Vertically oriented few-layered HfS2 nanosheets: growth mechanism and optical properties

For the first time, large-area, vertically oriented few-layered hafnium disulfide (V- HfS 2 ) nanosheets have been grown by chemical vapor deposition. The individual HfS 2 nanosheets are well [001] oriented, with highly crystalline quality. Far different from conventional van der Waals epitaxial growth mechanism for two-dimensional transition metal dichalcogenides, a novel dangling-bond-assisted self-seeding growth mechanism is proposed to describe the growth of V- HfS 2 nanosheets: difficult migration of HfS 2 adatoms on substrate surface results in HfS 2 seeds growing perpendicularly to the substrate; V- HfS 2 nanosheets inherit the growth direction of HfS 2 seeds; V- HfS 2 nanosheets further expand in the in-plane direction with time evolution. Moreover, the V- HfS 2 nanosheets show strong and broadened photons absorption from near infrared to ultraviolet; the V- HfS 2 -based photodetector exhibits an ultrafast photoresponse time of 24 ms, and a high photosensitivity ca. 103 for 405 nm laser.

In spite of their superiority in theoretic prediction, there are only limited experimental researches on HfS 2 . Very recently, Kai Xu et al have demonstrated few-layer HfS 2 phototransistors, which exhibit ultrahigh photoresponsivity and photogain [9]. They also have demonstrated high-performance top-gate HfS 2 field-effect transistors (FETs) with HfO 2 as dielectric and Al and Y as buffer layer [10]. Toru Kanazawa et al have fabricated few-layer HfS 2 FETs and observed high drain current and mobility [11]. Sang Hoon Chae et al have prepared few-layer AA-stacking HfS 2 FETs display excellent on/off current ratio and reasonable mobility [12]. All HfS 2 flakes used in above studies were mechanically exfoliated from HfS 2 single crystal using the scotch tape technique. The synthesis of large-area high-quality HfS 2 nanosheets still faces a great challenge.
It is noted that numerous studies of synthesis of large-area group-VIB TMD nanosheets have been reported by chemical vapor deposition (CVD) [13][14][15][16], which is more facile and efficient than mechanical exfoliation. However, it is extremely difficult to grow HfS 2 nanosheets by CVD, mainly lacking suitable Hfbased precursor compared to synthesize other TMD nanosheets, or mixed products (such as HfO x ) existing after the reaction. Furthermore, the CVD growth of vertically oriented HfS 2 (V-HfS 2 ) nanosheets is much more difficult. But it is of great significance to grow V-HfS 2 nanosheets with high active surface areas and predominately exposed edges, which will have multifarious applications in hydrogen evolution reactions, lithium-ion batteries, sodium-ion batteries, dye-sensitized solar cells, laser pulse generation and photodetectors, similar to other vertically oriented MX 2 nanosheets [17][18][19][20][21][22]. Unfortunately, so far, there are no reports on the CVD synthesis of large-area and highquality HfS 2 nanosheets, not to mention high-quality V-HfS 2 nanosheets; corresponding growth mechanism and applications of CVD grown HfS 2 have never been explored.
To address the issues mentioned above, herein, for the first time, few-layered V-HfS 2 nanosheets with large-area size and highly crystalline quality have been grown by CVD using HfCl 4 as Hf source instead of conventional Hf-based oxide or organic compound source. A novel dangling-bond-assisted self-seeding growth mechanism is proposed to describe the growth of V-HfS 2 nanosheets, far different from conventional van der Waals epitaxy for 2D TMDs. Furthermore, the V-HfS 2 -based photodetectors exhibit excellent ultrafast photoresponse and high photosensitivity.

Results and discussion
Unlike most of CVD synthesis of TMDs, the hafnium dioxide (HfO 2 ) was not considered a reactant as hafnium source since the evaporation temperature of HfO 2 is prohibitively high (ca. 2500 C). Instead, we chose hafnium tetrachloride (HfCl 4 ) with a low sublimation temperature (317 C) as precursor by reference to monolayer and few-layer MoS 2 [23] and ZrS 2 [24,25] prepared by the reaction of chloride and sulfur under appropriate conditions. Figures 1(a)-(b) typically illustrates the low-pressure CVD setup for V-HfS 2 nanosheets synthesis in a two-zone furnace and typical growth conditions. In the growth process with argon and hydrogen flowing, the substrates were placed at the second heating zone of furnace at temperature of 950 C. To avoid unintended reaction of sulfur evaporation and HfCl 4 powder, the quartz boats loaded with HfCl 4 powders and sulfur pieces were respectively put inside two smaller quartz tubes and placed at the first heating zone of the furnace at temperature of 160 C. The heating temperature of source is determined by the point where the vapor pressure data lines intersect in the inset of figure 1(b), which is calculated in supporting information. By designing the same vapor pressure, the evaporation of source is more controllable, which depends only on the amount of precursor and the flow rate of carrier gas (see experimental section for optimized synthesis parameters). It is notable to point out that hydrogen is essential in the sulfurization of HfCl 4 , which is different than sulfurization of WCl 6 [26], MoCl 5 [23,26] and ZrCl 4 [24]. In our experiment, V-HfS 2 cannot be obtained if there is no hydrogen gas served as the strong additional reductant. The possible reaction formulation is as follows Figure 1(c) shows an optical image of large-area V-HfS 2 nanosheets grown on a ca. 5 cm × 1 cm SiO 2 /Si substrate. The as-grown V-HfS 2 appears a homogeneous yellow-green color, demonstrating its good uniformity and continuity via our CVD method.
With the purpose of investigating the morphology, crystalline structure and elemental composition of the as-grown V-HfS 2 nanosheets on SiO 2 /Si substrate, a series of characterization techniques were used including scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX), Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). As demonstrated clearly in figures 2(a)-(b), the surface structure of the as-grown HfS 2 are composed of vertically oriented nanosheets with uniform thickness (along c-axis direction) of ca. 10 nm, height (perpendicular to substrate) of ca. 400 nm and width (parallel to substrate) of ca. 1 μm. We can figure out the existence of Hf and S elements with an atom ratio of 1:2.2 from EDX shown in the inset of figure 2(b), which is very close to the ideal value for HfS 2 . The Raman spectrum is shown in figure 2(c). Apparently, excepting for Si peak at 520 cm −1 , there are two peaks located at ca. 259 and 337 cm −1 , corresponding to E g mode which caused by the vibration of the S atoms in the basal plane, and A 1g mode which is consequence of the S atoms vibration out of the basal plane, respectively. The spectrum agrees well with the previous experimental studies of bulk [27,28] and few-layered HfS 2 [9][10][11][12], and no additional peaks are observed. XRD was used to characterize the crystalline structure of V-HfS 2 . As shown in figure 2(d), excepting diffraction peak at 2θ = 33.1°a nd 69.2°ascribed to the substrate of Si (200) and (400), we observed only (00l) diffraction peak of HfS 2 at 2θ = 15.1°, 30.5°, 46.6°and 63.5°, corresponding to (001), (002), (003) and (004) of HfS 2 (JCPDS card No.28-0444), respectively. It indicates that individual HfS 2 nanosheets are highly [001] oriented, as the same as reported of another 1T-structure ReS 2 nanosheets with vertically oriented structure [29]. The narrow and sharp diffraction peaks, all with <0.3°full-width-athalf-maximum (FWHM) values, indicate the highly crystalline quality of the as-grown V-HfS 2 nanosheets. The atomic model of 1T-HfS 2 is shown in the inset of figure 2(d), in which each layer consists of three atomic planes in the sequence of S-Hf-S with interlayer distance of is 0.585 nm calculated by XRD. The elemental and bond composition of V-HfS 2 was examined by the XPS measurements. Peaks are observed at 17.8 eV, 16.1 eV, 162.0 eV and 160.8 eV, corresponding to the Hf 4f 5 2 , Hf 4f 7 2 , S 2p 1 2 and 2p 3 2 , respectively (figure 2(e)) [30]. All above confirmed that the vertically oriented nanosheets grown via our CVD system are HfS 2 with highly crystalline quality. Based on previous report [31], 2D materials with thin and vertically oriented structure could serve as an ideal material for energy storage, catalysis, detector or edge emitter devices because of the  high active surface areas and predominately exposed edges. Therefore, it is reasonable that the V-HfS 2 nanosheets have similar potential multifarious applications in such fields.
In order to understand how and why the vertically oriented few-layered hafnium disulfide nanosheets grow, the growth evolution of V-HfS 2 were investigated. The time of growth was precisely controlled and set at 1, 3, 5, 10, 15 and 20 min. It is noticed that the surface color of the as-grown film on SiO 2 /Si substrate changes from magenta, purple, cerulean, cyan, chartreuse to olive with increasing growth time from 1 to 20 min (figures 3(a)-(f), top right corner). Empirically, the blue shift of color is caused by increasing amount of V-HfS 2 . This speculation is verified by the subsequent SEM and Raman analysis. As shown in figure 3(a), an ultrathin flat layer, instead of vertically oriented HfS 2 nanosheets, was observed. And a weak Raman peak located at 337 cm −1 , which belongs to A 1g of HfS 2 , was also noticed. That means, at the beginning of growth, HfS 2 indeed exists in some form rather than nanosheet perpendicular to the growth substrate. After growing for 3 min ( figure 3(b)), vertically oriented nanosheets start to appear, along with the enhancement of A 1g peak which exceed the intensity of 2TA(X) peak at 302 cm −1 and LO(X) peak at 433 cm −1 corresponding to Si substrate [32]. When the growth time reaches 5 min (figure 3(c)), the density of V-HfS 2 increases and the weak E g peak of HfS 2 at 259 cm −1 emerges. Then, with the extended the growth time (figures 3(d)-(f)), V-HfS 2 nanosheets become larger and larger in the in-plane direction prominently, as the same of the surface coverage of V-HfS 2 on SiO 2 /Si substrate. Meanwhile, the FWHM of the A 1g peaks decrease from 14.7 to 10.3 cm −1 (figure S1), which is attributed to a larger crystallite size and a smaller amount of defects [33,34]. The location positions of A 1g and E g keep unchanged and A 1g /E g intensity ratios remain approximately constant at 0.09. One can see that few HfS 2 nanosheets have hexagonal shape (yellow arrows in figure 3(e)), suggesting the vertical growth rate is near equilibrium but can be further optimized [35]. The sheet size evolution of V-HfS 2 visualizes the growth process and reveals two forms of HfS 2 during development.
For further studying the growth mode and explore the growth mechanism of V-HfS 2 , the morphologies, crystalline structures and element composition of the sample, which was at intermediate stage (sample grown for 15 min) with both vertically oriented and the other form of HfS 2 simultaneously, were characterized. Figure 4 Figure 4(e) is HRTEM of the nanoparticles which compose the bottom layer, corresponding to the region 2 marked in figure 4(b). The lattice figures located at the edge of nanoparticles demonstrate a structure consisting of bent planes with lattice spacing of 0.59 nm, which correspond to HfS 2 (001) planes. Lager-magnification HRTEM image of the denoted red window in figure 4(e) is given in figure 4(f). It distinctly indicates the nanoparticle is edge-oriented HfS 2 as demonstrated by the schematic structure model overlapped. Combining edge-oriented structure with approximate circle shape, two conceivable models of the nanoparticles, the other HfS 2 form, are illustrated in the inset of figure 4(f): edge-oriented closed and not-yetclosed polygonal nanoplates. The model of fullerenelike sphere is not suitable due to taking consideration of that the layer consisted of nanoparticles should have observable thickness of ca. 60 nm if the nanoparticles are near-spherical, but the fact is that the layer has hardly any thickness in the SEM image comparing with 300 nm SiO 2 layer. EDX compositional mappings of Hf, S, Cl and O distributions within HfS 2 nanoparticle are shown in figure 4(g). Referencing HRTEM image of HfS 2 nanoparticle, Hf and S elements distributions are essentially homogeneous, Cl element is hardly to observe, and very little O element is distributed to the whole area due to absorption of oxygen and slight oxidization of HfS 2 , which is reflected in ratio of Hf/S (1:1.8) from EDX spectrum and caused by transfer process to TEM gird. This result of EDX analysis is different from the growth mechanism of TMD monolayers (self-seeding fullerene nuclei) reported recently [36], the nanoparticle is not composed of a partially sulfurized hafnium tetrachloride, HfS x Cl 4 − 2x , or partially sulfurized hafnium dioxide, HfS x O 2−x wrapped in a fullerene-like shell of HfS 2 . But the mechanism in our experiment can also be deduced as 'self-seeding', in which HfS 2 nanoparticles act as the seed or buffer layer for the growth of vertically oriented HfS 2 nanosheets. It is noticed that the gaps between nanoparticles, bright area in figure 4(g), are not distributed any Hf or S elements. It means no horizontal HfS 2 film grown on the seed or buffer layer, consequently, the vertical growth mechanisms of TMDs based on horizontally aligned layers, such as the hierarchical growth [21], bending-growth resulted from fracturing of layers [37], collision and up-curving between two islands [38] are not suitable for our synthesis process.
To better understand the growth mechanism, the schematic illustration for the growth of V-HfS 2 nanosheets is shown in figure 5. Firstly, at the original reaction stage (i), the vapor of HfCl 4 , sulfur and hydrogen reacted to form HfS 2 species, which diffuse onto substrates simultaneously. Then, the HfS 2 species adsorbed on substrate try to migrate to form HfS 2 nanostructure. However, the existence of dangling bonds at the surface of SiO 2 /Si substrate increase the energy barrier for migration of HfS 2 adatoms along the substrate surface [39]. In addition, the surface migration strongly depends on the interaction of the adatoms with substrate and the concentration gradient of the adatoms [40]. Thereby, the large interlayer interaction energy of 1T-HfS 2 (1.33 eV/primitive cell), which is much larger than MoS 2 (ca. 0.46 eV/ primitive cell) [12], and low diffusion flux of gaseous HfS 2 species form the vapor phase at the beginning of reaction seriously shorten the migration length, leading to vertical growth. Subsequently, at the self-seeding state (ii), in the absence of strong interaction forces, the individual HfS 2 layers prefer to bend to form closed structures in order to decrease the number of marginal dangling bonds and the total energy of the system. The height of HfS 2 nanoparticles is restricted in limited nucleation time before the nanoparticles cover the whole surface of SiO 2 /Si substrate. At the preliminary vertical growth stage (iii), the exposed dangling bonds at the top surface edges of HfS 2 nanoparticles act as growth sites for growing of vertically oriented HfS 2 nanosheets, which inherit the growth direction from HfS 2 seeds. The growth kinetics changes from on-substrate migration to on-nanoplate migration [40], the HfS 2 adatoms on surface of HfS 2 nanoparticles migrate to the growth sites and are incorporated into crystal lattice of HfS 2 nanosheets. Afterwards, with abundant flux of HfS 2 species, the HfS 2 nanosheets expand prominently in in-plane direction at the extension stage (iv). Distinguishing from conventional two-dimensional TMD materials growth, which is parallel to substrate and based on van der Waals epitaxy, the novel growth of HfS 2 perpendicular to substrate is based on dangling bonds at the surface of substrate and the edges of self-seeding layer. In-depth experimental work will be needed to further elucidate this mechanism.
To explore the potential applications for V-HfS 2 , the optical properties of V-HfS 2 have been further studied. Figure 6(a) shows the UV-vis absorption spectrum of V-HfS 2 nanosheets transferred on optical quartz by fast dry-transfer process. Compared to quartz substrate, two peaks located at 3.2, and 6.2 eV and two slight shoulders located at 2.9, and 5.2 eV are observed, corresponding to E2, E4, E1, E3 optical gaps of HfS 2 , respectively [41]. The optical transitions in the absorption edges are contributed to the indirect allowed transitions with strongly excitonic effects [42]. The photons absorption of V-HfS 2 nanosheets from near infrared to ultraviolet makes them potential candidates for solar cell applications. Figure 6(b) shows the the top of valence band and bottom of conduction band of mono-, bi-, tri-, quad-layer and bulk HfS 2 data extracted from calculated band structures (figure S2), respectively. The direct excitonic transitions occur at Γ point and the indirect from Γ point to M point. With reduced layer thickness, both direct and indirect bandgap become larger, from 2.71 to 3.01eV and 1.74 to 2.27 eV, respectively. That means different thickness distribution of few-layered HfS 2 nanosheets broadens the absorption range, which is consistent with our absorbance spectrum. The strong optical absorption based on direct and indirect transitions suggests that V-HfS 2 has potential applications in optoelectronic devices.
As a typical application example, here the V-HfS 2 photodetectors was fabricated. Unlike MoS 2 or WS 2 , HfS 2 thin film is easily oxidized under ambient conditions [12]. To prevent HfS 2 from oxidization, the fast dry-transfer approach is more appropriate than the traditional PMMA-assisted wet-transfer method. Figure S3 illustrates the schematics of the dry-transfer process using thermal release tape. In brief, firstly, the tape was stuck on the surface of as-grown V-HfS 2 , then the tape was peeled off with V-HfS 2 from the growth substrate and pressed onto the target substrate, i.e. gold interdigitated electrode onSiO 2 /Si substrate.
After that, the target substrate was placed on a hotplate at 120 C. The tape lost the adhesion and automatically degummed, leaving the V-HfS 2 on the target substrate. The total time spend in try-transfer approach is less than 5 min so that to keep the sample from being oxidized during fabrication as possible. Figure 6(c) shows the long-term stability and repeatability of V-HfS 2 photodetector with periodically turning on and off of a commercial laser pointer (wavelength 405 nm, power 5 mW, spot size 0.2 cm 2 , power density ca. 250 W m −2 ) at the voltage of 1 V. The enlarged potion of one single photoresponse cycle is given in figure 6(d), which demonstrates a low off state current of ca. 1.5 pA and a high on state current of ca. 1.2 nA. The device exhibits high photosensitivity to 405 nm laser with a large on/off ratio of ca. 10 3 , a comparable value to the reported HfS 2 phototransistor without gate voltage [9]. As expected, In figure S4, the on/off ratio of device illuminated by 405 nm (3.1 eV) is larger than 532 nm (2.3 eV) and 650 nm (1.9 eV) laser, matching with the UV-vis absorption spectrum. More uplifting is the rise and fall times are both as short as 24 ms, limited by the precisions of measurement, demonstrating an ultrafast response and recovery performance. It is noticed that the rise time is shorter than photodetector based on mechanically exfoliated HfS 2 nanosheets without gate voltage (150 ms), even shorter than that with gate voltage at 80 V (38 ms) [9]. It is speculated that the fast response time is attributed to the less of Hf ion vacancies in analogy with that reported by Jun He group on GaTe phototransistor based on CVD grown GaTe and mechanically exfoliated GaTe [39,43]. It needs to be pointed out that the performance of device might be further improved by using a phototransistor mode due to the response time and responsivity are strongly dependent on gate voltage [9,10].

Conclusions
In summary, we have successfully synthesized largearea, vertically oriented few-layered HfS 2 nanosheets with well [001] textured and highly crystalline quality by CVD. The dangling-bond-assisted self-seeding growth mechanism was presented. UV-vis absorption spectrum indicates that the V-HfS 2 has strong optical absorption based on direct and indirect transitions, suggesting that it has potential applications in optoelectronic devices. The photodetectors based on V-HfS 2 nanosheets exhibited high photosensitivity to 405 nm laser with a large on/off ratio of ca. 10 3 and ultrafast response performance with rise and fall times of 24 ms. The high active surface areas and predominately exposed edges may make V-HfS 2 nanosheets potential multifarious applications in the field of energy storage, catalysis, detector or edge emitter.

Synthesis of V-HfS 2 nanosheets
Typically, Single crystal silicon substrates with a thermally deposited 300 nm thick silicon oxide layer (SiO 2 /Si) were ultrasonically cleaned for 5 min with acetone, ethyl alcohol and deionized water, respectively. After drying with compressed nitrogen gas, the substrates were put inside 1 inch quartz tube and placed at the second heating zone of the furnace. Two quartz boats loaded with 100 mg hafnium chloride (HfCl 4 ) powders (99.9%, Alfa Aesar) and 200 mg sulfur pieces (99.999%, Alfa Aesar) were respectively put inside two small quartz tubes and placed at the second heating zone of the furnace. The pressure in the chamber was reduced to 4×10 −3 Pa, and then, argon (10 sccm) and hydrogen (10 sccm) were pumped into the tube, reaching a pressure of 18 Pa, which keeps unchanged during the growth of V-HfS 2 nanosheets. The first heating zone was kept at room temperature before the second heating zone getting close to 950 C and then heated to a temperature of 160 C with heating rate of 25 C min −1 as fast as the second heating zone. After growing for 20 min, all heating was stopped and the furnace was opened immediately, then the quartz tube was taken out to rapidly cool down.

Calculation details
The calculations were performed mainly with pseudopotential code PWscf [44], with the van der Waals density functional module to treat van der Waals interaction between layers [45]. The generalized gradient approximation functional was parameterized by Perdew, Burke, and Ernzerhof [46]. The plane-wave kinetic energy cutoff was set to 70 Ry with the density cutoff 700 Ry, and shifted 15×15×1 Monkhorst-Pack meshes for the layered structures and 15×15×13 for the bulk were used to perform Brillouin zone integration in order to ensure the convergence of the results. The convergence of the total energy was set to be better than 10 −9 Hartree. A vacuum layer with thickness of Å 30 was used to model the 2D-nature of the compounds.

Device fabrication
To fabricate the HfS 2 photodetector, a Gold interdigitated electrode with a finger width and an interfinger spacing of ca. 200 mm was firstly patterned on SiO 2 /Si substrate by electron beam evaporation using metal hard mask, and then HfS 2 nanosheets were transferred to Au interdigitated electrode using dry transfer assisted by thermal release tape (Notto Denko No. 3193MS, 7.3 N mm −1 ).

Characterizations
The morphology and composition investigations were examined by SEM (JEOL JSM-7000F) with an EDX and TEM (FEI Tecnai F20 at 200 kV). TEM samples were prepared by scraping the as-grown V-HfS 2 off the substrate, then sending them to ethanol and dropdrying onto TEM grids. The crystalline structure of the obtained samples was characterized by XRD (Jordan Valley D1 Evolution) using Cu Kα radiation and HRTEM. Raman spectra were acquired at room temperature with an excitation laser line of 514.5 nm (Renishaw inVia Reflex). The electronic structure was performed by XPS (Kratos AXIS-Ultra). The UV-vis absorption was measured with spectrophotometer (Shimadzu UV-2550). The data of photocurrent were recorded using a semiconductor parameter analyzer (Agilent 4155B) at room temperature and atmospheric pressure.