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GaSe layered nanorods formed by liquid phase exfoliation for resistive switching memory applications
Journal Pre-proof GaSe layered nanorods formed by liquid phase exfoliation for resistive switching memory applications Ganlin Chen, Lei Zhang, Luying Li, Feng Cheng, Xiao Fu, Jinhua Li, Ruikun Pan, Wanqiang Cao, A. Sattar Chan, Gennady N. Panin, Jiaxian Wan, Heng Zhang, Chang Liu PII:
S0925-8388(20)30060-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153697
Reference:
JALCOM 153697
To appear in:
Journal of Alloys and Compounds
Received Date: 24 September 2019 Revised Date:
31 December 2019
Accepted Date: 6 January 2020
Please cite this article as: G. Chen, L. Zhang, L. Li, F. Cheng, X. Fu, J. Li, R. Pan, W. Cao, A.S. Chan, G.N. Panin, J. Wan, H. Zhang, C. Liu, GaSe layered nanorods formed by liquid phase exfoliation for resistive switching memory applications, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153697. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Graphical abstracts
A resistive switching device based on GaSe layered nanorods fabricated by LPE with a graphene/GaSe/graphene structure is demonstrated.
GaSe Layered Nanorods Formed by Liquid Phase Exfoliation for Resistive Switching Memory Applications
Ganlin Chen,1 Lei Zhang,1* Luying Li,2 Feng Cheng,2 Xiao Fu,3 Jinhua Li,1 Ruikun Pan,1 Wanqiang Cao,1 A. Sattar Chan,3 Gennady N. Panin,3,4* Jiaxian Wan,5 Heng Zhang5 and Chang Liu5
1
Ministry of Education Key Laboratory for Green Preparation and Application of
Functional Materials, Hubei Provincial Key Laboratory of Polymers, School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China 2
Center for Nanoscale Characterization and Devices, Wuhan National Laboratory for
Optoelectronics and School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China 3
Department of Physics, Nano Information Technology Academy, Dongguk
University, Seoul 04620, Republic of Korea 4
Institute for Microelectronics Technology and High Purity Materials, Chernogolovka,
Moscow Distr., 142432, Russia 5
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education,
School of Physics and Technology, Wuhan University, Wuhan 430072, China *
Corresponding author: Lei Zhang Email : [email protected]； Gennady N. Panin Email: [email protected]
1
Abstract: The increasing interest and rapid progress in the artificial neural networks have driven extensive research in resistive switching memory based on various insulators and two-dimensional nanomaterials. Herein, we have demonstrated a kind of resistive switching memory device based on GaSe layered nanorods fabricated by liquid phase exfoliation (LPE) method with a lateral graphene/GaSe/graphene structure. The single crystalline GaSe layered nanorods mixed with a few nanoflakes were synthesized by shear and ultrasonic (bath and probe) exfoliation methods, which showed high quality and hexagonal crystalline structure. The electrical properties and the resistive switching behavior were investigated, indicating the charge trapping/detrapping effect is the dominant resistive switching mechanism. This work suggests a novel strategy to develop the resistive switching devices based on various 2D materials by using LPE method, and these 2D-nanomaterials-based memristors have the potential to be used for constructing neuromorphic computing systems.
Keywords: Liquid phase exfoliation, two-dimensional nanomaterials, nanorods, resistive switching, charge trapping/detrapping
2
1. Introduction The past decade has seen extensive research efforts being devoted to two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDCs). Accordingly, great progress has been made in both fundamental research and technique development [1-4] due to their fascinating properties as well as the possibility to revolutionize the future electronic and optoelectronic technologies. Graphene is one of the most important 2D materials and considered as a promising candidate for nanoelectronics. However, its zero band gap limits the applications in logic electronics and field-effect transistors (FETs) [5-7]. Following graphene, TMDCs (such as MoS2, MoSe2, WS2, WSe2, etc) and black phosphorus (BP) have attracted extensive attention and revealed extraordinary capabilities as FETs [8, 9], photodetectors [10] and sensors [11]. Very recently, atomically thin metal monochalcogenides, such as GaSe, GaS, InSe, have been extensively reported [12-14] with significantly different electronic and optoelectronic properties from TMDCs [15]. GaSe is a typical layered AIIIBVI binary chalcogenides with strong covalent in-plane inter-atomic bonding[16] and weaker van der Waals inter-plane bonding [17]. From the side view, the single tetralayer with a hexagonal in-plane structure consists of four atoms: Se-Ga-Ga-Se. The band gap energy of bulk GaSe is about 2.1 eV [18], and that of the monolayer is predicted to be more than 3.5 eV. So far, few-layer 2D GaSe [19] has demonstrated a variety of interesting electrical and optical properties, including p-type behavior [20], a high on/off ratio [21], high responsivity [22], anisotropic Hall mobility [23], outstanding gas sensibility [24], and strong second-harmonic 3
generation [25], which makes it an appealing candidate for vast applications in nano-optoelectronics, nonlinear optics, piezo-photonics, and wearable devices [17]. Like other 2D materials, the layered GaSe can be exfoliated into mono- or fewlayers by micromechanical exfoliation [26], and also synthesized on various substrate using vapor phase deposition [26], molecular beam epitaxy (MBE) [21, 25, 27], chemical vapor deposition (CVD) [15, 28, 29], and liquid phase exfoliation (LPE) [30]. As is well known, LPE is considered as one of the most promising routes toward large-scale production of monolayer, few-layers and quantum dots 2D materials. LPE has already been used as a mass-scale approach for production of 2D materials which offers several advantages including cost reduction and scalability. Another important advantage of LPE is that it can be used to create inks made of nanosheets of different 2D crystals for printing devices [31]. Furthermore, this technique is compatible with low-cost and flexible substrates, which is expected to be applied for flexible and wearable electronics [32]. The LPE methods can be classified into four main sections: direct ultrasonic (bath and probe sonication) exfoliation in various solvents, shear exfoliation, electrochemical exfoliation and functionalization-assisted exfoliation [7]. Traditionally, N-methyl-2-pyrrolidone (NMP) and N, N-dimethylformamide (DMF) are the candidates as the organic solvents for exfoliating most of 2D materials. However, they have some disadvantages such as high boiling points and toxicity which deteriorate the device performances and hinder their vast applications [14]. Although GaSe is considered as the most promising 2D material according to its properties, and the LPE method has been applied extensively for synthesizing a 4
variety of 2D materials [33], the study on GaSe nanostructures formed by the LPE method hasn’t been systematically explored. In the present work, we combine the shear and direct ultrasonic exfoliation by using environmentally friendly solvent to exfoliate the bulk GaSe into few-layered GaSe nanostructures, and obtain GaSe nanoflakes mixed with layered nanorods, which hasn’t been reported previously. Here, we employ Isopropanol Alcohol (IPA) and DI water with low boiling point as the solvents to exfoliate GaSe nanostructures, and study their optical and electrical properties systematically.
2. Experimental 1.2 g of bulk GaSe (Gallium selenide, 99.99%, purchased from Alfa Aesar) was added in 120 mL solution with 80% IPA and 20% DI water and shear-mixed for 2 hours at the speed of 6000 rpm by using a L5M high-shear laboratory shear mixer (Silverson Machines). As shown in the schematic of the exfoliation procedure in Fig. 1, following the shear mixing, the dispersion was further bath ultrasonicated for 5 hours with ice-cooling. Then, half of the dispersion was centrifuged at 3000 rpm for 30 min directly, and the other half of the dispersion was taken for additional sonication for 15 hours by using a horn-probe tip sonicator operating at 60% amplitude with 5 s On and 5 s Off pulses with ice-cooling, and then was centrifuged at 10000 rpm for 60 min. Finally, the top 30% supernatants of each dispersion (labeled as GaSe 3000 and GaSe 10000) were retained for further analyses, respectively. Fabrication of GaSe based devices: 5
GaSe suspensions were deposited on SiO2/Si substrates with graphene electrodes. The graphene electrodes were grown on Cu foil by CVD method at 1020 oC under methane/hydrogen flow at 600 mTorr, and then transferred onto SiO2/Si substrates by using PMMA. Characterization: The morphology of GaSe nanostructures was obtained by scanning electron microscope (SEM, XL30 SFEG, FEI). Transmission electron microscopy (TEM) analysis was performed on a Tecnai G2 20 U-TWIN TEM operated at 200 kV. Raman analysis was carried out on micro-Raman spectrometer using an excitation wavelength of 532 nm. The optical properties of the GaSe nanostructures were recorded by employing a Varian Cary ultraviolet−visible (UV−vis) spectrometer. The current-voltage characteristics were studied by using a Keithley 617 semiconductor parameter analyzer.
3. Results and discussion In order to analyze the morphological and structural properties of the synthesized GaSe nanostructures, SEM and TEM images were recorded. Figure 2(a) shows the SEM image of GaSe nanostructure obtained by LPE method with centrifugation speed of 3000 rpm, which contains mostly nanoflakes just as other 2D materials synthesized by the LPE method [34], and some nanorods also appear with different sizes. As shown in Fig. 2(c), statistical analysis based on SEM characterization suggests that the distribution of length of the nanorods follows the Gaussian function with an average length around 1 µm. In contrast, GaSe nanostructures obtained by the shear 6
and direct ultrasonic (bath and probe) exfoliation followed by centrifuging at 10000 rpm contains nanoflakes stacked in plane with a chaotic array of longer nanorods spreading on it as shown in Fig. 2(b). The width of the nanorods is in the range of 100-200 nm, and the length distributes from 4 to 14 µm with an average value of 7.33 µm as statistically analyzed in Fig. 2(d). It’s well known that LPE of micrometer size 2D powder could provide the stacked nanoflakes [35]. In this work, it’s interesting to find that lots of chaotically-distributed nanorods mixed with the flakes were obtained when the GaSe bulk crystals with the size of centimeter were exfoliated with a Mixer. As shown in the supporting information (Fig. S1), instead of the micrometer-sized powders which can flow into the stator due to attraction, and can be shear-mixed directly by the rotor, the centimeter-sized GaSe bulk crystals that can’t flow into the stator may be attached to the interface of the stator during shear-mixing and exfoliation. We suggest that the GaSe layered nanorods may come from the shear-exfoliation of the larger-sized GaSe bulk crystals attached at the stator by the inside rotor as shown in Fig. S1. Figures 3(a) and (b) show the typical bright-field TEM images for the nanostructures and a specific nanorod with a higher magnification. The corresponding selected area electron diffraction (SAED) patterns are shown in Fig. 3(c) and (d). Figure 3(c) corresponds to the red square area in Fig. 3(a), and the polycrystalline rings indicate many GaSe nanoflakes with different orientations gathering together. The SAED pattern of the single nanorod taken along the [110] zone axis in Fig. 3(d) shows only one set of diffraction spots of six-fold symmetry, which conforms to the 7
space group P-6 of hexagonal GaSe, indicating that single crystalline GaSe nanorods with hexagonal symmetry have been successfully synthesized. The top and side views of the atomic structure for the hexagonal GaSe nanorod are illustrated in the inset of Fig. 3(c) and (d). The optical properties of the GaSe nanostructures were evaluated via the UV-vis absorption, as shown in Fig. 4(a). For GaSe 3000, a broad absorption peak occurs around the wavelength of 600 nm, which is in accordance with its bandgap [18] and can be attributed to the thick GaSe nanoflakes absorption. It was demonstrated that a room temperature emission at wavelength of 710 nm is related to the defects and traps at the surface of GaSe nanoribben [36]. Here with a further probe-sonication treatment for 15 h and a high centrifugation speed of 10000 rpm for the sample GaSe 10000, the GaSe nanorods become more defective, and a significant absorption peak appears around 710 nm (the red spectrum in Fig. 4(a)) due to the defects and traps from the longer and thinner nanorods as illustrated by SEM image in Fig. 2(b). At the same time, smaller and thinner nanoflakes were also selected due to the higher centrifugation speed of 10000 rpm, which is consistent with Fig. 3(a) and (c). As predicted previously, the bandgap of GaSe nanoflakes can be modulated from 2.08 to 3.57 eV depending on the thickness [37]. For the absorption spectrum of GaSe 10000 in Fig. 4(a), the confined thin GaSe nanoflakes contribute to the absorption occurred at 450 nm and the absorption peak around 600 nm may be attributed to the remaining thick GaSe nanoflakes. Micro-Raman spectroscopy is a powerful technique to explore the evolution of 2D 8
materials with different sizes [18]. Figure 4(b) shows Raman spectra acquired from bulk GaSe, and GaSe nanostructures at the centrifugation speeds of 3000 and 10000 rpm, respectively. As shown in Figure 4(b), two of these vibrational modes below 100 cm-1 were not observed because of the detection limit of the Raman system, while the other four modes were detected for bulk GaSe, and the corresponding four peaks located at 134.8 cm-1 ( cm-1 (
), 209.5 cm-1 (
), 244.0 cm-1 (
), and 304.6
) are consistent with the vibration modes previously reported [38]. Two A1g
peaks with much stronger intensities are associated with the out-of-plane vibrational mode of GaSe, while E2g and very weak E1g peaks correspond to the in-plane mode [30, 39, 40]. As shown in Table 1, the ratios of and
/
/
(19.93),
/
(11.67)
(5.51) refer to thick GaSe crystal. However, the intensity of E1g peak
increases prominently after exfoliation, the
peak disappears and the
hardly observed for GaSe 3000 sample. The ratio of
/
peak is
decreases from 19.93
to 0.52 which suggests that the out of plane vibration mode becomes very weak comparing to the GaSe bulk crystal, indicating the thickness of GaSe nanostructure was decreased. With further exfoliation at higher speed and longer time of centrifugation for GaSe 10000, all out of plane modes disappears. While in-plane mode E1g becomes more prominent compared with GaSe 3000 (the ratio ℎ
/
increases from 0.37 to 0.41). It indicates that the E1g peak increases may
because of the dominance of the in-plane vibrational mode for much thinner nanosheets and nanorods for GaSe 10000. It is noted that no other peaks were obtained for GaSe nanostructure in the range of 100-500 cm-1 except four peaks for 9
GaSe and one peak located at 414.2 cm-1 which comes from the underneath sapphire substrate, suggesting that no oxidation reaction was occurred during exfoliation [41]. To investigate the electrical properties of GaSe nanostructures, devices were prepared by depositing the GaSe dispersions onto the SiO2/Si substrates between graphene electrodes. The device setup is shown in the inset of Fig. 5(a). The graphene electrodes were transferred by common PMMA wet transfer method, and then fabricated using a nanotip probe like previous reports [42]. The channel between two graphene electrodes was around 15 µm. The GaSe dispersion was then deposited onto two graphene electrodes by drop casting, followed by heating the samples at 80 oC in ambient for 1 h. Fig. S2 shows a Raman spectrum of graphene after processing. The G and 2D peaks are located at 1586 cm-1 and 2683 cm-1, respectively. A full width at half maximum (FWHM) of 33.7 cm-1 for 2D peak and the I(2D)/I(G) ratio of 1.62 as well as a weak D peak at 1355 cm-1 indicate the high quality of the transferred monolayer graphene. The current-voltage characteristic of the device (graphene/GaSe 3000/graphene) is shown in Fig. 5(a). The device is described as the GaSe channel with more nanoflakes and less nanorods sandwiched laterally by two Schottky diodes in opposite directions, as shown in the inset of Fig. 5(a). The non-linear rectifying behavior and the negligible hysteresis of I-V curve indicate that a barrier is formed at the graphene/GaSe nanostructures interface accompanied with charging effect. When graphene is in contact with GaSe, at thermal equilibrium, a dipole layer is formed at
10
the interface due to the charge transfer between graphene and GaSe [19, 30, 43], and a charge accumulation region is formed in the GaSe region. The I-V characteristic of the device graphene/GaSe 10000/graphene is further explored. During a continuous sweeping of a bias voltage from 0 V → 10 V → 0 V → -10 V → 0 V, an obvious pinched hysteresis loop was obtained as shown in Fig. 5(b). Initially, the I-V curve of the device is in its virgin state with a relatively high resistance. As the voltage increases, the current suddenly increases at the voltage of ~ 9.5 V, switching to a low resistance state (ON state). Then, the device switches to the OFF state with a high resistance as the voltage decreases to 0 V. Here, an ON/OFF ratio above 104 can be achieved, and the device operates well throughout over 100 cycles of ON/OFF without severe degradation, indicating that the device possesses excellent memristive behavior and high stability. Note that the current intensity is much lower than that of GaSe 3000, which could be attributed to the much lower concentration of the GaSe 10000 nanostructures when higher speed and longer time of centrifugation was used. Figure 5(c) shows the endurance characteristics of the resistance switching with graphene/GaSe 10000/graphene structure. The endurance characteristics is an important parameter for the performance of the resistive memory device, which refers to the phenomenon that the resistance value of the high and low resistance states of the memory deviates from the original resistance value as the number of switching times increases. After 100 cycles of switching, the resistance value of the device has no significant change, as shown in Fig. 5(c). This shows that the prepared graphene/GaSe 10000/graphene structure memory device has better 11
endurance resistance characteristics [44]. It is well known that the memristive devices can retain a state of internal resistance based on the history of applied voltage and current, which can store and process information and offer several key performance characteristics exceeding conventional integrated circuit technology [45]. Networks of such memristors, in the form of crossbar structures, have been widely studied for the energy-efficient artificial neural networks [46]. Till now, the memristors have been made from various materials and operated under different principles [47, 48]. The microscopic nature of resistance switching in such devices is still controversial [49], and different models have been suggested [50]: the origin of resistance change under the electric field as stemmed from charge trapping/de-trapping at the interface or defects [49]; the localized metal-atom chains between two electrodes [45]; the drift of vacancies/ions within the insulator layer [2]; and the filamentary conduction model based on the thermal effect [51], etc. It is possible that different mechanisms could be dominant in different materials systems, and even different mechanisms may co-exist [50]. To understand the mechanism of resistive switching behavior in our work, the double-logarithmic I-V curve was obtained, as shown in Fig. 5(d). When the applied voltage runs from 0 to the positive voltage, the device is at OFF state with a negligible current, since the injected charges are captured by the defects of the nanorods, and the traps of the interfaces between GaSe nanoflakes and nanorods. Thus, the traps would be constantly filled as the voltage increases. As the I-V curve sweeps from 8.5 to 9.5 V, the I-V relationship is fitted as 12
∝
. According to the charge
trapping/detrapping theory, the traps in the channel have all been filled, and then the injected charges could be de-trapped as the voltage increases, leading to an abrupt increase of current and the switching from OFF to ON state. When the applied voltage sweeps back to 0 V, the I-V characteristics change in nature from partially-trap-filled
∝
∝
to
( > 2), and the slopes of the double-logarithmic I-V
curve changes from 2, 4 to 6. According to the fitting results, we suggest that the resistive switching mechanism could be explained by the charge trapping/de-trapping effect. Furthermore, we consider that the electron transports through nanoflakes and nanorods should co-exist, both of which play important roles in the graphene/GaSe 3000/graphene sample, because of their difference in size and the small concentrations of both nanoflakes and nanorods. At the same time, the weak absorption in the absorption spectrum of GaSe 3000 in the range of 700-800 nm indicates that the defects also exist, which contributes to the negligible hysteresis loop in I-V curve as shown in Fig. 5(a). However for the sample GaSe 10000, the size of nanoflakes is too small to form continuous conducting film, while the nanorods (2~16 µm) are long enough to connect two graphene electrodes (~ 15 µm). It suggests that the electrons transport mainly through the longer nanorods within the device made of GaSe 10000. Since more defects in the nanorods of GaSe 10000 are introduced by the probe-sonication treatment for a long time compared to GaSe 3000, as illustrated by the absorption spectra in Fig. 4(a), the device shows evident resistance switching behavior in Fig. 5(b). Therefore, we conclude that the resistive switching mechanism 13
in this work should be based on the charge trapping/de-trapping effect by the defects (traps) rather than filamentary conduction, Poole-Frenkel effect, and so on [52].
4. Conclusions In this work, we have demonstrated a resistive switching memory device with a lateral graphene/GaSe/graphene structure. The single crystalline GaSe layered nanorods mixed with nanoflakes are synthesized by shear and ultrasonic exfoliation methods, which show high quality and hexagonal crystalline structure. The memory device exhibits excellent memristive behavior and good stability that an ON/OFF ratio above 104 and the endurance cycles over 100 can be achieved. Our results indicate that the charge trapping/de-trapping effect is the dominant resistive switching mechanism in accordance with previous reports. This work suggests that the liquid phase exfoliation method is a novel strategy to develop the memristor devices with good performances for the neuromorphic computing systems based on various 2D materials.
Acknowledgements This work was financially supported by the National Key Research and Development Plan (MOST) under Grant Number 2017YFA0205802, the National Natural Science Foundation of China (Grant No. 11574075, No. 51871104, No. 11574235, and No. 11875212).
14
Table 1. Comparison of the ratios for different Raman modes. /
/
/
GaSe bulk
19.93
11.67
5.51
-
GaSe 3000
0.52
-
0.53
0.37
GaSe 10000
-
-
-
0.41
15
/
ℎ
Figure captions: Figure 1: Schematic of the exfoliation procedure of GaSe nanostructures.
Figure 2: SEM images of GaSe nanostructures obtained at centrifugation speeds of (a) 3000 and (b) 10000 rpm. (c) and (d) Statistical analyses of nanorods length referring to (a) and (b), respectively.
Figure 3: (a) and (b) Bright-field TEM images of GaSe nanostructures and a specific nanorod obtained at centrifugation speed of 10000 rpm. (c) Selected area electron diffraction (SAED) pattern for the square area in (a). (d) SAED pattern for the single nanorod taken along the [110] zone axis in (b). Inset: top view and side view of the atomic strucuture for the GaSe nanorods. Atoms in the structure: Ga, blue; Se, yellow.
Figure 4: (a) Absorption spectra of GaSe nanostructures obtained at centrifugation speeds of 3000 and 10000 rpm. (b) Raman spectra of bulk GaSe and GaSe nanostructures obtained at centrifugation speeds of 3000 and 10000 rpm.
Figure 5: (a) I-V curve of the device made by GaSe nanostructures obtained at centrifuge
speed
of
graphene/GaSe/graphene.
3000 (b)
rpm.
Inset:
Typical
I-V
device
setup
characteristic
with of
structure
of
graphene/GaSe
10000/graphene switch at the 1st (red), 50th (black), and 100th (blue) cycles at room temperature. (c) Endurance characteristics of the device with 100 switching cycles. (d) Double-logarithmic I-V curve of the resistive switching memory device.
16
Figure 1
17
Figure 2
18
Figure 3
19
Figure 4
20
Figure 5
21
References: [1] M. Xu, T. Liang, M. Shi, H. Chen, Graphene-like two-dimensional materials, Chem. Rev., 113 (2013) 3766-3798. [2] M.M. Rehman, G.U. Siddiqui, J.Z. Gul, S.W. Kim, J.H. Lim, K.H. Choi, Resistive Switching in All-Printed, Flexible and Hybrid MoS2-PVA Nanocomposite based Memristive Device Fabricated by Reverse Offset, Sci Rep, 6 (2016) 36195. [3] X. Duan, C. Wang, A. Pan, R. Yu, X. Duan, Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges, Chem. Soc. Rev., 44 (2015) 8859-8876. [4] F. Pan, S. Gao, C. Chen, C. Song, F. Zeng, Recent progress in resistive random access memories: Materials, switching mechanisms, and performance, Materials Science and Engineering: R: Reports, 83 (2014) 1-59. [5] N.M. Freitag, T. Reisch, L.A. Chizhova, P. Nemes-Incze, C. Holl, C.R. Woods, R.V. Gorbachev, Y. Cao, A.K. Geim, K.S. Novoselov, J. Burgdorfer, F. Libisch, M. Morgenstern, Large tunable valley splitting in edge-free graphene quantum dots on boron nitride, Nat. Nanotechnol., 13 (2018) 392-397. [6] T. Szkopek, A longer-lasting memory in layered semiconductors, Nat. Nanotechnol., 13 (2018) 357. [7] A. Amiri, M. Naraghi, G. Ahmadi, M. Soleymaniha, M. Shanbedi, A review on liquid-phase exfoliation for scalable production of pure graphene, wrinkled, crumpled and functionalized graphene and challenges, FlatChem, 8 (2018) 40-71. [8] C. Liu, X. Yan, X. Song, S. Ding, D.W. Zhang, P. Zhou, A semi-floating gate memory based on van der Waals heterostructures for quasi-non-volatile applications, Nat. Nanotechnol., 13 (2018) 404-410. [9] M. Si, C. Jiang, W. Chung, Y. Du, M.A. Alam, P.D. Ye, Steep-Slope WSe2 Negative Capacitance Field-Effect Transistor, Nano Lett., 18 (2018) 3682-3687. [10] G. Wang, Y. Zhang, C. You, B. Liu, Y. Yang, H. Li, A. Cui, D. Liu, H. Yan, Two dimensional materials based photodetectors, Infrared Physics & Technology, 88 (2018) 149-173. [11] Y. Xu, X. Wang, W.L. Zhang, F. Lv, S. Guo, Recent progress in two-dimensional inorganic quantum dots, Chem. Soc. Rev., 47 (2018) 586-625. [12] B. Chitara, A. Ya'akobovitz, Elastic properties and breaking strengths of GaS, GaSe and GaTe nanosheets, Nanoscale, 10 (2018) 13022-13027. [13] A. Islam, J. Lee, P.X.L. Feng, Atomic Layer GaSe/MoS2 van der Waals Heterostructure Photodiodes with Low Noise and Large Dynamic Range, ACS Photonics, 5 (2018) 2693-2700. [14] Y. Sun, S. Luo, X.G. Zhao, K. Biswas, S.L. Li, L. Zhang, InSe: a two-dimensional material with strong interlayer coupling, Nanoscale, 10 (2018) 7991-7998. [15] N. Zhou, R. Wang, X. Zhou, H. Song, X. Xiong, Y. Ding, J. Lu, L. Gan, T. Zhai, P-GaSe/N-MoS2 Vertical Heterostructures Synthesized by van der Waals Epitaxy for Photoresponse Modulation, Small, 14 (2018). [16] O.D. Pozo-Zamudio, S. Schwarz, M. Sich, I.A. Akimov, M. Bayer, R.C. Schofield, E.A. Chekhovich, B.J. Robinson, N.D. Kay, O.V. Kolosov, A.I. Dmitriev, G.V. Lashkarev, D.N. 22
Borisenko, N.N. Kolesnikov, A.I. Tartakovskii, Photoluminescence of two-dimensional GaTe and GaSe films, 2D Materials, 2 (2015) 035010. [17] X. Li, M.W. Lin, A.A. Puretzky, J.C. Idrobo, C. Ma, M. Chi, M. Yoon, C.M. Rouleau, Kravchenko, II, D.B. Geohegan, K. Xiao, Controlled vapor phase growth of single crystalline, two-dimensional GaSe crystals with high photoresponse, Sci Rep, 4 (2014) 5497. [18] A. Bergeron, J. Ibrahim, R. Leonelli, S. Francoeur, Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy, Applied Physics Letters, 110 (2017) 241901. [19] C. Si, Z. Lin, J. Zhou, Z. Sun, Controllable Schottky barrier in GaSe/graphene heterostructure: the role of interface dipole, 2D Materials, 4 (2016) 015027. [20] Y. Cao, K. Cai, P. Hu, L. Zhao, T. Yan, W. Luo, X. Zhang, X. Wu, K. Wang, H. Zheng, Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors, Sci Rep, 5 (2015) 8130. [21] X. Yuan, L. Tang, S. Liu, P. Wang, Z. Chen, C. Zhang, Y. Liu, W. Wang, Y. Zou, C. Liu, N. Guo, J. Zou, P. Zhou, W. Hu, F. Xiu, Arrayed van der Waals Vertical Heterostructures Based on 2D GaSe Grown by Molecular Beam Epitaxy, Nano Lett., 15 (2015) 3571-3577. [22] Y. Zhou, Y. Nie, Y. Liu, K. Yan, J. Hong, C. Jin, Y. Zhou, J. Yin, Z. Liu, H. Peng, Epitaxy and photoresponse of two-dimensional GaSe crystals on flexible transparent mica sheets, ACS Nano, 8 (2014) 1485-1490. [23] V. Augelli, C. Manfredotti, R. Murri, L. Vasanelli, Hall-mobility anisotropy in GaSe, Physical Review B, 17 (1978) 3221-3226. [24] Y. Wu, D. Zhang, K. Lee, G.S. Duesberg, A. Syrlybekov, X. Liu, M. Abid, M. Abid, Y. Liu, L. Zhang, C.Ó. Coileáin, H. Xu, J. Cho, M. Choi, B.S. Chun, H. Wang, H. Liu, H.-C. Wu, Quantum Confinement and Gas Sensing of Mechanically Exfoliated GaSe, Advanced Materials Technologies, 2 (2017) 1600197. [25] X. Zhou, J. Cheng, Y. Zhou, T. Cao, H. Hong, Z. Liao, S. Wu, H. Peng, K. Liu, D. Yu, Strong Second-Harmonic Generation in Atomic Layered GaSe, J. Am. Chem. Soc., 137 (2015) 7994-7997. [26] C.S. Jung, K. Park, F. Shojaei, J.Y. Oh, H.S. Im, J.A. Lee, D.M. Jang, J. Park, N. Myoung, C.-L. Lee, J.W. Lee, J.K. Song, H.S. Kang, Photoluminescence and Photocurrents of GaS1–xSex Nanobelts, Chemistry of Materials, 28 (2016) 5811-5820. [27] Z. Ben Aziza, D. Pierucci, H. Henck, M.G. Silly, C. David, M. Yoon, F. Sirotti, K. Xiao, M. Eddrief, J.-C. Girard, A. Ouerghi, Tunable quasiparticle band gap in few-layer GaSe/graphene van der Waals heterostructures, Physical Review B, 96 (2017). [28] X. Li, M.W. Lin, J. Lin, B. Huang, A.A. Puretzky, C. Ma, K. Wang, W. Zhou, S.T. Pantelides, M. Chi, I. Kravchenko, J. Fowlkes, C.M. Rouleau, D.B. Geohegan, K. Xiao, Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy, Sci Adv, 2 (2016) e1501882. [29] X. Wei, F. Yan, Q. Lv, C. Shen, K. Wang, Fast gate-tunable photodetection in the graphene sandwiched WSe2/GaSe heterojunctions, Nanoscale, 9 (2017) 8388-8392. [30] R. Lu, J. Liu, H. Luo, V. Chikan, J.Z. Wu, Graphene/GaSe-Nanosheet Hybrid: Towards High Gain and Fast Photoresponse, Sci Rep, 6 (2016) 19161. [31] L. Tang, Z. Zhao, S. Yuan, T. Yang, B. Zhou, H. Zhou, Self-catalytic VLS growth one dimensional layered GaSe nanobelts for high performance photodetectors, Journal of Physics and Chemistry of Solids, 118 (2018) 186-191. 23
[32] A.C. Ferrari, F. Bonaccorso, V. Fal'ko, K.S. Novoselov, S. Roche, P. Boggild, S. Borini, F.H. Koppens, V. Palermo, N. Pugno, J.A. Garrido, R. Sordan, A. Bianco, L. Ballerini, M. Prato, E. Lidorikis, J. Kivioja, C. Marinelli, T. Ryhanen, A. Morpurgo, J.N. Coleman, V. Nicolosi, L. Colombo, A. Fert, M. Garcia-Hernandez, A. Bachtold, G.F. Schneider, F. Guinea, C. Dekker, M. Barbone, Z. Sun, C. Galiotis, A.N. Grigorenko, G. Konstantatos, A. Kis, M. Katsnelson, L. Vandersypen, A. Loiseau, V. Morandi, D. Neumaier, E. Treossi, V. Pellegrini, M. Polini, A. Tredicucci, G.M. Williams, B.H. Hong, J.H. Ahn, J.M. Kim, H. Zirath, B.J. van Wees, H. van der Zant, L. Occhipinti, A. Di Matteo, I.A. Kinloch, T. Seyller, E. Quesnel, X. Feng, K. Teo, N. Rupesinghe, P. Hakonen, S.R. Neil, Q. Tannock, T. Lofwander, J. Kinaret, Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, Nanoscale, 7 (2015) 4598-4810. [33] D. McManus, S. Vranic, F. Withers, V. Sanchez-Romaguera, M. Macucci, H. Yang, R. Sorrentino, K. Parvez, S.K. Son, G. Iannaccone, K. Kostarelos, G. Fiori, C. Casiraghi, Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures, Nat. Nanotechnol., 12 (2017) 343-350. [34] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O'Neill, C. Boland, M. Lotya, O.M. Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed, M. Moebius, H. Pettersson, E. Long, J. Coelho, S.E. O'Brien, E.K. McGuire, B.M. Sanchez, G.S. Duesberg, N. McEvoy, T.J. Pennycook, C. Downing, A. Crossley, V. Nicolosi, J.N. Coleman, Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids, Nat. Mater., 13 (2014) 624-630. [35] A.G. Kelly, T. Hallam, C. Backes, A. Harvey, A.S. Esmaeily, I. Godwin, J. Coelho, V. Nicolosi, J. Lauth, A. Kulkarni, S. Kinge, L.D. Siebbeles, G.S. Duesberg, J.N. Coleman, All-printed thin-film transistors from networks of liquid-exfoliated nanosheets, Science, 356 (2017) 69-73. [36] X. Xiong, Q. Zhang, X. Zhou, B. Jin, H. Li, T. Zhai, One-step synthesis of p-type GaSe nanoribbons and their excellent performance in photodetectors and phototransistors, J. Mater. Chem. C, 4 (2016) 7817-7823. [37] D.V. Rybkovskiy, N.R. Arutyunyan, A.S. Orekhov, I.A. Gromchenko, I.V. Vorobiev, A.V. Osadchy, E.Y. Salaev, T.K. Baykara, K.R. Allakhverdiev, E.D. Obraztsova, Size-induced effects in gallium selenide electronic structure: The influence of interlayer interactions, Physical Review B, 84 (2011). [38] H. Huang, P. Wang, Y. Gao, X. Wang, T. Lin, J. Wang, L. Liao, J. Sun, X. Meng, Z. Huang, X. Chen, J. Chu, Highly sensitive phototransistor based on GaSe nanosheets, Applied Physics Letters, 107 (2015) 143112. [39] P. Hu, Z. Wen, L. Wang, P. Tan, K. Xiao, Synthesis of few-layer GaSe nanosheets for high performance photodetectors, ACS Nano, 6 (2012) 5988-5994. [40] M. Mahjouri-Samani, M. Tian, K. Wang, A. Boulesbaa, C.M. Rouleau, A.A. Puretzky, M.A. McGuire, B.R. Srijanto, K. Xiao, G. Eres, G. Duscher, D.B. Geohegan, Digital transfer growth of patterned 2D metal chalcogenides by confined nanoparticle evaporation, ACS Nano, 8 (2014) 11567-11575. [41] Y. Zhao, R.L. Frost, Raman spectroscopy and characterisation of α-gallium oxyhydroxide and β-gallium oxide nanorods, Journal of Raman Spectroscopy, 39 (2008) 1494-1501. [42] X. Fu, P. Ilanchezhiyan, G. Mohan Kumar, H.D. Cho, L. Zhang, A.S. Chan, D.J. Lee, 24
G.N. Panin, T.W. Kang, Tunable UV-visible absorption of SnS2 layered quantum dots produced by liquid phase exfoliation, Nanoscale, 9 (2017) 1820-1826. [43] Z. Ben Aziza, H. Henck, D. Pierucci, M.G. Silly, E. Lhuillier, G. Patriarche, F. Sirotti, M. Eddrief, A. Ouerghi, van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties, ACS Nano, 10 (2016) 9679-9686. [44] S. Gao, X. Yi, J. Shang, G. Liu, R.W. Li, Organic and hybrid resistive switching materials and devices, Chem. Soc. Rev., 48 (2019) 1531-1565. [45] R. Waser, M. Aono, Nanoionics-based resistive switching memories, Nat Mater, 6 (2007) 833-840. [46] P. Cheng, K. Sun, Y.H. Hu, Memristive Behavior and Ideal Memristor of 1T Phase MoS2 Nanosheets, Nano Lett., 16 (2016) 572-576. [47] L. Zhang, T. Gong, H. Wang, Z. Guo, H. Zhang, Memristive devices based on emerging two-dimensional materials beyond graphene, Nanoscale, 11 (2019) 12413-12435. [48] S. Bertolazzi, P. Bondavalli, S. Roche, T. San, S.Y. Choi, L. Colombo, F. Bonaccorso, P. Samori, Nonvolatile Memories Based on Graphene and Related 2D Materials, Adv. Mater., 31 (2019) 1806663. [49] J.J. Yang, M.D. Pickett, X. Li, D.A. Ohlberg, D.R. Stewart, R.S. Williams, Memristive switching mechanism for metal/oxide/metal nanodevices, Nat. Nanotechnol., 3 (2008) 429-433. [50] D.B. Strukov, G.S. Snider, D.R. Stewart, R.S. Williams, The missing memristor found, Nature, 453 (2008) 80-83. [51] C. Hu, Q. Wang, S. Bai, M. Xu, D. He, D. Lyu, J. Qi, The effect of oxygen vacancy on switching mechanism of ZnO resistive switching memory, Applied Physics Letters, 110 (2017) 073501. [52] G. H. Shin, C.-Ki Kim, G. S. Bang, J. Y. Kim, B. C. Jang, B.J. Koo, M.H.W. , Y. Choi., S.-Y. Choi., Multilevel resistive switching nonvolatile memory based on MoS2 nanosheet-embedded graphene oxide, 2D Mater., 3 (2016) 034002.
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Supporting Information: Figure S1: Close-up view of the mixing head with the stator and rotor for the L5M high-shear laboratory shear mixer (Silverson Machines).
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Figure S2: Raman spectra of graphene prepared by CVD method.
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Research highlights The single crystalline GaSe layered nanorods mixed with nanoflakes are synthesized by shear and ultrasonic exfoliation methods, which show high quality and hexagonal crystalline structure. This work suggests a novel strategy to develop the resistive switching devices based on various 2D materials by using LPE method, and these 2D-nanomaterials-based memristors have the potential to be used for constructing neuromorphic computing systems.
The following is the corresponding contribution for each author: Guarantor of integrity of entire study: Lei Zhang, Gennady N. Panin Study design: Ganlin Chen, Xiao Fu, Luying Li Data acquisition: Ganlin Chen, Feng Cheng, Jiaxian Wan, Heng Zhang Data analysis/interpretation: Ganlin Chen, Xiao Fu, Luying Li, Feng Cheng, Jiaxian Wan, Heng Zhang Manuscript definition of intellectual content: Lei Zhang, Gennady N. Panin Manuscript revision/review: Lei Zhang, Gennady N. Panin, Ruikun Pan, Wanqiang Cao, A. Sattar Chan, Chang Liu The contributions of the above authors are true and are hereby stated.
Lei Zhang, Dr. Lei Zhang, Associate Professor School of Materials science and Engineering, Hubei University Wuchang Youyi Avenue No.368, 430062 Wuhan, Hubei Province, China Email: [email protected]
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30060-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153697
Reference:
JALCOM 153697
To appear in:
Journal of Alloys and Compounds
Received Date: 24 September 2019 Revised Date:
31 December 2019
Accepted Date: 6 January 2020
Please cite this article as: G. Chen, L. Zhang, L. Li, F. Cheng, X. Fu, J. Li, R. Pan, W. Cao, A.S. Chan, G.N. Panin, J. Wan, H. Zhang, C. Liu, GaSe layered nanorods formed by liquid phase exfoliation for resistive switching memory applications, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153697. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Graphical abstracts
A resistive switching device based on GaSe layered nanorods fabricated by LPE with a graphene/GaSe/graphene structure is demonstrated.
GaSe Layered Nanorods Formed by Liquid Phase Exfoliation for Resistive Switching Memory Applications
Ganlin Chen,1 Lei Zhang,1* Luying Li,2 Feng Cheng,2 Xiao Fu,3 Jinhua Li,1 Ruikun Pan,1 Wanqiang Cao,1 A. Sattar Chan,3 Gennady N. Panin,3,4* Jiaxian Wan,5 Heng Zhang5 and Chang Liu5
1
Ministry of Education Key Laboratory for Green Preparation and Application of
Functional Materials, Hubei Provincial Key Laboratory of Polymers, School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China 2
Center for Nanoscale Characterization and Devices, Wuhan National Laboratory for
Optoelectronics and School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China 3
Department of Physics, Nano Information Technology Academy, Dongguk
University, Seoul 04620, Republic of Korea 4
Institute for Microelectronics Technology and High Purity Materials, Chernogolovka,
Moscow Distr., 142432, Russia 5
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education,
School of Physics and Technology, Wuhan University, Wuhan 430072, China *
Corresponding author: Lei Zhang Email : [email protected]； Gennady N. Panin Email: [email protected]
1
Abstract: The increasing interest and rapid progress in the artificial neural networks have driven extensive research in resistive switching memory based on various insulators and two-dimensional nanomaterials. Herein, we have demonstrated a kind of resistive switching memory device based on GaSe layered nanorods fabricated by liquid phase exfoliation (LPE) method with a lateral graphene/GaSe/graphene structure. The single crystalline GaSe layered nanorods mixed with a few nanoflakes were synthesized by shear and ultrasonic (bath and probe) exfoliation methods, which showed high quality and hexagonal crystalline structure. The electrical properties and the resistive switching behavior were investigated, indicating the charge trapping/detrapping effect is the dominant resistive switching mechanism. This work suggests a novel strategy to develop the resistive switching devices based on various 2D materials by using LPE method, and these 2D-nanomaterials-based memristors have the potential to be used for constructing neuromorphic computing systems.
Keywords: Liquid phase exfoliation, two-dimensional nanomaterials, nanorods, resistive switching, charge trapping/detrapping
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1. Introduction The past decade has seen extensive research efforts being devoted to two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDCs). Accordingly, great progress has been made in both fundamental research and technique development [1-4] due to their fascinating properties as well as the possibility to revolutionize the future electronic and optoelectronic technologies. Graphene is one of the most important 2D materials and considered as a promising candidate for nanoelectronics. However, its zero band gap limits the applications in logic electronics and field-effect transistors (FETs) [5-7]. Following graphene, TMDCs (such as MoS2, MoSe2, WS2, WSe2, etc) and black phosphorus (BP) have attracted extensive attention and revealed extraordinary capabilities as FETs [8, 9], photodetectors [10] and sensors [11]. Very recently, atomically thin metal monochalcogenides, such as GaSe, GaS, InSe, have been extensively reported [12-14] with significantly different electronic and optoelectronic properties from TMDCs [15]. GaSe is a typical layered AIIIBVI binary chalcogenides with strong covalent in-plane inter-atomic bonding[16] and weaker van der Waals inter-plane bonding [17]. From the side view, the single tetralayer with a hexagonal in-plane structure consists of four atoms: Se-Ga-Ga-Se. The band gap energy of bulk GaSe is about 2.1 eV [18], and that of the monolayer is predicted to be more than 3.5 eV. So far, few-layer 2D GaSe [19] has demonstrated a variety of interesting electrical and optical properties, including p-type behavior [20], a high on/off ratio [21], high responsivity [22], anisotropic Hall mobility [23], outstanding gas sensibility [24], and strong second-harmonic 3
generation [25], which makes it an appealing candidate for vast applications in nano-optoelectronics, nonlinear optics, piezo-photonics, and wearable devices [17]. Like other 2D materials, the layered GaSe can be exfoliated into mono- or fewlayers by micromechanical exfoliation [26], and also synthesized on various substrate using vapor phase deposition [26], molecular beam epitaxy (MBE) [21, 25, 27], chemical vapor deposition (CVD) [15, 28, 29], and liquid phase exfoliation (LPE) [30]. As is well known, LPE is considered as one of the most promising routes toward large-scale production of monolayer, few-layers and quantum dots 2D materials. LPE has already been used as a mass-scale approach for production of 2D materials which offers several advantages including cost reduction and scalability. Another important advantage of LPE is that it can be used to create inks made of nanosheets of different 2D crystals for printing devices [31]. Furthermore, this technique is compatible with low-cost and flexible substrates, which is expected to be applied for flexible and wearable electronics [32]. The LPE methods can be classified into four main sections: direct ultrasonic (bath and probe sonication) exfoliation in various solvents, shear exfoliation, electrochemical exfoliation and functionalization-assisted exfoliation [7]. Traditionally, N-methyl-2-pyrrolidone (NMP) and N, N-dimethylformamide (DMF) are the candidates as the organic solvents for exfoliating most of 2D materials. However, they have some disadvantages such as high boiling points and toxicity which deteriorate the device performances and hinder their vast applications [14]. Although GaSe is considered as the most promising 2D material according to its properties, and the LPE method has been applied extensively for synthesizing a 4
variety of 2D materials [33], the study on GaSe nanostructures formed by the LPE method hasn’t been systematically explored. In the present work, we combine the shear and direct ultrasonic exfoliation by using environmentally friendly solvent to exfoliate the bulk GaSe into few-layered GaSe nanostructures, and obtain GaSe nanoflakes mixed with layered nanorods, which hasn’t been reported previously. Here, we employ Isopropanol Alcohol (IPA) and DI water with low boiling point as the solvents to exfoliate GaSe nanostructures, and study their optical and electrical properties systematically.
2. Experimental 1.2 g of bulk GaSe (Gallium selenide, 99.99%, purchased from Alfa Aesar) was added in 120 mL solution with 80% IPA and 20% DI water and shear-mixed for 2 hours at the speed of 6000 rpm by using a L5M high-shear laboratory shear mixer (Silverson Machines). As shown in the schematic of the exfoliation procedure in Fig. 1, following the shear mixing, the dispersion was further bath ultrasonicated for 5 hours with ice-cooling. Then, half of the dispersion was centrifuged at 3000 rpm for 30 min directly, and the other half of the dispersion was taken for additional sonication for 15 hours by using a horn-probe tip sonicator operating at 60% amplitude with 5 s On and 5 s Off pulses with ice-cooling, and then was centrifuged at 10000 rpm for 60 min. Finally, the top 30% supernatants of each dispersion (labeled as GaSe 3000 and GaSe 10000) were retained for further analyses, respectively. Fabrication of GaSe based devices: 5
GaSe suspensions were deposited on SiO2/Si substrates with graphene electrodes. The graphene electrodes were grown on Cu foil by CVD method at 1020 oC under methane/hydrogen flow at 600 mTorr, and then transferred onto SiO2/Si substrates by using PMMA. Characterization: The morphology of GaSe nanostructures was obtained by scanning electron microscope (SEM, XL30 SFEG, FEI). Transmission electron microscopy (TEM) analysis was performed on a Tecnai G2 20 U-TWIN TEM operated at 200 kV. Raman analysis was carried out on micro-Raman spectrometer using an excitation wavelength of 532 nm. The optical properties of the GaSe nanostructures were recorded by employing a Varian Cary ultraviolet−visible (UV−vis) spectrometer. The current-voltage characteristics were studied by using a Keithley 617 semiconductor parameter analyzer.
3. Results and discussion In order to analyze the morphological and structural properties of the synthesized GaSe nanostructures, SEM and TEM images were recorded. Figure 2(a) shows the SEM image of GaSe nanostructure obtained by LPE method with centrifugation speed of 3000 rpm, which contains mostly nanoflakes just as other 2D materials synthesized by the LPE method [34], and some nanorods also appear with different sizes. As shown in Fig. 2(c), statistical analysis based on SEM characterization suggests that the distribution of length of the nanorods follows the Gaussian function with an average length around 1 µm. In contrast, GaSe nanostructures obtained by the shear 6
and direct ultrasonic (bath and probe) exfoliation followed by centrifuging at 10000 rpm contains nanoflakes stacked in plane with a chaotic array of longer nanorods spreading on it as shown in Fig. 2(b). The width of the nanorods is in the range of 100-200 nm, and the length distributes from 4 to 14 µm with an average value of 7.33 µm as statistically analyzed in Fig. 2(d). It’s well known that LPE of micrometer size 2D powder could provide the stacked nanoflakes [35]. In this work, it’s interesting to find that lots of chaotically-distributed nanorods mixed with the flakes were obtained when the GaSe bulk crystals with the size of centimeter were exfoliated with a Mixer. As shown in the supporting information (Fig. S1), instead of the micrometer-sized powders which can flow into the stator due to attraction, and can be shear-mixed directly by the rotor, the centimeter-sized GaSe bulk crystals that can’t flow into the stator may be attached to the interface of the stator during shear-mixing and exfoliation. We suggest that the GaSe layered nanorods may come from the shear-exfoliation of the larger-sized GaSe bulk crystals attached at the stator by the inside rotor as shown in Fig. S1. Figures 3(a) and (b) show the typical bright-field TEM images for the nanostructures and a specific nanorod with a higher magnification. The corresponding selected area electron diffraction (SAED) patterns are shown in Fig. 3(c) and (d). Figure 3(c) corresponds to the red square area in Fig. 3(a), and the polycrystalline rings indicate many GaSe nanoflakes with different orientations gathering together. The SAED pattern of the single nanorod taken along the [110] zone axis in Fig. 3(d) shows only one set of diffraction spots of six-fold symmetry, which conforms to the 7
space group P-6 of hexagonal GaSe, indicating that single crystalline GaSe nanorods with hexagonal symmetry have been successfully synthesized. The top and side views of the atomic structure for the hexagonal GaSe nanorod are illustrated in the inset of Fig. 3(c) and (d). The optical properties of the GaSe nanostructures were evaluated via the UV-vis absorption, as shown in Fig. 4(a). For GaSe 3000, a broad absorption peak occurs around the wavelength of 600 nm, which is in accordance with its bandgap [18] and can be attributed to the thick GaSe nanoflakes absorption. It was demonstrated that a room temperature emission at wavelength of 710 nm is related to the defects and traps at the surface of GaSe nanoribben [36]. Here with a further probe-sonication treatment for 15 h and a high centrifugation speed of 10000 rpm for the sample GaSe 10000, the GaSe nanorods become more defective, and a significant absorption peak appears around 710 nm (the red spectrum in Fig. 4(a)) due to the defects and traps from the longer and thinner nanorods as illustrated by SEM image in Fig. 2(b). At the same time, smaller and thinner nanoflakes were also selected due to the higher centrifugation speed of 10000 rpm, which is consistent with Fig. 3(a) and (c). As predicted previously, the bandgap of GaSe nanoflakes can be modulated from 2.08 to 3.57 eV depending on the thickness [37]. For the absorption spectrum of GaSe 10000 in Fig. 4(a), the confined thin GaSe nanoflakes contribute to the absorption occurred at 450 nm and the absorption peak around 600 nm may be attributed to the remaining thick GaSe nanoflakes. Micro-Raman spectroscopy is a powerful technique to explore the evolution of 2D 8
materials with different sizes [18]. Figure 4(b) shows Raman spectra acquired from bulk GaSe, and GaSe nanostructures at the centrifugation speeds of 3000 and 10000 rpm, respectively. As shown in Figure 4(b), two of these vibrational modes below 100 cm-1 were not observed because of the detection limit of the Raman system, while the other four modes were detected for bulk GaSe, and the corresponding four peaks located at 134.8 cm-1 ( cm-1 (
), 209.5 cm-1 (
), 244.0 cm-1 (
), and 304.6
) are consistent with the vibration modes previously reported [38]. Two A1g
peaks with much stronger intensities are associated with the out-of-plane vibrational mode of GaSe, while E2g and very weak E1g peaks correspond to the in-plane mode [30, 39, 40]. As shown in Table 1, the ratios of and
/
/
(19.93),
/
(11.67)
(5.51) refer to thick GaSe crystal. However, the intensity of E1g peak
increases prominently after exfoliation, the
peak disappears and the
hardly observed for GaSe 3000 sample. The ratio of
/
peak is
decreases from 19.93
to 0.52 which suggests that the out of plane vibration mode becomes very weak comparing to the GaSe bulk crystal, indicating the thickness of GaSe nanostructure was decreased. With further exfoliation at higher speed and longer time of centrifugation for GaSe 10000, all out of plane modes disappears. While in-plane mode E1g becomes more prominent compared with GaSe 3000 (the ratio ℎ
/
increases from 0.37 to 0.41). It indicates that the E1g peak increases may
because of the dominance of the in-plane vibrational mode for much thinner nanosheets and nanorods for GaSe 10000. It is noted that no other peaks were obtained for GaSe nanostructure in the range of 100-500 cm-1 except four peaks for 9
GaSe and one peak located at 414.2 cm-1 which comes from the underneath sapphire substrate, suggesting that no oxidation reaction was occurred during exfoliation [41]. To investigate the electrical properties of GaSe nanostructures, devices were prepared by depositing the GaSe dispersions onto the SiO2/Si substrates between graphene electrodes. The device setup is shown in the inset of Fig. 5(a). The graphene electrodes were transferred by common PMMA wet transfer method, and then fabricated using a nanotip probe like previous reports [42]. The channel between two graphene electrodes was around 15 µm. The GaSe dispersion was then deposited onto two graphene electrodes by drop casting, followed by heating the samples at 80 oC in ambient for 1 h. Fig. S2 shows a Raman spectrum of graphene after processing. The G and 2D peaks are located at 1586 cm-1 and 2683 cm-1, respectively. A full width at half maximum (FWHM) of 33.7 cm-1 for 2D peak and the I(2D)/I(G) ratio of 1.62 as well as a weak D peak at 1355 cm-1 indicate the high quality of the transferred monolayer graphene. The current-voltage characteristic of the device (graphene/GaSe 3000/graphene) is shown in Fig. 5(a). The device is described as the GaSe channel with more nanoflakes and less nanorods sandwiched laterally by two Schottky diodes in opposite directions, as shown in the inset of Fig. 5(a). The non-linear rectifying behavior and the negligible hysteresis of I-V curve indicate that a barrier is formed at the graphene/GaSe nanostructures interface accompanied with charging effect. When graphene is in contact with GaSe, at thermal equilibrium, a dipole layer is formed at
10
the interface due to the charge transfer between graphene and GaSe [19, 30, 43], and a charge accumulation region is formed in the GaSe region. The I-V characteristic of the device graphene/GaSe 10000/graphene is further explored. During a continuous sweeping of a bias voltage from 0 V → 10 V → 0 V → -10 V → 0 V, an obvious pinched hysteresis loop was obtained as shown in Fig. 5(b). Initially, the I-V curve of the device is in its virgin state with a relatively high resistance. As the voltage increases, the current suddenly increases at the voltage of ~ 9.5 V, switching to a low resistance state (ON state). Then, the device switches to the OFF state with a high resistance as the voltage decreases to 0 V. Here, an ON/OFF ratio above 104 can be achieved, and the device operates well throughout over 100 cycles of ON/OFF without severe degradation, indicating that the device possesses excellent memristive behavior and high stability. Note that the current intensity is much lower than that of GaSe 3000, which could be attributed to the much lower concentration of the GaSe 10000 nanostructures when higher speed and longer time of centrifugation was used. Figure 5(c) shows the endurance characteristics of the resistance switching with graphene/GaSe 10000/graphene structure. The endurance characteristics is an important parameter for the performance of the resistive memory device, which refers to the phenomenon that the resistance value of the high and low resistance states of the memory deviates from the original resistance value as the number of switching times increases. After 100 cycles of switching, the resistance value of the device has no significant change, as shown in Fig. 5(c). This shows that the prepared graphene/GaSe 10000/graphene structure memory device has better 11
endurance resistance characteristics [44]. It is well known that the memristive devices can retain a state of internal resistance based on the history of applied voltage and current, which can store and process information and offer several key performance characteristics exceeding conventional integrated circuit technology [45]. Networks of such memristors, in the form of crossbar structures, have been widely studied for the energy-efficient artificial neural networks [46]. Till now, the memristors have been made from various materials and operated under different principles [47, 48]. The microscopic nature of resistance switching in such devices is still controversial [49], and different models have been suggested [50]: the origin of resistance change under the electric field as stemmed from charge trapping/de-trapping at the interface or defects [49]; the localized metal-atom chains between two electrodes [45]; the drift of vacancies/ions within the insulator layer [2]; and the filamentary conduction model based on the thermal effect [51], etc. It is possible that different mechanisms could be dominant in different materials systems, and even different mechanisms may co-exist [50]. To understand the mechanism of resistive switching behavior in our work, the double-logarithmic I-V curve was obtained, as shown in Fig. 5(d). When the applied voltage runs from 0 to the positive voltage, the device is at OFF state with a negligible current, since the injected charges are captured by the defects of the nanorods, and the traps of the interfaces between GaSe nanoflakes and nanorods. Thus, the traps would be constantly filled as the voltage increases. As the I-V curve sweeps from 8.5 to 9.5 V, the I-V relationship is fitted as 12
∝
. According to the charge
trapping/detrapping theory, the traps in the channel have all been filled, and then the injected charges could be de-trapped as the voltage increases, leading to an abrupt increase of current and the switching from OFF to ON state. When the applied voltage sweeps back to 0 V, the I-V characteristics change in nature from partially-trap-filled
∝
∝
to
( > 2), and the slopes of the double-logarithmic I-V
curve changes from 2, 4 to 6. According to the fitting results, we suggest that the resistive switching mechanism could be explained by the charge trapping/de-trapping effect. Furthermore, we consider that the electron transports through nanoflakes and nanorods should co-exist, both of which play important roles in the graphene/GaSe 3000/graphene sample, because of their difference in size and the small concentrations of both nanoflakes and nanorods. At the same time, the weak absorption in the absorption spectrum of GaSe 3000 in the range of 700-800 nm indicates that the defects also exist, which contributes to the negligible hysteresis loop in I-V curve as shown in Fig. 5(a). However for the sample GaSe 10000, the size of nanoflakes is too small to form continuous conducting film, while the nanorods (2~16 µm) are long enough to connect two graphene electrodes (~ 15 µm). It suggests that the electrons transport mainly through the longer nanorods within the device made of GaSe 10000. Since more defects in the nanorods of GaSe 10000 are introduced by the probe-sonication treatment for a long time compared to GaSe 3000, as illustrated by the absorption spectra in Fig. 4(a), the device shows evident resistance switching behavior in Fig. 5(b). Therefore, we conclude that the resistive switching mechanism 13
in this work should be based on the charge trapping/de-trapping effect by the defects (traps) rather than filamentary conduction, Poole-Frenkel effect, and so on [52].
4. Conclusions In this work, we have demonstrated a resistive switching memory device with a lateral graphene/GaSe/graphene structure. The single crystalline GaSe layered nanorods mixed with nanoflakes are synthesized by shear and ultrasonic exfoliation methods, which show high quality and hexagonal crystalline structure. The memory device exhibits excellent memristive behavior and good stability that an ON/OFF ratio above 104 and the endurance cycles over 100 can be achieved. Our results indicate that the charge trapping/de-trapping effect is the dominant resistive switching mechanism in accordance with previous reports. This work suggests that the liquid phase exfoliation method is a novel strategy to develop the memristor devices with good performances for the neuromorphic computing systems based on various 2D materials.
Acknowledgements This work was financially supported by the National Key Research and Development Plan (MOST) under Grant Number 2017YFA0205802, the National Natural Science Foundation of China (Grant No. 11574075, No. 51871104, No. 11574235, and No. 11875212).
14
Table 1. Comparison of the ratios for different Raman modes. /
/
/
GaSe bulk
19.93
11.67
5.51
-
GaSe 3000
0.52
-
0.53
0.37
GaSe 10000
-
-
-
0.41
15
/
ℎ
Figure captions: Figure 1: Schematic of the exfoliation procedure of GaSe nanostructures.
Figure 2: SEM images of GaSe nanostructures obtained at centrifugation speeds of (a) 3000 and (b) 10000 rpm. (c) and (d) Statistical analyses of nanorods length referring to (a) and (b), respectively.
Figure 3: (a) and (b) Bright-field TEM images of GaSe nanostructures and a specific nanorod obtained at centrifugation speed of 10000 rpm. (c) Selected area electron diffraction (SAED) pattern for the square area in (a). (d) SAED pattern for the single nanorod taken along the [110] zone axis in (b). Inset: top view and side view of the atomic strucuture for the GaSe nanorods. Atoms in the structure: Ga, blue; Se, yellow.
Figure 4: (a) Absorption spectra of GaSe nanostructures obtained at centrifugation speeds of 3000 and 10000 rpm. (b) Raman spectra of bulk GaSe and GaSe nanostructures obtained at centrifugation speeds of 3000 and 10000 rpm.
Figure 5: (a) I-V curve of the device made by GaSe nanostructures obtained at centrifuge
speed
of
graphene/GaSe/graphene.
3000 (b)
rpm.
Inset:
Typical
I-V
device
setup
characteristic
with of
structure
of
graphene/GaSe
10000/graphene switch at the 1st (red), 50th (black), and 100th (blue) cycles at room temperature. (c) Endurance characteristics of the device with 100 switching cycles. (d) Double-logarithmic I-V curve of the resistive switching memory device.
16
Figure 1
17
Figure 2
18
Figure 3
19
Figure 4
20
Figure 5
21
References: [1] M. Xu, T. Liang, M. Shi, H. Chen, Graphene-like two-dimensional materials, Chem. Rev., 113 (2013) 3766-3798. [2] M.M. Rehman, G.U. Siddiqui, J.Z. Gul, S.W. Kim, J.H. Lim, K.H. Choi, Resistive Switching in All-Printed, Flexible and Hybrid MoS2-PVA Nanocomposite based Memristive Device Fabricated by Reverse Offset, Sci Rep, 6 (2016) 36195. [3] X. Duan, C. Wang, A. Pan, R. Yu, X. Duan, Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges, Chem. Soc. Rev., 44 (2015) 8859-8876. [4] F. Pan, S. Gao, C. Chen, C. Song, F. Zeng, Recent progress in resistive random access memories: Materials, switching mechanisms, and performance, Materials Science and Engineering: R: Reports, 83 (2014) 1-59. [5] N.M. Freitag, T. Reisch, L.A. Chizhova, P. Nemes-Incze, C. Holl, C.R. Woods, R.V. Gorbachev, Y. Cao, A.K. Geim, K.S. Novoselov, J. Burgdorfer, F. Libisch, M. Morgenstern, Large tunable valley splitting in edge-free graphene quantum dots on boron nitride, Nat. Nanotechnol., 13 (2018) 392-397. [6] T. Szkopek, A longer-lasting memory in layered semiconductors, Nat. Nanotechnol., 13 (2018) 357. [7] A. Amiri, M. Naraghi, G. Ahmadi, M. Soleymaniha, M. Shanbedi, A review on liquid-phase exfoliation for scalable production of pure graphene, wrinkled, crumpled and functionalized graphene and challenges, FlatChem, 8 (2018) 40-71. [8] C. Liu, X. Yan, X. Song, S. Ding, D.W. Zhang, P. Zhou, A semi-floating gate memory based on van der Waals heterostructures for quasi-non-volatile applications, Nat. Nanotechnol., 13 (2018) 404-410. [9] M. Si, C. Jiang, W. Chung, Y. Du, M.A. Alam, P.D. Ye, Steep-Slope WSe2 Negative Capacitance Field-Effect Transistor, Nano Lett., 18 (2018) 3682-3687. [10] G. Wang, Y. Zhang, C. You, B. Liu, Y. Yang, H. Li, A. Cui, D. Liu, H. Yan, Two dimensional materials based photodetectors, Infrared Physics & Technology, 88 (2018) 149-173. [11] Y. Xu, X. Wang, W.L. Zhang, F. Lv, S. Guo, Recent progress in two-dimensional inorganic quantum dots, Chem. Soc. Rev., 47 (2018) 586-625. [12] B. Chitara, A. Ya'akobovitz, Elastic properties and breaking strengths of GaS, GaSe and GaTe nanosheets, Nanoscale, 10 (2018) 13022-13027. [13] A. Islam, J. Lee, P.X.L. Feng, Atomic Layer GaSe/MoS2 van der Waals Heterostructure Photodiodes with Low Noise and Large Dynamic Range, ACS Photonics, 5 (2018) 2693-2700. [14] Y. Sun, S. Luo, X.G. Zhao, K. Biswas, S.L. Li, L. Zhang, InSe: a two-dimensional material with strong interlayer coupling, Nanoscale, 10 (2018) 7991-7998. [15] N. Zhou, R. Wang, X. Zhou, H. Song, X. Xiong, Y. Ding, J. Lu, L. Gan, T. Zhai, P-GaSe/N-MoS2 Vertical Heterostructures Synthesized by van der Waals Epitaxy for Photoresponse Modulation, Small, 14 (2018). [16] O.D. Pozo-Zamudio, S. Schwarz, M. Sich, I.A. Akimov, M. Bayer, R.C. Schofield, E.A. Chekhovich, B.J. Robinson, N.D. Kay, O.V. Kolosov, A.I. Dmitriev, G.V. Lashkarev, D.N. 22
Borisenko, N.N. Kolesnikov, A.I. Tartakovskii, Photoluminescence of two-dimensional GaTe and GaSe films, 2D Materials, 2 (2015) 035010. [17] X. Li, M.W. Lin, A.A. Puretzky, J.C. Idrobo, C. Ma, M. Chi, M. Yoon, C.M. Rouleau, Kravchenko, II, D.B. Geohegan, K. Xiao, Controlled vapor phase growth of single crystalline, two-dimensional GaSe crystals with high photoresponse, Sci Rep, 4 (2014) 5497. [18] A. Bergeron, J. Ibrahim, R. Leonelli, S. Francoeur, Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy, Applied Physics Letters, 110 (2017) 241901. [19] C. Si, Z. Lin, J. Zhou, Z. Sun, Controllable Schottky barrier in GaSe/graphene heterostructure: the role of interface dipole, 2D Materials, 4 (2016) 015027. [20] Y. Cao, K. Cai, P. Hu, L. Zhao, T. Yan, W. Luo, X. Zhang, X. Wu, K. Wang, H. Zheng, Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors, Sci Rep, 5 (2015) 8130. [21] X. Yuan, L. Tang, S. Liu, P. Wang, Z. Chen, C. Zhang, Y. Liu, W. Wang, Y. Zou, C. Liu, N. Guo, J. Zou, P. Zhou, W. Hu, F. Xiu, Arrayed van der Waals Vertical Heterostructures Based on 2D GaSe Grown by Molecular Beam Epitaxy, Nano Lett., 15 (2015) 3571-3577. [22] Y. Zhou, Y. Nie, Y. Liu, K. Yan, J. Hong, C. Jin, Y. Zhou, J. Yin, Z. Liu, H. Peng, Epitaxy and photoresponse of two-dimensional GaSe crystals on flexible transparent mica sheets, ACS Nano, 8 (2014) 1485-1490. [23] V. Augelli, C. Manfredotti, R. Murri, L. Vasanelli, Hall-mobility anisotropy in GaSe, Physical Review B, 17 (1978) 3221-3226. [24] Y. Wu, D. Zhang, K. Lee, G.S. Duesberg, A. Syrlybekov, X. Liu, M. Abid, M. Abid, Y. Liu, L. Zhang, C.Ó. Coileáin, H. Xu, J. Cho, M. Choi, B.S. Chun, H. Wang, H. Liu, H.-C. Wu, Quantum Confinement and Gas Sensing of Mechanically Exfoliated GaSe, Advanced Materials Technologies, 2 (2017) 1600197. [25] X. Zhou, J. Cheng, Y. Zhou, T. Cao, H. Hong, Z. Liao, S. Wu, H. Peng, K. Liu, D. Yu, Strong Second-Harmonic Generation in Atomic Layered GaSe, J. Am. Chem. Soc., 137 (2015) 7994-7997. [26] C.S. Jung, K. Park, F. Shojaei, J.Y. Oh, H.S. Im, J.A. Lee, D.M. Jang, J. Park, N. Myoung, C.-L. Lee, J.W. Lee, J.K. Song, H.S. Kang, Photoluminescence and Photocurrents of GaS1–xSex Nanobelts, Chemistry of Materials, 28 (2016) 5811-5820. [27] Z. Ben Aziza, D. Pierucci, H. Henck, M.G. Silly, C. David, M. Yoon, F. Sirotti, K. Xiao, M. Eddrief, J.-C. Girard, A. Ouerghi, Tunable quasiparticle band gap in few-layer GaSe/graphene van der Waals heterostructures, Physical Review B, 96 (2017). [28] X. Li, M.W. Lin, J. Lin, B. Huang, A.A. Puretzky, C. Ma, K. Wang, W. Zhou, S.T. Pantelides, M. Chi, I. Kravchenko, J. Fowlkes, C.M. Rouleau, D.B. Geohegan, K. Xiao, Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy, Sci Adv, 2 (2016) e1501882. [29] X. Wei, F. Yan, Q. Lv, C. Shen, K. Wang, Fast gate-tunable photodetection in the graphene sandwiched WSe2/GaSe heterojunctions, Nanoscale, 9 (2017) 8388-8392. [30] R. Lu, J. Liu, H. Luo, V. Chikan, J.Z. Wu, Graphene/GaSe-Nanosheet Hybrid: Towards High Gain and Fast Photoresponse, Sci Rep, 6 (2016) 19161. [31] L. Tang, Z. Zhao, S. Yuan, T. Yang, B. Zhou, H. Zhou, Self-catalytic VLS growth one dimensional layered GaSe nanobelts for high performance photodetectors, Journal of Physics and Chemistry of Solids, 118 (2018) 186-191. 23
[32] A.C. Ferrari, F. Bonaccorso, V. Fal'ko, K.S. Novoselov, S. Roche, P. Boggild, S. Borini, F.H. Koppens, V. Palermo, N. Pugno, J.A. Garrido, R. Sordan, A. Bianco, L. Ballerini, M. Prato, E. Lidorikis, J. Kivioja, C. Marinelli, T. Ryhanen, A. Morpurgo, J.N. Coleman, V. Nicolosi, L. Colombo, A. Fert, M. Garcia-Hernandez, A. Bachtold, G.F. Schneider, F. Guinea, C. Dekker, M. Barbone, Z. Sun, C. Galiotis, A.N. Grigorenko, G. Konstantatos, A. Kis, M. Katsnelson, L. Vandersypen, A. Loiseau, V. Morandi, D. Neumaier, E. Treossi, V. Pellegrini, M. Polini, A. Tredicucci, G.M. Williams, B.H. Hong, J.H. Ahn, J.M. Kim, H. Zirath, B.J. van Wees, H. van der Zant, L. Occhipinti, A. Di Matteo, I.A. Kinloch, T. Seyller, E. Quesnel, X. Feng, K. Teo, N. Rupesinghe, P. Hakonen, S.R. Neil, Q. Tannock, T. Lofwander, J. Kinaret, Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, Nanoscale, 7 (2015) 4598-4810. [33] D. McManus, S. Vranic, F. Withers, V. Sanchez-Romaguera, M. Macucci, H. Yang, R. Sorrentino, K. Parvez, S.K. Son, G. Iannaccone, K. Kostarelos, G. Fiori, C. Casiraghi, Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures, Nat. Nanotechnol., 12 (2017) 343-350. [34] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O'Neill, C. Boland, M. Lotya, O.M. Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed, M. Moebius, H. Pettersson, E. Long, J. Coelho, S.E. O'Brien, E.K. McGuire, B.M. Sanchez, G.S. Duesberg, N. McEvoy, T.J. Pennycook, C. Downing, A. Crossley, V. Nicolosi, J.N. Coleman, Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids, Nat. Mater., 13 (2014) 624-630. [35] A.G. Kelly, T. Hallam, C. Backes, A. Harvey, A.S. Esmaeily, I. Godwin, J. Coelho, V. Nicolosi, J. Lauth, A. Kulkarni, S. Kinge, L.D. Siebbeles, G.S. Duesberg, J.N. Coleman, All-printed thin-film transistors from networks of liquid-exfoliated nanosheets, Science, 356 (2017) 69-73. [36] X. Xiong, Q. Zhang, X. Zhou, B. Jin, H. Li, T. Zhai, One-step synthesis of p-type GaSe nanoribbons and their excellent performance in photodetectors and phototransistors, J. Mater. Chem. C, 4 (2016) 7817-7823. [37] D.V. Rybkovskiy, N.R. Arutyunyan, A.S. Orekhov, I.A. Gromchenko, I.V. Vorobiev, A.V. Osadchy, E.Y. Salaev, T.K. Baykara, K.R. Allakhverdiev, E.D. Obraztsova, Size-induced effects in gallium selenide electronic structure: The influence of interlayer interactions, Physical Review B, 84 (2011). [38] H. Huang, P. Wang, Y. Gao, X. Wang, T. Lin, J. Wang, L. Liao, J. Sun, X. Meng, Z. Huang, X. Chen, J. Chu, Highly sensitive phototransistor based on GaSe nanosheets, Applied Physics Letters, 107 (2015) 143112. [39] P. Hu, Z. Wen, L. Wang, P. Tan, K. Xiao, Synthesis of few-layer GaSe nanosheets for high performance photodetectors, ACS Nano, 6 (2012) 5988-5994. [40] M. Mahjouri-Samani, M. Tian, K. Wang, A. Boulesbaa, C.M. Rouleau, A.A. Puretzky, M.A. McGuire, B.R. Srijanto, K. Xiao, G. Eres, G. Duscher, D.B. Geohegan, Digital transfer growth of patterned 2D metal chalcogenides by confined nanoparticle evaporation, ACS Nano, 8 (2014) 11567-11575. [41] Y. Zhao, R.L. Frost, Raman spectroscopy and characterisation of α-gallium oxyhydroxide and β-gallium oxide nanorods, Journal of Raman Spectroscopy, 39 (2008) 1494-1501. [42] X. Fu, P. Ilanchezhiyan, G. Mohan Kumar, H.D. Cho, L. Zhang, A.S. Chan, D.J. Lee, 24
G.N. Panin, T.W. Kang, Tunable UV-visible absorption of SnS2 layered quantum dots produced by liquid phase exfoliation, Nanoscale, 9 (2017) 1820-1826. [43] Z. Ben Aziza, H. Henck, D. Pierucci, M.G. Silly, E. Lhuillier, G. Patriarche, F. Sirotti, M. Eddrief, A. Ouerghi, van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties, ACS Nano, 10 (2016) 9679-9686. [44] S. Gao, X. Yi, J. Shang, G. Liu, R.W. Li, Organic and hybrid resistive switching materials and devices, Chem. Soc. Rev., 48 (2019) 1531-1565. [45] R. Waser, M. Aono, Nanoionics-based resistive switching memories, Nat Mater, 6 (2007) 833-840. [46] P. Cheng, K. Sun, Y.H. Hu, Memristive Behavior and Ideal Memristor of 1T Phase MoS2 Nanosheets, Nano Lett., 16 (2016) 572-576. [47] L. Zhang, T. Gong, H. Wang, Z. Guo, H. Zhang, Memristive devices based on emerging two-dimensional materials beyond graphene, Nanoscale, 11 (2019) 12413-12435. [48] S. Bertolazzi, P. Bondavalli, S. Roche, T. San, S.Y. Choi, L. Colombo, F. Bonaccorso, P. Samori, Nonvolatile Memories Based on Graphene and Related 2D Materials, Adv. Mater., 31 (2019) 1806663. [49] J.J. Yang, M.D. Pickett, X. Li, D.A. Ohlberg, D.R. Stewart, R.S. Williams, Memristive switching mechanism for metal/oxide/metal nanodevices, Nat. Nanotechnol., 3 (2008) 429-433. [50] D.B. Strukov, G.S. Snider, D.R. Stewart, R.S. Williams, The missing memristor found, Nature, 453 (2008) 80-83. [51] C. Hu, Q. Wang, S. Bai, M. Xu, D. He, D. Lyu, J. Qi, The effect of oxygen vacancy on switching mechanism of ZnO resistive switching memory, Applied Physics Letters, 110 (2017) 073501. [52] G. H. Shin, C.-Ki Kim, G. S. Bang, J. Y. Kim, B. C. Jang, B.J. Koo, M.H.W. , Y. Choi., S.-Y. Choi., Multilevel resistive switching nonvolatile memory based on MoS2 nanosheet-embedded graphene oxide, 2D Mater., 3 (2016) 034002.
25
Supporting Information: Figure S1: Close-up view of the mixing head with the stator and rotor for the L5M high-shear laboratory shear mixer (Silverson Machines).
26
Figure S2: Raman spectra of graphene prepared by CVD method.
27
Research highlights The single crystalline GaSe layered nanorods mixed with nanoflakes are synthesized by shear and ultrasonic exfoliation methods, which show high quality and hexagonal crystalline structure. This work suggests a novel strategy to develop the resistive switching devices based on various 2D materials by using LPE method, and these 2D-nanomaterials-based memristors have the potential to be used for constructing neuromorphic computing systems.
The following is the corresponding contribution for each author: Guarantor of integrity of entire study: Lei Zhang, Gennady N. Panin Study design: Ganlin Chen, Xiao Fu, Luying Li Data acquisition: Ganlin Chen, Feng Cheng, Jiaxian Wan, Heng Zhang Data analysis/interpretation: Ganlin Chen, Xiao Fu, Luying Li, Feng Cheng, Jiaxian Wan, Heng Zhang Manuscript definition of intellectual content: Lei Zhang, Gennady N. Panin Manuscript revision/review: Lei Zhang, Gennady N. Panin, Ruikun Pan, Wanqiang Cao, A. Sattar Chan, Chang Liu The contributions of the above authors are true and are hereby stated.
Lei Zhang, Dr. Lei Zhang, Associate Professor School of Materials science and Engineering, Hubei University Wuchang Youyi Avenue No.368, 430062 Wuhan, Hubei Province, China Email: [email protected]
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: