Tantalum disulfide quantum dots: preparation, structure, and properties

Abstract Tantalum disulfide (TaS2) two-dimensional film material has attracted wide attention due to its unique optical and electrical properties. In this work, we report the preparation of 1 T-TaS2 quantum dots (1 T-TaS2 QDs) by top-down method. Herein, we prepared the TaS2 QDs having a monodisperse grain size of around 3 nm by an effective ultrasonic liquid phase exfoliation method. Optical studies using UV-Vis, PL, and PLE techniques on the as-prepared TaS2 QDs exhibited ultraviolet absorption at 283 nm. Furthermore, we found that dimension reduction of TaS2 has led to a modification of the band gap, namely a transition from indirect to direct band gap, which is explained using first-principle calculations. By using quinine as reference, the fluorescence quantum yield is 45.6%. Therefore, our results suggest TaS2 QDs have unique and extraordinary optical properties. Moreover, the low-cost, facile method of producing high quality TaS2 QDs in this work is ideal for mass production to ensure commercial viability of devices based on this material. Graphical abstract TaS2 quantum dots having a monodisperse grain size of around 3 nm have been prepared by an ultrasonic liquid phase exfoliation method, it has been found that the dimension reduction of TaS2 has led to a transition from indirect to direct band gap that results in the unique and extraordinary optical properties (PL QY: 45.6%).


Introduction
Recently, the family of layered transition metal dichalcogenides (LTMDs) [1] has drawn great attention in many fields, such as electronic devices [2], energy storage [3], catalysis [4], bio-imaging [5,6], and biosensing [7], due to its many interesting physical and electrical properties. Typically, the structure of LTMDs is formed [1] by covalently bonded monolayer, and each monolayer is linked by Van der Waals forces; hence the LTMDs can be easily cleaved along the layer plane by either chemical or physical methods. According to previous work, the band gaps of LTMDs can be modified from indirect to direct band gap by decreasing the number of layers [8]. In particular, the TaS 2 has been studied extensively for various applications, ranging from optical switch [9] to catalysis [10], as they exhibit tunable band gap, controllable size, and strong photoluminescence. Therefore, it is becoming a widely focused functional material.
At present, both top-down and bottom-up methods are adopted to prepare nanomaterials [11,12]. The bottom-up approach is based on atoms and molecules as building blocks, which are used to form nanoparticle structures according to relevant purposes. This method mainly involves gas-phase and liquid-phase reactions [13,14]. As for the top-down approach, electrochemical and etching methods [15,16] have been applied to prepare TaS 2 nanomaterials by weakening the Van der Waals interactions and cleaving the bonding force between the layers to obtain TaS 2 nanomaterials from its bulk materials. For example, Zeng et al. [17] prepared TaS 2 nanosheets through lithium interaction and exfoliation by controlling the cut-off voltage. Zhang et al. [18] prepared monodispersed TaS 2 nanodots by a facile topdown method. QDs [19][20][21][22] has attracted wide interest worldwide in recent years, but TaS 2 QDs are rarely reported. Therefore, facile methods are still in need to prepare industry applicable TaS 2 QDs with narrow size distribution and good dispersion.
In this study, TaS 2 QDs with a monodisperse grain size of around 3 nm were prepared by ultrasonic method. In this process, the Van der Waals interactions in between the TaS 2 layers were first weakened by intercalation of NMP and then followed by further exfoliation using high power ultrasonic energy. The size of TaS 2 QDs can be easily tuned by adjusting centrifugation rate and time; a higher centrifugation rate and a bigger centrifugation time result in a smaller size. This provides an efficient and practical method in the preparation of TaS 2 QDs. Its structural, electronic, and optical properties were characterized by experiments, as well as firstprinciple calculations.

Preparation of TaS 2 QDs
TaS 2 powder 0.5 g was grinded in the mortar for 2 h. Fifty milliliters of NMP solvent was added to the grinded powder sample. The mixture was then ultrasonic treated for 4 h with an ultrasonic power of 210 W. The suspension after ultrasonic treatment was centrifuged at the rate of 7000 rpm for 25 min. The supernatant, which obtains the TaS 2 QDs, was collected.

Results and Discussion
The process of TaS 2 QDs formation from its bulk crystal is depicted in Fig. 1a. The preparation process consisted of three steps, namely grinding, ultrasonic, and centrifugation. An enlarged schematic of TaS 2 QDs is shown within the dotted red square of Fig. 1a. The tawny solution in the sample bottle was TaS 2 QDs solution after centrifugation. Figure 1 b shows the TEM image of the TaS 2 QDs, which are spherical in shape with uniform size distribution. As shown in the inset, the size distribution of the TaS 2 QDs followed a Gaussian fitted curve with an average diameter of W C = 3.0 nm and full width at half maximum (FWHM) of 1.4 nm. It was reported that the thickness of the TaS 2 monolayer ranged from 0.6 to 1.2 nm [23]; hence, the QDs could comprise of 2-5 layers of TaS 2 . The number of TaS 2 layer can be reduced by increasing the centrifugal rate and time (as shown in Additional file 1: Fig. S1).
The FFT pattern is shown in the inset of Fig. 1c. It shows a hexagonal crystalline structure, which corresponds to the TaS 2 QDs structure in Fig. 1a. As shown in Fig. 1d, the TaS 2 QDs line profile exhibits obvious lattice stripe with a spacing of 0.207 nm. The SEM image in Fig. 1e shows a uniform distribution of TaS 2 QDs, thus indicating a good dispersibility and uniform size distribution. At higher SEM magnification, it is apparent that the surface consisted of rugged particles as shown in Fig. 1f. This indicates the formation of independent spherical TaS 2 QDs during the preparation process. EDS technique was used to characterize the elemental composition as shown in Fig. 1g. A film of TaS 2 QD S was deposited on copper sheet during the EDS characterization in order to avoid the overlapping of Ta and Si/SiO 2 substrate peaks, which could complicate the analysis. The measured atomic percentage ratio of Ta and S in the material was 33.9/66.1 ≈ 1:1.95, which is close to the theoretical value of 1:2. Figure 2 a shows AFM images of the TaS 2 QDs, labeled as A, B, and C, which were randomly selected and their heights were measured to be 2.30 nm, 2.03 nm, and 2.91 nm, respectively. The average height of 2.41 nm is consistent with the average diameter of 3.01 nm measured using TEM. The FTIR spectra, shown in Fig. 2b, reveal that the Ta-S bond stretching vibration absorption peak was situated at 616 cm −1 . Figure 2 c shows Ta 4f, S 2p, ★ C 1s, and ★ O 1s peaks from XPS full-scan spectrum. The ★ C 1s and ★ O 1s peaks were impurity  Figure 2 d shows the XPS spectrum of S 2p deconvoluted into three components, namely S 2p 3/2 (163.4 eV), S 2p 1/2 (166.7 eV), and oxidized sulfur (168.2 eV). The XPS spectrum of Ta 4f is shown in Fig. 2e and was deconvoluted into components, such as Ta 4f 7/2 (23.2 eV), Ta 4f 5/2 (25.6 eV), and Ta 4f 7/2 (27.2 eV). The Ta 4f 7/2 peak at 27.2 eV is associated with oxidized tantalum [24,25]; oxidation has also been observed in other QDs [26][27][28]. Figure 2 f shows the Raman vibration mode of TaS 2 QDs. The E 1 2g and A 1g modes relate to the in-plane and out-of-plane vibration respectively [29]. The A 1g and E 1 2g modes of the TaS 2 QDs were observed at 301.4 cm −1 and 242.3 cm −1 respectively. The Raman intensity of the E 1 2g vibration mode was much smaller than that of A 1g , which could due to the fact that A 1g mode is more sensitive to strain than the E 1 2g mode in TaS 2 QDs. It shows that the A 1g mode dominated during the preparation process of TaS 2 QDs. Figure 2 g shows XRD diffraction pattern of the TaS 2 QDs. When compared with space group P3m1(164), the pattern indicates trigonal structure of 1 T-TaS 2 [30]. According to the standard PDF#04-001-0068 card, the diffraction peak 2θ at 15.0°represented crystal plane (001) with d = 0.590 nm, which corresponds to the C-axis crystal surface spacing. The peak at 30.2°represented crystal plane (002) with d = 0.295 nm. The peak at 33.0°(asterisk peak) was originated from the Si (001) substrate [31]. The grain size can be calculated using the Debye-Scherrer (Eq. (1)) [32].
where D is grain size, β is FWHM of the diffraction peak of the measured sample, θ is diffraction angle, and λ is X-ray wavelength. The calculated grain size of 3.8 nm using the strongest diffraction peak (001) is close to the TEM result of 3.01 nm. Figure 3 a and d show the PL and PLE spectra of the TaS 2 QDs respectively, with excitation wavelength (λ Ex ) varied from 320 nm to 460 nm and the emission wavelength (λ Em ) changed from 400 to 520 nm at 20 nm step. The PL and PLE peaks were redshifted, as indicated by the dark blue lines in Fig. 3a and d respectively. As shown in Fig. 3b and e, the red-shift of the normalized intensity peaks is more noticeable for the PL (e.g., from 391 to 519 nm) than the PLE (e.g., from 324 to 380 nm) spectra. The wavelength and energydependent PL and PLE peaks are shown in greater details in Fig. 3c and f respectively. The peak energies of the redshifted excitation wavelength ranged from 3.17 to 2.39 eV, while the red-shifted emission energies ranged from 3.83 to 3.26 eV. It can be seen that a higher excitation energy (λ Ex = 320 nm) led to a larger Stokes shift (71 nm), whereas a lower excitation energy (λ Ex = 460 nm) resulted in a smaller Stokes shift (59 nm). The difference in Stokes shift is probably due to the size distribution of the prepared QDs, which has also been observed from Se QDs and Te QDs [27,28]. Comparing the PL and PLE peaks, Fig. 3 a, d The PL & PLE spectra of TaS 2 QDs [* interference peak (λ Ex and λ Em ) from the instrument], respectively; b, e The PL and PLE normalized spectra of TaS 2 QDs under different λ Ex and λ Em , respectively; c, f the relationship of peak and energy for TaS 2 QDs the PL peaks exhibited a greater red-shift than the PLE peaks, and with the increase of the peak wavelengths, the PL peak has a larger Stokes frequency shift than the PLE peak [33]. The red-shifted intensity peak indicates that the optical properties of TaS 2 QDs have an obvious dependence on the wavelength. Figure 4a shows the UV-Vis absorption spectra of TaS 2 QDs. An absorption peak was observed at 283 nm, which is caused by electron transition upon UV illumination. Based on the study of the reduction in the number of layers, a blue-shifted absorption peak was observed (as shown in Additional file 1: Fig. S2). In addition, the TaS 2 QDs solution appeared yellow in color under natural light, peony in color under ultraviolet at 254 nm, and blue in color at 365 nm. Tauc mapping method was used to calculate the TaS 2 QDs' band gap spectrum, according to Eq. (2) [17,34]: where α is absorption coefficient, A is a constant, hv is light energy, and E g is band gap energy. The TaS 2 QDs has a direct band gap (E g = 3.69 eV) as shown in Fig. 4b.
The results indicate a reduction in the number of layers in TaS 2 QDs would lead to a modification of the band gap, including indirect to direct band gap transitions. Based on the above results, TaS 2 QDs energy level structure is proposed as shown in Fig. 4c. During electron transition (E1-E2), the energy level of TaS 2 QDs is E g = 4.38 eV. Due to the presence of surface energy level, an emission wavelength of 401 nm is observed and this corresponds to E g = 3.09 eV. Therefore, the transition energy from E1 to surface state is about 1.3 eV. In order to study the influence of fluorescence effect caused by the increase in band gap, the fluorescence quantum yield (Q s ) of TaS 2 QDs was calculated using quinine (Q r = 0.54 in 24.9% ethanol) as reference, based on the following equation (Eq. 3) [35,36]: where the subscript s denotes sample and r indicates reference. Q is PL quantum yield, I is emitting peak area of fluorescence, A is absorbance at a specific excitation wavelength, and n is refractive index. The calculated fluorescenceyield of 45.6% indicates excellent fluorescence properties of the TaS 2 QDs. First-principle calculations were performed to further investigate the reasons for the increase in band gap of TaS 2 QDs. Figure 5 a and b show the bulk and monolayer structures of TaS 2 . For the monolayer TaS 2 , a vacuum of 29.5 Å in the Z direction was added when constructing the unit-cell. The calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab initio simulation package (VASP) [37][38][39]. The electronic exchange-correlation effects were treated using the generalized gradient  [40,41]. When using Heyd-Scuseria-Ernzerhof (HSE) hybrid functionals [42,43], 25% Hartree-Fock and 75% PBE-GGA were chosen for short-ranged exchange part in the HSE06 hybrid functionals. The projector augmented wave (PAW) method was utilized to treat the interactions between the ionic cores and the valence electrons [44,45], where valence electron configuration of S and Ta were set as 3S 2 3p 4 and 5d 3 6s 2 , respectively. The energy cut-off of plane wave basis was set to 520 eV. The Monkhorst-Pack grid mesh [46] of 11 × 11 × 7 and 11 × 11 × 1 were used to sample the Brillouin zone of the bulk and monolayer TaS 2 , respectively. The convergence in energy was set to 1 × 10 −5 eV during electronic structure calculations. The electronic structures of bulk and monolayer TaS 2 were calculated by PBE functional, as shown in Fig. 5c and e, respectively.
The results are in good agreement with previous calculations [47,48]. Both the bulk and monolayer TaS 2 have metallic in-phase states, and the band across the Fermi level is mostly composed of d x2 orbital of Ta atoms. The valence band is mainly composed of p orbitals of S atoms, while the conduction band is mainly from d orbitals of Ta atoms. At Γ point, an indirect gap is transiting to a direct gap from bulk to monolayer structure due to lacking of inter-plane interactions. The band structure was checked using HSE06 hybrid functionals (Fig. 6a, b). The results are similar to PBE except for a larger gap near Γ point for the HSE results, where the conduction band shifted toward lower energy for about 0.5 eV. The absorption spectrum of monolayer TaS 2 was also calculated and it contains mainly four peaks at 1.41 eV, 2.00 eV, 6.61 eV, and 7.23 eV. Comparing the absorption spectra and the PDOS, as shown in Fig. 6c and d, the two peaks in the 0~2 eV region are provided by the S 3p→Ta 5d electronic transitions, and the S 3p → Ta 6 s electronic transitions contribute to the peaks in the 6~8 eV region.
Next, the TaS 2 QDs were modeled as clusters with one and two-layer of Ta-2S unit and compared their spinpolarized DOS. As shown in Fig. 6e and f, due to the dangling bonds of S atoms at the surface, the spin-polarized DOS of the one-and two-layer models show half-metallic nature, where there is a~3 eV gap for the spin-up electrons that is twice the gap at Γ point of the infinite 2D monolayer in Fig. 6b. This demonstrates strong quantum confinement effect. The gap of spin-up electrons of the one-layer model is slightly larger than the two-layer model as a result of lacking inter-plane interactions.

Conclusions
TaS 2 QDs having an average size of about 3 nm were prepared by ultrasonic method. The morphology and structural studies performed on the nanomaterials show that they have controllable and hexagonal honeycomb shape. The optical properties of the TaS 2 QDs, including absorption and PL, were investigated. A red-shifted effect, compared to the bulk material, was observed and the QDs exhibited multicolor luminescence with strong absorption in near ultraviolet region. The band gap of the TaS 2 QDs increased to 3.69 eV from indirect to direct band gap, hence exhibiting extraordinary optical properties. The indirect to direct transition and quantum confinement effect in the electronic structures were confirmed by firstprinciple calculations of a simple model of the QDs. These results will extend the application of TaS 2 QDs in devices, such as photodetectors. Furthermore, the preparation method is also applicable to other layered materials to produce low-cost high-quality QDs from bulk materials.

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Competing Interests
The authors declare that they have no competing interests.