Excited-State Imaging of Single Particles on the Subnanometer Scale

Abstract

At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.

Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy

“…However, owing to their all-optical excitation and detection, their spatial resolution is limited by the diffraction limit of light. Therefore, novel ultrafast techniques such as ultrafast STM have been developed where the probing mechanism is nonoptical, which allows temporal dynamics to be resolved with a spatial resolution of just a few nanometers . Consequently, it is possible to measure the dynamics of single molecules, nanometer-sized particles, or highly heterogeneous low-dimensional materials with superior spatial precision .…”

“…Consequently, it is possible to measure the dynamics of single molecules, nanometer-sized particles, or highly heterogeneous low-dimensional materials with superior spatial precision . For example, ultrafast STM has been used to detect variations in the electronic structure of individual nanoparticles and interactions between adjacent particles with nanometer-scale resolution. , …”

“…Therefore, novel ultrafast techniques such as ultrafast STM have been developed where the probing mechanism is nonoptical, which allows temporal dynamics to be resolved with a spatial resolution of just a few nanometers. 143 Consequently, it is possible to measure the dynamics of single molecules, 144 nanometer-sized particles, 145 or highly heterogeneous low-dimensional materials with superior spatial precision. 146 For example, ultrafast STM has been used to detect variations in the electronic structure of individual nanoparticles and interactions between adjacent particles with nanometer-scale resolution.…”

“…146 For example, ultrafast STM has been used to detect variations in the electronic structure of individual nanoparticles and interactions between adjacent particles with nanometerscale resolution. 143,145 The principle of ultrafast time-resolved single molecule absorption microscopy by STM (tr-SMA-STM) is similar to that of TA microscopy in that it uses optical pulses in a pump− probe geometry to excite a sample. The pump and probe beams excite micrometer-scale areas of the surface of a sample, but the detected signal is the STM tunneling current as the STM tip is raster-scanned over the sample surface to generate an image (Figure 3).…”

Optically Induced Molecular Logic Operations

“…The implementation of near-field enhanced and scanning tunneling microscopy techniques to interrogate and manipulate complex systems makes super-resolution microscopy possible. Such near-field imaging and spectroscopy provide a method for a high degree of accuracy for different research areas, such as nanodevices that coherently control tunneling currents and measurement and processing techniques. − Recent advances in apertureless near-field enhanced techniques permit optical input and output with submolecular resolution. − In this extremely reduced dimensionality, the intense local electrical field is highly confined by the excitation of surface plasmon modes and can be used for nondestructive probing of molecular structures by means of Raman spectroscopy. − Moreover, asymmetric excitation of a surface plasmon can generate a localized angular momentum, − which can couple to nearby molecules. − The effect of this localized angular momentum on molecular junctions has never been studied.…”