7 research outputs found
Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip Electron Injection
We have studied the adsorption and tip-induced chemistry
of 2-chloroanthracene
on TiO<sub>2</sub>(110). STM images show that at 135 K and low coverage,
i.e., ∼0.1 ML, these molecules are physisorbed along the five-coordinated
titanium rows on the rutile(110) surface as a result of electrostatic
interaction. Applying electric pulses >2.5 V from the STM
tip
to individual molecules causes either desorption or dissociation of
the molecules, as indicated by the changes in the STM images. We have
observed dissociative electron capture of a single 2-chloroanthracene
molecule, which leaves behind a surface chlorine atom adsorbed in
the on-top configuration on a surface Ti atom. The threshold energy
required for the dissociation was found to be ∼2.7 eV
Photoreactions on a Single Isolated TiO<sub>2</sub> Nanocrystal on Au(111): Photodecomposition of TMAA
An atomically resolved study of an
adsorbate photoreaction on the
surface of an isolated TiO<sub>2</sub> nanocrystal is reported. The
crystal is grown <i>in situ</i> on Au(111) in an ultrahigh-vacuum
chamber. The experiments use scanning tunneling microscopy (STM) backed
by temperature-programmed desorption (TPD) to determine the surface
coverage of trimethylacetic acid (TMAA) on the nanocrystal surfaces
before and after irradiation with monochromated 305 nm UV light. A
detailed determination of the surface structure of the 1–3
nm thick and 10–30 nm wide nanocrystals is presented and the
importance of moiré effects in controlling the reaction sites
shown. The normalized TMAA photodesorption quantum efficiency from
Au-supported TiO<sub>2</sub> nanocrystals was found to be ∼4
times lower compared to the same reaction on rutile TiO<sub>2</sub>(110) surface, a result consistent with the lower fraction of light
adsorbed in the nanocrystals
Correlation of H Adsorption Energy and Nanoscale Elastic Surface Strain on Rutile TiO<sub>2</sub>(110)
Scanning tunneling microscopy (STM)
has been used to obtain the
aerial distribution of bridge-bonded hydroxyl groups (HO<sub>b</sub>) on a rutile TiO<sub>2</sub>(110) surface, modified with a well-defined
nanoscale strain field. Our study makes use of earlier findings that
5–30 nm wide locally strained areas of the surface can be formed
via low-energy Ar-ion bombardment combined with a thermal treatment.
These strained areas appear as protrusions in the STM images, resulting
from subsurface argon-filled cavities. Our STM images show that the
local surface concentration of OH<sub>b</sub> groups is lower on the
protrusions. This lowering of concentration has been interpreted as
a reduction in the local H absorption energy, Δ<i>E</i>, a result similar to that observed on metals. In this paper, analysis
of the reduction in this O–H bond energy across the surface
shows a strong correlation between Δ<i>E</i><sub>OH</sub> and the characteristic surface strain value, <i>S</i>.
The Δ<i>E</i><sub>OH</sub> values have been calculated
through a subtraction of the contribution of the repulsive dipole–dipole
interaction between OH<sub>b</sub> groups. This interaction has been
estimated from an analysis of the radial distribution of OH<sub>b</sub> pairs in the STM images. The measured linear relation between the
reduction in O–H bond energy and the surface strain has been
estimated to be Δ<i>E</i><sub>OH</sub> (meV) ≈ 11·<i>S</i> (%)
Supplement 1: Extrinsic photodiodes for integrated mid-infrared silicon photonics
Originally published in Optica on 20 October 2014 (optica-1-4-264
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)/Cu(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
Graphene Plasmon Enhanced Vibrational Sensing of Surface-Adsorbed Layers
We characterize the influence of graphene nanoribbon plasmon excitation
on the vibrational spectra of surface-absorbed polymers. As the detuning
between the graphene plasmon frequency and a vibrational frequency
of the polymer decreases, the vibrational peak intensity first increases
and is then transformed into a region of narrow optical transparency
as the frequencies overlap. Examples of this are provided by the carbonyl
vibration in thin films of poly(methyl methacrylate) and polyvinylpyrrolidone.
The signal depth of the plasmon-induced transparency is found to be
5 times larger than that of light attenuated by the carbonyl vibration
alone. The plasmon-vibrational mode coupling and the resulting fields
are analyzed using both a phenomenological model of electromagnetically
coupled oscillators and finite-difference time-domain simulations.
It is shown that this coupling and the resulting absorption enhancement
can be understood in terms of near-field electromagnetic interactions
Direct Measurement of the Tunable Electronic Structure of Bilayer MoS<sub>2</sub> by Interlayer Twist
Using
angle-resolved photoemission on micrometer-scale sample areas, we
directly measure the interlayer twist angle-dependent electronic band
structure of bilayer molybdenum-disulfide (MoS<sub>2</sub>). Our measurements,
performed on arbitrarily stacked bilayer MoS<sub>2</sub> flakes prepared
by chemical vapor deposition, provide direct evidence for a downshift
of the quasiparticle energy of the valence band at the Brillouin zone
center (Γ̅ point) with the interlayer twist angle, up
to a maximum of 120 meV at a twist angle of ∼40°. Our
direct measurements of the valence band structure enable the extraction
of the hole effective mass as a function of the interlayer twist angle.
While our results at Γ̅ agree with recently published
photoluminescence data, our measurements of the quasiparticle spectrum
over the full 2D Brillouin zone reveal a richer and more complicated
change in the electronic structure than previously theoretically predicted.
The electronic structure measurements reported here, including the
evolution of the effective mass with twist-angle, provide new insight
into the physics of twisted transition-metal dichalcogenide bilayers
and serve as a guide for the practical design of MoS<sub>2</sub> optoelectronic
and spin-/valley-tronic devices