The PI proposes to synthesize and characterize extremely thin layers of Ag thiolate (AgSR) with the support from the Solid State and Materials Chemistry program in the Division of Materials Research. Crystals form in a variety of shapes and sizes but some crystals are more special than others. These special sizes, often referred to as Magic Number Sizes, are of great interest in materials chemistry and physics because often they are evidence of unique underlying properties of the material. AgSR is an interesting polymer in that it grows as stacked flat bilayers. The PI envisions lamellar AgSR to be a new model building block for bottom-up self assembly of superstructures that can be used in a variety of new material applications in physics, chemistry, biology and microelectronic technologies. The PI recently developed a new synthesis method using nanoparticles of metal which generates large AgSR crystal platelets. These crystals have diameters of 1 micron and heights of up to ~30 layers thick with nearly atomically flat surfaces (Langmuir 2009). One major objective in this proposal is to exploit the unusual planar attributes of AgSR and to extend this new synthesis path to produce crystals of the fundamental minimum thickness, one-layer two-dimensional AgSR crystals. This one-layer crystal is the ultimate starting point for bottom-up self-assembly synthesis. Discovery in 2004 of graphene has defied the conventional wisdom that these thin crystals could not exist because of an inherent instability in part due to reduced melting temperature (size-dependent melting). A major objective of this proposal is to measure the melting of these 1-layer crystals which to date has never been done. This goal will be accomplished using Nanocalorimetry. These structures can be used to study thermodynamics and size-dependent melting phenomenon with single-layer control of crystal size which has been implausible except for cluster beams. The PI proposes to study size-effects in this system using NanoDSC in addition to the formation of liquid crystal and metastable phases.
NON-TECHNICAL SUMMARY: The scientific impact of this work will be in the development of generating new synthesis methods for applications in the microelectronics industry. Furthermore the new characterization techniques developed in thermal analysis will greatly expand our scientific ability to probe material properties at a very small size scale which is useful for advancement in Nanotechnology. The broad impact of this research will include the education and training of graduate students in the field of nanotechnology and the development of new synthesis methods. The students will be educated and trained in fabrication of MEMs devices using microelectronic techniques and in using self-assembly synthesis methods. Almost all of the students who have been trained by the PI are currently working in the microelectronics industry. In addition to formal journal articles, the dissemination of this work will be in the form of conference talks given by both the PI and graduate and undergraduate students along with Nanocalorimetry short courses in specialized Thermal Analysis conferences. By further developing the capabilities of the synthesis methods and NanoDSC technique, this work will add to the fabrication and instrumentation infrastructure of the country for basic science as well as for technology. Scientific collaborations are expected to be developed in the area of Thermal Analysis with groups in Spain and Canada as well as other research groups in the United States.
During this NSF funding period (2010-2014), the major accomplishment we achieved is the successful synthesis and characterization of two-dimensional (2D) silver alkanethiolate (AgSCn) lamellar crystals on inert substrates, especially for the thermodynamic properties of layered materials. This is achieved by the combination of a new layer-control synthesis method and nanocalorimetry technique developed in our group. The final product of the reaction between silver and alkanethiol depends on the initial silver amount (Fig. 1). Self-assembled monolayer is produced when the silver source is continuous films while monolayer protected clusters are grown from large discrete silver islands. If the amount of deposited silver decreased to be lower than 10 Å, AgSCn compounds starts to form. The thickness of AgSCn lamellar crystals can be changed by either modulating alkanethiol chain length (n=7-18) or by stacking single layer crystals to a specific number of layers (m=1-10), which, during synthesis, can be precisely controlled by tuning both the deposited silver amount and the annealing temperature after reaction. This method is highlighted by the successful synthesis of 2D single layer AgSCn with uniform thickness and in-plane ordering. Nanocalorimetry is a unique technique for the investigation of thermodynamic properties of very small amount of nanoscale materials, due to its ultrafast scanning rate (5k-200k K/s) and high sensitivity (0.1 nJ/K). This is the main characterization technique used in this research. The most important finding in this research is that the melting point of single layer crystals decreases as the layers become thinner ("Magic Number Size" Melting, Fig. 2). This effect is called "melting point depression". We found that the shorter the chain, the lower the melting point. Thus, the lowest melting point of these layers is a single layer of the shortest alkanethiol chain. It is irrelevant as to whether the alkanethiol chains have either an odd or even number of carbons to the melting process. The second most important finding in this work is the effect of stacking these single layers on top of each other. By stacking these single layers (i.e. making a double layer from two single layers) the melting point of the stacked layers is tremendously higher than the unstacked single layers. From a basic science point of view, this finding opens up new insight into studies of the effect of "joining/packing/mating" materials which is important in interface physics. From a more practical point of view, this effect has direct implications in the biological physics of the membranes which encapsulate biological cells. Based on this work we would expect that the membranes of cells or vesicles (e.g. DODAB) would melt at higher temperatures if they consisted of double layers instead of single layers. The third most important finding is the discovery that the melting point of stacked layer alkanethiols depends not only on the chain length but also on the nature of the chain. Chains with an odd number of carbons melt at higher temperatures than even chains (Fig. 3). This discovery brings new understanding of the energetics and local packing geometry of those interface molecules which form the bonding between the two mating layers. These findings provide a general methodology for the investigation of interfacial properties in 2D layered materials, which have attracted increasing attentions after the synthesis of graphene in 2004. Applications of these layered systems include electronics, optics, lithography and biophysics. The broad impact of this research mainly includes student mentoring and instrumentation infrastructure. Lito de la Rama, supported by this NSF funding from 2010-2013, received his PhD degree in June 2013 with an honor of MRS Graduate Student Award (2013) (Fig. 4). He is one of the only 3 Filipinos in UIUC Engineering. He then started his new job as a Senior Engineer at SanDisk Corp in July 2013. Zichao Ye, who was funded by this funding from 2011-2014, has been trained to be a senior graduate student and started his key research on this project. He received Racheff Teaching Award (2013) as an outstanding teaching assistant for the course of Electronic Materials Lab, which has been rated top 10% of all UIUC classes. Prof. Allen has been the instructor for this course for more than ten years and has been listed as an Excellent Teacher. NSF also provides me opportunities for mentoring many underrepresented undergraduate and graduate students, including Yiran Yan, Rui Wang, Anisa Nuanes (Undergrad), Lito de la Rama and Zisuh Zhang (Grad). As funded by NSF, our nanocalorimetry technique continued its leadership in the field of nanoscale thermal analysis as it has been adopted by several groups worldwide, including NIST, University of Montreal and University of Barcelona. The new commercialized Flash DSC (Mettler Toledo) and HyperDSC (Perkin-Elmer) were inspired by our fast scanning technique. We also have given many invited talks or short-courses on Nanocalorimetry at many conferences such as CALCON, JSCTA, NATAS and Laehnwitzseminar.
