Bandgap Engineering in OH‐Functionalized Silicon Nanocrystals: Interplay between Surface Functionalization and Quantum Confinement

Abstract

In this work, a systematic first-principles study of the quasi-band structure of silicon nanocrystals (Si-NCs) is provided, focusing on bandgap engineering by combining quantum confinement of the electronic states with OH surfacefunctionalization. A mapping between the bandgap, Si-NC diameter, and the degree of hydroxide coverage is provided, which can be used as a guideline for bandgap engineering. Complementary to first-principles calculations, the photoluminescence (PL) wavelength of Si-NCs in the quantum-confinement regime is measured with well-defined diameters between 1 and 4 nm. The Si-NCs are prepared by means of a microplasma technique, which allows a surfactant-free engineering of the Si-NCs surface with OH groups. The microplasma treatment technique allows us to gradually change the degree of OH coverage, enabling us, in turn, to gradually shift the emitted light in the PL spectra by up to 100 nm to longer wavelengths. The first-principles calculations are consistent with the experimentally observed dependence of the wavelengths on the OH coverage and show that the PL redshift is determined by the charge transfer between the Si-NC and the functional groups, while on the other hand surface strain plays only a minor part.

Tuning the Bandgap Character of Quantum‐Confined Si–Sn Alloyed Nanocrystals

“…Increasing the NC diameter decreases the degree of quantum confinement and hence the bandgap decreases. [ 7 ] For Sn NCs we observe a direct bandgap at Γ 1 ‐point (Γ −1 ‐point). Both the indirect bandgap for Si and the direct bandgap for α‐Sn are consistent with what our experimental findings suggest.…”

“…With increasing system size the discrete energy levels become more and more dense and band‐like structures start to emerge which are additionally broadened, in particular, by finite temperature which leads to the formation of band‐like electronic states. [ 7 ] In the following we will not further discriminate between the term quasi‐band structure and band structure and stick for simplicity to band structure even though that this is strictly speaking not completely accurate.…”

“…The corresponding NCs and band structures are summarized in Figure 1f–h. For 12% and 17% Sn the bandgap remains clearly indirect and the band structure becomes somewhat more “fuzzy” due to the induced strain, [ 7 ] but remains overall comparable to that of the pristine Si NC with additional bands starting to emerge around the Γ 1 ‐point (Γ −1 ‐point). With increasing Sn concentration these additional bands become more and more pronounced and shift toward lower energies, while the rest of the bands remain roughly constant in energy.…”

“…Nanocrystals (NC) in the quantum confinement regime bridge this gap between the band structure of bulk materials and discrete energy level in molecules, combining the best of both worlds. That is, the band structure description remains approximate valid and we gain a new degree of freedom namely the finite NC surface which can be controlled chemically by, e.g., introducing organic ligands for surface functionalization [ 5,6 ] or controlling the energy level alignment by chemical doping, [ 7 ] moreover the quantum confinement itself provides an efficient means to tailor the bandgap. [ 2,7,8 ] Especially quantum confined silicon nanocrystals have attracted significant interest in the past and synthesis, tailoring absorption and luminescence, and device integration have been extensively addressed.…”

“…However, for systems such as the NCs discussed here, which are in the intermediate regime between molecules with discrete energy levels and bulk with continuous energy bands, it is possible to obtain an approximate band structure in terms of quasibands that include surface and quantum confinements effects but still allows an intuitive interpretation following the traditional band structure theory. [ 7,25 ] Moreover the quasi‐band structure is closely related to angle‐resolved photoemission spectroscopy (ARPES) of electronic states, which can be used for molecular systems to image and determine the energy of molecular wavefunctions [ 26,27 ] as well as probing the band structure of bulk and thin‐film materials. [ 28,29 ]…”