<p></p><p></p><p>The unprecedented
properties of 2D materials such as graphene and MoS2 have been researched
extensively [1,2] for a range of applications including nanoscale electronic and
optoelectronic devices [3–6]. Their unique physical and electronic properties
promise them as the next generation materials for electrodes and other
functional units in nanostructured devices. However, successful incorporation
of 2D materials into devices entails development of high resolution patterning
techniques that are applicable to 2D materials. Patterning at the sub-10 nm
scale is particularly of great interest as the next technology nodes require
patterning of (semi)conductors and insulators at 7 nm and 5 nm scales for
nanoelectronics. It will also benefit organic photovoltaic cells as phase
segregation of p/n-type semiconducting polymers on 2D electrodes at
length scales smaller than the typical exciton diffusion length (10 nm)</p>
<p>is expected to improve
the charge separation efficiency [7].</p><br><p></p><p></p><p>Characterizing locally
modulated properties of non-ovalently functionalized 2D materials requires
high-resolution imaging techniques capable of extracting measurements of
various physical/chemical properties. One such method is scanning probe
microscopy (SPM) [18–21]. In Chapter 1, we present a brief review of SPM
modalities, some of which are used to characterize interfacial properties, such
as conductivity and local contact potential differences that can be modulated
by amphiphilic assemblies [17, 22]. Atomic force microscopy (AFM) is one of
main techniques that we use to determine topography. All imaging in this work
were performed in attractive AC mode [23,24] in order to minimize disruption to
the self-assembly of the amphiphiles by the scanning tip.</p><br><p></p><p></p><p>One challenge of using
SAMs for locally modulated functionalization is that the proximity to the
nonpolar interface can modify the behavior of the functionalities present on
the surface in conjunction with the steric hindrance of 2D molecular
assemblies. For instance, ionizable functional groups, one of the strongest
local modulators of surface chemistry, undergo substantial pKa shifts (in some
cases, > 5 units) at nonpolar interfaces, limiting their ability to ionize.
In order to apply molecular assembly to create 2D chemical patterns, we needed
to design alternative structures that can avoid such penalties against the
intrinsic properties of functionalities present in the assemblies. Among
amphiphiles, we observed that the chiral centers of phospholipids have the
potential of elevating the terminal functional group in the head from the surface
for improved accessibility. We refer to this type of assembly as a ’sitting’
phase. Chapter 2 describes sitting phase assembly of phospholipids; the
projection of the terminal functionality allows it to maintain solution
phase-like behavior while the dual alkyl tails provide additional stabilizing
interactions with the substrates. Given the diversity of phospholipid
architecture [25], the sitting phase assembly suggests the possibility of
greatly diversifying the orthogonality of the chemical patterns, allowing
highly precise control over surface functionalities.</p><br><p></p><p></p><p>While a variety of
methods including drop-casting [26–28] and microcontact printing [29] have been
used previously by others for noncovalent assembly of materials on the surface,
they mostly address patterning scale in the sub-μm range. Here, we utilize
Langmuir-Schaefer(LS) transfer, which has been historically used to transfer
standing phase multilayers [30], and lying-down domains of PCDA at < 100 nm
scales in the interest of molecular electronics [14, 31–33], as our sample
preparation technique. LS transfer is remarkable in that the transferred
molecules relinquish their pre-existing interactions in the standing phase at
air-water interface to undergo ∼ 90◦
rotation and assemble into the striped phase on a substrate. This introduces
the possibility of modulating local transfer rate across the substrate by
manipulating local environment of the molecules. Thus, LS transfer has the
potential to offer spatial control over the noncovalent chemical
functionalization of the 2D substrate, essential in device applications.</p><br><p></p><p></p><p>In Chapter 3 and 4, We
make comparative studies of various experimental factors such as surface pressure,
temperature and molecular interactions that affect the efficiency of LS
conversion. Considering the energetics of the transfer process, we predicted
that the rate of transfer from the air-water interface to the substrate should
be the highest from the regions around defects, which would be the
energetically</p>
<p>least stable regions of
the Langmuir film [34, 35]. In Langmuir films, two phases of lipid
assemblies—liquid expanded (LE) and liquid condensed (LC)—often coexist at the
low surface pressures (< 10 mN/m) used for sample preparation. Hence, we
hypothesized that the microscale structural heterogeneity of Langmuir films
could be translated into microscale patterns in the transferred film on HOPG.
We compare the transfer rates between LE and LC phases and investigate the
impacts of physical conditions during LS transfer such as temperature, packing
density, dipping rate and contact time to conclude that local destabilization
of Langmuir films leads to increased transfer efficiency. (Chapter 3)</p><p><br></p><p></p><p>As in the case of lipid
membranes that reorganize routinely based on the structure of the constituent
molecules [36–38], the structure of Langmuir films is strongly dependent on the
molecular structures of the constituent molecules [39–43]. Accordingly, we
expected the molecular structures/interactions to provide additional control
over the LS transfer process. In Chapter 4, we compare domain morphologies and
the average coverages between three single chain amphiphiles and two
phospholipids, each</p><p></p><p>
</p><p>of which contain
hydrogen bonding motifs of varying strengths. We show that by influencing the
adsorption and diffusion rates, molecular architecture indeed influences LS
conversion efficiency and subsequent assembly on the substrate. The presence of
strong lateral interactions limits transfer and diffusion, forming vacancies in
the transferred films with smaller domain sizes while weaker intermolecular
interactions enabled high transfer efficiencies.</p><p></p><p><br></p><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7492568 |
| Date | 20 December 2018 |
| Creators | Jae Jin Bang (5929496) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Tailoring_Nanoscopic_and_Macroscopic_Noncovalent_Chemical_Patterns_on_Layered_Materials_at_Sub-10_nm_Scales/7492568 |
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