The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a... more The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a non-negligible valence band offset and is sensitive to chemical solutions. In this work, we investigated a type-II superlattice (T2SL) barrier that is homogeneous with the T2SL absorber layer in order to resolve these drawbacks of the AlGaSb barrier. The lattice mismatch of the T2SL barrier was smaller than that of the AlGaSb barrier. At − 70 m V and 80 K, the dark current density and the noise equivalent temperature difference of the nBn devices with the T2SL barrier were 4.4 × 1 0 − 6 A / c m 2 and 33 mK, respectively.
IEEE Antennas and Wireless Propagation Letters, Apr 1, 2018
A novel design of electrically small, extremely thin and horizontally polarized magnetic dipole a... more A novel design of electrically small, extremely thin and horizontally polarized magnetic dipole antenna is proposed. It is composed of multiple folded arms packed in an extremely thin height of 0.0016λ, with the electrical size ka of 0.276. The radiation efficiency is 87 % from the simulation. The radiation properties are investigated and its operation as a small magnetic dipole antenna is confirmed. A 3D printing technology is used for building the prototype due to the complexity of the proposed structure. The experimental results show good agreement with the computed expectation values.
Generally, triboelectric generators (TENGs) demonstrate a considerably lower output current than ... more Generally, triboelectric generators (TENGs) demonstrate a considerably lower output current than output voltage; this has largely limited their performance enhancement. Thus, enormous research efforts have been made to address this problem. In this work, we present a simple method to enhance the triboelectric output current by burying an indium zinc oxide (IZO) layer under the triboelectric polymer friction layer. The IZO layer provides large interface density of states, which function as a charge reservoir. During frictional contactseparation motion of the TENG, electrons can be stored in or pumped out of these states. By optimizing the properties of the IZO layer, the output performance of the TENG is greatly improved, generating an output power density of ~25 mW/cm 2. Specifically, an output voltage and current density of ~140 V and ~180 A/cm 2 were obtained, which are 4-fold and 9-fold higher, respectively, than a TENG without an IZO layer. The method introduced here suffers less from friction layer wear-out and can effectively enhance the performance of TENGs.
Wearable mechanical energy harvesting technologies have been achieved much attention for the wire... more Wearable mechanical energy harvesting technologies have been achieved much attention for the wireless sustainable power source applications. In this study, we have fabricated polyvinylidene fluoride (PVDF)/graphene quantum dot (GQD) composite nanofibers (NFs), which showed improved triboelectric nanogenerator (TENG) performance. PVDF/GQD composite NFs were fabricated by an electrospinning method. Structural and chemical investigations show that the GQDs were embedded in the PVDF NFs and promoted the formation of polar β-phase when an
In the aeronautics industry, wireless avionics intracommunications have a tremendous potential to... more In the aeronautics industry, wireless avionics intracommunications have a tremendous potential to improve efficiency and flexibility while reducing the weight, fuel consumption, and maintenance costs over traditional wired avionics systems. This survey starts with an overview of the major benefits and opportunities in the deployment of wireless technologies for critical applications of an aircraft. The current state-of-art is presented in terms of system classifications based on data rate demands and transceiver installation locations. We then discuss major technical challenges in the design and realization of the envisioned aircraft applications. Although wireless avionics intra-communication has aspects and requirements similar to mission-critical applications of industrial automation, it also has specific issues such as complex structures, operations, and safety of the aircraft that make this area of research self-standing and challenging. To support the critical operations of an aircraft, existing wireless standards for mission-critical industrial applications are briefly discussed to investigate the applicability of the current solutions. Specifically, IEEE 802.15.4-based protocols and Bluetooth are discussed for low data rate applications, whereas IEEE 802.11based standards are considered for high data rate applications. Eventually, we propose fundamental schemes in terms of network architecture, protocol, and resource management to support the critical avionics applications and discuss the research directions in this emerging area.
We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V... more We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V)-doped ZnO nanosheets (NSs) and the polydimethylsiloxane (PDMS) composite structure. The V-doped ZnO NSs were synthesized to overcome the inherently low piezoelectric properties of intrinsic ZnO. Ferroelectric phase transition induced in the V-doped ZnO NSs contributed to signifi- cantly improve the performance of the PENGs after the poling process. Consequently, the PENGs exhibited high output voltage and current up to ∼32 V and ∼6.2 μA, respectively, under the applied strain, which are sufficient to directly turn on a number of light emitting diodes (LEDs). The composite approach for PENG fabrication is scalable, robust, and reproducible during periodic bending/releasing over extended cycles. The approach introduced here extends the performance limits of ZnO-based PENGs and demonstrates their potential as energy harvesting devices.
The fabrication of a piezoelectric generator (PEG) that could provide higher levels of durability... more The fabrication of a piezoelectric generator (PEG) that could provide higher levels of durability and stability while delivering higher output is an ever-present goal for self-powered and wearable electronic devices. This study resulted in a PEG based on a composite film composed of methylammonium lead iodide (MAPbI 3) perovskite, and poly(vinylidene fluoride) (PVDF), as a flexible polymer matrix that delivers high performance, flexibility and cost-effectiveness. These homogeneous MAPbI 3-PVDF composite films with various volume factions of MAPbI 3 were realized using simple solution technique, which show predominant dielectric and ferroelectric properties by varying the volume fraction of the MAPbI 3. In particular, composite films containing 25 vol % MAPbI 3 revealed a high dielectric constant of 56 at 1 kHz and remanent polarization of 0.83 µC/cm 2. This particular PEG showed a high open-circuit voltage (V oc) of 17.8 V and a short-circuit current density (J sc) of 2.1 µA/cm 2 , and the same PEG with an active layer thickness of 97.7 µm has shown an enhancement in V oc of 45.6 V and J sc of 4.7 µA/cm 2. This level of high-output power performance is sufficient for operating commercial light-emitting diodes even under finger tapping without the need of a storage device.
There is a growing interest in window air filters to protect indoor air quality from ultrafine pa... more There is a growing interest in window air filters to protect indoor air quality from ultrafine particulate matter (PM) in outdoor air. The filters for this purpose must achieve high filtering efficiency without compromising the original functions of the window, such as high air permeability and visibility. Several filters meeting these requirements have been developed and demonstrate a high PM2.5 filtering efficiency. However, these filters are installed outside the window or on the window screen guard, thereby requiring high levels of ultraviolet (UV), chemical, and thermal resistance. These requirements have been overlooked so far. In this study, we examine the fabrication and performance of a polybenzimidazole-benzophenone (PBI-BP) composite nanofiber air filter, which demonstrates superb UV resistance, and chemical and thermal durability. Owing to the UV absorbance of the BP in the nanofibers, the filter membrane is robust even under prolonged UV exposure, which is essential for filters for this purpose. The filter membrane is not damaged even after treatment in strong acids or annealing at high temperature up to 400 ºC. Thus, the PBI-BP composite filter is suitable for practical application in window air filters and can also be adapted to develop filters used under other harsh environments.
The threshold voltages at the onset of conduction for electron and hole branches can provide info... more The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or the interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values. From a little discrepancy an average density of interface states per energy within a band gap (Dit) is also estimated. The data clearly show that Dit decreases as a bilayer graphene band gap increases rather than being constant. Despite the decreasing trend of Dit, interestingly the total interface states within a gap increases linearly as a band gap increases. This is because of ~ 2 10 10 cm-2 interface states localized at band edges even without a band gap, and other gap states are equally spread over the gap.
A ZnO NR decorated PVDF-MXene composite NF EMI shielding material exhibiting superb absorption do... more A ZnO NR decorated PVDF-MXene composite NF EMI shielding material exhibiting superb absorption dominant EMI SE, flexibility, and multi-functionality.
The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a... more The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a non-negligible valence band offset and is sensitive to chemical solutions. In this work, we investigated a type-II superlattice (T2SL) barrier that is homogeneous with the T2SL absorber layer in order to resolve these drawbacks of the AlGaSb barrier. The lattice mismatch of the T2SL barrier was smaller than that of the AlGaSb barrier. At − 70 m V and 80 K, the dark current density and the noise equivalent temperature difference of the nBn devices with the T2SL barrier were 4.4 × 1 0 − 6 A / c m 2 and 33 mK, respectively.
IEEE Antennas and Wireless Propagation Letters, Apr 1, 2018
A novel design of electrically small, extremely thin and horizontally polarized magnetic dipole a... more A novel design of electrically small, extremely thin and horizontally polarized magnetic dipole antenna is proposed. It is composed of multiple folded arms packed in an extremely thin height of 0.0016λ, with the electrical size ka of 0.276. The radiation efficiency is 87 % from the simulation. The radiation properties are investigated and its operation as a small magnetic dipole antenna is confirmed. A 3D printing technology is used for building the prototype due to the complexity of the proposed structure. The experimental results show good agreement with the computed expectation values.
Generally, triboelectric generators (TENGs) demonstrate a considerably lower output current than ... more Generally, triboelectric generators (TENGs) demonstrate a considerably lower output current than output voltage; this has largely limited their performance enhancement. Thus, enormous research efforts have been made to address this problem. In this work, we present a simple method to enhance the triboelectric output current by burying an indium zinc oxide (IZO) layer under the triboelectric polymer friction layer. The IZO layer provides large interface density of states, which function as a charge reservoir. During frictional contactseparation motion of the TENG, electrons can be stored in or pumped out of these states. By optimizing the properties of the IZO layer, the output performance of the TENG is greatly improved, generating an output power density of ~25 mW/cm 2. Specifically, an output voltage and current density of ~140 V and ~180 A/cm 2 were obtained, which are 4-fold and 9-fold higher, respectively, than a TENG without an IZO layer. The method introduced here suffers less from friction layer wear-out and can effectively enhance the performance of TENGs.
Wearable mechanical energy harvesting technologies have been achieved much attention for the wire... more Wearable mechanical energy harvesting technologies have been achieved much attention for the wireless sustainable power source applications. In this study, we have fabricated polyvinylidene fluoride (PVDF)/graphene quantum dot (GQD) composite nanofibers (NFs), which showed improved triboelectric nanogenerator (TENG) performance. PVDF/GQD composite NFs were fabricated by an electrospinning method. Structural and chemical investigations show that the GQDs were embedded in the PVDF NFs and promoted the formation of polar β-phase when an
In the aeronautics industry, wireless avionics intracommunications have a tremendous potential to... more In the aeronautics industry, wireless avionics intracommunications have a tremendous potential to improve efficiency and flexibility while reducing the weight, fuel consumption, and maintenance costs over traditional wired avionics systems. This survey starts with an overview of the major benefits and opportunities in the deployment of wireless technologies for critical applications of an aircraft. The current state-of-art is presented in terms of system classifications based on data rate demands and transceiver installation locations. We then discuss major technical challenges in the design and realization of the envisioned aircraft applications. Although wireless avionics intra-communication has aspects and requirements similar to mission-critical applications of industrial automation, it also has specific issues such as complex structures, operations, and safety of the aircraft that make this area of research self-standing and challenging. To support the critical operations of an aircraft, existing wireless standards for mission-critical industrial applications are briefly discussed to investigate the applicability of the current solutions. Specifically, IEEE 802.15.4-based protocols and Bluetooth are discussed for low data rate applications, whereas IEEE 802.11based standards are considered for high data rate applications. Eventually, we propose fundamental schemes in terms of network architecture, protocol, and resource management to support the critical avionics applications and discuss the research directions in this emerging area.
We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V... more We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V)-doped ZnO nanosheets (NSs) and the polydimethylsiloxane (PDMS) composite structure. The V-doped ZnO NSs were synthesized to overcome the inherently low piezoelectric properties of intrinsic ZnO. Ferroelectric phase transition induced in the V-doped ZnO NSs contributed to signifi- cantly improve the performance of the PENGs after the poling process. Consequently, the PENGs exhibited high output voltage and current up to ∼32 V and ∼6.2 μA, respectively, under the applied strain, which are sufficient to directly turn on a number of light emitting diodes (LEDs). The composite approach for PENG fabrication is scalable, robust, and reproducible during periodic bending/releasing over extended cycles. The approach introduced here extends the performance limits of ZnO-based PENGs and demonstrates their potential as energy harvesting devices.
The fabrication of a piezoelectric generator (PEG) that could provide higher levels of durability... more The fabrication of a piezoelectric generator (PEG) that could provide higher levels of durability and stability while delivering higher output is an ever-present goal for self-powered and wearable electronic devices. This study resulted in a PEG based on a composite film composed of methylammonium lead iodide (MAPbI 3) perovskite, and poly(vinylidene fluoride) (PVDF), as a flexible polymer matrix that delivers high performance, flexibility and cost-effectiveness. These homogeneous MAPbI 3-PVDF composite films with various volume factions of MAPbI 3 were realized using simple solution technique, which show predominant dielectric and ferroelectric properties by varying the volume fraction of the MAPbI 3. In particular, composite films containing 25 vol % MAPbI 3 revealed a high dielectric constant of 56 at 1 kHz and remanent polarization of 0.83 µC/cm 2. This particular PEG showed a high open-circuit voltage (V oc) of 17.8 V and a short-circuit current density (J sc) of 2.1 µA/cm 2 , and the same PEG with an active layer thickness of 97.7 µm has shown an enhancement in V oc of 45.6 V and J sc of 4.7 µA/cm 2. This level of high-output power performance is sufficient for operating commercial light-emitting diodes even under finger tapping without the need of a storage device.
There is a growing interest in window air filters to protect indoor air quality from ultrafine pa... more There is a growing interest in window air filters to protect indoor air quality from ultrafine particulate matter (PM) in outdoor air. The filters for this purpose must achieve high filtering efficiency without compromising the original functions of the window, such as high air permeability and visibility. Several filters meeting these requirements have been developed and demonstrate a high PM2.5 filtering efficiency. However, these filters are installed outside the window or on the window screen guard, thereby requiring high levels of ultraviolet (UV), chemical, and thermal resistance. These requirements have been overlooked so far. In this study, we examine the fabrication and performance of a polybenzimidazole-benzophenone (PBI-BP) composite nanofiber air filter, which demonstrates superb UV resistance, and chemical and thermal durability. Owing to the UV absorbance of the BP in the nanofibers, the filter membrane is robust even under prolonged UV exposure, which is essential for filters for this purpose. The filter membrane is not damaged even after treatment in strong acids or annealing at high temperature up to 400 ºC. Thus, the PBI-BP composite filter is suitable for practical application in window air filters and can also be adapted to develop filters used under other harsh environments.
The threshold voltages at the onset of conduction for electron and hole branches can provide info... more The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or the interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values. From a little discrepancy an average density of interface states per energy within a band gap (Dit) is also estimated. The data clearly show that Dit decreases as a bilayer graphene band gap increases rather than being constant. Despite the decreasing trend of Dit, interestingly the total interface states within a gap increases linearly as a band gap increases. This is because of ~ 2 10 10 cm-2 interface states localized at band edges even without a band gap, and other gap states are equally spread over the gap.
A ZnO NR decorated PVDF-MXene composite NF EMI shielding material exhibiting superb absorption do... more A ZnO NR decorated PVDF-MXene composite NF EMI shielding material exhibiting superb absorption dominant EMI SE, flexibility, and multi-functionality.
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Papers by Junghyo Nah