5 research outputs found
Two-Dimensional Nanosheets from Redox-Active Superatoms
We
describe a new approach to synthesize two-dimensional (2D) nanosheets
from the bottom-up. We functionalize redox-active superatoms with
groups that can direct their assembly into multidimensional solids.
We synthesized Co<sub>6</sub>Se<sub>8</sub>Â[PEt<sub>2</sub>(4-C<sub>6</sub>H<sub>4</sub>COOH)]<sub>6</sub> and found that it forms a
crystalline assembly. The solid-state structure is a three-dimensional
(3D) network in which the carboxylic acids form intercluster hydrogen
bonds. We modify the self-assembly by replacing the reversible hydrogen
bonds that hold the superatoms together with zinc carboxylate bonds
via the solvothermal reaction of Co<sub>6</sub>Se<sub>8</sub>Â[PEt<sub>2</sub>(4-C<sub>6</sub>H<sub>4</sub>COOH)]<sub>6</sub> with ZnÂ(NO<sub>3</sub>)<sub>2</sub>. We obtain two types of crystalline materials
using this approach: one is a 3D solid and the other consists of stacked
layers of 2D sheets. The dimensionality is controlled by subtle changes
in reaction conditions. These 2D sheets can be chemically
exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After
they are deposited on a substrate, they can be imaged. We cast them
onto an electrode surface and show that they retain the redox activity
of the superatom building blocks due to the porosity in the sheets
Patterning Superatom Dopants on Transition Metal Dichalcogenides
This
study describes a new and simple approach to dope two-dimensional
transition metal dichalcogenides (TMDCs) using the superatom Co<sub>6</sub>Se<sub>8</sub>(PEt<sub>3</sub>)<sub>6</sub> as the electron
dopant. Semiconducting TMDCs are wired into field-effect transistor
devices and then immersed into a solution of these superatoms. The
degree of doping is determined by the concentration of the superatoms
in solution and by the length of time the films are immersed in the
dopant solution. Using this chemical approach, we are able to turn
mono- and few-layer MoS<sub>2</sub> samples from moderately to heavily
electron-doped states. The same approach applied on WSe<sub>2</sub> films changes their characteristics from hole transporting to electron
transporting. Moreover, we show that the superatom doping can be patterned
on specific areas of TMDC films. To illustrate the power of this technique,
we demonstrate the fabrication of a lateral p–n junction by
selectively doping only a portion of the channel in a WSe<sub>2</sub> device. Finally, encapsulation of the doped films with crystalline
hydrocarbon layers stabilizes their properties in an ambient environment
Coverage-Dependent Modification of the Surface Electronic Structure of an Organic-Semiconductor-Adsorbate Layer
The
electronic structure of a hexa-<i>cata</i>-hexabenzocoronene
(HBC)/CuÂ(111) interface is investigated by two-photon photoemission
over a range of coverage from 0 to 2 ML monolayers. It is found that
increasing the HBC coverage shifts the vacuum level of the Cu substrate
until this shift saturates at a coverage of ∼2 ML. Over this
same range of coverage, the Shockley and the bare-surface Cu(111)
image-potential states are shown to be quenched, while new unoccupied
states appear and grow in strength with coverage. The use of momentum-
and polarization-resolved photoemission spectra reveals that the new
states are modified image states
Single-Molecule Reaction Chemistry in Patterned Nanowells
A new approach to
synthetic chemistry is performed in ultraminiaturized, nanofabricated
reaction chambers. Using lithographically defined nanowells, we achieve
single-point covalent chemistry on hundreds of individual carbon nanotube
transistors, providing robust statistics and unprecedented spatial
resolution in adduct position. Each device acts as a sensor to detect,
in real-time and through quantized changes in conductance, single-point
functionalization of the nanotube as well as consecutive chemical
reactions, molecular interactions, and molecular conformational changes
occurring on the resulting single-molecule probe. In particular, we
use a set of sequential bioconjugation reactions to tether a single-strand
of DNA to the device and record its repeated, reversible folding into
a G-quadruplex structure. The stable covalent tether allows us to
measure the same molecule in different solutions, revealing the characteristic
increased stability of the G-quadruplex structure in the presence
of potassium ions (K<sup>+</sup>) versus sodium ions (Na<sup>+</sup>). Nanowell-confined reaction chemistry on carbon nanotube devices
offers a versatile method to isolate and monitor individual molecules
during successive chemical reactions over an extended period of time
van der Waals Solids from Self-Assembled Nanoscale Building Blocks
Traditional
atomic van der Waals materials such as graphene, hexagonal boron-nitride,
and transition metal dichalcogenides have received widespread attention
due to the wealth of unusual physical and chemical behaviors that
arise when charges, spins, and vibrations are confined to a plane.
Though not as widespread as their atomic counterparts, molecule-based
two-dimensional (2D) layered solids offer significant benefits; their
structural flexibility will enable the development of materials with
tunable properties. Here we describe a layered van der Waals solid
self-assembled from a structure-directing building block and C<sub>60</sub> fullerene. The resulting crystalline solid contains a corrugated
monolayer of neutral fullerenes and can be mechanically exfoliated.
The absorption spectrum of the bulk solid shows an optical gap of
390 ± 40 meV that is consistent with thermal activation energy
obtained from electrical transport measurement. We find that the dimensional
confinement of fullerenes significantly modulates the optical and
electronic properties compared to the bulk solid