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A nanoprobe is utilized to map surface- and edge-state conductivity and traps of a topological insulator at nanoscale resolutions. The surface- and edge-states exhibit different transport behavior using external stimuli such as light and electric field.

Herein, underlying factors for enabling efficient and stable performance of perovskite solar cells are studied through nanostructural controls of organic-inorganic halide perovskites. Namely, MAPbI3, (FA0.83MA0.17)Pb(I0.83Br0.17)3, and (Cs0.10FA0.75MA0.15)Pb(I0.85Br0.15)3 perovskites (abbreviated as MA, FAMA, and CsFAMA, respectively) are examined with a grain growth control through thermal annealing. FAMA- and CsFAMA-based cells result in stable photovoltaic performance, while MA cells are sensitively dependent on the perovskite grain size dominated by annealing time. Micro-/nanoscopic features are comprehensively analyzed to unravel the origin that is directly correlated to the cell performance with the applications of electronic-trap characterizations such as photoconductive noise microscopy and capacitance analyses. It is revealed that CsFAMA has a lower trap density compared to MA and FAMA through the analyses of 1/ f noises and trapping/detrapping capacitances. Also, an open-circuit voltage ( Voc) change is correlated to the variation of trap states during the shelf-life test: FAMA and CsFAMA cells with the negligible change of Voc over weeks exhibit trap states shifting toward the band edge, although the power-conversion efficiencies are clearly reduced. The origins that critically affect the solar cell performance through the characterizations of shallow/deep traps with additional mobile defects in the perovskite and interfaces are discussed.

We developed a “photoconductive noise microscopy” method to directly image electronic charge traps distributed on a methylammonium lead iodide perovskite film in a solar cell device. The method enabled quantitative imaging of trap densities along with local photocurrents on the solar cell 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.

We report a nanoscale mapping of noise-source-controlled transport characteristics in the domains of reduced graphene oxide by utilizing noise-source imaging strategies. In this method, current and noise images were measured simultaneously using a scanning noise microscopy and analyzed to map sheet−resistances (R□) and noise−source densities (neff). The maps showed the formation of conducting and insulating domains, where the insulating domains exhibited up to three-four orders of higher R□ and neff than those of conducting domains. Interestingly, the sheet−conductance (Σ□) and neff followed rather opposite power−law behaviors like Σ □∝ neff−0.5 and Σ□ ∝ neff0.5 in conducting and insulating domains, respectively, which could be attributed to the difference in mesoscopic charge transport mechanisms controlled by neff in domains. Notably, high biases resulted in the increased conductance (ΔΣ□) and decreased noise−source density (Δneff) following a relationship like ΔΣ□ ∝−Δneff0.5 for both conducting and insulting domains, which could be explained by the passivation of noise−sources at high biases. Furthermore, ΔΣ□ versus Δneff plot on the annealing also followed a power−law dependence (ΔΣ□ ∝−Δneff0.5) in conducting domains, which could be attributed to carrier generation on the annealing. Our results about mesoscopic charge transports could be significant advancements in fundamental researches and applications.

Organic–inorganic hybrid perovskite solar cells have emerged as promising candidates for next-generation solar cells. To attain high photovoltaic efficiency, reducing the defects in perovskites is crucial along with a uniform coating of the films. Also, evaluating the quality of synthesized perovskites via facile and adequate methods is important as well. Herein, CH3NH3PbI3 perovskites were synthesized by applying second solvent dripping to nonstoichiometric precursors containing excess CH3NH3I. The resulting perovskite films exhibited a larger average grain size with a better crystallinity compared to that from stoichiometric precursors. As a result, the performance of planar perovskite solar cells was significantly improved, achieving an efficiency of 14.3%. Furthermore, perovskite films were effectively analyzed using a conductive AFM and noise spectroscopy, which have been uncommon in the field of perovskite solar cells. Comparing the topography and photocurrent maps, the variation of photocurrents in...

An organic-inorganic hybrid perovskite is considered as a next generation solar energy harvester due to the high power conversion efficiency. The starting precursor solution for the organolead halide perovskite is of significant interests because the ionic components in the precursor can critically affect the nanostructures and thereby the optoelectronic properties. In this work, the basic and well-known precursor solution for CH3NH3PbI3(Cl) comprised of CH3NH3I and PbCl2, is specifically analyzed to unravel the phenomena in the Cl-mediated solutions. The shift in equilibrium between lead-halide complex and the solvent results into the CH3NH3PbI3(Cl) grain evolution with Cl incorporation, which is confirmed through x-ray fluorescence and diffraction. The effects of Cl on the optoelectronic properties are further verified by conductive atomic force microscopy, and the existing Cl leads to the 30-times-increased and inhomogeneously distributed photocurrent for CH3NH3PbI3(Cl) grains compared with CH3NH3PbI3. Moreover, photocurrent noise from the mixed-halide perovskite is reduced than that from the triiodide perovskite phase. Combining the microstructural evolution with the optoelectronic properties of mixed-halide perovskite, it is concluded that additional Cl reduces the defects of recombination centers resulting high photocurrent.

A method is developed to directly map nanoscale "noise-source switching" phenomena during the optoelectronic switching of phase-separated polymer nanocomposites of tetrathiafulvalene (TTF) and phenyl-C61 -butyric acid methyl ester (PCBM) molecules dispersed in a polystyrene (PS) matrix. In the method, electrical current and noise maps of the nanocomposite film are recorded using a conducting nanoprobe, enabling the mapping of a conductivity and a noise-source density. The results provide evidence for a repeated modulation in noise sources, a "noise-source switching," in each stage of a switching cycle. Interestingly, when the nanocomposite is "set" by a high bias, insulating PS-rich phases shows a drastic decrease in a noise-source density which becomes lower than that of conducting TTF-PCBM-rich phases. This can be attributed to a trap filling by charge carriers generated from a TTF (donor)-PCBM (acceptor) complex. In addition, when the film is exposed to UV, an optical switching occurs due to chemical reactions which lead to irreversible changes on the noise-source density and conductivity. The method provides a new insight on noise-source activities during the optoelectronic switching of polymer nanocomposites and thus can be a powerful tool for basic noise research and applications in organic memory devices.

We develolped a method to directly image the nanoscale effects of localized noise-source activities on photoconducting charge transports in domain structures of phase-separated polymer-blend films of Poly(9,9-di-n-octylfluorenyl-2,7-diyl) and Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole). For the imaging, current and noise maps of the polymer-blend were recorded using a conducting nanoprobe in contact with the surface, enabling the conductivity (σ) and noise-source density (N T) mappings under an external stimulus. The blend-films exhibited the phase-separation between the constituent polymers at domains level. Within a domain, high σ (low N T) and low σ (high N T) regions were observed, which could be associated with the ordered and disordered regions of a domain. In the N T maps, we observed that noise-sources strongly affected the conduction mechanism, resulting in a scaling behavior of σ ∝ [Formula: see text] in both ordered and disordered regions. When a blend film was under an influence of an external stimulus such as a high bias or an illumination, an increase in the σ was observed, but that also resulted in increases in the N T as a trade-off. Interestingly, the Δσ versus ΔN T plot exhibited an unusual scaling behavior of Δσ ∝ [Formula: see text] which is attributed to the de-trapping of carriers from deep traps by the external stimuli. In addition, we found that an external stimulus increased the conductivity at the interfaces without significantly increasing their N T, which can be the origin of the superior performances of polymer-blend based devices. These results provide valuable insight about the effects of noise-sources on nanoscale optoelectronic properties in polymer-blend films, which can be an important guideline for improving devices based on polymer-blend.

We have developed a "vibrational noise spectroscopy (VNS)" method to identify and map vibrational modes of molecular wires on a solid substrate. In the method, electrical-noises generated in molecules on a conducting substrate were measured using a conducting atomic force microscopy (AFM) with a nanoresolution. We found that the bias voltage applied to the conducting AFM probe can stimulate specific vibrational modes of measured molecules, resulting in enhanced electrical noises. Thus, by analyzing noise-voltage spectra, we could identify various vibrational modes of the molecular wires on the substrates. Further, we could image the distribution of vibrational modes on molecule patterns on the substrates. In addition, we found that VNS imaging data could be further analyzed to quantitatively estimate the density of a specific vibrational mode in the layers of different molecular species. The VNS method allows one to measure molecular vibrational modes under ambient conditions with a nanoresolution, and thus it can be a powerful tool for nanoscale electronics and materials researches in general.

We report a noise mapping strategy for the reliable identification and analysis of noise sources in molecular wire junctions. Here, different molecular wires were patterned on a gold substrate, and the current-noise map on the pattern was measured and analyzed, enabling the quantitative study of noise sources in the patterned molecular wires. The frequency spectra of the noise from the molecular wire junctions exhibited characteristic 1/f2 behavior, which was used to identify the electrical signals from molecular wires. This method was applied to analyze the molecular junctions comprising various thiol molecules on a gold substrate, revealing that the noise in the junctions mainly came from the fluctuation of the thiol bonds. Furthermore, we quantitatively compared the frequencies of such bond fluctuations in different molecular wire junctions and identified molecular wires with lower electrical noise, which can provide critical information for designing low-noise molecular electronic devices. Our method provides valuable insights regarding noise phenomena in molecular wires and can be a powerful tool for the development of molecular electronic devices.

We developed a method to prepare photoconductive channels of hybrid nanostructures based on Au nanohemispheres (Au NHs) and cadmium sulfide nanowires (CdS NWs) for photoconductive applications. The hybrid nanostructures exhibited enhanced absorbance, which was attributed to the strong electronic coupling between plasmons in Au NHs and excitons in CdS NWs. The photoconductive channels based on the hybrid nanostructures showed an increase in photocurrent level by ∼5× compared to bare CdS NWs. Because our strategy can be utilized to improve the photoconductivity of quite versatile photoconductive channels, it can be a powerful method for the development of various advanced optoelectronic and photovoltaic devices.

An electrical noise is one of the key parameters determining the performance of modern electronic devices. However, it has been extremely difficult, if not impossible, to image localized noise sources or their activities in such devices. We report a “noise spectral imaging” strategy to map the activities of localized noise sources in graphene domains. Using this method, we could quantitatively estimate sheet resistances and noise source densities inside graphene domains, on domain boundaries and on the edge of graphene. The results show high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. Additionally, we showed that the top layer in double-layer graphene had lower noises than single-layer graphene. This work provides valuable insights about the electrical noises of graphene. Furthermore, the capability to directly map noise sources in electronic channels can be a major breakthrough in electrical noise research in general.

The localized noise-sources and those induced by external-stimuli were directly mapped by using a conducting-AFM integrated with a custom-designed noise measurement set-up. In this method, current and noise images of a poly(9,9-dioctylfluorene)-polymer-film on a conducting-substrate were recorded simultaneously, enabling the mapping of the resistivity and noise source density (NT). The polymer-films exhibited separate regions with high or low resistivities, which were attributed to the ordered or disordered phases, respectively. A larger number of noise-sources were observed in the disordered-phase-regions than in the ordered-phase regions, due to structural disordering. Increased bias-voltages on the disordered-phase-regions resulted in increased NT, which is explained by the structural deformation at high bias-voltages. On photo-illumination, the ordered-phase-regions exhibited a rather large increase in the conductivity and NT. Presumably, the illumination released carriers from deep-traps which should work as additional noise-sources. These results show that our methods provide valuable insights into noise-sources and, thus, can be powerful tools for basic research and practical applications of conducting polymer films.

In this paper, a novel retinal prosthetic system, combining nanowire field-effect transistor (FET) switches with microelectrodes is developed. In order to minimize the wiring complexity of high-resolution microelectrode arrays, silicon nanowire-FET switches are integrated with microelectrodes. Also, in order to fit the curvature of an eyeball, the silicon nanowire-FET is transferred to a flexible substrate. A microelectrode array with 32 × 32 pixels is fabricated, which has 1,024 microelectrodes. Using the FET switches in a 2-dimensional array-addressing configuration, 1,024 microelectrodes are addressed by only 64 lines (32 for scan and 32 for data), as compared to requiring 1,024 lines in the conventional one-to-one configuration. The threshold voltage, current on/off ratio, and on-resistance of the fabricated silicon nanowire FET switch are measured to be -0.4 V, 1 × 107, and 43 KΩ at gate voltage of -5 V, respectively. The maximum allowable current injection limit of the silicon nanowire-FET switch integrated microelectrode is measured as 48 μA at pulse duration of 1 ms. The concept of FET switching of collocated retinal electrodes is completely novel, and highly scalable. The results have a high potential to realize high-resolution retinal prosthetic systems.

In the present study, nanoscale wavelength-selective photovoltaic activities in H- and J-aggregates of azo dye-based solar cell films were mapped by wavelength-dependent photoconductive noise microscopy. In this strategy, the local conductivities and charge traps in dye films were mapped by a conducting probe scanning the surface while illuminating the lights with selected wavelengths. We observed the formation of localized domains exhibiting wavelength-dependent photoconductivities. The individual domains could be identified as H- and J-aggregates, which showed dominant photoexcitations at wavelengths of 600 and 450 nm, respectively. Notably, the short-circuit currents Isc and photoconductivities ΔσPC of the film showed power-law dependencies with trap densities under illuminated conditions (NT_L) and the trap density change by the illumination (ΔNT), respectively, like ΔσPC ∝ |ΔNT|1/2 and Isc ∝ NT_L−3/4 for each wavelength illumination. These results revealed the carrier recombination process in cooperation with the traps which could be a major factor determining the performance of solar cells. Significantly, the J-aggregates showed lower trap densities than those of the H-aggregates, resulting in superior solar cell characteristics of the J-aggregates, such as a higher Isc and larger open circuit voltages. Since our method allows mapping the nanoscale photovoltaic activities of solar cell film aggregates, it can be a powerful tool for both basic research and in the application of photoelectronic devices.