Width and Crystal Orientation Dependent Band Gap Renormalization in Substrate-Supported Graphene Nanoribbons

The excitation energy levels of two-dimensional (2D) materials and their one-dimensional (1D) nanostructures, such as graphene nanoribbons (GNRs), are strongly affected by the presence of a substrate due to the long-range screening effects. We develop a first-principles approach combining density functional theory (DFT), the GW approximation, and a semiclassical image-charge model to compute the electronic band gaps in planar 1D systems in weak interaction with the surrounding environment. Application of our method to the specific case of GNRs yields good agreement with the range of available experimental data and shows that the band gap of substrate-supported GNRs are reduced by several tenths of an electronvolt compared to their isolated counterparts, with a width and orientation-dependent renormalization. Our results indicate that the band gaps in GNRs can be tuned by controlling screening at the interface by changing the surrounding dielectric materials

Aluminum Conducts Better than Copper at the Atomic Scale: A First-Principles Study of Metallic Atomic Wires

Using a first-principles density functional method, we have studied the electronic structure, electron–phonon coupling, and quantum transport properties of atomic wires of Ag, Al, Au, and Cu. Non-equilibrium Green’s function-based transport studies of finite atomic wires suggest that the conductivity of Al atomic wires is higher than that of Ag, Au, and Cu in contrast to the bulk where Al has the lowest conductivity among these systems. This is attributed to the higher number of eigenchannels in Al wires, which becomes the determining factor in the ballistic limit. On the basis of density functional perturbation theory, we find that the electron–phonon coupling constant of the Al atomic wire is lowest among the four metals studied, and more importantly, that the value is reduced by a factor of 50 compared to the bulk

Quantum Dots in Graphene Nanoribbons

Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics, and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap <i>N</i> = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of <i>N</i> = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over 1 order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices

Photoinduced Water Oxidation at the Aqueous GaN (101̅0) Interface: Deprotonation Kinetics of the First Proton-Coupled Electron-Transfer Step

Photoelectrochemical water splitting plays a key role in a promising path to the carbon-neutral generation of solar fuels. Wurzite GaN and its alloys (e.g., GaN/ZnO and InGaN) are demonstrated photocatalysts for water oxidation, and they can drive the overall water splitting reaction when coupled with co-catalysts for proton reduction. The present work investigates the water oxidation mechanism on the prototypical GaN (101̅0) surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface. The investigation of free-energy changes for the four proton-coupled electron-transfer (PCET) steps of the water oxidation mechanism shows that the first PCET step for the conversion of −Ga–OH to −Ga–O^•– requires the highest energy input. The study further examines the sequential PCETs, with the proton transfer (PT) following the electron transfer (ET), and finds that photogenerated holes localize on surface −NH sites, and the calculated free-energy changes indicate that PCET through −NH sites is thermodynamically more favorable than −OH sites. However, proton transfer from −OH sites with subsequent localization of holes on oxygen atoms is kinetically favored owing to hydrogen bonding interactions at the GaN (101̅0)–water interface. The deprotonation of surface −OH sites is found to be the limiting factor for the generation of reactive oxyl radical ion intermediates and consequently for water oxidation

Periodic Arrays of Phosphorene Nanopores as Antidot Lattices with Tunable Properties

A tunable band gap in phosphorene extends its applicability in nanoelectronic and optoelectronic applications. Here, we propose to tune the band gap in phosphorene by patterning antidot lattices, which are periodic arrays of holes or nanopores etched in the material, and by exploiting quantum confinement in the corresponding nanoconstrictions. We fabricated antidot lattices with radii down to 13 nm in few-layer black phosphorus flakes protected by an oxide layer and observed suppression of the in-plane phonon modes relative to the unmodified material <i>via</i> Raman spectroscopy. In contrast to graphene antidots, the Raman peak positions in few-layer BP antidots are unchanged, in agreement with predicted power spectra. We also use DFT calculations to predict the electronic properties of phosphorene antidot lattices and observe a band gap scaling consistent with quantum confinement effects. Deviations are attributed primarily to self-passivating edge morphologies, where each phosphorus atom has the same number of bonds per atom as the pristine material so that no dopants can saturate dangling bonds. Quantum confinement is stronger for the zigzag edge nanoconstrictions between the holes as compared to those with armchair edges, resulting in a roughly bimodal band gap distribution. Interestingly, in two of the antidot structures an unreported self-passivating reconstruction of the zigzag edge endows the systems with a metallic component. The experimental demonstration of antidots and the theoretical results provide motivation to further scale down nanofabrication of antidots in the few-nanometer size regime, where quantum confinement is particularly important

Revealing the Electronic Structure of Silicon Intercalated Armchair Graphene Nanoribbons by Scanning Tunneling Spectroscopy

The electronic properties of graphene nanoribbons grown on metal substrates are significantly masked by the ones of the supporting metal surface. Here, we introduce a novel approach to access the frontier states of armchair graphene nanoribbons (AGNRs). The in situ intercalation of Si at the AGNR/Au(111) interface through surface alloying suppresses the strong contribution of the Au(111) surface state and allows for an unambiguous determination of the frontier electronic states of both wide and narrow band gap AGNRs. First-principles calculations provide insight into substrate induced screening effects, which result in a width-dependent band gap reduction for substrate-supported AGNRs. The strategy reported here provides a unique opportunity to elucidate the electronic properties of various kinds of graphene nanomaterials supported on metal substrates

Controlled Sculpture of Black Phosphorus Nanoribbons

Black phosphorus (BP) is a highly anisotropic allotrope of phosphorus with great promise for fast functional electronics and optoelectronics. We demonstrate the controlled structural modification of few-layer BP along arbitrary crystal directions with sub-nanometer precision for the formation of few-nanometer-wide armchair and zigzag BP nanoribbons. Nanoribbons are fabricated, along with nanopores and nanogaps, using a combination of mechanical–liquid exfoliation and <i>in situ</i> transmission electron microscopy (TEM) and scanning TEM nanosculpting. We predict that the few-nanometer-wide BP nanoribbons realized experimentally possess clear one-dimensional quantum confinement, even when the systems are made up of a few layers. The demonstration of this procedure is key for the development of BP-based electronics, optoelectronics, thermoelectrics, and other applications in reduced dimensions

Controlled Sculpture of Black Phosphorus Nanoribbons

Black phosphorus (BP) is a highly anisotropic allotrope of phosphorus with great promise for fast functional electronics and optoelectronics. We demonstrate the controlled structural modification of few-layer BP along arbitrary crystal directions with sub-nanometer precision for the formation of few-nanometer-wide armchair and zigzag BP nanoribbons. Nanoribbons are fabricated, along with nanopores and nanogaps, using a combination of mechanical–liquid exfoliation and <i>in situ</i> transmission electron microscopy (TEM) and scanning TEM nanosculpting. We predict that the few-nanometer-wide BP nanoribbons realized experimentally possess clear one-dimensional quantum confinement, even when the systems are made up of a few layers. The demonstration of this procedure is key for the development of BP-based electronics, optoelectronics, thermoelectrics, and other applications in reduced dimensions

Heteroatom-Doped Perihexacene from a Double Helicene Precursor: On-Surface Synthesis and Properties

We report on the surface-assisted synthesis and spectroscopic characterization of the hitherto longest periacene analogue with oxygen–boron–oxygen (OBO) segments along the zigzag edges, that is, a heteroatom-doped perihexacene <b>1</b>. Surface-catalyzed cyclodehydrogenation successfully transformed the double helicene precursor <b>2</b>, i.e., 12a,26a-dibora-12,13,26,27-tetraoxa-benzo­[1,2,3-<i>hi</i>:4,5,6-<i>h</i>′<i>i</i>′]­dihexacene, into the planar perihexacene analogue <b>1</b>, which was visualized by scanning tunneling microscopy and noncontact atomic force microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy, together with theoretical modeling, on both precursor <b>2</b> and product <b>1</b>, provided further insights into the cyclodehydrogenation process. Moreover, the nonplanar precursor <b>2</b> underwent a conformational change upon adsorption on surfaces, and one-dimensional self-assembled superstructures were observed for both <b>2</b> and <b>1</b> due to the presence of OBO units along the zigzag edges