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Spatial Control of Photoluminescence at Room Temperature by Ferroelectric Domains in Monolayer WS2/PZT Hybrid Structures Connie H. Li,† Kathleen M. McCreary,† and Berend T. Jonker* Materials Science and Technology Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States S Supporting Information *
ABSTRACT: Single-monolayer transition metal dichalcogenides exhibit exceptionally strong photoluminescence (PL), dominated by a combination of distinct neutral and charged exciton contributions. We show here that the surface charge associated with ferroelectric domains patterned into a lead zirconium titanate film with an atomic force microscope laterally controls the spatial distribution of neutral and charged exciton populations in an adjacent WS2 monolayer. This is manifested by the intensity and spectral composition of the PL measured in air at room temperature from the areas of WS2 over a ferroelectric domain with a polarization dipole pointed either out of the surface plane or into the surface plane. This approach enables spatial modulation of PL intensity and trion/ neutral exciton populations and fabrication of lateral quantum dot arrays in any geometry, with potential applications in nonvolatile optically addressable memory or optical quantum computation.
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substrates2 or are chemically treated.24,25 Photoluminescence (PL) typically consists of contributions from the free exciton and the charged exciton or trion, depending upon the environment and surface treatment.7 Because the the exciton and trion binding energies are very large (∼600 and ∼30 meV, respectively), these characteristic emission features persist at room temperature. Here, we show that the intensity and spectral composition of PL from single-monolayer WS2, measured in air at room temperature, can be spatially controlled by the polarization domains in an adjacent film of the ferroelectric material, lead zirconium titanate (PZT). These domains are written and read in the PZT using a conductive atomic force microscope (C-AFM),26 and they produce local surface charge and electric fields arising from the polarization dipole. Because the polarization domain wall width in a ferroelectric material can be as low as 1−10 nm,27 this approach enables spatial modulation of the PL intensity and trion/neutral exciton populations, with potential for nanoscale resolution. This offers spatial control over their corresponding interactions, which have recently led, for example, to the observation of new phenomena arising from trion coupling to localized pockets of a 2D electron gas in WS2.28
INTRODUCTION Single-monolayer transition metal dichalcogenides (TMDs) exhibit striking optical properties arising from the evolution of their band structure to direct gap from indirect gap, which is characteristic of two or more layers.1−4 The dielectric screening of single-monolayer TMDs is very low due to their twodimensional (2D) character relative to that of bulk material because the screening that would normally occur due to carriers in the bulk is largely absent. Thus, their properties are strongly influenced by their immediate environment and can be modified and controlled by variations in the local charge density due to adsorbates5−7 or electrostatic gating,8−10 generating keen interest in a wide variety of electronic and optical device applications.11−14 This has stimulated efforts to develop large-area synthesis of these single-monolayer materials using various chemical vapor deposition (CVD) processes4,15−17 to facilitate emerging device technologies or incorporation into van der Waals heterostructures.18 There is significant interest in using TMDs in conventional field-effect or tunneling transistors,11,16 as well as in ferroelectric memory and photodetector applications.19−22 Although typical substrates include SiO2/Si and sapphire, recent work has demonstrated the growth of MoS2 on periodically poled lithium niobate wafers, with growth occurring preferentially on only one of the two polarization domains, resulting in laterally templated growth.23 The absorption and luminescence of TMD monolayers are highly sensitive to their environment, and their quantum efficiency can vary significantly if they are placed on different © 2016 American Chemical Society
Received: October 11, 2016 Accepted: November 22, 2016 Published: December 4, 2016 1075
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Figure 1. (a) Image of the poled PZT surface obtained with an AFM operated in the EFM phase mode. The same AFM was used to write the polarization domains into the PZT in a checkerboard pattern, with the tip polarities shown. The total image size is 30 × 30 μm, and each poled square is 10 × 10 μm. The dashed lines are guides to the eye, and the bias voltages applied to the C-AFM tip are indicated. (b) Horizontal EFM line scan averaged left to right across the top two panels of the checkerboard. (c) Schematic cross section of the PZT film illustrating the orientation of the polarization domains and the corresponding surface charge.
Figure 2. (a) Raw PL spectra and (b) PL peak intensity map of these spectra obtained from the WS2 monolayer over a 30 × 30 μm area in the sample plane, acquired from a region of the PZT that was not poled by AFM. The PL intensity and spectral distribution are very uniform over this area.
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areas. Polarization domains were then written into the PZT film using a C-AFM.26 A tip voltage of ±10 V direct current (dc) was applied in the contact mode, and 10 × 10 μm square polarization domains were written in a checkerboard pattern. A positive tip voltage generates polarization dipoles pointing into the sample plane and a negative charge at the PZT surface, whereas a negative tip voltage generates polarization dipoles pointing out of the sample plane and a positive surface charge. The polarization domains were imaged with the same instrument in the electrostatic force microscopy (EFM) mode, and an EFM phase image of such an area is shown in Figure 1a. A horizontal EFM line scan averaged across the top two poled domains is shown in Figure 1b. There is strong
RESULTS AND DISCUSSION The samples were fabricated by mechanically transferring largearea monolayer WS2 grown by a CVD process29 onto a 100 nm thick lead zirconium titanate (Pb[Zr0.2Ti0.8]O3) film on a conducting n-type strontium titanate wafer. The transfer was performed using standard techniques that have been described elsewhere30 and in the Methods section. AFM images of the PZT film before and after transfer of the WS2 monolayer are shown in Figures S1 and S2 (Supporting Information), respectively. Before transfer, a metal marker grid pattern (Ti/Au) was deposited onto the PZT using either a shadow mask or photolithography technique to assist in locating specific poled 1076
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Figure 3. (a) Raw PL spectra (solid black line) and fits (dashed green line) using two Lorentzians centered at 2.01 eV (red line) and 1.99 eV (blue line), with the difference between the raw spectra and the fit shown by the purple residual line. (b) PL peak intensity map of the raw PL spectra obtained from the WS2 monolayer over a 30 × 30 μm2 area in the sample plane, acquired from a region of the PZT that was intentionally poled by AFM in the checkerboard pattern illustrated in Figure 1a. (c) Spatial map of the raw PL line width (FWHM) corresponding to the data from (b).
contrast between the 10 × 10 μm squares written with opposite AFM tip polarities, indicating successful poling of the PZT film. There is little contrast between the areas of PZT that were not poled by the AFM and the 10 × 10 μm squares intentionally poled with a +10 V tip bias, due to the global poling of the entire PZT film before addressing it with the AFM. Figure 1c schematically illustrates the orientation of the poled domains within the PZT film and the corresponding charge distributions. After mechanical transfer of the WS2 monolayer onto the poled PZT films, PL spectra were acquired in air at room temperature with 1 μm spatial resolution using a PL microscope equipped with both incident laser beam scanning mirrors and a stepper motor-controlled sample stage for rastering in the xy sample plane, permitting both single-spot and scanned-area spectral acquisition. Four separate transfers of monolayer WS 2 onto the PZT were performed, and representative data are shown. Discrete spectra were typically acquired at 1 μm steps in both x and y directions. A 488 nm laser was used, with an incident power on the sample of less than 5 μW to avoid spurious effects such as photoexcitation of free carriers and inadvertent trion formation.7 For reference, the raw PL spectra and corresponding peak intensity map acquired from WS2 over a 30 × 30 μm region of the PZT film that was not poled with the AFM are shown in Figure 2. Figure 2a shows individual spectra acquired from the positions
indicated in Figure 2b. These spectra are generally attributed to the ground-state exciton, referred to as the “A” exciton in the literature, and they can be fit with two Lorentzian components, which will be discussed later. The PL intensity map in Figure 2b is reasonably uniform, demonstrating that any inhomogeneity in either the PZT or the transferred WS2 has a modest effect on the intensity and little effect on the spectral distribution. Similar data acquired from the WS2 over a 30 × 30 μm area of the PZT that was intentionally poled with the AFM in the checkerboard pattern described above (Figure 1a) are shown in Figure 3. Figure 3a shows typical raw PL spectra (solid black line) obtained from the regions labeled Box 1 and Box 2 in Figure 3b, where the underlying FE polarization dipole is pointed down into the surface plane (negative PZT surface charge) or up out of the surface plane (positive PZT surface charge). The spectrum from Box 1 over the “down” polarization domain in the PZT shows a lower intensity and a markedly asymmetric line shape, peaked at ∼1.99 eV, with a distinct low-energy tail. In contrast, the spectrum from Box 2 over the “up” polarization domain shows a much higher intensity and a narrower, nearly symmetric line shape, peaked at ∼2.01 eV. Figure 3b shows a spatial map of the peak intensities of the raw PL spectra, with significantly higher intensities observed from Boxes 2 and 3 over the up polarization domains than those from Boxes 1 and 4 over the down polarization domains. Figure 3c shows the spatial variation of the raw PL 1077
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line width (full width at half-maximum (FWHM)), where the line width in Boxes 2 and 3 is markedly narrower than that in Boxes 1 and 4. It is clear that both PL intensity and spectral distribution are strongly modulated by the polarization domains in the underlying PZT film, with a significantly higher intensity and narrower line widths observed over the domains with the polarization dipole pointing out of the sample plane (positive charge at the PZT surface). The PL spectra can be quantitatively fit using two Lorentzian components, shown by the red and blue lines in Figure 3a, with the fit and residual shown by green dashed and purple solid lines, respectively. The fits are obtained by first allowing the Lorentzian peak energy, height, and FWHM to vary to determine the optimal peak energies, 1.99 and 2.01 eV, for all spectra, and then allowing only the Lorentzian height and FWHM to vary to minimize the residual for individual spectra. The PL spectra of Box 1 (WS2 over the down polarization domain with a negative PZT surface charge) are dominated by the lower-energy Lorentzian, whereas the spectra of Box 2 (WS2 over the up polarization domain with a positive PZT surface charge) are dominated by the higher-energy component. To understand the origin of these changes in intensity and spectral distribution, we begin by noting that recent work has shown that WS2 monolayers on SiO2/Si substrates can be purposefully prepared so that their PL is dominated by either the neutral exciton, X°, or the charged exciton, X−, consisting of an electron bound to the exciton and referred to as a trion.7 PL from the air-exposed surface at room temperature is dominated by emission from the neutral ground-state exciton near 2.0 eV, attributed to the presence of surface adsorbates which act as electron acceptors. When the sample is placed in vacuum and illuminated with a laser, the PL spectrum is dominated by the trion which appears at a lower energy, attributed to the laserinduced desorption of these surface adsorbates, which leave behind excess electrons to form trions. The relative intensities of these two components are determined from the available surface charge due to adsorbates or laser power, and their energy positions and separation depend further on the particular substrate, strain, and total electron density.30 We note that the down polarization domains in Boxes 1 and 4 result in negative charge accumulation at the PZT surface, and our spectra obtained from WS2 over these regions have a more significant contribution from the lower-energy PL component. On the basis of the spatial correlation of the negative surface charge with the lower-energy PL component, reduced intensity, and broader line width, we conclude that the PL from Boxes 1 and 4 is dominated by the trion contribution. In contrast, the up polarization domains in Boxes 2 and 3 result in positive surface charge accumulation at the PZT surface, and the spectra obtained from WS2 over these regions have a more significant contribution from the higher-energy component. On the basis of the spatial correlation of the positive surface charge with the higher-energy PL component, higher intensity, and narrower line width, we conclude that the PL from Boxes 2 and 3 is dominated by the neutral exciton. These results demonstrate that the trion to exciton population ratio can be laterally modulated at room temperature, with a spatial resolution determined by the polarization domains in the underlying ferroelectric layer.
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CONCLUSIONS
In summary, reduced dielectric screening due to the singlemonolayer habit makes single-monolayer TMDs attractive as an active transport channel for conventional field-effect transistor applications16 because the channel can be easily pinched off by small gate voltages, enabling ultralow-power operation in a proven and highly scalable architecture. In a similar manner, TMDs are attractive as the channel material in ferroelectric transistors19 because the positive/negative surface charge associated with the polarization domains effectively gates the channel conductivity. Because these domains exhibit remnant polarization, such hybrid structures are very promising for nonvolatile memory applications,20 with the additional advantage of large-area optical erasing21 due to the strong optical absorption of the direct-gap TMD monolayer: the photogenerated carriers effectively erase the ferroelectric domain structure. As we have shown here, the surface charge distribution in the ferroelectric layer also dramatically affects the optical emission properties in the adjacent TMD monolayer, enabling lateral spatial control of the luminescence intensity and spectral composition. The lateral resolution is determined by a combination of the ferroelectric domain wall width (∼10 nm, material dependent) and the exciton/carrier diffusion length in the TMD, which is known to be exceedingly short due to the sub-picosecond exciton formation times and picosecond radiative lifetimes. The ability to purposefully design and modulate adjacent populations of trions and neutral excitons may have implications for nonvolatile optically addressable memory,21 single-photon emitters,31 optical quantum computation, and fabrication of lateral quantum dot arrays in any geometry.
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METHODS WS2 Monolayer Growth. Monolayer WS2 on SiO2/Si was prepared by CVD in a 2 in. tube furnace. WO3 powder and sulfur precursors are heated to 825 °C under a 100 sccm argon and 10 sccm hydrogen flow. Perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt is used as seed molecules to promote lateral growth. This procedure has been recently shown to produce large-grain (>20 μm) uniform-coverage monolayer WS2 that exhibits enhanced PL.29 The monolayer nature of the film was confirmed by Raman and PL mapping. Transfer of WS2 to PZT. The films were removed from the Si/SiO2 growth substrate and transferred to the desired wafer. In the first step of the transfer process, poly(methyl methacrylate) (PMMA) resist (950 A4) is spun onto WS2 and cured on a hot plate at 100 °C for 10 min. The sample is then submerged in buffered hydrofluoric acid (HF) to etch the oxide layer, freeing the WS2 from the growth substrate. Once released, the film is moved to deionized water, where it floats on the surface. WS2 is then lifted from the water with the desired substrate. Adhesion is improved by spinning at 2000 rpm and baking at 100 °C. PMMA is then removed using acetone and isopropanol. Ferroelectric Domains. Polarization domains were written into the PZT film using C-AFM (Park NX-10) cantilevers with dc voltages of up to ±10 V. Two types of cantilevers were used: Cr−Pt-coated (Multi75E, Budget Sensors) and Au-coated (PPP-NCSTAu, Nanosensors) Si cantilevers. Similar results were obtained with both. The line-scan densities of at least 512 lines per 10 μm were used. Dynamic contact electrostatic force 1078
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microscopy was used to image the polarization state of the poled regions, at a frequency of 75−160 kHz.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.6b00302. Experimental details of WS2/PZT sample preparation, including AFM images of the PZT surface before and after transfer of WS2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Berend T. Jonker: 0000-0001-8816-7857 Author Contributions
† C.H.L. and K.M.M. contributed equally to this work. C.H.L. performed the C-AFM/EFM measurements to write and image polarization domains in PZT. K.M.M. grew the WS 2 monolayers, transferred them to PZT, and performed the PL measurements and data analysis along with C.H.L. B.T.J. conceived and coordinated the experiments and wrote the manuscript with input from all authors. All authors reviewed and approved the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by core programs at the Naval Research Laboratory and by the Air Force Office of Scientific Research under contract AOARD 14IOA018-134141. The authors gratefully acknowledge the NRL Nanoscience Institute for the use of their facilities.
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