Chemical Vapor Deposition Growth of Vertical MoS2 Nanosheets on p

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Functional Nanostructured Materials (including low-D carbon)

Chemical vapor deposition growth of vertical MoS2 nanosheets on p-GaN nanorods for photodetector application Guofeng Yang, Yan Gu, Pengfei Yan, Jin Wang, Junjun Xue, Xiumei Zhang, Naiyan Lu, and Guoqing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22344 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Materials & Interfaces

Chemical Vapor Deposition Growth of Vertical MoS2 Nanosheets on p-GaN Nanorods for Photodetector Application Guofeng Yang1,*, Yan Gu1, Pengfei Yan1, Jin Wang1, Junjun Xue2, Xiumei Zhang1, Naiyan Lu1, Guoqing Chen1

1. School of Science, Jiangsu Provincial Research Center of Light Industrial Optoelectronic Engineering and Technology, Jiangnan University, Wuxi 214122, China 2. School of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China

*Email: [email protected] (G. F. Yang)

Abstract Vertically oriented multi-layered MoS2 nanosheets were successfully grown on p-GaN nanorod substrate using chemical vapor deposition (CVD) method. The p-GaN nanorod substrate was fabricated by dry etching employing self-assembled nickel (Ni) nanopartical as mask. Photoluminescence (PL) and Raman characterizations demonstrate the multi-layered structure of MoS2 nanosheet growth on p-GaN nanorods as compared with the referential monolayer MoS2 on GaN wafer substrate under the same growth procedure. The growth model of vertical MoS2 nanosheet formed on GaN nanorods is evidently proposed according to the first-principle calculations. More importantly, it is demonstrated here that the as-grown vertical MoS2 nanosheets/p-GaN nanorod heterostructure holds promising applications in photodetector device, where high optical gain and broad spectral response in the visible range have been obtained.

Key words: vertical MoS2 nanosheets; chemical vapor deposition (CVD); GaN nanorods; firstprinciple calculation; photodetector.

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1. Introduction Since the discovery of graphene by K.S. Novoselov et al.,1 two dimensional (2D) material family has accepted many members and attracted widespread interests in a host of fields, including transition metal dichalcogenides (TMDCs),2-4 transition metal oxides,5 BN,6 etc. A huge number of researches have been reported to use 2D materials for the applications of electronics,2 sensors,3 energy storage,4,5 and optoelectronics.6 Among these various kinds of 2D materials, molybdenum disulﬁde (MoS2) attracted enormous research interests due to its excellent and particular electronic and optical characteristics. The 2D MoS2 can accommodate lithium ions through conversion reaction at a low potential, form matrix of Mo nanoparticles and insoluble Li2S.7 Also it has been discovered potential applications in the areas of hydrogen evolution process8,9 and biological sensing.10,11 Moreover, field effect transistors (FETs) fabricated by monolayer or few-layer MoS2 crystals exhibited an ultrahigh current on/off ratio of 109 and mobility of more than 1980 cm2/Vs.12, 13 In addition, MoS2 crystals were also be used as a part of photodetectors14 due to its excellent optical properties. According to previous reports, the crystal defects, doping and thickness of MoS2 crystals had great effect on the optical property of MoS2, thus the spectral response of MoS2 could be modulated from visible to mid-infrared.15-17 However, most researches have tried to synthesize and employ 2D materials horizontally on the planar substrates, little efforts has been paid on another alternative structure which is producing 2D materials vertically.18-20 The high aspect ratio, specific surface area and extensively exposed edges21 of vertical 2D materials afford promising applications in diverse aspects. Shi J. et al.22 demonstrated that the catalytic reaction of hydrogen evolution revealed important relationship with the exposed edge sites of MoS2 crystals.23 Besides, vertical 1D nanowires and 2D nanosheets with atomically edge sites were demonstrated to enhance the optical emission properties significantly,24, 25 making vertical 2D materials prospective candidates in optoelectronic applications.26-28 Moreover, the successfully prepared multi-layered vertical MoS2 exhibited excellent optical absorption and quick longitudinal intra-layer transport, and realized the enhancement of photoresponsivity and high response speed for photo-detection.29 As for the methods of synthesizing vertical MoS2 crystals, the key point is proper original vertical growth condition. Shi Y. et al. 30 employed graphene film as the substrate 2


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to provide growth points and induce synthesis of MoS2 flowers. Additionally, nickel foam was used to grow vertical graphene nanosheets (VGNS) to support the growth of vertical MoS2 crystals.31 Shokhen V. et al.

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utilized a 0.5mm polished molybdenum foil as the

substrate and reactants, which caused vertical alignment of MoS2. Li H. et al. discovered a technology of using the forces from two monolayer MoS2 to support the formation of bilayer vertical MoS2.33 Though vertical MoS2 nanostructures have been successfully synthesized by different methods, these methods have their own drawbacks, which need special synthesis conditions different from the normal growth condition of horizontal 2D MoS2. Therefore, in this work, we propose a simple chemical vapor deposition (CVD) process of growing vertical MoS2 nanosheets using p-GaN nanorods as a substrate to form a heterostructure, with the deposition conditions the same as that of horizontal 2D MoS2 CVD growth. Various measurements were employed to analyze the structure of vertical MoS2 nanosheets. Additionally, a possible growth model of the vertical MoS2 nanosheets on GaN nanorods is proposed according to the experimental characterizations, and the biding energies calculated from the density functional theory (DFT) are provided to demonstrate the formation possibility of vertical MoS2 nanosheets on GaN nanorods. More importantly, a photodetector device based on the vertical 2D MoS2/p-GaN nanorod heterostructure is successfully prepared, which can be worked over the visible light ranges and demonstrates a typical spectral photoresponse. The results indicate that the vertical MoS2/p-GaN nanorod heterostructure grown by CVD would be promising for optoelectronic device applications.

2. Experimental details The GaN nanorod was fabricated by a dry etching method using nickel (Ni) nanoparticle mask on p-GaN wafer, which was similar to our previous report.34 Firstly, a thin film of Ni was deposited on p-GaN wafer by electron-beam (EB) evaporation. Then the Ni film was annealed at 850℃ for 1 min by rapid thermal annealing (RTA), forming the self-assembled Ni nanoparticle mask on the surface of p-GaN wafer. With the Ni nano particle used as an etching mask, GaN nanorods were fabricated by ICP-RIE dry etching using Cl2 and BCl3

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as reactive gases. After which the residual Ni masks on GaN nanorods were removed by dilute HCl solution. After the fabrication of p-GaN nanorod, a CVD furnace with two independent heating zones was used to grow MoS2 on the GaN nanorod substrate. For reference, another 2D MoS2 sample was grown on GaN wafer substrate under the same growth conditions. After ultrasonic cleaning of all the substrates, 0.1 g MoO3 power (99.9%) was dispersedly put in a quartz boat with the GaN wafer or GaN nanorod substrate above it, which was placed in the center of the downstream heating zone. The distance between MoO3 and surface of substrate was about 35 mm. Another quartz boat with 1 g (99.9%) sulfur powder was put in the central region of the upstream heating zone. High-purity nitrogen was utilized as the carrier gas with a constant flow rate of 70 sccm in the whole heating process. The downstream zone with MoO3 power was slowly heated to 800℃ in about 100 min. Meanwhile, sulfur powder in the upstream zone was initially kept at room temperature (RT) for 60 min, and then gradually rised to 180℃ in 40 min. Subsequently, the temperature of the downstream zone was set constant at 800℃ for 20 min, and then dropped to RT naturally. The schematic processes for the fabrication of p-GaN nanorods and CVD growth of MoS2 nanosheets are plotted in Fig. 1.

Fig. 1. The schematic diagrams of the p-GaN nanorod fabrication and MoS2 nanosheet growth process with the 3D vision of MoS2 nanosheets on GaN nanorods.

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Standard photolithography, drying etcing and lift-off processes were employed for the fabrication of vertical MoS2/p-GaN nanorod photodetector. The 50 nm Au top contact layer and the 100 nm Au bottom contact layer were fabricated on the surface of MoS2 and pGaN mesa area, respectively, by using EB evaporation. The surface morphologies of fabricated GaN nanorods and as-grown MoS2 nanosheets on GaN wafer and p-GaN nanorod were measured by the field-emission scanning electron microscope (FE-SEM: ZEISS SIGMA 04-03), and the monolayer structure of MoS2 was characterized by atomic force microscope. A Renishaw Inviamicro-Raman system with an excitation laser of 532 nm was employed to analyze the Raman and photoluminescence (PL) properties of the as-grown samples. The photoresponse property of the vertical MoS2/p-GaN nanorod heterostructure were measured by the probe station with Keithley 4200 electrometer under dark and illumination conditions, and using an illumination source of 488 continuous-wave laser.

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3. Results and discussion

Fig. 2. (a) Top-view SEM image of the Ni nanoparticle formed on GaN surface by RTA process. (b) SEM images of p-GaN nanorods fabricated by dry etching method using Ni nanoparticle mask. (c) PL spectra of the fabricated p-GaN nanorods using dry etching method.

Fig. 2(a) displays the top-view SEM image of Ni nanoparticle formed by RTA process. It can be seen that dispersive Ni nanoparticles with diameter of 100～300 nm are assembly distributed on p-GaN surface. According to our previous report, the Ostwald ripening process would contribute to the formation of Ni nanoparticles on GaN surface under thermal annealing conditions.34 Fig. 2(b) presents the side-view SEM image of p-GaN nanorod fabricated by the as-prepared Ni nanoparticle masks. It is observed that GaN nanorods with the vertical height of ～150 nm are formed using etching time of 0.5 min. Fig. 2(c) exhibits the PL spectra of the fabricated nanorod GaN employed as supporting substrate for MoS2 nanosheets growth. The PL spectra reveals an obvious emission peak at 362 nm with a narrow full width at half maximum (FWHM), corresponding to the near6


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band-edge exciton transition of nanorod GaN.35 The strong emission peak confirm that the GaN nanorods exhibit high crystallinity and excellent optical properties. The morphology of referential monolayer MoS2 grown on GaN wafer substrate is shown in Fig. S1. It is observed that most of the as-grown MoS2 sheets exhibit triangular structure with large coverage ratio on the p-GaN substrate, and the lateral sizes of the triangular domains are limited because of the certain lattice mismatch between MoS2 and the p-GaN substrates. Fig. S1(b) shows a typical AFM image of partial triangular MoS2 sheet on GaN substrate. The height profile extracted from the AFM image demonstrates that the thickness of the MoS2 sheet is approximately 0.8 nm, which agrees well with the monolayer MoS2 thickness (0.6 ～ 0.9 nm) on other substrates.36,

37

The results indicate that the CVD growth procedure in our

experiments can successfully obtain monolayer MoS2 on p-GaN substrate.

Fig. 3. (a) SEM image of MoS2 nanosheets grown on p-GaN nanorods, and (b) the enlarged SEM image of MoS2 nanosheets, the red dashed square inset indicates single vertical Mos2 nanosheet on the sidewall of GaN nanorod.

In comparison, by using the same CVD procedure to grown MoS2 on p-GaN nanorod (shown in Fig. 2(b)) substrates, it is found in Fig. 3(a) that large area of MoS2 nanosheets are produced on GaN nanorods. The enlarged SEM image shown in Fig. 3(b) obviously exhibits that the as-grown MoS2 nanosheets are almost perpendicular to the basal substrate. Accordingly, as the inset clearly shown in Fig. 3(b), it is proposed that the general morphologies of GaN nanorods surrounded by vertical MoS2 nanosheets are formed, and the formation model of the vertical structure will be further explored in the following part. The optical properties of MoS2 nanosheets on GaN nanorods, bulk and monolayer MoS2 7



on flat GaN wafer substrate are compared by Raman and PL spectra as shown in Fig. 4(a) and (b), respectively. Typical E12g and A1g peaks of the Raman vibration modes in MoS2 (Fig. 4(a)) are obviously seen. However, the estimated wave number difference between A1g and E12g peaks from the Raman spectra of monolayer MoS2 on GaN wafer is approximately 19 cm-1, indicating the single layer structure of the as-grown MoS2 on GaN wafer substrate.38 While the Raman wave number differences of MoS2 nanosheets on GaN nanorods and bulk MoS2 on GaN substrate are approximately 24 cm-1 and more than 26 cm-1, respectively, demonstrating that the as-grown MoS2 nanosheets on GaN nanorods are multi-layered structure. This is because the wave number difference between the E12g and A1g peaks varies with the MoS2 layer number in Raman spectra, and can be commonly applied to estimate the number of MoS2 layers.39 In addition, it is found in Fig. 4(b) that the PL emission peak of MoS2 nanosheets formed on GaN nanorods located between the emission peak of monolayer and bulk MoS2 on GaN substrate with a broad FWHM, which further verifies multi-layered structure of MoS2 nanosheets on GaN nanorods.

Fig. 4. Raman (a) and PL (b) spectra of MoS2 nanosheets on GaN nanorods, bulk and monolayer MoS2 on flat GaN wafer substrate.

Moreover, Fig. 5 and Fig. S2 show the power-dependent Raman and PL spectra of vertical MoS2 nanosheets on GaN nanorods and referential monolayer MoS2 on GaN respectively. While the peak position and peak intensity of their Raman and PL spectra as a function of incident laser power are depicted in Fig. S3. P0 denotes the maximum power of excitation laser. It is observed that the positions of E12g and A1g peaks for MoS2 nanosheets on GaN nanorods reveal larger shift than that of the monolayer MoS2 as 8


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excitation power increases, due to the changed restoring forces in the vibrations for nanorod substrates.40 Additionally, the power-dependent intensity of E12g and A1g peaks of MoS2 nanosheets exhibit much higher increase slope compared with the slope of referential monolayer MoS2, indicating the vibrational modes might soften caused by the reduced interlayer van der Waals interaction between the nanosheets and substrate. The PL emission peak position of MoS2 nanosheets on GaN nanorods shows a red-shift than the PL position of monolayer MoS2, originating from the reduced band-gap of multi-layered structure of MoS2 nanosheets. It is also notable that both PL emission peaks of MoS2 nanosheets and monolayer MoS2 display slight red-shift when the excitation power increases, the result might be attribute to the heating effect induced by laser irradiation.41, 42

Additionally, by fitting the curves of PL intensity as a function of excitation power, it is

found that the relationship of PL intensity and excitation power is similar to the power law of I ∝ Lk, where I, L, and k denote the PL intensity, excitation laser power, and increase factor, respectively. As a result, k is figured out to be 0.58 and 0.92 for MoS2 nanosheets and monolayer MoS2, respectively, verifying the direct recombination process of excitons for PL emission in 2D MoS2.43

Fig. 5. Power-dependent Raman (a) and PL (b) spectra of vertical MoS2 nanosheets on GaN nanorods.

To better propose the formation model of MoS2 nanosheets grown on GaN nanorods, schematic diagrams with atomic structure of monolayer MoS2 are displayed in Fig. 6. At the initial growth stage, reduction reaction occurred between sulfur vapor and partial MoO3 powder, with volatile MoO3 − x or gaseous MoS2 produced, which were subsequently adsorbed and diffused to the side walls of GaN nanorods, and formed the nucleation sites 9



MoS2 thin films. As the growth time increased, separated MoS2 layered nanosheets were gradually formed along the nanorod side walls due to the emerging nucleation sites MoS2. After which the multi-layered MoS2 thin films would be facilitated by the high concentration reactants resulted from fast evaporation of sulfur powder under higher temperature, and a trend of layer-by-layer growth was followed until reaching a certain critical thickness. With the growth continued, partial parts of MoS2 thin film might exceed the certain height of GaN nanorods, and extend to horizontal growth direction without the support of nanorod structure. Thus, energetic favorable morphologies of vertical multilayered MoS2 nanosheets with partial horizontal directions were formed according to the thermodynamics and kinetics factor for the MoS2 growth.44 It is worth to note that though most of the vertical MoS2 nanosheets could originate from the basal support of GaN side walls, the MoS2 (002) plane dominates the surface facets of multi-layered nanosheets (Fig. 6(d)), because of its lowest surface energy.45 Based on the above analysis, it is believed that GaN nanorod structure would be provided as the growth template for vertical formation of MoS2 nanosheets, and the GaN nanorod structure can induce the alignment of MoS2 nanosheets in vertical direction.

Fig. 6. Schematic diagrams of the proposed formation mechanism of MoS2 nanosheets on GaN nanorods at different stages (a-c), and the atomic structure of monolayer (002) MoS2 (d).

In order to further verify the above proposed growth model, a more detailed illustration from the atomic level was performed through first-principle calculations using DFT. 10


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Geometry optimizations and electronic property calculations of monolayer (002) MoS2 on GaN typical horizontal surface of (001) plane, and side wall surfaces of (010) and (100) planes were implemented in the ATOMISTIX TOOLKIT (ATK) code.46,

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The

generalized gradient approximation (GGA) was employed to consider the exchangecorrelation potential.48 Grimwe’s DFT-D2 dispersion correction approach was utilized to describe the van der Waals interaction.49 A vacuum region of 15Å was set to eliminate the interaction of adjacent layers. As for the geometry optimization, the Brillouin zone was sampled by 5×5×1 Monk horst-Pack k-point meshes, while 8×8×1 k-point meshes were used to calculate the electronic properties. During the calculation, ten atomic layer thickness of GaN were chosen to simulate the bulk GaN crystal, since the calculated results would reveal little change beyond this thickness, where the skill was commonly employed in the previous reports.50, 51

Fig. 7. The ELF plots of (002) MoS2 on GaN (a) (001), (b) (010), and (c) (100) planes.

The top- and side-views of the most stable atomic geometries of monolayer (002) MoS2 on GaN (001), (010), and (100) planes are illustrated in Fig. S4. The obtained optimized distances between MoS2 and the three GaN planes are 2.75 Å, 2.62 Å and 2.53 Å, respectively, which are larger than the S-Ga (2.34 Å) and S-N (1.62 Å) bond lengths,52,53 indicating the van der Walls interaction between MoS2 and GaN surfaces in the three strucutures. Moreover, the electron localization function (ELF) plots of MoS2 monolayer on GaN is shown in Fig. 7 in order to evaluate the binding property between MoS2 and GaN. The ELF was provided by:54, 55 11



1

𝐸𝐿𝐹(𝒓) = 1 + 𝜒2(𝒓),

(1)

𝜎

𝐷𝜎(𝒓)

with 𝜒𝜎(𝒓) = 𝐷𝑜(𝒓) ,

(2)

𝜎

2

3

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3

and 𝐷0𝜎(𝒓) = 5(6𝜋2)3𝜌𝜎5 (𝒓).

(3)

Here ρ represents the electron spin density. Accordingly, the ELF images obtained in Fig. 7 indicate that the values of ELF are almost zero for the three MoS2-GaN systems, which further demonstrates the van der Walls interactions between 2D MoS2 and GaN. In addition, stronger electron localization of MoS2-(010) GaN and MoS2-(100) GaN systems indicates the enhanced binding characteristic of 2D MoS2 on GaN side walls than that of on (001) GaN top surface, which proves that 2D MoS2 tends to grown vertically around the side facets rather than on top surface horizontally. Furthermore, the binding energies of monolayer MoS2-(001), -(010), and -(100) GaN systems are numerically investigated, with the binding energy defined as: Eb = (EM + EG–EM-G)/N,

(4)

Where EM, EG, and EM-G denote the total energy for pristine MoS2, GaN, and MoS2-GaN combined system, respectively. N denotes sulfur atom number in the sub-layer close to the GaN. Total energy of monolayer MoS2, GaN crystal and MoS2-GaN system are calculated by Etotal(n)=T(n)+ 𝐸(𝑛)-σS,

(5)

where n is the electron density, T(n) represents the kinetic energy of the Kohn-Sham orbitals, E(n) means interaction energy, and -σS expresses the entropy contribution due to smearing of the occupation function. Electron density of the many-electron system is given by the occupied eigenstates of the Kohn–Sham Hamiltonian: 𝑛 = 𝑛(𝒓) = ∑ 𝑓𝛼|𝜓𝛼(𝒓)|2,

(6)

𝛼

where fα is the occupation of the level denoted by α. The kinetic energy of the Kohn-Sham orbitals is obtained by: 𝑇(𝑛) =

∑

⟨ |

𝑓𝛼 𝜓𝛼 𝛼

| 𝜓 ⟩,

― ℏ2 2 2𝑚 ∇

(7)

𝛼

12


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And E(n) has three contribution terms, which is expressed as: 𝐸(𝑛) = 𝐸𝐻(𝑛) + 𝐸𝑋𝐶(𝑛) + 𝐸𝑒𝑥𝑡.

(8)

Where Exc(n) relates to the exchange-correlation energy, EH(n) includes the electrostatic terms of Hartree energy and the interaction energy with the pseudo potential ions, and Eext(n) is the interaction energy with an external electrostatic field. As a result, binding energies of monolayer MoS2 combined with (001), (010), and (100) GaN planes are 0.048 eV, 0.137 eV, and 0.092 eV, respectively, indicating the enhanced hybridization of 2D MoS2 on GaN side facets.

Fig. 8. (a) Schematic diagram of the photodetector device based on vertical MoS2 nanosheets/p-GaN nanorods. (b) The dark and illuminated I-V curves, with (c) spectral response as a function of wavelength of the fabricated device.

The basal facets of vertical 2D layered materials always exhibit as terminating surface, which have large numbers of dangling bonds and high aspect ratio. These properties are 13



proper for optoelectronic and electronic device applications, because the layered molecules are arranged vertically to expose the edge sites maximally and and exhibit large exchange current densities.23 Moreover, a conceptually ideal structure would be the whole plane covered with edges, which could be obtained by forming the alignment of vertical layers in contrast to flat films. As a result, the novel structure of vertical 2D MoS2 nanosheets with high density edge sites would bring about more effective device applications, and the distinctive application of our vertical 2D MoS2 nanosheets on p-GaN nanorod is demonstrated here. The schematic diagram of the fabricated photodetector based on vertical MoS2 nanosheets/p-GaN nanorod heterostructure is illustrated in Fig. 8(a). The 50 nm Au top contact layer and 100 nm Au bottom contact layer are deposited on MoS2 surface and p-GaN mesa area, respectively. Fig. 8(b) shows the current-voltage (I-V) curves of the photodetector in dark and under laser illumination. It is clearly observed that the device reveals a low dark current of about 10-4 mA, while the photo-current under illumination rises rapidly below the bias voltage of 1 V, and then reveals a slight increase. This trend is suggested to be caused by the depletion of the active region between the MoS2/p-GaN nanorod due to the p-n heterojunction formation. In addition, the magnitude of the photo-current reaches more than 10-1 mA, which is much higher than the dark current under the same bias voltage, indicating large optical gain of the as-fabricated photodectector. Fig. 8(c) shows the photoresponse of the photodectector. It is noted that the vertical MoS2/p-GaN nanorod heterostructure represents a broad spectral response ranging from 500 to 750 nm with a peak photoresponse at about 670 nm, which demonstrates the vertical MoS2/p-GaN nanorod heterostructure is prospective for the visible optoelectronic device applications.

4. Conclusions In conclusion, we have achieved a CVD growth of vertical MoS2 nanosheets on p-GaN nanorods, which are fabricated by dry etching process employing self-assembled Ni nanoparticals as mask. It is demonstrated that the multi-layered structure of MoS2 nanosheets is formed by comparing the optical properties of as-grown MoS2 on p-GaN nanorods with that of referential MoS2 monolayer on GaN wafer substrate. The based pGaN nanorods are suggested to provide as the growth substrate and induce the vertical 14


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morphology of MoS2 nanosheets according to the first-principle calculations of 2D MoS2 binding characteristics on typical GaN top and side facets. The calculated binding energies of monolayer MoS2 on GaN side facets are larger than that on the top surface, indicating the enhanced hybridization behavior of 2D MoS2 nano sheets on GaN side facets. Moreover, the as-grown vertical MoS2 nanosheets on p-GaN nanorods are employed as heterostructure for photodetector device application. The device exhibits low dark-current of about 10-4 mA, and a broad spectral response from 500 to 750 nm with a peak photoresponse at about 670 nm, which demonstrates the vertical MoS2/p-GaN nanorod heterostructure holds promising applications for visible optoelectronic devices.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 11604124), Natural Science Foundation of Jiangsu Province (Nos. BK20150158), Open Project Program of State Key Laboratory of Food Science and Technology, Jiangnan University (No. SKLF-KF-201706), the China Postdoctoral Science Foundation (No. 2017M621623), the Fundamental Research Funds for Central Universities (Nos. JUSRP51628B, JUSRP51517, JUSRP51716A), the national first-class discipline program of Food Science and Technology (No. JUFSTR20180302), and University Science Research Project of Jiangsu Province (No. 16KJB140011)

References (1) Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666669. (2) Mak, K.F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (3) Liu, B.; Chen, L.; Liu, G.; Abbas, A.N.; Fathi, M.; Zhou, C. High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors. ACS Nano 2014, 8, 5304-5314. (4) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10, 313-318. 15



(5) Putungan, D. B.;. Lin, S.-H; Wei, C.-M.; Kuo, J.-L. Li Adsorption, Hydrogen Storage and Dissociation Using Monolayer MoS2: An ab Initio Random Structure Searching Approach. Phy. Chem. Chem. Phy. 2015, 17, 11367-11374. (6) Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271-1275. (7) Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P.M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate. Small 2012, 8, 966-971. (8) Chang, Y.H.; Wu, F.Y.; Chen, T.Y.; Hsu, C.L.; Chen, C.H.; Wiryo, F.; Wei, K.H.; Chiang, C.Y.; Li, L.J. Three‐Dimensional Molybdenum Sulfide Sponges for Electrocatalytic Water Splitting. Small 2014, 10, 895-900. (9) Chang, Y.H.; Lin, C.T.; Chen, T.Y.; Hsu, C.L.; Lee, Y.H.; Zhang, W.; Wei, K.H.; Li, L.J. Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene‐Protected 3D Ni Foams. Adv. Mater. 2013, 25, 756-760. (10) Wang, L.; Wang, Y.; Wong, J.I.; Palacios, T.; Kong, J.; Yang, H.Y. Functionalized MoS2 Nanosheet-Based Field-Effect Biosensor for Label-Free Sensitive Detection of Cancer Marker Proteins in Solution. Small 2014, 10, 1101-1105. (11) Loan, P.T.K.; Zhang, W.; Lin, C.T.; Wei, K.H.; Li, L.J.; Chen, C.H. Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation. Adv. Mater. 2014, 26, 48384844. (12) Illarionov, Y.Y.; Smithe, K.K.H.; Waltl, M.; Knobloch, T.; Pop, E.; Grasser, T. Improved Hysteresis and Reliability of MoS2 FETs with High-Quality CVD Growth and Al2O3 Encapsulation. IEEE Electron Dev. Lett. 2017, 38,1763-1766. (13) Majd, S.M.; Salimi, A.; Ghasemi, F. An Ultrasensitive Detection of MiRNA-155 in Breast Cancer via Direct Hybridization Assay using Two-dimensional Molybdenum Disulfide Field-effect Transistor Biosensor. Biosens. Bioelectron. 2018, 105, 6-13. (14) Qiao, S.; Cong, R.; Liu, J.; Liang, B.; Fu, G.; Yu, W.; Ren, K.; Wang, S.; Pan, C. Vertical Layered MoS2/Si Heterojunction for Ultrahigh and Ultrafast Photoresponse Photodetector. J. Mater. Chem. C 2018, 6, 3233-3239. (15) Gomathi, P. T.; Sahatiya, P.; Badhulika, S. Large-area, Flexible Broadband Photodetector Based on ZnS-MoS2 Hybrid on Paper Substrate. Adv. Funct. Mater. 2017, 27, 1701611. (16) Xie, Y.; Zhang, B.; Wang, S.; Wang, D.; Wang, A.; Wang, Z.; Yu, H.; Zhang, H.; Chen, Y.; Zhao, M. Ultrabroadband MoS2 Photodetector with Spectral Response From 445 to 2717 nm. Adv. Mater. 2017, 29, 1605972. (17) Bernardi, M.; Palummo, M.; Grossman, J.C. Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-dimensional Monolayer Materials. Nano Lett. 2013, 13, 3664-3670. 16


Page 16 of 20

Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Wang, P.P.; Sun, H.; Ji, Y.; Li, W.; Wang, X. Three-Dimensional Assembly of Single-Layered MoS2. Adv. Mater. 2014, 26, 964-969. (19) Yu, J.H.; Lee, H.R.; Hong, S.S.; Kong, D.; Lee, H.W.; Wang, H.; Feng, X.; Wang, S.; Cui, Y. Vertical Heterostructure of Two-Dimensional MoS2 and WSe2 with Vertically Aligned Layers. Nano Lett. 2015, 15, 1031-1035. (20) Romo-Herrera, J.M.; Terrones, M.; Terrones, H.; Dag, S.; Meunier, V. Covalent 2D and 3D Networks from 1D Nanostructures: Designing New Materials. Nano Lett. 2007, 7, 570-576. (21) Jiang, S.; Yin, X.; Zhang, J.; Zhu, X.; Li, J.; He, M. Vertical Ultrathin MoS2 Nanosheets on a Flexible Substrate as an Efficient Counter Electrode for Dye-Sensitized Solar Cells. Nanoscale 2015, 7,10459-10464. (22) Shi, J.; Ma, D.; Han, G.F.; Zhang, Y.; Ji, Q.; Gao, T.; Sun, J.; Song, X.; Li, C.; Zhang, Y. Controllable Growth and Transfer of Monolayer MoS2 on Au Foils and Its Potential Application in Hydrogen Evolution Reaction. Acs Nano 2014, 8, 10196-10204. (23) Kong, D.; Wang, H.; Cha, J.J.; Pasta, M.; Koski, K.J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 2013, 13, 1341-1347. (24) Huang, H.; Li, Y.; Li, Q.; Li, B.; Song, Z.; Huang, W.; Zhao, C.; Zhang, H.; Wen, S.; Carroll, D. Field Electron Emission of Layered Bi₂Se₃ Nanosheets with Atom-Thick Sharp Edges. Nanoscale 2014, 6, 8306-8310. (25) Huang, C.K.; Ou, Y.; Bie, Y.; Zhao, Q.; Yu, D. Well-Aligned Graphene Arrays for Field Emission Displays. Appl. Phys. Lett. 2011, 98, 263104. (26) Kashid, R.V., Late, D.J.; Chou, S.S.; Huang, Y.K.; De, M.; Joag, D.S.; More, M.A.; Dravid, V.P. Enhanced Field-Emission Behavior of Layered MoS₂ Sheets. Small 2013, 9, 2730-2734. (27) Hallam, T.; Cole, M.T.; Milne, W.I.; Duesberg, G.S. Microtransfer Printing: Field Emission Characteristics of Contact Printed Graphene Fins. Small 2014, 10, 95-99. (28) Jiang, L.; Yang, T.; Liu, F.; Dong, J.; Yao, Z.; Shen, C.; Deng, S.; Xu, N.; Liu, Y.; Gao, H.J.; Controlled Synthesis of Large-Scale, Uniform, Vertically Standing Graphene for HighPerformance Field Emitters. Adv. Mater. 2013, 25, 250-255. (29) Gong, F.; Fang, H. H.; Wang, P.; Su, M.; Li, Q.; Ho, J. C.; Chen, X. S.; Lu, W.; Liao, L.; Wang, J.; Hu, W. D. Visible to Near-Infrared Photodetectors Based on MoS2 Vertical Schottky Junctions. Nanotechnology 2017, 28, 484002. (30) Shi, Y.; Li, H.; Wong, J.I.; Zhang, X.; Wang, Y.; Song, H.; Yang, H.Y. MoS2 Surface Structure Tailoring via Carbonaceous Promoter. Sci. Rep. 2015, 5, 10378. (31) Wang, Y.; Chen, B.; Seo, D.H.; Han, Z.J.; Wong, J.I.; Ostrikov, K.; Zhang, H.; Yang, H.Y. MoS2Coated Vertical Graphene Nanosheet for High-Performance Rechargeable Lithium-ion Batteries and Hydrogen Production. NPG Asia Mater. 2016, 8, e268. 17



(32) Shokhen, V.; Miroshnikov, Y.; Gershinsky, G.; Gotlib, N.; Stern, C.; Naveh, D.; Zitoun, D. On the Impact of Vertical Alignment of MoS2 for Efficient Lithium Storage. Sci. Rep. 2017, 7, 3280. (33) Li, H.; Wu, H. Q.; Yuan, S. G.; Qian, H. Synthesis and Characterization of Vertically Standing MoS2 Nanosheets. Sci. Rep. 2016, 6, 21171. (34) Yang, G. F.; Guo, Y.; Zhu, H. X.; Yan, D. W.; Li, G. H.; Gao, S. M. Fabrication of Nanorod InGaN/GaN Multiple Quantum Wells with Self-Assembled Ni Nano-Island Masks. Appl. Surf. Sci. 2013, 285P, 772-777. (35) Yang, G. F.; Xie, F.; Tong, Y. Y.; Chen, P.; Yu, Z. G.; Yan, D. W.; Xue, J. J.; Zhu, H. X.; Guo, Y.; Li, G. H.; Gao, S. M. Formation of Nanorod InGaN/GaN Multiple Quantum Wells Using Nickel Nano-Masks and Dry Etching for InGaN-Based Light-Emitting Diodes. Mater. Sci. in Semicon. Proc. 2015, 30, 694-706. (36) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147-150. (37) Robertson, J.; Liu, X.; Yue, C. L.; Escarra, M.; Wei, J. Wafer-Scale Synthesis of Monolayer and Few-Layer MoS2 via Thermal Vapor Sulfurization. 2D Mater. 2017, 4, 045007. (38) Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single and Few-Layer MoS2. Acs Nano 2010, 4, 2695-2700. (39) Dhakal, K. P.; Duong, D. L.; Lee, J.; Nam, H.; Kim, M.; Kan, M; Lee, Y. H.; Kim, J. Confocal Absorption Spectral Imaging of MoS2: Optical Transitions Depending on the Atomic Thickness of Intrinsic and Chemically doped MoS2. Nanoscale 2014, 6, 13028–13035. (40) Rice, C.; Young, R.J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K.S. Raman-Scattering Measurements and First-Principles Calculations of Strain-Induced Phonon Shifts in Monolayer MoS2. Phys. Rev. B 2013, 87, 081307. (41) Zhang, J.; Li, D.; Chen, R.; Xiong, Q. Laser Cooling of a Semiconductor by 40 Kelvin, Nature 2013, 493, 504-508. (42) Mitioglu, A.A.; Plochocka, P.; Jadczak, J.N.; Escoffier, W.; Rikken, G.L.J.A.; Kulyuk, L.; Maude, D.K. Optical Manipulation of the Exciton Charge State in Single Layer Tungsten Disulfide. Phys. Rev. B. 2013, 88, 245403. (43) Yan, T.F.; Qiao, X.F.; Liu, X.N.; Tan, P. H.; Zhang, X. H. Photoluminescence Properties and Exciton Dynamics in Monolayer WSe2. Appl. Phys. Lett. 2014, 105, 101901. (44) Lin, Z. Y.; Zhao, Y. D.; Zhou, C. J.; Zhang, R.; Wang, X. S.; Tsang, Y. H.; Chai, Y. Controllable Growth of Large-Size Crystalline MoS2 and Resist-Free Transfer Assisted with a Cu Thin Film. Sci. Rep. 2015, 5, 18596. (45) Weiss, K.; Phillips, J. M. Calculated Specific Surface-Energy of Molybdenite (MoS2). Phys. Rev. B 1976, 14, 5392-5395. (46) Atomistix Toolkit version 2017.0, Synopsys QuantumWise A/S (www.quantumwise.com). 18


Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47) Taylor, J.; Guo, H.; Wang, J. Ab Initio Modeling of Open Systems: Charge Transfer, Electron Conduction, and Molecular Switching of a C-60 Device. Phys. Rev. B 2001, 63, 121104. (48) Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (49) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (50) Wang, J.; Yang, G. F.; Sun, R.; Yan, P. F.; Lu, Y. N.; Xue, J. J.; Chen, G. Q. A Study on the Electronic and Interfacial Structures of Monolayer ReS2-Metal Contacts. Phys. Chem. Chem. Phys. 2017, 19, 27052-27058. (51) Sun, R.; Yang, G. F.; Wang, F. X.; Chu, G. Y.; Lu, N. Y.; Shen, X. W. A Theoretical Study on the Metal Contacts of Monolayer Gallium Nitride (GaN). Mat. Sci. Semicon. Proc. 2018, 84, 64-70. (52) Vaqueiro, P.; Romero, M. L.; Rowan, B. C.; Richards, B. S. Arrays of Chiral Nanotubes and a Layered Coordination Polymer Containing Gallium-Sulfide Supertetrahedral Clusters. Chem. Eur. J. 2010, 16, 4462-4465. (53) Kumar, B.G.; Muralidharan, K. S4N4 as an Intermediate in Ag2S Nanoparticle Synthesis. Rsc Adv. 2014, 4, 28219-28224. (54) Becke, A.D.; Edgecombe, K.E. A Simple Measure of Electron Localization in Atomic and Molecular Systems. J. Chem. Phys. 1990, 92, 5397-5403. (55) Yan, P. F.; Tian, Q. Q.; Yang G. F.; Weng, Y.Y.; Zhang, Y. Y.; Wang, J.; Xie, F.; Lu, N. Y. Epitaxial growth and interfacial property of monolayer MoS2 on gallium nitride. RSC Adv. 2018, 8, 33193.

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SEM and AFM images of the as-grown MoS2 triangular monolayers on p-GaN wafer substrate; Powerdependent Raman and PL spectra of the referential monolayer MoS2 on GaN substrate; Top and side views of the optimized atomic structure of monolayer (002) MoS2 on GaN (001), (010), and (100) planes. 85x67mm (300 x 300 DPI)


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Chemical Vapor Deposition Growth of Vertical MoS2 Nanosheets on p

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