Persistent photoconductivity in two-dimensional Mo1 xWxSe2–MoSe2 van der Waals heterojunctions À Xufan Li,b) Ming-Wei Lin,b) and Alexander A. Puretzky Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Leonardo Basile Departamento de Física, Escuela Politécnica Nacional, Quito, 17012759, Ecuador Kai Wang, Juan C. Idrobo, Christopher M. Rouleau, David B. Geohegan, and Kai Xiaoa) Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA (Received 22 September 2015; accepted 4 January 2016) Van der Waals (vdW) heterojunctions consisting of vertically-stacked individual or multiple layers of two-dimensional layered semiconductors, especially the transition metal dichalcogenides (TMDs), show novel optoelectronic functionalities due to the sensitivity of their electronic and optical properties to strong quantum confinement and interfacial interactions. Here, monolayers of n-type MoSe2 and p-type Mo1 xWxSe2 are grown by vapor transport methods, then transferred and stamped to form artificialÀ vdW heterostructures with strong interlayer coupling as proven in photoluminescence and low-frequency Raman spectroscopy measurements. Remarkably, the heterojunctions exhibit an unprecedented photoconductivity effect that persists at room temperature for several days. This persistent photoconductivity is shown to be tunable by applying a gate bias that equilibrates the charge distribution. These measurements indicate that such ultrathin vdW heterojunctions can function as rewritable optoelectronic switches or memory elements under time-dependent photo-illumination, an effect which appears promising for new monolayer TMDs-based optoelectronic devices applications. I. INTRODUCTION less attention, but has obvious potential for rewritable 15 Two-dimensional (2D) layered semiconductors, espe- photoresponsive switches and memory elements. PPC has been primarily observed in compound cially transition metal dichalcogenides (TMDs), have 16–18 emerged as exciting and versatile materials when their semiconductors and layered structures. However, thickness is reduced to a monolayer or few-layers due to 2D materials have suitable properties for PPC that fi include strong light-matterinteraction,highcarrier the emergence of quantum con nement effects and strong 15 interfacial interactions.1,2 Stacking different 2D semi- mobility, and gate tunability. While graphene has conductors into van der Waals (vdW) heterojunctions low photoresponsivity due to its intrinsic metallic creates a diverse palette of new, artificially-structured, property, hybrid graphene structures utilizing quantum dots or MoS2 atomic layers exhibit interesting PPC layered materials with tunable optoelectronic properties 15,19 depending on the stacking order, relative orientation properties. TMDs, especially MoS2 and MoSe2, angle, and atomic registry between the layers. Recently, are highly photoresponsive materials with strong opti- p–n vdW heterojunctions have been constructed by either cal absorption and band gaps in the visible spectrum. stacking or epitaxially growing different layered materi- vdW heterostructures constructed from these and other als on top of one another or growing them laterally, and TMDs are good candidates for photoresponsive hybrid have demonstrated desirable optoelectronic functional- materials with charge exchange across the atomically ities such as photodetectors, photovoltaics, light-emitting sharp interfaces governed by quantum tunneling diodes, and photodiodes.3–11 Persistent photoconductiv- transport. ity (PPC), an interesting optoelectronic effect where Doping or alloying have proven very effective ways to tune the optical and electrical properties in TMD mono- enhanced electrical conductivity persists after the 20,21 removal of light illumination,12–14 has so far received layers. Recently, we found that isoelectronic sub- stitution of Mo by W in the monolayer MoSe2 lattice can switch it from n-type to p-type behavior.22 The resulting Contributing Editor: Joshua Robinson a) alloy, i.e., Mo1 xWxSe2, has almost the same lattice Address all correspondence to this author. À e-mail: [email protected] constant as intrinsic MoSe2, but exhibits a tunable band b)These authors contribute equally to this work. gap with doping concentration, providing a tunable DOI: 10.1557/jmr.2016.35 candidate for the construction of vdW heterojunctions. J. Mater. Res., Vol. 31, No. 7, Apr 14, 2016 Ó Materials Research Society 2016 923 X. Li et al.: Persistent photoconductivity in two-dimensional Mo1 xWxSe2–MoSe2 van der Waals heterojunctions À In this study, vdW heterojunctions are fabricated by substrate with the monolayer Mo1 xWxSe2. The PMMA transferring and stacking monolayer single-crystals of was removed by acetone and bakingÀ at 300 °C in ;30 MoSe2 and Mo1 xWxSe2 that were grown by vapor Torr Ar/H2 (95%/5%) flowing for 2 h. transport methods.À The excellent heterojunction quality is assessed by aberration-corrected transmission electron C. Device fabrication microscopy (TEM), and strong interlayer coupling Electron beam lithography (FEI DB-FIB with Raith fi veri ed by low-frequency (LF) Raman spectroscopy pattern writing software; FEI Company, Hillsboro, Ore- and photoluminescence (PL). Electrical measurements gon) was used for monolayer MoSe2–Mo1 xWxSe2 are used to characterize the p–n junction formed by the heterojunction device fabrication. A layer ofÀ PMMA stacked MoSe2 and Mo1 xWxSe2, which are also found 495A4 was spun-coat on the SiO (250 nm)/Si substrate to exhibit extremely highÀ photoresponsivity and giant 2 with monolayer MoSe2–Mo1 xWxSe2 heterojunctions, PPC at room temperature. Under time-dependent photo- followed by a 180 °C annealing.À After pattern writing, illumination, these heterojunctions can function as rewrit- development, and lift off, a 10 nm layer of Ti followed by able optoelectronic switches or memory elements. a 50 nm layer of Au was deposited using electron beam evaporation. II. EXPERIMENTAL D. Characterizations A. Material growth The morphologies of the monolayer MoSe2 and Crystalline monolayers of MoSe2 and Mo1 xWxSe2 À Mo W Se crystals were characterized using optical were synthesized using a low pressure chemical vapor 1 x x 2 microscopyÀ (Leica DM4500 P, Wetzlar, Germany), deposition (CVD) approach that is similar to those scanning electron microscopy (SEM, Zeiss Merlin, described previously.23 The synthesis was conducted in Oberkochen, Germany), and atomic force microscopy a tube furnace CVD reactor equipped with a 2 in. quartz (AFM, Bruker Dimension Icon, Billerica, Massachu- tube. In a typical run, the growth substrates, i.e., Si wafer setts). The atomic structures of monolayer MoSe and with 250 nm SiO (SiO /Si) cleaned by acetone and 2 2 2 Mo W Se were investigated via annular dark field isopropanol, were placed face down above an alumina 1 x x 2 (ADF)À imaging using an aberration-corrected scanning crucible containing ;0.2 g of MoO powder (for the 3 transmission electron microscope (STEM) (ADF-STEM, growth of Mo1 xWxSe2, a mixture of MoO3 and WO3 À Nion UltraSTEM™ 100, Kirkland, Washington) opera- powder was used), which was then inserted into the ting at 100 kV, using a half-angle of the ADF detector center of the quartz tube. Another crucible containing that ranged from 86 to 200 mrad. ;1.2 g Se powder was located at the upstream side of the 3 Raman measurements were performed using a micro- tube. After evacuating the tube to ;5 10À Torr, flows Raman system (JobinYvon Horiba, T64000, Edison, of 40 sccm (standard cubic centimeter per minute) argon New Jersey) based on a triple spectrometer equipped and 4 sccm hydrogen gas were introduced into the tube, with three 1800 groves/mm gratings and a liquid nitrogen and the reaction was conducted at 780 °C (with a tem- cooled charge-coupled device (CCD) detector. The perature ramping rate of 30 °C/min) for 5 min at a reaction Raman spectra were acquired under a microscope in chamber pressure of 20 Torr. At 780 °C, the temperature backscattering configuration using 532 nm laser excita- at the location of Se powder was ;290 °C. After growth, tion (0.1 mW laser power). The excitation laser was the furnace was cooled naturally to room temperature. focused to a ;1 lm spot using a microscope objective (100x, numeric aperture, N/A 5 0.9). B. Monolayer crystal transfer and heterojunction PL measurements were conducted using a home-built fabrication micro-PL setup, which included an upright microscope For the heterojunction fabrication and TEM sample coupled to a spectrometer (Spectra Pro 2300i, Princeton preparation, poly(methyl methacrylate) (PMMA) was first Instruments, Acton, Massachusetts, f 5 0.3 m, 150 spun onto the monolayer crystals on the SiO2/Si substrate at grooves/mm grating) equipped with a CCD camera (Pixis 3500 rpm for 60 s. The PMMA-coated substrate was then 256BR, Princeton Instruments). The PL was collected floated on 1 M KOH solution that etched silica epilayer, through a 100x objective. leaving the PMMA film with the monolayer crystals The electrical properties and photoresponse of the floating on the solution surface. The film was transferred monolayer flakes and heterojunctions were measured in 6 to deionized water several times to remove residual KOH. vacuum (;10À Torr) under a probe station using For TEM samples, the washed film was captured on a Si a semiconductor analyzer (Keithley 4200, Keithley TEM grid covered by a 50 nm-thick amorphous SiN film Instruments, Cleveland, Ohio) and a laser driven white