Session K27: Carbon Nanotubes: Electronic, Transport & Sensing

Carbon nanotube based field-effect transistors: merits and fundamental limits

The development of even more powerful computer systems are made possible by scaling of CMOS transistors, and this simple process has afforded continuous improvement in both the device switch time and integration density. However, CMOS scaling has become very difficult at the 22-nm node and unlikely to be rewarding beyond the 14-nm node. Among other new approaches, carbon nanotube devices are emerging as the most promising technique with unique properties that are ideal for nanoelectronics. In particular, perfect n-type and p-type contacts are now available for controlled injection of electrons into the conduction band and holes into the valence band of the CNT, paving the way for a doping free fabrication of CNT based ballistic CMOS, high performance optoelectronic devices, and integrated circuits [1-3]. These results will be compared with data projected for Si CMOS toward the end of the roadmap at 2026, as well as with those thermodynamic and quantum limits. References: [1] L.-M. Peng, Z.Y. Zhang and S. Wang, Materials Today 17 (2014) 433 [2] L. Ding et al., Nature Communication Published 14 Feb 2012, DOI: 10.1038/ncomms1682; Tian Pei et al., Nano Letters 14 (2014) 3102 [3] S. Wang et al., Nano Letters 11 (2011) 23; L.J. Yang et al., Nature Photonics 5 (2011) 672; H. Xu et al., Nano Letters 14 (2014) 5382   

Integration of High-Purity Carbon Nanotube Solution into Electronic Devices

Due to their exceptional electronic properties, carbon nanotubes (cnt) are leading candidates to be employed as channel materials in future nanoelectronic devices. A key bottleneck to realizing device integration is the sorting of carbon nanotubes, namely the isolation of high-purity, semiconducting cnt solutions. This talk will describe our efforts in using polymer-based sorting methods to isolate high-density and high-purity semiconducting cnt solutions. We explore the dependence of starting material and polymer to cnt ratio on the effectiveness of the separation. We confirm optically and electrically that the semiconducting purity is \textgreater 99.99{\%} through several thousand individual device measurements. In addition to single-cnt devices, thin-film transistors were also fabricated and tested. Due to the high purity of the solutions, device switching (\textasciitilde 10$^{\mathrm{5}}$ I$_{\mathrm{ON}}$/I$_{\mathrm{OFF}})$ was observed at channel lengths below the percolation threshold (\textless 500 nm). Operating below the percolation threshold allows for devices with much higher current densities and effective mobilities as transport is now the result of direct transport as opposed to hopping between cnts.   

One-dimensional quantum transport in hybrid metal-semiconductor nanotube systems.

We study the inter-play between the intrinsic 1D conductance of metallic atomic wires (AWs) and plasmon mediated near-field effects for semiconducting single wall carbon nanotubes (CNs) that encapsulate AWs of finite length. We use the matrix Green's functions formalism to develop an electron transfer theory for such a hybrid quasi-1D metal-semiconductor nanotube system. The theory predicts Fano resonances in electron transmission through the system. That is the AW-CN near-field interaction blocks some of the pristine AW transmission band channels to open up new coherent channels in the CN forbidden gap outside the pristine AW transmission band. This makes the entire hybrid system transparent in the energy domain where neither of the individual pristine constituents, neither AW nor CN, are transparent. The effect can be used to control electron charge transfer in semiconducting CN based devices for nanoscale energy conversion, separation and storage [1-3]. -- [1]S.Nanot, et al., Sci. Rep. 3, 1335 (2013); [2]M.Barkelid and V.Zwiller, Nature Photo 8, 47 (2014); [3]A.Sharma, et al., Nature Nano DOI:10.1038/nnano.2015.220.   

Making End-Bonded Contacts to Carbon Nanotubes

As a promising candidate for post-Si era, the implementation of carbon nanotube (CNT)-based CMOS technology requires both high-quality channel and electrical contacts that can be scaled down to sub-10 nm. In the efforts of making scalable contacts to CNT, we have recently demonstrated low-resistance end-bonded carbide contacts, formed by the reaction of Mo with CNT through high-temperature annealing (\textgreater 800 $^{\mathrm{o}}$C) [1]. Such end-bonded contact scheme leads to a size-independent contact resistance of about 30 kilo-ohms, which overcomes the scaling limit of conventional side contacts. In this talk, we will present another strategy to make end-bonded contacts to CNTs through thermal annealing at much lower temperatures (400-600 $^{\mathrm{o}}$C). The contact metals are carefully chosen to have a high carbon solubility, so that the carbon atoms could dissolve into the contacts to inherently form end-bonded contacts. Experimental results, including Raman, SEM, and electrical measurements, with different annealing temperatures will be presented. The length-dependent contact resistance for this new end-bonded contact will be evaluated and compared with that of conventional side contact and also end-bonded carbide contact. [1] Q. Cao, et al., Science, 350, 68-72 (2015).   

Carbon nanotube transistor based high-frequency electronics

At the nanoscale carbon nanotubes (CNTs) have higher carrier mobility and carrier velocity than most incumbent semiconductors. Thus CNT based field-effect transistors (FETs) are being considered as strong candidates for replacing existing MOSFETs in digital applications. In addition, the predicted high intrinsic transit frequency and the more recent finding of ways to achieve highly linear transfer characteristics have inspired investigations on analog high-frequency (HF) applications. High linearity is extremely valuable for an energy efficient usage of the frequency spectrum, particularly in mobile communications. Compared to digital applications, the much more relaxed constraints for CNT placement and lithography combined with already achieved operating frequencies of at least 10 GHz for fabricated devices make an early entry in the low GHz HF market more feasible than in large-scale digital circuits. Such a market entry would be extremely beneficial for funding the development of production CNTFET based process technology. This talk will provide an overview on the present status and feasibility of HF CNTFET technology will be given from an engineering point of view, including device modeling, experimental results, and existing roadblocks.   

Transport properties of C60/nanotube heterostructures

Crystal structures determine the electronic properties. The fullerene C60 is insulating but can become superconducting with Tc above 30 K by inserting atoms or apply a pressure. In addition, Tc changes with the distance between fullerene molecules. Because of relatively weak intermolecular interactions, C60 molecules can even pack into nanowire structures with low dimensional constrains imposed by carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs). In this study, we will report the characterization of C60/nanotube heterostructures and their transport properties. By filling into nanotubes, C60 molecules form one-dimensional quasi-crystal structures, which are absent in bulk or film forms. The C60-C60 intermolecular distance changes with the tube diameters, which could potentially yield rich transport properties.   

Environmental effects on the electrical properties of narrow-gap carbon nanotubes

Observations of single-walled carbon nanotubes (CNTs) with band gaps of 50 - 100 meV and diameters of approximately 2 nm pose an intriguing puzzle. The orthodox theory of CNTs predicts that such CNTs should have band gaps between 0 and 25 meV, yet these ``narrow-gap'' CNTs are routinely observed (band gaps in the range 50 -- 100 meV). A possible explanation is that strong Coulomb interactions cause a Mott gap in nominally metallic CNTs (Deshpande et al., Science, 2009). To test this hypothesis, we have fabricated field-effect transistor devices from suspended narrow-gap CNTs. We have tested these devices in a variety of dielectric environments, including air, vacuum, TiO$_{\mathrm{2}}$ coatings, and molecular liquids such as oil, anisole, toluene, isopropanol, and water. In many cases we can relate changes in electrical properties to changes in electrostatic disorder, gate capacitance, mobility and band alignment. We will discuss the possibility of an interaction-driven effect that is changed by the dielectric environment.   

Absorption of CO$_{2}$ on Carbon-based Sensors: First-Principle Analysis.

We present first-principle investigation of the adsorption properties of CO and CO$_{2}$ molecules on both graphene and carbon nano-tubes (CNTs) in presence of metal catalysis, mainly iron (Fe). The relaxations were carried out using the self-consistent-charge density-functional tight-binding (SCC-DFTB) code in neglect of heat effects. The results show the following: (1) Defected graphene is found to have high sensitivity and high selectivity towards chemisorption of CO molecules and weak physisorption with CO$_{2}$ molecules. (2) In case of CNTs, the iron ``Fe'' catalyst plays an essential role in capturing CO$_{2}$ molecules. The Fe ad-atoms on the surface of CNT introduce huge density of states at Fermi level, but the capture of CO2 molecules would reduce that density and consequently reduce conductivity and increase sensitivity. Concerning the selectivity, we have studied the sensitivity versus various gas molecules (such as: O$_{2}$, N$_{2}$, H$_{2}$, H$_{2}$O, and CO). Furthermore, to assess the effect of catalysis on sensitivity, we have studied the sensitivity of other metal catalysts (such as: Ni, Co, Ti, and Sc). We found that CNT-Fe is highly sensitive and selective towards detection of CO and CO$_{2}$ molecules. CNT being conductive or semiconducting does not matter much on the adsorption properties.   

Detection of the Odor Signature of Ovarian Cancer using DNA-Decorated Carbon Nanotube Field Effect Transistor Arrays

Carbon nanotubes are low-dimensional materials that exhibit remarkable chemical and bio-sensing properties and have excellent compatibility with electronic systems. Here, we present a study that uses an electronic olfaction system based on a large array of DNA-carbon nanotube field effect transistors vapor sensors to analyze the VOCs of blood plasma samples collected from patients with malignant ovarian cancer, patients with benign ovarian lesions, and age-matched healthy subjects. Initial investigations involved coating each CNT sensor with single-stranded DNA of a particular base sequence. 10 distinct DNA oligomers were used to functionalize the carbon nanotube field effect transistors, providing a 10-dimensional sensor array output response. Upon performing a statistical analysis of the 10-dimensional sensor array responses, we showed that blood samples from patients with malignant cancer can be reliably differentiated from those of healthy control subjects with a p-value of 3 x 10$^{-5}$. The results provide preliminary evidence that the blood of ovarian cancer patients contains a discernable volatile chemical signature that can be detected using DNA-CNT nanoelectronic vapor sensors, a first step towards a minimally invasive electronic diagnostic technology for ovarian cancer.   

Monitoring DNA polymerase with nanotube-based nanocircuits

DNA polymerases play an important role in the process of life by accurately and efficiently replicating our genetic information. They use a single-stranded DNA as a template and incorporate nucleotides to create the full, double-stranded DNA. Recent experiments have successfully monitored this process by attaching a Klenow fragment of polymerase I to a carbon nanotube and measuring the current along the tube [1]. Follow-up experiments have shown promise for distinguishing between DNA base pairs when nucleotide analogs are used [2], thus opening a new avenue for DNA sequencing. In this talk, we present results from computational studies on DNA polymerase I nanocircuits. The enzyme was first equilibrated in molecular dynamics and then density functional theory and Keldysh non-equilibrium Green's function methods were used to calculate the ballistic transmission coefficients and currents for different enzymatic states. Our results show significant change in current when the enzyme alternates between open (idle) and closed (synthesizing) states. We can also differentiate between some template bases when modified nucleotides and gate scanning are used. [1] T. J. Olsen et. al., JACS 135, 7855 (2013) [2] K. M. Pugliese et. al., JACS 137, 9587 (2015)   

A photonic microscope for observing real-time vibrations of carbon nanotubes

Vibrational modes in suspended carbon nanotubes (CNTs) are incredibly responsive to small forces, which makes them a prime candidate as nano-mechanical sensors. However, transducing this mechanical motion into detectable signals is a considerable challenge. Electrical detection, which has been the prevailing method thus far, suffers a significant impedance mismatch to macroscopic electronics and is thus susceptible to noise. We demonstrate an alternative: optical readout of CNT vibrations in real-time. By combining a unique CNT microtweezer platform with a high-finesse optical microdisk resonator, we dramatically enhance the naturally small optical cross-section of CNTs and thereby achieve unprecedented detection sensitivity. With this novel photonic microscope, we directly measure the thermal Brownian motion of CNTs and observe marked spectral diffusion at room temperature, shedding light on CNTs unique thermal physics. By further enhancing the optical coupling, we demonstrate optical amplification of CNT vibrations and directly observe period-doubling in the amplified state.