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Probing Exciton Complexes and Charge Distribution in Inkslab-Like WSe Homojunction 2

Taishen Li, Mingling Li, Yue Lin, Hongbing Cai, Yiming Wu, Huaiyi Ding, Siwen Zhao, Nan Pan, and Xiaoping Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02060 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Probing Exciton Complexes and Charge Distribution in Inkslab-Like WSe2 Homojunction Taishen Li†, Mingling Li†, Yue Lin†, Hongbing Cai†, ,☨, Yiming Wu†, Huaiyi Ding†, ,☨, ‡

‡

Siwen Zhao†, Nan Pan†,‡,☨,*and Xiaoping Wang†,‡,☨,* †

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, ‡ University of Science and Technology of China, Hefei, Anhui 230026, P.R. China, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and

Technology of China, Hefei, Anhui 230026, P. R. China,☨Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences,School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China.

*

E-mail: [email protected] [email protected] ABSTRACT

By virtue of the layer-dependent band structure and valley-selected optical/electronic properties, atomically-layered transition-metal dichalcogenides (TMDs) exhibit great potentials such as in valleytronics and quantum devices, and have captured significant attentions. Precise control of the optical and electrical properties of TMDs is always the pursuing goal for real applications, and constructing advanced structures that allow playing with more degrees of freedom may hold the key. Here, we introduce a triangular inkslab-like WSe2 homojunction with a monolayer in the inner surrounded by a multilayer frame. Benefit from this interesting structure, the photoluminescence (PL) peaks redshift up to 50 meV and the charge density increases about 6 times from the centre to the edge region of the inner monolayer. We demonstrated that the Se-deficient multilayer frame offers the excessive free electrons for the generation of the electron density gradient inside the monolayer, which also results in the spatial variation and distribution gradient of a series of exciton complexes. Furthermore, we observed the strong rectifying characteristic and clear photovoltaic response across the homojunction through measuring and mapping the photocurrent of the devices. Our result provides another route for efficient modulation of the exciton-complex emissions of TMDs, which is exceptionally desirable for the “layer- and charge-engineered” photonic and optoelectronic devices.

KEYWORDS: homojunction, inkslab-like WSe2, excitons, trions, charged biexcitons Being the most representative prototype, two-dimensional (2D) transition metal dichalcogenides (TMDs), especially MX2 (M=Mo, W; X=S, Se, Te), have been placed at the centre of the stage. Not only because of their graphene-like laminar stacking but also their intriguing optical and electrical properties,1-3 the TMDs can provide an ideal platform to realize electronic and optical functionalities in ultrathin, flexible and transparent devices such astransistors4 and light-emitting


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diodes.5, 6 Unlike graphene which is gapless, TMDs possess natural bandgaps and exhibit a series of intriguing properties such as the strong photovoltaic responses,5, 7, 8 the fascinating valleytronic phenomena9-12 and the indirect-to-direct bandgap crossover.13-17 Moreover, due to the quantum confinement effect and the reduced dielectric charge screening, exploration and control of many-body effects in TMDs such as trions and biexcitons,18, 19 which are rarely accessible in bulk materials, has been long pursued. These quasi-particles of exciton species elementarily describe the electronic response to optical excitation in TMDs, and the deep understanding on which may shed light on many applications from solar cells to quantum logical devices.20 It is well known that, a junction consisting of two crystalline semiconductors with different bandgaps is the essential building block in electronic and optoelectronic devices.21 In recent, both lateral and vertical heterojunctions between two analogous monolayer TMDs with the different direct gaps have been realized.22-26 For example, Gong et al. reported a scalable single-step vapor phase method for the growth of vertically-stacked bilayer and in-plane interconnected WS2/MoS2 heterostructures.27 Additionally, Lee et al. demonstrated an van der Waals-bonded bilayer of MoS2/WSe2 junction, showing the photovoltaic response and strong rectifying characteristic.28 However, due to thermal and lattice mismatches, it is still an inherent challenge in fabricating such heterostructures for applications. If the junction could be realized in one homogeneous TMDs material, the problems of both mismatches could be naturally circumvented. Most recently, the electrical and photovoltaic properties of “demo” lateral homojunctions have been investigated through spatially-selective charge modulation (i.e., either doping or gate-tuning) of homogenous monolayer TMDs.29-31 Besides, since the bandgaps of TMDs are tunable with layer thickness,15, 16 the homojunctions constructed by the same type of TMDs, with two different layer thicknesses at the opposite sides, have becoming another promising and feasible choice. Unfortunately, although researchers have demonstrated the preparation and properties of this thickness-modulated TMDs’ homojunction,13, 32 no fascinating optical properties have been observed so far. In essence, playing with more controlling degrees of freedom is highly desirable for maximizing the performances of TMDs’ homojunction devices; if the charge and thickness/layer modulations could be effectively combined to the homojunction design, there should be a plenty of room for people to manipulate the optical and optoelectronic properties for these systems. In this work, we introduce a monolayer-multilayer WSe2 homojunction through the way of both layer engineering and charge doping effect. The physical vapor deposition (PVD) synthesized WSe2 homojunction samples possess a concentrically-triangular inkslab-like morphology with a monolayer in the inner surrounded by a multilayer frame. A series of fascinating optical and electrical properties of the inkslab-like samples, such as the redshift of PL emission, the efficient formation of exciton’s many-body complexes at room temperature, the strong rectifying character and the clear photovoltaic response, have been observed and attributed to charge doping and band alignment effects. This structure can offer an ideal platform to explore the rich phenomena of exciton-complex transitions and their potential photonic and optoelectronic applications.

RESULTS AND DISCUSSION The inkslab-like WSe2 samples were synthesized directly on SiO2/Si substrates by a modified PVD approach at normal pressure. In order to improve the growth controllability, we adopted a rapid heating-up approach: both WSe2 powder (upstream) and bare SiO2/Si wafer (downstream)


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were placed into a quartz tube with a proper distance, as long as the dual-temperature-zone tube furnace heats up in an ambiance of argon gas to the designed growth temperatures, specifically, 1100 °C for the source and 900 °C for the substrate, the quartz tube was pushed into the furnace centre immediately. The entire growth procedure and scheme are detailed in the Experimental Section and supplementary information (figure S1). Figure 1a shows the optical microscope (OM) photograph of the as-prepared inkslab-like WSe2 samples. Considering the very strange, uneven optical contrast, we employed atomic force microscopy (AFM) to characterize the topography of the individual WSe2 samples, as displayed in figure 1b. From the image, we can clearly define a triangular inkslab-like morphology of the WSe2 sample (with a thin basin in the centre surrounded by a thick closed frame as the barrier), so we name it the ‘inkslab-like’ WSe2. To pinpoint the thickness distribution of both the inner and the frame regions, figure 1c gives the line profile of AFM height along the red line marked in figure 1b. From the profile, the thickness of the inner triangular region is determined to be about 0.75 nm, revealing its monolayer nature; and the height of the frame is about 10 nm, which is over a dozen layers and corresponding to multilayer WSe2. To evaluate the crystal structure of the inkslab-like WSe2 samples, we performed the selected area electron diffraction (SAED) measurement in transmission electron microscope (TEM). Figure S2 shows the SAED patterns of the monolayer and the multilayer domains in a typical inkslab-like WSe2 sample. Through the well-defined patterns, we can find that the inkslab-like WSe2 possesses a consistent hexagonal lattice over the entire structure, corresponding to the 2h phase of WSe2; the crystallinity of the monolayer region is clearly better than the multilayer frame, indicating the existence of lattice distortions and defects in the latter. Owing to the capability of Raman for exploring grain boundaries, local defects and stresses in a variety of materials,33-37 we also employed µ-Raman to characterize the inkslab-like WSe2 sample. Figure 1d shows the Raman spectra from the different domains as pointed in figure 1a. The spectra reveal two features: (1) The E2g peaks from all of the different regions are centered at 249 cm-1 and display nearly no shift. Because the E2g mode is related to the in-plane shear vibration of tungsten and selenium atoms, which is sensitive to the change of stress and almost could not be influenced by charge doping and other factors,38 we can infer that there is no obvious stress in the inkslab-like WSe2 sample. (2) The E2g intensity of the monolayer region is much stronger than that of the multilayer frame. This is consistent with the previous report that the E2g vibration weakens as the number of layer increases, due to either the interlayer coupling effect39 or local defects40 in the multilayer frame. Figure 1e gives the E2g mode Raman intensity mapping of the entire sample corresponding to that in figure 1a. As seen, the intensity of E2g in the entire monolayer region is quite uniform and much stronger than that from the multilayer frame, indicating the homogeneity and high quality of the monolayer region. Due to the optical and electrical properties of TMDs can be modulated through thickness, stress, chemical doping, grain boundary and so on,7, 15, 18, 35, 41-44 it is natural to expect some interesting optical/electrical properties on the inkslab-like WSe2 sample considering its different architecture from others. Figure 2a plots the PL spectra obtained from a series of representative excitation spots on the sample (as pointed in the inset). The excitation laser is of 514 nm wavelength with a power of 0.1 mW. In the spectra, the PL emission from the inkslab-like WSe2 sample reveals a clear spatial nonuniformity. Firstly, the intensities and spectral positions of the PL peaks strongly depend on the locations of the inner monolayer region (the brown, red and green lines). Secondly, there is almost no PL emission from the multilayer frame (the orange line). Thirdly, the PL of the


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junction region (the blue line) has two maxima and its intensity is rather weak. Figure 2b gives the PL intensity map of the single inkslab-like sample. It also clearly manifests that the PL emissions mainly come from the inner monolayer region (the area between the two dashed purple triangles corresponds to the multilayer frame of the sample), because of the WSe2 bandgap transform from direct to indirect as the number of layer increasing from monolayer to multilayer. To reveal the nonuniform PL distribution of the monolayer region more clearly, figure 2c shows the PL line-scanning spectra from the centre to the edge of the monolayer (along the red arrow marked in figure 2b). The line-scan intensity clearly follows a first-increase-and-then-decrease tendency. What is more, a notable redshift about 50 meV (from 734 to 756 nm) can be clearly defined. In monolayer WS2, Kim et al. have found the PL enhancement and redshift near the edges and grain boundaries of the samples, they attributed the phenomena to the formation of biexcitons due to the large local population of charges favorable for the formation of exciton complexes.45 In this context, we speculate that the PL nonuniformity might originate from the natively nonuniform charge doping profile. To probe the charge carrier distribution, we employed Kelvin probe force microscopy (KPFM) to get the surface contact potential difference (CPD) of the different domains in the inkslab-like WSe2, where CPD=Φt-Φs, and Φt and Φs are the work functions of the tip and the WSe2 sample, respectively. In figure 2d, the colored dots and the black solid line represent the original and fitting data of the CPD along the yellow arrow marked in the 3D KPFM image (the inset), and the cyan solid line corresponds to the height profile along the same position. From the KPFM line profile, one can clearly divide the CPD of the inkslab-like WSe2 into three zones, the monolayer, homojunction, and multilayer frame zones, respectively; in particular, over the entire monolayer zone, CPD goes up gradually and monotonically about 40 meV from the centre to the edge. Being absent from thickness (layer) or stress effect on the bandgap, such a gradient of CPD within the monolayer could only result from the intrinsically-nonuniform distribution of charge carrier density. In other words, the increment of CPD from the centre to the edge of the monolayer clearly evidences the corresponding increase of the charge carrier density. Using ߟ = ݁‫ ݌ݔ‬ቀ∆஼௉஽ ቁ, ௞ ் ಳ

where η is relative enhancement of the charge density, ∆CPD is the difference value of CPD, we can estimate that the charge density increases about six times from the centre to the edge of the monolayer. Therefore, we can conclude that, it is the charge doping effect that results in the PL spatial nonuniformity in the inkslab-like WSe2 sample, which also implies the nonuniform spatial distributions of the different exciton complexes. To deconvolute the spectral components corresponding to the different exciton complexes, it is necessary to study the PL evolution from different positions as a function of excitation power, because the different exciton complexes follow the different power laws. For this purpose, we selected three representative positions: namely the centre, near the centre, and near the edge of the monolayer region of the inkslab-like WSe2 sample, and recorded their PL spectra under a series of different laser powers, as shown in figures 3a-c, respectively. We can find that the full width at half maximum (FWHM) of the PL near the edge broadens about twice comparing to that of the centre (being about 59, 85 and 113 meV in figures 3a, b, and c, respectively). It has been reported that the trions, biexcitons and biexciton’s excited states in TMDs can be efficiently modulated by the charge density,45, 46 however, the exact underlying mechanism is not yet very clear. Considering the simultaneous redshift and broadening of PL spectrum of the inkslab-like WSe2, we infer that the charge doping effect can efficiently promote the formation of trion and


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many-body complexes especially at higher excitation powers. With this consideration, we deconvoluted the PL spectra with three main components: the neutral exciton (A), trion/charged exciton (A-), and many-body complex (X), as detailed in figures S3 to S5.The deconvolutions are based on the PL spectra at maximum excitation powers for the three selected positions, and the results are shown in figures 3a-c with dashed lines, respectively. As seen, only two peaks, i.e., A and A-, are needed to fit the PL from the centre (figures 3a and S3). However, the PL near the centre has to be fitted by four peaks, being assigned to A, A-, X, and localized states (D), respectively (figures 3b and S4). In regard to the PL near the edge of the monolayer, three peaks, A-, X, and D, are required for the well-fitting. Note that D peak is probably related to defects, whose generation and monotonic- increase tendency indicates the emergence of defects from the centre to the edge of the monolayer region. This is also in good consistency with the results of SAED and Raman. In order to further verify the rationality of the deconvolution results, we plotted and fitted the integrated intensities of A, A- and X as the functions of excitation power by a power-law relation in the form of I~Pα, where I, P, and α represent the PL intensity, the excitation power, and the power exponent, respectively. As shown in figures 3d-f, two main features can be clearly found. Firstly, the A and A- represent a sublinear relationship between logarithmic PL intensity and laser power; however, on the contrary, the X displays a clear superlinear characteristic. Moreover, the power exponent for X (~1.26) is about twice as that for A (~0.67), well reflecting the fundamental feature of the many-body complexes.45, 47, 48 Secondly, the contributions from A, A- and X to the total PL emission intensity are clearly different along the radial direction of the monolayer region. Specifically, in the centre region, the intensity of A is comparable to that of A-, whereas no X component can be found. However, near the center region, the intensity of A- becomes dominant, and the signal of X superlinearly increases with the excitation power. As to near the edge region, the intensity of X becomes predominant and A component disappears completely. Considering the exciton-charge and exciton-trion interactions,49, 50 the above behavior can be understood by the charge distribution gradient in the inkslab-like WSe2. The conversion from an exciton to a trion, through the former bounding to a charge, can be dramatically enhanced at higher charge doping concentrations, resulting in the prevalence of A- at the positions away from the centre. Similarly, we infer that the X component could be the bound complex of exciton and trion (the charged biexciton), and its dominance certainly leads to the exhaustion of exciton (A) near the edge. Additionally, the binding energies for A- and X can be estimated to be about ∼30 and ∼51 meV, respectively, by inspecting the peak positions of the different exciton species in figure S6. The binding energy of 51 meV for X is larger than the previous calculation of 37 meV for WSe2 neutral biexciton,47 which also hints the formation of charged biexciton in the high charge doping regime for our case.51 Therefore, we believe that the spectral changes of the PL from the centre to the edge of the monolayer derive from the varied contributions and completely different radial distributions of the exciton, trion and many-body complex (most probably the charged biexcitons). In other words, the PL from the different positions corresponds to the emissions from the different kinds of exciton complexes, as shown in the schematic of figure 3g. Up to now, it is clear that the radial gradient of the charge doping causes the spectral change of the PL emissions through the formation of the different exciton complexes across the sample. In order to know whether this charge distribution arise from the chemical inhomogeneity within the inkslab-like samples, energy dispersive X-ray spectroscopy (EDX) was performed to analyze the


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elements’ distributions of the samples. Figures 4a and b show the W and Se EDX mappings of the inkslab-like WSe2, it is clear that both W and Se signals from the frame region are obviously stronger than those from the inner. For further semi-quantitative analysis and decreasing the experimental uncertainty, we selected nine pixel lines in the orange rectangles marked in figures 4a and b, the unprocessed intensity line profiles are shown in figures S7a-b, respectively. It is found that the element distribution between the inner monolayer region and the multilayer frame can be clearly distinguished. With this method, we can eventually obtain the averaged intensity profiles of W and Se, as shown in figure 4c. The average ratio of Se to W is about 2.12 (±0.17) for the monolayer and 1.77 (±0.12) for the frame, indicating that the multilayer and monolayer regions are Se-deficient and Se-rich domains, respectively. Considering that Se vacancy can introduce the native doping of WSe2, we conclude that: it is the Se-deficient multilayer frame that offers the excessive free electrons. These electrons can efficiently transfer to the monolayer region through the formation of homojunction and generate the in-plane electron density gradient throughout the monolayer, which results in the band alignment as schematically shown in figure 4d.13, 15, 45 Due to the unusually-expanded space charge region throughout the entire sample (figure 2d) and the favorable band alignment (figure 4d), this inkslab-like WSe2 sample must also possess good electrical properties. To verify this speculation, we used standard electron-beam lithography (EBL) process to make electrodes on the samples, and the diagram is shown in figure S8a. It is worth pointing out that, in order to eliminate the shunt effect from the highly-conductive frame (see figures S8a and c), we employed focused ion-beam etching (FIB) to cut off the bypass frame. The final device configuration is sketched in figure S8b. Figures 5a and b show the schematic and colored SEM image of a single inkslab-like WSe2 homojunction device, the I-V characteristics of which (figure 5c) clearly reveals a rectifying behavior of the junction. We also investigated the photocurrent response around the junction by scanning photocurrent microscopy (SPCM). Figure 5d plots the photocurrent mapping at zero bias. The photoresponse can only be clearly observed near the junction region (about 1.2 µm wide), indicating that the photocarriers are indeed separated efficiently by the built-in electric field (BEF) across the homojunction. Figures 5e and g (left) respectively show the photocurrent mapping and schematic band alignment of the junction under a reverse bias voltage of -1 V. As expected, since the total electric field is further strengthened at a reverse bias, it separates the photocarriers more efficiently, and the measured photocurrent enhances by about six times. However, in sharp contrast, under a +0.5 V forward bias, the BEF is cancelled out and the vector sum of the electric field eventually turns to the opposite direction (becomes the same direction as the forward bias), as sketched in figure 5g (right). While scanning within the junction region, due to the counteraction of the BEF against the forward bias, the total electric field is greatly weakened, resulting in the smallest photocurrent region in figure 5f. Whereas at positions away from the junction, the unimpaired applied field of the forward bias is free to drift the photocarriers, therefore, the photocurrent greatly improves and becomes notably larger than those around the junction. From this result, we can conclude that the observed rectifying behavior of the device exactly result from the junction between the mono- and multi-layer domains, rather than any of the Schottky junctions between WSe2 and electrodes. Besides the rectifying behavior, we also observed clear photovoltaic effect from the junction device, as shown in figure S8d (under a monochromatic excitation of 655 nm with a power density of about 3000 mW/cm2). The output power of the device gets a maximum value of 186 pW, with a


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short-circuit current of 4.6 nA, an open-circuit voltage of 151 mV, and a fill factor of 0.27. This result indicates that the inkslab-like WSe2 sample with the charge- and layer-engineered junction has the great potential for future applications, especially in energy-saving optoelectronic devices such as self-powered photodetectors and flexible electronic sensors. Finally, the intrinsic advantages of the inkslab-like WSe2 homostructure are prospected. From a fundamental viewpoint, probing and understanding the fundamental physics of quasi-particles’ many-body interactions and indirect excitons are among the most challenging goals in the related fields. To study the exciton’s many-body interactions in TMDs, a sample with an in-plane charge doping gradient can naturally provide the clear and panoramic view of the distribution/evolution gradient of different exciton species. As to the indirect exciton studies in TMDs, extending the lifetime is always an ultimate pursuing goal.52 In the proposed inkslab-like WSe2 homojunction, benefiting from the favorable band alignment structure, both real- and k-space separated indirect exciton probably could be formed near the junction, and the issue of interlayer coupling can be completely circumvented.27, 53 It might not only serve as an ideal “hotbed” for the real- and k-space separated indirect exciton, but also the k-space separation (decisive for the radiative recombination) could be readily tuned by applying a bias voltage to change the width of the depletion/accumulation region of the junction. From a practical point of view, in this inkslab-like WSe2 homojunction, besides the junction band alignment, the in-plane charge doping gradient in the entire monolayer region can offer another degree of freedom to manipulate the optoelectronic performance. Playing with more controlling degrees of freedom is certainly desirable to maximize the device performance. The studies on these topics are call for in the future.

CONCLUSION In summary, we have grown an inkslab-like WSe2 homojunction with a monolayer basin in the centre surrounded by a multilayer closed frame as the barrier. This inkslab-like structure exhibits substantially nonuniform PL emission (the peak position redshifts for 50 meV and the FWHM broadens about twice) and charge density gradient (increases by six times) from the centre to the edge of the inner monolayer region. It reveals that, the Se-deficient multilayer frame can generate the observed electron density gradient throughout the monolayer, and thus promote the formation of trion and many-body complex (most probably the charged biexcitons) at room temperature very efficiently. Moreover, this inkslab-like WSe2 homojunction possesses a strong rectifying behavior and a clear photovoltaic response. These results not only provide a platform to further explore the fundamental physics of many-body interactions in atomically-thin semiconductors, but also help to manipulate the electrical and optical properties of 2D TMDs devices with the more controlling degrees of freedom. We anticipate that the layer- and charge-engineering could prompt more applications for TMDs and various 2D materials, such as ultrafast optoelectronics and quantum devices.

METHODS Preparation of Inkslab-Like WSe2 Structure: During a typical growth, high-purity WSe2 powder (Alfa Aesar, 13084) of 0.2 g was added into a small quartz boat and served as the only evaporation precursor; whereas cleaned SiO2/Si wafers (1× 3 cm2 in size) were used as the growth


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substrates. Firstly, the small boat and the substrate were both placed into a long quartz tube (2 cm in diameter) with a horizontal separation of 15 cm with each other. Then, this long quartz tube was put into the horizontal tube furnace (with an inner diameter of 5 cm) in the manner that the substrate locates downstream of the boat (Figure S1). After this, the whole system (the chamber and pipes) was evacuated by a mechanical pump to a base vacuum of ~4.5 Pa, and then followed by the re-filling of ultrahigh-purity argon gas (∼99.999 %) with a constant flow of 120 sccm. Under the protective environment of this carrier gas, the two heating zones of the tube furnace were ramped to target temperatures of 1100 (for the precursor) and 900 °C (for the substrate), respectively; whereas the long quartz tube (including the precursor and the substrate) remained outside of the heating zones. Once the target temperatures reach, the long quartz tube was rapidly inserted into the centre of the furnace by a mechanical arm, with the precursor (substrate) exactly positioned at the 1100 (900) °C heating centre. From then on, the argon gas flow was turndown and kept at 90 sccm, the growth was typically lasted for 30 min, before it is finally stopped by shutting off the power of the furnace. The substrate was naturally cooled down to room temperature before the cutoff of the argon flow and system venting. Characterization of Inkslab-like WSe2 Structure: The surface topography and KPFM were both acquired by atomic force microscopy (Bruker, Demension Icon). The crystal structure (SAED) and composition (EDX) were determined by aberration-corrected (scanning) transmission electron microscopy (JEOL JEM-ARM 200F), for which the TEM samples were prepared using a standard PMMA-assisted transfer method.53 Raman and PL Measurements: A HORIBA LabRam HR800 system was used to record both the Raman and PL spectra of the inkslab-like WSe2 samples. The 514 nm laser was focused using a 100x objective lens, the spot size on the sample surface is down to ∼1 µm. This spot size is much smaller than the monolayer region of the inkslab-like WSe2, ensuring that the spatial mappings of Raman and PL are convincing. The laser power on the sample was controlled at 0.2 mW for the Raman excitation and varied from 1 to 1500 µW for the PL. All measurements were performed at room temperature. Fabrication and Optoelectronic Measurements of Single Inkslab-Like WSe2 Junction Devices: the homojunction device is fabricated by standard EBL (JEOL, JBX 6300FS) and FIB (FEI, Helios NanoLab650). The electrical and optoelectronic measurements were performed by using the attocube attodry 1000 test system and Keithley 4200A-SCS. The attocube system was used for the (1) optical excitation with an optical resolution of ∼700 nm for 635 nm laser excitation, (2) piezo-driven scan control of the sample for positioning and mapping, and (3) collection of the electrical signals by electric feedthrough. The Keithley 4200A-SCS was used for readout and analysis of the electrical and photocurrent signals. The laser power on the sample was controlled at 10 µW.

ASSOCIATED CONTENT Supporting Information. The supporting information mainly includes growth schematics of the inkslab-like WSe2 samples, the SAED patterns, the detailed results of PL deconvolution and fitting, Unprocessed Element Intensity Line Profiles of EDX, and the device fabrication and photovoltaic data of the inkslab-like WSe2 homojunction. The supporting information is available free of charge on the ACS publications website.


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AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] [email protected]

ACKNOWLEDGEMENTS We acknowledge the financial support from the MOST of China (2016YFA0200602), the National Natural Science Foundation of China (21421063, 11474260, 11374274, 11504364, 11504359), the Chinese Academy of Sciences (XDB01020200), the Fundamental Research Funds for the Central Universities (WK2030020027, WK2060190084, WK3510000004), and Natural Science Foundation of Anhui Province (1608085QA17). We also thank Anhui initiative in Quantum Information Technologies for its supporting. This work was partially carried out at the University of Science and Technology of China Center for Micro and Nanoscale Research and Fabrication.

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Figure 1. (a) Optical microscope image of an as-prepared inkslab-like WSe2 sample. (b) AFM topographic image of the same sample. (c) The height line profile along the red line marked in (b). (d) Raman spectra of the single inkslab-like WSe2 sample obtained from the different positions as pointed in the inset of (a). The spectra have been vertically shifted for clarity. The wavelength of the excitation laser is 514 nm. (e) Raman intensity mapping of E2g mode for the entire sample corresponding to (a). The red arrows in (a), (b) and (e) represent the forward direction of the same sample.


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Figure 2. The nonuniform PL and charge density distribution in the inkslab-like WSe2 sample. (a) PL spectra of a single inkslab-like WSe2 sample obtained from the different positions as pointed in the inset schematically. (b) PL intensity mapping of the same sample. The area between the two dashed triangles corresponds to the multilayer frame of the “inkslab”. (c) PL spectral mapping along the red arrow marked in (b). The excitation wavelength is 514 nm. (d) KPFM line scanning and AFM height profile along the arrow marked in the inset (the 3D KPFM image of the inkslab-like WSe2).

Figure 3. Properties of PL spectral components in the monolayer region of an inkslab-like WSe2 sample. (a)-(c): PL spectra under different excitation powers (solid lines) and the deconvolutions (dashed lines) of the PL at the maximum excitation powers from three selected positions, (a) at the centre, (b) near the centre, and (c) near the edge (about 2.5 µm away from the monolayer edge), respectively. The insets show the optical microscope images of the actual locations of the laser spot. (d-f) integrated intensities of the deconvoluted PL components of neutral exciton (A), trion (A-), and many-body complex (X) as the functions of laser power, corresponding to the positions in (a)-(c), respectively. The solid lines in (d)-(f) are the power law fittings and the slope values represent the power exponents. (g) Schematic of the spatial distributions and gradients of exciton complexes in the monolayer region and their evolutions under different excitation powers. The excitation wavelength of the laser is 514 nm.


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Figure 4. (a) and (b): EDX mappings of the inkslab-like WSe2 for (a) W and (b) Se, respectively. (c) The averaged element distribution line profiles of Se (red) and W (green) along the bars marked in (a) and (b). The average values of W (Se) are represented by green (red) numbers and lines. (d) Schematic diagram of the band alignment of the inkslab-like WSe2 homojunction.


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Figure 5. Electrical and photocurrent characterizations of the inkslab-like WSe2 homojunction. (a) and (b) are the schematic and colored SEM image of the WSe2 junction device, respectively. (c) I-V curves of the device at different gate voltages. (d)-(f): Scanning photocurrent microscopy (SPCM) mappings of the junction area, as marked in (b) by the red-dashed rectangle, under the bias voltages of 0 V (d), -1 V (e), and +0.5 V (f), respectively. The illumination laser is 655 nm in wavelength with the power of 10 µW. The white solid lines show the boundaries between the electrodes and the sample. The electrode connected to the multilayer is kept grounded. (g) Band alignment structures of the homojunction under reverse and forward bias, respectively.

TOC


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Figure 1. (a) Optical microscope image of an as-prepared inkslab-like WSe2 sample. (b) AFM topographic image of the same sample. (c) The height line profile along the red line marked in (b). (d) Raman spectra of the single inkslab-like WSe2 sample obtained from the different positions as pointed in the inset of (a). The spectra have been vertically shifted for clarity. The wavelength of the excitation laser is 514 nm. (e) Raman intensity mapping of E2g mode for the entire sample corresponding to (a). The red arrows in (a), (b) and (e) represent the forward direction of the same sample. 160x96mm (300 x 300 DPI)


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Figure 2. The nonuniform PL and charge density distribution in the inkslab-like WSe2 sample. (a) PL spectra of a single inkslab-like WSe2 sample obtained from the different positions as pointed in the inset schematically. (b) PL intensity mapping of the same sample. The area between the two dashed triangles corresponds to the multilayer frame of the “inkslab”. (c) PL spectral mapping along the red arrow marked in (b). The excitation wavelength is 514 nm. (d) KPFM line scanning and AFM height profile along the arrow marked in the inset (the 3D KPFM image of the inkslab-like WSe2). 160x121mm (300 x 300 DPI)


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

Figure 3. Properties of PL spectral components in the monolayer region of an inkslab-like WSe2 sample. (a)(c): PL spectra under different excitation powers (solid lines) and the deconvolutions (dashed lines) of the PL at the maximum excitation powers from three selected positions, (a) at the centre, (b) near the centre, and (c) near the edge (about 2.5 µm away from the monolayer edge), respectively. The insets show the optical microscope images of the actual locations of the laser spot. (d-f) integrated intensities of the deconvoluted PL components of neutral exciton (A), trion (A-), and many-body complex (X) as the functions of laser power, corresponding to the positions in (a)-(c), respectively. The solid lines in (d)-(f) are the power law fittings and the slope values represent the power exponents. (g) Schematic of the spatial distributions and gradients of exciton complexes in the monolayer region and their evolutions under different excitation powers. The excitation wavelength of the laser is 514 nm. 160x127mm (300 x 300 DPI)


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Figure 4. (a) and (b): EDX mappings of the inkslab-like WSe2 for (a) W and (b) Se, respectively. (c) The averaged element distribution line profiles of Se (red) and W (green) along the bars marked in (a) and (b). The average values of W (Se) are represented by green (red) numbers and lines. (d) Schematic diagram of the band alignment of the inkslab-like WSe2 homojunction. 160x136mm (300 x 300 DPI)


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Figure 5. Electrical and photocurrent characterizations of the inkslab-like WSe2 homojunction. (a) and (b) are the schematic and colored SEM image of the WSe2 junction device, respectively. (c) I-V curves of the device at different gate voltages. (d)-(f): Scanning photocurrent microscopy (SPCM) mappings of the junction area, as marked in (b) by the red-dashed rectangle, under the bias voltages of 0 V (d), -1 V (e), and +0.5 V (f), respectively. The illumination laser is 655 nm in wavelength with the power of 10 µW. The white solid lines show the boundaries between the electrodes and the sample. The electrode connected to the multilayer is kept grounded. (g) Band alignment structures of the homojunction under reverse and forward bias, respectively. 160x122mm (300 x 300 DPI)


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ToC 35x15mm (300 x 300 DPI)


Probing Exciton Complexes and Charge ... - ACS Publications

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