Exfoliation
of large-area monolayers is important for fundamental
research and technological implementation of transition-metal dichalcogenides.
Various techniques have been explored to increase the exfoliation
yield, but little is known about the underlying mechanism at the atomic
level. Here, we demonstrate gold-assisted mechanical exfoliation of
monolayer molybdenum disulfide, up to a centimeter scale. Detailed
spectroscopic, microscopic, and first-principles density functional
theory analyses reveal that strong van der Waals (vdW) interaction
between Au and the topmost MoS2 layer facilitates the exfoliation
of monolayers. However, the large-area exfoliation promoted by such
strong vdW interaction is only achievable on freshly prepared clean
and smooth Au surfaces, while rough surfaces and surfaces exposed
to air for more than 15 min result in negligible exfoliation yields.
This technique is successfully extended to MoSe2, WS2, WSe2, MoTe2, WTe2, and
GaSe. In addition, electrochemical characterization reveals intriguing
interactions between monolayer MoS2 and Au. A subnanometer-thick
MoS2 monolayer strongly passivates the chemical properties
of the underlying Au, and the Au significantly modulates the electronic
band structure of the MoS2, turning it from semiconducting
to metallic. This could find applications in many areas, including
electrochemistry, photovoltaics, and photocatalysis.
The charge carrier dynamics of opaque, aqueous
suspensions of Degussa P-25 TiO2 are probed
with
femtosecond time-resolved diffuse reflectance spectroscopy.
Comparison of ultrafast pump−probe diffuse
reflectance measurements of P-25 suspensions with dry P-25 powder and
the transient absorption of transparent,
aqueous Q-TiO2 solutions allows the observed kinetics to be
assigned to charge carrier recombination. The
electron−hole recombination kinetics are consistent with a
second-order process as demonstrated by a laser
fluence dependence study. Interfacial hole transfer dynamics of
the P-25 TiO2/SCN- complex are probed
as
a function of thiocyanate ion concentration. A dramatic increase
in the population of trapped charge carriers
is observed within the first few picoseconds, demonstrating that
interfacial charge transfer of an electron
from the SCN- to a hole on the photoexcited
TiO2 effectively competes with electron−hole
recombination
on an ultrafast time scale. The experimental dependence of the
charge carrier dynamics are shown to be
consistent with a kinetic model of competing second-order processes.
The implications of the results on the
use of nanoscale TiO2 for photocatalysis are
discussed.
In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement in dielectric constant was generally associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artifact produced by carrier migration to interfaces within the dielectric. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be attributed to the Maxwell–Wagner effect.
We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown by pulsedlaser deposition, on a variety of substrates and with a range of thicknesses. Results highlight the importance of epitaxy, and show that NdNiO 3 , with a sharp metal-insulator phase transition, can be fabricated without the need for high-pressure processing. The conductivity of the nickelate films was found to be well described by a linear sum of activated transport and Mott's variable range hopping in the entire measured temperature range of the semiconducting state, and this description was also found to provide an accurate fit for previously published transport properties of bulk ceramics. The transition was subsequently modeled using a percolative approach. It was found that the temperature of the metal-insulator phase transition, in both our films and in bulk, corresponded to a critical percolation threshold where the volume fraction of the semiconducting phase (V s ) was 2 3 , as expected for a three-dimensional cubic lattice. For the thinnest films grown on NdGaO 3 , a possible crossover to two-dimensional percolation was indicated by V s ϭ 1 2 .
The charge carrier dynamics of several T1O2 powders are investigated using femtosecond time-resolved diffusereflectance spectroscopy. Ultrafast pump-probe measurements of transparent solutions of 2-nm T1O2 nanoclusters and the transient diffuse reflection observed in dry powders of Ti02 are found to be qualitatively similar. The comparison allows (i) the decay kinetics observed by time-resolved diffuse reflection in powders to be attributed to electron-hole recombination and (ii) the unequivocal assignment of the long-lived absorption in nanocluster solutions of T1O2 to trapped electrons. The conclusions are further supported by an intensitydependent study of the electron-hole recombination kinetics of Ti02 powder which was consistent with a second-order process. The sensitivity of the technique is demonstrated by comparison of the recombination dynamics of powders of differing size and crystal structure and with Fe(III) dopants. The results are discussed in terms of the photoreactivities of the various Ti02 materials.
A Maxwell–Wagner series capacitor model is proposed to explain anomalous dielectric properties of ferroelectric superlattices. The results of the model show that a superlattice consisting of normal ferroelectric layers separated by low-resistivity interfacial regions can account for most experimental results reported to date, namely: dielectric enhancement for certain stacking periodicities, giant permittivities, and temperature migration of dielectric maxima as a function of frequency. The predictions of the model are discussed and compared to our own experimental results from thin film superlattice capacitors made by pulsed-laser deposition.
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