Grayscale electron-beam lithography

In a collaborative study with Prof. Akshay Singh’s group in Department of Physics at the Indian Institute of Science (IISc), we have demonstrated significant enhancement in optical signals from 2D materials using engineered substrates.

Using grayscale electron-beam lithography (g-EBL), the team fabricated substrates with precisely tuned trench depths. When monolayer MoS₂ was transferred onto these structures, a remarkable >2 orders of magnitude enhancement in both Raman and photoluminescence (PL) intensities was observed, compared to conventional flat substrates.

This work opens up new avenues in tailoring light-matter interaction in 2D materials through substrate engineering, with applications in nanophotonics, optoelectronics, and 2D material-based sensors.

Title: Enhancement of Raman and Photoluminescence Intensity of Monolayer MoS₂ Using Engineered Substrates via Grayscale Electron-Beam Lithography
Journal: ACS Applied Nano Materials
Authors: Manavendra Pratap Singh, Ranju Dalal, Akshay Singh, Akshay Naik
Read the paper here 

Using Graphene nanoresoantor as strain sensor

 

Sensors that produce a measurable response to the applied mechanical deformation are useful in many applications including personal and structural health monitoring. One of the ways to make these strain sensors by fabricating small string-like vibrating structures called nanoresonators.  These tiny (sub 10um) structures, made of materials such as graphene, vibrate at very high frequencies (10-100MHz) and are exquisitely sensitive to changes in the strain. However, introducing the strain stimuli into these sensors implies that the devices have to fabricated on flexible substrates such as PDMS and PET. The fabrication process on these substrates is complex and device yields are small.

Swapnil More has developed a simple method to fabricate these devices on a thin diaphragm on silicon substrates. He then utilized air pressure to deform the substrate on which NEMS are fabricated. This controlled deformation of the substrate induces strain in the nano resonator and changes the resonant frequency. The magnitude of this strain change can be deduced using the frequency tuning of the nano resonator. We estimate that strain changes as low as 10-6 can be measured using these devices. This method can be used with a wide variety of nanomechanical systems. Besides their utility as sensors, strain tuneable nanoresonators are interesting tools to study some of the most intriguing dynamical phenomena such as synchronization, mode coupling, internal resonance.

More information here.

Charge detection using nano-resonators

The separation of leaves in gold leaf electroscopes indicates the amount of charge on them. Detectors that can accurately measure small fluctuations in charges are extremely important in a lot of applications including quantum sensing. Most of these sensitive detectors such as the ones based on Josephson junctions and single-electron transistors operate at temperatures below 1 K. These detectors rely on rapid change in the output in response to minute charge fluctuations at the input. Nonlinear systems at bifurcation points do exhibit such abrupt changes in output for small changes in the input. Such bifurcation amplifiers have previously been demonstrated using Josephson junction and mechanical resonators. Previous demonstrations using mechanical resonators were able detect charge fluctuations on the order of 100 electrons at room temperature. In our work, we have taken advantage of the exquisite force sensitivity of 2D material nano-electromechanical resonators in conjunction with a bifurcation amplifier implementation to sense charges on the order of 10 electrons in real-time at room temperature. We have also implemented a set-reset flip-flop using the same device to record short-lived charge fluctuations until an erase/reset operation is performed.  

 

 

The paper is published in APL.

More details can be found here

Congratulations Aneesh, Nishta and Swapnil!

Ph.D. Admissions

 

Here is some information that might be useful to prospective students.

What programs at IISc are appropriate for me?

Students generally pick departments that are closest to their undergradaute major. While this is probably ok, you might miss out on some really exciting research going on in other departments. e.g. While ECE and DESE might seem like a very good choice for students from ECE  background, they might want to explore departments such as Physics and CeNSE. There is a lot of device physics that happens in these departments that might be of interest to you. So make a list of broad areas that you are interested in and then look up the websites of the departments.

What are the modes of entry in IISc? Continue reading “Ph.D. Admissions”

Large tunable cooperativity between nanoresonators

In a recently published research in Nano Letters, we study coupling mechanism between two modes of a nanoscale resonators. Strong coupling between nanoresonators have previously been used to demonstrate novel effects such as cooling and electromagnetically induced transparency. The nanoscale resonator in this work is drum shaped resonator made of few atomic layers thick membrane and the modes are accommodated in this single nano drum resonator. The two different mechanical vibration modes have frequencies in 100 MHz range, the frequency range where FM radio works. The modes interact with each other via tension in the membrane which can be controlled electrically. In our experiment, the coupling is manipulated to enhance or reduce the energy exchange between the vibrational modes. The coupling can be increased to such an extent that the energy between the modes is exchanged more than 500 times back and forth before the information is lost to the environment. This is more than an order of magnitude improvement of in coupling compared to previous demonstrations. This demonstration of strong tunable coupling between high-frequency vibrational modes at nanoscale could lead to improvements in sensitivity of nano-mechanical sensors and has major implications for mechanical logic circuits and quantum limited measurement.

More details can be found here

Congratulations Parmeshwar and Nishta!

 

DNA translocation work by Ramkumar

Ramkumar’s work on DNA translocation through hybrid bilayer nanopores is now published in  Journal of Physical Chemistry C.  The work present simulations of hybrid nanopore structure of single-layer graphene and a DNA origami layer. Specific interactions between the overhangs and the DNA provides interaction specific residence times. This hybrid nano pore design can significantly improve the signal to noise ratio.

Find the manuscript here