Tunable near-infrared light emission from layered TiS₃ nanoribbons

Abstract

The low-dimensional light source shows promise in photonic integrated circuits. Stable layered van der Waals material that exhibits luminescence in the near-infrared optical communication waveband is an essential component in on-chip light sources. Herein, the tunable near-infrared photoluminescence (PL) of the air-stable layered titanium trisulfide (TiS₃) is reported. Compared with iodine particles as a transport agent, TiS₃ grown by chemical vapor transport using sulfur powder as a transport agent has fewer sulfur vacancies, which increases the luminescence intensity by an order of magnitude. The PL emission wavelength can be regulated in the near-infrared regime by thickness control. In addition, we observed an interesting anisotropic strain response of PL in layered TiS₃ nanoribbon: a blue shift of PL was achieved when the uniaxial tensile strain was applied along the b-axis, while a negligible shift was observed when the strain was applied along the a-axis. Our work reveals the tunable near-infrared luminescent properties of TiS₃ nanoribbons, suggesting their potential applications as near-infrared light sources in photonic integrated circuits.

Graphical abstract

Keywords

titanium trisulfide / near-infrared luminescence / S-vacancy / tunability / strain engineering

1 Introduction

Photonic integrated circuits offer new opportunities for faster and energy-efficient communication and computing systems [1-6]. A low-dimensional light source, which has the advantages of miniaturization and compatibility, is a competitive optical component in photonic integrated circuits. In the last decade, light sources based on layered van der Waals materials have been reported [7, 8], such as transition metal dichalcogenides (TMDs) [9-16] and black phosphorus (BP) [17-19]. The semiconducting TMDs in monolayer form possess a direct bandgap with typical light emission in the visible waveband. Monolayer MoTe₂ exhibits light emission in the near-infrared waveband with a bandgap of ~1.1 eV [20, 21]. However, 2D materials with emissions in the near-infrared communication regime, such as the O and C bands, are limited. Trilayer and tetralayer BP exhibit excitonic emission at around 1280 and 1480 nm, respectively [22]. However, the air instability of BP, especially for the thin layers, hinders its practical application.

As a member of the transition metal trichalcogenides (TMTs), titanium trisulfide (TiS₃) has attracted much attention due to its anisotropy optical and electronic properties resulting from its quasi-one-dimensional (1D) crystal structure [23, 24]. Interestingly, TiS₃ shows a direct bandgap of about 0.9 eV [25-28], lying in the near-infrared communication waveband, which makes it a promising luminescent material for on-chip light sources. However, the studies on the optical emission properties of TiS₃ are limited, which may be hindered by the content of S vacancies and abundant nonradiative recombination process in TiS₃ [29-31]. Therefore, controlling and reducing the sulfur vacancies is of great significance to exploit the luminescent properties of TiS₃. In addition, the regulation of wavelength is desirable for the information application as an intelligent on-chip light source. The van der Waals semiconductors with versatile thickness control and strain engineering offer the opportunity to modulate their band structure and the light emission wavelengths.

Here, we report on the regulation of the light emission intensity and wavelength of TiS₃ in the near-infrared regime. By comparing the transport agent of S powder and iodine granule in the chemical vapor transport (CVT) growth method, we found that the photoluminescence (PL) emission of TiS₃ can be increased by an order of magnitude by changing the transport agent from iodine granule to S powder. Broad PL tunability from 1269 to 1385 nm near the O band in TiS₃ nanoribbon was demonstrated by controlling the flake thicknesses. In addition, by using uniaxial tensile strain, the center wavelength of PL from TiS₃ can be tuned reversibly from 1289 nm to 1342 nm. Interestingly, an anisotropic strain response of PL in TiS₃ nanoribbons has been observed. The tunable light emission from the van der Waals TiS₃ shows potential applications in photonic integrated circuits for optical fiber communication.

2 Experimental section

Materials synthesis. The TiS₃ and TiS_3−x bulk crystals were synthesized by the CVT method. For the growth of TiS₃ crystal, Ti powder (Alfa Aesar, 99.99%) and sulfur (S) powder (Alfa Aesar, 99.99%) with a Molar ratio of 1:3.05 were mixed and sealed in a clear quartz ampule under a vacuum of ~10⁻² Pa, the excess sulfur was considered as transport agents. The quartz ampule was placed in a two-zone tube furnace, slowly heated to 480−620 °C (the temperature corresponds to the cold zone and hot zone of the tube furnace, respectively), and maintained for five days. After the tube furnace was naturally cooled to room temperature, high-quality shiny ribbon-like TiS₃ was obtained in the cold zone. For the growth of TiS_3−x crystal, Ti powder (Alfa Aesar, 99.99%) and S powder (Alfa Aesar, 99.99%) with a molar ratio of 1:3 were used as precursors (the total mass was 1 g), and 15 mg of the iodine granule (Alfa Aesar, 99.99%) was used as transport agent, the subsequent growth process was similar to the aforementioned for TiS₃.

Materials characterization. The crystal structure of TiS₃ was characterized by XRD (Bruker, D8 Advance) in the 2θ range of 5°‒65°. The thickness of all flakes was characterized using an AFM (Bruker, Dimension ICON). The crystalline quality and chemical composition of TiS₃ were characterized using SEM (Quanta FEG 250) and STEM (Themis Z, operating at 300 kV).

Optical measurement. The Raman spectra and angle-dependent Raman spectra were measured by a micro-Raman system (LABRAM HR) with a 532 nm laser through a 100X objective lens. The optical absorption and optical bandgap were measured by ultraviolet-visible-near infrared (UV–vis-NIR) spectrophotometer (SHIMADZU, UV-1280). Near-infrared μ-PL measurement was carried out on a commercial confocal μ-PL system (Fuxiang, golite solution-PL). A single-mode fiber laser at 532 nm was used to excite the PL of TiS₃ nanoribbon. Angle-dependent near-infrared PL spectra were measured by using an analyzer (Glan‒Taylor polarizer) and a half-wave plate in the front of the integrated InGaAs detector. Near-infrared μ-PL measurement of strained TiS₃ was collected by a 50× infrared objective and fed to a spectrometer (LabRAM Odyssey, Horiba) equipped with a cryogenic InGaAs detector with a 532 nm laser for excitation.

3 Results and discussion

A top view of the atomic structure of TiS₃ is shown in Fig.1(a). Ti atom is encompassed by six sulfur atoms, forming bicapped distorted tetrahedrons. 1D chains are formed through strong covalent bonds along the crystallographic b-axis and these repeating 1D chains are covalently bonded within the a−b plane into a layered structure. The distorted in-plane crystal structure dictates its anisotropic optical and electronic properties. The layers are further stacked by weak van der Waals force with no dangling bonds out of the plane. The crystal structure of the experimentally grown TiS₃ was identified by X-ray diffraction (XRD), where the prominent diffraction peaks closely matched its crystal standard PDF card (PDF# 36-1337) [Fig.1(b)]. The detailed atomic crystal structure and element composition characterization can be seen in Fig. S1 of the electronic supplementary materials (ESM). The anisotropic lattice vibration of the TiS₃ nanoribbon was characterized by angle-resolved Raman spectroscopy in the parallel configuration, as shown in Fig.1(c). Four Raman-active vibrational peaks at 175.9 cm⁻¹, 300.6 cm⁻¹, 371.3 cm⁻¹ and 559.6 cm⁻¹ were observed, corresponding to the out-of-plane vibrational peak I-A_g^{rigid}, in-plane vibrational mode II-A_g^{internal} and III-A_g^{internal}, and in-plane S−S bond vibrational peak Ⅳ-A_g^{S-S}, respectively. We set the initial direction of the incident laser parallel to the b-axis (corresponding to 0°, as illustrated in Fig. S2 of the ESM) and the Raman intensity of different modes goes through maxima and minima with the variation of polarization angle with two- or four-lobed features, which can be explained from the Raman tensor analysis [32].

The Ultraviolet-Visable-Near infrared (UV-Vis-NIR) optical absorption of TiS₃ nanoribbons exhibits an absorption edge at around 1400 nm, as shown in Fig.1(d). The inset shows the corresponding Tauc’s plot, expressed as {\left(\alpha h\nu \right)}^{1/n}=A\left(h\nu -E_g\right), where \alpha, h, \nu, A, and E_g are the absorption coefficient, Planck’s constant, light frequency, proportionality constant, and band gap, respectively. We extracted n equals to 1/2 for TiS₃, which is consistent with its direct bandgap nature. The band gap of TiS₃ nanoribbon is estimated to be ~0.91 eV, which lies in the near-infrared O band region. The PL spectra of typical TiS₃ flakes at 300 K with various excitation powers are shown in Fig.1(e). PL emissions at around 1330 nm were observed, which are in agreement with the absorption results, suggesting its radiative recombination in the band edge. It is worth noting that the TiS₃ flakes show stable PL emission under the excitation, as shown in Fig. S3 of the ESM. The PL spectra at 77 K (Fig. S4 of the ESM) show distinct PL signals of silicon substrate and TiS₃. With the decreasing of temperature, the PL intensity increases and the FWHM narrows, which could be attributed to the inhibited nonradiative recombination center at low temperatures. The ratio of the logarithm of the spectral integral PL to the logarithm of the excited power density (slope c) can be obtained to be ~0.9 at 77 K (Fig. S4 of the ESM), and this sub-linear dependence implies contribution from possible defect states [29]. Furthermore, Fig.1(f) shows the linear polarization of the PL with a degree of polarization of about 77% and we elucidate that the polarization is along the b-axis (chain direction). Angle-resolved PL spectra of TiS₃ flake evolving with angle are shown in Fig. S5 of the ESM. Further, to clarify the stability of the sample, we examined the emission of the samples after two months in ambient conditions, and no degradation was observed, demonstrating its high environmental stability (Fig. S6 of the ESM).

One significant challenge for layered TiS₃ is its sulfur vacancy introduced in the growth process, which would result in defect states and reduce the electron−hole radiative recombination rate [30, 31]. To increase the emission properties of the layered TiS₃, we explored the growth method for TiS₃ to effectively reduce the S vacancies. Different from the typical CVT growth with iodine granule as a transport agent, we used sulfur powder as a transport agent instead. Compared with TiS_3−x nanosheets grown with iodine granule as a transport agent [Fig.2(a)], TiS₃ grown by S-self-transport has higher crystallinity and is more likely to grow into nanowires rather than nanosheets, as shown in the scanning electron microscope (SEM) image in Fig.2(b). The fact that TiS₃ with a nanowire shape tends to have a lower density of sulfur vacancies is consistent with previous reports [28]. X-ray photoelectron spectroscopy (XPS) was employed to further verify the difference in sulfur vacancy content of TiS₃ and TiS_3–x nanoribbons. The S/Ti atomic ratios of TiS₃ and TiS_3−x were obtained to be 2.97 and 3.19 respectively, as shown in Fig.2(c). In the S 2p spectra of TiS₃ and TiS_3−x, the S₂²⁻ peak decreases, as shown by the red curve in Fig.2(c), which suggests the presence of more S₂²⁻ vacancies in TiS_3−x. In addition, a slight shift to lower binding energy was observed in the Ti 2p spectra from TiS_3−x to TiS₃ [Fig.2(d)], which is consistent with the trend from TiS₂ to TiS₃ [30, 33-35]. More details of the XPS results are shown in Fig. S7 of the ESM. The reduction of S vacancies directly affects the near-infrared light emission of TiS₃, as shown in Fig.2(e). Under the same excitation optical power density (532 nm, 0.798 kW/cm²) and the identical detection configuration, TiS₃ (blue spectral curves) has a stronger PL than TiS_3−x (red spectral curves). The typical PL emission intensity of TiS₃ is an order of magnitude larger than TiS_3−x [Fig.2(f)], demonstrating the effective regulation of infrared PL in TiS₃ by reducing the sulfur vacancies.

In addition to increasing the near-infrared PL intensity from 2D semiconductors, it is desired to control the wavelength of the emission, especially in optical communication applications. Firstly, taking advantage of the van der Waals interaction in the layered TiS₃, we explored its thickness-dependent luminescence. Fig.3(a) shows the normalized PL spectra of TiS₃ nanoribbons at room temperature with thicknesses from 16 to 261 nm (The thicknesses were determined by an atomic force microscope (AFM) in Fig. S8 of the ESM). As the thickness decreases, the center wavelength of the PL spectra shows blue shift and the band gap increases, which are attributed to the quantum confinement effect [36]. A summary of the bandgap variation with the number of layers is shown in Fig.3(b), which decreases monotonously from 0.974 eV at 16 nm (∼13 layers) to 0.903 eV at 261 nm thick TiS₃ nanoribbons (∼217 layers). The optical bandgap is in good agreement with the experimental result obtained by scanning tunneling spectroscopy, photoelectrochemical measurements [37], and theoretical predictions [25, 38, 39]. In addition, the layer number dependent peak energy can be fitted by a one-dimensional tight-binding model [black curve of Fig.3(b)] [40], E = 0.345/N^0.403+0.858, where E and N are band gap and layer number of TiS₃, respectively. We also observed the distinct emission wavelengths in TiS₃ nanoribbon with a thickness gradient, as shown in Fig.3(c). The TiS₃ nanosheet with gradient thickness was exfoliated to the Si/SiO₂ substrate by the micromechanical cleavage technique. The optical diagram is shown in Fig. S9 of the ESM, and the corresponding height profile and AFM image are shown in Fig.3(d). From the right side of the TiS₃ nanosheet to the left side, the thickness decreases gradually, and the corresponding center wavelength of PL spectra exhibits a blue shift. This is consistent with the previous results at different thicknesses. In short, by increasing the thickness of TiS₃, the central wavelength of the TiS₃ nanoribbons PL can vary from 1289 nm to 1342 nm. The emission range of TiS₃ (counting the range of full width at half maximum) spans from 1200 nm to 1460 nm covering the O and E bands, unlocking great potential for applications as gain materials in on-chip light sources.

Strain engineering is a powerful method to dynamically modulate the properties of 2D materials. We further apply tensile strain to dynamically modulate the emission wavelength of TiS₃ deposited on flexible substrates with a home-built strain setup, as shown in Fig.4(a). Here, we use an elastomeric polyethylene terephthalate (PET) as a flexible substrate and encapsulate the TiS₃ nanoribbons with polymethyl methacrylate (PMMA) to ensure that strain is applied to the TiS₃ flakes [41, 42]. Tensile strain is calculated by \epsilon =\frac{d}{2R}, where d and R are the thickness and radius of curvature of the flexible PET substrate, respectively. We further apply uniaxial tensile strain along the b-axis and a-axis of the TiS₃ to explore its anisotropic response [Fig.4(a)]. When the strain is along the b-axis direction, the redshifts of the II-A_g^{internal} and III-A_g^{internal} modes are observed while the I-A_g^{rigid} and IV-A_g^{S-S} remain unchanged, as shown in Fig.4(b) and S10 of the ESM. With strain of 0.096%, redshifts of ~2.01 cm⁻¹ for the III-A_g^{internal} mode and ~0.65 cm⁻¹ for the II-A_g^{internal} mode was observed. Raman modes soften with the tensile strain implying that the energy of the corresponding phonon modes decreases, thus weakening the associated restoring force in these vibrations. The Grüneisen parameter [{\gamma }_m, {\gamma }_m=-\frac{1}{{\omega }_m}\frac{{\mathrm{\Delta }\omega }_m}{\mathrm{\Delta }\epsilon }, where {\omega }_m = 370.48 cm⁻¹, \mathrm{\Delta }\epsilon ={\epsilon }_b-\upsilon {\epsilon }_b (\upsilon = 0.33 for PET)], which represents the shift rate of the active mode as lattice expanding, is 5.65 for III-A_g^{internal}, consistent with previous results [42]. Fig.4(b) also shows the PL spectra measured without strain (blue curve) and with strain (khaki and red curve) applied along the b-axis. A blueshift of PL spectra can be seen from 1338 nm to 1316 nm with 0.048% tensile strain, and as the tensile strain continues to increase to 0.096%, the center wavelength of the PL continues to blueshift to ~1289 nm. In contrast, when 0.15% strain is applied along a-axis, a slight redshift of the III-A_g^{internal} Raman vibration peak was observed while the center wavelength of PL are almost unchanged [Fig.4(c)]. The results show that the strain response along the a-axis is different from the strain response along the b-axis. The giant anisotropy reflected in strain response is due to the quasi-one-dimensional structure of TiS₃. When tensile strain is applied along the b-axis, the Ti-S covalent bond length will be elongated, which will change the lattice structure and lead to an enlarged band gap. Whereas chains are connected by weak forces in the direction perpendicular to the chain, the tensile strain along the a-axis leads to a less sensitive response reflected from the Raman and PL spectra. Another sample with a strain-tuned bandgap is shown in Fig. S11 of the ESM.

To further apply higher strain to the TiS₃ nanoribbons, we prepared TiS₃ nanoribbons with wrinkles under uniaxial strain along the b-axis on the Si/SiO₂ substrate by mechanical exfoliation and dry transfer technique, with the schematic diagram and the optical micrograph exhibited in Fig.4(d). Fig.4(e) and S12 of the ESM demonstrate the Raman spectra measured on the planar region and the top of the wrinkle region for TiS₃. On the top of the wrinkle region, II-A_g^{internal} and III-A_g^{internal} modes show distinct redshifts of 2.4 cm⁻¹ and 3.62 cm⁻¹, respectively. A larger Raman mode redshift compared to Fig.4(b) implies a larger strain at this wrinkle. According to the {\gamma }_m value of 5.65 and the shift, the strain along the b-axis is around 0.26%. Fig.4(e) also shows the PL spectra measured on the planar region and on the top of the wrinkle region, where the PL spectrum on the top of the wrinkle (red curve) is blue-shifted by ~66 nm. The Raman and PL response of another TiS₃ nanosheet with the wrinkle under the uniaxial strain along the b-axis on the Si/SiO₂ substrate is shown in Fig. S13 of the ESM, in which a similar blue shift was observed on the top of the wrinkle.

4 Conclusion

In summary, we report on the tunable near-infrared light emission of layered TiS₃ nanoribbons. We developed a method to use sulfur as a transport agent to decrease the sulfur vacancy in the crystal and enhance its emission intensity. Further, PL emission from 1200 to 1370 nm in the near-infrared O band at 300 K was observed under the regulation of the thickness of the flakes. In addition, we demonstrated the emission wavelength tunability through uniaxial tensile strain by both the flexible substrates and flakes with wrinkles with interesting anisotropic strain response along the b-axis and a-axis. The enhanced PL intensity and broadband wavelength tunability in the near-infrared O band offer the prospect for intelligent integrated light sources for communication applications.

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