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Impact of PHI XPS instruments on scientific discovery

PHI's innovative XPS technologies have empowered researchers with unique tools to address intricate materials challenges and expedite the development of novel materials and products. In this spotlight article, we underscore the profound impact of PHI XPS instruments – VersaProbe, Quantera, and Quantes – on discoveries and scientific advancements in research throughout 2023.

The use of PHI XPS instruments contributed to 4180 publications, 43 of which found their place in flagship journals published by Nature and Science/AAAS. These publications have made significant contributions to diverse areas such as optoelectronics, batteries, catalysis, semiconductors, microfluidics, quantum dots, bio/sustainable materials, CO₂ capture/utilization/mitigation, cold-plasma applications, and soil remediation. This spotlight highlights a few of these publications, shedding light on the pivotal role played by PHI XPS instruments in advancing research and development.

The latest research by Professor Yoshida’s research group, published in J. Phys. Chem. Lett., involves a systematic comparison of exciton binding energy in organic semiconductors, including 42 organic solar cell materials. Exciton binding energy is a crucial factor that influences the performance of optoelectronic devices but lacks precise experimental data in organic semiconductors. The study reveals that the exciton binding energy is consistently one-quarter of the transport gap across all materials, which can be calculated directly from ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS) measurements. UPS measurements were conducted using PHI 5000 VersaProbe II, and the LEIPS measurements were done on a standalone instrument. The unique LEIPS technology can also be installed alongside UPS on a PHI Genesis XPS instrument.

The most effective way to find exciton binding energy (ΔE_EX) involves measuring the energy difference between the free carrier state and the exciton. Optical measurements like photoabsorption (PA) and photoluminescence (PL) experiments identify the exciton energy as the first excitation state (or the optical bandgap E_G^opt). The energy of the free electron state (or the transport gap E_G^trans) is calculated by finding the difference between ionization energy IE (also known as HOMO energy) and electron affinity EA (LUMO energy) using Equation 1. LEIPS measures EA with precision > 0.05 eV, enabling determination of ΔE_EX in organic semiconductors materials with a precision of 0.1 eV, surpassing the accuracy of previous studies.

Equation 1: ΔE_EX = E_G^trans - E_G^opt = (IE - EA) - E_G^opt     

Figures 1a and 1b display the UPS and LEIPS spectra of indacenodithiophene (IDT) derivatives known as ITIC. IE and EA are determined based on the onset energy with respect to the vacuum level E_VL. Statistical distributions reveal that the average EA for ITIC is determined to be 3.75 ± 0.05 eV. The same analysis is applied to the other materials. Derived from the modified hydrogen atom model for organic semiconductors, considering quantized energy levels within a Coulomb potential, equation 2 demonstrates remarkable agreement with the experimental data, consistently showing exciton binding energy as one-quarter of the transport gap across all materials.

Equation 2: ΔE_EX ≈ E_G^trans/4 ≈ E_G^opt/3

Figure 1: (a) UPS and (b) LEIPS spectra of ITIC.

This latest publication in Nature Communications utilizes XPS to study a strategy to minimize performance degradation of α-MoO₃, a promising electrode material for solid-state lithium-based batteries. The strategy utilizes η-Mo₄O₁₁ that forms a pinning interface with α-MoO₃ to suppress lattice expansion and irreversible phase transformation due to Li⁺ ion insertion. The designed heterostructure exhibits robust stability, retaining about 81% battery capacity after 3000 cycles. PHI Quantera was instrumental in monitoring the evolution of chemical states in the MoO₃/η-Mo₄O₁₁ heterostructure to assess its electrochemical reversibility.

The XPS analysis of α-MoO3/η-Mo4O11 indicates a notable increase in the Mo5+/Mo6+ ratio, confirming the formation of a heterostructure given that half of the Mo atoms in η-Mo4O11 are in the 5+ oxidation state. As illustrated in Figure 2, following complete delithiation at an applied voltage, the α-MoO3/η-Mo4O11 heterostructure exhibits only Mo6+ and Mo5+, while the pure α-MoO3 shows ~18% Mo4+ in addition to Mo6+ and Mo5+. This observation implies that the pure α-MoO3 undergoes a more pronounced initial irreversible phase transition probably due to the trapping of a higher proportion of inactive Li+ ions.

Figure 2: Mo 3d XPS spectra of α-MoO3 and α-MoO3/η-Mo4O11 at the delithiated state (3.5 V vs. Li/Li+).

A recent Scientific Reports article discusses a novel composite catalyst system comprising highly defective graphene quantum dots (HDGQDs)-doped 1T/2H-MoS₂ for efficient hydrogen evolution reactions (HER). The catalyst's high electrocatalytic activity arises from enhanced conductivity, many active sites in 1T-MoS₂, and improved charge transfer due to additional defects introduced by HDGQDs. This enhanced charge transfer improves MoS₂'s efficiency in adsorbing protons, significantly reducing charge transfer resistance (R_CT) in the catalyzed HER.

XPS analysis using PHI Quantera II was employed to estimate the phase compositions in 1T/2H-MoS₂ and further investigate GQDs-to-MoS₂ charge transfer in 1T/2H-MoS₂/HDGQDs. The analysis confirms the presence of 1T-MoS₂, 2H-MoS₂, and MoO₃ in the 1T/2H-MoS₂ sample as shown in Figure 3a. Mo exhibits three chemical states, with the Mo 3d XPS spectra revealing six peaks assigned to Mo⁴⁺ of 1T/2H-MoS₂ (228.3 and 231.5 eV), Mo⁴⁺ of 2H-MoS₂ (229.3 and 232.5 eV), and Mo⁶⁺ of MoO₃ impurity (231.6 and 234.8 eV). The S 2s XPS peak for MoS₂ at 225.61 eV is also observed. The fitted S 2p XPS peak (Figure 3b) reveals two chemical states corresponding to the 1T-MoS₂ phase (161.1 and 162.2 eV) and those for the 2H-MoS₂ phase (162.1 and 163.2 eV).

Figure 3: (a) Mo 3d and (b) S 2p XPS spectra of 1T/2H-MoS₂.

Comparative analysis of Mo 3d and S 2p spectra between 1T/2H-MoS₂ and 1T/2H-MoS₂/HDGQDs confirms successful charge transfer, evidenced by lowered peak positions (Mo 3d by 0.73 eV and S 2p by 0.48 eV) compared to those for 1T/2H-MoS₂, indicating charge transfer from HDGQDs to MoS₂.

In conclusion, the impact of PHI XPS instruments in 2023 is evident through their extensive contributions to scientific publications across diverse fields. PHI's pivotal role in the highlighted studies underscores its commitment to advancing research and development, reinforcing its position at the forefront of innovation.