Nanoscale Investigation of Defects and Oxidation of HfSe₂

Abstract

HfSe2 is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe2. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe2 surface using scanning tunneling microscopy and spectroscopy. Compared to MoS2 and WSe2, HfSe2 exhibits similar type of defects, albeit with a substantially higher density of 9 × 1011 cm–2. The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe2 surface leads to very low Schottky barrier heights. Conductive atomic force microscopy measurements reveal a very small dependence of the Schottky barrier height on the work function of the metals, suggesting a strong Fermi-level pinning. We attribute the observed Fermi-level pinning (pinning factor ∼0.1) to surface distortions and Se/Hf defects. In addition, we have also studied the HfSe2 surface after the exposure to air by scanning tunneling microscopy and conductive atomic force microscopy. Partly oxidized layers with band gaps of 2 eV and Schottky barrier heights of ∼0.6 eV were readily found on the surface. Our experiments reveal that HfSe2 is very air-sensitive, implying that capping or encapsulating of HfSe2, in order to protect it against oxidation, is a necessity for technological applications.

Revisiting the conformational adsorption of L‐ and D‐cysteine on Au nanoparticles by Raman spectroscopy

“…The vibrational study was performed by means of Raman spectroscopy instead of SERS to avoid misleading band assignment, reassuring Raman spectroscopy as a powerful analytical technique that can provide key information on the structure and bonding processes of the adsorbate molecules on metal surfaces in the solid and liquid phase. [4,40,41] The high quality of our Raman spectroscopy data provides very well resolved bands with higher definition than previous SERS results, [34][35][36][37] contrary to what can be expected when studying systems that involve metal surfaces. This quality data allowed us to determine the structural differences that the Au NP metallic surface induces on the cysteine molecule upon adsorption.…”

“…[62] Vibrational C S bond stretch band demonstrates the same behaviour for both molecules spec-tra, shifting from 637 to 690 cm −1 in the case of L-cys (53 cm −1 upshift) and from 658, 677 cm −1 doublet to also 689 cm −1 (3 cm −1 upshift) in the case of D-cys. Finally, the absence of the band at 490 cm −1 from the stretching vibration of the S-S bond [37] indicates that there was no formation of the disulfide bond at the moment of Raman spectroscopy examination.…”

“…[32,33] Several pioneer experimental investigations on the subject [34][35][36] claim that both the amino and carboxyl group are adsorbed on the gold surface, drawing conclusions based on surface enhanced Raman scattering (SERS) spectra, which show vibrational bands of both functional groups enhanced after cysteine adsorption on the metal surface. Conversely, other studies, also based on SERS and aided by density functional theory (DFT) calculations, [37,38] reported two possible adsorption modes: through the thiol group alone or via the thiol group together with the carboxyl group (one or two of the oxygen atoms from the COO − group). However, considering that SERS bands are shifted with respect to Raman bands due to the molecule adsorption, the wavenumber analysis and band assignment is currently considered a challenge using this technique.…”

“…However, considering that SERS bands are shifted with respect to Raman bands due to the molecule adsorption, the wavenumber analysis and band assignment is currently considered a challenge using this technique. [37,39] In order to shed light on the cysteine adsorption debate and elucidate which of the functional groups participate actively in the interaction with Au NPs, we performed vibrational analysis of cysteine-protected Au NPs synthesized by microwave irradiation that promotes partial hydrolyzing of HAuCl 4 . Our method produced colloidal neutral clusters in high yield with high affinity to the medium (H 2 O), allowing the attachment of the zwitterionic biosurfactant cysteine on their surface, conferring stability versus aggregation and coalescence that may cause particle growth.…”

“…The Au NPs used were synthesized to be smaller than 1 nm in diameter, and the theoretical models were calculated to have 34 Au atoms in order to increase the comparison accuracy with respect to previous results that report NPs of around 30 nm diameter and models with less than 10 atoms. [35,37,42] Experiments were performed with orthorhombic L-cysteine and monoclinic D-cysteine in order to complete an experimental description of the molecule prior and after adsorption on Au NPs, including polymorphism and enantiomerism to look for potential vibrational evidence of chiral selectivity, following previous predictions of an enantiospecific vibrational effect by theoretical calculations. [29] By the confrontation of the experimental vibrational data and the DFT computational calculations presented in this work, we aim to confirm unequivocally the adsorption sites and conformational rotamer of zwitterionic cysteine adsorbed on small Au NPs in solution.…”