Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip Electron Injection

We have studied the adsorption and tip-induced chemistry of 2-chloroanthracene on TiO₂(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 TiO₂ Nanocrystal on Au(111): Photodecomposition of TMAA

An atomically resolved study of an adsorbate photoreaction on the surface of an isolated TiO₂ 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 TiO₂ nanocrystals was found to be ∼4 times lower compared to the same reaction on rutile TiO₂(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 TiO₂(110)

Scanning tunneling microscopy (STM) has been used to obtain the aerial distribution of bridge-bonded hydroxyl groups (HO_b) on a rutile TiO₂(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_b 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>_OH and the characteristic surface strain value, <i>S</i>. The Δ<i>E</i>_OH values have been calculated through a subtraction of the contribution of the repulsive dipole–dipole interaction between OH_b groups. This interaction has been estimated from an analysis of the radial distribution of OH_b 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>_OH (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 MoS₂ 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 (MoS₂). Our measurements, performed on arbitrarily stacked bilayer MoS₂ 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 MoS₂ optoelectronic and spin-/valley-tronic devices