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Recent Submissions

  • UAV Assisted IoT Geo-positioning Solution Employing Low-Cost Bluetooth Enabled Tags

    Bilal, Rana Muhammad; Akhter, Zubair; Alsahli, Nawaf; Abdel-Aal, Muhammad; Shamim, Atif (IEEE, 2023-03-20) [Conference Paper]
    Technology integration has enabled value-added services and quality-of-life enhancement in almost all aspects of modern life. In this paper, we present a UAV and low-cost Bluetooth low energy (BLE) tags-based location search system which enables a cart take-home service for shoppers of a supermarket in a model smart colony. The presented system has quality-of-life enhancement as well as carbon footprint reduction effects and can be integrated with the existing security and/or transport system of the model smart colony. Conducted field trials on location accuracy of the system are also presented, showing that carts left by residents outside the home can be located within 6.58m and carts taken inside homes or buildings can be located within 16.43m.
  • Recent Advancements in Reconfigurable mmWave Devices Based on Phase-Change and Metal Insulator Transition Materials

    Singh, Tejinder; Hummel, Gwendolyn; Vaseem, Mohammad; Shamim, Atif (IEEE Journal of Microwaves, Institute of Electrical and Electronics Engineers (IEEE), 2023-03-17) [Article]
    Chalcogenide Phase Change Materials (PCM) and metal insulator transition (MIT) materials are a group of materials that are capable of switching between low resistance and high resistance states. These emerging materials have been widely used in optical storage media and memory devices. Over the past recent years, there have been interests in exploiting the PCM and MIT materials, especially germanium antimony telluride (GST) alloys and vanadium dioxide (VO 2 ), for radio frequency (RF) applications. The PCM and MIT-based RF devices are expected to bridge the gap between semiconductor switches and microelectromechanical system (MEMS) switches as they combine the low insertion loss performance of MEMS technology and the small size and reliability performance of semiconductor technology. This article presents an overview of the PCM and MIT materials for RF circuits and discusses the recent advancements in reconfigurable millimeter-wave (mmWave) devices based on PCM and MIT materials in depth.
  • Fully Printed Dual-Layer Depolarizing Chipless RFID Tag for Wearable Applications

    Wang, Renqi; Akhter, Zubair; Li, Weiwei; Shamim, Atif (IEEE Journal of Radio Frequency Identification, Institute of Electrical and Electronics Engineers (IEEE), 2023-03-16) [Article]
    This work presents a cross-polar dual-layer chipless radio-frequency identification (RFID) tag based on a ladder-shaped resonator design. An integrated ground plane enables direct attachment to human skin without performance deterioration. Simulations show that the ladder-shaped resonator provides several advantages over traditional L-shaped and straight resonators, including a strong cross-polar radar cross section (-23.4 dBsm), third-order harmonics, orientation insensitivity, and compact size (0.062 λ2). The effects of the ground plane shape on the surface current distribution are investigated, and a circular tag of 20 mm radius is designed using ladder resonator groups and frequency shift encoding to provide an active area of 96.45 bits/λ2 and a unit frequency of 6.03 bits/GHz. The tag substrate is three-dimensionally (3D) printed with metallic resonator patterns that are subsequently screen-printed on the substrate. The maximum read range is measured at 40 mm using a cross-shaped, dual-polarized Vivaldi antenna connected to a network analyzer. The measured characteristics in free space are in good agreement with the simulation results, and practical on-body performance tests for the manufactured prototype using simulation and direct measurements indicate that the tag performance remains stable for both free space and on-body cases. The fully printed fabrication process makes the proposed tag design suitable for mass production at a low cost.
  • On-chip photoacoustic transducer based on monolithic integration of piezoelectric micromachined ultrasonic transducers and metasurface lenses

    Zhai, Yanfen; Sasaki, Takashi; Moridi, Mohssen; Lin, Ronghui; Alnakhli, Zahrah; Shamim, Atif; Li, Xiaohang; Younis, Mohammad I.; Hane, Kazuhiro; Wu, Lixiang (SPIE, 2023-03-09) [Conference Paper, Poster]
    An on-chip photoacoustic transducer is proposed by monolithically integrating piezoelectric micromachined ultrasonic transducers (PMUTs) on metasurface lenses for applications such as single-cell metabolic photoacoustic microscopy (SCM-PAM)1 . As shown in Figure 1a, every PMUT cell has a ring-shaped top electrode, and the membrane center is transparent without piezoelectric and electrode materials. The laser beam, therefore, can travel through a PMUT cell after being focused by a metasurface lens bonded on the backside of the PMUT (see Figure.3). The on-chip photoacoustic transducer fully leverages current PMUT and metasurface technologies and does not rely on transparent piezoelectric and electrode materials like typical transparent ultrasonic transducers2 . Moreover, the on-chip photoacoustic transducer has a monolithic integrated achromatic metasurface lens (see Figure 3), which can easily and efficiently focus the visible light (wavelength range: 400-700 nm) at the same focus point. Design and process this and preliminarily test the performance of PMUT and metasurface.
  • Self-Isolated Multiple-Input-Multiple-Output Antenna for mm-Wave Applications

    Sokunbi, Oludayo; Attia, Hussein; Hamza, Abubakar; Shamim, Atif; Kishk, Ahmed A. (IEEE, 2023-02-20) [Conference Paper]
    A low-cost innovative MIMO antenna configuration with minimal separation between the radiating elements and high isolation over a wide frequency band is presented. Several precisely designed slots with various forms, locations, and sizes are etched on the radiating patches to improve inter-element isolation throughout the mm-wave band of 30–41 GHz impedance bandwidth. This achieved elements isolation better than 70 dB with an inter-element spacing of 0.2 mm (0.02A at 30 GHz). The suggested self-isolation method is validated by designing a 1x2 MIMO array configuration. The innovative mm-wave antenna has the following characteristics over the desired bandwidth: high impedance bandwidth (30%) and low mutual coupling (70 dB). To the authors' knowledge, the present design is the first to demonstrate such broadband isolation enhancement in the mm-wave frequency range without any sophisticated decoupling structure such as metamaterial or frequency-selective surface.
  • A Via-less Fully Screen-Printed Reconfigurable Intelligent Surface for 5G Millimeter Wave Communication

    Yang, Yiming; Wang, Renqi; Vaseem, Mohammad; Makki, Behrooz; Shamim, Atif (arXiv, 2023-02-07) [Preprint]
    In this paper, we propose a via-less fully screen-printed reconfigurable intelligent surface which can establish a second line-of-sight communication from 23.5GHz to 29.5GHz. By serially connecting the H shaped resonator along the H field of the incident wave, we minimize the effect of the biasing lines and make a via-less design, which reduces the fabrication difficulty and cost. The unit-cell simulation of the array with screen-printed VO2 switches shows a 215° to 160° phase shift difference between the ON and OFF states within bandwidth. During the field testing of the ideal arrays, we verify that the array can redirect the 45° incident wave to 0° reflection with a signal enhancement of at least 10 dB as compared to the array which has all unit cells in the OFF condition.
  • Inkjet-Printed Ferrite Substrate Based Vialess Waveguide Phase Shifter

    Myrzakhan, Ulan; Ghaffar, Farhan A.; Vaseem, Mohammad; Fariborzi, Hossein; Shamim, Atif (IEEE Transactions on Magnetics, Institute of Electrical and Electronics Engineers (IEEE), 2023-02-03) [Article]
    Magnetically controlled waveguide-based phase shifters are desirable for their high performance, but are bulky and heavy, thus cannot be easily integrated with printed circuit board (PCB) based circuits. To tackle this, substrate-integrated waveguide (SIW) technology has been utilized, which brings the waveguide to a standard PCB, but requires large numbers of vias as well as multiple cavities in the substrate for ferrite slab placement. This implementation technique involves complex fabrication and yields a relatively low figure of merit (FoM). To alleviate this, we present the first completely vialess ferrite substrate-based waveguide phase shifter realized through low-cost inkjet printing technique. All the four sides of a yttrium iron garnet (YIG) substrate have been metalized through inkjet printing, allowing the fabrication of a conventional rectangular waveguide on a standard magnetic substrate. The prototype has been tested in symmetric as well as antisymmetric modes of biasing and peak FoMs of 160°/dB at 7.22 GHz and 332°/dB at 7.46 GHz have been measured, which are higher than those of the previously reported designs. This all-around inkjet printing approach can open the door to low-cost, integrable magnetic phase shifters with excellent RF performances.
  • Highly Scalable, Flexible, and Frequency Reconfigurable Millimeter-Wave Absorber by Screen Printing VO2 Switch Array onto Large Area Metasurfaces

    Park, Eiyong; Li, Weiwei; Jung, Heijun; Lee, Minjae; Park, Joon-Ha; Shamim, Atif; Lim, Sungjoon (Advanced Materials Technologies, Wiley, 2023-01-29) [Article]
    Flexible and reconfigurable (FAR) electronics are in high demand for emerging applications, including wearable, bioelectronics, and internet of things. Highly scalable antenna arrays or periodic surfaces are required for high directivity or electromagnetic wave path control, particularly for 5G millimeter-wave (mm-wave) due to high path losses. Conventional lumped tuning components have limitations related to scalable FAR electronics and hence highly scalable and flexible vanadium dioxides (VO2) switch array is proposed for mm-wave applications. A frequency reconfigurable mm-wave absorber is designed by screen printing the VO2 switch array to demonstrate the proposed approach feasibility for large scale electronics, achieving high scalability, tunability, and flexibility because the 40 µm thick VO2 switch array satisfies radio frequency switch requirement. Flexibility and repeatability are tested up to 2000 bending cycles with 25 mm bending radius, and tunability and scalability are demonstrated with 300 ON/OFF ratio, and 98% product yield for 400 switches printed on 144 × 144 mm2 polyethylene terephthalate substrates. Absorption frequency is switchable from 14 to 28 GHz at 150 mm bend radius while retaining better than 90% absorptivity as a frequency reconfigurable mm-wave absorber. Therefore, the proposed VO2 switch array would be suitable for scalable 5G and 6G FAR electronics.
  • Improving the Performance of Antenna-on-Chip by Effectively Illuminating the Artificial Magnetic Conductors through Coupling Enhancement Structures

    Yu, Yiyang; Akhter, Zubair; Shamim, Atif (IEEE Transactions on Antennas and Propagation, Institute of Electrical and Electronics Engineers (IEEE), 2023-01-27) [Article]
    An antenna-on-chip (AoC) suffers from low radiation efficiency and gain because of the highly conductive silicon substrate used in standard complementary metal–oxide–semiconductor (CMOS) processes. Placing an artificial magnetic conductor (AMC) surface underneath the AoC not only isolates the antenna from the lossy substrate but also boosts its performance through in-phase reflection. Ideally, the AMC, which has a periodic structure, should be infinite in size, but practical chip sizes limit its dimensions. The gain of an AMC-backed AoC shows a positive correlation with the AMC lateral area. However, in the ultrathin stack-up of standard CMOS processes, the AMC cannot be irradiated uniformly by the antenna because of the physical illumination issue, so the AoC gain does not increase with the AMC area after the initial increase. To improve the illumination of the AMC by an AoC, this work introduces coupling enhancement structures (CES) that employ the available metal layers of the stack-up. The proposed AMC-backed AoC with CES demonstrates a gain of 9.8 dBi and radiation efficiency of 71% at 94 GHz, which are improvements of 4.3 dB in gain and 23% in radiation efficiency, respectively, compared to an AoC backed by a conventional AMC.
  • Synergistic multi-source ambient RF and thermal energy harvester for green IoT applications

    Bakytbekov, Azamat; Nguyen, Thang Q.; Zhang, Ge; Strano, Michael S.; Salama, Khaled N.; Shamim, Atif (Energy Reports, Elsevier BV, 2023-01-09) [Article]
    In a future green Internet of Things (IoT) reality, billions of devices of the IoT infrastructure should be self-powered. Harvesting ambient energy to power IoT devices is an attractive solution that can extend battery life or can completely replace batteries. Considering the global applications of IoT, ubiquitous and continuous availability is an important requirement for ambient energy sources. Radio frequency (RF) energy from mobile phone towers and thermal energy from diurnal cycle temperature fluctuations are good candidates. In this study, we present a synergistic multi-source energy harvester (MSEH) comprising an RF energy harvester (RFEH) and a thermal energy harvester (TEH) integrated through a dual-function component, heatsink antenna. Both harvesters collect ambient energy 24 h a day and are not location specific. The TEH, which is in the shape of a box, collects energy using heatsinks on its sidewalls. The same heatsinks are optimized to also serve as receiving antennas of the RFEH, which collects energy from the GSM900, GSM1800, and 3G bands. Due to the synergistic integration, radiation efficiency of the antenna doubled from 40% to 80% which resulted in ∼10% increase in power conversion efficiency of the RFEH. Similarly, the average power of the TEH without heatsinks 120 μW is doubled to 240 μW for TEH with heatsinks. Field tests have shown that the outputs of the TEH and RFEH have increased 4 and 3 times compared to the independent TEH and RFEH respectively. A temperature and humidity sensor based IoT node has been successfully powered through this energy harvesting system. Overall, the MSEH can collect 3680 μWh of energy per day which is sufficient to obtain the sensors data with a time interval of 3.5 s.
  • New Self-Isolated Wideband MIMO Antenna System for 5G mm-Wave Applications using Slot Characteristics

    Sokunbi, Oludayo; Attia, Hussein; Hamza, Abubakar; Shamim, Atif; Yu, Yiyang; Kishk, Ahmed (IEEE Open Journal of Antennas and Propagation, Institute of Electrical and Electronics Engineers (IEEE), 2023-01-05) [Article]
    This paper proposes a simple novel technique for self-isolating a MIMO antenna for mm-wave applications. MIMO antenna elements with inter-element separation of 0.2 mm (0.023λ at 35 GHz) and measured high isolation (> 50 dB) are presented. By observing the concentration of surface waves on the radiating patch, several rigorously optimized slots of different shapes, positions, and dimensions are etched on the patch to enhance the inter-element isolation and increase the bandwidth within 28-37.5 GHz. The circuit models of the reference and proposed antennas have been presented. The coupling is measured by the level of the differences in the output voltage in both antennas. The novel mm-wave antenna exhibits high impedance bandwidth (> 29%), high isolation (> 50 dB), high efficiency (> 90%), and low envelope correlation coefficient (<0.005). Two configurations of MIMO antenna (i.e, 1×2 and 1×4) are fabricated and measured to validate the simulation outcomes. The single reference antenna has dimensions of 10x12 mm2 while the 1x2 array has dimensions of 19x12 mm2. The presented design is the first to exhibit such wideband isolation improvement without any external decoupling structure at the mm-wave frequency range, to the best of the authors’ knowledge.
  • Remote Monitoring of Skin Temperature through a Wristband Employing a Printed VO2 Sensor

    Fatani, Firas; Vaseem, M.; Akhter, Zubair; Bilal, Rana Muhammad; Shamim, Atif (IEEE Sensors Journal, IEEE, 2022-11-29) [Article]
    The need for highly sensitive, environmentally stable, mechanically flexible, and low-cost temperature sensors for on-body measurements has been increasing with the wide adoption of personal Healthcare-Internet-of-Things (H-IoT) devices. Printed electronics (PE) is a good platform for such sensors because it enables the realization of flexible devices through simple and rapid methods at a relatively low cost. However, previously reported printed temperature sensors suffer from poor sensitivity and/or environmental instability. In this paper, we report a custom Tungsten (W)-doped Vanadium Dioxide (VO2) ink-based screen-printed temperature sensor having the highest Temperature-Coefficient-of-Resistance (TCR) of 2.78%∙°C-1 with a resolution of 0.1°C between 30°C and 40°C. To protect it from environmental effects, a fluoropolymer-based passivation layer is added for accurate temperature readings even in 90% relative humidity. The sensor is printed on a flexible substrate and shows minimal deterioration in performance over 1000 bending cycles. For wearability and remote monitoring, the sensor is integrated with a custom Bluetooth Low Energy (BLE) wireless readout in the form of a wristband. The BLE readout comprises an ultra-thin and flexible patch antenna optimized for both BLE bandwidth and human wearability. It demonstrates a minimal SAR value of only 0.068W/kg, making it safe to wear. Despite the antenna’s thin structure (0.004λ), it has a gain of 1.65dBi, enabling an excellent communication range. The proposed wristband is tested on ten volunteers and under daily activities, which shows promising results with a maximum error of 0.16°C with reference to those of a commercial thermometer.
  • A Planar SIW-Based mm-Wave Frequency-Scanning Slot Antenna Array with no Scan Blindness at Normal

    Klionovski, Kirill; Liao, Hanguang; Bankov, Sergey; Akhter, Zubair; Shamim, Atif (IEEE Transactions on Antennas and Propagation, IEEE, 2022-10-25) [Article]
    Planar antenna reflectors are the modern design trend of both multibeam and frequency-scanning antenna arrays. The planar implementation of reflectors is typically performed using substrate-integrated waveguide (SIW) technology. A reflector’s profile can be different from the canonical one (parabolic, elliptic, hyperbolic, etc.) because of the effect of spatial dispersion of the reflection coefficient of the SIW-based surface. It should be synthesized considering the magnitude and argument of the field reflected from such a surface to maximize the efficiency of the reflection. In this paper, we present a planar mm-wave slot antenna array with SIW-based horn-reflector feeding. We analytically formulate the optimization of the SIW surface dimensions while accounting for the spatial dispersion of the reflection coefficient. We minimize the dimensions of the planar horn-reflector feeding. Finally, we demonstrate that using a dual-slot radiating element, we can avoid the effects of scan blindness along the normal direction. A prototype has been built and a good agreement has been achieved between the measured results and the predicted results based on calculations. The prototype achieved ±17° beam scanning within 16% of the operational frequency range, with no scan blindness along the normal direction.
  • High-conductivity screen-printable silver nanowire Ink for optically transparent flexible radio frequency electronics

    Vaseem, Mohammad; Akhter, Zubair; Li, Weiwei; Yarali, Emre; Anthopoulos, Thomas D.; Jayaswal, Gaurav (Flexible and Printed Electronics, IOP Publishing, 2022-10-05) [Article]
    Optically transparent conductors have paved the way in various optoelectronic and radio frequency (RF) devices where high electrical conductivity and optical transparency with mechanical flexibility, as well as large area fabrication are deemed necessary. Printing techniques are viable for fabricating large-area devices with high mechanical flexibilities. However, the preparation of suitable inks and printing recipes is essential to achieve a high electrical conductivity and transparency. In this study, the best tradeoff between conductivity and optical transmittance was achieved through silver (Ag) nanowires (NWs)-based ink formulation with tuned Ag NW loading, solvent compositions and polymer weight percentages. The ink was deposited through screen-printing, which enabled a large-area and high-resolution patterning of the AgNWs. The washing time of the post-printed films exhibited a decisive effect on the initial conductivity, which was further improved through photonic sintering. During the photonic sintering, the voltages, pulse lengths (μs) and fire rates (Hz) were optimized to obtain the best conductivity of the printed films. Maximum optical transparencies of 78 % and 83 % were achieved for the conductivities of ~5.88 × 106 and ~6.25 × 106 S/m, respectively. As a proof of concept, a fully printed optically transparent antenna was realized that could operate in a wide frequency band suitable for high-data-rate wireless communication.
  • Quasi-Isotropic Antenna-in-Package with a Shielded Core and Wide Circular Polarization Coverage

    Bermudez Arboleda, Maria; Klionovski, Kirill; Shamim, Atif (IEEE, 2022-09-21) [Conference Paper]
    Given the dynamic nature of the Internet of Things (IoT) scenarios, peripheral devices’ radiation patterns must cover large areas to avoid disconnection in case of change of orientation relative to the base station. This broad coverage should be for both the power radiated as well as for the polarization of the electromagnetic wave, increasing the probabilities of a sturdy link between peripheral devices and the base stations. Current devices implement simple omnidirectional antennas that are linearly polarized and can be easily detuned due to the proximity of the electronics. This paper proposes an Antenna-in-Package design based on a microstrip patch volumetric array. The system consists of six circular patch antenna elements with perturbation segments. The patches work on the 2.4 GHz ISM band. Each element is placed on one face of a 3D hollow cube, and the internal walls are covered in metal, acting as a shielded core for the embedded electronics and ground for the patch elements. The design simultaneously demonstrates a 7 dB gain variation in 95% of the 3D sphere and a circular polarization coverage in 84%.
  • Additively Manufactured Compact UHF RFID Tag Employing Slow-Wave Structures

    Lopez Reyes, Zulma; Akhter, Zubair; Shamim, Atif (IEEE, 2022-09-21) [Conference Paper]
    Smart connected things that can sense, interact, and share data have a huge potential in enabling a smart environment. Due to the rising interest in realizing the internet of things to provide smart services in smart cities, there is a requirement of vast deployment; therefore, things/tags need to be low in cost and compact in size. Radio Frequency Identification (RFID) is one of the technologies that can enable sensing and wireless communication between things. In this paper, additive manufacturing is used to minimize the cost of RFID tags while slow-wave structures (SWS) are explored to make them compact. The largest element of RFID tags i.e. antenna is initially taken as a standard half-wave dipole then folded to employ SWS along the parallel conductors. The process of folding and SWS loading have been carefully performed to ensure decent radiation efficiency and read range. To fabricate the proposed RFID tag, silver nanowires-based conductive ink is employed while the substrate is 3D printed. The overall tag size is 54 x 56 x 0.5 mm3 (ka=0.71), and it has a read range of 6.4 m at 867 MHz.
  • Screen-Printed, Compact, Flexible ISM Antenna for Wrist Wearability

    Fatani, Firas; Akhter, Zubair; Shamim, Atif (IEEE, 2022-09-21) [Conference Paper]
    Wearables have been gaining popularity in the current decade, which needs a wearable antenna design that adheres to certain requirements including human tissue compatibility, flexibility, and low cost. In this work, a printed, compact, and flexible patch antenna is proposed for wrist wearables applications. The proposed antenna has compact dimensions of 36mmx44mmx0.5mm with a full ground plane of 50mmx100mm to be wrapped around the wrist. Although bending and human tissue proximity affect the bandwidth, the resulting bandwidth is 72MHz at the 2.4GHz ISM band. Meanwhile, the realized gain of the antenna at the center frequency is 1.65dBi and a specific absorption rate (SAR) of only 0.06W/kg.
  • Figure of Merit for Objective Assessment of mmWave 5G Phased Arrays

    Zhang, Haoran; Shamim, Atif (IEEE, 2022-09-21) [Conference Paper]
    Recently, considerable research efforts have been concentrated on the design of mmWave 5G phased array designs for base stations and small cells. However, it is difficult to compare the performance of different mmWave 5G phased array designs, as there are many design trade-offs, such as operational bandwidth, the number of elements, beam scanning range, realized antenna gain, polarization properties, and overall dimensions. Currently, there is no standard or figure of merit (FoM) for an objective assessment of the array performance. In this paper, we derive a FoM, which includes all the above factors to create a fair comparison of different mmWave 5G phased array designs. The proposed FoM is then applied to various mmWave 5G phased array designs from the literature to compare their performances.
  • 28nm Neck Width Graphene Geometric Diode for THz Harvesting

    Wang, Heng; Shamim, Atif (IEEE, 2022-08-29) [Conference Paper]
    Rectennas are widely used for energy harvesting purposes. For THz rectennas, traditional semiconductor-based diodes cannot be used as they are not fast enough. As an alternate, fast metal-insulator-metal (MIM) diodes have been considered in the recent past, however, they suffer from limited cut-off frequency due to high RC constant (especially high capacitance). Geometric diode, on the other hand, can solve the issue of high capacitance due to its planar structure. The neck width of the geometric diode affects its responsivity, so smaller the neck width, better is the responsivity. Previously, the lowest reported neck width for a funnel-shaped graphene geometric diode is 50 nm with a zero-bias responsivity of 0.06 A/W. In this work, we propose a better design with the lowest reported neck width of 28 nm, which shows a higher zero-bias responsivity of 0.17 A/W experimentally.
  • Single and Double Layer of Monoclinic VO2 Ink Based Printed and Interdigitated Supercapacitors

    Alhebshi, Nuha A.; Vaseem, Mohammad; Minyawi, Bashaer A.; AlAmri, Amal M.; Shamim, Atif (Energy Technology, Wiley, 2022-08-24) [Article]
    Screen printing has received significant attention for manufacturing energy storage devices. However, preparing high-quality inks is still one of the main challenges. Herein, a homogenous and stable ink based on the monoclinic phase of VO2(M) microparticles has been synthesized by a simple hydrothermal process in only 6 h and through non-toxic solutions. The VO2 ink is printed on Kapton substrate in a single-layer as well as in a double-layer arrangement (1-LP and 2-LP). The printed 1-LP VO2 electrode delivers a maximum areal capacitance of 20 mF cm−2 with a small equivalent series resistance of 4 ohms without conducting additives. The interdigitated full-cell VO2 supercapacitor adequately employs the electric double-layer and Faradic redox mechanisms in 1.4 V, which is the highest operating voltage reported for symmetric supercapacitors based on pure vanadium oxide electrodes in aqueous inorganic electrolytes. Moreover, the maximum areal energy of the 2-LP supercapacitor is 0.8 µWh cm−2 at an areal power of 21.0 µW cm−2, which is larger than the 1-LP (0.2 µWh cm−2 at 17.5 µW cm−2). This improvement is attributed to the homogenously printed double-layer of the porous VO2 microparticles. Integrating such supercapacitors into thin-film electronics could develop portable devices.

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