Session Z6: Focus Session: Carbon Nanotube Electronics, Properties, and Devices

Exploring adiabatic/non-adiabatic phase transitions in suspended metallic carbon nanotubes

We investigate the non-adiabatic Kohn anomaly in suspended pristine metallic single-walled carbon nanotubes by studying the dependence of the Raman $G$ band and $2D$ band frequency on Fermi energy. We find that by varying temperature, metallic nanotubes can switch between a regime in which the non-adiabatic Kohn anomaly is clearly observed, to a regime where the non-adiabatic Kohn anomaly is absent. Furthermore, we find that the non-adiabatic Kohn anomaly is always accompanied by a dramatic gate-induced modulation of the $G$ band Raman intensity. By establishing a quantitative correlation between the strength of the non-adiabatic Kohn anomaly and the modulation of Raman intensity, we determine that the underlying mechanism that leads to both these effects is the same.   

Current-Induced Cleaning of Adsorbates from Suspended Semiconducting Carbon Nanotube Diodes

Single-walled carbon nanotubes (SWNT) are prime candidates for future applications, including nanoelectronic and nanophotonic devices. Because of their large surface-to-volume ratio compared to that of a bulk seminconductor, however, SWNTs are very sensitive to their environment. Others have already established that their electronic properties can be dramatically changed by exposure to air, particularly oxygen or water. In this paper,\footnote{A. Malapanis, E. Comfort, and J. U. Lee, Appl. Phys. Lett., 98, 263108 (2011).} we show that the electronic and optical properties of $p-n$ diodes fabricated with suspended semiconducting SWNTs degrade over time with exposure to ambient conditions, mainly due to adsorption onto the tube's suspended part, which creates band-gap states. We provide the first correlation between adsorbate-generated electronic states and their impact on diode performance. Specifically, we show that the ideality factor, a fundamental parameter used to measure defect states in a $p-n$ diode, increases with the degree of adsorbate coverage. We also demonstrate a simple technique---current annealing---that can thermally reverse such degradation, returning device properties to their original characteristics.   

Electronic performance of flexible single-wall carbon nanotube films: The role of electronic type

Recent advances in the separation of single-wall carbon nanotubes (SWCNTs) by length and electronic type have made highly monodisperse SWCNT membranes a reality, opening up new realms of potential application in flexible electronics. By measuring the coupling between mechanical flexibility and electronic performance for thin transparent films of metallic and semiconducting SWCNTs assembled on elastic polymer substrates, we demonstrate a marked difference in the electronic manifestations of thin-film deformation for the two electronic SWCNT types. We relate these differences to mechanical and interfacial phenomena that stem from the distinct optical resonances characteristic of metallic or semiconducting nanotubes, and we evaluate the durability of each film type in response to repeated mechanical strain.   

Origin of Capacitance Change in Semiconducting and Metallic Carbon Nanotubes

Different mechanisms are reported for the capacitance changes in metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWCNTs) upon gas adsorption. The sensitivity, i.e., the change in capacitance, was high and reached approximately 2500{\%} in the aligned s-SWCNT network, whereas the sensitivity was found to be low in the m-SWCNT network. The charge transfer/Fermi level shift and quantum capacitance related to the localized electronic density of states near the Fermi level were key contributors to sensitivity changes upon gas adsorption, although the polarization effect also played a role in the capacitance changes, particularly in the aligned CNTs, under a strong electric field.   

Carbon based devices for molecular quantum spintronics

This presentation will address a new field called molecular quantum spintronics, which combines the concepts of spintronics, molecular electronics and quantum computing [1]. Various research groups are currently developing low-temperature scanning tunnelling microscopes to manipulate spins in single molecules, while others are working on molecular devices (such as molecular spin-transistors, spin valves and filters, and carbon-nanotube-based devices [1]) to read and manipulate the spin state and perform basic quantum operations. The talk will discuss the read-out of the spin states of single-molecule magnets using carbon based devises. In particular, carbon nanotube devices [2] and graphene nano-constrictions [3] will be discussed.\\[4pt] [1] L. Bogani \& W. Wernsdorfer, Molecular spintronics using single-molecule magnets, Nature Mat. 7, 179 (2008).\\[0pt] [2] M. Urdampilleta, S. Klyatskaya, J.-P. Cleuziou, M. Ruben, W. Wernsdorfer. Supramolecular Spin Valves. Nature Mat. 10, 502 (2011).\\[0pt] [3] A. Candini, S. Klyatskaya, M. Ruben, W. Wernsdorfer, M. Affronte. Graphene Spintronic Devices with Molecular Nanomagnets. Nano Lett. 11, 2634, (2011).   

Poole-Frenkel emission by carbon nanotube defect sites

Single walled carbon nanotubes (SWCNTs) have a conductance that is particularly sensitive to the presence of defects and disorder. Here, we fabricate field effect devices out of individual SWCNTs in order to investigate this effect. The bias- and gate-dependent conductance of SWCNT devices is measured over a temperature range of 77 -- 400 K. By performing these measurements on the same SWCNT before and after the incorporation of a point defect, we clearly discern the electronic consequences of the addition. Specifically, the initial recording of the pristine SWCNT determines the energy-dependent resistances of the SWCNT itself. After electrochemical point functionalization to introduce a defect site, the additional resistance and its energy-dependence is determined by properly accounting for the initial contributions. We find the defect scattering to be well fit by a Poole-Frenkel emission model, with the consequence that barrier widths and heights can be extracted for different defect types.   

Scanning Probe Characterization of Electronic Scattering by Carbon Nanotube Defects

Single-walled carbon nanotubes (SWCNTs) are quasi-ballistic, one-dimensional conductors with unique electronic properties. The controlled addition of a single covalent, sidewall defect interrupts this conduction pathway and produces a disproportionate change in the device properties. Here, we investigate the electronic effects in the immediate vicinity of a defect site using Kelvin force microscopy (KFM) and scanning gate spectroscopy (SGS). KFM images the local electrostatic surface potential along a SWCNT and thus can directly measure the potential drop at defects and other electronic surface features. Using KFM, we clearly observe the bias dependence of resistance at the site of defects and/or chemical attachments. The SGS technique provides complementary information by mapping their gate dependence. Combined, these two scanning probe techniques resolve the full energy dependence of scattering by a defect site and allow the determination of effective barrier widths and heights for different types.   

Fundamental Limits of Current Flow in One-dimensional Carbon Nanomaterials

Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) carry very high current densities given their strong sp2 bonds and high carrier mobility. However, the fundamental limitations to their maximum current flow are still not well understood. We measured the maximum current flow of CNTs and GNRs in substrate-supported configurations in ambient air where devices break from Joule heating at $\sim $600 C [1,2], revealing information about their power dissipation. Interestingly, thermal coupling with the substrate increases with CNT diameter but decreases with GNR width, due to competing roles of thermal boundary resistance and heat spreading into the substrate. To further study quasi-metallic single-wall CNTs at very high fields, we performed measurements in vacuum. We found some devices show current saturation as expected [3], but in many the current continues to increase with a constant slope of $\sim $~1~$\mu $A/V at fields $>$10 V/$\mu $m. This is observed even in small-diameter ($\sim $1.2 nm) CNTs whose dimensions were verified by AFM and Raman spectroscopy. We suggest such behavior can be explained by a combination of better heat coupling with the substrate and higher subband conduction. [1] A. Liao \textit{et al}, PRB \textbf{82}, 205406 (2010). [2] A. Liao \textit{et al}, PRL \textbf{106}, 256801 (2011). [3] Z. Yao \textit{et al}, PRL \textbf{84}, 2941 (2000).   

Terahertz Detection in an Individual Single-Walled Carbon Nanotube

Carbon nanotubes (CNTs) serve as a test experimental system for verification of physical models of one-dimensional (1-D) conduction. We aim to excite terahertz standing wave resonances on a CNT, which are predicted to display Luttinger-liquid behavior due to the lack of screening in 1-D. We describe measurements of terahertz (THz) absorption in individual single-walled carbon nanotubes and distinguish between two response mechanisms: bolometric detection due to heating a CNT with a temperature-dependent resistance and the response due to non-thermal electrical contact nonlinearities. This is the first frequency-domain demonstration of THz detection in an individual CNT. The effect of the contact nonlinearity is not decreased at THz frequencies and allows for analysis of the parallel contact capacitance to an individual CNT. Both detection mechanisms are expected to give evidence of the Luttinger-liquid resonant behavior. This experimental technique provides a method to study high-frequency charge excitations in the nanotube as a probe of the strength of the electron-electron interactions in this 1-D system.   

Individual Single-Walled Carbon Nanotube Device Fabrication for Terahertz Detection

We describe the fabrication process for the development of antenna-coupled single-walled carbon nanotube (CNT) terahertz (THz) detectors. This requires the development of a lithographically-defined gate electrode, as the device is fabricated on a THz-transparent insulating silicon substrate. Antenna development is geared towards accessing the largest possible bandwidth to facilitate the study of standing wave resonances in the CNT. These standing wave resonances are expected from the one-dimensional Tomonaga-Luttinger Liquid theory, which predicts collective charge oscillations, known as plasmons, which travel faster than the Fermi velocity. Using a Fourier transform interferometer, we excite the antenna-coupled CNT with a broadband blackbody source and probe the THz impedance of the CNT through Joule heating.   

In-situ TEM study of heat transfer between a carbon nanotube and contacting material

Due to their interesting intrinsic properties and 1D nature, carbon nanotubes may be (part of) the solution to the heat management challenge in nanoelectronics in multiple ways: e.g. as thermal interface material and/or interconnects in existing Si-based electronics, or as a building block in carbon-based electronics. Of crucial importance for any of these applications is the (in)ability to get heat in and out of the tube: the thermal contact resistance. Thermal contact resistance for carbon nanotubes is not fully understood yet, and theory is under construction. Measuring it is a challenge of its own as it is intimately connected to the thermal conductivity of the connected materials which are not necessarily known that well either. We study the thermal properties of carbon nanotubes with Electron Thermal Microscopy [T. Brintlinger et al., Nano Lett. 8, 582 (2008)], which allows for thermal imaging with a resolution of 150 nm, and combine that with finite-element modeling. With this approach we have already demonstrated that also on a substrate the CNT thermal conductivity is high, and that different heat transfer mechanisms (e.g. phonon-phonon, electron-surface polariton interaction) can be important. Our goal is to study and quantify thermal contact resistance between a carbon nanotube and its surroundings and we will present our latest results.   

Remote Joule heating of crossed nanotube

The high thermal conductivity of carbon nanotubes makes them an excellent candidate for thermal management and thermal logic devices. We have studied the thermal characteristics of Joule-heated MWNTs in a crossed geometry, using an established thermal measurement technique that relies on the solid to liquid phase transition of indium islands [1]. Our experimental observations show the efficient transfer of heat from CNT to substrate but inefficient heat transfer past the crossing point. This supports the presence of a nonlocal joule heating phenomenon in which the hot electrons from the biased CNT directly transfer their energy to a nearby material. This talk will cover the developed thermal characterization technique, experimental results and simulations. \\[4pt] [1] T. Brintlinger, et al., Nano Lett. \textbf{8}, 582 (2008).