Current Rectification through Vertical Heterojunctions between Two

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Current Rectification through Vertical Heterojunctions between Two Single-Layer Dichalcogenides (WSe|MoS pn-Junctions) 2

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Hrishikesh Bhunia, Abhijit Bera, and Amlan J. Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15740 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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ACS Applied Materials & Interfaces

Current Rectification through Vertical Heterojunctions between Two Single-Layer Dichalcogenides (WSe2|MoS2 pn-Junctions) Hrishikesh Bhunia, Abhijit Bera, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India *E-mail: [email protected]

KEYWORDS: 2D single-layer, p-type WSe2 and n-type MoS2, formation of pn-junction between two single layers, current-rectification, scanning tunneling spectroscopy, density of states. 1 ACS Paragon Plus Environment


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Abstract

We form junctions between two single layers of p-type WSe2 and n-type MoS2 in both sequences. The WSe2|MoS2 and MoS2|WSe2 junctions of ultimate thickness limit exhibit current rectification when characterized vertically with a scanning tunneling microscope (STM) tip. The direction of rectification in the pn-junction is opposite to that of the np-junction confirming occurrence of the rectification to be due to the junctions themselves. From scanning tunneling spectroscopy (STS) and correspondingly the density of states (DOS), we locate the conduction and valence band-edges (CB and VB, respectively) of the materials inferring their single-layer and 2H phase configuration. Band-edges of the semiconductors form a type-II band-alignment resulting in current rectification. In junctions of WSe2 and MoS2 single layers having a partial overlap, we map band-edges along different points on individual semiconductors and the overlapped region (junction). The results have inferred experimental evidence of current rectification through van der Waals vertical heterojunctions between two single layers.

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1. INTRODUCTION It is said that the two-dimensional (2D) atomic crystals have opened a new era in nanotechnology. Properties of a single layer of 2D materials vary largely as compared to their bilayer or multilayer configurations. Amongst graphene, hexagonal boron nitride and similar layered materials, and transition metal dichalcogenides (TMDs), the latter class of materials is considered to be a strong contender for a range of next-generation electronic and opto-electronic applications.1,2 Band-gap engineering, high carrier-mobility, and the ability to tune carrier concentration have made this class of atomically-thin 2Dmaterials extremely suitable for designed electronic, photonic, and sensor applications.3,4 Any such application per se requires at least one heterojunction as a building block, which is in general formed through covalent bonding in conventional compound semiconductors. With 2D materials in this direction, lateral (in-plane) heterojunctions have been formed in a monolayer between two TMDs, so that device functionalities can be imagined in a single atomically thin layer.5 The TMDs can moreover form vertical heterostructures through a weak van der Waals force without any dangling bonds that are known to be detrimental for carrier transport.6-8 Such artificial van der Waals homo- or hetero-structures between TMDs are now considered for a range electronic and optoelectronic devices, such as integrated circuits,9 transistor applications,10 light-emitting diodes,11 photovoltaics,12-15 and photodetectors.14-16 Formation of a monolayer of TMDs is generally manifested by observing a strong photoluminescence (PL) emission,17 shift in Raman peaks due to a weak coupling between electronic transitions and phonons,18 a direct and wider band-gap,4,19 and also through different electron/force microscopies.10,20 Parameters of synthesized and other possible TMDs have also been determined from first principle studies21 and were also simulated22 to envision newer heterojunctions for improved device performances. In the van der Waals heterostructures, a significant broadening of excitonic transitions was observed inferring a charge-transfer process due to a staggered band-alignment.23 Band-edges of the materials showed that WSe2 and MoS2 would truly form a pn-junction at the ultimate thickness limit;8 Philip Kim and his collaborators observed photoinduced charge separation in WSe2|MoS2 heterojunctions when 3 ACS Paragon Plus Environment


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sandwiched between two graphene layers inferring formation of a pn-junction between two single sheets.7 Such large-area heterojunctions were probed also through metal contacts formed on the atomic sheets away from the overlapped region;12,14,15,24 these junctions, due to their advantageous size, exhibited charge-transfer across the planar interface upon illumination and hence photovoltaic effect. In this work, we aimed for vertical measurements on WSe2|MoS2 pn-junctions, which were formed through a partial overlap between two single layers of WSe2 and MoS2. Such junctions would provide a new way to study the properties of van der Waals heterostructures excluding the influences of metal electrodes. The localized measurements were carried out with a scanning tunneling microscope (STM) tip operated at 80 K in an ultrahigh vacuum (UHV) condition. To do so, we first made scanning tunneling spectroscopy (STS) measurements to probe band-edges of the p-type WSe2 section and the n-type MoS2 section in order to depict the band-diagram of WSe2|MoS2 junctions and also to image the overlapped region between the two sheets. More importantly, we also aimed for localized measurement of tunneling current in the vertical direction through the overlapped region. We formed and characterized WSe2|MoS2 and MoS2|WSe2 heterojunctions both to sift electronic properties of the pn- and np-junctions from those arising out of interfaces between the semiconductor and the two electrodes. We envisaged in achieving rectifying current-voltage (I-V) characteristics in the pn- and np-junctions with the direction of rectification being opposite to each other and thereby experimentally manifesting thinnest possible current-rectifiers.

2. RESULTS AND DISCUSSION Characteristics of WSe2 and MoS2 Single Layers. Optical absorption spectroscopy,25-27 Raman spectra,18,28 energy-dispersive X-ray (EDX) analyses, and high-resolution transmission electron microscopy (HR-TEM) images29 have so far been utilized to evidence formation of WSe2 and MoS2 single- and multi-layered sheets. In WSe2 and MoS2 single layers that we have exfoliated, EDX analyses and optical absorption spectra of WSe2 and MoS2 2D-sheets are presented in Figures S1-S2 in the 4 ACS Paragon Plus Environment

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Supporting Information, respectively. While the optical spectra have been discussed in depth in the Supporting Information, it may be stated here that the peaks and shoulders matched reasonably well with the reported results.25-27 Raman and Fourier transform infrared spectroscopy (FTIR) of the exfoliated TMDs did not bring out any signature of NMP (N-methyl-2-pyrrolidone) (Figures S3 and S4 in the Supporting Information). Band-Edges of WSe2 and MoS2 through STS. In recent years, tunneling current and correspondingly density of states (DOS) through STS are being utilized to determine band-edges of a range of nanostructures and their heterostructures.30-32 We therefore first aimed to locate the band-edges of the WSe2 and MoS2 single layers through STS in order to confirm that the exfoliated WSe2 and MoS2 were semiconducting in nature (2H phase). To do so, we recorded tunneling current versus voltage characteristics on isolated single layers, which were identified through recording of line-profile in STM images. That is, the sheets that would yield a thickness of around 0.6 - 0.8 nm could be considered to be single layer in nature and have to be characterized.17,27 When the line profile would result a higher value, the sheet would be considered to be a multilayered one. Typical STM images and their corresponding line profiles of WSe2 and MoS2 single layers are shown in the panel above Figure 1. Thickness derived from the line-profiles evidenced that we indeed imaged single-layered sheets. The figures moreover showed atomic-resolution of the 2D dichalcogenides. Tunneling current was then measured on many different points on such single layers of the semiconductors so that an average of STS measurements, which are known to be localized in nature, could be achieved. In Figure 1a, we present a typical DOS of a WSe2 single layer. We recorded tunneling current and derived DOS accordingly at about 50 points. From each DOS, we could locate CB and VB edges in the form of peaks in the positive and negative voltages, respectively, closest to 0 V. The edges were with respect to the Fermi energy (EF) of the respective semiconductor that is considered to be aligned to 0 V. The energies have been summed up in the form of a histogram of CB and VB energies (Figure 1b). The figure first of all brings out 2H phase of the material and also the p-type nature of the 5 ACS Paragon Plus Environment


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semiconductor, since the VB edge was closer to its Fermi energy. A band-gap of 2.64 eV could be observed that agrees well with reported results.20 For MoS2, the DOS and histogram of CB and VB edges, as presented in Figure 1(c) and 1(d), respectively, similarly inferred 2H phase of the material. Since the CB-edge could be seen to be closer to the Fermi energy, the material’s intrinsic n-type nature can be inferred. Here the band-gap was 2.44 eV that agrees with results obtained from STS of a single layer of MoS2.4,33 Since band-gap of 2D TMDs depends on the number of layers,4 the agreement further confirms that we indeed characterized single layers in the STS. The spectra furthermore brought out some additional features above CB and below VB edges. The gap in MoS2 has however been reported to shrink with a decrease in CB-edge due to a strong interaction between a graphite substrate and the dichalcogenide when the latter was formed through vapor phase reaction.33 The band gap seems to be higher than some reported results;21 as such band-edges of TMDs derived from STS provide energies at both - and K-points. The energies depend on the mode of measurements, tip-to-sample distance, and nature of the substrate-semiconductor interaction amongst other factors. In general, the local VB-energy at the -point is significantly lower than the global VB at the K-point. The band-gap related to the local VBenergy is therefore higher than that involving VB at the K-point. The CB-energy at the -point is similarly higher than the CB at the K-point. The histograms allowed us to form a band-diagram of WSe2|MoS2 pn-junction (Figure 1e). Following approach of the tip, Fermi energies of the semiconductors and the work-functions of HOPG and Pt/Ir electrodes align energetically; the energy diagram of the heterojunction could be seen to have formed a type-II band-alignment at the interface inferring a possible current-rectification through the junction between two single layer TMDs.


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WSe2 monolayer 0.6 nm

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Figure 1. (a-d) A typical DOS and histogram of VB and CB edges of p-type WSe2 and n-type MoS2 single sheet measured at 77 K. (e) Band-diagram of WSe2|MoS2 heterojunction showing the band-edges of the semiconductors and work-function of HOPG and Pt/It. Fermi energy (EF) after alignment is shown as dashed black-line. Typical STM topography, line profile, and atomic image of WSe2 and of MoS2 single layers are shown in the panel above the respective DOS spectrum. While scale bar in the topographies were 20 and 50 nm, respectively, the atomic images had a scale bar of 200 pm. 7 ACS Paragon Plus Environment


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3.18 Å 3.30 Å Figure 2. HR-TEM images of (a) MoS2, (b) WSe2, and (c) interface of a WSe2|MoS2 junction. Characterization of Junctions based on WSe2|MoS2: HR-TEM and STM images. Since we aimed to form a junction between the two types of single layers, it is imperative to know if such junctions are formed through van der Waals interaction. To infer so, we recorded HR-TEM images of a junction at the interface so that the layer beneath is also visible along with the material above. HR-TEM images of WSe2 and MoS2 sheets, when imaged separately (Figures 2a and 2b) and also in areas outside the overlapped region of a junction, returned atomic resolution with an interlayer spacing of 0.330 and 0.318 nm, respectively. The spacing matched that of the (002) plane of both the crystals as shown in JCPDS Card Nos. 38-1388 and 17-0744, respectively. To record HR-TEM image of a junction, it was first identified through EDX analysis which yielded all four elements (Figure S1c in the Supporting Information). In the TEM image at the interface region (Figure 2c), where part of MoS2 had a WSe2 sheet underneath, the bright part represented MoS2; in the dark part of the image, lattice spacing of WSe2 could also be visible through the


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lattice of MoS2. Both layers appearing in HR-TEM images infer their compactness and hence activating van der Waals interaction between the p- and n-type single layers. Formation of a junction can further be evidenced through STM images (Figure 3). The STM images more importantly inferred that a junction could be formed between two single layers. When a line-profile was drawn along the junction, two-steps yielding a thickness of around 0.8 and 0.7 nm, respectively, could be observed ensuring formation of such a junction between two single layers (Figure 3e). The thickness of the upper layer was close to the thickness of MoS2 single layer again inferring compactness of the two layers in forming a van der Waals heterojunction. The line profile showed another step (0.37 nm) in addition; this value matched with the thickness of an atomic sheet of graphite34 implying that the substrate had an intrinsic step at the position of image. To confirm further, we recorded STS on both the steps of highly oriented pyrolytic graphite (HOPG); the corresponding DOS showed metallic behavior (Figure S5 in the Supporting Information) along with some additional features consolidating presence of a HOPG step in the STM image. The results in addition bring out the strength of STS in identifying lowerdimensional materials through localized measurement of electronic characteristics. The correspondence between the image and the line-profile moreover brings out p- and n-sections apart from the interface region between the two single layers. Atomic resolution of the two materials could be observed in the STM topography with lattice separation matching the values obtained from HR-TEM images (Figures 3a-c). Here, lattice of only the upper material could be seen when a junction was imaged, since tunneling of electrons occurred to the upper semiconductor followed by conduction process to the lower one and finally to the electrode (HOPG). The interface could moreover be nicely imaged illustrating planes of the two crystals; such an image again infers formation of a junction between WSe2 and MoS2 single layers of ultimate thickness limit.



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Figure 3. Atomic resolution observed in (a) WSe2, (b) WSe2|MoS2 interface, and (c) MoS2 in STM topographies. (d) STM topography of a typical WSe2|MoS2 interface. (e) Captured line profile drawn on along the STM topography (as shown) yields steps representing single sheets of WSe2 and of MoS2 in forming the junction. Mapping of Junctions based on Single Sheets of p-Type WSe2 and n-Type MoS2. We then proceeded to measure tunneling current through WSe2|MoS2 and also through MoS2|WSe2 junctions based on single layers of p-type WSe2 and n-type MoS2. STM topographies of such pn- and np-junctions including p- and n-sections outside the junction are shown in Figures 4a and 4b. We selected such junctions between two 10 ACS Paragon Plus Environment

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single layers through line-profile of STM images. We recorded DOS at many points along the p-section and the overlapped pn-junction region that had an n-type MoS2 single layer as the top-layer (Figure 4). In an np-junction having the n-type layer in contact with HOPG, we similarly recorded DOS along the n-type sheet and the overlapped np-junction region having the p-type WSe2 sheet facing the STM tip. In both the junctions, the spectra of p- and n-sections matched those of WSe2 and MoS2 single sheets, as presented in Figure 1a and 1c, respectively. When the tip progressed to the overlapped region of pn- and np-junctions, the DOS spectra mostly resembled that of the upper material depicting CB and VB edges of the semiconductor above. Absence of band-edges of the material underneath implied that the two single layers were bound through an interlayer coupling. Here, electrons were tunneled to the upper layer followed by conduction to the layer beneath and then to the base electrode. That is, along the line of measurement towards the junction, the tip always monitored the layer that it faced. There has been a small change in VB energies of the upper material in a junction as compared to the band-edge of the semiconductor in isolation (Figure 1) or outside the junction (traces 1-3 of Figure 4d). The deviation in energies can be explained if we consider strain arising out of lattice mismatch between the two layers. We may further add that the lattice mismatch has been reported to affect primarily the valence band keeping the other band mostly unaltered.4



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Figure 4. STM topography of (a) a WSe2|MoS2 junction and (b) a MoS2|WSe2 junction formed between two single layers of p- and n-type semiconductors showing the spots at which dI/dV spectra were recorded. (c & d) dI/dV spectra on the spots of STM images. The solid and dashed vertical lines indicate the location of valence and conduction band-edges, respectively. Scale bar in (a) and (b) were 10 and 20 nm, respectively.


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Rectification through pn-Junctions of Ultimate Thickness Limit. Tunneling current versus voltage characteristics of WSe2|MoS2 and MoS2|WSe2 junctions brought out interesting characteristics. In Figure 5a, we present many such characteristics of the two junctions. The characteristics were reproducible in nature enabling us to plot an average of the characteristics in each of the cases. Both of them expectedly passed through the set-point (2.0 V, 0.1 nA) used during approach of the STM tip. The magnitude of current in the negative voltage therefore depended on the tip-to-sample distance at the positive voltage or the set-point. The plots show that tunneling current though the overlapped regions returned rectifying I-V characteristics evidencing current rectifiers of ultimate thickness limit. We plotted I-V characteristics of individual components also, that is single layer of WSe2 and of MoS2 formed separately (Figures 5b and c, respectively). They also passed through the set-point. In contrast to the characteristics of WSe2|MoS2 and MoS2|WSe2 junctions, the I-Vs of the components were mostly non-rectifying. The little asymmetry in case of MoS2 single layer appeared due to a strong n-type nature of the semiconductor. This infers that the rectifying characteristics in Figure 5a appeared truly due to the heterojunctions between two single layers. 0.4

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Figure 5. (a) Tunneling current versus voltage characteristics of WSe2|MoS2 and of MoS2|WSe2 junctions formed between two single layers of the semiconductors. At least 10 characteristics measured on different points on each junction are shown in the figure with the average characteristics being presented as thick lines. Black circle is the set-point for approach of the tip forcing all I-Vs to pass through the point. (b & c) Tunneling current versus voltage characteristics of separate WSe2 and MoS2 single layers. 13 ACS Paragon Plus Environment


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Direction of rectification can be explained by considering band-diagram (Figure 1c). For example, in a HOPG|WSe2|MoS2|Pt/Ir pn-junction, a positive voltage to the p-type TMD amounted to a forward-biased diode resulting in a higher current as compared to the current at a negative voltage to the WSe2. Tunneling current versus sample bias plot for the WSe2|MoS2 heterojunction hence yielded a higher current in the positive sample voltage. In the same manner, the np-junction resulted in a higher current in the negative sample bias region. While all the characteristics passed through the set-point at +2.0 V, the magnitude of current in the negative voltage was higher in np-junction as compared that in the pn-junction or isolated single layer cases. This occurred since the voltage for approach of the tip (+2.0 V) meant a forward-bias in the pn- and a reverse-bias in the np-junction. The tip-to-sample distance was hence shorter in the npjunction as compared to that in the pn-one and also in isolated single layer cases. The direction of rectification in the pn-junction was opposite to that in the np-one; this confirmed that the current rectification occurred indeed due to the vertical heterojunctions formed between the two single layers themselves. The opposing rectification in WSe2|MoS2 and MoS2|WSe2 junctions rules out the role of the two other interfaces with the electrodes in yielding rectifying characteristics. Rectification ratio was 4 to 8 at 2.0 V and was expectedly voltage dependent; it was however low due to tunneling nature of current involved enabling a large flow of current. Characteristics of WSe2 and MoS2 single layers, when measured separately that involved the other two interfaces with the electrodes, was largely symmetric (the ratio between current at +2.0 and -2.0 V was less than 1.5). As compared to the MoS2|WSe2 junction, the ratio was higher in the WSe2|MoS2 one, since rectifying behavior appearing due to dissimilar workfunctions of HOPG and Pt/Ir tip might have added up to that due to the pn-junction. The ratio did not change when a different set-point was used during the approach of the tip, although the magnitude of current depended on the parameters of tip-approach. The ratio differed to some extent when junctions having another stacking pattern were characterized. Since the stacking pattern affects interlayer interaction,35 such a small variation in rectification ratio can be expected. The direction of current rectification however did not change with the interlayer interaction. The results hence infer experimental 14 ACS Paragon Plus Environment

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evidence of current rectification through van der Waals vertical heterojunctions, namely WSe2|MoS2 and MoS2|WSe2 between two single layers.

3. CONCLUSIONS In conclusion, we have used single layers of p-type WSe2 and of n-type MoS2 in both sequences to form pn- and np-junctions. The WSe2|MoS2 and MoS2|WSe2 junctions between the two single layers, when characterized in an UHV-STM, yielded current rectification through the junctions of ultimate thickness limit. The direction of rectification in the pn- and np-junctions was opposite to each other corroborating appearance of rectification due to the junctions themselves. dI/dV spectra and correspondingly DOS of individual semiconducting-sheets brought out CB and VB edges of the 2D materials and hence their 2H phase alongside a type-II band-alignment resulting in current rectification. While STM topography evidenced formation of heterojunctions between two single layers, we mapped the vertical junctions and regions outside them through STS imaging. The results inferred experimental evidence of current rectification through van der Waals vertical heterojunctions between two single layers of TMDs.

4. EXPERIMENTAL SECTION Exfoliation of WSe2 and MoS2 to Single Layers. Single layers of the TMDs were formed by chemical exfoliation method.29 To do so, powders of the TMDs were first dissolved in N-methyl-2-pyrrolidone (NMP) in a flat-bottomed beaker. To be specific, 0.5 g of the powder was added to 10 mL of NMP to achieve a concentration of 50 mg/mL. Such dispersed solutions were sonicated for 10 h. During the sonication, temperature of the solution was maintained below 5 °C to facilitate the exfoliation process. After sonication, the dispersed solution was allowed to settle for about 24 h before centrifuging them at 1500 rpm for 45 min. The repeated centrifuge process was carried out in ethanol, so that NMP (N-methyl2-pyrrolidone), which was used during the exfoliation process, gets replaced. We then dried the flakes in vacuum for 30 min to remove ethanol. The top three-fourth part of the dispersed solution was collected by 15 ACS Paragon Plus Environment


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a pipette; the rest portion was centrifuged for further exfoliation. Since the dispersed solution would contain single layered and also smaller multilayered sheets, the sheets were isolated by increasing the rotation speed in steps followed by separation. Finally, the dispersed solution was centrifuged at 3000 rpm for 15 min. During scanning tunneling spectroscopy (STS) measurements, single-layer sheets were selected through measurement of line profile keeping in mind that a single layer had a thickness of less than 0.9 nm depending on the material. Characterization of the 2D Materials. Optical absorption spectroscopy of the materials was performed with a UV-Vis spectrophotometer (Cary 5000, Agilent Technologies). FTIR and Raman spectroscopy were carried out with a Perkin Elmer Spectrum 100 FTIR spectrometer and a Horiba Jobin-Yvon Raman triple grating spectrometer system (model number T64000) using 514 nm excitation of a Spectra Physics laser source (model number Stabilite 2017), respectively. In addition, atomic force microscopy (AFM) and bright-field transmission electron microscopy (TEM) images were recorded with a Nanosurf Easyscan2 AFM and a JEOL transmission electron microscope operated at 200 kV, respectively. For TEM measurements, a drop of ultra-dilute solution of the sheets was placed onto a carbon coated copper grid in an ambient condition. Formation of Heterojunctions (WSe2|MoS2 and MoS2|WSe2). We aimed to form heterojunctions between a single layer of p-type WSe2 and another single layer of n-type MoS2 in both sequences, that is, WSe2|MoS2 and MoS2|WSe2. To do so, single layers of individual materials were dispersed in ethanol in separate vials to form ultra-dilute solutions. Considering the heterojunction that was aimed to form, one drop of the first material was drop-casted on a freshly-cleaved highly oriented pyrolytic graphite (HOPG) substrate. After drying the first layer, one drop of the second material was also drop-casted. This allowed formation of some heterojunctions randomly situated amongst single sheets of the two materials. The two heterojunctions thus formed were placed in an ultrahigh vacuum scanning tunneling microscope (UHVSTM) chamber for further characterization.


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Scanning Tunneling Spectroscopy (STS) of Single Layers and Heterojunctions. STS of the individual single layers and heterojunctions between two single sheets was recorded in a Pan-style UHV-STM of M/s RHK Technology. The pressure of the microscope was 1.2 × 10-10 Torr; temperature of the substrate and the tip both was 80 K. Pt/Ir (80%:20%) tips, which were formed through a mechanical cut of a wire having a diameter of 0.25 mm, was used to measure tunneling current. During approach of the tip, a current of 0.1 nA was targeted to achieve at 2.0 V. Thereafter, tunneling current versus sample voltage (IV) characteristics were recorded after disabling the feed-back loop. dI/dV spectra that have correspondence to the density of states (DOS) were recorded using a lock-in amplifier (20 mV rms 986 Hz). Since bias was applied to the sample, electron could be injected to the conduction band (CB) at positive voltages; DOS peaks at positive voltage hence corresponded to the location of CB band-edges. The peaks at negative voltages similarly represented conduction band (VB) edges. While recording tunneling current, we aimed to locate heterojunctions having some overlap between ptype WSe2 and n-type MoS2 single layers. Such heterojunctions allowed us to measure tunneling current through a junction and also through individual components. Although all the current-voltage (I-V) characteristics were forced to pass through the set-point (2.0 V, 0.1 nA) in STS, the measurement procedure allowed us to observe current rectification at a higher voltage at which electron-tunneling process is operative followed by conduction pathways through the heterojunction.

Supporting Information The Supporting Information material is available free of charge via the Internet at http://pubs.acs.org. Discussion and figures on optical absorbance spectra and FTIR and Raman spectroscopy, EDX spectra, STM topography of a WSe2|MoS2 junction and dI/dV spectra of HOPG.



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Corresponding Author *E-mail: [email protected] Acknowledgement. AJP acknowledges JC Bose Fellowship (SB/S2/JCB-001/2016) of SERB. The authors acknowledge financial support through Nano Mission projects. HB and AB acknowledge CSIR Fellowship Nos. 09/080(0958)/2014-EMR-I (Roll No. 521931) and 09/080(0779)/2011-EMR-I (Roll No. 510847), respectively.

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ToC Graphics

MoS2

WSe2 1

WSe2|MoS2 MoS2|WSe2

Current (nA)

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0

-1

-2

-1

0

1

2

Sample Bias (V)


Current Rectification through Vertical Heterojunctions between Two

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