Modeling the properties of carbon nanotubes for sensor-based devices

We acknowledge funding from the European Community through NoE Nanoquanta (NMP4-CT-2004-500198), SANES (NMP4-T-2006-017310), DNA-NANODEVICES (IST-2006-029192) and NANO-ERA Chemistry projects, UPV/EHU (SGIker Arina) and the Basque Governement.Peer reviewe

Analytical prediction of highly sensitive CNT-FET-based sensor performance for detection of gas molecule

In this study, a set of new analytical models to predict and investigate the impacts of gas adsorption on the electronic band structure and electrical transport properties of the single-wall carbon nanotube field-effect transistor (SWCNT-FET) based gas sensor are proposed. The sensing mechanism is based on introducing new hopping energy and on-site energy parameters for gas-carbon interactions representing the charge transfer between gas molecules (CO2, NH3, and H2O) and the hopping energies between carbon atoms of the CNT and gas molecule. The modeling starts from the atomic level to the device level using the tight-binding technique to formulate molecular adsorption effects on the energy band structure, density of states, carrier velocity, and I-V characteristics. Therefore, the variation of the energy bandgap, density of states and current-voltage properties of the CNT sensor in the presence of the gas molecules is discovered and discussed. The simulated results show that the proposed analytical models can be used with an electrical CNT gas sensor to predict the behavior of sensing mechanisms in gas sensors

Electrical and optical properties of carbon nanotube intra-connects and conductive polymers

Growth of individual carbon nanotube (CNT) intra-connects (bridges) between two prefabricated electrodes is a great challenge and a pre-requisite to the development of single electron devices. In this thesis, CNT intra-connects were fabricated and studied. Later on, the intra-connects were deposited with electrically conductive polymer (ECP) in the realization of CNT/ECP hybrids. The process started by realizing two electrodes with sharp end on a silicon wafer using e-beam lithography. The intra-connects were then grown by use of chemical vapor deposition (CVD) technique. The intra-connects were later electroplated by various conductive polymers. The morphology, electrical conductivity of these intra-connects as well as their Raman spectroscopy signatures were studied. Scanning electron microscopy (SEM) was also employed. The CNT intra-connects were well-aligned from tip to tip. Their Raman spectra indicated the existence of CNT channels between metal tips and nowhere else on the wafer. Enhancement of photoconductance has been shown to correlate with a novel negative differential resistance (NDR) effect. Electroplated intraconnects exhibited unique properties both for multi-walled and single-walled carbon nanotube channels

Point-of-care immunoassay system using carbon nanotube labels

The goal of this research was to develop enhanced signal detection mechanisms for immunosensing using carbon nanotubes (CNTs). The utilization of CNT labels for direct electrical measurement was implemented on lateral flow system and microfluidic integrated interdigitated array microelectrodes. These sensing mechanisms in simple and miniaturized system provided higher sensitivity and autonomous flow control for rapid detection aimed at point-of-care diagnostics. Specific functionalization protocols were carried out to chemically modify the surface of the CNTs for uniform dispersion and antibody conjugation in aqueous solution. Surfactant assisted dispersion of the CNTs was studied using PVP and PEG. Covalent conjugation of antibodies on the carboxyl groups of the CNTs was accomplished using EDC/Sulfo-NHS coupling chemistry. The adsorption of surfactant and antibodies were manipulated in order to optimize immunoassay detection capability based on electrical measurements. Following surface functionalization methods, CNTs as a sensing label were employed on a lateral flow system. Competitive and sandwich immunoassay formats were demonstrated based on antibody and antigen binding. The lateral flow system was used for immobilization of capture molecules and passive sample transport by capillary action. CNTs conjugated to antibodies formed conductive network at the capture zone providing a visual indication corresponding to the amount of binding. Most importantly, significant change in electrical conductance was measured for varying low antigen concentrations, detecting anti-human Immunoglobulin G concentration below 1 ng/ml. Research was also conducted to obtain on-chip immunoassay detection using CNT labels. An IDA microelectrode was used as a binding surface and integrated within a PDMS microfluidic system. The sample and reagents were delivered to the sensing area through a microchannel. The capture of target analyte was indicated by the conjugated CNTs that formed a conducting matrix across the IDA. The detection was based on the selective binding between HSA and anti-HSA, where the conductimetric signal of the binding reaction was monitored through the IDA. The developed miniaturized system provided simple and sensitive immunosensing with detection capability below 1 ng/ml concentration using only 5 ìl of sample volume. Simulation was performed in order to understand the influence of the parameters in the microfluidic detection system

Novel Two-dimensional Nanomaterials and Their Gas Sensing Properties

Graphene, an atomic thin two-dimensional (2D) material with C atoms arranged in a honeycomb lattice, has sparked an unprecedented research interest across various scientific communities since its initial mechanical isolation in 2004. The linear energy dispersion with respect to the momentum within 1 eV around the Fermi level at the high symmetric K (Dirac) points in the Brillouin zone renders graphene a wonder material for scientists. However, graphene’s semimetallic nature significantly limits its high-end applications, e.g., in digital logic circuits. Therefore, continued efforts in opening the band gap for graphene and in searching for novel 2D semiconducting materials are rewarding. Various methods have been proposed for generating band gaps in graphene and other related 2D nanomaterials; however, few can be utilized to tune the band gap over a wide range on the same device and many are realized at the cost of severe degradation of carrier mobility. Recently, a new graphene-based crystalline structure, graphene monoxide (GMO), has been discovered based on electron diffraction observations during in situ thermal reduction of multilayer graphene oxide (GO) under vacuum in a transmission electron microscope (TEM) chamber. Supported by infrared spectroscopy and first-principles calculations, the new 2D material was identified as a two-phase hybrid containing GMO domains that evolve in the graphene matrix. GMO extends the electronic property of a graphene derivative into the semiconductor world, enabling potential applications for nanoelectronics. Another route to address the graphene band gap bottleneck is to search for new 2D nanomaterial candidates, among which 2D transition metal dichalcogenides (e.g., MoS2) and black phosphorus (BP) are attracting significant attention. Although both are layered structures and have a tunable band gap, a higher carrier mobility and a wider band gap ranging from 0.3 eV for bulk-like BP to 1.8 eV of monolayer BP make BP an outstanding candidate for future electronic applications. Conductance-based nanoscale gas sensors based on these 2D nanomaterials are attractive due to their superior sensitivity/selectivity and relatively low cost. Experimental studies have shown that in general semiconducting materials exhibit better sensitivity than insulating/metallic materials. Thus, it is crucial to understand the gas sensing mechanism of semiconducting materials and to gain better insights into the performance enhancement. This thesis aims to explore the fundamental properties of novel 2D nanomaterials and to understand their gas sensing performance. Various GMO properties were calculated using density functional theory (DFT)-based techniques. Infrared (IR) spectra of GMO were calculated for both pure GMO and GMO domains embedded into the graphene matrix to facilitate its identification during formation. GMO has three IR active modes that are distinctive from those of graphene and GO. The electronic and mechanical properties of GMO were predicted to illuminate its potential applications in semiconductor devices. The band gap of GMO can be tuned over a wide range from 0 to 1.35 eV. The capability of heat removal in intrinsic GMO was also simulated with and without planar lattice strains and compared with that of graphene and silicon. GMO exhibits a superior thermal conductivity (\u3e3,000 Wm-1K-1), 80% of that of graphene along the armchair direction for large lateral sample sizes (\u3e5 µm). The magnetic properties of zigzag graphene nanoribbons (ZGNRs) induced by GMO domains (or epoxy pair chains) were investigated. The epoxy pair chains can generate finite spin moments in ZGNRs irrespective of the spin coupling between ribbon edges. The gas sensing properties of selected 2D nanomaterials were characterized both theoretically and experimentally. First, we developed statistical thermodynamics models with the gas binding energy from DFT calculations as the only input to characterize the monolayer gas adsorption density on graphene and BP thin films. Our statistical thermodynamics models can successfully predict the gas adsorption density with high accuracy compared with experimental data. Second, an analytical model was established to interpret why semiconducting materials are preferred for gas sensing applications using a BP thin film-based gas sensors as an example. The sensitivity model suggests that the optimum thickness of BP thin film is from several to 10+ nm, corresponding with a band gap of 0.3 to 0.6 eV. Third, van der Pauw and Hall measurements were performed to obtain the sheet resistance, the carrier concentration, and the carrier mobility for thermally-reduced GO (TRGO) at various temperatures to illuminate relative contributions from the carrier concentration and the carrier mobility to the sheet resistance change upon gas adsorption, which suggests that the conductance change upon gas adsorption mainly results from the carrier concentration change. Finally, the sensitivity enhancement from the nanocrystalline particles deposited on the surface of graphene-base materials was also investigated

FIRST-PRINCIPLES INVESTIGATION OF THE ELECTRONIC AND TRANSPORT PROPERTIES OF CARBON ATOM WIRES

The last years have witnessed an ever increasing interest in the field of molecular electronics, in which molecules connected with two or more electrodes are used as main building blocks to fabricate the components of integrated circuits. The achievement of high performance interconnects is one of the key step for the realization of such devices. In this context, conjugated carbon wires are the most promising candidates to this role due to their structural stability and their experimental feasibility. In particular we investigated the transport properties of systems where a freestanding conjugated carbon chain is contacted with two electrodes in order achieve a two-terminal device. Although some important issues had already been investigated in this field when starting this project, information were very scattered and a well-rounded perspective of the key aspects affecting the electron transport was missing. This thesis represents one attempt to fill this gap first in the case of Ag-chain-Ag systems and then for graphene-chain-graphene devices. Specifically we focused on the role that the chain-electrode junction plays in determining the characteristics of such systems, finding that the contact geometry strongly affect the transport properties. In fact, for Ag-chain-Ag systems, qualitative different transport properties have been found depending on the adsorption site on the silver surface, while results showed only a weak dependence on the chain length. Moreover we observed that those results can be extended to "tip-like" contacts, mimicking a more realistic electrodic surface. As opposite, linking atoms such as sulfur or silicon strongly influence the conductance. We further demonstrated that the vibrational motion of the atoms has negligible effects on the transport properties for a very large range of temperature. Finally we also critically assessed the reliability of our results using the optical properties of the gas-phase molecules to benchmark the adopted setup. The results obtained for graphene-chain-graphene devices confirmed that the junction plays a central role in determining the strength of the electrode-chain coupling, and in turn, the conductance. We first analyzed in details the transport properties of single chains connected to ideal electrodes, investigating their geometrical and electronic structure in order to establish a connection with their transport properties. Furthermore, at odds with the above mentioned metal-C-metal systems, we found that "tip-like" junctions here have dramatic effects on the transport properties. More importantly, we investigated the possibility to observe quantum interference, chemical gating and conductance switching behaviors in such systems

Design of gas sensors using carbon nanotubes

PhD ThesisThis thesis is concerned with the structural, chemical and physical properties of carbon nanotubes (CNTs) and composites of CNTs with conductive polymers and DNA. The application of these composites in electronic sensors for volatile organic compounds VOCs, carbon monoxide and ozone gas was also investigated. CNTs are promising materials for a gas sensor because of their unique properties; small size, large specific surface area and high aspect ratio. However, the conductance of bare CNTs gives a small response to many analytes and therefore the formation of CNT composites with other polymeric materials to enhance the sensing performance was explored. The first part of the study is based on coating CNTs by polypyrrole and using these composites to detect volatile organic compounds (VOCs: methanol, ethanol, acetone and chloroform). Polypyrrole (Ppy)-coated CNTs were prepared by an in situ oxidative polymerization method with FeCl3:6H2O as the oxidant. TEM and AFM images showed a significant change in the diameter of the nanotubes upon polymerization from the mean value of 10 nm for multi walled carbon nanotubes (MWCNTs) to 60 nm after coating and from 5 nm to 50 nm for single-walled carbon nanotubes (SWCNTs). FTIR and Raman spectra indicated successful coupling between CNTs and polypyrrole. In addition, I-V characterization of two terminal nanotube devices and impedance spectroscopy demonstrated the change in the electrical properties of drop-cast CNT films after coating with polymer. The electrical current decreased after coating for both MWCNTs and SWCNTs (8 mA to 0.027 μA) and (10 mA to 88 μA) at an applied voltage of 2 V. Uncoated CNTs had a small analytical sensitivity (S), where S is ii defined as the percentage change in resistance upon exposure to analyte. For 12.9 kPa of methanol vapour, typically S < 1% for bare CNTs, while the sensitivity of the nanocomposites was typically S > 50% for 12.9 kPa of MeOH at room temperature. The sensing mechanism was found to be reversible and the temperature dependence could be analyzed using a simple extension of the Van’t Hoff equation. This suggests that the temperature dependence of the sensitivity is controlled by the enthalpy of adsorption on the composite. The second part of this study used CNTs/boron nitride nanotube (BNNTs) composites as an ozone gas sensor. Ozone is a powerful oxidant and polymer additives are not sufficiently robust for this application. CNT/BNNT films were prepared by drop-casting from equimolar solutions of BNNTs/methanol and CNTs/methanol. The electrical properties of drop-cast CNTs were changed after adding the insulating BNNTs; the electrical current decreased from 8 mA to 1 mA at applied voltage of 2 V. The sensitivity was improved from 18% to 50% for 80 ppm of ozone. However, the problem with the CNT ozone sensor was a long recovery time which can be 25 min or more, depending on the gas concentration. For CNTs/BNNTs the recovery time was shorter, but still lies between (2-17) min at room temperature. The third part of the study was related to detection of CO gas by CNTs/Ppy at room temperature. It has been shown that the sensitivity of CNTs is enhanced after polypyrrole coating: > 20% for SWCNTs/Ppy and < 2% for SWCNTs at 1923 ppm CO in air. Again, the sensitivity of these nanotube composites decreased with increased temperature according to an adsorption equilibrium model.The last part of this study evaluated DNA@CNT composites as a VOCs sensor. Three samples of CNTs/DNA were prepared with three different amounts of -DNA 2μL, 5 μL and 10 μL of (500 μg mL-1 ) which were added to 50μL (0.001 mg mL-1 ) of an aqueous dispersion of CNTs. DNA@CNT Films were drop cast across microelectrodes and from I-V measurements, it was found that the current (at a bias of 2 V) decreased after coating with increasing amounts of DNA from 8 mA (bare CNTs) to 4.5 mA to 2 mA and finally to 1 mA for the 50:10 sample. AFM and TEM images showed the DNA coats the CNTs and this suggests that tunnel junctions are introduced between CNTs which account for the drop in conductance. These junctions are also suggested to be the origin of the improved sensing response: DNA@CNT composites have good sensitivity for VOCs (MeOH, EtOH, C3H6O and CHCl3) and are more sensitive to methanol vapour than other VOCs. Further, DNA/CNTs films show a larger response to chloroform vapour at 21.08 kPa than CNTs/Ppy films at room temperature. Interestingly, the sensitivity of CNTs/DNA films increased as the temperature was raised; this suggests that another mechanism apart from adsorption/desorption is involved in their response. Although CNTs have been suggested as transducers in various gas sensors, they show a poor sensitivity (fractional change in resistance upon exposure to analyte). However by preparing composites of CNTs and less conductive materials, the analytical sensitivity can be greatly increased even though the conductivity of the composite is usually much less than of the bare CNTs.Iraqi ministry of higher educatio

Graphene Versus MoS2: a Short Review

Graphene and MoS2 are two well-known quasi two-dimensional materials. This review is a comparative survey of the complementary lattice dynamical and mechanical properties of graphene and MoS2. This comparison facilitates the study of graphene/MoS2 heterostructures, which is expected to mitigate the negative properties of each individual constituent.Comment: Frontiers of Physics, published, Focus Revie

DESIGN OF GRAPHENE-BASED SENSORS FOR NUCLEIC ACIDS DETECTION AND ANALYSIS

DNA (Deoxyribonucleic Acid) is the blueprint of life as it encodes all genetic information. In genetic disorder such as gene fusion, Copy Number Variation (CNV), and single nucleotide polymorphism, Nucleic acids such as DNA bases detection and analysis is used as the gold standard for successful diagnosis. Researchers have been conducting rigorous studies to achieve genome sequences at low cost while maintaining high accuracy and high throughput. A quick, accurate, and low-cost DNA detection approach would revolutionize medicine. Genome sequence helps to enhance people’s perception of inheritance, disease, and individuality. This research aims to improve DNA bases detection accuracy, and efficiency, and reduce the production cost, thus novel based sensors were developed to detect and identify the DNA bases. This work aims at first to develop specialized field effect transistors which will acquire real-time detection for different concentrations of DNA. The sensor was developed with a channel of graphite oxide between gold electrodes on a substrate of a silicon wafer using Quantumwise Atomistix Toolkit (ATK) and its graphical user interfaces Virtual Nanolab (VNL). The channel was decorated with trimetallic nanoclusters that include gold, silver, and platinum which have high affinity to DNA. The developed sensor was investigated by both simulation and experiment. The second aim of this research was to analyze the tissue transcriptome through DNA bases detection, thus novel graphene-based sensors with a nanopore were designed and developed to detect the different DNA nucleobases (Adenine (A), Cytosine (C), Guanine (G), Thymine (T)). This research focuses on the simulation of charge transport properties for the developed sensors. This work includes experimental fabrication and software simulation studies of the electronic properties and structural characteristics of the developed sensors. Novel sensors were modeled using Quantumwise Atomistix Toolkit (ATK) and its graphical user Interface Virtual Nanolab (VNL) where several electronic properties were studied including transmission spectrum and electrical current of DNA bases inside the sensor’s nanopore. The simulation study resulted in a unique current for each of the DNA bases within the nanopore. This work suggests that the developed sensors could achieve DNA sequencing with high accuracy. The practical implementation of this work represents the ability to predict and cure diseases from the genetic makeup perspective

Current Researches on Novel Applications of Carbon Nanotubes

Nanostructured materials are of extraordinary intrigued within the vitality capacity and change field due to their great mechanical and electrical properties. Carbon nanotubes are substances which have been shown those properties. They are an interesting nanostructure that has promising possibilities for future applications. CNTs have different allotropes such as fullerenes, CNTs and graphene. They have been subjects of wide investigate intrigued due to their potential for novel applications spread over the logical range. Graphene to a great extent considered the essential of all carbon allotropes can be formed to make 0D fullerene or rolled to create 1D CNTs. CNTs come in numerous shapes which shift by diameter and by the course of action of their hexagonal clusters within the grid. These contrasts result in the changes to the thickness of electronic states and give each sort of CNTs one of a kind of electrical and basic properties. Progresses in synthesis and decontamination have given analysts get to higher quality materials which has empowered distant better and improved understanding of their properties and their guarantee for future electronic applications