Photoinduced Carrier Generation and Distribution in Solution-Deposited Titanyl Phthalocyanine Monolayers

Wang, Shenkai; Chiang, Naihao; Guo, Han; Wattanatorn, Natcha; Barr, Kristopher K.; Alexandrova, Anastassia N.; Weiss, Paul S.

doi:10.1021/acs.chemmater.9b03252

Cited by 8 publications

(11 citation statements)

References 83 publications

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“…Details about the instrumentation and theory of SMA–STM are previously reported in refs − for UHV conditions and, more recently, in ref for ambient conditions. SMA–STM is developed to probe the optical excited-state structure with a subnanometer resolution. ,, In an SMA–STM experiment, a laser beam is amplitude-modulated at a frequency faster than the response of the STM feedback loop and is used to optically excite the CD.…”

Section: Experimental Methodsmentioning

confidence: 99%

Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution

et al. 2020

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Unlike quantum dots, photophysical properties of carbon dots (CDs) are not strongly correlated with particle size. The origin of CD photoluminescence has been related to sp2 domain size and the abundance of oxidized surface defects. However, direct imaging of surface-accessible spatially localized oxidized defects is still lacking. In this work, solvothermal-synthesized CDs are fractionated into different colors by polarity-based chromatography. We then study the mechanism of CD fluorescence by directly imaging individual CDs with subparticle resolution by scanning tunneling microscopy. Density of states imaging of CDs reveals that the graphitic core has a large bandgap that is inconsistent with observed fluorescence wavelength, whereas localized defects have smaller electronic gaps for both red-emitting dots (rCDs) and blue-emitting dots (bCDs). For individual bCDs within our laser tuning range, we directly image optically active surface defects (ca. 1–3 nm in size) and their bandgaps, which agree with the emission wavelength of the ensemble from which the bCDs were taken. We find that the emissive defects are not necessarily the ones with the smallest gap, consistent with quantum yields less than unity (0.1–0.26). X-ray photoelectron spectroscopy and pH-dependent fluorescence titration show that oxygen-containing surface-accessible protonatable functional groups (e.g., phenolic −OH, −COOH) define the chemical identity of the defects. This observation explains why we detect neither long-lived optical excitation of the core nor a correlation between size and emission wavelength. Instead, control over the number of oxygen-containing defects defines the emission wavelength, with more oxidized defects at the surface producing redder emission wavelengths.

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Section: Experimental Methodsmentioning

confidence: 99%

Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution

et al. 2020

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“…Temperature. trSMA-STM works by modulating the pump and probe lasers and imaging the excited state via the STM tunneling current modulation at ≤1-nm spatial resolution (14,15): Excitation by the pump beam places an electron into a previously empty orbital near the tunneling energy and results in partial blockage of tunneling through the optically excited state (8), whereas subsequent excitation by the probe beam empties that spin orbital and again opens up the tunneling path. Fig.…”

Section: Detection Of Transient Absorption Of Single Nanoparticles At Roommentioning

confidence: 99%

Ultrafast nanometric imaging of energy flow within and between single carbon dots

Nguyen

Srivastava

Pan

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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Time- and space-resolved excited states at the individual nanoparticle level provide fundamental insights into heterogeneous energy, electron, and heat flow dynamics. Here, we optically excite carbon dots to image electron–phonon dynamics within single dots and nanoscale thermal transport between two dots. We use a scanning tunneling microscope tip as a detector of the optically excited state, via optical blocking of electron tunneling, to record movies of carrier dynamics in the 0.1–500-ps time range. The excited-state electron density migrates from the bulk to molecular-scale (∼1 nm2) surface defects, followed by heterogeneous relaxation of individual dots to either long-lived fluorescent states or back to the ground state. We also image the coupling of optical phonons in individual carbon dots with conduction electrons in gold as an ultrafast energy transfer mechanism between two nearby dots. Although individual dots are highly heterogeneous, their averaged dynamics is consistent with previous bulk optical spectroscopy and nanoscale heat transfer studies, revealing the different mechanisms that contribute to the bulk average.

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“…In contrast to previous detailed STM techniques, SMA-STM has shown its unique capability especially in regard to a variety of supported energy-related nanomaterials. Organic molecules, 84,85 carbon nanotubes (CNTs), 25 QDs 46,47,86 (Figures 3I and 3J), and plasmonic metal nanoparticles have been successfully imaged revealing important information on structural defects, 30 chiral junctions, 25 photoinduced charge generation and charge transfer across p-n junctions, 84,85 electronic excitation of different angular momentum states, 46,86 or indirect plasmonic particle excitations. 48 This technique additionally bears the potential for ''SMA-STM tomography'' images in which the STM tip can be used to manipulate the nanoscale-feature of interest to obtain three-dimensional images.…”

Section: Electromagnetic Radiation Electronically Excites Materialsmentioning

confidence: 99%

Probing Semiconductor Properties with Optical Scanning Tunneling Microscopy

Nienhaus

2020

Joule

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Studying nanoscale photophysical processes is mandatory to fully understand the complex optoelectronic properties in semiconductor materials used in photovoltaics and light emitting diodes. In this perspective, we target specific scanning probe techniques, which combine scanning tunneling microscopy (STM) with optical methods to unravel the localized optoelectronic properties of semiconductors under realistic electric and optical fields, down to the nanoscale. Combining optical spectroscopy with STM yields a powerful platform that allows for simultaneous imaging of the surface morphology and the electronic structure down to the atomic level, a resolution that is otherwise not accessible due to the optical diffraction limit. Incident wavelengths spanning the electromagnetic spectrum from the terahertz region to X-rays have been coupled into the STM tip-sample junction to investigate the nanoscale properties of semiconductor materials, whereas the reverse process of luminescence can give insight on local recombination processes. Imagine the potential of a tool capable of detecting both localized absorption and spontaneous and stimulated emission processes of semiconductor materials at the nanoscale. The role of every atom, defect, or electronic interaction could be disentangled, tailored, or harnessed to its maximum capacity.

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Photoinduced Carrier Generation and Distribution in Solution-Deposited Titanyl Phthalocyanine Monolayers

Cited by 8 publications

References 83 publications

Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution

Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution

Ultrafast nanometric imaging of energy flow within and between single carbon dots

Probing Semiconductor Properties with Optical Scanning Tunneling Microscopy

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