Graphene is widely used in various real-life applications due to its high sensitivity to the change in the carrier concentration. Here, we demonstrate that graphene can be used for implementing a reliable lightning detection network as it... more
Graphene is widely used in various real-life applications due to its high sensitivity to the change in the carrier concentration. Here, we demonstrate that graphene can be used for implementing a reliable lightning detection network as it shows excellent sensitivity to the electric field of both positive and negative polarities, with a wide range of magnitude. The lowest electric field detected by our graphene sensor is 67 V/m, which is much smaller than the detection limit of previously reported graphene sensors and comparable to that of field mill and MEMS-based sensors. We also present the results of outdoor experiments where the response of the graphene sensor to the atmospheric electric field on a lightning day was tested and found to be in good agreement with the existing field mill sensor.
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In recent years, ultrafine patterning of graphene b y a Helium ion microscope (HIM) has been actively explored. The main advantages of He + ion beam milling over conventional gallium focused ion beam (FIB) milling are higher precision and... more
In recent years, ultrafine patterning of graphene b y a Helium ion microscope (HIM) has been actively explored. The main advantages of He + ion beam milling over conventional gallium focused ion beam (FIB) milling are higher precision and less damage. Carvi ng suspended graphene into single-nanometer (< 10 nm) structures (Fig. 1(a)) is getting particularly interested for various advanced applications such a s single-molecular detection [1] and nanoscale phonon engineering [2].
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Carrier Transistor Operation JAIST, ZW. Wang, T. Iwasaki, M. Muruganathan and H. Mizuta E-mail: s1530005@jaist.ac.jp The random charge localization caused by defects, and edge roughness in graphene nanoribbons heavily influences the... more
Carrier Transistor Operation JAIST, ZW. Wang, T. Iwasaki, M. Muruganathan and H. Mizuta E-mail: s1530005@jaist.ac.jp The random charge localization caused by defects, and edge roughness in graphene nanoribbons heavily influences the electrical characteristics for graphene based single carrier transistors (SCTs) devices. However, with the current top-down process technique, it is difficult to achieve a perfect graphene edge, which limits the graphene SCT performance [1]. In order to overcome this issue, the total edge length of graphene SCTs narrower part has to be reduced. In this direction, a single constriction of a few tens of nanometer in which enables to realize the SCT operation with the quantized energy levels. The total edge length of this structure is shorter compared to the geometrically defined quantum dot structure. Moreover, graphene surface should be protected from external adsorbents to avoid spurious interaction. For this purpose, we have coated the top graphene surf...
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Coupling single charge transport to mechanical motion is useful for ground state cooling,[1] sensitive charge sensing,[2] and atomic scale mass detection.[3] This coupling can be realized by embedding a single electron transistor (SET) in... more
Coupling single charge transport to mechanical motion is useful for ground state cooling,[1] sensitive charge sensing,[2] and atomic scale mass detection.[3] This coupling can be realized by embedding a single electron transistor (SET) in a nanoelectromechanical resonator. In this field, carbon based materials, i.e., suspended carbon nanotubes (CNTs) and graphene nanoribbons (GNRs), are appealing due to their stiffness, widely tunable resonance frequency and exceptional transport properties.[2-5] To gain an insight view of the coupling mechanism, it is necessary to develop the single dot SET to double dots. Double quantum dots can be realized by nanopatterning graphene. However, these channels easily deform during the supporting substrate etching process. To guarantee such a fragile structure, it is better to define the dots after suspending the channel. Helium ion beam milling (HIBM) has been demonstrated as an efficient technique for shaping suspended graphene into sub-10 nm scale...
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As the Tunnelling Field Effect Transistor (TFET) overcomes the subthreshold slope thermal limitation of MOSFETs, they are a potential successor of MOSFETs [1]. Moreover silicon-based TFETs are the most attractive because of the... more
As the Tunnelling Field Effect Transistor (TFET) overcomes the subthreshold slope thermal limitation of MOSFETs, they are a potential successor of MOSFETs [1]. Moreover silicon-based TFETs are the most attractive because of the well-established silicon technology. However, band-to-band tunneling (BTBT) in Si requires assistance of phonons for momentum conservation due to its indirect bandgap characteristics. Recently, isoelectronic traps (IETs) showed the incrase in the inter-band tunneling current without phonon assistance [2]. In this research work, we report the role of co-dopants at the p-to-n interface of the tunnel diode in the tunneling current enhancement without any phonon assitance. These results are based on the first-principles simulations in comparison with our experimental results for nano-pn tunnel diodes [3].
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The interlayer transport between individual layers of 2D materials is an ideal subject for studying the fundamental aspects of charge conduction in layered materials [1-2]. We report on the experimental observation of incoherent... more
The interlayer transport between individual layers of 2D materials is an ideal subject for studying the fundamental aspects of charge conduction in layered materials [1-2]. We report on the experimental observation of incoherent conduction in twisted bilayer graphene and a significant reduction in the interlayer resistance by applying an in-situ annealing process. This process enables the reduction of wrinkles and improves homogeneities in the interlayer, thus lessen the distance between the bottom and top graphene layers.
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E-mail: schmidtm@jaist.ac.jp Graphene is a zero band gap two dimensional (2D) material with superior electronic properties. However, opening a bandgap in graphene is essential for graphene-based electronic devices. Opening an energy gap... more
E-mail: schmidtm@jaist.ac.jp Graphene is a zero band gap two dimensional (2D) material with superior electronic properties. However, opening a bandgap in graphene is essential for graphene-based electronic devices. Opening an energy gap (EG) in graphene can be achieved by patterning the 2D graphene sheet into a narrow ribbon, called GNR. For a width smaller than 10 nm the EG is more than 100 meV [1]. Many techniques and methods have been developed to fabricate GNRs, however the most common technique remains to be electron beam lithography (EBL) patterning of an etch mask followed by reactive ion etching (RIE). Nevertheless, using the RIE method leads to a high degree of edge roughness in addition to the difficulty of achieving sub-10-nm widths. Here we propose a combination of EBL and helium ion microscopy (HIM) milling for sub-10-nm GNRs [2] with smooth edges, where a ~80 nm wide RIE-patterned GNR is milled to the desired width using HIM (Figure 1a). The reason for using this two-s...
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E-mail: schmidtm@jaist.ac.jp Phononic crystals (PnCs), periodic structures where the period is of similar dimension as the phonon wavelength, promise the opening of phononic band gaps (PnBGs) that could improve thermoelectric conversion... more
E-mail: schmidtm@jaist.ac.jp Phononic crystals (PnCs), periodic structures where the period is of similar dimension as the phonon wavelength, promise the opening of phononic band gaps (PnBGs) that could improve thermoelectric conversion efficiency and allow thermal rectification, as shown in Figure 1a [1]. Graphene, the atomic layer of carbon atoms in a hexagonal lattice, has a high Young’s modulus of ~1000 GPa and Debye termperature of ~2800 K. Thus, the wavelength of phonons at a given temperature is greatly increased compared to other materials such as silicon. This reduces the requirements for PnC patterning with PnBGs at room temperature. Recently, we have demonstrated sub-10-nm milling of a 3x3 pore array into suspended graphene by helium ion beam milling, where a tightly focused beam of ions (beam diameter <0.5 nm) interacts with the graphene and carbon atoms are ejected through collisions [2]. Up to 300 nm long PnCs with pitch of 18 nm were demonstrated, as well. However,...
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Introduction Dopant-atom transistors offer the ability to control carrier transport to the level of single atoms and single electrons. However, typical dopants in Si (such as P) have small barrier height and cannot sustain tunneling... more
Introduction Dopant-atom transistors offer the ability to control carrier transport to the level of single atoms and single electrons. However, typical dopants in Si (such as P) have small barrier height and cannot sustain tunneling operation at practical temperatures. Here, we discuss an alternative of using strongly-coupled a few donors to form quantum dots (QDs) with larger barriers, allowing tunneling operation at room temperature.
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E-mail: a_hammam@jaist.ac.jp Tunnel Field Effect Transistors (TFETs) attract much interest of the scientific community as a promising alternative for MOSFETs. Low off current and small sub-threshold slope are their major advantages.... more
E-mail: a_hammam@jaist.ac.jp Tunnel Field Effect Transistors (TFETs) attract much interest of the scientific community as a promising alternative for MOSFETs. Low off current and small sub-threshold slope are their major advantages. However, the on current is currently much lower than that for MOSFETs. Therefore, ways to increase the on current are actively investigated. According to quantum mechanics, materials with a small band gap and small effective mass of charge carrier can show higher band-to-band tunnel current. It is therefore expected that with their massless Dirac fermions and tunable bandgap, graphene is one of the most suitable candidates for TFETs. In this direction, we previously introduced the double gate GNR that showed a slight band-to-band tunnel current contribution to the total current[1]. In this work we propose a triple gate GNR field effect transistor (TG-GNRFET)(see Fig.1). First, numerical simulation was done by using the three-dimensional device simulator ...
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Phonons are collective mechanical lattice vibrations that are responsible for transmission of sound (low frequency oscillation of mechanical waves in KHz regime) and heat (high frequency oscillation of mechanical waves in THz regime)... more
Phonons are collective mechanical lattice vibrations that are responsible for transmission of sound (low frequency oscillation of mechanical waves in KHz regime) and heat (high frequency oscillation of mechanical waves in THz regime) through materials. Phonon engineering has earned immense interest recently because of its potential application in the fields of hypersound and heat control, acoustic and thermal cloaking, thermal diodes and so on. The versatility of graphene has long been appreciated in the scientific community. Recently, periodic arrays of 3-4 nm sized pores have been successfully nanopatterned in suspended graphene by focused helium ion beam milling. Such structures have the potential to allow roomtemperature phononic operation thanks to the high Young’s modulus and Debye temperature of graphene, forming phononic bandgaps in the low THz regime. For 3x3 arrays, a pitch of 9 nm had been reported, while this dimension is 18 nm for a larger array (see Fig. 1a), mainly du...
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Controlling the heat transport and thermal conductivity through a material is of prime importance for thermoelectric applications. Phononic crystals, which are a nanostructured array of specially designed pores, can suppress heat... more
Controlling the heat transport and thermal conductivity through a material is of prime importance for thermoelectric applications. Phononic crystals, which are a nanostructured array of specially designed pores, can suppress heat transportation owing to the phonon wave interference, resulting in bandgap formation in their band structure. To control heat phonon propagation in thermoelectric devices, phononic crystals with a bandgap in the THz regime are desirable. In this study, we carried out simulation on snowflake shaped phononic crystal and obtained several phononic bandgaps in the THz regime, with the highest being at ≈2 THz. The phononic bandgap position and the width of the bandgap were found to be tunable by varying the neck-length of the snowflake structure. A unique bandgap map computed by varying the neck-length continuously provides enormous amounts of information as to the size and position of the phononic bandgap for various pore dimensions. We have also carried out tra...
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Research Interests: Biochemistry, Evolutionary Biology, Physiology, Biophysics, Materials Science, and 15 morePerformance, Carbon Nanotube, Space Science, Medicine, Multidisciplinary, Density Functional Theory, MWCNT, Capacity, Binding Energy, Physisorption, Multiwalled Carbon Nanotubes, Carbon Fibers, Sodium-ion Batteries, Sodium Ions, and interlayer
Research Interests: Chemical Engineering, Microbiology, Biophysics, Materials Science, Electrochemistry, and 15 moreBiotechnology, Ecology, Nanomaterials, Medicine, Cycles, Melamine, Nitrogen Sources, Carbon Nanostructures, KG, Carbon Fibers, Carrier Mobility, Graphitic Carbon nitride, Power Density, Electrochemical Activity, and Electrochemical Stability
Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an... more
Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an energy gap opening. GNMs also have potential for use in phonon control and water filtration. Furthermore, physical phenomena, such as spin qubit, are predicted at pitches below 10 nm fabricated with precise structural control. Current GNM patterning techniques suffer from either large dimensions or a lack of structural control. This work establishes reliable GNM patterning with a sub-10 nm pitch and an < 4 nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Due to the simplicity of the method, no postpatterning processing is required. Electrical transport measurements reveal an effective energy gap opening of up to ∼450 meV. The reported technique combines the highest resolution with structural control and opens ...
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ABSTRACTAlthough initially developed as an imaging tool, the helium ion microscope (HIM) is finding applications in nanofabrication as its focused ion beam is capable of highly-localized material modification. In this study, an external... more
ABSTRACTAlthough initially developed as an imaging tool, the helium ion microscope (HIM) is finding applications in nanofabrication as its focused ion beam is capable of highly-localized material modification. In this study, an external pattern generator is used to explore the capabilities of the HIM for localized milling of a ∼7 nm thick layer of silicon-on-insulator, with atomic force microscopy (AFM) used to characterize the resulting patterns. The dose and patterned area size are varied and milling to depths >7 nm is demonstrated. At high doses and large areas, protuberances form, primarily due to sub-surface swelling caused by the implanted helium. The results suggest this technique could enable the rapid prototyping of next-generation nanoelectronic devices in thin silicon.
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Optical properties of Graphene have recently attracted enormous attention from investigators [1-5]. A significant amount of work is devoted to the calculation of optical conductivity σ(ω) with the aim of probing the interaction of light... more
Optical properties of Graphene have recently attracted enormous attention from investigators [1-5]. A significant amount of work is devoted to the calculation of optical conductivity σ(ω) with the aim of probing the interaction of light with this material. These calculations take the ...
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This paper demonstrates that the electrical properties of suspended graphene nanomesh (GNM) can be tuned by systematically changing the porosity with helium ion beam milling (HIBM). The porosity of the GNM is well-controlled by defining... more
This paper demonstrates that the electrical properties of suspended graphene nanomesh (GNM) can be tuned by systematically changing the porosity with helium ion beam milling (HIBM). The porosity of the GNM is well-controlled by defining the pitch of the periodic nanopores. The defective region surrounding the individual nanopores after HIBM, which limits the minimum pitch achievable between nanopores for a certain dose, is investigated and reported. The exponential relationship between the thermal activation energy (EA) and the porosity is found in the GNM devices. Good EA tuneability observed from the GNMs provides a new approach to the transport gap engineering beyond the conventional nanoribbon method.
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Focused ion beam technology with light gas ions has recently gained attention with the commercial helium and neon ion beam systems. These ions are atomic, and thus, the beam/sample interaction is well understood. In the case of the... more
Focused ion beam technology with light gas ions has recently gained attention with the commercial helium and neon ion beam systems. These ions are atomic, and thus, the beam/sample interaction is well understood. In the case of the nitrogen ion beam, several questions remain due to the molecular nature of the source gas, and in particular, if and when the molecular bond is split. Here, the authors report a cross-sectional scanning transmission electron microscopy (STEM) study of irradiated single crystalline silicon by various doses and energies of nitrogen ionized in a gas field ion source. The shape and dimensions of the subsurface damage is compared to Monte Carlo simulations and show very good agreement with atomic nitrogen with half the initial energy. Thus, it is shown that the nitrogen molecule is ionized as such and splits upon impact and proceeds as two independent atoms with half of the total beam energy. This observation is substantiated by molecular dynamics calculations...
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The direct growth of graphene on insulating substrate is highly desirable for the commercial scale integration of graphene due to the potential lower cost and better process control. We report a simple, direct deposition of... more
The direct growth of graphene on insulating substrate is highly desirable for the commercial scale integration of graphene due to the potential lower cost and better process control. We report a simple, direct deposition of nanocrystalline graphene (NCG) on insulating substrates via catalyst-free plasma-enhanced chemical vapor deposition at relatively low temperature of ∼800 °C. The parametric study of the process conditions that we conducted reveals the deposition mechanism and allows us to grow high quality films. Based on such film, we demonstrate the fabrication of a large-scale array of nanoelectromechanical (NEM) switches using regular thin film process techniques, with no transfer required. Thanks to ultra-low thickness, good uniformity, and high Young's modulus of ∼0.86 TPa, NCG is considered as a promising material for high performance NEM devices. The high performance is highlighted for the NCG switches, e.g. low pull-in voltage <3 V, reversible operations, minimal ...
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We present the density functional theory calculations of the binding energy of the Phosphorus (P) donor electrons in extremely downscaled single P-doped Silicon (Si) nanorods. In past studies, the binding energy of donor electrons was... more
We present the density functional theory calculations of the binding energy of the Phosphorus (P) donor electrons in extremely downscaled single P-doped Silicon (Si) nanorods. In past studies, the binding energy of donor electrons was evaluated for the Si nanostructures as the difference between the ionization energy for the single P-doped Si nanostructures and the electron affinity for the un-doped Si nanostructures. This definition does not take into account the strong interaction of donor electron states and Si electron states explicitly at the conductive states and results in a monotonous increase in the binding energy by reducing the nanostructure's dimensions. In this paper, we introduce a new approach to evaluate the binding energy of donor electrons by combining the projected density of states (PDOS) analysis and three-dimensional analysis of associated electron wavefunctions. This enables us to clarify a gradual change of the spatial distribution of the 3D electron wave...
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ABSTRACT In this paper we first present a new fabrication process of downscaled graphene nanodevices based on direct milling of graphene using an atomic-size He+ ion beam. We then study the effects of the He+ ion exposure on the carrier... more
ABSTRACT In this paper we first present a new fabrication process of downscaled graphene nanodevices based on direct milling of graphene using an atomic-size He+ ion beam. We then study the effects of the He+ ion exposure on the carrier transport properties in a bilayer graphene nanoribbon (GNR) by varying the time of He ion bombardment, along with underlying carrier scattering mechanisms. Finally we study the effects of various point defects in extremely-scaled GNRs on the carrier transport properties using ab initio simulation.
Research Interests: Materials Science, Graphene, Fabrication, Ion, Bilayer graphene, and 5 moreScattering, Tk, QC, Ab Initio, and Graphene Nanoribbons
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Silicon single electron transistors were fabricated by using the highly doped silicon channel with dual recess structure along with two recess gates and one central island gate as a pattern. The transition of Coulomb oscillation... more
Silicon single electron transistors were fabricated by using the highly doped silicon channel with dual recess structure along with two recess gates and one central island gate as a pattern. The transition of Coulomb oscillation characteristics from a single dot to a strongly coupled multiple dot was demonstrated for the different oxidation times and recess dimensions. The multiple-dot characteristic in the longer post lithography oxidized sample is attributed to the formation of a single dot in each recess due to the stress induced pattern-dependent oxidation, which leads to multiple dot in the channel. The temperature variation measurement, which was performed after two thermal cycling of the same sample to 20 and 4.2K with 1month gap, revealed the highly stable nature of the multiple-dot device transport characteristics. The multiple-dot device can also be operated as a unique nonlinear tunable resistance single electron transistor.
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Research Interests: Engineering, Electrical Engineering, Electronic Engineering, Monte Carlo Simulation, Nanotechnology, and 14 moreHigh Frequency, Radio Frequency, Shot Noise, Equivalent Circuit, Real Time, Single Electron Transistor, SPICE, Analytical Model, Transistor, Flicker Noise, Amplifier, Signal to Noise Ratio, Electrical And Electronic Engineering, and Measurement System
Research Interests: Materials Science, Numerical Analysis, Development, Finite element simulation, Non-Volatile Memory Technologies, and 13 moreHigh Speed, MOSFET, Quantum Dot, Bistability, Three Dimensional, Electron Devices, Scaling Law, Floating Gate, Electrical And Electronic Engineering, Flip Flop, Switching Time, Nonvolatile memory, and Mechanical Property
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This paper reports the study of the uncontrolled multiple tunnel junctions (MTJs) induced by the random dopants in heavily doped silicon single electron transistors (SETs). The SETs are fabricated by making dual lateral constrictions in... more
This paper reports the study of the uncontrolled multiple tunnel junctions (MTJs) induced by the random dopants in heavily doped silicon single electron transistors (SETs). The SETs are fabricated by making dual lateral constrictions in the narrow doped silicon channel formed on a silicon on insulator substrate. The doped SETs with relatively long constriction regions invariably exhibit the MTJ characteristics. The influence of the MTJs is suppressed by tuning the Fermi level in the constriction region. Finally, we show that the formation of uncontrolled MTJs can be avoided by making extremely sharp constrictions.
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This paper reports the observation of stochastic Coulomb blockade for the coupled silicon dots. The device was fabricated from the highly doped dual recess structured silicon channel by means of stress induced pattern-dependent oxidation.... more
This paper reports the observation of stochastic Coulomb blockade for the coupled silicon dots. The device was fabricated from the highly doped dual recess structured silicon channel by means of stress induced pattern-dependent oxidation. Sparsely placed Coulomb oscillation characteristics were observed from the transport characteristics at a low temperature and these irregularities decreased linearly as temperature increased. These characteristics were interpreted as the stochastic Coulomb blockade effect, which occurs due to the mismatch between individual dots in the energy spectrum ladder of the serially connected dots.