Rolled-up nanomembranes as compact 3D architectures for field effect transistors and fluidic sensing applications.

We fabricate inorganic thin film transistors with bending radii of less than 5 µm maintaining their high electronic performance with on-off ratios of more than 10(5) and subthreshold swings of 160 mV/dec. The fabrication technology relies on the roll-up of highly strained semiconducting nanomembranes, which compacts planar transistors into three-dimensional tubular architectures opening intriguing potential for microfluidic applications. Our technique probes the ultimate limit for the bending radius of high performance thin film transistors.

Motion of light adatoms and molecules on the surface of few-layer graphene.

Low-voltage aberration-corrected transmission electron microscopy (TEM) is applied to investigate the feasibility of continuous electron beam cleaning of graphene and monitor the removal of residual species as present on few-layer graphene (FLG) surfaces. This combined approach allows us to detect light adatoms and evaluate their discontinuous sporadic motional behavior. Furthermore, the formation and dynamic behavior of isolated molecules on the FLG surface can be captured. The preferential source of adatoms and adsorbed molecules appeared to be carbonaceous clusters accumulated from residual solvents on the graphene surface. TEM image simulations provide potential detail on the observed molecular structures. Molecular dynamics simulations confirm the experimentally observed dynamics occurring on the energy scale imposed by the presence of the 80 kV electron beam and help elucidate the underlying mechanisms.

Structural distortions in few-layer graphene creases.

Folds and creases are frequently found in graphene grown by chemical vapor deposition (CVD), due to the differing thermal expansion coefficients of graphene from the growth catalyst and the flexibility of the sheet during transfer from the catalyst. The structure of a few-layer graphene (FLG) crease is examined by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). A study of 2D fast Fourier transforms (FFTs) taken about the region of the crease allowed for the crystal stacking structure of the system to be elucidated. It was found that strain-induced stacking faults were created in the AB Bernal-stacked FLG bulk around the region proximal to the crease termination; this is of interest as the stacking order of FLG is known to have an effect on its electronic properties and thus should be considered when transferring CVD-grown FLG to alternate substrates for electronic device fabrication. The FFTs, along with analysis of the real space images, were used to determine the configuration of the layers in the crease itself and were corroborated by multislice atomistic TEM simulations. The termination of the crease part way through the FLG sheet is also examined and is found to show strong out of plane distortions in the area about it.

Catalyst poisoning by amorphous carbon during carbon nanotube growth: fact or fiction?

The influence of amorphous carbon on FePt catalyst particles under chemical vapor deposition conditions typically applied for CNT growth is examined through two routes. In the first, FePt catalyst particles supported on alumina are exposed to a well-established cyclohexane thermal CVD reaction at various temperatures. At higher temperatures where self-pyrolysis leads to copious amorphous carbon and carbon tar formation, carbon nanotubes are still able to form. In the second route, an amorphous carbon film is first deposited over the catalyst particles prior to the CVD reaction. Even for reactions where further amorphous carbon is deposited due to self-pyrolysis, graphitization is still demonstrated. Our findings reveal that the presence of amorphous carbon does not prevent catalytic hydrocarbon decomposition and graphitization processes. We also show an additional catalytic reaction to be present, catalytic hydrogenation, a process in which carbon in contact with the catalyst surface reacts with H(2) to form CH(4).

Atomic structure of interconnected few-layer graphene domains.

The atomic structure at the boundary interface between interconnected few-layer graphene (FLG) domains, synthesized by atmospheric pressure chemical vapor deposition (AP-CVD), is examined using aberration-corrected high-resolution transmission electron microscopy. Moiré patterns in the HRTEM images reveal the presence of rotational stacking faults in the boundary region that extend over distances of ∼100 nm. We show that FLG domains interconnect via two principle processes: graphene sheets from one domain grow over the top of a neighboring domain, while other graphene domains interconnect by direct atomic bonding. Differentiating between these two types of interconnects was found to be possible by examining the HRTEM contrast profiles produced at the interface. Graphene sheets that terminate were found to produce strong edge contrast with increasing defocus values, as well as a broader edge cross section, whereas atomically bonded interfaces were found to not exhibit any contrast, even under large defocus values. These findings are reinforced by correlating with multi-slice TEM image simulations of appropriate structures.

Synthesis of carbon nanotubes with and without catalyst particles.

The initial development of carbon nanotube synthesis revolved heavily around the use of 3d valence transition metals such as Fe, Ni, and Co. More recently, noble metals (e.g. Au) and poor metals (e.g. In, Pb) have been shown to also yield carbon nanotubes. In addition, various ceramics and semiconductors can serve as catalytic particles suitable for tube formation and in some cases hybrid metal/metal oxide systems are possible. All-carbon systems for carbon nanotube growth without any catalytic particles have also been demonstrated. These different growth systems are briefly examined in this article and serve to highlight the breadth of avenues available for carbon nanotube synthesis.

Carbon nanotube nanoelectronic devices compatible with transmission electron microscopy.

We report on a novel method to fabricate carbon nanotube (CNT) nanoelectronic devices on silicon nitride membrane grids that are compatible with high resolution transmission electron microscopy (HRTEM). Resist-based electron beam lithography is used to fabricate electrodes on 50 nm thin silicon nitride membranes and focused-ion-beam milling is used to cut out a 200 nm gap across a gold electrode to produce the viewing window for HRTEM. Spin-coating and AC electrophoresis are used as methods to deposit small bundles of carbon nanotubes across the electrodes. We demonstrate the viability of this approach by performing both electrical measurements and HRTEM imaging of solution-processed CNTs in a device.

Utilizing boron nitride sheets as thin supports for high resolution imaging of nanocrystals.

We demonstrate the use of thin BN sheets as supports for imaging nanocrystals using low voltage (80 kV) aberration-corrected high resolution transmission electron microscopy. This provides an alternative to the previously utilized 2D crystal supports of graphene and graphene oxide. A simple chemical exfoliation method is applied to get few layer boron nitride (BN) sheets with micrometer-sized dimensions. This generic approach of using BN sheets as supports is shown by depositing Mn doped ZnSe nanocrystals directly onto the BN sheets and resolving the atomic structure from both the ZnSe nanocrystals and the BN support. Phase contrast images reveal moiré patterns of interference between the beams diffracted by the nanocrystals and the BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes. Double diffraction is observed and has been analyzed.

Atomic resolution imaging of the edges of catalytically etched suspended few-layer graphene.

Nanostructured graphene and graphene nanoribbons have been fabricated by catalytic hydrogenation, and the edge smoothness has been examined via direct imaging with atomic resolution. When abstaining from solvents during sample preparation, the prepared nanoribbons possess clean edges ready for inspection via transmission electron microscopy (TEM). Edges with subnanometer smoothness could be observed. A method has been developed to make catalytic hydrogenation experiments compatible with TEM, which enables monitoring of the nanoparticles prior to and after hydrogenation. In this way, etching of free-standing few-layer graphene could be demonstrated. Our results enable evaluation of the degree of edge control that can be achieved by means of catalytic hydrogenation.

High-performance field effect transistors from solution processed carbon nanotubes.

Nanoelectronic field effect transistors (FETs) are produced using solution processed individual carbon nanotubes (CNTs), synthesized by both arc discharge and laser ablation methods. We show that the performance of solution processed FETs approaches that of CVD-grown FETs if the nanotubes have minimal lattice defects and are free from surface contamination. This is achieved by treating the nanotubes to a high-temperature vacuum annealing process and using 1,2-dichloroethane for dispersion. We present CNT FETs with mobilities of up to 3546 cm(2)/(V s), transconductance of 4.22 µS, on-state conductance of 9.35 µS and on/off ratios as high as 10(6). High-resolution transmission electron microscopy is used to examine the presence of catalyst particles and amorphous carbon on the surface and Raman spectroscopy is used to examine the lattice defects, both of which lead to reduced device performance.

In situ observations of fullerene fusion and ejection in carbon nanotubes.

We present in situ experimental observations of fullerenes seamlessly fusing to single-walled carbon nanotubes. The morphing-entry of a fullerene to the interior of a nanotube is also captured. The confined (1D) motion of the newly-encapsulated fullerene within its host attests to the actual change from the exterior to interior.

Investigating the outskirts of Fe and Co catalyst particles in alumina-supported catalytic CVD carbon nanotube growth.

Using thermal CVD, the synthesis of multi-walled carbon nanotubes exhibiting roots anchored directly onto alpha-alumina supports, rather than the catalyst particle, is reported. At such roots, the alignment of the graphitic planes with the support lattice fringes depends on the support crystal structure and orientation. Surface defects may alter the reactivity of the surface or control the anchoring of supported atoms or nanoparticles. We argue this surface defect is provided by the catalyst particle's edge interaction with the support, in other words its circumference. The development of oxide-based catalysts is attractive in that they potentially provide an appropriate solution to directly integrate the synthesis of carbon nanotubes and graphene into silicon-based technology.

Investigating the graphitization mechanism of SiO(2) nanoparticles in chemical vapor deposition.

The use of SiO(2) as a catalyst for graphitic nanostructures, such as carbon nanotubes and graphene, is a new and rapidly developing catalyst system. A key question is whether carbide phases form in the reaction. We show the formation of SiC from SiO(2) nanoparticles for the synthesis of graphitic carbon nanostructures via chemical vapor deposition (CVD) at 900 degrees C. Our findings point to the carbothermal reduction of SiO(2) in the CVD reaction. The inclusion of triethyl borate apparently accelerates the process and leads to improved yields. The study helps better understand the growth mechanisms at play in carbon nanotube and carbon nanofiber formation when using SiO(2) catalysts.

Investigating the diameter-dependent stability of single-walled carbon nanotubes.

We investigate the long-standing question of whether electrons accelerated at 80 kV are below the knock-on damage threshold for single-walled carbon nanotubes (SWNTs). Aberration-corrected high-resolution transmission electron microscopy is used to directly image the atomic structure of the SWNTs and provides in situ monitoring of the structural modification induced by electron beam irradiation at 80 kV. We find that SWNTs with small diameters of 1 nm are damaged by the electron beam, and defects are produced in the side walls that can lead to their destruction. SWNTs with diameters of 1.3 nm and larger are more stable against degradation, and stability increases with diameter. The effect of diameter, defects, and exterior contamination on the inherent stability of SWNTs under electron beam irradiation is investigated.

Oxide-driven carbon nanotube growth in supported catalyst CVD.

Detailed HREM studies on carbon nanotubes (CNTs) synthesized via chemical vapor deposition (CVD) using nanoengineered Fe particles on oxide supports show capped tops and open-ended roots. We demonstrate that the pristine catalyst particle dictates the CNT diameter and number of walls at nucleation. The consecutive inward formation of concentric graphene caps during nucleation constricts and elongates the catalyst particle within the tube core. Continued growth stems from the oxide support.