Download Hi-Res ImageDownload to MS-PowerPointCite This:J. Phys. Chem. Lett. 2023, 14, 15, 3691-3697

A Unified View of Vibrational Spectroscopy Simulation through Kernel Density Estimations

Romain Botella*
Romain Botella
Nano and Molecular Systems Research Unit, University of Oulu, Oulu 90014, Finland
*Email: [email protected]
More by Romain Botella
https://orcid.org/0000-0003-1016-6349
and
Andrey A. Kistanov
Andrey A. Kistanov
Nano and Molecular Systems Research Unit, University of Oulu, Oulu 90014, Finland
More by Andrey A. Kistanov
https://orcid.org/0000-0001-6175-5296

Cite this: J. Phys. Chem. Lett. 2023, 14, 15, 3691–3697

Publication Date (Web):April 10, 2023

https://doi.org/10.1021/acs.jpclett.3c00665

CC-BY 4.0.

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Abstract

To date, vibrational simulation results constitute more of an experimental support than a predictive tool, as the simulated vibrational modes are discrete due to quantization. This is different from what is obtained experimentally. Here, we propose a way to combine outputs such as the phonon density of states surrogate and peak intensities obtained from ab initio simulations to allow comparison with experimental data by using machine learning. This work is paving the way for using simulated vibrational spectra as a tool to identify materials with defined stoichiometry, enabling the separation of genuine vibrational features of pure phases from morphological and defect-induced signals.

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Vibrational properties are fundamental in materials science to ascertaining their dynamic stability (1,2) and thermodynamic properties. (3) They also have valuable analytical importance in identifying the materials obtained experimentally and assigning the modes (4,5) through vibrational spectroscopy. Usually, experimental peaks, whether Raman or infrared (IR), have a spread around the wavenumber of the maximum intensity, the nominal wavenumber, and are usually described as bands. (6−8) This is well described with the phonon density of states (pDOS) usually analyzed in computational materials science. (9−11) While well-defined theoretically, it corresponds only to a statistical analysis of the population of energy levels by the different vibrational modes and can be qualitatively compared only to experimental spectra on the absorbance (or transmittance) scale but exhibits accurate wavenumber spread. (12) To date, these vibrational simulation results constitute more of an experimental support than a fully predictive tool. One of the main issues with this method is that the vibrational modes are theoretically discrete due to the quantization of the vibrations, which is different from what is obtained experimentally, (13) as mentioned above. This comes from different, nontrivially intertwined types of broadening. Several solutions are used to circumvent this issue such as the use of molecular dynamics and ab initio (14−17) or machine learning (ML) (18−20) to add some of the broadening to the vibrational modes obtained at T = 0 K; nonetheless, the supercell method remains widely used. (21−23) It has the advantage of separating the modes, enabling a precise interpretation of experimental spectra. Using ML to expand simulation methods that are deemed too computationally costly is an interesting path when exploring the material space further. In that case, the use of ML would help to compensate for the drawbacks of costly but more precise simulation methods. (24−26)

Among the different materials to study with IR, two-dimensional (2D) materials have drawn much interest so far because of their interesting properties such as high exciton binding energies and mechanical flexibility. (26−28) Metal dichlorides (MCl₂) constitute a family of 2D materials starting to attract interest both computationally (21,29−33) and experimentally. (34,35) They are starting to be synthesized experimentally, but their vibrational spectra have never been reported or analyzed to date. They are, therefore, an interesting system for advanced vibrational properties study.

Herein, we present such an implementation of ML and propose a way to combine all of the outputs, such as pDOS and peak intensities obtained with the supercell method (T = 0 K), to allow comparison with experimental data by using an unsupervised kernel density estimation (KDE) algorithm. Furthermore, we derive the Cartesian components of each mode, allowing for irreducible representation (irrep) assignment. This result paves the way for the IR/Raman mode separation and subsequent IR simulation that are shown herein. With all of these improvements, we promote the supercell method to the level of truly comparing with experimental data, separating the Raman and IR components of the vibrational modes. Finally, we provide a simulation of the IR spectrum of a compound that has never been explicitly vibrationally characterized before, making this work and this vibrational simulation method a predictive tool rather than just an experimental support.

After simulating the FeCl₂ monolayer with high accuracy using first-principles simulations, the vibrational modes were computed and subsequently analyzed, as shown in Figure 1. In this figure, it is worth noting that all of the vibrational modes are displayed and used for subsequent calculations. More details on the simulations are shown in Computational Methods and in ref (21). Figure 1a contains the vibrational modes of the FeCl₂ monolayer represented by Dirac peaks, with heights corresponding to their respective intensities. By comparing the number of distinguishable peaks, which is 23, with the number of degrees of freedom of the entire slab, which is 141, one can see that most of the peaks are actually clusters of modes. This observation shows that the adopted methodology is consistent with both the number of mechanical degrees of freedom (3) and the nature of the species involved (mass and interatomic constants). Whether the supercell has a size of 1 × 1 × 1 or 10 × 10 × 1, the material is still the same in terms of physicochemical identity. Thus, the distribution of the vibrational modes should be similar or else the experimental identification of materials through vibrational spectroscopy would be more complicated than it actually is experimentally, producing totally different spectra for different material sizes. Therefore, one observed peak in Figure 1a can account for more than one vibrational mode.

Figure 1. Different representations of all of the vibrational modes of the FeCl₂ monolayer with (a) a discrete (Dirac peaks) view, (b) a superposition of the discrete view and the KpDOS, and (c) IWKDE. Arrows in (b) and (c) highlight the main signals.

Another way to calculate pDOS is to use the kernel density estimation (KDE) method. KDE is an unsupervised ML algorithm that is mainly used for statistical analysis. (36,37) It is associated with each point with a kernel function and calculates the entire density profile based on this assumption. We used a Gaussian kernel in this work for availability and experimental accuracy purposes. (38−40) The results of kernel density estimation of the phonon density of states (KpDOS) along with the Dirac peaks for comparison are shown in Figure 1b. The enlarged image of KpDOS that can be used as a surrogate for PDOS is shown in Figure S1.

Seven bands are observed, the highest KpDOS values are associated with the wavenumbers 222.04, 254.93, and 293.09 cm^–1, and the other bands are more than 4 times less than the highest KpDOS bands. The use of the KDE algorithm gives a continuous curve from a discrete collection of points, which is observed in pDOS. The large width of the bands is enables a smooth distribution, which is generally not noticed in classical pDOS (8−10) but is often obtained in experimental work. (41,42) The width can be modified to enable better flexibility in comparison with the experimental results. As can be seen in Figure S2, the width is controlling the number of bands visible on the KpDOS. The distribution becomes similar to more usual pDOS when the width decreases (e.g., 0.5 cm^–1), and a potato-shaped band is obtained when the width increases (e.g., 18 cm^–1).

However, in experimental work, the population of the energy values is not the only thing observed. According to the Beer–Lambert law, for a concentration of 1 mol·L^–1 and a path length of 1 cm, the distribution of the extinction coefficient is due to both the population of the energy levels and the values (intensities). While pDOS is well-defined theoretically, it is not clear how to include this term theoretically. On the other hand, the KDE algorithm can calculate the weighted density of a data collection. (37) Provided the weights are equal to the intensity of the vibrational modes, the intensity-weighted kernel density estimation (IWKDE) spectrum can be computed, as shown in Figure 1c.

We can see from Figure 1c that a new set of six bands are obtained. The wavenumbers of the bands are very different from those observed in KpDOS. The highest-intensity band has wavenumber of 294.94 cm^–1, and the other bands have the same IWKDE ratio as in KpDOS. The morphology of the IWKDE spectrum is not identical to the KpDOS profile, which is also one drawback observed when comparing pDOS or KpDOS and experimental vibrational spectra.

The apparent differences can be rationalized through Figure 1b, showing the superposition of the Dirac intensities with KpDOS. We can see that the KpDOS band at ca. 62 cm^–1 disappears because of its very small intensity as well as the KpDOS bands at ca. 222 and 255 cm^–1 that are greatly reduced because of the large difference between intensities and KpDOS values. These discrepancies could explain the observation of certain peaks in experimental work, as others are not seen in the expected wavenumber range. By adding this feature, the results reported here are one step closer to accurate vibrational spectrum simulation only from the output of a well-known method, by unifying pDOS and discrete Dirac peak views.

Usually, the vibrational modes are designated by the irreps to which they belong. This sorting is performed through normal-mode analysis (NMA) using group theory. (43) The naming of the different modes is nontrivial, forcing the use of more straightforward approaches (7) in the absence of a clear guide for attribution. As experimentalists use polarization to assign bands to group theory predictions, Cartesian components of the IR intensities can also be calculated (eqs 9–11), (44) enabling the assignment of the different vibrational modes to their respective irreps. By doing so, simulated vibrational spectra of materials not yet synthesized are becoming utterly predictive rather than a simple support for experimental endeavors.

According to group theory, the FeCl₂ monolayer belongs to the P3̅m1 space group (number 164) with the symmetry operations shown in Figure 2a for its 1 × 1 × 1 unit cell. The six optical modes of this cell can be decomposed into irreps as follows

Γ_{1 \times 1 \times 1} = A_{1 g} + E_{g} + {2 A}_{2 u} + {2 E}_{u}

(1)

with A_1g, A_2u, E_g, and E_u irreps of the P3̅m1 space group associated with the different vibrational modes of FeCl₂. E modes are in-plane vibrations, A modes are out-of-plane, g are centrosymmetric modes, and u are noncentrosymmetric modes. More details on group theory can be found in ref (43).

Figure 2. (a) Representation of the FeCl₂ monolayer unit cell and its symmetry operations, (b) decision tree diagram for the classification of the modes in the different irreps, (c) z-polarized discrete (Dirac) intensities (blue) of the modes of the FeCl₂ monolayer, (d) x-polarized discrete (Dirac) intensities (red) of the modes of the FeCl₂ monolayer, and (e) y-polarized discrete (Dirac) intensities (green) of the modes of the FeCl₂ monolayer. The nomenclature is from ref (46).

However, the supercell used in this work consists of 4 × 4 × 1 unit cells. The 141 optical modes associated with such a cell can also be decomposed in irreps according to

Γ_{4 \times 4 \times 1} = 16 A_{1 g} + 16 E_{g} + 31 A_{2 u} + 31 E_{u}

(2)

From the character of Table S1, E and A differ in their orientations: E modes are in-plane (z component = 0) and A modes are out-of-plane. While group theory is very clear about the orientation of the atomic displacements, the results of the ab initio calculations clearly show a deviation from the theory, as the only way to achieve the irrep decomposition in eq 2 is to set a threshold (Figure 2b) on the Cartesian components’ values. This deviation comes from many factors related to the approximations made in the calculations such as the anharmonicity of the atomic displacements, the strain applied to the cell upon optimization, the nonideal position of the atoms in the as-calculated cell, and the supercell size. As NMA is based on the harmonic approximation, many works have been able to calculate the degree of anharmonicity (45) in the vibrational spectra simulated. While this result is clearly visible in the frequency calculation, the true deviation comes from the shape of the potential energy surface. Hence, a threshold is needed to assign their irrep properly.

After the first classification step, 94 E (E_g+ E_u) modes and 47 A (A_1g+ A_2u) modes are obtained. The second step is to classify the modes between g (E_g, A_1g) and u (E_u, A_2u) irreps. From the character table (Table S1) of the D_3d group, the A_1g irrep corresponds to the fully symmetric mode, hence the signal should be unchanged by any symmetry operation. That means that the x and y components should be equal due to the threshold applied while the z component is different from x and y, and a threshold is applied for the same reasons as discussed above (Figure 2b). The same holds true for E_g. Form this, the classification in eq 2 is obtained, along with the detailed attribution and values of the x (I_x), y (I_y), and z (I_z) components (Figure 2c–e). It is worth noting that the minimum value of the threshold needed to classify properly is presented in Figure 2b. Most of the modes were already assigned for thresholds below the values given here, nuancing somehow the shift from the harmonic predictions.

Following the irrep assignment, the different modes can be separated into Raman-active (E_g+ A_1g) and IR-active (E_u+ A_2u) modes, as shown in Figure 3a–c. As noticed in Figure 1, the number of bands is 6 (Figure 3c), which is smaller than the number of modes, and the bands are not necessarily centered at the same wavenumbers. Regarding the IWKDE spectrum (Figure 3c), the wavenumbers and relative intensities of the bands are also changing. The highest band has wavenumber 294.79 cm^–1 in the IR IWKDE. Regarding the wavenumbers of the bands, they do not vary significantly for most of them, which can be interpreted as the same mode being active in both kinds of spectroscopy.

Figure 3. Different representations of the IR-active vibrational modes of the FeCl₂ monolayer with (a) a discrete (Dirac peaks) view, (b) the KpDOS, and (c) the IWKDE. Arrows in (b) and (c) highlight the main signals.

The IWKDE spectrum and the Dirac peaks (discrete modes), presented in Figure 4, are superposed for the IR. Despite a strong resemblance of Figure 4c with Figure 1c, only the IR-active modes are displayed, hence the use of a different color. From this, one can see that the simulated bands ignore several modes that have smaller intensities but still contain information. These results recommend caution as to the quick and easy assignment of a complex system’s spectroscopic results without well-established justifications. While it is true that bands can hide several minor signals and therefore hide some information, it is still true that the most intense bands are expressed in the spectrum, enabling us to obtain the main information about the vibrational state of the sample. Figure 4a, along with Table 1, paves the way for the irrep assignment of the different bands observed in the simulated band spectra.

Figure 4. Superposition of (a) the IR Dirac modes, the associated IWKDE spectrum, and the associated IWKDE spectrum and (b) IR- and Raman-active mode IR IWKDE spectra. Arrows highlight the main bands, and the Dirac intensity peaks have been normalized to be compared with IWKDE spectra.

Table 1. Distribution of the Contribution of the Different irreps to the IWKDE IR^a

	Infrared
Wavenumber (cm^–1)	E_u	A_2u
102.06	100%	0%
156.77	62%	38%
176.22	100%	0%
218.49	75%	35%
261.46	58%	42%
294.79	69%	31%

^{^a}

See Computational Methods.

As D_3d is centrosymmetric, Raman and IR modes should be complementary (i.e., the IR intensity of Raman-active modes should be zero). To validate this theoretical conclusion, the IR intensities of the IR-active modes and the Raman-active modes have been computed separately and superposed in Figure 4b. While the comparison of the IWKDE for the IR and Raman bands (Figure 4b) shows a clear difference in intensity between both simulated spectra, it is worth noticing that the IR intensities of Raman-active bands overlap. The high intensity of IR-active modes with respect to the IR intensity of Raman-active modes can be explained by the high ionicity of the FeCl₂ monolayer samples, proposed in previous work. (21) As a consequence, the dipole moment upon vibration is much more likely to change than the polarizability. The IR activity coefficient is therefore going to be much stronger than the Raman activity coefficient, explaining the difference in activity and cross-validating the sorting methodology herein. This observation also explains the resemblance between the IWKDE shown in Figure 1c and the IR IWKDE in Figure 4a.

From this result, it can also be recommended to use IR to obtain a much stronger signal, although the information may be mixed, as the bands are less pure with respect to the irrep attribution. This can be observed experimentally and can be interpreted as a deviation from ideality, as the mode is being somewhat activated by a sample not ideally prepared. Here it is clearly shown that the Raman-active and IR-active modes can have an overlapping band to a certain degree despite the theoretical rule of them being incompatible. This possibly comes from the anharmonicity mentioned earlier. This comparison is possible only because of the irrep attribution of the different mode and, to the best of our knowledge, is the first of this kind in the literature.

As not all of the bands are purely assigned to one irrep, they should output mixed information and are therefore less trustworthy than pure ones without more advanced spectroscopic methods to selectively analyze one irrep or the other. The IR band at ca. 261.46 cm^–1 is a mixture of modes from E_u (58%) and A_2u (42%) irreps. It has the highest degree of mixing and, as a consequence, is the least reliable. On the other hand, the 176.22 cm^–1 band belongs entirely to E_u irrep, which makes it one of the most trustworthy bands in the IR spectrum. This work also gives insights into how to assess the reliability of band evolution (inverse trends due to different species), based on computational results.

In this work, the FeCl₂ monolayer vibrational spectra have been simulated and studied in the spectroscopic framework. An unsupervised ML KDE algorithm has been used to unify pDOS and intensity, getting a spectrum for the first time with this method with a realistic appearance and allowing for a consistent comparison of experimental data. Moreover, simulation does not suffer from the drawbacks of experimental synthesis (e.g., impurities, overtones, and nonlinear baselines) and therefore enables the separation of the true spectroscopic signal of the desired material from the above-mentioned experimental spectroscopic perturbations. It will also provide a way to estimate the presence of defects, stoichiometry variations, and other modifications of the material, enabling performance improvement quantitatively, provided that the right simulation outputs are given.

Computational Methods

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The FeCl₂ monolayer was geometrically optimized, and its stability was checked in previous work. (21) The geometry of the primitive unit cell of the FeCl₂ monolayer is also available in the 2DMatPedia database (ID dm-3574). (47) To ensure the reliability of the model, we also performed geometry optimization for the here-considered FeCl₂ monolayer primitive unit cell and the 4 × 4 × 1 supercell. The FeCl₂ monolayer stabilizes in a trigonal lattice having lattice parameters of a = b = 3.41 Å, which is in good agreement with previous works reporting on the FeCl₂ monolayer. (21) The geometry optimization was conducted using the plane-wave method as implemented in the Vienna Ab Initio Simulation Package (VASP). The vibrational modes were computed for the 4 × 4 × 1 supercell using the finite difference method as implemented in VASP. Zone-center (Γ-point) frequencies were calculated. For all calculations, the Perdew–Burke–Ernzerhof exchange-correlation functional under the generalized gradient approximation was utilized. (48) The periodic boundary conditions were applied for the two in-plane transverse directions, while a vacuum space of 20 Å was introduced in the direction perpendicular to the surface plane.

The ML part was performed using the sci-kit learn package. (49) The KDE algorithm associates each point with a kernel function. Mathematically, it can be presented as follows:

ρ_{K} (y) = \sum_{i = 1}^{N} K (y - x_{i}; h)

(3)

With ρ_K(y), the density estimate was calculated with the kernel function K for a collection of N points x_i at value y. This algorithm includes the possibility to weight each data point as follows

ρ_{K} (y) = \sum_{i = 1}^{N} w_{i} K (y - x_{i}; h)

(4)

with w_i, the weight of point x_i, enabling the combination of the distribution of the vibrational modes and their respective intensity, as will be discussed below.

To smooth the KDE profile obtained, the density of points throughout the wavenumber range must be increased to avoid having a profile resembling a discrete one. To this end, a baseline has been manually added to the wavenumber range of −22.5 to 472.5 cm^–1. The baseline is constituted of points spaced from 0.225 cm^–1. The value is not central, as the curves obtained have the same shapes but different intensities.

The Cartesian components of the intensity can be separated from the nonpolarized outputs of the supercell methods in a rigorous manner. The absorbance is defined by

A = {({A_{x}}^{1 / 2} + {A_{y}}^{1 / 2} + {A_{z}}^{1 / 2})}^{2}

(5)

with A_i (i = x, y, z) being the Cartesian components of the absorbance. After development, we obtain

A = A_{x} + A_{y} + A_{z} + 2 ({(A_{x} A_{y})}^{1 / 2} + {(A_{y} A_{z})}^{1 / 2} + {(A_{x} A_{z})}^{1 / 2})

(6)

On the other hand, the intensities of the vibrational modes calculated with the supercell method are defined by

I (ω) = \sum_{α = 1}^{3} {| \sum_{β = 1}^{3} \sum_{l = 1}^{M} Z_{α β}^{*} (l) e_{β} (l) |}^{2}

(7)

with I(ω) being the intensity of the spectrum, α and β being any of the Cartesian coordinates, l being an ion in the system containing M ions, Z_αβ^*(l) being the Born effective charge tensor for the lth ion in the α, β directions, and e_β(l) being the β component of the ionic displacement of the lth ion. After the development of eq 7, eq 8 is obtained

I (ω) = \sum_{l = 1}^{M} ({{(Z}_{x x}^{*} (l) e_{x} (l))}^{2} + {{(Z}_{y x}^{*} (l) e_{x} (l))}^{2} + {{(Z}_{z x}^{*} (l) e_{x} (l))}^{2} + {{(Z}_{x y}^{*} (l) e_{y} (l))}^{2} + {{(Z}_{y y}^{*} (l) e_{y} (l))}^{2} + {{(Z}_{z y}^{*} (l) e_{y} (l))}^{2} + {{(Z}_{x z}^{*} (l) e_{z} (l))}^{2} + {{(Z}_{y z}^{*} (l) e_{z} (l))}^{2} + {{(Z}_{z z}^{*} (l) e_{z} (l))}^{2} + 2 (Z_{x x}^{*} (l) e_{x} (l) Z_{x y}^{*} (l) e_{y} (l) + Z_{x x}^{*} (l) e_{x} (l) Z_{x z}^{*} (l) e_{z} (l) + Z_{x y}^{*} (l) e_{y} (l) Z_{x z}^{*} (l) e_{z} (l) + Z_{y x}^{*} (l) e_{x} (l) Z_{y y}^{*} (l) e_{y} (l) + Z_{y x}^{*} (l) e_{x} (l) Z_{y z}^{*} (l) e_{z} (l) + Z_{y y}^{*} (l) e_{y} (l) Z_{y z}^{*} (l) e_{z} (l) + Z_{z x}^{*} (l) e_{x} (l) Z_{z y}^{*} (l) e_{y} (l) + Z_{z x}^{*} (l) e_{x} (l) Z_{z z}^{*} (l) e_{z} (l) + Z_{z y}^{*} (l) e_{y} (l) Z_{z z}^{*} (l) e_{z} (l))

(8)

exhibiting nine squared terms also present in the following nine terms. By identification, grouping the terms having the same displacement vector, we obtain eqs 9–11

I_{x} (ω) = A_{x} = \sum_{l = 1}^{M} {{(Z}_{x x}^{*} (l) e_{x} (l))}^{2} + {{(Z}_{y x}^{*} (l) e_{x} (l))}^{2} + {{(Z}_{z x}^{*} (l) e_{x} (l))}^{2}

(9)

I_{y} (ω) = A_{y} = \sum_{l = 1}^{M} {{(Z}_{x y}^{*} (l) e_{y} (l))}^{2} + {{(Z}_{y y}^{*} (l) e_{y} (l))}^{2} + {{(Z}_{z y}^{*} (l) e_{y} (l))}^{2}

(10)

I_{z} (ω) = A_{z} = \sum_{l = 1}^{M} {{(Z}_{x z}^{*} (l) e_{z} (l))}^{2} + {{(Z}_{y z}^{*} (l) e_{z} (l))}^{2} + {{(Z}_{z z}^{*} (l) e_{z} (l))}^{2}

(11)

with I_i(ω) (i = x, y, z) being the Cartesian components of the intensity. The threshold for the classification of the vibrational modes is set to be the minimum values of A_x, A_y, and A_z allowing the correct number of modes in each irrep.

The band irreducible representation (irrep) percentage (in numbers) of modes in the different bands is calculated according to

b a n d % (mode i r r e p) = N_{i r r e p} / N_{t o t a l}

(12)

where N_irrep is the number of modes belonging to irrep that are included in the band observed in the IKWDE spectrum and N_total is the total number of modes included in the band.

For visualization of the results, the matplotlib was used. (50)

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c00665.

Kernel density estimated phonon density of states (Figure S1); KpDOS profiles for the vibrational modes of FeCl₂ for different bandwidths (Figure S2); and wavenumbers, Cartesian polarized intensities, and corresponding irreps for the 4 × 4 × 1 supercell of the FeCl₂ monolayer (Table S1) (PDF)

jz3c00665_si_001.pdf (312.17 kb)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Author
- Romain Botella - Nano and Molecular Systems Research Unit, University of Oulu, Oulu 90014, Finland; https://orcid.org/0000-0003-1016-6349; Email: [email protected]
Author
- Andrey A. Kistanov - Nano and Molecular Systems Research Unit, University of Oulu, Oulu 90014, Finland; https://orcid.org/0000-0001-6175-5296
Notes
The authors declare no competing financial interest.

Acknowledgments

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This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (grant agreement no. 101002219). The authors thank Dr. Shubo Wang for a preliminary suggestion, enabling them to complete the study.

References

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Fultz, Brent
Progress in Materials Science (2010), 55 (4), 247-352CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
A review. The literature on vibrational thermodn. of materials is reviewed. The emphasis is on metals and alloys, esp. on the progress over the last decade in understanding differences in the vibrational entropy of different alloy phases and phase transformations. Some results on carbides, nitrides, oxides, hydrides and lithium-storage materials are also covered. Principles of harmonic phonons in alloys are organized into thermodn. models for unmixing and ordering transformations on an Ising lattice, and extended for non-harmonic potentials. Owing to the high accuracy required for the phonon frequencies, quant. predictions of vibrational entropy with anal. models prove elusive. Accurate tools for such calcns. or measurements were challenging for many years, but are more accessible today. Ab initio methods for calcg. phonons in solids are summarized. The exptl. techniques of calorimetry, inelastic neutron scattering, and inelastic X-ray scattering are explained with enough detail to show the issues of using these methods for investigations of vibrational thermodn. The explanations extend to methods of data anal. that affect the accuracy of thermodn. information. It is sometimes possible to identify the structural and chem. origins of the differences in vibrational entropy of materials, and the no. of these assessments is growing. There has been considerable progress in our understanding of the vibrational entropy of mixing in solid solns., compd. formation from pure elements, chem. unmixing of alloys, order-disorder transformations, and martensitic transformations. Systematic trends are available for some of these phase transformations, although more examples are needed, and many results are less reliable at high temps. Nanostructures in materials can alter sufficiently the vibrational dynamics to affect thermodn. stability. Internal stresses in polycrystals of anisotropic materials also contribute to the heat capacity. Lanthanides and actinides show a complex interplay of vibrational, electronic, and magnetic entropy, even at low temps. A "quasiharmonic model" is often used to extend the systematics of harmonic phonons to high temps. by accounting for the effects of thermal expansion against a bulk modulus. Non-harmonic effects beyond the quasiharmonic approxn. originate from the interactions of thermally-excited phonons with other phonons, or with the interactions of phonons with electronic excitations. In the classical high temp. limit, the adiabatic electron-phonon coupling can have a surprisingly large effect in metals when temp. causes significant changes in the electron d. near the Fermi level. There are useful similarities in how temp., pressure, and compn. alter the conduction electron screening and the interat. force consts. Phonon-phonon "anharmonic" interactions arise from those non-harmonic parts of the interat. potential that cannot be accounted for by the quasiharmonic model. Anharmonic shifts in phonon frequency with temp. can be substantial, but trends are not well understood. Anharmonic phonon damping does show systematic trends, however, at least for fcc metals. Trends of vibrational entropy are often justified with at. properties such as at. size, electronegativity, electron-to-atom ratio, and mass. Since vibrational entropy originates at the level of electrons in solids, such rules of thumb prove no better than similar rules devised for trends in bonding and structure, and tend to be worse. Fortunately, the required tools for accurate exptl. investigations of vibrational entropy have improved dramatically over the past few years, and the required ab initio methods have become more accessible. Steady progress is expected for understanding the phenomena reviewed here, as investigations are performed with the new tools of expt. and theory, sometimes in integrated ways.
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Henriques, J. M.; Barboza, C. A.; Albuquerque, E. L.; Fulco, U. L.; Moreira, E. J. Structural, optoelectronic, infrared and Raman spectra from first principles calculations of γ-Cd(OH)₂. Phys. Chem. Solids 2015, 76, 45– 50, DOI: 10.1016/j.jpcs.2014.08.003
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Structural, optoelectronic, infrared and Raman spectra from first principles calculations of γ-Cd(OH)2
Henriques, J. M.; Barboza, C. A.; Albuquerque, E. L.; Fulco, U. L.; Moreira, E.
Journal of Physics and Chemistry of Solids (2015), 76 (), 45-50CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
We have investigated the phys. properties of the cadmium hydroxide polymorph γ-Cd(OH)2 using d.-functional theory (DFT) considering both the local d. and generalized gradient approxns., LDA and GGA, resp. The electronic band structure, the electronic d. of states, the dielec. function and the optical absorption are calcd. A comparison reveals good agreement of the calcd. lattice parameters with exptl. results. A direct band gap E(Γ→Γ) = 1.70eV (1.53 eV) was obtained within the GGA (LDA) level of calcn. The vibrational normal modes as well as the Raman and IR spectra of γ-Cd(OH)2 were obtained and assigned.
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Sanson, A.; Giarola, M.; Mariotto, G.; Hu, L.; Chen, J.; Xing, X. Lattice dynamics and anharmonicity of CaZrF₆ from Raman spectroscopy and ab initio calculations. Mater. Chem. Phys. 2016, 180, 213– 218, DOI: 10.1016/j.matchemphys.2016.05.067
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Lattice dynamics and anharmonicity of CaZrF6 from Raman spectroscopy and ab initio calculations
Sanson, Andrea; Giarola, Marco; Mariotto, Gino; Hu, Lei; Chen, Jun; Xing, Xianran
Materials Chemistry and Physics (2016), 180 (), 213-218CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
Very recently it has been found that CaZrF6 exhibits a very large and isotropic neg. thermal expansion (NTE), even greater than the current most popular NTE materials. In this work, the vibrational dynamics of CaZrF6 has been investigated by temp.-dependent Raman spectroscopy combined with ab initio calcns. As expected on the basis of the group theory for CaZrF6, three Raman-active modes were identified: the F2g mode peaked at about 236 cm-1, the Eg mode at around 550-555 cm-1, and the Ag mode peaked at about 637 cm-1. The temp. dependence of their frequencies follows an unusual trend: the F2g mode, due to bending vibrations of fluorine atoms in the linear Ca-F-Zr chain, is hardened with increasing temp., while the Ag mode, corresponding to Ca-F-Zr bond stretching vibrations, is softened. We explain this anomalous behavior by sepg. implicit and explicit anharmonicity for both F2g and Ag modes. In fact, cubic anharmonicity (three-phonon processes) is obsd. to dominate the higher-frequency Ag phonon-mode, quartic anharmonicity (four-phonon processes) is found to dominate the lower-frequency F2g phonon-mode. As a result, the large NTE of CaZrF6 cannot be accurately predicted through the quasi-harmonic approxn.
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Georgieva, I.; Trendafilova, N.; Dodoff, N.; Kovacheva, D. DFT study of the molecular and crystal structure and vibrational analysis of cisplatin. Spectrochim. Acta, Part A 2017, 176, 58– 66, DOI: 10.1016/j.saa.2017.01.008
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DFT study of the molecular and crystal structure and vibrational analysis of cisplatin
Georgieva, I.; Trendafilova, N.; Dodoff, N.; Kovacheva, D.
Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2017), 176 (), 58-66CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)
DFT and periodic-DFT (PAW-PBE method, code VASP) calcns. have been performed to study the structural and vibrational characteristics of cis-diamminedichloroplatinum(II) (cisplatin) at mol. and outside mol. level. To est. the effect of the intermol. interactions in crystal on the structural and vibrational properties of cisplatin, three theor. models are considered in the present study: monomer (isolated mol.), hydrogen bonded dimer and periodic solid state structures. The work focused on the role of the theor. models for correct modeling and prediction of geometrical and vibrational parameters of cisplatin. It has been found that the elaborate three-dimensional intermol. hydrogen bonding network in the cryst. cisplatin significantly influences the structural and vibrational pattern of cisplatin and therefore the isolated cisplatin mol. is not the correct computational model regardless of the theor. level used. To account for the whole intermol. hydrogen bonding network in direction of both a and c axis and for more reliable calcns. of structural and vibrational parameters periodic DFT calcns. were carried out in the full cryst. periodic environment with the known lattice parameters for each cisplatin polymorph phase. The model calcns. performed both at mol. level and for the periodic structures of alpha and beta cisplatin polymorph forms revealed the decisive role of the extended theor. model for reliable prediction of the structural and vibrational characteristics of cisplatin. The powder diffraction pattern and the calcd. IR and Raman spectra predicted beta polymorph form of our cisplatin sample freshly synthesized for the purposes of the present study using the Dhara's method. The various rotamers realized in the polymorph forms of cisplatin were explained by the low population of the large no. of rotamers in soln. as well as with the high rotamer interconversion rate due to the low energy barrier.
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Demichelis, R.; Noel, Y.; Civalleri, B.; Roetti, C.; Ferrero, M.; Dovesi, R. The vibrational spectrum of α-AlOOH diaspore: An ab initio study with the CRYSTAL code. J. Phys. Chem. B 2007, 111, 9337– 9346, DOI: 10.1021/jp072501d
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The Vibrational Spectrum of α-AlOOH Diaspore: An Ab Initio Study with the CRYSTAL Code
Demichelis, R.; Noel, Y.; Civalleri, B.; Roetti, C.; Ferrero, M.; Dovesi, R.
Journal of Physical Chemistry B (2007), 111 (31), 9337-9346CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The vibrational spectrum of α-AlOOH diaspore has been calcd. at the B3LYP level of theory with a double-ζ quality Gaussian-type basis set by using the periodic ab initio CRYSTAL code. Harmonic frequencies at the Γ point and the corresponding 48 normal modes are analyzed and classified in terms of simple models (octahedra modes, hydrogen stretching, bending, rotations) by direct inspection of eigenvectors, graphical representation, and isotopic substitution. Hydrogen modes are fully sepd. from the octahedra modes appearing under 800 cm-1; bending modes are located in the range of 1040-1290 cm-1, whereas stretching modes appear at 3130-3170 cm-1. The available exptl. IR and Raman spectra are characterized by broad bands, in some cases as large as 800 cm-1, and individual peaks are obtained by decompg. these bands in terms of Lorentz-Gauss product functions; such a fitting procedure is affected by a relatively large degree of arbitrariness. The comparison of our calcd. data with the most complete sets of exptl. data shows, nevertheless, a relatively good agreement for all but the H modes; the mean abs. differences for modes not involving H are 10.9 and 7.2 cm-1 for the IR and the Raman spectra, resp., the max. differences being 15.5 and 18.2 cm-1. For the H bending modes, differences increase to 30 and 37 cm-1, and for the stretching modes, the calcd. frequencies are about 200 cm-1 higher than the exptl. ones; this is not surprising, as anharmonicity is expected to red shift the OH stretching by about 150 cm-1 in isolated OH groups and even more when the latter is involved in strong hydrogen bonds, as is the case here.
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Kong, F.; Liu, Y.; Wang, B.; Wang, Y.; Wang, L. Lattice dynamics of PbTe polymorphs from first principles. Comput. Mater. Sci. 2012, 56, 18– 24, DOI: 10.1016/j.commatsci.2011.12.030
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Lattice dynamics of PbTe polymorphs from first principles
Kong, Fanjie; Liu, Yanhua; Wang, Baolin; Wang, Yanzong; Wang, Lili
Computational Materials Science (2012), 56 (), 18-24CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
The structure, electronic structure, lattice dynamics and thermodn. of PbTe polymorphs (B1, Pnma and B2 phase) are systematically investigated via first principle calcns. The calcd. lattice consts. of PbTe polymorphs are in agreement with the exptl. ones. The B1 and Pnma phases are semiconductors with band gap 0.866 and 0.331 eV resp. while the B2 phase is metal. The lattice dynamical results regarding the phonon dispersion curves, phonon d. of states and thermodn. properties are reported, which may provide useful data for further investigation of the phonon contribution to the thermoelec. properties. The vibrational modes at the T point are analyzed using group theory as well as the IR and Raman spectra activities of the optical phonon branches for the studied polymorphs. Thermodn. such as enthalpy, free energy, the entropy, heat capacity and Debye temp. in a temp. range of 0-300 K are detd. The present calcn. results are in good agreement with the exptl. results.
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Diery, W. A.; Moujes, E. A.; Nunes, R. W. Nature of localized phonon modes of tilt grain boundaries in graphene. Carbon 2018, 140, 250– 258, DOI: 10.1016/j.carbon.2018.08.045
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Nature of localized phonon modes of tilt grain boundaries in graphene
Diery, W. A.; Moujaes, Elie A.; Nunes, R. W.
Carbon (2018), 140 (), 250-258CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
In this work, it is reported an investigation of the phonon spectrum of polycryst. models of graphene, using ab-initio calcns. of the phonon dispersion and phonon modes of these systems. Four different models of polycryst. graphene are considered: one where the lattices of adjacent grains have a relative translation between them, with a translational grain boundary connecting the grains, and three models where the lattices of adjacent grains are tilted relative to each other, with tilt grain boundaries connecting adjacent grains. It is found that tilt grain boundaries introduce high-frequency non-dispersive phonon modes in the phonon spectrum of polycryst. graphene, and that these modes are strongly localized in the core of the grain boundaries, while no such high-frequency localized modes are found in the case of the translational boundary system. By computing phonon group velocities and sp. heats of each system in our study, and by analyzing in detail the patterns of at. displacements of these localized modes, a consistent interpretation is provided for the exptl. obsd. trends of the lattice thermal cond. of polycryst. graphene samples, as a function of the grain-boundary tilt angle.
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Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S. First-principles study of zinc oxide honeycomb structures. Phys. Rev. B 2009, 80, 235119, DOI: 10.1103/PhysRevB.80.235119
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First-principles study of zinc oxide honeycomb structures
Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S.
Physical Review B: Condensed Matter and Materials Physics (2009), 80 (23), 235119/1-235119/14CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
The authors present a 1st-principles study of the at., electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. To reveal the dimensionality effects, the authors' study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2-dimensional ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temp. ab initio mol.-dynamics calcns. 2-dimensional ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of Zn-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the O atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with H atoms. From the phonon calcns., the 4th acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons were examd. under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties.
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Stoffel, R. P.; Wessel, C.; Lumey, M.-W.; Dronskowski, R. Ab initio thermochemistry of solid-state materials. Angew. Chem., Int. Ed. 2010, 49, 5242– 5266, DOI: 10.1002/anie.200906780
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Ab Initio Thermochemistry of Solid-State Materials
Stoffel, Ralf Peter; Wessel, Claudia; Lumey, Marck-Willem; Dronskowski, Richard
Angewandte Chemie, International Edition (2010), 49 (31), 5242-5266CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. The authors introduce an electronic-structure-theory-based approach to a quantum-chem. thermochem. of solids. The authors first deal with local and collective at. displacements and explain how to calc. these. The fundamental importance of the phonons, their dispersion relations, their exptl. detn. as well as their calcn. is elucidated, followed by the systematic construction of the thermodn. potentials on this basis. Subsequently, an introduction is provided for practical computation as well as a crit. anal. of the level of accuracy obtainable. It is shown how different solid-state chem. problems can be solved using this approach. Among these are the calcn. of activation energies in perovskite-like oxides, but also the use of theor. vibrational frequencies for detg. crystal structures is considered. The pressure and temp. polymorphism of elemental tin which has often been classically described is also treated, and the authors energetically classify the metastable oxynitrides of tantalum. Using the case of high-temp. superconductors, it is demonstrated that such calcns. may be used for an independent evaluation of thermochem. data of unsatisfactory accuracy. Finally, the present limits and the future challenges of the theory are discussed.
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Homberg, J.; Weismann, A.; Markussen, T.; Berndt, R. Resonance-enhanced vibrational spectroscopy of molecules on a superconductor. Phys. Rev. Lett. 2022, 129, 116801, DOI: 10.1103/PhysRevLett.129.116801
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Resonance-Enhanced Vibrational Spectroscopy of Molecules on a Superconductor
Homberg, Jan; Weismann, Alexander; Markussen, Troels; Berndt, Richard
Physical Review Letters (2022), 129 (11), 116801CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
Mol. vibrational spectroscopy with the scanning tunneling microscope is feasible but usually detects few vibrational modes. We harness sharp Yu-Shiba-Rusinov states obsd. from mols. on a superconductor to significantly enhance the vibrational signal. From a lead phthalocyanine mol. 46 vibrational peaks are resolved enabling a comparison with calcd. modes. The energy resoln. is improved beyond the thermal broadening limit and shifts induced by neighbor mols. or the position of the microscope tip are detd. Vice versa, spectra of vibrational modes are used to measure the effect of an elec. field on the energy of Yu-Shiba-Rusinov states. The method may help to further probe the interaction of mols. with their environment and to better understand selection rules for vibrational excitations.
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Fuente, E.; Menéndez, J. A.; Diez, M. A.; Suárez, D.; Montes-Morán, M. A. Infrared spectroscopy of carbon materials: A quantum chemical study of model compounds. J. Phys. Chem. B 2003, 107, 6350– 6359, DOI: 10.1021/jp027482g
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Infrared Spectroscopy of Carbon Materials: A Quantum Chemical Study of Model Compounds
Fuente, E.; Menendez, J. A.; Diez, M. A.; Suarez, D.; Montes-Moran, M. A.
Journal of Physical Chemistry B (2003), 107 (26), 6350-6359CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The present work reports a theor. study of the IR spectra of chem. structures that are suitable to the description of the surface chem. of carbon materials. Prior to any consideration, the computational approach was tested and adapted by comparing the predicted IR spectra to those obtained exptl. for various ref. compds. Several models were considered, subsequently accounting for the most relevant functional groups that have been postulated to decorate the edges of graphene layers on carbon materials (i.e., anhydrides, carboxyls, lactones, phenolic, quinones, and pyrones). For each of the previous functional groups, different structures involving a different no. of fused rings were considered. This strategy allowed us to establish the effect of conjugation on the shift of the IR frequencies corresponding to a given functional group. Cooperative effects between different functional groups (phenol-carboxyl, phenol-lactone, and so on) were another aspect that revealed itself to be an interesting issue when assigning frequencies in the IR spectra of highly oxidized carbon materials. Thus, it was found that the frequencies of the C:O bonds present in acid functional groups were systematically lowered when phenolic groups were close enough to establish hydrogen bonds. Special attention was also paid to the elucidation of the origin of the 1600-cm-1 band of carbons. It was found that, in the case of acid carbons, this band can be assigned to C:C stretching of carbon rings decorated mainly with phenolic groups. Cyclic ethers in basic carbons would also promote absorption in the 1600-cm-1 region of the IR spectrum. Finally, the predicted assignments are employed to interpret the IR spectra obtained exptl. for several activated carbons.
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Pezzotti, S.; Serva, A.; Sebastiani, F.; Brigiano, F. S.; Galimberti, D. R.; Potier, L.; Alfarano, S.; Schwaab, G.; Havenith, M.; Gaigeot, M.-P. Molecular fingerprints of hydrophobicity at aqueous interfaces from theory and vibrational spectroscopies. J. Phys. Chem. Lett. 2021, 12, 3827– 3836, DOI: 10.1021/acs.jpclett.1c00257
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Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies
Pezzotti, Simone; Serva, Alessandra; Sebastiani, Federico; Brigiano, Flavio Siro; Galimberti, Daria Ruth; Potier, Louis; Alfarano, Serena; Schwaab, Gerhard; Havenith, Martina; Gaigeot, Marie-Pierre
Journal of Physical Chemistry Letters (2021), 12 (15), 3827-3836CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Hydrophobicity/hydrophilicity of aq. interfaces at the mol. level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via d. functional theory-mol. dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the no. of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of mol. hydrophobicity at aq. interfaces.
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Ngouana-Wakou, B. F.; Cornette, P.; Corral Valero, M.; Costa, D.; Raybaud, P. An atomistic description of the γ-alumina/water interface revealed by ab initio molecular dynamics. J. Phys. Chem. C 2017, 121, 10351– 10363, DOI: 10.1021/acs.jpcc.7b00101
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An Atomistic Description of the γ-Alumina/Water Interface Revealed by Ab Initio Molecular Dynamics
Ngouana-Wakou, B. F.; Cornette, P.; Corral Valero, M.; Costa, D.; Raybaud, P.
Journal of Physical Chemistry C (2017), 121 (19), 10351-10363CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The authors report ab initio mol. dynamics (AIMD) simulations of the (100) and (110) γ-Al2O3/water interfaces at 300 K, using two sets of supercell models for each surface and two time lengths of simulation (10 and 40 ps). The authors first show that the effect of liq. water on the vibrational frequencies of hydroxyl groups at the interface varies according to the type of surface. This trend is explained by two key parameters affecting the interaction of both surfaces with water: the nature of the OH groups (i.e., μ1-OH, μ1-H2O, μ2-OH, and μ3-OH) and H-bond network among surface OH groups. The hydroxylated (110) surface favors the local structuration of water at the interface and the solvation of its μ1-OH and μ1-H2O groups by water similarly as in bulk liq. water. By contrast, on the (100) surface, a stronger H-bond network among μ1-OH and μ1-H2O groups reduces the water/surface interaction. The authors illustrate also how the interfacial interacting sites are spatially organized on the surfaces by two-dimensional maps of O-H distances. On both surfaces, the interfacial water layer orientation is predominantly Hup-Hdown. For long AIMD simulation time, Grotthuss-like mechanisms are identified on the (110) surface.
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Nejad, A.; Meyer, E.; Suhm, M. A. Glycolic acid as a vibrational anharmonicity benchmark. J. Phys. Chem. Lett. 2020, 11, 5228– 5233, DOI: 10.1021/acs.jpclett.0c01462
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Glycolic Acid as a Vibrational Anharmonicity Benchmark
Nejad, Arman; Meyer, Enno; Suhm, Martin A.
Journal of Physical Chemistry Letters (2020), 11 (13), 5228-5233CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Three energetically close pairs of vibrational states in glycolic acid are investigated by Raman spectroscopy in a supersonic jet to provide challenging benchmarks for vibrational and electronic structure theory and to solve some open issues in this prototypical hydroxy acid. The alc. OH stretching fundamental is located only 8 cm-1 below the acidic OH stretch at 3586 cm-1, much less shifted than predicted by previous anharmonic calcns. and by exptl. analogy to a fluorene deriv. This and further near-degeneracies in the CH and C=O stretching region are used to assess the predictive power of an exploratory set of quantum chem. calcns. including anharmonic VPT2 corrections.
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Teodoro, T. Q.; Koenis, M. A. J.; Galembeck, S. E.; Nicu, V. P.; Buma, W. J.; Visscher, L. Frequency range selection method for vibrational spectra. J. Phys. Chem. Lett. 2018, 9, 6878– 6882, DOI: 10.1021/acs.jpclett.8b02963
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Frequency Range Selection Method for Vibrational Spectra
Teodoro, T. Q.; Koenis, M. A. J.; Galembeck, S. E.; Nicu, V. P.; Buma, W. J.; Visscher, L.
Journal of Physical Chemistry Letters (2018), 9 (23), 6878-6882CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Theor. calcns. of vibrational properties are widely used to explain and predict exptl. spectra. However, with std. quantum chem. methods all mol. motions are considered, which is rather time-consuming for large mols. Because typically only a specific spectral region is of exptl. interest, the authors propose here an efficient method that allows calcn. of only a selected frequency interval. After a computationally cheap low-level est. of the mol. motions, the computational time is proportional to the no. of normal modes needed to describe this frequency range. Results for a medium-sized mol. show a redn. in computational time of up to 1 order of magnitude with negligible loss in accuracy. Also still larger computational savings are possible by using an addnl. intensity-selection procedure.
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Beckmann, R.; Brieuc, F.; Schran, C.; Marx, D. Infrared spectra at coupled cluster accuracy from neural network representations. J. Chem. Theory Comput. 2022, 18, 5492– 5501, DOI: 10.1021/acs.jctc.2c00511
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Infrared Spectra at Coupled Cluster Accuracy from Neural Network Representations
Beckmann, Richard; Brieuc, Fabien; Schran, Christoph; Marx, Dominik
Journal of Chemical Theory and Computation (2022), 18 (9), 5492-5501CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
IR spectroscopy is key to elucidating mol. structures, monitoring reactions, and observing conformational changes, while providing information on both structural and dynamical properties. This makes the accurate prediction of IR spectra based on first-principle theories a highly desirable pursuit. Mol. dynamics simulations have proven to be a particularly powerful approach for this task, albeit requiring the computation of energies, forces and dipole moments for a large no. of mol. configurations as a function of time. This explains why highly accurate first-principles methods, such as coupled cluster theory, have so far been inapplicable for the prediction of fully anharmonic vibrational spectra of large systems at finite temps. Here, we push cutting-edge machine learning techniques forward by using neural network representations of energies, forces, and in particular dipoles to predict such IR spectra fully at "gold std." coupled cluster accuracy as demonstrated for protonated water clusters as large as the protonated water hexamer, in its extended Zundel configuration. Furthermore, we show that this methodol. can be used beyond the scope of the data considered during the development of the neural network models, allowing for the computation of finite-temp. IR spectra of large systems inaccessible to explicit coupled cluster calcns. This substantially expands the hitherto existing limits of accuracy, speed, and system size for theor. spectroscopy and opens up a multitude of avenues for the prediction of vibrational spectra and the understanding of complex intra- and intermol. couplings.
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Hu, W.; Ye, Sh.; Zhang, Y.; Li, T.; Zhang, G.; Luo, Y.; Mukamel, Sh.; Jiang, J. Machine learning protocol for surface-enhanced Raman spectroscopy. J. Phys. Chem. Lett. 2019, 10, 6026– 6031, DOI: 10.1021/acs.jpclett.9b02517
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Machine Learning Protocol for Surface-Enhanced Raman Spectroscopy
Hu, Wei; Ye, Sheng; Zhang, Yujin; Li, Tianduo; Zhang, Guozhen; Luo, Yi; Mukamel, Shaul; Jiang, Jun
Journal of Physical Chemistry Letters (2019), 10 (20), 6026-6031CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Surface-enhanced Raman spectroscopy (SERS) is a powerful technique that can capture the electronic-vibrational fingerprint of mols. on surfaces. Ab initio prediction of Raman response is a long-standing challenge because of the diversified interfacial structures. A cost-effective machine learning (ML) random forest method can predict SERS signals of a trans-1,2-bis(4-pyridyl)ethylene (BPE) mol. adsorbed on a Au substrate. Using geometric descriptors extd. from quantum chem. simulations of thousands of ab initio mol. dynamics conformations, the ML protocol predicts vibrational frequencies and Raman intensities. The resulting spectra agree with d. functional theory calcns. and expt. Predicted SERS responses of the mol. on different surfaces, or under external fields of elec. fields and solvent environment, demonstrate the good transferability of the protocol.
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Prezhdo, O. V. Advancing physical chemistry with machine learning. J. Phys. Chem. Lett. 2020, 11, 9656– 9658, DOI: 10.1021/acs.jpclett.0c03130
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Advancing Physical Chemistry with Machine Learning
Prezhdo, Oleg V.
Journal of Physical Chemistry Letters (2020), 11 (22), 9656-9658CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
There is no expanded citation for this reference.
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Kistanov, A. A.; Scherbinin, S. A.; Botella, R.; Davletshin, A.; Cao, W. Family of two-dimensional transition metal dichlorides. J. Phys. Chem. Lett. 2022, 13, 2165– 2172, DOI: 10.1021/acs.jpclett.2c00367
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Family of Two-Dimensional Transition Metal Dichlorides: Fundamental Properties, Structural Defects, and Environmental Stability
Kistanov, Andrey A.; Shcherbinin, Stepan A.; Botella, Romain; Davletshin, Artur; Cao, Wei
Journal of Physical Chemistry Letters (2022), 13 (9), 2165-2172CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
A large no. of novel two-dimensional (2D) materials are constantly being discovered and deposited in databases. Consolidated implementation of machine learning algorithms and d. functional theory (DFT)-based predictions have allowed the creation of several databases contg. an unimaginable no. of 2D samples. As the next step in this chain, the investigation leads to a comprehensive study of the functionality of the invented materials. In this work, a family of transition metal dichlorides have been screened out for systematic investigation of their structural stability, fundamental properties, structural defects, and environmental stability via DFT-based calcns. The work highlights the importance of using the potential of the invented materials and proposes a comprehensive characterization of a new family of 2D materials.
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Mounet, N.; Gibertini, M.; Schwaller, P. Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds. Nat. Nanotechnol. 2018, 13, 246– 252, DOI: 10.1038/s41565-017-0035-5
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Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds
Mounet, Nicolas; Gibertini, Marco; Schwaller, Philippe; Campi, Davide; Merkys, Andrius; Marrazzo, Antimo; Sohier, Thibault; Castelli, Ivano Eligio; Cepellotti, Andrea; Pizzi, Giovanni; Marzari, Nicola
Nature Nanotechnology (2018), 13 (3), 246-252CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compds. Starting from 108,423 unique, exptl. known 3D compds., we identify a subset of 5,619 compds. that appear layered according to robust geometric and bonding criteria. High-throughput calcns. using van der Waals d. functional theory, validated against exptl. structural data and calcd. RPA binding energies, further allowed the identification of 1,825 compds. that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compds. as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compds., we explore vibrational, electronic, magnetic and topol. properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.
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Lee, J.; Seko, A.; Shitara, K.; Nakayama, K.; Tanaka, I. Prediction model of band gap for inorganic compounds by combination of density functional theory calculations and machine learning techniques. Phys. Rev. B 2016, 93, 115104, DOI: 10.1103/PhysRevB.93.115104
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Prediction model of band gap for inorganic compounds by combination of density functional theory calculations and machine learning techniques
Lee, Joohwi; Seko, Atsuto; Shitara, Kazuki; Nakayama, Keita; Tanaka, Isao
Physical Review B (2016), 93 (11), 115104/1-115104/12CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
Machine learning techniques are applied to make prediction models of the G0W0 band gaps for 270 inorg. compds. using Kohn-Sham (KS) band gaps, cohesive energy, cryst. vol. per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least abs. shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approxn. of Perciew-Burke-Emzerhof (PBE) or modified Becke-Johnson (mat) is used as a single predictor, the OLSR model predicts the G0W0 band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mat methods are used together with a set of predictors representing constituent elements and compds., the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estd. in this way should be useful as predictors for virtual screening of a large set of materials.
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Zheng, P.; Zubatyuk, R.; Wu, W.; Isayev, O.; Dral, P. O. Artificial intelligence-enhanced quantum chemical method with broad applicability. Nat. Commun. 2021, 12, 7022, DOI: 10.1038/s41467-021-27340-2
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Artificial intelligence-enhanced quantum chemical method with broad applicability
Zheng, Peikun; Zubatyuk, Roman; Wu, Wei; Isayev, Olexandr; Dral, Pavlo O.
Nature Communications (2021), 12 (1), 7022CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
Abstr.: High-level quantum mech. (QM) calcns. are indispensable for accurate explanation of natural phenomena on the atomistic level. Their staggering computational cost, however, poses great limitations, which luckily can be lifted to a great extent by exploiting advances in artificial intelligence (AI). Here we introduce the general-purpose, highly transferable artificial intelligence-quantum mech. method 1 (AIQM1). It approaches the accuracy of the gold-std. coupled cluster QM method with high computational speed of the approx. low-level semiempirical QM methods for the neutral, closed-shell species in the ground state. AIQM1 can provide accurate ground-state energies for diverse org. compds. as well as geometries for even challenging systems such as large conjugated compds. (fullerene C60) close to expt. This opens an opportunity to investigate chem. compds. with previously unattainable speed and accuracy as we demonstrate by detg. geometries of polyyne mols.-the task difficult for both expt. and theory. Noteworthy, our method's accuracy is also good for ions and excited-state properties, although the neural network part of AIQM1 was never fitted to these properties.
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Keith, J. A.; Vassilev-Galindo, V.; Cheng, B.; Chmiela, S.; Gastegger, M.; Müller, K.-R.; Tkatchenko, A. Combining machine learning and computational chemistry for predictive insights into chemical systems. Chem. Rev. 2021, 121, 9816– 9872, DOI: 10.1021/acs.chemrev.1c00107
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Combining Machine Learning and Computational Chemistry for Predictive Insights Into Chemical Systems
Keith, John A.; Vassilev-Galindo, Valentin; Cheng, Bingqing; Chmiela, Stefan; Gastegger, Michael; Muller, Klaus-Robert; Tkatchenko, Alexandre
Chemical Reviews (Washington, DC, United States) (2021), 121 (16), 9816-9872CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Machine learning models are poised to make a transformative impact on chem. sciences by dramatically accelerating computational algorithms and amplifying insights available from computational chem. methods. However, achieving this requires a confluence and coaction of expertise in computer science and phys. sciences. This Review is written for new and experienced researchers working at the intersection of both fields. We first provide concise tutorials of computational chem. and machine learning methods, showing how insights involving both can be achieved. We, then, follow with a crit. review of noteworthy applications that demonstrate how computational chem. and machine learning can be used together to provide insightful (and useful) predictions in mol. and materials modeling, retrosyntheses, catalysis, and drug design.
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Choi, J.-H.; Cui, P.; Lan, H.; Zhang, Zh. Linear scaling of the exciton binding energy versus the band gap of two-dimensional materials. Phys. Rev. Lett. 2015, 115, 066403, DOI: 10.1103/PhysRevLett.115.066403
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Linear scaling of the exciton binding energy versus the band gap of two-dimensional materials
Choi, Jin-Ho; Cui, Ping; Lan, Haiping; Zhang, Zhenyu
Physical Review Letters (2015), 115 (6), 066403/1-066403/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
The exciton is one of the most crucial phys. entities in the performance of optoelectronic and photonic devices, and widely varying exciton binding energies have been reported in different classes of materials. Using first-principles calcns. within the GW-Bethe-Salpeter equation approach, here we investigate the excitonic properties of two recently discovered layered materials: phosphorene and graphene fluoride. We first confirm large exciton binding energies of, resp., 0.85 and 2.03 eV in these systems. Next, by comparing these systems with several other representative two-dimensional materials, we discover a striking linear relationship between the exciton binding energy and the band gap and interpret the existence of the linear scaling law within a simple hydrogenic picture. The broad applicability of this novel scaling law is further demonstrated by using strained graphene fluoride. These findings are expected to stimulate related studies in higher and lower dimensions, potentially resulting in a deeper understanding of excitonic effects in materials of all dimensionalities.
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Ahmad, S.; Zubair, M.; Jalil, O.; Mehmood, M. Q.; Younis, U.; Liu, X.; Ang, K. W.; Ang, L. K. Generalized scaling law for exciton binding energy in two-dimensional materials. Phys. Rev. Applied 2020, 13, 064062, DOI: 10.1103/PhysRevApplied.13.064062
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Generalized Scaling Law for Exciton Binding Energy in Two-Dimensional Materials
Ahmad, S.; Zubair, M.; Jalil, O.; Mehmood, M. Q.; Younis, U.; Liu, X.; Ang, K. W.; Ang, L. K.
Physical Review Applied (2020), 13 (6), 064062CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
Binding energy calcn. in two-dimensional (2D) materials is crucial in detg. their electronic and optical properties pertaining to enhanced Coulomb interactions between charge carriers due to quantum confinement and reduced dielec. screening. Based on full solns. of the Schrodinger equation in a screened hydrogen model with a modified Coulomb potential (1/rβ-2), we present a generalized and anal. scaling law for the exciton binding energy, Eβ=E0(αβb+c)(μ/ε2), where β is a fractional-dimension parameter that accounts for the reduced dielec. screening. The model is able to provide accurate binding energies, benchmarked using the reported Bethe-Salpeter equation and exptl. data, for 58 monolayer 2D and eight bulk materials, resp., through β. For a given material, β is varied from β = 3 for bulk three-dimensional materials to a value lying in the range 2.55-2.7 for 2D monolayer materials. With βmean = 2.625, our model improves the av. relative mean square error by a factor of 3 in comparison to existing models. The results can be used for Coulomb engineering of exciton binding energies in the optimal design of 2D materials.
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Saykin, D. R.; Kachorovskii, V. Yu.; Burmistrov, I. S. Phase diagram of a flexible two-dimensional material. Phys. Rev. Research 2020, 2, 043099, DOI: 10.1103/PhysRevResearch.2.043099
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Phase diagram of a flexible two-dimensional material
Saykin, D. R.; Kachorovskii, V. Yu.; Burmistrov, I. S.
Physical Review Research (2020), 2 (4), 043099CODEN: PRRHAI; ISSN:2643-1564. (American Physical Society)
Transport and elastic properties of freestanding two-dimensional materials are detd. by competition between dynamical and quenched out-of-plane deformations, i.e., between flexural phonons and ripples, resp. They both tend to crumple the system by overcoming the strong anharmonicity which stabilizes the flat phases. Despite active research, it still remains unclear whether the rippled phase exists in the thermodn. limit or is destroyed by thermal out-of-plane fluctuations. We demonstrate that a sufficiently strong short-range disorder stabilizes ripples, whereas in the case of a weak disorder the thermal flexural fluctuations dominate in the thermodn. limit. Therefore the phase diagram of a flexible two-dimensional material with a quenched short-range disorder has four distinct phases. These phases have drastically different elastic and transport properties that are of crucial importance for the emergent field of flexible nanoelectronics.
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Torun, E.; Sahin, H.; Singh, S. K.; Peters, F. M. Stable half-metallic monolayers of FeCl₂. Appl. Phys. Lett. 2015, 106, 192404, DOI: 10.1063/1.4921096
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Stable half-metallic monolayers of FeCl2
Torun, E.; Sahin, H.; Singh, S. K.; Peeters, F. M.
Applied Physics Letters (2015), 106 (19), 192404/1-192404/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
The structural, electronic, and magnetic properties of single layers of FeCl2 were calcd. using 1st principles calcns. The 1T phase of the single layer FeCl2 is 0.17 eV/unit cell more favorable than its 1H phase. The structural stability is confirmed by phonon calcns. 1T-FeCl2 possess three Raman-active (130, 179, and 237 cm-1) and one IR-active (279 cm-1) phonon branches. The electronic band dispersion of the 1T-FeCl2 is calcd. using both gradient approxn. of Perdew-Burke-Ernzerhof and DFT-HSE06 functionals. Both functionals reveal that the 1T-FeCl2 has a half-metallic ground state with a Curie temp. of 17 K. (c) 2015 American Institute of Physics.
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Yang, Y.; Guo, P.; Luo, Y. Strain modulated ferromagnetic phase transitions in monolayer FeCl₂ through exchange competitions: The first-principle and Monte Carlo simulations. Phys. Chem. Chem. Phys. 2020, 22, 17291– 17298, DOI: 10.1039/D0CP01422B
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Strain modulated ferromagnetic phase transitions in monolayer FeCl2 through exchange competitions: the first-principle and Monte Carlo simulations
Yang, Ya; Guo, Peiyin; Luo, Yongsong
Physical Chemistry Chemical Physics (2020), 22 (30), 17291-17298CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
Tunable magnetic phase transitions and novel emergent spin phases in two-dimensional materials are fascinating subjects of research. 1T-FeCl2 has been predicted to be a magnetic monolayer. We performed the first-principle calcns. based on d. functional theory to clarify the electronic structure and magnetic properties of the monolayer 1T-FeCl2 modulated by the uniaxial and biaxial strains. Based on the stable structure confirmed by the phonon calcns., we showed that the geometry and magnetic structures evolved with strain. In combination with the Monte Carlo simulation, we found that the strain could induce a phase transition between the in-plane ferromagnetic order and the out-of-plane anti-ferromagnetic order. Energy bands with the Hubburd U and spin-orbital couplings confirmed the insulator ground state. We identified the strain-magnetism behavior originating from the competition between the direct-exchange interaction and the super-exchange interaction. Meanwhile, the strains regulated the Curie temps. by selecting the d-p bonding along the x-direction or y-direction. Through strain engineering, the 1T-FeCl2 could be an intriguing platform for the two-dimensional systems and a potential spintronic material.
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Guo, H.; Chu, W.; Prezhdo, O. V.; Zheng, Q.; Zhao, J. Strong modulation of band gap, carrier mobility and lifetime in two-dimensional black phosphorene through acoustic phonon excitation. J. Phys. Chem. Lett. 2021, 12, 3960– 3967, DOI: 10.1021/acs.jpclett.1c00747
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Strong Modulation of Band Gap, Carrier Mobility and Lifetime in Two-Dimensional Black Phosphorene through Acoustic Phonon Excitation
Guo, Hongli; Chu, Weibin; Prezhdo, Oleg V.; Zheng, Qijing; Zhao, Jin
Journal of Physical Chemistry Letters (2021), 12 (16), 3960-3967CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Black phosphorene (BP) has been attracting intense attention due to its high charge mobility and potential applications in electronic, optical and optoelectronic devices. We demonstrate by ab initio mol. dynamics and nonadiabatic quantum dynamics simulations that the excitation of out-of-plane acoustic phonon (ZA) provides strong modulation of the band gap, carrier lifetime and carrier mobility in BP. A 1% tensile strain can significantly enhance ZA mode excitation at room temp., distinctly reducing the band gap, carrier mobility, and lifetime. These electronic properties can be tuned easily by influencing the excitation amplitude of the ZA mode. Unique to the family of two-dimensional materials, the ZA mode plays an essential role in controlling the electronic properties of BP. The results of our study provide valuable guidelines for design of functional nanodevices based on 2D BP.
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Katin, K. P.; Maslov, M. M.; Nikitenko, V. R.; Kochaev, A. I.; Kaya, S.; Prezhdo, O. V. Anisotropic carrier mobility and spectral fingerprints of two-dimensional γ-phosphorus carbide with antisite defects. J. Phys. Chem. Lett. 2023, 14, 214– 220, DOI: 10.1021/acs.jpclett.2c03297
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Anisotropic Carrier Mobility and Spectral Fingerprints of Two-Dimensional γ-Phosphorus Carbide with Antisite Defects
Katin, Konstantin P.; Maslov, Mikhail M.; Nikitenko, Vladimir R.; Kochaev, Alexey I.; Kaya, Savas; Prezhdo, Oleg V.
Journal of Physical Chemistry Letters (2023), 14 (1), 214-220CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
We apply d. functional theory to study carrier mobility in a γ-phosphorus carbide monolayer. Although previous calcns. predicted high and anisotropic mobility in this material, we show that the mobility can be significantly influenced by common antisite defects. We demonstrate that at equil. concns. defects do not inhibit carrier mobility up to temps. of 1000 K. However, defects can change the mobility at high nonequil. concns. of about 10-4 to 10-2 defects per atom. At the low end of this concn. range, defects act as traps for charge carriers and inhibit their mobility. At the high end of this range, defects change the effective carrier masses and deformation potentials, and they can lead to both an increase and a decrease in mobility. We also report the Raman and IR spectra assocd. with antisite defects. We predict new vibrational modes and shifts of the existing modes due to the defects.
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Kistanov, A. A.; Shcherbinin, S. A.; Korznikova, E. A.; Prezhdo, O. V. Prediction and Characterization of two-dimensional Zn₂VN₃. J. Phys. Chem. Lett. 2023, 14, 1148– 1155, DOI: 10.1021/acs.jpclett.2c03796
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Prediction and Characterization of Two-Dimensional Zn2VN3
Kistanov, Andrey A.; Shcherbinin, Stepan A.; Korznikova, Elena A.; Prezhdo, Oleg V.
Journal of Physical Chemistry Letters (2023), 14 (5), 1148-1155CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
A two-dimensional (2D) monolayer of a novel ternary nitride Zn2VN3 is computationally designed, and its dynamical and thermal stability is demonstrated. A synthesis strategy is proposed based on exptl. works on prodn. of ternary nitride thin films, calcns. of formation and exfoliation energies, and ab initio mol. dynamics simulations. A comprehensive characterization of 2D Zn2VN3, including investigation of its optoelectronic and mech. properties, is conducted. It is shown that 2D Zn2VN3 is a semiconductor with an indirect band gap of 2.75 eV and a high work function of 5.27 eV. Its light absorption covers visible and UV regions. The band gap of 2D Zn2VN3 is found to be well tunable by applied strain. At the same time 2D Zn2VN3 possesses high stability against mech. loads, point defects, and environmental impacts. Considering the unique properties found for 2D Zn2VN3, it can be used for application in optoelectronic and straintronic nanodevices.
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Choi, S. H.; Yun, S. J.; Won, Y. S.; Oh, Ch. S.; Kim, S. M.; Kim, K. K.; Lee, Y. H. Large-scale synthesis of graphene and other 2D materials towards industrialization. Nat. Commun. 2022, 13, 1484, DOI: 10.1038/s41467-022-29182-y
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Large-scale synthesis of graphene and other 2D materials towards industrialization
Choi, Soo Ho; Yun, Seok Joon; Won, Yo Seob; Oh, Chang Seok; Kim, Soo Min; Kim, Ki Kang; Lee, Young Hee
Nature Communications (2022), 13 (1), 1484CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
The effective application of graphene and other 2D materials is strongly dependent on the industrial-scale manufg. of films and powders of appropriate morphol. and quality. Here, we discuss three state-of-the-art mass prodn. techniques, their limitations, and opportunities for future improvement.
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An in situ attenuated total reflection (ATR)-Fourier transform IR (FTIR) method, in which the ATR element is coated with hematite particle layers in direct contact with the aq. phase, was used to measure sulfate adsorption as a function of aq. sulfate concn. and pH. Between pH 3 and 6, monitoring the spectral region 900-1300 cm-1, an IR spectrum with three bands between 950 and 1150 cm-1, indicative of C3υ symmetry and monodentate sulfate coordination, was obsd. The spectral amplitudes varied with sulfate concn. and soln. pH in agreement with independent measurements and as described by quant. surface complexation models. Previous studies on dried samples found a spectrum with a fourth max. (consistent with C2υ symmetry) at >1200 cm-1, which has been interpreted as evidence of bidentate coordination. In the present study, a fourth band at 1200 cm-1 appeared only on drying of the sulfated hematite layer or with an aq. phase pH <2. Based on these observations and on a comparison with various reported IR measurements, it is suggested that in the presence of an aq. phase between pH 3 and 5, predominantly monodentate sulfate surface complexes are formed on hematite. Spectral changes on removal of the solvent indicate formation of monodentate bisulfate or of bidentate sulfate on dry hematite.
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Scientific data (2019), 6 (1), 86 ISSN:.
Two-dimensional (2D) materials have been a hot research topic in the last decade, due to novel fundamental physics in the reduced dimension and appealing applications. Systematic discovery of functional 2D materials has been the focus of many studies. Here, we present a large dataset of 2D materials, with more than 6,000 monolayer structures, obtained from both top-down and bottom-up discovery procedures. First, we screened all bulk materials in the database of Materials Project for layered structures by a topology-based algorithm and theoretically exfoliated them into monolayers. Then, we generated new 2D materials by chemical substitution of elements in known 2D materials by others from the same group in the periodic table. The structural, electronic and energetic properties of these 2D materials are consistently calculated, to provide a starting point for further material screening, data mining, data analysis and artificial intelligence applications. We present the details of computational methodology, data record and technical validation of our publicly available data ( http://www.2dmatpedia.org/ ).
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Abstract
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Figure 1
Figure 1. Different representations of all of the vibrational modes of the FeCl₂ monolayer with (a) a discrete (Dirac peaks) view, (b) a superposition of the discrete view and the KpDOS, and (c) IWKDE. Arrows in (b) and (c) highlight the main signals.
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Figure 2
Figure 2. (a) Representation of the FeCl₂ monolayer unit cell and its symmetry operations, (b) decision tree diagram for the classification of the modes in the different irreps, (c) z-polarized discrete (Dirac) intensities (blue) of the modes of the FeCl₂ monolayer, (d) x-polarized discrete (Dirac) intensities (red) of the modes of the FeCl₂ monolayer, and (e) y-polarized discrete (Dirac) intensities (green) of the modes of the FeCl₂ monolayer. The nomenclature is from ref (46).
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Figure 3
Figure 3. Different representations of the IR-active vibrational modes of the FeCl₂ monolayer with (a) a discrete (Dirac peaks) view, (b) the KpDOS, and (c) the IWKDE. Arrows in (b) and (c) highlight the main signals.
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Figure 4
Figure 4. Superposition of (a) the IR Dirac modes, the associated IWKDE spectrum, and the associated IWKDE spectrum and (b) IR- and Raman-active mode IR IWKDE spectra. Arrows highlight the main bands, and the Dirac intensity peaks have been normalized to be compared with IWKDE spectra.
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This article references 50 other publications.
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  First-principles calcns. have become a powerful tool to exclude the Edisonian approach in search of novel two-dimensional (2D) materials. However, no universal first-principles criteria to examine the realizability of hypothetical 2D materials have been established in the literature yet. Because of this, and as the calcns. are always performed in an artificial simulation environment, one can unintentionally study compds. that do not exist in expts. Although investigations of physics and chem. of unrealizable materials can provide some fundamental knowledge, the discussion of their applications can mislead experimentalists for years and increase the gap between exptl. and theor. research. By analyzing energy convex hull, phonon spectra, and structure evolution during ab initio mol. dynamics simulations for a range of synthesized and recently proposed 2D materials, we construct energy, phonon, and dynamic stability filters that need to be satisfied before proposing novel 2D compds. We demonstrate the power of the suggested filters for several selected 2D systems, revealing that some of them cannot be ever realized exptl.
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  The Vibrational Spectrum of α-AlOOH Diaspore: An Ab Initio Study with the CRYSTAL Code
  Demichelis, R.; Noel, Y.; Civalleri, B.; Roetti, C.; Ferrero, M.; Dovesi, R.
  Journal of Physical Chemistry B (2007), 111 (31), 9337-9346CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The vibrational spectrum of α-AlOOH diaspore has been calcd. at the B3LYP level of theory with a double-ζ quality Gaussian-type basis set by using the periodic ab initio CRYSTAL code. Harmonic frequencies at the Γ point and the corresponding 48 normal modes are analyzed and classified in terms of simple models (octahedra modes, hydrogen stretching, bending, rotations) by direct inspection of eigenvectors, graphical representation, and isotopic substitution. Hydrogen modes are fully sepd. from the octahedra modes appearing under 800 cm-1; bending modes are located in the range of 1040-1290 cm-1, whereas stretching modes appear at 3130-3170 cm-1. The available exptl. IR and Raman spectra are characterized by broad bands, in some cases as large as 800 cm-1, and individual peaks are obtained by decompg. these bands in terms of Lorentz-Gauss product functions; such a fitting procedure is affected by a relatively large degree of arbitrariness. The comparison of our calcd. data with the most complete sets of exptl. data shows, nevertheless, a relatively good agreement for all but the H modes; the mean abs. differences for modes not involving H are 10.9 and 7.2 cm-1 for the IR and the Raman spectra, resp., the max. differences being 15.5 and 18.2 cm-1. For the H bending modes, differences increase to 30 and 37 cm-1, and for the stretching modes, the calcd. frequencies are about 200 cm-1 higher than the exptl. ones; this is not surprising, as anharmonicity is expected to red shift the OH stretching by about 150 cm-1 in isolated OH groups and even more when the latter is involved in strong hydrogen bonds, as is the case here.
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8. 8
  Kong, F.; Liu, Y.; Wang, B.; Wang, Y.; Wang, L. Lattice dynamics of PbTe polymorphs from first principles. Comput. Mater. Sci. 2012, 56, 18– 24, DOI: 10.1016/j.commatsci.2011.12.030
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  8
  Lattice dynamics of PbTe polymorphs from first principles
  Kong, Fanjie; Liu, Yanhua; Wang, Baolin; Wang, Yanzong; Wang, Lili
  Computational Materials Science (2012), 56 (), 18-24CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
  The structure, electronic structure, lattice dynamics and thermodn. of PbTe polymorphs (B1, Pnma and B2 phase) are systematically investigated via first principle calcns. The calcd. lattice consts. of PbTe polymorphs are in agreement with the exptl. ones. The B1 and Pnma phases are semiconductors with band gap 0.866 and 0.331 eV resp. while the B2 phase is metal. The lattice dynamical results regarding the phonon dispersion curves, phonon d. of states and thermodn. properties are reported, which may provide useful data for further investigation of the phonon contribution to the thermoelec. properties. The vibrational modes at the T point are analyzed using group theory as well as the IR and Raman spectra activities of the optical phonon branches for the studied polymorphs. Thermodn. such as enthalpy, free energy, the entropy, heat capacity and Debye temp. in a temp. range of 0-300 K are detd. The present calcn. results are in good agreement with the exptl. results.
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9. 9
  Diery, W. A.; Moujes, E. A.; Nunes, R. W. Nature of localized phonon modes of tilt grain boundaries in graphene. Carbon 2018, 140, 250– 258, DOI: 10.1016/j.carbon.2018.08.045
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  9
  Nature of localized phonon modes of tilt grain boundaries in graphene
  Diery, W. A.; Moujaes, Elie A.; Nunes, R. W.
  Carbon (2018), 140 (), 250-258CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
  In this work, it is reported an investigation of the phonon spectrum of polycryst. models of graphene, using ab-initio calcns. of the phonon dispersion and phonon modes of these systems. Four different models of polycryst. graphene are considered: one where the lattices of adjacent grains have a relative translation between them, with a translational grain boundary connecting the grains, and three models where the lattices of adjacent grains are tilted relative to each other, with tilt grain boundaries connecting adjacent grains. It is found that tilt grain boundaries introduce high-frequency non-dispersive phonon modes in the phonon spectrum of polycryst. graphene, and that these modes are strongly localized in the core of the grain boundaries, while no such high-frequency localized modes are found in the case of the translational boundary system. By computing phonon group velocities and sp. heats of each system in our study, and by analyzing in detail the patterns of at. displacements of these localized modes, a consistent interpretation is provided for the exptl. obsd. trends of the lattice thermal cond. of polycryst. graphene samples, as a function of the grain-boundary tilt angle.
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10. 10
  Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S. First-principles study of zinc oxide honeycomb structures. Phys. Rev. B 2009, 80, 235119, DOI: 10.1103/PhysRevB.80.235119
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  10
  First-principles study of zinc oxide honeycomb structures
  Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S.
  Physical Review B: Condensed Matter and Materials Physics (2009), 80 (23), 235119/1-235119/14CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
  The authors present a 1st-principles study of the at., electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. To reveal the dimensionality effects, the authors' study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2-dimensional ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temp. ab initio mol.-dynamics calcns. 2-dimensional ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of Zn-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the O atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with H atoms. From the phonon calcns., the 4th acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons were examd. under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties.
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11. 11
  Stoffel, R. P.; Wessel, C.; Lumey, M.-W.; Dronskowski, R. Ab initio thermochemistry of solid-state materials. Angew. Chem., Int. Ed. 2010, 49, 5242– 5266, DOI: 10.1002/anie.200906780
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  11
  Ab Initio Thermochemistry of Solid-State Materials
  Stoffel, Ralf Peter; Wessel, Claudia; Lumey, Marck-Willem; Dronskowski, Richard
  Angewandte Chemie, International Edition (2010), 49 (31), 5242-5266CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. The authors introduce an electronic-structure-theory-based approach to a quantum-chem. thermochem. of solids. The authors first deal with local and collective at. displacements and explain how to calc. these. The fundamental importance of the phonons, their dispersion relations, their exptl. detn. as well as their calcn. is elucidated, followed by the systematic construction of the thermodn. potentials on this basis. Subsequently, an introduction is provided for practical computation as well as a crit. anal. of the level of accuracy obtainable. It is shown how different solid-state chem. problems can be solved using this approach. Among these are the calcn. of activation energies in perovskite-like oxides, but also the use of theor. vibrational frequencies for detg. crystal structures is considered. The pressure and temp. polymorphism of elemental tin which has often been classically described is also treated, and the authors energetically classify the metastable oxynitrides of tantalum. Using the case of high-temp. superconductors, it is demonstrated that such calcns. may be used for an independent evaluation of thermochem. data of unsatisfactory accuracy. Finally, the present limits and the future challenges of the theory are discussed.
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12. 12
  Homberg, J.; Weismann, A.; Markussen, T.; Berndt, R. Resonance-enhanced vibrational spectroscopy of molecules on a superconductor. Phys. Rev. Lett. 2022, 129, 116801, DOI: 10.1103/PhysRevLett.129.116801
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  12
  Resonance-Enhanced Vibrational Spectroscopy of Molecules on a Superconductor
  Homberg, Jan; Weismann, Alexander; Markussen, Troels; Berndt, Richard
  Physical Review Letters (2022), 129 (11), 116801CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
  Mol. vibrational spectroscopy with the scanning tunneling microscope is feasible but usually detects few vibrational modes. We harness sharp Yu-Shiba-Rusinov states obsd. from mols. on a superconductor to significantly enhance the vibrational signal. From a lead phthalocyanine mol. 46 vibrational peaks are resolved enabling a comparison with calcd. modes. The energy resoln. is improved beyond the thermal broadening limit and shifts induced by neighbor mols. or the position of the microscope tip are detd. Vice versa, spectra of vibrational modes are used to measure the effect of an elec. field on the energy of Yu-Shiba-Rusinov states. The method may help to further probe the interaction of mols. with their environment and to better understand selection rules for vibrational excitations.
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13. 13
  Fuente, E.; Menéndez, J. A.; Diez, M. A.; Suárez, D.; Montes-Morán, M. A. Infrared spectroscopy of carbon materials: A quantum chemical study of model compounds. J. Phys. Chem. B 2003, 107, 6350– 6359, DOI: 10.1021/jp027482g
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  Infrared Spectroscopy of Carbon Materials: A Quantum Chemical Study of Model Compounds
  Fuente, E.; Menendez, J. A.; Diez, M. A.; Suarez, D.; Montes-Moran, M. A.
  Journal of Physical Chemistry B (2003), 107 (26), 6350-6359CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The present work reports a theor. study of the IR spectra of chem. structures that are suitable to the description of the surface chem. of carbon materials. Prior to any consideration, the computational approach was tested and adapted by comparing the predicted IR spectra to those obtained exptl. for various ref. compds. Several models were considered, subsequently accounting for the most relevant functional groups that have been postulated to decorate the edges of graphene layers on carbon materials (i.e., anhydrides, carboxyls, lactones, phenolic, quinones, and pyrones). For each of the previous functional groups, different structures involving a different no. of fused rings were considered. This strategy allowed us to establish the effect of conjugation on the shift of the IR frequencies corresponding to a given functional group. Cooperative effects between different functional groups (phenol-carboxyl, phenol-lactone, and so on) were another aspect that revealed itself to be an interesting issue when assigning frequencies in the IR spectra of highly oxidized carbon materials. Thus, it was found that the frequencies of the C:O bonds present in acid functional groups were systematically lowered when phenolic groups were close enough to establish hydrogen bonds. Special attention was also paid to the elucidation of the origin of the 1600-cm-1 band of carbons. It was found that, in the case of acid carbons, this band can be assigned to C:C stretching of carbon rings decorated mainly with phenolic groups. Cyclic ethers in basic carbons would also promote absorption in the 1600-cm-1 region of the IR spectrum. Finally, the predicted assignments are employed to interpret the IR spectra obtained exptl. for several activated carbons.
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14. 14
  Pezzotti, S.; Serva, A.; Sebastiani, F.; Brigiano, F. S.; Galimberti, D. R.; Potier, L.; Alfarano, S.; Schwaab, G.; Havenith, M.; Gaigeot, M.-P. Molecular fingerprints of hydrophobicity at aqueous interfaces from theory and vibrational spectroscopies. J. Phys. Chem. Lett. 2021, 12, 3827– 3836, DOI: 10.1021/acs.jpclett.1c00257
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  14
  Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies
  Pezzotti, Simone; Serva, Alessandra; Sebastiani, Federico; Brigiano, Flavio Siro; Galimberti, Daria Ruth; Potier, Louis; Alfarano, Serena; Schwaab, Gerhard; Havenith, Martina; Gaigeot, Marie-Pierre
  Journal of Physical Chemistry Letters (2021), 12 (15), 3827-3836CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Hydrophobicity/hydrophilicity of aq. interfaces at the mol. level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via d. functional theory-mol. dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the no. of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of mol. hydrophobicity at aq. interfaces.
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15. 15
  Ngouana-Wakou, B. F.; Cornette, P.; Corral Valero, M.; Costa, D.; Raybaud, P. An atomistic description of the γ-alumina/water interface revealed by ab initio molecular dynamics. J. Phys. Chem. C 2017, 121, 10351– 10363, DOI: 10.1021/acs.jpcc.7b00101
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  15
  An Atomistic Description of the γ-Alumina/Water Interface Revealed by Ab Initio Molecular Dynamics
  Ngouana-Wakou, B. F.; Cornette, P.; Corral Valero, M.; Costa, D.; Raybaud, P.
  Journal of Physical Chemistry C (2017), 121 (19), 10351-10363CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The authors report ab initio mol. dynamics (AIMD) simulations of the (100) and (110) γ-Al2O3/water interfaces at 300 K, using two sets of supercell models for each surface and two time lengths of simulation (10 and 40 ps). The authors first show that the effect of liq. water on the vibrational frequencies of hydroxyl groups at the interface varies according to the type of surface. This trend is explained by two key parameters affecting the interaction of both surfaces with water: the nature of the OH groups (i.e., μ1-OH, μ1-H2O, μ2-OH, and μ3-OH) and H-bond network among surface OH groups. The hydroxylated (110) surface favors the local structuration of water at the interface and the solvation of its μ1-OH and μ1-H2O groups by water similarly as in bulk liq. water. By contrast, on the (100) surface, a stronger H-bond network among μ1-OH and μ1-H2O groups reduces the water/surface interaction. The authors illustrate also how the interfacial interacting sites are spatially organized on the surfaces by two-dimensional maps of O-H distances. On both surfaces, the interfacial water layer orientation is predominantly Hup-Hdown. For long AIMD simulation time, Grotthuss-like mechanisms are identified on the (110) surface.
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16. 16
  Nejad, A.; Meyer, E.; Suhm, M. A. Glycolic acid as a vibrational anharmonicity benchmark. J. Phys. Chem. Lett. 2020, 11, 5228– 5233, DOI: 10.1021/acs.jpclett.0c01462
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  16
  Glycolic Acid as a Vibrational Anharmonicity Benchmark
  Nejad, Arman; Meyer, Enno; Suhm, Martin A.
  Journal of Physical Chemistry Letters (2020), 11 (13), 5228-5233CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Three energetically close pairs of vibrational states in glycolic acid are investigated by Raman spectroscopy in a supersonic jet to provide challenging benchmarks for vibrational and electronic structure theory and to solve some open issues in this prototypical hydroxy acid. The alc. OH stretching fundamental is located only 8 cm-1 below the acidic OH stretch at 3586 cm-1, much less shifted than predicted by previous anharmonic calcns. and by exptl. analogy to a fluorene deriv. This and further near-degeneracies in the CH and C=O stretching region are used to assess the predictive power of an exploratory set of quantum chem. calcns. including anharmonic VPT2 corrections.
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17. 17
  Teodoro, T. Q.; Koenis, M. A. J.; Galembeck, S. E.; Nicu, V. P.; Buma, W. J.; Visscher, L. Frequency range selection method for vibrational spectra. J. Phys. Chem. Lett. 2018, 9, 6878– 6882, DOI: 10.1021/acs.jpclett.8b02963
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  Frequency Range Selection Method for Vibrational Spectra
  Teodoro, T. Q.; Koenis, M. A. J.; Galembeck, S. E.; Nicu, V. P.; Buma, W. J.; Visscher, L.
  Journal of Physical Chemistry Letters (2018), 9 (23), 6878-6882CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Theor. calcns. of vibrational properties are widely used to explain and predict exptl. spectra. However, with std. quantum chem. methods all mol. motions are considered, which is rather time-consuming for large mols. Because typically only a specific spectral region is of exptl. interest, the authors propose here an efficient method that allows calcn. of only a selected frequency interval. After a computationally cheap low-level est. of the mol. motions, the computational time is proportional to the no. of normal modes needed to describe this frequency range. Results for a medium-sized mol. show a redn. in computational time of up to 1 order of magnitude with negligible loss in accuracy. Also still larger computational savings are possible by using an addnl. intensity-selection procedure.
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18. 18
  Beckmann, R.; Brieuc, F.; Schran, C.; Marx, D. Infrared spectra at coupled cluster accuracy from neural network representations. J. Chem. Theory Comput. 2022, 18, 5492– 5501, DOI: 10.1021/acs.jctc.2c00511
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  18
  Infrared Spectra at Coupled Cluster Accuracy from Neural Network Representations
  Beckmann, Richard; Brieuc, Fabien; Schran, Christoph; Marx, Dominik
  Journal of Chemical Theory and Computation (2022), 18 (9), 5492-5501CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
  IR spectroscopy is key to elucidating mol. structures, monitoring reactions, and observing conformational changes, while providing information on both structural and dynamical properties. This makes the accurate prediction of IR spectra based on first-principle theories a highly desirable pursuit. Mol. dynamics simulations have proven to be a particularly powerful approach for this task, albeit requiring the computation of energies, forces and dipole moments for a large no. of mol. configurations as a function of time. This explains why highly accurate first-principles methods, such as coupled cluster theory, have so far been inapplicable for the prediction of fully anharmonic vibrational spectra of large systems at finite temps. Here, we push cutting-edge machine learning techniques forward by using neural network representations of energies, forces, and in particular dipoles to predict such IR spectra fully at "gold std." coupled cluster accuracy as demonstrated for protonated water clusters as large as the protonated water hexamer, in its extended Zundel configuration. Furthermore, we show that this methodol. can be used beyond the scope of the data considered during the development of the neural network models, allowing for the computation of finite-temp. IR spectra of large systems inaccessible to explicit coupled cluster calcns. This substantially expands the hitherto existing limits of accuracy, speed, and system size for theor. spectroscopy and opens up a multitude of avenues for the prediction of vibrational spectra and the understanding of complex intra- and intermol. couplings.
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19. 19
  Hu, W.; Ye, Sh.; Zhang, Y.; Li, T.; Zhang, G.; Luo, Y.; Mukamel, Sh.; Jiang, J. Machine learning protocol for surface-enhanced Raman spectroscopy. J. Phys. Chem. Lett. 2019, 10, 6026– 6031, DOI: 10.1021/acs.jpclett.9b02517
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  Machine Learning Protocol for Surface-Enhanced Raman Spectroscopy
  Hu, Wei; Ye, Sheng; Zhang, Yujin; Li, Tianduo; Zhang, Guozhen; Luo, Yi; Mukamel, Shaul; Jiang, Jun
  Journal of Physical Chemistry Letters (2019), 10 (20), 6026-6031CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Surface-enhanced Raman spectroscopy (SERS) is a powerful technique that can capture the electronic-vibrational fingerprint of mols. on surfaces. Ab initio prediction of Raman response is a long-standing challenge because of the diversified interfacial structures. A cost-effective machine learning (ML) random forest method can predict SERS signals of a trans-1,2-bis(4-pyridyl)ethylene (BPE) mol. adsorbed on a Au substrate. Using geometric descriptors extd. from quantum chem. simulations of thousands of ab initio mol. dynamics conformations, the ML protocol predicts vibrational frequencies and Raman intensities. The resulting spectra agree with d. functional theory calcns. and expt. Predicted SERS responses of the mol. on different surfaces, or under external fields of elec. fields and solvent environment, demonstrate the good transferability of the protocol.
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20. 20
  Prezhdo, O. V. Advancing physical chemistry with machine learning. J. Phys. Chem. Lett. 2020, 11, 9656– 9658, DOI: 10.1021/acs.jpclett.0c03130
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  Advancing Physical Chemistry with Machine Learning
  Prezhdo, Oleg V.
  Journal of Physical Chemistry Letters (2020), 11 (22), 9656-9658CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  There is no expanded citation for this reference.
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21. 21
  Kistanov, A. A.; Scherbinin, S. A.; Botella, R.; Davletshin, A.; Cao, W. Family of two-dimensional transition metal dichlorides. J. Phys. Chem. Lett. 2022, 13, 2165– 2172, DOI: 10.1021/acs.jpclett.2c00367
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  Family of Two-Dimensional Transition Metal Dichlorides: Fundamental Properties, Structural Defects, and Environmental Stability
  Kistanov, Andrey A.; Shcherbinin, Stepan A.; Botella, Romain; Davletshin, Artur; Cao, Wei
  Journal of Physical Chemistry Letters (2022), 13 (9), 2165-2172CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  A large no. of novel two-dimensional (2D) materials are constantly being discovered and deposited in databases. Consolidated implementation of machine learning algorithms and d. functional theory (DFT)-based predictions have allowed the creation of several databases contg. an unimaginable no. of 2D samples. As the next step in this chain, the investigation leads to a comprehensive study of the functionality of the invented materials. In this work, a family of transition metal dichlorides have been screened out for systematic investigation of their structural stability, fundamental properties, structural defects, and environmental stability via DFT-based calcns. The work highlights the importance of using the potential of the invented materials and proposes a comprehensive characterization of a new family of 2D materials.
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22. 22
  Mounet, N.; Gibertini, M.; Schwaller, P. Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds. Nat. Nanotechnol. 2018, 13, 246– 252, DOI: 10.1038/s41565-017-0035-5
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  Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds
  Mounet, Nicolas; Gibertini, Marco; Schwaller, Philippe; Campi, Davide; Merkys, Andrius; Marrazzo, Antimo; Sohier, Thibault; Castelli, Ivano Eligio; Cepellotti, Andrea; Pizzi, Giovanni; Marzari, Nicola
  Nature Nanotechnology (2018), 13 (3), 246-252CODEN: NNAABX; ISSN:1748-3387. (Nature Research)
  Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compds. Starting from 108,423 unique, exptl. known 3D compds., we identify a subset of 5,619 compds. that appear layered according to robust geometric and bonding criteria. High-throughput calcns. using van der Waals d. functional theory, validated against exptl. structural data and calcd. RPA binding energies, further allowed the identification of 1,825 compds. that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compds. as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compds., we explore vibrational, electronic, magnetic and topol. properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.
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23. 23
  Lee, J.; Seko, A.; Shitara, K.; Nakayama, K.; Tanaka, I. Prediction model of band gap for inorganic compounds by combination of density functional theory calculations and machine learning techniques. Phys. Rev. B 2016, 93, 115104, DOI: 10.1103/PhysRevB.93.115104
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  Prediction model of band gap for inorganic compounds by combination of density functional theory calculations and machine learning techniques
  Lee, Joohwi; Seko, Atsuto; Shitara, Kazuki; Nakayama, Keita; Tanaka, Isao
  Physical Review B (2016), 93 (11), 115104/1-115104/12CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
  Machine learning techniques are applied to make prediction models of the G0W0 band gaps for 270 inorg. compds. using Kohn-Sham (KS) band gaps, cohesive energy, cryst. vol. per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least abs. shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approxn. of Perciew-Burke-Emzerhof (PBE) or modified Becke-Johnson (mat) is used as a single predictor, the OLSR model predicts the G0W0 band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mat methods are used together with a set of predictors representing constituent elements and compds., the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estd. in this way should be useful as predictors for virtual screening of a large set of materials.
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24. 24
  Zheng, P.; Zubatyuk, R.; Wu, W.; Isayev, O.; Dral, P. O. Artificial intelligence-enhanced quantum chemical method with broad applicability. Nat. Commun. 2021, 12, 7022, DOI: 10.1038/s41467-021-27340-2
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  Artificial intelligence-enhanced quantum chemical method with broad applicability
  Zheng, Peikun; Zubatyuk, Roman; Wu, Wei; Isayev, Olexandr; Dral, Pavlo O.
  Nature Communications (2021), 12 (1), 7022CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  Abstr.: High-level quantum mech. (QM) calcns. are indispensable for accurate explanation of natural phenomena on the atomistic level. Their staggering computational cost, however, poses great limitations, which luckily can be lifted to a great extent by exploiting advances in artificial intelligence (AI). Here we introduce the general-purpose, highly transferable artificial intelligence-quantum mech. method 1 (AIQM1). It approaches the accuracy of the gold-std. coupled cluster QM method with high computational speed of the approx. low-level semiempirical QM methods for the neutral, closed-shell species in the ground state. AIQM1 can provide accurate ground-state energies for diverse org. compds. as well as geometries for even challenging systems such as large conjugated compds. (fullerene C60) close to expt. This opens an opportunity to investigate chem. compds. with previously unattainable speed and accuracy as we demonstrate by detg. geometries of polyyne mols.-the task difficult for both expt. and theory. Noteworthy, our method's accuracy is also good for ions and excited-state properties, although the neural network part of AIQM1 was never fitted to these properties.
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25. 25
  Keith, J. A.; Vassilev-Galindo, V.; Cheng, B.; Chmiela, S.; Gastegger, M.; Müller, K.-R.; Tkatchenko, A. Combining machine learning and computational chemistry for predictive insights into chemical systems. Chem. Rev. 2021, 121, 9816– 9872, DOI: 10.1021/acs.chemrev.1c00107
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  Combining Machine Learning and Computational Chemistry for Predictive Insights Into Chemical Systems
  Keith, John A.; Vassilev-Galindo, Valentin; Cheng, Bingqing; Chmiela, Stefan; Gastegger, Michael; Muller, Klaus-Robert; Tkatchenko, Alexandre
  Chemical Reviews (Washington, DC, United States) (2021), 121 (16), 9816-9872CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Machine learning models are poised to make a transformative impact on chem. sciences by dramatically accelerating computational algorithms and amplifying insights available from computational chem. methods. However, achieving this requires a confluence and coaction of expertise in computer science and phys. sciences. This Review is written for new and experienced researchers working at the intersection of both fields. We first provide concise tutorials of computational chem. and machine learning methods, showing how insights involving both can be achieved. We, then, follow with a crit. review of noteworthy applications that demonstrate how computational chem. and machine learning can be used together to provide insightful (and useful) predictions in mol. and materials modeling, retrosyntheses, catalysis, and drug design.
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26. 26
  Choi, J.-H.; Cui, P.; Lan, H.; Zhang, Zh. Linear scaling of the exciton binding energy versus the band gap of two-dimensional materials. Phys. Rev. Lett. 2015, 115, 066403, DOI: 10.1103/PhysRevLett.115.066403
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  Linear scaling of the exciton binding energy versus the band gap of two-dimensional materials
  Choi, Jin-Ho; Cui, Ping; Lan, Haiping; Zhang, Zhenyu
  Physical Review Letters (2015), 115 (6), 066403/1-066403/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  The exciton is one of the most crucial phys. entities in the performance of optoelectronic and photonic devices, and widely varying exciton binding energies have been reported in different classes of materials. Using first-principles calcns. within the GW-Bethe-Salpeter equation approach, here we investigate the excitonic properties of two recently discovered layered materials: phosphorene and graphene fluoride. We first confirm large exciton binding energies of, resp., 0.85 and 2.03 eV in these systems. Next, by comparing these systems with several other representative two-dimensional materials, we discover a striking linear relationship between the exciton binding energy and the band gap and interpret the existence of the linear scaling law within a simple hydrogenic picture. The broad applicability of this novel scaling law is further demonstrated by using strained graphene fluoride. These findings are expected to stimulate related studies in higher and lower dimensions, potentially resulting in a deeper understanding of excitonic effects in materials of all dimensionalities.
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27. 27
  Ahmad, S.; Zubair, M.; Jalil, O.; Mehmood, M. Q.; Younis, U.; Liu, X.; Ang, K. W.; Ang, L. K. Generalized scaling law for exciton binding energy in two-dimensional materials. Phys. Rev. Applied 2020, 13, 064062, DOI: 10.1103/PhysRevApplied.13.064062
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  Generalized Scaling Law for Exciton Binding Energy in Two-Dimensional Materials
  Ahmad, S.; Zubair, M.; Jalil, O.; Mehmood, M. Q.; Younis, U.; Liu, X.; Ang, K. W.; Ang, L. K.
  Physical Review Applied (2020), 13 (6), 064062CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
  Binding energy calcn. in two-dimensional (2D) materials is crucial in detg. their electronic and optical properties pertaining to enhanced Coulomb interactions between charge carriers due to quantum confinement and reduced dielec. screening. Based on full solns. of the Schrodinger equation in a screened hydrogen model with a modified Coulomb potential (1/rβ-2), we present a generalized and anal. scaling law for the exciton binding energy, Eβ=E0(αβb+c)(μ/ε2), where β is a fractional-dimension parameter that accounts for the reduced dielec. screening. The model is able to provide accurate binding energies, benchmarked using the reported Bethe-Salpeter equation and exptl. data, for 58 monolayer 2D and eight bulk materials, resp., through β. For a given material, β is varied from β = 3 for bulk three-dimensional materials to a value lying in the range 2.55-2.7 for 2D monolayer materials. With βmean = 2.625, our model improves the av. relative mean square error by a factor of 3 in comparison to existing models. The results can be used for Coulomb engineering of exciton binding energies in the optimal design of 2D materials.
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28. 28
  Saykin, D. R.; Kachorovskii, V. Yu.; Burmistrov, I. S. Phase diagram of a flexible two-dimensional material. Phys. Rev. Research 2020, 2, 043099, DOI: 10.1103/PhysRevResearch.2.043099
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  Phase diagram of a flexible two-dimensional material
  Saykin, D. R.; Kachorovskii, V. Yu.; Burmistrov, I. S.
  Physical Review Research (2020), 2 (4), 043099CODEN: PRRHAI; ISSN:2643-1564. (American Physical Society)
  Transport and elastic properties of freestanding two-dimensional materials are detd. by competition between dynamical and quenched out-of-plane deformations, i.e., between flexural phonons and ripples, resp. They both tend to crumple the system by overcoming the strong anharmonicity which stabilizes the flat phases. Despite active research, it still remains unclear whether the rippled phase exists in the thermodn. limit or is destroyed by thermal out-of-plane fluctuations. We demonstrate that a sufficiently strong short-range disorder stabilizes ripples, whereas in the case of a weak disorder the thermal flexural fluctuations dominate in the thermodn. limit. Therefore the phase diagram of a flexible two-dimensional material with a quenched short-range disorder has four distinct phases. These phases have drastically different elastic and transport properties that are of crucial importance for the emergent field of flexible nanoelectronics.
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29. 29
  Torun, E.; Sahin, H.; Singh, S. K.; Peters, F. M. Stable half-metallic monolayers of FeCl₂. Appl. Phys. Lett. 2015, 106, 192404, DOI: 10.1063/1.4921096
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  Stable half-metallic monolayers of FeCl2
  Torun, E.; Sahin, H.; Singh, S. K.; Peeters, F. M.
  Applied Physics Letters (2015), 106 (19), 192404/1-192404/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  The structural, electronic, and magnetic properties of single layers of FeCl2 were calcd. using 1st principles calcns. The 1T phase of the single layer FeCl2 is 0.17 eV/unit cell more favorable than its 1H phase. The structural stability is confirmed by phonon calcns. 1T-FeCl2 possess three Raman-active (130, 179, and 237 cm-1) and one IR-active (279 cm-1) phonon branches. The electronic band dispersion of the 1T-FeCl2 is calcd. using both gradient approxn. of Perdew-Burke-Ernzerhof and DFT-HSE06 functionals. Both functionals reveal that the 1T-FeCl2 has a half-metallic ground state with a Curie temp. of 17 K. (c) 2015 American Institute of Physics.
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30. 30
  Yang, Y.; Guo, P.; Luo, Y. Strain modulated ferromagnetic phase transitions in monolayer FeCl₂ through exchange competitions: The first-principle and Monte Carlo simulations. Phys. Chem. Chem. Phys. 2020, 22, 17291– 17298, DOI: 10.1039/D0CP01422B
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  Strain modulated ferromagnetic phase transitions in monolayer FeCl2 through exchange competitions: the first-principle and Monte Carlo simulations
  Yang, Ya; Guo, Peiyin; Luo, Yongsong
  Physical Chemistry Chemical Physics (2020), 22 (30), 17291-17298CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  Tunable magnetic phase transitions and novel emergent spin phases in two-dimensional materials are fascinating subjects of research. 1T-FeCl2 has been predicted to be a magnetic monolayer. We performed the first-principle calcns. based on d. functional theory to clarify the electronic structure and magnetic properties of the monolayer 1T-FeCl2 modulated by the uniaxial and biaxial strains. Based on the stable structure confirmed by the phonon calcns., we showed that the geometry and magnetic structures evolved with strain. In combination with the Monte Carlo simulation, we found that the strain could induce a phase transition between the in-plane ferromagnetic order and the out-of-plane anti-ferromagnetic order. Energy bands with the Hubburd U and spin-orbital couplings confirmed the insulator ground state. We identified the strain-magnetism behavior originating from the competition between the direct-exchange interaction and the super-exchange interaction. Meanwhile, the strains regulated the Curie temps. by selecting the d-p bonding along the x-direction or y-direction. Through strain engineering, the 1T-FeCl2 could be an intriguing platform for the two-dimensional systems and a potential spintronic material.
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31. 31
  Guo, H.; Chu, W.; Prezhdo, O. V.; Zheng, Q.; Zhao, J. Strong modulation of band gap, carrier mobility and lifetime in two-dimensional black phosphorene through acoustic phonon excitation. J. Phys. Chem. Lett. 2021, 12, 3960– 3967, DOI: 10.1021/acs.jpclett.1c00747
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  Strong Modulation of Band Gap, Carrier Mobility and Lifetime in Two-Dimensional Black Phosphorene through Acoustic Phonon Excitation
  Guo, Hongli; Chu, Weibin; Prezhdo, Oleg V.; Zheng, Qijing; Zhao, Jin
  Journal of Physical Chemistry Letters (2021), 12 (16), 3960-3967CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Black phosphorene (BP) has been attracting intense attention due to its high charge mobility and potential applications in electronic, optical and optoelectronic devices. We demonstrate by ab initio mol. dynamics and nonadiabatic quantum dynamics simulations that the excitation of out-of-plane acoustic phonon (ZA) provides strong modulation of the band gap, carrier lifetime and carrier mobility in BP. A 1% tensile strain can significantly enhance ZA mode excitation at room temp., distinctly reducing the band gap, carrier mobility, and lifetime. These electronic properties can be tuned easily by influencing the excitation amplitude of the ZA mode. Unique to the family of two-dimensional materials, the ZA mode plays an essential role in controlling the electronic properties of BP. The results of our study provide valuable guidelines for design of functional nanodevices based on 2D BP.
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32. 32
  Katin, K. P.; Maslov, M. M.; Nikitenko, V. R.; Kochaev, A. I.; Kaya, S.; Prezhdo, O. V. Anisotropic carrier mobility and spectral fingerprints of two-dimensional γ-phosphorus carbide with antisite defects. J. Phys. Chem. Lett. 2023, 14, 214– 220, DOI: 10.1021/acs.jpclett.2c03297
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  Anisotropic Carrier Mobility and Spectral Fingerprints of Two-Dimensional γ-Phosphorus Carbide with Antisite Defects
  Katin, Konstantin P.; Maslov, Mikhail M.; Nikitenko, Vladimir R.; Kochaev, Alexey I.; Kaya, Savas; Prezhdo, Oleg V.
  Journal of Physical Chemistry Letters (2023), 14 (1), 214-220CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  We apply d. functional theory to study carrier mobility in a γ-phosphorus carbide monolayer. Although previous calcns. predicted high and anisotropic mobility in this material, we show that the mobility can be significantly influenced by common antisite defects. We demonstrate that at equil. concns. defects do not inhibit carrier mobility up to temps. of 1000 K. However, defects can change the mobility at high nonequil. concns. of about 10-4 to 10-2 defects per atom. At the low end of this concn. range, defects act as traps for charge carriers and inhibit their mobility. At the high end of this range, defects change the effective carrier masses and deformation potentials, and they can lead to both an increase and a decrease in mobility. We also report the Raman and IR spectra assocd. with antisite defects. We predict new vibrational modes and shifts of the existing modes due to the defects.
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33. 33
  Kistanov, A. A.; Shcherbinin, S. A.; Korznikova, E. A.; Prezhdo, O. V. Prediction and Characterization of two-dimensional Zn₂VN₃. J. Phys. Chem. Lett. 2023, 14, 1148– 1155, DOI: 10.1021/acs.jpclett.2c03796
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  Prediction and Characterization of Two-Dimensional Zn2VN3
  Kistanov, Andrey A.; Shcherbinin, Stepan A.; Korznikova, Elena A.; Prezhdo, Oleg V.
  Journal of Physical Chemistry Letters (2023), 14 (5), 1148-1155CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  A two-dimensional (2D) monolayer of a novel ternary nitride Zn2VN3 is computationally designed, and its dynamical and thermal stability is demonstrated. A synthesis strategy is proposed based on exptl. works on prodn. of ternary nitride thin films, calcns. of formation and exfoliation energies, and ab initio mol. dynamics simulations. A comprehensive characterization of 2D Zn2VN3, including investigation of its optoelectronic and mech. properties, is conducted. It is shown that 2D Zn2VN3 is a semiconductor with an indirect band gap of 2.75 eV and a high work function of 5.27 eV. Its light absorption covers visible and UV regions. The band gap of 2D Zn2VN3 is found to be well tunable by applied strain. At the same time 2D Zn2VN3 possesses high stability against mech. loads, point defects, and environmental impacts. Considering the unique properties found for 2D Zn2VN3, it can be used for application in optoelectronic and straintronic nanodevices.
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34. 34
  Choi, S. H.; Yun, S. J.; Won, Y. S.; Oh, Ch. S.; Kim, S. M.; Kim, K. K.; Lee, Y. H. Large-scale synthesis of graphene and other 2D materials towards industrialization. Nat. Commun. 2022, 13, 1484, DOI: 10.1038/s41467-022-29182-y
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  Large-scale synthesis of graphene and other 2D materials towards industrialization
  Choi, Soo Ho; Yun, Seok Joon; Won, Yo Seob; Oh, Chang Seok; Kim, Soo Min; Kim, Ki Kang; Lee, Young Hee
  Nature Communications (2022), 13 (1), 1484CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
  The effective application of graphene and other 2D materials is strongly dependent on the industrial-scale manufg. of films and powders of appropriate morphol. and quality. Here, we discuss three state-of-the-art mass prodn. techniques, their limitations, and opportunities for future improvement.
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35. 35
  Mannix, A. J.; Kiraly, B.; Hersam, M. C.; Guisinger, N. P. Synthesis and chemistry of elemental 2D materials. Nat. Rev. Chem. 2017, 1, 0014, DOI: 10.1038/s41570-016-0014
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  Synthesis and chemistry of elemental 2D materials
  Mannix, Andrew J.; Kiraly, Brian; Hersam, Mark C.; Guisinger, Nathan P.
  Nature Reviews Chemistry (2017), 1 (2), 0014CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)
  A review. 2D materials have attracted considerable attention in the past decade for their superlative phys. properties. These materials consist of atomically thin sheets exhibiting covalent in-plane bonding and weak interlayer and layer-substrate bonding. Following the example of graphene, most emerging 2D materials are derived from structures that can be isolated from bulk phases of layered materials, which form a limited library for new materials discovery. Entirely synthetic 2D materials provide access to a greater range of properties through the choice of constituent elements and substrates. Of particular interest are elemental 2D materials, because they provide the most chem. tractable case for synthetic exploration. In this Review, we explore the progress made in the synthesis and chem. of synthetic elemental 2D materials, and offer perspectives and challenges for the future of this emerging field.
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36. 36
  Rambaut, A.; Drummond, A. J; Xie, D.; Baele, G.; Suchard, M. A Posterior summarization in Bayesian phylogenetics using tracer 1.7. Syst. Biol. 2018, 67, 901– 904, DOI: 10.1093/sysbio/syy032
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  Posterior summarization in bayesian phylogenetics using tracer 1.7
  Rambaut, Andrew; Drummond, Alexei J.; Xie, Dong; Baele, Guy; Suchard, Marc A.
  Systematic Biology (2018), 67 (5), 901-904CODEN: SYBIER; ISSN:1076-836X. (Oxford University Press)
  Bayesian inference of phylogeny using Markov chain Monte Carlo (MCMC) plays a central role in understanding evolutionary history from mol. sequence data. Visualizing and analyzing the MCMC-generated samples from the posterior distribution is a key step in any non-trivial Bayesian inference. We present the software package Tracer (version 1.7) for visualizing and analyzing the MCMC trace files generated through Bayesian phylogenetic inference. Tracer provides kernel d. estn., multivariate visualization, demog. trajectory reconstruction, conditional posterior distribution summary, and more.
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37. 37
  DiNardo, J.; Fortin, N. M.; Lemieux, T. Labor market institutions and the distribution of wages, 1973–1992: A semiparametric approach. Econometrica 1996, 64, 1001– 1044, DOI: 10.2307/2171954
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  Yang, Y.; Du, J.; Jing, C. Dynamic adsorption process of phthalate at goethite/aqueous interface: An ATR-FTIR study. Colloids Surf., A Physicochem. Eng. Asp. 2014, 441, 504– 509, DOI: 10.1016/j.colsurfa.2013.10.021
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  Hug, S. J. In situ Fourier transform infrared measurements of sulfate adsorption on hematite in aqueous solutions. J. Colloid Interface Sci. 1997, 188, 415– 422, DOI: 10.1006/jcis.1996.4755
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  In situ Fourier transform infrared measurements of sulfate adsorption on hematite in aqueous solutions
  Hug, Stephan J.
  Journal of Colloid and Interface Science (1997), 188 (2), 415-422CODEN: JCISA5; ISSN:0021-9797. (Academic)
  An in situ attenuated total reflection (ATR)-Fourier transform IR (FTIR) method, in which the ATR element is coated with hematite particle layers in direct contact with the aq. phase, was used to measure sulfate adsorption as a function of aq. sulfate concn. and pH. Between pH 3 and 6, monitoring the spectral region 900-1300 cm-1, an IR spectrum with three bands between 950 and 1150 cm-1, indicative of C3υ symmetry and monodentate sulfate coordination, was obsd. The spectral amplitudes varied with sulfate concn. and soln. pH in agreement with independent measurements and as described by quant. surface complexation models. Previous studies on dried samples found a spectrum with a fourth max. (consistent with C2υ symmetry) at >1200 cm-1, which has been interpreted as evidence of bidentate coordination. In the present study, a fourth band at 1200 cm-1 appeared only on drying of the sulfated hematite layer or with an aq. phase pH <2. Based on these observations and on a comparison with various reported IR measurements, it is suggested that in the presence of an aq. phase between pH 3 and 5, predominantly monodentate sulfate surface complexes are formed on hematite. Spectral changes on removal of the solvent indicate formation of monodentate bisulfate or of bidentate sulfate on dry hematite.
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  Boily, J.-F.; Szanyi, J.; Felmy, A. R. A combined FTIR and TPD study on the bulk and surface dihydroxylation and decarbonation of synthetic goethite. Geochim. Cosmochim. Ac. 2006, 70, 3613– 3624, DOI: 10.1016/j.gca.2006.05.013
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  Chio, C. H.; Sharma, S. K.; Muenow, D. W. The hydrates and deuterates of ferrous sulfate (FeSO₄): a Raman spectroscopic study. J. Raman. Spectrosc. 2007, 38, 87– 99, DOI: 10.1002/jrs.1623
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  The hydrates and deuterates of ferrous sulfate (FeSO4): a Raman spectroscopic study
  Chio, Chi Hong; Sharma, Shiv K.; Muenow, David W.
  Journal of Raman Spectroscopy (2007), 38 (1), 87-99CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
  A comparative anal. was carried out on the Raman spectra of FeSO4·nH2O (n =1, 4, 7) including the 2D-analogs. The effects of changing the degrees of hydration were found from the lattice, SO42- internal, and H2O internal modes. Increasing degrees of hydration shift the intense ν1(SO4) peak to lower wave-nos. and reduce the amt. of splitting on the ν3(SO4) peaks. Some of the water librational bands cause the broadening of the ν4(SO4) peaks in FeSO4·7H2O and the ν2(SO4) peaks in FeSO4·7D2O. The ν2(H2O) band in FeSO4·H2O is red-shifted in excess of 100 cm-1 relative to the unperturbed H2O band. Between 240 and 190 K and between 140 and 90 K in the spectra of FeSO4·4H2O, 2 potential phase transitions were identified from the changes in the lattice and water-stretching regions. The resoln. of the ν1(H2O) and ν3(H2O) bands in FeSO4·4H2O and FeSO4·H2O also improved sharply at low temps. The capability of distinguishing various forms of FeSO4 hydrates unambiguously makes the Raman technique a potential anal. tool for the identification of sulfate minerals on planetary surfaces.
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  Nash, K. L.; Sully, J.; Horn, A. B. Observations on the interpretation and analysis of sulfuric acid hydrate infrared spectra. J. Phys. Chem. A 2001, 105, 9422– 9426, DOI: 10.1021/jp0114541
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  Observations on the Interpretation and Analysis of Sulfuric Acid Hydrate Infrared Spectra
  Nash, Karen L.; Sully, K. Jessica; Horn, Andrew B.
  Journal of Physical Chemistry A (2001), 105 (41), 9422-9426CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  New IR spectra of pure bulk mol. H2SO4 and its mono- and tetrahydrates under equil. H2O partial pressure conditions were obtained using an internal reflection geometry. For each of the species present (H2SO4, HSO4- and SO42-), a well-defined set of core SO4 vibrations was identified and assigned to individual vibrational modes. Using these data, earlier literature data was reexamd. Existing methods for the quantification of compn. and ion speciation in H2SO4/H2O mixts. are examd. and the need for a more sophisticated method for the detn. of sulfate aerosol compn. is proposed.
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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c00665.
- Kernel density estimated phonon density of states (Figure S1); KpDOS profiles for the vibrational modes of FeCl₂ for different bandwidths (Figure S2); and wavenumbers, Cartesian polarized intensities, and corresponding irreps for the 4 × 4 × 1 supercell of the FeCl₂ monolayer (Table S1) (PDF)
- jz3c00665_si_001.pdf (312.17 kb)
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A Unified View of Vibrational Spectroscopy Simulation through Kernel Density Estimations

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