4 research outputs found
Synthesis of Liquid CoreâShell Particles and Solid Patchy Multicomponent Particles by Shearing Liquids Into Complex Particles (SLICE)
We
report a simple method that uses (i) emulsion shearing with
oxidation to make coreâshell particles, and (ii) emulsion shearing
with surface-tension driven phase segregation to synthesize particles
with complex surface compositions and morphologies. Subjecting eutectic
galliumâindium, a liquid metal, to shear in an acidic carrier
fluid we synthesized smooth liquid coreâshell particles 6.4
nm to over 10 Îźm in diameter. Aggregates of these liquid particles
can be reconfigured into larger structures using a focused ion beam.
Using Fieldâs metal melts we synthesized homogeneous nanoparticles
and solid microparticles with different surface roughness and/or composition
through shearing and phase separation. This extension of droplet emulsion
technique, SLICE, applies fluidic shear to create micro- and nanoparticles
in a tunable, green, and low-cost approach
Role of Molecular Dipoles in Charge Transport across Large Area Molecular Junctions Delineated Using Isomorphic Self-Assembled Monolayers
Delineating
the role of dipoles in large area junctions that are
based on self-assembled monolayers (SAMs) is challenging due to molecular
tilt, surface defects, and interchain coupling among other features.
To mitigate SAM-based effects in study of dipoles, we investigated
tunneling rates across carboranesî¸isostructural molecules that
orient along the surface normal on Au (but bear different dipole moments)
without changing the thickness, packing density, or morphology of
the SAM. Using the Au-SAM//Ga<sub>2</sub>O<sub>3</sub>-EGaIn junction
(where â//â = physisorption, âââ
= chemisorption, and EGaIn is eutectic galliumâindium), we
observe that molecules with dipole moments oriented along the surface
normal (with dipole moment, <i>p</i> = 4.1D for both M9
and 1O2) gave lower currents than when the dipole is orthogonal (<i>p</i> = 1.1 D, M1) at Âą0.5 V applied bias. Similarly, from
transition voltage spectroscopy, the transition voltages, <i>V</i><sub>T</sub> (volt), are significantly different. (0.5,
0.43, and 0.4 V for M1, M9, and 1O2, respectively). We infer that
the magnitude and direction of a dipole moments significantly affect
the rate of charge transport across large area junctions with Î
log|J| â
0.4 per Debye. This difference is largely due to effect
of the dipole moment on the molecule-electrode coupling strength,
Î, hence effect of dipoles is likely to manifest in the contact
resistance, <i>J</i><sub>o</sub>, although in conformational
flexible molecules field-induced effects are expected
Limits to the Effect of Substrate Roughness or Smoothness on the OddâEven Effect in Wetting Properties of <i>n</i>âAlkanethiolate Monolayers
This
study investigates the effect of roughness on interfacial
properties of an <i>n</i>-alkanethiolate self-assembled
monolayer (SAM) and uses hydrophobicity to demonstrate the existence
of upper and lower limits. This article also sheds light on the origin
of the previously unexplained gradual increase in contact angles with
increases in the size of the molecule making the SAM. We prepared
Au surfaces with a root-mean-square (RMS) roughness of âź0.2â0.5
nm and compared the wetting properties of <i>n</i>-alkanethiolate
(C<sub>10</sub>âC<sub>16</sub>) SAMs fabricated on these surfaces.
Static contact angles, θ<sub>s</sub>, formed between the SAM
and water, diethylene glycol, and hexadecane showed an oddâeven
effect irrespective of the solvent properties. The average differences
in subsequent SAM<sup>E</sup> and SAM<sup>O</sup> are Îθ<sub>s|<i>n </i>âŻââŻ(<i>n</i>+1)|</sub> â 1.7° (<i>n</i> = even) and Îθ<sub>s|<i>nâŻ</i>ââŻ(<i>n</i>+1)|</sub> â 3.1° (<i>n</i> = odd). A gradual increase
in θ<sub>s</sub> with increasing length of the molecule was
observed, with values ranging from water 104.7â110.7°
(overall Îθ<sub>s</sub> = 6.0° while for the evens
Îθ<sub>s</sub><sup>E</sup> = 4.4° and odds Îθ<sub>s</sub><sup>O</sup> = 3.5°) to diethylene glycol 72.9â80.4°
(overall Îθ<sub>s</sub> = 7.5° while for the evens
Îθ<sub>s</sub><sup>E</sup> = 2.9° and odds Îθ<sub>s</sub><sup>O</sup> = 2.4°) and hexadecane 40.4â49.4°
(overall Îθ<sub>s</sub> = 9.0° while for the evens
Îθ<sub>s</sub><sup>E</sup> = 3.7° and odds Îθ<sub>s</sub><sup>O</sup> = 2.1°). This article establishes that
the gradual increase in θ<sub>s</sub> with increasing molecular
size in SAMs is due to asymmetry in the zigzag oscillation in the
oddâeven effect. Comparison of the magnitude and proportion
differences in this asymmetry allows us to establish the reduction
in interfacial dispersive forces, due to increasing SAM crystallinity
with increasing molecular size, as the origin of this asymmetry. By
comparing the dependence of θ<sub>s</sub> on surface roughness
we infer that (i) RMS roughness â 1 nm is a theoretical limit
beyond which the oddâeven effect cannot be observed and (ii)
on a hypothetically flat surface the maximum difference in hydrophobicity,
as expressed in θ<sub>s</sub>, is âź3°
OddâEven Effect in the Hydrophobicity of <i>n</i>âAlkanethiolate Self-Assembled Monolayers Depends upon the Roughness of the Substrate and the Orientation of the Terminal Moiety
The
origin of the oddâeven effect in properties of self-assembled
monolayers (SAMs) and/or technologies derived from them is poorly
understood. We report that hydrophobicity and, hence, surface wetting
of SAMs are dominated by the nature of the substrate (surface roughness
and identity) and SAM tilt angle, which influences surface dipoles/orientation
of the terminal moiety. We measured static contact angles (θ<sub>s</sub>) made by water droplets on <i>n</i>-alkanethiolate
SAMs with an odd (SAM<sup>O</sup>) or even (SAM<sup>E</sup>) number
of carbons (average θ<sub>s</sub> range of 105.8â112.1°).
When SAMs were fabricated on smooth âtemplate-strippedâ
metal (M<sup>TS</sup>) surfaces [root-mean-square (rms) roughness
= 0.36 Âą 0.01 nm for Au<sup>TS</sup> and 0.60 Âą 0.04 nm
for Ag<sup>TS</sup>], the oddâeven effect, characterized by
a zigzag oscillation in values of θ<sub>s</sub>, was observed.
We, however, did not observe the same effect with rougher âas-depositedâ
(M<sup>AD</sup>) surfaces (rms roughness = 2.27 Âą 0.16 nm for
Au<sup>AD</sup> and 5.13 Âą 0.22 nm for Ag<sup>AD</sup>). The
oddâeven effect in hydrophobicity inverts when the substrate
changes from Au<sup>TS</sup> (higher θ<sub>s</sub> for SAM<sup>E</sup> than SAM<sup>O</sup>, with average Îθ<sub>s |<i>n</i> â (<i>n</i> + 1)|</sub> â 3°) to Ag<sup>TS</sup> (higher θ<sub>s</sub> for SAM<sup>O</sup> than SAM<sup>E</sup>, with average Îθ<sub>s |<i>n</i> â (<i>n</i> + 1)|</sub> â 2°). A comparison of hydrophobicity
across Ag<sup>TS</sup> and Au<sup>TS</sup> showed a statistically
significant difference (Studentâs <i>t</i> test)
between SAM<sup>E</sup> (Îθ<sub>s |Ag evens â Au evens|</sub> â 5°; <i>p</i> < 0.01) but failed to show
statistically significant differences on SAM<sup>O</sup> (Îθ<sub>s |Ag odds â Au odds|</sub> â
1°; <i>p</i> > 0.1). From these results, we deduce
that the roughness of the metal substrate (from comparison of M<sup>AD</sup> versus M<sup>TS</sup>) and orientation of the terminal âCH<sub>2</sub>CH<sub>3</sub> (by comparing SAM<sup>E</sup> and SAM<sup>O</sup> on Au<sup>TS</sup> versus Ag<sup>TS</sup>) play major roles in the
hydrophobicity and, by extension, general wetting properties of <i>n</i>-alkanethiolate SAMs