Two-dimensional covalent organic frameworks (2D COFs) represent an ideal platform to develop novel technological applications. The integration of 2D COFs into thin-film device architectures requires ...a deep knowledge of their mechanical performance, especially as large-area ultrathin films. Here, we report the synthesis, transfer, and mechanical characterization of large-area freestanding 2D COF films with nanometer thicknesses. Imine-linked COF nanofilms are prepared by condensation reaction at air–water interface, which provides freestanding, uniform centimeter-scale 2D COF films with controlled thickness. The developed procedure enables the direct transfer of the synthetized large-area COF nanofilm onto patterned substrates for mechanical characterization. Tensile tests are performed on freestanding 2D COF films with 85 nm thickness and with a testing area as large as 0.3 mm2. The measured strength of the COF nanofilms is 188 ± 57 MPa, while the Young's modulus is 37 ± 15 GPa. Our findings not only demonstrate the high stiffness and strength of COF nanofilms over a large-area, which make them suitable for applications where high mechanical performance is required, but also pave the way for a fundamental understanding of the relationship between the structures and macroscopic mechanical properties of 2D COFs. Thus, the method that we developed herein will enable a broad exploration of the properties of large-area 2D COFs that will guide their engineering design toward the development of novel COF-based devices.
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•Uniform centimeter-scale imine-linked two-dimensional (2D) covalent organic framework (COF) films with 85 nm thickness were synthetized at air-water interface.•The COF nanofilms were transferred onto patterned silicon substrates without damage.•Tensile measurements were performed on freestanding 2D COF nanofilms with a testing area as large as 0.3 mm2.•2D COF revealed high strength of 188 ± 57 MPa and high stiffness, with a Young's modulus of 37 ± 15 GPa.
We present the results of a XAS experiment carried out at the Ti 2p edge on well-ordered TiO(x)@ ultrathin films grown on the Pt(1 1 1) surface. XAS at the Ti 2p edge has been extensively applied to ...the study of Ti bulk compounds, particularly to the study of titania. According to the literature, the corresponding Ti 2p edge spectral shape is related to the stoichiometry and crystal field symmetry at the Ti sites. In the present study we aim at extending the potential of the XAS technique by discussing Ti 2p spectra obtained on several, dimensionally confined, TiO(x)@ phases in the form of ultrathin films. One of the main features of these films is their high degree of structural order. Furthermore, the results of previous studies provide valuable information about the chemistry and structure of the films, so that we are able to analyse the current XAS data in detail and to compare them with the appropriate Ti 2p XAS data of bulk oxides. We find that in the case of ultrathin film with a fully oxidised Ti(4+) stoichiometry, the Ti 2p XAS data display features that are very similar to the ones observed in related bulk systems. The XAS data of the reduced TiO(x)@ films (with x < 2) show a rather different and specific shape. By comparing the experimental spectra with an atomic type of model calculation we show that the Ti 2p XAS profiles can be attributed mainly to stoichiometry-symmetry effects.
We present the results of a XAS experiment carried out at the Ti 2
p edge on well-ordered TiO
x
ultrathin films grown on the Pt(1
1
1) surface. XAS at the Ti 2
p edge has been extensively applied to ...the study of Ti bulk compounds, particularly to the study of titania. According to the literature, the corresponding Ti 2
p edge spectral shape is related to the stoichiometry and crystal field symmetry at the Ti sites. In the present study we aim at extending the potential of the XAS technique by discussing Ti 2
p spectra obtained on several, dimensionally confined, TiO
x
phases in the form of ultrathin films. One of the main features of these films is their high degree of structural order. Furthermore, the results of previous studies provide valuable information about the chemistry and structure of the films, so that we are able to analyse the current XAS data in detail and to compare them with the appropriate Ti 2
p XAS data of bulk oxides. We find that in the case of ultrathin film with a fully oxidised Ti
4+ stoichiometry, the Ti 2
p XAS data display features that are very similar to the ones observed in related bulk systems. The XAS data of the reduced TiO
x
films (with
x
<
2) show a rather different and specific shape. By comparing the experimental spectra with an atomic type of model calculation we show that the Ti 2
p XAS profiles can be attributed mainly to stoichiometry-symmetry effects.
We have measured the transformation of pseudomorphic Ni films on Pd(100) into
their bulk fcc phase as a function of the film thickness. We made use of x-ray
diffraction and x-ray induced ...photoemission to study the evolution of the Ni
film and its interface with the substrate. The growth of a pseudomorphic film
with tetragonally strained face centered symmetry (fct) has been observed by
out-of-plane x-ray diffraction up to a maximum thickness of 10 Ni layers (two
of them intermixed with the substrate), where a new fcc bulk-like phase is
formed. After the formation of the bulk-like Ni domains, we observed the
pseudomorphic fct domains to disappear preserving the number of layers and
their spacing. The phase transition thus proceeds via lateral growth of the
bulk-like phase within the pseudomorphic one, i.e. the bulk-like fcc domains
penetrate down to the substrate when formed. This large depth of the walls
separating the domains of different phases is also indicated by the strong
increase of the intermixing at the substrate-film interface, which starts at
the onset of the transition and continues at even larger thickness. The
bulk-like fcc phase is also slightly strained; its relaxation towards the
orthomorphic lattice structure proceeds slowly with the film thickness, being
not yet completed at the maximum thickness presently studied of 30 Angstrom
(i.e. about 17 layers).
We have measured the transformation of pseudomorphic Ni films on Pd(100) into their bulk fcc phase as a function of the film thickness. We made use of x-ray diffraction and x-ray induced ...photoemission to study the evolution of the Ni film and its interface with the substrate. The growth of a pseudomorphic film with tetragonally strained face centered symmetry (fct) has been observed by out-of-plane x-ray diffraction up to a maximum thickness of 10 Ni layers (two of them intermixed with the substrate), where a new fcc bulk-like phase is formed. After the formation of the bulk-like Ni domains, we observed the pseudomorphic fct domains to disappear preserving the number of layers and their spacing. The phase transition thus proceeds via lateral growth of the bulk-like phase within the pseudomorphic one, i.e. the bulk-like fcc domains penetrate down to the substrate when formed. This large depth of the walls separating the domains of different phases is also indicated by the strong increase of the intermixing at the substrate-film interface, which starts at the onset of the transition and continues at even larger thickness. The bulk-like fcc phase is also slightly strained; its relaxation towards the orthomorphic lattice structure proceeds slowly with the film thickness, being not yet completed at the maximum thickness presently studied of 30 Angstrom (i.e. about 17 layers).
Abstract
We examine the wavelength dependence of radial light profiles based on Sérsic index
n
measurements of 1067 galaxies with
M
*
≥ 10
9.5
M
⊙
and in the redshift range 0.5 <
z
< 3. The sample ...and rest-frame optical light profiles are drawn from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and 3D Hubble Space Telescope (HST); rest-frame near-infrared light profiles are inferred from images collected for the Cosmic Evolution Early Release Science (CEERS) program with the Near Infrared Camera (NIRCam) on board of the James Webb Space Telescope (JWST).
n
shows only a weak dependence on the wavelength, regardless of the redshift, galaxy mass, and type. On average, star-forming galaxies have
n
= 1–1.5 and quiescent galaxies have
n
= 3–4 in the rest-frame optical and near-infrared. The strong correlation at all wavelengths between
n
and star formation activity implies a physical connection between the radial stellar mass profile and star formation activity. The main caveat is that the current sample is too small to discern trends for the most massive galaxies (
M
*
> 10
11
M
⊙
).
Abstract
We present a detailed study of the molecular gas content and stellar population properties of three massive galaxies at 1 <
z
< 1.3 that are in different stages of quenching. The galaxies ...were selected to have quiescent optical/near-infrared spectral energy distribution and relatively bright emission at 24
μ
m, and show remarkably diverse properties. CO emission from each of the three galaxies is detected in deep NOEMA observations, allowing us to derive molecular gas fractions
M
gas
/
M
*
of 13%–23%. We also reconstruct the star formation histories by fitting models to the observed photometry and optical spectroscopy, finding evidence for recent rejuvenation in one object, slow quenching in another, and rapid quenching in the third system. To better constrain the quenching mechanism we explore the depletion times for our sample and other similar samples at
z
∼ 0.7 from the literature. We find that the depletion times are highly dependent on the method adopted to measure the star formation rate: using the UV+IR luminosity we obtain depletion times about 6 times shorter than those derived using dust-corrected O
ii
emission. When adopting the star formation rates from spectral fitting, which are arguably more robust, we find that recently quenched galaxies and star-forming galaxies have similar depletion times, while older quiescent systems have longer depletion times. These results offer new, important constraints for physical models of galaxy quenching.
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