Toh-Ming Lu

Bio: Toh-Ming Lu is an academic researcher from Rensselaer Polytechnic Institute. The author has contributed to research in topics: Thin film & Dielectric. The author has an hindex of 57, co-authored 629 publications receiving 15156 citations. Previous affiliations of Toh-Ming Lu include University of Wisconsin-Madison & University of Cambridge.

Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm

Abstract: Using a pump–probe method with a 150 fs laser at the wavelength of 1.55 μm, we have experimentally demonstrated that single-walled carbon nanotubes (SWNT) have an exciton decay time of less than 1 ps and a high third-order polarizability, which is reasonably interpreted as due to their azimuthal symmetry. These experimental results reveal that a SWNT polymer composite may be a candidate material for high-quality subpicosecond all-optical switches.

Nanostructured Silicon Anodes for Lithium Ion Rechargeable Batteries

Abstract: Rechargeable lithium ion batteries are integral to today's information-rich, mobile society. Currently they are one of the most popular types of battery used in portable electronics because of their high energy density and flexible design. Despite their increasing use at the present time, there is great continued commercial interest in developing new and improved electrode materials for lithium ion batteries that would lead to dramatically higher energy capacity and longer cycle life. Silicon is one of the most promising anode materials because it has the highest known theoretical charge capacity and is the second most abundant element on earth. However, silicon anodes have limited applications because of the huge volume change associated with the insertion and extraction of lithium. This causes cracking and pulverization of the anode, which leads to a loss of electrical contact and eventual fading of capacity. Nanostructured silicon anodes, as compared to the previously tested silicon film anodes, can help overcome the above issues. As arrays of silicon nanowires or nanorods, which help accommodate the volume changes, or as nanoscale compliant layers, which increase the stress resilience of silicon films, nanoengineered silicon anodes show potential to enable a new generation of lithium ion batteries with significantly higher reversible charge capacity and longer cycle life.

Defect-induced photoluminescence in monolayer semiconducting transition metal dichalcogenides.

Abstract: It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1g ring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2 or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ∼0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications.

Aging of Transition Metal Dichalcogenide Monolayers.

Abstract: Two-dimensional sheets of transition metal dichalcogenides are an emerging class of atomically thin semiconductors that are considered to be “air-stable”, similar to graphene. Here we report that, contrary to current understanding, chemical vapor deposited transition metal dichalcogenide monolayers exhibit poor long-term stability in air. After room-temperature exposure to the environment for several months, monolayers of molybdenum disulfide and tungsten disulfide undergo dramatic aging effects including extensive cracking, changes in morphology, and severe quenching of the direct gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy reveal that this effect is related to gradual oxidation along the grain boundaries and the adsorption of organic contaminants. These results highlight important challenges associated with the utilization of transition metal dichalcogenide monolayers in electronic and optoelectronic devices. We also demonstrate a potential solution to this problem, featur...

Characterization of Amorphous and Crystalline Rough Surface: Principles and Applications

Abstract: The structure of a growth or an etch front on a surface is not only a subject of great interest from the practical point of view but also is of fundamental scientific interest. Very often surfaces are created under non-equilibrium conditions such that the morphology is not always smooth. In addition to a detailed description of the characteristics of random rough surfaces, "Experimental Methods in the Physical Sciences, Volume 37", "Characterization of Amorphous and Crystalline Rough Surface-Principles and Applications" will focus on the basic principles of real and diffraction techniques for quantitative characterization of the rough surfaces. The book thus includes the latest development on the characterization and measurements of a wide variety of rough surfaces. The complementary nature of the real space and diffraction techniques is fully displayed. It includes an accessible description of quantitative characterization of random rough surfaces and growth/etch fronts. It has a detailed description of the principles, experimentation, and limitations of advanced real-space imaging techniques (such as atomic force microscopy) and diffraction techniques (such as light scattering, X-ray diffraction, and electron diffraction). It has characterization of a variety of rough surfaces (e.g., self-affine, mounded, anisotropic, and two-level surfaces) accompanied by quantitative examples to illustrate the essence of the principles. It is an insightful description of how rough surfaces are formed. There is presentation of the most recent examples of the applications of rough surfaces in various areas.

Materials for terahertz science and technology

Abstract: Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Abstract: Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries

Abstract: In this paper, the use of nanostructured anode materials for rechargeable lithium-ion batteries (LIBs) is reviewed. Nanostructured materials such as nano-carbons, alloys, metal oxides, and metal sulfides/nitrides have been used as anodes for next-generation LIBs with high reversible capacity, fast power capability, good safety, and long cycle life. This is due to their relatively short mass and charge pathways, high transport rates of both lithium ions and electrons, and other extremely charming surface activities. In this review paper, the effect of the nanostructure on the electrochemical performance of these anodes is presented. Their synthesis processes, electrochemical properties, and electrode reaction mechanisms are also discussed. The major goals of this review are to give a broad overview of recent scientific researches and developments of anode materials using novel nanoscience and nanotechnology and to highlight new progresses in using these nanostructured materials to develop high-performance LIBs. Suggestions and outlooks on future research directions in this field are also given.