Solution Processing for Lateral Transition Metal Dichalcogenides

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Solution Processing for Lateral Transition Metal Dichalcogenides Homojunction from Polymorphic Crystal Jiajing Wu, Jing Peng, Yuan Zhou, Yue Lin, Xiaolei Wen, Junchi Wu, Yingcheng Zhao, Yuqiao Guo, Changzheng Wu, and Yi Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11656 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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Journal of the American Chemical Society

Solution Processing for Lateral Transition Metal Dichalcogenides Homojunction from Polymorphic Crystal Jiajing Wu§†, Jing Peng§†, Yuan Zhou†, Yue Lin†, Xiaolei Wen‡, Junchi Wu†, Yingcheng Zhao†, Yuqiao Guo†, Changzheng Wu*† and Yi Xie† † Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei 230026, PR China. ‡ Center for Micro- and Nanoscale Research and Fabrication, University of Science and Technology of China, Hefei 230026, PR China. * Corresponding Authors. E-mails: [email protected] ABSTRACT: Homojunctions comprised of transition metal dichalcogenides (TMD) polymorphs are attractive building blocks for next-generation two-dimensional (2D) electronic circuitry. However, the synthesis of such homojunctions, which usually involves elaborate manipulation at nanoscale, still remains a great challenge. Herein, we demonstrated a solution-processing strategy to successfully harvest lateral semiconductor-metal homojunctions with high yield. Specially, through precisely controlled lithiation process, precursors of polymorphic crystal arranged with 1T-2H domains were successfully achieved. A programmed exfoliation procedure was further employed to orderly laminate each phase in the polymorphic crystal, thus leading to 1T-2H TMD homojunction monolayers with size up to tens of micrometers. Moreover, the atomically sharp boundaries and superior band alignment improved the device based on the semiconductor-metal homojunction with 50% decrease of electric field strength required in the derivation of state transition. We anticipate that solution processing based on programmed exfoliation would be a powerful tool to produce new configurations of 2D nanomaterials.

INTRODUCTION Layered transition metal dichalcogenides (TMD) nanomaterials with abundant electronic and structure properties have witnessed an explosive development for various applications in the past decade.1-14 As is rapidly emerging recently, TMD homojunctions, which combine different phases and properties within an isoelectronic monolayer, have drawn increasing interest and demonstrated vast potential in two-dimensional (2D) electronic and optoelectronic applications.7-13 In fact, atomically thin junctions, including p-n junctions, metal–semiconductor contacts and metal–insulator barriers have been proved excellent candidates for the design of devices with great performance and high efficiency.13-16 TaX2 (X=S, Se) is a canonical member of TMD family, with variety of polytypic phases originating from the distinct in-plane coordination. Among them, strongly-correlated 1T and 2H polytypes are attracting major attention in that they constitute ideal case studies for the investigation of competing orders, including superconductivity, charge density wave (CDW) and hidden phase.17,18 Therefore, junctions and interfaces in these systems can be a topic of cutting-edge research and further demonstrate novel electronic behaviors arising from many-body effect.19,20 However, feasible obtainment of such lateral homojunctions still remains elusive, especially through solution processing route. Generally, the synthesis of TMD homojunctions strongly requires controlled production of distinct phases arranging in the same atomically thin monolayer, which usually involves elaborate manipulation at nanoscale.14-16 To date, driven by post chemical or laser treatment, local phase conversion offered to fabricate homojunctions with desired geometry with the help of lithography patterning.10-12,21 Moreover, bottom-up growth of different phases by virtue of heteroepitaxy chemical vapor deposition has been developed to produce such structures with large lateral size.13,22 As a representative in top-

down method, solution-processing exfoliation has always been universally attractive for the synthesis of novel 2D materials,23-28 owing to its advantages in low cost, massive production and flexible substrate-independence. However, it still remains an open question whether solution processing can be extended to synthesize single-layer 2D TMD homojunctions. Herein, we highlight a solution-processing exfoliation strategy to harvest single-layer semiconductor-metal TaX2 (X=S, Se) homojunctions with high yield and large lateral size, representing the first top-down route toward large-sized TMD homojunctions. Through precise control of lithiation procedure, unique polymorphic crystals arranged with largesized 1T and 2H domains are successfully achieved. By taking advantage of the inherent difference of each individual ingredient phase, a programmed delithiation process is designed to implement gigantic expansion of each phase respectively in the polymorphic crystal, which efficiently yields high-quality TaX2 homojunction monolayers with size up to tens of micrometers. The atomically sharp boundaries of the semiconductor-metal TaS2 homojunction give rise to superior band alignment, leading to significantly enhanced device performance with more than 50% off electric field strength to drive the charge density wave transition. We believe that the programmed exfoliation sheds light on promising feasibility for the massive production of large-sized 2D homojunctions through solution processing exfoliation.

RESULTS AND DISCUSSION Solution processing produciton of 2D homojunctions. Solution-processing 2D semiconductor-metal homojunctions were achieved through programmatically exfoliating polymorphic crystals which was induced via elaborately controlled lithiation (Figure 1). Shown in Figure 1b and c, our solution processed TaS2 and TaSe2 1T-2H homojunctions exhibit extremely large lateral size and high homogeneity for each phase.

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As a typical member of top-down exfoliation realm, chemical

Figure 1. (a) Lateral 1T-2H TaX2 (X=S, Se) homojunction with atomically sharp interface. Optical images for solution processed (b) TaS2 and (c) TaSe2 homojunctions. (d) Phase engineering for 1T (octahedral) and 2H (trigonal) phases in 1T-TaS2 with the control of lithiation. (e) Solution processing for lateral TMD homojunction via programmed exfoliation of each phase with different solvents in a polymorphic crystal generated by controlled intercalation of lithium.

exfoliation has been well-known in high efficiency of monolayer yielding,29-32 with the key contribution from intercalation. Interestingly, besides prompting sufficient expansion of the interlayers, the intercalated lithium is also able to trigger phase conversions in most TMDs via changing their coordination configuration driven by charge transfer,11,31,32 entailing to phase engineering in 2D TMD material. Indeed, previous works reported that MoS2 can be transformed from semiconducting phase to metastable metallic one by electron donation from organolithium intercalation;31-33 while such metastable phase can be easily destroyed by annealing,34 laser or microwave irradiation.35,36 As a classic strongly correlated system, TaS2 also owns typical polymorph properties with various intriguing many-body effects.37-40 But unlike that for MoS2, both 1T and 2H TaS2 are stable at room temperature, which are supposed to bring much potential in theoretic as well as real-life applications.40,41 At the same time, due to slight lattice distortion after lithiation in TaS2, intact in-plane structure can survive without introducing obvious lattice strains.27 Thus, very large-sized TaS2 homojunctions with coplanar 1T-2H domains can be feasibly generated via phase conversion from the semiconducting 1T to metallic 2H by charge donation. By precisely manipulating the content of intercalated lithium, the phase of TaS2 can be readily conversed between 1T and 2H (see details in Figure S1). As demonstrated in

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Figure 1d, TaS2 maintains its parent octahedral coordination (1T) when intercalated with low Li content, whereas trigonal prismatic coordination (2H) emerges with enhanced lithium content, accompanying with a conversion from semiconducting 1T structure to metallic 2H phase.39 Therefore, through controlling lithium content at intermediate level, we can successfully realize polymorphic crystals arranged with 1T and 2H phases. The polymorphic bulk with separated domains provides great opportunity for massive production of homojunction by top-down exfoliation strategy. Here, we proposed the periphery to center path way in lithiation process as reported in TMDs.42,43 After controlled lithiation, most of TaS2 single crystals are comprised of low-lithium 1T region surrounded with high-lithium 2H region (Figure S2 and Figure S3). The unique polymorphic crystal can thus be exfoliated into homojunction monolayers with half 2H and half 1T phase. As shown in Figure 1e, programmed exfoliation was introduced to realize such lamination, and obtained massive 1T-2H TaS2 homojunctions with large size. Concretely, two steps were performed in the programmed exfoliation. For the first step, H2O was chosen to exfoliate the 1T part of the polymorphic crystal, while maintaining the highly lithiated 2H region intact due to the slightly soluble property of LiOH. After the removal of residual H2O, acids such as HCl, H2SO4 etc. were further utilized to exfoliate the 2H domains which were fulfilled with lithium atoms. Considering the inherent difference in lithium content of these two phases, different solvents were selected to conduct delithiation process to introduce great lattice expansion, which played crucial role in exfoliating very large-sized

Figure 2. XRD patterns and the enlargement of (001) peaks of 1T-TaS2 crystal and polymorphic 1T-2H LixTaS2 crystal.

nanomaterials with mild forces.27,44 As seen in Figure 2, the Xray diffraction (XRD) pattern of TaS2 crystals confirmed the transition from single crystal to polymorphic crystal after controlled lithiation. Before lithiation, 1T TaS2 single crystal exhibits high crystallinity and the observed peaks can be indexed into (00l) orientation. However, the peaks split into two sets after controlled insertion of lithium, which corresponds to the two phases of TaS2 respectively. Moreover, in addition to the shifts toward lower angles, the polymorphic crystal possesses the retention of highly crystalline characteristics, which would greatly benefit for the high quality and large size of the homojunctions. Thus, via deliberately arranging 1T and 2H domains into one matrix,


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Journal of the American Chemical Society very large-sized TMD semiconductor-metal homojunctions from solution-processing method could be realized.

2H TaS2 as lithium content increased (Figure S5), revealing that the content of 2H phase is feasibly controllable via regulating the Li content in our controlled lithiation process.

Figure 3. (a) Low-magnification optical microscope image of the exfoliated TaS2 homojunctions deposited on Si/SiO2 substrate, and optical image (inset) of the exfoliation homojunction dispersions. (b), (d), (e) Optical image and Raman mapping images of 1T-2H TaS2 homojunction with green and blue color corresponding to the intensity distributions of 1T E1g and 2H A1g peaks. (c), Raman spectra of 1T and 2H domains with peak positions denoted for Raman mapping analysis. (f)–(h), Optical and AFM images of an individual 1T-2H TaS2 monolayer with corresponding height profile.

Characterization of the solution-processing homojunctions. In our solution processing method, polymorphic TaS2 crystal was programmatically exfoliated into TaS2 homojunctions with very large lateral size and high yield of monolayers. As seen in Figure 3a, high-coverage ultrathin TaS2 homojunctions with very large size up to tens of micrometers were yielded. Due to their pristine difference in optical and electronic properties, the domains of 1T and 2H can be easily distinguished under optical microscopy. Moreover, the evolution in X-ray photoelectron spectroscopy and ultraviolet−visible (UV-Vis) spectra confirmed the phase transformation from 1T to 2H phase in TaS2 after adequate lithium insertion. According to Figure S4, with the increasing inserted lithium content, Ta 4f and S 2p peaks in corresponding TaS2 monolayers shifted to lower binding energy region, indicating the slight reduction of the material, which suggested the charge transfer from lithium atoms to TaS2 matrix. Similarly, the peak in UV-Vis spectra for nanosheet suspension gradually shifted toward shorter wavelength and got close to characteristic peak (~400 nm) of

In order to study the structure characterization of the lateral semiconductor-metal homojunction, the polymorphism of 1T and 2H TaS2 were characterized using Raman spectra and mapping. The slight blue region exhibited in-plane phonon modes at 190 cm-1 (𝐸1𝑔) and 303 cm−1 ( 𝐸12𝑔), and out-of-plane mode at 394 cm-1 (𝐴1𝑔), which are consistent with 2H TaS2 monolayers,44 whereas the slight violet region displayed the characteristic Raman peaks of the in-plane 𝐸1𝑔 and 𝐸12𝑔 mode in the vicinity of 240 and 303 cm−1 respectively, and the outof-plane 𝐴1𝑔 mode at 374 cm-1, corresponding well with pristine 1T phase (Figure 3c).27 Furthermore, Raman mapping signals of 1T (𝐸1𝑔) and 2H (𝐴1𝑔) in Figure 3b and 3d-e indicated the separated and homogeneous properties of the two phases. Atomic force microscopy (AFM) images for an individual 1T-2H homojunction were applied to investigate the height morphology of the homojunction monolayer in Figure 3f. As a result, the height profile (inset Figure 3g) showed extraordinary homogeneity of 0.85 nm across the whole nanosheet, which corresponds well with the thickness



of TaS2 monolayer. Additionally, it also directly substantiated the thickness consistency of the two domains; while the phase signals (Figure 3h), which usually relate to structure information, represented distinct difference between 1T and 2H domains. Moreover, we also found that this solution processing method toward lateral 1T-2H homojunction could be extended to other TMD system. Figure S6 exhibited the

be directly identified according to the profile of the two distinct scattering signals in Figure 4c, demonstrating clearly interface between 1T and 2H domains. Furthermore, highangle annular dark-field (HAADF) image with atomic resolution was employed to directly disclose the interface between two phases. The results are displayed in Figure 4d, superimposed with schematic trigonal and octahedral lattices

Figure 4. (a) SEM image of the exfoliated TaS2 homojunctions with distinct contrast of 1T and 2H domains. (b) Image of the nearfield scattering amplitude for the exfoliated TaS2 homojunction with sharp phase boundaries. (c) Line profile along the black dashed line in b. (d) HADDF image of the homojunction with interfaces outlined by a white dashed line and superimposed 1T and 2H lattice schematics. (e) Line profiles along the green and red lines denoted in d.

optical images of large-sized TaSe2 1T-2H homojunctions of different thickness obtained from solution processing method. The 1T and 2H structures of the separated domains were further confirmed by Raman spectra in Figure S7. Therefore, these observations confirmed that programmed exfoliation successfully produced lateral high-quality 1T-2H homojunctions with homogeneous 1T and 2H domains. Junctions formed at the interface of metal and semiconductor usually play crucial role in electronic and optoelectronic devices. Thus, we further conducted the investigation of the interfaces of the homojunctions from our top-down exfoliation method. Scanning electron microscopy (SEM) image of single-layered homojunction was displayed in Figure 4a, showing the coexistence of 1T (bright contrast) and 2H (dark contrast) phases in one nanosheet. Optical behavior differences nearby the phase boundary were also gained from scattering scanning near-field optical microscope (s-SNOM) with spatial resolution of about 10 nm (see Figure S8 for details), which enabled us to easily define the interface of the two domains according to their optical contrast. As displayed in Figure 4b, the representative near-field amplitude mapping of exfoliated homojunctions as a function of the position indicates that metallic 2H endows larger scattering amplitude than the insulating 1T counterpart. Of note, a sharp change can

on the corresponding 1T and 2H domains respectively. It reveals that the 1T and 2H phases of TaS2 can coexist in the same monolayer panel and that the interface of the two phases is atomically sharp as indicated by a white dashed line. Arising from the huge Z difference between Ta (73) and S (16), the line profiles for the two phases are required to analyze atom arrangement around interfaces of 1T-2H homojunctions. In Figure 4e, the profiles of the HAADF signal corresponds well with the atom arrangements of Ta-SS-Ta in 1T phase (the green profile) and Ta-S-Ta in 2H (the red profile) respectively.11 Thus, the solution processing of programmed exfoliation was proved to be an effective way to produce 1T-2H homojunctions with atomically sharp interfaces, which is significantly important for device applications. Device performance based on semiconductor-metal homojunctions. Given the fact that 1T TaS2 is a typical correlated system with first-order CDW phase transitions,45,46 the homojunction provides an opportunity in memristor application via electrically controlling resistive states, which is promising in neuromorphic circuits, ultra-dense information storage, and other applications.41,47 In our case, we investigated the electrically driven phase transition in a 2H1T-2H configurational electrical device at room temperature


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Journal of the American Chemical Society (see details in Method).40,48,49 As is shown in Figure 5a, two types of electrode contact in each device were tested: one with the Au metal pads contacted to the metallic 2H phase, and the other with Au pads contacted on the 1T phase directly. In both cases, the channel consists of semiconducting 1T phase. Two devices of

Figure 5d (see details in Figure S10). Obviously, metallic 2H region acts as a lateral connection between Au and 1T region where large barrier exists to increase carrier transport, which is experimentally observed in the transport properties. The decreased barrier in 2H-1T-2H homojunction helps to reduce the electric field strength that is necessary for triggering the CDW transition.49 Furthermore, considering that the interface between 1T and 2H phase is atomically sharp, lower interface trap density in 1T-2H interface can be generated, and thus results in lower electrical field for the phase transition.

CONCLUSION

Figure 5. (a) I-V curves and (b) electrical driven phase transition curves for devices based on exfoliated TaS2 homojunctions with Au pad deposited on 2H (solid lines) and 1T (dashed lines) phases. (c) KPFM image of exfoliated homojunction with Au pad deposited on 2H and 1T phases. (d) Band diagram for Au pad directly deposited on 1T (green dashed lines) and 2H (black solid lines) phase.

TaS2 junctions with thickness of 8.2 nm (device 1, the green line) and 6.5 nm (device 2, the red line) were tested as displayed in Figure 5a and b. The linear I-V characteristics in Figure 5a indicate the ohmic-like behavior of both two types contact, whereas a relatively larger current was observed in the 2H contact devices (solid lines) comparing to the 1T contact ones (dashed lines) in both two devices. The CDW phase transition at room temperature can be effectively driven by applying an electrical field as displayed in Figure 5b. Interestingly, it can be clearly observed that homojunctions with 2H-contact have halved the electric field to trigger the phase transition when compared to the 1T-contact. Therefore, by decreasing the critical electrical field needed for resistive states control, 2H-1T-2H TaS2 homojunctions from our programmed exfoliation greatly advanced its potential in memristor application. In order to investigate the contact behavior of the homojunctions and Au electrodes, Kelvin Probe Force Microscopy (KPFM) was conducted to study the surface potential of the devices. Figure 5c shows a KPFM image of a part of 1T-2H TaS2 homojunction with Au electrodes deposited on 1T and 2H phases respectively. The corresponding optical image and topography are exhibited in Figure S9. As the result shown in Figure 5c, the surface potential between TaS2 and Au is easily discernable, while the 1T and 2H parts share a similar value. According to the relationship of surface potential and work function,50,51 a proposed band diagram for Au pad directly deposited on 1T phase (dashed line) and 2H phase (solid line) are displayed in

In conclusion, we highlight a brand-new solution process, in which polymorphic crystals are programmatically exfoliated via a precisely controlled delithiation process, successfully harvesting 2D semiconductor-metal homojunctions with very large sizes and high yield. Our 1T-2H TaX2 (X=S, Se) homojunctions represent the first case of TMD junctions beyond group VI (Mo, W). Owing to the atomically sharp interface and superior band alignment, TaS2 semiconductormetal homojunctions show more than 50% off electric field strength for the derivation of a CDW transition. 2D semiconductor-metal homojunctions from solution processing route would inevitably bring synergic advantages of facile and green synthesis procedure as well as massive production, which would provide a robust pathway to exploring the full potential of 2D TMD materials and their vast applications in large-area electronics.

EXPERIMENTAL SECTION Preparation of polymorphic LixTaS2 crystals. 1T TaS2 single crystals with size up to 5 mm, were synthesized by chemical vapor transport followed by water quenching. The crystals mixed with 1.6 M n-BuLi in hexane were then sealed in a quartz-lined autoclave and kept at 120 °C for 0.5-2 hours. In order to prevent n-BuLi from being oxidized, the whole processes were then put into argon-filled atmosphere. It can be observed that the lithium intercalated products is well separated from solvent after reaction as well as maintaining its high crystallinity. The LixMX2 products were then quickly washed by n-hexane for several times and dried with Ar gas, and the excess of n-BuLi was consumed by ethyl alcohol. Programmed exfoliation of LixMX2 single crystals. Distilled water was first added to the LixMX2 crystals, after which no obvious expansion can be observed. Then, dilute acid solutions, such as HCl or H2SO4 with H+ of concentration at 0.05 M were added to react with the dense lithium, during which apparent lattice expansion can be easily obtained. After removal of the acid, distilled water was again added to achieve exfoliation just by slightly manual shaking, and the mixture turned into a uniform dispersion. Then, the dispersion was centrifuged at 1,000 r.p.m. for 10 min to remove very thick nanosheets and at 8,000 r.p.m. for 10 min to remove excess impurity. The sediment was finally re-dispersed into aqueous solution for other applications. Sample characterizations. The optical images were obtained on Olympus BX51M. Raman spectra and mapping were recorded at room temperature with a Renishaw Raman System, of which the excitation wavelength is 532nm. The sSNOM near-field images were performed on NeaSNOM (Neaspec GmbH Co.). AFM was conducted by AFM (Bruker,



Demension Icon) using tapping mode. TEM and HAADF were obtained on a JEM 2100F transmission electron microscopes (200 kV, field-emission gun) equipped with an Oxford INCA x-sight EDS Si (Li) detector. The SEM images were taken on a JEOL JSM-6700F SEM. Kelvin Probe Force Microscopy (KPFM) measurements were performed using AFM (Bruker, Demension Icon) with a Pt-Ir coated cantilever. XPS were carried out on an ESCALAB MKII with Mg Kα (hν = 1253.6 eV) as the excitation source, where the binding energies were corrected against the specimen charging by referencing C 1s to 284.6 eV. The ultraviolet−visible (UV−vis) spectra were measured on a PerkinElmer Lambda 950 UV−vis−NIR spectrophotometer. Device fabrication and characterization. UV lithography (Optical Aligner-SUSS MABA6) followed by e-beam evaporator (AdNaNotek, EBS-150) of gold (100 nm) was employed to create the source/drain electrodes for electrical measurements. Electric field driven phase transitions were tested on Keithly 4200 at room temperature. Current-voltage curves were measured at the scanning rate of 10 mV/s.

ASSOCIATED CONTENT Supporting Information Detailed experimental synthesis procedures and supplementary characterizations including statistics for 1T and 2H phase fractions of TaS2 nanosheets with various content of lithium, phase distribution in the homojunctions, XPS and UV-Vis spectra results, schematic of the scanning near-field optical microscope, morphology of the KPFM tested sample and comparison of contact potential difference are revealed in Supporting Information.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions §These authors contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We are grateful to Nan Pan and Huaiyi Ding for help with device data analysis. This work was financially supported by the National Basic Research Program of China (2015CB932302), the National Natural Science Foundation of China (21890751, 91745113), National Program for Support of Top-notch Young Professionals, the Fundamental Research Funds for the Central Universities (No. WK 2060190084), the Major Program of Development Foundation of Hefei Center for Physical Science and Technology (Grant No. 2016FXZY001). We appreciate the support from the USTC Center for Micro and Nanoscale Research and Fabrication.

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Solution Processing for Lateral Transition Metal Dichalcogenides

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