Nicholas J. Kybert

Intrinsic Response of Graphene Vapor Sensors

Graphene is a two-dimensional material with extremely favorable chemical sensor properties. Conventional nanolithography typically leaves a resist residue on the graphene surface, whose impact on the sensor characteristics has not yet been determined. Here we show that the contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrate a cleaning process that verifiably removes the contamination on the device structure and allows the intrinsic chemical responses of the graphene monolayer to be measured. These intrinsic responses are surprisingly small, even upon exposure to strong analytes such as ammonia vapor.

DNA-Decorated Graphene Chemical Sensors

Graphene is a two-dimensional material with exceptional electronic properties and enormous potential for applications. Graphene’s promise as a chemical sensor material has been noted but there has been little work on practical chemical sensing using graphene, and in particular, how chemical functionalization may be used to sensitize graphene to chemical vapors. Here we show one route towards improving the ability of graphene to work as a chemical sensor by using single stranded DNA as a sensitizing agent. The resulting devices show fast response times, complete and rapid recovery to baseline at room temperature, and discrimination between several similar vapor analytes.

Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations

Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS2) on insulating substrates, but to date, growth of isolated crystalline flakes has been demonstrated at random locations only. Here we use patterned seeds of molybdenum source material to grow flakes of MoS2 at predetermined locations with micrometre-scale resolution. MoS2 flakes are predominantly monolayers with high material quality, as confirmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the monolayer flakes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm(2) V(-1) s(-1) and 10(6), respectively. The technique provides a path for in-depth physical analysis of monolayer MoS2 and fabrication of MoS2-based integrated circuits.

Size-selective nanoparticle growth on few-layer graphene films.

We observe that gold atoms deposited by physical vapor deposition onto few-layer graphenes condense upon annealing to form nanoparticles with an average diameter that is determined by the graphene film thickness. The data are well described by a theoretical model in which the electrostatic interactions arising from charge transfer between the graphene and the gold particle limit the size of the growing nanoparticles. The model predicts a nanoparticle size distribution characterized by a mean diameter D that follows a D proportional, variant m(1/3) scaling law where m is the number of carbon layers in the few-layer graphene film.

Scalable, non-invasive glucose sensor based on boronic acid functionalized carbon nanotube transistors

We developed a scalable, label-free all-electronic sensor for D-glucose based on a carbon nanotube transistor functionalized with pyrene-1-boronic acid. This sensor responds to glucose in the range 1 μM–100 mM, which includes typical glucose concentrations in human blood and saliva. Control experiments establish that functionalization with the boronic acid provides high sensitivity and selectivity for glucose. The devices show better sensitivity than commercial blood glucose meters and could represent a general strategy to bloodless glucose monitoring by detecting low concentrations of glucose in saliva.

Differentiation of complex vapor mixtures using versatile DNA-carbon nanotube chemical sensor arrays.

Vapor sensors based on functionalized carbon nanotubes (NTs) have shown great promise, with high sensitivity conferred by the reduced dimensionality and exceptional electronic properties of the NT. Critical challenges in the development of NT-based sensor arrays for chemical detection include the demonstration of reproducible fabrication methods and functionalization schemes that provide high chemical diversity to the resulting sensors. Here, we outline a scalable approach to fabricating arrays of vapor sensors consisting of NT field effect transistors functionalized with single-stranded DNA (DNA-NT). DNA-NT sensors were highly reproducible, with responses that could be described through equilibrium thermodynamics. Target analytes were detected even in large backgrounds of volatile interferents. DNA-NT sensors were able to discriminate between highly similar molecules, including structural isomers and enantiomers. The sensors were also able to detect subtle variations in complex vapors, including mixtures of structural isomers and mixtures of many volatile organic compounds characteristic of humans.

Scalable arrays of chemical vapor sensors based on DNA-decorated graphene

Arrays of chemical vapor sensors based on graphene field effect transistors functionalized with single-stranded DNA have been demonstrated. Standard photolithographic processing was adapted for use on large-area graphene by including a metal protection layer, which protected the graphene from contamination and enabled fabrication of high quality field-effect transistors (GFETs). Processed graphene devices had hole mobilities of 1,640 ± 250 cm2·V−1·s−1 and Dirac voltages of 15 ± 10 V under ambient conditions. Atomic force microscopy was used to verify that the graphene surface remained uncontaminated and therefore suitable for controlled chemical functionalization. Single-stranded DNA was chosen as the functionalization layer due to its affinity to a wide range of target molecules and π-π stacking interaction with graphene, which led to minimal degradation of device characteristics. The resulting sensor arrays showed analyte- and DNA sequence-dependent responses down to parts-per-billion concentrations. DNA/GFET sensors were able to differentiate among chemically similar analytes, including a series of carboxylic acids, and structural isomers of carboxylic acids and pinene. Evidence for the important role of electrostatic chemical gating was provided by the observation of understandable differences in the sensor response to two compounds that differed only by the replacement of a (deprotonating) hydroxyl group by a neutral methyl group. Finally, target analytes were detected without loss of sensitivity in a large background of a chemically similar, volatile compound. These results motivate further development of the DNA/graphene sensor family for use in an electronic olfaction system.

DNA-decorated graphene nanomesh for detection of chemical vapors

The promise of graphene for use as a vapor sensor motivated exploration of the vapor responses of graphene nanomesh (GNM) functionalized with single stranded DNA. Devices detected different vapor types, including carboxylic acids, aldehydes, organophosphates, and explosives. As-fabricated GNM field effect transistors (FETs) had larger vapor responses than standard graphene FETs due to the effect of oxidized edges and lattice defects. DNA-GNM devices discriminated between homologous species with detection limits of a few parts per million, with fast response and recovery. Responses varied significantly when the base sequence of the DNA was changed, making the sensor class an intriguing candidate for use in an electronic nose system.

Scalable Production of Molybdenum Disulfide Based Biosensors

We demonstrate arrays of opioid biosensors based on chemical vapor deposition grown molybdenum disulfide (MoS2) field effect transistors (FETs) coupled to a computationally redesigned, water-soluble variant of the μ-opioid receptor (MOR). By transferring dense films of monolayer MoS2 crystals onto prefabricated electrode arrays, we obtain high-quality FETs with clean surfaces that allow for reproducible protein attachment. The fabrication yield of MoS2 FETs and biosensors exceeds 95%, with an average mobility of 2.0 cm(2) V(-1) s(-1) (36 cm(2) V(-1) s(-1)) at room temperature under ambient (in vacuo). An atomic length nickel-mediated linker chemistry enables target binding events that occur very close to the MoS2 surface to maximize sensitivity. The biosensor response calibration curve for a synthetic opioid peptide known to bind to the wild-type MOR indicates binding affinity that matches values determined using traditional techniques and a limit of detection ∼3 nM (1.5 ng/mL). The combination of scalable array fabrication and rapid, precise binding readout enabled by the MoS2 transistor offers the prospect of a solid-state drug testing platform for rapid readout of the interactions between novel drugs and their intended protein targets.

Facile fabrication of a ultraviolet tunable MoS2/p-Si junction diode

Chemical vapor deposition grown MoS2 single crystals were transferred onto the edge of a p-Si/SiO2 wafer, forming an abrupt heterogeneous junction diode at the MoS2/p-Si interface. When electrically characterized as a field effect transistor, MoS2 exhibits an n-type response and can be doped in the presence of ultraviolet (UV) light. As a diode, it operates satisfactorily in air, but has higher currents in vacuum with a turn on voltage of ∼1.3 V and an on/off ratio of 20 at ±2 V. UV irradiation increases the diode on state current, decreases the turn-on voltage, and reduces the ideality parameter below 2. These changes are reversible after annealing in air as desorption of electron trapping species like O2− and H2O− are believed responsible for this effect. A circuit integrating this diode was used to rectify a 1 kHz signal with an efficiency of 12%. Its simple design, coupled with the ability to clip AC signals, sense UV light, and reversibly tune these diodes, makes them inexpensive, multifunctional, an...