Ultrahigh-Performance Optoelectronics Demonstrated in Ultrathin

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Ultrahigh-Performance Optoelectronics Demonstrated in Ultrathin Perovskite-Based Vertical Semiconductor Heterostructures Tiefeng Yang,†,‡,# Xiao Wang,‡,# Biyuan Zheng,†,# Zhaoyang Qi,†,# Chao Ma,† Yuhao Fu,§ Yongping Fu,∥ Matthew P. Hautzinger,∥ Ying Jiang,‡ Ziwei Li,† Peng Fan,‡ Fang Li,‡ Weihao Zheng,‡ Ziyu Luo,† Jie Liu,⊥ Bin Yang,† Shula Chen,† Dong Li,† Lijun Zhang,§ Song Jin,∥ and Anlian Pan*,†,‡ †

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, PR China ‡ School of Physics and Electronics, Hunan University, Changsha, Hunan 410082, PR China § State Key Laboratory of Superhard Materials, Key Laboratory of Automobile Materials of MOE, and School of Materials Science and Engineering, Jilin University, Changchun 130012, PR China ∥ Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States ⊥ College of Electrical and Information Engineering, Hunan University, Changsha, Hunan 410082, PR China S Supporting Information *

ABSTRACT: Two-dimensional (2D) atomic layered semiconductor (e.g., transition metal dichalcogenides, TMDCs) heterostructures display diverse novel interfacial carrier properties and have potential applications in constructing next generation highly compact electronics and optoelectronics devices. However, the optoelectronic performance of this kind of semiconductor heterostructures has difficulty reaching the expectations of practical applications, due to the intrinsic weak optical absorption of the atomic-thick component layers. Here, combining the extraordinary optoelectronic properties of quantumconfined organic−inorganic hybrid perovskite (PVK), we design an ultrathin PVK/TMDC vertical semiconductor heterostructure configuration and realize the controlled vapor-phase growth of highly crystalline few-nanometer-thick PVK layers on TMDCs monolayers. The achieved ultrathin PVKs show strong thickness-induced quantum confinement effect, and simultaneously form band alignment-engineered heterointerfaces with the underlying TMDCs, resulting in highly efficient interfacial charge separation and transport. Electrical devices constructed with the as-grown ultrathin PVK/WS2 heterostructures show ambipolar transport originating from p-type PVK and n-type WS2, and exhibit outstanding optoelectronic characteristics, with the optimized response time and photoresponsivity reaching 64 μs and 11174.2 A/W, respectively, both of which are 4 orders of magnitude better than the heterostructures with a thick PVK layer, and also represent the best among all previously reported 2D layered semiconductor heterostructures. This work provides opportunities for 2D vertical semiconductor heterostructures via incorporating ultrathin PVK layers in high-performance integrated optoelectronics. KEYWORDS: 2D material, heterostructures, perovskite, vapor growth, band alignment, optoelectronic devices

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However, the optoelectronic performance of the previously reported TMDCs-based heterostructures is still unsatisfactory,10−24 since these ultrathin semiconductor layers cannot absorb sufficient light to bring efficient light-matter interactions.

wo-dimensional semiconductor heterostructures have recently attracted intense interest due to their novel interfacial properties and promising potential electronic and optoelectronic applications.1−8 Specifically, as the thickness of the heterostructure is scaled down to the atomically thin level, photoexcited charge carriers can be efficiently separated at the interface and are well confined in the component layers.6,9 This further facilitates long distance carrier transport, and greatly reduces the radiative and nonradiative recombination processes. © XXXX American Chemical Society

Received: April 8, 2019 Accepted: June 20, 2019 Published: June 20, 2019 A

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Figure 1. Synthesis of ultrathin PVK/WS2 vertical heterostructures. (a) Schematic illustration of the growth process. (b) Optical images of several representative PbI2/WS2 heterostructures obtained at different deposition temperature region and (c) the corresponding PVK/WS2 heterostructures after the vapor-phase intercalation reaction process with an emphasis on the thinner heterostructures. All scale bars are 50 μm. Superimposed in c are also the AFM images and line scan profiles showing the thickness of PVK. (d) The relationship between the thickness of PbI2 and PVK.

alignment transition from type-I to type-II depending on the thickness of the PVK layer. This modular band alignment leads to highly efficient interfacial charge carrier separation. Photodetectors based on the as-grown heterostructures were constructed, demonstrating fast photoresponse speed and ultrahigh photoresponsivity, both of which represent the best among all previously reported 2D layered semiconductor heterostructures. This work offers a general and reliable strategy for the controlled growth of a family of PVK-based ultrathin heterostructures, which marks an important step toward nextgeneration highly compact high-performance optoelectronics.

To this end, rationally selecting semiconductors and carefully engineering the interface are of great importance to design ideal 2D heterostructures for high-performance optoelectronics. Organic−inorganic hybrid PVKs (e.g., CH3NH3PbI3) have been demonstrated to have high light absorption coefficients,25,26 long carrier diffusion lengths,27 and a strong defect tolerance.28 Particularly, ultrathin quantum-confined PVK not only inherits the advantages of bulk PVK, but also possesses increased stability,29 large exciton binding energy,30 and tunable band structures.31 This makes ultrathin PVK an ideal material platform for investigating light-matter interactions.32,33 Therefore, heterostructures consisting of ultrathin PVK and TMDC layers could combine the advantages of the PVK layers and the highly efficient charge carrier interactions at the atomically sharp interfaces. Although several efforts have been devoted to the preparation of such PVK/TMDC heterostructures,34−36 the reported optoelectronic performance is still subpar due to the large thickness of the PVK layers. This thick PVK layer not only impedes the efficient light capture of the underlying TMDC layers, but also prolongs the charge carrier diffusion time in the vertical direction, thus detrimentally affecting the light-matter interaction at the heterojunction region. To achieve high performance optoelectronics of PVK/TMDC heterostructures, a central task is to realize controlled preparation of high-quality ultrathin PVK layers interfaced with TMDCs monolayers. This still remains a significant challenge due to the nonlayered features of CH3NH3PbI3 PVK, which has similar crystal growth rates in both horizontal and vertical directions. In this work, we developed a fully vapor-phase method to grow highly crystalline and thickness-tunable layers on top of TMDCs monolayers. The result of this growth was the formation of high-quality ultrathin PVK/TMDC heterostructures with atomically sharp interfaces. As the thickness of the top PVK layer decreases from tens of nanometers to a few nanometers, photoluminescence of the PVK blue-shifted due to a strong quantum confinement effect. First-principles calculations indicate the PVK/TMDC heterostructures band

RESULTS To rationally synthesize the PVK/TMDC heterostructures with an ultrathin PVK layer, we developed a three-step fully vaporphase growth process (Figure 1a). WS2 monolayers are initially grown on a SiO2/Si substrate37 as templates. These WS2 monolayers served as epitaxial substrates for a thickness-tunable vertical van der Waals (vdWs) growth of PbI2 via a temperatureselective deposition strategy. The formed PbI2/WS2 heterostructures were then converted into PVK/WS2 heterostructures through a vapor-phase intercalation reaction of CH3NH3I with the PbI2 layer. During the PbI2 growth, creating an appropriate deposition temperature is crucial to realize the ultrathin intermediate PbI2 layer. At low temperatures, the surface diffusion of PbI2 precursors can be inhibited and undesired nucleation of crystal seeds could be minimized, ensuring the exclusive lateral epitaxial growth38 to obtain ultrathin PbI2 layers. Figure 1b shows six representative PbI2/WS2 heterostructure flakes obtained at different deposition temperature regions ranging from 280 to 220 °C, with the thickness of PbI2 being modulated from 17.2 nm down to 1.2 nm. Finally, through a vapor-phase intercalation of CH3NH3I, the upper PbI2 layers are fully converted into the desired high-quality PVK layers. In this step, we found that the position of substrate is crucial for the crystallization quality of the obtained ultrathin PVK (see Figure S4 for more details). Figure 1c shows the corresponding PVK/ B

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Figure 2. Structural characterizations of the ultrathin PVK/WS2 vertical heterostructures. (a) Time evolution of the X-ray diffraction patterns of the resulting PVK/WS2 heterostructures. (b) High-resolution TEM image of a selected PVK/WS2 sample. (c) Indexed SAED patterns of the ultrathin PVK/WS2 heterostructure. (d) Elemental mapping obtained through energy-dispersive X-ray spectroscopy (EDS) characterization. (e) Cross-sectional HRTEM image (top) and atomic structure model (bottom) of a selected ultrathin PVK/WS2 heterostructure. (f) HAADF intensity file acquired from the blue dashed rectangle region in (e), showing the thickness of three-unit-cell PVK is 2.0 nm, corresponding to 0.67 nm for one-unit-cell PVK. (g) EDS spectrum measured at P1 (marked by red box in (e)) and P2 (blue box).

resolution TEM image (Figure 2b), where clear lattice fringes are observed, confirming highly crystalline quality of the converted PVK layers. The interplanar distances of ∼0.328 and ∼0.332 nm can be indexed to the (400) and (−220) planes of tetragonal (CH3NH3)n+1PbnI3n+1 PVK,40 respectively. It should be noted that no lattice signals corresponding to the bottom WS2 layer was observed. But the selected area electron diffraction (SAED) pattern clearly shows two sets of diffraction patterns (Figure 2c), where the hexagonal pattern (red) and the tetragonal pattern (green) are assigned to the bottom WS2 and the top PVK, respectively. Energy dispersive X-ray spectroscopy (EDS) mapping (Figure 2d) further confirms the spatially uniform elemental distribution of C, N, Pb, I, W, S in the PVK/ WS2 heterostructures. A high-resolution cross-sectional TEM image (Figure 2e) shows that the PVK layers form an atomically sharp interface intimately integrated with the monolayer WS2 without any obvious gaps or impurities between them. The intensity difference in the high angle annular dark-field (HAADF) STEM intensity profiles (Figure 2f), further confirms the vertical stack and allows us to accurately determine the thickness of one-unit-cell of PVK to be 0.67 nm.39,41 Meanwhile, EDS spectrum collected from point 1 (marked by red P1 in Figure 2e) and point 2 (blue) was used to further confirm the vertical spatial element distribution, in which obvious peaks of W and S were observed in P1, and peaks of Pb and I were found in P2. All of these results demonstrate the successful fabrication of high-quality ultrathin vertically stacked PVK/WS2 heterostructures. The achievement of such PVK-based ultrathin heterostructures provides a family of materials for investigating

WS2 heterostructures after conversion, featuring sub-10 nm thickness. The thicknesses of the top PVK layers are measured to be 1.3, 2.7, 4.3, and 5.2 nm, which correspond to 2, 4, 6, and 7 unit cells of (CH3NH3)n+1PbnI3n+1.39 After conversion from PbI2 to PVK, the thickness of the top layer increases by a factor of 1.1 (Figure 1d), consistent with the expected change in lattice constant values. These results demonstrate that the accurately controlled thickness of PbI2 results in desired converted PVK with well-defined thicknesses. The morphology of the heterostructures ((CH3NH3)n+1PbnI3n+1/WS2) was probed by AFM measurements, which show surfaces with atomic level smoothness, indicating the high uniformity. Furthermore, this method is also generally applicable to fabricate other ultrathin PVK/TMDC heterostructures with MoS2, MoSe2, and WSe2 (Figure S5). The resulting PVK/WS2 heterostructures were encapsulated by PDMS to prevent environmental degradation in subsequent measurements (Figure S6). The crystal quality of the achieved PVK/WS2 heterostructures was further characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. XRD (Figure 2a) illustrates the structural transformation from PbI2/ WS2 to PVK/WS2. For PbI2/WS2 heterostructures, a set of sharp XRD peaks for the (001) family of PbI2 planes (space group P3m1) are observed.36 While after 1 h of the intercalation reaction, additional diffraction peaks corresponding to tetragonal (CH3NH3)n+1PbnI3n+1 were observed.36 The diffraction peaks of PbI2 disappeared after 4 h of reaction, indicating complete conversion of PbI2 into (CH3NH3)n+1PbnI3n+1.36 Inplane structural information was characterized by highC

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Figure 3. Photophysical characterizations of the ultrathin PVK/WS2 vertical heterostructures. (a) PL spectra of the various 2D PVK/WS2 heterostructures with different thicknesses. (b) Calculated band structures and band alignments of the achieved heterostructures. (c) Pump− probe results of the heterostructures with different PVK thickness. (d,f) Zoomed-in transient absorptance signals at 0−200 ps range. (e,g) Schematic illustration of the corresponding band alignments.

type-II (n < 9) (Figure 3b). Such band alignment transition can also be confirmed by the different degrees of the PL quenching of WS2 emission in heterostructures with different PVK thicknesses (see the inset of Figure 3a), since the 2.7 nm thick corresponds to type II and the 12.2 nm thick corresponds to type I. The change of heterostructure band alignment with thickness has crucial effect on the interfacial charge transfer process. To investigate this, we conducted ultrafast pump−probe measurements of the WS2 optical band for the heterostructures with TPVK = 0 (pristine WS2), 2.7, 4.3, 5.2, and 12.2 nm (Figure 3c). In pristine WS2, the transient absorption signal decays with typically three lifetime components42 and the average decay lifetime is around 406 ps. In contrast, for the heterostructures with thin PVKs (2.7, 4.3, and 5.2 nm), the transient absorption signals show an additional rise-time component in addition to the decay components (Figure 3d). This rising-up behavior reflects the interlayer charge transfer characteristics in the heterostructures, where the electrons can spontaneously transfer from PVK to WS2 (Figure 3e). Furthermore, as the TPVK increases from 2.7 to 5.2 nm, the rise-time component slows down from 27.11 to 67.55 ps. This phenomenon is in good agreement with the band structure offset calculations, that is, the larger band offset in thinner PVK heterostructures results in the faster interlayer charge transfer rate thus shorter rise-time in the dynamics.43 The shorter diffusion distance of the carriers to the interface in thinner PVK heterostructures may also contribute to the shorter rise-time. The heterostructure with a relatively thicker PVK, TPVK = 12.2 nm, the decay signal exhibits behavior similar to pristine WS2, but exhibiting shorter decay time (Figure 3f). This can be understood within the type-I band alignment picture, in which both electrons and holes transfer from WS2 to

light-matter interactions, realizing high-quality interface and constructing high performance integrated optoelectronics. To investigate the photophysical properties of the achieved PVK/WS2 heterostructures, we performed steady-state photoluminescence (PL) and time-resolved pump−probe spectroscopic measurements. The PVK top layer exhibits a thickness dependent strong quantum confinement effect, which enables the precise engineering of the band structures and the resulting interfacial charge dynamics in heterostructures. Figure 3a shows the intensity-normalized steady-state PL spectra of the PVK/ WS2 heterostructures with different PVK thickness (TPVK) values. Besides the WS2 emission band (∼638 nm), additional emission peaks centered at 691, 712, 724, 738, 746, and 751 nm can be observed from the heterostructures with TPVK of 2.0, 2.7, 3.3, 4.3, 5.2, and 12.2 nm (corresponding to 3, 4, 5, 6, 7, and 19 PVK unit-cells), respectively. The emission at lower energy relative to the WS2 emission are assigned to the band-to-band recombination of photoexcited electrons and holes in the top PVK layer.32 For heterostructures with very thick PVK (e.g., TPVK = 36.2 nm), only one peak at 764 nm was observed, which corresponds to the emission of bulk PVK32 (Figure S9). The clear blue-shift of the PL emission peak with decreasing thickness of PVK demonstrates strong quantum confinement effect in the top PVK layer. The quantum confinement-induced band gap opening, seen by the blue-shifted PL, was reproduced by our first-principles calculations (Figures S10−S11). By calculating the band structure offsets between composed PVK and WS2 layers in the heterostructures, we strikingly found that as the thickness of the (CH3NH3)n+1PbnI3n+1 layer decreases, both the conduction band minima (CBM) and the valence band maxima (VBM) positions of PVK increase. As a result, the band alignments of the heterostructures tuned from type-I (n ≥ 9) to D

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Figure 4. Charge transport and optoelectronic device characterizations of the ultrathin PVK/WS2 vertical heterostructures. (a) Schematic illustration of the three-terminal back-gate device based on PVK/WS2 heterostructures. (b) Transfer characteristic curves, (c) net photocurrent, and (d) photoresponsivity as a function of illumination power intensities obtained from the PVK/WS2 samples with TPVK = 0, 2.7, 5.2, 7.1, and 36.2 nm. (e) Thickness dependent photoresponsivity. (f) Detailed transient rise and decay times of the photodetectors with different thickness. (g) Thickness dependent evolution of rise time and decay time.

clearly confirm the formation of p−n junctions in the vertical direction of the PVK/WS2 heterostructures. Significantly, the charge transport characteristics of these heterostructures can be systematically tuned by tailoring the thickness of PVK, from pure n-type, to n-type dominated ambipolar, p-type dominated ambipolar, and pure p-type nature, which has never been reported in previous 2D vertical heterostructure systems. We then examined the photosensing behaviors of our heterostructures with varying thicknesses of PVK. Figures 4c and 4d display the net photocurrent (Iph) and photoresponsivity (R) of the photodetector devices under different laser power densities, respectively. The photodetector device based on pristine WS2 monolayer has a low Iph value of ∼4 pA under a 4 pW incident power illumination, consequently yielding a small R value of 0.97 A/W. This poor photoresponse is primarily due to the weak absorption of the atomically thin WS2 monolayer. In addition, defects or charge impurities in WS2 monolayer can result in a slow photoresponse time (τr: 33 ms, τd: 50 ms, see Figure 4f).45 Significantly enhanced light harvesting is achieved in the PVK/WS2 heterostructures, which we attribute to the superior light absorption of PVK. Moreover, in the heterostructure-based photodetectors, the photoinduced electrons and holes can be efficiently separated at different sides of p−n junction, where the ultrathin thickness of the whole heterostructures ensures that the charge carriers can be efficiently

PVK, giving rise to the observed shorter carrier decay time (Figure 3g). Benefiting from the efficient interfacial charge carrier separation as well as the shorter diffusion distance in the thinner heterostructures with type-II band alignment, these ultrathin heterostructures are excellent candidates for use in high-performance optoelectronics. In order to explore the electric transport properties of our heterostructures, we fabricated back-gated field-effect transistors (FETs) based on PVK/WS2 heterostructures (see Figure 4a) using a metal electrode transferring method44 (see Figure S12 for details). Here both the source and drain electrodes are in direct contact with the top PVK layer exclusively and are not connected with the bottom WS2. Figure 4b presents the transfer characteristics of heterostructures with different TPVK values. In the control material of pristine WS2 (i.e., TPVK = 0 nm), a typical n-type behavior is observed. The heterostructure with 2.7 nm PVK on top of the WS2 exhibits a slight ambipolar behavior, where the p-type conduction can be reasonably attributed to the contribution of p-type PVK. When the thickness of the top PVK layer is increased to 5.2 nm, the device exhibits a more obvious ambipolar behavior with pronounced p-type conduction and subdued n-type conduction. For the device with a very thick PVK layer (e.g., 36.2 nm), pure p-type behavior originated from the bulk PVK is observed, with the contribution of the bottom ntype WS2 obscured (see Figure S13 for details). These results E

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substrate was placed at the downstream of a tube. Prior to heating, 400 SCCM Ar flow was introduced into the system for 15 min to purge the air in the tube, which ensured a favorable circumstance for the chemical reaction. The furnace was then rapidly heated to 1065 °C and held for 4 min for the growth, under a flow of 150 SCCM Ar carrier gas. The pressure inside the tube was maintained at a normal level during the growth process, and then furnace was slowly cooled to room temperature. Van der Waals Epitaxy Growth of PbI2 on Monolayer WS2. The PbI2 powder was placed at the center of a furnace. A piece of preprepared 1L-WS2/SiO2/Si was placed at the downstream of a tube as the growth substrate. Prior to heating, 400 SCCM Ar flow was introduced into the system for 15 min to purge the air in the tube. The furnace was then rapidly heated to 400 °C in 10 min, and the temperature was held for 10 min during the growth process. During the whole growth process, a flow of 30 SCCM Ar was introduced as the carrier gas. Similar to the previous step, the pressure inside the tube was maintained at a normal level during the growth, and the furnace was then slowly cooled to room temperature. Vapor-Phase Conversion of PbI2 to (CH3NH3)n+1PbnI3n+1. The CH3NH3I powder was placed at the center of a furnace. A piece of preprepared PbI2/WS2/SiO2/Si was placed at the center of a tube as the growth substrate. Prior to heating, 400 SCCM Ar flow was introduced into the system for 15 min to purge the air in the tube, which ensured a favorable environment for the chemical reaction. The furnace was then rapidly heated to 120 °C in 25 min, and the temperature was held for 4 h during the growth under a flow of 30 SCCM Ar carrier gas. During this growth process, the pressure inside the tube was maintained at 8 Torr, and then the furnace was slowly cooled to room temperature. Steady-State PL and Time-Resolved Pump−Probe Measurements. The morphologies of the PVK/WS2 vertical heterostructures were characterized using optical microscope (Zeiss Axio Scope A1) and Atomic Force Microscope (Bruker Multimode 8). All optical measurements were conducted by using a confocal microscope (WITec). A CW laser at 488 nm was employed as the excitation source for the PL measurements, and the PL signals were collected by a 50× objective and detected by a CCD spectrograph (alpha-300). Femtosecond pump−probe microscopy measurements were performed at room temperature with a sapphire laser/amplifier system (Spitfire-ace, Spectra-Physics, 800 nm wavelength, 120 fs pulse width, 250 Hz repetition rate). The PVK/WS2 heterostructures were encapsulated by PDMS after growth for optical characterization. First-Principles Calculations. First-principles density functional theory (DFT) calculations were performed by using plane-wave pseudopotential methods as implemented in the Vienna ab initio Simulation Package.46,47 The electron−ion interactions were described by the projected augmented wave pseudopotentials48 with the 1s (H), 2s and 2p (C), 2s and 2p (N), 3s and 3p (S), 5s and 5p (I), 5d and 6s (W), and 5d, 6s, and 6p (Pb) electrons treated explicitly as valence electrons. We used the generalized gradient approximation formulated by Perdew, Burke, and Ernzerhof49 as the exchange correlation functional. Kinetic energy cutoff for the plane-wave basis set was set to 400 eV, and the k-point meshes with grid spacing of 2π × 0.03 Å−1 were used for electronic Brillouin zone integration. The halide PVK layers with different thicknesses were simulated by using the (001)-oriented Ruddlesden−Popper phase of MAn+1PbnI3n+1 with different n. Both PVK layers and monolayer WS2 were embedded in a vacuum region of 30 Å. The structures were optimized through total energy minimization with the residual forces on the atoms converged to below 0.02 eV/Å. To properly take into account the long-range van der Waals interactions that play a nonignorable role in the hybrid PVKs involving organic molecules, the vdW-optB86b functional50 was adopted. The alignment of the band structures is made by using the vacuum energy as reference. To remedy the bandgap estimation problem in standard DFT calculations and take into account the exitonic effect, we adopt the PL emission peaks as the actual band gap values. Fabrication and Measurements of the PVK/WS2 Devices. The metal contact electrode for the devices were fabricated following a metal transferring method. Briefly, after patterning the device channels by a standard photolithography process, a 50 nm-thick layer of Au film

collected by the electrodes. As a result, the heterostructure photodetector with TPVK = 2.7 nm exhibits a significantly larger Iph of 11.7 nA and R of 2929.7 A/W, as well as an ultrafast rise time of 64 μs and a decay time of 102 μs. To the best of our knowledge, such photoresponse is nearly 3 orders of magnitude faster than that of the photodetectors based on pristine WS2 (Figure S14) and ultrathin CH3NH3PbI3,32 and is comparable to all 2D layered semiconductor heterostructures.10−24 As the TPVK in the heterostructure increases to 5.2 nm, the light absorption is further increased, and the Iph reaches a peak value of 44.7 nA, which is nearly 4 orders of magnitude higher than that of pristine WS2 device. In this device, we also achieved the highest R value of 11174.2 A/W, which is significantly higher than those of either components (WS2 or 2D PVK32), and represents the best performance among all 2D layered semiconductor heterostructures.10−24 On the other hand, the larger PVK thickness also brings in increased charge carrier transport time (confirmed by time-resolved pump−probe characterizations), and therefore a slower photoresponse speed is obtained (τr: 0.18 ms, τd: 0.34 ms). When TPVK further increases to 7.1 and 36.2 nm, the photoresponsivity monotonously decreases and the response time increases. Such deterioration of device performance can be explained by lower charge separation efficiency at the junction region and longer carrier transport distance in the thicker heterostructure systems (details in Figure S19). Moreover, the TPVK-dependent photoresponse behaviors (Figure 4e and Figure 4g) are also consistent with the band alignment arguments (Figure 3c). Heterostructures with thinner PVK (2.7 and 5.2 nm) possess type-II band alignment, which contributes to the higher photoresponsivity and faster photoresponse speed relative to type-I alignment structures.

CONCLUSIONS In summary, ultrathin PVK/TMDC vertical semiconductor heterostructures have been designed and realized through a controllable fully vapor-phase growth strategy. By engineering the thickness of quantum-confined PVK layer of the heterostructures, the band alignments of the heterointerface and subsequent interfacial charge transfer behaviors can be readily tailored. Field-effect transistors based on the as-grown ultrathin PVK/WS2 heterostructures show novel PVK thickness-dependent charge transport behaviors, involving pure ntype, ambipolar, and pure p-type transport. Furthermore, the ultrathin p−n junctions demonstrate a type-II band alignment, which enhances the photoexcited charge separation as well as the photosensing performance relative to heterostructures with a thick PVK layer. As a result, a high photoresponsivity of 11174.2 A/W and an ultrafast photoresponse time of 64 μs have been demonstrated based on photodetectors constructed with the ultrathin PVK/WS2 heterostructures, which are nearly 4 orders of magnitude better than those of thick devices. These results also represent the best performance among all previously reported 2D layered semiconductor heterostructures. This growth method is also universally applicable to the growth of PVK on other 2D layered semiconductors. The controlled growth of such heterostructures utilizing this method will offer an avenue toward high-performance integrated optoelectronic devices and systems. METHODS Vapor-Phase Growth of Monolayer WS2. Tungsten disulfide powder was placed at the center of a furnace. A piece of SiO2/Si F

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ACS Nano was deposited on silicon substrate by the electron beam evaporation method. Subsequently, the prefabricated patterned metal electrodes were functionalized in a hexamethyldisilazane (HMDS) steam for 10 min, followed by the spin-coating of PMMA. Then the metal electrodes were released and transferred to the target PVK/WS2 heterostructures. The electrical and optoelectronic properties of the heterostructures were measured in vacuum (∼10−4 mbar) with a Lake Shore Probe Station and an Agilent B1500A semiconductor analyzer at room temperature. The time response of the device was measured by switching the 520 nm laser on and off with an internal square-wave trigger source (Thorlabs ITC 4001) and recorded by a digital oscilloscope (Tektronix MDO 3014). During the measurement, a 520 nm laser was employed as the illumination source. For the timeresolved photoresponse test, a pulsed laser (520 nm, 67.14 mW cm−2) triggered by a square-wave (f = 0.5 Hz) was used as the input, and a high-speed oscilloscope was used to monitor the output signal; here, the photoresponse time is defined as the total time required for the output to rise from 10% to 90% (rise time, τr) and to fall from 90% to 10% (decay time, τd) of the pulse.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02676. Growth details, band structure calculations and device characterization results (PDF) Video S1 (AVI) Video S2 (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiao Wang: 0000-0002-2973-8215 Chao Ma: 0000-0001-8599-9340 Matthew P. Hautzinger: 0000-0002-4764-3076 Bin Yang: 0000-0002-5667-9126 Dong Li: 0000-0003-0391-7060 Lijun Zhang: 0000-0002-6438-5486 Song Jin: 0000-0001-8693-7010 Anlian Pan: 0000-0003-3335-3067 Author Contributions #

T. Yang, X. Wang, B. Zheng, and Z. Qi contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the NSF of China (Nos. 51525202, 51772084, 61574054, 91850116, 61505051, and 61635001), Innovation platform and talent plan of Hunan Province (2017RS3027), the Program for Youth Leading Talent and Science and Technology Innovation of Ministry of Science and Technology of China. Y.F., M.P.H. and S.J. thank the support by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award DE-FG02-09ER46664. REFERENCES (1) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and van der Waals Heterostructures. Science 2016, 353, aac9439. G

DOI: 10.1021/acsnano.9b02676 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.9b02676 ACS Nano XXXX, XXX, XXX−XXX