I am currently a researcher in Prof. Mark Hersam’s research group at Northwestern University. My Ph.D. work focuses on fabricating electronic devices from solution-processed nanomaterials. In particular, I work with 2D materials, a class of nanomaterials with a wide range of optical and electronic properties. 2D materials can easily be paired together through printing techniques to produce a wide range of common electronic devices, such as thin film transistors, photodetectors, and electrochemical sensors.

Published Work

Below are some layperson summaries of my published research work. For the original papers, please check out my Google Scholar profile.

Thermoreflectance Imaging of (Ultra)wide Band-Gap Devices with MoS2 Enhancement Coatings

Authors: Riley Hanus*, Sonal V. Rangnekar, Shahab Mollah, Kamal Hussain, Nicholas Hines, Eric Heller, Mark C. Hersam, Asif Khan, and Samuel Graham

General summary: High power electronic devices are extremely important in electric vehicles and for renewable energy conversion. These devices must be constructed from specific materials that won’t breakdown if you place a large voltage across or large amount of current through the device (most common electronics would burn out instantly at high power). A subset of materials called ultrawide band gap (UWBG) semiconductors are often used in power electronics devices. However, even these UWBG materials can have limitations – heat doesn’t flow easily through the materials so they develop “hot spots” that can eventually cause the device to fail. We know that the hot spots occur, but we don’t always have the right tools to find out where along the device they tend to occur. This is because high-resolution thermal imaging techniques often use light, and the ultrawide band gap material doesn’t absorb the typical types of light used (the energy of the light is much smaller than the energy of the band gap).
To circumvent this problem, we coated the UWBG device with a material that does absorb the light. First, we deposited a film of molybdenum disulfide (MoS2) on top of UWBG materials gallium nitride (GaN) and aluminum gallium nitride (AlGaN). Then, we turned on the device, and MoS2 absorbed the heat from the device below, developing a similar thermal profile as the UWBG device. Finally, we used a technique called thermoreflectance imaging (TTI) to scan a green laser across the device. In TTI, a light detector collects information about how much green light is reflected from the MoS2 surface. MoS2 reflects a different amount of light at different temperatures. This allows us to visualize a temperature change across the surface of the device. Thus, the combination of the MoS2 coating with TTI imaging allows us to identify the exact location of the hot spot in the UWBG device.

Aerosol-jet-printed graphene electrochemical histamine sensors for food safety monitoring

Authors: Kshama Parate, Cícero C Pola, Sonal V Rangnekar, Deyny L Mendivelso-Perez, Emily A Smith, Mark C Hersam, Carmen L Gomes and Jonathan C Claussen

Aerosol-Jet-Printed Graphene Immunosensor for Label-Free Cytokine Monitoring in Serum

Authors: Kshama Parate, Sonal V. Rangnekar, Dapeng Jing, Deyny L. Mendivelso-Perez, Shaowei Ding, Ethan B. Secor, Emily A. Smith, Jesse M. Hostetter, Mark C. Hersam*, and Jonathan C. Claussen*

Simple summary: In today’s world, inexpensive and rapid medical diagnostic tests are needed more than ever. We believe that disposable electrochemical sensors can meet this need. These sensors operate very similarly to blood glucose monitors, which require a small sample of body fluid (i.e., blood) and instantly give quantitative information about the user’s blood sugar levels. However, common sensor electrodes are made from gold, which becomes wasteful in a disposable context. As an alternative, conductive carbon-based electrodes can be utilized. Graphene, a highly conductive, two-dimensional form of carbon, is an excellent candidate for electrode materials. Graphene films are an ideal material for electrochemical biosensing due to their high electrical conductivity, large surface area, and biocompatibility.
In this work, we produced an inexpensive graphene electrode for the rapid electrochemical biosensing of proteins (cytokines) and small molecules (histamine). First, we produced graphene inks by taking a graphene powder and dispersing it in a specific blend of solvents and stabilizers. Then we used an emerging printing technique called aerosol jet printing (AJP) to pattern the inks into the specific electrode geometry we wanted (i.e., interdigitated electrodes). In AJP, the ink is aerosolized and then sprayed onto a surface in a very small area to create high resolution printed features. After printing, the electrodes undergo a series of chemical processes to attach antibodies to the surface of the graphene. Any type of antibody can be used depending on the biomolecule that needs to be detected. As a first demonstration, we detected cytokines, which are immune system proteins that become elevated in the blood during states of infection. We were able to detect cytokines at levels that were medically relevant for diagnosing paratuberculosis in cattle. We also detected the small molecule histamine, which creates an inflammatory response in the body if ingested at sufficiently high concentrations. Rotting fish products can produce histamine, so we developed our sensor to detect histamine in fish broth at medically relevant levels. Overall, the low cost of manufacturing and short testing time (~30 min to soak and sense) suggest that we can use this printed graphene biosensor platform for other sensing applications, including wearable health monitoring and human health diagnostics.