Polar-induced Selective Epitaxial Growth of Multi-junction

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Functional Inorganic Materials and Devices

Polar-induced Selective Epitaxial Growth of Multi-junction Nanoribbons for High-performance Optoelectronics Huawei Liu, Ying Jiang, Peng Fan, Yexin Feng, Jianyue Lan, Gengzhao Xu, Xiaoli Zhu, Xuehong Zhang, Xuelu Hu, Tiefeng Yang, Bin Yang, Qinglin Zhang, Dong Li, Xiao Wang, and Anlian Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04470 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Polar-induced Selective Epitaxial Growth of Multi-junction Nanoribbons for High-performance Optoelectronics Huawei Liu†||, Ying Jiang†||, Peng Fan†||, Yexin Feng†, Jianyue Lan§, Gengzhao Xu§, Xiaoli Zhu* †， Xuehong Zhang†, Xuelu Hu†, Tiefeng Yang†, Bin Yang‡, Qinglin Zhang†, Dong Li‡, Xiao Wang†, and Anlian Pan*†‡ †

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key

Laboratory of Chemo/Biosensing and Chemometrics, School of Physics and Electronics, Hunan University, Changsha, Hunan 410082, China ‡College

of Materials Science and Engineering, Hunan University, Changsha, Hunan

410082,China §Suzhou

Institute of Nano-tech and Nano-Bionics, Chinese Academy of Sciences Suzhou

215123, People’s Republic of China

ACS Paragon Plus Environment

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KEYWORDS: polar-induced, multi-junction, epitaxial growth, charge transfer, photodetector ABSTRACT Semiconductor heterostructures are basic building blocks for modern electronics and optoelectronics. However, it still remains a great challenge to combine different semiconductor materials in single nanostructures with tailored geometry and chemical composition. Here, a polar-induced selective epitaxial growth method is reported to alternately grow CdS and CdSxSe1-x heterostructure nanoribbons (NRs) side by side in the lateral direction, with the heterointerface (junction) number to be well controlled. Transmission electron microscopy (TEM) and spatial-resolved µ-PL spectra are employed to characterize the heterostructure NRs, which indicate that the achieved NRs are high quality heterostructures with sharp interfaces. Kelvin probe force microscopy (KPFM) and femtosecond pump-probe characterizations further confirm the efficient charge transfer process across the interfaces in the multi-junction NRs. Photodetectors based on the achieved NRs are realized and systematically investigated, demonstrating junction-number-dependent optoelectronic response behaviors. NRs with more junctions exhibit more superior device performances, reflecting the important roles of the highquality interface regions. Based on this multi-junction NRs device, high on-off ratio (107) and remarkable responsivity (1.5×105 A/W) are demonstrated, both of which represent the best compared to the ever reported CdS, CdSe and their heterostructures. These novel multi-junction NRs may find broad applications in future integrated photonics and optoelectronics devices and systems.


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INTRODUCTION Low dimensional semiconductor heterostructures have attracted significant attention in the past several years due to their intriguing physical properties, enabling diverse applications in photonics, electronics and optoelectronics.1-15 A semiconducting heterostructure is a junction made up of two dissimilar semiconducting materials. When the two semiconductors contact with each other, charge redistribution often takes place at the interface due to the difference of the Fermi levels, resulting in energy-band bending and an electrical field created across the interface. Such electrical field at the interface can effectively assist the separation of the photo-induced charge

carriers,

leading

to

excellent

photoelectric

properties.7,10,16-19

Theoretically,

heterostructures and superlattices with two or more junctions should possess more superior optoelectronic properties, owning to that the multiple interfaces can facilitate the charge separation efficiency and promote the photoelectric response.20-22 Vapor grown II-VI semiconductor nanowires and nanoribbons, like CdS, CdSe and their alloys, owning to excellent optical and optoelectrical properties, can act as high quality nanoscale lasers, waveguides and optoelectronic devices.23-27 However, due to the poor controllability of the conventional vapor growth approaches, the growth of high quality II-VI semiconductor heterostructures is still a great challenge. Herein, we developed a polar-induced temperature-modulated source switching vapor growth strategy, and thus demonstrated, for the first, the controllable growth of lateral CdS-CdSxSe1-x multi-junction NRs. NRs with continuous junction numbers can be achieved through the continuously repeatable growth. The interfaces


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along the junctions were sharp and high quality, confirmed by both the transmission electron microscopy and the spatial-resolved µ-PL measurements. Kelvin probe force microscopy (KPFM) and femtosecond pump-probe characterizations further confirmed the existence of efficient charge transfer process at the interfaces of the multi-junction NRs, which contributes dramatically to the high photodetecting performance with remarkable responsivity up to ~1.5×105 A/W and high Ion/Ioff ratio up to ~107, indicating that the as-grown multi-junction NRs are potential candidates for further photonics and optoelectronics.

RESULTS AND DISCUSSION

Figure 1a schematically illustrates the growth of the lateral CdS-CdSxSe1-x multi-junction NRs. Before growth, we firstly calculated the binding energies of CdxSx clusters (x=1~5) adsorbed on the Cd2+-terminated (0001) surface and S2--terminated (000-1) surface, respectively. The results are shown in Figure 1b. It could be found that the binding energy of the Cd2+terminated (0001) surface to absorb foreign clusters is about 0.5 eV lower than that of the S2-terminated (000-1) surface. This energy difference indicates that Cd2+-terminated (0001) surface and S2--terminated (000-1) surface have different polarities and selective epitaxial growth can be realized through temperature control.28-33 Figure S1 shows the detailed growth process of the multi-junction NRs, where CdS NRs were grown on a Si substrate in the first step, followed by epitaxial growth of CdSxSe1-x on the side surfaces of the CdS ribbons. Due to the different binding energy of the side surfaces depicted above, the lateral epitaxial of CdSxSe1-x exhibits a


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temperature-dependent selective behavior. At high temperature, CdSxSe1-x with high energy may overcome the chemically inert of Se2- terminated (000-1) surface and then epitaxial growth along both sides of CdS nanobelts to form CdSxSe1-x-CdS-CdSxSe1-x (3L, L is layer or segment along the lateral direction of the NRs) heterostructures. While at low temperature, the CdSxSe1-x with low energy preferentially epitaxial growth along the active Cd2+ terminated (0001) surface to form CdS-CdSxSe1-x (2L) heterostructures. When the growth time of CdSxSe1-x further reduced, the as-grown sample is the saw-shaped ribbon (Figure S3), with one side flat and the other side with sharp teeth. EDS results conformed that the flat side is CdS part, while the saw-shaped side is CdSxSe1-x segment. In experiment, the sharp teeth side have already been conformed that is chemical active side with Cd2+-terminated (0001) surface.32 With a similar temperaturedependent growth process, CdS epitaxial growth along the edges of the as-grown CdSxSe1-x layers can also be realized, resulting in the formation of CdSxSe1-x-CdS-CdSxSe1-x-CdS (4L) heterostructure

and

CdS-CdSxSe1-x-CdS-CdSxSe1-x-CdS

(5L)

heterostructure.

Similarly,

heterostructures or superlattices with more junctions can further be obtained through repeating the above growth processes. Figure 1c shows the real-color photographs of the representative NRs we achieved, with the layer number from 1 (pure CdS, zero junction) in turn increased to 7 (six junctions), taken with an optical microscope under a broad laser illumination of 405 nm. Based on the emission color contrast from the photographs, the multi-junction NRs exhibit fine periodic structures, with green color and red color alternatively presented. The well-defined


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interfaces indicate an abrupt change of the bandgap/composition between different layers of the multi-junction NRs. Transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) were further used to characterize the microstructure and composition of the obtained multi-junction NRs. Figure 2a shows a typical TEM image of an as-grown 5L NRs with a uniform width of ~8 μm and a thickness of 62.7 nm, and the corresponding SEM image is shown in Figure S2. Figure 2b displays a distribution map of elements Cd, S, and Se from the selected region marked with red rectangle in Figure 2a. It is evident that Cd element uniformly covers the entire region, while S element is mainly distributed in the center and two outermost sides, and Se element completely spreads between the center and edge part. The results indicate that these NRs are well-defined CdS-CdSxSe1-x-CdS-CdSxSe1-x-CdS multi-junction structures. EDX result as shown in Figure S6 also confirms the 5L lateral CdS-CdSxSe1-x multi-junction structures, which is in good agreement with the observations in the real-color photograph and the TEM elemental mapping. The high resolution TEM (HRTEM) was used to characterize the interfaces. As shown in Figure 2c, the red dashed arrow indicates the interface reign and the dspacings as suggested by parallel lines are 0.675 nm and 0.685 nm, which correspond to the distance of the (0001) plane of wurtzite CdS and CdSxSe1-x, respectively. Thus, S mole fraction x of the wurtzite CdSxSe1-x should be 0.49.26,34 These results clearly suggest that we have successfully achieved the lateral CdS-CdS0.49Se0.51 multi-junction NRs with the interface region less than 4 nm.


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The spatial-resolved µ-PL of the lateral multi-junction NRs was executed using Confocal Laser Scanning Microscopy System (WITec, alpha-300). Figure 2d gives the real color photograph of some dispersed 5L NRs under laser illumination (405 nm), which shows that all these NRs have fine multi-junction structures along their width direction. Figure 2f displays a two dimensional PL mapping of the selected 5L multi-junction NR as shown in Figure 2e, in which two different emission segments can be clearly observed. Figures 2g and 2h give the wavelength-selected PL emission mapping extracted from Figure 2e in the spectral regions of 513-516 nm (green) and 605-608 nm (red), respectively, which clearly show that the two different color emissions are well spatial separated and distributed in different regions along the width direction of the NR and the PL spectra collected from different position of the lateral NR have shown in Figure S4. PL line-scanning profiles along the lateral direction for 515 nm and 607 nm emission are shown in Figure 2i, in which sharp emission transition can be clearly observed. Figures 2j and 2k display the extracted PL spectra from the two different emission regions, with the peak wavelength centered at 515 nm and 607 nm, respectively, showing good agreements with the band edge emission of CdS (Eg=2.41eV) and CdS0.49Se0.51 (Eg=2.04 eV), respectively (Figure S5).35 The PL results further well demonstrate the formation of the lateral multiple junctions in the NRs, which will help to enable the interfacial charge transfer between different material regions with different band alignments along the width direction of the multijunction NR.


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To further verify the charge transfer process at the interfaces of the multi-junction NR, KPFM was employed to quantitatively analyze the changes in surface potential to reflect the work function information across the junction.36-40 The surface potential difference (SPD) between the probe tip and the sample can be acquired using the KPFM measurements, which can be written as SPD （Wtip  Wsample）/ e (Wtip and Wsample represent the work function of the probe tip and the sample, respectively).35,40,41 Therefore, the SPD difference between the CdS and CdS0.49Se0.51 (△SPD) can be defined as:

SPD  SPDCdS  SPDCdS0.49 Se0.51  ( WCdS 0.49 Se0.51  WCdS ) / e The KPFM surface potential profiles of the selected region (marked with a white rectangle in Figure 3a) in the dark and under illumination (488 nm) are shown in Figure 3b and Figure 3c, respectively. Clearly, the △SPD between the CdS and the CdS0.49Se0.51 is 40 mV in the dark, while under illumination (488 nm), the △SPD increase to 68 mV. Such obvious change of △SPD between dark and illumination conditions can be attributed to the charge transfer across the junctions and the redistribution of the charges in the multi-junction NR. 42 From the KPFM result (in the dark), a Fermi level difference of 40 meV between the CdS and the CdS0.49Se0.51 can be deduced, which can be expressed by the flat band diagram as shown in Figure 3d (left), in which the bandgap information is achieved from the PL spectra shown in Figure 2. While forming the heterojunction, charge transfer would occur at the interface (the electrons flow from CdS (higher level) to CdSxSe1-x (lower level), which will result in the Fermi level of CdSxSe1-x goes up until it


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is equal to the CdS), leading to energy-band bending and an electrical field created in the lateral direction as shown in Figure 3d (middle). Upon light illumination, the photo-induced electron−hole pairs are generated in both CdS and CdS0.49Se0.51, which can further be separated efficiently by the as-produced built-in electric field with electrons to the CdS side and holes to the CdS0.49Se0.51 side, leading to the shift of the Fermi-level (right in Figure 3d) in the two different part and the variation of the △SPD observed in Figure 3c. We further investigated the charge-transfer dynamics in CdS-CdS0.49Se0.51 multi-junction NR by a femtosecond pump-probe microscopy.44,45 Pump laser (400 nm) with power density of 50 μJ/cm2 was used to excite the NR, and the transient absorption (TA) spectra are depicted in Figure 4a. Two peaks can be observed at 500 nm and 586 nm, which stand for the ground bleaching (GB) signals of CdS and CdS0.49Se0.51, respectively (PL spectra of the NR and the GB of pure CdS as shown in Figure S7). After 0 ps, the GB of CdS (~500 nm) first decayed rapidly and then relaxed relatively slower until recovered completely, while that of CdS0.49Se0.51 (~586 nm) showed a competition between rise and decay for about 40 ps before it decayed for recovery. Accordingly, the logarithmic dynamics at both 500 nm (CdS) and 586 nm (CdS0.49Se0.51) are nonlinear (Figure 4c) and especially during the early tens of ps, the dynamics at 500 nm decayed quickly while that at 586 nm increased simultaneously (Figure 4d). Both the transient absorption spectra and the non-monoexponential dynamics for the two compositions show that instead of just single recombination process in CdS and CdS0.49Se0.51, respectively, the


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carriers of the NR should have experienced a more complicated process after the excitation. A global fitting was further done (Figure 4b) and the result demonstrates that there are three lifetime components (~1.67 ps, ~102.85 ps, and ~436.09 ps) in the NR.46 The two longer lifetime components with positive peaks can be assigned to the recombination processes of the excited carriers in CdS (green, ~102.85 ps) and CdS0.49Se0.51 (red, ~436.09 ps), respectively. It is worth noting that the shortest lifetime component (blue, ~1.67 ps) shows a positive peak at 500 nm (representing the decay of the GB of CdS) and a negative peak at ~575 nm (representing the increase of the GB of CdS0.49Se0.51), which indicates that there exists charge-transfer process between CdS and CdS0.49Se0.51 during this short time scale. Combining with the result of the surface potential test, it is in fact that the hole transfer from CdS to CdS0.49Se0.51 (electrons from CdS0.49Se0.51 to CdS) results in the decay of the GB of CdS while the increase of that of CdS0.49Se0.51 at the same time. Therefore, the TA results also prove the charge-transfer in the NR and this process is ultrafast occurring at ~1.67 ps. The efficient charge transfer process occurred at the interfaces can readily enable us to develop high performance photodetectors. Figure 5a schematically illustrates the configuration of the device, where electrodes are in contact with the two ends of the multi-junction NR, and Figure 5b shows a typical as-fabricated device based on a 5L NR. For comparison, photodetectors based on different structure NRs from single CdS to 5L heterostructure NRs were constructed simultaneously. The sizes of these NRs are kept almost the same with each other


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(length: 25 μm; width: 6 μm; thickness: 65 nm) for reasonable comparisons (Figure S9). Figure 5c systematically investigates the photoelectric properties of the NR-based devices with different junction numbers in the channel (light wavelength: 510 nm; power density: 2.82 mW/cm2). It is clear that the photocurrent increases with the increase of junction numbers from 0 to 4, and the 5L NR-based photodetector demonstrates the highest photocurrent response under similar conditions. Figure 5d plots the extracted photocurrent (Iph) at bias voltage of 1 V. Under the illumination power density of 2.82 mW/cm2, the 5L NR-based device shows an ultrahigh photocurrent up to ~12 μA, which is approximately 20 times higher than that of the pure CdS device (0.6 μA). These excellent properties are mainly attributed to built-in electric field produced at the multi-junction interface. When the heterostructure nanoribbon is excited by light with energy higher than its bandgap, the photo-generated electron-hole pairs will produced and separated by the built-in electric field (proved by the KPFM results), leading to the distribution of electrons in CdS parts and holes in CdSxSe1-x segments. Driven by the external electric field, the holes and electrons will be collected by the opposite electrodes, leading to the excellent photoresponse. Due to that the 5L NR has more junction interfaces than others, so its lightharvesting efficiency is higher and generating higher photocurrent under the same measurement conditions. Figure S8 shows the corresponding responsivity comparison, indicating that NRs with more junctions are more suitable for photodetection. More devices were fabricated to verify the trend as shown in Figure S9.


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We further deeply investigated the photoelectric properties of the 5L NR-based photodetectors. Figure 5e shows the wavelength dependent current-voltage (I-V) curves of the device with the highest photoresponse observed at 510 nm. Figure 5f demonstrates the photoresponse of the device under different illumination intensities with the wavelength of 510 nm. As we can see, the photocurrent gradually increases with the increasing of the illumination intensity. At the light power of 2.82 mW/cm2 and the bias voltage of 1 V, photocurrent of 12 μA can be observed in the device which is about seven orders higher than that in the dark, indicating a high Ion/Ioff ratio (107). Figure 5g presents the measured photoresponsivity versus the incident light power. With the light power decreasing, the responsivity of the device increases very rapidly. At the light power of 2.82 mW/cm2, the responsivity of the device is about 8.5×102 A/W; while the light power decreases to 0.08 mW/cm2, the responsivity increases up to 6×103 A/W, indicating that the device should be especially suitable for ultrasensitive photodetectors to weak light. The performance of the device can be further enhanced by applying a gate voltage (Figure S10). At the gate voltage of 30 V, a higher responsivity of 1.5×105 A/W can be achieved, which is orders of magnitude higher than that of CdS, CdSe and their alloys (Table S1).27,47-50 When a positive gate voltage is applied on the device, the contact barrier between electrode and the channel material will become lower, leading to the enhancement of current extraction by the applied bias voltage (Figure S11). Figure 5h shows the photocurrent response of the device to the light with different power for five cycles at the bias voltage of 1 V, clearly indicating that the photocurrent is generated when the light is turned on, and rapidly disappears when the light is


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turned off. The generated photocurrent increases with the power increasing. Response time is an important parameter for photodetectors. Photodetectors with fast response time indicate that they can response quickly and efficiently to the light irradiation. Figure 5i shows the photoresponse time of the device. Rise time of 750 µs and decay time of 3.4 ms are achieved in the device, indicating an excellent photodetection performance. CONCLUSIONS In conclusion, we have thus developed a temperature-modulated source switching vapor growth method to controllably grow lateral NRs with different junction numbers. TEM and spatial-resolved µ-PL spectra measurements indicate that the achieved NRs are high quality heterostructures with very sharp interfaces. In such NRs, efficient charge transfer process was observed across the interfaces, which dramatically contributes to the performance of the NRbased photodetectors. NRs with more junction numbers exhibit more superior photoelectric performances. With the junction number up to four (5L NRs), remarkable responsivity up to 1.5×105 A/W, fast response time (raise time 750 μs, decay time 3.4 ms) and high Ion/Ioff ratio (107) were demonstrated. This study may provide a practical way to produce multi-junction NRs and facilitate the applications of the NRs in high performance photonics and optoelectronics.

METHODS


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Materials Synthesis: The multi-junction NRs were synthesized by a source switching vapor growth strategy (schematically illustrated in Figure S1), where a tube furnace (Kejing Materials Technology CO, LTD, OTF-1200X) with quartz tube (210 cm length, 45 mm inner diameter and 50 mm outer diameter) was used. An alumina boat with CdS powder (99.999%, Alfa Aesar) was transferred into the quartz tube and located at the center of the furnace. Before growth, two other boats with CdSe/CdS mixed powder and CdS powder were located at the upstream out of the heating zone. The boats were separated by quartz rods, and a step motor was used to control the position of these boats in the tube through the magnetic force system. Several silicon wafers with pre-sputtered Au films (10 nm thickness) were placed downstream of the gas flow, 15 cm far away from the furnace center. Before heating, the system was flushed by high pure nitrogen for 40 mins at a rate of 150 sccm. Then the furnace center temperature was ramped to 840°C with a rate of 14°C/min, with the pressure of 300 mbar. CdS NRs were achieved after 50 mins (step1 in Figure S1), followed by CdSxSe1-x epitaxial growth along the edges of the CdS NR. At this stage, the temperature was descend to 740°C with a rate of 25°C/min and then pushing the alumina boat with the mixed powder (CdS and CdSe) into the center of the furnace quickly to replace the CdS boat (step2 in Figure S1). After 60 mins, unilateral epitaxial growth of the CdS-CdS0.49Se0.51 NRs (2L) can be achieved. In this step, if the growth temperature was set at 780°C, the CdS0.49Se0.51-CdS-CdS0.49Se0.51 (3L) NRs can be achieved. After that, the furnace temperature was rapidly increased to 800°C (/840°C) and then pushed the CdS boat into the center of the heating zone (step3 in Figure S1), the two other boats would be pushed into the downstream out


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of the furnace at the same time. After 60 mins CdS can epitaxial along one side (/both sides) of the 3L NRs, and 4L NRs (/5L NRs) can be achieved. Characterizations: Confocal μ-PL system (WITec, alpha-300), SEM (ZEISS. Sigma HD) and TEM (Tecnai G2 F20 S-TWIN) equipped with an EDX were employed to characterize the asgrown multi-junction NRs. The KPFM measurements were performed with a Bruker Dimension ICON AFM with Pt/Ir-coated tips (ACCESS EFM). 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). Device Fabrication and Measurements: The as-grown multi-junction NRs were transferred onto the pre-cleaned highly doped p-type silicon substrates with a thermally grown of 300 nm thick SiO2 layer. Then the substrates were spin-coated with PMMA, and the electron-beam Lithography (EBL, Raith 150 Two) was applied to define the source and drain patterns. Ti/Au (15 nm/60 nm) electrodes were completed by metal evaporation and lift-off processes. Current– voltage (I–V) characteristics of the multi-junction NRs-based photodetectors were measured using a Keithley 4200.


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Figure 1. Fabrication of the lateral multi-junction semiconductor nanoribbons. (a) Schematic growth of the lateral multi-junction semiconductor NRs. (b) Theoretical simulation of the binding energy difference between Cd2+ atom-terminated (0001) surface and S2- atomterminated (000-1) surface. (c) Real-color images of some representative samples from pure CdS to 7L NR (scale bar, 5 μm), which were removed from the initial grown substrate (Si) and dispersed onto a transparent MgF2 wafer under diffused 405 nm laser illumination.


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Figure 2. Structure characterization of the 5L NRs. (a) Low-resolution TEM image of a 5L lateral multi-junction semiconductor NR (scale bar, 5 μm). (b) 2D elemental mapping for the three detected elements (Cd, S, and Se). (c) HRTEM image taken from the interface of the 5L NR (scale bar, 2 nm). (d) Real-color photograph of some 5L NRs dispersed on a transparent MgF2 wafer under diffused 405 nm laser illumination (scale bar, 5 μm). (e) Real-color photograph of a selected 5L NR (scale bar, 5 μm) and (f) the corresponding PL mapping image. (g, h) 2D PL mapping images in the regions of 513–516 nm and 605–608 nm, respectively. (i) PL line-scanning profile of 515 nm and 607 nm emitting across the interface of the NR (the dotted line in panel f). (j, k) PL spectra for the CdS region and the CdS0.49Se0.51 region, respectively.


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Figure 3. Band alignment of a typical 5L lateral multi-junction semiconductor NR. (a) Optical image of a 5L NR and the scale bar is 5 μm. (b, c) Kelvin probe force microscopy (KPFM) characterization of the NR in the dark and under illumination of 488 nm monochromatic light. The scale bar is 1 μm. The corresponding line profiles are shown on the right. (d) Schematic diagram of the device for the flat band (left), band alignment in the dark (middle) and band alignment under illumination (right).


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Figure 4. Charge transfer dynamics of a typical 5L lateral multi-junction semiconductor NR. (a) Transient absorption spectra of a 5L lateral CdS-CdS0.49Se0.51 NR probed at different delay times under 400 nm excitation (pump density of 50 μJ/cm2). (b) Global analysis for the transient absorption data in Figure 4a. (c) Dynamics of GB at 500 nm (CdS) and 586 nm (CdS0.49Se0.51). (d) Normalized Dynamics of GB at 500 nm (CdS) and 586 nm (CdS0.49Se0.51) zoomed to the first 600 ps after excitation.


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Figure 5. Photoelectric characterization of the as-fabricated NR-based devices. (a) Schematic diagram and (b) an optical image of a typical as-fabricated device. Scale bar, 5 μm. (c) I-V curves and (d) the extracted I-Pin curves of different structure-based devices under 510 nm light illumination (power density: 2.82 mW/cm2) with same device area, in which the 5L NR-based device shows the highest photoresponse. (e) Wavelength dependent photocurrent response of the 5L NR-based device (power density: 2.82 mW/cm2), and highest photoresponse is observed at the wavelength of 510 nm. (f) Power density dependent photocurrent response and (g) the corresponding photocurrent responsivity of the 5L NR-based device. (h) Photocurrent response of the device to the light with different power switching on and off. (i) Time-resolved photoresponse of the 5L NR-based device, namely the rise time and decay time of the photocurrent.


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ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website. The schematic diagram of the growth process, SEM, EDS, and PL spectra of the multi-junction nanoribbons, transient absorption spectra, device images, photoresponse of different junction number devices (Figures S1-S11).

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]

Author Contributions ||

H. Liu, Y. Jiang, P. Fan contributed equally to this work as first authors.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors are grateful to the National Natural Science Foundation of China (Nos. 51525202, 51772084, 61574054, 61505051, 61474040, 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, the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and the Foundation for Innovative Research Groups of NSFC (Grant 21521063).

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Polar-induced Selective Epitaxial Growth of Multi-junction

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