\u3ci\u3eIn-situ\u3c/i\u3e electrical, mechanical and electrochemical characterizations of one-dimensional nanostructures

One-dimensional nanostructures initiated new aspects to the materials applications due to their superior properties compared to the bulk materials. Properties of nanostructures have been characterized by many techniques and used for various device applications. However, simultaneous correlation between the physical and structural properties of these nanomaterials has not been widely investigated. Therefore, it is necessary to perform in-situ study on the physical and structural properties of nanomaterials to understand their relation. In this work, we will use a unique instrument to perform real time atomic force microscopy (AFM) and scanning tunneling microscopy (STM) of nanomaterials inside a transmission electron microscopy (TEM) system. This AFM/STM-TEM system is used to investigate the mechanical, electrical, and electrochemical properties of boron nitride nanotubes (BNNTs) and Silicon nanorods (SiNRs). BNNTs are one of the subjects of this PhD research due to their comparable, and in some cases superior, properties compared to carbon nanotubes. Therefore, to further develop their applications, it is required to investigate these characteristics in atomic level. In this research, the mechanical properties of multi-walled BNNTs were first studied. Several tests were designed to study and characterize their real-time deformation behavior to the applied force. Observations revealed that BNNTs possess highly flexible structures under applied force. Detailed studies were then conducted to understand the bending mechanism of the BNNTs. Formations of reversible ripples were observed and described in terms of thermodynamic energy of the system. Fracture failure of BNNTs were initiated at the outermost walls and characterized to be brittle. Second, the electrical properties of individual BNNTs were studied. Results showed that the bandgap and electronic properties of BNNTs can be engineered by means of applied strain. It was found that the conductivity, electron concentration and carrier mobility of BNNTs can be tuned as a function of applied stress. Although, BNNTs are considered to be candidate for field emission applications, observations revealed that their properties degrade upon cycles of emissions. Results showed that due to the high emission current density, the temperature of the sample was increased and reached to the decomposition temperature at which the B-N bonds start to break. In addition to BNNTs, we have also performed in-situ study on the electrochemical properties of silicon nanorods (SiNRs). Specifically, lithiation and delithiation of SiNRs were studied by our STM-TEM system. Our observations showed the direct formation of Li22Si5 phases as a result of lithium intercalation. Radial expansion of the anode materials were observed and characterized in terms of size-scale. Later, the formation and growth of the lithium fibers on the surface of the anode materials were observed and studied. Results revealed the formation of lithium islands inside the ionic liquid electrolyte which then grew as Li dendrite toward the cathode material

Real-time fracture detection of individual boron nitride nanotubes in severe cyclic deformation processes

Real-time deformation of individual multiwalled boron nitride nanotubes (BNNTs) was investigated using an atomic force microscopy (AFM) stage installed inside the chamber of a transmission electron microscopy (TEM) system. These in situ AFM-TEM experiments were conducted in following two deformation regimes: a small-angle (∼65°) and a large-angle (∼120°) cyclic bending process. BNNTs survived from the low-angle test and their modulus was determined as ∼0.5 TPa. Fracture failure of individual BNNTs was discovered in the large-angle cyclic bending. The brittle failure mechanism was initiated from the outermost walls and propagated toward the tubular axis with discrete drops of applied force

In situ observation of reversible rippling in multi-walled boron nitride nanotubes

The recent observation of high flexibility in buckled boron nitride nanotubes (BNNTs) contradicts the pre-existing belief about BN nanotube brittleness due to the partially ionic character of bonding between the B and N atoms. However, the underlying mechanisms and relationships within the nanotube remained unexplored. This study reports for the first time the buckling mechanism in multi-walled BNNTs upon severe mechanical deformation. Individual BNNTs were deformed inside a transmission electron microscope (TEM) equipped with an in situ atomic force microscopy holder. High-resolution TEM images revealed that bent BNNTs form multiple rippling upon buckling. The critical strain to form the first ripple was measured as 4.1% and the buckling process was reversible up to 26% strain. As opposed to carbon nanotubes, the BNNTs buckled into V-shaped ripples rather than smooth wavy shapes. The rippling wavelength was quantified in terms of the outer diameter and thickness of the nanotubes. The BNNTs showed a larger rippling wavelength compared to that of CNTs with the same number of walls. This difference was explained by the tendency of BN structures to reduce the number of thermodynamically unfavorable B–B and N–N bonds at the sharp corners in the rippling regions. The BNNTs\u27 structure also exhibited a higher fracture strain compared to their counterpart

In situ TEM monitoring of thermal decomposition in individual boron nitride nanotubes

The future of microelectronic and nanoelectronics devices could lie in one-dimensional nanomaterials including boron nitride nanotubes (BNNTs). In such applications, however, the flow of electrical current may induce structural failure resulting in reduction of component Service life. Here, we utilized scanning tunneling microscopy inside a transmission electron microscope to study the thermal failure of individual multi-walled BNNTs via Joule heating. At elevated temperatures, the nanotube failed by the formation of amorphous nanoclusters and progression of structural defects. These Clusters have various sizes and initially form on the outermost shell layers of BNNTs

Field emission and strain engineering of electronic properties in boron nitride nanotubes

The electrical properties of boron nitride (BN) nanostructures, particularly BN nanotubes (NTs), have been studied less in comparison to the counterpart carbon nanotubes. The present work investigates the field emission (FE) behavior of BNNTs under multiple cycles of FE experiments and demonstrates a strain-engineering pathway to tune the electronic properties of BNNTs. The electrical probing of individual BNNTs were conducted inside a transmission electron microscope (TEM) using an in situ electrical holder capable of applying a bias voltage of up to 110 V. Our results indicate that in the first cycle a single BNNT can exhibit the current density of ~1 mA cm−2 at 110 V and the turn-on voltage of 325 V μm−1. However, field emission properties reduced considerably in subsequent cycles. Real-time imaging revealed the structural degradation of individual BNNTs during FE experiments. The electromechanical measurements show that the conductivity of BNNTs can be tuned by means of mechanical straining. The resistance of individual BNNTs reduced from 2000 to 769 MΩ and the carrier concentration increased from 0.35 × 1017 to 1.1 × 1017 cm−3 by straining the samples up to 2.5%

On the relation of mechanical deformation and electrical properties of BN nanotubes

Using a novel in-situ scanning tunneling microcopy integrated into a 200Kv transmission electron microscopy, we have shown that boron nitride nanotubes (BNNTs) posses remarkable flexibility and convert from insulator to semi-conductor upon bending. To measure the electrical properties, the BNNT was bent between two gold contacts constructing a metal-semiconductor-metal circuit. The resistivity of the BNNT under bending condition was measured to be ∼460 MΩ from the experimentally recorded current-voltage data. Our finding suggests that mechanical straining can improve the electrical transport in BN nanotubes via reducing the band gap

Real time observation of nanomaterials in energy harvesting and Li-ion battery systems

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011