Research Interest

AREA OF SPECIALIZATION 

Expertise: Advanced Materials and Devices, Nanoscience & Nanotechnology; Optoelectronic Nanodevice

Verticals: Electronics and Semiconductor Technologies, Advanced Manufacturing Technologies

Horizontals: Semiconductor Devices: Physics & Technology / Semiconductor Materials & Process Technologies, Precision/ Micro-nano Manufacturing/ Surface Engineering


Brief Info: Nanomaterials, 2D van-der-Waals Material, Electronic Materials & Devices, Optoelectronics, Advanced Device Applications


RESEARCH INTERESTS

# Dry Transfer Fabrication Technique and Ferroelectric Properties Study at 2D Hetero-interface 

# Large Scale Two-dimensional (2D) van-der-Waals (vdW) Heterostructure Fabrication

# Low Temperature Plasma (LTP) Application to Tune Surface Properties of 2D Material

# Phase transition based resistive random-access memory (PT-ReRAM): mechanism study and development 

# Quantum and Nano-electronics: Applications of 2D layered Transition Metal Dichalcogenides

# Electronic Transport: Opto-electrical transport in 2D TMDs based devices, van-der-Waals Junction

# Advanced Device Applications: Photodetectors, Sensors and Non-volatile Memory Heterostructures

# Nano-plasmonics: Application of Surface Plasmon Resonance (SPR) Sensors


RESEARCH FUNDING RAISED:

Project/Area Number:   17F17360 (https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-17F17360/)

Budget Amount: ¥2,200,000

Topic: Development of highly sensitive optical sensor with two-dimensional atomic layers


Research Projects Executed:

Two-Dimensional (2D) Layered Materials based Low Power Electronics, Optoelectronics & Plasmonics:

Graphene-like atomically thin two-dimensional layered materials (2DLMs) will be a crucial component of future nanotechnologies promising extremely small and ultra-fast devices and systems. Few layer TMDCs represent a class of semiconductors in the two-dimensional (2D) limit due to their large carrier effective mass and the reduced screening in 2D, electron-hole interactions are much stronger than in conventional semiconductors. Such thin materials can be made of transition metal dichalcogenides (TMDCs) that exhibit versatile physics and chemistry and offer opportunities for fundamental and technological research in a variety of fields including catalysis, energy storage, sensing and electronic devices such as field-effect transistors and logic circuits. 

Since March, 2014, as a postdoctoral researcher at GWU, Washington, D.C., I was involved in mainly experimental and simulation based research with 2D TMDCs material. My works involved in simulating, designing and fabricating two-dimensional thin TMDCs material based highly efficient devices in the area of nanoplasmonics. During my second Post-Doctoral studies at IIT Bombay, I am involved in various project works on enhancing scattering, absorption and luminescence in two-dimensional layered material systems with surface plasmons and periodicity and device application in photodetectors

Recently from November 2017, I am working in Quantum Device Engineering Group, NIMS, Japan as an independent researcher. I started with various 2D material and their heterostructure based electronic and optoelectronic devices for the application in low operating power transistors, tunnel diodes and memory devices. Finally, I explored various advanced optoelectronic device for fast photodiodes with high quantum efficiency and laser assisted non-volatile memory devices for smart technology like Internet-of-Thing (IoT). 

  

(i) Development of laser assisted multilevel flash memory using the heterostructure of 2D van-der-Waals materials: - 

Demonstration of low intensity laser communicated multibit memory storage via fast (10 ms) optical erasure operation of a photoelectronic memory device at a low operation bias of 50 mV with a good retention time and endurance, which can be optically communicated multilevel non-volatile memory using the advantage of various van-der-Waals 2D materials and forming the vertical heterostructure among the 2D layers of semiconductor (ReS2), insulator (h-BN) and conductor like (graphene) materials. This flash memory can be program/erase using +/- 10V gate pulse with fast speed of 10 ms range, which can be operated multilevel memory storage via a green laser pulses (532 nm) with a low laser intensity of 1- 4 mW/cm 2 for future all- optical logic and quantum information processing.

This study provides an overview of the use of all-2D atomically-flat layered materials to realize various electronic components including sensors, detectors and memory devices for the Internet-of-Things (IoT) era. The photonic signal storage that we propose in this work can potentially enable all-optical logic processing and quantum information processing. This work was funded by JSPS Grants-in-Aid for Scientific Research (KAKENHI Grant No. 17F17360) and Project sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. This work has been published in Advanced Functional Materials (2020) and highlighted in MANA, NIMS E-BULLETIN & Press release.


[Vol. 65]

Laser-Assisted Non-Volatile Memory Based on 2D van-der-Waals Heterostructures https://www.nims.go.jp/mana/research/highlights/vol65.html 

https://www.nims.go.jp/eng/news/press/2020/08/202008250.html 


(ii) Layered 2D material based van der Walls p-n heterostructure for wide spectral photoresponse: - 

To realize optoelectrical device application, high quality p-n junction diode is important to build the structure. Two-dimensional layered TMDs and their interface are interesting for future optoelectronic device application. Vertical geometry in p-n junction is extremely useful as it helps to apply high electric field strength in order to separate the generated excitons in the thin TMDs material to contribute in current before recombination.

We fabricated few layer ReS2 based on vertical van der Waals p-n junction for photo-sensing application. Dry transfer of n-ReS2 flake to patterned p++Si substrate forms photodiode and its performance under different light wavelength and power has studied. A part of this work has appeared in SSDM international conferences regarding science and technology in devices and materials. (2018). This van der Waals heterostructure based photodetector project work is carried out by the JSPS supported project “Developments of high sensitive photodetectors with 2D layered materials”. We are also working on MoTe2/ReS2 p-n junction based high sensitive photosensor device, where the reverse bias of the interface has greater role. Direct band gap ReS2 material is already a promising material for photosensing application, which acts as high absorber on top of MoTe2 layer.   

(iii) Enhancing Scattering, Luminescence and Absorption in Two-Dimensional Layered Material with Surface Plasmons and Periodicity: -

The electromagnetic waves scattered or absorbed by the structures having atomically thin layered materials can be enhanced by surface plasmon resonance and periodicity. In brief, our theoretical, numerical, and experimental studies investigating the light-matter interaction occurring in complex structures composed of atomically thin TMDs materials (i.e. MoS2 and WSe2), periodic and randomly distributed metal nanostructures. Surface enhanced Raman scattering due to incorporating Au nanoparticles on top of WSe2 surface has studied. A part of this work has appeared in Materials Research Express (2015).

Modulation of luminescence of atomically thin transition metal dichalcogenide two- dimensional materials is critical for their integration in optoelectronic and photonic device applications. By coupling with different plasmonic array geometries, we have shown that the photoluminescence intensity can be enhanced and quenched in comparison with pristine monolayer MoS2. The enhanced exciton emission intensity can be further tuned by varying the angle of polarized incident excitation. Through controlled variation of the structural parameters of the plasmonic array in our experiment, we demonstrate modulation of the photoluminescence intensity from nearly fourfold quenching to approximately threefold enhancement. Our data indicates that the plasmonic resonance couples to optical fields strongly and increases the spontaneous emission rate in a double spacing plasmonic array structure as compared with an equal spacing array structure. This work has appeared in Scientific Report (2017).

Absorption enhancement in 2D TMDs material by decorating Au nanopillars has been studied. Few layer thick ReS2 material based two-probe nanodevices were made, where one part of the flake was decorated with Au nanopillars to observe the enhancement in photocurrent due to enhancing the optical absorption by the help of Au nanopillars. This work is communicating in Applied Physics Letters (2018).   

(iv) Light Matter Interactions in Complex Media with 2D Materials and Metamaterials:-

We numerically study the possibility of using atomically thin TMDCs for applications requiring broadband absorption in the visible range of the electromagnetic spectrum. If the thickness of each stack is equal to the quarter wavelength, then the constructive interference guarantees a high reflectance at that particular wavelength. We demonstrate that when monolayer TMDCs are positioned into a finite-period of multilayer Bragg stack geometry, they make broadband, wide-angle, almost polarization-independent absorbers. We demonstrate that bandwidth of metamaterial absorbers can be expanded using monolayer TMDs, where the structures can be engineered to make perfect reflectors for saturable absorption applications. This work has appeared in Optics Communications (2016).

(v) Visibility of Atomically Thin Layered Materials Buried in Silicon Dioxide:-

In order to protect optoelectronic and mechanical properties of atomically thin layered materials (ATLMs) fabricated over SiO2/Si substrates, a secondary oxide or nitride layer can be capped over. However, such protective capping might decrease ATLMs’ visibility dramatically. We have worked on different atomically thin TMDCs material to conclude their best optical visibility when buried in silicon dioxide. We find that the capping layer should not be thicker than 60 nm. Furthermore the optimum capping layer thickness value can be calculated as a function of underlying oxide thickness, and vice versa. This work has appeared in Nanotechnology (2016).

(vi) Plasmonics Enhanced Average Broadband Absorption of Monolayer MoS2:-

The interactions between TMD excitons and surface plasmons that are excited on the surface of metal nanoparticles are not completely understood. When we illuminate a TMD film decorated with metal nanoparticles, three additional interactions happen compared to illuminating a bare TMD film: (a) since metal nanoparticles absorb some of the incident energy, TMDs receive less light; (b) induced dipole moment of metal nanoparticles make them act like local antennas and create secondary fields; and (c) this secondary field gets reflected back from the substrate to the metal nanoparticle. If we neglect the third interaction for the sake of simplicity, we can expect having no enhanced field if the metal nanoparticles are so small compared to the wavelength and their density is low to have a localized plasmonic resonance (LSPR). And if we think about the opposite scenario (i.e. metal nano-particles are not so small and the inter-particle distance is equal to or less than their diameter), we might observe an enhancement or a weakening. The question is under what condition(s) field enhancement occurs? This work has appeared in Plasmonics (2016).

(vii) Complex Electrical Permittivity of the Monolayer MoS2 in Near UV and Visible:-

The attention on two-dimensional transition-metal dichalcogenides (2D-TMDs) has been growing exponentially due to their unprecedented electrical, optical, and mechanical properties. For the design and simulation of 2D-TMD systems, electrical and optical properties (like permittivity and conductivity) are “must have” items. However, none of the existing permittivity models (found in the literature) was accurate enough to support our experimental studies with numerical results. This is why we measured the transmission and reflection spectra through and from MoS2 coated substrate and a developed a simple model to calculate MoS2’s temperature and Fermi energy dependent complex electrical permittivity. Numerical results compared to experimental results reveal that the developed permittivity model can successfully represent the monolayer MoS2 under different biasing conditions at different temperatures for the design and simulation of MoS2 based optoelectronic devices. This work has appeared in Materials Research Express (2015).


Doctoral Research

Understanding the optoelectrical properties of individual semiconducting nanostructure of different morphologies based devices. Experimental investigations of the followings:

(i) stepped-surfaced single GeSe2 nanobelt devices with high-gain photoconductivity, (ii) photocurrent characteristics of individual smooth-surfaced GeSe2 nanobelt with Schottky effects, (iii) near infrared Schottky photodetectors based on individual single-crystalline GeSe nanosheet, (iv) direct laser micropatterning of GeSe2 nanostructures with controlled optoelectrical properties. 

 

(i) Stepped-surfaced single GeSe2 nanobelt devices with high-gain photoconductivity:-

Single crystalline stepped-surfaced GeSe2 nanobelts (NBs) were synthesized by vapor transport and deposition method with the presence of Au catalyst for the first time. The dynamic reshaping of the catalyst particle leads to formation of steps along the NB. Photodetectors comprising individually isolated NBs were fabricated to study their photodetection properties. The photoresponsivity of the devices was investigated at four different excitation wavelengths of 405 nm, 532 nm, 808 nm and 1064 nm. High photoresponsivity of 1040 AW-1 and a photoconductive gain of 121800% was achieved at a wavelength of 1064 nm, suggesting that the excitation to defect-related energy states near or below the mid band-gap energy plays a major role in the generation of photocurrent in these highly stepped NB devices. 

 

(ii) Photocurrent characteristics of individual smooth-surfaced GeSe2 nanobelt with Schottky effects:-

High quality smooth-surfaced GeSe2 NBs were synthesized and device performance of individual NB were studied. The current increased by three orders of magnitude upon laser irradiation (wavelength 532nm and intensity 6.8 mW/cm2) with responsivity of 2764 AW-1 at fixed 4V bias. Localized photoconductivity study shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions. In addition, the electrically Schottky nature of nanobelt/Au contact and p-type conductivity nature of GeSe2 nanobelt are extracted from the current-voltage characteristics and spatially resolved photocurrent measurements. The high sensitivity and quick photoresponse in the visible wavelength range indicate potential applications of individual GeSe2 nanobelt devices in realizing optoelectronic switches.

 

(iii) Near infrared Schottky photodetectors based on individual single-crystalline GeSe nanosheet:-

We designed 2D photodetectors using mechanically exfoliated GeSe nanosheet. The nonlinearship, asymmetric, and unsaturated characteristics of the I−V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 102 % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 AW−1), which is much higher than single layer MoS2 devices, suggests its potential applications as photodetectors.

 

(iv) Direct laser micropatterning of GeSe2 nanostructures with controlled optoelectrical properties:-

Surface functionalization of nanostructures after synthesis is important to realize many more interesting properties, which are absent in pristine nanomaterials. We demonstrate that a direct focused laser beam irradiation is able to achieve localized modification on GeSe2 nanostructures (NSs) film. Using a scanning focused laser beam setup, micropatterns on GeSe2 NSs film are created directly on the substrate. Controlled structural and chemical changes of the NSs are achieved by varying laser power and the treatment environment. The laser modified GeSe2 NSs exhibit distinct optical, electrical and optoelectrical properties. Detailed characterization is carried out and the possible mechanisms for the laser induced changes are discussed. The laser modified NSs film shows superior photoconductivity properties as compared to the pristine nanostructure film. The construction of micropatterns with improved functionality could prove to be useful in miniature optoelectrical devices.