Materials characterization by x-rays requires a large number of atoms and reducing the material quantity for measurements is a long-standing goal. To date, attogram amount of sample can be detected by x-rays; however, this is still in the range of 10,000 atoms or more and gaining access to a much smaller samples is becoming extremely arduous.
Synchrotron x-ray scanning tunneling microscopy (SX-STM) combines the chemical contrast of synchrotron x-rays with the locality of STM. In this presentation, we show that x-rays can be used to characterize the elemental and chemical state of just one atom. Using a specialized tip as a detector, x-ray excited currents generated from an iron and a terbium atom coordinated to organic ligands are detected. The fingerprints of a single atom, the L2,3 and M4,5 absorption edge signals for iron and terbium respectively, are clearly observed in x-ray absorption spectra. X-ray excited resonance tunnelling is dominant for the iron atom. The x-ray signal can be sensed only when the tip detector is located directly above the atom in extreme proximity, which confirms atomically localized detection in the tunnelling regime. Our work connects synchrotron x-rays with a quantum tunnelling process and opens future x-rays experiments for simultaneous characterizations of elemental, and chemical properties of materials at the ultimate single atom limit.
This work was performed at the Advanced Photon Source and the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357.
In situ multimodal microscopic x-ray characterizations demonstrate their unique capabilities in revealing the mechanisms of material degradation and the pathways for mitigation in energy harvesting applications such as halide perovskite solar cells. Despite the excellent device performance exhibited by halide perovskites, their sensitive nature and material interfaces necessitate a precisely controlled and tunable characterization environment to identify the sources of device performance loss. In this work, we designed an in-situ sample chamber that allows the control of various environmental conditions, including heat, illumination, and bias, while simultaneously collecting chemical (X-ray fluorescence, XRF), optical (X-ray Excited Optical Luminescence, XEOL), and performance (X-ray Beam Induced Current, XBIC) measurements on functional devices. The integrated thermoelectric cooler module of the designed chamber enables controlled heating up to 100 °C and rapid cooling back down to room temperature. This allows simultaneous multimodal XRF, XEOL and XBIC signal collections on Cs0.05FA0.95PbI3 perovskite devices at various temperatures. The results show increasing homogeneity in the XBIC maps and continuous reduction in XEOL intensity, with a redshift in XEOL peak positions as sample temperatures increase. The results of the simultaneous multimodal study pave the way for improved in situ sample environments for future photovoltaic device characterizations.
Singlet fission can split a high energy singlet exciton and generate two lower energy triplet excitons. This process has shown near 200 percent triplet exciton yield. Sensitizing solar cells with singlet fission material, it can potentially increase the power conversion efficiency limit from 29 percent to 35 percent. Singlet fission in the tetracene is known to be efficient, and the energy of the triplet excitons are energetically matched to the silicon bandgap. In this work, we designed an optical measurement with an external magnetic field to determine the efficiencies of triplet exciton transfer from tetracene to silicon. Using this method, we have found that a passivation layer of 8 angstroms of hafnium oxynitride on silicon allows efficient triplet exciton transfer around 133 percent.
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