Percent mass gain for Al2O3 ALD deposition on polypropylene nonwoven fiber mats (at 50 °C) as a function of Al2O3 cycle number. Mass gain was measurable at 50 Al2O3 cycles, and increased as more cycles were applied. These data provide a reference mass to determine the mass gain after ZnO ALD onto Al2O3-coated fibers. A linear trend was fit to the data with an R-squared value of 0.99; it is noted that non-linear growth maybe occurring at low cycle numbers, although this was not investigated further due to sensitivity limitations of the measurement technique.
(a) The percent mass gain of 500 ZnO ALD cycles (at 155 °C) is plotted as a function of deposition temperature for (1) deposition on virgin polypropylene (◇); (2) deposition on polypropylene first coated with 200 Al2O3 ALD cycles at 50 °C (▪). The samples are labeled ZnO/PP and ZnO/AlO/PP, respectively; (b) the ZnO film thickness on silicon substrates deposited with the fiber samples in Fig. 2(a) . ZnO deposited on silicon is labeled ZnO/Si, and the ZnO deposited on alumina coated silicon is labeled ZnO/AlO/Si.
ZnO mass gain (%) plotted as a function of ZnO cycle number for ZnO ALD on (1) virgin polypropylene (Δ); (2) polypropylene coated with 50 (Δ), 100 (○), and 200 (▪) Al2O3 ALD cycles (at 50 °C) prior to ZnO deposition. By first coating the polypropylene with Al2O3, the ZnO nucleates more quickly on the Al2O3 coating than on virgin polypropylene, reducing the cycles before linear mass gain is achieved.
TEM images of polypropylene after 300 cycles of ZnO ALD at 110 °C for (a) untreated polypropylene and (b) polypropylene pre-treated with 150 cycles of Al2O3 ALD at 50 °C. The untreated polypropylene fiber shows small granular formation towards the center, growing larger towards the edge of the fiber. For the pre-treated fiber (b), two distinct layers can be seen, the Al2O3 pretreatment and the ZnO coating.
(a) Percent mass gain of ZnO ALD at 155 °C is plotted as a function of DEZ dose time for (1) 200 ZnO cycles on untreated polypropylene (ZnO/PP ○) and (2) 200 ZnO cycles on polypropylene pre-coated with 200 Al2O3 cycles at 50 °C (ZnO/AlO/PP •). (b) Film growth on simultaneously deposited silicon substrates shows ZnO deposition on bare Si, and Al2O3 coated Si, is achieved quickly. Similar to the silicon substrates, Al2O3 coated polypropylene shows saturation is achieved with short precursor doses.
Conductivity of polypropylene after a 300 cycle ZnO ALD coating deposited at various temperatures on (1) virgin polypropylene (○), and (2) polypropylene with a 200 cycle Al2O3 ALD barrier layer deposited at 50 °C (•), and corresponding silicon samples (◻, ▪), respectively. The Al2O3 barrier results in ZnO with higher conductivity; presumably because higher quality ZnO is deposited on the Al2O3 barrier layer. Fig. 4 shows higher density ZnO is deposited on top of the Al2O3 barrier coated polypropylene. Higher temperature ZnO ALD leads to higher conductivity, most likely due to a shift in preferred crystalline orientation of the deposited ZnO.
(a) Conductivity of ZnO deposited at 155 °C on (1) thermal oxide silicon; and (2) virgin polypropylene, as a function of DEZ dose time. The conductivity of ZnO on silicon plateaus with a 1 s dose. Conductivity of ZnO on virgin PP increases with dose times up to 3 s. (b) Conductivity of ZnO deposited at 155 °C on (1) Al2O3 coated thermal oxide silicon; and (2) ZnO on PP first treated with 200 Al2O3 cycles at 50 °C; both samples show saturation with a 1 s dose. This shows that Al2O3 is eliminating some of the non-ideality of ZnO ALD on PP that is shown in (a).
(a) Conductivity as a function of ZnO cycle number for (1) ZnO on virgin polypropylene at 155 °C (○); and (2) ZnO (at 155 °C) on polypropylene first coated with a 200 cycle Al2O3 barrier (•). (b) Conductivity as a function of zinc oxide ALD cycles for (1) zinc oxide deposited on a 150 nm thermal oxide silicon (◻), and (2) zinc oxide deposited on 200 Al2O3 ALD cycles on top of a 150 nm thermal oxide silicon wafer (▪). Conductivity of ZnO deposited on Al2O3 is noticeably higher than ZnO without the Al2O3 layer. A peak exists at 200 ZnO cycles (∼360 Å of ZnO), after which conductivity decreases towards a bulk value of ∼150 S/cm, in line with the value for ZnO deposited directly on thermal oxide.
Conductivity is plotted as a function of the number of ZnO ALD cycles, for ZnO/AlO/PP samples deposited at two different temperatures. As demonstrated in Fig. 6 , conductivity is higher for ZnO samples deposited at higher temperatures. This shows that conductivity exhibits similar behavior as a function of ZnO thickness, for two different deposition temperatures. Both samples show low conductivity for <200 ZnO cycles, and then exhibit “plateau” like behavior for thicker ZnO depositions.
(a) A picture and (b) a schematic of a ZnO coated polypropylene sample as it is being bent around a glass cylinder with 1.13 cm radius. As the cylinder is rolled over the sample, the sample becomes sandwiched between the cylinder wall and the paper support adhered to the cylinder. This causes the sample to roll up the side as the cylinder is rotated forward; the cylinder is then rolled in reverse, the sample is removed and the conductivity is measured.
The conductivity of ZnO/Al2O3 coated polypropylene samples (from Fig. 8(a) ) as a function of the inverse of the diameter of bending. The samples were tested prior to bending (“unbent”), as well as after bending around cylinders of decreasing diameters. Trend lines were fit for samples with 100, 200, 300, and 500 ZnO cycles (deposited at 155 °C on 200 cycle Al2O3 coated polypropylene). Data are shown for other samples, but was not fit, in order to reduce clutter in the figure. Because the samples sat in ambient for up to 16 months before testing, the unbent conductivity is lower than initially measured from Fig. 8(a) .
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