Limits...
Shrinking of Solid-state Nanopores by Direct Thermal Heating.

Asghar W, Ilyas A, Billo JA, Iqbal SM - Nanoscale Res Lett (2011)

Bottom Line: Direct heating results in shrinking of the silicon dioxide nanopores.The inbuilt stress in the oxide film is also reduced due to annealing.The surface composition of the pore walls remains the same during the shrinking process.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX 76019, USA. smiqbal@uta.edu.

ABSTRACT
Solid-state nanopores have emerged as useful single-molecule sensors for DNA and proteins. A novel and simple technique for solid-state nanopore fabrication is reported here. The process involves direct thermal heating of 100 to 300 nm nanopores, made by focused ion beam (FIB) milling in free-standing membranes. Direct heating results in shrinking of the silicon dioxide nanopores. The free-standing silicon dioxide membrane is softened and adatoms diffuse to a lower surface free energy. The model predicts the dynamics of the shrinking process as validated by experiments. The method described herein, can process many samples at one time. The inbuilt stress in the oxide film is also reduced due to annealing. The surface composition of the pore walls remains the same during the shrinking process. The linear shrinkage rate gives a reproducible way to control the diameter of a pore with nanometer precision.

No MeSH data available.


High temperature shrinking process. (a) TEM micrograph of ~270 nm diameter nanopore before shrinking. (b) TEM micrograph of nanopore after 4 min of thermal shrinking at 1250°C. The pore closed in just 4 min due to high shrinkage rate. The shrinking rate was about 70 nm/min.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3211463&req=5

Figure 3: High temperature shrinking process. (a) TEM micrograph of ~270 nm diameter nanopore before shrinking. (b) TEM micrograph of nanopore after 4 min of thermal shrinking at 1250°C. The pore closed in just 4 min due to high shrinkage rate. The shrinking rate was about 70 nm/min.

Mentions: We observed the nanopores shrinking or expanding when subjected to high temperature (1000 to 1250°C), contradicting previous findings [15]. The nanopores having an initial diameter of 250 nm were reduced to 3 nm at 1150°C as shown in Figure 2. The nanopores were imaged with TEM after each temperature processing step to characterize the process. After loading the dyes into the furnace, the temperature was allowed to stabilize for 30 s before counting the actual processing time. After the thermal process, the dyes were unloaded from furnace and cooled down to room temperature. When the dyes were processed at temperatures below 1000°C, it was observed that there was very little or no change in the diameter of the nanopore. This can be explained by the fact that at low temperature (<1000°C), the oxide layer would not be relaxed to an extent that it would start changing pore morphology. When the nanopores were processed at a higher temperature (>1250°C), the oxide membranes either broke due to very high thermal stress or the shrinking process was too fast to control. This was especially so for pores smaller than 20 nm diameter [24]. As an example, a nanopore with initial diameter of ~270 nm, processed at 1250°C, is shown in Figure 3. The TEM images of the nanopore show that the nanopore closed after 4 min due to an increased shrinking rate. The shrinking or expansion rate thus increased at higher temperature. When the pore diameter was larger than the membrane thickness, the nanopore started expanding in size instead of shrinking. A 350 nm nanopore in a 300 nm thick membrane was processed at 1150°C for 50 min. The pore expanded in size to 1.5 μm (Figure 4). It is interesting to note that direct heating can be used to shrink or expand the pore based only on the ratio of initial nanopore diameter to cylindrical length of the pore. The temperature itself had no effect on whether the nanopore would shrink or expand. The pore shrinking and expanding mechanism can be explained by the surface tension which induced viscous flow of oxide film as described below.


Shrinking of Solid-state Nanopores by Direct Thermal Heating.

Asghar W, Ilyas A, Billo JA, Iqbal SM - Nanoscale Res Lett (2011)

High temperature shrinking process. (a) TEM micrograph of ~270 nm diameter nanopore before shrinking. (b) TEM micrograph of nanopore after 4 min of thermal shrinking at 1250°C. The pore closed in just 4 min due to high shrinkage rate. The shrinking rate was about 70 nm/min.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211463&req=5

Figure 3: High temperature shrinking process. (a) TEM micrograph of ~270 nm diameter nanopore before shrinking. (b) TEM micrograph of nanopore after 4 min of thermal shrinking at 1250°C. The pore closed in just 4 min due to high shrinkage rate. The shrinking rate was about 70 nm/min.
Mentions: We observed the nanopores shrinking or expanding when subjected to high temperature (1000 to 1250°C), contradicting previous findings [15]. The nanopores having an initial diameter of 250 nm were reduced to 3 nm at 1150°C as shown in Figure 2. The nanopores were imaged with TEM after each temperature processing step to characterize the process. After loading the dyes into the furnace, the temperature was allowed to stabilize for 30 s before counting the actual processing time. After the thermal process, the dyes were unloaded from furnace and cooled down to room temperature. When the dyes were processed at temperatures below 1000°C, it was observed that there was very little or no change in the diameter of the nanopore. This can be explained by the fact that at low temperature (<1000°C), the oxide layer would not be relaxed to an extent that it would start changing pore morphology. When the nanopores were processed at a higher temperature (>1250°C), the oxide membranes either broke due to very high thermal stress or the shrinking process was too fast to control. This was especially so for pores smaller than 20 nm diameter [24]. As an example, a nanopore with initial diameter of ~270 nm, processed at 1250°C, is shown in Figure 3. The TEM images of the nanopore show that the nanopore closed after 4 min due to an increased shrinking rate. The shrinking or expansion rate thus increased at higher temperature. When the pore diameter was larger than the membrane thickness, the nanopore started expanding in size instead of shrinking. A 350 nm nanopore in a 300 nm thick membrane was processed at 1150°C for 50 min. The pore expanded in size to 1.5 μm (Figure 4). It is interesting to note that direct heating can be used to shrink or expand the pore based only on the ratio of initial nanopore diameter to cylindrical length of the pore. The temperature itself had no effect on whether the nanopore would shrink or expand. The pore shrinking and expanding mechanism can be explained by the surface tension which induced viscous flow of oxide film as described below.

Bottom Line: Direct heating results in shrinking of the silicon dioxide nanopores.The inbuilt stress in the oxide film is also reduced due to annealing.The surface composition of the pore walls remains the same during the shrinking process.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX 76019, USA. smiqbal@uta.edu.

ABSTRACT
Solid-state nanopores have emerged as useful single-molecule sensors for DNA and proteins. A novel and simple technique for solid-state nanopore fabrication is reported here. The process involves direct thermal heating of 100 to 300 nm nanopores, made by focused ion beam (FIB) milling in free-standing membranes. Direct heating results in shrinking of the silicon dioxide nanopores. The free-standing silicon dioxide membrane is softened and adatoms diffuse to a lower surface free energy. The model predicts the dynamics of the shrinking process as validated by experiments. The method described herein, can process many samples at one time. The inbuilt stress in the oxide film is also reduced due to annealing. The surface composition of the pore walls remains the same during the shrinking process. The linear shrinkage rate gives a reproducible way to control the diameter of a pore with nanometer precision.

No MeSH data available.