Biomass‐Derived Nanoporous Graphene Memory Cell

Nanoporous graphene (NPG) exhibits an apparent semiconductivity to solve the zero‐gap problem of graphene and also offers multifunctionalities that are directly associated with its structural and chemical nature. However, reliable, low‐cost, and large‐scale production of NPG is a major challenge for its practical applications. Here, a high‐performance resistive‐switching memory cell based on biomass‐derived NPG materials is demonstrated for the first time. A new processing method is suggested to create 3D NPG starting from Saccharum officinarum. The fabricated Au/NPG/Au two‐terminal devices achieve an excellent electrical performance characterized by an operating voltage below 5 V and an ON/OFF current ratio of over 106. A range of materials and device characterizations reveal the oxygen ion migration and charge‐injection modulation as a key mechanism behind the observed memory behaviors. This unconventional approach to high‐performance memory devices is an important step toward sustainable electronics and intelligent technologies.


Biomass-Derived Nanoporous Graphene Memory Cell
Seyed Mehdi Sattari-Esfahlan, Yvan Bonnassieux, Ioannis Kymissis, and Chang-Hyun Kim* DOI: 10.1002/admi.202200084 be efficient in breaking the intrinsic energetic symmetry of graphene in opening a meaningful energy bandgap. [1][2][3][4][5] For instance, a systematic structural modification in atomic-scale graphene superlattices (GSLs) [6,7] has been introduced to make use of their quantized energy states [8] and in situ electrical tunability. [9] Graphene nanoribbons (GNRs) have shown strongly structure-dependent transport regimes, [10] and graphene quantum dots (GQDs) have also exhibited a bandgap directly sensitive to the number of graphene layers and the number of atoms per layer. [11] Graphene heterostructures hybridized with other 2D materials [12][13][14][15] have manifested intriguing physical, chemical, and electrical properties that are not easily obtainable by homogeneous materials. In this regard, structural understanding and manipulation of graphene-based nanomaterials is now gaining an increasing interest, motivating a wide range of electronics [2,[16][17][18][19][20][21][22] and optoelectronics research. [23][24][25][26] Recently, nanoporous graphene (NPG) with a 3D structural motif was suggested as a new functional material. [27,28] In addition to the controlled semi-conductivity of NPG from partially broken π-conjugation, the large surface-to-volume ratio and the possibility of chemical pore-edge functionalization [29] seems to be key advantages of this material for a wide range of emerging electronic devices. For instance, a small sheet-tosheet distance in NPG can provide a large bandgap required for efficient switching, while an increasing pore density can facilitate the electrical current. [30] In terms of devices, fieldeffect transistors (FETs) based on NPG showed an ON/OFF current ratio and current levels comparable to those of GNR FETs. [31] Related theoretical investigation revealed the quantum transport regime and detailed band structures of NPG. [32] Also, NPG Esaki diodes were experimentally demonstrated, [33] which featured a negative differential resistance. However, we recognized the lack of efforts on applying NPG to memory devices and systems, [33][34][35][36][37] despite the rapidly increasing impact of nanomaterials-based memory technologies on both traditional electronics [33,38] and emerging neuromorphic computing systems. [39][40][41] Here, we report on an environmentally friendly chemical method to prepare versatile NPG and demonstrate high-performance resistive-switching memories that incorporate this NPG as an active channel medium. Our simple two-terminal NPG memory cells exhibited a controllable length-dependent threshold-switching characteristic, with a high ON/OFF current ratio and a competitively low operating voltage. The main performance metrics of our devices favorably compare to Nanoporous graphene (NPG) exhibits an apparent semiconductivity to solve the zero-gap problem of graphene and also offers multifunctionalities that are directly associated with its structural and chemical nature. However, reliable, low-cost, and large-scale production of NPG is a major challenge for its practical applications. Here, a high-performance resistive-switching memory cell based on biomass-derived NPG materials is demonstrated for the first time. A new processing method is suggested to create 3D NPG starting from Saccharum officinarum. The fabricated Au/NPG/Au two-terminal devices achieve an excellent electrical performance characterized by an operating voltage below 5 V and an ON/OFF current ratio of over 10 6 . A range of materials and device characterizations reveal the oxygen ion migration and charge-injection modulation as a key mechanism behind the observed memory behaviors. This unconventional approach to high-performance memory devices is an important step toward sustainable electronics and intelligent technologies.

Introduction
The large-scale fabrication of "semi-conducting" graphene nanostructures has been an important motivation in modern nanoelectronics and optoelectronics research. Specifically, a nanoscale structural manipulation of graphene was found to

Results and Discussion
We employed the Saccharum officinarum (also known as sugar cane) as a starting material and synthesized NPG through steps illustrated in Scheme 1. Briefly, the raw bagasse material was mechanically ground, dissolved in KOH, and dried to form an intermediate carbon mixture. It was then fully calcinated in an inert atmosphere, enabling a simple production of high-purity NPG in large quantities. After synthesis, a dry transfer technique was adopted to build electronic devices (further details in the Experimental Section). We fabricated NPG-based memory in a lateral two-terminal device configuration by realizing Au/NPG/ Au structure on an insulating substrate. A schematic device structure and an optical image of the single memory cell are presented in Figure 1a,b respectively. From a structural point of view, the synthesized solid-state material was shown to feature a highly 3D and hierarchical nature, which might be useful for various electronic and catalytic applications. For instance, a scanning-electron microscopy (SEM) image in Figure 1c gives evidence of a broad rupture of long-range sheet continuity, which is supposed to contribute to gap opening. For a more nanoscopic investigation, we took a high-resolution transmission-electron microscopy (HR-TEM) image of the NPG flake. Figure 1d clearly shows the existence of a local planar region that contains densely packed nanopores with an estimated size roughly ranging from a few nanometers to a few tens of nanometers.
Considering our highly unconventional NPG synthesis, there is an important need to directly probe its fundamental materials properties, which may differ from those reported for other graphene nanostructures. We therefore used a range of compositional, crystallographic, and surface analysis tools to unravel intrinsic properties of the biomass-derived NPG. As shown in Figure 2a, the Raman spectrum of NPG exhibits three dominant peaks. The D peak is at 1340 cm -1 and is attributed to disorders in sp 2 structure, defects, porosity, and structural imperfections present in this material. The honeycomb-like carbon arrangement dictates the appearance of the G peak at 1591 cm -1 . The peak at 2690 cm -1 indicates the formation of 2D graphene structure. Figure 2b shows the X-ray diffraction (XRD) patterns of NPG. Here, the strong peak at ≈26.9° is from the (002) plane of crystalline graphene and the weaker peak at ≈42.8° corresponds to the reflection of the (100) plane. It is inferred that the (002) peak is attributed to the dominant orientation of the individual graphene layers in a wholly 3D arrangement, while the (100) peak originates from the local condensation of the carbon rings. Further analyses of the width and intensity of these peaks provided the average number of stacked graphene layers in wellordered regions from three to eight in our NPG.
X-ray photoemission spectroscopy (XPS) measurement of the as-prepared NPG film shows the presence of oxygencontaining functional groups (which we consider to be a key element in memory mechanisms). As shown in Figure 2c, the peaks at 286.5, 288.4, and 289.3 eV stem from the CO, CO, and OCO, respectively. The XPS data for Au4f peak analysis of a Au-NPG interface sample showed peaks at 83.5 and 87 eV indicating the tight binding between the Au surface and the deposited NPG channel ( Figure S1, Supporting Information). [42] It is also important to note that, differently from graphene oxide (GO) junctions with low work-function metals, [43] no traces of Au-O bonding were found in our XPS data, allowing us to exclude any physical mechanisms based on electrode oxidation that are mostly relevant to devices with Al, Ag, or Cu contacts. [44] Our NPG film was further characterized by the Brunauer-Emmett-Teller (BET) method, which relies on the measured dependence of adsorbed N 2 molecules on the relative gas pressure for a model-based estimation of pore sizes [45] (Figure S2, Supporting Information). The measurement revealed a large specific surface area (SSA) of NPG over 2750 m 2 g −1 , and a minimum adsorption variation was observed in the relative pressure range of 0.4-1 with a negligible hysteresis, pointing to a high porosity of our sample. We infer that such a high SSA reflects an intrinsic cellulosic structure of the raw plant material. The pore size distribution of NPG from BET analysis is provided in Figure 2d. This result indicates an apparently hierarchical distribution of pores, which vary in size from a few nanometers to more than a few tens of nanometers. By integrating the obtained data, an average pore size is estimated as ≈2.5 nm with a total pore volume of ≈2.3 cm 3 g −1 .
We successfully fabricated electrical memories incorporating the biomass-derived NPG as an active channel, in a lateral Au/NPG/Au two-terminal device configuration depicted in Figure 1a. Figure 3a shows a semi-log plot current-voltage (I-V) curve measured on an NPG device with the channel length (L) of 100 µm. Overall, the observed cyclic I-V characteristics with a very large hysteresis originate from an efficient switching from the high-resistance state (HRS, or OFF-state) to the low-resistance state (LRS, or ON-state) of the NPG device, occurring at a threshold voltage (V TH ) of ≈4.9 V. Most importantly, the ON/OFF current ratio (I ON /I OFF ) above 10 6 here is a strong evidence of substantial gap opening and an excellent semi-conductivity of our NPG material. The same results were plotted on a double-log plot in Figure 3b for transport analysis. During the forward sweep (from 0 to 5 V), a crossover from an injection-limited (exponential) to trap-charge-limited current (TCLC) is clearly observed. [46] In contrast, the electrical current over the revere sweep (from 5 to 0 V) is found to be practically Ohmic (see also the excellent linearity in the inset figure), revealing a remarkable change in charge-transport behavior induced by the threshold switching. We fabricated a set of NPG memories with five varying L values, to gain further insights into their operating principles. The channel width of all the tested devices was kept as close as possible to ≈20 µm by choosing NPG flakes with a right physical dimension and by carefully placing them onto the device substrate. The thickness of an NPG flake locally varies due to its structural characteristics, while an average value of ≈450 nm was estimated from a cross-sectional SEM image ( Figure S3, Supporting Information). Figure 3c collectively visualizes the measured L-dependence of electrical memory characteristics. A particularly interesting feature is that the I OFF gets smaller when L increases (which seems to be predictable from a conventional scaling law), while the I ON tends to have a reverse trend. This observation will be further rationalized below after suggesting an operating mechanism of our systems. The V TH value and I ON /I OFF ratio of each cell were extracted from Figure 3c and directly plotted against L in Figure 3d. It was found that the V TH is relatively independent of L, allowing us to obtain an average V TH (V TH, avg ) of 4.3 V, a characteristic that we will also use to come up with our own working mechanism. On the other hand, the I ON /I OFF ratio substantially increases with increasing L, due to the bi-directional modulation in Figure 3c. We additionally checked the reproducibility and sustainability of the electrical states of the NPG memories. Our devices showed a stable hysteresis over repeated voltage sweeps ( Figure S4, Supporting Information), and also provided reproducible ON-and OFFstate resistance levels for over 200 switching cycles ( Figure S5, Supporting Information). A bipolar switching (hysteresis at both positive and negative V) was occasionally observed at memories with a small L featuring a high initial current and a low negative reset V TH ( Figure S6, Supporting Information).
We propose the key mechanism of our NPG memory devices in Figure 4. While the XPS data in Figure 2c revealed the presence of oxygenated chemical groups, the principles of our NPG memories are likely to fundamentally differ from those of most reported GO memories, because of the use of Au contacts that are stable against oxidation and/or filamentary conduction. [44,47] Also, as compared to GO, the NPG contains fewer carbonyl and residual functional groups that may affect a switching process. [48] Under such circumstances, negatively charged oxygen ions in NPG can play a key role. As we illustrated in Figure 4, these ions are initially distributed over the whole channel region in addition to other oxygen-containing groups (e.g., hydroxyl or carboxylic groups). However, when the applied V increases, these ions can be coulombically attracted toward the anode junction. These oxygens can therefore establish a space-charge layer rather than oxidizing an inter Au surface, forming an aligned surface dipole in locally altering the band energies and triggering an efficient hole tunneling to set the LRS. [49][50][51] Note that this unique mechanism is directly motivated by the structural and spectroscopic analyses in Figure 2, and that it can fully account for the important electrical characteristics in Figure 3. Especially, the negligible dependence of V TH on L provides a convincing evidence that the switching mechanism is essentially an interfacial effect, rather than a bulk phenomenon. Also, the increase of I ON /I OFF ratio with increasing L in our systems can be explained by a larger oxygen content in a longer-channel device, in which the barrier-lowering effect can be more pronounced due to the stronger accumulation of oxygen ions.
We emphasize that this combination of unique structural property and physical mechanisms allows us to create one of the best-performing graphene-based memory devices, not to mention the important environmental value of this technology. Table 1 shows a comparison with the previous reported graphene-based two-terminal memory devices, which may reflect different materials-and device-specific mechanisms. [38] First of all, our devices outperform all the cited devices in terms of the ON/OFF ratio, benefiting from the high porosity and restricted metallic conductivity of NPG. Also note that most reported devices are of a vertical configuration, while our memories were built upon a planar geometry ideal for simple material transfer and device fabrication. Although a vertical device is generally preferred to lower the operating voltage, Table 1 shows that our lateral NPG devices were operated at voltages comparable to or even smaller than those used for vertical devices. A vertical structure can also be required to achieve a high integration density in conventional semiconductor chips. In this regard, our devices seem to find a potential use rather in future Internet-of-Things (IoTs) applications where lost-cost fabrication of simple, low-power electronics components are highly desirable.

Conclusions
We have proposed the plant-based synthesis of NPG and the fabrication of high-performance electrical memory devices utilizing this highly functional nanomaterial. A wide range of experimental techniques were employed to probe the key materials and device characteristics, revealing the crystallinity, structural defects, chemical states, pore sizes, as well as dominant mechanisms of NPG materials and devices. Our method introduced hierarchical nanopores into the graphene network, which was responsible for a disrupted sp 2 landscape, a highly enhanced semi-conductivity, and a current ON/OFF ratio in excess of 10 6 . The use of planar Au contact geometry was also shown to be critical to an injection-modulated switching at a reasonably low voltage under 5 V. The cost-effective, scalable, and environmentally considered production of high-performance NPG and their devices will open the door to new sustainable electronics and next-generation neuromorphic technologies.

Experimental Section
Materials Synthesis: The NPG was prepared by starting from a commercially available raw bagasse material. This bio-waste was rinsed with the deionized water and dried at 120 °C, and then crushed and ground into a powder. The source powder was stirred and mixed with saturated copper (II) acetate monohydrate solution and KOH with the weight ratio of 2:12.5. The mixture was then dried at 110 °C and transferred into a vacuum furnace. The mixture was thermally activated (or calcinated) at 700-950 °C for 75 min, followed by a cool-down cycle. The impurities were removed by HCl, and the prepared materials were then filtered and neutralized to pH 7 by washing with an excessive amount of distilled water. Finally, NPG was obtained after drying at 100 °C for 48 h.
Device Fabrication: The 100-nm-thick Au films were vacuum-deposited on a SiO 2 /Si wafer at a base pressure of 10 -5 mTorr and micro-patterned by electron beam lithography, with various target L values. To remove possible metal residues, the substrate-electrode samples were placed in acetone overnight. A flake of NPG was picked up by an elastic polydimethylsiloxane (PDMS) stamp, and it was transferred onto the active region defined between a pair of Au film electrodes.
Material Characterization: The SEM and HR-TEM images were taken by Seron AIF 2100 and TECNAI F20, respectively. The NPG structure was studied using an Thermo Nicolet Almega Dispersive Raman spectrometer (Nd:YLF laser with an excitation wavelength of 532 nm and a power of 100 mW). The XRD data were obtained using INEL Equinox 3000, and the XPS data were measured using an Al Kα X-ray source at 1486.6 eV.
Electrical Characterization: The fabricated NPG memory devices were characterized using an electrical probe station equipped with a semiconductor source-measure unit Keithley model 2400 at a room temperature and under ambient air.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.