Synthesis of Large‐Size 1T′ ReS2xSe2(1−x) Alloy Monolayer...

CommuniCation

1705015 (1 of 9) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advmat.de

Synthesis of Large-Size 1T′ ReS2xSe2(1−x) Alloy Monolayer with Tunable Bandgap and Carrier Type

Fangfang Cui, Qingliang Feng, Jinhua Hong, Renyan Wang, Yu Bai, Xiaobo Li, Dongyan Liu, Yu Zhou, Xing Liang, Xuexia He, Zhongyue Zhang, Shengzhong Liu, Zhibin Lei, Zonghuai Liu, Tianyou Zhai,* and Hua Xu*

DOI: 10.1002/adma.201705015

Semiconducting 2D transition metal dichalcogenides (TMDs) such as MX2 (MoS2, WSe2, etc. called group VI-TMDs) have attracted considerable attentions in the nanoelectronic, optoelectronic applica-tions owing to their high mobility, layer-dependent indirect to direct bandgap transition and valley polarization.[1–6] To satisfy the requirements of the building blocks of functional devices, researchers are striving to modulate the bandgap and carrier type of TMDs through electrical gating, strain engineering, molecular adsorption, and alloying materials.[7–11] Among the various approaches, large-scale monolayer films of three or four element TMD alloys (MoSe2xS2(1−x), WSe2xS2(1−x), MoxW(1−x)Se2, Mo1−xWxSe2(1−x)S2x, etc.) synthesized by chemical vapor deposi-tion (CVD) growth are promising with the tunable bandgap and carrier type.[12–19] Although these TMD alloys crystallize in the semiconductor 2H structure with the

Chemical vapor deposition growth of 1T′ ReS2xSe2(1−x) alloy monolayers is reported for the first time. The composition and the corresponding bandgap of the alloy can be continuously tuned from ReSe2 (1.32 eV) to ReS2 (1.62 eV) by precisely controlling the growth conditions. Atomic-resolution scanning transmission electron microscopy reveals an interesting local atomic distri-bution in ReS2xSe2(1−x) alloy, where S and Se atoms are selectively occupied at different X sites in each Re-X6 octahedral unit cell with perfect matching between their atomic radius and space size of each X site. This structure is much attractive as it can induce the generation of highly desired localized electronic states in the 2D surface. The carrier type, threshold voltage, and carrier mobility of the alloy-based field effect transistors can be systemati-cally modulated by tuning the alloy composition. Especially, for the first time the fully tunable conductivity of ReS2xSe2(1−x) alloys from n-type to bipolar and p-type is realized. Owing to the 1T′ structure of ReS2xSe2(1−x) alloys, they exhibit strong anisotropic optical, electrical, and photoelectric properties. The controllable growth of monolayer ReS2xSe2(1−x) alloy with tunable bandgaps and electrical properties as well as superior anisotropic feature provides the feasibility for designing multifunctional 2D optoelectronic devices.

Chemical Vapor Deposition

F. F. Cui, X. B. Li, D. Y. Liu, X. Liang, X. X. He, Prof. S. Z. Liu, Prof. Z. B. Lei, Prof. Z. H. Liu, Prof. H. XuKey Laboratory of Applied Surface and Colloid ChemistryMinistry of EducationShaanxi Key Laboratory for Advanced Energy DevicesShaanxi Engineering Lab for Advanced Energy TechnologySchool of Materials Science and EngineeringShaanxi Normal UniversityXi’an 710119, P. R. ChinaE-mail: [email protected]. Q. L. FengKey Laboratory of Space Applied Physics and ChemistryMinistry of EducationShaanxi Key Laboratory of Optical Information TechnologySchool of ScienceNorthwestern Polytechnical UniversityXi’an 710072, P. R. ChinaDr. J. H. HongNanostructures Research LaboratoryJapan Fine Ceramics CenterAtsuta, Nagoya 456-8587, Japan

Dr. R. Y. Wang, Prof. T. Y. ZhaiState Key Laboratory of Material Processing and Die and Mould TechnologySchool of Materials Science and EngineeringHuazhong University of Science and TechnologyWuhan 430074, P. R. ChinaE-mail: [email protected]. Bai, Prof. Z. Y. ZhangSchool of Physics and Information TechnologyShaanxi Normal UniversityXi’an 710119, P. R. ChinaDr. Y. ZhouDepartment of Mechanical Engineering and Materials ScienceYale UniversityNew Haven, CT 06511, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201705015.

Adv. Mater. 2017, 29, 1705015

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1705015 (2 of 9)

www.advmat.dewww.advancedsciencenews.com

bandgap tunability covering the range of visible spectrum, the carrier-type control is just achieved in a few members.[17,18] In this regard, 1T′ semiconductor TMDs, which show quite dif-ferent structures and properties compared with 2H TMDs, provide new opportunity to enrich the properties of 2D TMD alloys.

Group VII-TMDs such as ReX2 (X = S, Se), unlike group VI-TMDs that stabilized in highly symmetric 2H structure, crystallizing in the 1T′ structure with reduced in-plane crystal symmetry have received surging research interest recently.[20–27] The lower lattice symmetry of ReX2 endows them unusual in-plane anisotropic optical, electrical, and phonon proper-ties.[28–32] The opposing conduction property of ReS2 (n-type) and ReSe2 (p-type) offers a reasonable route to control the car-rier type and electronic properties of such 1T′ 2D materials by alloying them.[33–37] Moreover, the bandgaps of monolayers ReS2 and ReSe2 are ≈1.6 eV and ≈1.3 eV,[20,38] respectively, allowing the bandgap engineering of their alloys in the near-infrared range (NIR). Especially the strong anisotropy feature similar to that of black phosphorus offers one more degree of freedom for designing conceptual electronic, optoelectronic, and ther-moelectricity devices.[39–42] The theoretically negative formation energy of ReS2xSe2(1−x) alloys (as explained in Table S1 in the Supporting information) and recent works of mechanically exfoliated ReS2xSe2(1−x) alloy nanosheets show the possibility of CVD-derived stable 1T′ phase alloy films.[43,44] However, direct synthesis of monolayer 1T′ ReS2xSe2(1−x) alloys is still challenging due to complicated growth dynamics induced by highly anisotropic structure.

Here, we report the successful synthesis of large-size 1T′ semiconductor alloys ReS2xSe2(1−x) monolayer film with a wide range of compositions (x = 0–1.0) by CVD growth. The bandgap is continuously modulated from 1.32 eV (940 nm, x = 0) to 1.62 eV (765 nm, x = 1). Atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) characterizations showed that S and Se atoms uniformly dis-tributed in the ReS2xSe2(1−x) alloys in the large area, while an interesting sub-nanometer-scale local atom enrichment is found in the lattice of alloy. Electrical transport measurements of ReS2xSe2(1−x) alloy based field effect transistors demonstrate that the carrier type, threshold voltage, and carrier mobility can be systematically tuned by adjusting the alloy composition. Especially, for the first time, we realized the fully tunable con-ductivity of 2D ReS2xSe2(1−x) alloys from n-type (x = 0.6–1.0) to bipolar (x = 0.3–0.6) and p-type (x = 0–0.3). Furthermore, the anisotropic optical, electrical, and photoelectric properties of these monolayer alloys are verified by polarization Raman spec-troscopy, direction dependent electrical, and photoelectrical measurements.

For the growth of monolayer ReS2xSe2(1−x) alloys, sulfur (S) and selenium (Se) powder were placed into two quartz boats at the temperatures of TS ≈ 200 °C and TSe ≈ 300 °C, respectively, as shown in Figure 1a and Figure S1 in the Supporting infor-mation. A ceramic boat loaded with ReO3 powder was placed at the heating center of a quartz tube, and freshly cleaved fluo-rophlogopite mica substrate was placed onto the ceramic boat. Typical growth conditions were carrier gas Ar with flow rate of 60 sccm, growth temperature of 700 °C, and growth time of 5 min. Figure 1c shows a typical optical microscope (OM)

image of ReS2xSe2(1−x) alloy grown on mica substrate. They can be completely transferred onto the SiO2/Si substrate via poly (methyl methacrylate) (PMMA)-assisted transfer method, as shown in Figure 1c,d. Clearly, most of the achieved sam-ples exhibit hexagonal shape with domain size up to 200 µm. Here, it should be pointed out that the individual hexagonal shape domain is composed of several sub-domains (Figure S2, Supporting Information), which is similar to that of CVD-grown pure ReS2 and ReSe2 reported recently.[36,45] Atomic force microscopy (AFM) analysis on ReS2xSe2(1−x) hexagonal domains exhibits an ultraflat surface with height of 0.78 nm (Figure 1e), indicating the monolayer feature. Notably, the outer edge of the flake displays a dendritic-like feature, which is attributed to the anisotropic growth of the low lattice sym-metry 2D materials as explained in previous works.[23,34] Both OM and AFM characterizations demonstrated that large-area ReS2xSe2(1−x) alloy film with highly uniform monolayer was synthesized, and the layer number, domain size, and shape were controlled through finely tuning the growth conditions (Figure S3, Supporting Information).

The elemental composition of the synthesized alloy was examined by X-ray photoelectron spectroscopy (XPS). The predominant signal of Re, S, and Se was clearly seen from the survey spectra of ReS2xSe2(1−x) samples along with Si and O signals from the Si/SiO2 substrate (Figure 1f). The high-res-olution spectra of Re 4f, S 2p, and Se 3d peaks are shown in the insets of Figure 1f. The couple peaks of Re 4f7/2 and 4f5/2 are located at around 39.4 and 41.8 eV. While the strong peaks at around 51.9 and 52.8 eV represent the Se 3d5/2 and Se 3d3/2, respectively. The prominent 3p3/2 and 3p1/2 level peaks of Se are located at 158.4 and 164.0 eV, respectively. The peaks at around 159.8 and 161.0 eV are attributed to the binding energy of S 2p3/2 and S 2p1/2. Furthermore, the atomic ratio between Re, S, and Se elements is 1:1.60:0.40, indicating that the composition of CVD-grown alloy is ReS1.60Se0.40. Figure 1g shows the low-magnification ADF-STEM image of a ReS1.60Se0.40 nanosheet and the corresponding selected area electron diffraction (SAED) pattern and energy dispersive spectrometer (EDS) elemental mapping, demonstrating the high quality of our CVD-grown ReS1.60Se0.40 alloy with Re, S, and Se elements uniformly dis-tributed throughout the entire sample.

A series of monolayer ReS2xSe2(1−x) alloys with continuously variable compositions (x from 0 to 1) have been successfully obtained by systematically adjusting the mass ratio of S and Se powders during synthesis process (see details in experi-ment section). The exact composition of each alloy samples was confirmed via the XPS analysis, as shown in Figure 2a. The intensities of S 2p3/2 and S 2p1/2 peaks (159.8 and 161.0 eV) monotonically decrease with the decrease of S composition in ReS2xSe2(1−x) alloys, while peaks for Se 3p3/2 and Se 3p1/2 (158.4 and 164.0 eV) appear and gradually become dominant. Photolu-minescence (PL) measurement was performed on ReS2xSe2(1−x) alloys to evaluate the composition-dependent bandgap evolu-tion (Figure 2b). As expected, the PL spectral peak positions are continuously tunable from 940 nm (for pure ReSe2) to 765 nm (for pure ReS2) as the x value varies from 0 to 1.0. The bandgap energy (Eg), corresponding to the PL peak posi-tion (Figure 2c), shows approximately linear dependence on S composition x between 1.32 eV (ReSe2) and 1.62 eV (ReS2). The

Adv. Mater. 2017, 29, 1705015



composition-dependent bandgaps of the achieved alloys can be described by the following function

ReS Se ReS 1 ReSe 1g 2 2 1 g 2 g 2E xE x E bx xX x( ) ( ) ( ) ( ) ( )= + − − −( )− (1)where b is the bowing parameter. The experimental results of monolayer alloys can be fitted using Equation (1), yielding bowing parameter of 0.25. Above results demonstrate that the composition and the corresponding bandgap of the alloy can be continually tuned from pure ReSe2 to pure ReS2 by precisely con-trolling the growth conditions. Notably, just one direct excitonic emission is observed in our monolayer alloy samples, indicating that the PL emission is originated from a single band-edge emis-sion. The results would be different in the thick layers as reported recently, where several direct and indirect excitonic emissions are detected.[46] In addition, different from recently reported 1T′ MoxW1−xTe2 alloy that is a Weyl semimetal,[47] 1T′ ReS2xSe2(1−x) alloy is semiconductor. These ReS2xSe2(1−x) alloy system with tun-able bandgap can greatly enrich 2D material family and enable construction of wavelength-selective optoelectronic devices.

Raman spectra were utilized to investigate the composi-tion-dependent lattice vibration modes of alloys. Figure 2d–f shows the Raman spectra of the as-grown ReS2xSe2(1−x) alloys with S composition x increased gradually from 0 to 1.0. For mono layer ReS2, the main vibration modes are in the range of 100–450 cm−1, while for monolayer ReSe2, the primary vibra-tion modes are located below 300 cm−1. The primary Raman modes of these alloys can be defined as Eg-like, Ag-like, or Cp-like modes based on the dominant direction of phonon vibra-tions. The Raman frequency for all vibration modes shows obvious blue shifts as S content increases (Figure 2d–f) as a result of smaller reduced mass for higher S content in the ReS2xSe2(1−x) alloys. For the lower-frequency Raman modes (from 100 to 150 cm−1), the intensity is nearly independent of the S and Se composition in ReS2xSe2(1−x) alloys, indicating that those modes are mainly attributed to the vibration modes related to Re atoms. For the Raman shift ranging from 150 to 300 cm−1, large amounts of peaks were observed for Se-rich alloys, while only few peaks shown in S-rich alloys. In higher-frequency region (from 300 to 450 cm−1), several Raman peaks

Adv. Mater. 2017, 29, 1705015

Figure 1. a) Schematic illustration of the atomic structure of ReS2xSe2(1−x) alloy and its synthesis approach. b–d) Typical optical microscopy image of monolayer ReS2xSe2(1−x) alloy b) grown on mica substrate and c,d) transferred onto SiO2/Si substrate. e) AFM image of as-grown ReS2xSe2(1−x) alloy on mica substrate. f) XPS full spectrum of monolayer ReS1.60Se0.40 alloy on SiO2/Si substrate. Inserts are the high-resolution XPS Re 4f (left), S 2p (middle), and Se 3d (right) spectra. g) ADF-STEM image, SAED pattern, and EDS mapping of Re, S and Se.



arise as S content increases, indicating that these Raman peaks correspond to the vibration modes related with S atoms. The Raman frequency of several main peaks shows a linear increase as the S composition increases (Figure 2g), giving quick and accurate way to determine the composition of alloys. Such sys-tematic Raman frequency shift and intensity evolution with S and Se content are well consistent with that of mechanical exfo-liated alloy samples,[44,46] and thus further confirm the expected structural and compositional modulation of the CVD-grown ReS2xSe2(1−x) alloys.

The crystal structures and atomic distribution of 1T′ ReS2xSe2(1−x) alloys were investigated using ADF-STEM imaging on an aberration-corrected TEM. Figure 3a–c shows the atomic-resolved ADF-STEM images of monolayer ReS2xSe2(1−x) alloys with different x. As S (Z = 16) is much lighter than Se (Z = 34) and Re (Z = 75), the brightness of S sites in the Z-contrast imaging (ADF-STEM imaging) is much dimmer than that of Se and Re sites. Clearly, the diamond-shaped Re4 chains can be visualized in these alloys, and the numbers of Se sites increase apparently with S composition x (x obtained from XPS) decreases. In the zoomed-in image

of Figure 3a, we can more accurately identify their positions in the alloy lattice structure and count the numbers of Re, S, and Se sites (Figure 3d). A statistical histogram of the intensi-ties at these sites (Figure 3e) has two separate regions, where the gray region with an intensity of ≈70–105 arbitrary units (a.u.) assigned to Se, the brightest region with an intensity of ≈205–256 a.u. assigned to Re. By counting individual sites, the Se atom percentage can be obtained (112/235 = 0.48), matching well with the XPS result of 2(1 − x) = 0.49 for the Se composition. In the STEM image color-coded by ADF intensity (Figure 3f), the Re and Se atoms can be more clearly differen-tiated as yellow and blue balls, respectively, with the pattern being further verified by the corresponding intensity line pro-file shown in Figure 3h,i. Such atomic arrangements are in per-fect accordance with the 1T′-phase atomic model (Figure 3g), in which S and Se atoms are coordinated to Re atoms with a distorted Re-X6 octahedron structure. In addition, due to the much small lattice mismatch (see Table S1 in the Supporting Information) between ReS2 and ReSe2 crystal,[48] no obvious lattice distortion or deformation of Re chains is observed in the ReS2xSe2(1−x) alloys.

Adv. Mater. 2017, 29, 1705015

Figure 2. a) XPS and b) PL spectra of all as-grown monolayer ReS2xSe2(1−x) alloys. c) Plots of fluorescence-emission energy as function of the S compo-sition x. d–f) Raman spectra of monolayer ReS2xSe2(1−x) alloys with S composition x from 0 to 1.0. g) Plots of Raman frequency of four primary Raman modes as function of the S composition x.



As demonstrated above that the atomic structure of 1T′ ReX2 is different from that of 2H MX2, it means that there should be some difference in the atomic distribution (a key factor of

alloys) between 1T′ and 2H alloys. Fortunately, we found that the bonding behavior of Re with S and Se atoms in the 1T′ ReS2xSe2(1−x) alloys is much different from that in group-VI

Adv. Mater. 2017, 29, 1705015

Figure 3. ADF-STEM image of monolayer ReS2xSe2(1−x) alloys with different S composition x: a) 0.76, b) 0.65, and c) 0.49. Here, the STEM images of these alloys were rotated to make their b-axis nearly in the same direction. d) The enlarged STEM image of the highlighted rectangle area in (a), the Re, S and Se atoms were marked by cycles in red, yellow, and blue color, respectively. e) Static numbers of Re and Se atoms in ReS2xSe2(1−x) alloy. f) Colored STEM image of monolayer ReS1.29Se0.71 alloy, and g) the corresponding atomic models for the alloy. h,i) Intensity profiles along the two white dashed lines in (f). j) Atomic structure of ReX2 shows the lattice constants. k) Relationship between the lattice constants (d1 and d2) and the alloy composition. l) Atomic distribution of S and Se atoms around Re atom at the six X sites with different space size.



TMD alloys where the six X atoms (S and Se) are randomly bond to each M (Mo or W) atom because they have equivalent space positions.[13] In the ReS2xSe2(1−x) alloys, the numbers of Se atom (S atom) bond to Re atom at the outside of Re4 chain are found to be more (less) than that at the inside of Re4 chain (Figure 3a–d; Figure S4, Supporting Information). Such results can be understood from the 1T′ structure of ReX2 where each Re atom is bonded with three adjacent Re atoms forming a rigid Re4 atom chain (Figure 3j), then the six X atoms (numbered with 1–6) bonded with each Re atom are different in the spatial position. More specifically, there are four different positions for all X atoms in ReX2 structure (Figure 3j; Figure S4, Supporting Information), site 1 (equivalent to 2) has the largest space size, sites 3 and 6 come second and third, while site 4 (equivalent to 5) has the smallest space size. This structure also leads to a large difference in the lattice distance between the two adja-cent Re atom chain along both (010) and (001) directions, for example, d2 is larger than d1, and d4 is larger than d3. In this case, the relative larger Se atom (atomic radius: 1.17 Å) should be preferred to bond to Re atom at the outside of Re4 chain, while the relative smaller S atom (atomic radius: 1.04 Å) should be inclined to bond to Re atom at the inside of Re4 chain. This tendency was further confirmed from the lattice distance varia-tion with alloy composition, as shown in Figure 3k and Figure S5 in the Supporting information. Clearly, d2 and d4 exhibit obvious decrease with S composition increases, while d1 and d3 display nearly negligible variation with composition changes. Of course, the rigid Re4 chain formed with three ReRe bonds at each Re atom hinders the variation of d1 and d3 with the X composition changes. The statistic atomic distribution of

S and Se atom around the Re atom at the six X positions in ReS2xSe2(1−x) alloy is shown in Figure 3l and Figure S6 in the Supporting information. Clearly, the order of numbers of Se atom that occupies the six X positions perfectly matches with the sequence of their space size variation; the relative large Se atom (relative small S atom) is preferred to occupy X sites with large (small) space size. Thus, the atomic distributions of ReS2xSe2(1−x) alloy exhibit high homogeneity in large area along with local nonuniformity (enrichment) in each Re-X6 octahe-dral unit cell, which should be a typical characteristic of 1T′ alloy. This sub-nanometer-scale local atomic distribution could induce the generation of localized electronic states in the 2D alloy film, which would bring many amusing properties but needs to be further verified.

To investigate composition dependence of the electrical proper-ties of the as-grown monolayer ReS2xSe2(1−x) alloy, back-gate field effect transistors (FETs) were fabricated on SiO2/Si substrates. Figure 4a,b shows the transfer characteristics of mono layer ReS0.84Se1.16 alloy FETs measured at room temperature in ambient air condition. It exhibits a bipolar conduction behavior with elec-tron and hole field-effect mobility of 0.46 and 0.16 cm2 V−1 s−1, respectively. The output curves (Ids–Vds) of ReS0.84Se1.16 alloy FET show nearly linear relationship (Figure 4c), which suggests that Ohmic-like contacts formed at the source and drain electrodes. Notably, the output characteristic of alloy transistor shows a linear to nonlinear evolution with the Se composition increases (Figure S7, Supporting Information), suggesting an increasing Schottky barrier induced by the band-edge variation.

The transfer characteristics of ReS2xSe2(1−x) alloys with dif-ferent S composition were systematically studied, as shown

Adv. Mater. 2017, 29, 1705015

Figure 4. Electrical transport properties of monolayer ReS2xSe2(1−x) alloys. a,b) Transfer curves (source−drain current vs. gate voltage (Ids–Vg)) of ReS0.84Se1.16 alloy transistors with source−drain voltage (Vds) varying from 0 to 1.0 V on a linear scale a) and a logarithmic scale b), respectively. c) Output curves (Ids–Vds) at varying back gate voltages. d) Transfer curves (Ids–Vg) of ReS2xSe2(1−x) alloy transistors with different S composition x. e) Threshold voltage (Vth) as function of S composition x in ReS2xSe2(1−x) alloys, with the red dots highlighting the p-type conduction behavior in ReSe2-rich alloys and blue dots indicating n-type conduction behavior in ReS2-rich alloys. f) Field effect mobility as function of S composition x in ReS2xSe2(1−x) alloys, with the red dots representing the hole mobility in Se-rich alloys and the blue dots representing the electron mobility in S-rich alloys.



in Figure S8 in the Supporting information. In Figure 4d, we present the transfer curves of several representative alloys. For the S-rich alloys (x = 0.6–1.0), the FETs predominantly display n-type semiconductor properties, whereas for the Se-rich alloys (x = 0–0.3), the p-type semiconductor properties are mainly observed. Notably, the electrical conduction for the intermediate composition of alloys region (x = 0.3–0.6) exhibits obvious bipolar behavior. The onset threshold voltage (Vth) of the ReS2xSe2(1−x) alloy transistor (Figure 4e) for both the electron conduction and hole conduction shifts to positive value, indicating that the valence and conduction bands shift as the S composition changes. As shown in Figure 4f, the hole mobility drops from 0.03 to 0.002 cm2 V−1 s−1 as the Se-ratio decreases in the Se-rich side. The electron mobility in S-rich alloys shows a similar trend, starting from a relatively high value of 0.8 cm2 V−1 s−1 in the pure ReS2, and decreasing to 0.03 cm2 V−1 s−1 in the increasingly alloying samples. The mobility of ReS2 FETs achieved in our work is comparable to that of recently reported CVD-grown ReS2 monolayer (0.44–4.3 cm2 V−1 s−1),[34,49] and the value of ReSe2 FETs is one order of magnitude higher than that of recently reported CVD-grown ReSe2 mono layer (1.36 × 10−3 cm2 V−1 s−1).[36] The rela-tive low mobility for intermediate composition of ReS2xSe2(1−x) alloys (x = 0.3–0.6) may be attributed partly to larger contact resistance due to decreased doping concentration (Figure S7, Supporting Information), and partly to the increased alloy scattering and ionized impurity scattering in the sample, which is commonly observed in the group-VI TMDs alloys.[17] Despite all this, the mobility of our ReS2xSe2(1−x) alloy FETs is

comparable to that of most group VI-TMD alloys (0.4–2.97 × 10−3 cm2 V−1 s−1),[13,18,50,51] and it could be further improved by using contact metal with appropriate work function and high-k top gate dielectrics.

To further assess the anisotropic feature of the 1T′ ReS2xSe2(1−x) alloys, we conducted angle-resolved polarized Raman spectra (ARPRS), angle-dependent electrical transport, and polarization-sensitive photoresponse measurements on alloys (Figure 5a). Figure 5b shows the evolution of the Raman spectra of ReS1.71Se0.29 alloy with respect to the polarization angle of incident light. It can be seen that the peak intensity of each Raman mode varies significantly with a period of 180°, while the peak positions remained unchanged. This depend-ence can be clearly visualized in the polar plots of the peak intensity as a function of the polarization angle (θ) of the laser (Figure 5c), where the polarized Raman intensity of three main peaks at 147, 208, and 307 cm−1 can be well fitted by Equation (2)[30]

cos sin cos sin2 2

I a d d bθ θ θ θ∝ + + + (2)

Notably, such polarization-sensitive Raman spectra can be observed in all alloys with different composition (Figure S9, Supporting Information), indicating the aniso-tropic lattice structure and phonon properties of these alloys. Similar to the mechanical exfoliated ReS2xSe2(1−x) alloys,[44] the maximum of polarized Raman intensity of Eg-like Raman mode at 175–213 cm−1 of the CVD-grown alloys is along their b-axis direction (Figure S10, Supporting Information), which can be

Adv. Mater. 2017, 29, 1705015

Figure 5. a) Schematic illustration of the anisotropic measurement for the Raman spectra, and electrical and photoelectrical properties. In reality, the laser beam irradiates perpendicularly to the sample. b) 2D mapping of the Raman spectra of ReS2xSe2(1−x) alloy with respect to the polarization angle of laser. (The intensity is normalized with respect to the peak intensity at 147 cm−1). c) Polar plots for the Raman intensity of peaks at 147, 208, and 307 cm−1 with respect to polarization angle of the incident light. d) Angle-dependent transfer curves (Vds = 1.0 V) of ReS1.23Se0.77 alloy device. e) Photo-current response of ReS1.06Se0.94 alloy device under light on and off irradiation, and under light with different polarization direction. The direction of b-axis is determined via ARPRS. f) Polar plots for the photocurrent with respect to the polarization angle of the incident light.



Adv. Mater. 2017, 29, 1705015

used to identify the lattice orientation of these alloys. In addi-tion, the electrical transport properties of the alloy FETs show obvious lattice direction dependence (Figure 5d), where both electron and hole mobility of the FET channel along the b-axis is larger than that perpendicular to the b-axis. The nearly three times difference in electron mobility along the two lattice direc-tions is equal to that of pure ReS2 FETs,[34,41] implying that the introduction of doping atoms does not cause obvious degenera-tion to its anisotropic electrical transport properties. This is of significant importance for the anisotropy-based device applica-tion of the 2D 1T′ alloys. Furthermore, the photoresponse of the alloy device depends on the polarization angle of the incident light (Figure 5e) with the photocurrent reaching the maximum and minimum values when the incident light polarized along and perpendicular to the b-axis, respectively (Figure 5f). This polarization-sensitive photodetection, which can realize in any composition of alloys, is highly desired for constructing future multifunctional 2D optoelectronic devices.

In summary, large-scale 1T′ semiconductor alloy ReS2xSe2(1−x) monolayer films with fully tunable chemical com-positions have been directly synthesized using CVD growth. The bandgap of these alloys can be continually modulated in the NIR range (1.3–1.6 eV) with the composition variations. An interesting sub-nanometer-scale local atomic distribution was observed in the 1T′ ReS2xSe2(1−x) alloys, which is different from that of 2H alloys, that might induce the generation of local electronic states and thus bring many amusing properties. Electrical transport studies revealed systematic modulation of the electronic properties, including the carrier type, threshold voltage, and carrier mobility, with the alloy composition being changed. Furthermore, the low lattice symmetry 2D alloys dis-played excellent anisotropic optical, electrical, and photoelectric properties. Such tunable bandgap and carrier type coupled with the strong anisotropic feature realized within a monolayer 1T′ semiconductor alloy will open up new prospects for TMDs in integrated electronic and optoelectronic devices.

Experimental SectionSample Growth and Transfer: Monolayer ReS2xSe2(1−x) alloy was

synthesized by using a home-made CVD system with ReO3 as Re precursor, and sulfur powder and selenium powder as S and Se source, respectively. The loading amount of the S and Se powders was usually 10–100 mg, and their mass ratio was turned according to the prepared alloy composition. The mica substrates were put onto the ceramic boat containing ReO3 powder (3 mg) at the heating center of tube furnace. The growth was conducted at temperature of 700 °C with carrier gas of 60 sccm argon (Ar) for about 5 min. The volatilization temperature of S and Se was strictly fixed at 200 and 300 °C, by controlling their distances away from the heating center, respectively. Note that the ReO3 powder was covered with a layer of molecular sieve, which is important for the controlled growth of alloy film with uniform monolayer, because it can control the release rate of ReO3 source in CVD growth system. The samples were transferred onto SiO2/Si (300 nm) substrate or copper grid via the PMMA-mediated transfer method for further characterization.

Structure and Composition Characterization: Optical images were taken on an Olympus BX51 microscope. AFM was done on a Bruker ICON microscope. Raman and PL spectra were measured on a Renishaw inVia micro-Raman spectroscope with 532 nm laser. Angle-resolved polarization Raman spectra were achieved with a half-wave being used to turn the polarization angle of excitation laser. XPS analysis was done on an Axis

Ultra system. High angle annular dark field (HAADF)-STEM imaging was performed on a probe aberration-corrected JEM-2100F operated at 200 kV.

Device Fabrication and Electrical Measurement: Electrode patterns were generated through electron beam lithography with 5 nm Cr and 50 nm Au being thermally evaporated for contacts. All alloy devices were measured by an Agilent B2912 semiconductor analyzer under ambient air at room temperature. Polarization-dependent photoresponse was performed under the 532 nm laser excitation with a half-wave being used to turn its polar directions.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsF.F.C. and Q.L.F. contributed equally to this work. The authors acknowledge the insightful suggestions and comments from Dr. N. N. Mao and N. Zhang at Peking University. This work was supported by the National Natural Science Foundation of China (51502167, 91622117, 11634010), the Fundamental Research Funds for the Central Universities in Shaanxi Normal University (GK201502003), the Funded Projects for the Academic Leaders and Academic Backbones, Shaanxi Normal University (16QNGG011), and the Fundamental Research Funds for the Central Universities in Northwestern Polytechnical University (3102016QD071).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsReS2xSe2(1−x) alloys, anisotropy, carrier-type modulation, chemical vapor deposition, tunable bandgap

Received: September 2, 2017Published online: October 23, 2017

[1] X. F. Qian, J. W. Liu, L. Fu, J. Li, Science 2014, 346, 1344.[2] K. F. Mak, J. Shan, Nat. Photonics 2016, 10, 216.[3] F. H. L. Koppens, T. Mueller, P. Avouris, A. C. Ferrari, M. S. Vitiello,

M. Polini, Nat. Nanotechnol. 2014, 9, 780.[4] X. Cui, G.-H. Lee, Y. D. Kim, G. Arefe, P. Y. Huang, C.-H. Lee,

D. A. Chenet, X. Zhang, L. Wang, F. Ye, F. Pizzocchero, B. S. Jessen, K. Watanabe, T. Taniguchi, D. A. Muller, T. Low, P. Kim, J. Hone, Nat. Nanotechnol. 2015, 10, 534.

[5] A. Splendiani, L. Sun, Y. B. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, F. Wang, Nano Lett. 2010, 10, 1271.

[6] H. L. Zeng, J. F. Dai, W. Yao, D. Xiao, X. D. Cui, Nat. Nanotechnol. 2012, 7, 490.

[7] T. Chu, H. Ilatikhameneh, G. Klimeck, R. Rahman, Z. H. Chen, Nano Lett. 2015, 15, 8000.

[8] S. B. Desai, G. Seol, J. S. Kang, H. Fang, C. Battaglia, R. Kapadia, J. W. Ager, J. Guo, A. Javey, Nano Lett. 2014, 14, 4592.

[9] Q. S. Zeng, H. Wang, W. Fu, Y. J. Gong, W. Zhou, P. M. Ajayan, J. Lou, Z. Liu, Small 2015, 11, 1868.

[10] Y. D. Zhao, K. Xu, F. Pan, C. J. Zhou, F. C. Zhou, Y. Chai, Adv. Funct. Mater. 2017, 27, https://doi.org/10.1002/adfm.201603484.

https://doi.org/10.1002/adfm.201603484



Adv. Mater. 2017, 29, 1705015

[11] Y. F. Chen, J. Y. Xi, D. O. Dumcenco, Z. Liu, K. Suenaga, D. Wang, Z. Q. Shuai, Y.-S. Huang, L. M. Xie, ACS Nano 2013, 7, 4610.

[12] J. Mann, Q. Ma, P. M. Odenthal, M. Isarraraz, D. Le, E. Preciado, D. Barroso, K. Yamaguchi, G. von Son Palacio, A. Nguyen, T. Tran, M. Wurch, A. Nguyen, V. Klee, S. Bobek, D. Z. Sun, T. F. Heinz, T. S. Rahman, R. Kawakami, L. Bartels, Adv. Mater. 2014, 26, 1399.

[13] Q. L. Feng, Y. M. Zhu, J. H. Hong, M. Zhang, W. J. Duan, N. N. Mao, J. X. Wu, H. Xu, F. L. Dong, F. Lin, C. H. Jin, C. M. Wang, J. Zhang, L. M. Xie, Adv. Mater. 2014, 26, 2648.

[14] H. L. Li, X. X. Duan, X. P. Wu, X. J. Zhuang, H. Zhou, Q. L. Zhang, X. L. Zhu, W. Hu, P. Y. Ren, P. F. Guo, L. Ma, X. P. Fan, X. X. Wang, J. Y. Xu, A. L. Pan, X. F. Duan, J. Am. Chem. Soc. 2014, 136, 3756.

[15] W. T. Zhang, X. D. Li, T. T. Jiang, J. Song, Y. Lin, L. X. Zhu, X. L. Xu, Nanoscale 2015, 7, 13554.

[16] Q. Fu, L. Yang, W. H. Wang, A. Han, J. Huang, P. W. Du, Z. Y. Fan, J. Y. Zhang, B. Xiang, Adv. Mater. 2015, 27, 4732.

[17] X. D. Duan, C. Wang, Z. Fan, G. L. Hao, L. Z. Kou, U. Halim, H. L. Li, X. P. Wu, Y. C. Wang, J. H. Jiang, A. L. Pan, Y. Huang, R. Q. Yu, X. F. Duan, Nano Lett. 2016, 16, 264.

[18] X. F. Li, M.-W. Lin, L. Basile, S. M. Hus, A. A. Puretzky, J. Lee, Y.-C. Kuo, L.-Y. Chang, K. Wang, J. C. Idrobo, A.-P. Li, C.-H. Chen, C. M. Rouleau, D. B. Geohegan, K. Xiao, Adv. Mater. 2016, 28, 8240.

[19] S. Susarla, A. Kutana, J. A. Hachtel, V. Kochat, A. Apte, R. Vajtai, J. C. Idrobo, B. I. Yakobson, C. S. Tiwary, P. M. Ajayan, Adv. Mater. 2017, 29, 1702457.

[20] S. Tongay, H. Sahin, C. Ko, A. Luce, W. Fan, K. Liu, J. Zhou, Y. S. Huang, C. H. Ho, J. Y. Yan, D. F. Ogletree, S. Aloni, J. Ji, S. S. Li, J. Li, F. M. Peeters, J. Q. Wu, Nat. Commun. 2014, 5, 3252.

[21] M. Rahman, K. Davey, S.-Z. Qiao, Adv. Funct. Mater. 2017, 27, 1606129.

[22] Q. Zhang, S. J. Tan, R. G. Mendes, Z. T. Sun, Y. T. Chen, X. Kong, Y. H. Xue, M. H. Rümmeli, X. J. Wu, S. L. Chen, L. Fu, Adv. Mater. 2016, 28, 2616.

[23] K. Keyshar, Y. J. Gong, G. Ye, G. Brunetto, W. Zhou, D. P. Cole, K. Hackenberg, Y. He, L. Machado, M. Kabbani, A. H. C. Hart, B. Li, D. S. Galvao, A. George, R. Vajtai, C. S. Tiwary, P. M. Ajayan, Adv. Mater. 2015, 27, 4640.

[24] E. F. Liu, M. S. Long, J. W. Zeng, W. Luo, Y. J. Wang, Y. M. Pan, W. Zhou, B. G. Wang, W. D. Hu, Z. H. Ni, Y. M. You, X. A. Zhang, S. Q. Qin, Y. Shi, K. Watanabe, T. Taniguchi, H. T. Yuan, H. Y. Hwang, Y. Cui, F. Miao, D. Y. Xing, Adv. Funct. Mater. 2016, 26, 1938.

[25] E. Zhang, Y. B. Jin, X. Yuan, W. Y. Wang, C. Zhang, L. Tang, S. S. Liu, P. Zhou, W. D. Hu, F. X. Xiu, Adv. Funct. Mater. 2015, 25, 4076.

[26] Y. C. Lin, H. P. Komsa, C. H. Yeh, T. Björkman, Z. Y. Liang, C. H. Ho, Y. S. Huang, P. W. Chiu, A. V. Krasheninnikov, K. Suenaga, ACS Nano 2015, 9, 11249.

[27] M. Hafeez, L. Gan, A. Saleem Bhatti, T. Y. Zhai, Mater. Chem. Front. 2017, 1, 1917.

[28] D. A. Chenet, O. B. Aslan, P. Y. Huang, C. Fan, A. M. van der Zande, T. F. Heinz, J. C. Hone, Nano Lett. 2015, 15, 5667.

[29] D. Wolverson, S. Crampin, A. S. Kazemi, A. Ilie, S. J. Bending, ACS Nano 2014, 8, 11154.

[30] H. Zhao, J. B. Wu, H. X. Zhong, Q. S. Guo, X. M. Wang, F. N. Xia, L. Yang, P. H. Tan, H. Wang, Nano Res. 2015, 8, 3651.

[31] S. Sim, D. Lee, M. Noh, S. Cha, C. H. Soh, J. H. Sung, M.-H. Jo, H. Choi, Nat. Commun. 2016, 7, 13569.

[32] A. Arora, J. Noky, M. Drüppel, B. Jariwala, T. Deilmann, R. Schneider, R. Schmidt, O. Del Pozo-Zamudio, T. Stiehm, A. Bhattacharya, P. Krüger, S. Michaelis de Vasconcellos, M. Rohlfing, R. Bratschitsch, Nano Lett. 2017, 17, 3202.

[33] X. X. He, F. C. Liu, P. Hu, W. Fu, X. L. Wang, Q. S. Zeng, W. Zhao, Z. Liu, Small 2015, 11, 5423.

[34] F. F. Cui, C. Wang, X. B. Li, G. Wang, K. Q. Liu, Z. Yang, Q. L. Feng, X. Liang, Z. Y. Zhang, S. Z. Liu, Z. Lei, Z. H. Liu, H. Xu, J. Zhang, Adv. Mater. 2016, 28, 5019.

[35] F. F. Cui, X. Li, Q. L. Feng, J. B. Yin, L. Zhou, D. Y. Liu, K. Q. Liu, X. X. He, X. Liang, S. Z. Liu, Z. B. Lei, Z. H. Liu, H. L. Peng, J. Zhang, J. Kong, H. Xu, Nano Res. 2017, 2732.

[36] M. Hafeez, L. Gan, H. Q. Li, Y. Ma, T. Y. Zhai, Adv. Mater. 2016, 28, 8296.

[37] B. Kang, Y. Kim, J. H. Cho, C. Lee, 2D Mater. 2017, 4, 025014.[38] B. Jariwala, D. Voiry, A. Jindal, B. A. Chalke, R. Bapat,

A. Thamizhavel, M. Chhowalla, M. Deshmukh, A. Bhattacharya, Chem. Mater. 2016, 28, 3352.

[39] F. N. Xia, H. Wang, Y. C. Jia, Nat. Commun. 2014, 5, 4458.[40] J. X. Wu, N. N. Mao, L. M. Xie, H. Xu, J. Zhang, Angew. Chem. Int.

Ed. 2015, 54, 2366.[41] E. F. Liu, Y. J. Fu, Y. J. Wang, Y. Q. Feng, H. M. Liu, X. G. Wan,

W. Zhou, B. G. Wang, L. B. Shao, C. H. Ho, Y. S. Huang, Z. Y. Cao, L. G. Wang, A. D. Li, J. W. Zeng, F. Q. Song, X. R. Wang, Y. Shi, H. T. Yuan, H. Y. Hwang, Y. Cui, F. Miao, D. Y. Xing, Nat. Commun. 2015, 6, 6991.

[42] F. C. Liu, S. J. Zheng, X. X. He, A. Chaturvedi, J. F. He, W. L. Chow, T. R. Mion, X. L. Wang, J. D. Zhou, Q. D. Fu, H. J. Fan, B. K. Tay, L. Song, R. H. He, C. Kloc, P. M. Ajayan, Z. Liu, Adv. Funct. Mater. 2015, 26, 1169.

[43] F. C. Liu, S. J. Zheng, A. Chaturvedi, V. Zolyomi, J. D. Zhou, Q. D. Fu, C. Zhu, P. Yu, Q. S. Zeng, N. D. Drummond, H. J. Fan, C. Kloc, V. I. Fal'ko, X. X. He, Z. Liu, Nanoscale 2016, 8, 5826.

[44] W. Wen, Y. M. Zhu, X. L. Liu, H.-P. Hsu, Z. Fei, Y. F. Chen, X. S. Wang, M. Zhang, K.-H. Lin, F.-S. Huang, Y.-P. Wang, Y.-S. Huang, C.-H. Ho, P.-H. Tan, C. H. Jin, L. Xie, Small 2017, 13, 1603788.

[45] K. D. Wu, B. Chen, S. J. Yang, G. Wang, W. Kong, H. Cai, T. Aoki, E. Soignard, X. Marie, A. Yano, A. Suslu, B. Urbaszek, S. Tongay, Nano Lett. 2016, 16, 5888.

[46] C. H. Ho, Z. Z. Liu, M. H. Lin, Nanotechnology 2017, 28, 235203.[47] I. Belopolski, D. S. Sanchez, Y. Ishida, X. X. Pan, P. Yu, S.-Y. Xu,

G. Q. Chang, T.-R. Chang, H. Zheng, N. Alidoust, G. Bian, M. Neupane, S.-M. Huang, C.-C. Lee, Y. Song, H. J. Bu, G. Wang, S. Li, G. Eda, H.-T. Jeng, T. Kondo, H. Lin, Z. Liu, F. Q. Song, S. Shin, M. Z. Hasan, Nat. Commun. 2016, 7, 13643.

[48] C. H. Ho, Y. S. Huang, P. C. Liao, K. K. Tiong, J. Phys. Chem. Solids 1999, 60, 1797.

[49] A. Dathbun, Y. Kim, S. Kim, Y. Yoo, M. S. Kang, C. Lee, J. H. Cho, Nano Lett. 2017, 17, 2999.

[50] M. Zhang, J. X. Wu, Y. M. Zhu, D. O. Dumcenco, J. H. Hong, N. N. Mao, S. B. Deng, Y. F. Chen, Y. L. Yang, C. H. Jin, S. H. Chaki, Y.-S. Huang, J. Zhang, L. M. Xie, ACS Nano 2014, 8, 7130.

[51] J. Huang, W. Wang, Q. Fu, L. Yang, K. Zhang, J. Zhang, B. Xiang, Nanotechnology 2016, 27, 13LT01.

Synthesis of Large‐Size 1T′ ReS2xSe2(1−x) Alloy Monolayer...

Documents

Transcript of Synthesis of Large‐Size 1T′ ReS2xSe2(1−x) Alloy Monolayer...