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X-ray spectroscopy unveils the electronic structure of wide bandgap semiconductors

30-01-2025

In situ “photon-in photon-out” X-ray spectroscopy at beamline ID26, combined with density functional theory calculations, has provided direct insights into the transformation of the highest occupied molecular orbital and lowest unoccupied molecular orbital in zinc molecular precursors, as well as the emergence of the electronic structure of zinc-sulfide nanorods in solution and at elevated temperatures.

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Understanding the processes that occur during the synthesis of nanoparticles in solution at atomic and molecular levels is crucial for advancing the design and optimization of nanomaterials with tailored properties for various applications.

X-ray spectroscopic and scattering methods, in particular, are powerful tools for probing numerous processes across different length scales and for revealing reaction pathways [1]. However, tracking the electronic structure during the synthesis and growth of wide-bandgap nanomaterials has rarely been reported.

Zinc sulfide (ZnS), one of the first semiconductors to be discovered, serves as an ideal model for such studies. With its large bandgap values at room temperature (3.54 eV in cubic form known as sphalerite, and 3.91 eV in hexagonal form, known as wurtzite), ZnS is a key material for various applications, such as optoelectronics, photocatalysis, and nanostructure protective shells. 

In this study, the reaction pathway from zinc(II) acetate (Zn(Ac)2) and elemental sulfur in oleylamine to the formation of ZnS nanorods was investigated using in situ X-ray spectroscopy and scattering techniques. High-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) and valence-to-core X-ray emission spectroscopy (vtc-XES) measurements performed at beamline ID26 were combined to study the electronic structure during the synthesis and growth of ZnS nanorods at elevated temperatures, which are conditions not readily accessible through other techniques.

Focusing on the Zn K-edge, HERFD-XAS, and vtc-XES measurements, combined with density functional theory (DFT) calculations, proved to be a powerful approach to resolve the structure of molecular clusters [2]. This methodology revealed the starting point of the reaction (Figure 1a).
 


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Fig. 1: a) Upon dissolving Zn(Ac)2 in oleylamine (OA), tetrahedral [Zn(OA)4]2+ and octahedral [Zn(OA)6]2+ complexes are formed. Addition of elemental sulfur leads to conversion into the tetrahedral [Zn(SOA)4]2+ complex. b) Experimental HERFD-XAS and vtc-XES spectra of Zn(Ac)2 (black line), Zn(Ac)2 dissolved in OA (light blue solid line), corresponding to the mixture of [Zn(OA)4]2+ and [Zn(OA)6]2+, and Zn(Ac)2 dissolved in OA with sulfur (dark blue solid line, assigned to the [Zn(SOA)4]2+ complex). c) Simulated XAS and vtc-XES spectra calculated with DFT.


A comparison of experimental and simulated spectra (as seen in Figure 1b and 1c) suggests that Zn(Ac)2 dissolves in oleylamine through the replacement of acetate ligands by oleylamine molecules. This ligand exchange, involving a change from Zn-O to Zn-N coordination, is evidenced by a 0.8 eV shift to lower energies in the XAS spectra and a 0.6 eV shift to higher energies in the Kβ2,5 peak of the vtc-XES spectra. These spectral changes support the formation of a mixture of tetrahedral [Zn(OA)4]2+ and octahedral [Zn(OA)6]2+ complexes, where OA represents oleylamine, as illustrated in Figures 1b and 1c.

Notably, the addition of elemental sulfur to the mixture induced a 0.6 eV shift of the E0 to lower energies in the HERFD-XAS spectrum, while the Kβ2,5 peak shifted by 0.6 eV to higher energies. These observations, combined with DFT simulations (Figure 1b and 1c), indicate a change in the coordination of the Zn2+ involving sulfur atoms of the thioamide derivative of oleylamine (SOA = oleylthioamide), resulting in the tetrahedral [Zn(SOA)4]2+ complex. These findings highlight the capability of high-energy photon-in photon-out spectroscopies to identify the formation of the thioamide derivative from the reaction of oleylamine and sulfur in the presence of Zn2+ ions at room temperature – a process previously observed only at elevated temperatures [3]

After identifying the starting point of the reaction, the synthesis from the initial complex [Zn(SOA)4]2+ to ZnS at 155°C was monitored by in situ vtc-XES and HERFD-XAS at the Zn-K edge, as shown in Figure 2a. HERFD-XAS data indicate that Zn atoms maintain a tetrahedral coordination throughout the reaction and reveal the emergence of ZnS in both wurtzite (w-ZnS) and sphalerite (s-ZnS) phases. Full conversion of the starting complex [Zn(SOA)4]2+ into w-ZnS occurs at 110°C, coinciding with the initial formation of the s-ZnS phase. The two phases persist until the reaction concludes, yielding a final composition of 60% w-ZnS and 40% s-ZnS. These findings were supported by complementary in situ powder X-ray diffraction (PXRD) measurements performed at PETRA III-DESY in Germany. 
 


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Fig. 2: a) In situ vtc-XES and HERFD-XAS spectra at the Zn-K edge during the synthesis of ZnS. b) Multivariate curve resolution-alternating least-squares analysis of the in situ HERFD-XAS data reveals three contributions: the initial [Zn(SOA)4]2+ complex, ZnS in the wurtzite (w-ZnS) phase, and ZnS in the sphalerite (s-ZnS) phase. c) Schematic representation of interstate transitions under non-resonant (left) and resonant (right) excitation, illustrating the bandgap calculations. d) The difference between the minimum of the derivative of the XES signal and the maximum of the derivative of the XAS signal defines the HOMO/LUMO gap during the preparation of s-ZnS. e) Comparison of the HOMO/LUMO gap values obtained from in situ XAS/XES analysis during the reaction in solution and the HOMO/LUMO optical gap values from ex situ UV–Visible light spectroscopy analysis of unwashed aliquots.


By combining vtc-XES data with in situ HERFD-XAS, the evolution of the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) during the reaction from the [Zn(SOA)4]2+ complex to ZnS was tracked. HERFD-XAS probed transitions from the 1s core state to the unoccupied 4p states (1s → 4p, corresponding to the LUMO), while vtc-XES examined electronic transitions from the valence band (occupied states), primarily 3p orbitals, into the 1s core hole of Zn (3p →1s, reflecting the HOMO).

Figure 2c illustrates the complete energy diagram for photon-in (Ω) and photon-out (ω) spectroscopy, including the ground, intermediate, and final states at the Zn K-edge. The HOMO/LUMO gap at each reaction stage was calculated as the difference between the minimum of the derivative of the XES data and the maximum of the derivative of the HERFD-XAS data (Figure 2d). 

As shown in Figure 2e, the HOMO-LUMO gap decreases as the reaction progresses, starting from 5.0 eV for the [Zn(SOA)4]2+ complex and dropping to 4.3 eV when w-ZnS forms. This relatively high bandgap value for the wurtzite phase suggests very small crystallite sizes due to carrier quantum confinement effects, with a diameter of approximately 2.1 nm. Ultimately, the electronic bandgap reduces further to 3.8 eV upon the initial formation of s-ZnS.  

In summary, HERFD-XAS, vtc-XES, and DFT calculations provided a comprehensive understanding of the coordination chemistry and the structural and electronic transformations taking place during the synthesis of ZnS nanocrystals. This work represents the first instance of in situ monitoring of the electronic structure during the synthesis of a nanoscale semiconductor. Notably, collecting in situ vtc-XES data with relevant time scales (approximately 10 minutes) during the synthesis of ZnS was made possible by the advanced capabilities of the ESRF’s Extremely Brilliant Source.

This novel approach opens up possibilities for investigating other semiconducting nanomaterials during their formation and growth in solution, where alternative methods are limited and element-specific techniques are required.


Principal publication and authors
Utilizing High X‑ray Energy Photon-In Photon-Out Spectroscopies and X‑ray Scattering to Experimentally Assess the Emergence of Electronic and Atomic Structure of ZnS Nanorods, L. Klemeyer (a), T.L.R. Gröne (a), C.A. Zito (a), O. Vasylieva (a), M.G. Akcaalan (a), S.Y. Harouna-Mayer (a,b), F. Caddeo (a), T. Steenbock (c), S.A. Hussak (a), J. Kopula Kesavan (a), A.C. Dippel (d), X. Sun (d,e), A. Köppen (c), V.A. Saveleva (f), S. Kumar (c), G. Bester (b,c), P. Glatzel (f), D. Koziej (a,b), J. Am. Chem. Soc. 146(49), 33475-33484 (2024); https://doi.org/10.1021/jacs.4c10257  
(a) Institute for Nanostructure and Solid-State Physics, University of Hamburg, Hamburg (Germany)
(b) The Hamburg Centre for Ultrafast Imaging, Hamburg (Germany) 
(c) Department of Chemistry, University of Hamburg, Hamburg (Germany)
(d) DESY, Hamburg (Germany)
(e) Institute of Integrated Natural Science, University of Koblenz, Koblenz (Germany)
(f) ESRF 


References
[1] L. Grote et al., Nat. Commun. 12, 4429 (2021).
[2] O.M. Stepanic et al., Inorg. Chem. 59(18), 13551-13560 (2020).
[3] J.W. Thomson et al., J. Am. Chem. Soc. 133(13), 5036-5041 (2011).

 

About the beamline: ID26

ID26 is designed for X-ray absorption and emission spectroscopy of complex systems in the tender and hard X-ray range. The high-brilliance X-ray beam allows for spectroscopic studies of samples with low analyte concentration and challenging matrices. X-ray emission spectroscopy is performed by means of crystal analyser spectrometers. By combining a tuneable incident energy with an emission spectrometer, it is possible to take advantage of resonance effects that can provide detailed information on the electronic structure.

The local coordination and electronic structure of an X-ray-absorbing atom are studied by extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) spectroscopy. The techniques probe occupied and unoccupied electron orbitals, providing a wealth of information. It is thus possible to study orbital splittings, spin and oxidation states, as well as the coordination symmetry and ligand type. RIXS gives access to element-specific excitations of only a few eV that may arise from local (e.g., d-d), nearest-neighbour (e.g., charge transfer), and collective excitations.

With the tender and hard X-ray probe, very few restrictions apply to the sample environment. ID26 can host cryostats and cells for in situ and operando studies to carry out experiments in applied sciences including coordination chemistry, (bio)catalysis, materials science, electro-chemistry and environmental sciences.