Cation Exchange in Single Nanowires
Final Report Abstract
In this project, electrical transport measurements were performed on individual nanowires before, during and after the cation exchange of CdSe to Ag2Se. The CdSe nanowires were prepared directly on Si/SiO2 substrates using a wet chemical solution-liquid-solid synthesis. Subsequently, individual CdSe nanowires were contacted by optical lithography. Titanium or gold/titanium materials were used for the electrodes. In this way, single nanowire field-effect transistors with different wire diameters could be produced. The capacitive coupling of the nanowires to the doped silicon substrate, which served as a back gate, could be calculated with the simulation program Comsol Multiphysics. With the capacitive coupling of the back gate to the nanowire, the specific electrical conductivity, the field effect mobility and the charge carrier concentration of individual nanowires in transistor geometry could be determined. After fabrication of the CdSe nanowire transistors, a complete cation exchange from CdSe to Ag2Se could be performed on individual nanowires. The material compositions were confirmed by EDX measurements. Continuous cation exchange was also used to investigate the transition from insulating CdSe to conducting Ag2Se in electronic transport measurements. Here, the CdSe nanowire transistors were immersed in a silver nitrate solution for short time intervals and then electronically characterized. The pure CdSe nanowires were highly insulating, while the fully exchanged Ag2Se nanowires exhibited metallic, diffuse 3D charge transport. The specific conductivity of the pure Ag2Se nanowires corresponded to that of macroscopic Ag2Se. The observations suggest that cation exchange occurs more slowly below the electrodes than in the center of the wire between the electrodes. Nonlinear current-voltage characteristics commonly observed in early stages of partial cation exchange support this hypothesis. Improved ohmic contacts between the lead electrodes and the nanowires, for example by diffusing the electrode metals, could therefore be helpful to further track the degree of cation exchange based on the conductivity of the nanowires. As part of the project, an apparatus for temperature-dependent transport measurements for the range -195°C to 420°C was established. In temperature-dependent conductivity measurements, a sudden increase in the conductivity of the Ag2Se nanowires could be observed below 100°C. Such an increase in conductivity is characteristic for the superionic transition from the beta to the alpha phase in Ag2Se nanwires. In macroscopic Ag2Se samples, the superionic transition temperature described in the literature is only at 133°C. The decrease of the superionic transition temperature in the Ag2Se nanowires can be partly explained by the high surface-to-volume ratio in nanowires compared to bulk samples. A similar effect is the lowering of the melting temperature in nanoparticles relative to bulk samples. An additional decrease in the superionic transition temperature in the Ag2Se nanowires could be due to phase boundaries in the crystal structure.
Publications
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Frühjahrstagung der Deutschen Physikalischen Gesellschaft Berlin, Germany, Talk: Transport Measurements on Single CdSe Nanowires during Cation Exchange. 2018
Maximilian Schwarz, Christian Strelow, August Dorn & Alf Mews
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Gordon Research Conference - Colloidal Semiconductor Nanocrystals Smithfield, RI, USA, Poster: Electrical Properties of Individual CdSe Nanowires Throughout Cation Exchange to Ag2Se. 2018
Smithfield, RI, USA, Maximilian Schwarz, Christian Strelow, August Dorn & Alf Mews
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DPG Summer School Bad Honnef, Germany Superionic Phase Transition in Single Ag2Se Nanowires. 2019
Maximilian Schwarz, August Dorn & Alf Mews
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Frühjahrstagung der Deutschen Physikalischen Gesellschaft Regensburg, Germany, Talk: Temperature Controlled Phase Transition in Single Ag2Se Nanowires. 2019
Maximilian Schwarz, August Dorn & Alf Mews
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NaNaX 9 Hamburg, Germany Poster: Superionic Phase Transition in Single Ag2Se Nanowires Obtained from CdSe Nanowires Through Cation Exchange
Maximilian Schwarz, August Dorn, Alf MewsMaximilian Schwarz, August Dorn & Alf Mews
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Superionic phase transition in individual silver selenide nanowires. Nanoscale, 13(17), 8017-8023.
Schwarz, Maximilian; Mews, Alf & Dorn, August
