Session P53: Physical properties of carbon nanotubes, 1D structures derived from 2D materials, and nanowires

Band-Gap-Dependent Electronic Compressibility of Carbon Nanotubes in the Wigner Crystal Regime

Electronic compressibility, the second derivative of ground-state energy with respect to total electron number, is a measurable quantity that reveals the interaction strength of a system and can be used to characterize the orderly crystalline lattice of electrons known as the Wigner crystal. Here, we measure the electronic compressibility of individual suspended ultraclean carbon nanotubes in the low-density, Wigner crystal regime. Using low-temperature quantum transport measurements, we determine the compressibility as a function of carrier number in nanotubes with varying band gaps. We observe two qualitatively different trends in compressibility versus carrier number, both of which can be explained using a theoretical model of a Wigner crystal that accounts for both the band gap and the confining potential experienced by charge carriers. We extract the interaction strength as a function of carrier number for individual nanotubes and show that the compressibility can be used to distinguish between strongly- and weakly- interacting regimes.   

Beating the Detailed Balance Limit in Ideal Carbon Nanotube pn diodes

The detailed balance (DB) limit developed by Shockley-Queisser has been the guiding principle for the design of modern solar cells. According to the DB limit, a single junction pn diode exhibits the maximum open-circuit voltage (V_OC) when it operates in the ideal diode limit. An ideal pn diode is characterized by an ideality factor n=1 which correlates to a specific generation and recombination (GR) process in a diode. Though V_OC is directly proportional to n, typically a higher n does not correspond to a larger V_OC. This is due to higher GR process for diodes with n〉1 that directly correspond to an increased diode leakage current, which reduces the diode’s open-circuit voltage. Here, we exploit a previously overlooked parameter, the diode ideality factor n, to increase the V_OC above the ideal diode limit. We use dynamic gate modulation in a single-walled carbon nanotube diode to engineer a digitally tunable effective ideality factor that is decoupled from the diode leakage current. We show that the open-circuit voltage can be tuned in direct proportion to n and achieve a V_OC that is 3 times higher than that given by the DB limit of an ideal pn diode.   

Electrical Transport of One-Dimensional Atomic Tellurium Nanowires Scaling Down to Two Nanometer Limit

Due to the capability of forming air-stable thin films with high carrier mobility up to 800 cm² V^-1 s^-1 at room temperature, tellurium (Te) as a chalcogen has shown its excellent potential in the field of semiconductor devices. As a narrow-bandgap p-type semiconductor, bulk Te has a direct bandgap of ~0.35 eV. Besides, Te has a unique crystal structure composed by helical chains. Each Te atom is covalently bonded with the other two adjacent Te atoms to form a spiral chain, and bulk crystal is consist of numerous chains stacked by van der Waals interaction. Here, we systematically studied the electronic transport properties of 1D Te devices scaling down to 2 nm limit in diameter. Since the native oxidation and degradation of 1D Te nanowires (NWs), the limit of bare Te NWs to exhibit stable electronic signal is around 6-7 nm. In order to further explore the performance of thinner Te NWs, we grew Te atomic chains into boron nitride nanotubes (BNNTs) and realized its field effect successfully down to 2 nm limit. Due to the encapsulation of BNNTs, the on-state current can even be dramatically increased. Moreover, the phonon responses of 1D Te NWs have also been investigated from bulk form down to a single atomic chain limit by encapsulation of carbon nanotubes (CNTs).   

Fabrication and characterization of ultrathin Ta2(Pd or Pt)3Se8 nanowires

Miniaturization of semiconductor devices has been a prime focus of the electronics industry to address the requirements of next-generation electronic devices. One possible solution to tackle this problem is semiconducting one-dimensional (1D) van der Waals(vdW) wires. Here we present quasi 1D semiconducting Ta₂(Pd or Pt)₃Se₈(TPS) crystals, composed of weakly bound molecular ribbons. Inter-ribbon bonding energy(0.34eV/atom) of TPS bulk is 17 times weaker than the intra ribbon bonding energy(5.7eV/atom). In this work, we used cost-effective and easily processable liquid phase exfoliation technique to achieve molecularly thin wires. The thinnest nanowire we have readily achieved is around 1 nm, corresponding to a bundle of one or two molecular ribbons. Optimum solvent for the dispersion is investigated and found Di-acetone alcohol gives strong dispersion. High-resolution TEM, SEM and AFM studies were done to investigate the morphology and crystallinity of the ultrathin wires obtained. Our electrical transport measurements have shown that these thin wires are gatable and preserve the intrinsic bulk characteristics. The fabricated 1D transistors exhibit high switching performance and excellent ambient environment stability.   

Carbon nanotube photocurrent quantum yield greater than 100%

Carbon nanotubes (CNTs) have been identified as a candidate material to surpass the Shockley-Queisser limit, however, early measurements of photocurrent quantum yield (PCQY) in CNT photodiodes found PCQY < 100%. We studied CNT photodiodes made from individual suspended CNTs with the goal of elucidating the role of nanotube diameter, dielectric environment, axial electric field, and the optical excitation energy. When photons of energy approximately twice the band gap are absorbed in the intrinsic region of a CNT photodiode, it is possible to extract a photocurrent that corresponds to more than one electron-hole pair per absorbed photon. We observed this high-quantum-yield process by increasing CNT diameter to > 2.6 nm and increasing the axial electric field to > 10 V/µm (a previously unstudied regime). Higher energy optical excitation gives even higher PCQY. The observed dependence on diameter and dielectric environment is consistent with theoretical models for exciton binding energy and exciton dissociation rates. Our work suggests there are conditions for which efficient exciton dissociation co-exists with efficient impact ionization.   

Temperature-Dependent Thermoelectric Transport in Polymer-Sorted Semiconducting Carbon Nanotube Networks with Different Diameter Distributions

We report carrier density and temperature dependent field-effect mobility and on-chip Seebeck coeff. measurements of small diameter (6,5) (0.76 nm), ttmgb treated (6,5) and large diameter plasma torch (RN, 1.17-1.55 nm) SWCNT networks with different network densities to provide insights into their charge transport mechanisms and potential for thermoelectric applications. The Seebeck coeff. offers insights into transport energetics, potential contributions from electron-electron and electron-phonon interactions and the relative contributions of inter- versus intra-CNT transport.¹ A pure percolation theory and VRH description cannot explain both transport coefficients and instead intra-CNT transport depending on chirality needs to be considered as well. In the high network density regime, the density influences neither electric nor thermoelectric transport strongly, indicating both are dominated by tubes rather than tube-tube junctions. Despite generally lower mobilities, (6,5) SWCNTs show comparable power factors to RN networks. These findings provide design guidelines towards narrow DoS distr., large diameter SWCNT networks for both electronic and thermoelectric applications.²
[1] Statz, M. et al. Commun. Phys. 1, 16 (2018).
[2] Statz, M. et al., in prep. (2019).
  

Applying Machine Learning to Thermal Conductance

Machine learning provides a new approach in materials design. Recent years have seen the use of generative models to algorithmically produce candidate molecules for a variety of tasks. One such model is the variational autoencoder (VAE), which can map discrete information to a continuous, low-dimensional space. We study the ability of these methods to design nanostructures for thermal conductivity: from idealistic harmonic chains to functionalized carbon nanotubes (CNTs). Designing the latter's side-chains is of particular interest for overcoming the severe Kapitza resistance of CNTs and accessing their promising thermal properties. We also highlight the ability of a genetic algorithm (GA) enhanced with a discriminating neural network to optimize molecules for their thermal conductance. Because discriminating GAs are forced to consider a diverse field of molecules, they are an effective way to generate a varied dataset for analysis. It can be shown that molecules sampled in this manner are clustered based on their thermal properties in the latent space learned by the VAE. In addition, studying the GA trajectory may reveal design rules that can narrow the search for good conductors.   

Carbon Nanotube Alignment Dynamics under Electric Fields in Different Solutions

Controlled macroscopic assembly of carbon nanotubes (CNTs) while preserving their excellent optoelectronic properties still remain a key challenge. Here, we investigated the molecular dynamics of single-wall carbon nanotube (SWCNT) alignment inside various viscous media under electric fields. An analytical model based on dieletrophoresis induced torque considering the viscosity and conductivity of the medium was used to obtain molecular dynamics of the SWCNTs. An alternating current (AC) electric field was applied to SWCNT containing liquid solutions of several solvents/surfactants such as DIW, DMF, DPSF, SDS, and DOC. Time required for the SWCNTs to get aligned to the applied AC electric field were calculated for different initial conditions for all the solutions. The effects of CNT length, CNT diameter, and frequency of the electric field on the SWCNT network formation were theoretically studied. Longer and thinner SWCNTs prompted to faster alignment in SWCNT network. Furthermore, the effect of concentration of surfactant on arrangement time was examined. Slower SWCNT alignment was observed in medium with higher viscosity. The findings of this report will be helpful for establishing an effective technique of producing large-scale aligned CNT films for diverse applications.   

Achieving Global Alignment of Single-Wall Carbon Nanotube Films through Electrostatic Control

Single-wall carbon nanotubes (SWCNTs), known for their exceptional mechanical, thermal, electrical, and optical properties, present a platform for next generation opto- and thermo-electronics. The difficulty, however, lies in the ability to reproducibly form aligned SWCNT films from aqueous nanotube solutions, which continues to be a significant scientific and technological challenge. Although multiple research groups have achieved high nematic ordering from SWCNT solutions, the mechanism driving this alignment, as well as optimization of it, remains elusive. Here, we use an automated filtration platform and superhydrophobic glass to create globally aligned films from solution-based SWCNTs. We show how SWCNT alignment is impacted by the filtration flow rate and SWCNT electrostatic environment. Additionally, we find that the SWCNT nematicity can be enhanced via buffing of the polymeric membrane coating and hindered by reducing the Debye interaction length of the SWCNTs, both of which show the importance of electrostatics in the alignment mechanism. These results have direct implications for achieving and optimizing the alignment of single- and few- chirality SWCNT films.   

Electron-phonon and phonon-phonon interaction in low-dimensional carbon materials

Low-dimensional (e.g. atomically thin) materials continue to gain prominence in applications ranging from electronics to photonics and alternative energy generation systems. Critical to efficiently developing these systems is the understanding of the fundamental processes related to the dynamics of charge carriers, phonons, and other excitations (i.e. excitons, polaritons). In this talk, I will focus on electron-phonon interactions in low dimensional carbon materials. Through these interactions the electrons lose all their excess energy above the band edge and become thermally equilibrated with the most strongly coupled optical phonon modes. Subsequently the optical phonons modes through unharmonic phonon-phonon scattering processes decay to lower-energy phonon modes.   

A Raman study of G-bands in isolated and bundled triple-walled carbon nanotubes

The G-band is a Raman-active phonon mode, caused by tangential and longitudinal stretching of the two dissimilar carbon atoms in the unit cell. This mode is well studied in other forms of carbon, e.g. graphite, graphene, single-walled and double-walled carbon nanotubes. Building on that knowledge, a study on bundled and isolated tripled-walled carbon nanotubes (TWNT) is presented. Of particular interest is how many G-band peaks are present, their lineshapes, and how the G-band frequencies of the constituent tubes are affected from being in a TWNT system compared to if they were just SWNTs.   

High pressure Raman spectroscopy of Linear carbon chain encapsulated in isolated multiwall carbon nanotube

Elusive measurements of the Longitudinal optical (LO) phonon mode frequency of isolated Cn@MWCNT systems (linear carbon chains encapsulated by multi-wall carbon nanotubes) were studied under extreme high pressure via Resonance Raman spectroscopy. The pressure-dependent frequency softening with increasing pressure is discussed in terms of a simple force constant model in which the spring constants present a dependence with pressure. The model allowed us to easily obtain both the Young’s modulus and the Gruneisen parameter associated to such LO mode. In particular, the Gruneisen parameter was found to be much higher than that of Graphene and CNTs. Additionally, the LO phonon lifetime was found to be decrease with increasing stress.   

Raman Spectra of Vertical Graphene Nanosheets

Raman spectroscopy studies have been demonstrated their importance in providing a quantitative measure on different carbon-based samples, such as graphene, carbon nanotube, and sp3 amorphous carbon. Recently, plasma-enhanced chemical vapor deposition grown vertical graphene nanosheets (VGNs) have attracted much attention owing to their abundant edges, surface, and enhanced electrochemical activity, which are potentially applicable to a wide of fields. However, due to its complexity of the vertical structures, the knowledge gained in previous carbon-based materials has not fully been extended to VGNs. In this work, we review the recent advance of Raman characterization results on PECVD-grown VGNs. With the insights from interference in graphene nanosheets and their electrical field distribution, we are able to extend the knowledge from the Raman spectroscopy of conventionally flat graphene layer to vertical graphene nanosheets. A feasible way to distinguish and evaluate the quality and structures of the VGNs from Raman spectra is presented in combination with the transmission electron microscopy characterization.   

Manipulating high-harmonic generation in single walled carbon nanotubes

High-harmonic generation (HHG), which is the generation of multiple optical harmonic light, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. The HHG in solid state materials is strongly influenced by the presence of nonlinearity in transport and optical transitions, but how to engineer these parameters has not yet been elucidated. Here, we demonstrate manipulation of HHG by tuning the electronic structure and carrier densities using single-wall carbon nanotubes with tuned electronic structures and Fermi levels. We reveal systematic changes in the HH spectra of carbon nanotubes with a series of electronic structures from a zero band-gap metal to a semiconductor. We demonstrate enhancement or reduction of harmonic generation by more than one order of magnitude by tuning electron and hole injection into the semiconductor carbon nanotubes through static electric field application.   

Polymer Host for Optical and Terahertz Spectroscopy of Low-Aggregation Nanoparticle Films

Preparation techniques for producing films of individualized nanoparticle for low-temperature optical spectroscopy are often challenging to execute and specific for a particular nanoparticle system. Here, we present a rapid, facile, and low-cost technique for producing 100 μm-thick nanoparticle-polymer films that exhibit high uniformity, low aggregation, excellent optical transparency, and low terahertz absorption. These films are both robust at cryogenic temperatures and exhibit a high laser damage threshold of 0.3 TW/cm², which make them ideal for pulsed laser measurements. Additionally, we show that free-standing flexible films can be made from 0D quantum dots and 1D single-wall carbon nanotubes (SWCNTs). Using absorption, Raman scattering, and photoluminescence excitation spectroscopy we show that SWCNT individualization is maintained from solution to film. Finally, we perform optical pump, terahertz probe measurements on SWCNT films to demonstrate pulsed spectroscopic investigation across a broad electromagnetic regime. This polymer host presents spectroscopists with a straightforward method for producing free-standing and flexible nanoparticle films with low aggregation.