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Controlling the Interface-Areas of Organic/Inorganic Semiconductors Heterojunction Nanowires for High Performance Diodes Zheng Xue, Hui Yang, Juan Gao, Jiaofu Li, Yanhuan Chen, Zhiyu Jia, Yongjun Li, Huibiao Liu, Wensheng Yang, Yuliang Li, and Dan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06274 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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ACS Applied Materials & Interfaces

Controlling the Interface-Areas of Organic/Inorganic Semiconductors Heterojunction Nanowires for High Performance Diodes Zheng Xue,1,2 Hui Yang,2 Juan Gao,2 Jiaofu Li,2 Yanhuan Chen,2 Zhiyu Jia,2 Yongjun Li,2 Huibiao Liu,*2 Wensheng Yang,*1 Yuliang Li2 and, Dan Li3 1. State Key Laboratory for Supramolecular Structures and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China 2. Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China 3. Department of Chemistry, Shantou University, Shantou, Guangdong 515063, P. R. China

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ABSTRACT: A new method of in-situ electrically induced self-assembly technology combined with electrochemical deposition has been developed for the controllable preparation of organic/inorganic core/shell semiconductor heterojunction nanowire arrays. The interface-size of the heterojunction nanowire can be tuned by growing parameter. The heterojunction nanowires of graphdiyne/CuS with core/shell structure showed the dependence strongly of interface-size with rectification ratio and perfect diode performance. It will be a new way for controlling the structures and properties of one-dimensional heterojunction nanomaterials.

KEYWORDS: graphdiyne, CuS, organic/inorganic, core/shell, heterojunction nanowires, rectification ratio


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1. INTRODUCTION Low-dimensional

organic/inorganic

heterojunction

nanomaterials

exhibit

unique

optoelectronic and electric properties and potential applications.1-38 Many approaches have been developed for preparing the organic/inorganic semiconductor heterojunction nanowires.5, 8, 10, 12, 16, 21-24, 28-35, 37, 38

It is well known that how to control the interface-size of the heterojunction is a

key factor for tuning the performances of heterojunction nanowires. There are some reports for growing organic/inorganic heterojunction nanowires with different heterojunctional interfacesize.1, 2, 21-26, 29, 30, 37 However, precisely controlling the heterojunctional interface-size is a crucial challenge to construct the organic/inorganic heterojunction nanowire with high performance. Electrochemical deposition technology with the help of porous anodized alumina oxide (AAO) templates has been discussed.21-24, 28-33, 37 As yet the possibilities are limited and it is desirable to develop further concepts for the formation of nanoscale heterojunction assemblies. Graphdiyne (GD) is a new allotrope of carbon with sp and sp2 hybridization states and has attracted many researchers to focus on it for its high π-conjunction, uniformly distributed pores and its perfect properties such as photocatalysis, lithium batteries, solar cells and so on after firstly synthesized by Li’s group in 2010.39-50 Recently, we have prepared 1D GD nanotubes by in-situ polymerized technology using AAO template and precisely controlled the wall thickness of GD nanotube.50 The concept is based on the combination of copper ionic induction and π–π interactions and thus can be applied to a wide variety of chemically and structurally different building blocks. Herein, we developed an in-situ electrically induced self-assembly technology combined with electrochemical deposition for fabricating organic (GD)/inorganic (CuS) core/shell heterojunction nanowires and archiving to control the heterojunction interface-area and the electrical property precisely. Preparation of the GD/CuS core-shell heterojunction


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Figure 1. The scheme of controlling fabrication the GD/CuS core-shell heterojunction nanowire. (a) AAO template. (b) GD nanowire in AAO template. (c) CuCl2 and S in DMSO at 130 °C. (d) In the electric field, the CuS grew around GD nanowire. (e) With extending the time of electrochemical deposition the GD/CuS nanowire was got. (c1) Cu2+ adsorbed on the surface of the AAO template and GD nanowire in DMSO at 130 °C. (c2) CuS nuclei aggregated to grow CuS nanocrystal on the surface of the Cu foil by electrically induced self-assembly technology combined with electrochemical deposition. nanowire is shown in Figure 1: First, GD nanowires are synthesized in the AAO template through in-situ polymerizing technology.50 Subsequently, the shell of CuS is fabricated by in-situ electrically induced self-assembly technology combined with electrochemical deposition for the first time. The interface-size of the heterojunction is controlled by tuning the length of CuS nanoshell, which is adjusted easily by changing the time of electrochemical deposition of


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growing CuS. The independent GD/CuS core-shell heterojunction nanowire exhibits perfect diode performance and its rectification ratio is up to 238.08 under forward bias of 0.5 V, which is higher than that of all organic/inorganic semiconductor heterojunction nanostructures reported and is comparable to some inorganic/inorganic semiconductor heterojunction nanowires. Their rectification ratios (RR) are rapidly falling with the increase of the heterojunction areas, and the quantitative relationship between RR and the areas of the heterojunction is confirmed at the first time. 2. EXPERIMENTAL 2.1 Synthesis of GD/CuS heterojunction core-shell nanowires (NWs) The GD/CuS heterojunction nanowires were synthesized by in-situ electrically induced selfassembly technology combined with electrochemical deposition. First, 15 mg the monomer of hexaethynylbenzene followed our previous work was dissolved in 5 mL THF and 5 mL TMEDA together and then added into a homemade electrolytic cell with AAO templates processed whose one side was fixed on a copper foil pretreated.39, 48 The mixture was left to stand at 60 °C under the nitrogen atmosphere. After 72 hours the reaction ended, the AAO template kept in the reactor was washed with acetone three times and dried under a flow of argon. After these, the GD nanowires were synthesized successfully and they were still reserved in the holes of the AAO templates. Second 0.05 M CuCl2 and 0.17 M S dispersed in 10 mL DMSO were heated to 130 °C and then transferred to the homemade electrolytic cell with the GD nanowires in the AAO template.51 The CuS shell was then grown around the GD nanowires and gradually formed the GD/CuS core-shell heterojunction nanowires at a current density of 4 mA/cm2 with the copper foil as a working electrode and a platinum wire as the counter electrode. The size of heterojunction core-shell nanowires can be controlled by adjusting the size of CuS nanoshell.


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The length of the CuS nanoshell increases with prolonging the time of electrochemical deposition. The length of CuS nanoshell is 1.42 µm, 4.72 µm, 11.07 µm, 23.55 µm, 27.93 µm, in the time of electrochemical deposition of 5 min, 10 min, 20 min, 30 min, 40 min, respectively. After fabricated the CuS nanoshell, the AAO template filled with the GD/CuS core-shell heterojunction nanowires was washed by hot DMSO, H2O, and acetone in turn and dried by a flow of argon. Last, the AAO templates with the GD/CuS core-shell heterojunction nanowires were etched by 6 M NaOH solution for 60 min at room temperature. After the template was dissolved absolutely, the nanowires were freed in the solution and then centrifuged with deionized water three times and ethanol two times and preserved in ethanol for the future characterizations and measurements. 2.2 Characterization Field emission scanning electron microscopy (SEM) images were observed from Hitachi S4800 FESEM microscope at an accelerating voltage of 15 kV. Transmission electronmicroscopy (TEM) images were taken from JEOL JEM 1011 microscope using an accelerating rate voltage of 100 kV. High resolution transmission electronmicroscopy (HRTEM) images, selective-area electron diffraction patterns (SAED) and energy dispersive X-ray spectrometry (EDS) element mapping were taken from JEOL JEM-2100F microscope using an accelerating rate voltage of 200 kV. Atomic force microscope (AFM) images and Raman spectra were recorded at room temperature using a NT-MDT NTEGRA Spectra system with excitation from an Ar laser at 473 nm. X-ray diffractometer (XRD) was characterized by RigakuDmax200 (Cu Kα), the scanning rate was 5°/min, and the 2θ range was from 5° to 60°. 2.3 Preparation of Nanowire Devices and I−V Measurements


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The single GD/CuS core-shell heterojunction nanowire devices were prepared as follows: First the microelectrodes whose Au electrodes were sputtered on SiO2/Si substrate were machined by First MEMS Co., Ltd.; Second the colloid of the GD/CuS core-shell heterojunction nanowires in ethanol was spread on the microelectrodes, and some of the nanowires remained on the two neighbouring electrodes; Third the current–voltage (I–V) characteristics of the single GD/CuS core-shell heterojunction nanowires was measured with a Keithley 4200 SCS and a shielded probe station with triaxconnectors was used to minimize noise at room temperature in air. 3. RESULTS AND DISCUSSION The independent GD/CuS core-shell heterojunction nanowire was characterized by scanning electron microscopy (SEM) first. Figure 2a to 2e showed the SEM images of the single GD/CuS core-shell heterojunction nanowire synthesized under different time of electrochemical deposition (the corresponding TEM images were shown in Figure S2). It is clearly observed that there are visible differences between the two segments of the nanowire in grey-scale contrast, in which the dark segment is GD nanowires and the grey segment is GD/CuS core-shell heterojunction nanowires. From Figure 2a to 2e, the lengths of CuS nanoshells are 1.42 µm, 4.72 µm, 11.07 µm, 23.55 µm, 27.93 µm, respectively. The amplifying junction of one independent nanowire is shown in Figure 2f, the diameter of GD nanowire is 170 nm while the diameter of the GD/CuS core-shell heterojunction nanowire is 225 nm, which suggested that the CuS grew on the surface of the GD nanowire. In Figure 2f, two different phases can be easily identified which is the same of Figure 2a. The GD nanowire is swaddled by a layer of CuS shell, and the GD nanowire is covered partially at the border, which indicates that CuS shell grew along the GD nanowire from the bottom. The results show that the heterojunction size can be controlled that by adjusting the time of electrochemical deposition for growing CuS shell.


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Figure 2. SEM images of the independent GD/CuS core-shell heterojunction nanowire under different time of electrochemical deposition for growing CuS nanoshell. (a) 5 min. (b) 10 min. (c) 20 min. (d) 30 min. (e) 40 min. (f) Large magnification of GD/CuS core-shell heterojunction nanowire under 5 min.

Transmission electron microscopy (TEM) was used to characterize the nanostructure of the single GD/CuS core-shell heterojunction nanowire further. TEM image of the border of a single GD/CuS core-shell heterojunction nanowire in Figure 3a shows that the diameter of GD nanowire is 130 nm and the diameter of GD/CuS core-shell heterojunction nanowire is 215 nm, in which it is clearly observed that GD nanowire is covered by CuS. The discrepancy of the diameter of GD/CuS nanowire measured by SEM and TEM was caused by the AAO templates. The same batch of the AAO templates was used for the experiment. High resolution transmission electron microscopy (HRTEM) in Figure 3b shows the lattice fringe spacing of CuS is 0.27 nm corresponding to the (006) plane of the hexagonal phase of a CuS crystal (JCPDS 79-2321).51 Selective-area electron diffraction patterns (SAED) were taken to characterize the GD/CuS core-


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shell heterojunction nanowires (Figure 3c and 3d). Figure 3c-d show that GD nanowire is amorphous and CuS is crystalline. The crystalline interplanar spacing of 0.27 nm is consistent with the interplanar spacing of the facet (006) of the CuS crystal (JCPDS 79-2321), which indicates that CuS is typical hexagonal lattice of hexagonal crystal system.51 The GD/CuS coreshell heterojunction nanowire is demonstrated by EDS elemental mapping (Figure 3e and 3f), which shows the dispersion of Cu element (Figure 3e) and S element (Figure 3f), and it is consistent with the results of TEM observed.

Figure 3. TEM images of the GD/CuS core-shell heterojunction nanowires. (a) Large magnification view of the interface of the GD/CuS core-shell heterojunction nanowires. (b) HRTEM images of the CuS segment, the fringes are separated by 0.27 nm. (c) SAED pattern


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taken from the GD segment. (d) SAED pattern taken from the CuS segment. (e, f) EDS elemental mapping images of the interface of a typical single GD/CuS core-shell heterojunction nanowires.

The results of X-ray diffractometer (XRD) patterns confirm that the GD was amorphous but CuS was crystalline (Figure S3) in the GD/CuS core-shell heterojunction nanowire. Figure S3 shows CuS shell is hexagonal structure (JCPDS 79-2321).51 As shown in Figure S3, the high intensity of peak (006) also indicates that [006] as the main growth direction for the component part of CuS, which is consistent with the HRTEM results. The highest peak is assigned to (200) of Pt which was evaporated on the bottom of the AAO template to get the GD/CuS nanowire arrays not scattered single nanowires after the AAO template was etched by NaOH solution (6 M).51 There were no any other phases such as elemental copper, sulfur and Cu2S.

Figure 4. AFM-Raman spectroscopies of a typical single GD/CuS core-shell heterojunction nanowire on SiO2/Si substrate. (a) AFM image of the interface of a typical single GD/CuS coreshell heterojunction nanowire. (b) Raman mapping of the same position. (c) The height profile is


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taken along the white line A in Figure 4a. (d) The height profile is taken along the white line B in Figure 4a. (e) Raman spectra of the positions C and D in Figure 4b.

The AFM-Raman spectroscopy was used to confirm the component of the GD/CuS core-shell heterojunction nanowires. Figure 4a is the AFM image of an independent GD/CuS core-shell heterojunction nanowire. And Figure 4c and Figure 4d show the height profiles taken along the white lines A and B in Figure 4a. Figure 4a shows that the right segment is brighter than that of the left segment, which means that the right segment is higher than that of the left segment. The 3D AFM image is shown in Figure S4, the clearly height difference is observed at the position of X axis 9.5 µm. The position at X axis 9.5 µm is border between the GD nanowire and the GD/CuS core-shell heterojunction nanowire, while the left segment is GD nanowire and the right segment is GD/CuS core-shell heterojunction nanowire. Figure 4c shows that the height is 178 nm along the white line A, which is corresponding to the GD nanowire part. Figure 4d shows that the height is 281 nm along the white line B, which is corresponding to the GD/CuS coreshell part. Raman mapping as shown in Figure 4b further confirms the formation of GD/CuS core-shell heterojunction nanowire, the bright segment is GD nanowire, the dark segment is GD/CuS core-shell nanostructures, which is corresponding to the results of AFM (Figure 4a). Raman spectroscopies of the different positions of the same nanowire are shown in Figure 4e. The band at 473 cm-1 of the GD/CuS core-shell heterojunction nanowire is the characteristic peak of CuS (the S-S stretching mode).52 Figure 4e displays that typical G band and D band of GD are observed in both of GD nanowire part and GD/CuS core-shell part of the GD/CuS coreshell heterojunction nanowire. The band of GD nanowire at 1597 cm-1 corresponds to the firstorder scattering of the E2g mode observed for in-phase stretching vibration sp2 carbon domains in


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aromatic rings (G band).39 The band at 1382 cm-1 is attributed to the breathing vibration of sp2 carbon domains in aromatic rings (D band).39 For the GD nanowire, the ratio of the intensities of D and G bands are 0.828, which indicates the GD nanowire is high order and low content of defects.50 The band of the vibration of conjugated diine links (-C≡C-C≡C-) is observed at 2123 cm-1 from the GD nanowire.39 The results confirm the formation of GD/CuS core-shell heterojunction nanowire. The band at 521 cm-1 is a typical Lorentzian representing the triple degenerate phonon of the bulk material,53 and the band at 971 cm-1 is assigned to the νs modes of the (SiO4)4- units of SiO2/Si substrate.54

The in-situ electrically induced self-assembly technology combined with electrochemical deposition method is proposed based on the results and discussion above. The growth process of GD/CuS nanowires is believed to the following (Figure 1): (i) The GD nanowires grew in the AAO templates from the bottom to the top by self-assembly process, in which the growth of GD nanowires began in the middle of hole bottom.50 This leads to there is a slight gap between GD nanowire and the wall of AAO hole, where the diameter of GD nanowire is slightly less than that of pore of AAO template (Figure 1b); (ii) Before turning on the DC power source, Cu2+ ions are easily adsorbed on the Al2O3 wall surface of the holes of the AAO templates due to the interaction between oxygen atoms and Cu2+ after the AAO template with GD nanowires is immersed in CuCl2 solution. Furthermore, there is also weak coordination of interactions between Cu2+ and alkyne,55 which induced there are many Cu2+ in the gap between GD nanowires and hole wall of AAO template (Figure 1c1); (iii) After turning on the DC power source the negative pole was lined with Cu foil, so the sulphur got electrons and formed S2- ions on the surface of the Cu foil (where is the bottom of the GD nanowires) first. Then Cu2+ ions


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react with S2- ions to form CuS nuclei in the gap and then CuS nuclei deposit on the bottom of the AAO template attached to the surface of the Cu foil (Figure 1c2). Induced by electrical field, CuS nuclei rapidly aggregate to grow CuS nanocrystal on the surface of the Cu foil, then further grow to form CuS shell around the surface of GD nanowires (Figure 1d).31, 51 The length of the CuS nanoshell increases with prolonging the time of electrochemical deposition (Figure 1e). In this case, the length of CuS nanoshell is controlled by adjusting the electrolytic time. The GD/CuS core-shell heterojunction nanowires with controllable length were fabricated and the junction-interface-area of GD/CuS core-shell heterojunction nanowire in a large range can be controlled.

Figure 5. The SEM and current–voltage (I–V) characteristics of the single GD/CuS core-shell heterojunction nanowire device. (a, b, c, d and e) SEM images of five independent GD/CuS coreshell heterojunction nanowire device with different heterojunctional interface-size. (f) The current–voltage (I–V) curves of the five single GD/CuS core-shell heterojunction nanowires at


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room temperature. (g) The relationship between the rectification ratios and the areas of the junctions. The scale bar in Figure 5a to 5e is 2 µm.

The current–voltage (I–V) characteristics of the single GD/CuS core-shell heterojunction nanowire device were measured with a Keithley 4200 SCS and a shielded probe station with triax connectors was used to minimize noise at room temperature in air. Figure 5a to 5e shows the SEM images of the five devices of a single GD/CuS core-shell heterojunction nanowire, SEM images of the single GD nanowire device and single CuS nanowire device are shown in Figure S5a and Figure S5c. Figure 5f shows the perfectly rectifying diode behavior curves of the GD/CuS core-shell heterojunction nanowires, while the rectifying diode behavior is not any observed on the single GD nanowire and CuS nanowire as shown in Figure S5b and Figure S5d. The current increases rapidly under forward bias and keeps almost the same under reverse bias for the whole five independent GD/CuS core-shell heterojunction nanowire device in Figure 5f. The current increases linearly in Figure S5b and Figure S5d from -0.5 V to 0.5 V which indicates that there is Ohmic contact between the GD nanowire and Au electrode and the CuS nanowire and Au electrode. The GD/CuS core-shell heterojunction nanowire exhibits a high rectification ratio at a low bias voltage. The rectification ratios (RR) (defined as RR=[current at 0.5 V]/[current at -0.5 V]) of the GD/CuS core-shell heterojunction nanowires is up to 238.08 at bias of 0.5 V when the area of the heterojunction was 0.0162 µm2 (Figure 5a), which is higher than that of all organic/inorganic semiconductor heterojunction nanostructures reported,21, 24, 28-30, 37 and is comparable to inorganic/inorganic semiconductor heterojunction nanowires.31,

56

The

rectification ratios decrease to 154.63, 77.92, 42.46 and 35.23, respectively, with increasing the areas of the heterojunctions to 0.0353 µm2, 0.207 µm2, 0.5865 µm2 and 0.7262 µm2 (Figure 5b to


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5e). As shown in Figure 5g, the relationship between RR and the areas of the junctions is signified by a mathematical formula, which means that the rectification ratio (RR) was quantitatively controlled by adjusting the heterojunction area (A) in heterojunction nanowire: RR = 23.22 − 33.88 × lnሺA − 0.014ሻ When GD nanowire is wrapped by CuS nanoshell, there will form p-n heterojunction (depletion regions in both components) along the sides, which will increase the resistance of both components. When the length of CuS nanoshell increased, the more the GD nanowire is wrapped by CuS nanoshell, and the more the depletion region areas along the sides, which induce to the increasing resistance of both components. As a result the resistance of whole system will increase, this will considerably decrease the forward current and hence the rectification ratio. 4. CONCLUSIONS In conclusion, we have developed an in-situ electrically induced self-assembly technology combined with electrochemical deposition for controlling preparation of GD/CuS core/shell heterojunction nanowires, which exhibit excellent performances of the diode. Furthermore, the diode rectification ratios of GD/CuS core-shell heterojunction nanowires have been easily tuned by controlling the growing parameters and the quantitative relationship between rectification ratios and the areas of the heterojunction is confirmed at the first time. It will be a new way for controlling the structures and properties of one-dimensional heterojunction nanomaterials. Resulting in heterojunction nanomaterials with tunable heterojunction-interface-size may for example be of interest for nanoscale devices in electronics, optoelectronics and catalysis. ASSOCIATED CONTENT


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Supporting Information. Schematic diagram of energy level diagram, TEM, XRD, 3D-AFM, SEM images and current–voltage (I–V) characteristics of the single GD nanowire device and the single CuS nanowire device. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel: +86-431-85168185. Fax: +86-431-85168186. * E-mail: [email protected] Tel: +86-10-82616576. Fax: +86-10-82615870. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research 973 Program of China (2012CB932901), the National Key Research and Development Program (2016YFA0200104), the National Nature Science Foundation of China (51573191, 21373235 and 21021091) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010100). REFERENCES (1) Penner, R. M.; Martin, C. R. Preparation and Electrochemical Characterization of Ultramicroelectrode Ensembles. Anal. Chem. 1987, 59 (21), 2625-2630. (2) Martin, C. R. Nanomaterials: A Membrane-Based Synthetic Approach. Science 1994, 266 (5193), 1961-1966. (3) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Epitaxial Core-Shell and Core-Multishell Nanowire Heterostructures. Nature 2002, 420 (6911), 57-61. (4) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Coaxial Silicon Nanowires as Solar Cells and Nanoelectronic Power Sources. Nature 2007, 449 (7164), 885-890.


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Inorganic Semiconductor

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