Hiroshi Mizuta | Japan Advanced Institute of Science and Technology (original) (raw)

Papers by Hiroshi Mizuta

AIP Advances

Graphene is widely used in various real-life applications due to its high sensitivity to the chan... 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.

The Japan Society of Applied Physics, 2017

In recent years, ultrafine patterning of graphene b y a Helium ion microscope (HIM) has been acti... 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].

The Japan Society of Applied Physics, 2017

Carrier Transistor Operation JAIST, ZW. Wang, T. Iwasaki, M. Muruganathan and H. Mizuta E-mail: s... 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...

The Japan Society of Applied Physics, 2019

Coupling single charge transport to mechanical motion is useful for ground state cooling,[1] sens... 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...

The Japan Society of Applied Physics, 2017

As the Tunnelling Field Effect Transistor (TFET) overcomes the subthreshold slope thermal limitat... 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].

The Japan Society of Applied Physics, 2017

The interlayer transport between individual layers of 2D materials is an ideal subject for studyi... 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.

The Japan Society of Applied Physics, 2017

The Japan Society of Applied Physics, 2016

E-mail: schmidtm@jaist.ac.jp Graphene is a zero band gap two dimensional (2D) material with super... 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...

The Japan Society of Applied Physics, 2018

E-mail: schmidtm@jaist.ac.jp Phononic crystals (PnCs), periodic structures where the period is of... 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,...

The Japan Society of Applied Physics, 2016

Introduction Dopant-atom transistors offer the ability to control carrier transport to the level ... 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.

The Japan Society of Applied Physics, 2016

E-mail: a_hammam@jaist.ac.jp Tunnel Field Effect Transistors (TFETs) attract much interest of the... 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 ...

The Japan Society of Applied Physics, 2017

Phonons are collective mechanical lattice vibrations that are responsible for transmission of sou... 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...

Micromachines, 2020

Controlling the heat transport and thermal conductivity through a material is of prime importance... 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...

Physical Review B, 2020

Berry curvature, which arises from the asymmetry in a system, is a paramount physical property th... more Berry curvature, which arises from the asymmetry in a system, is a paramount physical property that results in interesting phenomena such as valley Hall effect. There have been several experimental observations of valley Hall effect in hBN encapsulated bilayer graphene systems as well as hBN/single-layer graphene heterostructures. Although the intent of encapsulating graphene with hBN is to improve the electronic properties of the graphene, the effect of the hBN layer in breaking the layer symmetry and inducing Berry curvature in these systems is not studied explicitly. In this study, we show that for the commensurate state of the heterostructure, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature in bilayer graphene system as they break the layer symmetry. Also, the polarity and the magnitude of the Berry curvature can further be manipulated with the application of an out-of-plane electric field by tuning the layer asymmetry. Whereas, in hBN/single-layer graphene commensurate systems, the presence of an hBN layer breaks the sublattice symmetry and induces Berry curvature, whose magnitude and polarity depend on the configuration and the orientation of the hBN layer. Moreover, although the magnitude can be further tuned with the application of an out-of-plane electric field, the polarity is rather insensitive to the field as the sublattice asymmetry cannot be reversed with the applied field.

Nano Letters, 2018

We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active an... more We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active anode material in sodium ion battery by expanding the interlayer distance in few outer layers of multiwalled carbon nanotubes (MWCNT). The performance enhancement was investigated using Density functional tight binding (DFTB) molecular dynamics simulation. It is found that sodium atom forms a stable bonding with the partially expanded MWCNT (PECNT) with the binding energy of-1.50 eV based on density functional theory calculation with van der Waals correction, where sodium atom is caged between the two carbon hexagons in the two consecutive MWCNTs. Wavefunction and charge density analyses shows that this binding is physisorption in nature. This larger exothermic nature of binding energy favors the stable bonding between PECNT and sodium atom thereby it helps to enhance the electrochemical performance. In the experimental works, partially opening of MWCNT with the expanded interlayer has been designed by the well-known Hummer's method. It has been found that introduction of functional groups causes partial opening of the outer few layers of MWCNT, with the inner core remaining undisturbed. The enhanced performance is due to expanded interlayer of carbon nanotubes, which provide sufficient active sites for the sodium ions to adsorb as well as to intercalate into the carbon structure. PECNT shows a high specific capacity of 510 mAh g-1 at a current density of 20 mA g-1 which is about 2.3 times the specific capacity obtained for pristine MWCNT at the same current density. This specific capacity is higher when compared to other carbon-based materials. The PECNT also shows a satisfactory cyclic stability at a current density of 200 mA g-1 for 100 cycles. Based on our experimental and theoretical results, an alternative perspective for the storage of sodium ions in MWCNT is proposed.

ACS Omega, 2018

Present work demonstrates a single step process for simultaneous synthesis of metal-nanoparticlee... more Present work demonstrates a single step process for simultaneous synthesis of metal-nanoparticleencapsulated nitrogen-doped bamboo-shaped carbon nanotubes (M/N-BCNTs) and graphitic carbon nitride (G-C 3 N 3). The synthesis of two different carbon nanostructures in a single step is recognized for the first time. This process involves the use of inexpensive and nontoxic precursors such as melamine as carbon and nitrogen sources for the growth of G-C 3 N 3 and M/N-BCNTs. In this technique, the utilization of unwanted gases such as ammonia and hydrocarbons released during the decomposition of melamine is the key to grow M/N-BCNTs over the catalyst along with the formation of G-C 3 N 4. The implementation of M/N-BCNTs as the electrode material for all-solid-state symmetric supercapacitor results in a maximum specific capacitance of ∼368 F g −1 with excellent electrochemical stability with 97% capacity retention after 10 000 cycles. Furthermore, fabricated symmetric supercapacitor shows maximum high energy and power density up to 10.88 W h kg −1 and 2.06 kW kg −1 , respectively. The superior electrochemical activity of M/N-BCNTs can be attributed to its high surface to area volume ratio, unique structural characteristics, ultrahigh electrical conductivity, and carrier mobility.

ACS applied materials & interfaces, Jan 14, 2018

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by ... 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 ...

2014 Silicon Nanoelectronics Workshop (SNW), 2014

We investigated a method of forming tunnel barriers in monolayer graphene nanoribbon (GNR) using ... more We investigated a method of forming tunnel barriers in monolayer graphene nanoribbon (GNR) using controlled ion irradiation. By using a helium ion microscope (HIM), we are able to reduce the width of exposure area down to 5nm. Sourcedrain conductance of side-gated GNR has been measured and the gate capacitances were extracted.

AIP Advances

Graphene is widely used in various real-life applications due to its high sensitivity to the chan... 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.

The Japan Society of Applied Physics, 2017

In recent years, ultrafine patterning of graphene b y a Helium ion microscope (HIM) has been acti... 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].

The Japan Society of Applied Physics, 2017

Carrier Transistor Operation JAIST, ZW. Wang, T. Iwasaki, M. Muruganathan and H. Mizuta E-mail: s... 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...

The Japan Society of Applied Physics, 2019

Coupling single charge transport to mechanical motion is useful for ground state cooling,[1] sens... 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...

The Japan Society of Applied Physics, 2017

As the Tunnelling Field Effect Transistor (TFET) overcomes the subthreshold slope thermal limitat... 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].

The Japan Society of Applied Physics, 2017

The interlayer transport between individual layers of 2D materials is an ideal subject for studyi... 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.

The Japan Society of Applied Physics, 2017

The Japan Society of Applied Physics, 2016

E-mail: schmidtm@jaist.ac.jp Graphene is a zero band gap two dimensional (2D) material with super... 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...

The Japan Society of Applied Physics, 2018

E-mail: schmidtm@jaist.ac.jp Phononic crystals (PnCs), periodic structures where the period is of... 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,...

The Japan Society of Applied Physics, 2016

Introduction Dopant-atom transistors offer the ability to control carrier transport to the level ... 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.

The Japan Society of Applied Physics, 2016

E-mail: a_hammam@jaist.ac.jp Tunnel Field Effect Transistors (TFETs) attract much interest of the... 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 ...

The Japan Society of Applied Physics, 2017

Phonons are collective mechanical lattice vibrations that are responsible for transmission of sou... 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...

Micromachines, 2020

Controlling the heat transport and thermal conductivity through a material is of prime importance... 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...

Physical Review B, 2020

Berry curvature, which arises from the asymmetry in a system, is a paramount physical property th... more Berry curvature, which arises from the asymmetry in a system, is a paramount physical property that results in interesting phenomena such as valley Hall effect. There have been several experimental observations of valley Hall effect in hBN encapsulated bilayer graphene systems as well as hBN/single-layer graphene heterostructures. Although the intent of encapsulating graphene with hBN is to improve the electronic properties of the graphene, the effect of the hBN layer in breaking the layer symmetry and inducing Berry curvature in these systems is not studied explicitly. In this study, we show that for the commensurate state of the heterostructure, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature in bilayer graphene system as they break the layer symmetry. Also, the polarity and the magnitude of the Berry curvature can further be manipulated with the application of an out-of-plane electric field by tuning the layer asymmetry. Whereas, in hBN/single-layer graphene commensurate systems, the presence of an hBN layer breaks the sublattice symmetry and induces Berry curvature, whose magnitude and polarity depend on the configuration and the orientation of the hBN layer. Moreover, although the magnitude can be further tuned with the application of an out-of-plane electric field, the polarity is rather insensitive to the field as the sublattice asymmetry cannot be reversed with the applied field.

Nano Letters, 2018

We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active an... more We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active anode material in sodium ion battery by expanding the interlayer distance in few outer layers of multiwalled carbon nanotubes (MWCNT). The performance enhancement was investigated using Density functional tight binding (DFTB) molecular dynamics simulation. It is found that sodium atom forms a stable bonding with the partially expanded MWCNT (PECNT) with the binding energy of-1.50 eV based on density functional theory calculation with van der Waals correction, where sodium atom is caged between the two carbon hexagons in the two consecutive MWCNTs. Wavefunction and charge density analyses shows that this binding is physisorption in nature. This larger exothermic nature of binding energy favors the stable bonding between PECNT and sodium atom thereby it helps to enhance the electrochemical performance. In the experimental works, partially opening of MWCNT with the expanded interlayer has been designed by the well-known Hummer's method. It has been found that introduction of functional groups causes partial opening of the outer few layers of MWCNT, with the inner core remaining undisturbed. The enhanced performance is due to expanded interlayer of carbon nanotubes, which provide sufficient active sites for the sodium ions to adsorb as well as to intercalate into the carbon structure. PECNT shows a high specific capacity of 510 mAh g-1 at a current density of 20 mA g-1 which is about 2.3 times the specific capacity obtained for pristine MWCNT at the same current density. This specific capacity is higher when compared to other carbon-based materials. The PECNT also shows a satisfactory cyclic stability at a current density of 200 mA g-1 for 100 cycles. Based on our experimental and theoretical results, an alternative perspective for the storage of sodium ions in MWCNT is proposed.

ACS Omega, 2018

Present work demonstrates a single step process for simultaneous synthesis of metal-nanoparticlee... more Present work demonstrates a single step process for simultaneous synthesis of metal-nanoparticleencapsulated nitrogen-doped bamboo-shaped carbon nanotubes (M/N-BCNTs) and graphitic carbon nitride (G-C 3 N 3). The synthesis of two different carbon nanostructures in a single step is recognized for the first time. This process involves the use of inexpensive and nontoxic precursors such as melamine as carbon and nitrogen sources for the growth of G-C 3 N 3 and M/N-BCNTs. In this technique, the utilization of unwanted gases such as ammonia and hydrocarbons released during the decomposition of melamine is the key to grow M/N-BCNTs over the catalyst along with the formation of G-C 3 N 4. The implementation of M/N-BCNTs as the electrode material for all-solid-state symmetric supercapacitor results in a maximum specific capacitance of ∼368 F g −1 with excellent electrochemical stability with 97% capacity retention after 10 000 cycles. Furthermore, fabricated symmetric supercapacitor shows maximum high energy and power density up to 10.88 W h kg −1 and 2.06 kW kg −1 , respectively. The superior electrochemical activity of M/N-BCNTs can be attributed to its high surface to area volume ratio, unique structural characteristics, ultrahigh electrical conductivity, and carrier mobility.

ACS applied materials & interfaces, Jan 14, 2018

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by ... 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 ...

2014 Silicon Nanoelectronics Workshop (SNW), 2014

We investigated a method of forming tunnel barriers in monolayer graphene nanoribbon (GNR) using ... more We investigated a method of forming tunnel barriers in monolayer graphene nanoribbon (GNR) using controlled ion irradiation. By using a helium ion microscope (HIM), we are able to reduce the width of exposure area down to 5nm. Sourcedrain conductance of side-gated GNR has been measured and the gate capacitances were extracted.