RESUMO
Recent advances in the growth of III-V semiconductor nanowires (NWs) hold great promise for nanoscale optoelectronic device applications. It is established that a small amount of nitrogen (N) incorporation in III-V semiconductor NWs can effectively red-shift their wavelength of operation and tailor their electronic properties for specific applications. However, understanding the impact of N incorporation on non-equilibrium charge carrier dynamics and transport in semiconducting NWs is critical in achieving efficient semiconducting NW devices. In this work, ultrafast optical pump-terahertz probe spectroscopy has been used to study non-equilibrium carrier dynamics and transport in Te-doped GaAsSb and dilute nitride GaAsSbN NWs, with the goal of correlating these results with electrical characterization of their equilibrium photo-response under bias and low-frequency noise characteristics. Nitrogen incorporation in GaAsSb NWs led to a significant increase in the carrier scattering rate, resulting in a severe reduction in carrier mobility. Carrier recombination lifetimes of 33 ± 1 picoseconds (ps) and 147 ± 3 ps in GaAsSbN and GaAsSb NWs, respectively, were measured. The reduction in the carrier lifetime and photoinduced optical conductivities are due to the presence of N-induced defects, leading to deterioration in the electrical and optical characteristics of dilute nitride NWs relative to the non-nitride NWs. Finally, we observed a very fast rise time of â¼2 ps for both NW materials, directly impacting their potential use as high-speed photodetectors.
RESUMO
We present a combined approach that relies on computational simulations and scanning tunneling microscopy (STM) measurements to reveal morphological properties and stability criteria of carbon nanotube-DNA (CNT-DNA) constructs. Application of STM allows direct observation of very stable CNT-DNA hybrid structures with the well-defined DNA wrapping angle of 63.4° and a coiling period of 3.3 nm. Using force field simulations, we determine how the DNA-CNT binding energy depends on the sequence and binding geometry of a single strand DNA. This dependence allows us to quantitatively characterize the stability of a hybrid structure with an optimal π-stacking between DNA nucleotides and the tube surface and better interpret STM data. Our simulations clearly demonstrate the existence of a very stable DNA binding geometry for (6,5) CNT as evidenced by the presence of a well-defined minimum in the binding energy as a function of an angle between DNA strand and the nanotube chiral vector. This novel approach demonstrates the feasibility of CNT-DNA geometry studies with subnanometer resolution and paves the way towards complete characterization of the structural and electronic properties of drug-delivering systems based on DNA-CNT hybrids as a function of DNA sequence and a nanotube chirality.
RESUMO
We employ scanning tunneling microscopy (STM) to reveal the structure of DNA-carbon nanotube complexes with unprecedented spatial resolution and compare our experimental results to molecular dynamics simulations. STM images show strands of DNA wrapping around (6,5) nanotubes at approximately 63 degrees angle with a coiling period of 3.3 nm, in agreement with the theoretical predictions. In addition, we observe width modulations along the DNA molecule itself with characteristic lengths of 1.9 and 2.5 nm, which remain unexplained. In our modeling we use a helical coordinate system, which naturally accounts for tube chirality along with an orbital charge density distribution and allows us to simulate this hybrid system with the optimal pi-interaction between DNA bases and the nanotube. Our results provide novel insight into the self-assembling mechanisms of nanotube-DNA hybrids and can be used to guide the development of novel DNA-based nanotube separation and self-assembly methods, as well as drug delivery and cancer therapy techniques.
