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Abstract
Interest in two-dimensional (2D) transition metal dichalcogenide materials has recently grown due to their tunable band gap, large exciton binding energy, and flexibility in designing a wide range of optoelectronic devices. However, the presence of adsorbates, strain, and aging-induced heterogeneities can severely influence their excitonic photoluminescence (PL). Therefore, precise nanoscale correlation between the optical and structural heterogeneities is of paramount importance and needs further investigation. In this report, we used tip-enhanced photoluminescence (TEPL) imaging of the nanoscale exciton emission of air-aged monolayer MoSe2-WSe2 multijunction lateral heterostructures with subwavelength spatial resolution of 40 nm. Distinct regions corresponding to monolayer MoSe2 and WSe2 were identified. Near-field excitation was used for the quantitative evaluation of the optical transition width: 40 nm sharp transition (WSe2→MoSe2), and 230 nm smooth interface (MoSe2→WSe2). Nanoscale composition of the alloying properties of the junction was estimated using TEPL. Furthermore, localized optical fluctuations and the role of the aging-induced nanoparticles in the excitonic PL suppression were investigated. The observed linear dependence of the PL suppression as a function of the nanoparticle size indicates the contribution of the chemical aging effects. The correlation between the nanoscale optical and structural characteristics will be useful for controlling and manipulating excitons in the next-generation atomically thin devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) heterostructures composed of atomically thin sheets of transition metal dichalcogenide (TMD) layers in the form of MX2 (M = Mo,W and X = Se,S) [1,2] have emerged as a new class of functional materials for next-generation electronic devices such as tunneling field effect transistors [3], and flexible optoelectronic devices [4] such as light-emitting diodes, photodetectors, solar cells, and sensors [5,6]. Recently, new methods have been developed to synthesize a wide range of TMD heterostructures in the form of the vertical [5,7], and lateral [2,8] configurations. Several proof-of-principle TMD-based vertical heterostructure devices were recently demonstrated [5,9]. Interest in monolayer lateral TMD heterostructures has grown to realize atomic scale flexible devices with unprecedented characteristics [2,8,10–12]. The lateral heterointerface may allow to control excitons, phonons and polaritons [13] that will facilitate the design of ultra-smart devices [4,13]. The unique properties of 2D lateral heterostructures include the formation of the atomically sharp p-n junctions, which are promising for flexible 2D electronics [7,14–16]. In addition, lateral heterostructures may provide a larger degree of compactness to further reduce the size of electronic devices. In-plane electric conduction through heterojunctions formed by covalent chemical bonds is expected to be more efficient than through van der Waals stacks where the carriers have to tunnel through the junction. Lateral heterostructures also provide an easy access for physical contact of heterojunctions with external agents such as the electric and plasmonic probes, that will allow for the formation of hybrid devices with extended functionalities and improved tunability. However, the overall performance of monolayer TMD heterostructures is often affected by diverse factors such as strain and poor environmental stability [17,18]. Precise knowledge of nanoscale level correlations between the optical and structural characteristics of these materials is desirable [19].
Monolayer TMDs have tightly bound excitonic states at room temperature, large exciton binding energies and strong light mater interactions [20], which are beneficial for light emission and harvesting devices [21–24]. However, the presence of local adsorbates, strain and grain boundaries can severely influence the net quantum yield [17–19,25]. Although electron microscopy can probe atomic scale electronic properties [26,27], the direct investigation via the nano-optical microscopic techniques would have multifold benefits [27–30], In this context, near-field optical microscopy provides a way to explore excitonic properties below the diffraction limit [31–42], Here we show that tip-enhanced photoluminescence (TEPL) with the optical excitation confined to a few nanometers has the potential to achieve these objectives. We performed nanoscale optical imaging of monolayer MoSe2-WSe2 lateral multijunction heterostructures grown by chemical vapor deposition (CVD) using TEPL, which allowed for mapping the spatial distribution of exciton emission, local fluctuations and quantification on length scales that are comparable to the exciton diffusion length [33,43].
2. Materials and methods
Monolayer MoSe2-WSe2 lateral multijunction heterostructures were grown by water-assisted one-pot synthesis via sequential edge-epitaxy in the CVD system [2]. Briefly, 100 mg of MoSe2 powder (99.9% purity, Sigma Aldrich) mixed with 50 mg of WSe2 powder (99.9%, Sigma Aldrich) was placed in a high purity alumina boat at the center of a two-zone horizontal furnace within a quartz tube of 1” diameter. Cleaned Si substrates with 285 nm SiO2 layer were placed within the temperature range of 780-800 °C, whereas the solid sources were kept at 1060 °C. The selection of the domains was controlled independently by switching the appropriate carrier gases; N2 + H2O vapors favor the growth of the MoSe2 domains whereas Ar + H2 (5%) gas favors the growth of the WSe2 domains.2 The far-field Raman and PL experiments were performed using a confocal spectrometer with 532 nm laser excitation (LabRAM HR Evolution, Horiba Scientific). TEPL imaging was performed using a scanning probe microscope (OmegaScope-R coupled with LabRAM Evolution microscope, Horiba Scientific). Au-coated Ag plasmonic probe (~20 nm radius) was positioned in the focus of the laser spot [40]. Localized surface plasmons, formed at the apex of the metal probe, directly interact with the TMD heterostructures, significantly enhancing the population of the excitonic states. The nature of these plasmon-exciton interactions in TMD materials has been previously investigated [44,45]. The nanoscale optical imaging was performed with the step size of 40 nm, which corresponds to the spatial resolution of TEPL. This sets a limit to the observed junction width using TEPL. At every spatial location of the probe, the far-field (FF) PL spectra were collected in a backscattering geometry in which the tip-sample distance was maintained at ~20 nm, whereas the ~0.3 nm distance was maintained to collect the near-field (NF) PL spectra. The spatial resolution of the far-field measurements was ~1 μm. After the CVD growth, the heterostructures were kept under the ambient conditions in air for ~2 weeks.
3. Results and discussion
A scanning electron microscopy (SEM) image of a monolayer MoSe2-WSe2 lateral heterostructure is shown in Fig. 1(a). Alternating thin strips of MoSe2 monolayer can be seen between the inner (MoSe2 triangle) and outer WSe2 layer. A schematic representation showing the atomic ball model of the monolayer MoSe2-WSe2 lateral heterostructure is depicted in Fig. 1(d). Figures 1(b) and 1(c) show low and high magnification optical images, respectively, of a monolayer lateral multijunction heterostructure composed of MoSe2 (dark) and WSe2 (light) domains on the Si/SiO2 substrate. The heterostructures consist of nine in-plane heterojunctions with truncated triangular geometry. The lateral widths of the individual TMD domains were controlled by varying the growth time. Previously, two kinds of interfaces were identified using electron microscopy, smooth (MoSe2→WSe2) and sharp (WSe2→MoSe2) [2]. The formation of smooth and sharp interfaces is mainly the result of distinct oxidation/reduction kinetics of molybdenum and tungsten containing species [2]. This is additionally influenced by varying local experimental conditions during the CVD growth, such as the gas switching rates, gas composition homogeneity, etc [2,46,47]. Since the gas switching is performed manually, the structural width of the smooth interfaces may vary from a few nm to a few hundred nm with varying extent of alloying at the heterojunction. TEPL provides high resolution analysis of the corresponding optical properties of the heterojunctions, which are complimentary to the electron microscopic techniques. Notably, Fig. 1(c) (magnified image from a section in Fig. 1(b)) shows that the fine MoSe2 strips are difficult to visualize via confocal optical microscopy.
Fig. 1 (a) Scanning electron microscopy (SEM) image of monolayer MoSe2-WSe2 lateral heterostructure. (b) Low magnification optical image of a nine-junction monolayer lateral heterostructure with MoSe2 and WSe2 domains; and (c) larger magnification from a section of the heterostructure in (b); arrows indicate thin MoSe2 strips. Dark and bright regions correspond to MoSe2 and WSe2, respectively. (d) Atomic ball model showing the MoSe2-WSe2 lateral junctions. (e) Raman spectra and (f) photoluminescence (PL) spectra of individual MoSe2 and WSe2 domains.
Far-field Raman and PL spectroscopies were used to probe the structural and optical properties of the heterostructures. The Raman spectra collected from different individual domains are shown in Fig. 1(e), which correspond to the monolayer MoSe2 and WSe2 domains [2,46,48]. The monolayer nature of the heterostructure domains was further confirmed using the uniform AFM height image in Fig. 2(a). In the inner MoSe2 region, the Raman spectrum is mostly dominated by a strong peak at 240 cm−1, which belongs to the A1g mode of monolayer MoSe2. The additional shoulder at 249 cm−1 is assigned to the 2E22g shear mode of MoSe2 at the M point [49]. On the other hand, the strong peak at 250 cm−1 in the WSe2 domain is the characteristic signature of monolayer WSe2 (E12g + A1g) [48], and the peak at 260 cm−1 is due to the double resonance of the LA(M) phonon. Similarly, the FF PL spectra acquired from the MoSe2 domain show a strong peak around 810 nm, attributed to the excitonic emission of monolayer MoSe2 (Fig. 1(f)). On the other hand, the PL peak around 769 nm corresponds to the excitonic emission of monolayer WSe2. In the conventional PL microscopy, the spatial resolution is diffraction limited. Therefore, the PL measurements shown in Fig. 1(f) cannot provide detailed nanoscale optical properties as well as the nature of the interfaces of the thin MoSe2 strips, which are below the optical resolution limits.
Fig. 2 Nanoscale imaging of MoSe2-WSe2 lateral heterostructure. (a) AFM height image, (b) AFM phase image. (c) Far-field (FF) PL peak position map, and (d) near-field (NF) PL peak position map. (e) Line profiles showing FF and NF PL peak position variations across different interfaces as indicated by arrows in (c) and (d), respectively. The highlighted regions in (e) show the width of the two transitions: 40 nm sharp (WSe2→MoSe2) and 230 nm smooth (MoSe2→WSe2). (f) 3D plot of the NF PL peak position corresponding to the lower-left corner in (d), and (g) higher magnification 3D plot of the NF PL peak position across the WSe2 ← MoSe2 ←WSe2 (right-to-left side) transition corresponding to the box region in (f).
Atomic force microscopy (AFM) can provide the topographic information of the MoSe2-WSe2 heterostructure via the height and phase measurements shown in Figs. 2(a) and 2(b), respectively. Figure 2(a) shows the AFM height topography of the heterostructure that was kept for 2 weeks under the ambient conditions in air, demonstrating uniform thickness across the triangular island as well as the formation of nanoparticles (NPs). The MoSe2 strips are not visible in this image, and the lack of significant height differences across multiple interfaces indicates that the individual domains are grown epitaxially at the edges of the preceding TMD domains. However, the corresponding AFM phase map (Fig. 2(b)) shows the presence of multiple thin strips of MoSe2 (bright lines) laterally connected to WSe2 domains. Neither the AFM topography nor the phase mapping could provide the sufficient information related to the spatial distribution of individual TMD domains, interface quality and structural heterogeneity. Nevertheless, the presence of aging-induced NPs, formed via oxidation or physisorption process, on the surface of TMD domains can be located via topography maps as well as phase mapping.
Next, the nanoscale optical heterogeneity, mesoscopic extent of exciton suppression, and the role of aging effects in heterostructures were investigated using TEPL. For the TEPL maps, 660 nm laser was focused onto a Au-coated Ag tip, and simultaneously scanned a region of the TMD triangular island (Figs. 2(c) and 2(d)). The integrated FF PL position map in the spectral range from 765 to 816 nm shows the presence of two spatially separated emission regions which can be correlated with the pattern observed in the AFM phase map. The thin MoSe2 domains are not well resolved in the FF image due to the limited spatial resolution (Fig. 2(c)). However, NF nanoimaging provides enhanced resolution that can be visualized in the NF PL peak position variation (Fig. 2(d)). It confirms the presence of distinct boundaries and emission characteristics, which are unresolved in the FF map.
The line profiles across the junctions, as indicated by the arrows in NF and FF maps, reveal the spectral modulation of the PL emission across the heterointerface between the MoSe2 and WSe2 domains (Fig. 2(e)). The 3-dimensional (3D) maps corresponding to the lower left region of the NF image in Fig. 2(c), further confirm the presence of two distinct hetero-junctions (Figs. 2(f) and 2(g)). The regions with higher PL peak position correspond to the MoSe2 thin strips, while the regions with lower energy (~774 nm) are WSe2 domains. Notably, the PL peak position shifts gradually for the MoSe2 → WSe2 domain (optical width ~230 nm), whereas a sharp increase in the peak position can be seen for the case of the WSe2 → MoSe2 transition (optical width ~40 nm). TEPL shows different nanoscale optical widths during the transition from one TMD domain to the other. The magnified image in Fig. 2(g) (corresponding to the dotted square in Fig. 2(f)), showing the 3D view of the PL peak position corresponding to the WSe2 ←MoSe2 ← WSe2 transition, provides significant information related to the nanoscale interface heterogeneity. In addition, we observed a maximum of 3 nm peak shift (WSe2 domain) corresponding to the sharp interface (WSe2 → MoSe2 transition), whereas a 6 nm peak shift (WSe2 domain) was observed for the smooth interface (MoSe2 → WSe2 transition). The observed PL peak shift is presumably associated with the slight alloying across the interfaces. The composition of the W(1-x)MoxSe2 alloy can be estimated from the position of the PL peak according to [50,51],
By considering the band gap bowing parameter b = 0.15 [50], Eg(x) smooth interface = 781 nm and Eg(x) sharp interface = 778 nm, the estimated alloy configurations across the heterojunction vicinity in the form of W(1-x)MoxSe2 are W0.94Mo0.06Se2 (smooth interface) and W0.97Mo0.03Se2 (sharp interface), corresponding to x = 0.06, and x = 0.03, respectively. We assumed that only metal atom (W or Mo) incorporation occurs during the transition between different TMD domains, since the chalcogenide environment remains unchanged during the heterostructure fabrication [2].
The nanoscale local optical variation was further evaluated using the integrated PL intensity map over the TMD triangular island. The integrated FF PL intensity map in the spectral range from 770 to 810 nm shows the presence of the spatial intensity modulation that follows the spatial pattern of the lateral heterostructure domains (Fig. 3(a)). However, similarly to the peak position map in Fig. 2(c), the FF intensity map does not provide significant information related to the local optical properties of individual TMD domains (Fig. 3(a)). In contrast, the NF PL intensity map provides enhanced resolution that enables to quantitatively evaluate the local exciton-suppression phenomena across the monolayer junctions as well as at the aging-induced NPs (Fig. 3(b)). The NF PL suppression depends on several factors including the presence of grain boundaries, strain and effects of band-bending [17,52]. Also, the regions with lower PL intensity within the TMD island that correspond to the MoSe2 thin strips can be attributed to the fact that the PL intensity of WSe2 monolayer is nearly 4.5 times higher than that of the monolayer MoSe2, as shown in Fig. 1(f).
Fig. 3 Nanoscale imaging of MoSe2-WSe2 lateral heterostructure. (a) Integrated intensity FF PL map over the spectral range of 770-810 nm. (b) Integrated intensity NF PL image over the spectral range of 770-810 nm and (c) the corresponding NF PL spectra of WSe2, MoSe2 and WSe2-MoSe2 interface. (d) FF and (e) NF PL spectra which correspond to the selected areas highlighted by the square boxes in Figs. 3(a) and 3(b), respectively. Line profiles showing FF and NF PL intensity distributions across different interfaces, which correspond to the long (blue) (f) and short (orange) (g) arrows in Figs. 3(a) and 3(b), respectively.
Normalized spatially integrated NF intensity PL spectra corresponding to WSe2, MoSe2 and WSe2-MoSe2 interface regions are shown in Fig. 3(c). The PL peaks at 780 nm and 810 nm indicate the exciton transition energies in monolayer WSe2 and MoSe2 (Fig. 3(c)), respectively [2]. A red shift of 8.8 meV and a double-peak shaped spectral profile was observed in the NF PL spectra compared to the FF PL spectra obtained from the pristine WSe2 region. However, the FF PL spectra obtained from different points across the WSe2 (outer domain) ← MoSe2 (center)← WSe2 (inner domain) interface (square marks in Fig. 3(a)) show broad peaks centered between 775 to 800 nm corresponding to monolayer WSe2 (Fig. 3(d)). This shows that FF PL cannot be used distinguish MoSe2 and WSe2 domains neither spatially nor spectrally, but the NF PL can be used to achieve both. The observed PL peak broadening and peak shifts can be attributed to the formation of MoxW1-xSe2 alloy across the interface [53].
The integrated PL intensity line profiles across eight lateral heterojunctions, as shown in Figs. 3(f) and 3(g) (corresponding to the PL images in Figs. 3(a) and 3(b)), indicate the presence of different alloying ranges across the interfaces. The FF intensity line profiles are less distinctive than the NF line profiles. The average FF PL intensity of the WSe2 domain (138k ± 16k) is nearly 13 times higher than the average NF PL intensity (10.1k ± 1k), extracted from Figs. 3(f) and 3(g). In thinner MoSe2 strips, the FF PL intensity reduced by 1.3 times compared to the WSe2 PL intensity, and the corresponding 5.2 times reduction was observed in the NF PL intensity. This can be understood from the fact that the number of emitting photons as well as the collection area in the FF imaging process are significantly larger than that of the NF map.
The correlated AFM and NF 3D surface maps were further used to directly visualize the localized topography, phase and PL emission as signatures of the aging effects in 2D heterostructures (Fig. 4). No NPs were initially observed right after the CVD growth of the heterostructures, which showed initially smooth surfaces [2,33]. After exposing the heterostructure for 2 weeks to the air, we observed the formation of the NPs (Fig. 4). As expected, the topography as shownin the slanted 3D surface plot in Fig. 4(a) across the TMD heterostructure is uniform, except the regions having a few scattered aging-induced NPs. The corresponding 3D phase profile also shows the presence of alternating TMD domains (Fig. 4(b)). Interestingly, the NF 3D surface map provides more localized photophysical information over the selected heterostructure area, with 40 nm resolution (Fig. 4(c)). Spatial modulation of the NF PL intensity across different interfaces can be clearly visualized in the magnified image in Fig. 4(d). Furthermore, the optical transition behavior between the WSe2 ↔ MoSe2 domains is in accordance with the line profiles drawn across these heterojunctions (Figs. 3(f) and 3(g)). The enhanced resolution of the optically sharp (WSe2 → MoSe2 interface) and smooth (MoSe2→WSe2 interface) transitions of the multi-junction lateral heterostructure is substantially improved using TEPL. In addition, the local optical fluctuations, optical width and presence of NPs in the localized NF PL emission characteristics are effectively quantified.
Fig. 4 Three-dimensional nanoscale images of MoSe2-WSe2 lateral heterostructure. (a) AFM topography and (b) AFM phase image. (c) NF PL intensity variation map, and (d) magnified image of a section of (c) showing the heterogeneous spatial modulation of the NF PL intensity and the interface transition. Arrows indicate the localized PL suppression sites.
The influence of NPs on the local exciton-suppression characteristic has been quantitatively evaluated. There is a strong correlation between these adsorbates in the AFM topography as well as the phase map to the corresponding regions in the NF map. Notably, the NF PL intensity showed the suppression of the total intensity due to the presence of the NPs, appearing as localized holes in the NF PL map (indicated by arrows in Figs. 4(a) to 4(d) and dashed squares in Fig. 5(a)). In addition, at the localized NP sites on WSe2 (dashed squares in Fig. 5(a)), the NF PL peak position exhibited a 2 to 4 nm red shift as compared to the pristine region (Fig. 5(b)). The observed suppression of the PL intensity by the NPs might be due to the partial blocking of the tip-induced enhancement of the NF PL signals. On the other hand, the observed red shift of NF PL peak position (Fig. 5(b)) of the monolayer WSe2 in the presence of NPs suggests a possibility of the formation of different states or strain at the localized NP sites. The average NP size calculated from the AFM topography image (Fig. 2(a)) was found to be 1.7 ± 0.5 nm (histogram in Fig. 5(c)) and the corresponding reduction of the NF PL intensity was 1.9k ± 0.7k (histogram in Fig. 5(d)), which is nearly 28% ± 9% less than the average NF PL intensity (Fig. 6). Figures 6(b)–6(c) show the integrated NF intensity profiles across different NPs which correspond to the selected areas (A, B and C) highlighted by white arrows in Fig. 6(a). The nanoscale heterogeneous features observed in these line profiles reveal the local photophysical properties of the NP sites in the heterostructure.
Fig. 5 (a) NF nano-PL (n-PL) intensity map integrated over the spectral range of 770-810 nm (left panel) and peak position map (right panel); (b) the corresponding PL spectra from pristine monolayer (1L) WSe2, close to interface, and nanoparticle (NP) on monolayer WSe2, as indicated by the dashed squares in (a). Histograms of (c) the distribution of nanoparticles on MoSe2-WSe2 lateral heterostructure obtained from the AFM topography, and (d) the corresponding change (decrease) in n-PL NF total intensity. (e) linear correlation between nanoparticle height and area to the reduction of the NF n-PL intensity.
Fig. 6 (a) Integrated intensity NF PL image over the spectral range of 770-810 nm of MoSe2-WSe2 lateral heterostructure; (b), (c) and (d) line profiles showing NF PL intensity distributions across different nanoparticles which correspond to the selected areas highlighted by the arrows in (a).
A linear relationship was found between the size of the NPs and the corresponding extent of the total NF PL intensity suppression as shown in Fig. 5(e). Notably, the NF PL suppression behavior is significant in the NF map as compared to that of the FF images. This indicates that the presence of the NPs can only be revealed in the localized optical emission signals, whereas the bulk optical emission remains unaltered. The observed linear dependence of the NF PL suppression on the NP height supports the chemical aging effects of the PL suppression as opposed to the physical blocking mechanism. The physical blocking would lead to the exponential decrease of the NF PL intensity due to the exponential decrease of the optical field intensity in the vicinity of the plasmonic tip.
Recent theoretical [54] and experimental studies [43,55,56] evaluated the detrimental effects of air exposure towards the stability of TMD materials. It was found that prolong air exposure can degrade monolayer TMDs [55] via gradual oxidation along grain boundaries, defect sites as well as with the exposure to organic contaminates [54–56]. As a result, chains of oxidized metal nanoparticles often form in monolayer TMDs along the grain boundaries/cracks, which significantly suppresses the PL yield [56]. Air exposure leading to the formation of surface protrusions as well as increase in the roughness in monolayer TMDs were also reported [55]. We assume that the NPs observed in our samples are formed during the exposure to the ambient conditions. The origin of the low quantum yield observed at the localized NP sites in these TMD domains may be attributed to the local change in the chemical composition. It was also demonstrated that presence of adsorbates increases the active sites that can significantly improve the hydrogen evolution activity [57], catalysis and optical absorption thresholds [57]. Local heterogeneity engineering in 2D heterostructures can pave the way to exotic optoelectronic applications including quantum emitters and quantum logic [57,58].
4. Conclusions
We demonstrated tip-enhanced optical characterization of 2D monolayer multijunction MoSe2-WSe2 lateral heterostructures grown via CVD process with sub-diffraction resolution (~40 nm). NF imaging was used to locally map the distribution of excitonic emission across the heterostructures and found to be more effective to provide enhanced nanoscale optical characterization with high spatial resolution and local fluctuations as compared to the FF imaging. The integrated NF PL spectral position mapping was used to probe the optical nanoscale width of the lateral heterointerfaces and NP profiles with a significantly higher spatial resolution. In addition, the aging effects and their extent on the exciton-suppression process were quantitatively evaluated via the strong correlation between the AFM topography and the corresponding TEPL maps. The presence of aging-induced NPs significantly reduces the overall exciton emission yield. The detailed understanding of the nanoscale excitonic emission characteristics with the corresponding structural heterogeneity in 2D heterostructures would facilitate the use of these materials for atomically thin devices. The nanoscale optical information may be used to improve the electrical control of the PL of lateral heterojunctions [59,60].
Funding
National Science Foundation (NSF) (CHE-1609608, DMR-1557434).
Acknowledgements
We thank Prof. Marlan Scully for the use of the laser facility.
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