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Research of Interest: from RF/microwave Physics to Advanced RF/microwave Nano-&Bio-Electronics
1-1.Classical Electrodynamics-Fields & Waves Theory:
Michael Faraday (1791–1867, English Physicist): His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis. Although Faraday received little formal education, as a self-made man, he was one of the most influential scientists in history. It was by his research on the magnetic field around a conductor carrying a direct current that Faraday established the concept of the electromagnetic field in physics. Faraday also established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena. He similarly discovered the principles of electromagnetic induction, diamagnetism, and the laws of electrolysis. His inventions of electromagnetic rotary devices formed the foundation of electric motor technology, and it was largely due to his efforts that electricity became practical for use in technology.
Right-hand side figure: Michael Faraday (Ref.: https://en.wikipedia.org/wiki/Michael_Faraday).
James Clerk Maxwell (1831-1879, Scottish Physicist): His most notable achievement was to formulate the classical theory of electromagnetic radiation, bringing together for the first time electricity, magnetism, and light as different manifestations of the same phenomenon. Maxwell's equations for electromagnetism have been called the "second great unification in physics" where the first one had been realized by Isaac Newton. With the publication of "A dynamical theory of the electromagnetic field" in 1865, Maxwell demonstrated that electric and magnetic fields travel through space as waves moving at the speed of light. The unification of light and electrical phenomena led his prediction of the existence of radio waves. Maxwell is also regarded as a founder of the modern field of electrical engineering.
Left- & Right-hand side figure & formula: Maxwell's equations (Gauss' law in electrostatics, Faraday's law in electrodynamics, Guass' law in magnetostatics, Ampere's law in electrodynamics) & James Clerk Maxwell. (Ref.: https://en.wikipedia.org/wiki/James_Clerk_Maxwell).
1-2.Quantum Electrodynamics-Fields & Waves Theory:
Paul Adrien Maurice Dirac (1902–1984, English Physicist): He was an English mathematical and theoretical physicist who is considered to be one of the founders of quantum mechanics. Dirac laid the foundations for both quantum electrodynamics and quantum field theory. Among other discoveries, he formulated the Dirac equation in 1928, which describes the behaviour of fermions and predicted the existence of antimatter, which is one of the most important equations in physics, and is regarded by some physicists as the "real seed of modern physics". Dirac shared the 1933 Nobel Prize in Physics with Erwin Schrödinger for "the discovery of new productive forms of atomic theory".
Right-hand side figure: Paul A. M. Dirac (Ref.: https://en.wikipedia.org/wiki/Paul_Dirac).
2.Interaction of Electromagnetic Waves with Media & Microwave Absorbing Materials:
Research abstract: The microwave, i.e. 0.5- 10 GHz, transmission characteristics of carbon nanofiber (CNF) with three different micrometer-scale thicknesses were experimentally investigated using a coplanar waveguide (CPW) transmission line. In the experimental results, when the film of CNF was thick, the signal transmission level (S21-magnitude) was significantly lower and its phase (S21-phase) was shifted toward the low-frequency region. Based on the obtained S21-parameter (S21-magnitude and S21-phase), the electric permittivity (ε) of CNF was extracted and showed clear differences depending on the thickness. From the analysis of electromagnetic fields, the microwave conductivity of CNF linearly increased with the increasing thickness due to enhanced electromagnetic field coupling between the film of CNF and the CPW line. As a result, we demonstrated that the film of CNF has a significant attenuation effect on signal transmission in the microwave regime, depending on micrometer-scale changes in film thickness.
Right-hand side figure: Electromagnetic field and surface current distribution of the silver-printed CPW line with CNFs film via a full-electromagnetic solver based on finite element method. (Ref.: H.-J. Lee, J.H. Jeong, and B.-H. Kim, "Microwave transmission characteristics of carbon nanofibers film with different micrometer-scale thickness," Carbon, Vol. 173, pp.419-426, 2021y).
Research abstract: In this study, we report high-frequency transmission properties of the coplanar waveguide line (CPW line) with a CNF film synthesized at CTs of 700 ℃, 800 ℃, and 900 ℃ (denoted CNF-700, CNF-800, and CNF-900, respectively) in the frequency ranging from 0.5 to 10 GHz. For all samples, a bare CPW line, a CPW line with a tape, and a CPW line with three kinds of CNF films, the S11 magnitude (reflection coefficient) is less than −10 dB in the frequency range, and the S21 magnitude (transmission coefficient) significantly decreases linearly to −1.3 dB (CNF-700), −3.0 dB (CNF-800), and −5.5 dB (CNF-900) at 10 GHz. Furthermore, the effective permittivity (εeff) of CPW line with CNF films increases with CT, although εeff decreases with increasing frequency. Conversely, due to dielectric and conductive loss of CNF film, the attenuation constant (α) of CPW line with CNF film increases linearly with CTs and frequency. These results indicate that the CPW line with a CNF-900 (CPWCNF-900) shows lossier characteristic than the other CNF films because of finer three-dimensional network structure, higher conductivity, higher attenuation constant, higher shielding effectiveness, and larger effective permittivity. Furthermore, this study shows the possibility that thin CNF films with different CT variations can regulate signal transmission for high-frequency and high-speed circuit and device applications.
Left-hand side figure: Transmission coefficient (S21) for four sample configurations (CNF+T-tape, CNF-700, CNF-800, and CNF-900) and fitting results based on measured data. (Ref.: H.-J. Lee, J.H. Jeong, N. Jeong, and B.-H. Kim, "High-frequency transmission properties of carbon nanofibers with carbonization temperature variations," Applied Materials Today, Vol. 35, pp. 101989 (1-8), 2023y).
3.RF/microwave Characterization of Carbon Nanomaterials, e.g., Graphene, Graphene Oxide, Carbon Nanofibers:
Research abstract: We have quantitatively evaluated the effective surface conductivity of chemical vapor deposition-grown graphene through a full-wave electromagnetic method and also investigated the intrinsic characteristics of the transmission line (TL) of the graphene at frequency ranging from 0.5 to 40 GHz. According to the simulated data based on the measured S-parameters, the effective conductivity of single- and multi-layer graphene (MLG) was about 4.3 × 106 S/m and 1.2 × 106 S/m, respectively. Furthermore, we confirm that multi-layer graphene is more suitable for use in transmission lines compared to single-layer graphene in the observed frequency region.
Right-hand side figure: Coplanar waveguide electrode with a multi-layer graphene (~4 nm-thickness). (Ref.: H.-J. Lee,E.H. Kim, J.-G. Yook, and J.W. Jung "Intrinsic characteristics of transmission line of graphene at microwave frequencies," Applied Physics Letters, Vol. 100, pp.223102-1-223102-3, 2012y).
Research abstract: The radio-frequency, i.e. 0.5–40 GHz, characteristics of chemical vapor deposition-grown graphene monolayer via HNO3 doping is experimentally investigated. According to the obtained results, the sheet resistance of HNO3-treated graphene decreases about half compared to bare graphene. In the case of radio-frequency characteristics, the transmission coefficient and effective conductivity of the HNO3-treated graphene are more enhanced than those of the bare graphene. Moreover, the intrinsic resistance and inductance of the HNO3-treated graphene itself show diminishing tendency with frequency increase. As a result, it is verified that the direct current as well as high frequency characteristics of graphene are improved by using the chemical doping method.
Left-hand side figures: Raman spectra of graphene sample. (a) 2D (∼2700 cm−1), G (∼1580 cm−1), and D (∼1350 cm−1) peaks of graphene (Ref.:H.-J. Lee, E.H. Kim, J.H. Park, W.S. Song, K.-S. An, Y.S. Kim, J.-G. Yook, and J.W. Jung "Radio-frequency characteristics of graphene monolayer via nitric acid doping," Carbon, Vol. 78, pp. 532-539, 2014y).
4.Carbon Nanomaterials-Based RF/microwave Bio-materials & Gas molecules Sensing Platforms:
Review abstract: In the past decade, graphene has been widely researched to improve or overcome the performance of conventional radio-frequency (RF) nanodevices and circuits. In recent years, novel RF bio and gas sensors based on graphene and its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), have emerged as new RF sensing platforms using a wireless remote system. Although the sensing schemes are still immature, this review focuses on the recent trends and advances of graphene and GO (rGO)-based RF bio and gas sensors for a real-time and continuous wireless health care system.
Right-hand side figure: Graphene-based radio-frequency (RF) nanodevices and circuits. (a) graphene: a single atomic sheet of sp2-bonded carbon; (b) graphene-based radio-frequency identification (RFID) tag antenna; (c) graphene-based frequency mixer; and (d) graphene-based 100 GHz transistor. (Ref.:H.-J. Lee "Recent progress in radio-frequency sensing platforms with graphene/graphene oxide for wireless health care system," Applied Sciences, Vol. 11, pp. 1-16, 2021y).
Review abstract: In this paper, the advances in radio-frequency (RF)/microwave biosensors based on graphene nanomaterials including graphene, graphene oxide (GO), and reduced graphene oxide (rGO) are reviewed. From a few frontier studies, recently developed graphene nanomaterials-based RF/microwave biosensors are examined in-depth and discussed. Finally, the prospects and challenges of the next-generation RF/microwave biosensors for wireless biomedical applications are proposed.
Left-hand side figure: Graphene nanomaterials-based radio-frequency (RF) interdigital capacitor (IDC) circuit for detecting biomaterials such as DNA, biotin-streptavidin, and bacteria. (Ref.:H.-J. Lee and J.-G. Yook, "Graphene nanomaterials-based radio-frequency/microwave biosensors for biomaterials detection," Materials, Vol. 12, pp. 1-13, 2019y).
Research abstract: We have demonstrated CNT resonator‐based biosensors using the biotin–streptavidin system. The CNT resonators were fabricated by connecting the interdigital capacitor to the CNT in parallel. These CNT resonators exhibited a clear resonance at ≈10–13 GHz depending on the device. After the streptavidin was bound to the biotinylated CNT, the resonance frequency was lowered by ≈0.7–1.3 GHz and the Q factor was enhanced. According to the simulation results, the resonance frequency shift and the enhanced Q factor are explained by the increase in CNT and Rf. Since the CNT resonator‐based biosensors can detect biomolecules by measuring the capacitance change through the resonance frequency measurements, their sensitivity is not reduced even for devices with metallic and semiconducting CNTs. In addition, their resonance frequency can be set to a desired frequency range by appropriate design of the metal pad.
Right-hand side figures: AFM images of a CNT (a) before and (b) after immobilization of the streptavidin. (Ref.:H.S. Lee, H.-J. Lee, H.H. Choi, J.-G. Yook, and G.-H. Yoo "Carbon-nanotube-resonator-based biosensors," Small, Vol. 4, pp. 1723-1727, 2008y).
5.Development of High-Q RF/microwave Resonators & Novel RF/microwave Bio-Sensing Platforms:
Research abstract: The asymmetric split-ring resonator, metamaterial element, is presented as a biosensing transducer for detection of highly sensitive and label-free stress biomarkers. In particular, the two biomarkers, cortisol and a-amylase, are used for evaluating the sensitivity of the proposed biosensor. In case of cortisol detection, the competitive reaction between cortisol-bovine serum albumin and free cortisol is employed, while alpha-amylase is directly detected by its antigen-antibody reaction. From the experimental results, we find that the limit of detection and sensitivity of the proposed sensing device are about 1 ng/ml and 1.155 MHz/ng ml, respectively.
Right-hand side figures: Schematic of aSRR-based biosensor and its chip device-level. (a) the schematic of aSRR based on a high impedance microstrip line, (b) the geometric parameters of aSRR , (c) the current mode of aSRR by time-varying magnetic field, and (d) the photograph of a fabricated sample. (Ref.:H.-J. Lee, J.-H. Lee, S. Choi, I.-S. Jang, J.-S. Choi, and H.-I. Jung "Asymmetric split-ring resonator-based biosensor for detection of label-free stress biomarkers," Applied Physics Letters, Vol. 103, pp. 053702-1-053702-5, 2013y).
Review abstract: The radio-frequency (RF) biosensors based on passive and/or active devices and circuits are reviewed. In particular, we focus on RF biosensors designed for detection of various biomolecules such as biotin-streptavidin, DNA hybridization, IgG, and glucose. The performance of these biosensors has been enhanced by the introduction of various sensing schemes with diverse nanomaterials (e.g., carbon nanotubes, graphene oxide, magnetic and gold nanoparticles, etc.). In addition, the RF biosensing platforms that can be associated with an RF active system are discussed. Finally, the challenges of RF biosensors are presented and suggestions are made for their future direction and prospects.
Left-hand side figure: Each type of resonator-based RF biosensors. From left to right side: QCM, SAW, MC, NEMS, DR, NMR/RFIC, SRR, and RFID. (Ref.:H.-J. Lee and J.-G. Yook "Recent research trends of radio-frequency biosensors for biomolecular detection," Biosensors and Bioelectronics, Vol. 61, pp. 448-459, 2014y).
Research abstract: We demonstrate the optimal multi-modes for glucose droplet (GD) sensing depending on sample (glucose droplet) positions in a half-wavelength (λ/2) microstrip resonator. Using two different GD positions, i.e., an edge and a center position, in the resonator, the resonant behavior of microwave multi-modes for GD sensing was investigated. GDs with three different concentrations, i.e., 0.1, 0.2, and 0.3 g/ml, were tested in the present experiment. With increasing GD concentration, the S21-level of the edge-positional GD gradually was declined at all resonant modes. For a center-positional GD, the frequency not only was deviated into the high-frequency region, but its S21-level also gradually was declined for the second and fourth modes. Based on the experimental results, the resonant behavior indicated the edge-(center-) positional GD had the resolution (~0.01/ gmL-1 or ~0.05 dB/gmL-1) at the second (second) and third (fourth) modes, respectively. Furthermore, we showed that the edge-positional GD sensing is more suitable when the resonator is associated with an active circuit system, and simultaneous sensing is also possible for different positions in multi-modes.
Right-hand side figure: (a) Multi-modes based on coupled microstrip resonator and (b) surface current distribution of the corresponding mode. (Ref.:H.-J. Lee, S.K, Kim and Y.-P. Hong "On the optimal modes for glucose droplet sensing based on multi-modes," IEEE Sensors Journal, Vol. 21, No. 21, pp. 24048-24055, 2021y).
Research abstract: In this study, we report the transmission characteristics based on microwave multi-resonant modes (MMRMs) for the pathogenic bacteria, Escherichia coli and Bacillus cereus, in the frequency range from 0.5 GHz to 10 GHz. In particular, we deeply analyze the differentiation between these microorganisms via transmission coefficient (T) at a specific concentration (~0.5 OD600/ml). The bacteria were evaluated on a coupled microstrip line resonator with the four MMRMs in the observed frequency region. According to the measured results, the transmission coefficient difference (ΔT) between the bacteria in a Luria-Bertani medium indicated a distinct level and displayed linear characteristics for the MMRMs from the first to the third resonant mode. Based on the sensing results, these bacteria with lower concentrations (0.1, 0.05, and 0.01 OD600/ml) were detectable and distinguished from MMRMs. Consequently, we showed that the ΔT could discriminate harmful germs at a given concentration using the MMRMs technique. In the future, we anticipate that the sensing approach will lead to advancements in traditional, time-consuming, and expensive bio-sensing systems for harmful bacteria detection and identification.
Left-hand side figure: Image of (a) E. coli and (b) B. cereus under an inverted fluorescent microscope. (Ref.:H.-J. Lee, D. K. Dey and S. C. Kang "Transmission properties of microwave multiresonant modes to pathogenic bacteria and their discrimination," IEEE Sensors Journal, Vol. xx, No. xx, pp. xx-xx, 2024y).
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