Spin-Coated Heterogenous Stacked Electrodes for Performance Enhancement in CMOS-Compatible On-Chip Microsupercapacitors

Integration of microsupercapacitors (MSCs) with on-chip sensors and actuators with nanoenergy harvesters can improve the lifetime of wireless sensor nodes in an Internet-of-Things (IoT) architecture. However, to be easy to integrate with such harvester technology, MSCs should be fabricated through a complementary-metal-oxide-semiconductor (CMOS) compatible technology, ubiquitous in electrode choice with the capability of heterogeneous stacking of electrodes for modulation in properties driven by application requirements. In this article, we address both these issues through fabrication of multielectrode modular, high energy density microsupercapacitors (MSC) containing reduced graphene oxide (GO), GO-heptadecane-9-amine (GO-HD9A), rGO-octadecylamine (rGO-ODA), and rGO-heptadecane-9-amine (rGO-HD9A) that stack through a scalable, CMOS compatible, high-wafer-yield spin-coating process. Furthermore, we compare the performance of the stack with individual electrode MSCs fabricated through the same process. The individual electrodes, in the presence of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfony)imide (EMIM-TFSI), demonstrate a capacitance of 38, 30, 36, and 105 μF cm–2 at 20 mV s–1 whereas the fabricated stack of electrodes demonstrates a high capacitance of 280 μF cm–2 at 20 mV s–1 while retaining and enhancing the material-dependent capacitance, charge retention, and power density.


S1. Preparation of dispersion
Spin coating requires electrode material in solutions that have good dispersibility. To make graphene based composite material solutions, we started the process by using GNPs and performed the Hummer's process 1 to obtain aqueous solutions of GO. The powder GNP (0.5 g) solution was oxidized in H 2 SO 4 (30 ml) and KNO 3 (0.295 g) with KMnO 4 (3 g) after six hours of reaction time while being stirred at 450 rpm at a temperature below 15 °C. The solution was quenched with DI-water (100 ml) and H 2 O 2 (30 %, 6 ml). The yellow dispersion of GO was centrifuged for 10 min at 3500 rpm with HCl (10 %, 100 ml) added in the precipitate. The precipitate was centrifuged with DI-water three times to obtain a higher purity. The recovered GO is mixed with DI-water and stored as a solution. The functional group HD9A ((CH 3 (CH 2 ) 7 ) 2 CHNH 2 ) are mixed in ethanol and stirred with the GO-solution with solvent at 450 rpm at room temperature for 5 min to achieve GO-HD9A. The GO-HD9A solution is stored and used directly on the wafer substrate for MSC fabrication. Procured GO is mixed with ethanol in presence of ODA and HD9A to obtain GO-ODA and GO-HD9A respectively. Finally, both GO-ODA and GO-HD9A solutions were reduced to rGO using ascorbic acid-6-palmitate (100 ml) at 98 °C for 2 h at 450 rpm. Further information on the synthesis and characterization of the material can be found in the article reported by Mendez et al. 2

S2
was prepared for spin coating as a control. The details of the solution synthesis can be found in our previous research 3 .

S2. Material characterization
The synthesized electrode materials are characterized using Attenuated total reflection-Fouriertransform infrared spectroscopy (ATR-FTIR) for chemical modifications, X-ray photon spectroscopy (XPS) for surface chemical state, and UV-Vis spectroscopy for optical property analysis.
For investigating the chemical modification of the synthesized materials, an ATR-FTIR analysis was performed. To compare the original GO versus the rGOs, the common peaks of oxygen functional groups are presented in Figure 2(a). For instance, the GO spectrum (green line) presents various oxygen functional groups: a typical broadband from 3000 cm -1 to 3700 cm -1 with a peak at 3200 cm -1 that refer to wavenumbers of hydroxyl (OH), carbonyl (C=O) at 1719 cm -1 , aromatic (C=C) at 1618 cm -1 , alkoxy (C-O) at 1160 cm -1 and epoxy (C-O-C) at 1030 cm -1 . All features corresponding to highly oxidized material, i.e., graphene oxide, according to previous reports 4 . For rGO-ODA and rGO-HD9A, the prominent hydroxyl band is completely removed, as well as the carbonyl. On the other hand, the most perceptible bands for the two are at 2915 cm -1 and 2850 cm -1 corresponding to wavenumbers of C-H, respectively 5 , confirming a complete chemical reduction process. Furthermore, the vibrations of carbon-nitrogen bonding in the amine (C-NH-C) at 3444 cm -1 and at 1560 cm -1 , also the wavenumber for NH is recorded at 1470 cm -1 6,7 . The C-O and C-O-C at 1150 cm -1 and 1070 cm -1 in the rGOs present a small shift from the GO original positions 8 . Finally, N-O bonding can be observed at 718 cm -1 and 618 cm -1 , respectively 4 , suggesting that besides the reduction, the functionalization takes place with the carboxylic and hydroxyl groups. The most interesting feature to distinguish between the ODA and HD9A (linear vs branched alkyl chains), is that transmittance of rGO-HD9A is higher (30 % more), revealing a lower restacking level 9 due to the higher steric hindrance attributed to the branched chains. This feature is relevant for spin coating of rGO as it tends to S3 agglomerate in the solvent solution after synthesis which can lead to non-uniform electrode deposition.
Furthermore, for a deeper examination of the surface chemical state, the synthesized materials were analyzed by X-ray photoelectron spectroscopy (XPS). The C1s core level spectra are shown in Figure 2 The analysis of optical band gap reduction (E g opt ) values by UV-Vis spectroscopy is presented in Figure 2(c). The absorbance of rGO-ODA and rGO-HD9A follow a different behavior when compared to GO, i.e., zero absorbance at greater wavelengths than 700 nm. This behavior could be attributed to rGOs because of the linear dispersion by Dirac´s electrons [12][13][14] , which is also good evidence of a highly reduced graphene oxide, with recovered sp 2 hybridization that resembles a more graphene-like material instead of GO. To compare the structural quality of the graphene-based materials, Raman spectra ( Figure S2) recorded with 2.33 eV laser energy are presented. It is well known that the dispersion of  electrons in graphene offers powerful and efficient insights into their electronic properties, and therefore of their crystallinity. It can be noticed that all spectra exhibit an intense band from 1450-1660 cm -1 corresponding to the G band due to vibrational E 2g degenerative mode observed in sp 2 carbons. Furthermore, another band is observed at 1260-1400 cm -1 , assigned to the D band and related to the A 1g mode. The D peak is originated due to the interaction between phonons and defects, such as in-plane substitution heteroatoms, vacancies, or grain boundaries [15][16][17] . As expected, the material with the best crystallinity is the starting graphite, since it were not subjected to any process. The GO has the lowest crystallinity, but, the rGO-ODA does not recover the crystallinity, despite the highly chemical reduction shown in the XPS results [18][19][20] . MSC. The capacitance is then calculated as where is the discharging time for the MSC, is the maximum final potential, and is the current. The energy density is calculated as is the voltage window for the MSC. The power density is calculated as After GCD measurements, we performed electrochemical impedance spectroscopy (EIS) on the devices at a peak-to-peak a.c. signal of 0.5 mV over a range of frequencies ( ). The real /2 and imaginary capacitances for a frequency, and are calculated as Where , and are real and imaginary impedance of the system respectively. The phase constant of the device ( ) is calculated as = tan -1 ( ) (7) Finally, the devices undergo cyclic charge-discharge measurements where the MSCs are initially charged to a potential of 1 V at 5 μA cm -2 current density, then the current polarity is switched and the device is discharged to 0 V for 3000 cycles. The device capacitance at the end of each cycle is calculated using equation (2) and then plotted as a function of cycle number.
The limitation of the approach is that the tests have been conducted in open atmosphere, therefore, the electrolyte voltage window has not exceeded 1 V as ionic liquids are prone to chemical reactions in air above that potential 21 . References: