60th Annual Report on Research 2015

During the first funding period, we have mainly focused on investigating the effect of nanoscale confinements on CPE molecular conformation. This involves preparation of CPEs (both in the single molecule form and the thin film form) in nanochannels with pre-defined nanoscale confinements and characterization of the resulting CPE samples using optical spectroscopy.  Two ME Ph.D. students, Mohammad Alibakhshi and Yinxiao Li have worked on this project under my supervision. Mohammad only made his contribution to this project through device fabrication and passed the prospectus defense in 05/2015. Yinxiao was involved in the entire project and is expected to take his prospectus defense in 10/2015.

To conduct the proposed nanoconfinement studies, we first focused on creating 2-D nanochannel devices with desired nanoscale confinements. Two types of nanochannels devices (named type I and II in the original proposal, see Fig.1a) have been successfully fabricated. Type I devices were designed for single molecule conformation study and include local constrictions (channel height change) in the nanochannels to trap CPE molecules. We have developed a modified etching and bonding scheme that includes a special double-layer photoresist coating step to fabricate type I devices (Fig. 1b&1c). Type II devices were designed for thin film conformation study and has uniform channel geometry along the nanochannel length direction. Type II devices were fabricated using the classic etching and bonding scheme that we developed five years ago. The smallest confinement that we have achieved for these two devices were 7 nm (Fig. 1d).

Figure 1 a) Proposed type I & II devices. b) Fabrication steps for type I (i-vi) and type II (i-iii)  nanochannel device. c) Microscopic images of a type I device. d) AFM image of a type I device, showing 7-nm confinement.

After creating the desired nanochannel devices, we have studied capillary evaporation in these two types of nanochannels devices before using them to prepare CPE samples. Capillary evaporation is the main method that we proposed to drive CPE molecules into the nanochannels. A complete understanding of capillary evaporation in nanoscale confined spaces would enable us to estimate CPE sample quantity and/or sample preparation time. We discovered that type I devices were ideal devices to investigate the kinetic-limited capillary evaporation as this design ensures sufficient water supply and simple evaporation rates measurements. One can easily measure the evaporation rates in the shallow channel by monitoring the meniscus receding from the deep nanochannel (Fig. 2a).  Using type I devices, we have studied the effects of confinement, temperature and humidity on the kinetic-limited maximum evaporation rate and the corresponding evaporation coefficient. Our results showed that that the maximum evaporation rate per projected area decreases with the increasing channel height and relative humidity, but increases as the temperature decreases (Fig. 2b).  A 16-nm-high nanochannel showed a maximum evaporation rate of 21287 ± 2414 mm/s at 40 °C and 0% relative humidity, corresponding to a heat flux of 4804 ± 545 W/cm². The measure evaporation coefficient varies from 0.5 to 0.75, which is found to be independent on geometrical confinement, but shows a clear dependence on temperature and vapor pressure. These findings provide new insights and guidelines for further development of capillary evaporation-based technologies for material synthesis and electronics cooling.

Figure 2 a) Schematic of capillary evaporation measurement using type I device. b) Evaporation flux as a function of height. The inset shows the volumetric evaporation rate as a function of height. The dashed line shows the theoretical evaporation flux by assuming a perfectly circular meniscus shape and an evaporation coefficient of 1.

Starting from the beginning of this year, we have started investigating the effect of nanoscale confinements on CPE conformation. Poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene-vinylene] (MPS-PPV, a negatively charged CPE) was chosen as the target molecule. We prepared MPS-PPV thin films using type II devices and the proposed evaporation-induced self-assembly method (based on capillary evaporation). MPS-PPV thin films in five different confinements were prepared, including 60 nm, 90nm, 108nm, 121nm and 251nm.  We also prepared MPS-PPV thin films using the conventional drop-cast method for comparison. The emission spectra of these thin films were measured using a Raman spectrometer.  Fig. 3a shows the typical emission spectra of MPS-PPV prepared by the drop-cast method. The emission peak is not well controlled, ranging from 604nm to 623nm. In contrast, the emission spectra of MPS-PPV thin film formed in nanoconfinements are quite consistent (Fig. 3b). The minimum emission peak is 604nm at a height of 60nm, corresponding to a most collapsed form. As channel height increases, the emission peak quickly increases and reaches a maximum of 666nm at a height of 91nm, which corresponds to a most open and extended form, and then gradually decreases again. This result shows that nanoscale confinement can significantly and consistently change the molecular conformation of CPEs in the dried thin film form. We have also used type I devices to prepare CPE samples for single molecule conformation studies. However, it turned out that our current instrument did not have the required resolution to recognize emission signal from single MPS-PPV molecules, which requires further investigation in the second funding period. 

Figure 3 Emission spectra upon 532-nm excitation of evaporation induced self-assembled thin films from 0.25wt% MPS-PPV in water. a) Thin films prepared by drop-casting. b) Thin films prepared inside different confinements.