Chemical and Biological Sensing
DNA-Functionalized Carbon Nanotube Chemical Sensors
We have developed chemical sensors consisting of a single-walled carbon nanotube field effect transistor (swCN-FET) with a nanoscale layer of single stranded DNA (ssDNA) adsorbed to the tube's outerwall. The current through the swCN-FET show a characteristic response to gaseous analysis. This response varies depending on the base sequence of the adsorbed ssDNA. These sensors have been able to detect methanol, trimethylamine, propionic acid, dimethyl methylphosphonate (a simulant of sarin), and dinitrotoluene (a derivative of TNT) at the ppm level. The response and recovery of this biosensor is on the order of seconds.
Figure 1: DNA-Functionalized Carbon Nanotube Chemical Sensor
Figure 2: Self-assembly of a DNA-Nanotube Hybrid
DNA-Decorated Graphene Chemical Sensors
Chemical vapor sensors based on biomolecular functionalization of graphene field effect transistor arrays are demonstrated. Novel photolithographic methods were developed to fabricate high quality transistors from CVD-grown graphene. Atomic Force Microscopy was used to verify that the graphene surface remained uncontaminated and was thus suitable for controlled chemical functionalization. Single-stranded DNA was chosen as the functionalizing biomolecule due to its affinity to a wide range of target molecules as well as its π-π stacking interaction with graphene, which allowed functionalization with minimal impact on the transistor mobility. The resulting sensor arrays showed analyte and DNA sequence dependent responses down to parts-per-billion level concentrations. By using large arrays of differently functionalized devices, we distinguished chemically similar analytes and determined electronic signatures indicative of their presence.
Figure 3: DNA-Functionalized Graphene Chemical Sensor
Carbon Nanotube Hybrid Biosensors
We have designed and implemented a practical nanoelectronic interface to G-protein coupled receptors (GPCRs), a large family of membrane proteins whose roles in the detection of molecules outside eukaryotic cells make them important pharmaceutical targets. Specifically, we have coupled olfactory receptor proteins (ORs) with carbon nanotube transistors. The resulting devices transduce signals associated with odorant binding to ORs in the gas phase under ambient conditions and show responses that are in excellent agreement with results from established assays for OR–ligand binding. The work represents significant progress on a path toward a bioelectronic nose that can be directly compared to biological olfactory systems as well as a general method for the study of GPCR function in multiple domains using electronic readout.
Figure 4: Carbon nanotube functionalized with mouse olfactory receptor protein
Growth of Two-dimensional Materials
Growth Mechanism of Graphene
The properties of a graphene nanostructure are strongly influenced by the arrangement of the atoms on its edge. Growing graphene nanostructures with specified edge types in practical, scalable ways has proven challenging, with limited success to date. Here we report a method for producing graphene flakes with hexagonal shape over large areas, by a brief chemical vapor deposition growth at atmospheric pressure on polished Cu catalyst foil, with limited carbon feedstock. Raman spectra show evidence that the edges of the hexagonal crystallites are predominantly oriented along the zigzag direction. Density functional theory calculations demonstrate that the edge selectivity derives from favorable kinetics of sequential incorporation of carbon atoms to the vacancies in nonzigzag portions of the edges, driving the edges to pure zigzag geometry.
Figure 5: Growth of graphene on copper
Growth of Graphene-Boron Nitride Heterostructures
Graphene–boron nitride monolayer heterostructures contain adjacent electrically active and insulating regions in a continuous, single-atom thick layer. To date structures were grown at low pressure, resulting in irregular shapes and edge direction, so studies of the graphene–boron nitride interface were restricted to the microscopy of nanodomains. Here we report templated growth of single crystalline hexagonal boron nitride directly from the oriented edge of hexagonal graphene flakes by atmospheric pressure chemical vapor deposition, and physical property measurements that inform the design of in-plane hybrid electronics. Ribbons of boron nitride monolayer were grown from the edge of a graphene template and inherited its crystallographic orientation. The relative sharpness of the interface was tuned through control of growth conditions. The electronic functionality of monolayer heterostructures was demonstrated through fabrication of field effect transistors with boron nitride as an in-plane gate dielectric.
Figure 6: Growth of graphene and boron nitride heterostructures on copper
Performance of Sub-10 nm Graphene-based Devices
Atomic Structure and Transport in Graphene Nanoribbons
Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components, a fact that motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (< 10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. We found that the intrinsic conductance of sub-10 nm ribbons scaled with width as G(w) ≈ 0.75 (e^2/h)w[nm] for few-layer GNRs, where w is the width (measured in nm), while monolayer GNRs were roughly five times less conductive. Few-layer GNRs consistently formed bonded-bilayers and were robust structures that sustained currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry.
Figure 7: Graphene nanoribbon fabricated and characterized in an aberration-corrected TEM