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Noise Microscopy for Direct Mapping of Noise Sources and Materials Properties

Noise Microscopy Setup on a Graphene Device
(ACS Nano 10 10135 2016)

   Electrical current noises are generated by various sources such as charge traps and structural defects in electrical channels. Electrical noises often determine the reliability and performance of electronic devices and, thus, can be a critical issue in electronics applications. However, until now, electrical noises have been studied mostly by measuring noise spectra from multiple devices, and direct imaging of noise sources has been a very difficult, if not impossible, task. Our unique imaging system named as ¡°noise microscopy¡± enables nanoscale resolution measurements and imaging of electrical noise sources (e.g. charge traps) in electronic materials and devices. The noise microscopy is a combined system of a conducting atomic force microscopy (cAFM), a current-noise analyzer, and noise analysis software. For the noise microscopy measurement, a sharp conducting nano-probe makes a direct contact with a specific location on an electronic channel and measures local electrical currents and noises at the location. Further, by scanning the probe over the channel surface, we can obtain current and noise images showing the distribution of electrical currents and noises on the scanned area with a nanoscale resolution. Remarkably, by analyzing the current and noise maps, we can estimate quantitative distributions of resistivities and noise sources in electronic channels and study their properties with a nanoscale resolution. Noise microscopy is quite a unique and powerful tool to directly image the nanoscale distribution of materials properties (e.g. resistivity, noise source density, mobility) in an electronic channel with a nanoscale resolution.

  Following are a few examples of noise microscopy results.

 1) Nanoscale Imaging of Charge Trap Densities and Sheet Resistances in Graphene Domains

Noise Microscopy on Graphene
(ACS Nano 10 10135 2016)

  Using noise microscopy, we could map the nanoscale distributions of charge traps and sheet resistances in the domain structures of single-layer graphene. The results show high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. In double-layer graphene, we found that the bottom layer screens the activities of charge traps in the underlying substrate and, as a result, the top layer had lower electrical noises than single-layer graphene.

 2) Imaging of Electrical Fluctuations in Molecular Wire Junctions

Noise Microscopy on Molecular Wire Junctions
(Sci. Rep. 7 43411 2017)

  Molecular wires are nanoscale chains that conduct electric currents. They are the building blocks of molecular electronic devices and, thus, the investigation of their basic properties is essential for the further advance of molecular electronics. The noise microscopy could be utilized to analyze noises in molecular wire junctions. Here, different molecular wires were patterned on a gold substrate, and the current-noise map on the pattern was obtained via the noise microscopy. We found that the power spectral density of current noises from nanoscale molecular-wire junctions exhibited a unique 1/f2 behavior in frequency spectra unlike common nanoscale conducting channels exhibiting 1/f behaviors, which can be utilized to identify the electrical signals from molecular wires. By measuring molecular junctions comprising different thiol molecules on a gold substrate, we revealed that electrical noises in a molecular-wire system were mainly generated by the random fluctuations of thiol bonds between molecules and metal electrodes. Further, we quantitatively compared the frequencies of such bond fluctuations in different insulating and conducting molecular wires and identified molecular species generating lower electrical noises, which can provide valuable information for designing low-noise molecular electronic devices.

3) Trap Density Imaging on Domain Structures of Polymer

Noise Microscopy on Polymer Film
(Nanoscale 8 835 2016)

  Conducting polymers are current-carrying macromolecules composed of many repeated subunits. They have been drawing much attention as building blocks of flexible organic electronic devices. Electrical-noise sources in the conducting polymers were reported to seriously degrade their conduction characteristics, and thus the investigation and engineering of the noise sources should be essential to improve conducting polymer devices. We utilized the noise microscopy to study the localized noise-source activities and the generation of new noise sources induced by external-stimuli in poly(9,9-dioctylfluorene) (PFO) polymer films. The PFO has domain structures comprised of ordered (conducting) and disordered (insulating) phase regions. By analyzing the current and noise imaging data, resistivities and noise source densities in individual domains in PFO could be mapped. Interestingly, a larger number of noise-sources were observed in the disordered-phase-regions than in the ordered-phase-regions in the polymer film, due to structural disordering. Further, increased bias-voltages on the disordered regions increased noise sources more than on the ordered regions. On a photo-illumination, the ordered-phase-regions exhibited a rather large increase in the conductivity and noise source density. Presumably, the lights stimulated the release of charge carriers from deep-traps for the enhanced conductivity, and the remaining deep traps worked as additional noise sources.

4) Photoconductive Noise Microscopy on a Perovskite Solar Cell

Photoconductive Noise Microscopy on a Perovskite Solar Cell
(Nano Energy 43 29 2018)

  Perovskite such as a methylammonium lead iodide is a promising light-harvesting material. In the past few years, solar cells based on perovskites have progressed rapidly, reaching remarkable power conversion efficiencies of ~20%. Electrical-noise sources in perovskites were reported to significantly affect their photoconduction characteristics, and thus the investigation and engineering of the noise sources should be essential for the further improvement of perovskite-based devices. We developed a ¡°photoconductive noise microscopy¡± for the nanoscale imaging of electronic charge traps distributed on a perovskite film in a solar cell structure. The method enabled quantitative imaging of trap densities along with local photocurrents on the perovskite film. By analyzing the imaging data, we could reveal quantitative correlations between the trap distribution and local photocurrents. The results show that the spatial density of the charge traps has a power-law relationship with the short-circuit currents during a solar cell operation as well as localized photocurrents under a sample bias, indicating that a charge trap distribution in a perovskite film can be a major factor determining the performance of the perovskite-based solar cells.