Large Band Edge Tunability in Colloidal Nanoplatelets

Abstract

We study the impact of organic surface ligands on the electronic structure and electronic band edge energies of quasi-two-dimensional (2D) colloidal cadmium selenide nanoplatelets (NPLs) using density functional theory. We show how control of the ligand and ligand–NPL interface dipoles results in large band edge energy shifts, over a range of 5 eV for common organic ligands with a minor effect on the NPL band gaps. Using a model self-energy to account for the dielectric contrast and an effective mass model of the excitons, we show that the band edge tunability of NPLs together with the strong dependence of the optical band gap on NPL thickness can lead to favorable photochemical and optoelectronic properties.

Engineering the Electronic Structure of Two-Dimensional Materials with Near-Field Electrostatic Effects of Self-Assembled Organic Layers

“…In the case of bulk 3D materials, electrostatic modification occurs through far-field effects resulting from the monopole and areal dipole of the material/molecule interface [1][2][3][8][9][10][11][12][13], with consequences on observables such as work functions [14][15][16], band offsets [17] and superconducting transitions [18]. While also impacted by far-field effects [5,6,19], 2D electronic states such as surface states [20][21][22] and electronic states in 2D materials [18,[23][24][25][26][27][28][29][30] can also be impacted by near-field effects resulting from higher moments of the surface/interface electronic density, typically observed within ≈ 10 Å [31][32][33] of the interface. In particular, for these systems, near-field modulation of the electrostatic potential has been proposed as a means to engineer band structures, wave functions, and topological properties [20,21,[34][35][36][37][38][39][40].…”

Electronic Structure of Metallophthalocyanines, MPc (M = Fe, Co, Ni, Cu, Zn, Mg) and Fluorinated MPc

“…Similar results are found in the cases of H 2 Pc and ZnPc, see Figures S10 and S11, which have both a comparable electrostatic potential shift and a similar orbital energy shift as CoPc. This result indicates that the uniform orbital energy shifts induced by fluorine atoms are of electrostatic origin, and can be understood by examining the change in electronic density near the peripheral atoms when substituting electron-donating hydrogens by strongly electronegative fluorine atoms . As the value of this change is well approximated by the change in potential at the center of the molecule, as shown by the dashed lines in Figure S12, this effect can in principle be captured through a multipole expansion of the molecular charge densityspecifically its quadrupole moment, as the dipole moments are zero due to the D 4 h symmetry.…”

“…We note that this uniform shift in energy may allow for additional tunability of the band alignment in MPc-based heterojunctions, which is critical in controlling photophysics, and photocatalytic reactivity of mixed dimensional heterojunctions. − …”

“…This result indicates that the uniform orbital energy shifts induced by fluorine atoms are of electrostatic origin, and can be understood by examining the change in electronic density near the peripheral atoms when substituting electron-donating hydrogens by strongly electronegative fluorine atoms. 65 As the value of this change is well approximated by the change in potential at the center of the molecule, as shown by the dashed lines in Figure S12, this effect can in principle be captured through a multipole expansion of the molecular charge densityspecifically its quadrupole moment, as the dipole moments are zero due to the D We note that this uniform shift in energy may allow for additional tunability of the band alignment in MPc-based heterojunctions, which is critical in controlling photophysics, and photocatalytic reactivity of mixed dimensional heterojunctions. 65−67 Optical band gaps E g opt for MPcs from B3LYP and OT-RSH as well as experimental absorption spectroscopy results are included in Table 3, with similar values from B3LYP and OT-RSH, which are ∼0.2 eV to the blue of the experimental values.…”

Understanding Surface Modulation to Improve the Photo/Electrocatalysts for Water Oxidation/Reduction

“…As a result, the accumulation of charge carriers at the surface and charge transfer kinetics to the adsorbed electron/hole acceptors are controllable with various surface functionalizations. In addition, considering the surface-to-volume ratio, the band edge of materials with smaller dimensions, including zero-dimensional quantum dots and quasi-two-dimensional nanoplatelets, are strongly modulated by functionalization with molecular ligands [82][83][84]. Thus, surface functionalities on the nanostructured electrode materials become important for controlling the energetic states at the surface.…”