properties, such as quantum hall effect, ultrahigh carrier mobility, bright optical transparency, huge specific surface area, as well as remarkable photothermal effect [10], [11], [12]. Up to now, a lot of materials with 2D structure have been developed, including graphene, transition metal dichalcogenides, black phosphorus, III–VI layered semiconductor, and metal-organic frameworks [10], [13], [14], [15], [16]. Among them, di-indium tri-selenide (In₂Se₃) is a representative III–VI layered semiconductor that exists in five crystalline phases (α, β, γ, δ, and κ) [17], [18]. In addition, γ-In₂Se₃ is a direct band gap semiconductor with bandgap of about 1.9 eV (652 nm), which suggests that the emission is limited in the visible region. The only visible emission restricts the applications in photonics to a large extent. Therefore, it is significant to seek a strategy to modulate the emission of 2D In₂Se₃ semiconductor.

By comparison, lanthanide ions are normally doped into some insulators or semiconductor materials for realizing the luminescence from the ultraviolet (UV) to the infrared region, which originates from the 4f electron orbital transitions of lanthanide ions [19], [20], [21]. There are amounts of lanthanide ions such as Eu³⁺, Dy³⁺, Nd³⁺, Er³⁺, Ho³⁺, and Yb³⁺, whose luminescence properties contain abundant emission bands, narrow bandwidth, long lifetime, and high light stability [22], [23], [24]. Among them, Nd³⁺ ion is a typical lanthanide dopant with luminescence peaks at 900, 1100, and 1300 nm, which are widely applied to near-infrared (NIR) photonics areas [25]. Over recent research, Bai and coworkers incorporated the lanthanide ion Er³⁺ into MoS₂ thin film with bilayer thickness for the first time through chemical vapor deposition (CVD) method. They confirmed the possibility of lanthanide ion-doped 2D materials and doping lanthanide ions can modulate the emission of semiconductor nanosheets [26].

There are numerous methods of doping lanthanide ions into matrix, such as solid state reaction, hydrothermal process, sol-gel method, and microwave synthesis. It is a simple way of solid state reaction for doping lanthanide ions into the host on account of the advantages for high yield and good crystallinity. In addition, massive efforts have been used for fabrication of 2D materials, including micromechanical cleavage, CVD, wet-chemical synthesis, liquid exfoliation, and pulsed laser deposition [27], [28], [29], [30]. For instance, Gu and Tao fabricated 2D In₂Se₃ thin layers with a thickness of ~4–5 nm via mechanical exfoliation method [14]. Hao’s group synthesized the Yb/Er codoped layered WSe₂ nanosheets through the pulsed laser deposition process [31]. Unfortunately, the fabricated yields of existing approaches are still at an extremely low level; it is significant to adopt an effective approach to improve the yield of the doped 2D materials. Due to its superiority in terms of wide applicability, well controllability, and high yield, the liquid assistant exfoliation technique is the most widely used, practical approach for manufacture of 2D materials. Hence, it is crucial to find a tactic to dope the lanthanide ions into the host and make them in the 2D form as efficiently as possible. In addition, 2D materials fabricated by the liquid exfoliation method also have wide applications in photothermal areas and photoelectric nanodevices owing to their small particle size and convenient preparation [11], [32], [33], [34].

In this report, we adopted a two-step method of solid state reaction and liquid phase exfoliation to fabricate lanthanide-doped 2D In₂Se₃ nanosheets. The as-prepared material possessed not only good crystallinity after lanthanide ion doping but also 2D nanosheet structure. After the doping of lanthanide Nd³⁺ ions, the emission region of intrinsic In₂Se₃ semiconductor was extended from visible to NIR, which has promising applications in NIR photonic filed. A down-conversion process of single photon was also observed from the lanthanide Nd³⁺ ions in the 2D In₂Se₃ nanosheets under 808 nm excitation.

2 Experiment section

The bulk In₂Se₃ with a grain size of 80 μm was purchased from Foshan QIANLIMU Technology Co., Ltd (Foshan, China). NdF₃ powders with high purity of 99.99% were purchased from Aladdin Industrial Corporation (Shanghai, China). Analytically pure ethanol was provided by Hangzhou Gaojing Fine Chemical Industry Co., Ltd (Hangzhou, China). The 2D In₂Se₃:Nd³⁺ nanosheets were fabricated by a two-step method of solid state reaction and liquid phase exfoliation. First, the solid state reaction was executed for preparing bulk In₂Se₃ doped with lanthanide Nd ions. The raw materials of 99% In₂Se₃ and 1% NdF₃ with molar percentage were mixed and abraded in an agate mortar. Next, the mixed powders were placed in a tube furnace and heated to 650°C for 4 h in protective gas Ar. Finally, the Nd-doped In₂Se₃ powders were cooled to room temperature and grinded fine for further disposing. Posteriorly, we adopted the liquid phase exfoliation technique to acquire the In₂Se₃:Nd³⁺ nanosheets. The doped In₂Se₃ powders were dispersed in ethanol with a ratio of 2 mg/ml. Then, the mixture was exfoliated with probe sonication for 24 h. Ultimately, the nanosheets were gained from the supernatant after controlled centrifugation.

The crystal phase for microcrystals and nanosheets of Nd³⁺-doped In₂Se₃ was identified via X-ray diffraction (XRD; D2 PHASER, Bruker, Karlsruhe, Germany) measurement with Cu Kα radiation in the range of 10–65°. To investigate micromorphology, a Tecnai FEI transmission electron microscope (TEM), type of G2 F20, was used to observe the structure and size of as-prepared nanosheets at nanoscale. Atomic force microscopy (AFM) (D3100, Veeco, Plainview, NY, USA) was applied for thickness detection to figure out the morphology of nanosheets. X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Fisher, Waltham, MA, USA) detection with a monochromated Al Kα source (hν=1361 eV) was carried out to determine the situation for doping of Nd³⁺ ions. Raman spectra were acquired by a Renishaw inVia-Reflex Raman spectrograph with a 532 nm laser for investigation of microstructure. Absorbance spectra in the range of 300–1300 nm were collected by PerkinElmer Lambda 750S UV-VIS-NIR spectrophotometer (Perkin Elmer, Waltham, MA). The intrinsic luminescence spectra of as-prepared nanosheets were gained by Renishaw Raman spectrograph in the photoluminescence (PL) mode. And the NIR emission spectra of nanosheets were obtained through a Fluorolog-3 (Jobin Yvon, Paris, France) fluorescence spectrometer with an 808 nm laser diode.

3 Results and discussion

Structure and morphology play an important role in material properties. Figure 1A shows the XRD patterns of bulk and nanosheets of In₂Se₃ doped with Nd³⁺ ions. The doped In₂Se₃ bulk exhibits a perfect accordance with XRD patterns of the standard card (PDF#40-1407), which suggests the bulk belonging to hexagonal system. And it shows that the characteristic of the doped γ-phase is similar with former works of Marsillac and coworkers [17]. The XRD patterns of nanosheets also show the feature of γ-phase, which demonstrates that the exfoliation will not alter the phase of as-prepared materials. TEM was used to reveal the microscopic morphology of the prepared In₂Se₃:Nd³⁺ nanosheets. As shown in Figure 1B, the contrast of nanosheets is almost at the same level, which demonstrates that the nanosheets are quite thin to a large extent. And it can be observed that the size of as-prepared nanosheets is at tens to hundreds of nanometers scale. Figure 1C indicates the high-resolution TEM (HR-TEM) image of In₂Se₃:Nd³⁺ nanosheets. It reveals a lattice spacing of around 0.319 nm. According to the formula:

Figure 1:

Morphology and crystalline structure.

(A) XRD patterns of bulk and nanosheets of the In₂Se₃:Nd³⁺ samples. (B) TEM image of In₂Se₃:Nd³⁺ nanosheets. (C) HR-TEM image of In₂Se₃:Nd³⁺ nanosheets. (D) AFM image of In₂Se₃:Nd³⁺ nanosheets. Inset: Height profile of In₂Se₃:Nd³⁺ nanosheets.

which indicates the relationship between interplanar spacing (d) and Miller index (h, k, l) and lattice constant (a, c); the observed plane is consistent with (110) face of hexagonal γ-In₂Se₃ crystal after calculation.

The AFM image further shows that the as-prepared nanosheets are extremely flat. It can be obtained from the AFM image that the plane size of as-prepared nanosheets is around hundreds of nanometers, but the thickness is just 4.5 nm, which is far less than the flat size. According to the above results of XRD, TEM, and AFM, it can be concluded that 2D-structure nanosheets with good crystallinity have been successfully fabricated.

XPS was carried out to investigate whether Nd³⁺ ions have been already doped into In₂Se₃ nanosheets. Figure 2A shows the XPS pattern of In₂Se₃:Nd³⁺ nanosheets in full scale. Apparently, the In, Se and Nd elements can be observed in the full-scale spectrum; however, C and O are introduced from the measurement. Among them, HR-XPS spectrum of Nd 3d is presented in Figure 2B. Peaks located at 974.2 and 995.2 eV originated from Nd 3d_5/2 and Nd 3d_3/2 binding energies, respectively. Figure 2C and D expressed the HR-XPS spectra of In₂Se₃ nanosheets with undoped and doped Nd³⁺ ions. It can be observed that the core-level peaks of In and Se in the In₂Se₃:Nd³⁺ sample show a slightly shift toward higher binding energies compared with the undoped In₂Se₃ sample, which reveals the transformation of chemical microenvironment. Therefore, the intensities of In and Se elements are both lower than the undoped sample, which means the electric pairs of chemical bonds are attracted to them. This change suggests that Nd³⁺ ions have been introduced into the In₂Se₃ matrix. This may be due to facts that the electronegativities of In and Se are larger than that of the Nd element from Allred-Rochow scale [35]. Besides, no apparent binding energy peaks of NdF₃ can be discerned here. Therefore, it can be confirmed from the above results that the Nd³⁺ ions have been successfully doped into In₂Se₃ nanosheets.

Figure 2:

XPS spectra of In₂Se₃ nanosheets with and without Nd ions.

(A) Full-scale XPS spectrum of In₂Se₃:Nd³⁺ nanosheets. (B) HR-XPS spectrum of Nd 3d core levels in In₂Se₃:Nd³⁺ nanosheets. Contrastive HR-XPS spectra of (C) In 3d and (D) Se 3d core levels in undoped In₂Se₃ and In₂Se₃:Nd³⁺ nanosheets.

Raman and intrinsic PL spectra were further used to reveal the structural characteristics of the prepared In₂Se_3:Nd³⁺ nanosheets. As shown in Figure 3A, the Raman spectra of bulk and nanosheets of In₂Se₃:Nd³⁺ samples were collected under 532 nm excitation. Five Raman peaks located at around 81, 150, 179, 206, and 227 cm⁻¹ are observed in both bulk and nanosheets, which agree well with the previous reports of γ-In₂Se₃ [17], [36], [37]. The full width at half maximum of the main Raman peak located at about 150 cm⁻¹ is approximately 9 cm⁻¹, which is consistent with former research as well [37]. Furthermore, there is almost no peak position difference between bulk and nanosheets, which suggests that the sonification hardly changed the phase structure of In₂Se₃ nanosheets. Considering the above results, it can be further proved that the as-prepared nanosheets are γ-phase with 2D structure.

Figure 3:

Raman and intrinsic PL spectra of the prepared In₂Se₃:Nd³⁺ samples.

(A) Raman and (B) intrinsic photoluminescence spectra of bulk and nanosheets under 532 nm excitation.

Figure 3B shows the intrinsic PL spectra of bulk and nanosheets of In₂Se₃:Nd³⁺ under 532 nm (2.33 eV, which is larger than the band gap of 2D In₂Se₃) excitation. Apparently, the PL intensity of nanosheets is stronger than bulk, whose peaks are both located at around 675 nm (1.84 eV). According to previous reports, the intensity of PL emission is relevant with the thickness (z-axis) and plane size (xy-plane) of the prepared sample [38], [39], [40]. It can be speculated that the PL performance of bulk and nanosheets is related to the combined effects of carrier confinement through the xy-plane together with along the z-axis [41], [42]. Our intrinsic result is consistent with the previous report [42].

UV-VIS-NIR absorption spectroscopy was applied to investigate the band gap and optical response. Figure 4A indicates the absorption spectrum of In₂Se₃:Nd³⁺ nanosheets. It can be observed that the nanosheets possess a broadband absorption in the range from 300 nm (UV) to 1200 nm (NIR). After calculation with Tauc plot method (as shown in right insert), the band gap of as-prepared nanosheets is around 1.83 eV (677 nm), which is nearly consistent with the above intrinsic PL emission. As shown in Figure 4B, the absorption signals decrease with the increase in centrifugation speed. This is owing to a smaller particle size and thinner concentration of the obtained supernatant, which results in weaker absorption signals. Moreover, the colors of supernatants with various centrifugal speeds are not the same because of the different sizes of the nanoparticles, which is just like the optical photo displayed in the insert.

Figure 4:

Absorption spectra of In₂Se₃:Nd³⁺ nanosheets.

(A) UV-VIS-NIR absorption spectrum of In₂Se₃:Nd³⁺ nanosheets. Insert: Tauc plot with its calculation of band gap. (B) UV-VIS-NIR absorption spectra of In₂Se₃:Nd³⁺ nanosheets at different rates of centrifugation. Insert: photos with suspension of In₂Se₃:Nd³⁺ nanosheets at different centrifugal speeds.

The PL performance demonstrates that the In₂Se₃:Nd³⁺ nanosheets possess intrinsic emission at a range of visible. In order to extend the emission range, Nd³⁺ ions were doped into the In₂Se₃ matrix. We then investigated the luminescence properties of lanthanide Nd³⁺ ions in 2D In₂Se₃ nanosheets. Figure 5A illustrates the emission spectra of In₂Se₃ nanosheets with doped and undoped Nd³⁺ ions by pump laser of 808 nm. It can be clearly seen that three PL peaks appear after Nd³⁺ ion doping. The emission peaks located at 910, 1057, and 1324 nm are consistent with the energy transitions of Nd³⁺ ions from excited state ⁴F_3/2 toward ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2, respectively [25]. On the contrary, emission peaks of undoped nanosheets in the range of NIR are hardly observed. Therefore, the doping of Nd³⁺ ions did increase the emission region from visible to NIR spectra. Figure 5B indicates the emission spectra of In₂Se₃:Nd³⁺ nanosheets with different concentrations from 100 ppm to 500 ppm. The emission signal of nanosheets exhibits a stronger trend as the increment of concentration. This can be attributed to positive correlation between emission intensity and consistency of lanthanide Nd³⁺ under some conditions.

Figure 5:

NIR PL spectra of In₂Se₃ nanosheets with and without Nd ions.

(A) NIR down-shifting PL spectra of undoped and doped In₂Se₃ nanosheets under the pump at 808 nm. (B) NIR emission spectra of In₂Se₃:Nd³⁺ nanosheets in various concentrations by 808 nm excitation.

Additionally, optical characteristics and luminescent transition mechanism were further researched in In₂Se₃:Nd³⁺ nanosheets. Figure 6A shows the emission spectra of In₂Se₃:Nd³⁺ nanosheets in a concentration of 500 ppm with different pump powers under 808 nm excitation. It can be distinctly observed that the emission signal became strong with the increasing of pump power from 500 to 5000 mW, which is owing to the addition of population in unit time with excitation source. Moreover, the pump power (P) and the PL intensity (I) abide by this relation: I∝Pⁿ, where n represents the number of photons [43]. According to this relationship, the double 10 logarithm plot (log-log) was drawn, as shown in Figure 6B, and the slopes denote the number of photons. After linear fitting of data points, the slopes of emission peaks at 910, 1057, and 1324 nm are 0.35 ± 0.03, 0.64 ± 0.04, and 0.73 ± 0.03, respectively. That means that the PL emission of 910, 1057, and 1324 nm possesses one photon, i.e. single photon process. The reason why the value of slopes is all lower than 1 may be attributed to factors of saturated excitation, heat accumulation, or other effects. Figure 6C shows the decay curve for Nd³⁺ ions under 808 nm pump excitation. The lifetime of Nd³⁺ ions in In₂Se₃ nanosheets with emission peak located at 1057 nm (⁴F_3/2→⁴I_11/2) is determined to be 0.75 ms after calculating from the decay curve.

Figure 6:

NIR luminescence properties of Nd ions in In₂Se₃ nanosheets.

(A) NIR emission spectra of In₂Se₃:Nd³⁺ nanosheets excited at 808 nm in diverse pump power. (B) Pump power dependence of emission intensity at 910, 1057, and 1324 nm. (C) NIR decay curve for 1057 nm emission of In₂Se₃:Nd³⁺ nanosheets. (D) Energy level diagram of Nd³⁺ ions and the probable down-conversion mechanism.

The possible down-conversion mechanism for Nd³⁺ ions in 2D In₂Se₃ nanosheets is described in Figure 6D. The GSA, CR, and NR represent ground state absorption, cross relaxation, and nonradiative relaxation, respectively. Under the excitation of 808 nm pump, the ground state ⁴I_9/2 of Nd³⁺ ions are absorbed to excited state ⁴F_5/2. Next, it happens to the NR process from excited state ⁴F_5/2 to ⁴F_3/2. Lastly, the radiation transitions from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 are determined to be NIR emission of 910, 1057, and 1324 nm, respectively.

4 Conclusion

In summary, a reliable and effective method of solid state reaction and liquid phase exfoliation has been employed to fabricate lanthanide ion Nd³⁺-doped 2D In₂Se₃ nanosheets. The fabricated Nd³⁺ ion-doped In₂Se₃ nanosheets are determined to be 2D in structure with a thickness of 4.5 nm via TEM and AFM characterizations. The prepared 2D In₂Se₃ nanosheets doped with Nd³⁺ ions possess γ-phase after analyses by XRD and Raman detection. The XPS measurement, combined with PL spectra, indicate that the lanthanide ions Nd³⁺ have been successfully introduced into In₂Se₃ nanosheets. Under 808 nm excitation, several emissions in the NIR region can be observed from the lanthanide ions Nd³⁺ in 2D In₂Se₃ nanosheets. The emission peaks located at 910, 1057, and 1324 nm originated from the transitions of ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2, respectively. This down-conversion luminescence of Nd³⁺ ions in the NIR region is determined to be single photon process by analysis of power-dependent emission spectra. Moreover, the introduction of Nd³⁺ ions extends the emission region of intrinsic In₂Se₃ semiconductor from visible to NIR spectra. Our work indicates that the two-step method reported here is an effective approach to embed the lanthanide ions into 2D nanosheets. The prepared 2D In₂Se₃:Nd³⁺ nanosheets with excellent NIR luminescence have promising applications in NIR photonic nanodevices and biomedicine.