8 research outputs found
Urban land expansion model based on SLEUTH, a case study in Dongguan city, China
The SLEUTH urban model is developed with sets of predefined growing rules involving Spontaneous Growth, New Spreading Center Growth, Edge Growth, Road Influenced Growth and Self-modification. They are applied continuously to lead the urban simulation to a specific morphology. A SLEUTH land use model was set up to simulate urban growth trajectory of Dongguan city from 1997 to 2009. The accuracy of localized parameters was evaluated to illuminate the growth pattern of Dongguan. Two different scenarios were set to predict the urban development from 2022 to 2030. Edge Growth is the dominant force of Dongguan's urbanization: regions adjacent to growth centers are more likely to be urbanized than remote area in general. Rapid urban expansion takes up large amount of other land types, around 2030, urbanization will reach the critical state in spatial. Unlike excessive growth rate in scenario 1, the urbanization speed is obviously more reasonable and sustainable in scenario 2, which confirms SLEUTH urban model is a good assistant of urban planning to avoid willful expansion with a scenario forecast. To protect ecological environment and promoting sustainable development of the region, relevant decision makers should take effective strategies to control urban sprawl. By the set of forecast scenarios, SLEUTH can certainly predict future urban development as an auxiliary to urban planners and government.Dongguan is under rapid urbanization in these decades. SLEUTH is an urban land use model named after the six input layers (Slope, Land use, Excluded, Urban, Transportation and Hill shade), and it is applied for simulating how surrounding land use changes due to urban expansion. A SLEUTH model was coupled with multi-source GIS (Geographic Information Systems) and RS (Remote Sensing) data to simulate urban growth trajectory of Dongguan city from 1997 to 2009. The accuracy of localized parameters was evaluated to illuminate the growth pattern of Dongguan. Based on the hypothesis that the urbanization process is as fast as before, a historical scenario from 2010 to 2050 was built up to choose the suitable study periods. In order to prove SLEUTH is able to offer reasonable outcomes for urban plan, two different scenarios were set to predict the urban development from 2022 to 2030, which shows SLEUTH is able to offer reasonable outcomes to government policy makers. Finally, the dynamic mechanism of urban growth combined with local characteristics was discussed. Some suggestions were also proposed for future urban planning and policy making in this study
Multiplicative Jensen's formula and quantitative global theory of one-frequency Schr\"odinger operators
We introduce the concept of dual Lyapunov exponents, leading to a
multiplicative version of the classical Jensen's formula for one-frequency
analytic Schr\"odinger cocycles. This formula, in particular, gives a new proof
and a quantitative version of the fundamentals of Avila's global theory
\cite{avila}, fully explaining the behavior of complexified Lyapunov exponent
through the dynamics of the dual cocycle. In particular, concepts of
(sub/super) critical regimes and acceleration are all explained (in a
quantitative way) through the duality approach. This leads to a number of
powerful spectral and physics applicationsComment: 40 page
Pressure-Tailored Band Gap Engineering and Structure Evolution of Cubic Cesium Lead Iodide Perovskite Nanocrystals
Metal
halide perovskites (MHPs) have attracted increasing research
attention given the ease of solution processability with excellent
optical absorption and emission qualities. However, effective strategies
for engineering the band gap of MHPs to satisfy the requirements of
practical applications are difficult to develop. Cubic cesium lead
iodide (α-CsPbI<sub>3</sub>), a typical MHP with an ideal band
gap of 1.73 eV, is an intriguing optoelectric material owing to the
approaching Shockley–Queisser limit. Here, we carried out a
combination of in situ photoluminescence, absorption, and angle-dispersive
synchrotron X-ray diffraction spectra to investigate the pressure-induced
optical and structural changes of α-CsPbI<sub>3</sub> nanocrystals
(NCs). The α-CsPbI<sub>3</sub> NCs underwent a phase transition
from cubic (α) to orthorhombic phase and subsequent amorphization
upon further compression. The structural changes with octahedron distortion
to accommodate the Jahn–Teller effect were strongly responsible
for the optical variation with the increase of pressure. First-principles
calculations reveal that the band-gap engineering is governed by orbital
interactions within the inorganic Pb–I frame through the structural
modification. Our high-pressure studies not only established structure–property
relationships at the atomic scale of α-CsPbI<sub>3</sub> NCs,
but also provided significant clues in optimizing photovoltaic performance,
thus facilitating the design of novel MHPs with increased stimulus-resistant
capability
Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals
Metal
halide perovskites (MHPs) are gaining increasing interest
because of their extraordinary performance in optoelectronic devices
and solar cells. However, developing an effective strategy for achieving
the band-gap engineering of MHPs that will satisfy the practical applications
remains a great challenge. In this study, high pressure is introduced
to tailor the optical and structural properties of MHP-based cesium
lead bromide nanocrystals (CsPbBr<sub>3</sub> NCs), which exhibit
excellent thermodynamic stability. Both the pressure-dependent steady-state
photoluminescence and absorption spectra experience a stark discontinuity
at ∼1.2 GPa, where an isostructural phase transformation regarding
the <i>Pbnm</i> space group occurs. The physical origin
points to the repulsive force impact due to the overlap between the
valence electron charge clouds of neighboring layers. Simultaneous
band-gap narrowing and carrier-lifetime prolongation of CsPbBr<sub>3</sub> trihalide perovskite NCs were also achieved as expected,
which facilitates the broader solar spectrum absorption for photovoltaic
applications. Note that the values of the phase change interval and
band-gap red-shift of CsPbBr<sub>3</sub> nanowires are between those
for CsPbBr<sub>3</sub> nanocubes and the corresponding bulk counterparts,
which results from the unique geometrical morphology effect. First-principles
calculations unravel that the band-gap engineering is governed by
orbital interactions within the inorganic Pb–Br frame through
structural modification. Changes of band structures are attributed
to the synergistic effect of pressure-induced modulations of the Br–Pb
bond length and Pb–Br–Pb bond angle for the PbBr<sub>6</sub> octahedral framework. Furthermore, the significant distortion
of the lead–bromide octahedron to accommodate the Jahn–Teller
effect at much higher pressure would eventually lead to a direct to
indirect band-gap electronic transition. This study enables high pressure
as a robust tool to control the structure and band gap of CsPbBr<sub>3</sub> NCs, thus providing insight into the microscopic physiochemical
mechanism of these compressed MHP nanosystems