Maria Longobardi

Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals

Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials have attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm2 V−1s−1 at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, which allow the measurement of the indirect gap (Eind), and optical transmission spectroscopy on crystals of different thickness, which enables the determination of both the direct (Edir) and the indirect gap. We find that at room temperature Eind = 0.88 eV and Edir = 1.02 eV. Our results suggest that thin MoTe2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs.

Quantitative Analysis of Scanning Tunneling Microscopy Images of Mixed-Ligand-Functionalized Nanoparticles

Ligand-protected gold nanoparticles exhibit large local curvatures, features rapidly varying over small scales, and chemical heterogeneity. Their imaging by scanning tunneling microscopy (STM) can, in principle, provide direct information on the architecture of their ligand shell, yet STM images require laborious analysis and are challenging to interpret. Here, we report a straightforward, robust, and rigorous method for the quantitative analysis of the multiscale features contained in STM images of samples consisting of functionalized Au nanoparticles deposited onto Au/mica. The method relies on the analysis of the topographical power spectral density (PSD) and allows us to extract the characteristic length scales of the features exhibited by nanoparticles in STM images. For the mixed-ligand-protected Au nanoparticles analyzed here, the characteristic length scale is 1.2 ± 0.1 nm, whereas for the homoligand Au NPs this scale is 0.75 ± 0.05 nm. These length scales represent spatial correlations independent of scanning parameters, and hence the features in the PSD can be ascribed to a fingerprint of the STM contrast of ligand-protected nanoparticles. PSD spectra from images recorded at different laboratories using different microscopes and operators can be overlapped across most of the frequency range, proving that the features in the STM images of nanoparticles can be compared and reproduced.

Scalable Patterning of One-Dimensional Dangling Bond Rows on Hydrogenated Si(001)

Silicon dangling bonds exposed on the monohydride silicon (001) (Si(001):H) surface are highly reactive, thus enabling site-selective absorption of atoms and single molecules into custom patterns designed through the controlled removal of hydrogen atoms. Current implementations of high-resolution hydrogen lithography on the Si(001):H surface rely on sequential removal of hydrogen atoms using the tip of a scanning probe microscope. Here, we present a scalable thermal process that yields very long rows of single dimer wide silicon dangling bonds suitable for self-assembly of atoms and molecules into one-dimensional structures of unprecedented length on Si(001):H. The row consists of the standard buckled Si dimer and an unexpected flat dimer configuration.

Structure of self-assembled Mn atom chains on Si(001)

Mn has been found to self-assemble into atomic chains running perpendicular to the surface dimer reconstruction on Si(001). They differ from other atomic chains by a striking asymmetric appearance in filled state scanning tunneling microscopy (STM) images. This has prompted complicated structural models involving up to three Mn atoms per chain unit. Combining STM, atomic force microscopy, and density functional theory we find that a simple necklacelike chain of single Mn atoms reproduces all their prominent features, including their asymmetry not captured by current models. The upshot is a remarkably simpler structure for modeling the electronic and magnetic properties of Mn atom chains on Si(001).

Subatomic electronic feature from dynamic motion of Si dimer defects in Bi nanolines on Si(001)

Scanning tunneling microscopy (STM) reveals unusual sharp features in otherwise defect-free Bi nanolines self-assembled on Si(001). They appear as subatomic thin lines perpendicular to the Bi nanoline at positive biases and as atomic size beads at negative biases. Density functional theory (DFT) simulations show that these features can be attributed to buckled Si dimers substituting for Bi dimers in the nanoline, where the sharp feature is the counterintuitive signature of these dimers flipping during scanning. The perfect correspondence between the STM data and the DFT simulation demonstrated in this paper highlights the detailed understanding we have of the complex Bi-Si(001) Haiku system. This discovery has applications in the patterning of Si dangling bonds for nanoscale electronics.

Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study

Atomic nanolines are one-dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic p orbitals, and an electronic coupling between the Bi nanoline and neighboring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.

One-dimensional silicon nanolines in the Si(001):H surface

We present a detailed study of the structural and electronic properties of a self-assembled silicon nanoline embedded in the monohydride Si(001):H surface, known as the Haiku stripe. The nanoline is a perfectly straight and defect free endotaxial structure of huge aspect ratio; it can grow micrometer long at a constant width of exactly four Si dimers (1.54 nm). Another remarkable property is its capacity to be exposed to air without suffering any degradation. The nanoline grows independently of any step edges at tunable densities, from isolated nanolines to a dense array of nanolines. In addition to these unique structural characteristics, scanning tunnelling microscopy and density functional theory reveal a one-dimensional state confined along the Haiku core. This nanoline is a promising candidate for the long sought after electronic solid-state one-dimensional model system to explore the fascinating quantum properties emerging in such reduced dimensionality.