1,306 research outputs found
Two-dimensional carrier gas at a polar interface without surface band gap states: A first-principles perspective
We present first-principles calculations of the interface between GaN and
strained AlN, using a slab model in which polarization is compensated via
surface fractional-charge pseudo-hydrogen atoms. We show that an interface
two-dimensional carrier electron or hole gas emerges naturally in response to
different compensating surface charges, but that this need not involve in-gap
surface states
Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural
behaviour, with successive transitions between orthorhombic, tetragonal and
cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles
lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The
equilibrium structures compare well to solutions of temperature-dependent
powder neutron diffraction. By following the normal modes we calculate infrared
and Raman intensities of the vibrations, and compare them to the measurement of
a single crystal where the Raman laser is controlled to avoid degradation of
the sample. Despite a clear separation in energy between low frequency modes
associated with the inorganic PbI3 network and high-frequency modes of the
organic CH3NH3+ cation, significant coupling between them is found, which
emphasises the interplay between molecular orientation and the corner-sharing
octahedral networks in the structural transformations. Soft modes are found at
the boundary of the Brillouin zone of the cubic phase, consistent with
displacive instabilities and anharmonicity involving tilting of the PbI6
octahedra around room temperature.Comment: 9 pages, 4 figure
The preparation route and final form of V-MXenes override the effect of the O/F ratio on their magnetic properties
This work was supported by OP VVV “Excellent Research Teams” project no. CZ.02.1.01/0.0/0.0/15_003/0000417 – CUCAM. P. E. would like to also acknowledge the Czech Science Foundation for the ExPro project (19-27551X). Computational resources and low-temperature infrastructure were supplied by the projects “e-Infrastruktura CZ” (e-INFRA CZ LM2018140) and MGML (LM2023065) supported by the Ministry of Education, Youth and Sports of the Czech Republic.Transition metal carbides and nitrides (MXenes) show a high potential for electrochemical energy storage in batteries and supercapacitors and for electrocatalysis. Their excellent electronic and magnetic characteristics have been highlighted in several theoretical studies. However, experimental research on MXenes is yet to confirm their predicted properties as candidates for controllable magnetic 2D materials. Here, we report our theoretical and experimental study of V2CTx MXenes (T = O, OH, F), providing key insights into their magnetism. Based on our density functional theory (DFT) analysis, we predicted ferromagnetic (FM) and antiferromagnetic (AFM) states of V2CTx, which are determined by the O/F ratio of surface functional groups. Accordingly, we prepared V2CTx MXenes in the form of multilayered powders and thin films with different O/F ratios. No experimental evidence of FM or AFM properties was found in any material. Nevertheless, powders and films with almost identical chemical compositions (in terms of O/F ratio) displayed different magnetic properties, whereas films with disparate chemical compositions revealed a similar magnetic character. Therefore, the preparation route and form of the final V2CTx material override the effect of the O/F ratio, which is often overestimated in theoretical studies. Moreover, these findings underscore the importance of preparing MXene materials to experimentally confirm their theoretically predicted properties.Peer reviewe
Halogen effects on ordering and bonding of CH<sub>3</sub>NH<sub>3</sub> <sup>+</sup> in CH<sub>3</sub>NH<sub>3</sub>PbX<sub>3</sub> (X = Cl, Br, I) hybrid perovskites:a vibrational spectroscopic study
This study reports Raman and infrared
spectra of hybrid organic–inorganic
MAPbX<sub>3</sub> perovskites (MA = CH<sub>3</sub>NH<sub>3</sub>,
X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were
recorded at three laser wavelengths (514, 785, and 1064 nm) under
on- and off-resonance conditions, as well as at room temperature and
100 K. The use of different excitation wavelengths allowed the unambiguous
acquisition of “true” Raman spectra from the perovskites,
without degradation or photoinduced structural changes. Low-frequency
PbX vibrational modes were thoroughly identified by comparison of
Raman and far-IR results. Red Raman frequency shifts for almost all
MA vibrations from 200 to 3200 cm<sup>–1</sup>, and particularly
intense for the torsional mode, were observed toward heavy halide
derivatives, indicative of strengthening the interaction between halides
and the organic cation inside the inorganic cage. Different MA–X
bonding schemes are evidenced by torsional mode pairs emerging in
the orthorhombic phase. MAPbBr<sub>3</sub> was further characterized
by variable temperature Raman measurements (100–295 K). Broadening
of the MA rocking mode slightly above the tetragonal I to II phase
transition is connected with disorder of the MA cation. Our results
advance the understanding of perovksite materials properties (ferroelectric
domain formation, anomalous hysteresis) and their use as efficient
light absorbers in solar cells
Atomistic origins of high-performance in hybrid halide perovskite solar cells
The performance of organometallic perovskite solar cells has rapidly
surpassed that of both conventional dye-sensitised and organic photovoltaics.
High power conversion efficiency can be realised in both mesoporous and
thin-film device architectures. We address the origin of this success in the
context of the materials chemistry and physics of the bulk perovskite as
described by electronic structure calculations. In addition to the basic
optoelectronic properties essential for an efficient photovoltaic device
(spectrally suitable band gap, high optical absorption, low carrier effective
masses), the materials are structurally and compositionally flexible. As we
show, hybrid perovskites exhibit spontaneous electric polarisation; we also
suggest ways in which this can be tuned through judicious choice of the organic
cation. The presence of ferroelectric domains will result in internal junctions
that may aid separation of photoexcited electron and hole pairs, and reduction
of recombination through segregation of charge carriers. The combination of
high dielectric constant and low effective mass promotes both Wannier-Mott
exciton separation and effective ionisation of donor and acceptor defects. The
photoferroic effect could be exploited in nanostructured films to generate a
higher open circuit voltage and may contribute to the current-voltage
hysteresis observed in perovskite solar cells.Comment: 6 pages, 5 figure
Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites
We present Raman and terahertz absorbance spectra of methylammonium lead
halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and
370 K. These results show good agreement with density-functional-theory phonon
calculations.1 Comparison of experimental spectra and calculated vibrational
modes enables confident assignment of most of the vibrational features between
50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in
their cavities at the orthorhombic-to-tetragonal phase transition, plays a key
role in shaping the vibrational spectra of the different compounds.
Calculations show that these dynamics effects split Raman peaks and create more
structure than predicted from the independent harmonic modes. This explains the
presence of extra peaks in the experimental spectra that have been a source of
confusion in earlier studies. We discuss singular features, in particular the
torsional vibration of the C-N axis, which is the only molecular mode that is
strongly influenced by the size of the lattice. From analysis of the spectral
linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes,
which can be linked to low lattice thermal conductivity. We show that optical
rather than acoustic phonon scattering is likely to prevail at room temperature
in these materials.Comment: 40 pages (no supporting info uploaded
- …