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Charge transfer is a fundamental interface process that can be harnessed for light detection, photovoltaics, and photosynthesis. Recently, charge transfer was exploited in nanophotonics to alter plasmon polaritons by involving additional non-polaritonic materials to activate the charge transfer. Yet, direct charge transfer between polaritonic materials has not been demonstrated. We report the direct charge transfer in pure polaritonic van der Waals (vdW) heterostructures of α-MoO3/graphene. We extracted the Fermi energy of 0.6 eV for graphene by infrared nano-imaging of charge transfer hyperbolic polaritons in the vdW heterostructure. This unusually high Fermi energy is attributed to the charge transfer between graphene and α-MoO3. Moreover, we have observed charge transfer hyperbolic polaritons in multiple energy–momentum dispersion branches with a wavelength elongation of up to 150%. With the support from the density functional theory calculation, we find that the charge transfer between graphene and α-MoO3, absent in mechanically assembled vdW heterostructures, is attributed to the relatively pristine heterointerface preserved in the epitaxially grown vdW heterostructure. The direct charge transfer and charge transfer hyperbolic polaritons demonstrated in our work hold great promise for developing nano-optical circuits, computational devices, communication systems, and light and energy manipulation devices. [ABSTRACT FROM AUTHOR]

A single-step solid-state synthetic approach was developed for the synthesis of silicon-coated carbon nanotube (CNT) core/shell structures. This was achieved through laser-induced melting and evaporation of CNT-deposited Si substrates using a continuous wavelength CO2 laser. The synthesis location of the CNT/Si structures was defined by the laser-irradiated spots. The thickness of the coating was controlled by tuning the laser power and synthesis time during the coating process. This laser-based synthetic technique provides a convenient approach for solid-state, controllable, gas-free, simple and cost-effective fabrication of CNT/Si core/shell structures. [ABSTRACT FROM AUTHOR]

Plasmonic nanoantennas show significant potential in photodetection applications, but the extent to which their full potential can be realized is dictated by the volume and location of the active materials within the plasmonic structure. Carbon nanotubes (CNTs) have been used as a novel material in photodetection application due to their excellent electronic and optoelectronic properties. However, difficulties in the integration of CNTs in the gaps of nanoantennas have limited the investigation of antenna-coupled CNT detectors. Here, we demonstrate a unique plasmonic approach for selectively growing CNTs in the gap of nanoantenna arrays for fabrication of plasmonic infrared bolometers operating at room temperature. Strong concentration of light at the tips of nanoantennas was utilized for localized heating and growth of CNTs. Moreover, interaction of this strong optical field with the small volume of CNTs enhanced the photoresponse of the bolometers. Consequently, a high responsivity of about 800 V W-1 was achieved at room temperature. [ABSTRACT FROM AUTHOR]

We developed a process to form transparent interconnections using graphene patterns that were synthesized by laser chemical vapor deposition. The number of graphene layers was tightly controlled by laser scan speed. Graphene patterns were fabricated at a high scan speed of up to 200 μm/s with a single-step process. The process time is about a million times faster than the conventional chemical vapor deposition method. The fabricated graphene patterns on nickel foils were directly transferred to desired positions on patterned electrodes. The position-controlled transfer with rapid single-step fabrication of graphene patterns provides an innovative pathway for graphene-based interconnections. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) of different alignments, such as surface-bounded and vertically aligned arrays, enable applications in different fields. In this study, controlled growth of CNTs with different alignments was achieved by electrically biasing catalyzed electrodes with different polarities in a laser-assisted chemical vapor deposition process. CNT growth was suggested to be guided by the movement of electrically charged catalyst-nanoparticles under the influence of an external electric field. This discovery provides a convenient approach to control the alignment of CNT arrays for different applications. [ABSTRACT FROM AUTHOR]

Employing the photoluminescence (PL) properties of the semiconducting single-walled carbon nanotubes (SWNTs) in applications is limited by their low quantum yield. Here, we report a simple method for enhancing the efficiency of PL emission by modulating the diameter and hence the bandgap of semiconducting SWNTs along their axes. The PL enhancement is explained by the exciton energy transfer via exciting SWNT segments of large bandgaps which induces emission from the neighboring segments of small bandgaps. The diameter-modulated SWNTs were grown by periodic modulation of the laser beam, and hence, the temperature in a laser assisted chemical vapor deposition process. [ABSTRACT FROM AUTHOR]

Gold-coated horizontally aligned carbon nanotube (Au-HA-CNT) substrates were fabricated for surface-enhanced Raman spectroscopy (SERS). The Au-HA-CNT substrates, which are granular in nature, are easy-to-prepare with large SERS-active area. Enhancement factors (EFs) of ∼107 were achieved using the Au-HA-CNTs as substrates for rhodamine 6G (R6G) molecules. Maximum enhancement was found when the polarization direction (E-field) of the incident laser beam was parallel to the aligned direction of the HA-CNTs. Simulations using the finite-difference time-domain (FDTD) method were carried out for the granular Au-HA-CNT samples. Enhancement mechanisms and determination of EFs were analyzed. Biological samples, including 13C- and deuterium (D)-labeled fatty acids and Coccomyxa sp. c-169 microalgae cells, were also measured using this SERS substrate. The limits of detection (LODs) of D- and 13C-labeled fatty acids on the SERS substrate were measured to be around 10 nM and 20 nM, respectively. Significantly enhanced Raman signals from the microalgae cells were acquired using the SERS substrate. [ABSTRACT FROM AUTHOR]

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons-quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS 2 . The low dielectric medium serves a dual purpose-it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS 2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc.

The interest in the wafer-scale growth of two-dimensional (2D) materials, including transition metal dichalcogenides (TMDCs), has been rising for transitioning from lab-scale devices to commercial-scale systems. Among various synthesis techniques, physical vapor deposition, such as pulsed laser deposition (PLD), has shown promise for the wafer-scale growth of 2D materials. However, due to the high volatility of chalcogen atoms (e.g., S and Se), films deposited by PLD usually suffer from a lack of stoichiometry and chalcogen deficiency. To mitigate this issue, excess chalcogen is necessary during the deposition, which results in problems like uniformity or not being repeatable. This study demonstrates a condensed-phase or amorphous phase-mediated crystallization (APMC) approach for the wafer-scale synthesis of 2D materials. This method uses a room-temperature PLD process for the deposition and formation of amorphous precursors with controlled thicknesses, followed by a post-deposition crystallization process to convert the amorphous materials to crystalline structures. This approach maintains the stoichiometry of the deposited materials throughout the deposition and crystallization process and enables the large-scale synthesis of crystalline 2D materials (e.g., MoS 2 and WSe 2 ) on Si/SiO 2 substrates, which is critical for future wafer-scale electronics. We show that the thickness of the layers can be digitally controlled by the number of laser pulses during the PLD phase. Optical spectroscopy is used to monitor the crystallization dynamics of amorphous layers as a function of annealing temperature. The crystalline quality, domain sizes, and the number of layers were explored using nanoscale and atomistic characterization (e.g., AFM, STEM, and EDS) along with electrical characterization to explore process-structure-performance relationships. This growth technique is a promising method that could potentially be adopted in conventional semiconductor industries for wafer-scale manufacturing of next-generation electronic and optoelectronic devices.

Understanding and controlling the growth evolution of atomically thin monolayer two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) are vital for next-generation 2D electronics and optoelectronic devices. However, their growth kinetics are not fully observed or well understood due to the bottlenecks associated with the existing synthesis methods. This study demonstrates the time-resolved and ultrafast growth of 2D materials by a laser-based synthesis approach that enables the rapid initiation and termination of the vaporization process during crystal growth. The use of stoichiometric powder (e.g., WSe 2 ) minimizes the complex chemistry during the vaporization and growth process, allowing rapid initiation/termination control over the generated flux. An extensive set of experiments is performed to understand the growth evolution, achieving subsecond growth as low as 10 ms along with a 100 μm/s growth rate on a noncatalytic substrate such as Si/SiO 2 . Overall, this study allows us to observe and understand the 2D crystal evolution and growth kinetics with time-resolved and subsecond time scales.

Titanium is widely used in medical devices, such as dental and orthopedic implants, due to its excellent mechanical properties, low toxicity, and biocompatibility. However, the titanium surface has the risk of microbial biofilm formation, which results in infections from species such as Candida albicans ( C. albicans ). This kind of biofilm prevents antifungal therapy and complicates the treatment of infectious diseases associated with implanted devices. It is critical to developing a feasible surface to decrease microbial growth while not interfering with the growth of the host cells. This study reports the influence of titanium surface modification to titanium disulfide (TiS 2 ) on inhibiting C. albicans biofilm formation while allowing the attachment of human gingival fibroblasts (HGFs) on their surface. The surface of titanium parts is directly converted to structured titanium and TiS 2 using direct laser processing and crystal growth methods. C. albicans adhesion and colonization are then investigated on these surfaces by the colony counting assay and reactive oxygen species (ROS) assay, using 2',7'-dichlorofluorescin diacetate (DCFH-DA) and microscopy images. Also, the viability and adhesion of HGFs on these surfaces are investigated to show their adhesion and biocompatibility. Titanium samples with the TiS 2 surface show both C. albicans biofilm inhibition and HGF attachment. This study provides insight into designing and manufacturing titanium biomedical implants.

This work presents a novel, to the best of our knowledge, cross correlation technique for determining the laser heating-induced Raman shift laser power coefficient ψ required for energy transport state-resolved Raman (ET-Raman) methods. The cross correlation method determines the measure of similarity between the experimental intensity data and a varying test Gaussian signal. By circumventing the errors inherent in any curve fittings, the cross correlation method quickly and accurately determines the location where the test Gaussian signal peak is most like the Raman peak, thereby revealing the peak location and ultimately the value of ψ. This method improves the reliability of optothermal Raman-based methods for micro/nanoscale thermal measurements and offers a robust approach to data processing through a global treatment of Raman spectra.

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions. Moreover, these systems exhibit a dramatic slowing down of exciton dynamics (100s of picoseconds to few nanoseconds) with an increase in carrier densities, which strongly contrasts with the monolayers in their freestanding form as well as in comparison with more traditionally grown TMDs. The observations are related to the modifications of the electronic bands as expected from the strain and associated population of the intervalley dark excitons that can now interplay with intravalley excitations. These findings are consistent across both the Mo- and W-based TMD families, providing key information about the influence of the growth conditions on the nature of optical excitations and fostering emerging optoelectronic applications of monolayer TMDs., (© 2022 Wiley-VCH GmbH.)

The emergence of rapidly expanding infectious diseases such as coronavirus (COVID-19) demands effective biosensors that can promptly detect and recognize the pathogens. Field-effect transistors based on semiconducting two-dimensional (2D) materials (2D-FETs) have been identified as potential candidates for rapid and label-free sensing applications. This is because any perturbation of such atomically thin 2D channels can significantly impact their electronic transport properties. Here, we report the use of FET based on semiconducting transition metal dichalcogenide (TMDC) WSe 2 as a promising biosensor for the rapid and sensitive detection of SARS-CoV-2 in vitro . The sensor is created by functionalizing the WSe 2 monolayers with a monoclonal antibody against the SARS-CoV-2 spike protein and exhibits a detection limit of down to 25 fg/μL in 0.01X phosphate-buffered saline (PBS). Comprehensive theoretical and experimental studies, including density functional theory, atomic force microscopy, Raman and photoluminescence spectroscopies, and electronic transport properties, were performed to characterize and explain the device performance. The results demonstrate that TMDC-based 2D-FETs can potentially serve as sensitive and selective biosensors for the rapid detection of infectious diseases.

Two-dimensional transition metal dichalcogenides (2D-TMDs) hold a great potential to platform future flexible optoelectronics. The beating hearts of these materials are their excitons known as X A and X B , which arise from transitions between spin-orbit split (SOS) levels in the conduction and valence bands at the K-point. The functionality of 2D-TMD-based devices is determined by the dynamics of these excitons. One of the most consequential channels of exciton decay on the device functionality is the defect-assisted recombination (DAR). Here, we employ steady-state absorption and emission spectroscopies, and pump density-dependent femtosecond transient absorption spectroscopy to report on the effect of DAR on the lifetime of excitons in monolayers of tungsten disulfide (2D-WS 2 ) and diselenide (2D-WSe 2 ). These pump-probe measurements suggested that while exciton decay dynamics in both monolayers are driven by DAR, in 2D-WS 2 , defect states near the X B exciton fill up before those near the X A exciton. However, in the 2D-WSe 2 monolayer, the defect states fill up similarly. Understanding the contribution of DAR on the lifetime of excitons and the partition of this decay channel between X A and X B excitons may open new horizons for the incorporation of 2D-TMD materials in future optoelectronics.

Direct synthesis, large-scale integration, and patterning of two-dimensional (2D) quantum materials (e.g. MoS 2 , WSe 2 ) on flexible and transparent substrates are of high interest for flexible and conformal device applications. However, the growth temperatures (e.g. 850 °C) of the emerging 2D materials in the common gas-phase synthesis methods are well beyond the tolerances limit of flexible substrates, such as polydimethylsiloxane (PDMS). In addition, random nucleation and growth process in most growth systems limits the predicted integration and patterning freedoms. Here, we report a rapid direct laser crystallization and mask-free large-scale patterning of MoS 2 and WSe 2 crystals on PDMS substrates. A thin layer of stoichiometric amorphous 2D film is first laser-deposited via pulsed laser deposition (PLD) system onto the flexible substrates followed by a controlled crystallization and direct writing process using a tunable nanosecond laser (1064 nm). The influences of pulse duration, number of pulses, and the thickness of the deposited amorphous 2D layer on the crystallization of 2D materials are discussed. Optical spectroscopy and electrical characterizations are performed to confirm the quality of crystallized 2D materials on flexible substrates. This novel method opens up a new opportunity for the crystallization of complex patterns directly from computer-aided design models for the future 2D materials-based wearable, transparent, and flexible devices.

This work reports the interfacial thermal conductance ( G ) and radiative recombination efficiency (β), also known as photoluminescence quantum yield (PL QY), of monolayer WSe 2 flakes supported by fused silica substrates via energy-transport state-resolved Raman (ET-Raman). This is the first known work to consider the effect of radiative electron-hole recombination on the thermal transport characteristics of single-layer transition-metal dichalcogenides (TMDs). ET-Raman uses a continuous-wave laser for steady-state heating as well as nanosecond and picosecond lasers for transient energy transport to simultaneously heat the monolayer flakes and extract the Raman signal. The three lasers induce distinct heating phenomena that distinguish the interfacial thermal conductance and radiative recombination efficiency, which can then be determined in tandem with three-dimensional (3D) numerical modeling of the temperature rise from respective laser irradiation. For the five samples measured, G is found to range from 2.10 ± 0.14 to 15.9 ± 5.0 MW m -2 K -1 and β ranges from 36 ± 6 to 65 ± 7%. These values support the claim that interfacial phenomena such as surface roughness and two-dimensional (2D) material-substrate bonding strength play critical roles in interfacial thermal transport and electron-hole recombination mechanisms in TMD monolayers. It is also determined that low-level defect density enhances the radiative recombination efficiency of single-layer WSe 2 .

Excitons in two-dimensional transition metal dichalcogenide monolayers (2D-TMDs) are of essential importance due to their key involvement in 2D-TMD-based applications. For instance, exciton dissociation and exciton radiative recombination are indispensible processes in photovoltaic and light-emitting devices, respectively. These two processes depend drastically on the photogeneration efficiency and lifetime of excitons. Here, we incorporate femtosecond pump-probe spectroscopy to investigate the ultrafast dynamics of exciton formation and decay in a single crystal of monolayer 2D tungsten disulfide (WS2). Investigation of the formation dynamics of the lowest exciton (XA) indicated that the formation time linearly increases from ∼150 fs upon resonant excitation, to ∼500 fs following excitation that is ∼1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of XA. This is confirmed by infrared measurements of electron intraband relaxation dynamics. Furthermore, pump-probe experiments suggested that the XA ground state depletion recovery dynamics depend on the excitation energy as well. The average recovery time increased from ∼10 ps in the case of resonant excitation to ∼50 ps following excitation well above the band-gap. Having the ability to control whether generating short-lived or long-lived electron-hole pairs in 2D-TMD monolayers opens a new horizon for the application of these materials. For instance, long-lived electron-hole pairs are appropriate for photovoltaic devices, but short-lived excitons are more beneficial for lasers with ultrashort pulses.

Titanium has been the material of interest in biological implant applications due to its unique mechanical properties and biocompatibility. Their design is now growing rapidly due to the advent of additive manufacturing technology that enables the fabrication of complex and patient-customized parts. Titanium dioxides (TiO 2 ) coatings with different phases (e.g., anatase, rutile) and morphologies have shown to be effective in enhancing osteointegration and antibacterial behavior. This enhanced antibacterial behavior stems from the photocatalytic activity generated from crystalline TiO 2 coatings. Anatase has commonly been shown to be a more photocatalytic oxide phase compared to rutile despite its larger band gap. However, more recent studies have suggested that a synergistic effect leading to increased photocatalytic activity may be produced with a combination of oxides containing both anatase and rutile phases. Here, we demonstrate the selective and localized formation of TiO 2 nanostructures on additive and wrought titanium parts with anatase, rutile, and mixed phases by a laser-induced transformation approach. Compared to conventional coating processes, this technique produces desired TiO 2 phases simply by controlled laser irradiation of titanium parts in an oxygen environment, where needed. The effects of processing conditions such as laser power, scanning speed, laser pulse duration, frequency, and gas flow on the selective transformation were studied. The morphological and structural evolutions were investigated using various characterization techniques. This method is specifically of significant interest in creating phase-selective TiO 2 surfaces on titanium-based bioimplants, including those fabricated by additive manufacturing technologies., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Interest in layered two-dimensional (2D) materials has been escalating rapidly over the past few decades due to their promising optoelectronic and photonic properties emerging from their atomically thin 2D structural confinements. When these 2D materials are further confined in lateral dimensions toward zero-dimensional (0D) structures, 2D nanoparticles and quantum dots with new properties can be formed. Here, we report a nonequilibrium gas-phase synthesis method for the stoichiometric formation of gallium selenide (GaSe) nanoparticles ensembles that can potentially serve as quantum dots. We show that the laser ablation of a target in an argon background gas condenses the laser-generated plume, resulting in the formation of metastable nanoparticles in the gas phase. The deposition of these nanoparticles onto the substrate results in the formation of nanoparticle ensembles, which are then post-processed to crystallize or sinter the nanoparticles. The effects of background gas pressures, in addition to crystallization/sintering temperatures, are systematically studied. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, and time-correlated single-photon counting (TCSPC) measurements are used to study the correlations between growth parameters, morphology, and optical properties of the fabricated 2D nanoparticle ensembles.

Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem, which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here, we reveal how topographic curvature of lithographically designed substrate features govern the strain and growth dynamics of triangular WS 2 monolayer single crystals. Single crystals grow conformally without strain over deep trenches and other features with zero Gaussian curvature; however, features with nonzero Gaussian curvature can easily impart sufficient strain to initiate grain boundaries and fractured growth in different directions. Within a strain-tolerant regime, however, triangular single crystals can accommodate considerable (<1.1%) localized strain exerted by surface features that shift the bandgap up to 150 meV. Within this regime, the crystal growth accelerates in specific directions, which we describe using a growth model. These results present a previously unexplored strategy to strain-engineer the growth directions and optoelectronic properties of 2D crystals.

In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques. Here we show that transient absorption measurements with high spatial and temporal resolution can be used to directly monitor such transfer processes. We studied MoS 2 -MoSe 2 in-plane heterostructures fabricated by chemical vapor deposition and lithographic patterning followed by laser-generated vapor sulfurization. Transient absorption measurements in reflection geometry revealed evidence of exciton transfer from MoS 2 to MoSe 2 . By comparing the experimental data with a simulation, we extracted an exciton transfer velocity of 10 4 m s -1 . These results provide valuable information for understanding and controlling in-plane carrier transfer in two-dimensional lateral heterostructures for their electronic and optoelectronic applications.

Nonequilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gas-phase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally and spatially resolved gated-intensified charge couple device (ICCD) imaging and ion probe measurements are employed as in situ diagnostics to understand and control the plume expansion conditions for the synthesis of nearly pure fluxes of ultrasmall (∼3 nm) amorphous TiO 2 nanoparticles in background gases and their selective delivery to substrates. These amorphous nanoparticles assemble into loose, mesoporous assemblies on substrates at room temperature but dynamically crystallize by sequential particle attachment at higher substrate temperatures to grow nanostructures with different phases and morphologies. Molecular dynamics calculations are used to simulate and understand the crystallization dynamics. This work demonstrates that nonequilibrium crystallization by particle attachment of metastable ultrasmall nanoscale "building blocks" provides a versatile approach for exploring and controlling the growth of nanoarchitectures with desirable crystalline phases and morphologies.

We demonstrate that UV-light activation of polycrystalline ZnO films on flexible polyimide (Kapton) substrates can be used to detect and differentiate between environmental changes in oxygen and water vapor. The in-plane resistive and impedance properties of ZnO films, fabricated from bacteria-derived ZnS nanoparticles, exhibit unique resistive and capacitive responses to changes in O 2 and H 2 O. We propose that the distinctive responses to O 2 and H 2 O adsorption on ZnO could be utilized to statistically discriminate between the two analytes. Molecular dynamic simulations (MD) of O 2 and H 2 O adsorption energy on ZnO surfaces were performed using the large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) with a reactive force-field (ReaxFF). These simulations suggest that the adsorption mechanisms differ for O 2 and H 2 O adsorption on ZnO, and are governed by the surface termination and the extent of surface hydroxylation. Electrical response measurements, using DC resistance, AC impedance spectroscopy, and Kelvin Probe Force Microscopy (KPFM), demonstrate differences in response to O 2 and H 2 O, confirming that different adsorption mechanisms are involved. Statistical and machine learning approaches were applied to demonstrate that by integrating the electrical and kinetic responses the flexible ZnO sensor can be used for detection and discrimination between O 2 and H 2 O at low temperature.

We use amorphous titania nanoparticle networks produced by pulsed laser vaporization at room temperature as a model system for understanding the mechanism of formation of black titania. Here, we characterize the transformation of amorphous nanoparticles by annealing in pure Ar at 400 °C, the lowest temperature at which black titania was observed. Atomic resolution electron microscopy methods and electron energy loss spectroscopy show that the onset of crystallization occurs by nucleation of an anatase core that is surrounded by an amorphous Ti 2 O 3 shell. The formation of the metastable anatase core before the thermodynamically stable rutile phase occurs according to the Ostwald phase rule. In the second stage the particle size increases by coalescence of already crystallized particles by a self-organized mechanism of crystallization by particle attachment. We show that the Ti 2 O 3 shell plays a critical role in both black titania transformation and functionality. At 400 °C, Ti 2 O 3 hinders the agglomeration of neighboring particles to maintain a high surface-to-volume ratio that is beneficial for enhanced photocatalytic activity. In agreement with previous results, the thin Ti 2 O 3 surface layer acts as a narrow bandgap semiconductor in concert with surface defects to enhance the photocatalytic activity. Our results demonstrate that crystallization by particle attachment can be a highly effective mechanism for optimizing photocatalytic efficiency by controlling the phase, composition, and particle size distribution in a wide range of self-doped defective TiO 2 architectures simply by varying the annealing conditions of amorphous nanoparticles.

Photoinduced interfacial charge transfer is at the heart of many applications, including photovoltaics, photocatalysis, and photodetection. With the emergence of a new class of semiconductors, i.e., monolayer two-dimensional transition metal dichalcogenides (2D-TMDs), charge transfer at the 2D/2D heterojunctions has attracted several efforts due to the remarkable optical and electrical properties of 2D-TMDs. Unfortunately, in 2D/2D heterojunctions, for a given combination of two materials, the relative energy band alignment and the charge-transfer efficiency are locked. Due to their large variety and broad size tunability, semiconductor quantum dots (0D-QDs) interfaced with 2D-TMDs may become an attractive heterostructure for optoelectronic applications. Here, we incorporate femtosecond pump-probe spectroscopy to reveal the sub-45 fs charge transfer at a 2D/0D heterostructure composed of tungsten disulfide monolayers (2D-WS 2 ) and a single layer of cadmium selenide/zinc sulfide core/shell 0D-QDs. Furthermore, ultrafast dynamics and steady-state measurements suggested that, following electron transfer from the 2D to the 0D, hybrid excitons, wherein the electron resides in the 0D and the hole resides in the 2D-TMD monolayer, are formed with a binding energy on the order of ∼140 meV, which is several times lower than that of tightly bound excitons in 2D-TMDs.

A promising way to advance perovskite solar cells is to improve the quality of the electron transport material -e.g., titanium dioxide (TiO 2 ) - in a direction that increases electron transport and extraction. Although dense TiO 2 films are easily grown in solution, efficient electron extraction suffers due to a lack of interfacial contact area with the perovskites. Conversely, mesoporous films do offer high surface-area-to-volume ratios, thereby promoting efficient electron extraction, but their morphology is relatively difficult to control via conventional solution synthesis methods. Here, a pulsed laser deposition method was used to assemble TiO 2 nanoparticles into TiO 2 hierarchical architectures exhibiting an anatase crystal structure, and prototype solar cells employing these structures yielded power conversion efficiencies of ∼14%. Our approach demonstrates a way to grow high aspect-ratio TiO 2 nanostructures for improved interfacial contact between TiO 2 and perovskite materials, leading to high electron-hole pair separation and electron extraction efficiencies for superior photovoltaic performance. Compared to previous pulsed laser deposition-synthesized TiO 2 mesoporous crystalline networks that needed post-thermal annealing at 500 °C to form mesoporous crystalline networks, our relatively low temperature (300 °C) TiO 2 processing method may promote reduced energy-consumption during device fabrication, as well as enable compatibility with flexible polymer substrates such as polyimide.

Defect engineering has been a critical step in controlling the transport characteristics of electronic devices, and the ability to create, tune, and annihilate defects is essential to enable the range of next-generation devices. Whereas defect formation has been well-demonstrated in three-dimensional semiconductors, similar exploration of the heterogeneity in atomically thin two-dimensional semiconductors and the link between their atomic structures, defects, and properties has not yet been extensively studied. Here, we demonstrate the growth of MoSe2-x single crystals with selenium (Se) vacancies far beyond intrinsic levels, up to ∼20%, that exhibit a remarkable transition in electrical transport properties from n- to p-type character with increasing Se vacancy concentration. A new defect-activated phonon band at ∼250 cm(-1) appears, and the A1g Raman characteristic mode at 240 cm(-1) softens toward ∼230 cm(-1) which serves as a fingerprint of vacancy concentration in the crystals. We show that post-selenization using pulsed laser evaporated Se atoms can repair Se-vacant sites to nearly recover the properties of the pristine crystals. First-principles calculations reveal the underlying mechanisms for the corresponding vacancy-induced electrical and optical transitions.

Development of devices and structures based on the layered 2D materials critically hinges on the capability to induce, control, and tailor the electronic, transport, and optoelectronic properties via defect engineering, much like doping strategies have enabled semiconductor electronics and forging enabled introduction the of iron age. Here, we demonstrate the use of a scanning helium ion microscope (HIM) for tailoring the functionality of single layer MoSe2 locally, and decipher associated mechanisms at the atomic level. We demonstrate He(+) beam bombardment that locally creates vacancies, shifts the Fermi energy landscape and increases the Young's modulus of elasticity. Furthermore, we observe for the first time, an increase in the B-exciton photoluminescence signal from the nanoforged regions at the room temperature. The approach for precise defect engineering demonstrated here opens opportunities for creating functional 2D optoelectronic devices with a wide range of customizable properties that include operating in the visible region.

van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm(-1) that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe2 A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump-probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump-probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe2/WS2 bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.

Carbon nanotube yarn microelectrodes (CNTYMEs) exhibit rapid and selective detection of dopamine with fast-scan cyclic voltammetry (FSCV); however, the sensitivity limits their application in vivo . In this study, we introduce laser treatment as a simple, reliable, and efficient approach to improve the sensitivity of CNTYMEs by three fold while maintaining high temporal resolution. The effect of laser treatment on the microelectrode surface was characterized by scanning electron microscopy, Raman spectroscopy, energy dispersion spectroscopy, and laser confocal microscopy. Laser treatment increases the surface area and oxygen containing functional groups on the surface, which provides more adsorption sites for dopamine than at unmodified CNTYMEs. Moreover, similar to unmodified CNTYMEs, the dopamine signal at laser treated CNTYMEs is not dependent on scan repetition frequency, unlike the current at carbon fiber microelectrodes (CFMEs) which decreases with increasing scan repetition frequency. This frequency independence is caused by the significantly larger surface roughness which would trap dopamine-o-quinone and amplify the dopamine signal. CNTYMEs were applied as an in vivo sensor with FSCV for the first time and laser treated CNTYMEs maintained high dopamine sensitivity compared to CFMEs with an increased scan repetition frequency of 50 Hz, which is five-fold faster than the conventional frequency. CNTYMEs with laser treatment are advantageous because of their easy fabrication, high reproducibility, fast electron transfer kinetics, high sensitivity, and rapid in vivo measurement of dopamine and could be a potential alternative to CFMEs in the future.

The remarkable properties of black TiO2 are due to its disordered surface shell surrounding a crystalline core. However, the chemical composition and the atomic and electronic structure of the disordered shell and its relationship to the core remain poorly understood. Using advanced transmission electron microscopy methods, we show that the outermost layer of black TiO2 nanoparticles consists of a disordered Ti2O3 shell. The measurements show a transition region that connects the disordered Ti2O3 shell to the perfect rutile core consisting first of four to five monolayers of defective rutile, containing clearly visible Ti interstitial atoms, followed by an ordered reconstruction layer of Ti interstitial atoms. Our data suggest that this reconstructed layer presents a template on which the disordered Ti2O3 layers form by interstitial diffusion of Ti ions. In contrast to recent reports that attribute TiO2 band-gap narrowing to the synergistic action of oxygen vacancies and surface disorder of nonspecific origin, our results point to Ti2O3, which is a narrow-band-gap semiconductor. As a stoichiometric compound of the lower oxidation state Ti(3+) it is expected to be a more robust atomic structure than oxygen-deficient TiO2 for preserving and stabilizing Ti(3+) surface species that are the key to the enhanced photocatalytic activity of black TiO2.

The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an approach for the formation of lithographically patterned arrays of lateral semiconducting heterojunctions within a single two-dimensional crystal. Electron beam lithography is used to pattern MoSe2 monolayer crystals with SiO2, and the exposed locations are selectively and totally converted to MoS2 using pulsed laser vaporization of sulfur to form MoSe2/MoS2 heterojunctions in predefined patterns. The junctions and conversion process are studied by Raman and photoluminescence spectroscopy, atomically resolved scanning transmission electron microscopy and device characterization. This demonstration of lateral heterojunction arrays within a monolayer crystal is an essential step for the integration of two-dimensional semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin devices.

The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm(-1)) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries.

Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe2) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe2 are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD-VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications.

We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of single-walled carbon nanohorns (CNHs) from common solvents such as acetone and water onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams, in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on time scales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, including nonconductive substrates. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications, among others.

The direct formation of graphene on various dielectric surfaces is successful via a single-step rapid thermal processing (RTP) of substrates coated with amorphous carbon (C) and nickel (Ni) thin films. High-quality graphene is obtained uniformly on the whole surface of wafers with a controlled number of graphene layers. The monolayer graphene exhibits a low sheet resistance and a high optical transmittance in the visible range., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Diameter modulation by fast temperature control in laser-assisted chemical vapor deposition (LCVD) was successfully achieved to tune the diameters of single-walled carbon nanotubes (SWNTs) in different segments. Due to the inverse relationship between the SWNT diameter and the growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were integrated in electrodes to form field-effect transistors (FETs) and to investigate their electronic transport properties. The SWNTs in the FET structures have electronic properties similar to Schottky diodes, indicating clear evidence of different bandgap structures at the two ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of the SWNTs.

Where it starts and where it goes? Controlled integration of single-walled carbon nanotubes (SWNTs) into pre-designed nano-architectures is one of the major challenges to be overcome for extensive scientific research and technological applications. Various serial assembly techniques have been proposed and developed. However, they are still a long way from practical applications due to the drawbacks on reliability, yield and cost. Here we demonstrate a laser-based strategy to achieve parallel integration of SWNTs into pre-designed nano-architectures through an optically controlled in situ growth process. Optical driving forces originated from tip-induced optical near-field enhancement and laser beam polarization were applied in this study to realize the controlled placement of SWNTs at designated sites following wanted orientations on the nanometer scale. Parallel integration of SWNT arrays was achieved by adjusting laser beam diameter to cover interested nano-architectures. The laser-based process suggests an efficient and cost-effective approach for fabricating and integrating SWNT-based devices and circuits.

Distinguishing between carbon nanotubes (CNTs) according to their individual electronic properties is of significant importance for developing CNT-based electronics and devices. In this study, selective removal of metallic CNTs from CNT mixtures on silicon substrates was investigated using controlled laser irradiation. Free electron movement and eddy currents are induced within the metallic CNTs by the strong electric field and optical near-field effects caused by the laser irradiation. Selective heating of metallic CNTs in air results in selective removal of metallic CNTs when the laser fluence and wavelength are properly selected. Through this process, metallic nanotubes are successfully removed from the CNT mixtures. This technique provides an efficient single-step approach for selective removal of metallic CNTs from CNT mixtures.

Self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized by utilizing optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process. By introducing the optical near-field effects, bridge structures containing single suspended SWNT channels were successfully fabricated through the LCVD process at a relatively low substrate temperature. Raman spectroscopy and I-V analyses have been carried out to characterize the SWNT-bridge structures. Numerical simulations using a high-frequency structure simulator revealed that significant enhancement of local heating occurs at metallic electrode tips under laser irradiation; it is about one order of magnitude higher than that in the rest of the electrodes. This technique suggests a novel approach to in situ low-temperature fabrication of SWNT-based devices in a precisely controlled manner, based on the nanoscale heating enhancement induced by the optical near-field effects.