High-Speed Spectroscopic Transient Absorption Imaging of Defects in Graphene

Far-field super-resolution chemical microscopy

Far-field chemical microscopy providing molecular electronic or vibrational fingerprint information opens a new window for the study of three-dimensional biological, material, and chemical systems. Chemical microscopy provides a nondestructive way of chemical identification without exterior labels. However, the diffraction limit of optics hindered it from discovering more details under the resolution limit. Recent development of super-resolution techniques gives enlightenment to open this door behind far-field chemical microscopy. Here, we review recent advances that have pushed the boundary of far-field chemical microscopy in terms of spatial resolution. We further highlight applications in biomedical research, material characterization, environmental study, cultural heritage conservation, and integrated chip inspection.

Polycaprolactone-MXene Nanofibrous Scaffolds for Tissue Engineering

New conductive materials for tissue engineering are needed for the development of regenerative strategies for nervous, muscular, and heart tissues. Polycaprolactone (PCL) is used to obtain biocompatible and biodegradable nanofiber scaffolds by electrospinning. MXenes, a large class of biocompatible 2D nanomaterials, can make polymer scaffolds conductive and hydrophilic. However, an understanding of how their physical properties affect potential biomedical applications is still lacking. We immobilized Ti3C2Tx MXene in several layers on the electrospun PCL membranes and used positron annihilation analysis combined with other techniques to elucidate the defect structure and porosity of nanofiber scaffolds. The polymer base was characterized by the presence of nanopores. The MXene surface layers had abundant vacancies at temperatures of 305-355 K, and a voltage resonance at 8 × 104 Hz with the relaxation time of 6.5 × 106 s was found in the 20-355 K temperature interval. The appearance of a long-lived component of the positron lifetime was observed, which was dependent on the annealing temperature. The study of conductivity of the composite scaffolds in a wide temperature range, including its inductive and capacity components, showed the possibility of the use of MXene-coated PCL membranes as conductive biomaterials. The electronic structure of MXene and the defects formed in its layers were correlated with the biological properties of the scaffolds in vitro and in bacterial adhesion tests. Double and triple MXene coatings formed an appropriate environment for cell attachment and proliferation with mild antibacterial effects. A combination of structural, chemical, electrical, and biological properties of the PCL-MXene composite demonstrated its advantage over the existing conductive scaffolds for tissue engineering.

Rapid multiplex ultrafast nonlinear microscopy for material characterization

We demonstrate rapid imaging based on four-wave mixing (FWM) by assessing the quality of advanced materials through measurement of their nonlinear response, exciton dephasing, and exciton lifetimes. We use a WSe₂ monolayer grown by chemical vapor deposition as a canonical example to demonstrate these capabilities. By comparison, we show that extracting material parameters such as FWM intensity, dephasing times, excited state lifetimes, and distribution of dark/localized states allows for a more accurate assessment of the quality of a sample than current prevalent techniques, including white light microscopy and linear micro-reflectance spectroscopy. We further discuss future improvements of the ultrafast FWM techniques by modeling the robustness of exponential decay fits to different spacing of the sampling points. Employing ultrafast nonlinear imaging in real-time at room temperature bears the potential for rapid in-situ sample characterization of advanced materials and beyond.

Inspection of Line Defects in Transition Metal Dichalcogenides Using a Microscopic Hyperspectral Imaging Technique

The line defects of two-dimensional (2D) transition metal dichalcogenides (TMDs) play a vital role in determining their device performance. In this work, a microscopic hyperspectral imaging technique based on differential reflectance was introduced for the online inspection of line defects in TMDs. Upon comparison of the measurement results of imaging and spectra, the relationship between optical contrast and differential reflectance spectra was established. A light selection method was proposed to optimize the optical contrast of line defects. Via application of an image processing algorithm, an automatic detection of the line defects with a classification accuracy of 95% was achieved for WS₂, MoS₂, and MoSe₂. This work not only provides a microscopic hyperspectral imaging technique for detecting 2D material defects but also introduces a versatile design strategy for developing an advanced machine vision spectroscopic system.

Single 3.3 fs multiple plate compression light source in ultrafast transient absorption spectroscopy

Ultrafast transient absorption spectroscopy is a powerful tool to reveal excited state dynamics in various materials. Conventionally, probe pulses are generated via bulk supercontinuum generation or (noncollinear) optical parametric amplifiers whilst pump pulses are generated separately using (noncollinear) optical parametric amplifiers. These systems are limited by either their spectral density, stability, spectral range, and/or temporal compressibility. Recently, a new intense broadband light source is being developed, the multi-plate compression, which promises to overcome these limitations. In this paper, we analyze the supercontinuum generated by a single Multiple Plate Compression system to set a benchmark for its use in the field of ultrafast pump-probe spectroscopy. We have compressed the supercontinuum to 3.3 fs using chirp mirrors alone, making it an excellent candidate for pump-probe experiments requiring high temporal resolution. Furthermore, the single light source can be used to generate both probe and pump pulses due to its high spectral density (>14.5 nJ/nm) between 490 and 890 nm. The intensity has an average shot-to-shot relative standard deviation of 4.6 % over 490 to 890 nm, calculated over 2,000 sequential shots. By using only 1,000 shot pairs, a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta T/T$$\end{document}ΔT/T noise level of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2.6\times 10^{-4}$$\end{document}2.6×10-4 RMS is achieved. Finally, as a proof of concept, the transient absorption spectrum of a methylammonium lead iodide perovskite film is taken, showing great signal to noise with only 1,000 shot pairs. These results show great potential for the employment of this technique in other spectroscopic techniques such as coherent multidimensional spectroscopy.

Leaving the Limits of Linearity for Light Microscopy

Nonlinear microscopy has enabled additional modalities for chemical contrast, deep penetration into biological tissues, and the ability to collect dynamics on ultrafast timescales across heterogenous samples. The additional light fields introduced to a sample offer seemingly endless possibilities for variation to optimize and customize experimentation and the extraction of physical insight. This perspective highlights three areas of growth in this diverse field: the collection of information across multiple timescales, the selective imaging of interfacial chemistry, and the exploitation of quantum behavior for future imaging directions. Future innovations will leverage the work of the studies reviewed here as well as address the current challenges presented.

Oblique angle transient-reflectivity laser-scanning microscopy for mineral imaging in natural ores

The microscopic arrangement of different minerals in ores is of high interest for mine planning, mineral processing and extractive metallurgy. Many economically important, naturally occurring minerals are highly absorbing semiconductors. To characterize these materials, we have implemented pump-probe laser scanning microscopy (LSM) in a two-lens reflective configuration that offers efficient collection of signal light by using a combination of galvanometer and sample stage scanning. We show that the short-time (∼10 ps) pump-probe response of a material allows us to distinguish economically important sulfide minerals.

Transient absorption microscopy: Technological innovations and applications in materials science and life science

Transient absorption (TA) spectroscopy has been extensively used in the study of excited state dynamics of various materials and molecules. The transition from TA spectroscopy to TA microscopy, which enables the space-resolved measurement of TA, is opening new investigations toward a more complete picture of excited state dynamics in functional materials, as well as the mapping of crucial biopigments for precision diagnosis. Here, we review the recent instrumental advancement that is pushing the limit of spatial resolution, detection sensitivity, and imaging speed. We further highlight the emerging application in materials science and life science.

Plasmon-induced efficient hot carrier generation in graphene on gold ultrathin film with periodic array of holes: Ultrafast pump-probe spectroscopy

Using ultrafast pump-probe reflectivity with a 3.1 eV pump and coherent white light probe (1.1-2.6 eV), we show that graphene on gold nanostructures exhibits a strong coupling to the plasmonic resonances of the ordered lattice hole array, thus injecting a high density of hot carriers in graphene through plasmons. The system being studied is single-layer graphene on an ultrathin film of gold with periodic arrangements of holes showing anomalous transmission. A comparison is made with gold film with and without hole array. By selectively probing transient carrier dynamics in the spectral regions corresponding to plasmonic resonances, we show efficient plasmon induced hot carrier generation in graphene. We also show that due to high electromagnetic field intensities at the edge of the submicron holes, fast decay time (10-100 fs), and short decay length (1 nm) of plasmons, a highly confined density of hot carriers (very close to the edge of the holes) is generated by Landau damping of plasmons within the holey gold film. A contribution to transient decay dynamics due to the diffusion of the initial nonuniform distribution of hot carriers away from the hole edges is observed. Our results are important for future applications of novel hot carrier device concepts where hot carriers with tunable energy can be generated in different graphene regions connected seamlessly.

Inhomogeneous photocarrier dynamics and transport in monolayer MoS2 by ultrafast microscopy

Monolayer MoS₂ as a member of two-dimensional transition metal dichalcogenides (TMDs) has attracted considerable attention due to its superior optoelectronic properties. Understanding the photocarrier dynamics and transport in these two dimensional systems is beneficial for applications from photovoltaics to sensing. However, various structural defects strongly impact the dynamics and transport of photocarriers. Especially there lacks a precise measuring and understanding of photocarrier transport in TMDs. Here, femtosecond transient absorption spectroscopy and microscopy were employed to study the photocarrier dynamics and transport in monolayer MoS₂. Defect correlated photocarrier dynamics are observed across the monolayer MoS₂ where exciton formation and nonradiative recombination are the two dominant decay processes. To the best of our knowledge, we report two distinct photocarrier transport regimes in MoS₂ for the first time with diffusion coefficients of [Formula: see text] cm² s^-1 and [Formula: see text] cm² s^-1, by taking advantages of ultrafast microscopy with ∼20 nm spatial precision and ∼200 fs temporal resolution. These two regimes are ascribed to fast hot photocarrier diffusion and slow phonon-limited thermal diffusion, respectively. The results indicate that the initial fast photocarrier transport is less dependent on structural defects compared to photocarrier relaxation dynamics which may be useful for hot photocarrier extraction applications.

Path towards graphene commercialization from lab to market

The ground-breaking demonstration of the electric field effect in graphene reported more than a decade ago prompted the strong push towards the commercialization of graphene as evidenced by a wealth of graphene research, patents and applications. Graphene flake production capability has reached thousands of tonnes per year, while continuous graphene sheets of tens of metres in length have become available. Various graphene technologies developed in laboratories have now transformed into commercial products, with the very first demonstrations in sports goods, automotive coatings, conductive inks and touch screens, to name a few. Although challenges related to quality control in graphene materials remain to be addressed, the advancement in the understandings of graphene will propel the commercial success of graphene as a compelling technology. This Review discusses the progress towards commercialization of graphene for the past decade and future perspectives.

Toward Mass Production of CVD Graphene Films

Chemical vapor deposition (CVD) is considered to be an efficient method for fabricating large-area and high-quality graphene films due to its excellent controllability and scalability. Great efforts have been made to control the growth of graphene to achieve large domain sizes, uniform layers, fast growth, and low synthesis temperatures. Some attempts have been made by both the scientific community and startup companies to mass produce graphene films; however, there is a large difference in the quality of graphene synthesized on a laboratory scale and an industrial scale. Here, recent progress toward the mass production of CVD graphene films is summarized, including the manufacturing process, equipment, and critical process parameters. Moreover, the large-scale homogeneity of graphene films and fast characterization methods are also discussed, which are crucial for quality control in mass production.

Ultrafast measurements of the dynamics of single nanostructures: a review

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.