Atomically-thin two-dimensional (2D) metal chalcogenides have emerged as an exciting class of materials which have the potential to enable numerous new applications that range from electronics to photonics. Developing new methods for controlled synthesis and manipulation of these layered materials is crucial for emerging applications in functional devices. Here, we demonstrate non-equilibrium laser-based approaches to form and deliver atoms, cluster or stoichiometric nanoparticles with tunable kinetic energies for the synthesis and processing of 2D layered semiconductors. Utilizing the stoichiometric nanoparticles as feedstocks, we demonstrate the formation of either small domain nanosheet networks (~ 20 nm) or large crystalline domains (~100 µm). On the other hand, atomic precursors with tunable kinetic energies are used in doping, alloying and conversion of 2D monolayers. Patterned arrays of lateral heterojunctions between 2D layered semiconductors, MoSe₂/MoS₂, are formed by e-beam lithography and selective conversion processes. Moreover, we explore the nonequilibrium, bottom-up synthesis of single crystalline monolayers of MoSe_2-x with controllable levels of Se vacancies far beyond intrinsic levels (up to 20 %) exhibiting unique optical and electrical properties. These non-equilibrium laser-based approaches provide unique synthesis and processing opportunities that are not easily accessible through conventional methods.

Incorporating dopants in monolayer transition metal dichalcogenides (TMD) can enable manipulations of their electrical and optical properties. Previous attempts in amphoteric doping in monolayer TMDs have proven to be challenging. Here we report the incorporation of molybdenum (Mo) atoms in monolayer WS₂ during growth by chemical vapor deposition, and correlate the distribution of Mo atoms with the optical properties including photoluminescence and ultrafast transient absorption dynamics. Dark field scanning transmission electron microscopy imaging quantified the isoelectronic doping of Mo in WS₂ and revealed its gradual distribution along a triangular WS₂ monolayer crystal, increasing from 0% at the edge to 2% in the center of the triangular WS₂ triangular crystals. This agrees well with the Raman spectra data that showed two obvious modes between 360 cm^-1 and 400 cm^-1 that corresponded to MoS₂ in the center. This in-plane gradual distribution of Mo in WS₂ was found to account for the spatial variations in photoluminescence intensity and emission energy. Transition absorption spectroscopy further indicated that the incorporation of Mo in WS² regulate the amplitude ratio of XA and XB of WS2. The effect of Mo incorporation on the electronic structure of WS₂ was further elucidated by density functional theory. Finally, we compared the electrical properties of Mo incorporated and pristine WS₂ monolayers by fabricating field-effect transistors. The isoelectronic doping of Mo in WS₂ provides an alternative approach to engineer the bandgap and also enriches our understanding the influence of the doping on the excitonic dynamics.

Currently, two-dimensional (2D) layered materials are rapidly emerging as a new platform for many potential applications in nanoscale optoelectronics, optics, flexible electronics, energy, etc. Monolayers of 2D crystals [e.g., transition metals dichalcogenides (TMDs)] are basically surface and therefore, their optoelectronic properties are very sensitive to defects and environment including ambient gases and substrates. However, only limited number of studies is devoted to understanding of the effect of defects on their optical properties. It is not clear if the specific defects have their fingerprints in Raman, absorption, and PL spectra. Here, we report measurements of low temperature (4-150K) Raman and photoluminescence (PL) spectra of TMD monolayers (MoSe2, WS2) with variable and controlled concentrations of specific defects, i.e., chalcogenide atom vacancies, to reveal optical signatures of these defects. The defective TMD monolayers were synthesized using our new laser CVD approach. To identify the type of defects and their concentration the 2D crystals were transferred from a substrate to a TEM grid and atomic resolution STEM and EELS measurements were performed. Low temperature Raman and PL mapping were used to understand spatial distribution of the defects within the 2D crystals. The assignment of the observed spectral features in low temperature Raman and PL spectra was supported by ab initio theoretical modeling.

Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Characterization and computational science at CNMS was supported by the Scientific User Facilities Division, BES.

Two-dimensional transition metal dichalcogenide (2D-TMD) semiconductors are new class of functional materials with a great promise for optoelectronics. Despite their atomic thickness, they strongly interact with light. This allows 2D-TMDs to become suitable converters of photons into useful electric charges in heterostructures involving 2D-TMDs and metallic nano-plasmonics or semiconductor quantum dots (QDs). In this talk, I will illustrate how femtosecond pump-probe spectroscopy can reveal a sub-45 fs charge transfer at a 2D/QDs heterostructure composed of tungsten disulfide monolayers (2D-WS2) and a single layer of cadmium selenide (CdSe)/zinc sulfide (ZnS) core/shell 0D-QDs. In another heterostructure involving 2D-TMDs and plasmonics, I will describe how plasmons of an array of aluminum (Al) nanoantennas are excited indirectly via energy transfer from photoexcited exciton of 2D-WS2 semiconductor. In particular, femtosecond spectroscopy measurements indicated that the lifetime of the resulting plasmon-induced hot electrons in the Al array continue as long as that of the 2D-WS2 excitons. Conversely, the presence of these excited plasmons almost triples the lifetime of the 2D-WS2 excitons from ~15 to ~44 ps. This exciton-plasmon coupling enabled by such hybrid nanostructures may open new opportunities for optoelectronic applications.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Synthesis of the two-dimensional materials was supported by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

The bulk production of loose graphene flakes and its doped variants are important for energy applications including batteries, fuel cells, and supercapacitors as well as optoelectronic and thermal applications. While laser-based methods have been reported for large-scale synthesis of single-wall carbon nanohorns (SWNHs), similar large-scale production of graphene has not been reported. Here we explored the synthesis of doped few layered graphene by pulsed laser vaporization (PLV) with the goal of producing an oxidation resistant electrode support for solid acid fuel cells. PLV of graphite with various amounts of boron was carried out in mixtures in either Ar or Ar/H2 at 0.1 MPa at elevated temperatures under conditions typically used for synthesis of SWNHs. Both the addition of hydrogen to the background argon, or the addition of boron to the carbon target, was found to shift the formation of carbon nanohorns to two-dimensional flakes of a new form of few-layer graphene material, with sizes up to microns in dimension as confirmed by XRD and TEM. However, the materials made with boron exhibited superior resistance to carbon corrosion in the solid acid fuel cell and thermal oxidation resistance in air compared to similar product made without boron. Mechanisms for the synthesis and oxidation resistance of these materials will be discussed based upon detailed characterization and modeling. •Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Material processing and characterization science supported by ARPA-E under Cooperative Agreement Number 
DE-AR0000499 and as a user project at the Center for Nanophase Materials Sciences, a Department of Energy Office of Science User Facility. 


Key issues of the controlled synthesis of nanoparticles and nanostructures, as well as laser-particle interactions are considered in the context of the latest applications appearing in many fields such as photonics, medicine, 3D printing, etc. The results of a multi-physics numerical study of laser interaction with nanoparticles will be presented in the presence of several environments. In particular, attention will be paid to the numerical study of laser interactions with heterogeneous materials (eg. colloidal liquids and/or nanoparticles in a dielectric medium) and the aggregation/sintering/fragmentation processes induced by ultra-short laser pulses.

On-chip signal modulation, processing and routing are now largely carried out in hard-wired circuits occupying most of the area of CPU chips, creating speed bottlenecks and increasing thermal loading. We have demonstrated on-chip, all-optical modulation in hybrid silicon-vanadium dioxide (VO₂) ring resonators, in which a nanoscale patch of VO₂ acts as the switch, driven by the enormous change in index of refraction in the IMT (Δn~1.3). In principle, the femtosecond IMT transition time of VO₂ enables modulation speeds up to 500 GHz. However, the IMT is accompanied by a structural phase transition (SPT) from a monoclinic (M) to a rutile (R) phase, and nanosecond return to the M phase would reduce modulation frequencies below 1 GHz. Moreover, VO₂ is lossy, so that optimizing switching contrast and power consumption simultaneously requires that the VO₂ modulator should have lateral dimensions of order 100 nm, about the size of a single grain.

This talk highlights advances in fabrication and performance of ultrafast switching of the nanoscale VO₂ thin-film modulator. Studies of ultrafast excitation and relaxation of VO₂ confirm the existence of an excited monoclinic phase (mM) with a fast recovery time compatible with Tbps switching. We show how nanoscale modulator design, and particularly optimizing the resonant ring, can achieve power requirements compatible with systems specifications for all-optical modulators, and elaborate on the effects of dopants. Finally, we show how optical switching in the hybrid ring resonator can be achieved using near band-edge pumping of the VO₂ nanopatch at wavelengths in the telecommunications band.

The assembly of nanoscale building blocks in engineered mesostructures is one of the fundamental goals of nanotechnology. Among the various processes developed to date, self-assembly emerges as one of the most promising, since it relays solely on basic physico-chemical forces. Our research is focused on a new type of self-assembly strategy from the gas-phase: Scattered Ballistic Deposition (SBD). SBD arises from the interaction of a supersonic molecular beam with a static gas and enables the growth of quasi-1D hierarchical mesostructures. Overall, they resemble a forest composed of individual, high aspect-ratio, tree-like structures, assembled from amorphous or crystalline nanoparticles. SBD is a general occurring phenomenon and can be obtained with different vapour or cluster sources. In particular, SBD by Pulsed Laser Deposition is a convenient physical vapor technique that allows the generation of supersonic plasma jets from any inorganic material irrespective of melting temperature, preserving even the most complex stoichiometries. One of the advantages of PLD over other vapour deposition techniques is extremely wide operational pressure range, from UHV to ambient pressure. These characteristics allowed us to develop quasi-1D hierarchical nanostructures from different transition metal oxides, semiconductors and metals. The precise control offered by the SBD-PLD technique over material properties at the nanoscale allowed us to fabricate ultra-thin, high efficiency hierarchical porous photonic crystals with Bragg reflectivity up to 85%.

In this communication we will discuss the application of these materials to solar energy harvesting and storage, stimuli responsive photonic crystals and smart surfaces with digital control of their wettability behaviour.

With the proliferation of highly confined, nanophotonic waveguides and laser sources with increasing intensity, the effects of laser heating will begin to greatly impact the materials used in optical applications. In order to better understand the mechanism of laser heating, its timescales, and the dispersion of heat into the material, simulations of nanoparticles in various media are presented.

A generic model to describe a variety of nanoparticle shapes and sizes is desirable to describe complex phenomenon. These particles are dispersed into various solids, liquids, or gases depending on the application. To simulate nanoparticles and their interaction with their host material, the Finite Element Method (FEM) is used. Heat transfer following an absorption event is also described by a parabolic partial differential equation, and transient solutions are generated in response to continuous, pulsed, or modulated laser radiation.

The simplest physical system described by FEM is that of a broadly-absorbing round-shaped nanoparticle dispersed in viscous host fluid or solid. Many experimental and theoretical studies conveniently describe a very similar system: a carbon “black” nanoparticle suspended in water. This material is well-known to exhibit nonlinear behavior when a laser pulse carrying 0.7 J/cm² is incident on the material. For this process the FEM simulations agree with experimental results to show that a pulse of this fluence is capable of heating the solvent elements adjacent to the nanoparticle to their boiling point. This creates nonlinear scattering which is empirically observed as a nonlinear decrease in the transmitted power at this input fluence.

Laser Ablation in Liquids is an innovative method, which is used to obtain colloidal solutions of nanoparticles that show unique properties and are not achievable by conventional synthesis methods. However, this method lacks of key parameters and scaling factors. In this work we present a strategy which utilizes a 500W, 10 MHz ps-laser in combination with a polygon scanner (500m/s) to scale up the process by enhancing the productivity of the synthesis to up to 5 gram per hour in a continuous process.

Plasmonic biosensors form the core label-free technology for studies of biomolecular interactions, but they still need a drastic improvement of sensitivity and novel nano-architectural implementations to match modern trends of nanobiotechnology. Here, we consider the generation of resonances in light reflected from 3D woodpile plasmonic crystal metamaterials fabricated by Direct Laser Writing by Multi-Photon Polymerization, followed by silver electroless plating. We show that the generation of these resonances is accompanied by the appearance of singularities of phase of reflected light and examine the response of phase characteristics to refractive index variations inside the metamaterial matrix. The recorded phase sensitivity (3*10⁴ deg. of phase shift per RIU change) outperforms most plasmonic counterparts and is attributed to particular conditions of plasmon excitation in 3D plasmonic crystal geometry. Combined with a large surface for biomolecular immobilizations offered by the 3D woodpile matrix, the proposed sensor architecture promises a new important landmark in the advancement of plasmonic biosensing technology.

We have investigated mechanisms of photocorrosion of GaAs/Al_0.35Ga_0.65As heterostructures immersed in aqueous solutions and excited with above-bandgap radiation. The difference in photocorrosion rates of GaAs and Al_0.35Ga_0.65As appears weakly dependent on the bandgap energy of these materials. The intensity of an integrated PL signal from GaAs quantum wells or a buried GaAs epitaxial layer is found dominated by the surface states and chemical reactivity of heterostructure surfaces revealed during the photocorrosion process. Under optimized photocorrosion conditions, the method allows to resolve a 1 nm thick GaAs layer sandwiched between Al_0.35Ga_0.65As layers. We demonstrate that this approach can be used as an inexpensive, and simple room temperature tool for post-growth diagnostics of interface locations in PL emitting quantum wells and other nano-heterostructures.

In this study, plasmonic Au and Au/Ag nanostructures in soda-lime-silicate glasses have been generated by means of ArF-excimer laser irradiation (193 nm) below the ablation threshold of the glass. For this purpose pure and silver/sodium ion-exchanged float glasses have been coated by gold and then irradiated by the laser. The formation of Au and Au/Ag nanoparticles could be verified by the surface plasmon resonances between 420 and 620 nm, which were obtained by optical spectroscopy. Both, pure Au and Ag particles as well as bimetallic Au/Ag nanoparticles, could be observed by means of small angle X-ray scattering experiments. These results demonstrate that such procedures enable the spaceselected generation of plasmonic nanostructures in glass surfaces by excimer laser irradiation.

We investigated the interaction between femotsecond laser and polyimide with a high repetition femtosecond fiber laser and a precisely motorized 3D stage. We have found that high repetition femtosecond laser pulse train can effectively fabricate double-layer electrical conductive tracks inside a polyimide (PI) sheets by a single-time irradiation. This interaction comprised multi-photon absorption, dissociation of polymer molecules and the thermal accumulation. The experiment unveiled that dual-layer carbonization was a consequence of an inside micro-lens formed instantly as laser was just focused into the inside of polyimide. This micro-lens further focused the subsequent laser pulse to carbonize the polymer through multi-photon excitation, bond breaking and graphite layer reformation and eventually form the second electronic conductive layer. The second conductive layer was generated below the focal point. With the laser irradiating is kept at the same height, the top layer at the focused plane continued to absorb laser energy then carbonized into the conductive layer. We called the process as a kind of self-focusing phenomenon. We study the focus effect of inside microlenses under different laser powers and irradiation times. The gap of double electronic tracks embedded in the polyimide matrix can be adjusted with the laser processing parameters. When the gap is more than 30 micrometer, two conductive layers are electrically insulating. While the gap is smaller than 10 micrometer, two conductive layers are electrically connected. Various applications, such as, supercapacitors, capacitive sensors and the field effect transistors were investigated in the flexible PI sheets using this 3D double-layer electrical conductive architecture.

This study demonstrates the first known instance of the templating of titanium dioxide nanotube arrays by laser induced periodic surface structures (LIPSS) and subsequent electrochemical anodization. Titanium dioxide is an established photocatalyst, however it suffers from poor visible light absorption, thus limiting its use under solar irradiation. Thermal annealing can enhance the visible light absorption, with the downside of introducing more defect traps that reduce the lifetime of the charge separated state. Hence, this study proposes an alternative to chemical methods, by modulating the surface profile of the nanotube array to trap visible light. The enhanced visible light absorption is predicted via computational modelling and the morphological evolution of the anodization process was investigated. This study provides the basis for further work into LIPSS templating of other anodized transition metal oxide materials.

Titanium (Ti) is widely used as biomaterial, for example artificial bone, joint etcetera. Femtosecond laser can be used to form periodic nanostructures on Ti surface, and the structures help to control cell elongation. The period of the periodic nanostructures on Ti under atmospheric condition is about 70 to 80% compared with the laser wavelength. However, the mechanism of periodic nanostructure formation by femtosecond laser irradiation has not been clarified yet. Thus, we focused on Surface Plasmon Polariton (SPP) model, which was proposed as a model for formation of periodic nanostructures by femtosecond laser irradiation. In this model, standing waves are generated on the material surface caused by excited electrons on the material surface by laser irradiation. The wavelength of the standing waves depends on the permittivity of the surrounding medium, and the period of the periodic nanostructures also depends on the wavelength of the standing waves. Therefore, it is considered that the period of the nanostructures varies by changing the permittivity at the laser irradiation interface^2,3).In this study, a polyethylene terephthalate (PET) films which has permittivity of 3.0, and a polymethyl methacrylate (PMMA) films which has permittivity of 3.4 were contacted on Ti surface by using contact jig and then the femtosecond laser at a wavelength of 800 nm was irradiated to create periodic nanostructures. As a result, periodic nanostructures with a period of 440 nm was formed on Ti under PET adhesion condition, and periodic nanostructures with a period of 380 nm was formed on Ti under PMMA adhesion condition. On the other hand, periodic nanostructures with a period of 600 nm was formed on Ti under atmospheric condition. It was found that the period of periodic nanostructures can be controlled by changing the permittivity of the medium adhered to Ti.

We report on the development and application of a new laser diagnostic for the in situ detection of large molecules and nanoparticles. This four wave mixing diagnostic technique relies on the creation of an optical lattice in a medium due to the interaction between polarized particles and intense laser fields. Though this interaction, we can detect the temperature, pressure, relative density, polarizability and speed of sound of a gas and gas mixture. This diagnostic was already successfully demonstrated in atomic and molecular gaseous environments, where the different gas polarizabilities and pressures were successfully measured. We are currently conducting measurements with large molecules and nanoparticles, the results of which will be presented in this meeting.