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Archive for December, 2010

Characterization of Nanowires

Nanowire characterization is very important for optimizing the manufacturing process. You cannot improve, if you cannot properly measure something. Right now this process is not standardized, researchers select the “most suitable” portion of the nanowire collection, they measure the desired metric for arbitrary number of nanowires etc. That’s why nanomanufacturing process improvement results can be misleading.

I found an article about this topic: “A Statistics-Guided Approach to Precise Characterization of Nanowire Morphology” Article describes statistical methods for three metrics: length, density, diameter.

Nanowire length

They used three-dimensional geometric model to get a distribution of nanowire length (mean, variance). They measured projected lengths of nanowires for three different tilting angles.

Nanowire density

Nanowire density is usually defined to be the number of nanowires located per unit area. Method they propose: divide the substrate to n1 and also to n2 parts. Randomly pick n1 cells from n12 cells such that one region is selected from each row. Then from each chosen large cell, randomly pick one small cell inside the large cell to perform measurement. After this there is one more selection of cells such that there is one cell selected from each column.

Nanowire diameter

Using same logic as in length, they measure diameter projection for six different angles. For hexagonal nanowires, randomly measured tilt angles are between zero and 180/6 = 30

I will look for papers which cited this paper.

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Today I read the paper “Optimizing and Improving the Growth Quality of ZnO Nanowire Arrays Guided by Statistical Design of Experiments“.

High aspect ratio of ZnO nanowires increases antireflectivity of photovoltaic devices. Xu et al. managed to increase aspet ratio from 10 to 23.

Firstly they made 34-2 factorial design. Tested parameters:

  • temperature of furnace
  • growth time
  • zinc concentration
  • capping agent

Some of the runs resulted in poor aspect ratios. Aspect ratio was calculated by measuring lengths and widths of around 20 nanowires using Photoshop. In the second stage they used this information to narrow down focus.

In the third stage they aimed at reducing noise effects (decreasing noise means getting consistent results in each trial with same parameter settings). After that, they had 3 more stages.

I liked the stage by stage refinement of parameters. Will be helpful for my research. What I did not like was aspect ratio calculation process: 20 nanowires is not enough, how they selected these 20 nanowires is not clear.

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