Carbon nanotube deformation and collapse under metal contacts

Nonideality in Arrayed Carbon Nanotube Field Effect Transistors Revealed by High-Resolution Transmission Electron Microscopy

High density and high semiconducting-purity single-walled carbon nanotube array (A-CNT) have recently been demonstrated as promising candidates for high-performance nanoelectronics. Knowledge of the structures and arrangement of CNTs within the arrays and their interfaces to neighboring CNTs, metal contacts, and dielectrics, as the key components of an A-CNT field effect transistor (FET), is essential for device mechanistic understanding and further optimization, particularly considering that the current technologies for the fabrication of A-CNT wafers are mainly laboratory-level solution-based processes. Here, we conduct a systematic investigation into the microstructures of A-CNT FETs mainly via cross-sectional high-resolution transmission electron microscopy and tentatively establish a framework consisting of up to 11 parameters which can be used for structure-side quality evaluation of the A-CNT FETs. The parameter ensemble includes the diameter, length (or terminal), and density distribution of CNTs, radial deformation of CNTs, array alignment defects, surface crystallography facets of contact metal, thickness distribution of high-k dielectrics (HfO2), and the contact ratios for the CNT-CNT, CNT-metal, CNT-dielectric, and CNT-substrate interfaces. Enriched array alignment defects, i.e., bundle, stacking, misorientation, and voids, are observed with a total ratio sometimes up to ∼90% in pristine A-CNTs and even up to ∼95% after the device fabrication process. Thus, they are suggested as the prevalent performance-limiting factors for A-CNT FETs. Complex interfacial structures are observed at the CNT-CNT, CNT-metal contact, and CNT-high-k dielectric interfaces, making the local environment and the property of each component CNT involved in an A-CNT FET distinct from others in terms of the diameters, radial deformation, and interactions with the local surroundings (mainly through van der Waals interactions). The present study suggests further improvements on the fabrication technology of A-CNT wafers and devices and mechanistic investigations into the impacts of complex array alignment defects and interface structures on the electrical performance of A-CNT FETs as well.

Wetting and strain engineering of 2D materials on nanopatterned substrates

The fascinating realm of strain engineering and wetting transitions in two-dimensional (2D) materials takes place when placed on a two-dimensional array of nanopillars or one-dimensional rectangular grated substrates. Our investigation encompasses a diverse set of atomically thin 2D materials, including transition metal dichalcogenides, hexagonal boron nitride, and graphene, with a keen focus on the impact of van der Waals adhesion energies to the substrate on the wetting/dewetting behavior on nanopatterned substrates. We find a critical aspect ratio of the nanopillar or grating heights to the period of the pattern when the wetting/dewetting transition occurs. Furthermore, energy hysteresis analysis reveals dynamic detachment and re-engagement events during height adjustments, shedding light on energy barriers of 2D monolayer transferred on patterned substrates. Our findings offer avenues for strain engineering in 2D materials, leading to promising prospects for future technological applications.

Establishing Ohmic Contact of a Radial Compressed CNT Bundle with High Work Function Metal

Establishing low-resistance ohmic contact is critical for developing electronic devices based on traditional silicon and new low-dimensional materials. Due to unprecedented electronic and mechanical properties, the one-dimensional carbon nanotubes (CNTs) have been used as source/drain, gate, or tunnel to fabricate transistors. However, the mechanism causing low-resistance ohmic contact is not clear yet. Here, the hybrid atomic force microscopy-scanning electron microscopy (AFM-SEM) instrument was developed to establish lower-resistance ohmic contact between a radial compressed deformed multiwalled CNT bundle and high work function metal (platinum and gold). The radial compression structure under strong van der Waals attraction was in situ characterized through the SEM image to obtain the diameter and width and through AFM to get height and to perform nanoindentation, indicating that Pt has the smaller radial compression deformation. Molecular dynamics simulations exhibit that compared to Pt, a wider ribbon-like graphene layer formed when the radial compressed CNTs contacted with Au. The bond forming and electron orbital overlapping between C atoms of deformed CNTs and the high work function metal atom is beneficial for good electrical contact.

Metal Contact Induced Unconventional Field Effect in Metallic Carbon Nanotubes

One-dimensional carbon nanotubes (CNTs) are promising for future nanoelectronics and optoelectronics, and an understanding of electrical contacts is essential for developing these technologies. Although significant efforts have been made in this direction, the quantitative behavior of electrical contacts remains poorly understood. Here, we investigate the effect of metal deformations on the gate voltage dependence of the conductance of metallic armchair and zigzag CNT field effect transistors (FETs). We employ density functional theory calculations of deformed CNTs under metal contacts to demonstrate that the current-voltage characteristics of the FET devices are qualitatively different from those expected for metallic CNT. We predict that, in the case of armchair CNT, the gate-voltage dependence of the conductance shows an ON/OFF ratio of about a factor of two, nearly independent of temperature. We attribute the simulated behavior to modification of the band structure under the metals caused by deformation. Our comprehensive model predicts a distinct feature of conductance modulation in armchair CNTFETs induced by the deformation of the CNT band structure. At the same time, the deformation in zigzag metallic CNTs leads to a band crossing but not to a bandgap opening.

Advance in Close-Edged Graphene Nanoribbon: Property Investigation and Structure Fabrication

The absence of dangling bonds in close-edged graphene nanoribbons (CEGNRs) confers upon them a series of fascinating properties, especially when compared with cylindrical carbon nanotubes and open-edged GNRs. Here, the configuration of CEGNRs is described, followed by the structure-related properties, including mechanical, thermal, electrical, optical, and magnetic properties. Based on the unique structures and extraordinary properties, their potential applications in a variety of fields, such as field-effect transistors, energy suppliers, nanoactuators, and fibers, are discussed. Remarkably, the strategies applied for generating CEGNRs, mainly from the collapse of carbon nanotubes and graphene tubes, are depicted in detail. Finally, the prospects in the research area of CEGNRs are proposed.

Band Structure and Contact Resistance of Carbon Nanotubes Deformed by a Metal Contact

Capillary and van der Waals forces cause nanotubes to deform or even collapse under metal contacts. Using ab initio band structure calculations, we find that these deformations reduce the band gap by as much as 30%, while fully collapsed nanotubes become metallic. Moreover, degeneracy lifting due to the broken axial symmetry, and wave functions mismatch between the fully collapsed and the round portions of a CNT, lead to a 3 times higher contact resistance. The latter we demonstrate by contact resistance calculations within the tight-binding approach.

Self-assembled and intercalated film of reduced graphene oxide for a novel vacuum pressure sensor

We report a new method for measuring vacuum pressures using Van der Waals (VDW) interactions between reduced graphene oxide (RGO) sheets. For this purpose, we utilized a reaction-based self-assembly process to fabricate various intercalated RGO (i-RGO) films, and monitored their electrical behavior with changing pressure and temperature. Pumping to remove gas from a vacuum chamber produced a decrease in the sheet resistance of i-RGO. With further pumping, distinctly different sheet resistance behaivors were observed depending on the measurement temperature. With increasing vacuum pressure, the resistance increased at 100 °C, whereas it decreased at 30 °C. Two types of VDW interactions are proposed to explain these features: a local VDW interaction between RGO sheets that resulted in V-shaped curves of sheet resistance with pressure changes and broad VDW interactions that occur between RGO sheets when the elastic force required to bend carbon clusters on an RGO sheet exceeds their vibrational energy at low temperatures. On the basis of the results, we propose that the resistance behavior of i-RGO as a function of vacuum pressure can be interpreted as the sum of the two different VDW interactions.

The impact of Cr adhesion layer on CNFET electrical characteristics

The effect of a Cr adhesion layer on the transfer characteristics of Cr/Au-contacted carbon nanotube field-effect transistors (CNFETs) based on individual single-walled carbon nanotubes (SWNTs) is presented in this paper. We show that a very thin Cr layer (≈0.4 nm) already has an impact on the carrier transport in Schottky-barrier-modulated CNFETs. The ratio of the p- and n-branch current is reduced by eight times when the Cr adhesion layer thickness is increased from 0 to 8 nm. We suggest a change in Schottky barrier height at the contact as the determining mechanism for this result. Additionally, superior lifetime of devices is observed even for non-passivated CNFETs with preserved clean SWNT/Cr/Au-contacts using Cr layer thinner than 2 nm. Our experiments show that the role of the adhesion layer in metal/nanotube contacts should be explicitly considered when designing CNTFET-based circuits, developing CNFET fabrication processes, and analyzing the corresponding properties of the electrical contacts.

Wrapping of nanoparticles by the cell membrane: the role of interactions between the nanoparticles

A fundamental understanding of the interactions between nanoparticles (NPs) and the cell membrane is essential to improve the performance of the NP-based biomedical applications and assess the potential toxicity of NPs. Despite the great progress in understanding the interaction between individual NP and the membrane, little is known about the interaction between multiple NPs and the membrane. In this work, we investigate the wrapping of two parallel elongated NPs by the membrane, taking the NP-NP electrostatic interaction and van der Waals (vdW) interaction into consideration. Three types of NPs, namely the rigid NPs with circular and elliptic cross-sections and the deformable NPs, are systematically investigated. The results show that the electrostatic interaction would enhance the tendency of the independent wrapping and inhibit the rotation of the elongated and equally charged NPs with elliptic cross-sections. Under the vdW interaction, the competition of the NP-NP adhesion and the membrane elastic energies with the NP-membrane adhesion energy leads the NPs to be wrapped cooperatively or independently. For the system with elongated NPs with elliptic cross-sections, the NPs are more likely to be wrapped independently as the shapes become more anisotropic and the NPs would rotate to contact each other with the flat sides in the cooperative wrapping configuration. Moreover, the soft NPs are more likely to be wrapped cooperatively compared with the stiff NPs. These results may provide guidelines to control the internalization pathway of NPs and improve the efficiency of NP-based drug delivery systems.

Wetting transition for carbon nanotube arrays under metal contacts

Structural arrays with nanoscale spacing arise in many device concepts. Carbon nanotube transistors are an extreme example, where a practical technology will require arrays of parallel nanotubes with spacing of order 10 nm or less. We show that with decreasing pitch there is a first-order transition, from a robust structure in which the metal wets the substrate between tubes, to a poorly wetting structure in which the metal rides atop the nanotube array without touching the substrate. The latter is analogous to the superhydrophobic "lotus leaf effect." There is a sharp minimum in the delamination energy of metal contacts at the transition pitch. We discuss implications for contact resistance and possible mitigation strategies.

Adhesion and bending rigidity-mediated wrapping of carbon nanotubes by a substrate-supported cell membrane

The adhesion and bending rigidity-mediated wrapping of carbon nanotubes by a substrate-supported cell membrane has been explored and phase diagrams that characterize the effect of the energy competition on the equilibrium configuration have been presented.