Rajat K. Paul

Synthesis of a Pillared Graphene Nanostructure: A Counterpart of Three-Dimensional Carbon Architectures

Graphene is a single sheet of carbon atoms with outstanding electrical and physical properties and is being exploited for applications in electronics, sensors, photovoltaics, and energy storage. A novel 3D architecture called a pillared graphene nanostructure (PGN) is a combination of two allotropes of carbon, including graphene and carbon nanotubes. A one-step chemical vapor deposition process for large-area PGN fabrication via a combination of surface catalysis and in situ vapor-liquid-solid mechanisms is described. A process by which PGN layers can be transferred onto arbitrary substrates while keeping the 3D architecture intact is also described. Single and multilayer stacked PGNs are envisioned for future ultralarge and tunable surface-area applications in hydrogen storage and supercapacitors.

Graphene nanomesh as highly sensitive chemiresistor gas sensor

Graphene is a one atom thick carbon allotrope with all surface atoms that has attracted significant attention as a promising material as the conduction channel of a field-effect transistor and chemical field-effect transistor sensors. However, the zero bandgap of semimetal graphene still limits its application for these devices. In this work, ethanol-chemical vapor deposition (CVD) of a grown p-type semiconducting large-area monolayer graphene film was patterned into a nanomesh by the combination of nanosphere lithography and reactive ion etching and evaluated as a field-effect transistor and chemiresistor gas sensors. The resulting neck-width of the synthesized nanomesh was about ∼20 nm and was comprised of the gap between polystyrene (PS) spheres that was formed during the reactive ion etching (RIE) process. The neck-width and the periodicities of the graphene nanomesh (GNM) could be easily controlled depending on the duration/power of the RIE and the size of the PS nanospheres. The fabricated GNM transistor device exhibited promising electronic properties featuring a high drive current and an I(ON)/I(OFF) ratio of about 6, significantly higher than its film counterpart. Similarly, when applied as a chemiresistor gas sensor at room temperature, the graphene nanomesh sensor showed excellent sensitivity toward NO(2) and NH(3), significantly higher than their film counterparts. The ethanol-based graphene nanomesh sensors exhibited sensitivities of about 4.32%/ppm in NO(2) and 0.71%/ppm in NH(3) with limits of detection of 15 and 160 ppb, respectively. Our demonstrated studies on controlling the neck width of the nanomesh would lead to further improvement of graphene-based transistors and sensors.

Heterogeneous Graphene Nanostructures: ZnO Nanostructures Grown on Large‐Area Graphene Layers

In this work, the synthesis and characterization of three-dimensional hetergeneous graphene nanostructures (HGN) comprising continuous large-area graphene layers and ZnO nanostructures, fabricated via chemical vapor deposition, are reported. Characterization of large-area HGN demonstrates that it consists of 1-5 layers of graphene, and exhibits high optical transmittance and enhanced electrical conductivity. Electron microscopy investigation of the three-dimensional heterostructures shows that the morphology of ZnO nanostructures is highly dependent on the growth temperature. It is observed that ordered crystalline ZnO nanostructures are preferably grown along the <0001> direction. Ultraviolet spectroscopy and photoluminescence spectroscopy indicates that the CVD-grown HGN layers has excellent optical properties. A combination of electrical and optical properties of graphene and ZnO building blocks in ZnO-based HGN provides unique characteristics for opportunities in future optoelectronic devices.

Room temperature detection of NO2 using InSb nanowire

Room temperature detection of NO2 down to one part-per-million (ppm) using single crystalline n-type InSb nanowires (NWs) chemiresistive gas sensor is presented. These sensors were synthesized and fabricated by the combination of chemical vapor deposition and dielectrophoresis alignment techniques. The sensor devices showed an increase in resistance upon exposure to successive increments of NO2 concentration up to 10 ppm. The reduction in conductance of n-type InSb NWs when exposed to NO2 is made possible due to the charge transfer from the InSb NW surface to the adsorbed electron acceptor NO2 molecules. The demonstrated results suggest InSb NW as a promising candidate in sensing applications as well as being environmental friendly over existing arsenic and/or phosphorous-based III-V NW sensors.

InSb nanowire based field effect transistor

InSb nanowire field effect transistors (NWFET) were fabricated using electrochemically synthesized nanowires. To accurately extract transistor parameters, we introduced a model which takes into account the often ignored ungated nanowire segments. A significant improvement in extracted device parameters was observed which demonstrated that conventional models tend to underestimate the gate effect and therefore lead to lower carrier mobilities. Based on the model, we obtained a NWFET ON current of 11.8uA, an ION/IOFF ratio of 63.5 and hole mobility of 292.84 cm2V-1s-1.