Selective Direct Growth of Atomic Layered HfS2 on Hexagonal Boron

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Selective Direct Growth of Atomic Layered HfS on Hexagonal Boron Nitride for High Performance Photodetectors Denggui Wang, Junhua Meng, Xingwang Zhang, Gencai Guo, Zhigang Yin, Heng Liu, Likun Cheng, Menglei Gao, Jingbi You, and Ruzhi Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01091 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018

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Chemistry of Materials

Selective Direct Growth of Atomic Layered HfS2 on Hexagonal Boron Nitride for High Performance Photodetectors Denggui Wang,† Junhua Meng,† Xingwang Zhang,*,† Gencai Guo,‡ Zhigang Yin,† Heng Liu,† Likun Cheng,† Menglei Gao,† Jingbi You,† and Ruzhi Wang‡

†

Key Lab of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy

of Sciences, Beijing 100083, & College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡

College of Materials Science and Engineering, Beijing University of Technology, Beijing

100124, P. R. China

ABSTRACT: Hafnium disulfide (HfS2) has attracted significant interest because of the predicted excellent electronic properties superior to group VIB transition metal dichalcogenides. On the other hand, atomically thin hexagonal boron nitride (h-BN) is an ideal dielectric substrate for optoelectronic applications of other 2D materials. Here, for the first time we report the direct growth of high-quality atomic layered HfS2 on few-layer h-BN transferred on SiO2/Si substrates by chemical vapor deposition. It is found that the HfS2 layers are selectively grown on h-BN rather than on SiO2/Si. Density functional theory calculations are performed to help understand the mechanism of selective growth of HfS2. Furthermore, the photodetectors based 1 ACS Paragon Plus Environment

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on the HfS2/h-BN heterostructures exhibit excellent visible-light sensing performance, such as a high on/off ratio of more than 105, an ultrafast response rate of about 200 µs, a high responsivity of 26.5 mA W-1 and a competitive detectivity exceeding 3×1011 Jones, superior to the vast majority of the reported 2D materials based photodetectors. The remarkable performance of the HfS2/h-BN photodetector is attributed to the unique features of 2D h-BN substrate.

INTRODUCTION Inspired by the recent advancements in graphene, atomically thin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted significant interest because of their intriguing physical properties.1-8 Among the TMDs, hafnium disulfide (HfS2), where the Hf layer is sandwiched between two sulfur layers by covalent forces, possesses many interesting properties.9-13 For instance, the room-temperature acoustic-photon-limited mobility of HfS2 was calculated to be above 1800 cm2 V-1 s-1, which is much higher than that of widely studied MoS2 (340 cm2 V-1 s-1).11 Furthermore, it was reported that HfS2-based field-effect transistors (FETs) can have sheet current densities of up to 650 µA µm-1, approximately 85 times higher than that of MoS2.12 These extraordinary properties make HfS2 attractive for applications in logic and optoelectronic devices. Recently, based on micromechanical exfoliated few-layer HfS2, ultrasensitive phototransistors and high-performance FETs have been fabricated by several groups.14-17 However, the mechanical exfoliation method lacks in control in the number of layers, and has a low yield and limited flake sizes, hindering the study and practical applications of 2D HfS2. Chemical vapor deposition (CVD) method has initially been applied to synthesize 2D HfS2 on SiO2/Si substrates by Zheng et al..18 Nevertheless, due to the higher interlayer interaction energy of HfS2 and the influence of substrate surface energy, the vertically oriented 2 ACS Paragon Plus Environment

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thick HfS2 nanosheets were obtained, which is unsuitable for the construction of electronic devices. More recently, it was reported that 2D HfS2 nanoflakes have been synthesized on mica and sapphire substrates by CVD.19-21 Mica was seldom used as substrates in electronic devices, and bulk substrate appears to have only a limited application. Especially, recent reports reveal that heterostructures vertically aligned by two different 2D materials behave with novel properties, offering a promising approach to design and fabricate novel electronic devices.22-26 Thus, searching for a suitable 2D material as substrate to achieve high-quality 2D HfS2 is highly desirable. Hexagonal boron nitride (h-BN) has an atomically flat and dangling-bond-free surface and a wide band gap of 5.9 eV, which makes it an ideal dielectric substrate for optoelectronic applications of other 2D materials.27 Indeed, micrometer scale ballistic transport and quantum Hall effect have been observed in the h-BN-based 2D heterostructures.28-30 Graphene and MoS2 displayed a large increase in their mobility when h-BN was used as the dielectric substrate, leading to significantly improved device performance.31-34 It might be thought that the HfS2/hBN heterostructures will also exhibit fascinating properties and improved device performance. It is thus highly interesting to directly grow HfS2 on 2D h-BN while taking advantage of unique features of h-BN, which was never attempted previously. In this work, for the first time we report the direct growth of high-quality 2D HfS2 on fewlayer h-BN transferred on SiO2/Si substrates by CVD method. It is noteworthy that the HfS2 layers are selectively grown on h-BN rather than on SiO2/Si. The calculated cohesive energy provides a fundamental theoretical account for the selective growth of HfS2 on h-BN. Furthermore, a high-performance HfS2/h-BN photodetector was fabricated with a high on/off ratio of more than 105 and an ultrafast response rate of approximately 200 µs. The

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photoresponsivity and detectivity are up to 26.5 mA W-1 and 3.3×1011 Jones, respectively. The remarkable performance of the HfS2/h-BN photodetector is attributed to the 2D h-BN substrate. The results suggest that HfS2/h-BN heterostructures are highly promising for future applications in high-performance optoelectronics.

EXPERIMENTAL SECTION Synthesis and Transfer of h-BN. The h-BN domains and films were prepared on the Cu foils by ion beam sputtering deposition (IBSD) method, as reported previously.35 Prior to the hBN growth, a cleaned Cu foil was annealed at 1000 °C for 20 min under 20 sccm (standard cubic centimeters per minute) H2 atmosphere, then boron and nitrogen species were sputtered from a pure h-BN target by a 1.0 keV Ar ion beam. During the growth, the chamber was kept at a constant pressure of 3×10-2 Pa, and the typical substrate temperature was 1050 °C. After growth, the h-BN was transferred to a SiO2/Si substrate by a rosin-assisted wet-transfer method. Typically, a thin layer of rosin (30 wt. %, Alfa Aesar) was first spin-coated on the h-BN at 3000 rpm for 1 min. The rosin layer was then cured at room temperature, followed by etching the Cu foil in the Cu etchant solution (Alfa Aesar) to obtain a rosin/h-BN stack. Next, the rosin/h-BN stack was washed by DI water to remove the residual reagent and then transferred to a SiO2/Si substrate. Subsequently, the rosin layer was dissolved by isopropyl alcohol and ethyl alcohol in sequence. Finally, the h-BN was annealed at 700 °C in pure O2 atmosphere for 30 min to remove the residual organic matter. Growth of HfS2 on h-BN. The HfS2 layers were synthesized on h-BN using a CVD apparatus. HfCl4 (>99.9%, Alfa Aesar) and S (>99.999%, Sigma Aldrich) powder were used as the reactant sources. 1000 mg of S, 30 mg of HfCl4, and h-BN on SiO2/Si substrates were placed


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in the center of the low-, middle- and high-temperature zones of a quartz reactor, respectively. A mixture of Ar (20 sccm) and H2 (10 sccm) flows was used as the carrier gas. S was heated to 200 °C and HfCl4 was heated to 300 °C, respectively, while the substrate was keep at 950 °C. After the growth process, the tube furnace was cooled down naturally to room temperature under an Ar flow of 100 sccm. Characterizations of HfS2/h-BN Heterostructures. The morphology and structure of HfS2/h-BN were characterized using optical microscope (Olympus BX51M), SEM (FEI Quanta450), AFM (NTMDT Solver P47), TEM (JEOL JEM-2010, 120 kV) equipped with EDS. SEM images were taken at an accelerating voltage of 10 kV, a spot size of 3 and a working distance of 5 mm. XRD spectra were recorded by a Rigaku D/MAX-2500 system using Cu Kα as the Xray source. Raman spectroscopy was acquired with a confocal spectrometer (Renishaw Model in Via-Reflex) using a 532 nm laser as the excitation source. The FTIR spectra were taken in attenuated total reflection (ATR) mode by a Varian spectrometer (Excalibur 3100). The PL spectra were measured at room temperature with a HITACHI F-4500 spectrometer using the 380 nm excitation wavelength. Optical absorption spectra were obtained by using a Varian Cary 5000 UV-vis spectrophotometer in a double-beam mode. XPS measurements were carried out on an ESCALAB 250Xi instrument with a monochromated Al Kα source. Computational Method. First-principles calculations were performed based on the framework of DFT as implemented in the VASP. The projector augmented wave (PAW) method and a plane-wave expansion set with an energy cutoff of 450 eV were used in the calculations. In order to minimize the lattice mismatch effects, a super cell of 2×2×1 and 3×3×2 were used for HfS2 and h-BN in HfS2/h-BN, respectively. The amorphous nature of SiO2 makes it difficult to study by theoretical methods. However, some recent works opened the way to



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simulate the amorphous surface via periodic-DFT.36,37 Here, the SiO2 component was amorphized by melting and quenching using classical MD method, and the structural model was further relaxed through periodic-DFT. Although the atom sites and structures of our calculations are different with previous works, it is still reliable due to the similar method used to produce the amorphous SiO2 structure. To match the lattice, a 3×5×1 orthometric supercell was used for HfS2 in HfS2/SiO2. All the structures were fully relaxed using the conjugated gradient method, until the force was less than 10-2 eV Å-1 and the energy was less than 10-4 eV between two consecutive self-consistent steps. Device Fabrication and Characterization. For fabrication of photodetectors, the 100-nmthick Au interdigitated electrodes separated by 70 µm were deposited on the as-grown HfS2/hBN layers by thermal evaporation through a shadow mask under a high vacuum of 1×10-4 Pa. All the device measurements were performed under ambient conditions. The I-V characteristics were measured using a Keithley 2450 source meter in the dark and under illumination. A 450 nm laser with adjustable optical power was utilized as the light source and the output power was measured using a laser power meter. A broadband laser-driven light source (Energetiq, EQ-99X) together with a spectrometer was used to obtain the responsivity spectra. To accurately determine the photoresponse time, an oscilloscope assisted with a chopper working at 200 Hz was employed.

RESULTS AND DISCUSSION Synthesis of high-quality HfS2/h-BN heterostructures. The fabrication strategy for the HfS2/h-BN heterostructure is illustrated in Figure S1. High-quality h-BN domains or films were initially synthesized on Cu foils by IBSD method.35 Then, the h-BN was transferred onto


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SiO2/Si substrates by the rosin-assisted wet-transfer method.38 After that, HfS2 monolayer or few-layer was grown on the h-BN via CVD method in a dual-temperature-zone furnace.19 Finally, the samples were cooled down to room temperature under an argon atmosphere. By controlling the growth time varied from 3 to 15 min, separated HfS2 flakes or homogeneous films can be synthesized on the h-BN. Characterization. Figure 1a,b shows the optical microscopy images of h-BN on SiO2/Si substrates before and after the growth of HfS2 layer, respectively. Even the small optical contrast between h-BN and 300 nm SiO2 due to its negligible opacity in the visible spectrum,39 the h-BN/SiO2 border can still be recognized, as indicated by the dashed line in Figure 1a. Atomically-thin HfS2 exhibits the deepened color because of its narrower band gap, leading to a remarkable contrast across the border after the growth of HfS2 layer (Figure 1b). Both h-BN and HfS2 layers are quite uniform and continuous over a large area (Figure S2), which is favorable for the fabrication of photoelectric device. Furthermore, the FTIR spectra confirm that the rosin has been effectively removed by an additional annealing process during the transfer (Figure S3). Figure 1c shows a typical X-ray diffraction (XRD) pattern of the HfS2 layer grown on h-BN. Besides the Si (400) peak from the SiO2/Si substrate, only (00l) diffraction peaks of 1T phase HfS2 are clearly observed, as labeled in Figure 1c. Accordingly, the crystal lattice “c” is calculated to be 0.59 nm, which is consistent with the previously reported values of HfS2.17-19 The Raman spectrum of HfS2 layer (Figure 1d) presents two characteristic peaks located at 260.4 and 336.6 cm-1, corresponding to the Eg and A1g Raman active modes of HfS2, respectively, matching well with the mechanically exfoliated samples.14-17 Notably, the Raman characteristic peak at about 1373 cm-1, arising from the E2g vibrational mode of h-BN,27,35



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remains almost unchanged after the growth of HfS2 layer, which confirms that the h-BN had not been decomposed during the growth process and HfS2 was indeed grown on the h-BN.

Figure 1. Optical microscopy images of h-BN on SiO2/Si substrates (a) before and (b) after the growth of HfS2. White dashed lines indicate the h-BN/SiO2 border before and after depositing HfS2. (c) XRD pattern of the HfS2/h-BN layer on SiO2/Si. (d) Raman spectra of HfS2/h-BN heterostructure on SiO2/Si, h-BN on SiO2/Si, and SiO2/Si substrate. (e) PL spectra of HfS2/h-BN heterostructure and SiO2/Si substrate. (f) UV-vis absorption spectra of HfS2/h-BN and h-BN on quartz. Photoluminescence (PL) and UV-vis absorption spectroscopies were applied to characterize the optical properties of the HfS2/h-BN heterostructure. The upper line in Figure 1e displays the PL spectrum of HfS2 layer with a peak at 480 nm (2.58 eV), which is attributed to the near-band gap emission of HfS2. To measure the UV-vis absorption spectroscopy, the HfS2 layer was grown on the h-BN transferred onto a transparent quartz substrate. As shown in Figure 1f, two distinct absorption peaks are observed at 203 and 373 nm, respectively. The absorption peak at 8 ACS Paragon Plus Environment

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203 nm corresponds to the π-π* interband transition of h-BN,27,35 whereas the wider peak around 375 nm originates from the optical absorption of HfS2.19 The optical bandgap of HfS2 layer estimated from the absorption edge of UV-vis spectrum is about 2.64 eV. The presence of these two peaks clearly indicates the coexistence of h-BN and HfS2. X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition and bonding types in the HfS2/h-BN heterostructure. The full XPS spectrum (Figure S4) demonstrates that the as-grown HfS2 are of high chemical purity with negligible impurities. The XPS core level spectra of Hf and S are shown in Figure 2a. The peaks observed at 18.6, 16.9 eV and 162.5, 161.5 eV correspond to Hf 4f5/2, Hf 4f7/2 and S 2p1/2, S 2p3/2, respectively. The atomic ratio between Hf and S is determined to be 1:2.02, indicating that the CVD-grown HfS2 is stoichiometric. In addition, Figure 2b shows that the B 1s and N 1s core-level peaks are centered at 190.1 and 397.8 eV, respectively, consistent with the previously reported values.35,40,41 To provide additional evidence on the layered structure of the sample, the HfS2/h-BN film was characterized by angle-resolved XPS measurements, which is a surface-sensitive technique for non-destructive depth profile analysis. Figure 2c shows the emission angle dependence of the atomic concentrations of elements from the as-grown HfS2/h-BN heterostructure on SiO2/Si, while all the raw XPS data are given in Figure S5. The emission angle is defined as the angle between the substrate normal and the emission direction of the photoelectrons, as shown in Figure 2d. As expected, the atomic concentrations of Hf and S from the topmost HfS2 layer increase with the emission angle, on the contrary, the signal of Si from the bottom substrate monotonically decreases with increasing emission angle. Moreover, the concentration of N atom rises gradually at the low emission angles, however, it reaches saturation and even starts to



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decrease when the emission angle is greater than 70°. The results reveal the construction of vertically stacked HfS2-on-h-BN.

Figure 2. XPS core-level spectra of (a) S 2p and Hf 4f, and (b) N 1s and B 1s. (c) The the emission angle dependence of the atomic concentrations of elements from the as-grown HfS2/hBN heterostructure on SiO2/Si. (d) The schematic drawing of the angle-resolved XPS measurement system. The crystallographic structure and chemical composition of CVD-grown HfS2 were further characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS), respectively. Figure 3a shows the low-magnification TEM image of an HfS2 flake on h-BN transferred onto a Cu grid. The sample shows a typical hexagonal-shaped flake with uniform and transparent h-BN background. The lattice fringes indicate a perfect atomic structure with a lattice spacing of 0.31 nm (Figure 3b), corresponding to the (10-10) plane of 1T phase HfS2.14,19 Furthermore, the selected area electron diffraction (SAED) pattern (the inset of Figure 3b) presents a set of six-fold symmetric diffraction spots and a typical polycrystalline ring. The sharp hexagonal diffraction spots (denoted by orange circles), corresponding to HfS2 {10-10} planes, evidence the high-quality single-crystalline structure of 10 ACS Paragon Plus Environment

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HfS2. The polycrystalline ring, corresponding to the h-BN {10-10} planes, is observed because of the fact that HfS2 flakes are preferentially nucleated on h-BN domain boundaries. Figure 3c captured from the edge of HfS2 flakes reveals the layer structure, and the observed interlayer distance is nearly 0.61 nm, which agrees well with the above XRD results. To further identify the sample composition, Figure 3d-f presents the EDS mapping of Hf, S, and N elements corresponding to the area marked by the white square in Figure 3a, respectively. As displayed in Figure 3d, e, both Hf and S atoms are distributed in a manner similar to the shape of the HfS2 flake in Figure 3a, while Figure 3f reveals a uniform spatial distribution of N atoms in the whole area. All these characterizations indicate that the sample synthesized via our CVD system is of high-quality single-crystalline HfS2 on h-BN.

Figure 3. (a) Low-magnification TEM image of an HfS2 flake on h-BN transferred onto a Cu grid. (b) A typical HRTEM image taken from the HfS2 flake. The inset shows the SAED pattern of HfS2/h-BN heterostructure, and the scale bar is 5 nm-1. (c) HRTEM image of the folded edges of HfS2, showing that the interlayer distance is 0.61 nm. EDS mappings of (d) Hf-L, (e) S-K, and (f) N-K corresponding to the area marked by the white square in (a). 11 ACS Paragon Plus Environment


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Selective direct growth of HfS2 on h-BN. In this work, both h-BN domains and continuous films transferred on SiO2/Si were used as the substrate. Figure 4a-c presents typical atomic force microscopy (AFM) images of the h-BN continuous film on SiO2/Si before and after the growth of HfS2. Apart from the h-BN grain boundaries indicated by the white arrow in Figure 4a, a few wrinkles (as indicated by the dashed rectangle) can be clearly observed in the h-BN film, which arise from the differential thermal expansion coefficients of Cu and h-BN.42 The large-area scanning electron microscopy (SEM) image of h-BN on Cu foils also indicates that the overall contrast of the h-BN film is rather uniform (Figure S6). Figure 4b and Figure S7a show the AFM images of HfS2 grown at 950 °C for 3 min on a continuous h-BN film, and the sharp edges of the HfS2 flakes can be observed clearly. From the height profile of the AFM image (the inset of Figure 4b), the thickness of the HfS2 flake is determined to be about 1.0 nm, in accord with a monolayer HfS2.19-21 The slight increase in AFM thickness measurement for the monolayer HfS2 is likely due to the chemical contrast between the HfS2 and the substrate.43 It is clear that more nucleation occurs near or on the h-BN domain boundaries, and adjacent HfS2 flakes coalesce together to form larger flakes. By increasing the growth time to 7 min, the growing HfS2 flakes might merge into a continuous film, and the HfS2 layer is mainly monolayered as demonstrated in Figure S7b. Further increasing the growth time to 10 min, besides the monolayer HfS2, the few-layer HfS2 with the thicknesses of 1.6 nm can also be obtained as shown in Figure 4c. The surface of as-grown HfS2 layer is rather smooth with a root-mean-square (RMS) roughness of 0.9 nm, which is slightly higher than that of the bared h-BN layer (RRMS=0.6 nm). Figure 4d shows a typical AFM image of the h-BN domains transferred on a SiO2/Si substrate. The triangular h-BN single crystalline domains with the lateral size of ~2 µm can be recognized, and the 0.5 nm step in the AFM height profile (the inset of Figure 4d) corresponds


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Figure 4. (a) A typical AFM image of an h-BN continuous film transferred onto a 300 nm SiO2/Si substrate. The dashed rectangle and the white arrows in (a) indicate the wrinkle and the grain boundary of h-BN film, respectively. AFM images of HfS2 synthesized on h-BN film for (b) 3 min and (c) 10 min, and the height profiles along the blue lines drawn in the AFM images show thicknesses of 1.0 nm and 1.6 nm, respectively. (d) A typical AFM image of h-BN domains transferred on a SiO2/Si substrate. The inset shows the height profile along the blue line drawn in (d), showing a thickness of 0.5 nm for a monolayer h-BN. (e,g) The AFM height images and (f,h) the corresponding phase images of the HfS2 flakes grown on h-BN domains for (e,f) 3 min and (g,h) 7 min. to a monolayer h-BN. Figure 4e presents the AFM height image of the HfS2 flakes synthesized on the h-BN domains at 950 °C for 3 min. Some small HfS2 flakes can be observed clearly, however, it is hard to distinguish the h-BN domains from the SiO2/Si substrate due to the occasionally occurred particles with the large height. Fortunately, the AFM phase image can produce very high material contrast of fine structures that barely can be seen in topographical image. Any boundaries with material discontinuity caused by the difference in surface



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properties can be reflected by phase shift. As shown in Figure 4f, besides the HfS2 flakes, the hBN domains can be easily identified by the phase image. More interestingly, we find that all the HfS2 flakes are nucleated on the h-BN domains, while no HfS2 is observed on the bared SiO2 regions. This is quite different from the previous work, in which MoS2 crystals grown by CVD both on and off the h-BN layer.44 The selective growth of HfS2 on h-BN rather than on SiO2 is also demonstrated for the 7-min-grown sample, as shown in Figure 4g, h. A large number of HfS2 flakes almost merge into a continuous film on h-BN domains, while there are yet no HfS2 flakes on SiO2. In addition, the selective growth of HfS2 on h-BN can be further confirmed by the optical image and Raman spectra acquired from the edge of sample (Figure S8). DFT calculation. In order to understand the mechanism of selective growth of HfS2, the cohesive energies of HfS2/h-BN and HfS2/SiO2 were theoretically calculated via density functional theory (DFT) as implemented in the Vienna Ab-initio Simulation Package (VASP). The theoretical models are displayed in Figure 5. Basic units used for the calculation are composed of 2×2×1 HfS2 and 3×3×2 h-BN primitive cells in HfS2/h-BN, while a 3×5×1 orthometric supercell is used for HfS2 in HfS2/SiO2. To minimize the lattice mismatch effects between HfS2 and amorphous SiO2, SiO2 component was amorphized by melting and quenching using classical molecular dynamics (MD). The corresponding computational parameters are displayed

in

Table

S1.

The

cohesive

energy Ec is

given

by

the

formula

Ec = ( EHfS2 /sub - EHfS2 - Esub ) / n , where the EHfS2 /sub , EHfS 2 and Esub are the total energy of HfS2/h-BN (or HfS2/SiO2), HfS2 and h-BN (or SiO2), and n is the number of HfS2 units in HfS2 supercell. The computational results reveal that the cohesive energies of HfS2 on h-BN and SiO2 substrates are -0.21 and -0.07 eV, respectively, as shown in Figure 5. The CVD growth of HfS2 involves two major steps: one is the deposition of reactant vapor on substrates; the other is the 14 ACS Paragon Plus Environment

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diffusion of adsorbed species on substrates. At high substrate temperatures, the diffusion rate of HfS2 molecules is deemed to be faster than the deposition rate, which affords sufficient time for the adsorbed HfS2 molecules to explore the potential energy surface to reach a minimum energy state. Clearly, both the cohesive energy approaching zero for HfS2/SiO2 and the large energy difference of 140 meV between two cases provide a fundamental theoretical account for the selective growth of HfS2 on h-BN rather than on SiO2. Furthermore, because of the large migration energy, the dangling bonds at the surface of SiO2 produce a strong impediment to the nucleation and growth of layered chalcogenide materials on SiO2.45 Therefore, the HfS2 layer preferentially nucleates and grows on h-BN rather than on SiO2.

Figure 5. The calculated cohesive energy and theoretical models of HfS2 layers on h-BN (-0.21 eV) and SiO2 (-0.07 eV). Photodetectors based on HfS2/h-BN hetrostructures. To investigate the optoelectronic properties of the HfS2/h-BN heterostructures, the prototype photodetectors using HfS2/h-BN were fabricated on 300 nm SiO2/Si substrates. Briefly, we deposited two 100 nm-thick interdigitated Au electrodes separated by 70 µm on the top of the HfS2/h-BN layers through a shadow mask by thermal evaporation. The photograph and the optical microscope images of the 15 ACS Paragon Plus Environment


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HfS2/h-BN photodetector are shown in Figure S9, respectively. The representative spectral photoresponse curve in Figure 6a demonstrates that the 2.0-nm-thick HfS2 layer on h-BN has a detectable photocurrent for the incident light wavelength ranging from 325 to 500 nm. The cut off wavelength of ~500 nm (2.48 eV) is close to the band gap of HfS2 obtained from the aforementioned PL and UV-vis absorption spectra. It is much larger than the bandgap of ~2.0 eV reported in literatures.20,21 The big difference between this work and previous results may be related with the quantum confinement effect, as reported previously in MoS2.46 However, the exact reason for this dramatic difference in bandgap is not yet clear. Figure 6b presents the logarithmic I-V curves of HfS2/h-BN photodetector in the dark and under the 450 nm laser irradiation with different power densities. The dark current was the order of 10-2 nA, implying the high electrical resistance of the HfS2 layers. The light on/off ratio under 83.3 mW cm-2 illumination is determined to be >105, which is among the highest values reported for HfS2 photodetectors.15,16,18,19 The dependence of photocurrent Iph at 10 V on light power intensity P is plotted in Figure 6c. Fitting the plot as (Iph~Pθ) leads to the value of θ = 0.93, suggesting that the CVD-grown HfS2 layers on h-BN are of high quality with very few traps or defects.47,48 In contrast, a sublinear behavior was observed in the HfS2/mica or SnSe2/SiO2 photodetectors, which is attributed to the defects and charged impurities in TMDs or at the interface.27,49 The cycling behavior of the photodetector was checked at 10 V under different incident light intensities. As demonstrated in Figure 6d, the on/off switching behavior is preserved over multiple cycles, indicating long-term stability, good reproducibility and fast response times as well. To obtain the precise response time of photodetector, the I-t curve of the device was measured using an oscilloscope for shorter time interval collection. As shown in Figure 6e, the rise and decay times are estimated to be 225 and 240 µs, respectively, which are much faster


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than the mechanically exfoliated or CVD-grown HfS218,20,21 and other 2D materials like MoS21 and WS22.

Figure 6. Optoelectronic properties of the photodetectors made from the HfS2/h-BN hetrostructures. (a) Spectral response curve of the HfS2/h-BN photodetector. (b) The logarithmic I-V curves of HfS2/h-BN photodetector in the dark and under the 450 nm laser irradiation with different power densities. (c) The dependence of photocurrent at 10 V on the light power intensity. (d) The cycling behavior of the photodetector at 10 V under the 450 nm irradiation at various power densities. (e) Normalized high-resolution photoresponse for the rise and decay times. (f) The calculated responsivity and detectivity as a function of the power intensity at 10 V. Besides the response time, the photodetector performance can be evaluated by its figures of merit such as responsivity (R) and specific detectivity (D*). Responsivity is a measure of how efficiently the photodetector converts optical power into photocurrent and is given by R=Iph/(PS), where Iph is the photocurrent, P is the incident light intensity and S is the effective illuminated area of photodetectors. Accordingly, the responsivity is calculated to be 26.5 mA W17 ACS Paragon Plus Environment


1

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under the light intensity of 6.7 mW cm-2 at 10 V, and it slightly decrease with increasing power

intensity, as shown in Figure 6f. This value is substantially higher than most previously reported HfS2-based photodetectors,19,20 and is much better than the photodetectors based on other 2D materials.1-4 Furthermore, the D* characterizes how weak light it can detect, and is expressed as D*=RS1/2/(2qId)1/2, where q is the unit charge, and Id is the dark current. The D* at 450 nm is then calculated to be about 3×1011 Jones (Jones=cm Hz1/2 W-1). These values for our HfS2/h-BN photodetector are higher than those of most other 2D semiconductor photodetectors reported in literature,3,14,19 comparison is given in the Table 1. The combination of ultrahigh responsivity and detectivity, high on/off ratio and ultra-fast response time makes HfS2/h-BN heterostructure a promising candidate for future high performance nanophotodetectors. Due to its great specific surface area, substrate and interface play a critical role in the properties of 2D materials. For example, it was widely reported that the mobilities of graphene and MoS2 on h-BN can be enhanced more than one order of magnitude than that on SiO2. In this work, the remarkable performance of the HfS2/h-BN photodetector is attibuted to the 2D h-BN substrate, which possesses atomically smooth surface without any dangling bonds and trapped charges, resulting in ultralow density of defects and trap states at the interface between HfS2 and h-BN. We propose that the mobility of HfS2 can also be improved by using 2D h-BN as the substrate, which is favorable to enhance the performance of HfS2/h-BN photodetector. Indeed, it has been reported that the device performance was significantly improved when h-BN was used as a substrate or dielectric layer in MoS2 and graphene based FETs, usually due to the improved mobility.31-34 Moreover, the selective direct growth of HfS2 provides a facile approach for the preparation of HfS2-based nano-optoelectronics devices with well-defined structure by using patterned h-BN substrates.


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Table 1. Performance comparison of photodetectors based on HfS2/h-BN hetrostructures and other 2D materials produced by mechanical exfoliation or CVD method. Device 1L MoS2

Fabrication method

Responsivity -1 [mA W ]

Mechanical exfoliation

7.5

FL WS2

CVD method

FL SnS2

CVD method

-2

2.1×10 9

Detectivity On/off ratio Rise time Decay time [Jones] [ms] [ms] 3

-

10

50

50

-

-

5.3

5.3

9

-3

-

10

10

-

4

38

-

14

4

-

-

15

3

24

16.1

-


8.9×105

1010

10

FL HfS2


-

-

10

10

FL HfS2

CVD method

-

FL HfS2

CVD method

0.01

7×10

10

1.3×10

FL HfS2

CVD method

2.8

-

-

55

FL HfS2

CVD method

26.2

10

11

10

3

7×10

7

CVD method

FL HfS2

8

2

3

FL ReS2

-

1 -3

5×10 6

Ref.

3

5

10

24 -1

-1

2.25×10

18 -1

1.55×10

19

78

20 -1

2.4×10

This work

CONCLUSIONS In summary, for the first time we demonstrate the direct growth of high-quality HfS2 flakes on 2D h-BN by CVD method. It is found that the HfS2 flakes are selectively grown on h-BN rather than on SiO2/Si, which is attributed to the large difference in cohesive energy between HfS2/hBN and HfS2/SiO2. Impressively, the photodetectors based on the HfS2/h-BN heterostructures exhibit excellent visible-light sensing performance, such as a high on/off ratio of more than 105, an ultrafast response rate of 225 µs for the rise and 240 µs for the decay times, a high responsivity of 26.5 mA W-1 and a competitive detectivity exceeding 3×1011 Jones, superior to the vast majority of the reported 2D materials based photodetectors. The remarkable performance of the HfS2/h-BN photodetector is attributed to the unique features of 2D h-BN substrate. These results indicate that HfS2/h-BN heterostructures, as well as the selective direct growth of HfS2, are very promising for future applications in high performance nanooptoelectronics. 19 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxx. Schematic diagram of fabrication strategy, OM images, FTIR spectra, additional XPS, SEM, AFM and Raman characterizations, and Table S1 are included. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (No. 2016YFB0400802), the National Natural Science Foundation of China (No. 61674137, U1738114) and the Beijing Natural Science Foundation (No. 4184101).

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Photodetectors. Adv. Mater. 2015, 27, 8035-8041.

Table of Contents


Selective Direct Growth of Atomic Layered HfS2 on Hexagonal Boron

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