Deciphering the Adsorption Mechanisms of RGD Subunits: l‑Aspartic Acid on Cu(110)

In this work we present a detailed surface science characterization of l-aspartic acid adsorption on a Cu(110) surface. Aspartic acid is one of the main components of the tripeptide RGD (arginine–glycine–aspartic acid). We replaced the traditional sublimation method to obtain molecular films by dosing aspartic acid directly from an aqueous solution through an electrospray ionization (ESI) device. X-ray photoelectron spectroscopy (XPS) and polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS) evidenced different adsorption states ranging from a submonolayer regime up to multilayers. Molecule–substrate interactions guide the creation of the pattern observed in the submonolayer, but molecule–molecule interactions are prevailing from a certain coverage stage, promoting the overlayer growth while leaving exposed areas of bare copper. This is evidenced by scanning tunneling microscopy (STM) results, showing that single aspartic acid molecules self-organize in a two-dimensional (2D) chiral network at low coverage and start originating new molecular layers even before a saturated monolayer has been reached

Tuning the Surface Chirality of Adsorbed Gly-Pro Dipeptide/Cu(110) by Changing Its Chemical Form via Electrospray Deposition

By changing the ultrahigh vacuum (UHV) deposition method, classical sublimation versus electrospray ionization, one can tune the chemistry of a chiral dipeptide molecule (Gly-Pro, GP), when adsorbed on a Cu(110) surface, from anionic to zwitterionic. This chemical shift will influence the adsorption mode of the dipeptide, either in a three-point fashion in the case of anionic GP molecules with a strong interaction among the copper surface, both O atoms of the carboxylate moiety, and the nitrogen atoms, or in the case of zwitterions GP, the adsorption mode relies on the sole interaction of one carboxylate oxygen atom. These different anchoring modes strongly modify the expression of surface 2D chirality and the supramolecular assemblies with two very distinct unit cells

Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals

Colloidal nanocrystals are an interesting platform for the design of low cost optoelectronic devices especially in the infrared range of wavelengths. Mercury chalcogenides have reached high maturity to address wavelengths above the telecom range (1.5 μm). However, no screening of the surface chemistry influence has been conducted yet. In this paper, we systematically probe the influence of a series of ligands, Cl^–, SCN^–, 1,2-ethanedithiol, 1,4-benzenedithiol, 1-octanethiol, 1-butanethiol, As₂S₃, and S^2–, on the photoconductive properties of HgTe nanocrystal thin films. A high bandwidth, large dynamic transient photocurrent setup is used to determine the photocarrier dynamics. Two regimes are clearly identified. At the early stage (few nanoseconds) a fast decay of the photocurrent is resulting from recombination and trapping. Then transport enters in a multiple trapping regime where carriers present a continuously decreasing effective value of their mobility. The power law dependence of the conductance can be used to estimate the trap carrier density and determine the value of the Urbach energy (35–50 meV). We demonstrate that a proper choice of ligand is necessary for a trade-off between the material performance (μτ product) and the quality of the surface passivation (to keep a low Urbach energy)

Wave-Function Engineering in HgSe/HgTe Colloidal Heterostructures To Enhance Mid-infrared Photoconductive Properties

The use of intraband transition is an interesting alternative path for the design of optically active complex colloidal materials in the mid-infrared range. However, so far, the performance obtained for photodetection based on intraband transition remains much smaller than the one relying on interband transition in narrow-band-gap materials operating at the same wavelength. New strategies have to be developed to make intraband materials more effective. Here, we propose growing a heterostructure of HgSe/HgTe as a means of achieving enhanced intraband-based photoconduction. We first tackle the synthetic challenge of growing a heterostructure on soft (Hg-based) material. The electronic spectrum of the grown heterostructure is then investigated using a combination of numerical simulation, infrared spectroscopy, transport measurement, and photoemission. We report a type-II band alignment with reduced doping compared with a core-only object and boosted hole conduction. Finally, we probe the photoconductive properties of the heterostructure while resonantly exciting the intraband transition by using a high-power-density quantum cascade laser. Compared to the previous generation of material based on core-only HgSe, the heterostructures have a lower dark current, stronger temperature dependence, faster photoresponse (with a time response below 50 μs), and detectivity increased by a factor of 30

Intraband Mid-Infrared Transitions in Ag₂Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection

Infrared photodetection based on colloidal nanoparticles is a promising path toward low-cost devices. However, mid-infrared absorption relies on interband transitions in heavy metal-based materials, which is a major flaw for the development toward mass market. In the quest of mercury-free infrared active colloidal materials, we here investigate Ag₂Se nanoparticles presenting intraband transition between 3 and 15 μm. With photoemission and infrared spectroscopy, we are able to propose an electronic spectrum of the material in the absolute energy scale. We also investigate the origin of doping and demonstrate that it results from a cation excess under the Ag⁺ form. We demonstrate photoconduction into this material under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds) and (ii) some electrochemical instabilities at room temperature