Characterization of semiconductor nanocrystals using advanced solid-state NMR spectroscopy

Abstract

Semiconductor nanomaterials have attracted significant interest in various areas during the past several decades. Due to excellent size-tunable optical and electronic properties, they become ideal building blocks in photovoltaics, photocatalysis, light emitting, and biological imaging. Numerous studies have focused on “precise” control of the colloidal synthesis of semiconductor nanomaterials over surface ligands, composition, size, and morphology, as well as integrated them into devices. However, to rational design semiconductor nanomaterials, which advances the science and applications of nanomaterials, analytical techniques capable of probing surfaces, defects, and atomic scale disorder are required. Semiconductor nanomaterials are highly dynamic and reactive species, and their surfaces are the key parameters responsible for their optoelectronic properties due to high surface-to-volume ratios. Sadly, it is very challenging to obtain overall structural information from traditional structural analysis methods, such as microscopy (SEM, TEM, AFM, etc.), spectroscopy (IR, Absorbance, XPS, etc.), and powder XRD; and surface chemistry of semiconductor nanomaterials remains largely unexplained. In this dissertation, insights into the surface and the overall structure of semiconductor nanomaterials were provided using advanced nuclear magnetic resonance (NMR) spectroscopy methods. In general, NMR is a powerful technique for structure determination because NMR probes the local chemical environment by measuring chemical shifts or electric field gradients (EFG) and enables the connectivity/proximity of different chemical sites to be ascertained via scalar couplings or dipolar couplings. Most of the elements found in main-group inorganic semiconductors have NMR-active nuclei, potentially making solid-state NMR an ideal tool for probing the structure of semiconductor nanomaterials. However, NMR is an inherently insensitive technique due to the small Zeeman-splitting of spin energy states. This issue is further exacerbated by exotic and unreceptive isotopes found in semiconductor nanomaterials, including high-Z spin-½ nuclei with large chemical shift anisotropy (CSA), low-gamma spin-1/2, and half-integer quadrupolar nuclei, with/without low natural abundance. Therefore, developing sensitivity-enhanced solid-state NMR methods, including correlation methods, that enable the specific detection of dilute defects or interfacial sites and unreceptive NMR-active nuclei, is a must. Dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) NMR spectroscopy has been applied to greatly enhance the NMR signal by a factor of 10 to 100, corresponding to a shortening of the experiment time by 2 – 4 orders of magnitude. This should enable the rapid acquisition of NMR spectra of semiconductor nanomaterials, which was considered infeasible in previous studies. But before applying it to semiconductor nanomaterials, a formulation method that not only inhibited the aggregation of nanomaterials under experimental conditions but also had favorable dielectric properties and preserved the high concentration of nanomaterials had to be developed. To this end, we discovered that the powder impregnation procedure of h-BN with colloidal nanomaterials provides a factor of 9 improvements in NMR sensitivity in comparison to previously established DNP SENS procedures, enabling challenging homonuclear and heteronuclear 2D NMR experiments on CdS, Si, Cd3P2 and CdSe nanomaterials. These experiments allow NMR signals from the surface, subsurface, and core sites to be observed and assigned. With knowledge of surface atoms coordination number and two spins proximity and/or connectivity, NMR has been proven to shed light on semiconductor nanomaterials surface species and core structures. Currently, the most studied semiconductor nanocrystals (NCs) systems are the recently developed, highly luminescent lead-halide perovskite (LHP) NCs. In specific, highly luminescent colloidal CsPbBr3 NCs caught our attention due to their bright (QY = 50–90%), stable, spectrally narrow, and broadly tunable photoluminescence. We made use of all NMR-active isotopes in CsPbBr3 NCs to determine the atomic termination of the NCs, as well as the binding sites and modes of cationic (i.e., alkylammonium) and anionic ligands (i.e., oleate) at the surface via solid-state NMR spectroscopy. Besides, we employed solid-state NMR spectroscopy to probe the correlations between surface structure alteration and photoluminescence recovery with post-synthesis surface treatments of didodecyldimethylammonium bromide (DDAB) combined with/without cesium bromide (CsBr) or lead bromide (PbBr2). These findings in this study could help complete the puzzle of understanding LHP NCs by shedding light on recombination at the grain boundaries. Currently, a molten salt In-to-Ga cation exchange to prepare In1-xGaxP NCs and subsequent ZnS shelling protocol have achieved high radiative efficiency. To be more specific, bright In1-xGaxP/ZnS core-shell particles show decent emission color purity with PLQY exceeding 80% and PL emission linewidths as narrow as 50 nm FWHM. Here we utilized all NMR-active isotopes in In1-xGaxP NCs to offer insights into the geometry of the chemical surroundings of a spin within NCs, as well as the number, distribution, and nature of the species present in NCs. In summary, the overarching goal of this work is to advance the science of nanomaterials by using sensitivity-enhanced solid-state NMR spectroscopy to reveal the interplays between the properties of nanomaterials and the structures of their bulk, interfaces, and defects. As showcased above, our results highlight the utility and prospects of surface characterization of nanomaterials via solid-state NMR spectroscopy.