7 research outputs found
Benchmark Assessment of Density Functional Methods on Group IIāVI MX (M = Zn, Cd; X = S, Se, Te) Quantum Dots
In
this work, we build a benchmark data set of geometrical parameters,
vibrational normal modes, and low-lying excitation energies for MX
quantum dots, with M = Cd, Zn, and X = S, Se, Te. The reference database
has been constructed by <i>ab initio</i> resolution-of-identity
second-order approximate coupled cluster RI-CC2/def2-TZVPP calculations
on (MX)<sub>6</sub> model molecules in the wurtzite structure. We
have tested 26 exchange-correlation density functionals, ranging from
local generalized gradient approximation (GGA) and hybrid GGA to meta-GGA,
meta-hybrid, and long-range corrected. The best overall functional
is the hybrid PBE0 that outperforms all other functionals, especially
for excited state energies, which are of particular relevance for
the systems studied here. Among the DFT methodologies with no HartreeāFock
exchange, the M06-L is the best one. Local GGA functionals usually
provide satisfactory results for geometrical structures and vibrational
frequencies but perform rather poorly for excitation energies. Regarding
the CdSe cluster, we also present a test of several basis sets that
include relativistic effects via effective core potentials (ECPs)
or via the ZORA approximation. The best basis sets in terms of computational
efficiency and accuracy are the SBKJC and def2-SVĀ(P). The LANL2DZ
basis set, commonly employed nowadays on these types of nanoclusters,
performs very disappointingly. Finally, we also provide some suggestions
on how to perform calculations on larger systems keeping a balance
between computational load and accuracy
Photoinduced Energy Shift in Quantum-Dot-Sensitized TiO<sub>2</sub>: A First-Principles Analysis
We investigate the photoinduced dipole
(PID) phenomenon, which
holds enormous potential for the optimization of quantum dot-sensitized
solar cells (QDSSCs), by means of first-principles electronic structure
calculations. We demonstrate that the sensitization of the TiO<sub>2</sub> substrate with core/shell QDs produces almost no changes
in the ground state but decisively improves the performance upon photoexcitation.
In particular, the maximum attainable <i>V</i><sub>OC</sub> is predicted to increase by ā¼25 meV due to two additive effects:
(i) the displacement of the photoexcited hole away from the TiO<sub>2</sub> surface and (ii) the interfacial electrostatic interaction
established between the TiO<sub>2</sub>-injected electrons and the
holes residing in the QD core. We believe that this work, explaining
the mechanisms by which PID cells deliver better efficiencies, paves
the way for the design of new QDSSCs with improved efficiencies
<i>Ab Initio</i> Molecular Dynamics Simulations of Methylammonium Lead Iodide Perovskite Degradation by Water
Protecting organohalide perovskite
thin films from water and ambient
humidity represents a paramount challenge for the commercial uptake
of perovskite solar cells and, in general, of related optoelectronic
devices. Therefore, understanding the perovskite/water interface is
of crucial importance. As a step in this direction, here we present <i>ab initio</i> molecular dynamics simulations aimed at unraveling
the atomistic details of the interaction between the methylammonium
lead iodide (MAPbI<sub>3</sub>) perovskite surfaces and a liquid water
environment. According to our calculations, MAI-terminated surfaces
undergo a rapid solvation process, driven by the interaction of water
molecules with Pb atoms, which prompts the release of I atoms. PbI<sub>2</sub>-terminated surfaces, instead, seem to be more robust to degradation,
by virtue of the stronger (shorter) PbāI bonds formed on these
facets. We also observe the incorporation of a water molecule into
the PbI<sub>2</sub>-terminated slab, which could represent the first
step in the formation of an intermediate hydrated phase. Interestingly,
PbI<sub>2</sub> defects on the PbI<sub>2</sub>-terminated surface
promote the rapid dissolution of the exposed facet. Surface hydration,
which is spontaneous for both MAI- and PbI<sub>2</sub>-terminated
slabs, does not modify the electronic landscape of the former, while
the local band gap of the PbI<sub>2</sub>-exposing model widens by
ā¼0.3 eV in the interfacial region. Finally, we show that water
incorporation into bulk MAPbI<sub>3</sub> produces almost no changes
in the tetragonal structure of the perovskite crystal (ā¼1%
volume expansion) but slightly opens the band gap. We believe that
this work, unraveling some of the atomistic details of the perovskite/water
interface, may inspire new interfacial modifications and device architectures
with increased stabilities, which could in turn assist the commercial
uptake of perovskite solar cells and optoelectronic devices
Modeling Surface Passivation of ZnS Quantum Dots
We report on the interaction between ZnS quantum dots and several surface ligands by means of pure Quantum Mechanical (QM) and hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) methods. To shed light on the nature of the interaction, we focus our discussion on the structural and energetic aspects. The Zn<sub>6</sub>S<sub>6</sub> cluster has been chosen to model the quantum dot core, while trimethylamine (NMe<sub>3</sub>), trimethylphosphine (PMe<sub>3</sub>), trymethylphosphine oxide (OPMe<sub>3</sub>), methanol (MeOH), methanethiol (MeSH), and methaneselenol (MeSeH) have been employed to model the passivating ligands. Our results concerning the interaction between the cluster and one ligand of each type reveal that NMe<sub>3</sub>, PMe<sub>3</sub>, and OPMe<sub>3</sub> show a significantly greater affinity to Zn<sub>6</sub>S<sub>6</sub> than MeOH, MeSH, and MeSeH. We noticed that the softer the heteroatom of the ligand bonded to the cluster, the greater the interaction energy. A comparative study of different amines shows that the interaction is strengthened with the number and the length of the alkyl substituents in the ligand. We demonstrated that the interaction is mainly electrostatic, even if an important polarization of the charge density is observed. Fully passivated complexes have also been investigated, and our calculations point out that the bond is weaker than in the complexes with a single bonded ligand, suggesting that the repulsive interactions between the ligands and the diminished charge acceptor capacity of the cluster come into play
Effect of Structural Dynamics on the Opto-Electronic Properties of Bare and Hydrated ZnS QDs
Quantum
mechanical calculations on the structural and optoelectronic
features of two realistic wurtzite-like ZnS quantum dot (QD) models,
namely, (ZnS)<sub>33</sub> and (ZnS)<sub>116</sub>, are presented
both in vacuo and in an explicit water solution environment. CarāParrinello
molecular dynamics (CPMD) simulation and excited-state, Time-Dependent
Density Functional Theory (DFT/TDDFT) calculations on extended models
are combined to unravel hitherto inaccessible atomistic features of
the investigated systems. Ultrasmall QDs are predicted to exhibit
strong dynamical fluctuations. Accordingly, the bare (ZnS)<sub>33</sub> model undergoes a drastic structural rearrangement and evolves from
the starting bulk-like structure to an amorphous phase. The geometrical
changes occurring over the time are reflected on the opto-electronic
properties. The band-edge states and the optical absorption onset
both sizably vary along the CPMD trajectory. Eventually, the optical
gap decreases due to the emergence of high-lying occupied orbitals.
These midgap states are mainly localized in undercoordinated S sites
and could act as trap states for the photogenerated holes. Water molecules
are predicted to form strong ZnāOH<sub>2</sub> bonds with the
surface Zn atoms. Hydration seems to lower the surface energy, stabilize
the wurtzite polymorph, hinder the ZnāS bond breaking, and
largely prevent the appearance of trap states. Besides, adsorbed water
molecules produce a notable blue-shift of the optical gap. The electrostatic
field induced by the solvent shell and the electron-donor properties
of the water molecules are supposed to be responsible for the opening
of the gap. Moreover, capping the QDs with water molecules increases
the intensity of the lowest-lying electronic excitations. This study
sheds light on the important opto-electronic modifications occurring
for realistic QD in water solution and offers at the same time the
methodological framework to investigate photocatalytic reactions mediated
by ZnS
Quantum Dot Photoactivation of Pt(IV) Anticancer Agents: Evidence of an Electron Transfer Mechanism Driven by Electronic Coupling
Herein
we elucidate the mechanism of photoreduction of the PtĀ(IV)
complex <i>cis,cis,trans-</i>[PtĀ(NH<sub>3</sub>)<sub>2</sub>(Cl)<sub>2</sub>(O<sub>2</sub>CCH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H)<sub>2</sub>] (<b>1</b>) into PtĀ(II) species (among
which is cisplatin) by quantum dots (QDs), a process which holds potential
for photodynamic therapy. Density functional theory (DFT) and time-dependent
density functional theory (TDDFT) methodologies, integrated by selected
experiments, were employed to study the interaction and the light-induced
electron transfer (ET) process occurring between two QD models and <b>1</b>. Direct adsorption
of the complex on the nanomaterial surface results in large electronic
coupling between the LUMO (lowest unoccupied molecular orbital) of
the excited QD* and the LUMO+1 of <b>1</b>, providing the driving
force to the light-induced release of the succinate ligands from the
Pt derivative. As confirmed by photolysis experiments performed a
posteriori, DFT highlights that QD photoactivation of <b>1</b> can favor the formation of preferred PtĀ(II) photoproducts, paving
the way for the design of novel hybrid PtĀ(IV)āsemiconductor
systems where photochemical processes can be finely tuned
Carbo-Cages: A Computational Study
Inspired by their geometrical perfection,
intrinsic beauty, and
particular properties of polyhedranes, a series of carbo-cages is
proposed in silico via density functional theory computations. The
insertion of alkynyl units into the CāC bonds of polyhedranes
results in a drastic lowering of the structural strain. The induced
magnetic field shows a significant delocalization around the three-membered
rings. For larger rings, the response is paratropic or close to zero,
suggesting a nonaromatic behavior. In the carbo-counterparts, the
values of the magnetic response are shifted with respect to their
parent compounds, but the aromatic/nonaromatic character remains unaltered.
Finally, BornāOppenheimer molecular dynamics simulations at
900 K do not show any drastic structural changes up to 10 ps. In the
particular case of a carbo-prismane, no structural change is perceived
until 2400 K. Therefore, although carbo-cages have enthalpies of formation
1 order of magnitude higher than those of their parent compounds,
their future preparation and isolation should not be discarded, because
the systems are kinetically stable, explaining why the similar systems
like carbo-cubane have already been synthesized