Michel Bosman

Three-dimensional tubular arrays of MnO2–NiO nanoflakes with high areal pseudocapacitance

Transition metal oxide nanostructures are current research focus for energy storage applications. We herein report the synthesis of MnO2–NiO nanoflake-assembled tubular array on stainless steel substrate to function as pseudocapacitor electrode by programmed three-dimensional (3D) interfacial reactions, in which the ZnO nanowire array is employed as the low-cost in situ sacrificial template. In this 3D nanoelectrode, MnO2 and NiO nanoflakes share the same “root” and form an integrated hierarchical structure, which adheres robustly to the substrate. Importantly, both MnO2 and NiO contribute to the charge storage. The highly porous structure, which allows easy penetration of the electrolyte, gives additional merits. Detailed electrochemical characterization reveals that the assembled MnO2–NiO array exhibits good rate performance and cycle life. In particular, it displays an areal capacitance that is four orders of magnitude higher than that of carbonaceous materials and significantly superior to those of previous directly-grown pseudocapacitive nanostructure films.

Nanoplasmonics: Classical down to the Nanometer Scale

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light-matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms.

Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions

Controlling Quantum Plasmonics Electron tunneling across cavities could potentially induce a quantum mechanical plasmon mode that would be important in nano-electronics, catalysis, nonlinear optics, or single-molecule sensing, but has been expected to occur only at length scales beyond the reach of current state-of-the-art technology. Using a system of plasmonic dimers comprising silver nanocubes bridged by a molecular self-assembled monolayer, Tan et al. (p. 1496; see the Perspective by Nordlander) observed quantum plasmonic tunneling between the resonators and were able to tune the frequency of this tunneling plasmon resonance via selection of the molecular tunnel junctions. Moreover, the effects were observed at length scales that are technologically accessible. The optical properties of silver plasmonic dimers depend on the selection of bridging molecules. [Also see Perspective by Nordlander] Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. We describe the direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs). The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, we directly observe a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 terahertz) is dependent on the molecules bridging the gaps.

Au Nanoparticle‐Modified MoS2 Nanosheet‐Based Photoelectrochemical Cells for Water Splitting

The surface plasmon resonance of Au nanoparticle on MoS2 nanosheet is successfully measured by the electron energy-loss spectroscopy. Furthermore, Au-MoS2 composite is developed as the photoanode material in the photo-electronchemical cell (PEC) for water splitting. Compared to the pure MoS2 -based PEC, Au-MoS2 based PEC shows the enhanced performance in the photocatalytic water splitting.

One-Pot Synthesis of Cu1.94S−CdS and Cu1.94S−ZnxCd1−xS Nanodisk Heterostructures

Nanodisk heterostructures consisting of monoclinic Cu(1.94)S and wurtzite CdS have been colloidally synthesized for the first time. Initially, hexagonal-shaped nanodisks of Cu(1.94)S were produced upon thermolysis of a copper complex in a solvent mixture of HDA and TOA at 250 °C. Rapid addition of Cd precursor to the reaction mixture resulted in the partial conversion of Cu(1.94)S into CdS, yielding Cu(1.94)S-CdS nanoheterostructures. The original morphology of the Cu(1.94)S nanodisks was conserved during the transformation. When Zn precursor was added together with the Cd precursor, Cu(1.94)S-Zn(x)Cd(1-x)S nanodisks were generated. These two-component nanostructures are potentially useful in the fabrication of heterojunction solar cells.

Direct evidence of plasmon enhancement on photocatalytic hydrogen generation over Au/Pt-decorated TiO2 nanofibers.

Direct evidence of plasmon-enhanced H2 generation is observed in photocatalytic water reduction by using TiO2 electrospun nanofibers co-decorated with Au and Pt nanoparticles through dual-beam irradiation. The Au/Pt/TiO2 nanofibers exhibit certain activity for H2 generation under single irradiation at 420 nm that excites the defect/impurity states of TiO2. Significantly, when secondary irradiation at 550 nm is introduced to simultaneously excite Au SPR, we observed 2.5 times higher activity for H2 generation. Further investigation by finely controlling the irradiation wavelengths reveals that the enhancement factor on the photocatalytic activity for H2 generation is directly correlated with the plasmon absorption band of the Au nanoparticles in the Au/Pt/TiO2 nanofibers. The control experiments with different sacrificial agents suggest that the hot plasmonic electrons of Au are responsible for the enhanced photocatalytic activity that can be magnified when TiO2 is simultaneously excited.

Surface Plasmon Damping Quantified with an Electron Nanoprobe

Fabrication and synthesis of plasmonic structures is rapidly moving towards sub-nanometer accuracy in control over shape and inter-particle distance. This holds the promise for developing device components based on novel, non-classical electro-optical effects. Monochromated electron energy-loss spectroscopy (EELS) has in recent years demonstrated its value as a qualitative experimental technique in nano-optics and plasmonic due to its unprecedented spatial resolution. Here, we demonstrate that EELS can also be used quantitatively, to probe surface plasmon kinetics and damping in single nanostructures. Using this approach, we present from a large (>50) series of individual gold nanoparticles the plasmon Quality factors and the plasmon Dephasing times, as a function of energy/frequency. It is shown that the measured general trend applies to regular particle shapes (rods, spheres) as well as irregular shapes (dendritic, branched morphologies). The combination of direct sub-nanometer imaging with EELS-based plasmon damping analysis launches quantitative nanoplasmonics research into the sub-nanometer realm.

Surface modification-induced phase transformation of hexagonal close-packed gold square sheets

Conventionally, the phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). Here we report the complete phase transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can also be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs. Depending on the surfactants used during the metal coating process, two transformation pathways are observed, leading to the formation of (100)f-oriented fcc Au@Ag core-shell square sheets and (110)h/(101)f-oriented hcp/fcc mixed Au@Ag nanosheets. Furthermore, monochromated electron energy loss spectroscopy reveals the strong surface plasmon resonance absorption of fcc AuSS and Au@Ag square sheet in the infrared region. Our findings may offer a new route for the crystal-phase and shape-controlled synthesis of inorganic nanocrystals.

Fowler-nordheim tunneling induced charge transfer plasmons between nearly touching nanoparticles

Summary form only given. A plasmon resonant mode is the collective oscillation of free electrons in a structure stimulated by incident light. Reducing the gap between two metal nanoparticles down to atomic dimensions uncovers novel plasmon resonant modes. Of particular interest is a mode known as the charge transfer plasmon (CTP). This mode has been experimentally observed in touching nanoparticles, where charges can shuttle between the nanoparticles via a conductive path. However, the CTP mode for nearly touching nanoparticles has only been predicted theoretically to occur via direct tunneling when the gap is reduced to ~0.4 nm2. Because of challenges in fabricating and characterizing gaps at these dimensions, experiments have been unable to provide evidence for this plasmon mode that is supported by tunneling. In this work, we consider an alternative tunneling process, that is, the well-known Fowler-Nordheim (FN) tunneling that occurs at high electric fields, and apply it for the first time in the theoretical investigation of plasmon resonances between nearly touching nanoparticles. This new approach relaxes the requirements on gap dimensions, and intuitively suggests that with a sufficiently high-intensity irradiation, the CTP can be excited via FN tunneling for a range of subnanometer gaps. For instance, a plasmonic gap field of 1010 V/m (or an incident power of 3×1010 W/cm2) is needed when the gap length is 0.8 nm. The unique feature of FN tunneling induced CTP is the ability to turn on and off the charge transfer by varying the intensity of an external light source, and this could inspire the development of novel quantum devices, such as high speed switches and modulators.

Stabilization of 4H hexagonal phase in gold nanoribbons

Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.