Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers

Abstract

Conduction and valence band energies (ECB, EVB) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure EVB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure ECB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show EVB = 5.9-6.1 eV and ECB = 4.1-4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts EVB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above EVB. Spectroelectrochemical results confirm these trends: Au tipping shifts ECB closer to vacuum, by 0.4-0.6 eV, and introduces midgap states below ECB, which are assigned as metal-semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. The complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.

Electronic Processes within Quantum Dot-Molecule Complexes

“…Furthermore, the measured potentials of QDs appear to be dependent on their surface functionalization, and it is sometimes difficult to determine whether this dependence results from measurements of redox-active surface states or perturbation of core state energies by surface species. We present a list of experimentally measured oxidation and reduction potentials for several materials of QDs of different sizes with different capping molecules in Table . ,,− We reference all of the potentials listed in the table to SCE for easier comparison. Some literature reports list peak potentials ( E pc -potential for cathodic peak (reduction) and E pa -potential for anodic peak (oxidation), whereas the others list onset potentials or half-wave potentials.…”

“…Tuning the size of the QD, and therefore the dimensions of the particle-in-a-sphere potential well, or incorporating it within a semiconductor–semiconductor , or semiconductor–metal heterostructure, are straightforward means of changing the bandgap and potentials of a QD. The large surface area-to-volume ratio of a QD also makes the electronic structure of its core sensitive to surface functionalization in some cases.…”

Type I vs. quasi-type II modulation in CdSe@CdS tetrapods: ramifications for noble metal tipping

“…Among these II-VI semiconductor heterostructures, nanorod systems have been studied extensively for directed charge transfer, for which solution deposited metallic nanoparticle "tips" can be energetically aligned to promote electron transfer from photogenerated excitons in the semiconductor phase. [23][24][25][26][27][28][29][30][31][32][33][34][35] Such asymmetrically metaltipped semiconductor nanocrystals (NCs) were pioneered by Banin et al, where the seminal NR systems were terminally tipped with either Au or Pt nanoparticles. [36][37][38][39] More recently, CdSe seeded CdS (CdSe@CdS) TPs with noble metal NP tips have been developed, in particular with high selectivity to enable photodeposition of a single AuNP tip on the end of one arm of each tetrapod.…”

Electrogenerated Chemiluminescence of Near-Infrared-Emitting CdSe@CdSe Nanorods in Aqueous Solution

“…Characterization of CdSe@CdSe NRs.⎯ CdSe@CdSe NRs (length (L) ≈ 40 nm; D ≈ 9-10 nm) 17 As described earlier, ideal QDs will produce a narrow and sharp PL spectra and the QDs band gap width can be deduced from the central PL wavelength position in the absence of surface defects. On the other hand, non-ideal QDs will produce a broad, asymmetric, and red-shifted PL peak with a nonzero background tail attributed to their surface defects; these surface energetic states can trap the excited electrons/holes that migrate from the conduction band /valence band, resulting in the surface-state-related e−h recombination and transition (emission) 21,22 .…”

“…⎯ All reagents were of analytical grade, and the aqueous solutions were prepared with ultrapure water (Millipore Milli-Q water, 18.2 MΩ cm). CdSe@CdSe NRs were synthesized as described previously 17 . CdSe nanoparticles (NPs) were synthesized and used to seed the growth of CdSe@CdSe NRs based on a modified literature procedure for CdSe@CdSe NRs 18 .…”